Muscle stem cell quiescence and heterogeneity – SatNet

The capacity of the skeletal muscle to regenerate depends on satellite cells, the quiescent stem cells of the adult muscle. Quiescent satellite cells are activated in response to injury or in degenerative muscle diseases. During regeneration, a proportion of the activated satellite cells self-renews and regains quiescence while some will proliferate and generate new muscle fibers. Other cell populations in the muscle like interstitial, microvascular or inflammatory cells are thought to provide modulatory signals. Defining the signals that allow satellite cell maintenance, self-renewal and differentiation is a prerequisite for stimulating the endogenous stem cell-mediated repair in therapy. In the adult, an ill defined subpopulation of quiescent satellite cells will remain in the dormant state for long periods, with low metabolic rates, anaerobic metabolism, and are resistant against stress like hypothermia. The satellite cells population is therefore heterogeneous, with a population of dormant cells that do not divide (label-retaining cells, LRCs), as well as a second population that is thought to cycle at very low rates (non- LRCs). The two populations do not only differ in proliferative activity, but also have different self-renewing and regenerative capacities. Why these two populations behave differently is not understood and difficult to assess since the different satellite cell populations are small and difficult to isolate.
We propose here to join efforts of French and German teams to identify the molecular mechanisms allowing the establishment/maintenance of quiescence in satellite cells and during muscle regeneration. We will combine efforts to analyze the satellite cell heterogeneity, with the aim to characterize on a molecular level the enigmatic subpopulation of satellite cells that has superior engraftment and self-renewal properties. For this, sate of the art techniques like mass cytometry and single cell sequencing will be used. We will perform a systematic analysis of cell cycle and myogenic factors during skeletal muscle homeostasis and regeneration, with a specific focus upon factors and signals that control Cyclin-dependent kinase inhibitors. We will also analyze the oscillatory expression of components of the Notch signaling pathway and targets thereof, and assess the functional consequences of oscillatory expression for proliferation and cell cycle exit. Finally, we will characterize the molecular and functional characteristics of human satellite cells that are resilient to hypothermic treatment, and establish an animal model that provides cellular material to assess the functional consequences of hypothermic treatment in depth. The French and German partners have already ongoing collaborations and different interdependent expertise and address in the collaborative efforts complementary goals, which will provide ample opportunities for further collaborations and interactions.
We expect that our work will provide valuable information regarding the cellular and molecular details of satellite cell biology and skeletal muscle regeneration. In addition, we anticipate to unravel new mechanisms that control the long-term maintenance of satellite cells through the control of their quiescence and growth arrest. Our work will also shed light on an as yet enigmatic subpopulation of resilient satellite cells that have high self-renewal and regenerative capacities. This proposal will therefore improve basic knowledge on muscle stem cells and contribute to the development of therapies that rely on such cells.