FGF2 Has Distinct Molecular Functions from GDNF in the Mouse Germline Niche

Abstract

SummaryBoth glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2) are bona fide self-renewal factors for spermatogonial stem cells, whereas retinoic acid (RA) induces spermatogonial differentiation. In this study, we investigated the functional differences between FGF2 and GDNF in the germline niche by providing these factors using a drug delivery system in vivo. Although both factors expanded the GFRA1+ subset of undifferentiated spermatogonia, the FGF2-expanded subset expressed RARG, which is indispensable for proper differentiation, 1.9-fold more frequently than the GDNF-expanded subset, demonstrating that FGF2 expands a differentiation-prone subset in the testis. Moreover, FGF2 acted on the germline niche to suppress RA metabolism and GDNF production, suggesting that FGF2 modifies germline niche functions to be more appropriate for spermatogonial differentiation. These results suggest that FGF2 contributes to induction of differentiation rather than maintenance of undifferentiated spermatogonia, indicating reconsideration of the role of FGF2 in the germline niche.

Biology and manipulation technologies of male germline stem cells in mammals

“…These observations raise the possibility that FGF2 plays a role distinct from GDNF in controlling the fate of SSCs/undifferentiated spermatogonia in vivo. Our most recent studies revealed expression of Fgf2 in the germ cell population, while Gdnf is expressed in Sertoli cells and peritubular myoid cells . In these studies, germ cell depletion increased the relative expression of Gdnf , while Fgf2 was relatively suppressed .…”

“…Our most recent studies revealed expression of Fgf2 in the germ cell population, while Gdnf is expressed in Sertoli cells and peritubular myoid cells . In these studies, germ cell depletion increased the relative expression of Gdnf , while Fgf2 was relatively suppressed . Considering that germ cell‐depleted conditions are relatively appropriate for expansion rather than differentiation of transplanted undifferentiated spermatogonia/SSCs, the Gdnf/Fgf2 ratio might affect fate determination of undifferentiated spermatogonia/SSCs (Figure ).…”

“…In addition, the former cells survive and proliferate without MAP2K1 (dual specificity mitogen‐activated protein kinase 1) activation, whereas the latter cells require its activation, suggesting that FGF2 promotes survival and proliferation of SSCs via signals distinct from GDNF signals. In fact, we found that FGF2‐stimulated undifferentiated spermatogonia form colonies with characteristics distinct from those of GDNF‐induced undifferentiated spermatogonia in vivo . In this study, forced input of strong FGF2/GDNF signals was applied directly in mouse testes using intelligent biomaterials.…”

“…FGF2‐induced GFRA1 + spermatogonia are a differentiation‐prone subset because these cells tend to express RARG, the receptor for retinoic acid that induces spermatogonial differentiation. However, GDNF‐induced GFRA1 + spermatogonia tend to be negative for RARG, suggesting that these cells are a differentiation‐resistant subset . The Gdnf/Fgf2 ratio decreases along with postnatal testicular development and increases during regeneration .…”

Open niche regulation of mouse spermatogenic stem cells

“…Sertoli cells also play an important role in the retinoic acid (RA) signal that promotes irreversible differentiation to Kit + spermatogonia, as Sertoli cells and different stages of germ cells cooperatively express the enzymes involved in the retinoid metabolism (Endo, Freinkman, De Rooij, & Page, ; Sugimoto, Nabeshima, & Yoshida, ; Vernet et al., ). According to in vitro studies using cultured spermatogonia and in vivo studies using transient transfection and bead implantation, fibroblast growth factor (FGF) is another important regulator of SSCs (Hasegawa & Saga, ; Kanatsu‐Shinohara et al., ; Masaki et al., ; Takashima et al., ). This notion is also supported by the analyses of causal mutations for a group of human paternal‐age effect genetic disorders (e.g., Apert syndrome and achondroplasia), which has led to the concept of selfish selection of SSCs carrying gain‐of‐function mutations in the FGF receptor or downstream effector genes that occur in fathers’ seminiferous tubules (Goriely, Mcgrath, Hultman, Wilkie, & Malaspina, ; Goriely, Mcvean, Rojmyr, Ingemarsson, & Wilkie, ).…”