Cdx 2 determines the fate of postnatal intestinal endoderm

INTRODUCTION Cdx2 is expressed in intestinal endoderm posterior to the stomach throughout gestation and postnatally throughout life; it is never expressed in gastric tissues (Beck et al., 1995; Silberg et al., 2000). Previously, we disabled Cdx2 by homologous recombination (Chawengsaksophak et al., 1997). Homozygous Cdx2–/– embryos failed to implant because trophoblast did not differentiate (Strumpf et al., 2005) but heterozygotes were born alive. These had polyps in the paracaecal region of the intestine that consisted of normally organised stomach mucosa arranged in an anatomically sequential manner from the stratified squamous epithelium characteristic of forestomach through to pyloric antral mucosa (Beck et al., 1999). The mesodermally expressed stomach-specific gene Barx1 was found in the mesenchyme immediately deep to the developing polyps and Sox2 was expressed in the overlying endoderm (Stringer et al., 2008). It was postulated that local sporadic haploinsufficiency of Cdx2 caused an anterior homeotic shift in which undifferentiated intestinal endoderm defaulted to a rostral gastric phenotype (forestomach); the remaining gastric tissue types (cardia, corpus and pylorus) were subsequently intercalated between forestomach and surrounding colon, in order to maintain the orderly succession of tissue types that are established during normal stomach development. Chimaeric studies using Cdx2–/– cells in wild-type hosts confirmed these observations (Beck et al., 2003). Gao et al. (Gao et al., 2009) overcame the embryonic lethality of Cdx2 ablation by targeting its knock out specifically to embryonic endoderm early in development, thus inactivating gene expression throughout the intestine. They confirm that the intestinal lining reverted to a forestomach phenotype. We have now conditionally knocked out the Cdx2 gene in the adult mouse intestine and have monitored the phenotype over 62 weeks to determine whether it has retained the ability to give rise to histologically and organotypically normal stomach mucosa, and also to investigate whether differentiation of the intestinal cell types that normally arise from the stem cells is at all possible in the absence of Cdx2 expression. Neither of these matters have previously been investigated. We describe a novel phenotype that differs fundamentally from that following knock out at conception or early in development. While the adult intestinal stem cells have the ability to express gastric-type genes, they are fixed into maintaining an intestinal architecture and have lost the positional information cues that determine morphogenic organisation of stomach mucosa. Consequently, a variety of gastric genes are expressed in an intestinal setting throughout the small intestine; in the paracaecal region, these heterotopias occasionally exhibit a tubulo-glandular picture that is suggestive of pyloric antral glands but do not express the gastric marker Sox2 immunocytochemically. In this respect they differ fundamentally from normal antral mucosa. Cdx2-negative crypts give rise to subepithelial cystic vesicles that are devoid of Paneth cells. Similar thin-walled Cdx2-null empty cysts are produced from stem cell-targeted Cdx2 inactivation in cultured crypts instead of the intestinal organoids produced from control crypts. The results of our in vivo and in vitro studies indicate that, in the absence of Cdx2 expression, the stem cells lose the ability to Development 139, 465-474 (2012) doi:10.1242/dev.070722 © 2012. Published by The Company of Biologists Ltd