Unique chromatin remodeling and transcriptional regulation in spermatogenesis

Distinct Regions of the Mouse Cyclin A1 Gene, Ccna1, Confer Male Germ-Cell Specific Expression and Enhancer Function1

The gene encoding mouse cyclin A1, Ccna1, is expressed at highest levels in late pachytene-diplotene spermatocytes, where it is required for meiotic cell division. To begin to understand the mechanisms responsible for its highly restricted pattern of expression, transgenic mouse lines carrying constructs consisting of the cyclin A1 regulatory region fused with the reporter gene lacZ were generated. Analysis of tissue-specific and testicular cell-type-specific transgene expression indicated that sequences within -1.3 kilobases (kb) of the cyclin A1 putative transcriptional start site were sufficient to direct transgene expression uniquely to late spermatocytes while maintaining repression in other tissues. However, sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site were apparently required to transcribe the reporter at levels needed for consistent X-gal staining. Comparison of the mouse, rat, and human proximal promoters revealed regions of high sequence conservation and consensus sequences both for known transcription factors, some of which are coexpressed with Ccna1, such as A-myb and Hsf2, and for elements that control expression of genes in somatic cell cycles, such as CDE, CHR, and CCAAT elements. Thus, the promoter region within 1.3 kb upstream of the putative Ccna1 transcriptional start can direct expression of lacZ to spermatocytes, while sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site may enhance expression of lacZ.

A Short Core Promoter Drives Expression of the ALF Transcription Factor in Reproductive Tissues of Male and Female Mice1

The control of gene expression in reproductive tissues involves a number of unique germ cell-specific transcription factors. One such factor, ALF (TFIIA tau), encodes a protein similar to the large subunit of general transcription factor TFIIA. To understand how this factor is regulated, we characterized transgenic mice that contain the ALF promoter linked to either beta-galactosidase or green fluorescent protein (GFP) reporters. The results show that as little as 133 base pairs are sufficient to drive developmentally accurate and cell-specific expression. Transgene DNA was methylated and inactive in liver, but could be reactivated in vivo by system administration of 5-aza, 2'-deoxycytidine. Fluorescence-activated cell sorting allowed the identification of male germ cells that express the GFP transgene and provides a potential method to collect cells that might be under the control of a nonsomatic transcription system. Finally, we found that transcripts from the endogenous ALF gene and derived transgenes can also be detected in whole ovary and in germinal vesicle-stage oocytes of female mice. The ALF sequence falls into a class of germ cell promoters whose features include small size, high GC content, numerous CpG dinucleotides, and an apparent TATA-like element. Overall, the results define a unique core promoter that is active in both male and female reproductive tissues, and suggest mouse ALF may have a regulatory role in male and female gametogenic gene expression programs.

Metastasis-associated protein 1 interacts with NRIF3, an estrogen-inducible nuclear receptor coregulator

The transcriptional activity of estrogen receptor alpha (ER-alpha) is modified by regulatory action and interactions of coactivators and corepressors. Recent studies have shown that the metastasis-associated protein 1 (MTA1) represses estrogen receptor element (ERE)-driven transcription in breast cancer cells. With a yeast two-hybrid screen to clone MTA1-interacting proteins, we identified a known nuclear receptor coregulator (NRIF3) as an MTA1-binding protein. NRIF3 interacted with MTA1 both in vitro and in vivo. NRIF3 bound to the C-terminal region of MTA1, while MTA1 bound to the N-terminal region of NRIF3, containing one nuclear receptor interaction LXXLL motif. We showed that NRIF3 is an ER coactivator, hyperstimulated ER transactivation functions, and associated with the endogenous ER and its target gene promoter. MTA1 repressed NRIF3-mediated stimulation of ERE-driven transcription and interfered with NRIF3's association with the ER target gene chromatin. In addition, NRIF3 deregulation enhanced the responsiveness of breast cancer cells to estrogen-induced stimulation of growth and anchorage independence. Furthermore, we found that NRIF3 is an estrogen-inducible gene and activated ER associated with the ER response element in the NRIF3 gene promoter. These findings suggest that NRIF3, an MTA1-interacting protein, is an estrogen-inducible gene and that regulatory interactions between MTA1 and NRIF3 might be important in modulating the sensitivity of breast cancer cells to estrogen.

ATRX, a member of the SNF2 family of helicase/ATPases, is required for chromosome alignment and meiotic spindle organization in metaphase II stage mouse oocytes

ATRX is a centromeric heterochromatin binding protein belonging to the SNF2 family of helicase/ATPases with chromatin remodeling activity. Mutations in the human ATRX gene result in X-linked alpha-thalassaemia with mental retardation (ATRX) syndrome and correlate with changes in methylation of repetitive DNA sequences. We show here that ATRX also functions to regulate key stages of meiosis in mouse oocytes. At the germinal vesicle (GV) stage, ATRX was found associated with the perinucleolar heterochromatin rim in transcriptionally quiescent oocytes. Phosphorylation of ATRX during meiotic maturation is dependent upon calcium calmodulin kinase (CamKII) activity. Meiotic resumption also coincides with deacetylation of histone H4 at lysine 5 (H4K5 Ac) while ATRX and histone H3 methylated on lysine 9 (H3K9) remained bound to the centromeres and interstitial regions of condensing chromosomes, respectively. Inhibition of histone deacetylases (HDACs) with trichostatin A (TSA) disrupted ATRX binding to the centromeres of hyperacetylated chromosomes resulting in abnormal chromosome alignments at metaphase II (MII). Similarly, while selective ablation of ATRX by antibody microinjection and RNA interference (RNAi) had no effect on the progression of meiosis, it had severe consequences for the alignment of chromosomes on the metaphase II spindle. These results suggest that genome-wide epigenetic modifications such as global histone deacetylation are essential for the binding of ATRX to centromeric heterochromatin. Moreover, centromeric ATRX is required for correct chromosome alignment and organization of a bipolar meiotic metaphase II spindle.

Chromatin dynamics and Arabidopsis development

The plant life cycle involves a series of developmental phase transitions. These transitions require the regulation and highly co-ordinated expression of many genes. Epigenetic controls have now been shown to be a key element of this mechanism of regulation. In the model plant Arabidopsis, recent genetic and molecular studies on chromatin have begun to dissect the molecular basis of these epigenetic controls. Chromatin dynamics represent the emerging and exciting field of gene regulation notably involved in plant developmental transitions. By comparing plant and animal systems, new insights into the molecular complexes and mechanisms governing development can be delineated. We are now beginning to identify the components of chromatin complexes and their functions.

Abnormal sperm in mice with targeted deletion of the act (activator of cAMP-responsive element modulator in testis) gene

ACT [activator of cAMP-responsive element modulator (CREM) in testis] is a LIM-only protein that interacts with transcription factor CREM in postmeiotic male germ cells and enhances CREM-dependent transcription. CREM regulates many crucial genes required for spermatid maturation, and targeted mutation of the Crem gene in the mouse germ-line blocks spermatogenesis. Here we report the phenotype of mice in which targeted disruption of the act gene was obtained by homologous recombination. Whereas the seminiferous tubules of the act(-/-) mice contain all of the developmental stages of germ cells and the mice are fertile, the amount of mature sperm in the epididymis is drastically reduced. The residual sperm display severe abnormalities, including fully folded tails and aberrant head shapes. These results indicate that numerous postmeiotic genes under CREM control require the coactivator function of ACT. Thus, the fine-tuning of sperm development is achieved by the coordinated action of two transcriptional regulators.

Separation of spermatogenic cells from adult transgenic mouse testes using unit-gravity sedimentation

During the final stage of spermatogenesis (i.e., spermiogenesis), round spermatids differentiate into mature spermatozoa. This transformation is mediated by a suite of nuclear packaging proteins. These include the transition proteins and the protamines. The two human protamines PRM1 and PRM2, and transition protein TNP2, are encoded by a single chromatin domain bounded by two regions of matrix attachment. Previous transgenic studies in our laboratory have shown that mice harboring a 40-kb segment of human chromosome 16p13.13 containing the PRM1--> PRM2-->TNP2 domain express the transgene in a haploid-specific, copy number-dependent, and position-independent manner. While these results indicate that this segment of the genome is a complete structural and functional regulatory unit, the elements governing the haploid expression of this suite of genes remain to be clearly defined. The preparation of spermatogenic cells is required to begin to address this mechanism. The CELSEP (Wescor/Dupont Inc. Wilmington, DE) unit-gravity sedimentation apparatus provides a simple, efficient, and reproducible means to separate testicular germ cells at all stages along this differentiative pathway. The high quality and integrity of germ cells obtained by this means provides a valuable resource for characterizing the molecular mechanisms governing the regulation of the PRM1-->PRM2-->TNP2 domain during spermatogenesis. A discussion of the CELSEP apparatus and the application of this methodology in our laboratory are presented.

Temporal association of protamine 1 with the inner nuclear membrane protein lamin B receptor during spermiogenesis

During mammalian spermiogenesis, histones are replaced by transition proteins, which are in turn replaced by protamines P1 and P2. P1 protamine contains a short arginine/serine-rich (RS) domain that is highly phosphorylated before being deposited into sperm chromatin and almost completely dephosphorylated during sperm maturation. We now demonstrate that, in elongating spermatids, this phosphorylation is required for the temporal association of P1 protamine with lamin B receptor (LBR), an inner nuclear membrane protein that also possesses a stretch of RS dipeptides at its nucleoplasmic NH(2)-terminal domain. Previous studies have shown that the cellular protein p32 also binds tightly to the unmodified RS domain of LBR. Extending those findings, we now present evidence that p32 prevents phosphorylation of LBR and furthermore that dissociation of this protein precedes P1 protamine association. Our data suggest that docking of protamine 1 to the nuclear envelope is an important intermediate step in spermiogenesis and reveal a novel role for SR protein kinases and p32.

Role of follitropin receptor signaling in nuclear protein transitions and chromatin condensation during spermatogenesis

Follitropin receptor (FSHR) in testicular Sertoli cells mediates signaling by pituitary follitropin (FSH) promoting intercellular communication with germ cells for normal spermatogenesis. Using receptor knockout mice we examined changes in sperm nucleoproteins and chromatin architecture. The expressions of transition proteins 1/2 (TP1/2) and protamine-2 (PRM-2) were greatly diminished at 21 days, but returned to normal at 35 days and 3 months after birth. However, protein components in chromatin were quite different. Western blots detected a reduction in PRM1/2 and prolonged retention of mono-ubiquitinated histone 2A (uH2A) in the epididymal sperm from adult mutants. Two forms of mono- and poly-uH2A were present in sonication-resistant testicular spermatids in normal mice, whereas only an elevated mono-uH2A was detectable in mutants. Decrease in PRM1/2 and retention of mono-uH2A was coincident with reduction in TP1/2 in premature spermatids. Thus lack of FSHR signaling impairs expression of TP1/2 and PRM-2 at an early stage of post-natal development causing delayed spermatogenesis. In the adult, absence of FSHR signaling prolongs retention of mono-uH2A, leading to impair transition of basic nucleoproteins and chromatin remodeling during mouse spermatogenesis.

Production of fertile offspring from genetically infertile male mice

A number of recessive autosomal genes cause male infertility. Male mice homozygous for the blind-sterile (bs/bs) and quaking-sterile (qk/qk) gene mutations are sterile, because they either do not produce any spermatozoa or produce only a few abnormal spermatozoa. Mice lacking the cyclic AMP responsive-element modulator gene are sterile due to failure of spermiogenesis. All these mice, however, are able to produce fertile offspring when their spermatozoa or round spermatids are injected into oocytes of normal females. This implies that genetic and epigenetic elements necessary for syngamy and embryonic development are established in round spermatids and spermatozoa of these animals, even though their spermatogenic cells are destined to die (bs/bs and qk/qk) or are programmed to undergo apoptosis (cyclic AMP responsive-element modulator-null) without becoming functional spermatozoa.

Rosbin: a novel homeobox-like protein gene expressed exclusively in round spermatids

Mammalian spermiogenesis is a complex process occurring in a highly coordinated fashion within the seminiferous tubules. To elucidate the molecular mechanisms controlling haploid germ cell differentiation, we have isolated haploid germ cell- specific cDNA clones from a subtracted cDNA library of mouse testis. One of these cDNAs, Rosbin, is 3.2 kilobases (kb) long and has an open reading frame of 2385 nucleotides encoding a putative protein of 795 amino acid residues. A computer-mediated homology search revealed that it contained a domain similar to that of homeobox genes. Northern blot analysis revealed a 3.2-kb mRNA expressed exclusively in male germ cells. Transcription of the Rosbin gene was not observed in prepubertal testis but became detectable after Day 23. By Western blot analysis the protein encoded by this gene had a molecular mass of 89 kDa, expressing specifically in the testis and localized to the nucleus of stages IV-VIII haploid round spermatids, predominantly at stages VII-VIII of spermatogenesis. ROSBIN is associated with and is most likely phosphorylated by protein kinase A. We suggest that it plays an important role in transcriptional regulation in haploid germ cells.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

A specific programme of gene transcription in male germ cells

The differentiation of male germ cell requires spermatogenic stage and cell-specific gene expression that is achieved by unique chromatin remodelling, transcriptional control, and the expression of testis-specific genes or isoforms. Specialized transcription complexes that coordinate the differentiation programme of spermatogenesis have been found in germ cells, which display specific differences in the components of the general transcription machinery. The TATA-binding (TBP) protein family and its associated co-factors, for example, show upregulated expression in testis. In this physiological context, transcriptional control mediated by the activator CREM represents an established paradigm. In somatic cells, activation by CREM requires its phosphorylation at a unique regulatory site (Ser117) and subsequent interaction with the ubiquitous coactivator CBP. In testis, CREM transcriptional activity is controlled through interaction with a tissue-specific partner, ACT, which confers a powerful, phosphorylation-independent activation capacity. The function of ACT is regulated by a testis-specific kinesin, KIF17b. This study discusses some aspects of the testis-specific transcription machinery, the function of which is essential for the process of spermatogenesis.

Control of mouse hils1 gene expression during spermatogenesis: identification of regulatory element by transgenic mouse

Histone H1-like protein in spermatids 1 (Hils1) is a testis- specific histone H1-like protein exclusively expressed in haploid spermatids and should be involved in chromatin remodeling during mouse spermatogenesis. Spatial and temporal regulation of the hils1 gene expression would be critical for the formation of functional sperm, controlled at both transcriptional and translational levels. Previously, we reported that transcripts of the hils1 gene are exclusively expressed in mouse testis from 23 days of age whereas the Hils1 protein is not detected until 28 days of age, suggesting that hils1 is a member of a class of translationally regulated genes. By analyzing transgenic mice, we could demonstrate that 318-base pair (bp) 5'-proximal region corresponding to the first 70-bp proximal TATA-less promoter, and 248 bp of 5'-untranslated region is sufficient to confer testis- and spermatid-specific transcription as well as posttranscriptional control of the mouse hils1 gene in vivo.

Transcriptional Control in Male Germ Cells: General Factor TFIIA Participates in CREM-Dependent Gene Activation

Regulation of gene expression in haploid male germ cells follows a number of specific rules that differ from somatic cells. In this physiological context, transcriptional control mediated by the activator CREM (cAMP-responsive element modulator) represents an established paradigm. In somatic cells activation by CREM requires its phosphorylation at a unique regulatory site (Ser117) and subsequent interaction with the ubiquitous coactivator CBP (cAMP response element binding protein-binding protein). In testis, CREM transcriptional activity is controlled through interaction with a tissue-specific partner, ACT (activator of CREM in testis), which confers a powerful, phosphorylation-independent activation capacity. In addition to specialized transcription factors and coactivators, a variety of general factors of the basal transcriptional machinery, and their distinct tissue-specific isoforms, are highly expressed in testis, supporting the general notion that testis-specific gene expression requires specialized mechanisms. Here, we describe that CREM interacts with transcription factor IIA (TFIIA), a general transcription factor that stimulates RNA polymerase II-directed transcription. This association was identified by a two-hybrid screen, using a testis-derived cDNA library, and confirmed by coimmunoprecipitation. The interaction is restricted to the activator isoforms of CREM and does not require Ser117. Importantly, CREM does not interact with TFIIAtau-ALF, a testis-specific TFIIA homolog. CREM and TFIIA are expressed in a spatially and temporally coordinated fashion during the differentiation program of germ cells. The two proteins also colocalize intracellularly in spermatocyte and spermatid cells. These findings contribute to the understanding of the highly specialized rules of transcriptional regulation in haploid germ cells.

Intratesticular signals for progression of germ cell stages in vertebrates

The mechanisms underlying the complexity of spermatogenesis and spermiogenesis have deeply been studied in recent years. Transgenic animals, gene-targeting techniques, and lower vertebrate animal models have led to the discovery of some of the intratesticular signals involved in germ cell progression. This review wish to give the state of the art about it with particular emphasis on the comparative approach.

Hypomorphic mutation in an essential cell-cycle kinase causes growth retardation and impaired spermatogenesis

Cdc7 kinase is essential for initiation of DNA replication. Cdc7(-/-) mouse embryonic stem (ES) cells are non-viable but their growth can be rescued by an ectopically expressed transgene (Cdc7(-/-)tg). Here we report that, despite the normal growth capability of Cdc7(-/-)tg ES cells, the mice with the identical genetic background exhibit growth retardation. Concomi tantly, Cdc7(-/-)tg embryonic fibroblasts (MEFs) display delayed S phase entry and slow S phase progression. Furthermore, spermatogenesis of Cdc7(-/-)tg mice is disrupted prior to pachytene stage of meiotic prophase I. The impairment in spermatogenesis correlates with the extremely low level of Cdc7 protein in testes, and is rescued by introducing an additional allele of transgene, which results in increase of Cdc7 expression. The increased level of Cdc7 also recovers the growth of Cdc7(-/-)tg MEFs and mice, indicating that the developmental abnormalities of Cdc7(-/-)tg mice are due to insufficiency of Cdc7 protein. Our results indicate the requirement of a critical level of a cell-cycle regulator for mouse development and provide genetic evidence that Cdc7 plays essential roles in meiotic processes in mammals.

Expression of the germ cell-specific transcription factor ALF in Xenopus oocytes compensates for translational inactivation of the somatic factor TFIIA

The discovery of germ cell-specific general transcription factor and coactivator variants has suggested that reproductive tissues control gene expression somewhat differently than somatic tissues. One of these factors, ALF (TFIIAtau), was first described as a testis-specific counterpart of the large (alpha/beta) subunit of TFIIA. Here we characterize endogenous ALF and TFIIA activities in the African clawed frog Xenopus laevis. ALF is present in both testis and ovary in this organism, and it completely replaces TFIIA in immature oocytes. When oocytes undergo progesterone-induced maturation, ALF activity disappears, and TFIIA activity is restored. Reactivation occurs through the translational up-regulation of two maternal TFIIAalpha/beta mRNAs and involves polyadenylation of a conserved 3'-untranslated region module. The effects of ALF overexpression and ALF immunodepletion on a thymidine kinase promoter construct demonstrate that this factor serves as an active replacement for TFIIA. In contrast, overexpression of TFIIA inhibits transcription, indicating that the somatic factor fails to function properly in the context of the oocyte transcription machinery. Overall, the results show that the translationally regulated reciprocal expression of ALF and TFIIA allows for the production of an active TFIIA-like general transcription factor throughout oogenesis.

Dynamics of protamine 1 binding to single DNA molecules

Protamine molecules bind to and condense DNA in the sperm of most vertebrates, packaging the sperm genome in an inactive state until it can be reactivated following fertilization. By using methods that enable the analysis of protamine binding to individual DNA molecules, we have monitored the kinetics of DNA condensation and decondensation by protamine 1 (P1) and synthetic peptides corresponding to specific segments of the bull P1 DNA binding domain. Our results show that the number of clustered arginine residues present in the DNA binding domain is the most important factor affecting the condensation and stability of the DNA-protamine complex prior to the formation of inter-protamine disulfide cross-links. The high affinity of P1 for DNA is achieved by the coordinated binding of three anchoring domains, which together in bull P1 contain 19 Arg residues. The single DNA molecule experiments show that sequences containing two or more anchoring domains have an off-rate that is at least 3 orders of magnitude slower than those containing a single domain. The use of Arg, rather than Lys residues, and the inclusion of Tyr or Phe residues in the hinge regions between anchoring domains provide additional stability to the complex.

Identification of a short PIASx gene promoter that directs male germ cell-specific transcription in vivo

PIASx gene encodes two SUMO E3 ligases that are highly expressed in the testis. We have isolated and analyzed the promoter of the murine PIASx gene. Electrophoretic mobility shift assays with testicular nuclear extracts showed that the proximal promoter forms a major DNA-protein complex containing Sp1, Sp2, and Sp3 transcription factors. Reporter gene assays in cultured cells indicated that a fragment comprising nucleotides from -168 to +76 relative to transcription start site is sufficient for basal promoter activity in cultured cells, but these in vitro assays failed to reveal clear differences in promoter activity between testis- and non-testis-derived cell lines. Interestingly, however, the proximal promoter encompasses the elements necessary for a testis-specific transcription in vivo, as it directed beta-galactosidase expression exclusively to male germ cells in transgenic mice. In conclusion, we have characterized the minimal PIASx promoter that can be used for highly specific targeting of transgene expression to male germ cells.

The structural organization of sperm chromatin

The packaging of the male haploid genome within the differentiating spermatid nucleus is facilitated by small basic nuclear proteins called protamines. Although the majority of the DNA in human sperm chromatin is bound by these proteins, a small percentage retains a nucleosomal-like component. These histone-enriched regions may possess enhanced nuclease sensitivity and have been postulated to designate certain genes involved in early embryogenesis. We have shown previously that the chromatin domain containing the two human protamines PRM1 and PRM2 and the transition protein TNP2 forms a DNase I-sensitive conformation in pachytene spermatocytes, a requisite event prior to the haploid expression of its members in round spermatids (Kramer, J. A, McCarrey, J., Djakiew, D., and Krawetz, S. A. (1998) Development 125, 4749-4755). Interestingly, this configuration persists in mature spermatozoa subsequent to the transcriptional silencing of the locus. It was therefore postulated that the retained, enhanced DNase I-sensitive conformation of the PRM1-->PRM2-->TNP2 domain in human sperm may be preferentially histone-enriched. To address this tenet, we examined the chromatin structure of the human PRM1--> PRM2--> TNP2 domain using a PCR-based assay. The results show that this retained, enhanced DNase I sensitive domain reflects an enrichment of histones at discrete regions across the locus. In addition, a similar examination of other genes and repetitive sequences suggests the non-random distribution of histones and protamines within the sperm nucleus. A discussion of these results and their functional significance is presented.

SPAM1 (PH-20) protein and mRNA expression in the epididymides of humans and macaques: utilizing laser microdissection/RT-PCR

BACKGROUND

The Sperm Adhesion Molecule 1 (SPAM1) is an important sperm surface hyaluronidase with at least three functions in mammalian fertilization. Previously our laboratory reported that in the mouse, in addition to its expression in the testis, Spam1 is synthesized in the epididymis where it is found in membranous vesicles in the principal cells of the epithelium in all three regions. Since SPAM1 is widely conserved among mammals the aim of the study was to determine if its expression pattern in the epididymis is conserved in rodents and primates.

METHODS

We used laser microdissection (LM)/RT-PCR on frozen and paraffin-embedded epididymal sections of humans (n = 3) and macaques (n = 2) as well as in situ transcript hybridization to determine if transcripts are present in the epididymal epithelium. Western analysis and immunohistochemistry were used to detect and confirm the protein expression, and hyaluronic acid substrate gel electrophoresis analyzed its hyaluronidase activity. An in silico analysis of the proximal promoter of SPAM1 was also performed to identify relevant putative transcription binding sites for the androgen receptor.

RESULTS

We demonstrate that mRNA unique to SPAM1 is present in the principal cells of the epididymal epithelium in all individuals of both species studied. SPAM1 protein is present in all three regions of the epididymis, as well as the vas deferens, and is localized similarly to the transcripts. SPAM1 was shown to have hyaluronidase activity at pH 7.0. In the proximal promoter of SPAM1 were uncovered putative epididymal transcription factor binding sites including androgen receptor elements (AREs), consistent with epididymal expression.

CONCLUSIONS

These findings allow us to conclude that epididymal SPAM1 is conserved in at least two mammalian classes, rodents and primates. This conservation of expression suggests that the protein is likely to play an important function, possibly in sperm maturation.

Mouse Spam1 (PH-20) is a multifunctional protein: evidence for its expression in the female reproductive tract

Sperm adhesion molecule 1 (Spam1) is a widely conserved sperm surface protein with multiple roles in mammalian fertilization. Although the gene for this protein has been thought to be testis specific based on Northern blot analysis, there is evidence for nontesticular expression when transcripts are analyzed by more sensitive techniques. In the present investigation, results of a reverse transcription polymerase chain reaction assay, an RNase-protection assay (RPA), and an in situ transcript hybridization assay revealed that the murine Spam1 gene is transcribed in the female genital tract. RPA revealed that Spam1 transcripts are synthesized in a region-dependent manner, with the oviduct having lower transcript levels than the uterus and vagina. The transcripts levels were 3- to 10-fold lower in the female genital tract than in the testis. In situ transcript hybridization assay revealed RNA in the luminal epithelium in all three regions of the genital tract and in the uterine myometrium and the oviductal mesothelium. Western blot analysis and immunohistochemistry demonstrated that the protein concentration is 1.5- to 3-fold lower in female tissues than in sperm, and localization is similar to that of the transcripts. The protein has hyaluronidase activity at neutral pH, which is unique for sperm hyaluronidase, but not at acidic pH. In the uterus, Spam1 expression fluctuated during the estrous cycle. Its localization suggests that in addition to functioning as a secretory protein, it may be involved in hyaluronic acid metabolism or turnover in the female genital tract. Our results provide further evidence that Spam1 is a multifunctional protein and that it is less restricted in its expression than previously reported.

Analysis of regulatory elements of E-cadherin with reporter gene constructs in transgenic mouse embryos

Proper regulation of E-cadherin-mediated cell adhesion is important during early embryonic development and in organogenesis. In mice, E-cadherin is expressed from the fertilized egg onward and becomes down-regulated during gastrulation in mesoderm and its derivatives, but its expression is maintained in all epithelia. E-cadherin promoter analyses led to the identification of binding sites for two transcriptional repressors, Snail and SIP1, which are able to mediate down-regulation in vitro, but little is known about the regulatory elements that govern E-cadherin transcriptional activity in vivo. Here, we compared the developmentally regulated expression of a series of lacZ-reporter transgenes fused to different sequences of the murine E-cadherin gene between -6 kb, including the promoter, and +16 kb, covering one third of intron 2. Four different segments with distinct regulatory properties were identified. The promoter fragment from +0.1 to -1.5 kb remains inactive in most cases but occasionally induces ectopic expression in mesodermal tissues, although it contains binding sites for the repressors Snail and SIP1. This promoter fragment also lacks positive elements needed for the activation of transcription in ectoderm and endoderm. Sequences from -1.5 to -6 kb harbor regulatory elements for brain-specific expression and, in addition, insulator or silencer elements, because they are consistently inactive in the mesoderm. Only if sequences from +0.1 to +11 kb are combined with the promoter fragments is E-cadherin-specific transgene expression observed in endoderm and certain epithelia. Sequences between +11 and +16 kb contain cis-active elements that generally enhance transcription. Our analyses show that E-cadherin expression is governed by a complex interplay of multiple regulatory regions dispersed throughout large parts of the locus.

Genomic structure and promoter activity of the testis haploid germ cell-specific intronless genes, Tact1 and Tact2

The Tact1 and Tact2 genes, each of which encodes an actin-like protein, are exclusively expressed and translated in haploid germ cells in testis. To characterize the haploid germ cell-specific gene structure, a mouse genomic library was screened with a Tact1 cDNA as a probe, and four independent phage clones containing the Tact1 gene were isolated. Southern hybridization and sequencing analyses revealed that Tact1 and Tact2 were single copy genes contained on a common fragment in a head-to-head orientation, and that the distance between these genes was less than 2 kb. Comparison of the nucleotide sequences of genomic DNA and cDNA demonstrated that Tact1 and Tact2 lack introns, although all known actin or actin-related genes in mammals contain introns. Human Tact orthologues also lack introns and are located within 6.4 kb in a head-to-head orientation. These findings indicate that Tact1 and Tact2 or one of these genes arose by retroposition of a spliced mRNA transcribed from an actin progenitor gene prior to the divergence of rodents and primates. The Tact1 and Tact2 genes are unusual retroposons in that they have retained an open reading frame and are expressed in testicular germ cells, because almost all retroposons become pseudogenes. It was revealed that a 2kb sequence between the two genes bidirectionally controls haploid germ-cell specific expression by analyzing transgenic mice. Comparison of the murine Tact genes with their human orthologues showed a high level of identity between the two species in the 5'-upstream and non-coding sequences as well as in the coding region, indicating that conserved elements in these regions may be involved in the regulation of haploid germ cell-specific expression. The promoter region contains no TATA-, CCAAT- or GC-boxes, although there are potential cAMP response element (CRE)-like motifs in the 5'-upstream region and the 5'-untranslated region in Tact1 and Tact2, respectively. Transient promoter analyses indicate that CREMtau may activate Tact1 and Tact2 expression in germ cells.

MICoA, a novel metastasis-associated protein 1 (MTA1) interacting protein coactivator, regulates estrogen receptor-alpha transactivation functions

The transcriptional activity of estrogen receptor-alpha (ER-alpha) is modified by coactivators, corepressors, and chromatin remodeling complexes. We have previously shown that the metastasis-associated protein-1 (MTA1), a component of histone deacetylase and nucleosome remodeling complexes, represses ER-driven transcription by recruiting histone deacetylases to the estrogen receptor element (ERE)-containing target gene chromatin in breast cancer cells. Using a yeast two-hybrid screening to clone MTA1-interacting proteins, we identified a previously uncharacterized molecule, which we named as MTA1-interacting coactivator (MICoA). Our findings suggest that estrogen signaling promotes nuclear translocation of MICoA and that MICoA interacts with MTA1 both in vitro and in vivo. MICoA binds to the C-terminal region of MTA1, whereas MTA1 binds to the N-terminal MICoA containing one nuclear receptor interaction LSRLL motif. We showed that MICoA is an ER coactivator, cooperates with other ER coactivators, stimulates ER-transactivation functions, and associates with the endogenous ER and its target gene promoter chromatin. MTA1 also repressed MICoA-mediated stimulation of ERE-mediated transcription in the presence of ER and ER variants with naturally occurring mutations, such as D351Y and K303R, and that it interfered with the association of MICoA with the ER-target gene chromatin. Because chromatin is a highly dynamic structure and because MTA1 and MICoA could be detected within the same complex, these findings suggest that MTA1 and MICoA might transmodulate functions of each other and any potential deregulation of MTA1 is likely to contribute to the functional inactivation of the ER pathway, presumably by derecruitment of MICoA from ER target promoter chromatin.

The intracellular localisation of TAF7L, a paralogue of transcription factor TFIID subunit TAF7, is developmentally regulated during male germ-cell differentiation

Transcription regulation in male germ cells can involve specialised mechanisms and testis-specific paralogues of the general transcription machinery. Here we describe TAF7L, a germ-cell-specific paralogue of the TFIID subunit TAF7. TAF7L is expressed through most of the male germ-cell differentiation programme, but its intracellular localisation is dynamically regulated from cytoplasmic in spermatogonia and early spermatocytes to nuclear in late pachytene spermatocytes and haploid round spermatids. Import of TAF7L into the nucleus coincides with decreased TAF7 expression and a strong increase in nuclear TBP expression, which suggests that TAF7L replaces TAF7 as a TFIID subunit in late pachytene spermatocytes and in haploid cells. In agreement with this, biochemical experiments indicate that a subpopulation of TAF7L is tightly associated with TBP in both pachytene and haploid cells and TAF7L interacts with the TFIID subunit TAF1. We further show that TAF3, TAF4 and TAF10 are all strongly expressed in early spermatocytes, but that in contrast to TBP and TAF7L, they are downregulated in haploid cells. Hence, different subunits of the TFIID complex are regulated in distinct ways during male germ-cell differentiation. These results show for the first time how the composition of a general transcription factor such as TFIID and other TAF-containing complexes are modulated during a differentiation programme highlighting the unique nature of the transcription regulatory machinery in spermatogenesis.

Chromatin remodeling by nuclear receptors

The eukaryotic genome is structurally organized into nucleosomes to form chromatin, which regulates gene expression, in part, by controlling the accessibility of regulatory factors. When packaged as chromatin, many promoters are transcriptionally repressed, thus reducing the access of transcription factors to their binding sites. However, nuclear receptors (NRs) are a group of transcription factors that have the ability to access their binding sites in this repressive chromatin structure. Nuclear receptors are able to bind to their sites and recruit chromatin-remodeling proteins such as ATP-dependent chromatin-remodeling complexes and histone-modifying enzymes, resulting in transcriptional activation. In this review, we present the role of NRs in recruiting these chromatin-modifying enzymes by means of an extensively studied model system, the glucocorticoid receptor-mediated transactivation of the mouse mammary tumor virus (MMTV) promoter. We use these findings as a template to begin to understand the effect of chromatin changes on gene expression during spermatogenesis.

BIR-1, a Caenorhabditis elegans homologue of Survivin, regulates transcription and development

bir-1, a Caenorhabditis elegans inhibitor-of-apoptosis gene homologous to Survivin is organized in an operon with the transcription cofactor C. elegans SKIP (skp-1). Because genes arranged in operons are frequently linked functionally, we have asked whether BIR-1 also functions in transcription. bir-1 inhibition resulted in multiple developmental defects that overlapped with C. elegans SKIP loss-of-function phenotypes: retention of eggs, dumpy, movement defects, and lethality. bir-1 RNA-mediated interference decreased expression of several gfp transgenes and the endogenous genes dpy-7 and hlh-1. Immunoblot analysis revealed decreased phosphoacetylated histones in bir-1 RNA-mediated interference-treated worms. In a heterologous transfection system, BIR-1 augments thyroid hormone-regulated transcription and has an additive effect with SKIP. These results show that BIR-1 functions in the regulation of transcription and development.

Testis-specific expression of the nuclear form of phospholipid hydroperoxide glutathione peroxidase (PHGPx)

The selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx) is present in at least three different isoforms in testis: as a cytosolic, as a mitochondrial, and as a nuclear protein. We have recently shown that a sperm nucleus-specific glutathione peroxidase (snGPx) is identical to the mitochondrial and cytosolic forms of PHGPx apart from its N-terminus. This arginine-rich N-terminus of snGPx, reminiscent of protamines, is encoded by an alternative exon located in the first intron of the PHGPx gene and is responsible for nuclear localisation and chromatin binding of snGPx [Pfeifer et al., FASEB J. 15 (2001), pp. 1236-1238]. By using a combination of techniques including selective cloning of mRNA 5'-ends, RT-PCR, and S1 analyses, we provide evidence that the transcript encoding the nuclear form is generated by transcription initiation at an alternative promoter and not by alternative splicing. We show that the major transcription start region is located at -12 to -14 upstream of the AUG translation initiation site of the sperm nucleus-specific exon and lacks a TATA box. Two minor TATA-less transcription initiation sites are located at around -30 and -45. We have shown by in situ hybridisation that snGPx expression in testis, like protamine expression, is restricted to late stages of spermatogenesis whereas PHGPx expression is only found in spermatocytes and early spermatids. These findings have to be taken into account when studying either the differential regulation of PHGPx and snGPx expression in testis or the impact of putative mutations in snGPx on male fertility in man.

Impaired gametogenesis in mice that overexpress the RNA-binding protein HuR

A series of experiments, using cell culture models or in vitro assays, has shown that the RNA-binding protein HuR increases the half-life of some messenger RNAs that contain adenylate/uridylate-rich decay elements. However, its function in an integrated system has not yet been investigated. Here, using a mouse model, we report that misregulation of HuR, due to expression of an HuR transgene, prevents the production of fully functional gametes. This work provides the first evidence for a physiological function of HuR, and highlights its involvement in spermatogenesis.

Nuclear envelope dynamics in oocytes: from germinal vesicle breakdown to mitosis

We have recently gained new insight into the mechanisms involved in nuclear envelope breakdown, the irreversible step that commits a cell to the M phase. Results from mammalian cell and starfish oocyte studies suggest that mechanical forces of the cytoskeleton, as well as biochemical disassembly of nuclear envelope protein complexes, play important roles in this process.

Mouse germ cell-less as an essential component for nuclear integrity

A mouse homologue of the Drosophila melanogaster germ cell-less (mgcl-1) gene is expressed ubiquitously, and its gene product is localized to the nuclear envelope based on its binding to LAP2 beta (lamina-associated polypeptide 2 beta). To elucidate the role of mgcl-1, we analyzed two mutant mouse lines that lacked mgcl-1 gene expression. Abnormal nuclear morphologies that were probably due to impaired nuclear envelope integrity were observed in the liver, exocrine pancreas, and testis. In particular, functional abnormalities were observed in testis in which the highest expression of mgcl-1 was detected. Fertility was significantly impaired in mgcl-1-null male mice, probably as a result of severe morphological abnormalities in the sperm. Electron microscopic observations showed insufficient chromatin condensation and abnormal acrosome structures in mgcl-1-null sperm. In addition, the expression patterns of transition proteins and protamines, both of which are essential for chromatin remodeling during spermatogenesis, were aberrant. Considering that the first abnormality during the process of spermatogenesis was abnormal nuclear envelope structure in spermatocytes, the mgcl-1 gene product appears to be essential for appropriate nuclear-lamina organization, which in turn is essential for normal sperm morphogenesis and chromatin remodeling.

No circadian rhythms in testis: Period1 expression is clock independent and developmentally regulated in the mouse

Spermatogenesis is a process whereby haploid spermatozoa differentiate through meiosis from precursor stem cells. We examined the expression of circadian clock genes in the testis, to assess clock control over the timing of different developmental events. Clock genes are known to oscillate with circadian rhythmicity in the central clock structure, the suprachiasmatic nucleus of the hypothalamus, but also in peripheral tissues. Here we show that Per1 gene expression in the testis is constant over a 24-h period and that the Per1 transcript is expressed at a level higher than the peak values of the Per1 oscillations observed for other tissues. Bmal1, another clock gene whose expression oscillates in other tissues, also shows constant expression levels in the testis. In addition, the levels and phosphorylation state of the PER1 protein are not oscillating at all times of day. Strikingly, Per1 is restricted primarily to step 7 to 10 spermatids and thus appears to be developmentally regulated. The expression of the Clock transcript is also developmentally regulated, but it is found principally in spermatogonia and spermatocytes up until the time of the first meiotic division. Per1 expression is not altered in testes from Clock mutant mice, suggesting that CLOCK does not activate Per1 in male germ cells, in contrast to what it does in other mouse tissues. Taken together, our observations suggest that the testis, in contrast to all other peripheral tissues, lacks a functioning circadian clock.

CREM-dependent transcription in male germ cells controlled by a kinesin

ACT is a LIM-only protein expressed exclusively in round spermatids, where it cooperates with transcriptional activator CREM in regulating various postmeiotic genes. Targeted inactivation of CREM leads to a complete block of mouse spermiogenesis. We sought to identify the regulatory steps controlling the functional interplay between CREM and ACT. We found that ACT selectively associates with KIF17b, a kinesin highly expressed in male germ cells. The ACT-KIF17b interaction is restricted to specific stages of spermatogenesis and directly determines the intracellular localization of ACT. Sensitivity to leptomycin B indicates that KIF17b can be actively exported from the nucleus through the Crm1 receptor. Thus, a kinesin directly controls the activity of a transcriptional coactivator by a tight regulation of its intracellular localization.

Expression of proteasome subunit isoforms during spermatogenesis in Drosophila melanogaster

In this study, we sought to identify and characterize all the proteasome genes of Drosophila melanogaster. Earlier work led to the identification of two genes encoding alpha4-type 20S proteasome subunit isoforms that are expressed exclusively in the male germline. Here we extend these results and show that six of the 20S proteasome subunits, and four of the 19S regulatory cap subunits, have gene duplications encoding male-specific isoforms. More detailed analyses of two of these male-specific subunits (Prosalpha3T and Prosalpha6T), using GFP-tagged reporter transgenes, revealed that they are predominantly localized to the nucleus at later stages of spermatogenesis and are present there in mature, motile sperm. These results suggest a possible role of a 'spermatogenesis-specific' proteasome in sperm differentiation and/or function.