Compartmental specificity of cellular membrane fusion encoded in SNARE proteins

Cotton SNARE complex component GhSYP121 regulates salicylic acid signaling during defense against Verticillium dahliae

Syntaxin of plant (SYP) plays a crucial role in SNARE-mediated membrane trafficking during endocytic and secretory pathways, contributing to the regulation and execution of plant immunity against pathogens. Verticillium wilt is among the most destructive fungal diseases affecting cotton worldwide. However, information regarding SYP family genes in cotton is scarce. Through genome-wide identification and transcriptome profiling, we identified GhSYP121, a Qa SNARE gene in Gossypium hirsutum. GhSYP121 is notably induced by Verticillium dahliae, the causal agent of Verticillium wilt in cotton, and acts as a negative regulator of defense against V. dahliae. This is evidenced by the reduced resistance of GhSYP121-deficient cotton and the increased susceptibility of GhSYP121-overexpressing lines. Furthermore, the activation of the salicylic acid (SA) pathway by V. dahliae is inversely correlated with the expression level of GhSYP121. GhSYP121 interacts with its cognate SNARE component, GhSNAP33, which is required for the penetration resistance against V. dahliae in cotton. Collectively, GhSYP121, as a member of the cotton SNARE complex, is involved in regulating the SA pathway during plant defense against V. dahliae. This finding enhances our understanding of the potential role of GhSYP121 in these distinct pathways that contribute to plant defense against V. dahliae infection.

Specialized contact sites regulate the fusion of chlamydial inclusion membranes

The intracellular bacterial pathogen Chlamydia trachomatis replicates within a membrane-bound compartment called the inclusion. Upon infection with several chlamydiae, each bacterium creates its own inclusion, resulting in multiple inclusions within each host cell. Ultimately, these inclusions fuse together in a process that requires the chlamydial protein IncA. Here, we show that inclusions form unique contact sites (inclusion contact sites, ICSs) prior to fusion, that serve as fusogenic platforms in which specific lipids and chlamydial proteins concentrate. Fusion depends on IncA clustering within ICSs and is regulated by PI(3,4)P2 and sphingolipids. As IncA concentrates within ICSs, its C-terminus likely interacts in trans with IncA on the apposing membrane, securing a high concentration of IncA at fusion sites. This regulatory mechanism contrasts with eukaryotic or viral fusion systems that are either composed of multiple proteins or use a change in pH to initiate membrane fusion. Thus, our study demonstrates that Chlamydia-mediated membrane fusion is primarily regulated by specific structural domains in IncA and its local organization on the inclusion membrane, which is affected by the host cell lipid composition.

A look beyond the QR code of SNARE proteins

Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor (SNARE) proteins catalyze the fusion process of vesicles with target membranes in eukaryotic cells. To do this, they assemble in a zipper-like fashion into stable complexes between the membranes. Structural studies have shown that the complexes consist of four different helices, which we subdivide into Qa-, Qb-, Qc-, and R-helix on the basis of their sequence signatures. Using a combination of biochemistry, modeling and molecular dynamics, we investigated how the four different types are arranged in a complex. We found that there is a matching pattern in the core of the complex that dictates the position of the four fundamental SNARE types in the bundle, resulting in a QabcR complex. In the cell, several different cognate QabcR-SNARE complexes catalyze the different transport steps between the compartments of the endomembrane system. Each of these cognate QabcR complexes is compiled from a repertoire of about 20 SNARE subtypes. Our studies show that exchange within the four types is largely tolerated structurally, although some non-cognate exchanges lead to structural imbalances. This suggests that SNARE complexes have evolved for a catalytic mechanism, a mechanism that leaves little scope for selectivity beyond the QabcR rule.

Differential SNARE chaperoning by Munc13-1 and Munc18-1 dictates fusion pore fate at the release site

The regulated release of chemical messengers is crucial for cell-to-cell communication; abnormalities in which impact coordinated human body function. During vesicular secretion, multiple SNARE complexes assemble at the release site, leading to fusion pore opening. How membrane fusion regulators act on heterogeneous SNARE populations to assemble fusion pores in a timely and synchronized manner, is unknown. Here, we demonstrate the role of SNARE chaperones Munc13-1 and Munc18-1 in rescuing individual nascent fusion pores from their diacylglycerol lipid-mediated inhibitory states. At the onset of membrane fusion, Munc13-1 clusters multiple SNARE complexes at the release site and synchronizes release events, while Munc18-1 stoichiometrically interacts with trans-SNARE complexes to enhance N- to C-terminal zippering. When both Munc proteins are present simultaneously, they differentially access dynamic trans-SNARE complexes to regulate pore properties. Overall, Munc proteins' direct action on fusion pore assembly indicates their role in controlling quantal size during vesicular secretion.

Ykt6 functionally overlaps with vacuolar and exocytic R-SNAREs in the yeast Saccharomyces cerevisiae

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex forms a 4-helix coiled-coil bundle consisting of 16 layers of interacting side chains upon membrane fusion. The central layer (layer 0) is highly conserved and comprises three glutamines (Q) and one arginine (R), and thus SNAREs are classified into Qa-, Qb-, Qc-, and R-SNAREs. Homotypic vacuolar fusion in Saccharomyces cerevisiae requires the SNAREs Vam3 (Qa), Vti1 (Qb), Vam7 (Qc), and Nyv1 (R). However, the yeast strain lacking NYV1 (nyv1Δ) shows no vacuole fragmentation, whereas the vam3Δ and vam7Δ strains display fragmented vacuoles. Here, we provide genetic evidence that the R-SNAREs Ykt6 and Nyv1 are functionally redundant in vacuole homotypic fusion in vivo using a newly isolated ykt6 mutant. We observed the ykt6-104 mutant showed no defect in vacuole morphology, but the ykt6-104 nyv1Δ double mutant had highly fragmented vacuoles. Furthermore, we show the defect in homotypic vacuole fusion caused by the vam7-Q284R mutation was compensated by the nyv1-R192Q or ykt6-R165Q mutations, which maintained the 3Q:1R ratio in the layer 0 of the SNARE complex, indicating that Nyv1 is exchangeable with Ykt6 in the vacuole SNARE complex. Unexpectedly, we found Ykt6 assembled with exocytic Q-SNAREs when the intrinsic exocytic R-SNAREs Snc1 and its paralog Snc2 lose their ability to assemble into the exocytic SNARE complex. These results suggest that Ykt6 may serve as a backup when other R-SNAREs become dysfunctional and that this flexible assembly of SNARE complexes may help cells maintain the robustness of the vesicular transport network.

SNAREs: a double-edged sword for intravacuolar bacterial pathogens within host cells

In the tug-of-war between host and pathogen, both evolve to combat each other's defence arsenals. Intracellular phagosomal bacteria have developed strategies to modify the vacuolar niche to suit their requirements best. Conversely, the host tries to target the pathogen-containing vacuoles towards the degradative pathways. The host cells use a robust system through intracellular trafficking to maintain homeostasis inside the cellular milieu. In parallel, intracellular bacterial pathogens have coevolved with the host to harbour strategies to manipulate cellular pathways, organelles, and cargoes, facilitating the conversion of the phagosome into a modified pathogen-containing vacuole (PCV). Key molecular regulators of intracellular traffic, such as changes in the organelle (phospholipid) composition, recruitment of small GTPases and associated effectors, soluble N-ethylmaleimide-sensitive factor-activating protein receptors (SNAREs), etc., are hijacked to evade lysosomal degradation. Legionella, Salmonella, Coxiella, Chlamydia, Mycobacterium, and Brucella are examples of pathogens which diverge from the endocytic pathway by using effector-mediated mechanisms to overcome the challenges and establish their intracellular niches. These pathogens extensively utilise and modulate the end processes of secretory pathways, particularly SNAREs, in repurposing the PCV into specialised compartments resembling the host organelles within the secretory network; at the same time, they avoid being degraded by the host's cellular mechanisms. Here, we discuss the recent research advances on the host-pathogen interaction/crosstalk that involves host SNAREs, conserved cellular processes, and the ongoing host-pathogen defence mechanisms in the molecular arms race against each other. The current knowledge of SNAREs, and intravacuolar bacterial pathogen interactions, enables us to understand host cellular innate immune pathways, maintenance of homeostasis, and potential therapeutic strategies to combat ever-growing antimicrobial resistance.

The role of insect gut microbiota in host fitness, detoxification and nutrient supplementation

Insects are incredibly diverse, ubiquitous and have successfully flourished out of the dynamic and often unpredictable nature of evolutionary processes. The resident microbiome has accompanied the physical and biological adaptations that enable their continued survival and proliferation in a wide array of environments. The host insect and microbiome's bidirectional relationship exhibits their capability to influence each other's physiology, behavior and characteristics. Insects are reported to rely directly on the microbial community to break down complex food, adapt to nutrient-deficit environments, protect themselves from natural adversaries and control the expression of social behavior. High-throughput metagenomic approaches have enhanced the potential for determining the abundance, composition, diversity and functional activities of microbial fauna associated with insect hosts, enabling in-depth investigation into insect-microbe interactions. We undertook a review of some of the major advances in the field of metagenomics, focusing on insect-microbe interaction, diversity and composition of resident microbiota, the functional capability of endosymbionts and discussions on different symbiotic relationships. The review aims to be a valuable resource on insect gut symbiotic microbiota by providing a comprehensive understanding of how insect gut symbionts systematically perform a range of functions, viz., insecticide degradation, nutritional support and immune fitness. A thorough understanding of manipulating specific gut symbionts may aid in developing advanced insect-associated research to attain health and design strategies for pest management.

Some Examples of Bacterial Toxins as Tools

Pathogenic bacteria produce diverse protein toxins to disturb the host's defenses. This includes the opening of epithelial barriers to establish bacterial growth in deeper tissues of the host and to modulate immune cell functions. To achieve this, many toxins share the ability to enter mammalian cells, where they catalyze the modification of cellular proteins. The enzymatic activity is diverse and ranges from ribosyl- or glycosyl-transferase activity, the deamidation of proteins, and adenylate-cyclase activity to proteolytic cleavage. Protein toxins are highly active enzymes often with tight specificity for an intracellular protein or a protein family coupled with the intrinsic capability of entering mammalian cells. A broad understanding of their molecular mechanisms established bacterial toxins as powerful tools for cell biology. Both the enzymatic part and the pore-forming/protein transport capacity are currently used as tools engineered to study signaling pathways or to transport cargo like labeled compounds, nucleic acids, peptides, or proteins directly into the cytosol. Using several representative examples, this review is intended to provide a short overview of the state of the art in the use of bacterial toxins or parts thereof as tools.

Synaptobrevin2 monomers and dimers differentially engage to regulate the functional trans-SNARE assembly

The precise cell-to-cell communication relies on SNARE-catalyzed membrane fusion. Among ∼70 copies of synaptobrevin2 (syb2) in synaptic vesicles, only ∼3 copies are sufficient to facilitate the fusion process at the presynaptic terminal. It is unclear what dictates the number of SNARE complexes that constitute the fusion pore assembly. The structure-function relation of these dynamic pores is also unknown. Here, we demonstrate that syb2 monomers and dimers differentially engage in regulating the trans-SNARE assembly during membrane fusion. The differential recruitment of two syb2 structures at the membrane fusion site has consequences in regulating individual nascent fusion pore properties. We have identified a few syb2 transmembrane domain residues that control monomer/dimer conversion. Overall, our study indicates that syb2 monomers and dimers are differentially recruited at the release sites for regulating membrane fusion events.

The SNARE complex formed by RIC-4/SEC-22/SYX-2 promotes C. elegans epidermal wound healing

Maintaining cellular viability relies on the integrity of the plasma membrane, which must be repaired upon damage. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion is a crucial mechanism involved in membrane repair. In C. elegans epidermal cell hyp 7, syntaxin-2 (SYX-2) facilitates large membrane wound repair; however, the underlying molecular mechanism remains unclear. Here, we found that SNAP-25 protein RIC-4 and synaptobrevin protein SEC-22 are required for SYX-2 recruitment at the wound site. They interact to form a SNARE complex to promote membrane repair in vivo and fusion in vitro. Moreover, we found that SEC-22 localized in multiple intracellular compartments, including endosomes and the trans-Golgi network, which recruited to the wounds. Furthermore, inhibition of RAB-5 disrupted SEC-22 localization and prevented its interaction with SYX-2. Our findings suggest that RAB-5 facilitates the formation of the RIC-4/SEC-22/SYX-2 SNARE complex and provides valuable insights into the molecular mechanism of how cells repair large membrane wounds.

A palisade-shaped membrane reservoir is required for rapid ring cell inflation in Drechslerella dactyloides

Fusion of individual vesicles carrying membrane-building materials with the plasma membrane (PM) enables gradual cell expansion and shape change. Constricting ring (CR) cells of carnivorous fungi triple in size within 0.1-1 s to capture passing nematodes. Here, we investigated how a carnivorous fungus, Drechslerella dactyloides, executes rapid and irreversible PM expansion during CR inflation. During CR maturation, vesicles carrying membrane-building materials accumulate and fuse, forming a structure named the Palisade-shaped Membrane-building Structure (PMS) around the rumen side of ring cells. After CR inflation, the PMS disappears, with partially inflated cells displaying wavy PM and fully inflated cells exhibiting smooth PM, suggesting that the PMS serves as the reservoir for membrane-building materials to enable rapid and extensive PM expansion. The DdSnc1, a v-SNARE protein, accumulates at the inner side of ring cells and is necessary for PMS formation and CR inflation. This study elucidates the unique cellular mechanisms underpinning rapid CR inflation.

Stacks of monogalactolipid bilayers can transform into a lattice of water channels

The lipid matrix of thylakoid membranes is a lamellar bilayer, but under a certain condition it can convert locally into a nonlamellar structure. This is possible because one of the main membrane lipids, MGDG, promotes the formation of an inverse hexagonal phase. Here, the spontaneous transformation of aligned hydrated MGDG bilayers into nonlamellar structures is investigated using all-atom molecular dynamics simulation. Previous studies have demonstrated that MGDG polar head groups connect vertically across the interface. In this study, the evolution of the system's initial structure into a lattice of water channels and contacted surfaces created by numerous vertical MGDG connections depended on the width of the hydrating water layers. These widths controlled the bilayers' ability to bend, which was a prerequisite for channel formation. Locally, an intensive exchange of MGDG molecules between apposing bilayer leaflets occurred, although a stable semi-toroidal stalk did not develop.

Sec22b and Stx4 Depletion Has No Major Effect on Cross-Presentation of PLGA Microsphere-Encapsulated Antigen and a Synthetic Long Peptide In Vitro

The induction of CTL responses by vaccines is important to combat infectious diseases and cancer. Biodegradable poly(lactic-co-glycolic acid) (PLGA) microspheres and synthetic long peptides are efficiently internalized by professional APCs and prime CTL responses after cross-presentation of Ags on MHC class I molecules. Specifically, they mainly use the cytosolic pathway of cross-presentation that requires endosomal escape, proteasomal processing, and subsequent MHC class I loading of Ags in the endoplasmic reticulum (ER) and/or the endosome. The vesicle SNARE protein Sec22b has been described as important for this pathway by mediating vesical trafficking for the delivery of ER-derived proteins to the endosome. As this function has also been challenged, we investigated the role of Sec22b in cross-presentation of the PLGA microsphere-encapsulated model Ag OVA and a related synthetic long peptide. Using CRISPR/Cas9-mediated genome editing, we generated Sec22b knockouts in two murine C57BL/6-derived APC lines and found no evidence for an essential role of Sec22b. Although pending experimental evidence, the target SNARE protein syntaxin 4 (Stx4) has been suggested to promote cross-presentation by interacting with Sec22b for the fusion of ER-derived vesicles with the endosome. In the current study, we show that, similar to Sec22b, Stx4 knockout in murine APCs had very limited effects on cross-presentation under the conditions tested. This study contributes to characterizing cross-presentation of two promising Ag delivery systems and adds to the discussion about the role of Sec22b/Stx4 in related pathways. Our data point toward SNARE protein redundancy in the cytosolic pathway of cross-presentation.

Vesicle-associated membrane protein 2 is a cargo-selective v-SNARE for a subset of GPCRs

Vesicle fusion at the plasma membrane is critical for releasing hormones and neurotransmitters and for delivering the cognate G protein-coupled receptors (GPCRs) to the cell surface. The SNARE fusion machinery that releases neurotransmitters has been well characterized. In contrast, the fusion machinery that delivers GPCRs is still unknown. Here, using high-speed multichannel imaging to simultaneously visualize receptors and v-SNAREs in real time in individual fusion events, we identify VAMP2 as a selective v-SNARE for GPCR delivery. VAMP2 was preferentially enriched in vesicles that mediate the surface delivery of μ opioid receptor (MOR), but not other cargos, and was required selectively for MOR recycling. Interestingly, VAMP2 did not show preferential localization on MOR-containing endosomes, suggesting that v-SNAREs are copackaged with specific cargo into separate vesicles from the same endosomes. Together, our results identify VAMP2 as a cargo-selective v-SNARE and suggest that surface delivery of specific GPCRs is mediated by distinct fusion events driven by distinct SNARE complexes.

The Uso1 globular head interacts with SNAREs to maintain viability even in the absence of the coiled-coil domain

Uso1/p115 and RAB1 tether ER-derived vesicles to the Golgi. Uso1/p115 contains a globular-head-domain (GHD), a coiled-coil (CC) mediating dimerization/tethering, and a C-terminal region (CTR) interacting with golgins. Uso1/p115 is recruited to vesicles by RAB1. Genetic studies placed Uso1 paradoxically acting upstream of, or in conjunction with RAB1 (Sapperstein et al., 1996). We selected two missense mutations in uso1 resulting in E6K and G540S in the GHD that rescued lethality of rab1-deficient Aspergillus nidulans. The mutations are phenotypically additive, their combination suppressing the complete absence of RAB1, which emphasizes the key physiological role of the GHD. In living hyphae Uso1 recurs on puncta (60 s half-life) colocalizing partially with the Golgi markers RAB1, Sed5, and GeaA/Gea1/Gea2, and totally with the retrograde cargo receptor Rer1, consistent with Uso1 dwelling in a very early Golgi compartment from which ER residents reaching the Golgi recycle back to the ER. Localization of Uso1, but not of Uso1E6K/G540S, to puncta is abolished by compromising RAB1 function, indicating that E6K/G540S creates interactions bypassing RAB1. That Uso1 delocalization correlates with a decrease in the number of Gea1 cisternae supports that Uso1-and-Rer1-containing puncta are where the protein exerts its physiological role. In S-tag-coprecipitation experiments, Uso1 is an associate of the Sed5/Bos1/Bet1/Sec22 SNARE complex zippering vesicles with the Golgi, with Uso1E6K/G540S showing a stronger association. Using purified proteins, we show that Bos1 and Bet1 bind the Uso1 GHD directly. However, Bet1 is a strong E6K/G540S-independent binder, whereas Bos1 is weaker but becomes as strong as Bet1 when the GHD carries E6K/G540S. G540S alone markedly increases GHD binding to Bos1, whereas E6K causes a weaker effect, correlating with their phenotypic contributions. AlphaFold2 predicts that G540S increases the binding of the GHD to the Bos1 Habc domain. In contrast, E6K lies in an N-terminal, potentially alpha-helical, region that sensitive genetic tests indicate as required for full Uso1 function. Remarkably, this region is at the end of the GHD basket opposite to the end predicted to interact with Bos1. We show that, unlike dimeric full-length and CTR∆ Uso1 proteins, the GHD lacking the CC/CTR dimerization domain, whether originating from bacteria or Aspergillus extracts and irrespective of whether it carries or not E6K/G540S, would appear to be monomeric. With the finding that overexpression of E6K/G540S and wild-type GHD complement uso1∆, our data indicate that the GHD monomer is capable of providing, at least partially, the essential Uso1 functions, and that long-range tethering activity is dispensable. Rather, these findings strongly suggest that the essential role of Uso1 involves the regulation of SNAREs.

Syntaxin 11 Contributes to the Interferon-Inducible Restriction of Coxiella burnetii Intracellular Infection

There is a limited understanding of host defense mechanisms targeting intracellular pathogens that proliferate in a lysosome. Coxiella burnetii is a model bacterial pathogen capable of replicating in the hydrolytic and acidic environment of the lysosome. It has been shown that gamma interferon (IFNγ)-stimulated host cells restrict C. burnetii replication by a mechanism that involves host IDO1 depletion of tryptophan. Host cells deficient in IDO1 activity, however, retain the ability to restrict C. burnetii replication when stimulated with IFNγ, which suggests additional mechanisms of host defense. This study identified syntaxin 11 (STX11) as a host protein that contributes to IFNγ-mediated suppression of C. burnetii replication. STX11 is a SNARE protein; SNARE proteins are proteins that mediate fusion of host vesicles with specific subcellular organelles. Depletion of STX11 using either small interfering RNA (siRNA)- or CRISPR-based approaches enhanced C. burnetii replication intracellularly. Stable expression of STX11 reduced C. burnetii replication in epithelial cells and macrophages, which indicates that this STX11-dependent cell-autonomous response is operational in multiple cell types and can function independently of other IFNγ-induced factors. Fluorescently tagged STX11 localized to the Coxiella-containing vacuole (CCV), and STX11 restriction was found to involve an interaction with STX8. Thus, STX11 regulates a vesicle fusion pathway that limits replication of this intracellular pathogen in a lysosome-derived organelle. IMPORTANCE Cell intrinsic defense mechanisms are used by eukaryotic cells to restrict the replication and dissemination of pathogens. This study identified a human protein called syntaxin 11 (STX11) as a host restriction factor that inhibits the intracellular replication of Coxiella burnetii. Syntaxins regulate the delivery of cargo inside vesicles by promoting specific membrane fusion events between donor and acceptor vesicles. Data presented here demonstrate that STX11 regulates an immunological defense pathway that controls replication of pathogens in lysosome-derived organelles, which provides new insight into the function of this SNARE protein.

Designer Nanodiscs to Probe and Reprogram Membrane Biology in Synapses

Signal transduction at the synapse is mediated by a variety of protein-lipid interactions, which are vital for the spatial and temporal regulation of synaptic vesicle biogenesis, neurotransmitter release, and postsynaptic receptor activation. Therefore, our understanding of synaptic transmission cannot be completed until the elucidation of these critical protein-lipid interactions. On this front, recent advances in nanodiscs have vastly expanded our ability to probe and reprogram membrane biology in synapses. Here, we summarize the progress of the nanodisc toolbox and discuss future directions in this exciting field.

SNARE Proteins in Synaptic Vesicle Fusion

Neurotransmitters are stored in small membrane-bound vesicles at synapses; a subset of synaptic vesicles is docked at release sites. Fusion of docked vesicles with the plasma membrane releases neurotransmitters. Membrane fusion at synapses, as well as all trafficking steps of the secretory pathway, is mediated by SNARE proteins. The SNAREs are the minimal fusion machinery. They zipper from N-termini to membrane-anchored C-termini to form a 4-helix bundle that forces the apposed membranes to fuse. At synapses, the SNAREs comprise a single helix from syntaxin and synaptobrevin; SNAP-25 contributes the other two helices to complete the bundle. Unc13 mediates synaptic vesicle docking and converts syntaxin into the permissive "open" configuration. The SM protein, Unc18, is required to initiate and proofread SNARE assembly. The SNAREs are then held in a half-zippered state by synaptotagmin and complexin. Calcium removes the synaptotagmin and complexin block, and the SNAREs drive vesicle fusion. After fusion, NSF and alpha-SNAP unwind the SNAREs and thereby recharge the system for further rounds of fusion. In this chapter, we will describe the discovery of the SNAREs, their relevant structural features, models for their function, and the central role of Unc18. In addition, we will touch upon the regulation of SNARE complex formation by Unc13, complexin, and synaptotagmin.

The exocyst complex is required for developmental and regenerative neurite growth in vivo

The exocyst complex is an important regulator of intracellular trafficking and tethers secretory vesicles to the plasma membrane. Understanding of its role in neuron outgrowth remains incomplete, and previous studies have come to different conclusions about its importance for axon and dendrite growth, particularly in vivo. To investigate exocyst function in vivo we used Drosophila sensory neurons as a model system. To bypass early developmental requirements in other cell types, we used neuron-specific RNAi to target seven exocyst subunits. Initial neuronal development proceeded normally in these backgrounds, however, we considered this could be due to residual exocyst function. To probe neuronal growth capacity at later times after RNAi initiation, we used laser microsurgery to remove axons or dendrites and prompt regrowth. Exocyst subunit RNAi reduced axon regeneration, although new axons could be specified. In control neurons, a vesicle trafficking marker often concentrated in the new axon, but this pattern was disrupted in Sec6 RNAi neurons. Dendrite regeneration was also severely reduced by exocyst RNAi, even though the trafficking marker did not accumulate in a strongly polarized manner during normal dendrite regeneration. The requirement for the exocyst was not limited to injury contexts as exocyst subunit RNAi eliminated dendrite regrowth after developmental pruning. We conclude that the exocyst is required for injury-induced and developmental neurite outgrowth, but that residual protein function can easily mask this requirement.

Protein Misfolding and Aggregation in the Brain: Common Pathogenetic Pathways in Neurodegenerative and Mental Disorders

Mental disorders represent common brain diseases characterized by substantial impairments of social and cognitive functions. The neurobiological causes and mechanisms of psychopathologies still have not been definitively determined. Various forms of brain proteinopathies, which include a disruption of protein conformations and the formation of protein aggregates in brain tissues, may be a possible cause behind the development of psychiatric disorders. Proteinopathies are known to be the main cause of neurodegeneration, but much less attention is given to the role of protein impairments in psychiatric disorders' pathogenesis, such as depression and schizophrenia. For this reason, the aim of this review was to discuss the potential contribution of protein illnesses in the development of psychopathologies. The first part of the review describes the possible mechanisms of disruption to protein folding and aggregation in the cell: endoplasmic reticulum stress, dysfunction of chaperone proteins, altered mitochondrial function, and impaired autophagy processes. The second part of the review addresses the known proteins whose aggregation in brain tissue has been observed in psychiatric disorders (amyloid, tau protein, α-synuclein, DISC-1, disbindin-1, CRMP1, SNAP25, TRIOBP, NPAS3, GluA1, FABP, and ankyrin-G).

The role of SCAMP5 in central nervous system diseases

OBJECTIVE

Secretory carrier membrane proteins (SCAMPs) constitute a group of membrane transport proteins in plants, insects and mammals. The mammalian genome contains five types of SCAMP genes, namely, SCAMP1-SCAMP5. SCAMPs participate in the vesicle cycling fusion of vesicles and cell membranes and play roles in regulating exocytosis and endocytosis, activating synaptic function and transmitting nerve signals. Among these proteins, SCAMP5 is highly expressed in the brain and has direct or indirect effects on the function of the central nervous system. This paper may allow us to better understand the role of SCAMP5 in the central nervous system diseases. SCAMP5 regulates membrane transport, controls the exocytosis of SVs and is related to secretion carrier and membrane function. In addition, SCAMP5 plays a major role in the normal maintenance of the physiological functions of nerve cells. This article summarizes the effects of SCAMP5 on nerve cell exocytosis, endocytosis and synaptic function, as well as the relationship between SCAMP5 and various neurological diseases, to better understand the role of SCAMP5 in the pathogenesis of neurological diseases.

METHODS

Through PubMed, this paper examined and analyzed the role of SCAMP5 in the central nervous system, as well as the relationship between SCAMP5 and various neurological diseases using the key terms "secretory carrier membrane proteins"," SCAMP5"," exocytosis"," endocytosis", "synaptic function", "central nervous system diseases" up to 01 March 2022.

RESULTS

SCAMP5 regulates membrane transport, controls the exocytosis of SVs and is related to secretion carrier and membrane function. In addition, SCAMP5 plays a major role in the normal maintenance of the physiological functions of nerve cells.

CONCLUSION

This article summarizes the effects of SCAMP5 on nerve cell exocytosis, endocytosis and synaptic function, as well as the relationship between SCAMP5 and various neurological diseases, to better understand the role of SCAMP5 in the pathogenesis of neurological diseases.

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.

Stability profile of the neuronal SNARE complex reflects its potency to drive fast membrane fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) form the SNARE complex to mediate most fusion events of the secretory pathway. The neuronal SNARE complex is featured by its high stability and half-zippered conformation required for driving robust and fast synaptic exocytosis. However, these two features seem to be thermodynamically mutually exclusive. In this study, we have employed temperature-dependent disassociation assays and single-molecule Förster resonance energy transfer (FRET) experiments to analyze the stability and conformation of the neuronal SNARE complex. We reclassified the amino acids of the SNARE motif into four sub-groups (core, core-side I and II, and non-contact). Our data showed that the core residues predominantly contribute to the complex stability to meet a basal requirement for SNARE-mediated membrane fusion, while the core-side residues exert an unbalanced effect on the N- and C-half bundle stability that determines the half-zippered conformation of the neuronal SNARE complex, which would accommodate essential regulations by complexins and synaptotagmins for fast Ca2+-triggered membrane fusion. Furthermore, our data confirmed a strong coupling of folding energy between the N- and C-half assembly of the neuronal SNARE complex, which rationalizes the strong potency of the half-zippered conformation to conduct robust and fast fusion. Overall, these results uncovered that the stability profile of the neuronal SNARE complex reflects its potency to drive fast and robust membrane fusion. Based on these results, we also developed a new parameter, the stability factor (Fs), to characterize the overall stability of the neuronal SNARE complex and resolved a linear correlation between the stability and inter-residue coulombic interactions of the neuronal SNARE complex, which would help rationally design artificial SNARE complexes and remold functional SNARE complexes with desirable stability.

Potential Similarities in Sex Difference in Key Genes and Their Expression, Network, EQTL and Pathways between COVID-19 and Chronic Kidney Disease Based on Mouse Model

COVID-19 and chronic kidney disease (CKD) share similarity in sex bias and key genes in the disease pathway of sex difference. We investigated the sex difference of molecular pathways of four key players of these two diseases using an existing large set of whole genome expression profiles from the kidneys of female and male mouse models. Our data show that there is little to no correlation at the whole genome expression level between female and male mice among these four genes. There are considerable sex differences among genes in upstream regulation, Ace2 complex interaction, and downstream pathways. Snap25 and Plcb4 may play important roles in the regulation of the expression level of Adam17, Tmprss2, and Cd146 in females. In males, Adh4 is a candidate gene for the regulation of Adam17, while Asl, Auts2, and Rabger1 are candidates for Tmprss2. Within the Ace2 complex, Cd146 directly influences the expression level of Adam17 and Ace2 in the female, while in the male Adam potentially has a stronger influence on Ace2 than that of Tmprss2. Among the top 100 most related genes, only one or two genes from four key genes and 11 from the control B-Actin were found to be the same between sexes. Among the top 10 sets of genes in the downstream pathway of Ace2, only two sets are the same between the sexes. We concluded that these known key genes and novel genes in CKD may play significant roles in the sex difference in the CKD and COVID-19 disease pathways.

Molecular determinants of complexin clamping and activation function

Previously we reported that Synaptotagmin-1 and Complexin synergistically clamp the SNARE assembly process to generate and maintain a pool of docked vesicles that fuse rapidly and synchronously upon Ca2+ influx (Ramakrishnan et al., 2020). Here, using the same in vitro single-vesicle fusion assay, we determine the molecular details of the Complexin-mediated fusion clamp and its role in Ca2+-activation. We find that a delay in fusion kinetics, likely imparted by Synaptotagmin-1, is needed for Complexin to block fusion. Systematic truncation/mutational analyses reveal that continuous alpha-helical accessory-central domains of Complexin are essential for its inhibitory function and specific interaction of the accessory helix with the SNAREpins enhances this functionality. The C-terminal domain promotes clamping by locally elevating Complexin concentration through interactions with the membrane. Independent of their clamping functions, the accessory-central helical domains of Complexin also contribute to rapid Ca2+-synchronized vesicle release by increasing the probability of fusion from the clamped state.

SNARE proteins: zip codes in vesicle targeting?

Membrane traffic in eukaryotic cells is mediated by transport vesicles that bud from a precursor compartment and are transported to their destination compartment where they dock and fuse. To reach their intracellular destination, transport vesicles contain targeting signals such as Rab GTPases and polyphosphoinositides that are recognized by tethering factors in the cytoplasm and that connect the vesicles with their respective destination compartment. The final step, membrane fusion, is mediated by SNARE proteins. SNAREs are connected to targeting signals and tethering factors by multiple interactions. However, it is still debated whether SNAREs only function downstream of targeting and tethering or whether they also participate in regulating targeting specificity. Here, we review the evidence and discuss recent data supporting a role of SNARE proteins as targeting signals in vesicle traffic.

Membrane transport inspired hydrolysis of non-activated esters at near physiological pH

A positively charged micelle loaded with substrates was transported selectively to the reaction site (cathode) to promote the proximity and localization of the reactants (ester and hydroxide). The guided vehicular delivery coupled with electrolysis allows the hydrolysis of non-activated esters at near physiological pH with significant yields along with recyclability.

α-Synuclein kinetically regulates the nascent fusion pore dynamics

α-Synuclein (α-syn_FL) is central to the pathogenesis of Parkinson's disease (PD), in which its nonfunctional oligomers accumulate and result in abnormal neurotransmission. The normal physiological function of this intrinsically disordered protein is still unclear. Although several previous studies demonstrated α-syn_FL's role in various membrane fusion steps, they produced conflicting outcomes regarding vesicular secretion. Here, we assess α-syn_FL's role in directly regulating individual exocytotic release events. We studied the micromillisecond dynamics of single recombinant fusion pores, the crucial kinetic intermediate of membrane fusion that tightly regulates the vesicular secretion in different cell types. α-Syn_FL accessed v-SNARE within the trans-SNARE complex to form an inhibitory complex. This activity was dependent on negatively charged phospholipids and resulted in decreased open probability of individual pores. The number of trans-SNARE complexes influenced α-syn_FL's inhibitory action. Regulatory factors that arrest SNARE complexes in different assembly states differentially modulate α-syn_FL's ability to alter fusion pore dynamics. α-Syn_FL regulates pore properties in the presence of Munc13-1 and Munc18, which stimulate α-SNAP/NSF-resistant SNARE complex formation. In the presence of synaptotagmin1(syt1), α-syn_FL contributes with apo-syt1 to act as a membrane fusion clamp, whereas Ca²⁺•syt1 triggered α-syn_FL-resistant SNARE complex formation that rendered α-syn_FL inactive in modulating pore properties. This study reveals a key role of α-syn_FL in controlling vesicular secretion.

SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling

Membrane fusion is a universal feature of eukaryotic protein trafficking and is mediated by the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family. SNARE proteins embedded in opposing membranes spontaneously assemble to drive membrane fusion and cargo exchange in vitro. Evolution has generated a diverse complement of SNARE regulatory proteins (SRPs) that ensure membrane fusion occurs at the right time and place in vivo. While a core set of SNAREs and SRPs are common to all eukaryotic cells, a specialized set of SRPs within neurons confer additional regulation to synaptic vesicle (SV) fusion. Neuronal communication is characterized by precise spatial and temporal control of SNARE dynamics within presynaptic subdomains specialized for neurotransmitter release. Action potential-elicited Ca²⁺ influx at these release sites triggers zippering of SNAREs embedded in the SV and plasma membrane to drive bilayer fusion and release of neurotransmitters that activate downstream targets. Here we discuss current models for how SRPs regulate SNARE dynamics and presynaptic output, emphasizing invertebrate genetic findings that advanced our understanding of SRP regulation of SV cycling.

SCAMP2/5 as diagnostic and prognostic markers for acute myeloid leukemia

The secretory carrier-associated membrane proteins (SCAMPs) are associated with the development of multiple human cancers. The role of SCAMPs in acute myeloid leukemia (AML), however, remains to be identified. In the present study, we explored expression patterns and prognostic value of SCAMPs and network analysis of SCAMPs-related signaling pathways in AML using Oncomine, GEPIA, cBioPortal, LinkedOmics, DAVID and Metascape databases. Genetic alteration analysis revealed that the mutation rate of SCAMP genes was below 1% (9/1272) in AML, and there was no significant correlation between SCAMPs gene mutation and AML prognosis. However, the SCAMP2/5 mRNA levels were significantly higher in AML patients than in healthy controls. Moreover, high mRNA expressions of SCAMP2/4/5 were associated with poor overall survival, which might be due to that SCAMP2/4/5 and their co-expressed genes were associated with multiple pathways related to tumorigenesis and progression, including human T-cell leukemia virus 1 infection, acute myeloid leukemia, mTOR and NF-kappa B signaling pathways. These results suggest that SCAMP2/4/5 are potential prognostic markers for AML, and that SCAMP2 and SCAMP5 individually or in combination may be used as diagnostic markers for AML.

Molecular Determinants of Complexin Clamping in Reconstituted Single-Vesicle Fusion.. [DOI: 10.1101/2021.07.05.451112]

Previously we reported that Synaptotagmin-1 and Complexin synergistically clamp the SNARE assembly process to generate and maintain a pool of docked vesicles that fuse rapidly and synchronously upon Ca2+ influx (Ramakrishnan et al. 2020). Here using the same in vitro single-vesicle fusion assay, we establish the molecular details of the Complexin clamp and its physiological relevance. We find that a delay in fusion kinetics, likely imparted by Synaptotagmin-1, is needed for Complexin to block fusion. Systematic truncation/mutational analyses reveal that continuous alpha-helical accessory-central domains of Complexin are essential for its inhibitory function and specific interaction of the accessory helix with the SNAREpins, analogous to the trans clamping model, enhances this functionality. The c-terminal domain promotes clamping by locally elevating Complexin concentration through interactions with the membrane. Further, we find that Complexin likely contributes to rapid Ca2+-synchronized vesicular release by preventing un-initiated fusion rather than by directly facilitating vesicle fusion.

Parkinson mice show functional and molecular changes in the gut long before motoric disease onset

BACKGROUND

There is increasing evidence that Parkinson's disease (PD) might start in the gut, thus involving and compromising also the enteric nervous system (ENS). At the clinical onset of the disease the majority of dopaminergic neurons in the midbrain is already destroyed, so that the lack of early biomarkers for the disease represents a major challenge for developing timely treatment interventions. Here, we use a transgenic A30P-α-synuclein-overexpressing PD mouse model to identify appropriate candidate markers in the gut before hallmark symptoms begin to manifest.

METHODS

Based on a gait analysis and striatal dopamine levels, we defined 2-month-old A30P mice as pre-symptomatic (psA30P), since they are not showing any motoric impairments of the skeletal neuromuscular system and no reduced dopamine levels, but an intestinal α-synuclein pathology. Mice at this particular age were further used to analyze functional and molecular alterations in both, the gastrointestinal tract and the ENS, to identify early pathological changes. We examined the gastrointestinal motility, the molecular composition of the ENS, as well as the expression of regulating miRNAs. Moreover, we applied A30P-α-synuclein challenges in vitro to simulate PD in the ENS.

RESULTS

A retarded gut motility and early molecular dysregulations were found in the myenteric plexus of psA30P mice. We found that i.e. neurofilament light chain, vesicle-associated membrane protein 2 and calbindin 2, together with the miRNAs that regulate them, are significantly altered in the psA30P, thus representing potential biomarkers for early PD. Many of the dysregulated miRNAs found in the psA30P mice are reported to be changed in PD patients as well, either in blood, cerebrospinal fluid or brain tissue. Interestingly, the in vitro approaches delivered similar changes in the ENS cultures as seen in the transgenic animals, thus confirming the data from the mouse model.

CONCLUSIONS

These findings provide an interesting and novel approach for the identification of appropriate biomarkers in men.

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.

Molecule-to-Circuit Disease Mechanisms of a Synaptic SNAREopathy

Alten et al. present a detailed investigation of disease-causing SNAP25 mutations based on structural analysis, neurotransmitter release, and emerging circuit properties. They show that structurally clustered mutations within the SNAP25 SNARE motif cause similar functional defects and predict that alterations of spontaneous release are a novel disease mechanism.

The Small GTPase Arf6 Functions as a Membrane Tether in a Chemically-Defined Reconstitution System

Arf-family small GTPases are essential protein components for membrane trafficking in all eukaryotic endomembrane systems, particularly during the formation of membrane-bound, coat protein complex-coated transport carriers. In addition to their roles in the transport carrier formation, a number of Arf-family GTPases have been reported to physically associate with coiled-coil tethering proteins and multisubunit tethering complexes, which are responsible for membrane tethering, a process of the initial contact between transport carriers and their target subcellular compartments. Nevertheless, whether and how indeed Arf GTPases are involved in the tethering process remain unclear. Here, using a chemically-defined reconstitution approach with purified proteins of two representative Arf isoforms in humans (Arf1, Arf6) and synthetic liposomes for model membranes, we discovered that Arf6 can function as a bona fide membrane tether, directly and physically linking two distinct lipid bilayers even in the absence of any other tethering factors, whereas Arf1 retained little potency to trigger membrane tethering under the current experimental conditions. Arf6-mediated membrane tethering reactions require trans-assembly of membrane-anchored Arf6 proteins and can be reversibly controlled by the membrane attachment and detachment cycle of Arf6. The intrinsic membrane tethering activity of Arf6 was further found to be significantly inhibited by the presence of membrane-anchored Arf1, suggesting that the tethering-competent Arf6-Arf6 assembly in trans can be prevented by the heterotypic Arf1-Arf6 association in a cis configuration. Taken together, these findings lead us to postulate that self-assemblies of Arf-family small GTPases on lipid bilayers contribute to driving and regulating the tethering events of intracellular membrane trafficking.

Neuroendocrine neoplasia of the gastrointestinal tract revisited: towards precision medicine

Over the past 5 years, a number of notable research advances have been made in the field of neuroendocrine cancer, specifically with regard to neuroendocrine cancer of the gastrointestinal tract. The aim of this Review is to provide an update on current knowledge that has proven effective for the clinical management of patients with these tumours. For example, for the first time in the tubular gastrointestinal tract, well-differentiated high-grade (grade 3) tumours and mixed neuroendocrine-non-neuroendocrine neoplasms (MiNENs) are defined in the WHO classification. This novel classification enables efficient identification of the most aggressive well-differentiated neuroendocrine tumours and helps in defining the degree of aggressiveness of MiNENs. The Review also discusses updates to epidemiology, cell biology (including vesicle-specific components) and the as-yet-unresolved complex genetic background that varies according to site and differentiation status. The Review summarizes novel diagnostic instruments, including molecules associated with the secretory machinery, novel radiological approaches (including pattern recognition techniques), novel PET tracers and liquid biopsy combined with DNA or RNA assays. Surgery remains the treatment mainstay; however, peptide receptor radionuclide therapy with novel radioligands and new emerging medical therapies (including vaccination and immunotherapy) are evolving and being tested in clinical trials, which are summarized and critically reviewed here.

Membrane Tethering Potency of Rab-Family Small GTPases Is Defined by the C-Terminal Hypervariable Regions

Membrane tethering is a crucial step to determine the spatiotemporal specificity of secretory and endocytic trafficking pathways in all eukaryotic endomembrane systems. Recent biochemical studies by a chemically-defined reconstitution approach reveal that, in addition to the structurally-diverse classic tethering factors such as coiled-coil tethering proteins and multisubunit tethering complexes, Rab-family small GTPases also retain the inherent membrane tethering functions to directly and physically bridge two distinct lipid bilayers by themselves. Although Rab-mediated membrane tethering reactions are fairly efficient and specific in the physiological context, its mechanistic basis is yet to be understood. Here, to explore whether and how the intrinsic tethering potency of Rab GTPases is controlled by their C-terminal hypervariable region (HVR) domains that link the conserved small GTPase domains (G-domains) to membrane anchors at the C-terminus, we quantitatively compared tethering activities of two representative Rab isoforms in humans (Rab5a, Rab4a) and their HVR-deleted mutant forms. Strikingly, deletion of the HVR linker domains enabled both Rab5a and Rab4a isoforms to enhance their intrinsic tethering potency, exhibiting 5- to 50-fold higher initial velocities of tethering for the HVR-deleted mutants than those for the full-length, wild-type Rabs. Furthermore, we revealed that the tethering activity of full-length Rab5a was significantly reduced by the omission of anionic lipids and cholesterol from membrane lipids and, however, membrane tethering driven by HVR-deleted Rab5a mutant was completely insensitive to the headgroup composition of lipids. Reconstituted membrane tethering assays with the C-terminally-truncated mutants of Rab4a further uncovered that the N-terminal residues in the HVR linker, located adjacent to the G-domain, are critical for regulating the intrinsic tethering activity. In conclusion, our current findings establish that the non-conserved, flexible C-terminal HVR linker domains define membrane tethering potency of Rab-family small GTPases through controlling the close attachment of the globular G-domains to membrane surfaces, which confers the active tethering-competent state of the G-domains on lipid bilayers.

Roles of Mso1 and the SM protein Sec1 in efficient vesicle fusion during fission yeast cytokinesis

Membrane trafficking during cytokinesis is essential for the delivery of membrane lipids and cargoes to the division site. However, the molecular mechanisms are still incompletely understood. In this study, we demonstrate the importance of uncharacterized fission yeast proteins Mso1 and Sec1 in membrane trafficking during cytokinesis. Fission yeast Mso1 shares homology with budding yeast Mso1 and human Mint1, proteins that interact with Sec1/Munc18 family proteins during vesicle fusion. Sec1/Munc18 proteins and their interactors are important regulators of SNARE complex formation during vesicle fusion. The roles of these proteins in vesicle trafficking during cytokinesis have been barely studied. Here, we show that fission yeast Mso1 is also a Sec1-binding protein and Mso1 and Sec1 localize to the division site interdependently during cytokinesis. The loss of Sec1 localization in mso1Δ cells results in a decrease in vesicle fusion and cytokinesis defects such as slow ring constriction, defective ring disassembly, and delayed plasma membrane closure. We also find that Mso1 and Sec1 may have functions independent of the exocyst tethering complex on the plasma membrane at the division site. Together, Mso1 and Sec1 play essential roles in regulating vesicle fusion and cargo delivery at the division site during cytokinesis.

A Membranome-Centered Approach Defines Novel Biomarkers for Cellular Subtypes in the Intervertebral Disc

OBJECTIVE

Lack of specific marker-sets prohibits definition and functional distinction of cellular subtypes in the intervertebral disc (IVD), such as those from the annulus fibrosus (AF) and the nucleus pulposus (NP).

DESIGN

We recently generated immortalized cell lines from human NP and AF tissues; these comprise a set of functionally distinct clonal subtypes. Whole transcriptome analyses were performed of 12 phenotypically distinct clonal cell lines (4× NP-Responder, 4× NP-nonResponder, 2× AF-Sheet forming, and 2× AF-nonSheet forming). Data sets were filtered for membrane-associated marker genes and compared to literature.

RESULTS

Comparison of our immortal cell lines to published primary NP, AF, and articular chondrocytes (AC) transcriptome datasets revealed preservation of AF and NP phenotypes. NP-specific membrane-associated genes were defined by comparison to AF cells in both the primary dataset (46 genes) and immortal cell-lines (161 genes). Definition of AF-specific membrane-associated genes yielded 125 primary AF cell and 92 immortal cell-line markers. Overlap between primary and immortal NP cells yielded high-confidence NP-specific marker genes for NP-R (CLDN11, TMEFF2, CA12, ANXA2, CD44) and NP-nR (EFNA1, NETO2, SLC2A1). Overlap between AF and immortal AF subtypes yielded specific markers for AF-S (COLEC12, LPAR1) and AF-nS (CHIC1).

CONCLUSIONS

The current study provides a reference platform for preclinical evaluation of novel membrane-associated cell type-specific markers in the IVD. Future research will focus on their biological relevance for IVD function in development, homeostasis, and degenerate conditions.

Recent Insights into Beta-cell Exocytosis in Type 2 Diabetes

As one of the leading causes of morbidity and mortality worldwide, diabetes affects an estimated 422 million adults, and it is expected to continue expanding such that by 2050, 30% of the U.S. population will become diabetic within their lifetime. Out of the estimated 422 million people currently afflicted with diabetes worldwide, about 5% have type 1 diabetes (T1D), while the remaining ~95% of diabetics have type 2 diabetes (T2D). Type 1 diabetes results from the autoimmune-mediated destruction of functional β-cell mass, whereas T2D results from combinatorial defects in functional β-cell mass plus peripheral glucose uptake. Both types of diabetes are now believed to be preceded by β-cell dysfunction. T2D is increasingly associated with numerous reports of deficiencies in the exocytosis proteins that regulate insulin release from β-cells, specifically the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE protein's functionality is further regulated by a variety of accessory factors such as Sec1/Munc18 (SM), double C2-domain proteins (DOC2), and additional interacting proteins at the cell surface that influence the fidelity of insulin release. As new evidence emerges about the detailed mechanisms of exocytosis, new questions and controversies have come to light. This emerging information is also contributing to dialogue in the islet biology field focused on how to correct the defects in insulin exocytosis. Herein we present a balanced review of the role of exocytosis proteins in T2D, with thoughts on novel strategies to protect functional β-cell mass.

Resolving kinetic intermediates during the regulated assembly and disassembly of fusion pores

The opening of a fusion pore during exocytosis creates the first aqueous connection between the lumen of a vesicle and the extracellular space. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate the formation of these dynamic structures, and their kinetic transitions are tightly regulated by accessory proteins at the synapse. Here, we utilize two single molecule approaches, nanodisc-based planar bilayer electrophysiology and single-molecule FRET, to address the relationship between SNARE complex assembly and rapid (micro-millisecond) fusion pore transitions, and to define the role of accessory proteins. Synaptotagmin (syt) 1, a major Ca²⁺-sensor for synaptic vesicle exocytosis, drove the formation of an intermediate: committed trans-SNARE complexes that form large, stable pores. Once open, these pores could only be closed by the action of the ATPase, NSF. Time-resolved measurements revealed that NSF-mediated pore closure occurred via a complex ‘stuttering’ mechanism. This simplified system thus reveals the dynamic formation and dissolution of fusion pores.

SNAREs mediate the formation of a fusion pore during exocytosis which connects the lumen of a vesicle with the extracellular space. Here, authors use single molecule approaches to define the role of synaptotagmin 1 and NSF in synaptic pore formation and dissolution.

Depletion of SNAP-23 and Syntaxin 4 alters lipid droplet homeostasis during Chlamydia infection

Chlamydia trachomatis is an obligate intracellular pathogen that replicates inside a parasitic vacuole called the inclusion. The nascent inclusion is derived from the host plasma membrane and serves as a platform from which Chlamydia controls interactions with the host microenvironment. To survive inside the host cell, Chlamydia scavenges for nutrients and lipids by recruiting and/or fusing with various cellular compartments. The mechanisms by which these events occur are poorly understood but require host proteins such as the SNARE proteins (SNAP (Soluble N-ethylmaleimide-sensitive factor attachment protein) Receptor). Here, we show that SNAP-23 and Syntaxin 4, two plasma membrane SNAREs, are recruited to the inclusion and play an important role in Chlamydia development. Knocking down SNAP-23 and Syntaxin 4 by CRISPR-Cas9 reduces the amount of infectious progeny. We then demonstrate that the loss of both of these SNARE proteins results in the dysregulation of Chlamydia-induced lipid droplets, indicating that both SNAP-23 and Syntaxin 4 play a critical role in lipid droplet homeostasis during Chlamydia infection. Ultimately, our data highlights the importance of lipid droplets and their regulation in Chlamydia development.

Sec17 (α-SNAP) and Sec18 (NSF) restrict membrane fusion to R-SNAREs, Q-SNAREs, and SM proteins from identical compartments

Membrane fusion at each organelle requires conserved proteins: Rab-GTPases, effector tethering complexes, Sec1/Munc18 (SM)-family SNARE chaperones, SNAREs of the R, Qa, Qb, and Qc families, and the Sec17/α-SNAP and ATP-dependent Sec18/NSF SNARE chaperone system. The basis of organelle-specific fusion, which is essential for accurate protein compartmentation, has been elusive. Rab family GTPases, SM proteins, and R- and Q-SNAREs may contribute to this specificity. We now report that the fusion supported by SNAREs alone is both inefficient and promiscuous with respect to organelle identity and to stimulation by SM family proteins or complexes. SNARE-only fusion is abolished by the disassembly chaperones Sec17 and Sec18. Efficient fusion in the presence of Sec17 and Sec18 requires a tripartite match between the organellar identities of the R-SNARE, the Q-SNAREs, and the SM protein or complex. The functions of Sec17 and Sec18 are not simply negative regulation; they stimulate fusion with either vacuolar SNAREs and their SM protein complex HOPS or endoplasmic reticulum/cis-Golgi SNAREs and their SM protein Sly1. The fusion complex of each organelle is assembled from its own functionally matching pieces to engage Sec17/Sec18 for fusion stimulation rather than inhibition.

Overexpression of trans-Golgi network t-SNAREs rescues vacuolar trafficking and TGN morphology defects in a putative tethering factor mutant

The trans-Golgi network (TGN) is a major site for sorting of cargo to either the vacuole or apoplast. The TGN-localized coiled-coil protein TNO1 is a putative tethering factor that interacts with the TGN t-SNARE SYP41 and is required for correct localization of the SYP61 t-SNARE. An Arabidopsis thaliana tno1 mutant is hypersensitive to salt stress and partially mislocalizes vacuolar proteins to the apoplast, indicating a role in vacuolar trafficking. Here, we show that overexpression of SYP41 or SYP61 significantly increases SYP41-SYP61 complex formation in a tno1 mutant, and rescues the salt sensitivity and defective vacuolar trafficking of the tno1 mutant. The TGN is disrupted and vesicle budding from Golgi cisternae is reduced in the tno1 mutant, and these defects are also rescued by overexpression of SYP41 or SYP61. Our results suggest that the trafficking and Golgi morphology defects caused by loss of TNO1 can be rescued by increasing SYP41-SYP61 t-SNARE complex formation, implicating TNO1 as a tethering factor mediating efficient vesicle fusion at the TGN.

Retrograde trafficking and quality control of yeast synaptobrevin, Snc1, are conferred by its transmembrane domain

Synaptobrevin/vesicle-associated membrane protein 2 (VAMP2) is an essential soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) protein that has been extensively studied in its role in synaptic vesicle fusion. However, sorting and trafficking of VAMP2 within the endosomal system is not well understood. Here, we use the yeast VAMP2 homologue Snc1 to investigate the pathways and signals required for endocytic trafficking. We identify two genetically distinct retrieval pathways from the endosomal system: a plasma membrane recycling pathway that requires the Rcy1 F-box protein and a retrograde pathway originating from the multivesicular/prevacuole endosome dependent on the Snx4-Atg20 sorting nexin complex. Lysine residues within the transmembrane domain of Snc1 are necessary for presentation of a Snx4-Atg20-dependent sorting signal located within its juxtamembrane region. Mutations of the transmembrane lysine residues ablate retrograde sorting and subject Snc1 to quality control via sorting into the degradative multivesicular endosome pathway. Degradative sorting requires lysine residues in the juxtamembrane region of Snc1 and is mediated by the Rsp5 ubiquitin ligase and its transmembrane adapters, Ear1 and Ssh4, which localize to endosome and vacuole membranes. This study shows that Snc1 is trafficked between the endosomal system and the Golgi apparatus via multiple pathways and provides evidence for protein quality control surveillance of a SNARE protein in the endo-vacuolar system.

Mechanisms of Neurological Dysfunction in GOSR2 Progressive Myoclonus Epilepsy, a Golgi SNAREopathy

Successive fusion events between transport vesicles and their target membranes mediate trafficking of secreted, membrane- and organelle-localised proteins. During the initial steps of this process, termed the secretory pathway, COPII vesicles bud from the endoplasmic reticulum (ER) and fuse with the cis-Golgi membrane, thus depositing their cargo. This fusion step is driven by a quartet of SNARE proteins that includes the cis-Golgi t-SNARE Membrin, encoded by the GOSR2 gene. Mis-sense mutations in GOSR2 result in Progressive Myoclonus Epilepsy (PME), a severe neurological disorder characterised by ataxia, myoclonus and seizures in the absence of significant cognitive impairment. However, given the ubiquitous and essential function of ER-to-Golgi transport, why GOSR2 mutations cause neurological dysfunction and not lethality or a broader range of developmental defects has remained an enigma. Here we highlight new work that has shed light on this issue and incorporate insights into canonical and non-canonical secretory trafficking pathways in neurons to speculate as to the cellular and molecular mechanisms underlying GOSR2 PME. This article is part of a Special Issue entitled: SNARE proteins: a long journey of science in brain physiology and pathology: from molecular.

Homotypic and heterotypic trans-assembly of human Rab-family small GTPases in reconstituted membrane tethering

Membrane tethering is a highly regulated event occurring during the initial physical contact between membrane-bounded transport carriers and their target subcellular membrane compartments, thereby ensuring the spatiotemporal specificity of intracellular membrane trafficking. Although Rab-family small GTPases and specific Rab-interacting effectors, such as coiled-coil tethering proteins and multisubunit tethering complexes, are known to be involved in membrane tethering, how these protein components directly act upon the tethering event remains enigmatic. Here, using a chemically defined reconstitution system, we investigated the molecular basis of membrane tethering by comprehensively and quantitatively evaluating the intrinsic capacities of 10 representative human Rab-family proteins (Rab1a, -3a, -4a, -5a, -6a, -7a, -9a, -11a, -27a, and -33b) to physically tether two distinct membranes via homotypic and heterotypic Rab-Rab assembly. All of the Rabs tested, except Rab27a, specifically caused homotypic membrane tethering at physiologically relevant Rab densities on membrane surfaces (e.g. Rab/lipid molar ratios of 1:100-1:3,000). Notably, endosomal Rab5a retained its intrinsic potency to drive efficient homotypic tethering even at concentrations below the Rab/lipid ratio of 1:3,000. Comprehensive reconstitution experiments further uncovered that heterotypic combinations of human Rab-family isoforms, including Rab1a/6a, Rab1a/9a, and Rab1a/33b, can directly and selectively mediate membrane tethering. Rab1a and Rab9a in particular synergistically triggered very rapid and efficient membrane tethering reactions through their heterotypic trans-assembly on two opposing membranes. In conclusion, our findings establish that, in the physiological context, homotypic and heterotypic trans-assemblies of Rab-family small GTPases can provide the essential molecular machinery necessary to drive membrane tethering in eukaryotic endomembrane systems.