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

Synergistic effects of FGF and non-ridge ectoderm on gene expression involved in the formation of the anteroposterior axis of the chick limb bud in cell culture

Skeletal patterning of the vertebrate limb is controlled by the zone of polarizing activity (ZPA), apical ectodermal ridge (AER) and dorsal ectoderm. In the present study, to understand the involvement of fibroblast growth factor (FGF) and non-ridge ectoderm in anteroposterior (AP) axis formation, gene expression in chick limb bud mesenchymal cells in culture was investigated by reverse transcription-polymerase chain reaction and in situ hybridization. It was found that Shh expression was locally maintained in the mesenchymal cells underneath and near non-ridge ectoderm in coculture with the posterior mesenchymal cells and non-ridge ectoderm in the presence of FGF-4 by in situ hybridization. In Shh-expressing anterior limb bud mesenchymal cells cultured with non-ridge ectoderm, it was also discovered that Bmp-2 was activated in the presence of FGF-2, -4 and -8, while Hoxd-13 was activated in the presence of FGF-4 and that FGF-2 had a similar effect but FGF-8 did not. This result indicates that Hoxd-13 activation by SHH depends on non-ridge ectoderm and FGF-2 or FGF-4, and that there may be a difference in the effect on AP axis formation of the limb bud between FGF-2, -4 and -8. Possible roles of these genes and signal molecules in AP pattern formation are discussed.

The bHLH transcription factor dHAND controls Sonic hedgehog expression and establishment of the zone of polarizing activity during limb development

Limb outgrowth and patterning of skeletal elements are dependent on complex tissue interactions involving the zone of polarizing activity (ZPA) in the posterior region of the limb bud and the apical ectodermal ridge. The peptide morphogen Sonic hedgehog (SHH) is expressed specifically in the ZPA and, when expressed ectopically, is sufficient to mimic its functions, inducing tissue growth and formation of posterior skeletal elements. We show that the basic helix-loop-helix transcription factor dHAND is expressed posteriorly in the developing limb prior to Shh and subsequently occupies a broad domain that encompasses the Shh expression domain. In mouse embryos homozygous for a dHAND null allele, limb buds are severely underdeveloped and Shh is not expressed. Conversely, misexpression of dHAND in the anterior region of the limb bud of transgenic mice results in formation of an additional ZPA, revealed by ectopic expression of Shh and its target genes, and resulting limb abnormalities that include preaxial polydactyly with duplication of posterior skeletal elements. Analysis of mouse mutants in which Hedgehog expression is altered also revealed a feedback mechanism in which Hedgehog signaling is required to maintain the full dHAND expression domain in the developing limb. Together, these findings identify dHAND as an upstream activator of Shh expression and important transcriptional regulator of limb development.

Targeted misexpression of constitutively active BMP receptor-IB causes bifurcation, duplication, and posterior transformation of digit in mouse limb

Members of bone morphogenetic proteins (BMPs) play important roles in many aspects of vertebrate embryogenesis. In developing limbs, BMPs have been implicated in control of anterior-posterior patterning, outgrowth, chondrogenesis, and apoptosis. These diverse roles of BMPs in limb development are apparently mediated by different BMP receptors (BMPR). To identify the developmental processes in mouse limb possibly contributed by BMP receptor-IB (BMPR-IB), we generated transgenic mice misexpressing a constitutively active Bmpr-IB (caBmpr-IB). The transgene driven by the mouse Hoxb-6 promoter was ectopically expressed in the posterior mesenchyme of the forelimb bud, the lateral plate mesoderm, and the whole mesenchyme of the hindlimb bud. While the forelimbs appeared normal, the transgenic hindlimbs exhibited several phenotypes, including bifurcation, preaxial polydactyly, and posterior transformation of the anterior digit. However, the size of bones in the transgenic limbs seemed unaltered. Defects in sternum and ribs were also found. The bifurcation in the transgenic hindlimb occurred early in the limb development (E10.5) and was associated with extensive cell death in the mesenchyme and occasionally in the apical ectodermal ridge (AER). Sonic hedgehog (Shh) and Patched (Ptc) expression appeared unaffected in the transgenic limb buds, suggesting that the BMPR-IB mediated signaling pathway is downstream from Shh. However, ectopic Fgf4 expression was found in the anterior AER, which may account for the duplication of the anterior digit. An ectopic expression of Gremlin found in the transgenic limb bud would be responsible for the ectopic Fgf4 expression. The observations that Hoxd-12 and Hoxd-13 expression patterns were extended anteriorly provide a molecular basis for the posterior transformation of the anterior digit. Together these results suggest that BMPR-IB is the endogenous receptor to mediate the role of BMPs in anterior-posterior patterning and apoptosis in mouse developing limb. In addition, BMPR-IB may represent a critical component in the Shh/FGF4 feedback loop by regulating Gremlin expression.

Embryonic stem cell-derived chondrogenic differentiation in vitro: activation by BMP-2 and BMP-4

Differentiation of mouse embryonic stem (ES) cells via embryoid bodies was established as a suitable model to study development in vitro. Here, we show that differentiation of ES cells in vitro into chondrocytes can be modulated by members of the transforming growth factor-beta family (TGF-beta(1), BMP-2 and -4). ES cell differentiation into chondrocytes was characterized by the appearance of Alcian blue-stained areas and the expression of cartilage-associated genes and proteins. Different stages of cartilage differentiation could be distinguished according to the expression pattern of the transcription factor scleraxis, and the cartilage matrix protein collagen II. The number of Alcian-blue-stained areas decreased slightly after application of TGF-beta(1), whereas BMP-2 or -4 induced chondrogenic differentiation. The inducing effect of BMP-2 was found to be dependent on the time of application, consistent with its role to recruit precursor cells to the chondrogenic fate.

Influence of FGF4 on digit morphogenesis during limb development in the mouse

Much of what we currently know about digit morphogenesis during limb development is deduced from embryonic studies in the chick. In this study, we used ex utero surgical procedures to study digit morphogenesis during mouse embryogenesis. Our studies reveal some similarities; however, we have found considerable differences in how the chick and the mouse autopods respond to experimentation. First, we are not able to induce ectopic digit formation from interdigital cells as a result of wounding or TGFbeta-1 application in the mouse, in contrast to what is observed in the chick. Second, FGF4, which inhibits the formation of ectopic digits in the chick, induces a digit bifurcation response in the mouse. We demonstrate with cell marking studies that this bifurcation response results from a reorganization of the prechondrogenic tip of the digit rudiment. The FGF4 effect on digit morphogenesis correlates with changes in the expression of a number of genes, including Msx1, Igf2, and the posterior members of the HoxD cluster. In addition, the bifurcation response is digit-specific, being restricted to digit IV. We propose that FGF4 is an endogenous signal essential for skeletal branching morphogenesis in the mouse. This work stresses the existence of major differences between the chick and the mouse in how digit morphogenesis is regulated and is thus consistent with the view that vertebrate digit evolution is a relatively recent event. Finally, we discuss the relationship between the digit IV bifurcation restriction and the placement of the metapterygial axis in the evolution of the tetrapod limb.

Bone morphogenetic proteins regulate interdigital cell death in the avian embryo

The embryonic limb bud provides an excellent model for analyzing the mechanisms that regulate programmed cell death during development. At the time of digit formation in the developing autopod, the undifferentiated distal mesodermal cells may undergo or chondrogenic differentiation or apoptosis depending whether they are incorporated into the future digital rays or into the interdigital spaces. Both chondrogenesis or apoptosis are induced by local BMPS. However, whereas the chondrogenic-promoting activity of BMPs appears to be regulated through the BMPR-1b receptor, the mechanism by which the BMPs execute the death program remains unknown. The BMP proapoptotic activity requires the expression of members of the msx family of closely related homeobox-containing genes and is finally mediated by caspase activation, but the nature of the caspase(s) directly responsible for the cell death is also unknown. Finally, other growth factors present in the developing autopod at the stages of digit formation such as members of the FGF and TGF beta families modulate the ability of BMPs to induce cell death or chondrogenesis.

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

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

Expression of sprouty2 during early development of the chick embryo is coincident with known sites of FGF signalling

The Drosophila sprouty protein is a recently-identified intracellular modulator of FGF and EGF receptor tyrosine kinase activity which antagonises ras/MAP kinase signalling. In a differential display analysis to identify genes involved in patterning the mid/hindbrain region of the chick neural tube, we have identified a sprouty orthologue, sprouty2. Here we report expression of sprouty2 transcripts in the developing chick embryo. We find a close correlation with known sites of FGF activity but little correlation with expression patterns of members of the EGF family. Initially, transcripts are associated with the primitive streak. During the period of neural tube patterning expression is detected in the anterior neuropore, in the isthmic region and in neural plate and posterior spinal cord. Transcripts are also detected in the otic placode, tail bud, mesoderm of the branchial arches, somitic myotome, retina, limb buds and gut mesenchyme; all known sites of FGF action.

Apical epithelial cap morphology and fibronectin gene expression in regenerating axolotl limbs

Urodele amphibians (salamanders) are unique among adult vertebrates in their ability to regenerate limbs. The regenerated structure is often indistinguishable from the developmentally produced original. Thus, the two processes by which the limb is produced - development and regeneration - are likely to use many conserved biochemical and developmental pathways. Some of these limb features are also likely to be conserved across vertebrate families. The apical ectodermal ridge (AER) of the developing amniote limb and the larger apical epithelial cap (AEC) of the regenerating urodele limb are both found at the limb's distalmost tip and have been suggested to be functionally similar even though their morphology is quite different. Both structures are necessary for limb outgrowth. However, the AEC is uniformly smooth and thickly covers the entire limb-tip, unlike the AER, which is a protruding ridge covering only the dorsoventral boundary. Previous data from our laboratory suggest the multilayered AEC may be subdivided into separate functional compartments. We used hematoxylin and eosin (H+E) staining as well as in situ hybridization to examine the basal layer of the AEC, the layer that lies immediately over the distal limb mesenchyme. In late-stage regenerates, this basal layer expresses fibronectin (FN) message very strongly in a stripe of cells along the dorso-ventral boundary. H+E staining also reveals the unique shape of basal cells in this area. The stripe of cells in the basal AEC also contains the notch/groove structure previously seen in avian and reptilian AERs. In addition, AEC expression of FN message in the cells around the groove correlates with previous amniote AER localization of FN protein inside the groove. The structural and biochemical analyses presented here suggest that there is a specialized ridge-like compartment in the basal AEC in late-stage regenerates. The data also suggest that this compartment may be homologous to the AER of the developing amniote limb. Thus, the external differences between amniote limb development and urodele limb regeneration may be outweighed by internal similarities, which enable both processes to produce morphologically complete limbs. In addition, we propose that this basal layer of the AEC is uniquely responsible for AEC functions in regeneration, such as secreting molecules to promote mesenchymal cell cycling and dictating the direction of limb outgrowth. Finally, we include here a clarification of existing nomenclature to facilitate further discussion of the AEC and its basal layer.

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

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

Pattern formation and regulation of gene expressions in chick recombinant limbs

Recombinant limbs were performed by ensembling dissociated-reaggregated wing bud mesoderm inside an ectodermal hull. The zone of polarizing activity was excluded from the mesoderm used to perform the recombinant limbs (non-polarized recombinants), and grafted when desired (polarized recombinants). Reorganization of patterning progressively occurred in the newly formed progress zone under the influence of the apical ectodermal ridge (AER), explaining the proximo-distal gradient of morphogenesis observed in developed recombinant limbs. The AER, without the influence of the polarizing region (ZPA), was sufficient to direct outgrowth and appropriate proximo-distal patterning, as observed in the expression of the Hoxa-11 and Hoxa-13 genes. The development of the recombinant limbs coursed with symmetric AER and downregulation of Bmp expression in the mesoderm supporting a negative effect of Bmp signaling upon the apical ridge. The recombinant ectoderm maintained previously established compartments of gene expressions and organized a correct dorso-ventral patterning in the recombinant progress zone. Finally, the ZPA effect was only detected on Bmp expression and pattern formation along the antero-posterior axis.

Differential display of genes expressed at the midbrain - hindbrain junction identifies sprouty2: an FGF8-inducible member of a family of intracellular FGF antagonists

Specification and polarization of the midbrain and anterior hindbrain involve planar signals originating from the isthmus. Current evidence suggests that FGF8, expressed at the isthmus, provides this patterning influence. In this study, we have sought to identify novel genes which are involved in the process by which regional identity is imparted to midbrain and anterior hindbrain (rhombomere 1). An enhanced differential display reverse transcription method was used to clone cDNAs derived from transcripts expressed specifically in either rhombomere 1 or midbrain during the period of isthmic patterning activity. This gene expression screen identified 28 differentially expressed cDNAs. A clone upregulated in cDNA derived from rhombomere 1 tissue showed a 91% identity at the nucleotide level to the putative human receptor tyrosine kinase antagonist: sprouty2. In situ hybridization on whole chick embryos showed chick sprouty2 to be expressed initially within the isthmus and rhombomere 1, spatially and temporally coincident with Fgf8 expression. However, at later stages this domain was more extensive than that of Fgf8. Introduction of ligand-coated beads into either midbrain or hindbrain region revealed that sprouty2 could be rapidly induced by FGF8. These data suggest that sprouty2 participates in a negative feedback regulatory loop to modulate the patterning activity of FGF8 at the isthmus.

FGFR2 signaling in normal and limbless chick limb buds

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

Syndactyly of Ft/+ mice correlates with an imbalance in bmp4 and fgf8 expression

The most obvious phenotype of Ft/+ mice is a syndactyly of fore limbs characterised by a fusion of the tips of digits 1 to 4. The tempospatial expression of genes involved in limb development revealed that patterning of Ft/+ limb buds is not affected by the mutation. However, an upregulation of Bmp4 in the anterior-distal region of the limb bud at d12.0 of embryonic development is accompanied by a loss of Fgf8 expression in the distal part of the AER. Downstream target genes of Bmp action such as Msx1 and 2 are upregulated. This induction of the signalling cascade indicates ectopic expression of functional Bmp4. Nevertheless, analysis of physical parameters of bones from adult mice revealed a reduction of the bone mass of the autopod. The data suggest a negative effect of Bmp4 on Fgf8 expression and a positive influence on the induction of bone elements.

Control of vertebrate limb outgrowth by the proximal factor Meis2 and distal antagonism of BMPs by Gremlin

The mechanisms controlling growth and patterning along the proximal-distal axis of the vertebrate limb are yet to be understood. We show that restriction of expression of the homeobox gene Meis2 to proximal regions of the limb bud is essential for limb development, since ectopic Meis2 severely disrupts limb outgrowth. We also uncover an antagonistic relationship between the secreted factors Gremlin and BMPs required to maintain the Shh/FGF loop that regulates distal outgrowth. These proximal and distal factors have coordinated activities: Meis2 can repress distal genes, and Bmps and Hoxd genes restrict Meis2 expression to the proximal limb bud. Moreover, combinations of BMPs and AER factors are sufficient to distalize proximal limb cells. Our results unveil a novel set of proximal-distal regulatory interactions that establish and maintain outgrowth of the vertebrate limb.

The dominant hemimelia mutation uncouples epithelial-mesenchymal interactions and disrupts anterior mesenchyme formation in mouse hindlimbs

Epithelial-mesenchymal interactions are essential for both limb outgrowth and pattern formation in the limb. Molecules capable of communication between these two tissues are known and include the signaling molecules SHH and FGF4, FGF8 and FGF10. Evidence suggests that the pattern and maintenance of expression of these genes are dependent on a number of factors including regulatory loops between genes expressed in the AER and those in the underlying mesenchyme. We show here that the mouse mutation dominant hemimelia (Dh) alters the pattern of gene expression in the AER such that Fgf4, which is normally expressed in a posterior domain, and Fgf8, which is expressed throughout are expressed in anterior patterns. We show that maintenance of Shh expression in the posterior mesenchyme is not dependent on either expression of Fgf4 or normal levels of Fgf8 in the overlying AER. Conversely, AER expression of Fgf4 is not directly dependent on Shh expression. Also the reciprocal regulatory loop proposed for Fgf8 in the AER and Fgf10 in the underlying mesenchyme is also uncoupled by this mutation. Early during the process of limb initiation, Dh is involved in regulating the width of the limb bud, the mutation resulting in selective loss of anterior mesenchyme. The Dh gene functions in the initial stages of limb development and we suggest that these initial roles are linked to mechanisms that pattern gene expression in the AER.

Vertebrate Sprouty genes are induced by FGF signaling and can cause chondrodysplasia when overexpressed

The Drosophila sprouty gene encodes an antagonist of FGF and EGF signaling whose expression is induced by the signaling pathways that it inhibits. Here we describe a family of vertebrate Sprouty homologs and demonstrate that the regulatory relationship with FGF pathways has been conserved. In both mouse and chick embryos, Sprouty genes are expressed in intimate association with FGF signaling centers. Gain- and loss-of-function experiments demonstrate that FGF signaling induces Sprouty gene expression in various tissues. Sprouty overexpression obtained by infecting the prospective wing territory of the chick embryo with a retrovirus containing a mouse Sprouty gene causes a reduction in limb bud outgrowth and other effects consistent with reduced FGF signaling from the apical ectodermal ridge. At later stages of development in the infected limbs there was a dramatic reduction in skeletal element length due to an inhibition of chondrocyte differentiation. The results provide evidence that vertebrate Sprouty proteins function as FGF-induced feedback inhibitors, and suggest a possible role for Sprouty genes in the pathogenesis of specific human chondrodysplasias caused by activating mutations in Fgfr3.

Dlx-5 in limb initiation in the chick embryo

Dlx-5 is a vertebrate homolog of the Drosophila Distal-less gene, one of the first genetic signals for limb formation in the fly. In the present study we have explored the possible role of Dlx-5 in limb initiation in the chick embryo. At stage 14 which is well before the initial formation of limb buds Dlx-5 is highly and specifically expressed in the ectoderm of the presumptive wing and leg forming regions of the lateral plate, but not in the intervening non-limb forming prospective flank. Thus, Dlx-5 expression distinguishes the limb-forming territories prior to limb budding, and is one of the first molecular markers of vertebrate limb initiation. Furthermore, Dlx-5 expression is induced in the non-limb-forming flank within 12 hours after implantation of an FGF2-soaked bead, a procedure that results in the induction of an ectopic limb. The rapid induction of Dlx-5 expression in response to a signal which ultimately leads to supernumerary limb formation is consistent with a role for Dlx-5 in limb initiation. We have also examined the expression of Dlx-5 in the limb buds of amelic limbless mutant chick embryos, which undergo normal limb formation but do not form an AER and thus fail to undergo further outgrowth. Dlx-5 is transiently expressed by the ectoderm of emergent limbless limb buds, consistent with a role for Dlx-5 in limb initiation. Together, our results suggest that Dlx-5 may be involved in the specification of the limb territories of the lateral plate, and in the initial formation of the limb bud from these regions. Dev Dyn 1999;216:10-15.

RLIM inhibits functional activity of LIM homeodomain transcription factors via recruitment of the histone deacetylase complex

LIM domains are required for both inhibitory effects on LIM homeodomain transcription factors and synergistic transcriptional activation events. The inhibitory actions of the LIM domain can often be overcome by the LIM co-regulator known as CLIM2, LDB1 and NLI (referred to hereafter as CLIM2; refs 2-4). The association of the CLIM cofactors with LIM domains does not, however, improve the DNA-binding ability of LIM homeodomain proteins, suggesting the action of a LIM-associated inhibitor factor. Here we present evidence that LIM domains are capable of binding a novel RING-H2 zinc-finger protein, Rlim (for RING finger LIM domain-binding protein), which acts as a negative co-regulator via the recruitment of the Sin3A/histone deacetylase corepressor complex. A corepressor function of RLIM is also suggested by in vivo studies of chick wing development. Overexpression of the gene Rnf12, encoding Rlim, results in phenotypes similar to those observed after inhibition of the LIM homeodomain factor LHX2, which is required for the formation of distal structures along the proximodistal axis, or by overexpression of dominant-negative CLIM1. We conclude that Rlim is a novel corepressor that recruits histone deacetylase-containing complexes to the LIM domain.

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.

DIO-1 is a gene involved in onset of apoptosis in vitro, whose misexpression disrupts limb development

The DIO-1 (death inducer-obliterator-1) gene, identified by differential display PCR in pre-B WOL-1 cells undergoing apoptosis, encodes a putative transcription factor whose protein has two Zn finger motifs, nuclear localization signals, and transcriptional activation domains, expressed in the limb interdigitating webs during development. When overexpressed, DIO-1 translocates to the nucleus and activates apoptosis in vitro. Nuclear translocation as well as induction of apoptosis are lost after deletion of the nuclear localization sequences. DIO-1 apoptotic induction is prevented by caspase inhibitors and Bcl-2 overexpression. The in vivo role of DIO-1 was studied by misexpressing DIO-1 during chicken limb development. The most frequently observed phenotype was an arrest in limb outgrowth, an effect that correlates with the inhibition of mesodermal and ectodermal genes involved in this process. Our data demonstrate the ability of DIO-1 to trigger apoptotic processes in vitro and suggest a role for this gene in cell death during development.

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

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

EGF-dependent lobule formation and FGF7-dependent stalk elongation in branching morphogenesis of mouse salivary epithelium in vitro

When supplemented with appropriate growth factors, salivary gland epithelial explants isolated from mouse embryos undergo branching morphogenesis in vitro in the absence of mesenchyme. Epidermal growth factor (EGF) induces lobule formation, while fibroblast growth factor 7 (FGF7) promotes stalk elongation. A mixture of EGF and FGF7 produces an intermediate morphology, which resembles the branching pattern of salivary epithelium observed in vivo. To investigate how lobule formation and stalk elongation are related to the pattern of epithelial cell proliferation induced by EGF and FGF7, we performed a bromodeoxyuridine labeling study in whole-mount preparations. During the initial steps of lobule formation in EGF cultures, cleft and non-cleft regions had similar proliferative activity. However, once clefts had fully deepened, cells with low proliferative activity appeared at the bottom of the clefts. In contrast, during stalk elongation in FGF7 cultures, distal regions of the explants always showed higher proliferative activity than proximal regions. These results suggest that stalk elongation, but not cleft formation, may result from differential cell proliferation.

Tbx5 and Tbx4 genes determine the wing/leg identity of limb buds

Much progress has been made in understanding limb development. Most genes are expressed equally and in the same pattern in the fore- and hindlimbs, which nevertheless develop into distinct structures. The T-box genes Tbx5 and Tbx4, on the other hand, are expressed differently in chick wing (Tbx5) and leg (Tbx4) buds. Molecular analysis of the optomotor blind gene, which belongs to the same family of transcription factors, has revealed that this gene is involved in the transdetermination of Drosophila wing and leg imaginal discs. In addition, expression of Tbx5 and Tbx4 correlates well with the identity of ectopic limb buds induced by fibroblast growth factor. Thus, it is thought that Tbx5 and Tbx4 might be involved in determining limb identity. Another candidate is the Pitx1 gene, which encodes a bicoid-type homeodomain transcription factor that is expressed in leg buds. Here we determine the importance of these factors in establishing limb identity.

p63 is essential for regenerative proliferation in limb, craniofacial and epithelial development

The p63 gene, a homologue of the tumour-suppressor p53, is highly expressed in the basal or progenitor layers of many epithelial tissues. Here we report that mice homozygous for a disrupted p63 gene have major defects in their limb, craniofacial and epithelial development. p63 is expressed in the ectodermal surfaces of the limb buds, branchial arches and epidermal appendages, which are all sites of reciprocal signalling that direct morphogenetic patterning of the underlying mesoderm. The limb truncations are due to a failure to maintain the apical ectodermal ridge, a stratified epithelium, essential for limb development. The embryonic epidermis of p63-/- mice undergoes an unusual process of non-regenerative differentiation, culminating in a striking absence of all squamous epithelia and their derivatives, including mammary, lacrymal and salivary glands. Taken together, our results indicate that p63 is critical for maintaining the progenitor-cell populations that are necessary to sustain epithelial development and morphogenesis.

p63 is a p53 homologue required for limb and epidermal morphogenesis

The p53 tumour suppressor is a transcription factor that regulates the progression of the cell through its cycle and cell death (apoptosis) in response to environmental stimuli such as DNA damage and hypoxia. Even though p53 modulates these critical cellular processes, mice that lack p53 are developmentally normal, suggesting that p53-related proteins might compensate for the functions of p53 during embryogenesis. Two p53 homologues, p63 and p73, are known and here we describe the function of p63 in vivo. Mice lacking p63 are born alive but have striking developmental defects. Their limbs are absent or truncated, defects that are caused by a failure of the apical ectodermal ridge to differentiate. The skin of p63-deficient mice does not progress past an early developmental stage: it lacks stratification and does not express differentiation markers. Structures dependent upon epidermal-mesenchymal interactions during embryonic development, such as hair follicles, teeth and mammary glands, are absent in p63-deficient mice. Thus, in contrast to p53, p63 is essential for several aspects of ectodermal differentiation during embryogenesis.

Role of Pitx1 upstream of Tbx4 in specification of hindlimb identity

In spite of recent breakthroughs in understanding limb patterning, the genetic factors determining the differences between the forelimb and the hindlimb have not been understood. The genes Pitx1 and Tbx4 encode transcription factors that are expressed throughout the developing hindlimb but not forelimb buds. Misexpression of Pitx1 in the chick wing bud induced distal expression of Tbx4, as well as HoxC10 and HoxC11, which are normally restricted to hindlimb expression domains. Wing buds in which Pitx1 was misexpressed developed into limbs with some morphological characteristics of hindlimbs: the flexure was altered to that normally observed in legs, the digits were more toe-like in their relative size and shape, and the muscle pattern was transformed to that of a leg.

Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids

We describe here how the early limb bud of the quail embryo develops in the absence of retinoids, including retinoic acid. Retinoid-deficient embryos develop to about stage 20/21, thus allowing patterns of early gene activity in the limb bud to be readily examined. Genes representing different aspects of limb polarity were analysed. Concerning the anteroposterior axis, Hoxb-8 was up-regulated and its border was shifted anteriorly whereas shh and the mesodermal expression of bmp-2 were down-regulated in the absence of retinoids. Concerning the apical ectodermal genes, fgf-4 was down-regulated whereas fgf-8 and the ectodermal domain of bmp-2 were unaffected. Genes involved in dorsoventral polarity were all disrupted. Wnt-7a, normally confined to the dorsal ectoderm, was ectopically expressed in the ventral ectoderm and the corresponding dorsal mesodermal gene Lmx-1 spread into the ventral mesoderm. En-1 was partially or completely absent from the ventral ectoderm. These dorsoventral patterns of expression resemble those seen in En-1 knockout mouse limb buds. Overall, the patterns of gene expression are also similar to the Japanese limbless mutant. These experiments demonstrate that the retinoid-deficient embryo is a valuable tool for dissecting pathways of gene activity in the limb bud and reveal for the first time a role for retinoic acid in the organisation of the dorsoventral axis.

Sequential roles for Fgf4, En1 and Fgf8 in specification and regionalisation of the midbrain

Experiments involving tissue recombinations have implicated both early vertical and later planar signals in the specification and polarisation of the midbrain. Here we investigate the role of fibroblast growth factors in regulating these processes in the avian embryo. We show that Fgf4 is expressed in the notochord anterior to Hensen's node before transcripts for the earliest molecular marker of midbrain tissue in the avian embryo, En1, are detected. The presence of notochord is required for the expression of En1 in neural plate explants in vitro and FGF4 mimics this effect of notochord tissue. Subsequently, a second member of the fibroblast growth factor family, Fgf8, is expressed in the isthmus in a manner consistent with it providing a polarising signal for the developing midbrain. Using a retroviral vector to express En1 ectopically, we show that En1 can induce Fgf8 expression in midbrain and posterior diencephalon. Results of the introduction of FGF8 protein into the anterior midbrain or posterior diencephalon are consistent with it being at least part of the isthmic activity which can repolarise the former tissue and respecify the latter to a midbrain fate. However, the ability of FGF8 to induce expression of genes which have earlier onsets of expression than Fgf8 itself, namely En1 and Pax2, strongly suggests that the normal function of FGF8 is in maintaining patterns of gene expression in posterior midbrain. Finally, we provide evidence that FGF8 also provides mitogenic stimulation during avian midbrain development.

Fibroblast growth factor-8 expression is regulated by intronic engrailed and Pbx1-binding sites

Fibroblast growth factor-8 (FGF8) plays a critical role in vertebrate development and is expressed normally in temporally and spatially restricted regions of the vertebrate embryo. We now report on the identification of regions of Fgf8 important for its transcriptional regulation in murine ES cell-derived embryoid bodies. Stable transfection of ES cells, using a human growth hormone reporter gene, was employed to identify regions of the Fgf8 gene with promoter/enhancer activity. A 2-kilobase 5' region of Fgf8 was shown to contain promoter activity. A 0.8-kilobase fragment derived from the large intron of Fgf8 was found to enhance human growth hormone expressed from the Fgf8 promoter 3-4-fold in an orientation dependent manner. The intronic fragment contains DNA-binding sites for the AP2, Pbx1, and Engrailed transcription factors. Gel shift and Western blot experiments documented the presence of these transcription factors in nuclear extracts from ES cell embryoid bodies. In vitro mutagenesis of the Engrailed or Pbx1 site demonstrated that these sites modulate the activity of the intronic fragment. In addition, in vitro mutagenesis of both Engrailed and Pbx1 sites indicated that other unidentified sites are responsible for the transcriptional enhancement observed with the intronic fragment.

Expression of chicken fibroblast growth factor homologous factor (FHF)-1 and of differentially spliced isoforms of FHF-2 during development and involvement of FHF-2 in chicken limb development

Members of the fibroblast growth factor (FGF) family have been identified as signaling molecules in a variety of developmental processes, including important roles in limb bud initiation, growth and patterning. This paper reports the cloning and characterization of the chicken orthologues of fibroblast growth factor homologous factors-1 and −2 (cFHF-1/cFGF-12 and cFHF-2/cFGF-13, respectively). We also describe the identification of a novel, conserved isoform of FHF-2 in chickens and mammals. This isoform arises by alternative splicing of the first exon of the FHF-2 gene and is predicted to encode a polypeptide with a distinct amino-terminus. Whole-mount in situ hybridization reveals restricted domains of expression of cFHF-1 and cFHF-2 in the developing neural tube, peripheral sensory ganglia and limb buds, and shows that the two cFHF-2 transcript isoforms are present in non-overlapping spatial distributions in the neural tube and adjacent structures. In the developing limbs, cFHF-1 is confined to the posterior mesoderm in an area that encompasses the zone of polarizing activity and cFHF-2 is confined to the distal anterior mesoderm in a region that largely overlaps the progress zone. Ectopic cFHF-2 expression is induced adjacent to grafts of cells expressing Sonic Hedgehog and the zone of cFHF-2 expression is expanded in talpid2 embryos. In the absence of the apical ectodermal ridge or in wingless or limbless mutant embryos, expression of cFHF-1 and cFHF-2 is lost from the limb bud. A role for cFHF-2 in the patterning and growth of skeletal elements is implied by the observation that engraftment of developing limb buds with QT6 cells expressing a cFHF-2 isoform that is normally expressed in the limb leads to a variety of morphological defects. Finally, we show that a secreted version of cFHF-2 activates the expression of HoxD13, HoxD11, Fgf-4 and BMP-2 ectopically, consistent with cFHF-2 playing a role in anterior-posterior patterning of the limb.

Fgf10 is essential for limb and lung formation

The interactions between fibroblast growth factors (FGF) and their receptors have important roles in mediating mesenchymal-epithelial cell interactions during embryogenesis. In particular, Fgf10 is predicted to function as a regulator of brain, lung and limb development on the basis of its spatiotemporal expression pattern in the developing embryo. To define the role of Fgf10, we generated Fgf10-deficient mice. Fgf10-/- mice died at birth due to the lack of lung development. Trachea was formed, but subsequent pulmonary branching morphogenesis was disrupted. In addition, mutant mice had complete truncation of the fore- and hindlimbs. In Fgf10-/- embryos, limb bud formation was initiated but outgrowth of the limb buds did not occur; however, formation of the clavicles was not affected. Analysis of the expression of marker genes in the mutant limb buds indicated that the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA) did not form. Thus, we show here that Fgf10 serves as an essential regulator of lung and limb formation.

The development of the paired fins in the zebrafish (Danio rerio)

In the present study, we describe the structure and normal development of the zebrafish (Danio rerio) paired fins. Particularly, we focus on the structure of the apical epidermis and on endoskeletal morphogenesis. Endoskeletal development proceeds differently in the pectoral and pelvic fins. Whereas in both fins major parts of the endoskeletal girdle develop within the fin bud mesenchyme, the pattern of chondrogenic condensations observed in the pelvic fins directly reflects the adult endoskeletal pattern. In the pectoral fin, a morphogenetic detour is taken via a functional larval endoskeleton, the endoskeletal disc. It arises in the fin bud mesenchyme from a chondrogenic anlage common with the girdle. The disc chondrifies and represents the functional endoskeleton of the larval pectoral fin. The pectoral fin endoskeleton is expanded as well as restructured during larval stages in a process which involves decomposition of cartilage matrix in the endoskeletal disc. Our comparisons of apical fold morphology with reports on other teleosts and tetrapod apical ridges show them to be homologous on the structural level. Comparisons of endoskeletal development of the zebrafish with reports on teleosts, actinopterygians and chondrichthyans show that endoskeletal morphogenesis in the zebrafish pectoral fin follows a morphogenetic process which is wide-spread among actinopterygians.

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.

The role of Alx-4 in the establishment of anteroposterior polarity during vertebrate limb development

We have determined that Strong's Luxoid (lstJ) [corrected] mice have a 16 bp deletion in the homeobox region of the Alx-4 gene. This deletion, which leads to a frame shift and a truncation of the Alx-4 protein, could cause the polydactyly phenotype observed in lstJ [corrected] mice. We have cloned the chick homologue of Alx-4 and investigated its expression during limb outgrowth. Chick Alx-4 displays an expression pattern complementary to that of shh, a mediator of polarizing activity in the limb bud. Local application of Sonic hedgehog (Shh) and Fibroblast Growth Factor (FGF), in addition to ectodermal apical ridge removal experiments suggest the existence of a negative feedback loop between Alx-4 and Shh during limb outgrowth. Analysis of polydactylous mutants indicate that the interaction between Alx-4 and Shh is independent of Gli3, a negative regulator of Shh in the limb. Our data suggest the existence of a negative feedback loop between Alx-4 and Shh during vertebrate limb outgrowth.

The expression and regulation of chick EphA7 suggests roles in limb patterning and innervation

Eph receptors and their ligands, the ephrins, have been implicated in early patterning and axon guidance in vertebrate embryos. Members of these families play pivotal roles in the formation of topographic maps in the central nervous system, the formation of brain commissures, and in the guidance of neural crest cells and motor axons through the anterior half of the somites. Here, we report a highly dynamic expression pattern of the chick EphA7 gene in the developing limb. Expression is detected in discrete domains of the dorsal mesenchyme from 3 days of incubation. The expressing cells are adjacent to the routes where axons grow to innervate the limb at several key points: the region of plexus formation, the bifurcation between dorsal and ventral fascicles, and the pathway followed by axons innervating the dorsal muscle mass. These results suggested a role for EphA7 in cell-cell contact-mediated signalling in dorsal limb patterning and/or axon guidance. We carried out experimental manipulations in the chick embryo wing bud to alter the dorsoventral patterning of the limb. The analyses of EphA7 expression and innervation in the operated wings indicate that a signal emanating from the dorsal ectoderm regulates EphA7 in such a way that, in its absence, the wing bud lacks EphA7 expression and shows innervation defects at the regions where the gene was downregulated. EphA7 downregulation in the dorsal mesenchyme after dorsal ectoderm removal is more rapid than that of Lmx-1, the gene known to mediate dorsalisation in response to the ectodermal signal. These results add a new gene to the dorsalisation signalling pathway in the limb. Moreover, they implicate the Eph receptor family in the patterning and innervation of the developing limb, extending its role in axon pathfinding to the distal periphery.

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.

Lhx2, a vertebrate homologue of apterous, regulates vertebrate limb outgrowth

apterous specifies dorsal cell fate and directs outgrowth of the wing during Drosophila wing development. Here we show that, in vertebrates, these functions appear to be performed by two separate proteins. Lmx-1 is necessary and sufficient to specify dorsal identity and Lhx2 regulates limb outgrowth. Our results suggest that Lhx2 is closer to apterous than Lmx-1, yet, in vertebrates, Lhx2 does not specify dorsal cell fate. This implies that in vertebrates, unlike Drosophila, limb outgrowth can be dissociated from the establishment of the dorsoventral axis.

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

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

Roles of transforming growth factor-alpha and epidermal growth factor in chick limb development

We have examined the distribution of transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and the chicken EGF receptor (c-erbB), in embryonic chick limbs. Prior to limb budding, TGF-alpha is present in prospective limb-forming mesoderm and in prospective apical ectodermal ridge (AER)-forming ectoderm, but is not detected in non-limb-forming flank mesoderm or ectoderm, nor in presumptive non-AER-forming limb ectoderm, suggesting possible roles in initial limb formation and AER induction. Consistent with this possibility, TGF-alpha is present in the mesoderm of the wing buds of the amelic chick mutants limbless and wingless, which form and bud normally, but is absent from limbless and wingless ectoderm, which fails to form an AER. TGF-alpha and EGF are present in the AER of the developing limb, and TGF-alpha, EGF, and c-erbB are present in the underlying subridge mesoderm, suggesting possible roles in reciprocal AER/subridge mesoderm interactions required for limb outgrowth. We found that exogenous TGF-alpha and EGF can promote the outgrowth of limb mesoderm in the absence of the AER in vitro and can also promote the outgrowth of limbless and wingless wing bud explants. EGF is present in ventral but not dorsal limb ectoderm, suggesting a role for EGF in specification of ventral ectoderm. TGF-alpha and EGF are not detected in the differentiating cartilaginous elements or muscle primordia of the limb, suggesting that cessation of TGF-alpha and EGF expression may be required for cartilage and muscle formation. We have found that exogenous TGF-alpha and EGF inhibit chondrogenesis and myogenesis of limb mesenchyme in vitro. Together these results indicate that signaling through the EGF receptor via endogenous TGF-alpha and EGF may be important for initial limb formation, AER induction, outgrowth of limb mesoderm, and regulation of limb chondrogenic and myogenic differentiation.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

FGFR-3 and FGFR-4 function cooperatively to direct alveogenesis in the murine lung

Mammalian lungs begin as an outpocket of the foregut, and depend on multiple stages of branching morphogenesis and alveogenesis to reach their final form. An examination of fgf receptor gene expression indicated that all four receptors (fgfr-1 to fgfr-4) are expressed in postnatal lungs at varying levels. We show that mice homozygous for a targeted mutation of fgfr-4 exhibited no overt abnormalities in the lungs or any other organ. However, mice doubly homozygous for disruptions of the fgfr-3 and fgfr-4 genes display novel phenotypes not present in either single mutant, which include pronounced dwarfism and lung abnormalities. Lungs of fgfr-3(−/−)fgfr-4(−/−)animals, which are normal at birth, are completely blocked in alveogenesis and do not form secondary septae to delimit alveoli. Consequently, air spaces in the lung are expanded and no alveoli can be seen. The mutant lungs failed to downregulate postnatal elastin deposition despite their normal levels of surfactant expression and cell proliferation. These data revealed a cooperative function of FGFR-3 and FGFR-4 to promote the formation of alveoli during postnatal lung development.

Spatio-temporal expression of FGFR 1, 2 and 3 genes during human embryo-fetal ossification

Mutations in FGFR 1-3 genes account for various human craniosynostosis syndromes, while dwarfism syndromes have been ascribed exclusively to FGFR 3 mutations. However, the exact role of FGFR 1-3 genes in human skeletal development is not understood. Here we describe the expression pattern of FGFR 1-3 genes during human embryonic and fetal endochondral and membranous ossification. In the limb bud, FGFR 1 and FGFR 2 are initially expressed in the mesenchyme and in epidermal cells, respectively, but FGFR 3 is undetectable. At later stages, FGFR 2 appears as the first marker of prechondrogenic condensations. In the growing long bones, FGFR 1 and FGFR 2 transcripts are restricted to the perichondrium and periosteum, while FGFR 3 is mainly expressed in mature chondrocytes of the cartilage growth plate. Marked FGFR 2 expression is also observed in the periarticular cartilage. Finally, membranous ossification of the skull vault is characterized by co-expression of the FGFR 1-3 genes in preosteoblasts and osteoblasts. In summary, the simultaneous expression of FGFR 1-3 genes in cranial sutures might explain their involvement in craniosynostosis syndromes, whereas the specific expression of FGFR 3 in chondrocytes does correlate with the involvement of FGFR 3 mutations in inherited defective growth of human long bones.

Defect in the maintenance of the apical ectodermal ridge in the Dactylaplasia mouse

During vertebrate limb development the distal apex of the limb bud ectoderm is induced to form the apical ectodermal ridge (AER). The presence of the AER is required for the continued outgrowth of the limb bud. Classical embryological studies have led to the hypothesis that a secreted mesenchymal factor is required to maintain the AER. We have undertaken a detailed analysis of Dactylaplasia (Dac) mice, a semidominant mutant which displays missing central digits in the fore- and hindlimbs of heterozygous animals and monodactyly in homozygous animals. Our data show that Dac mice have a defect in the maintenance of the AER. At E10.5, the mutant AER is found to be morphologically normal. However, by E11.5 the central aspect of the AER degenerates leaving the anterior and posterior AER intact. In homozygous mice both the central and anterior AER degenerate, while the posterior extremity of the AER is unaffected. Analysis of BrdU incorporation reveals that degeneration of the AER is due to a lack of cell proliferation in the mutant AER. The loss of the AER leads to a reduction in cell proliferation in the subridge mesenchyme at E11.5. The data represent direct genetic evidence for the existence of an AER maintenance activity that is distinct from AER induction and differentiation. Moreover, the data suggest that the role of the AER maintenance factor is to promote cell proliferation in the ridge. Based on our findings, we propose a model for AER maintenance in the vertebrate limb.

Pitx2 determines left-right asymmetry of internal organs in vertebrates

The handedness of visceral organs is conserved among vertebrates and is regulated by asymmetric signals relayed by molecules such as Shh, Nodal and activin. The gene Pitx2 is expressed in the left lateral plate mesoderm and, subsequently, in the left heart and gut of mouse, chick and Xenopus embryos. Misexpression of Shh and Nodal induces Pitx2 expression, whereas inhibition of activin signalling blocks it. Misexpression of Pitx2 alters the relative position of organs and the direction of body rotation in chick and Xenopus embryos. Changes in Pitx2 expression are evident in mouse mutants with laterality defects. Thus, Pitx2 seems to serve as a critical downstream transcription target that mediates left-right asymmetry in vertebrates.

Differential regulation of T-box and homeobox transcription factors suggests roles in controlling chick limb-type identity

The wing and the leg of the chick, although homologous structures, have characteristic patterns of skeletal elements, muscles, tendons, featherbuds and scales. Despite recent advances in understanding the common genetic pathways patterning the wing and leg, the molecular nature of the specification of limb-type identity has remained elusive. Embryological experiments have indicated the existence of limb-specific territories in the flank. In the newt, deviation of nerves from the limb into the flank can induce ectopic limbs to form from this tissue. In the chick, Fibroblast growth factor (FGF)-soaked beads applied to the flank can induce ectopic formation of limbs from the surrounding tissue. In both cases, the type of limb that forms, either a wing/forelimb or leg/hindlimb, is dependent upon the location to which the limb-inducing signal is applied. We have isolated and characterised three candidate genes for controlling limb identity in the chick. Two T-box transcription factors, cTbx4 and cTbx5, are expressed in a restricted manner in the leg bud and wing buds, respectively. cPtx1, a member of the Otx-related subclass of paired-type homeodomain proteins, is expressed exclusively in the leg bud. Using FGF to induce ectopic limb buds of wing, leg and intermediate identity, we show that early expression of cTbx5, cTbx4 and cPtx1 in the induced limb buds correlates with later wing- or leg-type identity of ectopic limbs. We observe a general correlation between the location of an ectopic outgrowth induced by FGF and the identity of the resulting limb but, significantly, we report that there is no definitive rostral-caudal level that divides the ectopic wing and leg territories.

Shh, Bmp-2 and Hoxd-13 gene expression in chick limb bud cells in culture

The anteroposterior axis of the vertebrate limb bud is determined by signals from the zone of polarizing activity (ZPA). Sonic hedgehog (Shh) is expressed in the posterior mesoderm, which corresponds closely to ZPA activity. Moreover, Bmp-2 and HoxD genes are expressed in the broader posterior mesoderm, and it is thought that the ZPA signaling pathway consists of these gene products. Limb outgrowth and patterning, including expression of these genes, depend on the apical ectodermal ridge (AER). Fibroblast growth factors (FGF) have been identified as candidates for signal molecules from the AER. To further understand the ZPA signaling pathway and the participation of FGF, expressions of these genes were examined by reverse transcription-polymerase chain reaction in chick limb bud cells cultured with FGF-4. The present results indicate that FGF-4 cannot maintain Shh expression but can maintain Hoxd-13 expression in cultured posterior cells; moreover, Bmp-2 is expressed independently of FGF-4. These results suggest that Bmp-2 and Hoxd-13 expressions do not require a continuous expression of Shh. Further, it was demonstrated that posterior cells cultured with FGF-4 recovered Shh expression when grafted to the limb bud, indicating that FGF-4 maintains not Shh expression itself but competence of Shh expression.

Morphogenesis of digits in the avian limb is controlled by FGFs, TGFbetas, and noggin through BMP signaling

In the final stages of limb morphogenesis, autopodial cells leaving the progress zone differentiate into cartilage or undergo apoptotic cell death, depending on whether they are incorporated into the digital rays or interdigital spaces. Most evidence indicates that these two opposite fates of the autopodial mesoderm are controlled by BMP signaling. However, the molecular basis for these two distinct actions of BMPs, including the receptors involved in the process, is controversial. In this study we have addressed this question by exploring the presence in the developing autopod of diffusible signals able to modulate BMP function and by analyzing the effects of their exogenous administration on the pattern of expression of BMP receptor genes. Our findings show that tgfbeta2 and noggin genes are expressed in the condensing region of the developing digital rays in addition to the well-known distribution in the autopodial tissues of FGFs (apical ectodermal ridge, AER) and BMPs (AER, progress zone mesoderm, and interdigital regions). Exogenous administration of all the factors causes changes in the expression of the bmpR-1b gene which are followed by parallel alterations of the skeletal phenotype: FGFs inhibit the expression of bmpR-1b compatible with their function in the maintenance of the progress zone mesoderm in an undifferentiated state; and TGFbetas induce the expression of bmpR-1b and promote ectopic chondrogenesis, compatible with a function in the establishment of the position of the digital rays. In addition we provide evidence for the occurrence of an interactive loop between BMPs and noggin accounting for the spatial distribution of bmpR-1b which may control the size and shape of the skeletal pieces. In contrast to the bmpR-1b gene, the bmpR-1a gene is expressed at low levels in the autopodial mesoderm and its expression is not modified by any of the tested factors regardless of their effects on chondrogenesis or cell death. Finally, the role of BMPs in programmed cell death is confirmed here by the intense inhibitory effect of noggin on apoptosis, but the lack of correlation between changes in the pattern of cell death induced by treatment with the studied factors and the expression of either bmpR-1a or bmpR-1b genes suggest that a still-unidentified BMP receptor may account for this BMP function.