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Fibroblast growth factor receptor-3 (FGFR-3Despite the critical importance of Paneth cells in maintaining gut homeostasis, knowledge of the pathways regulating Paneth cell differentiation and function has only recently begun to emerge. Studies using transgenic mice or mice with targeted mutations in epithelial regulatory genes, including components of the Wnt signaling pathway (Tcf4, Fz5, Sox9, and Apc), FGFR-3, and PPAR␤/␦, have identified these genes as key regulators of Paneth cell differentiation and expression of defensin peptides in vivo (9 -14). However, the lack of an adequate intestinal epithelial cell culture system that can recapitulate features of Paneth cell differentiation and maturation has hampered direct investigation of how these signaling pathways may interact in regulating Paneth cell differentiation and functional maturation.We recently reported that signaling through FGFR-3 in crypt epithelial cells regulates both the number of epithelial stem cells and Paneth cell differentiation during postnatal gut development (13,15). FGFR-3 is highly expressed along the membranes of cells in the lower portion of the crypt in the developing mouse intestine (15). FGFRs are members of the receptor-tyrosine kinase family. Ligand binding induces dimerization, autophosphorylation, and initiation of a signal transduction cascade that ultimately results in modification of gene expression. Ligands for FGFR-3, FGF1, FGF2, and FGF9, are up-regulated in the postnatal murine small intestine from birth through the suckling weaning transition, when crypt mor-

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Fibroblast growth factor receptors are receptor tyrosine kinases that mediate cellular growth and differentiation during normal gut development and injury‐repair. Robust FGFR3 expression is restricted to the lower portion of the crypt where stem cells reside during normal intestinal development. Activation of the β‐catenin/Tcf‐4 complex is crucial for maintaining the intestinal stem cell compartment and for Paneth cell lineage allocation/differentiation. Previously we showed that FGFR3‐/‐ mice have fewer intestinal crypts, have a reduction in the total number of clonogenic stem cells, and decreased β‐catenin/Tcf‐4 activation. FGFR3‐/‐ mice at post‐natal days 14 and 21 display both a significant reduction in the number of intestinal Paneth cells and a reduction in mRNA levels of Paneth cell products. Since Paneth cell lineage allocation/differentiation is dependent upon β‐catenin/Tcf‐4 activation, we examined whether FGFR3 signaling can directly modulate Paneth cell specification. Caco2 cells treated with FGF9, a FGFR3 ligand, or co‐transfected with a constituitively active mutant (K650E) of FGFR3 had sustained β‐catenin/Tcf‐4 transcriptional activity. FGF9 increased mRNA levels of two Paneth cell markers, lysozyme and human defensin 5. These findings suggest that FGFR3 signaling can directly modulate Paneth cell specification possibly through the β‐catenin/Tcf‐4 signaling complex.

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