Chaperoning SNARE assembly and disassembly

Coxiella burnetii effector CvpE maintains biogenesis of Coxiella-containing vacuoles by suppressing lysosome tubulation through binding PI(3)P and perturbing PIKfyve activity on lysosomes

Coxiella burnetii (C. burnetii) is the causative agent of Q fever, a zoonotic disease. Intracellular replication of C. burnetii requires the maturation of a phagolysosome-like compartment known as the replication permissive Coxiella-containing vacuole (CCV). Effector proteins secreted by the Dot/Icm secretion system are indispensable for maturation of a single large CCV by facilitating the fusion of promiscuous vesicles. However, the mechanisms of CCV maintenance and evasion of host cell clearance remain to be defined. Here, we show that C. burnetii secreted Coxiella vacuolar protein E (CvpE) contributes to CCV biogenesis by inducing lysosome-like vacuole (LLV) enlargement. LLV fission by tubulation and autolysosome degradation is impaired in CvpE-expressing cells. Subsequently, we found that CvpE suppresses lysosomal Ca2+ channel transient receptor potential channel mucolipin 1 (TRPML1) activity in an indirect manner, in which CvpE binds phosphatidylinositol 3-phosphate [PI(3)P] and perturbs PIKfyve activity in lysosomes. Finally, the agonist of TRPML1, ML-SA5, inhibits CCV biogenesis and C. burnetii replication. These results provide insight into the mechanisms of CCV maintenance by CvpE and suggest that the agonist of TRPML1 can be a novel potential treatment that does not rely on antibiotics for Q fever by enhancing Coxiella-containing vacuoles (CCVs) fission.

The WDR11 complex is a receptor for acidic-cluster-containing cargo proteins

Vesicle trafficking is a fundamental process that allows for the sorting and transport of specific proteins (i.e., "cargoes") to different compartments of eukaryotic cells. Cargo recognition primarily occurs through coats and the associated proteins at the donor membrane. However, it remains unclear whether cargoes can also be selected at other stages of vesicle trafficking to further enhance the fidelity of the process. The WDR11-FAM91A1 complex functions downstream of the clathrin-associated AP-1 complex to facilitate protein transport from endosomes to the TGN. Here, we report the cryo-EM structure of human WDR11-FAM91A1 complex. WDR11 directly and specifically recognizes a subset of acidic clusters, which we term super acidic clusters (SACs). WDR11 complex assembly and its binding to SAC-containing proteins are indispensable for the trafficking of SAC-containing proteins and proper neuronal development in zebrafish. Our studies thus uncover that cargo proteins could be recognized in a sequence-specific manner downstream of a protein coat.

Smart Molecular Imaging and Theranostic Probes by Enzymatic Molecular In Situ Self-Assembly

Enzymatic molecular in situ self-assembly (E-MISA) that enables the synthesis of high-order nanostructures from synthetic small molecules inside a living subject has emerged as a promising strategy for molecular imaging and theranostics. This strategy leverages the catalytic activity of an enzyme to trigger probe substrate conversion and assembly in situ, permitting prolonging retention and congregating many molecules of probes in the targeted cells or tissues. Enhanced imaging signals or therapeutic functions can be achieved by responding to a specific enzyme. This E-MISA strategy has been successfully applied for the development of enzyme-activated smart molecular imaging or theranostic probes for in vivo applications. In this Perspective, we discuss the general principle of controlling in situ self-assembly of synthetic small molecules by an enzyme and then discuss the applications for the construction of "smart" imaging and theranostic probes against cancers and bacteria. Finally, we discuss the current challenges and perspectives in utilizing the E-MISA strategy for disease diagnoses and therapies, particularly for clinical translation.

Moniezia benedeni drives the SNAP-25 expression of the enteric nerves in sheep's small intestine

BACKGROUND

The neuroimmune network plays a crucial role in regulating mucosal immune homeostasis within the digestive tract. Synaptosome-associated protein 25 (SNAP-25) is a presynaptic membrane-binding protein that activates ILC2s, initiating the host's anti-parasitic immune response.

METHODS

To investigate the effect of Moniezia benedeni (M. benedeni) infection on the distribution of SNAP-25 in the sheep's small intestine, the recombinant plasmid pET-28a-SNAP-25 was constructed and expressed in BL21, yielding the recombinant protein. Then, the rabbit anti-sheep SNAP-25 polyclonal antibody was prepared and immunofluorescence staining was performed with it. The expression levels of SNAP-25 in the intestines of normal and M. benedeni-infected sheep were detected by ELISA.

RESULTS

The results showed that the SNAP-25 recombinant protein was 29.3 KDa, the titer of the prepared immune serum reached 1:128,000. It was demonstrated that the rabbit anti-sheep SNAP-25 polyclonal antibody could bind to the natural protein of sheep SNAP-25 specifically. The expression levels of SNAP-25 in the sheep's small intestine revealed its primary presence in the muscular layer and lamina propria, particularly around nerve fibers surrounding the intestinal glands. Average expression levels in the duodenum, jejunum, and ileum were 130.32 pg/mg, 185.71 pg/mg, and 172.68 pg/mg, respectively. Under conditions of M. benedeni infection, the spatial distribution of SNAP-25-expressing nerve fibers remained consistent, but its expression level in each intestine segment was increased significantly (P < 0.05), up to 262.02 pg/mg, 276.84 pg/mg, and 326.65 pg/mg in the duodenum, jejunum, and ileum, and it was increased by 101.06%, 49.07%, and 89.16% respectively.

CONCLUSIONS

These findings suggest that M. benedeni could induce the SNAP-25 expression levels in sheep's intestinal nerves significantly. The results lay a foundation for further exploration of the molecular mechanism by which the gastrointestinal nerve-mucosal immune network perceives parasites in sheep.

Structure of the endosomal CORVET tethering complex

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

Biomarker-triggered, spatiotemporal controlled DNA nanodevice simultaneous assembly and disassembly

Despite many advances in the use of DNA nanodevices as assembly or disassembly modules to build various complex structures, the simultaneous assembly and disassembly of DNA structures in living cells remains a challenge. In this study, we present a modular engineering approach for assembling and disassembling DNA nanodevices in response to endogenous biomarkers. As a result of pairwise prehybridization of original DNA strands, the DNA nanodevice is initially inert. In an effort to bind one of the paired strands and release its complement, nucleolin competes. Assembly of the DNA nanodevice is initiated when the released complement binds to it, and disassembly is initiated when APE1 shears the assembled binding site of the DNA nanodevice. Spatial-temporal logic control is achieved through our approach during the assembly and disassembly of DNA nanodevices. Furthermore, by means of this assembly and disassembly procedure, the sequential detection and imaging of two tumor markers can be achieved, thereby effectively reducing false-positive signal results and accelerating the detection time. This study emphasizes the simultaneous assembly and disassembly of DNA nanodevices controlled by biomarkers in a simple and versatile manner; it has the potential to expand the application scope of DNA nanotechnology and offers an idea for the implementation of precision medicine testing.

SNARE chaperone Sly1 directly mediates close-range vesicle tethering

The essential Golgi protein Sly1 is a member of the Sec1/mammalian Unc-18 (SM) family of SNARE chaperones. Sly1 was originally identified through remarkable gain-of-function alleles that bypass requirements for diverse vesicle tethering factors. Employing genetic analyses and chemically defined reconstitutions of ER-Golgi fusion, we discovered that a loop conserved among Sly1 family members is not only autoinhibitory but also acts as a positive effector. An amphipathic lipid packing sensor (ALPS)-like helix within the loop directly binds high-curvature membranes. Membrane binding is required for relief of Sly1 autoinhibition and also allows Sly1 to directly tether incoming vesicles to the Qa-SNARE on the target organelle. The SLY1-20 mutation bypasses requirements for diverse tethering factors but loses this ability if the tethering activity is impaired. We propose that long-range tethers, including Golgins and multisubunit tethering complexes, hand off vesicles to Sly1, which then tethers at close range to initiate trans-SNARE complex assembly and fusion in the early secretory pathway.

Sec18 binds the tethering/SM complex HOPS to engage the Qc-SNARE for membrane fusion

Membrane fusion is regulated by Rab GTPases, their tethering effectors such as HOPS, SNARE proteins on each fusion partner, SM proteins to catalyze SNARE assembly, Sec17 (SNAP), and Sec18 (NSF). Though concentrated HOPS can support fusion without Sec18, we now report that fusion falls off sharply at lower HOPS levels, where direct Sec18 binding to HOPS restores fusion. This Sec18-dependent fusion needs adenine nucleotide but neither ATP hydrolysis nor Sec17. Sec18 enhances HOPS recognition of the Qc-SNARE. With high levels of HOPS, Qc has a Km for fusion of a few nM. Either lower HOPS levels, or substitution of a synthetic tether for HOPS, strikingly increases the Km for Qc to several hundred nM. With dilute HOPS, Sec18 returns the Km for Qc to low nM. In contrast, HOPS concentration and Sec18 have no effect on Qb-SNARE recognition. Just as Qc is required for fusion but not for the initial assembly of SNAREs in trans, impaired Qc recognition by limiting HOPS without Sec18 still allows substantial trans-SNARE assembly. Thus, in addition to the known Sec18 functions of disassembling SNARE complexes, oligomerizing Sec17 for membrane association, and allowing Sec17 to drive fusion without complete SNARE zippering, we report a fourth Sec18 function, the Sec17-independent binding of Sec18 to HOPS to enhance functional Qc-SNARE engagement.

Biomarker-Driven DNA-Functionalized Colloidal Programmed Simultaneous Assembly and Disassembly in Cells

Complex structures and devices, both natural and artificial, can often undergo assembly and disassembly. Assembly and disassembly allow multiple stimuli to initiate, for example, the assembly and disassembly of primary cilia under the control of E3 ubiquitin ligases and deubiquitinases. Although biology relies on such schemes, they are rarely available in materials science. Here, we demonstrate a DNA-functionalized colloidal Au response to endogenous biomarkers to trigger simultaneous assembly and disassembly techniques. Colloidal Au is initially inert because the starting DNA strands are paired and prehybridized. TK1 mRNA competes to bind one of the paired strands and release its complement. The released complement binds to the next colloidal Au to initiate assembly, and APE1 can shear the colloidal Au assembly binding site to initiate disassembly. Our strategy provides temporal and spatial logic control during colloidal Au assembly and disassembly, and this simultaneous assembly and disassembly process can be used for sequential detection and cellular imaging of two biomarkers, effectively reducing signal false-positive results and shortening detection time. This work highlights biomarker-controlled colloidal Au simultaneous assembly and disassembly in ways that are simple and versatile, with the potential to enrich the application scope of DNA nanotechnology and provide an idea for the application of precision medicine testing.

A minimal presynaptic protein machinery mediating synchronous and asynchronous exocytosis and short-term plasticity

Neurotransmitters are released from synaptic vesicles with remarkable precision in response to presynaptic Ca2+ influx but exhibit significant heterogeneity in exocytosis timing and efficacy based on the recent history of activity. This heterogeneity is critical for information transfer in the brain, yet its molecular basis remains poorly understood. Here, we employ a biochemically-defined fusion assay under physiologically-relevant conditions to delineate the minimal protein machinery sufficient to account for different modes of Ca2+-triggered vesicle fusion and short-term facilitation. We find that Synaptotagmin-1, Synaptotagmin-7, and Complexin, synergistically restrain SNARE complex assembly, thus preserving vesicles in a stably docked state at rest. Upon Ca2+ activation, Synaptotagmin-1 induces rapid vesicle fusion, while Synaptotagmin-7 mediates delayed fusion. Competitive binding of Synaptotagmin-1 and Synaptotagmin-7 to the same SNAREs, coupled with differential rates of Ca2+-triggered fusion clamp reversal, govern the kinetics of vesicular fusion. Under conditions mimicking sustained neuronal activity, the Synaptotagmin-7 fusion clamp is destabilized by the elevated basal Ca2+ concentration, thereby enhancing the synchronous component of fusion. These findings provide a direct demonstration that a small set of proteins is sufficient to account for how nerve terminals adapt and regulate the Ca2+-evoked neurotransmitter exocytosis process to support their specialized functions in the nervous system.

Cargo selective vesicle tethering: The structural basis for binding of specific cargo proteins by the Golgi tether component TBC1D23

The Golgi-localized golgins golgin-97 and golgin-245 capture transport vesicles arriving from endosomes via the protein TBC1D23. The amino-terminal domain of TBC1D23 binds to the golgins, and the carboxyl-terminal domain of TBC1D23 captures the vesicles, but how it recognizes specific vesicles was unclear. A search for binding partners of the carboxyl-terminal domain unexpectedly revealed direct binding to carboxypeptidase D and syntaxin-16, known cargo proteins of the captured vesicles. Binding is via a threonine-leucine-tyrosine (TLY) sequence present in both proteins next to an acidic cluster. A crystal structure reveals how this acidic TLY motif binds to TBC1D23. An acidic TLY motif is also present in the tails of other endosome-to-Golgi cargo, and these also bind TBC1D23. Structure-guided mutations in the carboxyl-terminal domain that disrupt motif binding in vitro also block vesicle capture in vivo. Thus, TBC1D23 attached to golgin-97 and golgin-245 captures vesicles by a previously undescribed mechanism: the recognition of a motif shared by cargo proteins carried by the vesicle.

Munc18c accelerates SNARE-dependent membrane fusion in the presence of regulatory proteins α-SNAP and NSF

Intracellular vesicle fusion is driven by the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and their cofactors, including Sec1/Munc18 (SM), α-SNAP, and NSF. α-SNAP and NSF play multiple layers of regulatory roles in the SNARE assembly, disassembling the cis-SNARE complex and the prefusion SNARE complex. How SM proteins coupled with NSF and α-SNAP regulate SNARE-dependent membrane fusion remains incompletely understood. Munc18c, an SM protein involved in the exocytosis of the glucose transporter GLUT4, binds and activates target (t-) SNAREs to accelerate the fusion reaction through a SNARE-like peptide (SLP). Here, using an in vitro reconstituted system, we discovered that α-SNAP blocks the GLUT4 SNAREs-mediated membrane fusion. Munc18c interacts with t-SNAREs to displace α-SNAP, which overcomes the fusion inhibition. Furthermore, Munc18c shields the trans-SNARE complex from NSF/α-SNAP-mediated disassembly and accelerates SNARE-dependent fusion kinetics in the presence of NSF and α-SNAP. The SLP in domain 3a is indispensable in Munc18c-assisted resistance to NSF and α-SNAP. Together, our findings demonstrate that Munc18c protects the prefusion SNARE complex from α-SNAP and NSF, promoting SNARE-dependent membrane fusion through its SLP.

LIGHT regulated gene expression in rheumatoid synovial fibroblasts

BACKGROUND

Synovial hyperplasia caused by rheumatoid arthritis (RA), an autoimmune inflammatory disease, leads to the destruction of the articular cartilage and bone. A member of the tumor necrosis factor superfamily, Lymphotoxin-related inducible ligand that competes for glycoprotein D binding to herpes virus entry mediator on T cells (LIGHT) has been shown to correlate with the pathogenesis of RA.

METHODS

We used cDNA microarray analysis to compare the expression of genes in rheumatoid fibroblast-like synoviocytes with and without LIGHT stimulation.

RESULTS

Significant changes in gene expression (P-values < 0.05 and fold change ≥ 2.0) were associated mainly with biological function categories of glycoprotein, glycosylation site as N-linked, plasma membrane part, integral to plasma membrane, intrinsic to plasma membrane, signal, plasma membrane, signal peptide, alternative splicing, and topological domain as extracellular.

CONCLUSIONS

Our results indicate that LIGHT may regulate the expression in RA-FLS of genes which are important in the differentiation of several cell types and in cellular functions.

Structure of a membrane tethering complex incorporating multiple SNAREs

Most membrane fusion reactions in eukaryotic cells are mediated by multisubunit tethering complexes (MTCs) and SNARE proteins. MTCs are much larger than SNAREs and are thought to mediate the initial attachment of two membranes. Complementary SNAREs then form membrane-bridging complexes whose assembly draws the membranes together for fusion. Here we present a cryo-electron microscopy structure of the simplest known MTC, the 255-kDa Dsl1 complex of Saccharomyces cerevisiae, bound to the two SNAREs that anchor it to the endoplasmic reticulum. N-terminal domains of the SNAREs form an integral part of the structure, stabilizing a Dsl1 complex configuration with unexpected similarities to the 850-kDa exocyst MTC. The structure of the SNARE-anchored Dsl1 complex and its comparison with exocyst reveal what are likely to be common principles underlying MTC function. Our structure also implies that tethers and SNAREs can work together as a single integrated machine.

Mechanisms of SNARE proteins in membrane fusion

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are a family of small conserved eukaryotic proteins that mediate membrane fusion between organelles and with the plasma membrane. SNAREs are directly or indirectly anchored to membranes. Prior to fusion, complementary SNAREs assemble between membranes with the aid of accessory proteins that provide a scaffold to initiate SNARE zippering, pulling the membranes together and mediating fusion. Recent advances have enabled the construction of detailed models describing bilayer transitions and energy barriers along the fusion pathway and have elucidated the structures of SNAREs complexed in various states with regulatory proteins. In this Review, we discuss how these advances are yielding an increasingly detailed picture of the SNARE-mediated fusion pathway, leading from first contact between the membranes via metastable non-bilayer intermediates towards the opening and expansion of a fusion pore. We describe how SNARE proteins assemble into complexes, how this assembly is regulated by accessory proteins and how SNARE complexes overcome the free energy barriers that prevent spontaneous membrane fusion.

Exploring the conformational changes of the Munc18-1/syntaxin 1a complex

Neurotransmitters are released from synaptic vesicles, the membrane of which fuses with the plasma membrane upon calcium influx. This membrane fusion reaction is driven by the formation of a tight complex comprising the plasma membrane N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins syntaxin-1a and SNAP-25 with the vesicle SNARE protein synaptobrevin. The neuronal protein Munc18-1 forms a stable complex with syntaxin-1a. Biochemically, syntaxin-1a cannot escape the tight grip of Munc18-1, so formation of the SNARE complex is inhibited. However, Munc18-1 is essential for the release of neurotransmitters in vivo. It has therefore been assumed that Munc18-1 makes the bound syntaxin-1a available for SNARE complex formation. Exactly how this occurs is still unclear, but it is assumed that structural rearrangements occur. Here, we used a series of mutations to specifically weaken the complex at different positions in order to induce these rearrangements biochemically. Our approach was guided through sequence and structural analysis and supported by molecular dynamics simulations. Subsequently, we created a homology model showing the complex in an altered conformation. This conformation presumably represents a more open arrangement of syntaxin-1a that permits the formation of a SNARE complex to be initiated while still bound to Munc18-1. In the future, research should investigate how this central reaction for neuronal communication is controlled by other proteins.

Highly efficient overexpression and purification of multisubunit tethering complexes in Saccharomyces cerevisiae

Vesicle trafficking is a fundamental cellular process that ensures proper material exchange between organelles in eukaryotic cells, and multisubunit tethering complexes (MTCs) are essential in this process. The heterohexameric homotypic fusion and protein sorting (HOPS) complex, which functions in the endolysosomal pathway, is a member of MTCs. Despite its critical role, the complex composition and low-expression level of HOPS have made its expression and purification extremely challenging. In this study, we present a highly efficient strategy for overexpressing and purifying HOPS from Saccharomyces cerevisiae. We achieved HOPS overexpression by integrating a strong promoter TEF1 before each subunit using the gRNA-tRNA array for CRISPR-Cas9 (GTR-CRISPR) system. The HOPS complex was subsequently purified using Staphylococcus aureus protein A (ProtA) affinity purification and size-exclusion chromatography, resulting in high purity and homogeneity. We obtained two-fold more HOPS using this method than that obtained using the commonly used GAL1 promoter-controlled HOPS overexpression. Negative staining electron microscopy analysis confirmed the correct assembly of HOPS. Notably, we also successfully purified two other MTCs, class C core vacuole/endosome tethering (CORVET) and Golgi-associated retrograde protein (GARP) using this approach. Our findings facilitate further in vitro biochemical characterization and functional studies of MTCs and provide a useful guide for the preparation of other heterogenic multisubunit complexes.

Differences between migrasome, a 'new organelle', and exosome

The migrasome is a new organelle discovered by Professor Yu Li in 2015. When cells migrate, the membranous organelles that appear at the end of the retraction fibres are migrasomes. With the migration of cells, the retraction fibres which connect migrasomes and cells finally break. The migrasomes detach from the cell and are released into the extracellular space or directly absorbed by the recipient cell. The cytoplasmic contents are first transported to the migrasome and then released from the cell through the migrasome. This release mechanism, which depends on cell migration, is named 'migracytosis'. The main components of the migrasome are extracellular vesicles after they leave the cell, which are easy to remind people of the current hot topic of exosomes. Exosomes are extracellular vesicles wrapped by the lipid bimolecular layer. With extensive research, exosomes have solved many disease problems. This review summarizes the differences between migrasomes and exosomes in size, composition, property and function, extraction method and regulation mechanism for generation and release. At the same time, it also prospects for the current hotspot of migrasomes, hoping to provide literature support for further research on the generation and release mechanism of migrasomes and their clinical application in the future.

α-Synuclein is the major platelet isoform but is dispensable for activation, secretion, and thrombosis

Platelets play many roles in the vasculature ensuring proper hemostasis and maintaining integrity. These roles are facilitated, in part, by cargo molecules released from platelet granules via Soluble NSF Attachment Protein Receptor (SNARE) mediated membrane fusion, which is controlled by several protein-protein interactions. Chaperones have been characterized for t-SNAREs (i.e. Munc18b for Syntaxin-11), but none have been clearly identified for v-SNAREs. α-Synuclein has been proposed as a v-SNARE chaperone which may affect SNARE-complex assembly, fusion pore opening, and thus secretion. Despite its abundance and that it is the only isoform present, α-synuclein's role in platelet secretion is uncharacterized. In this study, immunofluorescence showed that α-synuclein was present on punctate structures that co-stained with markers for α-granules and lysosomes and in a cytoplasmic pool. We analyzed the phenotype of α-synuclein-/- mice and their platelets. Platelets from knockout mice had a mild, agonist-dependent secretion defect but aggregation and spreading in vitro were unaffected. Consistently, thrombosis/hemostasis were unaffected in the tail-bleeding, FeCl3 carotid injury and jugular vein puncture models. None of the platelet secretory machinery examined, e.g. the v-SNAREs, were affected by α-synuclein's loss. The results indicate that, despite its abundance, α-synuclein has only a limited role in platelet function and thrombosis.

MARCKS Effector Domain, a reversible lipid ligand, illuminates late stages of membrane fusion

Yeast vacuolar HOPS tethers membranes, catalyzes trans-SNARE assembly between R- and Q-SNAREs, and shepherds SNAREs past early inhibition by Sec17. After partial SNARE zippering, fusion is driven slowly by either completion of SNARE zippering or by Sec17/Sec18, but rapid fusion needs zippering and Sec17/Sec18. Using reconstituted-vacuolar fusion, we find that MARCKS Effector Domain (MED) peptide, a lipid ligand, blocks fusion reversibly at a late reaction stage. The MED fusion blockade is overcome by either salt extraction, inactivation with the MED ligand calmodulin, or addition of Sec17/Sec18. During incubation with MED, SNAREs assemble stable complexes in trans and fusion becomes resistant to antibody to the Qa SNARE. When Q-SNAREs are preassembled, a synthetic tether can replace HOPS for fusion. With a synthetic tether, fusion needs both complete SNARE zippering and Sec17/Sec18 to overcome a MED block. In contrast, when SNARE domains are only two-third zippered, only HOPS will support Sec17/Sec18 driven fusion without needing complete zippering. HOPS thus remains engaged with SNAREs during zippering. MED facilitates the study of distinct fusion stages: tethering, initial trans-SNARE assembly and its sensitivity to Sec17, SNARE zippering, Sec17/Sec18 engagement, and lipid and lumenal mixing.

Cathepsin B knockout confers significant brain protection in the mouse model of stroke

BACKGROUND

Significant advances have been made in our understanding of the endolysosomal cycle. Disruption of this cycle leads to cell death. The objective of this study aims to investigate the role of disrupted endolysosomal cycle in brain ischemia-reperfusion injury.

METHODS

A total of 57 mice were randomly assigned into four experimental groups: (i) wildtype (wt) sham control; (ii) wt middle cerebral artery occlusion (MCAO); (iii) cathepsin B (CTSB) knockout (KO) sham control; and (iv) CTSB KO MCAO. Mice were subjected either to 0 min (sham) or 40 min of MCAO, followed by reperfusion for 1 or 7 days. Physical and behavioral examinations were conducted in the 7-day reperfusion group for 7 consecutive days after MCAO. Confocal microscopy was used to assess the levels, redistributions, and co-localizations of key endolysosomal cycle-related proteins. Histopathology was examined by light microscopy.

RESULTS

Confocal microscopy revealed a significant accumulation of CTSB in post-ischemic penumbral neurons relative to those in the sham group. In addition, N-ethylmaleimide sensitive factor ATPase (NSF) was irreversibly depleted in these neurons. Furthermore, CTSB-immunostained structures were enlarged and diffusely distributed in both the cytoplasm and extracellular space, indicating the release of CTSB from post-ischemic neurons. Compared to wt mice, CTSB KO mice showed a significant decrease in hippocampal injury area, a significant increase in the number of survival neurons in the striatal core area, and a significant improvement in physical and functional performance in post-MCAO mice.

CONCLUSION

Brain ischemia leads to a cascade of events leading to inactivation of NSF, disruption of the endolysosomal cycle, endolysosomal structural buildup and damage, and the release of CTSB, eventually resulting in brain ischemia reperfusion injury. CTSB KO in mice protected the brain from ischemia-reperfusion injury.

The machinery of vesicle fusion

The compartmentalization of eukaryotic cells is reliant on the fidelity of vesicle-mediated intracellular transport. Vesicles deliver their cargo via membrane fusion, a process requiring membrane tethers, Sec1/Munc18 (SM) proteins, and SNAREs. These components function in concert to ensure that membrane fusion is efficient and accurate, but the mechanisms underlying their cooperative action are still in many respects mysterious. In this brief review, we highlight recent progress toward a more integrative understanding of the vesicle fusion machinery. We focus particular attention on cryo-electron microscopy structures of intact multisubunit tethers in complex with SNAREs or SM proteins, as well as a structure of an SM protein bound to multiple SNAREs. The insights gained from this work emphasize the advantages of studying the fusion machinery intact and in context.

Role of SNAP25 on the occurrence and development of eosinophilic gastritis

Eosinophilic gastritis is characterized by gastrointestinal symptoms accompanied by peripheral eosinophilia. This study aims to explore the association between eosinophilic gastritis and Synaptosome Associated Protein 25 (SNAP25), and provide a new direction for the diagnosis and treatment of eosinophilic gastritis. GSE54043 was downloaded from the gene expression omnibus database. Differentially expressed genes (DEGs) were screened. The functions of common DEGs were annotated by Database for Annotation, Visualization and Integrated Discovery and Metascape. The protein-protein interaction network of common DEGs was obtained by Search Tool for the Retrieval of Interacting Genes and visualized by Cytoscape. Significant modules were identified from the protein-protein interaction network. A total of 186 patients with eosinophilic gastritis were recruited. The clinical data were recorded and the expression levels of CPE, SST, PCSK2, SNAP25, and SYT4 were detected. Pearson chi-square test and Spearman correlation coefficient were used to analyze the relationship between eosinophilic gastritis and related parameters. Univariate and multivariate Logistic regression were used for further analysis. 353 DEGs were presented. The top 10 genes screened by cytoHubb were shown, and Veen diagram figured out 5 mutual genes. Pearson's chi-square test showed that SNAP25 (P < .001) was significantly associated with eosinophilic gastritis. Spearman correlation coefficient showed a significant correlation between eosinophilic gastritis and SNAP25 (ρ = -0.569, P < .001). Univariate logistic regression analysis showed that SNAP25 (OR = 0.046, 95% CI: 0.018-0.116, P < .001) was significantly associated with eosinophilic gastritis. Multivariate logistic regression analysis showed that SNAP25 (OR = 0.024, 95% CI: 0.007-0.075, P < .001) was significantly associated with eosinophilic gastritis. The low expression of SNAP25 gene in eosinophilic gastritis is associated with a higher risk of eosinophilic gastritis.

Small-conductance calcium-activated potassium channels in the heart: expression, regulation and pathological implications

Ca2+-activated K+ channels are critical to cellular Ca2+ homeostasis and excitability; they couple intracellular Ca2+ and membrane voltage change. Of these, the small, 4-14 pS, conductance SK channels include three, KCNN1-3 encoded, SK1/KCa2.1, SK2/KCa2.2 and SK3/KCa2.3, channel subtypes with characteristic, EC50 ∼ 10 nM, 40 pM, 1 nM, apamin sensitivities. All SK channels, particularly SK2 channels, are expressed in atrial, ventricular and conducting system cardiomyocytes. Pharmacological and genetic modification results have suggested that SK channel block or knockout prolonged action potential durations (APDs) and effective refractory periods (ERPs) particularly in atrial, but also in ventricular, and sinoatrial, atrioventricular node and Purkinje myocytes, correspondingly affect arrhythmic tendency. Additionally, mitochondrial SK channels may decrease mitochondrial Ca2+ overload and reactive oxygen species generation. SK channels show low voltage but marked Ca2+ dependences (EC50 ∼ 300-500 nM) reflecting their α-subunit calmodulin (CaM) binding domains, through which they may be activated by voltage-gated or ryanodine-receptor Ca2+ channel activity. SK function also depends upon complex trafficking and expression processes and associations with other ion channels or subunits from different SK subtypes. Atrial and ventricular clinical arrhythmogenesis may follow both increased or decreased SK expression through decreased or increased APD correspondingly accelerating and stabilizing re-entrant rotors or increasing incidences of triggered activity. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

NSF/αSNAP2-mediated cis-SNARE complex disassembly precedes vesicle fusion in Arabidopsis cytokinesis

Eukaryotic membrane fusion requires trans-SNARE complexes bridging the gap between adjacent membranes1. Fusion between a transport vesicle and its target membrane transforms the trans- into a cis-SNARE complex. The latter interacts with the hexameric AAA+-ATPase N-ethylmaleimide-sensitive factor (NSF) and its co-factor alpha-soluble NSF attachment protein (αSNAP), forming a 20S complex2,3. ATPase activity disassembles the SNAP receptor (SNARE) complex into Qa-SNARE, which folds back onto itself, and its partners4,5. The fusion of identical membranes has a different sequence of events6. The fusion partners each have cis-SNARE complexes to be broken up by NSF and αSNAP. The Qa-SNARE monomers are then stabilized by interaction with Sec1/Munc18-type regulators (SM proteins) to form trans-SNARE complexes, as shown for the yeast vacuole7. Membrane fusion in Arabidopsis cytokinesis is formally akin to vacuolar fusion8. Membrane vesicles fuse with one another to form the partitioning membrane known as the cell plate. Cis-SNARE complexes of cytokinesis-specific Qa-SNARE KNOLLE and its SNARE partners are assembled at the endoplasmic reticulum and delivered by traffic via the Golgi/trans-Golgi network to the cell division plane9. The SM protein KEULE is required for the formation of trans-SNARE complexes between adjacent membrane vesicles10. Here we identify NSF and its adaptor αSNAP2 as necessary for the disassembly of KNOLLE cis-SNARE complexes, which is a prerequisite for KNOLLE-KEULE interaction in cytokinesis. In addition, we show that NSF is required for other trafficking pathways and interacts with the respective Q-SNAREs. The SNARE complex disassembly machinery is conserved in plants and plays a unique essential role in cytokinesis.

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.

Regulation of phospholipid distribution in the lipid bilayer by flippases and scramblases

Cellular membranes function as permeability barriers that separate cells from the external environment or partition cells into distinct compartments. These membranes are lipid bilayers composed of glycerophospholipids, sphingolipids and cholesterol, in which proteins are embedded. Glycerophospholipids and sphingolipids freely move laterally, whereas transverse movement between lipid bilayers is limited. Phospholipids are asymmetrically distributed between membrane leaflets but change their location in biological processes, serving as signalling molecules or enzyme activators. Designated proteins - flippases and scramblases - mediate this lipid movement between the bilayers. Flippases mediate the confined localization of specific phospholipids (phosphatidylserine (PtdSer) and phosphatidylethanolamine) to the cytoplasmic leaflet. Scramblases randomly scramble phospholipids between leaflets and facilitate the exposure of PtdSer on the cell surface, which serves as an important signalling molecule and as an 'eat me' signal for phagocytes. Defects in flippases and scramblases cause various human diseases. We herein review the recent research on the structure of flippases and scramblases and their physiological roles. Although still poorly understood, we address the mechanisms by which they translocate phospholipids between lipid bilayers and how defects cause human diseases.

The Sec1/Munc18 protein VPS33B forms a uniquely bidirectional complex with VPS16B

Loss of function variants of VPS33B and VIPAS39 (encoding VPS16B) are causative for arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, where early lethality of patients indicates that VPS33B and VPS16B play essential cellular roles. VPS33B is a member of the Sec1/Munc18 (SM) protein family, and thus thought to facilitate vesicular fusion via interaction with SNARE complexes, as does its paralog VPS33A in the homotypic fusion and vacuole sorting (HOPS) complex. VPS33B and VPS16B have been shown to associate, but little is known about the composition, structure or function of the VPS33B/VPS16B complex. We show here that human VPS33B/VPS16B is a high molecular weight complex, which we expressed in yeast to obtain material for structural, composition and stability analysis. Circular dichroism data indicate VPS33B/VPS16B has a well-folded α-helical secondary structure, for which size exclusion chromatography-multi angle light scattering revealed a MW of ∼315 kDa. Quantitative immunoblotting indicated the complex has a VPS33B:VPS16B ratio of 2:3. Expression of ARC syndrome-causing VPS33B missense variants showed that L30P disrupts complex formation, but not S243F or H344D. Truncated VPS16B containing amino acids 143-316 was sufficient to form a complex with VPS33B. Small angle X-ray scattering and negative staining electron microscopy revealed a two-lobed shape for VPS33B/VPS16B. Avidin tagging indicated that each lobe contains a VPS33B molecule, and they are oriented in opposite directions. From this we propose a structure for VPS33B/VPS16B that allows the copies of VPS33B at each end to interact with separate SNARE bundles and/or SNAREpins, plus their associated membrane components. Thus our observations reveal the only known potentially bidirectional SM protein complex.

Rapamycin Affects the Hippocampal SNARE Complex to Alleviate Cognitive Dysfunction Induced by Surgery in Aged Rats

Delayed neurocognitive recovery (dNCR) is a common complication that occurs post-surgery, especially in elderly individuals. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex plays an essential role in various membrane fusion events, such as synaptic vesicle exocytosis and autophagosome-lysosome fusion. Although SNARE complex dysfunction has been observed in several neurodegenerative disorders, the causal link between SNARE-mediated membrane fusion and dNCR remains unclear. We previously demonstrated that surgical stimuli caused cognitive impairment in aged rats by inducing α-synuclein accumulation, inhibiting autophagy, and disrupting neurotransmitter release in hippocampal synaptosomes. Here, we evaluated the effects of propofol anesthesia plus surgery on learning and memory and investigated levels of SNARE proteins and chaperones in hippocampal synaptosomes. Aged rats that received propofol anesthesia and surgery exhibited learning and memory impairments in a Morris water maze test and decreased levels of synaptosome-associated protein 25, synaptobrevin/vesicle-associated membrane protein 2, and syntaxin 1. Levels of SNARE chaperones, including mammalian uncoordinated-18, complexins 1 and 2, cysteine string protein-α, and N-ethylmaleimide-sensitive factor, were all significantly decreased following anesthesia with surgical stress. However, the synaptic vesicle marker synaptophysin was unaffected. The autophagy-enhancer rapamycin attenuated structural and functional disturbances of the SNARE complex and ameliorated disrupted neurotransmitter release. Our results indicate that perturbations of SNARE proteins in hippocampal synaptosomes may underlie the occurrence of dNCR. Moreover, the protective effect of rapamycin may partially occur through recovery of SNARE structural and functional abnormalities. Our findings provide insight into the molecular mechanisms underlying dNCR.

PI3P regulates multiple stages of membrane fusion

The conserved catalysts of intracellular membrane fusion are Rab-family GTPases, effector complexes that bind Rabs for membrane tethering, SNARE proteins of the R, Qa, Qb, and Qc families, and SNARE chaperones of the SM, Sec17/SNAP, and Sec18/NSF families. Yeast vacuole fusion is regulated by phosphatidylinositol-3-phosphate (PI3P). PI3P binds directly to the vacuolar Qc-SNARE and to HOPS, the vacuolar tethering/SM complex. We now report several distinct functions of PI3P in fusion. PI3P binds the N-terminal PX domain of the Qc-SNARE to enhance its engagement for fusion. Even when Qc has been preassembled with the Qa- and Qb-SNAREs, PI3P still promotes trans-SNARE assembly and fusion between these 3Q proteoliposomes and those with R-SNARE, whether with the natural HOPS tether or with a synthetic tether. With HOPS, efficient trans-SNARE complex formation needs PI3P on the 3Q-SNARE proteoliposomes, in cis to the Qc. PI3P is also needed for HOPS to confer resistance to Sec17/Sec18. With a synthetic tether, fusion is supported by PI3P on either fusion partner membrane, but this fusion is blocked by Sec17/Sec18. PI3P thus supports multiple stages of fusion: the engagement of the Qc-SNARE, trans-SNARE complex formation with preassembled Q-SNAREs, HOPS protection of SNARE complexes from Sec17/Sec18, and fusion per se after tethering and Q-SNARE assembly.

VPS45 is required for both diffuse and tip growth of Arabidopsis thaliana cells

INTRODUCTION

VPS45 belongs to the Sec1/Munc18 family of proteins, which interact with and regulate Qa-SNARE function during membrane fusion. We have shown previously that Arabidopsis thaliana VPS45 interacts with the SYP61/SYP41/VTI12 SNARE complex, which locates on the trans-Golgi network (TGN). It is required for SYP41 stability, and it functions in cargo trafficking to the vacuole and in cell expansion. It is also required for correct auxin distribution during gravitropism and lateral root growth.

RESULTS

As vps45 knockout mutation is lethal in Arabidopsis, we identified a mutant, vps45-3, with a point mutation in the VPS45 gene causing a serine 284-to-phenylalanine substitution. The VPS45-3 protein is stable and maintains interaction with SYP61 and SYP41. However, vps45-3 plants display severe growth defects with significantly reduced organ and cell size, similar to vps45 RNAi transgenic lines that have reduced VPS45 protein levels. Root hair and pollen tube elongation, both processes of tip growth, are highly compromised in vps45-3. Mutant root hairs are shorter and thicker than those of wild-type plants, and are wavy. These root hairs have vacuolar defects, containing many small vacuoles, compared with WT root hairs with a single large vacuole occupying much of the cell volume. Pollen tubes were also significantly shorter in vps45-3 compared to WT.

DISCUSSION

We thus show that VPS45 is essential for proper tip growth and propose that the observed vacuolar defects lead to loss of the turgor pressure needed for tip growth.

MitoSNARE Assembly and Disassembly Factors Regulate Basal Autophagy and Aging in C. elegans

SNARE proteins reside between opposing membranes and facilitate vesicle fusion, a physiological process ubiquitously required for secretion, endocytosis and autophagy. With age, neurosecretory SNARE activity drops and is pertinent to age-associated neurological disorders. Despite the importance of SNARE complex assembly and disassembly in membrane fusion, their diverse localization hinders the complete understanding of their function. Here, we revealed a subset of SNARE proteins, the syntaxin SYX-17, the synaptobrevins VAMP-7, SNB-6 and the tethering factor USO-1, to be either localized or in close proximity to mitochondria, in vivo. We term them mitoSNAREs and show that animals deficient in mitoSNAREs exhibit increased mitochondria mass and accumulation of autophagosomes. The SNARE disassembly factor NSF-1 seems to be required for the effects of mitoSNARE depletion. Moreover, we find mitoSNAREs to be indispensable for normal aging in both neuronal and non-neuronal tissues. Overall, we uncover a previously unrecognized subset of SNAREs that localize to mitochondria and propose a role of mitoSNARE assembly and disassembly factors in basal autophagy regulation and aging.

Controlled sequential in situ self-assembly and disassembly of a fluorogenic cisplatin prodrug for cancer theranostics

Temporal control of delivery and release of drugs in tumors are important in improving therapeutic outcomes to patients. Here, we report a sequential stimuli-triggered in situ self-assembly and disassembly strategy to direct delivery and release of theranostic drugs in vivo. Using cisplatin as a model anticancer drug, we design a stimuli-responsive small-molecule cisplatin prodrug (P-CyPt), which undergoes extracellular alkaline phosphatase-triggered in situ self-assembly and succeeding intracellular glutathione-triggered disassembly process, allowing to enhance accumulation and elicit burst release of cisplatin in tumor cells. Compared with cisplatin, P-CyPt greatly improves antitumor efficacy while mitigates off-target toxicity in mice with subcutaneous HeLa tumors and orthotopic HepG2 liver tumors after systemic administration. Moreover, P-CyPt also produces activated near-infrared fluorescence (at 710 nm) and dual photoacoustic imaging signals (at 700 and 750 nm), permitting high sensitivity and spatial-resolution delineation of tumor foci and real-time monitoring of drug delivery and release in vivo. This strategy leverages the advantages offered by in situ self-assembly with those of intracellular disassembly, which may act as a general platform for the design of prodrugs capable of improving drug delivery for cancer theranostics.

Structure of a Membrane Tethering Complex Incorporating Multiple SNAREs

Most membrane fusion reactions in eukaryotic cells are mediated by membrane tethering complexes (MTCs) and SNARE proteins. MTCs are much larger than SNAREs and are thought to mediate the initial attachment of two membranes. Complementary SNAREs then form membrane-bridging complexes whose assembly draws the membranes together for fusion. Here, we present a cryo-EM structure of the simplest known MTC, the 255-kDa Dsl1 complex, bound to the two SNAREs that anchor it to the endoplasmic reticulum. N-terminal domains of the SNAREs form an integral part of the structure, stabilizing a Dsl1 complex configuration with remarkable and unexpected similarities to the 850-kDa exocyst MTC. The structure of the SNARE-anchored Dsl1 complex and its comparison with exocyst reveal what are likely to be common principles underlying MTC function. Our structure also implies that tethers and SNAREs can work together as a single integrated machine.

Molecular Mechanisms of Neuroinflammation in Aging and Alzheimer's Disease Progression

Aging is the most prominent risk factor for late-onset Alzheimer's disease. Aging associates with a chronic inflammatory state both in the periphery and in the central nervous system, the evidence thereof and the mechanisms leading to chronic neuroinflammation being discussed. Nonetheless, neuroinflammation is significantly enhanced by the accumulation of amyloid beta and accelerates the progression of Alzheimer's disease through various pathways discussed in the present review. Decades of clinical trials targeting the 2 abnormal proteins in Alzheimer's disease, amyloid beta and tau, led to many failures. As such, targeting neuroinflammation via different strategies could prove a valuable therapeutic strategy, although much research is still needed to identify the appropriate time window. Active research focusing on identifying early biomarkers could help translating these novel strategies from bench to bedside.

Molecular insights into endolysosomal microcompartment formation and maintenance

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

A dynamic template complex mediates Munc18-chaperoned SNARE assembly

Munc18 chaperones assembly of three membrane-anchored soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) into a four-helix bundle to mediate membrane fusion between vesicles and plasma membranes, leading to neurotransmitter or insulin release, glucose transporter (GLUT4) translocation, or other exocytotic processes. Yet, the molecular mechanism underlying chaperoned SNARE assembly is not well understood. Recent evidence suggests that Munc18-1 and Munc18-3 simultaneously bind their cognate SNAREs to form ternary template complexes - Munc18-1:Syntaxin-1:VAMP2 for synaptic vesicle fusion and Munc18-3:Syntaxin-4:VAMP2 for GLUT4 translocation and insulin release, which facilitate the binding of SNAP-25 or SNAP-23 to conclude SNARE assembly. Here, we further investigate the structure, dynamics, and function of the template complexes using optical tweezers. Our results suggest that the synaptic template complex transitions to an activated state with a rate of 0.054 s^-1 for efficient SNAP-25 binding. The transition depends upon the linker region of syntaxin-1 upstream of its helical bundle-forming SNARE motif. In addition, the template complex is stabilized by a poorly characterized disordered loop region in Munc18-1. While the synaptic template complex efficiently binds both SNAP-25 and SNAP-23, the GLUT4 template complex strongly favors SNAP-23 over SNAP-25, despite the similar stabilities of their assembled SNARE bundles. Together, our data demonstrate that a highly dynamic template complex mediates efficient and specific SNARE assembly.

SNARE Modulators and SNARE Mimetic Peptides

The soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) proteins play a central role in most forms of intracellular membrane trafficking, a key process that allows for membrane and biocargo shuffling between multiple compartments within the cell and extracellular environment. The structural organization of SNARE proteins is relatively simple, with several intrinsically disordered and folded elements (e.g., SNARE motif, N-terminal domain, transmembrane region) that interact with other SNAREs, SNARE-regulating proteins and biological membranes. In this review, we discuss recent advances in the development of functional peptides that can modify SNARE-binding interfaces and modulate SNARE function. The ability of the relatively short SNARE motif to assemble spontaneously into stable coiled coil tetrahelical bundles has inspired the development of reduced SNARE-mimetic systems that use peptides for biological membrane fusion and for making large supramolecular protein complexes. We evaluate two such systems, based on peptide-nucleic acids (PNAs) and coiled coil peptides. We also review how the self-assembly of SNARE motifs can be exploited to drive on-demand assembly of complex re-engineered polypeptides.

High-resolution secretory timeline from vesicle formation at the Golgi to fusion at the plasma membrane in S. cerevisiae

Most of the components in the yeast secretory pathway have been studied, yet a high-resolution temporal timeline of their participation is lacking. Here, we define the order of acquisition, lifetime, and release of critical components involved in late secretion from the Golgi to the plasma membrane. Of particular interest is the timing of the many reported effectors of the secretory vesicle Rab protein Sec4, including the myosin-V Myo2, the exocyst complex, the lgl homolog Sro7, and the small yeast-specific protein Mso1. At the trans-Golgi network (TGN) Sec4's GEF, Sec2, is recruited to Ypt31-positive compartments, quickly followed by Sec4 and Myo2 and vesicle formation. While transported to the bud tip, the entire exocyst complex, including Sec3, is assembled on to the vesicle. Before fusion, vesicles tether for 5 s, during which the vesicle retains the exocyst complex and stimulates lateral recruitment of Rho3 on the plasma membrane. Sec2 and Myo2 are rapidly lost, followed by recruitment of cytosolic Sro7, and finally the SM protein Sec1, which appears for just 2 s prior to fusion. Perturbation experiments reveal an ordered and robust series of events during tethering that provide insights into the function of Sec4 and effector exchange.

Plasma membrane flipping of Syntaxin-2 regulates its inhibitory action on insulin granule exocytosis

Enhancing pancreatic β-cell secretion is a primary therapeutic target for type-2 diabetes (T2D). Syntaxin-2 (Stx2) has just been identified to be an inhibitory SNARE for insulin granule exocytosis, holding potential as a treatment for T2D, yet its molecular underpinnings remain unclear. We show that excessive Stx2 recruitment to raft-like granule docking sites at higher binding affinity than pro-fusion syntaxin-1A effectively competes for and inhibits fusogenic SNARE machineries. Depletion of Stx2 in human β-cells improves insulin secretion by enhancing trans-SNARE complex assembly and cis-SNARE disassembly. Using a genetically-encoded reporter, glucose stimulation is shown to induce Stx2 flipping across the plasma membrane, which relieves its suppression of cytoplasmic fusogenic SNARE complexes to promote insulin secretion. Targeting the flipping efficiency of Stx2 profoundly modulates secretion, which could restore the impaired insulin secretion in diabetes. Here, we show that Stx2 acts to assist this precise tuning of insulin secretion in β-cells, including in diabetes.

The critical role of the endolysosomal system in cerebral ischemia

Cerebral ischemia is a serious disease that triggers sequential pathological mechanisms, leading to significant morbidity and mortality. Although most studies to date have typically focused on the lysosome, a single organelle, current evidence supports that the function of lysosomes cannot be separated from that of the endolysosomal system as a whole. The associated membrane fusion functions of this system play a crucial role in the biodegradation of cerebral ischemia-related products. Here, we review the regulation of and the changes that occur in the endolysosomal system after cerebral ischemia, focusing on the latest research progress on membrane fusion function. Numerous proteins, including N-ethylmaleimide-sensitive factor and lysosomal potassium channel transmembrane protein 175, regulate the function of this system. However, these proteins are abnormally expressed after cerebral ischemic injury, which disrupts the normal fusion function of membranes within the endolysosomal system and that between autophagosomes and lysosomes. This results in impaired "maturation" of the endolysosomal system and the collapse of energy metabolism balance and protein homeostasis maintained by the autophagy-lysosomal pathway. Autophagy is the final step in the endolysosomal pathway and contributes to maintaining the dynamic balance of the system. The process of autophagosome-lysosome fusion is a necessary part of autophagy and plays a crucial role in maintaining energy homeostasis and clearing aging proteins. We believe that, in cerebral ischemic injury, the endolysosomal system should be considered as a whole rather than focusing on the lysosome. Understanding how this dynamic system is regulated will provide new ideas for the treatment of cerebral ischemia.

Vesicle trafficking with snares: a perspective for autism

Vesicle-mediated membrane traffic is the mechanism fundamental to many biological events, especially the release of neurotransmitters. The main proteins of the mechanism that mediates membrane fusion in vesicle-mediated membrane traffic are N-ethylmaleimide sensitive factor (NSF) supplemental protein (SNAP) receptor (SNAREs) proteins. SNAREs are classified into vesicle-associated SNAREs (vesicle-SNAREs/v-SNAREs) and target membrane-associated SNAREs (target-SNARE/t-SNAREs). Autism spectrum disorders (ASD) are neurodevelopmental disorders characterized by many symptoms, especially complications in social communication and stereotypical behaviours. Defects in synaptogenesis and neurotransmission, oxidative stress, and developmental defects in the early stages of development are defined in the pathogenesis of the disease. SNARE proteins are on the basis of synaptogenesis and neurotransmission. Although the formation mechanisms and underlying causes of the SNARE complex are not fully understood, expression differences, polymorphisms, abnormal expressions or dysfunctions of the proteins that make up the SNARE complex have been associated with many neurodevelopmental diseases, including autism. Further understanding of SNARE mechanisms is crucial both for understanding ASD and for developing new treatments. In this review, the formation mechanisms of the SNARE complex and the roles of various factors involved in this process are explained. In addition, a brief evaluation of clinical and basic studies on the SNARE complex in autism spectrum disorders was made.

FLOWERING REPRESSOR AAA+ ATPase 1 is a novel regulator of perennial flowering in Arabis alpina

Arabis alpina is a polycarpic perennial, in which PERPETUAL FLOWERING1 (PEP1) regulates flowering and perennial traits in a vernalization-dependent manner. Mutagenesis screens of the pep1 mutant established the role of other flowering time regulators in PEP1-parallel pathways. Here we characterized three allelic enhancers of pep1 (eop002, 085 and 091) which flower early. We mapped the causal mutations and complemented mutants with the identified gene. Using quantitative reverse transcriptase PCR and reporter lines, we determined the protein spatiotemporal expression patterns and localization within the cell. We also characterized its role in Arabidopsis thaliana using CRISPR and in A. alpina by introgressing mutant alleles into a wild-type background. These mutants carried lesions in an AAA⁺ ATPase of unknown function, FLOWERING REPRESSOR AAA⁺ ATPase 1 (AaFRAT1). AaFRAT1 was detected in the vasculature of young leaf primordia and the rib zone of flowering shoot apical meristems. At the subcellular level, AaFRAT1 was localized at the interphase between the endoplasmic reticulum and peroxisomes. Introgression lines carrying Aafrat1 alleles required less vernalization to flower and reduced number of vegetative axillary branches. By contrast, A. thaliana CRISPR lines showed weak flowering phenotypes. AaFRAT1 contributes to flowering time regulation and the perennial growth habit of A. alpina.

Host Cell Signatures of the Envelopment Site within Beta-Herpes Virions

Beta-herpesvirus infection completely reorganizes the membrane system of the cell. This system is maintained by the spatiotemporal arrangement of more than 3000 cellular proteins that continuously adapt the configuration of membrane organelles according to cellular needs. Beta-herpesvirus infection establishes a new configuration known as the assembly compartment (AC). The AC membranes are loaded with virus-encoded proteins during the long replication cycle and used for the final envelopment of the newly formed capsids to form infectious virions. The identity of the envelopment membranes is still largely unknown. Electron microscopy and immunofluorescence studies suggest that the envelopment occurs as a membrane wrapping around the capsids, similar to the growth of phagophores, in the area of the AC with the membrane identities of early/recycling endosomes and the trans-Golgi network. During wrapping, host cell proteins that define the identity and shape of these membranes are captured along with the capsids and incorporated into the virions as host cell signatures. In this report, we reviewed the existing information on host cell signatures in human cytomegalovirus (HCMV) virions. We analyzed the published proteomes of the HCMV virion preparations that identified a large number of host cell proteins. Virion purification methods are not yet advanced enough to separate all of the components of the rich extracellular material, including the large amounts of non-vesicular extracellular particles (NVEPs). Therefore, we used the proteomic data from large and small extracellular vesicles (lEVs and sEVs) and NVEPs to filter out the host cell proteins identified in the viral proteomes. Using these filters, we were able to narrow down the analysis of the host cell signatures within the virions and determine that envelopment likely occurs at the membranes derived from the tubular recycling endosomes. Many of these signatures were also found at the autophagosomes, suggesting that the CMV-infected cell forms membrane organelles with phagophore growth properties using early endosomal host cell machinery that coordinates endosomal recycling.

Molecular Mechanism and Regulation of Autophagy and Its Potential Role in Epilepsy

Autophagy is an evolutionally conserved degradation mechanism for maintaining cell homeostasis whereby cytoplasmic components are wrapped in autophagosomes and subsequently delivered to lysosomes for degradation. This process requires the concerted actions of multiple autophagy-related proteins and accessory regulators. In neurons, autophagy is dynamically regulated in different compartments including soma, axons, and dendrites. It determines the turnover of selected materials in a spatiotemporal control manner, which facilitates the formation of specialized neuronal functions. It is not surprising, therefore, that dysfunctional autophagy occurs in epilepsy, mainly caused by an imbalance between excitation and inhibition in the brain. In recent years, much attention has been focused on how autophagy may cause the development of epilepsy. In this article, we overview the historical landmarks and distinct types of autophagy, recent progress in the core machinery and regulation of autophagy, and biological roles of autophagy in homeostatic maintenance of neuronal structures and functions, with a particular focus on synaptic plasticity. We also discuss the relevance of autophagy mechanisms to the pathophysiology of epileptogenesis.

Energetics, kinetics, and pathways of SNARE assembly in membrane fusion

Fusion of transmitter-containing vesicles with plasma membranes at the synaptic and neuromuscular junctions mediates neurotransmission and muscle contractions, respectively, thereby underlying all thoughts and actions. The fusion process is driven by the coupled folding and assembly of three synaptic SNARE proteins--syntaxin-1 and SNAP-25 on the target plasma membrane (t-SNAREs) and VAMP2 on the vesicular membrane (v-SNARE) into a four-helix bundle. Their assembly is chaperoned by Munc18-1 and many other proteins to achieve the speed and accuracy required for neurotransmission. However, the physiological pathway of SNARE assembly and its coupling to membrane fusion remains unclear. Here, we review recent progress in understanding SNARE assembly and membrane fusion, with a focus on results obtained by single-molecule manipulation approaches and electric recordings of single fusion pores. We describe two pathways of synaptic SNARE assembly, their associated intermediates, energetics, and kinetics. Assembly of the three SNAREs in vitro begins with the formation of a t-SNARE binary complex, on which VAMP2 folds in a stepwise zipper-like fashion. Munc18-1 significantly alters the SNARE assembly pathway: syntaxin-1 and VAMP2 first bind on the surface of Munc18-1 to form a template complex, with which SNAP-25 associates to conclude SNARE assembly and displace Munc18-1. During membrane fusion, multiple trans-SNARE complexes cooperate to open a dynamic fusion pore in a manner dependent upon their copy number and zippering states. Together, these results demonstrate that stepwise and cooperative SNARE assembly drive stagewise membrane fusion.

Synaptic Secretion and Beyond: Targeting Synapse and Neurotransmitters to Treat Neurodegenerative Diseases

The nervous system is important, because it regulates the physiological function of the body. Neurons are the most basic structural and functional unit of the nervous system. The synapse is an asymmetric structure that is important for neuronal function. The chemical transmission mode of the synapse is realized through neurotransmitters and electrical processes. Based on vesicle transport, the abnormal information transmission process in the synapse can lead to a series of neurorelated diseases. Numerous proteins and complexes that regulate the process of vesicle transport, such as SNARE proteins, Munc18-1, and Synaptotagmin-1, have been identified. Their regulation of synaptic vesicle secretion is complicated and delicate, and their defects can lead to a series of neurodegenerative diseases. This review will discuss the structure and functions of vesicle-based synapses and their roles in neurons. Furthermore, we will analyze neurotransmitter and synaptic functions in neurodegenerative diseases and discuss the potential of using related drugs in their treatment.

PACSIN1 is indispensable for amphisome-lysosome fusion during basal autophagy and subsets of selective autophagy

Autophagy is an indispensable process that degrades cytoplasmic materials to maintain cellular homeostasis. During autophagy, double-membrane autophagosomes surround cytoplasmic materials and either fuse with endosomes (called amphisomes) and then lysosomes, or directly fuse with lysosomes, in both cases generating autolysosomes that degrade their contents by lysosomal hydrolases. However, it remains unclear if there are specific mechanisms and/or conditions which distinguish these alternate routes. Here, we identified PACSIN1 as a novel autophagy regulator. PACSIN1 deletion markedly decreased autophagic activity under basal nutrient-rich conditions but not starvation conditions, and led to amphisome accumulation as demonstrated by electron microscopic and co-localization analysis, indicating inhibition of lysosome fusion. PACSIN1 interacted with SNAP29, an autophagic SNARE, and was required for proper assembly of the STX17 and YKT6 complexes. Moreover, PACSIN1 was required for lysophagy, aggrephagy but not mitophagy, suggesting cargo-specific fusion mechanisms. In C. elegans, deletion of sdpn-1, a homolog of PACSINs, inhibited basal autophagy and impaired clearance of aggregated protein, implying a conserved role of PACSIN1. Taken together, our results demonstrate the amphisome-lysosome fusion process is preferentially regulated in response to nutrient state and stress, and PACSIN1 is a key to specificity during autophagy.

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.