Normal reproductive and macrophage function in Pem homeobox gene-deficient mice

Mammal Reproductive Homeobox (Rhox) Genes: An Update of Their Involvement in Reproduction and Development

The reproductive homeobox on the X chromosome (RHOX) genes were first identified in the mouse during the 1990s and have a crucial role in reproduction. In various transcription factors with a key regulatory role, the homeobox sequence encodes a "homeodomain" DNA-binding motif. In the mouse, there are three clusters of Rhox genes (α, β, and γ) on the X chromosome. Each cluster shows temporal and/or quantitative collinearity, which regulates the progression of the embryonic development process. Although the RHOX family is conserved in mammals, the interspecies differences in the number of RHOX genes and pseudogenes testifies to a rich evolutionary history with several relatively recent events. In the mouse, Rhox genes are mainly expressed in reproductive tissues, and several have a role in the differentiation of primordial germ cells (Rhox1, Rhox6, and Rhox10) and in spermatogenesis (Rhox1, Rhox8, and Rhox13). Despite the lack of detailed data on human RHOX, these genes appear to be involved in the formation of germ cells because they are predominantly expressed during the early (RHOXF1) and late (RHOXF2/F2B) stages of germ cell development. Furthermore, the few variants identified to date are thought to induce or predispose to impaired spermatogenesis and severe oligozoospermia or azoospermia. In the future, research on the pathophysiology of the human RHOX genes is likely to confirm the essential role of this family in the reproductive process and might help us to better understand the various causes of infertility and characterize the associated human phenotypes.

Concordant Androgen-Regulated Expression of Divergent Rhox5 Promoters in Sertoli Cells

Concordant transcriptional regulation can generate multiple gene products that collaborate to achieve a common goal. Here we report a case of concordant transcriptional regulation that instead drives a single protein to be produced in the same cell type from divergent promoters. This gene product-the RHOX5 homeobox transcription factor-is translated from 2 different mRNAs with different 5' untranslated regions (UTRs) transcribed from alternative promoters. Despite the fact that these 2 promoters-the proximal promoter (Pp) and the distal promoter (Pd)-exhibit different patterns of tissue-specific activity, share no obvious sequence identity, and depend on distinct transcription factors for expression, they exhibit a remarkably similar expression pattern in the testes. In particular, both depend on androgen signaling for expression in the testes, where they are specifically expressed in Sertoli cells and have a similar stage-specific expression pattern during the seminiferous epithelial cycle. We report evidence for 3 mechanisms that collaborate to drive concordant Pp/Pd expression. First, both promoters have an intrinsic ability to respond to androgen receptor and androgen. Second, the Pp acts as an enhancer to promote androgen-dependent transcription from the Pd. Third, Pd transcription is positively autoregulated by the RHOX5 protein, which is first produced developmentally from the Pp. Together, our data support a model in which the Rhox5 homeobox gene evolved multiple mechanisms to activate both of its promoters in Sertoli cells to produce Rhox5 in an androgen-dependent manner during different phases of spermatogenesis.

Updates in Sertoli Cell-Mediated Signaling During Spermatogenesis and Advances in Restoring Sertoli Cell Function

Since their initial description by Enrico Sertoli in 1865, Sertoli cells have continued to enchant testis biologists. Testis size and germ cell carrying capacity are intimately tied to Sertoli cell number and function. One critical Sertoli cell function is signaling from Sertoli cells to germ cells as part of regulation of the spermatogenic cycle. Sertoli cell signals can be endocrine or paracrine in nature. Here we review recent advances in understanding the interplay of Sertoli cell endocrine and paracrine signals that regulate germ cell state. Although these findings have long-term implications for treating male infertility, recent breakthroughs in Sertoli cell transplantation have more immediate implications. We summarize the surge of advances in Sertoli cell ablation and transplantation, both of which are wedded to a growing understanding of the unique Sertoli cell niche in the transitional zone of the testis.

The Rhox gene cluster suppresses germline LINE1 transposition

Transposable elements (TEs) are mobile sequences that engender widespread mutations and thus are a major hazard that must be silenced. The most abundant active class of TEs in mammalian genomes is long interspersed element class 1 (LINE1). Here, we report that LINE1 transposition is suppressed in the male germline by transcription factors encoded by a rapidly evolving X-linked homeobox gene cluster. LINE1 transposition is repressed by many members of this RHOX transcription factor family, including those with different patterns of expression during spermatogenesis. One family member-RHOX10-suppresses LINE1 transposition during fetal development in vivo when the germline would otherwise be susceptible to LINE1 activation because of epigenetic reprogramming. We provide evidence that RHOX10 suppresses LINE transposition by inducing Piwil2, which encodes a key component in the Piwi-interacting RNA pathway that protects against TEs. The ability of RHOX transcription factors to suppress LINE1 is conserved in humans but is lost in RHOXF2 mutants from several infertile human patients, raising the possibility that loss of RHOXF2 causes human infertility by allowing uncontrolled LINE1 expression in the germline. Together, our results support a model in which the Rhox gene cluster is in an evolutionary arms race with TEs, resulting in expansion of the Rhox gene cluster to suppress TEs in different biological contexts.

Hormone-induced and DNA demethylation-induced relief of a tissue-specific and developmentally regulated block in transcriptional elongation

Genome-wide studies have revealed that genes commonly have a high density of RNA polymerase II just downstream of the transcription start site. This has raised the possibility that genes are commonly regulated by transcriptional elongation, but this remains largely untested in vivo, particularly in vertebrates. Here, we show that the proximal promoter from the Rhox5 homeobox gene recruits polymerase II and begins elongating in all tissues and cell lines that we tested, but it only completes elongation in a tissue-specific and developmentally regulated manner. Relief of the elongation block is associated with recruitment of the elongation factor P-TEFb, the co-activator GRIP1, the chromatin remodeling factor BRG1, and specific histone modifications. We provide evidence that two mechanisms relieve the elongation block at the proximal promoter: demethylation and recruitment of androgen receptor. Together, our findings support a model in which promoter proximal pausing helps confer tissue-specific and developmental gene expression through a mechanism regulated by DNA demethylation-dependent nuclear hormone receptor recruitment.

The RHOX homeodomain proteins regulate the expression of insulin and other metabolic regulators in the testis

Defects in cellular metabolism have been widely implicated in causing male infertility, but there has been little progress in understanding the underlying mechanism. Here we report that several key metabolism genes are regulated in the testis by Rhox5, the founding member of a large X-linked homeobox gene cluster. Among these Rhox5-regulated genes are insulin 2 (Ins2), resistin (Retn), and adiponectin (Adipoq), all of which encode secreted proteins that have profound and wide-ranging effects on cellular metabolism. The ability of Rhox5 to regulate their levels in the testis has the potential to dictate metabolism locally in this organ, given the existence of the blood-testes barrier. We demonstrate that Ins2 is a direct target of Rhox5 in Sertoli cells, and we show that this regulation is physiologically significant, because Rhox5-null mice fail to up-regulate Ins2 expression during the first wave of spermatogenesis and have insulin-signaling defects. We identify other Rhox family members that induce Ins2 transcription, define protein domains and homeodomain amino acid residues crucial for this property, and demonstrate that this regulation is conserved. Rhox5-null mice also exhibit altered expression of other metabolism genes, including those encoding the master transcriptional regulators of metabolism, PPARG and PPARGC1A, as well as SCD1, the rate-limiting enzyme for fatty acid metabolism. These results, coupled with the known roles of RHOX5 and its target metabolism genes in spermatogenesis in vivo, lead us to propose a model in which RHOX5 is a central transcription factor that promotes the survival of male germ cells via its effects on cellular metabolism.

Rhox in mammalian reproduction and development

Homeobox genes play essential roles in embryonic development and reproduction. Recently, a large cluster of homeobox genes, reproductive homeobox genes on the X chromosome (Rhox) genes, was discovered as three gene clusters, α, β, and γ in mice. It was found that Rhox genes were selectively expressed in reproduction-associated tissues, such as those of the testes, epididymis, ovaries, and placenta. Hence, it was proposed that Rhox genes are important for regulating various reproductive features, especially gametogenesis in male as well as in female mammals. It was first determined that 12 Rhox genes are clustered into α (Rhox1-4), β (Rhox5-9), and γ (Rhox10-12) subclusters, and recently Rhox13 has also been found. At present, 33 Rhox genes have been identified in the mouse genome, 11 in the rat, and three in the human. Rhox genes are also responsible for embryonic development, with considerable amounts of Rhox expression in trophoblasts, placenta tissue, embryonic stem cells, and primordial germ cells. In this article we summarized the current understanding of Rhox family genes involved in reproduction and embryonic development and elucidated a previously unreported cell-specific expression in ovarian cells.

Identification of an imprinted gene cluster in the X-inactivation center

Mammalian development is strongly influenced by the epigenetic phenomenon called genomic imprinting, in which either the paternal or the maternal allele of imprinted genes is expressed. Paternally expressed Xist, an imprinted gene, has been considered as a single cis-acting factor to inactivate the paternally inherited X chromosome (Xp) in preimplantation mouse embryos. This means that X-chromosome inactivation also entails gene imprinting at a very early developmental stage. However, the precise mechanism of imprinted X-chromosome inactivation remains unknown and there is little information about imprinted genes on X chromosomes. In this study, we examined whether there are other imprinted genes than Xist expressed from the inactive paternal X chromosome and expressed in female embryos at the preimplantation stage. We focused on small RNAs and compared their expression patterns between sexes by tagging the female X chromosome with green fluorescent protein. As a result, we identified two micro (mi)RNAs–miR-374-5p and miR-421-3p–mapped adjacent to Xist that were predominantly expressed in female blastocysts. Allelic expression analysis revealed that these miRNAs were indeed imprinted and expressed from the Xp. Further analysis of the imprinting status of adjacent locus led to the discovery of a large cluster of imprinted genes expressed from the Xp: Jpx, Ftx and Zcchc13. To our knowledge, this is the first identified cluster of imprinted genes in the cis-acting regulatory region termed the X-inactivation center. This finding may help in understanding the molecular mechanisms regulating imprinted X-chromosome inactivation during early mammalian development.

Regulated expression of Rhox8 in the mouse ovary: evidence for the role of progesterone and RHOX5 in granulosa cells

The gonadotropin surge is the essential trigger to stimulate ovulation and luteinization of ovarian follicles. While the hormone signals from the brain that initiate ovulation are known, the specific targets which regulate this process are not well known. In this study, we assessed the suitability of the Rhox homeobox gene cluster to serve as the master regulators of folliculogenesis. In superovulated (equine chorionic gonadotropin [eCG]/human chorionic gonadotropin [hCG]) mice, the Rhox genes exhibited four distinct windows of peak expression, suggesting that these genes may regulate specific events during the ovulatory cycle. Like many members of the cluster, Rhox8 mRNA and protein were induced by follicle stimulating hormone [FSH]/eCG in granulosa cells. However, Rhox8 displayed unique peak expression at 8 h post-hCG administration, implying it might be the lone member of the cluster regulated by progesterone. Subsequent promoter analysis in granulosa cells revealed relevant homeobox binding and progesterone response elements within Rhox8's 5'-flanking region. In superovulated mice, progesterone receptor (PGR) is recruited to the Rhox8 promoter, as assessed by chromatin immunoprecipitation. In Rhox5-null mice, Rhox8 mRNA was reduced at 2 h and 4 h post-hCG administration but recovered once the follicles passed the antral stage of development. Conversely, in progesterone receptor knockout mice, Rhox8 exhibited normal stimulation by eCG but failed to reach its peak mRNA level at 8 h post-hCG found in wild-type mice. This suggests a model in which Rhox8 transcription is dependent upon RHOX5 during early folliculogenesis and upon progesterone during the periovulatory window when RHOX5 normally wanes. In support of this model, transfection of RHOX5 and PGR expression plasmids stimulated, whereas dominant negative and mutant constructs inhibited, Rhox8 promoter activity.

The RHOX homeobox gene cluster is selectively expressed in human oocytes and male germ cells

STUDY QUESTION

What human tissues and cell types express the X-linked reproductive homeobox (RHOX) gene cluster?

SUMMARY ANSWER

The RHOX homeobox genes and proteins are selectively expressed in germ cells in both the ovary and testis.

WHAT IS KNOWN ALREADY

The RHOX homeobox transcription factors are encoded by an X-linked gene cluster whose members are selectively expressed in the male and female reproductive tract of mice and rats. The Rhox genes have undergone strong selection pressure to rapidly evolve, making it uncertain whether they maintain their reproductive tissue-centric expression pattern in humans, an issue we address in this report.

STUDY DESIGN, SIZE, DURATION

We examined the expression of all members of the human RHOX gene cluster in 11 fetal and 8 adult tissues. The focus of our analysis was on fetal testes, where we evaluated 16 different samples from 8 to 20 weeks gestation. We also analyzed fixed sections from fetal testes, adult testes and adult ovaries to determine the cell type-specific expression pattern of the proteins encoded by RHOX genes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We used quantitative reverse transcription-polymerase chain reaction analysis to assay human RHOX gene expression. We generated antisera against RHOX proteins and used them for western blotting, immunohistochemical and immunofluorescence analyses of RHOXF1 and RHOXF2/2B protein expression.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that the RHOXF1 and RHOXF2/2B genes are highly expressed in the testis and exhibit low or undetectable expression in most other organs. Using RHOXF1- and RHOXF2/2B-specific antiserum, we found that both RHOXF1 and RHOXF2/2B are primarily expressed in germ cells in the adult testis. Early stage germ cells (spermatogonia and early spermatocytes) express RHOXF2/2B, while later stage germ cells (pachytene spermatocytes and round spermatids) express RHOXF1. Both RHOXF1 and RHOXF2/2B are expressed in prespermatogonia in human fetal testes. Consistent with this, RHOXF1 and RHOXF2/2B mRNA expression increases in the second trimester during fetal testes development when gonocytes differentiate into prespermatogonia. In the human adult ovary, we found that RHOXF1 and RHOXF2/2B are primarily expressed in oocytes.

LIMITATIONS, REASONS FOR CAUTION

While the average level of expression of RHOX genes was low or undetectable in all 19 human tissues other than testes, it is still possible that RHOX genes are highly expressed in a small subset of cells in some of these non-testicular tissues. As a case in point, we found that RHOX proteins are highly expressed in oocytes within the human ovary, despite low levels of RHOX mRNA in the whole ovary.

WIDER IMPLICATIONS OF THE FINDINGS

The cell type-specific and developmentally regulated expression pattern of the RHOX transcription factors suggests that they perform regulatory functions during human fetal germ cell development, spermatogenesis and oogenesis. Our results also raise the possibility that modulation of RHOX gene levels could correct some cases of human infertility and that their encoded proteins are candidate targets for contraceptive drug design.

The Rhox5 homeobox gene regulates the region-specific expression of its paralogs in the rodent epididymis

The mechanisms by which the region-specific expression patterns of clustered genes evolve are poorly understood. The epididymis is an ideal organ to examine this, as it is a highly segmented tissue that differs significantly in structure between closely related species. Here we examined this issue through analysis of the rapidly evolving X-linked reproductive homeobox (Rhox) gene cluster, the largest known homeobox gene cluster in metazoans. In the mouse, we found that most Rhox genes are expressed primarily in the caput region of the epididymis, a site where sperm mature and begin acquiring forward motility. This region-specific expression pattern depends, in part, on the founding member of the Rhox cluster--Rhox5--as targeted mutation of Rhox5 greatly diminishes the expression of several other family members in the caput region. In the rat, Rhox5 expression switches from the caput to the site of sperm storage: the cauda. All Rhox genes under the control of Rhox5 in the mouse epididymis display a concomitant change in their regional expression in the rat epididymis. Our results lead us to propose that widespread changes in the region-specific expression pattern of genes over evolutionary time can be the result of alterations of one or only a few master regulatory genes.

Dynamic expression pattern and subcellular localization of the Rhox10 homeobox transcription factor during early germ cell development

Homeobox genes encode transcription factors that regulate diverse developmental events. The largest known homeobox gene cluster - the X-linked mouse reproductive homeobox (Rhox) cluster - harbors genes whose expression patterns and functions are largely unknown. Here, we report that a member of this cluster, Rhox10, is expressed in male germ cells. Rhox10 is highly transcribed in spermatogonia in vivo and is upregulated in response to the differentiation-inducing agent retinoic acid in vitro. Using a specific RHOX10 antiserum that we generated, we found that RHOX10 protein is selectively expressed in fetal gonocytes, germline stem cells, spermatogonia, and early spermatocytes. RHOX10 protein undergoes a dramatic shift in subcellular localization as germ cells progress from mitotically arrested gonocytes to mitotic spermatogonia and from mitotic spermatogonia to early meiotic spermatocytes, consistent with RHOX10 performing different functions in these stages.

Rapid evolution and copy number variation of primate RHOXF2, an X-linked homeobox gene involved in male reproduction and possibly brain function

Background

Homeobox genes are the key regulators during development, and they are in general highly conserved with only a few reported cases of rapid evolution. RHOXF2 is an X-linked homeobox gene in primates. It is highly expressed in the testicle and may play an important role in spermatogenesis. As male reproductive system is often the target of natural and/or sexual selection during evolution, in this study, we aim to dissect the pattern of molecular evolution of RHOXF2 in primates and its potential functional consequence.

Results

We studied sequences and copy number variation of RHOXF2 in humans and 16 nonhuman primate species as well as the expression patterns in human, chimpanzee, white-browed gibbon and rhesus macaque. The gene copy number analysis showed that there had been parallel gene duplications/losses in multiple primate lineages. Our evidence suggests that 11 nonhuman primate species have one RHOXF2 copy, and two copies are present in humans and four Old World monkey species, and at least 6 copies in chimpanzees. Further analysis indicated that the gene duplications in primates had likely been mediated by endogenous retrovirus (ERV) sequences flanking the gene regions. In striking contrast to non-human primates, humans appear to have homogenized their two RHOXF2 copies by the ERV-mediated non-allelic recombination mechanism. Coding sequence and phylogenetic analysis suggested multi-lineage strong positive selection on RHOXF2 during primate evolution, especially during the origins of humans and chimpanzees. All the 8 coding region polymorphic sites in human populations are non-synonymous, implying on-going selection. Gene expression analysis demonstrated that besides the preferential expression in the reproductive system, RHOXF2 is also expressed in the brain. The quantitative data suggests expression pattern divergence among primate species.

Conclusions

RHOXF2 is a fast-evolving homeobox gene in primates. The rapid evolution and copy number changes of RHOXF2 had been driven by Darwinian positive selection acting on the male reproductive system and possibly also on the central nervous system, which sheds light on understanding the role of homeobox genes in adaptive evolution.

The rhox homeobox gene cluster is imprinted and selectively targeted for regulation by histone h1 and DNA methylation

Histone H1 is an abundant and essential component of chromatin whose precise role in regulating gene expression is poorly understood. Here, we report that a major target of H1-mediated regulation in embryonic stem (ES) cells is the X-linked Rhox homeobox gene cluster. To address the underlying mechanism, we examined the founding member of the Rhox gene cluster-Rhox5-and found that its distal promoter (Pd) loses H1, undergoes demethylation, and is transcriptionally activated in response to loss of H1 genes in ES cells. Demethylation of the Pd is required for its transcriptional induction and we identified a single cytosine in the Pd that, when methylated, is sufficient to inhibit Pd transcription. Methylation of this single cytosine prevents the Pd from binding GA-binding protein (GABP), a transcription factor essential for Pd transcription. Thus, H1 silences Rhox5 transcription by promoting methylation of one of its promoters, a mechanism likely to extend to other H1-regulated Rhox genes, based on analysis of ES cells lacking DNA methyltransferases. The Rhox cluster genes targeted for H1-mediated transcriptional repression are also subject to another DNA methylation-regulated process: Xp imprinting. Remarkably, we found that only H1-regulated Rhox genes are imprinted, not those immune to H1-mediated repression. Together, our results indicate that the Rhox gene cluster is a major target of H1-mediated transcriptional repression in ES cells and that H1 is a candidate to have a role in Xp imprinting.

Transcription and post-transcriptional regulation of spermatogenesis

Spermatogenesis in mammals is achieved by multiple players that pursue a common goal of generating mature spermatozoa. The developmental processes acting on male germ cells that culminate in the production of the functional spermatozoa are regulated at both the transcription and post-transcriptional levels. This review addresses recent progress towards understanding such regulatory mechanisms and identifies future challenges to be addressed in this field. We focus on transcription factors, chromatin-associated factors and RNA-binding proteins necessary for spermatogenesis and/or sperm maturation. Understanding the molecular mechanisms that govern spermatogenesis has enormous implications for new contraceptive approaches and treatments for infertility.

The Rhox genes

Homeobox genes encode transcription factors that have crucial roles in embryogenesis. A recently discovered set of homeobox genes--the Rhox genes--are expressed during both embryogenesis and in adult reproductive tissues. The 33 known mouse Rhox genes are clustered together in a single region on the X chromosome, while likely descendents of the primodial Rhox cluster, Arx and Esx1, have moved to other positions on the X chromosome. Here, we summarize what is known about the regulation and function of Rhox cluster and Rhox-related genes during embryogenesis and gametogenesis. The founding member of the Rhox gene cluster--Rhox5 (previously known as Pem)--has been studied in the most depth and thus is the focus of this review. We also discuss the unusually rapid evolution of the Rhox gene cluster.

The X-linked imprinted gene family Fthl17 shows predominantly female expression following the two-cell stage in mouse embryos

Differences between male and female mammals are initiated by embryonic differentiation of the gonad into either a testis or an ovary. However, this may not be the sole determinant. There are reports that embryonic sex differentiation might precede and be independent of gonadal differentiation, but there is little molecular biological evidence for this. To test for sex differences in early-stage embryos, we separated male and female blastocysts using newly developed non-invasive sexing methods for transgenic mice expressing green fluorescent protein and compared the gene-expression patterns. From this screening, we found that the Fthl17 (ferritin, heavy polypeptide-like 17) family of genes was predominantly expressed in female blastocysts. This comprises seven genes that cluster on the X chromosome. Expression analysis based on DNA polymorphisms revealed that these genes are imprinted and expressed from the paternal X chromosome as early as the two-cell stage. Thus, by the time zygotic genome activation starts there are already differences in gene expression between male and female mouse embryos. This discovery will be important for the study of early sex differentiation, as clearly these differences arise before gonadal differentiation.

Androgen-induced Rhox homeobox genes modulate the expression of AR-regulated genes

Rhox5, the founding member of the reproductive homeobox on the X chromosome (Rhox) gene cluster, encodes a homeodomain-containing transcription factor that is selectively expressed in Sertoli cells, where it promotes the survival of male germ cells. To identify Rhox5-regulated genes, we generated 15P-1 Sertoli cell clones expressing physiological levels of Rhox5 from a stably transfected expression vector. Microarray analysis identified many genes altered in expression in response to Rhox5, including those encoding proteins controlling cell cycle regulation, apoptosis, metabolism, and cell-cell interactions. Fifteen of these Rhox5-regulated genes were chosen for further analysis. Analysis of Rhox5-null male mice indicated that at least nine of these are Rhox5-regulated in the testes in vivo. Many of them have distinct postnatal expression patterns and are regulated by Rhox5 at different postnatal time points. Most of them are expressed in Sertoli cells, indicating that they are candidates to be directly regulated by Rhox5. Transfection analysis with expression vectors encoding different mouse and human Rhox family members revealed that the regulatory response of a subset of these Rhox5-regulated genes is both conserved and redundant. Given that Rhox5 depends on androgen receptor (AR) for expression in Sertoli cells, we examined whether some Rhox5-regulated genes are also regulated by AR. We provide several lines of evidence that this is the case, leading us to propose that RHOX5 serves as a key intermediate transcription factor that directs some of the actions of AR in the testes.

Expression of Atp8b3 in murine testis and its characterization as a testis specific P-type ATPase

Spermatogenesis is a complex process that produces haploid motile sperms from diploid spermatogonia through dramatic morphological and biochemical changes. P-type ATPases, which support a variety of cellular processes, have been shown to play a role in the functioning of sperm. In this study, we isolated one putative androgen-regulated gene, which is the previously reported sperm-specific aminophospholipid transporter (Atp8b3, previously known as Saplt), and explored its expression pattern in murine testis and its biochemical characteristics as a P-type ATPase. Atp8b3 is exclusively expressed in the testis and its expression is developmentally regulated during testicular development. Immunohistochemistry of the testis reveals that Atp8b3 is expressed only in germ cells, especially haploid spermatids, and the protein is localized in developing acrosomes. As expected, from its primary amino acid sequence, ATP8B3 has an ATPase activity and is phosphorylated by an ATP-producing acylphosphate intermediate, which is a signature property of the P-Type ATPases. Together, ATP8B3 may play a role in acrosome development and/or in sperm function during fertilization.

Identification and characterization of Rhox13, a novel X-linked mouse homeobox gene

Homeobox genes encode transcription factors whose expression organizes programs of development. A number of homeobox genes expressed in reproductive tissues have been identified recently, including a colinear cluster on the X chromosome in mice. This has led to an increased interest in understanding the role(s) of homeobox genes in regulating development of reproductive tissues including the testis, ovary, and placenta. Here we report the identification and characterization of a novel homeobox gene of the paired-like class on the X chromosome distal to the reproductive homeobox (Rhox) cluster in mice. Transcripts are found in the testis and ovary as early as 13.5 days post coitum (dpc). Transcription ceases in the ovary by 3 days post partum (dpp), but continues in the testis through adulthood. The Rhox13 gene encodes a 25.3 kDa protein expressed in the adult testis in germ cells at the basal aspect of the seminiferous epithelium.

Epigenetic regulation and downstream targets of the Rhox5 homeobox gene

The discovery of the Rhox homeobox gene cluster on the X chromosome opens up new vistas in the regulation of reproductive processes in mammals. In mice, this cluster comprises more than 30 genes that are selectively expressed in reproductive tissues. A subset of Rhox genes are androgen and AR regulated in postnatal and adult Sertoli cells, making them candidates to mediate androgen-dependent steps during spermatogenesis. The best characterized of these androgen/AR-regulated genes is Rhox5 (Pem), the founding member of the Rhox gene cluster. Targeted deletion of Rhox5 in mice causes male subfertility marked by increased germ-cell apoptosis and decreased sperm count and motility. Microarray analyses identified a wide variety of genes regulated by Rhox5 in Sertoli cells. One of them is the tumour suppressor UNC5C, a pro-apoptotic molecule previously only known to be involved in brain development. Targeted deletion of Unc5c causes decreased germ-cell apoptosis in postnatal and adult testes, indicating that it also has a role in spermatogenesis and supporting a model in which Rhox5 promotes germ-cell survival by downregulating Unc5c. Rhox5 has two independently regulated promoters that have distinct expression patterns. The unique tissue-specific and developmentally regulated transcription pattern of these two promoters appear to be controlled by DNA methylation. Both promoters are methylated in tissues in which they are not expressed, suggesting that DNA methylation serves to repress Rhox5 expression in inappropriate cell types and tissues. In summary, the Rhox gene cluster is an epigenetically regulated set of genes encoding a large number of transcription factors that are strong candidates to regulate gametogenesis and other aspects of reproduction.

Krüppel-like factor 4 is involved in functional differentiation of testicular Sertoli cells

Krüppel-like factor 4 (KLF4) is a pleiotropic zinc finger transcription factor that regulates genes being involved in differentiation and cell-cycle control. Knockout studies revealed a critical function for KLF4 in the terminal differentiation of many epithelial cells. In testicular Sertoli cells, Klf4 is strongly inducible by the glycoprotein follicle stimulating hormone (FSH). Because KLF4 is essential for postnatal survival in mice, we deleted Klf4 specifically in Sertoli cells using the Cre/loxP system. Importantly, around postnatal day 18, a critical period of terminal Sertoli cell differentiation, mutant seminiferous tubules exhibited a disorganized germinal epithelium and delayed lumen formation. The ultrastructural finding of highly vacuolized Sertoli cell cytoplasm and the identification of differentially expressed genes, which are known to play roles during vesicle transport and fusion or for maintenance of the differentiated cell state, suggest impaired apical secretion of the Sertoli cell. Interestingly, a high proportion of all identified genes was localized in a small subregion of chromosome 7, suggesting coordinated regulation. Intriguingly, adult mutant mice are fertile and show normal testicular morphology, although the testosterone levels are decreased. In summary, KLF4 plays a significant role for proper and timely Sertoli cell differentiation in pubertal mice.

GATA factors and androgen receptor collaborate to transcriptionally activate the Rhox5 homeobox gene in Sertoli cells

How Sertoli-specific expression is initiated is poorly understood. Here, we address this issue using the proximal promoter (Pp) from the Rhox5 homeobox gene. Its Sertoli cell-specific expression is achieved, in part, through a negative regulatory element that inhibits Pp transcription in non-Sertoli cell lines. Complementing this negative regulation is positive regulation conferred by four androgen-response elements (AREs) that interact with the androgen receptor (AR), a nuclear hormone receptor expressed at high levels in Sertoli cells. A third control mechanism is provided by a consensus GATA-binding site that is crucial for Pp transcription both in vitro and in vivo. Several lines of evidence suggested that GATA factors and AR act cooperatively to activate Pp transcription: (i) the GATA-binding site crucial for Pp transcription is in close proximity to two of the AREs, (ii) GATA and AR form a complex with the Pp in vitro, (iii) overexpression of GATA factors rescued expression from mutant Pp constructs harboring defective AREs, and (iv) incubation of a Sertoli cell line with testosterone triggered corecruitment of AR and GATA4 to the Pp. Collectively, our results suggest that the Rhox5 gene achieves Sertoli cell-specific transcription using a combinatorial strategy involving negative and cooperative positive regulation.

The RHOX5 homeodomain protein mediates transcriptional repression of the netrin-1 receptor gene Unc5c

The X-linked mouse Rhox gene cluster contains more than 30 homeobox genes that are candidates to regulate multiple steps in male and female gametogenesis. The founding member of the Rhox gene cluster, Rhox5, is an androgen-dependent gene expressed in Sertoli cells that promotes the survival and differentiation of the adjacent male germ cells. Here, we report the first identification and characterization of a Rhox5-regulated gene. This gene, Unc5c, encodes a pro-apoptotic receptor with tumor suppressor activity that we found is negatively regulated by Rhox5 in the testis in vivo. Transfection analyses in cell lines of different origin indicated that Rhox5-dependent down-regulation of Unc5c requires another Sertoli cell-specific cofactor. Examination of other mouse Rhox family members revealed that mouse RHOX2 and RHOX3 also have the ability to down-regulate Unc5c expression. The human RHOX protein PEPP2 (RHOXF2) also had this ability, indicating that Unc5c repression is a conserved RHOX-dependent response. Deletion analysis identified a Rhox5-responsive element in the Unc5c 5'-untranslated region. Although 5'-untranslated regions typically house post-transcriptional elements, several lines of evidence indicated that Rhox5 down-regulates Unc5c at the transcriptional level. The repression of Unc5c expression by Rhox5 may, in part, mediate the pro-survival function of Rhox5 in the testis, as we found that Unc5c mutant mice have decreased germ cell apoptosis in the testis. Along with our other data, these findings led us to propose a model in which Rhox5 is a negative regulator upstream of Unc5c in a Sertoli-cell pathway that promotes germ-cell survival.

Loss of androgen receptor binding to selective androgen response elements causes a reproductive phenotype in a knockin mouse model

Androgens influence transcription of their target genes through the activation of the androgen receptor (AR) that subsequently interacts with specific DNA motifs in these genes. These DNA motifs, called androgen response elements (AREs), can be classified in two classes: the classical AREs, which are also recognized by the other steroid hormone receptors; and the AR-selective AREs, which display selectivity for the AR. For in vitro interaction with the selective AREs, the androgen receptor DNA-binding domain is dependent on specific residues in its second zinc-finger. To evaluate the physiological relevance of these selective elements, we generated a germ-line knockin mouse model, termed SPARKI (SPecificity-affecting AR KnockIn), in which the second zinc-finger of the AR was replaced with that of the glucocorticoid receptor, resulting in a chimeric protein that retains its ability to bind classical AREs but is unable to bind selective AREs. The reproductive organs of SPARKI males are smaller compared with wild-type animals, and they are also subfertile. Intriguingly, however, they do not display any anabolic phenotype. The expression of two testis-specific, androgen-responsive genes is differentially affected by the SPARKI mutation, which is correlated with the involvement of different types of response elements in their androgen responsiveness. In this report, we present the first in vivo evidence of the existence of two functionally different types of AREs and demonstrate that AR-regulated gene expression can be targeted based on this distinction.

Transcriptional profiling of androgen receptor (AR) mutants suggests instructive and permissive roles of AR signaling in germ cell development

The androgen receptor (AR) is a transcription factor that plays a critical role in male sexual development, spermatogenesis, and maintenance of hormonal homeostasis. Despite the extensive knowledge of the phenotypic consequences of mutations in Ar, very little is known about the transcriptional targets of AR within the testis. To identify potential targets of androgen signaling in the testis, we have analyzed the transcriptional profile of adult testes from Ar hypomorphs alone or in combination with Sertoli cell-specific Ar ablation. Using Affymetrix MOE430A mouse genome arrays we interrogated more than 22,000 transcripts. We found the expression level of 62 transcripts in the Ar mutants differed by greater than 2-fold compared with wild type. We also found that more transcripts were up-regulated than down-regulated, highlighting AR's role as a transcriptional repressor in the testis. Twelve transcripts were uniquely affected, and 16 transcripts were more severely affected in Sertoli cell-specific Ar ablation compared with hypomorphic Ar mutants. Using a comparative genomic approach, we analyzed the 6 kb around the transcriptional start sites of affected transcripts for conserved AREs (androgen response elements). We identified at least one conserved ARE in 65% of the genes misregulated in our microarray analysis where clear mouse-human orthologs were available. We used a reporter assay in cell culture to functionally verify the AREs for the kallikrein 27 gene. This suggests that the majority of the misregulated transcripts have a high probability of being direct AR targets. The transcripts affected by these Ar mutations encode a diverse array of proteins whose molecular functions support the contention that AR supports spermatogenesis in both a permissive and instructive fashion.

Rhox homeobox gene cluster: recent duplication of three family members

We recently reported the discovery of a homeobox gene cluster on the mouse X chromosome, Rhox, whose 12 members are selectively expressed in specific cell types in reproductive organs. Here we report the existence of 20 additional Rhox homeobox genes in this gene cluster. Most of the newly identified Rhox paralogs retain the same order and relative orientation as three of the originally described Rhox genes, suggesting that they arose from recent duplications of this trimer unit. Many of these new Rhox family members are expressed in the testis and placenta. Analysis of synonymous and nonsynonymous substitutions in their homeodomain region suggests that these new Rhox paralogs duplicated so recently that their encoded proteins have not yet acquired distinct DNA-binding specificities. The existence of these new Rhox genes provides an opportunity to examine the initial stages of gene cluster evolution.

Positive selection in the evolution of cancer

We hypothesize that forms of antagonistic coevolution have forged strong links between positive selection at the molecular level and increased cancer risk. By this hypothesis, evolutionary conflict between males and females, mothers and foetuses, hosts and parasites, and other parties with divergent fitness interests has led to rapid evolution of genetic systems involved in control over fertilization and cellular resources. The genes involved in such systems promote cancer risk as a secondary effect of their roles in antagonistic coevolution, which generates evolutionary disequilibrium and maladaptation. Evidence from two sources: (1) studies on specific genes, including SPANX cancer/testis antigen genes, several Y-linked genes, the pem homebox gene, centromeric histone genes, the breast cancer gene BRCA1, the angiogenesis gene ANG, cadherin genes, cytochrome P450 genes, and viral oncogenes; and (2) large-scale database studies of selection on different functional categories of genes, supports our hypothesis. These results have important implications for understanding the evolutionary underpinnings of cancer and the dynamics of antagonistically-coevolving molecular systems.

Intragenomic politics

The mammalian genome contains multiple genetic factions with distinct interests in the outcomes of interactions among kin. In the context of an offspring's relations with its mother, these factions are proposed to align into two 'parties', one favoring increased demand by offspring and the other favoring reduced demand. A possible alignment has inhibitors of demand located on the X chromosome and enhancers of demand located on autosomes, because X-linked loci are maternally derived two-thirds of the time by contrast to autosomal loci which are maternally derived half of the time.

Sex-specific gene expression in preimplantation mouse embryos

The 3.5-day-old blastocyst-stage mouse embryo consists of two tissues and contains approximately 60 cells. This tiny structure has now been observed to express nearly 600 genes in a sex-specific fashion, including at least one gene (Rhox/Pem) expressed only in females from their paternal X chromosome.

Regulation of the Rhox5 Homeobox Gene in Primary Granulosa Cells: Preovulatory Expression and Dependence on SP1/SP3 and GABP1

Homeobox genes encode transcription factors that regulate embryonic development and postnatal events. Rhox5 (previously called Pem), the founding member of a homeobox gene cluster that we recently identified on the X chromosome, is selectively expressed in granulosa cells in the ovary and other somatic-cell types in other reproductive organs. In this report, we investigate its regulation in granulosa cells in the rat ovary. We found that Rhox5 expression in the ovary is governed by the Rhox5 distal promoter and is expressed at least as early as Day 5 postpartum. Rhox5 mRNA levels are regulated during the ovarian cycle, peaking before ovulation. Deletion analysis revealed a 25-nt element essential for distal promoter transcription in primary granulosa cells. This distal promoter element contains two ETS and one SP1 transcription-factor family binding sites that mutagenesis analysis indicated were essential for high-level transcription. This element was both necessary and sufficient for transcription, because it activated transcription when placed upstream of a heterologous minimal promoter. Cold competition and electrophoretic mobility shift assay studies demonstrated that SP1, SP3, and the ETS family transcription factor GABP bound this element. Dominant-negative forms of GABP and SP3 repressed distal promoter expression in primary rat granulosa, showing that these factors are crucial for Rhox5 expression. Cotransfection of dominant-negative mutants indicated that Rhox5 expression in granulosa cells is regulated by the c-Jun N-terminal protein kinase (JNK, MAPK8) and RAS pathways, which are known to be upstream of ETS family transcription factors. The discovery that Rhox5 expression in granulosa cells is regulated by MAPK pathways and ETS and SP1 family members provides an opportunity to understand how these regulatory pathways and factors collaborate to regulate gene expression during the ovarian cycle.

Rhox: a new homeobox gene cluster

Homeobox genes encode transcription factors notable for their ability to regulate embryogenesis. Here, we report the discovery of a cluster of 12 related homeobox genes on the X chromosome expressed in male and female reproductive tissues in adult mice. These reproductive homeobox on the X chromosome (Rhox) genes are expressed in a cell type-specific manner; several are hormonally regulated, and their expression pattern during postnatal testis development corresponds to their chromosomal position. Most of the Rhox genes are expressed in Sertoli cells, the nurse cells in direct contact with developing male germ cells, suggesting that they regulate the expression of somatic-cell gene products critical for germ cell development. In support of this, targeted disruption of Rhox5 increased male germ cell apoptosis and reduced sperm production, sperm motility, and fertility. Identification of this family of homeobox genes provides an opportunity to study colinear gene regulation and the transcriptional control of reproduction.

Gene Regulation in Spermatogenesis

Mammalian spermatogenesis is a complex hormone-dependent developmental program in which a myriad of events must take place to ensure that germ cells reach their proper stage of development at the proper time. Many of these events are controlled by cell type- and stage-specific transcription factors. The regulatory mechanisms involved provide an intriguing paradigm for the field of developmental biology and may lead to the development of new contraceptives an and innovative routs to treat male infertility. In this review, we address three aspects of the genetic regulatory mechanism that drive spermatogenesis. First, we detail what is known about how steroid hormones (both androgens and estrogens) and their cognate receptors initiate and maintain mammalian spermatogenesis. Steroids act through three mechanistic routes: (i) direct activation of genes through hormone-dependent promoter elements, (ii) secondary transcriptional responses through activation of hormone-dependent transcription factors, and (iii) rapid, transcription-independent (nonclassical) events induced by steroid hormones. Second, we provide a survey of transcription factors that function in mammalian spermatogenesis, including homeobox, zinc-finger, heat-shock, and cAMP-response family members. Our survey is not intended to cover all examples but to give a flavor for the gamut of biological roles conferred by transcription factors in the testis, particularly those defined in knockout mice. Third, we address how testis-specific transcription is achieved. In particular, we cover the evidence for and against the idea that some testis-specific genes are transcriptionally silent in somatic tissues as a result of DNA methylation.

Hormonal regulation of spermatogenesis

Proper functioning of the mammalian testis is dependent upon an array of hormonal messengers acting through endocrine, paracrine, and autocrine pathways. Within the testis, the primary messengers are the gonadotrophins, follicle stimulating hormone and luteinizing hormone, and the androgens. Abundant evidence indicates that the role of the gonadotrophins is to maintain proper functioning of testicular somatic cells. It is the androgens, primarily testosterone, which act through the somatic cells to regulate germ cell differentiation. Despite extensive research in this area, little is known about the cell-specific requirements for androgens and even less is understood about the downstream effectors of androgen signalling. However, recent work using cell-specific ablation of androgen receptor function has demonstrated a clear requirement for androgen signalling at multiple, discrete time points during spermatogenesis. These models also provide useful tools for identifying the targets of androgen receptor activity. The purpose of this review is to provide a brief overview of recent advances in our understanding of hormonal regulation of spermatogenesis, with an emphasis on the role of testosterone within the testis, and to pose important questions for future research in this field.

The win locus involved in activation of the distal N-myc2 gene upon WHV integration in woodchuck liver tumors harbors S/MAR elements

Woodchuck hepatitis virus (WHV) and the woodchuck (Marmota monax) are models for hepatocellular carcinoma (HCC) induced by hepatitis B virus (HBV). In woodchuck liver tumors, the N-myc2 proto-oncogene is frequently activated by WHV integration either close to the gene or in the b3n and win downstream loci, located 10 and 150 kb from N-myc2, respectively. A scaffold/matrix attachment region (S/MAR) regulative element was shown to be in b3n, possibly mediating activation of the upstream N-myc2 gene upon WHV integration. To investigate if S/MAR elements are in win too, a 17-kb DNA fragment corresponding to the major region of WHV insertion in this locus was cloned and sequenced. Overlapping subcloned fragments spanning candidate S/MARs predicted by sequence analysis were tested by standard in vitro binding assays. Results showed the presence of two S/MAR elements in win. The distribution of previously described WHV insertions relative to the S/MARs reinforces the hypothesis that S/MARs nearby distal WHV insertions might be involved in long-range activation of N-myc2.

A Novel PEPP Homeobox Gene, TOX, Is Highly Glutamic Acid Rich and Specifically Expressed in Murine Testis and Ovary1

The homeobox gene superfamily has been highly conserved throughout evolution. These genes act as transcription factors during several important developmental processes. To explore the functional roles of homeobox genes in spermatogenesis, we performed a degenerate oligonucleotide polymerase chain reaction (PCR) screening of a testis cDNA library and isolated a novel mouse homeobox gene. This gene, which we named Tox, encodes a homeodomain protein distantly related to members of the Paired/Pax (Prd/Pax) family. A phylogenetic analysis revealed Tox to be a member of the recently defined PEPP subfamily of Paired-like homeobox genes. Tox was mapped to chromosome X, with its homeodomain organized into three exons. A special feature of Tox is that the encoded protein sequence contains two poly-glutamic acid (poly E) stretches, which make Tox highly acidic. Tox transcripts were detected predominately in the testis and ovary of mice. Tox expression in testes was initiated soon after birth, mainly in Sertoli cells and spermatogonia; however, in adult mice, Tox expression shifts to the spermatids and spermatozoa. Tox expression in ovaries was detected in somatic cells of follicles, early on in theca cells, and in both granulosa and theca cells at the later stages of follicular development. Based on these results, Tox may play an important role during gametogenesis.

Multiple endocrine neoplasia type 1 with unusual concomitance of various neoplastic disorders

A patient with multiple endocrine neoplasia type 1 (MEN1) who manifested various MEN1-unrelated tumors was reported. The patient was a 43-year-old woman who manifested typical features of MEN1 including primary hyperparathyroidism, prolactinoma, adrenal adenoma and visceral lipomas. During the course, she also manifested chondrosarcoma, B cell lymphoma and mesothelioma. The patient had no apparent family history of MEN1 or any other neoplastic diseases. Genetic analysis of DNA from peripheral mononuclear cells of the patient revealed no germline mutations in MEN1 gene. Genetic instability due to yet unidentified cause is the possible explanation of occurrence of multiple tumors. Careful periodic screening of endocrine and other disorders for her siblings and children as well as for the patient is warranted.

Continuing primordial germ cell differentiation in the mouse embryo is a cell-intrinsic program sensitive to DNA methylation

The initial cohort of mammalian gametes is established by the proliferation of primordial germ cells in the early embryo. Primordial germ cells first appear in extraembyronic tissues and subsequently migrate to the developing gonad. Soon after they arrive in the gonad, the germ cells cease dividing and undertake sexually dimorphic patterns of development. Male germ cells arrest mitotically, while female germ cells directly enter meiotic prophase I. These sex-specific differentiation events are imposed upon a group of sex-common differentiation events that are shared by XX and XY germ cells. We have studied the appearance of GCNA1, a postmigratory sex-common germ cell marker, in cultures of premigratory germ cells to investigate how this differentiation program is regulated. Cultures in which proliferation was either inhibited or stimulated displayed a similar extent of differentiation as controls, suggesting that some differentiation events are the result of a cell-intrinsic program and are independent of cell proliferation. We also found that GCNA1 expression was accelerated by agents which promote DNA demethylation or histone acetylation. These results suggest that genomic demethylation of proliferative phase primordial germ cells is a mechanism by which germ cell maturation is coordinated.

Genetic ablation of the tumor suppressor menin causes lethality at mid-gestation with defects in multiple organs

Patients suffering from multiple endocrine neoplasia type 1 (MEN1) are predisposed to multiple endocrine tumors. The MEN1 gene product, menin, is expressed in many embryonic, as well as adult tissues, and interacts with several proteins in vitro and in vivo. However, the biological function of menin remains largely unknown. Here we show that disruption of the Men1 gene in mice causes embryonic lethality at E11.5-E13.5. The Men1 null mutant embryos appeared smaller in size, frequently with body haemorrhages and oedemas, and a substantial proportion of them showed disclosure of the neural tube. Histological analysis revealed an abnormal development of the nervous system and heart hypotrophy in some Men1 null embryos. Furthermore, Men1 null livers generally displayed an altered organization of the epithelial and hematopoietic compartments associated with enhanced apoptosis. Chimerism analysis of embryos generated by injection of Men1 null ES cells, showed that cells lacking menin do not seem to have a general cell-autonomous defect. However, primary Men1 null embryonic fibroblasts entered senescence earlier than their wild-type counterparts. Despite normal proliferation ability, Men1 null ES cells exhibited a deficiency to form embryoid bodies, suggesting an impaired differentiation capacity in these cells. The present study demonstrates that menin plays an important role in the embryonic development of multiple organs in addition to its proposed role in tumor suppression.

Menin interacting proteins as clues toward the understanding of multiple endocrine neoplasia type 1

Multiple endocrine neoplasia type 1 (MEN1) is a familial cancer syndrome characterized mostly by tumors of the parathyroids, pancreas and anterior pituitary. The gene responsible, MEN1, encodes Menin, a 610 aminoacid nuclear protein with no sequence homology to other proteins. Although a mouse knock-out model is available, the function of Menin is still elusive. Proteins of known function are shown to interact with Menin: JunD, nuclear factor-KappaB, Smad3, Pem, Nm23H1, glial fibrillary acidic protein, Vimentin, and probably P53. Their partnership with Menin may correspond to a regulation of their activity, but their relevance to the various traits of MEN1 pathogenicity is not established. This raises fundamental issues on the regulation pathways implicated in this complex endocrine disease.

Pem homeobox gene regulatory sequences that direct androgen-dependent developmentally regulated gene expression in different subregions of the epididymis

The epididymis is a useful model system to understand the mechanisms that govern region-specific gene expression, as many gene products display spatially restricted expression within this organ. However, surprisingly little is known about how this regulation is achieved. Here, we report regulatory sequences from the Pem homeobox gene that drive expression in different subregions of the mouse epididymis in vivo. We found that the 0.3-kb 5'-flanking sequence (region I) from the Pem proximal promoter (Pem Pp) was sufficient to confer androgen-dependent and developmentally regulated expression in the caput region of the epididymis. Expression was restricted to the normal regions of expression of Pem in the caput (segments 2-4), but there was also aberrant expression in the corpus region. This corpus misexpression was extinguished when 0.6 kb of Pem Pp 5'-flanking sequence was included in the transgene, indicating that one or more negative regulatory elements exist between 0.6 and 0.3 kb upstream of the Pem Pp start site (region II). When heterologous sequences were introduced upstream of the Pem Pp, expression was further restricted, mainly to caput segment 3, implying that the Pem Pp has segment-specific regulatory elements. To our knowledge, the regulatory regions we have identified are the shortest so far defined that dictate regionally localized expression in the epididymis in vivo. They may be useful for identifying the factors that regulate region-specific expression in the epididymis, for expressing and conditionally knocking out genes in different subregions of the epididymis, for treating male infertility, and for generating novel methods of male contraception.

Two novel human X-linked homeobox genes, hPEPP1 and hPEPP2, selectively expressed in the testis

The PEPP genes are a recently described subfamily of mouse homeobox genes preferentially expressed in reproductive tissues. Pem, the founding member of the PEPP subfamily, has undergone rapid divergence due to positive selection, rendering the identification of its human orthologue difficult. Here we report the isolation and characterization of two human homeobox genes, hPEPP1 and hPEPP2, that are related to Pem and other PEPP family members. We identified these human genes based on their location in Xq24, which is syntenic to the mouse X-chromosome region containing three PEPP genes: Pem, Psx-1, and Psx-2. We found that hPEPP1 and hPEPP2 are selectively expressed in the testis, where the mouse and rat Pem genes are also expressed. However, unlike all mouse PEPP genes, hPEPP1 and hPEPP2 were not expressed in placenta, which suggests the possibility that the regulation of PEPP genes has significantly changed since the split between hominids and rodents. Although hPEPP1 exhibits highly selective expression in normal tissues, it is aberrantly expressed in tumor cell lines from several different organs, analogous to the expression pattern of mouse and rat Pem but not mouse Psx-1 or Psx-2. We conclude that we identified two human homeobox genes from the PEPP subfamily that are good candidates to encode transcription factors that regulate downstream genes and biological events in the human testis.

OTEX, an androgen-regulated human member of the paired-like class of homeobox genes

paired genes emerged early in evolution and code for homeobox transcription factors, having fundamental roles in various biological processes. We identified a novel human member of the paired-like class, which we named OTEX. A phylogenetic analysis revealed that OTEX belonged to the recently defined PEPP subfamily of paired-like homeobox genes. It was organized into three introns and, like the other PEPP genes, it was mapped to chromosome X. Its transcripts were detected mainly in the ovary, testis and epididymis, but also in the prostate and mammary gland. In the PC-3/ARwt prostate cell line, OTEX expression was stimulated dramatically following androgen treatment. Immunofluorescence studies revealed an exclusively nuclear localization of the OTEX protein. Mutation of the RARCRRHQRE amino acid sequence present at the C-terminus of the OTEX homeodomain resulted in a mainly cytoplasmic localization, indicating that this motif harboured the nuclear localization signal. No inherent transactivation function was seen for OTEX using the one-hybrid assay, and no homodimer formation was observed in the two-hybrid assay, suggesting that additional partners were needed for this activity. Taken together, the data show that OTEX represents a novel, androgen-regulated, paired-like homeobox protein, with possibly an important role in human reproduction.

A highly active homeobox gene promoter regulated by Ets and Sp1 family members in normal granulosa cells and diverse tumor cell types

One mechanism by which normal cells become converted to tumor cells involves the aberrant transcriptional activation of genes that are normally silent. We characterize a promoter that normally exhibits highly tissue- and stage-specific expression but displays ubiquitous expression when cells become immortalized or malignant, regardless of their lineage or tissue origin. This promoter normally drives the expression of the Pem homeobox gene in specific cell types in ovary and placenta but is aberrantly expressed in lymphomas, neuroblastomas, retinoblastomas, carcinomas, and sarcomas. By deletion analysis we identified a region between nucleotides -80 and -104 that was absolutely critical for the expression from this distal Pem promoter (Pem Pd). Site-specific mutagenesis and transfection studies revealed that this region contains two consensus Ets sites and a single Sp1 site that were necessary for Pem Pd expression. Gel shift analysis showed that Ets and Sp1 family members bound to these sites. Transfection studies demonstrated that the Ets family members Elf1 and Gabp and the Sp1 family members Sp1 and Sp3 transactivated the Pem Pd. Surprisingly, we found that Sp3 was a more potent activator of the Pem Pd than was Sp1; this is unusual, because Sp3 is either a weak activator or a repressor of most other promoters. Activation by either Elf1 or Gabp required an intact Sp1 family member binding site, suggesting that Ets and Sp1 family members cooperate to activate Pem Pd transcription. Expression from the Pem Pd (either transiently transfected or endogenous) depended on the Ras pathway, which could explain both its Ets- and Sp1-dependent expression in normal cells and its aberrant expression in tumor cells, in which ras protooncogenes are frequently mutated. We suggest that the Pem Pd may be a useful model system to understand the molecular mechanism by which a tissue-specific promoter can be corrupted in tumor cells.

Normal gonadal development in mice lacking GPBOX, a homeobox protein expressed in germ cells at the onset of sexual dimorphism

Gpbox is a paired-like homeobox gene that colocalizes with two other members of the family, PsxI and Pem, on the proximal portion of the mouse X chromosome. Gpbox is expressed in the extraembryonic placenta and within the germ cells of the embryonic gonad. Beginning with the onset of sexual dimorphism (embryonic day [E]11.5 to 12.5), GPBOX transcripts accumulate faster in female than in male germ cells but disappear later in embryogenesis (E16) and have not been reported in adult tissues. To investigate the function of Gpbox, mouse cell lines lacking GPBOX were established using targeted mutagenesis in embryonic stem cells. Both homozygous Gpbox null female and hemizygous Gpbox null male mice were fertile and reproduced normally. Additionally, the development of male and female gonads in the null background was indistinguishable from that observed in normal littermates. The lack of an obvious phenotype raises the possibility that another member of this homeobox gene family provides the absent Gpbox function.