Mouse autosomal homolog of DAZ, a candidate male sterility gene in humans, is expressed in male germ cells before and after puberty

Identification and expression analysis of Dazl homologue in Cynops cyanurus

The Dazl (deleted in azoospermia-like) gene encodes an RNA-binding protein containing an RNA recognition motif (RRM) and a DAZ motif. Dazl is essential for gametogenesis in vertebrates. In this study, we report the cloning of Dazl cDNA from Cynops cyanurus. Ccdazl mRNA showed a germline-specific expression pattern as expected. Ccdazl expression gradually decreased during oogenesis, suggesting that it may be involved in oocyte development. Phylogenetic analysis revealed that the Ccdazl protein shares conserved motifs/domains with Dazl proteins from other species. Cloning of Ccdazl provides a new tool to carry out comparative studies of germ cell development in amphibians.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

A premeiotic function for boule in the planarian Schmidtea mediterranea

Mutations in Deleted in Azoospermia (DAZ), a Y chromosome gene, are an important cause of human male infertility. DAZ is found exclusively in primates, limiting functional studies of this gene to its homologs: boule, required for meiotic progression of germ cells in invertebrate model systems, and Daz-like (Dazl), required for early germ cell maintenance in vertebrates. Dazl is believed to have acquired its premeiotic role in a vertebrate ancestor following the duplication and functional divergence of the single-copy gene boule. However, multiple homologs of boule have been identified in some invertebrates, raising the possibility that some of these genes may play other roles, including a premeiotic function. Here we identify two boule paralogs in the freshwater planarian Schmidtea mediterranea Smed-boule1 is necessary for meiotic progression of male germ cells, similar to the known function of boule in invertebrates. By contrast, Smed-boule2 is required for the maintenance of early male germ cells, similar to vertebrate Dazl To examine if Boule2 may be functionally similar to vertebrate Dazl, we identify and functionally characterize planarian homologs of human DAZL/DAZ-interacting partners and DAZ family mRNA targets. Finally, our phylogenetic analyses indicate that premeiotic functions of planarian boule2 and vertebrate Dazl evolved independently. Our study uncovers a premeiotic role for an invertebrate boule homolog and offers a tractable invertebrate model system for studying the premeiotic functions of the DAZ protein family.

Characterisation of the deleted in azoospermia like (Dazl)-green fluorescent protein mouse model generated by a two-step embryonic stem cell-based strategy to identify pluripotent and germ cells

The deleted in azoospermia like (Dazl) gene is preferentially expressed in germ cells; however, recent studies indicate that it may have pluripotency-related functions. We generated Dazl-green fluorescent protein (GFP) transgenic mice and assayed the ability of Dazl-driven GFP to mark preimplantation embryo development, fetal, neonatal and adult tissues, and in vitro differentiation from embryonic stem cells (ESCs) to embryoid bodies (EBs) and to primordial germ cell (PGC)-like cells. The Dazl-GFP mice were generated by a two-step ESC-based strategy, which enabled primary and secondary screening of stably transfected clones before embryo injection. During preimplantation embryo stages, GFP was detected from the zygote to blastocyst stage. At Embryonic Day (E) 12.5, GFP was expressed in gonadal ridges and in neonatal gonads of both sexes. In adult mice, GFP expression was found during spermatogenesis from spermatogonia to elongating spermatids and in the cytoplasm of oocytes. However, GFP mRNA was also detected in other tissues harbouring multipotent cells, such as the intestine and bone marrow. Fluorescence was maintained along in vitro Dazl-GFP ESC differentiation to EBs, and in PGC-like cells. In addition to its largely known function in germ cell development, Dazl could have an additional role in pluripotency, supporting these transgenic mice as a valuable tool for the prospective identification of stem cells from several tissues.

The roles of DAZL in RNA biology and development

RNA-binding proteins play an important role in the regulation of gene expression by modulating translation and localization of specific messenger RNAs (mRNAs) during early development and gametogenesis. The DAZ (Deleted in Azoospermia) family of proteins, which includes DAZ, DAZL, and BOULE, are germ cell-specific RNA-binding proteins that are implicated in translational regulation of several transcripts. Of particular importance is DAZL, which is present in vertebrates and arose from the duplication of the ancestral BOULE during evolution. Identification of DAZL target mRNAs and characterization of the RNA-binding sequence through in vitro binding assays and crystallographic studies revealed that DAZL binds to GUU triplets in the 3' untranslated region of target mRNAs. Although there is compelling evidence for the role of DAZL in translation stimulation of target mRNAs, recent studies indicate that DAZL can also function in translational repression and transport of specific mRNAs. Furthermore, apart from the well-characterized function of DAZL in gametogenesis, recent data suggest its role in early embryonic development and differentiation of pluripotent stem cells toward functional gametes. In light of the mounting evidence for the role of DAZL in various cellular and developmental processes, we summarize the currently characterized biological functions of DAZL in RNA biology and development.

Cloning and expression of boule and dazl in the Nile tilapia (Oreochromis niloticus)

The Deleted in Azoospermia (DAZ) family of RNA binding proteins consists of highly conserved genes boule, daz and daz-like (dazl) essential for germ cell development. boule is known for its unisexual meiotic expression in invertebrates and mammals, but meiotic-specific female expression plus meiosis-preferential male expression in trout, and meiosis-preferential bisexual expression in medaka. dazl shows highly conserved bisexual expression throughout gametogenesis in diverse species. Here we report the cloning and expression of boule and dazl in the Nile tilapia (Oreochromis niloticus), an important aquaculture fish. Molecular cloning and sequence analysis led to the identification of tilapia boule and dazl cDNAs. The predicted partial Boule contains a conserved RRM motif and Dazl has the C-terminal sequence. On a phylogenetic tree, tilapia Boule and Dazl are in separate clades of Boule and Dazl homologs from other species, indicating their divergence during early vertebrate evolution. By RT-PCR analysis, boule and dazl showed bisexual gonad-specific expression. By in situ hybridization analysis, both boule and dazl RNAs were restricted to female and male germ cells of adult gonads but absent in gonadal soma. In the ovary, boule and dazl RNAs were abundant in oocytes. In the testis, boule and dazl RNAs were prominent in meiotic spermatocytes but barely detectable in meiotic products. These data show that boule and dazl are expressed bisexually in germ cells and provide useful markers to study gametogenesis in the adult tilapia.

Toward a more precise and informative nomenclature describing fetal and neonatal male germ cells in rodents

The germ cell lineages are among the best characterized of all cell lineages in mammals. This characterization includes precise nomenclature that distinguishes among numerous, often subtle, changes in function or morphology as development and differentiation of germ cells proceed to form the gametes. In male rodents, there are at least 41 distinct cell types that occur during progression through the male germ cell lineage that gives rise to spermatozoa. However, there is one period during male germ cell development-that which occurs immediately following the primordial germ cell stage and prior to the spermatogonial stage-for which the system of precise and informative cell type terminology is not adequate. Often, male germ cells during this period are referred to simply as "gonocytes." However, this term is inadequate for multiple reasons, and it is suggested here that nomenclature originally proposed in the 1970s by Hilscher et al., which employs the terms M-, T1-, and T2-prospermatogonia, is preferable. In this Minireview, the history, proper utilization, and advantages of this terminology relative to that of the term gonocytes are described.

Genomics and genetics of human and primate y chromosomes

In mammals, the Y chromosome plays the pivotal role in male sex determination and is essential for normal sperm production. Yet only three Y chromosomes have been completely sequenced to date--those of human, chimpanzee, and rhesus macaque. While Y chromosomes are notoriously difficult to sequence owing to their highly repetitive genomic landscapes, these dedicated sequencing efforts have generated tremendous yields in medical, biological, and evolutionary insight. Knowledge of the complex structural organization of the human Y chromosome and a complete catalog of its gene content have provided a deeper understanding of the mechanisms that generate disease-causing mutations and large-scale rearrangements. Variation among human Y-chromosome sequences has been an invaluable tool for understanding relationships among human populations. Comprehensive comparisons of the human Y-chromosome sequence with those of other primates have illuminated aspects of Y-chromosome evolutionary dynamics over much longer timescales (>25 million years compared with 100,000 years). The future sequencing of additional Y chromosomes will provide a basis for a more comprehensive understanding of the evolution of Y chromosomes and their roles in reproductive biology.

The ontogeny of cKIT+ human primordial germ cells proves to be a resource for human germ line reprogramming, imprint erasure and in vitro differentiation

The generation of research-quality, clinically relevant cell types in vitro from human pluripotent stem cells requires a detailed understanding of the equivalent human cell types. Here we analysed 134 human embryonic and fetal samples from 6 to 20 developmental weeks and identified the stages at which cKIT(+) primordial germ cells (PGCs), the precursors of gametes, undergo whole-genome epigenetic reprogramming with global depletion of 5mC, H3K27me3 and H2A.Z, and the time at which imprint erasure is initiated and 5hmC is present. Using five alternative in vitro differentiation strategies combined with single-cell microfluidic analysis and a bona fide human cKIT(+) PGC signature, we show the stage of cKIT(+) PGC formation in the first 16 days of differentiation. Taken together, our study creates a resource of human germ line ontogeny that is essential for future studies aimed at in vitro differentiation and unveiling the mechanisms necessary to pass human DNA from one generation to the next.

Sequencing of rhesus macaque Y chromosome clarifies origins and evolution of the DAZ (Deleted in AZoospermia) genes

Studies of Y chromosome evolution often emphasize gene loss, but this loss has been counterbalanced by addition of new genes. The DAZ genes, which are critical to human spermatogenesis, were acquired by the Y chromosome in the ancestor of Old World monkeys and apes. We and our colleagues recently sequenced the rhesus macaque Y chromosome, and comparison of this sequence to human and chimpanzee enables us to reconstruct much of the evolutionary history of DAZ. We report that DAZ arrived on the Y chromosome about 38 million years ago via the transposition of at least 1.1 megabases of autosomal DNA. This transposition also brought five additional genes to the Y chromosome, but all five genes were subsequently lost through mutation or deletion. As the only surviving gene, DAZ experienced extensive restructuring, including intragenic amplification and gene duplication, and has been the target of positive selection in the chimpanzee lineage. Editor's suggested further reading in BioEssays Should Y stay or should Y go: The evolution of non-recombining sex chromosomes Abstract.

Proliferation of small number of human spermatogonial stem cells obtained from azoospermic patients

PURPOSE

This study aims to proliferate spermatogonial stem cells (SSCs) and compare the in-vitro effects of laminin and growth factors on the proliferation of adult human SSC.

METHODS

Isolated testicular cells were cultured in DMEM supplemented with 5 % fetal calf serum (FCS). During the culture, enriched spermatogonial cells were treated with a combination of glial cell line-derived neurotrophic factor (GDNF), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) and mouse leukemia inhibitory factor (LIF) in the presence or absence of human placental laminin-coated dishes. Cluster assay was performed during culture. Presence of spermatogonia was determined by an ultrastructural study of the cell clusters, reverse transcription polymerase chain reaction (RT-PCR) for spermatogonial markers and xenotransplantation to the testes of busulfan-treated recipient mice. Statistical significance between mean values was determined using statistical ANOVA tests.

RESULTS

The findings indicated that the addition of GDNF, bFGF, EGF and LIF on laminin-coated dishes significantly increased in-vitro spermatogonial cell cluster formation in comparison with the control group (p ≤ 0.001). The expression of spermatogonial markers was maintained throughout the culture period. Furthermore, a transplantation experiment showed the presence of SSC among the cultured cells. In addition, a transmission electron microscopy (TEM) study suggested the presence of spermatogonial cells of typical morphology among the cluster cells.

CONCLUSIONS

It can be concluded that human SSCs obtained from non-obstructive azoospermic (NOA) patients had the ability to self-renew in the culture system. This system can be used for the propagation of a small number of these cells from small biopsies.

Unique case reports associated with ovarian failure: necessity of two intact x chromosomes

Premature ovarian failure is defined as the loss of functional follicles below the age of 40 years and the incidence of this abnormality is 0.1% among the 30-40 years age group. Unexplained POF is clinically recognized as amenorrhoea (>6 months) with low level of oestrogen and raised level of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH > 20 IU/l) occurring before the age of 40. It has been studied earlier that chromosomal defects can impair ovarian development and its function. Since there is paucity of data on chromosomal defects in Indian women, an attempt is made to carry out cytogenetic evaluation in patients with ovarian failure. Cytogenetic analysis of women with ovarian defects revealed the chromosome abnormalities to be associated with 14% of the cases analyzed. Interestingly, majority of the abnormalities involved the X-chromosome and we report two unique abnormalities, (46,XXdel(Xq21-22) and q28) and (mos,45XO/46,X+ringX) involving X chromosome in association with ovarian failure. This study revealed novel X chromosome abnormalities associated with ovarian defects and these observations would be helpful in genetic counseling and apart from, infertility clinics using the information to decide suitable strategies to help such patients.

Differential conservation and divergence of fertility genes boule and dazl in the rainbow trout

Background

The genes boule and dazl are members of the DAZ (Deleted in Azoospermia) family encoding RNA binding proteins essential for germ cell development. Although dazl exhibits bisexual expression in mitotic and meiotic germ cells in diverse animals, boule shows unisexual meiotic expression in invertebrates and mammals but a bisexual mitotic and meiotic expression in medaka. How boule and dazl have evolved different expression patterns in diverse organisms has remained unknown.

Methodology and Principal Findings

Here we chose the fish rainbow trout (Oncorhynchus mykiss) as a second lower vertebrate model to investigate the expression of boule and dazl. By molecular cloning and sequence comparison, we identified cDNAs encoding the trout Boule and Dazl proteins, which have a conserved RNA-recognition motif and a maximal similarity to their homologs. By RT-PCR analysis, adult RNA expression of trout boule and dazl is restricted to the gonads of both sexes. By chromogenic and two-color fluorescence in situ hybridization, we revealed bisexual and germline-specific expression of boule and dazl. We found that dazl displays conserved expression throughout gametogenesis and concentrates in the Balbinani's body of early oocytes and the chromatoid body of sperm. Surprisingly, boule exhibits mitotic and meiotic expression in the male but meiosis-specific expression in the female.

Conclusions

Our data underscores differential conservation and divergence of DAZ family genes during vertebrate evolution. We propose a model in which the diversity of boule expression in sex and stage specificity might have resulted from selective loss or gain of its expression in one sex and mitotic germ cells.

The roles of the DAZ family in spermatogenesis: More than just translation?

The DAZ family of genes are important fertility factors in animals, including humans. The family consists of Y-linked DAZ, and autosomal homologs Boule and Dazl. All three genes encode RNA-binding proteins that are nearly exclusively expressed in germ cells. The DAZ family is highly conserved, with ancestral Boule present in sea anemones through humans, Dazl conserved among vertebrates, and DAZ present only in higher primates. Here we review studies on DAZ family genes from multiple organisms, and summarize the common features of each DAZ gene and their roles during spermatogenesis in animals. DAZ family proteins are thought to activate the translation of RNA targets, but recent work has uncovered additional functions. Boule, Dazl, and DAZ likely function through similar mechanisms, and we present known functions of the DAZ family in spermatogenesis, and discuss possible mechanisms in addition to translation activation.

Prepubertal human spermatogonia and mouse gonocytes share conserved gene expression of germline stem cell regulatory molecules

In the human testis, beginning at approximately 2 months of age, gonocytes are replaced by adult dark (Ad) and pale (Ap) spermatogonia that make up the spermatogonial stem cell (SSC) pool. In mice, the SSC pool arises from gonocytes approximately 6 days after birth. During puberty in both species, complete spermatogenesis is established by cells that differentiate from SSCs. Essentially pure populations of prepubertal human spermatogonia and mouse gonocytes were selected from testis biopsies and validated by confirming the presence of specific marker proteins in cells. Stem cell potential of germ cells was demonstrated by transplantation to mouse testes, following which the cells migrated to the basement membrane of the seminiferous tubule and were maintained similar to SSCs. Differential gene expression profiles generated between germ cells and testis somatic cells demonstrated that expression of genes previously identified as SSC and spermatogonial-specific markers (e.g., zinc-finger and BTB-domain containing 16, ZBTB16) was greatly elevated in both human spermatogonia and mouse gonocytes compared to somatic cells. Several genes were expressed at significantly higher levels in germ cells of both species. Most importantly, genes known to be essential for mouse SSC self-renewal (e.g., Ret proto-oncogene, Ret; GDNF-family receptor alpha1, Gfr alpha1; and B-cell CLL/lymphoma 6, member B, Bcl6b) were more highly expressed in both prepubertal human spermatogonia and mouse gonocytes than in somatic cells. The results indicate remarkable conservation of gene expression, notably for self-renewal genes, in these prepubertal germline cells between two species that diverged phylogenetically approximately 75 million years ago.

A spontaneous smc1b mutation causes cohesin protein dysfunction and sterility in mice

In this paper, we describe a novel spontaneous mutation of the Smc1b gene coding a cohesin component, which causes female and male sterility. We have discovered an ICR male mouse with a novel autosomal recessive gene that causes small gonads and sterility in both sexes. Mutant female and male mice homozygous for the novel sterility gene had normal body weights and showed normal mating behavior, but did not produce any offspring. Histological examination showed that Sertoli cells and spermatogonia were present in the testicular seminiferous tubules in 8-week-old mutant male mice, but no spermatids or spermatozoa were observed. Mutant females showed a markedly reduced number of oocytes with age. The novel sterility gene mapped between D15Mit105 (47.9 cM) and D15Mit171 (54.5 cM) on chromosome 15. Sequences of three candidate sterility genes, Dmc1, Mei1 and Smc1b, which are closely linked to these microsatellite markers, were compared between normal and mutant mice. The Dmc1 and Mei1 genes showed the same sequences in both normal and mutant mice, but the Smc1b gene had a deletion of 16 nucleotides in exon 5, in the mutant mice. We concluded that this deletion led to a frame-shift, which generated a stop codon at position 761 (amino acid 247) of the Smc1b cDNA in mutant mice.

Sterile testis complementation with spermatogonial lines restores fertility to DAZL-deficient rats and maximizes donor germline transmission

Despite remarkable advances in assisted reproductive capabilities approximately 4% of all couples remain involuntarily infertile. In almost half of these cases, a lack of conception can in some measure be attributed to the male partner, wherein de novo Y-chromosomal deletions of sperm-specific Deleted-in-Azoospermia (DAZ) genes are particularly prevalent. In the current study, long-term cultures of rat spermatogonial stem cells were evaluated after cryo-storage for their potential to restore fertility to rats deficient in the DAZ-like (DAZL) gene. Detailed histological analysis of DAZL-deficient rat testes revealed an apparently intact spermatogonial stem cell compartment, but clear failure to produce mature haploid gametes resulting in infertility. After proliferating >1 million-fold in cell number during culture post-thaw, as few as 50,000 donor spermatogonia transplanted into only a single testis/recipient effectively restored fecundity to DAZL-deficient rats, yielding 100% germline transmission to progeny by natural mating. Based on these results, the potency and efficacy of this donor stem cell line for restoring fertility to azoospermic rodents is currently unprecedented. Prospectively, similar successes in humans could be directly linked to the feasibility of obtaining enough fully functional spermatogonial stem cells from minimal testis biopsies to be therapeutically effective. Thus, regeneration of sperm production in this sterile recipient provides an advanced pre-clinical model for optimizing the efficacy of stem cell therapies to cure a paradoxically increasing number of azoospermic men. This includes males that are rendered infertile by cancer therapies, specific types of endocrine or developmental defects, and germline-specific de novo mutations; all of whom may harbor healthy sources of their own spermatogonial stem cells for treatment.

Boule is present in fish and bisexually expressed in adult and embryonic germ cells of medaka

Background

The DAZ family genes boule, daz and dazl encode RNA binding proteins essential for fertility of diverse animals including human. dazl has bisexual expression in both mitotic and meiotic germ cells, whereas daz has male premeiotic expression, and boule is largely a unisexual meiotic regulator. Although boule has been proposed as the ancestor for dazl/daz by gene duplication, it has been identified only in invertebrates and mammals. It has, however, remained unclear when and how the DAZ family has evolved in vertebrates.

Methodology and Principal Findings

This study was aimed at identifying and characterizing the DAZ family genes in fish as the basal vertebrate. We show that boule and dazl coexist in medaka and stickleback. Similar to the medaka dazl (Odazl), the medaka boule (Obol) is maternally supplied and segregates with primordial germ cells. Surprisingly, Obol is expressed in adult germ cells at pre-meiotic and meiotic stages of spermatogenesis and oogenesis. However, the maximal meiotic Obol expression in spermatocytes contrasts with the predominant pre-meiotic Odazl expression in spermatogonia, and the diffuse cytoplasmic Obol distribution in early oocytes contrasts with the Odazl concentration in the Balbinani's body.

Conclusions

The identification of fish boule and dazl genes provides direct evidence for the early gene duplication during vertebrate evolution. Our finding that Obol exhibits bisexual expression in both embryonic and adult germ cells considerably extends the diversity of boule expression patterns and offers a new insight into the evolutions of DAZ family members, expression patterns and functions in animal fertility.

Y Chromosome deletions and male infertility

Approximately one in ten couples experience infertility, and in about 40% of these infertile unions there are abnormalities in the fertility of the male partner. The clinical management of these infertile men is less than satisfactory because in 40% of such patients the cause of the abnormalities of sperm production and quality is unknown. The possibility that genetic disorders may account for a proportion of these disturbances of sperm production has been raised. It is well recognized that chromosomal abnormalities such as Klinefelter's syndrome cause azoospermia and that gene defects are the basis of testicular feminization, Kallman's syndrome and Reifenstein's syndrome. With the explosion in our knowledge of the human genome, the possibility exists that other genetic disorders may form the basis of other sperma-togenic abnormalities. The past decade has witnessed the accumulation of evidence linking abnormalities of the Y chromosome with disturbances in sperm production and these observations form the basis of this review.

Characterization of a Dazl-GFP germ cell-specific reporter

In this study, we characterized the promoter activity of a 1.7 kb sequence in the 5' flanking region of the mouse Deleted in Azoospermia-Like (Dazl) gene. We found the 1.7 kb sequence sufficient to drive robust germ cell-specific expression of green fluorescent protein (GFP) in adult mouse testis and lower levels of expression in adult ovary and in fetal and newborn gonads of both sexes. This expression pattern was confirmed in two independently-derived transgenic mouse lines. In adult testis, Dazl-GFP exhibited a developmentally-regulated, stage-specific expression pattern during spermatogenesis. GFP was highly expressed in spermatocyte stages, with strongest expression in pachytene spermatocytes. Weaker expression was observed in round and elongating spermatids, as well as spermatogonial cells. In the fetal gonad, GFP transcript was detected by e12.5 in both sexes; however, GFP fluorescence was only detected during later embryonic stages. In addition, we produced mouse embryonic stem cell (ESC) lines harboring the Dazl-GFP reporter and used this reporter to isolate putative germ cell populations derived from mouse ESCs following embryoid body differentiation and fluorescence activated cell sorting.

In vitro germ cell differentiation from cynomolgus monkey embryonic stem cells

Background

Mouse embryonic stem (ES) cells can differentiate into female and male germ cells in vitro. Primate ES cells can also differentiate into immature germ cells in vitro. However, little is known about the differentiation markers and culture conditions for in vitro germ cell differentiation from ES cells in primates. Monkey ES cells are thus considered to be a useful model to study primate gametogenesis in vitro. Therefore, in order to obtain further information on germ cell differentiation from primate ES cells, this study examined the ability of cynomolgus monkey ES cells to differentiate into germ cells in vitro.

Methods and Findings

To explore the differentiation markers for detecting germ cells differentiated from ES cells, the expression of various germ cell marker genes was examined in tissues and ES cells of the cynomolgus monkey (Macaca fascicularis). VASA is a valuable gene for the detection of germ cells differentiated from ES cells. An increase of VASA expression was observed when differentiation was induced in ES cells via embryoid body (EB) formation. In addition, the expression of other germ cell markers, such as NANOS and PIWIL1 genes, was also up-regulated as the EB differentiation progressed. Immunocytochemistry identified the cells expressing stage-specific embryonic antigen (SSEA) 1, OCT-4, and VASA proteins in the EBs. These cells were detected in the peripheral region of the EBs as specific cell populations, such as SSEA1-positive, OCT-4-positive cells, OCT-4-positive, VASA-positive cells, and OCT-4-negative, VASA-positive cells. Thereafter, the effect of mouse gonadal cell-conditioned medium and growth factors on germ cell differentiation from monkey ES cells was examined, and this revealed that the addition of BMP4 to differentiating ES cells increased the expression of SCP1, a meiotic marker gene.

Conclusion

VASA is a valuable gene for the detection of germ cells differentiated from ES cells in monkeys, and the identification and characterization of germ cells derived from ES cells are possible by using reported germ cell markers in vivo, including SSEA1, OCT-4, and VASA, in vitro as well as in vivo. These findings are thus considered to help elucidate the germ cell developmental process in primates.

An epigenetic mechanism regulates germ cell-specific expression of the porcine Deleted in Azoospermia-Like (DAZL) gene

The Deleted in Azoospermia-Like (DAZL) gene is specifically expressed in fetal and adult gonads. While DAZL is known to play a role during gametogenesis, the mechanisms governing its germ cell-specific expression remain unclear. We identified the 5' untranslated region (UTR) of the porcine DAZL gene and cloned and characterized 2 kilobase pairs of its TATA-less 5' flanking region, identifying CpG-rich regions within the proximal promoter. Nine of 18 CpG sites in proximity to one region were largely unmethylated in germ cells but hypermethylated in somatic cells, suggesting that DNA methylation may regulate DAZL promoter activity. Furthermore, DAZL expression was induced in fibroblasts treated with a demethylating agent. Deletion analyses revealed that the minimal 149 base pair promoter region was sufficient to activate transcription. In vitro methylation of a reporter construct corresponding to these 149 base pairs resulted in complete suppression of DAZL promoter activity in primordial germ cells, further supporting a role for methylation in regulating DAZL expression. Interestingly, the differentially methylated region was shown to harbor several putative Sp1-binding sites. Mutation of only the most highly conserved site significantly reduced promoter activity in a reporter assay. Furthermore, gel shift assays revealed that Sp1 was able to specifically bind to this site, and that complex formation was inhibited when CpG dinucleotides within this region were methylated. Chromatin immunoprecipitation (ChIP) assays revealed that in vivo Sp1 binding to the core DAZL promoter region was enriched in germ cells but not in fibroblasts. Our data suggests that DNA methylation may suppress DAZL expression in somatic cells by interfering with Sp1 binding. This study provides insights into the potential mechanisms underlying the regulation of germ cell-specific gene expression.

Sohlh2 knockout mice are male-sterile because of degeneration of differentiating type A spermatogonia

The spermatogenesis and oogenesis-specific transcription factor Sohlh2 is normally expressed only in premeiotic germ cells. In this study, Sohlh2 and several other germ cell transcripts were found to be induced in mouse embryonic stem cells when cultured on a feeder cell line that overexpresses bone morphogenetic protein 4. To study the function of Sohlh2 in germ cells, we generated mice harboring null alleles of Sohlh2. Male Sohlh2-deficient mice were infertile because of a block in spermatogenesis. Although normal prior to birth, Sohlh2-null mice had reduced numbers of intermediate and type B spermatogonia by postnatal day 7. By day 10, development to the preleptotene spermatocyte stage was severely disrupted, rendering seminiferous tubules with only Sertoli cells, undifferentiated spermatogonia, and degenerating colonies of differentiating spermatogonia. Degenerating cells resembled type A2 spermatogonia and accumulated in M-phase prior to death. A similar phenotype was observed in Sohlh2-null mice on postnatal days 14, 21, 35, 49, 68, and 151. In adult Sohlh2-mutant mice, the ratio of undifferentiated type A spermatogonia (DAZL+/PLZF+) to differentiating type A spermatogonia (DAZL+/PLZF-) was twice normal levels. In culture, undifferentiated type A spermatogonia isolated from Sohlh2-null mice proliferated normally but linked the mutant phenotype to aberrant cell surface expression of the receptor-tyrosine kinase cKit. Thus, Sohlh2 is required for progression of differentiating type A spermatogonia into type B spermatogonia. One conclusion originating from these studies would be that testicular factors normally regulate the viability of differentiating spermatogonia by signaling through Sohlh2. This regulation would provide a crucial checkpoint to optimize the numbers of spermatocytes entering meiosis during each cycle of spermatogenesis. Disclosure of potential conflicts of interest is found at the end of this article.

Histone H4 acetylation and AZFc involvement in germ cells of specimens of impaired spermatogenesis

OBJECTIVE

To measure histone-H4 acetylation and involvement of the AZFc region in testicular mixed atrophy.

DESIGN

Prospective study.

SETTING

University-affiliated medical center.

PATIENT(S)

Azoospermic men (n = 23) who underwent testicular sperm extraction and preparation for intracytoplasmic sperm injection (ICSI) divided into obstructive azoospermia with complete spermatogenesis (group A), testicular mixed atrophy (group B), and testicular mixed atrophy associated with AZFc deletion (group C).

INTERVENTION(S)

Testicular biopsy evaluation by Western blotting and quantitative immunohistochemistry of histone-H4 hyperacetylation (Hypac-H4) and lysine-12 acetylation (Lys12ac-H4).

MAIN OUTCOME MEASURE(S)

Percentage of spermatogonia and spermatids stained by Hypac-H4 and Lys12ac-H4 antibodies in retrieved specimens.

RESULT(S)

The percentage of spermatogonia stained for Hypac-H4 and Lys12ac-H4 in groups B and C was statistically significantly reduced. The percentage of elongated spermatids showing positive staining to Hypac-H4 was statistically significantly lower in group B than group A. The percentage of Lys12ac-H4-labeled spermatids was similar for all groups. Hypac-H4 and Lys12ac-H4 processes were highly correlated in spermatogonia but not in spermatids.

CONCLUSION(S)

The reduced percentage of spermatogonia with Hypac-H4 and Lys12ac-H4 in groups B and C may contribute to lower sperm production in mixed atrophy. Spermatids Hypac-H4 impairment in mixed atrophy did not deteriorate further by AZFc region deletion.

Cloning and expression of medaka dazl during embryogenesis and gametogenesis

The Deleted in azoospermia family consists of RNA-binding proteins Boule, Daz, and Daz-like (Dazl) that are expressed in the germline. Here, we report the cloning and expression of the medakafish (Oryzias latipes) dazl gene (odazl). Interestingly, although the predicted medaka Dazl protein (oDazl) contains a RRM motif and a DAZ repeat characteristic of its mammalian homologs, it lacks 80 aa at the C-terminus. By RT-PCR, RNA in situ hybridization, Western blotting and fluorescent immunohistochemistry using a rabbit anti-Dazl antibody (alphaDazl), we analyzed the expression patterns of odazl and its protein. The odazl transcript persists throughout embryogenesis and delineates with primordial germ cells. In adults, the expression of odazl RNA and its protein is restricted to germ cells of both the testis and ovary. We observed differential expression of RNA and protein at critical stages of gametogenesis. In the testis, the odazl RNA is low at premeiotic stages, abundant at meiotic stages, but absent in postmeiotic stages; whereas the oDazl protein is rich in premeiotic stages, reduced at meiotic stages, becomes barely detectable or absent in postmeiotic round spermatids or sperm, respectively. This is in sharp contrast to the human situation where the Dazl transcript and protein are present in mature spermatozoa. In the ovary, the odazl RNA and protein persist throughout oogenesis and also show differential expression at premeiotic, meiotic and postmeiotic stages. Thus, the odazl or its protein is a marker for germ cells during embryogenesis and at critical stages of gametogenesis in both sexes of medaka.

Dazl can bind to dynein motor complex and may play a role in transport of specific mRNAs

Male germ cell development includes mitotic and meiotic cell divisions that are followed by dramatic morphological changes resulting in the production of spermatozoa. Genetic evidence has indicated that the DAZ family genes are critical for successful male germ cell development in diverse animals as well as humans. In the present study, we investigated the cellular functions of Dazl in the mouse male germ cells. We identified a specific interaction of Dazl with the dynein light chain, a component of the dynein-dynactin motor complex. The subcellular distribution of Dazl was microtubule-dependent and a selected number of Dazl-bound mRNAs could accumulate in the perinuclear area. Based on these results, we propose that Dazl may play a role in transport of specific mRNAs via dynein motor complex. The Dazl-bound mRNAs may be stored at specific sites and would be available for future developmental processes. Our study revealed the presence of an active mRNA transport system in mouse male germ cells.

Characterization of structure and expression of the Dzip1 gene in the rat and mouse

A transcript encoding a rat homologue of DZIP1 (DAZ-interacting protein) was isolated from testis RNA. Like human DZIP1, it contains a C(2)H(2) zinc finger domain. A predicted mouse homologue of DZIP1 was found in the GenBank database. Genome analysis indicated that while DZIP1 and mouse Dzip1 contain 22 and 20 exons, respectively, the rat sequence was intronless, confirmed by PCR on genomic DNA. This rat Dzip1 sequence is homologous to mouse Dzip1 exons 1-6 and DZIP1 exons 5-9. As this rat sequence was shorter than DZIP1 it was designated rat Dzip1S. The rat genome also contained a further predicted homologue of DZIP1 displaying conserved linkage homology with mouse Dzip1 and DZIP1. This sequence, if expressed, is the true rat homologue of DZIP1, designated rat Dzip1. Rat Dzip1S mRNA was present in all tissues examined by qualitative RT-RCR, and in situ hybridization of rat testis confirmed that expression of rat Dzip1S mRNA was confined to the spermatogenic lineage, specifically premeiotic spermatogonia.

Capillary morphogenesis gene (CMG)-1 is among the genes differentially expressed in mouse male germ line stem cells and embryonic stem cells

We recently established a technique to expand male germ line stem (GS) cells in long-term culture without losing their spermatogenic capacity. To gain insight into the genetic program of these cells, we compared the mRNA expression profile of GS cells with that of embryonic stem (ES) cells using DNA microarrays. We found 79 genes that were upregulated in GS cells compared to ES cells, including synaptonemal complex protein-1, deleted in azoospermia-like, ubiquitin-conjugating enzyme E2B, and ubiquitin carboxy-terminal hydrolase L1, all of which are functionally important for spermatogenesis. In addition, we identified a cDNA encoding the mouse ortholog of capillary morphogenesis gene (CMG)-1. CMG-1 transcripts were predominantly produced in spermatogonia and spermatocytes in mouse testis. When CMG-1 expression was attenuated in a mouse spermatocyte-derived cell line, GC-2spd(ts), by a target-specific short interfering RNA, the morphology of the cells was changed and the expression of cyclin D2 was abrogated. A reporter assay using a genomic region upstream of the mouse cyclin D2 gene revealed that this downmodulation occurs at the transcriptional level. We detected FLAG-tagged CMG-1 protein in the nuclei of transfected COS7 cells, suggesting that CMG-1 may play a unique role in the transcriptional regulation of the cyclin D2 gene. The upregulated GS genes identified in this study will provide useful information for the future investigation of spermatogonial stem cells and the early phase of male germ cell differentiation.

The Y deletion gr/gr and susceptibility to testicular germ cell tumor

Testicular germ cell tumor (TGCT) is the most common cancer in young men. Despite a considerable familial component to TGCT risk, no genetic change that confers increased risk has been substantiated to date. The human Y chromosome carries a number of genes specifically involved in male germ cell development, and deletion of the AZFc region at Yq11 is the most common known genetic cause of infertility. Recently, a 1.6-Mb deletion of the Y chromosome that removes part of the AZFc region--known as the "gr/gr" deletion--has been associated with infertility. In epidemiological studies, male infertility has shown an association with TGCT that is out of proportion with what can be explained by tumor effects. Thus, we hypothesized that the gr/gr deletion may be associated with TGCT. Using logistic modeling, we analyzed this deletion in a large series of TGCT cases with and without a family history of TGCT. The gr/gr deletion was present in 3.0% (13/431) of TGCT cases with a family history, 2% (28/1,376) of TGCT cases without a family history, and 1.3% (33/2,599) of unaffected males. Presence of the gr/gr deletion was associated with a twofold increased risk of TGCT (adjusted odds ratio [aOR] 2.1; 95% confidence interval [CI] 1.3-3.6; P = .005) and a threefold increased risk of TGCT among patients with a positive family history (aOR 3.2; 95% CI 1.5-6.7; P = .0027). The gr/gr deletion was more strongly associated with seminoma (aOR 3.0; 95% CI 1.6-5.4; P = .0004) than with nonseminoma TGCT (aOR 1.5; 95% CI 0.72-3.0; P = .29). These data indicate that the Y microdeletion gr/gr is a rare, low-penetrance allele that confers susceptibility to TGCT.

Immortalization of mouse germ line stem cells

In the mammalian testis, the germ line stem cells are a small subpopulation of type A spermatogonia that proliferate and ultimately differentiate into sperm under the control of both endocrine and paracrine factors. To study the early phases of spermatogenesis at the molecular level, an in vitro system must be devised whereby germ line stem cells can be either cultured for a prolonged period of time or expanded as cell lines. In the study reported here, we chose to immortalize type A spermatogonia using the Simian virus large T-antigen gene (LTAg) under the control of an ecdysone-inducible promoter. While the cells escaped the hormonal control after a finite number of generations and expressed the LTAg constitutively, their growth remained slow and the cells exhibited morphological features typical of spermatogonia at the light microscopic level. Moreover, the cells expressed detectable levels of protein markers specific for germ cells such as Dazl, and specific for germ line stem cells such as Oct-4, a transcription factor, and GFRalpha-1, the receptor for glial cell line-derived neurotrophic factor (GDNF). Further analysis confirmed the spermatogonial phenotype and also revealed the expression of markers expressed in stem cells such as Piwi12 and Prame11. Since the cells respond to GDNF by a marked increase in their rate of proliferation, this cell line represents a good in vitro model for studying aspects of mouse germ line stem cell biology.

Interaction of the conserved meiotic regulators, BOULE (BOL) and PUMILIO-2 (PUM2)

Germ cell development is complex; it encompasses specification of germ cell fate, mitotic replication of early germ cell populations, and meiotic and postmeiotic development. Meiosis alone may require several hundred genes, including homologs of the BOULE (BOL) and PUMILIO (PUM) gene families. Both BOL and PUM homologs encode germ cell specific RNA binding proteins in diverse organisms where they are required for germ cell development. Here, we demonstrate that human BOL forms homodimers and is able to interact with a PUMILIO homolog, PUM2. We mapped the domain of BOL that is required for dimerization and for interaction with PUM2. We also show that BOL and PUM2 can form a complex on a subset of PUM2 RNA targets that is distinct from targets bound by PUM2 and another deleted in azoospermia (DAZ) family member, DAZ-like (DAZL). This suggests that RNA sequences bound by PUM2 may be determined by protein interactions. This data also suggests that although the BOL, DAZ, and DAZL proteins are all members of the same gene family, they may function in distinct molecular complexes during human germ cell development.

Expression profiling of the developing testis in wild-type and Dazl knockout mice

Genetic understanding of male-factor infertility requires knowledge of gene expression patterns associated with normal germ cell differentiation. The mouse is one of the best models of mammalian fertility due to its well-characterized genetics and the existence of many infertile mutants both naturally occurring and experimentally induced. We used cDNA microarrays firstly to investigate normal gene expression in the wild-type (wt) testis and secondly to gain a better insight into the effect of the disruption of the Dazl gene on spermatogenesis. We constructed a cDNA microarray from a subtracted and normalized adult testis library and focused on six developmental time-points during the initial synchronous wave of spermatogenesis. The results suggest that in the wild-type testis, 89.5% of genes on our chip change expression dramatically during the time-course. To identify patterns in the gene-expression data, a k-means clustering algorithm and principal component analysis were used. In the Dazl knockout testes, the majority of genes remain at baseline levels of expression, because absence of Dazl has a severe effect on cell-types present in the testis. Although in the prepubescent Dazl-null mice the final point reached in germ cell development is the leptotene-zygotene stage, the microarray results suggest that lack of Dazl expression has a detectable effect on the mRNA complement of germ cells as early as day 5 when only type A spermatogonia are present. Mol. Reprod. Dev. 67: 26-54, 2004.

Derivation of embryonic germ cells and male gametes from embryonic stem cells

Egg and sperm cells (gametes) of the mouse are derived from a founder population of primordial germ cells that are set aside early in embryogenesis. Primordial germ cells arise from the proximal epiblast, a region of the early mouse embryo that also contributes to the first blood lineages of the embryonic yolk sac. Embryonic stem cells differentiate in vitro into cystic structures called embryoid bodies consisting of tissue lineages typical of the early mouse embryo. Because embryoid bodies sustain blood development, we reasoned that they might also support primordial germ cell formation. Here we isolate primordial germ cells from embryoid bodies, and derive continuously growing lines of embryonic germ cells. Embryonic germ cells show erasure of the methylation markers (imprints) of the Igf2r and H19 genes, a property characteristic of the germ lineage. We show that embryoid bodies support maturation of the primordial germ cells into haploid male gametes, which when injected into oocytes restore the somatic diploid chromosome complement and develop into blastocysts. Our ability to derive germ cells from embryonic stem cells provides an accessible in vitro model system for studies of germline epigenetic modification and mammalian gametogenesis.

Evolution of predetermined germ cells in vertebrate embryos: implications for macroevolution

The germ line is established in animal embryos with the formation of primordial germ cells (PGCs), which give rise to gametes. Therefore, the need to form PGCs can act as a developmental constraint by inhibiting the evolution of embryonic patterning mechanisms that compromise their development. Conversely, events that stabilize the PGCs may liberate these constraints. Two modes of germ cell determination exist in animal embryos: (a) either PGCs are predetermined by the inheritance of germ cell determinants (germ plasm) or (b) PGCs are formed by inducing signals secreted by embryonic tissues (i.e., regulative determination). Surprisingly, among the major extant amphibian lineages, one mechanism is found in urodeles and the other in anurans. In anuran amphibians PGCs are predetermined by germ plasm; in urodele amphibians PGCs are formed by inducing signals. To determine which mechanism is ancestral to the tetrapod lineage and to understand the pattern of inheritance in higher vertebrates, we used a phylogenetic approach to analyze basic morphological processes in both groups and correlated these with mechanisms of germ cell determination. Our results indicate that regulative germ cell determination is a property of embryos retaining ancestral embryological processes, whereas predetermined germ cells are found in embryos with derived morphological traits. These correlations suggest that regulative germ cell formation is an important developmental constraint in vertebrate embryos, acting before the highly conserved pharyngula stage. Moreover, our analysis suggests that germ plasm has evolved independently in several lineages of vertebrate embryos.

Spermatogenetic expression of RNA-binding motif protein 7, a protein that interacts with splicing factors

We have previously shown that a ubiquitously expressed RNA splicing factor, RNA-binding motif 7 (RBM7), cloned from a testis complementary DNA library, enhances messenger RNA (mRNA) splicing in vitro and is expressed in a cell-restricted fashion. Herein, we detail its mRNA and protein expression in the rodent testis. RNA in situ hybridization shows that Rbm7 expression in rat germ cells closely parallels the entry and progression of meiosis. The expression commences in type B spermatogonia, it rises during the preleptotene stage, peaks in leptotene spermatocytes, and declines afterward, but increases again in stage-associated pachytene spermatocytes. An affinity-purified polyclonal antibody raised against a peptide corresponding to amino acids 202-224 of the mouse RBM7 recognized the predicted 35 kd protein both in testicular lysates and in in vitro translation reactions. Consistent with the in situ hybridization results, RBM7 immunoreactivity was also detected in type B spermatogonia, spanned the entire period of spermatocyte development, and extended to round and early elongated spermatids. Moreover, RBM7 appeared nuclear up to the mid pachytene stage and became cytoplasmic thereafter. Consistent with its role in RNA splicing, yeast 2-hybrid and glutathione S-transferase pull-down assays show that RBM7 interacts with splicing factor 3b subunit 2 (SAP145), and with the splicing regulator, SRp20. These interactions and the nuclear localization of RBM7 provide insights into its function in pre-mRNA processing in developing spermatocytes during entry into meiosis and progression through the meiotic prophase.

Human Pumilio-2 is expressed in embryonic stem cells and germ cells and interacts with DAZ (Deleted in AZoospermia) and DAZ-like proteins

Early in development, a part of the embryo is set aside to become the germ cell lineage that will ultimately differentiate to form sperm and eggs and transmit genetic information to the next generation. Men with deletions encompassing the Y-chromosome DAZ genes have few or no germ cells but are otherwise healthy, indicating they harbor specific defects in formation or maintenance of germ cells. A DAZ homolog, DAZL (DAZ-Like), is found in diverse organisms, including humans and is required for germ cell development in males and/or females. We identified proteins that interact with DAZ proteins to better understand their function in human germ cells. Here, we show that PUM2, a human homolog of Pumilio, a protein required to maintain germ line stem cells in Drosophila and Caenorhabditis elegans, forms a stable complex with DAZ through the same functional domain required for RNA binding, protein-protein interactions and rescue of Pumilio mutations in flies. We also show that PUM2 is expressed predominantly in human embryonic stem cells and germ cells and colocalizes with DAZ and DAZL in germ cells. These data implicate PUM2 as a component of conserved cellular machinery that may be required for germ cell development.

Production of transgenic rats by lentiviral transduction of male germ-line stem cells

Primary cultures of rat spermatogenic cells that did not bind to collagen matrices were able to colonize and form mature spermatozoa when transferred to testes of recipient males. Up to 73% of the progeny from matings with recipient males were derived from the transferred spermatogenic cells. Subsequently, two populations of germ cells were obtained by selection on laminin matrices. Both populations expressed the spermatogenic cell marker, DAZL, but not the somatic cell marker, vimentin. The cells that bound to laminin represented approximately 5% of the total population and were greatly enriched in ability to colonize a recipient testis, suggesting an enrichment in germ-line stem cells. The colonization potential was maintained for at least 7 days in culture. These cells were subsequently transduced with a lentiviral enhanced GFP reporter vector and then transferred to WT recipient males. After mating, 26 of 44 pups were derived from the cultured donor germ cells, and 13 pups carried the lentiviral transgene. Based on Southern analysis, the transgene was integrated at a different genetic locus in each animal and was transmitted to approximately 50% of pups in the F(2) generation. Thus, by using these procedures, approximately 30% of pups in the F(1) generation inherited and stably transmitted a lentiviral transgene that integrated at various genomic sites.

Y chromosome genes and male infertility

Normal spermatogenesis is a complex process that depends on many factors. Genetics plays a major role in many of these factors including providing a normal hormonal milieu, the development of the testis and ductal system, and control of the stepwise maturation of sperm in the testis. The Y chromosome plays a key role in testis determination and control of spermatogenesis. Understanding how these genes work together can elucidate of the exact cause of infertility in some patients once thought to have idiopathic infertility. It is not only important that patients understand the cause of their infertility. Using sperm from these men to attain pregnancies by assisted reproductive techniques will probably result in infertile male offspring. Additional consequences are currently unknown but are the topic of research investigations.

Y chromosome microdeletions and male infertility

Classical cytogenetic mapping has identified a locus on the long arm of the human Y chromosome that is required for spermatogenesis and is termed AZF, an acronym for the hypothetical azoospermia factor encoded by this locus. Recent molecular attempts to identify the gene corresponding to this locus have revealed that there are at least three genes in three separate microdeletion intervals. Two of these microdeletion intervals contain genes encoding proteins with potential roles in RNA metabolism. These genes are members of Y-encoded gene families with autosomal homologues. The cell biology of one of these genes, RBM (an acronym of RNA binding motif), is complex and suggests a role in pre-mRNA splicing.

Male infertility, genetic analysis of the DAZ genes on the human Y chromosome and genetic analysis of DNA repair

Many genes that are required for fertility have been identified in model organisms (). Mutations in these genes cause infertility due to defects in development of the germ cell lineage, but the organism is otherwise healthy. Although human reproduction is undoubtedly as complex as that of other organisms, very few fertility loci have been mapped (). This is in spite of the prevalence of human infertility, the lack of effective treatments to remedy germ cell defects, and the cost to couples and society of assisted reproductive techniques. Fifteen percent of couples are infertile and half of all cases can be traced to the male partner. Aside from defects in sperm production, most infertile men are otherwise healthy. This review is divided into two distinct parts to discuss work that: (i) led to the identification of several genes on the Y chromosome that likely function in sperm production; and (ii) implicates DNA repair in male infertility via increased frequency of mutations in DNA from men with meiotic arrest.

Molecular insights into the causes of male infertility

Infertility is a reproductive health problem that affects many couples in the human population. About 13-18% of couple suffers from it and approximately one-half of all cases can be traced to either partner. Regardless of whether it is primary or secondary infertility, affected couples suffer from enormous emotional and psychological trauma and it can constitute a major life crisis in the social context. Many cases of idiopathic infertility have a genetic or molecular basis. The knowledge of the molecular genetics of male infertility is developing rapidly, new "spermatogenic genes" are being discovered and molecular diagnostic approaches (DNA chips) established. This will immensely help diagnostic and therapeutic approaches to alleviate human infertility. The present review provides an overview of the causes of human infertility, particularly the molecular basis of male infertility and its implications for clinical practice.

Nature of the spermatogenic arrest in Dazl -/- mice

Dazl encodes an RNA-binding protein essential for spermatogenesis. Mice that are deficient for Dazl are infertile, lacking any formation of spermatozoa, and the only germ cells present are spermatogonia and a few spermatocytes. To gain more insight regarding the timing of the spermatogenic arrest in Dazl -/- mice, we studied the spermatogonial cell types present in testis sections and in seminiferous tubular whole mounts. Most of the seminiferous tubular cross-sections contained A spermatogonia as the most advanced cell type, with only very few containing cells up to pachytene spermatocytes. Both 5-bromodeoxy-uridine incorporation and mitotic index indicated that the remaining A spermatogonia were actively proliferating. C-kit immunohistochemical studies showed that most of the A spermatogonia were positively stained for the c-Kit protein ( approximately 80%). The clonal composition of the A spermatogonia in tubular whole mounts indicated these cells to be A(single) (A(s)), A(paired) (A(pr)), and A(aligned) (A(al)) spermatogonia. It is concluded that the prime spermatogenic defect in the Dazl -/- mice is a failure of the great majority of the A(al) spermatogonia to differentiate into A(1) spermatogonia. As a result, most seminiferous tubules of Dazl -/- mice only contain actively proliferating A(s), A(pr), and A(al) spermatogonia, with cell production being equaled by apoptosis of these cells.

A gene family required for human germ cell development evolved from an ancient meiotic gene conserved in metazoans

The Deleted in AZoospermia (DAZ) genes encode potential RNA-binding proteins that are expressed exclusively in prenatal and postnatal germ cells and are strong candidates for human fertility factors. Here we report the identification of an additional member of the DAZ gene family, which we have called BOULE. With the identification of this gene, it is clear that the human DAZ gene family contains at least three members: DAZ, a Y-chromosome gene cluster that arose 30-40 million years ago and whose deletion is linked to infertility in men; DAZL, the "father" of DAZ, a gene that maps to human chromosome 3 and has homologs required for both female and male germ cell development in other organisms; and BOULE, a gene that we propose is the "grandfather" of DAZ and maps to human chromosome 2. Human and mouse BOULE resemble the invertebrate meiotic regulator Boule, the proposed ortholog of DAZ, in sequence and expression pattern and hence likely perform a similar meiotic function. In contrast, the previously identified human DAZ and DAZL are expressed much earlier than BOULE in prenatal germ stem cells and spermatogonia; DAZL also is expressed in female germ cells. These data suggest that homologs of the DAZ gene family can be grouped into two subfamilies (BOULE and DAZL) and that members of the DAZ family evolved from an ancestral meiotic regulator, Boule, to assume distinct, yet overlapping, functions in germ cell development.

Y chromosome microdeletions and alterations of spermatogenesis

Three different spermatogenesis loci have been mapped on the Y chromosome and named "azoospermia factors" (AZFa, b, and c). Deletions in these regions remove one or more of the candidate genes (DAZ, RBMY, USP9Y, and DBY) and cause severe testiculopathy leading to male infertility. We have reviewed the literature and the most recent advances in Y chromosome mapping, focusing our attention on the correlation between Y chromosome microdeletions and alterations of spermatogenesis. More than 4,800 infertile patients were screened for Y microdeletions and published. Such deletions determine azoospermia more frequently than severe oligozoospermia and involve especially the AZFc region including the DAZ gene family. Overall, the prevalence of Y chromosome microdeletions is 4% in oligozoospermic patients, 14% in idiopathic severely oligozoospermic men, 11% in azoospermic men, and 18% in idiopathic azoospermic subjects. Patient selection criteria appear to substantially influence the prevalence of microdeletions. No clear correlation exists between the size and localization of the deletions and the testicular phenotype. However, it is clear that larger deletions are associated with the most severe testicular damage. Patients with Y chromosome deletions frequently have sperm either in the ejaculate or within the testis and are therefore suitable candidates for assisted reproduction techniques. This possibility raises a number of medical and ethical concerns, since the use of spermatozoa carrying Y chromosome deletions may produce pregnancies, but in such cases the genetic anomaly will invariably be passed on to male offspring.

An abundance of X-linked genes expressed in spermatogonia

Spermatogonia are the self-renewing, mitotic germ cells of the testis from which sperm arise by means of the differentiation pathway known as spermatogenesis. By contrast with hematopoietic and other mammalian stem-cell populations, which have been subjects of intense molecular genetic investigation, spermatogonia have remained largely unexplored at the molecular level. Here we describe a systematic search for genes expressed in mouse spermatogonia, but not in somatic tissues. We identified 25 genes (19 of which are novel) that are expressed in only male germ cells. Of the 25 genes, 3 are Y-linked and 10 are X-linked. If these genes had been distributed randomly in the genome, one would have expected zero to two of the genes to be X-linked. Our findings indicate that the X chromosome has a predominant role in pre-meiotic stages of mammalian spermatogenesis. We hypothesize that the X chromosome acquired this prominent role in male germ-cell development as it evolved from an ordinary, unspecialized autosome.

Gonadal development in mammals at the cellular and molecular levels

In mammals, although sex is determined chromosomally, gonads in both sexes begin development as similar structures. Until recently it was widely held that female development constituted a "default" pathway of development, which would occur in the absence of a testis-determining gene. This master gene on the Y chromosome, SRY in the human and Sry in the mouse, is thought to act in a cell-autonomous fashion to determine that cells in the gonadal somatic population develop as pre-Sertoli cells. Triggering of somatic cell differentiation along the Sertoli cell pathway is therefore a key event; it was thought that further steps in gonadal differentiation would follow in a developmental cascade. In the absence of Sertoli cells, the lack of anti-Mullerian hormone would allow development of the female Mullerian duct and absence of Leydig cells would prevent maintenance of the Wolffian duct. Recent findings that female signals not only maintain the Mullerian duct and repress the Wolffian duct but also suppress the development of Leydig cells and maintain meiotic germ cells, together with the finding that an X-linked gene is required for ovarian development and must be silenced in the male, have shown that the female default pathway model is an oversimplification. Morphological steps in gonadal differentiation can be correlated with emerging evidence of molecular mechanisms; growth factors, cell adhesion, and signaling molecules interact together, often acting within short time windows via reciprocal control relationships.

The role of human and mouse Y chromosome genes in male infertility

It was suggested by Ronald Fisher in 1931 that genes involved in benefit to the male (including spermatogenesis genes) would accumulate on the Y chromosome. The analysis of mouse Y chromosome deletions and the discovery of microdeletions of the human Y chromosome associated with diverse defective spermatogenic phenotypes has revealed the presence of intervals containing one or more genes controlling male germ cell differentiation. These intervals have been mapped, cloned and examined in detail for functional genes. This review discusses the genes mapping to critical spermatogenesis intervals and the evidence indicating which are the most likely candidates underlying Y-linked male infertility.