Single-cell RNAseq analysis of testicular germ and somatic cell development during the perinatal period

scRNA-seq and scATAC-seq reveal that Sertoli cell mediates spermatogenesis disorders through stage-specific communications in non-obstructive azoospermia

Non-obstructive azoospermia (NOA) belongs to male infertility due to spermatogenesis failure. However, evidence for cell type-specific abnormalities of spermatogenesis disorders in NOA remains lacking. We performed single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) on testicular tissues from patients with obstructive azoospermia (OA) and NOA. HE staining confirmed the structural abnormalities of the seminiferous tubules in NOA patients. We identified 12 germ cell subtypes (spermatogonial stem cell-0 [SSC0], SSC1, SSC2, diffing-spermatogonia [Diffing-SPG], diffed-spermatogonia [Diffed-SPG], pre-leptotene [Pre-Lep], leptotene-zygotene [L-Z], pachytene [Pa], diplotene [Di], spermatids-1 [SPT1], SPT2, and SPT3) and 8 Sertoli cell subtypes (SC1-SC8). Among them, three novel Sertoli cell subtype phenotypes were identified, namely SC4/immature, SC7/mature, and SC8/further mature Sertoli cells. For each germ or Sertoli cell subtype, we identified unique new markers, among which immunofluorescence confirmed co-localization of ST3GAL4, A2M, ASB9, and TEX19 and DDX4 (classical marker of germ cell). PRAP1, BST2, and CCDC62 were co-expressed with SOX9 (classical marker of Sertoli cell) in testes tissues also confirmed by immunofluorescence. The interaction between germ cell subtypes and Sertoli cell subtypes exhibits stage-specific-matching pattern, as evidenced by SC1/2/5/7 involving in SSC0-2 development, SC3 participating in the whole process of spermiogenesis, SC4/6 participating in Diffing and Diffed-SPG development, and SC8 involving in the final stage of SPT3. This pattern of specific interactions between subtypes of germ cell and Sertoli cell was confirmed by immunofluorescence of novel markers in testes tissues. The interaction was mainly regulated by the Notch1/2/3 signaling. Our study profiled the single-cell transcriptome of human spermatogenesis and provided many potential molecular markers for developing testicular puncture-specific marker kits for NOA patients.

Cell type- and factor-specific nonsense-mediated RNA decay

Nonsense-mediated RNA decay (NMD) is a highly conserved RNA turnover pathway that influences several biological processes. Specific features in messenger RNAs (mRNAs) have been found to trigger decay by NMD, leading to the assumption that NMD sensitivity is an intrinsic quality of a given transcript. Here, we provide evidence that, instead, an overriding factor dictating NMD sensitivity is the cell environment. Using several genome-wide techniques to detect NMD-target mRNAs, we find that hundreds of mRNAs are sensitized to NMD as human embryonic stem cells progress to form neural progenitor cells. Another class of mRNAs escape from NMD during this developmental progression. We show that the differential sensitivity to NMD extends to in vivo scenarios, and that the RNA-binding protein, HNRNPL, has a role in cell type-specific NMD. We also addressed another issue in the field-whether NMD factors are core or branch-specific in their action. Surprisingly, we found that UPF3B, an NMD factor critical for the nervous system, shares only 30% of NMD-target transcripts with the core NMD factor UPF2. Together, our findings have implications for how NMD is defined and measured, how NMD acts in different biological contexts, and how different NMD branches influence human diseases.

Functional Analysis of HSD17B3-Deficient Male Mice Reveals Roles for HSD17B7 and HSD17B12 in Testosterone Biosynthesis

Historically, 17β-hydroxysteroid dehydrogenase type 3 (HSD17B3) was thought to be the key enzyme responsible for testicular testosterone production. In humans, loss-of-function mutations in HSD17B3 impair testosterone production during prenatal life leading to impaired development of androgen-dependent tissues in 46,XY individuals. However, male mice with HSD17B3 deficiency exhibit normal testicular testosterone concentrations, normal development of reproductive organs and are fertile, suggesting that mice express other hydroxysteroid dehydrogenase enzymes capable of testicular testosterone synthesis. This study aimed to investigate whether 17β-hydroxysteroid dehydrogenase type 12 (HSD17B12), which can convert androstenedione to testosterone in mice but not in humans, compensates for the lack of HSD17B3 in Hsd17b3 knockout (KO) mice. We used CRISPR/Cas9 to substitute the amino acid in mouse HSD17B12 that is responsible for its ability to convert androstenedione to testosterone with the amino acid of the human enzyme that prevents androstenedione being used as a substrate. When this Hsd17b12 mutation was introduced into Hsd17b3 KO mice, males exhibited normal reproductive tracts but reduced testicular testosterone production with a consequential reduction in seminal vesicle weight. This suggests HSD17B12 contributes toward testosterone production in the absence of HSD17B3, but other enzymes must also contribute. We therefore quantified other testicular hydroxysteroid dehydrogenases, finding that HSD17B7 mRNA and protein was markedly upregulated in Hsd17b3 KO testes. We confirmed that mouse, but not human, HSD17B7 can produce testosterone in vitro. We conclude that compared to humans, mice exhibit increased plasticity in testosterone production via hydroxysteroid dehydrogenase enzymes to support androgen action and male fertility.

Dynamics of transcriptional programs and chromatin accessibility in mouse spermatogonial cells from early postnatal to adult life

In mammals, spermatogonial cells (SPGs) are undifferentiated male germ cells in testis that are quiescent until birth and then self-renew and differentiate to produce spermatogenic cells and functional sperm from early postnatal life throughout adulthood. The transcriptome of SPGs is highly dynamic and timely regulated during postnatal development. We examined if such dynamics involves changes in chromatin organization by profiling the transcriptome and chromatin accessibility of SPGs from early postnatal stages to adulthood in mice using deep RNA-seq, ATAC-seq and computational deconvolution analyses. By integrating transcriptomic and epigenomic features, we show that SPGs undergo massive chromatin remodeling during postnatal development that partially correlates with distinct gene expression profiles and transcription factors (TF) motif enrichment. We identify genomic regions with significantly different chromatin accessibility in adult SPGs that are marked by histone modifications associated with enhancers and promoters. Some of the regions with increased accessibility correspond to transposable element subtypes enriched in multiple TFs motifs and close to differentially expressed genes. Our results underscore the dynamics of chromatin organization in developing germ cells and complement existing datasets on SPGs by providing maps of the regulatory genome at high resolution from the same cell populations at early postnatal, late postnatal and adult stages collected from single individuals.

Applications of spatial transcriptomics in studying spermatogenesis

Spermatogenesis is a complex differentiation process that is facilitated by a series of cellular and molecular events. High-throughput genomics approaches, such as single-cell RNA sequencing, have begun to enable the systematic characterization of these events. However, the loss of tissue context because of tissue disassociations in the single-cell isolation protocols limits our ability to understand the regulation of spermatogenesis and how defects in spermatogenesis lead to infertility. The recent advancement of spatial transcriptomics technologies enables the studying of the molecular signatures of various cell types and their interactions in the native tissue context. In this review, we discuss how spatial transcriptomics has been leveraged to identify spatially variable genes, characterize cellular neighborhood, delineate cell‒cell communications, and detect molecular changes under pathological conditions in the mammalian testis. We believe that spatial transcriptomics, along with other emerging spatially resolved omics assays, can be utilized to further our understanding of the underlying causes of male infertility, and to facilitate the development of new treatment approaches.

Spatiotemporal single-cell architecture of gene expression in the Caenorhabditis elegans germ cells

Spermatogenesis is an intricate and tightly controlled process encompassing various layers of gene expression regulation. Despite the advance of our current understanding, the developmental trajectory and regulatory mechanisms dictating spermatogenesis remain elusive. In this study, we have generated single-cell gene expression profiles for Caenorhabditis elegans sperm cells and constructed gene regulatory networks alongside the developmental trajectories of these cells. Our findings indicate that each pre- and post-developmental stage is closely linked by co-expressed genes, while simultaneously being uniquely identified by the combined expression of specific gene families. To illustrate the applicability of this exhaustive gene expression catalog, we used gene regulatory networks to uncover potential transcription factors for (1) the expression of genes in the phosphorylation pathway, identifying NHR-23-to-phosphatase regulation for the meiotic cell division process; and (2) the expression of constituent components of small RNA pathways, identifying ELT-1-to-Argonaute protein regulation for siRNA maintenance and sperm activation. We expect that this sperm cell-specific gene expression directory will prompt investigations into the underlying mechanisms determining anatomy, differentiation, and function across the reproductive system. Finally, our expression data can be explored using the web application CelegansGermAtlas ( https://scgerm-atlas.sjtu.edu.cn/website/#/home ).

Transcriptomic and metabolic profiling reveals the effects of long-term microwave exposure on testicular tissue

The effect of electromagnetic exposure on health is becoming increasingly important as it affects many aspects of human life and health. However, the effects in environmental electromagnetic fields on the male reproductive system were still controversial, and the impacts of long-term microwave exposure on testicular tissue remain poorly defined. This study exposed rats to 30 mW/cm2 of microwave radiation (2.856 GHz) for six weeks and revealed that long-term microwave exposure damaged the testis structures, sperm motility, and morphology, affected hormone levels, energy metabolism, and induced oxidative stress. Assays for bulk RNA, metabonomics, single-cell RNA, and transposase-accessible chromatin with high-throughput sequencing were performed to analyze the transcriptional and metabolic atlas of testicular damage after microwave radiation. Differentially expressed genes were enriched in oxidative stress and energy metabolism pathways. Furthermore, ten subgroups were identified with scRNA-seq, including five developmental phases of germ cells, and radiation-associated changes in cell composition, especially stuck in round spermatids, were observed. Radiation significantly upregulated the expression of Atp6v1e2 in round spermatids and enriched the expression of many transcription factors by disturbing the accessibility profile of chromatin. This study provides effective insights into the long-term impacts of microwave radiation on male reproduction.

Single-Cell RNA Sequencing Reveals an Atlas of Meihua Pig Testis Cells

Mammalian spermatogenesis is a complex biological process that is regulated by multiple types of cells. The heterogeneity of these cells poses a challenge for analyzing different cell types at different developmental stages. To characterize the transcriptomic landscape of porcine spermatogenesis and identify potential marker genes for spermatogonia, an unbiased transcriptomic study of spermatogenesis in neonatal and sexually mature six-month-old Meihua pigs was performed using 10× Genomics single-cell RNA sequencing (scRNA-seq). Through the collection of scRNA-seq data from 13,839 cells from Meihua pig testes, three germ cells (spermatogonia, spermatocytes and spermatids) and eight somatic cells (Sertoli cells, Leydig cells, myoid/stromal cells, endothelial cells, T cells/macrophages and erythroblasts) were identified. Pseudo-timing analysis showed that myoid cells and stromal cells originated from common progenitors in Meihua pigs. Functional enrichment analysis revealed that the differentially expressed genes (DEGs) in testicular somatic cells were enriched in the pathways of Ribosome, Oxidative phosphorylation, Protein processing in endoplasmic reticulum, Retrograde endocannabinoid signaling, Cellular senescence and Insulin signaling. Meanwhile, in the three different germ cells, except for pathways which were the same as the first three pathways for somatic cells, DEGs were also enriched in the Spliceosome, Cell cycle, Autophagy and Mitophagy pathways. Furthermore, the candidate marker gene TKTL1 in spermatogonia was identified using immunohistochemistry and immunofluorescence. In conclusion, we collected transcription datasets and constructed single-cell developmental maps of germ cells and somatic cells during the testicular development of Meihua pigs, which provided new insights into the spermatogenesis of Meihua pigs and the development of various types of cells in their testes.

Cytological Effects of Cadmium Poisoning and the Protective Effect of Quercetin: A Mechanism Exploration based on the Testicular Lamina Propria

This comprehensive study delved into the detrimental effects of cadmium (Cd), a toxic heavy metal, on the testicular lamina propria (LP), a key player in spermatogenesis, and the maintenance of testicular stem cell niches. Utilizing transmission electron microscopy, immunohistochemistry, and double-labeling immunofluorescence, the research characterized the structural and cellular components of mouse testicular LP under Cd exposure and investigated the protective effects of quercetin. The findings illustrated that Cd exposure results in significant morphological and cellular modifications within the LP, including the apoptosis of peritubular myoid cells, an upsurge in CD34+ stromal cells displaying anti-apoptotic behaviors, and an excessive production of collagen Type I fibers and extracellular matrix. Remarkably, quercetin effectively counteracted these adverse changes by reversing apoptosis, reducing the proliferation of CD34+ stromal cells, and addressing fibrosis markers, thereby mitigating the cellular damage induced by Cd. This study not only highlighted the critical impact of apoptosis and fibrosis in Cd-related testicular damage but also elucidated the protective mechanism of quercetin, laying the groundwork for future clinical applications in addressing testicular damage from heavy metal poisoning through cellular therapeutics and pharmacological interventions.

A Review and New Insights Into Spermatogenesis From Single-Cell RNA Sequencing

As the most primitive germ cells in the testis, spermatogonial stem cells (SSCs) not only constantly renew themselves to ensure their quantity, but also differentiate into mature sperm cells to complete spermatogenesis and transmit genetic information to the next generation. Successful spermatogenesis is inseparable from niche regulation, which provides factors that enhance the self-renewal of SSCs to maintain their numbers and directs the appropriate differentiation of spermatogonia. Some progress has been achieved in the definition and isolation of SSCs. However, a high degree of cellular heterogeneity is found in the testis, revealing a combination of various cell types at different developmental stages and a lack of specific molecular markers (especially in domestic animals) for fully screening and purifying SSCs. These factors have considerably hindered further research into the mechanisms of maintenance, self-renewal, and differentiation of SSCs, as well as limited their isolation, purification, and applications. Accumulated studies have recently successfully employed single-cell RNA sequencing (scRNA-seq) as a novel approach to detailing the classification of cell subsets, mining specifically expressed genes in different cell types, and accurate identification of specific cell types. This review summarises the progress of SSCs identification and offers new insights into the SSCs developmental trajectory from single-cell RNA sequencing.

Dynamic transcriptomic and regulatory networks underpinning the transition from fetal primordial germ cells to spermatogonia in mice

The transition from fetal primordial germ cells (PGCs) to spermatogonia (SPG) is critical for male germ cell development; however, the detailed transcriptomic dynamics and regulation underlying this transition remain poorly understood. Here by interrogating the comprehensive transcriptome atlas dataset of mouse male germ cells and gonadal cells development, we elucidated the regulatory networks underlying this transition. Our single-cell transcriptome analysis revealed that the transition from PGCs to SPG was characterized by global hypertranscription. A total of 315 highly active regulators were identified to be potentially involved in this transition, among which a non-transcription factor (TF) regulator TAGLN2 was validated to be essential for spermatogonial stem cells (SSCs) maintenance and differentiation. Metabolism profiling analysis also revealed dynamic changes in metabolism-related gene expression during PGC to SPG transition. Furthermore, we uncovered that intricate cell-cell communication exerted potential functions in the regulation of hypertranscription in germ cells by collaborating with stage-specific active regulators. Collectively, our work extends the understanding of molecular mechanisms underlying male germ cell development, offering insights into the recapitulation of germ cell generation in vitro.

Insights from the single-cell level: lineage trajectory and somatic-germline interactions during spermatogenesis in dwarf surfclam Mulinia lateralis

BACKGROUND

Spermatogenesis is a complex process of cellular differentiation that commences with the division of spermatogonia stem cells, ultimately resulting in the production of functional spermatozoa. However, a substantial gap remains in our understanding of the molecular mechanisms and key driver genes that underpin this process, particularly in invertebrates. The dwarf surfclam (Mulinia lateralis) is considered an optimal bivalve model due to its relatively short generation time and ease of breeding in laboratory settings.

RESULTS

In this study, over 4,600 testicular cells from various samples were employed to identify single-cell heterogeneity on a more comprehensive scale. The four germ cell populations (spermatogonia, primary spermatocytes, secondary spermatocytes, and round spermatids/spermatozoa) and three somatic populations (follicle cell, hemocyte, and nerve cell) were characterized. The four types of germ cells exhibited disparate cell cycle statuses and an uninterrupted developmental trajectory, progressing from spermatogonia to spermatids/spermatozoa. Pseudotime analysis indicates that gene expression, translation, ATP metabolic process, and microtubule-based process are involved in the transition of germ cell types. Weighted gene coexpression network analysis (WGCNA) identified four modules corresponding to the four types of germ cells, as well as key transcription factors (e.g., MYC, SREBF1, SOXH) that may play a critical role in these cell types. Furthermore, our findings revealed that there is extensive bidirectional communication between the somatic cells and the germline cells, including the FGF and TGF-β signaling pathways, as well as other ligand-receptor pairs, such as NTN1-NEO1 and PLG-PLGRKT.

CONCLUSIONS

This study provides a comprehensive single-cell transcriptome landscape of the gonad, which will contribute to the understanding of germ cell fate transition during spermatogenesis, and the development of germ cell manipulation technologies in mollusks.

Transcriptomic dynamics and cell-to-cell communication during the transition of prospermatogonia to spermatogonia revealed at single-cell resolution

BACKGROUND

Spermatogonia are essential for the continual production of sperm and regeneration of the entire spermatogenic lineage after injury. In mammals, spermatogonia are formed in the neonatal testis from prospermatogonia (also termed gonocytes), which are established from primordial germ cells during fetal development. Currently, the molecular regulation of the prospermatogonial to spermatogonia transition is not fully understood.

RESULTS

In this study, we examined the gene expression patterns of prospermatogonia, spermatogonia and testicular somatic cells at 4 different stages, including embryonic day (E) 12.5, E17.5 and postnatal days (P) 1 and 6, using single-cell RNA sequencing (scRNA-seq). We identified 5 different molecular states in the prospermogonial population and revealed gene expression dynamics in corresponding testicular somatic cells. Specifically, we found that prospermatogonia mainly receive signals, while Leydig cells and peritubular myoid cells are the mediators for transmitting signals, indicating their potential roles in regulating the development and differentiation of prospermatogonia. Transcription regulon analyses revealed the involvement of basic helix-loop-helix (bHLH) transcription factors in directing prospermogonial fate decisions. We then disrupted this transcription network by ectopic expression of inhibitor of differentiation 2 (Id2), which is a negative regulator of bHLH transcription factors. The overexpression of Id2 in prospermatogonia caused severe defects in the progression of prospermatogonia to spermatogonia.

CONCLUSION

Together, these findings provide a crucial dataset for dissecting key genes that direct the establishment of the foundational spermatogonial pool and the fate transitions of different somatic cell lineages in the testis during fetal and neonatal periods of development.

Iron regulatory protein 1-deficient mice exhibit hypospermatogenesis

Imbalances in testicular iron levels are linked to compromised sperm production and male infertility. Iron regulatory proteins (IRP) 1 and 2 play crucial roles in cellular iron regulation. We investigated the role of IRP1 on spermatogenesis using Irp1-deficient mice (Irp1-/-). Histological analysis of the testis of Irp1-/- mice revealed hypospermatogenesis with a significant reduction in the number of elongated spermatids and daily sperm production compared to wild-type (WT) mice. Flow cytometry of germ cells from WT and Irp1-/- mice showed reduction in spermatocytes and round and elongated spermatids in Irp1-/- mice, which was confirmed by histological and immunofluorescence quantification. Finally, stage VIII of spermatogenesis, crucial for spermatid maturation, was less frequent in Irp1-/- testicular cross-sections. Hypospermatogenesis worsened with age despite unchanged intratesticular iron levels. Mechanistically, this was due to increased oxidative stress indicated by elevated 8-Oxoguanine (8-OxoG) levels, a DNA lesion resulting from reactive oxygen species (ROS). Furthermore, bulk RNA-seq data indicated compromised DNA damage repair and cell cycle processes, including mitosis and meiosis in Irp1-/- mice, which may explain hypospermatogenesis. Our results suggest that IRP1 deletion leads to hypospermatogenesis due to impaired cell cycle progression, decreased DNA damage repair capacity, and oxidative damage. Altogether, this study uncovers a role for IRP1, independent of traditional mechanisms of iron regulation.

Single-cell transcriptomes reveal spermatogonial stem cells and the dynamic heterogeneity of spermatogenesis in a seasonal breeding teleost

Seasonal spermatogenesis in fish is driven by spermatogonial stem cells (SSCs), which undergo a complex cellular process to differentiate into mature sperm. In this study, we characterized spermatogenesis in the large yellow croaker (Larimichthys crocea), a marine fish of significant commercial value, based on a high-resolution single-cell RNA-sequencing atlas of testicular cells from three distinct developmental stages: juvenile, adult differentiating and regressed testes. We detailed a continuous developmental trajectory of spermatogenic cells, from spermatogonia to spermatids, elucidating the molecular events involved in spermatogenesis. We uncovered dynamic heterogeneity in cellular compositions throughout the annual reproductive cycle, accompanied by strong molecular signatures within specific testicular cells. Notably, we identified a distinct population of SSCs and observed a critical metabolic transition from glycolysis to oxidative phosphorylation, enhancing our understanding of the biochemical and molecular characteristics of SSCs. Additionally, we elucidated the interactions between somatic cells and spermatogonia, illuminating the mechanisms that regulate SSC development. Overall, this work enhances our understanding of spermatogenesis in seasonal breeding teleosts and provides essential insights for the further conservation and culture of SSCs.

Mammalian Ste-20-like Kinase 1/2 (MST1/2) Inhibitor XMU-MP-1: A Potential Compound to Improve Spermatogenesis in Mouse Model of Diabetes Mellitus

Background/Objectives: Spermatogenesis is a key process in male reproduction that, if it does not happen correctly, can lead to infertility, with diabetes being one of the most prevalent causes of spermatogenesis disruption. Currently, there is a lack of research examining the potential benefits of targeting cell proliferation to enhance spermatogenesis in this condition. XMU-MP1 has been identified as an inhibitor of MST1, a core component of the Hippo pathway, which is anticipated to promote proliferation and regeneration. This study aims to evaluate the effects of XMU-MP1 treatment on sperm and testicular characteristics in mice. Methods: We used the STZ-induced diabetic mouse model to investigate the impact of administering XMU-MP1 on testicular tissue and sperm parameters. This study compared the seminiferous tubules, specifically focusing on the diameter of the seminiferous tubule, the thickness of the seminiferous tubule epithelium, the ratio of the thickness of the seminiferous tubule epithelium to the diameter of the seminiferous tubules, and the lumen diameter of the seminiferous tubules. We also conducted a comparison of sperm parameters, including sperm concentration, progressive motility, total motility, total motility, and morphology. Results: XMU-MP1-treated mice had a larger spermatogenesis area and better sperm motility than control mice. Diabetic mice treated with XMU-MP1 also showed a trend toward improvements in the spermatogenesis area, sperm concentration, sperm motility, and sperm morphology, although these improvements were not statistically significant. Conclusions: XMU-MP1 serves as a potential compound to improve spermatogenesis in mice.

Spermatogenic Stem Cells: Core Biology, Defining Features, and Utilities

The actions of spermatogenic stem cells (SSCs) provide the foundation for continual spermatogenesis and regeneration of the cognate lineage following cytotoxic insult or transplantation. Several decades of research with rodent models have yielded knowledge about the core biology, morphological features, and molecular profiles of mammalian SSCs. Translation of these discoveries to utilities for human fertility preservation, improving animal agriculture, and wildlife conservation are actively being pursued. Here, we provide overviews of these aspects covering both historical and current states of understanding.

Conditional ablation of DIS3L2 ribonuclease in pre-meiotic germ cells causes defective spermatogenesis and infertility in male mice

Rationale: Spermatogenesis is a highly organized cell differentiation process in mammals, involving mitosis, meiosis, and spermiogenesis. DIS3L2, which is primarily expressed in the cytoplasm, is an RNA exosome-independent ribonuclease. In female mice, Dis3l2-deficient oocytes fail to resume meiosis, resulting in arrest at the germinal vesicle stage and complete infertility. However, the role of DIS3L2 in germ cell development in males has remained largely unexplored. Methods: We established a pre-meiotic germ cell conditional knockout mouse model and investigated the biological function of DIS3L2 in spermatogenesis and male fertility through bulk RNA-seq and scRNA-seq analyses. Results: This study unveils that conditional ablation of Dis3l2 in pre-meiotic germ cells with Stra8-Cre mice impairs spermatogonial differentiation and hinders spermatocyte meiotic progression coupled with cell apoptosis. Such conditional ablation leads to defective spermatogenesis and sterility in adults. Bulk RNA-seq analysis revealed that Dis3l2 deficiency significantly disrupted the transcriptional expression pattern of genes related to the cell cycle, spermatogonial differentiation, and meiosis in Dis3l2 conditional knockout testes. Additionally, scRNA-seq analysis indicated that absence of DIS3L2 in pre-meiotic germ cells causes disrupted RNA metabolism, downregulated expression of cell cycle genes, and aberrant expression of spermatogonial differentiation genes, impeding spermatogonial differentiation. In meiotic spermatocytes, loss of DIS3L2 results in disturbed RNA metabolism, abnormal translation, and disrupted meiotic genes that perturb meiotic progression and induce cell apoptosis, leading to subsequent failure of spermatogenesis and male infertility. Conclusions: Collectively, these findings highlight the critical role of DIS3L2 ribonuclease-mediated RNA degradation in safeguarding the correct transcriptome during spermatogonial differentiation and spermatocyte meiotic progression, thus ensuring normal spermatogenesis and male fertility.

Effects of Leydig cell elimination on testicular interstitial cell populations: characterization by scRNA-seq and immunocytochemical techniques

Background

The mammalian testicular interstitial cells are not well-defined. The present study characterized the interstitial cell types and their turnover dynamics in adult rats. Additionally, the heterogeneity of the mesenchymal population and the effects of Leydig cell elimination on interstitial homeostasis were further analyzed by scRNA-seq datasets and immunocytochemical techniques.

Methods

Interstitial cells were defined at the transcriptomic level by scRNA-seq and then confirmed and quantified with protein markers. The dividing activity of the major cell types was determined by continuous EdU labeling of the animals for one week. Some of the rats were also treated with a dose of ethylenedimethylsulfonate (EDS) to examine how the loss of Leydig cells (LCs) could affect interstitial homeostasis for three weeks.

Results

Seven interstitial cell types were identified, including cell types (percentage of the whole interstitial population) as follows: Leydig (44.6%), macrophage and dendritic (19.1%), lymphoid (6.2%), vascular endothelial (7.9%), smooth muscle (10.7%), and mesenchymal (11.5%) cells. The EdU experiment indicated that most cell types were dividing at relatively low levels (<9%) except for the mesenchymal cells (MCs, 17.1%). Further analysis of the transcriptome of MCs revealed 4 subgroups with distinct functions, including 1) glutathione metabolism and xenobiotic detoxification, 2) ROS response and AP-1 signaling, 3) extracellular matrix synthesis and binding, and 4) immune response and regulation. Stem LCs (SLCs) are primarily associated with subgroup 3, expressing ARG1 and GAP43. EDS treatment not only eliminated LCs but also increased subgroup 3 and decreased subgroups 1 and 2 of the mesenchymal population. Moreover, EDS treatment increased the division of immune cells by more than tenfold in one week.

Conclusion

Seven interstitial cell types were identified and quantified for rat testis. Many may play more diversified roles than previously realized. The elimination of LCs led to significant changes in MCs and immune cells, indicating the importance of LCs in maintaining testicular interstitial homeostasis.

Elucidating the Transcriptional States of Spermatogenesis-Joint Analysis of Germline and Supporting Cell, Mice and Human, Normal and Perturbed, Bulk and Single-Cell RNA-Seq

In studying the molecular underpinning of spermatogenesis, we expect to understand the fundamental biological processes better and potentially identify genes that may lead to novel diagnostic and therapeutic strategies toward precision medicine in male infertility. In this review, we emphasized our perspective that the path forward necessitates integrative studies that rely on complementary approaches and types of data. To comprehensively analyze spermatogenesis, this review proposes four axes of integration. First, spanning the analysis of spermatogenesis in the healthy state alongside pathologies. Second, the experimental analysis of model systems (in which we can deploy treatments and perturbations) alongside human data. Third, the phenotype is measured alongside its underlying molecular profiles using known markers augmented with unbiased profiles. Finally, the testicular cells are studied as ecosystems, analyzing the germ cells alongside the states observed in the supporting somatic cells. Recently, the study of spermatogenesis has been advancing using single-cell RNA sequencing, where scientists have uncovered the unique stages of germ cell development in mice, revealing new regulators of spermatogenesis and previously unknown cell subtypes in the testis. An in-depth analysis of meiotic and postmeiotic stages led to the discovery of marker genes for spermatogonia, Sertoli and Leydig cells and further elucidated all the other germline and somatic cells in the testis microenvironment in normal and pathogenic conditions. The outcome of an integrative analysis of spermatogenesis using advanced molecular profiling technologies such as scRNA-seq has already propelled our biological understanding, with additional studies expected to have clinical implications for the study of male fertility. By uncovering new genes and pathways involved in abnormal spermatogenesis, we may gain insights into subfertility or sterility.

Understanding testicular single cell transcriptional atlas: from developmental complications to male infertility

Spermatogenesis is a multi-step biological process where mitotically active diploid (2n) spermatogonia differentiate into haploid (n) spermatozoa via regulated meiotic programming. The alarming rise in male infertility has become a global concern during the past decade thereby demanding an extensive profiling of testicular gene expression. Advancements in Next-Generation Sequencing (NGS) technologies have revolutionized our empathy towards complex biological events including spermatogenesis. However, despite multiple attempts made in the past to reveal the testicular transcriptional signature(s) either with bulk tissues or at the single-cell, level, comprehensive reviews on testicular transcriptomics and associated disorders are limited. Notably, technologies explicating the genome-wide gene expression patterns during various stages of spermatogenic progression provide the dynamic molecular landscape of testicular transcription. Our review discusses the advantages of single-cell RNA-sequencing (Sc-RNA-seq) over bulk RNA-seq concerning testicular tissues. Additionally, we highlight the cellular heterogeneity, spatial transcriptomics, dynamic gene expression and cell-to-cell interactions with distinct cell populations within the testes including germ cells (Gc), Sertoli cells (Sc), Peritubular cells (PTc), Leydig cells (Lc), etc. Furthermore, we provide a summary of key finding of single-cell transcriptomic studies that have shed light on developmental mechanisms implicated in testicular disorders and male infertility. These insights emphasize the pivotal roles of Sc-RNA-seq in advancing our knowledge regarding testicular transcriptional landscape and may serve as a potential resource to formulate future clinical interventions for male reproductive health.

DIS3 ribonuclease is essential for spermatogenesis and male fertility in mice

Spermatogonial stem cell (SSC) self-renewal and differentiation provide foundational support for long-term, steady-state spermatogenesis in mammals. Here, we have investigated the essential role of RNA exosome associated DIS3 ribonuclease in maintaining spermatogonial homeostasis and facilitating germ cell differentiation. We have established male germ-cell Dis3 conditional knockout (cKO) mice in which the first and subsequent waves of spermatogenesis are disrupted. This leads to a Sertoli cell-only phenotype and sterility in adult male mice. Bulk RNA-seq documents that Dis3 deficiency partially abolishes RNA degradation and causes significant increases in the abundance of transcripts. This also includes pervasively transcribed PROMoter uPstream Transcripts (PROMPTs), which accumulate robustly in Dis3 cKO testes. In addition, scRNA-seq analysis indicates that Dis3 deficiency in spermatogonia significantly disrupts RNA metabolism and gene expression, and impairs early germline cell development. Overall, we document that exosome-associated DIS3 ribonuclease plays crucial roles in maintaining early male germ cell lineage in mice.

A comparative analysis of genes differentially expressed between rete testis cells and Sertoli cells of the mouse testis

The rete testis (RT) is a region of the mammalian testis that plays an important role in testicular physiology. The RT epithelium consists of cells sharing some well-known gene markers with supporting Sertoli cells (SCs). However, little is known about the differences in gene expression between these two cell populations. Here, we used fluorescence-activated cell sorting (FACS) to obtain pure cultures of neonatal RT cells and SCs and identified differentially expressed genes (DEGs) between these cell types using RNA sequencing (RNA-seq). We then compared our data with the RNA-seq data of other studies that examined RT cells and SCs of mice of different ages and generated a list of DEGs permanently upregulated in RT cells throughout testis development and in culture, which included 86 genes, and a list of 79 DEGs permanently upregulated in SCs. The analysis of studies on DMRT1 function revealed that nearly half of the permanent DEGs could be regulated by this SC upregulated transcription factor. We suggest that useful cell lineage markers and candidate genes for the specification of both RT cells and SCs may be present among these permanent DEGs.

Core binding factor subunit β plays diverse and essential roles in the male germline

Much of the foundation for lifelong spermatogenesis is established prior to puberty, and disruptions during this developmental window negatively impact fertility long into adulthood. However, the factors that coordinate prepubertal germline development are incompletely understood. Here, we report that core-binding factor subunit-β (CBFβ) plays critical roles in prepubertal development and the onset of spermatogenesis. Using a mouse conditional knockout (cKO) approach, inactivation of Cbfb in the male germline resulted in rapid degeneration of the germline during the onset of spermatogenesis, impaired overall sperm production, and adult infertility. Utilizing a different Cre driver to generate another Cbfb cKO model, we determined that the function of CBFβ in the male germline is likely limited to undifferentiated spermatogonia despite expression in other germ cell types. Within undifferentiated spermatogonia, CBFβ regulates proliferation, survival, and overall maintenance of the undifferentiated spermatogonia population. Paradoxically, we discovered that CBFβ also distally regulates meiotic progression and spermatid formation but only with Cbfb cKO within undifferentiated spermatogonia. Spatial transcriptomics revealed that CBFβ modulates cell cycle checkpoint control genes associated with both proliferation and meiosis. Taken together, our findings demonstrate that core programs established within the prepubertal undifferentiated spermatogonia population are necessary for both germline maintenance and sperm production.

Transcription and chromatin regulation by TAF4b during cellular quiescence of developing prospermatogonia

Prospermatogonia (ProSpg) link the embryonic development of male primordial germ cells to the healthy establishment of postnatal spermatogonia and spermatogonial stem cells. While these spermatogenic precursor cells undergo the characteristic transitions of cycling and quiescence, the transcriptional events underlying these developmental hallmarks remain unknown. Here, we investigated the expression and function of TBP-associated factor 4b (Taf4b) in the timely development of quiescent mouse ProSpg using an integration of gene expression profiling and chromatin mapping. We find that Taf4b mRNA expression is elevated during the transition of mitotic-to-quiescent ProSpg and Taf4b-deficient ProSpg are delayed in their entry into quiescence. Gene ontology, protein network analysis, and chromatin mapping demonstrate that TAF4b is a direct and indirect regulator of chromatin and cell cycle-related gene expression programs during ProSpg quiescence. Further validation of these cell cycle mRNA changes due to the loss of TAF4b was accomplished via immunostaining for proliferating cell nuclear antigen (PCNA). Together, these data indicate that TAF4b is a key transcriptional regulator of the chromatin and quiescent state of the developing mammalian spermatogenic precursor lineage.

Testis exposure to unopposed/elevated activin A in utero affects somatic and germ cells and alters steroid levels mimicking phthalate exposure

Correct fetal testis development underpins adult male fertility, and TGFβ superfamily ligands control key aspects of this process. Transcripts encoding one such ligand, activin A, are upregulated in testes after sex determination and remain high until after birth. Testis development requires activin signalling; mice lacking activin A (Inhba KO) display altered somatic and germ cell proliferation, disrupted cord elongation and altered steroid synthesis. In human pregnancies with pre-eclampsia, the foetus is inappropriately exposed to elevated activin A. To learn how this affects testis development, we examined mice lacking the potent activin inhibitor, inhibin, (Inha KO) at E13.5, E15.5 and PND0. At E13.5, testes appeared similar in WT and KO littermates, however E15.5 Inha KO testes displayed two germline phenotypes: (1) multinucleated germ cells within cords, and (2) germ cells outside of cords, both of which are documented following in utero exposure to endocrine disrupting phthalates in rodents. Quantitation of Sertoli and germ cells in Inha KO (modelling elevated activin A) and Inhba KO (low activin A) testes using immunofluorescence demonstrated activin A bioactivity determines the Sertoli/germ cell ratio. The 50% reduction in gonocytes in Inha KO testes at birth indicates unopposed activin A has a profound impact on embryonic germ cells. Whole testis RNAseq on Inha KO mice revealed most transcripts affected at E13.5 were present in Leydig cells and associated with steroid biosynthesis/metabolism. In agreement, androstenedione (A4), testosterone (T), and the A4:T ratio were reduced in Inha KO testes at E17.5, confirming unopposed activin A disrupts testicular steroid production. E15.5 testes cultured with either activin A and/or mono-2-ethylhexyl phthalate (MEHP) generated common histological and transcriptional outcomes affecting germline and Leydig cells, recapitulating the phenotype observed in Inha KO testes. Cultures with activin A and MEHP together provided evidence of common targets. Lastly, this study extends previous work focussed on the Inhba KO model to produce a signature of activin A bioactivity in the fetal testis. These outcomes show the potential for elevated activin A signalling to replicate some aspects of fetal phthalate exposure prior to the masculinization programming window, influencing fetal testis growth and increasing the risk of testicular dysgenesis.

The developmental dynamics of the human male germline

Male germ cells undergo a complex sequence of developmental events throughout fetal and postnatal life that culminate in the formation of haploid gametes: the spermatozoa. Errors in these processes result in infertility and congenital abnormalities in offspring. Male germ cell development starts when pluripotent cells undergo specification to sexually uncommitted primordial germ cells, which act as precursors of both oocytes and spermatozoa. Male-specific development subsequently occurs in the fetal testes, resulting in the formation of spermatogonial stem cells: the foundational stem cells responsible for lifelong generation of spermatozoa. Although deciphering such developmental processes is challenging in humans, recent studies using various models and single-cell sequencing approaches have shed new insight into human male germ cell development. Here, we provide an overview of cellular, signaling and epigenetic cascades of events accompanying male gametogenesis, highlighting conserved features and the differences between humans and other model organisms.

Decoding the spermatogonial stem cell niche under physiological and recovery conditions in adult mice and humans

The intricate interaction between spermatogonial stem cell (SSC) and testicular niche is essential for maintaining SSC homeostasis; however, this interaction remains largely uncharacterized. In this study, to characterize the underlying signaling pathways and related paracrine factors, we delineated the intercellular interactions between SSC and niche cell in both adult mice and humans under physiological conditions and dissected the niche-derived regulation of SSC maintenance under recovery conditions, thus uncovering the essential role of C-C motif chemokine ligand 24 and insulin-like growth factor binding protein 7 in SSC maintenance. We also established the clinical relevance of specific paracrine factors in human fertility. Collectively, our work on decoding the adult SSC niche serves as a valuable reference for future studies on the aetiology, diagnosis, and treatment of male infertility.

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.

WWC2 expression in the testis: Implications for spermatogenesis and male fertility

The family of WWC proteins is known to regulate cell proliferation and organ growth control via the Hippo signaling pathway. As WWC proteins share a similar domain structure and a common set of interacting proteins, they are supposed to fulfill compensatory functions in cells and tissues. While all three WWC family members WWC1, WWC2, and WWC3 are found co-expressed in most human organs including lung, brain, kidney, and liver, in the testis only WWC2 displays a relatively high expression. In this study, we investigated the testicular WWC2 expression in spermatogenesis and male fertility. We show that the Wwc2 mRNA expression level in mouse testes is increased during development in parallel with germ cell proliferation and differentiation. The cellular expression of each individual WWC family member was evaluated in published single-cell mRNA datasets of murine and human testes demonstrating a high WWC2 expression predominantly in early spermatocytes. In line with this, immunohistochemistry revealed cytosolic WWC2 protein expression in primary spermatocytes from human testes displaying full spermatogenesis. In accordance with these findings, markedly lower WWC2 expression levels were detected in testicular tissues from mice and men lacking germ cells. Finally, analysis of whole-exome sequencing data of male patients affected by infertility and unexplained severe spermatogenic failure revealed several heterozygous, rare WWC2 gene variants with a proposed damaging function and putative impact on WWC2 protein structure. Taken together, our findings provide novel insights into the testicular expression of WWC2 and show its cell-specific expression in spermatocytes. As rare WWC2 variants were identified in the background of disturbed spermatogenesis, WWC2 may be a novel candidate gene for male infertility.

Single-Cell RNA Sequencing Reveals Atlas of Yak Testis Cells

Spermatogenesis is a complex process that involves proliferation and differentiation of diploid male germ cells into haploid flagellated sperm and requires intricate interactions between testicular somatic cells and germ cells. The cellular heterogeneity of this process presents a challenge in analyzing the different cell types at various developmental stages. Single-cell RNA sequencing (scRNA-seq) provides a useful tool for exploring cellular heterogeneity. In this study, we performed a comprehensive and unbiased single-cell transcriptomic study of spermatogenesis in sexually mature 4-year-old yak using 10× Genomics scRNA-seq. Our scRNA-seq analysis identified six somatic cell types and various germ cells, including spermatogonial stem cells, spermatogonia, early-spermatocytes, late-spermatocytes, and spermatids in yak testis. Pseudo-timing analysis showed that Leydig and myoid cells originated from common progenitor cells in yaks. Moreover, functional enrichment analysis demonstrated that the top expressed genes in yak testicular somatic cells were significantly enriched in the cAMP signaling pathway, PI3K-Akt signaling pathway, MAPK signaling pathway, and ECM receptor interactions. Throughout the spermatogenesis process, genes related to spermatogenesis, cell differentiation, DNA binding, and ATP binding were expressed. Using immunohistochemical techniques, we identified candidate marker genes for spermatogonial stem cells and Sertoli cells. Our research provides new insights into yak spermatogenesis and the development of various types of cells in the testis, and presents more reliable marker proteins for in vitro culture and identification of yak spermatogonial stem cells in the later stage.

The single-cell chromatin accessibility landscape in mouse perinatal testis development

Spermatogenesis depends on an orchestrated series of developing events in germ cells and full maturation of the somatic microenvironment. To date, the majority of efforts to study cellular heterogeneity in testis has been focused on single-cell gene expression rather than the chromatin landscape shaping gene expression. To advance our understanding of the regulatory programs underlying testicular cell types, we analyzed single-cell chromatin accessibility profiles in more than 25,000 cells from mouse developing testis. We showed that single-cell sequencing assay for transposase-accessible chromatin (scATAC-Seq) allowed us to deconvolve distinct cell populations and identify cis-regulatory elements (CREs) underlying cell-type specification. We identified sets of transcription factors associated with cell type-specific accessibility, revealing novel regulators of cell fate specification and maintenance. Pseudotime reconstruction revealed detailed regulatory dynamics coordinating the sequential developmental progressions of germ cells and somatic cells. This high-resolution dataset also unveiled previously unreported subpopulations within both the Sertoli and Leydig cell groups. Further, we defined candidate target cell types and genes of several genome-wide association study (GWAS) signals, including those associated with testosterone levels and coronary artery disease. Collectively, our data provide a blueprint of the 'regulon' of the mouse male germline and supporting somatic cells.

Single-cell transcriptome analysis and in vitro differentiation of testicular cells reveal novel insights into male sterility of the interspecific hybrid cattle-yak

BACKGROUND

Interspecific hybridization plays vital roles in enriching animal diversity, while male hybrid sterility (MHS) of the offspring commonly suffered from spermatogenic arrest constitutes the postzygotic reproductive isolation. Cattle-yak, the hybrid offspring of cattle (Bos taurus) and yak (Bos grunniens) can serve as an ideal MHS animal model. Although meiotic arrest was found to contribute to MHS of cattle-yak, yet the cellular characteristics and developmental potentials of male germline cell in pubertal cattle-yak remain to be systematically investigated.

RESULTS

Single-cell RNA-seq analysis of germline and niche cell types in pubertal testis of cattle-yak and yak indicated that dynamic gene expression of developmental germ cells was terminated at late primary spermatocyte (meiotic arrest) and abnormal components of niche cell in pubertal cattle-yak. Further in vitro proliferation and differentially expressed gene (DEG) analysis of specific type of cells revealed that undifferentiated spermatogonia of cattle-yak exhibited defects in viability and proliferation/differentiation potentials.

CONCLUSION

Comparative scRNA-seq and in vitro proliferation analysis of testicular cells indicated that not only meiotic arrest contributed to MHS of cattle-yak. Spermatogenic arrest of cattle-yak may originate from the differentiation stage of undifferentiated spermatogonia and niche cells of cattle-yak may provide an adverse microenvironment for spermatogenesis.

Prenatal and postnatal exposure to polystyrene microplastics induces testis developmental disorder and affects male fertility in mice

Polystyrene microplastics (PS-MPs) can threaten human health, especially male fertility. However, most existing studies have focused on the adulthood stage of male reproduction toxicity caused by relatively short-term PS-MP exposure. This study aimed to investigate the toxic effect of PS-MPs on testicular development and reproductive function upon prenatal and postnatal exposure. Pregnant mice and their offspring were exposed to 0, 0.5 mg/L, 5 mg/L, and 50 mg/L PS-MPs through their daily drinking water from gestational day 1 to postnatal day (PND) 35 or PND70. We found that PS-MP exposure induced testis development disorder by PND35 and spermatogenesis dysfunction by PND70. By combining RNA sequencing results and bioinformatics analysis, the hormone-mediated signaling pathway, G1/S transition of the mitotic cell cycle, coregulation of androgen receptor activity, and Hippo signaling pathway were shown to be involved in testis development on PND35. The meiotic cell cycle, regulation of the immune effector process, neutrophil degranulation, and inflammation mediated by chemokine and cytokine signaling pathways were associated with disturbed spermatogenesis on PND70. These findings show that prenatal and postnatal exposure to PS-MPs resulted in testis development disorder and male subfertility, which may be regulated by the Hippo signaling pathway and involve an immune reaction.

Multiomics approach to profiling Sertoli cell maturation during development of the spermatogonial stem cell niche

Spermatogonial stem cells (SSCs) are the basis of spermatogenesis, a complex process supported by a specialized microenvironment, called the SSC niche. Postnatal development of SSCs is characterized by distinct metabolic transitions from prepubertal to adult stages. An understanding of the niche factors that regulate these maturational events is critical for the clinical application of SSCs in fertility preservation. To investigate the niche maturation events that take place during SSC maturation, we combined different '-omics' technologies. Serial single cell RNA sequencing analysis revealed changes in the transcriptomes indicative of niche maturation that was initiated at 11 years of age in humans and at 8 weeks of age in pigs, as evident by Monocle analysis of Sertoli cells and peritubular myoid cell (PMC) development in humans and Sertoli cell analysis in pigs. Morphological niche maturation was associated with lipid droplet accumulation, a characteristic that was conserved between species. Lipidomic profiling revealed an increase in triglycerides and a decrease in sphingolipids with Sertoli cell maturation in the pig model. Quantitative (phospho-) proteomics analysis detected the activation of distinct pathways with porcine Sertoli cell maturation. We show here that the main aspects of niche maturation coincide with the morphological maturation of SSCs, which is followed by their metabolic maturation. The main aspects are also conserved between the species and can be predicted by changes in the niche lipidome. Overall, this knowledge is pivotal to establishing cell/tissue-based biomarkers that could gauge stem cell maturation to facilitate laboratory techniques that allow for SSC transplantation for restoration of fertility.

Zinc restores functionality in porcine prepubertal Sertoli cells exposed to subtoxic cadmium concentration via regulating the Nrf2 signaling pathway

INTRODUCTION

Among substances released into the environment by anthropogenic activities, the heavy metal cadmium (Cd) is known to induce severe testicular injury causing male subfertility/infertility. Zinc (Zn) is another heavy metal that, unlike Cd, is physiologically present in the testis, being essential for spermatogenesis. We aimed to examine the possibility that 50 µM ZnCl₂ could counteract the toxic effects induced by Cd in an in vitro model of porcine prepubertal Sertoli cells (SCs) exposed to both subtoxic (5 μM) and toxic (10 μM) concentrations of CdCl₂ for 48 h.

MATERIALS AND METHODS

Apoptosis, cell cycle, and cell functionality were assessed. The gene expression of Nrf2 and its downstream antioxidant enzymes, ERK1/2, and AKT kinase signaling pathways were evaluated.

MATERIALS AND RESULTS

We found that Zn, in co-treatment with subtoxic and toxic Cd concentration, increased the number of metabolically active SCs compared to Cd exposure alone but restored SC functionality only in co-treatment with subtoxic Cd concentration with respect to subtoxic Cd alone. Exposure of Cd disrupted cell cycle in SCs, and Zn co-treatment was not able to counteract this effect. Cd alone induced SC death through apoptosis and necrosis in a dose-dependent manner, and co-treatment with Zn increased the pro-apoptotic effect of Cd. Subtoxic and toxic Cd exposures activated the Nrf2 signaling pathway by increasing gene expression of Nrf2 and its downstream genes (SOD, HO-1, and GSHPx). Zn co-treatment with subtoxic Cd attenuated upregulation on the Nrf2 system, while with toxic Cd, the effect was more erratic. Studying ERK1/2 and AKT pathways as a target, we found that the phosphorylation ratio of p-ERK1/2 and p-AKT was upregulated by both subtoxic and toxic Cd exposure alone and in co-treatment with Zn.

DISCUSSION

Our results suggest that Zn could counteract Cd effects by increasing the number of metabolically active SCs, fully or partially restoring their functionality by modulating Nrf2, ERK1/2, and AKT pathways. Our SC model could be useful to study the effects of early Cd exposure on immature testis, evaluating the possible protective effects of Zn.

Overview of single-cell RNA sequencing analysis and its application to spermatogenesis research

Background

Single-cell transcriptomics allows parallel analysis of multiple cell types in tissues. Because testes comprise somatic cells and germ cells at various stages of spermatogenesis, single-cell RNA sequencing is a powerful tool for investigating the complex process of spermatogenesis. However, single-cell RNA sequencing analysis needs extensive knowledge of experimental technologies and bioinformatics, making it difficult for many, particularly experimental biologists and clinicians, to use it.

Methods

Aiming to make single-cell RNA sequencing analysis familiar, this review article presents an overview of experimental and computational methods for single-cell RNA sequencing analysis with a history of transcriptomics. In addition, combining the PubMed search and manual curation, this review also provides a summary of recent novel insights into human and mouse spermatogenesis obtained using single-cell RNA sequencing analyses.

Main Findings

Single-cell RNA sequencing identified mesenchymal cells and type II innate lymphoid cells as novel testicular cell types in the adult mouse testes, as well as detailed subtypes of germ cells. This review outlines recent discoveries into germ cell development and subtypes, somatic cell development, and cell-cell interactions.

Conclusion

The findings on spermatogenesis obtained using single-cell RNA sequencing may contribute to a deeper understanding of spermatogenesis and provide new directions for male fertility therapy.

Modeling development of genitourinary birth defects to understand disruption due to changes in gene dosage

Genitourinary development is a delicately orchestrated process that begins in the embryo. Once complete, the genitourinary system is a collection of functionally disparate organs spread throughout the abdominal and pelvic regions. These distinct organs are interconnected through an elaborate duct system which aggregates the organs' products to a common exit point. The complicated nature of the genitourinary system makes it highly susceptible to developmental disruptions that produce anomalies. In fact, genitourinary anomalies are among the most common class of human birth defects. Aside from congenital anomalies of the kidney and urinary tract (CAKUT), for males, these birth defects can also occur in the penis (hypospadias) and testis (cryptorchism), which impact male fertility and male mental health. As genetic technology has advanced, it has become clear that a subset of cases of genitourinary birth defects are due to gene variation causing dosage changes in critical regulatory genes. Here we first review the parallels between human and mouse genitourinary development. We then demonstrate how translational research leverages mouse models of human gene variation cases to advance mechanistic understanding of causation in genitourinary birth defects. We close with a view to the future highlighting upcoming technologies that will provide a deeper understanding of gene variation affecting regulation of genitourinary development, which should ultimately advance treatment options for patients.

Single-cell RNA sequencing uncovers dynamic roadmap and cell-cell communication during buffalo spermatogenesis

Spermatogenesis carries the task of precise intergenerational transmission of genetic information from the paternal genome and involves complex developmental processes regulated by the testicular microenvironment. Studies performed mainly in mouse models have established the theoretical basis for spermatogenesis, yet the wide interspecies differences preclude direct translation of the findings, and farm animal studies are progressing slowly. More than 32,000 cells from prepubertal (3-month-old) and pubertal (24-month-old) buffalo testes were analyzed by using single-cell RNA sequencing (scRNA-seq), and dynamic gene expression roadmaps of germ and somatic cell development were generated. In addition to identifying the dynamic processes of sequential cell fate transitions, the global cell-cell communication essential to maintain regular spermatogenesis in the buffalo testicular microenvironment was uncovered. The findings provide the theoretical basis for establishing buffalo germline stem cells in vitro or culturing organoids and facilitating the expansion of superior livestock breeding.

Decoding the Spermatogenesis Program: New Insights from Transcriptomic Analyses

Spermatogenesis is a complex differentiation process coordinated spatiotemporally across and along seminiferous tubules. Cellular heterogeneity has made it challenging to obtain stage-specific molecular profiles of germ and somatic cells using bulk transcriptomic analyses. This has limited our ability to understand regulation of spermatogenesis and to integrate knowledge from model organisms to humans. The recent advancement of single-cell RNA-sequencing (scRNA-seq) technologies provides insights into the cell type diversity and molecular signatures in the testis. Fine-grained cell atlases of the testis contain both known and novel cell types and define the functional states along the germ cell developmental trajectory in many species. These atlases provide a reference system for integrated interspecies comparisons to discover mechanistic parallels and to enable future studies. Despite recent advances, we currently lack high-resolution data to probe germ cell-somatic cell interactions in the tissue environment, but the use of highly multiplexed spatial analysis technologies has begun to resolve this problem. Taken together, recent single-cell studies provide an improvedunderstanding of gametogenesis to examine underlying causes of infertility and enable the development of new therapeutic interventions.

Mechanisms of Cadmium-Induced Testicular Injury: A Risk to Male Fertility

Cadmium is a heavy toxic metal with unknown biological functions in the human body. Over time, cadmium accretion in the different visceral organs (liver, lungs, kidney, and testis) is said to impair the function of these organs, which is associated with a relatively long biological half-life and a very low rate of excretion. Recently studies have revealed that the testes are highly sensitive to cadmium. In this review, we discussed the adverse effect of cadmium on the development and biological functions of the testis. The Sertoli cells (SCs), seminiferous tubules, and Blood Testis Barrier are severely structurally damaged by cadmium, which results in sperm loss. The development and function of Leydig cells are hindered by cadmium, which also induces Leydig cell tumors. The testis's vascular system is severely disturbed by cadmium. Cadmium also perturbs the function of somatic cells and germ cells through epigenetic regulation, giving rise to infertile or sub-fertile males. In addition, we also summarized the other findings related to cadmium-induced oxidative toxicity, apoptotic toxicity, and autophagic toxicity, along with their possible mechanisms in the testicular tissue of different animal species. Consequently, cadmium represents a high-risk factor for male fertility.

Tmsb10 triggers fetal Leydig differentiation by suppressing the RAS/ERK pathway

Leydig cells in fetal testes play crucial roles in masculinizing fetuses through androgen production. Gene knockout studies have revealed that growth factors are implicated in fetal Leydig cell (FLC) differentiation, but little is known about the mechanisms regulating this process. We investigate this issue by characterizing FLC progenitor cells using single-cell RNA sequencing. The sequence datasets suggest that thymosin β10 (Tmsb10) is transiently upregulated in the progenitors. While studying the function of Tmsb10, we reveal that platelet-derived growth factor (PDGF) regulates ciliogenesis through the RAS/ERK and PI3K/AKT pathways, and thereby promotes desert hedgehog (DHH)-dependent FLC differentiation. Tmsb10 expressed in the progenitor cells induces their differentiation into FLCs by suppressing the RAS/ERK pathway. Through characterizing the transiently expressed Tmsb10 in the FLC progenitors, this study unveils the molecular process of FLC differentiation and shows that it is cooperatively induced by DHH and PDGF.

Investigation of fetal Leydig progenitors shows that thymosin β10 (Tmsb10) suppresses the RAS/ERK pathway, inducing progenitor differentiation into fetal Leydig cells.

Regulation of both transcription and RNA turnover contribute to germline specification

The nuanced mechanisms driving primordial germ cells (PGC) specification remain incompletely understood since genome-wide transcriptional regulation in developing PGCs has previously only been defined indirectly. Here, using SLAMseq analysis, we determined genome-wide transcription rates during the differentiation of embryonic stem cells (ESCs) to form epiblast-like (EpiLC) cells and ultimately PGC-like cells (PGCLCs). This revealed thousands of genes undergoing bursts of transcriptional induction and rapid shut-off not detectable by RNAseq analysis. Our SLAMseq datasets also allowed us to infer RNA turnover rates, which revealed thousands of mRNAs stabilized and destabilized during PGCLC specification. mRNAs tend to be unstable in ESCs and then are progressively stabilized as they differentiate. For some classes of genes, mRNA turnover regulation collaborates with transcriptional regulation, but these processes oppose each other in a surprisingly high frequency of genes. To test whether regulated mRNA turnover has a physiological role in PGC development, we examined three genes that we found were regulated by RNA turnover: Sox2, Klf2 and Ccne1. Circumvention of their regulated RNA turnover severely impaired the ESC-to-EpiLC and EpiLC-to-PGCLC transitions. Our study demonstrates the functional importance of regulated RNA stability in germline development and provides a roadmap of transcriptional and post-transcriptional regulation during germline specification.

Functional Importance of Mini-Puberty in Spermatogenic Stem Cell Formation

Primordial germ cells nesting in the fetal testis give rise to gonocytes. The gonocytes then transform into spermatogenic stem cells (SSCs) during the neonatal period and thereafter serve as a lifetime source of spermatogenesis. Therefore, gonocyte to SSC transformation is quite an important process that supports fertility in males. During the gonocyte to SSC transformation, morphological and transcriptomic changes sequentially occur and gonocytes migrate from the center to the peripheral region of the seminiferous tubules. However, extrinsic signals which trigger the transcriptomic changes as well as the migration are not yet fully clarified. Recent studies have drawn attention to the temporal activation of the hypothalamic-pituitary-gonadal axis during the neonatal stage which occurs concurrently with SSC formation. This phenomenon is called mini-puberty, and recent studies on human cryptorchid patients as well as animal models partially support the hypothesis that mini-puberty plays pivotal roles in gonocyte-to-SSC transformation. Focusing on this point, here, we aimed to discuss the latest knowledge on the importance of mini-puberty in spermatogenesis in this review.

Single-cell RNA sequencing of adult rat testes after Leydig cell elimination and restoration

Spermatogenesis is an efficient, complex, and highly organized proliferation and differentiation process that relies on multiple factors including testosterone produced by the Leydig cells. Although the critical role played by testosterone in spermatogenesis is well recognized, the mechanism by which it works is still not completely understood, partially due to the inability to specifically and precisely monitor testosterone-dependent changes within developing germ cells. Here we present single-cell RNA sequencing data from10,983 adult rat testicular cells after the rats were treated with ethanedimethanesulfonate, which temporarily eliminates Leydig cells. The elimination and recovery of Leydig cells represented a complete testosterone depletion and restoration cycle. The dataset, which includes all developing germ cells from spermatogonia to spermatozoa, should prove useful for characterizing developing germ cells, their regulatory networks, and novel cell-specific markers. The dataset should be particularly useful for exploring the effects of the androgen environment on the regulation of spermatogenesis. As this is the first single-cell RNA-Seq dataset for rat testes, it can also serve as a reference for future studies.

Activin A and Sertoli Cells: Key to Fetal Testis Steroidogenesis

The long-standing knowledge that Sertoli cells determine fetal testosterone production levels is not widespread, despite being first reported over a decade ago in studies of mice. Hence any ongoing use of testosterone as a marker of Leydig cell function in fetal testes is inappropriate. By interrogating new scRNAseq data from human fetal testes, we demonstrate this situation is also likely to be true in humans. This has implications for understanding how disruptions to either or both Leydig and Sertoli cells during the in utero masculinization programming window may contribute to the increasing incidence of hypospadias, cryptorchidism, testicular germ cell tumours and adult infertility. We recently discovered that activin A levels directly govern androgen production in mouse Sertoli cells, because the enzymes that drive the conversion of the precursor androgen androstenedione to generate testosterone are produced exclusively in Sertoli cells in response to activin A. This minireview addresses the implications of this growing understanding of how in utero exposures affect fetal masculinization for future research on reproductive health, including during programming windows that may ultimately be relevant for organ development in males and females.

Sertoli Cell-Germ Cell Interactions Within the Niche: Paracrine and Juxtacrine Molecular Communications

Male germ cell development depends on multiple biological events that combine epigenetic reprogramming, cell cycle regulation, and cell migration in a spatio-temporal manner. Sertoli cells are a crucial component of the spermatogonial stem cell niche and provide essential growth factors and chemokines to developing germ cells. This review focuses mainly on the activation of master regulators of the niche in Sertoli cells and their targets, as well as on novel molecular mechanisms underlying the regulation of growth and differentiation factors such as GDNF and retinoic acid by NOTCH signaling and other pathways.

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

Cell-fate transition and determination analysis of mouse male germ cells throughout development

Mammalian male germ cell development is a stepwise cell-fate transition process; however, the full-term developmental profile of male germ cells remains undefined. Here, by interrogating the high-precision transcriptome atlas of 11,598 cells covering 28 critical time-points, we demonstrate that cell-fate transition from mitotic to post-mitotic primordial germ cells is accompanied by transcriptome-scale reconfiguration and a transitional cell state. Notch signaling pathway is essential for initiating mitotic arrest and the maintenance of male germ cells' identities. Ablation of HELQ induces developmental arrest and abnormal transcriptome reprogramming of male germ cells, indicating the importance of cell cycle regulation for proper cell-fate transition. Finally, systematic human-mouse comparison reveals potential regulators whose deficiency contributed to human male infertility via mitotic arrest regulation. Collectively, our study provides an accurate and comprehensive transcriptome atlas of the male germline cycle and allows for an in-depth understanding of the cell-fate transition and determination underlying male germ cell development.