Fibroblast growth factor receptor-3 (FGFR-3) regulates expression of paneth cell lineage-specific genes in intestinal epithelial cells through both TCF4/beta-catenin-dependent and -independent signaling pathways.

Fibroblast growth factor receptor-3 (FGFR-3) expression in the developing intestine is restricted to the undifferentiated epithelial cells within the lower portion of the crypt. We previously showed that mice lacking functional FGFR-3 have a significant decrease in the number of Paneth cells in the small intestine. Here, we used Caco2 cells to investigate whether FGFR-3 signaling can directly modulate expression of Paneth cell differentiation markers through its effects on TCF4/ß-catenin or through other signaling pathways downstream of this receptor. Caco2 cells treated with FGFR-3 ligands or expressing FGFR-3(K650E), a constitutively active mutant, resulted in a significantly increased expression of genes characteristic of mature Paneth cells, including human α-defensins 5 and 6 (HD5 and HD6) and Paneth cell lysozyme, whereas enterocytic differentiation markers were reduced. Activation of FGFR-3 signaling sustained high levels of ß-catenin mRNA expression, leading to increased TCF4/ß-catenin-regulated transcriptional activity in Caco2 cells. Sustained activity of the TCF4/ß-catenin pathway was required for the induction of Paneth cell markers. Activation of the MAPK pathway by FGFR-3 is also required for the induction of Paneth cell markers in addition to and independent of the effect of FGFR-3 on TCF4/ß-catenin activity. These studies suggest that coordinate activation of multiple independent signaling pathways downstream of FGFR-3 is involved in regulation of Paneth cell differentiation.

Fibroblast growth factor receptor-3 regulates Paneth cell lineage allocation and accrual of epithelial stem cells during murine intestinal development.

Fibroblast growth factor receptor 3 (FGFR-3) is expressed in the lower crypt epithelium, where stem cells of the intestine reside. The role of FGFR-3 signaling in regulating features of intestinal morphogenesis was examined in FGFR-3-null (FGFR-3(-/-)) mice. FGFR-3(-/-) mice had only about half the number of intestinal crypts and a marked decrease in the number of functional clonogenic stem cells, as assessed by an in vivo microcolony-forming assay, compared with wild-type littermates. A marked deficit in allocation of progenitor cells to Paneth cell differentiation was noted, although all the principal epithelial lineages were represented in FGFR-3(-/-) mice. The total cellular content and nuclear localization of beta-catenin protein were reduced in FGFR-3(-/-) mice, as was expression of cyclin D1 and matrix metalloproteinase-7, major downstream targets of beta-catenin/T cell factor-4 (Tcf-4) signaling. Activation of FGFR-3 in Caco-2 cells, an intestinal epithelial cell line, abrogated the fall in beta-catenin/Tcf-4 signaling activity that is normally observed in these cells as cultures become progressively more confluent. These findings are consistent with the hypothesis that, during intestinal development, FGFR-3 signaling regulates crypt epithelial stem cell expansion and crypt morphogenesis, as well as Paneth cell lineage specification, through beta-catenin/Tcf-4-dependent and -independent pathways.

Altered epithelial cell lineage allocation and global expansion of the crypt epithelial stem cell population are associated with ileitis in SAMP1/YitFc mice.

Crohn's disease is characterized by cycles of mucosal injury and ulceration followed by epithelial regeneration and restoration of normal epithelial function. In this study, we examined whether ileitis in SAMP1/YitFc mice, a recombinant-inbred line that spontaneously develops ileitis resembling human Crohn's disease, was associated with alterations in normal patterns of epithelial differentiation or changes in epithelial regeneration after experimental injury. Increased numbers of Paneth, goblet, and intermediate cells were present focally in the ileum of SAMP1/YitFc mice by 4 weeks of age, before any histological evidence of acute or chronic inflammation. This increase in secretory cells became more pronounced at sites of ileitis with increasing age and inflammation. Additionally, there was mispositioning of Paneth and intermediate cells along the crypt-to-villus unit. A concomitant reduction in the number of absorptive enterocytes was observed. In contrast to the ileal-specific changes in lineage allocation, crypt stem cell numbers began to increase in both the ileum and proximal jejunum at the onset of inflammation in SAMP1/YitFc mice. These data suggest that the alterations in epithelial cell differentiation and increases in the size of the crypt stem cell population observed in SAMP1/YitFc mice are regulated by distinct mechanisms. We speculate that these epithelial alterations may play a role in the pathogenesis of ileitis in this murine model of Crohn's disease.

Fibroblast growth factor receptor-3 is expressed in undifferentiated intestinal epithelial cells during murine crypt morphogenesis.

Prior studies have demonstrated that fibroblast growth factor receptor-3 (FGFR-3) regulates proliferation of undifferentiated intestinal epithelial cells in vitro. However, the function(s) of FGFR-3-mediated signaling during intestinal development and epithelial differentiation in vivo remain unknown. The goal of this study was to define the temporal, regional, and cell-specific patterns of FGFR-3 expression and its ligands during normal intestinal ontogeny and epithelial regeneration. Both the IIIb and IIIc isoforms of FGFR-3 mRNA, which result from differential splicing of the FGFR-3 primary transcript, were detected in mouse small intestine as early as embryonic day 16. FGFR-3 levels peaked in the small intestine from 7 to 21 days after birth and decreased thereafter to reach the low levels observed in adult mice. FGFR-3 IIIb and IIIc mRNA levels were highest in the duodenum and proximal jejunum with lower levels of both seen in the distal jejunum, ileum, and colon. FGFR-3 was expressed in a subset of proliferating undifferentiated crypt epithelial cells located in the intervillous epithelium and in the lower half of nascently forming crypts but not in differentiated epithelial cell types. FGFR-3 IIIb was the dominant isoform expressed in both small intestinal and colonic crypts. Expression of FGF1, FGF2, and FGF9, known ligands of FGFR-3, paralleled patterns of FGFR-3 expression during gut development. These data suggest that signaling through FGFR-3 plays a role in regulating morphogenic events involved in formation of intestinal crypts and/or the fate of epithelial stem cells.

Intestinal stem cells and mucosal gut development.

PURPOSE OF REVIEW: In the past year, the study of intestinal stem cell biology has realized significant progress toward understanding the mechanisms and pathways regulating crypt stem cell turnover, maintenance, and differentiation. RECENT FINDINGS: This review summarizes recent investigations that have contributed significantly to the elucidation of mechanisms operative during intestinal development and in the adult intestine that regulate maintenance of the stem cell niche, cell fate and lineage allocation, and establishment and maintenance of the architectural organization of the crypt-to-villus axis. SUMMARY: The relevance of the findings discussed in this review extends beyond the field of intestinal development to encompass the study of tissue remodeling and repair and intestinal neoplasia.