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The ability to identify and isolate lineage-specific stem cells from adult tissues could facilitate cell replacement therapy. Leydig cells （LCs） are the primary source of androgen in the mammalian testis, and the prospective iden- tification of stem Leydig cells （SLCs） may offer new opportunities for treating testosterone deficiency. Here, in a transgenic mouse model expressing GFP driven by the Nestin （Nes） promoter, we observed Nes-GFP~ cells located in the testicular interstitial compartment where SLCs normally reside. We showed that these Nes-GFP~ cells expressed LIFR and PDGFR-e, but not LC lineage markers. We further observed that these cells were capable of clonogenic self-renewal and extensive proliferation in vitro and could differentiate into neural or mesenchymal cell lineages, as well as LCs, with the ability to produce testosterone, under defined conditions. Moreover, when transplanted into the testes of LC-disrupted or aging models, the Nes-GFP＋ cells colonized the interstitium and partially increased testosterone production, and then accelerated meiotic and post-meiotic germ cell recovery. In addition, we further demonstrated that CD51 might be a putative cell surface marker for SLCs, similar with Nestin. Taken together, these results suggest that Nes-GFP~ cells from the testis have the characteristics of SLCs, and our study would shed new light on developing stem cell replacement therapy for testosterone deficiency.

Spermatogenesis is fundamental to the establishment and maintenance of male reproduction, whereas its abnormality results in male infertility. Somatic cells, including Leydig cells, myoid cells, and Sertoli cells, constitute the microenvironment or the niche of testis, which is essential for regulating normal spermatogenesis. Leydig cells are an important component of the testicular stroma, while peritubular myoid cells are one of the major cell types of seminiferous tubules. Here we addressed the roles and mechanisms of Leydig cells and myoid cells in the regulation of spermatogenesis. Specifically, we summarized the biological features of Leydig cells and peritubular myoid cells, and we introduced the process of testosterone production and its major regulation. We also discussed other hormones, cytokines, growth factors, transcription factors and receptors associated with Leydig cells and myoid cells in mediating spermatogenesis. Furthermore, we highlighted the issues that are worthy of further studies in the regulation of spermatogenesis by Leydig cells and peritubular myoid cells. This review would provide novel insights into molecular mechanisms of the somatic cells in controlling spermatogenesis, and it could offer new targets for developing therapeutic approaches of male infertility.

Abstract 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. Graphical Abstract Graphical Abstract

Fetal growth plays a role in programming of adult cardiometabolic disorders, which in men, are associated with lowered testosterone levels. Fetal growth and fetal androgen exposure can also predetermine testosterone levels in men, although how is unknown, because the adult Leydig cells (ALCs) that produce testosterone do not differentiate until puberty. To explain this conundrum, we hypothesized that stem cells for ALCs must be present in the fetal testis and might be susceptible to programming by fetal androgen exposure during masculinization. To address this hypothesis, we used ALC ablation/regeneration to identify that, in rats, ALCs derive from stem/progenitor cells that express chicken ovalbumin upstream promoter transcription factor II. These stem cells are abundant in the fetal testis of humans and rodents, and lineage tracing in mice shows that they develop into ALCs. The stem cells also express androgen receptors (ARs). Reduction in fetal androgen action through AR KO in mice or dibutyl phthalate (DBP) -induced reduction in intratesticular testosterone in rats reduced ALC stem cell number by ∼40% at birth to adulthood and induced compensated ALC failure (low/normal testosterone and elevated luteinizing hormone). In DBP-exposed males, this failure was probably explained by reduced testicular steroidogenic acute regulatory protein expression, which is associated with increased histone methylation (H3K27me3) in the proximal promoter. Accordingly, ALCs and ALC stem cells immunoexpressed increased H3K27me3, a change that was also evident in ALC stem cells in fetal testes. These studies highlight how a key component of male reproductive development can fundamentally reprogram adult hormone production (through an epigenetic change), which might affect lifetime disease risk.

Zinc oxide nanoparticles (ZnO NPs) have shown adverse health impact on the human male reproductive system, with evidence of inducing apoptosis. However, whether or not ZnO NPs could promote autophagy, and the possible role of autophagy in the progress of apoptosis, remain unclear. In the current study, in vitro and in vivo toxicological responses of ZnO NPs were explored by using a mouse model and mouse Leydig cell line. It was found that intragastrical exposure of ZnO NPs to mice for 28 days at the concentrations of 100, 200, and 400 mg/kg/day disrupted the seminiferous epithelium of the testis and decreased the sperm density in the epididymis. Furthermore, serum testosterone levels were markedly reduced. The induction of apoptosis and autophagy in the testis tissues was disclosed by up-regulating the protein levels of cleaved Caspase-8, cleaved Caspase-3, Bax, LC3-II, Atg 5, and Beclin 1, accompanied by down-regulation of Bcl 2. In vitro tests showed that ZnO NPs could induce apoptosis and autophagy with the generation of oxidative stress. Specific inhibition of autophagy pathway significantly decreased the cell viability and up-regulated the apoptosis level in mouse Leydig TM3 cells. In summary, ZnO NPs can induce apoptosis and autophagy via oxidative stress, and autophagy might play a protective role in ZnO NPs-induced apoptosis of mouse Leydig cells.

Cannabidiol (CBD), one of the major components extracted from the plant Cannabis sativa L., has been used as a prescription drug to treat seizures in many countries. CBD-induced male reproductive toxicity has been reported in animal models; however, the underlying mechanisms remain unclear. We previously reported that CBD induced apoptosis in primary human Leydig cells, which constitute the primary steroidogenic cell population in the testicular interstitium. In this study, we investigated the effects of CBD and its metabolites on TM3 mouse Leydig cells. CBD, at concentrations below 30 µM, reduced cell viability, induced G1 cell cycle arrest, and inhibited DNA synthesis. CBD induced apoptosis after exposure to high concentrations (≥ 50 µM) for 24 h or a low concentration (20 µM) for 6 days. 7-Hydroxy-CBD and 7-carboxy-CBD, the main CBD metabolites of CBD, exhibited the similar toxic effects as CBD. In addition, we conducted a time-course mRNA-sequencing analysis in both primary human Leydig cells and TM3 mouse Leydig cells to understand and compare the mechanisms underlying CBD-induced cytotoxicity. mRNA-sequencing analysis of CBD-treated human and mouse Leydig cells over a 5-day time-course indicated similar responses in both cell types. Mitochondria and lysosome dysfunction, oxidative stress, and autophagy were the major enriched pathways in both cell types. Taken together, these findings demonstrate comparable toxic effects and underlying mechanisms in CBD-treated mouse and primary human Leydig cells.

Simiao Biejia Decoction (SMBJ), a traditional Chinese herbal formula, has been clinically applied to treat diabetes mellitus (DM)-induced male dysfunction due to its kidney-tonifying, heat-clearing, and blood-enriching properties. However, the precise molecular mechanisms and therapeutic targets through which SMBJ confers protection against DM-induced testicular damage remain to be fully elucidated. Goto-Kakizaki (GK) and wild-type rats were randomly assigned to the wild-type, GK model, and SMBJ low-, medium-, and high-dose groups. SMBJ was administered at low, medium, and high doses (2.44, 4.88, and 9.76 g/kg/day) by gavage for 4 weeks. Serum testosterone, blood glucose, and oxidative stress levels were measured using ELISA and colorimetric assays. Histopathological staining was performed to evaluate testicular damage. In vitro experiments were conducted using the R2C cell line subjected to high-glucose conditions, with intervention using SMBJ-containing serum. RT-qPCR, Western blotting, immunofluorescence, MeRIP-qPCR, RNA stability were employed to elucidate the underlying mechanisms. SMBJ alleviates oxidative stress injury, thereby reducing apoptosis and enhancing testosterone secretion in the interstitial tissue and in high glucose-stimulated R2C cell of diabetic rats. METTL3 expression was downregulated in the testes of diabetic rat and in R2C cell exposed to high glucose. METTL3 knockdown mimicked the effects of high glucose, underscoring its protective role in Leydig cells. Si-METTL3 decreased the stability of PI3K/AKT mRNAs, whereas SMBJ upregulated METTL3 expression, modulating the PI3K/AKT pathway, reversing high glucose-induced damage in R2C cell, and highlighting its therapeutic potential in Leydig cell protection. SMBJ may upregulates METTL3 expression alongside PI3K/AKT activation to ameliorate diabetes mellitus-induced Leydig cell dysfunction. The potential regulatory role of m6A modification in diabetes-induced testicular injury was further elucidated.

Background: Certain phthalates can impair Leydig cell distribution and steroidogenesis in the fetal rat in utero, but it is unknown whether similar effects might occur in the human. Objectives: Our aim in this study was to investigate the effects of di(n-butyl) phthalate (DBP), or its metabolite monobutyl phthalate (MBP), on testosterone production and Leydig cell aggregation (LCA) in fetal testis explants from the rat and human, and to compare the results with in vivo findings for DBP-exposed rats. We also wanted to determine if DBP/MBP affects testosterone production in vivo in the neonatal male marmoset. Methods: Fetal testis explants obtained from the rat [gestation day (GD)19.5] and from the human (15-19 weeks of gestation) were cultured for 24-48 hr with or without human chorionic gonadotropin (hCG) or 22R-hydroxy-cholesterol (22R-OH), and with or without DBP/MBP. Pregnant rats and neonatal male marmosets were dosed with 500 mg/kg/day DBP or MBP. Results: Exposure of rats in utero to DBP (500 mg/kg/day) for 48 hr before GD21.5 induced major suppression of intratesticular testosterone levels and cytochrome P450 side chain cleavage enzyme (P450scc) expression; this short-term treatment induced LCA, but was less marked than longer term (GD13.5-20.5) DBP treatment. In vitro, MBP ($10^{-3} M$) did not affect basal or 22R-OH-stimulated testosterone production by fetal rat testis explants but slightly attenuated hCG-stimulated steroidogenesis; MBP induced minor LCA in vitro. None of these parameters were affected in human fetal testis explants cultured with$10^{3} M$MBP for up to 48 hr. Because the in vivo effects of DBP/MBP were not reproduced in vitro in the rat, the absence of MBP effects in vitro on fetal human testes is inconclusive. In newborn (Day 2-7) marmosets, administration of a single dose of 500 mg/kg MBP significantly (p = 0.019) suppressed blood testosterone levels 5 hr later. Similar treatment of newborn co-twin male marmosets for 14 days resulted in increased Leydig cell volume per testis (p = 0.011), compared with co-twin controls; this is consistent with MBP-induced inhibition of steroidogenesis followed by compensatory Leydig cell hyperplasia/hypertrophy. Conclusions: These findings suggest that MBP/DBP suppresses steroidogenesis by fetal-type Leydig cells in primates as in rodents, but this cannot be studied in vitro.

Reduced serum testosterone (T), or hypogonadism, affects millions of men and is associated with many pathologies, including infertility, cardiovascular diseases,metabolic syndrome, and decreased libido and sexual function. Administering T-replacement therapy (TRT) reverses many of the symptoms associated with low T levels. However, TRT is linked to side effects such as infertility and increased risk of prostate cancer and cardiovascular diseases. Thus, there is a need to obtain T-producing cells that could be used to treat hypogonadism via transplantation and reestablishment of Tproducing cell lineages in the body. T is synthesized by Leydig cells (LCs), proposed to derive from mesenchymal cells of mesonephric origin. Although mesenchymal cells have been successfully induced into LCs, the limited source and possible trauma to donors hinders their application to clinical therapies. Alternatively, human induced pluripotent stem cells (hiPSCs), which are expandable in culture and have the potential to differentiate into all somatic cell types, have become the emerging source of autologous cell therapies. We have successfully induced the differentiation of hiPSCs into either human Leydig-like (hLLCs) or adrenal-like cells (hALCs) using chemically defined culture conditions. Factors critical for the development of LCs were added to both culture systems. hLLCs expressed all steroidogenic genes and proteins important for T biosynthesis, synthesized T rather than cortisol, secreted steroid hormones in response to dibutyryl-cAMP and 22(R)-hydroxycholesterol, and displayed ultrastructural features resembling LCs. By contrast, hALCs synthesized cortisol rather than T. The success in generating hiPSCderived hLLCs with broad human LC (hLC) features supports the potential for hiPSC-based hLC regeneration.

Declining rates of male fertility pose a significant clinical challenge, while the mechanisms underlying testicular interstitial function remain incompletely understood. Here, we conducted a comprehensive analysis of the single-cell transcriptomic landscape of the murine testicular interstitium across the postnatal lifespan. The investigation unveiled a previously unrecognized population of Cd34 + /Sox4 + mesenchymal cells nestled within the interstitium, hinting at their potential as Leydig cell (LC) progenitors. With the aging process of Cd34 + /Sox4 + mesenchymal cells, we observed a decline in glutathione levels within the testicular interstitium. Remarkably, these Cd34 + /Sox4 + mesenchymal cells exhibited clonogenic self-renewal capacity and a robust propensity to differentiate into LCs. Intriguingly, when transplanted into LC-disrupted or failure models, Cd34 + /Sox4 + cells efficiently colonized the testicular interstitium, resulting in a notable increase in testosterone production. Exploring the epigenetic landscape, we identified critical transcription factors, most notably Sox4, governing the stem cell fate of Cd34 + /Sox4 + mesenchymal cells. Overall, this comprehensive lifespan-resolved single-cell atlas of testicular interstitial cells provides fundamental insights into LC progenitor biology and regenerative capacity during aging.