Hox gene expression in teleost fins and the origin of vertebrate digits

Hox genes are essential for growth and patterning of the tetrapod limb skeleton. Mice mutant for the Hoxd-13 gene have an important delay in morphogenesis owing to reduced proliferation. Based on the appearance of atavisms in such mice, we suggested that modifications of Hox gene regulation may have been a source of morphological variation during the evolution of tetrapod limbs. Pectoral and pelvic fins are homologous to fore- and hindlimbs, respectively. To compare the relative importance of Hox genes during fin versus limb morphogenesis, we cloned zebrafish (Danio rerio) HoxD and HoxA complex genes and analysed their expression during fin development. The results suggest a scheme for the fin-limb transition in which the distal autopods (digits) are neomorphic structures produced by unequal proliferation of the posterior part of an ancestral appendix.

Differential expression of two different homeobox gene families during mouse tegument morphogenesis

The expression of six genes belonging to two different homeobox gene families was studied during the embryonic and postnatal morphogenesis of head and body regions of the mouse integument. The first family included the Otx1 and Otx2 genes, both related to the orthodenticle Drosophila gene and the second was represented by four members of the Antennapedia class HOX genes: Hoxc8 and three Hoxd genes, d9, d11 and d13. In situ hybridizations with 35S labeled antisense RNA probes were performed on head serial frontonasal sections, as well as entire embryo and postnatal tail longitudinal sections. The expression of these genes shows a differential spatiotemporal pattern along the cephalo-caudal axis. In 12.5-day and 15.5-day embryos, the Otx2 gene expression is restricted to the nasal epithelium and its associated glands, while the Otx1 transcripts are present in both nasal and facial integuments, including nasal glands and hair vibrissa follicles. The Hoxc8 expression first appears in skin at 14.5 days of gestation in the sternal region and is extended at 16.5 days to the thoracic ventral and lumbar dorsal regions. The Hoxd9 and Hoxd11 genes are only expressed in the caudal skin from 14.5 days of gestation. The Hoxd13 transcripts are the last to appear, 2 days after birth, and are limited to the last epidermal cells to differentiate, i.e. those of the hair matrix of the caudal pelage hair follicles. Taken together, these observations strengthen the hypothesis that different homeobox gene families specify the regional identity of the skin in the cephalic and body regions.

Colinearity and functional hierarchy among genes of the homeotic complexes

Homeotic genes identify structures along the anterior to posterior axis during the development of most animals. These genes are clustered into complexes, and their positions within the cluster correlates with their time of expression and the positions of the anterioposterior boundaries of their expression domains. Functional analyses have revealed that this specific genetic order also coincides with a functional hierarchy among members of these complexes, so that the products of more posterior genes in the cluster tend to be prevalent over those of more anterior genes.

Conservation of a functional hierarchy between mammalian and insect Hox/HOM genes

We have generated several transgenic Drosophila strains containing different mouse Hox genes under heat shock control and studied how their generalized expression affects Drosophila larval patterns. We find that they have spatially restricted effects which correlate with their genetic order and expression pattern in the mouse; as they are expressed more posteriorly in the mouse, they have more extensive effects in Drosophila. The generalized expressions of Hoxd-8 and d-9 modify Drosophila anterior head segment(s), but have no effect in the rest of the body. Hoxd-10 expression affects head and thorax, but not the abdomen. Finally, Hoxd-11 alters head, thorax not the abdomen. Finally, Hoxd-11 alters head, thorax and abdomen. The developmental effect of the Hox genes consists of a homeotic transformation of the affected segment(s), which exhibit a 'ground' pattern similar to that obtained in the absence of homeotic information, suggesting that Hox genes are able to inactivate Drosophila homeotic genes, but do not specify a pattern of their own. A partial exception is Hoxd-11 which, even though it has a general suppressing effect, can also activate the resident Abdominal-B and empty spiracles genes in ectopic positions. Our results strongly suggest a general conservation of the functional hierarchy of homeotic genes that correlates with genetic order and expression patterns.

Expression of the zebrafish gene hlx-1 in the prechordal plate and during CNS development

The zebrafish hlx-1 gene belongs to the H2.0 subfamily of homeobox genes and is closely related to the mouse Dbx gene with respect to both homeodomain homology (96.7%) and neural expression during embryogenesis. Analysis of hlx-1 expression by in situ hybridization reveals several particularly interesting features. In late gastrula embryos, hlx-1 transcripts are detected within a circular area in the region of the presumptive rostral brain. Subsequently, the expression domain becomes restricted to the hypoblast and undergoes dynamic changes involving conversion into a longitudinal stripe which elongates and retracts following a temporal sequence. The site of transient hlx-1 expression along the ventral midline of the rostral neurectoderm, which in part corresponds to the prechordal plate, suggests a role in the determination of head mesoderm as well as in patterning of the rostral brain. As the midline stripe gradually disappears, the hlx-1 gene becomes regionally expressed within the diencephalon and at a specific dorsoventral level along the hindbrain and spinal cord. In the hindbrain, expression is initiated in dorsoventrally restricted transversal stripes which correlate with the segmental pattern of rhombomeres. The stripes fuse into bilateral columns that are later converted to two series of paired transversal stripes at the rhombomere borders. This pattern is consistent with the proposed subdivision of hindbrain segments into rhombomere centers separated by border regions.

Disruption of the Hoxd-13 gene induces localized heterochrony leading to mice with neotenic limbs

Vertebrate Hoxd genes are sequentially activated during the morphogenesis and pattern formation of the limb. Using the approach of gene disruption via homologous recombination in embryonic stem cells, we have assessed the function of the last gene of the complex, Hoxd-13. Mutant mice displayed skeletal alterations along all body axes suggesting the existence of a general multiaxial patterning system. In limbs, abnormalities such as a reduction in the length of some bony elements, loss of phalanges, bone fusions, and the presence of an extra element were observed. We propose that the mutation induces local heterochrony, as illustrated by an important retardation in limb morphogenesis. The relevance of these observations to our understanding of the development and evolution of the tetrapod limb is discussed.

Cooperation of regulatory elements involved in the activation of the Hoxd-11 gene

We have used lacZ reporter gene constructs to study the cis regulation of the murine Hoxd-11 gene (previously Hox-4.6) in transgenic mice. We identified a genomic region, which was able to simulate important aspects of the developmental regulation of the gene. A short regulatory region, located 3' to the Hoxd-11 transcription unit, was required to mimic initial activation. This regulatory region contains two stretches of DNA, that are highly conserved in the chicken Hoxd-11 3' flanking region. A chimeric construct containing these short homologous regions from the chicken gene could replace the complete murine fragment thus demonstrating that the conserved domains carry the main regulatory elements involved in this activation. The first half of this bipartite regulatory region has positive regulatory activity, while the second half is required to restrict gene expression to the proper posterior domain in the somitic mesoderm. Our results suggest that the control of the Hoxd-11 promoter involves tissue-specific cooperations between regulatory elements.

Structure and activity of regulatory elements involved in the activation of the Hoxd-11 gene during late gastrulation

We have used reporter gene constructs to study the cis regulation of the Hoxd-11 gene (previously Hox-4.6) in transgenic mice. We identified a 5 kb regulatory unit, which was able to reproduce important aspects of the initial activation of the gene along the major body axis. The comparison of the nucleotide sequence of this DNA fragment with the corresponding avian genomic region revealed the presence of seven highly homologous stretches of DNA outside the protein coding regions. In particular, the 3' flanking region contained two such domains that are required to mediate the embryonic activation. A chimeric construct containing the two short homologous regions from the chicken gene could replace the complete murine fragment thus demonstrating that the conserved domains carry the main regulatory elements involved in this activation. The first half of this bipartite regulatory region has enhancer activity when tested with a heterologous promoter, while the second half is required to restrict the enhancer activity to the proper expression domain. These results suggest that stage- and tissue-specific cooperation between regulatory elements is required to control properly the activity of the Hoxd-11 promoter.

Local alterations of Krox-20 and Hox gene expression in the hindbrain suggest lack of rhombomeres 4 and 5 in homozygote null Hoxa-1 (Hox-1.6) mutant embryos

It is unknown whether cross-regulatory interactions between homeotic genes, which have been shown to play an important role in the maintenance of their expression domains during Drosophila development, are also important during mammalian development. We have analyzed here the expression of Hox genes in Hoxa-1 (Hox-1.6) null mutant embryos to investigate the possible existence of regulatory interactions between Hoxa-1 and other Hox genes. We show that the absence of a functional Hoxa-1 gene product does not globally interfere with the expression of other Hox genes in terms of both spatial boundaries and transcript abundance. However, a limited area of the hindbrain shows a strong reduction in Hoxb-1 (Hox-2.9) and Krox-20 transcripts, which most likely reflects a marked reduction in size of the former fourth and fifth rhombomeres. These alterations coincide with the region that is subsequently affected in Hoxa-1 null mutant mice and suggest that the primary defects in this mutation are spatially restricted deletions of some rhombomeric structures.

Correlation of expression of Wnt-1 in developing limbs with abnormalities in growth and skeletal patterning

The Wnt genes are members of a family of vertebrate genes related to the Drosophila gene wingless (wg). They encode secreted molecules that are thought to be important in patterning and growth control during ontogenesis. Several such genes are transcribed in localized domains during limb budding and morphogenesis. We report here a congenital limb malformation in a mouse transgenic line that ectopically expresses Wnt-1 in the developing limbs. The hemizygote phenotype, which is inherited as an autosomal dominant trait, presents extensive distal truncations of skeletal elements, skeletal fusions and interdigital webbing. The data shown here demonstrate that abnormal Wnt-1 expression is correlated with retarded mesenchymal condensations replaced by highly proliferative cells in the limb bud. This seems to lead to an inability of the affected cells to participate in normal skeletal development leading to the adult defects.

Targeted misexpression of Hox-4.6 in the avian limb bud causes apparent homeotic transformations

In the limb bud the 5' members of the Hox-4 gene cluster are expressed in a nested set of overlapping domains which are progressively restricted in the posterior and distal directions. These domains arise early in limb bud development and come to approximate the primordia of the major structural elements of the limb along the anterior/posterior axis (Fig. 1). This pattern, and the fact that surgical manipulations which lead to mirror image duplications along the anterior/posterior axis give rise to mirror image duplications of the domains of expression of these genes, have led to the proposal that these transcription factors specify positional identity along the anterior/posterior axis. Here we test this hypothesis directly using replication-competent retroviral vectors to expand the domain of expression of the Hox-4.6 gene anteriorly during limb development in vivo. We report that alteration of the domain of expression of the Hox-4.6 gene in the developing limb leads to reproducible pattern alterations consistent with a posterior homeotic transformation.

Homeobox genes and pattern formation in the vertebrate limb

The developing vertebrate limb is a powerful system to study genes potentially involved in pattern formation. Many such candidate genes encode transcription factors belonging to the class of the "homeodomain" proteins. In this short review, we discuss the possible functions of different subfamilies of homeobox genes. Genes belonging to the Hox family (related to the Drosophila homoeotic genes), such as the HOX-1, HOX-3, and HOX-4, complexes are probably among those encoding the patterning information. Their differential expression in the mesenchymal compartment is proposed to be responsible for the determination of the various axial elements. Other homeobox-containing genes are expressed in both the mesenchyme of the progress zone and the ectodermal ridge. These genes, Hox-7.1 and Hox-8.1, are related to the Drosophila msh gene and could be involved in epithelial-mesenchymal interactions linking the growth of the system to its patterning.

Hox-4 gene expression in mouse/chicken heterospecific grafts of signalling regions to limb buds reveals similarities in patterning mechanisms

The products of Hox-4 genes appear to encode position in developing vertebrate limbs. In chick embryos, a number of different signalling regions when grafted to wing buds lead to duplicated digit patterns. We grafted tissue from the equivalent regions in mouse embryos to chick wing buds and assayed expression of Hox-4 genes in both the mouse cells in the grafts and in the chick cells in the responding limb bud using species specific probes. Tissue from the mouse limb polarizing region and anterior primitive streak respecify anterior chick limb bud cells to give posterior structures and lead to activation of all the genes in the complex. Mouse neural tube and genital tubercle grafts, which give much less extensive changes in pattern, do not activate 5′-located Hox-4 genes. Analysis of expression of Hox-4 genes in mouse cells in the grafted signalling regions reveals no relationship between expression of these genes and strength of their signalling activity. Endogenous signals in the chick limb bud activate Hox-4 genes in grafts of mouse anterior limb cells when placed posteriorly and in grafts of mouse anterior primitive streak tissue. The activation of the same gene network by different signalling regions points to a similarity in patterning mechanisms along the axes of the vertebrate body.

The vertebrate limb: A model system to study theHox/hom gene network during development and evolution

The potential of the vertebrate limb as a model system to study developmental mechanisms is particularly well illustrated by the analysis of the Hox gene network. These genes are probably involved in the establishment of patterns encoding positional information. Their functional organisation during both limb and trunk development are very similar and seem to involve the progressive activation in time, along the chromosome, of a battery of genes whose products could differentially instruct those cells where they are expressed. This process may be common to all organisms that develop according to an anterior-posterior morphogenetic progression. The possible linkage of this system to a particular mechanism of segmentation as well as its phylogenetic implications are discussed.

Expression of Hox-4 genes in the chick wing links pattern formation to the epithelial-mesenchymal interactions that mediate growth

The relationship between the expression of Hox-4 genes in the mesenchyme and the apical ectodermal ridge was investigated in both normal chick wing buds and wing buds treated with retinoic acid. Two conclusions emerge. One is that the activation of Hox-4 domains and the elaboration of Hox-4 gene expression patterns involve cooperation with a signal from the apical ridge. The second is that the domains of expression of 5'-located members of the complex correlate with the maintenance of the thickened ridge which is required for subsequent bud outgrowth.

Comparison of mouse and human HOX-4 complexes defines conserved sequences involved in the regulation of Hox-4.4

We have cloned and sequenced, in both mouse and human, regions of the HOX-4 complex which contain two Abd-B like genes, Hox-4.4 and Hox-4.5 (HOX4C and HOX4D in human, respectively). The high degree of conservation between the homeoprotein sequences extends to non-coding areas, which suggests that the mechanisms of regulation have been conserved. We show that the Hox-4.5/Hox-4.4 intergenic region can be broadly subdivided into three domains based on DNA conservation between rodents and primates. The presence of all these domains in association with sequences located 3' to the transcription termination site are required to mimick the spatial regulation of Hox-4.4 in transgenic mouse embryos. Several highly conserved short sequences located in this region were studied in gel retardation assays for their binding to potential regulatory factors. One such factor is detected in embryonal carcinoma cells but absent from other differentiated cell lines. This specific binding activity is down regulated upon retinoic acid treatment.

The mis-expression of posterior Hox-4 genes in talpid (ta3) mutant wings correlates with the absence of anteroposterior polarity

Developing chicken wings homozygous for the talpid (ta3/ta3) mutation are polydactylous and have defects in the establishment of their anteroposterior polarity. We analysed the expression domains of the posteriorly restricted homeobox Hox-4 genes in such mutant wings. The Hox-4 genes are now expressed right across the anteroposterior axis instead of being expressed just posteriorly. This correlates well with the absence of clear morphological differences between the talpid3 digits and reinforces the idea that vertebrate Hox-4 genes are involved in setting up the limb anteroposterior asymmetry.

Expression of the murine Dlx-1 homeobox gene during facial, ocular and limb development

We have analysed the expression pattern of the mouse homeobox containing gene Dlx-1. This gene harbors a homeodomain related to that found in the Drosophila distal-less (dll) gene. In addition to its expression in the developing forebrain, Dlx-1 is transcribed in several structures containing cells of neural crest origin such as the facial mesenchyme and various elements of the peripheral nervous system. Dlx-1 transcripts are also detected in the differentiating retina and during limb morphogenesis, in the apical ectodermal ridge. The possible involvement of Dlx-1 during facial, ocular and limb development is discussed.

Regional expression of the homeobox gene Nkx-2.2 in the developing mammalian forebrain

A novel mouse homeobox-containing gene, Nkx-2.2, has been isolated. Nkx-2.2 is a member of a family of genes whose homeodomains are homologous to that of the Drosophila NK-2 gene. Nkx-2.2 transcripts are found in localized domains of the brain during mouse embryogenesis. Nkx-2.2 expression in the brain abuts and partially overlaps with the expression domains of two other related homeobox-containing genes, TTF-1 and Dlx. The expression domains of the three genes in the developing prosencephalon coincide with anatomical boundaries, particularly apparent in the diencephalon. This result raises the possibility that these genes may specify regional differentiation of the developing diencephalon into its anatomically and functionally defined subregions. Nkx-2.2 may be involved in specifying diencephalic neuromeric boundaries.