FGF10 can induce Fgf8 expression concomitantly with En1 and R-fng expression in chick limb ectoderm, independent of its dorsoventral specification

The limb bud has a thickened epithelium at the dorsal-ventral boundary, the apical ectodermal ridge (AER), which sustains limb outgrowth and patterning. A secreted molecule fibroblast growth factor (FGF)10 is involved in inducing Fgf8 expression in the prospective AER and mutual interaction between mesenchymal FGF10 and FGF8 in the AER is essential for limb outgrowth. A secreted factor Wnt7a and a homeobox protein Lmx1 are involved in the dorsal patterning of the limb, whereas a homeobox protein Engrailed 1 (En1) is involved in the dorsal-ventral patterning as well as AER formation. Radical fringe (R-fng), a vertebrate homolog of Drosophila fringe was also found to elaborate AER formation in chicks. However, little is known about the molecular interactions between these factors during AER formation. The present study clarified the relationship between FGF10, Wnt7a, Lmx1, R-fng and En1 during limb development using a foil-barrier insertion experiment. It was found that a foil-barrier inserted into the chick prospective wing mesenchyme lateral to the mesonephric duct blocks AER induction. This experiment was expanded by implanting Fgf10-expressing cells lateral to the barrier and examined whether FGF10 could rescue the expression of the limb-patterning genes reported in AER formation. It was found that FGF10 is sufficient to induce Fgf8 expression in the ectoderm of the foil-inserted limb bud, concomitantly with R-fng and En1 expression. However, FGF10 could not rescue the expression of the dorsal marker genes, Wnt7a or Lmx1. Thus, it is suggested that epithelial factors of En1 and R-fng can induce Fgf8 expression in the limb ectoderm in cooperation with a mesenchymal factor FGF10. Some factor(s) other than FGF10, possibly from the paraxial structures medial to the limb mesoderm, is responsible for the initial dorsal-ventral specification of the limb bud.     

Distance between AER and ZPA Is Defined by Feed-Forward Loop and Is Stabilized by their Feedback Loop in Vertebrate Limb Bud

In the development of organs, multiple morphogen sources are often involved, and interact with each other. For example, the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA) are major morphogen sources in the limb bud formation of vertebrates. Fgf expression in the AER and Shh expression in the ZPA are maintained by their positive feedback regulation mediated by diffusible molecules, FGF and SHH. A recent experimental observation suggests that the FGF-signal regulates the Shh expression in a feed-forward manner with activation and repression regulatory pathways. We study the coupled dynamics of Shh expression in the ZPA and Fgf expression in the AER, and the relationship of the relative position between AER and ZPA. We first show that with the feed-forward regulation only, the peak of ZPA activity can be formed distant from the AER as observed experimentally. Then, we clarify that the robustness of the ZPA spatial pattern to changes in system parameters is enhanced by adding the feedback regulation between the AER and the ZPA. Furthermore, sensitivity analysis shows that there exists the optimal feedback strength where the robustness is the most improved.     

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.     

Beta-catenin-dependent Wnt signaling in apical ectodermal ridge induction and FGF8 expression in normal and limbless mutant chick limbs

The fibroblast growth factor (FGF) and beta-catenin-dependent Wnt signaling pathways are key regulators of vertebrate limb development. FGF10 induces expression of Wnt3a, which regulates the formation and FGF8 expression of the apical ectodermal ridge (AER). In amelic limbless limbs, an AER fails to form and FGF8 is not expressed, despite expression of FGF10. It has been found that Wnt3a is initially expressed in limbless ectoderm, although subsequently is drastically reduced. In addition, changes in the expression pattern or level of several Frizzled receptors, Axin, Lef1/Tcf1 and beta-catenin have been found in limbless limbs. Notably, while normal wing buds respond to LiCl-stimulated activation of beta-catenin-dependent signaling by forming ectopic, FGF8-expressing AER, LiCl was unable to induce an AER in limbless wing buds. The results of this study suggest that the limbless gene is required for beta-catenin-dependent Wnt signaling in limb ectoderm leading to FGF8 expression and AER formation.     

Mitogenic property of the apical ectodermal ridge

While the apical ectodermal ridge (AER) is well known for its required role in the development of distal parts of the limb and for its ability to stimulate limb duplications, the mechanism of its action is unknown. In this study we use a culture system previously developed by M. Globus and S. Vethamany-Globus (1976, Differentiation6, 91–96) in which an AER grafted onto a high-density cell culture of limb mesenchyme stimulates the formation of an outgrowth. Time-lapse movies taken during the outgrowth period demonstrated no cellular activities other than cell division. Both the mitotic index and labeling index in the mesenchyme were significantly elevated under the AER as compared to that without AER, indicating that the AER provides a growth-promoting stimulus which increases the proportion of dividing cells. On the other hand, nonridge ectoderm had no detectable effect on the mitotic index. Treatment of cultures with cytosine arabinoside both inhibited DNA synthesis and prevented AER-induced outgrowth. These results demonstrate a mitogenic capacity of AER tissue and suggest that mesenchymal outgrowth requires this activity. The mitogenic property of the AER is considered in relation to limb outgrowth in situ.     

Function of FGF-4 in limb development

The apical ectodermal ridge plays a central role in limb development through its interactions with the underlying mesenchyme. Removal of the AER results in cessation of limb outgrowth and leads to truncation of the limb along the proximo-distal axis. The many functions attributed to the ridge include maintenance of the progress zone mesenchyme. Here, cells are stimulated to proliferate, are maintained in an undifferentiated state, and are assigned progressively more distal positional values as the limb grows. The AER also functions to maintain the activity of the polarizing region, a region of mesenchyme which is thought to provide the primary signal for patterning along the antero-posterior axis. We have begun to explore the function of fibroblast growth factor-4 (FGF-4) during limb development. FGF-4, which encodes an efficiently secreted protein, is expressed in the AER. We have previously demonstrated that FGF-4 protein can stimulate limb mesenchyme proliferation and can induce the expression of a downstream homeobox gene, Evx-1 (homologue of the Drosophila even-skipped gene), that is normally regulated by a signal from the AER. To determine to what extent FGF-4 protein can substitute for the AER to allow normal limb outgrowth, we performed experiments on the developing chick limb in ovo. Remarkably, we find that after AER removal, the FGF-4 protein can provide all the signals required for virtually normal outgrowth and patterning of the limb. Further studies indicate that proliferation of progress zone cells is not sufficient, and that an additional signal is produced by the posterior mesenchyme in response to FGF-4 which enables progress zone cells to acquire progressively more distal fates. Thus FGF-4 maintains progress zone activity through a combination of at least two signals—one that acts directly on progress zone cells to stimulate their proliferation, and one that acts indirectly by maintaining the production of patterning signal(s) by the posterior mesenchyme. We further show that failure of the posterior mesenchyme to produce this signal correlates with failure to maintain polarizing activity. This raises the possibility that the signal produced by the posterior mesenchyme and required for progressive proximo-distal limb patterning is identical to the polarizing activity. Further experiments demonstrate that retinoic acid, which mimics the activity of the polarizing region, can supply this signal. In conclusion, the finding that a single growth factor can serve as both the direct and indirect signals required to maintain progress zone activity provides a simple mechanism for ensuring that growth and pattern formation are linked in the developing limb. © 1994 Wiley-Liss, Inc.     

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.     

Growth arrest specific gene 1 acts as a region-specific mediator of the Fgf10/Fgf8 regulatory loop in the limb

Proximal-to-distal growth of the embryonic limbs requires Fgf10 in the mesenchyme to activate Fgf8 in the apical ectodermal ridge (AER), which in turn promotes mesenchymal outgrowth. We show here that the growth arrest specific gene 1 (Gas1) is required in the mesenchyme for the normal regulation of Fgf10/Fgf8. Gas1 mutant limbs have defects in the proliferation of the AER and the mesenchyme and develop with small autopods, missing phalanges and anterior digit syndactyly. At the molecular level, Fgf10 expression at the distal tip mesenchyme immediately underneath the AER is preferentially affected in the mutant limb, coinciding with the loss of Fgf8 expression in the AER. To test whether FGF10 deficiency is an underlying cause of the Gas1 mutant phenotype, we employed a limb culture system in conjunction with microinjection of recombinant proteins. In this system, FGF10 but not FGF8 protein injected into the mutant distal tip mesenchyme restores Fgf8 expression in the AER. Our data provide evidence that Gas1 acts to maintain high levels of FGF10 at the tip mesenchyme and support the proposal that Fgf10 expression in this region is crucial for maintaining Fgf8 expression in the AER.     

Normal limb development in conditional mutants of Fgf4

Fibroblast growth factors (FGFs) mediate multiple developmental signals in vertebrates. Several of these factors are expressed in limb bud structures that direct patterning of the limb. FGF4 is produced in the apical ectodermal ridge (AER) where it is hypothesized to provide mitogenic and morphogenic signals to the underlying mesenchyme that regulate normal limb development. Mutation of this gene in the germline of mice results in early embryonic lethality, preventing subsequent evaluation of Fgf4 function in the AER. A conditional mutant of Fgf4, based on site-specific Cre/loxP-mediated excision of the gene, allowed us to bypass embryonic lethality and directly test the role of FGF4 during limb development in living murine embryos. This conditional mutation was designed so that concomitant with inactivation of the Fgf4 gene by excision of all Fgf4-coding sequences, a reporter gene was activated in Fgf4-expressing cells, allowing assessment of the site-specific recombination reaction. Although a large body of evidence led us to predict that ablation of Fgf4 gene function in the AER of developing mice would result in abnormal limb outgrowth and patterning, we found that Fgf4 conditional mutants had normal limbs. Furthermore, expression patterns of Shh, Bmp2, Fgf8 and Fgf10 were normal in the limb buds of the conditional mutants. These findings indicate that the previously proposed FGF4-SHH feedback loop is not essential for coordination of murine limb outgrowth and patterning. We suggest that some of the roles currently attributed to FGF4 during early vertebrate limb development may be performed by other AER factors in vivo.     

Distribution and possible function of an adrenomedullin-like peptide in the developing chick limb bud

Adrenomedullin (AM) is a multifunctional peptide that exhibits discrete domains of expression during mouse embryogenesis consistent with a role in regulating growth and differentiation during morphogenesis. Here we report that AM immunoreactivity is present at high levels throughout the apical ectodermal ridge (AER) of the chick limb bud as the AER is directing the outgrowth and patterning of underlying limb mesoderm. Immunostaining is particularly strong along the surfaces of the contiguous cells of the AER. AM immunoreactivity attenuates as the AER regresses and is absent from the distal apical ectoderm of stage 20 limbless mutant limb buds which fail to develop an AER. To explore the possible role of AM in AER activity, we examined the effect of exogenous AM and an AM inhibitor on the in vitro morphogenesis of limb mesoderm, cultured in the presence and absence of the AER. Although exogenous AM cannot substitute for the AER in promoting outgrowth of limb mesoderm in vitro, a specific AM antagonist, AM(22-52), impairs the outgrowth and proliferation of limb mesoderm cultured in the presence of the AER. This is consistent with the possibility that inhibition of endogenous AM activity in the AER impairs the ability of the AER to promote limb morphogenesis. Taken together, these studies suggest that an AM-like molecule may function in an autocrine fashion to regulate some aspect of AER activity.     

In the limb AER Bmp2 and Bmp4 are required for dorsal-ventral patterning and interdigital cell death but not limb outgrowth

The apical ectodermal ridge (AER) in the vertebrate limb is required for limb outgrowth and patterning. To investigate the role BMP ligands expressed in the AER play in limb development we selectively inactivated both Bmp2 and Bmp4 in this tissue. The autopods of mice lacking both of these genes contained extra digits, digit bifurcations and interdigital webbing due to a decrease in programmed cell death and an increase in cell proliferation in the underlying mesoderm. Upon removal of Bmp2 and Bmp4 in the AER, no defects in proximal-distal patterning were observed. At the molecular level, removal of Bmp2 and Bmp4 in the AER caused an increase in Fgf expression, which correlated with an increase in both the width and length of the AER. Investigation of Engrailed-1 (En1) expression in the AER of limb buds in which Bmp2 and Bmp4 had been removed indicated that En1 expression was absent from this tissue. Our data suggests that AER expression of Bmp2 and Bmp4 is required for digit and dorsal-ventral patterning but surprisingly not for limb outgrowth.