Functions of the Drosophila JAK-STAT pathway: Lessons from stem cells

Abstract

JAK-STAT signaling has been proposed to act in numerous stem cells in a variety of organisms. Here we provide an overview of its roles in three well characterized stem cell populations in Drosophila, in the intestine, lymph gland and testis. In flies, there is a single JAK and a single STAT, which has made the genetic dissection of pathway function considerably easier and facilitated the analysis of communication between stem cells, their niches and offspring. Studies in flies have revealed roles for this pathway as diverse as regulating bona fide intrinsic self-renewal, integrating response to environmental cues that control quiescence and promoting mitogenic responses to stress.

Keywords: CySC; Drosophila; STAT92E; hematopoiesis; intestinal stem cell; lymph gland; prohemocyte; testis.

Figure 1. The Drosophila JAK-STAT pathway. The Drosophila JAK-STAT pathway consists of three Unpaired (Upd) ligands here collectively referred to as Upd (orange). The receptor Domeless (Dome) (magenta) is activated when Upd binds. This results in activation of the JAK Hopscotch (Hop) (green), leading to tyrosine phosphorylation (brown circles) of Dome. The phosphorylated receptor serves as a docking site for a STAT92E dimer (blue). Once bound, STAT92E is phosphorylated on tyrosine 711, generating an active STAT92E dimer that translocates to the nucleus, binds to a consensus TTCNNNGAA site, and alters gene expression. Some of the best-characterized STAT92E target genes are SOCS36E, zfh1, chinmo. SOCS36E (pink) encodes a negative regulator of Dome/JAK activity. A second receptor Eye Transformer (Et) [also called Latran (Lat)] referred to as Et/Lat (red) forms heterodimers with Dome and inhibits JAK-STAT signaling.

Figure 2. The adult midgut. (A) Homeostasis: absorptive polyploid enterocytes (ECs, blue) comprise the majority of the intestinal epithelium. Interspersed are hormone-producing enteroendocrine (ee) cells (green). Intestinal stem cells (ISCs) (yellow) are found close to the basement membrane (BM, red line). Visceral muscle cells (VM, dark green) reside under the basement membrane. ISCs express high levels of the Notch ligand Delta (red circles). Yorkie (Yki) activity is repressed in ISCs and ECs by the Hippo (Hpo) pathway. Notch signaling is prominent in enteroblasts (EBs, orange), the ISC daughter cell that gives rise to EC and ee cells. Activity of the JAK-STAT pathway (denoted pSTAT) is highest in EBs. STAT92E function is required in EBs for differentiation (arrows from pSTAT to EC and ee cells). Whether ISCs require JAK-STAT signaling for self-renewal is still controversial, as is the cellular source of upd (see text for details). (B) Regeneration: under a variety of conditions, including basement membrane damage (DSS), bacterial infection or bleomycin, ISC proliferation rate is dramatically increased as a regenerative response in the intestinal epithelium. In ECs, JNK signaling is activated while Hpo is inactivated. This leads to the nuclear accumulation of Yki, which directly or indirectly induces upd genes, in particular upd3. Upd ligands act non-cell-autonomously on ISCs and activate STAT92E (pSTAT) in these stem cells, which rapidly increases their proliferation rate. Upd can also be produced by ISCs, which then acts in an autocrine manner to stimulate ISC mitoses. Some groups have reported that Yki is activated in ISCs during regeneration and that this drives ISC proliferation in parallel to JAK-STAT signaling.

Figure 3. Hematopoiesis. (A) The lymph gland is the larval hematopoietic organ. In the third larval instar, one large anterior lobe and several smaller posterior lobes are found along the dorsal vessel, interspersed with pericardial cells. In the anterior lobe, there are three distinct zones. Cells in the posterior signaling center (PSC, blue) form the niche for multipotent progenitors called prohemocytes that reside in the medullary zone (MZ, green). Prohemocytes are quiescent and eventually give rise to all Drosophila blood lineages. Differentiation of hemocytes occurs in the cortical zone (CZ, red). Under homeostatic conditions, prohemocytes give rise primarily to plasmatocytes, which are phagocytic cells, and crystal cells, which mediate melanization reactions. Under immune challenge, prohemoctytes give rise to lamellocytes, large flat cells that encapsulate foreign objects. Plasmatocytes constitute more than 90% of differentiated hemocytes, while crystal cells and lamellocytes make up less than 5%. (B) Cells in the PSC (blue) produce soluble proteins like Upd3 and Hedgehog (Hh). Prohemocytes in the MZ (green) respond to Upd and Hh by activating JAK-STAT (pSTAT) and Hh signaling (Ci^ACT), respectively, which keeps these progenitors quiescent. PSC cells also produce Pvf1, which signals to cells in the CZ (red). Pvf1 activates Pvr on differentiating cells in the CZ, which leads directly or indirectly to activation of STAT92E (pSTAT) independently of canonical JAK-STAT signaling. Activated STAT92E increases expression of Adgf-A, a secreted protein. The primary function of Adgf-A is to inactivate extracellular adenosine, which can bind the adenosine receptor AdoR to activate PKA. By contrast, Hh signaling inhibits PKA. Since the level of PKA activity regulates whether Ci^ACT is cleaved into a shorter repressor form (Ci^R), PKA is a node through which prohemocytes can be regulated to remain quiescent via Hh produced by the niche or a Pvr/STAT/Adgf-A cascade in CZ cells.

Figure 4. Testis stem cell niche. (A) Hub cells are post-mitotic and function as a niche to support two populations of stem cells, germline stem cells (GSCs) (pink/red) and cyst stem cells (CySCs) (dark blue). GSCs can divide asymmetrically to self-renew and give rise to gonialblasts (GB), which undergo four mitoses with incomplete cytokinesis to generate spermatogonia. CySCs also divide asymmetrically but their progeny, the cyst cells (light blue), are post-mitotic, and envelop the developing germ cells while providing required support for their developmental progression. (B) Hub cells secrete the JAK-STAT ligand Upd. STAT92E activation (pSTAT) is required for CySC self-renewal and for adhesion to the niche in GSCs but for not GSC self-renewal per se. The STAT92E target genes zfh1 and chinmo are also required for CySC self-renewal. In addition, hub cells produce bone morphogenetic proteins (BMPs) and activation of BMP signaling in GSCs is required for their self-renewal. Furthermore, sustained activation of the JAK-STAT pathway or mis-expression of its targets zfh1 and chinmo in CySCs causes BMP expression and expands both the CySC and GSC populations.