Mesendoderm and left-right brain, heart and gut development are differentially regulated by pitx2 isoforms

The pitx2 gene is a member of the bicoid-homeodomain class of transcription factors that has been implicated in the control of left-right asymmetry du ring organogenesis, Here we demonstrate that in zebrafish there are two pit x2 isoforms, pitx2a and pitx2c, which show distinct expression patterns and have non-overlapping functions during mesendoderm and asymmetric organ dev elopment, pitx2c is expressed symmetrically in presumptive mesendoderm duri ng late blastula stages and in the prechordal plate during late gastrulatio n, pitx2a expression is first detected at bud stage in the anterior prechor dal plate, The regulation of early mesendoderm pitx2c expression is depende nt on one-eyed pinhead (EGF-CFC-related gene) and spadetail (tbx-transcript ion factor) and can be induced by ectopic goosecoid expression, Maintenance of pitx2c midline expression is dependent on cyclops (nodal) and schmalspu r, but not no tail (brachyury). Ectopic expression of pitx2 isoforms result s in distinct morphological and molecular phenotypes, indicating that pitx2 a and pitx2c have divergent regulatory functions, Both isoforms downregulat e goosecoid on the dorsal side, but in contrast to earlier reports that nod al and lefty are upstream of pitx2, ectopic pitx2c in other regions induces cyclops, lefty2 and goosecoid expression. Asymmetric isoform expression occurs in overlapping domains, with pitx2c in left dorsal diencephalon and developing gut and pitx2a in left heart primo rdium. Targeted asymmetric expression in Xenopus shows that both isoforms c an alter left-right development, but pitx2a has a slightly stronger effect on heart laterality, Our results indicate that distinct genetic pathways re gulate pitx2a and pitx2c isoform expression, and each isoform regulates dif ferent downstream pathways during mesendoderm and asymmetric organ developm ent.