Abstract 1059: Glycogen Synthase Kinase 3-beta Regulates Cardiomyocyte Proliferation During Cardiac Development
BACKGROUND: Proliferation of cardiomyocytes during mid-gestation is driven by a number of growth factors, including neuregulin/Erb-B2, FGFs, IGF-1, and BMP10. While much is known about the role of these factors, and the cell cycle regulators and transcriptional activators that drive cardiomyocyte proliferation (e.g. cyclins A, D, and E, c-Myc, N-myc and GATA-4), very little is known about signaling pathways connecting growth factors to cell cycle regulators during cardiac development. Similarly, Wnt/β-catenin/Tcf pathway and one of its targets, Pitx2, have been shown to play key roles in the development of the outflow tract and valves, but the mechanisms regulating this pathway during cardiac development are not known.
METHODS AND RESULTS: Since GSK-3β is downstream of three of the growth factor pathways (Erb-B2, FGFs, and IGF-1), is a core component of the canonical Wnt pathway, and has been reported to regulate expression or activity of all the cell cycle regulators and transcription factors noted above, we hypothesized that GSK-3β might be a key regulator of cardiomyoblast proliferation and/or outflow tract development. We found that deletion of GSK-3β leads to near-obliteration of the right and left ventricular cavities due to massively thickened walls of the heart. The increased size of the ventricular walls is primarily caused by markedly increased proliferation of cardiomyocytes in the GSK-3β-null embryos and to lesser extent, by hypertrophy of cardiomyocytes and increased glycogen accumulation. This hyperproliferation is associated with a marked increase in nuclear-localized cyclin D1, cyclin A and c-Myc. Furthermore, we found a marked increase in nuclear GATA-4, a regulator of cardiomyocyte proliferation that is normally retained in the cytosol (and thus are inactive) when phosphorylated by GSK-3β. While valve development was normal, GSK-3β null embryos have double outlet right ventricle and ventricular septal defects, which together lead to the “packed ventricle,” likely accounting for the early post-natal lethality of the mice.
CONCLUSIONS: These results are the first to employ loss of function of GSK-3β in vivo and demonstrate a critical role for the kinase in cardiomyocyte proliferation and outflow tract development.