Alternative splicing alterations of Ca²⁺ handling genes are associated with Ca²⁺ signal dysregulation in myotonic dystrophy type 1 (DM1) and type 2 (DM2) myotubes

Aims: The pathogenesis of myotonic dystrophy type 1 (DM1) and type 2 (DM2) has been related to the aberrant splicing of several genes, including those encoding for ryanodine receptor 1 (RYR1), sarcoplasmatic/endoplasmatic Ca²⁺-ATPase (SERCA) and α_1S subunit of voltage-gated Ca²⁺ channels (Ca_v1.1). The aim of this study is to determine whether alterations of these genes are associated with changes in the regulation of intracellular Ca²⁺ homeostasis and signalling. Methods: We analysed the expression of RYR1, SERCA and Ca_v1.1 and the intracellular Ca²⁺ handling in cultured myotubes isolated from DM1, DM2 and control muscle biopsies by semiquantitative RT-PCR and confocal Ca²⁺ imaging respectively. Results: (i) The alternative splicing of RYR1, SERCA and Ca_v1.1 was more severely affected in DM1 than in DM2 myotubes; (ii) DM1 myotubes exhibited higher resting intracellular Ca²⁺ levels than DM2; (iii) the amplitude of intracellular Ca²⁺ transients induced by sustained membrane depolarization was higher in DM1 myotubes than in controls, whereas DM2 showed opposite behaviour; and (iv) in both DM myotubes, Ca²⁺ release from sarcoplasmic reticulum through RYR1 was lower than in controls. Conclusion: The aberrant splicing of RYR1, SERCA1 and Ca_v1.1 may alter intracellular Ca²⁺ signalling in DM1 and DM2 myotubes. The differing dysregulation of intracellular Ca²⁺ handling in DM1 and DM2 may explain their distinct sarcolemmal hyperexcitabilities.