Wu-Min Deng

Kibra Functions as a Tumor Suppressor Protein that Regulates Hippo Signaling in Conjunction with Merlin and Expanded

The Hippo signaling pathway regulates organ size and tissue homeostasis from Drosophila to mammals. Central to this pathway is a kinase cascade wherein Hippo (Hpo), in complex with Salvador (Sav), phosphorylates and activates Warts (Wts), which in turn phosphorylates and inactivates the Yorkie (Yki) oncoprotein, known as the YAP coactivator in mammalian cells. The FERM domain proteins Merlin (Mer) and Expanded (Ex) are upstream components that regulate Hpo activity through unknown mechanisms. Here we identify Kibra as another upstream component of the Hippo signaling pathway. We show that Kibra functions together with Mer and Ex in a protein complex localized to the apical domain of epithelial cells, and that this protein complex regulates the Hippo kinase cascade via direct binding to Hpo and Sav. These results shed light on the mechanism of Ex and Mer function and implicate Kibra as a potential tumor suppressor with relevance to neurofibromatosis.

Dystroglycan is required for polarizing the epithelial cells and the oocyte in Drosophila.

The transmembrane protein Dystroglycan is a central element of the dystrophin-associated glycoprotein complex, which is involved in the pathogenesis of many forms of muscular dystrophy. Dystroglycan is a receptor for multiple extracellular matrix (ECM) molecules such as Laminin, agrin and perlecan, and plays a role in linking the ECM to the actin cytoskeleton; however, how these interactions are regulated and their basic cellular functions are poorly understood. Using mosaic analysis and RNAi in the model organism Drosophila melanogaster, we show that Dystroglycan is required cell-autonomously for cellular polarity in two different cell types, the epithelial cells (apicobasal polarity) and the oocyte (anteroposterior polarity). Loss of Dystroglycan function in follicle and disc epithelia results in expansion of apical markers to the basal side of cells and overexpression results in a reduced apical localization of these same markers. In Dystroglycan germline clones early oocyte polarity markers fail to be localized to the posterior, and oocyte cortical F-actin organization is abnormal. Dystroglycan is also required non-cell-autonomously to organize the planar polarity of basal actin in follicle cells, possibly by organizing the Laminin ECM. These data suggest that the primary function of Dystroglycan in oogenesis is to organize cellular polarity; and this study sets the stage for analyzing the Dystroglycan complex by using the power of Drosophila molecular genetics.

Involvement of Lgl and Mahjong/VprBP in Cell Competition

Mahjong is a novel Lethal giant larvae-binding protein that plays a vital role in cell competition in both flies and mammals.

Perlecan and Dystroglycan act at the basal side of the Drosophila follicular epithelium to maintain epithelial organization

Dystroglycan (Dg) is a widely expressed extracellular matrix (ECM) receptor required for muscle viability, synaptogenesis, basementmembrane formation and epithelial development. As an integral component of the Dystrophin-associated glycoprotein complex, Dg plays a central role in linking the ECM and the cytoskeleton. Disruption of this linkage in skeletal muscle leads to various types of muscular dystrophies. In epithelial cells, reduced expression of Dg is associated with increased invasiveness of cancer cells. We have previously shown that Dg is required for epithelial cell polarity in Drosophila, but the mechanisms of this polarizing activity and upstream/downstream components are largely unknown. Using the Drosophila follicle-cell epithelium (FCE) as a model system, we show that the ECM molecule Perlecan (Pcan) is required for maintenance of epithelial-cell polarity. Follicle cells that lack Pcan develop polarity defects similar to those of Dg mutant cells. Furthermore, Dg depends on Pcan but not on Laminin A for its localization in the basal-cell membrane, and the two proteins bind in vitro. Interestingly, the Dg form that interacts with Pcan in the FCE lacks the mucin-like domain, which is thought to be essential for Dg ligand binding activity. Finally, we describe two examples of how Dg promotes the differentiation of the basal membrane domain: (1) by recruiting/anchoring the cytoplasmic protein Dystrophin; and (2) by excluding the transmembrane protein Neurexin. We suggest that the interaction of Pcan and Dg at the basal side of the epithelium promotes basal membrane differentiation and is required for maintenance of cell polarity in the FCE.

The hippo pathway promotes Notch signaling in regulation of cell differentiation, proliferation, and oocyte polarity.

Specification of the anterior-posterior axis in Drosophila oocytes requires proper communication between the germ-line cells and the somatically derived follicular epithelial cells. Multiple signaling pathways, including Notch, contribute to oocyte polarity formation by controlling the temporal and spatial pattern of follicle cell differentiation and proliferation. Here we show that the newly identified Hippo tumor-suppressor pathway plays a crucial role in the posterior follicle cells in the regulation of oocyte polarity. Disruption of the Hippo pathway, including major components Hippo, Salvador, and Warts, results in aberrant follicle-cell differentiation and proliferation and dramatic disruption of the oocyte anterior-posterior axis. These phenotypes are related to defective Notch signaling in follicle cells, because misexpression of a constitutively active form of Notch alleviates the oocyte polarity defects. We also find that follicle cells defective in Hippo signaling accumulate the Notch receptor and display defects in endocytosis markers. Our findings suggest that the interaction between Hippo and classic developmental pathways such as Notch is critical to spatial and temporal regulation of differentiation and proliferation and is essential for development of the body axes in Drosophila.

Notch-dependent downregulation of the homeodomain gene cut is required for the mitotic cycle/endocycle switch and cell differentiation in Drosophila follicle cells.

During Drosophila mid-oogenesis, follicular epithelial cells switch from the mitotic cycle to the specialized endocycle in which the M phase is skipped. The switch, along with cell differentiation in follicle cells, is induced by Notch signaling. We show that the homeodomain gene cut functions as a linker between Notch and genes that are involved in cell-cycle progression. Cut was expressed in proliferating follicle cells but not in cells in the endocycle. Downregulation of Cut expression was controlled by the Notch pathway and was essential for follicle cells to differentiate and to enter the endocycle properly. cut-mutant follicle cells entered the endocycle and differentiated prematurely in a cell-autonomous manner. By contrast, prolonged expression of Cut caused defects in the mitotic cycle/endocycle switch. These cells continued to express an essential mitotic cyclin, Cyclin A, which is normally degraded by the Fizzy-related-APC/C ubiquitin proteosome system during the endocycle. Cut promoted Cyclin A expression by negatively regulating Fizzy-related. Our data suggest that Cut functions in regulating both cell differentiation and the cell cycle, and that downregulation of Cut by Notch contributes to the mitotic cycle/endocycle switch and cell differentiation in follicle cells.

The homeobox gene mirror links EGF signalling to embryonic dorso-ventral axis formation through notch activation.

Recent studies in vertebrates and Drosophila melanogaster have revealed that Fringe-mediated activation of the Notch pathway has a role in patterning cell layers during organogenesis. In these processes, a homeobox-containing transcription factor is responsible for spatially regulating fringe (fng) expression and thus directing activation of the Notch pathway along the fng expression border. Here we show that this may be a general mechanism for patterning epithelial cell layers. At three stages in Drosophila oogenesis, mirror (mirr) and fng have complementary expression patterns in the follicle-cell epithelial layer, and at all three stages loss of mirr enlarges, and ectopic expression of mirr restricts, fng expression, with consequences for follicle-cell patterning. These morphological changes are similar to those caused by Notch mutations. Ectopic expression of mirr in the posterior follicle cells induces a stripe of rhomboid (rho) expression and represses pipe (pip), a gene with a role in the establishment of the dorsal-ventral axis, at a distance. Ectopic Notch activation has a similar long-range effect on pip. Our results suggest that Mirror and Notch induce secretion of diffusible morphogens and we have identified TGF-β (encoded by dpp) as such a molecule in germarium. We also found that mirr expression in dorsal follicle cells is induced by the EGF-receptor (EGFR) pathway and that mirr then represses pip expression in all but the ventral follicle cells, connecting EGFR activation in the dorsal follicle cells to repression of pip in the dorsal and lateral follicle cells. Our results suggest that the differentiation of ventral follicle cells is not a direct consequence of germline signalling, but depends on long-range signals from dorsal follicle cells, and provide a link between early and late events in Drosophila embryonic dorsal-ventral axis formation.

Notch-Dependent Fizzy-Related/Hec1/Cdh1 Expression Is Required for the Mitotic-to-Endocycle Transition in Drosophila Follicle Cells

During Drosophila oogenesis, Notch function regulates the transition from mitotic cell cycle to endocycle in follicle cells at stage 6. Loss of either Notch function or its ligand Delta (Dl) disrupts the normal transition; this disruption causes mitotic cycling to continue and leads to an overproliferation phenotype. In this context, the only known cell cycle component that responds to the Notch pathway is String/Cdc25 (Stg), a G2/M cell cycle regulator. We found that prolonged expression of string is not sufficient to keep cells efficiently in mitotic cell cycle past stage 6, suggesting that Notch also regulates other cell cycle components in the transition. By using an expression screen, we found such a component: Fizzy-related/Hec1/Cdh1 (Fzr), a WD40 repeat protein. Fzr regulates the anaphase-promoting complex/cyclosome (APC/C) and is expressed at the mitotic-to-endocycle transition in a Notch-dependent manner. Mutant clones of Fzr revealed that Fzr is dispensable for mitosis but essential for endocycles. Unlike in Notch clones, in Fzr mutant cells mitotic markers are absent past stage 6. Only a combined reduction of Fzr and ectopic Stg expression prolongs mitotic cycles in follicle cells, suggesting that these two cell cycle regulators, Fzr and Stg, are important mediators of the Notch pathway in the mitotic-to-endocycle transition.

At the crossroads of differentiation and proliferation: precise control of cell-cycle changes by multiple signaling pathways in Drosophila follicle cells.

Here, we discuss the findings to date about genes and pathways required for regulation of somatic follicle‐cell proliferation and differentiation during Drosophila oogenesis and demonstrate how loss of these genes contributes to the tumorigenic potential of mutant cells. Follicle cells undergo cell‐fate determination through stepwise activation of multiple signaling pathways, including the Notch, Hedgehog, Wingless, janus kinase/STAT, and JNK pathways. In addition, changes in DNA replication and cellular growth depend on the spatial and temporal activation of the mitotic cycle‐endocycle and endocycle‐gene amplification cell‐cycle switches and insulin‐dependent monitoring of cellular health; systemic loss of these pathways contributes to loss of controlled cellular proliferation, loss of differentiation/growth, and aberrant cell polarity in follicle cells. We also highlight the effects of the neoplastic and Hippo pathways on the cell cycle and cellular proliferation in promoting normal development and conclude that lack of coordination of multiple signaling pathways promotes conditions favorable for tumorigenesis.

Lgl and its phosphorylation by aPKC regulate oocyte polarity formation in Drosophila

Specification of the anteroposterior (AP) axis in Drosophila oocytes requires proper organization of the microtubule and actin cytoskeleton. The establishment and regulation of cytoskeletal polarity remain poorly understood, however. Here, we show important roles for the tumor suppressor Lethal (2) giant larvae (Lgl) and atypical protein kinase C (aPKC) in regulating microtubule polarity and setting up the AP axis of the oocyte. Lgl in the germline cells regulates the localization of axis-specifying morphogens. aPKC phosphorylation of Lgl restricts Lgl activity to the oocyte posterior, thereby dividing the cortex into different domains along the AP axis. Active Lgl promotes the formation of actin-rich projections at the oocyte cortex and the posterior enrichment of the serine/threonine kinase Par-1, a key step for oocyte polarization. Our studies suggest that Lgl and its phosphorylation by aPKC may form a conserved regulatory circuitry in polarization of various cell types.