Sox3 expression in undifferentiated spermatogonia is required for the progression of spermatogenesis

The Role of Retinoic Acid (RA) in Spermatogonial Differentiation

Retinoic acid (RA) directs the sequential, but distinct, programs of spermatogonial differentiation and meiotic differentiation that are both essential for the generation of functional spermatozoa. These processes are functionally and temporally decoupled, as they occur in distinct cell types that arise over a week apart, both in the neonatal and adult testis. However, our understanding is limited in terms of what cellular and molecular changes occur downstream of RA exposure that prepare differentiating spermatogonia for meiotic initiation. In this review, we describe the process of spermatogonial differentiation and summarize the current state of knowledge regarding RA signaling in spermatogonia.

VEGFA splicing: divergent isoforms regulate spermatogonial stem cell maintenance

Despite being well-known for regulating angiogenesis in both normal and tumorigenic environments, vascular endothelial growth factor A (VEGFA) has been recently implicated in male fertility, namely in the maintenance of spermatogonial stem cells (SSC). The VEGFA gene can be spliced into multiple distinct isoforms that are either angiogenic or antiangiogenic in nature. Although studies have demonstrated the alternative splicing of VEGFA, including the divergent roles of the two isoform family types, many investigations do not differentiate between them. Data concerning VEGFA in the mammalian testis are limited, but the various angiogenic isoforms appear to promote seminiferous cord formation and to form a gradient across which cells may migrate. Treatment with either antiangiogenic isoforms of VEGFA or with inhibitors to angiogenic signaling impair these processes. Serendipitously, expression of KDR, the primary receptor for both types of VEGFA isoforms, was observed on male germ cells. These findings led to further investigation of the way that VEGFA elicits avascular functions within testes. Following treatment of donor perinatal male mice with either antiangiogenic VEGFA165b or angiogenic VEGFA164 isoforms, seminiferous tubules were less colonized following transplantation with cells from VEGFA165b-treated donors. Thus, VEGFA165b and possibly other antiangiogenic isoforms of VEGFA reduce SSC number either by promoting premature differentiation, inducing cell death, or by preventing SSC formation. Thus, angiogenic isoforms of VEGFA are hypothesized to promote SSC self-renewal, and the divergent isoforms are thought to balance one another to maintain SSC homeostasis in vivo.

Induction of Pluripotency in Astrocytes through a Neural Stem Cell-like State

It remains controversial whether the routes from somatic cells to induced pluripotent stem cells (iPSCs) are related to the reverse order of normal developmental processes. Specifically, it remains unaddressed whether or not the differentiated cells become iPSCs through their original tissue stem cell-like state. Previous studies analyzing the reprogramming process mostly used fibroblasts; however, the stem cell characteristics of fibroblasts made it difficult to address this. Here, we generated iPSCs from mouse astrocytes, a type of glial cells, by three (OCT3/4, KLF4, and SOX2), two (OCT3/4 and KLF4), or four (OCT3/4, KLF4, and SOX2 plus c-MYC) factors. Sox1, a neural stem cell (NSC)-specific transcription factor, is transiently up-regulated during reprogramming, and Sox1-positive cells become iPSCs. The up-regulation of Sox1 is essential for OCT3/4- and KLF4-induced reprogramming. Genome-wide analysis revealed that the gene expression profile of Sox1-expressing intermediate-state cells resembles that of NSCs. Furthermore, the intermediate-state cells are able to generate neurospheres, which can differentiate into both neurons and glial cells. Remarkably, during fibroblast reprogramming, neither Sox1 up-regulation nor an increase in neurogenic potential occurs. Our results thus demonstrate that astrocytes are reprogrammed through an NSC-like state.

DNA methylation and gene expression dynamics during spermatogonial stem cell differentiation in the early postnatal mouse testis

Background

In the male germline, neonatal prospermatogonia give rise to spermatogonia, which include stem cell population (undifferentiated spermatogonia) that supports continuous spermatogenesis in adults. Although the levels of DNA methyltransferases change dynamically in the neonatal and early postnatal male germ cells, detailed genome-wide DNA methylation profiles of these cells during the stem cell formation and differentiation have not been reported.

Results

To understand the regulation of spermatogonial stem cell formation and differentiation, we examined the DNA methylation and gene expression dynamics of male mouse germ cells at the critical stages: neonatal prospermatogonia, and early postntal (day 7) undifferentiated and differentiating spermatogonia. We found large partially methylated domains similar to those found in cancer cells and placenta in all these germ cells, and high levels of non-CG methylation and 5-hydroxymethylcytosines in neonatal prospermatogonia. Although the global CG methylation levels were stable in early postnatal male germ cells, and despite the reported scarcity of differential methylation in the adult spermatogonial stem cells, we identified many regions showing stage-specific differential methylation in and around genes important for stem cell function and spermatogenesis. These regions contained binding sites for specific transcription factors including the SOX family members.

Conclusions

Our findings show a distinctive and dynamic regulation of DNA methylation during spermatogonial stem cell formation and differentiation in the neonatal and early postnatal testes. Furthermore, we revealed a unique accumulation and distribution of non-CG methylation and 5hmC marks in neonatal prospermatogonia. These findings contrast with the reported scarcity of differential methylation in adult spermatogonial stem cell differentiation and represent a unique phase of male germ cell development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1833-5) contains supplementary material, which is available to authorized users.

Comparative transcriptomics in East African cichlids reveals sex- and species-specific expression and new candidates for sex differentiation in fishes

Males and females of the same species differ largely in gene expression, which accounts for most of the morphological and physiological differences and sex-specific phenotypes. Here, we analyzed sex-specific gene expression in the brain and the gonads of cichlid fishes from Lake Tanganyika belonging to four different lineages, so-called tribes (Eretmodini, Ectodini, Haplochromini, and Lamprologini), using the outgroup Nile tilapia (Oreochromis niloticus) as reference. The comparison between male and female brains revealed few differences between the sexes, consistent in all investigated species. The gonads, on the other hand, showed a large fraction of differentially expressed transcripts with the majority of them showing the same direction of expression in all four species. All here-studied cichlids, especially the three investigated mouth-breeding species, showed a trend toward more male- than female biased transcripts. Transcripts, which were female-biased in expression in all four species, were overrepresented on linkage group (LG)1 in the reference genome and common male-biased transcripts showed accumulation on LG23, the presumable sex chromosomes of the Nile tilapia. Sex-specific transcripts contained candidate genes for sex determination and differentiation in fishes,especially members of the transforming growth factor-b-superfamily and the Wnt-pathway and also prominent members of the sox-, dm-domain-, and high mobility group-box families. We further confirmed our previous finding on species/lineage-specific gene expression shifts in the sex steroid pathway, including synthesizing enzymes as the aromatase cyp19a1 and estrogen and androgen receptors.

Sequencing and De Novo Assembly of the Gonadal Transcriptome of the Endangered Chinese Sturgeon (Acipenser sinensis)

Background

The Chinese sturgeon (Acipenser sinensis) is endangered through anthropogenic activities including over-fishing, damming, shipping, and pollution. Controlled reproduction has been adopted and successfully conducted for conservation. However, little information is available on the reproductive regulation of the species. In this study, we conducted de novo transcriptome assembly of the gonad tissue to create a comprehensive dataset for A. sinensis.

Results

The Illumina sequencing platform was adopted to obtain 47,333,701 and 47,229,705 high quality reads from testis and ovary cDNA libraries generated from three-year-old A. sinensis. We identified 86,027 unigenes of which 30,268 were annotated in the NCBI non-redundant protein database and 28,281 were annotated in the Swiss-prot database. Among the annotated unigenes, 26,152 and 7,734 unigenes, respectively, were assigned to gene ontology categories and clusters of orthologous groups. In addition, 12,557 unigenes were mapped to 231 pathways in the Kyoto Encyclopedia of Genes and Genomes Pathway database. A total of 1,896 unigenes, potentially differentially expressed between the two gonad types, were found, with 1,894 predicted to be up-regulated in ovary and only two in testis. Fifty-five potential gametogenesis-related genes were screened in the transcriptome and 34 genes with significant matches were found. Besides, more paralogs of 11 genes in three gene families (sox, apolipoprotein and cyclin) were found in A. sinensis compared to their orthologs in the diploid Danio rerio. In addition, 12,151 putative simple sequence repeats (SSRs) were detected.

Conclusions

This study provides the first de novo transcriptome analysis currently available for A. sinensis. The transcriptomic data represents the fundamental resource for future research on the mechanism of early gametogenesis in sturgeons. The SSRs identified in this work will be valuable for assessment of genetic diversity of wild fish and genealogy management of cultured fish.

Hierarchical differentiation competence in response to retinoic acid ensures stem cell maintenance during mouse spermatogenesis

Stem cells ensure tissue homeostasis through the production of differentiating and self-renewing progeny. In some tissues, this is achieved by the function of a definitive stem cell niche. However, the mechanisms that operate in mouse spermatogenesis are unknown because undifferentiated spermatogonia (Aundiff) are motile and intermingle with differentiating cells in an 'open' niche environment of seminiferous tubules. Aundiff include glial cell line-derived neurotrophic factor receptor α1 (GFRα1)(+) and neurogenin 3 (NGN3)(+) subpopulations, both of which retain the ability to self-renew. However, whereas GFRα1(+) cells comprise the homeostatic stem cell pool, NGN3(+) cells show a higher probability to differentiate into KIT(+) spermatogonia by as yet unknown mechanisms. In the present study, by combining fate analysis of pulse-labeled cells and a model of vitamin A deficiency, we demonstrate that retinoic acid (RA), which may periodically increase in concentration in the tubules during the seminiferous epithelial cycle, induced only NGN3(+) cells to differentiate. Comparison of gene expression revealed that retinoic acid receptor γ (Rarg) was predominantly expressed in NGN3(+) cells, but not in GFRα1(+) cells, whereas the expression levels of many other RA response-related genes were similar in the two populations. Ectopic expression of RARγ was sufficient to induce GFRα1(+) cells to directly differentiate to KIT(+) cells without transiting the NGN3(+) state. Therefore, RARγ plays key roles in the differentiation competence of NGN3(+) cells. We propose a novel mechanism of stem cell fate selection in an open niche environment whereby undifferentiated cells show heterogeneous competence to differentiate in response to ubiquitously distributed differentiation-inducing signals.

A novel transcriptional factor Nkapl is a germ cell-specific suppressor of Notch signaling and is indispensable for spermatogenesis

Spermatogenesis is an elaborately regulated system dedicated to the continuous production of spermatozoa via the genesis of spermatogonia. In this process, a variety of genes are expressed that are relevant to the differentiation of germ cells at each stage. Although Notch signaling plays a critical role in germ cell development in Drosophila and Caenorhabditis elegans, its function and importance for spermatogenesis in mammals is controversial. We report that Nkapl is a novel germ cell-specific transcriptional suppressor in Notch signaling. It is also associated with several molecules of the Notch corepressor complex such as CIR, HDAC3, and CSL. It was expressed robustly in spermatogonia and early spermatocytes after the age of 3 weeks. Nkapl-deleted mice showed complete arrest at the level of pachytene spermatocytes. In addition, apoptosis was observed in this cell type. Overexpression of NKAPL in germline stem cells demonstrated that Nkapl induced changes in spermatogonial stem cell (SSC) markers and the reduction of differentiation factors through the Notch signaling pathway, whereas testes with Nkapl deleted showed inverse changes in those markers and factors. Therefore, Nkapl is indispensable because aberrantly elevated Notch signaling has negative effects on spermatogenesis, affecting SSC maintenance and differentiation factors. Notch signaling should be properly regulated through the transcriptional factor Nkapl.

Identification of side population cells in chicken embryonic gonads

The side population (SP) phenotype, defined by the ability of a cell to efflux fluorescent dyes such as Hoechst, is common to several stem/progenitor cell types. In avian species, SP phenotype has been identified in pubertal and adult testes, but nothing is known about its expression during prenatal development of a male gonad. In this study, we characterized the Hoechst SP phenotype via the cytofluorimetric analysis of disaggregated testes on different days of chicken embryonic development. Male prenatal gonads contained a fraction of SP cells at each stage analyzed. At least two main SP fractions, named P3 and P4, were identified. The percentage of P3 fraction decreased as development proceeds, whereas P4 cell number was not affected by gonad growth. Functional inhibition of BCRP1 channel membrane using Verapamil and/or Ko143 showed that P3, but not P4 phenotype, was dependent on BCRP1 activity. Molecular analysis of both P3- and P4-sorted fractions revealed a differential RNA expression pattern, indicating that P3 cells mainly contained germinal stem cell markers, whereas P4 was preferentially composed of both Sertoli and Leydig cell progenitor markers. Finally, these findings provided evidence that the SP phenotype is a common feature of both germ and somatic cells detected in chicken developing testis.

Expression analysis of sox3 during testicular development, recrudescence, and after hCG induction in catfish, Clarias batrachus

In teleosts, the expression of steroidogenic enzymes and related transcription factor genes occurs in a stage- and tissue-specific manner causing sexual development. The role of sox3, an evolutionary ancestor of SRY, has not been studied in detail. Therefore, the full-length cDNA of sox3 (1,197 kb) was cloned from catfish testis, and mRNA expression was analyzed during gonadal development, during the seasonal reproductive cycle, and after human chorionic gonadotropin (hCG) induction. Tissue distribution analysis showed that sox3 expression was higher in testis, ovary, and brain compared to other tissues analyzed. Developing and mature testis showed higher sox3 expression than ovary of corresponding stages, and more sox3 transcripts were found during the spawning phase of the seasonal reproductive cycle. Expression of sox3 was upregulated by hCG after in vivo and in vitro induction, suggesting that gonadotropins might stimulate it. In situ hybridization and immunohistochemistry showed the presence of sox3 mRNA and protein in somatic and interstitial cell layers of the testis. Sox3 could also be found in the zona radiata of developing and mature oocytes. Exposure of methyltestosterone (1 µg/l) and ethinylestradiol (1 µg/l) for 21 days during testicular development showed lower sox3 expression levels in the testis and brain, indicating a certain feedback intervention. These results suggest a possible role for Sox3 in the regulation of testicular development and function.

The mammalian Doublesex homolog DMRT6 coordinates the transition between mitotic and meiotic developmental programs during spermatogenesis

In mammals, a key transition in spermatogenesis is the exit from spermatogonial differentiation and mitotic proliferation and the entry into spermatocyte differentiation and meiosis. Although several genes that regulate this transition have been identified, how it is controlled and coordinated remains poorly understood. Here, we examine the role in male gametogenesis of the Doublesex-related gene Dmrt6 (Dmrtb1) in mice and find that Dmrt6 plays a crucial role in directing germ cells through the mitotic-to-meiotic germ cell transition. DMRT6 protein is expressed in late mitotic spermatogonia. In mice of the C57BL/6J strain, a null mutation in Dmrt6 disrupts spermatogonial differentiation, causing inappropriate expression of spermatogonial differentiation factors, including SOHLH1, SOHLH2 and DMRT1 as well as the meiotic initiation factor STRA8, and causing most late spermatogonia to undergo apoptosis. In mice of the 129Sv background, most Dmrt6 mutant germ cells can complete spermatogonial differentiation and enter meiosis, but they show defects in meiotic chromosome pairing, establishment of the XY body and processing of recombination foci, and they mainly arrest in mid-pachynema. mRNA profiling of Dmrt6 mutant testes together with DMRT6 chromatin immunoprecipitation sequencing suggest that DMRT6 represses genes involved in spermatogonial differentiation and activates genes required for meiotic prophase. Our results indicate that Dmrt6 plays a key role in coordinating the transition in gametogenic programs from spermatogonial differentiation and mitosis to spermatocyte development and meiosis.

Establishment of a proteome profile and identification of molecular markers for mouse spermatogonial stem cells

Spermatogonial stem cells (SSCs) are undifferentiated cells that are required to maintain spermatogenesis throughout the reproductive life of mammals. Although SSC transplantation and culture provide a powerful tool to identify the mechanisms regulating SSC function, the precise signalling mechanisms governing SSC self-renewal and specific surface markers for purifying SSCs remain to be clearly determined. In the present study, we established a steady SSC culture according to the method described by Shinohara's lab. Fertile progeny was produced after transplantation of cultured SSCs into infertile mouse testis, and the red fluorescence exhibited by the culture cell membranes was stably and continuously transmitted to the offspring. Next, via advanced mass spectrometry and an optimized proteomics platform, we constructed the proteome profile, with 682 proteins expressed in SSCs. Furthermore bioinformatics analysis showed that the list contained several known molecules that are regulated in SSCs. Several nucleoproteins and membrane proteins were chosen for further exploration using immunofluorescence and RT-PCR. The results showed that SALL1, EZH2, and RCOR2 are possibly involved in the self-renewal mechanism of SSCs. Furthermore, the results of tissue-specific expression analysis showed that Gpat2 and Pld6 were uniquely and highly expressed in mouse testes and cultured SSCs. The cellular localization of PLD6 was further explored and the results showed it was primarily expressed in the spermatogonial membrane of mouse testes and cultured SSCs. The proteins identified in this study form the basis for further exploring the molecular mechanism of self-renewal in SSCs and for identifying specific surface markers of SSCs.

Fluorescence- and magnetic-activated cell sorting strategies to isolate and enrich human spermatogonial stem cells

OBJECTIVE

To determine the molecular characteristics of human spermatogonia and optimize methods to enrich spermatogonial stem cells (SSCs).

DESIGN

Laboratory study using human tissues.

SETTING

Research institute.

PATIENT(S)

Healthy adult human testicular tissue.

INTERVENTION(S)

Human testicular tissue was fixed or digested with enzymes to produce a cell suspension. Human testis cells were fractionated by fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS).

MAIN OUTCOME MEASURE(S)

Immunostaining for selected markers, human-to-nude mouse xenotransplantation assay.

RESULT(S)

Immunohistochemistry costaining revealed the relative expression patterns of SALL4, UTF1, ZBTB16, UCHL1, and ENO2 in human undifferentiated spermatogonia as well as the extent of overlap with the differentiation marker KIT. Whole mount analyses revealed that human undifferentiated spermatogonia (UCHL1+) were typically arranged in clones of one to four cells whereas differentiated spermatogonia (KIT+) were typically arranged in clones of eight or more cells. The ratio of undifferentiated-to-differentiated spermatogonia is greater in humans than in rodents. The SSC colonizing activity was enriched in the THY1dim and ITGA6+ fractions of human testes sorted by FACS. ITGA6 was effective for sorting human SSCs by MACS; THY1 and EPCAM were not.

CONCLUSION(S)

Human spermatogonial differentiation correlates with increased clone size and onset of KIT expression, similar to rodents. The undifferentiated-to-differentiated developmental dynamics in human spermatogonia is different than rodents. THY1, ITGA6, and EPCAM can be used to enrich human SSC colonizing activity by FACS, but only ITGA6 is amenable to high throughput sorting by MACS.

Elucidation of distinctive genomic DNA structures in patients with 46,XX testicular disorders of sex development using genome wide analyses

PURPOSE

Although several genes, including the SRY gene, are involved in testicular differentiation, the entire mechanism of this differentiation remains unclear. We performed genome wide analysis in patients with 46,XX testicular disorders of sex development to comprehensively elucidate the mechanisms of testicular differentiation.

MATERIALS AND METHODS

Whole genomic DNA was extracted from the peripheral blood of 4 patients with 46,XX testicular disorders of sex development who were SRY negative. Genomic DNA was hybridized to a GeneChip® human mapping 250K array set. Compared to normal female data, we detected common loss of heterozygosity and copy number variation regions in 4 patients using Genotyping Console™ software.

RESULTS

Loss of heterozygosity was detected in 19 regions of 11 chromosomes. A total of 27 genes or nearby genomic areas were included in the applicable regions. Copy number loss was recognized in 13 regions of 10 chromosomes, and these regions included 55 genes. Copy number gain was detected in 6 regions of 4 chromosomes, which included the upstream region of the SOX3 gene.

CONCLUSIONS

The regions with loss of heterozygosity did not contain genes associated with testicular differentiation. However, the upstream area of the SOX3 gene, which is located in Xq27.1, was included in the region of copy number gain. These results suggest that high expression of the SOX3 gene led to testicular differentiation despite SRY gene loss. As this applicable area is not within a coding region, genome wide analyses were valuable for detecting the novel regions associated with testicular differentiation.

Transcriptional control of spermatogonial maintenance and differentiation

Spermatogenesis is a multistep process that generates millions of spermatozoa per day in mammals. A key to this process is the spermatogonial stem cell (SSC), which has the dual property of continually renewing and undergoing differentiation into a spermatogonial progenitor that expands and further differentiates. In this review, we will focus on how these proliferative and early differentiation steps in mammalian male germ cells are controlled by transcription factors. Most of the transcription factors that have so far been identified as promoting SSC self-renewal (BCL6B, BRACHYURY, ETV5, ID4, LHX1, and POU3F1) are upregulated by glial cell line-derived neurotrophic factor (GDNF). Since GDNF is crucial for promoting SSC self-renewal, this suggests that these transcription factors are responsible for coordinating the action of GDNF in SSCs. Other transcription factors that promote SSC self-renewal are expressed independently of GDNF (FOXO1, PLZF, POU5F1, and TAF4B) and thus may act in non-GDNF pathways to promote SSC cell growth or survival. Several transcription factors have been identified that promote spermatogonial differentiation (DMRT1, NGN3, SOHLH1, SOHLH2, SOX3, and STAT3); some of these may influence the decision of an SSC to commit to differentiate while others may promote later spermatogonial differentiation steps. Many of these transcription factors regulate each other and act on common targets, suggesting they integrate to form complex transcriptional networks in self-renewing and differentiating spermatogonia.

The sox family of transcription factors: versatile regulators of stem and progenitor cell fate

Sox family transcription factors are well-established regulators of cell fate decisions during development. Accumulating evidence documents that they play additional roles in adult tissue homeostasis and regeneration. Remarkably, forced expression of Sox factors, in combination with other synergistic factors, reprograms differentiated cells into somatic or pluripotent stem cells. Dysregulation of Sox factors has been further implicated in diseases including cancer. Here, we review molecular and functional evidence linking Sox proteins with stem cell biology, cellular reprogramming, and disease with an emphasis on Sox2.

Expression of the murine transcription factor SOX3 during embryonic and adult neurogenesis

Previous studies have shown that Sox3 is expressed in nascent neuroprogenitor cells and is functionally required in mammals for development of the dorsal telencephalon and hypothalamus. However, Sox3 expression during embryonic and adult neurogenesis has not been examined in detail. Using a SOX3-specific antibody, we show that murine SOX3 expression is maintained throughout telencephalic neurogenesis and is restricted to progenitor cells with neuroepithelial and radial glial morphologies. We also demonstrate that SOX3 is expressed within the adult neurogenic regions and is coexpressed extensively with the neural stem cell marker SOX2 indicating that it is a lifelong marker of neuroprogenitor cells. In contrast to the telencephalon, Sox3 expression within the developing hypothalamus is upregulated in developing neurons and is maintained in a subset of differentiated hypothalamic cells through to adulthood. Together, these data show that Sox3 regulation is region-specific, consistent with it playing distinct biological roles in the dorsal telencephalon and hypothalamus.

Biotechnological approaches to the treatment of aspermatogenic men

Aspermatogenesis is a severe impairment of spermatogenesis in which germ cells are completely lacking or present in an immature form, which results in sterility in approximately 25% of patients. Because assisted reproduction techniques require mature germ cells, biotechnology is a valuable tool for rescuing fertility while maintaining biological fatherhood. However, this process involves, for instance, the differentiation of preexisting immature germ cells or the production/derivation of sperm from somatic cells. This review critically addresses four potential techniques: sperm derivation in vitro, germ stem cell transplantation, xenologous systems, and haploidization. Sperm derivation in vitro is already feasible in fish and mammals through organ culture or 3D systems, and it is very useful in conditions of germ cell arrest or in type II Sertoli-cell-only syndrome. Patients afflicted by type I Sertoli-cell-only syndrome could also benefit from gamete derivation from induced pluripotent stem cells of somatic origin, and human haploid-like cells have already been obtained by using this novel methodology. In the absence of alternative strategies to generate sperm in vitro, in germ cells transplantation fertility is restored by placing donor cells in the recipient germ-cell-free seminiferous epithelium, which has proven effective in conditions of spermatogonial arrest. Grafting also provides an approach for ex-vivo generation of mature sperm, particularly using prepubertal testis tissue. Although less feasible, haploidization is an option for creating gametes based on biological cloning technology. In conclusion, the aforementioned promising techniques remain largely experimental and still require extensive research, which should address, among other concerns, ethical and biosafety issues, such as gamete epigenetic status, ploidy, and chromatin integrity.

In vitro culture and morphological characterization of prepubertal buffalo (Bubalus bubalis) putative spermatogonial stem cell

PURPOSE

Spermatogonial stem cells (SSCs) have the unique ability both to self-renew and to produce progeny that undergo differentiation to spermatozoa. The present study has been carried out to develop a method to purify and enrich the pure populations of spermatogonial stem cell like cells in buffalo.

METHODS

The spermatogonial cells were isolated from testes of 3-7 month old buffalo calves and disaggregated by double enzymatic digestion. Mixed population of isolated cells were then plated on Datura stramonium agglutinin (DSA) lectin coated dishes for attachment of Sertoli cells. The desired cells were obtained from suspension medium after 18 h of incubation and then loaded on discontinuous density gradient using percoll (20-65 %) and different types of spermatogonia cells were obtained at interface of each layer. These cells were cultured in vitro.

RESULTS

Spermatogonial cells isolated have spherical outline and two or three eccentrically placed nucleoli, created a colony after proliferation during first week or immediately after passage. After 7-10 days of culture, the resulted developed colonies of spermatogonial cells expressed the spermatogonial specific genes like Plzf and VASA; and other pluripotency related markers viz. alkaline phosphtase, DBA, CD9, CD90, SSEA-1, OCT-4, NANOG and REX-1.

CONCLUSION

Our results show that the isolated putative spermatogonial stem cells exhibit the expression of pluripotency related and spermatogonial specific genes. This study may help to establish a long term culture system for buffalo spermatogonia.

Sertoli-cell-specific knockout of connexin 43 leads to multiple alterations in testicular gene expression in prepubertal mice

A significant decline in human male reproductive function has been reported for the past 20 years but the molecular mechanisms remain poorly understood. However, recent studies showed that the gap junction protein connexin-43 (CX43; also known as GJA1) might be involved. CX43 is the predominant testicular connexin (CX) in most species, including in humans. Alterations of its expression are associated with different forms of spermatogenic disorders and infertility. Men with impaired spermatogenesis often exhibit a reduction or loss of CX43 expression in germ cells (GCs) and Sertoli cells (SCs). Adult male transgenic mice with a conditional knockout (KO) of the Gja1 gene [referred to here as connexin-43 (Cx43)] in SCs (SCCx43KO) show a comparable testicular phenotype to humans and are infertile. To detect possible signaling pathways and molecular mechanisms leading to the testicular phenotype in adult SCCx43KO mice and to their failure to initiate spermatogenesis, the testicular gene expression of 8-day-old SCCx43KO and wild-type (WT) mice was compared. Microarray analysis revealed that 658 genes were significantly regulated in testes of SCCx43KO mice. Of these genes, 135 were upregulated, whereas 523 genes were downregulated. For selected genes the results of the microarray analysis were confirmed using quantitative real-time PCR and immunostaining. The majority of the downregulated genes are GC-specific and are essential for mitotic and meiotic progression of spermatogenesis, including Stra8, Dazl and members of the DM (dsx and map-3) gene family. Other altered genes can be associated with transcription, metabolism, cell migration and cytoskeleton organization. Our data show that deletion of Cx43 in SCs leads to multiple alterations of gene expression in prepubertal mice and primarily affects GCs. The candidate genes could represent helpful markers for investigators exploring human testicular biopsies from patients showing corresponding spermatogenic deficiencies and for studying the molecular mechanisms of human male sterility.

ENU mutagenesis in mice identifies candidate genes for hypogonadism

Genome-wide mutagenesis was performed in mice to identify candidate genes for male infertility, for which the predominant causes remain idiopathic. Mice were mutagenized using N-ethyl-N-nitrosourea (ENU), bred, and screened for phenotypes associated with the male urogenital system. Fifteen heritable lines were isolated and chromosomal loci were assigned using low-density genome-wide SNP arrays. Ten of the 15 lines were pursued further using higher-resolution SNP analysis to narrow the candidate gene regions. Exon sequencing of candidate genes identified mutations in mice with cystic kidneys (Bicc1), cryptorchidism (Rxfp2), restricted germ cell deficiency (Plk4), and severe germ cell deficiency (Prdm9). In two other lines with severe hypogonadism, candidate sequencing failed to identify mutations, suggesting defects in genes with previously undocumented roles in gonadal function. These genomic intervals were sequenced in their entirety and a candidate mutation was identified in SnrpE in one of the two lines. The line harboring the SnrpE variant retains substantial spermatogenesis despite small testis size, an unusual phenotype. In addition to the reproductive defects, heritable phenotypes were observed in mice with ataxia (Myo5a), tremors (Pmp22), growth retardation (unknown gene), and hydrocephalus (unknown gene). These results demonstrate that the ENU screen is an effective tool for identifying potential causes of male infertility.

SOHLH1 and SOHLH2 coordinate spermatogonial differentiation

Spermatogonial self-renewal and differentiation are essential for male fertility and reproduction. We discovered that germ cell specific genes Sohlh1 and Sohlh2, encode basic helix-loop-helix (bHLH) transcriptional regulators that are essential in spermatogonial differentiation. Sohlh1 and Sohlh2 individual mouse knockouts show remarkably similar phenotypes. Here we show that SOHLH1 and SOHLH2 proteins are co-expressed in the entire spermatogonial population except in the GFRA1(+) spermatogonia, which includes spermatogonial stem cells (SSCs). SOHLH1 and SOHLH2 are expressed in both KIT negative and KIT positive spermatogonia, and overlap Ngn3/EGFP and SOX3 expression. SOHLH1 and SOHLH2 heterodimerize with each other in vivo, as well as homodimerize. The Sohlh1/Sohlh2 double mutant phenocopies single mutants, i.e., spermatogonia continue to proliferate but do not differentiate properly. Further analysis revealed that GFRA1(+) population was increased, while meiosis commenced prematurely in both single and double knockouts. Sohlh1 and Sohlh2 double deficiency has a synergistic effect on gene expression patterns as compared to the single knockouts. SOHLH proteins affect spermatogonial development by directly regulating Gfra1, Sox3 and Kit gene expression. SOHLH1 and SOHLH2 suppress genes involved in SSC maintenance, and induce genes important for spermatogonial differentiation.

Spermatogonial stem cells, infertility and testicular cancer

The spermatogonial stem cells (SSCs) are responsible for the transmission of genetic information from an individual to the next generation. SSCs play critical roles in understanding the basic reproductive biology of gametes and treatments of human infertility. SSCs not only maintain normal spermatogenesis, but also sustain fertility by critically balancing both SSC self-renewal and differentiation. This self-renewal and differentiation in turn is tightly regulated by a combination of intrinsic gene expression within the SSC as well as the extrinsic gene signals from the niche. Increased SSCs self-renewal at the expense of differentiation result in germ cell tumours, on the other hand, higher differentiation at the expense of self-renewal can result in male sterility. Testicular germ cell cancers are the most frequent cancers among young men in industrialized countries. However, understanding the pathogenesis of testis cancer has been difficult because it is formed during foetal development. Recent studies suggest that SSCs can be reprogrammed to become embryonic stem (ES)-like cells to acquire pluripotency. In the present review, we summarize the recent developments in SSCs biology and role of SSC in testicular cancer. We believe that studying the biology of SSCs will not only provide better understanding of stem cell regulation in the testis, but eventually will also be a novel target for male infertility and testicular cancers.

Sry, more than testis determination?

The Sry locus on the mammalian Y chromosome is the developmental switch responsible for testis determination. Inconsistent with this important function, the Sry locus is transcribed in adult males at times and in tissues not involved with testis determination. Sry is expressed in multiple tissues of the peripheral and central nervous system. Sry is derived from Sox3 and is similar to other SOXB family loci. The SOXB loci are responsible for nervous system development. Sry has been demonstrated to modulate the catecholamine pathway, so it should have functional consequences in the central and peripheral nervous system. The nervous system expression and potential function are consistent with Sry as a SOXB family member. In mammals, Sox3 is X-linked and undergoes dosage compensation in females. The expression of Sry in adult males allows for a type of sexual differentiation independent of circulating gonadal hormones. A quantitative difference in Sox3 plus Sry expression in males vs. females could drive changes in the transcriptome of these cells, differentiating male and female cells. Sry expression and its transcriptional effects should be considered when investigating sexual dimorphic phenotypes.

Sox3 functions in a cell-autonomous manner to regulate spermatogonial differentiation in mice

The X-linked Sox3 gene encodes a member of the Sry high-mobility group box proteins, which play a role in many developmental processes including neurogenesis and testis development. This study further examined the role of Sox3 in spermatogenesis. Males without Sox3 expression exhibited a similar number of germ cell nuclear antigen-positive germ cells at 1, 5, and 10 d postpartum (dpp) compared to their wild-type littermates, but there was significant germ cell depletion by 20 dpp. However, spermatogenesis later resumed and postmeiotic germ cells were observed by 56 dpp. The VasaCre transgene was used to generate a germ cell-specific deletion of Sox3. The phenotype of the germ cell-specific Sox3 knockout was similar to the ubiquitous knockout, indicating an intrinsic role for Sox3 in germ cells. The residual germ cells in 20 dpp Sox3(-/Y) males were spermatogonia as indicated by their expression of neurogenin3 but not synaptonemal complex protein 3, which is expressed within cells undergoing meiosis. RNA expression analyses corroborated the histological analyses and revealed a gradual transition from relatively increased expression of spermatogonia genes at 20 dpp to near normal expression of genes characteristic of undifferentiated and meiotic germ cells by 84 dpp. Fluorescent-activated cell sorting of undifferentiated (ret tyrosine kinase receptor positive) and differentiated (kit receptor tyrosine kinase-positive) spermatogonia revealed depletion of differentiated spermatogonia in Sox3(-/Y) tubules. These results indicate that Sox3 functions in an intrinsic manner to promote differentiation of spermatogonia in prepubertal mice but it is not required for ongoing spermatogenesis in adults. The Sox3(-/Y) males provide a unique model for studying the mechanism of germ cell differentiation in prepubertal testes.

EGR4 displays both a cell- and intracellular-specific localization pattern in the developing murine testis

Spermatogenesis is an intricately regulated process of cellular differentiation transforming spermatogonial stem cells to spermatozoa. Elimination of the transcription factor EGR4 generates subfertile male mice yet the expression and function of EGR4 in the mammalian testis has yet to be fully investigated. We performed in situ hybridization and immunofluorescence to identify Egr4 transcript and protein localization in the developing murine testis. EGR4 was detected in both germ and somatic cells in the neonatal testis but was specific to germ cells inside the seminiferous epithelium from juvenile development onward. EGR4 also displayed distinct intracellular localization patterns within specific cell populations of the testis. In addition, Egr4-deficient testis tubules regress from relatively normal to Sertoli cell and undifferentiated spermatogonia only over time. Taken together, these data suggest that Egr4 may regulate spermatogenesis at multiple steps, with roles in the dividing Sertoli cells, peritubular myoid cells, and the meiotic and elongating haploid germ cell populations.

Identification of SOX3 as an XX male sex reversal gene in mice and humans

Sex in mammals is genetically determined and is defined at the cellular level by sex chromosome complement (XY males and XX females). The Y chromosome-linked gene sex-determining region Y (SRY) is believed to be the master initiator of male sex determination in almost all eutherian and metatherian mammals, functioning to upregulate expression of its direct target gene Sry-related HMG box-containing gene 9 (SOX9). Data suggest that SRY evolved from SOX3, although there is no direct functional evidence to support this hypothesis. Indeed, loss-of-function mutations in SOX3 do not affect sex determination in mice or humans. To further investigate Sox3 function in vivo, we generated transgenic mice overexpressing Sox3. Here, we report that in one of these transgenic lines, Sox3 was ectopically expressed in the bipotential gonad and that this led to frequent complete XX male sex reversal. Further analysis indicated that Sox3 induced testis differentiation in this particular line of mice by upregulating expression of Sox9 via a similar mechanism to Sry. Importantly, we also identified genomic rearrangements within the SOX3 regulatory region in three patients with XX male sex reversal. Together, these data suggest that SOX3 and SRY are functionally interchangeable in sex determination and support the notion that SRY evolved from SOX3 via a regulatory mutation that led to its de novo expression in the early gonad.

Identification and characterization of Sox genes in the silkworm, Bombyx mori

Sox genes encode a family of transcription factors with important roles in metazoan development, including sex-determination, embryogenesis, neurogenesis, and skeletogenesis. We identified Sox genes in the Bombyx mori genome and characterized their evolution and expression patterns. Nine Sox genes were annotated, and could be classified into five groups, B-F. Four Sox genes in the B group were tandemly clustered on one chromosome, a characteristic common to their orthologs in other insects. The intron number in the high-mobility group (HMG) box of Sox genes exhibited low diversity across surveyed insects. Based on 40 different silkworm variety genomes, we found a similar number of single nucleotide polymorphisms (SNPs) in the coding sequences of each Sox gene, for domesticated and wild groups. However, a gene-based examination showed that SoxB3 and SoxD might be evolving under positive selection during silkworm domestication. Phylogenetic analysis showed that SoxC, SoxD, and SoxF originated before the radiation of insects, and groups B and E evolved through gene duplication after the radiation of insects. Furthermore, BmSox21a, BmSoxB3, BmSoxD, and BmSoxE reveal stage, tissue, or sex-dependent expression patterns. These results provide a foundation for further surveying the functions of Sox genes during silkworm development and domestication.

Zmynd15 encodes a histone deacetylase-dependent transcriptional repressor essential for spermiogenesis and male fertility

Spermatogenesis is a complex process through which male germ line stem cells undergo a multi-step differentiation program and sequentially become spermatogonia, spermatocytes, spermatids, and eventually spermatozoa. In this process, transcription factors act as switches that precisely regulate the expression of genes that in turn control the developmental program of male germ cells. Transcription factors identified to be essential for normal haploid gene expression all display transcription-activating effects and thus serve as the "on" switch for haploid gene expression. Here, we report that ZMYND15 acts as a histone deacetylase-dependent transcriptional repressor and controls normal temporal expression of haploid cell genes during spermiogenesis. Inactivation of Zmynd15 results in early activation of transcription of numerous important haploid genes including Prm1, Tnp1, Spem1, and Catpser3; depletion of late spermatids; and male infertility. ZMYND15 represents the first transcriptional repressor identified to be essential for sperm production and male fertility.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Molecular mechanisms underlying sex change in hermaphroditic groupers

Groupers are widely distributed throughout the tropical and subtropical waters of the world and are regarded as a favourite marine food fish. However, their large-scale aquaculture has been hindered by the rarity of natural males. Being protogynous hermaphrodites, groupers have been considered as study model for development and reproduction, especially for sex determination or sex differentiation, owing to the advantage that grouper gonad development undergoes transition from ovary to intersexual gonad and then to testis, and primordial germ cells and different stages of gametic cells during oogenesis and spermatogenesis are synchronously observed in the transitional gonads. Recently, a series of genes related to the reproduction regulation or sex differentiation have been identified in the groupers, mainly by researchers in China. One important finding was that the grouper gene, doublesex/male abnormal 3-related transcription factor 1 (DMRT1), is not only differentially expressed in gonads at different stages, but that it is also restricted to specific stages and specific cells of spermatogenesis. Grouper DMRT1 protein exists only in spermatogonia, primary spermatocytes and secondary spermatocytes, but not in the supporting Sertoli cells. Moreover, no introns were found in the grouper DMRT1, and no duplicated DMRT1 genes were detected. The finding implies that the intronless DMRT1 that is able to undergo rapid transcriptional turnover might be a significant gene for stimulating spermatogenesis in the protogynous hermaphroditic gonad. Additionally, we have found that grouper expression of sex-determining region Y-related high-mobility group-box gene 3 (SOX3) is a significant time point for enterable gametogenesis of primordial germ cells, because SOX3 is obviously expressed and localized in primordial germ cells. As SOX3 continues to express, the SOX3-positive primordial germ cells develop toward oogonia and then oocytes, whereas, when SOX3 expression is ceased, the SOX3-positive primordial germ cells develop toward spermatogonia. Therefore, we suggest that SOX3, as a transcription factor, might have more important roles in oogenesis than in spermatogenesis. Based on the findings, a hypothetic molecular mechanism underlying sex change is proposed in the hermaphroditic groupers, and some candidate genes related to the grouper sex change are also suggested for further research.

Spermatogonial stem cell regulation and spermatogenesis

This article will provide an updated review of spermatogonial stem cells and their role in maintaining the spermatogenic lineage. Experimental tools used to study spermatogonial stem cells (SSCs) will be described, along with research using these tools to enhance our understanding of stem cell biology and spermatogenesis. Increased knowledge about the biology of SSCs improves our capacity to manipulate these cells for practical application. The chapter concludes with a discussion of future directions for fundamental investigation and practical applications of SSCs.

Expression of a testis-specific form of Gal3st1 (CST), a gene essential for spermatogenesis, is regulated by the CTCF paralogous gene BORIS

Previously, it was shown that the CTCF paralogous gene, BORIS (brother of the regulator of imprinted sites) is expressed in male germ cells, but its function in spermatogenesis has not been defined. To develop an understanding of the functional activities of BORIS, we generated BORIS knockout (KO) mice. Mice homozygous for the null allele had a defect in spermatogenesis that resulted in small testes associated with increased cell death. The defect was evident as early as postnatal day 21 and was manifested by delayed production of haploid cells. By gene expression profiling, we found that transcript levels for Gal3st1 (also known as cerebroside sulfotransferase [CST]), known to play a crucial role in meiosis, were dramatically reduced in BORIS KO testes. We found that CST is expressed in testis as a novel testis-specific isoform, CST form F(TS), that has a short exon 1f. We showed that BORIS bound to and activated the promoter of CST form F(TS). Mutation of the BORIS binding site in the promoter reduced the ability of BORIS to activate the promoter. These findings define transcriptional regulation of CST expression as a critical role for BORIS in spermatogenesis.

Quantitative mRNA expression of sox3 and DMRT1 during sex reversal, and expression profiles after GnRHa administration in black porgy, Acanthopagrus schlegeli

We cloned full-length sox3 cDNA from testis of black porgy, Acanthopagrus schlegeli. Black porgy sox3 cDNA consists of 897 base pairs (bp) and encodes a protein of 298 amino acids. We have investigated the expression pattern of sox3 and DMRT1 mRNA during the sex-reverse process from male to female (immature testis, mature testis, testicular portion of mostly testis, ovarian portion of mostly testis, testicular portion of mostly ovary, ovarian portion of mostly ovary and ovary). The expression of sox3 and DMRT1 mRNA was high in mature testis of black porgy during sex-reverse process. In a histological analysis, testicular portion of gonad was degenerated and the ovary portion was increased during sex reversal from male to female, and then oocytes were increased in ovary. Also we examined the expression of sox3 and DMRT1 mRNA after gonadotropin-releasing hormone analogue (GnRHa) treatment in immature black porgy. The expression of sox3 and DMRT1 mRNA was increased after GnRHa treatment (in vivo and in vitro experiment) in immature black porgy. Therefore, we concluded that sox3 and DMRT1 were involved in the development of testis than ovary in black porgy.

MILI, a PIWI-interacting RNA-binding protein, is required for germ line stem cell self-renewal and appears to positively regulate translation

The Argonaute/PIWI protein family consists of Argonaute and PIWI subfamilies. Argonautes function in RNA interference and micro-RNA pathways; whereas PIWIs bind to PIWI-interacting RNAs and regulate germ line development, stem cell maintenance, epigenetic regulation, and transposition. However, the role of PIWIs in mammalian stem cells has not been demonstrated, and molecular mechanisms mediated by PIWIs remain elusive. Here we show that MILI, a murine PIWI protein, is expressed in the cytoplasm of testicular germ line stem cells, spermatogonia, and early spermatocytes, where it is enriched in chromatoid bodies. MILI is essential for the self-renewing division and differentiation of germ line stem cells but does not affect initial establishment of the germ line stem cell population at 7 days postpartum. Furthermore, MILI forms a stable RNA-independent complex with eIF3a and associates with the eIF4E- and eIF4G-containing m7G cap-binding complex. In isolated 7 days postpartum seminiferous tubules containing mostly germ line stem cells, the mili mutation has no effect on the cellular mRNA level yet significantly reduces the rate of protein synthesis. These observations indicate that MILI may positively regulate translation and that such regulation is required for germ line stem cell self-renewal.

Mouse differentiating spermatogonia can generate germinal stem cells in vivo

In adults, stem cells are responsible for the maintenance of many actively renewing tissues, such as haematopoietic, skin, gut and germinal tissues. These stem cells can self-renew or be committed to becoming progenitors. Stem-cell commitment is thought to be irreversible but in male and female Drosophila melanogaster, it was shown recently that differentiating germ cells can revert to functional stem cells that can restore germinal lineage. Whether progenitors are also able to generate stem cells in mammals remains unknown. Here we show that purified mouse spermatogonial progenitors committed to differentiation can generate functional germinal stem cells that can repopulate germ-cell-depleted testes when transplanted into adult mice. We found that GDNF, a key regulator of the stem-cell niche, and FGF2 are able to reprogram in vitro spermatogonial progenitors for reverse differentiation. This study supports the emerging concept that the stem-cell identity is not restricted in adults to a definite pool of cells that self-renew, but that stemness could be acquired by differentiating progenitors after tissue injury and throughout life.

Sohlh2 affects differentiation of KIT positive oocytes and spermatogonia

The differentiation programs of spermatogenesis and oogenesis are largely independent. In the early stages, however, the mechanisms partly overlap. Here we demonstrated that a germ-cell-specific basic helix-loop-helix (bHLH) transcription factor gene, Sohlh2, is required for early spermatogenesis and oogenesis. SOHLH2 was expressed in mouse spermatogonia from the undifferentiated stage through differentiation and in primordial-to-primary oocytes. Sohlh2-null mice, produced by gene targeting, showed both male and female sterility, owing to the disrupted differentiation of mature (KIT(+)) spermatogonia and oocytes. The Sohlh2-null mice also showed the downregulation of genes involved in spermatogenesis and oogenesis, including the Sohlh1 gene, which is essential for these processes. Furthermore, we showed that SOHLH2 and SOHLH1 could form heterodimers. These observations suggested that SOHLH2 might coordinate with SOHLH1 to control spermatogonial and oocyte genes, including Sohlh1, to promote the differentiation of KIT(+) germ cells in vivo. This study lays the foundation for further dissection of the bHLH network that regulates early spermatogenesis and oogenesis.

Spermatogonial stem cell self-renewal requires OCT4, a factor downregulated during retinoic acid-induced differentiation

The long-term production of billions of spermatozoa relies on the regulated proliferation and differentiation of spermatogonial stem cells (SSCs). To date only a few factors are known to function in SSCs to provide this regulation. Octamer-4 (OCT4) plays a critical role in pluripotency and cell survival of embryonic stem cells and primordial germ cells; however, it is not known whether it plays a similar function in SSCs. Here, we show that OCT4 is required for SSC maintenance in culture and for colonization activity following cell transplantation, using lentiviral-mediated short hairpin RNA expression to knock down OCT4 in an in vitro model for SSCs ("germline stem" [GS] cells). Expression of promyelocytic leukemia zinc-finger (PLZF), a factor known to be required for SSC self-renewal, was not affected by OCT4 knockdown, suggesting that OCT4 does not function upstream of PLZF. In addition to developing a method to test specific gene function in GS cells, we demonstrate that retinoic acid (RA) triggers GS cells to shift to a differentiated, premeiotic state lacking OCT4 and PLZF expression and colonization activity. Our data support a model in which OCT4 and PLZF maintain SSCs in an undifferentiated state and RA triggers spermatogonial differentiation through the direct or indirect downregulation of OCT4 and PLZF. The current study has important implications for the future use of GS cells as an in vitro model for spermatogonial stem cell biology or as a source of embryonic stem-like cells. Disclosure of potential conflicts of interest is found at the end of this article.

Expression patterns of SP1 and SP3 during mouse spermatogenesis: SP1 down-regulation correlates with two successive promoter changes and translationally compromised transcripts

Because of their prominent roles in regulation of gene expression, it is important to understand how levels of Krüpple-like transcription factors SP1 and SP3 change in germ cells during spermatogenesis. Using immunological techniques, we found that both factors decreased sharply during meiosis. SP3 declined during the leptotene-to-pachytene transition, whereas SP1 fell somewhat later, as spermatocytes progressed beyond the early pachytene stage. SP3 reappeared for a period in round spermatids. For Sp1, the transition to the pachytene stage is accompanied by loss of the normal, 8.2-kb mRNA and appearance of a prevalent, 8.8-kb variant, which has not been well characterized. We have now shown that this pachytene-specific transcript contains a long, unspliced sequence from the first intron and that this sequence inhibits expression of a reporter, probably because of its many short open-reading frames. A second testis-specific Sp1 transcript in spermatids of 2.4 kb also has been reported previously. Like the 8.8-kb variant, it is compromised translationally. We have confirmed by Northern blotting that the 8.8-, 8.2-, and 2.4-kb variants account for the major testis Sp1 transcripts. Thus, the unexpected decline of SP1 protein in the face of continuing Sp1 transcription is explained, in large part, by poor translation of both novel testis transcripts. As part of this work, we also identified five additional, minor Sp1 cap sites by 5' rapid amplification of cDNA ends, including a trans-spliced RNA originating from the Glcci1 gene.

Expression and functional analysis of Gm114, a putative mammalian ortholog of Drosophila bam

In the testis, the continuous production of sperm is maintained by a small population of stem cells called germ line stem cells (GSCs) in Drosophila, or spermatogonial stem cells (SSCs) in mammals. This stem cell population can self-renew or produce daughter cells that differentiate into mature sperm. In Drosophila, BMP signals inhibit GSC differentiation by blocking transcription of the gene bag of marbles (bam). Once bam is activated, germ cells initiate differentiation. We identified a novel gene in mouse, Gm114, that shows homology to Drosophila bam. In male germ cells, expression of Gm114 begins at 12.5-13.5 days post coitum (dpc), the stage in mice when germ cells cease proliferation and begin differentiation into prospermatogonia. In the adult testis, Gm114 is highly expressed in differentiated spermatocytes and spermatids but not in undifferentiated spermatogonia, strongly suggesting that, similar to Bam, GM114 plays an important role in mammalian germ line stem cell self-renewal and differentiation. Interestingly, deletion of the majority of the GM114 protein does not affect mouse viability or fertility. This suggests that either there is a function for the remaining N-terminal of GM114, or that there are alternative mechanisms in the mammalian system that control germ cell differentiation.

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.

Mitotic, but not meiotic, oriented cell divisions in rat spermatogenesis

The process of mammalian spermatogenesis involves both mitosis and meiosis at the same developmental age. Most previous studies have focused on mitotic spindle orientation during development, but not during meiotic division. Therefore, we asked whether there is a difference between mitotic and meiotic germ cell spindle orientation during rat spermatogenesis. Our results showed that mitotic spindles of spermatogonia were mainly oriented with angles ranging from 60 to 90 degrees, perpendicular in relation to the basement membrane of the seminiferous tubules. On the other hand, meiotic spindles showed a random orientation. Nocodazole treatment (at a concentration that depolymerizes only astral microtubules) induced a significant increase in cells with an angle between 0 and 30 degrees (parallel) in relation to the basement membrane. Meiotic spindles did not show a significant change in their orientation after the Nocodazole treatment. Therefore, our results suggest differences between the mechanisms controlling positioning and orientation of mitotic and meiotic spindles during rat spermatogenesis. It seems that a phylogenetically conserved programme controls the mitotic spindle orientation in organisms ranging from worms to mammals.

Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes

In eutherian ('placental') mammals, sex is determined by the presence or absence of the Y chromosome-borne gene SRY, which triggers testis determination. Marsupials also have a Y-borne SRY gene, implying that this mechanism is ancestral to therians, the SRY gene having diverged from its X-borne homologue SOX3 at least 180 million years ago. The rare exceptions have clearly lost and replaced the SRY mechanism recently. Other vertebrate classes have a variety of sex-determining mechanisms, but none shares the therian SRY-driven XX female:XY male system. In monotreme mammals (platypus and echidna), which branched from the therian lineage 210 million years ago, no orthologue of SRY has been found. In this study we show that its partner SOX3 is autosomal in platypus and echidna, mapping among human X chromosome orthologues to platypus chromosome 6, and to the homologous chromosome 16 in echidna. The autosomal localization of SOX3 in monotreme mammals, as well as non-mammal vertebrates, implies that SRY is absent in Prototheria and evolved later in the therian lineage 210-180 million years ago. Sex determination in platypus and echidna must therefore depend on another male-determining gene(s) on the Y chromosomes, or on the different dosage of a gene(s) on the X chromosomes.