Biology subjects

Tyagarajan, S. K.

Publications and source records attributed to Tyagarajan, S. K..

6 recordsLinked to original sources

bioRxiv2025

Cryo-EM Structures of Higher Order Gephyrin OligomersReveal Principles of Inhibitory Postsynaptic Scaffold Organization

Gephyrin is the principal scaffolding protein of inhibitory postsynaptic densities, clustering glycine and GABAA receptors via multivalent interactions. It features structured N and C terminal domains connected by an intrinsically disordered linker. Although the structural and functional properties of its terminal domains are well characterized, the mechanism by which full-length gephyrin organizes into higher-order complexes remains unresolved. Here, we combine biochemical reconstitution, cryo-electron microscopy, and mutational analyses to elucidate the structural logic of gephyrin oligomerization. We demonstrate that gephyrin adopts a stable dimeric assembly which constitutes the basic unit for both linear and oblique tetramers as well as linear hexameric arrangements. High resolution structures reveal a critical segment of the flexible linker that adopts two distinct conformations, one of which occludes the receptor-binding site. This segment harbors key phosphorylation sites, providing a mechanistic link between structural conformation and regulatory control. Our findings redefine the architecture of inhibitory synapses and reconcile gephyrin oligomerization models with published in-situ post-synaptic densities characterized by cryo-electron tomography.

neuroscience↗

bioRxiv2022

Neuroprotective tissue adaptation induced by IL-12 attenuates CNS inflammation

IL-12 is a well-established driver of type 1 immune responses. Paradoxically, in several autoimmune conditions including neuroinflammation, IL-12 reduces pathology and exhibits regulatory properties. Yet, the mechanism and the involved cellular players behind this immune regulation remain elusive. To identify the IL-12-responsive elements which prevent immunopathology, we generated mouse models lacking a functional IL-12 receptor either in all cells or in specific populations within the immune or central nervous system (CNS) compartments, and induced experimental autoimmune encephalomyelitis (EAE), which models human Multiple Sclerosis (MS). This revealed that the CNS tissue-protective features of IL-12 are mediated by cells of the neuroectoderm, and not immune cells. Importantly, sections of brain from patients with MS show comparable patterns of expression, indicating parallel mechanisms in humans. By combining spectral flow cytometry, bulk and single-nucleus RNA sequencing, we uncovered an IL-12-induced neuroprotective adaption of the neuroectoderm critically involved in maintaining CNS tissue integrity during inflammation.

immunology↗

bioRxiv2022

A DARPin-based molecular toolset to probe gephyrin and inhibitory synapse biology

Neuroscience currently requires the use of antibodies to study synaptic proteins, where antibody binding is used as a correlate to define the presence, plasticity, and regulation of synapses. Gephyrin is an inhibitory synaptic scaffolding protein used to mark GABAergic and glycinergic postsynaptic sites. Despite the importance of gephyrin in modulating inhibitory transmission, its study is currently limited by the tractability of available reagents. Designed Ankyrin Repeat Proteins (DARPins) are a class of synthetic protein binder derived from diverse libraries by in vitro selection, and tested by high-throughput screening to produce specific binders. In order to generate a functionally diverse toolset for studying inhibitory synapses, we screened a DARPin library against gephyrin mutants representing both phosphorylated and dephosphorylated states. We validated the robust use of anti-gephyrin DARPin clones for morphological identification of gephyrin clusters in rodent neuron culture and brain tissue, discovering previously overlooked clusters. This DARPin-based toolset includes clones with heterogenous gephyrin binding modes that allowed for identification of the most extensive gephyrin interactome to date, and defined novel classes of putative interactors, creating a framework for understanding gephyrins non-synaptic functions. This study demonstrates anti-gephyrin DARPins as a versatile platform for studying inhibitory synapses in an unprecedented manner.

neuroscience↗

bioRxiv2020

Differential impact of GABAA Receptors on Inhibitory connectivity for Superficial Pyramidal Neuron Responsiveness In Vivo

A diverse set of GABAA receptors (GABAARs) enable synaptic plasticity adaptations at inhibitory postsynaptic sites in collaboration with the scaffolding protein gephyrin. Early studies helped to identify distinctions between GABAAR subtypes allocated within specific functional circuits, but their contribution to the changing dynamics of a microcircuit remains unclear. Here, using the whisker-barrel system in mouse, we assessed the contribution of specific synaptic GABAAR subtypes and gephyrin scaffolding changes to sensory processing in vivo. We monitored spontaneous and evoked Ca2+ transients in layer 2/3 pyramidal cells with the genetically encoded Ca2+ sensor RCaMP1.07. Using Gabra1 or Gabra2 global and conditional knockout mice, we uncovered that 1- and 2-GABAARs determine the sparseness of L2/3 pyramidal neuron encoding. In a cell-type dependent manner, 1-GABAARs and 2-GABAARs affected neuronal excitability and the reliability of neuronal responses after whisker stimulation. We also discerned that gephyrin with its diverse post-translational modifications (PTMs) shows preference for specific GABAAR subtype to facilitate microcircuit activity. Our results underscore the relevance of the diversity of GABAARs within a cortical microcircuit. Key pointsO_LIWhile GABAergic inhibition from interneuron subtypes regulates cortical microcircuit activity the molecular determinants have remain unclear. C_LIO_LIWe demonstrate that specific-GABAA receptor subtypes contribute differentially to layer 2/3 neuronal activities in mouse barrel cortex. C_LIO_LIImportantly, we link the GABAAR contributions to the scaffolding properties of its important postsynaptic density protein gephyrin. We show that different PTMs on gephyrin determines neuronal excitability via GABAAR recruitment and modulation of inhibition within layer 2/3 neurons. C_LIO_LISpecifically, 1 and 2 subunits containing GABAA receptors, along with their scaffolding protein gephyrin determine the distribution of high, medium and low activity pyramidal neurons during sensory encoding, whereby controlling the total activity of cortical microcircuit. C_LI

neuroscience↗

bioRxiv2020

Blocking gephyrin phosphorylation or microglia BDNF signaling prevents synapse loss and reduces infarct volume after ischemia

Microglia interact with neurons to facilitate synapse plasticity; however, signal transducers between microglia and neuron remain unknown. Here, using in vitro organotypic hippocampal slice cultures and transient MCAO in genetically-engineered mice in vivo, we report that at 24 h post-ischemia microglia release BDNF to downregulate glutamatergic and GABAergic synapses within the peri-infarct area. Analysis of the CA1 hippocampal formation in vitro shows that proBDNF and mBDNF downregulate glutamatergic dendritic spines and gephyrin scaffold stability through p75NTR and TrkB receptors respectively. Post-MCAO, we report that in the peri- infarct area and in the corresponding contralateral hemisphere similar neuroplasticity occur through microglia activation and gephyrin phosphorylation at Ser268, Ser270 in vivo. Targeted deletion of the Bdnf gene in microglia or GphnS268A/S270A (phospho-null) point-mutations protect against ischemic brain damage, neuroinflamation and synapse downregulation normally seen post-MCAO. Collectively, we report that gephyrin phosphorylation and microglia derived BDNF faciliate synapse plasticity after transient ischemia.

neuroscience↗

bioRxiv2019

Surface proteome dynamics during neuronal development and synaptic plasticity

Neurons are highly compartmentalized cells with tightly controlled subcellular protein organization. While broad brain transcriptome, connectome and global proteome maps are being generated, system-wide analysis of temporal protein dynamics at the subcellular level are currently lacking for neuronal development and synapse formation. We performed a temporally-resolved surfaceome analysis of developing primary neuron cultures to a depth of 1000 bona fide surface proteins and reveal dynamic surface protein clusters that reflect the functional requirements during distinct stages of neuronal development. Moreover, our data shows that synaptic proteins are globally trafficked to the surface prior to synapse formation. Direct comparison of surface and total protein pools demonstrates that, depending on the time scale, surface abundance changes can correlate or differ from total protein abundance. The uncoupling of surface and total abundance changes has direct functional implications as shown in the context of synaptic vesicle transport. To demonstrate the utility of our approach we analyzed the surfaceome modulation in response to homeostatic synaptic scaling and found dynamic remodeling of the neuronal surface, which was largely independent of global proteostasis, indicative of wide-spread regulation on the level of surface trafficking. Finally, we present a quantitative analysis of the neuronal surface during early-phase long-term potentiation (LTP) and reveal fast externalization of diverse classes of surface proteins beyond the AMPA receptor, providing new insights into the requirement of exocytosis for LTP. Our resource and finding of organizational principles highlight the importance of subcellular resolution for systems-level understanding of cellular processes, which are typically masked by broad omics-style approaches.

neuroscience↗