Endocytosis, endosome trafficking, and the regulation of Drosophila development

Rab1 and Syntaxin 17 regulate hematopoietic homeostasis through β-integrin trafficking in Drosophila

Hematopoiesis is crucial for organismal health, and Drosophila serves as an effective genetic model due to conserved regulatory mechanisms with vertebrates. In larvae, hematopoiesis primarily occurs in the lymph gland, which contains distinct zones, including the cortical zone, intermediate zone, medullary zone, and posterior signaling center (PSC). Rab1 is vital for membrane trafficking and maintaining the localization of cell adhesion molecules, yet its role in hematopoietic homeostasis is not fully understood. This study investigates the effects of Rab1 dysfunction on β-integrin trafficking within circulating hemocytes and lymph gland cells. Rab1 impairment disrupts the endosomal trafficking of β-integrin, leading to its abnormal localization on cell membranes, which promotes lamellocyte differentiation and alters progenitor dynamics in circulating hemocytes and lymph glands, respectively. We also show that the mislocalization of β-integrin is dependent on the adhesion protein DE-cadherin. The reduction of β-integrin at cell boundaries in PSC cells leads to fewer PSC cells and lamellocyte differentiation. Furthermore, Rab1 regulates the trafficking of β-integrin via the Q-SNARE protein Syntaxin 17 (Syx17). Our findings indicate that Rab1 and Syx17 regulate distinct trafficking pathways for β-integrin in different hematopoietic compartments and maintain hematopoietic homeostasis of Drosophila.

Endolysosomal trafficking controls yolk granule biogenesis in vitellogenic Drosophila oocytes

Endocytosis and endolysosomal trafficking are essential for almost all aspects of physiological functions of eukaryotic cells. As our understanding on these membrane trafficking events are mostly from studies in yeast and cultured mammalian cells, one challenge is to systematically evaluate the findings from these cell-based studies in multicellular organisms under physiological settings. One potentially valuable in vivo system to address this challenge is the vitellogenic oocyte in Drosophila, which undergoes extensive endocytosis by Yolkless (Yl), a low-density lipoprotein receptor (LDLR), to uptake extracellular lipoproteins into oocytes and package them into a specialized lysosome, the yolk granule, for storage and usage during later development. However, by now there is still a lack of sufficient understanding on the molecular and cellular processes that control yolk granule biogenesis. Here, by creating genome-tagging lines for Yl receptor and analyzing its distribution in vitellogenic oocytes, we observed a close association of different endosomal structures with distinct phosphoinositides and actin cytoskeleton dynamics. We further showed that Rab5 and Rab11, but surprisingly not Rab4 and Rab7, are essential for yolk granules biogenesis. Instead, we uncovered evidence for a potential role of Rab7 in actin regulation and observed a notable overlap of Rab4 and Rab7, two Rab GTPases that have long been proposed to have distinct spatial distribution and functional roles during endolysosomal trafficking. Through a small-scale RNA interference (RNAi) screen on a set of reported Rab5 effectors, we showed that yolk granule biogenesis largely follows the canonical endolysosomal trafficking and maturation processes. Further, the data suggest that the RAVE/V-ATPase complexes function upstream of or in parallel with Rab7, and are involved in earlier stages of endosomal trafficking events. Together, our study provides s novel insights into endolysosomal pathways and establishes vitellogenic oocyte in Drosophila as an excellent in vivo model for dissecting the highly complex membrane trafficking events in metazoan.

Clathrin-dependent endocytosis plays a critical role in larval and pupal development, and female oocyte production in the red flour beetle (Tribolium castaneum)

BACKGROUND

Clathrin-dependent endocytosis is a vesicular transport process by which cells take macromolecules from the extracellular space to the intracellular space. It plays important roles in various cellular functions, but its biological significance in insect development and reproduction has not been well studied.

RESULTS

We characterized and functionally analyzed four major clathrin-dependent endocytic pathway genes (TcChc, TcAP50, TcVhaSFD, TcRab7) in Tribolium castaneum. RNA interference (RNAi) by injecting double-stranded RNA (dsRNA) targeting each gene at three doses (50, 100, or 200 ng per insect) in 20-day-old larvae led to 100% larval mortality. When the expressions of TcChc, TcVhaSFD, and TcRab7 were suppressed by injecting their respective dsRNAs at each dose in 1-day-old pupae, the adults that emerged from the dsRNA-injected pupae were deformed, with the absence of wing development. The deformed adults died within 2 days after eclosion. When the expression of TcAP50 was suppressed by injecting its dsRNA into 1-day-old pupae, although no apparent deformed adults were observed, all the adults died within 35 days after eclosion. In addition, when the expressions of TcChc and TcVhaSFD were suppressed by injecting their respective dsRNAs at a reduced dose (10 ng per insect) in 5-day-old pupae, the ovarian development and oocyte production in the resultant females were completely inhibited.

CONCLUSION

Our results indicate that clathrin-dependent endocytosis is essential for insect development and reproduction. The results from this study can help researchers identify potential molecular targets for developing novel strategies for insect pest management. © 2023 Society of Chemical Industry.

Oral RNAi toxicity assay suggests clathrin heavy chain as a promising molecular target for controlling the 28-spotted potato ladybird, Henosepilachna vigintioctopunctata

BACKGROUND

Use of RNA interference (RNAi) technology in effective pest management has been explored for decades. Henosepilachna vigintioctopunctata is a major solanaceous crop pest in Asia. In this study, the effects of the RNAi-mediated silencing of clathrin heavy chain in H. vigintioctopunctata were investigated.

RESULTS

Feeding either the in vitro-synthesized or the bacterially expressed double-stranded RNAs (dsRNAs) significantly impaired the normal physiology of H. vigintioctopunctata instars and adults. However, the bacterially expressed dsHvChc caused higher mortality than the in vitro-synthesized ones in the larvae and adults. Moreover, on evaluating the potential risk of dsHvChc on Propylea japonica, significant transcriptional effects of dsHvChc1 were observed, while the organismal level effects were not significant. On the contrary, dsHvChc2 did not affect P. japonica at either level. A similar test revealed significant transcriptional effects of dsPjChc1 on H. vigintioctopunctata, while staying ineffective at the organismal levels. Conversely, dsPjChc2 did not affect H. vigintioctopunctata at either level. Importantly, no effect of dsPjChc1 exposure on H. vigintioctopunctata suggested that other factors besides the 21-nucleotide (nt) matches between sequences were responsible. Finally, ingestion of dsHvmChc1 derived from H. vigintioctomaculata, containing 265-nt matches with dsHvChc1, caused 100% mortality in H. vigintioctopunctata.

CONCLUSIONS

We conclude that (i) species with numerous 21-nt matches in homologous genes are more likely to be susceptible to dsRNA; (ii) dsRNA can be safely designed to avoid negative effects on non-target organisms at both transcriptional and organismal levels; (iii) HvChc can be used as an efficient RNAi target gene to effectively manage H. vigintioctopunctata. © 2021 Society of Chemical Industry.

VPS38/UVRAG and ATG14, the variant regulatory subunits of the ATG6/Beclin1-PI3K complexes, are crucial for the biogenesis of the yolk organelles and are transcriptionally regulated in the oocytes of the vector Rhodnius prolixus

In insects the reserve proteins are stored in the oocytes into endocytic-originated vesicles named yolk organelles. VPS38/UVRAG and ATG14 are the variant regulatory subunits of two class-III ATG6/Beclin1 PI3K complexes that regulate the recruitment of the endocytic (complex II) and autophagic (complex I) machineries. In a previous work from our group, we found that the silencing of ATG6/Beclin1 resulted in the formation of yolk-deficient oocytes due to defects in the endocytosis of the yolk proteins. Because ATG6/Beclin1 is present in the two above-described PI3K complexes, we could not identify the contributions of each complex to the yolk defective phenotypes. To address this, here we investigated the role of the variant subunits VPS38/UVRAG (complex II, endocytosis) and ATG14 (complex I, autophagy) in the biogenesis of the yolk organelles in the insect vector of Chagas Disease Rhodnius prolixus. Interestingly, the silencing of both genes phenocopied the silencing of ATG6/Beclin1, generating 1) accumulation of yolk proteins in the hemolymph; 2) white, smaller, and yolk-deficient oocytes; 3) abnormal yolk organelles in the oocyte cortex; and 4) unviable F1 embryos. However, we found that the similar phenotypes were the result of a specific cross-silencing effect among the PI3K subunits where the silencing of VPS38/UVRAG and ATG6/Beclin1 resulted in the specific silencing of each other, whereas the silencing of ATG14 triggered the silencing of all three PI3K components. Because the silencing of VPS38/UVRAG and ATG6/Beclin1 reproduced the yolk-deficiency phenotypes without the cross silencing of ATG14, we concluded that the VPS38/UVRAG PI3K complex II was the major contributor to the previously observed phenotypes in silenced insects. Altogether, we found that class-III ATG6/Beclin1 PI3K complex II (VPS38/UVRAG) is essential for the yolk endocytosis and that the subunits of both complexes are under an unknown transcriptional regulatory system.

Parasitoid wasp venom vesicles (venosomes) enter Drosophila melanogaster lamellocytes through a flotillin/lipid raft-dependent endocytic pathway

Venosomes are extracellular vesicles found in the venom of Leptopilina endoparasitoids wasps, which transport and target virulence factors to impair the parasitoid egg encapsulation by the lamellocytes of their Drosophila melanogaster host larva. Using the co-immunolocalization of fluorescent L. boulardi venosomes and one of the putative-transported virulence factors, LbGAP, with known markers of cellular endocytosis, we show that venosomes endocytosis by lamellocytes is not a process dependent on clathrin or macropinocytosis and internalization seems to bypass the early endosomal compartment Rab5. After internalization, LbGAP colocalizes strongly with flotillin-1 and the GPI-anchored protein Atilla/L1 (a lamellocyte surface marker) suggesting that entry occurs via a flotillin/lipid raft-dependent pathway. Once internalized, venosomes reach all intracellular compartments, including late and recycling endosomes, lysosomes, and the endoplasmic reticulum network. Venosomes therefore enter their target cells by a specific mechanism and the virulence factors are widely distributed in the lamellocytes' compartments to impair their functions.

Reduction of endocytic activity accelerates cell elimination during tissue remodeling of the Drosophila epidermal epithelium

Epithelial tissues undergo cell turnover both during development and for homeostatic maintenance. Cells that are no longer needed are quickly removed without compromising the barrier function of the tissue. During metamorphosis, insects undergo developmentally programmed tissue remodeling. However, the mechanisms that regulate this rapid tissue remodeling are not precisely understood. Here, we show that the temporal dynamics of endocytosis modulate physiological cell properties to prime larval epidermal cells for cell elimination. Endocytic activity gradually reduces as tissue remodeling progresses. This reduced endocytic activity accelerates cell elimination through the regulation of Myosin II subcellular reorganization, junctional E-cadherin levels, and caspase activation. Whereas the increased Myosin II dynamics accelerates cell elimination, E-cadherin plays a protective role against cell elimination. Reduced E-cadherin is involved in the amplification of caspase activation by forming a positive-feedback loop with caspase. These findings reveal the role of endocytosis in preventing cell elimination and in the cell-property switching initiated by the temporal dynamics of endocytic activity to achieve rapid cell elimination during tissue remodeling.

The SWI/SNF Complex Protein Snr1 Is a Tumor Suppressor in Drosophila Imaginal Tissues

Components of the SWI/SNF chromatin-remodeling complex are among the most frequently mutated genes in various human cancers, yet only SMARCB1/hSNF5, a core member of the SWI/SNF complex, is mutated in malignant rhabdoid tumors (MRT). How SMARCB1/hSNF5 functions differently from other members of the SWI/SNF complex remains unclear. Here, we use Drosophila imaginal epithelial tissues to demonstrate that Snr1, the conserved homolog of human SMARCB1/hSNF5, prevents tumorigenesis by maintaining normal endosomal trafficking-mediated signaling cascades. Removal of Snr1 resulted in neoplastic tumorigenic overgrowth in imaginal epithelial tissues, whereas depletion of any other members of the SWI/SNF complex did not induce similar phenotypes. Unlike other components of the SWI/SNF complex that were detected only in the nucleus, Snr1 was observed in both the nucleus and the cytoplasm. Aberrant regulation of multiple signaling pathways, including Notch, JNK, and JAK/STAT, was responsible for tumor progression upon snr1-depletion. Our results suggest that the cytoplasmic Snr1 may play a tumor suppressive role in Drosophila imaginal tissues, offering a foundation for understanding the pivotal role of SMARCB1/hSNF5 in suppressing MRT during early childhood. Cancer Res; 77(4); 862-73. ©2017 AACR.

Anaplasma marginale Actively Modulates Vacuolar Maturation during Intracellular Infection of Its Tick Vector, Dermacentor andersoni

Tick-borne transmission of bacterial pathogens in the order Rickettsiales is responsible for diverse infectious diseases, many of them severe, in humans and animals. Transmission dynamics differ among these pathogens and are reflected in the pathogen-vector interaction. Anaplasma marginale has been shown to establish and maintain infectivity within Dermacentor spp. for weeks to months while escaping the complex network of vacuolar peptidases that are responsible for digestion of the tick blood meal. How this prolonged maintenance of infectivity in a potentially hostile environment is achieved has been unknown. Using the natural vector Dermacentor andersoni, we demonstrated that A. marginale-infected tick vacuoles (AmVs) concurrently recruit markers of the early endosome (Rab5), recycling endosome (Rab4 and Rab11), and late endosome (Rab7), are maintained near neutral pH, do not fuse with lysosomes, exclude the protease cathepsin L, and engage the endoplasmic reticulum and Golgi apparatus for up to 21 days postinfection. Maintenance of this safe vacuolar niche requires active A. marginale protein synthesis; in its absence, the AmVs mature into acidic, protease-active phagolysosomes. Identification of this bacterially directed modeling of the tick midgut endosome provides a mechanistic basis for examination of the differences in transmission efficiency observed among A. marginale strains and among vector populations.

IMPORTANCE

Ticks transmit a variety of intracellular bacterial pathogens that cause significant diseases in humans and animals. For successful transmission, these bacterial pathogens must first gain entry into the tick midgut digestive cells, avoid digestion, and establish a replicative niche without harming the tick vector. Little is known about how this replicative niche is established and maintained. Using the ruminant pathogen A. marginale and its natural tick vector, D. andersoni, this study characterized the features of the A. marginale niche in the tick midgut and demonstrates that A. marginale protein synthesis is required for the maintenance of this niche. This work opens a new line of inquiry about the pathogen effectors and their targets within the tick that mediate tick-pathogen interactions and ultimately serve as the determinants of pathogen success.

ESCRT-0 complex modulates Rbf-mutant cell survival by regulating Rhomboid endosomal trafficking and EGFR signaling

The Rb tumor suppressor is conserved in Drosophila, and its inactivation can lead to cell proliferation or death depending on the specific cellular context. Therefore, identifying genes that affect the survival of Rb-mutant cells can potentially identify novel targets for therapeutic intervention in cancer. From a genetic screen in Drosophila, we identified synthetic lethal interactions between mutations of fly Rb (rbf) and the ESCRT-0 components stam and hrs We show that inactivation of ESCRT-0 sensitizes rbf-mutant cells to undergo apoptosis through inhibition of EGFR signaling and accumulation of Hid protein. Mutation of stam inhibits EGFR signaling upstream of secreted Spi and downstream of Rhomboid expression, and causes Rhomboid protein to accumulate in the abnormal endosomes labeled with both the early and late endosomal markers Rab5 and Rab7. These results reveal that ESCRT-0 mutants inhibit EGFR signaling by disrupting Rhomboid endosomal trafficking in the ligand-producing cells. Because ESCRT-0 also plays crucial roles in EGFR downregulation after ligand binding, this study provides new insights into how loss of ESCRT-0 function can either increase or decrease EGFR signaling.

Modeling biological gradient formation: combining partial differential equations and Petri nets

Both Petri nets and differential equations are important modeling tools for biological processes. In this paper we demonstrate how these two modeling techniques can be combined to describe biological gradient formation. Parameters derived from partial differential equation describing the process of gradient formation are incorporated in an abstract Petri net model. The quantitative aspects of the resulting model are validated through a case study of gradient formation in the fruit fly.

Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and Delta-Notch signaling to the air sac primordium

The flight muscles, dorsal air sacs, wing blades, and thoracic cuticle of the Drosophila adult function in concert, and their progenitor cells develop together in the wing imaginal disc. The wing disc orchestrates dorsal air sac development by producing decapentaplegic and fibroblast growth factor that travel via specific cytonemes in order to signal to the air sac primordium (ASP). Here, we report that cytonemes also link flight muscle progenitors (myoblasts) to disc cells and to the ASP, enabling myoblasts to relay signaling between the disc and the ASP. Frizzled (Fz)-containing myoblast cytonemes take up Wingless (Wg) from the disc, and Delta (Dl)-containing myoblast cytonemes contribute to Notch activation in the ASP. Wg signaling negatively regulates Dl expression in the myoblasts. These results reveal an essential role for cytonemes in Wg and Notch signaling and for a signal relay system in the myoblasts.

Maintenance of glia in the optic lamina is mediated by EGFR signaling by photoreceptors in adult Drosophila

The late onset of neurodegeneration in humans indicates that the survival and function of cells in the nervous system must be maintained throughout adulthood. In the optic lamina of the adult Drosophila, the photoreceptor axons are surrounded by multiple types of glia. We demonstrated that the adult photoreceptors actively contribute to glia maintenance in their target field within the optic lamina. This effect is dependent on the epidermal growth factor receptor (EGFR) ligands produced by the R1-6 photoreceptors and transported to the optic lamina to act on EGFR in the lamina glia. EGFR signaling is necessary and sufficient to act in a cell-autonomous manner in the lamina glia. Our results suggest that EGFR signaling is required for the trafficking of the autophagosome/endosome to the lysosome. The loss of EGFR signaling results in cell degeneration most likely because of the accumulation of autophagosomes. Our findings provide in vivo evidence for the role of adult neurons in the maintenance of glia and a novel role for EGFR signaling in the autophagic flux.

Degeneration of the nervous system can be viewed as a failure to maintain cell survival or function in the nervous system. The late onset of neurodegeneration in humans indicates that the cell survival in the nervous system must be maintained throughout our lives. Neuronal survival is maintained by neurotrophic factors in adults; however, it is unclear whether glia survival is also maintained throughout adulthood. Here, we use the Drosophila visual system as a model to address the role played by adult neurons for the active maintenance of glia. We demonstrated that the adult photoreceptors secrete a signaling molecule, which is transported to the brain to act on the lamina glia and maintain its integrity. When this signaling pathway is blocked, the lamina glia undergoes a progressive and irreversible degeneration. The primary defect occurs in the trafficking from the late endosome and autophagosome to the lysosome. This defect leads to an accumulation of autophagosomes and subsequent cell degeneration as a result of autophagy. Our findings provide in vivo evidence for a novel aspect of the neuron-glia interaction and a novel role for EGFR signaling in regulating the maintenance and degeneration of the nervous system.

E(y)1/TAF9 mediates the transcriptional output of Notch signaling in Drosophila

Transcriptional activation of Notch signaling targets requires the formation of a ternary complex that involves the intracellular domain of the Notch receptor (NICD), DNA-binding protein Suppressor of Hairless [Su(H), RPBJ in mammals] and coactivator Mastermind (Mam). Here, we report that E(y)1/TAF9, a component of the transcription factor TFIID complex, interacts specifically with the NICD-Su(H)-Mam complex to facilitate the transcriptional output of Notch signaling. We identified E(y)1/TAF9 in a large-scale in vivo RNA interference (RNAi) screen for genes that are involved in a Notch-dependent mitotic-to-endocycle transition in Drosophila follicle cells. Knockdown of e(y)1/TAF9 displayed Notch-mutant-like phenotypes and defects in target gene and activity reporter expression in both the follicle cells and wing imaginal discs. Epistatic analyses in these two tissues indicated that E(y)1/TAF9 functions downstream of Notch cleavage. Biochemical studies in S2 cells demonstrated that E(y)1/TAF9 physically interacts with the transcriptional effectors of Notch signaling Su(H) and NICD. Taken together, our data suggest that the association of the NICD-Su(H)-Mastermind complex with E(y)1/TAF9 in response to Notch activation recruits the transcription initiation complex to induce Notch target genes, coupling Notch signaling with the transcription machinery.

Imaging the dynamics of endocytosis in live mammalian tissues

In mammalian cells, endocytosis plays a pivotal role in regulating several basic cellular functions. Up to now, the dynamics and the organization of the endocytic pathways have been primarily investigated in reductionist model systems such as cell and organ cultures. Although these experimental models have been fully successful in unraveling the endocytic machinery at a molecular level, our understanding of the regulation and the role of endocytosis in vivo has been limited. Recently, advancements in intravital microscopy have made it possible to extend imaging in live animals to subcellular structures, thus revealing new aspects of the molecular machineries regulating membrane trafficking that were not previously appreciated in vitro. Here, we focus on the use of intravital microscopy to study endocytosis in vivo, and discuss how this approach will allow addressing two fundamental questions: (1) how endocytic processes are organized in mammalian tissues, and (2) how they contribute to organ physiopathology.

Cell competition may function either as tumour-suppressing or as tumour-stimulating factor in Drosophila

Drosophila endocytosis-defective cells develop tumour overgrowths in the imaginal discs. We have analysed the tumorigenic potential of cells mutant for Rab5, a gene involved in endocytosis. We found that while a compartment entirely made by Rab5 mutant cells can grow indefinitely, clones of Rab5 cells surrounded by normal cells are eliminated by cell competition. However, when a group of about 400 cells are simultaneously made mutant for Rab5, they form an overgrowing tumour: mutant cells in the periphery are eliminated, but those inside survive and continue proliferating because they are beyond the range of cell competition. These results identify group protection as a mechanism to evade the tumour-suppressing function of cell competition in Drosophila. Furthermore, we find that the growth of the tumour depends to a large extent on the presence of apoptosis inside the tumour: cells doubly mutant for Rab5 and the proapoptotic gene dronc do not form overgrowing tumours. These results suggest that the apoptosis caused by cell competition acts as a tumour-stimulating factor, bringing about high levels of Jun N-terminal kinase and subsequently Wg/Dpp signalling and high proliferation levels in the growing tumour. We conclude that under these circumstances cell competition facilitates the progression of the tumour, thus reversing its normal antitumour role.

Effects of intrinsic stochasticity on delayed reaction-diffusion patterning systems

Cellular gene expression is a complex process involving many steps, including the transcription of DNA and translation of mRNA; hence the synthesis of proteins requires a considerable amount of time, from ten minutes to several hours. Since diffusion-driven instability has been observed to be sensitive to perturbations in kinetic delays, the application of Turing patterning mechanisms to the problem of producing spatially heterogeneous differential gene expression has been questioned. In deterministic systems a small delay in the reactions can cause a large increase in the time it takes a system to pattern. Recently, it has been observed that in undelayed systems intrinsic stochasticity can cause pattern initiation to occur earlier than in the analogous deterministic simulations. Here we are interested in adding both stochasticity and delays to Turing systems in order to assess whether stochasticity can reduce the patterning time scale in delayed Turing systems. As analytical insights to this problem are difficult to attain and often limited in their use, we focus on stochastically simulating delayed systems. We consider four different Turing systems and two different forms of delay. Our results are mixed and lead to the conclusion that, although the sensitivity to delays in the Turing mechanism is not completely removed by the addition of intrinsic noise, the effects of the delays are clearly ameliorated in certain specific cases.

Drosophila GGA model: an ultimate gateway to GGA analysis

Golgi-localized, γ-ear-containing, ADP ribosylation factor-binding (GGA) proteins are monomeric adaptors implicated in clathrin-mediated vesicular transport between the trans Golgi network and endosomes, characterized mainly from cell culture analysis of lysosomal sorting. To provide the first demonstration of GGA's role in vivo, we used Drosophila which has a single GGA and a single lysosomal sorting receptor, lysosomal enzyme receptor protein (LERP). Using RNAi knockdowns, we show that the Drosophila GGA is required for lysosomal sorting. We further identified authentic components of the Drosophila lysosomal sorting system--the sorting receptor LERP, the sorting adaptor GGA and the lysosomal cargo cathepsins B1, D and L--to show that GGA depletion results in lysosomal dysfunction. Abnormal lysosomal morphology, missorting of lysosomal cathepsins and impaired lysosomal proteolysis show disturbed LERP trafficking after GGA depletion. GGA is highly expressed in the mushroom bodies and the pigment cells of the retina, and increasing or decreasing the levels of GGA in the eyes leads to retinal defects. Reduced GGA levels also enhance an eye defect caused by overexpression of the autophagy-associated protein Blue cheese (Bchs), implicating GGA in autophagic processes. This shows that Drosophila provides an excellent whole-animal model to gain new insights into the function of GGA in the physiological environment of a multicellular organism.

Endosomal signaling of the tomato leucine-rich repeat receptor-like protein LeEix2

Extracellular leucine-rich repeat (LRR) receptor-like proteins (RLPs) represent a unique class of cell-surface receptors, as they lack a functional cytoplasmic domain. Our knowledge of how RLPs that do not contain a kinase or Toll domain function is very limited. The tomato RLP receptor LeEix2 signals to induce defense responses mediated by the fungal protein ethylene-inducing xylanase (EIX). The movement of FYVE-positive endosomes before and after EIX application was examined using spinning disc confocal microscopy. We found that while FYVE-positive endosomes generally observe a random movement pattern, following EIX application a subpopulation of FYVE-positive endosomes follow a directional movement pattern. Further, cellular endosomes travel greater distances at higher speeds following EIX application. Time-course experiments conducted with specific inhibitors demonstrate the involvement of endosomal signaling in EIX-triggered defense responses. Abolishing the existence of endosomes or the endocytic event prevented EIX-induced signaling. Endocytosis/endosome inhibitors, such as Dynasore or 1-butanol, inhibit EIX-induced signaling. Moreover, treatment with Endosidin1, which inhibits an early step in plasma membrane/endosome trafficking, enhances the induction of defense responses by EIX. Our data indicate a distinct endosomal signaling mechanism for induction of defense responses in this RLP system.

At the crossroads of chemistry and cell biology: inhibiting membrane traffic by small molecules

Intracellular membrane traffic regulates cell physiology at multiple levels ranging from cell growth and development to the function of the nervous and immune systems. Multiple endocytic routes are used by distinct cargoes including ligands bound to their receptors but also viruses and pathogens to gain access to the cell interior. Within the endosomal system, proteins and lipids are sorted for degradation or recycling allowing cells to dynamically respond to environmental signals and to regulate cell shape and morphology. Some receptors or toxins are sorted along the retrograde pathway from endosomes to the Golgi complex, where they intersect with secretory cargo destined for exocytosis. Genetic manipulations of these pathways frequently cause problems with regard to data interpretation as the resulting phenotypes may be indirect consequences resulting from perturbation of multiple steps or trafficking routes. Hence, novel approaches are needed to acutely and reversibly perturb intracellular membrane traffic, e.g., by small molecule inhibitors. Such drugs may also be pharmacologically important as they offer new avenues to fight human diseases. Here, we provide an overview of the small molecules available to interfere with intracellular membrane traffic and outline strategies for future research.

Spinster controls Dpp signaling during glial migration in the Drosophila eye

The development of multicellular organisms requires the well balanced and coordinated migration of many cell types. This is of particular importance within the developing nervous system, where glial cells often move long distances to reach their targets. The majority of glial cells in the peripheral nervous system of the Drosophila embryo is derived from the CNS and migrates along motor axons toward their targets. In the developing Drosophila eye, CNS-derived glial cells move outward toward the nascent photoreceptor cells, but the molecular mechanisms coupling the migration of glial cells with the growth of the eye imaginal disc are mostly unknown. Here, we used an enhancer trap approach to identify the gene spinster, which encodes a multipass transmembrane protein involved in endosome-lysosome trafficking, as being expressed in many glial cells. spinster mutants are characterized by glial overmigration. Genetic experiments demonstrate that Spinster modulates the activity of several signaling cascades. Within the migrating perineurial glial cells, Spinster is required to downregulate Dpp (Decapentaplegic) signaling activity, which ceases migratory abilities. In addition, Spinster affects the growth of the carpet cell, which indirectly modulates glial migration.

Cell polarity in plants: Linking PIN polarity generation mechanisms to morphogenic auxin gradients

Auxin efflux carrier PIN proteins have been intensively investigated as they are the first polar cargos to be identified in plants with a direct relevance for plant patterning. Based on their polar localization; PIN proteins direct the intercellular flow of signaling molecule auxin and thus bear a rate limiting effect on the formation of auxin activity gradients. With this influence on directionality and extent of auxin transport PINs play crucial roles in plant body organization. Many factors such as vesicle trafficking regulator ARF-GEF GNOM, a kinase PINOID, a retromer complex and membrane sterol composition influence polar PIN localization. Recent work uncovers the mechanism that generates default PIN polarity. Real time PIN tracking reveals that PIN polarity is generated from initially non-polar secretion via endocytosis and subsequent polar recycling. In addition, the Rab5 endocytic pathway emerges to be important for polar PIN localization as Rab5 interference causes non-polar distribution of PINs. This non-polar distribution of PINs during embryogenesis transiently alters auxin activity gradients and changes organ identity by transforming embryonic leaf cells to root fates. These findings for the first time link PIN polarity-based auxin activity gradient to cell fate decisions and thus demonstrate morphogen (a substance influencing cell fates on its concentration gradient) characters of auxin. They also suggest an auxin activity distribution-dependent sensing module that executes differential apical and basal developmental program during plant embryogenesis.

The EHEC type III effector NleL is an E3 ubiquitin ligase that modulates pedestal formation

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 causes hemorrhagic colitis and may result in potentially fatal hemolytic uremia syndrome in humans. EHEC colonize the intestinal mucosa and promote the formation of actin-rich pedestals via translocated type III effectors. Two EHEC type III secreted effectors, Tir and EspFu/TccP, are key players for pedestal formation. We discovered that an EHEC effector protein called Non-LEE-encoded Ligase (NleL) is an E3 ubiquitin ligase. In vitro, we showed that the NleL C753 residue is critical for its E3 ligase activity. Functionally, we demonstrated that NleL E3 ubiquitin ligase activity is involved in modulating Tir-mediated pedestal formation. Surprisingly, EHEC mutant strain deficient in the E3 ligase activity induced more pedestals than the wild-type strain. The canonical EPEC strain E2348/69 normally lacks the nleL gene, and the ectopic expression of the wild-type EHEC nleL, but not the catalytically-deficient nleL(C753A) mutant, in this strain resulted in fewer actin-rich pedestals. Furthermore, we showed that the C. rodentium NleL homolog is a E3 ubiquitin ligase and is required for efficient infection of murine colonic epithelial cells in vivo. In summary, our study demonstrated that EHEC utilizes NleL E3 ubiquitin ligase activity to modulate Tir-mediated pedestal formation.

The dynamics of Turing patterns for morphogen-regulated growing domains with cellular response delays

Since its conception in 1952, the Turing paradigm for pattern formation has been the subject of numerous theoretical investigations. Experimentally, this mechanism was first demonstrated in chemical reactions over 20 years ago and, more recently, several examples of biological self-organisation have also been implicated as Turing systems. One criticism of the Turing model is its lack of robustness, not only with respect to fluctuations in the initial conditions, but also with respect to the inclusion of delays in critical feedback processes such as gene expression. In this work we investigate the possibilities for Turing patterns on growing domains where the morphogens additionally regulate domain growth, incorporating delays in the feedback between signalling and domain growth, as well as gene expression. We present results for the proto-typical Schnakenberg and Gierer-Meinhardt systems: exploring the dynamics of these systems suggests a reconsideration of the basic Turing mechanism for pattern formation on morphogen-regulated growing domains as well as highlighting when feedback delays on domain growth are important for pattern formation.

D-Cbl binding to Drk leads to dose-dependent down-regulation of EGFR signaling and increases receptor-ligand endocytosis

Proper control of Epidermal Growth Factor Receptor (EGFR) signaling is critical for normal development and regulated cell behaviors. Abnormal EGFR signaling is associated with tumorigenic process of various cancers. Complicated feedback networks control EGFR signaling through ligand production, and internalization-mediated destruction of ligand-receptor complexes. Previously, we found that two isoforms of D-Cbl, D-CblS and D-CblL, regulate EGFR signaling through distinct mechanisms. While D-CblL plays a crucial role in dose-dependent down-regulation of EGFR signaling, D-CblS acts in normal restriction of EGFR signaling and does not display dosage effect. Here, we determined the underlying molecular mechanism, and found that Drk facilitates the dose-dependent regulation of EGFR signaling through binding to the proline-rich motif of D-CblL, PR. Furthermore, the RING finger domain of D-CblL is essential for promoting endocytosis of the ligand-receptor complex. Interestingly, a fusion protein of the two essential domains of D-CblL, RING- PR, is sufficient to down-regulate EGFR signal in a dose-dependent manner by promoting internalization of the ligand, Gurken. Besides, RING-SH2^Drk, a fusion protein of the RING finger domain of D-Cbl and the SH2 domain of Drk, also effectively down-regulates EGFR signaling in Drosophila follicle cells, and suppresses the effects of constitutively activated EGFR. The RING-SH2^Drk suppresses EGFR signaling by promoting the endosomal trafficking of ligand-receptor complexes, suggesting that Drk plays a negative role in EGFR signaling by enhancing receptor endocytosis through cooperating with the RING domain of D-Cbl. Interfering the recruitment of signal transducer, Drk, to the receptor by the RING-SH2^Drk might further reduces EGFR signaling. The fusion proteins we developed may provide alternative strategies for therapy of cancers caused by hyper-activation of EGFR signaling.

Self-induced patched receptor down-regulation modulates cell sensitivity to the hedgehog morphogen gradient

Morphogens form signaling gradients that control patterning processes during development. Responding cells must perceive and interpret the concentration-dependent information provided by the morphogen to generate precise patterns of gene expression and cell differentiation in developing tissues. Generally, the absolute number of activated, ligand-bound receptors determines cell perception of the morphogen. In contrast, cells interpret the morphogen Hedgehog (Hh) by measuring the ratio of bound to unbound molecules of its receptor Patched (Ptc). This ratio depends on both the Hh concentration and the absolute number of Ptc molecules. Here, I describe a posttranscriptional process that controls the absolute amount of Ptc present in a cell, which regulates gradient interpretation, wherein self-induced receptor down-regulation that is independent of ligand binding dictates the cell response to a morphogen gradient.

The Hrs/Stam complex acts as a positive and negative regulator of RTK signaling during Drosophila development

BACKGROUND

Endocytosis is a key regulatory step of diverse signalling pathways, including receptor tyrosine kinase (RTK) signalling. Hrs and Stam constitute the ESCRT-0 complex that controls the initial selection of ubiquitinated proteins, which will subsequently be degraded in lysosomes. It has been well established ex vivo and during Drosophila embryogenesis that Hrs promotes EGFR down regulation. We have recently isolated the first mutations of stam in flies and shown that Stam is required for air sac morphogenesis, a larval respiratory structure whose formation critically depends on finely tuned levels of FGFR activity. This suggest that Stam, putatively within the ESCRT-0 complex, modulates FGF signalling, a possibility that has not been examined in Drosophila yet.

PRINCIPAL FINDINGS

Here, we assessed the role of the Hrs/Stam complex in the regulation of signalling activity during Drosophila development. We show that stam and hrs are required for efficient FGFR signalling in the tracheal system, both during cell migration in the air sac primordium and during the formation of fine cytoplasmic extensions in terminal cells. We find that stam and hrs mutant cells display altered FGFR/Btl localisation, likely contributing to impaired signalling levels. Electron microscopy analyses indicate that endosome maturation is impaired at distinct steps by hrs and stam mutations. These somewhat unexpected results prompted us to further explore the function of stam and hrs in EGFR signalling. We show that while stam and hrs together downregulate EGFR signalling in the embryo, they are required for full activation of EGFR signalling during wing development.

CONCLUSIONS/SIGNIFICANCE

Our study shows that the ESCRT-0 complex differentially regulates RTK signalling, either positively or negatively depending on tissues and developmental stages, further highlighting the importance of endocytosis in modulating signalling pathways during development.

Neuralized promotes basal to apical transcytosis of delta in epithelial cells

In this article, it is shown that a pool of Delta localizes at the basolateral membrane of sensory organ precursor cells of Drosophila and of polarized MDCK cells and that Delta is endocytosed in a Neuralized-dependent manner from this basolateral membrane to allow for relocalization to the apical domain where it can bind and activate Notch.

Notch receptors mediate short-range signaling controlling many developmental decisions in metazoans. Activation of Notch requires the ubiquitin-dependent endocytosis of its ligand Delta. How ligand endocytosis in signal-sending cells regulates receptor activation in juxtaposed signal-receiving cells remains largely unknown. We show here that a pool of Delta localizes at the basolateral membrane of signal-sending sensory organ precursor cells in the dorsal thorax neuroepithelium of Drosophila and that Delta is endocytosed in a Neuralized-dependent manner from this basolateral membrane. This basolateral pool of Delta is segregated from Notch that accumulates apically. Using a compartimentalized antibody uptake assay, we show that murine Delta-like 1 is similarly internalized by mNeuralized2 from the basolateral membrane of polarized Madin-Darby canine kidney cells and that internalized ligands are transcytosed to the apical plasma membrane where mNotch1 accumulates. Thus, endocytosis of Delta by Neuralized relocalizes Delta from the basolateral to the apical membrane domain. We speculate that this Neuralized-dependent transcytosis regulates the signaling activity of Delta by relocalizing Delta from a membrane domain where it cannot interact with Notch to another membrane domain where it can bind and activate Notch.

The influence of gene expression time delays on Gierer-Meinhardt pattern formation systems

There are numerous examples of morphogen gradients controlling long range signalling in developmental and cellular systems. The prospect of two such interacting morphogens instigating long range self-organisation in biological systems via a Turing bifurcation has been explored, postulated, or implicated in the context of numerous developmental processes. However, modelling investigations of cellular systems typically neglect the influence of gene expression on such dynamics, even though transcription and translation are observed to be important in morphogenetic systems. In particular, the influence of gene expression on a large class of Turing bifurcation models, namely those with pure kinetics such as the Gierer-Meinhardt system, is unexplored. Our investigations demonstrate that the behaviour of the Gierer-Meinhardt model profoundly changes on the inclusion of gene expression dynamics and is sensitive to the sub-cellular details of gene expression. Features such as concentration blow up, morphogen oscillations and radical sensitivities to the duration of gene expression are observed and, at best, severely restrict the possible parameter spaces for feasible biological behaviour. These results also indicate that the behaviour of Turing pattern formation systems on the inclusion of gene expression time delays may provide a means of distinguishing between possible forms of interaction kinetics. Finally, this study also emphasises that sub-cellular and gene expression dynamics should not be simply neglected in models of long range biological pattern formation via morphogens.

Endocytic internalization routes required for delta/notch signaling

The internalization of transmembrane receptors from the cell surface plays a central role in signal regulation. Receptor internalization can occur through different routes; however, because of the difficulty in selectively blocking these routes in vivo, their roles in signaling are poorly understood. Here we use null mutations in Drosophila dynamin, clathrin, and AP-2 adaptor subunits to analyze internalization requirements for the Delta ligand and its receptor, Notch. Bulk Notch is internalized via AP-2-dependent endocytosis, but signaling by Notch requires AP-2-independent clathrin-dependent endocytosis, highlighting a distinction between Notch endocytic routes required for degradation versus signaling activation. Signaling by Delta requires dynamin, but whether this generates a pulling force of Delta on Notch or allows for Delta entry into a recycling pathway to gain signaling competence is widely debated. Surprisingly, we show that signaling by Delta in germline cells can occur by clathrin-independent endocytosis, when endosomal entry is blocked, and when activity of Rab11 or its effectors is reduced, suggesting that Delta need not pass through a recognized recycling pathway to achieve signaling competence. The absolute requirement for dynamin-dependent endocytosis but not endosomal entry or Rab11 activity supports "pulling force" rather than "recycling" models for Delta activation.

Aberrant behaviours of reaction diffusion self-organisation models on growing domains in the presence of gene expression time delays

Turing's pattern formation mechanism exhibits sensitivity to the details of the initial conditions suggesting that, in isolation, it cannot robustly generate pattern within noisy biological environments. Nonetheless, secondary aspects of developmental self-organisation, such as a growing domain, have been shown to ameliorate this aberrant model behaviour. Furthermore, while in-situ hybridisation reveals the presence of gene expression in developmental processes, the influence of such dynamics on Turing's model has received limited attention. Here, we novelly focus on the Gierer-Meinhardt reaction diffusion system considering delays due the time taken for gene expression, while incorporating a number of different domain growth profiles to further explore the influence and interplay of domain growth and gene expression on Turing's mechanism. We find extensive pathological model behaviour, exhibiting one or more of the following: temporal oscillations with no spatial structure, a failure of the Turing instability and an extreme sensitivity to the initial conditions, the growth profile and the duration of gene expression. This deviant behaviour is even more severe than observed in previous studies of Schnakenberg kinetics on exponentially growing domains in the presence of gene expression (Gaffney and Monk in Bull. Math. Biol. 68:99-130, 2006). Our results emphasise that gene expression dynamics induce unrealistic behaviour in Turing's model for multiple choices of kinetics and thus such aberrant modelling predictions are likely to be generic. They also highlight that domain growth can no longer ameliorate the excessive sensitivity of Turing's mechanism in the presence of gene expression time delays. The above, extensive, pathologies suggest that, in the presence of gene expression, Turing's mechanism would generally require a novel and extensive secondary mechanism to control reaction diffusion patterning.

Endocytosis unplugged: multiple ways to enter the cell

Endocytosis occurs at the cell surface and involves internalization of the plasma membrane (PM) along with its constituent membrane proteins and lipids. Endocytosis is involved in sampling of the extracellular milieu and also serves to regulate various processes initiated at the cell surface. These include nutrient uptake, signaling from cell-surface receptors, and many other processes essential for cell and tissue functioning in metazoans. It is also central to the maintenance of PM lipid and protein homeostasis. There are multiple means of internalization that operate concurrently, at the cell surface. With advancement in high-resolution visualization techniques, it is now possible to track multiple endocytic cargo at the same time, revealing a remarkable diversity of endocytic processes in a single cell. A combination of live cell imaging and efficient genetic manipulations has also aided in understanding the functional hierarchy of molecular players in these mechanisms of internalization. Here we provide an account of various endocytic routes, their mechanisms of operation and occurrence across phyla.

Numb independently antagonizes Sanpodo membrane targeting and Notch signaling in Drosophila sensory organ precursor cells

How Numb regulates Notch signaling following asymmetric cell division is unclear. Numb directly binds and blocks membrane localization of Sanpodo, a protein essential for Notch signaling in Drosophila. Uncoupling Sanpodo from Numb results in accumulation of Sanpodo at the membrane, but this surprisingly does not appear to promote Notch signaling.

In Drosophila, mitotic neural progenitor cells asymmetrically segregate the cell fate determinant Numb in order to block Notch signaling in only one of the two daughter cells. Sanpodo, a membrane protein required for Notch signaling in asymmetrically dividing cells, is sequestered from the plasma membrane to intracellular vesicles in a Numb-dependent way after neural progenitor cell mitosis. However, the significance of Numb-dependent Sanpodo regulation is unclear. In this study, we conducted a structure–function analysis to identify the determinants of Sanpodo targeting in vivo. We identified an NPAF motif in the amino-terminal cytoplasmic tail of Sanpodo, which is conserved among insect Sanpodo homologues. The Sanpodo NPAF motif is predicted to bind directly to the Numb phosphotyrosine-binding domain and is critical for Numb binding in vitro. Deletion or mutation of the NPAF motif results in accumulation of Sanpodo at the plasma membrane in Numb-positive cells in vivo. Genetic analysis of Sanpodo NPAF mutants shows that Numb-dependent Sanpodo endocytic targeting can be uncoupled from Notch signaling regulation. Our findings demonstrate that Sanpodo contains an evolutionarily conserved motif that has been linked to Numb-dependent regulation in vertebrates and further support the model that Numb regulates Notch signaling independently of Sanpodo membrane trafficking in neural progenitor cells.

Identifying genes that interact with Drosophila presenilin and amyloid precursor protein

The gamma-secretase complex is involved in cleaving transmembrane proteins such as Notch and one of the genes targeted in Alzheimer's disease known as amyloid precursor protein (APP). Presenilins function within the catalytic core of gamma-secretase, and mutated forms of presenilins were identified as causative factors in familial Alzheimer's disease. Recent studies show that in addition to Notch and APP, numerous signal transduction pathways are modulated by presenilins, including intracellular calcium signaling. Thus, presenilins appear to have diverse roles. To further understand presenilin function, we searched for Presenilin-interacting genes in Drosophila by performing a genetic modifier screen for enhancers and suppressors of Presenilin-dependent Notch-related phenotypes. We identified 177 modifiers, including known members of the Notch pathway and genes involved in intracellular calcium homeostasis. We further demonstrate that 53 of these modifiers genetically interacted with APP. Characterization of these genes may provide valuable insights into Presenilin function in development and disease.

Expression and localization of clathrin heavy chain in Drosophila melanogaster

Clathrin-coated vesicles mediate cellular endocytosis of nutrients and molecules that are involved in a variety of biological processes. Basic components of the vesicle coat are clathrin heavy chain (Chc) and clathrin light chain molecules. In Drosophila melanogaster the chc gene function has been analyzed in a number of previous studies mainly using genetic approaches. However, the chc mRNA and protein expression patterns have not been studied systematically. We have generated an antibody that specifically recognizes Chc and we have analyzed chc RNA and protein expression patterns throughout embryonic and larval stages. We found that chc mRNA and protein are highly expressed from early stages of embryogenesis onwards, consistent with genetic studies predicting a maternal contribution of the gene function. During subsequent stages mRNA and protein are co-expressed in all embryonic cells; however we found an up-regulation in specific tissues including the gut, the salivary glands, tracheal system and the epidermis. In addition the central nervous system and the nephrocyte-like garland cells show strong Chc expression at late embryogenesis. In larvae Chc is highly expressed in garland cells, imaginal discs, fat body, salivary glands and the ring gland. Subcellularly, we found Chc protein in a vesicle-like pattern within the cytoplasm and at the plasma membrane. Co-labeling studies show that Chc is partially in contact with the trans-Golgi network and co-localizes with markers for early endocytosis. Together, the antibody may serve as a new tool to study the function of Chc in clathrin-dependent cellular processes, such as endocytosis.

Drosophila liquid facets-Related encodes Golgi epsin and is an essential gene required for cell proliferation, growth, and patterning

Epsin and epsin-Related (epsinR) are multi-modular proteins that stimulate clathrin-coated vesicle formation. Epsin promotes endocytosis at the plasma membrane, and epsinR functions at the Golgi and early endosomes for trans-Golgi network/endosome vesicle trafficking. In Drosophila, endocytic epsin is known as Liquid facets, and it is essential specifically for Notch signaling. Here, by generating and analyzing loss-of-function mutants in the liquid facets-Related (lqfR) gene of Drosophila, we investigated the function of Golgi epsin in a multicellular context. We found that LqfR is indeed a Golgi protein, and that like liquid facets, lqfR is essential for Drosophila viability. In addition, primarily by analyzing mutant eye discs, we found that lqfR is required for cell proliferation, insulin-independent cell growth, and cell patterning, consistent with a role in one or several signaling pathways. Epsins in all organisms share an ENTH (epsin N-terminal homology) domain, which binds phosphoinositides enriched at the plasma membrane or the Golgi membrane. The epsinR ENTH domain is also the recognition element for particular cargos. By generating wild-type and mutant lqfR transgenes, we found that all apparent LqfR functions are independent of its ENTH domain. These results suggest that LqfR transports specific cargo critical to one or more signaling pathways, and lays the foundation for identifying those proteins.

The single ENTH-domain protein of trypanosomes; endocytic functions and evolutionary relationship with epsin

Epsin N-terminal homology (ENTH) domains occur in proteins of either the epsin or epsin-related (epsinR) form. They principally function in clathrin-mediated trafficking and membrane deformation. Both epsin and epsinR possess clathrin-binding motifs, but only epsin incorporates a ubiquitin-interaction motif (UIM). To better understand the origins of ENTH-domain proteins and their functions, we performed detailed comparative genomics and phylogenetics on the epsin family. The epsin ENTH-UIM configuration is an architecture restricted to yeast and animals. Further, we undertook functional analysis in Trypanosoma brucei (T. brucei), a divergent organism possessing a single ENTH-domain protein (TbEpsinR). TbEpsinR has a cellular location similar to both epsin and epsinR at plasma membrane clathrin budding sites and endosomal compartments, and associates with clathrin, as demonstrated by coimmunoprecipitation. Knockdown of TbEpsinR leads to a significant decrease in the intracellular pools of multiple surface antigens, without affecting bulk membrane internalization. Therefore, despite lacking the UIM, TbEpsinR maintains a similar role to metazoan epsin in endocytosis and participates as a clathrin-associated adaptor. We suggest that recruitment of a UIM to the ENTH-domain proteins was not essential for participation in endocytosis of ubiquitylated molecules, and is presumably a specific innovation restricted to higher eukaryotes.

Regulation of Notch and Wingless signalling by phyllopod, a transcriptional target of the EGFR pathway

Spatial and temporal control of Notch and Wingless (Wg) pathways during development is regulated at multiple levels. Here, we present an analysis of Phyllopod as a coordinated regulator of these two critical signal transduction pathways. Phyl specifically helps traffic Notch and Wg pathway components within early endocytic vesicles, thereby controlling the amount of processed signal available for causing a transcriptional response within the nucleus. In Drosophila, the EGFR pathway transcriptionally activates phyl whose product then blocks Notch and Wg signalling pathways. This provides a mechanistic basis for an antagonistic relationship between receptor tyrosine kinase and Notch/Wg pathways during development. Furthermore, this study identifies a Phyl-regulated class of endosomal vesicles that specifically include components of Notch and Wg signalling.

Auxin transport through non-hair cells sustains root-hair development

The plant hormone auxin controls root epidermal cell development in a concentration-dependent manner. Root hairs are produced on a subset of epidermal cells as they increase in distance from the root tip. Auxin is required for their initiation and continued growth, but little is known about its distribution in this region of the root. Contrary to the expectation that hair cells might require active auxin influx to ensure auxin supply, we did not detect the auxin-influx transporter AUX1 in root-hair cells. A high level of AUX1 expression was detected in adjacent non-hair cell files. Non-hair cells were necessary to achieve wild-type root-hair length, although an auxin response was not required in these cells. Three-dimensional modelling of auxin flow in the root tip suggests that AUX1-dependent transport through non-hair cells maintains an auxin supply to developing hair cells as they increase in distance from the root tip, and sustains root-hair outgrowth. Experimental data support the hypothesis that instead of moving uniformly though the epidermal cell layer, auxin is mainly transported through canals that extend longitudinally into the tissue.

Interactions between the juvenile Batten disease gene, CLN3, and the Notch and JNK signalling pathways

Mutations in the gene CLN3 are responsible for the neurodegenerative disorder juvenile neuronal ceroid lipofuscinosis or Batten disease. CLN3 encodes a multi-spanning and hydrophobic transmembrane protein whose function is unclear. As a consequence, the cell biology that underlies the pathology of the disease is not well understood. We have developed a genetic gain-of-function system in Drosophila to identify functional pathways and interactions for CLN3. We have identified previously unknown interactions between CLN3 and the Notch and Jun N-terminal kinase signalling pathways and have uncovered a potential role for the RNA splicing and localization machinery in regulating CLN3 function.

Signaling from endosomes: location makes a difference

In all transmembrane receptor systems the kinetics of receptor trafficking upon ligand stimulation is maintained in a balance between degradative and recycling pathways in order to keep homeostasis and to strictly control receptor-mediated signaling. Endocytosis is commonly considered as an efficient mechanism of uptake and transport of membrane-associated signaling molecules leading to attenuation of ligand-induced responses. Accumulating evidence, however, shows that signaling from internalized receptors not only continues in endosomal compartments, but that there are also distinct signaling events that require endocytosis. Endocytic organelles form a dynamic network of subcellular compartments, which actively control the timing, amplitude, and specificity of signaling. In this review we provide examples in which signal transduction either requires an active endocytic machinery, or directly originates from various types of endosomes. Based on recent discoveries, we emphasize the close interdependence between signaling and endocytosis, and the physiological relevance of endocytic transport in health and disease.

Apoptosis-induced compensatory proliferation. The Cell is dead. Long live the Cell!

In multi-cellular organisms, activation of apoptosis can trigger compensatory proliferation in surrounding cells to maintain tissue homeostasis. Genetic studies in Drosophila have indicated that distinct mechanisms of compensatory proliferation are employed in apoptotic tissues of different developmental states. In proliferating eye and wing tissues, the initiator caspase Dronc coordinates cell death and compensatory proliferation through the Jun N-terminal kinase and p53. The mitogens Decapentaplegic and Wingless are induced in this process. By contrast, in differentiating eye tissues, the effector caspases DrICE and Dcp-1 activate the Hedgehog signaling pathway to induce compensatory proliferation. In this review, we summarize these findings and discuss how activation of apoptosis is linked to the process of compensatory proliferation. The developmental and pathological relevance of compensatory proliferation is also discussed.

Regulated trafficking of the MSP/Eph receptor during oocyte meiotic maturation in C. elegans

BACKGROUND

In C. elegans, a sperm-sensing mechanism regulates oocyte meiotic maturation and ovulation, tightly coordinating sperm availability and embryo production; sperm release the major sperm protein (MSP) signal to trigger meiotic resumption. Meiotic arrest depends on the parallel function of the oocyte VAB-1 MSP/Eph receptor and somatic G protein signaling. MSP promotes meiotic maturation by antagonizing Eph receptor signaling and counteracting inhibitory inputs from the gonadal sheath cells.

RESULTS

Here, we present evidence suggesting that in the absence of the MSP ligand, the VAB-1 Eph receptor inhibits meiotic maturation while either in or in transit to the endocytic-recycling compartment. VAB-1::GFP localization to the RAB-11-positive endocytic-recycling compartment is independent of ephrins but is antagonized by MSP signaling. Two negative regulators of oocyte meiotic maturation, DAB-1/Disabled and RAN-1, interact with the VAB-1 receptor and are required for its accumulation in the endocytic-recycling compartment in the absence of MSP or sperm (hereafter referred to as MSP/sperm). Inactivation of the endosomal recycling regulators rme-1 or rab-11.1 causes a vab-1-dependent reduction in the meiotic-maturation rate in the presence of MSP/sperm. Further, we show that Galpha(s) signaling in the gonadal sheath cells, which is required for meiotic maturation in the presence of MSP/sperm, affects VAB-1::GFP trafficking in oocytes.

CONCLUSIONS

Regulated endocytic trafficking of the VAB-1 MSP/Eph receptor contributes to the control of oocyte meiotic maturation in C. elegans. Eph receptor trafficking in other systems may be influenced by the conserved proteins DAB-1/Disabled and RAN-1 and by crosstalk with G protein signaling in neighboring cells.

Endosomal entry regulates Notch receptor activation in Drosophila melanogaster

Signaling through the transmembrane receptor Notch is widely used throughout animal development and is a major regulator of cell proliferation and differentiation. During canonical Notch signaling, internalization and recycling of Notch ligands controls signaling activity, but the involvement of endocytosis in activation of Notch itself is not well understood. To address this question, we systematically assessed Notch localization, processing, and signaling in a comprehensive set of Drosophila melanogaster mutants that block access of cargo to different endocytic compartments. We find that gamma-secretase cleavage and signaling of endogenous Notch is reduced in mutants that impair entry into the early endosome but is enhanced in mutants that increase endosomal retention. In mutants that block endosomal entry, we also uncover an alternative, low-efficiency Notch trafficking route that can contribute to signaling. Our data show that endosomal access of the Notch receptor is critical to achieve physiological levels of signaling and further suggest that altered residence in distinct endocytic compartments could underlie pathologies involving aberrant Notch pathway activation.

Endocytic downregulation of ErbB receptors: mechanisms and relevance in cancer

ErbB receptors (EGFR (ErbB1), ErbB2, ErbB3, and ErbB4) are important regulators of normal growth and differentiation, and they are involved in the pathogenesis of cancer. Following ligand binding and receptor activation, EGFR is endocytosed and transported to lysosomes where the receptor is degraded. This downregulation of EGFR is a complex and tightly regulated process. The functions of ErbB2, ErbB3, and ErbB4 are also regulated by endocytosis to some extent, although the current knowledge of these processes is sparse. Impaired endocytic downregulation of signaling receptors is frequently associated with cancer, since it can lead to increased and uncontrolled receptor signaling. In this review we describe the current knowledge of ErbB receptor endocytic downregulation. In addition, we outline how ErbB receptors can escape endocytic downregulation in cancer, and we discuss how targeted anti-cancer therapy may induce endocytic downregulation of ErbB receptors.