The Physiology of Store-Operated Channels in the Nervous System

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Project Summary Ca2+ signaling mediates many essential roles in the brain including neurotransmitter release, synaptic plasticity, and gene transcription. Neurons and glia have an extensive Ca2+ signaling toolkit that includes many types of ion channels and Ca2+ release pathways which can be mixed and matched to create signals with widely different spatial and temporal properties. One of the newest - and least understood - members of this toolkit in the brain is store-operated calcium entry (SOCE). SOCE is mediated by the opening of Orai channels (Orai1-3), which are activated by the endoplasmic reticulum Ca2+ sensors, STIM1 and STIM2. In immune cells where SOCE was first discovered, the pathway mediates critical functions including gene expression and cytokine release, and aberrant Orai/STIM function is implicated in the etiology of several diseases including immunodeficiency, inflammation, and myopathy. However, in the brain where multiple isoforms of Orai and STIM are expressed, the molecular mechanisms and physiological functions of SOCE remain very poorly understood. Previous work on the molecular choreography of SOCE has revealed that Orai channel opening is triggered by a unique inside-out mechanism where store depletion activates the ER Ca2+ sensors STIM1 and STIM2 which then translocate to ER-plasma membrane contact sites to directly gate Orai1 channels. Our previous mechanistic work has established a strong framework for understanding the gating mechanisms of Orai channels, and using conditional Orai1 and STIM1 knockout mice, we have now begun to discover vital roles for Orai channels in effector functions in the brain such as NFAT-mediated gene expression, synaptic plasticity, and memory. We have learnt that SOCE in neurons exhibits unique specializations, including rapid activation and unusual Ca2+ dependencies whose basis cannot be readily explained easily from existing activation models. In this application, we aim to build an integrated view of the SOCE mechanism in the nervous system, its micro and macro architecture, regulation, and gating, and elucidate how neurotransmitter and receptor signaling through SOCE impacts fundamental processes of synaptic communication, metabolism, learning, and cognition. To address this goal, we will use a full range of approaches from electrophysiology, structural analysis, and molecular dynamics simulations to behavioral analysis of cognition, depression, and disease to gain an unprecedented view of the SOCE mechanism in the brain. These studies will address the role of a poorly understood Ca2+ entry pathway in the nervous system with immense relevance for a range of functions from cognition to pain, and ultimately facilitate efforts to target Orai channels for drug discovery for neurological diseases.