Synaptogyrin-dependent modulation of synaptic neurotransmission in Caenorhabditis elegans

In vivo overexpression of synaptogyrin-3 promotes striatal synaptic dopamine uptake in LRRK2R1441G mutant mouse model of Parkinson's disease

BACKGROUND

Leucine-rich repeat kinase 2 (LRRK2) mutation is a common genetic risk factor of Parkinson's disease (PD). Presynaptic dysfunction is an early pathogenic event associated with dopamine (DA) dysregulation in striatum of the brain. DA uptake activity of DA uptake transporter (DAT) affects synaptic plasticity and motor and non-motor behavior. Synaptogyrin-3 (SYNGR3) is part of the synaptogyrin family, especially abundant in brain. Previous in vitro studies demonstrated interaction between SYNGR3 and DAT. Reduced SYNGR3 expression was observed in human PD brains with unclear reasons.

METHODS

Here, we further explored whether inducing SYNGR3 expression can influence (i) cellular DA uptake using differentiated human SH-SY5Y neuronal cells, (ii) striatal synaptosomal DA uptake in a mutant LRRK2^R1441G  knockin mouse model of PD, and (iii) innate rodent behavior using the marble burying test.

RESULTS

Young LRRK2 mutant mice exhibited significantly lower SYNGR3 levels in striatum compared to age-matched wild-type (WT) controls, resembling level in aged WT mice. SYNGR3 is spatially co-localized with DAT at striatal presynaptic terminals, visualized by immuno-gold transmission electron microscopy and immunohistochemistry. Their protein-protein interaction was confirmed by co-immunoprecipitation. Transient overexpression of SYNGR3 in differentiated SH-SY5Y cells increased cellular DA uptake activity without affecting total DAT levels. Inducing SYNGR3 overexpression by adeno-associated virus-7 (AAV7) injection in vivo into striatum increased ex vivo synaptosomal DA uptake in LRRK2 mutant mice and improved their innate marble burying behavior.

CONCLUSION

Brain SYNGR3 expression may be an important determinant to striatal DA homeostasis and synaptic function. Our preliminary behavioral test showed improved innate behavior after SYNGR3 overexpression in LRRK2 mutant mice, advocating further studies to determine the influence of SYNGR3 in the pathophysiology of DA neurons in PD.

Rapid and reversible optogenetic silencing of synaptic transmission by clustering of synaptic vesicles

Acutely silencing specific neurons informs about their functional roles in circuits and behavior. Existing optogenetic silencers include ion pumps, channels, metabotropic receptors, and tools that damage the neurotransmitter release machinery. While the former hyperpolarize the cell, alter ionic gradients or cellular biochemistry, the latter allow only slow recovery, requiring de novo synthesis. Thus, tools combining fast activation and reversibility are needed. Here, we use light-evoked homo-oligomerization of cryptochrome CRY2 to silence synaptic transmission, by clustering synaptic vesicles (SVs). We benchmark this tool, optoSynC, in Caenorhabditis elegans, zebrafish, and murine hippocampal neurons. optoSynC clusters SVs, observable by electron microscopy. Locomotion silencing occurs with tauon ~7.2 s and recovers with tauoff ~6.5 min after light-off. optoSynC can inhibit exocytosis for several hours, at very low light intensities, does not affect ion currents, biochemistry or synaptic proteins, and may further allow manipulating different SV pools and the transfer of SVs between them.

The presynaptic machinery at the synapse of C. elegans

Synapses are specialized contact sites that mediate information flow between neurons and their targets. Important physical interactions across the synapse are mediated by synaptic adhesion molecules. These adhesions regulate formation of synapses during development and play a role during mature synaptic function. Importantly, genes regulating synaptogenesis and axon regeneration are conserved across the animal phyla. Genetic screens in the nematode Caenorhabditis elegans have identified a number of molecules required for synapse patterning and assembly. C. elegans is able to survive even with its neuronal function severely compromised. This is in comparison with Drosophila and mice where increased complexity makes them less tolerant to impaired function. Although this fact may reflect differences in the function of the homologous proteins in the synapses between these organisms, the most likely interpretation is that many of these components are equally important, but not absolutely essential, for synaptic transmission to support the relatively undemanding life style of laboratory maintained C. elegans. Here, we review research on the major group of synaptic proteins, involved in the presynaptic machinery in C. elegans, showing a strong conservation between higher organisms and highlight how C. elegans can be used as an informative tool for dissecting synaptic components, based on a simple nervous system organization.

Inactivation of GABAA receptor is related to heat shock stress response in organism model Caenorhabditis elegans

The mechanisms underlying oxidative stress (OS) resistance are not completely clear. Caenorhabditis elegans (C. elegans) is a good organism model to study OS because it displays stress responses similar to those in mammals. Among these mechanisms, the insulin/IGF-1 signaling (IIS) pathway is thought to affect GABAergic neurotransmission. The aim of this study was to determine the influence of heat shock stress (HS) on GABAergic activity in C. elegans. For this purpose, we tested the effect of exposure to picrotoxin (PTX), gamma-aminobutyric acid (GABA), hydrogen peroxide, and HS on the occurrence of a shrinking response (SR) after nose touch stimulus in N2 (WT) worms. Moreover, the effect of HS on the expression of UNC-49 (GABAA receptor ortholog) in the EG1653 strain and the effect of GABA and PTX exposure on HSP-16.2 expression in the TJ375 strain were analyzed. PTX 1 mM- or H2O2 0.7 mM-exposed worms displayed a SR in about 80 % of trials. GABA exposure did not cause a SR. HS prompted the occurrence of a SR as did PTX 1 mM or H2O2 0.7 mM exposure. In addition, HS increased UNC-49 expression, and PTX augmented HSP-16.2 expression. Thus, the results of the present study suggest that oxidative stress, through either H2O2 exposure or application of heat shock, inactivates the GABAergic system, which subsequently would affect the oxidative stress response, perhaps by enhancing the activity of transcription factors DAF-16 and HSF-1, both regulated by the IIS pathway and related to hsp-16.2 expression.

Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNA-expressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection.

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.

Kinetic analysis of mouse brain proteome alterations following Chikungunya virus infection before and after appearance of clinical symptoms

Recent outbreaks of Chikungunya virus (CHIKV) infection have been characterized by an increasing number of severe cases with atypical manifestations including neurological complications. In parallel, the risk map of CHIKV outbreaks has expanded because of improved vector competence. These features make CHIKV infection a major public health concern that requires a better understanding of the underlying physiopathological processes for the development of antiviral strategies to protect individuals from severe disease. To decipher the mechanisms of CHIKV infection in the nervous system, a kinetic analysis on the host proteome modifications in the brain of CHIKV-infected mice sampled before and after the onset of clinical symptoms was performed. The combination of 2D-DIGE and iTRAQ proteomic approaches, followed by mass spectrometry protein identification revealed 177 significantly differentially expressed proteins. This kinetic analysis revealed a dramatic down-regulation of proteins before the appearance of the clinical symptoms followed by the increased expression of most of these proteins in the acute symptomatic phase. Bioinformatic analyses of the protein datasets enabled the identification of the major biological processes that were altered during the time course of CHIKV infection, such as integrin signaling and cytoskeleton dynamics, endosome machinery and receptor recycling related to virus transport and synapse function, regulation of gene expression, and the ubiquitin-proteasome pathway. These results reveal the putative mechanisms associated with severe CHIKV infection-mediated neurological disease and highlight the potential markers or targets that can be used to develop diagnostic and/or antiviral tools.

Abnormal synaptic vesicle biogenesis in Drosophila synaptogyrin mutants

Sustained neuronal communication relies on the coordinated activity of multiple proteins that regulate synaptic vesicle biogenesis and cycling within the presynaptic terminal. Synaptogyrin and synaptophysin are conserved MARVEL domain-containing transmembrane proteins that are among the most abundant synaptic vesicle constituents, although their role in the synaptic vesicle cycle has remained elusive. To further investigate the function of these proteins, we generated and characterized a synaptogyrin (gyr)-null mutant in Drosophila, whose genome encodes a single synaptogyrin isoform and lacks a synaptophysin homolog. We demonstrate that Drosophila synaptogyrin plays a modulatory role in synaptic vesicle biogenesis at larval neuromuscular junctions. Drosophila lacking synaptogyrin are viable and fertile and have no overt deficits in motor function. However, ultrastructural analysis of gyr larvae revealed increased synaptic vesicle diameter and enhanced variability in the size of synaptic vesicles. In addition, the resolution of endocytic cisternae into synaptic vesicles in response to strong stimulation is defective in gyr mutants. Electrophysiological analysis demonstrated an increase in quantal size and a concomitant decrease in quantal content, suggesting functional consequences for transmission caused by the loss of synaptogyrin. Furthermore, high-frequency stimulation resulted in increased facilitation and a delay in recovery from synaptic depression, indicating that synaptic vesicle exo-endocytosis is abnormally regulated during intense stimulation conditions. These results suggest that synaptogyrin modulates the synaptic vesicle exo-endocytic cycle and is required for the proper biogenesis of synaptic vesicles at nerve terminals.

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.