Early/recycling endosomes-to-TGN transport involves two SNARE complexes and a Rab6 isoform

Spontaneous and evoked synaptic vesicle release arises from a single releasable pool

The quantal content of an evoked postsynaptic response is typically determined by dividing it by the average spontaneous miniature response. However, this approach is challenged by the notion that different synaptic vesicle pools might drive spontaneous and evoked release. Here, we "silence" synaptic vesicles through pharmacological alkalinization and subsequently rescue them by optogenetic acidification. We find that such silenced synaptic vesicles, retrieved during evoked or spontaneous activity, cross-deplete the complementary release mode in a fully reversible manner. A fluorescently tagged version of the endosomal SNARE protein Vti1a, which has been suggested to identify a separate pool of spontaneously recycling synaptic vesicles, is trafficked to synaptic vesicles significantly only upon overexpression but not when endogenously tagged by CRISPR-Cas9. Thus, both release modes draw synaptic vesicles from the same readily releasable pool.

Rab6-Mediated Polarized Transport of Synaptic Vesicle Precursors Is Essential for the Establishment of Neuronal Polarity and Brain Formation

Neurons are highly polarized cells that are composed of a single axon and multiple dendrites. Axon-dendrite polarity is essential for proper tissue formation and brain functions. Intracellular protein transport plays an important role in the establishment of neuronal polarity. However, the regulatory mechanism of polarized transport remains unclear. Here, we show that Rab6, a small GTPase that acts on the regulation of intracellular vesicular trafficking, plays key roles in neuronal polarization and brain development. Central nervous system-specific Rab6a/b double knock-out (Rab6 DKO) mice of both sexes exhibit severe dysplasia of the neocortex and the cerebellum. In the Rab6 DKO neocortex, impaired axonal extension of neurons results in hypoplasia of the intermediate zone. In vitro, deletion of Rab6a and Rab6b in cultured neurons from both sexes causes the abnormal accumulation of synaptic vesicle precursors (SVPs) adjacent to the Golgi apparatus, which leads to defects in axonal extension and the loss of axon-dendrite polarity. Moreover, Rab6 DKO causes significant expansion of lysosomes in the soma in neurons. Overall, our results reveal that Rab6-mediated polarized transport of SVPs is crucial for neuronal polarization and subsequent brain formation.

Movement of the endoplasmic reticulum is driven by multiple classes of vesicles marked by Rab-GTPases

Peripheral endoplasmic reticulum (ER) tubules move along microtubules to interact with various organelles through membrane contact sites (MCS). Traditionally, ER moves by either sliding along stable microtubules via molecular motors or attaching to the plus ends of dynamic microtubules through tip attachment complexes (TAC). A recently discovered third process, hitchhiking, involves motile vesicles pulling ER tubules along microtubules. Previous research showed that ER hitchhikes on Rab5- and Rab7-marked endosomes, but it is uncertain if other Rab-vesicles can do the same. In U2OS cells, we screened Rabs for their ability to cotransport with ER tubules and found that ER hitchhikes on post-Golgi vesicles marked by Rab6 (isoforms a and b). Rab6-ER hitchhiking occurs independently of ER-endolysosome contacts and TAC-mediated ER movement. Disrupting either Rab6 or the motility of Rab6-vesicles reduces overall ER movement. Conversely, relocating these vesicles to the cell periphery causes peripheral ER accumulation, indicating that Rab6-vesicle motility is crucial for a subset of ER movements. Proximal post-Golgi vesicles marked by TGN46 are involved in Rab6-ER hitchhiking, while other post-Golgi vesicles (Rabs 8/10/11/13/14) are not essential for ER movement. Our further analysis finds that ER to Golgi vesicles marked by Rab1 are also capable of driving a subset of ER movements. Taken together, our findings suggest that ER hitchhiking on Rab-vesicles is a significant mode of ER movement.

SIGNIFICANCE STATEMENT

Peripheral endoplasmic reticulum tubules move on microtubules by either attaching to motors (cargo adaptor-mediated), dynamic microtubule-plus ends (tip attachment complexes) or motile vesicles (hitchhiking) but the prevalence of each mode is not clearPost-Golgi vesicles marked by Rab6/TGN46 and ER to Golgi vesicles marked by Rab1 drive ER movementsER hitchhiking on multiple classes of vesicles (endolysosomal, post-Golgi and ER to Golgi) marked by Rabs plays a prominent role in ER movement.

MR1 antigen presentation to MAIT cells and other MR1-restricted T cells

MHC antigen presentation plays a fundamental role in adaptive and semi-invariant T cell immunity. Distinct MHC molecules bind antigens that differ in chemical structure, origin and location and present them to specialized T cells. MHC class I-related protein 1 (MR1) presents a range of small molecule antigens to MR1-restricted T (MR1T) lymphocytes. The best studied MR1 ligands are derived from microbial metabolism and are recognized by a major class of MR1T cells known as mucosal-associated invariant T (MAIT) cells. Here, we describe the MR1 antigen presentation pathway: the known types of antigens presented by MR1, the location where MR1-antigen complexes form, the route followed by the complexes to the cell surface, the mechanisms involved in termination of MR1 antigen presentation and the accessory cellular proteins that comprise the MR1 antigen presentation machinery. The current road map of the MR1 antigen presentation pathway reveals potential strategies for therapeutic manipulation of MR1T cell function and provides a foundation for further studies that will lead to a deeper understanding of MR1-mediated immunity.

Coordination of RAB-8 and RAB-11 during unconventional protein secretion

Multiple physiology-pertinent transmembrane proteins reach the cell surface via the Golgi-bypassing unconventional protein secretion (UcPS) pathway. By employing C. elegans-polarized intestine epithelia, we recently have revealed that the small GTPase RAB-8/Rab8 serves as an important player in the process. Nonetheless, its function and the relevant UcPS itinerary remain poorly understood. Here, we show that deregulated RAB-8 activity resulted in impaired apical UcPS, which increased sensitivity to infection and environmental stress. We also identified the SNARE VTI-1/Vti1a/b as a new RAB-8-interacting factor involved in the apical UcPS. Besides, RAB-11/Rab11 was capable of recruiting RABI-8/Rabin8 to reduce the guanine nucleotide exchange activity of SMGL-1/GEF toward RAB-8, indicating the necessity of a finely tuned RAB-8/RAB-11 network. Populations of RAB-8- and RAB-11-positive endosomal structures containing the apical UcPS cargo moved toward the apical side. In the absence of RAB-11 or its effectors, the cargo was retained in RAB-8- and RAB-11-positive endosomes, respectively, suggesting that these endosomes are utilized as intermediate carriers for the UcPS.

Exploring Retrograde Trafficking: Mechanisms and Consequences in Cancer and Disease

Retrograde trafficking (RT) orchestrates the intracellular movement of cargo from the plasma membrane, endosomes, Golgi or endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) in an inward/ER-directed manner. RT works as the opposing movement to anterograde trafficking (outward secretion), and the two work together to maintain cellular homeostasis. This is achieved through maintaining cell polarity, retrieving proteins responsible for anterograde trafficking and redirecting proteins that become mis-localised. However, aberrant RT can alter the correct location of key proteins, and thus inhibit or indeed change their canonical function, potentially causing disease. This review highlights the recent advances in the understanding of how upregulation, downregulation or hijacking of RT impacts the localisation of key proteins in cancer and disease to drive progression. Cargoes impacted by aberrant RT are varied amongst maladies including neurodegenerative diseases, autoimmune diseases, bacterial and viral infections (including SARS-CoV-2), and cancer. As we explore the intricacies of RT, it becomes increasingly apparent that it holds significant potential as a target for future therapies to offer more effective interventions in a wide range of pathological conditions.

Cortical miR-709 links glutamatergic signaling to NREM sleep EEG slow waves in an activity-dependent manner

MicroRNAs (miRNAs) are key post-transcriptional regulators of gene expression that have been implicated in a plethora of neuronal processes. Nevertheless, their role in regulating brain activity in the context of sleep has so far received little attention. To test their involvement, we deleted mature miRNAs in post-mitotic neurons at two developmental ages, i.e., in early adulthood using conditional Dicer knockout (cKO) mice and in adult mice using an inducible conditional Dicer cKO (icKO) line. In both models, electroencephalographic (EEG) activity was affected and the response to sleep deprivation (SD) altered; while the rapid-eye-movement sleep (REMS) rebound was compromised in both, the increase in EEG delta (1 to 4 Hz) power during non-REMS (NREMS) was smaller in cKO mice and larger in icKO mice compared to controls. We subsequently investigated the effects of SD on the forebrain miRNA transcriptome and found that the expression of 48 miRNAs was affected, and in particular that of the activity-dependent miR-709. In vivo inhibition of miR-709 in the brain increased EEG power during NREMS in the slow-delta (0.75 to 1.75 Hz) range, particularly after periods of prolonged wakefulness. Transcriptome analysis of primary cortical neurons in vitro revealed that miR-709 regulates genes involved in glutamatergic neurotransmission. A subset of these genes was also affected in the cortices of sleep-deprived, miR-709-inhibited mice. Our data implicate miRNAs in the regulation of EEG activity and indicate that miR-709 links neuronal activity during wakefulness to brain synchrony during sleep through the regulation of glutamatergic signaling.

The role of SPP/SPPL intramembrane proteases in membrane protein homeostasis

Signal peptide peptidase (SPP) and the four SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 constitute a family of aspartyl intramembrane proteases with homology to presenilins. The different members reside in distinct cellular localisations within the secretory pathway and the endo-lysosomal system. Despite individual cleavage characteristics, they all cleave single-span transmembrane proteins with a type II orientation exhibiting a cytosolic N-terminus. Though the identification of substrates is not complete, SPP/SPPL-mediated proteolysis appears to be rather selective. Therefore, according to our current understanding cleavage by SPP/SPPL proteases rather seems to serve a regulatory function than being a bulk proteolytic pathway. In the present review, we will summarise our state of knowledge on SPP/SPPL proteases and in particular highlight recently identified substrates and the functional and/or (patho)-physiological implications of these cleavage events. Based on this, we aim to provide an overview of the current open questions in the field. These are connected to the regulation of these proteases at the cellular level but also in context of disease and patho-physiological processes. Furthermore, the interplay with other proteostatic systems capable of degrading membrane proteins is beginning to emerge.

Characterisation and prion transmission study in mice with genetic reduction of sporadic Creutzfeldt-Jakob disease risk gene Stx6

Sporadic Creutzfeldt-Jakob disease (sCJD), the most common human prion disease, is thought to occur when the cellular prion protein (PrPC) spontaneously misfolds and assembles into prion fibrils, culminating in fatal neurodegeneration. In a genome-wide association study of sCJD, we recently identified risk variants in and around the gene STX6, with evidence to suggest a causal increase of STX6 expression in disease-relevant brain regions. STX6 encodes syntaxin-6, a SNARE protein primarily involved in early endosome to trans-Golgi network retrograde transport. Here we developed and characterised a mouse model with genetic depletion of Stx6 and investigated a causal role of Stx6 expression in mouse prion disease through a classical prion transmission study, assessing the impact of homozygous and heterozygous syntaxin-6 knockout on disease incubation periods and prion-related neuropathology. Following inoculation with RML prions, incubation periods in Stx6-/- and Stx6+/- mice differed by 12 days relative to wildtype. Similarly, in Stx6-/- mice, disease incubation periods following inoculation with ME7 prions also differed by 12 days. Histopathological analysis revealed a modest increase in astrogliosis in ME7-inoculated Stx6-/- animals and a variable effect of Stx6 expression on microglia activation, however no differences in neuronal loss, spongiform change or PrP deposition were observed at endpoint. Importantly, Stx6-/- mice are viable and fertile with no gross impairments on a range of neurological, biochemical, histological and skeletal structure tests. Our results provide some support for a pathological role of Stx6 expression in prion disease, which warrants further investigation in the context of prion disease but also other neurodegenerative diseases considering syntaxin-6 appears to have pleiotropic risk effects in progressive supranuclear palsy and Alzheimer's disease.

Simpson J. Rab6-mediated retrograde trafficking from the Golgi: the trouble with tubules

Next year marks one-quarter of a century since the discovery of the so-called COPI-independent pathway, which operates between the Golgi apparatus and the endoplasmic reticulum (ER) in eukaryotic cells. Unlike almost all other intracellular trafficking pathways, this pathway is not regulated by the physical accumulation of multisubunit proteinaceous coat molecules, but instead by the small GTPase Rab6. What also sets it apart from other pathways is that the transport carriers themselves often take the form of tubules, rather than conventional vesicles. In this review, we assess the relevant literature that has accumulated to date, in an attempt to provide a concerted description of how this pathway is regulated. We discuss the possible cargo molecules that are carried in this pathway, and the likely mechanism of Rab6 tubule biogenesis, including how the cargo itself may play a critical role. We also provide perspective surrounding the various molecular motors of the kinesin, myosin and dynein families that have been implicated in driving Rab6-coated tubular membranes long distances through the cell prior to delivering their cargo to the ER. Finally, we also raise several important questions that require resolution, if we are to ultimately provide a comprehensive molecular description of how the COPI-independent pathway is controlled.

ADP Ribosylation Factor 4 (Arf4) Regulates Radial Migration through N-Cadherin Trafficking during Cerebral Cortical Development

During the development of the cerebral cortex, N-cadherin plays a crucial role in facilitating radial migration by enabling cell-to-cell adhesion between migrating neurons and radial glial fibers or Cajar-Reztius cells. ADP ribosylation factor 4 (Arf4) and Arf5, which belong to the Class II Arf small GTPase subfamily, control membrane trafficking in the endocytic and secretory pathways. However, their specific contribution to cerebral cortex development remains unclear. In this study, we sought to investigate the functional involvement of Class II Arfs in radial migration during the layer formation of the cerebral cortex using mouse embryos and pups. Our findings indicate that knock-down of Arf4, but not Arf5, resulted in the stalling of transfected neurons with disorientation of the Golgi in the upper intermediate zone (IZ) and reduction in the migration speed in both the IZ and cortical plate (CP). Migrating neurons with Arf4 knock-down exhibited cytoplasmic accumulation of N-cadherin, along with disturbed organelle morphology and distribution. Furthermore, supplementation of exogenous N-cadherin partially rescued the migration defect caused by Arf4 knock-down. In conclusion, our results suggest that Arf4 plays a crucial role in regulating radial migration via N-cadherin trafficking during cerebral cortical development.

Galectin-9 interacts with Vamp-3 to regulate cytokine secretion in dendritic cells

Intracellular vesicle transport is essential for cellular homeostasis and is partially mediated by SNARE proteins. Endosomal trafficking to the plasma membrane ensures cytokine secretion in dendritic cells (DCs) and the initiation of immune responses. Despite its critical importance, the specific molecular components that regulate DC cytokine secretion are poorly characterised. Galectin-9, a ß-galactoside-binding protein, has emerged as a novel cellular modulator although its exact intracellular roles in regulating (immune) cell homeostasis and vesicle transport are virtually unknown. We investigated galectin-9 function in primary human DCs and report that galectin-9 is essential for intracellular cytokine trafficking to the cell surface. Galectin-9-depleted DCs accumulate cytokine-containing vesicles in the Golgi complex that eventually undergo lysosomal degradation. We observed galectin-9 to molecularly interact with Vamp-3 using immunoprecipitation-mass-spectrometry and identified galectin-9 was required for rerouting Vamp-3-containing endosomes upon DC activation as the underlying mechanism. Overall, this study identifies galectin-9 as a necessary mechanistic component for intracellular trafficking. This may impact our general understanding of vesicle transport and sheds new light into the multiple roles galectins play in governing cell function.

Type III Secretion in Chlamydia

Type III secretion systems (T3SSs) are utilized by Gram-negative pathogens to enhance their pathogenesis. This secretion system is associated with the delivery of effectors through a needle-like structure from the bacterial cytosol directly into a target eukaryotic cell. These effector proteins then manipulate specific eukaryotic cell functions to benefit pathogen survival within the host. The obligate intracellular pathogens of the family Chlamydiaceae have a highly evolutionarily conserved nonflagellar T3SS that is an absolute requirement for their survival and propagation within the host with about one-seventh of the genome dedicated to genes associated with the T3SS apparatus, chaperones, and effectors. Chlamydiae also have a unique biphasic developmental cycle where the organism alternates between an infectious elementary body (EB) and replicative reticulate body (RB). T3SS structures have been visualized on both EBs and RBs. And there are effector proteins that function at each stage of the chlamydial developmental cycle, including entry and egress. This review will discuss the history of the discovery of chlamydial T3SS and the biochemical characterization of components of the T3SS apparatus and associated chaperones in the absence of chlamydial genetic tools. These data will be contextualized into how the T3SS apparatus functions throughout the chlamydial developmental cycle and the utility of heterologous/surrogate models to study chlamydial T3SS. Finally, there will be a targeted discussion on the history of chlamydial effectors and recent advances in the field.

MDM2 Influences ACE2 Stability and SARS-CoV-2 Uptake

Angiotensin-converting enzyme 2 (ACE2) is the central entry receptor for SARS-CoV-2. However, surprisingly little is known about the effects of host regulators on ACE2 localization, expression, and the associated influence on SARS-CoV-2 infection. Here we identify that ACE2 expression levels are regulated by the E3 ligase MDM2 and that MDM2 levels indirectly influence infection with SARS-CoV-2. Genetic depletion of MDM2 elevated ACE2 expression levels, which strongly promoted infection with all SARS-CoV-2 isolates tested. SARS-CoV-2 spike-pseudotyped viruses and the uptake of non-replication-competent virus-like particles showed that MDM2 affects the viral uptake process. MDM2 ubiquitinates Lysine 788 of ACE2 to induce proteasomal degradation, and degradation of this residue led to higher ACE2 expression levels and superior virus particle uptake. Our study illustrates that cellular regulators of ACE2 stability, such as MDM2, play an important role in defining the infection capabilities of SARS-CoV-2.

Secretion of VGF relies on the interplay between LRRK2 and post-Golgi v-SNAREs

The neuropeptide VGF was recently proposed as a neurodegeneration biomarker. The Parkinson's disease-related protein leucine-rich repeat kinase 2 (LRRK2) regulates endolysosomal dynamics, a process that involves SNARE-mediated membrane fusion and could regulate secretion. Here we investigate potential biochemical and functional links between LRRK2 and v-SNAREs. We find that LRRK2 directly interacts with the v-SNAREs VAMP4 and VAMP7. Secretomics reveals VGF secretory defects in VAMP4 and VAMP7 knockout (KO) neuronal cells. In contrast, VAMP2 KO "regulated secretion-null" and ATG5 KO "autophagy-null" cells release more VGF. VGF is partially associated with extracellular vesicles and LAMP1+ endolysosomes. LRRK2 expression increases VGF perinuclear localization and impairs its secretion. Retention using selective hooks (RUSH) assays show that a pool of VGF traffics through VAMP4+ and VAMP7+ compartments, and LRRK2 expression delays its transport to the cell periphery. Overexpression of LRRK2 or VAMP7-longin domain impairs VGF peripheral localization in primary cultured neurons. Altogether, our results suggest that LRRK2 might regulate VGF secretion via interaction with VAMP4 and VAMP7.

Mapping localization of 21 endogenous proteins in the Golgi apparatus of rodent neurons

The Golgi apparatus is the major sorting hub in the secretory pathway and particularly important for protein sorting in neurons. Knowledge about protein localization in Golgi compartments is largely based on work in cell lines. Here, we systematically compared protein localization of 21 endogenous proteins in the Golgi apparatus of mouse neurons using confocal microscopy and line scan analysis. We localized these proteins by measuring the distance relative to the canonical TGN marker TGN38. Based on this, proteins fell into three groups: upstream of, overlapping with or downstream of TGN38. Seven proteins showed complete overlap with TGN38, while proteins downstream of TGN38 were located at varying distances from TGN38. Proteins upstream of TGN38 were localized in between TGN38 and the cis-/medial Golgi markers Giantin and GM130. This localization was consistent with protein function. Our data provide an overview of the relative localization of endogenous proteins in the Golgi of primary mouse neurons.

Loci for insulin processing and secretion provide insight into type 2 diabetes risk

Insulin secretion is critical for glucose homeostasis, and increased levels of the precursor proinsulin relative to insulin indicate pancreatic islet beta-cell stress and insufficient insulin secretory capacity in the setting of insulin resistance. We conducted meta-analyses of genome-wide association results for fasting proinsulin from 16 European-ancestry studies in 45,861 individuals. We found 36 independent signals at 30 loci (p value < 5 × 10-8), which validated 12 previously reported loci for proinsulin and ten additional loci previously identified for another glycemic trait. Half of the alleles associated with higher proinsulin showed higher rather than lower effects on glucose levels, corresponding to different mechanisms. Proinsulin loci included genes that affect prohormone convertases, beta-cell dysfunction, vesicle trafficking, beta-cell transcriptional regulation, and lysosomes/autophagy processes. We colocalized 11 proinsulin signals with islet expression quantitative trait locus (eQTL) data, suggesting candidate genes, including ARSG, WIPI1, SLC7A14, and SIX3. The NKX6-3/ANK1 proinsulin signal colocalized with a T2D signal and an adipose ANK1 eQTL signal but not the islet NKX6-3 eQTL. Signals were enriched for islet enhancers, and we showed a plausible islet regulatory mechanism for the lead signal in the MADD locus. These results show how detailed genetic studies of an intermediate phenotype can elucidate mechanisms that may predispose one to disease.

Developmental program-independent secretory granule degradation in larval salivary gland cells of Drosophila

Both constitutive and regulated secretion require cell organelles that are able to store and release the secretory cargo. During development, the larval salivary gland of Drosophila initially produces high amount of glue-containing small immature secretory granules, which then fuse with each other and reach their normal 3-3.5 μm in size. Following the burst of secretion, obsolete glue granules directly fuse with late endosomes or lysosomes by a process called crinophagy, which leads to fast degradation and recycling of the secretory cargo. However, hindering of endosome-to-TGN retrograde transport in these cells causes abnormally small glue granules which are not able to fuse with each other. Here, we show that loss of function of the SNARE genes Syntaxin 16 (Syx16) and Synaptobrevin (Syb), the small GTPase Rab6 and the GARP tethering complex members Vps53 and Scattered (Vps54) all involved in retrograde transport cause intense early degradation of immature glue granules via crinophagy independently of the developmental program. Moreover, silencing of these genes also provokes secretory failure and accelerated crinophagy during larval development. Our results provide a better understanding of the relations among secretion, secretory granule maturation and degradation and paves the way for further investigation of these connections in other metazoans.

An imaging-based RNA interference screen for modulators of the Rab6-mediated Golgi-to-ER pathway in mammalian cells

In mammalian cells, membrane traffic pathways play a critical role in connecting the various compartments of the endomembrane system. Each of these pathways is highly regulated, requiring specific machinery to ensure their fidelity. In the early secretory pathway, transport between the endoplasmic reticulum (ER) and Golgi apparatus is largely regulated via cytoplasmic coat protein complexes that play a role in identifying cargo and forming the transport carriers. The secretory pathway is counterbalanced by the retrograde pathway, which is essential for the recycling of molecules from the Golgi back to the ER. It is believed that there are at least two mechanisms to achieve this - one using the cytoplasmic COPI coat complex, and another, poorly characterised pathway, regulated by the small GTPase Rab6. In this work, we describe a systematic RNA interference screen targeting proteins associated with membrane fusion, in order to identify the machinery responsible for the fusion of Golgi-derived Rab6 carriers at the ER. We not only assess the delivery of Rab6 to the ER, but also one of its cargo molecules, the Shiga-like toxin B-chain. These screens reveal that three proteins, VAMP4, STX5, and SCFD1/SLY1, are all important for the fusion of Rab6 carriers at the ER. Live cell imaging experiments also show that the depletion of SCFD1/SLY1 prevents the membrane fusion event, suggesting that this molecule is an essential regulator of this pathway.

Pleiotropic effects of Syntaxin16 identified by gene editing in cultured adipocytes

Adipocytes play multiple roles in the regulation of glucose metabolism which rely on the regulation of membrane traffic. These include secretion of adipokines and serving as an energy store. Central to their energy storing function is the ability to increase glucose uptake in response to insulin, mediated through translocation of the facilitative glucose transporter GLUT4 to the cell surface. The trans-Golgi reticulum localized SNARE protein syntaxin 16 (Sx16) has been identified as a key component of the secretory pathway required for insulin-regulated trafficking of GLUT4. We used CRISPR/Cas9 technology to generate 3T3-L1 adipocytes lacking Sx16 to understand the role of the secretory pathway on adipocyte function. GLUT4 mRNA and protein levels were reduced in Sx16 knockout adipocytes and insulin stimulated GLUT4 translocation to the cell surface was reduced. Strikingly, neither basal nor insulin-stimulated glucose transport were affected. By contrast, GLUT1 levels were upregulated in Sx16 knockout cells. Levels of sortilin and insulin regulated aminopeptidase were also increased in Sx16 knockout adipocytes which may indicate an upregulation of an alternative GLUT4 sorting pathway as a compensatory mechanism for the loss of Sx16. In response to chronic insulin stimulation, Sx16 knockout adipocytes exhibit elevated insulin-independent glucose transport and significant alterations in lactate metabolism. We further show that the adipokine secretory pathways are impaired in Sx16 knockout cells. Together this demonstrates a role for Sx16 in the control of glucose transport, the response to elevated insulin, cellular metabolic profiles and adipocytokine secretion.

Vesicle trafficking and vesicle fusion: mechanisms, biological functions, and their implications for potential disease therapy

Intracellular vesicle trafficking is the fundamental process to maintain the homeostasis of membrane-enclosed organelles in eukaryotic cells. These organelles transport cargo from the donor membrane to the target membrane through the cargo containing vesicles. Vesicle trafficking pathway includes vesicle formation from the donor membrane, vesicle transport, and vesicle fusion with the target membrane. Coat protein mediated vesicle formation is a delicate membrane budding process for cargo molecules selection and package into vesicle carriers. Vesicle transport is a dynamic and specific process for the cargo containing vesicles translocation from the donor membrane to the target membrane. This process requires a group of conserved proteins such as Rab GTPases, motor adaptors, and motor proteins to ensure vesicle transport along cytoskeletal track. Soluble N-ethyl-maleimide-sensitive factor (NSF) attachment protein receptors (SNARE)-mediated vesicle fusion is the final process for vesicle unloading the cargo molecules at the target membrane. To ensure vesicle fusion occurring at a defined position and time pattern in eukaryotic cell, multiple fusogenic proteins, such as synaptotagmin (Syt), complexin (Cpx), Munc13, Munc18 and other tethering factors, cooperate together to precisely regulate the process of vesicle fusion. Dysfunctions of the fusogenic proteins in SNARE-mediated vesicle fusion are closely related to many diseases. Recent studies have suggested that stimulated membrane fusion can be manipulated pharmacologically via disruption the interface between the SNARE complex and Ca²⁺ sensor protein. Here, we summarize recent insights into the molecular mechanisms of vesicle trafficking, and implications for the development of new therapeutics based on the manipulation of vesicle fusion.

Biallelic loss-of-function variants in RABGAP1 cause a novel neurodevelopmental syndrome

PURPOSE

RABGAP1 is a GTPase-activating protein implicated in a variety of cellular and molecular processes, including mitosis, cell migration, vesicular trafficking, and mTOR signaling. There are no known Mendelian diseases caused by variants in RABGAP1.

METHODS

Through GeneMatcher, we identified 5 patients from 3 unrelated families with homozygous variants in the RABGAP1 gene found on exome sequencing. We established lymphoblastoid cells lines derived from an affected individual and her parents and performed RNA sequencing and functional studies. Rabgap1 knockout mice were generated and phenotyped.

RESULTS

We report 5 patients presenting with a common constellation of features, including global developmental delay/intellectual disability, microcephaly, bilateral sensorineural hearing loss, and seizures, as well as overlapping dysmorphic features. Neuroimaging revealed common features, including delayed myelination, white matter volume loss, ventriculomegaly, and thinning of the corpus callosum. Functional analysis of patient cells revealed downregulated mTOR signaling and abnormal localization of early endosomes and lysosomes. Rabgap1 knockout mice exhibited several features in common with the patient cohort, including microcephaly, thinning of the corpus callosum, and ventriculomegaly.

CONCLUSION

Collectively, our results provide evidence of a novel neurodevelopmental syndrome caused by biallelic loss-of-function variants in RABGAP1.

VAMP4 regulates insulin levels by targeting secretory granules to lysosomes

Insulin levels are essential for the maintenance of glucose homeostasis, and deviations lead to pathoglycemia or diabetes. However, the metabolic mechanism controlling insulin quantity and quality is poorly understood. In pancreatic β cells, insulin homeostasis and release are tightly governed by insulin secretory granule (ISG) trafficking, but the required regulators and mechanisms are largely unknown. Here, we identified that VAMP4 controlled the insulin levels in response to glucose challenge. VAMP4 deficiency led to increased blood insulin levels and hyperresponsiveness to glucose. In β cells, VAMP4 is packaged into immature ISGs (iISGs) at trans-Golgi networks and subsequently resorted to clathrin-coated vesicles during granule maturation. VAMP4-positive iISGs and resorted vesicles then fuse with lysosomes facilitated by a SNARE complex consisting of VAMP4, STX7, STX8, and VTI1B, which ensures the breakdown of excess (pro)insulin and obsolete materials and thus maintenance of intracellular insulin homeostasis. Thus, VAMP4 is a key factor regulating the insulin levels and a potential target for the treatment of diabetes.

C910 chemical compound inhibits the traffiking of several bacterial AB toxins with cross-protection against influenza virus

The development of anti-infectives against a large range of AB-like toxin-producing bacteria includes the identification of compounds disrupting toxin transport through both the endolysosomal and retrograde pathways. Here, we performed a high-throughput screening of compounds blocking Rac1 proteasomal degradation triggered by the Cytotoxic Necrotizing Factor-1 (CNF1) toxin, which was followed by orthogonal screens against two toxins that hijack the endolysosomal (diphtheria toxin) or retrograde (Shiga-like toxin 1) pathways to intoxicate cells. This led to the identification of the molecule C910 that induces the enlargement of EEA1-positive early endosomes associated with sorting defects of CNF1 and Shiga toxins to their trafficking pathways. C910 protects cells against eight bacterial AB toxins and the CNF1-mediated pathogenic Escherichia coli invasion. Interestingly, C910 reduces influenza A H1N1 and SARS-CoV-2 viral infection in vitro. Moreover, parenteral administration of C910 to mice resulted in its accumulation in lung tissues and a reduction in lethal influenza infection.

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Screen for inhibitors disrupting bacterial AB toxins vesicular trafficking pathways

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C910 affects EEA1/Rab5-positive early endosome morphology and sorting functions

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C910 protects cells against eight AB toxins, SARS-CoV-2 and influenza A virus

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C910 accumulates in lung tissues and protects mice against influenza A virus

Integrating intracellular nanovesicles into integrin trafficking pathways and beyond

Membrane traffic controls the movement of proteins and lipids from one cellular compartment to another using a system of transport vesicles. Intracellular nanovesicles (INVs) are a newly described class of transport vesicles. These vesicles are small, carry diverse cargo, and are involved in multiple trafficking steps including anterograde traffic and endosomal recycling. An example of a biological process that they control is cell migration and invasion, due to their role in integrin recycling. In this review, we describe what is known so far about these vesicles. We discuss how INVs may integrate into established membrane trafficking pathways using integrin recycling as an example. We speculate where in the cell INVs have the potential to operate and we identify key questions for future investigation.

Ferritinophagy and α-Synuclein: Pharmacological Targeting of Autophagy to Restore Iron Regulation in Parkinson's Disease

A major hallmark of Parkinson’s disease (PD) is the fatal destruction of dopaminergic neurons within the substantia nigra pars compacta. This event is preceded by the formation of Lewy bodies, which are cytoplasmic inclusions composed of α-synuclein protein aggregates. A triad contribution of α-synuclein aggregation, iron accumulation, and mitochondrial dysfunction plague nigral neurons, yet the events underlying iron accumulation are poorly understood. Elevated intracellular iron concentrations up-regulate ferritin expression, an iron storage protein that provides cytoprotection against redox stress. The lysosomal degradation pathway, autophagy, can release iron from ferritin stores to facilitate its trafficking in a process termed ferritinophagy. Aggregated α-synuclein inhibits SNARE protein complexes and destabilizes microtubules to halt vesicular trafficking systems, including that of autophagy effectively. The scope of this review is to describe the physiological and pathological relationship between iron regulation and α-synuclein, providing a detailed understanding of iron metabolism within nigral neurons. The underlying mechanisms of autophagy and ferritinophagy are explored in the context of PD, identifying potential therapeutic targets for future investigation.

Arabidopsis RAB8A, RAB8B and RAB8D Proteins Interact with Several RTNLB Proteins and are Involved in the Agrobacterium tumefaciens Infection Process

Arabidopsis thaliana small GTP-binding proteins, AtRAB8s, associate with the endomembrane system and modulate tubulovesicular trafficking between compartments of the biosynthetic and endocytic pathways. There are five members in Arabidopsis, namely AtRAB8A-8E. Yeast two-hybrid assays, bimolecular fluorescence complementation assays and glutathione-S-transferase pull-down assays showed that RAB8A, 8B and 8D interacted with several membrane-associated reticulon-like (AtRTNLB) proteins in yeast, plant cells and in vitro. Furthermore, RAB8A, 8B and 8D proteins showed interactions with the Agrobacterium tumefaciens virulence protein, VirB2, a component of a type IV secretion system (T4SS). A. tumefaciens uses a T4SS to transfer T-DNA and Virulence proteins to plants, which causes crown gall disease in plants. The Arabidopsis rab8A, rab8B and rab8D single mutants showed decreased levels of Agrobacterium-mediated root and seedling transformation, while the RAB8A, 8B and 8D overexpression transgenic Arabidopsis plants were hypersusceptible to A. tumefaciens and Pseudomonas syringae infections. RAB8A-8E transcripts accumulated differently in roots, rosette leaves, cauline leaves, inflorescence and flowers of wild-type plants. In summary, RAB8A, 8B and 8D interacted with several RTNLB proteins and participated in A. tumefaciens and P. syringae infection processes.

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

Congenital disorder of glycosylation caused by starting site-specific variant in syntaxin-5

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein syntaxin-5 (Stx5) is essential for Golgi transport. In humans, the STX5 mRNA encodes two protein isoforms, Stx5 Long (Stx5L) from the first starting methionine and Stx5 Short (Stx5S) from an alternative starting methionine at position 55. In this study, we identify a human disorder caused by a single missense substitution in the second starting methionine (p.M55V), resulting in complete loss of the short isoform. Patients suffer from an early fatal multisystem disease, including severe liver disease, skeletal abnormalities and abnormal glycosylation. Primary human dermal fibroblasts isolated from these patients show defective glycosylation, altered Golgi morphology as measured by electron microscopy, mislocalization of glycosyltransferases, and compromised ER-Golgi trafficking. Measurements of cognate binding SNAREs, based on biotin-synchronizable forms of Stx5 (the RUSH system) and Förster resonance energy transfer (FRET), revealed that the short isoform of Stx5 is essential for intra-Golgi transport. Alternative starting codons of Stx5 are thus linked to human disease, demonstrating that the site of translation initiation is an important new layer of regulating protein trafficking.

A Brucella effector modulates the Arf6-Rab8a GTPase cascade to promote intravacuolar replication

Remodeling of host cellular membrane transport pathways is a common pathogenic trait of many intracellular microbes that is essential to their intravacuolar life cycle and proliferation. The bacterium Brucella abortus generates a host endoplasmic reticulum‐derived vacuole (rBCV) that supports its intracellular growth, via VirB Type IV secretion system‐mediated delivery of effector proteins, whose functions and mode of action are mostly unknown. Here, we show that the effector BspF specifically promotes Brucella replication within rBCVs by interfering with vesicular transport between the trans‐Golgi network (TGN) and recycling endocytic compartment. BspF targeted the recycling endosome, inhibited retrograde traffic to the TGN, and interacted with the Arf6 GTPase‐activating Protein (GAP) ACAP1 to dysregulate Arf6‐/Rab8a‐dependent transport within the recycling endosome, which resulted in accretion of TGN‐associated vesicles by rBCVs and enhanced bacterial growth. Altogether, these findings provide mechanistic insight into bacterial modulation of membrane transport used to promote their own proliferation within intracellular vacuoles.

The vSNAREs VAMP2 and VAMP4 control recycling and intracellular sorting of post-synaptic receptors in neuronal dendrites

The endosomal recycling system dynamically tunes synaptic strength, which underlies synaptic plasticity. Exocytosis is involved in the expression of long-term potentiation (LTP), as postsynaptic cleavage of the SNARE (soluble NSF-attachment protein receptor) protein VAMP2 by tetanus toxin blocks LTP. Moreover, induction of LTP increases the exocytosis of transferrin receptors (TfRs) and markers of recycling endosomes (REs), as well as post-synaptic AMPA type receptors (AMPARs). However, the interplay between AMPAR and TfR exocytosis remains unclear. Here, we identify VAMP4 as the vesicular SNARE that mediates most dendritic RE exocytosis. In contrast, VAMP2 plays a minor role in RE exocytosis. LTP induction increases the exocytosis of both VAMP2- and VAMP4-labeled organelles. Knock down (KD) of VAMP4 decreases TfR recycling but increases AMPAR recycling. Moreover, VAMP4 KD increases AMPAR-mediated synaptic transmission, which consequently occludes LTP expression. The opposing changes in AMPAR and TfR recycling upon VAMP4 KD reveal their sorting into separate endosomal populations.

The Parkinson's Disease Protein LRRK2 Interacts with the GARP Complex to Promote Retrograde Transport to the trans-Golgi Network

Mutations in Leucine-rich repeat kinase 2 (LRRK2) cause Parkinson's disease (PD). However, the precise function of LRRK2 remains unclear. We report an interaction between LRRK2 and VPS52, a subunit of the Golgi-associated retrograde protein (GARP) complex that identifies a function of LRRK2 in regulating membrane fusion at the trans-Golgi network (TGN). At the TGN, LRRK2 further interacts with the Golgi SNAREs VAMP4 and Syntaxin-6 and acts as a scaffolding platform that stabilizes the GARP-SNAREs complex formation. Therefore, LRRK2 influences both retrograde and post-Golgi trafficking pathways in a manner dependent on its GTP binding and kinase activity. This action is exaggerated by mutations associated with Parkinson's disease and can be blocked by kinase inhibitors. Disruption of GARP sensitizes dopamine neurons to mutant LRRK2 toxicity in C. elegans, showing that these pathways are interlinked in vivo and suggesting a link in PD.

The Protein Toxins Ricin and Shiga Toxin as Tools to Explore Cellular Mechanisms of Internalization and Intracellular Transport

Protein toxins secreted by bacteria and found in plants can be threats to human health. However, their extreme toxicity can also be exploited in different ways, e.g., to produce hybrid toxins directed against cancer cells and to study transport mechanisms in cells. Investigations during the last decades have shown how powerful these molecules are as tools in cell biological research. Here, we first present a partly historical overview, with emphasis on Shiga toxin and ricin, of how such toxins have been used to characterize processes and proteins of importance for their trafficking. In the second half of the article, we describe how one can now use toxins to investigate the role of lipid classes for intracellular transport. In recent years, it has become possible to quantify hundreds of lipid species using mass spectrometry analysis. Thus, it is also now possible to explore the importance of lipid species in intracellular transport. The detailed analyses of changes in lipids seen under conditions of inhibited toxin transport reveal previously unknown connections between syntheses of lipid classes and demonstrate the ability of cells to compensate under given conditions.

Data mining for traffic information

Modern cell biology is now rich with data acquired at the whole genome and proteome level. We can add value to this data through integration and application of specialist knowledge. To illustrate, we will focus on the SNARE and RAB proteins; key regulators of intracellular fusion specificity and organelle identity. We examine published mass spectrometry data to gain an estimate of protein copy number and organelle distribution in HeLa cells for each family member. We also survey recent global CRISPR/Cas9 screens for essential genes from these families. We highlight instances of co-essentiality with other genes across a large panel of cell lines that allows for the identification of functionally coherent clusters. Examples of such correlations include RAB10 with the SNARE protein Syntaxin4 (STX4) and RAB7/RAB21 with the WASH and the CCC (COMMD/CCDC22/CCDC93) complexes, both of which are linked to endosomal recycling pathways.

The fungal RABOME: RAB GTPases acting in the endocytic and exocytic pathways of Aspergillus nidulans (with excursions to other filamentous fungi)

RAB GTPases are major determinants of membrane identity that have been exploited as highly specific reporters to study intracellular traffic in vivo. A score of fungal papers have considered individual RABs, but systematic, integrated studies on the localization and physiological role of these regulators and their effectors have been performed only with Aspergillus nidulans. These studies have influenced the intracellular trafficking field beyond fungal specialists, leading to findings such as the maturation of trans-Golgi (TGN) cisternae into post-Golgi RAB11 secretory vesicles, the concept that these RAB11 secretory carriers are loaded with three molecular nanomotors, the understanding of the role of endocytic recycling mediated by RAB6 and RAB11 in determining the hyphal mode of life, the discovery that early endosome maturation and the ESCRT pathway are essential, the identification of specific adaptors of dynein-dynactin to RAB5 endosomes, the exquisite dependence that autophagy displays on RAB1 activity, the role of TRAPPII as a GEF for RAB11, or the conclusion that the RAB1-to-RAB11 transition is not mediated by TRAPP maturation. A remarkable finding was that the A. nidulans Spitzenkörper contains four RABs: RAB11, Sec4, RAB6, and RAB1. How these RABs cooperate during exocytosis represents an as yet outstanding question.

Control of synaptic vesicle release probability via VAMP4 targeting to endolysosomes

Synaptic vesicle (SV) release probability (Pr), determines the steady state and plastic control of neurotransmitter release. However, how diversity in SV composition arises and regulates the Pr of individual SVs is not understood. We found that modulation of the copy number of the noncanonical vesicular SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor), vesicle-associated membrane protein 4 (VAMP4), on SVs is key for regulating Pr. Mechanistically, this is underpinned by its reduced ability to form an efficient SNARE complex with canonical plasma membrane SNAREs. VAMP4 has unusually high synaptic turnover and is selectively sorted to endolysosomes during activity-dependent bulk endocytosis. Disruption of endolysosomal trafficking and function markedly increased the abundance of VAMP4 in the SV pool and inhibited SV fusion. Together, our results unravel a new mechanism for generating SV heterogeneity and control of Pr through coupling of SV recycling to a major clearing system that regulates protein homeostasis.

Ablation of Vti1a/1b Triggers Neural Progenitor Pool Depletion and Cortical Layer 5 Malformation in Late-embryonic Mouse Cortex

Cortical morphogenesis entails several neurobiological events, including proliferation and differentiation of progenitors, migration of neuroblasts, and neuronal maturation leading to functional neural circuitry. These neurodevelopmental processes are delicately regulated by many factors. Endosomal SNAREs have emerged as formidable modulators of neuronal growth, aside their well-known function in membrane/vesicular trafficking. However, our understanding of their influence on cortex formation is limited. Here, we report that the SNAREs Vti1a and Vti1b (Vti1a/1b) are critical for proper cortical development. Following null mutation of Vti1a/1b in mouse, the late-embryonic mutant cortex appeared dysgenic, and the cortical progenitors therein were depleted beyond normal. Notably, cortical layer 5 (L5) is distinctively disorganized in the absence of Vti1a/1b. The latter defect, coupled with an overt apoptosis of Ctip2-expressing L5 neurons, likely contributed to the substantial loss of corticospinal and callosal projections in the Vti1a/1b-deficient mouse brain. These findings suggest that Vti1a/1b serve key neurodevelopmental functions during cortical histogenesis, which when mechanistically elucidated, can lend clarity to how endosomal SNAREs regulate brain development, or how their dysfunction may have implications for neurological disorders.

Eukaryotic SNARE VAMP3 Dynamically Interacts with Multiple Chlamydial Inclusion Membrane Proteins

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs.

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs. As with all chlamydial proteins, Incs are temporally expressed, modifying the chlamydial inclusion during the early and mid-developmental cycle. VAMP3 and VAMP4 are eukaryotic SNARE proteins that mediate membrane fusion and are recruited to the inclusion to facilitate inclusion expansion. Their recruitment requires de novo chlamydial protein synthesis during the mid-developmental cycle. Thus, we hypothesize that VAMP3 and VAMP4 are recruited by Incs. In chlamydia-infected cells, identifying Inc binding partners for SNARE proteins specifically has been elusive. To date, most studies examining chlamydial Inc and eukaryotic proteins have benefitted from stable interacting partners or a robust interaction at a specific time postinfection. While these types of interactions are the predominant class that have been identified, they are likely the exception to chlamydia-host interactions. Therefore, we applied two separate but complementary experimental systems to identify candidate chlamydial Inc binding partners for VAMPs. Based on these results, we created transformed strains of C. trachomatis serovar L2 to inducibly express a candidate Inc-FLAG protein. In chlamydia-infected cells, we found that five Incs temporally and transiently interact with VAMP3. Further, loss of incA or ct813 expression altered VAMP3 localization to the inclusion. For the first time, our studies demonstrate the transient nature of certain host protein-Inc interactions that contribute to the chlamydial developmental cycle.

Loss of MUNC18-1 leads to retrograde transport defects in neurons

Loss of the exocytic Sec1/MUNC18 protein MUNC18-1 or its target-SNARE partners SNAP25 and syntaxin-1 results in rapid, cell-autonomous and unexplained neurodegeneration, which is independent of their known role in synaptic vesicle exocytosis. cis-Golgi abnormalities are the earliest cellular phenotypes before degeneration occurs. Here, we investigated whether loss of MUNC18-1 causes defects in intracellular membrane transport pathways in primary murine neurons that may explain neurodegeneration. Electron, confocal and super resolution microscopy confirmed that loss of MUNC18-1 expression results in a smaller cis-Golgi. In addition, we now show that medial-Golgi and the trans-Golgi Network are also affected. However, stacking and cisternae ultrastructure of the Golgi were normal. Overall, ultrastructure of null mutant neurons was remarkably normal just hours before cell death occurred. By synchronizing protein trafficking by conditional cargo retention in the endoplasmic reticulum using selective hooks (RUSH) and immunocytochemistry, we show that anterograde Endoplasmic Reticulum-to-Golgi and Golgi exit of endogenous and exogenous proteins were normal. In contrast, loss of MUNC18-1 caused reduced retrograde Cholera Toxin B-subunit transport from the plasma membrane to the Golgi. In addition, MUNC18-1-deficiency resulted in abnormalities in retrograde TrkB trafficking in an antibody uptake assay. We conclude that MUNC18-1 deficient neurons have normal anterograde but reduced retrograde transport to the Golgi. The impairments in retrograde pathways suggest a role of MUNC18-1 in endosomal SNARE-dependent fusion and provide a plausible explanation for the observed Golgi abnormalities and cell death in MUNC18-1 deficient neurons.

Herpes Simplex Virus Entry by a Nonconventional Endocytic Pathway

Herpes simplex virus 1 (HSV-1) causes significant morbidity and mortality in humans worldwide. HSV-1 enters epithelial cells via an endocytosis mechanism that is low-pH dependent. However, the precise intracellular pathway has not been identified, including the compartment where fusion occurs. In this study, we utilized a combination of molecular and pharmacological approaches to better characterize HSV entry by endocytosis. HSV-1 entry was unaltered in both cells treated with small interfering RNA (siRNA) to Rab5 or Rab7 and cells expressing dominant negative forms of these GTPases, suggesting entry is independent of the conventional endo-lysosomal network. The fungal metabolite brefeldin A (BFA) and the quinoline compound Golgicide A (GCA) inhibited HSV-1 entry via beta-galactosidase reporter assay and impaired incoming virus transport to the nuclear periphery, suggesting a role for trans-Golgi network (TGN) functions and retrograde transport in HSV entry. Silencing of Rab9 or Rab11 GTPases, which are involved in the retrograde transport pathway, resulted in only a slight reduction in HSV infection. Together, these results suggest that HSV enters host cells by an intracellular route independent of the lysosome-terminal endocytic pathway.IMPORTANCE Herpes simplex virus 1 (HSV-1), the prototype alphaherpesvirus, is ubiquitous in the human population and causes lifelong infection that can be fatal in neonatal and immunocompromised individuals. HSV enters many cell types by endocytosis, including epithelial cells, the site of primary infection in the host. The intracellular itinerary for HSV entry remains unclear. We probed the potential involvement of several Rab GTPases in HSV-1 entry and suggest that endocytic entry of HSV-1 is independent of the canonical lysosome-terminal pathway. A nontraditional endocytic route may be employed, such as one that intersects with the trans-Golgi network (TGN). These results may lead to novel targets for intervention.

The ZIP Code of Vesicle Trafficking in Apicomplexa: SEC1/Munc18 and SNARE Proteins

Apicomplexans are obligate intracellular parasites harboring three sets of unique secretory organelles termed micronemes, rhoptries, and dense granules that are dedicated to the establishment of infection in the host cell. Apicomplexans rely on the endolysosomal system to generate the secretory organelles and to ingest and digest host cell proteins. These parasites also possess a metabolically relevant secondary endosymbiotic organelle, the apicoplast, which relies on vesicular trafficking for correct incorporation of nuclear-encoded proteins into the organelle. Here, we demonstrate that the trafficking and destination of vesicles to the unique and specialized parasite compartments depend on SNARE proteins that interact with tethering factors. Specifically, all secreted proteins depend on the function of SLY1 at the Golgi. In addition to a critical role in trafficking of endocytosed host proteins, TgVps45 is implicated in the biogenesis of the inner membrane complex (alveoli) in both Toxoplasma gondii and Plasmodium falciparum, likely acting in a coordinated manner with Stx16 and Stx6. Finally, Stx12 localizes to the endosomal-like compartment and is involved in the trafficking of proteins to the apical secretory organelles rhoptries and micronemes as well as to the apicoplast.IMPORTANCE The phylum of Apicomplexa groups medically relevant parasites such as those responsible for malaria and toxoplasmosis. As members of the Alveolata superphylum, these protozoans possess specialized organelles in addition to those found in all members of the eukaryotic kingdom. Vesicular trafficking is the major route of communication between membranous organelles. Neither the molecular mechanism that allows communication between organelles nor the vesicular fusion events that underlie it are completely understood in Apicomplexa. Here, we assessed the function of SEC1/Munc18 and SNARE proteins to identify factors involved in the trafficking of vesicles between these various organelles. We show that SEC1/Munc18 in interaction with SNARE proteins allows targeting of vesicles to the inner membrane complex, prerhoptries, micronemes, apicoplast, and vacuolar compartment from the endoplasmic reticulum, Golgi apparatus, or endosomal-like compartment. These data provide an exciting look at the "ZIP code" of vesicular trafficking in apicomplexans, essential for precise organelle biogenesis, homeostasis, and inheritance.

Machine learning discriminates a movement disorder in a zebrafish model of Parkinson's disease

Animal models of human disease provide an in vivo system that can reveal molecular mechanisms by which mutations cause pathology, and, moreover, have the potential to provide a valuable tool for drug development. Here, we have developed a zebrafish model of Parkinson's disease (PD) together with a novel method to screen for movement disorders in adult fish, pioneering a more efficient drug-testing route. Mutation of the PARK7 gene (which encodes DJ-1) is known to cause monogenic autosomal recessive PD in humans, and, using CRISPR/Cas9 gene editing, we generated a Dj-1 loss-of-function zebrafish with molecular hallmarks of PD. To establish whether there is a human-relevant parkinsonian phenotype in our model, we adapted proven tools used to diagnose PD in clinics and developed a novel and unbiased computational method to classify movement disorders in adult zebrafish. Using high-resolution video capture and machine learning, we extracted novel features of movement from continuous data streams and used an evolutionary algorithm to classify parkinsonian fish. This method will be widely applicable for assessing zebrafish models of human motor diseases and provide a valuable asset for the therapeutics pipeline. In addition, interrogation of RNA-seq data indicate metabolic reprogramming of brains in the absence of Dj-1, adding to growing evidence that disruption of bioenergetics is a key feature of neurodegeneration.This article has an associated First Person interview with the first author of the paper.

SNAREs and developmental disorders

Members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediate membrane fusion processes associated with vesicular trafficking and autophagy. SNAREs mediate core membrane fusion processes essential for all cells, but some SNAREs serve cell/tissue type-specific exocytic/endocytic functions, and are therefore critical for various aspects of embryonic development. Mutations or variants of their encoding genes could give rise to developmental disorders, such as those affecting the nervous system and immune system in humans. Mutations to components in the canonical synaptic vesicle fusion SNARE complex (VAMP2, STX1A/B, and SNAP25) and a key regulator of SNARE complex formation MUNC18-1, produce variant phenotypes of autism, intellectual disability, movement disorders, and epilepsy. STX11 and MUNC18-2 mutations underlie 2 subtypes of familial hemophagocytic lymphohistiocytosis. STX3 mutations contribute to variant microvillus inclusion disease. Chromosomal microdeletions involving STX16 play a role in pseudohypoparathyroidism type IB associated with abnormal imprinting of the GNAS complex locus. In this short review, I discuss these and other SNARE gene mutations and variants that are known to be associated with a variety developmental disorders, with a focus on their underlying cellular and molecular pathological basis deciphered through disease modeling. Possible pathogenic potentials of other SNAREs whose variants could be disease predisposing are also speculated upon.

Role of the Sec22b-E-Syt complex in neurite growth and ramification

Axons and dendrites are long and often ramified neurites that need particularly intense plasma membrane (PM) expansion during the development of the nervous system. Neurite growth depends on non-fusogenic Sec22b-Stx1 SNARE complexes at endoplasmic reticulum (ER)-PM contacts. Here, we show that Sec22b interacts with members of the extended synaptotagmin (E-Syt) family of ER lipid transfer proteins (LTPs), and this interaction depends on the longin domain of Sec22b. Overexpression of E-Syts stabilizes Sec22b-Stx1 association, whereas silencing of E-Syts has the opposite effect. Overexpression of wild-type E-Syt2, but not mutants unable to transfer lipids or attach to the ER, increase the formation of axonal filopodia and ramification of neurites in developing neurons. This effect is inhibited by a clostridial neurotoxin cleaving Stx1, and expression of the Sec22b longin domain and a Sec22b mutant with an extended linker between the SNARE and transmembrane domains. We conclude that Sec22b-Stx1 ER-PM contact sites contribute to PM expansion by interacting with LTPs, such as E-Syts.This article has an associated First Person interview with the first author of the paper.

Megakaryocytes package contents into separate α-granules that are differentially distributed in platelets

In addition to their primary roles in hemostasis and thrombosis, platelets participate in many other physiological and pathological processes, including, but not limited to inflammation, wound healing, tumor metastasis, and angiogenesis. Among their most interesting properties is the large number of bioactive proteins stored in their α-granules, the major storage granule of platelets. We previously showed that platelets differentially package pro- and antiangiogenic proteins in distinct α-granules that undergo differential release upon platelet activation. Nevertheless, how megakaryocytes achieve differential packaging is not fully understood. In this study, we use a mouse megakaryocyte culture system and endocytosis assay to establish when and where differential packaging occurs during platelet production. Live cell microscopy of primary mouse megakaryocytes incubated with fluorescently conjugated fibrinogen and endostatin showed differential endocytosis and packaging of the labeled proteins into distinct α-granule subpopulations. Super-resolution microscopy of mouse proplatelets and human whole-blood platelet α-granules simultaneously probed for 2 different membrane proteins (VAMP-3 and VAMP-8), and multiple granular content proteins (bFGF, ENDO, TSP, VEGF) confirmed differential packaging of protein contents into α-granules. These data suggest that megakaryocytes differentially sort and package α-granule contents, which are preserved as α-granule subpopulations during proplatelet extension and platelet production.

Vesicle transport through interaction with t-SNAREs 1a (Vti1a)'s roles in neurons

The Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family mediates membrane fusion during membrane trafficking and autophagy in all eukaryotic cells, with a number of SNAREs having cell type-specific functions. The endosome-trans-Golgi network (TGN) localized SNARE, Vesicle transport through interaction with t-SNAREs 1A (Vti1a), is unique among SNAREs in that it has numerous neuron-specific functions. These include neurite outgrowth, nervous system development, spontaneous neurotransmission, synaptic vesicle and dense core vesicle secretion, as well as a process of unconventional surface transport of the Kv4 potassium channel. Furthermore, the human VT11A gene is known to form fusion products with neighboring genes in cancer tissues, and VT11A variants are associated with risk in cancers, including glioma. In this review, I highlight VTI1A's known physio-pathological roles in brain neurons, as well as unanswered questions in these regards.

A trafficome-wide RNAi screen reveals deployment of early and late secretory host proteins and the entire late endo-/lysosomal vesicle fusion machinery by intracellular Salmonella

The intracellular lifestyle of Salmonella enterica is characterized by the formation of a replication-permissive membrane-bound niche, the Salmonella-containing vacuole (SCV). As a further consequence of the massive remodeling of the host cell endosomal system, intracellular Salmonella establish a unique network of various Salmonella-induced tubules (SIT). The bacterial repertoire of effector proteins required for the establishment for one type of these SIT, the Salmonella-induced filaments (SIF), is rather well-defined. However, the corresponding host cell proteins are still poorly understood. To identify host factors required for the formation of SIF, we performed a sub-genomic RNAi screen. The analyses comprised high-resolution live cell imaging to score effects on SIF induction, dynamics and morphology. The hits of our functional RNAi screen comprise: i) The late endo-/lysosomal SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, consisting of STX7, STX8, VTI1B, and VAMP7 or VAMP8, which is, in conjunction with RAB7 and the homotypic fusion and protein sorting (HOPS) tethering complex, a complete vesicle fusion machinery. ii) Novel interactions with the early secretory GTPases RAB1A and RAB1B, providing a potential link to coat protein complex I (COPI) vesicles and reinforcing recently identified ties to the endoplasmic reticulum. iii) New connections to the late secretory pathway and/or the recycling endosome via the GTPases RAB3A, RAB8A, and RAB8B and the SNAREs VAMP2, VAMP3, and VAMP4. iv) An unprecedented involvement of clathrin-coated structures. The resulting set of hits allowed us to characterize completely new host factor interactions, and to strengthen observations from several previous studies.

The facultative intracellular pathogen Salmonella enterica serovar Typhimurium induces the reorganization of the endosomal system of mammalian host cells. This activity is dependent on translocated effector proteins of the pathogen. The host cell factors required for endosomal remodeling are only partially known. To identify such factors for the formation and dynamics of endosomal compartments in Salmonella-infected cells, we performed a live cell imaging-based RNAi screen to investigate the role of 496 mammalian proteins involved in cellular logistics. We identified that endosomal remodeling by intracellular Salmonella is dependent on host factors in the following functional classes: i) the late endo-/lysosomal SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, ii) the early secretory pathway, represented by regulator GTPases RAB1A and RAB1B, iii) the late secretory pathway and/or recycling endosomes represented by GTPases RAB3A, RAB8A, RAB8B, and the SNAREs VAMP2, VAMP3, and VAMP4, and iv) clathrin-coated structures. The identification of these new host factors provides further evidence for the complex manipulation of host cell transport functions by intracellular Salmonella and should enable detailed follow-up studies on the mechanisms involved.

Tetanus insensitive VAMP2 differentially restores synaptic and dense core vesicle fusion in tetanus neurotoxin treated neurons

The SNARE proteins involved in the secretion of neuromodulators from dense core vesicles (DCVs) in mammalian neurons are still poorly characterized. Here we use tetanus neurotoxin (TeNT) light chain, which cleaves VAMP1, 2 and 3, to study DCV fusion in hippocampal neurons and compare the effects on DCV fusion to those on synaptic vesicle (SV) fusion. Both DCV and SV fusion were abolished upon TeNT expression. Expression of tetanus insensitive (TI)-VAMP2 restored SV fusion in the presence of TeNT, but not DCV fusion. Expression of TI-VAMP1 or TI-VAMP3 also failed to restore DCV fusion. Co-transport assays revealed that both TI-VAMP1 and TI-VAMP2 are targeted to DCVs and travel together with DCVs in neurons. Furthermore, expression of the TeNT-cleaved VAMP2 fragment or a protease defective TeNT in wild type neurons did not affect DCV fusion and therefore cannot explain the lack of rescue of DCV fusion by TI-VAMP2. Finally, to test if two different VAMPs might both be required in the DCV secretory pathway, Vamp1 null mutants were tested. However, VAMP1 deficiency did not reduce DCV fusion. In conclusion, TeNT treatment combined with TI-VAMP2 expression differentially affects the two main regulated secretory pathways: while SV fusion is normal, DCV fusion is absent.

Loss of Rab6a in the small intestine causes lipid accumulation and epithelial cell death from lactation

Intestinal epithelial cells (IECs) are not only responsible for the digestion and absorption of dietary substrates but also function as a first line of host defense against commensal and pathogenic luminal bacteria. Disruption of the epithelial layer causes malnutrition and enteritis. Rab6 is a small GTPase localized to the Golgi, where it regulates anterograde and retrograde transport by interacting with various effector proteins. Here, we generated mice with IEC-specific deletion of Rab6a (Rab6a^∆IEC mice). While Rab6a^ΔIEC mice were born at the Mendelian ratio, they started to show IEC death, inflammation, and bleeding in the small intestine shortly after birth, and these changes culminated in early postnatal death. We further found massive lipid accumulation in the IECs of Rab6a^∆IEC neonates. In contrast to Rab6a^∆IEC neonates, knockout embryos did not show any of these abnormalities. Lipid accumulation and IEC death became evident when Rab6a^∆IEC embryos were nursed by a foster mother, suggesting that dietary milk-derived lipids accumulated in Rab6a-deficient IECs and triggered IEC death. These results indicate that Rab6a plays a crucial role in regulating the lipid transport and maintaining tissue integrity.

Diacylglycerol kinase and phospholipase D inhibitors alter the cellular lipidome and endosomal sorting towards the Golgi apparatus

The membrane lipids diacylglycerol (DAG) and phosphatidic acid (PA) are important second messengers that can regulate membrane transport by recruiting proteins to the membrane and by altering biophysical membrane properties. DAG and PA are involved in the transport from the Golgi apparatus to endosomes, and we have here investigated whether changes in these lipids might be important for regulation of transport to the Golgi using the protein toxin ricin. Modulation of DAG and PA levels using DAG kinase (DGK) and phospholipase D (PLD) inhibitors gave a strong increase in retrograde ricin transport, but had little impact on ricin recycling or degradation. Inhibitor treatment strongly affected the endosome morphology, increasing endosomal tubulation and size. Furthermore, ricin was present in these tubular structures together with proteins known to regulate retrograde transport. Using siRNA to knock down different isoforms of PLD and DGK, we found that several isoforms of PLD and DGK are involved in regulating ricin transport to the Golgi. Finally, by performing lipidomic analysis we found that the DGK inhibitor gave a weak, but expected, increase in DAG levels, while the PLD inhibitor gave a strong and unexpected increase in DAG levels, showing that it is important to perform lipidomic analysis when using inhibitors of lipid metabolism.