Down-regulation of the mitochondrial matrix peptidase CIpP in muscle cells causes mitochondrial dysfunction and decreases cell proliferation

The caseinolytic peptidase P (CIpP) is the endopeptidase component of the mitochondrial matrix ATP dependent CIpXP protease. CIpP degrades unfolded proteins to maintain mitochondrial protein homeostasis and is involved in the initiation of the mitochondrial unfolded protein response (UPRmt). Outside of an integral role in the UPImt, the cellular function of CIpP is not well characterized in mammalian cells. To investigate the role of CIpP in mitochondrial function, we generated C2C12 muscle cells that are deficient in CIpP using siRNA or stable knockdown using lentiviral transduction. Reduction of CIpP levels by similar to 70% in C2C12 muscle cells resulted in a number of mitochondrial alterations including reduced mitochondrial respiration and reduced oxygen consumption rate in response to electron transport chain (ETC) complex I and II substrates. The reduction in CIpP altered mitochondrial morphology, changed the expression level of mitochondrial fission protein Drpl and blunted UPRmt induction. In addition, CIpP deficient cells showed increased generation of reactive oxygen species (ROS) and decreased membrane potential. At the cellular level, reduction of CIpP impaired myoblast differentiation, cell proliferation and elevated phosphorylation of eukaryotic initiation factor 2 alpha (elF2o) suggesting an inhibition of translation. Our study is the first to define the effects of CIpP deficiency on mitochondrial function in muscle cells in vitro. In addition, we have uncovered novel effects of CIpP on mitochondrial morphology, cell proliferation and protein translation pathways in muscle cells. (C) 2015 Elsevier Inc. All rights reserved.