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

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.

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.

Perspectives: Methods for Evaluating Primate Spermatogonial Stem Cells

Mammalian spermatogenesis is a complex, highly productive process generating millions of sperm per day. Spermatogonial stem cells (SSCs) are at the foundation of spermatogenesis and can either self-renew, producing more SSCs, or differentiate to initiate spermatogenesis and produce sperm. The biological potential of SSCs to produce and maintain spermatogenesis makes them a promising tool for the treatment of male infertility. However, translating knowledge from rodents to higher primates (monkeys and humans) is challenged by different vocabularies that are used to describe stem cells and spermatogenic lineage development in those species. Furthermore, while rodent SSCs are defined by their biological potential to produce and maintain spermatogenesis in a transplant assay, there is no equivalent routine and accessible bioassay to test monkey and human SSCs or replicate their functions in vitro. This chapter describes progress characterizing, isolating, culturing, and transplanting SSCs in higher primates.

miR-106b-5p Intensifies the Proliferative Potential of Spermatogonial Stem Cells as a Prerequisite for Male Infertility Treatment

Although numerous studies have investigated the molecular basis of male infertility, various aspects of this area have remained uncovered. Over the past years, researchers have reported the significant potential of miRNAs in posttranscriptional regulatory roles. By targeting mRNAs, these notable molecules can modulate the processes related to male infertility. On the other side, the outstanding potential of male germline stem cells, SSCs, includes their application in infertility treatment. SSCs retain normal spermatogenesis and fertility by adjusting both SSC self-renewal and differentiation. Therefore, for the characterization and manipulation of SSCs, effective and efficient in vitro culture methods are essential in supporting their maintenance and development. In this regard, the present investigation was undertaken to evaluate the impact of one of the recently conspicuous miRNAs, miR-106b, in SSCs enrichment. As a result, we first found that the SSCs induced with miR-106b-5p highly express TGF-β1, which is known as a regulator of epigenetic modifiers and downstream genes. We next sought to show that self-renewal markers, including c-Myc, Oct-4, and Sox2, are increased in the induced SSC group. The intended miRNA also induced the inhibitor of differentiation 4 (ID4) and aided to remain unmethylated in SSCs. Additionally, for the tumorigenicity possibility of the manipulation, we indicated that PTEN, a tumor-suppressor gene, expressed remarkably in the induced SSCs. In conclusion, our findings showed that miR-106b-5p enhances the proliferative potential of SSCs, making it a substantial factor for therapeutic strategies of male infertility.

MAP4K4/JNK Signaling Pathway Stimulates Proliferation and Suppresses Apoptosis of Human Spermatogonial Stem Cells and Lower Level of MAP4K4 Is Associated with Male Infertility

Spermatogonial stem cells (SSCs) serve as a foundation for spermatogenesis and they are essential for male fertility. The fate of SSC is determined by genetic and epigenetic regulatory networks. Many molecules that regulate SSC fate determinations have been identified in mice. However, the molecules and signaling pathways underlying human SSCs remain largely unclear. In this study, we have demonstrated that MAP4K4 was predominantly expressed in human UCHL1-positive spermatogonia by double immunocytochemical staining. MAP4K4 knockdown inhibited proliferation of human SSCs and induced their apoptosis. Moreover, MAP4K4 silencing led to inhibition of JNK phosphorylation and MAP4K4 phosphorylation at Ser801. RNA sequencing indicated that MAP4K4 affected the transcription of SPARC, ADAM19, GPX7, GNG2, and COLA1. Interestingly, the phenotype of inhibiting JNK phosphorylation by SP600125 was similar to MAP4K4 knockdown. Notably, MAP4K4 protein was lower in the testes of patients with non-obstructive azoospermia than those with normal spermatogenesis as shown by Western blots and immunohistochemistry. Considered together, our data implicate that MAP4K4/JNK signaling pathway mediates proliferation and apoptosis of human SSCs, which provides a novel insight into molecular mechanisms governing human spermatogenesis and might offer new targets for gene therapy of male infertility.

Roles of Spermatogonial Stem Cells in Spermatogenesis and Fertility Restoration

Spermatogonial stem cells (SSCs) are a group of adult stem cells in the testis that serve as the foundation of continuous spermatogenesis and male fertility. SSCs are capable of self-renewal to maintain the stability of the stem cell pool and differentiation to produce mature spermatozoa. Dysfunction of SSCs leads to male infertility. Therefore, dissection of the regulatory network of SSCs is of great significance in understanding the fundamental molecular mechanisms of spermatogonial stem cell function in spermatogenesis and the pathogenesis of male infertility. Furthermore, a better understanding of SSC biology will allow us to culture and differentiate SSCs in vitro, which may provide novel stem cell-based therapy for assisted reproduction. This review summarizes the latest research progress on the regulation of SSCs, and the potential application of SSCs for fertility restoration through in vivo and in vitro spermatogenesis. We anticipate that the knowledge gained will advance the application of SSCs to improve male fertility. Furthermore, in vitro spermatogenesis from SSCs sets the stage for the production of SSCs from induced pluripotent stem cells (iPSCs) and subsequent spermatogenesis.

Integration and gene co-expression network analysis of scRNA-seq transcriptomes reveal heterogeneity and key functional genes in human spermatogenesis

Spermatogenesis is a complex process of cellular division and differentiation that begins with spermatogonia stem cells and leads to functional spermatozoa production. However, many of the molecular mechanisms underlying this process remain unclear. Single-cell RNA sequencing (scRNA-seq) is used to sequence the entire transcriptome at the single-cell level to assess cell-to-cell variability. In this study, more than 33,000 testicular cells from different scRNA-seq datasets with normal spermatogenesis were integrated to identify single-cell heterogeneity on a more comprehensive scale. Clustering, cell type assignments, differential expressed genes and pseudotime analysis characterized 5 spermatogonia, 4 spermatocyte, and 4 spermatid cell types during the spermatogenesis process. The UTF1 and ID4 genes were introduced as the most specific markers that can differentiate two undifferentiated spermatogonia stem cell sub-cellules. The C7orf61 and TNP can differentiate two round spermatid sub-cellules. The topological analysis of the weighted gene co-expression network along with the integrated scRNA-seq data revealed some bridge genes between spermatogenesis's main stages such as DNAJC5B, C1orf194, HSP90AB1, BST2, EEF1A1, CRISP2, PTMS, NFKBIA, CDKN3, and HLA-DRA. The importance of these key genes is confirmed by their role in male infertility in previous studies. It can be stated that, this integrated scRNA-seq of spermatogenic cells offers novel insights into cell-to-cell heterogeneity and suggests a list of key players with a pivotal role in male infertility from the fertile spermatogenesis datasets. These key functional genes can be introduced as candidates for filtering and prioritizing genotype-to-phenotype association in male infertility.

Loss of tyrosine kinase receptor Ephb2 impairs proliferation and stem cell activity of spermatogonia in culture†

Germline stem and progenitor cells can be extracted from the adult mouse testis and maintained long-term in vitro. Yet, the optimal culture conditions for preserving stem cell activity are unknown. Recently, multiple members of the Eph receptor family were detected in murine spermatogonia, but their roles remain obscure. One such gene, Ephb2, is crucial for maintenance of somatic stem cells and was previously found enriched at the level of mRNA in murine spermatogonia. We detected Ephb2 mRNA and protein in primary adult spermatogonial cultures and hypothesized that Ephb2 plays a role in maintenance of stem cells in vitro. We employed CRISPR-Cas9 targeting and generated stable mutant SSC lines with complete loss of Ephb2. The characteristics of Ephb2-KO cells were interrogated using phenotypic and functional assays. Ephb2-KO SSCs exhibited reduced proliferation compared to wild-type cells, while apoptosis was unaffected. Therefore, we examined whether Ephb2 loss correlates with activity of canonical pathways involved in stem cell self-renewal and proliferation. Ephb2-KO cells had reduced ERK MAPK signaling. Using a lentiviral transgene, Ephb2 expression was rescued in Ephb2-KO cells, which partially restored signaling and proliferation. Transplantation analysis revealed that Ephb2-KO SSCs cultures formed significantly fewer colonies than WT, indicating a role for Ephb2 in preserving stem cell activity of cultured cells. Transcriptome analysis of wild-type and Ephb2-KO SSCs identified Dppa4 and Bnc1 as differentially expressed, Ephb2-dependent genes that are potentially involved in stem cell function. These data uncover for the first time a crucial role for Ephb2 signaling in cultured SSCs.

The Neonatal and Adult Human Testis Defined at the Single-Cell Level

Spermatogenesis has been intensely studied in rodents but remains poorly understood in humans. Here, we used single-cell RNA sequencing to analyze human testes. Clustering analysis of neonatal testes reveals several cell subsets, including cell populations with characteristics of primordial germ cells (PGCs) and spermatogonial stem cells (SSCs). In adult testes, we identify four undifferentiated spermatogonia (SPG) clusters, each of which expresses specific marker genes. We identify protein markers for the most primitive SPG state, allowing us to purify this likely SSC-enriched cell subset. We map the timeline of male germ cell development from PGCs through fetal germ cells to differentiating adult SPG stages. We also define somatic cell subsets in both neonatal and adult testes and trace their developmental trajectories. Our data provide a blueprint of the developing human male germline and supporting somatic cells. The PGC-like and SSC markers are candidates to be used for SSC therapy to treat infertility.

Sohni et al. use scRNA-seq analysis to define cell subsets in the human testis. Highlights include the identification of primordial germ cell- and spermatogonial stem cell-like cell subsets in neonatal testes, numerous undifferentiated spermatogonial cell states in adult testes, and somatic cell subsets in both neonatal and adult testes.

Spermatogonial Stem Cells for In Vitro Spermatogenesis and In Vivo Restoration of Fertility

Spermatogonial stem cells (SSCs) are the only adult stem cells capable of passing genes onto the next generation. SSCs also have the potential to provide important knowledge about stem cells in general and to offer critical in vitro and in vivo applications in assisted reproductive technologies. After century-long research, proof-of-principle culture systems have been introduced to support the in vitro differentiation of SSCs from rodent models into haploid male germ cells. Despite recent progress in organotypic testicular tissue culture and two-dimensional or three-dimensional cell culture systems, to achieve complete in vitro spermatogenesis (IVS) using non-rodent species remains challenging. Successful in vitro production of human haploid male germ cells will foster hopes of preserving the fertility potential of prepubertal cancer patients who frequently face infertility due to the gonadotoxic side-effects of cancer treatment. Moreover, the development of optimal systems for IVS would allow designing experiments that are otherwise difficult or impossible to be performed directly in vivo, such as genetic manipulation of germ cells or correction of genetic disorders. This review outlines the recent progress in the use of SSCs for IVS and potential in vivo applications for the restoration of fertility.

Regulation of long non-coding RNAs and circular RNAs in spermatogonial stem cells

Spermatogonial stem cells (SSCs) are one of the most significant stem cells with the potentials of self-renewal, differentiation, transdifferentiation and dedifferentiation, and thus, they have important applications in reproductive and regenerative medicine. They can transmit the genetic and epigenetic information across generations, which highlights the importance of the correct establishment and maintenance of epigenetic marks. Accurate transcriptional and post-transcriptional regulation is required to support the highly coordinated expression of specific genes for each step of spermatogenesis. Increasing evidence indicates that non-coding RNAs (ncRNAs), including long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play essential roles in controlling gene expression and fate determination of male germ cells. These ncRNA molecules have distinct characteristics and biological functions, and they independently or cooperatively modulate the proliferation, apoptosis and differentiation of SSCs. In this review, we summarized the features, biological function and fate of mouse and human SSCs, and we compared the characteristics of lncRNAs and circRNAs. We also addressed the roles and mechanisms of lncRNAs and circRNAs in regulating mouse and human SSCs, which would add novel insights into the epigenetic mechanisms underlying mammalian spermatogenesis and provide new approaches to treat male infertility.

Spermatogonial stem cells and spermatogenesis in mice, monkeys and men

Continuous spermatogenesis in post-pubertal mammals is dependent on spermatogonial stem cells (SSCs), which balance self-renewing divisions that maintain stem cell pool with differentiating divisions that sustain continuous sperm production. Rodent stem and progenitor spermatogonia are described by their clonal arrangement in the seminiferous epithelium (e.g., A_single, A_paired or A_aligned spermatogonia), molecular markers (e.g., ID4, GFRA1, PLZF, SALL4 and others) and most importantly by their biological potential to produce and maintain spermatogenesis when transplanted into recipient testes. In contrast, stem cells in the testes of higher primates (nonhuman and human) are defined by description of their nuclear morphology and staining with hematoxylin as A_dark and A_pale spermatogonia. There is limited information about how dark and pale descriptions of nuclear morphology in higher primates correspond with clone size, molecular markers or transplant potential. Do the apparent differences in stem cells and spermatogenic lineage development between rodents and primates represent true biological differences or simply differences in the volume of research and the vocabulary that has developed over the past half century? This review will provide an overview of stem, progenitor and differentiating spermatogonia that support spermatogenesis; identifying parallels between rodents and primates where they exist as well as features unique to higher primates.

Human spermatogonial markers

In this review, we provide an up-to-date compilation of published human spermatogonial markers, with focus on the three nuclear subtypes A_dark, A_pale and B. In addition, we have extended our recently published list of putative spermatogonial markers with protein expression and RNA-sequencing data from the Human Protein Atlas and supported these by literature evidence. Most importantly, we have put substantial effort in acquiring a comprehensive list of new and potentially interesting markers by refiltering the raw data of 15 published germ cell expression datasets (four human, eleven rodent) and subsequent building of intersections to acquire a robust, cross-species set of spermatogonia-enriched or -specific transcripts.

Unraveling transcriptome dynamics in human spermatogenesis

Spermatogenesis is a dynamic developmental process that includes stem cell proliferation and differentiation, meiotic cell divisions and extreme chromatin condensation. Although studied in mice, the molecular control of human spermatogenesis is largely unknown. Here, we developed a protocol that enables next-generation sequencing of RNA obtained from pools of 500 individually laser-capture microdissected cells of specific germ cell subtypes from fixed human testis samples. Transcriptomic analyses of these successive germ cell subtypes reveals dynamic transcription of over 4000 genes during human spermatogenesis. At the same time, many of the genes encoding for well-established meiotic and post-meiotic proteins are already present in the pre-meiotic phase. Furthermore, we found significant cell type-specific expression of post-transcriptional regulators, including expression of 110 RNA-binding proteins and 137 long non-coding RNAs, most of them previously not linked to spermatogenesis. Together, these data suggest that the transcriptome of precursor cells already contains the genes necessary for cellular differentiation and that timely translation controlled by post-transcriptional regulators is crucial for normal development. These established transcriptomes provide a reference catalog for further detailed studies on human spermatogenesis and spermatogenic failure.

Highlighted Article: Using laser capture microscopy, a comprehensive transcriptomic dataset of well-defined and distinct germ cell subtypes based on morphology and localization in the human testis is generated.

Spermatogonial Stem Cells: Implications for Genetic Disorders and Prevention

Spermatogonial stem cells (SSCs) propagate mammalian spermatogenesis throughout male reproductive life by continuously self-renewing and differentiating, ultimately, into sperm. SSCs can be cultured for long periods and restore spermatogenesis upon transplantation back into the native microenvironment in vivo. Conventionally, SSC research has been focused mainly on male infertility and, to a lesser extent, on cell reprogramming. With the advent of genome-wide sequencing technology, however, human studies have uncovered a wide range of pathogenic alleles that arise in the male germ line. A subset of de novo point mutations was shown to originate in SSCs and cause congenital disorders in children. This review describes both monogenic diseases (eg, Apert syndrome) and complex disorders that are either known or suspected to be driven by mutations in SSCs. We propose that SSC culture is a suitable model for studying the origin and mechanisms of these diseases. Lastly, we discuss strategies for future clinical implementation of SSC-based technology, from detecting mutation burden by sperm screening to gene correction in vitro.

Spermatogonial cells: mouse, monkey and man comparison

In all mammals, spermatogonia are defined as constituting the mitotic compartment of spermatogenesis including stem, undifferentiated and differentiating cell types, possessing distinct morphological and molecular characteristics. Even though the real nature of the spermatogonial stem cell and its regulation is still debated the general consensus holds that in steady-state spermatogenesis the stem cell compartment needs to balance differentiation versus self-renewal. This review highlights current understanding of spermatogonial biology, the kinetics of amplification and the signals directing spermatogonial differentiation in mammals. The focus will be on relevant similarities and differences between rodents and non human and human primates.

Spermatogenesis: The Commitment to Meiosis

Mammalian spermatogenesis requires a stem cell pool, a period of amplification of cell numbers, the completion of reduction division to haploid cells (meiosis), and the morphological transformation of the haploid cells into spermatozoa (spermiogenesis). The net result of these processes is the production of massive numbers of spermatozoa over the reproductive lifetime of the animal. One study that utilized homogenization-resistant spermatids as the standard determined that human daily sperm production (dsp) was at 45 million per day per testis (60). For each human that means ∼1,000 sperm are produced per second. A key to this level of gamete production is the organization and architecture of the mammalian testes that results in continuous sperm production. The seemingly complex repetitious relationship of cells termed the "cycle of the seminiferous epithelium" is driven by the continuous commitment of undifferentiated spermatogonia to meiosis and the period of time required to form spermatozoa. This commitment termed the A to A1 transition requires the action of retinoic acid (RA) on the undifferentiated spermatogonia or prospermatogonia. In stages VII to IX of the cycle of the seminiferous epithelium, Sertoli cells and germ cells are influenced by pulses of RA. These pulses of RA move along the seminiferous tubules coincident with the spermatogenic wave, presumably undergoing constant synthesis and degradation. The RA pulse then serves as a trigger to commit undifferentiated progenitor cells to the rigidly timed pathway into meiosis and spermatid differentiation.

Spermatogonial stem cells: Current biotechnological advances in reproduction and regenerative medicine

Spermatogonial stem cells (SSCs) are the germ stem cells of the seminiferous epithelium in the testis. Through the process of spermatogenesis, they produce sperm while concomitantly keeping their cellular pool constant through self-renewal. SSC biology offers important applications for animal reproduction and overcoming human disease through regenerative therapies. To this end, several techniques involving SSCs have been developed and will be covered in this article. SSCs convey genetic information to the next generation, a property that can be exploited for gene targeting. Additionally, SSCs can be induced to become embryonic stem cell-like pluripotent cells in vitro. Updates on SSC transplantation techniques with related applications, such as fertility restoration and preservation of endangered species, are also covered on this article. SSC suspensions can be transplanted to the testis of an animal and this has given the basis for SSC functional assays. This procedure has proven technically demanding in large animals and men. In parallel, testis tissue xenografting, another transplantation technique, was developed and resulted in sperm production in testis explants grafted into ectopical locations in foreign species. Since SSC culture holds a pivotal role in SSC biotechnologies, current advances are overviewed. Finally, spermatogenesis in vitro, already demonstrated in mice, offers great promises to cope with reproductive issues in the farm animal industry and human clinical applications.

Regulation of spermatogonial stem cell self-renewal and spermatocyte meiosis by Sertoli cell signaling

Spermatogenesis is a continuous and productive process supported by the self-renewal and differentiation of spermatogonial stem cells (SSCs), which arise from undifferentiated precursors known as gonocytes and are strictly controlled in a special 'niche' microenvironment in the seminiferous tubules. Sertoli cells, the only somatic cell type in the tubules, directly interact with SSCs to control their proliferation and differentiation through the secretion of specific factors. Spermatocyte meiosis is another key step of spermatogenesis, which is regulated by Sertoli cells on the luminal side of the blood-testis barrier through paracrine signaling. In this review, we mainly focus on the role of Sertoli cells in the regulation of SSC self-renewal and spermatocyte meiosis, with particular emphasis on paracrine and endocrine-mediated signaling pathways. Sertoli cell growth factors, such as glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2), as well as Sertoli cell transcription factors, such as ETS variant 5 (ERM; also known as ETV5), nociceptin, neuregulin 1 (NRG1), and androgen receptor (AR), have been identified as the most important upstream factors that regulate SSC self-renewal and spermatocyte meiosis. Other transcription factors and signaling pathways (GDNF-RET-GFRA1 signaling, FGF2-MAP2K1 signaling, CXCL12-CXCR4 signaling, CCL9-CCR1 signaling, FSH-nociceptin/OPRL1, retinoic acid/FSH-NRG/ERBB4, and AR/RB-ARID4A/ARID4B) are also addressed.

Quantitative detection of human spermatogonia for optimization of spermatogonial stem cell culture

STUDY QUESTION

Can human spermatogonia be detected in long-term primary testicular cell cultures using validated, germ cell-specific markers of spermatogonia?

SUMMARY ANSWER

Germ cell-specific markers of spermatogonia/spermatogonial stem cells (SSCs) are detected in early (1-2 weeks) but not late (> 6 weeks) primary testicular cell cultures; somatic cell markers are detected in late primary testicular cell cultures.

WHAT IS KNOWN ALREADY

The development of conditions for human SSC culture is critically dependent on the ability to define cell types unequivocally and to quantify spermatogonia/SSCs. Growth by somatic cells presents a major challenge in the establishment of SSC cultures and therefore markers that define spermatogonia/SSCs, but are not also expressed by testicular somatic cells, are essential for accurate characterization of SSC cultures.

STUDY DESIGN, SIZE, DURATION

Testicular tissue from eight organ donors with normal spermatogenesis was used for assay validation and establishing primary testicular cell cultures.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immunofluorescence analysis of normal human testicular tissue was used to validate antibodies (UTF1, SALL4, DAZL and VIM) and then the antibodies were used to demonstrate that primary testicular cells cultured in vitro for 1-2 weeks were composed of somatic cells and rare germ cells. Primary testicular cell cultures were further characterized by comparing to testicular somatic cell cultures using quantitative reverse transcriptase PCR (UTF1, FGFR3, ZBTB16, GPR125, DAZL, GATA4 and VIM) and flow cytometry (CD9 and SSEA4).

MAIN RESULTS AND THE ROLE OF CHANCE

UTF1, FGFR3, DAZL and ZBTB16 qRT-PCR and SSEA4 flow cytometry were validated for the sensitive, quantitative and specific detection of germ cells. In contrast, GPR125 mRNA and CD9 were found to be not specific to germ cells because they were also expressed in testicular somatic cell cultures. While the germ cell-specific markers were detected in early primary testicular cell cultures (1-2 weeks), their expression steadily declined over time in vitro. After 6 weeks in culture only somatic cells were detected.

LIMITATIONS, REASONS FOR CAUTION

Different groups attempting SSC culture have utilized different sources of human testes and minor differences in the preparation and maintenance of the testicular cell cultures. Differences in outcome may be explained by genetic background of the source tissue or technical differences.

WIDER IMPLICATIONS OF THE FINDINGS

The ability to propagate human SSCs in vitro is a prerequisite for proposed autologous transplantation therapy aimed at restoring fertility to men who have been treated for childhood cancer. By applying the assays validated here it will be possible to quantitatively compare human SSC culture conditions. The eventual development of conditions for long-term propagation of human SSCs in vitro will greatly facilitate learning about the basic biology of these cells and in turn the ability to use human SSCs in therapy.

STUDY FUNDING/COMPETING INTERESTS

The experiments presented in this manuscript were funded by a Project Development Team within the ICTSI NIH/NCRR Grant Number TR000006. The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

Not applicable.

Enhanced fitness of adult spermatogonial stem cells bearing a paternal age-associated FGFR2 mutation

Pathogenic de novo mutations increase with fathers’ age and could be amplified through competition between genetically distinct subpopulations of spermatogonial stem cells (SSCs). Here, we tested the fitness of SSCs bearing wild-type human FGFR2 or an Apert syndrome mutant, FGFR2 (S252W), to provide experimental evidence for SSC competition. The S252W allele conferred enhanced FGFR2-mediated signaling, particularly at very low concentrations of ligand, and also subtle changes in gene expression. Mutant SSCs exhibited improved competitiveness in vitro and increased stem cell activity in vivo upon transplantation. The fitness advantage in vitro only occurred in low concentrations of fibroblast growth factor (FGF), was independent of FGF-driven proliferation, and was accompanied by increased response to glial cell line-derived neurotrophic factor (GDNF). Our studies provide experimental evidence of enhanced stem cell fitness in SSCs bearing a paternal age-associated mutation. Our model will be useful for interrogating other candidate mutations in the future to reveal mechanisms of disease risk.

•

FGFR2-mediated signaling regulates SSC self-renewal

•

Age-associated Apert syndrome FGFR2 mutation confers a fitness advantage to SSCs

•

Mutant FGFR2 enables SSCs to withstand limiting GDNF

•

Excessive growth factor exposure impairs SSC self-renewal signals