In vivo partial reprogramming of myofibers promotes muscle regeneration by remodeling the stem cell niche

Abstract

Short-term, systemic expression of the Yamanaka reprogramming factors (Oct-3/4, Sox2, Klf4 and c-Myc [OSKM]) has been shown to rejuvenate aging cells and promote tissue regeneration in vivo. However, the mechanisms by which OSKM promotes tissue regeneration are unknown. In this work, we focus on a specific tissue and demonstrate that local expression of OSKM, specifically in myofibers, induces the activation of muscle stem cells or satellite cells (SCs), which accelerates muscle regeneration in young mice. In contrast, expressing OSKM directly in SCs does not improve muscle regeneration. Mechanistically, expressing OSKM in myofibers regulates the expression of genes important for the SC microenvironment, including upregulation of p21, which in turn downregulates Wnt4. This is critical because Wnt4 is secreted by myofibers to maintain SC quiescence. Thus, short-term induction of the Yamanaka factors in myofibers may promote tissue regeneration by modifying the stem cell niche.

Introduction

Reprogramming of somatic cells to a pluripotent state by overexpressing the Yamanaka factors (Oct-3/4, Sox2, Klf4, and c-Myc [OSKM]) is a long and complex process^1,2,3. Cellular reprogramming is widely utilized for disease modeling in vitro⁴. However, reprogramming in vivo induces tumor development^5,6,7. Our lab showed that partial reprogramming by short-term expression of reprogramming factors ameliorated aging hallmarks without tumor formation⁸, opening a possible application of this approach in vivo. Recently, other reports have demonstrated rejuvenation of dentate gyrus cells, retinal ganglion cells, chondrocytes, and muscle stem cells using reprogramming factors^9,10,11, reinforcing its potential application in clinical settings. Besides amelioration of cellular aging hallmarks, reprogramming factors promote tissue regeneration in aged mice^8,10. However, it is unknown whether OSKM-improved regeneration is solely a result of its rejuvenating effect.

Muscle regeneration is primarily mediated by muscle stem cells or satellite cells (SCs), which reside in a characteristic niche located between the basal lamina and plasma membrane of myofibers¹². The regenerative capacity of SCs is influenced by both intrinsic modulators and the extrinsic microenvironment^13,14,15. We have shown that partial reprogramming promotes skeletal muscle regeneration in 12-month-old mice⁸, but these studies were performed by expressing OSKM systemically (i.e., in all cell types)⁸. It is therefore unclear whether intrinsic or niche-specific factors contributed to the observed improvement in muscle regeneration.

In this work, we generate myofiber- and SC-specific OSKM induction mouse models to investigate the effect of OSKM induction on extrinsic and intrinsic modulators of SCs, respectively. In addition, we chose young mice to investigate whether the improvement of regeneration can be achieved by OSKM induction regardless of its rejuvenating effect. Our data shows that myofiber-specific OSKM induction accelerates muscle regeneration through downregulating the myofiber-secreted niche factor, Wnt4, to induce the activation and proliferation of SCs. In contrast, SC-specific OSKM induction does not improve muscle regeneration in young mice. We conclude that partial reprogramming via OSKM can remodel the SC niche to induce SC activation and proliferation and accelerate muscle regeneration….