Characterization of a novel postacrosomal perinuclear theca-specific protein, CYPT1

Transcriptome analysis of highly purified mouse spermatogenic cell populations: gene expression signatures switch from meiotic-to postmeiotic-related processes at pachytene stage

Background

Spermatogenesis is a complex differentiation process that involves the successive and simultaneous execution of three different gene expression programs: mitotic proliferation of spermatogonia, meiosis, and spermiogenesis. Testicular cell heterogeneity has hindered its molecular analyses. Moreover, the characterization of short, poorly represented cell stages such as initial meiotic prophase ones (leptotene and zygotene) has remained elusive, despite their crucial importance for understanding the fundamentals of meiosis.

Results

We have developed a flow cytometry-based approach for obtaining highly pure stage-specific spermatogenic cell populations, including early meiotic prophase. Here we combined this methodology with next generation sequencing, which enabled the analysis of meiotic and postmeiotic gene expression signatures in mouse with unprecedented reliability. Interestingly, we found that a considerable number of genes involved in early as well as late meiotic processes are already on at early meiotic prophase, with a high proportion of them being expressed only for the short time lapse of lepto-zygotene stages. Besides, we observed a massive change in gene expression patterns during medium meiotic prophase (pachytene) when mostly genes related to spermiogenesis and sperm function are already turned on. This indicates that the transcriptional switch from meiosis to post-meiosis takes place very early, during meiotic prophase, thus disclosing a higher incidence of post-transcriptional regulation in spermatogenesis than previously reported. Moreover, we found that a good proportion of the differential gene expression in spermiogenesis corresponds to up-regulation of genes whose expression starts earlier, at pachytene stage; this includes transition protein-and protamine-coding genes, which have long been claimed to switch on during spermiogenesis. In addition, our results afford new insights concerning X chromosome meiotic inactivation and reactivation.

Conclusions

This work provides for the first time an overview of the time course for the massive onset and turning off of the meiotic and spermiogenic genetic programs. Importantly, our data represent a highly reliable information set about gene expression in pure testicular cell populations including early meiotic prophase, for further data mining towards the elucidation of the molecular bases of male reproduction in mammals.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2618-1) contains supplementary material, which is available to authorized users.

Mouse Y-Encoded Transcription Factor Zfy2 Is Essential for Sperm Formation and Function in Assisted Fertilization

Spermatogenesis is a key developmental process allowing for a formation of a mature male gamete. During its final phase, spermiogenesis, haploid round spermatids undergo cellular differentiation into spermatozoa, which involves extensive restructuring of cell morphology, DNA, and epigenome. Using mouse models with abrogated Y chromosome gene complements and Y-derived transgene we identified Y chromosome encoded Zfy2 as the gene responsible for sperm formation and function. In the presence of a Zfy2 transgene, mice lacking the Y chromosome and transgenic for two other Y-derived genes, Sry driving sex determination and Eif2s3y initiating spermatogenesis, are capable of producing sperm which when injected into the oocytes yield live offspring. Therefore, only three Y chromosome genes, Sry, Eif2s3y and Zfy2, constitute the minimum Y chromosome complement compatible with successful intracytoplasmic sperm injection in the mouse.

The mammalian Y chromosome was once thought to be a genetic wasteland with testis determinant Sry being the only gene of importance. We now know that there are many genes on this chromosome crucial for male reproduction but their specific roles are often undefined. Here, we investigated the function of the Y chromosome gene Zfy2 during a final step of male gamete formation. We demonstrated that Zfy2 is responsible for allowing sperm precursor cells, haploid round spermatids, to undergo transformation into spermatozoa, and that these sperm are capable of yielding live offspring when injected into the oocytes. Thus, we identified a novel role of the Zfy2 gene during spermatogenesis and fertilization. Considering that in human sperm formation is a prerequisite for male infertility treatment using assisted reproduction technologies, our finding bear translational significance.

Human and mouse ZFY genes produce a conserved testis-specific transcript encoding a zinc finger protein with a short acidic domain and modified transactivation potential

Mammalian ZFY genes are located on the Y chromosome, and code putative transcription factors with 12–13 zinc fingers preceded by a large acidic (activating) domain. In mice, there are two genes, Zfy1 and Zfy2, which are expressed mainly in the testis. Their transcription increases in germ cells as they enter meiosis, both are silenced by meiotic sex chromosome inactivation (MSCI) during pachytene, and Zfy2 is strongly reactivated later in spermatids. Recently, we have shown that mouse Zfy2, but not Zfy1, is involved in triggering the apoptotic elimination of specific types of sex chromosomally aberrant spermatocytes. In humans, there is a single widely transcribed ZFY gene, and there is no evidence for a specific role in the testis. Here, we characterize ZFY transcription during spermatogenesis in mice and humans. In mice, we define a variety of Zfy transcripts, among which is a Zfy2 transcript that predominates in spermatids, and a Zfy1 transcript, lacking an exon encoding approximately half of the acidic domain, which predominates prior to MSCI. In humans, we have identified a major testis-specific ZFY transcript that encodes a protein with the same short acidic domain. This represents the first evidence that ZFY has a conserved function during human spermatogenesis. We further show that, in contrast to the full acidic domain, the short domain does not activate transcription in yeast, and we hypothesize that this explains the functional difference observed between Zfy1 and Zfy2 during mouse meiosis.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 2: changes in spermatid organelles associated with development of spermatozoa

Spermiogenesis is a long process whereby haploid spermatids derived from the meiotic divisions of spermatocytes undergo metamorphosis into spermatozoa. It is subdivided into distinct steps with 19 being identified in rats, 16 in mouse and 8 in humans. Spermiogenesis extends over 22.7 days in rats and 21.6 days in humans. In this part, we review several key events that take place during the development of spermatids from a structural and functional point of view. During early spermiogenesis, the Golgi apparatus forms the acrosome, a lysosome-like membrane bound organelle involved in fertilization. The endoplasmic reticulum undergoes several topographical and structural modifications including the formation of the radial body and annulate lamellae. The chromatoid body is fully developed and undergoes structural and functional modifications at this time. It is suspected to be involved in RNA storing and processing. The shape of the spermatid head undergoes extensive structural changes that are species-specific, and the nuclear chromatin becomes compacted to accommodate the stream-lined appearance of the sperm head. Microtubules become organized to form a curtain or manchette that associates with spermatids at specific steps of their development. It is involved in maintenance of the sperm head shape and trafficking of proteins in the spermatid cytoplasm. During spermiogenesis, many genes/proteins have been implicated in the diverse dynamic events occurring at this time of development of germ cells and the absence of some of these have been shown to result in subfertility or infertility.

Proteome analysis of round-headed and normal spermatozoa by 2-D fluorescence difference gel electrophoresis and mass spectrometry

Globozoospermia is a severe form of teratozoospermia characterized by round-headed spermatozoa with an absent acrosome, an aberrant nuclear membrane and midpiece defects. Globozoospermia is diagnosed by the presence of 100% round-headed spermatozoa on semen analysis, and patients with this condition are absolutely infertile. The objective of this study was to investigate the differences in protein expression between human round-headed and normal spermatozoa. Two-dimensional (2-D) fluorescence difference gel electrophoresis (DIGE) coupled with mass spectrometry (MS) was used in this study. Over 61 protein spots were analysed in each paired normal/round-headed comparison, using DIGE technology along with an internal standard. In total, 35 protein spots identified by tandem mass spectrometry (MS/MS) exhibited significant changes (paired t-test, P < 0.05) in the expression level between normal and round-headed spermatozoa. A total of nine proteins were found to be upregulated and 26 proteins were found to be downregulated in round-headed spermatozoa compared with normal spermatozoa. The differentially expressed proteins that we identified may have important roles in a variety of cellular processes and structures, including spermatogenesis, cell skeleton, metabolism and spermatozoa motility.

Male infertility and the involvement of the X chromosome

BACKGROUND

Male infertility is a worldwide problem, keeping many researchers puzzled. Besides environmental factors, much attention is paid to single gene defects. In this view, the sex chromosomes are particularly interesting since men only have a single copy of these chromosomes. The involvement of the Y chromosome in male infertility is obvious since the detection of Yq microdeletions. The role of the X chromosome, however, remains less understood.

METHODS

Articles were obtained by searching PubMed until December 2008. A first search attempted to identify genes located on the X chromosome potentially important for spermatogenesis. A second part of the study was focused on those genes for which the role has already been studied in infertile patients.

RESULTS

Multiple genes located on the X chromosome are expressed in testicular tissues. The function of many genes, especially the cancer-testis genes, has not been studied so far. There were striking differences between mouse and human genes. In the second part of the study, the results of mutation analyses of seven genes (AR, SOX3, USP26, NXF2, TAF7L, FATE and AKAP4) are described. Except for AR, no infertility causing mutations have, thus far, been described. It cannot be excluded that some of the observed changes should be considered as risk factors for impaired spermatogenesis.

CONCLUSIONS

It can be concluded that, so far, the mutation analysis of X-linked genes in humans, presumed to be crucial for spermatogenesis or sperm quality, has been disappointing. Other approaches to learn more about male infertility are necessary.

A shared promoter region suggests a common ancestor for the human VCX/Y, SPANX, and CSAG gene families and the murine CYPT family

Many testis-specific genes from the sex chromosomes are subject to rapid evolution, which can make it difficult to identify murine genes in the human genome. The murine CYPT gene family includes 15 members, but orthologs were undetectable in the human genome. However, using refined homology search, sequences corresponding to the shared promoter region of the CYPT family were identified at 39 loci. Most loci were located immediately upstream of genes belonging to the VCX/Y, SPANX, or CSAG gene families. Sequence comparison of the loci revealed a conserved CYPT promoter-like (CPL) element featuring TATA and CCAAT boxes. The expression of members of the three families harboring the CPL resembled the murine expression of the CYPT family, with weak expression in late pachytene spermatocytes and predominant expression in spermatids, but some genes were also weakly expressed in somatic cells and in other germ cell types. The genomic regions harboring the gene families were rich in direct and inverted segmental duplications (SD), which may facilitate gene conversion and rapid evolution. The conserved CPL and the common expression profiles suggest that the human VCX/Y, SPANX, and CSAG2 gene families together with the murine SPANX gene and the CYPT family may share a common ancestor. Finally, we present evidence that VCX/Y and SPANX may be paralogs with a similar protein structure consisting of C terminal acidic repeats of variable lengths.

Hominoid-specific SPANXA/D genes demonstrate differential expression in individuals and protein localization to a distinct nuclear envelope domain during spermatid morphogenesis

Human sperm protein associated with the nucleus on the X chromosome consists of a five-member gene family (SPANXA1, SPANXA2, SPANXB, SPANXC and SPANXD) clustered at Xq27.1. Evolved from an ancestral SPANX-N gene family (at Xq27 and Xp11) present in all primates as well as in rats and mice, the SPANXA/D family is present only in humans, bonobos, chimpanzees and gorillas. Among hominoid-specific genes, the SPANXA/D gene family is considered to be undergoing rapid positive selection in its coding region. In this study, RT-PCR of human testis mRNA from individuals showed that, although all SPANXA/D genes are expressed in humans, differences are evident. In particular, SPANXC is expressed only in a subset of men. The SPANXa/d protein localized to the nuclear envelope of round, condensing and elongating spermatids, specifically to regions that do not underlie the developing acrosome. During spermiogenesis, the SPANXa/d-positive domain migrated into the base of the head as the redundant nuclear envelope that protrudes into the residual cytoplasm. Post-testicular modification of the SPANXa/d proteins was noted, as were PEST (proline, glutamic acid, serine, and threonine rich regions) domains. It is concluded that the duplication of the SPANX-N gene family that occurred 6-11 MYA resulted in a new gene family, SPANXA/D, that plays a role during spermiogenesis. The SPANXa/d gene products are among the few examples of X-linked nuclear proteins expressed following meiosis. Their localization to non-acrosomal domains of the nuclear envelope adjacent to regions of euchromatin and their redistribution to the redundant nuclear envelope during spermiogenesis provide a biomarker for the redundant nuclear envelope of spermatids and spermatozoa.

Identification and expression profiling of 10 novel spermatid expressed CYPT genes

To identify candidate genes for poor sperm morphology, we have screened for genes expressed during spermiogenesis. We identified 10 new members of the cysteine-rich perinuclear theca (CYPT) family showing that this family contains at least 15 members, which also includes the casein kinase II target genes. Based on similarity the CYPT sequences could be divided into two groups, Cypt1-10 and the novel members Cypt12-15. The 5'-end of the CYPT family is highly similar to exon1A and part of the first intron of Zfy2. Seven CYPT genes mapped to the X chromosome; six contained an intron and one was intron-less. One CYPT gene mapped to chromosome 3 and one mapped to chromosome 9 which were both intron-less. The upstream region of the CYPT family and Zfy2 genes is conserved. For some the conservation extended over a large region, however, only about 150 nucleotides is conserved among all CYPT members and Zfy2. Nevertheless, the short conserved promoter leads to essentially identical expression profiles for the CYPT family members and Zfy2, which was clearly different from the profile of Zfy1. Expression of the CYPT family and Zfy2 preceded the expression of other spermatid-specific genes such as the transition proteins and the protamines. In situ hybridization revealed a low expression in pachytene spermatocytes from stages IX-X followed by a strong upregulation in spermatids from stage VI with maximum expression in spermatids in stages VII-VIII. The CYPT family may function in the remodeling of the spermatid nucleus before condensation of the DNA.

Deletions on mouse Yq lead to upregulation of multiple X- and Y-linked transcripts in spermatids

Deletions on the mouse Y-chromosome long arm (MSYq) lead to teratozoospermia and in severe cases to infertility. We find that the downstream transcriptional changes in the testis resulting from the loss of MSYq-encoded transcripts involve upregulation of multiple X- and Y-linked spermatid-expressed genes, but not related autosomal genes. Therefore, this indicates that in normal males, there is a specific repression of X and Y (gonosomal) transcription in post-meiotic cells, which depends on MSYq-encoded transcripts. Together with the known sex ratio skew in favour of females in the offspring of fertile MSYqdel males, this strongly suggests the existence of an intragenomic conflict between X- and Y-linked genes. Two potential antagonists in this conflict are the X-linked multicopy gene Xmr and its multicopy MSYq-linked relative Sly, which are upregulated and downregulated, respectively, in the testes of MSYqdel males. Xmr is also expressed during meiotic sex chromosome inactivation (MSCI), indicating a link between the MSCI and the MSYq-dependent gonosomal repression in spermatids. We therefore propose that this repression and MSCI itself are evolutionary adaptations to maintain a normal sex ratio in the face of X/Y antagonism.