Signal transduction in the early Drosophila follicle stem cell lineage

Abstract

The follicle stem cell (FSC) lineage in the Drosophila ovary is a highly informative model of in vivo epithelial stem cell biology. Studies over the past 30 years have identified roles for every major signaling pathway in the early FSC lineage. These pathways regulate a wide variety of cell behaviors, including self-renewal, proliferation, survival and differentiation. Studies of cell signaling in the follicle epithelium have provided new insights into how these cell behaviors are coordinated within an epithelial stem cell lineage and how signaling pathways interact with each other in the native, in vivo context of a living tissue. Here, we review these studies, with a particular focus on how these pathways specify differences between the FSCs and their daughter cells. We also describe common themes that have emerged from these studies, and highlight new research directions that have been made possible by the detailed understanding of the follicle epithelium.

Figure 1:. The Drosophila Germarium

Diagram of the early stages of Drosophila oogenesis and overview of sources of selected signaling ligands implicated in follicle cell development. The Drosophila germarium is divided in four subregions (1, 2a, 2b and 3). The first budded cyst is referred to as stage 2. Anterior-most terminal filament (TF) and cap cells (CC) build the niche for germline stem cells (GSC). Together with the inner germarial sheath (IGS) cells TF and CC provide Hedgehog (Hh) ligand to follicle stem cells (FSC), which are located at the 2a/2b border. IGS cells further provide Wingless (Wg) to FSCs. In response, FSCs and pFCs produce Spitz (Spi). A subset of prefollicle cells (pFC) receives Delta (Dl) from germline cells (GC) and assumes polar cell fate. Polar fated cells produce the JAK-STAT ligand Unpaired (Upd), which specifies stalk cells. To date, no signaling pathways have been identified to induce the earliest steps towards main body (MB) cell fate.

Figure 2:. Wingless and EGFR signaling pathways

A) Wg is provided to FSCs by IGS cells leading to high levels of Wnt pathway activity in FSCs, which changes dynamically [17,19]. The glypican Dlp aids to concentrate Wg in the niche region and is counteracted by Matrix metalloproteinase 2 (Mmp2). In the absence of Wg ligand, Arm is subject to proteasomal degradation induced by a destruction complex consisting of Apc, Shaggy (Sgg) and Axin (Axn). When Wg binds to its receptor Frizzled (Fz), active Dishevelled (Dsh) represses destruction complex activity allowing arm to interacts with Pangolin (Pan) to activate target gene expression. B) The expression of the EGFR ligand Spitz (Spi) is induced by Wnt signaling in the FSC. It is unclear whether Gurken (Grk) may also function as an EGFR ligand in the FSCs. EGFR signaling is active in FSCs, uniformly low in newly produced pFCs and active again in older pFCs and main body follicle cells. In FSCs activated EGFR signaling induces the MAP-Kinase pathway resulting in the dual-phosphorylation and activation of ERK. One target of dpERK is Groucho (Gro), which is repressed by ERK-mediated phosphorylation and functions as a molecular timer regulating pFC differentiation. Active EGFR signaling is also required for the establishment of cell polarity via the Lkb1-AMPK pathway.

Figure 3:. Hedgehog and Hippo signaling pathways

A) Hh signaling in the early follicle cell lineage. Hh is provided for FSCs from terminal filament cells, cap cells, and IGS cells. Brother of ihog (Boi) regulates hh release by sequestration. Under rich diet, cholesterol binds and activates the negative regulator of boi, Hormone receptor 96 (Hr96), allowing hh release. The micro-RNA family mir-310 represses Hh release by targeting Hr96, and Rab23, which functions in Hh transport. In canonical Hh signaling extracellular hh binds to its receptor Patched (Ptc) and prevents Ptc-mediated repression of Smoothened (Smo). This allows Costal (Cos), which interacts with Fused (Fu) and Supressor of fused (Su(fu)), to induce proteolytic cleavage of the transcription factor Cubitus interruptus (Ci), producing the repressive form (Ci^R). Ci^R acts as a repressor of Hh target gene expression. In the presence of Hh, Smo represses Cos allowing the active form of Ci (Ci^A) to activate Hh target gene expression. B) Core components of the Hippo signaling pathway. When Hippo signaling is active the Hippo (Hpo) kinase phosphorylates itself and the components of the kinase cassette: Salvador (Sav), which acts as a scaffold between Hippo and Warts (Wts), as well as the Warts cofactor Mob as tumor suppressor (Mats). The activated Warts kinase phosphorylates Yorkie (Yki) and prevents its nuclear translocation. When the kinase cassette is inactive, Yki enters the nucleus and interacts acts as a transcriptional cofactor to induce target gene expression. In the FSC, Yki is regulated by the Hh signaling pathway both transcriptionally as well as post-transcriptionally and induces the expression of the cell cycle gene Cyclin E [24,76].

Figure 4:. Notch and JAK-STAT signaling

A) Notch signaling provides the earliest-known differentiation signal in pFCs but is inactive in FSCs. A subset of pFCs receives Delta (Dl) signal from neighboring germline cells. Dl interaction with Notch (N) is enhanced by high levels of the glycosyltransferase Fringe (Fng). Dl binding to N leads to proteolytic cleavage release of the Notch intracellular domain (NICD) by the gamma secretase complex subunits, including Presilin (Psn) and Nicastrin (Nct). NICD enters the nucleus and triggers the release of Suppressor of Hairless (Su(H)) from its corepressor Hairless (H). Together with Su(H) NICD induces the expression of target genes, including Enhancer of split (E(spl)) genes. B) JAK-STAT signaling functions in FSCs as well as in pFC differentiation. The ligand Unpaired (Upd) is produced by polar fated cells and binds to the receptor Dome. Upon unpaired binding, bound Hopscotch (Hop, Drosophila JAK) is autophosphorylated and phosphorylates Dome, allowing binding of STAT92E. STAT92E is then phosphorylated, dimerizes and transfers to the nucleus to induce gene expression.