Restriction of hepatic competence by Fgf signaling

Abstract

SUMMARYHepatic competence, or the ability to respond to hepatic-inducing signals, is regulated by a number of transcription factors broadly expressed in the endoderm. However, extrinsic signals might also regulate hepatic competence, as suggested by tissue explant studies. Here, we present genetic evidence that Fgf signaling regulates hepatic competence in zebrafish. We first show that the endoderm posterior to the liver-forming region retains hepatic competence: using transgenic lines that overexpress hepatic inducing signals following heat-shock, we found that at late somitogenesis stages Wnt8a, but not Bmp2b, overexpression could induce liver gene expression in pancreatic and intestinal bulb cells. These manipulations resulted in the appearance of ectopic hepatocytes in the intestinal bulb. Second, by overexpressing Wnt8a at various stages, we found that as embryos develop, the extent of the endodermal region retaining hepatic competence is gradually reduced. Most significantly, we found, using gainand loss-of-function approaches, that Fgf10a signaling regulates this gradual reduction of the hepatic-competent domain. These data provide in vivo evidence that endodermal cells outside the liver-forming region retain hepatic competence and show that an extrinsic signal, Fgf10a, negatively regulates hepatic competence.

Specification of hepatopancreas progenitors in zebrafish by hnf1ba and wnt2bb

“…According to this model, broad overexpression of Wnt ligands should lead to posterior expansion of Prox1 expression where hnf1ba is expressed. As predicted and consistent with previous studies (Poulain and Ober, 2011;Shin et al, 2011), broad overexpression of Wnt8a via heatshock of Tg(hsp70l:wnt8a) w34 embryos (Weidinger et al, 2005) beginning at 21 hpf resulted in a robust posterior expansion of Prox1 expression within the hnf1ba-positive domain of the foregut endoderm ( Fig. 5E,F; n=30).…”

“…To investigate this, we examined the expression of cmyc (myca), a direct Wnt/β-catenin target (He et al, 1998), which is expressed in hepatopancreas progenitors and whose expression domain can be induced posteriorly by ectopic Wnt8a expression (Fig. 5H) (Yamaguchi et al, 2005;Shin et al, 2011). Intriguingly, this posterior expansion also correlates with the hnf1ba expression domain (compare Fig.…”

“…Probes for hnf1ba, hhex, prox1, cmyc (myca -Zebrafish Information Network) and wnt2bb were donated and are described elsewhere (Ho et al, 1999;Sun and Hopkins, 2001;Yamaguchi et al, 2005;Ober et al, 2006;Shin et al, 2011). Experimental and control embryos were stained in the same tube and all experiments were carried out at least twice using distinct clutches.…”

“…Wnt signaling interacts with hnf1ba after midsomitogenesis to specify hepatopancreas progenitors Current models suggest that genetic events regulating liver and pancreas specification occur during early somitogenesis (Chung et al, 2008;Wandzioch and Zaret, 2009;Shin et al, 2011). However, the liver and ventral pancreas do not bud until after midsomitogenesis, suggesting that other genetic events may more directly regulate hepatopancreas fate in the foregut endoderm.…”

A Sox9/Fgf feed-forward loop maintains pancreatic organ identity

“…The observation that albumin expression is not induced in dorsal endoderm isolated at E13.5 and beyond (Bossard and Zaret, 2000) has led to the proposal that cues from the mesoderm actively repress hepatogenic gene activity in dorsal gut endoderm up until E13.5. Studies in zebrafish further suggest that hepatic competence of the fish posterior endoderm, which is thought to correspond to the dorsal gut endoderm in mice, is negatively regulated by Fgf10 signaling (Dong et al, 2007;Shin et al, 2011). To date, the identity of the mesenchymal signal(s) that suppress liver-specific genes in dorsal gut endoderm in mice has remained elusive.…”