Splice variants of the CaV1.3 L-type calcium channel regulate dendritic spine morphology

Potassium and calcium channels in different nerve cells act as therapeutic targets in neurological disorders

The central nervous system, information integration center of the body, is mainly composed of neurons and glial cells. The neuron is one of the most basic and important structural and functional units of the central nervous system, with sensory stimulation and excitation conduction functions. Astrocytes and microglia belong to the glial cell family, which is the main source of cytokines and represents the main defense system of the central nervous system. Nerve cells undergo neurotransmission or gliotransmission, which regulates neuronal activity via the ion channels, receptors, or transporters expressed on nerve cell membranes. Ion channels, composed of large transmembrane proteins, play crucial roles in maintaining nerve cell homeostasis. These channels are also important for control of the membrane potential and in the secretion of neurotransmitters. A variety of cellular functions and life activities, including functional regulation of the central nervous system, the generation and conduction of nerve excitation, the occurrence of receptor potential, heart pulsation, smooth muscle peristalsis, skeletal muscle contraction, and hormone secretion, are closely related to ion channels associated with passive transmembrane transport. Two types of ion channels in the central nervous system, potassium channels and calcium channels, are closely related to various neurological disorders, including Alzheimer's disease, Parkinson's disease, and epilepsy. Accordingly, various drugs that can affect these ion channels have been explored deeply to provide new directions for the treatment of these neurological disorders. In this review, we focus on the functions of potassium and calcium ion channels in different nerve cells and their involvement in neurological disorders such as Parkinson's disease, Alzheimer's disease, depression, epilepsy, autism, and rare disorders. We also describe several clinical drugs that target potassium or calcium channels in nerve cells and could be used to treat these disorders. We concluded that there are few clinical drugs that can improve the pathology these diseases by acting on potassium or calcium ions. Although a few novel ion-channel-specific modulators have been discovered, meaningful therapies have largely not yet been realized. The lack of target-specific drugs, their requirement to cross the blood-brain barrier, and their exact underlying mechanisms all need further attention. This review aims to explain the urgent problems that need research progress and provide comprehensive information aiming to arouse the research community's interest in the development of ion channel-targeting drugs and the identification of new therapeutic targets for that can increase the cure rate of nervous system diseases and reduce the occurrence of adverse reactions in other systems.

Deletion of the voltage-gated calcium channel gene, Ca V 1.3, reduces Purkinje cell dendritic complexity without altering cerebellar-mediated eyeblink conditioning

Genetic variation in CACNA1D , the gene that encodes the pore-forming subunit of the L-type calcium channel Ca V 1.3, has been associated with increased risk for neuropsychiatric disorders that display abnormalities in cerebellar structures. We sought to clarify if deletion of Ca V 1.3 in mice would induce abnormalities in cerebellar cortex cytoarchitecture or synapse morphology. Since Ca V 1.3 is highly expressed in cerebellar molecular layer interneurons (MLIs) and L-type channels appear to regulate GABA release from MLIs, we hypothesized that loss of Ca V 1.3 would alter GABAergic synapses between MLIs and Purkinje cells (PCs) without altering MLI numbers or PC structure. As expected, we did not observe changes in the numbers of MLIs or PCs. Surprisingly, Ca V 1.3 KO mice do have decreased complexity of PC dendritic arbors without differences in the number or structure of GABAergic synapses onto PCs. Loss of Ca V 1.3 was not associated with impaired acquisition of delay eyeblink conditioning. Therefore, our data suggest that Ca V 1.3 expression is important for PC structure but does not affect other measures of cerebellar cortex morphology or cerebellar function as assessed by delay eyeblink conditioning.

Activity dependent Clustering of Neuronal L-Type Calcium Channels by CaMKII

Neuronal excitation-transcription (E-T) coupling pathways can be initiated by local increases of Ca2+ concentrations within a nanodomain close to the L-type voltage-gated Ca2+ channel (LTCC). However, molecular mechanisms controlling LTCC organization within the plasma membrane that help creation these localized signaling domains remain poorly characterized. Here, we report that neuronal depolarization increases CaV1.3 LTCC clustering in cultured hippocampal neurons. Our previous work showed that binding of the activated catalytic domain of Ca2+/calmodulin-dependent protein kinase II (CaMKII) to an RKR motif in the N-terminal cytoplasmic domain of CaV1.3 is required for LTCC-mediated E-T coupling. We tested whether multimeric CaMKIIα holoenzymes can bind simultaneously to co-expressed CaV1.3 α1 subunits with two different epitope tags. Co-immunoprecipitation assays from HEK293T cell lysates revealed that CaMKIIα assembles multimeric CaV1.3 LTCC complexes in a Ca2+/calmodulin-dependent manner. CaMKII-dependent assembly of multi-CaV1.3 complexes is further facilitated by co-expression of the CaMKII-binding LTCC β2a subunit, relative to the β3 subunit, which cannot bind directly to CaMKII. Moreover, clustering of surface localized CaV1.3 α1 subunits in intact HEK293 cells was increased by pharmacological LTCC activation, but only in the presence of co-expressed wild-type CaMKIIα. Moreover, depolarization-induced clustering of surface-expressed CaV1.3 LTCCs in cultured hippocampal neurons was disrupted by suppressing the expression of CaMKIIα and CaMKIIβ using shRNAs. The CaMKII-binding RKR motif is conserved in the N-terminal domain of CaV1.2 α1 subunits and we found that activated CaMKIIα promoted the assembly of CaV1.2 homomeric complexes, as well as CaV1.3-CaV1.2 heteromeric complexes in vitro. Furthermore, neuronal depolarization enhanced the clustering of surface-expressed CaV1.2 LTCCs, and enhanced the colocalization of endogenous CaV1.2 LTCCs with surface-expressed CaV1.3, by CaMKII-dependent mechanisms. This work indicates that CaMKII activation-dependent LTCC clustering in the plasma membrane following neuronal depolarization may be essential for the initiation of a specific long-range signal to activate gene expression.

A biallelic mutation in CACNA2D2 associated with developmental and epileptic encephalopathy affects calcium channel-dependent as well as synaptic functions of α2δ-2

α2δ proteins serve as auxiliary subunits of voltage-gated calcium channels and regulate channel membrane expression and current properties. Besides their channel function, α2δ proteins regulate synapse formation, differentiation, and synaptic wiring. Considering these important functions, it is not surprising that CACNA2D1-4, the genes encoding for α2δ-1 to -4 isoforms, have been implicated in neurological, neurodevelopmental, and neuropsychiatric disorders. Mutations in CACNA2D2 have been associated with developmental and epileptic encephalopathy (DEE) and cerebellar atrophy. In our present study, we performed a detailed functional characterization of the p.R593P mutation in α2δ-2, a homozygous mutation previously identified in two siblings with DEE. Importantly, we analyzed both calcium channel-dependent as well as synaptic functions of α2δ-2. Our data show that the corresponding p.R596P mutation in mouse α2δ-2 drastically decreases membrane expression and synaptic targeting of α2δ-2. This defect correlates with altered biophysical properties of postsynaptic CaV1.3 channel but has no effect on presynaptic CaV2.1 channels upon heterologous expression in tsA201 cells. However, homologous expression of α2δ-2_R596P in primary cultures of hippocampal neurons affects the ability of α2δ-2 to induce a statistically significant increase in the presynaptic abundance of endogenous CaV2.1 channels and presynaptic calcium transients. Moreover, our data demonstrate that in addition to lowering membrane expression, the p.R596P mutation reduces the trans-synaptic recruitment of GABAA receptors and presynaptic synapsin clustering in glutamatergic synapses. Lastly, the α2δ-2_R596P reduces the amplitudes of glutamatergic miniature postsynaptic currents in transduced hippocampal neurons. Taken together, our data strongly link the human biallelic p.R593P mutation to the underlying severe neurodevelopmental disorder and highlight the importance of studying α2δ mutations not only in the context of channelopathies but also synaptopathies.

Inactivation induced by pathogenic Cav1.3 L-type Ca2+-channel variants enhances sensitivity for dihydropyridine Ca2+ channel blockers

BACKGROUND AND PURPOSE

Pathogenic gain-of-function mutations in Cav1.3 L-type voltage-gated Ca2+-channels (CACNA1D) cause neurodevelopmental disorders with or without endocrine symptoms. We aimed to confirm a pathogenic gain-of function phenotype of CACNA1D de novo missense mutations A749T and L271H, and investigated the molecular mechanism causing their enhanced sensitivity for the Ca2+-channel blocker isradipine, a potential therapeutic for affected patients.

EXPERIMENTAL APPROACH

Wildtype and mutant channels were expressed in tsA-201 cells and their gating analysed using whole-cell and single-channel patch-clamp recordings. The voltage-dependence of isradipine action was quantified using protocols inducing variable fractions of inactivated channels. The molecular basis for altered channel gating in the mutants was investigated using in silico modelling and molecular dynamics simulations.

KEY RESULTS

Both mutations were confirmed pathogenic due to characteristic shifts of voltage-dependent activation and inactivation towards negative potentials (~20 mV). At negative holding potentials both mutations showed significantly higher isradipine sensitivity compared to wildtype. The affinity for wildtype and mutant channels increased with channel inactivation as predicted by the modulated receptor hypothesis (30- to 40-fold). The IC50 was indistinguishable for wildtype and mutants when >50% of channels were inactivated.

CONCLUSIONS AND IMPLICATIONS

Mutations A749T and L271H induce pathogenic gating changes. Like wildtype, isradipine inhibition is strongly voltage-dependent. Our data explains their apparent higher drug sensitivity at a given negative voltage by the availability of more inactivated channels due to their more negative inactivation voltage range. Low nanomolar isradipine concentrations will only inhibit Cav1.3 channels in neurons during prolonged depolarized states without selectivity for mutant channels.

A Novel De Novo Gain-of-Function CACNA1D Variant in Neurodevelopmental Disease With Congenital Tremor, Seizures, and Hypotonia

Background and Objectives

De novo gain-of-function variants in the CACNA1D gene, encoding the L-type voltage-gated Ca2+ channel CaV1.3, cause a multifaceted syndrome. Patients show variable degrees of autism spectrum disorder, developmental delay, epilepsy, and other neurologic and endocrine abnormalities (primary aldosteronism and/or hyperinsulinemic hypoglycemia). We study here a novel variant [c.3506G>A, NM_000720.4, p.(G1169D)] in 2 children with the same CACNA1D mutation but different disease severity.

Methods

The clinical data of the study patients were collected. After molecular analysis and cloning by site-directed mutagenesis, patch-clamp recordings of transfected tsA201 cells were conducted in whole-cell configuration. The functional effects of wild-type and mutated channels were analyzed.

Results

One child is a severely affected boy with a novel de novo CACNA1D variant with additional clinical symptoms including prenatal-onset tremor, congenital respiratory insufficiency requiring continuous positive airway pressure ventilation, and sensorineural deafness. Despite episodes of hypoglycemia, insulin levels were normal. Aldosterone:renin ratios as a screening parameter for primary aldosteronism were variable. In the second patient, putative mosaicism of the p.(G1169D) variant was associated with a less severe phenotype. Patch-clamp electrophysiology of the p.(G1169D) variant in a heterologous expression system revealed pronounced activity-enhancing gating changes, including a shift of channel activation and inactivation to more hyperpolarized potentials, as well as impaired channel inactivation and deactivation. Despite retained sensitivity to the Ca2+ channel blocker isradipine in vitro, no beneficial effects of isradipine or nifedipine treatment were observed in the index case.

Discussion

Through this report, we expand the knowledge about the disease presentation in patients with CACNA1D variants and show the novel variant's modulatory effects on CaV1.3 gating.

CaM-dependent modulation of human CaV1.3 whole-cell and single-channel currents by C-terminal CaMKII phosphorylation site S1475

Phosphorylation enables rapid modulation of voltage-gated calcium channels (VGCC) in physiological and pathophysiological conditions. How phosphorylation modulates human CaV1.3 VGCC, however, is largely unexplored. We characterized modulation of CaV1.3 gating via S1475, the human equivalent of a phosphorylation site identified in the rat. S1475 is highly conserved in CaV1.3 but absent from all other high-voltage activating calcium channel types co-expressed with CaV1.3 in similar tissues. Further, it is located in the C-terminal EF-hand motif, which binds calmodulin (CaM). This is involved in calcium-dependent channel inactivation (CDI). We used amino acid exchanges that mimic either sustained phosphorylation (S1475D) or phosphorylation resistance (S1475A). Whole-cell and single-channel recordings of phosphorylation state imitating CaV1.3 variants in transiently transfected HEK-293 cells revealed functional relevance of S1475 in human CaV1.3. We obtained three main findings: (1) CaV1.3_S1475D, imitating sustained phosphorylation, displayed decreased current density, reduced CDI and (in-) activation kinetics shifted to more depolarized voltages compared with both wildtype CaV1.3 and the phosphorylation-resistant CaV1.3_S1475A variant. Corresponding to the decreased current density, we find a reduced open probability of CaV1.3_S1475D at the single-channel level. (2) Using CaM overexpression or depletion, we find that CaM is necessary for modulating CaV1.3 through S1475. (3) CaMKII activation led to CaV1.3_WT-current properties similar to those of CaV1.3_S1475D, but did not affect CaV1.3_S1475A, confirming that CaMKII modulates human CaV1.3 via S1475. Given the physiological and pathophysiological importance of CaV1.3, our findings on the S1475-mediated interplay of phosphorylation, CaM interaction and CDI provide hints for approaches on specific CaV1.3 modulation under physiological and pathophysiological conditions. KEY POINTS: Phosphorylation modulates activity of voltage-gated L-type calcium channels for specific cellular needs but is largely unexplored for human CaV1.3 channels. Here we report that S1475, a CaMKII phosphorylation site identified in rats, is functionally relevant in human CaV1.3. Imitating phosphorylation states at S1475 alters current density and inactivation in a calmodulin-dependent manner. In wildtype CaV1.3 but not in the phosphorylation-resistant variant S1475A, CaMKII activation elicits effects similar to constitutively mimicking phosphorylation at S1475. Our findings provide novel insights on the interplay of modulatory mechanisms of human CaV1.3 channels, and present a possible target for CaV1.3-specific gating modulation in physiological and pathophysiological conditions.

CaV1.3 channel clusters characterized by live-cell and isolated plasma membrane nanoscopy

A key player of excitable cells in the heart and brain is the L-type calcium channel CaV1.3. In the heart, it is required for voltage-dependent Ca2+-signaling, i.e., for controlling and modulating atrial cardiomyocyte excitation-contraction coupling. The clustering of CaV1.3 in functionally relevant channel multimers has not been addressed due to a lack of stoichiometric labeling combined with high-resolution imaging. Here, we developed a HaloTag-labeling strategy to visualize and quantify CaV1.3 clusters using STED nanoscopy to address the questions of cluster size and intra-cluster channel density. Channel clusters were identified in the plasma membrane of transfected live HEK293 cells as well as in giant plasma membrane vesicles derived from these cells that were spread on modified glass support to obtain supported plasma membrane bilayers (SPMBs). A small fraction of the channel clusters was colocalized with early and recycling endosomes at the membranes. STED nanoscopy in conjunction with live-cell and SPMB imaging enabled us to quantify CaV1.3 cluster sizes and their molecular density revealing significantly lower channel densities than expected for dense channel packing. CaV1.3 channel cluster size and molecular density were increased in SPMBs after treatment of the cells with the sympathomimetic compound isoprenaline, suggesting a regulated channel cluster condensation mechanism.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

The human channel gating-modifying A749G CACNA1D (Cav1.3) variant induces a neurodevelopmental syndrome-like phenotype in mice

Germline de novo missense variants of the CACNA1D gene, encoding the pore-forming α1 subunit of Cav1.3 L-type Ca2+ channels (LTCCs), have been found in patients with neurodevelopmental and endocrine dysfunction, but their disease-causing potential is unproven. These variants alter channel gating, enabling enhanced Cav1.3 activity, suggesting Cav1.3 inhibition as a potential therapeutic option. Here we provide proof of the disease-causing nature of such gating-modifying CACNA1D variants using mice (Cav1.3AG) containing the A749G variant reported de novo in a patient with autism spectrum disorder (ASD) and intellectual impairment. In heterozygous mutants, native LTCC currents in adrenal chromaffin cells exhibited gating changes as predicted from heterologous expression. The A749G mutation induced aberrant excitability of dorsomedial striatum-projecting substantia nigra dopamine neurons and medium spiny neurons in the dorsal striatum. The phenotype observed in heterozygous mutants reproduced many of the abnormalities described within the human disease spectrum, including developmental delay, social deficit, and pronounced hyperactivity without major changes in gross neuroanatomy. Despite an approximately 7-fold higher sensitivity of A749G-containing channels to the LTCC inhibitor isradipine, oral pretreatment over 2 days did not rescue the hyperlocomotion. Cav1.3AG mice confirm the pathogenicity of the A749G variant and point toward a pathogenetic role of altered signaling in the dopamine midbrain system.

Clustering of CaV 1.3 L-type calcium channels by Shank3

Clustering of L-type voltage-gated Ca2+ channels (LTCCs) in the plasma membrane is increasingly implicated in creating highly localized Ca2+ signaling nanodomains. For example, neuronal LTCC activation can increase phosphorylation of the nuclear CREB transcription factor by increasing Ca2+ concentrations within a nanodomain close to the channel, without requiring bulk Ca2+ increases in the cytosol or nucleus. However, the molecular basis for LTCC clustering is poorly understood. The postsynaptic scaffolding protein Shank3 specifically associates with one of the major neuronal LTCCs, the CaV 1.3 calcium channel, and is required for optimal LTCC-dependent excitation-transcription coupling. Here, we co-expressed CaV 1.3 α1 subunits with two distinct epitope-tags with or without Shank3 in HEK cells. Co-immunoprecipitation studies using the cell lysates revealed that Shank3 can assemble complexes containing multiple CaV 1.3 α1 subunits under basal conditions. Moreover, CaV 1.3 LTCC complex formation was facilitated by CaV β subunits (β3 and β2a), which also interact with Shank3. Shank3 interactions with CaV 1.3 LTCCs and multimeric CaV 1.3 LTCC complex assembly were disrupted following the addition of Ca2+ to cell lysates, perhaps simulating conditions within an activated CaV 1.3 LTCC nanodomain. In intact HEK293T cells, co-expression of Shank3 enhanced the intensity of membrane-localized CaV 1.3 LTCC clusters under basal conditions, but not after Ca2+ channel activation. Live cell imaging studies also revealed that Ca2+ influx through LTCCs disassociated Shank3 from CaV 1.3 LTCCs clusters and reduced the CaV 1.3 cluster intensity. Deletion of the Shank3 PDZ domain prevented both binding to CaV 1.3 and the changes in multimeric CaV 1.3 LTCC complex assembly in vitro and in HEK293 cells. Finally, we found that shRNA knock-down of Shank3 expression in cultured rat primary hippocampal neurons reduced the intensity of surface-localized CaV 1.3 LTCC clusters in dendrites. Taken together, our findings reveal a novel molecular mechanism contributing to neuronal LTCC clustering under basal conditions.

CACNA1D-Related Channelopathies: From Hypertension to Autism

Tightly controlled Ca²⁺ influx through voltage-gated Ca²⁺ channels (Cavs) is indispensable for proper physiological function. Thus, it is not surprising that Cav loss and/or gain of function have been implicated in human pathology. Deficiency of Cav1.3 L-type Ca²⁺ channels (LTCCs) causes deafness and bradycardia, whereas several genetic variants of CACNA1D, the gene encoding the pore-forming α1 subunit of Cav1.3, have been linked to various disease phenotypes, such as hypertension, congenital hypoglycemia, or autism. These variants include not only common polymorphisms associated with an increased disease risk, but also rare de novo missense variants conferring high risk. This review provides a concise summary of disease-associated CACNA1D variants, whereas the main focus lies on de novo germline variants found in individuals with a neurodevelopmental disorder of variable severity. Electrophysiological recordings revealed activity-enhancing gating changes induced by these de novo variants, and tools to predict their pathogenicity and to study the resulting pathophysiological consequences will be discussed. Despite the low number of affected patients, potential phenotype-genotype correlations and factors that could impact the severity of symptoms will be covered. Since increased channel activity is assumed as the disease-underlying mechanism, pharmacological inhibition could be a treatment option. In the absence of Cav1.3-selective blockers, dihydropyridine LTCC inhibitors clinically approved for the treatment of hypertension may be used for personalized off-label trials. Findings from in vitro studies and treatment attempts in some of the patients seem promising as outlined. Taken together, due to advances in diagnostic sequencing techniques the number of reported CACNA1D variants in human diseases is constantly rising. Evidence from in silico, in vitro, and in vivo disease models can help to predict the pathogenic potential of such variants and to guide diagnosis and treatment in the clinical practice when confronted with patients harboring CACNA1D variants.

Neuronal deletion of CaV1.2 is associated with sex-specific behavioral phenotypes in mice

The gene CACNA1C, which encodes the pore forming subunit of the L-type calcium channel CaV1.2, is associated with increased risk for neuropsychiatric disorders including schizophrenia, autism spectrum disorder, major depression, and bipolar disorder. Previous rodent work identified that loss or reduction of CaV1.2 results in cognitive, affective, and motor deficits. Most previous work has either included non-neuronal cell populations (haploinsufficient and Nestin-Cre) or investigated a discrete neuronal cell population (e.g. CaMKII-Cre, Drd1-Cre), but few studies have examined the effects of more broad neuron-specific deletion of CaV1.2. Additionally, most of these studies did not evaluate for sex-specific effects or used only male animals. Here, we sought to clarify whether there are sex-specific behavioral consequences of neuron-specific deletion of CaV1.2 (neuronal CaV1.2 cKO) using Syn1-Cre-mediated conditional deletion. We found that neuronal CaV1.2 cKO mice have normal baseline locomotor function but female cKO mice display impaired motor performance learning. Male neuronal CaV1.2 cKO display impaired startle response with intact pre-pulse inhibition. Male neuronal CaV1.2 cKO mice did not display normal social preference, whereas female neuronal CaV1.2 cKO mice did. Neuronal CaV1.2 cKO mice displayed impaired associative learning in both sexes, as well as normal anxiety-like behavior and hedonic capacity. We conclude that deletion of neuronal CaV1.2 alters motor performance, acoustic startle reflex, and social behaviors in a sex-specific manner, while associative learning deficits generalize across sexes. Our data provide evidence for both sex-specific and sex-independent phenotypes related to neuronal expression of CaV1.2.

α2δ-4 and Cachd1 Proteins Are Regulators of Presynaptic Functions

The α2δ auxiliary subunits of voltage-gated calcium channels (VGCC) were traditionally regarded as modulators of biophysical channel properties. In recent years, channel-independent functions of these subunits, such as involvement in synapse formation, have been identified. In the central nervous system, α2δ isoforms 1, 2, and 3 are strongly expressed, regulating glutamatergic synapse formation by a presynaptic mechanism. Although the α2δ-4 isoform is predominantly found in the retina with very little expression in the brain, it was recently linked to brain functions. In contrast, Cachd1, a novel α2δ-like protein, shows strong expression in brain, but its function in neurons is not yet known. Therefore, we aimed to investigate the presynaptic functions of α2δ-4 and Cachd1 by expressing individual proteins in cultured hippocampal neurons. Both α2δ-4 and Cachd1 are expressed in the presynaptic membrane and could rescue a severe synaptic defect present in triple knockout/knockdown neurons that lacked the α2δ-1-3 isoforms (α2δ TKO/KD). This observation suggests that presynaptic localization and the regulation of synapse formation in glutamatergic neurons is a general feature of α2δ proteins. In contrast to this redundant presynaptic function, α2δ-4 and Cachd1 differentially regulate the abundance of presynaptic calcium channels and the amplitude of presynaptic calcium transients. These functional differences may be caused by subtle isoform-specific differences in α1-α2δ protein-protein interactions, as revealed by structural homology modelling. Taken together, our study identifies both α2δ-4 and Cachd1 as presynaptic regulators of synapse formation, differentiation, and calcium channel functions that can at least partially compensate for the loss of α2δ-1-3. Moreover, we show that regulating glutamatergic synapse formation and differentiation is a critical and surprisingly redundant function of α2δ and Cachd1.

Small Molecules as Modulators of Voltage-Gated Calcium Channels in Neurological Disorders: State of the Art and Perspectives

Voltage-gated calcium channels (VGCCs) are widely expressed in the brain, heart and vessels, smooth and skeletal muscle, as well as in endocrine cells. VGCCs mediate gene transcription, synaptic and neuronal structural plasticity, muscle contraction, the release of hormones and neurotransmitters, and membrane excitability. Therefore, it is not surprising that VGCC dysfunction results in severe pathologies, such as cardiovascular conditions, neurological and psychiatric disorders, altered glycemic levels, and abnormal smooth muscle tone. The latest research findings and clinical evidence increasingly show the critical role played by VGCCs in autism spectrum disorders, Parkinson's disease, drug addiction, pain, and epilepsy. These findings outline the importance of developing selective calcium channel inhibitors and modulators to treat such prevailing conditions of the central nervous system. Several small molecules inhibiting calcium channels are currently used in clinical practice to successfully treat pain and cardiovascular conditions. However, the limited palette of molecules available and the emerging extent of VGCC pathophysiology require the development of additional drugs targeting these channels. Here, we provide an overview of the role of calcium channels in neurological disorders and discuss possible strategies to generate novel therapeutics.

The Hidden Side of NCAM Family: NCAM2, a Key Cytoskeleton Organization Molecule Regulating Multiple Neural Functions

Although it has been over 20 years since Neural Cell Adhesion Molecule 2 (NCAM2) was identified as the second member of the NCAM family with a high expression in the nervous system, the knowledge of NCAM2 is still eclipsed by NCAM1. The first studies with NCAM2 focused on the olfactory bulb, where this protein has a key role in axonal projection and axonal/dendritic compartmentalization. In contrast to NCAM1, NCAM2's functions and partners in the brain during development and adulthood have remained largely unknown until not long ago. Recent studies have revealed the importance of NCAM2 in nervous system development. NCAM2 governs neuronal morphogenesis and axodendritic architecture, and controls important neuron-specific processes such as neuronal differentiation, synaptogenesis and memory formation. In the adult brain, NCAM2 is highly expressed in dendritic spines, and it regulates synaptic plasticity and learning processes. NCAM2's functions are related to its ability to adapt to the external inputs of the cell and to modify the cytoskeleton accordingly. Different studies show that NCAM2 interacts with proteins involved in cytoskeleton stability and proteins that regulate calcium influx, which could also modify the cytoskeleton. In this review, we examine the evidence that points to NCAM2 as a crucial cytoskeleton regulation protein during brain development and adulthood. This key function of NCAM2 may offer promising new therapeutic approaches for the treatment of neurodevelopmental diseases and neurodegenerative disorders.

On the Origin of Paroxysmal Depolarization Shifts: The Contribution of Cav1.x Channels as the Common Denominator of a Polymorphous Neuronal Discharge Pattern

Since their discovery in the 1960s, the term paroxysmal depolarization shift (PDS) has been applied to a wide variety of reinforced neuronal discharge patterns. Occurrence of PDS as cellular correlates of electrographic spikes during latent phases of insult-induced rodent epilepsy models and their resemblance to giant depolarizing potentials (GDPs) nourished the idea that PDS may be involved in epileptogenesis. Both GDPs and - in analogy - PDS may lead to progressive changes of neuronal properties by generation of pulsatile intracellular Ca2+ elevations. Herein, a key element is the gating of L-type voltage gated Ca2+ channels (LTCCs, Cav1.x family), which may convey Ca2+ signals to the nucleus. Accordingly, the present study investigates various insult-associated neuronal challenges for their propensities to trigger PDS in a LTCC-dependent manner. Our data demonstrate that diverse disturbances of neuronal function are variably suited to induce PDS-like events, and the contribution of LTCCs is essential to evoke PDS in rat hippocampal neurons that closely resemble GDPs. These PDS appear to be initiated in the dendritic sub-compartment. Their morphology critically depends on the position of recording electrodes and on their rate of occurrence. These results provide novel insight into induction mechanisms, origin, variability, and co-existence of PDS with other discharge patterns and thereby pave the way for future investigations regarding the role of PDS in epileptogenesis.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Targeting microglia L-type voltage-dependent calcium channels for the treatment of central nervous system disorders

Calcium (Ca2+ ) is a ubiquitous mediator of a multitude of cellular functions in the central nervous system (CNS). Intracellular Ca2+ is tightly regulated by cells, including entry via plasma membrane Ca2+ permeable channels. Of specific interest for this review are L-type voltage-dependent Ca2+ channels (L-VDCCs), due to their pleiotropic role in several CNS disorders. Currently, there are numerous approved drugs that target L-VDCCs, including dihydropyridines. These drugs are safe and effective for the treatment of humans with cardiovascular disease and may also confer neuroprotection. Here, we review the potential of L-VDCCs as a target for the treatment of CNS disorders with a focus on microglia L-VDCCs. Microglia, the resident immune cells of the brain, have attracted recent attention for their emerging inflammatory role in several CNS diseases. Intracellular Ca2+ regulates microglia transition from a resting quiescent state to an "activated" immune-effector state and is thus a valuable target for manipulation of microglia phenotype. We will review the literature on L-VDCC expression and function in the CNS and on microglia in vitro and in vivo and explore the therapeutic landscape of L-VDCC-targeting agents at present and future challenges in the context of Alzheimer's disease, Parkinson's disease, Huntington's disease, neuropsychiatric diseases, and other CNS disorders.

Role of densin-180 in mouse ventral hippocampal neurons in 24-hr retention of contextual fear conditioning

INTRODUCTION

Densin-180 interacts with postsynaptic molecules including calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) but its function in learning and memory process has been unclear.

METHODS

To investigate a role of hippocampal densin-180 in contextual fear conditioning (CFC) learning and memory processes, knockdown (KD) of densin-180 in hippocampal subareas was applied.

RESULTS

First, ventral hippocampal (vHC) densin-180 KD impaired single-trial CFC (stCFC) memory one day later. stCFC caused freezing behaviors to reach the peak about one hour later in both control and KD mice, but then freezing was disappeared at 2 hr postshock in KD mice. Second, stCFC caused an immediate and transient reduction of vHC densin-180 in control mice, which was not observed in KD mice. Third, stCFC caused phosphorylated-T286 (p-T286) CaMKIIα to change similarly to densin-180, but p-T305 CaMKIIα was increased 1 hr later in control mice. In KD mice, these effects were gone. Moreover, both basal levels of p-T286 and p-T305 CaMKIIα were reduced without change in total CaMKIIα in KD mice. Fourth, we found double-trial CFC (dtCFC) memory acquisition and retrieval kinetics were different from those of stCFC in vHC KD mice. In addition, densin-180 in dorsal hippocampal area appeared to play its unique role during the very early retrieval period of both CFC memories.

CONCLUSION

This study shows that vHC densin-180 is necessary for stCFC memory formation and retrieval and suggests that both densin-180 and p-T305 CaMKIIα at 1 ~ 2 hr postshock are important for stCFC memory formation. We conclude that roles of hippocampal neuronal densin-180 in CFC are temporally dynamic and differential depending on the pattern of conditioning stimuli and its location along the dorsoventral axis of hippocampal formation.

Neuronal Inactivity Co-opts LTP Machinery to Drive Potassium Channel Splicing and Homeostatic Spike Widening

Homeostasis of neural firing properties is important in stabilizing neuronal circuitry, but how such plasticity might depend on alternative splicing is not known. Here we report that chronic inactivity homeostatically increases action potential duration by changing alternative splicing of BK channels; this requires nuclear export of the splicing factor Nova-2. Inactivity and Nova-2 relocation were connected by a novel synapto-nuclear signaling pathway that surprisingly invoked mechanisms akin to Hebbian plasticity: Ca²⁺-permeable AMPA receptor upregulation, L-type Ca²⁺ channel activation, enhanced spine Ca²⁺ transients, nuclear translocation of a CaM shuttle, and nuclear CaMKIV activation. These findings not only uncover commonalities between homeostatic and Hebbian plasticity but also connect homeostatic regulation of synaptic transmission and neuronal excitability. The signaling cascade provides a full-loop mechanism for a classic autoregulatory feedback loop proposed ∼25 years ago. Each element of the loop has been implicated previously in neuropsychiatric disease.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Neural Cell Adhesion Molecule 2 (NCAM2)-Induced c-Src-Dependent Propagation of Submembrane Ca2+ Spikes Along Dendrites Inhibits Synapse Maturation

The neural cell adhesion molecule 2 (NCAM2) is encoded by a gene on chromosome 21 in humans. NCAM2 accumulates in synapses, but its role in regulation of synapse formation remains poorly understood. We demonstrate that an increase in NCAM2 levels results in increased instability of dendritic protrusions and reduced conversion of protrusions to dendritic spines in mouse cortical neurons. NCAM2 overexpression induces an increase in the frequency of submembrane Ca2+ spikes localized in individual dendritic protrusions and promotes propagation of submembrane Ca2+ spikes over segments of dendrites or the whole dendritic tree. NCAM2-dependent submembrane Ca2+ spikes are L-type voltage-gated Ca2+ channel-dependent, and their propagation but not initiation depends on the c-Src protein tyrosine kinase. Inhibition of initiation or propagation of NCAM2-dependent submembrane Ca2+ spikes reduces the NCAM2-dependent instability of dendritic protrusions. Synaptic boutons formed on dendrites of neurons with elevated NCAM2 expression are enriched in the protein marker of immature synapses GAP43, and the number of boutons with mature activity-dependent synaptic vesicle recycling is reduced. Our results indicate that synapse maturation is inhibited in NCAM2-overexpressing neurons and suggest that changes in NCAM2 levels and altered submembrane Ca2+ dynamics can cause defects in synapse maturation in Down syndrome and other brain disorders associated with abnormal NCAM2 expression.

Striatal Nurr1 Facilitates the Dyskinetic State and Exacerbates Levodopa-Induced Dyskinesia in a Rat Model of Parkinson's Disease

The transcription factor Nurr1 has been identified to be ectopically induced in the striatum of rodents expressing l-DOPA-induced dyskinesia (LID). In the present study, we sought to characterize Nurr1 as a causative factor in LID expression. We used rAAV2/5 to overexpress Nurr1 or GFP in the parkinsonian striatum of LID-resistant Lewis or LID-prone Fischer-344 (F344) male rats. In a second cohort, rats received the Nurr1 agonist amodiaquine (AQ) together with l-DOPA or ropinirole. All rats received a chronic DA agonist and were evaluated for LID severity. Finally, we performed single-unit recordings and dendritic spine analyses on striatal medium spiny neurons (MSNs) in drug-naïve rAAV-injected male parkinsonian rats. rAAV-GFP injected LID-resistant hemi-parkinsonian Lewis rats displayed mild LID and no induction of striatal Nurr1 despite receiving a high dose of l-DOPA. However, Lewis rats overexpressing Nurr1 developed severe LID. Nurr1 agonism with AQ exacerbated LID in F344 rats. We additionally determined that in l-DOPA-naïve rats striatal rAAV-Nurr1 overexpression (1) increased cortically-evoked firing in a subpopulation of identified striatonigral MSNs, and (2) altered spine density and thin-spine morphology on striatal MSNs; both phenomena mimicking changes seen in dyskinetic rats. Finally, we provide postmortem evidence of Nurr1 expression in striatal neurons of l-DOPA-treated PD patients. Our data demonstrate that ectopic induction of striatal Nurr1 is capable of inducing LID behavior and associated neuropathology, even in resistant subjects. These data support a direct role of Nurr1 in aberrant neuronal plasticity and LID induction, providing a potential novel target for therapeutic development.SIGNIFICANCE STATEMENT The transcription factor Nurr1 is ectopically induced in striatal neurons of rats exhibiting levodopa-induced dyskinesia [LID; a side-effect to dopamine replacement strategies in Parkinson's disease (PD)]. Here we asked whether Nurr1 is causing LID. Indeed, rAAV-mediated expression of Nurr1 in striatal neurons was sufficient to overcome LID-resistance, and Nurr1 agonism exacerbated LID severity in dyskinetic rats. Moreover, we found that expression of Nurr1 in l-DOPA naïve hemi-parkinsonian rats resulted in the formation of morphologic and electrophysiological signatures of maladaptive neuronal plasticity; a phenomenon associated with LID. Finally, we determined that ectopic Nurr1 expression can be found in the putamen of l-DOPA-treated PD patients. These data suggest that striatal Nurr1 is an important mediator of the formation of LID.

A homozygous missense variant in CACNB4 encoding the auxiliary calcium channel beta4 subunit causes a severe neurodevelopmental disorder and impairs channel and non-channel functions

P/Q-type channels are the principal presynaptic calcium channels in brain functioning in neurotransmitter release. They are composed of the pore-forming CaV2.1 α1 subunit and the auxiliary α2δ-2 and β4 subunits. β4 is encoded by CACNB4, and its multiple splice variants serve isoform-specific functions as channel subunits and transcriptional regulators in the nucleus. In two siblings with intellectual disability, psychomotor retardation, blindness, epilepsy, movement disorder and cerebellar atrophy we identified rare homozygous variants in the genes LTBP1, EMILIN1, CACNB4, MINAR1, DHX38 and MYO15 by whole-exome sequencing. In silico tools, animal model, clinical, and genetic data suggest the p.(Leu126Pro) CACNB4 variant to be likely pathogenic. To investigate the functional consequences of the CACNB4 variant, we introduced the corresponding mutation L125P into rat β4b cDNA. Heterologously expressed wild-type β4b associated with GFP-CaV1.2 and accumulated in presynaptic boutons of cultured hippocampal neurons. In contrast, the β4b-L125P mutant failed to incorporate into calcium channel complexes and to cluster presynaptically. When co-expressed with CaV2.1 in tsA201 cells, β4b and β4b-L125P augmented the calcium current amplitudes, however, β4b-L125P failed to stably complex with α1 subunits. These results indicate that p.Leu125Pro disrupts the stable association of β4b with native calcium channel complexes, whereas membrane incorporation, modulation of current density and activation properties of heterologously expressed channels remained intact. Wildtype β4b was specifically targeted to the nuclei of quiescent excitatory cells. Importantly, the p.Leu125Pro mutation abolished nuclear targeting of β4b in cultured myotubes and hippocampal neurons. While binding of β4b to the known interaction partner PPP2R5D (B56δ) was not affected by the mutation, complex formation between β4b-L125P and the neuronal TRAF2 and NCK interacting kinase (TNIK) seemed to be disturbed. In summary, our data suggest that the homozygous CACNB4 p.(Leu126Pro) variant underlies the severe neurological phenotype in the two siblings, most likely by impairing both channel and non-channel functions of β4b.

Heme Oxygenase 1 Induces Tau Oligomer Formation and Synapse Aberrations in Hippocampal Neurons

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by behavioral changes and cognitive decline. Recent evidence suggests that it is the soluble forms of tau oligomers (Tau-O) and Aβ oligomers (oAβ) rather than the well-studied insoluble protein aggregates that possess the neurotoxicity, infectivity, and amplification underlying disease progression. Heme oxygenase 1 (HO-1), an inducible enzyme upregulated in the cortex and hippocampus of AD brains, was reported to damage neural structures and disrupt brain function, suggesting possible contributions to Tau-O-mediated neurodegeneration. In this study, we focused on the effects of HO-1 on Tau-O formation. In hippocampus of HO-1-overexpressing transgenic mice and neural 2a (N2a) cells, Tau-O was co-localized with HO-1 as visualized by immunofluorescence staining. Furthermore, primary cultured hippocampal neurons from HO-1 transgenic mice showed elevated Tau-O and concomitant reductions in spine density and length as well as dendritic length, diameter, and arborization. Blocking Tau-O formation by isoprenaline reversed these HO-1-induced morphological changes. These results indicated that HO-1 contributes to Tau-O formation and ensuing synaptic damage. Thus, HO-1 is a promising target for AD drug development.

Presynaptic α2δ-2 Calcium Channel Subunits Regulate Postsynaptic GABAA Receptor Abundance and Axonal Wiring

Presynaptic α2δ subunits of voltage-gated calcium channels regulate channel abundance and are involved in glutamatergic synapse formation. However, little is known about the specific functions of the individual α2δ isoforms and their role in GABAergic synapses. Using primary neuronal cultures of embryonic mice of both sexes, we here report that presynaptic overexpression of α2δ-2 in GABAergic synapses strongly increases clustering of postsynaptic GABAARs. Strikingly, presynaptic α2δ-2 exerts the same effect in glutamatergic synapses, leading to a mismatched localization of GABAARs. This mismatching is caused by an aberrant wiring of glutamatergic presynaptic boutons with GABAergic postsynaptic positions. The trans-synaptic effect of α2δ-2 is independent of the prototypical cell-adhesion molecules α-neurexins (α-Nrxns); however, α-Nrxns together with α2δ-2 can modulate postsynaptic GABAAR abundance. Finally, exclusion of the alternatively spliced exon 23 of α2δ-2 is essential for the trans-synaptic mechanism. The novel function of α2δ-2 identified here may explain how abnormal α2δ subunit expression can cause excitatory-inhibitory imbalance often associated with neuropsychiatric disorders.SIGNIFICANCE STATEMENT Voltage-gated calcium channels regulate important neuronal functions such as synaptic transmission. α2δ subunits modulate calcium channels and are emerging as regulators of brain connectivity. However, little is known about how individual α2δ subunits contribute to synapse specificity. Here, we show that presynaptic expression of a single α2δ variant can modulate synaptic connectivity and the localization of inhibitory postsynaptic receptors. Our findings provide basic insights into the development of specific synaptic connections between nerve cells and contribute to our understanding of normal nerve cell functions. Furthermore, the identified mechanism may explain how an altered expression of calcium channel subunits can result in aberrant neuronal wiring often associated with neuropsychiatric disorders such as autism or schizophrenia.

Single-Channel Resolution of the Interaction between C-Terminal CaV1.3 Isoforms and Calmodulin

Voltage-dependent calcium (Ca_V) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca_V1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca_V1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca_V1.3 splice isoforms: the long Ca_V1.3₄₂ with the full-length CTM and the short Ca_V1.3_42A with the C-terminus truncated before the CTM. We found that CaM regulation of Ca_V1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca_V1.3₄₂ channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca_V1.3_42A isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca_V1.3 channel activity for particular cellular needs.

Impaired chromaffin cell excitability and exocytosis in autistic Timothy syndrome TS2-neo mouse rescued by L-type calcium channel blockers

KEY POINTS

Tymothy syndrome (TS) is a multisystem disorder featuring cardiac arrhythmias, autism and adrenal gland dysfunction that originates from a de novo point mutation in the gene encoding the Cav1.2 (CACNA1C) L-type channel. To study the role of Cav1.2 channel signals in autism, the autistic TS2-neo mouse has been generated bearing the G406R point-mutation associated with TS type-2. Using heterozygous TS2-neo mice, we report that the G406R mutation reduces the rate of inactivation and shifts leftward the activation and inactivation of L-type channels, causing marked increase of resting Ca²⁺ influx ('window' Ca²⁺ current). The increased 'window current' causes marked reduction of Na_V channel density, switches normal tonic firing to abnormal burst firing, reduces mitochondrial metabolism, induces cell swelling and decreases catecholamine release. Overnight incubations with nifedipine rescue Na_V channel density, normal firing and the quantity of catecholamine released. We provide evidence that chromaffin cell malfunction derives from altered Cav1.2 channel gating.

ABSTRACT

L-type voltage-gated calcium (Cav1) channels have a key role in long-term synaptic plasticity, sensory transduction, muscle contraction and hormone release. A point mutation in the gene encoding Cav1.2 (CACNA1C) causes Tymothy syndrome (TS), a multisystem disorder featuring cardiac arrhythmias, autism spectrum disorder (ASD) and adrenal gland dysfunction. In the more severe type-2 form (TS2), the missense mutation G406R is on exon 8 coding for the IS6-helix of the Cav1.2 channel. The mutation causes reduced inactivation and induces autism. How this occurs and how Cav1.2 gating-changes alter cell excitability, neuronal firing and hormone release on a molecular basis is still largely unknown. Here, using the TS2-neo mouse model of TS we show that the G406R mutation altered excitability and reduced secretory activity in adrenal chromaffin cells (CCs). Specifically, the TS2 mutation reduced the rate of voltage-dependent inactivation and shifted leftward the activation and steady-state inactivation of L-type channels. This markedly increased the resting 'window' Ca²⁺ current that caused an increased percentage of CCs undergoing abnormal action potential (AP) burst firing, cell swelling, reduced mitochondrial metabolism and decreased catecholamine release. The increased 'window' Ca²⁺ current caused also decreased Na_V channel density and increased steady-state inactivation, which contributed to the increased abnormal burst firing. Overnight incubation with the L-type channel blocker nifedipine rescued the normal AP firing of CCs, the density of functioning Na_V channels and their steady-state inactivation. We provide evidence that CC malfunction derives from the altered Cav1.2 channel gating and that dihydropyridines are potential therapeutics for ASD.

Alcohol and the Developing Brain: Why Neurons Die and How Survivors Change

The consequences of alcohol drinking during pregnancy are dramatic and usually referred to as fetal alcohol spectrum disorders (FASD). This condition is one of the main causes of intellectual disability in Western countries. The immature fetal brain exposed to ethanol undergoes massive neuron death. However, the same mechanisms leading to cell death can also be responsible for changes of developmental plasticity. As a consequence of such a maladaptive plasticity, the functional damage to central nervous system structures is amplified and leads to permanent sequelae. Here we review the literature dealing with experimental FASD, focusing on the alterations of the cerebral cortex. We propose that the reciprocal interaction between cell death and maladaptive plasticity represents the main pathogenetic mechanism of the alcohol-induced damage to the developing brain.

Role of a conserved glutamine in the function of voltage-gated Ca2+ channels revealed by a mutation in human CACNA1D

Voltage-gated Ca_v Ca²⁺ channels play crucial roles in regulating gene transcription, neuronal excitability, and synaptic transmission. Natural or pathological variations in Ca_v channels have yielded rich insights into the molecular determinants controlling channel function. Here, we report the consequences of a natural, putatively disease-associated mutation in the CACNA1D gene encoding the pore-forming Ca_v1.3 α₁ subunit. The mutation causes a substitution of a glutamine residue that is highly conserved in the extracellular S1-S2 loop of domain II in all Ca_v channels with a histidine and was identified by whole-exome sequencing of an individual with moderate hearing impairment, developmental delay, and epilepsy. When introduced into the rat Ca_v1.3 cDNA, Q558H significantly decreased the density of Ca²⁺ currents in transfected HEK293T cells. Gating current analyses and cell-surface biotinylation experiments suggested that the smaller current amplitudes caused by Q558H were because of decreased numbers of functional Ca_v1.3 channels at the cell surface. The substitution also produced more sustained Ca²⁺ currents by weakening voltage-dependent inactivation. When inserted into the corresponding locus of Ca_v2.1, the substitution had similar effects as in Ca_v1.3. However, the substitution introduced in Ca_v3.1 reduced current density, but had no effects on voltage-dependent inactivation. Our results reveal a critical extracellular determinant of current density for all Ca_v family members and of voltage-dependent inactivation of Ca_v1.3 and Ca_v2.1 channels.

Altered function of neuronal L-type calcium channels in ageing and neuroinflammation: Implications in age-related synaptic dysfunction and cognitive decline

The rapid developments in science have led to an increase in human life expectancy and thus, ageing and age-related disorders/diseases have become one of the greatest concerns in the 21st century. Cognitive abilities tend to decline as we get older. This age-related cognitive decline is mainly attributed to aberrant changes in synaptic plasticity and neuronal connections. Recent studies show that alterations in Ca²⁺ homeostasis underlie the increased vulnerability of neurons to age-related processes like cognitive decline and synaptic dysfunctions. Dysregulation of Ca²⁺ can lead to dramatic changes in neuronal functions. We discuss in this review, the recent advances on the potential role of dysregulated Ca²⁺ homeostasis through altered function of L-type voltage gated Ca²⁺ channels (LTCC) in ageing, with an emphasis on cognitive decline. This review therefore focuses on age-related changes mainly in the hippocampus, and with mention of other brain areas, that are important for learning and memory. This review also highlights age-related memory deficits via synaptic alterations and neuroinflammation. An understanding of these mechanisms will help us formulate strategies to reverse or ameliorate age-related disorders like cognitive decline.

Molecular mimicking of C-terminal phosphorylation tunes the surface dynamics of CaV1.2 calcium channels in hippocampal neurons

L-type voltage-gated Ca_V1.2 calcium channels (Ca_V1.2) are key regulators of neuronal excitability, synaptic plasticity, and excitation-transcription coupling. Surface-exposed Ca_V1.2 distributes in clusters along the dendrites of hippocampal neurons. A permanent exchange between stably clustered and laterally diffusive extra-clustered channels maintains steady-state levels of Ca_V1.2 at dendritic signaling domains. A dynamic equilibrium between anchored and diffusive receptors is a common feature among ion channels and is crucial to modulate signaling transduction. Despite the importance of this fine regulatory system, the molecular mechanisms underlying the surface dynamics of Ca_V1.2 are completely unexplored. Here, we examined the dynamic states of Ca_V1.2 depending on phosphorylation on Ser-1700 and Ser-1928 at the channel C terminus. Phosphorylation at these sites is strongly involved in Ca_V1.2-mediated nuclear factor of activated T cells (NFAT) signaling, long-term potentiation, and responsiveness to adrenergic stimulation. We engineered Ca_V1.2 constructs mimicking phosphorylation at Ser-1700 and Ser-1928 and analyzed their behavior at the membrane by immunolabeling protocols, fluorescence recovery after photobleaching, and single particle tracking. We found that the phosphomimetic S1928E variant increases the mobility of Ca_V1.2 without altering the steady-state maintenance of cluster in young neurons and favors channel stabilization later in differentiation. Instead, mimicking phosphorylation at Ser-1700 promoted the diffusive state of Ca_V1.2 irrespective of the differentiation stage. Together, these results reveal that phosphorylation could contribute to the establishment of channel anchoring mechanisms depending on the neuronal differentiation state. Finally, our findings suggest a novel mechanism by which phosphorylation at the C terminus regulates calcium signaling by tuning the content of Ca_V1.2 at signaling complexes.

Bicistronic CACNA1A Gene Expression in Neurons Derived from Spinocerebellar Ataxia Type 6 Patient-Induced Pluripotent Stem Cells

Spinocerebellar ataxia type 6 (SCA6) is an autosomal-dominant neurodegenerative disorder that is caused by a CAG trinucleotide repeat expansion in the CACNA1A gene. As one of the few bicistronic genes discovered in the human genome, CACNA1A encodes not only the α1A subunit of the P/Q type voltage-gated Ca²⁺ channel Ca_V2.1 but also the α1ACT protein, a 75 kDa transcription factor sharing the sequence of the cytoplasmic C-terminal tail of the α1A subunit. Isoforms of both proteins contain the polyglutamine (polyQ) domain that is expanded in SCA6 patients. Although certain SCA6 phenotypes appear to be specific for Purkinje neurons, other pathogenic effects of the SCA6 polyQ mutation can affect a broad spectrum of central nervous system (CNS) neuronal subtypes. We investigated the expression and function of CACNA1A gene products in human neurons derived from induced pluripotent stem cells from two SCA6 patients. Expression levels of CACNA1A encoding α1A subunit were similar between SCA6 and control neurons, and no differences were found in the subcellular distribution of Ca_V2.1 channel protein. The α1ACT immunoreactivity was detected in the majority of cell nuclei of SCA6 and control neurons. Although no SCA6 genotype-dependent differences in Ca_V2.1 channel function were observed, they were found in the expression levels of the α1ACT target gene Granulin (GRN) and in glutamate-induced cell vulnerability.

Activity-Dependent Facilitation of CaV1.3 Calcium Channels Promotes KCa3.1 Activation in Hippocampal Neurons

Ca_V1 L-type calcium channels are key to regulating neuronal excitability, with the range of functional roles enhanced by interactions with calmodulin, accessory proteins, or CaMKII that modulate channel activity. In hippocampal pyramidal cells, a prominent elevation of Ca_V1 activity is apparent in late channel openings that can last for seconds following a depolarizing stimulus train. The current study tested the hypothesis that a reported interaction among Ca_V1.3 channels, the scaffolding protein densin, and CaMKII could generate a facilitation of channel activity that outlasts a depolarizing stimulus. We found that Ca_V1.3 but not Ca_V1.2 channels exhibit a long-duration calcium-dependent facilitation (L-CDF) that lasts up to 8 s following a brief 50 Hz stimulus train, but only when coexpressed with densin and CaMKII. To test the physiological role for Ca_V1.3 L-CDF, we coexpressed the intermediate-conductance KCa3.1 potassium channel, revealing a strong functional coupling to Ca_V1.3 channel activity that was accentuated by densin and CaMKII. Moreover, the Ca_V1.3-densin-CaMKII interaction gave rise to an outward tail current of up to 8 s duration following a depolarizing stimulus in both tsA-201 cells and male rat CA1 pyramidal cells. A slow afterhyperpolarization in pyramidal cells was reduced by a selective block of Ca_V1 channels by isradipine, a CaMKII blocker, and siRNA knockdown of densin, and spike frequency increased upon selective block of Ca_V1 channel conductance. The results are important in revealing a Ca_V1.3-densin-CaMKII interaction that extends the contribution of Ca_V1.3 calcium influx to a time frame well beyond a brief input train.SIGNIFICANCE STATEMENT Ca_V1 L-type calcium channels play a key role in regulating the output of central neurons by providing calcium influx during repetitive inputs. This study identifies a long-duration calcium-dependent facilitation (L-CDF) of Ca_V1.3 channels that depends on the scaffolding protein densin and CaMKII and that outlasts a depolarizing stimulus by seconds. We further show a tight functional coupling between Ca_V1.3 calcium influx and the intermediate-conductance KCa3.1 potassium channel that promotes an outward tail current of up to 8 s following a depolarizing stimulus. Tests in CA1 hippocampal pyramidal cells reveal that a slow AHP is reduced by blocking different components of the Ca_V1.3-densin-CaMKII interaction, identifying an important role for Ca_V1.3 L-CDF in regulating neuronal excitability.

New gain-of-function mutation shows CACNA1D as recurrently mutated gene in autism spectrum disorders and epilepsy

CACNA1D encodes the pore-forming α1-subunit of Cav1.3, an L-type voltage-gated Ca2+-channel. Despite the recent discovery of two de novo missense gain-of-function mutations in Cav1.3 in two individuals with autism spectrum disorder (ASD) and intellectual disability CACNA1D has not been considered a prominent ASD-risk gene in large scale genetic analyses, since such studies primarily focus on likely-disruptive genetic variants. Here we report the discovery and characterization of a third de novo missense mutation in CACNA1D (V401L) in a patient with ASD and epilepsy. For the functional characterization we introduced mutation V401L into two major C-terminal long and short Cav1.3 splice variants, expressed wild-type or mutant channel complexes in tsA-201 cells and performed whole-cell patch-clamp recordings. Mutation V401L, localized within the channel's activation gate, significantly enhanced current densities, shifted voltage dependence of activation and inactivation to more negative voltages and reduced channel inactivation in both Cav1.3 splice variants. Altogether, these gating changes are expected to result in enhanced Ca2+-influx through the channel, thus representing a strong gain-of-function phenotype. Additionally, we also found that mutant channels retained full sensitivity towards the clinically available Ca2+ -channel blocker isradipine. Our findings strengthen the evidence for CACNA1D as a novel candidate autism risk gene and encourage experimental therapy with available channel-blockers for this mutation. The additional presence of seizures and neurological abnormalities in our patient define a novel phenotype partially overlapping with symptoms in two individuals with PASNA (congenital primary aldosteronism, seizures and neurological abnormalities) caused by similar Cav1.3 gain-of-function mutations.

Reduction of Cav1.3 channels in dorsal hippocampus impairs the development of dentate gyrus newborn neurons and hippocampal-dependent memory tasks

Ca_v1.3 has been suggested to mediate hippocampal neurogenesis of adult mice and contribute to hippocampal-dependent learning and memory processes. However, the mechanism of Ca_v1.3 contribution in these processes is unclear. Here, roles of Ca_v1.3 of mouse dorsal hippocampus during newborn cell development were examined. We find that knock-out (KO) of Ca_v1.3 resulted in the reduction of survival of newborn neurons at 28 days old after mitosis. The retroviral eGFP expression showed that both dendritic complexity and the number and length of mossy fiber bouton (MFB) filopodia of newborn neurons at ≥ 14 days old were significantly reduced in KO mice. Both contextual fear conditioning (CFC) and object-location recognition tasks were impaired in recent (1 day) memory test while passive avoidance task was impaired only in remote (≥ 20 days) memory in KO mice. Results using adeno-associated virus (AAV)-mediated Ca_v1.3 knock-down (KD) or retrovirus-mediated KD in dorsal hippocampal DG area showed that the recent memory of CFC was impaired in both KD mice but the remote memory was impaired only in AAV KD mice, suggesting that Ca_v1.3 of mature neurons play important roles in both recent and remote CFC memory while Ca_v1.3 in newborn neurons is selectively involved in the recent CFC memory process. Meanwhile, AAV KD of Ca_v1.3 in ventral hippocampal area has no effect on the recent CFC memory. In conclusion, the results suggest that Ca_v1.3 in newborn neurons of dorsal hippocampus is involved in the survival of newborn neurons while mediating developments of dendritic and axonal processes of newborn cells and plays a role in the memory process differentially depending on the stage of maturation and the type of learning task.

From Gene to Behavior: L-Type Calcium Channel Mechanisms Underlying Neuropsychiatric Symptoms

The L-type calcium channels (LTCCs) Cav1.2 and Cav1.3, encoded by the CACNA1C and CACNA1D genes, respectively, are important regulators of calcium influx into cells and are critical for normal brain development and plasticity. In humans, CACNA1C has emerged as one of the most widely reproduced and prominent candidate risk genes for a range of neuropsychiatric disorders, including bipolar disorder (BD), schizophrenia (SCZ), major depressive disorder, autism spectrum disorder, and attention deficit hyperactivity disorder. Separately, CACNA1D has been found to be associated with BD and autism spectrum disorder, as well as cocaine dependence, a comorbid feature associated with psychiatric disorders. Despite growing evidence of a significant link between CACNA1C and CACNA1D and psychiatric disorders, our understanding of the biological mechanisms by which these LTCCs mediate neuropsychiatric-associated endophenotypes, many of which are shared across the different disorders, remains rudimentary. Clinical studies with LTCC blockers testing their efficacy to alleviate symptoms associated with BD, SCZ, and drug dependence have provided mixed results, underscoring the importance of further exploring the neurobiological consequences of dysregulated Cav1.2 and Cav1.3. Here, we provide a review of clinical studies that have evaluated LTCC blockers for BD, SCZ, and drug dependence-associated symptoms, as well as rodent studies that have identified Cav1.2- and Cav1.3-specific molecular and cellular cascades that underlie mood (anxiety, depression), social behavior, cognition, and addiction.

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

Shank is a dose-dependent regulator of Cav1 calcium current and CREB target expression

Shank is a post-synaptic scaffolding protein that has many binding partners. Shank mutations and copy number variations (CNVs) are linked to several psychiatric disorders, and to synaptic and behavioral defects in mice. It is not known which Shank binding partners are responsible for these defects. Here we show that the C. elegans SHN-1/Shank binds L-type calcium channels and that increased and decreased shn-1 gene dosage alter L-channel current and activity-induced expression of a CRH-1/CREB transcriptional target (gem-4 Copine), which parallels the effects of human Shank copy number variations (CNVs) on Autism spectrum disorders and schizophrenia. These results suggest that an important function of Shank proteins is to regulate L-channel current and activity induced gene expression.

DOI:http://dx.doi.org/10.7554/eLife.18931.001

Densin-180 Controls the Trafficking and Signaling of L-Type Voltage-Gated Cav1.2 Ca2+ Channels at Excitatory Synapses

Voltage-gated Ca_v1.2 and Ca_v1.3 (L-type) Ca²⁺ channels regulate neuronal excitability, synaptic plasticity, and learning and memory. Densin-180 (densin) is an excitatory synaptic protein that promotes Ca²⁺-dependent facilitation of voltage-gated Ca_v1.3 Ca²⁺ channels in transfected cells. Mice lacking densin (densin KO) exhibit defects in synaptic plasticity, spatial memory, and increased anxiety-related behaviors-phenotypes that more closely match those in mice lacking Ca_v1.2 than Ca_v1.3. Therefore, we investigated the functional impact of densin on Ca_v1.2. We report that densin is an essential regulator of Ca_v1.2 in neurons, but has distinct modulatory effects compared with its regulation of Ca_v1.3. Densin binds to the N-terminal domain of Ca_v1.2, but not that of Ca_v1.3, and increases Ca_v1.2 currents in transfected cells and in neurons. In transfected cells, densin accelerates the forward trafficking of Ca_v1.2 channels without affecting their endocytosis. Consistent with a role for densin in increasing the number of postsynaptic Ca_v1.2 channels, overexpression of densin increases the clustering of Ca_v1.2 in dendrites of hippocampal neurons in culture. Compared with wild-type mice, the cell surface levels of Ca_v1.2 in the brain, as well as Ca_v1.2 current density and signaling to the nucleus, are reduced in neurons from densin KO mice. We conclude that densin is an essential regulator of neuronal Ca_v1 channels and ensures efficient Ca_v1.2 Ca²⁺ signaling at excitatory synapses.SIGNIFICANCE STATEMENT The number and localization of voltage-gated Ca_v Ca²⁺ channels are crucial determinants of neuronal excitability and synaptic transmission. We report that the protein densin-180 is highly enriched at excitatory synapses in the brain and enhances the cell surface trafficking and postsynaptic localization of Ca_v1.2 L-type Ca²⁺ channels in neurons. This interaction promotes coupling of Ca_v1.2 channels to activity-dependent gene transcription. Our results reveal a mechanism that may contribute to the roles of Ca_v1.2 in regulating cognition and mood.

Microglial Intracellular Ca2+ Signaling in Synaptic Development and its Alterations in Neurodevelopmental Disorders

Autism spectrum disorders (ASDs) are neurodevelopmental disorders characterized by deficits in social interaction, difficulties with language and repetitive/restricted behaviors. Microglia are resident innate immune cells which release many factors including proinflammatory cytokines, nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) when they are activated in response to immunological stimuli. Recent in vivo imaging has shown that microglia sculpt and refine the synaptic circuitry by removing excess and unwanted synapses and be involved in the development of neural circuits or synaptic plasticity thereby maintaining the brain homeostasis. BDNF, one of the neurotrophins, has various important roles in cell survival, neurite outgrowth, neuronal differentiation, synaptic plasticity and the maintenance of neural circuits in the CNS. Intracellular Ca²⁺ signaling is important for microglial functions including ramification, de-ramification, migration, phagocytosis and release of cytokines, NO and BDNF. BDNF induces a sustained intracellular Ca²⁺ elevation through the upregulation of the surface expression of canonical transient receptor potential 3 (TRPC3) channels in rodent microglia. BDNF might have an anti-inflammatory effect through the inhibition of microglial activation and TRPC3 could play important roles in not only inflammatory processes but also formation of synapse through the modulation of microglial phagocytic activity in the brain. This review article summarizes recent findings on emerging dual, inflammatory and non-inflammatory, roles of microglia in the brain and reinforces the importance of intracellular Ca²⁺ signaling for microglial functions in both normal neurodevelopment and their potential contributing to neurodevelopmental disorders such as ASDs.