Characterization of recombinant human fibroblast growth factor (FGF)-10 reveals functional similarities with keratinocyte growth factor (FGF-7)

BMP4 rescues a non-cell-autonomous function of Msx1 in tooth development

The development of many organs depends on sequential epithelial-mesenchymal interactions, and the developing tooth germ provides a powerful model for elucidating the nature of these inductive tissue interactions. In Msx1-deficient mice, tooth development arrests at the bud stage when Msx1 is required for the expression of Bmp4 and Fgf3 in the dental mesenchyme (Bei, M. and Maas, R. (1998) Development 125, 4325–4333). To define the tissue requirements for Msx1 function, we performed tissue recombinations between wild-type and Msx1 mutant dental epithelium and mesenchyme. We show that through the E14.5 cap stage of tooth development, Msx1 is required in the dental mesenchyme for tooth formation. After the cap stage, however, tooth development becomes Msx1 independent, although our experiments identify a further late function of Msx1 in odontoblast and dental pulp survival. These results suggest that prior to the cap stage, the dental epithelium receives an Msx1-dependent signal from the dental mesenchyme that is necessary for tooth formation. To further test this hypothesis, Msx1 mutant tooth germs were first cultured with either BMP4 or with various FGFs for two days in vitro and then grown under the kidney capsule of syngeneic mice to permit completion of organogenesis and terminal differentiation. Previously, using an in vitro culture system, we showed that BMP4 stimulated the growth of Msx1 mutant dental epithelium (Chen, Y., Bei, M. Woo, I., Satokata, I. and Maas, R. (1996). Development 122, 3035–3044). Using the more powerful kidney capsule grafting procedure, we now show that when added to explanted Msx1-deficient tooth germs prior to grafting, BMP4 rescues Msx1 mutant tooth germs all the way to definitive stages of enamel and dentin formation. Collectively, these results establish a transient functional requirement for Msx1 in the dental mesenchyme that is almost fully supplied by BMP4 alone, and not by FGFs. In addition, they formally prove the postulated downstream relationship of BMP4 with respect to Msx1, establish the non-cell-autonomous nature of Msx1 during odontogenesis, and disclose an additional late survival function for Msx1 in odontoblasts and dental pulp.

Keratinocyte growth factor-2 (FGF-10) promotes healing of experimental small intestinal ulceration in rats

Keratinocyte growth factor-2 (KGF-2, repifermin) is a homolog of KGF-1 with epithelial mitogenic activities. We investigated the therapeutic role of KGF-2 in intestinal ulceration and its mechanisms of protection. KGF-2 (0.3-5 mg/kg) was administered before or after induction of small intestinal ulceration by indomethacin (Indo) in prevention and treatment protocols. In acute studies, KGF-2 was injected for up to 7 days before or daily for 5 days after Indo. In a 15-day chronic study, KGF-2 was injected intravenously daily beginning before or 7 days after Indo. Injury was evaluated by blinded macroscopic and microscopic inflammatory scores, epithelial BrdU staining, tissue IL-1beta, PGE(2), and hydroxyproline concentrations, and collagen type I RNA expression. In vitro effects of KGF-2 were evaluated by epithelial cellular proliferation, restitution of wounded monolayers, PGE(2) secretion, and expression of COX-2 and collagen mRNA. Intravenous KGF-2 significantly decreased acute intestinal injury by all parameters and significantly decreased chronic ulceration. Pretreatment, daily infusion, and delayed treatment were effective. KGF-2 promoted in vitro epithelial restitution with only modest effects on epithelial cell proliferation, stimulated COX-2 expression in cultured epithelial cells, and upregulated in vitro and in vivo PGE(2) production. KGF-2 did not affect in vivo fibrosis, although it induced collagen expression in cultured intestinal myofibroblasts. These results suggest that KGF-2 inhibits intestinal inflammation by stimulating epithelial restitution and protective PGs.

Associations of FGF-3 and FGF-10 with signaling networks regulating tooth morphogenesis

The morphogenesis and cell differentiation in developing teeth is governed by interactions between the oral epithelium and neural crest-derived ectomesenchyme. The fibroblast growth factors FGF-4, -8, and -9 have been implicated as epithelial signals regulating mesenchymal gene expression and cell proliferation during tooth initiation and later during epithelial folding morphogenesis and the establishment of tooth shape. To further evaluate the roles of FGFs in tooth development, we analyzed the roles of FGF-3, FGF-7, and FGF-10 in developing mouse teeth. In situ hybridization analysis showed developmentally regulated expression during tooth formation for Fgf-3 and Fgf-10 that was mainly restricted to the dental papilla mesenchymal cells. Fgf-7 transcripts were restricted to the developing bone surrounding the developing tooth germ. Fgf-10 expression was observed in the presumptive dental epithelium and mesenchyme during tooth initiation, whereas Fgf-3 expression appeared in the dental mesenchyme at the late bud stage. During the cap and bell stage, both Fgf-3 and Fgf-10 were intensely expressed in the dental papilla mesenchymal cells both in incisors and molars. It is of interest that Fgf-3 expression was also observed in the primary enamel knot, a putative signaling center of the tooth, whereas no transcripts were seen in the secondary enamel knots that appear in the tips of future cusps of the bell stage tooth germs. Down-regulation of Fgf-3 and Fgf-10 expression in postmitotic odontoblasts correlated with the terminal differentiation of the odontoblasts and the neighboring ameloblasts. In the incisors, mesenchymal cells of the cervical loop area showed partially overlapping expression patterns for all studied Fgfs. In vitro analyses showed that expression of Fgf-3 and Fgf-10 in the dental mesenchyme was dependent on dental epithelium and that epithelially expressed FGFs, FGF-4 and -8 induced Fgf-3 but not Fgf-10 expression in the isolated dental mesenchyme. Beads soaked in Shh, BMP-2, and TGF-beta 1 protein did not induce either Fgf-3 or Fgf-10 expression. Cells expressing Wnt-6 did not induce Fgf-10 expression. Furthermore, FGF-10 protein stimulated cell proliferation in the dental epithelium but not in the mesenchyme. These results suggest that FGF-3 and FGF-10 have redundant functions as mesenchymal signals regulating epithelial morphogenesis of the tooth and that their expressions appear to be differentially regulated. In addition, FGF-3 may participate in signaling functions of the primary enamel knot. The dynamic expression patterns of different Fgfs in dental epithelium and mesenchyme and their interactions suggest existence of regulatory signaling cascades between epithelial and mesenchymal FGFs during tooth development.

Defective terminal differentiation and hypoplasia of the epidermis in mice lacking the Fgf10 gene

Here, we characterized the skin and hair phenotype of mice lacking the fibroblast growth factor 10 gene (Fgf10), a newly identified member of the fibroblast growth factor family. Histological examination of Fgf10(-/-) newborn mouse skin revealed abnormalities in epidermal morphogenesis. The number of proliferating cells in the basal layer was decreased, the granular layer was hypoplastic and lacked distinctive keratohyaline granules and tonofibrils. The expression of loricrin, a marker of epidermal differentiation, was dramatically reduced. Despite the presence of Fgf10 transcripts in normal hair follicles, abnormalities of hair development were not observed in Fgf10(-/-) skin. These data suggest that Fgf10 is required for embryonic epidermal morphogenesis but is not essential for hair follicle development.

Fibroblast growth factor-10: a stromal mediator of epithelial function in the ovine uterus

Fibroblast growth factor-10 (FGF-10) is a stromal-derived paracrine growth factor considered to be important during embryogenesis; however, its expression by cells in the female reproductive tract has not been investigated. Therefore, an ovine FGF-10 cDNA was cloned from an ovine endometrial cDNA library to investigate expression and potential paracrine characteristics of FGF-10 in the ovine uterus. The ovine FGF-10 cDNA encodes a protein of 213 amino acids and possesses an unusually long 5' untranslated region (UTR). In situ hybridization demonstrated that ovine FGF-10 mRNA was expressed by endometrial stromal cells and by mesenchymal cells of the chorioallantoic placenta. The mRNA for FGF-7, a homologue of FGF-10, was localized in the tunica muscularis of blood vessels in endometrium and myometrium. In contrast, FGF receptor 2IIIb, the high-affinity receptor for both FGF-10 and FGF-7, was expressed exclusively in luminal epithelium, glandular epithelium, and placental trophectoderm. The in vivo spatial expression pattern suggests that FGF-10 is a novel endometrial stromal cell-derived mediator of uterine epithelial and conceptus trophectodermal functions. The nonoverlapping spatial patterns of expression for FGF-10 and FGF-7 in ovine uterus and conceptus suggest independent roles in uterine function and conceptus development.

FGF-10 is expressed at low levels in the human prostate

BACKGROUND

Fibroblast growth factors (FGFs) are known to play an important role in the growth of normal prostatic epithelial cells. FGF-10 is a secreted growth factor that binds to FGF receptor-2 IIIb, which is expressed in prostatic epithelial cells and thus can potentially act as a growth factor for these cells. Prior work has indicated that FGF10 may play an important role in the development of the rat prostate, but its role in the adult human prostate is unclear.

METHODS

Expression of FGF10 in human prostate tissue and primary cultures of prostatic epithelial and stromal cells was assessed by reverse-transcriptase PCR (RT-PCR) and Northern blotting. Growth response to FGF10 was assessed by the addition of recombinant FGF-10 to primary cultures of prostatic epithelial and stromal cells.

RESULTS

FGF10 is expressed at levels detectable by RT-PCR and can act as a growth factor for prostatic epithelial cells, but is not active as a growth factor for stromal cells. However, FGF10 is expressed at extremely low levels relative to FGF7, which has a similar biological activity.

CONCLUSIONS

While FGF10 may play a role in prostatic development, it is unlikely to play a major role in prostate growth in normal or hyperplastic adult human prostate, due to its extremely low expression compared to FGF7.

Pattern of expression of the KGF receptor and its ligands KGF and FGF-10 during postnatal mouse mammary gland development

The expression of the KGF receptor (KGFR) and its stromal ligands, KGF and FGF-10, was compared during mouse mammary gland development. KGFR expression in mammary parenchyma is maximal in mature virgin mice, declines during pregnancy and lactation, but rises after weaning. The rise in KGFR mRNA in the virgin animal corresponds to parenchymal growth. The fall in KGFR expression in pregnancy is driven by hormone-induced alveolar differentiation since the level of KGFR mRNA is 5-fold higher in isolated ductal cells compared to alveolar cells. KGF and FGF-10 expression patterns differ during ductal development. FGF-10 is also expressed at about a 15-fold higher molar level than KGF. During pregnancy and lactation, expression of KGF and FGF-10 decreases in intact fat pads but is unchanged in parenchyma-free fat pads. Thus, the decrease in KGF and FGF-10 expression observed in intact glands during pregnancy and lactation is not a direct consequence of the changing hormonal milieu but more likely reflects an increase in the ratio of epithelium to stroma. Differences in the level and pattern of expression of mRNA for KGF, FGF-10, and the KGFR during postnatal development of the mouse mammary gland are a result of morphological development, changes in the ratio of stroma to epithelium, and hormonal regulation of cell differentiation. These changes suggest that the biological roles that these growth factors play are regulated by fluctuations in both growth factor and growth factor receptor expression and that KGF and FGF-10 may have different regulatory functions.

Caveolin-1 and -2 expression is differentially regulated in cultured keratinocytes and within the regenerating epidermis of cutaneous wounds

Keratinocyte growth factor (KGF) and its receptor are involved in various types of epithelial repair processes. To gain insight into the molecular mechanisms of KGF action in the healing skin wound, we searched for genes which are regulated by this factor in cultured keratinocytes. Using the PCR-select technology we constructed a subtractive cDNA library. One of the KGF-regulated genes that we identified was shown to encode caveolin-1, a major component of caveolar membranes. Caveolin-1 is involved in a wide variety of cellular processes, particularly in the regulation of various signal transduction pathways. Caveolin-1 mRNA levels increased in cultured keratinocytes after KGF treatment. By in situ hybridization and immunohistochemistry we found a strong expression of caveolin-1 in the KGF-responsive basal keratinocytes of the epidermis and the hyperproliferative epithelium of the wound as well as in endothelial cells and in other cells of the granulation tissue. In 13-day wounds expression of caveolin-1 mRNA was restricted to the regenerated dermis. In addition to caveolin-1, the mRNA expression of caveolin-2, a second member of the caveolin family, was also induced in keratinocytes after stimulation with KGF but also with other growth factors and cytokines. In contrast to caveolin-1, caveolin-2 protein was expressed in all layers of the normal epidermis and in the suprabasal layers of the hyperproliferative wound epithelium. These results demonstrate a differential expression of caveolin-1 and -2 in proliferating versus differentiating keratinocytes.

Keratinocyte growth factor: expression by endometrial epithelia of the porcine uterus

Keratinocyte growth factor/fibroblast growth factor-7 (KGF/FGF-7) is an established paracrine mediator of hormone-regulated epithelial growth and differentiation. In all organs studied, KGF is uniquely expressed in cells of mesenchymal origin. To determine whether KGF and its receptor, keratinocyte growth factor receptor (KGFR) or fibroblast growth factor receptor-2IIIb, were expressed in the porcine uterus as a potential paracrine system mediating progesterone action, we cloned KGF and KGFR partial cDNAs from the porcine endometrium. KGF and KGFR expression was detected in endometrium by Northern blot hybridization. Interestingly, in situ hybridization results demonstrated that KGF was expressed by endometrial epithelia and was particularly abundant between Days 12 and 15 of the estrous cycle and pregnancy. KGF secretion into the lumen of the porcine uterus was also detected on Day 12 of the estrous cycle and pregnancy. KGFR was expressed in both endometrial epithelia and conceptus trophectoderm. These novel findings suggest that KGF may act on the uterine endometrial epithelium in an autocrine manner and on the conceptus trophectoderm in a paracrine manner in the pig, which is the only species possessing a true epitheliochorial type of placentation.

Fibroblast growth factor (FGF) receptor 1-IIIb is a naturally occurring functional receptor for FGFs that is preferentially expressed in the skin and the brain

Fibroblast growth factors (FGFs) transmit their signals through four transmembrane receptors that are designated FGFR1-4. Alternative splicing in the extracellular region of FGFR1-3 generates receptor variants with different ligand binding affinities. Thus two types of transmembrane receptors (IIIb and IIIc isoforms) have been identified for FGFR2 and FGFR3, and the existence of analogous variants has been postulated for FGFR1 based on its genomic structure. However, only a single full-length transmembrane FGFR1 variant (FGFR1-IIIc) has been identified so far. Here we describe the cloning of a full-length cDNA encoding FGFR1-IIIb from a mouse skin wound cDNA library. This receptor isoform was expressed at the highest levels in a subset of sebaceous glands of the skin and in neurons of the hippocampus and the cerebellum. FGFR1-IIIb was expressed in L6 rat skeletal muscle myoblasts and used in cross-linking and receptor binding studies. FGF-1 was found to bind the receptor with high affinity, whereas FGF-2, -10, and -7 bound with significantly lower affinities. Despite their apparently similar but low affinities, FGF-10 but not FGF-7 induced the activation of p44/42 mitogen-activated protein kinase in FGFR1-IIIb-expressing L6 myoblasts and stimulated mitogenesis in these cells, demonstrating that this new receptor variant is a functional transmembrane receptor for FGF-10.

Conditional expression of fibroblast growth factor-7 in the developing and mature lung

Effects of fibroblast growth factor-7 (FGF-7) on lung morphogenesis, respiratory epithelial cell differentiation, and proliferation were assessed in transgenic mice in which the human FGF-7 cDNA was controlled by a conditional promoter under the direction of regulatory elements from either the human surfactant protein-C (SP-C) or rat Clara cell secretory protein (ccsp) genes. Expression of FGF-7 was induced in respiratory epithelial cells of the fetal lung by administration of doxycycline to the dam. Prenatally, doxycycline induced FGF-7 mRNA in respiratory epithelial cells in both Sp-c and Ccsp transgenic lines, increasing lung size and causing cystadenomatoid malformation. Postnatally, mice bearing both Ccsp-rtta and (Teto)(7)-cmv-fgf-7 transgenes survived, and lung morphology was normal. Induction of FGF-7 expression by doxycycline in the Ccsp-rtta x (Teto)(7)-cmv-fgf-7 mice caused marked epithelial cell proliferation, adenomatous hyperplasia, and pulmonary infiltration with mononuclear cells. Epithelial cell hyperplasia caused by FGF-7 was largely resolved after removal of doxycycline. Surfactant proteins, TTF-1, and aquaporin 5 expression were conditionally induced by doxycycline. The Sp-c-rtta and Ccsp-rtta activator mice provide models in which expression is conditionally controlled in respiratory epithelial cells in the developing and mature lung, altering lung morphogenesis, differentiation, and proliferation.

Keratinocyte growth factor in the promotion of human chorionic gonadotropin production in human choriocarcinoma cells

OBJECTIVE

Several growth factors and cytokines appear to participate in the proliferation or differentiation of trophoblast cells. The purpose of this study was to investigate the extent to which keratinocyte growth factor participates in the development of human embryonic and trophoblast cells at the maternal-fetal interface.

STUDY DESIGN

The reverse transcriptase-polymerase chain reaction method was used to determine the gene expression of keratinocyte growth factor and keratinocyte growth factor receptor in human choriocarcinoma cells (BeWo), human teratocarcinoma cells (PA-1), and human endometrial stromal cells. We also examined the effects of keratinocyte growth factor on cell proliferation and production of human chorionic gonadotropin in BeWo and PA-1 cells.

RESULTS

Keratinocyte growth factor gene was expressed in all cell types. The expression was pronounced in stromal cells of the endometrium collected during the secretory phase and early pregnancy. The keratinocyte growth factor expression was also enhanced in the differentiated BeWo cells. The expression of keratinocyte growth factor receptor gene was observed only in the BeWo cells. The addition of keratinocyte growth factor to the medium did not affect cell proliferation of the BeWo and PA-1 cells. On the other hand, keratinocyte growth factor (100 ng/mL) significantly enhanced human chorionic gonadotropin production in the BeWo cells. Stimulatory action of keratinocyte growth factor on human chorionic gonadotropin production in the BeWo cells was markedly enhanced after forskolin-induced differentiation.

CONCLUSIONS

We conclude that keratinocyte growth factor may play an important role in promotion of human chorionic gonadotropin production in the trophoblast cells.

Mesenchyme with fgf-10 expression is responsible for regenerative capacity in Xenopus limb buds

A young tadpole of an anuran amphibian can completely regenerate an amputated limb, and it exhibits an ontogenetic decline in the ability to regenerate its limbs. However, whether mesenchymal or epidermal tissue is responsible for this decrease of the capacity remains unclear. Moreover, little is known about the molecular interactions between these two tissues during regeneration. The results of this study showed that fgf-10 expression in the limb mesenchymal cells clearly corresponds to the regenerative capacity and that fgf-10 and fgf-8 are synergistically reexpressed in regenerating blastemas. However, neither fgf-10 nor fgf-8 is reexpressed after amputation of a nonregenerative limb. Nevertheless, nonregenerative epidermal tissue can reexpress fgf-8 under the influence of regenerative mesenchyme, as was demonstrated by experiments using a recombinant limb composed of regenerative limb mesenchyme and nonregenerative limb epidermis. Taken together, our data demonstrate that the regenerative capacity depends on mesenchymal tissue and suggest that fgf-10 is likely to be involved in this capacity.

An important role for the IIIb isoform of fibroblast growth factor receptor 2 (FGFR2) in mesenchymal-epithelial signalling during mouse organogenesis

The fibroblast growth factor receptor 2 gene is differentially spliced to encode two transmembrane tyrosine kinase receptor proteins that have different ligand-binding specificities and exclusive tissue distributions. We have used Cre-mediated excision to generate mice lacking the IIIb form of fibroblast growth factor receptor 2 whilst retaining expression of the IIIc form. Fibroblast growth factor receptor 2(IIIb) null mice are viable until birth, but have severe defects of the limbs, lung and anterior pituitary gland. The development of these structures appears to initiate, but then fails with the tissues undergoing extensive apoptosis. There are also developmental abnormalities of the salivary glands, inner ear, teeth and skin, as well as minor defects in skull formation. Our findings point to a key role for fibroblast growth factor receptor 2(IIIb) in mesenchymal-epithelial signalling during early organogenesis.

A subset of fibroblast growth factors (Fgfs) promote survival, but Fgf-8b specifically promotes astroglial differentiation of rat cortical precursor cells

Fibroblast growth factor-2 (Fgf-2 or basic Fgf) is known to promote the survival, proliferation, and differentiation of neural precursor cells. We have examined and compared the effects of Fgf-2 with those of Fgf-1, -4, -6, -7, -9, and -10, as well as three isoforms of Fgf-8 (-8a, -8b, and -8c), on the fate of cultured embryonic day 15 (E15) rat cortical cells. Clonal analysis, using retroviral tagging, shows that only Fgf-2, -4, and -8b can efficiently promote the survival of cortical precursor cells, the majority of which give rise to neurons. Surprisingly, and in contrast to other Fgfs, Fgf-8b also promotes astroglial differentiation of a subpopulation of these cells, which would otherwise appear to yield neurons. We also show that E15 cortical cells initially express the IIIc isoforms of Fgf-receptors (R-1,-2, and -3 but within 16 h of culturing they down regulate FgfR2-IIIc. These studies demonstrate that cortical precursor cells respond to Fgf stimulation in different ways depending on the ligand and by inference the Fgf receptors activated.

FGFR2 signaling in normal and limbless chick limb buds

FGF10 and FGF8, which are reciprocally expressed by the mesoderm and AER of the developing limb bud, have been implicated in limb initiation, outgrowth, and patterning. FGF10 and FGF8 signal through the FGFR2b and FGFR2c alternative splice isoforms, respectively [Ornitz DM, et al. 1996. J Biol Chem 271:15292-15297; Igarashi M, et al. 1998. J Biol Chem 273:13230-13235]. A paracrine signaling loop model has been proposed whereby FGF10 expressed by limb mesoderm signals via ectodermally restricted FGFR2b to regulate FGF8 expression by the apical ectoderm; in turn, FGF8 signals via mesodermally restricted FGFR2c to maintain FGF10 expression [Ohuchi H, et al. 1997. Development 124:2235-2244; Xu X, et al. 1998. Development 125:753-765]. To explore this model, we have examined FGFR2b and FGFR2c mRNA expression, using isoform-specific probes during the early stages of development of the chick limb when limb initiation, AER induction, and outgrowth are occurring. We have found that FGFR2b is expressed by limb ectoderm, including the AER, consistent with paracrine signaling of FGF10. By contrast, FGFR2c is expressed by both mesoderm and ectoderm, indicating that FGF8 has the potential to function in an autocrine as well as paracrine fashion. Indeed, as the limb grows out in response to the AER, FGFR2c expression attenuates in the mesoderm of the progress zone, but is maintained in the AER itself, arguing against exclusive paracrine signaling of FGF8 during limb outgrowth. We also report that transcripts for FGF10, FGFR2b, and FGFR2c are expressed normally in the limb buds of limbless mutant embryos, which fail to form an AER and do not express FGF8. Furthermore, we detect no mutations in exons specific for the FGFR2c or FGFR2b isoforms in limbless embryos. Since gene targeting has shown that expression of FGF8 in limb ectoderm depends on FGF10 [Min H, et al. 1998. Genes Dev 12:3156-3161; Sekine K, et al. 1999. Nature Genet 21:138-141], these results indicate that the product of the limbless gene is required for FGF10 to induce expression of FGF8.

Growth factor-regulated expression of enzymes involved in nucleotide biosynthesis: a novel mechanism of growth factor action

Keratinocyte growth factor (KGF) is a potent and specific mitogen for epithelial cells, including the keratinocytes of the skin. We investigated the mechanisms of action of KGF by searching for genes which are regulated by this growth factor in cultured human keratinocytes. Using the differential display RT-PCR technology we identified the gene encoding adenylosuccinate lyase [EC 4.3.2.2] as a novel KGF-regulated gene. Adenylosuccinate lyase plays an important role in purine de novo synthesis. To gain further insight into the potential role of nucleotide biosynthesis in the mitogenic effect of KGF, we cloned cDNA fragments of the key regulatory enzymes involved in purine and pyrimidine metabolism (adenylosuccinate synthetase [EC 6.3.4.4], phosphoribosyl pyrophosphate synthetase [EC 2.7.6.1], amidophosphoribosyl transferase [EC 2.4.2.14], hypoxanthine guanine phosphoribosyl transferase [EC 2.4.2.8] and the multifunctional protein CAD which includes the enzymatic activities of carbamoyl-phosphate synthetase II [EC 6.3.5.59], aspartate transcarbamylase [EC 2.1.3.2] and dihydroorotase [EC 3.5.2.3]). Expression of all of these enzymes was upregulated after treatment with KGF and also with epidermal growth factor (EGF), indicating that these mitogens stimulate nucleotide production by induction of these enzymes. To determine a possible in vivo correlation between the expression of KGF, EGF and the enzymes mentioned above, we analysed the expression of the enzymes during cutaneous wound repair, where high levels of these mitogens are present. Indeed, we found a strong mRNA expression of all of these enzymes in the EGF- and KGF-responsive keratinocytes of the hyperproliferative epithelium at the wound edge, indicating that their expression might also be regulated by growth factors during wound healing.

Alveolar epithelial fluid transport can be simultaneously upregulated by both KGF and beta-agonist therapy

Although keratinocyte growth factor (KGF) protects against experimental acute lung injury, the mechanisms for the protective effect are incompletely understood. Therefore, the time-dependent effects of KGF on alveolar epithelial fluid transport were studied in rats 48-240 h after intratracheal administration of KGF (5 mg/kg). There was a marked proliferative response to KGF, measured both by in vivo bromodeoxyuridine staining and by staining with an antibody to a type II cell antigen. In controls, alveolar liquid clearance (ALC) was 23 +/- 3%/h. After KGF pretreatment, ALC was significantly increased to 30 +/- 2%/h at 48 h, to 39 +/- 2%/h at 72 h, and to 36 +/- 3%/h at 120 h compared with controls (P < 0.05). By 240 h, ALC had returned to near-control levels (26 +/- 2%/h). The increase in ALC was explained primarily by the proliferation of alveolar type II cells, since there was a good correlation between the number of alveolar type II cells and the increase in ALC (r = 0.92, P = 0.02). The fraction of ALC inhibited by amiloride was similar in control rats (33%) as in 72-h KGF-pretreated rats (38%), indicating that there was probably no major change in the apical pathways for Na uptake in the KGF-pretreated rats at this time point. However, more rapid ALC at 120 h, compared with 48 h after KGF treatment, may be explained by greater maturation of alpha-epithelial Na channel, since its expression was greater at 120 than at 48 h, whereas the number of type II cells was the same at these two time points. beta-Adrenergic stimulation with terbutaline 72 h after KGF pretreatment further increased ALC to 50 +/- 7%/h (P < 0.5). In summary, KGF induced a sustained increase over 120 h in the fluid transport capacity of the alveolar epithelium. This impressive upregulation in fluid transport was further enhanced with beta-adrenergic agonist therapy, thus providing evidence that two different treatments can simultaneously increase the fluid transport capacity of the alveolar epithelium.

KGF and FGF-10 stimulate liquid secretion in human fetal lung

During fetal life, the pulmonary epithelium secretes liquid that distends the airways and is important for normal lung growth and development. The factors regulating human fetal lung liquid secretion are poorly understood; however, recent studies in murine models show that keratinocyte growth factor (KGF, FGF-7) and fibroblast growth factor 10 (FGF-10) stimulate liquid secretion. We asked whether KGF and FGF-10 stimulate liquid secretion in human fetal lung. First trimester fetal lung explants developed dose-dependent increases in intraluminal volume in response to KGF and FGF-10. Although there were no acute changes in explant transepithelial potential difference in response to KGF (0.1-1000 ng/mL), exposure to 5-50 ng/mL KGF over 60 h depolarized transepithelial potential difference compared with controls. We used ribonuclease protection assays to quantitate the ontogeny and regulation of mRNA expression for KGF and its receptor. Both mRNA were expressed in fetal and postnatal lung. Because the promoter region of the human KGF gene contains cAMP and IL-6 response elements, we asked whether cAMP or IL-6 stimulated expression of KGF or its receptor. We have previously shown that cAMP stimulates liquid secretion in this model. Both cAMP and IL-6 significantly increased expression of KGF but not KGF receptor during a 48-h experiment. Thus, stimulation of liquid secretion in explant models by cAMP may be mediated in part by induction of KGF expression. KGF and FGF-10 may be important paracrine factors regulating liquid secretion in human fetal lung.

Ectopic gland induction by lens-specific expression of keratinocyte growth factor (FGF-7) in transgenic mice

During mammalian embryogenesis, epithelial-mesenchymal interactions play a determining role in normal tissue patterning and development. Keratinocyte growth factor (KGF), a member of the fibroblast growth factor (FGF) family, is a mesenchymally-derived mitogen for epithelial cells. As the KGF receptor is expressed by epithelial cells of numerous tissues and KGF is produced in adjacent stromal cells, KGF is thought to play a role in mediating epithelial cell behaviour. To further investigate the role of this molecule in the development of ocular epithelia we employed transgenic mice engineered to overexpress human KGF in the eye. The most striking phenotypic development was the hyperproliferation of embryonic corneal epithelial cells and their subsequent differentiation into functional lacrimal gland-like tissues. This indicates that stimulation of the KGF receptor early in development, in surface ectoderm normally destined to form corneal epithelium, is sufficient to alter the fate of these cells. Furthermore, this suggests that the correct spatial and temporal expression of FGFs plays a critical role in normal lacrimal gland induction. These transgenic mice provide a valuable model system to study the mechanisms underlying cell fate decisions during ocular morphogenesis.

Genomic organization and embryonic expression of the mouse fibroblast growth factor 9 gene

Fibroblast growth factor 9 (FGF9), originally cloned as glial-activating factor from human glioma cells, is expressed in adult rat brain and kidney. Here we report the chromosomal localization, genomic organization, and embryonic expression pattern of the mouse Fgf9 gene. Fgf9 maps to chromosome 14 near the Ctla6 locus. The gene spans more than 34 kb and contains three exons and two introns. Translation initiation occurs in exon 1, and translation termination occurs in exon 3. Fgf9 RNA was detected during mouse embryogenesis in several tissues in which Fgf gene expression has not been previously described, including intermediate mesoderm of late-stage gastrulation, ventricular myocardium, lung pleura, skeletal myoblasts in the early limb bud, spinal cord motor neurons, olfactory bulb, and gut lumenal epithelium. Fgf9 is coexpressed with other Fgf genes in some skeletal myoblasts, in limb apical ectoderm, in craniofacial ectoderm, and in the retina, inner ear, and tooth bud. Dev Dyn 1999;216:72-88.

Prostatic growth and development are regulated by FGF10

We have examined the role of Fibroblast Growth Factor 10 (FGF10) during the growth and development of the rat ventral prostate (VP) and seminal vesicle (SV). FGF10 transcripts were abundant at the earliest stages of organ formation and during neonatal organ growth, but were low or absent in growth-quiescent adult organs. In both the VP and SV, FGF10 transcripts were expressed only in a subset of mesenchymal cells and in a pattern consistent with a role as a paracrine epithelial regulator. In the neonatal VP, FGF10 mRNA was expressed initially in mesenchymal cells peripheral to the peri-urethral mesenchyme and distal to the elongating prostatic epithelial buds. At later stages, mesenchymal cells surrounding the epithelial buds also expressed FGF10 transcripts. During induction of the SV, FGF10 mRNA was present in mesenchyme surrounding the lower Wolffian ducts and, at later stages, FGF10 transcripts became restricted to mesenchymal cells subadjacent to the serosa. We investigated whether the FGF10 gene might be regulated by androgens by analysing the levels of FGF10 transcripts in SV and VP organs grown in serum-free organ culture. While FGF10 transcript levels increased after treatment with testosterone in the SV (but not VP), these changes were not sensitive to anti-androgen treatment, and thus it is likely that FGF10 mRNA was not directly regulated by testosterone. Also, FGF10 mRNA was observed in the embryonic female reproductive tract in a position analogous to that of the ventral prostate in males suggesting that FGF10 is not regulated by androgens in vivo. Recombinant FGF10 protein specifically stimulated growth of Dunning epithelial and BPH1 prostatic epithelial cell lines, but had no effect on growth of Dunning stromal cells or primary SV mesenchyme. Furthermore, FGF10 protein stimulated the development of ventral prostate and seminal vesicle organ rudiments in serum-free organ culture. When both FGF10 and testosterone were added to organs in vitro, there was no synergistic induction of development. Additionally, development induced by FGF10 was not inhibited by the addition of the anti-androgen Cyproterone Acetate demonstrating that the effects of FGF10 were not mediated by the androgen receptor. Taken together, our experiments suggest that FGF10 functions as a mesenchymal paracrine regulator of epithelial growth in the prostate and seminal vesicle and that the FGF10 gene is not regulated by androgens

Fibroblast growth factor interactions in the developing lung

Cellular activities that lead to organogenesis are mediated by epithelial-mesenchymal interactions, which ultimately result from local activation of complex gene networks. Fibroblast growth factor (FGF) signaling is an essential component of the regulatory network present in the embryonic lung, controlling proliferation, differentiation and pattern formation. However, little is known about how FGFs interact with other signaling molecules in these processes. By using cell and organ culture systems, we provide evidence that FGFs, Sonic hedgehog (Shh), bone morphogenetic protein 4 (BMP-4), and TGFbeta-1 form a regulatory circuit that is likely relevant for lung development in vivo. Our data show that FGF-10 and FGF-7, important for patterning and growth of the lung bud, are differentially regulated by FGF-1, -2 and Shh. In addition, we show that FGFs regulate expression of Shh, BMP-4 and other FGF family members. Our data support a model in which Shh, TGFbeta-1 and BMP-4 counteract the bud promoting effects of FGF-10, and where FGF levels are maintained throughout lung development by other FGFs and Shh.

Effects of keratinocyte growth factor-2 (KGF-2) on wound healing in an ischaemia-impaired rabbit ear model and on scar formation

Keratinocyte growth factor-2 (KGF-2), also described as fibroblast growth factor-10 (FGF-10), is a member of the fibroblast growth factor family. KGF-2 shares 57 per cent sequence homology to previously reported KGF-1 (FGF-7). In skin, both growth factors are expressed in the dermal compartment. KGF-1 and KGF-2 bind to the same receptor with high affinity, the KGFR isoform of FGFR2, which is exclusively expressed by epithelial cells. This study examines the in vivo function of topically applied KGF-2 on wound healing using an ischaemia-impaired rabbit dermal ulcer model, in young and aged animals. Histological analysis of the wounds showed that KGF-2 significantly promoted re-epithelialization in both young and old animals. Similar results have been observed with KGF-1 in this model. In addition, KGF-2 enhanced granulation tissue formation in both young and old rabbits, a biological effect not found with KGF-1, suggesting a possible indirect mechanism which enhances neo-granulation tissue formation. Immunohistological staining of day 7 wounds with proliferating cell nuclear antigen (PCNA) antibody demonstrated a significant increase of dermal cell proliferation in KGF-2-treated wounds compared with placebo wounds. These results suggest a mesenchymal-epithelial interaction that is mediated by a paracrine feedback loop of KGF-2. Because of the wound healing impairment observed with ageing, the wound healing response to KGF-2 was also studied in ischaemic wounds of aged animals. Administration of KGF-2 led to significant stimulation of epithelial growth and granulation tissue formation. The effects seen in the old animals were delayed compared with the young animals. Lastly, the effect of KGF-2 was examined in a rabbit model of scar formation. Quantification of scar elevation index showed no significant differences in scar formation when KGF-2 was compared with buffer placebo. Compared with other growth factors, including KGF-1 and TGF-beta which have previously been examined in these models, KGF-2 is the most effective and causes no obvious scarring.

FGF7 and FGF10 directly induce the apical ectodermal ridge in chick embryos

During vertebrate limb development, the apical ectodermal ridge (AER) plays a vital role in both limb initiation and distal outgrowth of the limb bud. In the early chick embryo the prelimb bud mesoderm induces the AER in the overlying ectoderm. However, the direct inducer of the AER remains unknown. Here we report that FGF7 and FGF10, members of the fibroblast growth factor family, are the best candidates for the direct inducer of the AER. FGF7 induces an ectopic AER in the flank ectoderm of the chick embryo in a different manner from FGF1, -2, and -4 and activates the expression of Fgf8, an AER marker gene, in a cultured flank ectoderm without the mesoderm. Remarkably, FGF7 and FGF10 applied in the back induced an ectopic AER in the dorsal median ectoderm. Our results suggest that FGF7 and FGF10 directly induce the AER in the ectoderm both of the flank and of the dorsal midline and that these two regions have the competence for AER induction. Formation of the AER of the dorsal median ectoderm in the chick embryo is likely to appear as a vestige of the dorsal fin of the ancestors.

Keratinocyte growth regulation in fibroblast cocultures via a double paracrine mechanism

Epithelial-mesenchymal interactions play an important role in regulating tissue homeostasis and repair. For skin, the regulatory mechanisms of epidermal-dermal interactions were studied in cocultures of normal human epidermal keratinocytes (NEK) and dermal fibroblasts (HDF) rendered postmitotic by alpha-irradiation (HDFi). The expression kinetics of different cytokines and their receptors with presumed signalling function in skin were determined at the RNA and protein level in mono- and cocultured NEK and HDFi. In cocultured HDFi, mRNA and protein synthesis of keratinocyte growth factor (KGF) (FGF-7) was strongly enhanced, whereas in cocultured keratinocytes interleukin (IL)-1alpha and -1beta mRNA expression increased compared to monocultures. Thus we postulated that IL-1, which had no effect on keratinocyte proliferation, induced in fibroblasts the expression of factors stimulating keratinocyte proliferation, such as KGF. The functional significance of this reciprocal modulation was substantiated by blocking experiments. Both IL-1alpha and -1beta-neutralizing antibodies and IL-1 receptor antagonist significantly reduced keratinocyte proliferation supposedly through abrogation of KGF production, because IL-1 antibodies blocked the induced KGF production. These data indicate a regulation of keratinocyte growth by a double paracrine mechanism through release of IL-1 which induces KGF in cocultured fibroblasts. Thus IL-1, in addition to its proinflammatory function in skin, may play an essential role in regulating tissue homeostasis.

Keratinocyte growth factor induces angiogenesis and protects endothelial barrier function

Keratinocyte growth factor (KGF), also called fibroblast growth factor-7, is widely known as a paracrine growth and differentiation factor that is produced by mesenchymal cells and has been thought to act specifically on epithelial cells. Here it is shown to affect a new cell type, the microvascular endothelial cell. At subnanomolar concentrations KGF induced in vivo neovascularization in the rat cornea. In vitro it was not effective against endothelial cells cultured from large vessels, but did act directly on those cultured from small vessels, inducing chemotaxis with an ED50 of 0.02-0.05 ng/ml, stimulating proliferation and activating mitogen activated protein kinase (MAPK). KGF also helped to maintain the barrier function of monolayers of capillary but not aortic endothelial cells, protecting against hydrogen peroxide and vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) induced increases in permeability with an ED50 of 0.2-0.5 ng/ml. These newfound abilities of KGF to induce angiogenesis and to stabilize endothelial barriers suggest that it functions in microvascular tissue as it does in epithelial tissues to protect them against mild insults and to speed their repair after major damage.

Signaling through fibroblast growth factor receptor 2b plays a key role in the development of the exocrine pancreas

The development of the pancreas depends on epithelial-mesenchymal interactions. Fibroblast growth factors (FGFs) and their receptors (FGFRs 1-4) have been identified as mediators of epithelial-mesenchymal interactions in different organs. We show here that FGFR-2 IIIb and its ligands FGF-1, FGF-7, and FGF-10 are expressed throughout pancreatic development. We also show that in mesenchyme-free cultures of embryonic pancreatic epithelium FGF-1, FGF-7, and FGF-10 stimulate the growth, morphogenesis, and cytodifferentiation of the exocrine cells of the pancreas. The role of FGFs signaling through FGFR-2 IIIb was further investigated by inhibiting FGFR-2 IIIb signaling in organocultures of pancreatic explants (epithelium + mesenchyme) by using either antisense FGFR-2 IIIb oligonucleotides or a soluble recombinant FGFR-2 IIIb protein. Abrogation of FGFR-2 IIIb signaling resulted in a considerable reduction in the size of the explants and in a 2-fold reduction of the development of the exocrine cells. These results demonstrate that FGFs signaling through FGFR-2 IIIb play an important role in the development of the exocrine pancreas.

In vivo characterization of keratinocyte growth factor-2 as a potential wound healing agent

Human keratinocyte growth factor-2 exerts a proliferative effect on epithelial cells and mediates keratinocyte migration. It has also been shown to increase both deposition of granulation tissue and collagen and maturation of collagen. Because these properties should affect the healing trajectory of wounds, this study set out to investigate the effects of keratinocyte growth factor-2 on the healing of three different types of wounds. Human meshed skin grafts explanted to athymic "nude" rats, surgical incisions in Sprague-Dawley rats, and acute excisional rat wounds inoculated with Escherichia coli were used. Two concentrations of recombinant human keratinocyte growth factor-2 were compared to a vehicle control and keratinocyte growth factor-1. Keratinocyte growth factor-2 significantly accelerated the rate of epithelialization in the meshed skin graft model and effected a modestly more rapid gain in breaking strength of surgical incisions than keratinocyte growth factor-1 or the vehicle control treatment. Neither keratinocyte growth factors accelerated wound closure by contraction of the excisional wounds. Based on these data, keratinocyte growth factor-2 may be useful in accelerating healing in wounds healing mainly by the process of epithelialization such as venous stasis ulcers, partial thickness burn wounds, and skin graft donor sites. It might also accelerate the gain in incisional wound strength in acute surgical or traumatic wounds.

Fibroblast growth factor-10. A second candidate stromal to epithelial cell andromedin in prostate

Fibroblast growth factor (FGF)-10, a homologue of FGF-7, is expressed significantly in normal rat prostate tissue, well differentiated rat prostate tumors with an epithelial and stromal compartment and only in derived prostate stromal cells in culture. Similar to FGF-7, recombinant rat FGF-10 was a specific mitogen for prostate epithelial cells. In contrast to FGF-7 which is widely expressed among stromal cells in tissues, the expression of FGF-10 correlated with the presence of stromal cells of muscle origin. Radioreceptor binding assays and covalent cross-linking analysis revealed that FGF-10 binds with an affinity equal to FGF-7 to resident epithelial cell receptor, FGFR2IIIb, but unlike FGF-7 also binds the IIIb splice variant of FGFR1. Analysis of mRNA expression by RNase protection revealed that, similar to FGF-7, the expression of FGF-10 was responsive to androgen in stromal cells from normal prostate and non-malignant differentiated tumors. Although FGF-10 cDNA exhibits a signal sequence for secretion, cultured stromal cells exhibit strictly a cell-associated FGF-10 antigen that correlates with an alternately translated intracellular isoform. FGF-10 requires 1.4 times higher NaCl for elution from immobilized heparin than does FGF-7 and binds to four times the number of sites on the pericellular matrix of epithelial cells. The results show that prostate stromal cell-derived FGF-10, like FGF-7, exhibits the properties of an andromedin which may indirectly mediate control of epithelial cell growth and function by androgen. Although FGF-10 and FGF-7 bind and activate the same resident epithelial cell receptor (FGFR2IIIb), differences in cell type of origin, compartmentation by alternate translation, the affinity for FGFR1IIIb, and access to FGFR by differential interaction with pericellular matrix heparan sulfate suggest they may play both independent and compensatory roles in prostate homeostasis.

FGF-10 is a growth factor for preadipocytes in white adipose tissue

FGF-10 is a mesenchymal factor affecting epithelial cells during pattern formation. However, the expression and physiological role of FGF-10 in adults remains to be elucidated. We examined the expression of FGF-10 mRNA in a variety of adult rat tissues, and found to be most abundant in white adipose tissue. In white adipose tissue, FGF-10 mRNA was expressed in preadipocytes but not in mature adipocytes. The expression in white adipose tissue during postnatal development was also examined. The expression level was low at postnatal day 10 (P10). However, FGF-10 mRNA was abundantly detected later on (P28 and P48) when white adipose tissue growth was stimulated. We also examined the activity of recombinant FGF-10 for primary rat preadipocytes. FGF-10 showed significant mitogenic activity for primary preadipocytes, but did not affect the differentiation of preadipocytes. The expression profile of FGF-10 mRNA and the activity of FGF-10 reported here indicate that FGF-10, a unique secreted factor produced in white adipose tissue, acts as a growth factor for preadipocytes in white adipose tissues.

Induction of alveolar type II cell differentiation in embryonic tracheal epithelium in mesenchyme-free culture

We have previously shown that fetal lung mesenchyme can reprogram embryonic rat tracheal epithelium to express a distal lung phenotype. We have also demonstrated that embryonic rat lung epithelium can be induced to proliferate and differentiate in the absence of lung mesenchyme. In the present study we used a complex growth medium to induce proliferation and distal lung epithelial differentiation in embryonic tracheal epithelium. Day-13 embryonic rat tracheal epithelium was separated from its mesenchyme, enrobed in growth factor-reduced Matrigel, and cultured for up to 7 days in medium containing charcoal-stripped serum, insulin, epidermal growth factor, hepatocyte growth factor, cholera toxin, fibroblast growth factor 1 (FGF1), and keratinocyte growth factor (FGF7). The tracheal epithelial cells proliferated extensively in this medium, forming lobulated structures within the extracellular matrix. Many of the cells differentiated to express a type II epithelial cell phenotype, as evidenced by expression of SP-C and osmiophilic lamellar bodies. Deletion studies showed that serum, insulin, cholera toxin, and FGF7 were necessary for maximum growth. While no single deletion abrogated expression of SP-C, deleting both FGF7 and FGF1 inhibited growth and prevented SP-C expression. FGF7 or FGF1 as single additions to the medium, however, were unable to induce SP-C expression, which required the additional presence of serum or cholera toxin. FGF10, which binds the same receptor as FGF7, did not support transdifferentiation when used in place of FGF7. These data indicate that FGF7 is necessary, but not sufficient by itself, to induce the distal rat lung epithelial phenotype, and that FGF7 and FGF10 play distinct roles in lung development.

FGF-7 modulates ureteric bud growth and nephron number in the developing kidney

The importance of proportioning kidney size to body volume was established by clinical studies which demonstrated that in-born defecits of nephron number predispose the kidney to disease. As the kidney develops, the expanding ureteric bud or renal collecting system induces surrounding metanephric mesenchyme to proliferate and differentiate into nephrons. Thus, it is likely that nephron number is related to ureteric bud growth. The expression patterns of mRNAs encoding Fibroblast Growth Factor-7 (FGF-7) and its high affinity receptor suggested that FGF-7 signaling may play a role in regulating ureteric bud growth. To test this hypothesis we examined kidneys from FGF-7-null and wild-type mice. Results of these studies demonstrate that the developing ureteric bud and mature collecting system of FGF-7-null kidneys is markedly smaller than wild type. Furthermore, morphometric analyses indicate that mature FGF-7-null kidneys have 30+/−6% fewer nephrons than wild-type kidneys. In vitro experiments demonstrate that elevated levels of FGF-7 augment ureteric bud growth and increase the number of nephrons that form in rodent metanephric kidney organ cultures. Collectively, these results demonstrate that FGF-7 levels modulate the extent of ureteric bud growth during development and the number of nephrons that eventually form in the kidney.

FGFs and BMP4 induce both Msx1-independent and Msx1-dependent signaling pathways in early tooth development

During early tooth development, multiple signaling molecules are expressed in the dental lamina epithelium and induce the dental mesenchyme. One signal, BMP4, has been shown to induce morphologic changes in dental mesenchyme and mesenchymal gene expression via Msx1, but BMP4 cannot substitute for all the inductive functions of the dental epithelium. To investigate the role of FGFs during early tooth development, we examined the expression of epithelial and mesenchymal Fgfs in wild-type and Msx1 mutant tooth germs and tested the ability of FGFs to induce Fgf3 and Bmp4 expression in wild-type and Msx1 mutant dental mesenchymal explants. Fgf8 expression is preserved in Msx1 mutant epithelium while that of Fgf3 is not detected in Msx1 mutant dental mesenchyme. Moreover, dental epithelium as well as beads soaked in FGF1, FGF2 or FGF8 induce Fgf3 expression in dental mesenchyme in an Msx1-dependent manner. These results indicate that, like BMP4, FGF8 constitutes an epithelial inductive signal capable of inducing the expression of downstream signaling molecules in dental mesenchyme via Msx1. However, the BMP4 and FGF8 signaling pathways are distinct. BMP4 cannot induce Fgf3 nor can FGFs induce Bmp4 expression in dental mesenchyme, even though both signaling molecules can induce Msx1 and Msx1 is necessary for Fgf3 and Bmp4 expression in dental mesenchyme. In addition, we have investigated the effects of FGFs and BMP4 on the distal-less homeobox genes Dlx1 and Dlx2 and we have clarified the relationship between Msx and Dlx gene function in the developing tooth. Dlx1,Dlx2 double mutants exhibit a lamina stage arrest in maxillary molar tooth development (Thomas B. L., Tucker A. S., Qiu M., Ferguson C. A., Hardcastle Z., Rubenstein J. L. R. and Sharpe P. T. (1997) Development 124, 4811–4818). Although the maintenance of molar mesenchymal Dlx2 expression at the bud stage is Msx1-dependent, both the maintenance of Dlx1 expression and the initial activation of mesenchymal Dlx1 and Dlx2 expression during the lamina stage are not. Moreover, in contrast to the tooth bud stage arrest observed in Msx1 mutants, Msx1,Msx2 double mutants exhibit an earlier phenotype closely resembling the lamina stage arrest observed in Dlx1,Dlx2 double mutants. These results are consistent with functional redundancy between Msx1 and Msx2 in dental mesenchyme and support a model whereby Msx and Dlx genes function in parallel within the dental mesenchyme during tooth initiation. Indeed, as predicted by such a model, BMP4 and FGF8, epithelial signals that induce differential Msx1 and Msx2 expression in dental mesenchyme, also differentially induce Dlx1 and Dlx2 expression, and do so in an Msx1-independent manner. These results integrate Dlx1, Dlx2 and Fgf3 and Fgf8 into the odontogenic regulatory hierarchy along with Msx1, Msx2 and Bmp4, and provide a basis for interpreting tooth induction in terms of transcription factors which, individually, are necessary but not sufficient for the expression of downstream signals and therefore must act in specific combinations.

Fgf-10 is required for both limb and lung development and exhibits striking functional similarity to Drosophila branchless

Fgf-10-deficient mice (Fgf-10(-/-)) were generated to determine the role(s) of Fgf-10 in vertebrate development. Limb bud initiation was abolished in Fgf-10(-/-) mice. Strikingly, Fgf-10(-/-) fetuses continued to develop until birth, despite the complete absence of both fore- and hindlimbs. Fgf-10 is necessary for apical ectodermal ridge (AER) formation and acts epistatically upstream of Fgf-8, the earliest known AER marker in mice. Fgf-10(-/-) mice exhibited perinatal lethality associated with complete absence of lungs. Although tracheal development was normal, main-stem bronchial formation, as well as all subsequent pulmonary branching morphogenesis, was completely disrupted. The pulmonary phenotype of Fgf-10(-/-) mice is strikingly similar to that of the Drosophila mutant branchless, an Fgf homolog.