Snapin facilitates the synchronization of synaptic vesicle fusion

New insights into the pathogenesis of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) is characterized by defects of multiple tissue-specific lysosome-related organelles (LROs), typically manifesting with oculocutaneous albinism or ocular albinism, bleeding tendency, and in some cases with pulmonary fibrosis, inflammatory bowel disease or immunodeficiency, neuropsychological disorders. Eleven HPS subtypes in humans and at least 15 subtypes in mice have been molecularly identified. Current understanding of the underlying mechanisms of HPS is focusing on the defective biogenesis of LROs. Compelling evidences have shown that HPS protein-associated complexes (HPACs) function in cargo transport, cargo recycling, and cargo removal to maintain LRO homeostasis. Further investigation on the molecular and cellular mechanism of LRO biogenesis and secretion will be helpful for better understanding of its pathogenesis and for the precise intervention of HPS.

Transient Receptor Potential Channels and Botulinum Neurotoxins in Chronic Pain

Pain afflicts more than 1.5 billion people worldwide, with hundreds of millions suffering from unrelieved chronic pain. Despite widespread recognition of the importance of developing better interventions for the relief of chronic pain, little is known about the mechanisms underlying this condition. However, transient receptor potential (TRP) ion channels in nociceptors have been shown to be essential players in the generation and progression of pain and have attracted the attention of several pharmaceutical companies as therapeutic targets. Unfortunately, TRP channel inhibitors have failed in clinical trials, at least in part due to their thermoregulatory function. Botulinum neurotoxins (BoNTs) have emerged as novel and safe pain therapeutics because of their regulation of exocytosis and pro-nociceptive neurotransmitters. However, it is becoming evident that BoNTs also regulate the expression and function of TRP channels, which may explain their analgesic effects. Here, we summarize the roles of TRP channels in pain, with a particular focus on TRPV1 and TRPA1, their regulation by BoNTs, and briefly discuss the use of BoNTs for the treatment of chronic pain.

Snapin Specifically Up-Regulates Cav1.3 Ca2+ Channel Variant with a Long Carboxyl Terminus

Ca²⁺ entry through Ca_v1.3 Ca²⁺ channels plays essential roles in diverse physiological events. We employed yeast-two-hybrid (Y2H) assays to mine novel proteins interacting with Ca_v1.3 and found Snapin2, a synaptic protein, as a partner interacting with the long carboxyl terminus (CT_L) of rat Ca_v1.3_L variant. Co-expression of Snapin with Ca_v1.3_L/Ca_vβ₃/α₂δ₂ subunits increased the peak current density or amplitude by about 2-fold in HEK-293 cells and Xenopus oocytes, without affecting voltage-dependent gating properties and calcium-dependent inactivation. However, the Snapin up-regulation effect was not found for rat Ca_v1.3_S containing a short CT (CT_S) in which a Snapin interaction site in the CT_L was deficient. Luminometry and electrophysiology studies uncovered that Snapin co-expression did not alter the membrane expression of HA tagged Ca_v1.3_L but increased the slope of tail current amplitudes plotted against ON-gating currents, indicating that Snapin increases the opening probability of Ca_v1.3_L. Taken together, our results strongly suggest that Snapin directly interacts with the CT_L of Ca_v1.3_L, leading to up-regulation of Ca_v1.3_L channel activity via facilitating channel opening probability.

Cytosolic PSD-95 interactor alters functional organization of neural circuits and AMPA receptor signaling independent of PSD-95 binding

Cytosolic PSD-95 interactor (cypin) regulates many aspects of neuronal development and function, ranging from dendritogenesis to synaptic protein localization. While it is known that removal of postsynaptic density protein-95 (PSD-95) from the postsynaptic density decreases synaptic N-methyl-D-aspartate (NMDA) receptors and that cypin overexpression protects neurons from NMDA-induced toxicity, little is known about cypin's role in AMPA receptor clustering and function. Experimental work shows that cypin overexpression decreases PSD-95 levels in synaptosomes and the PSD, decreases PSD-95 clusters/μm², and increases mEPSC frequency. Analysis of microelectrode array (MEA) data demonstrates that cypin or cypinΔPDZ overexpression increases sensitivity to CNQX (cyanquixaline) and AMPA receptor-mediated decreases in spike waveform properties. Network-level analysis of MEA data reveals that cypinΔPDZ overexpression causes networks to be resilient to CNQX-induced changes in local efficiency. Incorporating these findings into a computational model of a neural circuit demonstrates a role for AMPA receptors in cypin-promoted changes to networks and shows that cypin increases firing rate while changing network functional organization, suggesting cypin overexpression facilitates information relay but modifies how information is encoded among brain regions. Our data show that cypin promotes changes to AMPA receptor signaling independent of PSD-95 binding, shaping neural circuits and output to regions beyond the hippocampus.

SNARE Complex-Associated Proteins and Alcohol

Alcohol addiction causes major health problems throughout the world, causing numerous deaths and incurring a huge economic burden to society. To develop an intervention for alcohol addiction, it is necessary to identify molecular target(s) of alcohol and associated molecular mechanisms of alcohol action. The functions of many central and peripheral synapses are impacted by low concentrations of ethanol (EtOH). While the postsynaptic targets and mechanisms are studied extensively, there are limited studies on the presynaptic targets and mechanisms. This article is an endeavor in this direction, focusing on the effect of EtOH on the presynaptic proteins associated with the neurotransmitter release machinery. Studies on the effects of EtOH at the levels of gene, protein, and behavior are highlighted in this article.

Crosstalk between presynaptic trafficking and autophagy in Parkinson's disease

Parkinson's disease (PD) is a debilitating neurodegenerative disorder that profoundly affects one's motor functions. The disease is characterized pathologically by denervation of dopaminergic (DAergic) nigrostriatal terminal and degeneration of DAergic neurons in the substantia nigra par compacta (SNpc); however, the precise molecular mechanism underlying disease pathogenesis remains poorly understood. Animal studies in both toxin-induced and genetic PD models suggest that presynaptic impairments may underlie the early stage of DA depletion and neurodegeneration (reviewed in Schirinzi, T., et al. 2016). Supporting this notion, human genetic studies and genomic analysis have identified an increasing number of PD risk variants that are associated with synaptic vesicle (SV) trafficking, regulation of synaptic function and autophagy/lysosomal system (Chang, D., et al. 2017, reviewed in Trinh, J. & Farrer, M. 2013; Singleton, A.B., et al. 2013). Although the precise mechanism for autophagy regulation in neurons is currently unclear, many studies demonstrate that autophagosomes form at the presynaptic terminal (Maday, S. & Holzbaur, E.L. 2014; Vanhauwaert, R., et al. 2017; reviewed in Yue, Z. 2007). Growing evidence has revealed overlapping genes involved in both SV recycling and autophagy, suggesting that the two membrane trafficking processes are inter-connected. Here we will review emergent evidence linking SV endocytic genes and autophagy genes at the presynaptic terminal. We will discuss their potential relevance to PD pathogenesis.

Nanos genes and their role in development and beyond

The hallmark of Nanos proteins is their typical (CCHC)2 zinc finger motif (zf-nanos). Animals have one to four nanos genes. For example, the fruit fly and demosponge have only one nanos gene, zebrafish and humans have three, and Fugu rubripes has four. Nanos genes are mainly known for their evolutionarily preserved role in germ cell survival and pluripotency. Nanos proteins have been reported to bind the C-terminal RNA-binding domain of Pumilio to form a post-transcriptional repressor complex. Several observations point to a link between the miRNA-mediated repression complex and the Nanos/Pumilio complex. Repression of the E2F3 oncogene product is, indeed, mediated by cooperation between the Nanos/Pumilio complex and miRNAs. Another important interaction partner of Nanos is the CCR4-NOT deadenylase complex. Besides the tissue-specific contribution of Nanos proteins to normal development, their ectopic expression has been observed in several cancer cell lines and various human cancers. An inverse correlation between the expression levels of human Nanos1 and Nanos3 and E-cadherin was observed in several cancer cell lines. Loss of E-cadherin, an important cell-cell adhesion protein, contributes to tumor invasion and metastasis. Overexpression of Nanos3 induces epithelial-mesenchymal transition in lung cancer cell lines partly by repressing E-cadherin. Other than some most interesting data from Nanos knockout mice, little is known about mammalian Nanos proteins, and further research is needed. In this review, we summarize the main roles of Nanos proteins and discuss the emerging concept of Nanos proteins as oncofetal antigens.

Structural and Functional Characterization of the Interaction of Snapin with the Dopamine Transporter: Differential Modulation of Psychostimulant Actions

The importance of dopamine (DA) neurotransmission is emphasized by its direct implication in several neurological and psychiatric disorders. The DA transporter (DAT), target of psychostimulant drugs, is the key protein that regulates spatial and temporal activity of DA in the synaptic cleft via the rapid reuptake of DA into the presynaptic terminal. There is strong evidence suggesting that DAT-interacting proteins may have a role in its function and regulation. Performing a two-hybrid screening, we identified snapin, a SNARE-associated protein implicated in synaptic transmission, as a new binding partner of the carboxyl terminal of DAT. Our data show that snapin is a direct partner and regulator of DAT. First, we determined the domains required for this interaction in both proteins and characterized the DAT-snapin interface by generating a 3D model. Using different approaches, we demonstrated that (i) snapin is expressed in vivo in dopaminergic neurons along with DAT; (ii) both proteins colocalize in cultured cells and brain and, (iii) DAT and snapin are present in the same protein complex. Moreover, by functional studies we showed that snapin produces a significant decrease in DAT uptake activity. Finally, snapin downregulation in mice produces an increase in DAT levels and transport activity, hence increasing DA concentration and locomotor response to amphetamine. In conclusion, snapin/DAT interaction represents a direct link between exocytotic and reuptake mechanisms and is a potential target for DA transmission modulation.

Gene networks in neurodegenerative disorders

Three neurodegenerative diseases [Amyotrophic Lateral Sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD)] have many characteristics like pathological mechanisms and genes. In this sense some researchers postulate that these diseases share the same alterations and that one alteration in a specific protein triggers one of these diseases. Analyses of gene expression may shed more light on how to discover pathways, pathologic mechanisms associated with the disease, biomarkers and potential therapeutic targets. In this review, we analyze four microarrays related to three neurodegenerative diseases. We will systematically examine seven genes (CHN1, MDH1, PCP4, RTN1, SLC14A1, SNAP25 and VSNL1) that are altered in the three neurodegenerative diseases. A network was built and used to identify pathways, miRNA and drugs associated with ALS, AD and PD using Cytoscape software an interaction network based on the protein interactions of these genes. The most important affected pathway is PI3K-Akt signalling. Thirteen microRNAs (miRNA-19B1, miRNA-107, miRNA-124-1, miRNA-124-2, miRNA-9-2, miRNA-29A, miRNA-9-3, miRNA-328, miRNA-19B2, miRNA-29B2, miRNA-124-3, miRNA-15A and miRNA-9-1) and four drugs (Estradiol, Acetaminophen, Resveratrol and Progesterone) for new possible treatments were identified.

The BLOC-1 Subunit Pallidin Facilitates Activity-Dependent Synaptic Vesicle Recycling

Membrane trafficking pathways must be exquisitely coordinated at synaptic terminals to maintain functionality, particularly during conditions of high activity. We have generated null mutations in the Drosophila homolog of pallidin, a central subunit of the biogenesis of lysosome-related organelles complex-1 (BLOC-1), to determine its role in synaptic development and physiology. We find that Pallidin localizes to presynaptic microtubules and cytoskeletal structures, and that the stability of Pallidin protein is highly dependent on the BLOC-1 components Dysbindin and Blos1. We demonstrate that the rapidly recycling vesicle pool is not sustained during high synaptic activity in pallidin mutants, leading to accelerated rundown and slowed recovery. Following intense activity, we observe a loss of early endosomes and a concomitant increase in tubular endosomal structures in synapses without Pallidin. Together, our data reveal that Pallidin subserves a key role in promoting efficient synaptic vesicle recycling and re-formation through early endosomes during sustained activity.

Microtubule-associated protein 1B (MAP1B)-deficient neurons show structural presynaptic deficiencies in vitro and altered presynaptic physiology

Microtubule-associated protein 1B (MAP1B) is expressed predominantly during the early stages of development of the nervous system, where it regulates processes such as axonal guidance and elongation. Nevertheless, MAP1B expression in the brain persists in adult stages, where it participates in the regulation of the structure and physiology of dendritic spines in glutamatergic synapses. Moreover, MAP1B expression is also found in presynaptic synaptosomal preparations. In this work, we describe a presynaptic phenotype in mature neurons derived from MAP1B knockout (MAP1B KO) mice. Mature neurons express MAP1B, and its deficiency does not alter the expression levels of a subgroup of other synaptic proteins. MAP1B KO neurons display a decrease in the density of presynaptic and postsynaptic terminals, which involves a reduction in the density of synaptic contacts, and an increased proportion of orphan presynaptic terminals. Accordingly, MAP1B KO neurons present altered synaptic vesicle fusion events, as shown by FM4-64 release assay, and a decrease in the density of both synaptic vesicles and dense core vesicles at presynaptic terminals. Finally, an increased proportion of excitatory immature symmetrical synaptic contacts in MAP1B KO neurons was detected. Altogether these results suggest a novel role for MAP1B in presynaptic structure and physiology regulation in vitro.

Protein interactome mining defines melatonin MT1 receptors as integral component of presynaptic protein complexes of neurons

In mammals, the hormone melatonin is mainly produced by the pineal gland with nocturnal peak levels. Its peripheral and central actions rely either on its intrinsic antioxidant properties or on binding to melatonin MT1 and MT2 receptors, belonging to the G protein-coupled receptor (GPCR) super-family. Melatonin has been reported to be involved in many functions of the central nervous system such as circadian rhythm regulation, neurotransmission, synaptic plasticity, memory, sleep, and also in Alzheimer's disease and depression. However, little is known about the subcellular localization of melatonin receptors and the molecular aspects involved in neuronal functions of melatonin. Identification of protein complexes associated with GPCRs has been shown to be a valid approach to improve our understanding of their function. By combining proteomic and genomic approaches we built an interactome of MT1 and MT2 receptors, which comprises 378 individual proteins. Among the proteins interacting with MT1 , but not with MT2 , we identified several presynaptic proteins, suggesting a potential role of MT1 in neurotransmission. Presynaptic localization of MT1 receptors in the hypothalamus, striatum, and cortex was confirmed by subcellular fractionation experiments and immunofluorescence microscopy. MT1 physically interacts with the voltage-gated calcium channel Cav 2.2 and inhibits Cav 2.2-promoted Ca(2+) entry in an agonist-independent manner. In conclusion, we show that MT1 is part of the presynaptic protein network and negatively regulates Cav 2.2 activity, providing a first hint for potential synaptic functions of MT1.

Endolysosomal Deficits Augment Mitochondria Pathology in Spinal Motor Neurons of Asymptomatic fALS Mice

One pathological hallmark in ALS motor neurons (MNs) is axonal accumulation of damaged mitochondria. A fundamental question remains: does reduced degradation of those mitochondria by an impaired autophagy-lysosomal system contribute to mitochondrial pathology? We reveal MN-targeted progressive lysosomal deficits accompanied by impaired autophagic degradation beginning at asymptomatic stages in fALS-linked hSOD1(G93A) mice. Lysosomal deficits result in accumulation of autophagic vacuoles engulfing damaged mitochondria along MN axons. Live imaging of spinal MNs from the adult disease mice demonstrates impaired dynein-driven retrograde transport of late endosomes (LEs). Expressing dynein-adaptor snapin reverses transport defects by competing with hSOD1(G93A) for binding dynein, thus rescuing autophagy-lysosomal deficits, enhancing mitochondrial turnover, improving MN survival, and ameliorating the disease phenotype in hSOD1(G93A) mice. Our study provides a new mechanistic link for hSOD1(G93A)-mediated impairment of LE transport to autophagy-lysosomal deficits and mitochondrial pathology. Understanding these early pathological events benefits development of new therapeutic interventions for fALS-linked MN degeneration.

Regulation of synaptic activity by snapin-mediated endolysosomal transport and sorting

Recycling synaptic vesicles (SVs) transit through early endosomal sorting stations, which raises a fundamental question: are SVs sorted toward endolysosomal pathways? Here, we used snapin mutants as tools to assess how endolysosomal sorting and trafficking impact presynaptic activity in wild-type and snapin(-/-) neurons. Snapin acts as a dynein adaptor that mediates the retrograde transport of late endosomes (LEs) and interacts with dysbindin, a subunit of the endosomal sorting complex BLOC-1. Expressing dynein-binding defective snapin mutants induced SV accumulation at presynaptic terminals, mimicking the snapin(-/-) phenotype. Conversely, over-expressing snapin reduced SV pool size by enhancing SV trafficking to the endolysosomal pathway. Using a SV-targeted Ca(2+) sensor, we demonstrate that snapin-dysbindin interaction regulates SV positional priming through BLOC-1/AP-3-dependent sorting. Our study reveals a bipartite regulation of presynaptic activity by endolysosomal trafficking and sorting: LE transport regulates SV pool size, and BLOC-1/AP-3-dependent sorting fine-tunes the Ca(2+) sensitivity of SV release. Therefore, our study provides new mechanistic insights into the maintenance and regulation of SV pool size and synchronized SV fusion through snapin-mediated LE trafficking and endosomal sorting.

I2020T mutant LRRK2 iPSC-derived neurons in the Sagamihara family exhibit increased Tau phosphorylation through the AKT/GSK-3β signaling pathway

Leucine-rich repeat kinase 2 (LRRK2) is the causative molecule of the autosomal dominant hereditary form of Parkinson's disease (PD), PARK8, which was originally defined in a study of a Japanese family (the Sagamihara family) harboring the I2020T mutation in the kinase domain. Although a number of reported studies have focused on cell death mediated by mutant LRRK2, details of the pathogenetic effect of LRRK2 still remain to be elucidated. In the present study, to elucidate the mechanism of neurodegeneration in PD caused by LRRK2, we generated induced pluripotent stem cells (iPSC) derived from fibroblasts of PD patients with I2020T LRRK2 in the Sagamihara family. We found that I2020T mutant LRRK2 iPSC-derived neurons released less dopamine than control-iPSC-derived neurons. Furthermore, we demonstrated that patient iPSC-derived neurons had a lower phospho-AKT level than control-iPSC-derived neurons, and that the former showed an increased incidence of apoptosis relative to the controls. Interestingly, patient iPSC-derived neurons exhibited activation of glycogen synthase kinase-3β (GSK-3β) and high Tau phosphorylation. In addition, the postmortem brain of the patient from whom the iPSC had been established exhibited deposition of neurofibrillary tangles as well as increased Tau phosphorylation in neurons. These results suggest that I2020T LRRK2-iPSC could be a promising new tool for reproducing the pathology of PD in the brain caused by the I2020T mutation, and applicable as a model in studies of targeted therapeutics.

Behavioral and electrophysiological characterization of Dyt1 heterozygous knockout mice

DYT1 dystonia is an inherited movement disorder caused by mutations in DYT1 (TOR1A), which codes for torsinA. Most of the patients have a trinucleotide deletion (ΔGAG) corresponding to a glutamic acid in the C-terminal region (torsinA(ΔE)). Dyt1 ΔGAG heterozygous knock-in (KI) mice, which mimic ΔGAG mutation in the endogenous gene, exhibit motor deficits and deceased frequency of spontaneous excitatory post-synaptic currents (sEPSCs) and normal theta-burst-induced long-term potentiation (LTP) in the hippocampal CA1 region. Although Dyt1 KI mice show decreased hippocampal torsinA levels, it is not clear whether the decreased torsinA level itself affects the synaptic plasticity or torsinA(ΔE) does it. To analyze the effect of partial torsinA loss on motor behaviors and synaptic transmission, Dyt1 heterozygous knock-out (KO) mice were examined as a model of a frame-shift DYT1 mutation in patients. Consistent with Dyt1 KI mice, Dyt1 heterozygous KO mice showed motor deficits in the beam-walking test. Dyt1 heterozygous KO mice showed decreased hippocampal torsinA levels lower than those in Dyt1 KI mice. Reduced sEPSCs and normal miniature excitatory post-synaptic currents (mEPSCs) were also observed in the acute hippocampal brain slices from Dyt1 heterozygous KO mice, suggesting that the partial loss of torsinA function in Dyt1 KI mice causes action potential-dependent neurotransmitter release deficits. On the other hand, Dyt1 heterozygous KO mice showed enhanced hippocampal LTP, normal input-output relations and paired pulse ratios in the extracellular field recordings. The results suggest that maintaining an appropriate torsinA level is important to sustain normal motor performance, synaptic transmission and plasticity. Developing therapeutics to restore a normal torsinA level may help to prevent and treat the symptoms in DYT1 dystonia.

Impaired maturation of large dense core vesicles in muted-deficient adrenal chromaffin cells

The large dense-core vesicle (LDCV), a type of lysosome-related organelle, is involved in the secretion of hormones and neuropeptides in specialized secretory cells. The granin family is a driving force in LDCV biogenesis, but the machinery for granin sorting to this biogenesis pathway is largely unknown. The mu mutant mouse, which carries a spontaneous null mutation on the Muted gene (also known as Bloc1s5) that encodes a subunit of lysosome-related organelles complex-1 (BLOC-1), is a mouse model of Hermansky-Pudlak syndrome. We here found that LDCVs were enlarged in mu adrenal chromaffin cells. Chromogranin A (CgA) was increased in mu adrenals and muted-knockdown cells. The increased CgA in mu mice was likely due to the failure of its sorting-out, which impairs LDCV maturation and docking. In mu chromaffin cells, the size of readily releasable pool and the vesicle release frequency were reduced. Our studies suggest that the muted protein is involved in the sorting-out of CgA during the biogenesis of LDCVs.

Hepatic transcriptome profiles differ among maturing beef heifers supplemented with inorganic, organic, or mixed (50% inorganic:50% organic) forms of dietary selenium

Selenium (Se) is an important trace mineral that, due to deficiencies in the soil in many parts of the USA, must be supplemented directly to the diet of foraging cattle. Both organic and inorganic forms of dietary Se supplements are available and commonly used, and it is known that Se form affects tissue assimilation, bioavailability, and physiological responses. However, little is known about the effects of form of dietary Se supplements on gene expression profiles, which ostensibly account for Se form-dependent physiological processes. To determine if hepatic transcriptomes of growing beef (Angus-cross) heifers (0.5 kg gain/day) was altered by form of dietary supplemental Se, none (Control), or 3 mg Se/day as inorganic Se (ISe, sodium selenite), organic (OSe, Sel-Plex®), or a blend of ISe and OSe (1.5 mg:1.5 mg, Mix) Se was fed for 168 days, and the RNA expression profiles from biopsied liver tissues was compared by microarray analysis. The relative abundance of 139 RNA transcripts was affected by Se treatment, with 86 of these with complete gene annotations. Statistical and bioinformatic analysis of the annotated RNA transcripts revealed clear differences among the four Se treatment groups in their hepatic expression profiles, including (1) solely and commonly affected transcripts; (2) Control and OSe profiles being more similar than Mix and ISe treatments; (3) distinct OSe-, Mix-, and ISe-Se treatment-induced "phenotypes" that possessed both common and unique predicted physiological capacities; and (4) expression of three microRNAs were uniquely sensitive to OSe, ISe, or Mix treatments, including increased capacity for redox potential induced by OSe and Mix Se treatments resulting from decreased expression of MiR2300b messenger RNA. These findings indicate that the form of supplemental dietary Se consumed by cattle will affect the composition of liver transcriptomes resulting, presumably, in different physiological capacities.

Mutations in the BLOC-1 subunits dysbindin and muted generate divergent and dosage-dependent phenotypes

Post-mortem analysis has revealed reduced levels of the protein dysbindin in the brains of those suffering from the neurodevelopmental disorder schizophrenia. Consequently, mechanisms controlling the cellular levels of dysbindin and its interacting partners may participate in neurodevelopmental processes impaired in that disorder. To address this question, we studied loss of function mutations in the genes encoding dysbindin and its interacting BLOC-1 subunits. We focused on BLOC-1 mutants affecting synapse composition and function in addition to their established systemic pigmentation, hematological, and lung phenotypes. We tested phenotypic homogeneity and gene dosage effects in the mouse null alleles muted (Bloc1s5(mu/mu)) and dysbindin (Bloc1s8(sdy/sdy)). Transcripts of NMDA receptor subunits and GABAergic interneuron markers, as well as expression of BLOC-1 subunit gene products, were affected differently in the brains of Bloc1s5(mu/mu) and Bloc1s8(sdy/sdy) mice. Unlike Bloc1s8(sdy/sdy), elimination of one or two copies of Bloc1s5 generated indistinguishable pallidin transcript phenotypes. We conclude that monogenic mutations abrogating the expression of a protein complex subunit differentially affect the expression of other complex transcripts and polypeptides as well as their downstream effectors. We propose that the genetic disruption of different subunits of protein complexes and combinations thereof diversifies phenotypic presentation of pathway deficiencies, contributing to the wide phenotypic spectrum and complexity of neurodevelopmental disorders.

Synaptic vesicle recycling: steps and principles

Synaptic vesicle recycling is one of the best-studied cellular pathways. Many of the proteins involved are known, and their interactions are becoming increasingly clear. However, as for many other pathways, it is still difficult to understand synaptic vesicle recycling as a whole. While it is generally possible to point out how synaptic reactions take place, it is not always easy to understand what triggers or controls them. Also, it is often difficult to understand how the availability of the reaction partners is controlled: how the reaction partners manage to find each other in the right place, at the right time. I present here an overview of synaptic vesicle recycling, discussing the mechanisms that trigger different reactions, and those that ensure the availability of reaction partners. A central argument is that synaptic vesicles bind soluble cofactor proteins, with low affinity, and thus control their availability in the synapse, forming a buffer for cofactor proteins. The availability of cofactor proteins, in turn, regulates the different synaptic reactions. Similar mechanisms, in which one of the reaction partners buffers another, may apply to many other processes, from the biogenesis to the degradation of the synaptic vesicle.

Transmembrane prostatic acid phosphatase (TMPAP) interacts with snapin and deficient mice develop prostate adenocarcinoma

The molecular mechanisms underlying prostate carcinogenesis are poorly understood. Prostatic acid phosphatase (PAP), a prostatic epithelial secretion marker, has been linked to prostate cancer since the 1930's. However, the contribution of PAP to the disease remains controversial. We have previously cloned and described two isoforms of this protein, a secretory (sPAP) and a transmembrane type-I (TMPAP). The goal in this work was to understand the physiological function of TMPAP in the prostate. We conducted histological, ultra-structural and genome-wide analyses of the prostate of our PAP-deficient mouse model (PAP^−/−) with C57BL/6J background. The PAP^−/− mouse prostate showed the development of slow-growing non-metastatic prostate adenocarcinoma. In order to find out the mechanism behind, we identified PAP-interacting proteins byyeast two-hybrid assays and a clear result was obtained for the interaction of PAP with snapin, a SNARE-associated protein which binds Snap25 facilitating the vesicular membrane fusion process. We confirmed this interaction by co-localization studies in TMPAP-transfected LNCaP cells (TMPAP/LNCaP cells) and in vivo FRET analyses in transient transfected LNCaP cells. The differential gene expression analyses revealed the dysregulation of the same genes known to be related to synaptic vesicular traffic. Both TMPAP and snapin were detected in isolated exosomes. Our results suggest that TMPAP is involved in endo-/exocytosis and disturbed vesicular traffic is a hallmark of prostate adenocarcinoma.

LRRK2 phosphorylates Snapin and inhibits interaction of Snapin with SNAP-25

Leucine-rich repeat kinase 2 (LRRK2) is a gene that, upon mutation, causes autosomal-dominant familial Parkinson's disease (PD). Yeast two-hybrid screening revealed that Snapin, a SNAP-25 (synaptosomal-associated protein-25) interacting protein, interacts with LRRK2. An in vitro kinase assay exhibited that Snapin is phosphorylated by LRRK2. A glutathione-S-transferase (GST) pull-down assay showed that LRRK2 may interact with Snapin via its Ras-of-complex (ROC) and N-terminal domains, with no significant difference on interaction of Snapin with LRRK2 wild type (WT) or its pathogenic mutants. Further analysis by mutation study revealed that Threonine 117 of Snapin is one of the sites phosphorylated by LRRK2. Furthermore, a Snapin T117D phosphomimetic mutant decreased its interaction with SNAP-25 in the GST pull-down assay. SNAP-25 is a component of the SNARE (Soluble NSF Attachment protein REceptor) complex and is critical for the exocytosis of synaptic vesicles. Incubation of rat brain lysate with recombinant Snapin T117D, but not WT, protein caused decreased interaction of synaptotagmin with the SNARE complex based on a co-immunoprecipitation assay. We further found that LRRK2-dependent phosphorylation of Snapin in the hippocampal neurons resulted in a decrease in the number of readily releasable vesicles and the extent of exocytotic release. Combined, these data suggest that LRRK2 may regulate neurotransmitter release via control of Snapin function by inhibitory phosphorylation.

Pre-synaptic release deficits in a DYT1 dystonia mouse model

DYT1 early-onset generalized torsion dystonia (DYT1 dystonia) is an inherited movement disorder caused by mutations in one allele of DYT1 (TOR1A), coding for torsinA. The most common mutation is a trinucleotide deletion (ΔGAG), which causes a deletion of a glutamic acid residue (ΔE) in the C-terminal region of torsinA. Although recent studies using cultured cells suggest that torsinA contributes to protein processing in the secretory pathway, endocytosis, and the stability of synaptic proteins, the nature of how this mutation affects synaptic transmission remains unclear. We previously reported that theta-burst-induced long-term potentiation (LTP) in the CA1 region of the hippocampal slice is not altered in Dyt1 ΔGAG heterozygous knock-in (KI) mice. Here, we examined short-term synaptic plasticity and synaptic transmission in the hippocampal slices. Field recordings in the hippocampal Schaffer collaterals (SC) pathway revealed significantly enhanced paired pulse ratios (PPRs) in Dyt1 ΔGAG heterozygous KI mice, suggesting an impaired synaptic vesicle release. Whole-cell recordings from the CA1 neurons showed that Dyt1 ΔGAG heterozygous KI mice exhibited normal miniature excitatory post-synaptic currents (mEPSC), suggesting that action-potential independent spontaneous pre-synaptic release was normal. On the other hand, there was a significant decrease in the frequency, but not amplitude or kinetics, of spontaneous excitatory post-synaptic currents (sEPSC) in Dyt1 ΔGAG heterozygous KI mice, suggesting that the action-potential dependent pre-synaptic release was impaired. Moreover, hippocampal torsinA was significantly reduced in Dyt1 ΔGAG heterozygous KI mice. Although the hippocampal slice model may not represent the neurons directly associated with dystonic symptoms, impaired release of neurotransmitters caused by partial dysfunction of torsinA in other brain regions may contribute to the pathophysiology of DYT1 dystonia.

Potential molecular mechanisms for decreased synaptic glutamate release in dysbindin-1 mutant mice

Behavioral genetic studies of humans have associated variation in the DTNBP1 gene with schizophrenia and its cognitive deficit phenotypes. The protein encoded by DTNBP1, dysbindin-1, is expressed in forebrain neurons where it interacts with proteins mediating vesicular trafficking and exocytosis. It has been shown that loss of dysbindin-1 results in a decrease in glutamate release in the prefrontal cortex; however the mechanisms underlying this decrease are not fully understood. In order to investigate this question, we evaluated dysbindin-1 null mutant mice, using electrophysiological recordings of prefrontal cortical neurons, imaging studies of vesicles, calcium dynamics and Western blot measures of synaptic proteins and Ca(2+) channels. Dysbindin-1 null mice showed a decrease in the ready releasable pool of synaptic vesicles, decreases in quantal size, decreases in the probability of release and deficits in the rate of endo- and exocytosis compared with wild-type controls. Moreover, the dysbindin-1 null mice show decreases in the [Ca(2+)]i,expression of L- and N-type Ca(2+)channels and several proteins involved in synaptic vesicle trafficking and priming. Our results provide new insights into the mechanisms of action of dysbindin-1.

Differential roles for snapin and synaptotagmin in the synaptic vesicle cycle

Evoked synaptic transmission is dependent on interactions between the calcium sensor Synaptotagmin I and the SNARE complex, comprised of Syntaxin, SNAP-25, and Synaptobrevin. Recent evidence suggests that Snapin may be an important intermediate in this process, through simultaneous interactions of Snapin dimers with SNAP-25 and Synaptotagmin. In support of this model, cultured neurons derived from embryonically lethal Snapin null mutant mice exhibit desynchronized release and a reduced readily releasable vesicle pool. Based on evidence that a dimerization-defective Snapin mutation specifically disrupts priming, Snapin is hypothesized to stabilize primed vesicles by structurally coupling Synaptotagmin and SNAP-25. To explore this model in vivo we examined synaptic transmission in viable, adult C. elegans Snapin (snpn-1) mutants. The kinetics of synaptic transmission were unaffected at snpn-1 mutant neuromuscular junctions (NMJs), but the number of docked, fusion competent vesicles was significantly reduced. However, analyses of snt-1 and snt-1;snpn-1 double mutants suggest that the docking role of SNPN-1 is independent of Synaptotagmin. Based on these results we propose that the primary role of Snapin in C. elegans is to promote vesicle priming, consistent with the stabilization of SNARE complex formation through established interactions with SNAP-25 upstream of the actions of Synaptotagmin in calcium-sensing and endocytosis.

Botulinum toxins: mechanisms of action, antinociception and clinical applications

Botulinum toxin (BoNT) is a potent neurotoxin that is produced by the gram-positive, spore-forming, anaerobic bacterium, Clostridum botulinum. There are 7 known immunologically distinct serotypes of BoNT: types A, B, C1, D, E, F, and G. Clostridum neurotoxins are produced as a single inactive polypeptide chain of 150kDa, which is cleaved by tissue proteinases into an active di-chain molecule: a heavy chain (H) of ∼100 kDa and a light chain (L) of ∼50 kDa held together by a single disulfide bond. Each serotype demonstrates its own varied mechanisms of action and duration of effect. The heavy chain of each BoNT serotype binds to its specific neuronal ecto-acceptor, whereby, membrane translocation and endocytosis by intracellular synaptic vesicles occurs. The light chain acts to cleave SNAP-25, which inhibits synaptic exocytosis, and therefore, disables neural transmission. The action of BoNT to block the release of acetylcholine botulinum toxin at the neuromuscular junction is best understood, however, most experts acknowledge that this effect alone appears inadequate to explain the entirety of the neurotoxin's apparent analgesic activity. Consequently, scientific and clinical evidence has emerged that suggests multiple antinociceptive mechanisms for botulinum toxins in a variety of painful disorders, including: chronic musculoskeletal, neurological, pelvic, perineal, osteoarticular, and some headache conditions.

E3 ubiquitin ligase RNF13 involves spatial learning and assembly of the SNARE complex

Changes in the structure and number of synapses modulate learning, memory and cognitive disorders. Ubiquitin-mediated protein modification is a key mechanism for regulating synaptic activity, though the precise control of this process remains poorly understood. RING finger protein 13 (RNF13) is a recently identified E3 ubiquitin ligase, and its in vivo function remains completely unknown. We show here that genetic deletion of RNF13 in mice leads to a significant deficit in spatial learning as determined by the Morris water maze test and Y-maze learning test. At the ultrastructral level, the synaptic vesicle density was decreased and the area of the active zone was increased at hippocampal synapses of RNF13-null mice compared with those of wild-type littermates. We found no change in the levels of SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) complex proteins in the hippocampus of RNF13-null mice, but impaired SNARE complex assembly. RNF13 directly interacted with snapin, a SNAP-25-interacting protein. Interestingly, snapin was ubiquitinated by RNF13 via the lysine-29 conjugated polyubiquitin chain, which in turn promoted the association of snapin with SNAP-25. Consistently, we found an attenuated interaction between snapin and SNAP-25 in the RNF13-null mice. Therefore, these results suggest that RNF13 is involved in the regulation of the SNARE complex, which thereby controls synaptic function.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Snapin is critical for presynaptic homeostatic plasticity

The molecular mechanisms underlying the homeostatic modulation of presynaptic neurotransmitter release are largely unknown. We have previously used an electrophysiology-based forward genetic screen to assess the function of >400 neuronally expressed genes for a role in the homeostatic control of synaptic transmission at the neuromuscular junction of Drosophila melanogaster. This screen identified a critical function for dysbindin, a gene linked to schizophrenia in humans (Dickman and Davis, 2009). Biochemical studies in other systems have shown that Snapin interacts with Dysbindin, prompting us to test whether Snapin might be involved in the mechanisms of synaptic homeostasis. Here, we demonstrate that loss of snapin blocks the homeostatic modulation of presynaptic vesicle release following inhibition of postsynaptic glutamate receptors. This is true for both the rapid induction of synaptic homeostasis induced by pharmacological inhibition of postsynaptic glutamate receptors, and the long-term expression of synaptic homeostasis induced by the genetic deletion of the muscle-specific GluRIIA glutamate receptor subunit. Loss of snapin does not alter baseline synaptic transmission, synapse morphology, synapse growth, or the number or density of active zones, indicating that the block of synaptic homeostasis is not a secondary consequence of impaired synapse development. Additional genetic evidence suggests that snapin functions in concert with dysbindin to modulate vesicle release and possibly homeostatic plasticity. Finally, we provide genetic evidence that the interaction of Snapin with SNAP25, a component of the SNARE complex, is also involved in synaptic homeostasis.

Recruitment of sphingosine kinase to presynaptic terminals by a conserved muscarinic signaling pathway promotes neurotransmitter release

Sphingolipids are potent lipid second messengers that regulate cell differentiation, migration, survival, and secretion, and alterations in sphingolipid signaling have been implicated in a variety of diseases. However, how sphingolipid levels are regulated, particularly in the nervous system, remains poorly understood. Here, we show that the generation of sphingosine-1-phosphate by sphingosine kinase (SphK) promotes neurotransmitter release. Electrophysiological, imaging, and behavioral analyses of Caenorhabditis elegans mutants lacking sphingosine kinase sphk-1 indicate that neuronal development is normal, but there is a significant defect in neurotransmitter release from neuromuscular junctions. SPHK-1 localizes to discrete, nonvesicular regions within presynaptic terminals, and this localization is critical for synaptic function. Muscarinic agonists cause a rapid increase in presynaptic SPHK-1 abundance, whereas reduction of endogenous acetylcholine production results in a rapid decrease in presynaptic SPHK-1 abundance. Muscarinic regulation of presynaptic SPHK-1 abundance is mediated by a conserved presynaptic signaling pathway composed of the muscarinic acetylcholine receptor GAR-3, the heterotrimeric G protein Gαq, and its effector, Trio RhoGEF. SPHK-1 activity is required for the effects of muscarinic signaling on synaptic transmission. This study shows that SPHK-1 promotes neurotransmitter release in vivo and identifies a novel muscarinic pathway that regulates SphK abundance at presynaptic terminals.

Beclin-1-interacting autophagy protein Atg14L targets the SNARE-associated protein Snapin to coordinate endocytic trafficking

Autophagy is a highly regulated membrane remodeling process that allows the lysosome-mediated degradation of cytoplasmic entities by sequestrating them in double-membrane autophagosomes. Autophagy is hence highly intertwined with the endocytic trafficking pathway, sharing similar molecular machinery. Atg14L, also known as Beclin 1-associated autophagy-related key regulator (Barkor), directly interacts with Beclin 1 through its coiled-coil domain and enhances phosphatidylinositol 3-phosphate kinase class III (PI3KC3) activity to induce autophagosome membrane nucleation, highlighting its essential role in the early stage of mammalian autophagy. Here, we report a novel function of Atg14L in the endocytic trafficking pathway wherein Atg14L binds to and colocalizes with the fusogenic SNARE effector protein Snapin to facilitate endosome maturation. Atg14L specifically binds to Snapin and this interaction effectively facilitates endosomal maturation without affecting autophagic cargo degradation. Consequently, atg14l knockdown significantly delayed the late stage of endocytic trafficking, as evidenced by the retarded kinetics of internalized surface receptor degradation. This phenotype was effectively complemented by wild-type Atg14L or Beclin 1-binding mutant, but not by its Snapin-binding mutant. Taken together, our study demonstrates that Atg14L functions as a multivalent trafficking effector that regulates endosome maturation as well as autophagosome formation, reflecting the complexity of the crosstalk between autophagic and endocytic vesicle trafficking in higher eukaryotes.

Snapin recruits dynein to BDNF-TrkB signaling endosomes for retrograde axonal transport and is essential for dendrite growth of cortical neurons

Neurotrophin signaling is crucial for neuron growth. While the "signaling endosomes" hypothesis is one of the accepted models, the molecular machinery that drives retrograde axonal transport of TrkB signaling endosomes is largely unknown. In particular, mechanisms recruiting dynein to TrkB signaling endosomes have not been elucidated. Here, using snapin deficient mice and gene rescue experiments combined with compartmentalized cultures of live cortical neurons, we reveal that Snapin, as a dynein adaptor, mediates retrograde axonal transport of TrkB signaling endosomes. Such a role is essential for dendritic growth of cortical neurons. Deleting snapin or disrupting Snapin-dynein interaction abolishes TrkB retrograde transport, impairs BDNF-induced retrograde signaling from axonal terminals to the nucleus, and decreases dendritic growth. Such defects were rescued by reintroducing the snapin gene. Our study indicates that Snapin-dynein coupling is one of the primary mechanisms driving BDNF-TrkB retrograde transport, thus providing mechanistic insights into the regulation of neuronal growth and survival.

Miniature release events of glutamate from hippocampal neurons are influenced by the dystonia-associated protein torsinA

TorsinA is an evolutionarily conserved AAA+ ATPase, and human patients with an in-frame deletion of a single glutamate (ΔE) codon from the encoding gene suffer from autosomal-dominant, early-onset generalized DYT1 dystonia. Although only 30-40% of carriers of the mutation show overt motor symptoms, most experience enhanced excitability of the central nervous system. The cellular mechanism responsible for this change in excitability is not well understood. Here we show the effects of the ΔE-torsinA mutation on miniature neurotransmitter release from neurons. Neurotransmitter release was characterized in cultured hippocampal neurons obtained from wild-type, heterozygous, and homozygous ΔE-torsinA knock-in mice using two approaches. In the first approach, patch-clamp electrophysiology was used to record glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in the presence of the Na⁺ channel blocker tetrodotoxin (TTX) and absence of GABA(A) receptor antagonists. The intervals between mEPSC events were significantly shorter in neurons obtained from the mutant mice than in those obtained from wild-type mice. In the second approach, the miniature exocytosis of synaptic vesicles was detected by imaging the unstimulated release of FM dye from the nerve terminals in the presence of TTX. Cumulative FM dye release was higher in neurons obtained from the mutant mice than in those obtained from wild-type mice. The number of glutamatergic nerve terminals was also assessed, and we found that this number was unchanged in heterozygous relative to wild-type neurons, but slightly increased in homozygous neurons. Notably, in both heterozygous and homozygous neurons, the unitary synaptic charge during each mEPSC event was unchanged. Overall, our results suggest more frequent miniature glutamate release in neurons with ΔE-torsinA mutations. This change may be one of the underlying mechanisms by which the excitability of the central nervous system is enhanced in the context of DYT1 dystonia. Moreover, qualitative differences between heterozygous and homozygous neurons with respect to certain synaptic properties indicate that the abnormalities observed in homozygotes may reflect more than a simple gene dosage effect.

The SNARE complex in neuronal and sensory cells

Transmitter release at synapses ensures faithful chemical coding of information that is transmitted in the sub-second time frame. The brain, the central unit of information processing, depends upon fast communication for decision making. Neuronal and neurosensory cells are equipped with the molecular machinery that responds reliably, and with high fidelity, to external stimuli. However, neuronal cells differ markedly from neurosensory cells in their signal transmission at synapses. The main difference rests in how the synaptic complex is organized, with active zones in neuronal cells and ribbon synapses in sensory cells (such as photoreceptors and hair cells). In exocytosis/neurosecretion, SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) and associated proteins play a critical role in vesicle docking, priming, fusion and synchronization of neurotransmitter release. Recent studies suggest differences between neuronal and sensory cells with respect to the molecular components of their synaptic complexes. In this review, we will cover current findings on neuronal and sensory-cell SNARE proteins and their modulators. We will also briefly discuss recent investigations on how deficits in the expression of SNARE proteins in humans impair function in brain and sense organs.

The schizophrenia susceptibility factor dysbindin and its associated complex sort cargoes from cell bodies to the synapse

Dysbindin assembles into the biogenesis of lysosome-related organelles complex 1 (BLOC-1), which interacts with the adaptor protein complex 3 (AP-3), mediating a common endosome-trafficking route. Deficiencies in AP-3 and BLOC-1 affect synaptic vesicle composition. However, whether AP-3-BLOC-1-dependent sorting events that control synapse membrane protein content take place in cell bodies upstream of nerve terminals remains unknown. We tested this hypothesis by analyzing the targeting of phosphatidylinositol-4-kinase type II α (PI4KIIα), a membrane protein present in presynaptic and postsynaptic compartments. PI4KIIα copurified with BLOC-1 and AP-3 in neuronal cells. These interactions translated into a decreased PI4KIIα content in the dentate gyrus of dysbindin-null BLOC-1 deficiency and AP-3-null mice. Reduction of PI4KIIα in the dentate reflects a failure to traffic from the cell body. PI4KIIα was targeted to processes in wild-type primary cultured cortical neurons and PC12 cells but failed to reach neurites in cells lacking either AP-3 or BLOC-1. Similarly, disruption of an AP-3-sorting motif in PI4KIIα impaired its sorting into processes of PC12 and primary cultured cortical neuronal cells. Our findings indicate a novel vesicle transport mechanism requiring BLOC-1 and AP-3 complexes for cargo sorting from neuronal cell bodies to neurites and nerve terminals.

Type VI adenylyl cyclase regulates neurite extension by binding to Snapin and Snap25

3'-5'-Cyclic AMP (cAMP) is an important second messenger which regulates neurite outgrowth. We demonstrate here that type VI adenylyl cyclase (AC6), an enzyme which catalyzes cAMP synthesis, regulates neurite outgrowth by direct interaction with a binding protein (Snapin) of Snap25 at the N terminus of AC6 (AC6-N). We first showed that AC6 expression increased during postnatal brain development. In primary hippocampal neurons and Neuro2A cells, elevated AC6 expression suppressed neurite outgrowth, whereas the downregulation or genetic removal of AC6 promoted neurite extension. An AC6 variant (AC6-N5) that contains the N terminus of AC5 had no effect, indicating the importance of AC6-N. The downregulation of endogenous Snapin or the overexpression of a Snapin mutant (Snap(Δ33-51)) that does not bind to AC6, or another Snapin mutant (Snapin(S50A)) that does not interact with Snap25, reversed the inhibitory effect of AC6. Pulldown assays and immunoprecipitation-AC assays revealed that the complex formation of AC6, Snapin, and Snap25 is dependent on AC6-N and the phosphorylation of Snapin. The overexpression of Snap25 completely reversed the action of AC6. Collectively, in addition to cAMP production, AC6 plays a complex role in modulating neurite outgrowth by redistributing localization of the SNARE apparatus via its interaction with Snapin.

Endosome maturation. EMBO J. 2011;30:3481-3500. [PMID: 21878991 DOI: 10.1038/emboj.2011.286]

Being deeply connected to signalling, cell dynamics, growth, regulation, and defence, endocytic processes are linked to almost all aspects of cell life and disease. In this review, we focus on endosomes in the classical endocytic pathway, and on the programme of changes that lead to the formation and maturation of late endosomes/multivesicular bodies. The maturation programme entails a dramatic transformation of these dynamic organelles disconnecting them functionally and spatially from early endosomes and preparing them for their unidirectional role as a feeder pathway to lysosomes.

Cell biology of the BLOC-1 complex subunit dysbindin, a schizophrenia susceptibility gene

There is growing interest in the biology of dysbindin and its genetic locus (DTNBP1) due to genetic variants associated with an increased risk of schizophrenia. Reduced levels of dysbindin mRNA and protein in the hippocampal formation of schizophrenia patients further support involvement of this locus in disease risk. Here, we discuss phylogenetically conserved dysbindin molecular interactions that define its contribution to the assembly of the biogenesis of lysosome-related organelles complex-1 (BLOC-1). We explore fundamental cellular processes where dysbindin and the dysbindin-containing BLOC-1 complex are implicated. We propose that cellular, tissue, and system neurological phenotypes from dysbindin deficiencies in model genetic organisms, and likely individuals affected with schizophrenia, emerge from abnormalities in few core cellular mechanisms controlled by BLOC-1-dysbindin-containing complex rather than from defects in dysbindin itself.

Blue native/SDS-PAGE combined with iTRAQ analysis reveals advanced glycation end-product-induced changes of synaptosome proteins in C57 BL/6 mice

Evidence shows that administration of high-level D-galactose induces the production of advanced glycation end-products (AGEs) that have been implicated in the development of diabetic complications such as neuropathy. The deterioration of learning and memory during neuropathy might be associated with the altered expression of proteins in synapse. To evaluate AGE-induced protein network alterations in synapse, blue native/SDS-PAGE and iTRAQ proteomic methods were used to screen for differentially expressed synaptic proteins of cerebral cortex in D-galactose-induced C57 BL/6 mice. In total, the expression level of 84 proteins is changed during AGE accumulation. The significantly differentially expressed proteins mainly participate in neurotransmission, energy metabolism and signal transduction pathway, suggesting that energy metabolism is damaged and neurotransmission is attenuated in synapse. The results of in vivo activities of malondialdehyde and superoxide dismutase suggested that AGE accumulation in the brain leads to the generation of reactive oxygen species. Therefore, elucidating the differentially expressed proteins underlying the AGE accumulation will open a new window to the mechanism of learning and memory impairments in neuropathy.

Snapin deficiency is associated with developmental defects of the central nervous system

The autophagy-lysosomal pathway is an intracellular degradation process essential for maintaining neuronal homoeostasis. Defects in this pathway have been directly linked to a growing number of neurodegenerative disorders. We recently revealed that Snapin plays a critical role in co-ordinating dynein-driven retrograde transport and late endosomal-lysosomal trafficking, thus maintaining efficient autophagy-lysosomal function. Deleting snapin in neurons impairs lysosomal proteolysis and reduces the clearance of autolysosomes. The role of the autophagy-lysosomal system in neuronal development is, however, largely uncharacterized. Here, we report that snapin deficiency leads to developmental defects in the central nervous system. Embryonic snapin-/- mouse brain showed reduced cortical plates and intermediate zone cell density, increased apoptotic death in the cortex and third ventricle, enhanced membrane-bound LC3-II staining associated with autophagic vacuoles and an accumulation of polyubiquitinated proteins in the cortex and hippocampus. Thus our results provide in vivo evidence for the essential role of late endocytic transport and autophagy-lysosomal function in maintaining neuronal survival and development of the mammalian central nervous system. In addition, our study supports the existence of a functional interplay between the autophagy-lysosome and ubiquitin-proteasome systems in the protein quality-control process.

The role of synaptobrevin1/VAMP1 in Ca2+-triggered neurotransmitter release at the mouse neuromuscular junction

Synaptobrevin (Syb)/vesicle-associated membrane protein (VAMP) is a small, integral membrane protein of synaptic vesicles. Two homologous isoforms of synaptobrevin, Syb1/VAMP1 and Syb2/VAMP2, exhibit distinct but partially overlapping patterns of expression in adult mammalian neurons: Syb1 is predominantly expressed in the spinal cord, especially in motor neurons and motor nerve terminals of the neuromuscular junction (NMJ), whereas Syb2 is primarily expressed in central synapses in the brain. Whereas many studies have focused on the function of Syb2 in the brain, few studies have examined the role of Syb1. Here we report that Syb1 plays a critical role in neuromuscular synaptic transmission. A null mutation of Syb1 resulting from a spontaneous, nonsense mutation in mice significantly impairs the function, but not the structure, of the NMJ. In particular, both spontaneous and evoked synaptic activities in Syb1 mutant mice are reduced significantly relative to control mice. Short-term synaptic plasticity in Syb1-deficient NMJs is markedly altered: paired-pulse facilitation is significantly enhanced, suggesting a reduction in the initial release probability of synaptic vesicles. Furthermore, Syb1-deficient NMJs display a pronounced asynchrony in neurotransmitter release. These impairments are not due to an alteration of the size of the readily releasable pool of vesicles, but are attributable to reduced sensitivity and cooperativity to calcium (Ca2+) due to the absence of Syb1. Our findings demonstrate that Syb1 plays an essential, non-redundant role in Ca2+-triggered vesicle exocytosis at the mouse NMJ.

CSN complex controls the stability of selected synaptic proteins via a torsinA-dependent process

DYT1 dystonia is caused by an autosomal dominant mutation that leads to a glutamic acid deletion in torsinA (TA), a member of the AAA+ ATPase superfamily. In this study, we identified a novel-binding partner of TA, the subunit 4 (CSN4) of CSN signalosome. TA binds CSN4 and the synaptic regulator snapin in neuroblastoma cells and in brain synaptosomes. CSN4 and TA are required for the stability of both snapin and the synaptotagmin-specific endocytic adaptor stonin 2, as downregulation of CSN4 or TA reduces the levels of both proteins. Snapin is phosphorylated by the CSN-associated kinase protein kinase D (PKD) and its expression is decreased upon PKD inhibition. In contrast, the stability of stonin 2 is regulated by neddylation, another CSN-associated activity. Overexpression of the pathological TA mutant (ΔE-TA) reduces stonin 2 expression, causing the accumulation of the calcium sensor synaptotagmin 1 on the cell surface. Retrieval of surface-stranded synaptotagmin 1 is restored by overexpression of stonin 2 in ΔE-TA-expressing cells, suggesting that the DYT1 mutation compromises the role of TA in protein stabilisation and synaptic vesicle recycling.

Snapin snaps into the dynein complex for late endosome-lysosome trafficking and autophagy

Late endosome-lysosome trafficking plays a key role in regulating cell surface signaling and degradation of intracellular components by autophagy. New work by Cai and coworkers in this issue of Neuron provides evidence that snapin regulates the recruitment of late endosomes to the dynein motor complex for retrograde trafficking along microtubules and maturation of lysosomes.

Snapin-regulated late endosomal transport is critical for efficient autophagy-lysosomal function in neurons

Neuron maintenance and survival require late endocytic transport from distal processes to the soma where lysosomes are predominantly localized. Here, we report a role for Snapin in attaching dynein to late endosomes through its intermediate chain (DIC). snapin(-/-) neurons exhibit aberrant accumulation of immature lysosomes, clustering and impaired retrograde transport of late endosomes along processes, reduced lysosomal proteolysis due to impaired delivery of internalized proteins and hydrolase precursors from late endosomes to lysosomes, and impaired clearance of autolysosomes, combined with reduced neuron viability and neurodegeneration. The phenotypes are rescued by expressing the snapin transgene, but not the DIC-binding-defective Snapin-L99K mutant. Snapin overexpression in wild-type neurons enhances late endocytic transport and lysosomal function, whereas expressing the mutant defective in Snapin-DIC coupling shows a dominant-negative effect. Altogether, our study highlights new mechanistic insights into how Snapin-DIC coordinates retrograde transport and late endosomal-lysosomal trafficking critical for autophagy-lysosomal function, and thus neuronal homeostasis.

EHD1 is a synaptic protein that modulates exocytosis through binding to snapin

EHD1 is an EH (Eps15 homology) domain-containing protein involved in endosomal recycling. Our yeast two hybrid screening experiments showed that EHD1 interacts with a synaptic protein, snapin, and the present study was carried out to further elucidate the functional significance of this interaction. Immunoreactivity to EHD1 is observed in the cerebral cortex, hippocampus and striatum, in the rat brain. The protein is colocalized with the axon terminal marker synaptophysin in cultured neurons. EHD1 binds to the C terminus of snapin via its C terminus EH domain. It negatively affects the binding of a SNARE complex protein, SNAP-25, to snapin, probably due to the competition for overlapping binding sites on the C terminus of snapin. EHD1 affects the coupling of synaptotagmin-1 to the SNARE complex, and could be a negative regulator of exocytosis. This is supported by electrophysiological findings that PC-12 cells which overexpress EHD1 show reduced depolarization-induced exocytosis compared to controls, but the reduced exocytosis is not observed in cells which overexpress the N terminus of EHD1 that is unable to bind snapin. Together, the above results indicate that EHD1 is a synaptic protein that negatively affects exocytosis through binding to snapin.

TorsinA and DYT1 dystonia: a synaptopathy?

DYT1 dystonia is an autosomal dominant movement disorder, characterized by early onset of involuntary sustained muscle contractions. It is caused by a 3-bp deletion in the DYT1 gene, which results in the deletion of a single glutamate residue in the C-terminus of the protein TA (torsinA). TA is a member of the AAA+ (ATPase associated with various cellular activities) family of chaperones with multiple functions in the cell. There is no evidence of neurodegeneration in DYT1 dystonia, which suggests that mutant TA leads to functional neuronal abnormalities, leading to dystonic movements. In recent years, different functional roles have been attributed to TA, including being a component of the cytoskeleton and the NE (nuclear envelope), and involvement in the secretory pathway and SV (synaptic vesicle) machinery. The aim of the present review is to summarize these findings and the different models proposed, which have contributed to our current understanding of the function of TA, and also to discuss the evidence implicating TA in SV function.

Two modes of release shape the postsynaptic response at the inner hair cell ribbon synapse

Cochlear inner hair cells (IHCs) convert sounds into receptor potentials and via their ribbon synapses into firing rates in auditory nerve fibers. Multivesicular release at individual IHC ribbon synapses activates AMPA-mediated EPSCs with widely ranging amplitudes. The underlying mechanisms and specific role for multivesicular release in encoding sound are not well understood. Here we characterize the waveforms of individual EPSCs recorded from afferent boutons contacting IHCs and compare their characteristics in immature rats (postnatal days 8-11) and hearing rats (postnatal days 19-21). Two types of EPSC waveforms were found in every recording: monophasic EPSCs, with sharp rising phases and monoexponential decays, and multiphasic EPSCs, exhibiting inflections on rising and decaying phases. Multiphasic EPSCs exhibited slower rise times and smaller amplitudes than monophasic EPSCs. Both types of EPSCs had comparable charge transfers, suggesting that they were activated by the release of similar numbers of vesicles, which for multiphasic EPSCs occurred in a less coordinated manner. On average, a higher proportion of larger, monophasic EPSCs was found in hearing compared to immature rats. In addition, EPSCs became significantly faster with age. The developmental increase in size and speed could improve auditory signaling acuity. Multiphasic EPSCs persisted in hearing animals, in some fibers constituting half of the EPSCs. The proportion of monophasic versus multiphasic EPSCs varied widely across fibers, resulting in marked heterogeneity of amplitude distributions. We propose that the relative contribution of two modes of multivesicular release, generating monophasic and multiphasic EPSCs, may underlie fundamental characteristics of auditory nerve fibers.