Brack Andrew S, Murphy-Seiler Fabienne, Hanifi Jasmine, Deka Jürgen, Eyckerman Sven, Keller Charles, Aguet Michel, Rando Thomas A
Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, CA 94305-5235, USA.
Dev Biol. 2009 Nov 1;335(1):93-105. doi: 10.1016/j.ydbio.2009.08.014. Epub 2009 Aug 21.
Muscle stem cells and their progeny play a fundamental role in the regeneration of adult skeletal muscle. We have previously shown that activation of the canonical Wnt/beta-catenin signaling pathway in adult myogenic progenitors is required for their transition from rapidly dividing transient amplifying cells to more differentiated progenitors. Whereas Wnt signaling in Drosophila is dependent on the presence of the co-regulator Legless, previous studies of the mammalian ortholog of Legless, BCL9 (and its homolog, BCL9-2), have not revealed an essential role of these proteins in Wnt signaling in specific tissues during development. Using Cre-lox technology to delete BCL9 and BCL9-2 in the myogenic lineage in vivo and RNAi technology to knockdown the protein levels in vitro, we show that BCL9 is required for activation of the Wnt/beta-catenin cascade in adult mammalian myogenic progenitors. We observed that the nuclear localization of beta-catenin and downstream TCF/LEF-mediated transcription, which are normally observed in myogenic progenitors upon addition of exogenous Wnt and during muscle regeneration, were abrogated when BCL9/9-2 levels were reduced. Furthermore, reductions of BCL9/9-2 inhibited the promotion of myogenic differentiation by Wnt and the normal regenerative response of skeletal muscle. These results suggest a critical role of BCL9/9-2 in the Wnt-mediated regulation of adult, as opposed to embryonic, myogenic progenitors.
肌肉干细胞及其子代细胞在成年骨骼肌再生过程中发挥着重要作用。我们之前已经表明，成年肌源性祖细胞中经典Wnt/β-连环蛋白信号通路的激活是其从快速分裂的瞬时扩增细胞转变为更具分化能力的祖细胞所必需的。果蝇中的Wnt信号传导依赖于共调节因子无腿蛋白（Legless）的存在，而之前对无腿蛋白在哺乳动物中的直系同源物BCL9（及其同源物BCL9-2）的研究，并未揭示这些蛋白在发育过程中特定组织的Wnt信号传导中的关键作用。利用Cre-lox技术在体内肌源性谱系中删除BCL9和BCL9-2，并利用RNAi技术在体外敲低蛋白水平，我们发现BCL9是成年哺乳动物肌源性祖细胞中Wnt/β-连环蛋白级联反应激活所必需的。我们观察到，当BCL9/9-2水平降低时，β-连环蛋白的核定位以及下游TCF/LEF介导的转录（通常在添加外源性Wnt后以及肌肉再生过程中在肌源性祖细胞中观察到）被消除。此外，BCL9/9-2的减少抑制了Wnt对肌源性分化的促进作用以及骨骼肌的正常再生反应。这些结果表明，BCL9/9-2在Wnt介导的成年（而非胚胎）肌源性祖细胞调节中起着关键作用。
