Analysis of Hox gene expression in the chick limb bud

Perspectives on the evolutionary origin of tetrapod limbs

The study of the origin and evolution of the tetrapod limb has benefited enormously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponentially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan.

Targeted inversion of a polar silencer within the HoxD complex re-allocates domains of enhancer sharing

Mammalian Hox genes are clustered at four genomic loci. During development, neighbouring genes are coordinately regulated by global enhancer sequences, which control multiple genes at once, as exemplified by the expression of series of contiguous Hoxd genes in either limbs or gut. The link between vertebrate Hox gene transcription and their clustered distribution is poorly understood. Experimental and comparative approaches have revealed that various mechanisms, such as gene clustering or global enhancer sequences, might have constrained this genomic organization and stabilized it throughout evolution. To understand what restricts the effect of a particular enhancer to a precise set of genes, we generated a loxP/Cre-mediated targeted inversion within the HoxD cluster. Mice carrying the inversion showed a reciprocal re-assignment of the limb versus gut regulatory specificities, suggesting the presence of a silencer element with a unidirectional property. This polar silencer appears to limit the number of genes that respond to one type of regulation and thus indicates how separate regulatory domains may be implemented within intricate gene clusters.

Evolution of Hoxa-11 in lineages phylogenetically positioned along the fin-limb transition

HOXA11 is a transcription factor implicated in paired appendage development. To identify signatures of evolutionary change in the structural, and putative functional, domains of HOXA11, we studied its evolution in tetrapod and nontetrapod lineages that represent approximately 1.5 billion years of evolutionary time. Here, Hoxa-11 gene proper sequences were determined for frog (Xenopus tropicalis), coelacanth (Latimeria chalumnae), common zebrafish (Danio rerio; Hoxa-11a and Hoxa-11b paralogs), and giant zebrafish (D. aequipinnatus; Hoxa-11b) and aligned against previously published Hoxa-11 sequences of human, mouse, chick, and newt. Based on aligned Hoxa-11 amino acid sequences, the protein was demarcated into three segments: Domains I (N-terminal) and III (homeobox + C-terminal), which varied slightly in rates and patterns of evolution, and a variable, overall hydrophilic region (HyD), which partially overlaps with Domain I. As judged by character reconstructions of HOXA11 Domains I and III, no significant changes in rates of coding sequence evolution occurred in tetrapods (frog and chick), relative to coelacanth (a lobe-finned fish), i.e., across the fin-limb transition. Accelerated rates of Hoxa-11 coding sequence evolution were observed for the mammalian and newt lineages. This was shown to be a gene-specific phenomenon. The duplicated Hoxa-11a and Hoxa-11b genes of zebrafish exhibited accelerated rates of evolution and accumulated substitutions at sites that are conserved among coelacanth and all tetrapods examined. Amino acid sequence comparisons of the HyD of HOXA11 suggested that a putative repressor subdomain, containing stretches of consecutive alanine residues, emerged within the tetrapods. A high degree of nucleotide conservation in the 5' half of the Hoxa-11 intron was observed for tetrapod and nontetrapod lineages. Using electrophoretic mobility shift assays, a 35-bp intron sequence, which is 100% conserved in all Hoxa-11 loci except for the zebrafish Hoxa-11a paralog, was found to bind protein(s) in HeLa and chick whole-cell extracts.

Hox genes and morphological identity: axial versus lateral patterning in the vertebrate mesoderm

The successful organization of the vertebrate body requires that local information in the embryo be translated into a functional, global pattern. Somite cells form the bulk of the musculoskeletal system. Heterotopic transplants of segmental plate along the axis from quail to chick were performed to test the correlation between autonomous morphological patterning and Hox gene expression in somite subpopulations. The data presented strengthen the correlation of Hox gene expression with axial specification and focus on the significance of Hox genes in specific derivatives of the somites. We have defined two anatomical compartments of the body based on the embryonic origin of the cells making up contributing structures: the dorsal compartment, formed from purely somitic cell populations; and the ventral compartment comprising cells from somites and lateral plate. The boundary between these anatomical compartments is termed the somitic frontier. Somitic tissue transplanted between axial levels retains both original Hox expression and morphological identity in the dorsal compartment. In contrast, migrating lateral somitic cells crossing the somitic frontier do not maintain donor Hox expression but apparently adopt the Hox expression of the lateral plate and participate in the morphology appropriate to the host level. Dorsal and ventral compartments, as defined here, have relevance for experimental manipulations that influence somite cell behavior. The correlation of Hox expression profiles and patterning behavior of cells in these two compartments supports the hypothesis of independent Hox codes in paraxial and lateral plate mesoderm.

Distribution of polarizing activity and potential for limb formation in mouse and chick embryos and possible relationships to polydactyly

A central feature of the tetrapod body plan is that two pairs of limbs develop at specific positions along the head-to-tail axis. However, the potential to form limbs in chick embryos is more widespread. This could have implications for understanding the basis of limb abnormalities. Here we extend the analysis to mouse embryos and examine systematically the potential of tissues in different regions outside the limbs to contribute to limb structures. We show that the ability of ectoderm to form an apical ridge in response to FGF4 in both mouse and chick embryos exists throughout the flank as does ability of mesenchyme to provide a polarizing region signal. In addition, neck tissue has weak polarizing activity. We show, in chick embryos, that polarizing activity of tissues correlates with the ability either to express Shh or to induce Shh expression. We also show that cells from chick tail can give rise to limb structures. Taken together these observations suggest that naturally occurring polydactyly could involve recruitment of cells from regions adjacent to the limb buds. We show that cells from neck, flank and tail can migrate into limb buds in response to FGF4, which mimics extension of the apical ectodermal ridge. Furthermore, when we apply simultaneously a polarizing signal and a limb induction signal to early chick flank, this leads to limb duplications.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Developmental regulation and asymmetric expression of the gene encoding Cx43 gap junctions in the mouse limb bud

The Gja1 gene encoding the gap junction connexin 43 (Cx43) is dynamically regulated during limb morphogenesis. Transcript expression is found in many regions of the limb bud known to be important in regulating limb growth and patterning. In the newly emerged limb bud, Gja1 transcripts are first expressed in the ventrodistal margin of the ectoderm, and later transcript expression is localized to the apical ectodermal ridge (AER). Interestingly, transcript expression in the ventrodistal ectoderm is initiated left/right asymmetrically, with some strain backgrounds showing reverse sidedness in the fore vs. hindlimb buds. In legless, a mouse mutant exhibiting both limb and left/right patterning defects, Gja1 transcripts could not be detected in this region. However, in the i.v./i.v. embryo, a mutant with randomization of body situs the same pattern of Gja1 asymmetry was found in the limb ectoderm regardless of body situs. This suggests that Gja1 transcript expression is not directly linked to signaling pathways involved in specification of the left/right axis. In addition to transcript expression in the apical ectodermal ridge, Gja1 transcripts were also found at high levels in the ventral ectoderm. In the limb bud mesenchyme, Gja1 transcripts were distributed in a posterior distal gradient, coincident with tissue known to have polarizing activity. With limb outgrowth and the initiation of limb mesenchyme condensation. Gja1 transcripts were localized in the presumptive progress zone, and in the condensing mesenchyme. In more proximal regions of the limb where mesenchyme differentiation has been initiated, Gja1 transcripts were expressed only in the outer mesenchymal cells comprising the presumptive perichondrium. Further analysis of transgenic mice ectopically expressing Wnt-1 in the limb mesenchyme revealed alterations in the pattern of Gja1 transcript expression in conjunction with the perturbation of limb mesenchyme condensation and differentiation. Together, these findings indicate that Cx43 gap junctions may mediate cell-cell interactions important in cell signaling processes involved in limb growth and patterning.

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.

Limbs and tail as evolutionarily diverging duplicates of the main body axis

Contrasting hypotheses have been proposed to explain the pervasive parallels in the patterning of arthropod and vertebrate appendages. These hypotheses either call for a common ancestor already provided with patterned appendages or body outgrowths, or for the recruitment in limb patterning of single genes or genetic cassettes originally used for purposes other than axis patterning. I suggest instead that body appendages such as arthropod and vertebrate limbs and chordate tails are evolutionarily divergent duplicates (paramorphs) of the main body axis, that is, its duplicates, albeit devoid of endodermal component. Thus, vertebrate limbs and arthropod limbs are not historical homologs, but homoplastic features only transitively related to real historical homologs. Thus, the main body axis and the axis of the appendages have distinct but not independent evolutionary histories and may be involved in processes of homeotic co-option producing effects of morphological assimilation. For instance, chordate segmentation may have originated in the posterior appendage (tail) and subsequently extended to the trunk.

Regulatory evolution and the origin of the bilaterians

The adult body plan of bilaterians is achieved by imposing regional specifications on pluripotential cells. The establishment of spatial domains is governed in part by regulating expression of transcription factors. The key to understanding bilaterian evolution is contingent on our understanding of how the regulation of these transcription factors influenced bilaterian stem-group evolution.

Analysis of gene expressions during Xenopus forelimb regeneration

Xenopus laevis can regenerate an amputated limb completely at early limb bud stages, but the metamorphosed froglet gradually loses this capacity and can regenerate only a spike-like structure. We show that the spike formation in a Xenopus froglet is nerve dependent as is limb regeneration in urodeles, since denervation concomitant with amputation is sufficient to inhibit the initiation of blastema formation and fgf8 expression in the epidermis. Furthermore, in order to determine the cause of the reduction in regenerative capacity, we examined the expression patterns of several key genes for limb patterning during the spike-like structure formation, and we compared them with those in developing and regenerating limb buds that produce a complete limb structure. We cloned Xenopus HoxA13, a marker of the prospective autopodium region, and the expression pattern suggested that the spike-like structure in froglets is accompanied by elongation and patterning along the proximodistal (PD) axis. On the other hand, shh expression was not detected in the froglet blastema, which expresses fgf8 and msx1. Thus, although the wound epidermis probably induces outgrowth of the froglet blastema, the polarizing activity that organizes the anteroposterior (AP) axis formation is likely to be absent there. Our results demonstrate that the lost region in froglet limbs is regenerated along the PD axis and that the failure of organization of the AP pattern gives rise to a spike-like incomplete structure in the froglet, suggesting a relationship between regenerative capacity and AP patterning. These findings lead us to conclude that the spike formation in postometamorphic Xenopus limbs is epimorphic regeneration.

Loss of fibula in mice overexpressing Hoxc11

This study demonstrates severe malformations of the appendicular skeleton in mice overexpressing Hoxc11. Consistent with the endogenous expression pattern, the most conspicuous defect in Hoxc11 overexpressing neonates is aplasia/hypoplasia of the fibula. This is preceded at day 15.5 of embryonic development by marked reduction of chondrocyte proliferation, lack of PTHR expressing prehypertrophic cells, and the absence of hypertrophic and calcifying chondrocytes. Combined with the lack of an overt phenotype in the majority of Hoxc11 overexpressing embryos at day 13.5, the data suggest inhibition of chondrocyte differentiation during the elongation phase of the fibula bone as a primary effect of elevated Hoxc11 expression. This interpretation is further corroborated by Hoxc11 reporter gene expression in the joint areas at embryonic day 15.5, suggesting an involvement of the periarticular perichondrium in generating the mutant phenotype.

Early tetrapod evolution

Tetrapods include the only fully terrestrial vertebrates, but they also include many amphibious, aquatic and flying groups. They occupy the highest levels of the food chain on land and in aquatic environments. Tetrapod evolution has generated great interest, but the earliest phases of their history are poorly understood. Recent studies have questioned long-accepted hypotheses about the origin of the pentadactyl limb, the phylogeny of tetrapods and the environment in which the first tetrapods lived.

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.

Mechanisms of Hox gene colinearity: transposition of the anterior Hoxb1 gene into the posterior HoxD complex

Transposition of Hoxd genes to a more posterior (5′) location within the HoxD complex suggested that colinearity in the expression of these genes was due, in part, to the existence of a silencing mechanism originating at the 5′ end of the cluster and extending towards the 3′ direction. To assess the strength and specificity of this repression, as well as to challenge available models on colinearity, we inserted a Hoxb1/lacZtransgene within the posterior HoxD complex, thereby reconstructing a cluster with a copy of the most anterior gene inserted at the most posterior position. Analysis of Hoxb1 expression after ectopic relocation revealed that Hoxb1-specific activity in the fourth rhombomere was totally abolished. Treatment with retinoic acid, or subsequent relocations toward more 3′ positions in theHoxD complex, did not release this silencing in hindbrain cells. In contrast, however, early and anterior transgene expression in the mesoderm was unexpectedly not suppressed. Furthermore, the transgene induced a transient ectopic activation of the neighboringHoxd13 gene, without affecting other genes of the complex. Such a local and transient break in colinearity was also observed after transposition of the Hoxd9/lacZ reporter gene, indicating that it may be a general property of these transgenes when transposed at an ectopic location. These results are discussed in the context of existing models, which account for colinear activation of vertebrate Hox genes.

Regulation of a muscle-specific transgene by persistent expression of Hox genes in postnatal murine limb muscle

Homeobox genes are necessary for the generation of the embryonic body plan in both invertebrate and vertebrate organisms. To investigate the potential function of homeodomain proteins in normal and regenerating skeletal muscle, we analyzed patterns of clustered homeobox gene expression in neonatal and adult muscle tissue. Transcripts encoding 5' genes in the HoxA cluster were detected in muscles from both the fore- and hindlimbs of neonatal and adult mice, whereas expression of HoxC gene transcripts was generally restricted to the muscles of the hindlimb. In contrast, transcripts encoding genes of the HoxB or HoxD clusters were not detected in muscles from either fore- or hindlimbs. Although ectopic expression of select HOX proteins in muscle cell cultures had modest effects upon the activity of a co-transfected myosin light chain (MLC) enhancer, mutation of a Hox binding site in this enhancer elicited increased linked reporter gene expression. Induction of muscle damage and regeneration was accompanied by the down-regulation of at least one Hox gene, concurrent with the activation of the regenerative program. Moreover, targeted ablation of the Hoxc-8 gene, normally expressed in mature fore- and hindlimb muscles, resulted in reduced expression of an MLC enhancer-driven transgene only in specific leg muscles. These results indicate that members of the HoxA and C clusters may, in combination, mediate various aspects of differentiation and patterning in adult musculature.

Coordinated expression of 3' hox genes during murine embryonal gut development: an enteric Hox code

BACKGROUND & AIMS

Hox genes are highly conserved developmental control genes that may be organized and expressed in the form of a code required for correct morphogenesis. Little is known about their control of the embryonal gut. However, Hox paralogues 4 and 5, which are expressed at the sites of origin of vagal neural crest cells and splanchnic mesoderm, are likely to be important. We have studied the expression domains of these genes in the gut both spatially and temporally.

METHODS

CD1 mice embryos of embryonic days E8.5-E17.5 were studied. The spatial and temporal expression patterns of messenger RNA of Hoxa4, b4, c4, d4, a5, c5, and b5 homeoprotein were determined by in situ hybridization and immunohistochemistry in whole embryos, whole gastrointestinal tracts, and vibratome sections.

RESULTS

There were different spatial, temporal, and combinatorial expression patterns in different morphological regions: foregut, prececal gut, cecum, and postcecal gut. Two dynamic gradients, rostral and caudal, were coordinated with nested expression domains along the gut primordium. Region-specific domains were present in the stomach and cecum.

CONCLUSIONS

The expression patterns of genes in paralogous groups 4 and 5 suggest that they are organized to form a specific enteric Hox code required for correct enteric development.

Conserved regulation of proximodistal limb axis development by Meis1/Hth

Vertebrate limbs grow out from the flanks of embryos, with their main axis extending proximodistally from the trunk. Distinct limb domains, each with specific traits, are generated in a proximal-to-distal sequence during development. Diffusible factors expressed from signalling centres promote the outgrowth of limbs and specify their dorsoventral and anteroposterior axes. However, the molecular mechanism by which limb cells acquire their proximodistal (P-D) identity is unknown. Here we describe the role of the homeobox genes Meis1/2 and Pbx1 in the development of mouse, chicken and Drosophila limbs. We find that Meis1/2 expression is restricted to a proximal domain, coincident with the previously reported domain in which Pbx1 is localized to the nucleus, and resembling the distribution of the Drosophila homologues homothorax (hth) and extradenticle (exd); that Meis1 regulates Pbx1 activity by promoting nuclear import of the Pbx1 protein; and that ectopic expression of Meis1 in chicken and hth in Drosophila disrupts distal limb development and induces distal-to-proximal transformations. We suggest that restriction of Meis1/Hth to proximal regions of the vertebrate and insect limb is essential to specify cell fates and differentiation patterns along the P-D axis of the limb.

Morphological analysis of the mammalian postcranium: a developmental perspective

The past two decades have greatly improved our knowledge of vertebrate skeletal morphogenesis. It is now clear that bony morphology lacks individual descriptive specification and instead results from an interplay between positional information assigned during early limb bud deployment and its "execution" by highly conserved cellular response programs of derived connective tissue cells (e.g., chondroblasts and osteoblasts). Selection must therefore act on positional information and its apportionment, rather than on more individuated aspects of presumptive adult morphology. We suggest a trait classification system that can help integrate these findings in both functional and phylogenetic examinations of fossil mammals and provide examples from the human fossil record.

Role of N-cadherin in the sorting-out of mesenchymal cells and in the positional identity along the proximodistal axis of the chick limb bud

Mesenchymal cells from different stages of chick limb buds sort out in monolayer culture, suggesting the presence of different cell affinities dependent on their positions along the proximodistal axis. However, it is still not clear which molecules are responsible for the sorting-out. Here, we propose that N-cadherin, a cell-adhesion molecule, is involved in the sorting-out and is likely to be a component of the mechanism of proximodistal patterning in the developing limb. N-cadherin proteins accumulate in the distal region of the chick limb bud as limb development proceeds. In monolayer culture of distal mesenchymal cells, the stage-dependent levels of N-cadherin proteins are maintained during cell sorting. The results of this study have also demonstrated that an anti-N-cadherin monoclonal antibody, NCD-2, clearly inhibits the cell sorting. Moreover, removal of the apical ectodermal ridge or retinoic-acid treatment of distal cells, which results in a change in the pattern of sorting-out, inhibits the accumulation of N-cadherin proteins, suggesting that the distribution of these proteins is related to the positional identity that gives rise to the different shape and number of cartilage elements along the proximodistal axis.

Sectorial gene repression in the control of development

Some evidence suggests that a number of regulator genes and gene clusters will likely be found to share with HOX complexes the property of being repressible ('superrepressible') through factor-driven conformational changes over whole sectors of chromatin, and of being assigned body locations in which they are either stably superrepressed or poised for transcription, according to determinants that act vectorially across a morphological zone. Such a subpopulation of regulator genes is expected to include, notably, genes governing developmental processes and might be thought to number, in mammals, between one hundred and several hundreds. When superrepressed, regulator genes are anticipated either to block programs of gene action or to permit these programs to unfold. To a significant extent, development would be determined by successive intersections of the paths of gene action deployment with superrepressed genes. These intersections, in cell lines advancing toward terminal differentiation, would be responsible for the progressive narrowing of the range of gene action programs potentially still available for later development. One implication of this model is that mosaic and regulative embryos are distinct merely by virtue of the time of onset of superrepression in their different cell lineages. Determination and transdetermination are considered to express the differential distribution over the genome of bound regulatory factors that function as molecular tools of superrepression, notably polycomb-group-like proteins. In turn, superrepressed genes are anticipated to be differentially distributed over cell types and thus to furnish a major framework for progressive differentiation and for the progressive limitation of the developmental potential of cells.

Modes of morphogen cooperation for limb formation in vertebrates and insects

Diffusing morphogens in cooperation can control gene expression in developing limbs. Additive cooperation corresponds to the Boolean operator OR and implies the equivalent action of the (suitably scaled) concentrations of two morphogens, either by their alternative binding to the same receptor or by another way of convergence of their effects during the signal transduction procedure. This cooperation can explain the spatial and temporal collinearities of the expression of hoxd genes in the vertebrate limb bud. A multiplicative cooperation of morphogens (corresponding to the Boolean operator AND), produced at the DPP and WG domains in the Drosophila leg imaginal disc, may account for the expression domains observed for Dll and dac. A molecular interpretation of the multiplicative morphogen cooperation is proposed. Some experiments are suggested for further testing of the model.

Fate maps old and new

Fate mapping was once the province of classical experimental embryologists. Now a battery of new and sophisticated methods can be used to trace where cells go and what they do in embryos. Here we use examples from gastrulating fish and amphibian embryos and from the chick limb bud and central nervous system to show how this information has contributed to our understanding of developmental processes. This knowledge will become increasingly important in interpreting the complex patterns of gene expression that are being discovered during development, as well as in understanding the effects of genetic manipulations and in directing experimental interventions.

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.

Monodactylous limbs and abnormal genitalia are associated with hemizygosity for the human 2q31 region that includes the HOXD cluster

Vertebrates have four clusters of Hox genes (HoxA, HoxB, HoxC, and HoxD). A variety of expression and mutation studies indicate that posterior members of the HoxA and HoxD clusters play an important role in vertebrate limb development. In humans, mutations in HOXD13 have been associated with type II syndactyly or synpolydactyly, and, in HOXA13, with hand-foot-genital syndrome. We have investigated two unrelated children with a previously unreported pattern of severe developmental defects on the anterior-posterior (a-p) limb axis and in the genitalia, consisting of a single bone in the zeugopod, either monodactyly or oligodactyly in the autopod of all four limbs, and penoscrotal hypoplasia. Both children are heterozygous for a deletion that eliminates at least eight (HOXD3-HOXD13) of the nine genes in the HOXD cluster. We propose that the patients' phenotypes are due in part to haploinsufficiency for HOXD-cluster genes. This hypothesis is supported by the expression patterns of these genes in early vertebrate embryos. However, the involvement of additional genes in the region could explain the discordance, in severity, between these human phenotypes and the milder, non-polarized phenotypes present in mice hemizygous for HoxD cluster genes. These cases represent the first reported examples of deficiencies for an entire Hox cluster in vertebrates and suggest that the diploid dose of human HOXD genes is crucial for normal growth and patterning of the limbs along the anterior-posterior axis.

Developmental basis of limblessness and axial patterning in snakes

The evolution of snakes involved major changes in vertebrate body plan organization, but the developmental basis of those changes is unknown. The python axial skeleton consists of hundreds of similar vertebrae, forelimbs are absent and hindlimbs are severely reduced. Combined limb loss and trunk elongation is found in many vertebrate taxa, suggesting that these changes may be linked by a common developmental mechanism. Here we show that Hox gene expression domains are expanded along the body axis in python embryos, and that this can account for both the absence of forelimbs and the expansion of thoracic identity in the axial skeleton. Hindlimb buds are initiated, but apical-ridge and polarizing-region signalling pathways that are normally required for limb development are not activated. Leg bud outgrowth and signalling by Sonic hedgehog in pythons can be rescued by application of fibroblast growth factor or by recombination with chick apical ridge. The failure to activate these signalling pathways during normal python development may also stem from changes in Hox gene expression that occurred early in snake evolution.

Limb development: Farewell to arms

Forelimbs and hindlimbs are, clearly, quite different, and it has long been appreciated that their differences are assigned early in development; the genetic basis of these differences has been more mysterious, however. Recent work has now shown that the homeobox gene Pitx1 imparts identity to the developing hindlimb bud.

Control of digit formation by activin signalling

Major advances in the genetics of vertebrate limb development have been obtained in recent years. However, the nature of the signals which trigger differentiation of the mesoderm to form the limb skeleton remains elusive. Previously, we have obtained evidence for a role of TGFbeta2 in digit formation. Here, we show that activins A and B and/or AB are also signals involved in digit skeletogenesis. activin betaA gene expression correlates with the initiation of digit chondrogenesis while activin betaB is expressed coincidently with the formation of the last phalanx of each digit. Exogenous administration of activins A, B or AB into the interdigital regions induces the formation of extra digits. follistatin, a natural antagonist of activins, is expressed, under the control of activin, peripherally to the digit chondrogenic aggregates marking the prospective tendinous blastemas. Exogenous application of follistatin blocks physiological and activin-induced digit formation. Evidence for a close interaction between activins and other signalling molecules, such as BMPs and FGFs, operating at the distal tip of the limb at these stages is also provided. Chondrogenesis by activins is mediated by BMPs through the regulation of the BMP receptor bmpR-1b and in turn activin expression is upregulated by BMP signalling. In addition, AER hyperactivity secondary to Wnt3A misexpression or local administration of FGFs, inhibits activin expression. In correlation with the restricted expression of activins in the course of digit formation, neither activin nor follistatin treatment affects the development of the skeletal components of the stylopod or zeugopod indicating that the formation of the limb skeleton is regulated by segment-specific chondrogenic signals.

Prostate development requires Sonic hedgehog expressed by the urogenital sinus epithelium

The prostate gland develops from the urogenital sinus by a testosterone-dependent process of ductal morphogenesis. Sonic hedgehog (Shh) is expressed in the urogenital sinus epithelium and the time course of expression coincides with the formation of the main prostatic ducts. Expression is most abundant in the lumen of the urogenital sinus and in the contiguous proximal duct segments. The initial upregulation of Shh expression in the male urogenital sinus depends on the presence of testosterone. The function of Shh was examined in the male urogenital sinus which was transplanted under the renal capsule of an adult male host mouse. Blockade of Shh function by a neutralizing antibody interferes with Shh signaling and abrogates growth and ductal morphogenesis in the transplanted tissue. These observations show that testosterone-dependent Shh expression in the urogenital sinus is necessary for the initiation of prostate development.

Breaking colinearity in the mouse HoxD complex

Vertebrate Hox genes are activated in a spatiotemporal sequence that reflects their clustered organization. While this colinear relationship is a property of most metazoans with an anterior to posterior polarity, the underlying molecular mechanisms are unknown. Previous work suggested that Hox genes were made progressively available for transcription in the course of gastrulation, implying the existence of an element capable of initiating a repressive conformation, subsequently relieved from the clusters sequentially. We searched for this element by combining a genomic walk with successive transgene insertions upstream of the HoxD complex followed by a series of deletions. The largest deficiency induced posterior homeotic transformations coincidentally with an earlier activation of Hoxd genes. These data suggest that a regulatory element located upstream of the complex is necessary for setting up the early pattern of Hox gene colinear activation.

Tbx5 and Tbx4 genes determine the wing/leg identity of limb buds

Much progress has been made in understanding limb development. Most genes are expressed equally and in the same pattern in the fore- and hindlimbs, which nevertheless develop into distinct structures. The T-box genes Tbx5 and Tbx4, on the other hand, are expressed differently in chick wing (Tbx5) and leg (Tbx4) buds. Molecular analysis of the optomotor blind gene, which belongs to the same family of transcription factors, has revealed that this gene is involved in the transdetermination of Drosophila wing and leg imaginal discs. In addition, expression of Tbx5 and Tbx4 correlates well with the identity of ectopic limb buds induced by fibroblast growth factor. Thus, it is thought that Tbx5 and Tbx4 might be involved in determining limb identity. Another candidate is the Pitx1 gene, which encodes a bicoid-type homeodomain transcription factor that is expressed in leg buds. Here we determine the importance of these factors in establishing limb identity.

SWiP-1: novel SOCS box containing WD-protein regulated by signalling centres and by Shh during development

We describe a novel chick WD-protein, cSWiP-1, expressed in somitic mesoderm and developing limb buds as well as in other embryonic structures where Hedgehog signalling has been shown to play a role. Using embryonic manipulations we show that in somites cSWiP-1 expression integrates two signals originating from structures adjacent to the segmental mesoderm: a positive signal from the notochord and a negative signal from intermediate and/or lateral mesoderm. In explant cultures of somitic mesoderm, Shh protein induces cSWiP-1, while a blocking antibody to Shh inhibits the induction of cSWiP-1 by the notochord. These results show that the positive signal from the notochord is mediated by Shh. We also show that in limb buds cSWiP-1 is upregulated by ectopic Shh. This occurs in about the same time period as upregulation of BMP2, placing cSWiP-1 among the earliest markers for the change of limb pattern caused by ectopic Shh. We also describe a human homologue of cSWiP-1 and a mouse gene, mSWiP-2, that is more distantly related to SWiP-1, suggesting that SWiP-1 belongs to a novel subfamily of WD-proteins.

Novel regulation of the homeotic gene Scr associated with a crustacean leg-to-maxilliped appendage transformation

Homeotic genes are known to be involved in patterning morphological structures along the antero-posterior axis of insects and vertebrates. Because of their important roles in development, changes in the function and expression patterns of homeotic genes may have played a major role in the evolution of different body plans. For example, it has been proposed that during the evolution of several crustacean lineages, changes in the expression patterns of the homeotic genes Ultrabithorax and abdominal-A have played a role in transformation of the anterior thoracic appendages into mouthparts termed maxillipeds. This homeotic-like transformation is recapitulated at the late stages of the direct embryonic development of the crustacean Porcellio scaber (Oniscidea, Isopoda). Interestingly, this morphological change is associated with apparent novelties both in the transcriptional and post-transcriptional regulation of the Porcellio scaber ortholog of the Drosophila homeotic gene, Sex combs reduced (Scr). Specifically, we find that Scr mRNA is present in the second maxillary segment and the first pair of thoracic legs (T1) in early embryos, whereas protein accumulates only in the second maxillae. In later stages, however, high levels of SCR appear in the T1 legs, which correlates temporally with the transformation of these appendages into maxillipeds. Our observations provide further insight into the process of the homeotic leg-to-maxilliped transformation in the evolution of crustaceans and suggest a novel regulatory mechanism for this process in this group of arthropods.

Role of Pitx1 upstream of Tbx4 in specification of hindlimb identity

In spite of recent breakthroughs in understanding limb patterning, the genetic factors determining the differences between the forelimb and the hindlimb have not been understood. The genes Pitx1 and Tbx4 encode transcription factors that are expressed throughout the developing hindlimb but not forelimb buds. Misexpression of Pitx1 in the chick wing bud induced distal expression of Tbx4, as well as HoxC10 and HoxC11, which are normally restricted to hindlimb expression domains. Wing buds in which Pitx1 was misexpressed developed into limbs with some morphological characteristics of hindlimbs: the flexure was altered to that normally observed in legs, the digits were more toe-like in their relative size and shape, and the muscle pattern was transformed to that of a leg.

Gli3 (Xt) and formin (ld) participate in the positioning of the polarising region and control of posterior limb-bud identity

During initiation of limb-bud outgrowth in vertebrate embryos, the polarising region (limb-bud organizer) is established upon activation of the Sonic Hedgehog (SHH) signaling molecule at the posterior limb-bud margin. Another hallmark of establishing anteroposterior limb-bud identities is the colinear activation of HoxD genes located at the 5′ end of the cluster (5′HoxD genes). The unique and shared functions of Gli3 and formin in these determinative events were genetically analyzed using single and double homozygous Extra-toes (Xt; disrupting Gli3) and limb deformity (ld; disrupting formin) mouse embryos. Analysis of the limb skeletal phenotypes reveals genetic interaction of the two genes. In addition to loss of digit identity and varying degrees of polydactyly, proximal skeletal elements are severely shortened in Xt;ld double homozygous limbs. The underlying molecular defects affect both establishment of the polarising region and posterior limb-bud identity. In particular, the synergism between Gli3- and formin-mediated mesenchyme-AER interactions positions the SHH signaling center at the posterior limb-bud margin. The present study shows that establishment and positioning of the polarising region is regulated both by restriction of Shh through Gli3 and its positive feedback regulation through formin. Concurrently, Gli3 functions independently of formin during initial posterior nesting of 5′HoxD domains, whereas their subsequent distal restriction and anterior expansion depends on genetic interaction of Gli3 and formin.

A Wnt signaling pathway controls hox gene expression and neuroblast migration in C. elegans

The specification of body pattern along the anteroposterior (A/P) body axis is achieved largely by the actions of conserved clusters of Hox genes. Limiting expression of these genes to localized regional domains and controlling the precise patterns of expression within those domains is critically important for normal patterning. Here we report that egl-20, a C. elegans gene required to activate expression of the Hox gene mab-5 in the migratory neuroblast QL, encodes a member of the Wnt family of secreted glycoproteins. We have found that a second Wnt pathway gene, bar-1, which encodes a beta-catenin/Armadillo-like protein, is also required for activation of mab-5 expression in QL. In addition, we describe the gene pry-1, which is required to limit expression of the Hox genes lin-39, mab-5 and egl-5 to their correct local domains. We find that egl-20, pry-1 and bar-1 all function in a linear genetic pathway with conserved Wnt signaling components, suggesting that a conserved Wnt pathway activates expression of mab-5 in the migratory neuroblast QL. Moreover, we find that members of this Wnt signaling system play a major role in both the general and fine-scale control of Hox gene expression in other cell types along the A/P axis.

The development of the paired fins in the zebrafish (Danio rerio)

In the present study, we describe the structure and normal development of the zebrafish (Danio rerio) paired fins. Particularly, we focus on the structure of the apical epidermis and on endoskeletal morphogenesis. Endoskeletal development proceeds differently in the pectoral and pelvic fins. Whereas in both fins major parts of the endoskeletal girdle develop within the fin bud mesenchyme, the pattern of chondrogenic condensations observed in the pelvic fins directly reflects the adult endoskeletal pattern. In the pectoral fin, a morphogenetic detour is taken via a functional larval endoskeleton, the endoskeletal disc. It arises in the fin bud mesenchyme from a chondrogenic anlage common with the girdle. The disc chondrifies and represents the functional endoskeleton of the larval pectoral fin. The pectoral fin endoskeleton is expanded as well as restructured during larval stages in a process which involves decomposition of cartilage matrix in the endoskeletal disc. Our comparisons of apical fold morphology with reports on other teleosts and tetrapod apical ridges show them to be homologous on the structural level. Comparisons of endoskeletal development of the zebrafish with reports on teleosts, actinopterygians and chondrichthyans show that endoskeletal morphogenesis in the zebrafish pectoral fin follows a morphogenetic process which is wide-spread among actinopterygians.

Molecular and cellular basis of pattern formation during vertebrate limb development

The body plan is generated by cells and tissues that become arranged precisely in the embryo. This process, termed pattern formation, involves cell interactions in which a particular group of cells produce signals that specify new cell types or patterns of differentiation in responding cells. These patterning signals emanate from very discrete centers called "organizer centers," such as the Hensen's node or Spemann organizer, the midbrain-hindbrain junction, the notochord, or in the case of the limb, the zone of polarizing activity (ZPA) or the apical ectodermal ridge (AER). The developing vertebrate limb is an ideal model system for the study of pattern formation because, in addition to surgical manipulations, molecular manipulations are now feasible. In this review we summarize early experiments that established, by means of surgical manipulations, the different organizer centers of the vertebrate limb: the ectoderm covering the limb bud, the apical ectodermal ridge, the zone of polarizing activity, and the distal mesoderm (progress zone) underlying the AER. We then describe the domains of expression of various genes present during the development of the limb and discuss some of the functional approaches (overexpression and lack of function studies) undertaken to ascertain their role in limb outgrowth. The knowledge acquired in the last few years has had an enormous impact not only on our view of how limbs develop (perhaps now one of the most approachable vertebrate model systems) but also in a more general sense of how the embryo is organized in space and time.

Glycosylphosphatidylinositol-anchored cell surface proteins regulate position-specific cell affinity in the limb bud

Although regional differences in mesenchymal cell affinity in the limb bud represent positional identity, the molecular basis for cell affinity is poorly understood. We found that treatment of the cell surface with bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) could change cell affinity in culture. When PI-PLC was added to the culture medium, segregation of the progress zone (PZ) cells from different stage limb buds was inhibited. Similarly, sorting out of the cells from different positions along the proximodistal (PD) axis of the same stage limb buds was disturbed. Since PI-PLC can remove glycosylphosphatidylinositol (GPI)-anchored membrane bound proteins from the cell surface, the GPI-anchored cell surface proteins may be involved in sorting out. To define the GPI-anchored molecules that determine the segregation of limb mesenchymal cells, we examined the effect of neutralizing antibody on the EphA4 receptor that binds to GPI-anchored cell surface ligands, called ephrin-A. Sorting out of the PZ cells at different stages could be inhibited by the neutralizing antibody to EphA4. These results suggest that EphA4 and its GPI-anchored ligands are, at least in part, involved in sorting out of limb mesenchymal cells with different proximal-distal positional values, and that GPI-anchored cell surface proteins play important roles in determining cell affinity in the limb bud.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Deletions in HOXD13 segregate with an identical, novel foot malformation in two unrelated families

Synpolydactyly (SPD) is a dominantly inherited congenital limb malformation consisting of 3/4 syndactyly in the hands and 4/5 syndactyly in the feet, with digit duplication in the syndactylous web. The condition recently has been found to result from different-sized expansions of an amino-terminal polyalanine tract in HOXD13. We report a novel type of mutation in HOXD13, associated in some cases with features of classic SPD and in all cases with a novel foot phenotype. In two unrelated families, each with a different intragenic deletion in HOXD13, all mutation carriers have a rudimentary extra digit between the first and second metatarsals and often between the fourth and fifth metatarsals as well. This phenotype has not been reported in any mice with genetic modifications of the HoxD gene cluster. The two different deletions affect the first exon and the homeobox, respectively, in each case producing frameshifts followed by a long stretch of novel sequence and a premature stop codon. Although the affected genes may encode proteins that exert a dominant negative or novel effect, they are most likely to act as null alleles. Either possibility has interesting implications for the role of HOXD13 in human autopod development.

Genetic analysis of a conserved sequence in the HoxD complex: regulatory redundancy or limitations of the transgenic approach?

Extensive sequencing in the HoxD complex of several vertebrate species has revealed a set of conserved DNA sequences interspersed between neighboring Hox genes. Their high degree of conservation strongly suggested that they are used for regulatory purposes, a hypothesis that was largely confirmed by using "classical transgenesis" or in vivo mutagenesis through the embryonic stem (ES) cell technology. Here, we show that this is not always the case. We report that the deletion of a conserved regulatory sequence located in the HoxD complex gives different results, depending on the transgenic approach that was used. In "conventional" transgenesis, this sequence was necessary for proper expression in a subdomain of the developing limb. However, a deletion of this sequence in complexo did not confirm this effect, thereby creating an important discrepancy between the classical transgenic and the ES cell-based, targeted mutagenesis. This unexpected observation may show the limitations of the former technology. Alternatively, it could illustrate a redundancy in regulatory circuits and, thus, justify the combination of parallel strategies.

Expression of HoxD genes in developing and regenerating axolotl limbs

Hox genes play a critical role in the development of the vertebrate axis and limbs, and previous studies have implicated them in the specification of positional identity, the control of growth, and the timing of differentiation. Axolotl limbs offer an opportunity to distinguish these alternatives because the sequence of skeletal differentiation is reversed along the anterior-posterior axis relative to that of other tetrapods. We report that during early limb development, expression patterns of HoxD genes in axolotls resemble those in amniotes and anuran amphibians. At later stages, the anterior boundary of Hoxd-11 expression is conserved with respect to morphological landmarks, but there is no anterior-distal expansion of the posterior domain of Hoxd-11 expression similar to that observed in mice and chicks. Since axolotls do not form an expanded paddle-like handplate prior to digit differentiation, we suggest that anterior expansion of expression in higher vertebrates is linked to the formation of the handplate, but is clearly not necessary for digit differentiation. We also show that the 5' HoxD genes are reexpressed during limb regeneration. The change in the expression pattern of Hoxd-11 during the course of regeneration is consistent with the hypothesis that the distal tip of the regenerate is specified first, followed by intercalation of intermediate levels of the pattern. Both Hoxd-8 and Hoxd-10 are expressed in non-regenerating wounds, but Hoxd-11 is specific for regeneration. It is also expressed in the posterior half of nerve-induced supernumerary outgrowths.

Differential regulation of T-box and homeobox transcription factors suggests roles in controlling chick limb-type identity

The wing and the leg of the chick, although homologous structures, have characteristic patterns of skeletal elements, muscles, tendons, featherbuds and scales. Despite recent advances in understanding the common genetic pathways patterning the wing and leg, the molecular nature of the specification of limb-type identity has remained elusive. Embryological experiments have indicated the existence of limb-specific territories in the flank. In the newt, deviation of nerves from the limb into the flank can induce ectopic limbs to form from this tissue. In the chick, Fibroblast growth factor (FGF)-soaked beads applied to the flank can induce ectopic formation of limbs from the surrounding tissue. In both cases, the type of limb that forms, either a wing/forelimb or leg/hindlimb, is dependent upon the location to which the limb-inducing signal is applied. We have isolated and characterised three candidate genes for controlling limb identity in the chick. Two T-box transcription factors, cTbx4 and cTbx5, are expressed in a restricted manner in the leg bud and wing buds, respectively. cPtx1, a member of the Otx-related subclass of paired-type homeodomain proteins, is expressed exclusively in the leg bud. Using FGF to induce ectopic limb buds of wing, leg and intermediate identity, we show that early expression of cTbx5, cTbx4 and cPtx1 in the induced limb buds correlates with later wing- or leg-type identity of ectopic limbs. We observe a general correlation between the location of an ectopic outgrowth induced by FGF and the identity of the resulting limb but, significantly, we report that there is no definitive rostral-caudal level that divides the ectopic wing and leg territories.

Fetal lung mRNA levels of Hox genes are differentially altered by maternal diabetes and butyrate in rats

Diabetes is known to be associated with delayed lung development in humans and in experimental animals. This includes delayed expression of surfactant apoproteins. An important component of the metabolic abnormalities in diabetes is elevated levels of analogs of butyric acid, and the effects of diabetes on surfactant apoproteins can be reproduced by exposure of fetal rat lung explants to butyrate. Dexamethasone has the opposite effects on lung development. In humans, antenatal exposure to dexamethasone results in a lower incidence of RDS, whereas in experimental animals, dexamethasone increases the expression of surfactant apoproteins. A subset of Hox genes are expressed in developing lung, and their level of expression decreases with advancing gestation. We hypothesized that: 1) lungs of fetuses of rats with streptozotocin-induced diabetes would have altered levels of expression of Hox genes, 2) the effect would be mediated in part through elevated levels of butyrate, and 3) dexamethasone would reverse the effect. We tested our hypotheses in vivo using fetuses from streptozotocin-treated rats and in vitro by treating lung explants from normal rats with sodium butyrate. Streptozotocin treatment increased expression of Hoxb-5 at 18 d of gestation, but did not affect Hoxa-5 expression. This was associated with a 20-fold increase in alpha-aminobutyrate levels. Dexamethasone tended to reverse this effect. In contrast, butyrate treatment of explants decreased the expression of Hoxa-5 and Hoxb-5. We conclude that diabetes alters expression of Hox genes, but that the effect of butyrate on lung development, and in particular on surfactant apoprotein expression, is independent of its effects on Hox genes.