Role of c-kit in mouse spermatogenesis: identification of spermatogonia as a specific site of c-kit expression and function

Adrenocorticotropic hormone and its receptor as a novel testicular system involves in the development of spermatogenesis

AIMS

To identify functional membrane-associate-specific SSC markers and examine the development of these cells under in vitro conditions.

MATERIALS AND METHODS

Cells were enzymatically isolated from seminiferous tubules (STs) of immature mice. Spermatogonial cells (Thy1, alpha-6-integrin, and C-KIT) were sorted by FACS. RNA was extracted from these cells for RNAseq analysis. The effect of adrenocorticotropic hormone (ACTH) - the ligand of MC2R- on the development of mouse spermatogonial cells was performed in vitro using a methylcellulose culture system (MCS). Immunofluorescence staining was used to localize MC2R-positive cells in the testes of immature and adult humans and mice and testes of busulfan-treated immature mice.

KEY FINDINGS

Our RNAseq analysis revealed a high expression of melanocortin receptor 2 (MC2R) in Thy1-positive sorted cells. MC2R-positive cells were localized in the periphery of the STs of humans (prepubertal and adults) and mice at immature and adult ages (normal and busulfan-treated mice). MC2R was doubled stained with PLZF and CDH1 (SSC markers). ACTH was localized in mouse testicular germ cells (pre-meiotic, meiotic, and post-meiotic cells) and somatic cells (Sertoli, Leydig, and peritubular cells). The addition of ACTH to isolated cells from mouse STs in MCS significantly increased the development of pre-meiotic and meiotic/post-meiotic cells in vitro.

SIGNIFICANCE

We were able to identify, for the first time, a novel membrane-associated and functional SSC marker (MC2R) with relation to ACTH. This marker can be used in future male fertility preservation strategies. Furthermore, we explored a novel testicular system (ACTH system) that regulates the development of spermatogenesis.

hnRNPC Functions with HuR to Regulate Alternative Splicing in an m6A-Dependent Manner and is Essential for Meiosis

N6-methyladenosine (m6A) and its reader proteins are involved in pre-mRNA processing and play a variety of roles in numerous biological processes. However, much remains to be understood about the regulation of m6A and the function of its specific readers during meiotic processes. Here, this study shows that the potential m6A reader protein hnRNPC is essential for both male and female meiosis in mice. Germ cell-specific knockout of Hnrnpc causes meiotic arrest at pachynema in male mice. Specifically, hnRNPC-deficient males show abnormal meiosis initiation and defective meiotic progression, ultimately leading to meiotic arrest at the pachytene stage. Interestingly, hnRNPC-null females show similar meiotic defects to males. Mechanistically, this study discovers that in male germ cells, hnRNPC works with HuR to directly bind and modulate alternative splicing of meiotic-related genes (e.g., Sycp1, Brca1, and Smc5) in an m6A-dependent manner during spermatogenesis. Collectively, these findings reveal hnRNPC as a critical factor for meiosis and contribute to a mechanistic understanding of the hnRNPC-HuR interaction in alternative splicing of mRNAs during germ cell development.

Utilization of the QuPath open-source software platform for analysis of mammalian spermatogenesis†

The adult mammalian testis is filled with seminiferous tubules, which contain somatic Sertoli cells along with germ cells undergoing all phases of spermatogenesis. During spermatogenesis in postnatal mice, male germ cells undergo at least 17 different nomenclature changes as they proceed through mitosis as spermatogonia (=8), meiosis as spermatocytes (=6), and spermiogenesis as spermatids (=3). Adding to this complexity, combinations of germ cells at each of these stages of development are clumped together along the length of the seminiferous tubules. Due to this, considerable expertise is required for investigators to accurately analyze changes in spermatogenesis in animals that have spontaneous mutations, have been genetically modified (transgenic or knockout/knockin), or have been treated with pharmacologic agents. Here, we leverage our laboratory's expertise in spermatogenesis to optimize the open-source "Quantitative Pathology & Bioimage Analysis" software platform for automated analyses of germ and somatic cell populations in both the developing and adult mammalian testis.

Cdkn1c orchestrates a molecular network that regulates euploidy of male mouse germline stem cells

Karyotype instability in the germline leads to infertility. Unlike the female germline, the male germline continuously produces fertile sperm throughout life. Here, we present a molecular network responsible for maintaining karyotype stability in the male mouse germline. Loss of the cyclin-dependent kinase inhibitor Cdkn1c in undifferentiated spermatogonia induced degeneration of spermatogenesis prior to entry into the differentiating spermatogonia stage. In vitro analysis of mouse spermatogonial stem cells revealed that CDKN1C localized to spindle microtubules during metaphase, and that disrupted microtubule dynamics increased its phosphorylation. Cdkn1c deficiency activated the spindle assembly checkpoint and led to centrosome amplification, premature chromosome segregation, and loss of AURKB, and ultimately TRP53-dependent apoptosis. Trp53-deficient spermatogonial stem cells exhibited karyotype defects, but proliferated normally despite reduced CDKN1C and AURKB expression. In contrast, Aurkb depletion upregulated TRP53 and CDKN1C, suggesting a negative feedback loop to maintain euploidy. Thus, Cdkn1c regulates the male germline karyotype.

Claudin-11 regulates immunological barrier formation and spermatogonial proliferation through stem cell factor

Tight junctions (TJs) between adjacent Sertoli cells are believed to form immunological barriers that protect spermatogenic cells expressing autoantigens from autoimmune responses. However, there is no direct evidence that Sertoli cell TJs (SCTJs) do indeed form immunological barriers. Here, we analyzed male mice lacking claudin-11 (Cldn11), which encodes a SCTJ component, and found autoantibodies against antigens of spermatocytes/spermatids in their sera. Defective spermatogenesis in Cldn11-deficient mice was not restored on a recombination activating gene 2 (Rag2) knockout background lacking mature T and B lymphocytes. This suggests that adaptive immune responses to spermatogenic cells are not a cause of defective spermatogenesis in Cldn11-deficient mice. Further analyses showed that Cldn11 knockout impaired Sertoli cell polarization, localization of stem cell factor (SCF) (a key molecule for maintaining differentiating spermatogonia) to the basal compartment of seminiferous tubules, and also proliferation of differentiating spermatogonia. We propose that CLDN11 creates a microenvironment for SCF-mediated spermatogonial proliferation at the basal compartment via Sertoli cell polarization.

DNA methylation modulates nucleosome retention in sperm and H3K4 methylation deposition in early mouse embryos

In the germ line and during early embryogenesis, DNA methylation (DNAme) undergoes global erasure and re-establishment to support germ cell and embryonic development. While DNAme acquisition during male germ cell development is essential for setting genomic DNA methylation imprints, other intergenerational roles for paternal DNAme in defining embryonic chromatin are unknown. Through conditional gene deletion of the de novo DNA methyltransferases Dnmt3a and/or Dnmt3b, we observe that DNMT3A primarily safeguards against DNA hypomethylation in undifferentiated spermatogonia, while DNMT3B catalyzes de novo DNAme during spermatogonial differentiation. Failing de novo DNAme in Dnmt3a/Dnmt3b double deficient spermatogonia is associated with increased nucleosome occupancy in mature sperm, preferentially at sites with higher CpG content, supporting the model that DNAme modulates nucleosome retention in sperm. To assess the impact of altered sperm chromatin in formatting embryonic chromatin, we measure H3K4me3 occupancy at paternal and maternal alleles in 2-cell embryos using a transposon-based tagging approach. Our data show that reduced DNAme in sperm renders paternal alleles permissive for H3K4me3 establishment in early embryos, independently of possible paternal inheritance of sperm born H3K4me3. Together, this study provides evidence that paternally inherited DNAme directs chromatin formation during early embryonic development.

Non-alkylating agents-induced gonadotoxicity in pre-pubertal males: Insights on the clinical and pre-clinical front

Whilst chemotherapy regimens have proven to be more successful for pediatric cancer patients over the years, their influence on long-term side effects is relatively poorly understood. One of the possible targets is the gonads, with gonadotoxic agents representing those that threaten the patient's ability to have children post surviving the primary disease treatment. Many risk stratification guidelines have categorized these agents based on the severity of their effect on the pre-pubertal testis. While the consensus is that those agents factored with a cyclophosphamide equivalent dosage pose the highest threat to fertility (e.g. alkylating agents), other agents might still contribute to a reduced testis function; especially in the case of combination therapies. Besides, it is important to note that studies deciphering the effect of other non-alkylating agents on the pre-pubertal testis lack standardized conclusions for clinically relevant outcomes. This makes it imperative to ensure the knowledge gap is addressed between the clinic and pre-clinic to understand potential gonadotoxic effects, ultimately leading to improved patient care. Therefore, this review will summarize the key findings in understanding the gonadotoxic effects of the most commonly researched non-alkylating agents: vincristine, etoposide, doxorubicin, and imatinib on the pre-pubertal testis.

The action of retinoic acid on spermatogonia in the testis

For mammalian spermatogenesis to proceed normally, it is essential that the population of testicular progenitor cells, A undifferentiated spermatogonia (Aundiff), undergoes differentiation during the A to A1 transition that occurs at the onset of spermatogenesis. The commitment of the Aundiff population to differentiation and leaving a quiescent, stem-like state gives rise to all the spermatozoa produced across the lifespan of an individual, and ultimately determines male fertility. The action of all-trans retinoic acid (atRA) on the Aundiff population is the determining factor that induces this change. Sertoli cells, omnipresent, nurse cells within the mammalian testis are responsible for synthesizing the atRA that prompts this change in the neonatal testicular environment. The mechanism of atRA synthesis and signaling has been robustly explored and, in this review, we have summarized what is currently known about the action of testicular atRA at the onset of spermatogenesis. We have combined this with evidence gained from prominent genetic studies that have further elucidated the function of genes critical to atRA synthesis. We have additionally described the effects of the first pulse of atRA delivered to the germ cells of the testis, which has been investigated using WIN 18,446 treatment which prevents atRA synthesis and induces spermatogenic synchrony. This method provides unparalleled resolution into cell and stage specific testicular changes, and combined with transgenic animal models, has allowed researchers to elucidate much regarding the onset of spermatogenesis.

Imatinib decreases germ cell survival and germline stem cell proliferation in rodent testis ex vivo and in vitro

BACKGROUND

Imatinib and dasatinib are tyrosine kinase inhibitors (TKIs) increasingly used to treat several diseases in both children and adults at fertile age. We have previously shown that imatinib has adverse effects on developing testis, and imatinib-treated male patients have been reported to have reduced sperm counts. However, the cellular level effects of imatinib and dasatinib on adult male germ cells and germline stem cells (mGSCs) have not been thoroughly investigated.

OBJECTIVES

To analyze whether imatinib or dasatinib exposure ex vivo and in vitro is harmful to adult male rodent germ cells and mGSCs.

MATERIALS AND METHODS

Seminiferous tubule segments of adult male mouse or rat were cultured in the presence or the absence of imatinib or dasatinib. Stage-specific effects were monitored by 3H-thymidine incorporation assay (DNA synthesis), immunohistochemistry (cleaved Caspase-3; apoptosis), immunofluorescence (KI67, GFRα1, STRA8, c-KIT, LIN28A; proliferation and spermatogonial differentiation) and flow cytometry (Hoechst). Mouse mGSCs were exposed to imatinib and dasatinib to study proliferation, apoptosis, and differentiation.

RESULTS

Imatinib decreased stage-specific DNA synthesis, and induced apoptosis in cultured rat seminiferous tubule segments. Imatinib also had an adverse effect on mGSC proliferation both in vitro and ex vivo, but did not induce cell death in cultured mGSCs. Imatinib did not impinge on induction of spermatogonial differentiation but suppressed c-KIT expression in nascent differentiating spermatogonia, providing a plausible mechanism for its pro-apoptotic function in spermatogenic cells. Clinically relevant doses of dasatinib did not induce apoptosis in seminiferous tubules but decreased mGSC colony growth in vitro.

CONCLUSIONS

Imatinib exposure ex vivo and in vitro impinges on male rodent germ cell proliferation and survival. The plausible mechanism in spermatogenic cells is the inhibition of SCF/c-KIT signaling, and reduced expression of c-KIT. Dasatinib did not show significant adverse effects with clinical doses ex vivo but inhibited mGSC colony growth in vitro.

Destabilization of mRNAs enhances competence to initiate meiosis in mouse spermatogenic cells

The specialized cell cycle of meiosis transforms diploid germ cells into haploid gametes. In mammals, diploid spermatogenic cells acquire the competence to initiate meiosis in response to retinoic acid. Previous mouse studies revealed that MEIOC interacts with RNA-binding proteins YTHDC2 and RBM46 to repress mitotic genes and to promote robust meiotic gene expression in spermatogenic cells that have initiated meiosis. Here, we have used the enhanced resolution of scRNA-seq and bulk RNA-seq of developmentally synchronized spermatogenesis to define how MEIOC molecularly supports early meiosis in spermatogenic cells. We demonstrate that MEIOC mediates transcriptomic changes before meiotic initiation, earlier than previously appreciated. MEIOC, acting with YTHDC2 and RBM46, destabilizes its mRNA targets, including the transcriptional repressors E2f6 and Mga, in mitotic spermatogonia. MEIOC thereby derepresses E2F6- and MGA-repressed genes, including Meiosin and other meiosis-associated genes. This confers on spermatogenic cells the molecular competence to, in response to retinoic acid, fully activate the transcriptional regulator STRA8-MEIOSIN, which is required for the meiotic G1/S phase transition and for meiotic gene expression. We conclude that, in mice, mRNA decay mediated by MEIOC-YTHDC2-RBM46 enhances the competence of spermatogenic cells to initiate meiosis.

Spermatogonial stem cells in the 129 inbred strain exhibit unique requirements for self-renewal

Spermatogonial stem cells (SSCs) undergo self-renewal division to sustain spermatogenesis. Although it is possible to derive SSC cultures in most mouse strains, SSCs from a 129 background never proliferate under the same culture conditions, suggesting they have distinct self-renewal requirements. Here, we established long-term culture conditions for SSCs from mice of the 129 background (129 mice). An analysis of 129 testes showed significant reduction of GDNF and CXCL12, whereas FGF2, INHBA and INHBB were higher than in testes of C57BL/6 mice. An analysis of undifferentiated spermatogonia in 129 mice showed higher expression of Chrna4, which encodes an acetylcholine (Ach) receptor component. By supplementing medium with INHBA and Ach, SSC cultures were derived from 129 mice. Following lentivirus transduction for marking donor cells, transplanted cells re-initiated spermatogenesis in infertile mouse testes and produced transgenic offspring. These results suggest that the requirements of SSC self-renewal in mice are diverse, which has important implications for understanding self-renewal mechanisms in various animal species.

Loss of function of male-specific lethal 3 (Msl3) does not affect spermatogenesis in rodents

BACKGROUND

Male-specific lethal 3 (Msl3) is a member of the chromatin-associated male-specific lethal MSL complex, which is responsible for the transcriptional upregulation of genes on the X chromosome in males of Drosophila. Although the dosage complex operates differently in mammals, the Msl3 gene is conserved from flies to humans. Msl3 is required for meiotic entry during Drosophila oogenesis. Recent reports indicate that also in primates, Msl3 is expressed in undifferentiated germline cells before meiotic entry. However, if Msl3 plays a role in the meiotic entry of mammals has yet to be explored.

RESULTS

To understand, if Msl3a plays a role in the meiotic entry of mammals, we used mouse spermatogenesis as a study model. Analyses of single-cell RNA-seq data revealed that, in mice, Msl3 is mostly expressed in meiotic cells. To test the role of Msl3 in meiosis, we used a male germline-specific Stra8-iCre driver and a newly generated Msl3flox conditional knock-out mouse line. Msl3 conditional loss-of-function in spermatogonia did not cause spermatogenesis defects or changes in the expression of genes related to meiosis.

CONCLUSIONS

Our data suggest that, in mice, Msl3 exhibits delayed expression compared to Drosophila and primates, and loss-of-function mutations disrupting the chromodomain of Msl3 alone do not impede meiotic entry in rodents.

Stage-specific regulation of undifferentiated spermatogonia by AKT1S1-mediated AKT-mTORC1 signaling during mouse spermatogenesis

Undifferentiated spermatogonia are composed of a heterogeneous cell population including spermatogonial stem cells (SSCs). Molecular mechanisms underlying the regulation of various spermatogonial cohorts during their self-renewal and differentiation are largely unclear. Here we show that AKT1S1, an AKT substrate and inhibitor of mTORC1, regulates the homeostasis of undifferentiated spermatogonia. Although deletion of Akt1s1 in mouse appears not grossly affecting steady-state spermatogenesis and male mice are fertile, the subset of differentiation-primed OCT4+ spermatogonia decreased significantly, whereas self-renewing GFRα1+ and proliferating PLZF+ spermatogonia were sustained. Both neonatal prospermatogonia and the first wave spermatogenesis were greatly reduced in Akt1s1-/- mice. Further analyses suggest that OCT4+ spermatogonia in Akt1s1-/- mice possess altered PI3K/AKT-mTORC1 signaling, gene expression and carbohydrate metabolism, leading to their functionally compromised developmental potential. Collectively, these results revealed an important role of AKT1S1 in mediating the stage-specific signals that regulate the self-renewal and differentiation of spermatogonia during mouse spermatogenesis.

Restoration of fertility in nonablated recipient mice after spermatogonial stem cell transplantation

Spermatogonial stem cell (SSC) transplantation is a valuable tool for studying stem cell-niche interaction. However, the conventional approach requires the removal of endogenous SSCs, causing damage to the niche. Here we introduce WIN18,446, an ALDH1A2 inhibitor, to enhance SSC colonization in nonablated recipients. Pre-transplantation treatment with WIN18,446 induced abnormal claudin protein expression, which comprises the blood-testis barrier and impedes SSC colonization. Consequently, WIN18,446 increased colonization efficiency by 4.6-fold compared with untreated host. WIN18,446-treated testes remained small despite the cessation of WIN18,446, suggesting its irreversible effect. Offspring were born by microinsemination using donor-derived sperm. While WIN18,446 was lethal to busulfan-treated mice, cyclophosphamide- or radiation-treated animals survived after WIN18,446 treatment. Although WIN18,446 is not applicable to humans due to toxicity, similar ALDH1A2 inhibitors may be useful for SSC transplantation into nonablated testes, shedding light on the role of retinoid metabolism on SSC-niche interactions and advancing SSC research in animal models and humans.

KDM6A/UTX promotes spermatogenic gene expression across generations and is not required for male fertility†

Paternal chromatin undergoes extensive structural and epigenetic changes during mammalian spermatogenesis, producing sperm with an epigenome optimized for the transition to embryogenesis. Lysine demethylase 6a (KDM6A, also called UTX) promotes gene activation in part via demethylation of H3K27me3, a developmentally important repressive modification abundant throughout the epigenome of spermatogenic cells and sperm. We previously demonstrated increased cancer risk in genetically wild-type mice derived from a paternal germ line lacking Kdm6a (Kdm6a cKO), indicating a role for KDM6A in regulating heritable epigenetic states. However, the regulatory function of KDM6A during spermatogenesis is not known. Here, we show that Kdm6a is transiently expressed in spermatogenesis, with RNA and protein expression largely limited to late spermatogonia and early meiotic prophase. Kdm6a cKO males do not have defects in fertility or the overall progression of spermatogenesis. However, hundreds of genes are deregulated upon loss of Kdm6a in spermatogenic cells, with a strong bias toward downregulation coinciding with the time when Kdm6a is expressed. Misregulated genes encode factors involved in chromatin organization and regulation of repetitive elements, and a subset of these genes was persistently deregulated in the male germ line across two generations of offspring of Kdm6a cKO males. Genome-wide epigenetic profiling revealed broadening of H3K27me3 peaks in differentiating spermatogonia of Kdm6a cKO mice, suggesting that KDM6A demarcates H3K27me3 domains in the male germ line. Our findings highlight KDM6A as a transcriptional activator in the mammalian male germ line that is dispensable for spermatogenesis but important for safeguarding gene regulatory state intergenerationally.

Postnatal development of mouse spermatogonial stem cells as determined by immunophenotype, regenerative capacity, and long-term culture-initiating ability: a model for practical applications

Spermatogonial stem cells (SSCs) are the foundation of life-long spermatogenesis. While SSC research has advanced greatly over the past two decades, characterization of SSCs during postnatal development has not been well documented. Using the mouse as a model, in this study, we defined the immunophenotypic profiles of testis cells during the course of postnatal development using multi-parameter flow cytometry with up to five cell-surface antigens. We found that the profiles progress over time in a manner specific to developmental stages. We then isolated multiple cell fractions at different developmental stages using fluorescent-activated cell sorting (FACS) and identified specific cell populations with prominent capacities to regenerate spermatogenesis upon transplantation and to initiate long-term SSC culture. The data indicated that the cell fraction with the highest level of regeneration capacity exhibited the most prominent potential to initiate SSC culture, regardless of age. Interestingly, refinement of cell fractionation using GFRA1 and KIT did not lead to further enrichment of regenerative and culture-initiating stem cells, suggesting that when a high degree of SSC enrichment is achieved, standard markers of SSC self-renewal or commitment may lose their effectiveness to distinguish cells at the stem cell state from committed progenitors. This study provides a significant information resource for future studies and practical applications of mammalian SSCs.

Tissue engineering studies in male infertility disorder

Infertility is an important issue among couples worldwide which is caused by a variety of complex diseases. Male infertility is a problem in 7% of all men. In vitro spermatogenesis (IVS) is the experimental approach that has been developed for mimicking seminiferous tubules-like functional structures in vitro. Currently, various researchers are interested in finding and developing a microenvironmental condition or a bioartificial testis applied for fertility restoration via gamete production in vitro. The tissue engineering (TE) has developed new approaches to treat male fertility preservation through development of functional male germ cells. This makes TE a possible future strategy for restoration of male fertility. Although 3D culture systems supply the perception of the effect of cellular interactions in the process of spermatogenesis, formation of a native gradient of autocrine/paracrine factors in 3D culture systems have not been considered. These results collectively suggest that maintaining the microenvironment of testicular cells even in the form of a 3D-culture system is crucial in achieving spermatogenesis ex vivo. It is also possible to engineer the testicular structures using biomaterials to provide a supporting scaffold for somatic and stem cells. The insemination of these cells with GFs is possible for temporally and spatially adjusted release to mimic the microenvironment of the in situ seminiferous epithelium. This review focuses on recent studies and advances in the application of TE strategies to cell-tissue culture on synthetic or natural scaffolds supplemented with growth factors.

Effects of Growth Factors on In Vitro Culture of Neonatal Piglet Testicular Tissue Fragments

In vitro spermatogenesis (IVS) has important applications including fertility preservation of prepubertal cancer patients; however, thus far, IVS has only been achieved using mouse models. To study the effects of growth factors on the maintenance of testicular tissue integrity, germ cell numbers, and potential induction of IVS using a porcine model, we cultured small testicular fragments (~2 mg) from 1-wk-old piglets under six different media conditions (DMEM + 10%KSR alone or supplemented with GDNF, bFGF, SCF, EGF, or a combination of all) for 8 weeks. Overall, tissues supplemented with GDNF and bFGF had the greatest seminiferous tubule integrity and least number of apoptotic cells. GDNF-supplemented tissues had the greatest number of gonocytes per tubule, followed by bFGF-supplemented tissues. There was evidence of gradual Sertoli cell maturation in all groups. Moreover, histological examination and the expression of c-KIT (a marker of differentiating spermatogonia and spermatocytes) and STRA8 (a marker of the pre/meiotic stage germ cells) confirmed the induction of IVS in all groups. However, GDNF- and bFGF-supplemented tissue cultures had greater numbers of seminiferous tubules with spermatocytes compared to other groups. In conclusion, overall, GDNF and bFGF supplementation better maintained the tissue integrity and gonocyte numbers and induced IVS in cultured testicular tissues.

Spermatogonial fate in mice with increased activin A bioactivity and testicular somatic cell tumours

Adult male fertility depends on spermatogonial stem cells (SSCs) which undergo either self-renewal or differentiation in response to microenvironmental signals. Activin A acts on Sertoli and Leydig cells to regulate key aspects of testis development and function throughout life, including steroid production. Recognising that activin A levels are elevated in many pathophysiological conditions, this study investigates effects of this growth factor on the niche that determines spermatogonial fate. Although activin A can promote differentiation of isolated spermatogonia in vitro, its impacts on SSC and spermatogonial function in vivo are unknown. To assess this, we examined testes of Inha KO mice, which feature elevated activin A levels and bioactivity, and develop gonadal stromal cell tumours as adults. The GFRA1+ SSC-enriched population was more abundant and proliferative in Inha KO compared to wildtype controls, suggesting that chronic elevation of activin A promotes a niche which supports SSC self-renewal. Intriguingly, clusters of GFRA1+/EOMES+/LIN28A- cells, resembling a primitive SSC subset, were frequently observed in tubules adjacent to tumour regions. Transcriptional analyses of Inha KO tumours, tubules adjacent to tumours, and tubules distant from tumour regions revealed disrupted gene expression in each KO group increased in parallel with tumour proximity. Modest transcriptional changes were documented in Inha KO tubules with complete spermatogenesis. Importantly, tumours displaying upregulation of activin responsive genes were also enriched for factors that promote SSC self-renewal, including Gdnf, Igf1, and Fgf2, indicating the tumours generate a supportive microenvironment for SSCs. Tumour cells featured some characteristics of adult Sertoli cells but lacked consistent SOX9 expression and exhibited an enhanced steroidogenic phenotype, which could arise from maintenance or acquisition of a fetal cell identity or acquisition of another somatic phenotype. Tumour regions were also heavily infiltrated with endothelial, peritubular myoid and immune cells, which may contribute to adjacent SSC support. Our data show for the first time that chronically elevated activin A affects SSC fate in vivo. The discovery that testis stromal tumours in the Inha KO mouse create a microenvironment that supports SSC self-renewal but not differentiation offers a strategy for identifying pathways that improve spermatogonial propagation in vitro.

Loss Of Chromodomain of Male-Specific Lethal 3 (MSL3) Does Not Affect Spermatogenesis In Rodents

Msl3 is a member of the chromatin-associated male-specific lethal MSL complex which is responsible for the transcriptional upregulation of genes on the X chromosome in males Drosophila. Although the dosage complex operates differently in mammals, the Msl3 gene is conserved from flies to humans. Msl3 is required for meiotic entry during Drosophila oogenesis. Recent reports indicate that also in primates, Msl3 is expressed in undifferentiated germline cells before meiotic entry. However, if Msl3 plays a role in the meiotic entry of mammals has yet to be explored. To study this, we used mouse spermatogenesis as a study model. Analyses of single cells RNA-seq data revealed that, in mice, Msl3 is mostly expressed in meiotic cells. To test the role of Msl3 in meiosis, we used a male germline-specific Stra8-iCre driver and a newly generated Msl3flox conditional knock-out mouse line. Msl3 conditional loss-of-function in spermatogonia did not cause spermatogenesis defects or changes in the expression of genes related to meiosis. Our data suggest that, in mice, Msl3 exhibits delayed expression compared to Drosophila and primates, and loss-of-function mutations disrupting the chromodomain of Msl3 alone do not impede meiotic entry in rodents.

Chromatin Remodeling via Retinoic Acid Action during Murine Spermatogonial Development

Spermatogonial differentiation is a process that commits germ cells to the complex process of spermatogenesis. Spermatogonial differentiation is mediated by the action of retinoic acid, which triggers major morphological and transcriptional changes. While these transcriptional changes have been well explored, there has been little effort devoted to epigenetic regulation surrounding spermatogonial development. This study aimed to uncover the timing and dynamics of chromatin organization during spermatogonial development within the context of these transcriptional changes. Using germ cell synchrony and the assay for transposase accessible chromatin and next generation sequencing (ATAC-seq) to isolate subpopulations of developing spermatogonia and identify accessible regions within their genome, we found that 50% of accessible regions in undifferentiated spermatogonia were condensed following retinoic acid action within 18 h. Surprisingly, genes with known functional relevance during spermatogonial development were accessible at all times, indicating that chromatin state does not impact transcription at these sites. While there was an overall decrease in gene accessibility during spermatogonial development, we found that transcriptionally active regions were not predictive of chromatin state.

Perspectives: Approaches for Studying Livestock Spermatogonia

At present, the knowledge base on characteristics and biology of spermatogonia in livestock is limited in comparison to rodents, yet the importance of studying these cells for comparative species analysis and enhancing reproductive capacity in food animals is high. Previous studies have established that although many core attributes of organ physiology and mechanisms governing essential cellular functions are conserved across eutherians, significant differences exist between mice and higher order mammals. In this chapter, we briefly discuss distinguishing aspects of testicular anatomy and the spermatogenic lineage in livestock and critical considerations for studying spermatogonial stem cell biology in these species.

Cell therapy for the treatment of reproductive diseases and infertility: an overview from the mechanism to the clinic alongside diagnostic methods

Infertility is experienced by 8%-12% of adults in their reproductive period globally and has become a prevalent concern. Besides routine therapeutic methods, stem cells are rapidly being examined as viable alternative therapies in regenerative medicine and translational investigation. Remarkable progress has been made in understanding the biology and purpose of stem cells. The affected pluripotent stem cells (iPSCs) and mesenchymal stem cells (MSCs) are further studied for their possible use in reproductive medicine, particularly for infertility induced by premature ovarian insufficiency and azoospermia. Accordingly, this study discusses current developments in the use of some kinds of MSCs such as adipose-derived stem cells, bone marrow stromal cells, umbilical cord MSCs, and menstrual blood MSCs. These methods have been used to manage ovarian and uterine disorders, and each technique presents a novel method for the therapy of infertility.

Modeling mammalian spermatogonial differentiation and meiotic initiation in vitro

In mammalian testes, premeiotic spermatogonia respond to retinoic acid by completing an essential lengthy differentiation program before initiating meiosis. The molecular and cellular changes directing these developmental processes remain largely undefined. This wide gap in knowledge is due to two unresolved technical challenges: (1) lack of robust and reliable in vitro models to study differentiation and meiotic initiation; and (2) lack of methods to isolate large and pure populations of male germ cells at each stage of differentiation and at meiotic initiation. Here, we report a facile in vitro differentiation and meiotic initiation system that can be readily manipulated, including the use of chemical agents that cannot be safely administered to live animals. In addition, we present a transgenic mouse model enabling fluorescence-activated cell sorting-based isolation of millions of spermatogonia at specific developmental stages as well as meiotic spermatocytes.

SRSF10 is essential for progenitor spermatogonia expansion by regulating alternative splicing

Alternative splicing expands the transcriptome and proteome complexity and plays essential roles in tissue development and human diseases. However, how alternative splicing regulates spermatogenesis remains largely unknown. Here, using a germ cell-specific knockout mouse model, we demonstrated that the splicing factor Srsf10 is essential for spermatogenesis and male fertility. In the absence of SRSF10, spermatogonial stem cells can be formed, but the expansion of Promyelocytic Leukemia Zinc Finger (PLZF)-positive undifferentiated progenitors was impaired, followed by the failure of spermatogonia differentiation (marked by KIT expression) and meiosis initiation. This was further evidenced by the decreased expression of progenitor cell markers in bulk RNA-seq, and much less progenitor and differentiating spermatogonia in single-cell RNA-seq data. Notably, SRSF10 directly binds thousands of genes in isolated THY+ spermatogonia, and Srsf10 depletion disturbed the alternative splicing of genes that are preferentially associated with germ cell development, cell cycle, and chromosome segregation, including Nasp, Bclaf1, Rif1, Dazl, Kit, Ret, and Sycp1. These data suggest that SRSF10 is critical for the expansion of undifferentiated progenitors by regulating alternative splicing, expanding our understanding of the mechanism underlying spermatogenesis.

The germ cell-specific RNA binding protein RBM46 is essential for spermatogonial differentiation in mice

Control over gene expression is exerted, in multiple stages of spermatogenesis, at the post-transcriptional level by RNA binding proteins (RBPs). We identify here an essential role in mammalian spermatogenesis and male fertility for 'RNA binding protein 46' (RBM46). A highly evolutionarily conserved gene, Rbm46 is also essential for fertility in both flies and fish. We found Rbm46 expression was restricted to the mouse germline, detectable in males in the cytoplasm of premeiotic spermatogonia and meiotic spermatocytes. To define its requirement for spermatogenesis, we generated Rbm46 knockout (KO, Rbm46-/-) mice; although male Rbm46-/- mice were viable and appeared grossly normal, they were infertile. Testes from adult Rbm46-/- mice were small, with seminiferous tubules containing only Sertoli cells and few undifferentiated spermatogonia. Using genome-wide unbiased high throughput assays RNA-seq and 'enhanced crosslinking immunoprecipitation' coupled with RNA-seq (eCLIP-seq), we discovered RBM46 could bind, via a U-rich conserved consensus sequence, to a cohort of mRNAs encoding proteins required for completion of differentiation and subsequent meiotic initiation. In summary, our studies support an essential role for RBM46 in regulating target mRNAs during spermatogonia differentiation prior to the commitment to meiosis in mice.

Behavior and Functional Roles of CD34+ Mesenchymal Cells in Mammalian Testes

Mammalian testes consist of seminiferous tubules within which Sertoli cells line up at the periphery and nurse germ cells, and of interstitia that harbor various cells such as peritubular myoid cells (PMCs), Leydig cells (LCs), vascular endothelial cells, immune cells such as macrophages, and mesenchymal (stromal) cells. Morphological studies have recently reported the presence of telocytes with telopodes in the interstitium of adult mouse, rat, and human testes. CD34+PDGFRα+ telocytes with long and moniliform telopodes form reticular networks with various cell types such as LCs, PMCs, and vessels, indicating their potential functions in cell-cell communications and tissue homeostasis. Functional studies have recently been performed on testicular interstitial cells and CD34+ cells, using 3D re-aggregate cultures of dissociated testicular cells, and cell cultures. Direct observation of CD34+ cells and adult LCs (ALCs) revealed that CD34+ cells extend thin cytoplasmic processes (telopodes), move toward the LC-CD34+ cell-re-aggregates, and finally enter into the re-aggregates, indicating the chemotactic behavior of CD34+ telocytes toward ALCs. In mammalian testes, important roles of mesenchymal interstitial cells as stem/progenitors in the differentiation and regeneration of LCs have been reported. Here, reports on testicular telocytes so far obtained are reviewed, and future perspectives on the studies of testicular telocytes are noted.

The evolutionary history and mechanistic basis of female ornamentation in a tropical songbird

Ornamentation, such as the showy plumage of birds, is widespread among female vertebrates, yet the evolutionary pressures shaping female ornamentation remain uncertain. In part this is due to a poor understanding of the mechanistic route to ornamentation in females. To address this issue, we evaluated the evolutionary history of ornament expression in a tropical passerine bird, the White-shouldered Fairywren, whose females, but not males, strongly vary between populations in occurrence of ornamented black-and-white plumage. We first use phylogenomic analysis to demonstrate that female ornamentation is derived and that female ornamentation evolves independently of changes in male plumage. We then use exogenous testosterone in a field experiment to induce partial ornamentation in naturally unornamented females. By sequencing the transcriptome of experimentally induced ornamented and natural feathers, we identify genes expressed during ornament production and evaluate the degree to which female ornamentation in this system is associated with elevated testosterone, as is common in males. We reveal that some ornamentation in females is linked to testosterone and that sexes differ in ornament-linked gene expression. Lastly, using genomic outlier analysis we identify a candidate melanogenesis gene that lies in a region of high genomic divergence among populations that is also differentially expressed in feather follicles of different female plumages. Taken together, these findings are consistent with sex-specific selection favoring the evolution of female ornaments and demonstrate a key role for testosterone in generating population divergence in female ornamentation through gene regulation. More broadly, our work highlights similarities and differences in how ornamentation evolves in the sexes.

Temperature sensitivity of DNA double-strand break repair underpins heat-induced meiotic failure in mouse spermatogenesis

Mammalian spermatogenesis is a heat-vulnerable process that occurs at low temperatures, and elevated testicular temperatures cause male infertility. However, the current reliance on in vivo assays limits their potential to detail temperature dependence and destructive processes. Using ex vivo cultures of mouse testis explants at different controlled temperatures, we found that spermatogenesis failed at multiple steps, showing sharp temperature dependencies. At 38 °C (body core temperature), meiotic prophase I is damaged, showing increased DNA double-strand breaks (DSBs) and compromised DSB repair. Such damaged spermatocytes cause asynapsis between homologous chromosomes and are eliminated by apoptosis at the meiotic checkpoint. At 37 °C, some spermatocytes survive to the late pachytene stage, retaining high levels of unrepaired DSBs but do not complete meiosis with compromised crossover formation. These findings provide insight into the mechanisms and significance of heat vulnerability in mammalian spermatogenesis.

Xenotransplantation of Human Spermatogonia Into Various Mouse Recipient Models

Spermatogonial stem cells are the foundation of continuous spermatogenesis in adult mammals. Xenograft models have been established to define human SSCs, mostly using infertile and immune-deficient mice as the recipients for human germ cell transplantation. However, it is time-consuming to prepare such recipients using irradiation or chemotherapeutic agents, and this approach may also introduce confounding factors when residual endogenous germ cells recover in transplanted recipients. It remains to be determined whether immune-competent genetically infertile mice can be suitable recipients for xenotransplantation. In this study, we observed similar engraftment efficiencies when using spermatogonia from human biopsied testes across immune-deficient nude mice, immune-competent ICR mice, and genetically infertile Kit w/w-v mice, suggesting minimal immunological rejection from immune-competent mouse recipients upon xenotransplantation of human germ cells. More importantly, we derived EpCAM negative and TNAP positive spermatogonia-like cells (SLCs) from human pluripotent stem cells (PSCs), which highly expressed spermatogonial markers including PLZF, INTERGRINα6, TKTL1, CD90, and DRMT3. We found that upon transplantation, these SLCs proliferated and colonized at the basal membrane of seminiferous tubules in testes of both immune-deficient nude mice and Kit w/w-v mice, though complete spermatogenesis would likely require supporting human signaling factors and microenvironment. Taken together, our study functionally defined the cell identity of PSC-derived SLCs, and supported xenotransplantation using genetically infertile recipients as a convenient model for functionally evaluating spermatogonia derived from different species.

Anti-KIT monoclonal antibody CDX-0159 induces profound and durable mast cell suppression in a healthy volunteer study

Background

Mast cells (MC) are powerful inflammatory immune sentinel cells that drive numerous allergic, inflammatory, and pruritic disorders when activated. MC‐targeted therapies are approved in several disorders, yet many patients have limited benefit suggesting the need for approaches that more broadly inhibit MC activity. MCs require the KIT receptor and its ligand stem cell factor (SCF) for differentiation, maturation, and survival. Here we describe CDX‐0159, an anti‐KIT monoclonal antibody that potently suppresses MCs in human healthy volunteers.

Methods

CDX‐0159‐mediated KIT inhibition was tested in vitro using KIT‐expressing immortalized cells and primary human mast cells. CDX‐0159 safety and pharmacokinetics were evaluated in a 13‐week good laboratory practice (GLP)‐compliant cynomolgus macaque study. A single ascending dose (0.3, 1, 3, and 9 mg/kg), double‐blinded placebo‐controlled phase 1a human healthy volunteer study (n = 32) was conducted to evaluate the safety, pharmacokinetics, and pharmacodynamics of CDX‐0159.

Results

CDX‐0159 inhibits SCF‐dependent KIT activation in vitro. Fc modifications in CDX‐0159 led to elimination of effector function and reduced serum clearance. In cynomolgus macaques, multiple high doses were safely administered without a significant impact on hematology, a potential concern for KIT inhibitors. A single dose of CDX‐0159 in healthy human subjects was generally well tolerated and demonstrated long antibody exposure. Importantly, CDX‐0159 led to dose‐dependent, profound suppression of plasma tryptase, a MC‐specific protease associated with tissue MC burden, indicative of systemic MC suppression or ablation.

Conclusion

CDX‐0159 administration leads to systemic mast cell ablation and may represent a safe and novel approach to treat mast cell‐driven disorders.

Proteostasis regulated by testis-specific ribosomal protein RPL39L maintains mouse spermatogenesis

Maintaining proteostasis is important for animal development. How proteostasis influences spermatogenesis that generates male gametes, spermatozoa, is not clear. We show that testis-specific paralog of ribosomal large subunit protein RPL39, RPL39L, is required for mouse spermatogenesis. Deletion of Rpl39l in mouse caused reduced proliferation of spermatogonial stem cells, malformed sperm mitochondria and flagella, leading to sub-fertility in males. Biochemical analyses revealed that lack of RPL39L deteriorated protein synthesis and protein quality control in spermatogenic cells, partly due to reduced biogenesis of ribosomal subunits and ribosome homeostasis. RPL39/RPL39L is likely assembled into ribosomes via H/ACA domain containing NOP10 complex early in ribosome biogenesis pathway. Furthermore, Rpl39l null mice exhibited compromised regenerative spermatogenesis after chemical insult and early degenerative spermatogenesis in aging mice. These data demonstrate that maintaining proteostasis is important for spermatogenesis, of which ribosome homeostasis maintained by ribosomal proteins coordinates translation machinery to the regulation of cellular growth.

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Rpl39l deletion causes reduced spermatogenesis and subfertility in male mice

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SSC proliferation, mitochondria and sperm flagella compromised in Rpl39l^–/– mice

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Rpl39l deletion affects ribosomal LSU formation and protein quality control

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Aberrant proteostasis affects spermatogenesis and regeneration

Effects of Stem Cell Factor/c-Kit Signaling on In Vitro Maturation of Porcine Oocytes and Subsequent Developmental Competence After Fertilization

Stem cell factor (SCF), also known as c-Kit ligand, plays an important role in the proliferation of primordial germ cells and the survival of oocytes during follicular development. The aim of this study was to investigate the effect of SCF/c-Kit signaling on in vitro maturation (IVM) of porcine oocytes by analyzing nuclear and cytoplasmic maturation, oocyte size, cumulus cell expansion, and developmental competence to the blastocyst stage. Moreover, mRNA expression patterns of porcine cumulus cells and oocytes were evaluated using qRT-PCR. Following 42 h of IVM, 10 and 50 ng/mL SCF-treated groups exhibited significantly (P < 0.05) increased polar body extrusion rates and intracellular glutathione levels compared with the control group. The cumulus expansion index significantly (P < 0.05) increased in all SCF-treated groups compared with the control samples. mRNA levels of the proapoptotic gene Bax and apoptosis-related cysteine peptidase Caspase3 were lower in SCF-treated cumulus cells than in the control group. Notably, the diameter of oocytes after IVM, the mRNA expression of well-known oocyte-secreted factors (GDF9 and BMP15), and an oocyte-specific protein essential for ovulation and oocyte health (YBX2) were significantly (P < 0.05) higher in SCF-treated than in non-treated oocytes. Inhibition of c-Kit during porcine IVM using ACK2, an antagonistic blocker of c-Kit, significantly (P < 0.05) decreased the polar body extrusion rate compared with the control, as well as blastocyst formation rate compared with the 10 ng/mL SCF-treated group. In conclusion, the effect of SCF/c-Kit-mediated signaling during porcine IVM could be ascribed to the reduced expression of apoptosis-related genes and higher expression of oocyte-specific/secreted factors.

A multistate stem cell dynamics maintains homeostasis in mouse spermatogenesis

In mouse testis, a heterogeneous population of undifferentiated spermatogonia (Aundiff) harbors spermatogenic stem cell (SSC) potential. Although GFRα1+ Aundiff maintains the self-renewing pool in homeostasis, the functional basis of heterogeneity and the implications for their dynamics remain unresolved. Here, through quantitative lineage tracing of SSC subpopulations, we show that an ensemble of heterogeneous states of SSCs supports homeostatic, persistent spermatogenesis. Such heterogeneity is maintained robustly through stochastic interconversion of SSCs between a renewal-biased Plvap+/GFRα1+ state and a differentiation-primed Sox3+/GFRα1+ state. In this framework, stem cell commitment occurs not directly but gradually through entry into licensed but uncommitted states. Further, Plvap+/GFRα1+ cells divide slowly, in synchrony with the seminiferous epithelial cycle, while Sox3+/GFRα1+ cells divide much faster. Such differential cell-cycle dynamics reduces mitotic load, and thereby the potential to acquire harmful de novo mutations of the self-renewing pool, while keeping the SSC density high over the testicular open niche.

Altered Biology of Testicular VSELs and SSCs by Neonatal Endocrine Disruption Results in Defective Spermatogenesis, Reduced Fertility and Tumor Initiation in Adult Mice

Reproductive health of men has declined in recent past with reduced sperm count and increased incidence of infertility and testicular cancers mainly attributed to endocrine disruption in early life. Present study aims to evaluate whether testicular stem cells including very small embryonic-like stem cells (VSELs) and spermatogonial stem cells (SSCs) get affected by endocrine disruption and result in pathologies in adult life. Effect of treatment on mice pups with estradiol (20 μg on days 5-7) and diethylstilbestrol (DES, 2 μg on days 1-5) was studied on VSELs, SSCs and spermatogonial cells in adult life. Treatment affected spermatogenesis, tubules in Stage VIII & sperm count were reduced along with reduction of meiotic (4n) cells and markers (Prohibitin, Scp3, Protamine). Enumeration of VSELs by flow cytometry (2-6 μm, 7AAD-, LIN-CD45-SCA-1+) and qRT-PCR using specific transcripts for VSELs (Oct-4a, Sox-2, Nanog, Stella, Fragilis), SSCs (tOct-4, Gfra-1, Gpr-125) and early germ cells (Mvh, Dazl) showed several-fold increase but transition from c-Kit negative to c-Kit positive spermatogonial cells was blocked on D100 after treatment. Transcripts specific for apoptosis (Bcl2, Bax) remained unaffected but tumor suppressor (p53) and epigenetic regulator (NP95) transcripts showed marked disruption. 9 of 10 mice exposed to DES showed tumor-like changes. To conclude, endocrine disruption resulted in a tilt towards excessive self-renewal of VSELs (leading to testicular cancer after DES treatment) and blocked differentiation (reduced numbers of c-Kit positive cells, meiosis, sperm count and fertility). Understanding the underlying basis for infertility and cancer initiation from endogenous stem cells through murine modelling will hopefully improve human therapies in future.

A transgenic DND1GFP fusion allele reports in vivo expression and RNA-binding targets in undifferentiated mouse germ cells†

In vertebrates, the RNA-binding protein (RBP) dead end 1 (DND1) is essential for primordial germ cell (PGC) survival and maintenance of cell identity. In multiple species, Dnd1 loss or mutation leads to severe PGC loss soon after specification or, in some species, germ cell transformation to somatic lineages. Our investigations into the role of DND1 in PGC specification and differentiation have been limited by the absence of an available antibody. To address this problem, we used CRISPR/Cas9 gene editing to establish a transgenic mouse line carrying a DND1GFP fusion allele. We present imaging analysis of DND1GFP expression showing that DND1GFP expression is heterogeneous among male germ cells (MGCs) and female germ cells (FGCs). DND1GFP was detected in MGCs throughout fetal life but lost from FGCs at meiotic entry. In postnatal and adult testes, DND1GFP expression correlated with classic markers for the premeiotic spermatogonial population. Utilizing the GFP tag for RNA immunoprecipitation (RIP) analysis in MGCs validated this transgenic as a tool for identifying in vivo transcript targets of DND1. The DND1GFP mouse line is a novel tool for isolation and analysis of embryonic and fetal germ cells, and the spermatogonial population of the postnatal and adult testis.

What has single-cell RNA-seq taught us about mammalian spermatogenesis?

Mammalian spermatogenesis is a complex developmental program that transforms mitotic testicular germ cells (spermatogonia) into mature male gametes (sperm) for production of offspring. For decades, it has been known that this several-weeks-long process involves a series of highly ordered and morphologically recognizable cellular changes as spermatogonia proliferate, spermatocytes undertake meiosis, and spermatids develop condensed nuclei, acrosomes, and flagella. Yet, much of the underlying molecular logic driving these processes has remained opaque because conventional characterization strategies often aggregated groups of cells to meet technical requirements or due to limited capability for cell selection. Recently, a cornucopia of single-cell transcriptome studies has begun to lift the veil on the full compendium of gene expression phenotypes and changes underlying spermatogenic development. These datasets have revealed the previously obscured molecular heterogeneity among and between varied spermatogenic cell types and are reinvigorating investigation of testicular biology. This review describes the extent of available single-cell RNA-seq profiles of spermatogenic and testicular somatic cells, how those data were produced and evaluated, their present value for advancing knowledge of spermatogenesis, and their potential future utility at both the benchtop and bedside.

The mTORC1 component RPTOR is required for maintenance of the foundational spermatogonial stem cell pool in mice†

The self-renewal, proliferation, and differentiation of the spermatogonial populations must be finely coordinated in the mammalian testis, as dysregulation of these processes can lead to subfertility, infertility, or the formation of tumors. There are wide gaps in our understanding of how these spermatogonial populations are formed and maintained, and our laboratory has focused on identifying the molecular and cellular pathways that direct their development. Others and we have shown, using a combination of pharmacologic inhibitors and genetic models, that activation of mTOR complex 1 (mTORC1) is important for spermatogonial differentiation in vivo. Here, we extend those studies to directly test the germ cell-autonomous requirement for mTORC1 in spermatogonial differentiation. We created germ cell conditional knockout mice for "regulatory associated protein of MTOR, complex 1" (Rptor), which encodes an essential component of mTORC1. While germ cell KO mice were viable and healthy, they had smaller testes than littermate controls, and no sperm were present in their cauda epididymides. We found that an initial cohort of Rptor KO spermatogonia proliferated, differentiated, and entered meiosis (which they were unable to complete). However, no self-renewing spermatogonia were formed, and thus the entire germline was lost by adulthood, resulting in Sertoli cell-only testes. These results reveal the cell autonomous requirement for RPTOR in the formation or maintenance of the foundational self-renewing spermatogonial stem cell pool in the mouse testis and underscore complex roles for mTORC1 and its constituent proteins in male germ cell development.

Transgenesis and Genome Editing of Mouse Spermatogonial Stem Cells by Lentivirus Pseudotyped with Sendai Virus F Protein

Spermatogonial stem cells (SSCs) serve as a resource for producing genetically modified animals. However, genetic manipulation of SSCs has met with limited success. Here, we show efficient gene transfer into SSCs via a lentivirus (FV-LV) using a fusion protein (F), a Sendai virus (SV) envelope protein involved in virion/cell membrane fusion. FV-LVs transduced cultured SSCs more efficiently than conventional LVs. Although SSCs infected with SV failed to produce offspring, those transduced with FV-LVs were fertile. In vivo microinjection showed that FV-LVs could penetrate not only the basement membrane of the seminiferous tubules but also the blood-testis barrier, which resulted in successful transduction of both spermatogenic cells and testicular somatic cells. Cultured SSCs transfected with FV-LVs that express drug-inducible CRISPR/Cas9 against Kit or Sycp3 showed impaired spermatogenesis upon transplantation and drug treatment in vivo. Thus, FV-LVs provide an efficient method for functional analysis of genes involved in SSCs and spermatogenesis.

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Sendai virus-derived F protein enhances lentiviral infection of male germ cells

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Transfected spermatogonial stem cells undergo germline transmission

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Lentivirus pseudotyped with F protein penetrates the blood-testis barrier

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This method is compatible with in vivo conditional gene editing

Diverse roles for CDK-associated activity during spermatogenesis

The primary function of cyclin-dependent kinases (CDKs) in complex with their activating cyclin partners is to promote mitotic division in somatic cells. This canonical cell cycle-associated activity is also crucial for fertility as it allows the proliferation and differentiation of stem cells within the reproductive organs to generate meiotically competent cells. Intriguingly, several CDKs exhibit meiosis-specific functions and are essential for the completion of the two reductional meiotic divisions required to generate haploid gametes. These meiosis-specific functions are mediated by both known CDK/cyclin complexes and meiosis-specific CDK-regulators and are important for a variety of processes during meiotic prophase. The majority of meiotic defects observed upon deletion of these proteins occur during the extended prophase I of the first meiotic division. Importantly a lack of redundancy is seen within the meiotic arrest phenotypes described for many of these proteins, suggesting intricate layers of cell cycle control are required for normal meiotic progression. Using the process of male germ cell development (spermatogenesis) as a reference, this review seeks to highlight the diverse roles of selected CDKs their activators, and their regulators during gametogenesis.

Spermatogonial stem cells

Spermatogonial stem cells (SSCs) are the most primitive spermatogonia in the testis and have an essential role to maintain highly productive spermatogenesis by self-renewal and continuous generation of daughter spermatogonia that differentiate into spermatozoa, transmitting genetic information to the next generation. Since the 1950s, many experimental methods, including histology, immunostaining, whole-mount analyses, and pulse-chase labeling, had been used in attempts to identify SSCs, but without success. In 1994, a spermatogonial transplantation method was reported that established a quantitative functional assay to identify SSCs by evaluating their ability to both self-renew and differentiate to spermatozoa. The system was originally developed using mice and subsequently extended to nonrodents, including domestic animals and humans. Availability of the functional assay for SSCs has made it possible to develop culture systems for their ex vivo expansion, which dramatically advanced germ cell biology and allowed medical and agricultural applications. In coming years, SSCs will be increasingly used to understand their regulation, as well as in germline modification, including gene correction, enhancement of male fertility, and conversion of somatic cells to biologically competent male germline cells.

Developmental kinetics and transcriptome dynamics of stem cell specification in the spermatogenic lineage

Continuity, robustness, and regeneration of cell lineages relies on stem cell pools that are established during development. For the mammalian spermatogenic lineage, a foundational spermatogonial stem cell (SSC) pool arises from prospermatogonial precursors during neonatal life via mechanisms that remain undefined. Here, we mapped the kinetics of this process in vivo using a multi-transgenic reporter mouse model, in silico with single-cell RNA sequencing, and functionally with transplantation analyses to define the SSC trajectory from prospermatogonia. Outcomes revealed that a heterogeneous prospermatogonial population undergoes dynamic changes during late fetal and neonatal development. Differential transcriptome profiles predicted divergent developmental trajectories from fetal prospermatogonia to descendant postnatal spermatogonia. Furthermore, transplantation analyses demonstrated that a defined subset of fetal prospermatogonia is fated to function as SSCs. Collectively, these findings suggest that SSC fate is preprogrammed within a subset of fetal prospermatogonia prior to building of the foundational pool during early neonatal development.

In neonatal testes, prospermatogonia generate both spermatogonia for the first wave of spermatogenesis and spermatogonial stem cells (SSCs) for maintenance of spermatogenesis in males. Here the authors characterize the development of mouse SSCs from prospermatogonia using single-cell RNA-seq and transplantation assays.

Seminiferous tubule molecular imaging for evaluation of male fertility: Seeing is believing

The proper assessment of male fertility is essential for diagnosing and treating male infertility. Currently, spermiogram and Johnsen testicular biopsy score counts are used to assess male fertility. However, spermiogram is not a suitable option for non-obstructive azoospermia patients, and Johnsen testicular biopsy scores only represent localized and not the overall spermatogenesis. Whole-mount staining was a novel method for evaluating protein expression in the tissue. Thus, we explored its application in human seminiferous tubules. Testicular biopsies from 57 azoospermia patients were categorized as obstructive azoospermia (OA), maturation arrest (MA) and Sertoli-cells only syndrome (SCOS). We performed whole-mount staining of their seminiferous tubules and evaluated the spermatogonial stem cells (SSCs), differentiated spermatogonia (SG), spermatocytes (SPC) and spermatids (SD) with their respective markers (GFRA1, CD117, SYCP3, and PNA) to assess fertility. GFRA1, CD117, SYCP3, and PNA were not expressed in SCOS patients, whereas all of them were detected in OA patients. In MA patients with arrested spermatogenesis at the SPC stage, GFRA1, CD117, and SYCP3, but not PNA were expressed in the seminiferous tubules. In MA patients with arrested spermatogenesis at the spermatogonia stage, only GFRA1 was expressed in the seminiferous tubules. These results were consistent with the Johnsen testicular biopsy score counts except for one patient, where although only Sertoli cells were indicated by the score, SSCs were also detected in the whole-mounts. Collectively, whole-mount staining could be used to analyze the inherent spermatogenesis of seminiferous tubules through staining of germ cells at different stages. It offers a more accurate and promising faster method for assessing male fertility compared with traditional biopsy screening. And it could have potential value for the clinical purpose for male fertility management.

DDX5 plays essential transcriptional and post-transcriptional roles in the maintenance and function of spermatogonia

Mammalian spermatogenesis is sustained by mitotic germ cells with self-renewal potential known as undifferentiated spermatogonia. Maintenance of undifferentiated spermatogonia and spermatogenesis is dependent on tightly co-ordinated transcriptional and post-transcriptional mechanisms. The RNA helicase DDX5 is expressed by spermatogonia but roles in spermatogenesis are unexplored. Using an inducible knockout mouse model, we characterise an essential role for DDX5 in spermatogonial maintenance and show that Ddx5 is indispensable for male fertility. We demonstrate that DDX5 regulates appropriate splicing of key genes necessary for spermatogenesis. Moreover, DDX5 regulates expression of cell cycle genes in undifferentiated spermatogonia post-transcriptionally and is required for cell proliferation and survival. DDX5 can also act as a transcriptional co-activator and we demonstrate that DDX5 interacts with PLZF, a transcription factor required for germline maintenance, to co-regulate select target genes. Combined, our data reveal a critical multifunctional role for DDX5 in regulating gene expression programmes and activity of undifferentiated spermatogonia.

Sustained sperm production is dependent on activity of undifferentiated spermatogonia. Here, the authors demonstrate an essential role for RNA helicase DDX5 in maintenance of spermatogonia in adults through control of gene transcription plus RNA processing and export.

Germline deletion of Cdyl causes teratozoospermia and progressive infertility in male mice

Chromodomain Y (CDY) is one of the candidate genes for male dyszoospermia related to Y chromosome microdeletion (YCM). However, the function of CDY in regulating spermatogenesis has not been completely determined. The mouse Cdyl (CDY-like) gene is the homolog of human CDY. In the present study, we generated a germline conditional knockout (cKO) model of mouse Cdyl. Significantly, the Cdyl^cKO male mice suffered from the defects in spermatogonia maintenance and spermatozoon morphogenesis, demonstrating teratozoospermia and a progressive infertility phenotype in early adulthood. Importantly, patterns of specific histone methylation and acetylation were extensively changed, which disturbed the transcriptome in Cdyl^cKO testis. Our findings indicated that Cdyl is crucial for spermatogenesis and male fertility, which provides novel insights into the function of CDY gene, as well as the pathogenesis of YCM-related reproductive failure.

Heterogeneity of Spermatogonial Stem Cells

Germ cells transfer genetic materials from one generation to the next, which ensures the continuation of the species. Spermatogenesis, the process of male germ cell production, is one of the most productive systems in adult tissues. This high productivity depends on the well-coordinated differentiation cascade in spermatogonia, occurring via their synchronized cell division and proliferation. Spermatogonial stem cells (SSCs) are responsible for maintaining the spermatogonial population via self-renewal and the continuous generation of committed progenitor cells that differentiate into spermatozoa. Like other stem cells in the body, SSCs are defined by their self-renewal and differentiation abilities. A functional transplantation assay, in which these biological properties of SSCs can be quantitatively evaluated, was developed using mice, and the cell surface characteristics and intracellular marker gene expression of murine SSCs were successfully determined. Another approach to elucidate SSC identity is a cell lineage-tracing experiment using transgenic mice, which can track the SSC behavior in the testes. Recent studies using both these experimental approaches have revealed that the SSC identity changed depending upon the developmental, homeostatic, and regenerative circumstances. In addition, single-cell transcriptomic analyses have further indicated the instability of marker gene expression in SSCs. More studies are needed to unify the results of the determination of SSC identity based on the functional properties and accumulating transcriptomic data of SSCs, to elucidate the functional interaction between SSC behavior and gene products and illustrate the conserved features of SSCs amidst their heterogeneity. Furthermore, the deterministic roles of distinct SSC niches under different physiological conditions in the SSC heterogeneity and its causal regulators must also be clarified in future studies.

Role of c-Kit in Myocardial Regeneration and Aging

c-Kit, a type III receptor tyrosine kinase (RTK), is involved in multiple intracellular signaling whereby it is mainly considered a stem cell factor receptor, which participates in vital functions of the mammalian body, including the human. Furthermore, c-kit is a necessary yet not sufficient marker to detect and isolate several types of tissue-specific adult stem cells. Accordingly, c-kit was initially used as a marker to identify and enrich for adult cardiac stem/progenitor cells (CSCs) that were proven to be clonogenic, self-renewing and multipotent, being able to differentiate into cardiomyocytes, endothelial cells and smooth muscle cells in vitro as well as in vivo after myocardial injury. Afterwards it was demonstrated that c-kit expression labels a heterogenous cardiac cell population, which is mainly composed by endothelial cells while only a very small fraction represents CSCs. Furthermore, c-kit as a signaling molecule is expressed at different levels in this heterogenous c-kit labeled cardiac cell pool, whereby c-kit low expressers are enriched for CSCs while c-kit high expressers are endothelial and mast cells. This heterogeneity in cell composition and expression levels has been neglected in recent genetic fate map studies focusing on c-kit, which have claimed that c-kit identifies cells with robust endothelial differentiation potential but with minimal if not negligible myogenic commitment potential. However, modification of c-kit gene for Cre Recombinase expression in these Cre/Lox genetic fate map mouse models produced a detrimental c-kit haploinsufficiency that prevents efficient labeling of true CSCs on one hand while affecting the regenerative potential of these cells on the other. Interestingly, c-kit haploinsufficiency in c-kit-deficient mice causes a worsening myocardial repair after injury and accelerates cardiac aging. Therefore, these studies have further demonstrated that adult c-kit-labeled CSCs are robustly myogenic and that the adult myocardium relies on c-kit expression to regenerate after injury and to counteract aging effects on cardiac structure and function.

Heterogeneity of Adult Cardiac Stem Cells

Cardiac biology and heart regeneration have been intensively investigated and debated in the last 15 years. Nowadays, the well-established and old dogma that the adult heart lacks of any myocyte-regenerative capacity has been firmly overturned by the evidence of cardiomyocyte renewal throughout the mammalian life as part of normal organ cell homeostasis, which is increased in response to injury. Concurrently, reproducible evidences from independent laboratories have convincingly shown that the adult heart possesses a pool of multipotent cardiac stem/progenitor cells (CSCs or CPCs) capable of sustaining cardiomyocyte and vascular tissue refreshment after injury. CSC transplantation in animal models displays an effective regenerative potential and may be helpful to treat chronic heart failure (CHF), obviating at the poor/modest results using non-cardiac cells in clinical trials. Nevertheless, the degree/significance of cardiomyocyte turnover in the adult heart, which is insufficient to regenerate extensive damage from ischemic and non-ischemic origin, remains strongly disputed. Concurrently, different methodologies used to detect CSCs in situ have created the paradox of the adult heart harboring more than seven different cardiac progenitor populations. The latter was likely secondary to the intrinsic heterogeneity of any regenerative cell agent in an adult tissue but also to the confusion created by the heterogeneity of the cell population identified by a single cell marker used to detect the CSCs in situ. On the other hand, some recent studies using genetic fate mapping strategies claimed that CSCs are an irrelevant endogenous source of new cardiomyocytes in the adult. On the basis of these contradictory findings, here we critically reviewed the available data on adult CSC biology and their role in myocardial cell homeostasis and repair.