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05/08/21 | Activity-dependent Golgi satellite formation in dendrites reshapes the neuronal surface glycoproteome
Govind AP, Jeyifous O, Russell TA, Yi Z, Weigel AV, Ramaprasad A, Newell L, Ramos W, Valbuena FM, Casler JC, Yan JZ, Glick BS, Swanson GT, Lippincott-Schwartz J, Green WN
bioRxiv. 05/2021:. doi: 10.1101/2021.04.06.438745

Activity-driven changes in the neuronal surface glycoproteome are known to occur with synapse formation, plasticity and related diseases, but their mechanistic basis and significance are unclear. Here, we observed that N-glycans on surface glycoproteins of dendrites shift from immature to mature forms containing sialic acid in response to increased neuronal excitation. In exploring the basis of these N-glycosylation alterations, we discovered they result from the growth and proliferation of Golgi satellites scattered throughout the dendrite. Golgi satellites that formed with neuronal excitation were in close association with ER exit sites and early endosomes and contained glycosylation machinery without the Golgi structural protein, GM130. They functioned as distal glycosylation stations in dendrites, terminally modifying sugars either on newly synthesized glycoproteins passing through the secretory pathway, or on surface glycoproteins taken up from the endocytic pathway. These activities led to major changes in the dendritic surface of excited neurons, impacting binding and uptake of lectins, as well as causing functional changes in neurotransmitter receptors such as nicotinic acetylcholine receptors. Neural activity thus boosts the activity of the dendrite’s satellite micro-secretory system by redistributing Golgi enzymes involved in glycan modifications into peripheral Golgi satellites. This remodeling of the neuronal surface has potential significance for synaptic plasticity, addiction and disease.Competing Interest StatementThe authors have declared no competing interest.

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05/01/21 | RNA transport and local translation in neurodevelopmental and neurodegenerative disease.
Fernandopulle MS, Lippincott-Schwartz J, Ward ME
Nature Neuroscience. 2021 May 01;24(5):622-32. doi: 10.1038/s41593-020-00785-2

Neurons decentralize protein synthesis from the cell body to support the active metabolism of remote dendritic and axonal compartments. The neuronal RNA transport apparatus, composed of cis-acting RNA regulatory elements, neuronal transport granule proteins, and motor adaptor complexes, drives the long-distance RNA trafficking required for local protein synthesis. Over the past decade, advances in human genetics, subcellular biochemistry, and high-resolution imaging have implicated each member of the apparatus in several neurodegenerative diseases, establishing failed RNA transport and associated processes as a unifying pathomechanism. In this review, we deconstruct the RNA transport apparatus, exploring each constituent's role in RNA localization and illuminating their unique contributions to neurodegeneration.

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04/29/21 | ER-to-Golgi protein delivery through an interwoven, tubular network extending from ER.
Weigel AV, Chang CL, Shtengel G, Xu S, Hoffman DP, Freeman M, Iyer N, Aaron J, Khuon S, Bogovic J, Qiu W, Hess HF, Lippincott-Schwartz J
Cell. 2021 Apr 29;184(9):2412. doi: 10.1016/j.cell.2021.03.035

Cellular versatility depends on accurate trafficking of diverse proteins to their organellar destinations. For the secretory pathway (followed by approximately 30% of all proteins), the physical nature of the vessel conducting the first portage (endoplasmic reticulum [ER] to Golgi apparatus) is unclear. We provide a dynamic 3D view of early secretory compartments in mammalian cells with isotropic resolution and precise protein localization using whole-cell, focused ion beam scanning electron microscopy with cryo-structured illumination microscopy and live-cell synchronized cargo release approaches. Rather than vesicles alone, the ER spawns an elaborate, interwoven tubular network of contiguous lipid bilayers (ER exit site) for protein export. This receptacle is capable of extending microns along microtubules while still connected to the ER by a thin neck. COPII localizes to this neck region and dynamically regulates cargo entry from the ER, while COPI acts more distally, escorting the detached, accelerating tubular entity on its way to joining the Golgi apparatus through microtubule-directed movement.

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05/24/21 | A general method to improve fluorophores using deuterated auxochromes.
Grimm JB, Xie L, Casler JC, Patel R, Tkachuk AN, Falco N, Choi H, Lippincott-Schwartz J, Brown TA, Glick BS, Liu Z, Lavis LD
JACS Au. 2021 May 24;1(5):690-6. doi: 10.1021/jacsau.1c00006

Fluorescence microscopy relies on dyes that absorb and then emit photons. In addition to fluorescence, fluorophores can undergo photochemical processes that decrease quantum yield or result in spectral shifts and irreversible photobleaching. Chemical strategies that suppress these undesirable pathways—thereby increasing the brightness and photostability of fluorophores—are crucial for advancing the frontier of bioimaging. Here, we describe a general method to improve small-molecule fluorophores by incorporating deuterium into the alkylamino auxochromes of rhodamines and other dyes. This strategy increases fluorescence quantum yield, inhibits photochemically induced spectral shifts, and slows irreparable photobleaching, yielding next-generation labels with improved performance in cellular imaging experiments.

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03/03/21 | Actin cables and comet tails organize mitochondrial networks in mitosis.
Moore AS, Coscia SM, Simpson CL, Ortega FE, Wait EC, Heddleston JM, Nirschl JJ, Obara CJ, Guedes-Dias P, Boecker A, Chew TL, Theriot JA, Lippincott-Schwartz J, Holzbaur ELF
Nature. 2021 Mar 03;591(7851):659-664. doi: 10.1038/s41586-021-03309-5

Symmetric cell division requires the even partitioning of genetic information and cytoplasmic contents between daughter cells. Whereas the mechanisms coordinating the segregation of the genome are well known, the processes that ensure organelle segregation between daughter cells remain less well understood. Here we identify multiple actin assemblies with distinct but complementary roles in mitochondrial organization and inheritance in mitosis. First, we find a dense meshwork of subcortical actin cables assembled throughout the mitotic cytoplasm. This network scaffolds the endoplasmic reticulum and organizes three-dimensional mitochondrial positioning to ensure the equal segregation of mitochondrial mass at cytokinesis. Second, we identify a dynamic wave of actin filaments reversibly assembling on the surface of mitochondria during mitosis. Mitochondria sampled by this wave are enveloped within actin clouds that can spontaneously break symmetry to form elongated comet tails. Mitochondrial comet tails promote randomly directed bursts of movement that shuffle mitochondrial position within the mother cell to randomize inheritance of healthy and damaged mitochondria between daughter cells. Thus, parallel mechanisms mediated by the actin cytoskeleton ensure both equal and random inheritance of mitochondria in symmetrically dividing cells.

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02/10/21 | Biomolecular Condensates and Their Links to Cancer Progression.
Cai D, Liu Z, Lippincott-Schwartz J
Trends in Biochemical Sciences. 2021 Feb 10:. doi: 10.1016/j.tibs.2021.01.002

Liquid-liquid phase separation (LLPS) has emerged in recent years as an important physicochemical process for organizing diverse processes within cells via the formation of membraneless organelles termed biomolecular condensates. Emerging evidence now suggests that the formation and regulation of biomolecular condensates are also intricately linked to cancer formation and progression. We review the most recent literature linking the existence and/or dissolution of biomolecular condensates to different hallmarks of cancer formation and progression. We then discuss the opportunities that this condensate perspective provides for cancer research and the development of novel therapeutic approaches, including the perturbation of condensates by small-molecule inhibitors.

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02/01/21 | Image-based pooled whole-genome CRISPRi screening for subcellular phenotypes.
Kanfer G, Sarraf SA, Maman Y, Baldwin H, Dominguez-Martin E, Johnson KR, Ward ME, Kampmann M, Lippincott-Schwartz J, Youle RJ
Journal of Cell Biology. 2021 Feb 01;220(2):. doi: 10.1083/jcb.202006180

Genome-wide CRISPR screens have transformed our ability to systematically interrogate human gene function, but are currently limited to a subset of cellular phenotypes. We report a novel pooled screening approach for a wider range of cellular and subtle subcellular phenotypes. Machine learning and convolutional neural network models are trained on the subcellular phenotype to be queried. Genome-wide screening then utilizes cells stably expressing dCas9-KRAB (CRISPRi), photoactivatable fluorescent protein (PA-mCherry), and a lentiviral guide RNA (gRNA) pool. Cells are screened by using microscopy and classified by artificial intelligence (AI) algorithms, which precisely identify the genetically altered phenotype. Cells with the phenotype of interest are photoactivated and isolated via flow cytometry, and the gRNAs are identified by sequencing. A proof-of-concept screen accurately identified PINK1 as essential for Parkin recruitment to mitochondria. A genome-wide screen identified factors mediating TFEB relocation from the nucleus to the cytosol upon prolonged starvation. Twenty-one of the 64 hits called by the neural network model were independently validated, revealing new effectors of TFEB subcellular localization. This approach, AI-photoswitchable screening (AI-PS), offers a novel screening platform capable of classifying a broad range of mammalian subcellular morphologies, an approach largely unattainable with current methodologies at genome-wide scale.

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12/15/20 | In situ differentiation of iridophore crystallotypes underlies zebrafish stripe patterning.
Gur D, Bain EJ, Johnson KR, Aman AJ, Pasoili A, Flynn JD, Allen MC, Deheyn DD, Lee JC, Lippincott-Schwartz J, Parichy DM
Nature Communications. 2020 Dec 15;11(1):6391. doi: 10.1038/s41467-020-20088-1

Skin color patterns are ubiquitous in nature, impact social behavior, predator avoidance, and protection from ultraviolet irradiation. A leading model system for vertebrate skin patterning is the zebrafish; its alternating blue stripes and yellow interstripes depend on light-reflecting cells called iridophores. It was suggested that the zebrafish's color pattern arises from a single type of iridophore migrating differentially to stripes and interstripes. However, here we find that iridophores do not migrate between stripes and interstripes but instead differentiate and proliferate in-place, based on their micro-environment. RNA-sequencing analysis further reveals that stripe and interstripe iridophores have different transcriptomic states, while cryogenic-scanning-electron-microscopy and micro-X-ray diffraction identify different crystal-arrays architectures, indicating that stripe and interstripe iridophores are different cell types. Based on these results, we present an alternative model of skin patterning in zebrafish in which distinct iridophore crystallotypes containing specialized, physiologically responsive, organelles arise in stripe and interstripe by in-situ differentiation.

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12/01/20 | The evolution of a cell biologist.
Lippincott-Schwartz J
Molecular Biology of the Cell. 2020 Dec 01;31(25):2763-2767. doi: 10.1091/mbc.E20-09-0603

I am honored and humbled to receive the E. B. Wilson Medal and happy to share some reflections on my journey as a cell biologist. It took me a while to realize that my interest in biology would center on how cells are spatially and dynamically organized. From an initial fascination with cellular structures I came to appreciate that cells exhibit dynamism across all scales-from their molecules, to molecular complexes, to organelles. Uncovering the principles of this dynamism, including new ways to observe and quantify it, has been the guiding star of my work.

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11/01/20 | Mechanisms of procollagen and HSP47 sorting during ER-to-Golgi trafficking
Omari S, Makareeva E, Gorrell L, Jarnik M, Lippincott-Schwartz J, Leikin S
Matrix Biology. 2020 Nov 01;93:79-94. doi: 10.1016/j.matbio.2020.06.002

Efficient quality control and export of procollagen from the cell is crucial for extracellular matrix homeostasis, yet it is still incompletely understood. One of the debated questions is the role of a collagen-specific ER chaperone HSP47 in these processes. Most ER chaperones preferentially bind to unfolded polypeptide chains, enabling selective export of natively folded proteins from the ER after chaperone release. In contrast, HSP47 preferentially binds to the natively folded procollagen and is believed to be released only in the ER-Golgi intermediate compartment (ERGIC) or cis-Golgi. HSP47 colocalization with procollagen in punctate structures observed by immunofluorescence imaging of fixed cells has thus been interpreted as evidence for HSP47 export from the ER together with procollagen in transport vesicles destined for ERGIC or Golgi. To understand the mechanism of this co-trafficking and its physiological significance, we imaged the dynamics of fluorescently tagged type I procollagen and HSP47 punctate structures in live MC3T3 murine osteoblasts with up to 120 nm spatial and 500 ms time resolution. Contrary to the prevailing model, we discovered that most bona fide carriers delivering procollagen from ER exit sites (ERESs) to Golgi contained no HSP47, unless the RDEL signal for ER retention in HSP47 was deleted or mutated. These transport intermediates exhibited characteristic rapid, directional motion along microtubules, while puncta with colocalized HSP47 and procollagen similar to the ones described before had only limited, stochastic motion. Live cell imaging and fluorescence recovery after photobleaching revealed that the latter puncta (including the ones induced by ARF1 inhibition) were dilated regions of ER lumen, ERESs, or autophagic structures surrounded by lysosomal membranes. Procollagen was colocalized with HSP47 and ERGIC53 at ERESs. It was colocalized with ERGIC53 but not HSP47 in Golgi-bound transport intermediates. Our results suggest that procollagen and HSP47 sorting occurs at ERES before procollagen is exported from the ER in Golgi-bound transport intermediates, providing new insights into mechanisms of procollagen trafficking.

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07/27/20 | A general method to optimize and functionalize red-shifted rhodamine dyes.
Grimm JB, Tkachuk AN, Xie L, Choi H, Mohar B, Falco N, Schaefer K, Patel R, Zheng Q, Liu Z, Lippincott-Schwartz J, Brown TA, Lavis LD
Nature Methods. 2020 Jul 27:. doi: 10.1038/s41592-020-0909-6

Expanding the palette of fluorescent dyes is vital to push the frontier of biological imaging. Although rhodamine dyes remain the premier type of small-molecule fluorophore owing to their bioavailability and brightness, variants excited with far-red or near-infrared light suffer from poor performance due to their propensity to adopt a lipophilic, nonfluorescent form. We report a framework for rationalizing rhodamine behavior in biological environments and a general chemical modification for rhodamines that optimizes long-wavelength variants and enables facile functionalization with different chemical groups. This strategy yields red-shifted 'Janelia Fluor' (JF) dyes useful for biological imaging experiments in cells and in vivo.

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07/10/20 | Revisiting Membrane Microdomains and Phase Separation: A Viral Perspective
Sengupta P, Lippincott-Schwartz J
Viruses. 2020 Jul 10;12(7):745. doi: 10.3390/v12070745

Retroviruses selectively incorporate a specific subset of host cell proteins and lipids into their outer membrane when they bud out from the host plasma membrane. This specialized viral membrane composition is critical for both viral survivability and infectivity. Here, we review recent findings from live cell imaging of single virus assembly demonstrating that proteins and lipids sort into the HIV retroviral membrane by a mechanism of lipid-based phase partitioning. The findings showed that multimerizing HIV Gag at the assembly site creates a liquid-ordered lipid phase enriched in cholesterol and sphingolipids. Proteins with affinity for this specialized lipid environment partition into it, resulting in the selective incorporation of proteins into the nascent viral membrane. Building on this and other work in the field, we propose a model describing how HIV Gag induces phase separation of the viral assembly site through a mechanism involving transbilayer coupling of lipid acyl chains and membrane curvature changes. Similar phase-partitioning pathways in response to multimerizing structural proteins likely help sort proteins into the membranes of other budding structures within cells.

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03/31/20 | ER membranes exhibit phase behavior at sites of organelle contact.
King C, Sengupta P, Seo AY, Lippincott-Schwartz J
Proceedings of the National Academy of Sciences of the United States of America. 2020 March 31;117(13):7225-7235. doi: 10.1073/pnas.1910854117

The endoplasmic reticulum (ER) is the site of synthesis of secretory and membrane proteins and contacts every organelle of the cell, exchanging lipids and metabolites in a highly regulated manner. How the ER spatially segregates its numerous and diverse functions, including positioning nanoscopic contact sites with other organelles, is unclear. We demonstrate that hypotonic swelling of cells converts the ER and other membrane-bound organelles into micrometer-scale large intracellular vesicles (LICVs) that retain luminal protein content and maintain contact sites with each other through localized organelle tethers. Upon cooling, ER-derived LICVs phase-partition into microscopic domains having different lipid-ordering characteristics, which is reversible upon warming. Ordered ER lipid domains mark contact sites with ER and mitochondria, lipid droplets, endosomes, or plasma membrane, whereas disordered ER lipid domains mark contact sites with lysosomes or peroxisomes. Tethering proteins concentrate at ER–organelle contact sites, allowing time-dependent behavior of lipids and proteins to be studied at these sites. These findings demonstrate that LICVs provide a useful model system for studying the phase behavior and interactive properties of organelles in intact cells.

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02/26/20 | Nicotine exposure and neuronal activity regulate Golgi membrane dispersal and distribution
Govind AP, Jeyifous O, Russell TA, Vaasjo LO, Yi Z, Weigel AV, Newell L, Koranda JL, Singh K, Valbuena F, Glick BS, Mukherjee J, Lippincott-Schwartz J, Zhuang X, Green WN
bioRxiv. 2020 Feb 26:

How nicotine exposure produces long-lasting changes that remodel neural circuits with addiction is unknown. Here, we report that long-term nicotine exposure alters the trafficking of α4β2-type nicotinic acetylcholine receptors (α4β2Rs) by dispersing and redistributing the Golgi apparatus. In cultured neurons, dispersed Golgi membranes were distributed throughout somata, dendrites and axons. Small, mobile vesicles in dendrites and axons lacked standard Golgi markers and were identified by other Golgi enzymes that modify glycans. Nicotine exposure increased levels of dispersed Golgi membranes, which required α4β2R expression. Similar nicotine-induced changes occurred in vivo at dopaminergic neurons at mouse nucleus accumbens terminals, consistent with these events contributing to nicotine’s addictive effects. Characterization in vitro demonstrated that dispersal was reversible, that dispersed Golgi membranes were functional, and that membranes were heterogenous in size, with smaller vesicles emerging from larger “ministacks”, similar to Golgi dispersal induced by nocadazole. Protocols that increased cultured neuronal synaptic excitability also increased Golgi dispersal, without the requirement of α4β2R expression. Our findings reveal novel activity- and nicotine-dependent changes in neuronal intracellular morphology. These changes regulate levels and location of dispersed Golgi membranes at dendrites and axons, which function in local trafficking at subdomains.

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01/17/20 | Correlative three-dimensional super-resolution and block-face electron microscopy of whole vitreously frozen cells.
Hoffman DP, Shtengel G, Xu S, Campbell KR, Freeman M, Wang L, Milkie DE, Pasolli A, Iyer N, Bogovic JA, Stabley DR, Shirinifard A, Pang S, Peale D, Schaefer K, Pomp W, Chang CL, Lippincott-Schwartz J, Kirchhausen T, Solecki DJ, Betzig E, Hess HF
Science. 2020 Jan 17;367(6475):. doi: 10.1126/science.aaz5357

Within cells, the spatial compartmentalization of thousands of distinct proteins serves a multitude of diverse biochemical needs. Correlative super-resolution (SR) fluorescence and electron microscopy (EM) can elucidate protein spatial relationships to global ultrastructure, but has suffered from tradeoffs of structure preservation, fluorescence retention, resolution, and field of view. We developed a platform for three-dimensional cryogenic SR and focused ion beam-milled block-face EM across entire vitreously frozen cells. The approach preserves ultrastructure while enabling independent SR and EM workflow optimization. We discovered unexpected protein-ultrastructure relationships in mammalian cells including intranuclear vesicles containing endoplasmic reticulum-associated proteins, web-like adhesions between cultured neurons, and chromatin domains subclassified on the basis of transcriptional activity. Our findings illustrate the value of a comprehensive multimodal view of ultrastructural variability across whole cells.

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12/18/19 | Phase separation of YAP reorganizes genome topology for long-term YAP target gene expression.
Cai D, Feliciano D, Dong P, Flores E, Gruebele M, Porat-Shliom N, Sukenik S, Liu Z, Lippincott-Schwartz J
Nature Cell Biology. 2019 Dec;21(12):1578-1589. doi: 10.1038/s41556-019-0433-z

Yes-associated protein (YAP) is a transcriptional co-activator that regulates cell proliferation and survival by binding to a select set of enhancers for target gene activation. How YAP coordinates these transcriptional responses is unknown. Here, we demonstrate that YAP forms liquid-like condensates in the nucleus. Formed within seconds of hyperosmotic stress, YAP condensates compartmentalized the YAP transcription factor TEAD1 and other YAP-related co-activators, including TAZ, and subsequently induced the transcription of YAP-specific proliferation genes. Super-resolution imaging using assay for transposase-accessible chromatin with photoactivated localization microscopy revealed that the YAP nuclear condensates were areas enriched in accessible chromatin domains organized as super-enhancers. Initially devoid of RNA polymerase II, the accessible chromatin domains later acquired RNA polymerase II, transcribing RNA. The removal of the intrinsically-disordered YAP transcription activation domain prevented the formation of YAP condensates and diminished downstream YAP signalling. Thus, dynamic changes in genome organization and gene activation during YAP reprogramming is mediated by liquid-liquid phase separation.

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09/25/19 | Rational design of fluorogenic and spontaneously blinking labels for super-resolution imaging.
Zheng Q, Ayala AX, Chung I, Weigel AV, Ranjan A, Falco N, Grimm JB, Tkachuk AN, Wu C, Lippincott-Schwartz J, Singer RH, Lavis LD
ACS Central Science. 2019 Sep 25;5(9):1602-1613. doi: 10.1021/acscentsci.9b00676

Rhodamine dyes exist in equilibrium between a fluorescent zwitterion and a nonfluorescent lactone. Tuning this equilibrium toward the nonfluorescent lactone form can improve cell-permeability and allow creation of "fluorogenic" compounds-ligands that shift to the fluorescent zwitterion upon binding a biomolecular target. An archetype fluorogenic dye is the far-red tetramethyl-Si-rhodamine (SiR), which has been used to create exceptionally useful labels for advanced microscopy. Here, we develop a quantitative framework for the development of new fluorogenic dyes, determining that the lactone-zwitterion equilibrium constant () is sufficient to predict fluorogenicity. This rubric emerged from our analysis of known fluorophores and yielded new fluorescent and fluorogenic labels with improved performance in cellular imaging experiments. We then designed a novel fluorophore-Janelia Fluor 526 (JF)-with SiR-like properties but shorter fluorescence excitation and emission wavelengths. JF is a versatile scaffold for fluorogenic probes including ligands for self-labeling tags, stains for endogenous structures, and spontaneously blinking labels for super-resolution immunofluorescence. JF constitutes a new label for advanced microscopy experiments, and our quantitative framework will enable the rational design of other fluorogenic probes for bioimaging.

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09/10/19 | RNA granules hitchhike on lysosomes for long-distance transport, Using annexin A11 as a molecular tether.
Liao YC, Fernandopulle MS, Wang G, Choi H, Hao L, Drerup CM, Patel R, Qamar S, Nixon-Abell J, Shen Y, Meadows W, Vendruscolo M, Knowles TPJ, Nelson M, Czekalska MA, Musteikyte G, Gachechiladze MA, Stephens CA, Pasolli A, Forrest LR, St George-Hyslop P, Lippincott-Schwartz J, Ward ME
Cell. 2019 Sep 19;179(1):147-164.e20. doi: 10.1016/j.cell.2019.08.050

Long-distance RNA transport enables local protein synthesis at metabolically-active sites distant from the nucleus. This process ensures an appropriate spatial organization of proteins, vital to polarized cells such as neurons. Here, we present a mechanism for RNA transport in which RNA granules "hitchhike" on moving lysosomes. In vitro biophysical modeling, live-cell microscopy, and unbiased proximity labeling proteomics reveal that annexin A11 (ANXA11), an RNA granule-associated phosphoinositide-binding protein, acts as a molecular tether between RNA granules and lysosomes. ANXA11 possesses an N-terminal low complexity domain, facilitating its phase separation into membraneless RNA granules, and a C-terminal membrane binding domain, enabling interactions with lysosomes. RNA granule transport requires ANXA11, and amyotrophic lateral sclerosis (ALS)-associated mutations in ANXA11 impair RNA granule transport by disrupting their interactions with lysosomes. Thus, ANXA11 mediates neuronal RNA transport by tethering RNA granules to actively-transported lysosomes, performing a critical cellular function that is disrupted in ALS.

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09/01/19 | A neuron-glia Co-culture system for studying intercellular lipid transport.
Ioannou MS, Liu Z, Lippincott-Schwartz J
Curr Protoc Cell Biol. 2019 Sep 01;84(1):e95. doi: 10.1002/cpcb.95

Neurons and glia operate in a highly coordinated fashion in the brain. Although glial cells have long been known to supply lipids to neurons via lipoprotein particles, new evidence reveals that lipid transport between neurons and glia is bidirectional. Here, we describe a co-culture system to study transfer of lipids and lipid-associated proteins from neurons to glia. The assay entails culturing neurons and glia on separate coverslips, pulsing the neurons with fluorescently labeled fatty acids, and then incubating the coverslips together. As astrocytes internalize and store neuron-derived fatty acids in lipid droplets, analyzing the number, size, and fluorescence intensity of lipid droplets containing the fluorescent fatty acids provides an easy and quantifiable measure of fatty acid transport. © 2019 The Authors.

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06/21/19 | Spastin tethers lipid droplets to peroxisomes and directs fatty acid trafficking through ESCRT-III.
Chang CL, Weigel AV, Ioannou MS, Pasolli A, Xu S, Peale DR, Shtengel G, Freeman M, Hess HF, Blackstone C, Lippincott-Schwartz J
Journal of Cell Biology. 2019 Jun 21;218(8):2583-99. doi: 10.1101/544023

Lipid droplets (LDs) are neutral lipid storage organelles that transfer lipids to various organelles including peroxisomes. Here, we show that the hereditary spastic paraplegia protein M1 Spastin, a membrane-bound AAA ATPase found on LDs, coordinates fatty acid (FA) trafficking from LDs to peroxisomes through two inter-related mechanisms. First, M1 Spastin forms a tethering complex with peroxisomal ABCD1 to promote LD-peroxisome contact formation. Second, M1 Spastin recruits the membrane-shaping ESCRT-III proteins IST1 and CHMP1B to LDs via its MIT domain to facilitate LD-to-peroxisome FA trafficking, possibly through IST1 and CHMP1B modifying LD membrane morphology. Furthermore, M1 Spastin, IST1 and CHMP1B are all required to relieve LDs of lipid peroxidation. The roles of M1 Spastin in tethering LDs to peroxisomes and in recruiting ESCRT-III components to LD-peroxisome contact sites for FA trafficking may help explain the pathogenesis of diseases associated with defective FA metabolism in LDs and peroxisomes.

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06/14/19 | NDP52 tunes cortical actin interaction with astral microtubules for accurate spindle orientation.
Yu H, Yang F, Dong P, Liao S, Liu WR, Zhao G, Qin B, Dou Z, Liu Z, Liu W, Zang J, Lippincott-Schwartz J, Liu X, Yao X
Cell Research. 2019 Jun 14;29(8):666-79. doi: 10.1038/s41422-019-0189-9

Oriented cell divisions are controlled by a conserved molecular cascade involving Gαi, LGN, and NuMA. Here, we show that NDP52 regulates spindle orientation via remodeling the polar cortical actin cytoskeleton. siRNA-mediated NDP52 suppression surprisingly revealed a ring-like compact subcortical F-actin architecture surrounding the spindle in prophase/prometaphase cells, which resulted in severe defects of astral microtubule growth and an aberrant spindle orientation. Remarkably, NDP52 recruited the actin assembly factor N-WASP and regulated the dynamics of the subcortical F-actin ring in mitotic cells. Mechanistically, NDP52 was found to bind to phosphatidic acid-containing vesicles, which absorbed cytoplasmic N-WASP to regulate local filamentous actin growth at the polar cortex. Our TIRFM analyses revealed that NDP52-containing vesicles anchored N-WASP and shortened the length of actin filaments in vitro. Based on these results we propose that NDP52-containing vesicles regulate cortical actin dynamics through N-WASP to accomplish a spatiotemporal regulation between astral microtubules and the actin network for proper spindle orientation and precise chromosome segregation. In this way, intracellular vesicles cooperate with microtubules and actin filaments to regulate proper mitotic progression. Since NDP52 is absent from yeast, we reason that metazoans have evolved an elaborate spindle positioning machinery to ensure accurate chromosome segregation in mitosis.

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06/11/19 | Fate plasticity and reprogramming in genetically distinct populations of leucophores.
Lewis VM, Saunders LM, Larson TA, Bain EJ, Sturiale SL, Gur D, Chowdhury S, Flynn JD, Allen MC, Deheyn DD, Lee JC, Simon JA, Lippincott-Schwartz J, Raible DW, Parichy DM
Proceedings of the National Academy of Sciences of the United States of America. 2019 Jun 11;116(24):11806-11. doi: 10.1073/pnas.1901021116

Understanding genetic and cellular bases of adult form remains a fundamental goal at the intersection of developmental and evolutionary biology. The skin pigment cells of vertebrates, derived from embryonic neural crest, are a useful system for elucidating mechanisms of fate specification, pattern formation, and how particular phenotypes impact organismal behavior and ecology. In a survey of fishes, including the zebrafish , we identified two populations of white pigment cells-leucophores-one of which arises by transdifferentiation of adult melanophores and another of which develops from a yellow-orange xanthophore or xanthophore-like progenitor. Single-cell transcriptomic, mutational, chemical, and ultrastructural analyses of zebrafish leucophores revealed cell-type-specific chemical compositions, organelle configurations, and genetic requirements. At the organismal level, we identified distinct physiological responses of leucophores during environmental background matching, and we showed that leucophore complement influences behavior. Together, our studies reveal independently arisen pigment cell types and mechanisms of fate acquisition in zebrafish and illustrate how concerted analyses across hierarchical levels can provide insights into phenotypes and their evolution.

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05/30/19 | Neuron-astrocyte metabolic coupling protects against activity-induced fatty acid toxicity.
Ioannou MS, Jackson J, Sheu SH, Chang CL, Weigel AV, Liu H, Pasolli A, Xu S, Pang S, Matthies D, Hess HF, Lippincott-Schwartz J, Liu Z
Cell. 2019 May 30;177(6):1522-1535.e14. doi: 10.1016/j.cell.2019.04.001

Metabolic coordination between neurons and astrocytes is critical for the health of the brain. However, neuron-astrocyte coupling of lipid metabolism, particularly in response to neural activity, remains largely uncharacterized. Here, we demonstrate that toxic fatty acids (FAs) produced in hyperactive neurons are transferred to astrocytic lipid droplets by ApoE-positive lipid particles. Astrocytes consume the FAs stored in lipid droplets via mitochondrial β-oxidation in response to neuronal activity and turn on a detoxification gene expression program. Our findings reveal that FA metabolism is coupled in neurons and astrocytes to protect neurons from FA toxicity during periods of enhanced activity. This coordinated mechanism for metabolizing FAs could underlie both homeostasis and a variety of disease states of the brain.

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05/17/19 | De novo design of tunable, pH-driven conformational changes.
Boyken SE, Benhaim MA, Busch F, Jia M, Back MJ, Choi H, Klima JC, Chen Z, Walkey C, Mileant A, Sahasrabuddhe A, Wei KY, Hodge EA, Byron S, Quijano-Rubio A, Sankaran B, King NP, Lippincott-Schwartz J, Wysocki VH, al et
Science. 2019 May 17;364(6441):658-64. doi: 10.1126/science.aav7897

The ability of naturally occurring proteins to change conformation in response to environmental changes is critical to biological function. Although there have been advances in the de novo design of stable proteins with a single, deep free-energy minimum, the design of conformational switches remains challenging. We present a general strategy to design pH-responsive protein conformational changes by precisely preorganizing histidine residues in buried hydrogen-bond networks. We design homotrimers and heterodimers that are stable above pH 6.5 but undergo cooperative, large-scale conformational changes when the pH is lowered and electrostatic and steric repulsion builds up as the network histidine residues become protonated. The transition pH and cooperativity can be controlled through the number of histidine-containing networks and the strength of the surrounding hydrophobic interactions. Upon disassembly, the designed proteins disrupt lipid membranes both in vitro and after being endocytosed in mammalian cells. Our results demonstrate that environmentally triggered conformational changes can now be programmed by de novo protein design.

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04/01/19 | A lipid-based partitioning mechanism for selective incorporation of proteins into membranes of HIV particles.
Sengupta P, Seo AY, Pasolli A, Song YEum, Johnson M, Lippincott-Schwartz J
Nature Cell Biology. 2019 Apr;21(4):452-461. doi: 10.1038/s41556-019-0300-y

Particles that bud off from the cell surface, including viruses and microvesicles, typically have a unique membrane protein composition distinct from that of the originating plasma membrane. This selective protein composition enables viruses to evade the immune response and infect other cells. But how membrane proteins sort into budding viruses such as human immunodeficiency virus (HIV) remains unclear. Proteins could passively distribute into HIV-assembly-site membranes producing compositions resembling pre-existing plasma-membrane domains. Here, we demonstrate that proteins instead sort actively into HIV-assembly-site membranes, generating compositions enriched in cholesterol and sphingolipids that undergo continuous remodelling. Proteins are recruited into and removed from the HIV assembly site through lipid-based partitioning, initiated by oligomerization of the HIV structural protein Gag. Changes in membrane curvature at the assembly site further amplify this sorting process. Thus, a lipid-based sorting mechanism, aided by increasing membrane curvature, generates the unique membrane composition of the HIV surface.

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02/13/19 | Regulation of plasma membrane nanodomains of the water channel aquaporin-3 revealed by fixed and live photoactivated localization microscopy.
Arnspang EC, Sengupta P, Mortensen KI, Jensen HH, Hahn U, Jensen EBV, Lippincott-Schwartz J, Nejsum LN
Nano Letters. 2019 Feb 13;19(2):699-707. doi: 10.1021/acs.nanolett.8b03721

Several aquaporin (AQP) water channels are short-term regulated by the messenger cyclic adenosine monophosphate (cAMP), including AQP3. Bulk measurements show that cAMP can change diffusive properties of AQP3; however, it remains unknown how elevated cAMP affects AQP3 organization at the nanoscale. Here we analyzed AQP3 nano-organization following cAMP stimulation using photoactivated localization microscopy (PALM) of fixed cells combined with pair correlation analysis. Moreover, in live cells, we combined PALM acquisitions of single fluorophores with single-particle tracking (spt-PALM). These analyses revealed that AQP3 tends to cluster and that the diffusive mobility is confined to nanodomains with radii of ∼150 nm. This domain size increases by ∼30% upon elevation of cAMP, which, however, is not accompanied by a significant increase in the confined diffusion coefficient. This regulation of AQP3 organization at the nanoscale may be important for understanding the mechanisms of water AQP3-mediated water transport across plasma membranes.

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01/18/19 | Cortical column and whole-brain imaging with molecular contrast and nanoscale resolution.
Gao R, Asano SM, Upadhyayula S, Pisarev I, Milkie DE, Liu TL, Singh V, Graves AR, Huynh GH, Zhao Y, Bogovic JA, Colonell J, Ott CM, Zugates CT, Tappan S, Rodriguez A, Mosaliganti KR, Sheu SH, Pasolli A, al et
Science (New York, N.Y.). 2019 Jan 18;363(6424):eaau8302. doi: 10.1126/science.aau8302

Optical and electron microscopy have made tremendous inroads toward understanding the complexity of the brain. However, optical microscopy offers insufficient resolution to reveal subcellular details, and electron microscopy lacks the throughput and molecular contrast to visualize specific molecular constituents over millimeter-scale or larger dimensions. We combined expansion microscopy and lattice light-sheet microscopy to image the nanoscale spatial relationships between proteins across the thickness of the mouse cortex or the entire Drosophila brain. These included synaptic proteins at dendritic spines, myelination along axons, and presynaptic densities at dopaminergic neurons in every fly brain region. The technology should enable statistically rich, large-scale studies of neural development, sexual dimorphism, degree of stereotypy, and structural correlations to behavior or neural activity, all with molecular contrast.

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12/11/18 | MYC induces a hybrid energetics program early in cell reprogramming.
Prieto J, Seo AY, León M, Santacatterina F, Torresano L, Palomino-Schätzlein M, Giménez K, Vallet-Sánchez A, Ponsoda X, Pineda-Lucena A, Cuezva JM, Lippincott-Schwartz J, Torres J
Stem Cell Reports. 2018 Dec 11;11(6):1479-92. doi: 10.1016/j.stemcr.2018.10.018

Cell reprogramming is thought to be associated with a full metabolic switch from an oxidative- to a glycolytic-based metabolism. However, neither the dynamics nor the factors controlling this metabolic switch are fully understood. By using cellular, biochemical, protein array, metabolomic, and respirometry analyses, we found that c-MYC establishes a robust bivalent energetics program early in cell reprogramming. Cells prone to undergo reprogramming exhibit high mitochondrial membrane potential and display a hybrid metabolism. We conclude that MYC proteins orchestrate a rewiring of somatic cell metabolism early in cell reprogramming, whereby somatic cells acquire the phenotypic plasticity necessary for their transition to pluripotency in response to either intrinsic or external cues.

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11/15/18 | Visualizing intracellular organelle and cytoskeletal interactions at nanoscale resolution on millisecond timescales.
Guo Y, Li D, Zhang S, Yang Y, Liu JJ, Wang X, Liu C, Milkie DE, Moore RP, Tulu S, Kiehart DP, Hu J, Lippincott-Schwartz J, Betzig E, Li D
Cell. 2018 Nov 15;175(5):1430-42. doi: 10.1016/j.cell.2018.09.057

In eukaryotic cells, organelles and the cytoskeleton undergo highly dynamic yet organized interactions capable of orchestrating complex cellular functions. Visualizing these interactions requires noninvasive, long-duration imaging of the intracellular environment at high spatiotemporal resolution and low background. To achieve these normally opposing goals, we developed grazing incidence structured illumination microscopy (GI-SIM) that is capable of imaging dynamic events near the basal cell cortex at 97-nm resolution and 266 frames/s over thousands of time points. We employed multi-color GI-SIM to characterize the fast dynamic interactions of diverse organelles and the cytoskeleton, shedding new light on the complex behaviors of these structures. Precise measurements of microtubule growth or shrinkage events helped distinguish among models of microtubule dynamic instability. Analysis of endoplasmic reticulum (ER) interactions with other organelles or microtubules uncovered new ER remodeling mechanisms, such as hitchhiking of the ER on motile organelles. Finally, ER-mitochondria contact sites were found to promote both mitochondrial fission and fusion.

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10/04/18 | Noncanonical autophagy at ER exit sites regulates procollagen turnover.
Omari S, Makareeva E, Roberts-Pilgrim A, Mirigian L, Jarnik M, Ott C, Lippincott-Schwartz J, Leikin S
Proceedings of the National Academy of Sciences of the United States of America. 2018 Oct 04;115(43):E10099-108. doi: 10.1073/pnas.1814552115

Type I collagen is the main component of bone matrix and other connective tissues. Rerouting of its procollagen precursor to a degradative pathway is crucial for osteoblast survival in pathologies involving excessive intracellular buildup of procollagen that is improperly folded and/or trafficked. What cellular mechanisms underlie this rerouting remains unclear. To study these mechanisms, we employed live-cell imaging and correlative light and electron microscopy (CLEM) to examine procollagen trafficking both in wild-type mouse osteoblasts and osteoblasts expressing a bone pathology-causing mutant procollagen. We found that although most procollagen molecules successfully trafficked through the secretory pathway in these cells, a subpopulation did not. The latter molecules appeared in numerous dispersed puncta colocalizing with COPII subunits, autophagy markers and ubiquitin machinery, with more puncta seen in mutant procollagen-expressing cells. Blocking endoplasmic reticulum exit site (ERES) formation suppressed the number of these puncta, suggesting they formed after procollagen entry into ERESs. The punctate structures containing procollagen, COPII, and autophagic markers did not move toward the Golgi but instead were relatively immobile. They appeared to be quickly engulfed by nearby lysosomes through a bafilomycin-insensitive pathway. CLEM and fluorescence recovery after photobleaching experiments suggested engulfment occurred through a noncanonical form of autophagy resembling microautophagy of ERESs. Overall, our findings reveal that a subset of procollagen molecules is directed toward lysosomal degradation through an autophagic pathway originating at ERESs, providing a mechanism to remove excess procollagen from cells.

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08/17/18 | mTOR-dependent phosphorylation controls TFEB nuclear export.
Napolitano G, Esposito A, Choi H, Matarese M, Benedetti V, Di Malta C, Monfregola J, Medina DLuis, Lippincott-Schwartz J, Ballabio A
Nature Communications. 2018 Aug 17;9(1):3312. doi: 10.1038/s41467-018-05862-6

During starvation the transcriptional activation of catabolic processes is induced by the nuclear translocation and consequent activation of transcription factor EB (TFEB), a master modulator of autophagy and lysosomal biogenesis. However, how TFEB is inactivated upon nutrient refeeding is currently unknown. Here we show that TFEB subcellular localization is dynamically controlled by its continuous shuttling between the cytosol and the nucleus, with the nuclear export representing a limiting step. TFEB nuclear export is mediated by CRM1 and is modulated by nutrient availability via mTOR-dependent hierarchical multisite phosphorylation of serines S142 and S138, which are localized in proximity of a nuclear export signal (NES). Our data on TFEB nucleo-cytoplasmic shuttling suggest an unpredicted role of mTOR in nuclear export.

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08/17/18 | The development and enhancement of FRAP as a key tool for investigating protein dynamics.
Lippincott-Schwartz J, Snapp ELee, Phair RD
Biophysical Journal. 2018 Aug 17;115(7):1146-55. doi: 10.1016/j.bpj.2018.08.007

The saga of fluorescence recovery after photobleaching (FRAP) illustrates how disparate technical developments impact science. Starting with the classic 1976 Axelrod et al. work in Biophysical Journal, FRAP (originally fluorescence photobleaching recovery) opened the door to extraction of quantitative information from photobleaching experiments, laying the experimental and theoretical groundwork for quantifying both the mobility and the mobile fraction of a labeled population of proteins. Over the ensuing years, FRAP's reach dramatically expanded, with new developments in GFP technology and turn-key confocal microscopy, which enabled measurement of protein diffusion and binding/dissociation rates in virtually every compartment within the cell. The FRAP technique and data catalyzed an exchange of ideas between biophysicists studying membrane dynamics, cell biologists focused on intracellular dynamics, and systems biologists modeling the dynamics of cell activity. The outcome transformed the field of cellular biology, leading to a fundamental rethinking of long-held theories of cellular dynamism. Here, we review the pivotal FRAP studies that made these developments and conceptual changes possible, which gave rise to current models of complex cell dynamics.

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08/13/18 | Triggered cell-cell fusion assay for cytoplasmic and organelle intermixing studies.
Feliciano D, Nixon-Abell J, Lippincott-Schwartz J
Current Protocols in Cell Biology. 2018 Aug 13;81(1):e61. doi: 10.1002/cpcb.61

Different multicellular organisms undergo cell-cell fusion to form functional syncytia that support specialized functions necessary for proper development and survival. For years, monitoring the structural consequences of this process using live-cell imaging has been challenging due to the unpredictable timing of cell fusion events in tissue systems. Here we present a triggered vesicular stomatitis virus G-protein (VSV-G)-mediated cell-cell fusion assay that can be used to synchronize fusion between cells. This allows the study of cellular changes that occur during cell fusion. The process is induced using a fast wash of low pH isotonic buffer, promoting the fusion of plasma membranes of two or more adjacent cells within seconds. This approach is suitable for studying mixing of small cytoplasmic molecules between fusing cells as well as changes in organelle distribution and dynamics. © 2018 by John Wiley & Sons, Inc.

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08/01/18 | Interacting organelles.
Cohen S, Valm AM, Lippincott-Schwartz J
Current Opinion in Cell Biology. 2018 Aug;53:84-91. doi: 10.1016/j.ceb.2018.06.003

Eukaryotic cells are organized into membrane-bound organelles. These organelles communicate with one another through vesicular trafficking pathways and membrane contact sites (MCSs). MCSs are sites of close apposition between two or more organelles that play diverse roles in the exchange of metabolites, lipids and proteins. Organelle interactions at MCSs also are important for organelle division and biogenesis. For example, the division of several organelles, including mitochondria and endosomes, seem to be regulated by contacts with the endoplasmic reticulum (ER). Moreover, the biogenesis of autophagosomes and peroxisomes involves contributions from the ER and multiple other cellular compartments. Thus, organelle-organelle interactions allow cells to alter the shape and activities of their membrane-bound compartments, allowing them to cope with different developmental and environmental conditions.

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06/01/18 | Multispectral live-cell imaging.
Cohen S, Valm AM, Lippincott-Schwartz J
Current Protocols in Cell Biology. 2018 Jun;79(1):e46. doi: 10.1002/cpcb.46

Fluorescent proteins and vital dyes are invaluable tools for studying dynamic processes within living cells. However, the ability to distinguish more than a few different fluorescent reporters in a single sample is limited by the spectral overlap of available fluorophores. Here, we present a protocol for imaging live cells labeled with six fluorophores simultaneously. A confocal microscope with a spectral detector is used to acquire images, and linear unmixing algorithms are applied to identify the fluorophores present in each pixel of the image. We describe the application of this method to visualize the dynamics of six different organelles, and to quantify the contacts between organelles. However, this method can be used to image any molecule amenable to tagging with a fluorescent probe. Thus, multispectral live-cell imaging is a powerful tool for systems-level analysis of cellular organization and dynamics. © 2018 by John Wiley & Sons, Inc.

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06/01/18 | Monitoring the effects of pharmacological reagents on mitochondrial morphology.
Fu D, Lippincott-Schwartz J
Current Protocols in Cell Biology. 2018 Jun;79(1):e45. doi: 10.1002/cpcb.45

This protocol describes how to apply appropriate pharmacological controls to induce mitochondrial fusion or fission in studies of mitochondria morphology for four different mammalian cell types, HepG2 human liver hepatocellular carcinoma cells, MCF7 human breast adenocarcinoma cells, HEK293 human embryonic kidney cells, and collagen sandwich culture of primary rat hepatocytes. The protocol provides methods of treating cells with these pharmacological controls, staining mitochondria with commercially available MitoTracker Green and TMRE dyes, and imaging the mitochondrial morphology in live cells using a confocal fluorescent microscope. It also describes the cell culture methods needed for this protocol. © 2018 by John Wiley & Sons, Inc.

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02/20/18 | VPS4 is a dynamic component of the centrosome that regulates centrosome localization of γ-tubulin, centriolar satellite stability and ciliogenesis.
Ott C, Nachmias D, Adar S, Jarnik M, Sherman S, Birnbaum RY, Lippincott-Schwartz J, Elia N
Scientific Reports. 2018 Feb 20;8(1):3353. doi: 10.1038/s41598-018-21491-x

The hexameric AAA ATPase VPS4 facilitates ESCRT III filament disassembly on diverse intracellular membranes. ESCRT III components and VPS4 have been localized to the ciliary transition zone and spindle poles and reported to affect centrosome duplication and spindle pole stability. How the canonical ESCRT pathway could mediate these events is unclear. We studied the association of VPS4 with centrosomes and found that GFP-VPS4 was a dynamic component of both mother and daughter centrioles. A mutant, VPS4, which can't hydrolyze ATP, was less dynamic and accumulated at centrosomes. Centrosome localization of the VPS4mutant, caused reduced γ-tubulin levels at centrosomes and consequently decreased microtubule growth and altered centrosome positioning. In addition, preventing VPS4 ATP hydrolysis nearly eliminated centriolar satellites and paused ciliogensis after formation of the ciliary vesicle. Zebrafish embryos injected with GFP-VPS4mRNA were less viable, exhibited developmental defects and had fewer cilia in Kupffer's vesicle. Surprisingly, ESCRT III proteins seldom localized to centrosomes and their depletion did not lead to these phenotypes. Our data support an ESCRT III-independent function for VPS4 at the centrosome and reveal that this evolutionary conserved AAA ATPase influences diverse centrosome functions and, as a result, global cellular architecture and development.

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11/07/17 | Immature HIV-1 lattice assembly dynamics are regulated by scaffolding from nucleic acid and the plasma membrane.
Pak AJ, Grime JMA, Sengupta P, Chen AK, Durumeric AEP, Srivastava A, Yeager M, Briggs JAG, Lippincott-Schwartz J, Voth GA
Proceedings of the National Academy of Sciences of the United States of America. 2017 Nov 07;114(47):E10056-65. doi: 10.1073/pnas.1706600114

The packaging and budding of Gag polyprotein and viral RNA is a critical step in the HIV-1 life cycle. High-resolution structures of the Gag polyprotein have revealed that the capsid (CA) and spacer peptide 1 (SP1) domains contain important interfaces for Gag self-assembly. However, the molecular details of the multimerization process, especially in the presence of RNA and the cell membrane, have remained unclear. In this work, we investigate the mechanisms that work in concert between the polyproteins, RNA, and membrane to promote immature lattice growth. We develop a coarse-grained (CG) computational model that is derived from subnanometer resolution structural data. Our simulations recapitulate contiguous and hexameric lattice assembly driven only by weak anisotropic attractions at the helical CA-SP1 junction. Importantly, analysis from CG and single-particle tracking photoactivated localization (spt-PALM) trajectories indicates that viral RNA and the membrane are critical constituents that actively promote Gag multimerization through scaffolding, while overexpression of short competitor RNA can suppress assembly. We also find that the CA amino-terminal domain imparts intrinsic curvature to the Gag lattice. As a consequence, immature lattice growth appears to be coupled to the dynamics of spontaneous membrane deformation. Our findings elucidate a simple network of interactions that regulate the early stages of HIV-1 assembly and budding.

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09/25/17 | Cell volume change through water efflux impacts cell stiffness and stem cell fate.
Guo M, Pegoraro AF, Mao A, Zhou EH, Arany PR, Han Y, Burnette DT, Jensen MH, Kasza KE, Moore JR, Mackintosh FC, Fredberg JJ, Mooney DJ, Lippincott-Schwartz J, Weitz DA
Proceedings of the National Academy of Sciences of the United States of America. 2017 Sep 25;114(41):E8618-27. doi: 10.1073/pnas.1705179114

Cells alter their mechanical properties in response to their local microenvironment; this plays a role in determining cell function and can even influence stem cell fate. Here, we identify a robust and unified relationship between cell stiffness and cell volume. As a cell spreads on a substrate, its volume decreases, while its stiffness concomitantly increases. We find that both cortical and cytoplasmic cell stiffness scale with volume for numerous perturbations, including varying substrate stiffness, cell spread area, and external osmotic pressure. The reduction of cell volume is a result of water efflux, which leads to a corresponding increase in intracellular molecular crowding. Furthermore, we find that changes in cell volume, and hence stiffness, alter stem-cell differentiation, regardless of the method by which these are induced. These observations reveal a surprising, previously unidentified relationship between cell stiffness and cell volume that strongly influences cell biology.

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08/08/17 | Cortical actin recovery at the immunological synapse leads to termination of lytic granule secretion in cytotoxic T lymphocytes.
Ritter AT, Kapnick SM, Murugesan S, Schwartzberg PL, Griffiths GM, Lippincott-Schwartz J
Proceedings of the National Academy of Sciences of the United States of America. 2017 Aug 08;114(32):E6585-94. doi: 10.1073/pnas.1710751114

CD8(+) cytotoxic T lymphocytes (CTLs) eliminate virally infected cells through directed secretion of specialized lytic granules. Because a single CTL can kill multiple targets, degranulation must be tightly regulated. However, how CTLs regulate the termination of granule secretion remains unclear. Previous work demonstrated that centralized actin reduction at the immune synapse precedes degranulation. Using a combination of live confocal, total internal reflection fluorescence, and superresolution microscopy, we now show that, after granule fusion, actin recovers at the synapse and no further secretion is observed. Depolymerization of actin led to resumed granule secretion, suggesting that recovered actin acts as a barrier preventing sustained degranulation. Furthermore, RAB27a-deficient CTLs, which do not secrete cytotoxic granules, failed to recover actin at the synapse, suggesting that RAB27a-mediated granule secretion is required for actin recovery. Finally, we show that both actin clearance and recovery correlated with synaptic phosphatidylinositol 4,5-bisphosphate (PIP2) and that alterations in PIP2 at the immunological synapse regulate cortical actin in CTLs, providing a potential mechanism through which CTLs control cortical actin density. Our work provides insight into actin-related mechanisms regulating CTL secretion that may facilitate serial killing during immune responses.

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07/17/17 | A consensus view of ESCRT-mediated Human Immunodeficiency Virus Type 1 abscission.
Lippincott-Schwartz J, Freed EO, van Engelenburg SB
Annual Review of Virology. 2017 Jul 17;4(1):309-25. doi: 10.1146/annurev-virology-101416-041840

The strong dependence of retroviruses, such as human immunodeficiency virus type 1 (HIV-1), on host cell factors is no more apparent than when the endosomal sorting complex required for transport (ESCRT) machinery is purposely disengaged. The resulting potent inhibition of retrovirus release underscores the importance of understanding fundamental structure-function relationships at the ESCRT-HIV-1 interface. Recent studies utilizing advanced imaging technologies have helped clarify these relationships, overcoming hurdles to provide a range of potential models for ESCRT-mediated virus abscission. Here, we discuss these models in the context of prior work detailing ESCRT machinery and the HIV-1 release process. To provide a template for further refinement, we propose a new working model for ESCRT-mediated HIV-1 release that reconciles disparate and seemingly conflicting studies. Expected final online publication date for the Annual Review of Virology Volume 4 is September 29, 2017. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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06/29/17 | Rational engineering of photoconvertible fluorescent proteins for dual-color fluorescence nanoscopy enabled by a triplet-state mechanism of primed conversion.
Mohr MAlexander, Kobitski AYu, Sabater LRullan, Nienhaus K, Obara CJohn, Lippincott-Schwartz J, Nienhaus GUlrich, Pantazis P
Angewandte Chemie (International ed. in English). 2017 Jun 29;56(38):11628-33. doi: 10.1002/anie.201706121

Green-to-red photoconvertible fluorescent proteins (pcFPs) are powerful tools for super-resolution localization microscopy and protein tagging. Recently, they have been found to undergo efficient photoconversion not only by the traditional 400-nm illumination but also by an alternative method termed primed conversion, employing dual wavelength illumination with blue and far-red/near-infrared light. Primed conversion has been reported only for Dendra2 and its mechanism has remained elusive. Here, we uncover the molecular mechanism of primed conversion by reporting the intermediate "primed" state to be a triplet dark state formed by intersystem crossing. We show that formation of this state can be influenced by the introduction of serine or threonine at sequence position 69 (Eos notation) and use this knowledge to create "pr"- (for primed convertible) variants of most known green-to-red pcFPs.

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05/24/17 | Applying systems-level spectral imaging and analysis to reveal the organelle interactome.
Valm AM, Cohen S, Legant WR, Melunis J, Hershberg U, Wait E, Cohen AR, Davidson MW, Betzig E, Lippincott-Schwartz J
Nature. 2017 May 24:. doi: 10.1038/nature22369

The organization of the eukaryotic cell into discrete membrane-bound organelles allows for the separation of incompatible biochemical processes, but the activities of these organelles must be coordinated. For example, lipid metabolism is distributed between the endoplasmic reticulum for lipid synthesis, lipid droplets for storage and transport, mitochondria and peroxisomes for β-oxidation, and lysosomes for lipid hydrolysis and recycling. It is increasingly recognized that organelle contacts have a vital role in diverse cellular functions. However, the spatial and temporal organization of organelles within the cell remains poorly characterized, as fluorescence imaging approaches are limited in the number of different labels that can be distinguished in a single image. Here we present a systems-level analysis of the organelle interactome using a multispectral image acquisition method that overcomes the challenge of spectral overlap in the fluorescent protein palette. We used confocal and lattice light sheet instrumentation and an imaging informatics pipeline of five steps to achieve mapping of organelle numbers, volumes, speeds, positions and dynamic inter-organelle contacts in live cells from a monkey fibroblast cell line. We describe the frequency and locality of two-, three-, four- and five-way interactions among six different membrane-bound organelles (endoplasmic reticulum, Golgi, lysosome, peroxisome, mitochondria and lipid droplet) and show how these relationships change over time. We demonstrate that each organelle has a characteristic distribution and dispersion pattern in three-dimensional space and that there is a reproducible pattern of contacts among the six organelles, that is affected by microtubule and cell nutrient status. These live-cell confocal and lattice light sheet spectral imaging approaches are applicable to any cell system expressing multiple fluorescent probes, whether in normal conditions or when cells are exposed to disturbances such as drugs, pathogens or stress. This methodology thus offers a powerful descriptive tool and can be used to develop hypotheses about cellular organization and dynamics.

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04/10/17 | AMPK and vacuole-associated Atg14p orchestrate µ-lipophagy for energy production and long-term survival under glucose starvation.
Seo AYoung, Lau PW, Feliciano D, Sengupta P, Le Gros MA, Cinquin B, Larabell CA, Lippincott-Schwartz J
eLife. 2017 Apr 10;6:e21690. doi: 10.7554/eLife.21690

Dietary restriction increases the longevity of many organisms but the cell signaling and organellar mechanisms underlying this capability are unclear. We demonstrate that to permit long-term survival in response to sudden glucose depletion, yeast cells activate lipid-droplet (LD) consumption through micro-lipophagy (µ-lipophagy), in which fat is metabolized as an alternative energy source. AMP-activated protein kinase (AMPK) activation triggered this pathway, which required Atg14p. More gradual glucose starvation, amino acid deprivation or rapamycin did not trigger µ-lipophagy and failed to provide the needed substitute energy source for long-term survival. During acute glucose restriction, activated AMPK was stabilized from degradation and interacted with Atg14p. This prompted Atg14p redistribution from ER exit sites onto liquid-ordered vacuole membrane domains, initiating µ-lipophagy. Our findings that activated AMPK and Atg14p are required to orchestrate µ-lipophagy for energy production in starved cells is relevant for studies on aging and evolutionary survival strategies of different organisms.

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04/07/17 | Defects in ER-endosome contacts impact lysosome function in hereditary spastic paraplegia.
Allison R, Edgar JR, Pearson G, Rizo T, Newton T, Günther S, Berner F, Hague J, Connell JW, Winkler J, Lippincott-Schwartz J, Beetz C, Winner B, Reid E
The Journal of Cell Biology. 2017 Apr 07;216(5):1337-55. doi: 10.1083/jcb.201609033

Contacts between endosomes and the endoplasmic reticulum (ER) promote endosomal tubule fission, but the mechanisms involved and consequences of tubule fission failure are incompletely understood. We found that interaction between the microtubule-severing enzyme spastin and the ESCRT protein IST1 at ER-endosome contacts drives endosomal tubule fission. Failure of fission caused defective sorting of mannose 6-phosphate receptor, with consequently disrupted lysosomal enzyme trafficking and abnormal lysosomal morphology, including in mouse primary neurons and human stem cell-derived neurons. Consistent with a role for ER-mediated endosomal tubule fission in lysosome function, similar lysosomal abnormalities were seen in cellular models lacking the WASH complex component strumpellin or the ER morphogen REEP1. Mutations in spastin, strumpellin, or REEP1 cause hereditary spastic paraplegia (HSP), a disease characterized by axonal degeneration. Our results implicate failure of the ER-endosome contact process in axonopathy and suggest that coupling of ER-mediated endosomal tubule fission to lysosome function links different classes of HSP proteins, previously considered functionally distinct, into a unifying pathway for axonal degeneration.

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04/04/17 | Optical measurement of receptor tyrosine kinase oligomerization on live cells.
Chung I
Biochimica et Biophysica Acta (BBA) - Biomembranes. 2017 Apr 04;1859(9):1436-44. doi: 10.1016/j.bbamem.2017.03.026

Receptor tyrosine kinases (RTK) are important cell surface receptors that transduce extracellular signals across the plasma membrane. The traditional view of how these receptors function is that ligand binding to the extracellular domains acts as a master-switch that enables receptor monomers to dimerize and subsequently trans-phosphorylate each other on their intracellular domains. However, a growing body of evidence suggests that receptor oligomerization is not merely a consequence of ligand binding, but is instead part of a complex process responsible for regulation of receptor activation. Importantly, the oligomerization dynamics and subsequent activation of these receptors are affected by other cellular components, such as cytoskeletal machineries and cell membrane lipid characteristics. Thus receptor activation is not an isolated molecular event mediated by the ligand-receptor interaction, but instead involves orchestrated interactions between the receptors and other cellular components. Measuring receptor oligomerization dynamics on live cells can yield important insights into the characteristics of these interactions. Therefore, it is imperative to develop techniques that can probe receptor movements on the plasma membrane with optimal temporal and spatial resolutions. Various microscopic techniques have been used for this purpose. Optical techniques including single molecule tracking (SMT) and fluorescence correlation spectroscopy (FCS) measure receptor diffusion on live cells. Receptor-receptor interactions can also be assessed by detecting Förster resonance energy transfer (FRET) between fluorescently-labeled receptors situated in close proximity or by counting the number of receptors within a diffraction limited fluorescence spot (stepwise bleaching). This review will describe recent developments of optical techniques that have been used to study receptor oligomerization on living cells. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

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03/01/17 | Myosin VI facilitates connexin 43 gap junction accretion.
Waxse BJ, Sengupta P, Hesketh GG, Lippincott-Schwartz J, Buss F
Journal of Cell Science. 2017 03 01;130(5):827-840. doi: 10.1242/jcs.199083

In this study, we demonstrate myosin VI enrichment at Cx43 (also known as GJA1)-containing gap junctions (GJs) in heart tissue, primary cardiomyocytes and cell culture models. In primary cardiac tissue and in fibroblasts from the myosin VI-null mouse as well as in tissue culture cells transfected with siRNA against myosin VI, we observe reduced GJ plaque size with a concomitant reduction in intercellular communication, as shown by fluorescence recovery after photobleaching (FRAP) and a new method of selective calcein administration. Analysis of the molecular role of myosin VI in Cx43 trafficking indicates that myosin VI is dispensable for the delivery of Cx43 to the cell surface and connexon movement in the plasma membrane. Furthermore, we cannot corroborate clathrin or Dab2 localization at gap junctions and we do not observe a function for the myosin-VI-Dab2 complex in clathrin-dependent endocytosis of annular gap junctions. Instead, we found that myosin VI was localized at the edge of Cx43 plaques by using total internal reflection fluorescence (TIRF) microscopy and use FRAP to identify a plaque accretion defect as the primary manifestation of myosin VI loss in Cx43 homeostasis. A fuller understanding of this derangement may explain the cardiomyopathy or gliosis associated with the loss of myosin VI.

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12/30/16 | Live cell single molecule-guided Bayesian localization super resolution microscopy.
Xu F, Zhang M, He W, Han R, Xue F, Liu Z, Zhang F, Lippincott-Schwartz J, Xu P
Cell Research. 2016 Dec 30:. doi: 10.1038/cr.2015.160
12/14/16 | The nanoscale spatial organization of B cell receptors on IgM- and IgG-expressing human B cells.
Lee J, Sengupta P, Brzostowski J, Lippincott-Schwartz J, Pierce SK
Molecular Biology of the Cell. 2016 Dec 14;28(4):511-23. doi: 10.1091/mbc.E16-06-0452

B cell activation is initiated by the binding of antigen to the B cell receptor (BCR). Here we used dSTORM super resolution imaging to characterize the nanoscale spatial organization of IgM and IgG BCRs on the surfaces of resting and antigen-activated human peripheral blood B cells. We provide insights into both the fundamental process of antigen-driven BCR clustering as well as differences in the spatial organization of IgM and IgG BCRs that may contribute to the characteristic differences in the responses of naïve and memory B cells to antigen. We provide evidence that although both IgM and IgG BCRs reside in highly heterogeneous protein islands that vary in both size and number of BCR single molecule localizations, both resting and activated B cells intrinsically maintain a high frequency of single isolated BCR localizations, which likely represent BCR monomers. IgG BCRs are more clustered than IgM BCRs on resting cells and form larger protein islands following antigen activation. Small dense BCR clusters likely formed via protein-protein interactions are present on the surface of resting cells and antigen activation induces these to come together to form less dense, larger islands, a process likely governed, at least in part, by protein-lipid interactions.

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12/08/16 | Sonic Hedgehog Pathway activation increases mitochondrial abundance and activity in hippocampal neurons.
Yao PJ, Manor U, Petralia RS, Brose RD, T Y Wu R, Ott C, Wang YX, Charnoff A, Lippincott-Schwartz J, Mattson MP
Molecular Biology of the Cell. 2016 Dec 08:. doi: 10.1091/mbc.E16-07-0553

Mitochondria are essential organelles whose biogenesis, structure, and function are regulated by many signaling pathways. In this study we present evidence that, in hippocampal neurons, activation of the Sonic hedgehog (Shh) signaling pathway impacts multiple aspects of mitochondria. Mitochondrial mass was increased significantly in neurons treated with Shh. Using biochemical and fluorescence imaging analyses, we show that Shh signaling activity reduces mitochondrial fission and promotes mitochondrial elongation, at least in part, via suppression of the mitochondrial fission protein dynamin-like GTPase Drp1. Mitochondria from Shh-treated neurons were more electron-dense as revealed by electron microscopy, and had higher membrane potential and respiratory activity. We further show that Shh protects neurons against a variety of stresses, including the mitochondrial poison rotenone, amyloid β-peptide, hydrogen peroxide, and high levels of glutamate. Collectively, our data suggest a link between Shh pathway activity and the physiological properties of mitochondria in hippocampal neurons.

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10/28/16 | AMPK activation prevents and reverses drug-induced mitochondrial and hepatocyte injury by promoting mitochondrial fusion and function.
Kang SWoo Sophie, Haydar G, Taniane C, Farrell G, Arias IM, Lippincott-Schwartz J, Fu D
PLoS One. 2016 Oct 31;11(10):e0165638. doi: 10.1371/journal.pone.0165638

Mitochondrial damage is the major factor underlying drug-induced liver disease but whether conditions that thwart mitochondrial injury can prevent or reverse drug-induced liver damage is unclear. A key molecule regulating mitochondria quality control is AMP activated kinase (AMPK). When activated, AMPK causes mitochondria to elongate/fuse and proliferate, with mitochondria now producing more ATP and less reactive oxygen species. Autophagy is also triggered, a process capable of removing damaged/defective mitochondria. To explore whether AMPK activation could potentially prevent or reverse the effects of drug-induced mitochondrial and hepatocellular damage, we added an AMPK activator to collagen sandwich cultures of rat and human hepatocytes exposed to the hepatotoxic drugs, acetaminophen or diclofenac. In the absence of AMPK activation, the drugs caused hepatocytes to lose polarized morphology and have significantly decreased ATP levels and viability. At the subcellular level, mitochondria underwent fragmentation and had decreased membrane potential due to decreased expression of the mitochondrial fusion proteins Mfn1, 2 and/or Opa1. Adding AICAR, a specific AMPK activator, at the time of drug exposure prevented and reversed these effects. The mitochondria became highly fused and ATP production increased, and hepatocytes maintained polarized morphology. In exploring the mechanism responsible for this preventive and reversal effect, we found that AMPK activation prevented drug-mediated decreases in Mfn1, 2 and Opa1. AMPK activation also stimulated autophagy/mitophagy, most significantly in acetaminophen-treated cells. These results suggest that activation of AMPK prevents/reverses drug-induced mitochondrial and hepatocellular damage through regulation of mitochondrial fusion and autophagy, making it a potentially valuable approach for treatment of drug-induced liver injury.

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10/28/16 | Increased spatiotemporal resolution reveals highly dynamic dense tubular matrices in the peripheral ER.
Nixon-Abell J, Obara CJ, Weigel AV, Li D, Legant WR, Xu CS, Pasolli A, Harvey K, Hess HF, Betzig E, Blackstone C, Lippincott-Schwartz J
Science (New York, N.Y.). 2016 Oct 28;354(6311):433-46. doi: 10.1126/science.aaf3928

The endoplasmic reticulum (ER) is an expansive, membrane-enclosed organelle that plays crucial roles in numerous cellular functions. We used emerging superresolution imaging technologies to clarify the morphology and dynamics of the peripheral ER, which contacts and modulates most other intracellular organelles. Peripheral components of the ER have classically been described as comprising both tubules and flat sheets. We show that this system consists almost exclusively of tubules at varying densities, including structures that we term ER matrices. Conventional optical imaging technologies had led to misidentification of these structures as sheets because of the dense clustering of tubular junctions and a previously uncharacterized rapid form of ER motion. The existence of ER matrices explains previous confounding evidence that had indicated the occurrence of ER “sheet” proliferation after overexpression of tubular junction–forming proteins.

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10/24/16 | Bright photoactivatable fluorophores for single-molecule imaging.
Lavis LD, Grimm JB, English BP, Choi H, Muthusamy AK, Mehl BP, Dong P, Brown TA, Lippincott-Schwartz J, Liu Z, Lionnet T
Nature Methods. 2016 Oct 24;13(12):985-8. doi: 10.1038/nmeth.4034

Small molecule fluorophores are important tools for advanced imaging experiments. The development of self-labeling protein tags such as the HaloTag and SNAP-tag has expanded the utility of chemical dyes in live-cell microscopy. We recently described a general method for improving the brightness and photostability of small, cell-permeable fluorophores, resulting in the novel azetidine-containing "Janelia Fluor" (JF) dyes. Here, we refine and extend the utility of the JF dyes by synthesizing photoactivatable derivatives that are compatible with live cell labeling strategies. These compounds retain the superior brightness of the JF dyes once activated, but their facile photoactivation also enables improved single-particle tracking and localization microscopy experiments.

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08/01/16 | Midbody remnant licenses primary cilia formation in epithelial cells.
Ott CM
The Journal of Cell Biology. 2016 Aug 1;214(3):237-9. doi: 10.1083/jcb.201607046

Tethered midbody remnants dancing across apical microvilli, encountering the centrosome, and beckoning forth a cilium-who would have guessed this is how polarized epithelial cells coordinate the end of mitosis and the beginning of ciliogenesis? New evidence from Bernabé-Rubio et al. (2016. J. Cell Biol http://dx.doi.org/10.1083/jcb.201601020) supports this emerging model.

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02/06/16 | Dynamin regulates metaphase furrow formation and plasma membrane compartmentalization in the syncytial Drosophila embryo.
Rikhy R, Mavrakis M, Lippincott-Schwartz J
Biology open. 2015;4(3):301-11. doi: 10.1242/bio.20149936

The successive nuclear division cycles in the syncytial Drosophila embryo are accompanied by ingression and regression of plasma membrane furrows, which surround individual nuclei at the embryo periphery, playing a central role in embryo compartmentalization prior to cellularization. Here, we demonstrate that cell cycle changes in dynamin localization and activity at the plasma membrane (PM) regulate metaphase furrow formation and PM organization in the syncytial embryo. Dynamin was localized on short PM furrows during interphase, mediating endocytosis of PM components. Dynamin redistributed off ingressed PM furrows in metaphase, correlating with stabilized PM components and the associated actin regulatory machinery on long furrows. Acute inhibition of dynamin in the temperature sensitive shibire mutant embryo resulted in morphogenetic consequences in the syncytial division cycle. These included inhibition of metaphase furrow ingression, randomization of proteins normally polarized to intercap PM and disruption of the diffusion barrier separating PM domains above nuclei. Based on these findings, we propose that cell cycle changes in dynamin orchestrate recruitment of actin regulatory machinery for PM furrow dynamics during the early mitotic cycles in the Drosophila embryo.

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02/03/16 | Intracellular and extracellular forces drive primary cilia movement.
Battle C, Ott CM, Burnette DT, Lippincott-Schwartz J, Schmidt CF
Proceedings of the National Academy of Sciences of the United States of America. 2015 Feb 3;112(5):1410-5. doi: 10.1073/pnas.1421845112

Primary cilia are ubiquitous, microtubule-based organelles that play diverse roles in sensory transduction in many eukaryotic cells. They interrogate the cellular environment through chemosensing, osmosensing, and mechanosensing using receptors and ion channels in the ciliary membrane. Little is known about the mechanical and structural properties of the cilium and how these properties contribute to ciliary perception. We probed the mechanical responses of primary cilia from kidney epithelial cells [Madin-Darby canine kidney-II (MDCK-II)], which sense fluid flow in renal ducts. We found that, on manipulation with an optical trap, cilia deflect by bending along their length and pivoting around an effective hinge located below the basal body. The calculated bending rigidity indicates weak microtubule doublet coupling. Primary cilia of MDCK cells lack interdoublet dynein motors. Nevertheless, we found that the organelles display active motility. 3D tracking showed correlated fluctuations of the cilium and basal body. These angular movements seemed random but were dependent on ATP and cytoplasmic myosin-II in the cell cortex. We conclude that force generation by the actin cytoskeleton surrounding the basal body results in active ciliary movement. We speculate that actin-driven ciliary movement might tune and calibrate ciliary sensory functions.

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05/23/15 | Fatty acid trafficking in starved cells: regulation by lipid droplet lipolysis, autophagy, and mitochondrial fusion dynamics.
Rambold AS, Cohen S, Lippincott-Schwartz J
Developmental Cell. 2015 Mar 23;32(6):678-92. doi: 10.1016/j.devcel.2015.01.029

Fatty acids (FAs) provide cellular energy under starvation, yet how they mobilize and move into mitochondria in starved cells, driving oxidative respiration, is unclear. Here, we clarify this process by visualizing FA trafficking with a fluorescent FA probe. The labeled FA accumulated in lipid droplets (LDs) in well-fed cells but moved from LDs into mitochondria when cells were starved. Autophagy in starved cells replenished LDs with FAs, increasing LD number over time. Cytoplasmic lipases removed FAs from LDs, enabling their transfer into mitochondria. This required mitochondria to be highly fused and localized near LDs. When mitochondrial fusion was prevented in starved cells, FAs neither homogeneously distributed within mitochondria nor became efficiently metabolized. Instead, FAs reassociated with LDs and fluxed into neighboring cells. Thus, FAs engage in complex trafficking itineraries regulated by cytoplasmic lipases, autophagy, and mitochondrial fusion dynamics, ensuring maximum oxidative metabolism and avoidance of FA toxicity in starved cells.

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03/27/15 | Selective visualization of GLUT4 storage vesicles and associated Rab proteins using IRAP-pHluorin.
Chen Y, Lippincott-Schwartz J
Methods in molecular biology (Clifton, N.J.). 2015;1298:173-9. doi: 10.1007/978-1-4939-2569-8_14

Fluorescence microscopy and fluorescent protein (FP)-tagged GLUT4 molecule have been great tools to characterize GLUT4 localization and dynamics inside the cell. However, it was difficult to distinguish GLUT4 storage vesicles (GSVs) from other intracellular compartments containing GLUT4 in live cells. Here, we describe the use of IRAP-pHluorin and total internal reflection fluorescence (TIRF) microscopy to selectively visualize GSVs and Rab proteins that associate with GSVs. This assay is also valuable to further defining GSV identity by unraveling other GSV-associated proteins.

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03/03/15 | Profile of Eric Betzig, Stefan Hell, and W. E. Moerner, 2014 Nobel Laureates in Chemistry.
Lippincott-Schwartz J
Proceedings of the National Academy of Sciences of the United States of America. 2015 Mar 3;112(9):2630-2. doi: 10.1073/pnas.1500784112
02/16/15 | Early steps in primary cilium assembly require EHD1/EHD3-dependent ciliary vesicle formation.
Lu Q, Insinna C, Ott C, Stauffer J, Pintado PA, Rahajeng J, Baxa U, Walia V, Cuenca A, Hwang YS, Daar IO, Lopes S, Lippincott-Schwartz J, Jackson PK, Caplan S, Westlake CJ
Nature Cell Biology. 2015 Feb 16;17(4):228-40. doi: 10.1038/ncb3109

Membrane association with mother centriole (M-centriole) distal appendages is critical for ciliogenesis initiation. How the Rab GTPase Rab11–​Rab8 cascade functions in early ciliary membrane assembly is unknown. Here, we show that the membrane shaping proteins ​EHD1 and ​EHD3, in association with the Rab11–​Rab8 cascade, function in early ciliogenesis. ​EHD1 and ​EHD3 localize to preciliary membranes and the ciliary pocket. EHD-dependent membrane tubulation is essential for ciliary vesicle formation from smaller distal appendage vesicles (DAVs). Importantly, this step functions in M-centriole to basal body transformation and recruitment of transition zone proteins and ​IFT20. ​SNAP29, a SNARE membrane fusion regulator and ​EHD1-binding protein, is also required for DAV-mediated ciliary vesicle assembly. Interestingly, only after ciliary vesicle assembly is ​Rab8 activated for ciliary growth. Our studies uncover molecular mechanisms informing a previously uncharacterized ciliogenesis step, whereby ​EHD1 and ​EHD3 reorganize the M-centriole and associated DAVs before coordinated ciliary membrane and axoneme growth.

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02/05/15 | Deacetylation of nuclear LC3 drives autophagy initiation under starvation.
Huang R, Xu Y, Wan W, Shou X, Qian J, You Z, Liu B, Chang C, Zhou T, Lippincott-Schwartz J, Liu W
Molecular cell. 2015 Feb 5;57(3):456-66. doi: 10.1016/j.molcel.2014.12.013

Shuttling of macromolecules between different cellular compartments helps regulate the timing and extent of different cellular activities. Here, we report that LC3, a key initiator of autophagy that cycles between the nucleus and cytoplasm, becomes selectively activated in the nucleus during starvation through deacetylation by the nuclear deacetylase Sirt1. Deacetylation of LC3 at K49 and K51 by Sirt1 allows LC3 to interact with the nuclear protein DOR and return to the cytoplasm with DOR, where it is able to bind Atg7 and other autophagy factors and undergo phosphatidylethanolamine conjugation to preautophagic membranes. The association of deacetylated LC3 with autophagic factors shifts LC3's distribution from the nucleus toward the cytoplasm. Thus, an acetylation-deacetylation cycle ensures that LC3 effectively redistributes in an activated form from nucleus to cytoplasm, where it plays a central role in autophagy to enable the cell to cope with the lack of external nutrients.

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