p110α of PI3K is Indispensable for Quiescence Exit in Adult Muscle Satellite Cells

Adult muscle stem cells (MuSC) are quiescent with a localization between myofibers and basal lamina. Upon injury, MuSC exit quiescence, reenter cell cycle, expand and differentiate for muscle regeneration. By using genetic mouse model, we identified p110α/mTORC1 signaling as a indispensable pathway that permits quiescence exit and cell cycle reentry. In order to dig out the downstream effectors, we compared the transcriptome of freshly isolated MuSC from Ctrl (p110α-f/+:R26-YFP/YFP:Pax7-CreER/CreER) to MuSC-specific p110α-null (iKO, p110α-f/f:R26-YFP/YFP:Pax7-CreER/CreER) mice by RNA-sequencing, and AP1 target genes were dramatically down-regulated in iKO MuSC. Restoration of Jun could significantly rescue the cell cycle reentry defect in iKO MuSC. In summary, we provided a p110α/mTORC1/Jun axis required for quiecence exit and cell cycle reentry of MuSC. 2-month-old Ctrl and iKO mice (n=4 in each group) were injected with 5 consecutive doses of Tamoxifen (Tmx) followed by 7 doses of Tmx every 2-3 days. Total RNAs were extracted from freshly isolated MuSC from Ctrl and iKO mice and subjected to gene expression profiling.

成体肌肉干细胞（Muscle Stem Cells，下称MuSC）处于静息状态，定位于肌纤维与基膜之间。当机体遭受损伤时，MuSC会退出静息状态，重新进入细胞周期并增殖分化，以完成肌肉再生。本研究借助基因工程小鼠模型，鉴定出p110α/mTORC1信号通路是调控MuSC退出静息状态、重新进入细胞周期的不可或缺的途径。为挖掘该通路的下游效应分子，本研究通过RNA测序技术，对比了对照组（Control，下称Ctrl，基因型为p110α-f/+:R26-YFP/YFP:Pax7-CreER/CreER）与MuSC特异性诱导性p110α敲除小鼠（iKO，基因型为p110α-f/f:R26-YFP/YFP:Pax7-CreER/CreER）的新鲜分离MuSC转录组，发现激活蛋白1（Activator Protein 1，下称AP1）靶基因在iKO MuSC中显著下调。恢复Jun的表达可显著挽救iKO MuSC的细胞周期重进入缺陷。综上，本研究证实p110α/mTORC1/Jun信号轴是MuSC退出静息状态并重新进入细胞周期所必需的。本研究对每组各4只的2月龄Ctrl与iKO小鼠，连续注射5剂他莫昔芬（Tamoxifen，下称Tmx），随后每2-3天注射1剂，共注射7剂。随后从Ctrl与iKO小鼠的新鲜分离MuSC中提取总RNA，进行基因表达谱分析。