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

Multiple roles of bone morphogenetic protein signaling in the regulation of cortical cell number and phenotype

Members of the bone morphogenetic protein (BMP) family have been implicated in multiple aspects of neural development in both the CNS and peripheral nervous system. BMP ligands and receptors, as well as the BMP antagonist noggin, are expressed in the developing cerebral cortex, making the BMPs likely candidates for regulating cortical development. To define the role of these factors in the developing cerebral cortex, we examined the effects of BMP2 and BMP4 on cortical cells in vitro. Cells were cultured from embryonic day 13 (E13) and E16 rat cerebral cortex in the absence or presence of different concentrations of fibroblast growth factor 2, a known regulator of cortical cell proliferation and differentiation. At E13, the BMPs promoted cell death and inhibited proliferation of cortical ventricular zone cells, resulting in the generation of fewer neurons and no glia. At E16, the effects of the BMPs were more complex. Concentrations of BMP2 in the range of 1-10 ng/ml promoted neuronal and astroglial differentiation and inhibited oligodendroglial differentiation, whereas 100 ng/ml BMP2 promoted cell death and inhibited proliferation. Addition of the BMP antagonist noggin promoted oligodendrogliogenesis in vitro, demonstrating that endogenous BMP signaling influences the differentiation of cortical cells in vitro. The distribution of BMP2 and noggin within the developing cortex suggests that local concentrations of ligands and antagonists define gradients of BMP signaling during corticogenesis. Together, these results support the hypothesis that the BMPs and their antagonist noggin co-regulate cortical cell fate and morphogenesis.

Cell migration and chick limb development: chemotactic action of FGF-4 and the AER

Experiments have been carried out to investigate the role of the apical ectodermal ridge (AER) and FGF-4 on the control of cell migration during limb bud morphogenesis. By coupling DiI cell labeling with ectopic implantation of FGF-4 microcarrier beads we have found that FGF-4 acts as a potent and specific chemoattractive agent for mesenchymal cells of the limb bud. The response to FGF-4 is dose dependent in both the number of cells stimulated to migrate and the distance migrated. The cell migration response to FGF-4 appears to be independent of the known inductive activity of FGF-4 on Shh gene expression. We investigated the role of the AER in controlling cell migration by characterizing the migration pattern of DiI-labeled subapical cells during normal limb outgrowth and following partial AER removal. Subapical cells within 75 micrometer of the AER migrate to make contact with the AER and are found intermingled with nonlabeled cells. Thus, the progress zone is dynamic with cells constantly altering their neighbor relationships during limb outgrowth. AER removal studies show that cell migration is AER dependent and that subapical cells redirect their path of migration toward a functional AER. These studies indicate that the AER has a chemoattractive function and regulates patterns of cell migration during limb outgrowth. Our results suggest that the chemoattractive activity of the AER is mediated in part by the production of FGF-4.

Bone morphogenetic protein-2 acts synergistically with transforming growth factor-beta3 during endothelial-mesenchymal transformation in the developing chick heart

In the early embryonic heart, endothelial cells in atrioventricular (AV) and outflow tract (OT) regions are transformed into the invasive mesenchymal cells that form endocardial cushion tissue (endothelial-mesenchymal transformation). It has been reported that bone morphogenetic proteins (BMPs) are transcribed in the AV and OT regions of the embryonic mouse heart. We previously reported that transforming growth factor beta 3 (TGFbeta3) triggers the initial phenotypic changes seen in endothelial-mesenchymal transformation. We cloned BMP2 from embryonic chick hearts and examined its functional role during endocardial cushion tissue formation. In situ hybridization showed BMP2 transcripts in the myocardium of the AV and OT regions, but not in endothelial/mesenchymal cells. Antisense oligodeoxynucleotides to BMP2 inhibited mesenchyme formation in AV endocardium cocultured with associated myocardium. This inhibitory effect was reversed by the addition of recombinant BMP2. In cultured AV endothelial monolayers, recombinant BMP2 did not induce any cellular phenotypic changes characteristic of endothelial-mesenchymal transformation. However, BMP2 enhanced the TGFbeta-induced initial phenotypic changes associated with endothelial-mesenchymal transformation. These results suggest that BMP2 1) plays an important role in the formation of endocardial cushion tissue and 2) acts synergistically with TGFbeta3 in the regulation of this developmental event.

Molecular mechanisms controlling lung morphogenesis

The complex process of lung formation is determined by the action of numerous genes that influence cell commitment, differentiation, and proliferation. This review summarizes current knowledge of various factors involved in lung morphogenesis correlating their temporal and spatial expression with their proposed functions at various times during the developmental process. Rapid progress in understanding the pathways involved in lung morphogenesis will likely provide the framework with which to elucidate the mechanisms contributing to lung malformations and the pathogenesis of genetic and acquired lung diseases.

Differential regulation of distinct phenotypic features of serotonergic neurons by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), growth and differentiation factor 5 (GDF5) and glial cell line-derived neurotrophic factor (GDNF) are members of the transforming growth factor-beta superfamily that have been implicated in tissue growth and differentiation. Several BMPs are expressed in embryonic and adult brain. We show now that BMP-2, -6 and -7 and GDF5 are expressed in the embryonic rat hindbrain raphe. To start to define roles for BMPs in the regulation of serotonergic (5-HT) neuron development, we have generated serum-free cultures of 5-HT neurons isolated from the embryonic (E14) rat raphe. Addition of saturating concentrations (10 ng/mL) of BMP-6 and GDF5 augmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons and cells specifically taking up 5, 7-dihydroxytryptamine (5,7-DHT) by about two-fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of short-term treatments. Importantly, BMP-7 selectively induced 5, 7-DHT uptake without affecting TpOH immunoreactivity. BMP-6 and -7 also promoted DNA synthesis and increased numbers of cells immunoreactive for vimentin and glial fibrillary acidic protein (GFAP). Pharmacological suppression of cell proliferation or glial development abolished the induction of serotonergic markers by BMP-6 and -7, suggesting that BMPs act indirectly by stimulating synthesis or release of glial-derived serotonergic differentiation factors. Receptor bodies for the neurotrophin receptor trkB, but not trkC, abolished the BMP-mediated effects on serotonergic development, suggesting that the glia-derived factor is probably brain-derived neurotrophic factor (BDNF) or neurotrophin-4. In support of this notion, we detected increased levels of BDNF mRNA in BMP-treated cultures. Together, these data suggest both distinct and overlapping roles of several BMPs in regulating 5-HT neuron development.

Initiation of mammalian liver development from endoderm by fibroblast growth factors

The signaling molecules that elicit embryonic induction of the liver from the mammalian gut endoderm or induction of other gut-derived organs are unknown. Close proximity of cardiac mesoderm, which expresses fibroblast growth factors (FGFs) 1, 2, and 8, causes the foregut endoderm to develop into the liver. Treatment of isolated foregut endoderm from mouse embryos with FGF1 or FGF2, but not FGF8, was sufficient to replace cardiac mesoderm as an inducer of the liver gene expression program, the latter being the first step of hepatogenesis. The hepatogenic response was restricted to endoderm tissue, which selectively coexpresses FGF receptors 1 and 4. Further studies with FGFs and their specific inhibitors showed that FGF8 contributes to the morphogenetic outgrowth of the hepatic endoderm. Thus, different FGF signals appear to initiate distinct phases of liver development during mammalian organogenesis.

Interaction between FGF and BMP signaling pathways regulates development of metanephric mesenchyme

Nephrogenesis in the mouse kidney begins at embryonic day 11 and ends approximately 10 days postpartum. During this period, new nephrons are continually being generated from a stem-cell population-the nephrogenic mesenchyme-in response to signals emanating from the tips of the branching ureter. Relatively little is known about the mechanism by which the nephrogenic mesenchyme cell population is maintained at the tips of the ureter in the presence of signals promoting tubulogenesis. Previous studies have shown that a loss of Bmp7 function leads to kidney defects that are a likely result of progressive loss of nephrogenic mesenchyme by apoptosis. The studies presented here demonstrate that BMP7 signaling can prevent apoptosis in explants of metanephric mesenchyme. The surviving mesenchyme cell population, however, is not competent to respond to signals promoting tubulogenesis. In conjunction with FGF2, BMP7 promotes growth and maintains competence of the mesenchyme in vitro. In addition, FGF2 and BMP7 signaling, both independently and in combination, inhibit tubulogenesis. Interestingly, FGF2 and BMP7 also promote expansion of the stromal progenitor cell population in whole kidney culture. Because BMP7 is not produced by stromal progenitor cells, these data suggest a novel interaction between the nephrogenic mesenchyme and stromal progenitor cell populations. A model for the regulation of nephrogenesis by FGF and BMP signaling is presented.

The distribution of BrdU- and TUNEL-positive cells during odontogenesis in mouse lower first molars

This study investigated the minute distribution of both proliferating and non-proliferating cells, and cell death in the developing mouse lower first molars using 5-bromo-2'-deoxyuridine (BrdU) incorporation and the terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate (dUTP)-biotin nick end labeling (TUNEL) double-staining technique. The distribution pattern of the TUNEL-positive cells was more notable than that of the BrdU-positive cells. TUNEL-positive cells were localized in the following six sites: (1) in the most superficial layer of the dental epithelium during the initiation stage, (2) in the dental lamina throughout the period during which tooth germs grow after bud formation, (3) in the dental epithelium in the most anterior part of the antero-posterior axis of the tooth germ after bud formation, (4) in the primary enamel knot from the late bud stage to the late cap stage, (5) in the secondary enamel knots from the late cap stage to the late bell stage, and (6) in the stellate reticulum around the tips of the prospective cusps after the early bell stage. These peculiar distributions of TUNEL-positive cells seemed to have some effect on either the determination of the exact position of the tooth germ in the mandible or on the complicated morphogenesis of the cusps. The distribution of BrdU-negative cells was closely associated with TUNEL-positive cells, which thus suggested cell arrest and the cell death to be essential for the tooth morphogenesis.

Control of digit formation by activin signalling

Major advances in the genetics of vertebrate limb development have been obtained in recent years. However, the nature of the signals which trigger differentiation of the mesoderm to form the limb skeleton remains elusive. Previously, we have obtained evidence for a role of TGFbeta2 in digit formation. Here, we show that activins A and B and/or AB are also signals involved in digit skeletogenesis. activin betaA gene expression correlates with the initiation of digit chondrogenesis while activin betaB is expressed coincidently with the formation of the last phalanx of each digit. Exogenous administration of activins A, B or AB into the interdigital regions induces the formation of extra digits. follistatin, a natural antagonist of activins, is expressed, under the control of activin, peripherally to the digit chondrogenic aggregates marking the prospective tendinous blastemas. Exogenous application of follistatin blocks physiological and activin-induced digit formation. Evidence for a close interaction between activins and other signalling molecules, such as BMPs and FGFs, operating at the distal tip of the limb at these stages is also provided. Chondrogenesis by activins is mediated by BMPs through the regulation of the BMP receptor bmpR-1b and in turn activin expression is upregulated by BMP signalling. In addition, AER hyperactivity secondary to Wnt3A misexpression or local administration of FGFs, inhibits activin expression. In correlation with the restricted expression of activins in the course of digit formation, neither activin nor follistatin treatment affects the development of the skeletal components of the stylopod or zeugopod indicating that the formation of the limb skeleton is regulated by segment-specific chondrogenic signals.

Bone morphogenetic protein-2 inhibits MAPK-dependent Elk-1 transactivation and DNA synthesis induced by EGF in mesangial cells

Bone morphogenetic protein-2 (BMP-2) is a member of the TGFbeta superfamily of growth and differentiation factors. We investigated the effect of BMP-2 on epidermal growth factor (EGF)-induced mitogenic signaling in kidney glomerular mesangial cells. BMP-2 dose-dependently inhibits EGF-induced DNA synthesis. Maximum effect was obtained at a concentration of 100 ng/ml. BMP-2 had no inhibitory effect on the EGF receptor (EGFR)-associated tyrosine kinase activity indicating that inhibition of DNA synthesis is due to regulation of post-receptor signaling event(s). EGF stimulates MAPK activity in mesangial cells in a time-dependent manner. Inhibition of MAPK by the MEK inhibitor PD098059 blocks EGF-induced DNA synthesis indicating the requirement of this enzyme activity in EGF-mediated mitogenic signaling. Furthermore, we show that exposure of mesangial cells to BMP-2 blocks EGF-induced MAPK activity which leads to phosphorylattion of Elk-1 transcription factor. Using a GAL-4 DNA binding-domain-Elk-1 transactivation domain fusion protein-based reporter assay, we demonstrate that BMP-2 inhibits EGF-induced Elk-1-mediated transcription. These data provide the first evidence that BMP-2 signaling in mesangial cells initiates a negative regulatory cross-talk with MAPK-based transcription to inhibit EGF-induced DNA synthesis.

Bone morphogenetic protein 2 inhibits platelet-derived growth factor-induced c-fos gene transcription and DNA synthesis in mesangial cells. Involvement of mitogen-activated protein kinase

Bone morphogenetic proteins (BMPs) play an important role in nephrogenesis. The biologic effect and mechanism of action of these proteins in the adult kidney has not yet been studied. We investigated the effect of BMP2, a member of these growth and differentiation factors, on mitogenic signal transduction pathways induced by platelet-derived growth factor (PDGF) in glomerular mesangial cells. PDGF is a growth and survival factor for these cells in vitro and in vivo. Incubation of mesangial cells with increasing concentrations of BMP2 inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 250 ng/ml. Immune complex tyrosine kinase assay of PDGF receptor beta immunoprecipitates from lysates of mesangial cells treated with PDGF showed no inhibitory effect of BMP2 on PDGF receptor tyrosine phosphorylation. This indicates that the inhibition of DNA synthesis is likely due to postreceptor events. However, BMP2 significantly inhibited PDGF-stimulated mitogen-activated protein kinase (MAPK) activity that phosphorylates the Elk-1 transcription factor, a component of the ternary complex factor. Using a fusion protein-based reporter assay, we also show that BMP2 blocks PDGF-induced Elk-1-mediated transcription. Furthermore, we demonstrate that BMP2 inhibits PDGF-induced transcription of c-fos gene, a natural target of Elk-1 that normally forms a ternary complex that activates the serum response element of the c-fos gene. These data provide the first evidence that in mesangial cells, BMP2 signaling cross-talks with MAPK-based transcriptional events to inhibit PDGF-induced DNA synthesis. One target for this inhibition is the early response gene c-fos.

Opposite effects of FGF and BMP-4 on embryonic blood formation: roles of PV.1 and GATA-2

In adult vertebrates, fibroblast growth factor (FGF) synergizes with many hematopoietic cytokines to stimulate the proliferation of hematopoietic progenitors. In vertebrate development, the FGF signaling pathway is important in the formation of some derivatives of ventroposterior mesoderm. However, the function of FGF in the specification of the embryonic erythropoietic lineage has remained unclear. Here we address the role of FGF in the specification of the erythropoietic lineage in the Xenopus embryo. We report that ventral injection of embryonic FGF (eFGF) mRNA at as little as 10 pg at the four-cell stage suppresses ventral blood island (VBI) formation, whereas expression of the dominant negative form of the FGF receptor in the lateral mesoderm, where physiologically no blood tissue is formed, results in a dramatic expansion of the VBI. Similar results were observed in isolated ventral marginal zones and animal caps. Bone morphogenetic protein-4 (BMP-4) is known to induce erythropoiesis in the Xenopus embryo. Therefore, we examined how the BMP-4 and FGF signaling pathways might interact in the decision of ventral mesoderm to form blood. We observed that eFGF inhibits BMP-4-induced erythropoiesis by differentially regulating expression of the BMP-4 downstream effectors GATA-2 and PV.1. GATA-2, which stimulates erythropoiesis, is suppressed by FGF. PV.1, which we demonstrate to inhibit blood development, is enhanced by FGF. Additionally, PV.1 and GATA-2 negatively regulate transcription of each other. Thus, BMP-4 induces two transcription factors which have opposing effects on blood development. The FGF and BMP-4 signaling pathways interact to regulate the specification of the erythropoietic lineage.

Molecular genetics of tooth morphogenesis and patterning: the right shape in the right place

Development of the mammalian tooth has for many years served as a useful model system for the study of cell-cell interactions in organogenesis. Early development of teeth (tooth buds) shows many morphological and molecular similarities with early development of other organs such as the lung, hair, kidney, etc. There has been much progress toward understanding epithelial/mesenchymal cell signaling in tooth germ formation. Advances in understanding the formation of different shapes of teeth (morphogenesis) at their correct positions in the jaws (patterning) has, until recently, been less forthcoming. We review here the latest ideas on the control of odontogenic patterning and morphogenesis. The stages of early tooth development are well-defined histologically and have been described in numerous textbooks. The progression from localized thickenings of oral epithelium to bud, cap, and bell stages provides an adequate description of the gross morphological changes seen in the epithelial cells of early developing tooth germs. Less obvious are the concomitant changes taking place in the dental (ecto)mesenchymal cells which originate from the cranial neural crest and which condense around the tooth bud epithelium. However, it is very clear that these mesenchymal cells are equal partners with epithelium during the early stages of tooth germ formation and undergo complex changes which, although not obvious histologically, are revealed with molecular (gene) probes. Genes identified as being important for the early communication between the epithelial and ectomesenchymal cells mainly comprise those which code for proteins which act as secreted signals between the cells (ligands) and those that code for nuclear proteins that act to control gene expression in response to the signals. Little is presently known about the changes in structural proteins such as cell adhesion molecules which are involved in mediating the physical interactions between cells and generating the morphological changes.

Signaling specificity: the RTK/RAS/MAP kinase pathway in metazoans

The molecular basis by which commonly used signaling pathways are able to elicit tissue-specific responses in multicellular organisms is an important yet poorly understood problem. In this review, we use the receptor tyrosine kinase (RTK)/RAS/MAP kinase signaling cascade as a model to discuss various hypotheses that have been proposed to explain signaling specificity. Specificity can arise at the level of the receptor, through the modulation of signaling kinetics, through the interaction of different signaling pathways, and at the level of downstream signaling components. Mechanisms of specificity used by the RTK/RAS/MAP kinase signaling pathway might apply to other signaling pathways as well, and might help explain how multicellular organisms are able to generate tissues of diverse forms and functions from a small set of common signaling pathways.

Epidermal growth factor receptors: critical mediators of multiple receptor pathways

Recently, the receptor for epidermal growth factor (EGF) was identified as a downstream element in different signaling pathways. This expanded its classical function as a receptor for EGF-like ligands to a role as mediator of diverse signaling systems and as a switch point of a cellular communication network. In addition, several downstream targets, (e.g. Smad proteins and STATs) into which signals from synergistic and antagonistic signaling pathways converge, were identified.

BMPs negatively regulate structure and function of the limb apical ectodermal ridge

The apical ectodermal ridge (AER), a transient specialized epithelium at the distal limb tip, is essential for vertebrate embryonic limb outgrowth along the proximodistal axis. Among all the molecules expressed in the AER, only the Fibroblast Growth Factors (FGFs) have been shown to substitute for its function in limb outgrowth. After specification of the skeletal progenitors is complete, the AER regresses, having fulfilled its function. However, the cellular processes underlying AER regression remain largely unclear, and the molecular ones, totally unknown. Members of the Bone Morphogenetic Protein (BMP) family are expressed in the AER throughout its life and in the mesenchyme. Our studies using misexpression of Noggin, a BMP inhibitor, reveal an unsuspected role for BMPs in the negative regulation of Fgf expression and AER function. We find that BMPs limit limb outgrowth by promoting AER regression, as BMP inhibition results in persistence of the AER, prolonged Fgf expression and excess soft-tissue growth. In addition, the Noggin misexpression studies uncover an earlier role for BMPs in repression of AER function. Noggin overexpression results in extension of the AER anteriorly and loss of AER asymmetry. We show that overall the AER becomes taller, and its anterior half becomes more similar to a normal posterior AER. In addition, Fgf4 transcripts, which are usually restricted to the posterior half of the AER, are now also expressed anteriorly. Moreover, ectopicFgf4 expression is induced independently of Sonic Hedgehog, contrary to current models of Fgf4 regulation in the limb. Our studies also provide insight into the activity of the hypothesized apical ectodermal maintenance factor (AEMF), which is thought to maintain the tall shape of the posterior part of the AER. Our work shows that the AER is negatively regulated by BMP.

Retinoid-induced limb defects 2: involvement of TGF-beta 2 in retinoid-induced inhibition of limb bud development

We previously demonstrated that retinoid-induced inhibition of chondrogenesis in the forelimb bud may be mediated by TGF-beta2 (1). The present study was conducted to examine whether TGF-beta2 is involved in the inhibition of forelimb bud development caused by all-trans-retinoic acid (RA). Expression of TGF-beta2 was examined immunohistochemically in forelimb buds of embryos 24 h after dosing to the mother on Day 12 of gestation in the rat. In the control and 50 mg/kg group, TGF-beta2 was expressed in the epithelium and prechondrogenic area around dead cells in the forelimb bud. In the 100 mg/kg group, a dose at which RA caused reduction defects of forearm bones, TGF-beta2 expression was observed in the distal margin of forelimb buds, in which no expression was observed in the control and 50 mg/kg group. Immunohistologic studies also indicated that in the 100 mg/kg group, the expression of TGF-beta2 was enhanced in forearm-bone prechondrocytes around the dead cells. In a whole embryo culture system, exposure to RA for 24 h reduced the proximodistal length and protein content in forelimb buds at concentrations of 3 microg/mL or more. The whole embryo culture system also showed that the expression of TGF-beta2 was induced at the concentration of 3 microg/mL in the same region as found in forelimb buds of embryos from dams administered a teratogenic dosage of RA in vivo. Local application of TGF-beta2 to the distal margin of the forelimb bud in Day 12 embryos reduced proximodistal growth and protein content in forelimb buds for 24 h in culture even without RA treatment. We also found that exogenous TGF-beta2 inhibited DNA synthesis of forelimb bud cells in culture in a concentration-dependent manner. Neutralization of TGF-beta2 with its antibody in the distal margin of forelimb buds partially prevented the RA-induced inhibition of forelimb bud growth in the whole embryo culture system. These results suggest that RA-induced TGF-beta2 in the distal margin of forelimb buds may be involved in RA-induced inhibition of forelimb bud growth via reduction of cell proliferation in the distal margin, and RA-induced TGF-beta2 in the prechondrogenic area may inhibit chondrogenesis in the future forearm bones, followed by reduction defects of the forearm bones.

Teeth. Where and how to make them

Organs have to develop at precisely determined sites to ensure functionality of the whole organism. Organogenesis is typically regulated by a series of interactions between morphologically distinct tissues. The developing tooth of the mouse is an excellent model to study these processes and we are beginning to understand the networks regulating reciprocal tissue interactions at the molecular level. Synergistic and antagonistic effects of signaling molecules including FGFs and BMPs are recursively used to induce localized responses in the adjacent tissue layer (mesenchyme or epithelium). However, at different phases of odontogenesis these secreted growth factors have distinct effects and at the same time they are regulated by different upstream factors. The mesenchymal transcription factors Msx1 and Pax9 are initially regulated by epithelial FGFs and BMPs, but subsequently they function upstream of these signaling molecules. This cascade provides a molecular model by which reciprocal tissue interactions are controlled.

Brachydactyly type B: linkage to chromosome 9q22 and evidence for genetic heterogeneity

Brachydactyly type B (BDB), an autosomal dominant disorder, is the most severe of the brachydactylies and is characterized by hypoplasia or absence of the terminal portions of the index to little fingers, usually with absence of the nails. The thumbs may be of normal length but are often flattened and occasionally are bifid. The feet are similarly but less severely affected. We have performed a genomewide linkage analysis of three families with BDB, two English and one Portugese. The two English families show linkage to the same region on chromosome 9 (combined multipoint maximum LOD score 8.69 with marker D9S257). The 16-cM disease interval is defined by recombinations with markers D9S1680 and D9S1786. These two families share an identical disease haplotype over 18 markers, inclusive of D9S278-D9S280. This provides strong evidence that the English families have the same ancestral mutation, which reduces the disease interval to <12.7 cM between markers D9S257 and D9S1851 in chromosome band 9q22. In the Portuguese family, we excluded linkage to this region, a result indicating that BDB is genetically heterogeneous. Reflecting this, there were atypical clinical features in this family, with shortening of the thumbs and absence or hypoplasia of the nails of the thumb and hallux. These results enable a refined classification of BDB and identify a novel locus for digit morphogenesis in 9q22.

Mitogen-activated protein kinase and neural specification in Xenopus

We have investigated the activity and function of mitogen-activated protein kinase (MAPK) during neural specification in Xenopus. Ectodermal MAPK activity increased between late blastula and midgastrula stages. At midgastrula, MAPK activity in both newly induced neural ectoderm and ectoderm overexpressing the anterior neural inducer noggin was 5-fold higher than in uninduced ectoderm. Overexpression of MAPK phosphatase-1 (MKP-1) in ectoderm inhibited MAPK activity and prevented neurectoderm-specific gene expression when the ectoderm was recombined with dorsal mesoderm or treated with fibroblast growth factor (FGF). Neurectoderm-specific gene expression was observed, however, in ectoderm overexpressing both noggin and MKP-1. To evaluate the role of MAPK in posterior regionalization, ectodermal isolates were treated with increasing concentrations of FGF and assayed for MAPK activity and neurectoderm-specific gene expression. Although induction of posterior neural ectoderm by FGF was accompanied by an elevation of MAPK activity, relative MAPK activity associated with posterior neural fate was no higher than that of ectoderm specified to adopt an anterior neural fate. Thus, increasingly posterior neural fates are not correlated with quantitative increases in MAPK activity. Because MAPK has been shown to down-regulate Smad1, MAPK may disrupt bone morphogenetic protein 4 (BMP-4) signaling during neural specification. Our results suggest that MAPK plays an essential role in the establishment of neural fate in vivo.

BMP4 is essential for lens induction in the mouse embryo

Vertebrate lens development is a classical model system for studying embryonic tissue interactions. Little is known, however, about the molecules mediating such inductive events. Here, we show that Bmp4, which is expressed strongly in the optic vesicle and weakly in the surrounding mesenchyme and surface ectoderm, has crucial roles during lens induction. In Bmp4(tm1) homozygous null mutant embryos, lens induction is absent, but the process can be rescued by exogenous BMP4 protein applied into the optic vesicle in explant cultures. This is associated with rescue of ectodermal expression of Sox2, an early lens placode marker. Substituting the optic vesicle in explant cultures with BMP4-carrying beads, however, does not lead to lens induction, indicating that other factors produced by the optic vesicle are involved. BMP4 appears to regulate expression of a putative downstream gene, Msx2, in the optic vesicle. No change in Pax6 expression is seen in Bmp4(tm1) mutant eyes, and Bmp4 expression appears unaffected in the eyes of homozygous Pax6(Sey-1Neu), suggesting that PAX6 and BMP4 function independently. Based on these results we propose that BMP4 is required for the optic vesicle to manifest its lens-inducing activity, by regulating downstream genes and/or serving as one component of multiple inductive signals.

Distinct regulatory elements govern Fgf4 gene expression in the mouse blastocyst, myotomes, and developing limb

Embryonic development requires a complex program of events which are directed by a number of signaling molecules whose expression must be rigorously regulated. We previously showed that expression of Fgf4, which plays an important role in postimplantation development and growth and patterning of the limb, is regulated in EC cells by the synergistic interaction of Sox2 and Oct-3 with the Fgf4 EC cell-specific enhancer. To verify whether this mechanism was also operating in vivo, and to identify new elements controlling Fgf4 gene expression in distinct developmental stages, we have analyzed the expression of LacZ reporter plasmids containing different fragments of the Fgf4 gene in transgenic mouse embryos. Utilizing these transgenic constructs we have been able to recapitulate, for the most part, Fgf4 gene expression during embryonic development. We show here that most of the cis-acting regulatory elements determining Fgf4 embryonic expression are located in conserved regions within the 3' UTR of the gene. The EC cell-specific enhancer is required to drive gene expression in the ICM of the blastocyst, and its activity requires the Sox and Oct-proteins binding sites. We were also able to identify specific and distinct enhancer elements that govern postimplantation expression in the somitic myotomes and the limb bud AER. The myotome-specific elements contain binding sites for bHLH myogenic regulatory factors, which appear to be essential for myotome expression. Finally, we present evidence that the very restricted pattern of expression of Fgf4 transcripts in the AER results from the combined action of positive and negative regulatory elements located 3' of the Fgf4 coding sequences. Thus the Fgf4 gene relies on multiple and distinct regulatory elements to achieve stage- and tissue-specific embryonic expression.

Downstream factors in transforming growth factor-beta family signaling

The recently identified family of Smad proteins has given insight in the understanding of how members of the transforming growth factor-beta (TGF-beta) family relay their signal to the nucleus. Besides Smad proteins, G proteins and MAPKs are also involved in the downstream signaling of TGF-beta family members. The identification of elements that function downstream in the TGF-beta signaling pathway and the fact that these downstream players can interact with the signaling cascade of other growth factors, may give insight into the diverse biological responses evoked by the TGF-beta family members.

Role of fibroblast growth factors and their receptors in mouse primordial germ cell growth

Primordial germ cells (PGCs) are the embryonic progenitors of mature germ cells. During their proliferative stage, murine PGCs may be transiently cultured on mitotically inactive feeder layers. This culture system has permitted identification of several growth factors active toward PGCs. We and others have previously identified basic fibroblast growth factor (bFGF) as a powerful mitogen in this system. Here we characterize some of the functions of bFGF in PGC culture. Our data demonstrate that fibroblast growth factor (FGF) receptors I and II are present in the developing gonad and are consistent with expression of these receptors by PGCs. Moreover, PGCs can bind radiolabeled bFGF in vitro, demonstrating that the factor can act directly on these cells. While mitotic PGCs of either sex are shown to bind radiolabeled bFGF, oogonia that are undergoing meiotic arrest exhibit reduced bFGF binding, indicating potential developmental regulation of an FGF receptor.

Molecular and cellular basis of pattern formation during vertebrate limb development

The body plan is generated by cells and tissues that become arranged precisely in the embryo. This process, termed pattern formation, involves cell interactions in which a particular group of cells produce signals that specify new cell types or patterns of differentiation in responding cells. These patterning signals emanate from very discrete centers called "organizer centers," such as the Hensen's node or Spemann organizer, the midbrain-hindbrain junction, the notochord, or in the case of the limb, the zone of polarizing activity (ZPA) or the apical ectodermal ridge (AER). The developing vertebrate limb is an ideal model system for the study of pattern formation because, in addition to surgical manipulations, molecular manipulations are now feasible. In this review we summarize early experiments that established, by means of surgical manipulations, the different organizer centers of the vertebrate limb: the ectoderm covering the limb bud, the apical ectodermal ridge, the zone of polarizing activity, and the distal mesoderm (progress zone) underlying the AER. We then describe the domains of expression of various genes present during the development of the limb and discuss some of the functional approaches (overexpression and lack of function studies) undertaken to ascertain their role in limb outgrowth. The knowledge acquired in the last few years has had an enormous impact not only on our view of how limbs develop (perhaps now one of the most approachable vertebrate model systems) but also in a more general sense of how the embryo is organized in space and time.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

BMPs mediate lateral inhibition at successive stages in feather tract development

The spacing of feather buds in a tract is thought to arise from the interaction between an inducing signal from the dermis and an inhibitory signal generated in the nascent buds. Local BMP-2 expression in the ectoderm precedes the formation of the ectodermal placodes, which are the first morphological sign of bud differentiation. We have altered the activity of BMP-2 or BMP-4 in the ectoderm of the feather field by expressing them or their inhibitor noggin using retroviral vectors. These experiments demonstrate that BMP-2 is necessary and sufficient to mediate the lateral inhibition that positions buds in a tract. After buds are initiated, BMP-2 and BMP-4 continue to inhibit the adoption of bud fates and help to specify the size and shape of the bud. They may do so in part by their regulation of Fgf receptor expression in both the ectoderm and dermis. Additional insights into pattern formation in the feather bud can be inferred from the effects of altered BMP activity on bud morphogenesis.

Crouzon-like craniofacial dysmorphology in the mouse is caused by an insertional mutation at the Fgf3/Fgf4 locus

Retroviral insertional mutagenesis by means of ES cells has resulted in a new autosomal dominant mutation causing craniofacial dysmorphology in the mouse (Bulgy-eye, Bey). Heterozygous Bey mice are viable and fertile but show facial shortening with increased interorbital distance and precocious closure of several cranial sutures (craniosynostosis). These features provide a murine phenocopy for a large class of human craniofacial dysmorphology syndromes associated with craniosynostosis, particularly Crouzon syndrome. The retroviral vector integration responsible for the Bey mutation is inserted in the intragenic region between Fgf3 and Fgf4. Transcript analysis demonstrates that expression of both Fgf3 and Fgf4 is up-regulated in the cranial sutures of Bey mice. Many of these human craniosynostosis syndromes are caused by mutations in the extracellular domain of receptors for fibroblast growth factors that result in constitutive receptor activation. Our data confirm that fibroblast growth factor signalling pathways are involved in craniofacial development and suggest that some human malformation pedigrees or sporadic craniosynostosis may be caused by mutations that deregulate expression of the Fgf ligands.

Smad2 transduces common signals from receptor serine-threonine and tyrosine kinases

SMAD proteins mediate signals from receptor serine-threonine kinases (RSKs) of the TGF-beta superfamily. We demonstrate here that HGF and EGF, which signal through RTKs, can also mediate SMAD-dependent reporter gene activation and induce rapid phosphorylation of endogenous SMAD proteins by kinase(s) downstream of MEK1. HGF induces phosphorylation and nuclear translocation of epitope-tagged Smad2 and a mutation that blocks TGF-beta signaling also blocks HGF signal transduction. Smad2 may thus act as a common positive effector of TGF-beta- and HGF-induced signals and serve to modulate cross talk between RTK and RSK signaling pathways.

Fate map of the developing chick face: analysis of expansion of facial primordia and establishment of the primary palate

Developing facial primordia change shape substantially in stages leading up to primary palate formation. We investigated expansion of cell populations within each of the four facial primordia of chick embryos between HH-stages 20 and 28, by using DiI labelling. Populations of cells centred around the nasal pits in the upper face, the midline of the paired mandibular primordia in the lower face, and at sites of fusion contribute most to overall expansion. Abundant Msx-1 transcripts are found in regions of high expansion, and Fgf-8 transcripts are seen in ectoderm associated with some of these regions. Many cell populations display preferential expansion along one axis. Maxillary and mandibular primordia cell populations expand along the proximodistal axis, whereas at the distal tip of the frontonasal mass, cell populations expand mediolaterally. Thus outgrowth occurs at the tips of mandibular and maxillary primordia, but at the base of the frontonasal mass. At regions where adjacent primordia abut each other, we found bidirectional movement of cells between primordia, unidirectional movement or could detect no movement at all. Regions of highest expansion in each primordium have the highest percentage of S phase labelled cells. Cell death occurs in some regions of low expansion but it seems likely that cell rearrangements and intercalations also contribute to shaping. These rearrangements could be associated with stretching of the primordia by neighbouring tissues. Treatment of chick embryos with retinoic acid causes clefts of the primary palate (Tamarin et al. [1984] J. Embryol. Exp. Morphol. 84:105-123). We found a decrease in expansion of cell populations that normally contribute to primary palate formation but surprisingly little ectopic cell death. Expansion of other cell populations in the treated upper face was more even rather than directed. This further supports the idea that tension exerted by neighbouring tissues plays a major role in global shaping of the upper face.

Local inhibitory action of BMPs and their relationships with activators in feather formation: implications for periodic patterning

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

A splice variant of CD44 expressed in the apical ectodermal ridge presents fibroblast growth factors to limb mesenchyme and is required for limb outgrowth

Signals from the apical ectodermal ridge (AER) of the developing vertebrate limb, including fibroblast growth factor-8 (FGF-8), can maintain limb mesenchymal cells in a proliferative state. We report here that a specific CD44 splice variant is crucial for the proliferation of these mesenchymal cells. Epitopes carried by this variant colocalize temporally and spatially with FGF-8 in the AER throughout early limb development. A splice variant containing the same sequences expressed on model cells binds both FGF-4 and FGF-8 and stimulates mesenchymal cells in vitro. When applied to the AER, an antibody against a specific CD44 epitope blocks FGF presentation and inhibits limb outgrowth. Therefore, CD44 is necessary for limb development and functions in a novel growth factor presentation mechanism likely relevant in other physiological and pathological situations where a cell surface protein presents a signaling molecule to a neighboring cell.

Defects in limb, craniofacial, and thymic development in Jagged2 mutant mice

The Notch signaling pathway is a conserved intercellular signaling mechanism that is essential for proper embryonic development in numerous metazoan organisms. We have examined the in vivo role of the Jagged2 (Jag2) gene, which encodes a ligand for the Notch family of transmembrane receptors, by making a targeted mutation that removes a domain of the Jagged2 protein required for receptor interaction. Mice homozygous for this deletion die perinatally because of defects in craniofacial morphogenesis. The mutant homozygotes exhibit cleft palate and fusion of the tongue with the palatal shelves. The mutant mice also exhibit syndactyly (digit fusions) of the fore- and hindlimbs. The apical ectodermal ridge (AER) of the limb buds of the mutant homozygotes is hyperplastic, and we observe an expanded domain of Fgf8 expression in the AER. In the foot plates of the mutant homozygotes, both Bmp2 and Bmp7 expression and apoptotic interdigital cell death are reduced. Mutant homozygotes also display defects in thymic development, exhibiting altered thymic morphology and impaired differentiation of gamma delta lineage T cells. These results demonstrate that Notch signaling mediated by Jag2 plays an essential role during limb, craniofacial, and thymic development in mice.

Morphological diversity of the avian foot is related with the pattern of msx gene expression in the developing autopod

The formation of the digits in amniota embryos is accompanied by apoptotic cell death of the interdigital mesoderm triggered through BMP signaling. Differences in the intensity of this apoptotic process account for the establishment of the different morphological types of feet observed in amniota (i.e., free-digits, webbed digits, lobulated digits). The molecular basis accounting for the differential pattern of interdigital cell death remains uncertain since the reduction of cell death in species with webbed digits is not accompanied by a parallel reduction in the pattern of expression of bmp genes in the interdigital regions. In this study we show that the duck interdigital web mesoderm exhibits an attenuated response to both BMP-induced apoptosis and TGFbeta-induced chondrogenesis in comparison with species with free digits. The attenuated response to these signals is accompanied by a reduced pattern of expression of msx-1 and msx-2 genes. Local application of FGF in the duck interdigit expands the domain of msx-2 expression but not the domain of msx-1 expression. This change in the expression of msx-2 is followed by a parallel increase in spontaneous and exogenous BMP-induced interdigital cell death, while the chondrogenic response to TGFbetas is unchanged. The regression of AER, as deduced by the pattern of extinction of fgf-8 expression, takes place in a similar fashion in the chick and duck regardless of the differences in interdigital cell death and msx gene expression. Implantation of BMP-beads in the distal limb mesoderm induces AER regression in both the chick and duck. This finding suggests an additional role for BMPs in the physiological regression of the AER. It is proposed that the formation of webbed vs free-digit feet in amniota results from a premature differentiation of the interdigital mesoderm into connective tissue caused by a reduced expression of msx genes in the developing autopod.

Soluble dominant-negative receptor uncovers essential roles for fibroblast growth factors in multi-organ induction and patterning

Despite a wealth of experimental data implicating fibroblast growth factor (FGF) signaling in various developmental processes, genetic inactivation of individual genes encoding specific FGFs or their receptors (FGFRs) has generally failed to demonstrate their role in vertebrate organogenesis due to early embryonic lethality or functional redundancy. Here we show that broad mid-gestational expression of a novel secreted kinase-deficient receptor, specific for a defined subset of the FGF superfamily, caused agenesis or severe dysgenesis of kidney, lung, specific cutaneous structures, exocrine and endocrine glands, and craniofacial and limb abnormalities reminiscent of human skeletal disorders associated with FGFR mutations. Analysis of diagnostic molecular markers revealed that this soluble dominant-negative mutant disrupted early inductive signaling in affected tissues, indicating that FGF signaling is required for growth and patterning in a broad array of organs and in limbs. In contrast, transgenic mice expressing a membrane-tethered kinase-deficient FGFR were viable. Our results demonstrate that secreted FGFR mutants are uniquely effective as dominant-negative agents in vivo, and suggest that related soluble receptor isoforms expressed in wild-type mouse embryos may help regulate FGF activity during normal development.

Cytochemical localization of adenylate cyclase in the limb buds of Bufo bufo

The importance of cyclic nucleotides in the regulation of the processes of differentiation and embryonic development is known. The possible role that cyclic adenosine monophosphate (cAMP) plays during the development of the posterior limb of Bufo bufo is studied by the cytochemical localization of adenylate cyclase (AC), an enzyme that catalyzes the synthesis of the cyclic nucleotide. The method is based on the reaction between the enzyme AC and its specific substrate AMP-PNP (5'-adenylylimidodiphosphate) in the presence of lead. The lead precipitates that form as secondary reaction products are evidence of enzymatic activity. Reaction products are present only at the epithelial level in the limb bud; initially, such products are visible only at the base of the bud, particularly on the epithelial fascia located at the boundary with the body. During successive elongation and toe formation, AC activity is only present on the cells of the proximal portion of each new segment. Enzymatic activity is never present in correspondence to the ectodermal apical crest. cAMP is probably not involved in the processes of cellular proliferation but, rather, in the processes of inducing differentiation of the internal mesenchymal cells.

Integrated FGF and BMP signaling controls the progression of progenitor cell differentiation and the emergence of pattern in the embryonic anterior pituitary

The mechanisms by which inductive signals control the identity, proliferation and timing of differentiation of progenitor cells in establishing spatial pattern in developing vertebrate tissues remain poorly understood. We have addressed this issue in the embryonic anterior pituitary, an organ in which distinct hormone cell types are generated in a precise temporal and spatial order from an apparently homogenous ectodermal primordium. We provide evidence that in this tissue the coordinate control of progenitor cell identity, proliferation and differentiation is imposed by spatial and temporal restrictions in FGF- and BMP-mediated signals. These signals derive from adjacent neural and mesenchymal signaling centers: the infundibulum and ventral juxtapituitary mesenchyme. The infundibulum appears to have a dual signaling function, serving initially as a source of BMP4 and subsequently of FGF8. The ventral juxtapituitary mesenchyme appears to serve as a later source of BMP2 and BMP7. In vitro, FGFs promote the proliferation of progenitor cells, prevent their exit from the cell cycle and contribute to the specification of progenitor cell identity. BMPs, in contrast, have no apparent effect on cell proliferation but instead appear to act with FGFs to control the initial selection of thyrotroph and corticotroph progenitor identity.

Expression and function of FGFs-4, -8, and -9 suggest functional redundancy and repetitive use as epithelial signals during tooth morphogenesis

To elucidate the roles of fibroblast growth factors (FGF) in the regulation of tooth morphogenesis we have analyzed the expression patterns of Fgf-4, -8, and -9 in the developing mouse molar and incisor tooth germs from initiation to completion of morphogenesis by in situ hybridization analysis. The expression of these Fgfs was confined to dental epithelial cells at stages when epithelial-mesenchymal signaling regulates critical steps of tooth morphogenesis. Fgf-8 and Fgf-9 mRNAs were present in the oral epithelium of the first branchial arch at E10 and 1 day later expression became more restricted to the area of presumptive dental epithelium and persisted there until the start of epithelial budding. Fgf-8 mRNAs were not detected later in the developing tooth. Fgf-4 and Fgf-9 expression was upregulated in the primary enamel knot, which is a putative signaling center regulating tooth shape. Subsequently, Fgf-4 and Fgf-9 were expressed in the secondary enamel knots at the sites of tooth cusps. Fgf-9 expression spread from the primary enamel knot within the inner enamel epithelium where it remained until E18. In the continuously growing incisors Fgf-9 expression persisted in the epithelium of the cervical loops. The effects of FGFs were analyzed on the expression of the homeobox-containing transcription factors Msx-1 and Msx-2, which are associated with tissue interactions and regulated by the dental epithelium. Locally applied FGF-4, -8, and -9 stimulated intensely the expression of Msx-1 but not Msx-2 in the isolated dental mesenchyme. We suggest that the three FGFs act as epithelial signals mediating inductive interactions between dental epithelium and mesenchyme during several successive stages of tooth formation. This data suggest roles for FGF-8 and FGF-9 during initiation of tooth development, and for FGF-4 and FGF-9 during regulation of tooth shape. FGF-9 may also be involved in differentiation of odontoblasts. The coexpression of Fgfs with other signaling molecules including Shh and several Bmps and their partly similar effects suggest that the FGFs participate in the signaling networks during odontogenesis.

Fibroblast growth factor receptor 2 (FGFR2)-mediated reciprocal regulation loop between FGF8 and FGF10 is essential for limb induction

FGFR2 is a membrane-spanning tyrosine kinase that serves as a high affinity receptor for several members of the fibroblast growth factor (FGF) family. To explore functions of FGF/FGFR2 signals in development, we have mutated FGFR2 by deleting the entire immunoglobin-like domain III of the receptor. We showed that murine FGFR2 is essential for chorioallantoic fusion and placenta trophoblast cell proliferation. Fgfr2(DeltaIgIII/DeltaIgIII) embryos displayed two distinct defects that resulted in failures in formation of a functional placenta. About one third of the mutants failed to form the chorioallantoic fusion junction and the remaining mutants did not have the labyrinthine portion of the placenta. Consequently, all mutants died at 10–11 days of gestation. Interestingly, Fgfr2(DeltaIgIII/DeltaIgIII) embryos do not form limb buds. Consistent with this defect, the expression of Fgf8, an apical ectodermal factor, is absent in the mutant presumptive limb ectoderm, and the expression of Fgf10, a mesenchymally expressed limb bud initiator, is down regulated in the underlying mesoderm. These findings provide direct genetic evidence that FGF/FGFR2 signals are absolutely required for vertebrate limb induction and that an FGFR2 signal is essential for the reciprocal regulation loop between FGF8 and FGF10 during limb induction.

Apoptosis in the chick wing bud and the permanence of FGF-2 rescue

Two regions of programmed cell death that occur in the mesoderm of developing chick wing buds were studied in vitro. The opaque patch (OP) and posterior necrotic zone (PNZ) were examined for the presence of internucleosomal DNA degradation and for rescue by protein synthesis inhibition, two defining characteristics of apoptosis. Agarose gel electrophoresis showed that DNA from OP and PNZ tissue was cleaved into nucleosome size pieces and this cleavage was prevented by inhibition of protein synthesis with cycloheximide. Both regions showed rescue with cycloheximide as determined by the chromium release assay and examination of electron micrographs. Also, the permanence of basic fibroblast growth factor (EGF-2) rescue in the OP and NPZ was examined using the chromium release assay. While rescue in the OP was found to be permanent, rescue in the PNZ only delayed death while FGF-2 was present in the culture medium. This research shows that death in the OP and PNZ exhibits internucleosomal DNA fragmentation and is prevented by inhibition of protein synthesis with cycloheximide, biochemically characterizing this death as apoptosis. It also suggests that in vitro FGF-2 rescue is permanent in the OP but is merely a delay of cell death in the PNZ.

SMADs: mediators and regulators of TGF-beta signaling

The discovery of SMAD proteins has allowed the delineation of a mechanism by which TGF-beta and related growth factors convey their signals from membrane receptors all the way into the nucleus. SMADs are directly phosphorylated and activated by the receptors and then form heteromeric SMAD-SMAD complexes that move into the nucleus where they orchestrate transcriptional responses. In rapid succession, recent reports have identified different modes of SMAD regulation by phosphorylation and have defined the SMAD domains that mediate SMAD interactions, binding to DNA or transcriptional activation. The recent discovery of antagonistic SMADs and regulatory crosstalk with Ras/MAP-kinase pathways add to our rapidly expanding understanding of this major regulatory network.

Antagonistic effects of FGF4 on BMP induction of apoptosis and chondrogenesis in the chick limb bud

In an effort to define the roles of bone morphogenic proteins (BMPs) and fibroblast growth factors (FGFs) during chick limb development more closely, we have implanted beads impregnated with these growth factors into chick limb buds between stages 20 and 26. Embryos were sacrificed at the time the bone chondrocyte condensations first appear (stages 27-28). Implantation of beads containing BMPs at the earlier stages (20-22) caused apoptosis to occur, in the most severe cases leading to complete limb degeneration. Application of FGF4, either in the same, or in a different bead, prevented the BMP-induced apoptosis. We argue that the apoptosis observed on removal of the AER prior to stage 23 of development could be brought about by BMPs. The action of epithelial FGF in preventing BMP-mediated apoptosis in the mesenchyme would define a novel aspect of epithelial-mesenchymal interactions. Implanting the BMP4 beads into the core of the limb bud a day later (stages 25-26) caused intense chondrogenesis rather than apoptosis. FGF4 could again nullify this effect and by itself caused a reduction in bone size. This is the reverse of the functional relationship these growth factors have in mouse tooth specification (where it is BMP4 that inhibits the FGF8 function), and suggests that the balance between the effects of FGFs and BMPs could control the size of the chondrocyte precursor cell pool. In this way members of these two growth factor families could control the size of appendages when they are initially formed.

Bone morphogenetic protein signaling is required for maintenance of differentiated phenotype, control of proliferation, and hypertrophy in chondrocytes

To examine the role of bone morphogenetic protein (BMP) signaling in chondrocytes during endochondral ossification, the dominant negative (DN) forms of BMP receptors were introduced into immature and mature chondrocytes isolated from lower and upper portions of chick embryo sternum, respectively. We found that control sternal chondrocyte populations expressed type IA, IB, and II BMP receptors as well as BMP-4 and -7. Expression of a DN-type II BMP receptor (termed DN-BMPR-II) in immature lower sternal (LS) chondrocytes led to a loss of differentiated functions; compared with control cells, the DN-BMPR- II-expressing LS chondrocytes proliferated more rapidly, acquired a fibroblastic morphology, showed little expression of type II collagen and aggrecan genes, and upregulated type I collagen gene expression. Expression of DN-BMPR-II in mature hypertrophic upper sternal (US) chondrocytes caused similar effects. In addition, the DN-BMPR-II-expressing US cells exhibited little alkaline phosphatase activity and type X collagen gene expression, while the control US cells produced both alkaline phosphatase and type X collagen. Both DN-BMPR-II-expressing US and LS chondrocytes failed to respond to treatment with BMP-2 . When we examined the effects of DN forms of types IA and IB BMP receptors, we found that DN-BMPR-IA had little effect, while DN-BMPR-IB had similar but weaker effects compared with those of DN-BMPR-II. We conclude that BMP signaling, particularly that mediated by the type II BMP receptor, is required for maintenance of the differentiated phenotype, control of cell proliferation, and expression of hypertrophic phenotype.

Msx1 expressing mesoderm is important for the apical ectodermal ridge (AER)-signal transfer in chick limb development

The apical ectodermal ridge (AER) is a specialized thickening of the distal limb ectoderm, and its signals are known to support limb morphogenesis. The expression of a homeobox gene, Msx1, in the distal limb mesoderm depends on signals from the AER. In the present paper it is reported that Msx1 expression in the distal mesoderm is necessary for the transfer of AER signals in chick limb buds. Interruption of AER-mesoderm interaction by insertion of a thick filter led to the inhibition of pattern specification in the mesoderm just under the filter. In such cases, the expression of Msx1 disappeared in the mesoderm under the filter, suggesting that AER is able to signal over short ranges. In advanced limb buds, Msx1 is also expressed in the proximal mesoderm under the anterior ectoderm. However, it was found that a grafted antero-proximal mesoderm shows no inhibitory effects on pattern specification of the host mesoderm, as is the case with the distal mesoderm. On the other hand, grafted mesoderms without potent Msx1 re-expression, even underneath AER, disturbed normal limb development. In such cases, the expression of Msx1 disappeared in the mesoderm under the grafts, whereas Fgf-8 expression was maintained in the AER above the graft. These results indicate that the expression of Msx1 in the mesoderm is important for the transfer of AER signals.

Opposing BMP and EGF signalling pathways converge on the TGF-beta family mediator Smad1

The growth factor TGF-beta, bone morphogenetic proteins (BMPs) and related factors regulate cell proliferation, differentiation and apoptosis, controlling the development and maintenance of most tissues. Their signals are transmitted through the phosphorylation of the tumour-suppressor SMAD proteins by receptor protein serine/threonine kinases (RS/TKs), leading to the nuclear accumulation and transcriptional activity of SMAD proteins. Here we report that Smadl, which mediates BMP signals, is also a target of mitogenic growth-factor signalling through epidermal growth factor and hepatocyte growth factor receptor protein tyrosine kinases (RTKs). Phosphorylation occurs at specific serines within the region linking the inhibitory and effector domains of Smad1, and is catalysed by the Erk family of mitogen-activated protein kinases. In contrast to the BMP-stimulated phosphorylation of Smad1, which affects carboxy-terminal serines and induces nuclear accumulation of Smad1, Erk-mediated phosphorylation specifically inhibits the nuclear accumulation of Smad1. Thus, Smadl receives opposing regulatory inputs through RTKs and RS/TKs, and it is this balance that determines the level of Smad1 activity in the nucleus, and so possibly the role of Smad1 in the control of cell fate.