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

Tgf-β superfamily and limb regeneration: Tgf-β to start and Bmp to end

Axolotls represent a popular model to study how nature solved the problem of regenerating lost appendages in tetrapods. Our work over many years focused on trying to understand how these animals can achieve such a feat and not end up with a scarred up stump. The Tgf-β superfamily represents an interesting family to target since they are involved in wound healing in adults and pattern formation during development. This family is large and comprises Tgf-β, Bmps, activins and GDFs. In this review, we present work from us and others on Tgf-β & Bmps and highlight interesting observations between these two sub-families. Tgf-β is important for the preparation phase of regeneration and Bmps for the redevelopment phase and they do not overlap with one another. We present novel data showing that the Tgf-β non-canonical pathway is also not active during redevelopment. Finally, we propose a molecular model to explain how Tgf-β and Bmps maintain distinct windows of expression during regeneration in axolotls.

Fgf-signaling is compartmentalized within the mesenchyme and controls proliferation during salamander limb development

Although decades of studies have produced a generalized model for tetrapod limb development, urodeles deviate from anurans and amniotes in at least two key respects: their limbs exhibit preaxial skeletal differentiation and do not develop an apical ectodermal ridge (AER). Here, we investigated how Sonic hedgehog (Shh) and Fibroblast growth factor (Fgf) signaling regulate limb development in the axolotl. We found that Shh-expressing cells contributed to the most posterior digit, and that inhibiting Shh-signaling inhibited Fgf8 expression, anteroposterior patterning, and distal cell proliferation. In addition to lack of a morphological AER, we found that salamander limbs also lack a molecular AER. We found that amniote and anuran AER-specific Fgfs and their cognate receptors were expressed entirely in the mesenchyme. Broad inhibition of Fgf-signaling demonstrated that this pathway regulates cell proliferation across all three limb axes, in contrast to anurans and amniotes where Fgf-signaling regulates cell survival and proximodistal patterning.

Salamanders are a group of amphibians that are well-known for their ability to regenerate lost limbs and other body parts. At the turn of the twentieth century, researchers used salamander embryos as models to understand the basic concepts of how limbs develop in other four-limbed animals, including amphibians, mammals and birds, which are collectively known as “tetrapods”. However, the salamander’s amazing powers of regeneration made it difficult to carry out certain experiments, so researchers switched to using the embryos of other tetrapods – namely chickens and mice – instead.

Studies in chickens, later confirmed in mice and frogs, established that there are two major signaling centers that control how the limbs of tetrapod embryos form and grow: a small group of cells known as the “zone of polarizing activity” within a structure called the “limb bud mesenchyme”; and an overlying, thin ridge of cells called the “apical ectodermal ridge”. Both of these centers release potent signaling molecules that act on cells in the limbs. The cells in the zone of polarizing activity produce a molecule often called Sonic hedgehog, or Shh for short. The apical ectodermal ridge produces another group of signals commonly known as fibroblast growth factors, or simply Fgfs.

Several older studies reported that salamander embryos do not have an apical ectodermal ridge suggesting that these amphibian’s limbs may form differently to other tetrapods. Yet, contemporary models in developmental biology treated salamander limbs like those of chicks and mice. To address this apparent discrepancy, Purushothaman et al. studied how the forelimbs develop in a salamander known as the axolotl.

The experiments showed that, along with lacking an apical ectodermal ridge, axolotls did not produce fibroblast growth factors normally found in this tissue. Instead, these factors were only found in the limb bud mesenchyme. Purushothaman et al. also found that fibroblast growth factors played a different role in axolotls than previously reported in chick, frog and mouse embryos. On the other hand, the pattern and function of Shh activity in the axolotl limb bud was similar to that previously observed in chicks and mice.

These findings show that not all limbs develop in the same way and open up questions for evolutionary biologists regarding the evolution of limbs. Future studies that examine limb development in other animals that regenerate tissues, such as other amphibians and lungfish, will help answer these questions.

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.

Digit patterning during limb development as a result of the BMP-receptor interaction

Turing models have been proposed to explain the emergence of digits during limb development. However, so far the molecular components that would give rise to Turing patterns are elusive. We have recently shown that a particular type of receptor-ligand interaction can give rise to Schnakenberg-type Turing patterns, which reproduce patterning during lung and kidney branching morphogenesis. Recent knockout experiments have identified Smad4 as a key protein in digit patterning. We show here that the BMP-receptor interaction meets the conditions for a Schnakenberg-type Turing pattern, and that the resulting model reproduces available wildtype and mutant data on the expression patterns of BMP, its receptor, and Fgfs in the apical ectodermal ridge (AER) when solved on a realistic 2D domain that we extracted from limb bud images of E11.5 mouse embryos. We propose that receptor-ligand-based mechanisms serve as a molecular basis for the emergence of Turing patterns in many developing tissues.

BMP-2 functions independently of SHH signaling and triggers cell condensation and apoptosis in regenerating axolotl limbs

Background

Axolotls have the unique ability, among vertebrates, to perfectly regenerate complex body parts, such as limbs, after amputation. In addition, axolotls pattern developing and regenerating autopods from the anterior to posterior axis instead of posterior to anterior like all tetrapods studied to date. Sonic hedgehog is important in establishing this anterior-posterior axis of limbs in all tetrapods including axolotls. Interestingly, its expression is conserved (to the posterior side of limb buds and blastemas) in axolotl limbs as in other tetrapods. It has been suggested that BMP-2 may be the secondary mediator of sonic hedgehog, although there is mounting evidence to the contrary in mice. Since BMP-2 expression is on the anterior portion of developing and regenerating limbs prior to digit patterning, opposite to the expression of sonic hedgehog, we examined whether BMP-2 expression was dependent on sonic hedgehog signaling and whether it affects patterning of the autopod during regeneration.

Results

The expression of BMP-2 and SOX-9 in developing and regenerating axolotl limbs corresponded to the first digits forming in the anterior portion of the autopods. The inhibition of sonic hedgehog signaling with cyclopamine caused hypomorphic limbs (during development and regeneration) but did not affect the expression of BMP-2 and SOX-9. Overexpression of BMP-2 in regenerating limbs caused a loss of digits. Overexpression of Noggin (BMP inhibitor) in regenerating limbs also resulted in a loss of digits. Histological analysis indicated that the loss due to BMP-2 overexpression was the result of increased cell condensation and apoptosis while the loss caused by Noggin was due to a decrease in cell division.

Conclusion

The expression of BMP-2 and its target SOX-9 was independent of sonic hedgehog signaling in developing and regenerating limbs. Their expression correlated with chondrogenesis and the appearance of skeletal elements has described in other tetrapods. Overexpression of BMP-2 did not cause the formation of extra digits, which is consistent with the hypothesis that it is not the secondary signal of sonic hedgehog. However, it did cause the formation of hypomorphic limbs as a result of increased cellular condensation and apoptosis. Taken together, these results suggest that BMP-2 does not have a direct role in patterning regenerating limbs but may be important to trigger condensation prior to ossification and to mediate apoptosis.

Wnt/Lef1 signaling acts via Pitx2 to regulate somite myogenesis

Wnt signaling has been implicated in somite, limb, and branchial arch myogenesis but the mechanisms and roles are not clear. We now show that Wnt signaling via Lef1 acts to regulate the number of premyogenic cells in somites but does not regulate myogenic initiation in the limb bud or maintenance in the first or second branchial arch. We have also analysed the function and regulation of a putative downstream transcriptional target of canonical Wnt signaling, Pitx2. We show that loss-of-function of Pitx2 decreases the number of myogenic cells in the somite, whereas overexpression increases myocyte number particularly in the epaxial region of the myotome. Increased numbers of mitotic cells were observed following overexpression of Pitx2 or an activated form of Lef1, suggesting an effect on cell proliferation. In addition, we show that Pitx2 expression is regulated by canonical Wnt signaling in the epaxial somite and second branchial arch, but not in the limb or the first branchial arch. These results suggest that Wnt/Lef1 signaling regulates epaxial myogenesis via Pitx2 but that this link is uncoupled in other regions of the body, emphasizing the unique molecular networks that control the development of various muscles in vertebrates.

Building the world's best hearing aid; regulation of cell fate in the cochlea

In mammals, auditory perception is initially mediated through sensory cells located in a rigorously patterned mosaic of unique cell types located within the coiled cochlea. Identification of the factors that direct multipotent progenitor cells to develop as each of these specialized cell types has the potential to enhance our understanding of the development of the auditory system and to identify potential targets for regenerative therapies. Recent results have identified specific signaling molecules and pathways, including Notch, Hedgehog, Sox2 and Fgfs, that guide progenitor cells to develop first as a sensory precursor, referred to as a prosensory cell, and subsequently as one of the specialized cell types within the sensory mosaic.

Expression of chick Fgf19 and mouse Fgf15 orthologs is regulated in the developing brain by Fgf8 and Shh

Fibroblast growth factors (Fgfs) constitute a family of signaling molecules that play essential roles in development. We have studied the expression pattern of mouse Fgf15 in the developing brain. Fgf19 is another member of the FGF family that has been suggested as the chick and human ortholog of mouse and rat Fgf15. Here, we compare the expression pattern during neural development of chick Fgf19 with mouse Fgf15. Unlike Fgf15, Fgf19 presents an expression in the isthmic alar plate, diencephalic and mesencephalic parabasal plates, hindbrain basal plate, as well as in the zona limitans intrathalamica (zli). Moreover, we explored the regulation between Fgf19 and the signaling molecules of the isthmic and zli organizers: Fgf8 and Shh, respectively. Considering the possibility that Fgf19 plays a similar role in humans and chicks, this finding could explain the significant diencephalic phenotypic differences between humans and mice in models and diseases where the Shh pathway is affected.

HOX gene analysis of endothelial cell differentiation in human bone marrow-derived mesenchymal stem cells

Human bone marrow-derived mesenchymal stem cells (hMSCs) have been shown to possess multilineage differentiation potential. HOX genes function in transcriptional regulators, and are involved in stem cell differentiation. The aim of the present study was to demonstrate HOX genes that are related to angiogenesis. To identify the expression patterns of 37 HOX genes in the endothelial cell differentiation of hMSCs, we analyzed HOX genes through profiling with multiplex RT-PCR. The results showed that the expression patterns of four HOX genes, HOXA7, HOXB3, HOXA3, and HOXB13, significantly changed during angiogenesis. The expression levels of HOXA7 and HOXB3 were dramatically increased, whereas those of HOXA3 and HOXB13 were decreased during endothelial cell differentiation. When further analysis of the expressions of these HOX genes was performed with real-time PCR and an immunoblot assay, the expression patterns were also found to be well-matched with the results of multiplex RT-PCR. Here, we report that HOXA7, HOXB3, HOXA3, and HOXB13 might be involved in the angiogenesis of hMSCs.

Application of sonic hedgehog to the developing chick limb

Here, we describe methods for applying Sonic hedgehog (Shh) to developing chick limbs. The Sonic hedgehog gene is expressed in the polarizing region, a signaling region at the posterior margin of the limb bud and application of Shh-expressing cells or Shh protein to early limb buds mimics polarizing region signaling. The polarizing region (or zone of polarizing activity) is involved in one of the best known cell-cell interactions in vertebrate embryos and is pivotal in controlling digit number and pattern. At later stages of limb development, the application of Shh protein to the regions between digit primordia can induce changes in digit morphogenesis.

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.

Expression pattern of BMPs during chick limb development

In vertebrates, BMPs (bone morphogenic proteins) play critical roles in establishing the basic embryonic body plan and are involved in the development of a large variety of organs and tissues. Here, we analyzed the expression pattern of various BMPs (2, 4, 5 and 7) by whole mount in situ hybridization during chick limb development. In limb, expression of BMPs suggests evolutionary conserved mechanisms of BMP-dependent differentiation between lower and higher vertebrates. During the early developmental stages, BMP-2 and BMP-7 are expressed in the posterior distal mesenchyme leaving a less prominent expression anteriorly. BMP-4 is initially expressed in the anterior mesenchyme and spreads later to the whole mesenchyme leaving a stronger expression at the anterior side. From HH-stage 25, expression of BMP-4 is observed in the anterior-posterior margins of the limb bud. The BMPs 2, 4 and 7 are expressed strongly in the AER, whereas BMP-5 is expressed as a weak signal in the distal mesoderm during the early stages of limb development. Later from HH-stage 25 onwards, BMP-5 is expressed in the dorsal and ventral muscular mass of the developing limb. As digits become identifiable, expression of BMPs are observed in the interdigital mesenchyme and can also be detected along the contours of the developing phalanges and at the distal tips of the digits. All these BMPs are found to be expressed in the developing feather buds from day 8 onwards.

BMP signaling and skeletogenesis

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)-beta superfamily of signal molecules that mediate many diverse biological processes ranging from early embryonic tissue patterning to postnatal tissue homeostasis. BMPs trigger cell responses mainly through the canonical signaling pathway where intracellular Smads play central roles in delivering the extracellular signals to the nucleus. While the same Smads are used by BMPs in all types of cells, different transcription factors account in part for the functional diversity of BMPs. These transcription factors are recruited by Smads to regulate the expression of specific subsets of target genes depending on the cell types. Among the transcription factors are Hox proteins. Experimental gain and loss-of-function studies as well as naturally occurring mutations in Hox genes demonstrate their central roles in embryonic skeletal patterning. In addition to the interactions with Smads observed for several Hox proteins, there is also evidence that the expression of a number of Hox genes is regulated by BMPs. It is suggested that Hox proteins play an important role in the BMP pathway.

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.

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.

Smads oppose Hox transcriptional activities

BMPs and Hox proteins play crucial roles in developmental processes. Beyond their mutual regulation of gene expression, little is known about the relations between their mechanisms of actions. Previously, we have shown that Hoxc8 acts as a downstream repressor in the BMP signaling pathway. Smad1 and Smad6 interact with Hoxc8 and regulate its repression activities. The Hox family contains 39 genes divided into 13 paralogs. In this report, we systemically examined the potential functions of all the paralogous Hox proteins as BMP downstream transcription factors. Representative Hox proteins from each paralog were tested. In the gel-shift assay, we found that Smad1, Smad4, and Smad6 interacted with most of the Hox proteins in ways similar to their interactions with Hoxc8. The interactions were confirmed in mammalian cells. We also examined the effects of Smads on Hox-induced transactivation. Particularly, we determined that for Hoxd10 as a transcriptional activator, both Smad1 and Smad6 opposed its activity. In addition, Smad6 also inhibited Hoxc8- and Hoxb7-induced osteoprotegerin (OPG) transactivation. Furthermore, Smad1 inhibited Hoxb4-mediated target gene Irx5 expression during early Xenopus development. Our findings suggest that Hox proteins act as general downstream DNA-binding proteins in BMP signaling cascade and their transcriptional activities are regulated by Smads.

Morphogens and cell survival during development

The notion of "morphogens" is an important one in developmental biology. By definition, a morphogen is a molecule that emanates from a specific set of cells that is present in a concentration gradient and that specifies the fate of each cell along this gradient. The strongest candidate morphogens are members of the transforming growth factor-beta (TGF-beta), Hedgehog (Hh), and Wnt families. While these morphogens have been extensively described as differentiation inducers, some reports also suggest their possible involvement in cell death and cell survival. It is frequently speculated that the cell death induction that is found associated with experimental removal of morphogens is the manifestation of abnormal differentiation signals. However, several recent reports have raised controversy about this death by default, suggesting that cell death regulation is an active process for shaping tissues and organs. In this review, we will present morphogens, with a specific emphasis on Sonic Hedgehog, a mammalian member of the Hh family, not as a positive regulators of cell differentiation but as key regulators of cell survival.

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.

Spaciotemporal association and bone morphogenetic protein regulation of sclerostin and osterix expression during embryonic osteogenesis

Sclerostin (SOST), a member of the cystine-knot superfamily, is essential for proper skeletogenesis because a loss-of-function mutation in the SOST gene results in sclerosteosis featured with massive bone growth in humans. To understand the function of SOST in developmental skeletal tissue formation, we examined SOST gene expression in embryonic osteogenesis in vitro and in vivo. During osteoblastic differentiation in primary calvarial cells, the levels of SOST expression were increased along with those of alkaline phosphatase activity and nodule formation. In situ hybridization study revealed that SOST mRNA expression was observed in the digits in embryonic 13-d limb buds, and SOST expression was observed in osteogenic front in embryonic 16.5-d postcoitus embryonic calvariae, and this expression persisted in the peripheral area of cranial bone in the later developmental stage (embryonic 18.5-d post coitum). These temporal and spacial expression patterns in vivo and in vitro were in parallel to those of osterix (Osx), which is a critical transcriptional factor for bone formation. Similar coexpression of SOST and Osx mRNA was observed when the primary osteoblastic calvarial cells were cultured in the presence of bone morphogenetic protein (BMP)2 in vitro. Moreover, endogenous expression of SOST and Osx mRNA was inhibited by infection of noggin-expression adenovirus into the primary osteoblastic calvarial cells, suggesting that endogenous BMPs are required for these cells to express SOST and Osx mRNA. Thus, expression and regulation of SOST under the control of BMP were closely associated with those of Osx in vivo and in vitro.

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.

Function of BMPs in the apical ectoderm of the developing mouse limb

Several bone morphogenetic proteins (BMPs) are expressed in the apical ectodermal ridge (AER), a critical signaling center that directs the outgrowth and patterning of limb mesoderm, but little is known about their function. To study the functions of apical ectodermal BMPs, an AER-specific promoter element from the Msx2 gene was used to target expression of the potent BMP antagonist noggin to the apical ectoderm of the limbs of transgenic mice. Msx2-noggin mutant mice have severely malformed limbs characterized by syndactyly, postaxial polydactyly, and dorsal transformations of ventral structures indicated by absence of ventral footpads and presence of supernumerary ventral nails. Mutant limb buds exhibit a dorsoventral (DV) and anteroposterior (AP) expansion in the extent of the AER. AER activity persists longer than normal and is maintained in regions of the apical ectoderm where its activity normally ceases. Mutant limbs possess a broad band of mesodermal tissue along the distal periphery that is absent from normal limbs and which fails to undergo the apoptosis that normally occurs in the subectodermal mesoderm. Taken together, our results suggest that apical ectodermal BMPs may delimit the boundaries of the AER by preventing adjacent nonridge ectodermal cells from becoming AER cells; negatively modulate AER activity and thus fine-tune the strength of AER signaling; and regulate the apoptosis of the distal subectodermal mesoderm that occurs as AER activity attenuates, an event that is essential for normal limb development. Our results also confirm that ectodermal BMP signaling regulates DV patterning.

ALK2 functions as a BMP type I receptor and induces Indian hedgehog in chondrocytes during skeletal development

Growth plate chondrocytes integrate multiple signals during normal development. The type I BMP receptor ALK2 is expressed in cartilage and expression of constitutively active (CA) ALK2 and other activated type I BMP receptors results in maturation-independent expression of Ihh in chondrocytes in vitro and in vivo. The findings suggest that BMP signaling modulates the Ihh/PTHrP signaling pathway that regulates the rate of chondrocyte differentiation.

INTRODUCTION

Bone morphogenetic proteins (BMPs) have an important role in vertebrate limb development. The expression of the BMP type I receptors BMPR-IA (ALK3) and BMPR-IB (ALK6) have been more completely characterized in skeletal development than ALK2.

METHODS

ALK2 expression was examined in vitro in isolated chick chondrocytes and osteoblasts and in vivo in the developing chick limb bud. The effect of overexpression of CA ALK2 and the other type I BMP receptors on the expression of genes involved in chondrocyte maturation was determined.

RESULTS

ALK2 was expressed in isolated chick osteoblasts and chondrocytes and specifically mediated BMP signaling. In the developing chick limb bud, ALK2 was highly expressed in mesenchymal soft tissues. In skeletal elements, expression was higher in less mature chondrocytes than in chondrocytes undergoing terminal differentiation. CA ALK2 misexpression in vitro enhanced chondrocyte maturation and induced Ihh. Surprisingly, although parathyroid hormone-related peptide (PTHrP) strongly inhibited CA ALK2 mediated chondrocyte differentiation, Ihh expression was minimally decreased. CA ALK2 viral infection in stage 19-23 limbs resulted in cartilage expansion with joint fusion. Enhanced periarticular expression of PTHrP and delayed maturation of the cartilage elements were observed. In the cartilage element, CA ALK2 misexpression precisely colocalized with the expression with Ihh. These findings were most evident in partially infected limbs where normal morphology was maintained. In contrast, BMP-6 had a normal pattern of differentiation-related expression. CA BMPR-IA and CA BMPR-IB overexpression similarly induced Ihh and PTHrP.

CONCLUSIONS

The findings show that BMP signaling induces Ihh. Although the colocalization of the activated type I receptors and Ihh suggests a direct BMP-mediated signaling event, other indirect mechanisms may also be involved. Thus, while BMPs act directly on chondrocytes to induce maturation, this effect is counterbalanced in vivo by induction of the Ihh/PTHrP signaling loop. The findings suggest that BMPs are integrated into the Ihh/PTHrP signaling loop and that a fine balance of BMP signaling is essential for normal chondrocyte maturation and skeletal development.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Patterning systems--from one end of the limb to the other

A combination of embryology and gene identification has led us to the current view of vertebrate limb development, in which a series of three interlocking patterning systems operate sequentially over time. This review describes current understanding of these regulatory mechanisms and how they form a framework for future analysis of limb patterning.

A role for the BMP antagonist chordin in endochondral ossification

Bone morphogenetic proteins (BMPs) are ubiquitous regulators of cellular growth and differentiation. A variety of processes modulate BMP activity, including negative regulation by several distinct binding proteins. One such BMP antagonist chordin has a role in axis determination and neural induction in the early embryo. In this study, a role for chordin during endochondral ossification has been investigated. During limb development, Chordin expression was detected only at the distal ends of the skeletal elements. In cultured embryonic sternal chondrocytes, Chordin expression was related inversely to the stages of maturation. Further, treating cultured chondrocytes with chordin interfered with maturation induced by treatment with BMP-2. These results suggest that chordin may negatively regulate chondrocyte maturation and limb growth in vivo. To address this hypothesis, chordin protein was expressed ectopically in Hamburger-Hamilton (HH) stage 25-27 embryonic chick limbs. The phenotypic changes and alteration of gene expression in treated limbs revealed that overexpression of chordin protein delayed chondrocyte maturation in developing skeletal elements. In summary, these findings strongly support a role for chordin as a negative regulator of endochondral ossification.

FGF signaling is necessary for the specification of the odontogenic mesenchyme

Tooth development is initiated by signals from the oral ectoderm which induce gene expression required for tooth development in the underlying mesenchyme. In this study, we have used Su5402, an inhibitor of FGF receptor signaling, to analyze the requirement of FGF signaling during early tooth development. We show that FGF signaling is necessary for expression of Pax9, a transcription factor required for development of all teeth, in prospective incisor and molar mesenchyme until E11.0. Expression of the LIM homeobox gene Lhx7 also requires FGF signaling until E11.0 whereas expression of its homologue Lhx6 and the homeobox transcription factor Barx1 already becomes independent of FGF signaling at E10.75. In contrast, ectodermal expression of several genes thought to be important for tooth development was unaffected by the block of FGF signaling. Finally, we show that expression of the TGFbeta antagonist Dan in prospective tooth mesenchyme requires ectodermal signals and can be induced by FGF-soaked beads but is maintained in mandibular explants in the absence of FGF signaling. Together, these results demonstrate that FGF signaling is required for development of both molar and incisor teeth and suggest that specification of tooth mesenchyme involves at least two FGF-dependent steps.

"Fingering" the vertebrate limb

A detailed and precise picture is being pieced together about how the pattern of digits develops in vertebrate limbs. What is particularly exciting is that it will soon be possible to trace the process all the way from establishment of a signalling centre in a small bud of undifferentiated cells right through to final limb anatomy. The development of the vertebrate limb is a traditional model in which to explore mechanisms involved in pattern formation, and there is accelerating knowledge about the genes involved. One reason why the limb is holding its place in the post-genomic age is that it is rich in pre-genomic embryology. Here, we will focus on recent findings about the aspect of vertebrate limb development concerned with digit pattern across the anteroposterior axis of the limb. This process is controlled by a signalling region in the early limb bud known as the polarizing region. Interactions between polarizing region cells and other cells in the limb bud ensure that a thumb develops at one edge of the hand (anterior) and a little finger at the other (posterior).

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.

Cholesterol modification of sonic hedgehog is required for long-range signaling activity and effective modulation of signaling by Ptc1

Sonic hedgehog (Shh) signaling from the posterior zone of polarizing activity (ZPA) is the primary determinant of anterior-posterior polarity in the vertebrate limb field. An active signal is produced by an autoprocessing reaction that covalently links cholesterol to the N-terminal signaling moiety (N-Shh(p)), tethering N-Shh(p) to the cell membrane. We have addressed the role played by this lipophilic modification in Shh-mediated patterning of mouse digits. Both the distribution and activity of N-Shh(p) indicate that N-Shh(p) acts directly over a few hundred microns. In contrast, N-Shh, a form that lacks cholesterol, retains similar biological activity to N-Shh(p), but signaling is posteriorly restricted. Thus, cholesterol modification is essential for the normal range of signaling. It also appears to be necessary for appropriate modulation of signaling by the Shh receptor, Ptc1.

An acylatable residue of Hedgehog is differentially required in Drosophila and mouse limb development

The Drosophila Hedgehog protein and its vertebrate counterpart Sonic hedgehog are required for a wide variety of patterning events throughout development. Hedgehog proteins are secreted from cells and undergo autocatalytic cleavage and cholesterol modification to produce a mature signaling domain. This domain of Sonic hedgehog has recently been shown to acquire an N-terminal acyl group in cell culture. We have investigated the in vivo role that such acylation might play in appendage patterning in mouse and Drosophila; in both species Hedgehog proteins define a posterior domain of the limb or wing. A mutant form of Sonic hedgehog that cannot undergo acylation retains significant ability to repattern the mouse limb. However, the corresponding mutation in Drosophila Hedgehog renders it inactive in vivo, although it is normally processed. Furthermore, overexpression of the mutant form has dominant negative effects on Hedgehog signaling. These data suggest that the importance of the N-terminal cysteine of mature Hedgehog in patterning appendages differs between species.

Cell biology of limb patterning

Of vertebrate organ systems, the developing limb has been especially well characterized. Morphological studies have yielded a wealth of information describing limb outgrowth and have allowed for the identification of a multitude of important factors. In terms of the latter, key signaling pathways are known to control numerous aspects of limb development, including establishment of the early limb field, determination of limb identity, elongation of the limb bud, specification of digit pattern, and sculpting of the digits. Modification of underlying signaling pathways can thus result in dramatic alterations of the limb phenotype, accounting for many of the diverse limb patterns observed in nature. Given this, it is clear that signaling pathways regulate the highly orchestrated and tightly controlled sequence of cellular events necessary for limb outgrowth; however, exactly how molecular signals interface with the cell biology of limb development remains largely a mystery. In this review we first provide an overview of a number of the morphogenetic signaling pathways that have been identified in the developing limb and then review how a subset of these signals are known to modify cell behaviors important for limb outgrowth.

Dorsal dermis development depends on a signal from the dorsal neural tube, which can be substituted by Wnt-1

To investigate the origin and nature of the signals responsible for specification of the dermatomal lineage, excised axial organs in 2-day-old chick embryos were replaced by grafts of the dorsal neural tube, or the ventral neural tube plus the notochord, or aggregates of cells engineered to produce Sonic hedgehog (Shh), Noggin, BMP-2, Wnt-1, or Wnt-3a. By E10, grafts of the ventral neural tube plus notochord or of cells producing Shh led to differentiation of cartilage and muscles, and an impaired dermis derived from already segmented somites. In contrast, grafts of the dorsal neural tube, or of cells producing Wnt-1, triggered the formation of a feather-inducing dermis. These results show that the dermatome inducer is produced by the dorsal neural tube. The signal can be Wnt-1 itself, or can be mediated, or at least mimicked by Wnt-1.

Constructive antagonism in limb development

Several advances have been made in our understanding of the control of the growth and patterning of embryonic limbs. Development of the vertebrate limb is dependent on reciprocal interactions between the ectoderm and mesoderm that regulate the structure and function of the apical ectodermal ridge. One key component of this regulatory program appears to be the precise control of signaling by members of the bone morphogenetic protein family via multiple antagonistic interactions.

Erythroid differentiation in vitro is blocked by cyclopamine, an inhibitor of hedgehog signaling

Adult hematopoietic differentiation is a developmental process that employs many of the same molecular mechanisms as embryogenesis. To explore the possibility that hedgehog signaling is involved in the control of hematopoietic differentiation, we screened a panel of human leukemia cell lines for the expression of Patched1 and Smoothened, the receptor and coreceptor for hedgehog ligands. Expression was found in multiple cell lines, and Patched1 expression was detected in normal marrow. Induction of myeloid differentiation in cell lines downregulated expression of both genes. When normal marrow mononuclear cells were grown in semisolid medium in the presence of 10 microM cyclopamine, development of colonies of granulocytic/monocytic lineage was unaffected in terms of both number and morphology. The number of erythroid colonies, however, was significantly reduced (P < 0.01). Furthermore, hemoglobinization was substantially delayed relative to controls in those erythroid colonies that did form. Incubation of hematopoietic progenitors with Shh-N and GM-CSF resulted in increased granulocyte/monocyte colonies (P < 0.01); the increase was blocked by cyclopamine. Incubation of hematopoietic progenitors with Shh-N and stem cell factor resulted in larger erythroid colonies. These results suggest that elements of the hedgehog signaling pathway are involved in the control of hematopoietic differentiation.

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

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

Role of dHAND in the anterior-posterior polarization of the limb bud: implications for the Sonic hedgehog pathway

dHAND is a basic helix-loop-helix (bHLH) transcription factor essential for cardiovascular development. Here we analyze its pattern of expression and functional role during chick limb development. dHAND expression was observed in the lateral plate mesoderm prior to emergence of the limb buds. Coincident with limb initiation, expression of dHAND became restricted to the posterior half of the limb bud. Experimental procedures that caused mirror-image duplications of the limb resulted in mirror-image duplications of the pattern of dHAND expression along the anterior-posterior axis. Retroviral overexpression of dHAND in the limb bud produced preaxial polydactyly, corresponding to mild polarizing activity at the anterior border. At the molecular level, misexpression of dHAND caused ectopic activation of members of the Sonic hedgehog (Shh) pathway, including Gli and Patched, in the anterior limb bud. A subset of infected embryos displayed ectopic anterior activation of Shh. Other factors implicated in anterior-posterior polarization of the bud such as the most 5′ Hoxd genes and Bmp2 were also ectopically activated at the anterior border. Our results indicate a role for dHAND in the establishment of anterior-posterior polarization of the limb bud.

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

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

A new function of BMP4: dual role for BMP4 in regulation of Sonic hedgehog expression in the mouse tooth germ

The murine tooth development is governed by sequential and reciprocal epithelial-mesenchymal interactions. Multiple signaling molecules are expressed in the developing tooth germ and interact each other to mediate the inductive tissue interactions. Among them are Sonic hedgehog (SHH), Bone Morphogenetic Protein-2 (BMP2) and Bone Morphogenetic Protein-4 (BMP4). We have investigated the interactions between these signaling molecules during early tooth development. We found that the expression of Shh and Bmp2 is downregulated at E12.5 and E13.5 in the dental epithelium of the Msx1 mutant tooth germ where Bmp4 expression is significantly reduced in the dental mesenchyme. Inhibition of BMP4 activity by noggin resulted in repression of Shh and Bmp2 in wild-type dental epithelium. When implanted into the dental mesenchyme of Msx1 mutants, beads soaked with BMP4 protein were able to restore the expression of both Shh and Bmp2 in the Msx1 mutant epithelium. These results demonstrated that mesenchymal BMP4 represents one component of the signal acting on the epithelium to maintain Shh and Bmp2 expression. In contrast, BMP4-soaked beads repressed Shh and Bmp2 expression in the wild-type dental epithelium. TUNEL assay indicated that this suppression of gene expression by exogenous BMP4 was not the result of an increase in programmed cell death in the tooth germ. Ectopic expression of human Bmp4 to the dental mesenchyme driven by the mouse Msx1 promoter restored Shh expression in the Msx1 mutant dental epithelium but repressed Shh in the wild-type tooth germ in vivo. We further demonstrated that this regulation of Shh expression by BMP4 is conserved in the mouse developing limb bud. In addition, Shh expression was unaffected in the developing limb buds of the transgenic mice in which a constitutively active Bmpr-IB is ectopically expressed in the forelimb posterior mesenchyme and throughout the hindlimb mesenchyme, suggesting that the repression of Shh expression by BMP4 may not be mediated by BMP receptor-IB. These results provide evidence for a new function of BMP4. BMP4 can act upstream to Shh by regulating Shh expression in mouse developing tooth germ and limb bud. Taken together, our data provide insight into a new regulatory mechanism for Shh expression, and suggest that this BMP4-mediated pathway in Shh regulation may have a general implication in vertebrate organogenesis.

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.

Bmp-4 requires the presence of the digits to initiate programmed cell death in limb interdigital tissues

The effects of Bmp-4 on interdigital cell death were investigated in the mouse. Affi-Gel beads, loaded with recombinant Bmp-4 protein, were transplanted into the interdigital tissues of day 12.5 hindlimb, ex utero. It was established that Bmp-4 could induce precocious interdigital cell death. Using in situ hybridization, the expression patterns of bmp-4 and alk-6 receptor were established. Both genes were found coexpressed in the interdigital region of 12.5- and 13. 5-day hindlimbs. This suggests that Bmp-4 may act in an autocrine fashion. We have also studied the effects of Bmp-4 on 12.5-day interdigital tissue cultures. In all specimens examined, the interdigital tissues produced cartilage instead of participating in cell death. The addition of exogenous Bmp-4 to the interdigital cultures did not induce apoptosis but instead enhanced chondrogenesis. The discrepancy between the effects of Bmp-4 in vitro and ex utero was attributed to the presence of digits. When a flanking digit was left attached to the interdigital tissues, in vitro, Bmp-4 promoted apoptosis instead of chondrogenesis. In sum, the results suggest that Bmp-4 is a multifunctional protein and its effect on the interdigital tissues is dependent on the modulating influence of the digits.

Pattern formation and regulation of gene expressions in chick recombinant limbs

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

The BMP antagonist Gremlin regulates outgrowth, chondrogenesis and programmed cell death in the developing limb

In this study, we have analyzed the expression and function of Gremlin in the developing avian limb. Gremlin is a member of the DAN family of BMP antagonists highly conserved through evolution able to bind and block BMP2, BMP4 and BMP7. At early stages of development, gremlin is expressed in the dorsal and ventral mesoderm in a pattern complementary to that of bmp2, bmp4 and bmp7. The maintenance of gremlin expression at these stages is under the control of the AER, ZPA, and BMPs. Exogenous administration of recombinant Gremlin indicates that this protein is involved in the control of limb outgrowth. This function appears to be mediated by the neutralization of BMP function to maintain an active AER, to restrict the extension of the areas of programmed cell death and to confine chondrogenesis to the central core mesenchyme of the bud. At the stages of digit formation, gremlin is expressed in the proximal boundary of the interdigital mesoderm of the chick autopod. The anti-apoptotic influence of exogenous Gremlin, which results in the formation of soft tissue syndactyly in the chick, together with the expression of gremlin in the duck interdigital webs, indicates that Gremlin regulates the regression of the interdigital tissue. At later stages of limb development, gremlin is expressed in association with the differentiating skeletal pieces, muscles and the feather buds. The different expression of Gremlin in relation with other BMP antagonists present in the limb bud, such as Noggin, Chordin and Follistatin indicates that the functions of BMPs are regulated specifically by the different BMP antagonists, acting in a complementary fashion rather than being redundant signals.

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.

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

Outgrowth and patterning of the vertebrate limb are controlled by reciprocal interactions between the posterior mesenchyme (polarizing region) and a specialized ectodermal structure, the apical ectodermal ridge (AER). Sonic hedgehog (SHH) signalling by the polarizing region modulates fibroblast growth factor (FGF)4 signalling by the posterior AER, which in turn maintains the polarizing region (SHH/FGF4 feedback loop). Here we report that the secreted bone-morphogenetic-protein (BMP) antagonist Gremlin relays the SHH signal from the polarizing region to the AER. Mesenchymal Gremlin expression is lost in limb buds of mouse embryos homozygous for the limb deformity (Id) mutation, which disrupts establishment of the SHH/FGF4 feedback loop. Grafting Gremlin-expressing cells into ld mutant limb buds rescues Fgf4 expression and restores the SHH/FGF4 feedback loop. Analysis of Shh-null mutant embryos reveals that SHH signalling is required for maintenance of Gremlin and Formin (the gene disrupted by the ld mutations). In contrast, Formin, Gremlin and Fgf4 activation are independent of SHH signalling. This study uncovers the cascade by which the SHH signal is relayed from the posterior mesenchyme to the AER and establishes that Formin-dependent activation of the BMP antagonist Gremlin is sufficient to induce Fgf4 and establish the SHH/FGF4 feedback loop.