Abstract 9815: Role for Btg1 and Btg2 in Postnatal Cardiomyocyte Cell Cycle Arrest and Maturation

Abstract

Background: Adult mammalian cardiomyocytes (CM) are predominantly post-mitotic and cannot proliferatively repair the heart following myocardial infarction (MI). Overexpression of the transcription factor Tbx20 in adult mouse CMs promotes proliferative cardiac repair post-MI, via mechanisms including direct repression of anti-proliferative genes p21 , Meis1 , and Btg2 . Btg2 (B-cell translocation gene 2), a tumor suppressor and transcriptional co-regulator, exhibits high structural and functional similarity with Btg1. However, both Btg1 and Btg2 (Btg1/2) are virtually uncharacterized in the heart. Here, we investigate the role of Btg1/2 in postnatal cardiac maturation. Methods and Results: By immunostaining in embryonic, neonatal, and adult C57BL/6 mouse hearts, the highest expression of Btg1/2 was observed in late fetal and early neonatal ventricles, concurrent with upregulation of other CM cell cycle inhibitors. In neonatal mouse CMs in vitro, siRNA-mediated loss of Btg2 leads to increased CM proliferation. In vivo , Btg1/2 constitutive single- and double- knockout (SKO and DKO respectively) mice exhibit normal heart weight-to-body weight ratios compared to age-matched wildtype (WT) controls, at postnatal day (P)7, P30, and 1 year after birth. Interestingly, at P7, DKO mice have significantly higher CM mitotic activity, as indicated by pHH3 staining, compared to WT. In addition, DKO mice also exhibit significantly smaller CM cross-sectional area at P7 compared to WT. However, by P15, CM mitotic activity and cell size are comparable between WT and Btg1/2 KO mice. Currently, siRNA-mediated knockdown of Btg1/2 in neonatal rat ventricular cardiomyocyte cultures and RNAseq studies are being performed, to assess the transcriptional regulatory roles of Btg1/2 in rodent CMs. Conclusions: Here, we highlight two novel regulators of postnatal CM maturation, Btg1 and Btg2, which are upregulated coincident with CM mitotic arrest in mice. Similar to p21 and Meis1, Btg1/2 depletion in mice induces a brief period of increased CM proliferative activity before onset of CM cell cycle arrest. Our results provide evidence for Btg1/2 working in tandem with other cardiac transcription factors and cell cycle regulators, to control CM mitotic arrest after birth.