TAM-associated CASQ1 mutants diminish intracellular Ca²⁺ content and interfere with regulation of SOCE

Tubular aggregate myopathy (TAM) is a rare myopathy characterized by muscle weakness and myalgia. Muscle fibers from TAM patients show characteristic accumulation of membrane tubules that contain proteins from the sarcoplasmic reticulum (SR). Gain-of-function mutations in STIM1 and ORAI1, the key proteins participating in the Store-Operated Ca²⁺ Entry (SOCE) mechanism, were identified in patients with TAM. Recently, the CASQ1 gene was also found to be mutated in patients with TAM. CASQ1 is the main Ca²⁺ buffer of the SR and a negative regulator of SOCE. Previous characterization of CASQ1 mutants in non-muscle cells revealed that they display altered Ca²⁺dependent polymerization, reduced Ca²⁺storage capacity and alteration in SOCE inhibition. We thus aimed to assess how mutations in CASQ1 affect calcium regulation in skeletal muscles, where CASQ1 is naturally expressed. We thus expressed CASQ1 mutants in muscle fibers from Casq1 knockout mice, which provide a valuable model for studying the Ca²⁺ storage capacity of TAM-associated mutants. Moreover, since Casq1 knockout mice display a constitutively active SOCE, the effect of CASQ1 mutants on SOCE inhibition can be also properly examined in fibers from these mice. Analysis of intracellular Ca²⁺ confirmed that CASQ1 mutants have impaired ability to store Ca²⁺and lose their ability to inhibit skeletal muscle SOCE; this is in agreement with the evidence that alterations in Ca²⁺entry due to mutations in either STIM1, ORAI1 or CASQ1 represents a hallmark of TAM.