Fibroblast Growth Receptor 3 regulates crypt stem cell morphogenesis through a β‐catenin/Tcf‐4 dependent signaling mechanism

Abstract

Fibroblast growth factor receptors are tyrosine kinases that mediate cellular growth and differentiation during normal development and injury‐repair. To determine whether FGF receptor 3 (FGFR3) is involved in crypt morphogenesis, we examined the function of FGFR3 following intestinal injury and during gut development. FGFR3 is expressed in epithelial cells near the crypt base, where stem cells reside. Mice lacking FGFR3 develop fewer intestinal crypts (~49% of wt) and have reduced crypt survival following radiation‐injury suggesting that these mice have fewer clonogenic stem cells. β‐catenin/Tcf‐4 signaling is known to regulate stem cell proliferation in intestinal crypts. Crypt epithelial cells in FGFR3−/ − mice show decreased levels of β‐catenin protein and reduced transcription of some β‐catenin/Tcf‐4 target genes. To determine if FGFR3 affects Tcf‐4 signaling, Caco2 cells were transfected with a Tcf4 dependent reporter. Cells treated with FGF9, a FGFR3 ligand, or co‐transfected with the constituitively active mutant (K650E) of FGFR3 had sustained β‐catenin/Tcf‐4 transcriptional activity; whereas, in unstimulated cells and cells co‐transfected with a FGFR3 kinase dead mutant (K508R) activity fell ~4.7 fold by day 4 post‐confluence. These findings suggest FGFR3 mediates expansion of the stem cell pool by activating intermediate signaling pathways in a β‐catenin/Tcf‐4 dependent mechanism.