Pitx2 and Pitx3 transcription factors: two key regulators of the redox state in adult skeletal muscle stem cells and muscle regeneration

Adult tissue homeostasis and regeneration rely on tissue stem cell populations that generate committed precursors and differentiated cells while maintaining a pool of stem cells. In adult skeletal muscle, such cells, called satellite cells, remain quiescent at the periphery of muscle fibers. Upon injury they undergo activation, proliferation and differentiation to replace damaged fibers and also self-renew to reconstitute the muscle stem cell pool. During regeneration, the transition from a quiescent muscle stem cell to a differentiated fiber is accompanied by major metabolic changes. Such changes, and notably the switch from a glycolytic proliferative progenitor state to an oxidative post-mitotic differentiated state, require extensive mitochondrial biogenesis that takes place at the onset of differentiation and leads to increased ROS production. However, it is unclear whether this enhanced ROS production/mitochondrial content reflects an adaptation to the rising energy demand or whether it constitutes an essential regulation element of the differentiation program.To investigate the potential role of this metabolic switch and more specifically of reactive oxygen species during muscle regeneration, we took advantage of mouse mutants for Pitx2 and Pitx3 genes. Both genes are involved in foetal myogenesis where they have been identified as key regulators of the redox state preventing excessive ROS levels and DNA damage as cells undergo differentiation. We have now analyzed adult single and double Pitx2:Pitx3 conditional mutant mouse lines targeted to the muscle stem cell compartment. Double mutant satellite cells undergo senescence with impaired regeneration after injury, whereas in single Pitx3 mutants, premature differentiation occurs. We show that these effects are directly linked to dose-dependent changes in ROS levels and can be reversed by lowering ROS with the N-acetylcystein, supporting the notion that a controlled increase in ROS is required for differentiation of muscle stem cells.