Conserved regulation of proximodistal limb axis development by Meis1/Hth

MEIS and PBX homeobox proteins in ovarian cancer

Three amino-acid loop extension (TALE) homeobox proteins MEIS and PBX are cofactors for HOX-class homeobox proteins, which control growth and differentiation during embryogenesis and homeostasis. We showed that MEIS and PBX expression are related to cisplatin resistance in ovarian cancer cell lines. Therefore, MEIS1, MEIS2 and PBX expression were investigated immunohistochemically in a tissue microarray (N=232) of ovarian cancers and ovarian surface epithelium (N=15). Results were related to clinicopathologic characteristics and survival. All cancers expressed MEIS1, MEIS2 and PBX in nucleus and cytoplasm. MEIS1 and 2 only stained nuclear in surface epithelium. Nuclear MEIS2 was negatively related to stage, grade and overall survival in univariate analyses. Additionally, MEIS and PBX RNA expression in ovarian surface epithelium and other normal tissues and ovarian cancer versus other tumour types using public array data sets were studied. In ovarian cancer, MEIS1 is highly expressed compared to other cancer types. In conclusion, MEIS and PBX are extensively expressed in ovarian carcinomas and may play a role in ovarian carcinogenesis.

Genome-wide association study of restless legs syndrome identifies common variants in three genomic regions

Restless legs syndrome (RLS) is a frequent neurological disorder characterized by an imperative urge to move the legs during night, unpleasant sensation in the lower limbs, disturbed sleep and increased cardiovascular morbidity. In a genome-wide association study we found highly significant associations between RLS and intronic variants in the homeobox gene MEIS1, the BTBD9 gene encoding a BTB(POZ) domain as well as variants in a third locus containing the genes encoding mitogen-activated protein kinase MAP2K5 and the transcription factor LBXCOR1 on chromosomes 2p, 6p and 15q, respectively. Two independent replications confirmed these association signals. Each genetic variant was associated with a more than 50% increase in risk for RLS, with the combined allelic variants conferring more than half of the risk. MEIS1 has been implicated in limb development, raising the possibility that RLS has components of a developmental disorder.

Lateral motor column axons execute a ternary trajectory choice between limb and body tissues

Background

Neuronal topographic map formation requires appropriate selection of axonal trajectories at intermediate choice points prior to target innervation. Axons of neurons in the spinal cord lateral motor column (LMC), as defined by a transcription factor code, are thought to innervate limb target tissues exclusively. Axons of the medial and lateral LMC divisions appear to execute a binary decision at the base of the limb as they choose between ventral and dorsal limb trajectories. The cellular logic that guides motor axon trajectory choices into non-limb tissues such as the ventral flank remains unclear.

Results

We determined the spinal cord motor column origin of motor nerves that innervate ventral flank tissues at hindlimb level. We found unexpectedly that a subset of medial LMC axons innervates ventral non-limb mesenchyme at hindlimb level, rather than entering ventral limb mesenchyme. We also found that in a conditional BmprIa mutant where all ventral hindlimb mesenchyme is converted to a dorsal identity, all medial LMC axons are redirected into the ventral flank, while lateral LMC axons innervate the bidorsal limb.

Conclusion

We have found that medial LMC neurons innervate both ventral flank and limb targets. While normally only a subset of medial LMC axons innervate the flank, all are capable of doing so. Furthermore, LMC axons execute a ternary, rather than binary, choice at the base of the limb between ventral flank, ventral limb and dorsal limb trajectories. When making this choice, medial and lateral LMC axons exhibit different and asymmetric relative preferences for these three trajectories. These data redefine the LMC as a motor column that innervates both limb and body tissues.

Early steps of paired fin development in zebrafish compared with tetrapod limb development

The development of zebrafish paired fins and tetrapod forelimbs and hindlimbs show striking similarities at the molecular level. In recent years, the zebrafish, Danio rerio has become a valuable model for the study of the development of vertebrate paired appendages and several large-scale mutagenesis screens have identified novel fin mutants. This review summarizes recent advances in research into zebrafish paired fin development and highlights features that are shared with and distinct from limb development in other main animal models.

Shox2 is required for chondrocyte proliferation and maturation in proximal limb skeleton

Mutations in the short stature homeobox gene SHOX lead to growth retardation associated with Turner, Leri-Weill dyschondrosteosis, and Langer mesomelic dysplasia syndromes, which marked the shortening of the forearms and lower legs. We report here that in contrast to the SHOX mutations in humans, Shox2 deficiency in mice leads to a virtual elimination of the stylopod in the developing limbs, while the zeugopod and autopod appear relatively normal. This phenotype is consistent with the restriction of the Shox2 expression to the proximal mesenchyme in the limb bud and later to chondrocytes associated with the forming stylopod. In the Shox2(-/-) embryo, the mesenchymal condensation for the stylopod initiates normally but the cartilaginous element subsequently fails in growth, chondrogenesis and endochondral ossification. A dramatic down-regulation of Runx2 and Runx3 could account for the lack of chondrocyte hypertrophy, while a down-regulation of Ihh expression may be responsible for a significant reduction in chondrocyte proliferation in the mutant stylopod. We further demonstrate that an enhanced and ectopic Bmp4 expression in the proximal limb of the Shox2 embryo may underlie the down-regulation of Runx2, as ectopically applied exogenous BMP4 represses Runx2 expression in the early limb bud. Moreover, we show that mouse Shox2, similar to human SHOX, can perform opposite roles on gene expression: either as a transcription activator or a repressor in different cell types. Our results establish a key role for Shox2 in regulating the growth of stylopod by controlling chondrocyte maturation via Runx2 and Runx3.

Transcription profiling of adult and fetal human neuroprogenitors identifies divergent paths to maintain the neuroprogenitor cell state

Global gene expression profiling was performed using RNA from adult human hippocampus-derived neuroprogenitor cells (NPCs) and multipotent frontal cortical fetal NPCs compared with adult human mesenchymal stem cells (hMSCs) as a multipotent adult stem cell control, and adult human hippocampal tissue, to define a gene expression pattern that is specific for human NPCs. The results were compared with data from various databases. Hierarchical cluster analysis of all neuroectodermal cell/tissue types revealed a strong relationship of adult hippocampal NPCs with various white matter tissues, whereas fetal NPCs strongly correlate with fetal brain tissue. However, adult and fetal NPCs share the expression of a variety of genes known to be related to signal transduction, cell metabolism and neuroectodermal tissue. In contrast, adult NPCs and hMSCs overlap in the expression of genes mainly involved in extracellular matrix biology. We present for the first time a detailed transcriptome analysis of human adult NPCs suggesting a relationship between hippocampal NPCs and white matter-derived precursor cells. We further provide a framework for standardized comparative gene expression analysis of human brain-derived NPCs with other stem cell populations or differentiated tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Co-factors and co-repressors of Engrailed: expression in the central nervous system and cerci of the cockroach, Periplaneta americana

In the larval cockroach (Periplaneta americana), knockout of Engrailed (En) in the medial sensory neurons of the cercal sensory system changes their axonal arborization and synaptic specificity. Immunocytochemistry has been used to investigate whether the co-repressor Groucho (Gro; vertebrate homolog: TLE) and the co-factor Extradenticle (Exd; vertebrate homolog: Pbx) are expressed in the cercal system. Gro/TLE is expressed ubiquitously in cell nuclei in the embryo, except for the distal pleuropodia. Gro is expressed in all nuclei of the thoracic and abdominal central nervous system (CNS) of first instar larva, although some neurons express less Gro than others. Cercal sensory neurons express Gro protein, which might therefore act as a co-repressor with En. Exd/Pbx is expressed in the proximal portion of all segmental appendages in the embryo, with the exception of the cerci. In the first instar CNS, Exd protein is expressed in subsets of neurons (including dorsal unpaired medial neurons) in the thoracic ganglia, in the first two abdominal ganglia, and in neuromeres A8-A11 of the terminal ganglion. Exd is absent from the cerci. Because Ultrabithorax/Abdominal-A (Ubx/Abd-A) can substitute for Exd as En co-factors in Drosophila, Ubx/Abd-A immunoreactivity has also been investigated. Ubx/Abd-A immunostaining is present in abdominal segments of the embryo and first instar CNS as far caudal as A7 and faintly in the T3 segment. However, Ubx/Abd-A is absent in the cerci and their neurons. Thus, in contrast to its role in Drosophila segmentation, En does not require the co-factors Exd or Ubx/Abd-A in order to control the synaptic specificity of cockroach sensory neurons.

Expression of the short stature homeobox gene Shox is restricted by proximal and distal signals in chick limb buds and affects the length of skeletal elements

SHOX is a homeobox-containing gene, highly conserved among species as diverse as fish, chicken and humans. SHOX gene mutations have been shown to cause idiopathic short stature and skeletal malformations frequently observed in human patients with Turner, Leri-Weill and Langer syndromes. We cloned the chicken orthologue of SHOX, studied its expression pattern and compared this with expression of the highly related Shox2. Shox is expressed in central regions of early chick limb buds and proximal two thirds of later limbs, whereas Shox2 is expressed more posteriorly in the proximal third of the limb bud. Shox expression is inhibited distally by signals from the apical ectodermal ridge, both Fgfs and Bmps, and proximally by retinoic acid signaling. We tested Shox functions by overexpression in embryos and micromass cultures. Shox-infected chick limbs had normal proximo-distal patterning but the length of skeletal elements was consistently increased. Primary chick limb bud cell cultures infected with Shox showed an initial increase in cartilage nodules but these did not enlarge. These results fit well with the proposed role of Shox in cartilage and bone differentiation and suggest chick embryos as a useful model to study further the role of Shox in limb development.

Distinct functions of homeodomain-containing and homeodomain-less isoforms encoded by homothorax

The homothorax (hth) gene of Drosophila melanogaster is required for executing Hox functions, for head development, and for forming the proximodistal (PD) axis of the appendages. We show that alternative splicing of hth generates two types of protein isoforms, one that contains a DNA-binding homeodomain (HthFL) and one that does not contain a homeodomain (HDless). Both types of Hth isoforms include the evolutionarily conserved HM domain, which mediates a direct interaction with Extradenticle (Exd), another homeodomain protein. We show that although both HthFL and HDless isoforms of Hth can induce the nuclear localization of Exd, they carry out distinct sets of functions during development. Surprisingly, we find that many of hth's functions, including PD patterning and most Hox-related activities, can be executed by the HDless isoforms. In contrast, antennal development shows an absolute dependency on the HthFL isoform. Thus, alternative splicing of hth results in the generation of multiple transcription factors that execute unique functions in vivo. We further demonstrate that the mouse ortholog of hth, Meis1, also encodes a HDless isoform, suggesting that homeodomain-less variants of this gene family are evolutionarily ancient.

A group 13 homeodomain is neither necessary nor sufficient for posterior prevalence in the mouse limb

Posterior prevalence is the general property attributed to HOX proteins describing the dominant effect of more posterior HOX proteins over the function of anterior orthologs in common areas of expression. To explore the HOX group 13 protein domains required for this property, we used the mouse Prx-1 promoter to drive transgenic expression of Hox constructs throughout the entire limb bud during development. This system allowed us to conclusively demonstrate a hierarchy of Hox function in developing limbs. Furthermore, by substituting the HOXD11 or HOXA9 homeodomain for that of HOXD13, we show that a HOXD13 homeodomain is not necessary for posterior prevalence. Proximal expression of these chimeric proteins unexpectedly caused defects consistent with wild-type HOXD13 mediated posterior prevalence. Moreover, group 13 non-homeodomain residues appear to confer the property as proximal expression of HOXA9 containing the HOXD13 homeodomain did not result in limb reductions characteristic of HOXD13. These data are most compatible with models of posterior prevalence based on protein-protein interactions and support examination of the N-terminal non-homeodomain regions of Hox group 13 proteins as necessary agents for posterior prevalence.

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

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

Hox cofactors in vertebrate development

Hox genes encode homeodomain-containing transcription factors that pattern the body axes of animal embryos. It is well established that the exquisite DNA-binding specificity that allows different Hox proteins to specify distinct structures along the body axis is frequently dependent on interactions with other DNA-binding proteins which act as Hox cofactors. These include the PBC and MEIS classes of TALE (Three Amino acid Loop Extension) homeodomain proteins. The PBC class comprises fly Extradenticle (Exd) and vertebrate Pbx homeoproteins, whereas the MEIS class includes fly Homothorax (Hth) and vertebrate Meis and Prep homeoproteins. Exd was first implicated as a Hox cofactor based on mutant phenotypes in the fly. In vertebrates, PBC and MEIS homeobox proteins play important roles in development and disease. In this review, we describe the evidence that these functions reflect a requirement for Pbx and Meis/Prep proteins as Hox cofactors. However, there is mounting evidence that, like in the fly, Pbx and Meis/Prep proteins function more broadly, and we also discuss how "Hox cofactors" function as partners for other, non-Hox transcription factors during development. Conversely, we review the evidence that Hox proteins have functions that are independent of Pbx and Meis/Prep cofactors and discuss the possibility that other proteins may participate in the DNA-bound Hox complex, contributing to DNA-binding specificity in the absence of, or in addition to, Pbx and Meis/Prep.

Multiple roles of mesenchymal beta-catenin during murine limb patterning

Recently canonical Wnt signaling in the ectoderm has been shown to be required for maintenance of the apical ectodermal ridge (AER) and for dorsoventral signaling. Using conditional gain- and loss-of-function beta-catenin alleles, we have studied the role of mesenchymal beta-catenin activity during limb development. Here, we show that loss of beta-catenin results in limb truncations due to a defect in AER maintenance. Stabilization of beta-catenin also results in truncated limbs, caused by a premature regression of the AER. Concomitantly, in these limbs, the expression of Bmp2, Bmp4 and Bmp7, and of the Bmp target genes Msx1, Msx2 and gremlin, is expanded in the mesenchyme. Furthermore, we found that the expression of Lmx1b, a gene exclusively expressed in the dorsal limb mesenchyme and involved in dorsoventral patterning, is reduced upon loss of beta-catenin activity and is expanded ventrally in gain-of-function limbs. However, the known ectodermal regulators Wnt7a and engrailed 1 are expressed normally. This suggests that Lmx1b is also regulated, in part, by a beta-catenin-mediated Wnt signal, independent of the non-canoncial Wnt7a signaling pathway. In addition, loss of beta-catenin results in a severe agenesis of the scapula. Concurrently, the expression of two genes, Pax1 and Emx2, which have been implicated in scapula development, is lost in beta-catenin loss-of-function limbs; however, only Emx2 is upregulated in gain-of-function limbs. Mesenchymal beta-catenin activity is therefore required for AER maintenance, and for normal expression of Lmx1b and Emx2.

Role of retinoic acid receptors alpha1 and gamma in the response of murine limbs to retinol in vitro

BACKGROUND

Derivatives of retinol (vitamin A), commonly referred to as retinoids, signal through retinoic acid and retinoid X receptors (RARs/RXRs) and are essential for normal limb formation. Retinoid imbalances or perturbations in receptor function result in aberrant limb development. To examine the mechanisms underlying retinol-induced limb defects, we determined the responsiveness of limbs from RARalpha1-/-gamma mice to excess retinol in vitro.

METHODS

RARalpha1-/-gamma+/- mice were bred and their embryos were recovered at gestational day (GD) 12.5. The forelimbs were excised and cultured in vitro in the presence of all-trans retinol acetate (0, 1.25, 12.5, or 62.5 microM) for 6 days. The expression profiles of genes known to affect chondrogenesis (sox9 and col2a1) and limb outgrowth (meis1, meis2, and pbx1a) were examined by real-time qRT-PCR following retinol exposure for 3 hr.

RESULTS

Whereas RARalpha1-/-gamma+/+ and RARalpha1-/-gamma+/- limbs exhibited deleterious effects on limb outgrowth and chondrogenesis in the presence of exogenous retinol, this outcome was significantly attenuated in RARalpha1-/-gamma-/- limbs. The expressions of sox9 and col2a1 were significantly decreased in retinol-exposed RARalpha1-/-gamma+/+ limbs. In contrast, expression was not altered in limbs from their RARalpha1-/-gamma+/- or RARalpha1-/-gamma-/- littermates. Retinol exposure upregulated the expression of meis1 and meis2 in RARalpha1-/-gamma+/+ limbs; however, in RARalpha1-/-gamma-/- limbs the expression of both genes was unresponsive to retinol. Pbx1a remained unresponsive to retinol treatment in all genotypes.

CONCLUSION

In the absence of RARalpha1, RARgamma is a functionally important mediator of retinoid-induced limb dysmorphogenesis.

Global gene expression profiles reveal significant nuclear reprogramming by the blastocyst stage after cloning

Nuclear transfer (NT) has potential applications in agriculture and biomedicine, but the technology is hindered by low efficiency. Global gene expression analysis of clones is important for the comprehensive study of nuclear reprogramming. Here, we compared global gene expression profiles of individual bovine NT blastocysts with their somatic donor cells and fertilized control embryos using cDNA microarray technology. The NT embryos' gene expression profiles were drastically different from those of their donor cells and closely resembled those of the naturally fertilized embryos. Our findings demonstrate that the NT embryos have undergone significant nuclear reprogramming by the blastocyst stage; however, problems may occur during redifferentiation for tissue genesis and organogenesis, and small reprogramming errors may be magnified downstream in development.

Parallels between the proximal-distal development of vertebrate and arthropod appendages: homology without an ancestor?

Evolutionary studies suggest that the limbs of vertebrates and the appendages of arthropods do not share a common origin. However, recent genetic studies show new similarities in their developmental programmes. These similarities might be caused by the independent recruitment of homologous genes for similar functions or by the conservation of an ancestral proximal-distal development programme. This basic programme might have arisen in an ancestral outgrowth and been independently co-opted in vertebrate and arthropod appendages. It has subsequently diverged in both phyla to fine-pattern the limb and to control phylum-specific cellular events. We suggest that although vertebrate limbs and arthropod appendages are not strictly homologous structures they retain remnants of a common ancestral developmental programme.

Proximodistal identity during vertebrate limb regeneration is regulated by Meis homeodomain proteins

The mechanisms by which cells obtain instructions to precisely re-create the missing parts of an organ remain an unresolved question in regenerative biology. Urodele limb regeneration is a powerful model in which to study these mechanisms. Following limb amputation, blastema cells interpret the proximal-most positional identity in the stump to reproduce missing parts faithfully. Classical experiments showed the ability of retinoic acid (RA) to proximalize blastema positional values. Meis homeobox genes are involved in RA-dependent specification of proximal cell identity during limb development. To understand the molecular basis for specifying proximal positional identities during regeneration, we isolated the axolotl Meis homeobox family. Axolotl Meis genes are RA-regulated during both regeneration and embryonic limb development. During limb regeneration, Meis overexpression relocates distal blastema cells to more proximal locations, whereas Meis knockdown inhibits RA proximalization of limb blastemas. Meis genes are thus crucial targets of RA proximalizing activity on blastema cells.

Group 13 HOX proteins interact with the MH2 domain of R-Smads and modulate Smad transcriptional activation functions independent of HOX DNA-binding capability

Interactions with co-factors provide a means by which HOX proteins exert specificity. To identify candidate protein interactors of HOXA13, we created and screened an E11.5–E12.5, distal limb bud yeast two-hybrid prey library. Among the interactors, we isolated the BMP-signaling effector Smad5, which interacted with the paralogous HOXD13 but not with HOXA11 or HOXA9, revealing unique interaction capabilities of the AbdB-like HOX proteins. Using deletion mutants, we determined that the MH2 domain of Smad5 is necessary for HOXA13 interaction. This is the first report demonstrating an interaction between HOX proteins and the MH2 domain of Smad proteins. HOXA13 and HOXD13 also bind to other BMP and TGF-β/Activin-regulated Smad proteins including Smad1 and Smad2, but not Smad4. Furthermore, HOXD13 could be co-immunoprecipitated with Smad1 from cells. Expression of HOXA13, HOXD13 or a HOXD13 homeodomain mutant (HOXD13^IQN>AAA) antagonized TGF-β-stimulated transcriptional activation of the pAdtrack-3TP-Lux reporter vector in Mv1Lu cells as well as the Smad3/Smad4-activated pTRS₆-E1b promoter in Hep3B cells. Finally, using mammalian one-hybrid assay, we show that transcriptional activation by a GAL4/Smad3-C-terminus fusion protein is specifically inhibited by HOXA13. Our results identify a new co-factor for HOX group 13 proteins and suggest that HOX proteins may modulate Smad-mediated transcriptional activity through protein–protein interactions without the requirement for HOX monomeric DNA-binding capability.

Characterization of Mesenchyme Homeobox 2 (MEOX2) transcription factor binding to RING finger protein 10

The molecular mechanisms by which Mesenchyme Homeobox 2 (Meox2) regulates the proliferation, differentiation and migration of vascular smooth muscle cells and cardiomyocytes are not known. The discovery of MEOX2 binding proteins will aid in understanding how MEOX2 functions as a regulator of these key cellular processes. To identify MEOX2 binding proteins, a yeast two-hybrid screen of a human heart cDNA library was performed using a deleted MEOX2 bait protein that does not contain the histidine/glutamine rich region (MEOX2deltaHQ). Eleven putative interacting proteins were identified including RING finger protein 10 (RNF10). In vitro pull-down assays and co-immunoprecipitation studies in mammalian cells further supported the yeast data demonstrating RNF10 bound to MEOX2. The minimal RNF10 binding region of MEOX2 was determined to be a central region between the histidine/glutamine rich domain and the homeodomain (amino acids 101-185). The amino terminal region of RNF10, containing the RING finger domain, was not essential for the binding to MEOX2. Our results also demonstrated that MEOX2 activation of the p21WAF1 promoter was enhanced by RNF10 co-expression.

Comparative analysis of genes downstream of the Hoxd cluster in developing digits and external genitalia

Mammalian Hox genes encode transcription factors that are crucial for proper morphogenesis along the various body axes. Despite their extensive structural and functional characterization, the nature of their target genes remains elusive. We have addressed this question by using DNA microarrays to screen for genes whose expression in developing distal forelimbs and genital eminences was significantly modified in the absence of the full Hoxd gene complement. This comparative approach not only identified specific candidate genes, but also allowed the examination of whether a similar Hox expression pattern in distinct tissues leads to the modulation of the same or different downstream genes. We report here a set of potential target genes, most of which were not previously known to play a role in the early stages of either limb or genital bud development. Interestingly, we find that the majority of these candidate genes are differentially expressed in both structures,although often at different times. This supports the idea that both appendices involve similar genetic controls, both upstream and downstream of the Hox gene family. These results highlight the surprising mechanistic relationship between these rather different body parts and suggest a common developmental strategy to build up the most distal appendicular structures of the body, i.e. the digits and the penis/clitoris.

Early developmental arrest of mammalian limbs lacking HoxA/HoxD gene function

Vertebrate HoxA and HoxD cluster genes are required for proper limb development. However, early lethality, compensation and redundancy have made a full assessment of their function difficult. Here we describe mice that are lacking all Hoxa and Hoxd functions in their forelimbs. We show that such limbs are arrested early in their developmental patterning and display severe truncations of distal elements, partly owing to the absence of Sonic hedgehog expression. These results indicate that the evolutionary recruitment of Hox gene function into growing appendages might have been crucial in implementing hedgehog signalling, subsequently leading to the distal extension of tetrapod appendages. Accordingly, these mutant limbs may be reminiscent of an ancestral trunk extension, related to that proposed for arthropods.

Ultraconserved elements in insect genomes: a highly conserved intronic sequence implicated in the control of homothorax mRNA splicing

Recently, we identified a large number of ultraconserved (uc) sequences in noncoding regions of human, mouse, and rat genomes that appear to be essential for vertebrate and amniote ontogeny. Here, we used similar methods to identify ultraconserved genomic regions between the insect species Drosophila melanogaster and Drosophila pseudoobscura, as well as the more distantly related Anopheles gambiae. As with vertebrates, ultraconserved sequences in insects appear to occur primarily in intergenic and intronic sequences, and at intron-exon junctions. The sequences are significantly associated with genes encoding developmental regulators and transcription factors, but are less frequent and are smaller in size than in vertebrates. The longest identical, nongapped orthologous match between the three genomes was found within the homothorax (hth) gene. This sequence spans an internal exon-intron junction, with the majority located within the intron, and is predicted to form a highly stable stem-loop RNA structure. Real-time quantitative PCR analysis of different hth splice isoforms and Northern blotting showed that the conserved element is associated with a high incidence of intron retention in hth pre-mRNA, suggesting that the conserved intronic element is critically important in the post-transcriptional regulation of hth expression in Diptera.

The homeodomain protein Meis1 is essential for definitive hematopoiesis and vascular patterning in the mouse embryo

Homeodomain proteins of the Meis subfamily are expressed dynamically in several organs during embryogenesis and exert potent regulatory activity through their interaction with Hox proteins and other transcription factors. Here we show that Meis1 is expressed in the hematopoietic stem cell (HSC) compartment in the fetal liver, and in the primary sites of definitive hematopoiesis, including the aorta-gonad-mesonephros (AGM) mesenchyme, the hemogenic embryonic arterial endothelium, and hematopoietic clusters within the aorta, vitelline, and umbilical arteries. We inactivated the Meis1 gene in mice and found that Meis1 mutant mice die between embryonic days 11.5 and 14.5, showing internal hemorrhage, liver hypoplasia, and anemia. In Meis1 mutant mouse fetal liver and AGM, HSC compartments are severely underdeveloped and colony-forming potential is profoundly impaired. AGM mesenchymal cells expressing Runx1, an essential factor for definitive HSC specification, are almost absent in mutant mice. In addition, hematopoietic clusters in the dorsal aorta, vitelline, and umbilical arteries are reduced in size and number. These results show a requirement for Meis1 in the establishment of definitive hematopoiesis in the mouse embryo. Meis1 mutant mice also displayed complete agenesis of the megakaryocyte lineage and localized defects in vascular patterning, which may cause the hemorrhagic phenotype.

Range of HOX/TALE superclass associations and protein domain requirements for HOXA13:MEIS interaction

AbdB-like HOX proteins form DNA-binding complexes with the TALE superclass proteins MEIS1A and MEIS1B, and trimeric complexes have been identified in nuclear extracts that include a second TALE protein, PBX. Thus, soluble DNA-independent protein-protein complexes exist in mammals. The extent of HOX/TALE superclass interactions, protein structural requirements, and sites of in vivo cooperative interaction have not been fully explored. We show that Hoxa13 and Hoxd13 expression does not overlap with that of Meis1-3 in the developing limb; however, coexpression occurs in the developing male and female reproductive tracts (FRTs). We demonstrate that both HOXA13 and HOXD13 associate with MEIS1B in mammalian and yeast cells, and that HOXA13 can interact with all MEIS proteins but not more diverged TALE superclass members. In addition, the C-terminal domains (CTDs) of MEIS1A (18 amino acids) and MEIS1B (93 amino acids) are necessary for HOXA13 interaction; for MEIS1B, this domain was also sufficient. We also show by yeast two-hybrid assay that MEIS proteins can interact with anterior HOX proteins, but for some, additional N-terminal MEIS sequences are required for interaction. Using deletion mutants of HOXA13 and HOXD13, we provide evidence for multiple HOX peptide domains interacting with MEIS proteins. These data suggest that HOX:MEIS interactions may extend to non-AbdB-like HOX proteins in solution and that differences may exist in the MEIS peptide domains utilized by different HOX groups. Finally, the capability of multiple HOX domains to interact with MEIS C-terminal sequences implies greater complexity of the HOX:MEIS protein-protein interactions and a larger role for variation of HOX amino-terminal sequences in specificity of function.

Patterning function of homothorax/extradenticle in the thorax of Drosophila

In Drosophila, the morphological diversity is generated by the activation of different sets of active developmental regulatory genes in the different body subdomains. Here, we have investigated the role of the homothorax/extradenticle (hth/exd) gene pair in the elaboration of the pattern of the anterior mesothorax (notum). These two genes are active in the same regions and behave as a single functional unit. We find that their original uniform expression in the notum is downregulated during development and becomes restricted to two distinct, α and βsubdomains. This modulation appears to be important for the formation of distinct patterns in the two subdomains. The regulation of hth/exdexpression is achieved by the combined repressing functions of the Pax gene eyegone (eyg) and of the Dpp pathway. hth/exd is repressed in the body regions where eyg is active and that also contain high levels of Dpp activity. We also present evidence for a molecular interaction between the Hth and the Eyg proteins that may be important for the patterning of the α subdomain.

Ectopic expression of Hoxd10 in thoracic spinal segments induces motoneurons with a lumbosacral molecular profile and axon projections to the limb

Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS cord but not the thoracic cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in thoracic segments in chick embryos by means of in ovo electroporation. Regional motoneuron identity was assessed after the normal period of motoneuron differentiation. Subsets of motoneurons in transfected thoracic segments developed a molecular profile normally shown by LS motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior thoracic segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. At thoracic levels, we also found a decrease in motoneuron numbers and a reduction in gonad size. These last findings suggest that early and high levels of Hox expression impeded motoneuron development and neural-mesodermal interactions. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS region.

Bone Formation: Biological Aspects and Modelling Problems

In this work we succintly review the main features of bone formation in vertebrates. Out of the many aspects of this exceedingly complex process, some particular stages are selected for which mathematical modelling appears as both feasible and desirable. In this way, a number of open questions are formulated whose study seems to require interaction among mathematical analysis and biological experimentation.

Pbx3 deficiency results in central hypoventilation

Pbx proteins comprise a family of TALE (three amino acid loop extension) class homeodomain transcription factors that are implicated in developmental gene expression through their abilities to form hetero-oligomeric DNA-binding complexes and function as transcriptional regulators in numerous cell types. We demonstrate here that one member of this family, Pbx3, is expressed at high levels predominantly in the developing central nervous system, including a region of the medulla oblongata that is implicated in the control of respiration. Pbx3-deficient mice develop to term but die within a few hours of birth from central respiratory failure due to abnormal activity of inspiratory neurons in the medulla. This partially phenocopies the defect in mice deficient for Rnx, a metaHox homeodomain transcription factor, that we demonstrate here is capable of forming a DNA-binding complex with Pbx3. Rnx expression is unperturbed in Pbx3-deficient mice, but its ability to enhance transcription in vitro as a complex with TALE proteins is compromised in the absence of Pbx3. Thus, Pbx3 is essential for respiration and, like its DNA-binding partner Rnx, is critical for proper development of medullary respiratory control mechanisms. Pbx3-deficient mice provide a model for congenital central hypoventilation syndrome and suggest that Pbx3 mutations may promote the pathogenesis of this disorder.

Subcellular localization of multiple PREP2 isoforms is regulated by actin, tubulin, and nuclear export

The PREP, MEIS, and PBX families are mammalian members of the TALE (three amino acid loop extension) class of homeodomain-containing transcription factors. These factors have been implicated in cooperative DNA binding with the HOX class of homeoproteins, but PREP and MEIS interact with PBX in apparently non-HOX-dependent cooperative DNA binding as well. PREP, MEIS, and PBX have all been reported to reside in the cytoplasm in one or more tissues of the developing vertebrate embryo. In the case of PBX, cytoplasmic localization is due to the modulation of nuclear localization signals, nuclear export sequences, and interaction with a cytoplasmic anchoring factor, non-muscle myosin heavy chain II B. Here we report that murine PREP2 exists in multiple isoforms distinguished by interaction with affinity-purified antibodies raised to N- and C-terminal epitopes and by nuclear versus cytoplasmic localization. Alternative splicing gives rise to some of these PREP2 isoforms, including a 25-kDa variant lacking the C-terminal half of the protein and homeodomain and having the potential to act as dominant-negative. We further show that cytoplasmic localization is due to the concerted action of nuclear export, as evidenced by sensitivity to leptomycin B, and cytoplasmic retention by the actin and microtubule cytoskeletons. Cytoplasmic PREP2 colocalizes with both the actin and microtubule cytoskeletons and coimmunoprecipitates with actin and tubulin. Importantly, disruption of either cytoskeletal system redirects cytoplasmic PREP2 to the nucleus. We suggest that transcriptional regulation by PREP2 is modulated through the subcellular distribution of multiple isoforms and by interaction with two distinct cytoskeletal systems.

Retinoic Acid Synthesis Controlled by Raldh2 Is Required Early for Limb Bud Initiation and Then Later as a Proximodistal Signal during Apical Ectodermal Ridge Formation

We present evidence for the existence of two phases of retinoic acid (RA) signaling required for vertebrate limb development. Limb RA synthesis is under the control of retinaldehyde dehydrogenase-2 (Raldh2) expressed in the lateral plate mesoderm, which generates a proximodistal RA signal during limb outgrowth. We report that Raldh2(-/-) embryos lack trunk mesodermal RA activity and fail to initiate forelimb development. This is associated with deficient expression of important limb determinants Tbx5, Meis2, and dHand needed to establish forelimb bud initiation, proximal identity, and the zone of polarizing activity (ZPA), respectively. Limb expression of these genes can be rescued by maternal RA treatment limited to embryonic day 8 (E8) during limb field establishment, but the mutant forelimbs obtained at E10 display a significant growth defect associated with a smaller apical ectodermal ridge (AER), referred to here as an apical ectodermal mound (AEM). In these RA-deficient forelimbs, a ZPA expressing Shh forms, but it is located distally adjacent to the Fgf8 expression domain in the AEM rather than posteriorly as is normal. AER formation in Raldh2(-/-) forelimbs is rescued by continuous RA treatment through E10, which restores RA to distal ectoderm fated to become the AER. Our findings indicate the existence of an early phase of RA signaling acting upstream of Tbx5, Meis2, and dHand, followed by a late phase of RA signaling needed to expand AER structure fully along the distal ectoderm. During ZPA formation, RA acts early to activate expression of dHand, but it is not required later for Shh activation.

Regulation of retinoic acid distribution is required for proximodistal patterning and outgrowth of the developing mouse limb

Exogenous retinoic acid (RA) induces marked effects on limb patterning, but the precise role of endogenous RA in this process has remained unknown. We have studied the role of RA in mouse limb development by focusing on CYP26B1, a cytochrome P450 enzyme that inactivates RA. Cyp26b1 was shown to be expressed in the distal region of the developing limb bud, and mice that lack CYP26B1 exhibited severe limb malformation (meromelia). The lack of CYP26B1 resulted in spreading of the RA signal toward the distal end of the developing limb and induced proximodistal patterning defects characterized by expansion of proximal identity and restriction of distal identity. CYP26B1 deficiency also induced pronounced apoptosis in the developing limb and delayed chondrocyte maturation. Wild-type embryos exposed to excess RA phenocopied the limb defects of Cyp26b1(-/-) mice. These observations suggest that RA acts as a morphogen to determine proximodistal identity, and that CYP26B1 prevents apoptosis and promotes chondrocyte maturation, in the developing limb.

The mechanism of Drosophila leg development along the proximodistal axis

During development of higher organisms, most patterning events occur in growing tissues. Thus, unraveling the mechanism of how growing tissues are patterned into final morphologies has been an essential subject of developmental biology. Limb or appendage development in both vertebrates and invertebrates has attracted great attention from many researchers for a long time, because they involve almost all developmental processes required for tissue patterning, such as generation of the positional information by morphogen, subdivision of the tissue into distinct parts according to the positional information, localized cell growth and proliferation, and control of adhesivity, movement and shape changes of cells. The Drosophila leg development is a good model system, upon which a substantial amount of knowledge has been accumulated. In this review, the current understanding of the mechanism of Drosophila leg development is described.

Hematopoietic, angiogenic and eye defects in Meis1 mutant animals

Meis1 and Hoxa9 expression is upregulated by retroviral integration in murine myeloid leukemias and in human leukemias carrying MLL translocations. Both genes also cooperate to induce leukemia in a mouse leukemia acceleration assay, which can be explained, in part, by their physical interaction with each other as well as the PBX family of homeodomain proteins. Here we show that Meis1-deficient embryos have partially duplicated retinas and smaller lenses than normal. They also fail to produce megakaryocytes, display extensive hemorrhaging, and die by embryonic day 14.5. In addition, Meis1-deficient embryos lack well-formed capillaries, although larger blood vessels are normal. Definitive myeloerythroid lineages are present in the mutant embryos, but the total numbers of colony-forming cells are dramatically reduced. Mutant fetal liver cells also fail to radioprotect lethally irradiated animals and they compete poorly in repopulation assays even though they can repopulate all hematopoietic lineages. These and other studies showing that Meis1 is expressed at high levels in hematopoietic stem cells (HSCs) suggest that Meis1 may also be required for the proliferation/self-renewal of the HSC.

Prep1.1 has essential genetic functions in hindbrain development and cranial neural crest cell differentiation

In this study we analysed the function of the Meinox gene prep1.1 during zebrafish development. Meinox proteins form heterotrimeric complexes with Hox and Pbx members, increasing the DNA binding specificity of Hox proteins in vitro and in vivo. However, a role for a specific Meinox protein in the regulation of Hox activity in vivo has not been demonstrated. In situ hybridization showed that prep1.1 is expressed maternally and ubiquitously up to 24 hours post-fertilization (hpf), and restricted to the head from 48 hpf onwards. Morpholino-induced prep1.1 loss-of-function caused significant apoptosis in the CNS. Hindbrain segmentation and patterning was affected severely, as revealed by either loss or defective expression of several hindbrain markers (foxb1.2/mariposa, krox20, pax2.1 and pax6.1), including anteriorly expressed Hox genes (hoxb1a, hoxa2 and hoxb2), the impaired migration of facial nerve motor neurons, and the lack of reticulospinal neurons (RSNs) except Mauthner cells. Furthermore, the heads of prep1.1 morphants lacked all pharyngeal cartilages. This was not caused by the absence of neural crest cells or their impaired migration into the pharyngeal arches, as shown by expression of dlx2 and snail1, but by the inability of these cells to differentiate into chondroblasts. Our results indicate that prep1.1 has a unique genetic function in craniofacial chondrogenesis and, acting as a member of Meinox-Pbc-Hox trimers, it plays an essential role in hindbrain development.

Retinoic acid regulates the expression of PBX1, PBX2, and PBX3 in P19 cells both transcriptionally and post-translationally

Pre-B cell leukemia transcription factors (PBXs) are important co-factors for the transcriptional regulation mediated by a number of Hox proteins during embryonic development. It was previously shown that the expression of several Pbx genes is elevated in mouse embryo limb buds and embryonal carcinoma P19 cells upon retinoic acid (RA) treatment although the mechanism of this induction is not well understood. In this report, we demonstrate that PBX1a, PBX1b, PBX2, and PBX3 mRNAs and PBX1/2/3 proteins are induced during endodermal and neuronal differentiation of P19 cells in a RAR-dependent subtype-unspecific manner following RA treatment. The increases in both PBX1 mRNA and PBX3 mRNA levels are secondary responses to RA treatment requiring new proteins synthesis while the increase in PBX2 mRNA is a primary response. The RA-dependent increases in PBX1 mRNA, PBX2 mRNA, and PBX3 mRNA levels are likely to be transcriptionally regulated since the stability of these mRNAs does not change. In addition, the half-lives of PBX1/2/3 proteins are significantly extended by RA treatment. Two possible mechanisms could contribute to the stabilization of PBX proteins: PBX proteins associate with RA-dependent increased levels of MEIS proteins, and RA may decrease the proteasome dependent degradation of PBX proteins.

Ephrin-A2 regulates position-specific cell affinity and is involved in cartilage morphogenesis in the chick limb bud

In the developing limb bud, mesenchymal cells show position-specific affinity, suggesting that the positional identity of the cells is represented as their surface properties. Since the affinity is regulated by glycosylphosphatidylinositol (GPI)-anchored cell surface proteins, and by EphA4 receptor tyrosine kinase, we hypothesized that the GPI-anchored ligand, the ephrin-A family, also contributes to the affinity. Here, we describe the role of ephrin-A2 in the chick limb bud. Ephrin-A2 protein is uniformly distributed in the limb bud during early limb development. As the limb bud grows, expression of ephrin-A2 is strong in its proximal-to-intermediate regions, but weak distally. The position-dependent expression is maintained in vitro, and is regulated by FGF protein, which is produced in the apical ectodermal ridge. To investigate the role of ephrin-A2 in affinity and in cartilage morphogenesis of limb mesenchyme, we ectopically expressed ephrin-A2 in the limb bud using the retrovirus vector, RCAS. Overexpressed ephrin-A2 modulated the affinity of the mesenchymal cells that differentiate into autopod elements. It also caused malformation of the autopod skeleton and interfered with cartilage nodule formation in vitro without inhibiting chondrogenesis. These results suggest that ephrin-A2 regulates the position-specific affinity of limb mesenchyme and is involved in cartilage pattern formation in the limb.

Gene expression in spider appendages reveals reversal of exd/hth spatial specificity, altered leg gap gene dynamics, and suggests divergent distal morphogen signaling

Leg development in Drosophila has been studied in much detail. However, Drosophila limbs form in the larva as imaginal discs and not during embryogenesis as in most other arthropods. Here, we analyze appendage genes in the spider Cupiennius salei and the beetle Tribolium castaneum. Differences in decapentaplegic (dpp) expression suggest a different mode of distal morphogen signaling suitable for the specific geometry of growing limb buds. Also, expression of the proximal genes homothorax (hth) and extradenticle (exd) is significantly altered: in the spider, exd is restricted to the proximal leg and hth expression extends distally, while in insects, exd is expressed in the entire leg and hth is restricted to proximal parts. This reversal of spatial specificity demonstrates an evolutionary shift, which is nevertheless compatible with a conserved role of this gene pair as instructor of proximal fate. Different expression dynamics of dachshund and Distal-less point to modifications in the regulation of the leg gap gene system. We comment on the significance of this finding for attempts to homologize leg segments in different arthropod classes. Comparison of the expression profiles of H15 and optomotor-blind to the Drosophila patterns suggests modifications also in the dorsal-ventral patterning system of the legs. Together, our results suggest alterations in many components of the leg developmental system, namely proximal-distal and dorsal-ventral patterning, and leg segmentation. Thus, the leg developmental system exhibits a propensity to evolutionary change, which probably forms the basis for the impressive diversity of arthropod leg morphologies.

Nonmuscle myosin promotes cytoplasmic localization of PBX

In the absence of MEIS family proteins, two mechanisms are known to restrict the PBX family of homeodomain (HD) transcription factors to the cytoplasm. First, PBX is actively exported from the nucleus via a CRM1-dependent pathway. Second, nuclear localization signals (NLSs) within the PBX HD are masked by intramolecular contacts. In a screen to identify additional proteins directing PBX subcellular localization, we identified a fragment of murine nonmuscle myosin II heavy chain B (NMHCB). The interaction of NMHCB with PBX was verified by coimmunoprecipitation, and immunofluorescence staining revealed colocalization of NMHCB with cytoplasmic PBX in the mouse embryo distal limb bud. The interaction domain in PBX mapped to a conserved PBC-B region harboring a potential coiled-coil structure. In support of the cytoplasmic retention function, the NMHCB fragment competes with MEIS1A to redirect PBX, and the fly PBX homologue EXD, to the cytoplasm of mammalian and insect cells. Interestingly, MEIS1A also localizes to the cytoplasm in the presence of the NMHCB fragment. These activities are largely independent of nuclear export. We show further that the subcellular localization of EXD is deregulated in Drosophila zipper mutants that are depleted of nonmuscle myosin heavy chain. This study reveals a novel and evolutionarily conserved mechanism controlling the subcellular distribution of PBX and EXD proteins.

Patterning systems--from one end of the limb to the other

A combination of embryology and gene identification has led us to the current view of vertebrate limb development, in which a series of three interlocking patterning systems operate sequentially over time. This review describes current understanding of these regulatory mechanisms and how they form a framework for future analysis of limb patterning.

Distal antenna and distal antenna related encode nuclear proteins containing pipsqueak motifs involved in antenna development in Drosophila

Legs and antennae are considered to be homologous appendages. The fundamental patterning mechanisms that organize spatial pattern are conserved, yet appendages with very different morphology develop. A genetic hierarchy for specification of antennal identity has been partly elucidated. We report identification of a novel family of genes with roles in antennal development. The distal antenna (dan) and distal antenna-related (danr) genes encode novel nuclear proteins that are expressed in the presumptive distal antenna, but not in the leg imaginal disc. Ectopic expression of dan or danr causes partial transformation of distal leg structure toward antennal identity. Mutants that remove dan and danr activity cause partial transformation of antenna toward leg identity. Therefore we suggest that dan and danr contribute to differentiation of antenna-specific characteristics. Antenna-specific expression of dan and danr depends on a regulatory hierarchy involving homothorax and Distal-less, as well as cut and spineless. We propose that dan and danr are effector genes that act downstream of these genes to control differentiation of distal antennal structures.

PBX1 nuclear export is regulated independently of PBX-MEINOX interaction by PKA phosphorylation of the PBC-B domain

The regulation of PBC protein function through subcellular distribution is a crucial evolutionarily conserved mechanism for appendage patterning. We investigated the processes controlling PBX1 nuclear export. Here we show that in the absence of MEINOX proteins nuclear export is not a default pathway for PBX1 subcellular localization. In different cell backgrounds, PBX1 can be imported or exported from the nucleus independently of its capacity to interact with MEINOX proteins. The cell context-specific balance between nuclear export and import of PBX1 is controlled by the PBC-B domain, which contains several conserved serine residues corresponding to phosphorylation sites for Ser/Thr kinases. PBX1 subcellular localization correlates with the phosphorylation state of these residues whose dephosphorylation induces nuclear export. Protein kinase A (PKA) specifically phosphorylates PBX1 at these serines, and stimulation of endogenous PKA activity in vivo blocks PBX1 nuclear export in distal limb mesenchymal cells. Our results reveal a novel mechanism for the control of PBX1 nuclear export in addition to the absence of MEINOX protein, which involves the inhibition of PKA-mediated phosphorylation at specific sites within the PBC-B domain.

Distinct functions of homothorax in leg development in Drosophila

The Drosophila leg is subdivided into two mutually antagonistic proximal and distal domains. The proximal domain is defined by the activity of the homeobox genes homothorax and extradenticle and the distal one by the Dpp/Wg targets Distal-less (Dll) and dachshund (dac). It is known that hth/exd function prevents the activity of Dpp and Wg response genes and that cells deficient for exd activity in the proximal domain differentiate pattern elements corresponding to more distal leg regions. We report new results on the role of hth/exd antagonising the Dpp pathway. In cells expressing hth in the distal leg, there is a debilitation of the Dpp pathway which is reflected in lower levels of Mad phosphorylation and in increased levels of the receptor thick veins. Ectopic hth expression in the distal leg results in JNK-mediated apoptosis, decreased growth and pattern abnormalities. It also causes a general proximalisation of the appendage, which can be explained by interference with the Dpp and Wg pathways. We also report that the repression by hth/exd of the Dpp and Wg target Distal-less is not achieved at the level of transcription but preventing the activation of Dll target genes. We propose that hth/exd function contributes to the normal identity of proximal cells both by limiting the influence of the Dpp and Wg pathways and by activating proximal genes like teashirt (tsh) and aristaless (al).

PBX, MEIS, and IGF-I are potential mediators of retinoic acid-induced proximodistal limb reduction defects

BACKGROUND

Phocomelia, which is primarily due to a disruption in the proximodistal axis, is found in virtually all mouse embryos exposed to high doses of retinoic acid (RA) on 11 days post coitum (dpc).

METHODS

To identify genes that potentially mediate the effects of retinoic acid (RA) on limb development, we have examined the expression of 9,000 clones from the IMAGE consortium by microarray analysis of RNA isolated from 11 dpc mouse forelimbs exposed to RA or vehicle for 6 hr. Eight genes that demonstrated altered expression were chosen for further study of their mRNA levels using RT-PCR. Protein levels were determined by Western blot analysis.

RESULTS

Of the 9,000 genes examined in the microarray, approximately 111 demonstrated altered expression (33 known genes and 78 ESTs). Of the eight known genes chosen for further study using RT-PCR, four mRNAs (PBX1a, PBX1b, IGF-Ia, and IGF-Ib) demonstrated consistent elevation ( approximately 3-fold) in their levels after RA treatment in both the forelimbs and hindlimbs as early as 3 hr after RA treatment. In addition to the two PBX1 isoforms, the mRNA level of the other two subtypes (PBX2 and PBX3) and the level of PBX1/2/3 protein were also found to be elevated in limb buds after RA treatment. Finally, we examined the expression of MEIS1, MEIS2, and MEIS3 because these proteins are necessary for PBX nuclear localization. The mRNA level of all three subtypes of MEIS were elevated approximately three- to four-fold in both the forelimbs and hindlimbs after RA treatment.

CONCLUSIONS

Because both PBX and MEIS (and their orthologs) are believed to be involved in the control of proximodistal axis formation in mouse and fly limbs and IGFs in the development of limbs, we suggest that increases in PBX, MEIS and IGF-1 mRNA levels may contribute to proximodistal limb reduction defects caused by teratogenic doses of RA.

Molecular basis of vertebrate limb patterning

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

The newt ortholog of CD59 is implicated in proximodistal identity during amphibian limb regeneration

The proximodistal identity of a newt limb regeneration blastema is respecified by exposure to retinoic acid, but its molecular basis is unclear. We identified from a differential screen the cDNA for Prod 1, a gene whose expression in normal and regenerating limbs is regulated by proximodistal location and retinoic acid: Prod 1 is the newt ortholog of CD59. Prod 1/CD59 was found to be located at the cell surface with a GPI anchor which is cleaved by PIPLC. A proximal newt limb blastema engulfs a distal blastema after juxtaposition in culture, and engulfment is specifically blocked by PIPLC, and by affinity-purified antibodies to two distinct Prod 1/CD59 peptides. Prod 1 is therefore a cell surface protein implicated in the local cell-cell interactions mediating positional identity.