A muscular hypotonia-associated STIM1 mutant at R429 induces abnormalities in intracellular Ca2+ movement and extracellular Ca2+ entry in skeletal muscle

Stroma I interaction molecule 1 (STIM1) mediates extracellular Ca2+ entry into the cytosol through a store-operated Ca2+ entry (SOCE) mechanism, which is involved in the physiological functions of various tissues, including skeletal muscle. STIM1 is also associated with skeletal muscle diseases, but its pathological mechanisms have not been well addressed. The present study focused on examining the pathological mechanism(s) of a mutant STIM1 (R429C) that causes human muscular hypotonia. R429C was expressed in mouse primary skeletal myotubes, and the properties of the skeletal myotubes were examined using single-cell Ca2+ imaging of myotubes and transmission electron microscopy (TEM) along with biochemical approaches. R429C did not interfere with the terminal differentiation of myoblasts to myotubes. Unlike wild-type STIM1, there was no further increase of SOCE by R429C. R429C bound to endogenous STIM1 and slowed down the initial rate of SOCE that were mediated by endogenous STIM1. Moreover, R429C increased intracellular Ca2+ movement in response to membrane depolarization by eliminating the attenuation on dihydropyridine receptor-ryanodine receptor (DHPRRyR1) coupling by endogenous STIM1. The cytosolic Ca2+ level was also increased due to the reduction in SR Ca2+ level. In addition, R429C-expressing myotubes showed abnormalities in mitochondria! shape, a significant decrease in ATP levels, and the higher expression levels of mitochondria! fission-mediating proteins. Therefore, serial defects in SOCE, intracellular Ca2+ movement, and cytosolic Ca2+ level along with mitochondria! abnormalities in shape and ATP level could be a pathological mechanism of R429C for human skeletal muscular hypotonia. This study also suggests a novel clue that STIM1 in skeletal muscle could be related to mitochondria via regulating intra and extracellular Ca2+ movements.