Synaptotagmin I functions as a calcium regulator of release probability

Circularized fluorescent nanodiscs for probing protein-lipid interactions

Protein–lipid interactions are vital for numerous transmembrane signaling pathways. However, simple tools to characterize these interactions remain scarce and are much needed to advance our understanding of signal transduction across lipid bilayers. To tackle this challenge, we herein engineer nanodisc as a robust fluorescent sensor for reporting membrane biochemical reactions. We circularize nanodiscs via split GFP and thereby create an intensity-based fluorescent sensor (isenND) for detecting membrane binding and remodeling events. We show that isenND responds robustly and specifically to the action of a diverse array of membrane-interacting proteins and peptides, ranging from synaptotagmin and synuclein involved in neurotransmission to viral fusion peptides of HIV-1 and SARS-CoV-2. Together, isenND can serve as a versatile biochemical reagent useful for basic and translational research of membrane biology.

A fluorescent probe for detecting membrane protein binding and remodeling events is presented, which relies on split-GFP technology to generate circularized nanodiscs useful in membrane protein biophysics and structural biology.

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

NK cell-mediated cytotoxicity is a critical element of our immune system required for protection from microbial infections and cancer. NK cells bind to and eliminate infected or cancerous cells via direct secretion of cytotoxic molecules toward the bound target cells. In this review, we summarize the current understanding of the molecular regulations of NK cell cytotoxicity, focusing on lytic granule development and degranulation processes. NK cells synthesize apoptosis-inducing proteins and package them into specialized organelles known as lytic granules (LGs). Upon activation of NK cells, LGs converge with the microtubule organizing center through dynein-dependent movement along microtubules, ultimately polarizing to the cytotoxic synapse where they subsequently fuse with the NK plasma membrane. From LGs biogenesis to degranulation, NK cells utilize several strategies to protect themselves from their own cytotoxic molecules. Additionally, molecular pathways that enable NK cells to perform serial killing are beginning to be elucidated. These advances in the understanding of the molecular pathways behind NK cell cytotoxicity will be important to not only improve current NK cell-based anti-cancer therapies but also to support the discovery of additional therapeutic opportunities.

Molecular Mechanisms Underlying Neurotransmitter Release

Major recent advances and previous data have led to a plausible model of how key proteins mediate neurotransmitter release. In this model, the soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin form tight complexes that bring the membranes together and are crucial for membrane fusion. NSF and SNAPs disassemble SNARE complexes and ensure that fusion occurs through an exquisitely regulated pathway that starts with Munc18-1 bound to a closed conformation of syntaxin-1. Munc18-1 also binds to synaptobrevin, forming a template to assemble the SNARE complex when Munc13-1 opens syntaxin-1 while bridging the vesicle and plasma membranes. Synaptotagmin-1 and complexin bind to partially assembled SNARE complexes, likely stabilizing them and preventing fusion until Ca²⁺ binding to synaptotagmin-1 causes dissociation from the SNARE complex and induces interactions with phospholipids that help trigger release. Although fundamental questions remain about the mechanism of membrane fusion, these advances provide a framework to investigate the mechanisms underlying presynaptic plasticity.

Influence of T-Bar on Calcium Concentration Impacting Release Probability

The relation of form and function, namely the impact of the synaptic anatomy on calcium dynamics in the presynaptic bouton, is a major challenge of present (computational) neuroscience at a cellular level. The Drosophila larval neuromuscular junction (NMJ) is a simple model system, which allows studying basic effects in a rather simple way. This synapse harbors several special structures. In particular, in opposite to standard vertebrate synapses, the presynaptic boutons are rather large, and they have several presynaptic zones. In these zones, different types of anatomical structures are present. Some of the zones bear a so-called T-bar, a particular anatomical structure. The geometric form of the T-bar resembles the shape of the letter “T” or a table with one leg. When an action potential arises, calcium influx is triggered. The probability of vesicle docking and neurotransmitter release is superlinearly proportional to the concentration of calcium close to the vesicular release site. It is tempting to assume that the T-bar causes some sort of calcium accumulation and hence triggers a higher release probability and thus enhances neurotransmitter exocytosis. In order to study this influence in a quantitative manner, we constructed a typical T-bar geometry and compared the calcium concentration close to the active zones (AZs). We compared the case of synapses with and without T-bars. Indeed, we found a substantial influence of the T-bar structure on the presynaptic calcium concentrations close to the AZs, indicating that this anatomical structure increases vesicle release probability. Therefore, our study reveals how the T-bar zone implies a strong relation between form and function. Our study answers the question of experimental studies (namely “Wichmann and Sigrist, Journal of neurogenetics 2010”) concerning the sense of the anatomical structure of the T-bar.

Phosphoproteome Analysis Identifies a Synaptotagmin-1-Associated Complex Involved in Ischemic Neuron Injury

Cerebral stroke is one of the leading causes of death in adults worldwide. However, the molecular mechanisms of stroke-induced neuron injury are not fully understood. Here, we obtained phosphoproteomic and proteomic profiles of the acute ischemic hippocampus by LC–MS/MS analysis. Quantitative phosphoproteomic analyses revealed that the dysregulated phosphoproteins were involved in synaptic components and neurotransmission. We further demonstrated that phosphorylation of Synaptotagmin-1 (Syt1) at the Thr112 site in cultured hippocampal neurons aggravated oxygen-glucose deprivation–induced neuronal injury. Immature neurons with low expression of Syt1 exhibit slight neuronal injury in a cerebral ischemia model. Administration of the Tat-Syt1^T112A peptide protects neurons against cerebral ischemia-induced injury in vitro and in vivo. Surprisingly, potassium voltage-gated channel subfamily KQT member 2 (Kcnq2) interacted with Syt1 and Annexin A6 (Anxa6) and alleviated Syt1-mediated neuronal injury upon oxygen-glucose deprivation treatment. These results reveal a mechanism underlying neuronal injury and may provide new targets for neuroprotection after acute cerebral ischemia onset.

•

Established the phosphoproteome profiles of acute cerebral ischemic hippocampus.

•

Phosphoproteomic profile reveals phosphorylation of Syt1 and Kcnq2, which are upregulated.

•

Phosphorylation of Syt1 aggravates neuron injury, which is relieved by Tat-Syt1^T112A.

•

Kcnq2 interacts with Syt1 and Anxa6 and alleviates Syt1-mediated neuronal injury.

Forces, Kinetics, and Fusion Efficiency Altered by the Full-Length Synaptotagmin-1 -PI(4,5)P2 Interaction in Constrained Geometries

A mechanism for full-length synaptotagmin-1 (syt-1) to interact with anionic bilayers and to promote fusion in the presence of SNAREs is proposed. Colloidal probe force spectroscopy in conjunction with tethered particle motion monitoring showed that in the absence of Ca²⁺ the binding of syt-1 to membranes depends on the presence and content of PI(4,5)P₂. Addition of Ca²⁺ switches the interaction forces from weak to strong, eventually exceeding the cohesion of the C2A domain of syt-1 leading to partial unfolding of the protein. Fusion of single unilamellar vesicles equipped with syt-1 and synaptobrevin 2 with planar pore-spanning target membranes containing PS and PI(4,5)P₂ shows an almost complete suppression of stalled intermediate fusion states and an accelerated fusion kinetics in the presence of Ca²⁺, which is further enhanced upon addition of ATP.

Exosome: A novel neurotransmission modulator or non-canonical neurotransmitter?

Neurotransmission is the electrical impulse-triggered propagation of signals between neurons or between neurons and other cell types such as skeletal muscle cells. Recent studies point out the involvement of exosomes, a type of small bilipid layer-enclosed extracellular vesicles, in regulating neurotransmission. Through horizontally transferring proteins, lipids, and nucleic acids, exosomes can modulate synaptic activities rapidly by controlling neurotransmitter release or progressively by regulating neural plasticity including synapse formation, neurite growth & removal, and axon guidance & elongation. In this review, we summarize the similarities and differences between exosomes and synaptic vesicles in their biogenesis, contents, and release. We also highlight the recent progress made in demonstrating the biological roles of exosome in regulating neurotransmission, and propose a modified model of neurotransmission, in which exosomes act as novel neurotransmitters. Lastly, we provide a comprehensive discussion of the enlightenment of the current knowledge on neurotransmission to the future directions of exosome research.

Similarity and Diversity of Presynaptic Molecules at Neuromuscular Junctions and Central Synapses

Synaptic transmission is essential for controlling motor functions and maintaining brain functions such as walking, breathing, cognition, learning, and memory. Neurotransmitter release is regulated by presynaptic molecules assembled in active zones of presynaptic terminals. The size of presynaptic terminals varies, but the size of a single active zone and the types of presynaptic molecules are highly conserved among neuromuscular junctions (NMJs) and central synapses. Three parameters play an important role in the determination of neurotransmitter release properties at NMJs and central excitatory/inhibitory synapses: the number of presynaptic molecular clusters, the protein families of the presynaptic molecules, and the distance between presynaptic molecules and voltage-gated calcium channels. In addition, dysfunction of presynaptic molecules causes clinical symptoms such as motor and cognitive decline in patients with various neurological disorders and during aging. This review focuses on the molecular mechanisms responsible for the functional similarities and differences between excitatory and inhibitory synapses in the peripheral and central nervous systems, and summarizes recent findings regarding presynaptic molecules assembled in the active zone. Furthermore, we discuss the relationship between functional alterations of presynaptic molecules and dysfunction of NMJs or central synapses in diseases and during aging.

Presynaptic mechanisms underlying GABAB-receptor-mediated inhibition of spontaneous neurotransmitter release

Inhibition of neurotransmitter release by neurotransmitter substances constitutes a fundamental means of neuromodulation. In contrast to well-delineated mechanisms that underlie inhibition of evoked release via suppression of voltage-gated Ca²⁺ channels, processes that underlie neuromodulatory inhibition of spontaneous release remain unclear. Here, we interrogated inhibition of spontaneous glutamate and GABA release by presynaptic metabotropic GABA_B receptors. Our findings show that this inhibition relies on Gβγ subunit action at the membrane, and it is largely independent of presynaptic Ca²⁺ signaling for both forms of release. In the case of spontaneous glutamate release, inhibition requires Gβγ interaction with the C terminus of the key fusion machinery component SNAP25, and it is modulated by synaptotagmin-1. Inhibition of spontaneous GABA release, on the other hand, is independent of these pathways and likely requires alternative Gβγ targets at the presynaptic terminal.

OXER1 mediates testosterone-induced calcium responses in prostate cancer cells

In prostate cancer, calcium homeostasis plays a significant role in the disease's development and progression. Intracellular calcium changes are an important secondary signal, triggered by a variety of extracellular stimuli, that controls many cellular functions. One of the main events affecting calcium is androgen signaling. Indeed, via calcium changes, androgens regulate cell processes like cell growth, differentiation and motility. In the present work we explored the nature of the receptor involved in calcium response induced by membrane-acting testosterone in prostate cancer cells. We report that testosterone, independently of the presence of the classical androgen receptor, can rapidly increase intracellular calcium from calcium stores, through the oxoeicosanoid receptor 1 (OXER1) and a specific signaling cascade that triggers calcium release from the endoplasmic reticulum. These findings reveal for the first time the receptor involved in the rapid calcium changes induced by androgens. Moreover, they further support the notion that androgens, even in the absence of AR, can still exert specific effects that regulate cancer cell fate.

Variance of filtered signals: Characterization for linear reaction networks and application to neurotransmission dynamics

Neurotransmission at chemical synapses relies on the calcium-induced fusion of synaptic vesicles with the presynaptic membrane. The distance of the synaptic vesicle to the calcium channels determines the release probability and consequently the postsynaptic signal. Suitable models of the process need to capture both the mean and the variance observed in electrophysiological measurements of the postsynaptic current. In this work, we propose a method to directly compute the exact first- and second-order moments for signals generated by a linear reaction network under convolution with an impulse response function, rendering computationally expensive numerical simulations of the underlying stochastic counting process obsolete. We show that the autocorrelation of the process is central for the calculation of the filtered signal's second-order moments, and derive a system of PDEs for the cross-correlation functions (including the autocorrelations) of linear reaction networks with time-dependent rates. Finally, we employ our method to efficiently compare different spatial coarse graining approaches for a specific model of synaptic vesicle fusion. Beyond the application to neurotransmission processes, the developed theory can be applied to any linear reaction system that produces a filtered stochastic signal.

Control of neurotransmitter release by two distinct membrane-binding faces of the Munc13-1 C1C2B region

Munc13-1 plays a central role in neurotransmitter release through its conserved C-terminal region, which includes a diacyglycerol (DAG)-binding C1 domain, a Ca2+/PIP2-binding C2B domain, a MUN domain and a C2C domain. Munc13-1 was proposed to bridge synaptic vesicles to the plasma membrane through distinct interactions of the C1C2B region with the plasma membrane: (i) one involving a polybasic face that is expected to yield a perpendicular orientation of Munc13-1 and hinder release; and (ii) another involving the DAG-Ca2+-PIP2-binding face that is predicted to result in a slanted orientation and facilitate release. Here, we have tested this model and investigated the role of the C1C2B region in neurotransmitter release. We find that K603E or R769E point mutations in the polybasic face severely impair Ca2+-independent liposome bridging and fusion in in vitro reconstitution assays, and synaptic vesicle priming in primary murine hippocampal cultures. A K720E mutation in the polybasic face and a K706E mutation in the C2B domain Ca2+-binding loops have milder effects in reconstitution assays and do not affect vesicle priming, but enhance or impair Ca2+-evoked release, respectively. The phenotypes caused by combining these mutations are dominated by the K603E and R769E mutations. Our results show that the C1-C2B region of Munc13-1 plays a central role in vesicle priming and support the notion that two distinct faces of this region control neurotransmitter release and short-term presynaptic plasticity.

Introduction of synaptotagmin 7 promotes facilitation at the climbing fiber to Purkinje cell synapse

Synaptotagmin 7 (Syt7) is a high-affinity calcium sensor that is implicated in multiple aspects of synaptic transmission. Here, we study the influence of Syt7 on the climbing fiber (CF) to Purkinje cell (PC) synapse. We find that small facilitation and prominent calcium-dependent recovery from depression at this synapse do not rely on Syt7 and that Syt7 is not normally present in CFs. We expressed Syt7 in CFs to assess the consequences of introducing Syt7 to a synapse that normally lacks Syt7. Syt7 expression does not promote asynchronous release or accelerate recovery from depression. Syt7 decreases the excitatory postsynaptic current (EPSC) magnitude, consistent with a decrease in the initial probability of release (PR). Syt7 also increases synaptic facilitation to such a large extent that it could not arise solely as an indirect consequence of decreased PR. Thus, the primary consequence of Syt7 expression in CFs, which normally lack Syt7, is to promote synaptic facilitation.

Evaluation of the tert-butyl group as a probe for NMR studies of macromolecular complexes

The development of methyl transverse relaxation optimized spectroscopy has greatly facilitated the study of macromolecular assemblies by solution NMR spectroscopy. However, limited sample solubility and stability has hindered application of this technique to ongoing studies of complexes formed on membranes by the neuronal SNAREs that mediate neurotransmitter release and synaptotagmin-1, the Ca²⁺ sensor that triggers release. Since the ¹H NMR signal of a ^tBu group attached to a large protein or complex can be observed with high sensitivity if the group retains high mobility, we have explored the use of this strategy to analyze presynaptic complexes involved in neurotransmitter release. For this purpose, we attached ^tBu groups at single cysteines of fragments of synaptotagmin-1, complexin-1 and the neuronal SNAREs by reaction with 5-(tert-butyldisulfaneyl)-2-nitrobenzoic acid (BDSNB), ^tBu iodoacetamide or ^tBu acrylate. The ^tBu resonances of the tagged proteins were generally sharp and intense, although ^tBu groups attached with BDSNB had a tendency to exhibit somewhat broader resonances that likely result because of the shorter linkage between the ^tBu and the tagged cysteine. Incorporation of the tagged proteins into complexes on nanodiscs led to severe broadening of the ^tBu resonances in some cases. However, sharp ^tBu resonances could readily be observed for some complexes of more than 200 kDa at low micromolar concentrations. Our results show that tagging of proteins with ^tBu groups provides a powerful approach to study large biomolecular assemblies of limited stability and/or solubility that may be applicable even at nanomolar concentrations.

A biallelic SNIP1 Amish founder variant causes a recognizable neurodevelopmental disorder

SNIP1 (Smad nuclear interacting protein 1) is a widely expressed transcriptional suppressor of the TGF-β signal-transduction pathway which plays a key role in human spliceosome function. Here, we describe extensive genetic studies and clinical findings of a complex inherited neurodevelopmental disorder in 35 individuals associated with a SNIP1 NM_024700.4:c.1097A>G, p.(Glu366Gly) variant, present at high frequency in the Amish community. The cardinal clinical features of the condition include hypotonia, global developmental delay, intellectual disability, seizures, and a characteristic craniofacial appearance. Our gene transcript studies in affected individuals define altered gene expression profiles of a number of molecules with well-defined neurodevelopmental and neuropathological roles, potentially explaining clinical outcomes. Together these data confirm this SNIP1 gene variant as a cause of an autosomal recessive complex neurodevelopmental disorder and provide important insight into the molecular roles of SNIP1, which likely explain the cardinal clinical outcomes in affected individuals, defining potential therapeutic avenues for future research.

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

Synaptotagmins at the endoplasmic reticulum-plasma membrane contact sites maintain diacylglycerol homeostasis during abiotic stress

Endoplasmic reticulum-plasma membrane contact sites (ER-PM CS) play fundamental roles in all eukaryotic cells. Arabidopsis thaliana mutants lacking the ER-PM protein tether synaptotagmin1 (SYT1) exhibit decreased PM integrity under multiple abiotic stresses, such as freezing, high salt, osmotic stress, and mechanical damage. Here, we show that, together with SYT1, the stress-induced SYT3 is an ER-PM tether that also functions in maintaining PM integrity. The ER-PM CS localization of SYT1 and SYT3 is dependent on PM phosphatidylinositol-4-phosphate and is regulated by abiotic stress. Lipidomic analysis revealed that cold stress increased the accumulation of diacylglycerol at the PM in a syt1/3 double mutant relative to wild-type while the levels of most glycerolipid species remain unchanged. In addition, the SYT1-green fluorescent protein fusion preferentially binds diacylglycerol in vivo with little affinity for polar glycerolipids. Our work uncovers a SYT-dependent mechanism of stress adaptation counteracting the detrimental accumulation of diacylglycerol at the PM produced during episodes of abiotic stress.

Single-cell atlas of domestic pig cerebral cortex and hypothalamus

The brain of the domestic pig (Sus scrofa domesticus) has drawn considerable attention due to its high similarities to that of humans. However, the cellular compositions of the pig brain (PB) remain elusive. Here we investigated the single-nucleus transcriptomic profiles of five regions of the PB (frontal lobe, parietal lobe, temporal lobe, occipital lobe, and hypothalamus) and identified 21 cell subpopulations. The cross-species comparison of mouse and pig hypothalamus revealed the shared and specific gene expression patterns at the single-cell resolution. Furthermore, we identified cell types and molecular pathways closely associated with neurological disorders, bridging the gap between gene mutations and pathogenesis. We reported, to our knowledge, the first single-cell atlas of domestic pig cerebral cortex and hypothalamus combined with a comprehensive analysis across species, providing extensive resources for future research regarding neural science, evolutionary developmental biology, and regenerative medicine.

Comprehensive analysis on the expression profile and prognostic values of Synaptotagmins (SYTs) family members and their methylation levels in gastric cancer

Synaptotagmins (SYTs), constitute a family of 17 membrane-trafficking protein, palying crucial roles in the development and progression of human cancers. However, only very few studies have investigated the expression profile and prognostic values of SYTs family members in gastric cancer (GC). Therefore, we comprehensively evaluated the expression, methylation, prognosis and immune significance of SYTs family members through bioinformatics analysis from the online databases in GC. The expressions of SYT4, SYT9, and SYT14 were up-regulated, and negatively associated with their methylation levels in GC. Both the over-expression of SYT4, SYT9 and SYT14 and their hypomethylation levels contributed to an unsatisfactory overall survival (OS) and progression-free survival (PFS) in GC. Moreover, the low expressions of several methylation cg sites (cg02795029, cg07581146, cg15149095, cg19922137, cg25371503, cg26158959, cg02269161, cg03226737, cg08185661, cg16437728, cg22723056 and cg24678137) were significantly correlated with an unfavorable OS and PFS in GC. Furthermore, the expression of SYT4, SYT9 and SYT14 played a pivotal role in immune cells infiltration in GC. Collectively, our current finding suggested that SYT4, SYT9 and SYT14 might be potent prognostic indictors and promising immunotherapeutic targets for GC patients.

Brain Acetyl-CoA Production and Phosphorylation of Cytoskeletal Proteins Are Targets of CYP46A1 Activity Modulation and Altered Sterol Flux

Cholesterol and 24-hydroxycholesterol are the most abundant brain sterols and represent the substrate and product, respectively, of cytochrome P450 46A1 (CYP46A1), a CNS-specific enzyme. CYP46A1 controls cholesterol elimination and turnover in the brain, the two processes that determine the rate of brain sterol flux through the plasma membranes and thereby the properties of these membranes. Brain sterol flux is decreased in Cyp46a1-/- mice compared to wild-type mice and increased in 5XFAD mice (a model of Alzheimer's disease) when they are treated with a small dose of efavirenz, a CYP46A1 activator. Herein, we first assessed the brain proteome (synaptosomal fractions) and phospho-proteome (synaptosomal fractions and brain homogenates) of efavirenz-treated and control 5XFAD mice. Then, based on the pattern of protein abundance change, we conducted acetyl-CoA measurements (brain homogenates and mitochondria) and metabolic profiling (brain homogenates). The phospho-proteomics datasets were used for comparative analyses with the datasets obtained by us previously on mice with the same changes (efavirenz-treated and control 5XFAD mice from a different treatment paradigm) or with changes in the opposite direction (Cyp46a1-/- vs wild-type mice) in brain sterol flux. We found that CYP46A1 activity or the rate of brain sterol flux affects acetyl-CoA-related metabolic pathways as well as phosphorylation of cytoskeletal and other proteins. Knowledge of the key roles of acetyl-CoA and cytoskeletal phosphorylation in cell biology expands our understanding of the significance of CYP46A1-mediated cholesterol 24-hydroxylation in the brain and provides an additional explanation for why CYP46A1 activity modulations are beneficial in mouse models of different brain diseases.

Synaptotagmin-1-, Munc18-1-, and Munc13-1-dependent liposome fusion with a few neuronal SNAREs

Neurotransmitter release is governed by eight central proteins among other factors: the neuronal SNAREs syntaxin-1, synaptobrevin, and SNAP-25, which form a tight SNARE complex that brings the synaptic vesicle and plasma membranes together; NSF and SNAPs, which disassemble SNARE complexes; Munc18-1 and Munc13-1, which organize SNARE complex assembly; and the Ca²⁺ sensor synaptotagmin-1. Reconstitution experiments revealed that Munc18-1, Munc13-1, NSF, and α-SNAP can mediate Ca²⁺-dependent liposome fusion between synaptobrevin liposomes and syntaxin-1-SNAP-25 liposomes, but high fusion efficiency due to uncontrolled SNARE complex assembly did not allow investigation of the role of synaptotagmin-1 on fusion. Here, we show that decreasing the synaptobrevin-to-lipid ratio in the corresponding liposomes to very low levels leads to inefficient fusion and that synaptotagmin-1 strongly stimulates fusion under these conditions. Such stimulation depends on Ca²⁺ binding to the two C₂ domains of synaptotagmin-1. We also show that anchoring SNAP-25 on the syntaxin-1 liposomes dramatically enhances fusion. Moreover, we uncover a synergy between synaptotagmin-1 and membrane anchoring of SNAP-25, which allows efficient Ca²⁺-dependent fusion between liposomes bearing very low synaptobrevin densities and liposomes containing very low syntaxin-1 densities. Thus, liposome fusion in our assays is achieved with a few SNARE complexes in a manner that requires Munc18-1 and Munc13-1 and that depends on Ca²⁺ binding to synaptotagmin-1, all of which are fundamental features of neurotransmitter release in neurons.

Inhibitory synaptic transmission is impaired at higher extracellular Ca2+ concentrations in Scn1a+/- mouse model of Dravet syndrome

Dravet syndrome (DS) is an intractable form of childhood epilepsy that occurs in infancy. More than 80% of all patients have a heterozygous abnormality in the SCN1A gene, which encodes a subunit of Na+ channels in the brain. However, the detailed pathogenesis of DS remains unclear. This study investigated the synaptic pathogenesis of this disease in terms of excitatory/inhibitory balance using a mouse model of DS. We show that excitatory postsynaptic currents were similar between Scn1a knock-in neurons (Scn1a+/- neurons) and wild-type neurons, but inhibitory postsynaptic currents were significantly lower in Scn1a+/- neurons. Moreover, both the vesicular release probability and the number of inhibitory synapses were significantly lower in Scn1a+/- neurons compared with wild-type neurons. There was no proportional increase in inhibitory postsynaptic current amplitude in response to increased extracellular Ca2+ concentrations. Our study revealed that the number of inhibitory synapses is significantly reduced in Scn1a+/- neurons, while the sensitivity of inhibitory synapses to extracellular Ca2+ concentrations is markedly increased. These data suggest that Ca2+ tethering in inhibitory nerve terminals may be disturbed following the synaptic burst, likely leading to epileptic symptoms.

Complexin Suppresses Spontaneous Exocytosis by Capturing the Membrane-Proximal Regions of VAMP2 and SNAP25

The neuronal protein complexin contains multiple domains that exert clamping and facilitatory functions to tune spontaneous and action potential-triggered synaptic release. We address the clamping mechanism and show that the accessory helix of complexin arrests assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex that forms the core machinery of intracellular membrane fusion. In a reconstituted fusion assay, site-and stage-specific photo-cross-linking reveals that, prior to fusion, the complexin accessory helix laterally binds the membrane-proximal C-terminal ends of SNAP25 and VAMP2. Corresponding complexin interface mutants selectively increase spontaneous release of neuro-transmitters in living neurons, implying that the accessory helix suppresses final zippering/assembly of the SNARE four-helix bundle by restraining VAMP2 and SNAP25.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Identification of miRNA-Target Gene Pairs in the Parietal and Frontal Lobes of the Brain in Patients with Alzheimer's Disease Using Bioinformatic Analyses

Alzheimer's disease (AD) is a growing health concern worldwide. MicroRNAs (miRNAs) have been extensively studied in many diseases, including AD. To identify differentially expressed miRNAs (DEmiRNAs) and genes specific to AD, we used bioinformatic analyses to investigate candidate miRNA-mRNA pairs involved in the pathogenesis of AD. We focused on differentially expressed genes (DEGs) that are targets of DEmiRNAs. The GEO2R tool and the HISAT2-DESeq2 software were used to identify DEmiRNAs and DEGs. Bioinformatic tools available online, such as TAM and the Database for Annotation, Visualization and Integrated Discovery (DAVID), were used to perform functional annotation and enrichment analysis. Targets of miRNAs were predicted using the miRTarBase. The Search Tool for the Retrieval of Interacting Genes (STRING) and Cytoscape, which are available online, were utilized to construct protein-protein interaction (PPI) networks and identify hub genes. Furthermore, transcription factors (TFs) encoded by the DEGs were predicted using the TransmiR database and TF-miRNA-mRNA networks were constructed. Finally, the expression profile of a hub gene in peripheral blood mononuclear cells was compared between healthy individuals and AD patients. We identified 26 correlated miRNA-mRNA pairs. In the parietal lobe, miRNA-mRNA pairs involved in protein folding were enriched, and in the frontal lobe, miRNA-mRNA pairs involved in synaptic transmission, abnormal protein degradation, and apoptosis were enriched. In addition, HSP90AB1 in peripheral blood mononuclear cells was found to be significantly downregulated in AD patients, and this was consistent with its expression profile in the parietal lobe of AD patients. Our results provide brain region-specific changes in miRNA-mRNA associations in AD patients, further our understanding of potential underlying molecular mechanisms of AD, and reveal promising diagnostic and therapeutic targets for AD.

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.

SNARE complex alters the interactions of the Ca2+ sensor synaptotagmin 1 with lipid bilayers

Release of neuronal transmitters from nerve terminals is triggered by the molecular Ca²⁺ sensor synaptotagmin 1 (Syt1). Syt1 is a transmembrane protein attached to the synaptic vesicle (SV), and its cytosolic region comprises two domains, C2A and C2B, which are thought to penetrate into lipid bilayers upon Ca²⁺ binding. Before fusion, SVs become attached to the presynaptic membrane (PM) by the four-helical SNARE complex, which is thought to bind the C2B domain in vivo. To understand how the interactions of Syt1 with lipid bilayers and the SNARE complex trigger fusion, we performed molecular dynamics (MD) simulations at a microsecond scale. We investigated how the isolated C2 modules and the C2AB tandem of Syt1 interact with membranes mimicking either SV or PM. The simulations showed that the C2AB tandem can either bridge SV and PM or insert into PM with its Ca²⁺-bound tips and that the latter configuration is more favorable. Surprisingly, C2 domains did not cooperate in penetrating into PM but instead mutually hindered their insertion into the bilayer. To test whether the interaction of Syt1 with lipid bilayers could be affected by the C2B-SNARE attachment, we performed systematic conformational analysis of the C2AB-SNARE complex. Notably, we found that the C2B-SNARE interface precludes the coupling of C2 domains and promotes their insertion into PM. We performed the MD simulations of the prefusion protein complex positioned between the lipid bilayers mimicking PM and SV, and our results demonstrated in silico that the presence of the Ca²⁺ bound C2AB tandem promotes lipid merging. Altogether, our MD simulations elucidated the role of the Syt1-SNARE interactions in the fusion process and produced the dynamic all-atom model of the prefusion protein-lipid complex.

Multi-Ancestry Genome-wide Association Study Accounting for Gene-Psychosocial Factor Interactions Identifies Novel Loci for Blood Pressure Traits

Psychological and social factors are known to influence blood pressure (BP) and risk of hypertension and associated cardiovascular diseases. To identify novel BP loci, we carried out genome-wide association meta-analyses of systolic, diastolic, pulse, and mean arterial BP taking into account the interaction effects of genetic variants with three psychosocial factors: depressive symptoms, anxiety symptoms, and social support. Analyses were performed using a two-stage design in a sample of up to 128,894 adults from 5 ancestry groups. In the combined meta-analyses of Stages 1 and 2, we identified 59 loci (p value <5e-8), including nine novel BP loci. The novel associations were observed mostly with pulse pressure, with fewer observed with mean arterial pressure. Five novel loci were identified in African ancestry, and all but one showed patterns of interaction with at least one psychosocial factor. Functional annotation of the novel loci supports a major role for genes implicated in the immune response (PLCL2), synaptic function and neurotransmission (LIN7A, PFIA2), as well as genes previously implicated in neuropsychiatric or stress-related disorders (FSTL5, CHODL). These findings underscore the importance of considering psychological and social factors in gene discovery for BP, especially in non-European populations.

Calcium signaling: breast cancer's approach to manipulation of cellular circuitry

Calcium is a versatile element that participates in cell signaling for a wide range of cell processes such as death, cell cycle, division, migration, invasion, metabolism, differentiation, autophagy, transcription, and others. Specificity of calcium in each of these processes is achieved through modulation of intracellular calcium concentrations by changing the characteristics (amplitude/frequency modulation) or location (spatial modulation) of the signal. Breast cancer utilizes calcium signaling as an advantage for survival and progression. This review integrates evidence showing that increases in expression of calcium channels, GPCRs, pumps, effectors, and enzymes, as well as resulting intracellular calcium signals, lead to high calcium and/or an elevated calcium- mobilizing capacity necessary for malignant functions such as migratory, invasive, proliferative, tumorigenic, or metastatic capacities.

Palisade Endings Have an Exocytotic Machinery But Lack Acetylcholine Receptors and Distinct Acetylcholinesterase Activity

Purpose

The purpose of this work was to test whether palisade endings express structural and molecular features of exocytotic machinery, and are associated with acetylcholine receptors, and enzymes for neurotransmitter breakdown.

Methods

Extraocular rectus muscles from six cats were studied. Whole-mount preparations of extraocular muscles (EOMs) were immunolabeled with markers for exocytotic proteins, including synaptosomal-associated protein of 25 kDa (SNAP25), syntaxin, synaptobrevin, synaptotagmin, and complexin. Acetylcholine receptors (AChRs) were visualized with α-bungarotoxin and with an antibody against AChRs, and acetylcholinesterase (AChE) was tagged with anti-AChE. Molecular features of multicolor labeled palisade endings were analyzed in the confocal scanning microscope, and their ultrastructural features were revealed in the transmission electron microscope.

Results

All palisade endings expressed the exocytotic proteins SNAP25, syntaxin, synaptobrevin, synaptotagmin, and complexin. At the ultrastructural level, vesicles docked at the plasma membrane of terminal varicosities of palisade endings. No AChRs were associated with palisade endings as demonstrated by the absence of α-bungarotoxin and anti-AChR binding. AChE, the degradative enzyme for acetylcholine exhibited low, if any, activity in palisade endings. Axonal tracking showed that axons with multiple en grappe motor terminals were in continuity with palisade endings.

Conclusions

This study demonstrates that palisade endings exhibit structural and molecular characteristics of exocytotic machinery, suggesting neurotransmitter release. However, AChRs were not associated with palisade endings, so there is no binding site for acetylcholine, and, due to low/absent AChE activity, insufficient neurotransmitter removal. Thus, the present findings indicate that palisade endings belong to an effector system that is very different from that found in other skeletal muscles.

Role of Aberrant Spontaneous Neurotransmission in SNAP25-Associated Encephalopathies

SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex, composed of synaptobrevin, syntaxin, and SNAP25, forms the essential fusion machinery for neurotransmitter release. Recent studies have reported several mutations in the gene encoding SNAP25 as a causative factor for developmental and epileptic encephalopathies of infancy and childhood with diverse clinical manifestations. However, it remains unclear how SNAP25 mutations give rise to these disorders. Here, we show that although structurally clustered mutations in SNAP25 give rise to related synaptic transmission phenotypes, specific alterations in spontaneous neurotransmitter release are a key factor to account for disease heterogeneity. Importantly, we identified a single mutation that augments spontaneous release without altering evoked release, suggesting that aberrant spontaneous release is sufficient to cause disease in humans.

Open syntaxin overcomes exocytosis defects of diverse mutants in C. elegans

Assembly of SNARE complexes that mediate neurotransmitter release requires opening of a ‘closed’ conformation of UNC-64/syntaxin. Rescue of unc-13/Munc13 mutant phenotypes by overexpressed open UNC-64/syntaxin suggested a specific function of UNC-13/Munc13 in opening UNC-64/ syntaxin. Here, we revisit the effects of open unc-64/syntaxin by generating knockin (KI) worms. The KI animals exhibit enhanced spontaneous and evoked exocytosis compared to WT animals. Unexpectedly, the open syntaxin KI partially suppresses exocytosis defects of various mutants, including snt-1/synaptotagmin, unc-2/P/Q/N-type Ca²⁺ channel alpha-subunit and unc-31/CAPS, in addition to unc-13/Munc13 and unc-10/RIM, and enhanced exocytosis in tom-1/Tomosyn mutants. However, open syntaxin aggravates the defects of unc-18/Munc18 mutants. Correspondingly, open syntaxin partially bypasses the requirement of Munc13 but not Munc18 for liposome fusion. Our results show that facilitating opening of syntaxin enhances exocytosis in a wide range of genetic backgrounds, and may provide a general means to enhance synaptic transmission in normal and disease states.

Opening of the UNC-64/syntaxin closed conformation by UNC-13/Munc13 to form the neuronal SNARE complex is critical for neurotransmitter release. Here the authors show that facilitating the opening of syntaxin enhances exocytosis not only in unc-13 nulls as well as in diverse C. elegans mutants.

Synaptotagmin-1 membrane binding is driven by the C2B domain and assisted cooperatively by the C2A domain

Synaptotagmin interaction with anionic lipid (phosphatidylserine/phosphatidylinositol) containing membranes, both in the absence and presence of calcium ions (Ca²⁺), is critical to its central role in orchestrating neurotransmitter release. The molecular surfaces involved, namely the conserved polylysine motif in the C2B domain and Ca²⁺-binding aliphatic loops on both C2A and C2B domains, are known. Here we use surface force apparatus combined with systematic mutational analysis of the functional surfaces to directly measure Syt1-membrane interaction and fully map the site-binding energetics of Syt1 both in the absence and presence of Ca²⁺. By correlating energetics data with the molecular rearrangements measured during confinement, we find that both C2 domains cooperate in membrane binding, with the C2B domain functioning as the main energetic driver, and the C2A domain acting as a facilitator.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

SNAP23 is required for constitutive and regulated exocytosis in mouse oocytes†

Mammalian oocytes are stored in the ovary for prolonged periods, and arrested in meiotic prophase. During this period, their plasma membranes are constantly being recycled by endocytosis and exocytosis. However, the function of this membrane turnover is unknown. Here, we investigated the requirement for exocytosis in the maintenance of meiotic arrest. Using Trim-away, a newly developed method for rapidly and specifically depleting proteins in oocytes, we have identified the SNARE protein, SNAP23, to be required for meiotic arrest. Degradation of SNAP23 causes premature meiotic resumption in follicle-enclosed oocytes. The reduction in SNAP23 is associated with loss of gap junction communication between the oocyte and surrounding follicle cells. Reduction of SNAP23 protein also inhibits regulated exocytosis in response to a Ca2+ stimulus (cortical granule exocytosis), as measured by lectin staining and cleavage of ZP2. Our results show an essential role for SNAP23 in two key processes that occur in mouse oocytes and eggs.

Glucose Overload Inhibits Glutamatergic Synaptic Transmission: A Novel Role for CREB-Mediated Regulation of Synaptotagmins 2 and 4

Glucose metabolism derangement is critically involved in the age-related memory loss but the underlying molecular mechanisms are still poorly understood. In a mouse model of type 1 diabetes we found memory impairment associated with inhibition of the transcription factor CREB and alteration of pre- and post-synaptic protein expression in the hippocampus. Accordingly, glucose excess negatively affected activity-dependent CREB phosphorylation and CREB-mediated mRNA expression of synaptic proteins in hippocampal primary neurons. Specifically, glucose excess inhibited the activity-dependent recruitment of CREB on the regulatory sequences of synaptotagmin (SYT) 2 and 4 promoters and the expression of SYT4 protein. As a result, high glucose affected both the frequency of miniature excitatory postsynaptic currents and NMDA receptor-mediated currents in autaptic hippocampal neuronal cultures. Collectively, our findings highlight novel mechanisms underlying hyperglycaemia-related memory loss, including CREB-dependent downregulation of synaptotagmin expression.

Arrangements of proteins at reconstituted synaptic vesicle fusion sites depend on membrane separation

Synaptic vesicle proteins, including N‐ethylmaleimide‐sensitive factor attachment protein receptors (SNAREs), Synaptotagmin‐1 and Complexin, are responsible for controlling the synchronised fusion of synaptic vesicles with the presynaptic plasma membrane in response to elevated cytosolic calcium levels. A range of structures of SNAREs and their regulatory proteins have been elucidated, but the exact organisation of these proteins at synaptic junction membranes remains elusive. Here, we have used cryoelectron tomography to investigate the arrangement of synaptic proteins in an in vitro reconstituted fusion system. We found that the separation between vesicle and target membranes strongly correlates with the organisation of protein complexes at junctions. At larger membrane separations, protein complexes assume a ‘clustered’ distribution at the docking site, inducing a protrusion in the target membrane. As the membrane separation decreases, protein complexes become displaced radially outwards and assume a ‘ring‐like’ arrangement. Our findings indicate that docked vesicles can possess a wide range of protein complex numbers and be heterogeneous in their protein arrangements.

Local Design Principles at Hippocampal Synapses Revealed by an Energy-Information Trade-Off

Synapses across different brain regions display distinct structure-function relationships. We investigated the interplay of fundamental design constraints that shape the transmission properties of the excitatory CA3-CA1 pyramidal cell connection, a prototypic synapse for studying the mechanisms of learning in the mammalian hippocampus. This small synapse is characterized by probabilistic release of transmitter, which is markedly facilitated in response to naturally occurring trains of action potentials. Based on a physiologically motivated computational model of the rat CA3 presynaptic terminal, we show how unreliability and short-term dynamics of vesicular release work together to regulate the trade-off of information transfer versus energy use. We propose that individual CA3-CA1 synapses are designed to operate near the maximum possible capacity of information transmission in an efficient manner. Experimental measurements reveal a wide range of vesicular release probabilities at hippocampal synapses, which may be a necessary consequence of long-term plasticity and homeostatic mechanisms that manifest as presynaptic modifications of the release probability. We show that the timescales and magnitude of short-term plasticity (STP) render synaptic information transfer nearly independent of differences in release probability. Thus, individual synapses transmit optimally while maintaining a heterogeneous distribution of presynaptic strengths indicative of synaptically-encoded memory representations. Our results support the view that organizing principles that are evident on higher scales of neural organization percolate down to the design of an individual synapse.

Post-tetanic potentiation lowers the energy barrier for synaptic vesicle fusion independently of Synaptotagmin-1

Previously, we showed that modulation of the energy barrier for synaptic vesicle fusion boosts release rates supralinearly (Schotten, 2015). Here we show that mouse hippocampal synapses employ this principle to trigger Ca²⁺-dependent vesicle release and post-tetanic potentiation (PTP). We assess energy barrier changes by fitting release kinetics in response to hypertonic sucrose. Mimicking activation of the C2A domain of the Ca²⁺-sensor Synaptotagmin-1 (Syt1), by adding a positive charge (Syt1^D232N) or increasing its hydrophobicity (Syt1^4W), lowers the energy barrier. Removing Syt1 or impairing its release inhibitory function (Syt1^9Pro) increases spontaneous release without affecting the fusion barrier. Both phorbol esters and tetanic stimulation potentiate synaptic strength, and lower the energy barrier equally well in the presence and absence of Syt1. We propose a model where tetanic stimulation activates Syt1-independent mechanisms that lower the energy barrier and act additively with Syt1-dependent mechanisms to produce PTP by exerting multiplicative effects on release rates.

Ca2+-dependent release of synaptotagmin-1 from the SNARE complex on phosphatidylinositol 4,5-bisphosphate-containing membranes

The Ca²⁺ sensor synaptotagmin-1 and the SNARE complex cooperate to trigger neurotransmitter release. Structural studies elucidated three distinct synaptotagmin-1-SNARE complex binding modes involving 'polybasic', 'primary' and 'tripartite' interfaces of synaptotagmin-1. We investigated these interactions using NMR and fluorescence spectroscopy. Synaptotagmin-1 binds to the SNARE complex through the polybasic and primary interfaces in solution. Ca²⁺-free synaptotagmin-1 binds to SNARE complexes anchored on PIP₂-containing nanodiscs. R398Q/R399Q and E295A/Y338W mutations at the primary interface, which strongly impair neurotransmitter release, disrupt and enhance synaptotagmin-1-SNARE complex binding, respectively. Ca²⁺ induces tight binding of synaptotagmin-1 to PIP₂-containing nanodiscs, disrupting synaptotagmin-1-SNARE interactions. Specific effects of mutations in the polybasic region on Ca²⁺-dependent synaptotagmin-1-PIP₂-membrane interactions correlate with their effects on release. Our data suggest that synaptotagmin-1 binds to the SNARE complex through the primary interface and that Ca²⁺ releases this interaction, inducing PIP₂/membrane binding and allowing cooperation between synaptotagmin-1 and the SNAREs in membrane fusion to trigger release.

Membrane Association and Functional Mechanism of Synaptotagmin-1 in Triggering Vesicle Fusion

Upon Ca²⁺ influx, synaptic vesicles fuse with the presynaptic plasma membrane (PM) to release neurotransmitters. Membrane fusion is triggered by synaptotagmin-1, a transmembrane protein in the vesicle membrane (VM), but the mechanism is under debate. Synaptotagmin-1 contains a single transmembrane helix (TM) and two tandem C2 domains (C2A and C2B). This study aimed to use molecular dynamics simulations to elucidate how Ca²⁺-bound synaptotagmin-1, by simultaneously associating with VM and PM, brings them together for fusion. Although C2A stably associates with VM via two Ca²⁺-binding loops, C2B has a propensity to partially dissociate. Importantly, an acidic motif in the TM-C2A linker competes with VM for interacting with C2B, thereby flipping its orientation to face PM. Subsequently, C2B readily associates with PM via a polybasic cluster and a Ca²⁺-binding loop. The resulting mechanistic model for the triggering of membrane fusion by synaptotagmin-1 reconciles many experimental observations.

Neurotrophic factors and target-specific retrograde signaling interactions define the specificity of classical and neuropeptide cotransmitter release at identified Lymnaea synapses

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network. However, the molecular mechanisms that produce synaptic heterogeneity by regulating the cotransmitter release characteristics of individual presynaptic terminals remain poorly defined. In particular, we know little about the regulation of neuropeptide corelease, despite the fact that they mediate synaptic transmission, plasticity and neuromodulation. Here, we report that an identified Lymnaea neuron selectively releases its classical small molecule and peptide neurotransmitters, acetylcholine and FMRFamide-derived neuropeptides, to differentially influence the activity of distinct postsynaptic targets that coordinate cardiorespiratory behaviour. Using a combination of electrophysiological, molecular, and pharmacological approaches, we found that neuropeptide cotransmitter release was regulated by cross-talk between extrinsic neurotrophic factor signaling and target-specific retrograde arachidonic acid signaling, which converged on modulation of glycogen synthase kinase 3. In this context, we identified a novel role for the Lymnaea synaptophysin homologue as a specific and synapse-delimited inhibitory regulator of peptide neurotransmitter release. This study is among the first to define the cellular and molecular mechanisms underlying the differential release of cotransmitter substances from individual presynaptic terminals, which allow for context-dependent tuning and plasticity of the synaptic networks underlying patterned motor behaviour.

Molecular Evolution and Characterization of Fish Stathmin Genes

Simple Summary

Stathmin is a highly conserved microtubule remodeling protein. Here, 175 putative stathmin genes were identified in 27 species of fish. Gene organization, motif distribution, divergence of duplicated genes, functional divergence, synteny relationship, and protein-protein interaction were performed to investigate their evolutionary history. In addition, expression profiles of some stathmins were examined under dimethoate treatment. The results will provide useful references for further functional analyses.

Abstract

Stathmin is a highly conserved microtubule remodeling protein, involved in many biological processes such as signal transduction, cell proliferation, neurogenesis and so on. However, little evolutional information has been reported about this gene family in fish. In this study, 175 stathmin genes were identified in 27 species of fish. Conserved exon-intron structure and motif distributions were found in each group. Divergence of duplicated genes implied the species’ adaptation to the environment. Functional divergence suggested that the evolution of stathmin is mainly influenced by purifying selection, and some residues may undergo positive selection. Moreover, synteny relationship near the stathmin locus was relatively conserved in some fish. Network analyses also exhibited 74 interactions, implying functional diversity. The expression pattern of some stathmin genes was also investigated under pesticide stress. These will provide useful references for their functional research in the future.

12q21 deletion syndrome: Narrowing the critical region down to 1.6 Mb including SYT1 and PPP1R12A

Deletions in the 12q21 region are rare and non-recurrent CNVs. To date, only 11 patients with deletions in this region have been reported in the literature. These patients most often presented with syndromic intellectual deficiency, ventriculomegaly or hydrocephalus, ectodermal abnormalities, growth retardation and renal and cardiac malformations, suggesting a recognizable microdeletion syndrome. We report three new patients with overlapping deletions of the 12q21 region, including the smallest deletion reported to date and the first case characterized by array CGH during pregnancy. We describe specific clinical findings and shared facial features as developmental delay, ectodermal abnormalities, ventriculomegaly or hydrocephalus, axial hypotonia or spastic diplegia, growth retardation, heart defect, hydronephrosis, ureteral reflux or horseshoe kidney, large thorax or pectus excavatum, syndactyly of 2-3 toes, pterygium coli or excess nuchal skin, large anterior fontanel, low set ears, prominent forehead, short-upturned nose with nostril hypoplasia, microretrognathia and hypertelorism. These new patients and a comprehensive review of the literature allow us to define a minimum critical region spanning 1.6 Mb in 12q21. By screening the critical region using prediction tools, we identified two candidate genes: SYT1and PPP1R12A.

Presynaptic dysfunction in neurodevelopmental disorders: Insights from the synaptic vesicle life cycle

The activity-dependent fusion, retrieval and recycling of synaptic vesicles is essential for the maintenance of neurotransmission. Until relatively recently it was believed that most mutations in genes that were essential for this process would be incompatible with life, because of this fundamental role. However, an ever-expanding number of mutations in this very cohort of genes are being identified in individuals with neurodevelopmental disorders, including autism, intellectual disability and epilepsy. This article will summarize the current state of knowledge linking mutations in presynaptic genes to neurodevelopmental disorders by sequentially covering the various stages of the synaptic vesicle life cycle. It will also discuss how perturbations of specific stages within this recycling process could translate into human disease. Finally, it will also provide perspectives on the potential for future therapy that are targeted to presynaptic function.

Synaptotagmin-1 is the Ca2+ sensor for fast striatal dopamine release

Dopamine powerfully controls neural circuits through neuromodulation. In the vertebrate striatum, dopamine adjusts cellular functions to regulate behaviors across broad time scales, but how the dopamine secretory system is built to support fast and slow neuromodulation is not known. Here, we set out to identify Ca²⁺-triggering mechanisms for dopamine release. We find that synchronous dopamine secretion is abolished in acute brain slices of conditional knockout mice in which Synaptotagmin-1 is removed from dopamine neurons. This indicates that Synaptotagmin-1 is the Ca²⁺ sensor for fast dopamine release. Remarkably, dopamine release induced by strong depolarization and asynchronous release during stimulus trains are unaffected by Synaptotagmin-1 knockout. Microdialysis further reveals that these modes and action potential-independent release provide significant amounts of extracellular dopamine in vivo. We propose that the molecular machinery for dopamine secretion has evolved to support fast and slow signaling modes, with fast release requiring the Ca²⁺ sensor Synaptotagmin-1.

Autaptic Cultures: Methods and Applications

Neurons typically form daisy chains of synaptic connections with other neurons, but they can also form synapses with themselves. Although such self-synapses, or autapses, are comparatively rare in vivo, they are surprisingly common in dissociated neuronal cultures. At first glance, autapses in culture seem like a mere curiosity. However, by providing a simple model system in which a single recording electrode gives simultaneous access to the pre- and postsynaptic compartments, autaptic cultures have proven to be invaluable in facilitating important and elegant experiments in the area of synaptic neuroscience. Here, I provide detailed protocols for preparing and recording from autaptic cultures (also called micro-island or microdot cultures). Variations on the basic procedure are presented, as well as practical tips for optimizing the outcomes. I also illustrate the utility of autaptic cultures by reviewing the types of experiments that have used them over the past three decades. These examples serve to highlight the power and elegance of this simple model system, and will hopefully inspire new experiments for the interrogation of synaptic function.

Loss of Doc2b does not influence transmission at Purkinje cell to deep nuclei synapses under physiological conditions

Doc2a and Doc2b are high-affinity calcium-binding proteins that interact with SNARE proteins and phospholipids. Experiments performed on cultured cells indicated that Doc2 proteins promote spontaneous vesicle fusion and asynchronous neurotransmitter release, regulate vesicle priming, mediate augmentation, and regulate transmission during sustained activity. Here, we assess the role of Doc2 proteins in synaptic transmission under physiological conditions at mature synapses made by Purkinje cells onto neurons in the deep cerebellar nuclei (PC to DCN synapses). PCs express Doc2b but not Doc2a. Surprisingly, spontaneous neurotransmitter release, synaptic strength, the time course of evoked release, responses evoked by sustained high-frequency stimulation, and short-term plasticity were normal in Doc2b KO mice. Thus, in stark contrast to numerous functions previously proposed for Doc2, here we find that Doc2b removal does not influence transmission at PC-to-DCN synapses, indicating that conclusions based on studies of Doc2b in cultured cells do not necessarily generalize to mature synapses under physiological conditions.