Somatic cell lineage is required for differentiation and not maintenance of germline stem cells in Drosophila testes

Adult stem cells are believed to be maintained by a specialized microenvironment, the niche, which provides short-range signals that either instruct stem cells to self-renew or inhibit execution of preprogrammed differentiation pathways. In Drosophila testes, somatic cyst stem cells (CySCs) and the apical hub form the niche for neighboring germline stem cells (GSCs), with CySCs as the proposed source of instructive self-renewal signals Leatherman JL, Dinardo S (2010) Nat Cell Biol 12(8):806–811]. In contrast to this model, we show that early germ cells with GSC characteristics can be maintained over time after ablation of CySCs and their cyst cell progeny. Without CySCs and cyst cells, early germ cells away from the hub failed to initiate differentiation. Our results suggest that CySCs do not have a necessary instructive role in specifying GSC self-renewal and that the differentiated progeny of CySCs provide an environment necessary to trigger GSC differentiation. This work highlights the complex interaction between different stem cell populations in the same niche and how the state of one stem cell population can influence the fate of the other.The ability of a stem cell niche to maintain a population of stem cells ensures the continued availability of adult stem cells to replenish and repair specific tissues throughout the lifetime of an organism (1, 2). Failure of a niche to maintain its appropriate stem cell population may lead to degeneration, aging, or an inability to repair tissue damage (3). Conversely, failure of a niche to properly regulate differentiation versus proliferation may contribute to the genesis of cancer in adult stem cell lineages (4). A comprehensive understanding of how the local microenvironment of the stem cell niche functions suggests strategies for expansion of adult stem cell populations in vitro, facilitates design of artificial niches for transplantation, and provides ideas for increasing maintenance and functionality of endogenous adult stem cell populations used for regenerative medicine.The Drosophila testis stem cell niche, a key model for understanding how the microenvironment regulates stem cell behavior (5–7), supports two distinct adult stem cell populations—germline stem cells (GSCs) and cyst stem cells (CySCs)—both of which attach to a cluster of postmitotic somatic cells that form the apical hub (Fig. 1A). GSCs and CySCs normally divide with oriented spindles to produce daughters that remain next to the hub and self-renew and daughters displaced away from the hub that initiate differentiation (8, 9). GSCs give rise to gonialblasts (Gb) and CySCs give rise to postmitotic cyst cells (10), a pair of which encapsulates each Gb to form a cyst. The encapsulated Gb undergoes four rounds of synchronous transit-amplifying (TA) divisions before entering meiosis and terminal differentiation (Fig. 1A).Open in a separate windowFig. 1.Early germ cells can be maintained after ablation of CySCs and cyst cells. (A) Diagram of Drosophila spermatogenesis at the apical tip of the testis. (Red) fusomes; (green) Bam protein expression. (B–G) Immunofluorescence images of c587Gal4; UAS Grim; tubGal80^ts testes stained with anti-FasIII (white, hub), anti-Vasa (red, germ cells), and anti-Tj (green) nuclei of hub, CySCs, and cyst cells). (B) Newly eclosed flies before shift to 30 °C. (Arrowheads) CySCs; (arrows) cyst cells. (C) Flies shifted to 30 °C for 1 d. (D) Flies shifted to 30 °C for 1 d and back to 18 °C for 7 d. (E) Flies shifted to 30 °C for 1 d and back to 18 °C for 7 d. (F) Flies shifted to 30 °C for 1 d and back to 18 °C for 14 d. (G) Flies shifted to 30 °C for 1 d and back to 18 °C for 21 d. (H) Bar graph depicting phenotype distribution at different time points. (Blue bar) Testes with CySCs and/or cyst cells (incomplete ablation); (red bar) testes with early germ cells but lacked CySCs or cyst cells; (green bar) testes lacking early germ cells, CySCs, and cyst cells. No significant difference in phenotype distribution was observed among the 7-, 14-, and 21-d time points. (Scale bar: B–G, 10 μm.)Both the apical hub and the CySCs influence the GSC state. A cytokine-like signal from the hub activates the JAK-STAT signaling pathway in both GSCs and CySCs (11, 12). Although JAK-STAT signaling is required cell autonomously for CySC maintenance, it is not necessary to retain GSCs in their stem cell state. Rather, activity of Stat in the germline is essential for continued attachment of GSCs to the hub and retains GSCs in their niche (13). Several lines of evidence suggest that CySCs provide a niche for maintenance of GSCs (13–15). Consistent with this model, it has been proposed that self-renewal of GSCs is specified by instructive signal(s) from the CySCs, with a likely candidate being TGF-β signaling (13).Here we show that early germ cells can be maintained next to the hub in testes in which CySCs and cyst cells had been permanently ablated. We further show that the progeny of GSC-like cells displaced from the hub failed to initiate the TA program in the absence of CySCs and cyst cells, and instead continued to proliferate as undifferentiated cells. Our findings suggest that CySCs do not play a required instructive role in GSC self-renewal and that cyst cells, the differentiated progeny of CySCs, are required for proper onset of the germline differentiation.