Regulation of early endosomal entry by the Drosophila tumor suppressors Rabenosyn and Vps45

Rabenosyn-5 suppresses non-small cell lung cancer metastasis via inhibiting CDC42 activity

Metastasis, the primary cause of death in lung cancer patients, is facilitated by cytoskeleton remodeling, which plays a crucial role in cancer cell migration and invasion. However, the precise regulatory mechanisms of intracellular trafficking proteins involved in cytoskeleton remodeling remain unclear. In this study, we have identified Rabenosyn-5 (Rbsn) as an inhibitor of filopodia formation and lung cancer metastasis. Mechanistically, Rbsn interacts with CDC42 and functions as a GTPase activating protein (GAP), thereby inhibiting CDC42 activity and subsequent filopodia formation. Furthermore, we have discovered that Akt phosphorylates Rbsn at the Thr253 site, and this phosphorylation negates the inhibitory effect of Rbsn on CDC42 activity. Additionally, our analysis reveals that Rbsn expression is significantly downregulated in lung cancer, and this decrease is associated with a worse prognosis. These findings provide strong evidence supporting the role of Rbsn in suppressing lung cancer progression through the inhibition of metastasis.

Role of Rabenosyn-5 and Rab5b in host cell cytosol uptake reveals conservation of endosomal transport in malaria parasites

Vesicular trafficking, including secretion and endocytosis, plays fundamental roles in the unique biology of Plasmodium falciparum blood-stage parasites. Endocytosis of host cell cytosol (HCC) provides nutrients and room for parasite growth and is critical for the action of antimalarial drugs and parasite drug resistance. Previous work showed that PfVPS45 functions in endosomal transport of HCC to the parasite's food vacuole, raising the possibility that malaria parasites possess a canonical endolysosomal system. However, the seeming absence of VPS45-typical functional interactors such as rabenosyn 5 (Rbsn5) and the repurposing of Rab5 isoforms and other endolysosomal proteins for secretion in apicomplexans question this idea. Here, we identified a parasite Rbsn5-like protein and show that it functions with VPS45 in the endosomal transport of HCC. We also show that PfRab5b but not PfRab5a is involved in the same process. Inactivation of PfRbsn5L resulted in PI3P and PfRab5b decorated HCC-filled vesicles, typical for endosomal compartments. Overall, this indicates that despite the low sequence conservation of PfRbsn5L and the unusual N-terminal modification of PfRab5b, principles of endosomal transport in malaria parasite are similar to that of model organisms. Using a conditional double protein inactivation system, we further provide evidence that the PfKelch13 compartment, an unusual apicomplexa-specific endocytosis structure at the parasite plasma membrane, is connected upstream of the Rbsn5L/VPS45/Rab5b-dependent endosomal route. Altogether, this work indicates that HCC uptake consists of a highly parasite-specific part that feeds endocytosed material into an endosomal system containing more canonical elements, leading to the delivery of HCC to the food vacuole.

Domain 3a mutation of VPS33A suppresses larval arrest phenotype in the loss of VPS45 in Caenorhabditis elegans

The Sec1/Munc18 (SM) protein VPS45 is a key regulator of SNARE-mediated membrane fusion in endosomal trafficking, but its precise role remains unknown. To understand the function of VPS45 in vivo , we performed a genetic suppressor screen in Caenorhabditis elegans . We found that the temperature-sensitive lethality caused by the loss of VPS-45 can be suppressed by a mutation in another SM protein, VPS33A. The VPS33A M376I mutation is located in domain 3a, which is predicted to be essential for SNARE complex assembly. These results highlight the functional importance of domain 3a in endosomal SM proteins and its role in specific membrane fusion.

A humanized Caenorhabditis elegans model for studying pathogenic mutations in VPS45, a protein essential for membrane trafficking, associated with severe congenital neutropenia

VPS45, one of the essential membrane trafficking factors, has been identified as a cause of severe congenital neutropenia 5 (SCN5), but its pathophysiological role remains unknown. Here, we developed a humanized C. elegans model for three pathogenic VPS45 variants. We found that wild-type human VPS45 functionally complemented the loss of C. elegans VPS-45 , and the pathogenic human VPS45 variants functioned almost normally with respect to larval development and endocytosis in C. elegans . These results suggest that SCN5-associated mutations have little effect on the core function of VPS45, and/or that the degree of VPS45 requirement varies, depending on the cell/tissue.

WASH activation controls endosomal recycling and EGFR and Hippo signaling during tumor-suppressive cell competition

Cell competition is a conserved homeostatic mechanism whereby epithelial cells eliminate neighbors with lower fitness. Cell communication at the interface of wild-type "winner" cells and polarity-deficient (scrib^-/-) "losers" is established through Sas-mediated Ptp10D activation in polarity-deficient cells. This tumor-suppressive cell competition restrains EGFR and Hippo signaling and enables Eiger-JNK mediated apoptosis in scrib^-/- clones. Here, we show that the activation state of the endosomal actin regulator WASH is a central node linking EGFR and Hippo signaling activation. The tyrosine kinase Btk29A and its substrate WASH are required downstream of Ptp10D for "loser" cell elimination. Constitutively active, phosphomimetic WASH is sufficient to induce both EGFR and Yki activation leading to overgrowth. On the mechanistic level we show that Ptp10D is recycled by the WASH/retromer complex, while EGFR is recycled by the WASH/retriever complex. Constitutive WASH activation selectively interferes with retromer function leading to Ptp10D mistargeting while promoting EGFR recycling and signaling activation. Phospho-WASH also activates aberrant Arp2/3 actin polymerization, leading to cytoskeletal imbalance, Yki activation and reduced apoptosis. Selective manipulation of WASH phosphorylation on sorting endosomes may restrict epithelial tumorous growth.

The PH Domain and C-Terminal polyD Motif of Phafin2 Exhibit a Unique Concurrence in Animals

Phafin2, a member of the Phafin family of proteins, contributes to a plethora of cellular activities including autophagy, endosomal cargo transportation, and macropinocytosis. The PH and FYVE domains of Phafin2 play key roles in membrane binding, whereas the C-terminal poly aspartic acid (polyD) motif specifically autoinhibits the PH domain binding to the membrane phosphatidylinositol 3-phosphate (PtdIns3P). Since the Phafin2 FYVE domain also binds PtdIns3P, the role of the polyD motif remains unclear. In this study, bioinformatics tools and resources were employed to determine the concurrence of the PH-FYVE module with the polyD motif among Phafin2 and PH-, FYVE-, or polyD-containing proteins from bacteria to humans. FYVE was found to be an ancient domain of Phafin2 and is related to proteins that are present in both prokaryotes and eukaryotes. Interestingly, the polyD motif only evolved in Phafin2 and PH- or both PH-FYVE-containing proteins in animals. PolyD motifs are absent in PH domain-free FYVE-containing proteins, which usually display cellular trafficking or autophagic functions. Moreover, the prediction of the Phafin2-interacting network indicates that Phafin2 primarily cross-talks with proteins involved in autophagy, protein trafficking, and neuronal function. Taken together, the concurrence of the polyD motif with the PH domain may be associated with complex cellular functions that evolved specifically in animals.

Rab21 in enterocytes participates in intestinal epithelium maintenance

Membrane trafficking is defined as the vesicular transport of proteins into, out of, and throughout the cell. In intestinal enterocytes, defects in endocytic/recycling pathways result in impaired function and are linked to diseases. However, how these trafficking pathways regulate intestinal tissue homeostasis is poorly understood. Using the Drosophila intestine as an in vivo system, we investigated enterocyte-specific functions for the early endosomal machinery. We focused on Rab21, which regulates specific steps in early endosomal trafficking. Depletion of Rab21 in enterocytes led to abnormalities in intestinal morphology, with deregulated cellular equilibrium associated with a gain in mitotic cells and increased cell death. Increases in apoptosis and Yorkie signaling were responsible for compensatory proliferation and tissue inflammation. Using an RNAi screen, we identified regulators of autophagy and membrane trafficking that phenocopied Rab21 knockdown. We further showed that Rab21 knockdown-induced hyperplasia was rescued by inhibition of epidermal growth factor receptor signaling. Moreover, quantitative proteomics identified proteins affected by Rab21 depletion. Of these, we validated changes in apolipoprotein ApoLpp and the trehalose transporter Tret1-1, indicating roles for enterocyte Rab21 in lipid and carbohydrate homeostasis, respectively. Our data shed light on an important role for early endosomal trafficking, and Rab21, in enterocyte-mediated intestinal epithelium maintenance. [Media: see text] [Media: see text].

Cell proliferation control by Notch signalling during imaginal discs development in Drosophila

The Notch signalling pathway is evolutionary conserved and participates in numerous developmental processes, including the control of cell proliferation. However, Notch signalling can promote or restrain cell division depending on the developmental context, as has been observed in human cancer where Notch can function as a tumor suppressor or an oncogene. Thus, the outcome of Notch signalling can be influenced by the cross-talk between Notch and other signalling pathways. The use of model organisms such as Drosophila has been proven to be very valuable to understand the developmental role of the Notch pathway in different tissues and its relationship with other signalling pathways during cell proliferation control. Here we review recent studies in Drosophila that shed light in the developmental control of cell proliferation by the Notch pathway in different contexts such as the eye, wing and leg imaginal discs. We also discuss the autonomous and non-autonomous effects of the Notch pathway on cell proliferation and its interactions with different signalling pathways.

Mammalian VPS45 orchestrates trafficking through the endosomal system

Vacuolar protein sorting 45 homolog (VPS45), a member of the Sec1/Munc18 (SM) family, has been implicated in the regulation of endosomal trafficking. VPS45 deficiency in human patients results in congenital neutropenia, bone marrow fibrosis, and extramedullary renal hematopoiesis. Detailed mechanisms of the VPS45 function are unknown. Here, we show an essential role of mammalian VPS45 in maintaining the intracellular organization of endolysosomal vesicles and promoting recycling of cell-surface receptors. Loss of VPS45 causes defective Rab5-to-Rab7 conversion resulting in trapping of cargos in early endosomes and impaired delivery to lysosomes. In this context, we demonstrate aberrant trafficking of the granulocyte colony-stimulating factor receptor in the absence of VPS45. Furthermore, we find that lack of VPS45 in mice is not compatible with embryonic development. Thus, we identify mammalian VPS45 as a critical regulator of trafficking through the endosomal system and early embryogenesis of mice.

Identification of New Interactions between Endolysosomal Tethering Factors

Proper functioning of the precisely controlled endolysosomal system is essential for maintaining the homeostasis of the entire cell. Tethering factors play pivotal roles in mediating the fusion of different transport vesicles, such as endosomes or autophagosomes with each other or with lysosomes. In this work, we uncover several new interactions between the endolysosomal tethering factors Rabenosyn-5 (Rbsn) and the HOPS and CORVET complexes. We find that Rbsn binds to the HOPS/CORVET complexes mainly via their shared subunit Vps18 and we mapped this interaction to the 773-854 region of Vps18. Based on genetic rescue experiments, the binding between Rbsn and Vps18 is required for endosomal transport and is dispensable for autophagy. Moreover, Vps18 seems to be important for β1 integrin recycling by binding to Rbsn and its known partner Vps45.

RAB11-Mediated Trafficking and Human Cancers: An Updated Review

Simple Summary

The small GTPase RAB11 is a master regulator of both vesicular trafficking and membrane dynamic defining the surface proteome of cellular membranes. As a consequence, the alteration of RAB11 activity induces changes in both the sensory and the transduction apparatuses of cancer cells leading to tumor progression and invasion. Here, we show that this strictly depends on RAB11′s ability to control the sorting of signaling receptors from endosomes. Therefore, RAB11 is a potential therapeutic target over which to develop future therapies aimed at dampening the acquisition of aggressive traits by cancer cells.

Abstract

Many disorders block and subvert basic cellular processes in order to boost their progression. One protein family that is prone to be altered in human cancers is the small GTPase RAB11 family, the master regulator of vesicular trafficking. RAB11 isoforms function as membrane organizers connecting the transport of cargoes towards the plasma membrane with the assembly of autophagic precursors and the generation of cellular protrusions. These processes dramatically impact normal cell physiology and their alteration significantly affects the survival, progression and metastatization as well as the accumulation of toxic materials of cancer cells. In this review, we discuss biological mechanisms ensuring cargo recognition and sorting through a RAB11-dependent pathway, a prerequisite to understand the effect of RAB11 alterations in human cancers.

Capturing the transcription factor interactome in response to sub-lethal insecticide exposure

The increasing levels of pesticide resistance in agricultural pests and disease vectors represents a threat to both food security and global health. As insecticide resistance intensity strengthens and spreads, the likelihood of a pest encountering a sub-lethal dose of pesticide dramatically increases. Here, we apply dynamic Bayesian networks to a transcriptome time-course generated using sub-lethal pyrethroid exposure on a highly resistant Anopheles coluzzii population. The model accounts for circadian rhythm and ageing effects allowing high confidence identification of transcription factors with key roles in pesticide response. The associations generated by this model show high concordance with lab-based validation and identifies 44 transcription factors putatively regulating insecticide-responsive transcripts. We identify six key regulators, with each displaying differing enrichment terms, demonstrating the complexity of pesticide response. The considerable overlap of resistance mechanisms in agricultural pests and disease vectors strongly suggests that these findings are relevant in a wide variety of pest species.

A Genetic Screen Links the Disease-Associated Nab2 RNA-Binding Protein to the Planar Cell Polarity Pathway in Drosophila melanogaster

Mutations in the gene encoding the ubiquitously expressed RNA-binding protein ZC3H14 result in a non-syndromic form of autosomal recessive intellectual disability in humans. Studies in Drosophila have defined roles for the ZC3H14 ortholog, Nab2 (aka Drosophila Nab2 or dNab2), in axon guidance and memory due in part to interaction with a second RNA-binding protein, the fly Fragile X homolog Fmr1, and coregulation of shared Nab2-Fmr1 target mRNAs. Despite these advances, neurodevelopmental mechanisms that underlie defective axonogenesis in Nab2 mutants remain undefined. Nab2 null phenotypes in the brain mushroom bodies (MBs) resemble defects caused by alleles that disrupt the planar cell polarity (PCP) pathway, which regulates planar orientation of static and motile cells via a non-canonical arm of the Wnt/Wg pathway. A kinked bristle phenotype in surviving Nab2 mutant adults additionally suggests a defect in F-actin polymerization and bundling, a PCP-regulated processes. To test for Nab2-PCP genetic interactions, a collection of PCP mutant alleles was screened for modification of a rough-eye phenotype produced by Nab2 overexpression in the eye (GMR> Nab2) and, subsequently, for modification of a viability defect among Nab2 nulls. Multiple PCP alleles dominantly modify GMR> Nab2 eye roughening and a subset rescue low survival and thoracic bristle kinking in Nab2 zygotic nulls. Collectively, these genetic interactions identify the PCP pathway as a potential target of the Nab2 RNA-binding protein in developing eye and wing tissues and suggest that altered PCP signaling could contribute to neurological defects that result from loss of Drosophila Nab2 or its vertebrate ortholog ZC3H14.

Class III PI3K Vps34 Controls Thyroid Hormone Production by Regulating Thyroglobulin Iodination, Lysosomal Proteolysis, and Tissue Homeostasis

Background: The production of thyroid hormones [triiodothyronine (T3), thyroxine (T4)] depends on the organization of the thyroid in follicles, which are lined by a monolayer of thyrocytes with strict apicobasal polarity. This polarization supports vectorial transport of thyroglobulin (Tg) for storage into, and recapture from, the colloid. It also allows selective addressing of channels, transporters, ion pumps, and enzymes to their appropriate basolateral [Na⁺/I^- symporter (NIS), SLC26A7, and Na⁺/K⁺-ATPase] or apical membrane domain (anoctamin, SLC26A4, DUOX2, DUOXA2, and thyroperoxidase). How these actors of T3/T4 synthesis reach their final destination remains poorly understood. The PI 3-kinase isoform Vps34/PIK3C3 is now recognized as a main component in the general control of vesicular trafficking and of cell homeostasis through the regulation of endosomal trafficking and autophagy. We recently reported that conditional Vps34 inactivation in proximal tubular cells in the kidney prevents normal addressing of apical membrane proteins and causes abortive macroautophagy. Methods:Vps34 was inactivated using a Pax8-driven Cre recombinase system. The impact of Vps34 inactivation in thyrocytes was analyzed by histological, immunolocalization, and messenger RNA expression profiling. Thyroid hormone synthesis was assayed by ¹²⁵I injection and plasma analysis. Results:Vps34 conditional knockout (Vps34^cKO) mice were born at the expected Mendelian ratio and showed normal growth until postnatal day 14 (P14), then stopped growing and died at ∼1 month of age. We therefore analyzed thyroid Vps34^cKO at P14. We found that loss of Vps34 in thyrocytes causes (i) disorganization of thyroid parenchyma, with abnormal thyrocyte and follicular shape and reduced PAS⁺ colloidal spaces; (ii) severe noncompensated hypothyroidism with extremely low T4 levels (0.75 ± 0.62 μg/dL) and huge thyrotropin plasma levels (19,300 ± 10,500 mU/L); (iii) impaired ¹²⁵I organification at comparable uptake and frequent occurrence of follicles with luminal Tg but nondetectable T4-bearing Tg; (iv) intense signal in thyrocytes for the lysosomal membrane marker, LAMP-1, as well as Tg and the autophagy marker, p62, indicating defective lysosomal proteolysis; and (v) presence of macrophages in the colloidal space. Conclusions: We conclude that Vps34 is crucial for thyroid hormonogenesis, at least by controlling epithelial organization, Tg iodination as well as proteolytic T3/T4 excision in lysosomes.

Acquisition of a side population fraction augments malignant phenotype in ovarian cancer

Side population (SP) cells harbor malignant phenotypes in cancer. The aim of this study was to identify genes that modulate the proportion of ovarian cancer SP cells. Using a shRNA library targeting 15,000 genes, a functional genomics screen was performed to identify genes whose suppression increased the SP percentage. The biological effects caused by alteration of those identified genes were investigated in vitro and in vivo. We found that suppression of MSL3, ZNF691, VPS45, ITGB3BP, TLE2, and ZNF498 increased the proportion of SP cells. Newly generated SP cells exhibit greater capacity for sphere formation, single cell clonogenicity, and in vivo tumorigenicity. On the contrary, overexpression of MSL3, VPS45, ITGB3BP, TLE2, and ZNF498 decreased the proportion of SP cells, sphere formation capacity and single cell clonogenicity. In ovarian cancer cases, low expression of MSL3, ZNF691 and VPS45 was related to poor prognosis. Suppression of these six genes enhanced activity of the hedgehog pathway. Cyclopamine, a hedgehog pathway inhibitor, significantly decreased the number of SP cells and their sphere forming ability. Our results provide new information regarding molecular mechanisms favoring SP cells and suggest that Hedgehog signaling may provide a viable target for ovarian cancer.

WASH phosphorylation balances endosomal versus cortical actin network integrities during epithelial morphogenesis

Filamentous actin (F-actin) networks facilitate key processes like cell shape control, division, polarization and motility. The dynamic coordination of F-actin networks and its impact on cellular activities are poorly understood. We report an antagonistic relationship between endosomal F-actin assembly and cortical actin bundle integrity during Drosophila airway maturation. Double mutants lacking receptor tyrosine phosphatases (PTP) Ptp10D and Ptp4E, clear luminal proteins and disassemble apical actin bundles prematurely. These defects are counterbalanced by reduction of endosomal trafficking and by mutations affecting the tyrosine kinase Btk29A, and the actin nucleation factor WASH. Btk29A forms protein complexes with Ptp10D and WASH, and Btk29A phosphorylates WASH. This phosphorylation activates endosomal WASH function in flies and mice. In contrast, a phospho-mimetic WASH variant induces endosomal actin accumulation, premature luminal endocytosis and cortical F-actin disassembly. We conclude that PTPs and Btk29A regulate WASH activity to balance the endosomal and cortical F-actin networks during epithelial tube maturation.

Endocytic trafficking factor VPS45 is essential for spatial regulation of lens fiber differentiation in zebrafish

In vertebrate lens, lens epithelial cells cover the anterior half of the lens fiber core. Lens epithelial cells proliferate, move posteriorly and start to differentiate into lens fiber cells at the lens equator. Although FGF signaling promotes this equatorial commencement of lens fiber differentiation, the underlying mechanism is not fully understood. Here, we show that lens epithelial cells abnormally enter lens fiber differentiation without passing through the equator in zebrafish vps45 mutants. VPS45 belongs to the Sec1/Munc18-like protein family and promotes endosome trafficking, which differentially modulates signal transduction. Ectopic lens fiber differentiation in vps45 mutants does not depend on FGF, but is mediated through activation of TGFβ signaling and inhibition of canonical Wnt signaling. Thus, VPS45 normally suppresses lens fiber differentiation in the anterior region of lens epithelium by modulating TGFβ and canonical Wnt signaling pathways. These data indicate a novel role of endosome trafficking to ensure equator-dependent commencement of lens fiber differentiation.

Summary: The endocytic regulator VPS45 suppresses FGF-independent lens fiber differentiation and ensures the spatial pattern of lens development.

Molecular and functional characterization of the American cockroach, Periplaneta americana, Rab5: the first exopterygotan low molecular weight ovarian GTPase during oogenesis

The small Rab GTPases are key regulators of membrane vesicle trafficking. Ovaries of Periplaneta americana (Linnaeus) (Blattodea: Blattidae) have small molecular weight GTP/ATP-binding proteins during early and late vitellogenic periods of oogenesis. However, the identification and characterization of the detected proteins have not been yet reported. Herein, we cloned a cDNA encoding Rab5 from the American cockroach, P. americana, ovaries (PamRab5). It comprises 796 bp, encoding a protein of 213 amino acid residues with a predicted molecular weight of 23.5 kDa. PamRab5 exists as a single-copy gene in the P. americana genome, as revealed by Southern blot analysis. An approximate 2.6 kb ovarian mRNA was transcribed especially at high levels in the previtellogenic ovaries, detected by Northern blot analysis. The muscle and head tissues also showed high levels of PamRab5 transcript. PamRab5 protein was localized, via immunofluorescence labeling, to germline-derived cells of the oocytes, very early during oocyte differentiation. Immunoblotting detected a ∼25 kDa signal as a membrane-associated form revealed after application of detergent in the extraction buffer, and 23 kDa as a cytosolic form consistent with the predicted molecular weight from amino acid sequence in different tissues including ovary, muscles and head. The PamRab5 during late vitellogenic periods is required to regulate the endocytotic machinery during oogenesis in this cockroach. This is the first report on Rab5 from a hemimetabolan, and presents an inaugural step in probing the molecular premises of insect oocyte endocytotic trafficking important for oogenesis and embryonic development.

Endosome maturation factors Rabenosyn-5/VPS45 and caveolin-1 regulate ciliary membrane and polycystin-2 homeostasis

Primary cilium structure and function relies on control of ciliary membrane homeostasis, regulated by membrane trafficking processes that deliver and retrieve ciliary components at the periciliary membrane. However, the molecular mechanisms controlling ciliary membrane establishment and maintenance, especially in relation to endocytosis, remain poorly understood. Here, using Caenorhabditis elegans, we describe closely linked functions for early endosome (EE) maturation factors RABS-5 (Rabenosyn-5) and VPS-45 (VPS45) in regulating cilium length and morphology, ciliary and periciliary membrane volume, and ciliary signalling-related sensory behaviour. We demonstrate that RABS-5 and VPS-45 control periciliary vesicle number and levels of select EE/endocytic markers (WDFY-2, CAV-1) and the ciliopathy membrane receptor PKD-2 (polycystin-2). Moreover, we show that CAV-1 (caveolin-1) also controls PKD-2 ciliary levels and associated sensory behaviour. These data link RABS-5 and VPS-45 ciliary functions to the processing of periciliary-derived endocytic vesicles and regulation of ciliary membrane homeostasis. Our findings also provide insight into the regulation of PKD-2 ciliary levels via integrated endosomal sorting and CAV-1-mediated endocytosis.

Multivalency in Rab effector interactions

Rab proteins regulate vesicular transport in eukaryotic cells and establish connections to various cellular structures and processes by interacting with so-called effector molecules. Several of these effectors are known to not only bind a single Rab protein, but to be able to bind multiple different Rabs simultaneously. In this review we will give a short overview of effectors in general and (putative) functions of the aforementioned multivalent Rab:effector interactions.

A functional endosomal pathway is necessary for lysosome biogenesis in Drosophila

Background

Lysosomes are the major catabolic compartment within eukaryotic cells, and their biogenesis requires the integration of the biosynthetic and endosomal pathways. Endocytosis and autophagy are the primary inputs of the lysosomal degradation pathway. Endocytosis is specifically needed for the degradation of membrane proteins whereas autophagy is responsible for the degradation of cytoplasmic components. We previously identified the deubiquitinating enzyme UBPY/USP8 as being necessary for lysosomal biogenesis and productive autophagy in Drosophila. Because UBPY/USP8 has been widely described for its function in the endosomal system, we hypothesized that disrupting the endosomal pathway itself may affect the biogenesis of the lysosomes.

Results

In the present study, we blocked the progression of the endosomal pathway at different levels of maturation of the endosomes by expressing in fat body cells either dsRNAs or dominant negative mutants targeting components of the endosomal machinery: Shibire, Rab4, Rab5, Chmp1 and Rab7. We observed that inhibition of endosomal trafficking at different steps in vivo is systematically associated with defects in lysosome biogenesis, resulting in autophagy flux blockade.

Conclusion

Our results show that the integrity of the endosomal system is required for lysosome biogenesis and productive autophagy in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-016-0115-7) contains supplementary material, which is available to authorized users.

Complex Polarity: Building Multicellular Tissues Through Apical Membrane Traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level.

MiniCORVET is a Vps8-containing early endosomal tether in Drosophila

Yeast studies identified two heterohexameric tethering complexes, which consist of 4 shared (Vps11, Vps16, Vps18 and Vps33) and 2 specific subunits: Vps3 and Vps8 (CORVET) versus Vps39 and Vps41 (HOPS). CORVET is an early and HOPS is a late endosomal tether. The function of HOPS is well known in animal cells, while CORVET is poorly characterized. Here we show that Drosophila Vps8 is highly expressed in hemocytes and nephrocytes, and localizes to early endosomes despite the lack of a clear Vps3 homolog. We find that Vps8 forms a complex and acts together with Vps16A, Dor/Vps18 and Car/Vps33A, and loss of any of these proteins leads to fragmentation of endosomes. Surprisingly, Vps11 deletion causes enlargement of endosomes, similar to loss of the HOPS-specific subunits Vps39 and Lt/Vps41. We thus identify a 4 subunit-containing miniCORVET complex as an unconventional early endosomal tether in Drosophila.

DOI:http://dx.doi.org/10.7554/eLife.14226.001

Extracellular Vesicles in Brain Tumor Progression

Brain tumors can be viewed as multicellular 'ecosystems' with increasingly recognized cellular complexity and systemic impact. While the emerging diversity of malignant disease entities affecting brain tissues is often described in reference to their signature alterations within the cellular genome and epigenome, arguably these cell-intrinsic changes can be regarded as hardwired adaptations to a variety of cell-extrinsic microenvironmental circumstances. Conversely, oncogenic events influence the microenvironment through their impact on the cellular secretome, including emission of membranous structures known as extracellular vesicles (EVs). EVs serve as unique carriers of bioactive lipids, secretable and non-secretable proteins, mRNA, non-coding RNA, and DNA and constitute pathway(s) of extracellular exit of molecules into the intercellular space, biofluids, and blood. EVs are also highly heterogeneous as reflected in their nomenclature (exosomes, microvesicles, microparticles) attempting to capture their diverse origin, as well as structural, molecular, and functional properties. While EVs may act as a mechanism of molecular expulsion, their non-random uptake by heterologous cellular recipients defines their unique roles in the intercellular communication, horizontal molecular transfer, and biological activity. In the central nervous system, EVs have been implicated as mediators of homeostasis and repair, while in cancer they may act as regulators of cell growth, clonogenicity, angiogenesis, thrombosis, and reciprocal tumor-stromal interactions. EVs produced by specific brain tumor cell types may contain the corresponding oncogenic drivers, such as epidermal growth factor receptor variant III (EGFRvIII) in glioblastoma (and hence are often referred to as 'oncosomes'). Through this mechanism, mutant oncoproteins and nucleic acids may be transferred horizontally between cellular populations altering their individual and collective phenotypes. Oncogenic pathways also impact the emission rates, types, cargo, and biogenesis of EVs, as reflected by preliminary analyses pointing to differences in profiles of EV-regulating genes (vesiculome) between molecular subtypes of glioblastoma, and in other brain tumors. Molecular regulators of vesiculation can also act as oncogenes. These intimate connections suggest the context-specific roles of different EV subsets in the progression of specific brain tumors. Advanced efforts are underway to capture these events through the use of EVs circulating in biofluids as biomarker reservoirs and to guide diagnostic and therapeutic decisions.

Three mechanisms control E-cadherin localization to the zonula adherens

E-cadherin localization to the zonula adherens is fundamental for epithelial differentiation but the mechanisms controlling localization are unclear. Using the Drosophila follicular epithelium we genetically dissect E-cadherin transport in an in vivo model. We distinguish three mechanisms mediating E-cadherin accumulation at the zonula adherens. Two membrane trafficking pathways deliver newly synthesized E-cadherin to the plasma membrane. One is Rab11 dependent and targets E-cadherin directly to the zonula adherens, while the other transports E-cadherin to the lateral membrane. Lateral E-cadherin reaches the zonula adherens by endocytosis and targeted recycling. We show that this pathway is dependent on RabX1, which provides a functional link between early and recycling endosomes. Moreover, we show that lateral E-cadherin is transported to the zonula adherens by an apically directed flow within the plasma membrane. Differential activation of these pathways could facilitate cell shape changes during morphogenesis, while their misregulation compromises cell adhesion and tissue architecture in differentiated epithelia.

An in vivo screen identifies ependymoma oncogenes and tumor-suppressor genes

Cancers are characterized by non-random chromosome copy number alterations that presumably contain oncogenes and tumor-suppressor genes (TSGs). The affected loci are often large, making it difficult to pinpoint which genes are driving the cancer. Here we report a cross-species in vivo screen of 84 candidate oncogenes and 39 candidate TSGs, located within 28 recurrent chromosomal alterations in ependymoma. Through a series of mouse models, we validate eight new ependymoma oncogenes and ten new ependymoma TSGs that converge on a small number of cell functions, including vesicle trafficking, DNA modification and cholesterol biosynthesis, identifying these as potential new therapeutic targets.

Rme-8 depletion perturbs Notch recycling and predisposes to pathogenic signaling

The retromer-associated DNAJ protein Rme-8 is necessary for normal Notch recycling, and reductions in Rme-8 sensitize cells so that additional loss-of-sorting retromer or ESCRT-0 components have catastrophic effects.

Notch signaling is a major regulator of cell fate, proliferation, and differentiation. Like other signaling pathways, its activity is strongly influenced by intracellular trafficking. Besides contributing to signal activation and down-regulation, differential fluxes between trafficking routes can cause aberrant Notch pathway activation. Investigating the function of the retromer-associated DNAJ protein Rme-8 in vivo, we demonstrate a critical role in regulating Notch receptor recycling. In the absence of Rme-8, Notch accumulated in enlarged tubulated Rab4-positive endosomes, and as a consequence, signaling was compromised. Strikingly, when the retromer component Vps26 was depleted at the same time, Notch no longer accumulated and instead was ectopically activated. Likewise, depletion of ESCRT-0 components Hrs or Stam in combination with Rme-8 also led to high levels of ectopic Notch activity. Together, these results highlight the importance of Rme-8 in coordinating normal endocytic recycling route and reveal that its absence predisposes toward conditions in which pathological Notch signaling can occur.

Drosophila Strip serves as a platform for early endosome organization during axon elongation

Early endosomes are essential for regulating cell signalling and controlling the amount of cell surface molecules during neuronal morphogenesis. Early endosomes undergo retrograde transport (clustering) before their homotypic fusion. Small GTPase Rab5 is known to promote early endosomal fusion, but the mechanism linking the transport/clustering with Rab5 activity is unclear. Here we show that Drosophila Strip is a key regulator for neuronal morphogenesis. strip knockdown disturbs the early endosome clustering and Rab5-positive early endosomes become smaller and scattered. Strip genetically and biochemically interacts with both Glued (the regulator of dynein-dependent transport) and Sprint (the guanine nucleotide exchange factor for Rab5), suggesting that Strip is a molecular linker between retrograde transport and Rab5 activation. Overexpression of an active form of Rab5 in strip mutant neurons suppresses the axon elongation defects. Thus, Strip acts as a molecular platform for the early endosome organization that plays important roles in neuronal morphogenesis.

Loss of the Sec1/Munc18-family proteins VPS-33.2 and VPS-33.1 bypasses a block in endosome maturation in Caenorhabditis elegans

Evidence is presented for the existence of HOPS and CORVET tethering complexes in metazoans. A role is shown for the SM protein components of tethers in controlling the flux of material through the endosomal system.

The end of the life of a transport vesicle requires a complex series of tethering, docking, and fusion events. Tethering complexes play a crucial role in the recognition of membrane entities and bringing them into close opposition, thereby coordinating and controlling cellular trafficking events. Here we provide a comprehensive RNA interference analysis of the CORVET and HOPS tethering complexes in metazoans. Knockdown of CORVET components promoted RAB-7 recruitment to subapical membranes, whereas in HOPS knockdowns, RAB-5 was found also on membrane structures close to the cell center, indicating the RAB conversion might be impaired in the absence of these tethering complexes. Unlike in yeast, metazoans have two VPS33 homologues, which are Sec1/Munc18 (SM)-family proteins involved in the regulation of membrane fusion. We assume that in wild type, each tethering complex contains a specific SM protein but that they may be able to substitute for each other in case of absence of the other. Of importance, knockdown of both SM proteins allowed bypass of the endosome maturation block in sand-1 mutants. We propose a model in which the SM proteins in tethering complexes are required for coordinated flux of material through the endosomal system.

Single point mutation in Rabenosyn-5 in a female with intractable seizures and evidence of defective endocytotic trafficking

BACKGROUND

We report a 6.5 year-old female with a homozygous missense mutation in ZFYVE20, encoding Rabenosyn-5 (Rbsn-5), a highly conserved multi-domain protein implicated in receptor-mediated endocytosis. The clinical presentation includes intractable seizures, developmental delay, microcephaly, dysostosis, osteopenia, craniofacial dysmorphism, macrocytosis and megaloblastoid erythropoiesis. Biochemical findings include transient cobalamin deficiency, severe hypertriglyceridemia upon ketogenic diet, microalbuminuria and partial cathepsin D deficiency.

METHODS AND RESULTS

Whole exome sequencing followed by Sanger sequencing confirmed a rare (frequency:0.003987) homozygous missense mutation, g.15,116,371 G > A (c.1273G > A), in ZFYVE20 resulting in an amino acid change from Glycine to Arginine at position 425 of the Rbsn protein (p.Gly425Arg), as the only mutation segregating with disease in the family. Studies in fibroblasts revealed expression and localization of Rbsn-5G425R in wild-type manner, but a 50% decrease in transferrin accumulation, which is corrected by wild-type allele transfection. Furthermore, the patient's fibroblasts displayed an impaired proliferation rate, cytoskeletal and lysosomal abnormalities.

CONCLUSION

These results are consistent with a functional defect in the early endocytic pathway resulting from mutation in Rbsn-5, which secondarily disrupts multiple cellular functions dependent on endocytosis, leading to a severe multi-organ disorder.

Loss of Rab5 drives non-autonomous cell proliferation through TNF and Ras signaling in Drosophila

Deregulation of the endocytic machinery has been implicated in human cancers. However, the mechanism by which endocytic defects drive cancer development remains to be clarified. Here, we find through a genetic screen in Drosophila that loss of Rab5, a protein required for early endocytic trafficking, drives non-autonomous cell proliferation in imaginal epithelium. Our genetic data indicate that dysfunction of Rab5 leads to cell-autonomous accumulation of Eiger (a TNF homolog) and EGF receptor (EGFR), which causes activation of downstream JNK and Ras signaling, respectively. JNK signaling and its downstream component Cdc42 cooperate with Ras signaling to induce upregulation of a secreted growth factor Upd (an IL-6 homolog) through inactivation of the Hippo pathway. Such non-autonomous tissue growth triggered by Rab5 defect could contribute to epithelial homeostasis as well as cancer development within heterogeneous tumor microenvironment.

The F-box protein Slmb restricts the activity of aPKC to polarize epithelial cells

The Par-3/Par-6/aPKC complex is the primary determinant of apical polarity in epithelia across animal species, but how the activity of this complex is restricted to allow polarization of the basolateral domain is less well understood. In Drosophila, several multiprotein modules antagonize the Par complex through a variety of means. Here we identify a new mechanism involving regulated protein degradation. Strong mutations in supernumerary limbs (slmb), which encodes the substrate adaptor of an SCF-class E3 ubiquitin ligase, cause dramatic loss of polarity in imaginal discs accompanied by tumorous proliferation defects. Slmb function is required to restrain apical aPKC activity in a manner that is independent of endolysosomal trafficking and parallel to the Scribble module of junctional scaffolding proteins. The involvement of the Slmb E3 ligase in epithelial polarity, specifically limiting Par complex activity to distinguish the basolateral domain, points to parallels with polarization of the C. elegans zygote.

Phagocytic ability declines with age in adult Drosophila hemocytes

Most multicellular organisms show a physiological decline in immune function with age. However, little is known about the mechanisms underlying these changes. We examined Drosophila melanogaster, an important model for identifying genes affecting innate immunity and senescence, to explore the role of phagocytosis in age-related immune dysfunction. We characterized the localized response of immune cells at the dorsal vessel to bacterial infection in 1-week- and 5-week-old flies. We developed a quantitative phagocytosis assay for adult Drosophila and utilized this to characterize the effect of age on phagocytosis in transgenic and natural variant lines. We showed that genes necessary for bacterial engulfment in other contexts are also required in adult flies. We found that blood cells from young and old flies initially engulf bacteria equally well, while cells from older flies accumulate phagocytic vesicles and thus are less capable of destroying pathogens. Our results have broad implications for understanding how the breakdown in cellular processes influences immune function with age.

Seamless tube shape is constrained by endocytosis-dependent regulation of active Moesin

Most tubes have seams (intercellular or autocellular junctions that seal membranes together into a tube), but "seamless" tubes also exist. In Drosophila, stellate-shaped tracheal terminal cells make seamless tubes, with single branches running through each of dozens of cellular extensions. We find that mutations in braided impair terminal cell branching and cause formation of seamless tube cysts. We show that braided encodes Syntaxin7 and that cysts also form in cells deficient for other genes required either for membrane scission (shibire) or for early endosome formation (Rab5, Vps45, and Rabenosyn-5). These data define a requirement for early endocytosis in shaping seamless tube lumens. Importantly, apical proteins Crumbs and phospho-Moesin accumulate to aberrantly high levels in braided terminal cells. Overexpression of either Crumbs or phosphomimetic Moesin induced lumenal cysts and decreased terminal branching. Conversely, the braided seamless tube cyst phenotype was suppressed by mutations in crumbs or Moesin. Indeed, mutations in Moesin dominantly suppressed seamless tube cyst formation and restored terminal branching. We propose that early endocytosis maintains normal steady-state levels of Crumbs, which recruits apical phosphorylated (active) Moe, which in turn regulates seamless tube shape through modulation of cortical actin filaments.

Principles of membrane tethering and fusion in endosome and lysosome biogenesis

Endosomes and lysosomes receive cargo via vesicular carriers that arrive along multiple trafficking routes. On both organelles, tethering proteins have been identified that interact specifically with Rab5 on endosomes and Rab7 on late endosomes/lysosomes and that facilitate the SNARE-driven membrane fusion. Even though the structure and stoichiometry of the involved proteins and protein complexes differ strongly, they may operate by similar principles. Within this review, we will provide insights into their common functions and discuss the open questions in the field.

Clathrin heavy chain plays multiple roles in polarizing the Drosophila oocyte downstream of Bic-D

Bicaudal-D (Bic-D), Egalitarian (Egl), microtubules and their motors form a transport machinery that localizes a remarkable diversity of mRNAs to specific cellular regions during oogenesis and embryogenesis. Bic-D family proteins also promote dynein-dependent transport of Golgi vesicles, lipid droplets, synaptic vesicles and nuclei. However, the transport of these different cargoes is still poorly understood. We searched for novel proteins that either mediate Bic-D-dependent transport processes or are transported by them. Clathrin heavy chain (Chc) co-immunopurifies with Bic-D in embryos and ovaries, and a fraction of Chc colocalizes with Bic-D. Both proteins control posterior patterning of the Drosophila oocyte and endocytosis. Although the role of Chc in endocytosis is well established, our results show that Bic-D is also needed for the elevated endocytic activity at the posterior of the oocyte. Apart from affecting endocytosis indirectly by its role in osk mRNA localization, Bic-D is also required to transport Chc mRNA into the oocyte and for transport and proper localization of Chc protein to the oocyte cortex, pointing to an additional, more direct role of Bic-D in the endocytic pathway. Furthermore, similar to Bic-D, Chc also contributes to proper localization of osk mRNA and to oocyte growth. However, in contrast to other endocytic components and factors of the endocytic recycling pathway, such as Rabenosyn-5 (Rbsn-5) and Rab11, Chc is needed during early stages of oogenesis (from stage 6 onwards) to localize osk mRNA correctly. Moreover, we also uncovered a novel, presumably endocytosis-independent, role of Chc in the establishment of microtubule polarity in stage 6 oocytes.

JAK/STAT pathway dysregulation in tumors: a Drosophila perspective

Sustained activation of the JAK/STAT pathway is causal to human cancers. This pathway is less complex in Drosophila, and its dysregulation has been linked to several tumor models in this organism. Here, we discuss models of metastatic epithelial and hematopoietic tumors that are causally linked to dysregulation of JAK/STAT signaling in Drosophila. First, we focus on cancer models in imaginal discs where ectopic expression of the JAK/STAT pathway ligand Unpaired downstream of distinct tumor suppressors has emerged as an unexpected mediator of neoplastic transformation. We also discuss the collaboration between STAT and oncogenic Ras in epithelial transformation. Second, we examine hematopoietic tumors, where mutations that cause hyperactive JAK/STAT signaling are necessary and sufficient for "fly leukemia". We highlight the important contributions that genetic screens in Drosophila have made to understanding the JAK/STAT pathway, its developmental roles, and how its function is co-opted during tumorigenesis.

Function and regulation of the endosomal fusion and fission machineries

Organelles within the endomembrane system are connected via vesicle flux. Along the endocytic pathway, endosomes are among the most versatile organelles. They sort cargo through tubular protrusions for recycling or through intraluminal vesicles for degradation. Sorting involves numerous machineries, which mediate fission of endosomal transport intermediates and fusion with other endosomes or eventually with lysosomes. Here we review the recent advances in our understanding of these processes with a particular focus on the Rab GTPases, tethering factors, and retromer. The cytoskeleton has also been recently recognized as a central player in membrane dynamics of endosomes, and this review covers the regulation of the machineries that govern the formation of branched actin networks through the WASH and Arp2/3 complexes in relation with cargo recycling and endosomal fission.

Endophilin B is required for the Drosophila oocyte to endocytose yolk downstream of Oskar

The nutritional environment is crucial for Drosophila oogenesis in terms of controlling hormonal conditions that regulate yolk production and the progress of vitellogenesis. Here, we discovered that Drosophila Endophilin B (D-EndoB), a member of the endophilin family, is required for yolk endocytosis as it regulates membrane dynamics in developing egg chambers. Loss of D-EndoB leads to yolk content reduction, similar to that seen in yolkless mutants, and also causes poor fecundity. In addition, mutant egg chambers exhibit an arrest at the previtellogenic stage. D-EndoB displayed a crescent localization at the oocyte posterior pole in an Oskar-dependent manner; however, it did not contribute to pole plasm assembly. D-EndoB was found to partially colocalize with Long Oskar and Yolkless at the endocytic membranes in ultrastructure analysis. Using an FM4-64 dye incorporation assay, D-EndoB was also found to promote endocytosis in the oocyte. When expressing the full-length D-endoB(FL) or D-endoB(ΔSH3) mutant transgenes in oocytes, the blockage of vitellogenesis and the defect in fecundity in D-endoB mutants was restored. By contrast, a truncated N-BAR domain of the D-EndoB only partially rescued these defects. Taken together, these results allow us to conclude that D-EndoB contributes to the endocytic activity downstream of Oskar by facilitating membrane dynamics through its N-BAR domain in the yolk uptake process, thereby leading to normal progression of vitellogenesis.

A central role for vesicle trafficking in epithelial neoplasia: intracellular highways to carcinogenesis

Epithelial cell carcinogenesis involves the loss of cell polarity, alteration of polarized protein presentation, dynamic cell morphology changes, increased proliferation, and increased cell motility and invasion. Membrane vesicle trafficking underlies all of these processes. Specific membrane trafficking regulators, including RAB small GTPases, through the coordinated dynamics of intracellular trafficking along cytoskeletal pathways, determine the cell surface presentation of proteins and the overall function of both differentiated and neoplastic cells. Although mutations in vesicle trafficking proteins may not be direct drivers of transformation, components of the machinery of vesicle movement have crucial roles in the phenotypes of neoplastic cells. Therefore, the regulators of membrane vesicle trafficking decisions are essential mediators of the full range of cell physiologies that drive cancer cell biology, including initial loss of cell polarity, invasion and metastasis. Targeting of these fundamental intracellular processes may permit the manipulation of cancer cell behaviour.

Expression of mutant CHMP2B, an ESCRT-III component involved in frontotemporal dementia, causes eye deformities due to Notch misregulation in Drosophila

Endosomal sorting complexes required for transport (ESCRTs) mediate sorting of ubiquitinated membrane proteins into multivesicular bodies en route to lysosomes for degradation. A mutation in CHMP2B (CHMP2B(Intron5), an ESCRT-III component) that is associated with a hereditary form of frontotemporal dementia (FTD3) disrupts the endosomal-lysosomal pathway and causes accumulation of autophagosomes and multilamellar structures. We previously demonstrated that expression of CHMP2B(Intron5) in the Drosophila eye using GMR-Gal4 causes misregulation of the Toll receptor pathway. Here, we show that ectopic expression of CHMP2B(Intron5) using eyeless-Gal4 (ey>CHMP2B(Intron5)), a driver with different spatiotemporal expression attributes than GMR-Gal4 in the Drosophila eye, causes eye deformities when compared to expression of wild-type CHMP2B (CHMP2B(WT)) and the Drosophila homologue of CHMP2B (CG4618). In addition, ey>CHMP2B(Intron5) flies showed defects in photoreceptor cell patterning and phototactic behavior. Furthermore, ey>CHMP2B(Intron5) flies showed accumulation of Notch in enlarged endosomes and up-regulation of Notch activity. Partial loss of Notch activity in ey>CHMP2B(Intron5) flies significantly rescued eye deformities, photoreceptor patterning defect, and phototactic behavior defect, indicating that these defects are primarily due to Notch misregulation. These results demonstrate that CHMP2B(Intron5) preferentially affects different receptor signaling pathways in a cellular and developmental context-dependent manner.

Rabaptin-5 and Rabex-5 are neoplastic tumour suppressor genes that interact to modulate Rab5 dynamics in Drosophila melanogaster

Endocytosis plays an important role in the regulation of tumour growth and metastasis. In Drosophila, a number of endocytic neoplastic tumour suppressor genes have been identified that when mutated cause epithelial disruption and over-proliferation. Here we characterise the Drosophila homologue of the Rab5 effector Rabaptin-5, and show that it is a novel neoplastic tumour suppressor. Its ability to bind Rab5 and modulate early endosomal dynamics is conserved in Drosophila, as is its interaction with the Rab5 GEF Rabex5, for which we also demonstrate neoplastic tumour suppressor characteristics. Surprisingly, we do not observe disruption of apico-basal polarity in Rabaptin-5 and Rabex-5 mutant tissues; instead the tumour phenotype is associated with upregulation of Jun N-terminal Kinase (JNK) and Janus Kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) signalling.

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Drosophila Rabaptin-5 and Rabex-5 are endocytic neoplastic tumour suppressor genes.

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The Rab5 effector function of Rabaptin-5 is highly conserved in Drosophila.

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Rabaptin-5 interacts with Rabex-5 to modulate early endosomal dynamics in vivo.

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Tumour phenotypes are associated with upregulation of JNK and JAK/STAT signalling.

Presynaptic membrane retrieval and endosome biology: defining molecularly heterogeneous synaptic vesicles

The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.

Sorting of GLUT4 into its insulin-sensitive store requires the Sec1/Munc18 protein mVps45

Insulin stimulates glucose transport in fat and muscle cells by regulating delivery of the facilitative glucose transporter, glucose transporter isoform 4 (GLUT4), to the plasma membrane. In the absence of insulin, GLUT4 is sequestered away from the general recycling endosomal pathway into specialized vesicles, referred to as GLUT4-storage vesicles. Understanding the sorting of GLUT4 into this store is a major challenge. Here we examine the role of the Sec1/Munc18 protein mVps45 in GLUT4 trafficking. We show that mVps45 is up-regulated upon differentiation of 3T3-L1 fibroblasts into adipocytes and is expressed at stoichiometric levels with its cognate target-soluble N-ethylmaleimide-sensitive factor attachment protein receptor, syntaxin 16. Depletion of mVps45 in 3T3-L1 adipocytes results in decreased GLUT4 levels and impaired insulin-stimulated glucose transport. Using sub-cellular fractionation and an in vitro assay for GLUT4-storage vesicle formation, we show that mVps45 is required to correctly traffic GLUT4 into this compartment. Collectively our data reveal a crucial role for mVps45 in the delivery of GLUT4 into its specialized, insulin-regulated compartment.

A congenital neutrophil defect syndrome associated with mutations in VPS45

BACKGROUND

Neutrophils are the predominant phagocytes that provide protection against bacterial and fungal infections. Genetically determined neutrophil disorders confer a predisposition to severe infections and reveal novel mechanisms that control vesicular trafficking, hematopoiesis, and innate immunity.

METHODS

We clinically evaluated seven children from five families who had neutropenia, neutrophil dysfunction, bone marrow fibrosis, and nephromegaly. To identify the causative gene, we performed homozygosity mapping using single-nucleotide polymorphism arrays, whole-exome sequencing, immunoblotting, immunofluorescence, electron microscopy, a real-time quantitative polymerase-chain-reaction assay, immunohistochemistry, flow cytometry, fibroblast motility assays, measurements of apoptosis, and zebrafish models. Correction experiments were performed by transfecting mutant fibroblasts with the nonmutated gene.

RESULTS

All seven affected children had homozygous mutations (Thr224Asn or Glu238Lys, depending on the child's ethnic origin) in VPS45, which encodes a protein that regulates membrane trafficking through the endosomal system. The level of VPS45 protein was reduced, as were the VPS45 binding partners rabenosyn-5 and syntaxin-16. The level of β1 integrin was reduced on the surface of VPS45-deficient neutrophils and fibroblasts. VPS45-deficient fibroblasts were characterized by impaired motility and increased apoptosis. A zebrafish model of vps45 deficiency showed a marked paucity of myeloperoxidase-positive cells (i.e., neutrophils). Transfection of patient cells with nonmutated VPS45 corrected the migration defect and decreased apoptosis.

CONCLUSIONS

Defective endosomal intracellular protein trafficking due to biallelic mutations in VPS45 underlies a new immunodeficiency syndrome involving impaired neutrophil function. (Funded by the National Human Genome Research Institute and others.).

The Thr224Asn mutation in the VPS45 gene is associated with the congenital neutropenia and primary myelofibrosis of infancy

VPS45 is a new gene associated with severe infections and bone marrow failure in infancy that can be treated by bone marrow transplantation. The mutation affects intracellular storage and transport and results in increased programmed cell death in neutrophils and bone marrow.

Cell polarity and patterning by PIN trafficking through early endosomal compartments in Arabidopsis thaliana

PIN-FORMED (PIN) proteins localize asymmetrically at the plasma membrane and mediate intercellular polar transport of the plant hormone auxin that is crucial for a multitude of developmental processes in plants. PIN localization is under extensive control by environmental or developmental cues, but mechanisms regulating PIN localization are not fully understood. Here we show that early endosomal components ARF GEF BEN1 and newly identified Sec1/Munc18 family protein BEN2 are involved in distinct steps of early endosomal trafficking. BEN1 and BEN2 are collectively required for polar PIN localization, for their dynamic repolarization, and consequently for auxin activity gradient formation and auxin-related developmental processes including embryonic patterning, organogenesis, and vasculature venation patterning. These results show that early endosomal trafficking is crucial for cell polarity and auxin-dependent regulation of plant architecture.

Auxin is a unique plant hormone, which is actively and directionally transported in plant tissues. Transported auxin locally accumulates in the plant body and triggers a multitude of responses, including organ formation and patterning. Therefore, regulation of the directional auxin transport is very important in multiple aspects of plant development. The PIN-FORMED (PIN) family of auxin transporters is known to localize at specific sides of cells and export auxin from the cells, enabling the directional transport of auxin in the tissues. PIN proteins are rapidly shuttling between the plasma membrane and intracellular compartments, potentially allowing dynamic changes of the asymmetric localization according to developmental and environmental cues. Here, we discovered that a mutation in the Sec1/Munc18 family protein VPS45 abolishes its own early endosomal localization and compromises intracellular trafficking of PIN proteins. By genetic and pharmacological inhibition of early endosomal trafficking, we also revealed that another early endosomal protein, ARF GEF BEN1, is involved in early endosomal trafficking at a distinct step. Furthermore, we showed that these components play crucial roles in polar localization and dynamic repolarization of PIN proteins, which underpin various developmental processes. These findings highlight the indispensable roles of early endosomal components in regulating PIN polarity and plant architecture.

De-regulation of JNK and JAK/STAT signaling in ESCRT-II mutant tissues cooperatively contributes to neoplastic tumorigenesis

Multiple genes involved in endocytosis and endosomal protein trafficking in Drosophila have been shown to function as neoplastic tumor suppressor genes (nTSGs), including Endosomal Sorting Complex Required for Transport-II (ESCRT-II) components vacuolar protein sorting 22 (vps22), vps25, and vps36. However, most studies of endocytic nTSGs have been done in mosaic tissues containing both mutant and non-mutant populations of cells, and interactions among mutant and non-mutant cells greatly influence the final phenotype. Thus, the true autonomous phenotype of tissues mutant for endocytic nTSGs remains unclear. Here, we show that tissues predominantly mutant for ESCRT-II components display characteristics of neoplastic transformation and then undergo apoptosis. These neoplastic tissues show upregulation of c-Jun N-terminal Kinase (JNK), Notch, and Janus Kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) signaling. Significantly, while inhibition of JNK signaling in mutant tissues partially inhibits proliferation, inhibition of JAK/STAT signaling rescues other aspects of the neoplastic phenotype. This is the first rigorous study of tissues predominantly mutant for endocytic nTSGs and provides clear evidence for cooperation among de-regulated signaling pathways leading to tumorigenesis.

Class III phosphatidylinositol 3-kinase and its catalytic product PtdIns3P in regulation of endocytic membrane traffic

Endocytosis and subsequent membrane traffic through endosomes are cellular processes that are integral to eukaryotic evolution, and numerous human diseases are associated with their dysfunction. Consequently, it is important to untangle the molecular machineries that regulate membrane dynamics and protein flow in the endocytic pathway. Central in this context is class III phosphatidylinositol 3-kinase, an evolutionarily conserved enzyme complex that phosphorylates phosphatidylinositol into phosphatidylinositol 3-phosphate. Phosphatidylinositol 3-phosphate recruits specific effector proteins, most of which contain FYVE or PX domains, to promote endocytosis, endosome fusion, endosome motility and endosome maturation, as well as cargo sorting to lysosomes, the biosynthetic pathway or the plasma membrane. Here we review the functions of key phosphatidylinositol 3-phosphate effectors in regulation of endocytic membrane dynamics and protein sorting.