Signal relay by BMP antagonism controls the SHH/FGF4 feedback loop in vertebrate limb buds

Dlx homeobox gene control of mammalian limb and craniofacial development

The Dlx homeobox gene family is of ancient origin and crucial for embryonic development in invertebrates and vertebrates. The Dlx proteins are thought to function as DNA-binding transcriptional regulators, likely controlling large numbers of downstream effector genes. In mammals gene expression analysis of the six members of the Dlx gene family has been demonstrated in the nervous system, neural crest derivatives, branchial arches, and developing appendages. Through genetic approaches the Dlx genes have been implicated in patterning and development of the brain, craniofacial structures, and the axial and appendicular skeleton. Substantial functional redundancy within the Dlx gene family has prevented the analysis of single gene mutations from demonstrating the full developmental control exerted by the Dlx proteins. Here, we will discuss data from recent combined loss and gain-of-function genetic mutations, which have given greater insight into the role of Dlx homeobox genes in craniofacial, limb, and bone development.

Anteroposterior patterning in the limb and digit specification: contribution of mouse genetics

The limb has been a privileged object of investigation and reflection for scientists over the past two centuries and continues to provide a heuristic framework to analyze vertebrate development. Recently, accumulation of new data has significantly changed our view on the mechanisms of limb patterning, in particular along the anterior-posterior axis. These data have led us to revisit the mode of action of the zone of polarizing activity. They shed light on the molecular and cellular mechanisms of patterning linked to the Shh-Gli3 signaling pathway and give insights into the mechanism of activation of these cardinal factors, as well as the consequences of their activity. These new data are in good part the result of systematic Application of tools used in contemporary mouse molecular genetics. These have extended the power of mouse genetics by introducing mutational strategies that allow fine-tuned modulation of gene expression, interchromosomal deletions and duplication. They have even made the mouse embryo amenable to cell lineage analysis that used to be the realm of chick embryos. In this review, we focus on the data acquired over the last five years from the analysis of mouse limb development and discuss new perspectives opened by these results.

Genetic analysis of the roles of BMP2, BMP4, and BMP7 in limb patterning and skeletogenesis

Bone morphogenetic protein (BMP) family members, including BMP2, BMP4, and BMP7, are expressed throughout limb development. BMPs have been implicated in early limb patterning as well as in the process of skeletogenesis. However, due to complications associated with early embryonic lethality, particularly for Bmp2 and Bmp4, and with functional redundancy among BMP molecules, it has been difficult to decipher the specific roles of these BMP molecules during different stages of limb development. To circumvent these issues, we have constructed a series of mouse strains lacking one or more of these BMPs, using conditional alleles in the case of Bmp2 and Bmp4 to remove them specifically from the limb bud mesenchyme. Contrary to earlier suggestions, our results indicate that BMPs neither act as secondary signals downstream of Sonic Hedghog (SHH) in patterning the anteroposterior axis nor as signals from the interdigital mesenchyme in specifying digit identity. We do find that a threshold level of BMP signaling is required for the onset of chondrogenesis, and hence some chondrogenic condensations fail to form in limbs deficient in both BMP2 and BMP4. However, in the condensations that do form, subsequent chondrogenic differentiation proceeds normally even in the absence of BMP2 and BMP7 or BMP2 and BMP4. In contrast, we find that the loss of both BMP2 and BMP4 results in a severe impairment of osteogenesis.

A group of related signaling molecules called bone morphogenetic proteins (BMPs) are known to play important roles in the formation of the structures such as the limbs. However, because different members of this group often have similar effects on target cells and are produced in overlapping regions of the embryo and hence can be redundant with one another, removal of any single member of the BMP family may not reveal the full extent of the roles they play during development. We have therefore improved on this type of analysis by removing pairs of these factors (BMP2 and BMP4 or BMP2 and BMP7) specifically from the developing limb. Although some have speculated that these signals play an early role in organizing or “patterning” the different tissues of the limb, we find no evidence for such a role. We do find, however, that a minimal amount of BMP signal is required to form cartilage, and hence some cartilaginous elements fail to form in limbs deficient in both BMP2 and BMP4. Moreover, in the absence of these two BMP family members, there is a severe impairment in the development of bone tissue, resulting in severely deformed limbs. This study gives important new insight into the roles of these BMP signals in making skeletal tissues in the embryo.

Bone morphogenic protein antagonist Drm/gremlin is a novel proangiogenic factor

Angiogenesis plays a key role in various physiologic and pathologic conditions, including tumor growth. Drm/gremlin, a member the Dan family of bone morphogenic protein (BMP) antagonists, is commonly thought to affect different processes during growth, differentiation, and development by heterodimerizing various BMPs. Here, we identify Drm/gremlin as a novel proangiogenic factor expressed by endothelium. Indeed, Drm/gremlin was purified to homogeneity from the conditioned medium of transformed endothelial cells using an endothelial-cell sprouting assay to follow protein isolation. Accordingly, recombinant Drm/gremlin stimulates endothelial-cell migration and invasion in fibrin and collagen gels, binds with high affinity to various endothelial cell types, and triggers tyrosine phosphorylation of intracellular signaling proteins. Also, Drm/gremlin induces neovascularization in the chick embryo chorioallantoic membrane. BMP4 does not affect Drm/gremlin interaction with endothelium, and both molecules exert a proangiogenic activity in vitro and in vivo when administered alone or in combination. Finally, Drm/gremlin is produced by the stroma of human tumor xenografts in nude mice, and it is highly expressed in endothelial cells of human lung tumor vasculature when compared with non-neoplastic lung. Our observations point to a novel, previously unrecognized capacity of Drm/gremlin to interact directly with target endothelial cells and to modulate angiogenesis.

Regulation of myogenic progenitor proliferation in human fetal skeletal muscle by BMP4 and its antagonist Gremlin

Skeletal muscle side population (SP) cells are thought to be “stem”-like cells. Despite reports confirming the ability of muscle SP cells to give rise to differentiated progeny in vitro and in vivo, the molecular mechanisms defining their phenotype remain unclear. In this study, gene expression analyses of human fetal skeletal muscle demonstrate that bone morphogenetic protein 4 (BMP4) is highly expressed in SP cells but not in main population (MP) mononuclear muscle-derived cells. Functional studies revealed that BMP4 specifically induces proliferation of BMP receptor 1a–positive MP cells but has no effect on SP cells, which are BMPR1a-negative. In contrast, the BMP4 antagonist Gremlin, specifically up-regulated in MP cells, counteracts the stimulatory effects of BMP4 and inhibits proliferation of BMPR1a-positive muscle cells. In vivo, BMP4-positive cells can be found in the proximity of BMPR1a-positive cells in the interstitial spaces between myofibers. Gremlin is expressed by mature myofibers and interstitial cells, which are separate from BMP4-expressing cells. Together, these studies propose that BMP4 and Gremlin, which are highly expressed by human fetal skeletal muscle SP and MP cells, respectively, are regulators of myogenic progenitor proliferation.

Differential regulation of gene expression in the digit forming area of the mouse limb bud by SHH and gremlin 1/FGF-mediated epithelial-mesenchymal signalling

Spatially and temporally coordinated changes in gene expression are crucial to orderly progression of embryogenesis. We combine mouse genetics with experimental manipulation of signalling to analyze the kinetics by which the SHH morphogen and the BMP antagonist gremlin 1 (GREM1) control gene expression in the digit-forming mesenchyme of mouse limb buds. Although most mesenchymal cells respond rapidly to SHH signalling, the transcriptional upregulation of specific SHH target signals in the mesenchyme occurs with differential temporal kinetics and in a spatially restricted fashion. In particular, the expression of the BMP antagonist Grem1 is always upregulated in mesenchymal cells located distal to the SHH source and acts upstream of FGF signalling by the apical ectodermal ridge. GREM1/FGF-mediated feedback signalling is, in turn, required to propagate SHH and establish the presumptive digit expression domains of the Notch ligand jagged 1(Jag1) and 5′Hoxd genes in the distal limb bud mesenchyme. Their establishment is significantly delayed in Grem1-deficient limb buds and cannot be rescued by specific restoration of SHH signalling in mutant limb buds. This shows that GREM1/FGF feedback signalling is required for regulation of the temporal kinetics of the mesenchymal response to SHH signalling. Finally, inhibition of SHH signal transduction at distinct time points reveals the differential temporal dependence of Grem1, Jag1and 5′Hoxd gene expression on SHH signalling. In particular, the expression of Hoxd13 depends on SHH signal transduction significantly longer than does Hoxd11 expression, revealing that the reverse co-linear establishment, but not maintenance of their presumptive digit expression domains, depends on SHH signalling.

Mechanistic insight into how Shh patterns the vertebrate limb

The hands and feet of a newborn baby are a beautiful reminder of the complexity of embryonic patterning. Classical studies on how these structures form have led to a theoretical framework for understanding, in general, how discrete groups of cells can instruct differential fates across a wider field through the action of long-range signals. The discovery just more than a decade ago that localized expression of Sonic hedgehog (Shh) differentially patterns structures across the limb field, resulting in digits with unique characteristics, provided a starting point for readdressing these models at a molecular level. Current research has revealed unexpected complexity in how a gradient of Shh activity is both established and received, prompting re-evaluation of the nature of patterning mechanisms within the limb.

Regulation of Gremlin expression in the posterior limb bud

Proper outgrowth of the limb bud requires a positive feedback loop between Sonic hedgehog (Shh) in the zone of polarizing activity (ZPA) and Fgfs in the overlying apical ectodermal ridge. The Bmp antagonist Gremlin is expressed in a domain anterior to the ZPA and is thought to act as a signaling intermediate between Shh and Fgf. It is currently unclear whether Shh acts directly or indirectly to initiate and maintain Gremlin. In this study, we confirm that Bmp activity is necessary and sufficient for induction of Gremlin. Beads soaked in the Bmp antagonist Noggin downregulate Gremlin, while beads soaked in Bmp2 cause its upregulation. Furthermore, Bmp2 is also capable of upregulating Gremlin in oligozeugodactyly mutant limbs that lack Shh activity, demonstrating that Gremlin expression does not depend on the combined exposure to both these factors. In spite of the ability of Bmp2 to induce Gremlin, beads soaked in high concentrations of Bmp2 downregulate Gremlin around the bead without apparent induction of cell death, whereas another target gene Msx2 is upregulated around the bead. Consistent with this concentration-dependent effect, we find that low concentrations of Bmp2 upregulate Gremlin while high concentrations of Bmp2 downregulate Gremlin in limb mesenchyme cultures. These data implicate Bmp activity as a required intermediate in the Shh-Fgf4 signaling loop. Though we show that Bmp activity is sufficient to upregulate Gremlin, Gremlin expression is excluded from a posterior domain of the limb, and expansion of this domain as limb outgrowth proceeds is important in terminating the Shh-Fgf4 signaling loop. We find that the posterior limb is refractory to Gremlin induction in response to Bmp2, suggesting that termination of the Shh-Fgf4 signaling loop results from inability of Bmp activity to induce Gremlin in the posterior. In contrast, in the oligozeugodactyly limb, we find that beads soaked in Bmp2 can induce Gremlin in the posterior, demonstrating that Shh activity is required for exclusion of Gremlin in the posterior. Finally, by blocking Shh activity with cyclopamine, we find evidence that continued Shh activity is also required to maintain refractoriness to Gremlin expression in response to Bmp activity.

BMP receptor type IA in limb bud mesenchyme regulates distal outgrowth and patterning

The mesenchyme of the developing vertebrate limb responds to inductive signals, giving rise to skeletal elements that define limb shape and size. Several signals emanate from the limb ectoderm and in particular from the specialized epithelium of the apical ectodermal ridge (AER), including three members of the bone morphogenetic protein (BMP) family of signaling molecules, BMP2, BMP4 and BMP7. Using the Cre/loxP system in mice, we rendered limb bud mesenchyme insensitive to BMP signals through the type I receptor, BMPR-IA. Conditional mutants had shortened limbs and almost complete agenesis of the autopod because of reduced cell proliferation. Reduced expression of downstream BMP signaling targets, Msx1, Msx2 and gremlin in the distal mesenchyme (progress zone) correlated with decreased levels of cyclin D1 and Wnt5a. Ectopic anterior activation of sonic hedgehog (SHH) signaling and Hox expression revealed alterations in anterior-posterior (AP) patterning. Abnormal localization of Lmx1b-expressing cells in the ventral mesenchyme, along with histological alterations and an abnormal melanization pattern of the limb, indicate altered dorsal-ventral (DV) boundaries. These findings suggest that signaling through BMPR-IA in limb mesenchyme is essential for distal outgrowth and also influences AP and DV patterning.

Cholesterol modification restricts the spread of Shh gradient in the limb bud

Sonic hedgehog (Shh) produced in the zone of polarizing activity is the major determinant of anteroposterior development of the amniote limb. The mature and active Shh protein is cholesterol-modified at its C terminus, and the hydrophobic nature of the modification requires the function of Dispatched (mDispA), a seven-pass transmembrane protein, for Shh release from its source. The current model suggests that the cholesterol moiety promotes the spread of Shh gradient in the limb bud. However, this model is inconsistent with findings in Drosophila and not in line with current thoughts on the role of the cholesterol moiety in Shh multimerization. Therefore, it remains unclear how the cholesterol moiety affects the postrelease extracellular behavior of Shh that relates to the shape of its activity gradient in responsive tissues. Here, we report functional analyses in mice showing that Shh lacking cholesterol modification (ShhN) has an increased propensity to spread long-distance, eliciting ectopic Shh pathway activation consistent with target gene expressions and modulating the level of Gli3 processing in the anterior limb mesoderm. These molecular alterations are reflected in the mispatterning of digits in ShhN mutants. Additionally, we provide direct evidence for the long-distance movement of ShhN across the anteroposterior axis of the limb bud. Our findings suggest that the cholesterol moiety regulates the range and shape of the Shh morphogen gradient by restricting rather than promoting the postrelease spread of Shh across the limb bud during early development.

The bone morphogenetic protein antagonist gremlin 1 is overexpressed in human cancers and interacts with YWHAH protein

BACKGROUND

Basic studies of oncogenesis have demonstrated that either the elevated production of particular oncogene proteins or the occurrence of qualitative abnormalities in oncogenes can contribute to neoplastic cellular transformation. The purpose of our study was to identify an unique gene that shows cancer-associated expression, and characterizes its function related to human carcinogenesis.

METHODS

We used the differential display (DD) RT-PCR method using normal cervical, cervical cancer, metastatic cervical tissues, and cervical cancer cell lines to identify genes overexpressed in cervical cancers and identified gremlin 1 which was overexpressed in cervical cancers. We determined expression levels of gremlin 1 using Northern blot analysis and immunohistochemical study in various types of human normal and cancer tissues. To understand the tumorigenesis pathway of identified gremlin 1 protein, we performed a yeast two-hybrid screen, GST pull down assay, and immunoprecipitation to identify gremlin 1 interacting proteins.

RESULTS

DDRT-PCR analysis revealed that gremlin 1 was overexpressed in uterine cervical cancer. We also identified a human gremlin 1 that was overexpressed in various human tumors including carcinomas of the lung, ovary, kidney, breast, colon, pancreas, and sarcoma. PIG-2-transfected HEK 293 cells exhibited growth stimulation and increased telomerase activity. Gremlin 1 interacted with homo sapiens tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein, eta polypeptide (14-3-3 eta; YWHAH). YWHAH protein binding site for gremlin 1 was located between residues 61-80 and gremlin 1 binding site for YWHAH was found to be located between residues 1 to 67.

CONCLUSION

Gremlin 1 may play an oncogenic role especially in carcinomas of the uterine cervix, lung, ovary, kidney, breast, colon, pancreas, and sarcoma. Over-expressed gremlin 1 functions by interaction with YWHAH. Therefore, Gremlin 1 and its binding protein YWHAH could be good targets for developing diagnostic and therapeutic strategies against human cancers.

Making digit patterns in the vertebrate limb

The vertebrate limb has been a premier model for studying pattern formation - a striking digit pattern is formed in human hands, with a thumb forming at one edge and a little finger at the other. Classic embryological studies in different model organisms combined with new sophisticated techniques that integrate gene-expression patterns and cell behaviour have begun to shed light on the mechanisms that control digit patterning, and stimulate re-evaluation of the current models.

Hedgehog signaling in the normal and diseased pancreas

The hedgehog (Hh) family of genes, sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh) encode signaling molecules that regulate multiple functions during organ development and in adult tissues. Altered hedgehog signaling has been implicated in disturbed organ development as well as in different degenerative and neoplastic human diseases. Hedgehog signaling plays an important role in determination the fate of the mesoderm of the gut tube, as well as in early pancreatic development, and islet cell function. Recently, it has been shown that deregulation of hedgehog signaling molecules contributes to the pathogenesis and progression of pancreatic cancer and of chronic pancreatitis. Inhibition of hedgehog signaling using hedgehog antagonists reduces pancreatic cancer cell growth in vitro and in vivo, thus holding promise of novel agents in the treatment of this devastating disease. In this review, we discuss the role of hedgehog signaling during pancreatic development, its role in the pathogenesis of both chronic pancreatitis and pancreatic cancer, and lastly, the implications of this newly available information with regards to treatment of pancreatic cancer.

Multiple roles of mesenchymal beta-catenin during murine limb patterning

Recently canonical Wnt signaling in the ectoderm has been shown to be required for maintenance of the apical ectodermal ridge (AER) and for dorsoventral signaling. Using conditional gain- and loss-of-function beta-catenin alleles, we have studied the role of mesenchymal beta-catenin activity during limb development. Here, we show that loss of beta-catenin results in limb truncations due to a defect in AER maintenance. Stabilization of beta-catenin also results in truncated limbs, caused by a premature regression of the AER. Concomitantly, in these limbs, the expression of Bmp2, Bmp4 and Bmp7, and of the Bmp target genes Msx1, Msx2 and gremlin, is expanded in the mesenchyme. Furthermore, we found that the expression of Lmx1b, a gene exclusively expressed in the dorsal limb mesenchyme and involved in dorsoventral patterning, is reduced upon loss of beta-catenin activity and is expanded ventrally in gain-of-function limbs. However, the known ectodermal regulators Wnt7a and engrailed 1 are expressed normally. This suggests that Lmx1b is also regulated, in part, by a beta-catenin-mediated Wnt signal, independent of the non-canoncial Wnt7a signaling pathway. In addition, loss of beta-catenin results in a severe agenesis of the scapula. Concurrently, the expression of two genes, Pax1 and Emx2, which have been implicated in scapula development, is lost in beta-catenin loss-of-function limbs; however, only Emx2 is upregulated in gain-of-function limbs. Mesenchymal beta-catenin activity is therefore required for AER maintenance, and for normal expression of Lmx1b and Emx2.

Gremlin gene expression in bovine retinal pericytes exposed to elevated glucose

AIM

To assess the influence of high extracellular glucose on the expression of the bone morphogenetic protein (BMP) antagonist, gremlin, in cultured bovine retinal pericytes (BRPC).

METHODS

BRPC were cultured under conditions of 5 mM and 30 mM d-glucose for 7 days and total RNA was isolated. Gremlin mRNA levels were correlated, by RT-PCR, with other genes implicated in the pathogenesis of diabetic retinopathy and the signalling pathways in high glucose induced gremlin expression were probed using physiological inhibitors. Gremlin expression was also examined in the retina of streptozotocin induced diabetic mice.

RESULTS

High glucose stimulated a striking increase in BRPC gremlin mRNA levels in parallel with increases in mRNA for the growth factors vascular endothelial growth factor (VEGF), transforming growth factor beta (TGFbeta), and connective tissue growth factor (CTGF) and changes in other genes including fibronectin and plasminogen activator inhibitor-1 (PAI-1). High glucose triggered gremlin expression was modulated by anti-TGFbeta antibody, by the uncoupler of oxidative phosphorylation, CCCP, and by inhibition of MAP-kinase (MAPK) activation. Striking gremlin expression was observed in the outer retina of diabetic mice and also at the level of the vascular wall.

CONCLUSIONS

Gremlin gene expression is induced in BRPC in response to elevated glucose and in the retina of the streptozotocin induced diabetic mouse. Its expression is modulated by hyperglycaemic induction of the MAPK, reactive oxygen species, and TGFbeta pathways, all of which are reported to have a role in diabetic fibrotic disease. This implicates a role for gremlin in the pathogenesis of diabetic retinopathy.

DNA methylation-associated inactivation of TGFbeta-related genes DRM/Gremlin, RUNX3, and HPP1 in human cancers

The transforming growth factor β (TGFβ)-signalling pathway is deregulated in many cancers. We examined the role of gene silencing via aberrant methylation of DRM/Gremlin and HPP1, which inhibit TGFβ signalling, and RUNX3, which facilitates TGFβ-signalling, of all genes that are thought to be tumour suppressors, are aberrantly expressed, and are thus thought to have important role in human cancers. We examined DRM/Gremlin mRNA expression in 44 cell lines and the promoter methylation status of DRM/Gremlin, HPP1, and RUNX3 in 44 cell lines and 511 primary tumours. The loss of DRM/Gremlin mRNA expression in human cancer cell lines is associated with DNA methylation, and treatment with the methylation inhibitor-reactivated mRNA expression (n=13). Methylation percentages of the three genes ranged from 0–83% in adult tumours and 0–50% in paediatric tumours. Methylation of DRM/Gremlin was more frequent in lung tumours in smokers, and methylation of all three genes was inversely correlated with prognosis in patients with bladder or prostate cancer. Our results provide strong evidence that these TGFβ-related genes are frequently deregulated through aberrant methylation in many human malignancies.

Reciprocal epithelial-mesenchymal FGF signaling is required for cecal development

Fibroblast growth factor (FGF) signaling mediates reciprocal mesenchymal-epithelial cell interactions in the developing mouse lung and limb. In the gastrointestinal (GI) tract, FGF10 is expressed in the cecal mesenchyme and signals to an epithelial splice form of FGF receptor (FGFR) 2 to regulate epithelial budding. Here, we identify FGF9 as a reciprocal epithelial-mesenchymal signal required for cecal morphogenesis. Fgf9 null (Fgf9(-/-)) mouse embryos have agenesis of the embryonic cecum, lacking both mesenchymal expansion and an epithelial bud. In the cecal region of Fgf9(-/-) embryos, mesenchymal expression of Fgf10 and Bmp4 is notably absent, whereas the expression of epithelial markers, such as sonic hedgehog, is not affected. Using epithelial and whole explant cultures, we show that FGF9 signals to mesenchymal FGFRs and that FGF10 signals to epithelial FGFRs. Taken together, these data show that an epithelial FGF9 signal is necessary for the expansion of cecal mesenchyme and the expression of mesenchymal genes that are required for epithelial budding. Thus, these data add to our understanding of FGF-mediated reciprocal epithelial-mesenchymal signaling.

Increasing Fgf4 expression in the mouse limb bud causes polysyndactyly and rescues the skeletal defects that result from loss of Fgf8 function

A major function of the limb bud apical ectodermal ridge (AER) is to produce fibroblast growth factors (FGFs) that signal to the underlying mesenchyme. Previous studies have suggested that of the four FGF genes specifically expressed in the mouse AER, Fgf8 is unique not only in its expression pattern, but also because it is the only such FGF gene that causes limb skeletal abnormalities when individually inactivated. However, when both Fgf8 and Fgf4 are simultaneously inactivated in the AER, the limb does not develop. One possible explanation for these observations is that although both of these FGF family members contribute to limb development, Fgf8 has functions that Fgf4 cannot perform. To test this hypothesis, we used a novel method to substitute Fgf4 for Fgf8 expression in the developing limb bud by concomitantly activating a conditional Fgf4 gain-of-function allele and inactivating an Fgf8 loss-of-function allele in the same cells via Cre-mediated recombination. Our data show that when Fgf4 is expressed in place of Fgf8, all of the skeletal defects caused by inactivation of Fgf8 are rescued, conclusively demonstrating that FGF4 can functionally replace FGF8 in limb skeletal development. We also show that the increase in FGF signaling that occurs when the Fgf4 gain-of-function allele is activated in a wild-type limb bud causes formation of a supernumerary posterior digit (postaxial polydactyly), as well as cutaneous syndactyly between all the digits. These data underscore the importance of controlling the level of FGF gene expression for normal limb development.

Fgf8 expression defines a morphogenetic center required for olfactory neurogenesis and nasal cavity development in the mouse

In vertebrate olfactory epithelium (OE), neurogenesis proceeds continuously, suggesting that endogenous signals support survival and proliferation of stem and progenitor cells. We used a genetic approach to test the hypothesis that Fgf8 plays such a role in developing OE. In young embryos, Fgf8 RNA is expressed in the rim of the invaginating nasal pit (NP), in a small domain of cells that overlaps partially with that of putative OE neural stem cells later in gestation. In mutant mice in which the Fgf8 gene is inactivated in anterior neural structures, FGF-mediated signaling is strongly downregulated in both OE proper and underlying mesenchyme by day 10 of gestation. Mutants survive gestation but die at birth, lacking OE, vomeronasal organ (VNO), nasal cavity, forebrain, lower jaw, eyelids and pinnae. Analysis of mutants indicates that although initial NP formation is grossly normal, cells in the Fgf8-expressing domain undergo high levels of apoptosis, resulting in cessation of nasal cavity invagination and loss of virtually all OE neuronal cell types. These findings demonstrate that Fgf8 is crucial for proper development of the OE, nasal cavity and VNO, as well as maintenance of OE neurogenesis during prenatal development. The data suggest a model in which Fgf8 expression defines an anterior morphogenetic center, which is required not only for the sustenance and continued production of primary olfactory (OE and VNO) neural stem and progenitor cells, but also for proper morphogenesis of the entire nasal cavity.

STAR proteins quaking-6 and GLD-1 regulate translation of the homologues GLI1 and tra-1 through a conserved RNA 3'UTR-based mechanism

The binding of the STAR protein GLD-1 to an element in the tra-2 3' untranslated region (3'UTR), called the TGE (tra GLI element), represses tra-2 translation, allowing for hermaphrodite spermatogenesis in Caenorhabditis elegans. GLD-1 is a member of the STAR family that includes the mammalian quaking (Qk) proteins. Here, we show that the 3'UTR of the nematode homologue of GLI1, called tra-1, also contains a TGE, through which translation is regulated by GLD-1. We find that GLD-1 activity is required for proper TRA-1 protein expression in hermaphrodites. RNA gel shift assays show that GLD-1 binds the predicted sites. Using reporter transgenes, we show that the human GLI1 (hGLI1) 3'UTR controls translation in the mouse embryo. We demonstrate that the addition of the mouse QK isoform-6 (QKI-6) protein to a mammalian cell line that lacks QKI-6 can confer regulation on reporter and GLI1 mRNAs in a TGE-specific manner, and reduction of QKI-6 activity with siRNA disrupts translational control. Further, siRNA knockdown of QKI-6 increases the activity of a reporter transgene that monitors the transcriptional activity of mouse Gli1 (mGli1) and increases mouse Gli1 protein. We show by immunoprecipitation that QKI-6 antibody specifically co-purifies TGE-containing mRNAs in ribonucleoproteins. Thus, we find that the mouse QKI-6 protein acts through the mGli1 and hGLI1 RNAs to repress translation. Our results suggest that STAR family-dependent translational control of GLI mRNAs is ancient, and that it existed before the division of nematodes and mammals.

Studies on epidermal growth factor receptor signaling in vertebrate limb patterning

The epidermal growth factor receptor (EGFR) regulates multiple patterning events in Drosophila limb development, but its role in vertebrate limb morphogenesis has received little attention. The EGFR and several of its ligands are expressed in developing vertebrate limbs in manners consistent with potential patterning roles. To gain insight into functions of EGFR signaling in vertebrate limb development, we expressed a constitutively active EGFR in developing chick limbs in ovo. Expression of activated EGFR causes pre- and postaxial polydactyly, including mirror-image-type digit duplication, likely due to induction of ectopic expression and/or modulation of genes involved in anterior-posterior (AP) patterning such as Sonic hedgehog (Shh), dHand, Patched (Ptc), Gli3, Hoxd13, Hoxd11, bone morphogenetic protein 2 (Bmp2), Gremlin, and FGF4. Activation of EGFR signaling dorsalizes the limb and alters expression of the dorsal-ventral (DV) patterning genes Wnt7a, Lmx, and En1. Ectopic and/or extended FGF8 expressing apical ectodermal ridges (AERs) are also seen. Interdigital regression is inhibited and the digits fail to separate, leading to syndactyly, likely due to antiapoptotic and pro-proliferative effects of activated EGFR signaling on limb mesoderm, and/or attenuation of interdigital Bmp4 expression. These findings suggest potential roles for EGFR signaling in AP and DV patterning, AER formation, and cell survival during limb morphogenesis.

Globalisation reaches gene regulation: the case for vertebrate limb development

Analysis of key regulators of vertebrate limb development has revealed that the cis-regulatory regions controlling their expression are often located several hundred kilobases upstream of the transcription units. These far up- or down-stream cis-regulatory regions tend to reside within rather large, functionally and structurally unrelated genes. Molecular analysis is beginning to reveal the complexity of these large genomic landscapes, which control the co-expression of clusters of diverse genes by this novel type of long-range and globally acting cis-regulatory region. An increasing number of spontaneous mutations in vertebrates, including humans, are being discovered inactivating or altering such global control regions. Thereby, the functions of a seemingly distant but essential gene are disrupted rather than the closest.

Function and regulation of Alx4 in limb development: complex genetic interactions with Gli3 and Shh

The role of the aristaless-related homeobox gene Alx4 in antero-posterior (AP-) patterning of the developing vertebrate limb has remained somewhat elusive. Polydactyly of Alx4 mutant mice is known to be accompanied by ectopic anterior expression of genes like Shh, Fgf4 and 5'Hoxd. We reported previously that polydactyly in Alx4 mutant mice requires SHH signaling, but we now show that in early Alx4-/- limb buds the anterior ectopic expression of Fgf4 and Hoxd13, and therefore disruption of AP-patterning, occurs independently of SHH signaling. To better understand how Alx4 functions in the pathways that regulate AP-patterning, we also studied genomic regulatory sequences that are capable of directing expression of a reporter gene in a pattern corresponding to endogenous Alx4 expression in anterior limb bud mesenchyme. We observed, as expected for authentic Alx4 expression, expansion of reporter construct expression in a Shh-/- background. Total lack of reporter expression in a Gli3-/- background confirms the existence of Gli3-dependent and -independent Alx4 expression in the limb bud. Apparently, these two modules of Alx4 expression are linked to dissimilar functions.

Factors involved in the determination of the neurotransmitter phenotype of developing neurons of the CNS: Applications in cell replacement treatment for Parkinson's disease

The developmental stages involved in the conversion of stem cells to fully functional neurons of specific neurotransmitter phenotype are complex and not fully understood. Over the past decade many studies have been published that demonstrate that in vitro manipulation of the epigenetic environment of the stem cells allows experimental control of final neuronal phenotypic choice. This review presents the evidence for the involvement of a number of endogenous neurobiochemicals, which have been reported to potently influence DAergic (and other neurotransmitter) phenotype expression in vitro. They act at different stages on the pathway to neurotransmitter phenotype determination, and in different ways. Many are better known for their involvement in other aspects of development, and in other biochemical roles. Their proper place, and precise roles, in neurotransmitter phenotype determination in vivo will no doubt be determined in the future. Meanwhile, considerable medical benefits are offered from producing large, long-term, viable cryostores of self-regenerating multipotential neural precursor cells (i.e., brain stem cells), which can be used for cell replacement therapies in the treatment of degenerative brain diseases, such as Parkinson's disease.

Analysis of Msx1; Msx2 double mutants reveals multiple roles for Msx genes in limb development

The homeobox-containing genes Msx1 and Msx2 are highly expressed in the limb field from the earliest stages of limb formation and, subsequently, in both the apical ectodermal ridge and underlying mesenchyme. However, mice homozygous for a null mutation in either Msx1 or Msx2 do not display abnormalities in limb development. By contrast, Msx1; Msx2 double mutants exhibit a severe limb phenotype. Our analysis indicates that these genes play a role in crucial processes during limb morphogenesis along all three axes. Double mutant limbs are shorter and lack anterior skeletal elements (radius/tibia, thumb/hallux). Gene expression analysis confirms that there is no formation of regions with anterior identity. This correlates with the absence of dorsoventral boundary specification in the anterior ectoderm, which precludes apical ectodermal ridge formation anteriorly. As a result, anterior mesenchyme is not maintained, leading to oligodactyly. Paradoxically, polydactyly is also frequent and appears to be associated with extended Fgf activity in the apical ectodermal ridge, which is maintained up to 14.5 dpc. This results in a major outgrowth of the mesenchyme anteriorly, which nevertheless maintains a posterior identity, and leads to formation of extra digits. These defects are interpreted in the context of an impairment of Bmp signalling.

Conditional BMP inhibition in Xenopus reveals stage-specific roles for BMPs in neural and neural crest induction

Bone morphogenetic protein (BMP) inhibition has been proposed as the primary determinant of neural cell fate in the developing Xenopus ectoderm. The evidence supporting this hypothesis comes from experiments in explanted "animal cap" ectoderm and in intact embryos using BMP antagonists that are unregulated and active well before gastrulation. While informative, these experiments cannot answer questions regarding the timing of signals and the behavior of cells in the more complex environment of the embryo. To examine the effects of BMP antagonism at defined times in intact embryos, we have generated a novel, two-component system for conditional BMP inhibition. We find that while blocking BMP signals induces ectopic neural tissue both in animal caps and in vivo, in intact embryos, it can only do so prior to late blastula stage (stage 9), well before the onset of gastrulation. Later inhibition does not induce neural identity, but does induce ectopic neural crest, suggesting that BMP antagonists play temporally distinct roles in establishing neural and neural crest identity. By combining BMP inhibition with fibroblast growth factor (FGF) activation, the neural inductive response in whole embryos is greatly enhanced and is no longer limited to pre-gastrula ectoderm. Thus, BMP inhibition during gastrulation is insufficient for neural induction in intact embryos, arguing against a BMP gradient as the sole determinant of ectodermal cell fate in the frog.

Sprouty1 is a critical regulator of GDNF/RET-mediated kidney induction

Intercellular signaling molecules and their receptors, whose expression must be tightly regulated in time and space, coordinate organogenesis. Regulators of intracellular signaling pathways provide an additional level of control. Here we report that loss of the receptor tyrosine kinase (RTK) antagonist, Sprouty1 (Spry1), causes defects in kidney development in mice. Spry1(-/-) embryos have supernumerary ureteric buds, resulting in the development of multiple ureters and multiplex kidneys. These defects are due to increased sensitivity of the Wolffian duct to GDNF/RET signaling, and reducing Gdnf gene dosage correspondingly rescues the Spry1 null phenotype. We conclude that the function of Spry1 is to modulate GDNF/RET signaling in the Wolffian duct, ensuring that kidney induction is restricted to a single site. These results demonstrate the importance of negative feedback regulation of RTK signaling during kidney induction and suggest that failures in feedback control may underlie some human congenital kidney malformations.

The contribution of chicken embryology to the understanding of vertebrate limb development

The chicken is an excellent model organism for studying vertebrate limb development, mainly because of the ease of manipulating the developing limb in vivo. Classical chicken embryology has provided fate maps and elucidated the cell-cell interactions that specify limb pattern. The first defined chemical that can mimic one of these interactions was discovered by experiments on developing chick limbs and, over the last 15 years or so, the role of an increasing number of developmentally important genes has been uncovered. The principles that underlie limb development in chickens are applicable to other vertebrates and there are growing links with clinical genetics. The sequence of the chicken genome, together with other recently assembled chicken genomic resources, will present new opportunities for exploiting the ease of manipulating the limb.

Hoxd13 expression in the developing limbs of the short-tailed fruit bat, Carollia perspicillata

Bat forelimbs are highly specialized for sustained flight, providing a unique model to explore the genetic programs that regulate vertebrate limb diversity. Hoxd9-13 genes are important regulators of stylopodium, zeugopodium, and autopodium development and thus evolutionary changes in their expression profiles and biochemical activities may contribute to divergent limb morphologies in vertebrates. We have isolated the genomic region that includes Hoxd12 and Hoxd13 from Carollia perspicillata, the short-tailed fruit bat. The bat Hoxd13 gene encodes a protein that shares 95% identity with human and mouse HOXD13. The expression pattern of bat Hoxd13 mRNA during limb development was compared with that of mouse. In bat and mouse hindlimbs, the expression patterns of Hoxd13 are relatively similar. However, although the forelimb Hoxd13 expression patterns in both organisms during early limb bud stages are similar, at later stages they diverge; the anterior expression boundary of bat Hoxd13 is posterior-shifted relative to the mouse. These findings, compared with the Hoxd13 expression profiles of other vertebrates, suggest that divergent Hoxd13 expression patterns may contribute to limb morphological variation.

Genetic analysis of interactions between the somitic muscle, cartilage and tendon cell lineages during mouse development

Proper formation of the musculoskeletal system requires the coordinated development of the muscle, cartilage and tendon lineages arising from the somitic mesoderm. During early somite development, muscle and cartilage emerge from two distinct compartments, the myotome and sclerotome, in response to signals secreted from surrounding tissues. As the somite matures, the tendon lineage is established within the dorsolateral sclerotome, adjacent to and beneath the myotome. We examine interactions between the three lineages by observing tendon development in mouse mutants with genetically disrupted muscle or cartilage development. Through analysis of embryos carrying null mutations in Myf5 and Myod1, hence lacking both muscle progenitors and differentiated muscle, we identify an essential role for the specified myotome in axial tendon development, and suggest that absence of tendon formation in Myf5/Myod1 mutants results from loss of the myotomal FGF proteins, which depend upon Myf5 and Myod1 for their expression, and are required, in turn, for induction of the tendon progenitor markers. Our analysis of Sox5/Sox6 double mutants, in which the chondroprogenitors are unable to differentiate into cartilage,reveals that the two cell fates arising from the sclerotome, axial tendon and cartilage are alternative lineages, and that cartilage differentiation is required to actively repress tendon development in the dorsolateral sclerotome.

Skeletal overexpression of gremlin impairs bone formation and causes osteopenia

Skeletal cells synthesize bone morphogenetic proteins (BMPs) and BMP antagonists. Gremlin, a BMP antagonist, is expressed in osteoblasts and opposes BMP effects on osteoblastic differentiation and function in vitro. However, its effects in vivo are not known. To investigate the actions of gremlin on bone remodeling in vivo, we generated transgenic mice overexpressing gremlin under the control of the osteocalcin promoter. Gremlin transgenics exhibited bone fractures and reduced bone mineral density by 20-30%, compared with controls. Static and dynamic histomorphometry of femurs revealed that gremlin overexpression caused reduced trabecular bone volume and the appearance of woven bone. Polarized light microscopy revealed disorganized collagen bundles at the endosteal cortical surface. Gremlin transgenic mice displayed a 70% decrease in the number of osteoblasts/trabecular area and reduced mineral apposition and bone formation rates. In vivo bromodeoxyuridine labeling and marrow stromal cell cultures demonstrated an inhibitory effect of gremlin on osteoblastic cell replication, but no change on apoptosis was detected. Marrow stromal cells from gremlin transgenics displayed a reduced response to BMP on phosphorylated mothers against decapentaplegic 1/5/8 phosphorylation and reduced free cytosolic beta-catenin levels. In conclusion, transgenic mice overexpressing gremlin in the bone microenvironment have decreased osteoblast number and function leading to osteopenia and spontaneous fractures.

The developing limb and the control of the number of digits

Congenital malformations of the limbs are among the most frequent congenital anomalies found in humans, and they preferentially affect the distal part--the hand or foot. The presence of extra digits, a condition called polydactyly, is the most common limb deformity of the human hand and is the consequence of disturbances in the normal program of limb development. However, despite the extensive use of the developing limb as a classical developmental model, the cellular and genetic mechanisms that control the number and identity of the digits are not completely understood. The aim of this review is to introduce the reader to the current state of knowledge in limb development and to provide the necessary background for an understanding of how deviations from the normal developmental program may lead to polydactyly.

Levels of Gli3 repressor correlate with Bmp4 expression and apoptosis during limb development

Removal of the posterior wing bud leads to massive apoptosis of the remaining anterior wing bud mesoderm. We show here that this finding correlates with an increase in the level of the repressor form of the Gli3 protein, due to the absence of the Sonic hedgehog (Shh) protein signaling. Therefore, we used the anterior wing bud mesoderm as a model system to analyze the relationship between the repressor form of Gli3 and apoptosis in the developing limb. With increased Gli3R levels, we demonstrate a concomitant increase in Bmp4 expression and signaling in the anterior mesoderm deprived of Shh signaling. Several experimental approaches show that the apoptosis can be prevented by exogenous Noggin, indicating that Bmp signaling mediates it. The analysis of Bmp4 expression in several mouse and chick mutations with defects in either expression or processing of Gli3 indicates a correlation between the level of the repressor form of Gli3 and Bmp4 expression in the distal mesoderm. Our analysis adds new insights into the way Shh differentially controls the processing of Gli3 and how, subsequently, BMP4 expression may mediate cell survival or cell death in the developing limb bud in a position-dependent manner.

Shh signaling in limb bud ectoderm: Potential role in teratogen-induced postaxial ectrodactyly

A variety of teratogens induce the loss of postaxial forelimb structures when administered during mid-gestation to the mouse. Previous studies demonstrated that teratogen exposure is associated with a reduction in zone of polarizing activity (ZPA) -related polarizing activity without a noticeable loss of Shh expression. Herein, we quantitatively confirm that expression of Shh, Ptch1, and Gli3 are unaltered by teratogen exposure and demonstrate that sonic hedgehog (Shh) translation is unaffected. Examination of the polarizing response of host chick wings to teratogen-exposed ZPA tissue revealed an induced growth response and ectopic induction of Fgf4, Bmp2, Ptch1, and Gli1 expression similar to control ZPA tissue. Control ZPA tissue altered the fate of cells destined to die in the anterior necrotic zone, whereas cell death ensued in hosts receiving teratogen-exposed grafts. Immunohistochemical studies localized Shh protein in the mouse limb to the posterior mesoderm and overlying ectoderm. We postulate that teratogen exposure alters the ability of Shh to signal to the ectoderm and present microarray and reverse transcriptase-polymerase chain reaction data, indicating that Shh signaling could occur in the limb bud ectoderm.

Bmp4 in limb bud mesoderm regulates digit pattern by controlling AER development

In the developing limb, Bmp4 is expressed in the apical ectodermal ridge (AER) and underlying mesoderm. Insight into the function of Bmp4 in limb development has been hampered by the early embryonic lethality of Bmp4 null embryos. We directly investigated Bmp4 using a conditional null allele of Bmp4 and the Prx1(cre) transgene to inactivate Bmp4 in limb bud mesoderm. The limb bud mesoderm of Prx1(cre);Bmp4 mutants was defective in production of Bmp4 but still competent to respond to Bmp signaling. Prx1(cre);Bmp4 mutant embryos had defective digit patterning including hindlimb preaxial polydactyly with posterior digit transformations. The Prx1(cre);Bmp4 mutants also had postaxial polydactyly with digit five duplications. Bmp4 mutant limbs had delayed induction and maturation of the AER that resulted in expanded Shh signaling. Moreover, the AER persisted longer in the Bmp4 mutant limb buds exposing the forming digits to prolonged Fgf8 signaling. Our data show that Bmp4 in limb mesoderm regulates AER induction and maturation and implicate signaling from the AER in regulation of digit number and identity.

It takes time to make a pinky: unexpected insights into how SHH patterns vertebrate digits

It is widely accepted that the diffusible Sonic Hedgehog (SHH) morphogen signal establishes a spatial gradient that patterns embryonic structures by long-range signaling. In response, cell fates are determined by linear thresholds according to the position of cells within the gradient field. Two recent studies of SHH signaling during vertebrate limb development challenge this spatial gradient model. They establish that a large fraction of limb bud cells patterned by SHH are descendants of cells that have previously expressed Shh. These cells are endowed with a kinetic memory that integrates their exposure to SHH rather than sensing their position in a SHH gradient. In addition, a fraction of cells changes their SHH responsiveness progressively during limb bud pattering, which is indicative of local nonlinear modulation of cell fate specification.

Cutting Edge: Bone Morphogenetic Protein Antagonists Drm/Gremlin and Dan Interact with Slits and Act as Negative Regulators of Monocyte Chemotaxis

Drm/Gremlin and Dan, two homologous secreted antagonists of bone morphogenic proteins, have been shown to regulate early development, tumorigenesis, and renal pathophysiology. In this study, we report that Drm and Dan physically and functionally interact with Slit1 and Slit2 proteins. Drm binding to Slits depends on its glycosylation and is not interfered with by bone morphogenic proteins. Importantly, Drm and Dan function as inhibitors for monocyte migration induced by stromal cell-derived factor 1alpha (SDF-1alpha) or fMLP. The inhibition of SDF-1alpha-induced monocyte chemotaxis by Dan is not due to blocking the binding of SDF-1alpha to its receptor. Thus, the results identify that Drm and Dan can interact with Slit proteins and act as inhibitors of monocyte chemotaxis, demonstrating a previously unidentified biological role for these proteins.

Pax6 Interacts with cVax and Tbx5 to Establish the Dorsoventral Boundary of the Developing Eye

Dorsoventral pattern formation of the optic cup is essential for vertebrate eye morphogenesis and retinotectal topographic mapping. Dorsal and ventral aspects of the eye are distinct at early stages of development; cVax homeodomain protein expression is confined to the ventral optic cup, whereas Tbx5 (T-box transcription factor) expression domain becomes restricted to the dorsal region. Misexpression of cVax or Tbx5 induces profound defects in eye morphology and abnormal visual projections. In the Pax6-/- mutant Tbx5 fails to be expressed, and Vax1 and -2 are abnormally present in the entire optic vesicle. During eye development Pax6 becomes expressed in a gradient at the optic cup stage due to the specific activation of a highly conserved intronic alpha enhancer in the Pax6 locus. We observed that the highest level of Pax6 in the optic cup corresponds to the boundary between non-overlapping cVax and Tbx5 territories. To further investigate how these transcription factors control the patterning of the eye, we overexpressed Pax6 in the chick optic cup (E2) using in ovo electroporation. We observed that overexpression of Pax6 extends the Tbx5 and Bmp4 domains but reduces the cVax expression domains in the E3 chick eye. This results in an abnormal eye phenotype at E4. In addition, we showed that cVax and Tbx5 interact with Pax6 and modulate in an opposite manner the activity of the Pax6 alpha enhancer. Moreover, the Pax6/cVax interaction inhibits the transactivation properties of Pax6. These results demonstrate that Pax6 together with cVax and Tbx5 mediate dorsoventral patterning of the eye.

Comparison of maf gene expression patterns during chick embryo development

Maf proteins are basic-leucine zipper transcription factors belonging to the AP1 superfamily. Several developmental processes require Maf proteins yet, the redundancy or complementarity of their respective roles in common processes has been only partially investigated. We present for the first time a complete comparative analysis of maf gene expression patterns in vertebrates. Expression of c-maf, mafB/kreisler, mafA/L-maf, mafF, mafG and mafK was analyzed by whole-mount in situ hybridization within chick embryos and their extraembryonic tissues ranging from embryonic day (E) 1 to 7. We carefully examined the extent of overlap between distinct maf genes and report that the developing lens, kidney, pancreas and apoptotic zones of limb buds show sustained co-expression of large maf genes. Small maf genes also exhibit overlap, for example in the dermomyotome. We also describe so far unidentified sites of maf gene expression. mafA is found in the developing neural tube and dorsal root ganglia. c-maf hybridization is detected in the neuroretina, the notochord and the endothelium of extraembryonic blood vessels.

Early neural cell death: dying to become neurons

The importance of programmed cell death (PCD) during vertebrate development has been well established. During the development of the nervous system in particular, neurotrophic cell death in innervating neurons matches the number of neurons to the size of their target field. However, PCD also occurs during earlier stages of neural development, within populations of proliferating neural precursors and newly postmitotic neuroblasts, all of which are not yet fully differentiated. This review addresses early neural PCD, which is distinct from neurotrophic death in differentiated neurons. Although early neural PCD is observed in a range of organisms, from Caenorhabditis elegans to mouse, the role and the regulation of early neural PCD are not well understood. The regulation of early neural PCD can be inferred from the function of factors such as bone morphogenetic proteins (BMPs), Wnts, fibroblast growth factors (FGFs), and Sonic Hedgehog (Shh), which regulate both early neural development and PCD occurring in other developmental processes. Cell number control, removal of damaged or misspecified cells (spatially or temporally), and selection are the proposed roles early neural PCDs play during neural development. Data from developmental PCD in C. elegans and Drosophila provide insights into the possible signaling pathways integrating PCD with other processes during early neural development and the roles they might play.

Alpha-catenin: at the junction of intercellular adhesion and actin dynamics

Alpha-catenin has often been considered to be a non-regulatory intercellular adhesion protein, in contrast to beta-catenin, which has well-documented dual roles in cell-cell adhesion and signal transduction. Recently, however, alpha-catenin has been found to be important not only in connecting the E-cadherin-beta-catenin complex to the actin cytoskeleton, but also in coordinating actin dynamics and inversely correlating cell adhesion with proliferation. As the number of alpha-catenin-interacting partners increases, intriguing new connections imply even more complex regulatory functions for this protein.

Twist is an integrator of SHH, FGF, and BMP signaling

Development of vertebrate embryos is regulated by a number of different signaling pathways. These pathways are frequently not independent of each other but are connected by crosstalk between cells and tissues. Furthermore, different signaling pathways have been found to interact at the cellular level. Development of cranial and limb structures is an example, in which FGF, BMP, and SHH signaling interact. Mutations in the different signaling pathways may therefore result in complex but similar phenotypes. This indicates the existence of integrator molecules, which depend in their expression or activity on the combination of different signaling pathways. Here we show that expression of the bHLH transcription factor Twist in the paraxial mesoderm requires an induction from the notochord. This induction can only be substituted by a combination of FGF and SHH signaling, but not by individual application of FGF8 or SHH alone. Furthermore, the expression of Twist can be modified by BMP2 in a complex, age-dependent manner. We propose that Twist is one of the integrating parts of the three signaling pathways and mediates some of the common effects.

Chick Dach1 interacts with the Smad complex and Sin3a to control AER formation and limb development along the proximodistal axis

Based on recent data, a new view is emerging that vertebrate Dachshund(Dach) proteins are components of Six1/6 transcription factor-dependent signaling cascades. Although Drosophila data strongly suggest a tight link between Dpp signaling and the Dachshund gene, a functional relationship between vertebrate Dach and BMP signaling remains undemonstrated. We report that chick Dach1 interacts with the Smad complex and the corepressor mouse Sin3a, thereby acting as a repressor of BMP-mediated transcriptional control. In the limb, this antagonistic action regulates the formation of the apical ectodermal ridge (AER) in both the mesenchyme and the AER itself, and also controls pattern formation along the proximodistal axis of the limb. Our data introduce a new paradigm of BMP antagonism during limb development mediated by Dach1, which is now proven to function in different signaling cascades with distinct interacting partners.

The limb bud Shh-Fgf feedback loop is terminated by expansion of former ZPA cells

Vertebrate limb outgrowth is driven by a positive feedback loop involving Sonic Hedgehog (Shh), Gremlin, and Fgf4. By overexpressing individual components of the loop at a time after these genes are normally down-regulated in chicken embryos, we found that Shh no longer maintains Gremlin in the posterior limb. Shh-expressing cells and their descendants cannot express Gremlin. The proliferation of these descendants forms a barrier separating the Shh signal from Gremlin-expressing cells, which breaks down the Shh-Fgf4 loop and thereby affects limb size and provides a mechanism explaining regulative properties of the limb bud.

Bmp7 regulates branching morphogenesis of the lacrimal gland by promoting mesenchymal proliferation and condensation

The lacrimal gland provides an excellent model with which to study the epithelial-mesenchymal interactions that are crucial to the process of branching morphogenesis. In the current study, we show that bone morphogenetic protein 7 (Bmp7) is expressed with a complex pattern in the developing gland and has an important role in regulating branching. In loss-of-function analyses, we find that Bmp7-null mice have distinctive reductions in lacrimal gland branch number, and that inhibition of Bmp activity in gland explant cultures has a very similar consequence. Consistent with this, exposure of whole-gland explants to recombinant Bmp7 results in increased branch number. In determining which cells of the gland respond directly to Bmp7, we have tested isolated mesenchyme and epithelium. We find that, as expected, Bmp4 can suppress bud extension in isolated epithelium stimulated by Fgf10, but interestingly, Bmp7 has no discernible effect. Bmp7 does, however, stimulate a distinct response in mesenchymal cells. This manifests as a promotion of cell division and formation of aggregates, and upregulation of cadherin adhesion molecules, the junctional protein connexin 43 and of alpha-smooth muscle actin. These data suggest that in this branching system, mesenchyme is the primary target of Bmp7 and that formation of mesenchymal condensations characteristic of signaling centers may be enhanced by Bmp7. Based on the activity of Bmp7 in promoting branching, we also propose a model suggesting that a discrete region of Bmp7-expressing head mesenchyme may be crucial in determining the location of the exorbital lobe of the gland.

Gremlin-mediated BMP antagonism induces the epithelial-mesenchymal feedback signaling controlling metanephric kidney and limb organogenesis

Epithelial-mesenchymal feedback signaling is the key to diverse organogenetic processes such as limb bud development and branching morphogenesis in kidney and lung rudiments. This study establishes that the BMP antagonist gremlin (Grem1) is essential to initiate these epithelial-mesenchymal signaling interactions during limb and metanephric kidney organogenesis. A Grem1 null mutation in the mouse generated by gene targeting causes neonatal lethality because of the lack of kidneys and lung septation defects. In early limb buds, mesenchymal Grem1 is required to establish a functional apical ectodermal ridge and the epithelial-mesenchymal feedback signaling that propagates the sonic hedgehog morphogen. Furthermore, Grem1-mediated BMP antagonism is essential to induce metanephric kidney development as initiation of ureter growth,branching and establishment of RET/GDNF feedback signaling are disrupted in Grem1-deficient embryos. As a consequence, the metanephric mesenchyme is eliminated by apoptosis, in the same way as the core mesenchymal cells of the limb bud.

Protein phosphatase 2A as a new target for morphogenetic studies in the chick limb

The family of ser/thr protein phosphatases 2A (PP2A) is a major regulator of cell proliferation and cell death and is critically involved in the maintenance of homeostasis. In order to analyse the importance of PP2A proteins in apoptotic and developmental processes, this review focuses on previous studies concerning the role of PP2A in morphogenesis. We first analyse wing formation in Drosophila, a model for invertebrates, then chick limb bud, a model for vertebrates. We also present a pioneer experiment to illustrate the potential relevance of PP2A studies in BMP signalling during chicken development and we finally discuss the BMP downstream signalling pathways.

Cardiac outflow tract defects in mice lacking ALK2 in neural crest cells

Cardiac neural crest cells are multipotent migratory cells that contribute to the formation of the cardiac outflow tract and pharyngeal arch arteries. Neural crest-related developmental defects account for a large proportion of congenital heart disorders. Recently, the genetic bases for some of these disorders have been elucidated, and signaling pathways required for induction,migration and differentiation of cardiac neural crest have emerged. Bone morphogenetic proteins comprise a family of secreted ligands implicated in numerous aspects of organogenesis, including heart and neural crest development. However, it has remained generally unclear whether BMP ligands act directly on neural crest or cardiac myocytes during cardiac morphogenesis,or function indirectly by activating other cell types. Studies on BMP receptor signaling during organogenesis have been hampered by the fact that receptor knockouts often lead to early embryonic lethality. We have used a Cre/loxP system for neural crest-specific deletion of the type I receptor, ALK2, in mouse embryos. Mutant mice display cardiovascular defects, including persistent truncus arteriosus, and abnormal maturation of the aortic arch reminiscent of common forms of human congenital heart disease. Migration of mutant neural crest cells to the outflow tract is impaired, and differentiation to smooth muscle around aortic arch arteries is deficient. Moreover, in Alk2 mutants, the distal outflow tract fails to express Msx1, one of the major effectors of BMP signaling. Thus, the type I BMP receptor ALK2 plays an essential cell-autonomous role in the development of the cardiac outflow tract and aortic arch derivatives.

FGF18 represses noggin expression and is induced by calcineurin

Fibroblast growth factors (FGFs) and bone morphogenetic proteins strongly regulate chondrogenesis and chondrocyte gene expression. The interactions of the signaling pathways initiated by these factors are central to the control of chondrocyte differentiation. Here we show that calcium-dependent signals induce expression of FGF18, an essential regulator of bone and cartilage differentiation. The induction of FGF18 expression required the calcium-dependent phosphatase, calcineurin. The activated forms of calcineurin or the calcineurin-dependent transcription factor, NFAT4 (nuclear factor of activated T-cell 4), induced FGF18 expression. FGF18 or a constitutive active FGF receptor suppressed noggin gene induction and thereby increased chondrocyte gene expression and chondrogenesis by facilitating bone morphogenetic protein-dependent signals. These findings reinforce the interdependence of bone morphogenetic protein and FGF signaling and provide a rational explanation for abnormal bone development occurring in humans or mice with constitutively active FGF receptors.