Presynaptic Ca2+ channels compete for channel type-preferring slots in altered neurotransmission arising from Ca2+ channelopathy

Stiff substrates enhance cultured neuronal network activity

The mechanical property of extracellular matrix and cell-supporting substrates is known to modulate neuronal growth, differentiation, extension and branching. Here we show that substrate stiffness is an important microenvironmental cue, to which mouse hippocampal neurons respond and integrate into synapse formation and transmission in cultured neuronal network. Hippocampal neurons were cultured on polydimethylsiloxane substrates fabricated to have similar surface properties but a 10-fold difference in Young's modulus. Voltage-gated Ca(2+) channel currents determined by patch-clamp recording were greater in neurons on stiff substrates than on soft substrates. Ca(2+) oscillations in cultured neuronal network monitored using time-lapse single cell imaging increased in both amplitude and frequency among neurons on stiff substrates. Consistently, synaptic connectivity recorded by paired recording was enhanced between neurons on stiff substrates. Furthermore, spontaneous excitatory postsynaptic activity became greater and more frequent in neurons on stiff substrates. Evoked excitatory transmitter release and excitatory postsynaptic currents also were heightened at synapses between neurons on stiff substrates. Taken together, our results provide compelling evidence to show that substrate stiffness is an important biophysical factor modulating synapse connectivity and transmission in cultured hippocampal neuronal network. Such information is useful in designing instructive scaffolds or supporting substrates for neural tissue engineering.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Bassoon specifically controls presynaptic P/Q-type Ca(2+) channels via RIM-binding protein

Voltage-dependent Ca(2+) channels (CaVs) represent the principal source of Ca(2+) ions that trigger evoked neurotransmitter release from presynaptic boutons. Ca(2+) influx is mediated mainly via CaV2.1 (P/Q-type) and CaV2.2 (N-type) channels, which differ in their properties. Their relative contribution to synaptic transmission changes during development and tunes neurotransmission during synaptic plasticity. The mechanism of differential recruitment of CaV2.1 and CaV2.2 to release sites is largely unknown. Here, we show that the presynaptic scaffolding protein Bassoon localizes specifically CaV2.1 to active zones via molecular interaction with the RIM-binding proteins (RBPs). A genetic deletion of Bassoon or an acute interference with Bassoon-RBP interaction reduces synaptic abundance of CaV2.1, weakens P/Q-type Ca(2+) current-driven synaptic transmission, and results in higher relative contribution of neurotransmission dependent on CaV2.2. These data establish Bassoon as a major regulator of the molecular composition of the presynaptic neurotransmitter release sites.

Large somatic synapses on neurons in the ventral lateral lemniscus work in pairs

In the auditory system, large somatic synapses convey strong excitation that supports temporally precise information transfer. The information transfer of such synapses has predominantly been investigated in the endbulbs of Held in the anterior ventral cochlear nucleus and the calyx of Held in the medial nucleus of the trapezoid body. These large synapses either work as relays or integrate over a small number of inputs to excite the postsynaptic neuron beyond action potential (AP) threshold. In the monaural system, another large somatic synapse targets neurons in the ventral nucleus of the lateral lemniscus (VNLL). Here, we comparatively analyze the mechanisms of synaptic information transfer in endbulbs in the VNLL and the calyx of Held in juvenile Mongolian gerbils. We find that endbulbs in the VNLL are functionally surface-scaled versions of the calyx of Held with respect to vesicle availability, release efficacy, and synaptic peak currents. This functional scaling is achieved by different calcium current kinetics that compensate for the smaller AP in VNLL endbulbs. However, the average postsynaptic current in the VNLL fails to elicit APs in its target neurons, even though equal current suffices to generate APs in neurons postsynaptic to the calyx of Held. In the VNLL, a postsynaptic A-type outward current reduces excitability and prevents AP generation upon a single presynaptic input. Instead, coincidence detection of inputs from two converging endbulbs is ideal to reliably trigger APs. Thus, even large endbulbs do not guarantee one-to-one AP transfer. Instead, information flow appears regulated by circuit requirements.

RNAi silencing of P/Q-type calcium channels in Purkinje neurons of adult mouse leads to episodic ataxia type 2

Episodic ataxia type-2 (EA2) is a dominantly inherited human neurological disorder caused by loss of function mutations in the CACNA1A gene, which encodes the CaV2.1 subunit of P/Q-type voltage-gated calcium channels. It remains however unknown whether the deficit of cerebellar CaV2.1 in adult is in direct link with the disease. To address this issue, we have used lentiviral based-vector RNA interference (RNAi) to knock-down CaV2.1 expression in the cerebellum of adult mice. We show that suppression of the P/Q-type channels in Purkinje neurons induced motor abnormalities, such as imbalance and ataxic gait. Interestingly, moderate channel suppression caused no basal ataxia, while β-adrenergic activation and exercise mimicked stress induced motor disorders. Moreover, stress-induced ataxia was stable, non-progressive and totally abolished by acetazolamide, a carbonic anhydrase inhibitor used to treat EA2. Altogether, these data reveal that P/Q-type channel suppression in adult mice supports the episodic status of EA2 disease.

Down-regulation of endogenous KLHL1 decreases voltage-gated calcium current density

The actin-binding protein Kelch-like 1 (KLHL1) can modulate voltage-gated calcium channels in vitro. KLHL1 interacts with actin and with the pore-forming subunits of Cav2.1 and CaV3.2 calcium channels, resulting in up-regulation of P/Q and T-type current density. Here we tested whether endogenous KLHL1 modulates voltage gated calcium currents in cultured hippocampal neurons by down-regulating the expression of KLHL1 via adenoviral delivery of shRNA targeted against KLHL1 (shKLHL1). Control adenoviruses did not affect any of the neuronal properties measured, yet down-regulation of KLHL1 resulted in HVA current densities ~68% smaller and LVA current densities 44% smaller than uninfected controls, with a concomitant reduction in α(1A) and α(1H) protein levels. Biophysical analysis and western blot experiments suggest Ca(V)3.1 and 3.3 currents are also present in shKLHL1-infected neurons. Synapsin I levels, miniature postsynaptic current frequency, and excitatory and inhibitory synapse number were reduced in KLHL1 knockdown. This study corroborates the physiological role of KLHL1 as a calcium channel modulator and demonstrates a novel, presynaptic role.

Calcium channels and synaptic transmission in familial hemiplegic migraine type 1 animal models

One of the outstanding developments in clinical neurology has been the identification of ion channel mutations as the origin of a wide variety of inherited disorders like migraine, epilepsy, and ataxia. The study of several channelopathies has provided crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological diseases. This review addresses the mutations underlying familial hemiplegic migraine (FHM) with particular interest in Cav2.1 (i.e., P/Q-type) voltage-activated Ca²⁺ channel FHM type-1 mutations (FHM1). Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of Cav2.1 channels. FHM1 Cav2.1 channels activate at more hyperpolarizing potentials and show an increased open probability. These biophysical alterations may lead to a gain-of-function on synaptic transmission depending upon factors such as action potential waveform and/or Cav2.1 splice variants and auxiliary subunits. Analysis of FHM knock-in mouse models has demonstrated a deficient regulation of the cortical excitation/inhibition (E/I) balance. The resulting excessive increases in cortical excitation may be the mechanisms that underlie abnormal sensory processing together with an increase in the susceptibility to cortical spreading depression (CSD). Increasing evidence from FHM KI animal studies support the idea that CSD, the underlying mechanism of aura, can activate trigeminal nociception, and thus trigger the headache mechanisms.

Distinct roles for Cav2.1-2.3 in activity-dependent synaptic dynamics

Synaptic transmission throughout most of the CNS is steeply dependent on presynaptic calcium influx through the voltage-gated calcium channels Cav2.1-Cav2.3. In addition to triggering exocytosis, this calcium influx also recruits short-term synaptic plasticity. During the complex patterns of presynaptic activity that occur in vivo, several forms of plasticity combine to generate a synaptic output that is dynamic, in which the size of a given excitatory postsynaptic potential (EPSP) in response to a given spike depends on the short-term history of presynaptic activity. It remains unclear whether the different Cav2 channels play distinct roles in defining these synaptic dynamics and, if so, under what conditions different Cav2 family members most effectively determine synaptic output. We examined these questions by measuring the effects of calcium channel-selective toxins on synaptic transmission at the Schaffer collateral synapse in hippocampal slices from adult mice in response to both low-frequency stimulation and complex stimulus trains derived from in vivo recordings. Blockade of Cav2.1 had a greater inhibitory effect on synaptic transmission during low-frequency components of the stimulus train than on synaptic transmission during high-frequency components of the train, indicating that Cav2.1 had a greater fractional contribution to synaptic transmission at low frequencies than at high frequencies. Relative to Cav2.1, Cav2.2 had a disproportionately reduced contribution to synaptic transmission at frequencies >20 Hz, while Cav2.3 had a disproportionately increased contribution to synaptic transmission at frequencies >1 Hz. These activity-dependent effects of different Cav2 family members shape the filtering characteristics of GABAB receptor-mediated presynaptic inhibition. Thus different Cav2 channels vary in their coupling to synaptic transmission over different frequency ranges, with consequences for the frequency tuning of both synaptic dynamics and presynaptic neuromodulation.

Emerging links between homeostatic synaptic plasticity and neurological disease

Homeostatic signaling systems are ubiquitous forms of biological regulation, having been studied for hundreds of years in the context of diverse physiological processes including body temperature and osmotic balance. However, only recently has this concept been brought to the study of excitatory and inhibitory electrical activity that the nervous system uses to establish and maintain stable communication. Synapses are a primary target of neuronal regulation with a variety of studies over the past 15 years demonstrating that these cellular junctions are under bidirectional homeostatic control. Recent work from an array of diverse systems and approaches has revealed exciting new links between homeostatic synaptic plasticity and a variety of seemingly disparate neurological and psychiatric diseases. These include autism spectrum disorders, intellectual disabilities, schizophrenia, and Fragile X Syndrome. Although the molecular mechanisms through which defective homeostatic signaling may lead to disease pathogenesis remain unclear, rapid progress is likely to be made in the coming years using a powerful combination of genetic, imaging, electrophysiological, and next generation sequencing approaches. Importantly, understanding homeostatic synaptic plasticity at a cellular and molecular level may lead to developments in new therapeutic innovations to treat these diseases. In this review we will examine recent studies that demonstrate homeostatic control of postsynaptic protein translation, retrograde signaling, and presynaptic function that may contribute to the etiology of complex neurological and psychiatric diseases.

Nanoscale-targeted patch-clamp recordings of functional presynaptic ion channels

Direct electrical access to presynaptic ion channels has hitherto been limited to large specialized terminals such as the calyx of Held or hippocampal mossy fiber bouton. The electrophysiology and ion-channel complement of far more abundant small synaptic terminals (≤1 μm) remain poorly understood. Here we report a method based on superresolution scanning ion conductance imaging of small synapses in culture at approximately 100–150 nm 3D resolution, which allows presynaptic patch-clamp recordings in all four configurations (cell-attached, inside-out, outside-out, and whole-cell). Using this technique, we report presynaptic recordings of K⁺, Na⁺, Cl⁻, and Ca²⁺ channels. This semiautomated approach allows direct investigation of the distribution and properties of presynaptic ion channels at small central synapses.

Video Abstract

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Topographic imaging of live synaptic boutons at nanoscale resolution

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Cell-attached patch-clamp recordings of ion channels in small central synapses

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Whole-cell small presynaptic bouton recordings

The E1015K variant in the synprint region of the CaV2.1 channel alters channel function and is associated with different migraine phenotypes

Mutations in the CACNA1A gene, which encodes the pore-forming α1A subunit of the CaV2.1 voltage-gated calcium channel, cause a number of human neurologic diseases including familial hemiplegic migraine. We have analyzed the functional impact of the E1015K amino acid substitution located in the "synprint" domain of the α1A subunit. This variant was identified in two families with hemiplegic migraine and in one patient with migraine with aura. The wild type (WT) and the E1015K forms of the GFP-tagged α1A subunit were expressed in cultured hippocampal neurons and HEK cells to understand the role of the variant in the transport activity and physiology of CaV2.1. The E1015K variant does not alter CaV2.1 protein expression, and its transport to the cell surface and synaptic terminals is similar to that observed for WT channels. Electrophysiological data demonstrated that E1015K channels have increased current density and significantly altered inactivation properties compared with WT. Furthermore, the SNARE proteins syntaxin 1A and SNAP-25 were unable to modulate voltage-dependent inactivation of E1015K channels. Overall, our findings describe a genetic variant in the synprint site of the CaV2.1 channel which is characterized by a gain-of-function and associated with both hemiplegic migraine and migraine with aura in patients.

Dominance of P/Q-type calcium channels in depolarization-induced presynaptic FM dye release in cultured hippocampal neurons

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We analyzed depolarization-induced synaptic FM dye release in hippocampal neurons.

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We pharmacologically isolated the contribution of voltage-gated Ca²⁺ channels.

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85% of synapses utilize N- and P/Q-type channels, 15% only P/Q-type channels.

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In both groups of synapses release kinetics are determined by P/Q-type channels.

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We propose a more direct coupling of P/Q-type channels to synaptic release.

Neurotransmitter release probability is related by high power to the local concentration of calcium in presynaptic terminals, which in turn is controlled by voltage-gated calcium channels. P/Q- and N-type channels trigger synaptic transmission in the majority of neurons of the central nervous system. However, whether and under which conditions both channel types act cooperatively or independently is still insufficiently understood. Previous studies suggested either a dominance of N- or P/Q-type channels, or a synergistic action of both channels, depending on the experimental paradigms. Thus, to provide insight into the properties of neurotransmitter release in cultured mouse hippocampal neurons, we used quantitative analysis of FM dye release from presynaptic boutons induced by high potassium membrane depolarization. Increasing extracellular potassium concentrations revealed a sigmoid dependence of FM dye release to the stimulation strength. Individual and combined application of the P/Q- and N-type channel-specific blockers ω-agatoxin-IVA and ω-conotoxin-GVIA, respectively, allowed us to specifically isolate the contribution of both channel types to release triggered with 40 mM KCl. Analysis of the release kinetics and the fractional release amplitude demonstrate that, whereas in only 15% of the synapses release depended exclusively on P/Q-type channels, the majority of synapses (85%) contained both N- and P/Q-type channels. Nevertheless, the kinetics of FM dye release in synapses containing both channel types was determined by the P/Q-type channels. Together, our data suggest a more direct coupling of P/Q-type channels to synaptic release compared to N-type channels, which may explain the high prevalence of neurological P/Q-type channelopathies.

Functional analysis of a migraine-associated TRESK K+ channel mutation

Recent genetic and functional studies suggest that migraine may result from abnormal activities of ion channels and transporters. A frameshift mutation in the human TWIK-related spinal cord K(+) (TRESK) channel has been identified in migraine with aura patients in a large pedigree. In Xenopus oocytes, mutant TRESK subunits exert a dominant-negative effect on whole-cell TRESK currents. However, questions remain as to whether and how mutant TRESK subunits affect the membrane properties and the excitability of neurons in the migraine circuit. Here, we investigated the functional consequences of the mutant TRESK subunits in HEK293T cells and mouse trigeminal ganglion (TG) neurons. First, we found that mutant TRESK subunits exhibited dominant-negative effects not only on the size of the whole-cell TRESK currents, but also on the level of TRESK channels on the plasma membrane in HEK293T cells. This likely resulted from the heterodimerization of wild-type and mutant TRESK subunits. Next, we expressed mutant TRESK subunits in cultured TG neurons and observed a significant decrease in the lamotrigine-sensitive K(+) current, suggesting that the mutant TRESK subunits have a dominant-negative effect on currents through the endogenous TRESK channels. Current-clamp recordings showed that neurons expressing mutant TRESK subunits had a higher input resistance, a lower current threshold for action potential initiation, and a higher spike frequency in response to suprathreshold stimuli, indicating that the mutation resulted in hyperexcitability of TG neurons. Our results suggest a possible mechanism through which the TRESK mutation increases the susceptibility of migraine headache.

Homeostatic competition between phasic and tonic inhibition

The GABAA receptors are the major inhibitory receptors in the brain and are localized at both synaptic and extrasynaptic membranes. Synaptic GABAA receptors mediate phasic inhibition, whereas extrasynaptic GABAA receptors mediate tonic inhibition. Both phasic and tonic inhibitions regulate neuronal activity, but whether they regulate each other is not very clear. Here, we investigated the functional interaction between synaptic and extrasynaptic GABAA receptors through various molecular manipulations. Overexpression of extrasynaptic α6β3δ-GABAA receptors in mouse hippocampal pyramidal neurons significantly increased tonic currents. Surprisingly, the increase of tonic inhibition was accompanied by a dramatic reduction of the phasic inhibition, suggesting a possible homeostatic regulation of the total inhibition. Overexpressing the α6 subunit alone induced an up-regulation of δ subunit expression and suppressed phasic inhibition similar to overexpressing the α6β3δ subunits. Interestingly, blocking all GABAA receptors after overexpressing α6β3δ receptors could not restore the synaptic GABAergic transmission, suggesting that receptor activation is not required for the homeostatic interplay. Furthermore, insertion of a gephyrin-binding-site (GBS) into the α6 and δ subunits recruited α6(GBS)β3δ(GBS) receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission. Thus, it is not the positional effect of extrasynaptic α6β3δ receptors that causes the down-regulation of phasic inhibition. Overexpressing α5β3γ2 subunits similarly reduced synaptic GABAergic transmission. We propose a working model that both synaptic and extrasynaptic GABAA receptors may compete for limited receptor slots on the plasma membrane to maintain a homeostatic range of the total inhibition.

Pearls and pitfalls in genetic studies of migraine

Purpose of review

Migraine is a prevalent neurovascular brain disorder with a strong genetic component, and different methodological approaches have been implemented to identify the genes involved. This review focuses on pearls and pitfalls of these approaches and genetic findings in migraine.

Summary

Common forms of migraine (i.e. migraine with and without aura) are thought to have a polygenic make-up, whereas rare familial hemiplegic migraine (FHM) presents with a monogenic pattern of inheritance. Until a few years ago only studies in FHM yielded causal genes, which were identified by a classical linkage analysis approach. Functional analyses of FHM gene mutations in cellular and transgenic animal models suggest abnormal glutamatergic neurotransmission as a possible key disease mechanism. Recently, a number of genes were discovered for the common forms of migraine using a genome-wide association (GWA) approach, which sheds first light on the pathophysiological mechanisms involved.

Conclusions

Novel technological strategies such as next-generation sequencing, which can be implemented in future genetic migraine research, may aid the identification of novel FHM genes and promote the search for the missing heritability of common migraine.

The familial hemiplegic migraine type 1 mutation K1336E affects direct G protein-mediated regulation of neuronal P/Q-type Ca2+ channels

BACKGROUND

Familial hemiplegic migraine type 1 (FHM-1) is an autosomal dominant form of migraine with aura characterized by recurrent migraine, hemiparesis and ataxia. FHM-1 has been linked to missense mutations in the CACNA1A gene encoding the pore-forming subunit of the neuronal voltage-gated P/Q-type Ca(2+) channel (CaV2.1α1).

METHODS

Here, we explored the effects of the FHM-1 K1336E mutation on G protein-dependent modulation of the recombinant P/Q-type channel. The mutation was introduced into the human CaV2.1α1 subunit and its functional consequences investigated after heterologous expression in HEK-293 cells using patch-clamp recordings.

RESULTS

Functional analysis of the K1336E mutation revealed a reduction of Ca(2+) current densities, a ∼10 mV left-shift in the current-voltage relationship, and the slowing of current inactivation kinetics. When co-expressed along with the human μ-opioid receptor, application of the agonist DAMGO inhibited whole-cell currents through both the wild-type and the mutant channels. Prepulse facilitation was also reduced by the K1336E mutation. Likewise, the kinetic analysis of the onset and decay of facilitation showed that the mutation affects the apparent dissociation and reassociation rates of the Gβγ dimer from the channel complex.

CONCLUSIONS

These results suggest that the extent of G-protein-mediated inhibition is significantly reduced in the K1336E mutant CaV2.1 Ca(2+) channels. This alteration would contribute to render the neuronal network hyperexcitable, possibly as a consequence of reduced presynaptic inhibition, and may help to explain some aspects of the FHM-1 pathophysiology.

Calcium channels and migraine

Missense mutations in CACNA1A, the gene that encodes the pore-forming α1 subunit of human voltage-gated Ca(V)2.1 (P/Q-type) calcium channels, cause a rare form of migraine with aura (familial hemiplegic migraine type 1: FHM1). Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache that may be preceded by transient neurological aura symptoms. This review, first, briefly summarizes current understanding of the pathophysiological mechanisms that are believed to underlie migraine headache, migraine aura and the onset of a migraine attack, and briefly describes the localization and function of neuronal Ca(V)2.1 channels in the brain regions that have been implicated in migraine pathogenesis. Then, the review describes and discusses i) the functional consequences of FHM1 mutations on the biophysical properties of recombinant human Ca(V)2.1 channels and native Ca(V)2.1 channels in neurons of knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene, and ii) the functional consequences of these mutations on neurophysiological processes in the cerebral cortex and trigeminovascular system thought to be involved in the pathophysiology of migraine, and the insights into migraine mechanisms obtained from the functional analysis of these processes in FHM1 knockin mice. This article is part of a Special Issue entitled: Calcium channels.

Independent regulation of basal neurotransmitter release efficacy by variable Ca²+ influx and bouton size at small central synapses

Concurrent imaging of vesicular release and calcium dynamics in small presynaptic boutons shows that the fusion probability of readily releasable vesicles is a major determinant of the overall variability in release probability.

The efficacy of action potential evoked neurotransmitter release varies widely even among synapses supplied by the same axon, and the number of release-ready vesicles at each synapse is a major determinant of this heterogeneity. Here we identify a second, equally important, mechanism for release heterogeneity at small hippocampal synapses, the inter-synaptic variation of the exocytosis probability of release-ready vesicles. Using concurrent measurements of vesicular pool sizes, vesicular exocytosis rates, and presynaptic Ca²⁺ dynamics, in the same small hippocampal boutons, we show that the average fusion probability of release-ready vesicles varies among synapses supplied by the same axon with the size of the spike-evoked Ca²⁺ concentration transient. We further show that synapses with a high vesicular release probability exhibit a lower Ca²⁺ cooperativity, arguing that this is a direct consequence of increased Ca²⁺ influx at the active zone. We conclude that variability of neurotransmitter release under basal conditions at small central synapses is accounted for not only by the number of release-ready vesicles, but also by their fusion probabilities, which are set independently of bouton size by variable spike-evoked presynaptic Ca²⁺ influx.

Touch responsiveness in zebrafish requires voltage-gated calcium channel 2.1b

The molecular and physiological basis of the touch-unresponsive zebrafish mutant fakir has remained elusive. Here we report that the fakir phenotype is caused by a missense mutation in the gene encoding voltage-gated calcium channel 2.1b (CACNA1Ab). Injection of RNA encoding wild-type CaV2.1 restores touch responsiveness in fakir mutants, whereas knockdown of CACNA1Ab via morpholino oligonucleotides recapitulates the fakir mutant phenotype. Fakir mutants display normal current-evoked synaptic communication at the neuromuscular junction but have attenuated touch-evoked activation of motor neurons. NMDA-evoked fictive swimming is not affected by the loss of CaV2.1b, suggesting that this channel is not required for motor pattern generation. These results, coupled with the expression of CACNA1Ab by sensory neurons, suggest that CaV2.1b channel activity is necessary for touch-evoked activation of the locomotor network in zebrafish.

Trafficking and stability of voltage-gated calcium channels

Voltage-gated calcium channels are important mediators of calcium influx into electrically excitable cells. The amount of calcium entering through this family of channel proteins is not only determined by the functional properties of channels embedded in the plasma membrane but also by the numbers of channels that are expressed at the cell surface. The trafficking of channels is controlled by numerous processes, including co-assembly with ancillary calcium channel subunits, ubiquitin ligases, and interactions with other membrane proteins such as G protein coupled receptors. Here we provide an overview about the current state of knowledge of calcium channel trafficking to the cell membrane, and of the mechanisms regulating the stability and internalization of this important ion channel family.

Neuronal P/Q-type calcium channel dysfunction in inherited disorders of the CNS

The past two decades have witnessed the emergence of a new and expanding field of neurological diseases--the genetic ion channelopathies. These disorders arise from mutations in genes that encode ion channel subunits, and manifest as paroxysmal attacks involving the brain or spinal cord, and/or muscle. The voltage-gated P/Q-type calcium channel (P/Q channel) is highly expressed in the cerebellum, hippocampus and cortex of the mammalian brain. The P/Q channel has a fundamental role in mediating fast synaptic transmission at central and peripheral nerve terminals. Autosomal dominant mutations in the CACNA1A gene, which encodes voltage-gated P/Q-type calcium channel subunit α(1) (the principal pore-forming subunit of the P/Q channel) are associated with episodic and progressive forms of cerebellar ataxia, familial hemiplegic migraine, vertigo and epilepsy. This Review considers, from both a clinical and genetic perspective, the various neurological phenotypes arising from inherited P/Q channel dysfunction, with a focus on recent advances in the understanding of the pathogenetic mechanisms underlying these disorders.

GPCR and voltage-gated calcium channels (VGCC) signaling complexes

Voltage-gated ion channels are transmembrane proteins that control nerve impulses and cell homeostasis. Signaling molecules that regulate ion channel activity and density at the plasma membrane must be specifically and efficiently coupled to these channels in order to control critical physiological functions such as action potential propagation. Although their regulation by G-protein receptor activation has been extensively explored, the assembly of ion channels into signaling complexes of GPCRs plays a fundamental role, engaging specific downstream -signaling pathways that trigger precise downstream effectors. Recent work has confirmed that GPCRs can intimately interact with ion channels and serve as -chaperone proteins that finely control their gating and trafficking in subcellular microdomains. This chapter aims to describe examples of GPCR-ion channel co-assembly, focusing mainly on signaling complexes between GPCRs and voltage-gated calcium channels.

Effects of familial hemiplegic migraine type 1 mutation T666M on voltage-gated calcium channel activities in trigeminal ganglion neurons

Familial hemiplegic migraine type 1 (FHM-1), a rare hereditary form of migraine with aura and hemiparesis, serves as a good model for exploring migraine pathophysiology. The FHM-1 gene encodes the pore-forming Ca(V)2.1 subunit of human P/Q-type voltage-gated Ca(2+) channels (VGCCs). Some FHM-1 mutations result in a decrease of whole cell P/Q-type current density in transfected cells/neurons. Questions remain as to whether and how these mutations may increase the gain of the trigeminal nociceptive pathway underlying migraine headache. Here, we investigated the effects of T666M, the most frequently occurring FHM-1 mutation, on VGCC currents and neuronal excitability in trigeminal ganglion (TG) neurons. We expressed human wild-type and T666M Ca(V)2.1 subunits in cultured TG neurons from Ca(V)2.1 knockout mice and recorded whole cell VGCC currents in transfected neurons. Currents mediated by individual VGCC subtypes were dissected according to their pharmacological and biophysical properties. TG neurons were sorted into three subpopulations based on their soma size and their affinity to isolectin B4 (IB4). We found that the T666M mutation did not affect total or surface expression of Ca(V)2.1 proteins but caused a profound reduction of P/Q-type current in all subtypes of TG neurons. Interestingly, a compensatory increase in Ca(V)3.2-mediated low-voltage-activated T-type currents only occurred in small IB4-negative (IB4(-)) TG neurons expressing T666M subunits. Current-clamp recordings showed that the T666M mutation resulted in hyperexcitability of the small IB4(-) TG population. Taken together, our results suggest a possible scenario through which FHM-1 mutations might increase the gain of the trigeminal nociceptive pathway.

Nanodomain coupling between Ca²⁺ channels and sensors of exocytosis at fast mammalian synapses

The physical distance between presynaptic Ca(2+) channels and the Ca(2+) sensors that trigger exocytosis of neurotransmitter-containing vesicles is a key determinant of the signalling properties of synapses in the nervous system. Recent functional analysis indicates that in some fast central synapses, transmitter release is triggered by a small number of Ca(2+) channels that are coupled to Ca(2+) sensors at the nanometre scale. Molecular analysis suggests that this tight coupling is generated by protein-protein interactions involving Ca(2+) channels, Ca(2+) sensors and various other synaptic proteins. Nanodomain coupling has several functional advantages, as it increases the efficacy, speed and energy efficiency of synaptic transmission.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.

Transsynaptic channelosomes: non-conducting roles of ion channels in synapse formation

Recent findings demonstrate that synaptic channels are directly involved in the formation and maintenance of synapses by interacting with synapse organizers. The synaptic channels on the pre- and postsynaptic membranes possess non-conducting roles in addition to their functional roles as ion-conducting channels required for synaptic transmission. For example, presynaptic voltage-dependent calcium channels link the target-derived synapse organizer laminin β2 to cytomatrix of the active zone and function as scaffolding proteins to organize the presynaptic active zones. Furthermore, postsynaptic δ2-type glutamate receptors organize the synapses by forming transsynaptic protein complexes with presynaptic neurexins through synapse organizer cerebellin 1 precursor proteins. Interestingly, the synaptic clustering of AMPA receptors is regulated by neuronal activity-regulated pentraxins, while postsynaptic differentiation is induced by the interaction of postsynaptic calcium channels and thrombospondins. This review will focus on the non-conducting functions of ion-channels that contribute to the synapse formation in concert with synapse organizers and active-zone-specific proteins.

Properties of Na+ currents conducted by a skeletal muscle L-type Ca2+ channel pore mutant (SkEIIIK)

Four glutamate residues residing at corresponding positions within the four conserved membrane-spanning repeats of L-type Ca(2+) channels are important structural determinants for the passage of Ca(2+) across the selectivity filter. Mutation of the critical glutamate in Repeat III in the a 1S subunit of the skeletal L-type channel (Ca(v)1.1) to lysine virtually eliminates passage of Ca(2+) during step depolarizations. In this study, we examined the ability of this mutant Ca(v)1.1 channel (SkEIIIK) to conduct inward Na(+) current. When 150 mM Na(+) was present as the sole monovalent cation in the bath solution, dysgenic (Ca(v)1.1 null) myotubes expressing SkEIIIK displayed slowly-activating, non-inactivating, nifedipine-sensitive inward currents with a reversal potential (45.6 ± 2.5 mV) near that expected for Na(+). Ca(2+) block of SkEIIIK-mediated Na(+) current was revealed by the substantial enhancement of Na(+) current amplitude after reduction of Ca(2+) in the external recording solution from 10 mM to near physiological 1 mM. Inward SkEIIIK-mediated currents were potentiated by either ±Bay K 8644 (10 mM) or 200-ms depolarizing prepulses to +90 mV. In contrast, outward monovalent currents were reduced by ±Bay K 8644 and were unaffected by strong depolarization, indicating a preferential potentiation of inward Na(+) currents through the mutant Ca(v)1.1 channel. Taken together, our results show that SkEIIIK functions as a non-inactivating, junctionally-targeted Na(+) channel when Na(+) is the sole monvalent cation present and urge caution when interpreting the impact of mutations designed to ablate Ca(2+) permeability mediated by Ca(v) channels on physiological processes that extend beyond channel gating and permeability.

Neurological channelopathies

Inherited ion channel mutations can affect the entire nervous system. Many cause paroxysmal disturbances of brain, spinal cord, peripheral nerve or skeletal muscle function, with normal neurological development and function in between attacks. To fully understand how mutations of ion channel genes cause disease, we need to know the normal location and function of the channel subunit, consequences of the mutation for biogenesis and biophysical properties, and possible compensatory changes in other channels that contribute to cell or circuit excitability. Animal models of monogenic channelopathies increasingly help our understanding. An important challenge for the future is to determine how more subtle derangements of ion channel function, which arise from the interaction of genetic and environmental influences, contribute to common paroxysmal disorders, including idiopathic epilepsy and migraine, that share features with rare monogenic channelopathies.

Severe and progressive neurotransmitter release aberrations in familial hemiplegic migraine type 1 Cacna1a S218L knock-in mice

Familial hemiplegic migraine type 1 (FHM1) is caused by mutations in the CACNA1A gene, encoding neuronal presynaptic Ca(V)2.1 (P/Q-type) Ca(2+) channels. These channels mediate neurotransmitter release at many central synapses and at the neuromuscular junction (NMJ). Mutation S218L causes a severe neurological phenotype of FHM and, additionally, ataxia and susceptibility to seizures, delayed brain edema, and fatal coma after minor head trauma. Recently, we generated a Cacna1a S218L knock-in mutant mouse, displaying these features and reduced survival. A first electrophysiological study showed high susceptibility for cortical spreading depression, enhanced neuronal soma Ca(2+) influx, and at diaphragm NMJs, a considerable increase of neurotransmitter release. We here assessed the function of S218L knock-in NMJs at several muscle types in great detail. Pharmacological analyses using specific Ca(V) subtype-blocking toxins excluded compensatory contribution of non-Ca(V)2.1 channels. Endplate potentials were considerably broadened at many NMJs. High rate (40 Hz)-evoked acetylcholine release was slightly reduced; however, it was not associated with block of neurotransmission causing weakness, as assessed with grip strength measurements and in vitro muscle contraction experiments. The synaptopathy clearly progressed with age, including development of an increased acetylcholine release at low-rate nerve stimulation at physiological extracellular Ca(2+) concentration and further endplate potential broadening. Our results suggest enhanced Ca(2+) influx into motor nerve terminals through S218L-mutated presynaptic Ca(V)2.1 channels, likely because of the earlier reported negative shift of activation potential and reduced inactivation. Similar severe aberrations at central synapses of S218L mutant mice and humans may underlie or contribute to the drastic neurological phenotype.

Different relationship of N- and P/Q-type Ca2+ channels to channel-interacting slots in controlling neurotransmission at cultured hippocampal synapses

Synaptic transmission at CNS synapses is often mediated by joint actions of multiple Ca(2+) channel subtypes, most prominently, P/Q- and N-type. We have proposed that P/Q-type Ca(2+) channels saturate type-preferring slots at presynaptic terminals, which impose a ceiling on the synaptic efficacy of the channels. To test for analogous interactions for presynaptic N-type Ca(2+) channels, we overexpressed their pore-forming Ca(V)2.2 subunit in cultured mouse hippocampal neurons, recorded excitatory synaptic transmission from transfected cells, and dissected the contributions of N-, P/Q-, and R-type channels with subtype-specific blockers. Overexpression of Ca(V)2.2 did not increase the absolute size of the EPSC even though somatic N-type current was augmented by severalfold. Thus, the strength of neurotransmission is saturated with regard to levels of Ca(2+) channel expression for both N-type and P/Q-type channels. Overexpression of Ca(2+)-impermeable Ca(V)2.2 subunits decreased EPSC size, corroborating competition for channel slots. Striking asymmetries between N- and P/Q-type channels emerged when their relative contributions were compared with channel overexpression. Overexpressed N-type channels could competitively displace P/Q-type channels from P/Q-preferring slots and take over the role of supporting transmission. The converse was not found with overexpression of P/Q-type channels, regardless of their C-terminal domain. We interpret these findings in terms of two different kinds of presynaptic slots at excitatory synapses, one accepting N-type channels but rejecting P/Q-type (N(specific)) and the other preferring P/Q-type but also accepting N-type (PQ(preferring)). The interaction between channels and slots governs the respective contributions of multiple channel types to neurotransmission and, in turn, the ability of transmission to respond to various stimulus patterns and neuromodulators.

High cortical spreading depression susceptibility and migraine-associated symptoms in Ca(v)2.1 S218L mice

OBJECTIVE

The CACNA1A gene encodes the pore-forming subunit of neuronal Ca(V)2.1 Ca2+ channels. In patients, the S218L CACNA1A mutation causes a dramatic hemiplegic migraine syndrome that is associated with ataxia, seizures, and severe, sometimes fatal, brain edema often triggered by only a mild head trauma.

METHODS

We introduced the S218L mutation into the mouse Cacna1a gene and studied the mechanisms for the S218L syndrome by analyzing the phenotypic, molecular, and electrophysiological consequences.

RESULTS

Cacna1a(S218L) mice faithfully mimic the associated clinical features of the human S218L syndrome. S218L neurons exhibit a gene dosage-dependent negative shift in voltage dependence of Ca(V)2.1 channel activation, resulting in enhanced neurotransmitter release at the neuromuscular junction. Cacna1a(S218L) mice also display an exquisite sensitivity to cortical spreading depression (CSD), with a vastly reduced triggering threshold, an increased propagation velocity, and frequently multiple CSD events after a single stimulus. In contrast, mice bearing the R192Q CACNA1A mutation, which in humans causes a milder form of hemiplegic migraine, typically exhibit only a single CSD event after one triggering stimulus.

INTERPRETATION

The particularly low CSD threshold and the strong tendency to respond with multiple CSD events make the S218L cortex highly vulnerable to weak stimuli and may provide a mechanistic basis for the dramatic phenotype seen in S218L mice and patients. Thus, the S218L mouse model may prove a valuable tool to further elucidate mechanisms underlying migraine, seizures, ataxia, and trauma-triggered cerebral edema.

Headache-type adverse effects of NO donors: vasodilation and beyond

Although nitrate therapy, used in the treatment of cardiovascular disorders, is frequently associated with side-effects, mainly headaches, the summaries of product characteristics of nitrate-containing medicines do not report detailed description of headaches and even do not highlight the possibility of nitrate-induced migraine. Two different types of nitrate-induced headaches have been described: (i) immediate headaches that develop within the first hour of the application, are mild or medium severity without characteristic symptoms for migraine, and ease spontaneously; and (ii) delayed, moderate or severe migraine-type headaches (occurring mainly in subjects with personal or family history of migraine), that develop 3-6 h after the intake of nitrates, with debilitating, long-lasting symptoms including nausea, vomiting, photo- and/or phono-phobia. These two types of headaches are remarkably different, not only in their timing and symptoms, but also in the persons who are at risk. Recent studies provide evidence that the two headache types are caused by different mechanisms: immediate headaches are connected to vasodilation caused by nitric oxide (NO) release, while migraines are triggered by other actions such as the release of calcitonin gene-related peptide or glutamate, or changes in ion channel function mediated by cyclic guanosine monophosphate or S-nitrosylation. Migraines usually need anti-attack medication, such as triptans, but these drugs are contraindicated in most medical conditions that are treated using nitrates. In conclusion, these data recommend the correction of summaries of nitrate product characteristics, and also suggest a need to develop new types of anti-migraine drugs, effective in migraine attacks, that could be used in patients with risk for angina pectoris.

CaV2.1 channelopathies

Mutations in the CACNA1A gene that encodes the pore-forming alpha1 subunit of human voltage-gated CaV2.1 (P/Q-type) Ca2+ channels cause several autosomal-dominant neurologic disorders, including familial hemiplegic migraine type 1 (FHM1), episodic ataxia type 2, and spinocerebellar ataxia type 6 (SCA6). For each channelopathy, the review describes the disease phenotype as well as the functional consequences of the disease-causing mutations on recombinant human CaV2.1 channels and, in the case of FHM1 and SCA6, on neuronal CaV2.1 channels expressed at the endogenous physiological level in knockin mouse models. The effects of FHM1 mutations on cortical spreading depression, the phenomenon underlying migraine aura, and on cortical excitatory and inhibitory synaptic transmission in FHM1 knockin mice are also described, and their implications for the disease mechanism discussed. Moreover, the review describes different ataxic spontaneous cacna1a mouse mutants and the important insights into the cerebellar mechanisms underlying motor dysfunction caused by mutant CaV2.1 channels that were obtained from their functional characterization.

Genetics of headaches

Insight into the molecular mechanisms involved in primary headaches is important to identify drug targets for improving treatment of patients, but essentially lacking. Genetic research is increasingly successful in pinpointing these mechanisms. Most progress has been made for Familial Hemiplegic Migraine, a rare subtype of migraine with aura. Three genes (CACNA1A, ATP1A2 and SCN1A) have been identified that all encode ion transporters. Cellular and transgenic mouse studies suggest that neuronal hyperexcitability and increased susceptibility to cortical spreading depression, the correlate of migraine aura, are important molecular mechanisms in migraine. Investigating monogenic diseases in which migraine is a prominent feature such as CADASIL, which is caused by mutations in the NOTCH3 gene, can help understanding the pathology of migraine. Candidate gene association studies and linkage studies in the common forms of migraine were less successful. Except for the MTHFR gene no gene variant has been identified yet. Convincingly demonstrated genetic findings in other primary headaches such as cluster headache and tension-type headache are even rarer. However, with current technical possibilities of massive genotyping and international efforts to collect large well-phenotyped patient cohorts, the first gene variants for various primary headache types are likely to be discovered in the coming decade.

ELKS2alpha/CAST deletion selectively increases neurotransmitter release at inhibitory synapses

The presynaptic active zone is composed of a protein network that contains ELKS2alpha (a.k.a. CAST) as a central component. Here we demonstrate that in mice, deletion of ELKS2alpha caused a large increase in inhibitory, but not excitatory, neurotransmitter release, and potentiated the size, but not the properties, of the readily-releasable pool of vesicles at inhibitory synapses. Quantitative electron microscopy revealed that the ELKS2alpha deletion did not change the number of docked vesicles or other ultrastructural parameters of synapses, except for a small decrease in synaptic vesicle numbers. The ELKS2alpha deletion did, however, alter the excitatory/inhibitory balance and exploratory behaviors, possibly as a result of the increased synaptic inhibition. Thus, as opposed to previous studies indicating that ELKS2alpha is essential for mediating neurotransmitter release, our results suggest that ELKS2alpha normally restricts release and limits the size of the readily-releasable pool of synaptic vesicles at the active zone of inhibitory synapses.

An atypical role for collapsin response mediator protein 2 (CRMP-2) in neurotransmitter release via interaction with presynaptic voltage-gated calcium channels

Collapsin response mediator proteins (CRMPs) specify axon/dendrite fate and axonal growth of neurons through protein-protein interactions. Their functions in presynaptic biology remain unknown. Here, we identify the presynaptic N-type Ca(2+) channel (CaV2.2) as a CRMP-2-interacting protein. CRMP-2 binds directly to CaV2.2 in two regions: the channel domain I-II intracellular loop and the distal C terminus. Both proteins co-localize within presynaptic sites in hippocampal neurons. Overexpression in hippocampal neurons of a CRMP-2 protein fused to enhanced green fluorescent protein caused a significant increase in Ca(2+) channel current density, whereas lentivirus-mediated CRMP-2 knockdown abolished this effect. Interestingly, the increase in Ca(2+) current density was not due to a change in channel gating. Rather, cell surface biotinylation studies showed an increased number of CaV2.2 at the cell surface in CRMP-2-overexpressing neurons. These neurons also exhibited a significant increase in vesicular release in response to a depolarizing stimulus. Depolarization of CRMP-2-enhanced green fluorescent protein-overexpressing neurons elicited a significant increase in release of glutamate compared with control neurons. Toxin block of Ca(2+) entry via CaV2.2 abolished this stimulated release. Thus, the CRMP-2-Ca(2+) channel interaction represents a novel mechanism for modulation of Ca(2+) influx into nerve terminals and, hence, of synaptic strength.

Maturation of active zone assembly by Drosophila Bruchpilot

Synaptic vesicles fuse at active zone (AZ) membranes where Ca²⁺ channels are clustered and that are typically decorated by electron-dense projections. Recently, mutants of the Drosophilamelanogaster ERC/CAST family protein Bruchpilot (BRP) were shown to lack dense projections (T-bars) and to suffer from Ca²⁺ channel–clustering defects. In this study, we used high resolution light microscopy, electron microscopy, and intravital imaging to analyze the function of BRP in AZ assembly. Consistent with truncated BRP variants forming shortened T-bars, we identify BRP as a direct T-bar component at the AZ center with its N terminus closer to the AZ membrane than its C terminus. In contrast, Drosophila Liprin-α, another AZ-organizing protein, precedes BRP during the assembly of newly forming AZs by several hours and surrounds the AZ center in few discrete punctae. BRP seems responsible for effectively clustering Ca²⁺ channels beneath the T-bar density late in a protracted AZ formation process, potentially through a direct molecular interaction with intracellular Ca²⁺ channel domains.

Molecular genetics of migraine

Migraine is an episodic neurovascular disorder that is clinically divided into two main subtypes that are based on the absence or presence of an aura: migraine without aura (MO) and migraine with aura (MA). Current molecular genetic insight into the pathophysiology of migraine predominantly comes from studies of a rare monogenic subtype of migraine with aura called familial hemiplegic migraine (FHM). Three FHM genes have been identified, which all encode ion transporters, suggesting that disturbances in ion and neurotransmitter balances in the brain are responsible for this migraine type, and possibly the common forms of migraine. Cellular and animal models expressing FHM mutations hint toward neuronal hyperexcitability as the likely underlying disease mechanism. Additional molecular insight into the pathophysiology of migraine may come from other monogenic syndromes (for instance cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, which is caused by NOTCH3 mutations), in which migraine is prominent. Investigating patients with common forms of migraine has had limited successes. Except for 5',10'-methylenetetrahydrolate reductase, an enzyme in folate metabolism, the large majority of reported genetic associations with candidate migraine genes have not been convincingly replicated. Genetic linkage studies using migraine subtypes as an end diagnosis did not yield gene variants thus far. Clinical heterogeneity in migraine diagnosis may have hampered the identification of such variants. Therefore, the recent introduction of more refined methods of phenotyping, such as latent-class analysis and trait component analysis, may be certainly helpful. Combining the new phenotyping methods with genome-wide association studies may be a successful strategy toward identification of migraine susceptibility genes. Likely the identification of reliable biomarkers for migraine diagnosing will make these efforts even more successful.

The headache of a hyperactive calcium channel

Migraine is thought to be triggered by excessive neocortical neuronal excitability that leads to cortical spreading depression. In this issue of Neuron, Tottene et al. study a mouse model of familial hemiplegic migraine type 1, and provide evidence for the hyperactivity of P/Q-type calcium channel-mediated cortical glutamatergic synaptic transmission as an underlying mechanism for the susceptibility of cortical spreading depression initiation in migraine disorders.

Enhanced excitatory transmission at cortical synapses as the basis for facilitated spreading depression in Ca(v)2.1 knockin migraine mice

Migraine is a common disabling brain disorder. A subtype of migraine with aura (familial hemiplegic migraine type 1: FHM1) is caused by mutations in Ca(V)2.1 (P/Q-type) Ca(2+) channels. Knockin mice carrying a FHM1 mutation show increased neuronal P/Q-type current and facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura and may activate migraine headache mechanisms. We studied cortical neurotransmission in neuronal microcultures and brain slices of FHM1 mice. We show gain of function of excitatory neurotransmission due to increased action-potential-evoked Ca(2+) influx and increased probability of glutamate release at pyramidal cell synapses but unaltered inhibitory neurotransmission at fast-spiking interneuron synapses. Using an in vitro model of CSD, we show a causative link between enhanced glutamate release and CSD facilitation. The synapse-specific effect of FHM1 mutations points to disruption of excitation-inhibition balance and neuronal hyperactivity as the basis for episodic vulnerability to CSD ignition in migraine.

A presynaptic homeostatic signaling system composed of the Eph receptor, ephexin, Cdc42, and CaV2.1 calcium channels

The molecular mechanisms underlying the homeostatic modulation of presynaptic neurotransmitter release remain largely unknown. In a screen, we isolated mutations in Drosophila ephexin (Rho-type guanine nucleotide exchange factor) that disrupt the homeostatic enhancement of presynaptic release following impairment of postsynaptic glutamate receptor function at the Drosophila neuromuscular junction. We show that Ephexin is sufficient presynaptically for synaptic homeostasis and localizes in puncta throughout the nerve terminal. However, ephexin mutations do not alter other aspects of neuromuscular development, including morphology or active zone number. We then show that, during synaptic homeostasis, Ephexin functions primarily with Cdc42 in a signaling system that converges upon the presynaptic CaV2.1 calcium channel. Finally, we show that Ephexin binds the Drosophila Eph receptor (Eph) and Eph mutants disrupt synaptic homeostasis. Based on these data, we propose that Ephexin/Cdc42 couples synaptic Eph signaling to the modulation of presynaptic CaV2.1 channels during the homeostatic enhancement of presynaptic release.

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.

The role of MAP1A light chain 2 in synaptic surface retention of Cav2.2 channels in hippocampal neurons

Ca(v)2.2 channels are localized at nerve terminals where they play a critical role in neurotransmission. However, the determinant that controls surface retention of these channels has not been identified. Here, we report that presynaptic surface localization of Ca(v)2.2 is mediated through its interaction with light chain 2 (LC2) of microtubule-associated protein MAP1A. Deletion of a 23-residue binding domain within the Ca(v)2.2 C terminus resulted in reduced synaptic distribution of the mutant channels. Using an antibody generated against an extracellular epitope of Ca(v)2.2, we demonstrate that interfering the interaction with LC2 reduced surface expression of endogenous Ca(v)2.2 at presynaptic boutons. In addition, the disruption of LC2-Ca(v)2.2 coupling reduced Ca(2+)-influx into nerve terminals through Ca(v)2.2 and impaired activity-dependent FM4-64 uptake. The treatments of neurons with Latrunculin A to disrupt actin filaments resulted in reduced density of surface Ca(v)2.2-positive boutons. Furthermore, LC2NT, a LC2 truncated mutant lacking the actin-binding domain, could not rescue Ca(v)2.2 surface expression after suppressing LC2 expression with RNAi. Because actin filaments are major cytomatric components at the presynaptic boutons, these observations suggest a mechanism by which LC2 provides anchoring of surface Ca(v)2.2 to the actin cytoskeleton, thus contributing to presynaptic function.