Alternative splicing of jnk1a in zebrafish determines first heart field ventricular cardiomyocyte numbers through modulation of hand2 expression

Author summary The planar cell polarity pathway is a genetic cascade important for normal development of the heart. The jnk gene family is known to act within this pathway but its specific role in heart development has been difficult to establish using mouse mutants because genes within the family overlap in function and embryos mutant for more than one jnk gene die before the heart is fully formed. In these studies, we used zebrafish to determine the role of jnk1 in heart development and found that a specific gene transcript from zebrafish jnk1a is needed to produce the full complement of cardiomyocytes that form the first part of the ventricular heart chamber. We show that this happens because jnk1a is required for the normal expression of hand2, a gene required to make ventricular heart cells. Furthermore, we can fully reverse the abnormalities in jnk1a embryos by increasing hand2 gene activity. This part of the zebrafish ventricular chamber is equivalent to the left ventricle in humans and understanding how jnk genes work may help understand why some human heart malformations are associated with an underdeveloped left ventricle. The planar cell polarity pathway is required for heart development and whilst the functions of most pathway members are known, the roles of the jnk genes in cardiac morphogenesis remain unknown as mouse mutants exhibit functional redundancy, with early embryonic lethality of compound mutants. In this study zebrafish were used to overcome early embryonic lethality in mouse models and establish the requirement for Jnk in heart development. Whole mount in-situ hybridisation and RT-PCR demonstrated that evolutionarily conserved alternative spliced jnk1a and jnk1b transcripts were expressed in the early developing heart. Maternal zygotic null mutant zebrafish lines for jnk1a and jnk1b, generated using CRISPR-Cas9, revealed a requirement for jnk1a in formation of the proximal, first heart field (FHF)-derived portion of the cardiac ventricular chamber. Rescue of the jnk1a mutant cardiac phenotype was only possible by injection of the jnk1a EX7 Lg alternatively spliced transcript. Analysis of mutants indicated that there was a reduction in the size of the hand2 expression field in jnk1a mutants which led to a specific reduction in FHF ventricular cardiomyocytes within the anterior lateral plate mesoderm. Moreover, the jnk1a mutant ventricular defect could be rescued by injection of hand2 mRNA. This study reveals a novel and critical requirement for Jnk1 in heart development and highlights the importance of alternative splicing in vertebrate cardiac morphogenesis. Genetic pathways functioning through jnk1 may be important in human heart malformations with left ventricular hypoplasia.