Single-cell transcriptome analyses reveal critical regulators of spermatogonial stem cell fate transitions.

BACKGROUND: Spermatogonial stem cells (SSCs) are the foundation cells for continual spermatogenesis and germline regeneration in mammals. SSC activities reside in the undifferentiated spermatogonial population, and currently, the molecular identities of SSCs and their committed progenitors remain unclear. RESULTS: We performed single-cell transcriptome analysis on isolated undifferentiated spermatogonia from mice to decipher the molecular signatures of SSC fate transitions. Through comprehensive analysis, we delineated the developmental trajectory and identified candidate transcription factors (TFs) involved in the fate transitions of SSCs and their progenitors in distinct states. Specifically, we characterized the Asingle spermatogonial subtype marked by the expression of Eomes. Eomes+ cells contained enriched transplantable SSCs, and more than 90% of the cells remained in the quiescent state. Conditional deletion of Eomes in the germline did not impact steady-state spermatogenesis but enhanced SSC regeneration. Forced expression of Eomes in spermatogenic cells disrupted spermatogenesis mainly by affecting the cell cycle progression of undifferentiated spermatogonia. After injury, Eomes+ cells re-enter the cell cycle and divide to expand the SSC pool. Eomes+ cells consisted of 7 different subsets of cells at single-cell resolution, and genes enriched in glycolysis/gluconeogenesis and the PI3/Akt signaling pathway participated in the SSC regeneration process. CONCLUSIONS: In this study, we explored the molecular characteristics and critical regulators of subpopulations of undifferentiated spermatogonia. The findings of the present study described a quiescent SSC subpopulation, Eomes+ spermatogonia, and provided a dynamic transcriptional map of SSC fate determination.

Pancreatic lipase-related protein 2 is selectively expressed by peritubular myoid cells in the murine testis and sustains long-term spermatogenesis.

Spermatogenesis is a complicated process of germ cell differentiation that occurs within the seminiferous tubule in the testis. Peritubular myoid cells (PTMCs) produce major components of the basement membrane that separates and ensures the structural integrity of seminiferous tubules. These cells secrete niche factors to promote spermatogonial stem cell (SSC) maintenance and mediate androgen signals to direct spermatid development. However, the regulatory mechanisms underlying the identity and function of PTMCs have not been fully elucidated. In the present study, we showed that the expression of pancreatic lipase-related protein 2 (Pnliprp2) was restricted in PTMCs in the testis and that its genetic ablation caused age-dependent defects in spermatogenesis. The fertility of Pnliprp2 knockout animals (Pnliprp2-/-) was normal at a young age but declined sharply beginning at 9 months. Pnliprp2 deletion impaired the homeostasis of undifferentiated spermatogonia and severely disrupted the development and function of spermatids. Integrated analyses of single-cell RNA-seq and metabolomics data revealed that glyceride metabolism was changed in PTMCs from Pnliprp2-/- mice. Further analysis found that 60 metabolites were altered in the sperm of the Pnliprp2-/- animals; notably, lipid metabolism was significantly dysregulated. Collectively, these results revealed that Pnliprp2 was exclusively expressed in PTMCs in the testis and played a novel role in supporting continual spermatogenesis in mice. The outcomes of these findings highlight the function of lipid metabolism in reproduction and provide new insights into the regulation of PTMCs in mammals.

Single-cell transcriptomic characterization of sheep conceptus elongation and implantation.

Bidirectional communication between the developing conceptus and endometrium is essential for pregnancy recognition and establishment in ruminants. We dissect the transcriptomic dynamics of sheep conceptus and corresponding endometrium at pre- and peri-implantation stages using single-cell RNA sequencing. Spherical blastocysts contain five cell types, with 68.62% trophectoderm cells. Strikingly, elongated conceptuses differentiate into 17 cell types, indicating dramatic cell fate specifications. Cell-type-specific gene expression delineates the features of distinctive trophectoderm lineages and indicates that the transition from polar trophectoderm to trophoblast increases interferon-tau expression and likely drives elongation initiation. We identify 13 endometrium-derived cell types and elucidate their molecular responses to conceptus development. Integrated analyses uncover multiple paired transcripts mediating the dialogues between extraembryonic membrane and endometrium, including IGF2-IGF1R, FGF19-FGFR1, NPY-NPY1R, PROS1-AXL, and ADGRE5-CD55. These data provide insight into the molecular regulation of conceptus elongation and represent a valuable resource for functional investigations of pre- and peri-implantation ruminant development.

Localization and expression of SLX4 in the testis of sterile male cattle-yak.

Cattle-yak, the hybrid offspring of yak (Bos grunniens) and cattle (Bos taurus), serves as a unique model to dissect the molecular mechanisms underlying reproductive isolation. While female cattle-yaks are fertile, the males are completely sterile due to spermatogenic arrest at the meiosis stage and massive germ cell apoptosis. Interestingly, meiotic defects are partially rescued in the testes of backcrossed offspring. The genetic basis of meiotic defects in male cattle-yak remains unclear. Structure-specific endonuclease subunit (SLX4) participates in meiotic double-strand break (DSB) formation in mice, and its deletion results in defects in spermatogenesis. In the present study, we examined the expression patterns of SLX4 in the testes of yak, cattle-yak, and backcrossed offspring to investigate its potential roles in hybrid sterility. The results showed that the relative abundances of SLX4 mRNA and protein were significantly reduced in the testis of cattle-yak. The results of immunohistochemistry revealed that SLX4 was predominately expressed in spermatogonia and spermatocytes. Chromosome spreading experiments showed that SLX4 was significantly decreased in the pachytene spermatocytes of cattle-yak compared with yak and backcrossed offspring. These findings suggest that SLX4 expression was dysregulated in the testis of cattle-yak, potentially resulting in the failure of crossover formation and collapses of meiosis in hybrid males.

Transcriptome analysis reveals dysregulated gene expression networks in Sertoli cells of cattle-yak hybrids.

Cattle-yak, the hybrid offspring of yak and taurine cattle, exhibits male sterility with normal female fertility. Spermatogenesis is arrested in adult cattle-yak, and apoptosis is elevated in spermatogenic cells. Currently, the mechanisms underlying these defects remain elusive. Sertoli cells are the only somatic cells that directly interact with spermatogenic cells in the seminiferous tubules and play essential roles in spermatogenesis. The present study was designed to investigate gene expression signatures and potential roles of Sertoli cells in hybrid sterility in cattle-yak. Immunohistochemical analysis showed that the 5 mC and 5hmC signals in Sertoli cells of cattle-yaks were significantly different from those of age-matched yaks (P < 0.05). Transcriptome profiling of isolated Sertoli cells identified 402 differentially expressed genes (DEGs) between cattle-yaks and yaks. Notably, niche factor glial cell derived neurotrophic factor (GDNF) was upregulated, and genes involved in retinoic acid (RA) biogenesis were changed in Sertoli cells of cattle-yak, suggesting possible impairments of spermatogonial fate decisions. Further studies showed that the numbers of proliferative gonocytes and undifferentiated spermatogonia in cattle-yak were significantly higher than those in yak (P < 0.01). Exogenous GDNF significantly promoted the proliferation of UCHL1-positive spermatogonia in yaks. Therefore, we concluded that altered GDNF expression and RA signaling impacted the fate decisions of undifferentiated spermatogonia in cattle-yak. Together, these findings highlight the role of Sertoli cells and their derived factors in hybrid sterility.

Paternal hypoxia exposure impairs fertilization process and preimplantation embryo development.

Environmental hypoxia exposure causes fertility problems in human and animals. Compelling evidence suggests that chronic hypoxia impairs spermatogenesis and reduces sperm motility. However, it is unclear whether paternal hypoxic exposure affects fertilization and early embryo development. In the present study, we exposed male mice to high altitude (3200 m above sea level) for 7 or 60 days to evaluate the effects of hypoxia on sperm quality, zygotic DNA methylation and blastocyst formation. Compared with age-matched controls, hypoxia-treated males exhibited reduced fertility after mating with normoxic females as a result of defects in sperm motility and function. Results of in vitro fertilization (IVF) experiments revealed that 60 days' exposure significantly reduced cleavage and blastocyst rates by 30% and 70%, respectively. Immunohistochemical staining of pronuclear formation indicated that the pronuclear formation process was disturbed and expression of imprinted genes was reduced in early embryos after paternal hypoxia. Overall, the findings of this study suggested that exposing male mice to hypoxia impaired sperm function and affected key events during early embryo development in mammals.

Genetic diversity and population structure of Tibetan sheep breeds determined by whole genome resequencing.

Tibetan sheep is one of primitive Chinese sheep breeds, which achieved the divergence about 2500 years ago in Qinghai plateau region. According to different geographic conditions, especially altitudes, Tibetan sheep evolved into different breeds. In this study, we performed whole genome resequencing of 5 representative Tibetan sheep breeds. Comparative genomic analysis showed that they can be divided into different clades with a close genetic relationship. However, some genes with common selective regions were enriched for hypoxic adaptability in different breeds living at higher altitude, including GHR, BMP15, and CPLANE1. Furthermore, breed-specific selective regions about physical characteristics, especially wool growth, were found in genes such as BSND, USP24, NCAPG, and LCORL. This study could contribute to our understanding about trait formation and offer a reference for breeding of Tibetan sheep.

WTAP Function in Sertoli Cells Is Essential for Sustaining the Spermatogonial Stem Cell Niche.

Sertoli cells are the major component of the spermatogonial stem cell (SSC) niche; however, regulatory mechanisms in Sertoli cells that dictate SSC fate decisions remain largely unknown. Here we revealed features of the N6-methyladenosine (m6A) mRNA modification in Sertoli cells and demonstrated the functions of WTAP, the key subunit of the m6A methyltransferase complex in spermatogenesis. m6A-sequencing analysis identified 21,909 m6A sites from 15,365 putative m6A-enriched transcripts within 6,122 genes, including many Sertoli cell-specific genes. Conditional deletion of Wtap in Sertoli cells resulted in sterility and the progressive loss of the SSC population. RNA sequencing and ribosome nascent-chain complex-bound mRNA sequencing analyses suggested that alternative splicing events of transcripts encoding SSC niche factors were sharply altered and translation of these transcripts were severely dysregulated by Wtap deletion. Collectively, this study uncovers a novel regulatory mechanism of the SSC niche and provide insights into molecular interactions between stem cells and their cognate niches in mammals.

Transcriptome analysis revealed key signaling networks regulating ovarian activities in the domestic yak.

Domestic yaks are the most important livestock species on the Qinghai-Tibetan Plateau. Adult female yaks normally breed in warm season and enter anestrous in cold season. Currently, how the ovarian activity is regulated at the molecular level remains to be determined. This study was conducted to investigate follicular development and gene expression patterns of yak ovarian tissues in the warm and cold seasons. Dynamics of follicular development was evaluated based on histological analyses and global gene expression was examined by using RNA-sequencing (RNA-seq) technology. Firstly, we found that follicle development of yak cows in cold season was different from that in warm season. Interestingly, ovaries collected from yaks in cold season contained a significant higher number of antral follicles and some of these follicles showed signs of polycystic structure, indicating abnormal granulosa cell function. RNA-seq analyses of ovarian tissues from non-pregnant adult yaks in cold and warm season revealed that a list of 320 transcripts were differentially expressed, specifically, 79 were up-regulated and 241 were down-regulated in the ovaries from yaks during the cold season. Further analysis demonstrated that transcripts associated with estrogen secretion and metabolism signaling pathway were altered, including FST, CYP1A1, PIK3R1 and PIK3R2. This study showed histological features of follicle development and revealed candidate genes that may have important roles in regulating ovarian activities in the yak seasonal reproduction.

Testosterone-retinoic acid signaling directs spermatogonial differentiation and seasonal spermatogenesis in the Plateau pika (Ochotona curzoniae).

During evolution, animals optimize their reproductive strategies to increase offspring survival. Seasonal breeders reproduce only during certain times of the year. In mammals, reproduction is tightly controlled by hypothalamus-pituitary-gonad axis. Although pathways regulating gametogenesis in non-seasonal model species have been well established, molecular insights into seasonal reproduction are severely limited. Using the Plateau pika (Ochotona curzoniae), a small rodent animal species native to the Qinghai-Tibetan plateau, as a model, here we report that seasonal spermatogenesis is governed at the level of spermatogonial differentiation. In testis of the reproductively dormant animals, undifferentiated spermatogonia failed to differentiate and accumulated in the seminiferous tubules. RNA-seq analyses of the active and dormant testes revealed that genes modulating retinoic acid biogenesis and steriodogenesis were differentially regulated. A single injection of all-trans retinoic acid (ATRA) reinitiated spermatogenesis and inhibition the function of RA-degrading enzyme CYP26B1 for 10 days induced spermatogonial differentiation. Strikingly, testosterone injection reinitiated spermatogenesis in short day adapted animals. Testosterone provides a permissive environment of RA biogenesis and actions in testis, therefore, indirectly controls spermatogonial differentiation. Collectively, these findings provide a key mechanistic insight regarding the molecular regulation of seasonal reproduction in mammals.

The CXCL12-CXCR4 signaling promotes oocyte maturation by regulating cumulus expansion in sheep.

Gonadotropins and growth factors synergistically regulate folliculogenesis and oocyte development. C-X-C motif chemokine 12 (CXCL12) and its receptor CXCR4 are expressed in ovaries of sheep, cattle and other species, however, roles of this multifunctional signal axis in oocyte maturation are not defined. Using sheep as a model, we examined the expression patterns and functions of the CXCL12-CXCR4 axis during oocyte maturation. CXCL12 and CXCR4 mRNA and protein were present in oocytes and granulosa cells. Relative abundance of CXCR4 transcript was controlled by epidermal growth factor (EGF). Transient inhibition of CXCR4 suppressed oocyte nuclear maturation while supplementing recombination CXCL12 significantly increased percent of oocyte undergone metaphase I phase. Inhibition of CXCR4 function decreased cumulus expansion growth rate. Furthermore, granulosa cell migration was impaired and expression of hyaluronan synthase 2 (HAS2) and hyaluronan binding protein tumor necrosis factor-alpha-induced protein 6 (TNFAIP6) were downregulated by CXCR4 inhibition. These findings revealed a novel role of the CXCL12-CXCR4 signaling in oocyte development in sheep.

Effect of oocyte vitrification on deoxyribonucleic acid methylation of H19, Peg3, and Snrpn differentially methylated regions in mouse blastocysts.

OBJECTIVE: To examine whether mouse oocytes vitrification could alter the deoxyribonucleic acid (DNA) methylation of differentially methylated regions (DMRs) of three imprinted genes in in vitro fertilized blastocysts. DESIGN: In vitro experiments using murine model. SETTING: State key laboratory and university research laboratory. ANIMAL(S): Kunming white mice. INTERVENTION(S): The mouse metaphase II oocytes were vitrified. After thawing, the surviving oocytes were fertilized in vitro to produce blastocysts. The blastocysts derived in vitro from fresh oocytes were used as a control. The DNA methylation patterns of the DMRs of imprinted genes in oocytes and blastocysts and the relative expression of DNMTs (Dnmt1, Dnmt3a, Dnmt3b, and Dnmt3l) in oocytes and blastocysts were detected. MAIN OUTCOME MEASURE(S): Methylation patterns of DMRs of H19, Peg3, and Snrpn analyzed by bisulfite mutagenesis and sequencing. Expression levels of messenger ribonucleic acid as measured by real-time reverse-transcriptase polymerase chain reaction. RESULT(S): After oocytes vitrification, the methylation levels at H19, Peg3, and Snrpn DMRs in blastocysts were decreased. However, there was no significant difference in the percentage of hypermethylated strands at Peg3 DMRs between the vitrified and control groups. DNMTs expression in vitrified oocytes and the expression of Dnmt3b in blastocysts derived from vitrified oocytes were significantly reduced. CONCLUSION(S): Oocytes vitrification could lead to the loss of DNA methylation of imprinted genes (H19, Peg3, and Snrpn) in mouse blastocysts, which is mainly caused by the reductions of DNMTs after vitrification of oocytes.