A possible meiotic function of the peculiar patterns of gene expression in mammalian spermatogenic cells

Cloning and expression profiling of testis-expressed microRNAs

Using a new small RNA cloning method, we identified 141 miRNAs from the mouse testis, of which 29 were novel. The 141 miRNAs were mapped onto all chromosomes except the Y chromosome and 2/3 of these miRNA genes exist as clusters. approximately 70% of these miRNA genes were located in intronic or intergenic regions, whereas the remaining miRNAs were derived from exonic sequences. We further validated these cloned miRNAs by examining their expression in multiple mouse organs including developing testes and also in purified spermatogenic cells using semi-quantitative PCR analyses. Our expression profiling assays revealed that 60% of the testis-expressed miRNAs were ubiquitously expressed and the remaining are either preferentially (35%) or exclusively (5%) expressed in the testis. We also observed a lack of strand selection during testicular miRNA biogenesis, characterized by paired expression of both the 5' strands and 3' strands derived from the same precursor miRNAs. The present work identified numerous miRNAs preferentially or exclusively expressed in the testis, which would be interesting targets for further functional studies.

Two-Dimensional Fluorescence Difference Gel Electrophoresis Analysis of Spermatogenesis in the Rat

The molecular mechanisms underlying normal and pathological spermatogenesis remain poorly understood. We compared protein concentrations in different germ cell types to identify those proteins specifically or preferentially expressed at each stage of rat spermatogenesis. Crude cytosolic protein extracts and reversed-phase HPLC prefractionated cytosolic extracts from spermatogonia, pachytene spermatocytes, and early spermatids were subjected to two-dimensional difference gel electrophoresis (2-D DIGE). By comparing gels and carrying out statistical analyses, we were able to identify 1274 protein spots with relative abundances differing significantly between the three cell types. We found that 265 of these spots displaying highly differential expression (ratio > or = 2.5 between two cell types), identified by mass fingerprinting, corresponded to 123 nonredundant proteins. The proteins clustered into three clades, corresponding to mitotic, meiotic, and post-meiotic cell types. The differentially expressed proteins identified by 2-D DIGE were confirmed and validated by western blotting and immunohistochemistry, in the few cases in which antibodies were available. 2-D DIGE appears a relevant proteomics approach for studying rat germ cell differentiation, allowing the establishment of the precise expression profiles for a relatively large number of proteins during normal spermatogenesis.

Expression and localization of androgen receptor-interacting protein-4 in the testis

Androgen receptor-interacting protein 4 (ARIP4) belongs to the SNF2 family of proteins involved in chromatin remodeling, DNA excision repair, and homologous recombination. It is a DNA-dependent ATPase, binds to DNA and mononucleosomes, and interacts with androgen receptor (AR) and modulates AR-dependent transactivation. We have examined in this study the expression and cellular localization of ARIP4 during postnatal development of mouse testis. ARIP4 was detected by immunohistochemistry in Sertoli cell nuclei at all ages studied, starting on day 5, and exhibited the highest expression level in adult mice. At the onset of spermatogenesis, ARIP4 expression became evident in spermatogonia, pachytene, and diplotene spermatocytes. Immunoreactive ARIP4 antigen was present in Leydig cell nuclei. In Sertoli cells ARIP4 was expressed in a stage-dependent manner, with high expression levels at stages II-VI and VII-VIII. ARIP4 expression patterns did not differ significantly in testes of wild-type, follicle-stimulating hormone receptor knockout, and luteinizing hormone receptor knockout mice. In testes of hypogonadal mice, ARIP4 was found mainly in interstitial cells and exhibited lower expression in Sertoli and germ cells. In vitro stimulation of rat seminiferous tubule segments with testosterone, FSH, or forskolin did not significantly change stage-specific levels of ARIP4 mRNA. Heterozygous ARIP4(+/-) mice were haploinsufficient and had reduced levels of Sertoli-cell specific androgen-regulated Rhox5 (also called Pem) mRNA. Collectively, ARIP4 is an AR coregulator in Sertoli cells in vivo, but the expression in the germ cells implies that it has also AR-independent functions in spermatogenesis.

Novel noncoding RNA from human Y distal heterochromatic block (Yq12) generates testis-specific chimeric CDC2L2

The human Y chromosome, because it is enriched in repetitive DNA, has been very intractable to genetic and molecular analyses. There is no previous evidence for developmental stage- and testis-specific transcription from the male-specific region of the Y (MSY). Here, we present evidence for the first time for a developmental stage- and testis-specific transcription from MSY distal heterochromatic block. We isolated two novel RNAs, which localize to Yq12 in multiple copies, show testis-specific expression, and lack active X-homologs. Experimental evidence shows that one of the above Yq12 noncoding RNAs (ncRNAs) trans-splices with CDC2L2 mRNA from chromosome 1p36.3 locus to generate a testis-specific chimeric beta sv13 isoform. This 67-nt 5'UTR provided by the Yq12 transcript contains within it a Y box protein-binding CCAAT motif, indicating translational regulation of the beta sv13 isoform in testis. This is also the first report of trans-splicing between a Y chromosomal and an autosomal transcript.

Sex-specific promoters regulate Dnmt3L expression in mouse germ cells

BACKGROUND

Dnmt3L, a member of the DNA methyltransferase 3 family, lacks enzymatic activity but is required for de-novo methylation of imprinted genes in oocytes and for transposon repression in male germ cells.

METHODS

We used northern blots, RT-PCR, 5' rapid amplification of complementary DNA (cDNA) ends (RACE), RNase H mapping, real-time/quantitative RT-PCR and in situ hybridization to identify and characterize Dnmt3L transcripts produced during germ cell development.

RESULTS

Mouse Dnmt3L uses three sex-specific promoters, not the single promoter previously thought. A promoter active in prospermatogonia drives transcription of an mRNA encoding the full-length protein in perinatal testis, where de-novo methylation occurs. Late pachytene spermatocytes activate a second promoter in intron 9 of the Dnmt3L gene. After this stage, the predominant transcripts are three truncated mRNAs, which appear to be non-coding. We could also detect similar adult testis transcripts in humans. In the mouse ovary, an oocyte-specific promoter located in an intron of the neighbouring autoimmune regulator (Aire) gene produces a transcript with the full open reading frame (ORF). This is the only Dnmt3L transcript found in growing oocytes and is absent in the oocytes of Dnmt3L-/- females.

CONCLUSIONS

Sex-specific promoters control Dnmt3L expression in the mouse germ line, mirroring the situation at the Dnmt1 and Dnmt3A loci.

Evolution of primate gene expression

It has been suggested that evolutionary changes in gene expression account for most phenotypic differences between species, in particular between humans and apes. What general rules can be described governing expression evolution? We find that a neutral model where negative selection and divergence time are the major factors is a useful null hypothesis for both transcriptome and genome evolution. Two tissues that stand out with regard to gene expression are the testes, where positive selection has exerted a substantial influence in both humans and chimpanzees, and the brain, where gene expression has changed less than in other organs but acceleration might have occurred in human ancestors.

Identification, molecular cloning, and cellular distribution of the rat homolog of minichromosome maintenance protein 7 (MCM7) in the rat testis

As part of a program to decipher the rat testicular proteome, we studied spermatogonia and identified numerous proteins including the human homolog of the Minichromosome Maintenance Protein 7 (MCM7). MCM7 has been implicated in DNA replication in various species, but had not been detected in the testis. Here we describe the cellular distribution of MCM7 transcripts and protein, and their testicular ontogenetic expression. The full-length coding region of the rat MCM7 was also characterized. Northern blot analyses showed that MCM7 transcripts are more abundant in the testis than other organs and confirmed the presence of the 2.4 kb MCM7 transcript at all ages studied. Interestingly, two additional transcripts of 3.2 and 1.6 kb were found from 26 days post partum onwards, when spermatocytes and spermatids accumulate within the tubules. This was confirmed in isolated cell types: the three MCM7 transcripts were observed in meiotic and post-meiotic germ cells. The 3.2 kb isoform has an extended 5' untranslated region (UTR) and the 1.6 kb transcript is the result of alternative splicing of five exons. Western blot and immunohistochemistry experiments evidenced abundant MCM7 in proliferating gonocytes and Sertoli cells in the fetal testis. In the adult testis, an intense signal was observed in spermatogonia and primary spermatocytes. We conclude that the Mcm7 is one example of genes that are differently transcribed and translated in somatic and spermatogenetic cells in mammals. Further work is required to determine the roles of MCM7 in spermatogonia and germ lineage.

The mRNA encoding tauCstF-64 is expressed ubiquitously in mouse tissues

Polyadenylation is a process of endonucleolytic cleavage of the mRNA, followed by addition of up to 250 adenosine residues to the 3' end of the mRNA. Polyadenylation is essential for eukaryotic mRNA expression, and CstF-64 is a subunit of the CstF polyadenylation factor that is required for accurate polyadenylation. We discovered that there are two forms of the CstF-64 protein in mammalian male germ cells, one of which (CstF-64) is expressed in all tissues, the other of which (tauCstF-64) is expressed only in male germ cells and in brain (albeit at significantly lower levels in the brain). Therefore, we were surprised to find that, using reverse transcription-PCR, cDNA cloning, and RNA blot analyses, tauCstF-64 mRNA was expressed at higher levels in brain than in testis. Also, tauCstF-64 mRNA was expressed at lower but detectable levels in all tissues tested, including epididymis, heart, kidney, liver, lung, muscle, ovary, spleen, thymus, and uterus. These results suggest the hypothesis that tauCstF-64 mRNA is regulated at the translational or post-translational level.

Transcriptome analyses of male germ cells with serial analysis of gene expression (SAGE)

Serial analysis of gene expression (SAGE) provides an alternative with additional advantages to microarrays for studying gene expression during spermatogenesis. The digitized transcriptome provided by SAGE of purified mouse germ cells identified 27,504 species of transcripts expressed in type A spermatogonia, pachytene spermatocytes, and round spermatids. Over 2700 of these transcripts were novel. Computational analyses allowed the identification of clusters of co-regulated genes, cell-specific promoter modules, cell-specific biological processes, as well as "preferential" biological networks in different cell types. These analyses provided potential drug targets for interference of specific pathways at different stages of spermatogenesis. Analyses of the transcriptomes revealed the prominent role of cytochrome c oxidase in germ cells and suggest a novel role for this enzyme in cytochrome c-mediated apoptosis in spermatogonia. A number of genes were shown to undergo differential splicing during spermatogenesis giving rise to cell-specific splice variants.

Expression profiling reveals meiotic male germ cell mRNAs that are translationally up- and down-regulated

Gametes rely heavily on posttranscriptional control mechanisms to regulate their differentiation. In eggs, maternal mRNAs are stored and selectively activated during development. In the male, transcription ceases during spermiogenesis, necessitating the posttranscriptional regulation of many paternal mRNAs required for spermatozoan assembly and function. To date, most of the testicular mRNAs known to be translationally regulated are initially transcribed in postmeiotic cells. Because protein synthesis occurs on polysomes and translationally inactive mRNAs are sequestered as ribonucleoproteins (RNPs), movement of mRNAs between these fractions is indicative of translational up- and down-regulation. Here, we use microarrays to analyze mRNAs in RNPs and polysomes from testis extracts of prepuberal and adult mice to characterize the translation state of individual mRNAs as spermatogenesis proceeds. Consistent with published reports, many of the translationally delayed postmeiotic mRNAs shift from the RNPs into the polysomes, establishing the validity of this approach. In addition, we detect another 742 mouse testicular transcripts that show dramatic shifts between RNPs and polysomes. One subgroup of 35 genes containing the known, translationally delayed phosphoglycerate kinase 2 (Pgk2) is initially transcribed during meiosis and is translated in later-stage cells. Another subgroup of 82 meiotically expressed genes is translationally down-regulated late in spermatogenesis. This high-throughput approach defines the changing translation patterns of populations of genes as male germ cells differentiate and identifies groups of meiotic transcripts that are translationally up- and down-regulated.

Uneven evolutionary rates of bradykinin B1 and B2 receptors in vertebrate lineages

Bradykinin acts through two receptor subtypes in mammals and generates a variety of responses including pain, inflammation and hypotension. The evolutionary history of the bradykinin system has been unclear due to shortage of information outside mammals. We describe here two receptor subtypes and the bradykinin precursor in three species of bony fish (the zebrafish Danio rerio, the Japanese pufferfish Takifugu rubripes, and the green spotted pufferfish Tetraodon nigroviridis) and chicken and analyze the relationships to mammals by a combination of phylogeny, conserved synteny and exon-intron organization. All of these species have two receptor genes located close to each other in a tandem formation, with the B2 gene 5' to the B1 gene, in chromosomal regions displaying conserved synteny between the species (albeit conservation of synteny in zebrafish is still unclear due to poor genome assembly). The evolutionary rate differs between the two genes as well as between lineages leading to differing pharmacological properties for both B1 and B2 across vertebrate classes. Also the bradykinin precursor gene was identified in all of these species in a chromosome region with conserved synteny. The tissue distribution of mRNA in T. rubripes is similar for B1 and B2, suggesting more similar regulation for the two genes than in mammals. In conclusion, the receptor tandem duplication predates the divergence of ray-finned fish and tetrapods and no additional duplicates of the receptors or bradykinin seem to have survived the ray-finned fish tetraploidization.

Developmental and cell type-specific regulation of core promoter transcription factors in germ cells of frogs and mice

This article reports on the comparative cell type-specific expression profiles of selected core promoter-associated transcription factors during gametogenesis and embryogenesis in frogs and mice. In frogs we tested TBP, TRF2/TLF, TRF3, TFIIAalphabeta, and ALF, as well as variant forms of TAFs 4, 5, and 6. Four of these factors, TRF3, TAF4L, TAF5L, and the previously-characterized ALF gene, are preferentially expressed in testis and ovary. In mice we tested TBP, TRF2/TLF, TRF3, TFIIAalphabeta, and ALF. The results showed that while ALF was present in testis and ovary, as expected, TRF3 could only be detected in the ovary. RT-PCR experiments using RNAs from microdissected ovary tissue, together with in situ hybridization analysis, showed that TRF3 and ALF genes are specifically expressed in oocytes in both adult and prepubertal animals, whereas, their somatic counterparts, TBP and TFIIAalphabeta, are present in oocytes and in surrounding somatic cells of the follicle. Furthermore, both mice and frogs displayed a reduction in TRF3 and ALF transcript levels around the time of fertilization. In mice, transcripts from these genes could again be detected at low levels in embryonic reproductive tissues, but only reached maximal levels in adult animals. Finally, the results of protein-DNA interaction assays show that all combinations of core promoter complexes can be formed in vitro using recombinant TBP, TRF3, TFIIA, and ALF, including a TRF3-ALF complex. Overall, the diverse gene regulatory patterns observed here and in earlier reports indicate precise control over which transcription factor complexes can be formed in vivo during gametogenesis and early embryogenesis.

Expression and subcellular localization of GSE protein in germ cells and preimplantation embryos

We previously identified a novel gonad-specific expression gene (Gse) and investigated its expression during gametogenesis in the mouse testis and ovary. In this study, we generated a polyclonal antibody to GSE protein and determined the profiles of the protein's expression in germ cells and preimplantation embryos in detail using immunocytochemical and immunofluorescence staining. In a Western blot analysis, the anti-GSE antibody recognized long and short isoforms (approximately 27.6 kDa and 23.1 kDa) of the protein in the mouse testis and the long isoform in the ovary. In the mouse testis, GSE protein was expressed in spermatocytes I in the pachytene stage, round spermatids, and elongated spermatids. In the mouse ovary, the protein was located in the cytoplasm and nucleus of all oocytes regardless of the stage of the ovarian follicles. In preimplantation embryos from the pronuclear to blastocyst stage, however, GSE protein was mainly detected in the nuclei of cells. At the blastocyst stage, the protein was confirmed to have accumulated in the inner cell mass (ICM), whereas it had mostly disappeared from the trophectoderm (TE). These findings suggest that GSE protein may play a role in the establishment of nuclear totipotency and may be associated with early lineage specification.

Diversification of CDK11 transcripts during chicken testis development and regression

CDK11 (cyclin-dependent kinase 11, formerly known as PITSLRE) is a serine/threonine kinase that associates with the cyclin L2 regulatory partner. CDK11 catalytic activity has been associated with apoptosis, transcription, and RNA processing. Here, we identify novel chicken testis CDK11 transcripts that differ in their 5'UTR, 3'UTR, splicing of the exon 6, and polyadenylation. We have also characterized the differential expression of CDK11 in somatic tissues, during testis development and upon testicular regression by diethylstilbestrol (DES) treatment. The heterogeneity of CDK11 transcripts presented in this study suggests new possibilities for post-transcriptional regulation.

Comparative genomics of the sperm mitochondria-associated cysteine-rich protein gene

The sperm mitochondrial cysteine-rich protein (SMCP) is a rapidly evolving cysteine- and proline-rich protein that is localized in the mitochondrial capsule and enhances sperm motility. The sequences of the SMCP protein, gene, and mRNA in a variety of mammals have been compared to understand their evolution and regulation. SMCP can now be reliably identified by its tripartite structure including a short amino-terminal segment; a central segment containing short tandem repeats rich in cysteine, proline, glutamine, and lysine; and a C-terminal segment containing no repeats, few cysteines, and a C-terminal lysine. The SMCP gene is located in the epidermal differentiation complex (EDC), a large gene cluster that functions in forming epithelial barriers. Similarities in chromosomal location, molecular function, intron-exon structure, and protein organization argue that SMCP originated from an EDC gene and acquired spermatogenic cell-specific transcriptional and translational regulation and a novel cellular function in sperm motility. The SMCP 5' UTR and 3' UTR contain conserved elements and uORFs that may function in cytoplasmic regulation of gene expression, and the levels of SMCP mRNA in human are much lower than in other mammals, a feature of male-biased expression. The evolution of SMCP has been accompanied by changes in the sequence, number, and length of repeat units, including three alleles in dogs. The major proteins associated with the mitochondrial capsule, SMCP and phospholipid hydroperoxide glutathione peroxidase, provide outstanding examples of changes in cellular function driven by selective pressures on sperm motility, an important determinant of male reproductive success.

Localization of a long-chain acyl-CoA hydrolase in spermatogenic cells in mice

Brain acyl-CoA hydrolase (BACH) hydrolyzes long-chain acyl-CoAs to free fatty acids and CoA-SH. BACH is highly distributed in brain and is localized in neurons, but not glial cells. This suggests that BACH plays a specific role in neurons. BACH is also detected in testis, although the expression profile of BACH is unknown in testis. In this study, developmental changes and cellular distribution of BACH were examined in mouse testis. Before postnatal day (P) 10, BACH was detected at very low levels by Western blotting. Then, BACH content rapidly increased from P14 and reached maximum levels at P21, remaining high until at least P70. The increase in BACH content corresponded to the appearance of pachytene spermatocytes, which was confirmed by immunohistochemistry. BACH was also detectable in spermatids, but not in spermatogonia, mature spermatozoa. These results suggest that BACH is expressed in a cell-specific manner and plays a role in spermatogenesis.

Basic mechanisms for the control of germ cell gene expression

The patterns of gene expression in spermatocytes and oocytes are quite different from those in somatic cells. The messenger RNAs produced by these cells are not only required to support germ cell development but, in the case of oocytes, they are also used for maturation, fertilization, and early embryogenesis. Recent studies have begun to provide an explanation for how germ-cell-specific programs of gene expression are generated. Part of the answer comes from the observation that germ cells express core promoter-associated regulatory factors that are different from those expressed in somatic cells. These factors supplement or replace their somatic counterparts to direct expression during meiosis and gametogenesis. In addition, germ cell transcription involves the recognition and use of specialized core promoter sequences. Finally, transcription must occur on chromosomal DNA templates that are reorganized into new chromatin-packaging configurations using alternate histone subunits. This article will review recent advances in our understanding of the factors and mechanisms that control transcription in ovary and testis and will discuss models for germ cell gene expression.

Transgene transmission to progeny by oMt1a-oGH transgenic mice

Most studies utilizing transgenic technology focus on the impact to traits of interest, rather than propagation of the transgene to offspring. In animals containing growth hormone constructs, transgene transmission to progeny follows a Mendelian pattern of inheritance in the first few generations following generation of a founder animal, but decreases in subsequent generations. In the present study, the ovine metallothionein 1a-ovine growth hormone (oMt1a-oGH) transgenic mouse was used to determine whether transgene transmission rate to progeny was affected by overexpression of ovine growth hormone in the transgenic parent. The oMt1a-oGH mouse is a useful model for assessing transgene transmission, as the construct is easily regulatable and transgene inactivation results in a return of plasma GH to basal levels. Male and female hemizygous oMt1a-oGH mice were assigned to 1 of 3 treatment groups: (1) mice never actively expressing the transgene, (2) mice actively expressing the transgene from 3 weeks of age, and (3) mice actively expressing the transgene from 3 to 11 (males) or 3 to 8 (females) weeks of age. Transgenic mice were mated to wild type animals and the resulting progeny were genotyped. Males never actively expressing the transgene passed on the transgene to progeny in a Mendelian fashion, while males actively expressing the transgene transmitted the transgene to a smaller than expected number of progeny. However, following inactivation of the oMt1a-oGH construct in transgenic males, subsequent offspring demonstrated Mendelian inheritance of the transgene. In contrast, females expressing the transgene from 3 to 8 weeks of age were able to pass on the oMt1a-oGH construct in a Mendelian fashion, but females from other treatment groups were not. In oMt1a-oGH males, reduced transgene transmission appears to be due to selection against transgenic gametes. In females, however, selection against the transgenic genotype likely occurs at the embryonic level.

Expression profiling of purified male germ cells: stage-specific expression patterns related to meiosis and postmeiotic development

Gene expression profiling was performed using the National Institute on Aging 15,000-cDNA microarray to reveal the differential expression pattern of 160 genes between meiotic pachytene spermatocytes and postmeiotic round spermatids of the mouse. Our results indicate that more genes are expressed in spermatids than in spermatocytes. Genes participating in cell cycle regulation and chromatin structure and dynamics are preferentially expressed in spermatocytes, while genes for protein turnover, signal transduction, energy metabolism, and intracellular transport are prevalent in spermatids. This suggests that a switch of functional requirement occurs when meiotic germ cells differentiate into haploid spermatids. Concordant expression patterns were obtained when quantitative real-time polymerase chain reaction was performed to verify the microarray data. Interestingly, the majority of the differentially expressed genes were underrepresented in mitotic type A spermatogonia, and they were preferentially expressed in the testis. Our results suggest that an even higher proportion of the mouse genome is devoted to male gamete development from meiosis than was previously estimated. We also provide evidence that underscores the advantage of using purified germ cells over whole testes in profiling spermatogenic gene expression to identify transcripts that demonstrate stage-specific expression patterns.

Emergence of young human genes after a burst of retroposition in primates

The origin of new genes through gene duplication is fundamental to the evolution of lineage- or species-specific phenotypic traits. In this report, we estimate the number of functional retrogenes on the lineage leading to humans generated by the high rate of retroposition (retroduplication) in primates. Extensive comparative sequencing and expression studies coupled with evolutionary analyses and simulations suggest that a significant proportion of recent retrocopies represent bona fide human genes. We estimate that at least one new retrogene per million years emerged on the human lineage during the past ∼63 million years of primate evolution. Detailed analysis of a subset of the data shows that the majority of retrogenes are specifically expressed in testis, whereas their parental genes show broad expression patterns. Consistently, most retrogenes evolved functional roles in spermatogenesis. Proteins encoded by X chromosome−derived retrogenes were strongly preserved by purifying selection following the duplication event, supporting the view that they may act as functional autosomal substitutes during X-inactivation of late spermatogenesis genes. Also, some retrogenes acquired a new or more adapted function driven by positive selection. We conclude that retroduplication significantly contributed to the formation of recent human genes and that most new retrogenes were progressively recruited during primate evolution by natural and/or sexual selection to enhance male germline function.

In humans, retroposition--integration into the genome of DNA reverse transcribed from mRNA--has contributed to the formation of recent functional genes selected to enhance male germline function.

Mechanisms and consequences of paternally-transmitted chromosomal abnormalities

Paternally-transmitted chromosomal damage has been associated with pregnancy loss, developmental and morphological defects, infant mortality, infertility, and genetic diseases in the offspring, including cancer. There is epidemiological evidence linking paternal exposure to occupational or environmental agents with an increased risk of abnormal reproductive outcomes. There is also a large body of literature on germ cell mutagenesis in rodents showing that treatment of male germ cells with mutagens has dramatic consequences on reproduction, producing effects such as those observed in human epidemiological studies. However, we know very little about the etiology, transmission, and early embryonic consequences of paternally-derived chromosomal abnormalities. The available evidence suggests that: 1) there are distinct patterns of germ cell-stage differences in the sensitivity of induction of transmissible genetic damage, with male postmeiotic cells being the most sensitive; 2) cytogenetic abnormalities at first metaphase after fertilization are critical intermediates between paternal exposure and abnormal reproductive outcomes; and 3) there are maternal susceptibility factors that may have profound effects on the amount of sperm DNA damage that is converted into chromosomal aberrations in the zygote and that directly affect the risk for abnormal reproductive outcomes.

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.

Evolutionary implications of pericentromeric gene expression in humans

Human pericentromeric sequences are enriched for recent sequence duplications. The continual creation and shuffling of these duplications can create novel intron-exon structures and it has been suggested that these regions have a function as gene nurseries. However, these sequences are also rich in satellite repeats which can repress transcription, and analyses of chromosomes 10 and 21 have suggested that they are transcript poor. Here, we investigate the relationship between pericentromeric duplication and transcription by analyzing the in silico transcriptional profiles within the proximal 1.5 Mb of genomic sequence on all human chromosome arms in relation to duplication status. We identify an approximately 5x excess of transcripts specific to cancer and/or testis in pericentromeric duplications compared to surrounding single copy sequence, with the expression of >50% of all transcripts in duplications being restricted to these tissues. We also identify an approximately 5x excess of transcripts in duplications which contain large quantities of interspersed repeats. These results indicate that the transcriptional profiles of duplicated and single copy sequences within pericentromeric DNA are distinct, suggesting that pericentromeric instability is unlikely to represent a common route for gene creation but may have a disproportionate effect upon genes whose function is restricted to the germ line.

Absolute mRNA levels and transcriptional regulation of the mouse testis-specific thioredoxins

Thioredoxins function as general protein disulphide reductases. Mammalian male germ cells are equipped with a set of three testis-specific thioredoxins (named Sptrx-1, -2, and -3, respectively) that are expressed either in different structures within the sperm cell or at different stages of sperm development. Previous studies based on qualitative northern-blot and in situ hybridization analyses restricted the presence of Sptrx mRNAs to adult testis, but nothing is known about their transcriptional regulation or relative expression levels in this tissue. In this report, we investigate the transcriptional profiles of the mouse Sptrx genes in terms of the germ cell-specific regulation by promoter analysis in GC-2spd(ts) cells. Besides, we perform a comprehensive quantification of the Sptrx mRNA molecules by real-time PCR in whole-animal experiments. By these means, we show that transcription is differentially regulated for each Sptrx gene and identify the 5'-flanking regions anticipated to contain the cis-regulatory elements responsible, at least in part, for the transcriptional silencing and/or activation of the Sptrx genes. In addition, we show remarkable age-associated variations between the Sptrx mRNA expression patterns.

Expression of 25 kDa thiamine triphosphatase in rodent tissues using quantitative PCR and characterization of its mRNA

Thiamine triphosphate (ThTP) is found in most organisms, but its biological role remains unclear. In mammalian tissues, cellular ThTP concentrations remain low, probably because of hydrolysis by a specific 25 kDa thiamine triphosphatase (ThTPase). The aim of the present study was to use quantitative PCR, for comparing the 25 kDa ThTPase mRNA expression in various mouse tissues with its enzyme activities. ThTPase mRNA was expressed at only a few copies per cell. The highest amount of mRNA was found in testis, followed by lung and muscle, while the highest enzyme activities were found in liver and kidney. The poor correlation between mRNA levels and enzyme activities might result either from tissue-specific post-transcriptional regulation of mRNA processing and/or translation or from the regulation of enzyme activities by post-translational mechanisms. Purified recombinant human ThTPase was phosphorylated by casein kinase II, but this phosphorylation did not modify the enzyme activity. However, the characterization of the 3'-untranslated mRNA region revealed a unique, highly conserved, 200-nucleotide sequence that might be involved in translational control. In situ hybridization studies in testis suggest a predominant localization of ThTPase mRNA in poorly differentiated spermatogenic cells. This is the first study demonstrating a cell-specific 25 kDa ThTPase mRNA expression, suggesting that this enzyme might be related to the degree of differentiation or the metabolic state of the cell.

Nucleotide excision repair activity varies among murine spermatogenic cell types

Germ cells perform a unique and critical biological function: they propagate the DNA that will be used to direct development of the next generation. Genetic integrity of germ cell DNA is essential for producing healthy and reproductively fit offspring, and yet germ cell DNA is damaged by endogenous and exogenous agents. Nucleotide excision repair (NER) is an important mechanism for coping with a variety of DNA lesions. Little is known about NER activity in spermatogenic cells. We expected that germ cells would be more efficient at DNA repair than somatic cells, and that this efficiency may be reduced with age when the prevalence of spontaneous mutations increases. In the present study, NER was measured in defined spermatogenic cell types, including premeiotic cells (A and B type spermatogonia), meiotic cells (pachytene spermatocytes), and postmeiotic haploid cells (round spermatids) and compared with NER in keratinocytes. Global genome repair and transcription-coupled repair subpathways of NER were examined. All spermatogenic cell types from young mice displayed good repair of (6-4) pyrimidone photoproducts, although the repair rate was slower than in primary keratinocytes. In aged mice, repair of 6-4 pyrimidone photoproducts was depressed in postmeiotic cells. While repair of cyclobutane pyrimidine dimers was not detected in spermatogenic cells or in keratinocytes, the transcribed strands of active genes were repaired with greater efficiency than nontranscribed strands or inactive genes in keratinocytes and in meiotic and postmeiotic cells; spermatogonia displayed low to moderate ability to repair cyclobutane pyrimidine dimers on both DNA strands regardless of transcriptional status. Overall, the data suggest cell type-specific NER activity during murine spermatogenesis, and our results have possible implications for germ cell aging.

Disruption of murine Tenr results in teratospermia and male infertility

Gametes rely heavily on posttranscriptional control for their differentiation. Translational control, alternative splicing, and alternative processing of the 3' end of mRNAs are all common during spermatogenesis. Tenr, which encodes a highly conserved 72-kDa protein, is expressed solely in germ cells of the testis from the mid-pachytene stage until the elongating spermatid stage. TENR contains a double-stranded RNA binding domain, is localized to the nucleus, and is phylogenetically related to a family of adenosine deaminases involved in RNA editing. We show here that targeted mutation of the Tenr gene causes male sterility. Tenr mutant males have a reduced sperm count, and Tenr-/- sperm show a decrease in motility and an increase in malformed heads. Despite their sterility, some epididymal sperm from Tenr mutants have normal morphology. The ability of Tenr mutant sperm to fertilize zona pellucida-free oocytes and to bind to, but not fertilize, zona pellucida-intact oocytes suggests that the normal-appearing sperm are not able to penetrate the zona pellucida. These data suggest that TENR plays an essential function in spermatid morphogenesis.

Establishment of male-specific epigenetic information

The setting of male-specific epigenetic information is a complex process, which involves a major global re-organisation, as well as localized changes of the nucleus structure during the pre-meiotic, meiotic and post-meiotic stages of the male germ cell differentiation. Although it has long been known that DNA methylation in targeted regions of the genome is associated with male-specific genomic imprinting, or that most core histones are hyperacetylated and then replaced by sperm-specific proteins during the post-meiotic condensation of the nucleus, many questions remain unanswered. How these changes interact, how they affect the epigenetic information and how the paternal epigenetic marks contribute to the future genome are indeed major issues remaining to be explored.

Genetic control of spermiogenesis: insights from the CREM gene and implications for human infertility

Male germ cell differentiation requires a highly cell-specific gene expression programme that is achieved by unique chromatin remodelling, transcriptional control, and the expression of testis-specific genes or isoforms. The regulatory processes governing gene expression in spermatogenesis have fundamentally unique requirements, including meiosis, ongoing cellular differentiation and a peculiar chromatin organization. The signalling cascades and the downstream effectors contributing to the programme of spermatogenesis are currently being unravelled, revealing the unique features of germ cell regulatory circuits. This paper reports on the unique role that CREM exerts as a master regulator. Targeted inactivation of the genes encoding CREM and ACT has been achieved. ACT selectively associates with KIF17b, a kinesin motor protein highly expressed in germ cells. It has been found that KIF17b directly determines the intracellular localization of ACT. Thus, the activity of a transcriptional co-activator is intimately coupled to the function of a kinesin via tight regulation of its intracellular localization. The conservation of these elements and of their regulatory functions in human spermatogenesis indicates that they are likely to provide important insights into understanding the molecular mechanisms of human infertility.

Sexual selection, genetic conflict, selfish genes, and the atypical patterns of gene expression in spermatogenic cells

This review proposes that the peculiar patterns of gene expression in spermatogenic cells are the consequence of powerful evolutionary forces known as sexual selection. Sexual selection is generally characterized by intense competition of males for females, an enormous variety of the strategies to maximize male reproductive success, exaggerated male traits at all levels of biological organization, co-evolution of sexual traits in males and females, and conflict between the sexual advantage of the male trait and the reproductive fitness of females and the individual fitness of both sexes. In addition, spermatogenesis is afflicted by selfish genes that promote their transmission to progeny while causing deleterious effects. Sexual selection, selfish genes, and genetic conflict provide compelling explanations for many atypical features of gene expression in spermatogenic cells including the gross overexpression of certain mRNAs, transcripts encoding truncated proteins that cannot carry out basic functions of the proteins encoded by the same genes in somatic cells, the large number of gene families containing paralogous genes encoding spermatogenic cell-specific isoforms, the large number of testis-cancer-associated genes that are expressed only in spermatogenic cells and malignant cells, and the overbearing role of Sertoli cells in regulating the number and quality of spermatozoa.

Effects of scrotal heating on sperm surface protein PH-20 expression in sheep

Sperm surface protein PH-20 expression was studied during spermatogenesis in pubertal and adult sheep, using molecular and histological methods. The effects of 24 hr of insulation raising scrotal temperatures to 39 degrees C on PH-20 expression in ejaculated sheep sperm were also determined. A 282 nt cDNA fragment of ovine PH-20 was identified in total RNA extracts of sheep testes, which exhibited 76% identity at the nucleotide level with the equivalent region of the human sequence. Ovine PH-20 mRNA and immunoreactivity were identified only in adult ram testis and not in peri-pubertal ram testis tubules lacking round spermatids, nor in adult sheep brain, pituitary, heart, spleen, lung, liver, kidney, epididymis, or ovary. Ovine PH-20 protein was distributed predominantly on the postacrosomal membrane and was also present on the anterior membrane of the sperm head in fresh, unheated sheep semen. Scrotal heating caused a significant, transient decrease in the percentage of PH-20 immunoreactive sperm, but did not change the pattern of PH-20 staining on the sperm head. The results strongly suggest that ovine PH-20 is postmeiotically expressed in haploid germ cells in sheep testis and is arrayed on the membrane of the mature ovine spermatozoon. Scrotal heating appears to have few effects on PH-20 expression and distribution on ejaculated sperm.

Differential distribution of splice variants of estrogen receptor beta in human testicular cells suggests specific functions in spermatogenesis

A growing number of estrogen receptor beta (ER beta) splice variants are reported. Several of these have been discovered in testis, but with few exceptions little is known about their cellular localization. The aim of this study was to identify and elucidate the mRNA expression pattern of the different ER beta splice variants in human testicular cells. Northern analysis was performed on whole testis and fractions enriched in germ cells from untreated men and from estrogen-treated men undergoing sex change surgery. Probes were constructed in order to systematically screen for and identify various ER beta splice variants. Several ER beta bands were observed in the human testis, in which splice variants constituted the major part of total ER beta transcripts. Interestingly, only two ER beta wild-type transcripts were detected. These seem to be virtually absent from the haploid germ cells and are probably mainly located in somatic cells and/or primary spermatocytes. Several novel ER beta deletion variants were found in high levels in the haploid germ cell fractions and were nearly absent in testicular cells from the estrogen-treated men. The cell-dependent distribution raises the question whether splice variants may have specific functions in spermatogenesis, and whether the differential splicing of ER beta is regulated in a cell-specific manner.

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.

Patterns, mechanisms, and functions of translation regulation in mammalian spermatogenic cells

Translational regulation is a fundamental aspect of the atypical patterns of gene expression in mammalian meiotic and haploid spermatogenic cells. Every mRNA is at least partially translationally repressed in meiotic and haploid spermatogenic cells, but the extent of repression of individual mRNA species is regulated individually and varies greatly. Many mRNA species, such as protamine mRNAs, are stored in translationally repressed free-mRNPs in early haploid cells and translated actively in late haploid cells. However, translation does not regulate developmental expression of all mRNAs. Some mRNAs appear to be partially repressed for the entire period that the mRNA is expressed in meiotic and haploid cells, while other mRNAs, some of which are expressed at high levels, are almost totally inactivated in free-mRNPs and/or generate little or no protein. This distinctive phenomenon can be explained by the hypothesis that translational repression is used to prevent the potentionally deleterious effects of overproduction of proteins encoded by overexpressed mRNAs. Translational regulation also appears to be frequently altered by the widespread usage of alternative transcription start sites in spermatogenic cells. Many ubiquitously expressed genes generate novel transcripts in somatic spermatogenic cells containing elements, uORFs and secondary structure that are inhibitory to mRNA translation, while the ribosomal proten L32 mRNA lacks a repressive element that is present in somatic cells. Very little is known about the mechanisms that regulate mRNA translation in spermatogenic cells, largely because few labs have utilized in vivo genetic approaches, although there have been important insights into the repression and activation of protamine 1 mRNA, and the role of Y-box proteins and poly(A) lengthening in mRNA-specific translational activation mediated by the cytoplasmic poly(A) element binding protein and a testis-specific isoform of poly(A) polymerase. A very large literature by evolutionary biologists suggests that the atypical patterns of gene expression in spermatogenic cells are the consequence of the powerful and unusual selective pressures on male reproductive success.

A Novel Human β1,3-N-Acetylgalactosaminyltransferase That Synthesizes a Unique Carbohydrate Structure, GalNAcβ1-3GlcNAc

We found, using a BLAST search, a novel human gene (GenBank trade mark accession number BC029564) that possesses beta3-glycosyltransferase motifs. The full-length open reading frame consists of 500 amino acids and encodes a typical type II membrane protein. This enzyme has a domain containing beta1,3-glycosyltransferase motifs, which are widely conserved in the beta1,3-galactosyltransferase and beta1,3-N-acetylglucosaminyltransferase families. The putative catalytic domain was expressed in human embryonic kidney 293T cells as a soluble protein. Its N-acetylgalactosaminyltransferase activity was observed when N-acetylglucosamine (GlcNAc) beta1-O-benzyl was used as an acceptor substrate. The enzyme product was determined to have a beta1,3-linkage by NMR spectroscopic analysis, and was therefore named beta1,3-N-acetylgalactosaminyltransferase-II (beta3GalNAc-T2). The acceptor substrate specificity of beta3GalNAc-T2 was examined using various oligosaccharide substrates. Galbeta1-3(GlcNAcbeta1-6)GalNAcalpha1-O-para-nitrophenyl (core 2-pNP) was the best acceptor substrate for beta3GalNAc-T2, followed by GlcNAcbeta1-4GlcNAcbeta1-O-benzyl, and GlcNAcbeta1-6GalNAcalpha1-O-para-nitrophenyl (core 6-pNP), among the tested oligosaccharide substrates. Quantitative real time PCR analysis revealed that the beta3Gal-NAc-T2 transcripts was restricted in its distribution mainly to the testis, adipose tissue, skeletal muscle, and ovary. Its putative orthologous gene, mbeta3GalNAc-T2, was also found in a data base of mouse expressed sequence tags. In situ hybridization analysis with mouse testis showed that the transcripts are expressed in germ line cells. beta3GalNAc-T2 efficiently transferred GalNAc to N-glycans of fetal calf fetuin, which was treated with neuraminidase and beta-galactosidase. However, it showed no activity toward any glycolipid examined. Although the GalNAcbeta1-3GlcNAcbeta1-R structure has not been reported in humans or other mammals, we have discovered a novel human glycosyltransferase producing this structure on N- and O-glycans.

Post-meiotic gene products as targets for male contraception

Post-meiotic stages of male germ cell maturation represent an interesting target system for the development of novel male contraceptive agents. In the human, these stages represent a period of only about 16 days differentiation, and thus targeting these cells would represent a contraceptive approach with a relatively rapid onset and equivalent recovery. Results from the Human Genome Project suggest that these cells also express a high number of very specific transcripts, though whether all of these are functional and/or essential for sperm differentiation and function requires more research. Until recently, however, these haploid stages were relatively inaccessible to molecular research because of the lack of appropriate model systems and methods. This situation has recently improved, with several new techniques involving manipulation of primary cells and seminiferous tubules, germ cell transplantation and the development of new immortalized cell-lines. Also, new biochemical approaches are yielding more information about haploid-specific transcription factors, such as GCNF. It is therefore to be expected that soon several new targets for a potential post-meiotic male contraceptive will become available for pharmaceutical development.

X chromosomes, retrogenes and their role in male reproduction

Retrogenes originate from their progenitor genes by retroposition. Several retrogenes reported in recent studies are autosomal, originating from X-linked progenitor genes, and have evolved a testis-specific expression pattern. During male meiosis, sex chromosomes are segregated into a so-called 'XY' body and are silenced transcriptionally. It has been widely hypothesized that the silencing of the X chromosome during male meiosis is the driving force behind the retroposition of X-linked genes to autosomes during evolution. With the advent of sequenced genomes of many species, many retrogenes can be identified and characterized. The testis-specific retrogenes might be associated with human male infertility. My goal here is to integrate recent findings, highlight controversies in the field and identify areas for further study.

Transient expression analysis of the mouse ornithine decarboxylase antizyme haploid-specific promoter using in vivo electroporation

The testicular isoform of the ornithine decarboxylase antizyme (OAZt) gene is expressed exclusively in the haploid spermatids of mice. The 357-bp region, which includes a TATA-less promoter and an untranslated region, is sufficient for OAZt gene expression in the spermatids of transgenic mice. In this study, in vivo transient transfection to living mouse testes was used to define the transcriptional regulatory elements of the OAZt gene promoter. We found that the 10-bp element that contains an initiator (Inr) plays a central role as the core promoter, in combination with a downstream element, while two cyclic adenosine monophosphate-responsive element (CRE)-like sites in the upstream region also contribute to promoter activity. The electrophoretic mobility shift assay showed binding of the testis-specific factors to these elements. Our results show that the in vivo DNA transfer technique enables detailed analysis of haploid germ cell-specific gene regulation in mice.

Tektin3 encodes an evolutionarily conserved putative testicular microtubules-related protein expressed preferentially in male germ cells

Tektins are microtubule-associated cytoskeletal proteins that are expressed primarily in the male germ cell-lineage in centrioles and basal bodies and within ciliary and flagellar doublet microtubules. They are proposed to be important for axonemal architecture and microtubule stability in the sperm tail and in other ciliated and flagellar structures. Using an in silico (electronic database) subtractive approach to identify germ cell-specific genes in vertebrates, we isolated a new member of the Tektin gene family from mice, Tektin3. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot analyses confirm that Tektin3 encodes a 1.7 kb transcript detectable preferentially in the testes of adult mice. In situ hybridization analysis in the testes revealed Tektin3 mRNA expression exclusively in late pachytene spermatocytes and early round spermatids. The putative TEKTIN3 protein shares 83.5% overall sequence identity with the human ortholog and includes a fully conserved carboxy terminal nonapeptide signature sequence present in all TEKTIN family members. In addition, using database mining, highly conserved TEKTIN3 orthologs were identified in puffer fish (Fugu rubripes) and rats (Rattus norvegicus) which shared 60.9 and 91.4% identity, respectively, with mouse TEKTIN3. The Tekt3 gene maps to murine chromosome 11 in a region that is syntenic to the human 17p12 chromosomal region containing the human TEKTIN3 gene. Our studies demonstrate that TEKTIN3 is a novel evolutionarily conserved male germ cell-enriched protein and suggest that it might perform important roles in male reproductive development and physiology.

TESSP-1: A novel serine protease gene expressed in the spermatogonia and spermatocytes of adult mouse testes

A cDNA encoding a novel type of serine protease, designated testis-specific serine protease 1 (TESSP-1), was cloned using mRNA isolated from the adult mouse testis. The open reading frame of this cDNA codes for a protein of 322 amino acids, which includes a hydrophobic signal peptide of 18 amino acids and an N-terminal activation peptide of 34 amino acids. The protein has an additional hydrophobic amino acid sequence at the C-terminus. Expression of the TESSP-1 gene was restricted to the testis. TESSP-1 mRNA expression initiated in the mouse testis at 2 weeks after birth, and its level increased steadily with sexual maturation of the animal. In situ hybridization analysis revealed that TESSP-1 mRNA was expressed in type B spermatogonia and spermatocytes at stages between preleptotene and pachytene. The testis contained at least five distinct forms of TESSP-1 transcript, which presumably resulted from alternative splicing of the mRNA, but only one of these transcripts encodes a complete, functional enzyme. Expression experiments using COS-7 cells showed that TESSP-1 was synthesized as a glycoprotein with N-glycosylated carbohydrates. Tests also showed that the C-terminal hydrophobic region of TESSP-1 was important upon its binding to the membrane by anchoring through glycosylphosphatidylinositol (GPI).

'Side Population' cells in adult mouse testis express Bcrp1 gene and are enriched in spermatogonia and germinal stem cells

Stem cells in various somatic tissues (bone marrow, skeletal muscle) can be identified by the 'Side Population' marker based on Hoechst 33342 efflux. We show that mouse testicular cells also display a 'Side Population' that express Bcrp1 mRNA, the ABC transporter responsible for Hoechst efflux in hematopoietic cells. Inhibition of Hoechst efflux by specific BCRP1 inhibitor Ko143 show that germinal 'Side Population' phenotype is dependent on BCRP1 activity. Analysis of two well-defined models of altered spermatogenesis (W/Wv mutants and cryptorchid male mice) and RNA expression studies of differentiation markers demonstrate that germinal 'Side Population' contains spermatogonial cells. In addition, alpha 6-integrin and Stra8 germinal stem cell markers, are expressed in the 'Side Population'. In vivo repopulation assay clearly establishes that testis 'Side Population' in adult mice is highly enriched in male germ stem cells.

Alternative patterns of transcription and translation of the ribosomal protein L32 mRNA in somatic and spermatogenic cells in mice

The patterns of transcription and translation of the ribosomal protein L32 (Rpl32) mRNA differ greatly in adult testis and somatic tissues. Northern blots reveal that the levels of Rpl32 mRNA are four- to five-fold higher in prepubertal and adult testes, and purified pachytene spermatocytes and round spermatids than in a variety of nongrowing adult somatic tissues. 5' RACE demonstrates that transcription in 8-day prepubertal testis, which lacks meiotic and haploid cells, strongly prefers the same start site in the 5' terminal oligopyrimidine tract (5' TOP) that is used is somatic cells. The 5' TOP is a cis element that inhibits translation of many mRNAs in nongrowing somatic cells. Although the sizes of deadenylated Rpl32 mRNAs are indistinguishable in somatic and spermatogenic cells, transcription initiates at 11 sites over a 31-nt segment in adult testis and approximately 62% of Rpl32 mRNAs lack a 5' TOP. In agreement with previous studies, low levels of cycloheximide increase the proportions and sizes of polysomes in absorbance profiles, and increase the proportions and sizes of polysomes translating four 5' TOP mRNA species including the Rpl32 mRNA in 8-day seminiferous tubules. In contrast, cycloheximide has little or no effect on the absorbance profiles and distribution of Rpl32 mRNA and 5' TOP mRNAs in adult seminiferous tubules. The failure of cycloheximide to increase the size of polysomes in adult seminiferous tubules implies a block in the pathway by which ribosomes are recruited onto translationally active mRNAs.

Alternative promoters and polyadenylation regulate tissue-specific expression ofHemogen isoforms during hematopoiesis and spermatogenesis

Hemogen is a nuclear protein encoded by HEMGN (also known as hemogen in mouse, EDAG in human and RP59 in rat). It is considered to be a hematopoiesis-specific gene that is expressed during the ontogeny of hematopoiesis. Herein, we characterize two distinct splicing variants of HEMGN mRNA with restricted expression to hematopoietic cells and to round spermatids in the testis, respectively. Expression of the testis-specific HEMGN mRNA (HEMGN-t) is developmentally regulated and is concurrent with the first wave of meiosis in prepuberal mice. Sequence analysis reveals that HEMGN-t and the hematopoietic HEMGN mRNA (HEMGN-h) share a common coding sequence with distinct 5' and 3' untranslated regions and that these two isoforms are transcribed from the same gene locus, HEMGN, through the use of alternative promoters and polyadenylation sites. Thus, HEMGN expression exemplifies a developmental regulatory mechanism by which the diversification of gene expression is achieved through using distinct regulatory sequences in different cell types. Moreover, the existence of a testis-specific isoform of HEMGN suggests a role in spermatogenesis. Finally, fluorescence in situ hybridization demonstrates that HEMGN is localized to chromosome 4 A5-B2 in mouse and to chromosome 9q22 in human, which is a region known to harbor a cluster of leukemia breakpoints.

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.

MZF6D, a novel KRAB zinc-finger gene expressed exclusively in meiotic male germ cells

Spermatogenesis takes place in the seminiferous tubule in the testes and culminates in the production of spermatozoa (male gametes). Here we report the identification of a novel mouse zinc-finger gene, MZF6D, which is selectively expressed in meiotic spermatocytes. The MZF6D protein contains an N-terminally located repressor domain, a KRAB domain, followed by at least seven successive Krüppel zinc-finger motifs. The KRAB domain of MZF6D, which consists of a KRAB A box and the newly identified KRAB C box, has previously been shown to interact with TIF1beta, which is the common corepressor of all KRAB zinc-finger proteins. Northern blot analysis shows that the expression of MZF6D is restricted to testes. This was confirmed by RT-PCR analysis of a panel of mouse tissues. In situ hybridization of sections from adult mouse testes localizes the expression to meiotic spermatocytes, suggesting a specific role for MZF6D in the regulation of spermatogenesis.

Gene expression and nuclear architecture during development and differentiation

Development requires a precise program of gene expression to be carried out. Much work has focussed on the regulatory networks that control gene expression, for example in response to external cues. However, it is important to recognize that these regulatory events take place within the physical context of the nucleus, and that the physical position of a gene within the nuclear volume can have strong influences on its regulation and interactions. The first part of this review will summarize what is currently known about nuclear architecture, that is, the large-scale three-dimensional arrangement of chromosome loci within the nucleus. The remainder of the review will examine developmental processes from the point of view of the nucleus.

The low gonadotropin-independent constitutive production of testicular testosterone is sufficient to maintain spermatogenesis

Spermatogenesis is thought to critically depend on the high intratesticular testosterone (T) levels induced by gonadotropic hormones. Strategies for hormonal male contraception are based on disruption of this regulatory mechanism through blockage of gonadotropin secretion. Although exogenous T or T plus progestin treatments efficiently block gonadotropin secretion and suppress testicular T production, only approximately 60% of treated Caucasian men reach contraceptive azoospermia. We now report that in luteinizing hormone receptor knockout mice, qualitatively full spermatogenesis, up to elongated spermatids of late stages 13-16, is achieved at the age of 12 months, despite absent luteinizing hormone action and very low intratesticular T (2% of control level). However, postmeiotic spermiogenesis was blocked by the antiandrogen flutamide, indicating a crucial role of the residual low testicular T level in this process. The persistent follicle-stimulating hormone action in luteinizing hormone receptor knockout mice apparently stimulates spermatogenesis up to postmeiotic round spermatids, as observed in gonadotropin-deficient rodent models on follicle-stimulating hormone supplementation. The finding that spermatogenesis is possible without a luteinizing hormone-stimulated high level of intratesticular T contradicts the current dogma. Extrapolated to humans, it may indicate that only total abolition of testicular androgen action will result in consistent azoospermia, which is necessary for effective male contraception.

Fine structural cytochemical analysis of homologous chromosome recognition, alignment, and pairing in Guinea pig spermatogonia and spermatocytes

The nuclei of guinea pig spermatogonia and spermatocytes were studied by means of quantitative autoradiography and electron microscopic methods such as high-resolution cytochemistry, immunocytochemistry, and in situ hybridization. Our observations reveal, in the nucleus of spermatogonia type B, small lampbrush structures of extended chromatin not found in nonmeiotic cells. During meiotic interphase, pairs of parallel lampbrush structures become associated by numerous filaments. The formation of the synaptonemal complex is simultaneous with the extension of chromosomal axes in a continuous leptotene-zygotene stage. Some chromosomes do not recognize their homologs before the onset of the leptotene-zygotene stage and undergo classical leptotene and zygotene stages. The immunocytochemical localization of Dmc1 and Rad51 supports the idea that these proteins are not involved in homology search and final pairing. Immunolocalization of DNA, RNA polymerase II, heterogeneous nuclear ribonucleoproteins, small nuclear ribonucleoproteins, and the trimethyl-guanosin cap of small nuclear RNAs suggests that the chromatin of lampbrush structures transcribe hnRNA and that splicing is scarce. The results of quantitative autoradiography after [3H]uridine labeling show an intense transcription accompanied by a very slow export of RNA. In situ hybridization demonstrates the presence of RNA in the regions of homology recognition and pairing. These results lead us to propose that the RNA synthesized in the lampbrush structures is involved in the process of homology searching and recognition.

De-orphanization of cytochrome P450 2R1: a microsomal vitamin D 25-hydroxilase

The conversion of vitamin D into an active ligand for the vitamin D receptor requires 25-hydroxylation in the liver and 1alpha-hydroxylation in the kidney. Mitochondrial and microsomal vitamin D 25-hydroxylase enzymes catalyze the first reaction. The mitochondrial activity is associated with sterol 27-hydroxylase, a cytochrome P450 (CYP27A1); however, the identity of the microsomal enzyme has remained elusive. A cDNA library prepared from hepatic mRNA of sterol 27-hydroxylase-deficient mice was screened with a ligand activation assay to identify an evolutionarily conserved microsomal cytochrome P450 (CYP2R1) with vitamin D 25-hydroxylase activity. Expression of CYP2R1 in cells led to the transcriptional activation of the vitamin D receptor when either vitamin D2 or D3 was added to the medium. Thin layer chromatography and radioimmunoassays indicated that the secosteroid product of CYP2R1 was 25-hydroxyvitamin D3. Co-expression of CYP2R1 with vitamin D 1alpha-hydroxylase (CYP27B1) elicited additive activation of vitamin D3, whereas co-expression with vitamin D 24-hydroxylase (CYP24A1) caused inactivation. CYP2R1 mRNA is abundant in the liver and testis, and present at lower levels in other tissues. The data suggest that CYP2R1 is a strong candidate for the microsomal vitamin D 25-hydroxylase.

Members of the CDY family have different expression patterns: CDY1 transcripts have the best correlation with complete spermatogenesis

The CDY family of genes is of special interest because some of them are included in chromosome-Y microdeletions detected among infertile men and are apparently involved in the spermiogenetic process. In this study, we employed the reverse transcriptase/polymerase chain reaction technique to test the RNA expression of the various transcripts of these genes in testicular biopsies of 84 azoospermic men who had been classified by comprehensive histology and cytology analyses. We also evaluated the feasibility of detecting CDY expression in biopsies taken by testicular sperm extraction versus acquisition by aspiration. There was a significant association between the type of testicular impairment and the expression of CDY1 and CDY2 transcripts. CDY2 was expressed whenever germ cells were present, but CDY1 major and especially CDY1 minor and short transcripts were identified almost exclusively when mature spermatids/spermatozoa were detected. The expression of CDY1 minor and short transcripts detected in aspirated specimens was less efficient than that in testicular tissue acquired by extraction. It is suggested that CDY2 is apparently required in the early stages of spermatogenesis, whereas CDY1 transcripts are required later on in the process. The findings of this study imply different functional roles for CDY isoforms during spermatogenesis. However, in consideration of the high levels of identity between CDY1 and CDY2 (98% at the protein level), the delayed up-regulation of CDY1 transcripts could be attributable to temporal changes in dosage requirements.