Kathryn J. Mitchell

RATIONALE: Pericytes are key regulators of vascular maturation, but their value for cardiac repair remains unknown. OBJECTIVE: We investigated the therapeutic activity and mechanistic targets of saphenous vein-derived pericyte progenitor cells (SVPs) in a mouse myocardial infarction (MI) model. METHODS AND RESULTS: SVPs have a low immunogenic profile and are resistant to hypoxia/starvation (H/S). Transplantation of SVPs into the peri-infarct zone of immunodeficient CD1/Foxn-1(nu/nu) or immunocompetent CD1 mice attenuated left ventricular dilatation and improved ejection fraction compared to vehicle. Moreover, SVPs reduced myocardial scar, cardiomyocyte apoptosis and interstitial fibrosis, improved myocardial blood flow and neovascularization, and attenuated vascular permeability. SVPs secrete vascular endothelial growth factor A, angiopoietin-1, and chemokines and induce an endogenous angiocrine response by the host, through recruitment of vascular endothelial growth factor B expressing monocytes. The association of donor- and recipient-derived stimuli activates the proangiogenic and prosurvival Akt/eNOS/Bcl-2 signaling pathway. Moreover, microRNA-132 (miR-132) was constitutively expressed and secreted by SVPs and remarkably upregulated, together with its transcriptional activator cyclic AMP response element-binding protein, on stimulation by H/S or vascular endothelial growth factor B. We next investigated if SVP-secreted miR-132 acts as a paracrine activator of cardiac healing. In vitro studies showed that SVP conditioned medium stimulates endothelial tube formation and reduces myofibroblast differentiation, through inhibition of Ras-GTPase activating protein and methyl-CpG-binding protein 2, which are validated miR-132 targets. Furthermore, miR-132 inhibition by antimiR-132 decreased SVP capacity to improve contractility, reparative angiogenesis, and interstitial fibrosis in infarcted hearts. CONCLUSION: SVP transplantation produces long-term improvement of cardiac function through a novel paracrine mechanism involving the secretion of miR-132 and inhibition of its target genes.

HDAC inhibitors (HDACi) exert beneficial effects in mdx mice, by promoting endogenous regeneration; however, the cellular determinants of HDACi activity on dystrophic muscles have not been determined. We show that fibroadipogenic progenitors (FAP) influence the regeneration potential of satellite cells during disease progression in mdx mice and mediate HDACi ability to selectively promote regeneration at early stages of disease. FAPs from young mdx mice promote, while FAPs from old mdx mice repress, satellite cell-mediated formation of myotubes. In young mdx mice HDACi inhibited FAP adipogenic potential, while enhancing their ability to promote differentiation of adjacent satellite cells, through upregulation of the soluble factor follistatin. By contrast, FAPs from old mdx mice were resistant to HDACi-mediated inhibition of adipogenesis and constitutively repressed satellite cell-mediated formation of myotubes. We show that transplantation of FAPs from regenerating young muscles restored HDACi ability to increase myofibre size in old mdx mice. These results reveal that FAPs are key cellular determinants of disease progression in mdx mice and mediate a previously unappreciated stage-specific beneficial effect of HDACi in dystrophic muscles.

The role of dense core secretory vesicles in the control of cytosolic-free Ca(2+) concentrations ([Ca(2+)](c)) in neuronal and neuroendocrine cells is enigmatic. By constructing a vesicle-associated membrane protein 2-synaptobrevin.aequorin chimera, we show that in clonal pancreatic islet beta-cells: (a) increases in [Ca(2+)](c) cause a prompt increase in intravesicular-free Ca(2+) concentration ([Ca(2+)]SV), which is mediated by a P-type Ca(2+)-ATPase distinct from the sarco(endo) plasmic reticulum Ca(2+)-ATPase, but which may be related to the PMR1/ATP2C1 family of Ca(2+) pumps; (b) steady state Ca(2+) concentrations are 3-5-fold lower in secretory vesicles than in the endoplasmic reticulum (ER) or Golgi apparatus, suggesting the existence of tightly bound and more rapidly exchanging pools of Ca(2+); (c) inositol (1,4,5) trisphosphate has no impact on [Ca(2+)](SV) in intact or permeabilized cells; and (d) ryanodine receptor (RyR) activation with caffeine or 4-chloro-3-ethylphenol in intact cells, or cyclic ADPribose in permeabilized cells, causes a dramatic fall in [Ca(2+)](SV). Thus, secretory vesicles represent a dynamic Ca(2+) store in neuroendocrine cells, whose characteristics are in part distinct from the ER/Golgi apparatus. The presence of RyRs on secretory vesicles suggests that local Ca(2+)-induced Ca(2+) release from vesicles docked at the plasma membrane could participate in triggering exocytosis.

We have demonstrated recently (Mitchell, K. J., Pinton, P., Varadi, A., Tacchetti, C., Ainscow, E. K., Pozzan, T., Rizzuto, R., and Rutter, G. A. (2001) J. Cell Biol. 155, 41-51) that ryanodine receptors (RyR) are present on insulin-containing secretory vesicles. Here we show that pancreatic islets and derived beta-cell lines express type I and II, but not type III, RyRs. Purified by subcellular fractionation and membrane immuno-isolation, dense core secretory vesicles were found to possess a similar level of type I RyR immunoreactivity as Golgi/endoplasmic reticulum (ER) membranes but substantially less RyR II than the latter. Monitored in cells expressing appropriately targeted aequorins, dantrolene, an inhibitor of RyR I channels, elevated free Ca(2+) concentrations in the secretory vesicle compartment from 40.1 +/- 6.7 to 90.4 +/- 14.8 microm (n = 4, p < 0.01), while having no effect on ER Ca(2+) concentrations. Furthermore, nicotinic acid adenine dinucleotide phosphate (NAADP), a novel Ca(2+)-mobilizing agent, decreased dense core secretory vesicle but not ER free Ca(2+) concentrations in permeabilized MIN6 beta-cells, and flash photolysis of caged NAADP released Ca(2+) from a thapsigargin-insensitive Ca(2+) store in single MIN6 cells. Because dantrolene strongly inhibited glucose-stimulated insulin secretion (from 3.07 +/- 0.51-fold stimulation to no significant glucose effect; n = 3, p < 0.01), we conclude that RyR I-mediated Ca(2+)-induced Ca(2+) release from secretory vesicles, possibly potentiated by NAADP, is essential for the activation of insulin secretion.