Negative feedback regulation of Nodal signaling in zebrafish mesendoderm patterning

Embryo development is a complex and highly regulated process. Early in development cell fate is specified and cell movements are guided by the intricate interaction of morphogen gradients. In zebrafish, Nodal signaling induces and patterns the mesendoderm (a precursor tissue to the mesoderm and endoderm germ layers), guides cell movements during gastrulation, and later is involved in the establishment of left-right asymmetry. Stringent temporal and spatial control of target gene expression is thought to be essential for proper embryo patterning. Although several positive and negative feedback regulatory mechanisms are known for Nodal signaling, additional feedback mechanisms controlling the ultimate outcome of Nodal signaling are likely to exist but are as yet unknown.;The zinc finger transcription factor odd skipped related 1 (osr1) is known to negatively regulate endoderm gene expression in zebrafish and has a dynamic expression pattern during gastrulation. Its role in the Nodal signaling cascade is revealed by demonstrating that osr1 gene expression in mesendoderm is directly induced by Nodal signaling. Osr1 functions as a negative regulator of Nodal target genes in endoderm, defining a novel transcriptional regulatory mechanism. Moreover I provide evidence that Osr1 represses endoderm through a pathway parallel to FGF signaling.;Post-transcriptional regulation of mRNA stability plays an important role in the temporal regulation of gene expression. Nodal target genes such as bon, gata5, and mezzo are transiently expressed in the mesendoderm, and their transcripts show rapid decay upon Nodal signaling inhibition by the pharmacological compound SB505124, suggesting a post-transcriptional mechanism for regulating mRNA half-life. TTP/Zfp36 is a family of RNA binding proteins that recognize AU-rich elements (AREs) found in the 3'-UTRs of many primary response genes. The zebrafish TPP/Zfp36 homologues cth1, zfp36l2 and zfp36a, are expressed during mesendoderm patterning in the zebrafish embryo. Combined knockdown of these three genes stabilized bon transcript, preventing its decay following inhibition of Nodal signaling. These results reveal a previously unreported role for TPP/Zfp36-mediated regulation of mRNA stability during embryogenesis.