Interactions between HOXD and Gli3 genes control the limb apical ectodermal ridge via Fgf10

Sall4 regulates downstream patterning genes during limb regeneration

Many salamanders can completely regenerate a fully functional limb. Limb regeneration is a carefully coordinated process involving several defined stages. One key event during the regeneration process is the patterning of the blastema to inform cells of what they must differentiate into. Although it is known that many genes involved in the initial development of the limb are re-used during regeneration, the exact molecular circuitry involved in this process is not fully understood. Several large-scale transcriptional profiling studies of axolotl limb regeneration have identified many transcription factors that are up-regulated after limb amputation. Sall4 is a transcription factor that has been identified to play essential roles in maintaining cells in an undifferentiated state during development and also plays a unique role in limb development. Inactivation of Sall4 during limb bud development results in defects in anterior-posterior patterning of the limb. Sall4 has been found to be up-regulated during limb regeneration in both Xenopus and salamanders, but to date it function has been untested. We confirmed that Sall4 is up-regulated during limb regeneration in the axolotl using qRT-PCR and identified that it is present in the skin cells and also in cells within the blastema. Using CRISPR technology we microinjected gRNAs specific for Sall4 complexed with cas9 protein into the blastema to specifically knockout Sall4 in blastema cells only. This resulted in limb regenerate defects, including missing digits, fusion of digit elements, and defects in the radius and ulna. This suggests that during regeneration Sall4 may play a similar role in regulating the specification of anterior-proximal skeletal elements.

Rethinking positional information and digit identity: The role of late interdigit signaling

Seminal work from John Fallon's lab has illuminated how digit identity determination involves ongoing late regulation and occurs progressively during phalanx formation. Complementary genetic analyses in mice and several papers in this special issue have begun to flesh out how interdigit signaling accomplishes this, but major questions remain unaddressed, including how uncommitted progenitors from which phalanges arise are maintained, and what factors set limits on digit extension and phalanx number, particularly in mammals. This review summarizes what has been learned in the two decades since control of digit identity by late interdigit signals was first identified and what remains poorly understood, and will hopefully spark renewed interest in a process that is critical to evolutionary limb adaptations but nevertheless remains enigmatic.

Genetic basis for an evolutionary shift from ancestral preaxial to postaxial limb polarity in non-urodele vertebrates

In most tetrapod vertebrates, limb skeletal progenitors condense with postaxial dominance. Posterior elements (such as ulna and fibula) appear prior to their anterior counterparts (radius and tibia), followed by digit-appearance order with continuing postaxial polarity. The only exceptions are urodele amphibians (salamanders), whose limb elements develop with preaxial polarity and who are also notable for their unique ability to regenerate complete limbs as adults. The mechanistic basis for this preaxial dominance has remained an enigma and has even been proposed to relate to the acquisition of novel genes involved in regeneration. However, recent fossil evidence suggests that preaxial polarity represents an ancestral rather than derived state. Here, we report that 5'Hoxd (Hoxd11-d13) gene deletion in mouse is atavistic and uncovers an underlying preaxial polarity in mammalian limb formation. We demonstrate this shift from postaxial to preaxial dominance in mouse results from excess Gli3 repressor (Gli3R) activity due to the loss of 5'Hoxd-Gli3 antagonism and is associated with cell-cycle changes promoting precocious cell-cycle exit in the anterior limb bud. We further show that Gli3 knockdown in axolotl results in a shift to postaxial dominant limb skeleton formation, as well as expanded paddle-shaped limb-bud morphology and ensuing polydactyly. Evolutionary changes in Gli3R activity level, which also played a key role in the fin-to-limb transition, appear to be fundamental to the shift from preaxial to postaxial polarity in formation of the tetrapod limb skeleton.

The Shh/Gli3 gene regulatory network precedes the origin of paired fins and reveals the deep homology between distal fins and digits

In this study, we show that the inactivation of the gli3 gene in medaka fish results in the formation of larger dorsal and paired fins. These mutant fins display multiple radial bones and fin rays which resemble polydactyly in Gli3-deficient mice. Our molecular and genetic analyses indicate that the size of fish fins is controlled by an ancient mechanism mediated by SHH-GLI signaling that appeared prior to the evolutionary appearance of paired fins. We also show that the key regulatory networks that mediate the expansion of digit progenitor cells in tetrapods were already in place in the fins of the last common ancestor between ray and lobe-finned fishes, suggesting an ancient similarity between distal fins and digits.

One of the central problems of vertebrate evolution is understanding the relationship among the distal portions of fins and limbs. Lacking comparable morphological markers of these regions in fish and tetrapods, these relationships have remained uncertain for the past century and a half. Here we show that Gli3 functions in controlling the proliferative expansion of distal progenitors are shared among dorsal and paired fins as well as tetrapod limbs. Mutant knockout gli3 fins in medaka (Oryzias latipes) form multiple radials and rays, in a pattern reminiscent of the polydactyly observed in Gli3-null mutant mice. In limbs, Gli3 controls both anterior–posterior patterning and cell proliferation, two processes that can be genetically uncoupled. In situ hybridization, quantification of proliferation markers, and analysis of regulatory regions reveal that in paired and dorsal fins, gli3 plays a main role in controlling proliferation but not in patterning. Moreover, gli3 down-regulation in shh mutant fins rescues fin loss in a manner similar to how Gli3 deficiency restores digits in the limbs of Shh mutant mouse embryos. We hypothesize that the Gli3/Shh gene pathway preceded the origin of paired appendages and was originally involved in modulating cell proliferation. Accordingly, the distal regions of dorsal fins, paired fins, and limbs retain a deep regulatory and functional homology that predates the origin of paired appendages.

Geminin is required for Hox gene regulation to pattern the developing limb

Development of the complex structure of the vertebrate limb requires carefully orchestrated interactions between multiple regulatory pathways and proteins. Among these, precise regulation of 5' Hox transcription factor expression is essential for proper limb bud patterning and elaboration of distinct limb skeletal elements. Here, we identified Geminin (Gmnn) as a novel regulator of this process. A conditional model of Gmnn deficiency resulted in loss or severe reduction of forelimb skeletal elements, while both the forelimb autopod and hindlimb were unaffected. 5' Hox gene expression expanded into more proximal and anterior regions of the embryonic forelimb buds in this Gmnn-deficient model. A second conditional model of Gmnn deficiency instead caused a similar but less severe reduction of hindlimb skeletal elements and hindlimb polydactyly, while not affecting the forelimb. An ectopic posterior SHH signaling center was evident in the anterior hindlimb bud of Gmnn-deficient embryos in this model. This center ectopically expressed Hoxd13, the HOXD13 target Shh, and the SHH target Ptch1, while these mutant hindlimb buds also had reduced levels of the cleaved, repressor form of GLI3, a SHH pathway antagonist. Together, this work delineates a new role for Gmnn in modulating Hox expression to pattern the vertebrate limb.

Accelerated Evolution of Limb-Related Gene Hoxd11 in the Common Ancestor of Cetaceans and Ruminants (Cetruminantia)

Reduced numbers of carpal and tarsal bones (wrist and ankle joints) are extensively observed in the clade of Cetacea and Ruminantia (Cetruminantia). Homebox D11 (Hoxd11) is one of the important genes required for limb development in mammals. Mutations in Hoxd11 can lead to defects in particular bones of limbs, including carpus and tarsus. To test whether evolutionary changes in Hoxd11 underlie the loss of these bones in Cetruminantia, we sequenced and analyzed Hoxd11 coding sequences and compared them with other 5' HoxA and HoxD genes in a taxonomic coverage of Cetacea, Ruminantia and other mammalian relatives. Statistical tests on the Hoxd11 sequences found an accelerated evolution in the common ancestor of cetaceans and ruminants, which coincided with the reduction of carpal and tarsal bones in this clade. Five amino acid substitutions (G222S, G227A, G229S, A240T and G261V) and one amino acid deletion (G254Del) occurred in this lineage. In contrast, other 5' HoxA and HoxD genes do not show this same evolutionary pattern, but instead display a highly conserved pattern of evolution in this lineage. Accelerated evolution of Hoxd11, but not other 5' HoxA and HoxD genes, is probably related to the reduction of the carpal and tarsal bones in Cetruminantia. Moreover, we found two amino acid substitutions (G110S and D223N) in Hoxd11 that are unique to the lineage of Cetacea, which coincided with hindlimb loss in the common ancestor of cetaceans. Our results give molecular evidence of Hoxd11 adaptive evolution in cetaceans and ruminants, which could be correlated with limb morphological adaptation.

Hoxa13 regulates expression of common Hox target genes involved in cartilage development to coordinate the expansion of the autopodal anlage

To elucidate the role of Hox genes in limb cartilage development, we identified the target genes of HOXA11 and HOXA13 by ChIP‐Seq. The ChIP DNA fragment contained evolutionarily conserved sequences and multiple highly conserved HOX binding sites. A substantial portion of the HOXA11 ChIP fragment overlapped with the HOXA13 ChIP fragment indicating that both factors share common targets. Deletion of the target regions neighboring Bmp2 or Tshz2 reduced their expression in the autopod suggesting that they function as the limb bud‐specific enhancers. We identified the Hox downstream genes as exhibiting expression changes in the Hoxa13 knock out (KO) and Hoxd11‐13 deletion double mutant (Hox13 dKO) autopod by Genechip analysis. The Hox downstream genes neighboring the ChIP fragment were defined as the direct targets of Hox. We analyzed the spatial expression pattern of the Hox target genes that encode two different categories of transcription factors during autopod development and Hox13dKO limb bud. (a) Bcl11a, encoding a repressor of cartilage differentiation, was expressed in the E11.5 autopod and was substantially reduced in the Hox13dKO. (b) The transcription factors Aff3, Bnc2, Nfib and Runx1t1 were expressed in the zeugopodal cartilage but not in the autopod due to the repressive or relatively weak transcriptional activity of Hox13 at E11.5. Interestingly, the expression of these genes was later observed in the autopodal cartilage at E12.5. These results indicate that Hox13 transiently suspends the cartilage differentiation in the autopodal anlage via multiple pathways until establishing the paddle‐shaped structure required to generate five digits.

Fibroblast Growth Factor 10 and Vertebrate Limb Development

Early limb development requires fibroblast growth factor (Fgf)-mediated coordination between growth and patterning to ensure the proper formation of a functional organ. The apical ectodermal ridge (AER) is a domain of thickened epithelium located at the distal edge of the limb bud that coordinates outgrowth along the proximodistal axis. Considerable amount of work has been done to elucidate the cellular and molecular mechanisms underlying induction, maintenance and regression of the AER. Fgf10, a paracrine Fgf that elicits its biological responses by activating the fibroblast growth factor receptor 2b (Fgfr2b), is crucial for governing proximal distal outgrowth as well as patterning and acts upstream of the known AER marker Fgf8. A transgenic mouse line allowing doxycycline-based inducible and ubiquitous expression of a soluble form of Fgfr2b has been extensively used to identify the role of Fgfr2b ligands at different time points during development. Overexpression of soluble Fgfr2b (sFgfr2b) post-AER induction leads to irreversible loss of cellular β-catenin organization and decreased Fgf8 expression in the AER. A similar approach has been carried out pre-AER induction. The observed limb phenotype is similar to the severe proximal truncations observed in human babies exposed to thalidomide, which has been proposed to block the Fgf10-AER-Fgf8 feedback loop. Novel insights on the role of Fgf10 signaling in limb formation pre- and post-AER induction are summarized in this review and will be integrated with possible future investigations on the role of Fgf10 throughout limb development.

Alterations in anterior-posterior patterning and its accompanying changes along the proximal-distal axis during the fin-to-limb transition

The evolution from fins to limbs was one of the most successful innovations for vertebrates, allowing them to vastly expand their behaviors and habitats. Fossil records suggest that morphological changes occurred not only along the proximal-distal axis included appearance of the autopod, but also occurred along the anterior-posterior axis included reductions in the size and number of basal bones and digits. This review focuses on recent progress in developmental and genetic studies aimed at elucidating the mechanisms underlying alteration of anterior-posterior patterning and its accompanying changes along the proximal-distal axis during the fin-to-limb transition.

Regeneration and Regrowth Potentials of Digit Tips in Amphibians and Mammals

Tissue regeneration and repair have received much attention in the medical field over the years. The study of amphibians, such as newts and salamanders, has uncovered many of the processes that occur in these animals during full-limb/digit regeneration, a process that is highly limited in mammals. Understanding these processes in amphibians could shed light on how to develop and improve this process in mammals. Amputation injuries in mammals usually result in the formation of scar tissue with limited regrowth of the limb/digit; however, it has been observed that the very tips of digits (fingers and toes) can partially regrow in humans and mice under certain conditions. This review will summarize and compare the processes involved in salamander limb regeneration, mammalian wound healing, and digit regeneration in mice and humans.

GLI3-related polydactyly: a review

GLI3 mutations are known to be associated with nine syndromes/conditions in which polydactyly is a feature. In this review, the embryology, pathogenesis, and animal models of GLI3-related polydactyly are discussed first. This is followed by a detailed review of the genotype-phenotype correlations. Based on our review of the literature and our clinical experiences, we recommend viewing GLI3-related syndromes/conditions as four separate entities; each characterized by a specific pattern of polydactyly. These four entities are: the preaxial polydactyly type IV-Greig-acrocallosal spectrum, postaxial polydactyly types A/B, Pallister-Hall syndrome (PHS), and oral-facial-digital overlap syndrome. We also provide illustrative clinical examples from our practice including a family with a novel GLI3 mutation causing PHS. The review also introduces the term 'Forme Fruste' preaxial polydactyly and gives several conclusions/recommendations including the recommendation to revise the current criteria for the clinical diagnosis of PHS.

Hox genes in the adult skeleton: Novel functions beyond embryonic development

Hox genes encode evolutionarily conserved transcription factors that control skeletal patterning in the developing embryo. They are expressed in regionally restricted domains and function to regulate the morphology of specific vertebral and long bone elements. Recent work has provided evidence that Hox genes continue to be regionally expressed in adult tissues. Fibroblasts cultured from adult tissues show broadly maintained Hox gene expression patterns. In the adult skeleton, Hox genes are expressed in progenitor-enriched populations of mesenchymal stem/stromal cells (MSCs), and genetic loss-of-function analyses have provided evidence that Hox genes function during the fracture healing process. This review will highlight our current understanding of Hox expression in the adult animal and its function in skeletal regeneration. Developmental Dynamics 246:310-317, 2017. © 2016 Wiley Periodicals, Inc.

An interdigit signalling centre instructs coordinate phalanx-joint formation governed by 5'Hoxd-Gli3 antagonism

The number of phalanges and joints are key features of digit 'identity' and are central to limb functionality and evolutionary adaptation. Prior chick work indicated that digit phalanges and their associated joints arise in a different manner than the more sparsely jointed long bones, and their identity is regulated by differential signalling from adjacent interdigits. Currently, there is no genetic evidence for this model, and the molecular mechanisms governing digit joint specification remain poorly understood. Using genetic approaches in mouse, here we show that functional 5'Hoxd-Gli3 antagonism acts indirectly, through Bmp signalling from the interdigital mesenchyme, to regulate specification of joint progenitors, which arise in conjunction with phalangeal precursors at the digit tip. Phalanx number, although co-regulated, can be uncoupled from joint specification. We propose that 5'Hoxd genes and Gli3 are part of an interdigital signalling centre that sets net Bmp signalling levels from different interdigits to coordinately regulate phalanx and joint formation.

Gata6-Dependent GLI3 Repressor Function is Essential in Anterior Limb Progenitor Cells for Proper Limb Development

Gli3 is a major regulator of Hedgehog signaling during limb development. In the anterior mesenchyme, GLI3 is proteolytically processed into GLI3R, a truncated repressor form that inhibits Hedgehog signaling. Although numerous studies have identified mechanisms that regulate Gli3 function in vitro, it is not completely understood how Gli3 function is regulated in vivo. In this study, we show a novel mechanism of regulation of GLI3R activities in limb buds by Gata6, a member of the GATA transcription factor family. We show that conditional inactivation of Gata6 prior to limb outgrowth by the Tcre deleter causes preaxial polydactyly, the formation of an anterior extra digit, in hindlimbs. A recent study suggested that Gata6 represses Shh transcription in hindlimb buds. However, we found that ectopic Hedgehog signaling precedes ectopic Shh expression. In conjunction, we observed Gata6 and Gli3 genetically interact, and compound heterozygous mutants develop preaxial polydactyly without ectopic Shh expression, indicating an additional prior mechanism to prevent polydactyly. These results support the idea that Gata6 possesses dual roles during limb development: enhancement of Gli3 repressor function to repress Hedgehog signaling in the anterior limb bud, and negative regulation of Shh expression. Our in vitro and in vivo studies identified that GATA6 physically interacts with GLI3R to facilitate nuclear localization of GLI3R and repressor activities of GLI3R. Both the genetic and biochemical data elucidates a novel mechanism by Gata6 to regulate GLI3R activities in the anterior limb progenitor cells to prevent polydactyly and attain proper development of the mammalian autopod.

Gli3 is a major regulator of Hedgehog signaling in the limb, where Gli3 counteracts Sonic hedgehog (Shh) for patterning and proliferative expansion of limb progenitor cells. In the anterior limb mesenchyme, GLI3 is proteolytically processed into GLI3R, a truncated repressor form that inhibits Hedgehog signaling. In this study, we show a novel mechanism of regulation of GLI3R activities in limb buds by Gata6, a member of GATA transcription factor family. Conditional inactivation of Gata6 in mice caused formation of an extra digit in the anterior hindlimbs, a common congenital limb malformation. This phenotype was associated with ectopic Hedgehog signaling activation, and later ectopic Shh expression, in the anterior of hindlimb buds. We show that Gata6; Gli3 compound heterozygous mutants developed anterior extradigit without ectopic Shh expression, indicating there to be an additional and prior mechanism before ectopic Shh activation that induces extradigit formation. We identified that GATA6 physically interacts with GLI3R and that the interaction facilitates nuclear localization of GLI3R and repressor activities of GLI3R. Therefore, our study identified a novel mechanism by Gata6 to regulate GLI3R activities in the anterior limb mesenchyme to prevent extra digit formation and proper development of the mammalian autopod.

Role of Genetic Factors in the Pathogenesis of Radial Deficiencies in Humans

Radial deficiencies (RDs), defined as under/abnormal development or absence of any of the structures of the forearm, radial carpal bones and thumb, occur with a live birth incidence ranging from 1 out of 30,000 to 1 out 6,000 newborns and represent about one third/one fourth of all the congenital upper limb anomalies. About half of radial disorders have a mendelian cause and pattern of inheritance, whereas the remaining half appears sporadic with no known gene involved. In sporadic forms certain anomalies, such as thumb or radial hypoplasia, may occur either alone or in association with systemic conditions, like vertebral abnormalities or renal defects. All the cases with a mendelian inheritance are syndromic forms, which include cardiac defects (in Holt-Oram syndrome), bone marrow failure (in Fanconi anemia), platelet deficiency (in thrombocytopenia-absent-radius syndrome), ocular motility impairment (in Okihiro syndrome). The genetics of radial deficiencies is complex, characterized by genetic heterogeneity and high inter- and intra-familial clinical variability: this review will analyze the etiopathogenesis and the genotype/phenotype correlations of the main radial deficiency disorders in humans.

Conservation and divergence of regulatory strategies at Hox Loci and the origin of tetrapod digits

During development, expression of the Hoxa and Hoxd genes in zebrafish fins and mouse limbs are regulated via a conserved chromatin structure. However, zebrafish lack certain regulatory elements required to produce digits, revealing that radials—the fin's bony elements—are likely not homologous to tetrapod digits.

The evolution of tetrapod limbs from fish fins enabled the conquest of land by vertebrates and thus represents a key step in evolution. Despite the use of comparative gene expression analyses, critical aspects of this transformation remain controversial, in particular the origin of digits. Hoxa and Hoxd genes are essential for the specification of the different limb segments and their functional abrogation leads to large truncations of the appendages. Here we show that the selective transcription of mouse Hoxa genes in proximal and distal limbs is related to a bimodal higher order chromatin structure, similar to that reported for Hoxd genes, thus revealing a generic regulatory strategy implemented by both gene clusters during limb development. We found the same bimodal chromatin architecture in fish embryos, indicating that the regulatory mechanism used to pattern tetrapod limbs may predate the divergence between fish and tetrapods. However, when assessed in mice, both fish regulatory landscapes triggered transcription in proximal rather than distal limb territories, supporting an evolutionary scenario whereby digits arose as tetrapod novelties through genetic retrofitting of preexisting regulatory landscapes. We discuss the possibility to consider regulatory circuitries, rather than expression patterns, as essential parameters to define evolutionary synapomorphies.

Our upper limbs differ from fish fins, notably by their subdivision into arm and hand regions, which are separated by a complex articulation, the wrist. The development of this anatomy is associated with two distinct waves of expression of the Hoxa and Hoxd genes during development. Would such a shared expression pattern be sufficient to infer homology between fish fins and mouse limbs? We investigated this question here, looking at whether the two phases of Hox gene transcription that are observed during tetrapod limb development also occur during zebrafish fin development. We find the answer is “not quite.” For although the mechanisms that regulate the expression of Hoxa and Hoxd are comparable between zebrafish fins and mouse limbs, when the genomic regions that regulate Hox gene expression in fish fins are introduced into transgenic mice, they trigger Hox gene expression in only the proximal limb segment (the segment nearest the body) and not in the presumptive digits. We conclude that although fish have the Hox regulatory toolkit to produce digits, this potential is not utilized as it is in tetrapods, and as a result we propose that fin radials—the bony elements of fins—are not homologous to tetrapod digits.

Hox5 interacts with Plzf to restrict Shh expression in the developing forelimb

To date, only the five most posterior groups of Hox genes, Hox9-Hox13, have demonstrated loss-of-function roles in limb patterning. Individual paralog groups control proximodistal patterning of the limb skeletal elements. Hox9 genes also initiate the onset of Hand2 expression in the posterior forelimb compartment, and collectively, the posterior HoxA/D genes maintain posterior Sonic Hedgehog (Shh) expression. Here we show that an anterior Hox paralog group, Hox5, is required for forelimb anterior patterning. Deletion of all three Hox5 genes (Hoxa5, Hoxb5, and Hoxc5) leads to anterior forelimb defects resulting from derepression of Shh expression. The phenotype requires the loss of all three Hox5 genes, demonstrating the high level of redundancy in this Hox paralogous group. Further analyses reveal that Hox5 interacts with promyelocytic leukemia zinc finger biochemically and genetically to restrict Shh expression. These findings, along with previous reports showing that point mutations in the Shh limb enhancer lead to similar anterior limb defects, highlight the importance of Shh repression for proper patterning of the vertebrate limb.

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

Decoupling the function of Hox and Shh in developing limb reveals multiple inputs of Hox genes on limb growth

Limb development relies on an exquisite coordination between growth and patterning, but the underlying mechanisms remain elusive. Anterior-posterior and proximal-distal specification initiates in early limb bud concomitantly with the proliferative expansion of limb cells. Previous studies have shown that limb bud growth initially relies on fibroblast growth factors (FGFs) produced in the apical ectodermal ridge (AER-FGFs), the maintenance of which relies on a positive-feedback loop involving sonic hedgehog (Shh) and the BMP antagonist gremlin 1 (Grem1). The positive cross-regulation between Shh and the HoxA and HoxD clustered genes identified an indirect effect of Hox genes on the maintenance of AER-FGFs but the respective function of Shh and Hox genes in this process remains unknown. Here, by uncoupling Hox and Shh function, we show that HoxA and HoxD genes are required for proper AER-FGFs expression, independently of their function in controlling Shh expression. In addition, we provide evidence that the Hox-dependent control of AER-FGF expression is achieved through the regulation of key mesenchymal signals, namely Grem1 and Fgf10, ensuring proper epithelial-mesenchymal interactions. Notably, HoxA and HoxD genes contribute to both the initial activation of Grem1 and the subsequent anterior expansion of its expression domain. We propose that the intricate interactions between Hox genes and the FGF and Shh signaling pathways act as a molecular network that ensures proper limb bud growth and patterning, probably contributing to the coordination of these two processes.

Reduced BMP signaling results in hindlimb fusion with lethal pelvic/urogenital organ aplasia: a new mouse model of sirenomelia

Sirenomelia, also known as mermaid syndrome, is a developmental malformation of the caudal body characterized by leg fusion and associated anomalies of pelvic/urogenital organs including bladder, kidney, rectum and external genitalia. Most affected infants are stillborn, and the few born alive rarely survive beyond the neonatal period. Despite the many clinical studies of sirenomelia in humans, little is known about the pathogenic developmental mechanisms that cause the complex array of phenotypes observed. Here, we provide new evidences that reduced BMP (Bone Morphogenetic Protein) signaling disrupts caudal body formation in mice and phenocopies sirenomelia. Bmp4 is strongly expressed in the developing caudal body structures including the peri-cloacal region and hindlimb field. In order to address the function of Bmp4 in caudal body formation, we utilized a conditional Bmp4 mouse allele (Bmp4^flox/flox) and the Isl1 (Islet1)-Cre mouse line. Isl1-Cre is expressed in the peri-cloacal region and the developing hindimb field. Isl1Cre;Bmp4^flox/flox conditional mutant mice displayed sirenomelia phenotypes including hindlimb fusion and pelvic/urogenital organ dysgenesis. Genetic lineage analyses indicate that Isl1-expressing cells contribute to both the aPCM (anterior Peri-Cloacal Mesenchyme) and the hindlimb bud. We show Bmp4 is essential for the aPCM formation independently with Shh signaling. Furthermore, we show Bmp4 is a major BMP ligand for caudal body formation as shown by compound genetic analyses of Bmp4 and Bmp7. Taken together, this study reveals coordinated development of caudal body structures including pelvic/urogenital organs and hindlimb orchestrated by BMP signaling in Isl1-expressing cells. Our study offers new insights into the pathogenesis of sirenomelia.

GLI3 constrains digit number by controlling both progenitor proliferation and BMP-dependent exit to chondrogenesis

Inactivation of Gli3, a key component of Hedgehog signaling in vertebrates, results in formation of additional digits (polydactyly) during limb bud development. The analysis of mouse embryos constitutively lacking Gli3 has revealed the essential GLI3 functions in specifying the anteroposterior (AP) limb axis and digit identities. We conditionally inactivated Gli3 during mouse hand plate development, which uncoupled the resulting preaxial polydactyly from known GLI3 functions in establishing AP and digit identities. Our analysis revealed that GLI3 directly restricts the expression of regulators of the G(1)-S cell-cycle transition such as Cdk6 and constrains S phase entry of digit progenitors in the anterior hand plate. Furthermore, GLI3 promotes the exit of proliferating progenitors toward BMP-dependent chondrogenic differentiation by spatiotemporally restricting and terminating the expression of the BMP antagonist Gremlin1. Thus, Gli3 is a negative regulator of the proliferative expansion of digit progenitors and acts as a gatekeeper for the exit to chondrogenic differentiation.

Regionalized Twist1 activity in the forelimb bud drives the morphogenesis of the proximal and preaxial skeleton

Development of the mouse forelimb bud depends on normal Twist1 activity. Global loss of Twist1 function before limb bud formation stops limb development and loss of Twist1 throughout the mesenchyme after limb bud initiation leads to polydactyly, the ulnarization or loss of the radius and malformations and reductions of the shoulder girdle. Here we show that conditional deletion of Twist1 by Mesp1-Cre in the mesoderm that migrates into the anterior-proximal part of the forelimb bud results in the development of supernumerary digits and carpals, the acquisition of ulna-like characteristics by the radius and malformations of the humerus and scapula. The mirror-like duplications and posteriorization of pre-axial tissues are preceded by disruptions to anterior-posterior Shh, Bmp and Fgf signaling gradients and dysregulation of transcription factors that regulate anterior-posterior limb patterning.

Ezh2 regulates anteroposterior axis specification and proximodistal axis elongation in the developing limb

Specification and determination (commitment) of positional identities precedes overt pattern formation during development. In the limb bud, it is clear that the anteroposterior axis is specified at a very early stage and is prepatterned by the mutually antagonistic interaction between Gli3 and Hand2. There is also evidence that the proximodistal axis is specified early and determined progressively. Little is known about upstream regulators of these processes or how epigenetic modifiers influence axis formation. Using conditional mutagenesis at different time points, we show that the histone methyltransferase Ezh2 is an upstream regulator of anteroposterior prepattern at an early stage. Mutants exhibit posteriorised limb bud identity. During later limb bud stages, Ezh2 is essential for cell survival and proximodistal segment elongation. Ezh2 maintains the late phase of Hox gene expression and cell transposition experiments suggest that it regulates the plasticity with which cells respond to instructive positional cues.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

Developmental biology and classification of congenital anomalies of the hand and upper extremity

Recent investigations into the mechanism of limb development have clarified the roles of several molecules, their pathways, and interactions. Characterization of the molecular pathways that orchestrate limb development has provided insight into the etiology of many limb malformations. In this review, we describe how the insights from developmental biology are related to clinically relevant anomalies and the current classification schemes used to define, categorize, and communicate patterns of upper limb malformations. We advocate an updated classification scheme for upper limb anomalies that incorporates our current molecular perspective of limb development and the pathogenetic basis for malformations using dysmorphology terminology. We anticipate that this scheme will improve the utility of a classification as a basis for diagnosis, treatment, and research.

Distinct roles of Hand2 in initiating polarity and posterior Shh expression during the onset of mouse limb bud development

The polarization of nascent embryonic fields and the endowment of cells with organizer properties are key to initiation of vertebrate organogenesis. One such event is antero-posterior (AP) polarization of early limb buds and activation of morphogenetic Sonic Hedgehog (SHH) signaling in the posterior mesenchyme, which in turn promotes outgrowth and specifies the pentadactylous autopod. Inactivation of the Hand2 transcriptional regulator from the onset of mouse forelimb bud development disrupts establishment of posterior identity and Shh expression, which results in a skeletal phenotype identical to Shh deficient limb buds. In wild-type limb buds, Hand2 is part of the protein complexes containing Hoxd13, another essential regulator of Shh activation in limb buds. Chromatin immunoprecipitation shows that Hand2-containing chromatin complexes are bound to the far upstream cis-regulatory region (ZRS), which is specifically required for Shh expression in the limb bud. Cell-biochemical studies indicate that Hand2 and Hoxd13 can efficiently transactivate gene expression via the ZRS, while the Gli3 repressor isoform interferes with this positive transcriptional regulation. Indeed, analysis of mouse forelimb buds lacking both Hand2 and Gli3 reveals the complete absence of antero-posterior (AP) polarity along the entire proximo-distal axis and extreme digit polydactyly without AP identities. Our study uncovers essential components of the transcriptional machinery and key interactions that set-up limb bud asymmetry upstream of establishing the SHH signaling limb bud organizer.

During early limb bud development, posterior mesenchymal cells are selected to express Sonic Hedgehog (Shh), which controls antero-posterior (AP) limb axis formation (axis from thumb to little finger). We generated a conditional loss-of-function Hand2 allele to inactivate Hand2 specifically in mouse limb buds. This genetic analysis reveals the pivotal role of Hand2 in setting up limb bud asymmetry as initiation of posterior identity and establishment of the Shh expression domain are completely disrupted in Hand2 deficient limb buds. The resulting loss of the ulna and digits mirror the skeletal malformations observed in Shh-deficient limbs. We show that Hand2 is part of the chromatin complexes that are bound to the cis-regulatory region that controls Shh expression specifically in limb buds. In addition, we show that Hand2 is part of a protein complex containing Hoxd13, which also participates in limb bud mesenchymal activation of Shh expression. Indeed, Hand2 and Hoxd13 stimulate ZRS–mediated transactivation in cells, while the Gli3 repressor form (Gli3R) interferes with this up-regulation. Interestingly, limb buds lacking both Hand2 and Gli3 lack AP asymmetry and are severely polydactylous. Molecular analysis reveals some of the key interactions and hierarchies that govern establishment of AP limb asymmetries upstream of SHH.

An examination of the Chiropteran HoxD locus from an evolutionary perspective

Duplications of Hox gene clusters have been suggested as a mechanism whereby new Hox functions can be developed while preserving critical ancestral roles. However, in tetrapods, particularly in mammals, there is great variability in limb structure morphologies that are known to be affected by Hox genes without further Hox cluster duplications. The lack of further duplications suggests that if Hox genes have played a direct role in the morphological elaboration of tetrapod limbs, the changes must have come about from Hox protein sequence changes or from changes regarding the amount, time, and place of Hox gene expression. To investigate whether such changes to Hox genes could play a role in limb elaboration, we examined the HoxD locus in bats, which have both highly elaborated fore- and hindlimbs. We found that while the Chiropteran HoxD13 protein was highly conserved, there was an expansion of HoxD13 expression in the posterior portion of the Chiropteran forelimb and into the leading edge of the wing membrane. We were also able to uncover a number of unique lineage-specific sequence changes to a known HoxD limb enhancer, the Global Control Region (GCR). Further, mouse transgenic assays showed that the Chiropteran GCR has new limb enhancer activity domains beyond that reported for the Human GCR. These results suggest that modulation of Hox gene expression may be a mechanism for effecting morphological change in lineage-specific manner while maintaining ancestral constraints and cluster integrity.

Hox gene function in vertebrate gut morphogenesis: the case of the caecum

The digestive tract is made of different subdivisions with various functions. During embryonic development, the developing intestine expresses combinations of Hox genes along its anterior to posterior axis, suggesting a role for these genes in this regionalization process. In particular, the transition from small to large intestine is labelled by the transcription of all Hoxd genes except Hoxd12 and Hoxd13, the latter two genes being transcribed only near the anus. Here, we describe two lines of mice that express Hoxd12 ectopically within this morphological transition. As a consequence, budding of the caecum is impeded, leading to complete agenesis in homozygous individuals. This effect is concurrent with a dramatic reduction of both Fgf10 and Pitx1 expression. Furthermore, the interactions between ;anterior' Hox genes and ectopic Hoxd12 suggest a model whereby anterior and posterior Hox products compete in controlling Fgf10 signalling, which is required for the growth of this organ in mice. These results illuminate components of the genetic cascade necessary for the emergence of this gut segment, crucial for many vertebrates.

Hoxd and Gli3 interactions modulate digit number in the amniote limb

During limb development, Sonic hedgehog (SHH) and HOX proteins are considered among the most important factors regulating digit number and identity. SHH signaling prevents the processing of GLI3 into a short form that functions as a strong transcriptional repressor. Gli3 mutant limbs are characterized by a severe polydactyly and associated ectopic anterior expression of 5'Hoxd genes. To genetically determine the involvement of 5'Hoxd genes in the polydactyly of Gli3 mutants, we have generated a compound mutant that simultaneously removes the three most 5'-located Hoxd genes and Gli3. Remarkably, the limbs that form in the absence of all four of these genes show the most severe polydactyly so far reported in the mouse. The analysis of gene expression performed in compound mutants allows us to propose that the increase in the number of digits is mediated by the gain in function of Hoxd10 and Hoxd9. Our results also support the notion that an adequate balance between positive and negative effects of different Hoxd genes is required for pentadactyly.

Biphasic Hoxd gene expression in shark paired fins reveals an ancient origin of the distal limb domain

The evolutionary transition of fins to limbs involved development of a new suite of distal skeletal structures, the digits. During tetrapod limb development, genes at the 5' end of the HoxD cluster are expressed in two spatiotemporally distinct phases. In the first phase, Hoxd9-13 are activated sequentially and form nested domains along the anteroposterior axis of the limb. This initial phase patterns the limb from its proximal limit to the middle of the forearm. Later in development, a second wave of transcription results in 5' HoxD gene expression along the distal end of the limb bud, which regulates formation of digits. Studies of zebrafish fins showed that the second phase of Hox expression does not occur, leading to the idea that the origin of digits was driven by addition of the distal Hox expression domain in the earliest tetrapods. Here we test this hypothesis by investigating Hoxd gene expression during paired fin development in the shark Scyliorhinus canicula, a member of the most basal lineage of jawed vertebrates. We report that at early stages, 5'Hoxd genes are expressed in anteroposteriorly nested patterns, consistent with the initial wave of Hoxd transcription in teleost and tetrapod paired appendages. Unexpectedly, a second phase of expression occurs at later stages of shark fin development, in which Hoxd12 and Hoxd13 are re-expressed along the distal margin of the fin buds. This second phase is similar to that observed in tetrapod limbs. The results indicate that a second, distal phase of Hoxd gene expression is not uniquely associated with tetrapod digit development, but is more likely a plesiomorphic condition present the common ancestor of chondrichthyans and osteichthyans. We propose that a temporal extension, rather than de novo activation, of Hoxd expression in the distal part of the fin may have led to the evolution of digits.

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.