The Trimmer lab received funding from the NIH National Institute of Neurological Disorders and Stroke for their project titled "Defining the Proteomic Composition of ER:Plasma Membrane Junctions in Brain Neurons." This award is part of the R21 grant program that aims encourage exploratory/developmental research by providing support for the early and conceptual stages of project development, and builds on their recent success in identifying VAP proteins as fundamental components of brain ER-PM junctions.
Sites of contact between the endoplasmic reticulum (ER) and the plasma membrane (PM), termed ER-PM junctions or EPJs, are specialized membrane contact sites present in all cells, and at which physiologically important Ca2+ signaling events, lipid exchange, membrane protein trafficking, and other crucial cell biological processes occur. In many brain neurons, such as hippocampal pyramidal neurons (HPNs) and striatal medium spiny neurons (MSNs), EPJs represent the major Ca2+ signaling microdomain in aspiny regions of the neuron. Neuroproteomic analyses of the macromolecular signaling complexes at dendritic spines has provided information crucial to determining the specific molecular events that underlie normal synaptic signaling, and its dysregulation in neurodevelopmental and adult neurological and psychiatric disorders. A systematic dissection of the macromolecular protein complex present at EPJs at the proteomic level in any cell type, but especially in brain neurons, has not been pursued, due to the lack of appropriate methods and suitable tools. The lack of fundamental information, beginning with a molecular catalog of the protein constituents of these prominent extrasynaptic Ca2+ signaling microdomains, represents a major barrier to our understanding of basic neurophysiology and pathophysiology. We have found that an abundant and broadly expressed neuronal voltage-gated K+ channel, Kv2.1, is specifically localized to large clusters in the PM precisely at sites where EPJs form. Moreover, recent findings show that Kv2.1 actively promotes the formation and/or stabilization of EPJs through direct interaction with a resident ER protein. We propose in this exploratory research proposal to take advantage of the robust and widespread association of Kv2.1 with EPJs to undertake a concerted neuroproteomics effort to identify the protein constituents of this Ca2+ signaling microdomain in HPNs and MSNs. We will immunopurify and/or proximity label protein constituents of Kv2.1-containing EPJs in these neurons, and determine their identify by tandem mass spectrometry. These complementary neuroproteomics analyses will provide a molecular catalog of the protein constituents of these important Ca2+ signaling microdomains in HPNs and MSNs. This information will inform future studies to define the functional role of these constituents in the Ca2+ signaling events that shape the physiology and plasticity of these neurons. Lastly, as dysregulation of protein constituents of EPJs may contribute to the aberrant Ca2+ signaling that leads to degeneration of these important neurons, for example of HPNs in Alzheimer's disease and after stroke, and MSNs in Huntington's disease, they may represent important targets for therapeutic modulation.
Public health relevance statement
This study aims to better understand basic mechanisms controlling brain function in health and disease. It focuses on specialized structures present in all cells, where the intracellular membrane organelle the endoplasmic reticulum (ER) comes near the plasma membrane to form specialized membrane junctions, called ER-PM junctions. Many key biological processes, especially those involving Ca2+ signaling, occur at these sites. Although ER-PM junctions are abundant and play an important role in the normal physiology of brain neurons, and when dysfunctional underlie the pathophysiology of brain disease, remarkably little is known of their proteomic composition. We will define for the first time the molecular catalog or “parts list” of this specialized membrane domain, a crucial first step into defining the principles that control its crucial Ca2+ signaling function. These studies will provide important insights into neuronal function and how it is altered in disease.