OVER-EXPRESSION OF GATA-6 IN XENOPUS EMBRYOS BLOCKS DIFFERENTIATION OF HEART PRECURSORS

Xenopus GATA-6 transcripts are first detected at the beginning of gast rulation in the mesoderm, and subsequent domains of expression include the field of cells shown to have heart-forming potential, In this reg ion, GATA-6 expression continues only in those cells that go on to for m the heart; however, a decrease occurs prior to terminal differentiat ion. Artificial elevation of GATA-6, but not GATA-1, prevents expressi on of both cardiac actin and heart-specific myosin light chain, This e ffect is heart-specific because cardiac actin expression is unaffected in somites, Expression of the earlier marker XNkx-2.5 was unaffected and morphological development of the heart was initiated independently of the establishment of the contractile machinery, We conclude that a reduction in the level of GATA-6 is important for the progression of the cardiomyogenic differentiation programme and that GATA-6 may act t o maintain heart cells in the precursor state, At later stages, when t he elevated GATA-6 levels had decayed, differentiation ensued but the number of cells contributing to the myocardium had increased, suggesti ng either that the blocked cells had proliferated or that additional c ells had been recruited.