Myocardial infarction is the leading cause of morbidity and mortality in the United States. Compromised myocardial function, due to the lack of self-renewal capacity in mature hearts, is a major reason for heart failure. Available therapies can only ameliorate, but not reverse the loss of functional myocardium. With heart transplantation as the only available cure, design of an effective regenerative therapy has become imperative for cardiovascular research. To repopulate the heart with de novo cardiomyocytes, most attempts have been based on the transplantation of cardiac, non-cardiac stem cells or their derivatives, however a more profound knowledge of stem cells is required for achieving significant progress. Meanwhile, triggering endogenous regenerative capacity is a compelling strategy for cardiac repair. It has been reported that proliferation of pre-existing cardiomyocytes strongly contributes to regeneration. Thus, efforts have been made to reintroduce mature cardiomyocytes into mitotic cycle. The mechanisms underlying the proliferation of cardiomyocytes during development and their homeostasis during adulthood are not fully understood, but likely require tight regulation of transcription factors in specific cell types. We have previously shown that the mouse Hippo kinase cascade is a major heart-size control pathway during development. In addition, activation of Yap, a transcriptional cofactor inhibited by Hippo, by genetically disrupting Hippo signaling is sufficient to induce juvenile and adult myocardial regeneration after surgery-induced myocardial infarction. Here we identified the paired-like homeodomain transcription factor 2 (pitx2) as a potential downstream target and cofactor of Yap in mouse heart. Our data indicates that Pitx2 expression is induced by myocardial injury, and is required for neonatal myocardial regeneration in a postnatal day 1 (P1) apex resection model. Further studies show that over-expression of pitx2 in adult cardiomyocytes is sufficient to promote the restoration of myocardial structure and function after myocardial infarction. Together, we show that pitx2 is a new manipulator of myocardial regeneration and could serve as a novel therapeutic target in cardiac regenerative therapy.
Ex-Vivo Expanded Umbilical Cord Blood Stem Cells Retain Capacity for Myocardial Regeneration