Col6 deficiency in a zebrafish model of Bethlem myopathy leads to dysfunction of the muscle dihydropyridine receptor
Bethlem myopathy (BM) results from mutations in genes encoding one of the three chains of collagen VI (ColVI). This muscle disease is characterized by skeletal muscle weakness and wasting worsening with age. How alteration in ColVI present outside muscle fibers in the extracellular matrix induces dysfunction within muscle fibers is still misunderstood. Here we explored intracellular Ca2+ handling properties in isolated fast skeletal muscle fibers from adult zebrafish harboring a mutation (col6a1{Delta}ex14) that is the most frequently found in BM patients. Col6a1{Delta}ex14 fish muscle exhibited progressive loss of ColVI deposition, defects in basement membrane and ColVI intracellular accumulation. By combining voltage-clamp and intracellular Ca2+ measurements on isolated fibers, we showed that voltage-dependence of intramembrane charge movements produced by the activation of CaV1.1 controlling sarcoplasmic reticulum (SR) Ca2+ release and voltage-dependence of depolarization-induced SR Ca2+ release were shifted toward negative potentials in col6a1{Delta}ex14 fish. These changes in voltage-dependence gave rise to larger SR Ca2+ leak at voltages close to resting values and to higher frequency of spontaneous SR Ca2+ release elementary events in mutant fish. Trunk muscle force and swimming performance were also found to be reduced in col6a1{Delta}ex14 fish and mis-localization of CaV1.1 subunits clusters was observed in mutant fibers t-tubules. These data indicate that ColVI deficiency in BM leads to CaV1.1 dysfunction that contributes to promote a pathogenic SR Ca2+ leak responsible for progressive muscle weakness and wasting. CaV1.1 could represent the still elusive transmembrane link allowing altered myomatrix to transduce pathogenic signals within muscle.