The Drosophila Hippo pathway transcription factor Scalloped and its co-factors alter each other's chromatin binding dynamics and transcription in vivo.

The Hippo pathway is an important regulator of organ growth and cell fate. The major mechanism by which Hippo is known to control transcription is by dictating the nucleo-cytoplasmic shuttling rate of Yorkie, a transcription co-activator, which promotes transcription with the DNA binding protein Scalloped. The nuclear biophysical behavior of Yorkie and Scalloped, and whether this is regulated by the Hippo pathway, remains unexplored. Using multiple live-imaging modalities on Drosophila tissues, we found that Scalloped interacts with DNA on a broad range of timescales, and enrichment of Scalloped at sites of active transcription is mediated by longer DNA dwell times. Further, Yorkie increased Scalloped's DNA dwell time, whereas the repressors Nervous fingers 1 (Nerfin-1) and Tondu-domain-containing growth inhibitor (Tgi) decreased it. Therefore, the Hippo pathway influences transcription not only by controlling nuclear abundance of Yorkie but also by modifying the DNA binding kinetics of the transcription factor Scalloped.

Basal spot junctions of Drosophila epithelial tissues respond to morphogenetic forces and regulate Hippo signaling.

Organ size is controlled by numerous factors including mechanical forces, which are mediated in part by the Hippo pathway. In growing Drosophila epithelial tissues, cytoskeletal tension influences Hippo signaling by modulating the localization of key pathway proteins to different apical domains. Here, we discovered a Hippo signaling hub at basal spot junctions, which form at the basal-most point of the lateral membranes and resemble adherens junctions in protein composition. Basal spot junctions recruit the central kinase Warts via Ajuba and E-cadherin, which prevent Warts activation by segregating it from upstream Hippo pathway proteins. Basal spot junctions are prominent when tissues undergo morphogenesis and are highly sensitive to fluctuations in cytoskeletal tension. They are distinct from focal adhesions, but the latter profoundly influences basal spot junction abundance by modulating the basal-medial actomyosin network and tension experienced by spot junctions. Thus, basal spot junctions couple morphogenetic forces to Hippo pathway activity and organ growth.

The Hippo pathway uses different machinery to control cell fate and organ size.

The Hippo pathway is a conserved signaling network that regulates organ growth and cell fate. One such cell fate decision is that of R8 photoreceptor cells in the Drosophila eye, where Hippo specifies whether cells sense blue or green light. We show that only a subset of proteins that control organ growth via the Hippo pathway also regulate R8 cell fate choice, including the STRIPAK complex, Tao, Pez, and 14-3-3 proteins. Furthermore, key Hippo pathway proteins were primarily cytoplasmic in R8 cells rather than localized to specific membrane domains, as in cells of growing epithelial organs. Additionally, Warts was the only Hippo pathway protein to be differentially expressed between R8 subtypes, while central Hippo pathway proteins were expressed at dramatically lower levels in adult and pupal eyes than in growing larval eyes. Therefore, we reveal several important differences in Hippo signaling in the contexts of organ growth and cell fate.

Drosophila Sex Peptide controls the assembly of lipid microcarriers in seminal fluid.

Seminal fluid plays an essential role in promoting male reproductive success and modulating female physiology and behavior. In the fruit fly, Drosophila melanogaster, Sex Peptide (SP) is the best-characterized protein mediator of these effects. It is secreted from the paired male accessory glands (AGs), which, like the mammalian prostate and seminal vesicles, generate most of the seminal fluid contents. After mating, SP binds to spermatozoa and is retained in the female sperm storage organs. It is gradually released by proteolytic cleavage and induces several long-term postmating responses, including increased ovulation, elevated feeding, and reduced receptivity to remating, primarily signaling through the SP receptor (SPR). Here, we demonstrate a previously unsuspected SPR-independent function for SP. We show that, in the AG lumen, SP and secreted proteins with membrane-binding anchors are carried on abundant, large neutral lipid-containing microcarriers, also found in other SP-expressing Drosophila species. These microcarriers are transferred to females during mating where they rapidly disassemble. Remarkably, SP is a key microcarrier assembly and disassembly factor. Its absence leads to major changes in the seminal proteome transferred to females upon mating. Males expressing nonfunctional SP mutant proteins that affect SP's binding to and release from sperm in females also do not produce normal microcarriers, suggesting that this male-specific defect contributes to the resulting widespread abnormalities in ejaculate function. Our data therefore reveal a role for SP in formation of seminal macromolecular assemblies, which may explain the presence of SP in Drosophila species that lack the signaling functions seen in Dmelanogaster.

Glutamine deprivation alters the origin and function of cancer cell exosomes.

Exosomes are secreted extracellular vesicles carrying diverse molecular cargos, which can modulate recipient cell behaviour. They are thought to derive from intraluminal vesicles formed in late endosomal multivesicular bodies (MVBs). An alternate exosome formation mechanism, which is conserved from fly to human, is described here, with exosomes carrying unique cargos, including the GTPase Rab11, generated in Rab11-positive recycling endosomal MVBs. Release of Rab11-positive exosomes from cancer cells is increased relative to late endosomal exosomes by reducing growth regulatory Akt/mechanistic Target of Rapamycin Complex 1 (mTORC1) signalling or depleting the key metabolic substrate glutamine, which diverts membrane flux through recycling endosomes. Vesicles produced under these conditions promote tumour cell proliferation and turnover and modulate blood vessel networks in xenograft mouse models in vivo. Their growth-promoting activity, which is also observed in vitro, is Rab11a-dependent, involves ERK-MAPK-signalling and is inhibited by antibodies against amphiregulin, an EGFR ligand concentrated on these vesicles. Therefore, glutamine depletion or mTORC1 inhibition stimulates release from Rab11a compartments of exosomes with pro-tumorigenic functions, which we propose promote stress-induced tumour adaptation.

The regulation of Yorkie, YAP and TAZ: new insights into the Hippo pathway.

The Hippo pathway is a highly conserved signalling pathway that regulates multiple biological processes, including organ size control and cell fate. Since its discovery, genetic and biochemical studies have elucidated several key signalling steps important for pathway activation and deactivation. In recent years, technical advances in microscopy and genome modification have allowed new insights into Hippo signalling to be revealed. These studies have highlighted that the nuclear-cytoplasmic shuttling behaviour of the Hippo pathway transcriptional co-activators Yorkie, YAP and TAZ is far more dynamic than previously appreciated, and YAP and TAZ are also regulated by liquid-liquid phase separation. Here, we review our current understanding of Yorkie, YAP and TAZ regulation, with a focus on recent microscopy-based studies.

The dPix-Git complex is essential to coordinate epithelial morphogenesis and regulate myosin during Drosophila egg chamber development.

How biochemical and mechanical information are integrated during tissue development is a central question in morphogenesis. In many biological systems, the PIX-GIT complex localises to focal adhesions and integrates both physical and chemical information. We used Drosophila melanogaster egg chamber formation to study the function of PIX and GIT orthologues (dPix and Git, respectively), and discovered a central role for this complex in controlling myosin activity and epithelial monolayering. We found that Git's focal adhesion targeting domain mediates basal localisation of this complex to filament structures and the leading edge of migrating cells. In the absence of dpix and git, tissue disruption is driven by contractile forces, as reduction of myosin activators restores egg production and morphology. Further, dpix and git mutant eggs closely phenocopy defects previously reported in pak mutant epithelia. Together, these results indicate that the dPix-Git complex controls egg chamber morphogenesis by controlling myosin contractility and Pak kinase downstream of focal adhesions.

Rab-mediated trafficking in the secondary cells of Drosophila male accessory glands and its role in fecundity.

The male seminal fluid contains factors that affect female post-mating behavior and physiology. In Drosophila, most of these factors are secreted by the two epithelial cell types that make up the male accessory gland: the main and secondary cells. Although secondary cells represent only ~4% of the cells of the accessory gland, their contribution to the male seminal fluid is essential for sustaining the female post-mating response. To better understand the function of the secondary cells, we investigated their molecular organization, particularly with respect to the intracellular membrane transport machinery. We determined that large vacuole-like structures found in the secondary cells are trafficking hubs labeled by Rab6, 7, 11 and 19. Furthermore, these organelles require Rab6 for their formation and many are essential in the process of creating the long-term postmating behavior of females. In order to better serve the intracellular membrane and protein trafficking communities, we have created a searchable, online, open-access imaging resource to display our complete findings regarding Rab localization in the accessory gland.

Regulation of Dense-Core Granule Replenishment by Autocrine BMP Signalling in Drosophila Secondary Cells.

Regulated secretion by glands and neurons involves release of signalling molecules and enzymes selectively concentrated in dense-core granules (DCGs). Although we understand how many secretagogues stimulate DCG release, how DCG biogenesis is then accelerated to replenish the DCG pool remains poorly characterised. Here we demonstrate that each prostate-like secondary cell (SC) in the paired adult Drosophila melanogaster male accessory glands contains approximately ten large DCGs, which are loaded with the Bone Morphogenetic Protein (BMP) ligand Decapentaplegic (Dpp). These DCGs can be marked in living tissue by a glycophosphatidylinositol (GPI) lipid-anchored form of GFP. In virgin males, BMP signalling is sporadically activated by constitutive DCG secretion. Upon mating, approximately four DCGs are typically released immediately, increasing BMP signalling, primarily via an autocrine mechanism. Using inducible knockdown specifically in adult SCs, we show that secretion requires the Soluble NSF Attachment Protein, SNAP24. Furthermore, mating-dependent BMP signalling not only promotes cell growth, but is also necessary to accelerate biogenesis of new DCGs, restoring DCG number within 24 h. Our analysis therefore reveals an autocrine BMP-mediated feedback mechanism for matching DCG release to replenishment as secretion rates fluctuate, and might explain why in other disease-relevant systems, like pancreatic ß-cells, BMP signalling is also implicated in the control of secretion.