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Reconstitution of germ cell fate from pluripotent stem cells provides an opportunity to understand the molecular underpinnings of germ cell development. Here, we established robust methods for pluripotent stem cells (iPSCs) culture in the common marmoset (Callithrix jacchus, cj), which stably propagate in an undifferentiated state. Notably, iPSCs cultured on a feeder layer in the presence of a WNT signaling inhibitor upregulated genes related to ubiquitin-dependent protein catabolic processes and enter a permissive state that enables differentiation into primordial germ cell-like cells (PGCLCs) bearing immunophenotypic and transcriptomic similarities to pre-migratory cjPGCsin vivo. Induction of cjPGCLCs is accompanied by transient upregulation of mesodermal genes culminating in the establishment of a primate specific germline transcriptional network. Moreover, cjPGCLCs can be expanded in monolayer while retaining the germline state. Upon co-culture with mouse testicular somatic cells, these cells acquire an early prospermatogonia-like phenotype. Our findings provide a framework for understanding and reconstituting marmoset germ cell development in vitro, thus providing a comparative tool and foundation for a preclinical modeling of human in vitro gametogenesis.

In the developing mammalian testis, only a small proportion of fetal and neonatal prospermatogonia give rise to the foundational pool of spermatogonial stem cells (SSCs). Multiple lines of evidence have suggested the determination of which prospermatogonia give rise to foundational SSCs is not random, but is rather predetermined, such that foundational SSCs are ensured to develop advantageous characteristics such as enhanced genetic integrity. Here I suggest that differential epigenetic programing contributes to the molecular mechanisms by which an early subset of developing prospermatogonia becomes predetermined to form the foundational pool of SSCs. This would include epigenetic programing that promotes active expression of genes needed to develop advantageous characteristics, as well as differential epigenetic priming, which bookmarks genes that comprise the SSC-specific transcriptome to become activated when foundational SSCs appear in the postnatal testis. I suggest that, together, differential epigenetic programing and epigenetic priming contribute to the molecular mechanisms by which an early subset of developing prospermatogonia becomes predetermined to form the foundational pool of SSCs.

The sterility or inviability of hybrid offspring produced from an interspecific mating result from incompatibilities between parental genotypes that are thought to result from divergence of loci involved in epistatic interactions. However, attributes contributing to the rapid evolution of these regions also complicates their assembly, thus discovery of candidate hybrid sterility loci is difficult and has been restricted to a small number of model systems. Here we reported rapid interspecific divergence at the DXZ4 macrosatellite locus in an interspecific cross between two closely related mammalian species: the domestic cat (Felis silvestris catus) and the Jungle cat (Felis chaus). DXZ4 is an interesting candidate due to its structural complexity, copy number variability, and described role in the critical yet complex biological process of X-chromosome inactivation. However, the full structure of DXZ4 was absent or incomplete in nearly every available mammalian genome assembly given its repetitive complexity. We compared highly continuous genomes for three cat species, each containing a complete DXZ4 locus, and discovered that the felid DXZ4 locus differs substantially from the human ortholog, and that it varies in copy number between cat species. Additionally, we reported expression, methylation, and structural conformation profiles of DXZ4 and the X chromosome during stages of spermatogenesis that have been previously associated with hybrid male sterility. Collectively, these findings suggest a new role for DXZ4 in male meiosis and a mechanism for feline interspecific incompatibility through rapid satellite divergence.

Initiation of spermatogonial differentiation in the mouse testis begins with the response to retinoic acid (RA) characterized by activation of KIT and STRA8 expression. In the adult, spermatogonial differentiation is spatiotemporally coordinated by a pulse of RA every 8.6 days that is localized to stages VII–VIII of the seminiferous epithelial cycle. Dogmatically, progenitor spermatogonia that express retinoic acid receptor gamma (RARG) at these stages will differentiate in response to RA, but this has yet to be tested functionally. Previous single-cell RNA-seq data identified phenotypically and functionally distinct subsets of spermatogonial stem cells (SSCs) and progenitor spermatogonia, where late progenitor spermatogonia were defined by expression of RARG and Dppa3. Here, we found late progenitor spermatogonia (RARGhigh KIT−) were further divisible into two subpopulations based on Dppa3 reporter expression (Dppa3-ECFP or Dppa3-EGFP) and were observed across all stages of the seminiferous epithelial cycle. However, nearly all Dppa3+ spermatogonia were differentiating (KIT+) late in the seminiferous epithelial cycle (stages X–XII), while Dppa3− late progenitors remained abundant, suggesting that Dppa3+ and Dppa3− late progenitors differentially responded to RA. Following acute RA treatment (2–4 h), significantly more Dppa3+ late progenitors induced KIT, including at the midpoint of the cycle (stages VI–IX), than Dppa3− late progenitors. Subsequently, single-cell analyses indicated a subset of Dppa3+ late progenitors expressed higher levels of Rxra, which we confirmed by RXRA whole-mount immunostaining. Together, these results indicate RARG alone is insufficient to initiate a spermatogonial response to RA in the adult mouse testis and suggest differential RXRA expression may discriminate responding cells.

Background Analyzing whole-genome bisulfite sequencing (WGBS) datasets is a time-intensive process due to the complexity and size of the input raw sequencing files and lengthy read alignment step during downstream data processing. This is particularly challenging with WGBS data because the conversion of all unmethylated Cs to Ts genome-wide renders read alignment a cumbersome computational process that can take up to a full work week of computing time. The objective of the study described here was to modify the read alignment algorithm associated with the wg-blimp pipeline to shorten the time required to complete this phase while retaining overall read alignment accuracy. Results Here we report improvements upon the recently published pipeline wg-blimp (whole-genome bisulfite sequencing methylation analysis pipeline) achieved by replacing the use of the bwa-meth aligner with the faster gemBS aligner. This improvement to the wg-blimp pipeline has led to a > 7x acceleration in the processing speed of samples when scaled to larger publicly available FASTQ datasets containing 80–160 million (M) reads. Importantly, this acceleration was achieved while maintaining nearly identical accuracy of properly mapped reads when compared to data from the original pipeline. Conclusion The modifications to the wg-blimp pipeline reported here merge the speed and accuracy of the gemBS aligner with the comprehensive analysis and data visualization assets of the wg-blimp pipeline to provide a significantly accelerated pipeline that can produce high-quality data much more rapidly without compromising read accuracy.

Although reactive oxygen species (ROS) are required for spermatogonial stem cell (SSC) self-renewal, they induce DNA damage and are harmful to SSCs. However, little is known about how SSCs protect their genome during self-renewal. Here, we report that Ogg1 is essential for SSC protection against ROS. While cultured SSCs exhibited homologous recombination-based DNA double-strand break repair at levels comparable with those in pluripotent stem cells, they were significantly more resistant to hydrogen peroxide than pluripotent stem cells or mouse embryonic fibroblasts, suggesting that they exhibit high levels of base excision repair (BER) activity. Consistent with this observation, cultured SSCs showed significantly lower levels of point mutations than somatic cells, and showed strong expression of BER-related genes. Functional screening revealed that Ogg1 depletion significantly impairs survival of cultured SSCs upon hydrogen peroxide exposure. Thus, our results suggest increased expression of BER-related genes, including Ogg1, protects SSCs from ROS-induced damage.Summary sentenceOGG1 suppresses ROS-induced damages in spermatogonial stem cells.

Summary: Because epigenetics is a critical component for gene expression, the hypothesis was tested that DNA methylation alterations are dynamic and continually change throughout gametogenesis to generate the mature sperm. Developmental alterations and stage-specific DNA methylation during gametogenesis from primordial germ cells (PGCs) to mature sperm are investigated. Individual developmental stage germ cells were isolated and analyzed for differential DNA methylation regions (DMRs). The number of DMRs was highest in the first three comparisons with mature PGCs, prospermatogonia, and spermatogonia. The most statistically significant DMRs were present at all stages of development and had variations involving both increases or decreases in DNA methylation. DMR-associated genes were identified and correlated with gene functional categories, pathways, and cellular processes. Observations identified a dynamic cascade of epigenetic changes during development that is dramatic during the early developmental stages. Complex epigenetic alterations are required to regulate genome biology and gene expression during gametogenesis.

More than a decade ago, the ENCODE and NIH Epigenomics Roadmap consortia organized large multilaboratory efforts to profile the epigenomes of >110 different mammalian somatic cell types. This generated valuable publicly accessible datasets that are being mined to reveal genome-wide patterns of a variety of different epigenetic parameters. This consortia approach facilitated the powerful and comprehensive multiparametric integrative analysis of the epigenomes in each cell type. However, no germ cell types were included among the cell types characterized by either of these consortia. Thus, comprehensive epigenetic profiling data are not generally available for the most evolutionarily important cells, male and female germ cells. We discuss the need for reproductive biologists to generate similar multiparametric epigenomic profiling datasets for both male and female germ cells at different developmental stages and summarize our recent effort to derive such data for mammalian spermatogonial stem cells and progenitor spermatogonia. Summary sentence The ENCODE and NIH Epigenome Roadmap consortia have generated valuable datasets profiling epigenomes of >110 different mammalian somatic cell types, but none of these have included profiles of even a single germ cell type.

A number of environmental factors from nutrition to toxicants have been shown to promote the epigenetic transgenerational inheritance of disease and phenotypic variation. This requires alterations in the germline (sperm or egg) epigenome. Previously, the agricultural fungicide vinclozolin was found to promote the transgenerational inheritance of sperm differential DNA methylation regions (DMRs) termed epimutations that help mediate this epigenetic inheritance. The current study was designed to investigate the developmental origins of the transgenerational DMRs during gametogenesis. Male control and vinclozolin lineage F3 generation rats were used as a source of embryonic day 13 (E13) primordial germ cells, embryonic day 16 (E16) prospermatogonia, postnatal day 10 (P10) spermatogonia, adult pachytene spermatocytes, round spermatids, caput epididymal spermatozoa, and caudal sperm. The DMRs between the control versus vinclozolin lineage samples were determined for each developmental stage. The top 100 statistically significant DMRs for each stage were compared. The developmental origins of the caudal epididymal sperm DMRs were assessed. The chromosomal locations and genomic features of the different stage DMRs were investigated. In addition, the DMR associated genes were identified. Previous studies have demonstrated alterations in the DMRs of primordial germ cells (PGCs). Interestingly, the majority of the DMRs identified in the current study for the caudal sperm originated during the spermatogenic process in the testis. A cascade of epigenetic alterations initiated in the PGCs appears to be required to alter the epigenetic programming during spermatogenesis to modify the sperm epigenome involved in the transgenerational epigenetic inheritance phenomenon.

Summary: Spermatogenesis is a complex and dynamic cellular differentiation process critical to male reproduction and sustained by spermatogonial stem cells (SSCs). Although patterns of gene expression have been described for aggregates of certain spermatogenic cell types, the full continuum of gene expression patterns underlying ongoing spermatogenesis in steady state was previously unclear. Here, we catalog single-cell transcriptomes for >62,000 individual spermatogenic cells from immature (postnatal day 6) and adult male mice and adult men. This allowed us to resolve SSC and progenitor spermatogonia, elucidate the full range of gene expression changes during male meiosis and spermiogenesis, and derive unique gene expression signatures for multiple mouse and human spermatogenic cell types and/or subtypes. These transcriptome datasets provide an information-rich resource for studies of SSCs, male meiosis, testicular cancer, male infertility, or contraceptive development, as well as a gene expression roadmap to be emulated in efforts to achieve spermatogenesis in vitro. : Hermann et al. present single-cell transcriptomes from >62,000 individual spermatogenic cells from immature and adult male mice and adult men. Their analysis facilitates resolution of SSCs and progenitor spermatogonia, elucidates the full range of gene expression changes during male meiosis and spermiogenesis, and derives unique gene expression signatures for eleven mouse and human spermatogenic cell types. Keywords: male germ cells, RNA, heterogeneity, meiosis, spermiogenesis, continuum, human, mouse

Chromatin of male and female gametes undergoes a number of reprogramming events during the transition from germ cell to embryonic developmental programs. Although the rearrangement of DNA methylation patterns occurring in the zygote has been extensively characterized, little is known about the dynamics of DNA modifications during spermatid maturation. Here, we demonstrate that the dynamics of 5-carboxylcytosine (5caC) correlate with active transcription of LINE-1 retroelements during murine spermiogenesis. We show that the open reading frames of active and evolutionary young LINE-1s are 5caC-enriched in round spermatids and 5caC is eliminated from LINE-1s and spermiogenesis-specific genes during spermatid maturation, being simultaneously retained at promoters and introns of developmental genes. Our results reveal an association of 5caC with activity of LINE-1 retrotransposons suggesting a potential direct role for this DNA modification in fine regulation of their transcription., Blythe et al investigate the patterns of oxidized forms of the 5-methylcytosine DNA modification during spermatogenesis. They find that open reading frames of evolutionarily young and transcriptionally active LINE-1 retrotransposons are enriched in 5-carboxylcytosine (5caC) during spermatid maturation, suggesting a link between 5cac and LINE-1 activity.

Summary Spermatogonial stem cells (SSCs) both self-renew and give rise to progenitors that initiate spermatogenic differentiation in the mammalian testis. Questions remain regarding the extent to which the SSC and progenitor states are functionally distinct. Here we provide the first multiparametric integrative analysis of mammalian germ cell epigenomes comparable with that done for >100 somatic cell types by the ENCODE Project. Differentially expressed genes distinguishing SSC- and progenitor-enriched spermatogonia showed distinct histone modification patterns, particularly for H3K27ac and H3K27me3. Motif analysis predicted transcription factors that may regulate spermatogonial subtype-specific fate, and immunohistochemistry and gene-specific chromatin immunoprecipitation analyses confirmed subtype-specific differences in target gene binding of a subset of these factors. Taken together, these results show that SSCs and progenitors display distinct epigenetic profiling consistent with these spermatogonial subtypes being differentially programmed to either self-renew and maintain regenerative capacity as SSCs or lose regenerative capacity and initiate lineage commitment as progenitors., Graphical Abstract, Highlights • Spermatogonial stem cells both self-renew and give rise to progenitor spermatogonia • Differential gene expression reflects stem versus differentiating cell fates • Differentially expressed genes show differential epigenetic programming • Motif analysis predicts regulators of cell type-specific epigenetic programming, Developmental Genetics; Omics

Spermatogonial stem cells (SSCs) both self-renew and give rise to progenitor spermatogonia that initiate spermatogenic differentiation in the mammalian testis. Questions remain regarding the extent to which the SSC and progenitor states are functionally distinct. Here we provide the first multiparametric integrative analysis of mammalian germ cell epigenomes comparable to that done for >100 somatic cell types by the ENCODE Project. Differentially expressed genes distinguishing SSC- and progenitor-enriched spermatogonia showed distinct histone modification patterns, particularly related to enrichment of H3K27ac and H3K27me3. Motif-enrichment analysis predicted transcription factors that may regulate spermatogonial subtype-specific fate, and immunohistochemistry and gene-specific chromatin immunoprecipitation analyses confirmed subtype-specific differences in target gene binding of a subset of these factors. Taken together, these results suggest that SSCs and progenitors represent distinct spermatogonial subtypes differentially programmed to either self-renew and maintain regenerative capacity as SSCs, or lose regenerative capacity and initiate lineage commitment as progenitors.

OBJECTIVE To provide an overview of the history of the North American Testis Workshop (NATW), of its relationship to the American Society of Andrology (ASA), and of the publications that resulted from the first 25 workshops. METHODS The collection of volumes and journal articles that relate to the NATW was searched. DISCUSSION AND CONCLUSION During the first twenty-five meetings of the NATW, a remarkable number of breakthroughs regarding every aspect of the testis were presented. We anticipate that with the acceleration of new genetic, epigenetic, and molecular knowledge of the functions of testicular cells, we will continue to learn about the discovery of new and clinically important aspects of testicular function during the next twenty-five NATWs.

There is now considerable evidence indicating the potential for endocrine disrupting chemicals to alter the epigenome and for subsets of these epigenomic changes or “epimutations” to be heritably transmitted to offspring in subsequent generations. While there have been many studies indicating how exposure to endocrine disrupting chemicals can disrupt various organs associated with the body’s endocrine systems, there is relatively limited information regarding the relative susceptibility of different specific organs, tissues, or cell types to endocrine disrupting chemical-induced epimutagenesis. Here we review available information about different organs, tissues, cell types, and/or cell lines which have been shown to be susceptible to specific endocrine disrupting chemical-induced epimutations. In addition, we discuss possible mechanisms that may be involved, or impacted by this tissue- or cell type-specific, differential susceptibility to different endocrine disrupting chemicals. Finally, we summarize available information indicating that certain periods of development display elevated susceptibility to endocrine disrupting chemical exposure and we describe how this may affect the extent to which germline epimutations can be transmitted inter- or transgenerationally. We conclude that cell type-specific differential susceptibility to endocrine disrupting chemical-induced epimutagenesis is likely to directly impact the extent to, or manner in, which endocrine disrupting chemical exposure initially induces epigenetic changes to DNA methylation and/or histone modifications, and how these endocrine disrupting chemical-induced epimutations can then subsequently impact gene expression, potentially leading to the development of heritable disease states.

Pluripotent cells have been reported to exhibit lower frequencies of point mutations and higher levels of DNA repair than differentiated cells. This predicts that pluripotent cells are less susceptible to mutagenic exposures than differentiated cells. To test this prediction, we used a lacI mutation-reporter transgene system to assess the frequency of point mutations in multiple lines of mouse pluripotent embryonic stem cells and induced pluripotent cells, as well as in multiple lines of differentiated fibroblast cells, before and after exposure to a moderate dose of the mutagen, methyl methanesulfonate. We also measured levels of key enzymes in the base excision repair (BER) pathway in each cell line before and after exposure to the mutagen. Our results confirm that pluripotent cells normally maintain lower frequencies of point mutations than differentiated cells, and show that differentiated cells exhibit a large increase in mutation frequency following a moderate mutagenic exposure, whereas pluripotent cells subjected to the same exposure show no increase in mutations. This result likely reflects the higher levels of BER proteins detectable in pluripotent cells prior to exposure and supports our thesis that maintenance of enhanced genetic integrity is a fundamental characteristic of pluripotent cells.

Numerous environmental toxicants have been shown to induce the epigenetic transgenerational inheritance of disease and phenotypic variation. Alterations in the germline epigenome are necessary to transmit transgenerational phenotypes. In previous studies, the pesticide DDT (dichlorodiphenyltrichloroethane) and the agricultural fungicide vinclozolin were shown to promote the transgenerational inheritance of sperm differential DNA methylation regions, non-coding RNAs and histone retention, which are termed epimutations. These epimutations are able to mediate this epigenetic inheritance of disease and phenotypic variation. The current study was designed to investigate the developmental origins of the transgenerational differential histone retention sites (called DHRs) during gametogenesis of the sperm. Vinclozolin and DDT were independently used to promote the epigenetic transgenerational inheritance of these DHRs. Male control lineage, DDT lineage and vinclozolin lineage F3 generation rats were used to isolate round spermatids, caput epididymal spermatozoa, and caudal sperm. The DHRs distinguishing the control versus DDT lineage or vinclozolin lineage samples were determined at these three developmental stages. DHRs and a reproducible core of histone H3 retention sites were observed using an H3 chromatin immunoprecipitation-sequencing (ChIP-Seq) analysis in each of the germ cell populations. The chromosomal locations and genomic features of the DHRs were analyzed. A cascade of epigenetic histone retention site alterations was found to be initiated in the round spermatids and then further modified during epididymal sperm maturation. Observations show that in addition to alterations in sperm DNA methylation and ncRNA expression previously identified, the induction of differential histone retention sites (DHRs) in the later stages of spermatogenesis also occurs. This novel component of epigenetic programming during spermatogenesis can be environmentally altered and transmitted to subsequent generations through epigenetic transgenerational inheritance.

Spermatogonial stem cells (SSCs) both self-renew and give rise to progenitor spermatogonia that enter steady-state spermatogenesis in the mammalian testis. However, questions remain regarding the extent to which SSCs and progenitors represent stably distinct spermatogonial subtypes. Here we provide the first multiparametric integrative analysis of mammalian germ cell epigenomes comparable to that done by the ENCODE Project for >100 somatic cell types. Differentially expressed genes distinguishing SSCs and progenitors showed distinct histone modification patterns as well as differences in distal intergenic low-methylated regions. Motif-enrichment analysis predicted transcription factors that regulate this spermatogonial subtype-specific epigenetic programming, and gene-specific chromatin immunoprecipitation analyses confirmed subtype-specific differences in binding of a subset of these factors to target genes. Collectively, these results suggest that SSCs and progenitors are stably distinct spermatogonial subtypes differentially programmed to either self-renew and maintain regenerative capacity as SSCs, or lose regenerative capacity and initiate lineage commitment as progenitors.

SUMMARY Spermatogonial stem cells (SSCs) sustain spermatogenesis by balancing self-renewal and initiation of differentiation to produce progenitor spermatogonia committed to forming sperm. To define the regulatory logic among SSCs and progenitors, we performed single-cell RNA velocity analyses and validated results in vivo. A predominant quiescent SSC population spawns a small subset of cell-cycle-activated SSCs via mitogen-activated protein kinase (MAPK)/AKT signaling. Activated SSCs form early progenitors and mTORC1 inhibition drives activated SSC accumulation consistent with blockade to progenitor formation. Mechanistically, mTORC1 inhibition suppresses transcription among spermatogonia and specifically alters expression of insulin growth factor (IGF) signaling in early progenitors. Tex14−/− testes lacking intercellular bridges do not accumulate activated SSCs following mTORC1 inhibition, indicating that steady-state mTORC1 signaling drives activated SSCs to produce progenitor clones. These results are consistent with a model of SSC self-renewal dependent on interconversion between activated and quiescent SSCs, and mTORC1-dependent initiation of differentiation from SSCs to progenitor clones., Graphical Abstract, In Brief Suzuki et al. define relationships between subsets of adult mouse SSCs and progenitor spermatogonia using single-cell RNA velocity analyses and in vivo validations. Quiescent SCCs convert to cell-cycle-activated SCCs via MAPK/AKT signaling. Activated SCCs are driven to become early progenitor clones ready to initiate differentiation through mTORC1 signaling.

The derivation of dopaminergic neurons from induced pluripotent stem cells brings new hope for a patient-specific, stem cell-based replacement therapy to treat Parkinson's disease (PD) and related neurodegenerative diseases; and this novel cell-based approach has already proven effective in animal models. However, there are several aspects of this procedure that have yet to be optimized to the extent required for translation to an optimal cell-based transplantation protocol in humans. These challenges include pinpointing the optimal graft location, appropriately scaling up the graft volume, and minimizing the risk of chronic immune rejection, among others. To advance this procedure to the clinic, it is imperative that a model that accurately and fully recapitulates characteristics most pertinent to a cell-based transplantation to the human brain is used to optimize key technical aspects of the procedure. Nonhuman primates mimic humans in multiple ways including similarities in genomics, neuroanatomy, neurophysiology, immunogenetics, and age-related changes in immune function. These characteristics are critical to the establishment of a relevant model in which to conduct preclinical studies to optimize the efficacy and safety of cell-based therapeutic approaches to the treatment of PD. Here we review previous studies in rodent models, and emphasize additional advantages afforded by nonhuman primate models in general, and the baboon model in particular, for preclinical optimization of cell-based therapeutic approaches to the treatment of PD and other neurodegenerative diseases. We outline current unresolved challenges to the successful application of stem cell therapies in humans and propose that the baboon model in particular affords a number of traits that render it most useful for preclinical studies designed to overcome these challenges.

This study differentiated baboon fibroblast-derived induced pluripotent stem cells into dopaminergic neurons with the application of specific morphogens and growth factors. The results show the utility of the baboon model for testing and optimizing the efficacy and safety of stem cell-based therapeutic approaches for the treatment of Parkinson's disease., The progressive death of dopamine producing neurons in the substantia nigra pars compacta is the principal cause of symptoms of Parkinson’s disease (PD). Stem cells have potential therapeutic use in replacing these cells and restoring function. To facilitate development of this approach, we sought to establish a preclinical model based on a large nonhuman primate for testing the efficacy and safety of stem cell-based transplantation. To this end, we differentiated baboon fibroblast-derived induced pluripotent stem cells (biPSCs) into dopaminergic neurons with the application of specific morphogens and growth factors. We confirmed that biPSC-derived dopaminergic neurons resemble those found in the human midbrain based on cell type-specific expression of dopamine markers TH and GIRK2. Using the reverse transcriptase quantitative polymerase chain reaction, we also showed that biPSC-derived dopaminergic neurons express PAX6, FOXA2, LMX1A, NURR1, and TH genes characteristic of this cell type in vivo. We used perforated patch-clamp electrophysiology to demonstrate that biPSC-derived dopaminergic neurons fired spontaneous rhythmic action potentials and high-frequency action potentials with spike frequency adaption upon injection of depolarizing current. Finally, we showed that biPSC-derived neurons released catecholamines in response to electrical stimulation. These results demonstrate the utility of the baboon model for testing and optimizing the efficacy and safety of stem cell-based therapeutic approaches for the treatment of PD. Significance Functional dopamine neurons were produced from baboon induced pluripotent stem cells, and their properties were compared to baboon midbrain cells in vivo. The baboon has advantages as a clinically relevant model in which to optimize the efficacy and safety of stem cell-based therapies for neurodegenerative diseases, such as Parkinson's disease. Baboons possess crucial neuroanatomical and immunological similarities to humans, and baboon pluripotent stem cells can be differentiated into functional neurons that mimic those in the human brain, thus laying the foundation for the utility of the baboon model for evaluating stem cell therapies.

Epigenetic alterations in the germline can be triggered by a number of different environmental factors from diet to toxicants. These environmentally induced germline changes can promote the epigenetic transgenerational inheritance of disease and phenotypic variation. In previous studies, the pesticide DDT was shown to promote the transgenerational inheritance of sperm differential DNA methylation regions (DMRs), also called epimutations, which can in part mediate this epigenetic inheritance. In the current study, the developmental origins of the transgenerational DMRs during gametogenesis have been investigated. Male control and DDT lineage F3 generation rats were used to isolate embryonic day 16 (E16) prospermatogonia, postnatal day 10 (P10) spermatogonia, adult pachytene spermatocytes, round spermatids, caput epididymal spermatozoa, and caudal sperm. The DMRs between the control versus DDT lineage samples were determined at each developmental stage. The top 100 statistically significant DMRs at each stage were compared and the developmental origins of the caudal epididymal sperm DMRs were assessed. The chromosomal locations and genomic features of the different stage DMRs were analyzed. Although previous studies have demonstrated alterations in the DMRs of primordial germ cells (PGCs), the majority of the DMRs identified in the caudal sperm originated during the spermatogonia stages in the testis. Interestingly, a cascade of epigenetic alterations initiated in the PGCs is required to alter the epigenetic programming during spermatogenesis to obtain the sperm epigenetics involved in the epigenetic transgenerational inheritance phenomenon.

Induced pluripotent stem cells (iPSCs) offer the possibility of cell replacement therapies using patient-matched cells to treat otherwise intractable diseases and debilitations. To successfully realize this potential, several factors must be optimized including i) selection of the appropriate cell type and numbers to transplant, ii) determination of the means of transplantation and the location into which the transplanted cells should be delivered, and iii) demonstration of the safety and efficacy of the cell replacement protocol to mitigate each targeted disease state. A majority of diseases or debilitations likely to be targeted by cell-based therapeutic approaches represent complex conditions or physiologies manifest predominantly in primates including humans. Nonhuman primates afford the most clinically relevant model system for biomedical studies and testing of cell-based therapies. Baboons have 92% genomic similarity with humans overall and especially significant similarities in their immunogenetic system, rendering this species a particularly valuable model for testing procedures involving cell transplants into living individuals. To maximize the utility of the baboon model, standardized protocols must be developed for the derivation of induced pluripotent stem cells from living adults and the long-term maintenance of these cells in culture. Here we tested four commercially available culture systems (ReproFF, mTeSR1, E8 and Pluristem) for competence to maintain baboon iPSCs in a pluripotent state over multiple passages, and to support the derivation of new lines of baboon iPSCs. Of these four media only Pluristem was able to maintain baboon pluripotency as assessed by morphological characteristics, immunocytochemistry and RT-qPCR. Pluristem also facilitated the derivation of new lines of iPSCs from adult baboon somatic cells, which had previously not been accomplished. We derived multiple iPS cell lines from adult baboon peripheral blood mononuclear cells cultured in Pluristem. These were validated by expression of the pluripotency markers OCT4, NANOG, SOX2, SSEA4 and TRA181, as well as the ability to differentiate into tissues from all three germ layers when injected into immunocompromised mice. These findings further advance the utility of the baboon as an ideal preclinical model system for optimizing iPS cell-based, patient-specific replacement therapies in humans.

Spermatogenesis is an ongoing differentiation process that occurs in the seminiferous epithelium in the testis in males to produce spermatozoa (sperm) and is sustained by a tissue-specific stem cell termed the “spermatogonial stem cell.” Spermatogonial stem cells (SSCs) must maintain the spermatogenic stem cell population via self-renewal while also providing an ongoing supply of cells that enter the terminal differentiation process of spermatogenesis. This article describes current evidence and theories regarding the manner in which a unique subset of developing spermatogonia become allocated to form the SSCs.

This article provides a brief overview of Volume 3 of this encyclopedia entitled, “Gametogenesis, Fertilization and Development”—three processes that occur sequentially and are central to reproduction. Together these processes include the production of gametes in each sex (gametogenesis), the union of a male and female gamete (fertilization), and the initial development of the resulting new embryo (development).

Recent studies have described numerous environmentally-induced disruptions of the epigenome (epimutations) in mammals. While some of these appear to be corrected by normal germline-specific epigenetic reprogramming and are therefore not transmitted transgenerationally, others are not corrected and are transmitted over multiple subsequent generations. The mechanism(s) that distinguish transgenerational and non-transgenerational epimutations have not been delineated. This review examines several potential molecular and developmental distinctions between transgenerational and non-transgenerational epimutations in the context of the likelihood that any of these may or may not contribute to transgenerational inheritance of epimutations.

PIWI-interacting RNAs (piRNAs) are small noncoding RNAs that bind PIWI family proteins exclusively expressed in the germ cells of mammalian gonads. MIWI2-associated piRNAs are essential for silencing transposons during primordial germ cell development, and MIWI-bound piRNAs are required for normal spermatogenesis during adulthood in mice. Although piRNAs have long been regarded as germ cell-specific, increasing lines of evidence suggest that somatic cells also express piRNA-like RNAs (pilRNAs). Here, we report the detection of abundant pilRNAs in somatic cells, which are similar to MIWI-associated piRNAs mainly expressed in pachytene spermatocytes and round spermatids in the testis. Based on small RNA deep sequencing and quantitative PCR analyses, pilRNA expression is dynamic and displays tissue specificity. Although pilRNAs are similar to pachytene piRNAs in both size and genomic origins, they have a distinct ping-pong signature. Furthermore, pilRNA biogenesis appears to utilize a yet to be identified pathway, which is different from all currently known small RNA biogenetic pathways. In addition, pilRNAs appear to preferentially target the 3'-UTRs of mRNAs in a partially complementary manner. Our data suggest that pilRNAs, as an integral component of the small RNA transcriptome in somatic cell lineages, represent a distinct population of small RNAs that may have functions similar to germ cell piRNAs.

The Disposable Soma Theory holds that genetic integrity will be maintained at more pristine levels in germ cells than in somatic cells because of the unique role germ cells play in perpetuating the species. We tested the hypothesis that the same concept applies to pluripotent cells compared to differentiated cells. Analyses of transcriptome and cistrome databases, along with canonical pathway analysis and chromatin immunoprecipitation confirmed differential expression of DNA repair and cell death genes in embryonic stem cells and induced pluripotent stem cells relative to fibroblasts, and predicted extensive direct and indirect interactions between the pluripotency and genetic integrity gene networks in pluripotent cells. These data suggest that enhanced maintenance of genetic integrity is fundamentally linked to the epigenetic state of pluripotency at the genomic level. In addition, these findings demonstrate how a small number of key pluripotency factors can regulate large numbers of downstream genes in a pathway-specific manner.

Mammalian reproduction requires that males and females produce functional haploid germ cells through complex cellular differentiation processes known as spermatogenesis and oogenesis, respectively. While numerous studies have functionally characterized protein-coding genes and small noncoding RNAs (microRNAs and piRNAs) that are essential for gametogenesis, the roles of regulatory long noncoding RNAs (lncRNAs) are yet to be fully characterized. Previously, we and others have demonstrated that intergenic regions of the mammalian genome encode thousands of long noncoding RNAs, and many studies have now demonstrated their critical roles in key biological processes. Thus, we postulated that some lncRNAs may also impact mammalian spermatogenesis and fertility. In this study, we identified a dynamic expression pattern of lncRNAs during murine spermatogenesis. Importantly, we identified a subset of lncRNAs and very few mRNAs that appear to escape meiotic sex chromosome inactivation, an epigenetic process that leads to the silencing of the X- and Y-chromosomes at the pachytene stage of meiosis. Further, some of these lncRNAs and mRNAs show a strong testis expression pattern suggesting that they may play key roles in spermatogenesis. Lastly, we generated a mouse knockout of one X-linked lncRNA, Tslrn1 (testis-specific long noncoding RNA 1), and found that males carrying a Tslrn1 deletion displayed normal fertility but a significant reduction in spermatozoa. Our findings demonstrate that dysregulation of specific mammalian lncRNAs is a novel mechanism of low sperm count or infertility, thus potentially providing new biomarkers and therapeutic strategies.

In the mouse, the prospermatogonial phase of male germline development begins during the fetal stages and extends into the neonatal stages when the initial development of spermatogonia then occurs. In the immature testis, undifferentiated spermatogonia are heterogeneous with at least three distinct subpopulations—those spermatogonia that do not self-renew and give rise directly and only to the first spermatogenic wave, those spermatogonia that form spermatogonial stem cells (SSCs) that are capable of undergoing either self-renewal or differentiation such that they can sustain steady-state spermatogenesis throughout the reproductive lifespan of the male, and those spermatogonia that will undergo cell death. The mechanism that regulates which of these fates will be adopted by each developing spermatogonium remains unresolved. However, there is growing evidence that those prospermatogonia that ultimately give rise to SSCs may become predetermined to this fate during the early fetal stages of male germline development, such that these cells follow a unique developmental program that promotes accumulation of characteristics that are particularly advantageous to SSCs. This notion is supported by studies of the maintenance of genetic integrity in developing prospermatogonia and early undifferentiated spermatogonia that do or do not give rise to SSCs.

Pluripotent stem cells hold the potential to form the basis of novel approaches to treatment of disease in vivo as well as to facilitate the generation of models for human disease, providing powerful avenues to discovery of novel diagnostic biomarkers and/or innovative drug regimens in vitro. However, this will require extensive maintenance, expansion, and manipulation of these cells in culture, which raises a concern regarding the extent to which genetic integrity will be preserved throughout these manipulations. We used a mutation reporter (lacI) transgene approach to conduct direct comparisons of mutation frequencies in cell populations that shared a common origin and genetic identity, but were induced to undergo transitions in cell fate between pluripotent and differentiated states, or vice versa. We confirm that pluripotent cells normally maintain enhanced genetic integrity relative to that in differentiated cells, and we extend this finding to show that dynamic transformations in the relative stringency at which genetic integrity is maintained are associated with transitions between pluripotent and differentiated cellular states. These results provide insight into basic biological distinctions between pluripotent and differentiated cell types that impact genetic integrity in a manner that is directly relevant to the potential clinical use of these cell types.

Development of effective pluripotent stem cell-based therapies will require safety and efficacy testing in a clinically relevant preclinical model such as nonhuman primates (NHPs). Baboons and macaques are equally similar to humans genetically and both have been extensively used for biomedical research. Macaques are preferred for human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) research whereas baboons are preferred for transplantation studies because of the greater similarity of their anatomy and immunogenetic system to those of humans. We generated four induced pluripotent stem cell (iPSC) lines from skin cells of the olive baboon (Papio anubis). Each line shows the distinct morphology of primate pluripotent stem cells, including flat colonies with well-defined borders and a high nuclear/cytoplasm ratio. Each is positive for the pluripotency markers OCT4, SOX2, NANOG, and SSEA4. Pluripotency was confirmed in two lines by teratoma formation with representative tissues from each germ layer, whereas a third produced cells from all three germ layers following embryoid body differentiation. Three lines have a normal male karyotype and the fourth is missing the short arm of one copy of chromosome 18. This may serve as an in vitro model for the human developmental disorder 18p-, which impacts 1 in 50,000 births/year. These iPSC lines represent the first step toward establishing the baboon as a NHP model for developing stem cell-based therapies.

The phosphoglycerate kinase-2 (Pgk2) gene is regulated in a tissue-, cell type-, and developmental stage-specific manner during spermatogenesis and is required for normal sperm motility and fertility in mammals. Activation ofPgk2transcription is regulated by testis-specific demethylation of DNA and binding of testis-specific transcription factors to enhancer and core promoter elements. Here, we show that chromatin remodeling including reconfiguration of nucleosomes and changes in histone modifications is also associated with transcriptional activation of thePgk2gene during spermatogenesis. Developmental studies indicate that the order of events involved in transcriptional activation of thePgk2gene includes demethylation of DNA in T1- and T2-prospermatogonia, binding of a factor to the CAAT box in type A and B spermatogonia, followed by recruitment of chromatin remodeling factors, displacement of a nucleosome from thePgk2promoter region, binding of factors to thePgk2core promoter and enhancer regions, and, finally, initiation of transcription in primary spermatocytes. Transgene studies show thatPgk2core promoter elements are required to direct demethylation of DNA and reconfiguration of nucleosomes, whereas both enhancer and core promoter elements are required to direct changes in histone modifications and initiation of transcription. These results provide novel insight into the developmental order of molecular events required to activate tissue-specific transcription of thePgk2gene, the distinct elements in the 5′-regulatory region of thePgk2gene that regulate each of these events, and the relationship among these events in that each step in this process appears to be a necessary prerequisite for the subsequent step.

In multicellular organisms, germ cells carry the hereditary material from one generation to the next. Developing germ cells are unipotent gamete precursors, and mature gametes are highly differentiated, specialized cells. However, upon gamete union at fertilization, their genomes drive a totipotent program, giving rise to a complete embryo as well as extraembryonic tissues. The biochemical basis for the ability to transition from differentiated cell to totipotent zygote is unknown. Here we report that a set of developmentally critical genes is maintained in an epigenetically poised (bivalent) state from embryonic stages through the end of meiosis. We performed ChIP-seq and RNA-seq analysis on flow-sorted male and female germ cells during embryogenesis at three time points surrounding sexual differentiation and female meiotic initiation, and then extended our analysis to meiotic and postmeiotic male germ cells. We identified a set of genes that is highly enriched for regulators of differentiation and retains a poised state (high H3K4me3, high H3K27me3, and lack of expression) across sexes and across developmental stages, including in haploid postmeiotic cells. The existence of such a state in embryonic stem cells has been well described. We now demonstrate that a subset of genes is maintained in a poised state in the germ line from the initiation of sexual differentiation during fetal development and into postmeiotic stages. We propose that the epigenetically poised condition of these developmental genes is a fundamental property of the mammalian germ-line nucleus, allowing differentiated gametes to unleash a totipotent program following fertilization.