Masters or slaves? Vesicle release machinery and the regulation of presynaptic calcium channels

Long-term low-dose lamotrigine for paroxysmal kinesigenic dyskinesia: a two-year investigation of cognitive function in children

Objective

While low-dose lamotrigine has shown effectiveness in managing paroxysmal kinesigenic dyskinesia (PKD) in pediatric populations, the cognitive consequences of extended use are yet to be fully elucidated. This study seeks to assess the evolution of cognitive functions and the amelioration of attention deficit and hyperactivity disorder (ADHD) symptoms following a two-year lamotrigine treatment in children.

Methods

This investigation employed an open-label, uncontrolled trial design. Between January 2008 and December 2021, thirty-one participants, ranging in age from 6.5 to 14.1 years, were enrolled upon receiving a new diagnosis of PKD, as defined by the clinical diagnostic criteria set by Bruno in 2004. Comprehensive evaluation of PRRT2 variants and 16p11.2 microdeletion was achieved using whole-exome sequencing (WES) and bioinformatics analysis of copy number variant (CNV) for all subjects. Immediately after diagnosis, participants commenced treatment with low-dose lamotrigine. Cognitive function was assessed using the Wechsler Intelligence Scale for Children-Chinese Revised (WISC-CR) at baseline and after 2 years, with ADHD diagnoses and symptom severity simultaneously assessed by experts in accordance with the DSM-IV diagnostic criteria for ADHD and the ADHD Rating Scale-IV (ADHD-RS-IV).

Results

Initially, twelve out of 31 patients (38.7%) presented with comorbid ADHD. The latency to treatment initiation was notably longer in PKD patients with ADHD (30.75 ± 12.88 months) than in those without ADHD (11.66 ± 9.08 months), t = 4.856, p<0.001. Notably, patients with a latency exceeding 2 years exhibited a heightened risk for comorbid ADHD (OR = 4.671, P=0.015) in comparison to those with shorter latency. Out of the cohort, twenty-five patients saw the clinical trial to its completion. These individuals demonstrated a marked elevation in WISC-CR scores at the 2-year mark relative to the outset across FSIQ (baseline mean: 108.72 ± 10.45 vs 24 months: 110.56 ± 10.03, p=0.001), VIQ (baseline mean: 109.44 ± 11.15 vs 24 months: 110.80 ± 10.44, p=0.028), and PIQ domains (baseline mean: 106.52 ± 9.74 vs 24 months: 108.24 ± 9.38, p=0.012). Concurrently, a substantial mitigation was observed in ADHD inattention at 2 years compared to baseline (p<0.001), with an average total subscale scores decrement from 9.04 ± 4.99 to 6.24 ± 4.05.

Conclusion

Prolonged duration of untreated PKD in children may elevate the risk of ADHD comorbidity. Notably, following a 2-year lamotrigine regimen, enhancements were observed in both cognitive test outcomes and ADHD symptomatology.

Modulation of GABA release by 5-HT1B receptors: An interplay with AMPA-receptors and voltage-gated Ca2+ channels

Obesity has become a worldwide health challenge and commonly results from the intake of more calories than the body requires. The brain represents the master controller of food intake and as such has been the target of obesity medications. However, key mechanisms of druggable targets remain to be defined. Neurons within the arcuate nucleus of the hypothalamus co-expressing neuropeptide Y (NPY), agouti-related protein (AgRP) and GABA (NAG) are fundamental stimulators of hunger and food intake. NAG neurons also inhibit local satiety-promoting pro-opiomelanocortin (POMC) neurons. Agonists of the 1B subtype of metabotropic serotonin receptor (5-HT1BR) reduce food intake in part through the inhibition of hunger-promoting NAG neurons. We first confirmed that 5-HT1BR activation suppressed intake of a palatable Western diet in a mouse model of common dietary-induced obesity and genetically prone obesity. Next, we combined several electrophysiological approaches to analyse the effect of 5-HT1BRs in NAG neuron cell activity and GABA release. 5-HT1BR activation reduced NAG neuron action potential frequency and neurotransmitter release. We found that 5-HT1BR impact on GABA release from NAG neurons is mediated through voltage-gated Ca2+ channels with a critical input from glutamate receptors of AMPA subtype (AMPARs). As a fundamental outcome, this type of interplay provides an uncommon example of metabotropic action of AMPARs which regulates inhibitory signalling due to modulation of GABA release. As a translational outcome, our results provide a key mechanism through which 5-HT1BR drugs inhibit appetite-stimulating neurons within the brain to suppress food intake. This article is part of the Special Issue on "Ukrainian Neuroscience".

Specificities of Gβγ subunits for the SNARE complex before and after stimulation of α2a-adrenergic receptors

Ligand binding to G protein–coupled receptors (GPCRs), such as the α2a-adrenergic receptor (α2aAR), results in the activation of heterotrimeric G proteins, which consist of functionally distinct Gα subunits and Gβγ dimers. α2aAR-dependent inhibition of synaptic transmission regulates functions such as spontaneous locomotor activity, anesthetic sparing, and working memory enhancement and requires the soluble NSF attachment protein receptor (SNARE) complex, a Gβγ effector. To understand how the Gβγ-SNARE complex underlies the α2aAR-dependent inhibition of synaptic transmission, we examined the specificity of Gβγ subunits for the SNARE complex in adrenergic neurons, in which auto-α2aARs respond to epinephrine released from these neurons, and nonadrenergic neurons, in which hetero-α2aARs respond to epinephrine released from other neurons. We performed a quantitative, targeted multiple reaction monitoring proteomic analysis of Gβ and Gγ subunits bound to the SNARE complex in synaptosomes from mouse brains. In the absence of stimulation of auto-α2aARs, Gβ1 and Gγ3 interacted with the SNARE complex. However, Gβ1, Gβ2, and Gγ3 were found in the complex when auto-α2aARs were activated by epinephrine. Further understanding of the specific usage of distinct Gβγ subunits in vivo may provide insights into the homeostatic regulation of synaptic transmission and the mechanisms of dysfunction that occur in neurological diseases.

Cerebellar spreading depolarization mediates paroxysmal movement disorder

Paroxysmal kinesigenic dyskinesia (PKD) is the most common paroxysmal dyskinesia, characterized by recurrent episodes of involuntary movements provoked by sudden changes in movement. Proline-rich transmembrane protein 2 (PRRT2) has been identified as the major causative gene for PKD. Here, we report that PRRT2 deficiency facilitates the induction of cerebellar spreading depolarization (SD) and inhibition of cerebellar SD prevents the occurrence of dyskinetic movements. Using Ca²⁺ imaging, we show that cerebellar SD depolarizes a large population of cerebellar granule cells and Purkinje cells in Prrt2-deficient mice. Electrophysiological recordings further reveal that cerebellar SD blocks Purkinje cell spiking and disturbs neuronal firing of the deep cerebellar nuclei (DCN). The resultant aberrant firing patterns in DCN are tightly, temporally coupled to dyskinetic episodes in Prrt2-deficient mice. Cumulatively, our findings uncover a pivotal role of cerebellar SD in paroxysmal dyskinesia, providing a potent target for treating PRRT2-related paroxysmal disorders.

Clinical features of patients with paroxysmal kinesigenic dyskinesia, mutation screening of PRRT2 and the effects of morning draughts of oxcarbazepine

Background

The objective of this study was to summarize clinical features and PRRT2 mutations of paediatric paroxysmal kinesigenic dyskinesia (PKD) patients and observe the tolerability and effects of morning draughts of oxcarbazepine.

Methods

Twenty patients diagnosed with PKD at Children’s Hospital of Fudan University between January 2011 and December 2015 were enrolled. These patients’ medical records were reviewed. Peripheral venous blood was obtained from all enrolled patients, and polymerase chain reaction (PCR) and Sanger sequencing were used to sequence proline-rich transmembrane protein 2 (PRRT2) gene mutations. Clinical features of PKD patients with and without PRRT2 mutations were compared. All enrolled patients were treated with morning draughts of oxcarbazepine (OXC). The starting dose was 5 mg/kg·d, and the dose was increased by 5 mg/kg·d each week until attacks stopped. Effective doses and adverse effects were recorded.

Results

For all enrolled patients, dyskinesia was triggered by sudden movement. Dyskinetic movement usually involved the limbs and was bilateral; the majority of enrolled patients exhibited both dystonia and choreoathetosis. We identified PRRT2 mutations in 5 patients, including 4 familial patients and 1 sporadic patient. All 20 patients took low doses of OXC (5–20 mg/kg·d) as draughts in the morning, and dyskinesia attacks stopped in 19 patients.

Conclusions

Paediatric PKD patients have various phenotypes. PRRT2 mutations are common in familial cases. OXC taken as morning draughts can be a treatment option for paediatric PKD patients.

Dopamine-mediated calcium channel regulation in synaptic suppression in L. stagnalis interneurons

D2 dopamine receptor-mediated suppression of synaptic transmission from interneurons plays a key role in neurobiological functions across species, ranging from respiration to memory formation. In this study, we investigated the mechanisms of D2 receptor-dependent suppression using soma-soma synapse between respiratory interneuron VD4 and LPeD1 in the mollusk Lymnaea stagnalis (L. stagnalis). We studied the effects of dopamine on voltage-dependent Ca²⁺ current and synaptic vesicle release from the VD4. We report that dopamine inhibits voltage-dependent Ca²⁺ current in the VD4 by both voltage-dependent and -independent mechanisms. Dopamine also suppresses synaptic vesicle release downstream of activity-dependent Ca²⁺ influx. Our study demonstrated that dopamine acts through D2 receptors to inhibit interneuron synaptic transmission through both voltage-dependent Ca²⁺ channel-dependent and -independent pathways. Taken together, these findings expand our understanding of dopamine function and fundamental mechanisms that shape the dynamics of neural circuit.

Spontaneous Vesicle Fusion Is Differentially Regulated at Cholinergic and GABAergic Synapses

The locomotion of C. elegans is balanced by excitatory and inhibitory neurotransmitter release at neuromuscular junctions. However, the molecular mechanisms that maintain the balance of synaptic transmission remain enigmatic. Here, we investigated the function of voltage-gated Ca²⁺ channels in triggering spontaneous release at cholinergic and GABAergic synapses. Recordings of the miniature excitatory/inhibitory postsynaptic currents (mEPSCs and mIPSCs, respectively) showed that UNC-2/CaV2 and EGL-19/CaV1 channels are the two major triggers for spontaneous release. Notably, however, Ca²⁺-independent spontaneous release was observed at GABAergic but not cholinergic synapses. Functional screening led to the identification of hypomorphic unc-64/Syntaxin-1A and snb-1/VAMP2 mutants in which mEPSCs are severely impaired, whereas mIPSCs remain unaltered, indicating differential regulation of these currents at cholinergic and GABAergic synapses. Moreover, Ca²⁺-independent spontaneous GABA release was nearly abolished in the hypomorphic unc-64 and snb-1 mutants, suggesting distinct mechanisms for Ca²⁺-dependent and Ca²⁺-independent spontaneous release.

[Clinical manifestations and genetic diagnosis of paroxysmal kinesigenic dyskinesia]

The clinical manifestations of five children with paroxysmal kinesigenic dyskinesia (PKD) were retrospectively analyzed and their gene mutations were analyzed by high-throughput sequencing and chromosome microarray. The 5 patients consisted of 4 males and 1 female and the age of onset was 6-9 years. Dyskinesia was induced by sudden turn movement, scare, mental stress, or other factors. These patients were conscious and had abnormal posture of unilateral or bilateral extremities, athetosis, facial muscle twitching, and abnormal body posture. The frequency of onset ranged from 3-5 times a month to 2-7 times a day, with a duration of <30 seconds every time. Electroencephalography showed no abnormality in these patients. Three patients had a family history of similar disease. The high-throughput sequencing results showed that a heterozygous mutation in the PRRT2 gene, c.649_650insC (p.R217PfsX8), was found in two patients; the mutation c.436C>T (p.P146S) was found in one patient; a splice site mutation, IVS2-1G>A, was found in one patient. The two mutations c.436C>T and IVS2-1G>A had not been reported previously. The chromosome microarray analysis was performed in one patient with negative results of gene detection, and the chromosome 16p11.2 deletion (0.55 Mb) was observed. Low-dose carbamazepine was effective for treatment of the 5 patients. PKD is a rare neurological disease. The detection of the PRRT2 gene by multiple genetic analysis can help the early diagnosis of PKD.

Native metabotropic glutamate receptor 4 depresses synaptic transmission through an unusual Gαq transduction pathway

In cerebellar cortex, mGlu₄ receptors located on parallel fibers play an essential role in normal motor function, but the molecular mechanisms involved are not yet completely understood. Using a strategy combining biochemical and electrophysiological approaches in the rodent cerebellum, we demonstrate that presynaptic mGlu₄ receptors control synaptic transmission through an atypical activation of Gα_q proteins. First, the Gα_q subunit, PLC and PKC signaling proteins present in cerebellar extracts are retained on affinity chromatography columns grafted with different sequences of the cytoplasmic domain of mGlu₄ receptor. The i2 loop and the C terminal domain were used as baits, two domains that are known to play a pivotal role in coupling selectivity and efficacy. Second, in situ proximity ligation assays show that native mGlu₄ receptors and Gα_q subunits are in close physical proximity in cerebellar cortical slices. Finally, electrophysiological experiments demonstrate that the molecular mechanisms underlying mGlu₄ receptor-mediated inhibition of transmitter release at cerebellar Parallel Fiber (PF) - Molecular Layer Interneuron (MLI) synapses involves the Gα_q-PLC signaling pathway. Taken together, our results provide compelling evidence that, in the rodent cerebellar cortex, mGlu₄ receptors act by coupling to the Gα_q protein and PLC effector system to reduce glutamate synaptic transmission.

Trafficking of neuronal calcium channels

Neuronal voltage-gated calcium channels (VGCCs) serve complex yet essential physiological functions via their pivotal role in translating electrical signals into intracellular calcium elevations and associated downstream signalling pathways. There are a number of regulatory mechanisms to ensure a dynamic control of the number of channels embedded in the plasma membrane, whereas alteration of the surface expression of VGCCs has been linked to various disease conditions. Here, we provide an overview of the mechanisms that control the trafficking of VGCCs to and from the plasma membrane, and discuss their implication in pathophysiological conditions and their potential as therapeutic targets.

Paroxysmal movement disorders: An update

Paroxysmal movement disorders comprise both paroxysmal dyskinesia, characterized by attacks of dystonic and/or choreic movements, and episodic ataxia, defined by attacks of cerebellar ataxia. They may be primary (familial or sporadic) or secondary to an underlying cause. They can be classified according to their phenomenology (kinesigenic, non-kinesigenic or exercise-induced) or their genetic cause. The main genes involved in primary paroxysmal movement disorders include PRRT2, PNKD, SLC2A1, ATP1A3, GCH1, PARK2, ADCY5, CACNA1A and KCNA1. Many cases remain genetically undiagnosed, thereby suggesting that additional culprit genes remain to be discovered. The present report is a general overview that aims to help clinicians diagnose and treat patients with paroxysmal movement disorders.

Five cases of paroxysmal kinesigenic dyskinesia by genetic diagnosis

Paroxysmal kinesigenic dyskinesia (PKD) is an autosomal dominant disorder and PRRT2 is the causative gene of PKD. The aim of this study was to investigate PRRT2 mutations in patients who were clinically diagnosed with PKD. Nine PKD cases, including four familial cases and five sporadic cases, were selected. Peripheral blood was drawn after obtaining informed consent, and genomic DNA was extracted by a standard protocol. Sanger sequencing was performed for the screening of PRRT2 mutations. A total of five cases were detected to harbor PRRT2 mutations. Four familial cases carried a c.649dupC (p.Arg217Profs^*8) mutation, while one sporadic case and his asymptomatic father carried a c.133-136delCCAG (p.Pro45Argfs^*44) mutation. PRRT2 mutations were not identified in the remaining cases. The study further confirmed that PRRT2 was a causative gene of PKD and implied that PRRT2 mutation has incomplete penetrance.

Paroxysmal kinesigenic dyskinesia and myotonia congenita in the same family: coexistence of a PRRT2 mutation and two CLCN1 mutations

Paroxysmal kinesigenic dyskinesia (PKD) and myotonia congenita (MC) are independent disorders that share some clinical features. We aimed to investigate the sequences of PRRT2 and CLCN1 in a proband diagnosed with PKD and suspected MC. Clinical evaluation and auxiliary examinations were performed. Direct sequencing of the entire coding regions of the PRRT2 and CLCN1 genes was conducted. Haplotype analysis confirmed the relationships among the family members. The proband suffered choreoathetosis attacks triggered by sudden movements, and lower-limb weakness and stiffness that worsened in cold weather. Carbamazepine monotherapy completely controlled his choreoathetosis and significantly relieved his limb weakness and stiffness. His father, when young, had similar limb stiffness, while his mother and brother were asymptomatic. Genetic analysis revealed that the proband and his father harbored a PRRT2 c.649dupC mutation, and CLCN1 c.1723C>T and c.2492A>G mutations. His brother carried only the two CLCN1 mutations. None of these mutations were identified in his mother and 150 unrelated controls. This is the first report showing the coexistence of PRRT2 and CLCN1 mutations. Our results also indicate that both the PRRT2 and CLCN1 genes need to be screened if we fail to identify PRRT2 mutations in PKD patients or CLCN1 mutations in MC patients.

The immediately releasable pool of mouse chromaffin cell vesicles is coupled to P/Q-type calcium channels via the synaptic protein interaction site

It is generally accepted that the immediately releasable pool is a group of readily releasable vesicles that are closely associated with voltage dependent Ca²⁺ channels. We have previously shown that exocytosis of this pool is specifically coupled to P/Q Ca²⁺ current. Accordingly, in the present work we found that the Ca²⁺ current flowing through P/Q-type Ca²⁺ channels is 8 times more effective at inducing exocytosis in response to short stimuli than the current carried by L-type channels. To investigate the mechanism that underlies the coupling between the immediately releasable pool and P/Q-type channels we transiently expressed in mouse chromaffin cells peptides corresponding to the synaptic protein interaction site of Cav2.2 to competitively uncouple P/Q-type channels from the secretory vesicle release complex. This treatment reduced the efficiency of Ca²⁺ current to induce exocytosis to similar values as direct inhibition of P/Q-type channels via ω-agatoxin-IVA. In addition, the same treatment markedly reduced immediately releasable pool exocytosis, but did not affect the exocytosis provoked by sustained electric or high K⁺ stimulation. Together, our results indicate that the synaptic protein interaction site is a crucial factor for the establishment of the functional coupling between immediately releasable pool vesicles and P/Q-type Ca²⁺ channels.

Control of low-threshold exocytosis by T-type calcium channels

Low-voltage-activated (LVA) T-type Ca²⁺ channels differ from their high-voltage-activated (HVA) homologues by unique biophysical properties. Hence, whereas HVA channels convert action potentials into intracellular Ca²⁺ elevations, T-type channels control Ca²⁺ entry during small depolarizations around the resting membrane potential. They play an important role in electrical activities by generating low-threshold burst discharges that occur during various physiological and pathological forms of neuronal rhythmogenesis. In addition, they mediate a previously unrecognized function in the control of synaptic transmission where they directly trigger the release of neurotransmitters at rest. In this review, we summarize our present knowledge of the role of T-type Ca²⁺ channels in vesicular exocytosis, and emphasize the critical importance of localizing the exocytosis machinery close to the Ca²⁺ source for reliable synaptic transmission. This article is part of a Special Issue entitled: Calcium channels.

Differential expression of SNAP-25 family proteins in the mouse brain

Soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP)-25 is a neuronal SNARE protein essential for neurotransmitter release from presynaptic terminals. Three palmitoylated SNAP-25 family proteins: SNAP-25a, SNAP-25b, and SNAP-23, are expressed in the brain, but little is known about their distributions and functions. In the present study, we generated specific antibodies to distinguish these three homologous proteins. Immunoblot and immunohistochemical analyses revealed that SNAP-25b was distributed in synapse-enriched regions throughout almost the entire brain, whereas SNAP-25a and SNAP-23 were expressed in relatively specific brain regions with partially complementary expression patterns. SNAP-25a and SNAP-25b, but not SNAP-23, were also present in the axoplasm of nerve fibers. The intracellular localization was also different, and although SNAP-25b and SNAP-23 were found primarily in membrane and lipid raft-enriched fractions of mouse brain homogenates, a substantial amount of SNAP-25a was recovered in soluble fractions. In PC12 cells, SNAP-25b was localized to the plasma membrane, but SNAP-25a and SNAP-23 were distributed throughout the cytoplasm. The expression and distribution of these three proteins were also differentially regulated in the early postnatal period. These results indicate that the three SNAP-25 family proteins display a differential distribution in the brain as well as in neuronal cells, and possibly play distinct roles.

G protein modulation of CaV2 voltage-gated calcium channels

Voltage-gated Ca(2+) channels translate the electrical inputs of excitable cells into biochemical outputs by controlling influx of the ubiquitous second messenger Ca(2+) . As such the channels play pivotal roles in many cellular functions including the triggering of neurotransmitter and hormone release by CaV2.1 (P/Q-type) and CaV2.2 (N-type) channels. It is well established that G protein coupled receptors (GPCRs) orchestrate precise regulation neurotransmitter and hormone release through inhibition of CaV2 channels. Although the GPCRs recruit a number of different pathways, perhaps the most prominent, and certainly most studied among these is the so-called voltage-dependent inhibition mediated by direct binding of Gβγ to the α1 subunit of CaV2 channels. This article will review the basics of Ca(2+) -channels and G protein signaling, and the functional impact of this now classical inhibitory mechanism on channel function. It will also provide an update on more recent developments in the field, both related to functional effects and crosstalk with other signaling pathways, and advances made toward understanding the molecular interactions that underlie binding of Gβγ to the channel and the voltage-dependence that is a signature characteristic of this mechanism.

Signaling role of the voltage-gated calcium channel as the molecular on/off-switch of secretion

Voltage-gated calcium channels (VGCC) are involved in a large variety of cellular Ca(2+) signaling processes, including exocytosis, a Ca(2+) dependent release of neurotransmitters and hormones. Great progress has been made in understanding the mode of action of VGCC in exocytosis, a process distinguished by two sequential yet independent Ca(2+) binding reactions. First, Ca(2+) binds at the selectivity filter, the EEEE motif of the VGCC, and second, subsequent to a brief and intense Ca(2+) inflow to synaptotagmin, a vesicular protein. Inquiry into the functional and physical interactions of the channels with synaptic proteins has demonstrated that exocytosis is triggered during the initial Ca(2+) binding at the channel pore, prior to Ca(2+) entry. Accordingly, a cycle of secretion begins by an incoming stimulus that releases vesicles from a releasable pool upon Ca(2+) binding at the pore, and at the same time, the transient increase in [Ca(2+)](i) primes a fresh set of non-releasable vesicles, to be fused by the next incoming stimulus. We propose a model, in which the Ca(2+) binding at the EEEE motif and the consequent conformational changes in the channel are the primary event in triggering secretion, while synaptotagmin acts as a vesicle docking protein. Thus, the channel serves as the molecular On/Off signaling switch, where the predominance of a conformational change in Ca(2+)-bound channel provides for the fast secretory process.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Activity-dependent phosphorylation of Ser187 is required for SNAP-25-negative modulation of neuronal voltage-gated calcium channels

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a SNARE protein that regulates neurotransmission by the formation of a complex with syntaxin 1 and synaptobrevin/VAMP2. SNAP-25 also reduces neuronal calcium responses to stimuli, but neither the functional relevance nor the molecular mechanisms of this modulation have been clarified. In this study, we demonstrate that hippocampal slices from Snap25(+/-) mice display a significantly larger facilitation and that higher calcium peaks are reached after depolarization by Snap25(-/-) and Snap25(+/-) cultured neurons compared with wild type. We also show that SNAP-25b modulates calcium dynamics by inhibiting voltage-gated calcium channels (VGCCs) and that PKC phosphorylation of SNAP-25 at ser187 is essential for this process, as indicated by the use of phosphomimetic (S187E) or nonphosphorylated (S187A) mutants. Neuronal activity is the trigger that induces the transient phosphorylation of SNAP-25 at ser187. Indeed, enhancement of network activity increases the levels of phosphorylated SNAP-25, whereas network inhibition reduces the extent of protein phosphorylation. A transient peak of SNAP-25 phosphorylation also is detectable in rat hippocampus in vivo after i.p. injection with kainate to induce seizures. These findings demonstrate that differences in the expression levels of SNAP-25 impact on calcium dynamics and neuronal plasticity, and that SNAP-25 phosphorylation, by promoting inhibition of VGCCs, may mediate a negative feedback modulation of neuronal activity during intense activation.

Presynaptic calcium channels: structure, regulators, and blockers

The central and peripheral nervous systems express multiple types of ligand and voltage-gated calcium channels (VGCCs), each with specific physiological roles and pharmacological and electrophysiological properties. The members of the Ca(v)2 calcium channel family are located predominantly at presynaptic nerve terminals, where they are responsible for controlling evoked neurotransmitter release. The activity of these channels is subject to modulation by a number of different means, including alternate splicing, ancillary subunit associations, peptide and small organic blockers, G-protein-coupled receptors (GPCRs), protein kinases, synaptic proteins, and calcium-binding proteins. These multiple and complex modes of calcium channel regulation allow neurons to maintain the specific, physiological window of cytoplasmic calcium concentrations which is required for optimal neurotransmission and proper synaptic function. Moreover, these varying means of channel regulation provide insight into potential therapeutic targets for the treatment of pathological conditions that arise from disturbances in calcium channel signaling. Indeed, considerable efforts are presently underway to identify and develop specific presynaptic calcium channel blockers that can be used as analgesics.

Depolarization-evoked secretion requires two vicinal transmembrane cysteines of syntaxin 1A

Background

The interactions of the voltage-gated Ca²⁺ channel (VGCC) with syntaxin 1A (Sx 1A), Synaptosome-associated protein of 25 kD (SNAP-25), and synaptotagmin, couple electrical excitation to evoked secretion. Two vicinal Cys residues, Cys 271 and Cys 272 in the Sx 1A transmembrane domain, are highly conserved and participate in modulating channel kinetics. Each of the Sx1A Cys mutants, differently modify the kinetics of Cav1.2, and neuronal Cav2.2 calcium channel.

Methodology/Principle Findings

We examined the effects of various Sx1A Cys mutants and the syntaxin isoforms 2, 3, and 4 each of which lack vicinal Cys residues, on evoked secretion, monitoring capacitance transients in a functional release assay. Membrane capacitance in Xenopus oocytes co-expressing Cav1.2, Sx1A, SNAP-25 and synaptotagmin, which is Bot C- and Bot A-sensitive, was elicited by a double 500 ms depolarizing pulse to 0 mV. The evoked-release was obliterated when a single Cys Sx1A mutant or either one of the Sx isoforms were substituted for Sx 1A, demonstrating the essential role of vicinal Cys residues in the depolarization mediated process. Protein expression and confocal imaging established the level of the mutated proteins in the cell and their targeting to the plasma membrane.

Conclusions/Significance

We propose a model whereby the two adjacent transmembranal Cys residues of Sx 1A, lash two calcium channels. Consistent with the necessity of a minimal fusion complex termed the excitosome, each Sx1A is in a complex with SNAP-25, Syt1, and the Ca²⁺ channel. A Hill coefficient >2 imply that at least three excitosome complexes are required for generating a secreting hetero-oligomer protein complex. This working model suggests that a fusion pore that opens during membrane depolarization could be lined by alternating transmembrane segments of Sx1A and VGCC. The functional coupling of distinct amino acids of Sx 1A with VGCC appears to be essential for depolarization-evoked secretion.

Trafficking and regulation of neuronal voltage-gated calcium channels

The importance of voltage-gated calcium channels is underscored by the multitude of intracellular processes that depend on calcium, notably gene regulation and neurotransmission. Given their pivotal roles in calcium (and hence, cellular) homeostasis, voltage-gated calcium channels have been the subject of intense research, much of which has focused on channel regulation. While ongoing research continues to delineate the myriad of interactions that govern calcium channel regulation, an increasing amount of work has focused on the trafficking of voltage-gated calcium channels. This includes the mechanisms by which calcium channels are targeted to the plasma membrane, and, more specifically, to their appropriate loci within a given cell. In addition, we are beginning to gain some insights into the mechanisms by which calcium channels can be removed from the plasma membrane for recycling and/or degradation. Here we highlight recent advances in our understanding of these fundamentally important mechanisms.

The sequence of events that underlie quantal transmission at central glutamatergic synapses

The properties of synaptic transmission were first elucidated at the neuromuscular junction. More recent work has examined transmission at synapses within the brain. Here we review the remarkable progress in understanding the biophysical and molecular basis of the sequential steps in this process. These steps include the elevation of Ca2+ in microdomains of the presynaptic terminal, the diffusion of transmitter through the fusion pore into the synaptic cleft and the activation of postsynaptic receptors. The results give insight into the factors that control the precision of quantal transmission and provide a framework for understanding synaptic plasticity.

Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.

Calcium: a fundamental regulator of intracellular membrane fusion?

For many years, it has been known that an increase in cytosolic calcium triggers the fusion of secretory granules and synaptic vesicles with the plasma membrane. However, the role of calcium in the intracellular membrane-fusion reactions that coordinate the secretory and endocytic pathways has been less clear. Initially, there was accumulating evidence to indicate that a focally localized and transient calcium signal is required to trigger even those fusion events formerly classified as 'constitutive'-that is, those that normally occur in the absence of global cytosolic calcium increases. Therefore, calcium seemed to be a required fundamental co-factor underlying all biological membrane-fusion steps, perhaps with a conserved mechanism of action. However, although such unification would be gratifying, new data indicate that several intracellular fusion events do not require calcium after all. In this review, the evidence for calcium requirements and its modes of action in constitutive trafficking are discussed. As a challenging perspective, I suggest that the specific absence of calcium requirements for some transport steps in fact expands the function of calcium in trafficking, because divergent luminal calcium concentrations and requirements for fusion might increase the specificity with which intracellular membrane-fusion partners are determined.

Spontaneous neurotransmitter release and Ca2+--how spontaneous is spontaneous neurotransmitter release?

Neurotransmitter release from neurons takes place at specialized structures called synapses. Action potential-evoked exocytosis requires Ca(2+) influx through voltage-gated Ca(2+) channels. Spontaneous vesicle fusion occurs both in the absence of action potentials and without any apparent stimulus and is hence thought to be Ca(2+)-independent. However, increasing evidence shows that this form of neurotransmitter discharge can be modulated by changes in intracellular Ca(2+) concentration, suggesting that it is not truly spontaneous. This idea is supported by the fact that spontaneous release can be modulated by interfering with proteins involved in the exocytotic process. Interestingly, modulation of spontaneous discharge at the level of the release machinery is not always accompanied by corresponding modulation of action potential-evoked release, suggesting that two independent processes may underlie spontaneous and action potential-evoked exocytosis, at least at some synapses. This provides an attractive model whereby cells can modulate the two forms of neurotransmitter liberation, which often serve different physiological roles, independently of each other.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Calcium signaling in physiology and pathophysiology

Calcium ions are the most ubiquitous and pluripotent cellular signaling molecules that control a wide variety of cellular processes. The calcium signaling system is represented by a relatively limited number of highly conserved transporters and channels, which execute Ca2+ movements across biological membranes and by many thousands of Ca2+-sensitive effectors. Molecular cascades, responsible for the generation of calcium signals, are tightly controlled by Ca2+ ions themselves and by genetic factors, which tune the expression of different Ca2+-handling molecules according to adaptational requirements. Ca2+ ions determine normal physiological reactions and the development of many pathological processes.

Spike-mediated and graded inhibitory synaptic transmission between leech interneurons: evidence for shared release sites

Inhibitory synaptic transmission between leech heart interneurons consist of two components: graded, gated by Ca2+ entering by low-threshold [low-voltage-activated (LVA)] Ca channels and spike-mediated, gated by Ca2+ entering by high-threshold [high-voltage-activated (HVA)] Ca channels. Changes in presynaptic background Ca2+ produced by Ca2+ influx through LVA channels modulate spike-mediated transmission, suggesting LVA channels have access to release sites controlled by HVA channels. Here we explore whether spike-mediated and graded transmission can use the same release sites and thus how Ca2+ influx by HVA and LVA Ca channels might interact to evoke neurotransmitter release. We recorded pre- and postsynaptic currents from voltage-clamped heart interneurons bathed in 0 mM Na+/5 mM Ca2+ saline. Using different stimulating paradigms and inorganic Ca channel blockers, we show that strong graded synaptic transmission can occlude high-threshold/spike-mediated synaptic transmission when evoked simultaneously. Suppression of LVA Ca currents diminishes graded release and concomitantly increases the ability of Ca2+ entering by HVA channels to release transmitter. Uncaging of Ca chelator corroborates that graded release occludes spike-mediated transmission. Our results indicate that both graded and spike-mediated synaptic transmission depend on the same readily releasable pool of synaptic vesicles. Thus Ca2+, entering cells through different Ca channels (LVA and HVA), acts to gate release of the same synaptic vesicles. The data argue for a closer location of HVA Ca channels to release sites than LVA Ca channels. The results are summarized in a conceptual model of a heart interneuron release site.

Ion interaction at the pore of Lc-type Ca2+ channel is sufficient to mediate depolarization-induced exocytosis

The coupling of voltage-gated Ca2+ channel (VGCC) to exocytotic proteins suggests a regulatory function for the channel in depolarization-evoked exocytosis. To explore this possibility we have examined catecholamine secretion in PC12 and chromaffin cells. We found that replacing Ca2+ with La3+ or other lanthanide ions supported exocytosis in divalent ion-free solution. Cd2+, nifedipine, or verapamil inhibited depolarization-evoked secretion in La3+, indicating specific binding of La3+ at the pore of L-type VGCC, probably at the poly-glutamate (EEEE) locus. Lanthanide efficacy was stringently dependent on ionic radius with La3+>Ce3+>Pr3+, consistent with a size-selective binding interface of trivalent cations at the channel pore. La3+ inward currents were not detected and the highly sensitive La3+/fura-2 imaging assay (approximately 1 pm) detected no La3+ entry, cytosolic La3+ build-up, or alterations in cytosolic Ca2. These results provide strong evidence that occupancy of the pore of the channel by an impermeable cation leads to a conformational change that is transmitted to the exocytotic machinery upstream of intracellular cation build-up (intracellular Ca2+ concentration). Our model allows for a tight temporal and spatial coupling between the excitatory stimulation event and vesicle fusion. It challenges the conventional view that intracellular Ca2+ ion build-up via VGCC permeation is required to trigger secretion and establishes the VGCC as a plausible Ca2+ sensor protein in the process of neuroendocrine secretion.