Role of cell-autonomous regulation of metabolism on muscle stem cell fate and skeletal muscle homeostasis

Description

During muscle regeneration following injury, muscle stem cells, also called satellite cells, leave their quiescent state and activate. MuSCs are capable of both differentiating to repair muscle tissue after an injury and self-renewing to replenish the pool of stem cells. The regulation of their fate is modulated by several signaling pathways such as Wnt, Notch or TGFß pathway. However, there are few data concerning the involvement of metabolism in the fate of satellite cells. Yet it has been shown that the activation of satellite cells is closely related to cellular metabolism, which one of the main players is AMPK protein kinase. This heterotrimeric complex, composed of three subunits α, ß et 𝜸, is responsible for the balance between energy consumption and energy production within the cell. With the modulation of mTORC1, AMPK 1 has also been shown to be responsible for cell growth and proliferation of myogenic precursors. Using different mouse models, primary lines and sorted satellite cells, we determined the role that each isoform, AMPKα1 and AMPKα2, could play within the cell, on myogenesis and on the homeostasis of the regenerated muscle. First, we demonstrated that AMPKα1-LDH signaling pathway regulates the satellite cells self-renewal by controlling metabolism. Indeed, at the time of cell fate choice between commitment into terminal differentiation versus self-renewal, the AMPK 1 pathway induces a decrease in LDH activity, allowing cells to adopt an oxidative phosphorylation metabolism responding to their energy needs. In a second time, we demonstrated that the AMPKα2 isoform, expressed during myogenesis only after the induction of muscle cell differentiation, was responsible for a modulation of the muscular regeneration and that its absence induced a lack of differentiation and a delay in maturation of the new formed myofibers. Our work allowed us to confirm the central role of AMPK protein kinase in the regulation, by the modulation of metabolism, of muscle stem cell fate in a context of skeletal muscle regeneration in a mouse model.

Isabella Scionti, Team Schaeffer November 20, 2017

Epigenetic regulation of skeletal muscle differentiation

LSD1 and PHF2 are lysine de-methylases that can de-methylate both histone proteins, influencing gene expressionand non-histone proteins, affecting their activity or stability. Functional approaches using Lsd1or Phf2 inactivation in mouse have demonstrated the involvement of these enzymes in the engagement of progenitor cells into differentiation.

One of the best-characterized examples of how progenitor cells multiply and differentiate to form functional organ is myogenesis. It is initiated by the specific timing expression of the specific regulatory genes; among these factors, MYOD is a key regulator of the engagement into differentiation of muscle progenitor cells. Although the action of MYOD during muscle differentiation has been extensively studied, still little is known about the chromatin remodeling events associated with the activation of MyoDexpression. Among the regulatory regions of MyoDexpression, the Core Enhancer region (CE), which transcribes for a non-coding enhancer RNA (CEeRNA), has been demonstrated to control the initiation of MyoDexpression during myoblast commitment.

We identified LSD1 and PHF2 as key activators of the MyoDCE. In vitroand in vivoablation of LSD1 or inhibition of LSD1 enzymatic activity impaired the recruitment of RNA PolII on the CE, resulting in a failed expression of the CEeRNA. According to our results, forced expression of the CEeRNA efficiently rescue MyoDexpression and myoblast fusion in the absence of LSD1. Moreover PHF2 interacts with LSD1 regulating its protein stability. Indeed in vitroablation of PHF2 results in a massive LSD1 degradation and thus absence of CEeRNA expression. However, all the histone modifications occurring on the CE region upon activation cannot be directly attributed to LSD1 or PHF2 enzymatic activity.

These results raise the question of the identity of LSD1 and PHF2 partners, which co-participate to CEeRNA expression and thus to the engagement of myoblast cells into differentiation.