Differential expression of Hox 3.1 protein in subregions of the embryonic and adult spinal cord

Timing and precision of rattlesnake spinal motoneurons are determined by the KV72/3 potassium channel

The evolution of novel motor behaviors requires modifications in the central pattern generators (CPGs) controlling muscle activity. How such changes gradually lead to novel behaviors remains enigmatic due to the long time course of evolution. Rattlesnakes provide a unique opportunity to investigate how a locomotor CPG was evolutionarily modified to generate a novel behavior-in this case, acoustic signaling. We show that motoneurons (MNs) in the body and tail spinal cord of rattlesnakes possess fundamentally different physiological characteristics, which allow MNs in the tail to integrate and transmit CPG output for controlling superfast muscles with high temporal precision. Using patch-clamp electrophysiology, we demonstrate that these differences in locomotor and rattle MNs are mainly determined by KV72/3 potassium channels. However, although KV72/3 exerted a significantly different influence on locomotor and rattle MN physiology, single-cell RNA-seq unexpectedly did not reveal any differences in KV72/3 channels' expression. VIDEO ABSTRACT.

Transgenic studies on homeobox genes in nervous system development: spina bifida in Isl1 transgenic mice

To develop in vivo assays for homeobox gene function in neural development, we generated transgenic mice in which the expression of a homeobox gene is altered only within the nervous system, in neurons or neuronal precursor cells. Transgenic expression of Hoxc8 did not result in gross abnormalities, while a Hoxd4 transgene caused death shortly after birth. In neural progenitor cells, the motorneuron-specific homeodomain transcription factor Isl1 induced early developmental defects, including absence of anterior neural structures, profound defects in the neuroepithelium and defective neural tube closure. A fraction of Isl1 transgenic mice exhibited spina bifida. Isl1 transgene expression was also associated with decreased proliferation and increased Pbx1 expression in the ventral neural tube. Our results suggest a function for some homeobox genes in development of the nervous system, and that cell-type- and region-specific transgenic models will be useful to identify the cellular and molecular targets of homeobox transcription factors in nervous system development.

Hoxc8 downregulates Mgl1 tumor suppressor gene expression and reduces its concomitant function on cell adhesion

Hoxc8 is a homeobox gene family member, which is essential for growth and differentiation. Mgl1, a mouse homologue of the Drosophila tumor suppressor gene lgl, was previously identified as a possible target of Hoxc8. However, the biological effects and underlying molecular mechanism of Hoxc8 regulation on Mgl1 has not been fully established. The endogenous expression patterns of Hoxc8 were inversely correlated with those of Mgl1 in different types of cells and tissues. Here we showed that Hoxc8 overexpression downregulated the Mgl1 mRNA expression. Characterization of the ~2 kb Mgl1 promoter region revealed that the upstream sequence contains several putative Hox core binding sites and chromatin immunoprecipitation assay confirmed that Hoxc8 directly binds to the 5' upstream region of Mgl1. The promoter activity of this region was diminished by Hoxc8 expression but resumed by knockdown of Hoxc8 using siRNA against Hoxc8. Functional study of Mgl1 in C3H10T1/2 cells revealed a significant reduction in cell adhesion upon expression of Hoxc8. Taken together, our data suggest that Hoxc8 downregulates Mgl1 expression via direct binding to the promoter region, which in turn reduces cell adhesion and concomitant cell migration.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

Dynamic expression pattern of Hoxc8 during mouse early embryogenesis

The Hoxc8 expression pattern was examined in mouse embryos 7.5-12.5 days postcoitum (dpc) using whole-mount in situ hybridization and RT-PCR. The expression of Hoxc8 started between 7.5 and 8.5 dpc. A strong expression was detected in the ectoderm and mesoderm at 8.5 dpc. At 9.5 dpc, a distinct anterior boundary of Hoxc8 expression was established at the 10th and 16th somites in the neural tube and the paraxial mesoderm, respectively. This staggered expression pattern was maintained throughout the later stages. By 12.5 dpc, the forward progression of the Hoxc8 expression pattern was observed and the stain was weakened. In the ectoderm-derived neural tube, strong Hoxc8 expression was observed in the ventral horn and later in the ventral and mediolateral region of the mantle layer, indicating a possible association with the onset and progression of neural differentiation. In the case of the mesoderm-derivative cells, strong Hoxc8 expression was detected in the sclerotome on the way to the notochord and neural tube and mesonephros, suggesting a role of Hoxc8 in the formation of the vertebrae and ribs and the possible involvement in the differentiation into the kidney.

Transgenic mice ectopically expressing HOXA5 in the dorsal spinal cord show structural defects of the cervical spinal cord along with sensory and motor defects of the forelimb

Mutation of murine Hoxa5 has shown that HOXA5 controls lung, gastrointestinal tract and vertebrae development. Hoxa5 is also expressed in the spinal cord, yet no central nervous system phenotype has been described in Hoxa5 knockouts. To identify the role of Hoxa5 in spinal cord development, we developed transgenic mice that express HOXA5 in the dorsal spinal cord in the brachial region. Using HOXA5-specific antibodies, we show this expression pattern is ectopic as the endogenous protein is expressed only in the ventral spinal cord at this anterio-posterior level. This transgenic line (Hoxa5SV2) also displays forelimb-specific motor and sensory defects. Hoxa5SV2 transgenic mice cannot support their body weight in a forelimb hang, and forelimb strength is decreased. However, Rotarod performance was not impaired in Hoxa5SV2 mice. Hoxa5SV2 mice also show a delayed forelimb response to noxious heat, although hindlimb response time was normal. Administration of an analgesic significantly reduced the hang test defect and decreased the transgene effect on forelimb strength, indicating that pain pathways may be affected. The morphology of transgenic cervical (but not lumbar) spinal cord is highly aberrant. Nissl staining indicates superficial laminae of the dorsal horn are severely disrupted. The distribution of cells and axons immunoreactive for substance P, neurokinin-B, and their primary receptors were aberrant only in transgenic cervical spinal cord. Further, we see increased levels of apoptosis in transgenic spinal cord at embryonic day 13.5. Our evidence suggests apoptosis due to HOXA5 misexpression is a major cause of loss of superficial lamina cells in Hoxa5SV2 mice.

Hoxa10 and Hoxd10 coordinately regulate lumbar motor neuron patterning

The paralogous Hox genes Hoxa10 and Hoxd10 are expressed in overlapping domains in the developing lumbar spinal cord and surrounding mesoderm. Independent inactivation of these two genes alters the trajectory of spinal nerves and decreases the complement of motor neurons present in the lumbar spinal cord, whereas dual inactivation of these two genes has been shown to alter peripheral nerve growth and development in the mouse hindlimb. We have examined the organization and distribution of lumbar motor neurons in the spinal cords of Hoxa10/Hoxd10 double mutant animals. Double mutant animals have decreased numbers of lumbar motor neurons in both the medial and lateral motor columns. The anteroposterior position of the lumbar motor column is shifted caudally in double mutant animals, and the distribution of motor neurons is altered across individual spinal segments. Distinctions between classes of motor neurons based on positional specificity appear disrupted in double mutants. Double mutants also demonstrate abnormal spinal cord vasculature and altered kidney placement and size. Our observations suggest that Hoxa10 and Hoxd10 activity is required to specify the position of the lumbar motor column and to provide segmental specification and identity for the lumbar motor neurons.

Xenopus hoxc8 during early development

Vertebrate hoxc8 homologous genes have been shown to be involved in the formation of lower thoracic/lumbar vertebrae during early embryonic development. We report the isolation of a Xenopus hoxc8 (Xhoxc8), which shows 94% amino acid sequence identity to the mouse counterpart. Xhoxc8 is initially expressed in a broad region of blastopore lip at gastrular stage; however, at later stages, the region of expression is progressively restricted to the dorsal region caudal to the third somite and to the central trunk region of abdomen. Retinoic acid treatment that caused a severe malformation in antero-posterior axis did not induce any significant change in the spatio-temporal expression pattern of Xhoxc8 mRNA. Antisense RNA injection into 2- or 4-cell stage embryos resulted in a severe malformation in the abdominal structure leading to embryonic death. The results strongly indicate that Xhoxc8 expression is critical for the formation of abdominal structure.

Testing for genetic associations in a spina bifida population: analysis of the HOX gene family and human candidate gene regions implicated by mouse models of neural tube defects

Neural tube defects (NTDs) are among the most common severely disabling birth defects in the United States, affecting approximately 1-2 of every 1,000 live births. The etiology of NTDs is multifactorial, involving the combined action of both genetic and environmental factors. HOX genes play a central role in establishing the initial body plan by providing positional information along the anterior-posterior body and limb axis and have been implicated in neural tube closure. There are many mouse models that exhibit both naturally occurring NTDs in various mouse strains as well as NTDs that have been created by "knocking out" various genes. A nonparametric linkage method, the transmission disequilibrium test (TDT), was utilized to test the HOX gene family and human equivalents of genes (when known) or the syntenic region in humans to those in mouse models which could play a role in the formation of NTDs. DNA from 459 spina bifida (SB) affected individuals and their parents was tested for linkage and association utilizing polymorphic markers from within or very close to the HOXA, HOXB, HOXC, and HOXD genes as well as from within the genes/gene regions of eight mouse models that exhibit NTDs. No significant findings were obtained for the tested markers.

Changes in HOXB6 homeodomain protein structure and localization during human epidermal development and differentiation

HOX homeodomain proteins are master developmental regulators, which are now thought to function as transcription factors by forming cooperative DNA binding complexes with PBX or other protein partners. Although PBX proteins exhibit regulated subcellular localization and function in the nucleus in other tissues, little data exists on HOX and PBX protein localization during skin development. We now show that the HOXB6 protein is expressed in the suprabasal layer of the early developing epidermis and throughout the upper layers of late fetal and adult human skin. HOXB6 signal is cytoplasmic throughout fetal epidermal development, but substantially nuclear in normal adult skin. HOXB6 protein is also partially nuclear in hyperproliferative skin conditions, but appears to be cytoplasmic in basal and squamous cell carcinomas. Although all three PBX genes are expressed in fetal epidermis, none of the three PBX proteins exhibit nuclear co-localization with HOXB6 in either fetal or adult epidermis. RNA and protein data suggest that a truncated HOXB6 protein, lacking the homeodomain, is expressed in undifferentiated keratinocytes and that the full-length protein is induced by differentiation. GFP-fusion proteins were used to demonstrate that the full-length HOXB6 protein is localized to the nucleus while the truncated protein is largely cytoplasmic. Taken together, these data suggest that during epidermal development the truncated HOXB6 isoform may function by a mechanism other than as DNA binding protein, and that most of the nuclear, homeodomain-containing HOXB6 protein does not utilize PBX proteins as DNA binding partners in the skin. Published 2000 Wiley-Liss, Inc.

Differential expression of Hoxa-2 protein along the dorsal-ventral axis of the developing and adult mouse spinal cord

We have used synthetic oligopeptides derived from the coding sequence of the murine Hoxa-2 protein to produce polyclonal antibodies that specifically recognize the Hoxa-2 recombinant protein. Immunohistochemical studies reveal a distinct pattern of spatial and temporal expression of Hoxa-2 protein within the mouse spinal cord which is concomitant with the cytoarchitectural changes occurring in the developing cord. Hoxa-2 protein is predominantly detected in the nuclei of cells in the ventral mantle region of 10-day-old mouse embryos. Islet-1, a marker for motor neurons was also shown to be co-localized with Hoxa-2 in nuclei of cells in this region. As development progresses from 10-days to 14-days of gestation, Hoxa-2 protein expression gradually extends to the dorsal regions of the mantle layer. The Hoxa-2 protein expression pattern changes at 16-days of embryonic development with strong expression visible throughout the dorsal mantle layer. In 18-day-old and adult mouse spinal cords, Hoxa-2 protein was expressed predominantly by cells of the dorsal horn and only by a few cells of the ventral horn. Double labeling studies with an antibody against glial fibrillary acidic protein (GFAP, an astrocyte-specific intermediate filament protein) showed that within the adult spinal cord, astrocytes rarely expressed the Hoxa-2 protein. However, Hoxa-2 and GFAP double-labeled astrocytes were found in the neopallial cultures, although not all astrocytes expressed Hoxa-2. Hoxa-2 expressing oligodendrocyte progenitor cells were also identified after double-labeling with O4 and Hoxa-2 antibodies; although cells in this lineage that have begun to develop a more extensive array of cytoplasmic processes were less likely to be Hoxa-2 positive. The early pattern of Hoxa-2 protein expression across transverse sections of the neural tube is temporally and spatially modified as each major class of neuron is generated. This congruence in the expression of the Hoxa-2 protein and the generation of neurons in the cord suggests that the Hoxa-2 protein may contribute to dorsal-ventral patterning and/or to the specification of neuronal phenotype. Dev Dyn 1999;216:201-217.

The control of rostrocaudal pattern in the developing spinal cord: specification of motor neuron subtype identity is initiated by signals from paraxial mesoderm

The generation of distinct classes of motor neurons is an early step in the control of vertebrate motor behavior. To study the interactions that control the generation of motor neuron subclasses in the developing avian spinal cord we performed in vivo grafting studies in which either the neural tube or flanking mesoderm were displaced between thoracic and brachial levels. The positional identity of neural tube cells and motor neuron subtype identity was assessed by Hox and LIM homeodomain protein expression. Our results show that the rostrocaudal identity of neural cells is plastic at the time of neural tube closure and is sensitive to positionally restricted signals from the paraxial mesoderm. Such paraxial mesodermal signals appear to control the rostrocaudal identity of neural tube cells and the columnar subtype identity of motor neurons. These results suggest that the generation of motor neuron subtypes in the developing spinal cord involves the integration of distinct rostrocaudal and dorsoventral patterning signals that derive, respectively, from paraxial and axial mesodermal cell groups.

Increased apoptosis of motoneurons and altered somatotopic maps in the brachial spinal cord of Hoxc-8-deficient mice

Mice deficient for the homeotic gene Hoxc-8 suffer from a congenital prehension deficiency of the forepaw. During embryogenesis, Hoxc-8 is highly expressed in motoneurons within spinal cord segments C7 to T1. These motoneurons innervate forelimb distal muscles that move the forepaw. In Hoxc-8 mutant embryos, formation of these muscles is normal, but their innervation is perturbed. From E13.5 onwards, distal muscles normally supplied by C(7–8) MNs also receive ectopic projections from C(5–6) and T1 motoneurons. Coordinates of motor pools are altered along the rostrocaudal and also the mediolateral axes. Following this aberrant connectivity pattern and during the time of naturally occurring cell death, apoptosis is specifically enhanced in C7-T1 motoneurons. Loss of Hox-encoded regional specifications subsequently leads to a numerical deficit of motoneurons and an irreversible disorganization of motor pools. In Hoxc-8 null mutants, C(7–8) motoneurons lose their selective advantage in growth cone pathfinding behavior and/or target recognition, two essential steps in the establishment and maintenance of a functional nervous system.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

Spatial and temporal regulation of a lacZ reporter transgene in a binary transgenic mouse system

The transgenic mouse system is a powerful tool for the study of gene function. However, when the analysis involves genes that are critical for the normal developmental process, the usefulness of transgenic mouse systems is limited (for review see Hanahan, 1989; Westphal and Gruss, 1989; Byrne et al., 1991). This is due to potential transgene interference with development in case of ectopic or high level expression. As a result, establishing permanent transgenic mouse lines expressing these types of genes has proven difficult. To circumvent these difficulties, a binary transgenic mouse system has been established, termed the Multiplex System (Byrne and Ruddle, 1989). This is a two-tiered gene activation system in which expression of the gene of interest occurs only in offspring carrying transgenes encoding both components: transactivator and transresponder. Transactivator lines contain the gene encoding the VP16 protein of herpes simplex virus. Transresponder lines harbour the gene of interest linked to the IE promoter which includes recognition sequences for the VP16 transactivator. Previously, the inducibility of a chloramphenicol acetyltransferase reporter gene in newborn offspring that carried both a transactivator and transresponder transgene (Byrne and Ruddle, 1989) has been shown. Moreover, it has been demonstrated that expression of the VP16 protein was not detrimental to development and that transactivation appeared to be tissue specific. Here, the potential of the system for the expression of transgenes in early mouse embryogenesis was examined, using the Escherichia coli beta-galactosidase gene as a reporter in the transresponder mouse strain. To direct expression of VP16, the murine Hoxc-8 promoter, which is known to be active during early development, was used. Embryos from crosses of transactivators to transresponders were isolated at different stages of development and stained for beta-galactosidase activity. Transactivation, as demonstrated by strong beta-galactosidase staining, could be detected as early as eight days of development. At all stages examined, the pattern of lacZ transresponder gene expression accurately reflected the activity of the Hoxc-8 promoter controlling VP16 expression. It is demonstrated that the Multiplex System can be used to express transresponder transgenes in a spatially and temporally defined manner in multiple cell types early during mouse embryogenesis.

Regulation of Hoxc-8 during mouse embryonic development: identification and characterization of critical elements involved in early neural tube expression

We have characterized cis-acting elements that direct the early phase of Hoxc-8 expression using reporter gene analysis in transgenic mice. By deletion we show that a 135 bp DNA fragment, located approximately 3 kb upstream of the coding region of Hoxc-8, is capable of directing posterior neural tube expression. This early neural tube (ENT) enhancer consists of four separate elements, designated A, B, C and D, whose nucleotide sequences are similar to binding sites of known transcription factors. Nucleotide substitutions suggest that element A is an essential component of the ENT enhancer. However element A by itself is incapable of directing neural tube expression. This element requires interactions at any two of the other three elements, B, C or D. Thus, the components of the ENT enhancer direct neural tube expression in an interdependent manner. We propose that Hoxc-8 is activated in the neural tube by combinatorial interactions among several proteins acting within a small region. Our transgenic analyses provide a means to identify transcription factors that regulate Hoxc-8 expression during embryogenesis.

Primitive streak mesoderm-like cell lines expressing Pax-3 and Hox gene autoinducing activities

Differentiating P19 embryonal carcinoma (EC) cells transiently express an endogenous activity capable of inducing Pax-3 expression in adjacent P19 stem cells (Pruitt, Development 116, 573–583, 1992). In the present study, expression of this activity in mesodermal cell lineages is demonstrated. First, expression of the mesodermal marker Brachyury correlates with expression of Pax-3-inducing activity. Second, the ability of leukemia inhibitory factor (LIF) to block mesoderm differentiation at two different points is demonstrated and correlated with the inhibition of Pax-3-inducing activity. Finally, two mesodermal cell lines that express Pax-3-inducing activity were derived from P19 EC cells. Each of these lines expresses high levels of the mesodermal marker Brachyury and high levels of Oct-3/4 (which is down-regulated at early times during mesoderm differentiation) suggesting that these lines are early mesodermal derivatives. Unlike EC or embryonic stem cell lines, each of the two mesodermal derivatives autoinduces Hox gene expression on aggregation even in the presence of LIF. Following aggregation, anterior-specific genes are expressed more rapidly than more posterior genes. These observations directly demonstrate the ability of murine mesodermal derivatives to autoinduce Hox gene expression in the absence of signals from other cell lineages. Similar to the Pax-3-inducing activity, signals from mesodermal cell lines were sufficient to induce HOX expression in adjacent P19 stem cells in cell mixing assays. These observations are consistent with the previous suggestion (Blum, M., Gaunt, S. J., Cho, K. W. Y., Steinbeisser, H., Blumberg, B., Bittner, D. and De Robertis, E. M. (1992) Cell 69, 1097–1106) that signals responsible for anterior-posterior organizer activity are localized to the anterior primitive streak mesoderm of the mouse embryo.

A mouse homologue of the Drosophila tumour-suppressor gene l(2)gl controlled by Hox-C8 in vivo

The homeobox is a 183-base-pair DNA sequence originally found in Drosophila segmentation and homeotic genes. In Drosophila, homeotic genes are clustered in the Antennapedia and Bithorax complexes, collectively called the homeotic gene complex (HOM-C). In the mouse genome, about 40 homeobox genes (Hox) are clustered in four chromosomal regions (Hox A to D). The Hox genes are arranged in the same order and have the same anteroposterior pattern of expression as their structural homologue in the HOM-C, suggesting that they control mouse pattern formation in the same way that HOM-C members do in Drosophila. Homeobox gene products are believed to be transcription factors that regulate expression of target genes. A few candidate target genes have been identified in Drosophila by various approaches but the Hox gene targets are poorly understood, mostly because of limitations in the available approaches. Here we identify several candidate Hox gene targets, including a mouse homologue of the Drosophila tumour-suppressor gene l(2)gl, by immunopurification of DNA sequences bound to a Hox protein in native chromatin.

Altering the boundaries of Hox3.1 expression: evidence for antipodal gene regulation

To investigate the function of region-specific patterns of mouse homeobox gene expression during embryogenesis, we programmed a minimal change in the distribution of Hox3.1 transcripts along the anteroposterior body axis in transgenic mice. Regulatory sequences from Hox1.4, a gene normally expressed more anteriorly than Hox3.1, were chosen to direct expression of a Hox3.1 transgene. Offspring of independent transgenic lines expressed the transgene more anteriorly than the Hox3.1 gene. Rather than predicted posterior transformations, we observed anterior transformations of vertebrae in newborn mice. Transgenic mice also developed profound gastrointestinal tissue malformations, which may provide a molecular explanation for human developmental disorders often involving these same two regions. Paradoxically, vertebral transformations in the transgenic mice were strikingly similar to those reported in mice homozygous for a null mutation of the Hox3.1 gene. This observation suggests that Hox genes may be regulated antipodally, with over- or underexpression resulting in similar phenotypes.

Characterization of the Xenopus Hox 2.4 gene and identification of control elements in its intron

We report on the Xenopus homolog of the Hox 2.4 gene. This gene occupies the next to 5'-most position in the Xenopus Hox2 complex. Hox 2.4 RNA is first detected at the early neurula stage, reaching a peak at the early tailbud stage, and is localized in the middle and posterior portions of the embryos. Antibodies raised against a fusion protein show expression of Hox 2.4 protein in Xenopus embryos in a band located in the mid spinal cord. Thus, the protein is expressed in a narrower domain than that of Hox 2.4 mRNA. The Xenopus Hox 2.4 antibody cross-reacts readily with mouse embryonic tissue, where the protein is detected in migrating neural crest cells, the dorsal portion of the spinal cord, somites, lateral plate mesoderm, and in the forelimb bud. The Xenopus Hox 2.4 intron shares considerable sequence identity with the intron in the mouse homolog. A reporter gene containing an element from this intron which can bind homeodomain proteins is activated following microinjection into Xenopus embryos. The short distance between the end of the Hox 2.4 cDNA and the start site of the neighboring gene in the complex raises the possibility that this transcriptional element might be shared by two Hox genes.

Homeotic transformations in the mouse induced by overexpression of a human Hox3.3 transgene

A permanent transgenic mouse line was generated carrying 40 copies of the human Hox3.3 gene. The resulting mice express large amounts of Hox3.3 protein in posterior regions of the embryo where this homeodomain protein is normally not expressed. The transgene causes homeotic transformations of the skeleton, in particular the appearance of an extra pair of ribs in the lumbar region, transformation of the shape of posterior ribs into that of more anterior ones, and the joining of an additional pair of ribs to the sternum. The phenotype of this line resembles that obtained by the targeted loss-of-function mutation of Hox3.1 (Le Mouellic et al., 1992). In transient assays, the human Hox3.3 transgene leads to the formation of additional ribs in more posterior vertebrae as well.

Deformed autoregulatory element from Drosophila functions in a conserved manner in transgenic mice

The striking similarities in the structure, organization and anterior-posterior expression patterns between the murine Hox gene system and the Drosophila homeotic gene complexes, called HOM-C (ref. 3), may point to highly conserved mechanisms for specifying positional identities (reviewed in ref. 4). Strong support for this concept lies in the observation of conserved colinearity between the genomic order of the Hox/HOM genes and their unique successive expression domains along the anterior-posterior axes of both mouse and fly embryos. These unique and precise expression patterns appear to be facilitated by multiple cis-regulatory elements (reviewed in ref. 5). One of the few elements characterized in detail is the autoregulatory enhancer of the homeotic gene Deformed (Dfd), which supports expression in subregions of posterior head segments of Drosophila embryos. Here we present evidence that this enhancer is capable of conferring reporter gene expression to a discrete subregion of the hindbrain in transgenic mouse embryos. Remarkably, this anterior-posterior subregion lies within the common anterior expression domain of the Dfd cognate Hox genes in the postotic hindbrain. Our results indicate that the Dfd autoregulatory enhancer is part of a highly conserved mechanism for establishing region-specific gene expression along the anterior-posterior axis of the embryo.

Expression and modification of Hox 2.1 protein in mouse embryos

A polyclonal antibody, alpha Hox 2.1a, has been generated and used to immunolocalize Hox 2.1 protein in mouse embryos. Protein is present in nuclei of all tissues previously shown to express Hox 2.1 RNA. In addition, protein is seen in somites and proximal regions of the limb buds, tissues in which Hox 2.1 RNA expression was not clearly detected previously by in situ hybridization. At the 7 somite stage, protein is detectable in the neural tube up to the level of somite 1, but later retracts to a more posterior position. Immunoblot, in vitro translation, and immunoprecipitation experiments were carried out to characterize the Hox 2.1 protein. The results show that the Hox 2.1 gene produces at least two related phosphorylated proteins present in different proportions in different tissues.

Hox-3.6: isolation and characterization of a new murine homeobox gene located in the 5′ region of the Hox-3 cluster

Most members of the murine Hox gene system can be grouped into two subclasses based on their structural similarity to either one of the Drosophila homeotic genes Antennapedia (Antp) or Abdominal B (AbdB). All the AbdB-like genes reported thus far are located in the 5' region of their respective cluster. We describe here the isolation, structural characterization and spatio-temporal expression pattern of a new AbdB-like homeobox gene designated Hox-3.6 that is located in the 5' region of the Hox-3 cluster. Hox-3.6 has an extreme posterior expression domain in embryos of 12.5 days of gestation, a feature that has thus far only been observed for the 5' most genes of the Hox-4 cluster. Like the other members of the AbdB subfamily, Hox-3.6 exhibits spatially restricted expression in the hindlimb bud, but the expression domain is antero-proximal in contrast to the postero-distal domain reported for its cognate gene Hox-4.5. Structural analysis of the 5' region revealed the presence of a 35 bp sequence which shares homology and relative 5' position with an upstream sequence present in its two nearest downstream neighbors, Hox-3.2 and -3.1.

Homeosis in the mouse induced by a null mutation in the Hox-3.1 gene

We have replaced the Hox-3.1 coding sequence with the E. coli lacZ gene by means of homologous recombination in embryonic stem cells and thus produced null mutant mice. Homozygous mice were born alive, but most of them died within a few days. In the trunk region of homozygotes, several skeletal segments were transformed into the likeness of more anterior ones, as observed in Drosophila with loss-of-function homeotic mutations. The most obvious transformations were the attachment of the 8th pair of ribs to the sternum and the appearance of a 14th pair of ribs on the 1st lumbar vertebra. The pattern of beta-galactosidase activity was identical in heterozygotes and homozygotes and reflected faithfully the Hox-3.1 expression pattern. Thus, the mutation modified the identity, rather than the position, of embryonic cells that would normally express Hox-3.1.