FGF-4 and BMP-2 have opposite effects on limb growth

Effect of fibroblast growth factors on outgrowth of facial mesenchyme

The ectoderm is required for outgrowth of facial prominences and 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 stimulated an increase in length of cartilage rods in mandibular grafts compared to mandibular mesenchyme grafted without ectoderm (P < 0.05). FGF-4 stimulated a small increase in length of frontonasal mass mesenchyme (P < 0.05) and both FGFs increased the frequency of egg tooth formation in frontonasal mass mesenchyme 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 were significantly longer than mesenchyme grafts treated with FGF-soaked beads (P < 0.05). The addition of a second FGF-soaked bead did not significantly increase the length of the frontonasal mass or the mandibular mesenchyme. We have determined that FGF-2 protein is expressed in facial ectoderm and could 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. Our results indicate that FGFs are part of an endogenous signaling pathway involved in distal outgrowth and chondrogenesis of the facial prominences.

Bone morphogenetic proteins: neurotrophic roles for midbrain dopaminergic neurons and implications of astroglial cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily that have been implicated in tissue growth and remodelling. Recent evidence suggests that several BMPs are expressed in the developing and adult brain. Specifically, we show that BMP 2 and BMP 6 are expressed in the developing midbrain floor of the rat. We studied potential neurotrophic effects of BMPs on the in vitro survival, transmitter uptake and protection against MPP+ toxicity of mesencephalic dopaminergic neurons cultured from the embryonic midbrain floor at embryonic day (E) 14. At 10 ng/ml and under serum-free conditions, most BMPs promoted the survival of dopaminergic neurons visualized by tyrosine hydroxylase immunocytochemistry during an 8-day culture period, but to varying extents (relative potencies: BMP 6 = 12 > 2, 4, 7). BMPs 6 and 12 were as effective as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor, promoting survival 1.7-fold compared with controls. BMPs 9 and 11 were not effective. Dose-response curves revealed an EC50 for BMPs 2, 6 and 12 of 2 ng/ml. BMPs 2, 4, 6, 7, 9 and 12 also promoted DNA synthesis and astroglial cell differentiation, visualized by 5-bromodeoxyuridine (BrdU) incorporation and glial fibrillary acidic protein (GFAP) immunocytochemistry respectively. Suppression of cell proliferation and subsequent maturation of GFAP-positive cells by 5-fluorodeoxyuridine or aminoadipic acid abolished the neuron survival-promoting effect of BMP 2. This suggests that BMPs, like other non-TGF-beta factors affecting dopaminergic neuron survival, act indirectly, probably by stimulating the synthesis and/or release of glial-derived trophic factors. BMP 6 and BMP 7 also increased the uptake of [3H]dopamine without affecting the uptake of [3H]5-hydroxytryptamine and [3H]GABA, underscoring the specificity of the trophic effect. We conclude that several BMPs share a neurotrophic capacity for dopaminergic midbrain neurons with other members of the TGF-beta superfamily, but act indirectly, possibly through glial cells.

Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation

Vertebrate organogenesis is initiated at sites that are often morphologically indistinguishable from the surrounding region. Here we have identified Pax9 as a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. We provide evidence that 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. We suggest that a similar mechanism may be used for specifying the sites of development of other organs.

Opposing effects of activin A and follistatin on developing skeletal muscle cells

Activin and the activin-binding protein follistatin modulate a variety of biological processes and are abundant at sites of muscle development. Activin and follistatin were expressed in developing chick pectoral muscle in vivo and in primary cell culture. Addition of recombinant activin inhibited muscle development in a dose-dependent manner as measured by the number of nuclei in myosin heavy chain positive cells and creatine phosphokinase activity. Conversely, follistatin potentiated muscle development. The effects of activin were found to be distinct from those of the related protein transforming growth factor (TGF) beta1. Muscle development was repressed by activin at all time points investigated and did not recover with the removal of activin following a limited exposure. In contrast, while myogenic differentiation in TGFbeta1 was initially repressed, muscle marker expression recovered to control levels--even in the continued presence of TGFbeta1. Fibroblast growth factor (FGF) had little effect on inhibiton of muscle development caused by activin A. However, inhibition of development produced by TGFbeta increased with increasing concentrations of FGF. Finally, early expression of myoD and myf5 mRNA by muscle cultures in the presence of activin and follistatin was analyzed. Activin-treated cultures expressed reduced myoD and myf5 levels at 1.5 days after plating. Myf5 levels in follistatin-treated cultures were elevated, but, surprisingly, these cultures showed a reduction in myoD levels. These data suggest that endogenously expressed activin and follistatin are important modulators of muscle development.

Fibroblast growth factors 2 and 4 stimulate migration of mouse embryonic limb myogenic cells

Fibroblast growth factors (FGFs) are believed to be vital for limb outgrowth and patterning during embryonic development. Although the effect of FGFs on the formation of the skeletal elements has been studied in detail, their effect on the development of the limb musculature is still uncertain. In this study, we used Blindwell chemotactic chambers to examine the effect of FGF-2 and FGF-4 on the motility of myogenic cells obtained from the proximal region of the day 11.5 mouse forelimbs. The limb myogenic cells were found to be chemotactically attracted to FGF-2 and FGF-4 at 10-50 ng/ml. Both FGFs increased myogenic cell migration in a dose-dependent manner, with maximal responses attained at 1-50 ng/ml for FGF-2 and at 10 ng/ml for FGF-4; however, FGF-2 was found to be a more potent chemoattractant than FGF-4. It was possible to inhibit the myogenic cells' response to FGF-2 and FGF-4 by the addition of the appropriate neutralizing antibody. The effects of FGF-2 on cell migration were further investigated by loading this cytokine into Affi-Gel blue beads and transplanting them into day 11.5 forelimb buds. The results showed that FGF-2 attracted DiI-labelled proximal cells to migrate toward the implanted beads and that the migration was more extensive than that observed in the absence of FGF-2. A checkerboard assay was performed in which various concentrations of FGF-2 and FGF-4 were introduced to both the upper and lower wells of the Blindwell chambers. The results indicated that both FGF isoforms can stimulate chemokinesis as well as chemotaxis in myogenic cells. In addition, the effect of FGF-2 at 0.1-10 ng/ml stimulated a significant increase in the number of myocytes expressing sarcomeric myosin on examination after 48 hr in culture, but the effect of FGF-4 was negligible at all concentrations analyzed; however, both FGF-2 and FGF-4 inhibited myocyte fusion compared with the spontaneous fusion observed in control cultures. Finally, we used in situ hybridization and immunohistochemical techniques to determine the distribution of myogenic cells and FGF-2 protein in the day 11.5 mouse forelimbs.

Backfoot is a novel homeobox gene expressed in the mesenchyme of developing hind limb

Homeobox genes play important roles in pattern formation during development. Here, we report the cloning and temporal and spatial expression patterns of a novel homeobox gene Backfoot (BFT for the human gene, and Bft for the mouse gene), whose expression reveals an early molecular distinction between forelimb and hind limb. BFT was identified as a sequence-specific DNA-binding protein. In addition to the homeodomain, it shares a carboxyl-terminal peptide motif with other paired-like homeodomain proteins. Northern hybridization analysis of RNAs from human tissues revealed that human BFT is highly expressed in adult skeletal muscle and bladder. During midgestation embryogenesis, mouse Bft is expressed in the developing hind limb buds, mandibular arches, and Rathke's pouch. The expression of Bft begins prior to the appearance of hind limb buds in mesenchyme but is never observed in forelimbs. At later stages of limb development, the expression is progressively restricted to perichondrial regions, most likely in tendons and ligaments. The timing and pattern of expression suggest that Bft plays multiple roles in hind limb patterning, branchial arch development, and pituitary development. Bft is likely identical to a mouse gene, Ptx1, that was recently isolated by Lamonerie et al. ([1996] Genes Dev. 10:1284-1295) and that has been suggested to play a role in pituitary development.

Inhibition of chondrogenesis by Wnt gene expression in vivo and in vitro

The Wnt family of secreted signaling proteins are implicated in regulating morphogenesis and tissue patterning in a wide variety of organ systems. Several Wnt genes are expressed in the developing limbs and head, implying roles in skeletal development. To explore these functions, we have used retroviral gene transfer to express Wnt-1 ectopically in the limb buds and craniofacial region of chick embryos. Infection of wing buds at stage 17 and tissues in the head at stage 10 resulted in skeletal abnormalities whose most consistent defects suggested a localized failure of cartilage formation. To test this hypothesis, we infected micromass cultures of prechondrogenic mesenchyme in vitro and found that expression of Wnt-1 caused a severe block in chondrogenesis. Wnt-7a, a gene endogenously expressed in the limb and facial ectoderm, had a similar inhibitory effect. Further analysis of this phenomenon in vitro showed that Wnt-1 and Wnt-7a had mitogenic effects only in early prechondrogenic mesenchyme, that cell aggregation and formation of the prechondrogenic blastema occurred normally, and that the block to differentiation was at the late-blastema/early-chondroblast stage. These results indicate that Wnt signals can have specific inhibitory effects on cytodifferentiation and suggest that one function of endogenous Wnt proteins in the limbs and face may be to influence skeletal morphology by localized inhibition of chondrogenesis.

FGF-stimulated outgrowth and proliferation of limb mesoderm is dependent on syndecan-3

The outgrowth of the mesoderm of the developing limb bud in response to the apical ectodermal ridge (AER) is mediated at least in part by members of the FGF family. Recent studies have indicated that FGFs need to interact with heparan sulfate proteoglycans in order to bind to and activate their specific cell surface receptors. Syndecan-3 is an integral membrane heparan sulfate proteoglycan that is highly expressed by the distal mesodermal cells of the chick limb bud that are undergoing proliferation and outgrowth in response to the AER. Here we report that maintenance of high-level syndecan-3 expression by the subridge mesoderm of the chick limb bud is directly or indirectly dependent on the AER, since its expression is severely impaired in the distal mesoderm of the limb buds of limbless and wingless mutant embryos which lack functional AERs capable of directing the outgrowth of limb mesoderm. We have also found that exogenous FGF-2 maintains a domain of high-level syndecan-3 expression in the outgrowing mesodermal cells of explants of the posterior mesoderm of normal limb buds cultured in the absence of the AER and in the outgrowing subapical mesoderm of explants of limbless mutant limb buds which lack a functional AER. These results suggest that the domain of high-level syndecan-3 expression in the subridge mesoderm of normal limb buds is maintained by FGFs produced by the AER. Finally, we report that polyclonal antibodies against a syndecan-3 fusion protein inhibit the ability of FGF-2 to promote the proliferation and outgrowth of the posterior subridge mesoderm of limb buds cultured in the absence of the AER. These results suggest that syndecan-3 plays an essential role in limb outgrowth by mediating the interaction of FGFs produced by the AER with the underlying mesoderm of the limb bud.

Requirement of fibroblast growth factor signaling for regeneration of epiphyseal morphology in rabbit full-thickness defects of articular cartilage

The involvement of fibroblast growth factor-2 (FGF-2) during the repair process in rabbit full-thickness defects of articular cartilage was studied. Fibroblast growth factor-2 (50 pg/h) was administered for 2 weeks in a 5 mm defect of articular cartilage, which is large enough not to repair spontaneously. The administration of FGF-2 resulted in the regeneration of the articular cartilage and the subchondral bone within 8 weeks. In these defects, undifferentiated mesenchymal cells initiated chondrogenic differentiation coupled with replacement by subchondral bone, resulting in the resurfacing of the defects with hyaline cartilage and the recovery of subchondral bone up to the original bone-articular cartilage junction. In rabbits, full-thickness defects are capable of regenerating articular cartilage as long as the defect size is limited to < or = 3 mm in diameter. In the defects, strong immunoreactivity for FGF-2 was observed in the granulation tissue filling the defects in the early stage of repair, in association with the expression of FGF-2 mRNA shown by in situ hybridization. Once the undifferentiated mesenchymal cells had differentiated into chondrocytes, both the immunoreactivity and the in situ hybridization signal declined significantly. Upon the local administration of a monoclonal antibody against FGF-2 (bFM-1, 50 ng/h), the defects were filled with fibrous tissue and no resurfacing hyaline cartilage was formed. Compared to the non-treated defects, there were marked increases in FGF-2 immunoreactivity and the overexpression of FGF-2 mRNA in the reparative tissue in the bFM-1-treated defects. This rebound phenomenon indicates that the autocrine FGF-2 signaling is critically important for the regeneration of articular cartilage.

Role of BMP-2 and OP-1 (BMP-7) in programmed cell death and skeletogenesis during chick limb development

Bone Morphogenetic Protein 2 (BMP-2) and Osteogenic Protein 1 (OP-1, also termed BMP-7) are members of the transforming growth factor beta superfamily. In the present study, we have analyzed the effects of administering them locally at different stages and locations of the chick limb bud using heparin beads as carriers. Our results show that these BMPs are potent apoptotic signals for the undifferentiated limb mesoderm but not for the ectoderm or the differentiating chondrogenic cells. In addition, they promote intense radial growth of the differentiating cartilages and disturb the formation of joints accompanied by alterations in the pattern of Indian hedgehog and ck-erg expression. Interestingly, the effects of these two BMPs on joint formation were found to be different. While the predominant effect of BMP-2 is alteration in joint shape, OP-1 is a potent inhibitory factor for joint formation. In situ hybridizations to check whether this finding was indicative of specific roles for these BMPs in the formation of joints revealed a distinct and complementary pattern of expression of these genes during the formation of the skeleton of the digits. While Op-1 exhibited an intense expression in the perichondrium of the developing cartilages with characteristic interruptions in the zones of joint formation, Bmp-2 expression was a positive marker for the articular interspaces. These data suggest that, in addition to the proposed role for BMP-2 and OP-1 in the establishment of the anteroposterior axis of the limb, they may also play direct roles in limb morphogenesis: (i) in regulating the amount and spatial distribution of the undifferentiated prechondrogenic mesenchyme and (ii) in controlling the location of the joints and the diaphyses of the cartilaginous primordia of the long bones once the chondrogenic aggregates are established.

Activation of protein kinase A is a pivotal step involved in both BMP-2- and cyclic AMP-induced chondrogenesis

We studied the roles of protein kinase A (PKA) activation and cyclic AMP response element binding protein (CREB) phosphorylation in chondrogenesis using serum-free chicken limb bud micromass cultures as a model system. We showed the following points: 1) in micromass cultures, activation of PKA enhances chondrogenesis and increases the phosphorylation of CREB; 2) BMP-2, a chondrogenic stimulator, increases PKA activity and the level of phosphorylated CREB (P-CREB); 3) H8, a PKA inhibitor, inhibits chondrogenesis; 4) the chondrogenic activities of BMP-2 and cAMP are suppressed by H8; and 5) long-term TPA treatment (a protein kinase C (PKC) modulator) inhibits chondrogenesis and decreases the levels of CREB and P-CREB. These results suggest that activation of PKA is a physiological event during chondrogenesis that is involved in the chondrogenic effects of both BMP-2 and cyclic AMP (cAMP)-dependent pathways.

Ectopic application of recombinant BMP-2 and BMP-4 can change patterning of developing chick facial primordia

The facial primordia initially consist of buds of undifferentiated mesenchyme, which give rise to a variety of tissues including cartilage, muscle and nerve. These must be arranged in a precise spatial order for correct function. The signals that control facial outgrowth and patterning are largely unknown. The bone morphogenetic proteins Bmp-2 and Bmp-4 are expressed in discrete regions at the distal tips of the early facial primordia suggesting possible roles for BMP-2 and BMP-4 during chick facial development. We show that expression of Bmp-4 and Bmp-2 is correlated with the expression of Msx-1 and Msx-2 and that ectopic application of BMP-2 and BMP-4 can activate Msx-1 and Msx-2 gene expression in the developing facial primordia. We correlate this activation of gene expression with changes in skeletal development. For example, activation of Msx-1 gene expression across the distal tip of the mandibular primordium is associated with an extension of Fgf-4 expression in the epithelium and bifurcation of Meckel's cartilage. In the maxillary primordium, extension of the normal domain of Msx-1 gene expression is correlated with extended epithelial expression of shh and bifurcation of the palatine bone. We also show that application of BMP-2 can increase cell proliferation of the mandibular primordia. Our data suggest that BMP-2 and BMP-4 are part of a signalling cascade that controls outgrowth and patterning of the facial primordia.

Limb deformity proteins: role in mesodermal induction of the apical ectodermal ridge

During early limb development, distal tip ectoderm is induced by the underlying mesenchyme to form the apical ectodermal ridge. Subsequent limb growth and patterning depend on reciprocal signaling between the mesenchyme and ridge. Mice that are homozygous for mutations at the limb deformity (ld) locus do not form a proper ridge and the anteroposterior axis of the limb is shortened. Skeletal analyses reveal shortened limbs that involve loss and fusion of distal bones and digits, defects in both anteroposterior and proximodistal patterning. Using molecular markers and mouse-chick chimeras we examined the ridge-mesenchymal interactions to determine the origin of the ld patterning defects. In the ld ridge, fibroblast growth factor 8 (Fgf8) RNA is decreased and Fgf4 RNA is not detected. In the ld mesenchyme, Sonic hedgehog (Shh), Evx1 and Wnt5a expression is decreased. In chimeras between ld ectoderm and wild-type mesenchyme, a ridge of normal morphology and function is restored, Fgf8 and Shh are expressed normally, Fgf4 is induced and a normal skeletal pattern arises. These results suggest that the ld mesenchyme is unable to induce the formation of a completely functional ridge. This primary defect causes a disruption of ridge function and subsequently leads to the patterning defects observed in ld limbs. We propose a model in which ridge induction requires at least two phases: an early competence phase, which includes induction of Fgf8 expression, and a later differentiation phase in which Fgf4 is induced and a morphological ridge is formed. Ld proteins appear to act during the differentiation phase.

Functions of fibroblast growth factors in vertebrate development

Fibroblast growth factors (FGFs) are a class of secreted polypeptide ligands which mediate diverse cellular responses during embryonic, fetal, and postnatal vertebrate development. The purposes of this review are to provide a condensed overview of FGFs and their receptors, to catalog and categorize the functions of FGFs in vertebrate development, to present recent discoveries relating to the interplay of FGFs with other secreted ligands in the control of tissue growth and patterning, and to discuss several potential directions for future research in the field.

BMP-2/-4 mediate programmed cell death in chicken limb buds

During limb development, the mesenchymal cells in restricted areas of limb bud, anterior necrotic zone, posterior necrotic zone, opaque zone and interdigital necrotic zones, are eliminated by programmed cell death. The transcripts of bone morphogenetic protein (Bmp)-2 and −4 were first detected in the areas where cell death was observed, then showed overlapping expression with the programmed cell death zones except the opaque zone. To investigate the function of BMP-2 and BMP-4 during limb pattern formation, the dominant negative form of BMP receptor was overexpressed in chick leg bud via a replication-competent retrovirus to block the endogenous BMP-2/-4 signaling pathway. This resulted in excess web formation at the anterior and posterior regions of limb buds in addition to marked suppression of the regression of webbing at the interdigital regions. Significant reductions in the number of apoptotic cells in these three necrotic zones were found in the limb buds which received the virus carrying dominant negative BMP receptor. This indicates that extra tissue formation is due to suppression of programmed cell death in the three necrotic zones. Moreover, BMP-2/-4 protein induced apoptosis of mesenchymal cells isolated from the interdigital region in vitro. Other TGFbeta family proteins as TGFbeta1 and Activin did not show this effect. These results suggest that BMP-2 and BMP-4 are the apoptotic signal molecules of the programmed cell death process in the chick limb buds.

Transcripts for two members of the transforming growth factor-beta superfamily BMP-3 and BMP-7 are expressed in developing rat embryos

Bone morphogenetic protein-3 (BMP-3) and BMP-7 are members of the transforming growth factor beta superfamily that have been implicated in the formation of cartilage and bone. Using in situ hybridization, we localized mRNAs for BMP-3 and BMP-7 during organogenesis in rats. Both mRNAs were expressed in a variety of cells, in particular, in the developing hair follicle, tooth, kidney, and lung tissues, in which reciprocal epithelial-mesenchymal interactions are essential. In some tissues, the distribution of BMP-3 and BMP-7 mRNAs overlapped. In other tissues, the patterns of expression were quite different. Moreover, the site of expression of the transcripts changed from one cell type to another during organogenesis. These results suggest that BMP-3 and BMP-7 play important roles in organogenesis and that the differential patterns of their expression might reflect their distinct roles in embryogenesis.

Mice deficient for BMP2 are nonviable and have defects in amnion/chorion and cardiac development

To address the function of bone morphogenetic protein-2 (BMP2) in mammalian development, mice with a targeted deletion of the Bmp2 mature region were generated using embryonic stem cell technology. This mutation caused embryonic lethality when homozygous. Mutant embryos failed to close the proamniotic canal, which caused the malformation of the amnion/chorion. BMP2-deficient embryos also exhibited a defect in cardiac development, manifested by the abnormal development of the heart in the exocoelomic cavity. These defects are consistent with the expression of Bmp2 in the extraembryonic mesoderm cells and promyocardium. Thus BMP2 is a critical factor for both extraembryonic and embryonic development.

Ability of FGFs to promote the outgrowth and proliferation of limb mesoderm is dependent on IGF-I activity

IGF-I and members of the FGF family have been implicated in the reciprocal interactions between the apical ectodermal ridge (AER) and underlying subridge mesoderm that are required for outgrowth of the developing limb bud. Several FGFs are expressed by the AER and appear to mediate its outgrowth and patterning effects on the subridge mesoderm. IGF-I is expressed by the subridge mesoderm that is growing out in response to the AER, and exogenous IGF-I can promote the outgrowth of the subridge mesoderm in the absence of the AER. Maintenance of IGF-I expression by the subridge mesoderm is dependent on the AER. Here we report that exogenous FGF-2 and FGF-4 can substitute for the AER in maintaining IGF-I expression by the posterior subridge mesoderm of the chick limb bud. Furthermore, we have found that neutralizing antibodies against IGF-I inhibit the ability of FGFs to promote the outgrowth and proliferation of the posterior subridge mesoderm cultured in the absence of the AER. These results indicate that FGFs secreted by the AER maintain IGF-I expression by the subridge mesoderm, and that the ability of FGFs to promote the outgrowth and proliferation of limb mesoderm is dependent on IGF-I activity. Interestingly, however, the ability of FGFs to maintain the expression of the homeobox containing gene Msx-1 by the subridge mesoderm is not dependent on IGF-I activity.

Bone morphogenetic proteins: multifunctional regulators of vertebrate development

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Bone morphogenetic proteins and C-FOS: early signals in endochondral bone formation

The earliest events in endochondral ossification, whether in the formation of the skeleton during embryogenesis or in the regeneration of a bone during fracture healing, involve the establishment of a tissue-specific plan of gene expression arising from pluripotent mesenchymal cells. Recent data from studies of vertebrate limb bud development implicate gradients of diffusable morphogens that specify complex patterns of position-specific gene expression. Diffusable signaling molecules involved at the earliest stages of tissue development include fibroblast growth factors, platelet derived growth factors, bone morphogenetic proteins, transforming growth factor beta, and sonic hedgehog; there are undoubtedly others. An equally elaborate array of trans-membrane receptors in the tyrosine kinase and serine-threonine kinase signaling pathways are responsible for transducing these morphogenetic signals and for establishing boundaries of gene expression. Nuclear signals for transcriptional activation, such as the fos-jun proto-oncogene family and homeobox genes, are also involved in the specification and modification of morphogenetic plans. While much has been learned recently about the presence and interaction of these growth factors and transcription factors in the development of the vertebrate limb bud, very little is known about their presence and interaction in the earliest stages of post-natal bone formation at a fracture site or in heterotopic osteogenesis. Fibrodysplasia ossificans progressiva (FOP) a rare genetic disorder of heterotopic ossification, provides a unique opportunity to study the role of these morphogens and proto-oncogenes in the earliest events of endochondral bone formation.

Cloning of rat type I receptor cDNA for bone morphogenetic protein-2 and bone morphogenetic protein-4, and the localization compared with that of the ligands

A rat homologue cDNA of mouse (Koenig et al. [1994] Mol. Cell Biol. 14:5961-5974; Suzuki et al. [1994] Proc. Natl. Acad. Sci. USA 91: 10255-10259) and human (ten Dijke et al. [1994] J. Biol. Chem. 269:16985-16988) type I receptors for BMP-2 and BMP-4 was cloned. Tissue distribution of the receptor mRNA was studied by in situ hybridization using rats at embryonic days 9, 13, 15, and 18 as well as 1- and 5-day-old postnatal rats. In the rats at embryonic days 9, 13, and 15, the receptor mRNA was diffusely expressed over the embryonic bodies. At embryonic day 18, the receptor mRNA expression was high in the hair and whisker follicles, tooth bud, cartilage, bone, digestive organs, lung, kidney, heart, and meninges. The receptor mRNA was expressed over a much wider area than those of the ligands in many organs. In the lung and digestive organs, the receptor mRNA was diffusely expressed and most highly expressed in the bronchial epithelium and muscle layer, respectively, in both of which mRNA expression of the ligands was undetectable. The receptor mRNA was highly expressed in the meninges, although neither of the ligands was expressed in or near this region. These results suggest that this receptor participates in both mesoderm formation in early embryogenesis and differentiation of mesodermal cells during maturation of organs, and further suggest the presence of another factor(s) that binds the type I receptor.

Lef1 expression is activated by BMP-4 and regulates inductive tissue interactions in tooth and hair development

Targeted inactivation of the murine gene encoding the transcription factor LEF-1 abrogates the formation of organs that depend on epithelial-mesenchymal tissue interactions. In this study we have recombined epithelial and mesenchymal tissues from normal and LEF-1-deficient embryos at different stages of development to define the LEF-1-dependent steps in tooth and whisker organogenesis. At the initiation of organ development, formation of the epithelial primordium of the whisker but not tooth is dependent on mesenchymal Lef1 gene expression. Subsequent formation of a whisker and tooth mesenchymal papilla and completion of organogenesis require transient expression of Lef1 in the epithelium. These experiments indicate that the effect of Lef1 expression is transmitted from one tissue to the other. In addition, the finding that the expression of Lef1 can be activated by bone morphogenetic protein 4 (BMP-4) suggests a regulatory role of this transcription factor in BMP-mediated inductive tissue interactions.

Involvement of FGF-8 in initiation, outgrowth and patterning of the vertebrate limb

Fibroblast Growth Factors (FGFs) are signaling molecules that are important in patterning and growth control during vertebrate limb development. Beads soaked in FGF-1, FGF-2 and FGF-4 are able to induce additional limbs when applied to the flank of young chick embryos (Cohn, M.J., Izpisua-Belmonte, J-C., Abud, H., Heath, J. K., Tickle, C. (1995) Cell 80, 739–746). However, biochemical and expression studies suggest that none of these FGFs is the endogenous signal that initiates limb development. During chick limb development, Fgf-8 transcripts are detected in the intermediate mesoderm and subsequently in the prelimb field ectoderm prior to the formation of the apical ectodermal ridge, structures required for limb initiation and outgrowth, respectively. Later on, Fgf-8 expression is restricted to the ridge cells and expression disappears when the ridge regresses. Application of FGF-8 protein to the flank induces the development of additional limbs. Moreover, we show that FGF-8 can replace the apical ectodermal ridge to maintain Shh expression and outgrowth and patterning of the developing chick limb. Furthermore, continuous and widespread misexpression of FGF-8 causes limb truncations and skeletal alterations with phocomelic or achondroplasia phenotype. Thus, FGF-8 appears to be a key signal involved in initiation, outgrowth and patterning of the developing vertebrate limb.

Activation of Fgf-4 and HoxD gene expression by BMP-2 expressing cells in the developing chick limb

Bone morphogenetic protein-2 (BMP-2) has been implicated in the polarizing region signalling pathway, which specifies pattern across the antero-posterior of the developing vertebrate limb. Retinoic acid and Sonic Hedgehog (SHH) can act as polarizing signals; when applied anteriorly in the limb bud, they induce mirror-image digit duplications and ectopic Bmp-2 expression in anterior mesenchyme. In addition, the two signals can activate Fgf-4 expression in anterior ridge and HoxD expression in anterior mesenchyme. We tested the role of BMP-2 in this signalling cascade by ectopically expressing human BMP-2 (hBMP-2) at the anterior margin of the early wing bud using a replication defective retroviral vector, and found that ectopic expression of Fgf-4 was induced in the anterior part of the apical ectodermal ridge, followed later by ectopic expression of Hoxd-11 and Hoxd-13 in anterior mesenchyme. This suggests that BMP-2 is involved in regulating Fgf-4 and HoxD gene expression in the normal limb bud. Ectopically expressed hBMP-2 also induced duplication of digit 2 and bifurcation of digit 3, but could not produce the mirror-image digit duplications obtained with SHH-expressing cells. These results suggest that BMP-2 may be involved primarily in maintenance of the ridge, and in the link between patterning and outgrowth of the limb bud.

Retinoic acid signaling is required during early chick limb development

In the chick limb bud, the zone of polarizing activity controls limb patterning along the anteroposterior and proximodistal axes. Since retinoic acid can induce ectopic polarizing activity, we examined whether this molecule plays a role in the establishment of the endogenous zone of polarizing activity. Grafts of wing bud mesenchyme treated with physiologic doses of retinoic acid had weak polarizing activity but inclusion of a retinoic acid-exposed apical ectodermal ridge or of prospective wing bud ectoderm evoked strong polarizing activity. Likewise, polarizing activity of prospective wing mesenchyme was markedly enhanced by co-grafting either a retinoic acid-exposed apical ectodermal ridge or ectoderm from the wing region. This equivalence of ectoderm-mesenchyme interactions required for the establishment of polarizing activity in retinoic acid-treated wing buds and in prospective wing tissue, suggests a role of retinoic acid in the establishment of the zone of polarizing activity. We found that prospective wing bud tissue is a high-point of retinoic acid synthesis. Furthermore, retinoid receptor-specific antagonists blocked limb morphogenesis and down-regulated a polarizing signal, sonic hedgehog. Limb agenesis was reversed when antagonist-exposed wing buds were treated with retinoic acid. Our results demonstrate a role of retinoic acid in the establishment of the endogenous zone of polarizing activity.

Polarizing activity, Sonic hedgehog, and tooth development in embryonic and postnatal mouse

Tooth development involves reciprocal epithelial-mesenchymal interactions, polarized growth, mesenchyme condensation, and complex morphogenetic events. Because these processes bear similarities to those occurring in the developing limb, we asked whether morphogenetic signals found in the limb also occur in the developing tooth. We grafted mouse embryo tooth germs to the anterior margin of host chick embryo wing buds and determined whether the dental tissues had polarizing activity. Indeed, the grafts induced supernumerary digits. Activity of both molar and incisor tooth germs increased from bud to cap stages and was maximal at late bell stage in newborn. With further development the polarizing activity began to decrease, became undetectable in adult molar mesenchyme but persisted in incisor mesenchyme, correlating with the fact that incisors grow throughout postnatal life while molars do not. When different portions of neonatal incisors were assayed, a clear proximo-distal gradient of activity was apparent, with maximal activity restricted to the most proximal portion where undifferentiated mesenchyme and enamel organ reside. In situ hybridizations demonstrated that prior to induction of supernumerary digits, the tooth germ grafts induced expression in host tissue of Hoxd-12 and Hoxd-13. In addition, whole-mount in situ hybridizations and immunohistochemistry showed that developing tooth germs express Sonic hedgehog (Shh). Shh expression was first detected in bud stage tooth germs; at later stages Shh transcripts were prominent in enamel knot and differentiating ameloblasts at the cuspal region. We concluded that tooth germs possess polarizing activity and produce polarizing factors such as Shh. As in the limb, these factor(s) and activity probably play key roles in establishing polarity and regulating morphogenesis during early tooth development. Given its subsequent association with differentiating ameloblasts, Shh probably participates also in cytogenetic events during odontogenesis.

Human bone morphogenetic protein 2 contains a heparin-binding site which modifies its biological activity

Bone morphogenetic protein 2 (BMP-2) plays a decisive role during bone regeneration and repair as well as during various stages of embryonal development. A cDNA encoding mature human BMP-2 could be efficiently expressed in Escherichia coli, and after renaturation a dimeric BMP-2 protein of M(r) 26,000 was prepared with a purity greater 98%. The recombinant BMP-2 was functionally active as demonstrated by the induction of alkaline phosphatase activity in the C3H10T1/2 fibroblast cell line (EC50 of 70 nM) and proteoglycan synthesis in embryonic chicken limb bud cells (EC50 of 15-20 nM). A peptide 1-17 representing the N-terminal basic part of BMP-2 as well as heparin increased the specific activity of the protein about fivefold in the limb bud assay. These observations suggested that the N-terminai reduce the specific activity of BMP-2, probably by interacting with heparinic sites in the extracellular matrix. This conclusion was supported by a variant EHBMP-2, where the N-terminal residues 1-12 of BMP-2 had been substituted by a dummy sequence of equal length and which showed an EC50 value of around 1 nM which was affected neither by heparin nor by peptide 1-17. A physical interaction between BMP-2 and heparin could be seen in biosensor experiments, where BMP-2 bound to immobilized heparin with a dissociation constant, Kd, of approximately 20 nM, whereas the heparin-binding of variant EHBMP-2 was negligible. These results identify the basic N-terminal domains of dimeric BMP-2 as heparin-binding sites that are not obligatory for receptor activation but modulate its biological activity.

Basic fibroblast growth factor for stimulation of bone formation in osteoinductive or conductive implants

Basic Fibroblast Growth Factor (bFGF) is one of the endogenous factors found in bone matrix. bFGF is a mitogen for many cell types, including osteoblasts and chondrocytes. It can stimulate angiogenesis and osteoblast gene expression. The purpose of this study was to investigate whether exogenous bFGF can stimulate the formation of bone in bone grafts and in a bone graft substitute. In a model using demineralized bone matrix implants for bone induction, a dose of 15 ng bFGF per implant increased the number of chondrocytes and the amount of bone, whereas 1900 ng greatly inhibited cartilage and bone formation. These results are consistent with previous studies with this model, showing that a lower dose of bFGF increased bone calcium content and a higher dose reduced it. Thus, exogenous bFGF can stimulate proliferation during early phases of bone induction. A new device, the bone conduction chamber, was developed for the application of bFGF to bone conductive materials. This model made it possible to demonstrate a difference between the conductive properties of bone grafts and porous hydroxyapatite. bFGF increased bone ingrowth into bone graft inside the chamber and showed a biphasic dose-response curve, so that 8-200 ng per implant (0.4-10 ng/mm3) increased bone ingrowth, but higher or lower doses had no effect. The same doses had the same effects in porous hydroxyapatite. In both bone grafts and porous hydroxyapatite, the highest dose still caused an increase in ingrowth of fibrous tissue. The effect on bone ingrowth was first detected after 6 weeks, regardless if administration of bFGF started at implantation or 2 weeks later, using an implanted minipump. Hyaluronate gel was effective as a slow-release carrier for bFGF. In conclusion, bFGF stimulates bone formation in bone implants, depending on dose and method for administration.

IGF-I, insulin and FGFs induce outgrowth of the limb buds of amelic mutant chick embryos

IGF-I, insulin, FGF-2 and FGF-4 have been implicated in the reciprocal interactions between the apical ectodermal ridge (AER) and underlying mesoderm required for outgrowth and patterning of the developing limb. To study further the roles of these growth factors in limb outgrowth, we have examined their effects on the in vitro morphogenesis of limb buds of the amelic mutant chick embryos wingless (wl) and limbless (ll). Limb buds of wl and ll mutant embryos form at the proper time in development, but fail to undergo further outgrowth and subsequently degenerate. Wl and ll limb buds lack thickened AERs capable of promoting limb outgrowth, and their thin apical ectoderms fail to express the homeobox-containing gene Msx-2, which is highly expressed by normal AERs and has been implicated in regulating AER activity. Here we report that exogenous IGF-I and insulin, and, to a lesser extent, FGF-2 and FGF-4 induce the proliferation and directed outgrowth of explanted wl and ll mutant limb buds, which in vitro, like in vivo, normally fail to undergo outgrowth and degenerate. IGF-I and insulin, but not FGFs, also cause the thin apical ectoderms of wl and ll limb buds to thicken and form structures that grossly resemble normal AERs and, moreover, induce high level expression of Msx-2 in these thickened AER-like structures. Neither IGF-I, insulin nor FGFs induce expression of the homeobox-containing gene Msx-1 in the subapical mesoderm of wl or ll limb buds, although FGFs, but not IGF-I or insulin, maintain Msx-1 expression in normal (non-mutant) limb bud explants lacking an AER. The implications of these results to the relationships among the wl and ll genes, IGF-I/insulin, FGFs, Msx-2 and Msx-1 in the regulation of limb outgrowth is discussed.

Bcl-2 inhibits retinoic acid-induced apoptosis during the neural differentiation of embryonal stem cells

We report here that all trans-retinoic acid (RA), a classical morphogen, induces apoptosis during the neural differentiation of the embryonic stem cell line P19. The apoptotic cells showed, in addition to DNA cleavage, typical morphological changes including chromatin condensation, nuclear fragmentation, and cytoplasmic vacuolation. These apoptotic changes became obvious by 12 h after the addition of RA. The endogenous expression of bcl-2 in surviving cells was down-regulated during this process, and the compelled expression of bcl-2 by retroviral vectors reduced the number of apoptotic cells. Apoptosis was partially inhibited by adding antisense oligonucleotides against RA receptors (RARs) simultaneously or by transfecting a plasmid vector flanked with a RA-responsive element. Antisense oligonucleotides against retinoid X receptors (RXRs), the receptors for 9 cis-RA, did not inhibit apoptosis induced by all trans-RA. Cycloheximide and actinomycin D, inhibitors of protein and RNA syntheses, respectively, suppressed apoptosis. No changes were seen in the expression of tumor necrosis factors, their receptors, Fas, FasL, p53, or c-myc, molecules which have been suggested to participate in the apoptotic process. Addition of neurotrophins to the culture medium did not affect apoptosis. These findings suggest that the signals themselves, promote expression of molecules essential for apoptosis. Furthermore, we observed that RA induced apoptosis of cerebral neurons from murine embryos in primary culture, which suggests that RA might participate in cell death which occurs during neural development.

FGF-2 influences cell movements and gene expression during limb development

FGF-2 is proposed to be an important ectodermal signal directing limb outgrowth and patterning. Consistent with this hypothesis we show that ectopic application of FGF-2 can maintain the apical ectodermal ridge (AER)-dependent expression of Sonic hedgehog (Shh), and AER-dependent zone of polarizing activity (ZPA) signaling. We also find that ectopic FGF-2 applied to the posterior wing bud caused a dramatic change in the morphology of the limb bud, and results in limbs that display a reduction in the length of individual skeletal elements and loss of digits. Associated with these morphological changes was an FGF-2-stimulated expansion and bifurcation of the expression domains of two posteriorly expressed genes, Shh and HoxD13. Applying FGF-2 at a central or anterior location in the limb bud did not alter the Shh expression domain or cause digit loss. To test whether ectopic application of FGF-2 into the posterior limb bud was influencing the movement of limb bud cells, we used the lipophilic dye DiI to map the behavior of posterior cells in response to FGF-2. In response to FGF-2 posterior limb bud cells move in both a proximal and a distal direction, causing the initially labeled cell population to bifurcate into two distinct domains. Our data suggest that FGF-2 is influencing limb outgrowth by modifying cell movements and subsequent position-specific cell-cell interactions that are important for limb morphogenesis.

BMP-like signals are required after the midblastula transition for blood cell development

We have investigated the process by which the primitive erythroid cells develop during early vertebrate embryogenesis. Cultured Xenopus animal cap (AC) cells transiently activate the transcription of blood cell regulatory genes GATA-1 and GATA-2 but fail to commit stably to the blood lineage. By contrast, cells of the presumptive ventral marginal zone (VMZ), are committed by the midblastula transition (MBT) to express fully on erythroid program. Growth factor BMP-4, a member of the TGF-beta family of signaling molecules, has been implicated in the process of ventral mesoderm patterning. We show that expression of BMP-4 after MBT is sufficient to induce the blood program fully in AC cells. This includes high level expression of the blood markers SCL and globin, which are not activated in AC cells from uninjected embryos. Likewise, expression of a dominant negative receptor after MBT results in relatively normal embryos, which, however, completely lack differentiated blood cells. Our results are consistent with a role for BMP or BMP-like signaling during gastrulation in the differentiation of embryonic blood.

The enamel knot as a signaling center in the developing mouse tooth

Mammalian tooth forms are produced during development by folding of the enamel epithelium but the molecular mechanisms involved in the formation and patterning of tooth cusps are not understood. We now report that several key signaling molecules found in well-known vertebrate signaling tissues such as the node, the notochord, the apical ectodermal ridge, and the zone of polarizing activity in the limb bud are specifically expressed in cells of the enamel knot, which is a transient cluster of dental epithelial cells. By comparing three-dimensional reconstructions of serial sections following in situ hybridization we localized Sonic hedgehog, Bone morphogenetic proteins-2, -4 and -7, as well as Fibroblast growth factor-4 in nested domains within the enamel knot. We suggest that the enamel knot acts as a signaling or organizing center, which provides positional information for tooth morphogenesis and regulates the growth of tooth cusps.

Effects of all-trans-retinoic acid on skeletal pattern, 5'HoxD gene expression, and RAR beta 2/beta 4 promoter activity in embryonic mouse limbs

Mouse embryos were exposed to all-trans-retinoic acid on day 11 or day 12 of development and the resulting skeletal pattern alterations compared with early effects on Hoxd-11 and Hoxd-13 expression domains and RAR-beta 2/beta 4 promoter activity. The effects on skeletal pattern showed a clear correlation between the timing of retinoic acid exposure and the sequence of mesenchymal condensation. Ectopic RAR-beta 2/beta 4 promoter activity was detected within 2 hr of exposure to retinoic acid, and was present throughout the limb bud after 5 hr; it remained high in the apical ectodermal ridge and proximal mesenchyme after 12 hr, by which time the abnormal digital pattern could be seen. HoxD gene expression domains in the distal handplate were narrowed by 5 hr after maternal retinoic acid administration on day 11. Following retinoic acid treatment on both day 11 and day 12, the normal downregulation of Hoxd-11 and Hoxd-13 in the digital mesenchymal condensations was retarded. There was no evidence to suggest that RAR-beta 2/beta 4 promoter activity mediates the effects of RA on HoxD gene expression, but ectopic promoter activity is a useful indicator of at least some of the sites in which RA levels are raised. We suggest (1) that the apical ectodermal ridge is the most functionally significant of these sites, (2) that raised retinoic acid levels in the ridge result in altered gene expression and/or altered cell proliferation within this epithelium, (3) that both altered HoxD gene expression domains and altered skeletal pattern formation are secondary to this effect. There was a good correlation between the effects of retinoic acid on Hoxd-11 and Hoxd-13 expression and delay of skeletal differentiation, suggesting that this may be a direct effect.

Apoptosis in the developing tooth: association with an embryonic signaling center and suppression by EGF and FGF-4

Apoptosis was localized in developing mouse teeth from initiation of morphogenesis to completion of cusp formation by using modified TUNEL method for serial sections and Nile Blue staining for whole mounts. Apoptosis was first detected at bud stage (E12-E13) in the central cells of the invaginating dental epithelium suggesting involvement of cell death in epithelial budding morphogenesis. During cusp development, apoptotic cells were located in the enamel knots, which are transient clusters of dental epithelial cells proposed to act as signaling centers directing the morphogenesis of tooth cusps. Apoptosis was also detected in other restricted epithelial cell populations including the dental lamina, ameloblasts, as well as stratum intermedium and stellate reticulum cells suggesting that the removal of these epithelial cells occurs by apoptosis. Apoptotic cells, presumably osteoclasts, were also located on the surfaces of the developing alveolar bone. When dissected E13 dental epithelium or mesenchyme were cultured in isolation, apoptotic cells were abundant throughout the tissues, whereas when cultured together, apoptosis was inhibited in both tissues close to their interface indicating that epithelial-mesenchymal tissue interactions prevent apoptosis. Epidermal growth factor (EGF) and fibroblast growth factor-4 (FGF-4) inhibited apoptosis in the dental mesenchyme when applied locally using agarose or heparin-coated acrylic beads, suggesting involvement of these or related growth factors in the prevention of apoptosis in dental tissues in vivo. The spatially and temporally restricted distribution patterns of apoptotic cells suggest multiple roles for programmed cell death in dental development. Of particular interest is the removal of the enamel knots by apoptosis which may terminate their tasks as regulators of the patterning of the tooth cusps. The apical ectodermal ridge (AER) of the limb bud has similar signaling characteristics as the enamel knot, and it also undergoes apoptosis. Hence, apoptosis may be a general mechanism for the silencing of embryonic signaling centers.

Polydactyly in the Strong's luxoid mouse is suppressed by limb deformity alleles

The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.

Overexpression of a membrane protein, neuropilin, in chimeric mice causes anomalies in the cardiovascular system, nervous system and limbs

Neuropilin is a type 1 membrane protein, which is highly conserved among Xenopus frog, chicken and mouse. The extracellular part of the neuropilin protein is composed of three unique domains, each of which is thought to be involved in molecular and/or cellular interactions. In mice, neuropilin is expressed in the cardiovascular system, nervous system and limbs at particular developmental stages. To clarify the roles of neuropilin in morphogenesis in vivo, we generated mouse embryonic stem (ES) cell clones that constitutively expressed exogenous neuropilin, then produced chimeras using these ES cell clones. The chimeras overexpressed neuropilin and were embryonic lethal. The chimeric embryos exhibited several morphological abnormalities; excess capillaries and blood vessels, dilation of blood vessels, malformed hearts, ectopic sprouting and defasciculation of nerve fibers, and extra digits. All of these abnormalities occurred in the organs in which neuropilin is expressed in normal development. The variety of abnormalities occurring in these chimeric embryos suggested diverse functions of neuropilin in embryonic morphogenesis, which may be ascribed to multiple interaction domains identified in the molecule. Correct spatiotemporal expression of neuropilin seems to be essential for normal development of the cardiovascular system, nervous system and limbs.

Serrate signals through Notch to establish a Wingless-dependent organizer at the dorsal/ventral compartment boundary of the Drosophila wing

Growth and patterning of the Drosophila wing is controlled by organizing centers located at the anterior-posterior and dorsal-ventral compartment boundaries. Interaction between cells in adjacent compartments establish the organizer. We report here that Serrate and Notch mediate the interaction between dorsal and ventral cells to direct localized expression of Wingless at the D/V boundary. Serrate serves as a spatially localized ligand which directs Wg expression through activation of Notch. Ligand independent activation of Notch is sufficient to direct Wg expression, which in turn mediates the organizing activity of the D/V boundary.

Changes in the expression of fibroblast growth factor receptors mark distinct stages of chondrogenesis in vitro and during chick limb skeletal patterning

Members of the fibroblast growth factor (FGF) family of growth factors are key regulators of limb skeletal patterning and growth. Abnormal expression of FGFs or mutations in their receptors (fgfrs) result in skeletal disorders. Here we show that changes in the expression of fgfrs are intrinsic properties of differentiating cartilage. In mesenchymal micromass cultures differentiating into cartilage, as in ovo, fgfr 1 mRNA was found predominantly in undifferentiated, proliferating mesenchyme, fgfr 2 in precartilage cell aggregates, and fgfr 3 in differentiating cartilage nodules. Thus, our data suggest that switches in the expression of fgfr 1, 2, and 3 mRNAs are associated with phases of cartilage patterning both in vitro and in ovo, and mark distinct stages in the development of the limb skeleton.

BMP-7 is an inducer of nephrogenesis, and is also required for eye development and skeletal patterning

Bone morphogenetic proteins (BMPs) are multifunctional growth factors originally identified by their ability to induce ectopic bone formation. To investigate the function of one of the BMPs, BMP-7, we have generated BMP-7-deficient mice using embryonic stem cell technology. BMP-7-deficient mice die shortly after birth because of poor kidney development. Histological analysis of mutant embryos at several stages of development revealed that metanephric mesenchymal cells fail to differentiate, resulting in a virtual absence of glomerulus in newborn kidneys. In situ hybridization analysis showed that the absence of BMP-7 affects the expression of molecular markers of nephrogenesis, such as Pax-2 and Wnt-4 between 12.5 and 14.5 days postcoitum (dpc). This identifies BMP-7 as an inducer of nephrogenesis. In addition, BMP-7-deficient mice have eye defects that appear to originate during lens induction. Finally, BMP-7-deficient mice also have skeletal patterning defects restricted to the rib cage, the skull, and the hindlimbs.

A requirement for bone morphogenetic protein-7 during development of the mammalian kidney and eye

BMP-7/OP-1, a member of the transforming growth factor-beta (TGF-beta) family of secreted growth factors, is expressed during mouse embryogenesis in a pattern suggesting potential roles in a variety of inductive tissue interactions. The present study demonstrates that mice lacking BMP-7 display severe defects confined to the developing kidney and eye. Surprisingly, the early inductive tissue interactions responsible for establishing both organs appear largely unaffected. However, the absence of BMP-7 disrupts the subsequent cellular interactions required for their continued growth and development. Consequently, homozygous mutant animals exhibit renal dysplasia and anophthalmia at birth. Overall, these findings identify BMP-7 as an essential signaling molecule during mammalian kidney and eye development.

Expression of bone morphogenetic protein-4 (BMP-4), bone morphogenetic protein-7 (BMP-7), fibroblast growth factor-8 (FGF-8) and sonic hedgehog (SHH) during branchial arch development in the chick

Expression of Fgf-8, Bmp-4, Bmp-7, and shh in the branchial arches of the chick embryo is examined by in situ hybridization. Fgf-8 expression is initially broad and diffuse, becoming more tightly restricted, particularly in the epithelium of the posterior ectodermal margin (PEM) of the 2nd branchial arch. Bmp-7 transcripts, first seen at stage 12 in discrete regions corresponding to the developing branchial clefts, are later detected in both clefts and arches, including the PEM of the 2nd arch while Bmp-4 transcripts are detected at stage 18 in the distal tips of the arches. Shh expression remains localized, overlapping with both Bmp-7 and Fgf-8 in the PEM of the 2nd arch at stages 16 and 18. Based on these data, a model is proposed for the role of these signalling molecules in branchial arch development.

Evaluation of candidate genes for familial brachydactyly

Type A1 brachydactyly in humans is a recognisable syndrome characterised by shortening of the middle phalanx of all digits with occasional fusion of the middle and terminal phalanges. The purpose of this study was to evaluate candidate genes for type A1 brachydactyly in two families with multiple affected members. Several classes of genes have been implicated in the control of distal limb development including homeobox containing genes (MSX1, MSX2) some members of the homeobox gene family, and genes encoding growth factors of the FGF, TGF, and PDGF families. Homeobox (Hox) genes are a family of developmental control genes activated early in embryogenesis that encode positional information along the anterior-posterior body axis and specify distinct spatial domains within developing limbs. Growth factor genes can regulate the proliferation and differentiation of various embryonic structures including limb buds and have been shown to influence Hox gene expression. Candidate genes HOXD, MSX1, MSX2, FGF-1, and FGF-2 were excluded in one family. The brachydactyly type A1 gene or locus was not found in either of the two families studied.

Interstitial deletion of the long arm of chromosome 6 associated with unusual limb anomalies: report of two new patients and review of the literature

We report on unusual manifestations in 2 unrelated children with interstitial deletion of 6q, with nearly identical breakpoints of 6q16.2q23.1 and 6q16.3q22.3. Major findings include growth retardation, profound developmental delay, microcephaly, facial anomalies, sparse hair, congenital heart defects, and striking hand malformations. Discordant anomalies were duodenal atresia and hypoplastic genitalia in 1 child. Split-hand defect, polydactyly, gastrointestinal anomalies, and ectodermal dysplasia have not been described previously in children with 6q deletion. The presence of hand malformations in 2 children with similar deletion breakpoints strongly suggests that this is a candidate region for one or more genes involved in limb development. Comparison of the clinical findings of other patients with 6q2 deletion suggests a recognizable phenotype.

Ectodermal stimulation of the production of hyaluronan-dependent pericellular matrix by embryonic limb mesodermal cells

Interaction of ectoderm and underlying mesoderm is essential for normal vertebrate limb morphogenesis. One of the functions of limb bud ectoderm is its influence on the composition of extracellular matrix in subectodermal mesoderm, which in turn participates in morphogenesis of this region of the limb. This matrix is highly enriched in hyaluronan, even at the time when the level of hyaluronan in the chondrogenic and myogenic regions of the limb decreases, due to secretion of a stimulatory factor by the ectoderm. In this study we show that limb bud ectoderm not only stimulates hyaluronan synthesis but induces formation of large hyaluronan-dependent, pericellular matrices around cultured limb bud mesodermal cells. The ectodermal activity is mimicked in great part by fibroblast growth factor-2 and transforming growth factor-beta, and antibodies to these proteins inhibit induction of mesodermal pericellular matrix by the ectodermal factor. It has been shown by other investigators that fibroblast growth factor-2 is produced by limb ectoderm whereas transforming growth factor-2 is produced by limb ectoderm whereas transforming growth factor-beta is present in limb mesodermal tissues. Thus we conclude that the unique properties of mesodermally produced matrix underlying limb bud ectoderm are regulated, at least in part, by ectodermal fibroblast growth factor-2, probably in concert with mesodermal transforming growth factor-beta.