Zebrafish cdx1b regulates expression of downstream factors of Nodal

INTRODUCTION The digestive system is important for the maintenance of vertebrate physiology. After the endoderm progenitors are specified, they differentiate into epithelial cells of the embryonic gut. In amniotes such as mice, the endodermal layer of the mouse embryo begins to form a gut tube at embryonic day 8.5 (E8.5) by folding at the anterior end, resulting in the anterior intestinal portal (AIP), followed by creation of the caudal intestinal portal (CIP) at the posterior end (reviewed by Wells and Melton, 1999; Grapin-Botton and Melton, 2000). Next, a fully extended gut tube forms by extension and fusion of the AIP and CIP. Organ buds form during the E10.5-14.5 stages. Eventually, the esophagus, stomach, thyroid, lungs, pancreas and liver are derived from the foregut region. In amniotic vertebrates, various paracrine and transcription factors have been shown to be essential for specifying endoderm progenitors, patterning and morphogenesis during gastrointestinal tract development (Harmon et al., 2002; de Santa Barbara et al., 2003; Schier, 2003; Murtaugh et al., 2003). Among these, Nodal paracrine factors play crucial roles in mesendoderm induction in vertebrates (reviewed by Schier, 2003). In the absence of Nodal signaling, no endodermor mesoderm-derived organs or tissues can develop in zebrafish embryos (Feldman et al., 1998). The molecular pathway leading to early endoderm development in several vertebrates has been established (Alexander and Stainier, 1999; Shivdasani, 2002; Stainier, 2002; Tam et al., 2003). In zebrafish embryos, two Nodal factors (Squint and Cyclops; also known as Nodal-related 1 and 2, respectively – ZFIN) interact with the TGF related type I receptor, Taram-a (Tar; Acvr1b – ZFIN), and the Oneeyed pinhead (Oep) EGF-CFC co-receptor. Nodal signaling can be transduced either by association of the phosphorylated Smad2Smad4 complex with Bonnie and clyde (Bon) or with Gata5 to activate the HMG domain transcription factor, Casanova (Cas; also known as Sox32 – ZFIN). Alternatively, Cas may function in parallel with Gata5/Bon. Subsequent cooperation between Cas and the POU domain protein, Spg (Pou5f1 – ZFIN), activates the HMG domain transcription factor, Sox17, leading to endoderm formation (Alexander and Stainier, 1999; Alexander et al., 1999; Kikuchi et al., 2000; Kikuchi et al., 2001; Aoki et al., 2002; Lunde et al., 2004; Reim et al., 2004). Successive patterning and morphogenesis of the gut tube are regulated by coordinated transcriptional activity (reviewed by Well and Melton, 1999). Both mouse Cdx1 and Cdx2 are expressed in the embryonic and adult intestine and colon. In the adult intestine and colon, Cdx1 expression increases along the anteroposterior axis, with the highest expression level in the distal colon, whereas Cdx2 expression increases progressively from the duodenum to the distal intestine, with the highest level observed in the proximal colon (Silberg et al., 2000) (reviewed by Guo et al., 2004). Conversion of the gastric mucosa to intestinal metaplasia was detected in either Cdx1or Cdx2-expressing transgenic mice (Mutoh et al., 2002; Mutoh et al., 2004). In heterozygote Cdx2 mutant mice and Cdx2-null Zebrafish cdx1b regulates expression of downstream factors of Nodal signaling during early endoderm formation Pei-Yi Cheng*, Chia-Chi Lin, Chun-Shiu Wu, Yu-Fen Lu, Che Yi Lin, Chih-Ching Chung, Cheng-Ying Chu, Chang-Jen Huang, Chun-Yen Tsai, Svetlana Korzh, Jen-Leih Wu and Sheng-Ping L. Hwang