Retinaldehyde dehydrogenase 2 is down-regulated during duodenal atresia formation in Fgfr2IIIb-/- mice

Large intestine embryogenesis: Molecular pathways and related disorders (Review)

The large intestine, part of the gastrointestinal tract (GI), is composed of all three germ layers, namely the endoderm, the mesoderm and the ectoderm, forming the epithelium, the smooth muscle layers and the enteric nervous system, respectively. Since gastrulation, these layers develop simultaneously during embryogenesis, signaling to each other continuously until adult age. Two invaginations, the anterior intestinal portal (AIP) and the caudal/posterior intestinal portal (CIP), elongate and fuse, creating the primitive gut tube, which is then patterned along the antero‑posterior (AP) axis and the radial (RAD) axis in the context of left‑right (LR) asymmetry. These events lead to the formation of three distinct regions, the foregut, midgut and hindgut. All the above‑mentioned phenomena are under strict control from various molecular pathways, which are critical for the normal intestinal development and function. Specifically, the intestinal epithelium constitutes a constantly developing tissue, deriving from the progenitor stem cells at the bottom of the intestinal crypt. Epithelial differentiation strongly depends on the crosstalk with the adjacent mesoderm. Major molecular pathways that are implicated in the embryogenesis of the large intestine include the canonical and non‑canonical wingless‑related integration site (Wnt), bone morphogenetic protein (BMP), Notch and hedgehog systems. The aberrant regulation of these pathways inevitably leads to several intestinal malformation syndromes, such as atresia, stenosis, or agangliosis. Novel theories, involving the regulation and homeostasis of intestinal stem cells, suggest an embryological basis for the pathogenesis of colorectal cancer (CRC). Thus, the present review article summarizes the diverse roles of these molecular factors in intestinal embryogenesis and related disorders.

The Role of Fibroblast Growth Factor 10 Signaling in Duodenal Atresia

Introduction

Duodenal atresia (DA) is a congenital bowel obstruction requiring major surgery in the first week of life. Three morphological phenotypes are described, reflecting increasing degrees of obstruction and discontinuity of the duodenum. The cause of DA is not known. Tandler’s original “solid cord” hypothesis conflicts with recent biological evidence, and is unable to account for differing DA types. In humans, a genetic etiology is supported by the association between Trisomy 21 and DA, and reports of familial inheritance patterns. Interruption of FGF10/FGFR2b signaling is the best demonstrated genetic link to DA in mice, with 35–75% of homozygous knockout embryos developing DA.

Purpose

This review examines the current evidence surrounding the etiology of DA. We focus on research regarding FGF10/FGFR2b signaling and its role in duodenal and other intestinal atresia. Further, we outline planned future research in this area, that we consider necessary to validate and better understand this murine model in order to successfully translate this research into clinical practice.

Conclusion

Determining the etiology of DA in humans is a clinical and scientific imperative. Fgf10/Fgfr2b murine models represent current science’s best key to unlocking this mystery. However, further research is required to understand the complex role of FGF10/FGFR2b signaling in DA development. Such complexity is expected, given the lethality of their associated defects makes ubiquitous interruption of either Fgf10 or Fgfr2b genes an unlikely cause of DA in humans. Rather, local or tissue-specific mutation in Fgf10, Fgfr2b, or their downstream targets, is the hypothesized basis of DA etiology.

Analysis of duodenojejunal flexure formation in mice: implications for understanding the genetic basis for gastrointestinal morphology in mammals

The mammalian gut undergoes morphological changes during development. We studied the developing mouse duodenojejunal flexure (DJF) to elucidate the mechanism of formation. During embryonic days 10.75-13.75, DJF formation was morphologically classified into three stages: the expansion stage, flexure formation stage, and flexure elongation stage. From the expansion to the flexure formation stages, the DJF wall showed asymmetric morphology and proliferation along the left-right intestinal axis. From the flexure formation to the flexure elongation stage, the DJF started to bend dorsally with counterclockwise rotation along the antero-caudal intestinal axis, indicating that the original right side of the duodenum was rotated towards the dorsal body wall during development of the DJF. The direction of attachment of the dorsal mesentery to the DJF did not correspond to the bending direction of the DJF during flexure formation, and this finding indicated that the dorsal mesentery contributed very little to DJF formation. During DJF formation, Aldh1a2 and hedgehog mRNAs were detected at the DJF, and their expression levels differed along the bending axis. In conclusion, DJF formation might be triggered by asymmetric morphology and proliferation along the left-right intestinal axis under the control of retinoic acid and hedgehog signaling.

Haploinsufficiency of retinaldehyde dehydrogenase 2 decreases the severity and incidence of duodenal atresia in the fibroblast growth factor receptor 2IIIb-/- mouse model

BACKGROUND

Homozygous null mutation of the fibroblast growth factor receptor 2IIIb (Fgfr2IIIb) gene in mice results in 42% of embryos developing duodenal atresias. Retinaldehyde dehydrogenase 2 (Raldh2, a gene critical for the generation of retinoic acid) is expressed in the mouse duodenum during the temporal window when duodenal atresias form. Raldh2 is critical for the normal development of the pancreatoduodenal region; therefore, we were interested in the effect of a Raldh2 mutation on duodenal atresia formation. To test this, we rendered Fgfr2IIIb(-/-) embryos haploinsufficient for the Raldh2 and examined these embryos for the incidence and severity of duodenal atresia.

METHODS

Control embryos, Fgfr2IIIb(-/-) mutants, and Fgfr2IIIb(-/-); Raldh2(+/-) mutants were harvested at embryonic day 18.5, genotyped, and fixed overnight. Intestinal tracts were isolated. The type and severity of duodenal atresia was documented.

RESULTS

A total of 97 Fgfr2IIIb(-/-) embryos were studied; 44 had duodenal atresias, and 41 of these presented as type III. In the 70 Fgfr2IIIb(-/-); Raldh2(+/-) embryos studied, a lesser incidence of duodenal atresia was seen (15 of 70; P = .0017; Fisher exact test). Atresia severity was also decreased; there were 12 embryos with type I atresias, 3 with type II atresias, and 0 with type III atresias (P < 2.81E-013; Fisher exact test).

CONCLUSION

Haploinsufficiency of Raldh2 decreases the incidence and severity of duodenal atresia in the Fgfr2IIIb(-/-) model. The ability to alter defect severity through manipulation of a single gene in a specific genetic background has potentially important implications for understanding the mechanisms by which intestinal atresias arise.

Formation of intestinal atresias in the Fgfr2IIIb-/- mice is not associated with defects in notochord development or alterations in Shh expression

PURPOSE

The etiology of intestinal atresia remains elusive but has been ascribed to a number of possible events including in utero vascular accidents, failure of recanalization of the intestinal lumen, and mechanical compression. Another such event that has been postulated to be a cause in atresia formation is disruption in notochord development. This hypothesis arose from clinical observations of notochord abnormalities in patients with intestinal atresias as well as abnormal notochord development observed in a pharmacologic animal model of intestinal atresia. Atresias in this model result from in utero exposure to Adriamycin, wherein notochord defects were noted in up to 80% of embryos that manifested intestinal atresias. Embryos with notochord abnormalities were observed to have ectopic expression of Sonic Hedgehog (Shh), which in turn was postulated to be causative in atresia formation. We were interested in determining whether disruptions in notochord development or Shh expression occurred in an established genetic model of intestinal atresia and used the fibroblast growth factor receptor 2IIIb homozygous mutant (Fgfr2IIIb-/-) mouse model. These embryos develop colonic atresias (100% penetrance) and duodenal atresias (42% penetrance).

METHODS

Wild-type and Fgfr2IIIb-/- mouse embryos were harvested at embryonic day (E) 10.5, E11.5, E12.5, and E13.5. Whole-mount in situ hybridization was performed on E10.5 embryos for Shh. Embryos at each time point were harvested and sectioned for hematoxylin-eosin staining. Sections were photographed specifically for the notochord and resulting images reconstructed in 3-D using Amira software. Colons were isolated from wild-type and Fgfr2IIIb-/- embryos at E10.5, then cultured for 48 hours in Matrigel with FGF10 in the presence or absence of exogenous Shh protein. Explants were harvested, fixed in formalin, and photographed.

RESULTS

Fgfr2IIIb-/- mouse embryos exhibit no disruptions in Shh expression at E10.5, when the first events in atresia formation are known to occur. Three-dimensional reconstructions failed to demonstrate any anatomical disruptions in the notochord by discontinuity or excessive branching. Culture of wild-type intestines in the presence of Shh failed to induce atresia formation in either the duodenum or colon. Cultured Fgfr2IIIb-/- intestines developed atresias of the colon in either the presence or absence of Shh protein.

CONCLUSIONS

Although disruptions in notochord development can be associated with intestinal atresia formation, in the Fgfr2IIIb-/- genetic animal model neither disruptions in notochord development nor the presence of exogenous Shh protein are causative in the formation of these defects.