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: GSK3̤ (51 kDa) and GSK3̤ (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 GSK3̤ and GSK3̤, making it difficult to identify an inhibitor that can be selective against GSK3̤ or GSK3̤. Thus, it is relatively difficult to address the functions of GSK3 isoforms during embryogenesis. At this study, we attempt to specifically inhibit either GSK3̤ or GSK3̤ and uncover the isoform-specific roles that GSK3 plays during cardiogenesis. Results: We blocked gsk3 ̤ and gsk3 ̤ translations by injection of morpholino antisense oligonucleotides (MO). Both gsk3 ̤- and gsk3 ̤-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 ̤- and gsk3 ̤-MO-induced heart defects, we found that the reduced number of cardiomyocytes in gsk3 ̤ morphants during the heart-ring stage was due to apoptosis. On the contrary, gsk3 ̤ 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 ̤ morphants. bmp4 expression in gsk3 ̤ morphants was upregulated and disrupted the asymmetry pattern in the heart. The cardiac valve defects in gsk3 ̤ morphants were similar to those observed in axin1 and apcmcr mutants, suggesting that GSK3̤ might play a role in cardiac valve development through the Wnt/̤-catenin pathway. Finally, the phenotypes of gsk3 ̤ mutant embryos cannot be rescued by gsk3 ̤ mRNA, and vice versa, demonstrating that GSK3̤ and GSK3̤ are not functionally redundant. Conclusion: We conclude that (1) GSK3̤, but not GSK3̤, is necessary in cardiomyocyte survival; (2) the GSK3̤ plays important roles in modulating the left-right asymmetry and affecting heart positioning; and (3) GSK3̤ and GSK3̤ play distinct roles during zebrafish cardiogenesis. [ABSTRACT FROM AUTHOR]