The Impact of Parkin Dependent Mitophagy on Muscle Stem Cell Fate Decision, Lineage Progression, and Muscle Regeneration

Abstract

Skeletal muscles are considered one of the most dynamic tissues due to their capabilities of remodelling, repair, and regeneration attributed to their unique population of stem cells known as muscle stem cells (MuSCs). Mitochondrial function, maintenance, and remodelling has been linked with the ability of the MuSCs to maintain quiescence, undergo self-renewal, and differentiate in order to regenerate muscle. Subsequently, mechanisms affecting mitochondrial properties in MuSCs, including mitophagy, are suggested to play an important role. To determine the role of mitophagy in MuSC behavior, we developed a mouse model harboring a conditional MuSC specific inactivation of the key mitophagy gene Parkin. In vitro single myofiber experiments showed that Parkin deficiency triggered premature expression of activation and differentiation MuSC markers and lowered the proportion of quiescent cells indicating premature activation/commitment at the expense of the quiescent/self-renewing MuSC pool. Furthermore, cell cycle re-entry and proliferation were impaired in absence of Parkin, as evidenced by a reduction of MuSC cluster size, and the number of Ki67 (proliferation marker) expressing cells. Moreover, immunofluorescent experiments in the Tibialis Anterior (TA) muscle form cardiotoxin injured mice showed reduction in the Pax7+ and Ki67+ MuSCs at 7- and 14- days post injury indicating defects in the MuSC self-renewal and proliferation during muscle regeneration. The injured muscle also had a significant reduction in fiber size post injury indicating improper muscle regeneration and a defect in the regenerative potential. Furthermore, post-injury the Parkin deficient muscle also had a significant decrease in the number of Pax7+ self-renewing MuSCs signifying depletion of the MuSC pool. Together, our data suggests that manipulation of Parkin-mediated mitophagy alters muscle stem cell fate and lineage progression causing an imbalance in MuSC self-renewal and proliferation. Alterations in Parkin mediated mitophagy also impairs muscle regeneration and promotes the activation and commitment of MuSCs at the expense of MuSC self-renewal.