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

Translational regulation of a novel testis-specific RNF4 transcript

The RING-finger protein SNURF/RNF4, a modulator of both steroid receptor dependent and basal transcription, is expressed at very high levels in testis and at much lower levels in several other tissues. In somatic tissues, the RNF4 gene is expressed as a 3-kb transcript while an additional shorter sized transcript (1.6 kb) was found in mouse testis. In murine germ cells, RNF4 protein expression is strongly modulated during progression of spermatogonia to spermatids, with a peak in spermatocytes. The expression of 3-kb transcript correlated with protein levels in the different germ cell populations. Conversely, the 1.6-kb transcript was abundantly and specifically expressed in spermatids, in which RNF4 protein was detected at very low levels. We have then examined possible mechanisms underlying this discrepancy. Primer extension and RNase protection analyses demonstrated that the 1.6- and 3.0-kb transcripts originate from the same promoter, encode for the same protein and differ in the 3' UTR. In vitro assays showed that protein degradation is not involved in the regulation of RNF4 protein level. Finally, polysome analysis revealed that only a slight fraction of the testis-specific transcript is engaged in translation, thus providing a feasible mechanism for the quantitative differences of RNF4 mRNA and protein levels. Present results demonstrate that RNF4 short transcript is poorly translated suggesting that this mechanism could be essential for normal spermatogenesis.

Rapid evolution of male-biased gene expression in Drosophila

A number of genes associated with sexual traits and reproduction evolve at the sequence level faster than the majority of genes coding for non-sex-related traits. Whole genome analyses allow this observation to be extended beyond the limited set of genes that have been studied thus far. We use cDNA microarrays to demonstrate that this pattern holds in Drosophila for the phenotype of gene expression as well, but in one sex only. Genes that are male-biased in their expression show more variation in relative expression levels between conspecific populations and two closely related species than do female-biased genes or genes with sexually monomorphic expression patterns. Additionally, elevated ratios of interspecific expression divergence to intraspecific expression variation among male-biased genes suggest that differences in rates of evolution may be due in part to natural selection. This finding has implications for our understanding of the importance of sexual dimorphism for speciation and rates of phenotypic evolution.

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.

Identification and characterization of SRSP1, a rat gene differentially expressed during spermatogenesis and coding for a serine stretch-containing protein

By means of mRNA differential display and cDNA library screening we have characterized a novel gene of the rat that is differentially expressed during spermatogenesis. Northern blot analyses showed that its mRNA reaches the highest level during the first meiotic prophase. The transcript appears to be testis-specific as it was not detectable in any of the nine other tissues tested. The full length ORF encodes a putative phosphoprotein containing a serine stretch and a bipartite nuclear localization signal, which we call Srsp1 ("serin-rich spermatogenic protein 1"). Comparison of the cDNA from SRSP1 with genomic sequences in databases allowed us to determine the number of introns; some putative regulatory sequences are proposed. This is the first report of a gene differentially expressed during spermatogenesis that codes for a protein containing a serine stretch.

CPEB2, a novel putative translational regulator in mouse haploid germ cells

Translational control of specific mRNAs by cytoplasmic polyadenylation has fundamental roles in gametogenesis. The cytoplasmic polyadenylation element binding (CPEB) protein regulates cytoplasmic polyadenylation of mRNAs as a trans factor in oogenesis and spermatogenesis. The CPEB protein contains two RNA recognition motifs and a Zn-finger structure. Proteins (KIAA0940 and KIAA1673) with similar structures are predicted from the genome database, but nothing is known about their expression and function. Here, we report another novel member of the CPEB protein family, CPEB2. Comparison of the amino acid sequences of CPEB family members suggests that the family can be divided structurally and, perhaps, functionally into two groups: the CPEB group, and the CPEB2-KIAA0940-KIAA1673 group. The CPEB2 maps to mouse chromosome distal 5B and is abundantly expressed in testis. However, it was detected by reverse transcription-polymerase chain reaction in all tissues that we examined. It preferentially binds to poly(U) and localizes to the cytoplasm in transfected HeLa cells. The CPEB2 is expressed postmeiotically in mouse spermatogenesis, suggesting a possible role in translational regulation of stored mRNAs in transcriptionally inactive haploid spermatids.

SPEER--a new family of testis-specific genes from the mouse

Differential cloning revealed a partial mRNA sequence expressed in the mouse testis, which on further molecular characterization proved to be a member of a new family of 14 transcribed genes. Six of the genes appear to be expressed pseudogenes. The remainder indicate an open reading frame of approximately 200-220 amino acids encoding proteins with a very high proportion of alpha helical secondary structure, comprising approximately 15% glutamate residues. Because of this property, the family has been named SPErm-associated glutamate (E)-Rich protein (SPEER). Three members were chosen for more detailed characterization: SPEER-1 (pseudogene), SPEER-2, and SPEER-4D. All three are expressed tissue specifically in the testis of mice, with only very weak expression evident in the rat testis but in no other species tested. Using reverse transcription-polymerase chain reaction (RT-PCR), all three transcripts can be detected also in the epididymis, presumably due to the presence of spermatozoa. All three transcripts are expressed to high levels in haploid germ cells at the spermatocyte-spermatid transition. SPEER-1 mRNA is present in the cytoplasm as a sense transcript, SPEER-2 appears to be made mostly as an antisense transcript, whereas SPEER-4D appears to be localized within a subcellular compartment as a conventional sense transcript. Codon usage analysis suggests that all but the pseudogenes can be expressed as protein, confirmed for SPEER-2 and SPEER-4D by in vitro transcription/translation. An antibody raised against a peptide region of SPEER-4D, which probably cross-reacts with other SPEER members, immunohistochemically stains the nuclei of early round spermatids. While there are no true homologies to other proteins in the genome databases, some motifs are present that suggest a relationship to nuclear matrix proteins, implying that the SPEER family is a new group of haploid sperm-specific nuclear factors.

Analysis of germ cell nuclear factor transcripts and protein expression during spermatogenesis

Germ cell nuclear factor (GCNF), an orphan receptor in the nuclear receptor superfamily, is expressed predominantly in developing germ cells in the adult mouse. Two Gcnf transcripts (7.4 and 2.1 kilobase [kb]) encoded by a single copy gene are expressed in the testis of several mammalian species. To identify features that regulate Gcnf expression, we characterized the structure and sequence of the mouse gene and its two transcripts and determined the expression profile of the GCNF protein during spermatogenesis. Genomic fragments spanning part of the 5'-untranslated region (UTR), the coding sequence, and the complete 3'-UTR (approximately 80 kb) were isolated and sequenced. The 3'-UTRs of the two transcripts are quite distinct. The 7.4 kb transcript, which appears earlier in spermatogenesis, has a very long 3'-UTR of 4451 nucleotides. In contrast, the 2.1 kb transcript, which is expressed predominantly during the haploid phase of spermatogenesis, has a 3'-UTR that is only 202 nucleotides in length. Additional analyses indicate that both transcripts share the same coding region and are associated with polysomes. A single GCNF protein band was detected in testis extracts by Western blotting with a specific antiserum. Immunohistochemical analysis showed that GCNF is localized in the nuclei of pachytene spermatocytes and round spermatids. GCNF is first detectable in early pachytene spermatocytes (stage II) and is continuously expressed until spermatids begin to elongate in stage IX. Although GCNF is generally distributed throughout the nucleus, it is particularly prominent in heterochromatic regions at some stages and in condensed chromosomes undergoing the meiotic divisions. This expression profile suggests that GCNF plays a role in transcriptional regulation during meiosis and the early haploid phase of spermatogenesis.

Cancer/testis-associated genes: identification, expression profile, and putative function

Cancer/testis-associated genes (CTAs) are a subgroup of tumor antigens with a restricted expression in testis and malignancies. During the last decade, many of these immunotherapy candidate genes have been discovered using various approaches. Most of these genes are localized on the X-chromosome, often as multigene families. Methylation status seems to be the main, but not the only regulator of their specific expression pattern. In testis, CTAs are exclusively present in cells of the germ cell lineage, though there is a lot of variation in the moment of expression during different stages of sperm development. Likewise, there is also a lot of heterogeneity in the expression of CTAs in melanoma samples. Clues regarding functionality of CTAs for many of these proteins point to a role in cell cycle regulation or transcriptional control. Better insights in the function of these genes may shed light on the link between spermatogenesis and tumor growth and could be of use in anti-tumor therapies. This review outlines the CTA family and focuses on their expression and putative function during male germ cell development and melanocytic tumor progression.

Genomic sequence and transcriptional profile of the boundary between pericentromeric satellites and genes on human chromosome arm 10p

Contiguous finished sequence from highly duplicated pericentromeric regions of human chromosomes is needed if we are to understand the role of pericentromeric instability in disease, and in gene and karyotype evolution. Here, we have constructed a BAC contig spanning the transition from pericentromeric satellites to genes on the short arm of human chromosome 10, and used this to generate 1.4 Mb of finished genomic sequence. Combining RT-PCR, in silico gene prediction, and paralogy analysis, we can identify two domains within the sequence. The proximal 600 kb consists of satellite-rich pericentromerically duplicated DNA which is transcript poor, containing only three unspliced transcripts. In contrast, the distal 850 kb contains four known genes (ZNF248, ZNF25, ZNF33A, and ZNF37A) and up to 32 additional transcripts of unknown function. This distal region also contains seven out of the eight intrachromosomal duplications within the sequence, including the p arm copy of the approximately 250-kb duplication which gave rise to ZNF33A and ZNF33B. By sequencing orthologs of the duplicated ZNF33 genes we have established that ZNF33A has diverged significantly at residues critical for DNA binding but ZNF33B has not, indicating that ZNF33B has remained constrained by selection for ancestral gene function. These results provide further evidence of gene formation within intrachromosomal duplications, but indicate that recent interchromosomal duplications at this centromere have involved transcriptionally inert, satellite rich DNA, which is likely to be heterochromatic. This suggests that any novel gene structures formed by these interchromosomal events would require relocation to a more open chromatin environment to be expressed.

Genes associated with the development of the male germ line

The development of the mammalian germ line has been well studied, from the designation of primordial germ cells and their migration in the embryo to their progression through gametogenesis. The pattern of germ cell development, as established through classical studies, is now being overlaid with molecular, genetic and epigenetic data. Eventually, proteonomics will lead to a deeper understanding of the function of these genes. Through knowledge of germ cell gene expression patterns, it is now possible to develop transgenic molecular tools for the isolation of germ cells at different stages of development. By linking stage-specific germ cell promoter regions to the green fluorescent protein (GFP) reporter gene it is possible to tag these cells genetically for histological identification and cell sorting. Our long-term goal is to develop male germ cells as stem cells for therapeutic purposes. It is hoped that this goal will be achieved by purifying germ cells at different stages in development and gaining a deeper understanding of them by studying their gene expression patterns, potency and plasticity, both in vivo and in vitro.

Meiosis in the golden hamster: a confocal microscopy and flow cytometric analysis

In this study, confocal microscopy and flow-cytometry were utilized to follow meiosis in hamster spermatogenesis. Confocal microscopy was used as an analytical tool to observe spermatocytes inside the tubules following meiotic progression consecutively at defined spermatogenic stages. To study spermatocyte differentiation, the structure of the synaptonemal complex was studied in detail at various stages of hamster spermatogenesis using the antibody against SC3 (the protein of axial/lateral element). The synaptonemal complex was observed from the leptotene stage until the first meiotic division with maximal staining in mid-pachytene spermatocytes, suggesting a role for SC3 at this postrecombinational stage. In addition, 3-dimensional (3D) images of synaptonemal complex were observed, providing information about spatial distribution of the chromosomes within the nuclei of spermatocytes at different stages of meiosis. Changes in spermatocyte sizes and DNA condensation allowed assessment of meiosis by flow cytometry. Changes in chromatin condensation at different stages of hamster meiosis were followed, revealing decondensation from early to late pachytene stages. The analysis also allowed a comparing of chromatin status of mitotic and meiotic chromosomes, confirming the less compact structure of the latter, possibly connected to increased transcriptional activity during meiosis.

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.

Mouse models of male infertility

Spermatogenesis is a complex process that involves stem-cell renewal, genome reorganization and genome repackaging, and that culminates in the production of motile gametes. Problems at all stages of spermatogenesis contribute to human infertility, but few of them can be modelled in vitro or in cell culture. Targeted mutagenesis in the mouse provides a powerful method to analyse these steps and has provided new insights into the origins of male infertility.

A novel brain-expressed protein related to carnitine palmitoyltransferase I

Malonyl-CoenzymeA acts as a fuel sensor, being both an intermediate of fatty acid synthesis and an inhibitor of the two known isoforms of carnitine palmitoyltransferase I (CPT I), which control mitochondrial fatty acid oxidation. We describe here a novel CPT1 family member whose mRNA is present predominantly in brain and testis. Chromosomal locations and genome organization are reported for the mouse and human genes. The protein sequence contains all the residues known to be important for both carnitine acyltransferase activity and malonyl-CoA binding in other family members. Yeast expressed protein has no detectable catalytic activity with several different acyl-CoA esters that are good substrates for other carnitine acyltransferases, including the liver isoform of CPT I, which is also expressed in brain; however, it displays high-affinity malonyl-CoA binding. Thus this new CPT I related protein may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.

The germ cell-specific transcription factor ALF. Structural properties and stabilization of the TATA-binding protein (TBP)-DNA complex

The assembly and stability of the RNA polymerase II transcription preinitiation complex on a eukaryotic core promoter involves the effects of TFIIA on the interaction between TATA-binding protein (TBP) and DNA. To extend our understanding of these interactions, we characterized properties of ALF, a germ cell-specific TFIIA-like factor. ALF was able to stabilize the binding of TBP to DNA, but it could not stabilize TBP mutants A184E, N189E, E191R, and R205E nor could it facilitate binding of the TBP-like factor TRF2/TLF to a consensus TATA element. However, phosphorylation of ALF with casein kinase II resulted in the partial restoration of complex formation using mutant TBPs. Studies of ALF-TBP complexes formed on the Adenovirus Major Late (AdML) promoter revealed protection of the TATA box and upstream sequences from -38 to -20 (top strand) and -40 to -22 (bottom strand). The half-life and apparent K(D) of this complex was determined to be 650 min and 4.8 +/- 2.7 nm, respectively. The presence of ALF or TFIIA did not significantly alter the ability of TBP to bind TATA elements from several testis-specific genes. Finally, analysis of the distinct, nonhomologous internal regions of ALF and TFIIAalpha/beta using circular dichroism spectroscopy provided the first evidence to suggest that these domains are unordered, a result consistent with other genetic and biochemical properties. Overall, the results show that while the sequence and regulation of the ALF gene are distinct from its somatic cell counterpart TFIIAalpha/beta, the TFIIAgamma-dependent interactions of these factors with TBP are nearly indistinguishable in vitro. Thus, a role for ALF in the assembly and stabilization of initiation complexes in germ cells is likely to be similar or identical to the role of TFIIA in somatic cells.

Exceptional sperm cooperation in the wood mouse

Spermatozoa from a single male will compete for fertilization of ova with spermatozoa from another male when present in the female reproductive tract at the same time. Close genetic relatedness predisposes individuals towards altruism, and as haploid germ cells of an ejaculate will have genotypic similarity of 50%, it is predicted that spermatozoa may display cooperation and altruism to gain an advantage when inter-male sperm competition is intense. We report here the probable altruistic behaviour of spermatozoa in an eutherian mammal. Spermatozoa of the common wood mouse, Apodemus sylvaticus, displayed a unique morphological transformation resulting in cooperation in distinctive aggregations or 'trains' of hundreds or thousands of cells, which significantly increased sperm progressive motility. Eventual dispersal of sperm trains was associated with most of the spermatozoa undergoing a premature acrosome reaction. Cells undergoing an acrosome reaction in aggregations remote from the egg are altruistic in that they help sperm transport to the egg but compromise their own fertilizing ability.

Unique chromatin remodeling and transcriptional regulation in spermatogenesis

Most of our knowledge of transcriptional regulation comes from studies in somatic cells. However, increasing evidence reveals that gene regulation mechanisms are different in haploid germ cells. A number of highly specialized strategies operate during spermatogenesis. These include a unique chromatin reorganization program and the use of distinct promoter elements and specific transcription factors. Deciphering the rules governing transcriptional control during spermatogenesis will provide valuable insights of biomedical importance.

Regulation of ALF gene expression in somatic and male germ line tissues involves partial and site-specific patterns of methylation

ALF (TFIIAalpha/beta-like factor) is a germ cell-specific counterpart of the large (alpha/beta) subunit of general transcription factor TFIIA. Here we isolated homologous GC-rich promoters from the mouse and human ALF genes and used promoter deletion analysis to identify sequences active in COS-7 and 293 cells. Further, bisulfite sequence analysis of the mouse ALF promoter showed that all 21 CpG dinucleotides between -179 and +207 were partially methylated in five somatic tissues, brain, heart, liver, lung, and muscle, and in epididymal spermatozoa from adult mice. In contrast, DNA from prepubertal mouse testis and from purified spermatocytes were unmethylated except at C(+19)G and C(+170)G. We also found that ALF expression correlates with a strong promoter-proximal DNase I-hypersensitive site present in nuclei from testis but not from liver. Finally we show that in vitro methylation of the ALF promoter inhibits activity and that 5-aza-2'-deoxycytidine treatment reactivates the endogenous ALF gene in a panel of seven different mouse and human somatic cell lines. Overall the results show that silencing in somatic cells is methylation-dependent and reversible and that a unique CpG-specific methylation pattern at the ALF promoter precedes expression in pachytene spermatocytes. This pattern is transient as remethylation of the ALF promoter in haploid germ cell DNA has occurred by the time spermatozoa are present in the epididymis.

Reexamining the polyadenylation signal: were we wrong about AAUAAA?

Polyadenylation is the process by which most eukaryotic mRNAs form their 3' ends. It was long held that polyadenylation required the sequence AAUAAA and that 90% of mRNAs had AAUAAA within 30 nucleotides of the site of poly(A) addition. More recent studies, aided by computer analysis of sequences made available in GenBank and expressed sequence tag (EST) databases, have suggested that the actual incidence of AAUAAA is much lower, perhaps as low as 50-60%. Reproductive biologists have long recognized that a large number of mRNAs in male germ cells of mammals lack AAUAAA but are otherwise normally polyadenylated. Recent research in our laboratory has uncovered a new form of an essential polyadenylation protein, tauCstF-64, that is most highly expressed in male germ cells, and to a smaller extent in the brain, and which we propose plays a significant role in AAUAAA-independent mRNA polyadenylation in germ cells.