Development of the mammalian urethra is controlled by Fgfr2-IIIb.

Development of external genitalia in mammalian embryos requires tight coordination of a complex series of morphogenetic events involving outgrowth, proximodistal and dorsoventral patterning, and epithelial tubulogenesis. Hypospadias is a congenital defect of the external genitalia that results from failure of urethral tube closure. Although this is the second most common birth defect in humans, affecting one in every 250 children, the molecular mechanisms that regulate morphogenesis of the mammalian urethra are poorly understood. We report that mice lacking the IIIb isoform of fibroblast growth factor receptor 2 (Fgfr2) exhibit severe hypospadias. Urethral signaling regions, as indicated by Shh and Fgf8 expression, are established in Fgfr2-IIIb null mice; however, cell proliferation arrests prematurely and maturation of the urethral epithelium is disrupted. Fgfr2-IIIb-/- mutants fail to maintain the progenitor cell population required for uroepithelial renewal during tubular morphogenesis. In addition, we show that antagonism of the androgen receptor (AR) leads to loss of Fgfr2-IIIb and Fgf10 expression in the urethra, and an associated hypospadias phenotype, suggesting that these genes are downstream targets of AR during external genital development. Genitourinary defects resulting from disruption of AR activity, by either genetic or environmental factors, may therefore involve negative regulation of the Fgfr2 pathway. This represents the first example of how the developing genitourinary system integrates cues from systemically circulating steroid hormones with a locally expressed growth factor pathway.

Induction of chondrogenesis in neural crest cells by mutant fibroblast growth factor receptors.

Activating mutations in human fibroblast growth factor receptors (FGFR) result in a range of skeletal disorders, including craniosynostosis. Because the cranial bones are largely neural crest derived, the possibility arises that increased FGF signalling may predispose to premature/excessive skeletogenic differentiation in neural crest cells. To test this hypothesis, we expressed wild-type and mutant FGFRs in quail embryonic neural crest cells. Chondrogenesis was consistently induced when mutant FGFR1-K656E or FGFR2-C278F were electroporated in ovo into stage 8 quail premigratory neural crest, followed by in vitro culture without FGF2. Neural crest cells electroporated with wild-type FGFR1 or FGFR2 cDNAs exhibited no chondrogenic differentiation in culture. Cartilage differentiation was accompanied by expression of Sox9, Col2a1, and osteopontin. This closely resembled the response of nonelectroporated neural crest cells to FGF2 in vitro: 10 ng/ml induces chondrogenesis, Sox9, Col2a1, and osteopontin expression, whereas 1 ng/ml FGF2 enhances cell survival and Sox9 and Col2a1 expression, but never induces chondrogenesis or osteopontin expression. Transfection of neural crest cells with mutant FGFRs in vitro, after their emergence from the neural tube, in contrast, produced chondrogenesis at a very low frequency. Hence, mutant FGFRs can induce cartilage differentiation when electroporated into premigratory neural crest cells but this effect is drastically reduced if transfection is carried out after the onset of neural crest migration.

A crucial role for Fgfr2-IIIb signalling in epidermal development and hair follicle patterning.

To understand the role Fgf signalling in skin and hair follicle development, we analysed the phenotype of mice deficient for Fgfr2-IIIb and its main ligand Fgf10. These studies showed that the severe epidermal hypoplasia found in mice null for Fgfr2-IIIb is caused by a lack of the basal cell proliferation that normally results in a stratified epidermis. Although at term the epidermis of Fgfr2-IIIb null mice is only two to three cells thick, it expresses the classical markers of epidermal differentiation and establishes a functional barrier. Mice deficient for Fgf10 display a similar but less severe epidermal hypoplasia. By contrast, Fgfr2-IIIb-/-, but not Fgf10-/-, mice produced significantly fewer hair follicles, and their follicles were developmentally retarded. Following transplantation onto nude mice, grafts of Fgfr2-IIIb-/- skin showed impaired hair formation, with a decrease in hair density and the production of abnormal pelage hairs. Expression of Lef1, Shh and Bmp4 in the developing hair follicles of Fgfr2-IIIb-/- mice was similar to wild type. These results suggest that Fgf signalling positively regulates the number of keratinocytes needed to form a normal stratified epidermis and to initiate hair placode formation. In addition, Fgf signals are required for the growth and patterning of pelage hairs.