RNA Binding Protein Nanos2 Organizes Post-transcriptional Buffering System to Retain Primitive State of Mouse Spermatogonial Stem Cells

Abstract

In many adult tissues, homeostasis relies on self-renewing stem cells that are primed for differentiation. The reconciliation mechanisms of these characteristics remain a fundamental question in stem cell biology. We propose that regulation at the post-transcriptional level is essential for homeostasis in murine spermatogonial stem cells (SSCs). Here, we show that Nanos2, an evolutionarily conserved RNA-binding protein, works with other cellular messenger ribonucleoprotein (mRNP) components to ensure the primitive status of SSCs through a dual mechanism that involves (1) direct recruitment and translational repression of genes that promote spermatogonial differentiation and (2) repression of the target of rapamycin complex 1 (mTORC1), a well-known negative pathway for SSC self-renewal, by sequestration of the core factor mTOR in mRNPs. This mechanism links mRNA turnover to mTORC1 signaling through Nanos2-containing mRNPs and establishes a post-transcriptional buffering system to facilitate SSC homeostasis in the fluctuating environment within the seminiferous tubule.

Paracrine Wnt/β-catenin signaling mediates proliferation of undifferentiated spermatogonia in the adult mouse testis

“…Combined with the data from lineagetracing analysis (Fig. 3D), these results raise the possibility that Wnt/β-catenin signaling is required for the generation of progenitor cells that are committed to differentiation from undifferentiated spermatogonia, whereas the maintenance of the stem cell pool via self-renewal is regulated by other mechanisms, such as GDNF (8) and Nanos2 (46). This finding agrees with in vitro experiments that indicate that Wnt/β-catenin signaling does not act primarily on SSCs but instead on progenitors (14).…”

Retinoid signaling controls spermatogonial differentiation by regulating expression of replication-dependent core histone genes

“…The undifferentiated spermatogonia can be subdivided into A single (A s ), A paired (A p ) and A aligned (A al ) spermatogonia and include spermatogonial stem cells (SSCs) and progenitor spermatogonia. This cohort of undifferentiated spermatogonia express self-renewal-and proliferation-associated genes such as Pou5f1 (Pesce et al, 1998), Lin28a (Tong et al, 2011;Zheng et al, 2009), Mir-21 (Niu et al, 2011), Mir-17∼92 (Mirc1) (Tong et al, 2012), Foxo1 (Goertz et al, 2011), Nanos2 (Sada et al, 2009;Zhou et al, 2015), Neurog 3 (Ngn3) (Nakagawa et al, 2007), Sox3 (Laronda and Jameson, 2011), Taf4b (Falender et al, 2005), and Zbtb16 (Plzf ) (Buaas et al, 2004;Costoya et al, 2004) to maintain the capacity for self-renewal and proliferation. During spermatogonial differentiation, the undifferentiated spermatogonia downregulate these self-renewal associated genes and upregulate genes associated with differentiation such as Sohlh1 (Ballow et al, 2006), Sohlh2 (Hao et al, 2008), Stra8 (Endo et al, 2015;Zhou et al, 2008), Kit (Schrans-Stassen et al, 1999), Ccnd2 (Beumer et al, 2000) and Sall4 (Hobbs et al, 2012;Gely-Pernot et al, 2015).…”

NEDD4 controls spermatogonial stem cell homeostasis and stress response by regulating messenger ribonucleoprotein complexes

“…This RNA-based regulation influences many biological processes, including stem cell homeostasis, embryogenesis and stress response123. The post-transcriptional regulation of mRNA is controlled by a complicated repertoire of messenger ribonucleoprotein (mRNP) complexes4.…”

“…PBs usually consist of mRNAs aggregated with mRNA degradation machinery; in general, these mRNPs are present at a low level, but can be upregulated if a large pool of nontranslated mRNAs appears, for example, in spermatogonial stem cells (SSC), satellite cells or neural stem cells136. SGs are aggregates formed under stress conditions such as a low nutrient supply, heat or hypoxia; SGs are thought to represent a pool of mRNPs in a state of translational repression5.…”

“…These environmental signals synchronize differentiation of germ cells and generate a stage-dependent distribution pattern of germ cells13. Recently, we provided evidence of a post-transcriptional buffer system controlled by NANOS2-mRNP complexes that protect GFRα1 + NANOS2 + stem cells from differentiation signals in the seminiferous tubules3. Little is known, however, about the mechanism of removal of this NANOS2-mRNP barrier and the eventual induction of SSC differentiation.…”