Pathophysiology of Tubular Aggregate Myopathy

Tubular aggregate myopathy (TAM) is an inherited muscle disease associated with progressive weakness, cramps, myalgia, and exercise intolerance that primarily affects proximal muscles of the lower limbs. The extensive presence of tubular aggregates (TAs), arrays of ordered and densely packed sarcoplasmic reticulum (SR) tubes, in muscle biopsies from TAM patients represents a key histopathological hallmark of this disease. Recent studies have linked TAM to gain-of-function mutations in the Stim1, Orai1, and Casq1 genes. The proteins encoded by these genes coordinate store operated Ca2+ entry (SOCE), a mechanism that enables muscle to replenish SR Ca2+ stores needed to maintain force production. TAM mutations in Stim1, Orai1 and Casq1 are thought to promote constitutively active Ca2+ entry that serves as an upstream driver for an age- dependent myopathy with TAs. However, the precise downstream cellular and molecular mechanisms involved remain to be elucidated. To date, there is no cure or effective treatment for patients suffering from TAM. To identify downstream pathogenic mechanisms, as well as test and validate effective therapeutic interventions, we generated knock-in mice with a G100S TAM mutation in Orai1 (Orai1G100S/+) and a separate line of mice with a D44N TAM mutation in Casq1 (Casq1D44N/+). Consistent with that observed in TAM patients, Orai1G100S/+ mice exhibit a myopathy characterized by weakness, exercise intolerance, elevated creatine kinase levels and TAs, while Casq1D44N/+ mice exhibit a milder, later onset phenotype with an age-dependent increase in TAs. We also found that TAs are reduced and force production increased after housing Orai1G100S/+ mice for 6 months in cages with voluntary running wheels. We will use these TAM mouse models to investigate mechanisms of disease pathogenesis, muscle adaptations, and test novel therapeutic interventions. The Overall Hypothesis of this proposal is that gain-of-function TAM mutations in Orai1 and Casq1 result in uncontrolled, constitutive Ca2+ entry during early muscle development that drives a transcriptional program that initially reduces aberrant Orai1 function, but ultimately, leads to a myopathy characterized by TAs. We further hypothesize that the myopathy and TAs in Orai1G100S/+ mice can be mitigated by early and late intervention with exercise mimetic therapy. Aim 1 will characterize age- and sex-dependent changes in SOCE function, muscle phenotype, TAs, muscle proteome and mitochondrial function in Orai1G100S/+ mice. Aim 2 will characterize age- and sex-dependent changes in SOCE function, muscle phenotype, TAs, muscle proteome, and mitochondrial function in Casq1D44N/+ mice. Aim 3 will assess the therapeutic potential of early and late intervention with an exercise mimetic (150-500 mg/kg/day AICAR using osmotic minipumps) in preventing and reversing, respectively, the myopathy observed in Orai1G100S/+ mice. Overall, the results will provide important new insights into the pathogenesis of TAM and assess the therapeutic potential of exercise mimetic therapy in mitigating TAM disease progression.