The expression pattern of Xenopus Mox-2 implies a role in initial mesodermal differentiation

A mosaic of conserved and novel modes of gene expression and morphogenesis in mesoderm and muscle formation of a larval bivalve

The mesoderm gives rise to several key morphological features of bilaterian animals including endoskeletal elements and the musculature. A number of regulatory genes involved in mesoderm and/or muscle formation (e.g., Brachyury (Bra), even-skipped (eve), Mox, myosin II heavy chain (mhc)) have been identified chiefly from chordates and the ecdysozoans Drosophila and Caenorhabditis elegans, but data for non-model protostomes, especially those belonging to the ecdysozoan sister clade, Lophotrochozoa (e.g., flatworms, annelids, mollusks), are only beginning to emerge. Within the lophotrochozoans, Mollusca constitutes the most speciose and diverse phylum. Interestingly, however, information on the morphological and molecular underpinnings of key ontogenetic processes such as mesoderm formation and myogenesis remains scarce even for prominent molluscan sublineages such as the bivalves. Here, we investigated myogenesis and developmental expression of Bra, eve, Mox, and mhc in the quagga mussel Dreissena rostriformis, an invasive freshwater bivalve and an emerging model in invertebrate evodevo. We found that all four genes are expressed during mesoderm formation, but some show additional, individual sites of expression during ontogeny. While Mox and mhc are involved in early myogenesis, eve is also expressed in the embryonic shell field and Bra is additionally present in the foregut. Comparative analysis suggests that Mox has an ancestral role in mesoderm and possibly muscle formation in bilaterians, while Bra and eve are conserved regulators of mesoderm development of nephrozoans (protostomes and deuterostomes). The fully developed Dreissena veliger larva shows a highly complex muscular architecture, supporting a muscular ground pattern of autobranch bivalve larvae that includes at least a velum muscle ring, three or four pairs of velum retractors, one or two pairs of larval retractors, two pairs of foot retractors, a pedal plexus, possibly two pairs of mantle retractors, and the muscles of the pallial line, as well as an anterior and a posterior adductor. As is typical for their molluscan kin, remodelling and loss of prominent larval features such as the velum musculature and various retractor systems appear to be also common in bivalves.

Supplementary information

The online version contains supplementary material available at 10.1007/s13127-022-00569-5.

Combined p53- and PTEN-deficiency activates expression of mesenchyme homeobox 1 (MEOX1) required for growth of triple-negative breast cancer

Triple-negative breast cancer (TNBC) is an aggressive cancer subtype for which effective therapies are unavailable. TNBC has a high frequency of tumor protein p53 (Tp53/p53)- and phosphatase and tensin homolog (PTEN) deficiencies, and combined p53- and PTEN-deficiency is associated with poor prognosis and poor response to anticancer therapies. In this study, we discovered that combined p53- and PTEN-deficiency in TNBC activates expression of the transcription factor mesenchyme homeobox 1 (MEOX1). We found that MEOX1 is expressed only in TNBC cells with frequent deficiencies in p53 and PTEN, and that its expression is undetectable in luminal A, luminal B, and HER2+ subtypes, as well as in normal breast cells with wild-type (WT) p53 and PTEN. Notably, siRNA knockdown of both p53 and PTEN activated MEOX1 expression in breast cancer cells, whereas individual knockdowns of either p53 or PTEN had only minimal effects on MEOX1 expression. MEOX1 knockdown abolished cell proliferation of p53- and PTEN-deficient TNBC in vitro and inhibited tumor growth in vivo, but had no effect on the proliferation of luminal and HER2+ cancer cells and normal breast cells. RNA-Seq and immunoblotting analyses showed that MEOX1 knockdown decreased expression of tyrosine kinase 2 (TYK2), signal transducer and activator of transcription 5B (STAT5B), and STAT6 in p53- and PTEN-deficient TNBC cells. These results reveal the effects of combined p53- and PTEN-deficiency on MEOX1 expression and TNBC cell proliferation, suggesting that MEOX1 may serve as a potential therapeutic target for managing p53- and PTEN-deficient TNBC.

Conserved Noncoding Elements in the Most Distant Genera of Cephalochordates: The Goldilocks Principle

Cephalochordates, the sister group of vertebrates + tunicates, are evolving particularly slowly. Therefore, genome comparisons between two congeners of Branchiostoma revealed so many conserved noncoding elements (CNEs), that it was not clear how many are functional regulatory elements. To more effectively identify CNEs with potential regulatory functions, we compared noncoding sequences of genomes of the most phylogenetically distant cephalochordate genera, Asymmetron and Branchiostoma, which diverged approximately 120-160 million years ago. We found 113,070 noncoding elements conserved between the two species, amounting to 3.3% of the genome. The genomic distribution, target gene ontology, and enriched motifs of these CNEs all suggest that many of them are probably cis-regulatory elements. More than 90% of previously verified amphioxus regulatory elements were re-captured in this study. A search of the cephalochordate CNEs around 50 developmental genes in several vertebrate genomes revealed eight CNEs conserved between cephalochordates and vertebrates, indicating sequence conservation over >500 million years of divergence. The function of five CNEs was tested in reporter assays in zebrafish, and one was also tested in amphioxus. All five CNEs proved to be tissue-specific enhancers. Taken together, these findings indicate that even though Branchiostoma and Asymmetron are distantly related, as they are evolving slowly, comparisons between them are likely optimal for identifying most of their tissue-specific cis-regulatory elements laying the foundation for functional characterizations and a better understanding of the evolution of developmental regulation in cephalochordates.

Mutations in MEOX1, encoding mesenchyme homeobox 1, cause Klippel-Feil anomaly

Klippel-Feil syndrome (KFS) is a segmentation malformation of the cervical spine; clinically, it manifests as a short neck with reduced mobility and a low posterior hairline. Several genes have been proposed as candidates for KFS when it is present with other associated anomalies, but the genetics of isolated KFS have been difficult to study because of the syndrome's mostly sporadic occurrence. We describe a multiplex consanguineous family in which isolated KFS maps to a single 17q21.31 locus that harbors a homozygous frameshift deletion in MEOX1; this deletion results in complete instability of the transcript. Direct sequencing of this gene in two siblings from another consanguineous family affected by isolated KFS uncovered another homozygous truncating (nonsense) MEOX1 mutation that also leads to complete degradation of the transcript. This gene encodes a transcription factor with a well-established and nonredundant role in somite development, and homozygous null alleles of Meox1 in mice have a cervical skeletal defect that is remarkably similar to the one we observe in human individuals with MEOX1 mutations. Our data strongly suggest that KFS is the human phenotypic equivalent of the sclerotome polarity defect that results from Meox1 deficiency in mice.

Mef2d acts upstream of muscle identity genes and couples lateral myogenesis to dermomyotome formation in Xenopus laevis

Xenopus myotome is formed by a first medial and lateral myogenesis directly arising from the presomitic mesoderm followed by a second myogenic wave emanating from the dermomyotome. Here, by a series of gain and loss of function experiments, we showed that Mef2d, a member of the Mef2 family of MADS-box transcription factors, appeared as an upstream regulator of lateral myogenesis, and as an inducer of dermomyotome formation at the beginning of neurulation. In the lateral presomitic cells, we showed that Mef2d transactivates Myod expression which is necessary for lateral myogenesis. In the most lateral cells of the presomitic mesoderm, we showed that Mef2d and Paraxis (Tcf15), a member of the Twist family of transcription factors, were co-localized and activate directly the expression of Meox2, which acts upstream of Pax3 expression during dermomyotome formation. Cell tracing experiments confirm that the most lateral Meox2 expressing cells of the presomitic mesoderm correspond to the dermomyotome progenitors since they give rise to the most dorsal cells of the somitic mesoderm. Thus, Xenopus Mef2d couples lateral myogenesis to dermomyotome formation before somite segmentation. These results together with our previous works reveal striking similarities between dermomyotome and tendon formation in Xenopus: both develop in association with myogenic cells and both involve a gene transactivation pathway where one member of the Mef2 family, Mef2d or Mef2c, cooperates with a bHLH protein of the Twist family, Paraxis or Scx (Scleraxis) respectively. We propose that these shared characteristics in Xenopus laevis reflect the existence of a vertebrate ancestral mechanism which has coupled the development of the myogenic cells to the formation of associated tissues during somite compartmentalization.

A global view of gene expression in lithium and zinc treated sea urchin embryos: new components of gene regulatory networks

BACKGROUND

The genome of the sea urchin Strongylocentrotus purpuratus has recently been sequenced because it is a major model system for the study of gene regulatory networks. Embryonic expression patterns for most genes are unknown, however.

RESULTS

Using large-scale screens on arrays carrying 50% to 70% of all genes, we identified novel territory-specific markers. Our strategy was based on computational selection of genes that are differentially expressed in lithium-treated embryos, which form excess endomesoderm, and in zinc-treated embryos, in which endomesoderm specification is blocked. Whole-mount in situ hybridization (WISH) analysis of 700 genes indicates that the apical organ region is eliminated in lithium-treated embryos. Conversely, apical and specifically neural markers are expressed more broadly in zinc-treated embryos, whereas endomesoderm signaling is severely reduced. Strikingly, the number of serotonergic neurons is amplified by at least tenfold in zinc-treated embryos. WISH analysis further indicates that there is crosstalk between the Wnt (wingless int), Notch, and fibroblast growth factor signaling pathways in secondary mesoderm cell specification and differentiation, similar to signaling cascades that function during development of presomitic mesoderm in mouse embryogenesis. We provide differential expression data for more than 4,000 genes and WISH patterns of more than 250 genes, and more than 2,400 annotated WISH images.

CONCLUSION

Our work provides tissue-specific expression patterns for a large fraction of the sea urchin genes that have not yet been included in existing regulatory networks and await functional integration. Furthermore, we noted neuron-inducing activity of zinc on embryonic development; this is the first observation of such activity in any organism.

The homeobox gene GAX activates p21WAF1/CIP1 expression in vascular endothelial cells through direct interaction with upstream AT-rich sequences

Tumors secrete pro-angiogenic factors to induce the ingrowth of blood vessels from the surrounding stroma, the end targets of which are vascular endothelial cells (ECs). The homeobox gene GAX inhibits angiogenesis and induces p21(WAF1/CIP1) expression in vascular ECs. To elucidate the mechanism through which GAX activates p21(WAF1/CIP1) expression, we constructed GAX cDNAs with deletions of the N-terminal domain, the homeodomain, or the C-terminal domain and then assessed these constructs for their ability to activate p21(WAF1/CIP1). There was an absolute requirement for the homeodomain, whereas deleting the C-terminal domain decreased but did not abolish transactivation of the p21(WAF1/CIP1) promoter by GAX. Deleting the N-terminal domain did abolish transactivation. Next, we performed chromatin immunoprecipitation and found, approximately 15 kb upstream of the p21(WAF1/CIP1) ATG codon, an ATTA-containing GAX-binding site (designated A6) with a sequence similar to that of other homeodomain-binding sites. GAX was able to bind to A6 in a homeodomain-dependent manner and thereby activate the expression of a reporter gene coupled to this sequence, and this activation was abolished by mutating specific residues in this sequence. On the basis of the sequence of A6, we were then able to locate other ATTA-containing sequences that also bound GAX and activated transcription in reporter constructs. Finally, we found that the ability of these GAX deletions to induce G(0)/G(1) arrest correlates with their ability to transactivate the p21(WAF1/CIP1) promoter. We conclude that GAX activates p21(WAF1/CIP1) through multiple upstream AT-rich sequences. Given the multiple biological activities of GAX in regulating EC function, identification of a putative GAX-binding site will allow the study of how GAX activates or represses other downstream targets to inhibit angiogenesis.

Vitamin K-dependent proteins in Ciona intestinalis, a basal chordate lacking a blood coagulation cascade

We have isolated and sequenced several cDNAs derived from the sea squirt Ciona intestinalis that encode vitamin K-dependent proteins. Four of these encode gamma-carboxyglutamic acid (Gla) domain-containing proteins, which we have named Ci-Gla1 through Ci-Gla4. Two additional cDNAs encode the apparent orthologs of gamma-glutamyl carboxylase and vitamin K epoxide reductase. Ci-Gla1 undergoes gamma-glutamyl carboxylation when expressed in CHO cells and is homologous to Gla-RTK, a putative receptor tyrosine kinase previously identified in a related ascidian. The remaining three Gla domain proteins are similar to proteins that participate in fundamental developmental processes, complement regulation, and blood coagulation. These proteins are generally expressed at low levels throughout development and exhibit either relatively constant expression (Ci-Gla1, gamma-glutamyl carboxylase, and vitamin K epoxide reductase) or spatiotemporal regulation (Ci-Gla2, -3, and -4). These results demonstrate the evolutionary emergence of the vitamin K-dependent Gla domain before the divergence of vertebrates and urochordates and suggest novel functions for Gla domain proteins distinct from their roles in vertebrate hemostasis. In addition, these findings highlight the usefulness of C. intestinalis as a model organism for investigating vitamin K-dependent physiological phenomena, which may be conserved among the chordate subphyla.

The homeobox gene Gax inhibits angiogenesis through inhibition of nuclear factor-kappaB-dependent endothelial cell gene expression

The growth and metastasis of tumors are heavily dependent on angiogenesis, but much of the transcriptional regulation of vascular endothelial cell gene expression responsible for angiogenesis remains to be elucidated. The homeobox gene Gax is expressed in vascular endothelial cells and inhibits proliferation and tube formation in vitro. We hypothesized that Gax is a negative transcriptional regulator of the endothelial cell angiogenic phenotype and studied its regulation and activity in vascular endothelial cells. Several proangiogenic factors caused a rapid down-regulation of Gax mRNA in human vascular endothelial cells, as did conditioned media from breast cancer cell lines. In addition, Gax expression using a replication-deficient adenoviral vector inhibited human umbilical vein endothelial cell migration toward proangiogenic factors in vitro and inhibited angiogenesis in vivo in Matrigel plugs. To identify putative downstream targets of Gax, we examined changes in global gene expression in endothelial cells due to Gax activity. Gax expression resulted in changes in global gene expression consistent with a quiescent, nonangiogenic phenotype, with increased expression of cyclin kinase inhibitors and decreased expression of genes implicated in endothelial cell activation and angiogenesis. Further analysis revealed that Gax down-regulated numerous nuclear factor-kappaB (NF-kappaB) target genes and decreased the binding of NF-kappaB to its target sequence in electrophoretic mobility shift assays. To our knowledge, Gax is the first homeobox gene described that inhibits NF-kappaB activity in vascular endothelial cells. Because NF-kappaB has been implicated in endothelial cell activation and angiogenesis, the down-regulation of NF-kappaB-dependent genes by Gax suggests one potential mechanism by which Gax inhibits the angiogenic phenotype.

Inhibition of endothelial cell activation by the homeobox gene Gax

BACKGROUND

cardiovascular system, strongly inhibits growth factor-stimulated phenotypic modulation of vascular smooth muscle cells in vitro and in vivo. The function of Gax in vascular endothelium is unknown, but we hypothesized that it may play a similar role there. We therefore studied Gax expression in vascular endothelial cells and its effects on proliferation and tube formation.

MATERIALS AND METHODS

Gax expression in normal endothelial cells was examined in vitro by Northern blot and reverse transcriptase polymerase chain reaction and in vivo by immunohistochemistry. A replication-deficient adenovirus was then used to express Gax in human umbilical vein endothelial cells (HUVECs). HUVEC proliferation, 3H-thymidine uptake, p21 expression, and tube formation on reconstituted basement membrane were measured at different viral multiplicities of infection.

RESULTS

Gax mRNA was detected in HUVECs by reverse transcriptase polymerase chain reaction and Northern blot analysis and in normal vascular endothelium by immunohistochemistry. Compared with controls transduced with a virus expressing beta-galactosidase, Gax strongly inhibited HUVEC proliferation and mitogen-stimulated 3H-thymidine uptake. p21 expression in HUVECs transduced with Gax was increased up to 5-fold as measured by Northern blot, and p21 promoter activity was activated by 4- to 5-fold. Tube formation on Matrigel was strongly inhibited by Gax expression.

CONCLUSIONS

Gax is expressed in vascular endothelium and strongly inhibits endothelial cell activation in response to growth factors and tube formation in vitro. These observations suggest that Gax inhibits endothelial cell transition to the angiogenic phenotype in response to proangiogenic growth factors and, as a negative regulator of angiogenesis, may represent a target for the antiangiogenic therapy of cancer.

The single amphioxus Mox gene: insights into the functional evolution of Mox genes, somites, and the asymmetry of amphioxus somitogenesis

Mox genes are members of the "extended" Hox-cluster group of Antennapedia-like homeobox genes. Homologues have been cloned from both invertebrate and vertebrate species, and are expressed in mesodermal tissues. In vertebrates, Mox1 and Mox2 are distinctly expressed during the formation of somites and differentiation of their derivatives. Somites are a distinguishing feature uniquely shared by cephalochordates and vertebrates. Here, we report the cloning and expression of the single amphioxus Mox gene. AmphiMox is expressed in the presomitic mesoderm (PSM) during early amphioxus somitogenesis and in nascent somites from the tail bud during the late phase. Once a somite is completely formed, AmphiMox is rapidly downregulated. We discuss the presence and extent of the PSM in both phases of amphioxus somitogenesis. We also propose a scenario for the functional evolution of Mox genes within chordates, in which Mox was co-opted for somite formation before the cephalochordate-vertebrate split. Novel expression sites found in vertebrates after somite formation postdated Mox duplication in the vertebrate stem lineage, and may be linked to the increase in complexity of vertebrate somites and their derivatives, e.g., the vertebrae. Furthermore, AmphiMox expression adds new data into a long-standing debate on the extent of the asymmetry of amphioxus somitogenesis.

Homeodomain proteins Mox1 and Mox2 associate with Pax1 and Pax3 transcription factors

Mox1 and Mox2 homeobox genes have been shown to be critical in axial skeleton and in limb muscle development respectively. Pax1 and Pax3 gene products are also implicated in these processes. Mox and Pax expression patterns are highly overlapping both spatially and temporally during embryonic development. We show here for the first time that Mox proteins physically interact with Pax1 and Pax3 using the yeast two-hybrid protein interaction assay as well as in vitro biochemical assays. There is a strong preference of Mox1 to associate with Pax1 rather than Pax3 and of Mox2 to associate with Pax3 rather than Pax1. The observed interactions are mediated through the homeodomain of Mox.

Isolation of the avian homologue of the homeobox gene Mox2 and analysis of its expression pattern in developing somites and limbs

We have isolated the cDNA of avian Mox2 and analyzed its expression pattern during somitogenesis and limb bud formation. Mox2 plays an important role in limb muscle differentiation in the mouse. Mox2 is expressed in the somites of developing chick embryos and in presumptive migrating myoblasts from the dermomyotome to the limb buds. It is also expressed in the ventral and dorsal part of limb buds and is associated with non-proliferating myoblasts. Significant differences were observed in chick and mouse expression patterns, namely in the chick dermomyotome and limb.

The homeobox genes MSX2 and MOX2 are candidates for regulating epithelial-mesenchymal cell interactions in the human placenta

Homeobox genes of the Msx and Mox families are coexpressed in the vertebrate embryo in regions of epithelial-mesenchymal interactions. Here we show that a member of each family is expressed in extra-embryonic structures where epithelial and mesenchymal cell layers contact. In situ hybridization studies on first trimester human placental sections reveal that MSX2 and MOX2 are expressed predominantly in the cytotrophoblast cell layer. In term placenta, MSX2 and MOX2 are expressed in the syncytiotrophoblast. This is the first study to describe the expression of MOX2 in human tissues and to show that members of the Msx and Mox families of homeobox genes are expressed where epithelial and mesenchymal cell layers contact in the human placenta. A combinatorial code of homeobox genes that includes members of the Msx, Mox and Dlx families has been predicted to regulate epithelial-mesenchymal cell interactions in the vertebrate embryo. We have shown that MSX2, MOX2, DLX4 and the HB24 homeobox gene are expressed in the epithelial and mesenchymal cell types that form the placenta. We predict that this combination of homeobox genes is involved in regulating epithelial-mesenchymal cell interactions in extraembryonic tissues.

Homeobox genes in cardiovascular development

As summarized earlier, a surprisingly large number of different homeobox genes are expressed in the developing heart. Some are clearly important, as demonstrated by mouse gene ablation studies. For example, knockout of Nkx2-5 or Hoxa-3 function is embryonic lethal due to defects in cardiovascular development. However, gene ablation studies indicate that other homeobox genes that show cardiovascular expression are either not required for heart development or their function is effectively complemented by a redundant gene activity. Given the number of closely related homeobox genes that are expressed in the heart (and the rate at which new genes are being discovered), this is very likely to be the case for at least some homeobox gene activities. At present little is known of the precise mechanism of action of homeobox genes in embryonic development. This statement applies to homeobox genes in general, not just to genes involved in cardiovascular development. There is a popular view that homeobox genes are master regulators that control expression of a large number of downstream genes. In at least some cases, e.g., the eyeless gene of Drosophila (Holder et al., 1995), homeobox genes appear to be capable of activating and maintaining a very complex developmental program. Significantly, the eyeless gene is able to initiate eye development at numerous ectopic locations. Increasing evidence, however, suggests that genes of this type may be rather rare. Certainly there is no evidence to date that any of the homeobox genes expressed in the heart are able to initiate the complete heart development pathway. This is probably best understood in the case of the tinman gene in Drosophila, which, although absolutely required for heart development, is not capable of initiating the cardiac development pathway in ectopic locations (Bodmer, 1993). This conclusion is supported by studies of the vertebrate tinman-related gene Nkx2-5. Gene ablation studies show that Nkx2-5 is essential for correct cardiac development (Lyons et al., 1995) but is not able to initiate the regulatory pathway leading to cardiac development when expressed ectopically (Cleaver et al., 1996; Chen and Fishman, 1996). If most homeodomain proteins are not direct regulators of a differentiation pathway, what is their role during organogenesis? The cardiovascular homeobox gene about which most is known at the mechanistic level is gax (Smith et al., 1997). A number of experiments indicate that the Gax protein is involved in the regulation of cell proliferation and that it interacts with components of the cell cycle regulation machinery. Indeed, over recent years, the idea that at least some homeobox genes play their role in organogenesis through regulation of proliferation has been developed in some detail by Duboule (1995). Further evidence that this mechanism of homeobox activity is important, especially during organogenesis, comes from studies of the Hox11 homeobox gene, which is absolutely required for development of the spleen in mouse (Roberts et al., 1994). Studies indicate that Hox11 is able to interact with at least two different protein phosphatases, PP2A and PP1, which in turn, are involved in cell cycle regulation (Kawabe et al., 1997). It is quite clear that research in future years will need to focus on the precise mode of action of the different homeodomain proteins if we are to understand their role in the development of the cardiovascular system.

A distal-less class homeobox gene, DLX4, is a candidate for regulating epithelial-mesenchymal cell interactions in the human placenta

Homeobox genes of the Distal-less (Dlx) family are expressed in the vertebrate embryo in regions where epithelial cell layers contact adjacent mesenchymal cells. This study shows that the human Dlx family member, DLX4, is expressed in the placenta, primarily in regions where epithelial and mesenchymal cell layers contact. In situ hybridization studies at first trimester human placental sections revealed that DLX4 was expressed predominantly in the cytotrophoblast stem cell layer. In term placenta, DLX4 was expressed in the syncytiotrophoblast. Northern analysis revealed two DLX4 transcripts in first trimester placenta of 2.8 and 3.0 kb. Elevated levels of DLX4 mRNA were detected in a choriocarcinoma derived cell line when compared with a cytotrophoblast cell line and normal placenta. This is the first study to show that a member of the Dlx family of homeobox genes is expressed in regions of epithelial and mesenchymal cell layer contact in the human. Accumulated evidence from several studies suggest that a combinatorial code of homeobox genes is required to regulate epithelial-mesenchymal cell interactions in the vertebrate embryo. It is predicted that a similar combination of homeobox genes, that includes DLX4, is involved in regulating epithelial-mesenchymal cell interactions in extraembryonic tissues. DLX4 may also have a role in the regulation of the genes important for trophoblast invasion since the level of expression in trophoblast cell lines reflects invasive potential.

Forced expression of the homeodomain protein Gax inhibits cardiomyocyte proliferation and perturbs heart morphogenesis

The development of the tubular heart into a complex four-chambered organ requires precise temporal and region-specific regulation of cell proliferation, migration, death and differentiation. While the regulatory mechanisms in heart morphogenesis are not well understood, increasing attention has focused on the homeodomain proteins, which are generally linked to morphogenetic processes. The homeodomain containing gene Gax has been shown to be expressed in heart and smooth muscle tissues. In this study, the Gax protein was detected in the nuclei of myocardial cells relatively late in chicken heart development, at a time when myocyte proliferation is declining. To test the hypothesis that the Gax protein functions as a negative regulator of cardiomyocyte proliferation, a replication-defective adenovirus was used to force its precocious nuclear expression during chicken heart morphogenesis. In experiments in which Gax- and beta-galactosidase-expressing adenoviruses were co-injected, clonal expansion of myocytes was reduced, consistent with inhibition of myocyte proliferation. This effect on proliferation was corroborated by the finding that the percentage of exogenous Gax-expressing myocytes that were positive for the cell cycle marker PCNA decreased over time and was lower than in control myocytes. The precocious nuclear expression of Gax in tubular hearts resulted in abnormal heart morphology, including small ventricles with rounded apices, a thinned compact zone and coarse trabeculae. These results suggest a role for the Gax protein in heart morphogenesis causing proliferating cardiomyocytes to withdraw from the cell cycle, thus influencing the size and shape that the heart ultimately attains.

Markers of vertebrate mesoderm induction

Mesoderm formation is the first major differentiative event in vertebrate development. Many new mesoderm-specific genes have recently been described in the mouse, chick, frog and fish and belong to classes comprising T-domain genes, homeobox genes and those encoding secreted proteins. The T-domain genes have different but overlapping expression patterns and, in Xenopus, can ectopically activate nearly all other mesodermal genes. Several new homebox genes seem to mediate the ventralising activity of bone morphogenetic protein. New genes encoding secreted proteins induce dorsal mesoderm, in some cases by antagonizing ventralising factors.

The human homeobox genes MSX-1, MSX-2, and MOX-1 are differentially expressed in the dermis and epidermis in fetal and adult skin

In order to identify homeobox genes which may regulate skin development and possibly mediate scarless fetal wound healing we have screened amplified human fetal skin cDNAs by polymerase chain reaction (PCR) using degenerate oligonucleotide primers designed against highly conserved regions within the homeobox. We identified three non-HOX homeobox genes, MSX-1, MSX-2, and MOX-1, which were differentially expressed in fetal and adult human skin. MSX-1 and MSX-2 were detected in the epidermis, hair follicles, and fibroblasts of the developing fetal skin by in situ hybridization. In contrast, MSX-1 and MSX-2 expression in adult skin was confined to epithelially derived structures. Immunohistochemical analysis of these two genes suggested that their respective homeoproteins may be differentially regulated. While Msx-1 was detected in the cell nucleus of both fetal and adult skin; Msx-2 was detected as a diffuse cytoplasmic signal in fetal epidermis and portions of the hair follicle and dermis, but was localized to the nucleus in adult epidermis. MOX-1 was expressed in a pattern similar to MSX early in gestation but then was restricted exclusively to follicular cells in the innermost layer of the outer root sheath by 21 weeks of development. Furthermore, MOX-1 expression was completely absent in adult cutaneous tissue. These data imply that each of these homeobox genes plays a specific role in skin development.

A Mox homeobox gene in the gastropod mollusc Haliotis rufescens is differentially expressed during larval morphogenesis and metamorphosis

We have isolated a homeobox-containing cDNA from the gastropod mollusc Haliotis rufescens that is most similar to members of the Mox homeobox gene class. The derived Haliotis homeodomain sequence is 85% identical to mouse and frog Mox-2 homeodomains and 88.9% identical to the partial cnidarian cnox5-Hm homeodomain. Quantitative reverse transcription-polymerase chain reaction analysis of mRNA accumulation reveals that this gene, called HruMox, is expressed in the larva, but not in the early embryo. Transcripts are most prevalent during larval morphogenesis from trochophore to veliger. There are also transient increases in transcript prevalence 1 and 3 days after the intitiation of metamorphosis from veliger to juvenile. The identification of a molluscan Mox homeobox gene that is more closely related to vertebrate genes than other protostome (e.g. Drosophila) genes suggests the Mox class of homeobox genes may consist of several different families that have been conserved through evolution.

Antagonizing the Spemann organizer: role of the homeobox gene Xvent-1

We have identified a novel homeobox gene, Xvent-1, that is differentially expressed in the ventral marginal zone of the early Xenopus gastrula. Evidence is presented from mRNA microinjection experiments for a role for this gene in dorsoventral patterning of mesoderm. First, Xvent-1 is induced by BMP-4, a gene known to be a key regulator of ventral mesoderm development. Second, Xvent-1 and the organizer-specific gene goosecoid are able to interact, directly or indirectly, in a cross-regulatory loop suppressing each other's expression, consistent with their mutually exclusive expression in the marginal zone. Third, microinjection of Xvent-1 mRNA ventralizes dorsal mesoderm. The results suggest that Xvent-1 functions in a ventral signaling pathway that maintains the ventral mesodermal state and antagonizes the Spemann organizer.