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Hedgehog homologs and digits

Anteroposterior polarity in the vertebrate limb is thought to be regulated in response to signals derived from a specialized region of distal posterior mesenchyme, the zone of polarizing activity. Sonic Hedgehog (Shh) is expressed in the zone of polarizing activity and appears to mediate the action of the zone of polarizing activity. Shh signal in the limb was manipulated to assess whether it acts as a long-range signal to directly pattern all the digits. Alterations in digit development are dependent on the dose of Shh applied. Cells giving rise to the extra digits lie within a 300 micron radius of a single Shh bead and the most posterior digits come from cells that lie very close to the bead. A response to Shh involves a 12-16 hour period in which no irreversible changes in digit pattern occur. Increasing the time of exposure to Shh leads to specification of additional digits, first digit 2, then 3, then 4. Cell marking experiments demonstrate that cells giving rise to posterior digits are first specified as anterior digits and later adopt a more posterior character. Long-range diffusion across the anteroposterior axis of the limb is possible. However, despite a dramatic difference in their diffusibility in the limb mesenchyme, the two forms of alkaline phosphatase-tagged Shh proteins share similar polarizing activity. Beasds coated with Shh-N (the aminoterminal peptide of Shh) and Shh-expressing cells also exhibit similar patterning activity despite a significant difference in the diffusibility of Shh from these two sources. When Shh-N is attached to an integral membrane protein, cells transfected with this anchored signal also induce mirror-image pattern duplications in a dose-dependent fashion similar to the zone of polarizing activity itself. These data suggest that it is unlikely that Shh itself signals digit formation at a distance. Beads soaked in Shh-N do not induce Shh in anterior limb mesenchyme, ruling out direct propagation of the Shh signal. However, Shh induces dose-dependent expression of Bmp genes in anterior mesenchyme at the start of the promotion phase. Taken together, these results argue that the dose-dependent effects of Shh in the regulation of anteroposterior pattern in the limb may be mediated by some other signal(s). BMPs are plausible candidates (Yang, 1997).

Patterning of the vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) located at the posterior limb margin. One of the vertebrate Hh family members, Shh, has been shown to be able to mediate the function of the ZPA. Several naturally occurring mouse mutations with the phenotype of preaxial polydactyly exhibit ectopic Shh expression at the anterior limb margin. The molecular characterization of a spontaneous mouse mutation, Doublefoot (Dbf) is reported. Dbf is a dominant mutation that maps to chromosome 1. Heterozygous and homozygous embryos display a severe polydactyly with 6 to 8 digits on each limb. Shh is expressed normally in Dbf mutants. In contrast, a second Hh family member, Indian hedgehog (Ihh) which maps close to Dbf, is ectopically expressed in the distal limb bud. Ectopic Ihh expression in the distal and anterior limb bud results in the ectopic activation of several genes associated with anterior-posterior and proximal-distal patterning (Fgf4, Hoxd13, Bmp2). In addition, specific components in the Hedgehog pathway are either ectopically activated (Ptc, Ptc-2, Gli1) or repressed (Gli2). It is proposed that misexpression of Ihh is responsible for the Dbf phenotype. Ihh is considered to have a similar activity to Shh when expressed in the early Shh-responsive limb bud. To determine whether Dbf maps to the Ihh locus, which is also on chromosome 1, an interspecific backcross was performed. These results demonstrate that Dbf and Ihh are genetically separated by approximately 1.3 centimorgans, suggesting that Dbf mutation may cause an exceptionally long-range disruption of Ihh regulation. Although this leads to ectopic activation of Ihh, normal expression of Ihh in the cartilaginous elements is retained (Yang, 1998).

Gli genes (Drosophila homolog: cubitus interruptus) are members of a small family, encoding zinc-finger proteins of the Kruppel-type. The family consists of Gli(1), Gli2, and Gli3, all of which are expressed in the developing mouse limb bud. To assess the role of the Gli family and Sonic hedgehog (Shh) in mouse limb development, the expression domains of all three Gli genes and of Shh were compared. Although each Gli gene has its own distinct expression pattern in limb buds, at 10.5-11.5 dpc, none of the three genes were found to be expressed in the posterior region, the presumptive Shh expression domain. This transient mutually exclusive expression suggests a potential interaction between Gli genes and Shh. To address this matter, the expression of Gli genes and Shh were examined in two polydactyly mouse mutants, Extra toes (Xt) and Hemimelic-extra toes (Hx), both of which express Shh ectopically in the anterior region of the limb field. Since Xt mice lack Gli3 expression, the ectopic Shh expression is genetically linked to the absence of Gli3. In Hx mice a down-regulation of Gli3 in the anterior region of the limb bud was found. In both mutants Gli2 expression pattern is not altered, whereas Gli1 expression is anteriorly up-regulated adjacent to the ectopic Shh domain. These results strongly suggest a positive regulation of Gli1 by Shh and a negative interaction between Shh and Gli3 (Buscher, 1998).

Development of the musculature in chick limbs involves tissue and cellular patterning. Patterning at the tissue level leads to the precise arrangement of specific muscles; at the cellular level patterning gives rise to fiber type diversity in muscles. Although the data suggests that the information controlling muscle patterning is localized within the limb mesenchyme and not in the somitic myogenic precursor cells themselves, the mechanisms underlying muscle organization have still to be elucidated. The anterior-posterior axis of the limb is specified by a group of cells in the posterior region of the limb mesenchyme, called the zone of polarizing activity (ZPA). When polarizing-region cells are grafted to the anterior margin of the bud, they cause mirror-image digit duplications to be produced. The effect of ZPA grafts can be reproduced by application of retinoic acid (RA) beads and by grafting sonic hedgehog (SHH)-expressing cells to the anterior margin of the limb. Although most previous studies have looked at changes of the skeletal patterning, ZPA and RA also affect muscle patterning. The role of SHH in tissue and cellular patterning of forearm wing muscles has been investigated. Ectopic application of a localized source of SHH to the anterior margin of the wing, leading to complete digit duplication, is able to transform anterior forearm muscles into muscles with a posterior identity. Moreover, the ectopic source of SHH induces in the new posterior muscles a mirror image duplication of the normal fiber types found in posterior muscles. The reorganization of the slow fibers can be detected before muscle mass cleavage has started, suggesting that the appropriate fiber type arrangement is in place before the splitting process can be observed (Duprez, 1999).

The developmental properties of the polydactylous chicken mutant, talpid(2) have been examined. Ptc, Gli1, Bmp2, Hoxd13, and Fgf4 are expressed throughout the anteroposterior axis of the mutant limb bud, despite normal Shh expression. The expression of Gli3, Ihh, and Dhh appears to be normal, suggesting that the Shh signaling pathway is constitutively active in talpid(2) mutants. Preaxial talpid(2) limb bud mesoderm has polarizing activity in the absence of detectable Shh mRNA. When the postaxial talpid(2) limb bud (including all Shh-expressing cells) is removed, the preaxial cells reform a normal-shaped talpid(2) limb bud. However, a Shh-expressing region (zone of polarizing activity) does not reform; nevertheless Fgf4 expression in the apical ectodermal ridge is maintained. Such reformed talpid(2) limb buds develop complete talpid(2) limbs. After similar treatment, normal limb buds downregulate Fgf4, the preaxial cells do not reform a normal-shaped talpid(2) limb bud, and a truncated anteroposterior deficient limb forms. In talpid(2) limbs, distal outgrowth is independent of Shh and correlates with Fgf4, but not Fgf8, expression by the apical ectodermal ridge. A model for talpid(2) is proposed in which leaky activation of the Shh signaling pathway occurs in the absence of Shh ligand (Caruccio, 1999).

It has been proposed that digit identity in chick limb bud is specified in a dose-dependent fashion by a long-range morphogen, produced by the polarizing region. One candidate is Sonic hedgehog (Shh) protein, but it is not clear whether Shh acts long or short range or via Bmps. The relationship between Shh and Bmp signaling is dissected in this study. Shh is necessary not only for initiating bmp2 expression but also for sustaining its expression during the period when additional digits are being specified. Much of the effect of Shh during this period can be reproduced by applying only Bmp2. It has been demonstrated, by transiently adding Noggin or Bmp antibodies to limbs treated with Shh, that Bmps are responsible for digit specification. In such limbs, multiple additional digits still form but they all have the same identity. Time dependency and range of Shh signaling is explored by examining ptc expression. High-level ptc expression is induced rapidly when either Shh beads or polarizing regions are grafted to a host limb. Furthermore, high-level ptc expression is first widespread but later more restricted. All these data lead to the proposal of a new model for digit patterning. It is suggested that Shh initially acts long range to prime the region of the limb competent to form digits and thus control digit number. Then later, Shh acts short range to induce expression of Bmps, whose morphogenetic action specifies digit identity (Drossopoulou, 2000).

These findings suggest that Shh acts in two steps, first, long range, and then, short range, to control digit pattern. Whether Shh acts short range or long range will be determined by whether or not high-level expression of binding molecules such as ptc has been induced in responding cells. It is proposed that, in the first step, Shh acts long range to 'prime' limb mesenchyme cells and thus make them competent to form digits. This in essence will determine the number of digits that can form and will be related to the length of the apical ridge. In the second step, Shh acts short range to induce and maintain bmp2 expression. Bmp2 then acts in a dose-dependent fashion on the competent limb mesenchyme cells to specify digit identity. Thus, in terms of classical models for polarizing region signaling, it is proposed that Bmps act as polarizing morphogens and progressively diffuse into adjacent mesenchyme. This establishes a concentration gradient and cells that are first specified as anterior digits are later promoted to form posterior digits. This model accounts for the truncations found in the shh knockout, because, in the absence of Shh, no cells will be competent to form digits. The model also seems to be able to account for all previous results on the polarizing region and Shh induction of digits. For example, the dose-dependent effects of Shh on digit pattern are mediated by dose-and time-dependent induction of bmp2 expression, just as the dose-dependent effects of retinoic acid are mediated by dose-dependent induction of shh expression. The only exception is the finding that shh expressed tethered to the membrane protein CD4 appears to be able to induce full duplications. However, it is still possible that there is some kind of active diffusion of this Shh::CD4 involving cleavage and/or intracellular transport (Drossopoulou, 2000 and references therein).

This model could be widely applicable. The two phases of Sonic Hedgehog signaling are reminiscent of the way in which Hedgehog patterns the insect eye and sequential signaling, short range by Hh, followed by Dpp long range, is a well-established model in Drosophila wing patterning. In vertebrates, it has been shown that Shh signaling acts in two different phases to specify motor neuron identity during neural tube development; furthermore, sequential signaling by Shh and Bmps has been proposed to be involved in sclerotome specification in somites (Drossopoulou, 2000 and references therein).

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. Genetic analyses in mice shows 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. The effects of Shh signalling on skeletal patterning and ridge maintenance are necessarily mediated through Gli3. It is proposed 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 (Litingtung, 2002).

Despite extensive studies on the anterior-posterior (AP) axis formation of limb buds, mechanisms that specify digit identities along the AP axis remain obscure. Using the four-digit chick leg as a model, Tbx2 and Tbx3 are shown to specify the digit identities of digits IV and III, respectively. Misexpression of Tbx2 and Tbx3 induced posterior homeotic transformation of digit III to digit IV and digit II to digit III, respectively. Conversely, misexpression of their mutants VP16ΔTbx2 and VP16ΔTbx3 induced anterior transformation. In both cases, alterations in the expression of several markers (e.g., BMP2, Shh, and HoxD genes) were observed. In addition, Tbx2 and Tbx3 rescued Noggin-mediated inhibition of interdigital BMP signaling, signaling which is pivotal in establishing digit identities. Hence, it is concluded that Tbx3 specifies digit III, and the combination of Tbx2 and Tbx3 specifies digit IV, acting together with the interdigital BMP signaling cascade (Suzuki, 2004).

Thus chick Tbx3 and Tbx2 specify posterior digit identities by regulating interdigital BMP signaling. Misexpression of Tbx3 and Tbx2 induced posterior homeotic transformation of digit II to III and digit III to IV, respectively. In contrast, misexpression of VP16ΔTbx3 and VP16ΔTbx2 induced anterior transformation, thereby converting digit III to II and digit IV to I or II. In some cases, truncation of the posterior digits was observed, indicating that Tbx3 and Tbx2 also control the development of the posterior digits. Tbx2 and Tbx3 are known to have specific expression patterns in the interdigital autopod regions; namely, chick Tbx3 is expressed in ID3 and 4, and Tbx2 in ID4. Since the interdigit BMP level regulates its anterior digit identity, these expression patterns suggest that Tbx2 and Tbx3 might be direct regulators of the posterior digit identities. More specially, Tbx2 acts upstream of Shh and BMP2, and Tbx3 regulates BMP2. Conversely, Shh and BMP4 upregulate the posterior expression of Tbx2 and Tbx3. These lines of evidence suggest that the feedback and feedforward regulation between Tbx2/3 and the Shh and BMP signaling cascades is pivotal for the specification of posterior digit identities (Suzuki, 2004). -----------------------

Mutations that affect vertebrate limb development provide insight into pattern formation, evolutionary biology and human birth defects. Patterning of the limb axes depends on several interacting signaling centers; one of these, the zone of polarizing activity (ZPA), comprises a group of mesenchymal cells along the posterior aspect of the limb bud that expressed sonic hedgehog (Shh) and plays a key role in patterning the anterior-posterior (AP) axis. The mechanisms by which the ZPA and Shh expression are confined to the posterior aspect of the limb bud mesenchyme are not well understood. The polydactylous mouse mutant Strong's luxoid (lst) exhibits an ectopic anterior ZPA and expression of Shh that results in the formation of extra anterior digits. Alx4 (Drosophila homolog: Aristaless) haploinsufficiency and the importance of strain-specific modifiers leading to polydactyly are indicative of a critical threshold requirement for Alx4 in a genetic program operating to restrict polarizing activity and Shh expression in the anterior mesenchyme of the limb bud, and suggest that mutations in Alx4 may also underlie human polydactyly (Qu, 1998).

Strong's Luxoid (lstD) mice have a 16 bp deletion in the homeobox region of the Alx-4 gene. This deletion, which leads to a frame shift and a truncation of the Alx-4 protein, could cause the polydactyly phenotype observed in lstD mice. The chick homolog of Alx-4 was cloned and its expression was investigated during limb outgrowth. Chick Alx-4 displays an expression pattern complementary to that of Shh, a mediator of polarizing activity in the limb bud. Local application of Sonic hedgehog and fibroblast growth factor, in addition to ectodermal apical ridge removal experiments suggest the existence of a negative feedback loop between Alx-4 and Shh during limb outgrowth. Analysis of polydactylous mutants indicate that the interaction between Alx-4 and Shh is independent of Gli3, a negative regulator of Shh in the limb. These data suggest the existence of a negative feedback loop between Alx-4 and Shh during vertebrate limb outgrowth (Takahashi, 1998).

During vertebrate evolution the transition of fins into limbs has been proposed to have involved a developmental elaboration of digits as novel distal autopod elements. The molecular changes underlying this evolutionary process are not clear, but may reflect distal cell growth in response to distal Hox gene expression. Regardless of mechanism, the fossil record suggests that the ancestral Devonian tetrapods had limbs with up to eight digits. The digits of polydactylous limbs of Acanthostega and Ichthyostega appear at some level to resemble the mirror-image polydactyly seen in Alx4 alleles. This resemblance suggests that during the fin-to-limb transition, a branch of ancestral tetrapod limbs were in fact polydactylous due to the presence of symmetric anterior and posterior ZPAs. Subsequently, during evolution of the pentadactyl limb, the presence of an anterior ZPA would have become actively repressed. Based on this notion, the Alx4 mutant alleles would represent atavistic mutations, suggesting that during tetrapod evolution Alx4 may have been recruited or co-opted from a role in axial patterning as part of a genetic program to repress anterior ZPA formation. Alternatively, as may be argued from the restriction of Shh to the proximal posterior region of the fin bud in the teleost zebrafish, the polydactylous specimens in the fossil record may represent the 'capture' of an evolutionary offshoot and Alx4 expression in the anterior limb bud may instead be ancestral. An ancestral role for Alx4 is also suggested by the expression of Drosophila relatives of Alx4 and Gli3 (aristaless and cubitus interruptus, respectively) in the anterior compartment of the wing imaginal disc. Whereas in AP patterning of the fly wing ci appears to act downstream of hedgehog, the function of aristaless and its relationship to hedgehog and AP patterning is less well defined. Analysis of molecular markers such as Shh, Gli3 and Alx4 expression in zebrafish and in more primitive species such as lungfish and coelacanths should provide information by which to evaluate these speculations (Qu, 1998 and references).

The polarizing region expresses the signaling molecule Sonic hedgehog (Shh), and is an embryonic signaling center essential for outgrowth and patterning of the vertebrate limb. Previous work has suggested that there is a buffering mechanism that regulates polarizing activity. Little is known about how the number of Shh-expressing cells is controlled but, paradoxically, the polarizing region appears to overlap with the posterior necrotic zone, a region of programmed cell death. How Shh expression and cell death respond when levels of polarizing activity are altered has been investigated: an autoregulatory effect of Shh on Shh expression has been shown and Shh affects cell death in the posterior necrotic zone. When Shh signaling is increased, by grafting polarizing region cells or applying Shh protein beads, this leads to a reduction in the endogenous Shh domain and an increase in posterior cell death. In contrast, cells in other necrotic regions of the limb bud, including the interdigital areas, are rescued from death by Shh protein. Application of Shh protein to late limb buds also causes alterations in digit morphogenesis. When the number of Shh-expressing cells in the polarizing region are reduced by surgery or drug-induced killing, this leads to an expansion of the Shh domain and a decrease in the number of dead cells. Furthermore, direct prevention of cell death using a retroviral vector expressing Bcl2 leads to an increase in Shh expression. Finally, evidence is provided that the fate of some of the Shh-expressing cells in the polarizing region is to undergo apoptosis and contribute to the posterior necrotic zone during normal limb development. Taken together, these results show that there is a buffering system that regulates the number of Shh-expressing cells and thus polarizing activity during limb development. They also suggest that cell death induced by Shh could be the cellular mechanism involved. Such an autoregulatory process based on cell death could represent a general way for regulating patterning signals in embryos (Sanz-Ezquerro, 2000).

Myogenic regulatory factors (MRFs) comprise a family of transcription factors that when expressed in a cell reflects the commitment of that cell to a myogenic fate before any cytological sign of muscle differentiation is detectable. Myogenic cells in limb skeletal muscles originate from the lateral half of the somites. Cells that migrate away from the lateral part of the somites to the limb bud do not initially express any member of the MRF family. Expression of MRFs in the muscle precursor cells starts after the migration process is completed. The extracellular signals involved in activating the myogenic program in muscle precursor cells in the in vivo limb are not known. Sonic Hedgehog (SHH) expressed in the posterior part of the limb bud (the zone of polarizing activity) could be involved in the differentiation of muscle precursor cells in the limb. Retrovirally overexpressed SHH in the limb bud first induces the extension of the expression domain of the Pax-3 gene, then that of the MyoD gene, and finally that of the myosin protein. This leads to a hypertrophy of the muscles in vivo. Addition of SHH to primary cultures of myoblasts results in an increase in the proportion of myoblasts that incorporate bromodeoxyuridine, resulting in an increase in the number of myotubes. These data show that SHH is able to activate myogenesis in vivo and in vitro in already committed myoblasts and suggest that the stimulation of the myogenic programme by SHH involves activation of cell proliferation. It is suggested that SHH may activate Pax-3 expression, which in turn activates MyoD (Duprez, 1998).

dHAND is a basic helix-loop-helix (bHLH) transcription factor essential for cardiovascular development. Its pattern of expression and functional role during chick limb development has been analyzed. dHAND expression is observed in the lateral plate mesoderm prior to emergence of the limb buds. Coincident with limb initiation, expression of dHAND becomes restricted to the posterior half of the limb bud. Experimental procedures that caused mirror-image duplications of the limb result in mirror-image duplications of the pattern of dHAND expression along the anterior-posterior axis. Retroviral overexpression of dHAND in the limb bud produces preaxial polydactyly, corresponding to mild polarizing activity at the anterior border. At the molecular level, misexpression of dHAND causes ectopic activation of members of the Sonic hedgehog (Shh) pathway, including Gli and Patched, in the anterior limb bud. A subset of infected embryos displays ectopic anterior activation of Shh. Other factors implicated in anterior-posterior polarization of the bud, such as the most 5' Hoxd genes and Bmp2, are also ectopically activated at the anterior border. These results indicate a role for dHAND in the establishment of anterior-posterior polarization of the limb bud (Fernandez-Teran, 2000).

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. 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 reveal 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 (Charite, 2000).

The secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior-posterior limb polarity. In the Shh-/- mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. Shh-/- mutant limbs have been analyzed in detail. Each mutant embryo has four limbs with recognizable humerus/femur bones that have anterior-posterior polarity. Distal to the elbow/knee joints, skeletal elements representing the zeugopod form, but they lack identifiable anterior-posterior polarity. Therefore, Shh specifically becomes necessary for normal limb development at or just distal to the stylopod/zeugopod junction (elbow/knee joints) during mouse limb development. The forelimb autopod is represented by a single distal cartilage element, while the hindlimb autopod is invariably composed of a single digit with well-formed interphalangeal joints and a dorsal nail bed at the terminal phalanx. Analysis of GDF5 and Hoxd11-13 expression in the hindlimb autopod suggests that the forming digit has a digit-one identity. This finding is corroborated by the formation of only two phalangeal elements that are unique to digit one on the foot. The apical ectodermal ridge (AER) is induced in the Shh-/- mutant buds with relatively normal morphology. The architecture of the Shh-/- AER is gradually disrupted over developmental time in parallel with a reduction of Fgf8 expression in the ridge. Concomitantly, abnormal cell death in the Shh-/- limb bud occurs in the anterior mesenchyme of both fore- and hind-limb. It is notable that the AER changes and mesodermal cell death occur earlier in the Shh-/- forelimb than the hindlimb bud. This provides an explanation for the hindlimb-specific competence to form autopodial structures in the mutant. Finally, unlike the wild-type mouse limb bud, the Shh-/- mutant posterior limb bud mesoderm does not cause digit duplications when grafted to the anterior border of chick limb buds, and therefore lacks polarizing activity. It is proposed that a prepattern exists in the limb field for the three axes of the emerging limb bud as well as specific limb skeletal elements. According to this model, the limb bud signaling centers, including the zone of polarizing activity (ZPA) acting through Shh, are required to elaborate upon the axial information provided by the native limb field prepattern (Chiang, 2001).

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 polarizing region, which controls anteroposterior pattern. Tissue transplantations and grafting with beads soaked in Shh, Bmps, and Noggin have been carried out in chick limb buds, and Tbx3 expression has been analyzed. Tbx3 expression was also analyzed in limb buds of chicken and mouse mutants and retinoid-deficient quail in which anteroposterior patterning is abnormal. Tbx3 expression in anterior and posterior stripes is regulated differently. Posterior Tbx3 expression is stable and depends on the signalling cascade centered 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. These 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 (Tümpel, 2002).

A unique limb phenotype is described in a radiation-induced mutant mouse resulting from an inversion of a proximal segment of chromosome 5. The limb phenotype in the homozygous mutant presents with two anterior skeletal elements in the zeugopod but no posterior bone, hence the name replicated anterior zeugopod, raz. The zeugopod phenotype is accompanied by symmetrical central polydactyly of hand and foot. The chromosomal inversion includes the Shh gene and the regulatory locus, located ~1 Mb away, within the Lmbr1 gene. In homozygous mutants, the expression of Shh mRNA and Shh protein is severely downregulated to about 20% of wild-type limb buds, but Shh expression appears normal throughout the remainder of the embryo. Correspondingly, Gli3 expression is upregulated and posteriorly expanded in the raz/raz limb bud. It is proposed that the double anterior zeugopod and symmetrical central polydactyly are due to an increased and uniform concentration of the Gli3 repressor form because of lowered Shh signaling (Krebs, 2003).

The zone of polarizing activity (ZPA) in the posterior limb bud produces Sonic Hedgehog (Shh) protein, which plays a critical role in establishing distinct fates along the anterior-posterior axis. This activity has been modeled as a concentration-dependent response to a diffusible morphogen. Using recombinase base mapping in the mouse, the ultimate fate of the Shh-producing cells was determined. Strikingly, the descendants of the Shh-producing cells encompass all cells in the two most posterior digits and also contribute to the middle digit. This analysis suggests that, while specification of the anterior digits depends upon differential concentrations of Shh, the length of time of exposure to Shh is critical in the specification of the differences between the most posterior digits. Genetic studies of the effects of limiting accessibility of Shh within the limb support this model, in which the effect of the Shh morphogen is dictated by a temporal as well as a spatial gradient (Harfe, 2004).

The data suggest a model in which expansion of the posterior limb bud cell population affects the length of time a digit primordium is within the Shh-expressing domain and hence is exposed to maximal Shh signaling. The model explicitly depends on the ability of cells to respond differentially based on their time of exposure to Shh. Exposing chick limb bud mesenchyme to a high concentration of Shh for 10 hr has no effect on limb pattern. However, the treated cells exhibit a memory of this exposure, revealed when they were exposed to the same concentration for 16 more hours. The digit primordia preexposed to Shh adopted a more posterior fate than digit primordia treated in parallel but without the preexposure. These experiments indicate that time of exposure to Shh can indeed determine digit identity. Previous studies have also provided evidence that this is the case. Beads soaked in high concentrations of Shh were implanted into the anterior of chick limb buds. The cells adjacent to the bead were marked with DiI. The beads were then removed after different lengths of time. Depending on the length of exposure to Shh, the equivalent, marked cells developed into an ectopic digit 2 (with short exposure) or digit 3 or 4 (with longer exposure). These data supported a 'promotion' model in which digit primordia are first specified to an anterior fate and are then promoted to more posterior cell fates with longer times of exposure. It was also postulated that different digit primordia received different lengths of exposure to Shh because of the expansion of the posterior tissue (Harfe, 2004).

Conservation within intergenic DNA often highlights regulatory elements that control gene expression from a long range. How conservation within a single element relates to regulatory information and how internal composition relates to function is unknown. This study examined the structural features of the highly conserved ZRS (also called MFCS1) cis-regulator responsible for the spatiotemporal control of Shh in the murine limb bud. By systematically dissecting the ZRS, both in transgenic assays and within in the endogenous locus, the ZRS was shown, in effect, to be composed of two distinct domains of activity: one domain directs spatiotemporal activity but functions predominantly from a short range, whereas a second domain is required to promote long-range activity. These two domains encode activities that are highly integrated, and the second domain is crucial in promoting the chromosomal conformational changes correlated with gene activity. During limb bud development, these activities encoded by the ZRS are interpreted differently by the fore limbs and the hind limbs; in the absence of the second domain there is no Shh activity in the fore limb, and in the hind limb low levels of Shh lead to a variant digit pattern ranging from two to four digits. Hence, in the embryo, the second domain stabilises the developmental programme providing a buffer for SHH morphogen activity and this ensures that five digits form in both sets of limbs (Lettice, 2014).

Limb skeletal pattern relies heavily on graded Sonic hedgehog (Shh) signaling. As a morphogen and growth cue, Shh regulates identities of posterior limb elements, including the ulna/fibula and digits 2 through 5. In contrast, proximal and anterior structures, including the humerus/femur, radius/tibia, and digit 1, are regarded as Shh independent, and mechanisms governing their specification are unclear. This study shows that patterning of the proximal and anterior limb skeleton involves two phases. Irx3 and Irx5 (Irx3/5) are essential in the initiating limb bud to specify progenitors of the femur, tibia, and digit 1. However, these skeletal elements can be restored in Irx3/5 null mice when Shh signaling is diminished, indicating that Shh negatively regulates their formation after initiation. These data provide genetic evidence supporting the concept of early specification and progressive determination of anterior limb pattern (Li, 2014).

Table of contents

hedgehog continued: Biological Overview | Regulation | Targets of Activity | Protein Interactions | Developmental Biology | Effects of Mutation | References

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