Zebrafish hhex-null mutant develops an intrahepatic intestinal tube due to de-repression of cdx1b and pdx1

Area of Science:

• Developmental biology
• Genetics
• Molecular biology

Background:

• The hepatopancreatic duct (HPD) system is vital for connecting the liver and pancreas to the intestine.
• The precise molecular mechanisms governing HPD development, particularly the role of Haematopoietically expressed-homeobox (Hhex) protein, are not fully understood.

Purpose of the Study:

• To elucidate the molecular mechanism by which Hhex regulates the development of the hepatopancreatic duct (HPD) system in zebrafish.
• To identify the genetic network involved in HPD patterning and morphogenesis.

Main Methods:

• Analysis of zebrafish hhex-null mutants.
• Immunohistochemistry using biliary and intestinal markers (Annexin A4, sox9b, fabp2a).
• Cell lineage tracing.
• Gene knockdown experiments (cdx1b, pdx1).
• Chromatin-immunoprecipitation (ChIP) assays.

Main Results:

• Zebrafish hhex-null mutants lack the HPD system, with biliary and HPD markers (Annexin A4, sox9b) absent.
• An intrahepatic intestinal tube expressing fabp2a replaces the HPD in mutants, originating from non-hepatocyte/cholangiocyte sources.
• Ectopic expression of cdx1b and pdx1 was observed in the aberrant intestinal tube.
• Knockdown of cdx1b and pdx1 partially restored sox9b expression.
• Hhex directly binds to and represses the promoters of pdx1 and cdx1b.

Conclusions:

• Hhex acts as a repressor of pdx1 and cdx1b, preventing the formation of an intestinal tube in place of the HPD.
• A genetic network involving Hhex, Cdx1b, Pdx1, and Sox9b is essential for the correct patterning and morphogenesis of the HPD and digestive tract systems in zebrafish.