COP-coated vesicles

Vps41 functions as a molecular ruler for HOPS tethering complex-mediated membrane fusion

Fusion at the lysosome (or the yeast vacuole) requires the conserved hexameric HOPS tethering complex. In the yeast Saccharomyces cerevisiae, HOPS binds to the vacuolar Rab7-like GTPase Ypt7 via its subunits Vps41 and Vps39 and supports fusion by promoting SNARE assembly. In contrast to its sister complex CORVET, the Ypt7-interacting domain of Vps41 in the HOPS complex is connected to the core by a long, extended α-solenoid domain. Here, we show that this solenoid acts as a molecular ruler to position the Ypt7-interacting region of Vps41 relative to the core of HOPS to support function. Mutant complexes with a shortened or extended α-solenoid region in Vps41 still tethered membranes, but failed to efficiently support their fusion. In vivo, Vps41 mutants grew poorly and showed defects in vacuolar morphology, endolysosomal sorting and autophagy. Importantly, when a length-compensating linker was inserted instead of the shortened α-solenoid domain, these defects were rescued. This suggests that the Rab-specific Vps41 subunit requires the exact length of the α-solenoid domain but not the α-solenoid architecture for functionality, suggesting a revised model of how HOPS supports fusion.

Adaptation mechanism of clam Ruditapes philippinarum under polycyclic aromatic hydrocarbon stresses: Accumulation and excretion

Persistent organic pollutants (POPs) are easily accumulated in organisms due to its high lipophilicity. Bivalves are still one of the marine invertebrates with high species diversity despite experiencing with environmental pollution. However, studies investigating the adaptation mechanisms of bivalves towards POPs have been lacking. In this study, we chose benzo[a]pyrene (B[a]P), a typical POPs, to investigate the accumulation and excretion mechanism of clam Ruditapes philippinarum towards B[a]P. Laser confocal microscopy images of ovaries and testes confirmed significant colocalization between lipid droplets and B[a]P. However, B[a]P in the testes might mainly excrete with the excretory fluid. Furthermore, lipidomics results indicated that the upregulated TGs were TGs with 48-58 total carbon atoms and 6-12 double bonds in acyl side chains in ovaries, while the upregulated TGs mainly distributed in 48-58 total carbon atoms and 4-14 double bonds in acyl side chains in testes. In ovaries, gene expression of lipid metabolism and phospholipid metabolism basically presented an upward trend at Pro and Mat stages, while entirely performed a downregulation trend at Gro stage. Testes had the opposite regulation with that. Additionally, B[a]P prompted the excretion ability of female clams, but exhibited a complex effect in the male. This study provides a scientific basis for bivalve toxicology and healthy aquaculture, which is of great significance for marine ecological environment protection and aquatic product safety of China.

Roles of lipid droplets and related proteins in metabolic diseases

Lipid droplets (LDs), which are active organelles, derive from the monolayer membrane of the endoplasmic reticulum and encapsulate neutral lipids internally. LD-associated proteins like RAB, those in the PLIN family, and those in the CIDE family participate in LD formation and development, and they are active players in various diseases, organelles, and metabolic processes (i.e., obesity, non-alcoholic fatty liver disease, and autophagy). Our synthesis on existing research includes insights from the formation of LDs to their mechanisms of action, to provide an overview needed for advancing research into metabolic diseases and lipid metabolism.

Structure of the endosomal CORVET tethering complex

Cells depend on their endolysosomal system for nutrient uptake and downregulation of plasma membrane proteins. These processes rely on endosomal maturation, which requires multiple membrane fusion steps. Early endosome fusion is promoted by the Rab5 GTPase and its effector, the hexameric CORVET tethering complex, which is homologous to the lysosomal HOPS. How these related complexes recognize their specific target membranes remains entirely elusive. Here, we solve the structure of CORVET by cryo-electron microscopy and revealed its minimal requirements for membrane tethering. As expected, the core of CORVET and HOPS resembles each other. However, the function-defining subunits show marked structural differences. Notably, we discover that unlike HOPS, CORVET depends not only on Rab5 but also on phosphatidylinositol-3-phosphate (PI3P) and membrane lipid packing defects for tethering, implying that an organelle-specific membrane code enables fusion. Our data suggest that both shape and membrane interactions of CORVET and HOPS are conserved in metazoans, thus providing a paradigm how tethering complexes function.

Chemoproteomics enables identification of coatomer subunit zeta-1 targeted by a small molecule for enterovirus A71 inhibition

Human enterovirus A71 (EV-A71) is a significant etiological agent responsible for epidemics of hand, foot, and mouth disease (HFMD) in Asia-Pacific regions. There are presently no licensed antivirals against EV-A71, and the druggable target for EV-A71 remains very limited. The phenotypic hit 10,10'-bis(trifluoromethyl) marinopyrrole A derivative, herein termed MPA-CF3, is a novel potent small-molecule inhibitor against EV-A71, but its pharmacological target(s) and antiviral mechanisms are not defined. Here, quantitative chemoproteomics deciphered the antiviral target of MAP-CF3 as host factor coatomer subunit zeta-1 (COPZ1). Mechanistically, MPA-CF3 disrupts the interaction of COPZ1 with the EV-A71 nonstructural protein 2C by destabilizing COPZ1 upon binding. The destruction of this interaction blocks the coatomer-mediated transport of 2C to endoplasmic reticulum, and ultimately inhibits EV-A71 replication. Taken together, our study disclosed that MPA-CF3 can be a structurally novel host-targeting anti-EV-A71 agent, providing a structural basis for developing the COPZ1-targeting broad-spectrum antivirals against enteroviruses. The mechanistic elucidation of MPA-CF3 against EV-A71 may offer an alternative COPZ1-involved therapeutic pathway for enterovirus infection.

Vesicle formation-related protein CaSec16 and its ankyrin protein partner CaANK2B jointly enhance salt tolerance in pepper

Vesicle transport plays important roles in plant tolerance against abiotic stresses. However, the contribution of a vesicle formation related protein CaSec16 (COPII coat assembly protein Sec16-like) in pepper tolerance to salt stress remains unclear. In this study, we report that the expression of CaSec16 was upregulated by salt stress. Compared to the control, the salt tolerance of pepper with CaSec16-silenced was compromised, which was shown by the corresponding phenotypes and physiological indexes, such as the death of growing point, the aggravated leaf wilting, the higher increment of relative electric leakage (REL), the lower content of total chlorophyll, the higher accumulation of dead cells, H2O2, malonaldehyde (MDA), and proline (Pro), and the inhibited induction of marker genes for salt-tolerance and vesicle transport. In contrast, the salt tolerance of pepper was enhanced by the transient overexpression of CaSec16. In addition, heterogeneously induced CaSec16 protein did not enhance the salt tolerance of Escherichia coli, an organism lacking the vesicle transport system. By yeast two-hybrid method, an ankyrin protein, CaANK2B, was identified as the interacting protein of CaSec16. The expression of CaANK2B showed a downward trend during the process of salt stress. Compared with the control, pepper plants with transient-overexpression of CaANK2B displayed increased salt tolerance, whereas those with CaANK2B-silenced exhibited reduced salt tolerance. Taken together, both the vesicle formation related protein CaSec16 and its interaction partner CaANK2B can improve the pepper tolerance to salt stress.

Golgi-to-ER retrograde transport prevents premature differentiation of Drosophila type II neuroblasts via Notch-signal-sending daughter cells

Stem cells are heterogeneous to generate diverse differentiated cell types required for organogenesis; however, the underlying mechanisms that differently maintain these heterogeneous stem cells are not well understood. In this study, we identify that Golgi-to-endoplasmic reticulum (ER) retrograde transport specifically maintains type II neuroblasts (NBs) through the Notch signaling. We reveal that intermediate neural progenitors (INPs), immediate daughter cells of type II NBs, provide Delta and function as the NB niche. The Delta used by INPs is mainly produced by NBs and asymmetrically distributed to INPs. Blocking retrograde transport leads to a decrease in INP number, which reduces Notch activity and results in the premature differentiation of type II NBs. Furthermore, the reduction of Delta could suppress tumor formation caused by type II NBs. Our results highlight the crosstalk between Golgi-to-ER retrograde transport, Notch signaling, stem cell niche, and fusion as an essential step in maintaining the self-renewal of type II NB lineage.

COPI Vesicle Disruption Inhibits Mineralization via mTORC1-Mediated Autophagy

Bone mineralization is a sophisticated regulated process composed of crystalline calcium phosphate and collagen fibril. Autophagy, an evolutionarily conserved degradation system, whereby double-membrane vesicles deliver intracellular macromolecules and organelles to lysosomes for degradation, has recently been shown to play an essential role in mineralization. However, the formation of autophagosomes in mineralization remains to be determined. Here, we show that Coat Protein Complex I (COPI), responsible for Golgi-to-ER transport, plays a pivotal role in autophagosome formation in mineralization. COPI vesicles were increased after osteoinduction, and COPI vesicle disruption impaired osteogenesis. Mechanistically, COPI regulates autophagy activity via the mTOR complex 1 (mTORC1) pathway, a key regulator of autophagy. Inhibition of mTOR1 rescues the impaired osteogenesis by activating autophagy. Collectively, our study highlights the functional importance of COPI in mineralization and identifies COPI as a potential therapeutic target for treating bone-related diseases.

Synergism of vesicle trafficking and cytoskeleton during regulation of plant growth and development: A mechanistic outlook

The cytoskeleton is a fundamental component found in all eukaryotic organisms, serving as a critical factor in various essential cyto-biological mechanisms, particularly in the locomotion and morphological transformations of plant cells. The cytoskeleton is comprised of three main components: microtubules (MT), microfilaments (MF), and intermediate filaments (IF). The cytoskeleton plays a crucial role in the process of cell wall formation and remodeling throughout the growth and development of cells. It is a highly organized and regulated network composed of filamentous components. In the basic processes of intracellular transport, such as mitosis, cytokinesis, and cell polarity, the plant cytoskeleton plays a crucial role according to recent studies. The major flaws in the organization of the cytoskeletal framework are at the root of the aberrant organogenesis currently observed in plant mutants. The regulation of protein compartmentalization and abundance within cells is predominantly governed by the process of vesicle/membrane transport, which plays a crucial role in several signaling cascades.The regulation of membrane transport in eukaryotic cells is governed by a diverse array of proteins. Recent developments in genomics have provided new tools to study the evolutionary relationships between membrane proteins in different plant species. It is known that members of the GTPases, COP, SNAREs, Rabs, tethering factors, and PIN families play essential roles in vesicle transport between plant, animal, and microbial species. This Review presents the latest research on the plant cytoskeleton, focusing on recent developments related to the cytoskeleton and summarizing the role of various proteins in vesicle transport. In addition, the report predicts future research direction of plant cytoskeleton and vesicle trafficking, potential research priorities, and provides researchers with specific pointers to further investigate the significant link between cytoskeleton and vesicle trafficking.

An evolutionary perspective on Arf family GTPases

The Arf family GTPases are regulators of eukaryotic cellular organization, functioning in the secretory and endocytic pathways, in cilia and flagella, in cytoskeleton dynamics, and in lipid metabolism. We describe the evolution of this protein family and its well-studied regulators. The last eukaryotic common ancestor had fifteen members, and the current complement of Arf GTPases has been sculpted by gene loss and gene duplications since that point. Some Arf family GTPases (such as those that recruit vesicle coats in the secretory pathway) are present in virtually all eukaryotes, whereas others (such as those functioning in cilia/flagella) have a more limited distribution. A challenge for the future is understanding the full spectrum of Arf family functions throughout eukaryotes.

Vesicle trafficking in rice: too little is known

The vesicle trafficking apparatus is a fundamental machinery to maintain the homeostasis of membrane-enclosed organelles in eukaryotic cells. Thus, it is broadly conserved in eukaryotes including plants. Intensive studies in the model organisms have produced a comprehensive picture of vesicle trafficking in yeast and human. However, with respect to the vesicle trafficking of plants including rice, our understanding of the components and their coordinated regulation is very limited. At present, several vesicle trafficking apparatus components and cargo proteins have been identified and characterized in rice, but there still remain large unknowns concerning the organization and function of the rice vesicle trafficking system. In this review, we outline the main vesicle trafficking pathways of rice based on knowledge obtained in model organisms, and summarize current advances of rice vesicle trafficking. We also propose to develop methodologies applicable to rice and even other crops for further exploring the mysteries of vesicle trafficking in plants.

Quantitative proteomics reveals key pathways in the symbiotic interface and the likely extracellular property of soybean symbiosome

An effective symbiosis between legumes and rhizobia relies largely on diverse proteins at the plant-rhizobium interface for material transportation and signal transduction during symbiotic nitrogen fixation. Here, we report a comprehensive proteome atlas of the soybean symbiosome membrane (SM), peribacteroid space (PBS), and root microsomal fraction (RMF) using state-of-the-art label-free quantitative proteomic technology. In total, 1759 soybean proteins with diverse functions are detected in the SM, and 1476 soybean proteins and 369 rhizobial proteins are detected in the PBS. The diversity of SM proteins detected suggests multiple origins of the SM. Quantitative comparative analysis highlights amino acid metabolism and nutrient uptake in the SM, indicative of the key pathways in nitrogen assimilation. The detection of soybean secretory proteins in the PBS and receptor-like kinases in the SM provides evidence for the likely extracellular property of the symbiosome and the potential signaling communication between both symbionts at the symbiotic interface. Our proteomic data provide clues for how some of the sophisticated regulation between soybean and rhizobium at the symbiotic interface is achieved, and suggest approaches for symbiosis engineering.

Generation of nanoscopic membrane curvature for membrane trafficking

Curved membranes are key features of intracellular organelles, and their generation involves dynamic protein complexes. Here we describe the fundamental mechanisms such as the hydrophobic insertion, scaffolding and crowding mechanisms these proteins use to produce membrane curvatures and complex shapes required to form intracellular organelles and vesicular structures involved in endocytosis and secretion. For each mechanism, we discuss its cellular functions as well as the underlying physical principles and the specific membrane properties required for the mechanism to be feasible. We propose that the integration of individual mechanisms into a highly controlled, robust process of curvature generation often relies on the assembly of proteins into coats. How cells unify and organize the curvature-generating factors at the nanoscale is presented for three ubiquitous coats central for membrane trafficking in eukaryotes: clathrin-coated pits, caveolae, and COPI and COPII coats. The emerging theme is that these coats arrange and coordinate curvature-generating factors in time and space to dynamically shape membranes to accomplish membrane trafficking within cells.

Multifaceted functions of STING in human health and disease: from molecular mechanism to targeted strategy

Since the discovery of Stimulator of Interferon Genes (STING) as an important pivot for cytosolic DNA sensation and interferon (IFN) induction, intensive efforts have been endeavored to clarify the molecular mechanism of its activation, its physiological function as a ubiquitously expressed protein, and to explore its potential as a therapeutic target in a wide range of immune-related diseases. With its orthodox ligand 2'3'-cyclic GMP-AMP (2'3'-cGAMP) and the upstream sensor 2'3'-cGAMP synthase (cGAS) to be found, STING acquires its central functionality in the best-studied signaling cascade, namely the cGAS-STING-IFN pathway. However, recently updated research through structural research, genetic screening, and biochemical assay greatly extends the current knowledge of STING biology. A second ligand pocket was recently discovered in the transmembrane domain for a synthetic agonist. On its downstream outputs, accumulating studies sketch primordial and multifaceted roles of STING beyond its cytokine-inducing function, such as autophagy, cell death, metabolic modulation, endoplasmic reticulum (ER) stress, and RNA virus restriction. Furthermore, with the expansion of the STING interactome, the details of STING trafficking also get clearer. After retrospecting the brief history of viral interference and the milestone events since the discovery of STING, we present a vivid panorama of STING biology taking into account the details of the biochemical assay and structural information, especially its versatile outputs and functions beyond IFN induction. We also summarize the roles of STING in the pathogenesis of various diseases and highlight the development of small-molecular compounds targeting STING for disease treatment in combination with the latest research. Finally, we discuss the open questions imperative to answer.

Molecular insights into endolysosomal microcompartment formation and maintenance

The endolysosomal system of eukaryotic cells has a key role in the homeostasis of the plasma membrane, in signaling and nutrient uptake, and is abused by viruses and pathogens for entry. Endocytosis of plasma membrane proteins results in vesicles, which fuse with the early endosome. If destined for lysosomal degradation, these proteins are packaged into intraluminal vesicles, converting an early endosome to a late endosome, which finally fuses with the lysosome. Each of these organelles has a unique membrane surface composition, which can form segmented membrane microcompartments by membrane contact sites or fission proteins. Furthermore, these organelles are in continuous exchange due to fission and fusion events. The underlying machinery, which maintains organelle identity along the pathway, is regulated by signaling processes. Here, we will focus on the Rab5 and Rab7 GTPases of early and late endosomes. As molecular switches, Rabs depend on activating guanine nucleotide exchange factors (GEFs). Over the last years, we characterized the Rab7 GEF, the Mon1-Ccz1 (MC1) complex, and key Rab7 effectors, the HOPS complex and retromer. Structural and functional analyses of these complexes lead to a molecular understanding of their function in the context of organelle biogenesis.

Recent advances in the activation and regulation of the cGAS-STING pathway

The cGAS-STING pathway is responsible for cytoplasmic double-stranded DNA (dsDNA) -triggered innate immunity and involved in the pathology of various diseases including infection, autoimmune diseases, neurodegeneration and cancer. Understanding the activation and regulatory mechanisms of this pathway is critical to develop therapeutic strategies toward these diseases. Here, we review the signal transduction, cellular functions and regulations of cGAS and STING, particularly highlighting the latest understandings on the activation of cGAS by dsDNA and/or Manganese (Mn²⁺), STING trafficking, sulfated glycosaminoglycans (sGAGs)-induced STING polymerization and activation, and also regulation of the cGAS-STING pathway by different biocondensates formed via phase separation of proteins from host cells and viruses.

Stage-Specific COPII-Mediated Cargo Selectivity in African Trypanosomes

A hallmark of eukaryotic cells is the ability to form a secretory pathway connecting many intracellular compartments. In the early secretory pathway, coated protein complex II (COPII)-coated vesicles mediate the anterograde transport of newly synthesized secretory cargo from the endoplasmic reticulum to the Golgi apparatus. The COPII coat complex is comprised of an inner layer of Sec23/Sec24 heterodimers and an outer layer of Sec13/Sec31 heterotetramers. In African trypanosomes, there are two paralogues each of Sec23 and Sec24, that form obligate heterodimers (TbSec23.2/TbSec24.1, TbSec23.1/TbSec24.2). It is not known if these form distinct homotypic classes of vesicles or one heterotypic class, but it is known that TbSec23.2/TbSec24.1 specifically mediate forward trafficking of GPI-anchored proteins (GPI-APs) in bloodstream-form trypanosomes (BSF). Here, we showed that this selectivity was lost in insect procyclic stage parasites (PCF). All isoforms of TbSec23 and TbSec24 are essential in PCF parasites as judged by RNAi knockdowns. RNAi silencing of each subunit had equivalent effects on the trafficking of GPI-APs and p67, a transmembrane lysosomal protein. However, silencing of the TbSec23.2/TbSec24.1 had heterodimer had a significant impact on COPII mediated trafficking of soluble TbCatL from the ER to the lysosome. This finding suggests a model in which selectivity of COPII transport was altered between the BSF and PCF trypanosomes, possibly as an adaptation to a digenetic life cycle.

IMPORTANCE African trypanosomes synthesize dense surface coats composed of stage-specific glycosylphosphatidylinositol lipid anchored proteins. We previously defined specific machinery in bloodstream stage parasites that mediate the exit of these proteins from the endoplasmic reticulum. Here, we performed similar analyses in the procyclic insect stage and found significant differences in this process. These findings contribute to our understanding of secretory processes in this unusual eukaryotic model system.

COPII vesicles in plant autophagy and endomembrane trafficking

In eukaryotes, the endomembrane system allows for spatiotemporal compartmentation of complicated cellular processes. The plant endomembrane system consists of the endoplasmic reticulum (ER), the Golgi apparatus (GA), the trans-Golgi network (TGN), the multivesicular body (MVB), and the vacuole. Anterograde traffic from the ER to GA is mediated by coat protein complex II (COPII) vesicles. Autophagy, an evolutionarily conserved catabolic process that turns over cellular materials upon nutrient deprivation or in adverse environments, exploits double-membrane autophagosomes to recycle unwanted constituents in the lysosome/vacuole. Accumulating evidence reveals novel functions of plant COPII vesicles in autophagy and their regulation by abiotic stresses. Here, we summarize current knowledge about plant COPII vesicles in the endomembrane trafficking and then highlight recent findings showing their distinct roles in modulating the autophagic flux and stress responses.

The yeast LYST homolog Bph1 is a Rab5 effector and prevents Atg8 lipidation at endosomes

Lysosomes mediate degradation of macromolecules to their precursors for their cellular recycling. Additionally, lysosome-related organelles mediate cell type-specific functions. The Chédiak-Higashi syndrome is an autosomal, recessive disease, in which loss of the protein LYST causes defects in lysosomes and lysosome-related organelles. The molecular function of LYST, however, is largely unknown. Here, we dissected the function of the yeast LYST homolog, Bph1. We show that Bph1 is an endosomal protein, and an effector of the minor Rab5 isoform Ypt52. Strikingly, the bph1▵ mutant has lipidated Atg8 on their endosomes, which is sorted via late endosomes into the vacuole lumen under non-autophagy inducing conditions. In agreement, proteomics of bph1▵ vacuoles reveal an accumulation of Atg8, reduced flux via selective autophagy, and defective endocytosis. Additionally, bph1▵ cells have reduced autophagic flux under starvation conditions. Our observations suggest that Bph1 is a novel Rab5 effector that maintains endosomal functioning. When lost, Atg8 is lipidated at endosomes even during normal growth and ends up in the vacuole lumen. Thus, our results contribute to the understanding of the role of LYST-related proteins and associated diseases.

Spontaneous local membrane curvature induced by transmembrane proteins

The (local) curvature of cellular membranes acts as a driving force for the targeting of membrane-associated proteins to specific membrane domains, as well as a sorting mechanism for transmembrane proteins, e.g., by accumulation in regions of matching spontaneous curvature. The latter measure was previously experimentally employed to study the curvature induced by the potassium channel KvAP and by aquaporin AQP0. However, the direction of the reported spontaneous curvature levels as well as the molecular driving forces governing the membrane curvature induced by these integral transmembrane proteins could not be addressed experimentally. Here, using both coarse-grained and atomistic molecular dynamics (MD) simulations, we report induced spontaneous curvature values for the homologous potassium channel Kv 1.2/2.1 Chimera (KvChim) and AQP0 embedded in unrestrained lipid bicelles that are in very good agreement with experiment. Importantly, the direction of curvature could be directly assessed from our simulations: KvChim induces a strong positive membrane curvature (≈0.036 nm^-1) whereas AQP0 causes a comparably small negative curvature (≈-0.019 nm^-1). Analyses of protein-lipid interactions within the bicelle revealed that the potassium channel shapes the surrounding membrane via structural determinants. Differences in shape of the protein-lipid interface of the voltage-gating domains between the extracellular and cytosolic membrane leaflets induce membrane stress and thereby promote a protein-proximal membrane curvature. In contrast, the water pore AQP0 displayed a high structural stability and an only faint effect on the surrounding membrane environment that is connected to its wedge-like shape.

Single-cell analysis reveals androgen receptor regulates the ER-to-Golgi trafficking pathway with CREB3L2 to drive prostate cancer progression

Androgen receptor (AR) plays a central role in driving prostate cancer (PCa) progression. How AR promotes this process is still not completely clear. Herein, we used single-cell transcriptome analysis to reconstruct the transcriptional network of AR in PCa. Our work shows AR directly regulates a set of signature genes in the ER-to-Golgi protein vesicle-mediated transport pathway. The expression of these genes is required for maximum androgen-dependent ER-to-Golgi trafficking, cell growth, and survival. Our analyses also reveal the signature genes are associated with PCa progression and prognosis. Moreover, we find inhibition of the ER-to-Golgi transport process with a small molecule enhanced antiandrogen-mediated tumor suppression of hormone-sensitive and insensitive PCa. Finally, we demonstrate AR collaborates with CREB3L2 in mediating ER-to-Golgi trafficking in PCa. In summary, our findings uncover a critical role for dysregulation of ER-to-Golgi trafficking expression and function in PCa progression, provide detailed mechanistic insights for how AR tightly controls this process, and highlight the prospect of targeting the ER-to-Golgi pathway as a therapeutic strategy for advanced PCa.

Flexible open conformation of the AP-3 complex explains its role in cargo recruitment at the Golgi

Vesicle formation at endomembranes requires the selective concentration of cargo by coat proteins. Conserved adapter protein complexes at the Golgi (AP-3), the endosome (AP-1), or the plasma membrane (AP-2) with their conserved core domain and flexible ear domains mediate this function. These complexes also rely on the small GTPase Arf1 and/or specific phosphoinositides for membrane binding. The structural details that influence these processes, however, are still poorly understood. Here we present cryo-EM structures of the full-length stable 300 kDa yeast AP-3 complex. The structures reveal that AP-3 adopts an open conformation in solution, comparable to the membrane-bound conformations of AP-1 or AP-2. This open conformation appears to be far more flexible than AP-1 or AP-2, resulting in compact, intermediate, and stretched subconformations. Mass spectrometrical analysis of the cross-linked AP-3 complex further indicates that the ear domains are flexibly attached to the surface of the complex. Using biochemical reconstitution assays, we also show that efficient AP-3 recruitment to the membrane depends primarily on cargo binding. Once bound to cargo, AP-3 clustered and immobilized cargo molecules, as revealed by single-molecule imaging on polymer-supported membranes. We conclude that its flexible open state may enable AP-3 to bind and collect cargo at the Golgi and could thus allow coordinated vesicle formation at the trans-Golgi upon Arf1 activation.

Evidence for nutrient-dependent regulation of the COPII coat by O-GlcNAcylation

O-linked β-N-acetylglucosamine (O-GlcNAc) is a dynamic form of intracellular glycosylation common in animals, plants and other organisms. O-GlcNAcylation is essential in mammalian cells and is dysregulated in myriad human diseases, such as cancer, neurodegeneration and metabolic syndrome. Despite this pathophysiological significance, key aspects of O-GlcNAc signaling remain incompletely understood, including its impact on fundamental cell biological processes. Here, we investigate the role of O-GlcNAcylation in the coat protein II complex (COPII), a system universally conserved in eukaryotes that mediates anterograde vesicle trafficking from the endoplasmic reticulum. We identify new O-GlcNAcylation sites on Sec24C, Sec24D and Sec31A, core components of the COPII system, and provide evidence for potential nutrient-sensitive pathway regulation through site-specific glycosylation. Our work suggests a new connection between metabolism and trafficking through the conduit of COPII protein O-GlcNAcylation.

Collagen transport and related pathways in Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) comprises a heterogeneous group of patients who share bone fragility and deformities as the main characteristics, albeit with different degrees of severity. Phenotypic variation also exists in other connective tissue aspects of the disease, complicating disease classification and disease course prediction. Although collagen type I defects are long established as the primary cause of the bone pathology, we are still far from comprehending the complete mechanism. In the last years, the advent of next generation sequencing has triggered the discovery of many new genetic causes for OI, helping to draw its molecular landscape. It has become clear that, in addition to collagen type I genes, OI can be caused by multiple proteins connected to different parts of collagen biosynthesis. The production of collagen entails a complex process, starting from the production of the collagen Iα1 and collagen Iα2 chains in the endoplasmic reticulum, during and after which procollagen is subjected to a plethora of posttranslational modifications by chaperones. After reaching the Golgi organelle, procollagen is destined to the extracellular matrix where it forms collagen fibrils. Recently discovered mutations in components of the retrograde transport of chaperones highlight its emerging role as critical contributor of OI development. This review offers an overview of collagen regulation in the context of recent gene discoveries, emphasizing the significance of transport disruptions in the OI mechanism. We aim to motivate exploration of skeletal fragility in OI from the perspective of these pathways to identify regulatory points which can hint to therapeutic targets.

Caged lipids for subcellular manipulation

We present recently developed strategies to manipulate lipid levels in live cells by light. We focus on photoremovable protecting groups that lead to subcellular restricted localization and activation and discuss alternative techniques. We emphasize the development of organelle targeting of caged lipids and discuss recent advances in chromatic orthogonality of caging groups for future applications.

Motif-based endomembrane trafficking

Endomembrane trafficking, which allows proteins and lipids to flow between the different endomembrane compartments, largely occurs by vesicle-mediated transport. Transmembrane proteins intended for transport are concentrated into a vesicle or carrier by undulation of a donor membrane. This is followed by vesicle scission, uncoating, and finally, fusion at the target membrane. Three major trafficking pathways operate inside eukaryotic cells: anterograde, retrograde, and endocytic. Each pathway involves a unique set of machinery and coat proteins that pack the transmembrane proteins, along with their associated lipids, into specific carriers. Adaptor and coatomer complexes are major facilitators that function in anterograde transport and in endocytosis. These complexes recognize the transmembrane cargoes destined for transport and recruit the coat proteins that help form the carriers. These complexes use either linear motifs or posttranslational modifications to recognize the cargoes, which are then packaged and delivered along the trafficking pathways. In this review, we focus on the different trafficking complexes that share a common evolutionary branch in Arabidopsis (Arabidopsis thaliana), and we discuss up-to-date knowledge about the cargo recognition motifs they use.

Extracellular Vesicles: An Emerging Mechanism Governing the Secretion and Biological Roles of Tenascin-C

ECM composition and architecture are tightly regulated for tissue homeostasis. Different disorders have been associated to alterations in the levels of proteins such as collagens, fibronectin (FN) or tenascin-C (TnC). TnC emerges as a key regulator of multiple inflammatory processes, both during physiological tissue repair as well as pathological conditions ranging from tumor progression to cardiovascular disease. Importantly, our current understanding as to how TnC and other non-collagen ECM components are secreted has remained elusive. Extracellular vesicles (EVs) are small membrane-bound particles released to the extracellular space by most cell types, playing a key role in cell-cell communication. A broad range of cellular components can be transported by EVs (e.g. nucleic acids, lipids, signalling molecules and proteins). These cargoes can be transferred to target cells, potentially modulating their function. Recently, several extracellular matrix (ECM) proteins have been characterized as bona fide EV cargoes, exosomal secretion being particularly critical for TnC. EV-dependent ECM secretion might underpin diseases where ECM integrity is altered, establishing novel concepts in the field such as ECM nucleation over long distances, and highlighting novel opportunities for diagnostics and therapeutic intervention. Here, we review recent findings and standing questions on the molecular mechanisms governing EV–dependent ECM secretion and its potential relevance for disease, with a focus on TnC.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

AP-3 vesicle uncoating occurs after HOPS-dependent vacuole tethering

Heterotetrameric adapter (AP) complexes cooperate with the small GTPase Arf1 or lipids in cargo selection, vesicle formation, and budding at endomembranes in eukaryotic cells. While most AP complexes also require clathrin as the outer vesicle shell, formation of AP-3-coated vesicles involved in Golgi-to-vacuole transport in yeast has been postulated to depend on Vps41, a subunit of the vacuolar HOPS tethering complex. HOPS has also been identified as the tether of AP-3 vesicles on vacuoles. To unravel this conundrum of a dual Vps41 function, we anchored Vps41 stably to the mitochondrial outer membrane. By monitoring AP-3 recruitment, we now show that Vps41 can tether AP-3 vesicles to mitochondria, yet AP-3 vesicles can form in the absence of Vps41 or clathrin. By proximity labeling and mass spectrometry, we identify the Arf1 GTPase-activating protein (GAP) Age2 at the AP-3 coat and show that tethering, but not fusion at the vacuole can occur without complete uncoating. We conclude that AP-3 vesicles retain their coat after budding and that their complete uncoating occurs only after tethering at the vacuole.

A miRNA screen procedure identifies garz as an essential factor in adult glia functions and validates Drosophila as a beneficial 3Rs model to study glial functions and GBF1 biology

Invertebrate glia performs most of the key functions controlled by mammalian glia in the nervous system and provides an ideal model for genetic studies of glial functions. To study the influence of adult glial cells in ageing we have performed a genetic screen in Drosophila using a collection of transgenic lines providing conditional expression of micro-RNAs (miRNAs). Here, we describe a methodological algorithm to identify and rank genes that are candidate to be targeted by miRNAs that shorten lifespan when expressed in adult glia. We have used four different databases for miRNA target prediction in Drosophila but find little agreement between them, overall. However, top candidate gene analysis shows potential to identify essential genes involved in adult glial functions. One example from our top candidates' analysis is gartenzwerg ( garz). We establish that garz is necessary in many glial cell types, that it affects motor behaviour and, at the sub-cellular level, is responsible for defects in cellular membranes, autophagy and mitochondria quality control. We also verify the remarkable conservation of functions between garz and its mammalian orthologue, GBF1, validating the use of Drosophila as an alternative 3Rs-beneficial model to knock-out mice for studying the biology of GBF1, potentially involved in human neurodegenerative diseases.

A miRNA screen procedure identifies garz as an essential factor in adult glia functions and validates Drosophila as a beneficial 3Rs model to study glial functions and GBF1 biology

Invertebrate glia performs most of the key functions controlled by mammalian glia in the nervous system and provides an ideal model for genetic studies of glial functions. To study the influence of adult glial cells in ageing we have performed a genetic screen in Drosophila using a collection of transgenic lines providing conditional expression of micro-RNAs (miRNAs). Here, we describe a methodological algorithm to identify and rank genes that are candidate to be targeted by miRNAs that shorten lifespan when expressed in adult glia. We have used four different databases for miRNA target prediction in Drosophila but find little agreement between them, overall. However, top candidate gene analysis shows potential to identify essential genes involved in adult glial functions. One example from our top candidates' analysis is gartenzwerg ( garz). We establish that garz is necessary in many glial cell types, that it affects motor behaviour and, at the sub-cellular level, is responsible for defects in cellular membranes, autophagy and mitochondria quality control. We also verify the remarkable conservation of functions between garz and its mammalian orthologue, GBF1, validating the use of Drosophila as an alternative 3Rs-beneficial model to knock-out mice for studying the biology of GBF1, potentially involved in human neurodegenerative diseases.

Ritonavir and Lopinavir Suppress RCE1 and CAAX Rab Proteins Sensitizing the Liver to Organelle Stress and Injury

Organelle stress and Liver injuries often occur in human immunodeficiency virus (HIV) infected patients under anti-HIV therapies, yet few molecular off-targets of anti-HIV drugs have been identified in the liver. Here, we found through total RNA sequencing that the transcription of a host protease Ras converting CAAX endopeptidase 1 (RCE1) was altered in HepG2 cells treated with anti-HIV protease inhibitors, ritonavir and lopinavir. Levels of RCE1 protein were inhibited in HepG2 and primary mouse hepatocytes and in the liver of mice treated with the anti-HIV drugs, which were accompanied with inhibition of two potential substrates of RCE1, small GTP binding protein Rab13 and Rab18, which are with a common CAAX motif and known to regulate the ER-Golgi traffic or lipogenesis. Neither Rce1 transcription nor RCE1 protein level was inhibited by Brefeldin A, which is known to interfere with the ER-Golgi traffic causing Golgi stress. Knocking down Rce1 with RNA interference increased ritonavir and lopinavir-induced cell death as well as expression of Golgi stress response markers, TFE3, HSP47 and GCP60, in both primary mouse hepatocytes and mouse liver, and deteriorated alcohol-induced alanine aminotransferase (ALT) and fatty liver injury in mice. In addition, overexpressing Rab13 or Rab18 in primary human hepatocytes reduced partially the anti-HIV drugs and alcohol-induced Golgi fragmentation, Golgi stress response, and cell death injury. Conclusion: We identified a mechanism linking a host protease and its substrates, small guanosine triphosphate-binding proteins, to the anti-HIV drug-induced Golgi dysfunction, organelle stress response, and fatty liver injury.

A Model for ERD2 Function in Higher Plants

ER lumenal proteins have a K(H)DEL motif at their C-terminus. This is recognized by the ERD2 receptor (KDEL receptor in animals), which localizes to the Golgi apparatus and serves to capture escaped ER lumenal proteins. ERD2-ligand complexes are then transported back to the ER via COPI coated vesicles. The neutral pH of the ER causes the ligands to dissociate with the receptor being returned to the Golgi. According to this generally accepted scenario, ERD2 cycles between the ER and the Golgi, although it has been found to have a predominant Golgi localization. In this short article, we present a model for the functioning of ERD2 receptors in higher plants that explains why it is difficult to detect fluorescently tagged ERD2 proteins in the ER. The model assumes that the residence time for ERD2 in the ER is very brief and restricted to a specific domain of the ER. This is the small disc of ER immediately subjacent to the first cis-cisterna of the Golgi stack, representing specialized ER export and import sites and therefore constituting part of what is known as the "secretory unit", a mobile aggregate of ER domain plus Golgi stack. ERD2 molecules in the ER domain of the secretory unit may be small in number, transient and optically difficult to differentiate from the larger population of ERD2 molecules in the overlying Golgi stack in the confocal microscope.

Mutations in the coat complex II component SEC23B promote colorectal cancer metastasis

Metastasis is the leading cause of death for colorectal cancer (CRC). However, the protein transport process involved in CRC metastasis remains unclear. In this report, we use whole-exome sequencing and bioinformatics analysis to identify somatic mutations in CRC samples and found mutations of the protein transport gene Sec23 homolog B (SEC23B) in patients with metachronous liver metastasis. We show that deletion of SEC23B suppresses the membrane localization of adhesion proteins and augments cell mobility. SEC23B mutations either cause a premature stop (C649T) or impair its protein transport activity (C1467G and T488C + G791A + G2153A). Furthermore, SEC23B mutations inhibit the transport of epithelial cell adhesion molecule (EPCAM) and CD9 molecule, thereby attenuating cell adhesion and promoting invasiveness both in vitro and in vivo. Taken together, these data demonstrate the important impact of SEC23B mutations on metastasis, and we propose that SEC23B is a potential suppressor of CRC metastasis.

Poor old pores-The challenge of making and maintaining nuclear pore complexes in aging

The nuclear pore complex (NPC) is the sole gateway to the nuclear interior, and its function is essential to all eukaryotic life. Controlling the functionality of NPCs is a tremendous challenge for cells. Firstly, NPCs are large structures, and their complex assembly does occasionally go awry. Secondly, once assembled, some components of the NPC persist for an extremely long time and, as a result, are susceptible to accumulate damage. Lastly, a significant proportion of the NPC is composed of intrinsically disordered proteins that are prone to aggregation. In this review, we summarize how the quality of NPCs is guarded in young cells and discuss the current knowledge on the fate of NPCs during normal aging in different tissues and organisms. We discuss the extent to which current data supports a hypothesis that NPCs are poorly maintained during aging of nondividing cells, while in dividing cells the main challenge is related to the assembly of new NPCs. Our survey of current knowledge points toward NPC quality control as an important node in aging of both dividing and nondividing cells. Here, the loss of protein homeostasis during aging is central and the NPC appears to both be impacted by, and to drive, this process.

Diffuse or hitch a ride: how photoreceptor lipidated proteins get from here to there

Photoreceptors are polarized neurons, with specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment (OS) where vision begins, an inner segment (IS) where protein synthesis occurs and a synaptic terminal for signal transmission to second-order neurons. The OS is a large, modified primary cilium attached to the IS by a slender connecting cilium (CC), the equivalent of the transition zone (TZ). Daily renewal of ~10% of the OS requires massive protein biosynthesis in the IS with reliable transport and targeting pathways. Transport of lipidated (‘sticky’) proteins depends on solubilization factors, phosphodiesterase δ (PDEδ) and uncoordinated protein-119 (UNC119), and the cargo dispensation factor (CDF), Arf-like protein 3-guanosine triphosphate (ARL3-GTP). As PDE6 and transducin still reside prominently in the OS of PDEδ and UNC119 germline knockout mice, respectively, we propose the existence of an alternate trafficking pathway, whereby lipidated proteins migrate in rhodopsin-containing vesicles of the secretory pathway.

Protein quality control in the secretory pathway

Protein folding is inherently error prone, especially in the endoplasmic reticulum (ER). Even with an elaborate network of molecular chaperones and protein folding facilitators, misfolding can occur quite frequently. To maintain protein homeostasis, eukaryotes have evolved a series of protein quality-control checkpoints. When secretory pathway quality-control pathways fail, stress response pathways, such as the unfolded protein response (UPR), are induced. In addition, the ER, which is the initial hub of protein biogenesis in the secretory pathway, triages misfolded proteins by delivering substrates to the proteasome or to the lysosome/vacuole through ER-associated degradation (ERAD) or ER-phagy. Some misfolded proteins escape the ER and are instead selected for Golgi quality control. These substrates are targeted for degradation after retrieval to the ER or delivery to the lysosome/vacuole. Here, we discuss how these guardian pathways function, how their activities intersect upon induction of the UPR, and how decisions are made to dispose of misfolded proteins in the secretory pathway.

Atlastin-mediated membrane tethering is critical for cargo mobility and exit from the endoplasmic reticulum

In the early secretory pathway, newly synthesized proteins undergo folding and modifications and then leave the ER through COPII-coated vesicles. How these processes are coordinated and maintained are important but mostly unclear. We show here that ATL, a GTPase that connects ER tubules, controls ER protein mobility and regulates cargo packaging and coat assembly of COPII vesicles. The tethering and fusion activity by ATL likely maintains tension and other necessary parameters for COPII formation in ER membranes. These findings reveal a role of ER shaping in the early secretory pathway and provide insight into behaviors of ER exportation.

Endoplasmic reticulum (ER) membrane junctions are formed by the dynamin-like GTPase atlastin (ATL). Deletion of ATL results in long unbranched ER tubules in cells, and mutation of human ATL1 is linked to hereditary spastic paraplegia. Here, we demonstrate that COPII formation is drastically decreased in the periphery of ATL-deleted cells. ER export of cargo proteins becomes defective; ER exit site initiation is not affected, but many of the sites fail to recruit COPII subunits. The efficiency of cargo packaging into COPII vesicles is significantly reduced in cells lacking ATLs, or when the ER is transiently fragmented. Cargo is less mobile in the ER in the absence of ATL, but the cargo mobility and COPII formation can be restored by ATL R77A, which is capable of tethering, but not fusing, ER tubules. These findings suggest that the generation of ER junctions by ATL plays a critical role in maintaining the necessary mobility of ER contents to allow efficient packaging of cargo proteins into COPII vesicles.

Role of TREK-1 in Health and Disease, Focus on the Central Nervous System

TREK-1 is the most studied background K_2P channel. Its main role is to control cell excitability and maintain the membrane potential below the threshold of depolarization. TREK-1 is multi-regulated by a variety of physical and chemical stimuli which makes it a very promising and challenging target in the treatment of several pathologies. It is mainly expressed in the brain but also in heart, smooth muscle cells, endocrine pancreas, and prostate. In the nervous system, TREK-1 is involved in many physiological and pathological processes such as depression, neuroprotection, pain, and anesthesia. These properties explain why many laboratories and pharmaceutical companies have been focusing their research on screening and developing highly efficient modulators of TREK-1 channels. In this review, we summarize the different roles of TREK-1 that have been investigated so far in attempt to characterize pharmacological tools and new molecules to modulate cellular functions controlled by TREK-1.

Antiviral Functions of Monoclonal Antibodies against Chikungunya Virus

Chikungunya virus (CHIKV) is the most common alphavirus infecting humans worldwide. Antibodies play pivotal roles in the immune response to infection. Increasingly, therapeutic antibodies are becoming important for protection from pathogen infection for which neither vaccine nor treatment is available, such as CHIKV infection. The new generation of ultra-potent and/or broadly cross-reactive monoclonal antibodies (mAbs) provides new opportunities for intervention. In the past decade, several potent human and mouse anti-CHIKV mAbs were isolated and demonstrated to be protective in vivo. Mechanistic studies of these mAbs suggest that mAbs exert multiple modes of action cooperatively. Better understanding of these antiviral mechanisms for mAbs will help to optimize mAb therapies.

Identification of Suppressor of Clathrin Deficiency-1 (SCD1) and Its Connection to Clathrin-Mediated Endocytosis in Saccharomyces cerevisiae

Clathrin is a major coat protein involved in vesicle formation during endocytosis and transport in the endosomal/trans Golgi system. Clathrin is required for normal growth of yeast (Saccharomyces cerevisiae) and in some genetic backgrounds deletion of the clathrin heavy chain gene (CHC1) is lethal. Our lab defined a locus referred to as “suppressor of clathrin deficiency” (SCD1). In the presence of the scd1-v allele (“v” – viable), yeast cells lacking clathrin heavy chain survive but grow slowly, are morphologically abnormal and have many membrane trafficking defects. In the presence of scd1-i (“i”- inviable), chc1∆ causes lethality. As a strategy to identify SCD1, we used pooled linkage analysis and whole genome sequencing. Here, we report that PAL2 (YHR097C) is the SCD1 locus. pal2∆ is synthetic lethal with chc1∆; whereas a deletion of its paralog, PAL1, is not synthetic lethal with clathrin deficiency. Like Pal1, Pal2 has two NPF motifs that are potential binding sites for EH domain proteins such as the early endocytic factor Ede1, and Pal2 associates with Ede1. Also, GFP-tagged Pal2p localizes to cortical patches containing other immobile phase endocytic coat factors. Overall, our data show that PAL2 is the SCD1 locus and the Pal2 protein has characteristics of an early factor involved in clathrin-mediated endocytosis.

Dynamics and functions of lipid droplets

Lipid droplets are storage organelles at the centre of lipid and energy homeostasis. They have a unique architecture consisting of a hydrophobic core of neutral lipids, which is enclosed by a phospholipid monolayer that is decorated by a specific set of proteins. Originating from the endoplasmic reticulum, lipid droplets can associate with most other cellular organelles through membrane contact sites. It is becoming apparent that these contacts between lipid droplets and other organelles are highly dynamic and coupled to the cycles of lipid droplet expansion and shrinkage. Importantly, lipid droplet biogenesis and degradation, as well as their interactions with other organelles, are tightly coupled to cellular metabolism and are critical to buffer the levels of toxic lipid species. Thus, lipid droplets facilitate the coordination and communication between different organelles and act as vital hubs of cellular metabolism.

Cargo induces retromer-mediated membrane remodeling on membranes

Endosomes serve as a central sorting station of lipids and proteins that arrive via vesicular carrier from the plasma membrane and the Golgi complex. At the endosome, retromer complexes sort selected receptors and membrane proteins into tubules or vesicles that bud off the endosome. The mature endosome finally fuses with the lysosome. Retromer complexes consist of a cargo selection complex (CSC) and a membrane remodeling part (sorting nexin [SNX]-Bin/amphiphysin/Rvs [BAR], or Snx3 in yeast) and different assemblies of retromer mediate recycling of different cargoes. Due to this complexity, the exact order of events that results in carrier formation is not yet understood. Here, we reconstituted this process on giant unilamellar vesicles together with purified retromer complexes from yeast and selected cargoes. Our data reveal that the membrane remodeling activity of both Snx3 and the SNX-BAR complex is strongly reduced at low concentrations, which can be reactivated by CSC. At even lower concentrations, these complexes still associate with membranes, but only remodel membranes in the presence of their specific cargoes. Our data thus favor a simple model, where cargo functions as a specific trigger of retromer-mediated sorting on endosomes.

Regulation of axon repulsion by MAX-1 SUMOylation and AP-3

During neural development, growing axons navigate over long distances to reach their targets. A critical step in this process is the regulation of its surface receptors on the axon’s growth cone in response to environmental cues. We focus on how the UNC-5 receptor in Caenorhabditis elegans motor axons is regulated during axon repulsion. By combining C. elegans genetics, biochemistry, and imaging, we found that MAX-1 SUMOylation and AP-3 complex have significant roles in UNC-5–mediated axon repulsion. Our findings reveal how SUMOylation and AP-3–mediated trafficking and degradation interact to help the growing axon find its final target.

During neural development, growing axons express specific surface receptors in response to various environmental guidance cues. These axon guidance receptors are regulated through intracellular trafficking and degradation to enable navigating axons to reach their targets. In Caenorhabditis elegans, the UNC-5 receptor is necessary for dorsal migration of developing motor axons. We previously found that MAX-1 is required for UNC-5–mediated axon repulsion, but its mechanism of action remained unclear. Here, we demonstrate that UNC-5–mediated axon repulsion in C. elegans motor axons requires both max-1 SUMOylation and the AP-3 complex β subunit gene, apb-3. Genetic interaction studies show that max-1 is SUMOylated by gei-17/PIAS1 and acts upstream of apb-3. Biochemical analysis suggests that constitutive interaction of MAX-1 and UNC-5 receptor is weakened by MAX-1 SUMOylation and by the presence of APB-3, a competitive interactor with UNC-5. Overexpression of APB-3 reroutes the trafficking of UNC-5 receptor into the lysosome for protein degradation. In vivo fluorescence recovery after photobleaching experiments shows that MAX-1 SUMOylation and APB-3 are required for proper trafficking of UNC-5 receptor in the axon. Our results demonstrate that SUMOylation of MAX-1 plays an important role in regulating AP-3–mediated trafficking and degradation of UNC-5 receptors during axon guidance.

Rab GTPase Function in Endosome and Lysosome Biogenesis

Eukaryotic cells maintain a highly organized endolysosomal system. This system regulates the protein and lipid content of the plasma membrane, it participates in the intracellular quality control machinery and is needed for the efficient removal of damaged organelles. This complex network comprises an endosomal membrane system that feeds into the lysosomes, yet also allows recycling of membrane proteins, and probably lipids. Moreover, lysosomal degradation provides the cell with macromolecules for further growth. In this review, we focus primarily on the role of the small Rab GTPases Rab5 and Rab7 as organelle markers and interactors of multiple effectors on endosomes and lysosomes and highlight their role in membrane dynamics, particularly fusion along the endolysosomal pathway.

ESCRT-mediated vesicle concatenation in plant endosomes

ESCRT proteins play essential functions by remodeling cellular membranes. Buono et al. report on a novel ESCRT-dependent mechanism in plant endosomes that leads to sequential concatenation of vesicle buds by temporally uncoupling membrane constriction from membrane fission. During this process, ESCRT-III proteins remain inside endosomes after intralumenal vesicle release.

Ubiquitinated plasma membrane proteins (cargo) are delivered to endosomes and sorted by endosomal sorting complex required for transport (ESCRT) machinery into endosome intralumenal vesicles (ILVs) for degradation. In contrast to the current model that postulates that ILVs form individually from inward budding of the endosomal limiting membrane, plant ILVs form as networks of concatenated vesicle buds by a novel vesiculation mechanism. We ran computational simulations based on experimentally derived diffusion coefficients of an ESCRT cargo protein and electron tomograms of Arabidopsis thaliana endosomes to measure cargo escape from budding ILVs. We found that 50% of the ESCRT cargo would escape from a single budding profile in 5–20 ms and from three concatenated ILVs in 80–200 ms. These short cargo escape times predict the need for strong diffusion barriers in ILVs. Consistent with a potential role as a diffusion barrier, we find that the ESCRT-III protein SNF7 remains associated with ILVs and is delivered to the vacuole for degradation.

Endoplasmic reticulum stress and inflammation in the central nervous system

Persistent endoplasmic reticulum (ER) stress is thought to drive the pathology of many chronic disorders due to its potential to elicit aberrant inflammatory signaling and facilitate cell death. In neurodegenerative diseases, the accumulation of misfolded proteins and concomitant induction of ER stress in neurons contributes to neuronal dysfunction. In addition, ER stress responses induced in the surrounding neuroglia may promote disease progression by coordinating damaging inflammatory responses, which help fuel a neurotoxic milieu. Nevertheless, there still remains a gap in knowledge regarding the cell-specific mechanisms by which ER stress mediates neuroinflammation. In this review, we will discuss recently uncovered inflammatory pathways linked to the ER stress response. Moreover, we will summarize the present literature delineating how ER stress is generated in Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, and Multiple Sclerosis, and highlight how ER stress and neuroinflammation intersect mechanistically within the central nervous system. The mechanisms by which stress-induced inflammation contributes to the pathogenesis and progression of neurodegenerative diseases remain poorly understood. Further examination of this interplay could present unappreciated insights into the development of neurodegenerative diseases, and reveal new therapeutic targets.