Characterization of ORAI1G100s/+ mouse model of tubular aggregate myopathy.

Abstract

Tubular aggregate myopathy (TAM) is a heritable, progressive myopathy characterized by weakness, cramps and muscle pain. A key histologically feature of TAM is the presence of tubular aggregates (TAs), highly-ordered and tightly packed tubules of sarcoplasmic reticulum origin that appear as honeycomb-like structures in muscle cross sections. TAM is caused by gain-of-function mutations in STIM1 and ORAI1, which lead to constitutively-activated store-operated calcium entry (SOCE) that serves as an upstream trigger driving the myopathy and TA formation. Though 3 STIM1 knock-in mouse models that phenocopy several multi-systemic aspects of TAM were previously reported, none exhibit TAs and there are no mouse models of TAM based on Orai1 mutations. Here, we generated a new mouse model with a knock-in mutation in ORAI1 linked to TAM (ORAI1G100S/+ mice). Adult ORAI1G100S/+ mice exhibit significant muscle weakness (both in vivo and in vitro) and TAs in extensor digitorum longus muscles, validating ORAI1G100S/+ mice as the first TAM model with both functional and histological features of TAM. As expected for a gain of function mutation in ORAI1, constitutive Ca2+ entry is observed in myotubes and muscle fibers from young (≤5-week-old) ORAI1G100S/+ mice. However, both constitutive and store-operated Ca2+ entry are essentially abolished in muscle fibers from ≥2-month-old ORAI1G100S/+ mice. Since homozygous Orai1G100S/G100S mice are embryonic lethal, homotypic expression of G100S Orai1 was accomplished by generating MCK-Cre+:ORAI1G100S/fl mice (one G100S allele and one floxed ORAI1 allele). MCK-Cre+:ORAI1G100S/fl mice exhibit more severe phenotypes than ORAI1G100S/+ mice as assessed by both behavioral and physiological measures. Together, these findings indicate ORAI1G100S/+ mice represent an ideal model to investigate the pathophysiological mechanisms underlying muscle weakness and TA formation in TAM, as well as the compensatory responses of muscle and relevance, if any, of TAs in the disease.