The heart is a notorious non-regenerative organ, composed of terminally differentiated cardiac muscle cells, which has a very low renewal rate during adulthood, incapable of efficient organ regeneration after massive insults. This view is gradually changing as heart repair mechanisms have been discovered that are initiated in adult mammals following injuries. Clinically, the picture is less bright and, frustratingly, most clinical trials failed to yield novel treatments for this unmet need, illustrating the necessity to develop novel therapeutic approaches.

Mammalian cardiomyocytes proliferate during embryogenic heart development but loose their proliferative potential shortly after birth concomitant with morphological maturation and a profound switch of the cellular metabolism. Reversal of this process, essentially rewinding the developmental program, to achieve a more immature, fetal state allows division of cardiomyocytes seems essential for replacement of lost contractile tissue and successful organ regeneration. We have pursued several different strategies to rewind the development program to stimulate heart regeneration. We discovered that microRNAs of the miR-1/133a family suppress two crucial regulatory circuits controlling postnatal cardiomyocyte proliferation and dedifferentiation, the FGFR and OSMR pathways. Concomitant inactivation of both miR gene clusters in postnatal cardiomyocytes induces expression of cell cycle regulatory genes and cell-cycle re-entry of adult cardiomyocytes. We also found that heart-specific expression of OSKM (Oct4°, Sox2(S), Klf4(K) and c-Myc(M)) converts cardiomyocytes to a fetal-like state, conferring regenerative capacity to adult hearts. Short-term OSKM expression before and during myocardial infarction ameliorated myocardial damage and improved cardiac function, demonstrating that temporally controlled dedifferentiation and reprogramming enables cell cycle reentry of cardiomyocytes and facilitates heart regeneration. We also asked whether abrogation of fatty acid oxidation (FAO) in cardiomyocytes by inactivation of Cpt1b is sufficient to induce cardiomyocyte de-differentiation, proliferation and eventually heart regeneration. Our results indicate that this is indeed the case, revealing that inhibition of FAO specifically in adult hearts after ischemic injury promotes nearly full recovery of cardiac function. Metabolic studies reveal profound changes in energy metabolism and accumulation of α-ketoglutarate in Cpt1b-mutant cardiomyocytes, leading to activation of the α-ketoglutarate-dependent lysine demethylase KDM5. Activated KDM5 demethylates broad H3K4me3 domains in genes that drive cardiomyocyte maturation, lowering their transcription levels and shifting cardiomyocytes into a less mature state, thereby promoting proliferation. Our studies provide fascinating perspectives for future clinical applications.