Microarray-based analysis of cell-cycle gene expression during spermatogenesis in the mouse

Mammalian spermatogenesis is a continuum of cellular differentiation in a lineage that features three principal stages: 1) a mitotically active stage in spermatogonia, 2) a meiotic stage in spermatocytes, and 3) a postreplicative stage in spermatids. We used a microarray-based approach to identify changes in expression of cell-cycle genes that distinguish 1) mitotic type A spermatogonia from meiotic pachytene spermatocytes and 2) pachytene spermatocytes from postreplicative round spermatids. We detected expression of 550 genes related to cell-cycle function in one or more of these cell types. Although a majority of these genes were expressed during all three stages of spermatogenesis, we observed dramatic changes in levels of individual transcripts between mitotic spermatogonia and meiotic spermatocytes and between meiotic spermatocytes and postreplicative spermatids. Our results suggest that distinct cell-cycle gene regulatory networks or subnetworks are associated with each phase of the cell cycle in each spermatogenic cell type. In addition, we observed expression of different members of certain cell-cycle gene families in each of the three spermatogenic cell types investigated. Finally, we report expression of 221 cell-cycle genes that have not previously been annotated as part of the cell cycle network expressed during spermatogenesis, including eight novel genes that appear to be testis-specific.

Phosphoglycerate kinase 2 (PGK2) is essential for sperm function and male fertility in mice

Phosphoglycerate kinase 2 (PGK2), an isozyme that catalyzes the first ATP-generating step in the glycolytic pathway, is encoded by an autosomal retrogene that is expressed only during spermatogenesis. It replaces the ubiquitously expressed phosphoglycerate kinase 1 (PGK1) isozyme following repression of Pgk1 transcription by meiotic sex chromosome inactivation during meiotic prophase and by postmeiotic sex chromatin during spermiogenesis. The targeted disruption of Pgk2 by homologous recombination eliminates PGK activity in sperm and severely impairs male fertility, but does not block spermatogenesis. Mating behavior, reproductive organ weights (testis, excurrent ducts, and seminal vesicles), testis histology, sperm counts, and sperm ultrastructure were indistinguishable between Pgk2(-/-) and wild-type mice. However, sperm motility and ATP levels were markedly reduced in males lacking PGK2. These defects in sperm function were slightly less severe than observed in males lacking glyceraldehyde-3-phosphate dehydrogenase, spermatogenic (GAPDHS), the isozyme that catalyzes the step preceding PGK2 in the sperm glycolytic pathway. Unlike Gapdhs(-/-) males, the Pgk2(-/-) males also sired occasional pups. Alternative pathways that bypass the PGK step of glycolysis exist. We determined that one of these bypass enzymes, acylphosphatase, is active in mouse sperm, perhaps contributing to phenotypic differences between mice lacking GAPDHS or PGK2. This study determined that PGK2 is not required for the completion of spermatogenesis, but is essential for sperm motility and male fertility. In addition to confirming the importance of the glycolytic pathway for sperm function, distinctive phenotypic characteristics of Pgk2(-/-) mice may provide further insights into the regulation of sperm metabolism.

Sex chromosome inactivation in the male

Mammalian females have two X chromosomes, while males have only one X plus a Y chromosome. In order to balance X-linked gene dosage between the sexes, one X chromosome undergoes inactivation during development of female embryos. This process has been termed X-chromosome inactivation (XCI). Inactivation of the single X chromosome also occurs in the male, but is transient and is confined to the late stages of first meiotic prophase during spermatogenesis. This phenomenon has been termed meiotic sex chromosome inactivation (MSCI). A substantial portion ( approximately 15-25%) of X-linked mRNA-encoding genes escapes XCI in female somatic cells. While no mRNA genes are known to escape MSCI in males, approximately 80% of X-linked miRNA genes have been shown to escape this process. Recent results have led to the proposal that the RNA interference mechanism may be involved in regulating XCI in female cells. We suggest that some MSCI-escaping miRNAs may play a similar role in regulating MSCI in male germ cells.

Key features of the X inactivation process are conserved between marsupials and eutherians

In female marsupials, X chromosome inactivation (XCI) is imprinted, affecting the paternal X chromosome. One model, supported by recent studies, proposes that XCI in marsupials is achieved through inheritance of an already silent X chromosome from the father, with XCI initiated by meiotic sex chromosome inactivation (MSCI). This model is appealing because marsupials have no Xist gene and the marsupial inactive X chromosome is epigenetically dissimilar to that of mice, apparently lacking repressive histone marks such as H3K27 trimethylation. A central prediction of the meiotic inactivation model of XCI is that silencing of genes on the X chromosome, initiated during male meiosis, is stably maintained during subsequent spermiogenesis. Here we characterize XCI in the male germline and female soma of the marsupial Monodelphis domestica. Contrary to the meiotic inactivation model, we find that X genes silenced by MSCI are reactivated after meiosis and are subsequently inactivated in the female. A reexamination of the female somatic inactive marsupial X chromosome reveals that it does share common properties with that of eutherians, including H3K27 trimethylation and targeting to the perinucleolar compartment. We conclude that aspects of the XCI process are more highly conserved in therian mammals than previously thought.

Maintenance of genetic integrity during natural and assisted reproduction

The essence of reproduction involves propagation of genetic information from parents to offspring. In mammals, the frequency of spontaneously acquired mutations is lower in germ-line cells than in somatic cells, reflecting the role played by germline cells in the propagation of genetic information and the importance of maintaining genetic integrity in these cells. The Big Blue((R)) transgenic mouse model was used to investigate the frequency and spectrum of: (i) spontaneous point mutations in germ cells as they develop naturally during the life cycle of the mouse; and (ii) acquired mutations that are normally transmitted from parents to offspring during natural and assisted reproduction. The study found that germ cells normally maintain a frequency of spontaneous point mutations that is 5-10-fold lower than that observed in somatic cells from the same individual, leading to embryos with very low frequencies of point mutations in the next generation. No significant differences in the frequency or spectrum of mutations between naturally conceived fetuses and assisted-conception fetuses were observed, indicating that, with respect to maintenance of genetic integrity, these methods are safe. Preliminary analysis of fetuses produced by somatic cell nuclear transfer indicates that maintenance of genetic integrity is regulated in a tissue-specific manner by epigenetic mechanisms that are subject to reprogramming during cloning.

Many X-linked microRNAs escape meiotic sex chromosome inactivation

Meiotic sex chromosome inactivation (MSCI) during spermatogenesis is characterized by transcriptional silencing of genes on both the X and Y chromosomes in mid to late pachytene spermatocytes1. MSCI is believed to result from meiotic silencing of unpaired DNA because the X and Y chromosomes remain largely unpaired throughout first meiotic prophase2. However, unlike X-chromosome inactivation in female embryonic cells, where 25–30% of X-linked structural genes have been reported to escape inactivation3, previous microarray4- and RT-PCR5-based studies of expression of >364 X-linked mRNA-encoding genes during spermatogenesis have failed to reveal any X-linked gene that escapes the silencing effects of MSCI in primary spermatocytes. Here we show that many X-linked miRNAs are transcribed and processed in pachytene spermatocytes. This unprecedented escape from MSCI by these X-linked miRNAs suggests that they may participate in a critical function at this stage of spermatogenesis, including the possibility that they contribute to the process of MSCI itself, and/or that they may be essential for post-transcriptional regulation of autosomal mRNAs during the late meiotic and early postmeiotic stages of spermatogenesis.

Dynamic nuclear organization of constitutive heterochromatin during fetal male germ cell development in mice

In mice, male germ cells enter mitotic arrest beginning at 13.5 days postcoitum (dpc), and remain suspended in the G(0)/G(1) cell cycle stage until after birth. During this period, male germ cells undergo extensive epigenetic reprogramming, which is essential for their subsequent function as male gametes. A global reorganization and spatial clustering of constitutive heterochromatin has been implicated in epigenetic plasticity during cellular differentiation. Here, we have studied the dynamics of heterochromatin in fetal (12.5-19.5 dpc) and neonatal (4 days postpartum) male germ cells. We monitored constitutive heterochromatin-specific markers, and observed changes in the association of histone H3 trimethylation of lysine 9 (H3K9me3), binding of heterochromatin protein 1, and patterns of 4',6-diamino-2-phenylindole staining in pericentric regions of chromosomes, along with a coincident loss of chromocenters in fetal prospermatogonia during mitotic arrest. We also observed a transient loss of H3K9me3 associated with major and minor satellite repeat sequences, plus inactivation of histone methyltransferases (Suv39h1 and Suv39h2), and transient activation of histone demethylase (Jmjd2b) in these same cells. These epigenetic changes were correlated with relocation of centromeric regions toward the nuclear periphery in prospermatogonia during mitotic arrest. Taken together, these results show that constitutive heterochromatin undergoes dramatic reorganization during prespermatogenesis. We suggest that these dynamic changes in heterochromatin contribute to normal epigenetic reprogramming of the paternal genome in fetal prospermatogonia suspended in the G(0)/G(1) stage, and that this also represents an epigenomic state that is particularly amenable to reprogramming.

A cytoplasmic variant of the KH-type splicing regulatory protein serves as a decay-promoting factor for phosphoglycerate kinase 2 mRNA in murine male germ cells

Phosphoglycerate kinase 2 (PGK2) is a germ cell-specific protein whose mRNA is translationally regulated in the mammalian testis. Using RNA affinity chromatography with the 3'-untranslated region (UTR) of Pgk2 mRNA and adult testis extracts, several associated proteins including a novel isoform of the AU-rich element RNA-binding protein and KH-type splicing regulatory protein (KSRP) were identified. KSRP, a protein of approximately 75 kDa, is widely expressed in somatic and germ cells where it is primarily nuclear. In addition to the approximately 75-kDa KSRP, a approximately 52-kD KSRP, t-KSRP, is present in the cytoplasm of a subpopulation of germ cells. t-KSRP binds directly to a 93-nt sequence (designated the F1 region) of the 3'-UTR of the Pgk2 mRNA and destabilizes Pgk2 mRNA constructs in testis extracts and in transfected cells. We conclude that this testicular variant of the multifunctional nucleic acid-binding protein, KSRP, serves as a decay-promoting factor for Pgk2 mRNA in male germ cells.

A conserved E2F6-binding element in murine meiosis-specific gene promoters

During gametogenesis, germ cells must undergo meiosis in order to become viable haploid gametes. Successful completion of this process is dependent upon the expression of genes whose protein products function specifically in meiosis. Failure to express these genes in meiotic cells often results in infertility, whereas aberrant expression in somatic cells may lead to mitotic catastrophe. The mechanisms responsible for regulating the timely expression of meiosis-specific genes have not been fully elucidated. Here we demonstrate that E2F6, a member of the E2F family of transcription factors, is essential for the repression of the newly identified meiosis-specific gene, Slc25a31 (also known as Ant4, Aac4), in somatic cells. This discovery, along with previous studies, prompted us to investigate the role of E2F6 in the regulation of meiosis-specific genes in general. Interestingly, the core E2F6-binding element (TCCCGC) was highly conserved in the proximal promoter regions of 19 out of 24 (79.2%) meiosis-specific genes. This was significantly higher than the frequency found in the promoters of all mouse genes (15.4%). In the absence of E2F6, only a portion of these meiosis-specific genes was derepressed in somatic cells. However, endogenous E2F6 bound to the promoters of these meiosis-specific genes regardless of whether they required E2F6 for their repression in somatic cells. Further, E2F6 overexpression was capable of reducing their transcription. These findings indicate that E2F6 possesses a broad ability to bind to and regulate the meiosis-specific gene population.

Conservation of the H19 noncoding RNA and H19-IGF2 imprinting mechanism in therians

Comparisons between eutherians and marsupials suggest limited conservation of the molecular mechanisms that control genomic imprinting in mammals. We have studied the evolution of the imprinted IGF2-H19 locus in therians. Although marsupial orthologs of protein-coding exons were easily identified, the use of evolutionarily conserved regions and low-stringency Bl2seq comparisons was required to delineate a candidate H19 noncoding RNA sequence. The therian H19 orthologs show miR-675 and exon structure conservation, suggesting functional selection on both features. Transcription start site sequences and poly(A) signals are also conserved. As in eutherians, marsupial H19 is maternally expressed and paternal methylation upstream of the gene originates in the male germline, encompasses a CTCF insulator, and spreads somatically into the H19 gene. The conservation in all therians of the mechanism controlling imprinting of the IGF2-H19 locus suggests a sequential model of imprinting evolution.

Recovery of a low mutant frequency after ionizing radiation-induced mutagenesis during spermatogenesis

Humans are exposed to ionizing radiation (IR) under various circumstances, e.g. cosmic radiation, diagnostic X-rays and radiotherapy for cancer. It has been shown that IR can impair spermatogenesis and can cause mutations in germ cells. However, the mutagenic responses of germ cells exposed to IR at different stages of testicular maturation have not been examined by directly assessing the mutant frequency in defined spermatogenic cell types. This study was performed to address whether preadult exposure to IR can increase mutations in adult germ cells that could in turn have a major impact on adult reproductive function and the health of ensuing offspring. Male Lac I transgenic mice were irradiated with a single dose of 2.5 Gy of gamma-ray at different ages before adulthood, reflecting different stages of testicular maturation, and then mutant frequency (MF) was determined directly in spermatogenic cell types emanating from the irradiated precursor cells. The results showed that (1) preadult exposure to IR did not significantly increase MF in adult epididymal spermatozoa; (2) spermatogenic stages immediately following the irradiated stage(s) displayed an elevated mutant frequency; but (3) the mutant frequency was restored to unirradiated levels in later stages of spermatogenesis. These findings provide evidence that there is a mechanism(s) to prevent spermatogenic cells with elevated mutant frequencies from progressing through spermatogenesis.

Expression of stimulated by retinoic acid gene 8 (Stra8) and maturation of murine gonocytes and spermatogonia induced by retinoic acid in vitro

Vitamin A deficiency in the mouse results in an arrest in the progression of undifferentiated spermatogonia to differentiating spermatogonia. The supplement of retinol to vitamin-A-deficient mice reinitiates spermatogenesis in a synchronous manner throughout the testes. It is unclear whether the effects of retinoids are the result of a direct action on germ cells or are indirectly mediated through Sertoli cells. The expression of Stimulated by retinoic acid gene 8 (Stra8), which is required for spermatogenesis, is directly related to the availability of retinoic acid (RA). Analysis of gene expression by microarrays revealed moderate levels of Stra8 transcript in gonocytes and high levels in A and B spermatogonia. Stra8 mRNA levels were greatly reduced or absent in germ cells once they entered meiosis. This study examined the effect of retinoic acid on cultured neonatal testes and isolated gonocytes/spermatogonia in vitro. THY1(+) and KIT(+) germ cells were isolated by magnetic-activated cell sorting from the testes of mice of different ages. Isolated germ cells were cultured and treated with either vehicle (ethanol) or RA without feeder cells. We found that 1) Stra8 is predominantly expressed in premeiotic germ cells, 2) RA stimulates gonocyte DNA replication and differentiation in cultured neonatal testes, 3) in the absence of feeder cells, RA directly induces the transition of undifferentiated spermatogonia to differentiating spermatogonia by stimulating Stra8 and Kit gene expression, 4) RA dramatically stimulates Stra8 expression in undifferentiated spermatogonia but has a lesser impact in differentiating spermatogonia, 5) endogenous Stra8 gene expression is higher in differentiating spermatogonia than in undifferentiated spermatogonia and could mediate the RA effects on spermatogonial maturation, and 6) RA stimulates a group of genes involved in the metabolism, storage, transport, and signaling of retinoids.

Cloning and expression profiling of testis-expressed piRNA-like RNAs

Using a novel small RNA cloning method, we identified 630 piRNA-like RNAs (pilRNAs) from the mouse testis, and 498 of them are novel. These pilRNA genes were mapped to all chromosomes as 71 clusters, and the majority of them ( approximately 84%) are derived from intergenic, intronic, and exonic sequences. One of the structural characteristics for pilRNAs is that a single locus can encode numerous homologous pilRNAs with overlapping sequences. Hundreds or even thousands of pilRNAs from a single pilRNA gene cluster are all produced from a single long transcript. Expression profiling for 64 pilRNAs revealed that approximately 14% of all the pilRNAs analyzed displayed a ubiquitous expression pattern, although the majority of ( approximately 86%) pilRNAs were preferentially or exclusively expressed in meiotic and haploid male germ cells of the testis. Our semiquantitative analyses also suggest that the testis is the organ with the highest expression of pilRNAs both in number and in abundance. The large number, high abundance, unique genomic locations, and biogenesis all suggest that pilRNAs have important regulatory roles not only in spermatogenesis but also in other biological processes.

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.

In vivo analysis of developmentally and evolutionarily dynamic protein-DNA interactions regulating transcription of the Pgk2 gene during mammalian spermatogenesis

Transcription of the testis-specific Pgk2 gene is selectively activated in primary spermatocytes to provide a source of phosphoglycerate kinase that is critical to normal motility and fertility of mammalian spermatozoa. We examined dynamic changes in protein-DNA interactions at the Pgk2 gene promoter during murine spermatogenesis in vivo by performing genomic footprinting and chromatin immunoprecipitation assays with enriched populations of murine spermatogenic cells at stages prior to, during, and following transcription of this gene. We found that genes encoding the testis-specific homeodomain factor PBX4 and its coactivator, PREP1, are expressed in patterns that mirror expression of the Pgk2 gene and that these factors become bound to the Pgk2 enhancer in cells in which this gene is actively expressed. We therefore suggest that these factors, along with CREM and SP3, direct stage- and cell type-specific transcription of the Pgk2 gene during spermatogenesis. We propose that binding of PBX4, plus its coactivator PREP1, is a rate-limiting step leading to the initiation of tissue-specific transcription of the Pgk2 gene. This study provides insight into the developmentally dynamic establishment of tissue-specific protein-DNA interactions in vivo. It also allows us to speculate about the events that led to tissue-specific regulation of the Pgk2 gene during mammalian evolution.

Evolutionarily conserved mammalian adenine nucleotide translocase 4 is essential for spermatogenesis

The adenine nucleotide translocases (Ant) facilitate the transport of ADP and ATP by an antiport mechanism across the inner mitochondrial membrane, thus playing an essential role in cellular energy metabolism. We recently identified a novel member of the Ant family in mouse, Ant4, of which gene configuration as well as amino acid homology is well conserved among mammals. The conservation of Ant4 in mammals, along with the absence of Ant4 in nonmammalian species, suggests a unique and indispensable role for this ADP/ATP carrier in mammalian development. Of interest, in contrast to its paralog Ant2, which is encoded by the X chromosome and ubiquitously expressed in somatic cells, Ant4 is encoded by an autosome and selectively expressed in testicular germ cells. Immunohistochemical examination as well as RNA expression analysis using separated spermatogenic cell types revealed that Ant4 expression was particularly high in spermatocytes. When we generated Ant4-deficient mice by targeted disruption, a significant reduction in testicular size was observed without any other distinguishable abnormalities in the mice. Histological examination as well as stage-specific gene expression analysis in adult and neonatal testes revealed a severe reduction of spermatocytes accompanied by increased apoptosis. Subsequently, the Ant4-deficient male mice were infertile. Taken together, these data elucidated the indispensable role of Ant4 in murine spermatogenesis. Considering the unique conservation and chromosomal location of the Ant family genes in mammals, the Ant4 gene may have arisen in mammalian ancestors and been conserved in mammals to serve as the sole and essential mitochondrial ADP/ATP carrier during spermatogenesis where the sex chromosome-linked Ant2 gene is inactivated.

Autonomous regulation of sex-specific developmental programming in mouse fetal germ cells

In mice, unique events regulating epigenetic programming (e.g., genomic imprinting) and replication state (mitosis versus meiosis) occur during fetal germ cell development. To determine whether these processes are autonomously programmed in fetal germ cells or are dependent upon ongoing instructive interactions with surrounding gonadal somatic cells, we isolated male and female germ cells at 13.5 days postcoitum (dpc) and maintained them in culture for 6 days, either alone or in the presence of feeder cells or gonadal somatic cells. We examined allele-specific DNA methylation in the imprinted H19 and Snrpn genes, and we also determined whether these cells remained mitotic or entered meiosis. Our results show that isolated male germ cells are able to establish a characteristic "paternal" methylation pattern at imprinted genes in the absence of any support from somatic cells. On the other hand, cultured female germ cells maintain a hypomethylated status at these loci, characteristic of the normal "maternal" methylation pattern in endogenous female germ cells before birth. Further, the surviving female germ cells entered first meiotic prophase and reached the pachytene stage, whereas male germ cells entered mitotic arrest. These results indicate that mechanisms controlling both epigenetic programming and replication state are autonomously regulated in fetal germ cells that have been exposed to the genital ridge prior to 13.5 dpc.

Sex chromosome silencing in the marsupial male germ line

In marsupials, dosage compensation involves silencing of the father's X-chromosome. Because no XIST orthologue has been found, how imprinted X-inactivation occurs is unknown. In eutherians, the X is subject to meiotic sex chromosome inactivation (MSCI) in the paternal germ line and persists thereafter as postmeiotic sex chromatin (PMSC). One hypothesis proposes that the paternal X is inherited by the eutherian zygote as a preinactive X and raises the possibility of a similar process in the marsupial germ line. Here we demonstrate that MSCI and PMSC occur in the opossum. Surprisingly, silencing occurs before X-Y association. After MSCI, the X and Y fuse through a dense plate without obvious synapsis. Significantly, sex chromosome silencing continues after meiosis, with the opossum PMSC sharing features of eutherian PMSC. These results reveal a common gametogenic program in two diverse clades of mammals and support the idea that male germ-line silencing may have provided an ancestral form of mammalian dosage compensation.

The search for a marsupial XIC reveals a break with vertebrate synteny

X-chromosome inactivation (XCI) evolved in mammals to deal with X-chromosome dosage imbalance between the XX female and the XY male. In eutherian mammals, random XCI of the soma requires a master regulatory locus known as the 'X-inactivation center' (XIC/Xic), wherein lies the noncoding XIST/Xist silencer RNA and its regulatory antisense Tsix gene. By contrast, marsupial XCI is imprinted to occur on the paternal X chromosome. To determine whether marsupials and eutherians share the XIC-driven mechanism, we search for the sequence equivalents in the genome of the South American opossum, Monodelphis domestica. Positional cloning and bioinformatic analysis reveal several interesting findings. First, protein-coding genes that flank the eutherian XIC are well-conserved in M. domestica, as well as in chicken, frog, and pufferfish. However, in M. domestica we fail to identify any recognizable XIST or TSIX equivalents. Moreover, cytogenetic mapping shows a surprising break in synteny with eutherian mammals and other vertebrates. Therefore, during the evolution of the marsupial X chromosome, one or more rearrangements broke up an otherwise evolutionarily conserved block of vertebrate genes. The failure to find XIST/TSIX in M. domestica may suggest that the ancestral XIC is too divergent to allow for detection by current methods. Alternatively, the XIC may have arisen relatively late in mammalian evolution, possibly in eutherians with the emergence of random XCI. The latter argues that marsupial XCI does not require XIST and opens the search for alternative mechanisms of dosage compensation.