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132 Publications

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    Looger Lab
    11/07/22 | Chemically stable fluorescent proteins for advanced microscopy.
    Campbell BC, Paez-Segala MG, Looger LL, Petsko GA, Liu CF
    Nature Methods. 2022 Nov 07:. doi: 10.1038/s41592-022-01660-7

    We report the rational engineering of a remarkably stable yellow fluorescent protein (YFP), 'hyperfolder YFP' (hfYFP), that withstands chaotropic conditions that denature most biological structures within seconds, including superfolder green fluorescent protein (GFP). hfYFP contains no cysteines, is chloride insensitive and tolerates aldehyde and osmium tetroxide fixation better than common fluorescent proteins, enabling its use in expansion and electron microscopies. We solved crystal structures of hfYFP (to 1.7-Å resolution), a monomeric variant, monomeric hyperfolder YFP (1.6 Å) and an mGreenLantern mutant (1.2 Å), and then rationally engineered highly stable 405-nm-excitable GFPs, large Stokes shift (LSS) monomeric GFP (LSSmGFP) and LSSA12 from these structures. Lastly, we directly exploited the chemical stability of hfYFP and LSSmGFP by devising a fluorescence-assisted protein purification strategy enabling all steps of denaturing affinity chromatography to be visualized using ultraviolet or blue light. hfYFP and LSSmGFP represent a new generation of robustly stable fluorescent proteins developed for advanced biotechnological applications.

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    Looger Lab
    08/12/22 | Selective Serotonin Reuptake Inhibitors Within Cells: Temporal Resolution in Cytoplasm, Endoplasmic Reticulum, and Membrane
    Aaron L. Nichols , Zack Blumenfeld , Laura Luebbert , Hailey J. Knox , Anand K. Muthusamy , Jonathan S. Marvin , Charlene H. Kim , Stephen N. Grant , David P. Walton , Bruce N. Cohen , Rebekkah Hammar , Loren L. Looger , Per Artursson , Dennis A. Dougherty , Henry A. Lester
    bioRxiv. 2022 Aug 12:. doi: 10.1101/2022.08.09.502705

    Selective serotonin reuptake inhibitors (SSRIs) are the most prescribed treatment for individuals experiencing major depressive disorder (MDD). The therapeutic mechanisms that take place before, during, or after SSRIs bind the serotonin transporter (SERT) are poorly understood, partially because no studies exist of the cellular and subcellular pharmacokinetic properties of SSRIs in living cells. We studied escitalopram and fluoxetine using new intensity- based drug-sensing fluorescent reporters (“iDrugSnFRs”) targeted to the plasma membrane (PM), cytoplasm, or endoplasmic reticulum (ER) of cultured neurons and mammalian cell lines. We also employed chemical detection of drug within cells and phospholipid membranes. The drugs attain equilibrium in neuronal cytoplasm and ER, at approximately the same concentration as the externally applied solution, with time constants of a few s (escitalopram) or 200-300 s (fluoxetine). Simultaneously, the drugs accumulate within lipid membranes by ≥ 18-fold (escitalopram) or 180-fold (fluoxetine), and possibly by much larger factors. Both drugs leave cytoplasm, lumen, and membranes just as quickly during washout. We synthesized membrane-impermeant quaternary amine derivatives of the two SSRIs. The quaternary derivatives are substantially excluded from membrane, cytoplasm, and ER for > 2.4 h. They inhibit SERT transport-associated currents 6- or 11-fold less potently than the SSRIs (escitalopram or fluoxetine derivative, respectively), providing useful probes for distinguishing compartmentalized SSRI effects. Although our measurements are orders of magnitude faster than the “therapeutic lag” of SSRIs, these data suggest that SSRI-SERT interactions within organelles or membranes may play roles during either the therapeutic effects or the “antidepressant discontinuation syndrome”.

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    Looger Lab
    07/01/22 | Many dissimilar protein domains switch between α-helix and β-sheet folds
    Lauren L. Porter , Allen K. Kim , Swechha Rimal , Loren L. Looger , Ananya Majumdar , Brett D. Mensh , Mary Starich
    Nature Communications. 2022 Jul01;13(1):. doi: 10.1101/2021.06.10.447921

    Hundreds of millions of structured proteins sustain life through chemical interactions and catalytic reactions1. Though dynamic, these proteins are assumed to be built upon fixed scaffolds of secondary structure, α-helices and β-sheets. Experimentally determined structures of over >58,000 non-redundant proteins support this assumption, though it has recently been challenged by ∼100 fold-switching proteins2. These “metamorphic3” proteins, though ostensibly rare, raise the question of how many uncharacterized proteins have shapeshifting–rather than fixed–secondary structures. To address this question, we developed a comparative sequence-based approach that predicts fold-switching proteins from differences in secondary structure propensity. We applied this approach to the universally conserved NusG transcription factor family of ∼15,000 proteins, one of which has a 50-residue regulatory subunit experimentally shown to switch between α-helical and β-sheet folds4. Our approach predicted that 25% of the sequences in this family undergo similar α-helix ⇌ β-sheet transitions, a frequency two orders of magnitude larger than previously observed. Our predictions evade state-of-the-art computational methods but were confirmed experimentally by circular dichroism and nuclear magnetic resonance spectroscopy for all 10 assiduously chosen dissimilar variants. These results suggest that fold switching is a pervasive mechanism of transcriptional regulation in all kingdoms of life and imply that numerous uncharacterized proteins may also switch folds.

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    Looger Lab
    05/18/22 | Volume-transmitted GABA waves pace epileptiform rhythms in the hippocampal network
    Vincent Magloire , Leonid P. Savtchenko , Sergyi Sylantyev , Thomas P. Jensen , Nicholas Cole , Jonathan S. Marvin , Loren L. Looger , Dimitri M. Kullmann , Matthew C. Walker , Ivan Pavlov , Dmitri A. Rusakov
    bioRxiv. 2022 May 18:. doi: 10.1101/2021.03.25.437016

    Mechanisms that entrain and drive rhythmic epileptiform discharges remain debated. Traditionally, this quest has been focusing on interneuronal networks driven by GABAergic connections that activate synaptic or extrasynaptic receptors. However, synchronised interneuronal discharges could also trigger a transient elevation of extracellular GABA across the tissue volume, thus raising tonic GABAA receptor conductance (Gtonic) in multiple cells. Here, we use patch-clamp GABA ‘sniffer’ and optical GABA sensor to show that periodic epileptiform discharges are preceded by region-wide, rising waves of extracellular GABA. Neural network simulations that incorporate volume-transmitted GABA signals point to mechanistic principles underpinning this relationship. We validate this hypothesis using simultaneous patch-clamp recordings from multiple nerve cells, selective optogenetic stimulation of fast-spiking interneurons. Critically, we manipulate GABA uptake to suppress extracellular GABA waves but not synaptic GABAergic transmission, which shows a clear effect on rhythm generation. Our findings thus unveil a key role of extrasynaptic, volume-transmitted GABA actions in pacing regenerative rhythmic activity in brain networks.

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    Looger Lab
    02/19/22 | Volume-transmitted GABA waves drive epileptiform rhythms in the hippocampal network.
    Vincent Magloire , Leonid P. Savtchenko , Sergyi Sylantyev , Thomas P. Jensen , Nicholas Cole , Jonathan S. Marvin , Loren L. Looger , Dimitri M. Kullmann , Matthew C. Walker , Ivan Pavlov , Dmitri A. Rusakov
    bioRxiv. 2022 Feb 19:. doi: 10.1101/2021.03.25.437016

    Synchronised rhythmic activity of the brain is thought to arise from neuronal network behaviours that rely on synaptic signalling between individual cells. This notion has been a basis to explain periodic epileptiform discharges that are driven by interneuronal networks. However, interneuronal discharges not only engage cell-cell GABAergic transmission but also control the extracellular GABA concentration ([GABA]e) and thus tonic GABAA receptor conductance (Gtonic) across the cell population. At the same time, the firing activity of interneurons shows a bell-shaped dependence on Gtonic, suggesting an innate susceptibility to self-sustained oscillations. Here, we employ patch-clamp GABA ‘sniffer’ and fast two-photon excitation imaging of GABA sensor to show that periodic epileptiform discharges are preceded by a region-wide, rising wave of extracellular GABA. Neural network simulations based on such observations reveal that it is the volume-transmitted, extrasynaptic actions of GABA targeting multiple off-target cells that drives synchronised interneuronal spiking prompting periodic epileptiform bursts. We validate this hypothesis using simultaneous patch-clamp recordings from multiple nerve cells, selective optogenetic stimulation of fast-spiking interneurons, and by revealing the role of GABA uptake. Our findings thus unveil a key role of extrasynaptic, volume-transmitted GABA actions in enabling and pacing regenerative rhythmic activity in brain networks.

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    02/15/22 | Glutamate indicators with improved activation kinetics and localization for imaging synaptic transmission
    Abhi Aggarwal , Rui Liu , Yang Chen , Amelia J Ralowicz , Samuel J Bergerson , Filip Tomaska , Timothy L Hanson , Jeremy P Hasseman , Daniel Reep , Getahun Tsegaye , Pantong Yao , Xiang Ji , Marinus Kloos , Deepika Walpita , Ronak Patel , Paul W Tilberg , Boaz Mohar , GENIE , Loren L Looger , Jonathan S Marvin , Michael B Hoppa , Arthur Konnerth , David Kleinfeld , Eric R Schreiter , Kaspar Podgorski
    bioRxiv PrePrint. 2022 Feb 15:. doi: 10.1101/2022.02.13.480251

    The fluorescent glutamate indicator iGluSnFR enables imaging of neurotransmission with genetic and molecular specificity. However, existing iGluSnFR variants exhibit saturating activation kinetics and are excluded from post-synaptic densities, limiting their ability to distinguish synaptic from extrasynaptic glutamate. Using a multi-assay screen in bacteria, soluble protein, and cultured neurons, we generated novel variants with improved kinetics and signal-to-noise ratios. We also developed surface display constructs that improve iGluSnFR’s nanoscopic localization to post-synapses. The resulting indicator, iGluSnFR3, exhibits rapid non-saturating activation kinetics and reports synaptic glutamate release with improved linearity and increased specificity versus extrasynaptic signals in cultured neurons. In mouse visual cortex, imaging of iGluSnFR3 at individual boutons reported single electrophysiologically-observed action potentials with high specificity versus non-synaptic transients. In vibrissal sensory cortex Layer 4, we used iGluSnFR3 to characterize distinct patterns of touch-evoked feedforward input from thalamocortical boutons and both feedforward and recurrent input onto L4 cortical neuron dendritic spines.

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    Looger Lab
    02/01/22 | Many sequence-diverse domains switch between alpha-helix and beta-sheet folds
    Porter LL, Kim A, Looger L, Majumdar AK, Starich M
    Biophysical Journal. 2022 Feb 01;121(3):156a. doi: 10.1016/j.bpj.2021.11.1945

    The protein folding paradigm asserts that the three-dimensional structure of a protein is determined by its amino acid sequence. Here we show that a substantial population of proteins from the NusG superfamily of transcription factors do not adhere to this paradigm. Previous work demonstrated that one member of this superfamily has a regulatory domain that completely switches between α-helical and β-sheet folds, but the pervasiveness of this fold-switching mechanism is uncertain. To address this question, we developed a sequence-based predictor, which revealed that thousands of proteins from this superfamily switch folds. Circular dichroism and nuclear magnetic resonance spectroscopies of 10 sequence-diverse variants confirmed our predictions. By contrast, state-of-the-art methods based on the protein folding paradigm assume that related sequences adopt the same fold and thus predicted that the regulatory domains of all variants adopt only the β-sheet fold. Removal of this bias revealed that residue-residue contacts from both α-helical and β-sheet folds are conserved in a large subpopulation of fold-switching domains, poising them to assume disparate conformations. Our results suggest that fold switching is a pervasive mechanism of transcriptional regulation in all kingdoms of life and indicate that expanding the protein folding paradigm may reveal the involvement of fold-switching proteins in diverse biological processes.

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    Looger Lab
    01/04/22 | Fluorescence activation mechanism and imaging of drug permeation with new sensors for smoking-cessation ligands.
    Nichols AL, Blumenfeld Z, Fan C, Luebbert L, Blom AE, Cohen BN, Marvin JS, Borden PM, Kim CH, Muthusamy AK, Shivange AV, Knox HJ, Campello HR, Wang JH, Dougherty DA, Looger LL, Gallagher T, Rees DC, Lester HA
    eLife. 2022 Jan 04;11:. doi: 10.7554/eLife.74648

    Nicotinic partial agonists provide an accepted aid for smoking cessation and thus contribute to decreasing tobacco-related disease. Improved drugs constitute a continued area of study. However, there remains no reductionist method to examine the cellular and subcellular pharmacokinetic properties of these compounds in living cells. Here, we developed new intensity-based drug sensing fluorescent reporters ('iDrugSnFRs') for the nicotinic partial agonists dianicline, cytisine, and two cytisine derivatives - 10-fluorocytisine and 9-bromo-10-ethylcytisine. We report the first atomic-scale structures of liganded periplasmic binding protein-based biosensors, accelerating development of iDrugSnFRs and also explaining the activation mechanism. The nicotinic iDrugSnFRs detect their drug partners in solution, as well as at the plasma membrane (PM) and in the endoplasmic reticulum (ER) of cell lines and mouse hippocampal neurons. At the PM, the speed of solution changes limits the growth and decay rates of the fluorescence response in almost all cases. In contrast, we found that rates of membrane crossing differ among these nicotinic drugs by > 30 fold. The new nicotinic iDrugSnFRs provide insight into the real-time pharmacokinetic properties of nicotinic agonists and provide a methodology whereby iDrugSnFRs can inform both pharmaceutical neuroscience and addiction neuroscience.

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    Lavis LabLooger Lab
    12/23/20 | Directed Evolution of a Selective and Sensitive Serotonin Sensor via Machine Learning.
    Unger EK, Keller JP, Altermatt M, Liang R, Matsui A, Dong C, Hon OJ, Yao Z, Sun J, Banala S, Flanigan ME, Jaffe DA, Hartanto S, Carlen J, Mizuno GO, Borden PM, Shivange AV, Cameron LP, Sinning S, Underhill SM, Olson DE, Amara SG, Temple Lang D, Rudnick G, Marvin JS, Lavis LD, Lester HA, Alvarez VA, Fisher AJ, Prescher JA, Kash TL, Yarov-Yarovoy V, Gradinaru V, Looger LL, Tian L
    Cell. 2020 Dec 23;183(7):1986-2002.e26. doi: 10.1016/j.cell.2020.11.040

    Serotonin plays a central role in cognition and is the target of most pharmaceuticals for psychiatric disorders. Existing drugs have limited efficacy; creation of improved versions will require better understanding of serotonergic circuitry, which has been hampered by our inability to monitor serotonin release and transport with high spatial and temporal resolution. We developed and applied a binding-pocket redesign strategy, guided by machine learning, to create a high-performance, soluble, fluorescent serotonin sensor (iSeroSnFR), enabling optical detection of millisecond-scale serotonin transients. We demonstrate that iSeroSnFR can be used to detect serotonin release in freely behaving mice during fear conditioning, social interaction, and sleep/wake transitions. We also developed a robust assay of serotonin transporter function and modulation by drugs. We expect that both machine-learning-guided binding-pocket redesign and iSeroSnFR will have broad utility for the development of other sensors and in vitro and in vivo serotonin detection, respectively.

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    Looger Lab
    12/01/21 | Lupus susceptibility region containing CDKN1B rs34330 mechanistically influences expression and function of multiple target genes, also linked to proliferation and apoptosis.
    Singh B, Maiti GP, Zhou X, Fazel-Najafabadi M, Bae S, Sun C, Terao C, Okada Y, Chua KH, Kochi Y, Guthridge JM, Zhang H, Weirauch M, James JA, Harley JB, Varshney GK, Looger LL, Nath SK
    Arthritis Rheumatology. 2021 Dec 01;73(12):2303-13. doi: 10.1002/art.41799

    OBJECTIVE: A recent genome-wide association study (GWAS) reported a significant genetic association between rs34330 of cyclin-dependent kinase inhibitor 1B (CDKN1B) and risk of systemic lupus erythematosus (SLE) in Han Chinese. This study aims to validate the reported association and elucidate the biochemical mechanisms underlying the variant's effect.

    METHODS: We performed allelic association with SLE followed by meta-analysis across 11 independent cohorts (n=28,872). We applied in silico bioinformatics and experimental validation in SLE-relevant cell lines to determine the functional consequences of rs34330.

    RESULTS: We replicated genetic association between SLE and rs34330 (P =5.29x10 , OR (95% CI)=0.84 (0.81-0.87)). Follow-up bioinformatics and eQTL analysis suggest that rs34330 is located in active chromatin and potentially regulates several target genes. Using luciferase and ChIP-qPCR, we demonstrated substantial allele-specific promoter and enhancer activity, and allele-specific binding of three histone marks (H3K27ac, H3K4me3, H3K4me1), RNA pol II, CTCF, and a critical immune transcription factor (IRF-1). Chromosome conformation capture (3C) detected long-range chromatin interactions between rs34330 and the promoters of neighboring genes APOLD1 and DDX47, and effects on CDKN1B and the other target genes were directly validated by CRISPR-based genome editing. Finally, CRISPR-dCas9-based epigenetic activation/silencing confirmed these results. Gene-edited cell lines also showed higher levels of proliferation and apoptosis.

    CONCLUSION: Collectively, these findings suggest a mechanism whereby the rs34330 risk allele (C) influences the presence of histone marks, RNA pol II, and the IRF-1 transcription factor to regulate expression of several target genes linked to proliferation and apoptosis, which potentially underlie the association of rs34330 with SLE.

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