Hypoxia promotes satellite cell self-renewal and enhances the efficiency of myoblast transplantation

Abstract

SUMMARYMicroenvironmental oxygen (O 2 ) regulates stem cell activity, and a hypoxic niche with low oxygen levels has been reported in multiple stem cell types. Satellite cells are muscle-resident stem cells that maintain the homeostasis and mediate the regeneration of skeletal muscles. We demonstrate here that hypoxic culture conditions favor the quiescence of satellite cell-derived primary myoblasts by upregulating Pax7, a key regulator of satellite cell self-renewal, and downregulating MyoD and myogenin. During myoblast division, hypoxia promotes asymmetric self-renewal divisions and inhibits asymmetric differentiation divisions without affecting the overall rate of proliferation. Mechanistic studies reveal that hypoxia activates the Notch signaling pathway, which subsequently represses the expression of miR-1 and miR-206 through canonical Hes/Hey proteins, leading to increased levels of Pax7. More importantly, hypoxia conditioning enhances the efficiency of myoblast transplantation and the self-renewal of implanted cells. Given the robust effects of hypoxia on maintaining the quiescence and promoting the self-renewal of cultured myoblasts, we predict that oxygen levels in the satellite cell niche play a central role in precisely balancing quiescence versus activation, and self-renewal versus differentiation, in muscle stem cells in vivo.

Regulation of myogenesis by environmental hypoxia

“…By contrast, the asymmetric divisions that generate the self-renewing and proliferating progeny are increased. Conversely the asymmetric divisions that generate a differentiating and a proliferating progeny are decreased (Liu et al, 2012). Analysis of satellite cells derived from 31-month-old rats cultured at 3% O 2 indicate an increase in the number of large colonies of satellite cells.…”

“…Hypoxia has been shown to induce quiescence of satellite cells that derive from mouse primary myoblasts of hindlimb muscles by upregulating Pax7 and by downregulating MyoD and myogenin (Liu et al, 2012). In cultured cells at 1% O 2 , the overall rate of proliferation is not affected.…”

“…However, only few of the transplanted myoblasts survive and contribute to new satellite cells, resulting in limited therapeutic applications (Kuang and Rudnicki, 2008). In contrast, hypoxia-conditioned myoblasts show improved homing and regenerative efficiencies when they are grafted into mdx mice, an animal model of Duchenne muscular dystrophy (DMD) that exhibits muscle degeneration (Liu et al, 2012). These myoblasts show reduced expression levels of MyoD, which decreases their ability to differentiate and so enhances their self-renewal potential after transplantation.…”

“…In mouse primary myoblasts isolated from hindlimb muscle, hypoxia maintains myoblasts in their undifferentiated state with a rapid increase in Pax7 expression, suggesting that post-transcriptional regulation of Pax7 might take place. As miR-1 and miR-206 can recognize the 3′UTR of Pax7 mRNA and downregulate Pax7 protein production during normoxia (Chen et al, 2010), Liu et al addressed their role and showed that miR-1 and miR-206 are downregulated in hypoxic cultures (Liu et al, 2012). These data also show that hypoxia upregulates protein levels of Pax7 through the downregulation of miR-1 and miR-206, as knockdown of miR-1 and miR206 abolishes the effect of hypoxia on Pax7.…”

New Strategies for Myocardial Infarction Treatment

“…Autologous transplantation of SkMCs into the infarcted heart has the advantages in cell isolation, in vitro culture, high survival rate under hypoxic conditions [24] , the unipotent to differentiate into contractile cells, without immune rejection, and no moral dilemma. Transplantation of SkMCs in human and animal models of MI suggests that the beneficial effect of this cell therapy is reducing left ventricle dilation by altering the expression profile of cardiac genes associated with left ventricle remodeling [25,26] .…”