Skeletal muscle stem cells (MuSCs), also known as satellite cells, persist in adult mammals by entering a state of quiescence (G₀) during the early postnatal period. Quiescence is reversed during damage-induced regeneration and re-established after regeneration. Entry of cultured myoblasts into G₀ is associated with a specific, reversible induction of Wnt target genes, thus implicating members of the Tcf and Lef1 (Tcf/Lef) transcription factor family, which mediate transcriptional responses to Wnt signaling, in the initiation of quiescence. We found that the canonical Wnt effector β-catenin, which cooperates with Tcf/Lef, was dispensable for myoblasts to enter quiescence. Using pharmacological and genetic approaches in cultured C2C12 myoblasts and in MuSCs, we demonstrated that Tcf/Lef activity during quiescence depended not on β-catenin but on the transforming growth factor–β (TGF-β) effector and transcriptional coactivator Smad3, which colocalized with Lef1 at canonical Wnt-responsive elements and directly interacted with Lef1 specifically in G₀. Depletion of Smad3, but not β-catenin, reduced Lef1 occupancy at target promoters, Tcf/Lef target gene expression, and self-renewal of myoblasts. In vivo, MuSCs underwent a switch from β-catenin–Lef1 to Smad3-Lef1 interactions during the postnatal switch from proliferation to quiescence, with β-catenin–Lef1 interactions recurring during damage-induced reactivation. Our findings suggest that the interplay of Wnt-Tcf/Lef and TGF-β–Smad3 signaling activates canonical Wnt target promoters in a manner that depends on β-catenin during myoblast proliferation but is independent of β-catenin during MuSC quiescence.

骨骼肌干细胞（MuSCs），也称为卫星细胞，在产后早期进入静止状态（G ₀），在成年哺乳动物中持续存在。在损伤诱导的再生期间，静态被逆转，并且在再生之后重新建立。培养的成肌细胞进入G ₀Wnt与Wnt靶基因的特异性，可逆诱导相关，因此牵涉Tcf和Lef1（Tcf / Lef）转录因子家族的成员，后者在静止开始时介导对Wnt信号的转录反应。我们发现与Tcf / Lef协同作用的经典Wnt效应子β-连环蛋白对于成肌细胞进入静止状态是必不可少的。使用药理学和遗传学方法在培养的C2C12成肌细胞和MuSCs中，我们证明了静止时的Tcf / Lef活性不依赖于β-catenin，而是依赖于转化生长因子-β（TGF-β）效应子和转录共激活因子Smad3，后者与在典型的Wnt反应元件上的Lef1，并且直接与G _0中的Lef1直接相互作用。Smad3而非β-catenin的消耗减少了目标启动子上的Lef1占有率，Tcf / Lef目标基因表达和成肌细胞的自我更新。在体内，MuSC在出生后从增殖转变为静止的过程中经历了从β-catenin–Lef1到Smad3-Lef1相互作用的转变，在损伤诱导的再激活过程中，β-catenin–Lef1相互作用不断发生。我们的发现表明，Wnt-Tcf / Lef和TGF-β-Smad3信号的相互作用以成肌细胞增殖过程中依赖于β-catenin的方式激活了经典Wnt目标启动子，但在MuSC静止过程中独立于β-catenin。