Dicer-1-dependent Dacapo suppression acts downstream of Insulin receptor in regulating cell division of Drosophila germline stem cells.

It is important to understand the regulation of stem cell division because defects in this process can cause altered tissue homeostasis or cancer. The cyclin-dependent kinase inhibitor Dacapo (Dap), a p21/p27 homolog, acts downstream of the microRNA (miRNA) pathway to regulate the cell cycle in Drosophila melanogaster germline stem cells (GSCs). Tissue-extrinsic signals, including insulin, also regulate cell division of GSCs. We report that intrinsic and extrinsic regulators intersect in GSC division control; the Insulin receptor (InR) pathway regulates Dap levels through miRNAs, thereby controlling GSC division. Using GFP-dap 3'UTR sensors in vivo, we show that in GSCs the dap 3'UTR is responsive to Dicer-1, an RNA endonuclease III required for miRNA processing. Furthermore, the dap 3'UTR can be directly targeted by miR-7, miR-278 and miR-309 in luciferase assays. Consistent with this, miR-278 and miR-7 mutant GSCs are partially defective in GSC division and show abnormal cell cycle marker expression, respectively. These data suggest that the GSC cell cycle is regulated via the dap 3'UTR by multiple miRNAs. Furthermore, the GFP-dap 3'UTR sensors respond to InR but not to TGF-beta signaling, suggesting that InR signaling utilizes Dap for GSC cell cycle regulation. We further demonstrate that the miRNA-based Dap regulation may act downstream of InR signaling; Dcr-1 and Dap are required for nutrition-dependent cell cycle regulation in GSCs and reduction of dap partially rescues the cell cycle defect of InR-deficient GSCs. These data suggest that miRNA- and Dap-based cell cycle regulation in GSCs can be controlled by InR signaling.

Stage-specific differences in the requirements for germline stem cell maintenance in the Drosophila ovary.

In this study, we uncover a role for microRNAs in Drosophila germline stem cell (GSC) maintenance. Disruption of Dicer-1 function in GSCs during adult life results in GSC loss. Surprisingly, however, loss of Dicer-1 during development does not result in a GSC maintenance defect, although a defect is seen if both Dicer-1 and Dicer-2 function are disrupted. Loss of the bantam microRNA mimics the Dicer-1 maintenance defect when induced in adult GSCs, suggesting that bantam plays a key role in GSC self-renewal. Mad, a component of the TGF-beta pathway, behaves similarly to Dicer-1: adult GSC maintenance requires Mad if it is lost during adult life, but not if it is lost during pupal development. Overall, these results show stage-specific differential sensitivity of GSC maintenance to certain perturbations, and suggest that there may be Dcr-2 dependent redundancy of GSC maintenance mechanisms during development that is lost in later life.

Stem cells signal to the niche through the Notch pathway in the Drosophila ovary.

Stem cells are maintained and retain their capacity to continue dividing because of the influence of a niche. Although niches are important to maintain "stemness" in a wide variety of tissues, control of these niches is poorly understood. The Drosophila germline stem cells (GSCs) reside in a somatic cell niche. We show that Notch activation can induce the expression of niche-cell markers even in an adult fly; overexpression of Delta in the germline, or activated Notch in the somatic cells, results in extra niche cells, up to 10-fold over the normal number. In turn, these ectopic niche cells induce ectopic GSCs. Conversely, when GCSs do not produce functional Notch ligands, Delta and Serrate, the TGF-beta pathway is not activated in the GSCs, and they differentiate and subsequently leave the niche. Importantly, clonal analysis reveals that the receiving end of the Notch pathway is required in the somatic cells. These data show that a feedback loop exists between the stem cells and niche cells. Demonstration that stem cells can contribute to niche function has far-reaching consequences for stem cell therapies and may provide insight into how cancer can spread throughout an organism via populations of cancer stem cells.

Border of Notch activity establishes a boundary between the two dorsal appendage tube cell types.

Boundaries establish and maintain separate populations of cells critical for organ formation. We show that Notch signaling establishes the boundary between two types of post-mitotic epithelial cells, the Rhomboid- and the Broad-positive cells. These cells will undergo morphogenetic movements to generate the two sides of a simple organ, the dorsal appendage tube of the Drosophila egg chamber. The boundary forms due to a difference in Notch levels in adjacent cells. The Notch expression pattern mimics the boundary; Notch levels are high in Rhomboid cells and low in Broad cells. Notch(-) mutant clones generate an ectopic boundary: ectopic Rhomboid cells arise in Notch(+) cells adjacent to the Notch(-) mutant cells but not further away from the clonal border. Pangolin, a component of the Wingless pathway, is required for Broad expression and for rhomboid repression. We further show that Broad represses rhomboid cell autonomously. Our data provide a foundation for understanding how a single row of Rhomboid cells arises adjacent to the Broad cells in the dorsal appendage primordia. Generating a boundary by the Notch pathway might constitute an evolutionarily conserved first step during organ formation in many tissues.

Genome wide analysis of transcript levels after perturbation of the EGFR pathway in the Drosophila ovary.

Defects in the epidermal growth factor receptor (EGFR) pathway can lead to aggressive tumor formation. Activation of this pathway during normal development produces multiple outcomes at the cellular level, leading to cellular differentiation and cell cycle activation. To elucidate the downstream events induced by this pathway, we used genome-wide cDNA microarray technology to identify potential EGFR targets in Drosophila oogenesis. We focused on genes for which the transcriptional responses due to EGFR pathway activation and inactivation were in opposite directions, as this is expected for genes that are directly regulated by the pathway in this tissue type. We perturbed the EGFR pathway in epithelial follicle cells using seven different genetic backgrounds. To activate the pathway, we overexpressed an activated form of the EGFR (UAS-caEGFR), and an activated form of the signal transducer Raf (UAS-caRaf); we also over- or ectopically expressed the downstream homeobox transcription factor Mirror (UAS-mirr) and the ligand-activating serine protease Rhomboid (UAS-rho). To reduce pathway activity we used loss-of-function mutations in the ligand (gurken) and receptor (torpedo). From microarrays containing 6,255 genes, we found 454 genes that responded in an opposite manner in gain-of-function and loss-of-function conditions among which are many Wingless signaling pathway components. Further analysis of two such components, sugarless and pangolin, revealed a function for these genes in late follicle cell patterning. Of interest, components of other signaling pathways were also enriched in the EGFR target group, suggesting that one reason for the pleiotropic effects seen with EGFR activity in cancer progression and development may be its ability to regulate many other signaling pathways.

Juxtaposition between two cell types is necessary for dorsal appendage tube formation.

The Drosophila egg chamber provides an excellent model for studying the link between patterning and morphogenesis. Late in oogenesis, a portion of the flat follicular epithelium remodels to form two tubes; secretion of eggshell proteins into the tube lumens creates the dorsal appendages. Two distinct cell types contribute to dorsal appendage formation: cells expressing the rhomboid-lacZ (rho-lacZ) marker form the ventral floor of the tube and cells expressing high levels of the transcription factor Broad form a roof over the rho-lacZ cells. In mutants that produce defective dorsal appendages (K10, Ras and ectopic decapentaplegic) both cell types are specified and reorganize to occupy their stereotypical locations within the otherwise defective tubes. Although the rho-lacZ and Broad cells rearrange to form a tube in wild type and mutant egg chambers, they never intermingle, suggesting that a boundary exists that prevents mixing between these two cell types. Consistent with this hypothesis, the Broad and rho-lacZ cells express different levels of the homophilic adhesion molecule Fasciclin 3. Furthermore, in the anterior of the egg, ectopic rhomboid is sufficient to induce both cell types, which reorganize appropriately to form an ectopic tube. We propose that signaling across a boundary separating the rho-lacZ and Broad cells choreographs the cell shape-changes and rearrangements necessary to transform an initially flat epithelium into a tube.