Cyclophilin‐D is dispensable for atrophy and mitochondrial apoptotic signalling in denervated muscle

Non‐technical summary Activation of apoptotic cell death signalling by mitochondria is involved in mediating skeletal muscle atrophy following denervation. However, the underlying molecular mechanisms remain unclear, in particular the role of the permeability transition pore (PTP), a high conductance non specific channel of the mitochondrial inner membrane involved in cell death. Using Ppif−/− mice, which are devoid of the PTP sensitizing protein cyclophilin‐D and thus more resistant to pore opening, we directly tested the hypothesis that this pore is involved in atrophy and activation of apoptotic proteolytic signalling following denervation. Our results demonstrate that cyclophilin‐D, and by extension opening of the PTP, is dispensable for atrophy and activation of apoptotic proteolytic signalling induced by denervation. In the present study, we specifically determined whether the regulatory protein cyclophilin‐D (CypD), and by extension opening of the permeability transition pore (PTP), is involved in the activation of mitochondria‐derived apoptotic signalling previously described in skeletal muscle following loss of innervation. For this purpose, CypD‐defficient (CypD‐KO) mice and their littermate controls were submitted to unilateral sciatic nerve transection, and mitochondrial resistance to Ca2+‐induced opening of the PTP, and muscle apoptotic signalling were investigated 14 days post‐surgery. Denervation caused atrophy, facilitated Ca2+‐induced opening of the PTP in vitro in permeabilized muscle fibres, and activation of the apoptotic proteolytic cascade in the whole muscle of both mouse strains. In CypD‐KO mice, mitochondrial resistance to Ca2+‐induced PTP opening was greater than in WT mice, in both the normal and the denervated state, indicating that lack of CypD desensitized to PTP opening. However, denervation in CypD‐KO mice still resulted in a facilitation of PTP opening compared to normally innervated contralateral muscle, indicating that in vitro additional factors could poise mitochondria from denervated muscle toward PTP opening. At the whole muscle level, lack of CypD, despite conferring greater resistance to PTP opening, did not protect against atrophy, release of mitochondrial pro‐apoptotic factors and activation of caspases following denervation. Altogether, these results provide direct evidence that CypD‐dependent PTP opening is dispensable for atrophy and apoptotic signalling in skeletal muscle following denervation.