Calpains are family of calcium-activated neutral proteases involved with signal transduction and cell motility,but whose overactivation is strongly implicated in neurodegeneration following traumatic brain injury andother CMS insults and disorders. Although studies over the past 30 years have provided a wealth ofknowledge regarding calpains, important questions remain: Which calpain isoforms are responsible for theneurodegeneration? How is calpain activated postinjury? What are the critical substrates? Are m- and u-calpain located in the same subcellular compartment? We recently found that mitochondria contain u-calpain(calpain 1 plus calpain small subunit 1). Based on the existing data and our preliminary results, our workingmodel is that mitochondrial u-calpain is situated within the intermembrane space, is activated followingopening of the mitochondrial permeability transition pore (mPTP), and that mitochondrial substrates of u-calpain include apoptosis inducing factor (AIF), which when cleaved by calpain is released into the cytosoland translocates to the nucleus to contribute to caspase-independent cell death. In addition, we propose thatactivated u-calpain is released from the mitochondria into the cytosol, where its substrates are similar tothose of m-calpain. In summary, mitochondrial u-calpain is hypothesized to become activated followingmPTP opening and function as a death-related protease. We therefore hypothesize that neurons deficient incalpain 1will exhibit resistance to excitotoxic death and that neurodegeneration and functional impairmentfollowing traumatic brain injury will be markedly attenuated in mice deficient in u-calpain. Interactions withother projects include evaluating the ability of calpastatin (Project 1) and small molecule calpain inhibitors(Project 2) to inhibit mitochondrial u-calpain. This project will use the same injury models, methods ofassessing functional impairment, and methods for evaluating calpain activity as other projects, enablingdirect comparison of results. In addition, we will collaborate with the Proteomics and Biomarker Core C toidentify novel substrates of mitochondrial u-calpain. Together the experiments outlined in this project willenhance our understanding of the function of the u-calpain isoform, mechanisms involved in its activation,and its role in neuron death following traumatic brain injury and other CNS insults.
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