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Lee Tzumin Lab / Publications
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89 Publications

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    12/08/23 | Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons.
    Ott CM, Constable S, Nguyen TM, White K, Lee WA, Lippincott-Schwartz J, Mukhopadhyay S
    bioRxiv. 2023 Dec 08:. doi: 10.1101/2023.12.07.565988

    Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While essential during early developmental stages, the fate of granule cell cilia is unknown. Here, we provide nanoscopic resolution of ciliary dynamics by studying developmental changes in granule cell cilia using large-scale electron microscopy volumes and immunostaining of mouse cerebella. We found that many granule cell primary cilia were intracellular and concealed from the external environment. Cilia were disassembed in differentiating granule cell neurons in a process we call cilia deconstruction that was distinct from pre-mitotic cilia resorption in proliferating progenitors. In differentiating granule cells, ciliary loss involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Cilia did not reform from the docked centrioles, rather, in adult mice granule cell neurons remained unciliated. Many neurons in other brain regions require cilia to regulate function and connectivity. In contrast, our results show that granule cell progenitors had concealed cilia that underwent deconstruction potentially to prevent mitogenic hedgehog responsiveness. The ciliary deconstruction mechanism we describe could be paradigmatic of cilia removal during differentiation in other tissues.

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    11/01/23 | Nanometer-scale views of visual cortex reveal anatomical features of primary cilia poised to detect synaptic spillover
    Carolyn M Ott , Russel Torres , Tung-Sheng Kuan , Aaron T Kuan , JoAnn Buchanan , Leila Elabbady , Sharmishtaa Seshamani , Agnes L Bodor , Forrest C Collman , Davi D Bock , Wei-Chung Allen Lee , Nuno Macarico da Costa , Jennifer Lippincott-Schwartz
    bioRxiv. 2023 Nov 01:. doi: 10.1101/2023.10.31.564838

    A primary cilium is a thin membrane-bound extension off a cell surface that contains receptors for perceiving and transmitting signals that modulate cell state and activity. While many cell types have a primary cilium, little is known about primary cilia in the brain, where they are less accessible than cilia on cultured cells or epithelial tissues and protrude from cell bodies into a deep, dense network of glial and neuronal processes. Here, we investigated cilia frequency, internal structure, shape, and position in large, high-resolution transmission electron microscopy volumes of mouse primary visual cortex. Cilia extended from the cell bodies of nearly all excitatory and inhibitory neurons, astrocytes, and oligodendrocyte precursor cells (OPCs), but were absent from oligodendrocytes and microglia. Structural comparisons revealed that the membrane structure at the base of the cilium and the microtubule organization differed between neurons and glia. OPC cilia were distinct in that they were the shortest and contained pervasive internal vesicles only occasionally observed in neuron and astrocyte cilia. Investigating cilia-proximal features revealed that many cilia were directly adjacent to synapses, suggesting cilia are well poised to encounter locally released signaling molecules. The internal anatomy, including microtubule changes and centriole location, defined key structural features including cilium placement and shape. Together, the anatomical insights both within and around neuron and glia cilia provide new insights into cilia formation and function across cell types in the brain.

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    08/03/23 | Lysosomal release of amino acids at ER three-way junctions regulates transmembrane and secretory protein mRNA translation.
    Choi H, Liao Y, Yoon YJ, Grimm J, Lavis LD, Singer RH, Lippincott-Schwartz J
    bioRxiv. 2023 Aug 03:. doi: 10.1101/2023.08.01.551382

    One-third of the mammalian proteome is comprised of transmembrane and secretory proteins that are synthesized on endoplasmic reticulum (ER). Here, we investigate the spatial distribution and regulation of mRNAs encoding these membrane and secretory proteins (termed "secretome" mRNAs) through live cell, single molecule tracking to directly monitor the position and translation states of secretome mRNAs on ER and their relationship to other organelles. Notably, translation of secretome mRNAs occurred preferentially near lysosomes on ER marked by the ER junction-associated protein, Lunapark. Knockdown of Lunapark reduced the extent of secretome mRNA translation without affecting translation of other mRNAs. Less secretome mRNA translation also occurred when lysosome function was perturbed by raising lysosomal pH or inhibiting lysosomal proteases. Secretome mRNA translation near lysosomes was enhanced during amino acid deprivation. Addition of the integrated stress response inhibitor, ISRIB, reversed the translation inhibition seen in Lunapark knockdown cells, implying an eIF2 dependency. Altogether, these findings uncover a novel coordination between ER and lysosomes, in which local release of amino acids and other factors from ER-associated lysosomes patterns and regulates translation of mRNAs encoding secretory and membrane proteins.

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    07/22/23 | Towards Generalizable Organelle Segmentation in Volume Electron Microscopy.
    Heinrich L, Patton W, Bennett D, Ackerman D, Park G, Bogovic JA, Eckstein N, Petruncio A, Clements J, Pang S, Shan Xu C, Funke J, Korff W, Hess H, Lippincott-Schwartz J, Saalfeld S, Weigel A, CellMap Project Team
    Microscopy and Microanalysis. 2023 Jul 22;29(Supplement_1):975. doi: 10.1093/micmic/ozad067.487
    06/01/23 | Structural Diversity within the Endoplasmic Reticulum-From the Microscale to the Nanoscale.
    Obara CJ, Moore AS, Lippincott-Schwartz J
    Cold Spring Harbor Perspectives in Biology. 2023 Jun 01;15(6):. doi: 10.1101/cshperspect.a041259

    The endoplasmic reticulum (ER) is a continuous, highly dynamic membrane compartment that is crucial for numerous basic cellular functions. The ER stretches from the nuclear envelope to the outer periphery of all living eukaryotic cells. This ubiquitous organelle shows remarkable structural complexity, adopting a range of shapes, curvatures, and length scales. Canonically, the ER is thought to be composed of two simple membrane elements: sheets and tubules. However, recent advances in superresolution light microscopy and three-dimensional electron microscopy have revealed an astounding diversity of nanoscale ER structures, greatly expanding our view of ER organization. In this review, we describe these diverse ER structures, focusing on what is known of their regulation and associated functions in mammalian cells.

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    04/25/23 | Simultaneous photoactivation and high-speed structural tracking reveal diffusion-dominated motion in the endoplasmic reticulum
    Matteo Dora , Christopher J. Obara , Tim Abel , Jennifer Lippincott-Schwarz , David Holcman
    bioRxiv. 2023 Apr 25:. doi: 10.1101/2023.04.23.537908

    The endoplasmic reticulum (ER) is a structurally complex, membrane-enclosed compartment that stretches from the nuclear envelope to the extreme periphery of eukaryotic cells. The organelle is crucial for numerous distinct cellular processes, but how these processes are spatially regulated within the structure is unclear. Traditional imaging-based approaches to understanding protein dynamics within the organelle are limited by the convoluted structure and rapid movement of molecular components. Here, we introduce a combinatorial imaging and machine learning-assisted image analysis approach to track the motion of photoactivated proteins within the ER of live cells. We find that simultaneous knowledge of the underlying ER structure is required to accurately analyze fluorescently-tagged protein redistribution, and after appropriate structural calibration we see all proteins assayed show signatures of Brownian diffusion-dominated motion over micron spatial scales. Remarkably, we find that in some cells the ER structure can be explored in a highly asymmetric manner, likely as a result of uneven connectivity within the organelle. This remains true independently of the size, topology, or folding state of the fluorescently-tagged molecules, suggesting a potential role for ER connectivity in driving spatially regulated biology in eukaryotes.

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    03/15/23 | Cristae formation is a mechanical buckling event controlled by the inner membrane lipidome
    Kailash Venkatraman , Christopher T Lee , Guadalupe C. Garcia , Arijit Mahapatra , Guy Perkins , Keun-Young Kim , Hilda Amalia Pasolli , Sebastien Phan , Jennifer Lippincott-Schwartz , Mark Ellisman , Padmini Rangamani , Itay Budin
    bioRxiv. 2023 Mar 15:. doi: 10.1101/2023.03.13.532310

    The inner mitochondrial membrane (IMM) is the site of bulk ATP generation in cells and has a broadly conserved lipid composition enriched in unsaturated phospholipids and cardiolipin (CL). While proteins that shape the IMM and its characteristic cristae membranes (CM) have been defined, specific mechanisms by which mitochondrial lipids dictate its structure and function have yet to be elucidated. Here we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions shape CM morphology and ATP generation. When modulating fatty acid unsaturation in engineered yeast strains, we observed that loss of di-unsaturated phospholipids (PLs) led to a breakpoint in IMM topology and respiratory capacity. We found that PL unsaturation modulates the organization of ATP synthases that shape cristae ridges. Based on molecular modeling of mitochondrial-specific membrane adaptations, we hypothesized that conical lipids like CL buffer against the effects of saturation on the IMM. In cells, we discovered that loss of CL collapses the IMM at intermediate levels of PL saturation, an effect that is independent of ATP synthase oligomerization. To explain this interaction, we employed a continuum modeling approach, finding that lipid and protein-mediated curvatures are predicted to act in concert to form curved membranes in the IMM. The model highlighted a snapthrough instability in cristae tubule formation, which could drive IMM collapse upon small changes in composition. The interaction between CL and di-unsaturated PLs suggests that growth conditions that alter the fatty acid pool, such as oxygen availability, could define CL function. While loss of CL only has a minimal phenotype under standard laboratory conditions, we show that its synthesis is essential under microaerobic conditions that better mimic natural yeast fermentation. Lipid and protein-mediated mechanisms of curvature generation can thus act together to support mitochondrial architecture under changing environments.

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    03/01/23 | Rationalized deep learning super-resolution microscopy for sustained live imaging of rapid subcellular processes.
    Qiao C, Li D, Liu Y, Zhang S, Liu K, Liu C, Guo Y, Jiang T, Fang C, Li N, Zeng Y, He K, Zhu X, Lippincott-Schwartz J, Dai Q, Li D
    Nature Biotechnology. 2023 Mar 01;41(3):367-77. doi: 10.1038/s41587-022-01471-3

    The goal when imaging bioprocesses with optical microscopy is to acquire the most spatiotemporal information with the least invasiveness. Deep neural networks have substantially improved optical microscopy, including image super-resolution and restoration, but still have substantial potential for artifacts. In this study, we developed rationalized deep learning (rDL) for structured illumination microscopy and lattice light sheet microscopy (LLSM) by incorporating prior knowledge of illumination patterns and, thereby, rationally guiding the network to denoise raw images. Here we demonstrate that rDL structured illumination microscopy eliminates spectral bias-induced resolution degradation and reduces model uncertainty by five-fold, improving the super-resolution information by more than ten-fold over other computational approaches. Moreover, rDL applied to LLSM enables self-supervised training by using the spatial or temporal continuity of noisy data itself, yielding results similar to those of supervised methods. We demonstrate the utility of rDL by imaging the rapid kinetics of motile cilia, nucleolar protein condensation during light-sensitive mitosis and long-term interactions between membranous and membrane-less organelles.

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    02/20/23 | Phase separation of Hippo signalling complexes.
    Bonello TT, Cai D, Fletcher GC, Wiengartner K, Pengilly V, Lange KS, Liu Z, Lippincott-Schwartz J, Kavran JM, Thompson BJ
    EMBO Journal. 2023 Feb 20;42(6):e112863. doi: 10.15252/embj.2022112863

    The Hippo pathway was originally discovered to control tissue growth in Drosophila and includes the Hippo kinase (Hpo; MST1/2 in mammals), scaffold protein Salvador (Sav; SAV1 in mammals) and the Warts kinase (Wts; LATS1/2 in mammals). The Hpo kinase is activated by binding to Crumbs-Expanded (Crb-Ex) and/or Merlin-Kibra (Mer-Kib) proteins at the apical domain of epithelial cells. Here we show that activation of Hpo also involves the formation of supramolecular complexes with properties of a biomolecular condensate, including concentration dependence and sensitivity to starvation, macromolecular crowding, or 1,6-hexanediol treatment. Overexpressing Ex or Kib induces formation of micron-scale Hpo condensates in the cytoplasm, rather than at the apical membrane. Several Hippo pathway components contain unstructured low-complexity domains and purified Hpo-Sav complexes undergo phase separation in vitro. Formation of Hpo condensates is conserved in human cells. We propose that apical Hpo kinase activation occurs in phase separated "signalosomes" induced by clustering of upstream pathway components.

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    11/29/22 | Oligodendrocyte precursor cells ingest axons in the mouse neocortex.
    Buchanan J, Elabbady L, Collman F, Jorstad NL, Bakken TE, Ott C, Glatzer J, Bleckert AA, Bodor AL, Brittain D, Bumbarger DJ, Mahalingam G, Seshamani S, Schneider-Mizell C, Takeno MM, Torres R, Yin W, Hodge RD, Castro M, Dorkenwald S, Ih D, Jordan CS, Kemnitz N, Lee K, Lu R, Macrina T, Mu S, Popovych S, Silversmith WM, Tartavull I, Turner NL, Wilson AM, Wong W, Wu J, Zlateski A, Zung J, Lippincott-Schwartz J, Lein ES, Seung HS, Bergles DE, Reid RC, da Costa NM
    Proceedings of the National Academies of Science of the U.S.A.. 2022 Nov 29;119(48):e2202580119. doi: 10.1073/pnas.2202580119

    Neurons in the developing brain undergo extensive structural refinement as nascent circuits adopt their mature form. This physical transformation of neurons is facilitated by the engulfment and degradation of axonal branches and synapses by surrounding glial cells, including microglia and astrocytes. However, the small size of phagocytic organelles and the complex, highly ramified morphology of glia have made it difficult to define the contribution of these and other glial cell types to this crucial process. Here, we used large-scale, serial section transmission electron microscopy (TEM) with computational volume segmentation to reconstruct the complete 3D morphologies of distinct glial types in the mouse visual cortex, providing unprecedented resolution of their morphology and composition. Unexpectedly, we discovered that the fine processes of oligodendrocyte precursor cells (OPCs), a population of abundant, highly dynamic glial progenitors, frequently surrounded small branches of axons. Numerous phagosomes and phagolysosomes (PLs) containing fragments of axons and vesicular structures were present inside their processes, suggesting that OPCs engage in axon pruning. Single-nucleus RNA sequencing from the developing mouse cortex revealed that OPCs express key phagocytic genes at this stage, as well as neuronal transcripts, consistent with active axon engulfment. Although microglia are thought to be responsible for the majority of synaptic pruning and structural refinement, PLs were ten times more abundant in OPCs than in microglia at this stage, and these structures were markedly less abundant in newly generated oligodendrocytes, suggesting that OPCs contribute substantially to the refinement of neuronal circuits during cortical development.

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