Abstract: Stem cell niches are well-characterized factories of signaling information, but niche cells themselves also rely on their neighbors for fate maintenance. In this issue of Developmental Cell, Herrera et al. reveal bi-directional communication between Drosophila testis niche "hub" cells and somatic cyst stem cells. Stem cell niches are well-characterized factories of signaling information, but niche cells themselves also rely on their neighbors for fate maintenance. In this issue of Developmental Cell, Herrera et al. reveal bi-directional communication between Drosophila testis niche "hub" cells and somatic cyst stem cells. Stem cell populations frequently rely on non-dividing signaling centers, known as "niches," in order to balance the proliferation and differentiation requirements of cell lineages. Two stem cell populations in the apical tip of the Drosophila testis are regulated in such a niche environment: germline stem cells (GSCs) that ultimately produce sperm, and somatic cyst stem cells (CySCs) that produce somatic support cells (Figure 1A). Prior work has shown that niche cells, called "hub cells", go "above and beyond" in order to maintain the CySC population. After genetic ablation of all CySCs, hub cells have the capacity to exit quiescence and transdifferentiate into CySCs, regenerating the CySC population (Hétié et al., 2014Hétié P. de Cuevas M. Matunis E. Conversion of quiescent niche cells to somatic stem cells causes ectopic niche formation in the Drosophila testis.Cell Rep. 2014; 7: 715-721Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar). However, this regeneration is short-lived because the CySC population cannot be sufficiently supported by the now-reduced number of hub cells, and this likely contributes to age-dependent physiological decay of the niche. Therefore, there must be a mechanism that protects the hub cell population to prevent unnecessary transdifferentiation. Now, in this issue of Developmental Cell, Herrera et al. beautifully demonstrate the underlying mechanism of hub cell fate control by CySCs (Herrera et al., 2021Herrera S.C. de la Maza D.S. Grmai L. Margolis S. Plessel R. Burel M. O'Connor M. Amoyel M. Bach E.A. Proliferative stem cells maintain quiescence of their niche by secreting the Activin inhibitor Follistatin.Dev. Cell. 2021; 56 (2284–2294.e6)Abstract Full Text Full Text PDF Scopus (4) Google Scholar). Herrera et al. first found that the transcription factor complex E2f1/Dp is required for maintenance of the CySC population. Surprisingly, while depletion of E2f1/Dp from all CySCs led to CySC loss, CySCs that were mutant for Dp were maintained when they had non-mutant neighboring CySCs. The authors showed that the primary defect caused by depletion of E2f1/Dp from all CySCs was the loss of hub cells, which in turn led to the loss of CySCs. Based on this observation, they hypothesized that CySCs secrete a factor downstream of E2f1/Dp that maintains hub cells, and in the absence of such a factor, hub cells are lost, depleting stemness self-renewing signals and leading to the loss of CySCs. A screen for secreted factors expressed in CySCs revealed that knockdown of the Activin inhibitor Follistatin phenocopied the Dp phenotype, suggesting that Follistatin acts downstream of Dp. Consistently, the authors observed a reduction in Follistatin expression in Dp-RNAi animals. Furthermore, addition of exogenous Follistatin fully rescued the hub cell loss found in Dp-RNAi animals. From these findings, the authors speculated that secretion of Follistatin from CySCs maintains an Activin-low environment for hub cell maintenance. They further found that the type I Activin receptor Baboon is present at the membrane of hub cells, and this suggests that hub cells are receptive to surrounding Activin levels. The authors then autonomously and constitutively activated Baboon in hub cells and showed that such autonomous Activin-high signaling led to hub cell loss. Could Activin signaling regulate the identity switch of hub cells into CySCs? Lineage tracing revealed that after autonomous Activin signaling in the hub cells, many CySCs had originated from hub cell transdifferentiation. Similarly, when Dp was depleted in CySCs, hub-lineage CySCs were detected at an approximately 9-fold higher rate than in control samples. These CySCs that arose from a hub cell transdifferentiation event showed many of the hallmarks of normal CySCs, complete with CySC localization, marker expression, morphology, and cycling behavior. Under normal physiological conditions, total CySC ablation is unlikely. What, therefore, is the functional significance of CySC-to-hub signaling? The authors observed that Follistatin expression significantly declined over the course of the fly lifetime, and this indicates that it may be responsible for the age-related decrease in hub cell numbers (Wallenfang et al., 2006Wallenfang M.R. Nayak R. DiNardo S. Dynamics of the male germline stem cell population during aging of Drosophila melanogaster.Aging Cell. 2006; 5: 297-304Crossref PubMed Scopus (100) Google Scholar). They found that the overexpression of Follistatin in CySCs could block age-related hub cell loss, as did knockdown of baboon or the Activin transcriptional effector, Smox, in hub cells. Therefore, the regular replacement of CySCs by transdifferentiating hub cells could be a necessary homeostatic mechanism for regulating CySC number over the course of aging. Lastly, the authors looked at which of the three Activin ligands present in Drosophila are inhibited by CySC-secreted Follistatin. The Activin ligand Dawdle, unlike the ligands Activinβ and Myoglianin, is expressed in the testis stem cell niche. Dawdle transcripts were found in both hub cells and CySCs, and dawdle knockdown in hub cells suppressed hub cell loss through transdifferentiation. Together, these data—including many well-designed controls—present an elegant view of bi-directional niche-stem-cell signaling in the Drosophila testis. As controlled by the transcription factor complex Dp/E2f1, CySCs will typically express and secrete Follistatin, inhibiting local Dawdle ligands and maintaining an Activin-low environment (Figure 1B). In the absence of CySCs, insufficient Follistatin is produced, and Dawdle ligands activate Baboon receptors present on hub cells. Responding to this Activin signal, hub cells will transdifferentiate, adopting a CySC fate: simultaneously resulting in a regeneration of lost CySCs and a loss of necessary hub cells (Figure 1C). Follistatin/Activin signaling has been demonstrated to play a role in stem cell and regenerative contexts in many species, such as in axolotl limb regeneration (Bryant et al., 2017Bryant D.M. Johnson K. DiTommaso T. Tickle T. Couger M.B. Payzin-Dogru D. Lee T.J. Leigh N.D. Kuo T.H. Davis F.G. et al.A Tissue-Mapped Axolotl De Novo Transcriptome Enables Identification of Limb Regeneration Factors.Cell Rep. 2017; 18: 762-776Abstract Full Text Full Text PDF PubMed Scopus (411) Google Scholar), zebrafish cardiomyocyte repair (Dogra et al., 2017Dogra D. Ahuja S. Kim H.T. Rasouli S.J. Stainier D.Y.R. Reischauer S. Opposite effects of Activin type 2 receptor ligands on cardiomyocyte proliferation during development and repair.Nat. Commun. 2017; 8: 1902Crossref PubMed Scopus (39) Google Scholar), and planarian whole-body regeneration (Cloutier et al., 2021Cloutier J.K. McMann C.L. Oderberg I.M. Reddien P.W. activin-2 is required for regeneration of polarity on the planarian anterior-posterior axis.PLoS Genet. 2021; 17: e1009466Crossref PubMed Scopus (3) Google Scholar; Tewari et al., 2018Tewari A.G. Stern S.R. Oderberg I.M. Reddien P.W. Cellular and Molecular Responses Unique to Major Injury Are Dispensable for Planarian Regeneration.Cell Rep. 2018; 25: 2577-2590.e3Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar). Though under-studied relative to other TGFβ ligands, Follistatin/Activin are frequently uniquely capable of generating tight signaling environments through secreted activator-inhibitor networks. Herrera et. al. now add regulation of niche cell fate to the list of important stem cell behaviors mediated by Follistatin/Activin. Could other quiescent niche cells not only be capable of maintaining their resident stem cell populations, but also serve as the cellular source for stem cell regeneration after loss? There is precedent elsewhere: Paneth cells, which serve as niches for Lgr5+ intestinal stem cells (Lgr5+SCs), can generate Lgr5+SCs after irradiation-induced loss (Yu et al., 2018Yu S. Tong K. Zhao Y. Balasubramanian I. Yap G.S. Ferraris R.P. Bonder E.M. Verzi M.P. Gao N. Paneth Cell Multipotency Induced by Notch Activation following Injury.Cell Stem Cell. 2018; 23: 46-59.e5Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar). Notably, hub cells and CySCs share a common developmental lineage (Okegbe and DiNardo, 2011Okegbe T.C. DiNardo S. The endoderm specifies the mesodermal niche for the germline in Drosophila via Delta-Notch signaling.Development. 2011; 138: 1259-1267Crossref PubMed Scopus (24) Google Scholar), and Paneth cells are direct differentiation progeny of Lgr5+SCs. Such lineage relationships could potentially prime these pairs for some degree of fate interconversion in the adult – interconversion that is not available to developmentally divergent niche/stem cell pairs (e.g., hub cells and GSCs). Nevertheless, this work highlights the necessity of questioning the exclusive primacy of stem cell self-renewal in maintenance of many stem cell populations. These and other questions remain, but the advances described here by Herrera et al. will provide an excellent framework for truly understanding the complexities of stem cell niche dynamics. The authors declare no competing interests. Proliferative stem cells maintain quiescence of their niche by secreting the Activin inhibitor FollistatinHerrera et al.Developmental CellAugust 6, 2021In BriefHow stem cells support niches remains largely unknown. Herrera et al. show that testicular stem cells maintain niche quiescence by secreting Follistatin, an Activin antagonist. Loss of Follistatin in stem cells or gain of Activin signaling in niche cells causes niche-to-stem-cell transdifferentiation, and this process underlies niche decay during aging. Full-Text PDF Open Access