In contrast to the attention
In contrast to the attention devoted to GDNF, little work has focused on exploration of the role played by fibroblast growth factor 2 (FGF2), which is thought to be essential for SSC self-renewal (Kanatsu-Shinohara and Shinohara, 2013). The effects of FGF2 have been analyzed in vitro. FGF2 induces both MAPK1/3 and AKT phosphorylation in GS cells, and what does cytoskeleton do expressing activated MAP2K1 not only induced MAPK1/3 phosphorylation but also proliferated without FGF2, albeit at a slower rate than with FGF2 and GDNF (Ishii et al., 2012). In contrast, constitutively active AKT can replace GDNF and GS cells transfected with activated AKT proliferate in the absence of GDNF (Lee et al., 2007). Studies in humans have shown that spermatogonia carrying FGF receptor mutations preferentially transmit abnormal genetic haplotypes to the next generation. Mutations in Fgfr2 (in patients with Apert syndrome: C755G) or Fgfr3 (in patients with Achondroplasia: G380R) are thought to occur at the SSC level because sperm mutation frequencies increase with age; mutations in progenitor cells disappear due to the lack of self-renewal activity (Bellus et al., 1995; Goriely et al., 2005). Such results suggest that hyperactivation of FGF signaling enhances SSC self-renewal; however, the relevant in vivo mechanism remains unclear.
It is generally believed that progressive loss of spermatogenesis and development of “empty” tubules, as found in Gdnf/Ret/Gfra1 KO mice, are caused by reduced SSC self-renewal. However, we hypothesized that cessation of spermatogenesis would not necessarily indicate that SSCs were deficient. As we worked to confirm the role played by GDNF in vivo, we found that a small number of undifferentiated spermatogonia survived and formed colonies in Ret mutant mice, encouraging us to examine the role played by FGF2 in vivo and to seek to recapitulate SSC self-renewal in vitro in the absence of GDNF signaling.
Discussion Both GDNF and FGF2 are expressed by Sertoli cells, but very little is known about the roles played by these materials in vivo or the spatial relationship among such cells and the SSCs of seminiferous tubules. In the present study, we used a Ret mutant mouse strain to explore whether SSCs survived in the absence of GDNF signaling. Although spermatogonial transplantation is usually the best approach to testing for the presence of SSCs, we reasoned that analysis of the outcomes of germ cell transplantation from Ret or Gfra1 mutant mice might not be useful in the present context. As GDNF is apparently involved in spermatogonial proliferation, it was possible that lack of such signaling would limit colony expansion. In addition, GDNF has been implicated in migration of SSCs to their niches (Kanatsu-Shinohara et al., 2012). Therefore, we decided to analyze the in vivo development of mutant testes by transplanting testis fragments of Ret mutant mice into surrogate animals. We clearly showed that undifferentiated spermatogonia survived despite the absence of Y1062 phosphorylation, which is thought to be required for SSC self-renewal (Jain et al., 2004; Jijiwa et al., 2008). Although the frequency of germ cell clusters obtained was low, seven of eight transplanted fragments contained areas of mutant tubules with CDH1+ cells. Such results were unexpected because clusters of this type have not been reported in previous analyses of Gdnf/Ret/Gfra1 KO mice. The failure to detect germ cells in previous studies might be attributable to the lack of surrounding host Leydig cells in subcutaneous tissue (Naughton et al., 2006); such cells are thought to synthesize niche factors (Oatley et al., 2009). In this context, transplantation of testis fragments to busulfan-treated testes, as in the present study, may have afforded a better proliferative environment in that Leydig cells were present. Thus, our finding raise the possibility that a subset of As spermatogonia survive and proliferate in the absence of GDNF signaling.