Progressive restriction in the distribution of the Hox-1.3 homeodomain protein during embryogenesis

Dynamic expression of murine HOXA5 protein in the central nervous system

The Hox genes encode transcription factors that are indispensable for proper spatio-temporal patterning of the vertebrate body axes. As for other Hox genes, region-specific expression of Hoxa5 appears to be important for correct function during development. In mouse, Hoxa5 transcripts are differentially expressed in specific mesoderm-derived structures and in the most anterior domain of expression in the central nervous system (CNS), in contrast to indistinct patterns seen in the posterior CNS. However, the functional significance of any pattern of protein-coding RNAs must be verified by correlating the presence of the protein(s) and RNAs. Here, we describe the dynamic pattern of HOXA5 protein during mouse embryogenesis. The HOXA5 protein is detected as early as embryonic day (E) 9.0, and is found, as development proceeds, in several mesoderm-derived structures such as prevertebrae (pv), proximal forelimb bud, scapula, lung, trachea, and gut. In addition, the protein shows a strikingly restricted and dynamic expression pattern in the developing CNS, and is detected in both motor neurons and interneurons between E10.5 and E13.5. Moreover, this CNS region-specific HOXA5 protein pattern is more restricted than the pattern observed for the Hoxa5 transcripts. In many mesoderm-derived tissues affected by the Hoxa5 mutation, the expression pattern of HOXA5 protein corresponds to that of the putative functional Hoxa5 transcript. However, in the CNS, this correlation is exclusively demonstrated in the most anterior domain of expression. Overall, the HOXA5 protein pattern is consistent with its proposed role in positional specification in mesodermal structures, as well as in the embryonic neuraxis.

Ectopic HOXA5 expression results in abnormal differentiation, migration and p53-independent cell death of superficial dorsal horn neurons

Previously, we reported a line of mice (Hoxa5SV2) that ectopically expresses HOXA5 in the developing cervical and brachial dorsal spinal cord. Animals from this line exhibited a clear loss of cells in the outer lamina of the mature dorsal horn that coincided with an adult phenotype of sensory and motor defects of the forelimb. In this report, we examined the etiology of lost dorsal horn cells. Cells normally fated to populate the outer laminae I-III of the dorsal horn migrated inappropriately, as the percentage of laterally positioned cells in the dorsal horn was significantly reduced in Hoxa5SV2 transgenics. Apoptosis was a major cause of cell loss while proliferation of neurons was not affected in Hoxa5SV2 animals. Although Hoxa5 has been shown in vitro to regulate p53 expression and cause p53-dependent apoptosis, p53 was not required in vivo for the inappropriate apoptosis seen in Hoxa5SV2 mice, or for the normal death of motor neurons. Normal apoptosis is not dependent on Hoxa5, as the level of ventral horn motor neuron apoptosis was not changed in Hoxa5 null animals. As a possible cause of aberrant migration and/or apoptosis of dorsal neurons, misexpression of cell type markers was demonstrated. Further, the expression pattern of laminar markers was altered and sensory fibers aberrantly penetrated the outer lamina of mutants. Our evidence suggests that the loss of dorsal horn neurons in Hoxa5SV2 mutants was due to misexpression of dorsal horn neuronal markers, aberrant migration, and inappropriate apoptosis.

Identification of transcriptional targets of HOXA5

The homeobox gene HOXA5 encodes a transcription factor that has been shown to play important roles in embryogenesis, hematopoiesis, and tumorigenesis. In order to decipher downstream signaling pathways of HOXA5, we utilized oligonucleotide microarray analysis to identify genes that are differentially expressed in HOXA5-induced cells compared with uninduced cells. Comparative analysis of gene expression changes after 9 h of HOXA5 induction in Hs578T breast cancer cells identified 306 genes whose expression was modulated at least 2-fold. Ten of these 306 genes were also up-regulated by at least 2-fold at 6 h post-induction. The expression of all of these 10 genes was confirmed by semiquantitative reverse transcription-PCR. Among these 10 genes, which are most likely to be direct targets of HOXA5, we initiated an investigation into the pleiotrophin gene by first cloning its promoter. Transient transfection assays indicated that HOXA5 can specifically activate the pleiotrophin promoter. Promoter deletion, chromatin immunoprecipitation assay, and gel-shift assays were performed to show that HOXA5 can directly bind to one binding site on the pleiotrophin promoter. These data strongly suggest that microarray analysis can successfully identify many potential direct downstream genes of HOXA5. Further functional analysis of these targets will allow us to better understand the diverse functions of HOXA5 in embryonic development and tumorigenesis.

Homeobox protein, Hmx3, in postnatally developing rat submandibular glands

Homeobox-containing (Hox) genes play important roles in development, particularly in the development of neurons and sensory organs, and in specification of body plan. The Hmx gene family is a new class of homeobox-containing genes defined by a conserved homeobox region and a characteristic pattern of expression in the central nervous system that is more rostral than that of the Hox genes. To date, three closely related members of the Hmx family, Hmx1, Hmx2, and Hmx3, have been described. All three Hmx genes are expressed in the craniofacial region of developing embryos. Here we show, for the first time, the expression of the transcription factor Hmx3 in postnatally developing salivary glands. Hmx3 protein is expressed in a cell type-specific manner in rat salivary glands. Hmx3 is present in both the nuclei and cytoplasm of specific groups of duct cells of the submandibular, parotid, and sublingual glands. Hmx3 expression increases during postnatal development of the submandibular gland. The duct cells show increasing concentrations of Hmx3 protein with progressive development of the submandibular gland. In contrast, the acinar cells of the three salivary glands do not exhibit detectable levels of Hmx3 protein.

A proximal promoter domain containing a homeodomain-binding core motif interacts with multiple transcription factors, including HoxA5 and Phox2 proteins, and critically regulates cell type-specific transcription of the human norepinephrine transporter gene

Expression of the norepinephrine transporter (NET), which mediates the reuptake of norepinephrine into presynaptic nerve terminals, is restricted to noradrenergic (NA) neurons. We have demonstrated previously that the 9.0 kb upstream sequences and the first intron residing in the 5' untranslated area are critical for high-level and NA cell-specific transcription. Here, using transient transfection assays, we show that 4.0 kb of the 5' upstream sequences contains sufficient genetic information to drive reporter gene expression in an NA cell type-specific manner. Three functional domains appear to be potentially important for the regulation of human NET (hNET) gene transcription: an upstream enhancer region at -4.0 to -3.1 kb, a proximal domain at -133 to -75 bp, and a middle silencer region between these two domains. DNase I footprinting analysis of the proximal promoter region shows that a subdomain at -128 to -80 bp is protected in a cell-specific manner. We provide evidence that multiple protein factors interact with the proximal promoter domain to critically regulate the transcriptional activity of the hNET gene. In the middle of this proximal subdomain resides a homeodomain (HD)-binding core motif, which interacts with HD factors, including Phox2a and HoxA5, in an NA-specific manner. Cotransfection analyses suggest that HoxA5 and Phox2a may transactivate the hNET gene promoter. Together with previous studies indicating direct activation of dopamine beta-hydroxylase transcription by Phox2a/2b, the present results support a model whereby Phox2 proteins may coordinately regulate the phenotypic specification of NA neurons by activating both NA biosynthetic and reuptake genes.

A novel PF/PN motif inhibits nuclear localization and DNA binding activity of the ESX1 homeoprotein

Despite their significance for mammalian embryogenesis, the molecular mechanisms that regulate placental growth and development have not been well defined. The Esx1 homeobox gene is of particular interest because it is among the few regulatory genes that have specific expression and function in the placenta during murine development. In addition, the ESX1 protein contains several notable features that are not often associated with homeoproteins, including an atypical homeodomain of the paired-like class, a proline-rich region that contains an SH3 binding motif, and a novel repeat region consisting of prolines alternating with phenylalanines or asparagines that we term the PF/PN motif. We have found that the ESX1 protein is expressed in the labyrinth layer of the placenta in vivo, where its subcellular localization is primarily cytoplasmic. Our results suggest that this unexpected subcellular localization is conferred by the PF/PN motif, which inhibits nuclear localization of ESX1 in cell culture, as well as its DNA binding activity in vitro. Finally, we show that the proline-rich region of ESX1 mediates interactions in vitro with the c-abl SH3 domain as well as with certain WW domains. We propose that the PF/PN motif provides a novel mechanism for regulating nuclear entry and that the essential function of ESX1 during placental development is mediated by its ability to couple cytoplasmic signal transduction events with transcriptional regulation in the nucleus.

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.

Salivary glands: a paradigm for diversity of gland development

The major salivary glands of mammals are represented by three pairs of organs that cooperate functionally to produce saliva for the oral cavity. While each type of gland produces a signature secretion that complements the secretions from the other glands, there is also redundancy as evidenced by secretion of functionally similar and, in some cases, identical products in the three glands. This, along with their common late initiation of development, in fetal terms, their similarities in developmental pattern, and their proximate sites of origin, suggests that a common regulatory cascade may have been shared until shortly before the onset of overt gland development. Furthermore, occasional ectopic differentiation of individual mature secretory cells in the "wrong" gland suggests that control mechanisms responsible for the distinctive cellular composition of each gland also share many common steps, with only minor differences providing the impetus for diversification. To begin to address this area, we examine here the origins of the salivary glands by reviewing the expression patterns of several genes with known morphogenetic potential that may be involved based on developmental timing and location. The possibility that factors leading to determination of the sites of mammalian salivary gland development might be homologous to the regulatory cascade leading to salivary gland formation in Drosophila is also evaluated. In a subsequent section, cellular phenotypes of neonatal and adult glands are compared and evaluated for insights into the mechanisms and lineages leading to cellular diversification. Finally, the phenomena of proliferation, repair, and regeneration in adult salivary glands are reviewed, with emphasis on the extent to which the cellular diversity is reversible and which cell type other than stem cells has the ability to redifferentiate into other cell types.

Nucleolar localisation of three Hox homeoproteins

Homeoproteins encoded by genes of the Hox family are nuclear proteins believed to act as transcription factors and to participate in the determination of the body plan. Here we show that in several vertebrate cells, they exhibit a subnuclear localisation associated with the nucleolus. We used monoclonal antibodies to study the distribution of three homeoproteins, namely HOXB7, HOXC6 and HOXD4. The immunoreactivity to antibodies against HOXC6 protein in Xenopus laevis embryonic tissues is restricted to one or two spots within the nucleus; this distribution partially overlaps that of fibrillarin, a protein of the fibrillar zone of the nucleoli. Indirect immunofluorescence analysis of the distribution of HOXB7 protein in 3T3 cells, and of HOXD4 protein in human neuroblastoma and Raji lymphoma cell lines and activated lymphocytes, results invariably in a nucleolar localisation. Purified nucleoli from stimulated T lymphocytes, and Raji cells contain an activity capable of binding, in a gel retardation assay, to an oligonucleotide specifically recognised by the HOXD4 homeoprotein. This activity is specifically removed by anti-HOXD4 antibodies and is found associated in southwestern blots with a single band with an apparent M(r) of 30,000, corresponding to that of recombinant HOXD4. The functional significance of the nucleolar localisation of Hox proteins remains to be determined.

Plant morphogenesis. More knots untied

Knox genes are expressed within restricted domains of the maize shoot apical meristem. The patterns observed suggest that this homeobox gene family plays a pivotal role in the control of plant morphogenesis.

Homeodomain proteins in development and therapy

Homeobox genes encode transcriptional regulators found in all organisms ranging from yeast to humans. In Drosophila, a specific class of homeobox genes, the homeotic genes, specifies the identity of certain spatial units of development. Their genomic organization, in Drosophila, as well as in vertebrates, is uniquely connected with their expression which follows a 5'-posterior-3'-anterior rule along the longitudinal body axis. The 180-bp homeobox is part of the coding sequence of these genes, and the sequence of 60 amino acids it encodes is referred to as the homeodomain. Structural analyses have shown that homeodomains consist of a helix-turn-helix motif that binds the DNA by inserting the recognition helix into the major groove of the DNA and its amino-terminal arm into the adjacent minor groove. Developmental as well as gene regulatory functions of homeobox genes are discussed, with special emphasis on one group, the Antennapedia (Antp) class homeobox genes and a representative 60-amino acid Antennapedia peptide (pAntp). In cultured neuronal cells, pAntp translocates through the membrane specifically and efficiently and accumulates in the nucleus. The internalization process is followed by a strong induction of neuronal morphological differentiation, which raises the possibility that motoneuron growth is controlled by homeodomain proteins. It has been demonstrated that chimeric peptide molecules encompassing pAntp are also captured by cultured neurons and conveyed to their nuclei. This may be of enormous interest for the internalization of drugs.

The mouse Hox-1.3 gene is functionally equivalent to the Drosophila Sex combs reduced gene

To test whether the mouse Hox-1.3 gene is a cognate of the Drosophila Sex combs reduced (Scr) gene, we inserted a hsp 70-Hox-1.3 fusion gene into the Drosophila genome. Transgenic flies displayed Scr-like homeotic transformations after ectopic expression of Hox-1.3 induced by heat shock. In larvae, the thoracic segments T2 and T3 are transformed toward T1. In adults, head structures are dramatically disrupted, including transformation of antenna towards leg. Transformations are not the result of ectopic activation of the endogenous Scr gene. Rather, Hox-1.3 appears to directly regulate Scr target genes, as demonstrated by the ectopic activation of fork head by Hox-1.3. The results suggest that mouse Hox-1.3 cannot only substitute functionally for Drosophila Scr in the determination of external structures but also can participate in the regulatory hierarchy of insect organogenesis.

Antennapedia homeobox peptide regulates neural morphogenesis

We synthesized the 60-amino acid polypeptide corresponding to the sequence of the Drosophila antennapedia gene homeobox. This peptide (pAntp) recognized the consensus motif for binding to the promoter region of Hox-1.3. pAntp mechanically introduced into mammalian nerve cells provoked a dramatic morphological differentiation of the neuronal cultures. Moreover, pAntp directly added to already differentiated neuronal cultures penetrated the cells and further augmented their morphological differentiation. Examination of live and fixed neurons in classical and confocal fluorescence microscopy demonstrated that pAntp was captured at all regions of the nerve cells and accumulated in the nuclei. In addition, the effect of pAntp on neurite extension was blocked in the presence of the protein synthesis inhibitor cycloheximide. Thus, our results demonstrate that neurons possess an efficient uptake system for the antennapedia homeobox peptide and suggest that binding of pAntp to consensus motifs present in nerve cell nuclei influences neuronal morphogenetic programs.