Hox patterning of the vertebrate rib cage

Axial Hox9 activity establishes the posterior field in the developing forelimb

Current models hold that the early limb field becomes polarized into anterior and posterior domains by the opposing activities of Hand2 and Gli3. This polarization is essential for the initiation of Shh expression in the posterior margin of the limb bud, but how this polarity is established is not clear. Here we show that initial anteroposterior polarization of the early forelimb field requires the function of all four Hox9 paralogs (Hoxa9, Hoxb9, Hoxc9, and Hoxd9). This is unexpected, given that only HoxA and HoxD AbdB group genes have been shown to play a role in forelimb patterning, regulating the activation and maintenance of Shh expression and subsequent proximal-distal patterning of the forelimb. Our analysis of Hox9 quadruple mutants demonstrates that Hox9 function is required for the expression of Hand2 in the posterior limb field. Subsequently, Gli3 expression is not repressed posteriorly, Shh expression is not initiated, and collinear expression of HoxA/D10-13 is not established, resulting in severely malformed forelimbs lacking all posterior, Shh-regulated elements. This Hox9 mutant phenotype is restricted to the forelimbs; mutant hindlimbs are normal, revealing fundamental differences in the patterning mechanisms governing the establishment of forelimb and hindlimb fields.

Hox11 genes establish synovial joint organization and phylogenetic characteristics in developing mouse zeugopod skeletal elements

Hox11 genes are essential for zeugopod skeletal element development but their roles in synovial joint formation remain largely unknown. Here, we show that the elbow and knee joints of mouse embryos lacking all Hox11 paralogous genes are specifically remodeled and reorganized. The proximal ends of developing mutant ulna and radius elements became morphologically similar and formed an anatomically distinct elbow joint. The mutant ulna lacked the olecranon that normally attaches to the triceps brachii muscle tendon and connects the humerus to the ulna. In its place, an ulnar patella-like element developed that expressed lubricin on its ventral side facing the joint and was connected to the triceps muscle tendon. In mutant knees, both tibia and fibula fully articulated with an enlarged femoral epiphyseal end that accommodated both elements, and the neo-tripartite knee joint was enclosed in a single synovial cavity and displayed an additional anterior ligament. The mutant joints also exhibited a different organization of the superficial zone of articular cartilage that normally exerts an anti-friction function. In conclusion, Hox11 genes co-regulate and coordinate the development of zeugopod skeletal elements and adjacent elbow and knee joints, and dictate joint identity, morphogenesis and anatomical and functional organization. Notably, the ulnar patella and tripartite knee joints in the mouse mutants actually characterize several lower vertebrates, including certain reptiles and amphibians. The re-emergence of such anatomical structures suggests that their genetic blueprint is still present in the mouse genome but is normally modified to the needs of the mammalian joint-formation program by distinct Hox11 function.

Skeletal development in sloths and the evolution of mammalian vertebral patterning

Mammals show a very low level of variation in vertebral count, particularly in the neck. Phenotypes exhibited at various stages during the development of the axial skeleton may play a key role in testing mechanisms recently proposed to explain this conservatism. Here, we provide osteogenetic data that identify developmental criteria with which to recognize cervical vs. noncervical vertebrae in mammals. Except for sloths, all mammals show the late ossification of the caudal-most centra in the neck after other centra and neural arches. In sloths with 8-10 ribless neck vertebrae, the caudal-most neck centra ossify early, matching the pattern observed in cranial thoracic vertebrae of other mammals. Accordingly, we interpret the ribless neck vertebrae of three-toed sloths caudal to V7 as thoracic based on our developmental criterion. Applied to the unusual vertebral phenotype of long-necked sloths, these data support the interpretation that elements of the axial skeleton with origins from distinct mesodermal tissues have repatterned over the course of evolution.

Global control of motor neuron topography mediated by the repressive actions of a single hox gene

In the developing spinal cord, regional and combinatorial activities of Hox transcription factors are critical in controlling motor neuron fates along the rostrocaudal axis, exemplified by the precise pattern of limb innervation by more than fifty Hox-dependent motor pools. The mechanisms by which motor neuron diversity is constrained to limb levels are, however, not well understood. We show that a single Hox gene, Hoxc9, has an essential role in organizing the motor system through global repressive activities. Hoxc9 is required for the generation of thoracic motor columns, and in its absence, neurons acquire the fates of limb-innervating populations. Unexpectedly, multiple Hox genes are derepressed in Hoxc9 mutants, leading to motor pool disorganization and alterations in the connections by thoracic and forelimb-level subtypes. Genome-wide analysis of Hoxc9 binding suggests that this mode of repression is mediated by direct interactions with Hox regulatory elements, independent of chromatin marks typically associated with repressed Hox genes.

Growth differentiation factor 11 signaling controls retinoic acid activity for axial vertebral development

Mice deficient in growth differentiation factor 11 (GDF11) signaling display anterior transformation of axial vertebrae and truncation of caudal vertebrae. However, the in vivo molecular mechanisms by which GDF11 signaling regulates the development of the vertebral column have yet to be determined. We found that Gdf11 and Acvr2b mutants are sensitive to exogenous RA treatment on vertebral specification and caudal vertebral development. We show that diminished expression of Cyp26a1, a retinoic acid inactivating enzyme, and concomitant elevation of retinoic acid activity in the caudal region of Gdf11(-/-) embryos may account for this phenomenon. Reduced expression or function of Cyp26a1 enhanced anterior transformation of axial vertebrae in wild-type and Acvr2b mutants. Furthermore, a pan retinoic acid receptor antagonist (AGN193109) could lessen the anterior transformation phenotype and rescue the tail truncation phenotype of Gdf11(-/-) mice. Taken together, these results suggest that GDF11 signaling regulates development of caudal vertebrae and is involved in specification of axial vertebrae in part by maintaining Cyp26a1 expression, which represses retinoic acid activity in the caudal region of embryos during the somitogenesis stage.

Predicting embryonic patterning using mutual entropy fitness and in silico evolution

During vertebrate embryogenesis, the expression of Hox genes that define anterior-posterior identity follows general rules: temporal colinearity and posterior prevalence. A mathematical measure for the quality or fitness of the embryonic pattern produced by a gene regulatory network is derived. Using this measure and in silico evolution we derive gene interaction networks for anterior-posterior (AP) patterning under two developmental paradigms. For patterning during growth (paradigm I), which is appropriate for vertebrates and short germ-band insects, the algorithm creates gene expression patterns reminiscent of Hox gene expression. The networks operate through a timer gene, the level of which measures developmental progression (a candidate is the widely conserved posterior morphogen Caudal). The timer gene provides a simple mechanism to coordinate patterning with growth rate. The timer, when expressed as a static spatial gradient, functions as a classical morphogen (paradigm II), providing a natural way to derive the AP patterning, as seen in long germ-band insects that express their Hox genes simultaneously, from the ancestral short germ-band system. Although the biochemistry of Hox regulation in higher vertebrates is complex, the actual spatiotemporal expression phenotype is not, and simple activation and repression by Hill functions suffices in our model. In silico evolution provides a quantitative demonstration that continuous positive selection can generate complex phenotypes from simple components by incremental evolution, as Darwin proposed.

Microarray Analysis of Defective Cartilage in Hoxc8- and Hoxd4-Transgenic Mice

OBJECTIVE

Homeobox genes of the Hox class are required for proper patterning of skeletal elements and play a role in cartilage differentiation. In transgenic mice with overexpression of Hoxc8 and Hoxd4 during cartilage development, the authors observed severe defects, namely, physical instability of cartilage, accumulation of immature chondrocytes, and decreased maturation to hypertrophy. To define the molecular basis underlying these defects, the authors performed gene expression profiling using the Affymetrix microarray platform.

RESULTS

Primary chondrocytes were isolated from Hoxc8- and Hoxd4-transgenic mouse embryo rib cartilage at 18.5 days of gestation. In both cases, differentially expressed genes were identified that have a role in cell proliferation and cell cycle regulation. A comparison between the controls for both experimental groups did not reveal significant differences, as expected. However, the repertoires of differentially expressed genes were found not to overlap between Hoxc8- and Hoxd4-transgenic cartilage. This included different Wnt genes, cell cycle, and apoptosis regulators.

CONCLUSION

Overexpression of Hoxc8 and Hoxd4 transcription factors alters transcriptional profiles in chondrocytes at E18.5. The differences in repertoires of altered gene expression between the 2 transgenic conditions suggest that the molecular mechanisms underlying the cartilage defects may be different in both transgenic paradigms, despite apparently similar phenotypes.

Hox genes and regional patterning of the vertebrate body plan

Several decades have passed since the discovery of Hox genes in the fruit fly Drosophila melanogaster. Their unique ability to regulate morphologies along the anteroposterior (AP) axis (Lewis, 1978) earned them well-deserved attention as important regulators of embryonic development. Phenotypes due to loss- and gain-of-function mutations in mouse Hox genes have revealed that the spatio-temporally controlled expression of these genes is critical for the correct morphogenesis of embryonic axial structures. Here, we review recent novel insight into the modalities of Hox protein function in imparting specific identity to anatomical regions of the vertebral column, and in controlling the emergence of these tissues concomitantly with providing them with axial identity. The control of these functions must have been intimately linked to the shaping of the body plan during evolution.

Additive and global functions of HoxA cluster genes in mesoderm derivatives

Hox genes encode transcription factors that play a central role in the specification of regional identities along the anterior to posterior body axis. In the developing mouse embryo, Hox genes from all four genomic clusters are involved in range of developmental processes, including the patterning of skeletal structures and the formation of several organs. However, the functional redundancy observed either between paralogous genes, or among neighboring genes from the same cluster, has hampered functional analyses, in particular when synergistic, cluster-specific functions are considered. Here, we report that mutant mice lacking the entire HoxA cluster in mesodermal lineages display the expected spectrum of postnatal respiratory, cardiac and urogenital defects, previously reported for single gene mutations. Likewise, mild phenotypes are observed in both appendicular and axial skeleton. However, a striking effect was uncovered in the hematopoietic system, much stronger than that seen for Hoxa9 inactivation alone, which involves stem cells (HSCs) as well as the erythroid lineage, indicating that several Hoxa genes are necessary for normal hematopoiesis to occur. Finally, the combined deletions of Hoxa and Hoxd genes reveal abnormalities in axial elongation as well as skin morphogenesis that are likely the results of defects in epithelial-mesenchymal interactions.

Targeting HOX and PBX transcription factors in ovarian cancer

BACKGROUND

Ovarian cancer still has a relatively poor prognosis due to the frequent occurrence of drug resistance, making the identification of new therapeutic targets an important goal. We have studied the role of HOX genes in the survival and proliferation of ovarian cancer cells. These are a family of homeodomain-containing transcription factors that determine cell and tissue identity in the early embryo, and have an anti-apoptotic role in a number of malignancies including lung and renal cancer.

METHODS

We used QPCR to determine HOX gene expression in normal ovary and in the ovarian cancer cell lines SK-OV3 and OV-90. We used a short peptide, HXR9, to disrupt the formation of HOX/PBX dimers and alter transcriptional regulation by HOX proteins.

RESULTS

In this study we show that the ovarian cancer derived line SK-OV3, but not OV-90, exhibits highly dysregulated expression of members of the HOX gene family. Disrupting the interaction between HOX proteins and their co-factor PBX induces apoptosis in SK-OV3 cells and retards tumour growth in vivo.

CONCLUSION

HOX/PBX binding is a potential target in ovarian cancer.

Hox genes and segmentation of the hindbrain and axial skeleton

Segmentation is an important process that is frequently used during development to segregate groups of cells with distinct features. Segmental compartments provide a mechanism for generating and organizing regional properties along an embryonic axis and within tissues. In vertebrates the development of two major systems, the hindbrain and the paraxial mesoderm, displays overt signs of compartmentalization and depends on the process of segmentation for their functional organization. The hindbrain plays a key role in regulating head development, and it is a complex coordination center for motor activity, breathing rhythms, and many unconscious functions. The paraxial mesoderm generates somites, which give rise to the axial skeleton. The cellular processes of segmentation in these two systems depend on ordered patterns of Hox gene expression as a mechanism for generating a combinatorial code that specifies unique identities of the segments and their derivatives. In this review, we compare and contrast the signaling inputs and transcriptional mechanisms by which Hox gene regulatory networks are established during segmentation in these two different systems.

Control of vertebrate Hox clusters by remote and global cis-acting regulatory sequences

Despite apparently shared structural organisation and functional roles, vertebrate Hox genes are controlled by regulatory mechanisms rather distinct from those of the prototypic Drosophila Antennapedia (ANT-C) and Bithorax (BX-C) Complexes. If individual regulatory modules have been shown to recapitulate specific Hox expression patterns, other experimental studies underscore that vertebrate Hox clusters are controlled in many of their functions in a global manner, through distinct mechanisms. We will discuss the different models that have been proposed to account for these global regulatory modes. In this context, the studies of the regulation of the HoxD complex during limb development highlighted the role of global regulatory elements and the different mechanisms associated to transform a structural organisation into distinct temporal and spatial expression domains. We will further discuss how these mechanisms may have benefited from the structure of the vertebrate homeotic clusters and reciprocally contribute to shape their evolution towards an increased level of organisation and compaction.

Vertebrate skeletogenesis

Vertebrate skeletogenesis consists in elaborating an edifice of more than 200 pieces of bone and cartilage. Each skeletal piece is crafted at a distinct location in the body, is articulated with others, and reaches a specific size, shape, and tissue composition according to both species instructions and individual determinants. This complex, customized body frame fulfills multiple essential tasks. It confers morphological features, allows controlled postures and movements, protects vital organs, houses hematopoiesis, stores minerals, and adsorbs toxins. This review provides an overview of the multiple facets of this ingenious process for experts as well as nonexperts of skeletogenesis. We explain how the developing vertebrate uses both specific and ubiquitously expressed genes to generate multipotent mesenchymal cells, specify them to a skeletogenic fate, control their survival and proliferation, and direct their differentiation into cartilage, bone, and joint cells. We review milestone discoveries made toward uncovering the intricate networks of regulatory factors that are involved in these processes, with an emphasis on signaling pathways and transcription factors. We describe numerous skeletal malformation and degeneration diseases that occur in humans as a result of mutations in regulatory genes, and explain how these diseases both help and motivate us to further decipher skeletogenic processes. Upon discussing current knowledge and gaps in knowledge in the control of skeletogenesis, we highlight ultimate research goals and propose research priorities and approaches for future endeavors.

The lateral somitic frontier in ontogeny and phylogeny

The vertebrate musculoskeletal system comprises the axial and appendicular systems. The postcranial axial system consists of the vertebrae, ribs and associated muscles, and the appendicular system comprises the muscles and skeleton of the paired appendages and their respective girdles. The morphology, proportions, and arrangements of these parts have undergone tremendous variation during vertebrate history. Despite this vertebrate diversity, the cells that form all of the key parts of the musculoskeletal system during development arise from two populations of embryonic mesoderm, the somites and somatic lateral plate. Nowicki et al. (2003. Mech Dev 120:227-240) identified two dynamic domains in the developing chick embryo. The primaxial domain is populated exclusively by cells from the somites. The abaxial domain includes muscle and bone that develop within lateral plate-derived connective tissue. The boundary between the two domains is the lateral somitic frontier. We hypothesize that the primaxial and abaxial domains are patterned independently and that morphological evolution of the musculoskeletal system is facilitated by partially independent developmental changes in the abaxial and primaxial domain. Here we present our hypothesis in detail and review recent experimental and comparative studies that use the concept of the lateral somitic frontier in the analysis of the evolution of the highly derived chelonian and limbless squamate body plans.

Non-homeodomain regions of Hox proteins mediate activation versus repression of Six2 via a single enhancer site in vivo

Hox genes control many developmental events along the AP axis, but few target genes have been identified. Whether target genes are activated or repressed, what enhancer elements are required for regulation, and how different domains of the Hox proteins contribute to regulatory specificity are poorly understood. Six2 is genetically downstream of both the Hox11 paralogous genes in the developing mammalian kidney and Hoxa2 in branchial arch and facial mesenchyme. Loss-of-function of Hox11 leads to loss of Six2 expression and loss-of-function of Hoxa2 leads to expanded Six2 expression. Herein we demonstrate that a single enhancer site upstream of the Six2 coding sequence is responsible for both activation by Hox11 proteins in the kidney and repression by Hoxa2 in the branchial arch and facial mesenchyme in vivo. DNA-binding activity is required for both activation and repression, but differential activity is not controlled by differences in the homeodomains. Rather, protein domains N- and C-terminal to the homeodomain confer activation versus repression activity. These data support a model in which the DNA-binding specificity of Hox proteins in vivo may be similar, consistent with accumulated in vitro data, and that unique functions result mainly from differential interactions mediated by non-homeodomain regions of Hox proteins.

Anatomical transformation in mammals: developmental origin of aberrant cervical anatomy in tree sloths

Mammalian cervical count has been fixed at seven for more than 200 million years. The rare exceptions to this evolutionary constraint have intrigued anatomists since the time of Cuvier, but the developmental processes that generate them are unknown. Here we evaluate competing hypotheses for the evolutionary origin of cervical variants in Bradypus and Choloepus, tree sloths that have broken the seven cervical vertebrae barrier independently and in opposite directions. Transitional and mediolaterally disjunct anatomy characterizes the cervicothoracic vertebral boundary in each genus, although polarities are reversed. The thoracolumbar, lumbosacral, and sacrocaudal boundaries are also disrupted, and are more extreme in individuals with more extreme cervical counts. Hypotheses of homologous, homeotic, meristic, or associational transformations of traditional vertebral column anatomy are not supported by these data. We identify global homeotic repatterning of abaxial relative to primaxial mesodermal derivatives as the origin of the anomalous cervical counts of tree sloths. This interpretation emphasizes the strong resistance of the "rule of seven" to evolutionary change, as morphological stasis has been maintained primaxially coincident with the generation of a functionally longer (Bradypus) or shorter (Choloepus) neck.

A distinct role for secreted fibroblast growth factor-binding proteins in development

FGFs modulate diverse biological processes including embryonic development. Secreted FGF-binding proteins (BPs) can release FGFs from their local extracellular matrix storage, chaperone them to their cognate receptors, and thus modulate FGF signaling. Here we describe 2 chicken BP homologs (chBP) that show distinct expression peaks at embryonic days E7.5 (chBP2) and E11.5 (chBP1), although their tissue distribution is similar (skin = intestine>lung>heart, liver). Embryos were grown ex ovo to monitor the phenotypic impact of a timed in vivo knockdown of expression peaks by microinjection of specific siRNAs targeted to either of the chBPs. Knockdown of peak expression of chBP2 caused embryonic lethality within <5 days. Surviving embryos showed defective ventral wall closure indicative of altered dorsoventral patterning. This defect coincided with reduced expression of HoxB7 but not HoxB8 that are involved in the control of thoracic/abdominal segment morphology. Also, MAPK phosphatase 3, a negative regulator of FGF signaling, and sonic hedgehog that can participate in feedback control of the FGF pathway were reduced, reflecting altered FGF signaling. Knockdown of the chBP1 expression peak caused embryonic lethality within <3 days although no distinct morphologic phenotype or pathways alterations were apparent. We conclude that BPs play a significant role in fine-tuning the complex FGF signaling network during distinct phases of embryonic development.

Axial patterning in snakes and caecilians: evidence for an alternative interpretation of the Hox code

It is generally assumed that the characteristic deregionalized body plan of species with a snake-like morphology evolved through a corresponding homogenization of Hox gene expression domains along the primary axis. Here, we examine the expression of Hox genes in snake embryos and show that a collinear pattern of Hox expression is retained within the paraxial mesoderm of the trunk. Genes expressed at the anterior and most posterior, regionalized, parts of the skeleton correspond to the expected anatomical boundaries. Unexpectedly however, also the dorsal (thoracic), homogenous rib-bearing region of trunk, is regionalized by unconventional gradual anterior limits of Hox expression that are not obviously reflected in the skeletal anatomy. In the lateral plate mesoderm we also detect regionalized Hox expression yet the forelimb marker Tbx5 is not restricted to a rudimentary forelimb domain but is expressed throughout the entire flank region. Analysis of several Hox genes in a caecilian amphibian, which convergently evolved a deregionalized body plan, reveals a similar global collinear pattern of Hox expression. The differential expression of posterior, vertebra-modifying or even rib-suppressing Hox genes within the dorsal region is inconsistent with the homogeneity in vertebral identity. Our results suggest that the evolution of a deregionalized, snake-like body involved not only alterations in Hox gene cis-regulation but also a different downstream interpretation of the Hox code.

[Hoxa5: a master gene with multifaceted roles]

The Hox gene family occupies a central position in the control of body patterning by regulating the transcription of downstream effectors that, in turn, direct the morphogenetic events leading to the complex body forms along the axes. Analysis of Hox mutant mouse lines has revealed a panoply of phenotypes indicative of the broad range of Hox genes action throughout embryonic and postnatal life. Although Hox genes have been the subject of extensive research in the last two decades, the comprehension of the mechanisms involved in their regulation and function still remains elusive. Here, we present an overview of our current knowledge about one Hox gene family member, Hoxa5. The phenotypic survey of Hoxa5 mutant mice has unveiled the crucial role of this gene in regulating morphogenesis and specifying regional identity along the embryo. A majority of Hoxa5 mutant pups die at birth from defective respiratory tract. Surviving mutants present deficient alveolar septation revealing the importance of Hoxa5 during formation and maturation of the lung. Hoxa5 also participates in the morphogenesis of the digestive tract as well as that of the thyroid and mammary glands. Hoxa5 expression is restricted to the mesenchyme, and its action appears to be mediated through the control of mesenchymal-epithelial interactions during organogenesis. The implication of Hoxa5 in tumorigenesis has also been documented. In breast cancer, Hoxa5 down-regulation may impact on p53 gene expression, contributing to the oncogenic process. In contrast, the loss of Hoxa5 function limits leukaemia associated with specific chromosomal translocations. Thus, inappropriate Hoxa5 gene expression may disrupt normal growth and differentiation programs causing neoplasia. Hox gene function is intimately linked to its correct expression. Regulation of Hoxa5 expression requires multiple cis-acting regions, some encompassing coding sequences from neighboring genes. Moreover, it is complicated by the presence of several transcription units. Together these data enlighten the importance of Hox cluster organization in Hoxa5 function.

Regulation of Hoxb4 induction after neurulation by somite signal and neural competence

BACKGROUND

While the body axis is largely patterned along the anterior-posterior (A-P) axis during gastrulation, the central nervous system (CNS) shows dynamic changes in the expression pattern of Hox genes during neurulation, suggesting that the CNS refines the A-P pattern continuously after neural tube formation. This study aims at clarifying the role of somites in up-regulating Hoxb4 expression to eventually establish its final pattern and how the neural tube develops a competence to respond to extrinsic signals.

RESULTS

We show that somites are required for the up-regulation of Hoxb4 in the neural tube at the level of somites 1 to 5, the anterior-most domain of expression. However, each somite immediately adjacent to the neural tube is not sufficient at each level; planar signaling is additionally required particularly at the anterior-most segments of the expression domain. We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals. BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.

CONCLUSION

In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

Generation and expression of a Hoxa11eGFP targeted allele in mice

Hox genes are crucial for body axis specification during embryonic development. Hoxa11 plays a role in anteroposterior patterning of the axial skeleton, development of the urogenital tract of both sexes, and proximodistal patterning of the limbs. Hoxa11 expression is also observed in the neural tube. Herein, we report the generation of a Hoxa11eGFP targeted knock-in allele in mice in which eGFP replaces the first coding exon of Hoxa11 as an in-frame fusion. This allele closely recapitulates the reported mRNA expression patterns for Hoxa11. Hoxa11eGFP can be visualized in the tail, neural tube, limbs, kidneys, and reproductive tract of both sexes. Additionally, homozygous mutants recapitulate reported phenotypes for Hoxa11 loss of function mice, exhibiting loss of fertility in both males and females. This targeted mouse line will prove useful as a vital marker for Hoxa11 protein localization during control (heterozygous) or mutant organogenesis.

HOX transcription factors are potential therapeutic targets in non-small-cell lung cancer (targeting HOX genes in lung cancer)

The HOX genes are a family of homeodomain-containing transcription factors that determine the identity of cells and tissues during embryonic development. They are also known to behave as oncogenes in some haematological malignancies. In this study, we show that the expression of many of the HOX genes is highly elevated in primary non-small-cell lung cancers (NSCLCs) and in the derived cell lines A549 and H23. Furthermore, blocking the activity of HOX proteins by interfering with their binding to the PBX co-factor causes these cells to undergo apoptosis in vitro and reduces the growth of A549 tumours in vivo. These findings suggest that the interaction between HOX and PBX proteins is a potential therapeutic target in NSCLC.

Hoxb5b acts downstream of retinoic acid signaling in the forelimb field to restrict heart field potential in zebrafish

How adjacent organ fields communicate during development is not understood. Here, we identify a mechanism in which signaling within the forelimb field restricts the potential of the neighboring heart field. In zebrafish embryos deficient in retinoic acid (RA) signaling, the pectoral fins (forelimbs) are lost while both chambers of the heart are enlarged. We provide evidence that both of these phenotypes are due to RA signaling acting directly within the forelimb field. hoxb5b, an RA-responsive gene expressed within the forelimb field, is required to restrict the number of atrial cells arising from the adjacent heart field, although its function is dispensable for forelimb formation. Together, these data indicate nonautonomous influences downstream of RA signaling that act to limit individual chamber size. Therefore, our results offer new perspectives on the mechanisms regulating organ size and the possible causes of congenital syndromes affecting both the heart and forelimb.

Establishment of Hox vertebral identities in the embryonic spine precursors

The vertebrate spine exhibits two striking characteristics. The first one is the periodic arrangement of its elements-the vertebrae-along the anteroposterior axis. This segmented organization is the result of somitogenesis, which takes place during organogenesis. The segmentation machinery involves a molecular oscillator-the segmentation clock-which delivers a periodic signal controlling somite production. During embryonic axis elongation, this signal is displaced posteriorly by a system of traveling signaling gradients-the wavefront-which depends on the Wnt, FGF, and retinoic acid pathways. The other characteristic feature of the spine is the subdivision of groups of vertebrae into anatomical domains, such as the cervical, thoracic, lumbar, sacral, and caudal regions. This axial regionalization is controlled by a set of transcription factors called Hox genes. Hox genes exhibit nested expression domains in the somites which reflect their linear arrangement along the chromosomes-a property termed colinearity. The colinear disposition of Hox genes expression domains provides a blueprint for the regionalization of the future vertebral territories of the spine. In amniotes, Hox genes are activated in the somite precursors of the epiblast in a temporal colinear sequence and they were proposed to control their progressive ingression into the nascent paraxial mesoderm. Consequently, the positioning of the expression domains of Hox genes along the anteroposterior axis is largely controlled by the timing of Hox activation during gastrulation. Positioning of the somitic Hox domains is subsequently refined through a crosstalk with the segmentation machinery in the presomitic mesoderm. In this review, we focus on our current understanding of the embryonic mechanisms that establish vertebral identities during vertebrate development.

Hox, Cdx, and anteroposterior patterning in the mouse embryo

Cdx and Hox gene families descend from the same ProtoHox cluster, already present in the common ancestors of bilaterians and cnidarians, and thought to act by providing anteroposterior (A-P) positional identity to axial tissues in all bilaterians. Mouse Cdx and Hox genes still exhibit common features in their early expression and function. The initiation and early shaping of Hox and Cdx transcriptional domains in mouse embryos are very similar, in keeping with their common involvement in conveying A-P information to the nascent tissues during embryonic axial elongation. Considerations of the impact on axial patterning of the early expression phase of these genes that correlates with the temporally collinear expression of 3'-5'Hox genes suggest that it is concerned with the acquisition of A-P information by the three germ layers as the axis extends. This early A-P information acquired by all cells emerging from the primitive streak or tailbud and their neighbors in the caudal neural plate gets further modulated by the second phase of gene expression occurring later as the tissues mature and differentiate along the growing axis. We discuss the possibility that regulatory phase 1, common to all Cdx and Hox genes, is inherent to the concerted mechanism sequentially turning on 3'-5'Hox genes at early stages, and keeping expression of the initiated genes subsequently in the new materials added posteriorly at the axis extends. The posterior Hox gene expression domain would be subsequently complemented by Hox regulatory phase 2, consisting in a variety of gene-specific, region-specific, and/or tissue-specific gene expression controls. We also touch on the unanswered question whether vertebrate Cdx gene expression delivers A-P positional information in its own right, as Caudal does in Drosophila, or whether it does so exclusively by upregulating Hox genes.

Unique spatial and cellular expression patterns of Hoxa5, Hoxb4, and Hoxb6 proteins in normal developing murine lung are modified in pulmonary hypoplasia

BACKGROUND

Hox transcription factors modulate signaling pathways controlling organ morphogenesis and maintain cell fate and differentiation in adults. Retinoid signaling, key in regulating Hox expression, is altered in pulmonary hypoplasia. Information on pattern-specific expression of Hox proteins in normal lung development and in pulmonary hypoplasia is minimal. Our objective was to determine how pulmonary hypoplasia alters temporal, spatial, and cellular expression of Hoxa5, Hoxb4, and Hoxb6 proteins compared to normal lung development.

METHODS

Temporal, spatial, and cellular Hoxa5, Hoxb4, and Hoxb6 expression was studied in normal (untreated) and nitrofen-induced hypoplastic (NT-PH) lungs from gestational day 13.5, 16, and 19 fetuses and neonates using Western blot and immunohistochemistry.

RESULTS

Modification of protein levels and spatial and cellular Hox expression patterns in NT-PH lungs was consistent with delayed lung development. Distinct protein isoforms were detected for each Hox protein. Expression levels of the Hoxa5 and Hoxb6 protein isoforms changed with development and were altered further in NT-PH lungs. Compared to normal lungs, GD19 and neonatal NT-PH lungs had decreased Hoxb6 and increased Hoxa5 and Hoxb4. Hoxa5 cellular localization changed from mesenchyme to epithelia earlier in normal lungs. Hoxb4 was expressed in mesenchyme and epithelial cells throughout development. Hoxb6 remained mainly in mesenchymal cells around distal airways.

CONCLUSIONS

Unique spatial and cellular expression of Hoxa5, Hoxb4, and Hoxb6 participates in branching morphogenesis and terminal sac formation. Altered Hox protein temporal and cellular balance of expression either contributes to pulmonary hypoplasia or functions as a compensatory mechanism attempting to correct abnormal lung development and maturation in this condition.

Pbx1/Pbx2 govern axial skeletal development by controlling Polycomb and Hox in mesoderm and Pax1/Pax9 in sclerotome

The post-cranial axial skeleton consists of a metameric series of vertebral bodies and intervertebral discs, as well as adjoining ribs and sternum. Patterning of individual vertebrae and distinct regions of the vertebral column is accomplished by Polycomb and Hox proteins in the paraxial mesoderm, while their subsequent morphogenesis depends partially on Pax1/Pax9 in the sclerotome. In this study, we uncover that Pbx1/Pbx2 are co-expressed during successive stages of vertebral and rib development. Next, by exploiting a Pbx1/Pbx2 loss-of-function mouse, we show that decreasing Pbx2 dosage in the absence of Pbx1 affects axial development more severely than single loss of Pbx1. Pbx1/Pbx2 mutants exhibit a homogeneous vertebral column, with loss of vertebral identity, rudimentary ribs, and rostral hindlimb shifts. Of note, these axial defects do not arise from perturbed notochord function, as cellular proliferation, apoptosis, and expression of regulators of notochord signaling are normal in Pbx1/Pbx2 mutants. While the observed defects are consistent with loss of Pbx activity as a Hox-cofactor in the mesoderm, we additionally establish that axial skeletal patterning and hindlimb positioning are governed by Pbx1/Pbx2 through their genetic control of Polycomb and Hox expression and spatial distribution in the mesoderm, as well as of Pax1/Pax9 in the sclerotome.

Hox patterning of the vertebrate axial skeleton

The axial skeleton in all vertebrates is composed of similar components that extend from anterior to posterior along the body axis: the occipital skull bones and cervical, thoracic, lumbar, sacral, and caudal vertebrae. Despite significant changes in the number and size of these elements during evolution, the basic character of these anatomical elements, as well as the order in which they appear in vertebrate skeletons, have remained remarkably similar. Through extensive expression analyses, classic morphological perturbation experiments in chicken and targeted loss-of-function analyses in mice, Hox genes have proven to be critical regulators in the establishment of axial skeleton morphology. The convergence of these studies to date allows an emerging understanding of Hox gene function in patterning the vertebrate axial skeleton. This review summarizes genetic and embryologic findings regarding the role of Hox genes in establishing axial morphology and how these combined results impact our current understanding of the vertebrate Hox code.