Special Reviews

Male and Female Drosophila Germline Stem Cells: Two Versions of Immortality

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Science  20 Apr 2007:
Vol. 316, Issue 5823, pp. 402-404
DOI: 10.1126/science.1140861

Abstract

Drosophila male and female germline stem cells (GSCs) are sustained by niches and regulatory pathways whose common principles serve as models for understanding mammalian stem cells. Despite striking cellular and genetic similarities that suggest a common evolutionary origin, however, male and female GSCs also display important differences. Comparing these two stem cells and their niches in detail is likely to reveal how a common heritage has been adapted to the differing requirements of male and female gamete production.

The germ line is emerging as one of the best models for studying the biology of adult stem cells in vivo. Many organisms produce sperm and eggs throughout their reproductive lives by classic high-throughput stem cell systems, although the active contribution of germline stem cells (GSCs) to oogenesis in adult mammals remains doubtful. Above all, GSCs fascinate as the ultimate manifestation of stemness: maintaining the only cell type capable of contributing to the next generation and thereby recapitulating a species' entire repertoire of cellular diversity.

A Tale of Two Niches

Drosophila GSCs are currently among the best-understood adult stem cells (1, 2). A primary lesson from their study is the importance of the microenvironment (the stem cell niche) in regulating decisions between self-renewal and differentiation (35). The observation that self-renewal depends on short-range signals from a supporting niche can account for many properties of stem cell populations, including their ability to expand after transplantation to a stem cell–depleted host. Dependence on niches might also defend against cancer, capping stem cell numbers and preventing these potent undifferentiated precursors from self-renewing in the wrong location (6).

In males, the niche is limited to the space adjacent to a tightly packed plug of somatic hub cells at the testis tip, a region that is sufficient to accommodate 6 to 12 GSCs. In females, the niche is adjacent to four to seven cap cells and supports two to three GSCs (Fig. 1). In both sexes, GSCs are maintained by direct attachment to niche cells via adherens junctions and by receiving local signals that fail to stimulate germ cells located even one cell diameter away from the niche. Both niches also house somatic stem cells whose daughters enclose early germ cells. Internally, GSCs share other distinctive features, including the spectrosome: an aggregate of endoplasmic reticulum–like vesicles inherited asymmetrically during mitosis. Male and female GSC daughters, known as gonialblasts or cystoblasts, respectively, undergo synchronous incomplete divisions to generate analogous 16-cell germline cysts (Fig. 1, A and B). Also, many genes are required in common, including piwi, bam, bgcn, and stet. Despite these similarities, male and female GSC niches exhibit considerable differences.

Fig. 1.

Male and female germline and somatic stem cells in their gonadal niches. (A) Ovariole tip (16 per ovary) and (B) testis tip. Germ cells, dark pink (GSCs) and light pink (cysts); somatic cells, green, blue (ESCs or CPCs), turquoise, and yellow; spectrosome (S) and fusome (F), red. (A) Female. CB, cystoblast; TF, terminal filament; CC, cap cell; EC, escort cell; FSC, follicle stem cell; FC, follicle cell. (B) Male. GB, gonialblast; C, cyst cell; black asterisks, centrosomes. (C and D) Key regulatory pathways of the niche. (C) Female. BMP signaling maintains GSCs by repressing expression of bam. Activation of STAT maintains somatic escort stem cells. Dpp, decapentaplegic. (D) Male. Hub cells (H) express Upd, which activates STAT in GSCs. Yellow, localized APC2 and β-catenin; black asterisks, mother centrosomes; green asterisks, daughter centrosomes.

Different Signals

The biggest known difference between the sexes is in the short-range signals used to specify GSC self-renewal. Female GSCs are strictly governed by bone morphogenetic protein (BMP) signaling from the niche, mediated by the ligands decapentaplegic and glass-bottomed boat (GBB) expressed in somatic niche cells (1). BMP signaling represses transcription of bam (7), a key differentiation regulator that is normally turned off in GSCs (Fig. 1C). Bam acts with its partner, benign gonial cell neoplasm (BGCN), a large DExH box protein, to drive GSC daughters that leave the niche to initiate differentiation by an unknown mechanism. The cell-fate switch may be stabilized by activation of d-smurf, an E3 ubiquitin ligase that blocks residual BMP signaling by promoting degradation of the SMAD Medea (8). Forced expression of BAM in GSCs flips the switch and causes GSCs to differentiate. Germ cell regulation by somatic BMP signals is probably an ancient mechanism (1, 9); however, BAM and BGCN orthologs are unknown outside Drosophila.

The roles of BAM, BGCN, and BMP signaling are all substantially different in males, where BAM and BGCN function to end rather than initiate mitotic cyst divisions. BMP signal reception is still required to maintain GSCs and repress bam transcription, but GBB is expressed in cyst cells and the hub, and bam is repressed in gonialblasts as well as GSCs (Fig. 1D). BAM misexpression can be toxic to male GSCs, rather than just promoting differentiation, as it does in females (10).

The primary task of controlling GSC self-renewal in males is performed by a different signal, the cytokine-like ligand Unpaired (UPD) (4, 5). UPD expressed by hub cells activates the Janus kinase–signal transducers and activators of transcription (JAK-STAT) pathway in GSCs to specify stem cell self-renewal. When a male GSC divides, the daughter retaining contact with the hub maintains stem cell identity, while the daughter displaced away experiences a weaker signal and initiates differentiation. In contrast, female GSCs have no autonomous requirement for JAK-STAT signaling (1, 11). Thus, both male and female niches maintain GSCs and stimulate asymmetric division by limiting access to a local signal, but they rely on different signals for this function.

Sexual Differences in Orientation

For niches based on local signaling to reliably program asymmetric fate outcomes, one daughter cell must preferentially receive the signal after stem cell division. This is accomplished by oriented division: Normally, only one daughter cell remains in the niche. In both sexes, GSCs attach to niche cells via localized adherens junctions rich in E-cadherin (Fig. 1, C and D). Genetically disrupting the junctional components causes stem cell loss (1, 2). The mechanism used to displace exactly one of the daughters from the niche differs between males and females, however. In males, anaphase-promoting complex 2 (APC2) colocalized with E-cadherin at the cortex where GSCs contact the hub (Fig. 1D) mediates a key polarity cue that orients the spindles of mitotic GSCs so that they lie perpendicular to the hub. There is no evidence that such a system acts in the female niche, where space limitations may constrain a daughter cell from exiting the niche. Male and female GSCs also differ in the location of the spectrosome, which may reflect as yet uncharacterized differences in centrosome behavior.

Centrosome positioning plays a key role in ensuring the correct spindle orientation in male GSCs (2, 12). Early in interphase, the single GSC centrosome localizes near the cortex where it attaches to the hub (Fig. 1D). When the duplicated centrosomes separate, unusually early in G2, one stays next to the hub, while the other migrates to the opposite side of the cell. The stereotyped position of the centrosomes reliably orients the mitotic spindle perpendicular to the interface with the hub. Perhaps because of its greater capacity to hold astral microtubules, the mother centrosome normally remains next to the hub and is retained by the GSC through many GSC generations (Fig. 1D) (12). It remains an interesting question whether preferential inheritance of the mother centrosome is important for stem cell fate or is a side effect of its tightly oriented divisional program.

Stem Cell Partnerships

Both male and female niches also maintain morphologically similar somatic stem cells interspersed among the GSCs (Fig. 1). Known as cyst progenitor cells (CPCs) in males or escort stem cells (ESCs) in females, both produce squamous nondividing daughters (cyst or escort cells) that enclose the GSC daughter destined for differentiation and persist to envelop its progeny (2, 11). In the niche, thin processes from the CPCs or ESCs cover most GSC surfaces, isolating GSCs from each other but not from hub or cap cells. The close physical association of somatic and germline stem cells within a common niche probably facilitates their spatially coordinated production of differentiating daughters that must work together.

In both sexes, the proper interaction of germline and somatic cells to form cysts requires signals processed by the germline-specific rhomboid-class protease STET and received via the epidermal growth factor receptor in the adjacent cyst or escort cells (13), and the relationship between these germline and somatic partners in a cyst appears to be important for proper differentiation. JAK-STAT signaling plays a role in the female niche: ESCs require Stat to preserve niche function (11). Thus, although both male and female niches use BMP and JAK-STAT signaling, these signals play sex-specific roles, perhaps ultimately because of the very different functions carried out by bam in the two stem cell lineages.

Rules of Succession

Given their role as cellular reservoirs, it was surprising to learn that individual male and female GSCs turn over regularly (1, 2). It is now clear that replacement GSCs for the affected niches can arise by at least two distinct mechanisms. New female GSCs are produced after GSC loss when a partner GSC divides parallel to the cap cells, causing both daughters to remain in the niche and become stem cells (3) (Fig. 2). The ability to undergo such symmetric divisions under the right circumstances has been postulated to explain the expansion in stem cell numbers observed after transplantation of mammalian GSCs into depleted seminiferous tubules (14). However, new GSCs can also arise from the reversion of transit-amplifying cells in germline cysts up to at least the eight-cell stage into fully functional stem cells (15, 16) (Fig. 2). Reversion of differentiating cyst cells may normally be prevented by BAM, an insulating layer of escort or somatic cyst cells, or other factors. Symmetric division is the only mechanism that has been shown to replace female GSCs in adults. In contrast, there is no evidence at present for symmetric male GSC divisions. Furthermore, in mammalian testes, a breakdown of the youngest undifferentiated spermatogonial cysts is thought to replenish GSCs (17). Thus, male and female GSCs may employ different lines of succession when their reigning GSCs exit the niche.

Fig. 2.

Two modes for the replacement of lost stem cells. (A) Equal stem cell division. (B) Reversion of transit-amplifying cells in germline cysts. Dark pink, GSCs; light pink, differentiating transit-amplifying cell; green, somatic cells that provide the niche.

Conclusion

Finer aspects of regulation that differ between male and female GSCs lie beneath common general features, such as adhesion to stromal cells and dependence on close range signals. We can only speculate about the selective forces that may be responsible for these differences. The fact that there is only 1 niche per testis and 16 niches per ovary may explain the larger number of GSCs per niche in males. The more robust mechanism of spindle orientation in male GSCs may be a consequence of the larger niche. This, in turn, may cause a preferential use of cyst reversion rather than symmetric GSC division to generate replacement stem cells. Nonetheless, it is probably safe to expect that each stem cell type has become more or less specialized to the particular requirements of the cells it supports. Thus, while searching for the common laws of stem cells, we must continue to expect the unusual and even the unique.

References and Notes

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