Retinoic acid availability drives the asynchronous initiation of spermatogonial differentiation in the mouse

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

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

Carotenoids in female and male reproduction

Carotenoids are among the best-known pigments in nature, confer color to plants and animals, and are mainly derived from photosynthetic bacteria, fungi, algae, plants. Mammals cannot synthesize carotenoids. Carotenoids' source is only alimentary and after their assumption, they are mainly converted in retinal, retinol and retinoic acid, collectively known also as pro-vitamins and vitamin A, which play an essential role in tissue growth and regulate different aspects of the reproductive functions. However, their mechanisms of action and potential therapeutic effects are still unclear. This review aims to clarify the role of carotenoids in the male and female reproductive functions in species of veterinary interest. In female, carotenoids and their derivatives regulate not only folliculogenesis and oogenesis but also steroidogenesis. Moreover, they improve fertility by decreasing the risk of embryonic mortality. In male, retinol and retinoic acids activate molecular pathways related to spermatogenesis. Deficiencies of these vitamins have been correlated with degeneration of testis parenchyma with consequent absence of the mature sperm. Carotenoids have also been considered anti-antioxidants as they ameliorate the effect of free radicals. The mechanisms of action seem to be exerted by activating Kit and Stra8 pathways in both female and male. In conclusion, carotenoids have potentially beneficial effects for ameliorating ovarian and testes function.

Identification and Expression Pattern of cyp26b1 Gene in Gonad of the Chinese Tongue Sole (Cynoglossus semilaevis)

Simple Summary

In fish, it is obvious that the asynchronous development of the gonads and sexual dimorphism limit the development of aquaculture, so the research into sex-differentiation and gonadal growth is very important. Due to the sexual reversal phenomenon (genetic females becoming phenotypic males), the Chinese tongue sole (Cynoglossus semilaevis) is a great model for investigating sex-differentiation. Herein, we report one gene involved in sex-differentiation and gonadal growth of the Chinese Tongue Sole. The gene cyp26b1 (cytochrome P450 family 26 subfamily b member 1) is a metabolizing Retinoic Acid (RA) enzyme. Since it regulates RA to control sex determination and differentiation, cyp26b1 is considered a critical part of mammals’ ovary-antagonizing and testis-determining downstream passageway of Sry (sex-determining region Y) and Sox9 (sry-box transcription factor 9). In fish, the related research is reported only on the Japanese flounder (Paralichthys olivaceus) and zebrafish (Danio rerio). In the current investigation, the identification and expression pattern of the cyp26b1 gene in the Chinese tongue sole suggested that cyp26b1 might impact sex-differentiation and gonadal development.

Abstract

As an RA-metabolizing enzyme, cyp26b1 has a substantial impact on RA-signaling pathways. The cyp26b1 gene from the Chinese tongue sole was cloned and identified in this investigation. The cyp26b1 ORF was 1536 bp in length and encoded a 512 amino acid protein. A quantitative real-time PCR (qPCR) indicated that the cyp26b1 expression is no significant sexual dimorphism in the gonads at the 80 days post-hatching (dph) stages. After 4 months post-hatching (mph), the expression of cyp26b1 showed sexual dimorphism and lower level of expression in the ovaries than in the testes. An in situ hybridization demonstrated that cyp26b1 mRNA was primarily located in the testis. Interestingly, the cyp26b1 mRNA probe was also detected in the ovaries. These results suggested that cyp26b1 participates in the sex-differentiation and gonadal development of the Chinese tongue sole.

RBP4 regulates androgen receptor expression and steroid synthesis in Sertoli cells from Bactrian camels

Retinol-binding protein (RBP4) plays an important role in the transport and metabolism of retinol. In addition, RBP4 contributes to testicular homeostasis, including maintenance of spermatogenesis and synthesis of androgens that mediate their physiological functions through the androgen receptor. RBP4 in Sertoli cells regulates testosterone and dihydrotestosterone synthesis and secretion, although the mechanisms have yet to be revealed. In this study, we examined the expression and function of RBP4 in Sertoli cells isolated from Bactrian camels. qRT-PCR analysis of various Bactrian camel tissues revealed high expression of RBP4 in the testis and epididymis. To examine RBP4 function, Sertoli cells isolated from testes were transfected with an RBP4 overexpression plasmid or RBP4-targeting siRNA. RBP4 overexpression resulted in significant inhibition of transcription and translation of the steroidogenic enzymes 3βHSD and SRD5A1 concomitant with a significant decrease in androgen receptor expression and dihydrotestosterone secretion. Conversely, RBP4 knockdown significantly increased the expression of 3βHSD, SRD5A1, and androgen receptor and enhanced the secretion of dihydrotestosterone and testosterone. These data reveal a novel role for RBP4 in regulating steroid synthesis in Sertoli cells from Bactrian camels.

Disrupted spermatogenesis in a metabolic syndrome model: the role of vitamin A metabolism in the gut-testis axis

OBJECTIVE

Effects of the diet-induced gut microbiota dysbiosis reach far beyond the gut. We aim to uncover the direct evidence involving the gut-testis axis in the aetiology of impaired spermatogenesis.

DESIGN

An excessive-energy diet-induced metabolic syndrome (MetS) sheep model was established. The testicular samples, host metabolomes and gut microbiome were analysed. Faecal microbiota transplantation (FMT) confirmed the linkage between gut microbiota and spermatogenesis.

RESULTS

We demonstrated that the number of arrested spermatogonia was markedly elevated by using 10× single-cell RNA-seq in the MetS model. Furthermore, through using metabolomics profiling and 16S rDNA-seq, we discovered that the absorption of vitamin A in the gut was abolished due to a notable reduction of bile acid levels, which was significantly associated with reduced abundance of Ruminococcaceae_NK4A214_group. Notably, the abnormal metabolic effects of vitamin A were transferable to the testicular cells through the circulating blood, which contributed to abnormal spermatogenesis, as confirmed by FMT.

CONCLUSION

These findings define a starting point for linking the testicular function and regulation of gut microbiota via host metabolomes and will be of potential value for the treatment of male infertility in MetS.

Unique metabolic phenotype and its transition during maturation of juvenile male germ cells

Human male reproductive development has a prolonged prepubertal period characterized by juvenile quiescence of germ cells with immature spermatogonial stem cell (SSC) precursors (gonocytes) present in the testis for an extended period of time. The metabolism of gonocytes is not defined. We demonstrate with mitochondrial ultrastructure studies via TEM and IHC and metabolic flux studies with UHPLC-MS that a distinct metabolic transition occurs during the maturation to SSCs. The mitochondrial ultrastructure of prepubertal human spermatogonia is shared with prepubertal pig spermatogonia. The metabolism of early prepubertal porcine spermatogonia (gonocytes) is characterized by the reliance on OXPHOS fuelled by oxidative decarboxylation of pyruvate. Interestingly, at the same time, a high amount of the consumed pyruvate is also reduced and excreted as lactate. With maturation, prepubertal spermatogonia show a metabolic shift with decreased OXHPOS and upregulation of the anaerobic metabolism-associated uncoupling protein 2 (UCP2). This shift is accompanied with stem cell specific promyelocytic leukemia zinc finger protein (PLZF) protein expression and glial cell-derived neurotropic factor (GDNF) pathway activation. Our results demonstrate that gonocytes differently from mature spermatogonia exhibit unique metabolic demands that must be attained to enable their maintenance and growth in vitro.

Roles of Stra8 and Tcerg1l in retinoic acid induced spermatogonial differentiation in mouse†

Retinoic acid (RA) induces spermatogonial differentiation, but the mechanism by which it operates remains largely unknown. We developed a germ cell culture assay system to study genes involved in spermatogonial differentiation triggered by RA. Stimulated by RA 8 (Stra8), a RA-inducible gene, is indispensable for meiosis initiation, and its deletion results in a complete block of spermatogenesis at the pre-leptotene/zygotene stage. To interrogate the role of Stra8 in RA mediated differentiation of spermatogonia, we derived germ cell cultures from the neonatal testis of both wild type and Stra8 knock-out mice. We provide the first evidence that Stra8 plays a crucial role in modulating the responsiveness of undifferentiated spermatogonia to RA and facilitates transition to a differentiated state. Stra8-mediated differentiation is achieved through the downregulation of a large portfolio of genes and pathways, most notably including genes involved in the spermatogonial stem cell self-renewal process. We also report here for the first time the role of transcription elongation regulator-1 like (Tcerg1l) as a downstream effector of RA-induced spermatogonial differentiation.

Identification of regulatory elements required for Stra8 expression in fetal ovarian germ cells of the mouse

In mice, the entry of germ cells into meiosis crucially depends on the expression of stimulated by retinoic acid gene 8 (Stra8). Stra8 is expressed specifically in pre-meiotic germ cells of females and males, at fetal and postnatal stages, respectively, but the mechanistic details of its spatiotemporal regulation are yet to be defined. In particular, there has been considerable debate regarding whether retinoic acid is required, in vivo, to initiate Stra8 expression in the mouse fetal ovary. We show that the distinctive anterior-to-posterior pattern of Stra8 initiation, characteristic of germ cells in the fetal ovary, is faithfully recapitulated when 2.9 kb of the Stra8 promoter is used to drive eGFP expression. Using in vitro transfection assays of cutdown and mutant constructs, we identified two functional retinoic acid responsive elements (RAREs) within this 2.9 kb regulatory element. We also show that the transcription factor DMRT1 enhances Stra8 expression, but only in the presence of RA and the most proximal RARE. Finally, we used CRISPR/Cas9-mediated targeted mutation studies to demonstrate that both RAREs are required for optimal Stra8 expression levels in vivo.

FSH regulates RA signaling to commit spermatogonia into differentiation pathway and meiosis

BACKGROUND

Spermatogenesis is a complex process that is controlled by interactions between germ cells and somatic cells. The commitment of undifferentiated spermatogonia to differentiating spermatogonia and normal spermatogenesis requires the action of gonadotropins. Additionally, numerous studies revealed the role of retinoic acid signaling in induction of germ cell differentiation and meiosis entry.

MAIN TEXT

Recent studies have shown that expression of several RA signaling molecules including Rdh10, Aldh1a2, Crabp1/2 are influenced by changes in gonadotropin levels. Components of signaling pathways that are regulated by FSH signaling such as GDNF, Sohlh1/2, c-Kit, DMRT, BMP4 and NRGs along with transcription factors that are important for proliferation and differentiation of spermatogonia are also affected by retinoic acid signaling.

CONCLUSION

According to all studies that demonstrate the interface between FSH and RA signaling, we suggest that RA may trigger spermatogonia differentiation and initiation of meiosis through regulation by FSH signaling in testis. Therefore, to the best of our knowledge, this is the first time that the correlation between FSH and RA signaling in spermatogenesis is highlighted.

Activation of the cannabinoid receptor type 2 by the agonist JWH133 promotes the first wave of in vitro spermatogenesis

BACKGROUND

Oncological procedures have irreversible side effects on germ cells for childhood cancer survival boys. In vitro culture of prepubertal testicular tissue has been proposed to restore fertility; however, recent data on animal models showed that meiotic and post-meiotic progression was impaired.

OBJECTIVES

As potential key inducers of the mitosis-meiosis switch, type 2 cannabinoid receptor (CB₂ ) has been proposed to play a central role in the meiotic entry of male germ cells. Herein, the in vitro first spermatogenesis wave in mice was used to understand the impact of CB₂ activation on the differentiation of spermatogonia until elongated spermatids.

MATERIALS AND METHODS

A first set of cultured testicular explants of 6.5 days post-partum (dpp) mice was performed to assess the impact of a range of JWH133 supplementation (10 nm, 100 nm, 1 µm, 10 µm). Then, the progressive development of germ cells at key timepoints of spermatogenesis was evaluated throughout (i) in vitro culture (day 2 [D2], D3, D6, D10, D18, and D30) coupled with (ii) in vivo counterparts (8.5, 9.5, 12.5, 16.5, 24.5, and 36.5 dpp).

RESULTS

CB₂ was detected at the plasma membrane of cells, and a successful completion of spermatogenesis was obtained in vitro. One day after the activation of CB₂ by 1 μm of the agonist JWH133, percentage of zygotene spermatocyte I increased.

CONCLUSION

After 30 days of culture, (i) an enrichment of haploid germ cells detected by flow cytometry, (ii) a reduced necrotic area, and (iii) an increase in the density of post-meiotic germ cells were observed. We showed that the activation of CB₂ improves in vitro entry into meiosis and differentiation of spermatogonia, mimicking physiological meiotic transition.

The rapamycin analog Everolimus reversibly impairs male germ cell differentiation and fertility in the mouse†

Sirolimus, also known as rapamycin, and its closely related rapamycin analog (rapalog) Everolimus inhibit "mammalian target of rapamycin complex 1" (mTORC1), whose activity is required for spermatogenesis. Everolimus is Food and Drug Administration approved for treating human patients to slow growth of aggressive cancers and preventing organ transplant rejection. Here, we test the hypothesis that rapalog inhibition of mTORC1 activity has a negative, but reversible, impact upon spermatogenesis. Juvenile (P20) or adult (P>60) mice received daily injections of sirolimus or Everolimus for 30 days, and tissues were examined at completion of treatment or following a recovery period. Rapalog treatments reduced body and testis weights, testis weight/body weight ratios, cauda epididymal sperm counts, and seminal vesicle weights in animals of both ages. Following rapalog treatment, numbers of differentiating spermatogonia were reduced, with concomitant increases in the ratio of undifferentiated spermatogonia to total number of remaining germ cells. To determine if even low doses of Everolimus can inhibit spermatogenesis, an additional group of adult mice received a dose of Everolimus ∼6-fold lower than a human clinical dose used to treat cancer. In these animals, only testis weights, testis weight/body weight ratios, and tubule diameters were reduced. Return to control values following a recovery period was variable for each of the measured parameters and was duration and dose dependent. Together, these data indicate rapalogs exerted a dose-dependent restriction on overall growth of juvenile and adult mice and negative impact upon spermatogenesis that were largely reversed; following treatment cessation, males from all treatment groups were able to sire offspring.

ADAD1 and ADAD2, testis-specific adenosine deaminase domain-containing proteins, are required for male fertility

Adenosine-to-inosine RNA editing, a fundamental RNA modification, is regulated by adenosine deaminase (AD) domain containing proteins. Within the testis, RNA editing is catalyzed by ADARB1 and is regulated in a cell-type dependent manner. This study examined the role of two testis-specific AD domain proteins, ADAD1 and ADAD2, on testis RNA editing and male germ cell differentiation. ADAD1, previously shown to localize to round spermatids, and ADAD2 had distinct localization patterns with ADAD2 expressed predominantly in mid- to late-pachytene spermatocytes suggesting a role for both in meiotic and post-meiotic germ cell RNA editing. AD domain analysis showed the AD domain of both ADADs was likely catalytically inactive, similar to known negative regulators of RNA editing. To assess the impact of Adad mutation on male germ cell RNA editing, CRISPR-induced alleles of each were generated in mouse. Mutation of either Adad resulted in complete male sterility with Adad1 mutants displaying severe teratospermia and Adad2 mutant germ cells unable to progress beyond round spermatid. However, mutation of neither Adad1 nor Adad2 impacted RNA editing efficiency or site selection. Taken together, these results demonstrate ADAD1 and ADAD2 are essential regulators of male germ cell differentiation with molecular functions unrelated to A-to-I RNA editing.

Retinoic Acid and Germ Cell Development in the Ovary and Testis

Retinoic acid (RA), a derivative of vitamin A, is critical for the production of oocytes and sperm in mammals. These gametes derive from primordial germ cells, which colonize the nascent gonad, and later undertake sexual differentiation to produce oocytes or sperm. During fetal development, germ cells in the ovary initiate meiosis in response to RA, whereas those in the testis do not yet initiate meiosis, as they are insulated from RA, and undergo cell cycle arrest. After birth, male germ cells resume proliferation and undergo a transition to spermatogonia, which are destined to develop into haploid spermatozoa via spermatogenesis. Recent findings indicate that RA levels change periodically in adult testes to direct not only meiotic initiation, but also other key developmental transitions to ensure that spermatogenesis is precisely organized for the prodigious output of sperm. This review focuses on how female and male germ cells develop in the ovary and testis, respectively, and the role of RA in this process.

Cycles, waves, and pulses: Retinoic acid and the organization of spermatogenesis

Background

Spermatogenesis in mammals is organized in a manner that maximizes sperm production. The central aspect of this organization is the cycle of the seminiferous epithelium that is characterized by an asynchronous repeating series of germ cell associations. These cell associations are the result of a fixed point of entry into the cycle at regular short time intervals and the longer time required for cells to fully differentiate and exit the cycle.

Objective

This review will examine the current information on the action and metabolism of retinoic acid in the testis, the interaction of retinoic acid (RA) with the cycle and the spermatogenic wave, and the mechanisms that can lead to synchronous spermatogenesis. Finally, the unique applications of synchronous spermatogenesis to the study of the cycle and the mass isolation of specific germ cell populations are described.

Materials and methods

Retinoic acid metabolism and spermatogonial differentiation have been examined by gene deletions, immunocytochemistry, chemical inhibitors, and mass spectrometry.

Results, discussion, and conclusion

Both the Sertoli cells and the germ cells have the capacity to synthesize retinoic acid from retinol and in the mouse the entry into the cycle of the seminiferous epithelium, and the subsequent conversion of undifferentiated spermatogonia into differentiating spermatogonia is governed by a peak of RA synthesis occurring at stages VIII‐IX of the cycle. Normal asynchronous spermatogenesis can be modified by altering RA levels, and as a result the entire testis will consist of a few closely related stages of the cycle.

Testis Development

Production of sperm and androgens is the main function of the testis. This depends on normal development of both testicular somatic cells and germ cells. A genetic program initiated from the Y chromosome gene sex-determining region Y (SRY) directs somatic cell specification to Sertoli cells that orchestrate further development. They first guide fetal germ cell differentiation toward spermatogenic destiny and then take care of the full service to spermatogenic cells during spermatogenesis. The number of Sertoli cells sets the limits of sperm production. Leydig cells secrete androgens that determine masculine development. Testis development does not depend on germ cells; that is, testicular somatic cells also develop in the absence of germ cells, and the testis can produce testosterone normally to induce full masculinization in these men. In contrast, spermatogenic cell development is totally dependent on somatic cells. We herein review germ cell differentiation from primordial germ cells to spermatogonia and development of the supporting somatic cells. Testicular descent to scrota is necessary for normal spermatogenesis, and cryptorchidism is the most common male birth defect. This is a mild form of a disorder of sex differentiation. Multiple genetic reasons for more severe forms of disorders of sex differentiation have been revealed during the last decades, and these are described along with the description of molecular regulation of testis development.

50 years of spermatogenesis: Sertoli cells and their interactions with germ cells

The complex morphology of the Sertoli cells and their interactions with germ cells has been a focus of investigators since they were first described by Enrico Sertoli. In the past 50 years, information on Sertoli cells has transcended morphology alone to become increasingly more focused on molecular questions. The goal of investigators has been to understand the role of the Sertoli cells in spermatogenesis and to apply that information to problems relating to male fertility. Sertoli cells are unique in that they are a nondividing cell population that is active for the reproductive lifetime of the animal and cyclically change morphology and gene expression. The numerous and distinctive junctional complexes and membrane specializations made by Sertoli cells provide a scaffold and environment for germ cell development. The increased focus of investigators on the molecular components and putative functions of testicular cells has resulted primarily from procedures that isolate specific cell types from the testicular milieu. Products of Sertoli cells that influence germ cell development and vice versa have been characterized from cultured cells and from the application of transgenic technologies. Germ cell transplantation has shown that the Sertoli cells respond to cues from germ cells with regard to developmental timing and has furthered a focus on spermatogenic stem cells and the stem cell niche. Very basic and universal features of spermatogenesis such as the cycle of the seminiferous epithelium and the spermatogenic wave are initiated by Sertoli cells and maintained by Sertoli-germ cell cooperation.

The roles of retinoic acid in the differentiation of spermatogonia and spermatogenic disorders

Male fertility depends on the regulatory balance between germ cell self-renewal and differentiation, and the spatial and temporal patterns of this balance must be maintained throughout the life cycle. Retinoic acid and its receptors are important factors in spermatogenesis. Spermatogonia cells can self-proliferate and differentiate and have unique meiotic capabilities; they halve their genetic material and produce monomorphic sperm to pass genetic material to the next generation. A number of studies have found that the spermatogenesis process is halted in animals with vitamin A deficiency and that most germ cells are degraded, but they tend to recover after treatment with RA or vitamin A. This literature review discusses our understanding of how RA regulates sperm cell differentiation and meiosis and also reviews the functional information and details of RA.

Retinoic acid signaling pathways

Retinoic acid (RA), a metabolite of retinol (vitamin A), functions as a ligand for nuclear RA receptors (RARs) that regulate development of chordate animals. RA-RARs can activate or repress transcription of key developmental genes. Genetic studies in mouse and zebrafish embryos that are deficient in RA-generating enzymes or RARs have been instrumental in identifying RA functions, revealing that RA signaling regulates development of many organs and tissues, including the body axis, spinal cord, forelimbs, heart, eye and reproductive tract. An understanding of the normal functions of RA signaling during development will guide efforts for use of RA as a therapeutic agent to improve human health. Here, we provide an overview of RA signaling and highlight its key functions during development.

Differential RA responsiveness directs formation of functionally distinct spermatogonial populations at the initiation of spermatogenesis in the mouse

In the mammalian testis, sustained spermatogenesis relies on spermatogonial stem cells (SSCs); their progeny either remain as stem cells (self-renewal) or proliferate and differentiate to enter meiosis in response to retinoic acid (RA). Here, we sought to uncover elusive mechanisms regulating a key switch fundamental to spermatogonial fate: the capacity of spermatogonia to respond to RA. Using the developing mouse testis as a model, we found that spermatogonia and precursor prospermatogonia exhibit a heterogeneous capacity to respond to RA with at least two underlying causes. First, progenitor spermatogonia are prevented from responding to RA by catabolic activity of cytochrome P450 family 26 enzymes. Second, a smaller subset of undifferentiated spermatogonia enriched for SSCs exhibit catabolism-independent RA insensitivity. Moreover, for the first time, we observed that precursor prospermatogonia are heterogeneous and comprise subpopulations that exhibit the same differential RA responsiveness found in neonatal spermatogonia. We propose a novel model by which mammalian prospermatogonial and spermatogonial fates are regulated by their intrinsic capacity to respond (or not) to the differentiation signal provided by RA before, and concurrent with, the initiation of spermatogenesis.

Identification of candidate genes in regulation of spermatogenesis in sheep testis following dietary vitamin E supplementation

Dietary vitamin E supplementation is beneficial to semen quality in different sheep and goat breeds. The aim of this research was to further investigate the effect of vitamin E in sheep on spermatogenesis and its regulatory mechanisms using RNA-seq. Thirty male Hu lambs were randomly divided into three groups. The animals received 0, 200 or 2000 IU/day vitamin E dietary supplementation for 105 days, and its effects were subsequently evaluated. The results indicate vitamin E supplementation increased the number of germ cells in the testes and epididymides. The positive effects were reduced, however, in animals that received 2000 IU/d vitamin E. Using the RNA-seq procedure, there was detection of a number of differentially expressed genes such as NDRG1, FSCN3 and CYP26B1 with these genes being mainly related to the regulation of spermatogenesis. Supplementation with 2000 IU/d vitamin E supplementation resulted in a lesser abundance of skeleton-related transcripts such as TUBB, VIM and different subtypes of collagen, and there was also an effect on the ECM-receptor interaction pathway. These changes appear to be responsible for the lesser beneficial effect of the greater vitamin E concentrations. The results provide a novel insight into the regulation of spermatogenesis by vitamin E at the molecular level, however, for a precise understanding of functions of the affected genes there needs to be further study.

Sexually dimorphic germ cell identity in mammals

Germ cells are the stem cells of the species. Thus, it is critical that we have a good understanding of how they are specified, how the somatic cells instruct and support them, how they commit to one or other sex, and how they ultimately develop into functional gametes. Here, we focus on specifics of how sexual fate is determined during fetal life. Because the majority of relevant experimental work has been done using the mouse model, we focus on that species. We review evidence regarding the identity of instructive signals from the somatic cells, and the molecular responses that occur in germ cells in response to those extrinsic signals. In this way we aim to clarify progress to date regarding the mechanisms underlying the mitotic to meiosis switch in germ cells of the fetal ovary, and those involved in adopting and securing male fate in germ cells of the fetal testis.

Isolating mitotic and meiotic germ cells from male mice by developmental synchronization, staging, and sorting

Isolating discrete populations of germ cells from the mouse testis is challenging, because the adult testis contains germ cells at every step of spermatogenesis, in addition to somatic cells. We present a novel method for isolating precise, high-purity populations of male germ cells. We first synchronize germ cell development in vivo by manipulating retinoic acid metabolism, and perform histological staging to verify synchronization. We use fluorescence-activated cell sorting to separate the synchronized differentiating germ cells from contaminating somatic cells and undifferentiated spermatogonia. We achieve ~90% purity at each step of development from undifferentiated spermatogonia through late meiotic prophase. Utilizing this "3 S" method (synchronize, stage, and sort), we can separate germ cell types that were previously challenging or impossible to distinguish, with sufficient yield for epigenetic and biochemical studies. 3 S expands the toolkit of germ cell sorting methods, and should facilitate detailed characterization of molecular and biochemical changes that occur during the mitotic and meiotic phases of spermatogenesis.

Retinoic acid receptor signaling is necessary in steroidogenic cells for normal spermatogenesis and epididymal function

Spermatogenesis in mammals is a very complex, highly organized process, regulated in part by testosterone and retinoic acid (RA). Much is known about how RA and testosterone signaling pathways independently regulate this process, but there is almost no information regarding whether these two signaling pathways directly interact and whether RA is crucial for steroidogenic cell function. This study uses a transgenic mouse line that expresses a dominant-negative form of RA receptor α (RAR-DN) and the steroidogenic cell-specific Cre mouse line, Cyp17iCre, to generate male mice with steroidogenic cells unable to perform RA signaling. Testes of mutant mice displayed increased apoptosis of pachytene spermatocytes, an increased number of macrophages in the interstitium and a loss of advanced germ cells. Additionally, blocking RA signaling in Leydig cells resulted in increased permeability of the blood-testis barrier, decreased levels of the steroidogenic enzyme cytochrome P450 17a1 and decreased testosterone levels. Surprisingly, the epididymides of the mutant mice also displayed an abnormal phenotype. This study demonstrates that RA signaling is required in steroidogenic cells for their normal function and, thus, for male fertility.

Beyond stem cells: Commitment of progenitor cells to meiosis

The first step in established spermatogenesis is the production of progenitor cells by the stem cell population. The progenitor cells (undifferentiated A spermatogonia) expand in number via the formation of syncytial chains by mitosis. The mechanism by which these progenitor cells commit to meiosis and spermatogenesis is tightly controlled and results in complex morphological organization all of which is designed to efficiently achieve large numbers of spermatozoa. The major extrinsic factor that triggers the commitment to meiosis and establishes the structural complexity is retinoic acid (RA). Retinoic acid is produced from retinol via two oxidation steps in low abundance near its site of action. The action of RA on undifferentiated A spermatogonia results in the timed progression of these progenitor cells into the cycle of the seminiferous epithelium. We have utilized a drug WIN 18,446 that inhibits the second oxidation step in RA biosynthesis to block the progression of undifferentiated A spermatogonia in the mouse testis. As a result of this block the undifferentiated progenitor cells accumulate but do not differentiate into A1 spermatogonia. When the block is released and a bolus of RA is simultaneously administered the accumulated spermatogonia progress through the differentiation pathway in complete synchrony and maintain that synchrony with regard to stages of the cycle of the seminiferous epithelium for several months. This procedure allowed us to accumulate sufficient material to measure retinoic acid levels across the cycle and will allow us to isolate and analyze large number of progenitor cells proceeding synchronously down the pathway to meiosis. We have been able to show that the cycle of the seminiferous epithelium is established and maintained by pulses of RA that appear at stages VIII and IX of the cycle.

MAFB is dispensable for the fetal testis morphogenesis and the maintenance of spermatogenesis in adult mice

The transcription factor MAFB is an important regulator of the development and differentiation of various organs and tissues. Previous studies have shown that MAFB is expressed in embryonic and adult mouse testes and is expected to act as the downstream target of retinoic acid (RA) to initiate spermatogenesis. However, its exact localization and function remain unclear. Here, we localized MAFB expression in embryonic and adult testes and analyzed its gene function using Mafb-deficient mice. We found that MAFB and c-MAF are the only large MAF transcription factors expressed in testes, while MAFA and NRL are not. MAFB was localized in Leydig and Sertoli cells at embryonic day (E) 18.5 but in Leydig cells, Sertoli cells, and pachytene spermatocytes in adults. Mafb-deficient testes at E18.5 showed fully formed seminiferous tubules with no abnormal structure or differences in testicular somatic cell numbers compared with those of control wild-type mice. Additionally, the expression levels of genes related to development and function of testicular cells were unchanged between genotypes. In adults, the expression of MAFB in Sertoli cells was shown to be stage specific and induced by RA. By generating Mafb^fl/fl CAG-CreER^™ (Mafb-cKO) mice, in which Cre recombinase was activated upon tamoxifen treatment, we found that the neonatal cKO mice died shortly upon Mafb deletion, but adult cKO mice were alive upon deletion. Adult cKO mice were fertile, and spermatogenesis maintenance was normal, as indicated by histological analysis, hormone levels, and germ cell stage-specific markers. Moreover, there were no differences in the proportion of seminiferous stages between cKO mice and controls. However, RNA-Seq analysis of cKO Sertoli cells revealed that the down-regulated genes were related to immune function and phagocytosis activity but not spermatogenesis. In conclusion, we found that MAFB is dispensable for fetal testis morphogenesis and spermatogenesis maintenance in adult mice, despite the significant gene expression in different cell types, but MAFB might be critical for phagocytosis activity of Sertoli cells.

Testicular adenosine to inosine RNA editing in the mouse is mediated by ADARB1

Adenosine to inosine (A-to-I) RNA editing occurs in a wide range of tissues and cell types and can be catalyzed by one of the two adenosine deaminase acting on double-stranded RNA enzymes, ADAR and ADARB1. Editing can impact both coding and noncoding regions of RNA, and in higher organisms has been proposed to function in adaptive evolution. Neither the prevalence of A-to-I editing nor the role of either ADAR or ADARB1 has been examined in the context of germ cell development in mammals. Computational analysis of whole testis and cell-type specific RNA-sequencing data followed by molecular confirmation demonstrated that A-to-I RNA editing occurs in both the germ line and in somatic Sertoli cells in two targets, Cog3 and Rpa1. Expression analysis demonstrated both Adar and Adarb1 were expressed in both Sertoli cells and in a cell-type dependent manner during germ cell development. Conditional ablation of Adar did not impact testicular RNA editing in either germ cells or Sertoli cells. Additionally, Adar ablation in either cell type did not have gross impacts on germ cell development or male fertility. In contrast, global Adarb1 knockout animals demonstrated a complete loss of A-to-I RNA editing in spite of normal germ cell development. Taken together, these observations demonstrate ADARB1 mediates A-to-I RNA editing in the testis and these editing events are dispensable for male fertility in an inbred mouse strain in the lab.

Periodic production of retinoic acid by meiotic and somatic cells coordinates four transitions in mouse spermatogenesis

Mammalian spermatogenesis is an elaborately organized differentiation process, starting with diploid spermatogonia, which include germ-line stem cells, and ending with haploid spermatozoa. The process involves four pivotal transitions occurring in physical proximity: spermatogonial differentiation, meiotic initiation, initiation of spermatid elongation, and release of spermatozoa. We report how the four transitions are coordinated in mice. Two premeiotic transitions, spermatogonial differentiation and meiotic initiation, were known to be coregulated by an extrinsic signal, retinoic acid (RA). Our chemical manipulations of RA levels in mouse testes now reveal that RA also regulates the two postmeiotic transitions: initiation of spermatid elongation and spermatozoa release. We measured RA concentrations and found that they changed periodically, as also reflected in the expression patterns of an RA-responsive gene, STRA8; RA levels were low before the four transitions, increased when the transitions occurred, and remained elevated thereafter. We found that pachytene spermatocytes, which express an RA-synthesizing enzyme, Aldh1a2, contribute directly and significantly to RA production in testes. Indeed, chemical and genetic depletion of pachytene spermatocytes revealed that RA from pachytene spermatocytes was required for the two postmeiotic transitions, but not for the two premeiotic transitions. We conclude that the premeiotic transitions are coordinated by RA from Sertoli (somatic) cells. Once germ cells enter meiosis, pachytene spermatocytes produce RA to coordinate the two postmeiotic transitions. In combination, these elements underpin the spatiotemporal coordination of spermatogenesis and ensure its prodigious output in adult males.

Retinoic acid deficiency leads to an increase in spermatogonial stem number in the neonatal mouse testis, but excess retinoic acid results in no change

The onset of spermatogenesis occurs in response to retinoic acid (RA), the active metabolite of vitamin A. However, whether RA plays any role during establishment of the spermatogonial stem cell (SSC) pool is unknown. Because designation of the SSC population and the onset of RA signaling in the testis that induces differentiation have similar timing, this study asked whether RA influenced SSC establishment. Whole mount immunofluorescence and flow cytometric analysis using the Id4-eGfp transgenic reporter mouse line revealed an enrichment for ID4-EGFP+ cells within the testis following inhibition of RA synthesis by WIN 18,446 treatment. Transplantation analyses confirmed a significant increase in the number of SSCs in testes from RA-deficient animals. Conversely, no difference in the ID4-EGFP+ population or change in SSC number were detected following exposure to an excess of RA. Collectively, reduced RA altered the number of SSCs present in the neonatal testis but precocious RA exposure in the neonatal testis did not, suggesting that RA deficiency causes a greater proportion of progenitor undifferentiated spermatogonia to retain their SSC state past the age when the pool is thought to be determined.

Retinoic acid signaling is dispensable for somatic development and function in the mammalian ovary

Retinoic acid (RA) is a potent inducer of cell differentiation and plays an essential role in sex-specific germ cell development in the mammalian gonad. RA is essential for male gametogenesis and hence fertility. However, RA can also disrupt sexual cell fate in somatic cells of the testis, promoting transdifferentiation of male Sertoli cells to female granulosa-like cells when the male sexual regulator Dmrt1 is absent. The feminizing ability of RA in the Dmrt1 mutant somatic testis suggests that RA might normally play a role in somatic cell differentiation or cell fate maintenance in the ovary. To test for this possibility we disrupted RA signaling in somatic cells of the early fetal ovary using three genetic strategies and one pharmaceutical approach. We found that deleting all three RA receptors (RARs) in the XX somatic gonad at the time of sex determination did not significantly affect ovarian differentiation, follicle development, or female fertility. Transcriptome analysis of adult triple mutant ovaries revealed remarkably little effect on gene expression in the absence of somatic RAR function. Likewise, deletion of three RA synthesis enzymes (Aldh1a1-3) at the time of sex determination did not masculinize the ovary. A dominant-negative RAR transgene altered granulosa cell proliferation, likely due to interference with a non-RA signaling pathway, but did not prevent granulosa cell specification and oogenesis or abolish fertility. Finally, culture of fetal XX gonads with an RAR antagonist blocked germ cell meiotic initiation but did not disrupt sex-biased gene expression. We conclude that RA signaling, although crucial in the ovary for meiotic initiation, is not required for granulosa cell specification, differentiation, or reproductive function.

TSPAN8 Expression Distinguishes Spermatogonial Stem Cells in the Prepubertal Mouse Testis

Precise separation of spermatogonial stem cells (SSCs) from progenitor spermatogonia that lack stem cell activity and are committed to differentiation remains a challenge. To distinguish between these spermatogonial subtypes, we identified genes that exhibited bimodal mRNA levels at the single-cell level among undifferentiated spermatogonia from Postnatal Day 6 mouse testes, including Tspan8, Epha2, and Pvr, each of which encode cell surface proteins useful for cell selection. Transplantation studies provided definitive evidence that a TSPAN8-high subpopulation is enriched for SSCs. RNA-seq analyses identified genes differentially expressed between TSPAN8-high and -low subpopulations that clustered into multiple biological pathways potentially involved in SSC renewal or differentiation, respectively. Methyl-seq analysis identified hypomethylated domains in the promoters of these genes in both subpopulations that colocalized with peaks of histone modifications defined by ChIP-seq analysis. Taken together, these results demonstrate functional heterogeneity among mouse undifferentiated spermatogonia and point to key biological characteristics that distinguish SSCs from progenitor spermatogonia.

Control of Meiotic Crossovers: From Double-Strand Break Formation to Designation

Meiosis, the mechanism of creating haploid gametes, is a complex cellular process observed across sexually reproducing organisms. Fundamental to meiosis is the process of homologous recombination, whereby DNA double-strand breaks are introduced into the genome and are subsequently repaired to generate either noncrossovers or crossovers. Although homologous recombination is essential for chromosome pairing during prophase I, the resulting crossovers are critical for maintaining homolog interactions and enabling accurate segregation at the first meiotic division. Thus, the placement, timing, and frequency of crossover formation must be exquisitely controlled. In this review, we discuss the proteins involved in crossover formation, the process of their formation and designation, and the rules governing crossovers, all within the context of the important landmarks of prophase I. We draw together crossover designation data across organisms, analyze their evolutionary divergence, and propose a universal model for crossover regulation.

Characterizing the Spermatogonial Response to Retinoic Acid During the Onset of Spermatogenesis and Following Synchronization in the Neonatal Mouse Testis

Retinoic acid (RA), the active metabolite of vitamin A, is known to be required for the differentiation of spermatogonia. The first round of spermatogenesis initiates in response to RA and occurs in patches along the length of the seminiferous tubule. However, very little is known about the individual differentiating spermatogonial populations and their progression through the cell cycle due to the heterogeneous nature of the onset of spermatogenesis. In this study, we utilized WIN 18,446 and RA as tools to generate testes enriched with different populations of spermatogonia to further investigate 1) the undifferentiated to differentiating spermatogonial transition, 2) the progression of the differentiating spermatogonia through the cell cycle, and 3) Sertoli cell number in response to altered RA levels. WIN 18,446/RA-treated neonatal mice were used to determine when synchronous S phases occurred in the differentiating spermatogonial population following treatment. Five differentiating spermatogonial S phase windows were identified between spermatogonial differentiation and formation of preleptotene spermatocytes. In addition, a slight increase in Sertoli cell number was observed following RA treatment, possibly implicating a role for RA in Sertoli cell cycle progression. This study has enhanced our understanding of the spermatogonial populations present in the neonatal testis during the onset of spermatogenesis by mapping the cell cycle kinetics of both the undifferentiated and the differentiating spermatogonial populations and identifying the precise timing of when specific individual differentiating spermatogonial populations are enriched within the testis following synchrony, thus providing an essential tool for further study of the differentiating spermatogonia.

Spermatogonial cells: mouse, monkey and man comparison

In all mammals, spermatogonia are defined as constituting the mitotic compartment of spermatogenesis including stem, undifferentiated and differentiating cell types, possessing distinct morphological and molecular characteristics. Even though the real nature of the spermatogonial stem cell and its regulation is still debated the general consensus holds that in steady-state spermatogenesis the stem cell compartment needs to balance differentiation versus self-renewal. This review highlights current understanding of spermatogonial biology, the kinetics of amplification and the signals directing spermatogonial differentiation in mammals. The focus will be on relevant similarities and differences between rodents and non human and human primates.

Spermatogenesis: The Commitment to Meiosis

Mammalian spermatogenesis requires a stem cell pool, a period of amplification of cell numbers, the completion of reduction division to haploid cells (meiosis), and the morphological transformation of the haploid cells into spermatozoa (spermiogenesis). The net result of these processes is the production of massive numbers of spermatozoa over the reproductive lifetime of the animal. One study that utilized homogenization-resistant spermatids as the standard determined that human daily sperm production (dsp) was at 45 million per day per testis (60). For each human that means ∼1,000 sperm are produced per second. A key to this level of gamete production is the organization and architecture of the mammalian testes that results in continuous sperm production. The seemingly complex repetitious relationship of cells termed the "cycle of the seminiferous epithelium" is driven by the continuous commitment of undifferentiated spermatogonia to meiosis and the period of time required to form spermatozoa. This commitment termed the A to A1 transition requires the action of retinoic acid (RA) on the undifferentiated spermatogonia or prospermatogonia. In stages VII to IX of the cycle of the seminiferous epithelium, Sertoli cells and germ cells are influenced by pulses of RA. These pulses of RA move along the seminiferous tubules coincident with the spermatogenic wave, presumably undergoing constant synthesis and degradation. The RA pulse then serves as a trigger to commit undifferentiated progenitor cells to the rigidly timed pathway into meiosis and spermatid differentiation.

The Sertoli cell: one hundred fifty years of beauty and plasticity

It has been one and a half centuries since Enrico Sertoli published the seminal discovery of the testicular 'nurse cell', not only a key cell in the testis, but indeed one of the most amazing cells in the vertebrate body. In this review, we begin by examining the three phases of morphological research that have occurred in the study of Sertoli cells, because microscopic anatomy was essentially the only scientific discipline available for about the first 75 years after the discovery. Biochemistry and molecular biology then changed all of biological sciences, including our understanding of the functions of Sertoli cells. Immunology and stem cell biology were not even topics of science in 1865, but they have now become major issues in our appreciation of Sertoli cell's role in spermatogenesis. We end with the universal importance and plasticity of function by comparing Sertoli cells in fish, amphibians, and mammals. In these various classes of vertebrates, Sertoli cells have quite different modes of proliferation and epithelial maintenance, cystic vs. tubular formation, yet accomplish essentially the same function but in strikingly different ways.

Germ Cell Commitment to Oogenic Versus Spermatogenic Pathway: The Role of Retinoic Acid

The core of the decision to commit to either oogenesis or spermatogenesis lies in the timing of meiotic entry. Primordial germ cells within the fetal ovary become committed to the female pathway prior to birth and enter meiosis during embryonic development. In the fetal testis, however, the germ cells are protected from this signal before birth and instead receive this trigger postnatally. There is a growing body of evidence to indicate that RA is the meiosis-inducing factor in both sexes, with the gender-specific timing of meiotic entry controlled via degradation of this molecule only within the fetal testis. This chapter will review our current understanding of how RA controls germ cell fate in both the embryonic ovary and postnatal testis, highlighting the key studies that have led to the hypothesis that RA can drive the commitment to meiosis in both sexes and discussing the current debate over whether RA truly is the meiosis-inducing factor in the fetal ovary.

A Role of MicroRNAs in Cell Differentiation During Gonad Development

MicroRNAs (miRNAs) are a group of small noncoding RNA molecules that play a major role in posttranscriptional regulation of gene expression and are expressed in an organ-specific manner. One miRNA can potentially regulate the expression of several genes, depending on cell type and differentiation stage. miRNAs are differentially expressed in the male and female gonads and have an organ-specific reproductive function. Exerting their affect through germ cells and gonadal somatic cells, miRNAs regulate key proteins necessary for gonad development. The role of miRNAs in the testes is only starting to emerge though they have been shown to be required for adequate spermatogenesis. Widely explored in the ovary, miRNAs were suggested to play a fundamental role in follicles' assembly, growth, differentiation, and ovulation. In this chapter, we focus on data obtained from mice in which distinct proteins that participate in the biosynthesis of miRNAs were conditionally knocked out from germ cells (spermatogonial cells or oocytes) or gonadal somatic cells (Sertoli or granulosa cells). We detail recent advances in identification of particular miRNAs and their significance in the development and function of male and female gonads. miRNAs can serve as biomarkers and therapeutic agents of pathological conditions; thus, elucidating the branched and complex network of reproduction-related miRNAs will aid understanding of gonads' physiology and managing reproduction disorders.

Anti-inflammatory potential of β-cryptoxanthin against LPS-induced inflammation in mouse Sertoli cells

β-cryptoxanthin (CX), a major carotenoid pigment, can inhibit inflammatory gene expression in mice with nonalcoholic steatohepatitis. In the present study, we examined the anti-inflammatory effects of CX on lipopolysaccharide (LPS)-induced inflammation in mouse primary Sertoli cells and the possible molecular mechanisms behind its effects. The results showed that CX significantly inhibited LPS-induced decreases in cell viability and in the percentage of apoptotic cells. Moreover, CX inhibited the LPS-induced up-regulation of tumor necrosis factor α (TNF-α), interleukin-10 (IL-10), interleukin-6 (IL-6) and interleukin-1β (IL-1β) in Sertoli cells. In addition, CX significantly limited the LPS-induced down-regulation of AR, HSF2, CREB, FSHR, INHBB and ABP in Sertoli cells. Western blot analysis showed that CX significantly suppressed NF-κB (p65) activation as well as MAPK phosphorylation. All the results suggested that CX suppressed inflammation, possibly associated with the NF-κB activation and MAPK of phosphorylation. Thus, CX may possess therapeutic potential against inflammation-related diseases.

Retinoic Acid Receptors Control Spermatogonia Cell-Fate and Induce Expression of the SALL4A Transcription Factor

All-trans retinoic acid (ATRA) is instrumental to male germ cell differentiation, but its mechanism of action remains elusive. To address this question, we have analyzed the phenotypes of mice lacking, in spermatogonia, all rexinoid receptors (RXRA, RXRB and RXRG) or all ATRA receptors (RARA, RARB and RARG). We demonstrate that the combined ablation of RXRA and RXRB in spermatogonia recapitulates the set of defects observed both upon ablation of RAR in spermatogonia. We also show that ATRA activates RAR and RXR bound to a conserved regulatory region to increase expression of the SALL4A transcription factor in spermatogonia. Our results reveal that this major pluripotency gene is a target of ATRA signaling and that RAR/RXR heterodimers are the functional units driving its expression in spermatogonia. They add to the mechanisms through which ATRA promote expression of the KIT tyrosine kinase receptor to trigger a critical step in spermatogonia differentiation. Importantly, they indicate also that meiosis eventually occurs in the absence of a RAR/RXR pathway within germ cells and suggest that instructing this process is either ATRA-independent or requires an ATRA signal originating from Sertoli cells.

Mammalian target of rapamycin complex 1 (mTORC1) Is required for mouse spermatogonial differentiation in vivo

Spermatogonial stem cells (SSCs) must balance self-renewal with production of transit-amplifying progenitors that differentiate in response to retinoic acid (RA) before entering meiosis. This self-renewal vs. differentiation spermatogonial fate decision is critical for maintaining tissue homeostasis, as imbalances cause spermatogenesis defects that can lead to human testicular cancer or infertility. A great deal of effort has been exerted to understand how the SSC population is maintained. In contrast, little is known about the essential program of differentiation initiated by retinoic acid (RA) that precedes meiosis, and the pathways and proteins involved are poorly defined. We recently reported a novel role for RA in stimulating the PI3/AKT/mTOR kinase signaling pathway to activate translation of repressed mRNAs such as Kit. Here, we examined the requirement for mTOR complex 1 (mTORC1) in mediating the RA signal to direct spermatogonial differentiation in the neonatal testis. We found that in vivo inhibition of mTORC1 by rapamycin blocked spermatogonial differentiation, which led to an accumulation of undifferentiated spermatogonia. In addition, rapamycin also blocked the RA-induced translational activation of mRNAs encoding KIT, SOHLH1, and SOHLH2 without affecting expression of STRA8. These findings highlight dual roles for RA in germ cell development - transcriptional activation of genes, and kinase signaling to stimulate translation of repressed messages required for spermatogonial differentiation.