Mechanisms of couplon-linked skeletal muscle myopathies

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The skeletal muscle L-type Ca2+ channel (CaV1.1) in the transverse tubules, the ryanodine receptor (RyR1) in the sarcoplasmic reticulum (SR), and calsequestrin (Casq1) in the lumen of junctional SR (jSR) are key components of a macromolecular complex termed the couplon which regulates excitation-contraction coupling (ECC). Mutations in RyR1, CaV1.1, and Casq1 underlie human myopathies with overlapping pathological features. With the exception of dantrolene for malignant hyperthermia, there are no FDA approved interventions for any of these myopathies. Mice with a mutations CaV1.1 (E1014K), RyR1 (I4895T) and Casq1 (D244G) develop myopathies that increase in severity with age and their muscles display Casq1 mislocalization and persistent ER stress. This application is designed to elucidate the mechanisms by which mutation-associated alterations in a CaMKII-dependent pathway lead to ER stress/UPR and muscle disease. We will test the general hypothesis that the similarities in couplon myopathies arising from mutations in different couplon proteins are due to highly cooperative, bidirectional functional coupling between CaV1.1, CaMKII, RyR1 and Casq1. Our aims are to: 1. Define the roles of altered CaV1.1 functional state transitions and CaMKII activation in couplon myopathies. 2. Quantify the effects of couplon disease-associated mutations on RyR1 phosphorylation and the phosphorylation-mediated effects on CaV1.1 function and Casq1 retention at the jSR. 3. Define the roles of Casq1 mislocalization and ER stress in the couplon myopathies. We will also test the ability of 4PBA, which alleviates ER stress, to improve muscle function in myopathies that arise from mutations in couplon proteins.