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

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.

(Non)Parallel developmental mechanisms in vertebrate appendage reduction and loss

Appendages have been reduced or lost hundreds of times during vertebrate evolution. This phenotypic convergence may be underlain by shared or different molecular mechanisms in distantly related vertebrate clades. To investigate, we reviewed the developmental and evolutionary literature of appendage reduction and loss in more than a dozen vertebrate genera from fish to mammals. We found that appendage reduction and loss was nearly always driven by modified gene expression as opposed to changes in coding sequences. Moreover, expression of the same genes was repeatedly modified across vertebrate taxa. However, the specific mechanisms by which expression was modified were rarely shared. The multiple routes to appendage reduction and loss suggest that adaptive loss of function phenotypes might arise routinely through changes in expression of key developmental genes.

The formation of the thumb requires direct modulation of Gli3 transcription by Hoxa13

In the tetrapod limb, the digits (fingers or toes) are the elements most subject to morphological diversification in response to functional adaptations. However, despite their functional importance, the mechanisms controlling digit morphology remain poorly understood. Here we have focused on understanding the special morphology of the thumb (digit 1), the acquisition of which was an important adaptation of the human hand. To this end, we have studied the limbs of the Hoxa13 mouse mutant that specifically fail to form digit 1. We show that, consistent with the role of Hoxa13 in Hoxd transcriptional regulation, the expression of Hoxd13 in Hoxa13 mutant limbs does not extend into the presumptive digit 1 territory, which is therefore devoid of distal Hox transcripts, a circumstance that can explain its agenesis. The loss of Hoxd13 expression, exclusively in digit 1 territory, correlates with increased Gli3 repressor activity, a Hoxd negative regulator, resulting from increased Gli3 transcription that, in turn, is due to the release from the negative modulation exerted by Hox13 paralogs on Gli3 regulatory sequences. Our results indicate that Hoxa13 acts hierarchically to initiate the formation of digit 1 by reducing Gli3 transcription and by enabling expansion of the 5'Hoxd second expression phase, thereby establishing anterior-posterior asymmetry in the handplate. Our work uncovers a mutual antagonism between Gli3 and Hox13 paralogs that has important implications for Hox and Gli3 gene regulation in the context of development and evolution.

Hierarchical pattern formation during amphibian limb regeneration

In 1901 T.H. Morgan proposed in "Regeneration" that pattern formation in amphibian limb regeneration is a stepwise process. Since, biologist have continued to piece together the molecular components of this process to better understand the "patterning code" responsible for regenerate formation. Within this context, several different models have been proposed; however, all are based on one of two underlying hypotheses. The first is the "morphogen hypothesis" that dictates that pattern emerges from localized expression of signaling molecules, which produce differing position-specific cellular responses in receptive cells depending on the intensity of the signal. The second hypothesis is that cells in the remaining tissues retain memory of their patterning information, and use this information to generate new cells with the missing positional identities. A growing body of evidence supports the possibility that these two mechanisms are not mutually exclusive. Here, we propose our theory of hierarchical pattern formation, which consists of 4 basic steps. The first is the existence of cells with positional memory. The second is the communication of positional information through cell-cell interactions in a regeneration-permissive environment. The third step is the induction of molecular signaling centers. And the last step is the interpretation of these signals by specialized cell types to ultimately restore the limb in its entirety. Biological codes are intertwined throughout this model, and we will discuss their multiple roles and mechanisms.

Analysis of transcription factors expressed at the anterior mouse limb bud

Limb bud patterning, outgrowth, and differentiation are precisely regulated in a spatio-temporal manner through integrated networks of transcription factors, signaling molecules, and downstream genes. However, the exact mechanisms that orchestrate morphogenesis of the limb remain to be elucidated. Previously, we have established EMBRYS, a whole-mount in situ hybridization database of transcription factors. Based on the findings from EMBRYS, we focused our expression pattern analysis on a selection of transcription factor genes that exhibit spatially localized and temporally dynamic expression patterns with respect to the anterior-posterior axis in the E9.5–E11.5 limb bud. Among these genes, Irx3 showed a posteriorly expanded expression domain in Shh^-/- limb buds and an anteriorly reduced expression domain in Gli3^-/- limb buds, suggesting their importance in anterior-posterior patterning. To assess the stepwise EMBRYS-based screening system for anterior regulators, we generated Irx3 transgenic mice in which Irx3 was expressed in the entire limb mesenchyme under the Prrx1 regulatory element. The Irx3 gain-of-function model displayed complex phenotypes in the autopods, including digit loss, radial flexion, and fusion of the metacarpal bones, suggesting that Irx3 may contribute to the regulation of limb patterning, especially in the autopods. Our results demonstrate that gene expression analysis based on EMBRYS could contribute to the identification of genes that play a role in patterning of the limb mesenchyme.

Upstream regulation for initiation of restricted Shh expression in the chick limb bud

BACKGROUND

The organizing center, which serves as a morphogen source, has crucial functions in morphogenesis in animal development. The center is necessarily located in a certain restricted area in the morphogenetic field, and there are several ways in which an organizing center can be restricted. The organizing center for limb morphogenesis, the ZPA (zone of polarizing activity), specifically expresses the Shh gene and is restricted to the posterior region of the developing limb bud.

RESULTS

The pre-pattern along the limb anteroposterior axis, provided by anterior Gli3 expression and posterior Hand2 expression, seems insufficient for the initiation of Shh expression restricted to a narrow, small spot in the posterior limb field. Comparison of the spatiotemporal patterns of gene expression between Shh and some candidate genes (Fgf8, Hoxd10, Hoxd11, Tbx2, and Alx4) upstream of Shh expression suggested that a combination of these genes' expression provides the restricted initiation of Shh expression.

CONCLUSIONS

Taken together with results of functional assays, we propose a model in which positive and negative transcriptional regulatory networks accumulate their functions in the intersection area of their expression regions to provide a restricted spot for the ZPA, the source of morphogen, Shh. Developmental Dynamics 246:417-430, 2017. © 2017 Wiley Periodicals, Inc.

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.

Suppressor of Fused Is Required for Determining Digit Number and Identity via Gli3/Fgfs/Gremlin

The anterior-posterior patterning of the vertebrate limb bud requires closely coordinated signaling interactions, including Sonic Hedgehog (Shh)-mediated counteraction of the Gli3 transcription factor in the distal and posterior mesenchyme of the limb bud. Suppressor of Fused (Sufu), an intracellular negative regulator of Shh signaling via Gli2 and Gli3, is implicated in early development of the mouse limb bud. However, how Sufu is involved in the genetic regulation of limb bud patterning still remains elusive. In this study, we show that the conditional deletion of Sufu in the mesenchyme of the early limb bud results in polydactyly with loss of digit identity and supernumerary bones in the wrist and the ankle. These pattern alterations are associated with anterior expansion of HoxD genes located at the 5’ end of the cluster. By focusing on gene expression analysis of Shh/Gremlin1/Fgf signaling critical for the establishment and maintenance of anterior-posterior patterning, we show that early response to loss of Sufu involves anterior prolongation of Fgf4 and Fgf8 expression in the apical ectodermal ridge at E10.5. We also reveal the anterior activation of Shh-dependent posterior markers Ptc1, Gli1 and Gremlin in limb buds lacking Sufu. Furthermore, we find that loss of Sufu leads to attenuated levels of repressor Gli2 and repressor Gli3 in the early limb bud. Moreover, expression of Hand2 is activated in the entire limb bud at the early outgrowth stage in the mutant lacking Sufu. Thus, we provide evidence that Sufu is involved in the genetic network that restricts the posterior expression of Gli2/3/Hand2 and Gremlin/Fgf in limb bud patterning.

A Gli silencer is required for robust repression of gremlin in the vertebrate limb bud

The transcriptional response to the Hedgehog (Hh) pathway is mediated by Gli proteins, which function as context-dependent transcriptional activators or repressors. However, the mechanism by which Gli proteins regulate their target genes is poorly understood. Here, we have performed the first genetic characterization of a Gli-dependent cis-regulatory module (CRM), focusing on its regulation of Grem1 in the mouse limb bud. The CRM, termed GRE1 (Gli responsive element 1), can act as both an enhancer and a silencer. The enhancer activity requires sustained Hh signaling. As a Gli-dependent silencer, GRE1 prevents ectopic transcription of Grem1 driven through additional CRMs. In doing so, GRE1 works with additional GREs to robustly regulate Grem1. We suggest that multiple Gli CRMs may be a general mechanism for mediating a robust transcriptional response to the Hh pathway.

Review of the molecular development of the thumb: digit primera

The thumb, or digit 1, is not a typical digit. In addition to its unusual mobility and function, its formation is also unusual. It is the last digit to form and the most commonly targeted when limb development is disrupted. The thumb domain is defined by the overlapping expression of HOXA13, TBX5, GLI3R, and HOXD13 and, importantly, by an absence of other distal HOXD transcription factors. This brief review, combining developmental biology and clinical genetics, discusses the current understanding of how the thumb domain is established.

Heterochrony in the regulation of the developing marsupial limb

BACKGROUND

At birth, marsupial neonates have precociously developed forelimbs. The development of the tammar wallaby (Macropus eugenii) hindlimbs lags significantly behind that of the forelimbs. This differs from the grey short-tailed opossum, Monodelphis domestica, which has relatively similar fore- and hindlimbs at birth. This study examines the expression of the key patterning genes TBX4, TBX5, PITX1, FGF8, and SHH in developing limb buds in the tammar wallaby.

RESULTS

All genes examined were highly conserved with orthologues from opossum and mouse. TBX4 expression appeared earlier in development than in the mouse, but later than in the opossum. SHH expression is restricted to the zone of polarising activity, while TBX5 (forelimb) and PITX1 (hindlimb) showed diffuse mRNA expression. FGF8 is specifically localised to the apical ectodermal ridge, which is more prominent than in the opossum.

CONCLUSIONS

The most marked divergence in limb size in marsupials occurs in the kangaroos and wallabies. The faster development of the fore limb compared to that of the hind limb correlates with the early timing of the expression of the key patterning genes in these limbs.

Thumbs down: a molecular-morphogenetic approach to avian digit homology

Avian forelimb digit homology remains one of the standard themes in comparative biology and EvoDevo research. In order to resolve the apparent contradictions between embryological and paleontological evidence a variety of hypotheses have been presented in recent years. The proposals range from excluding birds from the dinosaur clade, to assignments of homology by different criteria, or even assuming a hexadactyl tetrapod limb ground state. At present two approaches prevail: the frame shift hypothesis and the pyramid reduction hypothesis. While the former postulates a homeotic shift of digit identities, the latter argues for a gradual bilateral reduction of phalanges and digits. Here we present a new model that integrates elements from both hypotheses with the existing experimental and fossil evidence. We start from the main feature common to both earlier concepts, the initiating ontogenetic event: reduction and loss of the anterior-most digit. It is proposed that a concerted mechanism of molecular regulation and developmental mechanics is capable of shifting the boundaries of hoxD expression in embryonic forelimb buds as well as changing the digit phenotypes. Based on a distinction between positional (topological) and compositional (phenotypic) homology criteria, we argue that the identity of the avian digits is II, III, IV, despite a partially altered phenotype. Finally, we introduce an alternative digit reduction scheme that reconciles the current fossil evidence with the presented molecular-morphogenetic model. Our approach identifies specific experiments that allow to test whether gene expression can be shifted and digit phenotypes can be altered by induced digit loss or digit gain.

Development of the marsupial shoulder girdle complex: a case study in Monodelphis domestica

During their embryogenesis, marsupials transiently develop a unique structure, the shoulder arch, which provides the structural support and muscle-attachments necessary for the newborn's crawl to the teat. One of the most pronounced and functionally important aspects of the shoulder arch is an enlarged coracoid. The goal of this study is to determine the molecular basis of shoulder arch formation in marsupials. To achieve this goal, this study investigates the relative expression of several genes with known roles in shoulder girdle morphogenesis in a marsupial-the opossum, Monodelphis domestica-and a placental, the mouse, Mus musculus. Results indicate that Hoxc6, a gene involved in coracoid patterning, is expressed for a longer period of time and at higher levels in opossum relative to mouse. Functional manipulation suggests that these differences in Hoxc6 expression are independent of documented differences in retinoic acid signaling in opossum and mouse forelimbs. Results also indicate that Emx2, a gene involved in scapular blade condensation, is upregulated in opossum relative to mouse. However, several other genes involved in shoulder girdle patterning (e.g., Gli3, Pax1, Pbx1, Tbx15) are comparably expressed in these species. These findings suggest that the upregulation of Hoxc6 and Emx2 occurs through independent genetic modifications in opossum relative to mouse. In summary, this study documents a correlation between gene expression and the divergent shoulder girdle morphogenesis of marsupial (i.e., opossum) and placental (i.e., mouse) mammals, and thereby provides a foundation for future research into the genetic basis of shoulder girdle morphogenesis in marsupials. Furthermore, this study supports the hypothesis that the mammalian shoulder girdle is a highly modular structure whose elements are relatively free to evolve independently.

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

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

Limb anterior-posterior polarity integrates activator and repressor functions of GLI2 as well as GLI3

Anterior-posterior (AP) limb patterning is directed by sonic hedgehog (SHH) signaling from the posteriorly located zone of polarizing activity (ZPA). GLI3 and GLI2 are the transcriptional mediators generally utilized in SHH signaling, and each can function as an activator (A) and repressor (R). Although GLI3R has been suggested to be the primary effector of SHH signaling during limb AP patterning, a role for GLI3A or GLI2 has not been fully ruled out, nor has it been determined whether Gli3 plays distinct roles in limb development at different stages. By conditionally removing Gli3 in the limb at multiple different time points, we uncovered four Gli3-mediated functions in limb development that occur at distinct but partially over-lapping time windows: AP patterning of the proximal limb, AP patterning of the distal limb, regulation of digit number and bone differentiation. Furthermore, by removing Gli2 in Gli3 temporal conditional knock-outs, we uncovered an essential role for Gli2 in providing the remaining posterior limb patterning seen in Gli3 single mutants. To test whether GLIAs or GLIRs regulate different aspects of AP limb patterning and/or digit number, we utilized a knock-in allele in which GLI1, which functions solely as an activator, is expressed in place of the bifunctional GLI2 protein. Interestingly, we found that GLIAs contribute to AP patterning specifically in the posterior limb, whereas GLIRs predominantly regulate anterior patterning and digit number. Since GLI3 is a more effective repressor, our results explain why GLI3 is required only for anterior limb patterning and why GLI2 can compensate for GLI3A in posterior limb patterning. Taken together, our data suggest that establishment of a complete range of AP positional identities in the limb requires integration of the spatial distribution, timing, and dosage of GLI2 and GLI3 activators and repressors.

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.

HOXA13 and HOXD13 expression during development of the syndactylous digits in the marsupial Macropus eugenii

Background

Kangaroos and wallabies have specialised limbs that allow for their hopping mode of locomotion. The hindlimbs differentiate much later in development but become much larger than the forelimbs. The hindlimb autopod has only four digits, the fourth of which is greatly elongated, while digits two and three are syndactylous. We investigated the expression of two genes, HOXA13 and HOXD13, that are crucial for digit patterning in mice during formation of the limbs of the tammar wallaby.

Results

We describe the development of the tammar limbs at key stages before birth. There was marked heterochrony and the hindlimb developed more slowly than the forelimb. Both tammar HOXA13 and HOXD13 have two exons as in humans, mice and chickens. HOXA13 had an early and distal mRNA distribution in the tammar limb bud as in the mouse, but forelimb expression preceded that in the hindlimb. HOXD13 mRNA was expressed earlier in the forelimb than the hindlimb and was predominantly detected in the interdigital tissues of the forelimb. In contrast, the hindlimb had a more restricted expression pattern that appeared to be expressed at discrete points at both posterior and anterior margins of the limb bud, and was unlike expression seen in the mouse and the chicken.

Conclusions

This is the first examination of HOXA and HOXD gene expression in a marsupial. The gene structure and predicted proteins were highly conserved with their eutherian orthologues. Interestingly, despite the morphological differences in hindlimb patterning, there were no modifications to the polyalanine tract of either HOXA13 or HOXD13 when compared to those of the mouse and bat but there was a marked difference between the tammar and the other mammals in the region of the first polyserine tract of HOXD13. There were also altered expression domains for both genes in the developing tammar limbs compared to the chicken and mouse. Together these findings suggest that the timing of HOX gene expression may contribute to the heterochrony of the forelimb and hindlimb and that alteration to HOX domains may influence phenotypic differences that lead to the development of marsupial syndactylous digits.

Homeobox genes in obsessive-compulsive disorder

BACKGROUND

Despite evidence that obsessive-compulsive disorder (OCD) is a familial neuropsychiatric condition, progress aimed at identifying genetic determinants of the disorder has been slow. The OCD Collaborative Genetics Study (OCGS) has identified several OCD susceptibility loci through linkage analysis.

METHODS

In this study we investigate two regions on chromosomes 15q and 1q by first refining the linkage region using additional short tandem repeat polymorphic (STRP) markers. We then performed association analysis on single nucleotide polymorphisms (SNP) genotyped (markers placed every 2-4 kb) in the linkage regions in the OCGS sample of 376 rigorously phenotyped affected families.

RESULTS

Three SNPs are most strongly associated with OCD: rs11854486 (P = 0.00005 [0.046 after adjustment for multiple tests]; genetic relative risk (GRR) = 11.1 homozygous and 1.6 heterozygous) and rs4625687 [P = 0.00007 (after adjustment = 0.06); GRR = 2.4] on 15q; and rs4387163 (P = 0.0002 (after adjustment = 0.08); GRR = 1.97) on 1q. The first SNP is adjacent to NANOGP8, the second SNP is in MEIS2, and the third is 150 kb between PBX1 and LMX1A.

CONCLUSIONS

All the genes implicated by association signals are homeobox genes and are intimately involved in neurodevelopment. PBX1 and MEIS2 exert their effects by the formation of a heterodimeric complex, which is involved in development of the striatum, a brain region involved in the pathophysiology of OCD. NANOGP8 is a retrogene of NANOG, a homeobox transcription factor known to be involved in regulation of neuronal development. These findings need replication; but support the hypothesis that genes involved in striatal development are implicated in the pathogenesis of OCD.

The temporal dynamics of vertebrate limb development, teratogenesis and evolution

Recent genetic and functional analysis of vertebrate limb development begins to reveal how the functions of particular genes and regulatory hierarchies can drastically change over time. The temporal and spatial interplay of the two instructive signalling centres are part of a larger signalling system that orchestrates limb bud morphogenesis in a rather self-regulatory manner. It appears that mesenchymal cells are specified early and subsequently, the progenitors for the different skeletal elements are expanded and determined progressively during outgrowth. Mutations and teratogens that disrupt distal progression of limb development most often cause death of the early-specified progenitors rather than altering their fates. The proliferative expansion and distal progression of paired appendage development was one of the main driving forces behind the transition from fin to limb buds during paired appendage evolution. Finally, the adaptive diversification or loss of modern tetrapod limbs in particular phyla or species appear to be a consequence of evolutionary tampering with the regulatory systems that control distal progression of limb development.

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.

The molecular regulation of vertebrate limb patterning

The limb has long been considered a paradigm for organogenesis because of its simplicity and ease of manipulation. However, it has become increasingly clear that the processes required to produce a perfectly formed limb involve complex molecular interactions across all three axes of limb development. Old models have evolved with acquisition of molecular knowledge, and in more recent times mathematical modeling approaches have been invoked to explain the precise spatio-temporal regulation of gene networks that coordinate limb patterning and outgrowth. This review focuses on recent advances in our understanding of vertebrate limb development, highlighting the signaling interactions required to lay down the pattern on which the processes of differentiation will act to ultimately produce the final limb.

Vertebrate limb bud development: moving towards integrative analysis of organogenesis

The limb bud is of paradigmatic value to understanding vertebrate organogenesis. Recent genetic analysis in mice has revealed the existence of a largely self-regulatory limb bud signalling system that involves many of the pathways that are known to regulate morphogenesis. These findings contrast with the prevailing view that the main limb bud axes develop largely independently of one another. In this Review, we discuss models of limb development and attempt to integrate the current knowledge of the signalling interactions that govern limb skeletal development into a systems model. The resulting integrative model provides insights into how the specification and proliferative expansion of the anteroposterior and proximodistal limb bud axes are coordinately controlled in time and space.

Heterochronic shift in Hox-mediated activation of sonic hedgehog leads to morphological changes during fin development

We explored the molecular mechanisms of morphological transformations of vertebrate paired fin/limb evolution by comparative gene expression profiling and functional analyses. In this study, we focused on the temporal differences of the onset of Sonic hedgehog (Shh) expression in paired appendages among different vertebrates. In limb buds of chick and mouse, Shh expression is activated as soon as there is a morphological bud, concomitant with Hoxd10 expression. In dogfish (Scyliorhinus canicula), however, we found that Shh was transcribed late in fin development, concomitant with Hoxd13 expression. We utilized zebrafish as a model to determine whether quantitative changes in hox expression alter the timing of shh expression in pectoral fins of zebrafish embryos. We found that the temporal shift of Shh activity altered the size of endoskeletal elements in paired fins of zebrafish and dogfish. Thus, a threshold level of hox expression determines the onset of shh expression, and the subsequent heterochronic shift of Shh activity can affect the size of the fin endoskeleton. This process may have facilitated major morphological changes in paired appendages during vertebrate limb evolution.

Modification of the zone of polarizing activity signal by trypsin

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

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.

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

The development of the vertebrate limb is dependent upon two signaling centers, the apical ectodermal ridge (AER), which provides the underlying mesenchyme with essential growth factors, and the zone of polarizing activity (ZPA), the source of the Sonic hedgehog (SHH) product. Recent work involving gain and loss of function of Hox genes has emphasized their impact both on AER maintenance and Shh transcriptional activation. Here, we describe antagonistic interactions between posterior Hoxd genes and Gli3, suggesting that the latter product protects the AER from the deleterious effect of the formers, and we present evidence that Fgf10 is the mediator of HOX-dependent AER expansion. Furthermore, the striking similarity between some of the hereby observed Hox/Gli3-dependent morphogenetic defects and those displayed by fetuses with severely altered retinoic acid metabolism suggests a tight connection between these various pathways. The nature of these potential interactions is discussed in the context of proximal-distal growth and patterning.

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

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

Control of Hoxd genes' collinearity during early limb development

Hoxd genes are essential for limb growth and patterning. They are activated following a complex transcriptional regulation, leading to expression domains that are collinear in both space and time. To understand the mechanism(s) underlying collinearity, we produced and analyzed a set of mouse strains containing systematic deletions and duplications within the HoxD cluster. We show that two waves of transcriptional activation, controlled by different mechanisms, generate the observed developmental expression patterns. The first wave is time-dependent, involves the action of opposite regulatory modules, and is essential for the growth and polarity of the limb up to the forearm. The second phase involves a different regulation and is required for the morphogenesis of digits. We propose that these two phases reflect the different phylogenetic histories of proximal versus distal limb structures and discuss the biological relevance of these collinear patterns, particularly for the origin of the anterior-to-posterior limb polarity.

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

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

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

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

Vertebrate limb development: from Harrison's limb disk transplantations to targeted disruption of Hox genes

Various animal organs have long been used to investigate the cellular and molecular nature of embryonic growth and morphogenesis. Among those organs, the tetrapod limb has been preferentially used as a model system for elucidating general patterning mechanisms. At the appropriate time during the embryonic period, the limb territories are first determined at the right positions along the cephalocaudal axis of the animal body, and soon the limb buds grow out from the flanks as mesenchymal cell masses covered by simple ectoderm. The position, number, and identity of the limbs depend on the expression of specific Hox genes. Limb morphogenesis occurs along three axes, which become gradually fixed: first the anteroposterior axis, then the dorsoventral, and finally the proximodistal axis, along which the bulk of limb growth occurs. Growth of the limb in amniotes depends on the formation of the apical ectodermal ridge, which, by secreting many members of the fibroblast growth factors family, attracts lateral plate and somitic mesodermal cells, keeps these cells in the progress zone proliferating, and prevents their differentiation until an appropriate time period. Mutual interactions between mesoderm and ectoderm are important in the growth process, and signaling regions have been identified, such as the zone of polarizing activity, the dorsal limb ectoderm, and the apical ectodermal ridge. Several molecules have been found to play leading roles in various biological processes relevant to morphogenesis. Besides its intrinsic merit as a model for unraveling the mechanisms of development, the limb deserves considerable clinical interest because defects of limb development are the most common single category of congenital abnormalities.

A Dual Role for Hox Genes in Limb Anterior-Posterior Asymmetry

Anterior-to-posterior patterning, the process whereby our digits are differently shaped, is a key aspect of limb development. It depends on the localized expression in posterior limb bud of Sonic hedgehog (Shh) and the morphogenetic potential of its diffusing product. By using an inversion of and a large deficiency in the mouse HoxD cluster, we found that a perturbation in the early collinear expression of Hoxd11, Hoxd12, and Hoxd13 in limb buds led to a loss of asymmetry. Ectopic Hox gene expression triggered abnormal Shh transcription, which in turn induced symmetrical expression of Hox genes in digits, thereby generating double posterior limbs. We conclude that early posterior restriction of Hox gene products sets up an anterior-posterior prepattern, which determines the localized activation of Shh. This signal is subsequently translated into digit morphological asymmetry by promoting the late expression of Hoxd genes, two collinear processes relying on opposite genomic topographies, upstream and downstream Shh signaling.

Mammalian formin-1 participates in adherens junctions and polymerization of linear actin cables

During epithelial sheet formation, linear actin cables assemble at nascent adherens junctions. This process requires alpha-catenin and actin polymerization, although the underlying mechanism is poorly understood. Here, we show that formin-1 interacts with alpha-catenin, localizes to adherens junctions and nucleates unbranched actin filaments. Furthermore, disruption of the alpha-catenin-formin-1 interaction blocks assembly of radial actin cables and perturbs intercellular adhesion. A fusion protein of the beta-catenin-binding domain of alpha-catenin with the actin polymerization domains of formin-1 rescues formation of adherens junctions and associated actin cables in alpha-catenin-null keratinocytes. These findings provide new insight into how alpha-catenin orchestrates actin dynamics during intercellular junction formation.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

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

Regulation of Tbx3 expression by anteroposterior signalling in vertebrate limb development

Tbx3, a T-box gene family member related to the Drosophila gene optomotor blind (omb) and encoding a transcription factor, is expressed in anterior and posterior stripes in developing chick limb buds. Tbx3 haploinsufficiency has been linked with the human condition ulnar-mammary syndrome, in which predominantly posterior defects occur in the upper limb. Omb is expressed in Drosophila wing development in response to a signalling cascade involving Hedgehog and Dpp. Homologous vertebrate signals Sonic hedgehog (Shh) and bone morphogenetic protein 2 (Bmp2) are associated in chick limbs with signalling of the polarising region which controls anteroposterior pattern. Here we carried out tissue transplantations, grafted beads soaked in Shh, Bmps, and Noggin in chick limb buds, and analysed Tbx3 expression. We also investigated Tbx3 expression in limb buds of chicken and mouse mutants and retinoid-deficient quail in which anteroposterior patterning is abnormal. We show that Tbx3 expression in anterior and posterior stripes is regulated differently. Posterior Tbx3 expression is stable and depends on the signalling cascade centred on the polarising region involving Shh and Bmps, while anterior Tbx3 expression is labile and depends on the balance between positive Bmp signals, produced anteriorly, and negative Shh signals, produced posteriorly. Our results are consistent with the idea that posterior Tbx3 expression is involved in specifying digit pattern and thus provides an explanation for the posterior defects in human patients. Anterior Tbx3 expression appears to be related to the width of limb bud, which determines digit number.

Progression of vertebrate limb development through SHH-mediated counteraction of GLI3

Distal limb development and specification of digit identities in tetrapods are under the control of a mesenchymal organizer called the polarizing region. Sonic Hedgehog (SHH) is the morphogenetic signal produced by the polarizing region in the posterior limb bud. Ectopic anterior SHH signaling induces digit duplications and has been suspected as a major cause underlying congenital malformations that result in digit polydactyly. Here, we report that the polydactyly of Gli3-deficient mice arises independently of SHH signaling. Disruption of one or both Gli3 alleles in mouse embryos lacking Shh progressively restores limb distal development and digit formation. Our genetic analysis indicates that SHH signaling counteracts GLI3-mediated repression of key regulator genes, cell survival, and distal progression of limb bud development.

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

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

Mouse Twist is required for fibroblast growth factor-mediated epithelial-mesenchymal signalling and cell survival during limb morphogenesis

Mouse Twist is essential for cranial neural tube, limb and somite development. [Genes Dev. 9 (1995) 686]. To identify the molecular defects disrupting limb morphogenesis, we have analysed expression of mesenchymal transcription factors involved in patterning and the cell-cell signalling cascades controlling limb bud development. These studies establish that Twist is essential for maintenance and progression of limb bud morphogenesis. In particular, the SHH/FGF signalling feedback loop operating between the polarizing region and the apical ectodermal ridge (AER) is disrupted. These defects in epithelial-mesenchymal signalling are most likely a direct consequence of disrupted fibroblast growth factor (FGF) signalling in Twist-deficient limb buds. In early limb buds, down-regulation of Fgf receptor 1 and Fgf10 expression in the mesenchyme occurs concurrent with loss of Fgf4 and Fgf8 expression in the AER. Finally, Twist function, most likely by regulating FGF signalling, is required for cell survival as apoptotic cells are detected in posterior and distal limb bud mesenchyme.

Suppression of polydactyly of the Gli3 mutant (extra toes) by deltaEF1 homozygous mutation

Digit patterning is established through multiple genetic interactions. Delta-crystallin enhancer/E2-box factor (deltaEF1) is a zinc finger and homeodomain containing repressor protein, and is expressed in the posterior half of the forelimb bud and in the entire hindlimb bud during the early stage of limb development. The 6EF1-deficient mutant mice display various skeletal abnormalities, among which inferior ossification and abnormal patterning of autopodial bones are similar to those observed in Hox and Gli gene mutants. Gli3 mutant mice, extra toes (Xt), exhibit pre-axial polydactyly losing the identity of digit I. It is demonstrated here that deltaEF1null(lacZ) homozygosity suppressed formation of the extra digit, uniquely of the hindlimb, in both Gli3XtJ heterozygous and homozygous mutants, but with no restoration of digit I identity. In Gli3XtJ mutants, the Hoxd13 expression domain was expanded more dramatically in homozygotes. In Gli3XtJ;deltaEF1null(lacZ) double homozygous mutants, Hoxd13 expression once expanded in Gli3XtJ homozygous mutant was reduced, more conspicuously in the hindlimbs, which may account for hindlimb-restricted suppression of formation of the extra digit. The data suggest the possibility that the extent of Hoxd13 expression along the distal margin of the limb bud is determinative in defining the digit number.

The bHLH transcription factor dHAND controls Sonic hedgehog expression and establishment of the zone of polarizing activity during limb development

Limb outgrowth and patterning of skeletal elements are dependent on complex tissue interactions involving the zone of polarizing activity (ZPA) in the posterior region of the limb bud and the apical ectodermal ridge. The peptide morphogen Sonic hedgehog (SHH) is expressed specifically in the ZPA and, when expressed ectopically, is sufficient to mimic its functions, inducing tissue growth and formation of posterior skeletal elements. We show that the basic helix-loop-helix transcription factor dHAND is expressed posteriorly in the developing limb prior to Shh and subsequently occupies a broad domain that encompasses the Shh expression domain. In mouse embryos homozygous for a dHAND null allele, limb buds are severely underdeveloped and Shh is not expressed. Conversely, misexpression of dHAND in the anterior region of the limb bud of transgenic mice results in formation of an additional ZPA, revealed by ectopic expression of Shh and its target genes, and resulting limb abnormalities that include preaxial polydactyly with duplication of posterior skeletal elements. Analysis of mouse mutants in which Hedgehog expression is altered also revealed a feedback mechanism in which Hedgehog signaling is required to maintain the full dHAND expression domain in the developing limb. Together, these findings identify dHAND as an upstream activator of Shh expression and important transcriptional regulator of limb development.

Formin-2, a novel formin homology protein of the cappuccino subfamily, is highly expressed in the developing and adult central nervous system

Formin-1 is the founding member of a family of genes of emerging biological and medical importance that share specific domains of homology, allowing them to be classified together as the formin homology proteins. Although deficiency mutations in formin-1 lead to profound developmental defects in limb and kidney formation, similar deficiency mutations in more distantly related members of this family (diaphanous and cappuccino in Drosophila and BNI1 in yeast) have ostensibly unrelated phenotypes. Here we describe murine and human formin-2 (Fmn2), a gene which bears a high degree of similarity to formin-1 and cappuccino. The mouse gene, which encodes a putative 1567-amino-acid open reading frame and maps to mouse Chromosome 1, is expressed almost exclusively in the developing and mature central nervous system. Expression begins at embryonic day 9. 5 in the developing spinal cord and brain structures and continues in neonatal and adult brain structures including the olfactory bulb, cortex, thalamus, hypothalamus, hippocampus and cerebellum. Human formin-2 has a similar expression pattern.