Retroviral expression of FGF-2 (bFGF) affects patterning in chick limb bud

Fibroblast growth factor 2

Fibroblast Growth Factor 2 (FGF2), also known as basic fibroblast growth factor, is a potent stimulator of growth and differentiation in multiple tissues. Its discovery traces back over 50 years ago when it was first isolated from bovine pituitary extracts due to its ability to stimulate fibroblast proliferation. Subsequent studies investigating the genomic structure of FGF2 identified multiple protein isoforms, categorized as the low molecular weight and high molecular weight FGF2. These isoforms arise from alternative translation initiation events and exhibit unique molecular and cellular functions. In this concise review, we aim to provide an overview of what is currently known about the structure, expression, and functions of the FGF2 isoforms within the contexts of development, homeostasis, and disease.

On the Formation of Digits and Joints during Limb Development

Critical steps in forming the vertebrate limb include the positioning of digits and the positioning of joints within each digit. Recent studies have proposed that the iterative series of digits is established by a Turing-like mechanism generating stripes of chondrogenic domains. However, re-examination of available data suggest that digits are actually patterned as evenly spaced spots, not stripes, which then elongate into rod-shaped digit rays by incorporating new cells at their tips. Moreover, extension of the digit rays and the patterning of the joints occur simultaneously at the distal tip, implying that an integrated model is required to fully understand these processes.

Coadministration of basic fibroblast growth factor-loaded polycaprolactone beads and autologous myoblasts in a dog model of fecal incontinence

PURPOSE

Basic fibroblastic growth factor (bFGF), a member of the heparin-binding growth factor family, regulates muscle differentiation. We investigated whether coadministration of autologous myoblasts and bFGF-loaded polycaprolactone beads could improve sphincter recovery in a dog model of fecal incontinence (FI).

METHODS

FI was induced by resecting 25% of the posterior anal sphincter in ten mongrel dogs. One month later, the dogs were randomized to receive either PKH-26-labeled autologous myoblasts alone (M group, five dogs) or autologous myoblasts and bFGF-loaded polycaprolactone beads (MBG group, five dogs). The outcomes included anal manometry, compound muscle action potentials (CMAPs) of the pudendal nerve, and histology.

RESULTS

The increase in anal contractile pressure over 3 months was significantly greater in the MBG group (from 4.85 to 6.83 mmHg) than that in the M group (from 4.94 to 4.25 mmHg), with a coefficient for the difference in recovery rate of 2.672 (95% confidence interval [CI] 0.962 to 4.373, p = 0.002). The change in the CMAP amplitude was also significantly greater in the MBG group (from 0.59 to 1.56 mV) than that in the M group (from 0.81 to 0.67 mV) (coefficient 1.114, 95% CI 0.43 to 1.80, p = 0.001). Labeled cells were detected in 2/5 (40%) and 5/5 (100%) dogs in the M and MBG groups, respectively.

CONCLUSION

Coadministration of bFGF-loaded PCL beads and autologous myoblasts improved the recovery of sphincter function in a dog model of FI and had better outcomes than cell-based therapy alone.

Biphasic effects of FGF2 on adipogenesis

Although stem cells from mice deficient of FGF2 have been reported to display enhanced capacity for adipogenesis, the literature using in vitro cell culture system has so far reported conflicting results on the role of FGF2 in adipogenesis. We here demonstrate that FGF2, depending on concentration, can function as either a positive or negative factor of in vitro adipogenesis by regulating activation of the ERK signaling pathway. FGF2 at concentrations lower than 2 ng/ml enhanced in vitro adipogenesis of human adipose-derived stem cells (hASCs). However, FGF2 at concentrations higher than 10 ng/ml was able to suppress adipogenesis by maintaining sustained phosphorylation of ERK and function as a dominant negative adipogenic factor toward BMP ligands. Expression levels of FGF2 in the fat tissues from high fat diet induced obese C57BL/6 mice were lower than those from normal chow diet mice, indicating that expression levels of FGF2 in the fat tissues might be in reverse correlation with the size of fat tissues. Our observation of concentration dependent biphasic effect as well as dominant negative effect of FGF2 on adipogenesis provides a mechanistic basis to understand roles of FGF2 in adipogenesis and development of fat tissues.

Aberrant FGF signaling, independent of ectopic hedgehog signaling, initiates preaxial polydactyly in Dorking chickens

The formation of supernumerary digits, or polydactyly, is a common congenital malformation. Although mutations in a number of genes have been linked to polydactyly, the molecular etiology for a third of human disorders with polydactyly remains unknown. To increase our understanding of the potential causes for polydactyly, we characterized a spontaneous chicken mutant, known as Dorking. The hindlimbs of Dorkings form a preaxial supernumerary digit. During the early stages of limb development, ectopic expression of several genes, including Sonic Hedgehog (Shh) and Fibroblast Growth Factor 4 (Fgf4), was found in Dorking hindlimbs. In addition to ectopic gene expression, a decrease in cell death in the anterior of the developing Dorking hindlimb was observed. Further molecular investigation revealed that ectopic Fgf4 expression was initiated and maintained independent of ectopic Shh. Additionally, inhibition of Fgf signaling but not hedgehog signaling was capable of restoring the normal anterior domain of cell death in Dorking hindlimbs. Our data indicates that in Dorking chickens, preaxial polydactyly is initiated independent of Shh.

A computational model of FGF-2 binding and HSPG regulation under flow

A novel convection-diffusion-reaction model is developed to simulate fibroblast growth factor (FGF-2) binding to cell surface receptors (FGFRs) and heparan sulfate proteoglycans (HSPGs) under flow conditions within a cylindrical-shaped vessel or capillary. The model consists of a set of coupled nonlinear partial differential equations (PDEs) and a set of coupled nonlinear ordinary differential equations (ODEs). The time-dependent PDE system is discretized and solved by a second-order implicit Euler scheme using the finite volume method. The ODE system is solved by a stiff ODE solver VODE using backward differencing formulation (BDF). The transient solution of FGF-2, FGFR, HSPG, and their bound complexes for three different flow rates are computed and presented. Simulation results indicate that the model can predict growth factor transport and binding to receptors with/without the presence of heparan sulfate, as well as the effect of flow rate on growth factor-receptor binding. Our computational model may provide a useful means to investigate the impact of fluid flow on growth factor dynamics, and ultimately, signaling within the circulation.

Differential regulation of GDF-5 and FGF-2/4 by immobilisation in ovo exposes distinct roles in joint formation

Members of the fibroblast growth factor (FGF) family and growth and differentiation factor 5 (GDF-5) have been implicated in joint specification, but their roles in subsequent cavity formation are not defined. Cavity formation (cavitation) depends upon limb movement in embryonic chicks and factors involved in joint formation are often identified by their expression at the joint-line. We have sought support for the roles of FGF-2, FGF-4, and GDF-5 in cavitation by defining expression patterns, immunohistochemically, during joint formation and establishing whether these are modified by in ovo immobilisation. We found that FGF-2 exhibited low level nuclear expression in chondrocytes and fibrocartilage cells close to presumptive joints, but showed significantly higher expression levels in cells at, and directly bordering, the forming joint cavity. This high-level joint line FGF-2 expression was selectively diminished in immobilised limbs. In contrast, we show that FGF-4 does not exhibit differential joint-line expression and was unaffected by immobilisation. GDF-5 protein also failed to show joint-line selective labelling, and although immobilisation induced a cartilaginous fusion across presumptive joints, it did not affect cellular GDF-5 expression patterns. Examining changes in GDF-5 expression in response to a direct mechanical strain stimulus in primary embryonic chick articular surface (AS) cells in vitro discloses only small mechanically-induced reductions in GDF-5 expression, suggesting that GDF-5 does not exert a direct positive contribution to the mechano-dependent joint cavitation process. This notion was supported by retroviral overexpression of UDPGD, a characteristic factor involved in hyaluronan (HA) accumulation at presumptive joint lines, which was also found to produce small decreases in AS cell GDF-5 expression. These findings support a direct mechano-dependent role for FGF-2, but not FGF-4, in the cavitation process and indicate that GDF-5 is likely to influence chondrogenesis positively without contributing directly to joint cavity formation.

Region-specific expression ofmario reveals pivotal function of the anterior nondigit region on digit formation in chick wing bud

We report the region-specific expression of a novel gene, named mario, whose expression domain is in the distal tip of the presumptive and developing digit 2 region in the developing chick wing bud. The anterior region-specific expression of mario corresponds well with the presence of digit 2, and fate map analysis showed that mario expression at early stages represents the presumptive digit 2 region. Using mario expression as a region-specific marker for the digit 2 region, several surgical operations were performed to obtain insights into digit 2 development in the chick wing. Cell fate tracing concomitant with a zone of polarizing activity (ZPA) implantation revealed that an additional digit 2 in the ZPA implantation into the anterior or middle region of wing bud is derived from the original digit 2 region (mario-positive region). Surgical manipulations revealed that the anterior nondigit region has an inhibitory effect on digit 2 formation. Taken together, these results suggest that the most-anterior region, including the anterior necrotic zone, restricts the position of digit 2 region by limiting the anterior border of the digit 2 region and preventing its expansion.

Ventral axial organs regulate expression of myotomal Fgf-8 that influences rib development

Fgf-8 encodes a secreted signaling molecule mediating key roles in embryonic patterning. This study analyzes the expression pattern, regulation, and function of this growth factor in the paraxial mesoderm of the avian embryo. In the mature somite, expression of Fgf-8 is restricted to a subpopulation of myotome cells, comprising most, but not all, epaxial and hypaxial muscle precursors. Following ablation of the notochord and floor plate, Fgf-8 expression is not activated in the somites, in either the epaxial or the hypaxial domain, while ablation of the dorsal neural tube does not affect Fgf-8 expression in paraxial mesoderm. Contrary to the view that hypaxial muscle precursors are independent of regulatory influences from axial structures, these findings provide the first evidence for a regulatory influence of ventral, but not dorsal axial structures on the hypaxial muscle domain. Sonic hedgehog can substitute for the ventral neural tube and notochord in the initiation of Fgf-8 expression in the myotome. It is also shown that Fgf-8 protein leads to an increase in sclerotomal cell proliferation and enhances rib cartilage development in mature somites, whereas inhibition of Fgf signaling by SU 5402 causes deletions in developing ribs. These observations demonstrate: (1) a regulatory influence of the ventral axial organs on the hypaxial muscle compartment; (2) regulation of epaxial and hypaxial expression of Fgf-8 by Sonic hedgehog; and (3) independent regulation of Fgf-8 and MyoD in the hypaxial myotome by ventral axial organs. It is postulated that the notochord and ventral neural tube influence hypaxial expression of Fgf-8 in the myotome and that, in turn, Fgf-8 has a functional role in rib formation.

The short-lived exostosis induced surgically versus the lasting genetic hereditary multiple exostoses

Hereditary osteochondromas are often caused by mutation in the EXT1 gene. The lesions are typified by formation of a "pseudo" growth plate like lesion growing at 60 degrees to the normal growth direction of the bone. Such lesions can be mimicked surgically by reverting the position--the polarity of the zone of LaCroix. The current study attempts to compare the pathology between EXT1 gene expression in humans and surgically created osteochondroma pathology in a rat model. Tissues of human bunion, human embryonal tissue, and human adult cartilage as well as normal rat epiphyses served as controls. Rats were operated on and a 60 degree span of the ring of LaCroix was inverted as described by Delgado (Delgado, E., Rodriguez, J. I., Serada, A., Tellez, M., and Pariagoa, R.. Clin. Orthop. 201, 251-258 (1985)). The surgically created osteochondromas were assessed by histology, histochemistry, and immunohistochemistry. The findings show that the surgically created lesions contain only a small amount of FGF receptor 3 (FGFR3) expressed on mesenchymal stem cells located in the perichondrium, as compared to the cell population carrying FGFR3 in the contralateral limb. Indian hedgehog and Bcl2 are downregulated, while BMP-2 is overexpressed in the operated limb, compared to the LaCroix ring of the contralaetral limb. The shortage, as well as the disturbed migration routes of the residual mesenchymal stem cells in surgically created osteochondromas leads eventually to resorption of the pathological elements. In search of additional markers characterizing such pathological structures composed of mesenchymal stem cells and cartilaginous and bony cells, EXT1 gene was found to be expressed in the surgically created osteochondromas, like in normal growth plates. Nitric oxide synthase was also expressed like in adult cartilage, though tumor necrosis factor alpha typifying Bunion formation was absent. In summary, surgically created osteochondromas lack the massive and continuous population of mesenchymal stem cells with Bcl2 expression. However, the small residual mesenchymal cell population gives rise to short-lived EXT1-expressing cells that disappear eventually due to spontaneous resorption.

Inhibition of polarizing activity in the anterior limb bud is regulated by extracellular factors

Anterior-posterior patterning of the developing limb is largely viewed as a function of polarizing activity. Recent evidence in polydactylous mutants, however, indicates that development of proper pattern also requires the involvement of inhibitory pathways in the anterior limb that prevent secondary polarizing zone formation, thus limiting the number of digits produced. We report the novel finding that grafts of extracellular matrix from the Mouse Posterior Limb Bud-4 cell line can induce supernumerary digits, including digits with posterior phenotype, from anterior chick limb mesenchyme. Unlike previously described mechanisms of pattern specification during limb development, it is shown that the extracellular matrix effect is not associated with release of an active signal. Rather, evidence is presented suggesting that heparan sulfate moieties in extracellular matrix grafts bind an endogenous, extracellular factor involved in inhibition of anterior polarizing activity, leading to derepression of the anterior limb and induction of polarizing zone marker genes including Sonic hedgehog and Bone morphogenetic protein-2.

Role of FGF9 and FGF receptor 3 in osteochondroma formation

Osteochondromas are chondro-osseous protuberances that occur in metaphyses of long bones. The cartilaginous cap is assumed to be responsible for the growth of the lesions during childhood and adolescence, but mitotic figures are rarely seen in the cap. Therefore, another cell population, probably mesenchymal cells, is responsible for proliferation and growth. Residual mesenchymal cells capable of rapid proliferation are difficult to detect due to lack of specific histologic features. Two specific markers for mesenchymal cells, FGF receptor 3 (FGFR3) and collagen type IIa, have been described. Osteochondroma mesenchymal cells are found in the soft tissues overlying the cartilage cap. The surrounding areas of typical cartilage are negative for both mesenchymal cell associated antigens. The soft tissues overlying the cartilage do not have cartilaginous features. The undifferentiated cells overlying the exostosis yield in culture a rapidly proliferating homogenous population of fibroblast-like cells. Expression at the mRNA level of FGF9, FGFR3, and collagen type IIa is found in these cells, but not in skin fibroblasts from afflicted or healthy individuals. Exogenous administration of TGFbeta1 to cultures of hereditary multiple exostosis eliminates FGF9 expression. These results indicate fibrous regions contain the mesenchymal cells responsible for osteochondroma growth.

Differential response of Shh expression between chick forelimb and hindlimb buds by FGF-4

The interactions of Sonic hedgehog (Shh) and fibroblast growth factor (FGF) play important roles in vertebrate limb pattern formation. In the posterior region of the chick limb bud, Shh and FGF-4 each maintain expression in a positive feedback loop. In the anterior region, Shh can also induce Fgf-4 expression in the anterior apical ectodermal ridge. However, the possibility of Shh induction by FGF protein is unclear. Because many experiments to analyze gene expression have been carried out by using the forelimb bud of the chick embryo, we investigated gene expression of the cells in the anterior region of the chick hindlimb bud after FGF-4 application and compared the results with those for the forelimb bud. When an FGF-4-containing bead was implanted into the anterior region of a stage 20 hindlimb bud, ectopic expression of Shh was induced in the mesenchyme beneath the anterior end of the apical ectodermal ridge at 36 hr after implantation. Subsequent to Shh activation, Hoxd13 was also observed in the anterior-distal region of the limb bud. Furthermore, FGF-4 implantation to the hindlimb bud caused additional digit formation accompanying respecification of positional value in the anterior tissue. Ectopic Shh was induced in cells located distal to the FGF-4 bead, and the cells of the flank region did not contribute to ectopic Shh induction. On the other hand, no ectopic Shh and Hoxd13 expression was detected by grafting an FGF-4 bead into the forelimb bud. Although FGF-4 implantation to the forelimb bud occasionally induced extra digit 2 formation, no embryos had an extra digit 3 or digit 4, and many specimens exhibited normal skeletal pattern. These results demonstrate the difference between the fore- and hindlimb buds in the cell competence of Shh induction in response to FGF-4, suggesting the possibility that the responsiveness of mesenchymal cells in signaling molecules is not the same in the fore- and hindlimb buds.

Misexpression of Fgf-4 in the chick limb inhibits myogenesis by down-regulating Frek expression

Skeletal muscle development involves an initial period of myoblast replication followed by a phase in which some myoblasts continue to proliferate while others undergo terminal differentiation. The latter process involves the permanent cessation of DNA synthesis, activation of muscle-specific gene expression, and fusion of single cells to generate multinucleated muscle fibres. The in vivo signals regulating the progression through all these steps remain unknown. Fibroblast growth factors (Fgfs) and Fgf receptors comprise a large family whose members have been shown to play multiple roles in the development of skeletal muscle in vitro. Exogenously applied Fgfs are able to stimulate proliferation and suppress myogenic differentiation in cell culture. We sought to determine the role played by Fgf-4 during limb myogenesis in vivo. Fgf-4 transcripts are located at both extremities of myotubes whereas the mRNAs of one of the Fgf receptors, Frek, are detected in mononucleated proliferating myoblasts surrounding the multinucleated fibres. Overexpression of mouse Fgf-4 (mFgf-4) using a replication-competent retrovirus, RCAS, leads to a down-regulation of muscle markers followed by an inhibition of terminal differentiation in limb muscles. Using quail/chick transplantations we were able to follow the muscle cells and found a dramatic decrease in their number after exposure to mFgf-4. Interestingly ectopic mFgf-4 down-regulates Frek transcripts in limb muscle areas. We conclude that overexpression of mFgf-4 inhibits myoblast proliferation, probably by down-regulating Frek mRNAs. This suggests a role for Fgf-4, located at the extremities of the myotubes, where it could be responsible for the absence of Frek mRNA in the muscle fibre.

Induction of inner ear fate by FGF3

Loss-of-function experiments in avians and mammals have provided conflicting results on the capacity of fibroblast growth factor 3 (FGF3) to act as a secreted growth factor responsible for induction and morphogenesis of the vertebrate inner ear. Using a novel technique for gene transfer into chicken embryos, we have readdressed the role of FGF3 during inner ear development in avians. We find that ectopic expression of FGF3 results in the formation of ectopic placodes which express otic marker genes. The ectopically induced placodes form vesicles which show the characteristic gene expression pattern of a developing inner ear. Ectopic expression of FGF3 also influences the formation of the normal orthotopic inner ear, whereas another member of the FGF family, FGF2, shows no effects on inner ear induction. These results demonstrate that a single gene can induce inner ear fate and reveal an unexpectedly widespread competence of the surface ectoderm to form sensory placodes in higher vertebrates.

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.

A model for anteroposterior patterning of the vertebrate limb based on sequential long- and short-range Shh signalling and Bmp signalling

It has been proposed that digit identity in chick limb bud is specified in a dose-dependent fashion by a long-range morphogen, produced by the polarising region. One candidate is Sonic hedgehog (Shh) protein, but it is not clear whether Shh acts long or short range or via Bmps. Here we dissect the relationship between Shh and Bmp signalling. We show that Shh is necessary not only for initiating bmp2 expression but also for sustaining its expression during the period when additional digits are being specified. We also show that we can reproduce much of the effect of Shh during this period by applying only Bmp2. We further demonstrate that it is Bmps that are responsible for digit specification by transiently adding Noggin or Bmp antibodies to limbs treated with Shh. In such limbs, multiple additional digits still form but they all have the same identity. We also explored time dependency and range of Shh signalling by examining ptc expression. We show that high-level ptc expression is induced rapidly when either Shh beads or polarising regions are grafted to a host limb. Furthermore, we find that high-level ptc expression is first widespread but later more restricted. All these data lead us to propose a new model for digit patterning. We suggest that Shh initially acts long range to prime the region of the limb competent to form digits and thus control digit number. Then later, Shh acts short range to induce expression of Bmps, whose morphogenetic action specifies digit identity.

Loss of FGF receptor 1 signaling reduces skeletal muscle mass and disrupts myofiber organization in the developing limb

The identities of extracellular growth factors that regulate skeletal muscle development in vivo are largely unknown. We asked if FGFs, which act as repressors of myogenesis in culture, play a similar role in vivo by ectopically expressing in the developing limb a truncated FGF receptor 1 (dnFGFR1) that acts as a dominant negative mutant. Hind limbs and the adjacent somites of Hamburger and Hamilton (HH) stage 17 chickens were infected with a replication-competent RCAS virus encoding dnFGFR1. By ED5, the virus had spread extensively within the limb and the adjacent somites with little rostral or caudal expansion of the infection along the axial midline. Viral infection and mutant receptor expression were coincident as revealed by the distribution of a viral coat protein and an HA epitope tag present on the carboxy terminus of dnFGFR1. Within 48 h following injection of dnFGFR1, we could detect no obvious changes in skeletal muscle precursor cell migration into the hind limb as compared to control limbs infected with an empty RCAN virus. However, by 3 days following infection of RCAS-dnFGFR1 virus, the level of skeletal muscle-specific myosin heavy chain was decreased and the expression pattern altered, suggesting disruption of skeletal muscle development. Two striking muscular phenotypes were observed in dnFGFR1-expressing limbs, including an average loss of 30% in skeletal muscle wet weight and a 50% decrease in myofiber density. At all ages examined the loss of skeletal muscle mass was accompanied by a loss of myoblasts and an unexpected concomitant loss of fibroblasts. Consistent with these observations, explants of infected cells revealed a reduction in the number of myonuclei in myotubes. Although the myofiber density per unit area was decreased over 50% compared to controls there were no detectable effects on myofiber diameter. The loss in myofiber density was, however, accompanied by an increase in the space surrounding individual myofibers and a generalized loss of myofiber integrity. It is noteworthy that long-bone development was unaffected by RCAS-dnFGFR1 infection, suggesting that FGFR2 and FGFR3 signaling was not disrupted. Our data provide conclusive evidence that FGFR1 signaling is necessary to maintain myoblast number and plays a role in myofiber organization.

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.

Sonic hedgehog signaling during digit pattern duplication after application of recombinant protein and expressing cells

HoxD expression and cartilage pattern formation were compared after application of a recombinant amino-terminal peptide of Sonic hedgehog protein (Shh-N) and implantation of cells expressing the Sonic hedgehog (Shh) gene. During digit duplication after implantation of a Shh-N-soaked bead, BMP-2 and Patched expression was transiently induced in the anterior limb mesenchyme 20 h after grafting, but was reduced to the basal level 48 h after grafting. On the contrary, when Shh-expressing cells were grafted to the anterior limb bud, expression domains of the BMP-2 and Patched genes were initially induced in the restricted region in close proximity to the grafted cells. Induced expression of BMP-2 and Patched was maintained in the anterior-peripheral region of the limb bud for 42 h after grafting. In either case, HoxD12 and HoxD13 were consistently induced in the anterior-distal limb mesenchyme, accompanying mirror-image duplication of the digit pattern. Induction and maintenance of HoxD expression were consistent with the resultant digit pattern. A steep gradient of Shh activity provided by Shh-expressing cells is most adequate to induce complete digit pattern, as compared to the shallow gradient provided by Shh-N protein released from a bead. These results suggest that positional identity is respecified by Shh-N activity within the first 24 h during digit duplication, and that Shh-N on its own is not acting as a long-range signaling molecule to determine positional identity at a distance in the limb bud.

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.

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.

Notochord repression of endodermal Sonic hedgehog permits pancreas development

Notochord signals to the endoderm are required for development of the chick dorsal pancreas. Sonic hedgehog (SHH) is normally absent from pancreatic endoderm, and we provide evidence that notochord, in contrast to its effects on adjacent neuroectoderm where SHH expression is induced, represses SHH expression in adjacent nascent pancreatic endoderm. We identify activin-betaB and FGF2 as notochord factors that can repress endodermal SHH and thereby permit expression of pancreas genes including Pdx1 and insulin. Endoderm treatment with antibodies that block hedgehog activity also results in pancreatic gene expression. Prevention of SHH expression in prepancreatic dorsal endoderm by intercellular signals, like activin and FGF, may be critical for permitting early steps of chick pancreatic development.

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.

Apical ectodermal ridge-dependent expression of the chick 67 kDa laminin binding protein gene (cLbp) in developing limb bud

Apical ectodermal ridge (AER)-mesoderm interaction is important for morphogenesis in the developing chick limb bud. Genes whose expression is dependent upon the presence of AER, are likely to play important roles in the AER-mesoderm interaction. We report here the gene expression pattern of the chick homolog of the 67 kDa laminin binding protein (LBP), which is a non-integrin laminin receptor whose function relates to cell attachment, spreading, and polarization. Northern analysis showed that a single 1.4 kb transcript exists in stage 20 limb buds and which is dramatically reduced 24 hr after removal of AER. In situ hybridization analysis revealed that the chick 67 kDa laminin binding protein gene (cLbp) was expressed in the mesodermal region overlapping the Msx1-expressing domain and in the AER in early stage limb buds. Expression in the mesoderm was gradually restricted to the distal region underneath the AER as development proceeds. The expression in the limb mesoderm could be induced by local application of FGF-2 which could thus mimic the AER functions. These results indicated that the expression of cLbp depends on AER signals and that the 67 kDa non-integrin receptor binding to laminin plays a role in the AER-mesoderm interaction.

Overexpression of FGF-2 modulates fiber cell differentiation and survival in the mouse lens

During mammalian embryogenesis, the ocular lens forms through a temporally and spatially regulated pattern of differentiation which is thought to be coordinated at least in part by the FGF-1 and FGF-2 members of the fibroblast growth factor (FGF) family. Previous transgenic experiments in which FGF-1 or dominant negative FGF receptors were overexpressed in the lens indicated that FGF-1 could induce differentiation while differentiated lens cells rely upon FGF signaling for their survival. In this study, we asked if the 17.5 kDa FGF-2 protein was capable of inducing differentiation of lens cells in transgenic mice. Unexpectedly, differentiation was inhibited by lens-specific expression of a transgene encoding a secreted form of the 17.5 kDa bovine FGF-2 protein under the transcriptional control of the murine alphaA-crystallin promoter (alphaAIgFGF-2 transgenic mice). To address the possibility that FGF-2 functions as a modulator of fiber cell survival, alphaAIgFGF-2 transgenic mice were crossed to transgenic mice exhibiting extensive apoptosis in the lens due to the functional inactivation of the retinoblastoma protein (alphaAE7 transgenic mice). The level of apoptosis in the lenses of double transgenic mice was substantially reduced as compared to the level in lenses from alphaAE7 only mice. These studies indicate that FGF-2 can act as a modulator of the later stages of differentiation including fiber cell survival. Additionally, they imply that control of lens development by FGFs is a complex process in which FGF-1 and FGF-2 play distinct roles.

Polydactyly and ectopic ZPA formation in Alx-4 mutant mice

Correct development of the limb is dependent on coordination between three distinct signaling centers. Recently, fibroblast growth factor-4 has been identified as a crucial determinant of AER function, which directs limb bud outgrowth, and Sonic hedgehog has been identified as a signaling molecule that mediates ZPA function, which specifies anterior-posterior patterning in the developing limb bud. In addition, Shh and FGF-4 reciprocally reinforce each other's expression via a positive feedback loop, providing a molecular basis for the coordination of limb bud outgrowth and anterior-posterior patterning. The mechanisms by which these signaling centers come to occupy their normal positions in the posterior limb bud during development are not understood. Here we identify and characterize Alx-4, a gene that encodes a paired-type homeodomain protein. Alx-4 is expressed in several populations of mesenchymal cells, including mesenchymal cells in the anterior limb bud, and mice homozygous for targeted disruption of the Alx-4 gene have multiple abnormalities, including preaxial polydactyly. The polydactyly is associated with the formation of an ectopic anterior ZPA, as indicated by anterior expression of Sonic hedgehog, HoxD13 and fibroblast growth factor-4. The expression of other candidate regulators of anterior-posterior positional information in the limb bud, including HoxB8 and Gli3, is not altered in Alx-4 mutant embryos. By chromosomal mapping experiments, Alx-4 is tightly linked to Strong's luxoid, a polydactylous mouse mutant. The results identify Alx-4 as a determinant of anterior-posterior positional identity in the limb and a component of a regulatory program that restricts ZPA formation to the posterior limb bud mesenchyme.

A minimal murine Msx-1 gene promoter. Organization of its cis-regulatory motifs and their role in transcriptional activation in cells in culture and in transgenic mice

To dissect the cis-regulatory elements of the murine Msx-1 promoter, which lacks a conventional TATA element, a putative Msx-1 promoter DNA fragment (from -1282 to +106 base pairs (bp)) or its congeners containing site-specific alterations were fused to luciferase reporter and introduced into NIH3T3 and C2C12 cells, and the expression of luciferase was assessed in transient expression assays. The functional consequences of the sequential 5' deletions of the promotor revealed that multiple positive and negative regulatory elements participate in regulating transcription of the Msx-1 gene. Surprisingly, however, the optimal expression of Msx-1 promoter in either NIH3T3 or C2C12 cells required only 165 bp of the upstream sequence to warrant detailed examination of its structure. Therefore, the functional consequences of site-specific deletions and point mutations of the cis-acting elements of the minimal Msx-1 promoter were systematically examined. Concomitantly, potential transcriptional factor(s) interacting with the cis-acting elements of the minimal promoter were also studied by gel electrophoretic mobility shift assays and DNase I footprinting. Combined analyses of the minimal promoter by DNase I footprinting, electrophoretic mobility shift assays, and super shift assays with specific antibodies revealed that 5'-flanking regions from -161 to -154 and from -26 to -13 of the Msx-1 promoter contains an authentic E box (proximal E box), capable of binding a protein immunologically related to the upstream stimulating factor 1 (USF-1) and a GC-rich sequence motif which can bind to Sp1 (proximal Sp1), respectively. Additionally, we observed that the promoter activation was seriously hampered if the proximal E box was removed or mutated, and the promoter activity was eliminated completely if the proximal Sp1 site was similarly altered. Absolute dependence of the Msx-1 minimal promoter on Sp1 could be demonstrated by transient expression assays in the Sp1-deficient Drosophila cell line cotransfected with Msx-1-luciferase and an Sp1 expression vector pPacSp1. The transgenic mice embryos containing -165/106-bp Msx-1 promoter-LacZ DNA in their genomes abundantly expressed beta-galactosidase in maxillae and mandibles and in the cellular primordia involved in the formation of the meninges and the bones of the skull. Thus, the truncated murine Msx-1 promoter can target expression of a heterologous gene in the craniofacial tissues of transgenic embryos known for high level of expression of the endogenous Msx-1 gene and found to be severely defective in the Msx-1 knock-out mice.

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

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

Control of the limb bud outgrowth in quail-chick chimera

It is well known that the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA) play a major role in growth and patterning of the limb. But a mechanism underlying species-specific growth of the limb has not yet been fully elucidated. To investigate the role of AER and ZPA in limb size control, we constructed quail-chick limb chimeras. When we grafted a whole forelimb bud from one species to another, the size of the developed grafted limb was comparable to the limb of the donor species. Moreover, we demonstrated that neither the interspecific substitution of the posterior half region of the limb bud containing the ZPA nor the exchange of the ectodermal component of the limb involving the AER could alter the species-specific size of the limb. These results indicate that the factors affecting the size of the limb are already involved in the mesodermal component of the limb bud at stage 20 of chick embryo. Thus, the mesoderm dictates limb specificity including size.

The ectodermal control in chick limb development: Wnt-7a, Shh, Bmp-2 and Bmp-4 expression and the effect of FGF-4 on gene expression

We have manipulated the chick limb bud by dorsoventrally inverting the ectoderm, by grafting the AER to the dorsal or ventral ectoderm and by insertion of an FGF-4 soaked heparin bead into the mesoderm. After dorso-ventral reversal of the ectoderm, Wnt-7a expression is autonomous from an early stage of limb development in the original dorsal ectoderm. Exogenous FGF-4 causes ectopic Wnt-7a expression and induces ectopic Shh. In addition, exogenous FGF-4 increases the thickness of cartilages and also shortens them, and both Bmp-2 and Bmp-4 may mediate this effect. The ectoderm outside the AER can regulate not only the dorso-ventral polarity of the underlying mesenchyme cells but also the cartilage formation, and both Bmp-2 and Bmp-4 may mediate this control.

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

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

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

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

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

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

Gene expression, polarising activity and skeletal patterning in reaggregated hind limb mesenchyme

The developing chick limb has two major signalling centres; the apical ectodermal ridge maintains expression of several important genes and outgrowth of the limb, and the polarising region specifies the pattern of skeletal elements along the anteroposterior axis. We have used reaggregated leg grafts (mesenchyme dissociated into single cells, placed in an ectodermal jacket and grafted to a host) to study patterning in a system where the developmental axes are severely disrupted. Reaggregates from different regions of leg mesenchyme developed correspondingly different digits, giving a system in which skeletal phenotype could be compared with the expression of genes thought to be important in patterning. We found that posterior third and whole leg reaggregates gave rise to different digits, yet expressed the same combination of HoxD, Bmp-2 and shh genes throughout their development. Anterior thirds initially only express the 3′ end of the HoxD cluster but activate the more 5′ members of the cluster sequentially over a period of 48 hours, a period during which Bmp-2 is activated but no shh or Fgf-4 expression could be detected. Our results suggest that there are two independent mechanisms for activating the HoxD complex, one polarising region-dependent and one independent, and that shh expression may not be necessary to maintain outgrowth and patterning once a ridge has been established.

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

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

A role for FGF-8 in the initiation and maintenance of vertebrate limb bud outgrowth

BACKGROUND

The outgrowth of the vertebrate limb bud is the result of a reciprocal interaction between the mesenchyme and a specialized region of the ectoderm, the apical ectodermal ridge (AER), which overlies it. Signals emanating from the AER act to maintain the underlying mesenchyme, called the progress zone, in a highly proliferative and undifferentiated state. Removal of the AER results in the cessation of limb bud growth, thus causing limb truncation. The best candidates for this AER-derived signal are members of the fibroblast growth factor (FGF) family, in particular FGF-4, which can maintain limb bud outgrowth following removal of the AER. However, FGF-4 is only expressed after considerable outgrowth has occurred and a well-developed limb bud has formed, and then only in the posterior part of the AER. Likewise, the other FGFs studied to date are not candidates for this activity.

RESULTS

We report evidence that a recently identified member of this family, FGF-8, is expressed in the ectoderm of the prospective limb territory prior to morphological outgrowth of the limb bud in both mouse and chick. Thereafter, expression is maintained throughout the AER during limb development. We have produced and purified the FGF-8 protein, and shown that it will substitute for the AER in maintaining limb bud outgrowth in mouse embryos from which the AER has been surgically removed. FGF-8 does not, however, maintain expression of the sonic hedgehog gene.

CONCLUSIONS

These results indicate that FGF-8 is an AER-derived mitogen that stimulates limb bud outgrowth. Moreover, our data suggest that FGF-8 may also be an ectodermally derived mitogen that stimulates the onset of limb bud outgrowth (budding) in the absence of a morphological AER, and indicate the possible involvement of FGF-8 in the establishment of the limb field.