Stem Leydig cell differentiation: gene expression during development of the adult rat population of Leydig cells

The role of platelet-derived growth factor BB signaling pathway in the regulation of stem and progenitor Leydig cell proliferation and steroidogenesis in male rats

Platelet-derived growth factor BB (BB) regulates cell proliferation and function. However, the roles of BB on proliferation and function of Leydig stem (LSCs) and progenitor cells (LPCs) and the underlying signaling pathways remain unclear. This study aimed to analyze the roles of PI3K and MAPK pathways in the regulation of proliferation-related and steroidogenesis-related gene expression in rat LSCs/LPCs. In this experiment, BB receptor antagonist, tyrosine kinase inhibitor IV (PKI), the PI3K inhibitor, LY294002, and the MEK inhibitor, U0126, were used to measure the effects of these pathways on the expression of cell cycle-related genes (Ccnd1 and Cdkn1b) and steroidogenesis-related genes (Star, Cyp11a1, Hsd3b1, Cyp17a1, and Srd5a1), as well as Leydig cell maturation gene Pdgfra [1]. These results showed that BB (10 ng/mL)-stimulated EdU-incorporation into LSCs and BB-mediated inhibition on its differentiation was mediated through the activation of its receptor, PDGFRB, as well as MAPK and PI3K pathways. The results of LPC experiment also showed that LY294002 and U0126 decreased BB (10 ng/mL)-upregulated Ccnd1 expression while only U0126 reversed BB (10 ng/mL)-downregulated Cdkn1b expression. U0126 significantly reversed BB (10 ng/mL)-mediated downregulation of Cyp11a1, Hsd3b1, and Cyp17a1 expression. On the other hand, LY294002 reversed the expression of Cyp17a1 and Abca1. In conclusion, BB-mediated induction of proliferation and suppression of steroidogenesis of LSCs/LPCs are dependent on the activation of both MAPK and PI3K pathways, which show distinct regulation of gene expression.

Bone morphogenetic protein 4 inhibits rat stem/progenitor Leydig cell development and regeneration via SMAD-dependent and SMAD-independent signaling

Bone morphogenetic protein 4 (BMP4) is an important member of the transforming growth factor-β superfamily. BMP4 is expressed in the Leydig cell lineage. We hypothesized that BMP4 might regulate the development of stem/progenitor Leydig cells. The BMP4 receptors, BMPR1A, BMPR1B, and BMPR2 were found to be expressed in progenitor Leydig cells of prepubertal testis and isolated cells. BMP4 at 1 and 10 ng/mL significantly reduced androgen production and down-regulated steroidogenesis-related gene and protein expression possibly by activating the SMAD signaling pathway (increasing SMAD1/5 phosphorylation and SMAD4) at 24 h treatment. BMP4 at 0.1 ng/mL and higher concentrations markedly reduced the EdU labeling index of CD90⁺ stem Leydig cells after 24 h treatment and significantly reduced the number of EdU⁺ stem Leydig cells on the surface of seminiferous tubules after 7 days of culture. BMP4 at 0.01 ng/mL and higher concentrations significantly blocked the differentiation of stem Leydig cells into adult cells, as shown by the reduction of testosterone secretion and the downregulation of Lhcgr, Scarb1, Cyp11a1, Hsd11b1, and Insl3 and their function after 3D seminiferous tubule culture for 3 weeks, and this effect was reversed by co-treatment with the BMP4 antagonists noggin and doxomorphine. In addition, BMP4 also blocked stem Leydig cell differentiation through SMAD-independent signaling pathways (ERK1/2 and AMPK). Ethanedimethane sulfonate (EDS) single injection can result in reduction of testosterone, restoration can happen post treatment. In an in vivo model of Leydig cell regeneration following EDS treatment, intratesticular injection of BMP4 from day 14 to day 28 post-elimination significantly reduced serum testosterone levels and down-regulated the expression of Scarb1, Star, Hsd11b1, and Insl3 and its proteins, possibly through SMAD-dependent and SMAD-independent (ERK1/2 and AMPK) signaling pathways. In conclusion, BMP4 is expressed in cells of the Leydig cell lineage and blocks entry of stem/progenitor Leydig cells into adult Leydig cells through SMAD-dependent and SMAD-independent signaling pathways.

Involvement of calmodulin-dependent protein kinase I in the regulation of the expression of connexin 43 in MA-10 tumor Leydig cells

Connexin 43 (Cx43, also known as Gja1) is the most abundant testicular gap junction protein. It has a crucial role in the support of spermatogenesis by Sertoli cells in the seminiferous tubules as well as in androgen synthesis by Leydig cells. The multifunctional family of Ca²⁺/calmodulin-dependent protein kinases (CaMK) is composed of CaMK I, II, and IV and each can serve as a mediator of nuclear Ca²⁺ signals. These kinases can control gene expression by phosphorylation of key regulatory sites on transcription factors. Among these, AP-1 members cFos and cJun are interesting candidates that seem to cooperate with CaMKs to regulate Cx43 expression in Leydig cells. In this study, the Cx43 promoter region important for CaMK-dependent activation is characterized using co-transfection of plasmid reporter-constructs with different plasmids coding for CaMKs and/or AP-1 members in MA-10 Leydig cells. Here we report that the activation of Cx43 expression by cFos and cJun is increased by CaMKI. Furthermore, results from chromatin immunoprecipitation suggest that the recruitment of AP-1 family members to the proximal region of the Cx43 promoter may involve another uncharacterized AP-1 DNA regulatory element and/or protein-protein interactions with other partners. Thus, our data provide new insights into the molecular regulatory mechanisms that control mouse Cx43 transcription in testicular Leydig cells.

The Fate of Leydig Cells in Men with Spermatogenic Failure

The steroidogenic cells in the testicle, Leydig cells, located in the interstitial compartment, play a vital role in male reproductive tract development, maintenance of proper spermatogenesis, and overall male reproductive function. Therefore, their dysfunction can lead to all sorts of testicular pathologies. Spermatogenesis failure, manifested as azoospermia, is often associated with defective Leydig cell activity. Spermatogenic failure is the most severe form of male infertility, caused by disorders of the testicular parenchyma or testicular hormone imbalance. This review covers current progress in knowledge on Leydig cells origin, structure, and function, and focuses on recent advances in understanding how Leydig cells contribute to the impairment of spermatogenesis.

Leukemia inhibitory factor-receptor signalling negatively regulates gonadotrophin-stimulated testosterone production in mouse Leydig Cells

Testicular Leydig cells (LCs) are the principal source of circulating testosterone in males. LC steroidogenesis maintains sexual function, fertility and general health, and is influenced by various paracrine factors. The leukemia inhibitory factor receptor (LIFR) is expressed in the testis and activated by different ligands, including leukemia inhibitory factor (LIF), produced by peritubular myoid cells. LIF can modulate LC testosterone production in vitro under certain circumstances, but the role of consolidated signalling through LIFR in adult LC function in vivo has not been established. We used a conditional Lifr allele in combination with adenoviral vectors expressing Cre-recombinase to generate an acute model of LC Lifr-KO in the adult mouse testis, and showed that LC Lifr is not required for short term LC survival or basal steroidogenesis. However, LIFR-signalling negatively regulates steroidogenic enzyme expression and maximal gonadotrophin-stimulated testosterone biosynthesis, expanding our understanding of the intricate regulation of LC steroidogenic function.

Identification of Rat Testicular Leydig Precursor Cells by Single-Cell-RNA-Sequence Analysis

Stem Leydig cells (SLCs) play a critical role in the development and maintenance of the adult Leydig cell (ALC) population. SLCs also are present in the adult testis. Their identification, characteristics, and regulation in the adult testis remain uncertain. Using single-cell RNA-seq, we found that the mesenchymal stromal population may be involved in ALC regeneration. Upon ALC elimination, a fraction of stromal cells begins to proliferate while a different fraction begins to differentiate to ALCs. Transcriptomic analysis identified five stromal clusters that can be classified into two major groups representing proliferation and differentiation populations. The proliferating group represents stem cells expressing high levels of CD90, Nes, Lum, Fn and Gap43. The differentiating group represents a progenitor stage that is ready to form ALCs, and specifically expresses Vtn, Rasl11a, Id1 and Egr2. The observation that the actively dividing cells after ALC loss were not those that formed ALCs suggests that stem cell proliferation and differentiation are regulated separately, and that the maintenance of the stromal stem cell pool occurs at the population level. The study also identified specific markers for the major interstitial cell groups and potential paracrine factors involved in the regulation of SLCs. Our data suggest a new theory about SLC identity, proliferation, differentiation, and regulation.

Comparative transcriptome analysis reveals potential testosterone function-related regulatory genes/pathways of Leydig cells in immature and mature buffalo (Bubalus bubalis) testes

Leydig cells (LCs) are testosterone-generating endocrine cells that are located outside the seminiferous tubules in the testis, and testosterone is fundamental for retaining spermatogenesis and male fertility. In buffalo, adult Leydig cells (ALCs) are developed by immature Leydig cells (ILCs) in the postnatal testes. However, the genes/pathways associated to the regulation of testosterone secretion function during the development of postnatal LCs remains comprehensively unidentified. The present study comparatively analyzed the transcriptome profiles of ILC and ALC in buffalo with significant differences in testosterone secretion. Differentially expressed genes (DEGs) analysis identified 972 and 1,091 annotated genes that were significantly up- and down-regulated in buffalo ALC. Functional enrichment analysis showed that cAMP signaling being the most significantly enriched pathway, and testosterone synthesis and lipid transport-related genes/pathways were upregulated in ALC. Furthermore, gene set enrichment analysis (GSEA) shows that cAMP signaling and steroid hormone biosynthesis were activated in ALC, demonstrating that cAMP signaling may serve as a positive regulatory pathway in the maintenance of testosterone function during postnatal development of LCs. Protein-protein interaction (PPI) networks analysis highlighted that ADCY8, ADCY2, POMC, CHRM2, SST, PTGER3, SSTR2, SSTR1, NPY1R, and HTR1D as hub genes in the cAMP signaling pathway. In conclusion, this study identified key genes and pathways associated in the regulation of testosterone secretion function during the ILC-ALC transition in buffalo based on bioinformatics analysis, and these key genes might be deeply involved in cAMP generation to influencing testosterone levels in LCs. The results suggest that ALCs might increase testosterone levels by enhancing cAMP production than ILCs. Our data will enhance the understanding of developmental mechanism studies related to testosterone function and provide preliminary evidence for molecular mechanisms of LCs regulating spermatogenesis.

Specific Transcriptomic Signatures and Dual Regulation of Steroidogenesis Between Fetal and Adult Mouse Leydig Cells

Leydig cells (LC) are the main testicular androgen-producing cells. In eutherian mammals, two types of LCs emerge successively during testicular development, fetal Leydig cells (FLCs) and adult Leydig cells (ALCs). Both display significant differences in androgen production and regulation. Using bulk RNA sequencing, we compared the transcriptomes of both LC populations to characterize their specific transcriptional and functional features. Despite similar transcriptomic profiles, a quarter of the genes show significant variations in expression between FLCs and ALCs. Non-transcriptional events, such as alternative splicing was also observed, including a high rate of intron retention in FLCs compared to ALCs. The use of single-cell RNA sequencing data also allowed the identification of nine FLC-specific genes and 50 ALC-specific genes. Expression of the corticotropin-releasing hormone 1 (Crhr1) receptor and the ACTH receptor melanocortin type 2 receptor (Mc2r) specifically in FLCs suggests a dual regulation of steroidogenesis. The androstenedione synthesis by FLCs is stimulated by luteinizing hormone (LH), corticotrophin-releasing hormone (CRH), and adrenocorticotropic hormone (ACTH) whereas the testosterone synthesis by ALCs is dependent exclusively on LH. Overall, our study provides a useful database to explore LC development and functions.

Stem Leydig cells: Current research and future prospects of regenerative medicine of male reproductive health

Stem cells are specialized cells that can renew themselves through cell division and can differentiate into multi-lineage cells. Mesenchymal stem cells are adult stem cells that exist in animal and human tissues. Mesenchymal stem cells have the ability to differentiate into mesodermal lineages, such as Leydig cells, adipocytes, osteocytes, and chondrocytes. Mesenchymal stem cells express cell surface markers, such as cluster of differentiation (CD) 29, CD44, CD73, CD90, CD105, and lack the expression of CD14, CD34, CD45 and HLA (human leukocyte antigen)-DR. Stem Leydig cells are one kind of mesenchymal stem cells, which are present in the interstitial compartment of testis. Stem Leydig cells are multipotent and can differentiate into Leydig cells, adipocytes, osteocytes, and chondrocytes. Stem Leydig cells have been isolated from rodent and human testes. Stem Leydig cells may have potential therapeutic values in several clinical applications, such as the treatment of male hypogonadism and infertility. In this review, we focus on the latest research on stem Leydig cells of both rodents and human, the expression of cell surface markers, culture, differentiation potential, and their applications.

Differentiation of seminiferous tubule-associated stem cells into leydig cell and myoid cell lineages

Peritubular stem Leydig cells (SLCs) have been identified from rat testicular seminiferous tubules. However, no stem cells for peritubular myoid cells have been reported in the adult testis so far. In the present study, we tested the hypothesis that the peritubular SLCs are multipotent and able to form either Leydig or myoid cells. Using cultured tubules, we show that in the presence of PDGFAA and luteinizing hormone, SLCs became testosterone-producing Leydig cells, while in the presence of PDGFBB and TGFB, the cells formed α-smooth muscle actin-expressing myoid cells. This multipotency was also confirmed by culture of isolated CD90+ SLCs. These results suggest that these stem cells outside the myoid layer are multipotent and give rise to either Leydig or myoid cells, depending on the inducing factors. These cells may serve as a common precursor population for maintaining homeostasis of both Leydig and myoid cell populations in the adult testis.

Xylene delays the development of Leydig cells in pubertal rats by inducing reactive oxidative species

Xylene is a cyclic hydrocarbon, which is commonly used as a solvent in dyes, paints, polishes, and industrial solutions. It is a potential environmental pollutant. Here, we report the effect of xylene exposure on Leydig cell development in male rats during puberty. Xylene (0, 150, 750, and 1500 mg/kg) was gavaged to 35-day-old male Sprague Dawley rats for 21 days. Xylene significantly reduced serum testosterone levels at 750 and 1500 mg/kg without affecting serum luteinizing hormone and follicle-stimulating hormone levels. Xylene reduced the number of HSD11B1-positive Leydig cells at the advanced stage at 1500 mg/kg. At 750 and 1500 mg/kg, xylene also reduced the cell size and cytoplasm size. It down-regulated the expression of Leydig cell-specific genes (Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, and Hsd11b1) and proteins. In addition, xylene significantly reduced the ratio of phosphorus-GSK-3β (pGSK-3β/GSK-3β), phosphorus-ERK1/2 (pERK)/ERK1/2, and phosphorus-AKT1 (pAKT1)/AKT1, and SIRT1 levels in the testes. In vitro Leydig cell culture showed that xylene induced oxidative stress by increasing the production of reactive oxygen species and lowing antioxidant (Sod2), and inhibited the production of testosterone, and down-regulated the expression of genes related to steroidogenesis, while vitamin E reversed the xylene-mediated effect as an antioxidant. In conclusion, xylene exposure may disrupt the development of pubertal Leydig cells by increasing reactive oxygen species production and reducing the expression of GSK-3β, ERK1/2, AKT1, and SIRT1.

Monocyte Chemoattractant Protein-1 stimulates the differentiation of rat stem and progenitor Leydig cells during regeneration

Background

Monocyte chemoattractant protein-1(MCP-1) is a chemokine secreted by Leydig cells and peritubular myoid cells in the rat testis. Its role in regulating the development of Leydig cells via autocrine and paracrine is still unclear. The objective of the current study was to investigate the effects of MCP-1 on Leydig cell regeneration from stem cells in vivo and on Leydig cell development in vitro.

Results

Intratesticular injection of MCP-1(10 ng/testis) into Leydig cell-depleted rat testis from post-EDS day 14 to 28 significantly increased serum testosterone and luteinizing hormone levels, up-regulated the expression of Leydig cell proteins, LHCGR, SCARB1, CYP11A1, HSD3B1, CYP17A1, and HSD17B3 without affecting progenitor Leydig cell proliferation, as well as increased ERK1/2 phosphorylation. MCP-1 (100 ng/ml) significantly increased medium testosterone levels and up-regulated LHCGR, CYP11A1, and HSD3B1 expression without affecting EdU incorporation into stem cells after in vitro culture for 7 days. RS102895, a CCR2 inhibitor, reversed MCP-1-mediated increase of testosterone level after culture in combination with MCP-1.

Conclusion

MCP-1 stimulates the differentiation of stem and progenitor Leydig cells without affecting their proliferation.

Mesenchymal Stromal Cells as Critical Contributors to Tissue Regeneration

Adult stem cells that are tightly regulated by the specific microenvironment, or the stem cell niche, function to maintain tissue homeostasis and regeneration after damage. This demands the existence of specific niche components that can preserve the stem cell pool in injured tissues and restore the microenvironment for their subsequent appropriate functioning. This role may belong to mesenchymal stromal cells (MSCs) due to their resistance to damage signals and potency to be specifically activated in response to tissue injury and promote regeneration by different mechanisms. Increased amount of data indicate that activated MSCs are able to produce factors such as extracellular matrix components, growth factors, extracellular vesicles and organelles, which transiently substitute the regulatory signals from missing niche cells and restrict the injury-induced responses of them. MSCs may recruit functional cells into a niche or differentiate into missing cell components to endow a niche with ability to regulate stem cell fates. They may also promote the dedifferentiation of committed cells to re-establish a pool of functional stem cells after injury. Accumulated evidence indicates the therapeutic promise of MSCs for stimulating tissue regeneration, but the benefits of administered MSCs demonstrated in many injury models are less than expected in clinical studies. This emphasizes the importance of considering the mechanisms of endogenous MSC functioning for the development of effective approaches to their pharmacological activation or mimicking their effects. To achieve this goal, we integrate the current ideas on the contribution of MSCs in restoring the stem cell niches after damage and thereby tissue regeneration.

Stem Leydig Cells in the Adult Testis: Characterization, Regulation and Potential Applications

Androgen deficiency (hypogonadism) affects males of all ages. Testosterone replacement therapy (TRT) is effective in restoring serum testosterone and relieving symptoms. TRT, however, is reported to have possible adverse effects in part because administered testosterone is not produced in response to the hypothalamic-pituitary-gonadal (HPG) axis. Progress in stem cell biology offers potential alternatives for treating hypogonadism. Adult Leydig cells (ALCs) are generated by stem Leydig cells (SLCs) during puberty. SLCs persist in the adult testis. Considerable progress has been made in the identification, isolation, expansion and differentiation of SLCs in vitro. In addition to forming ALCs, SLCs are multipotent, with the ability to give rise to all 3 major cell lineages of typical mesenchymal stem cells, including osteoblasts, adipocytes, and chondrocytes. Several regulatory factors, including Desert hedgehog and platelet-derived growth factor, have been reported to play key roles in the proliferation and differentiation of SLCs into the Leydig lineage. In addition, stem cells from several nonsteroidogenic sources, including embryonic stem cells, induced pluripotent stem cells, mature fibroblasts, and mesenchymal stem cells from bone marrow, adipose tissue, and umbilical cord have been transdifferentiated into Leydig-like cells under a variety of induction protocols. ALCs generated from SLCs in vitro, as well as Leydig-like cells, have been successfully transplanted into ALC-depleted animals, restoring serum testosterone levels under HPG control. However, important questions remain, including: How long will the transplanted cells continue to function? Which induction protocol is safest and most effective? For translational purposes, more work is needed with primate cells, especially human.

Fibroblast growth factor homologous factor 1 stimulates Leydig cell regeneration from stem cells in male rats

Fibroblast growth factor homologous factor 1 (FHF1) is an intracellular protein that does not bind to cell surface fibroblast growth factor receptor. Here, we report that FHF1 is abundantly present in Leydig cells with up‐regulation during its development. Adult male Sprague Dawley rats were intraperitoneally injected with 75 mg/kg ethane dimethane sulphonate (EDS) to ablate Leydig cells to initiate their regeneration. Then, rats daily received intratesticular injection of FHF1 (0, 10 and 100 ng/testis) from post‐EDS day 14 for 14 days. FHF1 increased serum testosterone levels without affecting the levels of luteinizing hormone and follicle‐stimulating hormone. FHF1 increased the cell number staining with HSD11B1, a biomarker for Leydig cells at the advanced stage, without affecting the cell number staining with CYP11A1, a biomarker for all Leydig cells. FHF1 did not affect PCNA‐labelling index in Leydig cells. FHF1 increased Leydig cell mRNA (Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, Hsd17b3, Insl3, Nr5a1 and Hsd11b1) and their protein levels in vivo. FHF1 increased preadipocyte biomarker Dlk1 mRNA level and decreased fully differentiated adipocyte biomarker (Fabp4 and Lpl) mRNA and their protein levels. In conclusion, FHF1 promotes Leydig cell regeneration from stem cells while inhibiting the differentiation of preadipocyte/stem cells into adipocytes in EDS‐treated testis.

Aberrant gene expression by Sertoli cells in infertile men with Sertoli cell-only syndrome

Sertoli cell-only (SCO) syndrome is a severe form of human male infertility seemingly characterized by the lack all spermatogenic cells. However, tubules of some SCO testes contain small patches of active spermatogenesis and thus spermatogonial stem cells. We hypothesized that these stem cells cannot replicate and seed spermatogenesis in barren areas of tubule because as-of-yet unrecognized deficits in Sertoli cell gene expression disable most stem cell niches. Performing the first thorough comparison of the transcriptomes of human testes exhibiting complete spermatogenesis with the transcriptomes of testes with SCO syndrome, we defined transcripts that are both predominantly expressed by Sertoli cells and expressed at aberrant levels in SCO testes. Some of these transcripts encode proteins required for the proper assembly of adherent and gap junctions at sites of contact with other cells, including spermatogonial stem cells (SSCs). Other transcripts encode GDNF, FGF8 and BMP4, known regulators of mouse SSCs. Thus, most SCO Sertoli cells can neither organize junctions at normal sites of cell-cell contact nor stimulate SSCs with adequate levels of growth factors. We propose that the critical deficits in Sertoli cell gene expression we have identified contribute to the inability of spermatogonial stem cells within small patches of spermatogenesis in some SCO testes to seed spermatogenesis to adjacent areas of tubule that are barren of spermatogenesis. Furthermore, we predict that one or more of these deficits in gene expression are primary causes of human SCO syndrome.

Fibroblast growth factor 16 stimulates proliferation but blocks differentiation of rat stem Leydig cells during regeneration

OBJECTIVES

We aim to investigate the effects of fibroblast growth factor 16 (FGF16) on Leydig cell regeneration in ethane dimethane sulphonate (EDS)-treated rat testis.

METHODS

We intraperitoneally inject 75 mg/kg EDS to adult male Sprague Dawley rats and then intratesticularly inject FGF16 (0, 10 and 100 ng/testis/day) from post-EDS day 14 for 14 days. We investigate serum hormone levels, Leydig cell number, gene and protein expression in vivo. We also explore the effects of FGF16 treatment on stem Leydig cell proliferation in vitro.

RESULTS

FGF16 lowers serum testosterone levels (21.6% of the control at a dose of 100 ng/testis) without affecting the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) on post-EDS day 28 in vivo. FGF16 increases Leydig cell number at doses of 10 and 100 ng/mg without affecting Sertoli cell number, increases the percentage of PCNA-positive Leydig cells, and down-regulates the expression of Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Cyp17a1 and Hsd17b3) and Sertoli cell genes (Fshr, Dhh and Sox9) and their proteins in vivo. FGF16 increases phosphorylation of AKT1 and AKT2 as well as EKR1/2 in vivo, indicating that it possibly acts via AKT1/ATK2 and ERK1/2 pathways. FGF16 also lowers medium testosterone levels and down-regulates the expression of Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Cyp17a1 and Hsd17b3) but increases EdU incorporation into stem Leydig cells in vitro.

CONCLUSIONS

These data suggest that FGF16 stimulates stem and progenitor Leydig cell proliferation but blocks their differentiation, thus lowering testosterone biosynthesis.

Effects of spermatogenic cycle on Stem Leydig cell proliferation and differentiation

We reported previously that stem Leydig cells (SLC) on the surfaces of rat testicular seminiferous tubules are able to differentiate into Leydig cells. The proliferation and differentiation of SLCs seem likely to be regulated by niche cells, including nearby germ and Sertoli cells. Due to the cyclical nature of spermatogenesis, we hypothesized that the changes in the germ cell composition of the seminiferous tubules as spermatogenesis proceeds may affect tubule-associated SLC functions. To test this hypothesis, we compared the ability of SLCs associated with tubules at different stages of the cycle to differentiate into Leydig cells in vitro. SLCs associated with stages IX-XI were more active in proliferation and differentiation than SLCs associated with stages VII-VIII. However, when the SLCs were isolated from each of the two groups of tubules and cultured in vitro, no differences were seen in their ability to proliferate or differentiate. These results suggested that the stage-dependent local factors, not the SLCs themselves, explain the stage-dependent differences in SLC function. TGFB, produced in stage-specific fashion by Sertoli cells, is among the factors shown in previous studies to affect SLC function in vitro. When TGFB inhibitors were included in the cultures of stages IX-XI and VII-VIII tubules, stage-dependent differences in SLC development were reduced, suggesting that TGFB may be among the paracrine factors involved in the stage-dependent differences in SLC function. Taken together, the findings suggest that there is dynamic interaction between SLCs and germ/Sertoli cells within the seminiferous tubules that may affect SLC proliferation and differentiation.

The Pluripotent Microvascular Pericytes Are the Adult Stem Cells Even in the Testis

The pericytes of the testis are part of the omnipresent population of pericytes in the vertebrate body and are the only true pluripotent adult stem cells able to produce structures typical for the tree primitive germ layers: ectoderm, mesoderm, and endoderm. They originate very early in the embryogenesis from the pluripotent epiblast. The pericytes become disseminated through the whole vertebrate organism by the growing and differentiating blood vessels where they remain in specialized periendothelial vascular niches as resting pluripotent adult stem cells for tissue generation, maintenance, repair, and regeneration. The pericytes are also the ancestors of the perivascular multipotent stromal cells (MSCs). The variable appearance of the pericytes and their progeny reflects the plasticity under the influence of their own epigenetic and the local environmental factors of the host organ. In the testis the pericytes are the ancestors of the neuroendocrine Leydig cells. After activation the pericytes start to proliferate, migrate, and build transit-amplifying cells that transdifferentiate into multipotent stromal cells. These represent progenitors for a number of different cell types in an organ. Finally, it becomes evident that the pericytes are a brilliant achievement of the biological nature aiming to supply every organ with an omnipresent population of pluripotent adult stem cells. Their fascinating features are prerequisites for future therapy concepts supporting cell systems of organs.

Fibroblast Growth Factor 1 Promotes Rat Stem Leydig Cell Development

Fibroblast growth factor 1 (FGF1) is reported to be expressed in the testis. How FGF1 affects stem Leydig cell development remains unclear. Here, we report the effects of FGF1 on rat stem Leydig cell development in an ethane dimethane sulfonate (EDS)-treated model. FGF1 (100 ng/testis) significantly increased serum testosterone level, increased PCNA-positive Leydig cell percentage and Leydig cell number, but down-regulated the expression of Lhcgr, Star, Cyp11a1, Hsd3b1, Cyp17a1, and Hsd11b1 in Leydig cells per se, after its daily intratesticular injection from post-EDS day 14 for 14 days. Primary culture of the seminiferous tubules showed that FGF1 stimulated EdU incorporation to stem Leydig cells but blocked the differentiation into the Leydig cell lineage, possibly via FGFR1-mediated mechanism. In conclusion, FGF1 promotes stem Leydig cell proliferation but blocks its differentiation.

Interleukin 6 inhibits the differentiation of rat stem Leydig cells

Inflammation causes male hypogonadism. Several inflammatory cytokines, including interleukin 6 (IL-6), are released into the blood and may suppress Leydig cell development. The objective of the present study was to investigate whether IL-6 affected the proliferation and differentiation of rat stem Leydig cells. Leydig cell-depleted rat testis (in vivo) and seminiferous tubules (in vitro) with ethane dimethane sulfonate (EDS) were used to explore the effects of IL-6 on stem Leydig cell development. Intratesticular injection of IL-6 (10 and 100 ng/testis) from post-EDS day 14 to 28 blocked the regeneration of Leydig cells, as shown by the lower serum testosterone levels (21.6% of the control at 100 ng/testis dose), the down-regulated Leydig cell gene (Lhcgr, Star, Cyp11a1, Cyp17a1, and Hsd17b3) expressions, and the reduced Leydig cell number. Stem Leydig cells on the surface of the seminiferous tubules were induced to enter the Leydig cell lineage in vitro in the medium containing luteinizing hormone and lithium. IL-6 (1, 10, and 100 ng/ml) concentration-dependently decreased testosterone production and Lhcgr, Cyp11a1, Cyp17a1, Hsd17b3 and Insl3 mRNA levels. The IL-6 mediated effects were antagonized by Janus kinase 1 (JAK) inhibitor (filgotinib) and Signal Transducers and Activators of Transcription 3 (STAT3) inhibitor (S3I-201), indicating that a JAK-STAT3 signaling pathway is involved. In conclusion, our results demonstrated that IL-6 was an inhibitory factor of stem Leydig cell development.

Modulation of the transcriptomic profile of the R2C tumor Leydig cell line by the adipose tissue derived hormone leptin

Leptin is an important adipose derived hormone being secreted by adipocytes and involved in appetite regulation. Disruption of normal plasma levels of leptin has been associated with the pathogenesis of obesity. It is now well established that obesity is associated to an increased risk of cancer development, including testicular cancers. Recently, we have shown that high levels of leptin have inhibitory effects on cAMP-dependent steroidogenic genes expression in MA-10 Leydig cells. Hence, we examined if leptin could alter the transcriptome of the constitutively steroidogenic rat tumor Leydig cell line R2C. These cells were treated with high levels of leptin (1000 ng/ml) for 4 h, followed by mRNA extraction and RNA-Seq analysis. Interestingly, leptin had no effect on steroidogenic gene expression or pathways promoting tumorigenesis of this cell line. Genes being upregulated or downregulated by leptin were enriched in biological processes that were non-relevant to Leydig cell function. Surprisingly, lepr isoforms were weakly expressed in R2C cells and may explain in part the low levels of leptin's response for tumor Leydig cells. Hence, the current findings suggest that acute treatments of tumor Leydig cells with high levels of leptin have negligible effects on transcriptomics and rather supports that leptin would have a central effect on the hypothalamus to influence testicular function.

Triphenyltin Chloride Delays Leydig Cell Maturation During Puberty in Rats

Triphenyltin chloride (TPT) is present in a wide range of human foods. TPT could disrupt testis function as a potential endocrine disruptor of Leydig cells. However, the effect of TPT on pubertal Leydig cell development is still unclear. The objective of the current study was to explore whether exposure to TPT affected Leydig cell developmental process and to clarify the underlying mechanisms. Male Sprague-Dawley rats at 35 days of age were randomly divided into four groups and received normal corn oil (control), 0.5, 1, or 2 mg/kg/day TPT for 18 days. Immature Leydig cells isolated from 35-day-old rat testes were treated with TPT (10 and 100 nM) for 24 h in vitro. In vivo exposure to ≥0.5 mg/kg TPT lowered serum testosterone levels and lowered Star mRNA. TPT at 2 mg/kg also lowered Lhcgr, Cyp11a1, Hsd3b1, Hsd17b3 as well as pAKT1/AKT1, pAKT2/AKT2, and pERK1/2/ERK1/2 ratios. In vitro exposure to TPT (100 nM) increased ROS production and induced cell apoptosis rate in rat immature Leydig cells. In conclusion, TPT exposure disrupts Leydig cell development possibly via interfering with the phosphorylation of AKT1, AKT2, and ERK1/2 kinases.

Previously claimed male germline stem cells from porcine testis are actually progenitor Leydig cells

Background

Male germline stem cells (mGSCs) offer great promise in regenerative medicine and animal breeding due to their capacity to maintain self-renewal and to transmit genetic information to the next generation following spermatogenesis. Human testis-derived embryonic stem cell-like cells have been shown to possess potential of mesenchymal progenitors, but there remains confusion about the characteristics and origin of porcine testis-derived stem cells.

Methods

Porcine testis-derived stem cells were obtained from primary testicular cultures of 5-day old piglets, and selectively expanded using culture conditions for long-term culture and induction differentiation. The stem cell properties of porcine testis-derived stem cells were subsequently assessed by determining the expression of pluripotency-associated markers, alkaline phosphatase (AP) activity, and capacity for sperm and multilineage differentiation in vitro. The gene expression profile was compared via microarray analysis.

Results

We identified two different types of testis-derived stem cells (termed as C1 and C2 here) during porcine testicular cell culture. The gene expression microarray analysis showed that the transcriptome profile of C1 and C2 differed significantly from each other. The C1 appeared to be morphologically similar to the previously described mouse mGSCs, expressed pluripotency- and germ cell-associated markers, maintained the paternal imprinted pattern of H19, displayed alkaline phosphatase activity, and could differentiate into sperm. Together, these data suggest that C1 represent the porcine mGSC population. Conversely, the C2 appeared similar to the previously described porcine mGSCs with three-dimensional morphology, abundantly expressed Leydig cell lineage and mesenchymal cell-specific markers, and could differentiate into testosterone-producing Leydig cells, suggesting that they are progenitor Leydig cells (PLCs).

Conclusion

Collectively, we have established the expected characteristics and markers of authentic porcine mGSCs (C1). We found for the first time that, the C2, equivalent to previously claimed porcine mGSCs, are actually progenitor Leydig cells (PLCs). These findings provide new insights into the discrepancies among previous reports and future identification and analyses of testis-derived stem cells.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0931-0) contains supplementary material, which is available to authorized users.

Influence of fetal Leydig cells on the development of adult Leydig cell population in rats

Leydig cells are the main endogenous testosterone synthesis cells in the body. Testosterone is an essential hormone in males that affects metabolism, emotion, and pubertal development. However, little is known about the development of Leydig cells and relationship between fetal Leydig cells (FLCs) and adult Leydig cells (ALCs). The aims of this study were to investigate the effect of (FLCs) on ALC development. Our study showed that FLCs in neonatal rat testis can be eliminated by 100 mg/kg ethane dimethane sulfonate (EDS) treatment without affecting the health of newborn rats. Immunohistological results showed that eliminating FLCs led to early re-generation of the ALC population (progenitor Leydig cells [PLCs] and ALCs) accompanied at first by increased and then by decreased serum testosterone, indicating that ALCs which appeared after neonatal EDS treatment were degenerated or had attenuated functions. Our results showed that FLCs were eliminated 4 days after EDS treatment, the ALC population regenerated by 21 days, and serum testosterone levels dramatically decreased at 56 days. Collectively, our results indicate that the ablation of FLCs in neonatal rat results in abnormal development of ALCs. Our study further indicates that abnormal development of Leydig cells in the fetal stage leads to steroid hormone disorders, such as testosterone deficiency, in the adult stage. Therefore, studies of Leydig cell development are important for understanding the pathogenesis of testosterone deficiency or pubertas praecox.

Exposure to cadmium during gestation and lactation affects development and function of Leydig cells in male offspring

Toxic effects of maternal exposure to Cadmium (Cd) on Leydig cells of male offspring arises much concern recently, but its toxic effects on the development of Leydig cells and androgen synthesis have not been elucidated. In this study, female rats were exposed to Cd during gestation and lactation, and the development of Leydig cells in the first filial-generation (F1) male rats was investigated. The steroidogenic signaling pathway and biomarkers related to the development of Leydig cells were detected to disclose how maternal Cd-exposure caused reproductive damage. F1 male rats with maternal Cd-exposure gained a low relative weight of testis and declined levels of steroid hormones. Maternal Cd-exposure interrupted the development of Leydig cells with high expression of SRD5α and cell morphology of immature Leydig cells in adulthood, inhibited the activation of cyclic adenosine monophosphate/ protein kinase A signaling pathway and down-regulated the steroidogenic enzymes. These results would help to disclose the origin of male sexual dysfunction in the developmental stages of Leydig cells.

Origin of a rapidly evolving homeostatic control system programming testis function

Mammals share common strategies for regulating reproduction, including a conserved hypothalamic-pituitary-gonadal axis; yet, individual species exhibit differences in reproductive performance. In this report, we describe the discovery of a species-restricted homeostatic control system programming testis growth and function. Prl3c1 is a member of the prolactin gene family and its protein product (PLP-J) was discovered as a uterine cytokine contributing to the establishment of pregnancy. We utilized mouse mutagenesis of Prl3c1 and revealed its involvement in the regulation of the male reproductive axis. The Prl3c1-null male reproductive phenotype was characterized by testiculomegaly and hyperandrogenism. The larger testes in the Prl3c1-null mice were associated with an expansion of the Leydig cell compartment. Prl3c1 locus is a template for two transcripts (Prl3c1-v1 and Prl3c1-v2) expressed in a tissue-specific pattern. Prl3c1-v1 is expressed in uterine decidua, while Prl3c1-v2 is expressed in Leydig cells of the testis. 5'RACE, chromatin immunoprecipitation and DNA methylation analyses were used to define cell-specific promoter usage and alternative transcript expression. We examined the Prl3c1 locus in five murid rodents and showed that the testicular transcript and encoded protein are the result of a recent retrotransposition event at the Mus musculus Prl3c1 locus. Prl3c1-v1 encodes PLP-J V1 and Prl3c1-v2 encodes PLP-J V2. Each protein exhibits distinct intracellular targeting and actions. PLP-J V2 possesses Leydig cell-static actions consistent with the Prl3c1-null testicular phenotype. Analysis of the biology of the Prl3c1 gene has provided insight into a previously unappreciated homeostatic setpoint control system programming testicular growth and function.

Insights into the Development of the Adult Leydig Cell Lineage from Stem Leydig Cells

Adult Leydig cells (ALCs) are the steroidogenic cells in the testes that produce testosterone. ALCs develop postnatally from a pool of stem cells, referred to as stem Leydig cells (SLCs). SLCs are spindle-shaped cells that lack steroidogenic cell markers, including luteinizing hormone (LH) receptor and 3β-hydroxysteroid dehydrogenase. The commitment of SLCs into the progenitor Leydig cells (PLCs), the first stage in the lineage, requires growth factors, including Dessert Hedgehog (DHH) and platelet-derived growth factor-AA. PLCs are still spindle-shaped, but become steroidogenic and produce mainly androsterone. The next transition in the lineage is from PLC to the immature Leydig cell (ILC). This transition requires LH, DHH, and androgen. ILCs are ovoid cells that are competent for producing a different form of androgen, androstanediol. The final stage in the developmental lineage is ALC. The transition to ALC involves the reduced expression of 5α-reductase 1, a step that is necessary to make the cells to produce testosterone as the final product. The transitions along the Leydig cell lineage are associated with the progressive down-regulation of the proliferative activity, and the up-regulation of steroidogenic capacity, with each step requiring unique regulatory signaling.

Leydig cell stem cells: Identification, proliferation and differentiation

Adult Leydig cells develop from undifferentiated mesenchymal-like stem cells (stem Leydig cells, SLCs) present in the interstitial compartment of the early postnatal testis. Putative SLCs also have been identified in peritubular and perivascular locations of the adult testis. The latter cells, which normally are quiescent, are capable of regenerating new Leydig cells upon the loss of the adult cells. Recent studies have identified several protein markers to identify these cells, including nestin, PDGFRα, COUP-TFII, CD51 and CD90. We have shown that the proliferation of the SLCs is stimulated by DHH, FGF2, PDGFBB, activin and PDGFAA. Suppression of proliferation occurred with TGFβ, androgen and PKA signaling. The differentiation of the SLCs into testosterone-producing Leydig cells was found to be regulated positively by DHH (Desert hedgehog), lithium-induced signaling and activin; and negatively by TGFβ, PDGFBB, FGF2, Notch and Wnt signaling. DHH, by itself, was found to induce SLC differentiation into LH-responsive steroidogenic cells, suggesting that DHH plays a critical role in the commitment of SLC into the Leydig lineage. These studies, taken together, address the function and regulation of low turnover stem cells in a complex, adult organ, and also have potential application to the treatment of androgen deficiency.

Steroidogenic genes expressions are repressed by high levels of leptin and the JAK/STAT signaling pathway in MA-10 Leydig cells

The adipose tissue is an important endocrine organ secreting numerous peptide hormones, including leptin. Increased circulating levels of leptin, as a result of hormonal resistance in obese individuals, may contribute to lower androgen production in obese males. However, the molecular mechanisms involved need to be better defined. Androgens are mainly produced by Leydig cells within the testis. In male rodents, activation of the leptin receptor modulates a cascade of intracellular signal transduction pathways which may lead to regulation of transcription factors having influences on steroidogenesis in Leydig cells. Thus, as a result of high leptin levels interacting with its receptor and modulating the activity of the JAK/STAT signaling pathway, the activity of transcription factors important for steroidogenic genes expressions may be inhibited in Leydig cells. Here we show that Lepr is increasingly expressed within Leydig cells according to postnatal development. Although high levels of leptin (corresponding to obesity condition) alone had no effect on Leydig cells' steroidogenic genes expression, it downregulated cAMP-dependent activations of the cholesterol transporter Star and of the rate-limiting steroidogenic enzyme Cyp11a1. Our results suggest that STAT transcriptional activity is downregulated by high levels of leptin, leading to reduced cAMP-dependent steroidogenic genes (Star and Cyp11a1) expressions in MA-10 Leydig cells. However, other transcription factors such as members of the SMAD and NFAT families may be involved and need further investigation to better define how leptin regulates their activities and their relevance for Leydig cells function.

Differentiation of human umbilical cord mesenchymal stem cells into steroidogenic cells in vitro

Although previous studies have shown that stem cells can be differentiated into Leydig cells by gene transfection, a simple, safe and effective induction method has not yet been reported. Therefore, the present study investigated novel methods for the induction of human umbilical cord mesenchymal stem cell (HUMSC) differentiation into Leydig-like, steroidogenic cells. HUMSCs were acquired using the tissue block culture attachment method, and the expression of MSC surface markers was evaluated by flow cytometry. Leydig cells were obtained by enzymatic digestion and identified by lineage-specific markers via immunofluorescence. Third-passage HUMSCs were cultured with differentiation-inducing medium (DIM) or Leydig cell-conditioned medium (LC-CM), and HUMSCs before induction were used as the control group. Following the induction of HUMSCs, Leydig cell lineage-specific markers (CYP11A1, CYP17A1 and 3β-HSD) were positively identified using immunofluorescence analysis. Additionally, reverse transcription-quantitative polymerase chain reaction and western blot analysis were performed to evaluate the expression levels of these genes and enzymes. In contrast, the control group cells did not show the characteristics of Leydig cells. Collectively, these results indicate that, under in vitro conditions, LC-CM can achieve a comparable effect to that of DIM on inducing HUMSCs differentiation into steroidogenic cells.

Regulation of development of rat stem and progenitor Leydig cells by activin

Stem Leydig cells have been demonstrated to differentiate into adult Leydig cells via intermediate stages of progenitor and immature Leydig cells. However, the exact regulatory mechanisms are unclear. We hypothesized that the development of stem or progenitor Leydig cells depends upon locally produced growth factors. Microarray analysis revealed that the expression levels of activin type I receptor (Acvr1) and activin A receptor type II-like 1 (Acvrl1) were stem > progenitor = immature = adult Leydig cells. This indicates that their ligand activin might play an important role in stem and progenitor Leydig cell proliferation and differentiation. When seminiferous tubules were incubated with 1 or 10 ng/mL activin A for 3 days, it concentration-dependently increased EdU incorporation into stem Leydig cells by up to 20-fold. When progenitor Leydig cells were incubated with 1 or 10 ng/mL activin A for 2 days, it concentration-dependently increased ³ H-thymidine incorporation into progenitor Leydig cells by up to 200%. Real-time PCR analysis showed that activin A primarily increased Pcna expression but reduced Star, Hsd3b1, and Cyp17a1 expression levels. Activin A also significantly inhibited the basal and luteinizing hormone-stimulated androgen production. In conclusion, activin A primarily stimulates the proliferation of stem and progenitor Leydig cells, but inhibits the differentiation of stem and progenitor Leydig cells into the Leydig cell lineage in rat testis.

Identification and characterization of Xenopus tropicalis common progenitors of Sertoli and peritubular myoid cell lineages

The origin of somatic cell lineages during testicular development is controversial in mammals. Employing basal amphibian tetrapod Xenopus tropicalis we established a cell culture derived from testes of juvenile male. Expression analysis showed transcription of some pluripotency genes and Sertoli cell, peritubular myoid cell and mesenchymal cell markers. Transcription of germline-specific genes was downregulated. Immunocytochemistry revealed that a majority of cells express vimentin and co-express Sox9 and smooth muscle α-actin (Sma), indicating the existence of a common progenitor of Sertoli and peritubular myoid cell lineages. Microinjection of transgenic, red fluorescent protein (RFP)-positive somatic testicular cells into the peritoneal cavity of X. tropicalis tadpoles resulted in cell deposits in heart, pronephros and intestine, and later in a strong proliferation and formation of cell-to-cell net growing through the tadpole body. Immunohistochemistry analysis of transplanted tadpoles showed a strong expression of vimentin in RFP-positive cells. No co-localization of Sox9 and Sma signals was observed during the first three weeks indicating their dedifferentiation to migratory-active mesenchymal cells recently described in human testicular biopsies.

Summary: We identified cells co-expressing differentiation markers of Sertoli and peritubular myoid cell lineages in X. tropicalis through the establishment and characterization of cell culture derived from juvenile testis.

ACBD2/ECI2-Mediated Peroxisome-Mitochondria Interactions in Leydig Cell Steroid Biosynthesis

Fatty acid metabolism and steroid biosynthesis are 2 major pathways shared by peroxisomes and mitochondria. Both organelles are in close apposition to the endoplasmic reticulum, with which they communicate via interorganelle membrane contact sites to promote cellular signaling and the exchange of ions and lipids. To date, no convincing evidence of the direct contact between peroxisomes and mitochondria was reported in mammalian cells. Hormone-induced, tightly controlled steroid hormone biosynthesis requires interorganelle interactions. Using immunofluorescent staining and live-cell imaging, we found that dibutyryl-cAMP treatment of MA-10 mouse tumor Leydig cells rapidly induces peroxisomes to approach mitochondria and form peroxisome-mitochondrial contact sites/fusion, revealed by the subcellular distribution of the endogenous acyl-coenzyme A-binding domain (ACBD)2/ECI2 isoform A generated by alternative splicing, and further validated using a proximity ligation assay. This event occurs likely via a peroxisome-like structure, which is mediated by peroxisomal and mitochondrial matrix protein import complexes: peroxisomal import receptor peroxisomal biogenesis factor 5 (PEX5), and the mitochondrial import receptor subunit translocase of outer mitochondrial membrane 20 homolog (yeast) protein. Similar results were obtained using the mLTC-1 mouse tumor Leydig cells. Ectopic expression of the ACBD2/ECI2 isoform A in MA-10 cells led to increased basal and hormone-stimulated steroid formation, indicating that ACBD2/ECI2-mediated peroxisomes-mitochondria interactions favor in the exchange of metabolites and/or macromolecules between these 2 organelles in support of steroid biosynthesis. Considering the widespread occurrence of the ACBD2/ECI2 protein, we propose that this protein might serve as a tool to assist in understanding the contact between peroxisomes and mitochondria.

Leydig cells contribute to the inhibition of spermatogonial differentiation after irradiation of the rat

Irradiation with 6 Gy produces a complete block of spermatogonial differentiation in LBNF1 rats that would be permanent without treatment. Subsequent suppression of gonadotropins and testosterone (T) restores differentiation to the spermatocyte stage; however, this process requires 6 weeks. We evaluated the role of Leydig cells (LCs) in maintenance of the block in spermatogonial differentiation after exposure to radiation by specifically eliminating functional LCs with ethane dimethane sulfonate (EDS). EDS (but not another alkylating agent), given at 10 weeks after irradiation, induced spermatogonial differentiation in 24% of seminiferous tubules 2 weeks later. However, differentiation became blocked again at 4 weeks as LCs recovered. When EDS was followed by treatment with GnRH antagonist and flutamide, sustained spermatogonial differentiation was induced in >70% of tubules within 2 weeks. When EDS was followed by GnRH antagonist plus exogenous T, which also inhibits LC recovery but restores follicle stimulating hormone (FSH) levels, the spermatogonial differentiation was again rapid but transient. These results confirm that the factors that block spermatogonial differentiation are indirectly regulated by T, and probably FSH, and that adult and possibly immature LCs contribute to the production of such inhibitory factors. We tested whether insulin-like 3 (INSL3), a LC-produced protein whose expression correlated with the block in spermatogonial differentiation, was indeed responsible for the block by injecting synthetic INSL3 into the testes and knocking down its expression in vivo with siRNA. Neither treatment had any effect on spermatogonial differentiation. The Leydig cell products that contribute to the inhibition of spermatogonial differentiation in irradiated rats remain to be elucidated.

Regulation of seminiferous tubule-associated stem Leydig cells in adult rat testes

Testicular Leydig cells are the primary source of testosterone in males. Adult Leydig cells have been shown to arise from stem cells present in the neonatal testis. Once established, adult Leydig cells turn over only slowly during adult life, but when these cells are eliminated experimentally from the adult testis, new Leydig cells rapidly reappear. As in the neonatal testis, stem cells in the adult testis are presumed to be the source of the new Leydig cells. As yet, the mechanisms involved in regulating the proliferation and differentiation of these stem cells remain unknown. We developed a unique in vitro system of cultured seminiferous tubules to assess the ability of factors from the seminiferous tubules to regulate the proliferation of the tubule-associated stem cells, and their subsequent entry into the Leydig cell lineage. The proliferation of the stem Leydig cells was stimulated by paracrine factors including Desert hedgehog (DHH), basic fibroblast growth factor (FGF2), platelet-derived growth factor (PDGF), and activin. Suppression of proliferation occurred with transforming growth factor β (TGF-β). The differentiation of the stem cells was regulated positively by DHH, lithium- induced signaling, and activin, and negatively by TGF-β, PDGFBB, and FGF2. DHH functioned as a commitment factor, inducing the transition of stem cells to the progenitor stage and thus into the Leydig cell lineage. Additionally, CD90 (Thy1) was found to be a unique stem cell surface marker that was used to obtain purified stem cells by flow cytometry.

Origin and Differentiation of Androgen-Producing Cells in the Gonads

Sexual reproduction is dependent on the activity of androgenic steroid hormones to promote gonadal development and gametogenesis. Leydig cells of the testis and theca cells of the ovary are critical cell types in the gonadal interstitium that carry out steroidogenesis and provide key androgens for reproductive organ function. In this chapter, we will discuss important aspects of interstitial androgenic cell development in the gonad, including: the potential cellular origins of interstitial steroidogenic cells and their progenitors; the molecular mechanisms involved in Leydig cell specification and differentiation (including Sertoli-cell-derived signaling pathways and Leydig-cell-related transcription factors and nuclear receptors); the interactions of Leydig cells with other cell types in the adult testis, such as Sertoli cells, germ cells, peritubular myoid cells, macrophages, and vascular endothelial cells; the process of steroidogenesis and its systemic regulation; and a brief discussion of the development of theca cells in the ovary relative to Leydig cells in the testis. Finally, we will describe the dynamics of steroidogenic cells in seasonal breeders and highlight unique aspects of steroidogenesis in diverse vertebrate species. Understanding the cellular origins of interstitial steroidogenic cells and the pathways directing their specification and differentiation has implications for the study of multiple aspects of development and will help us gain insights into the etiology of reproductive system birth defects and infertility.

Steroidogenic factor 1 differentially regulates fetal and adult leydig cell development in male mice

The nuclear receptor steroidogenic factor 1 (SF-1, AD4BP, NR5A1) is a key regulator of the endocrine axes and is essential for adrenal and gonad development. Partial rescue of Nr5a1(-/-) mice with an SF-1-expressing transgene caused a hypomorphic phenotype that revealed its roles in Leydig cell development. In contrast to controls, all male rescue mice (Nr5a1(-/-);tg(+/0)) showed varying signs of androgen deficiency, including spermatogenic arrest, cryptorchidism, and poor virilization. Expression of various Leydig cell markers measured by immunohistochemistry, Western blot analysis, and RT-PCR indicated fetal and adult Leydig cell development were differentially impaired. Whereas fetal Leydig cell development was delayed in Nr5a1(-/-);tg(+/0) embryos, it recovered to control levels by birth. In contrast, Sult1e1, Vcam1, and Hsd3b6 transcript levels in adult rescue testes indicated complete blockage in adult Leydig cell development. In addition, between Postnatal Days 8 and 12, peritubular cells expressing PTCH1, SF-1, and CYP11A1 were observed in control testes but not in rescue testes, indicating SF-1 is needed for either survival or differentiation of adult Leydig cell progenitors. Cultured prepubertal rat peritubular cells also expressed SF-1 and PTCH1, but Cyp11a1 was expressed only after treatment with cAMP and retinoic acid. Together, data show SF-1 is needed for proper development of fetal and adult Leydig cells but with distinct primary functions; in fetal Leydig cells, it regulates differentiation, whereas in adult Leydig cells it regulates progenitor cell formation and/or survival.

Basic fibroblast growth factor promotes stem Leydig cell development and inhibits LH-stimulated androgen production by regulating microRNA expression

Leydig cells are the primary source of testosterone in the testes, and their steroidogenic function is strictly controlled by the hypothalamus-pituitary-gonad axis. Emerging evidence has indicated that fibroblast growth factors play a role in regulating stem Leydig cell development and steroidogenesis, but little is known about the regulatory mechanism. Using a seminiferous tubule culture system, we demonstrated that basic fibroblast growth factor (bFGF) can promote stem Leydig cell proliferation and commitment toward differentiation in testosterone-producing Leydig cells. However, these promoting effects decreased with an increase in the bFGF dose. Previous studies have reported that bFGF inhibits luteinizing hormone (LH)-stimulated androgen production by downregulating the mRNA expression of steroidogenic genes in immature Leydig cells. However, the expression levels of 677 microRNAs did not change significantly during the LH-mediated process of testosterone synthesis. Five microRNAs (miR-29a, -29c, -142-3p, -451 and -335) were identified, and their expression in immature Leydig cells was regulated simultaneously by bFGF and LH. These results suggested that the inhibition of LH-stimulated androgen production may be modulated by a change in bFGF-mediated microRNA expression, which further impacts the signaling pathway of testosterone biosynthesis and steroidogenic gene expression.

Human testicular peritubular cells host putative stem Leydig cells with steroidogenic capacity

AIM

We aim to examine the steroidogenic phenotype and the differentiation potential of human testicular peritubular cells (HTPCs) and to explore their possible relationship to the adult Leydig cell lineage.

BACKGROUND

The cells of the adult Leydig cell lineage may reside in the peritubular compartment of the testis. This suggestion is supported by the facts that the rodent peritubular cells can be differentiated toward this lineage and that cAMP enhances their steroidogenic potential.

METHODS

Human testicular biopsies, and derived HTPCs, were analyzed by immunohistochemistry, RT-PCR, and Western blotting. After stimulation by forskolin or platelet-derived growth factor-BB, quantitative RT-PCR was used to compare the levels of mRNAs encoding proteins involved in steroidogenesis and steroid production was analyzed by liquid chromatography and tandem mass spectrometry.

RESULTS

Immunohistochemical analysis revealed that the peritubular cells that form the outer part of the tubular wall express platelet derived growth factor receptor-α. Furthermore, the pluripotency markers (POU domain class 5 transcription factor 1, GATA-binding protein 4), stem Leydig cell markers (platelet derived growth factor receptor-A, leukemia inhibitory factor receptor), and mRNAs encoding proteins involved in steroidogenesis (nuclear receptor subfamily 5, group A, member 1; steroidogenic acute regulatory protein; CYP11A1; CYP17A1; 3β-hydroxysteroid dehydrogenase) were expressed by the HTPCs. Stimulation with forskolin increased the expression of the steroidogenic markers, which was accompanied by the production of pregnenolone and progesterone by HTPCs in vitro. Treatment with platelet-derived growth factor-BB induced expression of steroidogenic acute regulatory protein.

CONCLUSIONS

Our results indicate that the tubular wall of the human testis is a reservoir for cells of the adult Leydig cell lineage and that the steroidogenic potential of these cells can be activated in culture.

Regulation of the proliferation and differentiation of Leydig stem cells in the adult testis

We reported previously that stem cells associated with adult rat testis seminiferous tubules are able to give rise to differentiated Leydig cells in vitro. The regulatory mechanisms by which they do so, however, are uncertain. Herein, we hypothesized that the proliferation and differentiation of Leydig cell stem cells (stem Leydig cells, SLCs) depend upon locally produced factors from the seminiferous tubules. Microarray analysis revealed that platelet-derived growth factor receptor alpha (PDGFRalpha) is up-regulated and PDGFRbeta is down-regulated with postnatal differentiation of SLCs. This suggested that their ligands, PDGF-AA and PDGF-BB, respectively, might have important roles in SLC proliferation and differentiation. To test this, we developed a unique in vitro culture system in which SLCs proliferate on the surfaces of cultured seminiferous tubules largely during Week 1 of culture and their progeny subsequently differentiate to testosterone-forming Leydig cells during Weeks 2 through 4. Using this system, seminiferous tubules from adult rat testes were cultured with PDGF-AA or PDGF-BB for up to 4 wk. Both ligands stimulated SLC proliferation during the first week of culture, with PDGF-BB significantly more potent than PDGF-AA. Furthermore, PDGF-AA had a stimulatory effect on SLC differentiation from Weeks 2 through 4 of culture. In contrast, PDGF-BB, which stimulated cell proliferation during Week 1, had a significant inhibitory effect on differentiation during Weeks 2 through 4. These findings, made possible by the development of the seminiferous tubule culture system, reveal distinct roles by locally produced PDGFs in SLC regulation.

Identification of Leydig cell-specific mRNA transcripts in the adult rat testis

The adult population of Leydig cells acts to secrete testosterone which is essential for reproductive health and fertility in the adult male. However, other physiological functions of these cells are uncertain, and to address this issue a cell ablation model has been used to identify Leydig cell-specific mRNA transcripts. Ethane dimethane sulphonate (EDS) was synthesised by a novel process and was used to ablate Leydig cells in adult male rats previously treated with butane dimethane sulphonate (busulphan) to delete the germ cell population. Levels of mRNA transcripts were measured in the testis using microarrays 1, 3, 5, 8 and 12 days after EDS injection. During this period, there was a significant change in the levels of 2200 different transcripts with a marked decline in the levels of canonical Leydig cell transcripts, such as Cyp11a1, Cyp17a1 and Insl3. A total of 95 transcripts showed a similar decline in expression after EDS treatment, suggesting that they have a Leydig cell-specific origin. Analysis of selected transcripts confirmed that they were expressed specifically in Leydig cells and showed that most had a late onset of expression during adult Leydig cell development. Apart from transcripts encoding components of the steroidogenic apparatus, the most common predicted function of translated proteins was endogenous and xenotoxicant metabolism. In addition, a number of transcripts encode acute-phase proteins involved in reduction of oxidative stress. Results show that, in addition to androgen secretion, Leydig cells may have a critical role to play in protecting the testis from damage caused by toxicants or stress.

Sertoli cell androgen receptor expression regulates temporal fetal and adult Leydig cell differentiation, function, and population size

We recently created a mouse model displaying precocious Sertoli cell (SC) and spermatogenic development induced by SC-specific transgenic androgen receptor expression (TgSCAR). Here we reveal that TgSCAR regulates the development, function, and absolute number of Leydig cells (LCs). Total fetal and adult type LC numbers were reduced in postnatal and adult TgSCAR vs control testes, despite normal circulating LH levels. Normal LC to SC ratios found in TgSCAR testes indicate that SC androgen receptor (SCAR)-mediated activity confers a quorum-dependent relationship between total SC and LC numbers. TgSCAR enhanced LC differentiation, shown by elevated ratios of advanced to immature LC types, and reduced LC proliferation in postnatal TgSCAR vs control testes. Postnatal TgSCAR testes displayed up-regulated expression of coupled ligand-receptor transcripts (Amh-Amhr2, Dhh-Ptch1, Pdgfa-Pdgfra) for potential SCAR-stimulated paracrine pathways, which may coordinate LC differentiation. Neonatal TgSCAR testes displayed normal T and dihydrotestosterone levels despite differential changes to steroidogenic gene expression, with down-regulated Star, Cyp11a1, and Cyp17a1 expression contrasting with up-regulated Hsd3b1, Hsd17b3, and Srd5a1 expression. TgSCAR males also displayed elevated postnatal and normal adult serum testosterone levels, despite reduced LC numbers. Enhanced adult-type LC steroidogenic output was revealed by increased pubertal testicular T, dihydrotestosterone, 3α-diol and 3β-diol levels per LC and up-regulated steroidogenic gene (Nr5a1, Lhr, Cyp11a1, Cyp17a1, Hsd3b6, Srd5a1) expression in pubertal or adult TgSCAR vs control males, suggesting regulatory mechanisms maintain androgen levels independently of absolute LC numbers. Our unique gain-of-function TgSCAR model has revealed that SCAR activity controls temporal LC differentiation, steroidogenic function, and population size.

Phenotype and steroidogenic potential of PDGFRα-positive rat neonatal peritubular cells

Platelet-derived growth factor receptor α (PDGFRα)-positive peritubular cells (PTCs) are suggested to be putative stem Leydig cells. At present little is known about their phenotype and steroidogenic potential. We isolated highly purified PDGFRα-positive neonatal PTCs by magnetic cell sorting (MACS) from 8dpp rat testes and characterized them in vitro. We have demonstrated that PDGFRα-positive PTCs have a mixed phenotype. They expressed PTC-specific genes (αSma, Myh11), pluripotency markers (Pou5f1, nestin, Lifr) and genes encoding steroidogenic enzymes. Treatment with the cAMP-analog (Bu)2cAMP for 7 days upregulated steroidogenic enzyme gene expression and significantly increased their steroidogenic potential. The main end-point steroid was progesterone due to rapid inactivation of CYP17 and 17βHSD. Long-term culturing of PDGFRα-positive PTCs increased the expression of Myh11, and treatment with (Bu)2cAMP attenuated this process. All together, our findings support the hypothesis that neonatal PDGFRα-positive PTCs are steroidogenic competent progeny of stem Leydig cells (SLCs) which give rise to the adult Leydig cell lineage.

The ERK1/2 pathway regulates testosterone synthesis by coordinately regulating the expression of steroidogenic genes in Leydig cells

Adult mice with a Leydig cell specific deletion of MAPK kinase (MEK) 1 and 2 (Mek1(f)(/)(f);Mek2(-/-);Cre(+)) mice display Leydig cell hypoplasia and hypergonadotropic hypogonadism. We used radioimmunoassays and quantitative PCR to evaluate the function and expression of the Leydig cell genes involved in the conversion of cholesterol to testosterone (Star, Cyp11a1, Hsd3b6, Cyp17a1 and Hsd17b3), androgen metabolism (Srda1 and Dhrs9), and four transcription factors (Creb1, Nr5a1, Nr4a1 and Nr0b1) that regulate the expression of steroidogenic genes. We show that Star, Hsd3b6, Cyp17a1 and Hsd17b3 are downregulated in Ledyig cells of adult Mek1(f)(/)(f);Mek2(-/-);Cre(+) mice whereas Srda1 and Dhrs9 are upregulated and Creb1, Nr5a1, Nr4a1 and Nr0b1 are unchanged or upregulated. Functionally, all the downregulated genes but none of the upregulated genes contribute to the decrease in testosterone synthesis in Leydig cells of adult Mek1(f)(/)(f);Mek2(-/-);Cre(+) mice because they produce low testosterone and dihydrotestosterone when stimulated with hCG or when incubated with testosterone precursors such as progesterone or androstenedione.