The C-terminal transmembrane region of synaptobrevin binds synaptophysin from adult synaptic vesicles

Synaptophysin-dependent synaptobrevin-2 trafficking at the presynapse-Mechanism and function

Synaptobrevin-2 (Syb2) is a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) that is essential for neurotransmitter release. It is the most numerous protein on a synaptic vesicle (SV) and drives SV fusion via interactions with its cognate SNARE partners on the presynaptic plasma membrane. Synaptophysin (Syp) is the second most abundant protein on SVs; however, in contrast to Syb2, it has no obligatory role in neurotransmission. Syp interacts with Syb2 on SVs, and the molecular nature of its interaction with Syb2 and its physiological role has been debated for decades. However, recent studies have revealed that the sole physiological role of Syp at the presynapse is to ensure the efficient retrieval of Syb2 during SV endocytosis. In this review, current theories surrounding the role of Syp in Syb2 trafficking will be discussed, in addition to the debate regarding the molecular nature of their interaction. A unifying model is presented that describes how Syp controls Syb2 function as part of an integrated mechanism involving key molecular players such as intersectin-1 and AP180/CALM. Finally, key future questions surrounding the role of Syp-dependent Syb2 trafficking will be posed, with respect to brain function in health and disease.

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.

Synaptophysin controls synaptobrevin-II retrieval via a cryptic C-terminal interaction site

The accurate retrieval of synaptic vesicle (SV) proteins during endocytosis is essential for the maintenance of neurotransmission. Synaptophysin (Syp) and synaptobrevin-II (SybII) are the most abundant proteins on SVs. Neurons lacking Syp display defects in the activity-dependent retrieval of SybII and a general slowing of SV endocytosis. To determine the role of the cytoplasmic C terminus of Syp in the control of these two events, we performed molecular replacement studies in primary cultures of Syp knockout neurons using genetically encoded reporters of SV cargo trafficking at physiological temperatures. Under these conditions, we discovered, 1) no slowing in SV endocytosis in Syp knockout neurons, and 2) a continued defect in SybII retrieval in knockout neurons expressing a form of Syp lacking its C terminus. Sequential truncations of the Syp C-terminus revealed a cryptic interaction site for the SNARE motif of SybII that was concealed in the full-length form. This suggests that a conformational change within the Syp C terminus is key to permitting SybII binding and thus its accurate retrieval. Furthermore, this study reveals that the sole presynaptic role of Syp is the control of SybII retrieval, since no defect in SV endocytosis kinetics was observed at physiological temperatures.

Comprehensive Characterization of Nanosized Extracellular Vesicles from Central and Peripheral Organs : Implications for Preclinical and Clinical Applications

Extracellular vesicles (EV) are nano-sized vesicles that have been garnering a lot of attention for their valuable role as potential diagnostic markers and therapeutic vehicles for a plethora of pathologies. Whilst EV markers from biofluids such as plasma, serum, urine, cerebrospinal fluid and in vitro cell culture based platforms have been extensively studied, a significant knowledge gap that remains is the characterization of specific organ derived EVs (ODE). Here, we present a standardized protocol for isolation and characterization of purified EV isolated from brain, heart, lung, kidney and liver from rat and postmortem human tissue. Next, using quantitative mass spectrometry based proteomics, we characterized the respective tissue EV proteomes that identified synaptophysin (SYP), caveolin-3 (CAV3), solute carrier family 22 member 2 (SLC22A2), surfactant protein B (SP-B), and fatty acid-binding protein 1 (FABP1) as potential markers for the brain, heart, kidney, lung, and liver-EV, respectively. These respective tissue specific markers were further validated using both immunoblotting and a nanoplasmonic platform- single EV imaging analysis in the two species. To summarize, our study for the first time using traditional biochemical and high precision technology platforms provide a valuable proof of concept approach in defining specific ODE markers which further could be developed as potential therapeutic candidates for respective end-organ associated pathologies.

Altered synaptobrevin-II trafficking in neurons expressing a synaptophysin mutation associated with a severe neurodevelopmental disorder

Following exocytosis, synaptic vesicles (SVs) have to be reformed with the correct complement of proteins in the correct stoichiometry to ensure continued neurotransmission. Synaptophysin is a highly abundant, integral SV protein necessary for the efficient retrieval of the SV SNARE protein, synaptobrevin II (sybII). However the molecular mechanism underpinning synaptophysin-dependent sybII retrieval is still unclear. We recently identified a male patient with severe intellectual disability, hypotonia, epilepsy and callosal agenesis who has a point mutation in the juxtamembrane region of the fourth transmembrane domain of synaptophysin (T198I). This mutation had no effect on the activity-dependent retrieval of synaptophysin that was tagged with the genetically-encoded pH-sensitive reporter (pHluorin) in synaptophysin knockout hippocampal cultures. This suggested the mutant has no global effect on SV endocytosis, which was confirmed when retrieval of a different SV cargo (the glutamate transporter vGLUT1) was examined. However neurons expressing this T198I mutant did display impaired activity-dependent sybII retrieval, similar to that observed in synaptophysin knockout neurons. Interestingly this impairment did not result in an increased stranding of sybII at the plasma membrane. Screening of known human synaptophysin mutations revealed a similar presynaptic phenotype between T198I and a mutation found in X-linked intellectual disability. Thus this novel human synaptophysin mutation has revealed that aberrant retrieval and increased plasma membrane localisation of SV cargo can be decoupled in human disease.

A Fine Balance of Synaptophysin Levels Underlies Efficient Retrieval of Synaptobrevin II to Synaptic Vesicles

Synaptobrevin II (sybII) is a vesicular soluble NSF attachment protein receptor (SNARE) protein that is essential for neurotransmitter release, and thus its correct trafficking to synaptic vesicles (SVs) is critical to render them fusion competent. The SV protein synaptophysin binds to sybII and facilitates its retrieval to SVs during endocytosis. Synaptophysin and sybII are the two most abundant proteins on SVs, being present in a 1:2 ratio. Synaptophysin and sybII are proposed to form a large multimeric complex, and the copy number of the proteins in this complex is also in a 1:2 ratio. We investigated the importance of this ratio between these proteins for the localisation and trafficking of sybII in central neurons. SybII was overexpressed in mouse hippocampal neurons at either 1.6 or 2.15–2.35-fold over endogenous protein levels, in the absence or presence of varying levels of synaptophysin. In the absence of exogenous synaptophysin, exogenous sybII was dispersed along the axon, trapped on the plasma membrane and retrieved slowly during endocytosis. Co-expression of exogenous synaptophysin rescued all of these defects. Importantly, the expression of synaptophysin at nerve terminals in a 1:2 ratio with sybII was sufficient to fully rescue normal sybII trafficking. These results demonstrate that the balance between synaptophysin and sybII levels is critical for the correct targeting of sybII to SVs and suggests that small alterations in synaptophysin levels might affect the localisation of sybII and subsequent presynaptic performance.

The iTRAPs: Guardians of Synaptic Vesicle Cargo Retrieval During Endocytosis

The reformation of synaptic vesicles (SVs) during endocytosis is essential for the maintenance of neurotransmission in central nerve terminals. Newly formed SVs must be generated with the correct protein cargo in the correct stoichiometry to be functional for exocytosis. Classical clathrin adaptor protein complexes play a key role in sorting and clustering synaptic vesicle cargo in this regard. However it is becoming increasingly apparent that additional “fail-safe” mechanisms exist to ensure the accurate retrieval of essential cargo molecules. For example, the monomeric adaptor proteins AP180/CALM and stonin-2 are required for the efficient retrieval of synaptobrevin II (sybII) and synaptotagmin-1 respectively. Furthermore, recent studies have revealed that sybII and synaptotagmin-1 interact with other SV cargoes to ensure a high fidelity of retrieval. These cargoes are synaptophysin (for sybII) and SV2A (for synaptotagmin-1). In this review, we summarize current knowledge regarding the retrieval mechanisms for both sybII and synaptotagmin-1 during endocytosis. We also define and set criteria for a new functional group of SV molecules that facilitate the retrieval of their interaction partners. We have termed these molecules intrinsic trafficking partners (iTRAPs) and we discuss how the function of this group impacts on presynaptic performance in both health and disease.

Exocytosis and synaptic vesicle function

Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.

The Sybtraps: control of synaptobrevin traffic by synaptophysin, α-synuclein and AP-180

Synaptobrevin II (sybII) is a key fusogenic molecule on synaptic vesicles (SVs) therefore the active maintenance of both its conformation and location in sufficient numbers on this organelle is critical in both mediating and sustaining neurotransmitter release. Recently three proteins have been identified having key roles in the presentation, trafficking and retrieval of sybII during the fusion and endocytosis of SVs. The nerve terminal protein α-synuclein catalyses sybII entry into SNARE complexes, whereas the monomeric adaptor protein AP-180 is required for sybII retrieval during SV endocytosis. Overarching these events is the tetraspan SV protein synaptophysin, which is a major sybII interaction partner on the SV. This review will evaluate recent studies to propose working models for the control of sybII traffic by synaptophysin and other Sybtraps (sybII trafficking partners) and suggest how dysfunction in sybII traffic may contribute to human disease.

X-linked intellectual disability-associated mutations in synaptophysin disrupt synaptobrevin II retrieval

Synaptophysin is an integral synaptic vesicle (SV) protein that accounts for ∼10% of total SV protein cargo. Deletion of synaptophysin results in the defective retrieval of synaptobrevin II (sybII) from the plasma membrane during endocytosis, coupled with a slowing in the speed of endocytosis. Synaptophysin has been implicated in X-linked intellectual disability, with a recent study identifying four separate synaptophysin gene mutations in families affected by the disorder. To determine how these mutations may affect synaptophysin function, we expressed them in cultured neurons derived from synaptophysin knock-out mice. Two distinct truncating mutants were mislocalized throughout the axon and phenocopied the arrest of sybII retrieval in synaptophysin knock-out cultures. The remaining two mutants displayed a nerve terminal localization but did not support efficient sybII retrieval. Interestingly, one mutant fully rescued SV endocytosis kinetics, suggesting that sybII retrieval and endocytosis speed are independent from each other. These studies suggest that the efficient retrieval of sybII by synaptophysin may be key to maintaining synaptic health and perturbation of this event may contribute to the pathogenesis underlying neurodevelopmental disorders such as X-linked intellectual disability.

HIV-1 gp41 transmembrane domain interacts with the fusion peptide: implication in lipid mixing and inhibition of virus-cell fusion

Fusion of the human immunodeficiency virus (HIV) with target cells is mediated by the gp41 subunit of the envelope protein. Mutation and deletion studies within the transmembrane domain (TMD) of intact gp41 influenced its fusion activity. In addition, current models suggest that the TMD is in proximity with the fusion peptide (FP) at the late fusion stages, but there are no direct experimental data to support this hypothesis. Here, we investigated the TMD focusing on two regions: the N-terminal containing the GxxxG motif and the C-terminal containing the GLRI motif, which is conserved among the TMDs of HIV and the T-cell receptor. Studies utilizing the ToxR expression system combined with synthetic peptides and their fluorescent analogues derived from TMD revealed that the GxxxG motif is important for TMD self-association, whereas the C-terminal region is for its heteroassociation with FP. Functionally, all three TMD peptides induced lipid mixing that was enhanced significantly upon mixing with FP. Furthermore, the TMD peptides inhibited virus-cell fusion apparently through their interaction with their endogenous counterparts. Notably, the R2E mutant (in the GLRI) was significantly less potent than the two others. Overall, our findings provide experimental evidence that HIV-1 TMD contributes to membrane assembly and function of the HIV-1 envelope. Owing to similarities between functional domains within viruses, these findings suggest that the TMDs and FPs may contribute similarly in other viruses as well.

Synaptophysin is required for synaptobrevin retrieval during synaptic vesicle endocytosis

The integral synaptic vesicle (SV) protein synaptophysin forms ∼10% of total SV protein content, but has no known function in SV physiology. Synaptobrevin (sybII) is another abundant integral SV protein with an essential role in SV exocytosis. Synaptophysin and sybII form a complex in nerve terminals, suggesting this interaction may have a key role in presynaptic function. To determine how synaptophysin controls sybII traffic in nerve terminals, we used a combination of optical imaging techniques in cultures derived from synaptophysin knock-out mice. We show that synaptophysin is specifically required for the retrieval of the pH-sensitive fluorescent reporter sybII-pHluorin from the plasma membrane during endocytosis. The retrieval of other SV protein cargo reporters still occurred; however, their recapture proceeded with slower kinetics. This slowing of SV retrieval kinetics in the absence of synaptophysin did not impact on global SV turnover. These results identify a specific and selective requirement for synaptophysin in the retrieval of sybII during SV endocytosis and suggest that their interaction may act as an adjustable regulator of SV retrieval efficiency.

Patterns of neural differentiation in melanomas

Background

Melanomas, highly malignant tumors arise from the melanocytes which originate as multipotent neural crest cells during neural tube genesis. The purpose of this study is to assess the pattern of neural differentiation in relation to angiogenesis in VGP melanomas using the tumor as a three dimensional system.

Methods

Tumor-vascular complexes [TVC] are formed at the tumor-stroma interphase, by tumor cells ensheathing angiogenic vessels to proliferate into a mantle of 5 to 6 layers [L1 to L5] forming a perivascular mantle zone [PMZ]. The pattern of neural differentiation is assessed by immunopositivity for HMB45, GFAP, NFP and synaptophysin has been compared in: [a] the general tumor [b] tumor-vascular complexes and [c] perimantle zone [PC] on serial frozen and paraffin sections. Statistical Analysis: ANOVA: Kruskal-Wallis One Way Analysis of Variance; All Pairwise Multiple Comparison Procedures [Tukey Test].

Results

The cells abutting on the basement membrane acquire GFAP positivity and extend processes. New layers of tumor cells show a transition between L2 to L3 followed by NFP and Syn positivity in L4&L5. The level of GFAP+vity in L1&L2 directly proportionate to the percentage of NFP/Syn+vity in L4&L5, on comparing pigmented PMZ with poorly pigmented PMZ. Tumor cells in the perimantle zone show high NFP [65%] and Syn [35.4%] positivity with very low GFAP [6.9%] correlating with the positivity in the outer layers.

Discussion

From this study it is seen that melanoma cells revert to the embryonic pattern of differentiation, with radial glial like cells [GFAP+ve] which further differentiate into neuronal positive cells [NFP&Syn+ve] during angiogenic tumor-vascular interaction, as seen during neurogenesis, to populate the tumor substance.

Sphingosine facilitates SNARE complex assembly and activates synaptic vesicle exocytosis

Synaptic vesicles loaded with neurotransmitters fuse with the plasma membrane to release their content into the extracellular space, thereby allowing neuronal communication. The membrane fusion process is mediated by a conserved set of SNARE proteins: vesicular synaptobrevin and plasma membrane syntaxin and SNAP-25. Recent data suggest that the fusion process may be subject to regulation by local lipid metabolism. Here, we have performed a screen of lipid compounds to identify positive regulators of vesicular synaptobrevin. We show that sphingosine, a releasable backbone of sphingolipids, activates synaptobrevin in synaptic vesicles to form the SNARE complex implicated in membrane fusion. Consistent with the role of synaptobrevin in vesicle fusion, sphingosine upregulated exocytosis in isolated nerve terminals, neuromuscular junctions, neuroendocrine cells and hippocampal neurons, but not in neurons obtained from synaptobrevin-2 knockout mice. Further mechanistic insights suggest that sphingosine acts on the synaptobrevin/phospholipid interface, defining a novel function for this important lipid regulator.

Association of botulinum neurotoxins with synaptic vesicle protein complexes

Botulinum neurotoxins (BoNTs) elicit flaccid paralysis by cleaving SNARE proteins within peripheral neurons. BoNTs are classified into seven serotypes, termed A-G, based on antibody cross-neutralization. Clostridia produce BoNTs as single-chain toxins that are cleaved into a di-chain protein that comprises an N-terminal zinc metalloprotease domain that is linked by a disulfide bond to the C-terminal translocation/receptor-binding domain. BoNT/A and BoNT/B utilize synaptic vesicle protein 2 (SV2) and synaptotagmin, respectively, as receptors for entry into neurons. Using affinity chromatography, BoNT/A and BoNT/B were found to bind a synaptic vesicle protein complex in CHAPS extracts of synaptic vesicles. Mass spectroscopy identified synaptic vesicle protein 2, synaptotagmin I, synaptophysin, vesicle-associated membrane protein 2, and the vacuolar ATPase-proton pump as components of the BoNT-synaptic vesicle protein complex. BoNT/A and BoNT/B possessed unique density-gradient profiles when bound to synaptic vesicle protein complexes. The identification of BoNT/A and BoNT/B bound to synaptic vesicle protein complexes provides insight into the interactions of BoNT and neuronal receptors.

Serotonin content of normal and inflamed appendix: a possible role of serotonin in acute appendicitis

The appendix is lined by a mucosa which has many neuroendocrine cells containing serotonin. Local release of serotonin can act as a mediator of inflammation. In this study we explored the serotonin content of the neuroendocrine cells of the appendixes removed for clinical diagnosis of appendicitis. Appendix specimens were divided into three groups: Acute appendicitis (AA), non-appendicitis (NA), and follicular hyperplasia (FH). Normal appendix specimens from patients undergoing elective abdominal surgery were used as the control group (NL). All sections were exposed to proteinase K, incubated with anti-serotonin, chromogranin A, and synaptophysin antibodies, and treated with the LSAB kit. Polygonal cells were seen within the crypt epithelium (enterochromaffin cell, EC) and within the lamina propria (subepithelial neuroendocrine cell, SNC). In AA, only 16 cases (64%) showed serotonin staining in non-destructed glands. There was a significant reduction in the number of ECs in AA compared to the FH (96%), NA (100%) and NL (100%) groups (P<0.001). Chromogranin and synaptophysin immunostaining also showed a significant reduction in the number of ECs in AA compared with the other three groups (P<0.001). SNC serotonin reactivity was lower in the AA group compared with the other groups (p<0.001). The inflamed appendix is markedly depleted of serotonin in the epithelium and lamina propria. Local serotonin release from ECs and SNCs in the appendix may act as an inflammatory mediator in appendicitis and is likely to be the source of raised blood serotonin in AA.

Determinants of synaptobrevin regulation in membranes

Neuronal exocytosis is driven by the formation of SNARE complexes between synaptobrevin 2 on synaptic vesicles and SNAP-25/syntaxin 1 on the plasma membrane. It has remained controversial, however, whether SNAREs are constitutively active or whether they are down-regulated until fusion is triggered. We now show that synaptobrevin in proteoliposomes as well as in purified synaptic vesicles is constitutively active. Potential regulators such as calmodulin or synaptophysin do not affect SNARE activity. Substitution or deletion of residues in the linker connecting the SNARE motif and transmembrane region did not alter the kinetics of SNARE complex assembly or of SNARE-mediated fusion of liposomes. Remarkably, deletion of C-terminal residues of the SNARE motif strongly reduced fusion activity, although the overall stability of the complexes was not affected. We conclude that although complete zippering of the SNARE complex is essential for membrane fusion, the structure of the adjacent linker domain is less critical, suggesting that complete SNARE complex assembly not only connects membranes but also drives fusion.

Association of botulinum neurotoxin serotypes a and B with synaptic vesicle protein complexes

Botulinum neurotoxins (BoNTs) elicit flaccid paralysis through cleavage of SNARE proteins within peripheral neurons. There are seven serotypes of the BoNTs, termed A-G, which differ in the SNARE protein and/or site that is cleaved. BoNTs are single-chain toxins that comprise an N-terminal zinc metalloprotease domain that is disulfide linked to the C-terminal translocation/receptor binding domain. SV2 and synaptotagmin have been identified as receptors for BoNT serotypes A and B, respectively. Using affinity chromatography, BoNTs A and B were observed to bind synaptic vesicle protein complexes in synaptosome lysates. Tandem LC-MS/MS identified SV2, synaptotagmin I, synaptophysin, vesicle-associated membrane protein 2 (VAMP2), and the vacuolar proton pump as components of the BoNT-receptor complex. Density gradient analysis showed that BoNT serotypes A and B exhibited unique interactions with the synaptic vesicle protein complexes. The association of BoNT serotypes A and B with synaptic vesicle protein complexes implicates a physiological role for protein complexes in synaptic vesicle biology and provides insight into the interactions of BoNT and neuronal receptors.

Tyrosine phosphorylation of synaptophysin in synaptic vesicle recycling

The integral SV (synaptic vesicle) protein synaptophysin was one of the first nerve terminal proteins identified. However its role, if any, in the SV life cycle remains undetermined. One of the most prominent features of synaptophysin is that its cytoplasmic C-terminus largely consists of pentapeptide repeats initiated by a tyrosine residue. Synaptophysin is heavily phosphorylated by tyrosine kinases in the nerve terminal, suggesting that this phosphorylation is central to its function. This review will cover the evidence for tyrosine phosphorylation of synaptophysin and how this phosphorylation may control its function in the SV life cycle.

Tyrosine phosphorylation of synaptophysin in synaptic vesicle recycling

The integral SV (synaptic vesicle) protein synaptophysin was one of the first nerve terminal proteins identified. However its role, if any, in the SV life cycle remains undetermined. One of the most prominent features of synaptophysin is that its cytoplasmic C-terminus largely consists of pentapeptide repeats initiated by a tyrosine residue. Synaptophysin is heavily phosphorylated by tyrosine kinases in the nerve terminal, suggesting that this phosphorylation is central to its function. This review will cover the evidence for tyrosine phosphorylation of synaptophysin and how this phosphorylation may control its function in the SV life cycle.