Glycogen synthase kinase 3α and 3β have distinct functions during cardiogenesis of zebrafish embryo

Background: Glycogen synthase kinase 3 (GSK3) encodes a serine/threonine protein kinase, is known to play roles in many biological processes. Two closely related GSK3 isoforms encoded by distinct genes: GSK3 alpha ( 51 kDa) and GSK3 beta ( 47 kDa). In previously studies, most GSK3 inhibitors are not only inhibiting GSK3, but are also affecting many other kinases. In addition, because of highly similarity in amino acid sequence between GSK3a and GSK3 beta, making it difficult to identify an inhibitor that can be selective against GSK3 alpha or GSK3 beta. Thus, it is relatively difficult to address the functions of GSK3 isoforms during embryogenesis. At this study, we attempt to specifically inhibit either GSK3 alpha or GSK3 beta and uncover the isoform-specific roles that GSK3 plays during cardiogenesis. Results: We blocked gsk3 alpha and gsk3 beta translations by injection of morpholino antisense oligonucleotides ( MO). Both gsk3 alpha- and gsk3 beta-MO-injected embryos displayed similar morphological defects, with a thin, string-like shaped heart and pericardial edema at 72 hours post-fertilization. However, when detailed analysis of the gsk3 alpha- and gsk3 beta-MO-induced heart defects, we found that the reduced number of cardiomyocytes in gsk3 alpha morphants during the heart-ring stage was due to apoptosis. On the contrary, gsk3 beta morphants did not exhibit significant apoptosis in the cardiomyocytes, and the heart developed normally during the heart-ring stage. Later, however, the heart positioning was severely disrupted in gsk3 beta morphants. bmp4 expression in gsk3 beta morphants was upregulated and disrupted the asymmetry pattern in the heart. The cardiac valve defects in gsk3 beta morphants were similar to those observed in axin I and apc(mcr) mutants, suggesting that GSK3 beta might play a role in cardiac valve development through the Wnt/beta-catenin pathway. Finally, the phenotypes of gsk3 alpha mutant embryos cannot be rescued by gsk3 beta mRNA, and vice versa, demonstrating that GSK3 alpha and GSK3 beta are not functionally redundant. Conclusion: We conclude that ( 1) GSK3 alpha, but not GSK3 beta, is necessary in cardiomyocyte survival; (2) the GSK3 beta plays important roles in modulating the left-right asymmetry and affecting heart positioning; and ( 3) GSK3 alpha and GSK3 beta play distinct roles during zebrafish cardiogenesis.