Live imaging of the Drosophila spermatogonial stem cell niche reveals novel mechanisms regulating germline stem cell output

INTRODUCTION Tissue maintenance relies on the output of adult stem cells that divide to self-renew and give rise to differentiating progeny. Residing in localized cellular micro-environments, or niches, stem cells receive molecular signals that maintain their fate and regulate their function (Morrison and Spradling, 2008). Although stem cells are considered to be long-lived relative to their differentiating progeny, they also undergo turnover (Doupe et al., 2010; Klein et al., 2010; Lopez-Garcia et al., 2010; Snippert et al., 2010; Wallenfang et al., 2006). Lost stem cells must be replaced to sustain the tissue, as depletion of the stem cell pool could result in tissue degeneration (Rossi et al., 2008). Contrary to previous models, where stem cells are considered to have low turnover rates, recent lineage tracing in the mouse testis, epidermis and intestine indicate that the stem cell pool is fairly dynamic in actively cycling tissues, and the fate of an individual stem cell cannot be reliably predicted (Doupe et al., 2010; Klein et al., 2010; Lopez-Garcia et al., 2010; Snippert et al., 2010). Formally, a stem cell can be replenished either by the division of an existing stem cell to produce two stem cells (symmetric division) or by the reversion of a differentiating cell into a stem cell (de-differentiation) (Fuller and Spradling, 2007; Morrison and Kimble, 2006). Both are thought to occur in Drosophila and mammalian stem cell systems (Barroca et al., 2009; Jiang et al., 2009; Nakagawa et al., 2007; Nakagawa et al., 2010). However, direct visualization of these events in real time in both normal and regenerating tissues has remained elusive. Live imaging of stem cells in intact tissues has yielded significant advances towards understanding stem cell function. Tracking of marked undifferentiated spermatogonia in the intact mouse testis revealed the putative niche at blood vessel branching points (Yoshida et al., 2007), and live imaging of hematopoietic stem and progenitor cells within the bone marrow identified anatomical locations involved in proper stem cell function during engraftment (Lo Celso et al., 2009; Xie et al., 2009). Therefore, live imaging has tremendous potential to enhance our understanding of stem cells, particularly when applied to tissues, such as the Drosophila gonads, where stem cells can be unambiguously distinguished from their differentiating daughters (Fuller, 1998; Fuller and Spradling, 2007). Drosophila spermatogenesis is initiated at the testis apex where ~10 germline stem cells (GSCs) are attached to a group of stromal cells called the hub (Fig. 1A). GSCs always divide with their mitotic spindles positioned perpendicularly with respect to the hub; tethering of the older (mother) centrosome at the GSC-hub interface ensures this. Thus, upon division, one GSC daughter remains adherent to the hub, and the other is displaced away from the hub, becoming a gonialblast (Hardy et al., 1979; Yamashita et al., 2003; Yamashita et al., 2007; Yamashita et al., 2010). Gonialblasts undergo four rounds of mitotic amplification with incomplete cytokinesis to produce clusters of interconnected spermatogonia that eventually form sperm (Fuller, 1998). This niche also maintains ~20 somatic stem cells known as cyst stem cells (CySCs) that flank GSCs and are also anchored to the hub (Gonczy and DiNardo, 1996; Hardy et al., 1979). CySCs divide asymmetrically to self-renew and produce cyst cells, two of which envelop each gonialblast. As the gonialblast divides, its accompanying cyst cells enlarge and sustain encystment. Both GSCs and CySCs are maintained by Janus Kinase-Signal Development 138, 3367-3376 (2011) doi:10.1242/dev.065797 © 2011. Published by The Company of Biologists Ltd