REPRODUCTIONREVIEW Regulation of spermatogonial stem cell self-renewal and spermatocyte meiosis by Sertoli cell signaling Su-Ren Chen and Yi-Xun Liu State Key Laboratory of Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China Correspondence should be addressed to Y-X Liu; Email: [email protected] Abstract Spermatogenesis is a continuous and productive process supported by the self-renewal and differentiation of spermatogonial stem cells (SSCs), which arise from undifferentiated precursors known as gonocytes and are strictly controlled in a special ‘niche’ microenvironment in the seminiferous tubules. Sertoli cells, the only somatic cell type in the tubules, directly interact with SSCs to control their proliferation and differentiation through the secretion of specific factors. Spermatocyte meiosis is another key step of spermatogenesis, which is regulated by Sertoli cells on the luminal side of the blood–testis barrier through paracrine signaling. In this review, we mainly focus on the role of Sertoli cells in the regulation of SSC self-renewal and spermatocyte meiosis, with particular emphasis on paracrine and endocrine-mediated signaling pathways. Sertoli cell growth factors, such as glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2), as well as Sertoli cell transcription factors, such as ETS variant 5 (ERM; also known as ETV5), nociceptin, neuregulin 1 (NRG1), and androgen receptor (AR), have been identified as the most important upstream factors that regulate SSC self-renewal and spermatocyte meiosis. Other transcription factors and signaling pathways (GDNF–RET–GFRA1 signaling, FGF2–MAP2K1 signaling, CXCL12–CXCR4 signaling, CCL9–CCR1 signaling, FSH–nociceptin/OPRL1, retinoic acid/FSH–NRG/ERBB4, and AR/RB–ARID4A/ARID4B) are also addressed. Reproduction (2015) 149 R159–R167 Introduction (see Fig. 1). Although PLZF (Costoya et al. 2004), SALL4 (Gassei & Orwig 2013), and CDH1 (Tokuda et al. 2007) Spermatogenesis is precisely controlled by intrinsic gene are expressed in all undifferentiated A-spermatogonia expression and extrinsic stimuli, as shown by many loss- at all stages, GFRA1 (Grasso et al. 2012), LIN28 (Zheng and gain-of-function studies. Spermatogonial stem cells et al. 2009), NANOS2 (Suzuki et al. 2009), and NGN3 (SSCs) are undifferentiated spermatogonia that maintain (Yoshida et al. 2004) are primarily expressed in specific the potential to self-renew and differentiate into types of undifferentiated A-spermatogonia. In addition, committed progenitors, which sustains spermatogenesis ID4 and PAX7 are specific to type Asingle spermatogonia (Kanatsu-Shinohara & Shinohara 2013). Although self- (Oatley et al. 2011, Aloisio et al. 2014). NANOS3 is renewal through cell division continues through the life detectable in most undifferentiated type A-spermatogo- of the organism, little is known about how germline cells nia and differentiating type A1 spermatogonia (Suzuki acquire and maintain their self-renewal activity. In mice, et al. 2009). c-KIT, a tyrosine kinase receptor for KITL, is SSCs (A ) and committed progenitor spermatogonia single expressed in some type Aaligned spermatogonia, differ- (Apaired and Aaligned) are collectively described as entiating spermatogonia, and the earliest preleptotene undifferentiated A-spermatogonia based on morpho- spermatocytes (Yoshinaga et al. 1991). SOHLH1 and logical analyses (Clermont & Bustos-Obregon 1968). SOHLH2 are expressed in both c-KIT-negative and c-KIT- C These undifferentiated A-spermatogonia undergo a series positive spermatogonia, with the exception of GFRA1 of cell divisions to form differentiating spermatogonia spermatogonia (Suzuki et al. 2012). PLZF is the first (A1, A2, A3, A4, intermediate, and B spermatogonia) transcription factor to be identified as being involved in before entering meiosis (de Rooij 2001; see Fig. 1). The SSC self-renewal. SALL4 functions by removing PLZF SSCs are defined by their ability to self-renew, and it is from its cognate targets (e.g. c-KIT). Thus, the compe- difficult to distinguish them from the more abundant tition between PLZF and SALL4 is an important topic of committed progenitors based on only morphology investigation. It has been recently shown by Liao et al. alone. Several well-defined markers can be used to (2014) that reduced levels of PLZF in Dnmt3l-knockout C identify SSCs and other undifferentiated spermatogonia Thy1 cells results in the releases of the PLZF antagonist q 2015 Society for Reproduction and Fertility DOI: 10.1530/REP-14-0481 ISSN 1470–1626 (paper) 1741–7899 (online) Online version via www.reproduction-online.org Downloaded from Bioscientifica.com at 09/25/2021 01:45:23PM via free access R160 S-R Chen and Y-X Liu understand why spermatogenesis is negatively affected onia Self-renewal g by heat. Our previous studies demonstrated that PAX7 ID4 GFRA1 NANOS2 LIN28 NGN3 PLZF CDH1 SALL4 NANOS3 A SOHLH1/2 SSCs single temporal heat stress in the testes resulted in a dramatic A Progenitor spermatogonia = paired c-KIT reduction in the expression of upstream factors, such as A aligned (4–16) A androgen receptor (AR), Wilms’ tumor gene 1 (WT1), 1 (16) A 2 (32) and liver receptor homolog 1 (LHR1) in monkey (Zhang A Differentiating spermatogonia = 3 (64) A Undifferentiated A-spermato et al. 2006, Guo et al. 2007, Chen et al. 2008), rat (Guo 4 (128) Int (256) et al. 2007), and mouse (Cai et al. 2011); however, these B (512) Leptotene effects were reversible. The reversible dedifferentiation Spermatocytes (prophase l) = Distinguishing marker of spermatogonia of Sertoli cells is followed by a temporary reversible disruption of the blood–testis barrier (BTB), which leads Figure 1 Characteristics of mammalian spermatogonial development and protein markers of SSCs. Whether SSC division is a symmetric or to dramatic germ cell apoptosis in the testis (Lue et al. asymmetric process in mammals remains a topic of debate. Regardless 2006). Importantly, heat-induced changes in Sertoli cells of division pathways, SSCs undergo either self-renewal, to maintain and the failed spermatogenesis were found to recover a pool of SSCs, or differentiation, committing to differentiating into over time. Upstream factors may act as transcription spermatogonia and then entering meiosis. A number of well-defined factors to regulate the reversible change in Sertoli cell markers of SSCs in the mammalian germline have been summarized, of differentiation and the process of spermatogenesis via which ID4 and PAX7 selectively mark type Asingle spermatogonia. BTB-associated proteins. For example, when the AR was Undifferentiated type A-spermatogonia may be basically classified into C C K K C three categories: GFRA1 NANOS2 LIN28 NGN3 , GFRA1 over-expressed, the heat-induced downregulation of C C K K K K C NANOS2 LIN28 NGN3 , and GFRA1 NANOS2 LIN28 NGN3 BTB-associated proteins was rescued. AR knockdown (Suzuki et al. 2009). The dashed and colored arrows indicate the period by RNA interference (RNAi) or treatment with an AR of expression. The succession of the various types of germ cells during antagonist (flutamide) inhibited the recovery of BTB- the early stage of mouse spermatogenesis is represented in the image: associated proteins. Furthermore, the co-localization Asingle, a single spermatogonia; Apaired, a paired spermatogonia; and interactions of components of partitioning-defective Aaligned, an aligned spermatogonia; A1,A2,A3,A4, types A1–A4 spermatogonia; Int, intermediate spermatogonia; B, type B protein 6 (PAR6)-PAR3-aPKC-Cdc42 polarity complex spermatogonia. components were disrupted in both AR-knockdown and heat-treated Sertoli cells (see Fig. 2; Li et al. 2013). Our SALL4, which is associated with maintaining the delicate study suggests that a small number of undifferentiated balance of cycling and quiescent SSCs/progenitor cells. spermatogonia survival in the disrupted niche after heat C Further study is required to investigate whether SALL4 / treatment and the SSC niche and BTB are re-established K PLZF A and A spermatogonia are destined to after the heat stress is withdrawn. Sertoli cells, which are single paired the principal contributors to the SSC niche and the BTB differentiate. The human ortholog of the mouse SSC structure, play a central role in both the disruption and maintenance factor PLZF is expressed in spermatogonia, recovery process. Absalan et al. (2011) demonstrated and its biallelic loss in humans is associated with genital that spermatogonia isolated from bilateral cryptorchid hypoplasia (Fischer et al. 2008). To our knowledge, PAX7 mice have the ability to regenerate spermatogenesis. is the only protein known to be expressed in SSCs in a Based on these findings, we support the notion that variety of mammals, including human. Further studies spermatogenesis is regulated by a complex paracrine are needed to identify additional markers of human SSCs and endocrine system within a structurally well- and other undifferentiated spermatogonia. It is also organized tissue. Central to this system are the Sertoli important to test the classic studies of SSCs through cells, which provide nutrient and structural support for spermatogonial transplantation techniques and other the differentiating germ cells. functional SSCs assays. In this review, we summarize findings regarding the Meiosis is essential for the production of haploid paracrine regulation of SSC self-renewal and spermato- gametes from spermatocytes and maintaining genome cyte meiosis by Sertoli cells. The well-defined