Quantitative proteomics of the Cav2 channel nano-environments in the mammalian brain

MAP6 interacts with Tctex1 and Cav 2.2/N-type calcium channels to regulate calcium signalling in neurons

MAP6 proteins were first described as microtubule‐stabilizing agents, whose properties were thought to be essential for neuronal development and maintenance of complex neuronal networks. However, deletion of all MAP6 isoforms in MAP6 KO mice does not lead to dramatic morphological aberrations of the brain but rather to alterations in multiple neurotransmissions and severe behavioural impairments. A search for protein partners of MAP6 proteins identified Tctex1 – a dynein light chain with multiple non‐microtubule‐related functions. The involvement of Tctex1 in calcium signalling led to investigate it in MAP6 KO neurons. In this study, we show that functional Ca_v2.2/N‐type calcium channels are deficient in MAP6 KO neurons, due to improper location. We also show that MAP6 proteins interact directly with both Tctex1 and the C‐terminus of Ca_v2.2/N‐type calcium channels. A balance of these two interactions seems to be crucial for MAP6 to modulate calcium signalling in neurons.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Native KCC2 interactome reveals PACSIN1 as a critical regulator of synaptic inhibition

KCC2 is a neuron-specific K+-Cl- cotransporter essential for establishing the Cl- gradient required for hyperpolarizing inhibition in the central nervous system (CNS). KCC2 is highly localized to excitatory synapses where it regulates spine morphogenesis and AMPA receptor confinement. Aberrant KCC2 function contributes to human neurological disorders including epilepsy and neuropathic pain. Using functional proteomics, we identified the KCC2-interactome in the mouse brain to determine KCC2-protein interactions that regulate KCC2 function. Our analysis revealed that KCC2 interacts with diverse proteins, and its most predominant interactors play important roles in postsynaptic receptor recycling. The most abundant KCC2 interactor is a neuronal endocytic regulatory protein termed PACSIN1 (SYNDAPIN1). We verified the PACSIN1-KCC2 interaction biochemically and demonstrated that shRNA knockdown of PACSIN1 in hippocampal neurons increases KCC2 expression and hyperpolarizes the reversal potential for Cl-. Overall, our global native-KCC2 interactome and subsequent characterization revealed PACSIN1 as a novel and potent negative regulator of KCC2.

Involvement of Parkin in the ubiquitin proteasome system-mediated degradation of N-type voltage-gated Ca2+ channels

N-type calcium (CaV2.2) channels are widely expressed in the brain and the peripheral nervous system, where they play important roles in the regulation of transmitter release. Although CaV2.2 channel expression levels are precisely regulated, presently little is known regarding the molecules that mediate its synthesis and degradation. Previously, by using a combination of biochemical and functional analyses, we showed that the complex formed by the light chain 1 of the microtubule-associated protein 1B (LC1-MAP1B) and the ubiquitin-proteasome system (UPS) E2 enzyme UBE2L3, may interact with the CaV2.2 channels promoting ubiquitin-mediated degradation. The present report aims to gain further insights into the possible mechanism of degradation of the neuronal CaV2.2 channel by the UPS. First, we identified the enzymes UBE3A and Parkin, members of the UPS E3 ubiquitin ligase family, as novel CaV2.2 channel binding partners, although evidence to support a direct protein-protein interaction is not yet available. Immunoprecipitation assays confirmed the interaction between UBE3A and Parkin with CaV2.2 channels heterologously expressed in HEK-293 cells and in neural tissues. Parkin, but not UBE3A, overexpression led to a reduced CaV2.2 protein level and decreased current density. Electrophysiological recordings performed in the presence of MG132 prevented the actions of Parkin suggesting enhanced channel proteasomal degradation. Together these results unveil a novel functional coupling between Parkin and the CaV2.2 channels and provide a novel insight into the basic mechanisms of CaV channels protein quality control and functional expression.

Cell-Specific RNA Binding Protein Rbfox2 Regulates CaV2.2 mRNA Exon Composition and CaV2.2 Current Size

The majority of multiexon mammalian genes contain alternatively spliced exons that have unique expression patterns in different cell populations and that have important cell functions. The expression profiles of alternative exons are controlled by cell-specific splicing factors that can promote exon inclusion or exon skipping but with few exceptions we do not know which specific splicing factors control the expression of alternatively spliced exons of known biological function. Many ion channel genes undergo extensive alternative splicing including Cacna1b that encodes the voltage-gated CaV2.2 α1 subunit. Alternatively spliced exon 18a in Cacna1b RNA encodes 21 amino acids in the II-III loop of CaV2.2, and its expression differs across the nervous system and over development. Genome-wide, protein-RNA binding analyses coupled to high-throughput RNA sequencing show that RNA binding Fox (Rbfox) proteins associate with CaV2.2 (Cacna1b) pre-mRNAs. Here, we link Rbfox2 to suppression of e18a. We show increased e18a inclusion in CaV2.2 mRNAs: (1) after siRNA knockdown of Rbfox2 in a neuronal cell line and (2) in RNA from sympathetic neurons of adult compared to early postnatal mice. By immunoprecipitation of Rbfox2-RNA complexes followed by qPCR, we demonstrate reduced Rbfox2 binding upstream of e18a in RNA from sympathetic neurons of adult compared to early postnatal mice. CaV2.2 currents in cell lines and in sympathetic neurons expressing only e18a-CaV2.2 are larger compared to currents from those expressing only Δ18a-CaV2.2. We conclude that Rbfox2 represses e18a inclusion during pre-mRNA splicing of CaV2.2, limiting the size of CaV2.2 currents early in development in certain neuronal populations.

A novel region in the CaV2.1 α1 subunit C-terminus regulates fast synaptic vesicle fusion and vesicle docking at the mammalian presynaptic active zone

In central nervous system (CNS) synapses, action potential-evoked neurotransmitter release is principally mediated by Ca_V2.1 calcium channels (Ca_V2.1) and is highly dependent on the physical distance between Ca_V2.1 and synaptic vesicles (coupling). Although various active zone proteins are proposed to control coupling and abundance of Ca_V2.1 through direct interactions with the Ca_V2.1 α1 subunit C-terminus at the active zone, the role of these interaction partners is controversial. To define the intrinsic motifs that regulate coupling, we expressed mutant Ca_V2.1 α₁ subunits on a Ca_V2.1 null background at the calyx of Held presynaptic terminal. Our results identified a region that directly controlled fast synaptic vesicle release and vesicle docking at the active zone independent of Ca_V2.1 abundance. In addition, proposed individual direct interactions with active zone proteins are insufficient for Ca_V2.1 abundance and coupling. Therefore, our work advances our molecular understanding of Ca_V2.1 regulation of neurotransmitter release in mammalian CNS synapses.

A Genome-Wide Association Study Using a Custom Genotyping Array Identifies Variants in GPR158 Associated With Reduced Energy Expenditure in American Indians

Pima Indians living in Arizona have a high prevalence of obesity, and we have previously shown that a relatively lower energy expenditure (EE) predicts weight and fat mass gain in this population. EE is a familial trait (heritability = 0.52); therefore, in the current study, we aimed to identify genetic variants that affect EE and thereby influence BMI and body fatness in Pima Indians. Genotypic data from 491,265 variants were analyzed for association with resting metabolic rate (RMR) and 24-h EE assessed in a whole-room calorimeter in 507 and 419 Pima Indians, respectively. Variants associated with both measures of EE were analyzed for association with maximum BMI and percent body fat (PFAT) in 5,870 and 912 Pima Indians, respectively. rs11014566 nominally associated with both measures of EE and both measures of adiposity in Pima Indians, where the G allele (frequency: Pima Indians = 0.60, Europeans <0.01) associated with lower 24-h EE (β = -33 kcal/day per copy), lower RMR (β = -31 kcal/day), higher BMI (β = +0.6 kg/m2), and higher PFAT (β = +0.9%). However, the association of rs11014566 with BMI did not directionally replicate when assessed in other ethnic groups. rs11014566 tags rs144895904, which affected promoter function in an in vitro luciferase assay. These variants map to GPR158, which is highly expressed in the brain and interacts with two other genes (RGS7 and CACNA1B) known to affect obesity in knockout mice. Our results suggest that common ethnic-specific variation in GPR158 may influence EE; however, its role in weight gain remains controversial, as it either had no association with BMI or associated with BMI but in the opposite direction in other ethnic groups.

SUMOylation and calcium signalling: potential roles in the brain and beyond

Small ubiquitin-like modifier (SUMO) conjugation (or SUMOylation) is a post-translational protein modification implicated in alterations to protein expression, localization and function. Despite a number of nuclear roles for SUMO being well characterized, this process has only started to be explored in relation to membrane proteins, such as ion channels. Calcium ion (Ca²⁺) signalling is crucial for the normal functioning of cells and is also involved in the pathophysiological mechanisms underlying relevant neurological and cardiovascular diseases. Intracellular Ca²⁺ levels are tightly regulated; at rest, most Ca²⁺ is retained in organelles, such as the sarcoplasmic reticulum, or in the extracellular space, whereas depolarization triggers a series of events leading to Ca²⁺ entry, followed by extrusion and reuptake. The mechanisms that maintain Ca²⁺ homoeostasis are candidates for modulation at the post-translational level. Here, we review the effects of protein SUMOylation, including Ca²⁺ channels, their proteome and other proteins associated with Ca²⁺ signalling, on vital cellular functions, such as neurotransmission within the central nervous system (CNS) and in additional systems, most prominently here, in the cardiac system.

Fusion Competent Synaptic Vesicles Persist upon Active Zone Disruption and Loss of Vesicle Docking

In a nerve terminal, synaptic vesicle docking and release are restricted to an active zone. The active zone is a protein scaffold that is attached to the presynaptic plasma membrane and opposed to postsynaptic receptors. Here, we generated conditional knockout mice removing the active zone proteins RIM and ELKS, which additionally led to loss of Munc13, Bassoon, Piccolo, and RIM-BP, indicating disassembly of the active zone. We observed a near-complete lack of synaptic vesicle docking and a strong reduction in vesicular release probability and the speed of exocytosis, but total vesicle numbers, SNARE protein levels, and postsynaptic densities remained unaffected. Despite loss of the priming proteins Munc13 and RIM and of docked vesicles, a pool of releasable vesicles remained. Thus, the active zone is necessary for synaptic vesicle docking and to enhance release probability, but releasable vesicles can be localized distant from the presynaptic plasma membrane.

Vertebrate Presynaptic Active Zone Assembly: a Role Accomplished by Diverse Molecular and Cellular Mechanisms

Among all the biological systems in vertebrates, the central nervous system (CNS) is the most complex, and its function depends on specialized contacts among neurons called synapses. The assembly and organization of synapses must be exquisitely regulated for a normal brain function and network activity. There has been a tremendous effort in recent decades to understand the molecular and cellular mechanisms participating in the formation of new synapses and their organization, maintenance, and regulation. At the vertebrate presynapses, proteins such as Piccolo, Bassoon, RIM, RIM-BPs, CAST/ELKS, liprin-α, and Munc13 are constant residents and participate in multiple and dynamic interactions with other regulatory proteins, which define network activity and normal brain function. Here, we review the function of these active zone (AZ) proteins and diverse factors involved in AZ assembly and maintenance, with an emphasis on axonal trafficking of precursor vesicles, protein homo- and hetero-oligomeric interactions as a mechanism of AZ trapping and stabilization, and the role of F-actin in presynaptic assembly and its modulation by Wnt signaling.

AMPA-receptor specific biogenesis complexes control synaptic transmission and intellectual ability

AMPA-type glutamate receptors (AMPARs), key elements in excitatory neurotransmission in the brain, are macromolecular complexes whose properties and cellular functions are determined by the co-assembled constituents of their proteome. Here we identify AMPAR complexes that transiently form in the endoplasmic reticulum (ER) and lack the core-subunits typical for AMPARs in the plasma membrane. Central components of these ER AMPARs are the proteome constituents FRRS1l (C9orf4) and CPT1c that specifically and cooperatively bind to the pore-forming GluA1-4 proteins of AMPARs. Bi-allelic mutations in the human FRRS1L gene are shown to cause severe intellectual disability with cognitive impairment, speech delay and epileptic activity. Virus-directed deletion or overexpression of FRRS1l strongly impact synaptic transmission in adult rat brain by decreasing or increasing the number of AMPARs in synapses and extra-synaptic sites. Our results provide insight into the early biogenesis of AMPARs and demonstrate its pronounced impact on synaptic transmission and brain function.

The biogenesis of AMPA-type glutamate receptor (AMPAR) complexes is only partially understood. Here the authors identify transient assemblies of GluA1-4 proteins and proteins FRRS1l/CPT1c that drive formation of mature AMPAR complexes in the ER. Mutations in FRRS1l are associated with intellectual disability and epilepsy in three families.

Proximal clustering between BK and CaV1.3 channels promotes functional coupling and BK channel activation at low voltage

Ca_V-channel dependent activation of BK channels is critical for feedback control of both calcium influx and cell excitability. Here we addressed the functional and spatial interaction between BK and Ca_V1.3 channels, unique Ca_V1 channels that activate at low voltages. We found that when BK and Ca_V1.3 channels were co-expressed in the same cell, BK channels started activating near −50 mV, ~30 mV more negative than for activation of co-expressed BK and high-voltage activated Ca_V2.2 channels. In addition, single-molecule localization microscopy revealed striking clusters of Ca_V1.3 channels surrounding clusters of BK channels and forming a multi-channel complex both in a heterologous system and in rat hippocampal and sympathetic neurons. We propose that this spatial arrangement allows tight tracking between local BK channel activation and the gating of Ca_V1.3 channels at quite negative membrane potentials, facilitating the regulation of neuronal excitability at voltages close to the threshold to fire action potentials.

DOI:http://dx.doi.org/10.7554/eLife.28029.001

Identification of Cav2-PKCβ and Cav2-NOS1 complexes as entities for ultrafast electrochemical coupling

Voltage-activated calcium (Cav) channels couple intracellular signaling pathways to membrane potential by providing Ca²⁺ ions as second messengers at sufficiently high concentrations to modulate effector proteins located in the intimate vicinity of those channels. Here we show that protein kinase Cβ (PKCβ) and brain nitric oxide synthase (NOS1), both identified by proteomic analysis as constituents of the protein nano-environment of Cav2 channels in the brain, directly coassemble with Cav2.2 channels upon heterologous coexpression. Within Cav2.2-PKCβ and Cav2.2-NOS1 complexes voltage-triggered Ca²⁺ influx through the Cav channels reliably initiates enzymatic activity within milliseconds. Using BK_Ca channels as target sensors for nitric oxide and protein phosphorylation together with high concentrations of Ca²⁺ buffers showed that the complex-mediated Ca²⁺ signaling occurs in local signaling domains at the plasma membrane. Our results establish Cav2-enzyme complexes as molecular entities for fast electrochemical coupling that reliably convert brief membrane depolarization into precisely timed intracellular signaling events in the mammalian brain.

C-terminal splice variants of P/Q-type Ca2+ channel CaV2.1 α1 subunits are differentially regulated by Rab3-interacting molecule proteins

Voltage-dependent Ca²⁺ channels (VDCCs) mediate neurotransmitter release controlled by presynaptic proteins such as the scaffolding proteins Rab3-interacting molecules (RIMs). RIMs confer sustained activity and anchoring of synaptic vesicles to the VDCCs. Multiple sites on the VDCC α₁ and β subunits have been reported to mediate the RIMs-VDCC interaction, but their significance is unclear. Because alternative splicing of exons 44 and 47 in the P/Q-type VDCC α₁ subunit Ca_V2.1 gene generates major variants of the Ca_V2.1 C-terminal region, known for associating with presynaptic proteins, we focused here on the protein regions encoded by these two exons. Co-immunoprecipitation experiments indicated that the C-terminal domain (CTD) encoded by Ca_V2.1 exons 40-47 interacts with the α-RIMs, RIM1α and RIM2α, and this interaction was abolished by alternative splicing that deletes the protein regions encoded by exons 44 and 47. Electrophysiological characterization of VDCC currents revealed that the suppressive effect of RIM2α on voltage-dependent inactivation (VDI) was stronger than that of RIM1α for the Ca_V2.1 variant containing the region encoded by exons 44 and 47. Importantly, in the Ca_V2.1 variant in which exons 44 and 47 were deleted, strong RIM2α-mediated VDI suppression was attenuated to a level comparable with that of RIM1α-mediated VDI suppression, which was unaffected by the exclusion of exons 44 and 47. Studies of deletion mutants of the exon 47 region identified 17 amino acid residues on the C-terminal side of a polyglutamine stretch as being essential for the potentiated VDI suppression characteristic of RIM2α. These results suggest that the interactions of the Ca_V2.1 CTD with RIMs enable Ca_V2.1 proteins to distinguish α-RIM isoforms in VDI suppression of P/Q-type VDCC currents.

The Auxiliary Calcium Channel Subunit α2δ4 Is Required for Axonal Elaboration, Synaptic Transmission, and Wiring of Rod Photoreceptors

Neural circuit wiring relies on selective synapse formation whereby a presynaptic release apparatus is matched with its cognate postsynaptic machinery. At metabotropic synapses, the molecular mechanisms underlying this process are poorly understood. In the mammalian retina, rod photoreceptors form selective contacts with rod ON-bipolar cells by aligning the presynaptic voltage-gated Ca2+ channel directing glutamate release (CaV1.4) with postsynaptic mGluR6 receptors. We show this coordination requires an extracellular protein, α2δ4, which complexes with CaV1.4 and the rod synaptogenic mediator, ELFN1, for trans-synaptic alignment with mGluR6. Eliminating α2δ4 in mice abolishes rod synaptogenesis and synaptic transmission to rod ON-bipolar cells, and disrupts postsynaptic mGluR6 clustering. We further find that in rods, α2δ4 is crucial for organizing synaptic ribbons and setting CaV1.4 voltage sensitivity. In cones, α2δ4 is essential for CaV1.4 function, but is not required for ribbon organization, synaptogenesis, or synaptic transmission. These findings offer insights into retinal pathologies associated with α2δ4 dysfunction.

Dynamic association of calcium channel subunits at the cellular membrane

High voltage gated calcium channels (VGCCs) are composed of at least three subunits, one pore forming [Formula: see text]-subunit, an intracellular [Formula: see text]-variant, and a mostly extracellular [Formula: see text]-variant. Interactions between these subunits determine the kinetic properties of VGCCs. It is unclear whether these interactions are stable over time or rather transient. Here, we used single-molecule tracking to investigate the surface diffusion of [Formula: see text]- and [Formula: see text]-subunits at the cell surface. We found that [Formula: see text]-subunits show higher surface mobility than [Formula: see text]-subunits, and that they are only transiently confined together, suggesting a weak association between [Formula: see text]- and [Formula: see text]-subunits. Moreover, we observed that different [Formula: see text]-subunits engage in different degrees of association with the [Formula: see text]-subunit, revealing the tighter interaction of [Formula: see text] with [Formula: see text]. These data indicate a distinct regulation of the [Formula: see text] interaction in VGCC subtypes. We modeled their membrane dynamics in a Monte Carlo simulation using experimentally determined diffusion constants. Our modeling predicts that the ratio of associated [Formula: see text]- and [Formula: see text]-subunits mainly depends on their expression density and confinement in the membrane. Based on the different motilities of particular [Formula: see text]-subunit combinations, we propose that their dynamic assembly and disassembly represent an important mechanism to regulate the signaling properties of VGCC.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

Keeping Our Calcium in Balance to Maintain Our Balance

Calcium is a key signaling molecule and ion involved in a variety of diverse processes in our central nervous system (CNS) which include gene expression, synaptic transmission and plasticity, neuronal excitability and cell maintenance. Proper control of calcium signaling is not only vital for neuronal physiology but also cell survival. Mutations in fundamental channels, transporters and second messenger proteins involved in orchestrating the balance of our calcium homeostasis can lead to severe neurodegenerative disorders, such as Spinocerebellar (SCA) and Episodic (EA) ataxias. Hereditary ataxias make up a remarkably diverse group of neurological disorders clinically characterized by gait ataxia, nystagmus, dysarthria, trunk and limb ataxia and often atrophy of the cerebellum. The largest family of hereditary ataxias is SCAs which consists of a growing family of 42 members. A relatively smaller family of 8 members compose the EAs. The gene mutations responsible for half of the EA members and over 35 of the SCA subtypes have been identified, and several have been found to be responsible for cerebellar atrophy, abnormal intracellular calcium levels, dysregulation of Purkinje cell pacemaking, altered cerebellar synaptic transmission and/or ataxia in mouse models. Although the genetic diversity and affected cellular pathways of hereditary ataxias are broad, one common theme amongst these genes is their effects on maintaining calcium balance in primarily the cerebellum. There is emerging evidence that the pathogenesis of hereditary ataxias may be caused by imbalances in intracellular calcium due to genetic mutations in calcium-mediating proteins. In this review we will discuss the current evidence supporting the role of deranged calcium as the culprit to neurodegenerative diseases with a primary focus on SCAs and EAs.

ELKS controls the pool of readily releasable vesicles at excitatory synapses through its N-terminal coiled-coil domains

In a presynaptic nerve terminal, synaptic strength is determined by the pool of readily releasable vesicles (RRP) and the probability of release (P) of each RRP vesicle. These parameters are controlled at the active zone and vary across synapses, but how such synapse specific control is achieved is not understood. ELKS proteins are enriched at vertebrate active zones and enhance P at inhibitory hippocampal synapses, but ELKS functions at excitatory synapses are not known. Studying conditional knockout mice for ELKS, we find that ELKS enhances the RRP at excitatory synapses without affecting P. Surprisingly, ELKS C-terminal sequences, which interact with RIM, are dispensable for RRP enhancement. Instead, the N-terminal ELKS coiled-coil domains that bind to Liprin-α and Bassoon are necessary to control RRP. Thus, ELKS removal has differential, synapse-specific effects on RRP and P, and our findings establish important roles for ELKS N-terminal domains in synaptic vesicle priming.

High-throughput sequencing of the synaptome in major depressive disorder

Major depressive disorder (MDD) is among the leading causes of worldwide disability. Despite its significant heritability, large-scale genome-wide association studies (GWASs) of MDD have yet to identify robustly associated common variants. Although increased sample sizes are being amassed for the next wave of GWAS, few studies have as yet focused on rare genetic variants in the study of MDD. We sequenced the exons of 1742 synaptic genes previously identified by proteomic experiments. PLINK/SEQ was used to perform single variant, gene burden and gene set analyses. The GeneMANIA interaction database was used to identify protein-protein interaction-based networks. Cases were selected from a familial collection of early-onset, recurrent depression and were compared with screened controls. After extensive quality control, we analyzed 259 cases with familial, early-onset MDD and 334 controls. The distribution of association test statistics for the single variant and gene burden analyses were consistent with the null hypothesis. However, analysis of prioritized gene sets showed a significant association with damaging singleton variants in a Cav2-adaptor gene set (odds ratio=2.6; P=0.0008) that survived correction for all gene sets and annotation categories tested (empirical P=0.049). In addition, we also found statistically significant evidence for an enrichment of rare variants in a protein-based network of 14 genes involved in actin polymerization and dendritic spine formation (nominal P=0.0031). In conclusion, we have identified a statistically significant gene set and gene network of rare variants that are over-represented in MDD, providing initial evidence that calcium signaling and dendrite regulation may be involved in the etiology of depression.

Surface dynamics of voltage-gated ion channels

Neurons encode information in fast changes of the membrane potential, and thus electrical membrane properties are critically important for the integration and processing of synaptic inputs by a neuron. These electrical properties are largely determined by ion channels embedded in the membrane. The distribution of most ion channels in the membrane is not spatially uniform: they undergo activity-driven changes in the range of minutes to days. Even in the range of milliseconds, the composition and topology of ion channels are not static but engage in highly dynamic processes including stochastic or activity-dependent transient association of the pore-forming and auxiliary subunits, lateral diffusion, as well as clustering of different channels. In this review we briefly discuss the potential impact of mobile sodium, calcium and potassium ion channels and the functional significance of this for individual neurons and neuronal networks.

Proteomic analysis of native cerebellar iFGF14 complexes

Intracellular Fibroblast Growth Factor 14 (iFGF14) and the other intracellular FGFs (iFGF11-13) regulate the properties and densities of voltage-gated neuronal and cardiac Na(+) (Nav) channels. Recent studies have demonstrated that the iFGFs can also regulate native voltage-gated Ca(2+) (Cav) channels. In the present study, a mass spectrometry (MS)-based proteomic approach was used to identify the components of native cerebellar iFGF14 complexes. Using an anti-iFGF14 antibody, native iFGF14 complexes were immunoprecipitated from wild type adult mouse cerebellum. Parallel control experiments were performed on cerebellar proteins isolated from mice (Fgf14(-/-)) harboring a targeted disruption of the Fgf14 locus. MS analyses of immunoprecipitated proteins demonstrated that the vast majority of proteins identified in native cerebellar iFGF14 complexes are Nav channel pore-forming (α) subunits or proteins previously reported to interact with Nav α subunits. In contrast, no Cav channel α or accessory subunits were revealed in cerebellar iFGF14 immunoprecipitates. Additional experiments were completed using an anti-PanNav antibody to immunoprecipitate Nav channel complexes from wild type and Fgf14(-/-) mouse cerebellum. Western blot and MS analyses revealed that the loss of iFGF14 does not measurably affect the protein composition or the relative abundance of Nav channel interacting proteins in native adult mouse cerebellar Nav channel complexes.

Role of Bassoon and Piccolo in Assembly and Molecular Organization of the Active Zone

Bassoon and Piccolo are two very large scaffolding proteins of the cytomatrix assembled at the active zone (CAZ) where neurotransmitter is released. They share regions of high sequence similarity distributed along their entire length and seem to share both overlapping and distinct functions in organizing the CAZ. Here, we survey our present knowledge on protein-protein interactions and recent progress in understanding of molecular functions of these two giant proteins. These include roles in the assembly of active zones (AZ), the localization of voltage-gated Ca²⁺ channels (VGCCs) in the vicinity of release sites, synaptic vesicle (SV) priming and in the case of Piccolo, a role in the dynamic assembly of the actin cytoskeleton. Piccolo and Bassoon are also important for the maintenance of presynaptic structure and function, as well as for the assembly of CAZ specializations such as synaptic ribbons. Recent findings suggest that they are also involved in the regulation activity-dependent communication between presynaptic boutons and the neuronal nucleus. Together these observations suggest that Bassoon and Piccolo use their modular structure to organize super-molecular complexes essential for various aspects of presynaptic function.

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

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

RIM-BPs Mediate Tight Coupling of Action Potentials to Ca(2+)-Triggered Neurotransmitter Release

Ultrafast neurotransmitter release requires tight colocalization of voltage-gated Ca(2+) channels with primed, release-ready synaptic vesicles at the presynaptic active zone. RIM-binding proteins (RIM-BPs) are multidomain active zone proteins that bind to RIMs and to Ca(2+) channels. In Drosophila, deletion of RIM-BPs dramatically reduces neurotransmitter release, but little is known about RIM-BP function in mammalian synapses. Here, we generated double conditional knockout mice for RIM-BP1 and RIM-BP2, and analyzed RIM-BP-deficient synapses in cultured hippocampal neurons and the calyx of Held. Surprisingly, we find that in murine synapses, RIM-BPs are not essential for neurotransmitter release as such, but are selectively required for high-fidelity coupling of action potential-induced Ca(2+) influx to Ca(2+)-stimulated synaptic vesicle exocytosis. Deletion of RIM-BPs decelerated action-potential-triggered neurotransmitter release and rendered it unreliable, thereby impairing the fidelity of synaptic transmission. Thus, RIM-BPs ensure optimal organization of the machinery for fast release in mammalian synapses without being a central component of the machinery itself.

Modular composition and dynamics of native GABAB receptors identified by high-resolution proteomics

GABAB receptors, the most abundant inhibitory G protein-coupled receptors in the mammalian brain, display pronounced diversity in functional properties, cellular signaling and subcellular distribution. We used high-resolution functional proteomics to identify the building blocks of these receptors in the rodent brain. Our analyses revealed that native GABAB receptors are macromolecular complexes with defined architecture, but marked diversity in subunit composition: the receptor core is assembled from GABAB1a/b, GABAB2, four KCTD proteins and a distinct set of G-protein subunits, whereas the receptor's periphery is mostly formed by transmembrane proteins of different classes. In particular, the periphery-forming constituents include signaling effectors, such as Cav2 and HCN channels, and the proteins AJAP1 and amyloid-β A4, both of which tightly associate with the sushi domains of GABAB1a. Our results unravel the molecular diversity of GABAB receptors and their postnatal assembly dynamics and provide a roadmap for studying the cellular signaling of this inhibitory neurotransmitter receptor.

Knock-down of synapsin alters cell excitability and action potential waveform by potentiating BK and voltage-gated Ca(2+) currents in Helix serotonergic neurons

Synapsins (Syns) are an evolutionarily conserved family of presynaptic proteins crucial for the fine-tuning of synaptic function. A large amount of experimental evidences has shown that Syns are involved in the development of epileptic phenotypes and several mutations in Syn genes have been associated with epilepsy in humans and animal models. Syn mutations induce alterations in circuitry and neurotransmitter release, differentially affecting excitatory and inhibitory synapses, thus causing an excitation/inhibition imbalance in network excitability toward hyperexcitability that may be a determinant with regard to the development of epilepsy. Another approach to investigate epileptogenic mechanisms is to understand how silencing Syn affects the cellular behavior of single neurons and is associated with the hyperexcitable phenotypes observed in epilepsy. Here, we examined the functional effects of antisense-RNA inhibition of Syn expression on individually identified and isolated serotonergic cells of the Helix land snail. We found that Helix synapsin silencing increases cell excitability characterized by a slightly depolarized resting membrane potential, decreases the rheobase, reduces the threshold for action potential (AP) firing and increases the mean and instantaneous firing rates, with respect to control cells. The observed increase of Ca(2+) and BK currents in Syn-silenced cells seems to be related to changes in the shape of the AP waveform. These currents sustain the faster spiking in Syn-deficient cells by increasing the after hyperpolarization and limiting the Na(+) and Ca(2+) channel inactivation during repetitive firing. This in turn speeds up the depolarization phase by reaching the AP threshold faster. Our results provide evidence that Syn silencing increases intrinsic cell excitability associated with increased Ca(2+) and Ca(2+)-dependent BK currents in the absence of excitatory or inhibitory inputs.

Molecular Basis of Regulating High Voltage-Activated Calcium Channels by S-Nitrosylation

Nitric oxide (NO) is involved in a variety of physiological processes, such as vasoregulation and neurotransmission, and has a complex role in the regulation of pain transduction and synaptic transmission. We have shown previously that NO inhibits high voltage-activated Ca(2+) channels in primary sensory neurons and excitatory synaptic transmission in the spinal dorsal horn. However, the molecular mechanism involved in this inhibitory action remains unclear. In this study, we investigated the role of S-nitrosylation in the NO regulation of high voltage-activated Ca(2+) channels. The NO donor S-nitroso-N-acetyl-DL-penicillamine (SNAP) rapidly reduced N-type currents when Cav2.2 was coexpressed with the Cavβ1 or Cavβ3 subunits in HEK293 cells. In contrast, SNAP only slightly inhibited P/Q-type and L-type currents reconstituted with various Cavβ subunits. SNAP caused a depolarizing shift in voltage-dependent N-type channel activation, but it had no effect on Cav2.2 protein levels on the membrane surface. The inhibitory effect of SNAP on N-type currents was blocked by the sulfhydryl-specific modifying reagent methanethiosulfonate ethylammonium. Furthermore, the consensus motifs of S-nitrosylation were much more abundant in Cav2.2 than in Cav1.2 and Cav2.1. Site-directed mutagenesis studies showed that Cys-805, Cys-930, and Cys-1045 in the II-III intracellular loop, Cys-1835 and Cys-2145 in the C terminus of Cav2.2, and Cys-346 in the Cavβ3 subunit were nitrosylation sites mediating NO sensitivity of N-type channels. Our findings demonstrate that the consensus motifs of S-nitrosylation in cytoplasmically accessible sites are critically involved in post-translational regulation of N-type Ca(2+) channels by NO. S-Nitrosylation mediates the feedback regulation of N-type channels by NO.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Genetic disruption of voltage-gated calcium channels in psychiatric and neurological disorders

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Voltage-gated calcium channel classification—genes and proteins.

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Genetic analysis of neuropsychiatric syndromes.

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Calcium channel genes identified from GWA studies of psychiatric disorders.

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Rare mutations in calcium channel genes in psychiatric disorders.

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Pathophysiological sequelae of CACNA1C mutations and polymorphisms.

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Monogenic disorders resulting from harmful mutations in other voltage-gated calcium channel genes.

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Changes in calcium channel gene expression in disease.

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Involvement of voltage-gated calcium channels in early brain development.

This review summarises genetic studies in which calcium channel genes have been connected to the spectrum of neuropsychiatric syndromes, from bipolar disorder and schizophrenia to autism spectrum disorders and intellectual impairment. Among many other genes, striking numbers of the calcium channel gene superfamily have been implicated in the aetiology of these diseases by various DNA analysis techniques. We will discuss how these relate to the known monogenic disorders associated with point mutations in calcium channels. We will then examine the functional evidence for a causative link between these mutations or single nucleotide polymorphisms and the disease processes. A major challenge for the future will be to translate the expanding psychiatric genetic findings into altered physiological function, involvement in the wider pathology of the diseases, and what potential that provides for personalised and stratified treatment options for patients.

Cardiac ion channels

Ion channels are critical for all aspects of cardiac function, including rhythmicity and contractility. Consequently, ion channels are key targets for therapeutics aimed at cardiac pathophysiologies such as atrial fibrillation or angina. At the same time, off-target interactions of drugs with cardiac ion channels can be the cause of unwanted side effects. This manuscript aims to review the physiology and pharmacology of key cardiac ion channels. The intent is to highlight recent developments for therapeutic development, as well as elucidate potential mechanisms for drug-induced cardiac side effects, rather than present an in-depth review of each channel subtype.

Inhibitory and excitatory axon terminals share a common nano-architecture of their Cav2.1 (P/Q-type) Ca(2+) channels

Tuning of the time course and strength of inhibitory and excitatory neurotransmitter release is fundamental for the precise operation of cortical network activity and is controlled by Ca²⁺ influx into presynaptic terminals through the high voltage-activated P/Q-type Ca²⁺ (Ca_v2.1) channels. Proper channel-mediated Ca²⁺-signaling critically depends on the topographical arrangement of the channels in the presynaptic membrane. Here, we used high-resolution SDS-digested freeze-fracture replica immunoelectron microscopy together with automatized computational analysis of Ca_v2.1 immunogold labeling to determine the precise subcellular organization of Ca_v2.1 channels in both inhibitory and excitatory terminals. Immunoparticles labeling the pore-forming α1 subunit of Ca_v2.1 channels were enriched over the active zone of the boutons with the number of channels (3–62) correlated with the area of the synaptic membrane. Detailed analysis showed that Ca_v2.1 channels are non-uniformly distributed over the presynaptic membrane specialization where they are arranged in clusters of an average five channels per cluster covering a mean area with a diameter of about 70 nm. Importantly, clustered arrangement and cluster properties did not show any significant difference between GABAergic and glutamatergic terminals. Our data demonstrate a common nano-architecture of Ca_v2.1 channels in inhibitory and excitatory boutons in stratum radiatum of the hippocampal CA1 area suggesting that the cluster arrangement is crucial for the precise release of transmitters from the axonal boutons.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.

GABAB receptor deficiency causes failure of neuronal homeostasis in hippocampal networks

Stabilization of neuronal activity by homeostatic control systems is fundamental for proper functioning of neural circuits. Failure in neuronal homeostasis has been hypothesized to underlie common pathophysiological mechanisms in a variety of brain disorders. However, the key molecules regulating homeostasis in central mammalian neural circuits remain obscure. Here, we show that selective inactivation of GABAB, but not GABA(A), receptors impairs firing rate homeostasis by disrupting synaptic homeostatic plasticity in hippocampal networks. Pharmacological GABA(B) receptor (GABA(B)R) blockade or genetic deletion of the GB(1a) receptor subunit disrupts homeostatic regulation of synaptic vesicle release. GABA(B)Rs mediate adaptive presynaptic enhancement to neuronal inactivity by two principle mechanisms: First, neuronal silencing promotes syntaxin-1 switch from a closed to an open conformation to accelerate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex assembly, and second, it boosts spike-evoked presynaptic calcium flux. In both cases, neuronal inactivity removes tonic block imposed by the presynaptic, GB(1a)-containing receptors on syntaxin-1 opening and calcium entry to enhance probability of vesicle fusion. We identified the GB(1a) intracellular domain essential for the presynaptic homeostatic response by tuning intermolecular interactions among the receptor, syntaxin-1, and the Ca(V)2.2 channel. The presynaptic adaptations were accompanied by scaling of excitatory quantal amplitude via the postsynaptic, GB(1b)-containing receptors. Thus, GABA(B)Rs sense chronic perturbations in GABA levels and transduce it to homeostatic changes in synaptic strength. Our results reveal a novel role for GABA(B)R as a key regulator of population firing stability and propose that disruption of homeostatic synaptic plasticity may underlie seizure's persistence in the absence of functional GABA(B)Rs.

Modulation of nociceptive ion channels and receptors via protein-protein interactions: implications for pain relief

In the last 2 decades biomedical research has provided great insights into the molecular signatures underlying painful conditions. However, chronic pain still imposes substantial challenges to researchers, clinicians and patients alike. Under pathological conditions, pain therapeutics often lack efficacy and exhibit only minimal safety profiles, which can be largely attributed to the targeting of molecules with key physiological functions throughout the body. In light of these difficulties, the identification of molecules and associated protein complexes specifically involved in chronic pain states is of paramount importance for designing selective interventions. Ion channels and receptors represent primary targets, as they critically shape nociceptive signaling from the periphery to the brain. Moreover, their function requires tight control, which is usually implemented by protein-protein interactions (PPIs). Indeed, manipulation of such PPIs entails the modulation of ion channel activity with widespread implications for influencing nociceptive signaling in a more specific way. In this review, we highlight recent advances in modulating ion channels and receptors via their PPI networks in the pursuit of relieving chronic pain. Moreover, we critically discuss the potential of targeting PPIs for developing novel pain therapies exhibiting higher efficacy and improved safety profiles.

An Exclusion Zone for Ca2+ Channels around Docked Vesicles Explains Release Control by Multiple Channels at a CNS Synapse

The spatial arrangement of Ca²⁺ channels and vesicles remains unknown for most CNS synapses, despite of the crucial importance of this geometrical parameter for the Ca²⁺ control of transmitter release. At a large model synapse, the calyx of Held, transmitter release is controlled by several Ca²⁺ channels in a "domain overlap" mode, at least in young animals. To study the geometrical constraints of Ca²⁺ channel placement in domain overlap control of release, we used stochastic MCell modelling, at active zones for which the position of docked vesicles was derived from electron microscopy (EM). We found that random placement of Ca²⁺ channels was unable to produce high slope values between release and presynaptic Ca²⁺ entry, a hallmark of domain overlap, and yielded excessively large release probabilities. The simple assumption that Ca²⁺ channels can be located anywhere at active zones, except below a critical distance of ~ 30 nm away from docked vesicles ("exclusion zone"), rescued high slope values and low release probabilities. Alternatively, high slope values can also be obtained by placing all Ca²⁺ channels into a single supercluster, which however results in significantly higher heterogeneity of release probabilities. We also show experimentally that high slope values, and the sensitivity to the slow Ca²⁺ chelator EGTA-AM, are maintained with developmental maturation of the calyx synapse. Taken together, domain overlap control of release represents a highly organized active zone architecture in which Ca²⁺ channels must obey a certain distance to docked vesicles. Furthermore, domain overlap can be employed by near-mature, fast-releasing synapses.

Ca²⁺ channels provide the rise in intracellular Ca²⁺ concentration necessary to initiate the membrane fusion of transmitter—filled vesicles at synapses. Because Ca²⁺ diffuses away from Ca²⁺ channels, the distance between Ca²⁺ channels and vesicles on the range of tens of nanometers is a crucial determinant of the vesicle fusion probability. However, there is still little experimental evidence on how Ca²⁺ channels and vesicles co-localize in the nanospace of a single synapse. We show by computational modelling that the channels should be located at some distance to vesicles (~ 30 nm), to allow for release control by several channels, a release mechanism found at many synapses. In realistic synapses with a high density of docked vesicles, this translates into a likely localization of Ca²⁺ channels at membrane sites not occupied by docked vesicles. Thus, we present a computational model of how Ca²⁺ channels can be localized in an active zone with several docked vesicles, to enable control of release by several Ca²⁺ channels.

Proteogenomics of the human hippocampus: The road ahead

The hippocampus is one of the most essential components of the human brain and plays an important role in learning and memory. The hippocampus has drawn great attention from scientists and clinicians due to its clinical importance in diseases such as Alzheimer's disease (AD), non-AD dementia, and epilepsy. Understanding the function of the hippocampus and related disease mechanisms requires comprehensive knowledge of the orchestration of the genome, epigenome, transcriptome, proteome, and post-translational modifications (PTMs) of proteins. The past decade has seen remarkable advances in the high-throughput sequencing techniques that are collectively called next generation sequencing (NGS). NGS enables the precise analysis of gene expression profiles in cells and tissues, allowing powerful and more feasible integration of expression data from the gene level to the protein level, even allowing "-omic" level assessment of PTMs. In addition, improved bioinformatics algorithms coupled with NGS technology are finally opening a new era for scientists to discover previously unidentified and elusive proteins. In the present review, we will focus mainly on the proteomics of the human hippocampus with an emphasis on the integrated analysis of genomics, epigenomics, transcriptomics, and proteomics. Finally, we will discuss our perspectives on the potential and future of proteomics in the field of hippocampal biology. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Quantitative proteomics reveals protein-protein interactions with fibroblast growth factor 12 as a component of the voltage-gated sodium channel 1.2 (nav1.2) macromolecular complex in Mammalian brain

Voltage-gated sodium channels (Nav1.1–Nav1.9) are responsible for the initiation and propagation of action potentials in neurons, controlling firing patterns, synaptic transmission and plasticity of the brain circuit. Yet, it is the protein–protein interactions of the macromolecular complex that exert diverse modulatory actions on the channel, dictating its ultimate functional outcome. Despite the fundamental role of Nav channels in the brain, information on its proteome is still lacking. Here we used affinity purification from crude membrane extracts of whole brain followed by quantitative high-resolution mass spectrometry to resolve the identity of Nav1.2 protein interactors. Of the identified putative protein interactors, fibroblast growth factor 12 (FGF12), a member of the nonsecreted intracellular FGF family, exhibited 30-fold enrichment in Nav1.2 purifications compared with other identified proteins. Using confocal microscopy, we visualized native FGF12 in the brain tissue and confirmed that FGF12 forms a complex with Nav1.2 channels at the axonal initial segment, the subcellular specialized domain of neurons required for action potential initiation. Co-immunoprecipitation studies in a heterologous expression system validate Nav1.2 and FGF12 as interactors, whereas patch-clamp electrophysiology reveals that FGF12 acts synergistically with CaMKII, a known kinase regulator of Nav channels, to modulate Nav1.2-encoded currents. In the presence of CaMKII inhibitors we found that FGF12 produces a bidirectional shift in the voltage-dependence of activation (more depolarized) and the steady-state inactivation (more hyperpolarized) of Nav1.2, increasing the channel availability. Although providing the first characterization of the Nav1.2 CNS proteome, we identify FGF12 as a new functionally relevant interactor. Our studies will provide invaluable information to parse out the molecular determinant underlying neuronal excitability and plasticity, and extending the relevance of iFGFs signaling in the normal and diseased brain.

More than a pore: ion channel signaling complexes

Voltage- and ligand-gated ion channels form the molecular basis of cellular excitability. With >400 members and accounting for ∼1.5% of the human genome, ion channels are some of the most well studied of all proteins in heterologous expression systems. Yet, ion channels often exhibit unexpected properties in vivo because of their interaction with a variety of signaling/scaffolding proteins. Such interactions can influence the function and localization of ion channels, as well as their coupling to intracellular second messengers and pathways, thus increasing the signaling potential of these ion channels in neurons. Moreover, functions have been ascribed to ion channels that are largely independent of their ion-conducting roles. Molecular and functional dissection of the ion channel proteome/interactome has yielded new insights into the composition of ion channel complexes and how their dysregulation leads to human disease.

Mechanisms of fast desensitization of GABA(B) receptor-gated currents

GABA(B) receptors (GABA(B)Rs) regulate the excitability of most neurons in the central nervous system by modulating the activity of enzymes and ion channels. In the sustained presence of the neurotransmitter γ-aminobutyric acid, GABA(B)Rs exhibit a time-dependent decrease in the receptor response-a phenomenon referred to as homologous desensitization. Desensitization prevents excessive receptor influences on neuronal activity. Much work focused on the mechanisms of GABA(B)R desensitization that operate at the receptor and control receptor expression at the plasma membrane. Over the past few years, it became apparent that GABA(B)Rs additionally evolved mechanisms for faster desensitization. These mechanisms operate at the G protein rather than at the receptor and inhibit G protein signaling within seconds of agonist exposure. The mechanisms for fast desensitization are ideally suited to regulate receptor-activated ion channel responses, which influence neuronal activity on a faster timescale than effector enzymes. Here, we provide an update on the mechanisms for fast desensitization of GABA(B)R responses and discuss physiological and pathophysiological implications.

Extracellular regulation of type IIa receptor protein tyrosine phosphatases: mechanistic insights from structural analyses

The receptor protein tyrosine phosphatases (RPTPs) exhibit a wide repertoire of cellular signalling functions. In particular, type IIa RPTP family members have recently been highlighted as hubs for extracellular interactions in neurons, regulating neuronal extension and guidance, as well as synaptic organisation. In this review, we will discuss the recent progress of structural biology investigations into the architecture of type IIa RPTP ectodomains and their interactions with extracellular ligands. Structural insights, in combination with biophysical and cellular studies, allow us to begin to piece together molecular mechanisms for the transduction and integration of type IIa RPTP signals and to propose hypotheses for future experimental validation.

The synapse in schizophrenia

It has been several decades since synaptic dysfunction was first suggested to play a role in schizophrenia, but only in the last few years has convincing evidence been obtained as progress has been made in elucidating the genetic underpinnings of the disorder. In the intervening years much has been learned concerning the complex macromolecular structure of the synapse itself, and genetic studies are now beginning to draw upon these advances. Here we outline our current understanding of the genetic architecture of schizophrenia and examine the evidence for synaptic involvement. A strong case can now be made that disruption of glutamatergic signalling pathways regulating synaptic plasticity contributes to the aetiology of schizophrenia.

Synaptic vesicle glycoprotein 2A modulates vesicular release and calcium channel function at peripheral sympathetic synapses

Synaptic vesicle glycoprotein (SV)2A is a transmembrane protein found in secretory vesicles and is critical for Ca(2+) -dependent exocytosis in central neurons, although its mechanism of action remains uncertain. Previous studies have proposed, variously, a role of SV2 in the maintenance and formation of the readily releasable pool (RRP) or in the regulation of Ca(2+) responsiveness of primed vesicles. Such previous studies have typically used genetic approaches to ablate SV2 levels; here, we used a strategy involving small interference RNA (siRNA) injection to knockdown solely presynaptic SV2A levels in rat superior cervical ganglion (SCG) neuron synapses. Moreover, we investigated the effects of SV2A knockdown on voltage-dependent Ca(2+) channel (VDCC) function in SCG neurons. Thus, we extended the studies of SV2A mechanisms by investigating the effects on vesicular transmitter release and VDCC function in peripheral sympathetic neurons. We first demonstrated an siRNA-mediated SV2A knockdown. We showed that this SV2A knockdown markedly affected presynaptic function, causing an attenuated RRP size, increased paired-pulse depression and delayed RRP recovery after stimulus-dependent depletion. We further demonstrated that the SV2A-siRNA-mediated effects on vesicular release were accompanied by a reduction in VDCC current density in isolated SCG neurons. Together, our data showed that SV2A is required for correct transmitter release at sympathetic neurons. Mechanistically, we demonstrated that presynaptic SV2A: (i) acted to direct normal synaptic transmission by maintaining RRP size, (ii) had a facilitatory role in recovery from synaptic depression, and that (iii) SV2A deficits were associated with aberrant Ca(2+) current density, which may contribute to the secretory phenotype in sympathetic peripheral neurons.

Preassociated apocalmodulin mediates Ca2+-dependent sensitization of activation and inactivation of TMEM16A/16B Ca2+-gated Cl- channels

Ca(2+)-activated chloride currents carried via transmembrane proteins TMEM16A and TMEM16B regulate diverse processes including mucus secretion, neuronal excitability, smooth muscle contraction, olfactory signal transduction, and cell proliferation. Understanding how TMEM16A/16B are regulated by Ca(2+) is critical for defining their (patho)/physiological roles and for rationally targeting them therapeutically. Here, using a bioengineering approach--channel inactivation induced by membrane-tethering of an associated protein (ChIMP)--we discovered that Ca(2+)-free calmodulin (apoCaM) is preassociated with TMEM16A/16B channel complexes. The resident apoCaM mediates two distinct Ca(2+)-dependent effects on TMEM16A, as revealed by expression of dominant-negative CaM1234. These effects are Ca(2+)-dependent sensitization of activation (CDSA) and Ca(2+)-dependent inactivation (CDI). CDI and CDSA are independently mediated by the N and C lobes of CaM, respectively. TMEM16A alternative splicing provides a mechanism for tuning apoCaM effects. Channels lacking splice segment b selectively lost CDI, and segment a is necessary for apoCaM preassociation with TMEM16A. The results reveal multidimensional regulation of TMEM16A/16B by preassociated apoCaM and introduce ChIMP as a versatile tool to probe the macromolecular complex and function of Ca(2+)-activated chloride channels.