The molecular ZPA

Postaxial limb hypoplasia (PALH): the classification, clinical features, and related developmental biology

Postaxial limb hypoplasia (PALH) is a group of nonhereditary diseases with congenital lower limb deficiency affecting the fibular ray, including fibular hemimelia, proximal femoral focal deficiency, and tarsal coalition. The etiology and the developmental biology of the anomaly are still not fully understood. Here, we review the previous classification systems, present the clinical features, and discuss the developmental biology of PALH.

Modification of the zone of polarizing activity signal by trypsin

Patterning of the developing vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) via the expression of Sonic hedgehog (Shh) and along the proximal-distal axis by the apical ectodermal ridge (AER) through the production of fibroblast growth factors (FGFs). ZPA grafting, as well as ectopic application of SHH to the anterior chick limb bud, demonstrate that digit patterning is largely influenced by these secreted factors. Although signal transduction pathways have been well characterized for SHH and for FGFs, little is known of how these signals are regulated extracellularly in the limb. The present study shows that alteration of the extracellular environment through trypsin treatment can have profound effects on digit patterning. These effects appear to be mediated by the induction of Shh in host tissues and by ectopic AER formation, implicating the extracellular matrix in regulating the signaling activities of key patterning genes in the limb.

Growth and differentiation of the developing limb bud from the perspective of chondrogenesis

Limb skeletal elements develop from a cartilage template, which is formed by the process termed chondrogenesis. This process is crucial in determining the shape and size of definitive bones in vertebrates. During chondrogenesis, aggregated mesenchymal cells undergo a highly organized process of proliferation and maturation along with secretion of extracellular matrix followed by programmed cell death and replacement by bone. The molecular mechanisms underlying this sophisticated process have been extensively studied. It has been demonstrated that several transcription factors such as Sox genes and Runx genes are indispensable for the major steps in chondrogenesis. Additionally, a number of signaling molecules including Bmps, Fgfs and Ihh/PTHrP are known to regulate chondrogenesis through highly coordinated interactions. This review is meant to provide an overview of the current knowledge of chondrogenesis with particular emphasis on the cellular and molecular aspects.

The spectrum of hand and foot malformations in patients with Greig cephalopolysyndactyly

PURPOSE

Greig cephalopolysyndactyly (GCPS) (OMIM 175700), a rare autosomal dominant disorder, is characterized by a distinct combination of craniofacial, hand and foot malformations. The hand and foot malformations often require orthopedic assessment and treatment. The disorder is caused by point mutations or deletions in the GLI3 gene, located on chromosome 7p14.3. Herewith, we review the hand and foot malformations in a cohort of 13 patients referred for genetic testing.

METHODS

We reviewed the medical files of 13 patients with GCPS seen at the Center for Human Genetics in Leuven between 2003 and 2005. Clinical, molecular and radiological findings, when available, were recorded.

RESULTS

We identified six different point mutations in the GLI3 gene, two microdeletions and three larger chromosomal deletions. In the hands, preaxial polydactyly was never observed, but the malformations included postaxial polydactyly, broad thumbs, clinodactyly of the thumbs and various degrees of syndactyly. In the feet the spectrum of malformations included preaxial polydactyly, postaxial polydactyly, different degrees of syndactyly and broad halluces. Syndactyly of the toes and hallux abnormalities were present in all patients. Most frequently, syndactyly was present between toes 1-2-3. The broadening of the hallux was either due to a complete or partial duplication of the first toe or to broadening of the distal phalanx. Mental retardation was found in three cases and was associated with a large chromosomal deletion of the GLI3 region.

CONCLUSION

We found the classic hand and foot malformations associated with GCPS in our cohort of patients. Patients with a large chromosomal deletion had mental retardation, but no structural brain anomalies were found.

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.

Molecular pathways regulating pro-migratory effects of Hedgehog signaling

The Hedgehog proteins play a crucial role in metazoan embryo development. Constitutive activation of the pathway is associated with multiple types of cancer. Recent experimental data suggest involvement of Hedgehog signaling in vascular remodeling, germ cell migration, and axon guidance. The molecular mechanisms underlying these effects remain elusive. Here we show that yolk sac-derived endothelial cells and embryonic fibroblasts can directly respond to the Hedgehog signal by increased migration in an in vitro scratch (wound) assay. We also identify Hedgehog transcriptional target genes in these cells, many of which participate in cell migration, axon guidance, and angiogenesis processes. Inhibition of one such molecular pathway, neuropilin-flavomonooxygenase, blocks Hedgehog-induced cell migration. These findings suggest that Hedgehog signaling directly affects embryonic endothelial and fibroblast cell migration via molecules and pathways known to regulate cell migration in response to a variety of environmental cues.

Pbx1/Pbx2 requirement for distal limb patterning is mediated by the hierarchical control of Hox gene spatial distribution and Shh expression

Vertebrate limb development occurs along three cardinal axes-proximodistal, anteroposterior and dorsoventral-that are established via the organization of signaling centers, such as the zone of polarizing activity (ZPA). Distal limb development, in turn, requires a molecular feedback loop between the ZPA expression of sonic hedgehog (Shh) and the apical ectodermal ridge. The TALE homeoprotein Pbx1 has been shown to be essential for proximal limb development. In this study, we first uncover that Pbx1 and Pbx2 are co-expressed in the lateral plate and early limb field mesoderm. Later, Pbx2 is expressed throughout the limb, unlike Pbx1, which is expressed only in the proximal bud. By exploiting a Pbx1/Pbx2 loss-of-function mouse model, we demonstrate that, despite the lack of limb abnormalities in Pbx2-deficient (Pbx2(-/-)) embryos, compound Pbx1(-/-); Pbx2(+/-) mutants, in addition to their exacerbated proximal limb defects, exhibit novel and severe distal abnormalities. Additionally, we reveal that Pbx1(-/-); Pbx2(-/-) embryos lack limbs altogether. Furthermore, we establish that, unlike in flies, where the leg develops independently of Hox and where the Pbx ortholog Exd is required for specification of proximal (but not distal) limbs, in vertebrates, distal limb patterning is Pbx1/Pbx2 dependent. Indeed, we demonstrate that Pbx genetic requirement is mediated, at least in part, through their hierarchical control of Hox spatial distribution and Shh expression. Overall, we establish that, by controlling the spatial expression of Hox genes in the posterior limb and regulating ZPA function, Pbx1/Pbx2 exert a primary hierarchical function on Hox genes, rather than behaving merely as Hox ancillary factors.

Société Française d’Orthopédie Pédiatrique

Limb malformations are frequent. These malformations are isolated or associated with anomalies of other developmental fields and accurate diagnostic is essential for prognosis evaluation, treatment and genetic counseling. Animal embryology and molecular biology techniques, have given us a better understanding of the processes of growth and patterning of the limb buds. The key genes that are involved in these processes have been identified and their interactions recognized. Human genetics has been able to identify, or at least localize, several genes implicated in limb development. We here review the present knowledge on these genes and their mutations responsible for limb anomalies.

Analysis of the regulation oflin-41during chick and mouse limb development

We have cloned the chicken and mouse orthologues of the Caenorhabditis elegans heterochronic gene lin-41. During limb development, lin-41 is expressed in three phases over developmental time and most notably is associated with the developing autopod. Using chicken and mouse mutants and bead implantations, we report that lin-41 is genetically and biochemically downstream of both the Shh and Fgf signaling pathways. In C. elegans, it is proposed that lin-41 activity is temporally regulated by miRNAs (let-7 and lin-4) that bind to complementary sites in the lin-41 3'-untranslated region (UTR). Taking a bioinformatics approach, we also report the presence of potential miRNA binding sites in the 3'-UTR of chicken lin-41, including sites for the chicken orthologues of both C. elegans let-7 and lin-4. Finally, we show that these miRNAs and others are expressed in the chick limb consistent with the hypothesis that they regulate chicken Lin-41 activity in vivo.

Genomic characterisation of a Fgf-regulated gradient-based neocortical protomap

Recent findings support a model for neocortical area formation in which neocortical progenitor cells become patterned by extracellular signals to generate a protomap of progenitor cell areas that in turn generate area-specific neurons. The protomap is thought to be underpinned by spatial differences in progenitor cell identity that are reflected at the transcriptional level. We systematically investigated the nature and composition of the protomap by genomic analyses of spatial and temporal neocortical progenitor cell gene expression. We did not find gene expression evidence for progenitor cell organisation into domains or compartments, instead finding rostrocaudal gradients of gene expression across the entire neocortex. Given the role of Fgf signalling in rostrocaudal neocortical patterning, we carried out an in vivo global analysis of cortical gene expression in Fgfr1 mutant mice, identifying consistent alterations in the expression of candidate protomap elements. One such gene, Mest, was predicted by those studies to be a direct target of Fgf8 signalling and to be involved in setting up, rather than implementing, the progenitor cell protomap. In support of this, we confirmed Mest as a direct transcriptional target of Fgf8-regulated signalling in vitro. Functional studies demonstrated that this gene has a role in establishing patterned gene expression in the developing neocortex, potentially by acting as a negative regulator of the Fgf8-controlled patterning system.

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.

Hedgehog signaling in murine vasculogenesis and angiogenesis

Vasculogenesis-the formation of blood vessels de novo from endothelial cells-and angiogenesis-the process of blood vessel remodeling-are regulated by a number of signal transduction pathways, some specific to the vascular system and others used more broadly during embryogenesis. Recent evidence in both zebrafish and mouse suggests a role for Hedgehog (Hh) signaling in both vasculogenesis and angiogenesis. Hh signaling can target endothelial cells directly or can stimulate blood vessel support cells to produce vascular growth factors. Current studies are aimed at determining how the Hh cascade interacts with the other signaling pathways to promote vessel differentiation.

Biology of developmental and regenerative skeletogenesis

Embryonic skeletal development involves the recruitment, commitment, differentiation, and maturation of mesenchymal cells into those in the skeletal tissue lineage, specifically cartilage and bone along the intramembranous and endochondral ossification pathways. The exquisite control of skeletal development is regulated at the level of gene transcription, cellular signaling, cell-cell and cell-matrix interactions, as well as systemic modulation. Mediators include transcription factors, growth factors, cytokines, metabolites, hormones, and environmentally derived influences. Understanding the mechanisms underlying developmental skeletogenesis is crucial to harnessing the inherent regenerative potential of skeletal tissues for wound healing and repair, as well as for functional skeletal tissue engineering. In this review, a number of key issues are discussed concerning the current and future challenges of the scientific investigation of developmental skeletogenesis in the embryo, specifically limb cartilage development, and how these challenges relate to regenerative or reparative skeletogenesis in the adult. Specifically, a more complete understanding the biology of skeletogenic progenitor cells and the cellular and molecular mechanisms governing tissue patterning and morphogenesis should greatly facilitate the development of regenerative approaches to cartilage repair.

Deciphering skeletal patterning: clues from the limb

Even young children can distinguish a Tyrannosaurus rex from a Brontosaurus by observing differences in bone size, shape, number and arrangement, that is, skeletal pattern. But despite our extensive knowledge about cartilage and bone formation per se, it is still largely a mystery how skeletal pattern is established. Much of what we do know has been learned from studying limb development in chicken and mouse embryos. Based on the data from such studies, models for how limb skeletal pattern is established have been proposed and continue to be hotly debated.

Loss of the Tg737 protein results in skeletal patterning defects

Tg737 mutant mice exhibit pathologic conditions in numerous tissues along with skeletal patterning defects. Herein, we characterize the skeletal pathologic conditions and confirm a role for Tg737 in skeletal patterning through transgenic rescue. Analyses were conducted in both the hypomorphic Tg737(orpk) allele that results in duplication of digit one and in the null Tg737(delta2-3betaGal) allele that is an embryonic lethal mutation exhibiting eight digits per limb. In early limb buds, Tg737 expression is detected throughout the mesenchyme becoming concentrated in precartilage condensations at later stages. In situ analyses indicate that the Tg737(orpk) mutant limb defects are not associated with changes in expression of Shh, Ihh, HoxD11-13, Patched, BMPs, or Glis. Likewise, in Tg737(delta2-3betaGal) mutant embryos, there was no change in Shh expression. However, in both alleles, Fgf4 was ectopically expressed on the anterior apical ectodermal ridge. Collectively, the data argue for a dosage effect of Tg737 on the limb phenotypes and that the polydactyly is independent of Shh misexpression.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Discovery of sonic hedgehog expression in postnatal growth plate chondrocytes: differential regulation of sonic and Indian hedgehog by retinoic acid

Sonic hedgehog (Shh) is a key signal protein in early embryological patterning of limb bud development. Its analog, Indian hedgehog (Ihh), primarily expressed during early cartilage development in prehypertrophic chondrocytes, regulates proliferation and suppresses terminal differentiation of postnatal growth plate (GP) chondrocytes. We report here for the first time that both Shh and Ihh mRNA are expressed in the GP of rapidly growing 6-week-old broiler-strain chickens. They are also expressed in other tissues such as articular chondrocytes, kidney, and bone. In situ hybridization and RT-PCR analyses reveal Shh in all zones of the GP, with peak expression in late hypertrophy. Using primary cultures of GP chondrocytes in serum-containing medium, we followed the patterns of Shh and Ihh mRNA expression as the cultures matured and mineralized. We find a cyclical expression of both hedgehog genes during the early period of culture development between day 10 and 14; when one is elevated, the other tended to be suppressed, suggesting that the two hedgehogs may play complementary roles during GP development. Retinoic acid (RA), a powerful modulator of gene expression in cell differentiation, stimulates GP chondrocytes toward terminal differentiation, enhancing mineral formation. We find that RA strongly suppresses Ihh, but enhances expression of Shh in this system. While Ihh suppresses maturation of GP chondrocytes to hypertrophy, we hypothesize that Shh acts to push these cells toward hypertrophy.

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb

We have analyzed a new limb mutant in the chicken that we name oligozeugodactyly (ozd). The limbs of this mutant have a longitudinal postaxial defect, lacking the posterior element in the zeugopod (ulna/fibula) and all digits except digit 1 in the leg. Classical recombination experiments show that the limb mesoderm is the defective tissue layer in ozd limb buds. Molecular analysis revealed that the ozd limbs develop in the absence of Shh expression, while all other organs express Shh and develop normally. Neither Ptc1 nor Gli1 are detectable in mutant limb buds. However, Bmp2 and dHAND are expressed in the posterior wing and leg bud mesoderm, although at lower levels than in normal embryos. Activation of Hoxd11-13 occurs normally in ozd limbs but progressively declines with time. Phase III of expression is more affected than phase II, and expression is more severely affected in the more 5' genes. Interestingly, re-expression of Hoxd13 occurs at late stages in the distal mesoderm of ozd leg buds, correlating with formation of digit 1. Fgf8 and Fgf4 expression are initiated normally in the mutant AER but their expression is progressively downregulated in the anterior AER. Recombinant Shh protein or ZPA grafts restore normal pattern to ozd limbs; however, retinoic acid fails to induce Shh in ozd limb mesoderm. We conclude that Shh function is required for limb development distal to the elbow/knee joints, similar to the Shh(-/-) mouse. Accordingly we classify the limb skeletal elements as Shh dependent or independent, with the ulna/fibula and digits other than digit 1 in the leg being Shh dependent. Finally we propose that the ozd mutation is most likely a defect in a regulatory element that controls limb-specific expression of Shh.

Molecular basis of vertebrate limb patterning

Mechanisms of limb development are common to all higher vertebrates. The current understanding of how vertebrate limbs develop comes mainly from studies on chick embryos, which are classical models for experimental manipulation, and mouse embryos, which can be genetically manipulated. Work on chick and mouse embryos is often complementary and has direct implications for human limb development. Analysis has moved to the molecular level, which allows direct links to genetics. Even though genes involved in limb development have been discovered by basic scientists through different routes to that taken by clinical geneticists, many of the same genes have been identified. Thus, the fields of embryology and clinical medicine increasingly converge. The next challenge will be to go back to animal models to begin to dissect how particular gene mutations lead to specific limb phenotypes.

How trematodes cause limb deformities in amphibians

We used trematode cyst infestation to induce limb deformities in two species of frogs of the genus Rana and compared them to deformities induced by surgical limb bud rotations. The specific deformities produced by both treatments closely resemble those of wild-caught deformed amphibians and are consistent with a known developmental response to disruption of the spatial organization of cells in developing limb buds. Histological analysis showed that trematode cysts cause massive disruption and abnormal cellular growth involving the limb buds of infected individuals. Our results indicate that trematode cyst infestation causes deformities in frogs by perturbation of the positional relationships of cells in developing limb buds. The crippling effects of cyst-infection on frogs may reflect complex co-evolutionary interactions among trematodes, frogs, and other hosts in the trematode's life cycle.

Shh and Gli3 are dispensable for limb skeleton formation but regulate digit number and identity

Most current models propose Sonic hedgehog (Shh) as the primary determinant of anteroposterior development of amniote limbs. Shh protein is said to be required to direct the formation of skeletal elements and to specify digit identity through dose-dependent activation of target gene expression. However, the identity of genes targeted by Shh, and the regulatory mechanisms controlling their expression, remain poorly understood. Gli3 (the gene implicated in human Greig cephalopolysyndactyly syndrome) is proposed to negatively regulate Shh by restricting its expression and influence to the posterior mesoderm. Here we report genetic analyses in mice showing that Shh and Gli3 are dispensable for formation of limb skeletal elements: Shh(-/-) Gli3(-/-) limbs are distally complete and polydactylous, but completely lack wild-type digit identities. We show that the effects of Shh signalling on skeletal patterning and ridge maintenance are necessarily mediated through Gli3. We propose that the function of Shh and Gli3 in limb skeletal patterning is limited to refining autopodial morphology, imposing pentadactyl constraint on the limb's polydactyl potential, and organizing digit identity specification, by regulating the relative balance of Gli3 transcriptional activator and repressor activities.

Sonic hedgehog signaling from the urethral epithelium controls external genital development

External genital development begins with formation of paired genital swellings, which develop into the genital tubercle. Proximodistal outgrowth and axial patterning of the genital tubercle are coordinated to give rise to the penis or clitoris. The genital tubercle consists of lateral plate mesoderm, surface ectoderm, and endodermal urethral epithelium derived from the urogenital sinus. We have investigated the molecular control of external genital development in the mouse embryo. Previous work has shown that the genital tubercle has polarizing activity, but the precise location of this activity within the tubercle is unknown. We reasoned that if the tubercle itself is patterned by a specialized signaling region, then polarizing activity may be restricted to a subset of cells. Transplantation of urethral epithelium, but not genital mesenchyme, to chick limbs results in mirror-image duplication of the digits. Moreover, when grafted to chick limbs, the urethral plate orchestrates morphogenetic movements normally associated with external genital development. Signaling activity is therefore restricted to urethral plate cells. Before and during normal genital tubercle outgrowth, urethral plate epithelium expresses Sonic hedgehog (Shh). In mice with a targeted deletion of Shh, external genitalia are absent. Genital swellings are initiated, but outgrowth is not maintained. In the absence of Shh signaling, Fgf8, Bmp2, Bmp4, Fgf10, and Wnt5a are downregulated, and apoptosis is enhanced in the genitalia. These results identify the urethral epithelium as a signaling center of the genital tubercle, and demonstrate that Shh from the urethral epithelium is required for outgrowth, patterning, and cell survival in the developing external genitalia.

Disruption of a long-range cis-acting regulator for Shh causes preaxial polydactyly

Preaxial polydactyly (PPD) is a common limb malformation in human. A number of polydactylous mouse mutants indicate that misexpression of Shh is a common requirement for generating extra digits. Here we identify a translocation breakpoint in a PPD patient and a transgenic insertion site in the polydactylous mouse mutant sasquatch (Ssq). The genetic lesions in both lie within the same respective intron of the LMBR1/Lmbr1 gene, which resides approximately 1 Mb away from Shh. Genetic analysis of Ssq reveals that the Lmbr1 gene is incidental to the phenotype and that the mutation directly interrupts a cis-acting regulator of Shh. This regulator is most likely the target for generating PPD mutations in human.

The art and design of genetic screens: Drosophila melanogaster

The success of Drosophila melanogaster as a model organism is largely due to the power of forward genetic screens to identify the genes that are involved in a biological process. Traditional screens, such as the Nobel-prize-winning screen for embryonic-patterning mutants, can only identify the earliest phenotype of a mutation. This review describes the ingenious approaches that have been devised to circumvent this problem: modifier screens, for example, have been invaluable for elucidating signal-transduction pathways, whereas clonal screens now make it possible to screen for almost any phenotype in any cell at any stage of development.

Limb development in a "nonmodel" vertebrate, the direct-developing frog Eleutherodactylus coqui

Mechanisms that mediate limb development are regarded as highly conserved among vertebrates, especially tetrapods. Yet, this assumption is based on the study of relatively few species, and virtually none of those that display any of a large number of specialized life-history or reproductive modes, which might be expected to affect developmental pattern or process. Direct development is an alternative life history found in many anuran amphibians. Many adult features that form after hatching in metamorphic frogs, such as limbs, appear during embryogenesis in direct-developing species. Limb development in the direct-developing frog Eleutherodactylus coqui presents a mosaic of apparently conserved and novel features. The former include the basic sequence and pattern of limb chondrogenesis, which are typical of anurans generally and appear largely unaffected by the gross shift in developmental timing; expression of Distal-less protein (Dlx) in the distal ectoderm; expression of the gene Sonic hedgehog (Shh) in the zone of polarizing activity (ZPA); and the ability of the ZPA to induce supernumerary digits when transplanted to the anterior region of an early host limb bud. Novel features include the absence of a morphologically distinct apical ectodermal ridge, the ability of the limb to continue distal outgrowth and differentiation following removal of the distal ectoderm, and earlier cessation of the inductive ability of the ZPA. Attempts to represent tetrapod limb development as a developmental "module" must allow for this kind of evolutionary variation among species.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration. We report the results of a pilot study that supports this hypothesis. In this study, the distal aspect of the right hindlimb buds of X. laevis tadpoles was amputated at the pre-foot paddle stage. The tadpoles were raised in water from a pond in Minnesota at which 7% of surveyed newly metamorphosed feral frogs had malformations. Six percent (6 of 100) of the right limbs of the tadpoles raised in pond water developed abnormally. One truncated right limb was the only malformation in the control group, which was raised in dechlorinated municipal water. All unamputated limbs developed normally in both groups. Three major factors under consideration for effecting the limb malformations are discussed. These factors include environmental chemicals (primarily agrichemicals), encysted larvae (metacercariae) of trematode parasites, and increased levels of ultraviolet light. Emphasis is placed on the necessary intersection of environmental stressors and developmental events to bring about the specific malformations that are observed in free-living frog populations.

Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function

The secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior-posterior limb polarity. In the Shh(-/-) mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. In this study, we report the analysis of Shh(-/-) mutant limbs in detail. Each mutant embryo has four limbs with recognizable humerus/femur bones that have anterior-posterior polarity. Distal to the elbow/knee joints, skeletal elements representing the zeugopod form but lack identifiable anterior-posterior polarity. Therefore, Shh specifically becomes necessary for normal limb development at or just distal to the stylopod/zeugopod junction (elbow/knee joints) during mouse limb development. The forelimb autopod is represented by a single distal cartilage element, while the hindlimb autopod is invariably composed of a single digit with well-formed interphalangeal joints and a dorsal nail bed at the terminal phalanx. Analysis of GDF5 and Hoxd11-13 expression in the hindlimb autopod suggests that the forming digit has a digit-one identity. This finding is corroborated by the formation of only two phalangeal elements which are unique to digit one on the foot. The apical ectodermal ridge (AER) is induced in the Shh(-/-) mutant buds with relatively normal morphology. We report that the architecture of the Shh(-/-) AER is gradually disrupted over developmental time in parallel with a reduction of Fgf8 expression in the ridge. Concomitantly, abnormal cell death in the Shh(-/-) limb bud occurs in the anterior mesenchyme of both fore- and hindlimb. It is notable that the AER changes and mesodermal cell death occur earlier in the Shh(-/-) forelimb than the hindlimb bud. This provides an explanation for the hindlimb-specific competence to form autopodial structures in the mutant. Finally, unlike the wild-type mouse limb bud, the Shh(-/-) mutant posterior limb bud mesoderm does not cause digit duplications when grafted to the anterior border of chick limb buds, and therefore lacks polarizing activity. We propose that a prepattern exists in the limb field for the three axes of the emerging limb bud as well as specific limb skeletal elements. According to this model, the limb bud signaling centers, including the zone of polarizing activity (ZPA) acting through Shh, are required to elaborate upon the axial information provided by the native limb field prepattern.

Differential expression of chicken CYP26 in anterior versus posterior limb bud in response to retinoic acid

Multiple studies indicate that quantitative control of the levels of all-trans-retinoic acid (RA) in the vertebrate embryo is necessary for correct development. The function of RA in cells is regulated by a number of coordinated mechanisms. One of those mechanisms involves controls on the rate of RA catabolism. Recently, enzymes capable of catabolizing RA were found to constitute a new family, called CYP26, within the cytochrome P450 superfamily. CYP26 homologues have been isolated from human, mouse, zebra fish, and recently from the chick. In this study, we examined the regulation of chicken CYP26 (cCYP26) expression by RA during the early phase of chick limb outgrowth. In the anterior limb mesenchyme and apical ectodermal ridge (AER), cCYP26 expression was induced in a concentration dependent manner by implanting beads soaked in 0.1, 1, and 5 mg/ml RA. The RA-induced expression of cCYP26 in anterior limb mesenchyme and the AER was detected as early as 1 hr after treatment and was not affected by the presence of cycloheximide. In contrast to the anterior limb, the induction of cCYP26 was dramatically reduced (or absent) when RA beads were implanted in the posterior limb mesenchyme. Furthermore, induction of cCYP26 expression in the anterior mesenchyme was inhibited by transplantations of the zone of polarizing activity (ZPA) and by Shh-soaked beads. Our data suggest that different mechanisms regulate retinoid homeostasis in the AER and mesenchyme during limb bud outgrowth. J. Exp. Zool. 290:136-147, 2001.

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.

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.

Hind limb malformations in free-living northern leopard frogs (Rana pipiens) from Maine, Minnesota, and Vermont suggest multiple etiologies

BACKGROUND

Reports of malformed frogs have increased throughout the North American continent in recent years. Most of the observed malformations have involved the hind limbs. The goal of this study was to accurately characterize the hind limb malformations in wild frogs as an important step toward understanding the possible etiologies.

METHODS

During 1997 and 1998, 182 recently metamorphosed northern leopard frogs (Rana pipiens) were collected from Minnesota, Vermont, and Maine. Malformed hind limbs were present in 157 (86%) of these frogs, which underwent necropsy and radiographic evaluation at the National Wildlife Health Center. These malformations are described in detail and classified into four major categories: (1) no limb (amelia); (2) multiple limbs or limb elements (polymelia, polydactyly, polyphalangy); (3) reduced limb segments or elements (phocomelia, ectromelia, ectrodactyly, and brachydactyly; and (4) distally complete but malformed limb (bone rotations, bridging, skin webbing, and micromelia).

RESULTS

Amelia and reduced segments and/or elements were the most common finding. Frogs with bilateral hind limb malformations were not common, and in only eight of these 22 frogs were the malformations symmetrical. Malformations of a given type tended to occur in frogs collected from the same site, but the types of malformations varied widely among all three states, and between study sites within Minnesota.

CONCLUSIONS

Clustering of malformation type suggests that developmental events may produce a variety of phenotypes depending on the timing, sequence, and severity of the environmental insult. Hind limb malformations in free-living frogs transcend current mechanistic explanations of tetrapod limb development.

Interdigital regulation of digit identity and homeotic transformation by modulated BMP signaling

The developmental mechanisms specifying digital identity have attracted 30 years of intense interest, but still remain poorly understood. Here, through experiments on chick foot development, we show digital identity is not a fixed property of digital primordia. Rather, digital identity is specified by the interdigital mesoderm, demonstrating a patterning function for this tissue before its regression. More posterior interdigits specify more posterior digital identities, and each primordium will develop in accordance with the most posterior cues received. Furthermore, inhibition of interdigital bone morphogenetic protein (BMP) signaling can transform digit identity, suggesting a role for BMPs in this process.

Inhibition of sonic hedgehog autoprocessing in cultured mammalian cells by sterol deprivation

Sonic hedgehog (Shh) is a signaling molecule that is important for defining patterning in the developing vertebrate central nervous system. After translation, Shh autoproteolyzes and covalently attaches cholesterol to the newly formed carboxyl terminus, a modification crucial for normal Shh signaling. Presented here is evidence that acute severe sterol deprivation in cultured Chinese hamster ovary cells expressing mouse Shh (mShh) inhibits autoprocessing of the protein. These conditions allowed the first detailed kinetic analysis of mShh autoprocessing and turnover rates revealing that cells rapidly degrade both precursor and mature mShh regardless of sterol content and sterol deprivation increases the rate of precursor degradation. Inhibition of mShh autoprocessing also allowed the determination of the subcellular localization of mShh precursor which accumulates in a pre-medial Golgi intracellular compartment. Finally, the precursor form of mShh that results from autoprocessing inhibition appears to accumulate as an amide rather than a stable thioester.

Plzf regulates limb and axial skeletal patterning

The promyelocytic leukaemia zinc finger (Plzf) protein (encoded by the gene Zfp145) belongs to the POZ/zinc-finger family of transcription factors. Here we generate Zfp145-/- mice and show that Plzf is essential for patterning of the limb and axial skeleton. Plzf inactivation results in patterning defects affecting all skeletal structures of the limb, including homeotic transformations of anterior skeletal elements into posterior structures. We demonstrate that Plzf acts as a growth-inhibitory and pro-apoptotic factor in the limb bud. The expression of members of the abdominal b (Abdb) Hox gene complex, as well as genes encoding bone morphogenetic proteins (Bmps), is altered in the developing limb of Zfp145-/- mice. Plzf regulates the expression of these genes in the absence of aberrant polarizing activity and independently of known patterning genes. Zfp145-/- mice also exhibit anterior-directed homeotic transformation throughout the axial skeleton with associated alterations in Hox gene expression. Plzf is therefore a mediator of anterior-to-posterior (AP) patterning in both the axial and appendicular skeleton and acts as a regulator of Hox gene expression.

Life, death and Sonic hedgehog

The secreted glycoprotein Sonic hedgehog (SHH), a vertebrate homologue of the Drosophila segment polarity gene Hedgehog, is essential for the development of diverse tissues during embryogenesis. Studies of SHH function during neural tube and somite development have focused on its role in specifying the dorsoventral polarity of these structures, but a recent report by Ahlgren and Bronner-Fraser(1) supports the possibility that SHH has additional functions in cell survival and cell proliferation. Perturbation of SHH signaling after the early dorsoventral specification of the cranial neural tube leads to increased cell death in both the neural tube and the neural crest. This implies that SHH is continually required as a trophic and/or mitogenic factor during brain development, and expands the variety of cellular responses to SHH signaling. BioEssays 22:499-502, 2000.

Forkhead transcription factor FoxF2 is expressed in mesodermal tissues involved in epithelio-mesenchymal interactions

The growing family of forkhead transcription factors plays many important roles during embryonic development. In this study we have used in situ hybridization to explore the expression pattern of the forkhead transcription factor gene FoxF2 (FREAC-2, LUN) during mouse and rat embryogenesis, postnatal development, and in adult tissues. We demonstrate that FoxF2 is expressed in the mesenchyme adjacent to the epithelium in alimentary, respiratory, and urinary tracts, similar to FoxF1 (FREAC-1, HFH-8). FoxF2 mRNA was also observed in organs that do not express FoxF1 during embryogenesis, e.g., in the central nervous system, eye, ear, and limb buds. In organs that express both FoxF2 and FoxF1, these transcription factors may have similar functions in epithelio-mesenchymal cross-talk, but the fact that FoxF2 is more widely expressed than FoxF1 suggests that FoxF2 also has an independent role as a developmental regulator.

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.

Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification

During neural development in vertebrates, a spatially ordered array of neurons is generated in response to inductive signals derived from localized organizing centres. One organizing centre that has been proposed to have a role in the control of neural patterning is the roof plate. To define the contribution of signals derived from the roof plate to the specification of neuronal cell types in the dorsal neural tube, we devised a genetic strategy to ablate the roof plate selectively in mouse embryos. Embryos without a roof plate lack all the interneuron subtypes that are normally generated in the dorsal third of the neural tube. Using a genetically based lineage analysis and in vitro assays, we show that the loss of these neurons results from the elimination of non-autonomous signals provided by the roof plate. These results reveal that the roof plate is essential for specifying multiple classes of neurons in the mammalian central nervous system.

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

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

Limbiting outgrowth: BMPs as negative regulators in limb development

Rapid progress is being made in understanding how integrated signaling pathways direct patterned outgrowth of the vertebrate limb. In contrast, the mechanisms that constrain limb outgrowth, and thus delimit adult morphology, remain poorly understood. Two recent pioneering reports have implicated bone morphogenetic proteins (BMPs) in negatively regulating the function of the apical ectodermal ridge (AER), an inductive structure required for continued proximodistal specification of limb elements. These studies provide the first insights into how the termination of a limb bud signaling center is accomplished, and intriguingly suggest how distinct aspects of limb morphogenesis are regulated.

Constitutive activation of sonic hedgehog signaling in the chicken mutant talpid(2): Shh-independent outgrowth and polarizing activity

We have examined the developmental properties of the polydactylous chicken mutant, talpid(2). Ptc, Gli1, Bmp2, Hoxd13, and Fgf4 are expressed throughout the anteroposterior axis of the mutant limb bud, despite normal Shh expression. The expression of Gli3, Ihh, and Dhh appears to be normal, suggesting that the Shh signaling pathway is constitutively active in talpid(2) mutants. We show that preaxial talpid(2) limb bud mesoderm has polarizing activity in the absence of detectable Shh mRNA. When the postaxial talpid(2) limb bud (including all Shh-expressing cells) is removed, the preaxial cells reform a normal-shaped talpid(2) limb bud (regulate). However, a Shh-expressing region (zone of polarizing activity) does not reform; nevertheless Fgf4 expression in the apical ectodermal ridge is maintained. Such reformed talpid(2) limb buds develop complete talpid(2) limbs. After similar treatment, normal limb buds downregulate Fgf4, the preaxial cells do not regulate, and a truncated anteroposterior deficient limb forms. In talpid(2) limbs, distal outgrowth is independent of Shh and correlates with Fgf4, but not Fgf8, expression by the apical ectodermal ridge. We propose a model for talpid(2) in which leaky activation of the Shh signaling pathway occurs in the absence of Shh ligand.