MicroRNAs (miRNAs) are a group of small noncoding RNAs that regulate the stability or translation of cognate mRNAs at the post-transcriptional level. Accumulating evidence indicates that miRNAs play important roles in many aspects of muscle function, including muscle growth and development, regeneration, contractility, and muscle fiber type plasticity. In the current study, we examined the function of miR-151-3p in myoblast proliferation and differentiation. Results show that overexpression of miR-151-3p not only upregulates myoblast proliferation, but also decreases slow muscle gene expression (such as MHC-β/slow and slow muscle troponin I) in both C2C12 myotubes and in primary cultures. Alternatively, inhibition of miR-151-3p by antisense RNA was found to upregulate MHC-β/slow expression, indicating that miR-151-3p plays a role in muscle fiber type determination. Further investigation into the underlying mechanisms revealed for the first time that miR-151-3p directly targets ATP2a2, a gene encoding for a slow skeletal and cardiac muscle specific Ca(2+) ATPase, SERCA2 thus downregulating slow muscle gene expression. Mechanisms by which the alteration in SERCA2 expression induces changes in other slow muscle gene expression levels needs to be defined in future research.

Retaining muscle stem satellite cell (SC) quiescence is important for the maintenance of stem cell population and tissue regeneration. Accumulating evidence supports the model where key extracellular signals play crucial roles in maintaining SC quiescence or activation, however, the intracellular mechanisms that mediate niche signals to control SC behavior are not fully understood. Here, we reported that KLF7 functioned as a key mediator involved in low-level TGF-b signaling and canonical Notch signaling-induced SC quiescence and myoblast arrest. The data obtained showed that KLF7 was upregulated in quiescent SCs and nonproliferating myoblasts. Silence of KLF7 promoted SCs activation and myoblasts proliferation, but overexpression of KLF7 induced myogenic cell arrest. Notably, the expression of KLF7 was regulated by TGF-b and Notch3 signaling. Knockdown of KLF7 diminished low-level TGF-b and canonical Notch signalinginduced SC quiescence. Investigation into the mechanism revealed that KLF7 regulation of SC function was dependent on p21 and acetylation of Lys227 and/or 231 in the DNA binding domain of KLF7. Our study provides new insights into the regulatory network of muscle stem cell quiescence. STEM CELLS 2016;34:1310-1320 SIGNIFICANCE STATEMENTIn this study, we provided evidences to gain insights into the niche signaling regulation of muscle stem cell quiescence and explored the requirement for KLF7 in TGF-b signaling blockage and canonical Notch signaling-induced myogenic cell cycle withdrawal. These findings provided new insights into the regulatory network of muscle stem cell quiescence.

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Background: Most bone marrow mesenchymal stem cell (BMSC) death is caused by the harsh ischemia and hypoxic microenvironment, which impacts the therapeutic effects of transplanted BMSCs. Fibronectin type III domain-containing protein 5 (FNDC5) and its cleaved product, irisin, are reportedly involved in cerebral protective effect. Research into whether FNDC5 plays a key role in the survival rate of BMSCs and cerebral infarction (CI) remains inadequate. The present study aimed to clarify the protective role of FNDC5 on the low viability of transplanted BMSCs and improve CI treatment outcomes. Methods: A lentivirus vector, which drives the expression of FNDC5, was constructed and used to transfect BMSCs. Cell Counting Kit-8 (CCK8), flow cytometry, immunofluorescence, and western blot were performed to evaluate the function of FNDC5-overexpressing BMSCs (BMSCs-OE-FNDC5) exposed to hypoxic and serum deprivation (H/SD) stress. Transmission electron microscopy (TEM) was used to monitor autophagy. In addition, BMSCs were engrafted into a middle cerebral artery occlusion (MCAO) rat model with or without FNDC5-overexpression (OE-FNDC5). The survival rate of transplanted BMSCs was evaluated by 5-ethynyl-2'-deoxyuridine (EdU) labeling. The CI volume was assessed by 2,3,5-triphenyl tetrazolium chloride (TTC) staining.Results: H/SD stress caused increased cell autophagy, apoptosis, and decreased cell viability of BMSCs, while OE-FNDC5 alleviated these injuries. The in vivo results showed that transplantation of BMSCs-OE-FNDC5 reduced the infarct volume in the rat MCAO model. Furthermore, OE-FNDC5 decreased neuronal apoptosis. The improved therapeutic efficacy of BMSCs-OE-FNDC5 may be attributable to the obviously increased cell survival number after transplantation.Conclusions: These results indicated that FNDC5 overexpression promotes BMSC survival in a CI model, which might provide a potential therapeutic target.

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