Defects in the coronary vascular system have significant impact on heart function and disease. Ischemic myocardial infarctions cause irreversible cell loss and scarring and are major source of morbidity and mortality in humans. A proper angiogenic response after infarction is critical for healing and repair.
A variety of stimuli can initiate the formation of new blood vessels in the heart, presumably through common downstream signaling cascades that trigger quiescent endothelial or other progenitor cells to form nascent tubular structures [1
]. Although many of the cellular and molecular mechanisms of embryonic coronary development are well investigated, the molecular basis of angiogenesis in the embryo seems to differ from the pathological vessel regeneration in adults [2
]. Blood vessels in the embryo form primarily through vasculogenesis, a differentiation of precursor cells (angioblasts) to endothelial cells that assemble into a vascular network. The critical cellular events include the formation of the primordium, the proepicardial organ and the epicardium, generation of the subepicardial space and mesenchymal cells and development and remodeling of the vascular plexus [3
]. New vessels in adults arise mainly through angiogenic sprouting, although vasculogenesis may also occur [2
]. Many studies have attempted to reveal the molecular and cellular mechanisms that support cardiac regeneration in adult hearts and to identify progenitor cells capable of cardiac repair [5
]. In zebrafish, epicardial cells invade the myocardium and create a vascular network likely to encourage cardiac regeneration in adults [12
]. Thus, the injured adult zebrafish heart can recall signaling pathways essential during embryonic coronary development, and the ability to mobilize epicardial cells may be the primary reason they effectively regenerate myocardium. Since adult mammalian hearts typically show insufficient neovascularization after myocardial infarction, experimental attempts to modify this deficiency by directly utilizing epicardial cells or their progenitors could prove favorable for cardiac regeneration.
We previously demonstrated that thymosin β4 (TB4), a 43-amino-acid G-actin-sequestering peptide is expressed in the embryonic heart, stimulates cardiomyocyte migration in vitro
and increases cardiac function while promoting the survival of cardiomyocytes in adult mice in vivo
]. More recently, analysis of heart-specific TB4 knockdown mouse embryos revealed a critical role for the peptide in epicardial development and coronary artery formation [14
]. However, the effects of TB4 on the adult epicardium and coronary growth in vivo
have not been discussed.
Here we show that TB4 initiates capillary-like tube formation of adult coronary endothelial cells, induces endothelial cell migration and proliferation in embryonic cardiac explants in vitro and supports revascularization in vivo. Importantly, it induces an organ-wide epicardial thickening and progenitor cell activation in adults similar to the changes in developing embryos and in regenerating adult zebrafish, while initiating the expression of numerous pro-angiogenic developmental genes. TB4 initiated protein kinase C (PKC) activity revealed to be essential for the epicardial activation.
Thus, TB4 supports cardiac regeneration not only by inhibiting myocardial cell death after infarction, but also through induction of vessel growth, myocardial progenitor mobilization and by reactivating the embryonic developmental program in adult epicardium in vivo.