Role of Mitochondrial Calcium in the Maintenance of Skeletal Muscle Homeostasis

Calcium accumulates into the mitochondrial matrix through the mitochondrial calcium uniporter (MCU). Few years ago a MCU homolog has been discovered, which has been called MCUb. MCU and MCUb share a 50% sequence and structure similarity although some conserved differences in the primary sequence prevent MCUb from forming a Ca2+‐permeable channel. Interestingly, MCUb/MCU expression ratio varies greatly among tissues, suggesting that it might contribute to the spatiotemporal control of mitochondrial calcium uptake. We found that MCUb is highly expressed during skeletal muscle regeneration, a process that is mainly regulated by phenotypic skewing of tissue resident and freshly recruited macrophages. We have demonstrated that MCUb expression is specifically induced in macrophages, the most abundant cell population in a regenerating muscle that drive both the proliferation and differentiation of the muscle stem cells, satellite cells, thus affecting the progression of healing after muscle damage. In vitro and in vivo experiments demonstrated that the absence of MCU affects macrophages skewing from a pro (M1) to an anti‐inflammatory profile (M2). Indeed, macrophages of mice lacking MCUb are more prone to acquire a pro‐inflammatory M1 profile. Since M2 macrophages have a pro‐regenerative role and promote myogenic precursor cell differentiation, we hypothesized that MCUb absence, by affecting M2 polarization, might influence the correct remodeling and reorganization of skeletal muscle structure after damage. Our results clearly show that regenerating muscles lacking MCUb show a decrease in the expression level of myogenic regulatory transcription factors involved in satellite cells activation and differentiation. We also observed a decrease in the percentage of regenerating myofibers and a decrease in collagen deposition. In addition, co‐culture experiments demonstrated that macrophage‐conditioned medium from M2 MCUb KO macrophages drastically decreases satellite cell differentiation. Altogether these results demonstrate the role of mitochondrial Ca 2+ uptake in promoting the polarization of macrophages towards an M2 pro‐regenerative profile during skeletal muscle regeneration. Moreover, we found that skeletal muscle mitochondria express a unique MCU complex containing an alternative splice isoform of MICU1, MICU1.1, characterized by the addition of a micro‐exon that is sufficient to greatly modify the properties of the MCU. Indeed, MICU1.1 binds Ca 2+ one order magnitude more efficiently than MICU1. In skeletal muscle in vivo , MICU1.1 is required for sustained mitochondrial Ca 2+ uptake and ATP production. Furthermore, we observed that the MICU1 forced expression in skeletal muscle not only affects energy production but also influences muscle trophism. These results highlight a novel mechanism of the molecular plasticity of the MCU Ca 2+ uptake machinery and suggest a previously unexplored pathogenic mechanism in skeletal muscle diseases. Support or Funding Information This work is supported by the Italian Telethon ONLUS Foundation (Rome, Italy): GGP16026 to AR and the Italian Ministry of Health (Ricerca Finalizzata) (GR‐2016‐02362779 to AR).