The PIDDosome controls cardiomyocyte polyploidization during postnatal heart development

The adult mammalian heart is characterized by post-mitotic polyploid cardiomyocytes (CMs). Understanding how CMs regulate cell cycle exit and ploidy can help developing new heart regenerative therapies. Here, we uncover that the PIDDosome, a multi-protein complex activating the endopeptidase Caspase-2, helps to implement a CM-specific differentiation program that limits ploidy during postnatal heart development. DNA content analyses show that PIDDosome-loss causes a cell-autonomous increase in nuclear and cellular CM ploidy. Remarkably, increased ploidy does not affect cardiac structure nor function. PIDDosome-imposed ploidy restriction commences at postnatal day 7 (P7), reaching a plateau on P14. PIDDosome activation requires ANKRD26, targeting PIDD1 to mother centrioles. Opposite to prior observations in liver development, the PIDDosome limits CM polyploidization in a p53-independent manner but reliant on p21/Cdkn1a, a notion supported by nuclear RNA sequencing and genetic deletion experiments. Our results provide new insights how proliferation of polyploid CMs is restricted during postnatal heart development.