Introduction
Because of the work of the Encyclopedia of DNA Elements (ENCODE) Project Consortium, we have a comprehensive understanding of DNA sequences in the human genome (The ENCODE Project Consortium, 2004, 2012; Birney et al., Reference Birney, Stamatoyannopoulos, Dutta, Guigo, Gingeras, Margulies, Weng, Snyder, Dermitzakis and Thurman2007). Eighty per cent of the genome is transcribed into RNA, while only a small proportion encodes proteins (The ENCODE Project Consortium, 2012). Noncoding RNAs, formerly called ‘junk’, have been proven to conduct ‘pervasive transcription’ and contain a great deal of functional regulatory elements (Ohno, Reference Ohno1972; Kapranov et al., Reference Kapranov, Cheng, Dike, Nix, Duttagupta, Willingham, Stadler, Hertel, Hackermuller and Hofacker2007; Neph et al., Reference Neph, Vierstra, Stergachis, Reynolds, Haugen, Vernot, Thurman, John, Sandstrom and Johnson2012). According to the length of noncoding RNA, they are divided into long noncoding RNAs (lncRNAs; >200 nt) and small noncoding RNAs. Small noncoding RNAs include microRNAs (miRNAs), endogenous small interfering RNAs (endosiRNAs) and PIWI-interacting RNAs (piRNAs) (Lucas and Raikhel, Reference Lucas and Raikhel2013). Noncoding RNAs (ncRNAs) play important roles in gene expression and stability in processes from embryonic development to adult homeostasis (Geisler and Coller, Reference Geisler and Coller2013; Patil et al., Reference Patil, Zhou and Rana2014). According to their functional features, ncRNAs are divided into housekeeping ncRNAs and regulatory ncRNAs. The regulatory ncRNAs comprise miRNAs, siRNAs, lncRNAs, piRNAs and intermediate ncRNAs such as small nucleolar RNAs. They are expressed in specific cells or a specific stage during cell development and differentiation or in response to environmental stimuli (Brosnan and Voinnet, Reference Brosnan and Voinnet2009; Guan et al., Reference Guan, Zhang, Zhang, Liu and Belmonte2013). Increasing evidence shows that ncRNAs are involved in spermatogenesis and maintenance of male fertility in germ cells (de Mateo and Sassone-Corsi, Reference de Mateo and Sassone-Corsi2014; Salviano-Silva et al., Reference Salviano-Silva, Lobo-Alves, Almeida, Malheiros and Petzl-Erler2018). Here we summarize the findings of studies on the noncoding RNAs involved in male fertility in somatic cells.
Sertoli cells
Niche cells, which are devoted to subtle coordination of the testicular microenvironment, are mostly composed of SCs. SCs, which are proximal to spermatogonial stem cells (SSCs), play auxiliary roles in spermatogenesis as ‘mother’ or ‘nurse’ cells for SSCs by supplying structural, immunological and nutritional support (Oatley and Brinster, Reference Oatley and Brinster2012; Hai et al., Reference Hai, Hou, Liu, Liu, Yang, Li and He2014). As supportive cells, SCs also produce a great number of growth factors and define the fate of SSCs, as stem cell factors (SCFs), bone morphogenetic proteins (BMPs) or glial cell line-derived neurotrophic factors (GDNFs) (Jan et al., Reference Jan, Hamer, Repping, de Rooij, van Pelt and Vormer2012; Hai et al., Reference Hai, Hou, Liu, Liu, Yang, Li and He2014). Moreover, SCs are important constituents of the blood–testis barrier, which is an essential ultrastructure for male fertility (Setchell, Reference Setchell2008; McCabe et al., Reference McCabe, Tarulli, Laven-Law, Matthiesson, Meachem, McLachlan, Dinger and Stanton2016). The biological functions of SCs have been summarized as follows: they expand SSCs as feeder cells and activate SSC differentiation, phagocytosis and immunoprotection (Zhang et al., Reference Zhang, Shao and Meistrich2007; Hai et al., Reference Hai, Hou, Liu, Liu, Yang, Li and He2014). In 2013, a study using computer-assisted annotation of the small RNA transcriptome in murine SCs was completed (Ortogero et al., Reference Ortogero, Hennig, Langille, Ro, McCarrey and Yan2013). Large numbers of studies have also shown that the disruption in the expression of noncoding RNAs in SCs might effect male fertility and these studies also made efforts to determine the functions of SCs in spermatogenesis and male fertility.
microRNAs in Sertoli cells
A microRNA is a small RNA of approximately 22 nucleotides in length (Kim, Reference Kim2005). Growing evidence has shown that microRNAs regulate many biological processes by influencing post-transcriptional gene expression, such as mRNA degradation, translational repression, DNA methylation and chromatin modification (Krol et al., Reference Krol, Loedige and Filipowicz2010). MicroRNAs target specific mRNAs and stimulate the degeneration or inhibition of translated mRNAs (Tay et al., Reference Tay, Zhang, Thomson, Lim and Rigoutsos2008). Recently, some studies have shown that miRNAs are also involved in modulating many functions in SCs.
Apoptosis and phagocytosis
Phagocytosis is indispensable for the maintenance of tissue homeostasis. Sufficient evidence has shown that phagocytosis by SCs plays an essential role in the development and differentiation of germ cells. More than one-half of spermatogenic stem cells was cleared and degraded by SCs (Wang et al., Reference Wang, Wang, Xiong, Chen, Ma, Ma, Ge and Han2006). Several miRNAs have been reported to be involved in conventional phagocytosis or LC3-associated phagocytosis (LAP) or cell apoptosis (Jovanovic and Hengartner, Reference Jovanovic and Hengartner2006; Niu et al., Reference Niu, Goodyear, Rao, Wu, Tobias, Avarbock and Brinster2011). miR-471-5p regulates the level of Dock180, which interacts with autophagy-related proteins and make up LC3-dependent phagocytic complexes. It has been proven that SCs recruit autophagy-related proteins via LAP and that these proteins play crucial roles in the clearance of apoptotic germ cells. Overexpression of miR-471-5p in SCs from transgenic mice increased the number of apoptotic germ cells and damaged male fertility as miR-471-5p targets Dock180, LC3, Atg12, Rab5, Rubicon and Becn1 and represses their expression. This evidence suggested that SC phagocytosis and the clearance of apoptotic germ cells are regulated by miR-471-5p and its target proteins (Panneerdoss et al., Reference Panneerdoss, Viswanadhapalli, Abdelfattah, Onyeagucha, Timilsina, Mohammad, Chen, Drake, Vuori and Kumar2017).
miR-758 and miR-98-5p were predicted to be participants in germ cell apoptosis by binding to the 3′UTR of mitogen-activated protein kinase 11 (MAPK11, p38 β isoform) gene. The expression of MAPK11 in SCs could induce the expression of tumour necrosis factor α (TNF-α), which interacts with TNF receptor 1 (TNFR1) and leads to germ cell apoptosis (Chen et al., Reference Chen, Zhou, Wang, Wang, Xiang, Li and Han2016). In the SCs of underfed sheep, the lack of let-7/miR-98 increased the expression of Fas mRNA and Fas protein and the presence of let-7/miR-98 reduced cell sensitivity to Fas-induced apoptosis (Wang et al., Reference Wang, Tang, Cui, Zhao, Luo, Pan, Huang and Shen2011). Overexpressed miR-202-3p increased the number of apoptotic SCs and inhibited the proliferation and synthesis function of SCs by targeting LRP6 and cyclin D1 (Yang C et al., Reference Yang, Yao, Tian, Zhu, Zhao, Li, Chen, Huang, Zhi and Gong2019). In addition, miR-125a-3p, miR-872 and miR-24 possibly induced cell apoptosis by targeting SOD-1, a Cu/Zn superoxide dismutase in SCs (Papaioannou et al., Reference Papaioannou, Lagarrigue, Vejnar, Rolland, Kuhne, Aubry, Schaad, Fort, Descombes and Neerman-Arbez2011).
Proliferation and development:
Dicer, an RNaseIII endonuclease, has been deemed crucial for miRNA production (Bernstein et al., Reference Bernstein, Caudy, Hammond and Hannon2001). Studies have shown that it is highly correlated with maturation and survival of SCs, which suggests that the development, survival and function of SCs might be regulated by miRNAs related to Dicer (Papaioannou et al., Reference Papaioannou, Pitetti, Ro, Park, Aubry, Schaad, Vejnar, Kuhne, Descombes and Zdobnov2009, Reference Papaioannou, Lagarrigue, Vejnar, Rolland, Kuhne, Aubry, Schaad, Fort, Descombes and Neerman-Arbez2011). Dicer is critical to SCs as well as to spermatogenesis. The absence of Dicer leads to infertility. Several miRNAs (miR-299, miR-381, miR-409-5p, miR-376a and miR-674) in SCs were suppressed in Dicer ablated mice, indicating that these RNAs may take part in the development and function of SCs (Papaioannou et al., Reference Papaioannou, Pitetti, Ro, Park, Aubry, Schaad, Vejnar, Kuhne, Descombes and Zdobnov2009).
miR-34c has been proved to be involved in the development of male germ cells (Lian et al., Reference Lian, Sun, Niu, Yang, Liu, Lu, Meng, Qiu, Zhang and Zhao2012). In fact, the development of seminiferous tubules also requires the participation of miR-34c. Platelet-derived growth factor receptor alpha (PDGFRA), which is mainly expressed in support cells such as SCs, has been identified as one target gene of miR-34c in swine. In addition, the expression level of PDGFRA in 2 days was much higher than it was in 5 months, indicating that PDGFRA may be involved in the early stages of SC development (Zhang X et al., Reference Zhang, Zhao, Li, Yu, Qiao, Li, Lu, Zhao and Sun2015). miR-762 also played an important role in pig testis. miR-762 promoted immature SC proliferation and controlled apoptosis by targeting the 3′UTR of ring finger protein 4 (RNF4), thereby decreasing AR protein expression and the transcriptional regulatory AR activity in SCs (Ma C et al., Reference Ma, Song, Yu, Guan, Hu, Li, Xia, Li, Jiang and Li2016). In addition, miR-762 could also promote DNA damage repair in SCs (Ma C et al., Reference Ma, Song, Yu, Guan, Hu, Li, Xia, Li, Jiang and Li2016). As mentioned above, miR-202-3p also inhibited the proliferation and synthesis functions of SCs (Yang C et al., Reference Yang, Yao, Tian, Zhu, Zhao, Li, Chen, Huang, Zhi and Gong2019).
Compared with those of patients with obstructive azoospermia (OA), the SCs of patients with Sertoli-cell-only syndrome (SCOS) expressed higher levels of miRNA-133b. Studies have shown that miR-133b promotes SC proliferation in humans by targeting GLI3 and activating cyclin B1 and cyclin D1 (Yao et al., Reference Yao, Sun, Yuan, Niu, Chen, Hou, Wang, Wen, Liu and Li2016). In addition, the level of miR-375 expression is negatively correlated with the mRNA levels of rearranged L-myc fusion (RLF) and hypoxia-induced gene domain protein 1A (HIGD1A) in pig SCs. MTS analysis showed that miR-375 might inhibit SC proliferation (Guo et al., Reference Guo, Liu, Yang, Liang, Bai, Zhao and Sun2018).
Junction of the blood–testis barrier
The BTB is one of the most impermeable blood–tissue barriers in the living body, It is which was composed of tight junctions (TJs), gap junctions (GJs) and desmosome-like junctions and adherens junctions (AJs). The AJs include the basal tubulobulbar complex (basal TBC) and the basal ectoplasmic specialization (basal ES) (Wong and Cheng, Reference Wong and Cheng2005; Cheng and Mruk, Reference Cheng and Mruk2012). The BTB separates the seminiferous epithelium into basal and apical parts. Spermatogenesis involves five orderly processes and the BTB provides a physical barrier that compartmentalizes three of these processes: the cell cycle that enables the spermatocyte transition from the zygotene to the diplotene stage, the procession of round spermatids to spermatozoa and spermiation. In addition, it enables these processes to progress in an immune-privileged environment, indicating that the BTB plays an important role in spermatogenesis and male fertility (Cheng and Mruk, Reference Cheng and Mruk2012). In mice, considerable evidence has proven that the miR-17-92 cluster plays an important role in spermatogenesis (Bjork et al., Reference Bjork, Sandqvist, Elsing, Kotaja and Sistonen2010; Tong et al., Reference Tong, Mitchell, McGowan, Evanoff and Griswold2012). miR-20a, a member of the miR-17–92 cluster, might be related to genistein (GEN)-induced abnormal spermatogenesis (Gu et al., Reference Gu, Wu, Yuan, Tang, Guo, Chen, Xia, Hu, Chen and Sha2017). As the target gene of miR-20a, Limk1 (Gu et al., Reference Gu, Wu, Yuan, Tang, Guo, Chen, Xia, Hu, Chen and Sha2017), is involved in the RhoB/ROCK/LIMK1 pathway and regulates the adherens junction dynamics of Sertoli germ cells (Lui et al., Reference Lui, Lee and Cheng2003).
Other findings about microRNAs in Sertoli cells
Circular RNAs (circRNAs) have loop structures and perform important functions in many biological processes (Rybak-Wolf et al., Reference Rybak-Wolf, Stottmeister, Glazar, Jens, Pino, Giusti, Hanan, Behm, Bartok and Ashwal-Fluss2015; Ebbesen et al., Reference Ebbesen, Kjems and Hansen2016), including those of inflammatory reactions (Ng et al., Reference Ng, Marinov, Liau, Lam, Lim and Ea2016). Evidence has shown that circRNA-9119 acts as a miRNA sponge and inhibits miR-136 and miR-26a expression. miR-136 and miR-26a inhibited the expression of retinoic acid inducible gene-I (RIG-I) and Toll-like receptor 3 (TLR3) by binding to the 3′UTR of RIG-I and TLR3 respectively in Sertoli cells and Leydig cells (LCs). By targeting TLR3 and RIG-I during orchitis in SCs and LCs, miR-136 and miR-26a modulate circRNA-9119-mediated inflammatory reactions, indicating that miRNAs and circRNAs are crucial to the immune microenvironment (Qin et al., Reference Qin, Lin and Xie2019).
miR-202-5p, regulated by the testis-determining factor SOX9, is a let-7 family member (Wainwright et al., Reference Wainwright, Jorgensen, Kim, Truong, Bagheri-Fam, Davidson, Svingen, Fernandez-Valverde, McClelland and Taft2013). miR-202-5p is selectively expressed in SCs. The expression of miR-202-5p is related to testicular development and maturation (Dabaja et al., Reference Dabaja, Mielnik, Robinson, Wosnitzer, Schlegel and Paduch2015). However, no miR-202-5p expression was detected in SCs from SCOS patients. The distinct expression of levels of miR-202-5p in fertile and infertile men suggests that it might play crucial roles in normal male fertility (Dabaja et al., Reference Dabaja, Mielnik, Robinson, Wosnitzer, Schlegel and Paduch2015).
A study on the expression of miRNAs and their target genes in SCs after exposure to nonylphenol (NP) has been completed. The expression of 186 miRNAs is significantly distinct from that of the control group. In addition, it has been proven that miR-135a* can mediate the generation of reactive oxygen species (ROS) by regulating the Wnt/beta-catenin signalling pathway (Choi et al., Reference Choi, Oh, Park, Choi, Park, Kang, Oh, Kim, Hwang and Yoon2011). In 2018, a study showing the regulation of microRNA signalling by doxorubicin in LCs and SCs was also reported (Akinjo et al., Reference Akinjo, Gant and Marczylo2018).
Long noncoding RNAs
LncRNAs are longer than 200 nucleotides, with tissue- or cell-type specificity and without protein-coding capacity (Derrien et al., Reference Derrien, Johnson, Bussotti, Tanzer, Djebali, Tilgner, Guernec, Martin, Merkel and Knowles2012). Large numbers of lncRNAs have been identified, however few of these have been deeply explored (Ma L et al., Reference Ma, Li, Zou, Xu, Xia, Yu, Bajic and Zhang2015; Reon et al., Reference Reon, Anaya, Zhang, Mandell, Purow, Abounader and Dutta2016; Liu et al., Reference Liu, Wang, Mao, Zhao and Wang2019). The main functions of lncRNAs were summarized as mediators of nuclear trafficking, altering splicing, changing mRNA stability and translation at both the transcriptional and post-transcriptional levels (Ayupe et al., Reference Ayupe, Tahira, Camargo, Beckedorff, Verjovski-Almeida and Reis2015). For post-transcriptional regulation, lncRNAs also organized protein complexes to influence cell signalling and regulate allosteric proteins (Geisler and Coller, Reference Geisler and Coller2013). The Catsper1 gene, expressed in male germ cells, is essential for sperm motility and fertilization. Evidence suggests that the promoter of the Catsper1 gene can regulate a new gene, named Catsper1au (Catsper1 antisense upstream transcript). Analysis of the whole genome sequence revealed that Catsper1au has 1402 bp and is a polyadenylated lncRNA with no intron. It is found in the nucleus of SCs and germ cells of adult male mouse testis as well as in LCs, suggesting that it might have an effect on spermatogenesis and male fertility (Jimenez-Badillo et al., Reference Jimenez-Badillo, Oviedo, Hernandez-Guzman, Gonzalez-Mariscal and Hernandez-Sanchez2017). Nevertheless, the detailed mechanism is still unclear and remains to be explored. Notably, many lncRNAs have been found in SCs, but their specific targets are also unknown (Yang et al., Reference Yang, Wang, Li, Ren, Pang, Wan, Wang, Feng and Zhang2018).
PiRNA-like RNAs
PIWI-interacting RNAs (piRNAs) are small noncoding RNAs that are exclusively expressed in the germ cells of mammalian gonads. They have been regarded for a long time as germ-cell-specific small RNAs. The piRNAs are categorized into repetitive sequence-derived piRNAs and non-repetitive sequence-derived piRNAs (Unhavaithaya et al., Reference Unhavaithaya, Hao, Beyret, Yin, Kuramochi-Miyagawa, Nakano and Lin2009). The former silences transposons by DNA methylation and the latter is intergenic or intragenic and some have other non-repetitive regions with relatively unclear functions (Aravin et al., Reference Aravin, Sachidanandam, Girard, Fejes-Toth and Hannon2007; Carmell et al., Reference Carmell, Girard, van de Kant, Bourc’his, Bestor, de Rooij and Hannon2007). However, in 2014, a novel class of somatic small RNAs, which were similar to piRNAs, were detected in somatic cells and named piRNA-like RNAs (pilRNAs). The pilRNAs have a distinct ping–pong signature and might target mRNAs 3′UTRs in a unique and complementary way (Ortogero et al., Reference Ortogero, Schuster, Oliver, Riordan, Hong, Hennig, Luong, Báo, Bhetwal and Ro2014). These data provided one possibility that pilRNAs, whose functions are similar to those of germ cell piRNAs in SCs, might play a role in male fertility.
Leydig cells
Androgen greatly influences male health. Lack of testosterone can influence general health in males, such as by downregulating bone density, impairing muscle mass, injuring cognitive function and damaging immunity (Huhtaniemi, Reference Huhtaniemi2014). As males age, serum testosterone levels decrease gradually following an increased in serum follicle stimulating hormone (FSH) levels that either enhance or do not affect LH levels, indicating that the loss of testosterone results from changes in the ability of LCs to respond to LH. The loss is at the gonadal level rather than the hypothalamic–pituitary level (Wang et al., Reference Wang, Chen, Ye, Zirkin and Chen2017). Many chronic and age-related clinical symptoms are associated with a low level of androgen, including cardiovascular diseases, obesity and metabolic syndrome (Kupelian et al., Reference Kupelian, Page, Araújo, Travison, Bremner and McKinlay2006; Saad and Gooren, Reference Saad and Gooren2009; Kloner et al., Reference Kloner, Carson, Dobs, Kopecky and Mohler2016). Testosterone is mainly produced by LCs. It plays a critical role in maintaining secondary sexual characteristics and spermatogenesis regulation in adults (Matzkin et al., Reference Matzkin, Yamashita and Ascoli2013).
The LCs are divided into fetal LCs and postnatal LCs. The LH and hypothalamic–pituitary–gonadal axis regulates testosterone synthesis in adult interstitial LCs (Huhtaniemi I, Reference Huhtaniemi2015). However, fetal LCs produce androstenedione instead of testosterone directly without the presence of 17β-hydroxysteroid dehydrogenase-type 3 and androstenedione is converted into testosterone by fetal SCs (Shima et al., Reference Shima, Miyabayashi, Haraguchi, Arakawa, Otake, Baba, Matsuzaki, Shishido, Akiyama and Tachibana2013). Recent studies have shown that large numbers of ncRNAs are also expressed in LCs, Here, we summarized the findings of ncRNA function in LCs.
MicroRNAs
MicroRNAs related to LC steroidogenesis
Basic fibroblast growth factors (bFGF), including acidic and basic fibroblast growth factors, play diverse and specific roles in specific stages of LC steroidogenesis (Laslett et al., Reference Laslett, McFarlane and Risbridger1997). It was reported that five miRNAs (miR-29a, miR-29c, miR-142-3p, miR-451 and miR-335) are regulated by both bFGF and LH and are involved in the regulation of androgen production in immature LCs (Liu et al., Reference Liu, Yang, Zhang, Liang, Ge, Zhang, Zhang, Xiang, Huang and Su2014). miR-142-3p plays a vital role in cAMP production and PKA biological function to influence the cAMP/PKA signalling cascade, a secondary messenger pathway for steroid synthesis (Huang et al., Reference Huang, Zhao, Lei, Shen, Li, Shen, Zhang and Feng2009; Manna et al., Reference Manna, Slominski, King, Stetson and Stocco2014). Scavenger receptor class B type I (SR-BI), a HDL (high-density lipoprotein) receptor, is essential for the selective uptake of HDL CEs (cholesteryl esters) in steroidogenic cells (Shen et al., Reference Shen, Azhar and Kraemer2018). The expression of SR-BI and the selective uptake of HDL CEs were inhibited after the transfection of pre-miRNA-125a and pre-miRNA-455 in LCs, implying that miRNA-125a and miRNA-455 also play roles in steroidogenesis. Evidence has shown that miRNA-125a and miRNA-455 can bind to the 3′UTR of the SR-BI gene and negatively regulate SR-BI functions in rat steroidogenic cells. The two miRNAs were sensitive to changes in trophic hormones (ACTH or gonadotropin) and cAMP (Hu et al., Reference Hu, Shen, Kraemer and Azhar2012). In addition, treatment with Bt2cAMP increased the levels of miRNA-96, miRNA-132, miRNA-182 miRNA-183 and miRNA-212 and decreased the expression levels of miRNA-19a and miRNA-138 in MLTC-1 cells. All of these miRNAs can be found in the adrenal glands and are sensitive hormones similar to ACTH. In addition, miRNA-132 and miRNA-214 could inhibit the expression of SREBP-1c and LDLR by combining with the 3′UTR of SREBP-1c and LDLR respectively (Hu et al., Reference Hu, Shen, Cortez, Tang, Liu, Kraemer and Azhar2013).
MicroRNAs associated with Leydig cell development
miR-140-3p was the most highly expressed miRNA expressed in a sexually dimorphic pattern, while the expression level of miR-140-5p in the testis was low. Evidence showed that the absence of miR-140-5p/miR-140-3p increased the number of mouse LCs, indicating that miR-140-3p and miR-140-5p might be related to the development of gonad as well as testis differentiation in mice (Rakoczy et al., Reference Rakoczy, Fernandez-Valverde, Glazov, Wainwright, Sato, Takada, Combes, Korbie, Miller and Grimmond2013). As explained above, miR-136 and miR-26a played roles in the immune microenvironment, which is crucial to LC development (Qin et al., Reference Qin, Lin and Xie2019).
Other findings of microRNAs in Leydig cells
Currently, in the zearalenone (ZEN)-exposed TM3 LC line, the analysis of miRNAs expression has been completed. Approximately 197 miRNAs were found to be significantly distinct from those of the control group. The predicted target genes participate in many signalling pathways, suggesting that ZEN, an important environmental pollutant, is regulated by miRNAs in LCs.(Wang M et al., Reference Wang, Wu, Li, He, Huang, Chen, Chen, Long, Yang and Li2019)
Long noncoding RNAs
In 2018, in total, 33,883 lncRNAs were identified from sheep testes. The sheep LCs with knocked out lncRNA TCONS_00863147 expressed lower levels of PRKCD (protein kinase C,delta), indicating that the PRKCD could interact with lncRNA TCONS_00863147 in a trans-activation mechanism and then played a role in spermatogenesis (Yang et al., Reference Yang, Wang, Li, Ren, Pang, Wan, Wang, Feng and Zhang2018). This study also revealed a large number of lncRNAs in the LCs, however their specific roles in male fertility remain unknown.
Tesra, a novel testis-specific lncRNA in mice, has been proved to be present in germ cells and the cytoplasm of LCs, as shown by in situ hybridization. Tesra activated Prss42/Tessp-2 gene expression by binding to the Prss42/Tessp-2 promoter and then enhancing promoter activity. Prss42/Tessp-2 played important roles in the progression of meiosis as well as in germ cell survival. It was found that Tesra, similar to other lncRNAs, might recruit histone modification enzymes or transcription factors such as GClnc1 to the Prss42/Tessp-2 promoter region. However the specific mechanism of Tesra in LCs is still unknown (Satoh et al., Reference Satoh, Takei, Kawamura, Takahashi, Kotani and Kimura2019).
Natural antisense transcripts
Natural antisense transcripts (NAT) are RNA sequences that complement a sense transcript and either encode a protein or do not encode a protein (Balbin et al., Reference Balbin, Malik, Dhanasekaran, Prensner, Cao, Wu, Robinson, Wang, Chen and Beer2015; Latge et al., Reference Latge, Poulet, Bours, Josse and Jerusalem2018). In fact, many NATs were mistakenly regarded as lncRNAs (Latge et al., Reference Latge, Poulet, Bours, Josse and Jerusalem2018). Similar to lncRNAs, NAT expression was regulated by promoters and enhancers. Notably, their sense genes or the neighbouring genes are closely connected to their expression levels (Lin et al., Reference Lin, Zhang, Luo and Zhang2015). Growing evidence implicates NATs as participants with a unique mechanism of action in gene expression (Pelechano and Steinmetz, Reference Pelechano and Steinmetz2013; Nishizawa et al., Reference Nishizawa, Ikeya, Okumura and Kimura2015; Latge et al., Reference Latge, Poulet, Bours, Josse and Jerusalem2018).
Translocator protein (Tspo), with rate-limiting step activity in steroidogenesis in LC steroidogenesis, can transport cholesterol into mitochondria (Chung et al., Reference Chung, Chen, Midzak, Burnett, Papadopoulos and Zirkin2013). Evidence showed that the expression of the Tspo gene and its function in steroidogenesis were regulated by a NAT that was specific for Tspo (Tspo-NAT) in LCs (Fan and Papadopoulos, Reference Fan and Papadopoulos2012). The extension of the SINE (short interspersed repetitive element) B2 element-mediated transcript formed Tspo-NAT in mouse tumour LCs. It has been proven that endogenous Tspo-NAT was more likely to suppress endogenous Tspo levels. In addition, the evidence also revealed that the expression of Tspo-NAT was regulated by cAMP and in this way maintained Tspo at a proper level for optimal LCs steroid production (Fan and Papadopoulos, Reference Fan and Papadopoulos2012).
The steroidogenic acute regulatory (StAR) protein is a key protein that transports cholesterol located in mitochondria from outer membrane to the inner membrane (Manna et al., Reference Manna, Cohen-Tannoudji, Counis, Garner, Huhtaniemi, Kraemer and Stocco2013). StAR NAT was complementary to the spliced StAR sense 3.5-kb transcript and was highly expressed in LC and steroidogenic tissues. Evidence has shown that the StAR RNAs sense strands and the StAR RNAs antisense strands might be regulated in coordination as they were both expressed in the same cells. It has been proven that StAR NAT could downregulate the expression of StAR protein, as well as progesterone, by regulating cAMP (Castillo et al., Reference Castillo, Fan, Papadopoulos and Podesta2011). In this way, StAR RNAs play a role in regulating steroid biosynthesis.
Conclusion
With the development of large-scale genomic technologies and bioinformatics analyses, an increasing number of ncRNAs have been identified in SCs and LCs. Noncoding RNAs especially miRNAs including Dicer-dependent and Dicer-independent miRNAs in SCs play key roles in phagocytosis, immunoprotection and SCs development. These miRNAs are essential for the junction of BTB which maintains the testicular microenvironment for spermatogenesis LC ncRNAs are involved in steroidogenesis and the production of testosterone as well as development of LCs. Many miRNAs directly target genes involved in steroidogenesis and many of these are regulated by cAMP. Recently several novel lncRNAs such as Tesra have been identified. In addition, the discovery of NATs has provided another prospect for the regulation of gene expression. Compared with the vast number of ncRNAs in somatic cells, the numbers of current functional studies are exploring only the tip of the iceberg. In addition, the discovery of pilRNAs, which are similar to piRNAs and are present in somatic cells, open new horizons for researchers.
Acknowledgements
The author thank members of the Basic Medical College of Nanchang University, China for their help.
Author contribution
Chunjie Li designed and wrote the review. Baiqi Chen assisted in writing the manuscript. Jing Wang carefully revised the article.
Financial support
This study was supported by National Natural Science Foundation of China (no. 81660332), Natural Science Foundation of Jiangxi province (no. 20151BAB205057) and Health and Family Planning Project of Jiangxi province (no. 20155634).
Conflicts of interest
The authors have no conflicts of interest.
Ethical standards
Not applicable.
