Fibroblast growth factors in gastrointestinal development, homeostasis and injury

• Main Author: Ding, Su Sin
• Other Authors: Spencer-Dene, Bradley
• Published: Imperial College London 2009
• Subjects: 612.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.513469

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are essential for controlling cell growth and proliferation, angiogenesis, wound healing and tumourigenesis. In mammals, there are twenty-three FGFs and five FGFRs, with each FGFR having different ligand binding specificities. FGFs are thought to act in a paracrine manner, in which they are secreted by one cell and activate FGFRs on another cell type, and such interaction helps to establish the fundamental crosstalk between epithelium and mesenchyme during development, homeostasis and tumourigenesis. This thesis aims to investigate the requirement of FGFR2-isoform IIIb (Fgfr2b) and one of its ligands, Fgf10, in gastrointestinal (GI) development, homeostasis and injury, using loss-of-function animal models. Fgfr2b-/- and Fgf10-/- exhibit similar multi-organ defects, including intestinal atresia. While caecal and colonic atresia have been previously described, the mechanism of duodenal atresia and the role of Fgf signalling in duodenal development have not been fully established. We demonstrate that absence of Fgfr2b or Fgf10 leads to decreased tissue proliferation and increased apoptosis in the duodenum, contributing to duodenal atresia. These mutants also develop gastric heterotopia in the rostral duodenum due to loss of gastric-intestinal boundary specification. In addition, we demonstrate reduced expression of Wnt targets Tcf1 and Tcf4 in the small intestine, with corresponding downregulation of Lgr5, an intestinal stem cell marker, and Cdx1, a homeobox gene involved in anterior-posterior patterning. We show by in vitro that Fgf10-Fgfr2b signalling is able to regulate Tcf4 expression via the Grb2/Sos/Ras/MAPK pathway. In order to study the requirement of Fgfr2b in intestinal homeostasis and injury, we crossed a transgenic line bearing a progesterone antagonist (RU486)-dependent Cre recombinase (A33-CrePR) expressed under the control of the intestinal-specific A33 antigen promoter, with an existing conditional Fgfr2b line. Following RU486 administration with/without administration of dextran sodium sulphate (DSS) thereafter to induce ulcerative colitis (UC), the A33-CrePR+/Fgfr2bflox/flox targeted Fgfr2b ablation with high efficiency across the small and large intestine. We demonstrate that significant downregulation of Fgfr2b does not affect intestinal homeostasis, but significantly increases the susceptibility of colonic epithelium to UC and delays wound healing, contributed by reduced epithelial proliferation. In order to study the requirement of Fgfr2b in gastric homeostasis, we characterised two stomach-specific minimal promoters for trefoil factor 1 (Tff1) and H+K+-ATPase (Atp4b) to generate two inducible Cre recombinase (CreERT2) lines. Gastric surface mucous cells and acidproducing parietal cells express Tff1 and Atp4b respectively, as well as Fgfr2b. We demonstrate by in vitro that Atp4b nucleotide -1,035bp to +24 bp and Tff1 nucleotide -641 bp to +28 bp are optimal for driving Cre expression in the mouse stomach. Thus, our novel data provides evidence that Fgfr2b and Fgf10 are required for normal duodenal morphogenesis and differentiation, and Fgfr2b confers protection against DSS-induced colonic injury and promotes wound repair.