The microRNA miR-202 prevents precocious spermatogonial differentiation and meiotic initiation during mouse spermatogenesis

Harnessing the power of miRNAs for precision diagnosis and treatment of male infertility

Infertility is a multifactorial reproductive system disorder, and most infertility cases occur in men. Semen testing is now thought to be the most important diagnostic test for infertile men; nonetheless, because of its limitations, the cause of infertility remains unknown for 40% of infertile men. Semen assessment's shortcomings indicate the need for improved and innovative diagnostic techniques and biomarkers worldwide. Non-coding RNAs with a length of roughly 18-22 nucleotides are called microRNAs (miRNAs). Most of our protein-coding genes are post-transcriptionally regulated by them. These molecules are unusual in bodily fluids, and aberrant variations in their expression can point to specific conditions like infertility. As a result, fresh potential biomarkers for the diagnosis and prognosis of various forms of male infertility may be represented by miRNAs. This review examined the most recent research revealing the association between different miRNAs' functions in male infertility and their expression patterns. Also, it aims to figure out the most recent strategies that could be applied for using such miRNAs as possible therapeutic targets for infertility treatment.

Mechanisms of Hormonal, Genetic, and Temperature Regulation of Germ Cell Proliferation, Differentiation, and Death During Spermatogenesis

Spermatogenesis is a complex and highly regulated process involving the proliferation, differentiation, and apoptosis of germ cells. This process is controlled by various hormonal, genetic, and environmental factors, including temperature. In hormonal regulation, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) are essential for correct spermatogenesis development from the early stages and spermatogonia proliferation to germ cell maturation. Other hormones, like inhibin and activin, finely participate tuning the process of spermatogenesis. Genetic regulation involves various transcription factors, such as SOX9, SRY, and DMRT1, which are crucial for the development and maintenance of the testis and germ cells. MicroRNAs (miRNAs) play a significant role by regulating gene expression post-transcriptionally. Epigenetic modifications, including DNA methylation, histone modifications, and chromatin remodelling, are also vital. Temperature regulation is another critical aspect, with the testicular temperature maintained around 2-4 °C below body temperature, essential for efficient spermatogenesis. Heat shock proteins (HSPs) protect germ cells from heat-induced damage by acting as molecular chaperones, ensuring proper protein folding and preventing the aggregation of misfolded proteins during thermal stress. Elevated testicular temperature can impair spermatogenesis, increasing germ cell apoptosis and inducing oxidative stress, DNA damage, and the disruption of the blood-testis barrier, leading to germ cell death and impaired differentiation. The cellular mechanisms of germ cell proliferation, differentiation, and death include the mitotic divisions of spermatogonia to maintain the germ cell pool and produce spermatocytes. Spermatocytes undergo meiosis to produce haploid spermatids, which then differentiate into mature spermatozoa. Apoptosis, or programmed cell death, ensures the removal of defective germ cells and regulates the germ cell population. Hormonal imbalance, genetic defects, and environmental stress can trigger apoptosis during spermatogenesis. Understanding these mechanisms is crucial for addressing male infertility and developing therapeutic interventions. Advances in molecular biology and genetics continue to uncover the intricate details of how spermatogenesis is regulated at multiple levels, providing new insights and potential targets for treatment.

The Biology and Regulation of Spermatogonial Stem Cells in the Niche

Spermatogenesis, the process responsible for the daily production of millions of sperm, originates from spermatogonial stem cells (SSCs). Dysregulation of spermatogenesis is a major contributing factor to male infertility. Additionally, cryopreservation of SSCs followed by transplantation is a viable approach to restore spermatogenesis after sterilizing treatments such as chemotherapy and radiotherapy for cancer treatment. Therefore, investigating the biology and regulatory mechanisms involved in maintaining SSCs will provide valuable insights into the etiology of male fertility disorders and inform clinical strategies for fertility preservation and restoration. In this chapter, we will review the origin of SSCs, their biological and functional properties, and the various types of cells that contribute to the SSC niche. Additionally, we will discuss the regulation of SSC self-renewal and differentiation by niche factors, cell-cell and cell-extracellular matrix interactions, intrinsic gene regulation, and emerging intercellular communication mechanisms.

Profiling of circulating extracellular vesicle microRNAs reveals diagnostic potential and pathways in non-obstructive and obstructive azoospermia†

The accurate diagnosis of non-obstructive azoospermia and obstructive azoospermia is crucial for selecting appropriate clinical treatments. This study aimed to investigate the pivotal role of microRNAs in circulating plasma extracellular vesicles in distinguishing between non-obstructive azoospermia and obstructive azoospermia, as well as uncovering the signaling pathways involved in azoospermia pathogenesis. In this study, differential expression of extracellular vesicle miR-513c-5p and miR-202-5p was observed between non-obstructive azoospermia and obstructive azoospermia patients, while the selenocompound metabolism pathway could be affected in azoospermia through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis. The predictive power of these microRNAs was evaluated using receiver characteristic operator-area under the curve analysis, demonstrating promising sensitivity, specificity, and area under the curve values. A binomial regression equation incorporating circulating plasma levels of extracellular vesicles miR-202-5p and miR-513c-5p along with follicle-stimulating hormone was calculated to provide a clinically applicable method for diagnosing non-obstructive azoospermia and obstructive azoospermia. This study presents a potentially non-invasive testing approach for distinguishing between non-obstructive azoospermia and obstructive azoospermia, offering a possibly valuable tool for clinical practice.

miR-34 negatively regulates the expression of Dmrt and related genes in the testis of mud crab Scylla paramamosain

The mud crab (Scylla paramamosain) is a commercially significant marine decapod crustacean. Due to its obvious sexual dimorphism, the mechanism of sex differentiation and gonadal development has attracted significant research interest. The Dmrt (double-sex and mab-3 related transcription factor) genes are vital in animal gonadal development and sex differentiation. In the present study, miR-34 was predicted to target the 3' end of Dmrt-1, idmrt-2, Dmrt-3, Dsx and Dmrt-like genes by prediction software, and the interactions between miR-34 and these Dmrt genes were validated by in vivo and in vitro experiments. Dual luciferase assay results indicated that miR-34 mimics/inhibitors co-transfected with plasmid vectors with 3' end of Dmrt-1, idmrt-2, Dmrt-3, Dsx and Dmrt-like, respectively, led to a significant decrease/increase of fluorescence activity in HEK293T cells. In vivo experiments showed that injection of agomir-34 significantly inhibited Dmrt-1, idmrt-2, Dsx and Dmrt-like expression, while injection of antagomir-34 caused the opposite result. However, Dmrt-3 expression was not affected by injection of miR-34 reagents. Meanwhile, the expression of spermatogenesis and testicular development-related molecular marker genes (IAG, foxl2 and vasa) in mud crabs was significantly changed after injecting the miR-34 reagent in vivo. Furthermore, the result of immunoblotting proved that the expression level of Dmrt-like protein can be regulated by miR-34. These results imply that miR-34 is indirectly involved in sex differentiation and testicular development of S. paramamosain by regulating Dmrt-1, idmrt-2, Dsx and Dmrt-like genes.

Mechanism of LncRNA Gm2044 in germ cell development

Germ cell development in mammals is a complex physiological process that involves the proliferation of primordial germ cells, meiosis, and the formation of male and female gametes. Long non-coding RNA (lncRNA) is a type of RNA with more than 200 nucleotides that does not code for proteins. A small number of lncRNAs have been shown to participate in spermatogenesis in the testes and in follicular development in the ovaries, but the role of the vast majority of lncRNAs and their molecular mechanisms still need further study. LncRNA Gm2044 was identified as a differentially expressed lncRNA in mouse spermatogenesis by microarray technology. In mouse testis, lncRNA Gm2044 can act as competing endogenous RNA to regulate SYCP1 expression in GC-2 cells derived from mouse spermatocyte cells, and it can also act as a host gene for miR-202 to regulate RBFOX2 protein expression. In female mouse ovaries, lncRNA Gm2044 regulates 17β-estradiol synthesis through the miRNA-138-5p-Nr5a1 pathway or by interacting with EEF2. In addition, studies suggest that lncRNA Gm2044 is also involved in the progression of reproductive system diseases such as male nonobstructive azoospermia. Here, we summarize the roles and molecular mechanisms of lncRNA Gm2044 in male and female gametogenesis and its potential role in some infertility disorders.

Gene expression programs in mammalian spermatogenesis

Mammalian spermatogenesis, probably the most complex of all cellular developmental processes, is an ideal model both for studying the specific mechanism of gametogenesis and for understanding the basic rules governing all developmental processes, as it entails both cell type-specific and housekeeping molecular processes. Spermatogenesis can be viewed as a mission with many tasks to accomplish, and its success is genetically programmed and ensured by the collaboration of a large number of genes. Here, I present an overview of mammalian spermatogenesis and the mechanisms underlying each step in the process, covering the cellular and molecular activities that occur at each developmental stage and emphasizing their gene regulation in light of recent studies.

MicroRNAs in spermatogenesis dysfunction and male infertility: clinical phenotypes, mechanisms and potential diagnostic biomarkers

Infertility affects approximately 10-15% of couples worldwide who are attempting to conceive, with male infertility accounting for 50% of infertility cases. Male infertility is related to various factors such as hormone imbalance, urogenital diseases, environmental factors, and genetic factors. Owing to its relationship with genetic factors, male infertility cannot be diagnosed through routine examination in most cases, and is clinically called 'idiopathic male infertility.' Recent studies have provided evidence that microRNAs (miRNAs) are expressed in a cell-or stage-specific manner during spermatogenesis. This review focuses on the role of miRNAs in male infertility and spermatogenesis. Data were collected from published studies that investigated the effects of miRNAs on spermatogenesis, sperm quality and quantity, fertilization, embryo development, and assisted reproductive technology (ART) outcomes. Based on the findings of these studies, we summarize the targets of miRNAs and the resulting functional effects that occur due to changes in miRNA expression at various stages of spermatogenesis, including undifferentiated and differentiating spermatogonia, spermatocytes, spermatids, and Sertoli cells (SCs). In addition, we discuss potential markers for diagnosing male infertility and predicting the varicocele grade, surgical outcomes, ART outcomes, and sperm retrieval rates in patients with non-obstructive azoospermia (NOA).

Small noncoding RNAs and sperm nuclear basic proteins reflect the environmental impact on germ cells

BACKGROUND

Molecular techniques can complement conventional spermiogram analyses to provide new information on the fertilizing potential of spermatozoa and to identify early alterations due to environmental pollution.

METHODS

Here, we present a multilevel molecular profiling by small RNA sequencing and sperm nuclear basic protein analysis of male germ cells from 33 healthy young subjects residing in low and high-polluted areas.

RESULTS

Although sperm motility and sperm concentration were comparable between samples from the two sites, those from the high-pollution area had a higher concentration of immature/immune cells, a lower protamine/histone ratio, a reduced ability of sperm nuclear basic proteins to protect DNA from oxidative damage, and an altered copper/zinc ratio in sperm. Sperm levels of 32 microRNAs involved in intraflagellar transport, oxidative stress response, and spermatogenesis were different between the two areas. In parallel, a decrease of Piwi-interacting RNA levels was observed in samples from the high-polluted area.

CONCLUSIONS

This comprehensive analysis provides new insights into pollution-driven epigenetic alterations in sperm not detectable by spermiogram.

A medaka gonad-specific lncRNA may act as pri-miR-202 to regulate testicular endocrine homeostasis and spermatogenesis

A large number of long non-coding RNAs (lncRNAs) are expressed in animal gonads, but their functions are poorly understood. In this study, a gonad-specific lncRNA, termed lnc4, was identified and characterized in the model fish medaka (Oryzias latipes). The expression pattern and in vitro functional analyses indicated that lnc4 was likely to be a primary transcript of miR-202 (pri-miR-202). Results of single-molecule fluorescence in situ hybridization demonstrated that the precursor miR-202 (pre-miR-202) was highly expressed in the nuclei of testicular somatic cells, including Leydig and Sertoli cells, whereas only a small amount of lnc4 molecules could be detected co-expressed with pre-miR-202 in Sertoli cells due to its low expression level. Deletion of the lnc4 locus led to a significant reduction in testis size and a dramatic decrease in the number of male germ cells, as well as a reduction in sperm viability. Moreover, lnc4 knockout resulted in enhanced synthesis and secretion of testicular somatic cells and accelerated differentiation of immature male germ cells. Taken together, functional studies of lnc4 and its mature transcript miR-202 will contribute to the understanding of the important role of non-coding RNAs in animal or human reproductive disorders.

The Role of microRNA in Spermatogenesis: Is There a Place for Fertility Preservation Innovation?

Oncological treatments have dramatically improved over the last decade, and as a result, survival rates for cancer patients have also improved. Quality of life, including concerns about fertility, has become a major focus for both oncologists and patients. While oncologic treatments are often highly effective at suppressing neoplastic growth, they are frequently associated with severe gonadotoxicity, leading to infertility. For male patients, the therapeutic option to preserve fertility is semen cryopreservation. In prepubertal patients, immature testicular tissue can be sampled and stored to allow post-cure transplantation of the tissue, immature germ cells, or in vitro spermatogenesis. However, experimental techniques have not yet been proven effective for restoring sperm production for these patients. MicroRNAs (miRNAs) have emerged as promising molecular markers and therapeutic tools in various diseases. These small regulatory RNAs possess the unique characteristic of having multiple gene targets. MiRNA-based therapeutics can, therefore, be used to modulate the expression of different genes involved in signaling pathways dysregulated by changes in the physiological environment (disease, temperature, ex vivo culture, pharmacological agents). This review discusses the possible role of miRNA as an innovative treatment option in male fertility preservation-restoration strategies and describes the diverse applications where these new therapeutic tools could serve as fertility protection agents.

Small Non-Coding RNAs in Male Reproduction

Male reproductive functions are strictly regulated in order to maintain sperm production and fertility. All processes are controlled by precise regulation of gene expression, which creates specific gene expression programs for different developmental stages and cell types, and forms the functional basis for the reproductive system. Small non-coding RNAs (sncRNAs) are involved in gene regulation by targeting mRNAs for translational repression and degradation through complementary base pairing to recognize their targets. This review article summarizes the current knowledge on the function of different classes of sncRNAs, in particular microRNAs (miRNAs) and PIWI-interacting RNAs (piRNAs), during male germ cell differentiation, with the focus on sncRNAs expressed in the germline. Although transcriptionally inactive, mature spermatozoa contain a complex population of sncRNAs, and we also discuss the recently identified role of sperm sncRNAs in the intergenerational transmission of epigenetic information on father's environmental and lifestyle exposures to offspring. Finally, we summarize the current information on the utility of sncRNAs as potential biomarkers of infertility that may aid in the diagnosis and prediction of outcomes of medically assisted reproduction.

Classical cadherins in the testis: how are they regulated?

Cadherins (CDH) are crucial intercellular adhesion molecules, contributing to morphogenesis and creating tissue barriers by regulating cells' movement, clustering and differentiation. In the testis, classical cadherins such as CDH1, CDH2 and CDH3 are critical to gonadogenesis by promoting the migration and the subsequent clustering of primordial germ cells with somatic cells. While CDH2 is present in both Sertoli and germ cells in rodents, CDH1 is primarily detected in undifferentiated spermatogonia. As for CDH3, its expression is mainly found in germ and pre-Sertoli cells in developing gonads until the establishment of the blood-testis barrier (BTB). This barrier is made of Sertoli cells forming intercellular junctional complexes. The restructuring of the BTB allows the movement of early spermatocytes toward the apical compartment as they differentiate during a process called spermatogenesis. CDH2 is among many junctional proteins participating in this process and is regulated by several pathways. While cytokines promote the disassembly of the BTB by enhancing junctional protein endocytosis for degradation, testosterone facilitates the assembly of the BTB by increasing the recycling of endocytosed junctional proteins. Mitogen-activated protein kinases (MAPKs) are also mediators of the BTB kinetics in many chemically induced damages in the testis. In addition to regulating Sertoli cell functions, follicle stimulating hormone can also regulate the expression of CDH2. In this review, we discuss the current knowledge on regulatory mechanisms of cadherin localisation and expression in the testis.

In vivo functions of miRNAs in mammalian spermatogenesis

MicroRNAs (miRNAs) are believed to play important roles in mammalian spermatogenesis mainly because spermatogenesis is more or less disrupted when genes encoding key enzymes for miRNA biogenesis are mutated. However, it is challenging to study the functions of individual miRNAs due to their family-wise high sequence similarities and the clustered genomic distributions of their genes, both of which expose difficulties in using genetic methods. Accumulating evidence shows that a number of miRNAs indeed play important roles in mammalian spermatogenesis and the underlying mechanisms start to be understood. In this mini review, we focus on highlighting the roles of miRNAs in mammalian spermatogenesis elucidated mainly by using in vivo genetic methods and on discussing the underlying mechanisms. We propose that studies on the roles of miRNAs in spermatogenesis should and can be conducted in a more fruitful way given the progress in traditional methods and the birth of new technologies.

Circulating microRNAs as promising testicular translatable safety biomarkers: current state and future perspectives

Drug-induced testicular injury (DITI) is one of the often-observed and challenging safety issues seen during drug development. Semen analysis and circulating hormones currently utilized have significant gaps in their ability to detect testicular damage accurately. In addition, no biomarkers enable a mechanistic understanding of the damage to the different regions of the testis, such as seminiferous tubules, Sertoli, and Leydig cells. MicroRNAs (miRNAs) are a class of non-coding RNAs that modulate gene expression post-transcriptionally and have been indicated to regulate a wide range of biological pathways. Circulating miRNAs can be measured in the body fluids due to tissue-specific cell injury/damage or toxicant exposure. Therefore, these circulating miRNAs have become attractive and promising non-invasive biomarkers for assessing drug-induced testicular injury, with several reports on their use as safety biomarkers for monitoring testicular damage in preclinical species. Leveraging emerging tools such as 'organs-on-chips' that can emulate the human organ's physiological environment and function is starting to enable biomarker discovery, validation, and clinical translation for regulatory qualification and implementation in drug development.

The Role of Long Noncoding RNAs on Male Infertility: A Systematic Review and In Silico Analysis

Male infertility is a complex disorder affecting many couples worldwide. Long noncoding RNAs (lncRNAs) regulate important cellular processes; however, a comprehensive understanding of their role in male infertility is limited. This systematic review investigates the differential expressions of lncRNAs in male infertility or variations in lncRNA regions associated with it. The PRISMA guidelines were used to search Pubmed and Web of Science (1 June 2022). Inclusion criteria were human participants, patients diagnosed with male infertility, and English language speakers. We also performed an in silico analysis investigating lncRNAs that are reported in many subtypes of male infertility. A total of 625 articles were found, and after the screening and eligibility stages, 20 studies were included in the final sample. Many lncRNAs are deregulated in male infertility, and interactions between lncRNAs and miRNAs play an important role. However, there is a knowledge gap regarding the impact of variants found in lncRNA regions. Furthermore, eight lncRNAs were identified as differentially expressed in many subtypes of male infertility. After in silico analysis, gene ontology (GO) and KEGG enrichment analysis of the genes targeted by them revealed their association with bladder and prostate cancer. However, pathways involved in general in tumorigenesis and cancer development of all types, such as p53 pathways, apoptosis, and cell death, were also enriched, indicating a link between cancer and male infertility. This evidence, however, is preliminary. Future research is needed to explore the exact mechanism of action of the identified lncRNAs and investigate the association between male infertility and cancer.

MiR-202-3p determines embryo viability during mid-blastula transition

Developmental growth is an intricate process involving the coordinated regulation of the expression of various genes, and microRNAs (miRNAs) play crucial roles in diverse processes throughout animal development. The mid-blastula transition (MBT) is a developmental milestone when maternal RNAs are cleared and the zygotic genome programmed asynchronous cell division begins to drive embryogenesis. While mechanisms underlying MBT have been intensively revealed, factors regulating cell proliferation at the transition remain largely unknown. We report here a microRNA, miR-202-3p to be a key factor that determines embryonic fate during MBT in zebrafish. A miR-202-3p antagomir specifically terminated embryo development at the mid-blastula stage. In vivo deletion of the miR-202 locus recapitulated the fatal phenotypes, which were rescued only by miR-202-3p or its precursor. Transcriptome comparison revealed >250 RNAs including both maternal and zygotic origins were dysregulated at MBT in the miR-202^−/− embryos, corresponding with arrays of homeostatic disorders leading to massive apoptosis. A trio of genes: nfkbiaa, perp and mgll, known to be intimately involved with cell proliferation and survival, were identified as direct targets of miR-202-3p. Importantly, over- or under-expression of any of the trio led to developmental delay or termination at the blastula or gastrula stages. Furthermore, nfkbiaa and perp were shown to inter-regulate each other. Thus, miR-202-3p mediates a regulatory network whose components interact closely during MBT to determine embryonic viability and development.

MicroRNA-202 safeguards meiotic progression by preventing premature SEPARASE-mediated REC8 cleavage

MicroRNAs (miRNAs) are believed to play important roles in mammalian spermatogenesis but the in vivo functions of single miRNAs in this highly complex developmental process remain unclear. Here, we report that miR-202, a member of the let-7 family, plays an important role in spermatogenesis by phenotypic evaluation of miR-202 knockout (KO) mice. Loss of miR-202 results in spermatocyte apoptosis and perturbation of the zygonema-to-pachynema transition. Multiple processes during meiosis prophase I including synapsis and crossover formation are disrupted, and inter-sister chromatid synapses are detected. Moreover, we demonstrate that Separase mRNA is a miR-202 direct target and provides evidence that miR-202 upregulates REC8 by repressing Separase expression. Therefore, we have identified miR-202 as a new regulating noncoding gene that acts on the established SEPARASE-REC8 axis in meiosis.

Roles of Spermatogonial Stem Cells in Spermatogenesis and Fertility Restoration

Spermatogonial stem cells (SSCs) are a group of adult stem cells in the testis that serve as the foundation of continuous spermatogenesis and male fertility. SSCs are capable of self-renewal to maintain the stability of the stem cell pool and differentiation to produce mature spermatozoa. Dysfunction of SSCs leads to male infertility. Therefore, dissection of the regulatory network of SSCs is of great significance in understanding the fundamental molecular mechanisms of spermatogonial stem cell function in spermatogenesis and the pathogenesis of male infertility. Furthermore, a better understanding of SSC biology will allow us to culture and differentiate SSCs in vitro, which may provide novel stem cell-based therapy for assisted reproduction. This review summarizes the latest research progress on the regulation of SSCs, and the potential application of SSCs for fertility restoration through in vivo and in vitro spermatogenesis. We anticipate that the knowledge gained will advance the application of SSCs to improve male fertility. Furthermore, in vitro spermatogenesis from SSCs sets the stage for the production of SSCs from induced pluripotent stem cells (iPSCs) and subsequent spermatogenesis.

Members of the AP-1 Family of Transcription Factors Regulate the Expression of Gja1 in Mouse GC-1 Spermatogonial Cells

Gap junctions, mainly formed by Gja1 (Connexin43), play an essential role in the regulation of proliferation and differentiation of spermatogonia in the testis. Regulation of the abundance of Gja1 in spermatogonia involves various processes, including gene transcription, mRNA maturation, protein synthesis, post-translational modifications, plasma membrane integration and protein degradation. However, gene expression of Gja1 is abnormally decreased in most testicular germ cell tumors. Hence, a better understanding of the mechanisms of transcriptional regulation of Gja1 in spermatogonia is essential to understand how the loss of its expression occurs during the development of testicular cancer. As in other cell types, activator protein-1 (AP-1) transcription factors may be involved in such regulatory process. Thus, AP-1 members were overexpressed in GC-1 cells to assess their impact on Gja1 expression. We showed that Jun and Fosl2 cooperate to activate the Gja1 promoter in GC-1 cells. Furthermore, the recruitment of Jun to the proximal region (−153 to +46 bp) of the Gja1 promoter has been confirmed via chromatin immunoprecipitation. Protein kinase A and calcium-calmodulin protein kinase I also contribute to the activation of Gja1 expression by improving the cooperation between AP-1 factors. Therefore, the reduction in Gja1 expression in testicular germ cell tumors may involve a loss of cooperation between AP-1 factors.