Plzf regulates germline progenitor self-renewal by opposing mTORC1

Effective production of recipient male pigs for spermatogonial stem cell transplantation by intratesticular injection with busulfan

Germ cell transplantation has facilitated spermatogonial stem cell (SSC) and spermatogenesis research and shown great potential in the seed-breeding of domestic livestock. However, little progress has been made in large animals, primarily reflecting the difficulties in preparing sterile recipients. Here, we developed a novel protocol to prepare recipient pigs through the direct injection of busulfan into the cavum vaginale of the scrotums of Landrace-Large bi-crossbreeding male pigs and Seghers male pigs, two economically-important types of pigs, to eliminate endogenous spermatogonia. No severe diseases or weight loss was observed in either pig type after the injection with busulfan. Histologic analysis showed an advanced and dose-dependent germ cell loss, with complete germ cell loss observed in the highest dose group, 3.0 mg/kg in the Landrace-Large bi-crossbreeding pigs and 2.0 mg/kg in the Seghers pigs. A smaller seminiferous tubule diameter, a vacuolized seminiferous epithelium and the overproliferation interstitial cells, frequently observed in mouse germ cell deficiency models, were present in the most of the high-dose busulfan-treated groups. Molecular markers detected in Seghers pigs further confirmed the depletion of endogenous germ cells, providing an accessible niche for exogenous SSCs. This study provides a basis to prepare the transplantation recipients of SSCs in pigs.

Immunolocalization of proteins in the spermatogenesis process of canine

Spermatogenesis is a process in which differentiated cells are produced and the adult stem cell population-known as spermatogonial stem cells (SSCs)-is continuously replenished. However, the molecular mechanisms underlying these processes are not fully understood in the canine species. We addressed this in this study by analysing the expression of specific markers in spermatogonia of seminiferous tubules of canine testes. SSCs at different stages of reproductive development (prepubertal and adult) were examined by immunohistochemistry and flow cytometry. Glial cell-derived neurotrophic factor family receptor alpha-1 (GFRA1), deleted in azoospermia-like (DAZL) and promyelocytic leukaemia zinc finger (PLZF) were expressed in SSCs, while stimulated by retinoic acid gene 8 (STRA8) was detected only in undifferentiated spermatogonia in prepubertal testis and differentiated spermatogonia and spermatocytes in adult canine. Octamer-binding transcription factor 4 (OCT4) showed an expression pattern, and the levels did not differ between the groups examined. However, C-kit expression varied as a function of reproductive developmental stage. Our results demonstrate that these proteins play critical roles in the self-renewal and differentiation of SSCs and can serve as markers to identify canine spermatogonia at specific stages of development.

Somatic stem cell differentiation is regulated by PI3K/Tor signaling in response to local cues

Stem cells reside in niches that provide signals to maintain self-renewal, and differentiation is viewed as a passive process that depends on loss of access to these signals. Here, we demonstrate that the differentiation of somatic cyst stem cells (CySCs) in the Drosophila testis is actively promoted by PI3K/Tor signaling, as CySCs lacking PI3K/Tor activity cannot differentiate properly. We find that an insulin peptide produced by somatic cells immediately outside of the stem cell niche acts locally to promote somatic differentiation through Insulin-like receptor (InR) activation. These results indicate that there is a local 'differentiation' niche that upregulates PI3K/Tor signaling in the early daughters of CySCs. Finally, we demonstrate that CySCs secrete the Dilp-binding protein ImpL2, the Drosophila homolog of IGFBP7, into the stem cell niche, which blocks InR activation in CySCs. Thus, we show that somatic cell differentiation is controlled by PI3K/Tor signaling downstream of InR and that the local production of positive and negative InR signals regulates the differentiation niche. These results support a model in which leaving the stem cell niche and initiating differentiation are actively induced by signaling.

Suppression of REDD1 in osteoarthritis cartilage, a novel mechanism for dysregulated mTOR signaling and defective autophagy

OBJECTIVE

Aging is a main risk factor for the development of osteoarthritis (OA) and the molecular mechanisms underlying the aging-related changes in articular cartilage include increased mammalian target of rapamycin (mTOR) signaling and defective autophagy. REDD1 is an endogenous inhibitor of mTOR that regulates cellular stress responses. In this study we measured REDD1 expression in normal, aged and OA cartilage and assessed REDD1 function in human and mouse articular chondrocytes.

METHODS

REDD1 expression was analyzed in human and mouse articular cartilage by qPCR, western blotting, and immunohistochemistry. For functional studies, REDD1 and TXNIP knockdown or overexpression was performed in chondrocytes in the presence or absence of rapamycin and chloroquine, and mTOR signaling and autophagy were measured by western blotting. REDD1/TXNIP protein interaction was assessed by co-immunoprecipitation experiments.

RESULTS

Human and mouse cartilage from normal knee joints expressed high levels of REDD1. REDD1 expression was significantly reduced in aged and OA cartilage. In cultured chondrocytes, REDD1 knockdown increased whereas REDD1 overexpression decreased mTOR signaling. In addition, REDD1 activated autophagy by an mTOR independent mechanism that involved protein/protein interaction with TXNIP. The REDD1/TXNIP complex was required for autophagy activation in chondrocytes.

CONCLUSION

The present study shows that REDD1 is highly expressed in normal human articular cartilage and reduced during aging and OA. REDD1 in human chondrocytes negatively regulates mTOR activity and is essential for autophagy activation. Reduced REDD1 expression thus represents a novel mechanism for the increased mTOR activation and defective autophagy observed in OA.

Quantitative analysis of male germline stem cell differentiation reveals a role for the p53-mTORC1 pathway in spermatogonial maintenance

p53 protects cells from DNA damage by inducing cell-cycle arrest upon encountering genomic stress. Among other pathways, p53 elicits such an effect by inhibiting mammalian target of rapamycin complex 1 (mTORC1), the master regulator of cell proliferation and growth. Although recent studies have indicated roles for both p53 and mTORC1 in stem cell maintenance, it remains unclear whether the p53-mTORC1 pathway is conserved to mediate this process under normal physiological conditions. Spermatogenesis is a classic stem cell-dependent process in which undifferentiated spermatogonia undergo self-renewal and differentiation to maintain the lifelong production of spermatozoa. To better understand this process, we have developed a novel flow cytometry (FACS)-based approach that isolates spermatogonia at consecutive differentiation stages. By using this as a tool, we show that genetic loss of p53 augments mTORC1 activity during early spermatogonial differentiation. Functionally, loss of p53 drives spermatogonia out of the undifferentiated state and causes a consistent expansion of early differentiating spermatogonia until the stage of preleptotene (premeiotic) spermatocyte. The frequency of early meiotic spermatocytes is, however, dramatically decreased. Thus, these data suggest that p53-mTORC1 pathway plays a critical role in maintaining the homeostasis of early spermatogonial differentiation. Moreover, our FACS approach could be a valuable tool in understanding spermatogonial differentiation.

Alteration of protein prenylation promotes spermatogonial differentiation and exhausts spermatogonial stem cells in newborn mice

Spermatogenesis in adulthood depends on the successful neonatal establishment of the spermatogonial stem cell (SSC) pool and gradual differentiation during puberty. The stage-dependent changes in protein prenylation in the seminiferous epithelium might be important during the first round of spermatogenesis before sexual maturation, but the mechanisms are unclear. We have previous found that altered prenylation in Sertoli cells induced spermatogonial apoptosis in the neonatal testis, resulting in adult infertility. Now we further explored the role of protein prenylation in germ cells, using a conditional deletion of geranylgeranyl diphosphate synthase (Ggpps) in embryonic stage and postmeiotic stage respectively. We observed infertility of Ggpps(-/-) Ddx4-Cre mice that displayed a Sertoli-cell-only syndrome phenotype, which resulted from abnormal spermatogonial differentiation and SSC depletion during the prepubertal stage. Analysis of morphological characteristics and cell-specific markers revealed that spermatogonial differentiation was enhanced from as early as the 7(th) postnatal day in the first round of spermatogenesis. Studies of the molecular mechanisms indicated that Ggpps deletion enhanced Rheb farnesylation, which subsequently activated mTORC1 and facilitated spermatogonial differentiation. In conclusion, the prenylation balance in germ cells is crucial for spermatogonial differentiation fate decision during the prepubertal stage, and the disruption of this process results in primary infertility.

mTOR Regulates Gap Junction Alpha-1 Protein Trafficking in Sertoli Cells and Is Required for the Maintenance of Spermatogenesis in Mice

The mammalian target of rapamycin (Mtor) gene encodes a serine/threonine kinase that acts as a master regulator of processes as diverse as cell growth, protein synthesis, cytoskeleton reorganization, and cell survival. In the testis, physiological roles for Mtor have been proposed in perinatal Sertoli cell proliferation and blood-testis barrier (BTB) remodeling during spermatogenesis, but no in vivo studies of Mtor function have been reported. Here, we used a conditional knockout approach to target Mtor in Sertoli cells. The resulting Mtor(flox/flox); Amhr2(cre/+) mice were characterized by progressive, adult-onset testicular atrophy associated with disorganization of the seminiferous epithelium, loss of Sertoli cell polarity, increased germ cell apoptosis, premature release of germ cells, decreased epididymal sperm counts, increased sperm abnormalities, and infertility. Histopathologic analysis and quantification of the expression of stage-specific markers showed a specific loss of pachytene spermatocytes and spermatids. Although the BTB and the ectoplasmic specializations did not appear to be altered in Mtor(flox/flox);Amhr2(cre/+) mice, a dramatic redistribution of gap junction alpha-1 (GJA1) was detected in their Sertoli cells. Phosphorylation of GJA1 at Ser373, which is associated with its internalization, was increased in the testes of Mtor(flox/flox); Amhr2(cre/+) mice, as was the expression and phosphorylation of AKT, which phosphorylates GJA1 at this site. Together, these results indicate that Mtor expression in Sertoli cells is required for the maintenance of spermatogenesis and the progression of germ cell development through the pachytene spermatocyte stage. One mechanism of mTOR action may be to regulate gap junction dynamics by inhibiting AKT, thereby decreasing GJA1 phosphorylation and internalization. mTOR regulates gap junction alpha-1 protein distribution in Sertoli cells and is necessary for progression through the pachytene spermatocyte stage.

Regulation of hematopoietic development by ZBTB transcription factors

Hematopoietic development is governed by the coordinated expression of lineage- and differentiation stage-specific genes. Transcription factors play major roles in this process and their perturbation may underlie hematologic and immunologic disorders. Nearly 1900 transcription factors are encoded in the human genome: of these, 49 BTB (for broad-complex, tram-track and bric à brac)-zinc finger transcription factors referred to as ZBTB or POK proteins have been identified. ZBTB proteins, including BCL6, PLZF, ThPOK and LRF, exhibit a broad spectrum of functions in normal and malignant hematopoiesis. This review summarizes developmental and molecular functions of ZBTB proteins relevant to hematology.

Hypermaintenance and hypofunction of aged spermatogonia: insight from age-related increase of Plzf expression

Like stem cells in other tissues, spermatogonia, including spermatogonial stem cells (SSCs) at the foundation of differentiation hierarchy, undergo age-related decline in function. The promyelocytic leukemia zinc finger (Plzf) protein plays an essential role in spermatogonia maintenance by preventing their differentiation. To evaluate whether there is an age-related change in Plzf expression, we found that aged mouse testes exhibited a robust “Plzf overexpression” phenotype, in that they showed not only a higher frequency of Plzf-expressing cells but also an increased level of Plzf expression in these cells. Moreover, some Plzf-expressing cells in aged testes even aberrantly appeared in the differentiating spermatogonia compartment, which is usually low or negative for Plzf expression. Importantly, ectopic Plzf expression in F9 cells suppressed retinoic acid (RA)-induced Stra8 activation, a gene required for meiosis initiation. These data, together with our observation of a lack of meiosis-initiating spermatocytes associated with high Plzf-expressing spermatogonia in the aged testes, particularly in the degenerative seminiferous tubules, suggest that age-related increase in Plzf expression represents a novel molecular signature of spermatogonia aging by functionally arresting their differentiation.

The regulatory repertoire of PLZF and SALL4 in undifferentiated spermatogonia

Spermatogonial stem cells (SSCs) maintain spermatogenesis throughout adulthood through balanced self-renewal and differentiation, yet the regulatory logic of these fate decisions is poorly understood. The transcription factors Sal-like 4 (SALL4) and promyelocytic leukemia zinc finger (PLZF; also known as ZBTB16) are known to be required for normal SSC function, but their targets are largely unknown. ChIP-seq in mouse THY1(+) spermatogonia identified 4176 PLZF-bound and 2696 SALL4-bound genes, including 1149 and 515 that were unique to each factor, respectively, and 1295 that were bound by both factors. PLZF and SALL4 preferentially bound gene promoters and introns, respectively. Motif analyses identified putative PLZF and SALL4 binding sequences, but rarely both at shared sites, indicating significant non-autonomous binding in any given cell. Indeed, the majority of PLZF/SALL4 shared sites contained only PLZF motifs. SALL4 also bound gene introns at sites containing motifs for the differentiation factor DMRT1. Moreover, mRNA levels for both unique and shared target genes involved in both SSC self-renewal and differentiation were suppressed following SALL4 or PLZF knockdown. Together, these data reveal the full profile of PLZF and SALL4 regulatory targets in undifferentiated spermatogonia, including SSCs, which will help elucidate mechanisms controlling the earliest cell fate decisions in spermatogenesis.

The Histone Demethylase FBXL10 Regulates the Proliferation of Spermatogonia and Ensures Long-Term Sustainable Spermatogenesis in Mice

The F-box and leucine-rich repeat protein 10 (Fbxl10) gene encodes a protein that catalyzes demethylation of H3K4 and H3K36. In this study, we show the important roles of FBXL10 as a histone demethylase in sustainable sperm production using mice in which the JmjC domain of Fbxl10 was deleted (Fbxl10(DeltaJ/DeltaJ)). In histological analysis, testis sections from 10-wk-old Fbxl10(DeltaJ/DeltaJ) mice appeared normal. On the other hand, testes from 7-mo-old Fbxl10(DeltaJ/DeltaJ) mice contained a greater ratio of seminiferous tubules exhibiting degeneration of spermatogenesis. Further analysis using an in vitro spermatogonia culture system, that is, germline stem cells (GSCs), revealed that Fbxl10(DeltaJ/DeltaJ) GSCs expressed a significantly higher level of P21 and P19 mRNA, cyclin-dependent kinase inhibitors and also known as cellular senescence markers, than wild-type (WT) GSCs. Furthermore, the ratio of Fbxl10(DeltaJ/DeltaJ) GSCs in G0/G1 phase was higher and the ratios in S and G2/M phases were lower than the corresponding ratios of WT GSCs, and the doubling speed of Fbxl10(DeltaJ/DeltaJ) GSCs was significantly slower than that of WT GSCs. In addition to these in vitro results, an in vivo study indicated that recovery of spermatogenesis after a transient reduction in the number of testicular germ cells by busulfan treatment was significantly slower in Fbxl10(DeltaJ/DeltaJ) mice than in WT mice. These data suggest that Fbxl10 plays important roles in long-term sustainable spermatogenesis via regulating cell cycle.

Low-dose ionizing irradiation triggers a 53BP1 response to DNA double strand breaks in mouse spermatogonial stem cells

The present study aims to examine the effect of low-dose ionizing irradiation on DNA double strand breaks (DSB) in mouse spermatogonial stem cells (SSCs) and reveal the underlying pathways for the DNA repair for DSB in SSCs. Eighteen one-month-old mice were divided into 6 groups and sacrificed separately at 45 minutes, 2 hours, 24 hours, 48 hours, and 72 hours after 0.1Gy X-ray irradiation (mice without receiving ionizing irradiation served as control). After perfusion fixation, testes were removed, sectioned, and followed by staining of γH2AX, 53BP1, Caspase 3, and promyelocytic leukemia zinc-finger (PLZF) for analysis among the different groups. The staining was observed by immunofluorescence visualized by confocal laser scanning. After low-dose irradiation, only 53BP1, but not Caspase3 or γH2AX was upregulated in PLZF positive SSCs within 45 minutes. The expression level of 53BP1 gradually decreased 24 hours after irradiation. Moreover, low-dose irradiation had no effect on the cell number and apoptotic status of SSCs. However other spermatogenic cells highly expressed γH2AX shortly after irradiation which was dramatically reduced following the events of DNA repair. It appears that low-dose ionizing irradiation may cause the DNA DSB of mouse spermatogenic cells. 53BP1, but not γH2AX, is involved in the DNA repair for DSB in SSCs. Our data indicates that 53BP1 plays an important role in the pathophysiological repair of DNA DSB in SSCs. This may open a new avenue to understanding the mechanisms of DNA repair of SSCs and male infertility.

Mechanisms Regulating Spermatogonial Differentiation

Mammalian spermatogenesis is a complex and highly ordered process by which male germ cells proceed through a series of differentiation steps to produce haploid flagellated spermatozoa. Underlying this process is a pool of adult stem cells, the spermatogonial stem cells (SSCs), which commence the spermatogenic lineage by undertaking a differentiation fate decision to become progenitor spermatogonia. Subsequently, progenitors acquire a differentiating spermatogonia phenotype and undergo a series of amplifying mitoses while becoming competent to enter meiosis. After spermatocytes complete meiosis, post-meiotic spermatids must then undergo a remarkable transformation from small round spermatids to a flagellated spermatozoa with extremely compacted nuclei. This chapter reviews the current literature pertaining to spermatogonial differentiation with an emphasis on the mechanisms controlling stem cell fate decisions and early differentiation events in the life of a spermatogonium.

CRISPR/Cas9 Promotes Functional Study of Testis Specific X-Linked Gene In Vivo

Mammalian spermatogenesis is a highly regulated multistage process of sperm generation. It is hard to uncover the real function of a testis specific gene in vitro since the in vitro model is not yet mature. With the development of the CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated 9) system, we can now rapidly generate knockout mouse models of testis specific genes to study the process of spermatogenesis in vivo. SYCP3-like X-linked 2 (SLX2) is a germ cell specific component, which contains a Cor1 domain and belongs to the XLR (X-linked, lymphocyte regulated) family. Previous studies suggested that SLX2 might play an important role in mouse spermatogenesis based on its subcellular localization and interacting proteins. However, the function of SLX2 in vivo is still elusive. Here, to investigate the functions of SLX2 in spermatogenesis, we disrupted the Slx2 gene by using the CRISPR/Cas9 system. Since Slx2 is a testis specific X-linked gene, we obtained knockout male mice in the first generation and accelerated the study process. Compared with wild-type mice, Slx2 knockout mice have normal testis and epididymis. Histological observation of testes sections showed that Slx2 knockout affected none of the three main stages of spermatogenesis: mitosis, meiosis and spermiogenesis. In addition, we further confirmed that disruption of Slx2 did not affect the number of spermatogonial stem cells, meiosis progression or XY body formation by immunofluorescence analysis. As spermatogenesis was normal in Slx2 knockout mice, these mice were fertile. Taken together, we showed that Slx2 itself is not an essential gene for mouse spermatogenesis and CRISPR/Cas9 technique could speed up the functional study of testis specific X-linked gene in vivo.

PLZF-Induced Upregulation of CXCR4 Promotes Dairy Goat Male Germline Stem Cell Proliferation by Targeting Mir146a

Previous studies have shown that promyelocytic leukemia zinc finger (PLZF), chemokine (C-X-C motif) receptor 4 (CXCR4) and mir146a were associated with the self-renewal of mouse spermatogonial stem cells (SSCs); however, there is little information on their effects on the fate of livestock SSCs. Here, we have identified a regulatory pathway in dairy goat mGSCs, involving PLZF, mir146a and the SDF-1 receptor CXCR4. PLZF overexpression downregulated mir146a and simultaneously upregulated the expression of CXCR4 protein, whereas PLZF knockdown (siPLZF) induced the specifically opposite effects. The in vitro assays demonstrated that PLZF specifically interacts with and suppresses the mir146a promoter, and mir146a targets CXCR4 to impede its translation. The levels of ERK1/2 phosphorylation in the mGSCs overexpressed CXCR4 and PLZF were upregulated, respectively, whereas mir146a expression was decreased and CXCR4 protein was increased. Mir146a overexpression and siPLZF impaired mGSC proliferation and differentiation, however, Mir146a knockdown induced the opposite effects. The effects of PLZF and mir146a were mediated regulation by mir146a and CXCR4, respectively. Overexpression of CXCR4 or addition of CXCL12 in cultures of dairy goat mGSCs resulted in the upregulation of their signaling, and the phosphorylation of ERK1/2 was increased. Collectively, these findings indicate that PLZF is an important transcription factor in the regulation of the expression of CXCR4 to promote dairy goat mGSC proliferation by targeting mir146a.

Inhibition of Akt/mTOR attenuates age-related changes in mesenchymal stem cells

The decline in mesenchymal stem cell (MSC) self-renewal and function with aging contributes to diseases associated with impaired osteogenesis. MSC donor age in prolonged culture also limits the therapeutic potential of these cells for tissue engineering and regenerative medicine. Here, we demonstrate an intervention to preserve the immature state MSC and consequently maintain self-renewal and differentiation capacity during in vitro aging. We showed that blocking of phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin (mTOR) prevents the development of an age-related phenotype and maintains MSC morphology of early passage cells with high clonogenic frequency and enhanced proliferative capacity. MSC cultured in the presence of inhibitors of Akt or mTOR also robustly maintain their osteogenic potential, that is otherwise lost during in vitro aging. We further report that these effects may be mediated by induction of expression of pluripotency genes Nanog and Oct-4 and by the reduction in the production of cytoplasmic reactive oxygen species (ROS). Additionally, loss of Akt/mTOR and ROS was accompanied with lower levels of DNA damage. These results provide an insight into mechanisms involved in MSC aging and suggest possible interventions to maintain quiescence and function of MSC prior to in vivo transplantation or as pharmacological agents in diseases associated with loss of MSC function.

Mammalian target of rapamycin complex 1 (mTORC1) Is required for mouse spermatogonial differentiation in vivo

Spermatogonial stem cells (SSCs) must balance self-renewal with production of transit-amplifying progenitors that differentiate in response to retinoic acid (RA) before entering meiosis. This self-renewal vs. differentiation spermatogonial fate decision is critical for maintaining tissue homeostasis, as imbalances cause spermatogenesis defects that can lead to human testicular cancer or infertility. A great deal of effort has been exerted to understand how the SSC population is maintained. In contrast, little is known about the essential program of differentiation initiated by retinoic acid (RA) that precedes meiosis, and the pathways and proteins involved are poorly defined. We recently reported a novel role for RA in stimulating the PI3/AKT/mTOR kinase signaling pathway to activate translation of repressed mRNAs such as Kit. Here, we examined the requirement for mTOR complex 1 (mTORC1) in mediating the RA signal to direct spermatogonial differentiation in the neonatal testis. We found that in vivo inhibition of mTORC1 by rapamycin blocked spermatogonial differentiation, which led to an accumulation of undifferentiated spermatogonia. In addition, rapamycin also blocked the RA-induced translational activation of mRNAs encoding KIT, SOHLH1, and SOHLH2 without affecting expression of STRA8. These findings highlight dual roles for RA in germ cell development - transcriptional activation of genes, and kinase signaling to stimulate translation of repressed messages required for spermatogonial differentiation.

The histone demethylase KDM1A is essential for the maintenance and differentiation of spermatogonial stem cells and progenitors

Little is known of the fundamental processes governed by epigenetic mechanisms in the supplier cells of spermatogenesis, the spermatogonial stem cells (SSCs). The histone H3 lysine demethylase KDM1A is expressed in spermatogonia. We hypothesized that KDM1A serves in transcriptional regulation of SSCs and fertility. Using a conditional deletion of Kdm1a [conditional knockout (cKO)] in mouse spermatogonia, we determined that Kdm1a is essential for spermatogenesis as adult cKO males completely lack germ cells. Analysis of postnatal testis development revealed that undifferentiated and differentiating spermatogonial populations form in Kdm1a-cKO animals, yet the majority fail to enter meiosis. Loss of germ cells in the cKO was rapid with none remaining by postnatal day (PND) 21. To gain insight into the mechanistic implications of Kdm1a ablation, we isolated PND 6 spermatogonia enriched for SSCs and analyzed their transcriptome by RNA sequencing. Loss of Kdm1a was associated with altered transcription of 1206 genes. Importantly, differentially expressed genes between control and Kdm1a-cKO animals included those that are essential for SSC and progenitor maintenance and spermatogonial differentiation. The complete loss of fertility and failure to establish spermatogenesis indicate that Kdm1a is a master controller of gene transcription in spermatogonia and is required for SSC and progenitor maintenance and differentiation.

The SCL/TAL1 Transcription Factor Represses the Stress Protein DDiT4/REDD1 in Human Hematopoietic Stem/Progenitor Cells

Hematopoietic stem/progenitor cells (HSPCs) are regulated through numerous molecular mechanisms that have not been interconnected. The transcription factor stem cell leukemia/T-cell acute leukemia 1 (TAL1) controls human HSPC but its mechanism of action is not clarified. In this study, we show that knockdown (KD) or short-term conditional over-expression (OE) of TAL1 in human HSPC ex vivo, respectively, blocks and maintains hematopoietic potentials, affecting proliferation of human HSPC. Comparative gene expression analyses of TAL1/KD and TAL1/OE human HSPC revealed modifications of cell cycle regulators as well as previously described TAL1 target genes. Interestingly an inverse correlation between TAL1 and DNA damage-induced transcript 4 (DDiT4/REDD1), an inhibitor of the mammalian target of rapamycin (mTOR) pathway, is uncovered. Low phosphorylation levels of mTOR target proteins in TAL1/KD HSPC confirmed an interplay between mTOR pathway and TAL1 in correlation with TAL1-mediated effects of HSPC proliferation. Finally chromatin immunoprecipitation experiments performed in human HSPC showed that DDiT4 is a direct TAL1 target gene. Functional analyses showed that TAL1 represses DDiT4 expression in HSPCs. These results pinpoint DDiT4/REDD1 as a novel target gene regulated by TAL1 in human HSPC and establish for the first time a link between TAL1 and the mTOR pathway in human early hematopoietic cells. Stem Cells 2015;33:2268-2279.

Ras/ERK1/2 pathway regulates the self-renewal of dairy goat spermatogonia stem cells

Spermatogonia stem cells (SSCs), also named the male germline stem cells (mGSCs), which is located at the base of the seminiferous tubules of testis, is the basis for generating sperm steadily in male animals. Currently, there are some preliminary study on the self-renewal and differentiation of SSCs, but further mechanism, especially in large animals, has not been clearly understood. Ras/ERK1/2 pathway is widely distributed in multiple cells in vivo. It plays an important role in cell proliferation, differentiation and so on. However, the study on the function for the self-renewal of dairy goats SSCs has not been investigated. In this study, the dairy goat SSCs characterization were evaluated by semi-RT-PCR, alkaline phosphatase (AP) staining, and immunofluorescence staining. Then, Ras/ERK1/2 pathway was blocked by specific MEK1/2 inhibitor PD0325901. We analyzed the proliferation by cell number, cell growth curve, Brdu incorporation assay, and cell cycle analysis. The results showed that the proliferation was significantly inhibited by PD0325901. Cell apoptosis induced by blocking the Ras/ERK1/2 pathway was analyzed by TUNEL. The expression of ETV5 and BCL6B, the downstream gene of Ras/ERK1/2 pathway, was downregulated. This study suggest that the Ras/ERK1/2 pathway plays a critical role in maintaining the self-renewal of dairy goat SSCs via regulation of ETV5 and BCL6B. This study laid a foundation for insights into the mechanism of SSCs self-renewal comprehensively.

Mammalian gonocyte and spermatogonia differentiation: recent advances and remaining challenges

The production of spermatozoa relies on a pool of spermatogonial stem cells (SSCs), formed in infancy from the differentiation of their precursor cells, the gonocytes. Throughout adult life, SSCs will either self-renew or differentiate, in order to maintain a stem cell reserve while providing cells to the spermatogenic cycle. By contrast, gonocytes represent a transient and finite phase of development leading to the formation of SSCs or spermatogonia of the first spermatogenic wave. Gonocyte development involves phases of quiescence, cell proliferation, migration, and differentiation. Spermatogonia, on the other hand, remain located at the basement membrane of the seminiferous tubules throughout their successive phases of proliferation and differentiation. Apoptosis is an integral part of both developmental phases, allowing for the removal of defective cells and the maintenance of proper germ–Sertoli cell ratios. While gonocytes and spermatogonia mitosis are regulated by distinct factors, they both undergo differentiation in response to retinoic acid. In contrast to postpubertal spermatogenesis, the early steps of germ cell development have only recently attracted attention, unveiling genes and pathways regulating SSC self-renewal and proliferation. Yet, less is known on the mechanisms regulating differentiation. The processes leading from gonocytes to spermatogonia have been seldom investigated. While the formation of abnormal gonocytes or SSCs could lead to infertility, defective gonocyte differentiation might be at the origin of testicular germ cell tumors. Thus, it is important to better understand the molecular mechanisms regulating these processes. This review summarizes and compares the present knowledge on the mechanisms regulating mammalian gonocyte and spermatogonial differentiation.

Distinct germline progenitor subsets defined through Tsc2-mTORC1 signaling

Adult tissue maintenance is often dependent on resident stem cells; however, the phenotypic and functional heterogeneity existing within this self-renewing population is poorly understood. Here, we define distinct subsets of undifferentiated spermatogonia (spermatogonial progenitor cells; SPCs) by differential response to hyperactivation of mTORC1, a key growth-promoting pathway. We find that conditional deletion of the mTORC1 inhibitor Tsc2 throughout the SPC pool using Vasa-Cre promotes differentiation at the expense of self-renewal and leads to germline degeneration. Surprisingly, Tsc2 ablation within a subset of SPCs using Stra8-Cre did not compromise SPC function. SPC activity also appeared unaffected by Amh-Cre-mediated Tsc2 deletion within somatic cells of the niche. Importantly, we find that differentiation-prone SPCs have elevated mTORC1 activity when compared to SPCs with high self-renewal potential. Moreover, SPCs insensitive to Tsc2 deletion are preferentially associated with mTORC1-active committed progenitor fractions. We therefore delineate SPC subsets based on differential mTORC1 activity and correlated sensitivity to Tsc2 deletion. We propose that mTORC1 is a key regulator of SPC fate and defines phenotypically distinct SPC subpopulations with varying propensities for self-renewal and differentiation.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Essential role for autophagy during invariant NKT cell development

Autophagy is an evolutionarily conserved cellular homeostatic pathway essential for development, immunity, and cell death. Although autophagy modulates MHC antigen presentation, it remains unclear whether autophagy defects impact on CD1d lipid loading and presentation to invariant natural killer T (iNKT) cells and on iNKT cell differentiation in the thymus. Furthermore, it remains unclear whether iNKT and conventional T cells have similar autophagy requirements for differentiation, survival, and/or activation. We report that, in mice with a conditional deletion of the essential autophagy gene Atg7 in the T-cell compartment (CD4 Cre-Atg7(-/-)), thymic iNKT cell development--unlike conventional T-cell development--is blocked at an early stage and mature iNKT cells are absent in peripheral lymphoid organs. The defect is not due to altered loading of intracellular iNKT cell agonists; rather, it is T-cell-intrinsic, resulting in enhanced susceptibility of iNKT cells to apoptosis. We show that autophagy increases during iNKT cell thymic differentiation and that it developmentally regulates mitochondrial content through mitophagy in the thymus of mice and humans. Autophagy defects result in the intracellular accumulation of mitochondrial superoxide species and subsequent apoptotic cell death. Although autophagy-deficient conventional T cells develop normally, they show impaired peripheral survival, particularly memory CD8(+) T cells. Because iNKT cells, unlike conventional T cells, differentiate into memory cells while in the thymus, our results highlight a unique autophagy-dependent metabolic regulation of adaptive and innate T cells, which is required for transition to a quiescent state after population expansion.

Retinoic acid regulates Kit translation during spermatogonial differentiation in the mouse

In the testis, a subset of spermatogonia retains stem cell potential, while others differentiate to eventually become spermatozoa. This delicate balance must be maintained, as defects can result in testicular cancer or infertility. Currently, little is known about the gene products and signaling pathways directing these critical cell fate decisions. Retinoic acid (RA) is a requisite driver of spermatogonial differentiation and entry into meiosis, yet the mechanisms activated downstream are undefined. Here, we determined a requirement for RA in the expression of KIT, a receptor tyrosine kinase essential for spermatogonial differentiation. We found that RA signaling utilized the PI3K/AKT/mTOR signaling pathway to induce the efficient translation of mRNAs for Kit, which are present but not translated in undifferentiated spermatogonia. Our findings provide an important molecular link between a morphogen (RA) and the expression of KIT protein, which together direct the differentiation of spermatogonia throughout the male reproductive lifespan.

Dietary and metabolic control of stem cell function in physiology and cancer

Organismal diet has a profound impact on tissue homeostasis and health in mammals. Adult stem cells are a keystone of tissue homeostasis that alters tissue composition by balancing self-renewal and differentiation divisions. Because somatic stem cells may respond to shifts in organismal physiology to orchestrate tissue remodeling and some cancers are understood to arise from transformed stem cells, there is a likely possibility that organismal diet, stem cell function, and cancer initiation are interconnected. Here we will explore the emerging effects of diet on nutrient-sensing pathways active in mammalian tissue stem cells and their relevance to normal and cancerous growth.

RTP801/REDD1: a stress coping regulator that turns into a troublemaker in neurodegenerative disorders

Mechanistic target of Rapamycin (mTOR) pathway regulates essential processes directed to preserve cellular homeostasis, such as cell growth, proliferation, survival, protein synthesis and autophagy. Importantly, mTOR pathway deregulation has been related to many diseases. Indeed, it has become a hallmark in neurodegenerative disorders, since a fine-tuned regulation of mTOR activities is crucial for neuron function and survival. RTP801/REDD1/Dig2 has become one of the most puzzling regulators of mTOR. Although the mechanism is not completely understood, RTP801 inactivates mTOR and Akt via the tuberous sclerosis complex (TSC1/TSC2) in many cellular contexts. Intriguingly, RTP801 protects dividing cells from hypoxia or H₂O₂-induced apoptosis, while it sensitizes differentiated cells to stress. Based on experimental models of Parkinson’s disease (PD), it has been proposed that at early stages of the disease, stress-induced RTP801 upregulation contributes to mTOR repression, in an attempt to maintain cell function and viability. However, if RTP801 elevation is sustained, it leads to neuron cell death by a sequential inhibition of mTOR and Akt. Here, we will review RTP801 deregulation of mTOR in a context of PD and other neurodegenerative disorders.

Biology and identity of fish spermatogonial stem cell

Although present at relatively low number in the testis, spermatogonial stem cells (SSCs) are crucial for the establishment and maintenance of spermatogenesis in eukaryotes and, until recently, those cells were investigated in fish using morphological criteria. The isolation and characterization of these cells in fish have been so far limited by the lack of specific molecular markers, hampering the high SSCs biotechnological potential for aquaculture. However, some highly conserved vertebrate molecular markers, such as Gfra1 and Pou5f1/Oct4, are now available representing important candidates for studies evaluating the regulation of SSCs in fish and even functional investigations using germ cells transplantation. A technique already used to demonstrate that, different from mammals, fish germ stem cells (spermatogonia and oogonia) present high sexual plasticity that is determined by the somatic microenvironment. As relatively well established in mammals, and demonstrated in zebrafish and dogfish, this somatic environment is very important for the preferential location and regulation of SSCs. Importantly, a long-term in vitro culture system for SSCs has been now established for some fish species. Therefore, besides the aforementioned possibilities, such culture system would allow the development of strategies to in vitro investigate key regulatory and functional aspects of germline stem cells (ex: self-renewal and/or differentiation) or to amplify SSCs of rare, endangered, or commercially valuable fish species, representing an important tool for transgenesis and the development of new biotechnologies in fish production.

G9a co-suppresses LINE1 elements in spermatogonia

Background

Repression of retrotransposons is essential for genome integrity and the development of germ cells. Among retrotransposons, the establishment of CpG DNA methylation and epigenetic silencing of LINE1 (L1) elements and the intracisternal A particle (IAP) endogenous retrovirus (ERV) is dependent upon the piRNA pathway during embryonic germ cell reprogramming. Furthermore, the Piwi protein Mili, guided by piRNAs, cleaves expressed L1 transcripts to post-transcriptionally enforce L1 silencing in meiotic cells. The loss of both DNA methylation and the Mili piRNA pathway does not affect L1 silencing in the mitotic spermatogonia where histone H3 lysine 9 dimethylation (H3K9me2) is postulated to co-repress these elements.

Results

Here we show that the histone H3 lysine 9 dimethyltransferase G9a co-suppresses L1 elements in spermatogonia. In the absence of both a functional piRNA pathway and L1 DNA methylation, G9a is both essential and sufficient to silence L1 elements. In contrast, H3K9me2 alone is insufficient to maintain IAP silencing in spermatogonia. The loss of all three repressive mechanisms has a major impact on spermatogonial populations inclusive of spermatogonial stem cells, with the loss of all germ cells observed in a high portion of seminiferous tubules.

Conclusions

Our study identifies G9a-mediated H3K9me2 as a novel and important L1 repressive mechanism in the germ line. We also demonstrate fundamental differences in the requirements for the maintenance of L1 and IAP silencing during adult spermatogenesis, where H3K9me2 is sufficient to maintain L1 but not IAP silencing. Finally, we demonstrate that repression of retrotransposon activation in spermatogonia is important for the survival of this population and testicular homeostasis.

Evaluation of candidate spermatogonial markers ID4 and GPR125 in testes of adult human cadaveric organ donors

The optimal markers for human spermatogonial stem cells (SSCs) are not known. Among the genes recently linked to SSCs in mice and other animals are the basic helix-loop-helix transcription factor ID4 and the orphan G-protein-coupled receptor GPR125. While ID4 and GPR125 are considered putative markers for SSCs, they have not been evaluated for coexpression in human tissue. Furthermore, neither the size nor the character of the human spermatogonial populations that express ID4 and GPR125, respectively, are known. A major barrier to addressing these questions is the availability of healthy adult testis tissue from donors with no known reproductive health problems. To overcome this obstacle, we have employed healthy testicular tissue from a novel set of organ donors (n = 16; aged 17-68 years) who were undergoing post-mortem clinical organ procurement. Using immunolabelling, we found that ID4 and GPR125 are expressed on partially overlapping spermatogonial populations and are more broadly expressed in the normal adult human testis. In addition, we found that expression of ID4 remained stable during ageing. These findings suggest that ID4 and GPR125 could be efficacious for identifying previously unrecognized human spermatogonial subpopulations in conjunction with other putative human stem cell markers, both in younger and older donors.

Skp1-Cullin-F-box (SCF)-type ubiquitin ligase FBXW7 negatively regulates spermatogonial stem cell self-renewal

Spermatogonial stem cells (SSCs) undergo self-renewal divisions to support spermatogenesis throughout life. Although several positive regulators of SSC self-renewal have been discovered, little is known about the negative regulators. Here, we report that F-box and WD-40 domain protein 7 (FBXW7), a component of the Skp1-Cullin-F-box-type ubiquitin ligase, is a negative regulator of SSC self-renewal. FBXW7 is expressed in undifferentiated spermatogonia in a cell cycle-dependent manner. Although peptidyl-prolyl cis/trans isomerase NIMA-interacting 1 (PIN1), essential for spermatogenesis, is thought to destroy FBXW7, Pin1 depletion decreased FBXW7 expression. Spermatogonial transplantation showed that Fbxw7 overexpression compromised SSC activity whereas Fbxw7 deficiency enhanced SSC colonization and caused accumulation of undifferentiated spermatogonia, suggesting that the level of FBXW7 is critical for self-renewal and differentiation. Screening of putative FBXW7 targets revealed that Fbxw7 deficiency up-regulated myelocytomatosis oncogene (MYC) and cyclin E1 (CCNE1). Although depletion of Myc/Mycn or Ccne1/Ccne2 compromised SSC activity, overexpression of Myc, but not Ccne1, increased colonization of SSCs. These results suggest that FBXW7 regulates SSC self-renewal in a negative manner by degradation of MYC.

DNMT3L promotes quiescence in postnatal spermatogonial progenitor cells

The ability of adult stem cells to reside in a quiescent state is crucial for preventing premature exhaustion of the stem cell pool. However, the intrinsic epigenetic factors that regulate spermatogonial stem cell quiescence are largely unknown. Here, we investigate in mice how DNA methyltransferase 3-like (DNMT3L), an epigenetic regulator important for interpreting chromatin context and facilitating de novo DNA methylation, sustains the long-term male germ cell pool. We demonstrated that stem cell-enriched THY1(+) spermatogonial stem/progenitor cells (SPCs) constituted a DNMT3L-expressing population in postnatal testes. DNMT3L influenced the stability of promyelocytic leukemia zinc finger (PLZF), potentially by downregulating Cdk2/CDK2 expression, which sequestered CDK2-mediated PLZF degradation. Reduced PLZF in Dnmt3l KO THY1(+) cells released its antagonist, Sal-like protein 4A (SALL4A), which is associated with overactivated ERK and AKT signaling cascades. Furthermore, DNMT3L was required to suppress the cell proliferation-promoting factor SALL4B in THY1(+) SPCs and to prevent premature stem cell exhaustion. Our results indicate that DNMT3L is required to delicately balance the cycling and quiescence of SPCs. These findings reveal a novel role for DNMT3L in modulating postnatal SPC cell fate decisions.

Chromatin and transcription transitions of mammalian adult germline stem cells and spermatogenesis

Adult germline stem cells (AGSCs) self-renew (Thy1(+) enriched) or commit to gametogenesis (Kit(+) enriched). To better understand how chromatin regulates AGSC biology and gametogenesis, we derived stage-specific high-resolution profiles of DNA methylation, 5hmC, histone modifications/variants, and RNA-seq in AGSCs and during spermatogenesis. First, we define striking signaling and transcriptional differences between AGSC types, involving key self-renewal and proliferation pathways. Second, key pluripotency factors (e.g., Nanog) are silent in AGSCs and bear particular chromatin/DNAme attributes that may "poise" them for reactivation after fertilization. Third, AGSCs display chromatin "poising/bivalency" of enhancers and promoters for embryonic transcription factors. Remarkably, gametogenesis occurs without significant changes in DNAme and instead involves transcription of DNA-methylated promoters bearing high RNAPol2, H3K9ac, H3K4me3, low CG content, and (often) 5hmC. Furthermore, key findings were confirmed in human sperm. Here, we reveal AGSC signaling asymmetries and chromatin/DNAme strategies in AGSCs to poise key transcription factors and to activate DNA-methylated promoters during gametogenesis.

The small GTPase Rheb is required for spermatogenesis but not oogenesis

The process of germ cell development is under the tight control of various signaling pathways, among which the PI3K-Akt-mTOR pathway is of critical importance. Previous studies have demonstrated sex-specific roles for several components of this pathway. In the current study, we aimed to evaluate the role of Rheb, a member of the small GTPase superfamily and a critical component for mTORC1 activation, in male and female gametogenesis. The function of Rheb in development and the nervous system has been extensively studied, but little is known about its role in the germ line. We have exploited genetic approaches in the mouse to study the role of Rheb in the germ line and have identified an essential role in spermatogenesis. Conditional knockout (cKO) of Rheb in the male germ line resulted in severe oligoasthenoteratozoospermia and male sterility. More detailed phenotypic analyses uncovered an age-dependent meiotic progression defect combined with subsequent abnormalities in spermiogenesis as evidenced by abnormal sperm morphology. In the female, however, germ-cell specific inactivation of Rheb was not associated with any discernible abnormality; these cKO mice were fertile with morphologically unremarkable ovaries, normal primordial follicle formation, and subsequent follicle maturation. The absence of an abnormal ovarian phenotype is striking given previous studies demonstrating a critical role for the mTORC1 pathway in the maintenance of primordial follicle pool. In conclusion, our findings demonstrate an essential role of Rheb in diverse aspects of spermatogenesis but suggest the existence of functionally redundant factors that can compensate for Rheb deficiency within oocytes.

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.

Mechanistic target of rapamycin complex 1 is critical for invariant natural killer T-cell development and effector function

The mechanisms that control invariant natural killer T (iNKT)-cell development and function are still poorly understood. The mechanistic or mammalian target of rapamycin (mTOR) integrates various environmental signals/cues to regulate cell growth, proliferation, metabolism, and survival. We report here that ablation of mTOR complex 1 (mTORC1) signaling by conditionally deleting Raptor causes severe defects in iNKT-cell development at early stages, leading to drastic reductions in iNKT-cell numbers in the thymus and periphery. In addition, loss of Raptor impairs iNKT-cell proliferation and production of cytokines upon α-galactosylceramide stimulation in vitro and in vivo, and inhibits liver inflammation in an iNKT cell-mediated hepatitis model. Furthermore, Raptor deficiency and rapamycin treatment lead to aberrant intracellular localization and functional impairment of promyelocytic leukemia zinc-finger, a transcription factor critical for iNKT-cell development and effector programs. Our findings define an essential role of mTORC1 to direct iNKT-cell lineage development and effector function.

Promyelocytic leukaemia zinc finger maintains self-renewal of male germline stem cells (mGSCs) and its expression pattern in dairy goat testis

OBJECTIVES

Previous studies have shown that promyelocytic leukaemia zinc finger (PLZF) is a spermatogonia-specific transcription factor in the testis, required to regulate self-renewal and maintenance of the spermatogonia stem cell. Up to now, expression and function of PLZF in the goat testis has not been known. The objectives of this study were to investigate PLZF expression pattern in the dairy goat and its effect on male goat germline stem cell (mGSC) self-renewal and differentiation.

MATERIALS AND METHODS

Testis development and expression patterns of PLZF in the dairy goat were analysed by haematoxylin and eosin staining, immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, effects of PLZF overexpression on mGSC self-renewal and differentiation were evaluated by quantitative RT-PCR (QRT-PCR), immunofluorescence and BrdU incorporation assay.

RESULTS

Promyelocytic leukaemia zinc finger was essential for dairy goat testis development and expression of several proliferation and pluripotency-associated proteins including OCT4, C-MYC were upregulated by PLZF overexpression. The study demonstrated that PLZF played a key role in maintaining self-renewal of mGSCs and its overexpression enhanced expression of proliferation-associated genes.

CONCLUSIONS

Promyelocytic leukaemia zinc finger could function in the dairy goat as well as in other species in maintaining self-renewal of germline stem cells and this study provides a model to study the mechanism on self-renewal and differentiation of mGSCs in livestock.

Spermatogonial stem cell functions in physiological and pathological conditions

Sperm have a vital role in the continuity of a species by contributing genetic information to the next generation. Production of these specialized gametes in numbers sufficient to confer normal fertility occurs via cycling of the spermatogenic lineage, a process referred to as spermatogenesis. Continuity relies on the activities of a self-renewing reservoir of spermatogonial stem cells (SSCs) from which progenitors will arise that transiently amplify in number before committing to a pathway of terminal differentiation. A primary population of SSCs is established during neonatal development from a pool of quiescent gonocyte precursors that forms in embryogenesis. Disruption of this process has dire consequences on maintenance of a cycling spermatogenic lineage in adulthood. At present, the molecular mechanisms underlying initial formation of the SSC pool are largely undefined. However, several transcription factors and posttranscriptional regulators have been identified as important regulators of SSC self-renewal from studies with mutant mouse models and experimental manipulation within primary cultures of mouse SSCs. Importantly, loss of function of these self-renewal factors may be underlying causes of infertility. Furthermore, disruption in the establishment of the SSC state within gonocytes or misregulation of self-renewal may manifest as testicular germ cell tumors in postnatal life.

Akt modes of stem cell regulation: more than meets the eye?

Akt signaling regulates many cellular functions that are essential for the proper balance between self-renewal and differentiation of tissue-specific and embryonic stem cells (SCs). However, the roles of Akt and its downstream signaling in SC regulation are rather complex, as Akt activation can either promote SC self-renewal or depletion in a context-dependent manner. In this review we have evidenced three "modes" of Akt-dependent SC regulation, which can be exemplified by three different SC types. In particular, we will discuss: 1) the integration of Akt signaling within the "core" SC signaling circuitry in the maintenance of SC self-renewal and pluripotency (embryonic SCs); 2) quantitative changes in Akt signaling in SC metabolic activity and exit from quiescence (hematopoietic SCs); 3) qualitative changes of Akt signaling in SC regulation: signaling compartment-talization and isoform-specific functions of Akt proteins in SC self-renewal and differentiation (limbal-corneal keratinocyte SCs). These diverse modes of action are not to be intended as mutually exclusive. Rather, it is likely that Akt proteins participate with multiple parallel mechanisms to regulation of the same SC type. We propose that under specific circumstances dictated by distinct developmental stages, differentiation programs or tissue culture conditions, one mode of Akt action prevails over the others in determining SC fates.

Spermatogonial stem cell self-renewal and development

Spermatogenesis originates from spermatogonial stem cells (SSCs). Development of the spermatogonial transplantation technique in 1994 provided the first functional assay to characterize SSCs. In 2000, glial cell line-derived neurotrophic factor was identified as a SSC self-renewal factor. This discovery not only provided a clue to understand SSC self-renewing mechanisms but also made it possible to derive germline stem (GS) cell cultures in 2003. In vitro culture of GS cells demonstrated their potential pluripotency and their utility in germline modification. However, in vivo SSC analyses have challenged the traditional concept of SSC self-renewal and have revealed their relationship with the microenvironment. An improved understanding of SSC self-renewal through functional assays promises to uncover fundamental principles of stem cell biology and will enable us to use these cells for applications in animal transgenesis and medicine.