Natriuretic peptides and Forkhead O transcription factors act in a cooperative manner to promote cardiomyocyte cell cycle re-entry in the postnatal mouse heart.

Cardiomyocytes proliferate rapidly during fetal life but lose their ability of proliferation soon after birth. However, before terminal withdrawal from the cell cycle, cardiomyocytes undergo another round of cell cycle during early postnatal life in mice. While a transient wave of increased DNA synthesis in cardiomyocyte has been observed in postnatal mouse hearts, the molecular mechanisms describing cardiomyocyte cell cycle re-entry remain poorly understood. Atrial and B-type natriuretic peptides (ANP and BNP) are abundantly expressed in embryonic heart ventricles. After birth, the expression of both genes is strongly reduced in the ventricular myocardium. Forkhead O (FOXO) transcription factors are expressed in both embryonic and postnatal heart ventricles. Their transcriptional activity negatively affects cardiomyocyte proliferation. Upon phosphorylation, FOXO is translocated to the cytoplasm and is transcriptionally inactive. Despite these important findings, it remains largely unknown whether natriuretic peptides and FOXO cooperatively play a role in regulating cardiomyocyte cell cycle activity during early postnatal life. We observed that the expression of ANP and BNP and the level of phosphorylated FOXO were transiently increased in the postnatal mouse heart ventricles, which coincided with the burst of cardiomyocyte cell cycle re-entry during early postnatal life in mice. Cell culture studies showed that ANP/BNP signaling and FOXO cooperatively promoted cell cycle activity in neonatal mouse cardiomyocytes. The enhanced cell cycle activity observed in combined treatment of ANP/BNP and dominant-negative FOXO (DN-FOXO), which can bind FOXO recognition sites on DNA but cannot activate transcription, was primarily mediated through natriuretic peptide receptor 3 (Npr3). In mice, simultaneous application of ANP and DN-FOXO in postnatal hearts reactivated cell cycle in cardiomyocytes, resulting in reduced scar formation after experimental myocardial infarction. Our data demonstrate the cooperative effects of natriuretic peptide and DN-FOXO on promoting cardiomyocyte cell cycle activity and mouse cardiac repair and regeneration after injury.