A-single spermatogonia heterogeneity and cell cycles synchronize with rat seminiferous epithelium stages VIII-IX

Male infertility

Clinical infertility is the inability of a couple to conceive after 12 months of trying. Male factors are estimated to contribute to 30-50% of cases of infertility. Infertility or reduced fertility can result from testicular dysfunction, endocrinopathies, lifestyle factors (such as tobacco and obesity), congenital anatomical factors, gonadotoxic exposures and ageing, among others. The evaluation of male infertility includes detailed history taking, focused physical examination and selective laboratory testing, including semen analysis. Treatments include lifestyle optimization, empirical or targeted medical therapy as well as surgical therapies that lead to measurable improvement in fertility. Although male infertility is recognized as a disease with effects on quality of life for both members of the infertile couple, fewer data exist on specific quantification and impact compared with other health-related conditions.

Rattus norvegicus Spermatogenesis Colony-Forming Assays

Knowledge gaps persist on signaling pathways and metabolic states in germ cells sufficient to support spermatogenesis independent of a somatic environment. Consequently, methods to culture mammalian stem cells through spermatogenesis in defined systems have not been established. Lack of success at culturing mammalian stem cells through spermatogenesis in defined systems reflects an inability to experimentally recapitulate biochemical events that develop in germ cells within the testis-specific seminiferous epithelium. Complex germ and somatic cell associations that develop each seminiferous epithelial cycle support such a hypothesis, conceivably explaining why highly pure mammalian spermatogonia do not effectively develop into and through meiosis without somatic cells. Here, we outline an in vitro spermatogenesis colony-forming assay to study how differentiating spermatogonial syncytia develop from rat spermatogonial stem cell lines. Robust spermatogonial differentiation under defined culture conditions, once established, is anticipated to facilitate molecular biology studies on pre-meiotic steps in gametogenesis by providing soma-free bioassays to systematically identify spermatogenic factors that promote meiotic progression in vitro.

Spermatogonial Gene Networks Selectively Couple to Glutathione and Pentose Phosphate Metabolism but Not Cysteine Biosynthesis

In adult males, spermatogonia maintain lifelong spermatozoa production for oocyte fertilization. To understand spermatogonial metabolism we compared gene profiles in rat spermatogonia to publicly available mouse, monkey, and human spermatogonial gene profiles. Interestingly, rat spermatogonia expressed metabolic control factors Foxa1, Foxa2, and Foxa3. Germline Foxa2 was enriched in Gfra1Hi and Gfra1Low undifferentiated A-single spermatogonia. Foxa2-bound loci in spermatogonial chromatin were overrepresented by conserved stemness genes (Dusp6, Gfra1, Etv5, Rest, Nanos2, Foxp1) that intersect bioinformatically with conserved glutathione/pentose phosphate metabolism genes (Tkt, Gss, Gc l c , Gc l m, Gpx1, Gpx4, Fth), marking elevated spermatogonial GSH:GSSG. Cystine-uptake and intracellular conversion to cysteine typically couple glutathione biosynthesis to pentose phosphate metabolism. Rat spermatogonia, curiously, displayed poor germline stem cell viability in cystine-containing media, and, like primate spermatogonia, exhibited reduced transsulfuration pathway markers. Exogenous cysteine, cysteine-like mercaptans, somatic testis cells, and ferroptosis inhibitors counteracted the cysteine-starvation-induced spermatogonial death and stimulated spermatogonial growth factor activity in vitro.

PLZFposc-KITpos-delineated A1-A4-differentiating spermatogonia by subset and stage detection upon Bouin fixation

While hallmarks of rodent spermatogonia stem cell biomarkers' heterogeneity have recently been identified, their stage and subset distributions remain unclear. Furthermore, it is currently difficult to accurately identify subset-specific SSC marker distributions due to the poor nuclear morphological characteristics associated with fixation in 4% paraformaldehyde. In the present study, testicular cross-sections and whole-mount samples were Bouin fixed to optimize nuclear resolution and visualized by immunohistochemistry (IHC) and immunofluorescence (IF). The results identified an expression pattern of PLZF^highc-KIT^pos in A₁ spermatogonia, while A₂-A₄-differentiating spermatogonia were PLZF^lowc-KIT^pos. Additionally, this procedure was used to examine asymmetrically expressing GFRA1 and PLZF clones, asymmetric A_pr and false clones were distinguished based on the presence or absence of TEX14, a molecular maker of intercellular bridges, despite having identical nuclear morphology and intercellular distances that were <25 μm. In conclusion, this optimized Bouin fixation procedure facilitates the accurate identification of spermatogonium subsets based on their molecular profiles and is capable of distinguishing asymmetric and false clones. Therefore, the findings presented herein will facilitate further morphological and functional analysis studies and provide further insight into spermatogonium subtypes.

Acute exposure to bis(2-ethylhexyl)phthalate disrupts calcium homeostasis, energy metabolism and induces oxidative stress in the testis of Danio rerio

Bis(2-ethylhexyl)phthalate (BEHP) negatively affects testicular functions in different animal species, disturbing reproductive physiology and male fertility. The present study investigated the in vitro acute effect of BEHP on the mechanism of action of ionic calcium (Ca²⁺) homeostasis and energy metabolism. In addition, the effect of BEHP on oxidative stress was studied in vitro and in vivo in the testis of Danio rerio (D. rerio). Testes were treated in vitro for 30 min with 1 μM BEHP for ⁴⁵Ca²⁺ influx measurements. Testes were also incubated with 1 μM BEHP for 1 h (in vitro) or 12 h (in vivo) for the measurements of lactate content, ¹⁴C-deoxy-d-glucose uptake, lactate dehydrogenase (LDH) and gamma-glutamyl transpeptidase (GGT) activity, total reactive oxygen species (ROS) production and lipid peroxidation. In addition, the effect of BEHP (1 μM) on GGT, glutamic oxaloacetic transferase (GOT) and glutamic pyruvic transferase (GPT) activity in the liver was evaluated after in vivo treatment for 12 h. BEHP disturbs the Ca²⁺ balance in the testis when given acutely in vitro. BEHP stimulated Ca²⁺ influx occurs through L-type voltage-dependent Ca²⁺ channels (L-VDCC), transitory receptor potential vaniloid (TRPV1) channels, reverse-mode Na⁺/Ca²⁺ exchanger (NCX) activation and inhibition of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA). BEHP affected energy metabolism in the testis by decreasing the lactate content and LDH activity. In vitro and in vivo acute effects of BEHP promoted oxidative stress by increasing ROS production, lipid peroxidation and GGT activity in the testis. Additionally, BEHP caused liver damage by increasing GPT activity.

Stem Cells, Self-Renewal, and Lineage Commitment in the Endocrine System

The endocrine system coordinates a wide array of body functions mainly through secretion of hormones and their actions on target tissues. Over the last decades, a collective effort between developmental biologists, geneticists, and stem cell biologists has generated a wealth of knowledge related to the contribution of stem/progenitor cells to both organogenesis and self-renewal of endocrine organs. This review provides an up-to-date and comprehensive overview of the role of tissue stem cells in the development and self-renewal of endocrine organs. Pathways governing crucial steps in both development and stemness maintenance, and that are known to be frequently altered in a wide array of endocrine disorders, including cancer, are also described. Crucially, this plethora of information is being channeled into the development of potential new cell-based treatment modalities for endocrine-related illnesses, some of which have made it through clinical trials.

Effects of 220 MHz Pulsed Modulated Radiofrequency Field on the Sperm Quality in Rats

Under some occupational conditions, workers are inevitably exposed to high-intensity radiofrequency (RF) fields. In this study, we investigated the effects of one-month exposure to a 220 MHz pulsed modulated RF field at the power density of 50 W/m² on the sperm quality in male adult rats. The sperm quality was evaluated by measuring the number, abnormality and survival rate of sperm cells. The morphology of testis was examined by hematoxylin-eosin (HE) staining. The levels of secreting factors by Sertoli cells (SCs) and Leydig cells (LCs) were determined by enzyme linked immunosorbent assay (ELISA). The level of cleaved caspase 3 in the testis was detected by immunofluorescence staining. Finally, the expression levels of the apoptosis-related protein (caspase 3, BAX and BCL2) in the testis were assessed by Western blotting. Compared with the sham group, the sperm quality in the RF group decreased significantly. The levels of secreting factors of SCs and the morphology of the testis showed an obvious change after RF exposure. The level of the secreting factor of LCs decreased significantly after RF exposure. The levels of cleaved caspase 3, caspase 3, and the BAX/BCL2 ratio in the testis increased markedly after RF exposure. These data collectively suggested that under the present experimental conditions, 220 MHz pulsed modulated RF exposure could impair sperm quality in rats, and the disruption of the secreting function of LCs and increased apoptosis of testis cells induced by the RF field might be accounted for by this damaging effect.

Dynamic cytoplasmic projections connect mammalian spermatogonia in vivo

Throughout the male reproductive lifespan, spermatogonial stem cells (SSCs) produce committed progenitors that proliferate and then remain physically connected in growing clones via short cylindrical intercellular bridges (ICBs). These ICBs, which enlarge in meiotic spermatocytes, have been demonstrated to provide a conduit for postmeiotic haploid spermatids to share sex chromosome-derived gene products. In addition to ICBs, spermatogonia exhibit multiple thin cytoplasmic projections. Here, we have explored the nature of these projections in mice and find that they are dynamic, span considerable distances from their cell body (≥25 μm), either terminate or physically connect multiple adjacent spermatogonia, and allow for sharing of macromolecules. Our results extend the current model that subsets of spermatogonia exist as isolated cells or clones, and support a model in which spermatogonia of similar developmental fates are functionally connected through a shared dynamic cytoplasm mediated by thin cytoplasmic projections.

Visualization and Lineage Tracing of Pax7+ Spermatogonial Stem Cells in the Mouse

The precise identity of spermatogonial stem cells-the germline stem cell of the adult testis-remains a controversial topic. Technical limitations have included the lack of specific markers and methods for lineage tracing of A_single spermatogonia and their subsets. Immunolocalization of proteins in tissue sections has been a standard tool for the in situ identification and visualization of rare cellular subsets. However, these studies are limited by the need for faithful and reliable protein markers to define these cell types, as well as the availability of specific antibodies to these markers. Here we describe the use of a monoclonal antibody to Pax7 as a means to detect spermatogonial stem cells (SSCs) both in tissue sections and in intact seminiferous tubules. Furthermore, we describe methods for lineage tracing as an alternative method to visualize Pax7⁺ spermatogonial stem cells and their progeny.

Neuregulin 1 Regulates Proliferation of Leydig Cells to Support Spermatogenesis and Sexual Behavior in Adult Mice

Adult Leydig cells are derived from proliferating stem/progenitor Leydig cells in the infant testis and subsequent differentiation to steroidogenic cells in adult mice. Leydig cell proliferation in the infant testis occurs primarily in response to increased levels of LH that induce Leydig cell expression of neuregulin 1 (NRG1). Depletion of NRG1 in Nrg1 mutant mice (Nrg1^flox;flox;Cyp19a1Cre mice) dramatically reduces Leydig cell proliferation in the infant testes, leading to a reduction of testis weight, epididymial weight, and serum T in the adult mutant mice. The mutant mice are subfertile due to impaired sexual behavior and abnormal elongation of the spermatogenic cells. These defects were reversed by T treatment of the mutant mice in vivo. Furthermore, NRG1 alone induces the proliferation of Leydig cells in cultures of infant (d 10) testes obtained from mutant mice. Collectively these results show that LH induction of NRG1 directly drives the proliferation of Leydig cells in the infant testis, leading to an obligatory number of adult Leydig cells required for the production of sufficient androgen to support and maintain spermatogenesis and sexual behavior of adult male mice.

Epigenetic Remodeling in Male Germline Development

In mammals, germ cells guarantee the inheritance of genetic and epigenetic information across generations and are the origin of a new organism. During embryo development, the blastocyst is formed in the early stage, is comprised of an inner cell mass which is pluripotent, and could give rise to the embryonic stem cells (ESCs). The inner cell mass undergoes demethylation processes and will reestablish a methylated state that is similar to that of somatic cells later in epiblast stage. Primordial germ cells (PGCs) will be formed very soon and accompanied by the process of genome-wide demethylation. With the input of male sex determination genes, spermatogonial stem cells (SSCs) are generated and undergo the process of spermatogenesis. Spermatogenesis is a delicately regulated process in which various regulations are launched to guarantee normal mitosis and meiosis in SSCs. During all these processes, especially during spermatid development, DNA methylation profile and histone modifications are of crucial importance. In this review, we will discuss the epigenetic modifications from zygote formation to mature sperm generation and their significance to these development processes.

From cyst to tubule: innovations in vertebrate spermatogenesis

Although vertebrates share many common traits, their germline development and function exhibit significant divergence. In particular, this article focuses on their spermatogenesis. The fundamental elements that constitute vertebrate spermatogenesis and the evolutionary changes that occurred upon transition from water to land will be discussed. The life-long continuity of spermatogenesis is supported by the function of stem cells. Series of mitotic and meiotic germ cell divisions are 'incomplete' due to incomplete cytokinesis, forming syncytia interconnected via intercellular bridges (ICBs). Throughout this process, germ cells are supported by appropriate microenvironments established primarily by somatic Sertoli cells. In anamniotes (fish and amphibians) spermatogenesis progresses in cysts, in which developing germ cell syncytia are individually encapsulated by Sertoli cells. Accordingly, Sertoli cells undergo turnover with germ cells that they nourish. This mode of cystic spermatogenesis is also observed in nonvertebrates as insects. In amniotes (reptiles, birds, and mammals), however, Sertoli cells do not turn over but comprise a persistent structure of seminiferous tubules. Sertoli cells nourish different stages of germ cells simultaneously in distinct regions of their surface. This function of Sertoli cells is spatiotemporally orchestrated, and the seminiferous epithelial cycle and spermatogenic wave make the seminiferous tubules a high-throughput factory for sperm production. Furthermore, contrary to the organized differentiating cells, undifferentiated spermatogonia that comprise the stem cell compartment exhibit active motion over the basal layer of seminiferous tubules and the frequent breakdown of ICBs. Thus, amniote seminiferous tubules represent a typical facultative (or open) niche environment without a stem cell tethering anatomically defined niche. WIREs Dev Biol 2016, 5:119-131. doi: 10.1002/wdev.204 For further resources related to this article, please visit the WIREs website.

NRG1 and KITL Signal Downstream of Retinoic Acid in the Germline to Support Soma-Free Syncytial Growth of Differentiating Spermatogonia

Defined culture systems supporting spermatogonial differentiation will provide experimental platforms to study spermatogenesis. However, germline-intrinsic signaling mechanisms sufficient to support spermatogonial differentiation without somatic cells remain largely undefined. Here, we analyzed EGF superfamily receptor and ligand diversity in rat testis cells, and delineated germline-intrinsic signaling via an ERBB3 co-transducer, ERBB2, as essential for retinoic acid-induced syncytial growth by differentiating spermatogonia. Like the ERBB2/3 agonist NRG1, we found KIT Ligand (KITL) robustly supported spermatogonial differentiation without serum or somatic cells. ERBB2 inhibitors failed to disrupt KITL-dependent spermatogonial development, and, KITL prevented ERBB3-deficient spermatogonial degeneration upon differentiation. Thus, we report NRG1 and KITL activate alternative pathways downstream of retinoic acid signaling in the germline that are essential for stem cells to undergo pre-meiotic steps of spermatogenesis in culture. Robust serum/soma-free spermatogonial differentiation opens new doors to study mammalian germ cell biology in culture, which will facilitate the discovery of spermatogenic factors that can drive meiotic progression in vitro.

Transcriptional and translational heterogeneity among neonatal mouse spermatogonia

Spermatogonial stem cells (SSCs) are a subset of undifferentiated spermatogonia responsible for ongoing spermatogenesis in mammalian testes. Spermatogonial stem cells arise from morphologically homogeneous prospermatogonia, but growing evidence suggests that only a subset of prospermatogonia develops into the foundational SSC pool. This predicts that subtypes of undifferentiated spermatogonia with discrete mRNA and protein signatures should be distinguishable in neonatal testes. We used single-cell quantitative RT-PCR to examine mRNA levels of 172 genes in individual spermatogonia from 6-day postnatal (P6) mouse testes. Cells enriched from P6 testes using the StaPut or THY1(+) magnetic cell sorting methods exhibited considerable heterogeneity in the abundance of specific germ cell and stem cell mRNAs, segregating into one somatic and three distinct spermatogonial clusters. However, P6 Id4-eGFP(+) transgenic spermatogonia, which are known to be enriched for SSCs, were more homogeneous in their mRNA levels, exhibiting uniform levels for the majority of genes examined (122 of 172). Interestingly, these cells displayed nonuniform (50 of 172) expression of a smaller cohort of these genes, suggesting there is substantial heterogeneity even within the Id4-eGFP(+) population. Further, although immunofluorescence staining largely demonstrated conformity between mRNA and protein levels, some proteins were observed in patterns that were disparate from those detected for the corresponding mRNAs in Id4-eGFP(+) spermatogonia (e.g., Kit, Sohlh2, Stra8), suggesting additional heterogeneity is introduced at the posttranscriptional level. Taken together, these data demonstrate the existence of multiple spermatogonial subtypes in P6 mouse testes and raise the intriguing possibility that these subpopulations may correlate with the development of functionally distinct spermatogenic cell types.

Biology and identity of fish spermatogonial stem cell

Although present at relatively low number in the testis, spermatogonial stem cells (SSCs) are crucial for the establishment and maintenance of spermatogenesis in eukaryotes and, until recently, those cells were investigated in fish using morphological criteria. The isolation and characterization of these cells in fish have been so far limited by the lack of specific molecular markers, hampering the high SSCs biotechnological potential for aquaculture. However, some highly conserved vertebrate molecular markers, such as Gfra1 and Pou5f1/Oct4, are now available representing important candidates for studies evaluating the regulation of SSCs in fish and even functional investigations using germ cells transplantation. A technique already used to demonstrate that, different from mammals, fish germ stem cells (spermatogonia and oogonia) present high sexual plasticity that is determined by the somatic microenvironment. As relatively well established in mammals, and demonstrated in zebrafish and dogfish, this somatic environment is very important for the preferential location and regulation of SSCs. Importantly, a long-term in vitro culture system for SSCs has been now established for some fish species. Therefore, besides the aforementioned possibilities, such culture system would allow the development of strategies to in vitro investigate key regulatory and functional aspects of germline stem cells (ex: self-renewal and/or differentiation) or to amplify SSCs of rare, endangered, or commercially valuable fish species, representing an important tool for transgenesis and the development of new biotechnologies in fish production.

PAX7 expression defines germline stem cells in the adult testis

Spermatogenesis is a complex, multistep process that maintains male fertility and is sustained by rare germline stem cells. Spermatogenic progression begins with spermatogonia, populations of which express distinct markers. The identity of the spermatogonial stem cell population in the undisturbed testis is controversial due to a lack of reliable and specific markers. Here we identified the transcription factor PAX7 as a specific marker of a rare subpopulation of A(single) spermatogonia in mice. PAX7+ cells were present in the testis at birth. Compared with the adult testis, PAX7+ cells constituted a much higher percentage of neonatal germ cells. Lineage tracing in healthy adult mice revealed that PAX7+ spermatogonia self-maintained and produced expanding clones that gave rise to mature spermatozoa. Interestingly, in mice subjected to chemotherapy and radiotherapy, both of which damage the vast majority of germ cells and can result in sterility, PAX7+ spermatogonia selectively survived, and their subsequent expansion contributed to the recovery of spermatogenesis. Finally, PAX7+ spermatogonia were present in the testes of a diverse set of mammals. Our data indicate that the PAX7+ subset of A(single) spermatogonia functions as robust testis stem cells that maintain fertility in normal spermatogenesis in healthy mice and mediate recovery after severe germline injury, such as occurs after cancer therapy.