Synaptotagmins: why so many?

Chemoproteomics Sheds Light on Epigenetic Targets of [11C]Martinostat in the Human Brain

Initiation of research programs to investigate binding specificity based on in vivo positron emission tomography (PET) imaging results can provide rich opportunities to improve data interpretation, gain biological insight, and inform hypothesis development. Here, we profile the binding specificity of the neuroepigenetic imaging probe, [11C]Martinostat. In vivo neuroimaging studies using [11C]Martinostat have uncovered differential regional uptake in relation to age and biological sex and in patients with schizophrenia, bipolar disorder, Alzheimer's disease, and low-back pain compared to healthy controls. Previous studies using recombinant proteins and thermal shift assays in postmortem tissue indicate that [11C]Martinostat engages class I and putatively class IIb histone deacetylases (HDACs). While HDACs serve multiple functions, including regulation of chromatin remodeling and gene transcription, it is not known how differences in HDAC expression may arise across brain regions. HDACs functionally interact with a diverse array of multisubunit complexes, and engagement with associated binding partners may contribute to these differences. To further assess target engagement of [11C]Martinostat, we designed a synthetic probe based on the inhibitor structural scaffold for use in competition experiments followed by proteomic analysis in postmortem tissue. The synthetic probe, called Compound 4, appears to interact with the class I HDAC paralog HDAC2 and the class IIb paralog HDAC6 in a robust manner. We also uncovered unique interacting partners, including synaptic proteins from the synaptotagmin (SYT) family of proteins and neuronal pentraxin 2 (NPTX2). Further work to investigate HDAC associations with interacting proteins across regions of the human brain is needed to better understand neuroepigenetic dysregulation in psychiatric and neurological conditions.

The Cell States of Sea Urchin During Metamorphosis Revealed by Single-Cell RNA Sequencing

Metamorphosis is a key process in the life history of sea urchin Heliocidaris crassispina. However, the understanding of its molecular mechanisms is still lacking, especially the basic cell biology pre-metamorphosis and post-metamorphosis. Therefore, we employed single-cell RNA sequencing to delineate the cellular states of larvae and juveniles of H. crassispina. Our investigation revealed that the cell composition in sea urchins comprises six primary populations, encompassing nerve cells, skeletogenic cells, immune cells, digestive cells, germ cells, and muscle cells. Subsequently, we identified subpopulations within these cells. Our findings indicated that the larval peripheral nerves were discarded during metamorphosis. A decrease in the number of spicules was observed during this process. Additionally, we examined the differences between larval and adult pigment cells. Meanwhile, cellulase is highlighted as an essential factor for the development of competent juveniles. In summary, this study not only serves as a valuable resource for future research on sea urchins but also deepens our understanding of the intricate metamorphosis process.

Evaluation of synaptotagmin-1 action models by all-atom molecular dynamics simulations

Neurotransmitter release is triggered in microseconds by the two C2 domains of the Ca2+ sensor synaptotagmin-1 and by SNARE complexes, which form four-helix bundles that bridge the vesicle and plasma membranes. The synaptotagmin-1 C2B domain binds to the SNARE complex via a 'primary interface', but the mechanism that couples Ca2+-sensing to membrane fusion is unknown. Widespread models postulate that the synaptotagmin-1 Ca2+-binding loops accelerate membrane fusion by inducing membrane curvature, perturbing lipid bilayers or helping bridge the membranes, but these models do not seem compatible with SNARE binding through the primary interface, which orients the Ca2+-binding loops away from the fusion site. To test these models, we performed molecular dynamics simulations of SNARE complexes bridging a vesicle and a flat bilayer, including the synaptotagmin-1 C2 domains in various configurations. Our data do not support the notion that insertion of the synaptotagmin-1 Ca2+-binding loops causes substantial membrane curvature or major perturbations of the lipid bilayers that could facilitate membrane fusion. We observed membrane bridging by the synaptotagmin-1 C2 domains, but such bridging or the presence of the C2 domains near the site of fusion hindered the action of the SNAREs in bringing the membranes together. These results argue against models predicting that synaptotagmin-1 triggers neurotransmitter release by inducing membrane curvature, perturbing bilayers or bridging membranes. Instead, our data support the hypothesis that binding via the primary interface keeps the synaptotagmin-1 C2 domains away from the site of fusion, orienting them such that they trigger release through a remote action.

Synaptotagmins 3 and 7 mediate the majority of asynchronous release from synapses in the cerebellum and hippocampus

Neurotransmitter release consists of rapid synchronous release followed by longer-lasting asynchronous release (AR). Although the presynaptic proteins that trigger synchronous release are well understood, the mechanisms for AR remain unclear. AR is sustained by low concentrations of intracellular Ca2+ and Sr2+, suggesting the involvement of sensors with high affinities for both ions. Synaptotagmin 7 (SYT7) partly mediates AR, but substantial AR persists in the absence of SYT7. The closely related SYT3 binds Ca2+ and Sr2+ with high affinity, making it a promising candidate to mediate AR. Here, we use knockout mice to study the contribution of SYT3 and SYT7 to AR at cerebellar and hippocampal synapses. AR is dramatically reduced when both isoforms are absent, which alters the number and timing of postsynaptic action potentials. Our results confirm the long-standing prediction that SYT3 mediates AR and show that SYT3 and SYT7 act as dominant mechanisms for AR at three central synapses.

A single base pair substitution in zebrafish distinguishes between innate and acute startle behavior regulation

Behavioral thresholds define the lowest stimulus intensities sufficient to elicit a behavioral response. Establishment of baseline behavioral thresholds during development is critical for proper responses throughout the animal's life. Despite the relevance of such innate thresholds, the molecular mechanisms critical to establishing behavioral thresholds during development are not well understood. The acoustic startle response is a conserved behavior whose threshold is established during development yet is subsequently acutely regulated. We have previously identified a zebrafish mutant line (escapist) that displays a decreased baseline or innate acoustic startle threshold. Here, we identify a single base pair substitution on Chromosome 25 located within the coding sequence of the synaptotagmin 7a (syt7a) gene that is tightly linked to the escapist acoustic hypersensitivity phenotype. By generating animals in which we deleted the syt7a open reading frame, and subsequent complementation testing with the escapist line, we demonstrate that loss of syt7a function is not the cause of the escapist behavioral phenotype. Nonetheless, escapist mutants provide a powerful tool to decipher the overlap between acute and developmental regulation of behavioral thresholds. Extensive behavioral analyses reveal that in escapist mutants the establishment of the innate acoustic startle threshold is impaired, while regulation of its acute threshold remains intact. Moreover, our behavioral analyses reveal a deficit in baseline responses to visual stimuli, but not in the acute regulation of responses to visual stimuli. Together, this work eliminates loss of syt7a as causative for the escapist phenotype and suggests that mechanisms that regulate the establishment of behavioral thresholds in escapist larvae can operate independently from those regulating acute threshold regulation.

Synaptotagmin-11 regulates immune functions of microglia in vivo

Membrane trafficking pathways mediate key microglial activities such as cell migration, cytokine secretion, and phagocytosis. However, the underlying molecular mechanism remains poorly understood. Previously, we found that synaptotagmin-11 (Syt11), a non-Ca2+ -binding Syt associated with Parkinson's disease (PD) and schizophrenia, inhibits cytokine release and phagocytosis in primary microglia. Here we reported the in vivo function of Syt11 in microglial immune responses using an inducible microglia-specific Syt11-conditional-knockout (cKO) mouse strain. Syt11-cKO resulted in activation of microglia and elevated mRNA levels of IL-6, TNF-α, IL-1β, and iNOS in various brain regions under both resting state and LPS-induced acute inflammation state in adult mice. In a PD mouse model generated by microinjection of preformed α-synuclein fibrils into the striatum, a reduced number of microglia migrated toward the injection sites and an enhanced phagocytosis of α-synuclein fibrils by microglia were found in Syt11-cKO mice. To understand the molecular mechanism of Syt11 function, we identified its direct binding proteins vps10p-tail-interactor-1a (vti1a) and vti1b. The linker domain of Syt11 interacted with both proteins and a peptide derived from it competitively inhibited the interaction of Syt11 with vti1a/vti1b in vitro and in cells. Importantly, application of this peptide induced more cytokine secretion in wild-type microglia upon LPS treatment, phenocopying defects in Syt11 knockdown cells. Altogether, we propose that Syt11 inhibits microglial activation in vivo and regulates cytokine secretion through interactions with vti1a and vti1b.

Neuroligin 2 governs synaptic morphology and function through RACK1-cofilin signaling in Drosophila

Neuroligins are transmembrane cell adhesion proteins well-known for their genetic links to autism spectrum disorders. Neuroligins can function by regulating the actin cytoskeleton, however the factors and mechanisms involved are still largely unknown. Here, using the Drosophila neuromuscular junction as a model, we reveal that F-Actin assembly at the Drosophila NMJ is controlled through Cofilin signaling mediated by an interaction between DNlg2 and RACK1, factors not previously known to work together. The deletion of DNlg2 displays disrupted RACK1-Cofilin signaling pathway with diminished actin cytoskeleton proteo-stasis at the terminal of the NMJ, aberrant NMJ structure, reduced synaptic transmission, and abnormal locomotion at the third-instar larval stage. Overexpression of wildtype and activated Cofilin in muscles are sufficient to rescue the morphological and physiological defects in dnlg2 mutants, while inactivated Cofilin is not. Since the DNlg2 paralog DNlg1 is known to regulate F-actin assembly mainly via a specific interaction with WAVE complex, our present work suggests that the orchestration of F-actin by Neuroligins is a diverse and complex process critical for neural connectivity.

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

A single base pair substitution on Chromosome 25 in zebrafish distinguishes between development and acute regulation of behavioral thresholds

Behavioral thresholds define the lowest stimulus intensities sufficient to elicit a behavioral response. Establishment of baseline behavioral thresholds during development is critical for proper responses throughout the animal's life. Despite the relevance of such innate thresholds, the molecular mechanisms critical to establishing behavioral thresholds during development are not well understood. The acoustic startle response is a conserved behavior whose threshold is established during development yet is subsequently acutely regulated. We have previously identified a zebrafish mutant line ( escapist ) that displays a decreased baseline or innate acoustic startle threshold. Here, we identify a single base pair substitution on Chromosome 25 located within the coding sequence of the synaptotagmin 7a ( syt7a ) gene that is tightly linked to the escapist acoustic hypersensitivity phenotype. By generating animals in which we deleted the syt7a open reading frame, and subsequent complementation testing with the escapist line, we demonstrate that loss of syt7a function is not the cause of the escapist behavioral phenotype. Nonetheless, escapist mutants provide a powerful tool to decipher the overlap between acute and developmental regulation of behavioral thresholds. Extensive behavioral analyses reveal that in escapist mutants the establishment of the innate acoustic startle threshold is impaired, while regulation of its acute threshold remains intact. Moreover, our behavioral analyses reveal a deficit in baseline responses to visual stimuli, but not in the acute regulation of responses to visual stimuli. Together, this work eliminates loss of syt7a as causative for the escapist phenotype and suggests that mechanisms that regulate the establishment of behavioral thresholds in escapist larvae can operate largely independently from those regulating acute threshold regulation.

Genomic signatures suggesting adaptation to ocean acidification in a coral holobiont from volcanic CO2 seeps

Ocean acidification, caused by anthropogenic CO₂ emissions, is predicted to have major consequences for reef-building corals, jeopardizing the scaffolding of the most biodiverse marine habitats. However, whether corals can adapt to ocean acidification and how remains unclear. We addressed these questions by re-examining transcriptome and genome data of Acropora millepora coral holobionts from volcanic CO₂ seeps with end-of-century pH levels. We show that adaptation to ocean acidification is a wholistic process involving the three main compartments of the coral holobiont. We identified 441 coral host candidate adaptive genes involved in calcification, response to acidification, and symbiosis; population genetic differentiation in dinoflagellate photosymbionts; and consistent transcriptional microbiome activity despite microbial community shifts. Coral holobionts from natural analogues to future ocean conditions harbor beneficial genetic variants with far-reaching rapid adaptation potential. In the face of climate change, these populations require immediate conservation strategies as they could become key to coral reef survival.

Targeting the Cysteine Redox Proteome in Parkinson's Disease: The Role of Glutathione Precursors and Beyond

Encouraging recent data on the molecular pathways underlying aging have identified variants and expansions of genes associated with DNA replication and repair, telomere and stem cell maintenance, regulation of the redox microenvironment, and intercellular communication. In addition, cell rejuvenation requires silencing some transcription factors and the activation of pluripotency, indicating that hidden molecular networks must integrate and synchronize all these cellular mechanisms. Therefore, in addition to gene sequence expansions and variations associated with senescence, the optimization of transcriptional regulation and protein crosstalk is essential. The protein cysteinome is crucial in cellular regulation and plays unexpected roles in the aging of complex organisms, which show cumulative somatic mutations, telomere attrition, epigenetic modifications, and oxidative dysregulation, culminating in cellular senescence. The cysteine thiol groups are highly redox-active, allowing high functional versatility as structural disulfides, redox-active disulfides, active-site nucleophiles, proton donors, and metal ligands to participate in multiple regulatory sites in proteins. Also, antioxidant systems control diverse cellular functions, including the transcription machinery, which partially depends on the catalytically active cysteines that can reduce disulfide bonds in numerous target proteins, driving their biological integration. Since we have previously proposed a fundamental role of cysteine-mediated redox deregulation in neurodegeneration, we suggest that cellular rejuvenation of the cysteine redox proteome using GSH precursors, like N-acetyl-cysteine, is an underestimated multitarget therapeutic approach that would be particularly beneficial in Parkinson's disease.

Neurotransmitter release progressively desynchronizes in induced human neurons during synapse maturation and aging

Rapid release of neurotransmitters in synchrony with action potentials is considered a key hardwired property of synapses. Here, in glutamatergic synapses formed between induced human neurons, we show that action potential-dependent neurotransmitter release becomes progressively desynchronized as synapses mature and age. In this solely excitatory network, the emergence of NMDAR-mediated transmission elicits endoplasmic reticulum (ER) stress leading to downregulation of key presynaptic molecules, synaptotagmin-1 and cysteine string protein α, that synchronize neurotransmitter release. The emergence of asynchronous release with neuronal maturity and subsequent aging is maintained by the high-affinity Ca2+ sensor synaptotagmin-7 and suppressed by the introduction of GABAergic transmission into the network, inhibition of NMDARs, and ER stress. These results suggest that long-term disruption of excitation-inhibition balance affects the synchrony of excitatory neurotransmission in human synapses.

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

Genome Wide Association Study with Imputed Whole Genome Sequence Data Identifies a 431 kb Risk Haplotype on CFA18 for Congenital Laryngeal Paralysis in Alaskan Sled Dogs

Congenital laryngeal paralysis (CLP) is an inherited disorder that affects the ability of the dog to exercise and precludes it from functioning as a working sled dog. Though CLP is known to occur in Alaskan sled dogs (ASDs) since 1986, the genetic mutation underlying the disease has not been reported. Using a genome-wide association study (GWAS), we identified a 708 kb region on CFA 18 harboring 226 SNPs to be significantly associated with CLP. The significant SNPs explained 47.06% of the heritability of CLP. We narrowed the region to 431 kb through autozygosity mapping and found 18 of the 20 cases to be homozygous for the risk haplotype. Whole genome sequencing of two cases and a control ASD, and comparison with the genome of 657 dogs from various breeds, confirmed the homozygous status of the risk haplotype to be unique to the CLP cases. Most of the dogs that were homozygous for the risk allele had blue eyes. Gene annotation and a gene-based association study showed that the risk haplotype encompasses genes implicated in developmental and neurodegenerative disorders. Pathway analysis showed enrichment of glycoproteins and glycosaminoglycans biosynthesis, which play a key role in repairing damaged nerves. In conclusion, our results suggest an important role for the identified candidate region in CLP.

SYT1-Associated Neurodevelopmental Disorder: A Narrative Review

Synaptic dysregulations often result in damaging effects on the central nervous system, resulting in a wide range of brain and neurodevelopment disorders that are caused by mutations disrupting synaptic proteins. SYT1, an identified synaptotagmin protein, plays an essential role in mediating the release of calcium-triggered neurotransmitters (NT) involved in regular synaptic vesicle exocytosis. Considering the significant role of SYT1 in the physiology of synaptic neurotransmission, dysfunction and degeneration of this protein can result in a severe neurological impairment. Genetic variants lead to a newly discovered rare disorder, known as SYT1-associated neurodevelopment disorder. In this review, we will discuss in depth the function of SYT1 in synapse and the underlying molecular mechanisms. We will highlight the genetic basis of SYT1-associated neurodevelopmental disorder along with known phenotypes, with possible interventions and direction of research.

Allosteric stabilization of calcium and phosphoinositide dual binding engages several synaptotagmins in fast exocytosis

Synaptic communication relies on the fusion of synaptic vesicles with the plasma membrane, which leads to neurotransmitter release. This exocytosis is triggered by brief and local elevations of intracellular Ca²⁺ with remarkably high sensitivity. How this is molecularly achieved is unknown. While synaptotagmins confer the Ca²⁺ sensitivity of neurotransmitter exocytosis, biochemical measurements reported Ca²⁺ affinities too low to account for synaptic function. However, synaptotagmin’s Ca²⁺ affinity increases upon binding the plasma membrane phospholipid PI(4,5)P₂ and, vice versa, Ca²⁺ binding increases synaptotagmin’s PI(4,5)P₂ affinity, indicating a stabilization of the Ca²⁺/PI(4,5)P₂ dual-bound state. Here, we devise a molecular exocytosis model based on this positive allosteric stabilization and the assumptions that (1.) synaptotagmin Ca²⁺/PI(4,5)P₂ dual binding lowers the energy barrier for vesicle fusion and that (2.) the effect of multiple synaptotagmins on the energy barrier is additive. The model, which relies on biochemically measured Ca²⁺/PI(4,5)P₂ affinities and protein copy numbers, reproduced the steep Ca²⁺ dependency of neurotransmitter release. Our results indicate that each synaptotagmin engaging in Ca²⁺/PI(4,5)P₂ dual-binding lowers the energy barrier for vesicle fusion by ~5 k_BT and that allosteric stabilization of this state enables the synchronized engagement of several (typically three) synaptotagmins for fast exocytosis. Furthermore, we show that mutations altering synaptotagmin’s allosteric properties may show dominant-negative effects, even though synaptotagmins act independently on the energy barrier, and that dynamic changes of local PI(4,5)P₂ (e.g. upon vesicle movement) dramatically impact synaptic responses. We conclude that allosterically stabilized Ca²⁺/PI(4,5)P₂ dual binding enables synaptotagmins to exert their coordinated function in neurotransmission.

For our brains and nervous systems to work properly, the nerve cells within them must be able to ‘talk’ to each other. They do this by releasing chemical signals called neurotransmitters which other cells can detect and respond to.

Neurotransmitters are packaged in tiny membrane-bound spheres called vesicles. When a cell of the nervous system needs to send a signal to its neighbours, the vesicles fuse with the outer membrane of the cell, discharging their chemical contents for other cells to detect. The initial trigger for neurotransmitter release is a short, fast increase in the amount of calcium ions inside the signalling cell. One of the main proteins that helps regulate this process is synaptotagmin which binds to calcium and gives vesicles the signal to start unloading their chemicals.

Despite acting as a calcium sensor, synaptotagmin actually has a very low affinity for calcium ions by itself, meaning that it would not be efficient for the protein to respond alone. Synpatotagmin is more likely to bind to calcium if it is attached to a molecule called PIP₂, which is found in the membranes of cells The effect also occurs in reverse, as the binding of calcium to synaptotagmin increases the protein’s affinity for PIP₂. However, how these three molecules – synaptotagmin, PIP₂, and calcium – work together to achieve the physiological release of neurotransmitters is poorly understood.

To help answer this question, Kobbersmed, Berns et al. set up a computer simulation of ‘virtual vesicles’ using available experimental data on synaptotagmin’s affinity with calcium and PIP₂. In this simulation, synaptotagmin could only trigger the release of neurotransmitters when bound to both calcium and PIP₂. The model also showed that each ‘complex’ of synaptotagmin/calcium/PIP₂ made the vesicles more likely to fuse with the outer membrane of the cell – to the extent that only a handful of synaptotagmin molecules were needed to start neurotransmitter release from a single vesicle.

These results shed new light on a biological process central to the way nerve cells communicate with each other. In the future, Kobbersmed, Berns et al. hope that this insight will help us to understand the cause of diseases where communication in the nervous system is impaired.

Synaptotagmin 11 scaffolds MKK7-JNK signaling process to promote stem-like molecular subtype gastric cancer oncogenesis

Background

Identifying biomarkers related to the diagnosis and treatment of gastric cancer (GC) has not made significant progress due to the heterogeneity of tumors. Genes involved in histological classification and genetic correlation studies are essential to develop an appropriate treatment for GC.

Methods

In vitro and in vivo lentiviral shRNA library screening was performed. The expression of Synaptotagmin (SYT11) in the tumor tissues of patients with GC was confirmed by performing Immunohistochemistry, and the correlation between the expression level and the patient’s survival rate was analyzed. Phospho-kinase array was performed to detect Jun N-terminal kinase (JNK) phosphorylation. SYT11, JNK, and MKK7 complex formation was confirmed by western blot and immunoprecipitation assays. We studied the effects of SYT11 on GC proliferation and metastasis, real-time cell image analysis, adhesion assay, invasion assay, spheroid formation, mouse xenograft assay, and liver metastasis.

Results

SYT11 is highly expressed in the stem-like molecular subtype of GC in transcriptome analysis of 527 patients with GC. Moreover, SYT11 is a potential prognostic biomarker for histologically classified diffuse-type GC. SYT11 functions as a scaffold protein, binding both MKK7 and JNK1 signaling molecules that play a role in JNK1 phosphorylation. In turn, JNK activation leads to a signaling cascade resulting in cJun activation and expression of downstream genes angiopoietin-like 2 (ANGPTL2), thrombospondin 4 (THBS4), Vimentin, and junctional adhesion molecule 3 (JAM3), which play a role in epithelial-mesenchymal transition (EMT). SNU484 cells infected with SYT11 shRNA (shSYT11) exhibited reduced spheroid formation, mouse tumor formation, and liver metastasis, suggesting a pro-oncogenic role of SYT11. Furthermore, SYT11-antisense oligonucleotide (ASO) displayed antitumor activity in our mouse xenograft model and was conferred an anti-proliferative effect in SNU484 and MKN1 cells.

Conclusion

SYT11 could be a potential therapeutic target as well as a prognostic biomarker in patients with diffuse-type GC, and SYT11-ASO could be used in therapeutic agent development for stem-like molecular subtype diffuse GC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02420-3.

Ciliate Research. From Myth to Trendsetting Science

This special issue of the Journal of Eukaryotic Microbiology (JEM) summarizes achievements obtained by generations of researchers with ciliates in widely different disciplines. In fact, ciliates range among the first cells seen under the microscope centuries ago. Their beauty made them an object of scientia amabilis and their manifold reactions made them attractive for college experiments and finally challenged causal analyses at the cellular level. Some of this work was honored by a Nobel Prize. Some observations yielded a baseline for additional novel discoveries, occasionally facilitated by specific properties of some ciliates. This also offers some advantage in the exploration of closely related parasites (malaria). Articles contributed here by colleagues from all over the world encompass a broad spectrum of ciliate life, from genetics to evolution, from molecular cell biology to ecology, from intercellular signaling to epigenetics etc. This introductory chapter, largely based on my personal perception, aims at integrating work presented in this special issue of JEM into a broader historical context up to current research.

Synaptotagmins 1 and 7 Play Complementary Roles in Somatodendritic Dopamine Release

The molecular mechanisms underlying somatodendritic dopamine (DA) release remain unresolved, despite the passing of decades since its discovery. Our previous work showed robust release of somatodendritic DA in submillimolar extracellular Ca2+ concentration ([Ca2+]o). Here we tested the hypothesis that the high-affinity Ca2+ sensor synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca2+ dependence. Somatodendritic DA release from SNc DA neurons was assessed using whole-cell recording in midbrain slices from male and female mice to monitor evoked DA-dependent D2 receptor-mediated inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse train-evoked D2ICs, revealing a functional role for Syt7. The assessment of the Ca2+ dependence of pulse train-evoked D2ICs confirmed robust DA release in submillimolar [Ca2+]o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knock-out (KO) mice. In millimolar [Ca2+]o, pulse train-evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca2+]o than seen in WT mice; the effect on single pulse-evoked DA release, however, did not differ between genotypes. Single-cell application of a Syt1 Ab had no effect on train-evoked D2ICs in WT SNc DA neurons, but did cause a decrease in D2IC amplitude in Syt7 KOs, indicating a functional substitution of Syt1 for Syt7. In addition, Syt1 Ab decreased single pulse-evoked D2ICs in WT cells, indicating the involvement of Syt1 in tonic DA release. Thus, Syt7 and Syt1 play complementary roles in somatodendritic DA release from SNc DA neurons.SIGNIFICANCE STATEMENT The respective Ca2+ dependence of somatodendritic and axonal dopamine (DA) release differs, resulting in the persistence of somatodendritic DA release in submillimolar Ca2+ concentrations too low to support axonal release. We demonstrate that synaptotagmin7 (Syt7), a high-affinity Ca2+ sensor, underlies phasic somatodendritic DA release and its Ca2+ sensitivity in the substantia nigra pars compacta. In contrast, we found that synaptotagmin 1 (Syt1), the Ca2+ sensor underlying axonal DA release, plays a role in tonic, but not phasic, somatodendritic DA release in wild-type mice. However, Syt1 can facilitate phasic DA release after Syt7 deletion. Thus, we show that both Syt1 and Syt7 act as Ca2+ sensors subserving different aspects of somatodendritic DA release processes.

Tricalbin proteins regulate plasma membrane phospholipid homeostasis

The evolutionarily conserved extended synaptotagmin (E-Syt) proteins are calcium-activated lipid transfer proteins that function at contacts between the ER and plasma membrane (ER-PM contacts). However, roles of the E-Syt family members in PM lipid organisation remain incomplete. Among the E-Syt family, the yeast tricalbin (Tcb) proteins are essential for PM integrity upon heat stress, but it is not known how they contribute to PM maintenance. Using quantitative lipidomics and microscopy, we find that the Tcb proteins regulate phosphatidylserine homeostasis at the PM. Moreover, upon heat-induced membrane stress, Tcb3 co-localises with the PM protein Sfk1 that is implicated in PM phospholipid asymmetry and integrity. The Tcb proteins also control the PM targeting of the known phosphatidylserine effector Pkc1 upon heat-induced stress. Phosphatidylserine has evolutionarily conserved roles in PM organisation, integrity, and repair. We propose that phospholipid regulation is an ancient essential function of E-Syt family members required for PM integrity.

Expression and distribution of synaptotagmin family members in the zebrafish retina

Synaptotagmins belong to a large family of proteins. Although various synaptotagmins have been implicated as Ca²⁺ sensors for vesicle replenishment and release at conventional synapses, their roles at retinal ribbon synapses remain incompletely understood. Zebrafish is a widely used experimental model for retinal research. We therefore investigated the homology between human, rat, mouse, and zebrafish synaptotagmins 1-10 using a bioinformatics approach. We also characterized the expression and distribution of various synaptotagmin (syt) genes in the zebrafish retina using RT-PCR, qPCR, and in situhybridization, focusing on the family members whose products likely underlie Ca²⁺ -dependent exocytosis in the central nervous system (synaptotagmins 1, 2, 5, and 7). Most zebrafish synaptotagmins are well conserved and can be grouped in the same classes as mammalian synaptotagmins, based on crucial amino acid residues needed for coordinating Ca²⁺ binding and determining phospholipid binding affinity. The only exception is synaptotagmin 1b, which lacks 34 amino acid residues in the C2B domain and is therefore unlikely to bind Ca²⁺ there. Additionally, the products of zebrafish syt5a and syt5b genes share identity with mammalian class 1 and 5 synaptotagmins. Zebrafish syt1, syt2, syt5, and syt7 paralogues are found in the zebrafish brain, eye, and retina, excepting syt1b, which is only present in the brain. The complementary expression pattern of the remaining paralogues in the retina suggests that syt1a and syt5a may underlie synchronous release and syt7a and syt7b may mediate asynchronous release or other Ca²⁺ -dependent processes in different retinal neurons.

N-Acetyl-Cysteine: Modulating the Cysteine Redox Proteome in Neurodegenerative Diseases

In the last twenty years, significant progress in understanding the pathophysiology of age-associated neurodegenerative diseases has been made. However, the prevention and treatment of these diseases remain without clinically significant therapeutic advancement. While we still hope for some potential genetic therapeutic approaches, the current reality is far from substantial progress. With this state of the issue, emphasis should be placed on early diagnosis and prompt intervention in patients with increased risk of neurodegenerative diseases to slow down their progression, poor prognosis, and decreasing quality of life. Accordingly, it is urgent to implement interventions addressing the psychosocial and biochemical disturbances we know are central in managing the evolution of these disorders. Genomic and proteomic studies have shown the high molecular intricacy in neurodegenerative diseases, involving a broad spectrum of cellular pathways underlying disease progression. Recent investigations indicate that the dysregulation of the sensitive-cysteine proteome may be a concurrent pathogenic mechanism contributing to the pathophysiology of major neurodegenerative diseases, opening new therapeutic opportunities. Considering the incidence and prevalence of these disorders and their already significant burden in Western societies, they will become a real pandemic in the following decades. Therefore, we propose large-scale investigations, in selected groups of people over 40 years of age with decreased blood glutathione levels, comorbidities, and/or mild cognitive impairment, to evaluate supplementation of the diet with low doses of N-acetyl-cysteine, a promising and well-tolerated therapeutic agent suitable for long-term use.

Membrane Traffic and Ca2+ -Signals in Ciliates

A Paramecium cell has as many types of membrane interactions as mammalian cells, as established with monoclonal antibodies by R. Allen and A. Fok. Since then, we have identified key-players, such as SNARE-proteins, Ca²⁺ -regulating proteins, including Ca²⁺ -channels, Ca²⁺ -pumps, Ca²⁺ -binding proteins of different affinity etc. at the molecular level, probed their function and localized them at the light and electron microscopy level. SNARE-proteins, in conjunction with a synaptotagmin-like Ca²⁺ -sensor protein, mediate membrane fusion. This interaction is additionally regulated by monomeric GTPases whose spectrum in Tetrahymena and Paramecium has been established by A. Turkewitz. As known from mammalian cells, GTPases are activated on membranes in conjunction with lumenal acidification by an H⁺ -ATPase. For these complex molecules we found in Paramecium an unsurpassed number of 17 a-subunit paralogs which connect the polymeric head and basis part, V1 and V0. (This multitude may reflect different local functional requirements.) Together with plasmalemmal Ca²⁺ -influx-channels, locally enriched intracellular InsP₃ -type (InsP₃ R, mainly in osmoregulatory system) and ryanodine receptor-like Ca²⁺ -release channels (ryanodine receptor-like proteins, RyR-LP), this complexity mediates Ca²⁺ signals for most flexible local membrane-to-membrane interactions. As we found, the latter channel types miss a substantial portion of the N-terminal part. Caffeine and 4-chloro-meta-cresol (the agent used to probe mutations of RyRs in man during surgery in malignant insomnia patients) initiate trichocyst exocytosis by activating Ca²⁺ -release channels type CRC-IV in the peripheral part of alveolar sacs. This is superimposed by Ca²⁺ -influx, i.e. a mechanism called "store-operated Ca²⁺ -entry" (SOCE). For the majority of key players, we have mapped paralogs throughout the Paramecium cell, with features in common or at variance in the different organelles participating in vesicle trafficking. Local values of free Ca²⁺ -concentration, [Ca²⁺ ]_i , and their change, e.g. upon exocytosis stimulation, have been registered by flurochromes and chelator effects. In parallel we have registered release of Ca²⁺ from alveolar sacs by quenched-flow analysis combined with cryofixation and x-ray microanalysis.

Distribution of Copy Number Variation in SYT11 Gene and Its Association with Growth Conformation Traits in Chinese Cattle

Simple Summary

It is known that many different breeds of cattle are widely distributed in China. However, due to a lengthy selection of draught direction, there are obvious shortcomings in Chinese cattle, such as less meat production, slow weight gain, poor meat quality, and a lack of specialized beef cattle breeds. Animal breeding heavily benefits from molecular technologies, among which molecular genetic markers were widely used to improve the economic traits of beef cattle. Because the copy number variation (CNV) involves a longer DNA sequence or even the entire functional gene, it may have a greater impact on the phenotype. Recent studies have indicated that CNVs are widespread in the Chinese cattle genome. By investigating the effects of CNVs on gene expression and cattle traits, we aim to find those genomic variations which could significantly affect cattle traits, and which could provide a basis for genetic selection and molecular breeding of local Chinese cattle.

Abstract

Currently, studies of the SYT11 gene mainly focus on neurological diseases such as schizophrenia and Parkinson’s disease. However, some studies have shown that the C2B domain of SYT11 can interact with RISC components and affect the gene regulation of miRNA, which is important for cell differentiation, proliferation, and apoptosis, and therefore has an impact on muscle growth and development in animals. The whole-genome resequencing data detected a CNV in the SYT11 gene, and this may affect cattle growth traits. In this study, CNV distribution of 672 individuals from four cattle breeds, Yunling, Pinan, Xianan, and Qinchuan, were detected by qPCR. The relationship between CNV, gene expression and growth traits was further investigated. The results showed that the proportion of multiple copy types was the largest in all cattle breeds, but there were some differences among different breeds. The normal type had higher gene expression than the abnormal copy type. The CNVs of the SYT11 gene were significantly correlated with body length, cannon circumference, chest depth, rump length, and forehead size of Yunling cattle, and was significantly correlated with the bodyweight of Xianan cattle, respectively. These data improve our understanding of the effects of CNV on cattle growth traits. Our results suggest that the CNV of SYT11 gene is a protentional molecular marker, which may be used to improve growth traits in Chinese cattle.

Hypothesis Relating the Structure, Biochemistry and Function of Active Zone Material Macromolecules at a Neuromuscular Junction

This report integrates knowledge of in situ macromolecular structures and synaptic protein biochemistry to propose a unified hypothesis for the regulation of certain vesicle trafficking events (i.e., docking, priming, Ca²⁺-triggering, and membrane fusion) that lead to neurotransmitter secretion from specialized “active zones” of presynaptic axon terminals. Advancements in electron tomography, to image tissue sections in 3D at nanometer scale resolution, have led to structural characterizations of a network of different classes of macromolecules at the active zone, called “Active Zone Material’. At frog neuromuscular junctions, the classes of Active Zone Material macromolecules “top-masts”, “booms”, “spars”, “ribs” and “pins” direct synaptic vesicle docking while “pins”, “ribs” and “pegs” regulate priming to influence Ca²⁺-triggering and membrane fusion. Other classes, “beams”, “steps”, “masts”, and “synaptic vesicle luminal filaments’ likely help organize and maintain the structural integrity of active zones. Extensive studies on the biochemistry that regulates secretion have led to comprehensive characterizations of the many conserved proteins universally involved in these trafficking events. Here, a hypothesis including a partial proteomic atlas of Active Zone Material is presented which considers the common roles, binding partners, physical features/structure, and relative positioning in the axon terminal of both the proteins and classes of macromolecules involved in the vesicle trafficking events. The hypothesis designates voltage-gated Ca²⁺ channels and Ca²⁺-gated K⁺ channels to ribs and pegs that are connected to macromolecules that span the presynaptic membrane at the active zone. SNARE proteins (Syntaxin, SNAP25, and Synaptobrevin), SNARE-interacting proteins Synaptotagmin, Munc13, Munc18, Complexin, and NSF are designated to ribs and/or pins. Rab3A and Rabphillin-3A are designated to top-masts and/or booms and/or spars. RIM, Bassoon, and Piccolo are designated to beams, steps, masts, ribs, spars, booms, and top-masts. Spectrin is designated to beams. Lastly, the luminal portions of SV2 are thought to form the bulk of the observed synaptic vesicle luminal filaments. The goal here is to help direct future studies that aim to bridge Active Zone Material structure, biochemistry, and function to ultimately determine how it regulates the trafficking events in vivo that lead to neurotransmitter secretion.

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

Synaptotagmin-13 Is a Neuroendocrine Marker in Brain, Intestine and Pancreas

Synaptotagmin-13 (Syt13) is an atypical member of the vesicle trafficking synaptotagmin protein family. The expression pattern and the biological function of this Ca²⁺-independent protein are not well resolved. Here, we have generated a novel Syt13-Venus fusion (Syt13-VF) fluorescence reporter allele to track and isolate tissues and cells expressing Syt13 protein. The reporter allele is regulated by endogenous cis-regulatory elements of Syt13 and the fusion protein follows an identical expression pattern of the endogenous Syt13 protein. The homozygous reporter mice are viable and fertile. We identify the expression of the Syt13-VF reporter in different regions of the brain with high expression in tyrosine hydroxylase (TH)-expressing and oxytocin-producing neuroendocrine cells. Moreover, Syt13-VF is highly restricted to all enteroendocrine cells in the adult intestine that can be traced in live imaging. Finally, Syt13-VF protein is expressed in the pancreatic endocrine lineage, allowing their specific isolation by flow sorting. These findings demonstrate high expression levels of Syt13 in the endocrine lineages in three major organs harboring these secretory cells. Collectively, the Syt13-VF reporter mouse line provides a unique and reliable tool to dissect the spatio-temporal expression pattern of Syt13 and enables isolation of Syt13-expressing cells that will aid in deciphering the molecular functions of this protein in the neuroendocrine system.

Synaptotagmin 7 is targeted to the axonal plasma membrane through γ-secretase processing to promote synaptic vesicle docking in mouse hippocampal neurons

Synaptotagmin 7 (SYT7) has emerged as a key regulator of presynaptic function, but its localization and precise role in the synaptic vesicle cycle remain the subject of debate. Here, we used iGluSnFR to optically interrogate glutamate release, at the single-bouton level, in SYT7KO-dissociated mouse hippocampal neurons. We analyzed asynchronous release, paired-pulse facilitation, and synaptic vesicle replenishment and found that SYT7 contributes to each of these processes to different degrees. 'Zap-and-freeze' electron microscopy revealed that a loss of SYT7 diminishes docking of synaptic vesicles after a stimulus and inhibits the recovery of depleted synaptic vesicles after a stimulus train. SYT7 supports these functions from the axonal plasma membrane, where its localization and stability require both γ-secretase-mediated cleavage and palmitoylation. In summary, SYT7 is a peripheral membrane protein that controls multiple modes of synaptic vesicle (SV) exocytosis and plasticity, in part, through enhancing activity-dependent docking of SVs.

Microtubules and Gαo-signaling modulate the preferential secretion of young insulin secretory granules in islet β cells via independent pathways

For sustainable function, each pancreatic islet β cell maintains thousands of insulin secretory granules (SGs) at all times. Glucose stimulation induces the secretion of a small portion of these SGs and simultaneously boosts SG biosynthesis to sustain this stock. The failure of these processes, often induced by sustained high-insulin output, results in type 2 diabetes. Intriguingly, young insulin SGs are more likely secreted during glucose-stimulated insulin secretion (GSIS) for unknown reasons, while older SGs tend to lose releasability and be degraded. Here, we examine the roles of microtubule (MT) and Gαo-signaling in regulating the preferential secretion of young versus old SGs. We show that both MT-destabilization and Gαo inactivation results in more SGs localization near plasma membrane (PM) despite higher levels of GSIS and reduced SG biosynthesis. Intriguingly, MT-destabilization or Gαo-inactivation results in higher secretion probabilities of older SGs, while combining both having additive effects on boosting GSIS. Lastly, Gαo inactivation does not detectably destabilize the β-cell MT network. These findings suggest that Gαo and MT can modulate the preferential release of younger insulin SGs via largely parallel pathways.

A Latent Gaussian Copula Model for Mixed Data Analysis in Brain Imaging Genetics

Recent advances in imaging genetics make it possible to combine different types of data including medical images like functional magnetic resonance imaging (fMRI) and genetic data like single nucleotide polymorphisms (SNPs) for comprehensive diagnosis of mental disorders. Understanding complex interactions among these heterogeneous data may give rise to a new perspective, while at the same time demand statistical models for their integration. Various graphical models have been proposed for the study of interaction or association networks with continuous, binary, and count data as well as the mixture of them. However, limited efforts have been made for the multinomial case, for instance, SNP data. Our goal is therefore to fill the void by developing a graphical model for the integration of fMRI image and SNP data, which can provide deeper understanding of the unknown neurogenetic mechanism. In this article, we propose a latent Gaussian copula model for mixed data containing multinomial components. We assume that the discrete variable is obtained by discretizing a latent (unobserved) continuous variable and then create a semi-rank based estimator of the graph structure. The simulation results demonstrate that the proposed latent correlation has more steady and accurate performance than several existing methods in detecting graph structure. When applying to a real schizophrenia data consisting of SNP array and fMRI image collected by the Mind Clinical Imaging Consortium (MCIC), the proposed method reveals a set of distinct SNP-brain associations, which are verified to be biologically significant. The proposed model is statistically promising in handling mixed types of data including multinomial components, which can find widespread applications. To promote reproducible research, the R code is available at https://github.com/Aiying0512/LGCM.

Ca2+ sensor proteins in spontaneous release and synaptic plasticity: Limited contribution of Doc2c, rabphilin-3a and synaptotagmin 7 in hippocampal glutamatergic neurons

Presynaptic neurotransmitter release is strictly regulated by SNARE proteins, Ca2+ and a number of Ca2+ sensors including synaptotagmins (Syts) and Double C2 domain proteins (Doc2s). More than seventy years after the original description of spontaneous release, the mechanism that regulates this process is still poorly understood. Syt-1, Syt7 and Doc2 proteins contribute predominantly, but not exclusively, to synchronous, asynchronous and spontaneous phases of release. The proteins share a conserved tandem C2 domain architecture, but are functionally diverse in their subcellular location, Ca2+-binding properties and protein interactions. In absence of Syt-1, Doc2a and -b, neurons still exhibit spontaneous vesicle fusion which remains Ca2+-sensitive, suggesting the existence of additional sensors. Here, we selected Doc2c, rabphilin-3a and Syt-7 as three potential Ca2+ sensors for their sequence homology with Syt-1 and Doc2b. We genetically ablated each candidate gene in absence of Doc2a and -b and investigated spontaneous and evoked release in glutamatergic hippocampal neurons, cultured either in networks or on microglial islands (autapses). The removal of Doc2c had no effect on spontaneous or evoked release. Syt-7 removal also did not affect spontaneous release, although it altered short-term plasticity by accentuating short-term depression. The removal of rabphilin caused an increased spontaneous release frequency in network cultures, an effect that was not observed in autapses. Taken together, we conclude that Doc2c and Syt-7 do not affect spontaneous release of glutamate in hippocampal neurons, while our results suggest a possible regulatory role of rabphilin-3a in neuronal networks. These findings importantly narrow down the repertoire of synaptic Ca2+ sensors that may be implicated in the spontaneous release of glutamate.

Evolutionary diversity of the dual Ca2+ sensor system for neurotransmitter release

Several proteins containing C2 domains have been identified as Ca²⁺ sensors for neurotransmitter release. In several cases, multiple C2 domain containing proteins function together to sustain evoked synchronous and asynchronous release as well as Ca²⁺-dependent forms of spontaneous release. Most recent publication by Li and colleagues have identified a novel Ca²⁺ sensor at the C. elegans neuromuscular junction [8] that complements the fast Ca²⁺ sensor synaptotagmin-1 in mediating a slower form of evoked release. Here, we discuss these results as well as earlier work suggesting an evolutionarily conserved diversity of Ca²⁺ sensors mediating distinct forms of neurotransmitter release.

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca²⁺-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca²⁺-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca²⁺-dependent priming as a prelude to fast and slow exocytosis triggering.

Late onset of Synaptotagmin 2a expression at synapses relevant to social behavior

As they form, synapses go through various stages of maturation and refinement. These steps are linked to significant changes in synaptic function, potentially resulting in emergence and maturation of behavioral outputs. Synaptotagmins are calcium-sensing proteins of the synaptic vesicle exocytosis machinery, and changes in Synaptotagmin proteins at synapses have significant effects on vesicle release and synaptic function. Here, we examined the distribution of the synaptic vesicle protein Synaptotagmin 2a (Syt2a) during development of the zebrafish nervous system. Syt2a is widely distributed throughout the midbrain and hindbrain early during larval development but very weakly expressed in the forebrain. Later in development, Syt2a expression levels in the forebrain increase, particularly in regions associated with social behavior, and most intriguingly, around the time social behavior becomes apparent. We provide evidence that Syt2a localizes to synapses onto neurons implicated in social behavior in the ventral forebrain and show that Syt2a is colocalized with tyrosine hydroxylase, a biosynthetic enzyme in the dopamine pathway. Our results suggest a developmentally important role for Syt2a in maturing synapses in the forebrain, coinciding with the emergence of social behavior.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Dysfunction of parvalbumin neurons in the cerebellar nuclei produces an action tremor

Essential tremor is a common brain disorder affecting millions of people, yet the neuronal mechanisms underlying this prevalent disease remain elusive. Here, we showed that conditional deletion of synaptotagmin-2, the fastest Ca2+ sensor for synaptic neurotransmitter release, from parvalbumin neurons in mice caused an action tremor syndrome resembling the core symptom of essential tremor patients. Combining brain region-specific and cell type-specific genetic manipulation methods, we found that deletion of synaptotagmin-2 from excitatory parvalbumin-positive neurons in cerebellar nuclei was sufficient to generate an action tremor. The synaptotagmin-2 deletion converted synchronous into asynchronous neurotransmitter release in projections from cerebellar nuclei neurons onto gigantocellular reticular nucleus neurons, which might produce an action tremor by causing signal oscillations during movement. The tremor was rescued by completely blocking synaptic transmission with tetanus toxin in cerebellar nuclei, which also reversed the tremor phenotype in the traditional harmaline-induced essential tremor model. Using a promising animal model for action tremor, our results thus characterized a synaptic circuit mechanism that may underlie the prevalent essential tremor disorder.

The HERC1 ubiquitin ligase regulates presynaptic membrane dynamics of central synapses

HERC1 is a ubiquitin ligase protein, which, when mutated, induces several malformations and intellectual disability in humans. The animal model of HERC1 mutation is the mouse tambaleante characterized by: (1) overproduction of the protein; (2) cerebellar Purkinje cells death by autophagy; (3) dysregulation of autophagy in spinal cord motor neurons, and CA3 and neocortical pyramidal neurons; (4) impairment of associative learning, linked to altered spinogenesis and absence of LTP in the lateral amygdala; and, (5) motor impairment due to delayed action potential transmission, decrease synaptic transmission efficiency and altered myelination in the peripheral nervous system. To investigate the putative role of HERC1 in the presynaptic dynamics we have performed a series of experiments in cultured tambaleante hippocampal neurons by using transmission electron microscopy, FM1-43 destaining and immunocytochemistry. Our results show: (1) a decrease in the number of synaptic vesicles; (2) reduced active zones; (3) less clathrin immunoreactivity and less presynaptic endings over the hippocampal main dendritic trees; which contrast with (4) a greater number of endosomes and autophagosomes in the presynaptic endings of the tambaleante neurons relative to control ones. Altogether these results show an important role of HERC1 in the regulation of presynaptic membrane dynamics.

Exocytosis in mouse vestibular Type II hair cells shows a high-order Ca2+ dependence that is independent of synaptotagmin-4

Mature hair cells transduce information over a wide range of stimulus intensities and frequencies for prolonged periods of time. The efficiency of such a demanding task is reflected in the characteristics of exocytosis at their specialized presynaptic ribbons. Ribbons are electron-dense structures able to tether a large number of releasable vesicles allowing them to maintain high rates of vesicle release. Calcium entry through rapidly activating, non-inactivating CaV 1.3 (L-type) Ca2+ channels in response to cell depolarization causes a local increase in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. The Ca2+ dependence of vesicle exocytosis at mammalian vestibular hair cell (VHC) ribbon synapses is believed to be linear, similar to that observed in mature cochlear inner hair cells (IHCs). The linear relation has been shown to correlate with the presence of the Ca2+ sensor synaptotagmin-4 (Syt-4). Therefore, we studied the exocytotic Ca2+ dependence, and the release kinetics of different vesicle pool populations, in Type II VHCs of control and Syt-4 knockout mice using patch-clamp capacitance measurements, under physiological recording conditions. We found that exocytosis in mature control and knockout Type II VHCs displayed a high-order dependence on Ca2+ entry, rather than the linear relation previously observed. Consistent with this finding, the Ca2+ dependence and release kinetics of the ready releasable pool (RRP) of vesicles were not affected by an absence of Syt-4. However, we did find that Syt-4 could play a role in regulating the release of the secondary releasable pool (SRP) in these cells. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at mature Type II VHC ribbon synapses is faithfully described by a nonlinear relation that is likely to be more appropriate for the accurate encoding of low-frequency vestibular information, consistent with that observed at low-frequency mammalian auditory receptors.

Synaptotagmin 13 is neuroprotective across motor neuron diseases

In amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), spinal and lower brainstem motor neurons degenerate, but some motor neuron subtypes are spared, including oculomotor neurons (OMNs). The mechanisms responsible for this selective degeneration are largely unknown, but the molecular signatures of resistant and vulnerable motor neurons are distinct and offer clues to neuronal resilience and susceptibility. Here, we demonstrate that healthy OMNs preferentially express Synaptotagmin 13 (SYT13) compared to spinal motor neurons. In end-stage ALS patients, SYT13 is enriched in both OMNs and the remaining relatively resilient spinal motor neurons compared to controls. Overexpression of SYT13 in ALS and SMA patient motor neurons in vitro improves their survival and increases axon lengths. Gene therapy with Syt13 prolongs the lifespan of ALS mice by 14% and SMA mice by 50% by preserving motor neurons and delaying muscle denervation. SYT13 decreases endoplasmic reticulum stress and apoptosis of motor neurons, both in vitro and in vivo. Thus, SYT13 is a resilience factor that can protect motor neurons and a candidate therapeutic target across motor neuron diseases.

Synaptotagmin 1 Is Involved in Neuropathic Pain and Electroacupuncture-Mediated Analgesic Effect

Numerous studies have verified that electroacupuncture (EA) can relieve neuropathic pain through a variety of mechanisms. Synaptotagmin 1 (Syt-1), a synaptic vesicle protein for regulating exocytosis of neurotransmitters, was found to be affected by EA stimulation. However, the roles of Syt-1 in neuropathic pain and EA-induced analgesic effect remain unclear. Here, the effect of Syt-1 on nociception was assessed through an antibody blockade, siRNA silencing, and lentivirus-mediated overexpression of spinal Syt-1 in rats with spared nerve injury (SNI). EA was used for stimulating bilateral "Sanjinjiao" and "Zusanli" acupoints of the SNI rats to evaluate its effect on nociceptive thresholds and spinal Syt-1 expression. The mechanically and thermally nociceptive behaviors were assessed with paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) at different temperatures, respectively, at day 0, 7, 8, 14, and 20. Syt-1 mRNA and protein levels were determined with qRT-PCR and Western blot, respectively, and its distribution was observed with the immunohistochemistry method. The results demonstrated Syt-1 antibody blockade and siRNA silencing increased ipsilateral PWTs and PWLs of SNI rats, while Syt-1 overexpression decreased ipsilateral PWTs and PWLs of rats. EA significantly attenuated nociceptive behaviors and down-regulated spinal Syt-1 protein levels (especially in laminae I-II), which were reversed by Syt-1 overexpression. Our findings firstly indicate that Syt-1 is involved in the development of neuropathic pain and that EA attenuates neuropathic pain, probably through suppressing Syt-1 protein expression in the spinal cord.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Going Too Far Is the Same as Falling Short†: Kinesin-3 Family Members in Hereditary Spastic Paraplegia

Proper intracellular trafficking is essential for neuronal development and function, and when any aspect of this process is dysregulated, the resulting "transportopathy" causes neurological disorders. Hereditary spastic paraplegias (HSPs) are a family of such diseases attributed to over 80 spastic gait genes (SPG), specifically characterized by lower extremity spasticity and weakness. Multiple genes in the trafficking pathway such as those relating to microtubule structure and function and organelle biogenesis are representative disease loci. Microtubule motor proteins, or kinesins, are also causal in HSP, specifically mutations in Kinesin-I/KIF5A (SPG10) and two kinesin-3 family members; KIF1A (SPG30) and KIF1C (SPG58). KIF1A is a motor enriched in neurons, and involved in the anterograde transport of a variety of vesicles that contribute to pre- and post-synaptic assembly, autophagic processes, and neuron survival. KIF1C is ubiquitously expressed and, in addition to anterograde cargo transport, also functions in retrograde transport between the Golgi and the endoplasmic reticulum. Only a handful of KIF1C cargos have been identified; however, many have crucial roles such as neuronal differentiation, outgrowth, plasticity and survival. HSP-related kinesin-3 mutants are characterized mainly as loss-of-function resulting in deficits in motility, regulation, and cargo binding. Gain-of-function mutants are also seen, and are characterized by increased microtubule-on rates and hypermotility. Both sets of mutations ultimately result in misdelivery of critical cargos within the neuron. This likely leads to deleterious cell biological cascades that likely underlie or contribute to HSP clinical pathology and ultimately, symptomology. Due to the paucity of histopathological or cell biological data assessing perturbations in cargo localization, it has been difficult to positively link these mutations to the outcomes seen in HSPs. Ultimately, the goal of this review is to encourage future academic and clinical efforts to focus on "transportopathies" through a cargo-centric lens.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Differential expression and subcellular localization of Copines in mouse retina

Combinatorial expression of Brn3 transcription factors is required for the development of cell-specific morphologies in retinal ganglion cells (RGCs). The molecular mechanisms by which Brn3s regulate RGC type specific features are largely unexplored. We previously identified several members of the Copine (Cpne) family of molecules as potential targets of Brn3 transcription factors in the retina. We now use in situ hybridization and immunohistochemistry to characterize Copine expression in the postnatal and adult mouse retina. We find that Cpne5, 6, and 9 are expressed in the ganglion cell layer (GCL) and inner nuclear layer (INL) in both amacrine cells and RGCs. Cpne4 expression is restricted to one amacrine cell population of the INL, but is specifically expressed in RGCs in the GCL. Cpne4 expression in RGCs is regulated by Brn3b both cell autonomously (in Brn3b⁺ RGCs) and cell nonautonomously (in Brn3b^- RGCs). Copines exhibit a variety of subcellular distributions when overexpressed in tissue culture cells (HEK293), and can induce the formation of elongated processes reminiscent of neurites in these non-neuronal cells. Our results suggest that Copines might be involved in a combinatorial fashion in Brn3b-dependent specification of RGC types. Given their expression profile and previously proven role as Ca²⁺ sensors, they may participate in the morphogenetic processes that shape RGC dendrite and axon formation at early postnatal ages.

Synaptotagmin-11 mediates a vesicle trafficking pathway that is essential for development and synaptic plasticity

Synaptotagmin-11 (Syt11) is a Synaptotagmin isoform that lacks an apparent ability to bind calcium, phospholipids, or SNARE proteins. While human genetic studies have linked mutations in the Syt11 gene to schizophrenia and Parkinson's disease, the localization or physiological role of Syt11 remain unclear. We found that in neurons, Syt11 resides on abundant vesicles that differ from synaptic vesicles and resemble trafficking endosomes. These vesicles recycle via the plasma membrane in an activity-dependent manner, but their exocytosis is slow and desynchronized. Constitutive knockout mice lacking Syt11 died shortly after birth, suggesting Syt11-mediated membrane transport is required for survival. In contrast, selective ablation of Syt11 in excitatory forebrain neurons using a conditional knockout did not affect life span but impaired synaptic plasticity and memory. Syt11-deficient neurons displayed normal secretion of fast neurotransmitters and peptides but exhibited a reduction of long-term synaptic potentiation. Hence, Syt11 is an essential component of a neuronal vesicular trafficking pathway that differs from the well-characterized synaptic vesicle trafficking pathway but is also essential for life.

Recovery from tachyphylaxis of TRPV1 coincides with recycling to the surface membrane

The transient receptor potential vanilloid-1 (TRPV1) ion channel is essential for sensation of thermal and chemical pain. TRPV1 activation is accompanied by Ca²⁺-dependent desensitization; acute desensitization reflects rapid reduction in channel activity during stimulation, whereas tachyphylaxis denotes the diminution in TRPV1 responses to repetitive stimulation. Acute desensitization has been attributed to conformational changes of the TRPV1 channel; however, the mechanisms underlying the establishment of tachyphylaxis remain to be defined. Here, we report that the degree of whole-cell TRPV1 tachyphylaxis is regulated by the strength of inducing stimulation. Using light-sheet microscopy and pH-sensitive sensor pHluorin to follow TRPV1 endocytosis and exocytosis trafficking, we provide real-time information that tachyphylaxis of different degrees concurs with TRPV1 recycling to the plasma membrane in a proportional manner. This process controls TRPV1 surface expression level thereby the whole-cell nociceptive response. We further show that activity-gated TRPV1 trafficking associates with intracellular Ca²⁺ signals of distinct kinetics, and recruits recycling routes mediated by synaptotagmin 1 and 7, respectively. These results suggest that activity-dependent TRPV1 recycling contributes to the establishment of tachyphylaxis.