Regulation of proliferation and differentiation in spermatogonial stem cells: the role of c-kit and its ligand SCF

The SCF/c-KIT system in the male: Survival strategies in fertility and cancer

Maintaining the delicate balance between cell survival and death is of the utmost importance for the proper development of germ cells and subsequent fertility. On the other hand, the fine regulation of tissue homeostasis by mechanisms that control cell fate is a factor that can prevent carcinogenesis. c-KIT is a type III receptor tyrosine kinase activated by its ligand, stem cell factor (SCF). c-KIT signaling plays a crucial role in cell fate decisions, specifically controlling cell proliferation, differentiation, survival, and apoptosis. Indeed, deregulating the SCF/c-KIT system by attenuation or overactivation of its signaling strength is linked to male infertility and cancer, and rebalancing its activity via c-KIT inhibitors has proven beneficial in treating human tumors that contain gain-of-function mutations or overexpress c-KIT. This review addresses the roles of SCF and c-KIT in the male reproductive tract, and discusses the potential application of c-KIT target therapies in disorders of the reproductive system.

MiRNA-20 and mirna-106a regulate spermatogonial stem cell renewal at the post-transcriptional level via targeting STAT3 and Ccnd1

Studies on spermatogonial stem cells (SSCs) are of unusual significance because they are the unique stem cells that transmit genetic information to subsequent generations and they can acquire pluripotency to become embryonic stem-like cells that have therapeutic applications in human diseases. MicroRNAs (miRNAs) have recently emerged as critical endogenous regulators in mammalian cells. However, the function and mechanisms of individual miRNAs in regulating SSC fate remain unknown. Here, we report for the first time that miRNA-20 and miRNA-106a are preferentially expressed in mouse SSCs. Functional assays in vitro and in vivo using miRNA mimics and inhibitors reveal that miRNA-20 and miRNA-106a are essential for renewal of SSCs. We further demonstrate that these two miRNAs promote renewal at the post-transcriptional level via targeting STAT3 and Ccnd1 and that knockdown of STAT3, Fos, and Ccnd1 results in renewal of SSCs. This study thus provides novel insights into molecular mechanisms regulating renewal and differentiation of SSCs and may have important implications for regulating male reproduction.

Cisplatin-induced alterations in the functional spermatogonial stem cell pool and niche in C57/BL/6J mice following a clinically relevant multi-cycle exposure

A typical clinical cis-diamminedichloroplatinum(II) (cisplatin) dosing regimen consists of repeated treatment cycles followed by a recovery period. While effective, this dosing structure results in a prolonged, often permanent, infertility in men. Spermatogonial stem cells (SSCs) are theoretically capable of repopulating the seminiferous tubules after exposure has ceased. We propose that an altered spermatogonial environment during recovery from the initial treatment cycle drives an increase in SSC mitotic cell activity, rendering the SSC pool increasingly susceptible to cisplatin-induced injury from subsequent cycles. To test this hypothesis, the undifferentiated spermatogonia population and niche of the adult mouse (C57/BL/6J) were examined during the recovery periods of a clinically-relevant cisplatin exposure paradigm. Histological examination revealed a disorganization of spermatogenesis correlating with the number of exposure cycles. Quantification of terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick end labeling (TUNEL) staining indicated an increase in apoptotic frequency following exposure. Immunohistochemical examination of Foxo1 and incorporated BrdU showed an increase in the undifferentiated spermatogonial population and mitotic activity in the recovery period in mice exposed to one cycle, but not two cycles of cisplatin. Immunohistochemical investigation of glial cell line-derived neurotrophic factor (GDNF) revealed an increase in production along the basal Sertoli cell membrane throughout the recovery period in all treatment groups. Taken together, these data establish that the impact of cisplatin exposure on the functional stem cell pool and niche correlates with: (1) the number of dosing cycles; (2) mitotic activity of early germ cells; and (3) alterations in the basal Sertoli cell GDNF expression levels after cisplatin-induced testicular injury.

Spatial and temporal expression of c-Kit in the development of the murine submandibular gland

The c-Kit pathway is important in the development of many mammalian cells and organs and is indispensable for the development of hematopoiesis, melanocytes, and primordial germ cells. Loss-of-function mutations in c-Kit lead to perinatal death in mouse embryos. Previously, c-Kit has been used as one of salivary epithelial stem or progenitor cell markers in mouse, its specific temporo-spatial expression pattern and function in developing murine submandibular gland (SMG) is still unclear. Here we used quantitative real-time PCR, in situ hybridization, and immunohistochemistry analysis to detect c-Kit expression during the development of the murine SMG. We found that c-Kit was expressed in the epithelia of developing SMGs from embryonic day 11.5 (E11.5; initial bud stage) to postnatal day 90 (P90; when the SMG is completely mature). c-Kit expression in the end bud epithelium increased during prenatal development and then gradually decreased after birth until its expression was undetectable in mature acini at P30. Moreover, c-Kit was expressed in the SMG primordial cord at the initial bud, pseudoglandular, canacular, and terminal end bud stages. c-Kit was also expressed in the presumptive ductal cells adjacent to the developing acini. By the late terminal end bud stage on P14, c-Kit expression could not be detected in ductal cells. However, c-Kit expression was detected in ductal cells at P30, and its expression had increased dramatically at P90. Taken together, these findings describe the spatial and temporal expression pattern of c-Kit in the developing murine SMG and suggest that c-Kit may play roles in epithelial histo-morphogenesis and in ductal progenitor cell homeostasis in the SMG.

c-kit expression profile and regulatory factors during spermatogonial stem cell differentiation

Background

It has been proven that c-kit is crucial for proliferation, migration, survival and maturation of spermatogenic cells. A periodic expression of c-kit is observed from primordial germ cells (PGCs) to spermatogenetic stem cells (SSCs), However, the expression profile of c-kit during the entire spermatogenesis process is still unclear. This study aims to reveal and compare c-kit expression profiles in the SSCs before and after the anticipated differentiation, as well as to examine its relationship with retinoic acid (RA) stimulation.

Results

We have found that there are more than 4 transcripts of c-kit expressed in the cell lines and in the testes. The transcripts can be divided into short and long categories. The long transcripts include the full-length canonical c-kit transcript and the 3′ end short transcript. Short transcripts include the 3.4 kb short transcript and several truncated transcripts (1.9-3.2 kb). In addition, the 3.4 kb transcript (starting from intron 9 and covering exons 10 ~ 21) is discovered to be specifically expressed in the spermatogonia. The extracellular domain of Kit is obtained in the spermatogonia stage, but the intracellular domain (50 kDa) is constantly expressed in both SSCs and spermatogonia. The c-kit expression profiles in the testis and the spermatogonial stem cell lines vary after RA stimulation. The wave-like changes of the quantitative expression pattern of c-kit (increase initially and decrease afterwards) during the induction process are similar to that of the in vivo male germ cell development process.

Conclusions

There are dynamic transcription and translation changes of c-kit before and after SSCs’ anticipated differentiation and most importantly, RA is a significant upstream regulatory factor for c-kit expression.

Spermatogonial behavior in rats during radiation-induced arrest and recovery after hormone suppression

Ionizing radiation has been shown to arrest spermatogenesis despite the presence of surviving stem spermatogonia, by blocking their differentiation. This block is a result of damage to the somatic environment and is reversed when gonadotropins and testosterone are suppressed, but the mechanisms are still unknown. We examined spermatogonial differentiation and Sertoli cell factors that regulate spermatogonia after irradiation, during hormone suppression, and after hormone suppression combined with Leydig cell elimination with ethane dimethane sulfonate. These results showed that the numbers and cytoplasmic structure of Sertoli cells are unaffected by irradiation, only a few type A undifferentiated (Aund) spermatogonia and even fewer type A1 spermatogonia remained, and immunohistochemical analysis showed that Sertoli cells still produced KIT ligand (KITLG) and glial cell line-derived neurotrophic factor (GDNF). Some of these cells expressed KIT receptor, demonstrating that the failure of differentiation was not a result of the absence of the KIT system. Hormone suppression resulted in an increase in Aund spermatogonia within 3 days, a gradual increase in KIT-positive spermatogonia, and differentiation mainly to A3 spermatogonia after 2 weeks. KITL (KITLG) protein expression did not change after hormone suppression, indicating that it is not a factor in the stimulation. However, GDNF increased steadily after hormone suppression, which was unexpected since GDNF is supposed to promote stem spermatogonial self-renewal and not differentiation. We conclude that the primary cause of the block in spermatogonial development is not due to Sertoli cell factors such (KITL\GDNF) or the KIT receptor. As elimination of Leydig cells in addition to hormone suppression resulted in differentiation to the A3 stage within 1 week, Leydig cell factors were not necessary for spermatogonial differentiation.

SNPs in KIT and KITLG genes may be associated with oligospermia in Chinese population

KIT/KITLG signaling system is crucial for spermatogenesis, which suggests that KIT and KITLG genes may be involved in spermatogenesis impairment and male infertility. To explore the possible association of KIT and KITLG genes with male infertility having spermatogenesis impairment, polymorphism distributions of SNP rs3819392 in KIT gene as well as rs995030 and rs4474514 in KITLG gene were investigated in 372 patients with idiopathic azoospermia or oligospermia and 205 fertile controls. As a result, the significant differences in polymorphism distributions of SNP rs3819392 in KIT gene and rs4474514 in KITLG gene were observed between the patients with oligospermia and controls. The frequencies of allele G (94.2% versus 90.0% p = 0.022) and genotype GG (89.2% versus 82.0% p = 0.042) in patients with oligospermia were significantly higher than those in controls at rs3819392 locus in KIT gene. In addition, the genotype CC of rs4474514 in KITLG (8.2% versus 3.4%, p = 0.034) also significantly increased in oligospermic patients in comparison to controls. These findings indicated that SNP rs3819392 in KIT gene and rs4474514 in KITLG gene may be associated with oligospermia, suggesting that polymorphism of KIT and KITLG genes may play a role in oligospermia.

Lgr4-mediated Wnt/β-catenin signaling in peritubular myoid cells is essential for spermatogenesis

Peritubular myoid cells (PMCs) are myofibroblast-like cells that surround the seminiferous tubules and play essential roles in male fertility. How these cells modulate spermatogenesis and the signaling pathways that are involved are largely unknown. Here we report that Lgr4 is selectively expressed in mouse PMCs in the testes, and loss of Lgr4 leads to germ cells arresting at meiosis I and then undergoing apoptosis. In PMCs of Lgr4 mutant mice, the expression of androgen receptor, alpha-smooth muscle actin and extracellular matrix proteins was dramatically reduced. Malfunctioning PMCs further affected Sertoli cell nuclear localization and functional protein expression in Lgr4(-/-) mice. In addition, Wnt/β-catenin signaling was activated in wild-type PMCs but attenuated in those of Lgr4(-/-) mice. When Wnt/β-catenin signaling was reactivated by crossing with Apc(min/+) mice or by Gsk3β inhibitor treatment, the Lgr4 deficiency phenotype in testis was partially rescued. Together, these data demonstrate that Lgr4 signaling through Wnt/β-catenin regulates PMCs and is essential for spermatogenesis.

Mast cells rescue implantation defects caused by c-kit deficiency

Various physiologically relevant processes are regulated by the interaction of the receptor tyrosine kinase (c-Kit) and its ligand stem cell factor (SCF), with SCF known to be the most important growth factor for mast cells (MCs). In spite of their traditional role in allergic disorders and innate immunity, MCs have lately emerged as versatile modulators of a variety of physiologic and pathologic processes. Here we show that MCs are critical for pregnancy success. Uterine MCs presented a unique phenotype, accumulated during receptivity and expanded upon pregnancy establishment. Kit^W-sh/W-sh mice, whose MC deficiency is based on restricted c-Kit gene expression, exhibited severely impaired implantation, which could be completely rescued by systemic or local transfer of wild-type bone marrow-derived MCs. Transferred wild-type MCs favored normal implantation, induced optimal spiral artery remodeling and promoted the expression of MC proteases, transforming growth factor-β and connective tissue growth factor. MCs contributed to trophoblast survival, placentation and fetal growth through secretion of the glycan-binding protein galectin-1. Our data unveil unrecognized roles for MCs at the fetomaternal interface with critical implications in reproductive medicine.

Functional Analysis of the Cooled Rat Testis

Direct cooling of the testis results in the depletion of most germ cells in vivo. Germ cell-depleted testes are now commonly used to investigate spermatogenic regeneration and can serve as recipients for germ cell transplantation. The present study explored the effects of cooling rat testes on the depletion of endogenous germ cells, spermatogenic regeneration, and Sertoli cell function. Adult rat testes were cooled with iced Ringer's solution for 60 minutes, which results in the initiation of apoptotic germ cell loss within 8 hours. Pachytene spermatocytes at stages XII-I were the cells most sensitive to cooling. In 46%-67% of seminiferous tubule cross-sections, only Sertoli cells remained in the cooled testes 3-10 weeks after treatment. Germ cell loss was accompanied by a significant decrease in circulating inhibin B and an increase in follicle-stimulating hormone concentrations, which indicated a change in Sertoli cell function. Quantitative analysis of mRNA expression associated with apoptotic signals showed no significant uniform changes among the cooled testes, although some individuals had a distinct up-regulation of FAS mRNA at 24 hours. Attempts to use the cooled testes as recipient testes for mouse-to-rat germ cell transplantation were undertaken, but none of the mouse germ cells transplanted into the testes 15-34 days after cooling appeared to have undergone spermatogenesis 64-92 days after transplantation. These data suggest that modifications to Sertoli cell function resulting from testicular cooling create an environment that is unable to support spermatogenesis by donor germ cells.

Testis tissue explantation cures spermatogenic failure in c-Kit ligand mutant mice

Male infertility is most commonly caused by spermatogenic defects or insufficiencies, the majority of which are as yet cureless. Recently, we succeeded in cultivating mouse testicular tissues for producing fertile sperm from spermatogonial stem cells. Here, we show that one of the most severe types of spermatogenic defect mutant can be treated by the culture method without any genetic manipulations. The Sl/Sl(d) mouse is used as a model of such male infertility. The testis of the Sl/Sl(d) mouse has only primitive spermatogonia as germ cells, lacking any sign of spermatogenesis owing to mutations of the c-kit ligand (KITL) gene that cause the loss of membrane-bound-type KITL from the surface of Sertoli cells. To compensate for the deficit, we cultured testis tissues of Sl/Sl(d) mice with a medium containing recombinant KITL and found that it induced the differentiation of spermatogonia up to the end of meiosis. We further discovered that colony stimulating factor-1 (CSF-1) enhances the effect of KITL and promotes spermatogenesis up to the production of sperm. Microinsemination of haploid cells resulted in delivery of healthy offspring. This study demonstrated that spermatogenic impairments can be treated in vitro with the supplementation of certain factors or substances that are insufficient in the original testes.

Spermatogonia differentiation requires retinoic acid receptor γ

Vitamin A is instrumental to mammalian reproduction. Its metabolite, retinoic acid (RA), acts in a hormone-like manner through binding to and activating three nuclear receptor isotypes, RA receptor (RAR)α (RARA), RARβ, and RARγ (RARG). Here, we show that 1) RARG is expressed by A aligned (A(al)) spermatogonia, as well as during the transition from A(al) to A(1) spermatogonia, which is known to require RA; and 2) ablation of Rarg, either in the whole mouse or specifically in spermatogonia, does not affect meiosis and spermiogenesis but impairs the A(al) to A(1) transition in the course of some of the seminiferous epithelium cycles. Upon ageing, this phenomenon yields seminiferous tubules containing only spermatogonia and Sertoli cells. Altogether, our findings indicate that RARG cell-autonomously transduces, in undifferentiated spermatogonia of adult testes, a RA signal critical for spermatogenesis. During the prepubertal spermatogenic wave, the loss of RARG function can however be compensated by RARA, as indicated by the normal timing of appearance of meiotic cells in Rarg-null testes. Accordingly, RARG- and RARA-selective agonists are both able to stimulate Stra8 expression in wild-type prepubertal testes. Interestingly, inactivation of Rarg does not impair expression of the spermatogonia differentiation markers Kit and Stra8, contrary to vitamin A deficiency. This latter observation supports the notion that the RA-signaling pathway previously shown to operate in Sertoli cells also participates in spermatogonia differentiation.

Foxo1 is required in mouse spermatogonial stem cells for their maintenance and the initiation of spermatogenesis

Spermatogonial stem cells (SSCs) capable of self-renewal and differentiation are the foundation for spermatogenesis. Although several factors important for these processes have been identified, the fundamental mechanisms regulating SSC self-renewal and differentiation remain unknown. Here, we investigated a role for the Foxo transcription factors in mouse spermatogenesis and found that Foxo1 specifically marks mouse gonocytes and a subset of spermatogonia with stem cell potential. Genetic analyses showed that Foxo1 was required for both SSC homeostasis and the initiation of spermatogenesis. Combined deficiency of Foxo1, Foxo3, and Foxo4 resulted in a severe impairment of SSC self-renewal and a complete block of differentiation, indicating that Foxo3 and Foxo4, although dispensable for male fertility, contribute to SSC function. By conditional inactivation of 3-phosphoinositide-dependent protein kinase 1 (Pdk1) and phosphatase and tensin homolog (Pten) in the male germ line, we found that PI3K signaling regulates Foxo1 stability and subcellular localization, revealing that the Foxos are pivotal effectors of PI3K-Akt signaling in SSCs. We also identified a network of Foxo gene targets--most notably Ret--that rationalized the maintenance of SSCs by the Foxos. These studies demonstrate that Foxo1 expression in the spermatogenic lineage is intimately associated with the stem cell state and revealed what we believe to be novel Foxo-dependent mechanisms underlying SSC self-renewal and differentiation, with implications for common diseases, including male infertility and testicular cancer, due to abnormalities in SSC function.

Surgical Therapy of End-Stage Heart Failure: Understanding Cell-Mediated Mechanisms Interacting with Myocardial Damage

Worldwide, cardiovascular disease results in an estimated 14.3 million deaths per year, giving rise to an increased demand for alternative and advanced treatment. Current approaches include medical management, cardiac transplantation, device therapy, and, most recently, stem cell therapy. Research into cell-based therapies has shown this option to be a promising alternative to the conventional methods. In contrast to early trials, modern approaches now attempt to isolate specific stem cells, as well as increase their numbers by means of amplifying in a culture environment. The method of delivery has also been improved to minimize the risk of micro-infarcts and embolization, which were often observed after the use of coronary catheterization. The latest approach entails direct, surgical, transepicardial injection of the stem cell mixture, as well as the use of tissue-engineered meshes consisting of embedded progenitor cells.

c-kit and its related genes in spermatogonial differentiation

Spermatogenesis is the process of production of male gametes from SSCs. The SSCs are the stem cells that differentiate into male gametes in the testis. in the mean time, the Spg are remarkable for their potential multiple trans-differentiations, which make them greatly invaluable for clinical applications. However, the molecular mechanism controlling differentiation of the Spg is still not clear. Among the discovered spermatogenesis-related genes, c-kit seems to be expressed first by the Spgs thus may play a central role in switching on the differentiation process. Expression of Kit and the activation of the Kit/Kitl pathway coincide with the start of differentiation of Spgs. Several genes have been discovered to be related to the Kit/Kitl pathway. in this review, we have summarized the recent discoveries of c-kit and the Kit/Kitl pathway-related genes in the spermatogenic cells during different stages of spermatogenesis.

Imatinib has deleterious effects on differentiating spermatogonia while sparing spermatogonial stem cell self renewal

Imatinib mesylate is among a growing number of effective cancer drugs that provide molecularly targeted therapy; however, imatinib causes reproductive defects in rodents. The availability of an in vitro system for screening the effect of drugs on spermatogenesis would be beneficial. The imatinib targets, KIT and platelet derived growth factor receptor beta (PDGFRB), were shown here to be expressed in "germline stem" (GS) cell cultures that contain spermatogonia, including spermatogonial stem cells (SSCs). GS cell cultures were utilized to determine whether imatinib affects SSC self renewal or differentiation. GS cells grown in imatinib retained self renewal based on multiple assays, including transplantation. However, growth in imatinib led to decreased numbers of differentiated spermatogonia and reduced culture growth consistent with the known requirement for KIT in survival and proliferation of spermatogonia. These results build upon the in vivo studies and support the possibility of utilizing GS cell cultures for preclinical drug tests.

Epigenetic mechanisms regulate stem cell expressed genes Pou5f1 and Gfra1 in a male germ cell line

Male fertility is declining and an underlying cause may be due to environment-epigenetic interactions in developing sperm, yet nothing is known of how the epigenome controls gene expression in sperm development. Histone methylation and acetylation are dynamically regulated in spermatogenesis and are sensitive to the environment. Our objectives were to determine how histone H3 methylation and acetylation contribute to the regulation of key genes in spermatogenesis. A germ cell line, GC-1, was exposed to either the control, or the chromatin modifying drugs tranylcypromine (T), an inhibitor of the histone H3 demethylase KDM1 (lysine specific demethylase 1), or trichostatin (TSA), an inhibitor of histone deacetylases, (HDAC). Quantitative PCR (qPCR) was used to identify genes that were sensitive to treatment. As a control for specificity the Myod1 (myogenic differentiation 1) gene was analyzed. Chromatin immunoprecipitation (ChIP) followed by qPCR was used to measure histone H3 methylation and acetylation at the promoters of target genes and the control, Myod1. Remarkably, the chromatin modifying treatment specifically induced the expression of spermatogonia expressed genes Pou5f1 and Gfra1. ChIP-qPCR revealed that induction of gene expression was associated with a gain in gene activating histone H3 methylation and acetylation in Pou5f1 and Gfra1 promoters, whereas CpG DNA methylation was not affected. Our data implicate a critical role for histone H3 methylation and acetylation in the regulation of genes expressed by spermatogonia – here, predominantly mediated by HDAC-containing protein complexes.

Effects of combined administration of low-dose gossypol with steroid hormones on the mitotic phase of spermatogenesis of rat

Our previous studies suggested that combined low-dose gossypol with steroid hormones possesses a reversible antifertility role on adult male rat. Spermatocyte apoptosis during the meiosis of spermatogenesis is the main reason for the antifertility. This study evaluates the effects of the regimen on the mitosis of spermatogenesis. Thirty-two adult male rats were divided into four groups randomly, Group GH: rats were fed orally with gossypol acetic acid (GA, 12.5 mg/kg d) and desogestrel (DSG, 0.125 mg/kg d)/ethinylestradiol (EE, 0.025 mg/kg d)/testosterone undecanoate (TU, 100 mg/kg d); Group G: a single dose of GA (12.5 mg/kg d); Group H: the same dosage of DSG/EE/TU as in Group GH; Group C: rats were treated with vehicle (1% methyl cellulose) as control. Testes were removed at the 8th week after treatment to assess the weight, volumes, diameter, and degenerated rate of seminiferous tubule, spermatogonia is quantitated by stereological assay. TUNEL assay and proliferating cell nuclear antigen immunohistochemistry were used to determine the germ cell apoptosis and proliferation. The changes of gene expression of glial cell line-derived neurotrophic factor (GDNF) and Ets variant gene 5 (Etv5) were analyzed. The results manifested that the spermatogenesis was suppressed, the type A and intermediate spermatogonia reduced, and spermatocyte and spermatid apoptosis increased both in Group GH and Group H. The expressions of GDNF protein in these two groups were downregulated, but no change was observed in Etv5 expression. However, the numbers of spermatogonia, germ cell apoptosis and proliferation, and the expressions of GDNF and Etv5 were unchanged in Group G compared with Group C. Above all, we concluded that the mitotic phase of spermatogenesis is suppressed by the combination regimen, it might be the steroid hormones in the regimen that caused the reduction of spermatogonia through the downregulation of GDNF.

Local signalling environments and human male infertility: what we can learn from mouse models

Infertility is one of the most prevalent public health problems facing young adult males in today's society. A clear, treatable cause of infertility cannot be determined in a large number of these patients, and a growing body of evidence suggests that infertility in many of these men may be due to genetic causes. Studies using mouse knockout technology have been integral for examination of normal spermatogenesis and to identify proteins essential for this process, which in turn are candidate genes for human male infertility. Successful spermatogenesis depends on a delicate balance of local signalling factors, and this review focuses on the genes that encode these factors. Normal functioning of all testicular cell types is essential for fertility and might also be crucial to prevent germ cell oncogenesis. Analysis of these signalling processes in spermatogenesis using mouse models has provided investigators with an invaluable tool to effectively translate basic science research to the research of human disease and infertility.

Phenotypic plasticity of mouse spermatogonial stem cells

Background

Spermatogonial stem cells (SSCs) continuously undergo self-renewal division to support spermatogenesis. SSCs are thought to have a fixed phenotype, and development of a germ cell transplantation technique facilitated their characterization and prospective isolation in a deterministic manner; however, our in vitro SSC culture experiments indicated heterogeneity of cultured cells and suggested that they might not follow deterministic fate commitment in vitro.

Methodology and Principal Findings

In this study, we report phenotypic plasticity of SSCs. Although c-kit tyrosine kinase receptor (Kit) is not expressed in SSCs in vivo, it was upregulated when SSCs were cultured on laminin in vitro. Both Kit⁻ and Kit⁺ cells in culture showed comparable levels of SSC activity after germ cell transplantation. Unlike differentiating spermatogonia that depend on Kit for survival and proliferation, Kit expressed on SSCs did not play any role in SSC self-renewal. Moreover, Kit expression on SSCs changed dynamically once proliferation began after germ cell transplantation in vivo.

Conclusions/Significance

These results indicate that SSCs can change their phenotype according to their microenvironment and stochastically express Kit. Our results also suggest that activated and non-activated SSCs show distinct phenotypes.

Signaling molecules and pathways regulating the fate of spermatogonial stem cells

Spermatogenesis is the process that involves the division and differentiation of spermatogonial stem cells (SSCs) into mature spermatozoa. SSCs are a subpopulation of type A spermatogonia resting on the basement membrane in the mammalian testis. Self-renewal and differentiation of SSCs are the foundation of normal spermatogenesis, and thus a better understanding of molecular mechanisms and signaling pathways in the SSCs is of paramount importance for the regulation of spermatogenesis and may eventually lead to novel targets for male contraception as well as for gene therapy of male infertility and testicular cancer. Uncovering the molecular mechanisms is also of great interest to a better understanding of SSC aging and for developing novel therapeutic strategies for degenerative diseases in view of the recent work demonstrating the pluripotent potential of the SSC. Progress has recently been made in elucidating the signaling molecules and pathways that determine cell fate decisions of SSCs. In this review, we first address the morphological features, phenotypic characteristics, and the potential of SSCs, and then we focus on the recent advances in defining the key signaling molecules and crucial signaling pathways regulating self-renewal and differentiation of SSCs. The association of aberrant expression of signaling molecules and cascades with abnormal spermatogenesis and testicular cancer are also discussed. Finally, we point out potential future directions to pursue in research on signaling pathways of SSCs.

The heterogeneity of spermatogonia is revealed by their topology and expression of marker proteins including the germ cell-specific proteins Nanos2 and Nanos3

Spermatogonial stem cells (SSCs) reside in undifferentiated type-A spermatogonia and contribute to continuous spermatogenesis by maintaining the balance between self-renewal and differentiation, thereby meeting the biological demand in the testis. Spermatogonia have to date been characterized principally through their morphology, but we herein report the detailed characterization of undifferentiated spermatogonia in mouse testes based on their gene expression profiles in combination with topological features. The detection of the germ cell-specific proteins Nanos2 and Nanos3 as markers of spermatogonia has enabled the clear dissection of complex populations of these cells as Nanos2 was recently shown to be involved in the maintenance of stem cells. Nanos2 is found to be almost exclusively expressed in A(s) to A(pr) cells, whereas Nanos3 is detectable in most undifferentiated spermatogonia (A(s) to A(al)) and differentiating A(1) spermatogonia. In our present study, we find that A(s) and A(pr) can be basically classified into three categories: (1) GFRalpha1(+)Nanos2(+)Nanos3(-)Ngn3(-), (2) GFRalpha1(+)Nanos2(+)Nanos3(+)Ngn3(-), and (3) GFRalpha1(-)Nanos2(+/-)Nanos3(+)Ngn3(+). We propose that the first of these groups is most likely to include the stem cell population and that Nanos3 may function in transit amplifying cells.

Mice deficient in the serine/threonine protein kinase VRK1 are infertile due to a progressive loss of spermatogonia

The VRK1 protein kinase has been implicated as a pro-proliferative factor. Genetic analyses of mutant alleles of the Drosophila and Caenorhabditis elegans VRK1 homologs have revealed phenotypes ranging from embryonic lethality to mitotic and meiotic defects with resultant sterility. Herein, we describe the first genetic analysis of murine VRK1. Two lines of mice containing distinct gene-trap integrations into the Vrk1 locus were established. Insertion into intron 12 (GT12) spared VRK1 function, enabling the examination of VRK1 expression in situ. Insertion into intron 3 (GT3) disrupted VRK1 function, but incomplete splicing to the gene trap rendered this allele hypomorphic (approximately 15% of wild-type levels of VRK1 remain). GT3/GT3 mice are viable, but both males and females are infertile. In testes, VRK1 is expressed in Sertoli cells and spermatogonia. The infertility of GT3/GT3 male mice results from a progressive defect in spermatogonial proliferation or differentiation, culminating in the absence of mitotic and meiotic cells in adult testis. These data demonstrate an important role for VRK1 in cell proliferation and confirm that the need for VRK1 during gametogenesis is evolutionarily conserved.

Vascular endothelial growth factor regulates germ cell survival during establishment of spermatogenesis in the bovine testis

Vascular endothelial growth factor-A (VEGFA) is a hypoxia-inducible peptide essential for angiogenesis and targets nonvascular cells in a variety of tissues and cell types. The objective of the current study was to determine the function of VEGF during testis development in bulls. We used an explant tissue culture and treatment approach to test the hypothesis that VEGFA-164 could regulate the biological activity of bovine germ cells. We demonstrate that VEGFA, KDR, and FLT1 proteins are expressed in germ and somatic cells in the bovine testis. Treatment of bovine testis tissue with VEGFA in vitro resulted in significantly more germ cells following 5 days of culture when compared with controls. Quantitative real-time RT-PCR analysis determined that VEGF treatment stimulated an intracellular response that prevents germ cell death in bovine testis tissue explants, as indicated by increased expression of BCL2 relative to BAX and decreased expression of BNIP3 at 3, 6, and 24 h during culture. Blocking VEGF activity in vitro using antisera against KDR and VEGF significantly reduced the number of germ cells in VEGF-treated testis tissue to control levels at 120 h. Testis grafting provided in vivo evidence that bovine testis tissue treated with VEGFA for 5 days in culture contained significantly more differentiating germ cells compared with controls. These findings support the conclusion that VEGF supports germ cell survival and sperm production in bulls.

Control of KIT signalling in male germ cells: what can we learn from other systems?

The KIT ligand (KITL)/KIT-signalling system is among several pathways known to be essential for fertility. In the postnatal testis, the KIT/KITL interaction is crucial for spermatogonial proliferation, differentiation, survival and subsequent entry into meiosis. Hence, identification of endogenous factors that regulate KIT synthesis is important for understanding the triggers driving germ cell maturation. Although limited information is available regarding local factors in the testicular microenvironment that modulate KIT synthesis at the onset of spermatogenesis, knowledge from other systems could be used as a basis for identifying how KIT function is regulated in germ cells. This review describes the known regulators of KIT, including transcription factors implicated in KIT promoter regulation. In addition, specific downstream outcomes in biological processes that KIT orchestrates are addressed. These are discussed in relationship to current knowledge of mammalian germ cell development.

Asymmetric distribution of UCH-L1 in spermatogonia is associated with maintenance and differentiation of spermatogonial stem cells

Asymmetric division of germline stem cells in vertebrates was proposed a century ago; however, direct evidence for asymmetric division of mammalian spermatogonial stem cells (SSCs) has been scarce. Here, we report that ubiquitin carboxy-terminal hydrolase 1 (UCH-L1) is expressed in type A (A(s), A(pr), and A(al)) spermatogonia located at the basement membrane (BM) of seminiferous tubules at high and low levels, but not in differentiated germ cells distant from the BM. Asymmetric segregation of UCH-L1 was associated with self-renewal versus differentiation divisions of SSCs as defined by co-localization of UCH-L1(high) and PLZF, a known determinant of undifferentiated SSCs, versus co-localization of UCH-L1(low/-) with proteins expressed during SSC differentiation (DAZL, DDX4, c-KIT). In vitro, gonocytes/spermatogonia frequently underwent asymmetric divisions characterized by unequal segregation of UCH-L1 and PLZF. Importantly, we could also demonstrate asymmetric segregation of UCH-L1 and PLZF in situ in seminiferous tubules. Expression level of UCH-L1 in the immature testis where spermatogenesis was not complete was not affected by the location of germ cells relative to the BM, whereas UCH-L1-positive spermatogonia were exclusively located at the BM in the adult testis. Asymmetric division of SSCs appeared to be affected by interaction with supporting somatic cells and extracelluar matrix. These findings for the first time provide direct evidence for existence of asymmetric division during SSCs self-renewal and differentiation in mammalian spermatogenesis.

Wnt signaling promotes proliferation and stemness regulation of spermatogonial stem/progenitor cells

Spermatogonial stem cells (SSCs) self-renew throughout life to produce progenitor cells that are able to differentiate into spermatozoa. However, the mechanisms underlying the cell fate determination between self-renewal and differentiation have not yet been delineated. Culture conditions and growth factors essential for self-renewal and proliferation of mouse SSCs have been investigated, but no information is available related to growth factors that affect fate determination of human spermatogonia. Wnts form a large family of secreted glycoproteins, the members of which are involved in cell proliferation, differentiation, organogenesis, and cell migration. Here, we show that Wnts and their receptors Fzs are expressed in mouse spermatogonia and in the C18-4 SSC line. We demonstrate that WNT3A induces cell proliferation, morphological changes, and cell migration in C18-4 cells. Furthermore, we show that beta-catenin is activated during testis development in 21-day-old mice. In addition, our study demonstrates that WNT3A sustained adult human embryonic stem (ES)-like cells derived from human germ cells in an undifferentiated stage, expressing essential human ES cell transcription factors. These results demonstrate for the first time that Wnt/beta-catenin pathways, especially WNT3A, may play an important role in the regulation of mouse and human spermatogonia.

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.

Generation of mice by transplantation of an adult spermatogonial cell line after cryopreservation

OBJECTIVES

The key to fertility in adult males is production of mature spermatogenic cells. Spermatogonial stem cells (SSC) have the dual capacity of self-renewal and of differentiation into mature sperm. SSC transplantation may provide potential treatment for specific male infertilities. However, until now, there has been no evidence of offspring produced by transplantation of adult SSC line cells in humans or other mammals.

MATERIALS AND METHODS

A new line of SSCs from adult C57BL/6 mouse was established by using magnetic-activated cell sorting. The cell line was characterized by immunocytochemistry, karyotype analysis and telomeric repeat amplification protocol (TRAP) telomerase activity assay. Spermatogenic function was examined by allograft into germ cell-ablated recipient mice.

RESULTS

For more than 14 months with more than 65 maintenance passages, the cell line showed a normal karyotype (40, XY) and high telomerase activity. It represented a Thy-1+, Oct4+, SSEA-1-, c-kit- (99 +/- 1%) cell subpopulation. We cryopreserved these SSCs and successfully produced normal offspring after transplanting them into testes of busulphan-sterilized mice.

CONCLUSIONS

We established and long-term maintained an adult SSC line with normal spermatogenic function, without the need of genetic modification; thus, this study provides a model system for basic research and clinical application.

Cluster characterization of mouse embryonic stem cell-derived pluripotent embryoid bodies in four distinct developmental stages

The formation of embryoid bodies (EBs) is the principal step in the differentiation of embryonic stem (ES) cells. In this study, the morphological characteristics and gene expression patterns of EBs related to the sequential stages of embryonic development were well defined in four distinct developmental groups over 112 days of culture: early-stage EBs groups (1-7 days of differentiation), mid-stage EBs groups (9-15 days of differentiation), maturing EBs groups (17-45 days of differentiation) and matured EBs groups (50 days of differentiation). We first determined definite histological location of apoptosis within EBs and the sequential expression of molecular markers representing stem cells (Oct4, SSEA-1, Sox-2 and AKP), germ cells (Fragilis, Dazl, c-kit, StellaR, Mvh and Stra8), ectoderm (Neurod, Nestin and Neurofilament), mesoderm (Gata-1, Flk-1 and Hbb) and endoderm (AFP and Transthyretin). Our results revealed that developing EBs possess either pluripotent stem cell or germ cell states and that three-dimensional aggregates of EBs initiate mES cell differentiation during prolonged culture in vitro. Therefore, we suggest that this EB system to some extent recapitulates the early developmental processes occurring in vivo.

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.

Male germ cell transplantation

Transplantation of male germ line stem cells from a donor animal to the testes of an infertile recipient was first described in 1994. Donor germ cells colonize the recipient's testis and produce donor-derived sperm, such that the recipient male can distribute the genetic material of the germ cell donor. Germ cell transplantation represents a functional reconstitution assay for male germ line stem cells and as such has vastly increased our ability to study the biology of stem cells in the testis and define phenotypes of infertility. First developed in rodents, the technique has now been used in a number of animal species, including domestic mammals, chicken and fish. There are three major applications for this technology in animals: first, to study fundamental aspects of male germ line stem cell biology and male fertility; second, to preserve the reproductive potential of genetically valuable individuals by male germ cell transplantation within or between species; third, to produce transgenic sperm by genetic manipulation of isolated germ line stem cells and subsequent transplantation. Transgenesis through the male germ line has tremendous potential in species in which embryonic stem cells are not available and somatic cell nuclear transfer has limited success. Therefore, transplantation of male germ cells is a uniquely valuable approach for the study, preservation and manipulation of male fertility in animals.

Platelet-derived growth factor receptor beta-subtype regulates proliferation and migration of gonocytes

Proliferation and migration of gonocytes, the precursors of spermatogonial stem cells, to the germline niche in the basal membrane of the seminiferous tubules, are two crucial events that take place between postnatal d 0.5 (P0.5) and P5.0 in the mouse and involve a selection of the cells that are committed to the germline stem cells lineage. Here we show that from embryonic d 18.0 (E18) and up to P5, the gonocytes express platelet-derived growth factor (PDGF) receptor beta-subtype (PDGFR-beta) and that during the same time period, the Sertoli cells express PDGF-B and PDGF-D, both ligands for PDGFR-beta. Inhibition of the PDGFR-beta tyrosine kinase activity during the first five postnatal days provokes a profound reduction of gonocyte number through inhibition of their proliferation and induction of apoptosis. Moreover, we found that PDGFR-beta ligands are chemotactic for gonocytes. These data suggest that PDGFR-beta activation has the remarkable capability to drive the selection, survival, and migration of the gonocytes from the center of the seminiferous tubules to the testicular germline niche on the basal membrane.

Gene silencing by RNAi in mouse Sertoli cells

BACKGROUND

RNA interference (RNAi) is a valuable tool in the investigation of gene function. The purpose of this study was to examine the availability, target cell types and efficiency of RNAi in the mouse seminiferous epithelium.

METHODS

The experimental model was based on transgenic mice expressing EGFP (enhanced green fluorescent protein). RNAi was induced by in vivo transfection of plasmid vectors encoding for short hairpin RNAs (shRNAs) targeting EGFP. shRNAs were transfected in vivo by microinjection into the seminiferous tubules via the rete testis followed by square wave electroporation. As a transfection reporter, expression of red fluorescent protein (HcRed 1) was used. Cell types, the efficiency of both transfections and RNAi were all evaluated.

RESULTS

Sertoli cells were the main transfected cells. A reduction of about 40% in the level of EGFP protein was detected in cells successfully transfected both in vivo and in vitro. However, the efficiency of in vivo transfection was low.

CONCLUSION

In adult seminiferous epithelial cells, in vivo post-transcriptional gene silencing mediated by RNAi via shRNA is efficient in Sertoli cells. Similar levels of RNAi were detected both in vivo and in vitro. This also indicates that Sertoli cells have the necessary silencing machinery to repress the expression of endogenous genes via RNAi.

A plasma membrane-anchored fluorescent protein fusion illuminates sieve element plasma membranes in Arabidopsis and tobacco

Rapid acquisition of quantitative anatomical data from the sieve tubes of angiosperm phloem has been confounded by their small size, their distance from organ surfaces, and the time-consuming nature of traditional methods, such as transmission electron microscopy. To improve access to these cells, for which good anatomical data are critical, a monomeric yellow fluorescent protein (mCitrine) was N-terminally fused to a small (approximately 6 kD) membrane protein (AtRCI2A) and stably expressed in Arabidopsis thaliana (Columbia-0 ecotype) and Nicotiana tabacum ('Samsun') under the control of a companion cell-specific promoter (AtSUC2p). The construct, called by its abbreviation SUmCR, yielded stable sieve element (SE) plasma membrane fluorescence labeling, even after plastic (methacrylate) embedding. In conjunction with wide-field fluorescence measurements of sieve pore number and position using aniline blue-stained callose, mCitrine-labeled material was used to calculate rough estimates of sieve tube-specific conductivity for both species. The SUmCR construct also revealed a hitherto unknown expression domain of the AtSUC2 Suc-H(+) symporter in the epidermis of the cell division zone of developing root tips. The success of this construct in targeting plasma membrane-anchored fluorescent proteins to SEs could be attributable to the small size of AtRCI2A or to the presence of other signals innate to AtRCI2A that permit the protein to be trafficked to SEs. The construct provides a hitherto unique entrée into companion cell-to-SE protein targeting, as well as a new tool for studying whole-plant phloem anatomy and architecture.

Study of the potential spermatogonial stem cell compartment in dogfish testis, Scyliorhinus canicula L

In the lesser-spotted dogfish (Scyliorhinus canicula), spermatogenesis takes place within spermatocysts made up of Sertoli cells associated with stage-synchronized germ cells. As shown in testicular cross sections, cysts radiate in maturational order from the germinative area, where they are formed, to the opposite margin of the testis, where spermiation occurs. In the germinative zone, which is located in a specific area between the tunica albuginea of the testis and the dorsal testicular vessel, individual large spermatogonia are surrounded by elongated somatic cells. The aim of this study has been to define whether these spermatogonia share characteristics with spermatogonial stem cells described in vertebrate and non-vertebrate species. We have studied their ultrastructure and their mitotic activity by 5'-bromo-2'-deoxyuridine (BrdU) incorporation and proliferating cell nuclear antigen (PCNA) immunodetection. Additionally, immunodetection of c-Kit receptor, a marker of differentiating spermatogonia in rodents, and of alpha- and beta-spectrins, as constituents of the spectrosome and the fusome, has been performed. Ultrastructurally, nuclei of stage I spermatogonia present the same mottled aspect in dogfish as undifferentiated spermatogonia nuclei in rodents. Moreover, intercellular bridges are not observed in dogfish spermatogonia, although they are present in stage II spermatogonia. BrdU and PCNA immunodetection underlines their low mitotic activity. The presence of a spectrosome-like structure, a cytological marker of the germline stem cells in Drosophila, has been observed. Our results constitute the first step in the study of spermatogonial stem cells and their niche in the dogfish.