Interactive Fly, Drosophila

FGF effects on epidermal growth and patterning

Keratinocyte growth factor (KGF) is another member of the fibroblast growth factor (FGF) family. Synthesized by cells of the dermal component of skin, KGF's potent mitogenic activity is on the epidermal component, which harbors the receptors for this factor. Mice expressing KGF ectopically in their epidermis typically appear frail and weak, often with grossly wrinkled skin. These mice exhibit a gross increase in epidermal thickness accompanied by alterations in epidermal growth and differentiation. Most remarkably, animals display several striking and unexpected changes, including a marked suppression of hair follicle morphogenesis and suppression of adipogenesis. With age, some animals develop gross transformations in the tongue epithelium and in epidermis. In addition, they exhibit elevated salivation and their salivary glands showed signs of altered differentiation. These findings provide new and important insights into the roles of KGF, implicating this potent growth factor in eliciting global effects not only on growth, but also on the development and differentiation of skin and other tissues. In particular, KGF seems to interfere with signaling of some mesenchymal-epithelial interactions (Guo, 1993).

The ectoderm is required for outgrowth of facial prominences; facial ectoderm from all facial prominences is interchangeable. Signals provided by the ectoderm may include members of the fibroblast growth factor family (FGF). In order to test whether FGFs could replace facial ectoderm and promote outgrowth, stage 24 frontonasal mass or mandibular mesenchyme was grafted to a host chick limb and a bead soaked in FGF-2 or FGF-4 was placed on top of the mesenchyme. Following 7 days of incubation, the amount of outgrowth was quantified by measuring the rods of cartilage that formed from the grafts. FGF-2 and FGF-4 stimulate an increase in length of cartilage rods in mandibular grafts, as compared to mandibular mesenchyme grafted without ectoderm (P < 0.05). FGF-4 stimulates a small increase in length of frontonasal mass mesenchyme (P < 0.05) and both FGFs increase the frequency of egg tooth formation in frontonasal mass mesenchyme, as compared to frontonasal mass mesenchyme grafted without ectoderm. FGFs can partially but not completely replace facial ectoderm, since homotypic recombinations of frontonasal mass and mandibular tissues are significantly longer than mesenchyme grafts treated with FGF-soaked beads (P < 0.05). The addition of a second FGF-soaked bead does not significantly increase the length of the frontonasal mass or the mandibular mesenchyme. FGF-2 protein is expressed in facial ectoderm and can be an endogenous signal for outgrowth. In contrast, FGF-8 transcripts are not expressed in the ectoderm covering the areas of the face that were grafted; thus, it is less likely that FGF-8 is required for outgrowth. These results indicate that FGFs are part of an endogenous signaling pathway involved in distal outgrowth and chondrogenesis of the facial prominences (Richman, 1997).

Keratinocyte growth factor (alternatively designated FGF-7) is a paracrine growth factor produced by mesenchymal cells; it is mitogenic specifically for epithelial cells. The potential effect of KGF on wound healing was assessed in vitro by measuring randomized migration and plasminogen activator (PA) activity of keratinocytes in response to the growth factor. Incubation of normal human keratinocytes with KGF significantly stimulates cell migration and PA activity. When tested in these assays on an equimolar basis, 1 nM KGF is at least as potent as transforming growth factor alpha and more active than basic FGF. None of these effects are observed when KGF is administered to fibroblasts or endothelial cells. Stimulation of keratinocyte migration by KGF is dose dependent, and a neutralizing monoclonal antibody against KGF reduces KGF-stimulated migration and cell growth. There is an increased PA activity, mainly attributable to an elevated level of urokinase-type PA. These in vitro results suggest that KGF may have an important role in stimulating reepithelialization during the process of wound repair (Tsuboi, 1993).

Expression of human keratinocyte growth factor (KGF/FGF-7) was directed to hepatocytes during the later period of mouse gestation using a human apolipoprotein E (ApoE) gene promoter and its associated liver-specific enhancer. Human KGF is detectable in liver extracts and serum prepared from e17.5-e19.5 embryos, concomitant with the appearance of morphological abnormalities in several organs that express KGF receptor. The most striking phenotypic aberration in the ApoE-hKGF transgenic embryos is marked hyperplasia and cystic dilation of the cortical and medullary kidney collecting duct system, a phenotype resembling infantile polycystic kidney disease in humans. Transgenic embryos have enlarged livers, with prominent biliary epithelial hyperplasia; they also exhibit enhanced bronchiolar epithelial and type II pneumocyte proliferation. There is variable hyperplasia of intestinal epithelia, and urothelium of the urinary bladder and ureters. When compared to age-matched littermate controls, marked epidermal papillomatous acanthosis and hyperkeratosis in the skin, with a notable decrease in the number of developing hair follicles is seen in transgenic embryos. The pancreas exhibits significant ductal hyperplasia, with an increase in the number of ductal epithelial cells staining positive for insulin expression. High systemic levels of KGF during the later stages of embryogenesis causes abnormalities in epithelial growth and differentiation within multiple organ systems, and results in perinatal lethality. Correct temporal and spatial expression of KGF during the later stages of organ development is likely to play a critical role in mesenchymal-epithelial signaling required for normal embryonic growth and development (Nguyen, 1996).

Murine FGF-7 is transiently detected in the developing myocardium, differentially regulated between the atrium and ventricle. The gene is also expressed in the myotomes of the somites, coincident with FGF-4 and FGF-5 transcripts, and is detected transiently in cleaved muscles. Regional expression is detected in the ventricular zone of the developing forebrain at 14.5 d.p.c. Later in development, FGF-7 RNA is detected in mesenchymal tissues, suggesting a role in epithelial-mesenchymal interactions and in the dermis consistent with its proposed role as a keratinocyte mitogen (Mason, 1994).

Pax9 is a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. The sites of Pax9 expression in the mandibular arch are positioned by the combined activity of two signals, one (FGF8) that induces Pax9 expression and the other (BMP2 and BMP4) that prevents this induction. Thus it appears that the position of the teeth is determined by a combination of two different types of signaling molecules produced in wide but overlapping domains rather than by a single localized inducer. It is suggested that a similar mechanism may be used for specifying the sites of development of other organs. For example, BMP2 and BMP4 can antagonize FGF function in the developing mouse limb bud (Neubuser, 1997).

The formation of periodic patterns is fundamental in biology. Theoretical models describing these phenomena have been proposed for feather patterning, however, no molecular candidates have been identified. The feather tract is initiated by a continuous stripe of Shh, Fgf-4, and Ptc expression in the epithelium, which then segregates into discrete feather primordia that are more strongly Shh and Fgf-4 positive. The primordia also become Bmp-2 and Bmp-4 positive. Bead-mediated delivery of BMPs inhibits local feather formation in contrast with the activators, Shh and Fgf-4, which induce feather formation. Both Fgf-4 and Shh induce local expression of Bmp-4, while Bmp-4 suppresses local expression of both. Fgf-4 also induces Shh. Based on these findings, a model is proposed that involves (1) homogeneously distributed global activators that define the field; (2) a position-dependent activator of competence that propagates across the field, and (3) local activators and inhibitors triggered in sites of individual primordia that act in a reaction-diffusion mechanism. A computer simulation model for feather pattern formation is also presented (Jung, 1998).

Spacing patterns are of fundamental importance in various repeated structures that develop at regular intervals such as feathers, teeth and insect ommatidia. The mouse tongue develops a regular papilla pattern and provides a good model to study pattern formation. The expression patterns of the signaling molecules, sonic hedgehog (Shh), bone morphogenetic proteins -2 and -4 (Bmp-2 and Bmp-4), and fibroblast growth factor-8 (Fgf-8) were studied in mouse embryos between E 10.5 and 15. All four genes are expressed uniformly in the tongue epithelium between E 10.5 and 11. At E 13, before morphologically detectable gustatory papillae initiation, Shh, Bmp-2 and Bmp-4 expression segregates into discrete spots, whereas, Fgf-8 is downregulated. At E 14, small eminences in the anterior part of the tongue are the first morphological indications of fungiform papillae, and they express Shh and Bmp-2, whereas, Bmp-4 is almost absent in the tongue. It is concluded that these conserved signaling molecules are associated with the initiation and early morphogenesis of the tongue papillae (Jung, 1999).

The mechanism of tissue induction and specification was examined using the lacrimal gland as a model system. This structure begins its morphogenesis as a bud-like outgrowth of the conjunctival epithelium and ultimately forms a branched structure with secretory function. Using a reporter transgene as a specific marker for gland epithelium, it has been shown that the transcription factor Pax6 is required for normal development of the gland and is probably an important competence factor. In investigating the cell-cell signaling required, it has been shown that FGF10 is sufficient to stimulate ectopic lacrimal bud formation in ocular explants. Expression of FGF10 in the mesenchyme adjacent to the presumptive lacrimal bud and absence of lacrimal gland development FGF10-null mice strongly suggest that it is an endogenous inducer. This was supported by the observation that inhibition of signaling by a receptor for FGF10 (receptor 2 IIIb) suppressed development of the endogenous lacrimal bud. In explants of mesenchyme-free gland epithelium, FGF10 stimulates growth but not branching morphogenesis. This suggests that its role in induction is to stimulate proliferation and, in turn, that FGF10 combines with other factors to provide the instructive signals required for lacrimal gland development. The lack of normal lacrimal gland formation in the Sey heterozygous mice indicates that the Pax6 transcription factor is required. Pax6 is expressed in the conjunctival epithelium but not in the neural crest-derived periocular mesenchyme. This suggests that the requirement for Pax6 in gland formation is autonomous to the cells of the precursor conjunctival epithelium and is consistent with an autonomous requirement for Pax6 in the formation of the lens. It is likely that Pax6 is one factor that establishes competence and permits gland development from conjunctival epithelium in response to an FGF ligand (Makarenkova, 2000).

To understand the role Fgf signalling in skin and hair follicle development, the phenotype was analyzed of mice deficient for Fgfr2-IIIb and its main ligand Fgf10. These studies show 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 produce significantly fewer hair follicles, and their follicles were developmentally retarded. Following transplantation onto nude mice, grafts of Fgfr2-IIIb^–/– skin show 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 is 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 (Petiot, 2003).

Morphogenesis of hairs and feathers is initiated by an as yet unknown dermal signal that induces placode formation in the overlying ectoderm. To determine whether FGF signals are required for this process soluble versions of FGFR1 or FGFR2 were overexpressed in the skin of chicken embryos. This produced a complete failure of feather formation prior to any morphological or molecular signs of placode development. Fgf10 is expressed in the dermis of nascent feather primordia, and anti-FGF10 antibodies block feather placode development in skin explants. In addition FGF10 can induce expression of positive and negative regulators of feather development and can induce its own expression under conditions of low BMP signaling. Together these results demonstrate that FGF signaling is required for the initiation of feather placode development and implicate FGF10 as an early dermal signal involved in this process (Mandler, 2004).

FGFs and external genitalia

Coordinated growth and differentiation of the genital tubercle (GT), an embryonic anlage of external genitalia, generates the proximodistally elongated structure suitable for copulation, erection, uresis and ejaculation. Despite recent progress in molecular embryology, few attempts have been made to elucidate the molecular developmental processes of external genitalia formation. Bone morphogenetic protein genes (Bmp genes) and their antagonists are spatiotemporally expressed during GT development. Exogenously applied BMP increases apoptosis of GT and inhibits its outgrowth. The distal urethral epithelium (DUE), distal epithelia marked by the Fgf8 expression, may control the initial GT outgrowth. Exogenously applied BMP4 downregulates the expression of Fgf8 and Wnt5a, concomitant with increased apoptosis and decreases cell proliferation of the GT mesenchyme. Furthermore, noggin mutants and Bmpr1a conditional mutant mice display hypoplasia and hyperplasia of the external genitalia respectively. noggin mutant mice exhibited downregulation of Wnt5a and Fgf8 expression with decreased cell proliferation. Consistent with such findings, Wnt5a mutant mice display GT agenesis with decreased cell proliferation. By contrast, Bmpr1a mutant mice display decreased apoptosis and augmented Fgf8 expression in the DUE associated with GT hyperplasia. These results suggest that some of the Bmp genes could negatively affect proximodistally oriented outgrowth of GT with regulatory functions on cell proliferation and apoptosis. The DUE region can be marked only until 14.0 dpc (days post coitum) in mouse development, while GT outgrowth continues thereafter. Possible signaling crosstalk among the whole distal GT regions were also investigated (Suzuki, 2003).

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branchless: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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