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The endothelial cell-specific microRNA (miRNA), miR-126, is considered a master regulator of physiological angiogenesis. Transplanted mesenchymal stem cells (MSCs) release soluble factors contributing to neoangiogenesis and cardiac repair. Therefore, we hypothesized that the over-expression of miR-126 may prolong MSC survival and enhance the cell secretome, thereby improving post-infarction angiogenesis and cardiac function. In this study, MSCs harvested from male C57BL/6 mouse bone marrow were infected in vitro with miR-126 (MSC(miR-126)) by using recombinant lentiviral vectors; the control cells were either non-transfected or transduced with mock vectors (MSC(null)). The results showed an increased secretion of angiogenic factors and a higher resistance against hypoxia in MSC(miR-126) compared with the control cells. The expression of the Notch ligand Delta-like (Dll)-4 in the MSC(miR-126) group was also increased. For in vivo experiments, MSC(miR-126) cultures were intramyocardially injected into the infarct region of the hearts of female C57BL/6 mice (an acute myocardial infarction model) who had undergone ligation of the left anterior descending coronary artery. The survival of MSC(miR-126) cultures, determined by Sry expression, was increased at 7 days after transplantation. MSC(miR-126)-treated animals showed significantly improved cardiac function as assessed by echocardiography 2 weeks later. The expression levels of angiogenic factors and Dll-4 in the infarcted myocardium were further increased by MSC(miR-126) compared with MSCs or MSC(null) cultures. Furthermore, fluorescent microsphere and histological studies revealed that myocardial blood flow and microvessel density were significantly increased in the MSC(miR-126)-transplanted animals. In addition, we found increased immature vessel proliferation following the transplantation of MSC(miR-126) cultures in which the expression of Dll-4 had been knocked down. However, blood vessels with lumen were barely detected, which indicated that Dll-4 plays a key role in tubulogenesis. We conclude that the transplantation of MSCs overexpressing miR-126 can further enhance functional angiogenesis in the ischemic myocardium possibly by the secretion of angiogenic factors and the activation of Dll-4, thus increasing MSC survival. Therefore, MSCs modified with miR-126 may represent a novel and efficient therapeutic approach for ischemic angiogenesis and the improvement of cardiac function.

The HOXA9 homeodomain protein is a key regulator of hematopoiesis and embryonic development. HOXA9 is expressed in primitive hematopoietic cells, and its prompt downregulation is associated with myelocytic maturation. Although transcriptional inactivation of HOXA9 during hematopoietic differentiation has been established, little is known about the biochemical mechanisms underlying the subsequent removal of HOXA9 protein. Here we report that the CUL-4A ubiquitylation machinery controls the stability of HOXA9 by promoting its ubiquitylation and proteasome-dependent degradation. The homeodomain of HOXA9 is responsible for CUL-4A-mediated degradation. Interfering CUL-4A biosynthesis by ectopic expression or by RNA-mediated interference resulted in alterations of the steady-state levels of HOXA9, mirrored by impairment of the ability of 32D myeloid progenitor cells to undergo proper terminal differentiation into granulocytes. These results revealed a novel regulatory mechanism of hematopoiesis by ubiquitindependent proteolysis.

Cochlear supporting cells (SCs), which include the cochlear progenitor cells, have been shown to be a promising resource for hair cell (HC) regeneration, but the mechanisms underlying the initiation and regulation of postnatal cochlear SC proliferation are not yet fully understood. Bmi1 is a member of the Polycomb protein family and has been reported to regulate the proliferation of stem cells and progenitor cells in multiple organs. In this study, we investigated the role of Bmi1 in regulating SC and progenitor cell proliferation in neonatal mice cochleae. We first showed that knockout of Bmi1 significantly inhibited the proliferation of SCs and Lgr5-positive progenitor cells after neomycin injury in neonatal mice in vitro, and we then showed that Bmi1 deficiency significantly reduced the sphere-forming ability of the organ of Corti and Lgr5-positive progenitor cells in neonatal mice. These results suggested that Bmi1 is required for the initiation of SC and progenitor cell proliferation in neonatal mice. Next, we found that DKK1 expression was significantly upregulated, while beta-catenin and Lgr5 expression were significantly downregulated in neonatal Bmi1 mice compared to wild-type controls. The observation that Bmi1 knockout downregulates Wnt signaling provides compelling evidence that Bmi1 is required for the Wnt signaling pathway. Furthermore, the exogenous Wnt agonist BIO overcame the downregulation of SC proliferation in Bmi1 mice, suggesting that Bmi1 knockout might inhibit the proliferation of SCs via downregulation of the canonical Wnt signaling pathway. Our findings demonstrate that Bmi1 plays an important role in regulating the proliferation of cochlear SCs and Lgr5-positive progenitor cells in neonatal mice through the Wnt signaling pathway, and this suggests that Bmi1 might be a new therapeutic target for HC regeneration.