Transiently Active Wnt/β-Catenin Signaling Is Not Required but Must Be Silenced for Stem Cell Function during Muscle Regeneration

摘要

class="head no_bottom_margin" id="sec1title">IntroductionAdult vertebrate muscle has an exceptional capacity for regeneration, mediated by a dedicated population of muscle stem cells. These muscle stem cells, termed satellite cells, were first identified by their unique anatomical position between the sarcolemma and basement membrane of myofibers (). Subsequently, satellite cells were found to express the transcription factor Pax7 (), and Pax7 is required for their maintenance in adult mice (). Recent genetic labeling and ablation studies in mouse, using Pax7^CreER mice, have definitively established that satellite cells are the endogenous stem cells necessary and sufficient for muscle regeneration (). During regeneration, satellite cells activate, proliferate, and give rise to transit-amplifying myoblasts, which differentiate into myocytes that fuse with one another to form multinucleate myofibers. In addition, like other stem cells, satellite cells self-renew.Canonical Wnt/β-catenin signaling is an important regulator of many adult stem cells () and has been proposed to be critical for satellite cells and muscle regeneration. Wnts are secreted glycoproteins that function as ligands, and β-catenin is the central mediator of canonical Wnt signaling (). In the absence of Wnts, β-catenin is phosphorylated and targeted for degradation. The binding of Wnts to their receptors leads to the formation of stabilized, unphosphorylated β-catenin that translocates to the nucleus, where it binds to TCF/LEF proteins and activates transcription of Wnt-responsive genes. Many studies have identified Wnt pathway components as being active during muscle regeneration (). Based largely on gain-of-function, primarily in vitro experiments, multiple labs have proposed that Wnt/β-catenin signaling is essential for muscle regeneration, although the conclusions of these papers are often contradictory (reviewed in ). However, no studies have explicitly examined in vivo whether Wnt/β-catenin signaling is necessary and sufficient specifically within satellite cells and their derivatives for muscle regeneration.In this study, we use a highly sensitive reporter of Wnt/β-catenin signaling (TCF/Lef:H2B-GFP^Tg; ), as well as a reagent (Pax7^CreERT2) that our lab has generated to genetically manipulate satellite cells with high specificity and efficiency (), to test the role of this signaling pathway specifically within satellite cells and their derivatives during muscle regeneration. We find that Wnt/β-catenin signaling is transiently active in myoblasts during regeneration. However, β-catenin is not required cell autonomously for muscle regeneration. Instead, downregulation of transiently activated β-catenin is critical for limiting the regenerative response, as continuous regeneration deleteriously leads to increased fibrosis and an increased number of small myofibers. Thus, surprisingly, we show that it is not activation of Wnt/β-catenin signaling but rather silencing of this activation that is important for muscle regeneration.