Preferential closed-state inactivation of neuronal calcium channels

Inactivation of CaV1 and CaV2 channels

Voltage-gated Ca2+ channels (VGCCs) are highly expressed throughout numerous biological systems and play critical roles in synaptic transmission, cardiac excitation, and muscle contraction. To perform these various functions, VGCCs are highly regulated. Inactivation comprises a critical mechanism controlling the entry of Ca2+ through these channels and constitutes an important means to regulate cellular excitability, shape action potentials, control intracellular Ca2+ levels, and contribute to long-term potentiation and depression. For CaV1 and CaV2 channel families, inactivation proceeds via two distinct processes. Voltage-dependent inactivation (VDI) reduces Ca2+ entry through the channel in response to sustained or repetitive depolarization, while Ca2+-dependent inactivation (CDI) occurs in response to elevations in intracellular Ca2+ levels. These processes are critical for physiological function and undergo exquisite fine-tuning through multiple mechanisms. Here, we review known determinants and modulatory features of these two critical forms of channel regulation and their role in normal physiology and pathophysiology.

Structural biology of voltage-gated calcium channels

Voltage-gated calcium (Cav) channels mediate Ca2+ influx in response to membrane depolarization, playing critical roles in diverse physiological processes. Dysfunction or aberrant regulation of Cav channels can lead to life-threatening consequences. Cav-targeting drugs have been clinically used to treat cardiovascular and neuronal disorders for several decades. This review aims to provide an account of recent developments in the structural dissection of Cav channels. High-resolution structures have significantly advanced our understanding of the working and disease mechanisms of Cav channels, shed light on the molecular basis for their modulation, and elucidated the modes of actions (MOAs) of representative drugs and toxins. The progress in structural studies of Cav channels lays the foundation for future drug discovery efforts targeting Cav channelopathies.

Structural biology and molecular pharmacology of voltage-gated ion channels

Voltage-gated ion channels (VGICs), including those for Na+, Ca2+ and K+, selectively permeate ions across the cell membrane in response to changes in membrane potential, thus participating in physiological processes involving electrical signalling, such as neurotransmission, muscle contraction and hormone secretion. Aberrant function or dysregulation of VGICs is associated with a diversity of neurological, psychiatric, cardiovascular and muscular disorders, and approximately 10% of FDA-approved drugs directly target VGICs. Understanding the structure-function relationship of VGICs is crucial for our comprehension of their working mechanisms and role in diseases. In this Review, we discuss how advances in single-particle cryo-electron microscopy have afforded unprecedented structural insights into VGICs, especially on their interactions with clinical and investigational drugs. We present a comprehensive overview of the recent advances in the structural biology of VGICs, with a focus on how prototypical drugs and toxins modulate VGIC activities. We explore how these structures elucidate the molecular basis for drug actions, reveal novel pharmacological sites, and provide critical clues to future drug discovery.

Reverse-engineered models reveal differential membrane properties of autonomic and cutaneous unmyelinated fibers

Unmyelinated C-fibers constitute the vast majority of axons in peripheral nerves and play key roles in homeostasis and signaling pain. However, little is known about their ion channel expression, which controls their firing properties. Also, because of their small diameters (~ 1 μm), it has not been possible to characterize their membrane properties using voltage clamp. We developed a novel library of isoform-specific ion channel models to serve as the basis functions of our C-fiber models. We then developed a particle swarm optimization (PSO) framework that used the isoform-specific ion channel models to reverse engineer C-fiber membrane properties from measured autonomic and cutaneous C-fiber conduction responses. Our C-fiber models reproduced experimental conduction velocity, chronaxie, action potential duration, intracellular threshold, and paired pulse recovery cycle. The models also matched experimental activity-dependent slowing, a property not included in model optimization. We found that simple conduction responses, characterizing the action potential, were controlled by similar membrane properties in both the autonomic and cutaneous C-fiber models, but complicated conduction response, characterizing the afterpotenials, were controlled by differential membrane properties. The unmyelinated C-fiber models constitute important tools to study autonomic signaling, assess the mechanisms of pain, and design bioelectronic devices. Additionally, the novel reverse engineering approach can be applied to generate models of other neurons where voltage clamp data are not available.

A Rich Conformational Palette Underlies Human CaV2.1-Channel Availability

Depolarization-evoked opening of CaV2.1 (P/Q-type) Ca2+-channels triggers neurotransmitter release, while voltage-dependent inactivation (VDI) limits channel availability to open, contributing to synaptic plasticity. The mechanism of CaV2.1 response to voltage is unclear. Using voltage-clamp fluorometry and kinetic modeling, we optically tracked and physically characterized the structural dynamics of the four CaV2.1 voltage-sensor domains (VSDs). VSD-I seems to directly drive opening and convert between two modes of function, associated with VDI. VSD-II is apparently voltage-insensitive. VSD-III and VSD-IV sense more negative voltages and undergo voltage-dependent conversion uncorrelated with VDI. Auxiliary β -subunits regulate VSD-I-to-pore coupling and VSD conversion kinetics. CaV2.1 VSDs are differentially sensitive to voltage changes brief and long-lived. Specifically the voltage-dependent conformational changes of VSD-I are linked to synaptic release and plasticity.

Asymmetric contribution of a selectivity filter gate in triggering inactivation of CaV1.3 channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultrahighly conserved tryptophan residues in the interdomain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudotetrameric CaV pore domain for feedback inhibition.

Asymmetric Contribution of a Selectivity Filter Gate in Triggering Inactivation of CaV1.3 Channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultra-highly conserved tryptophan residues in the inter-domain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudo-tetrameric CaV pore domain for feedback inhibition.

Inactivation influences the extent of inhibition of voltage-gated Ca+2 channels by Gem-implications for channelopathies

Voltage-gated Ca2+ channels (VGCC) directly control muscle contraction and neurotransmitter release, and slower processes such as cell differentiation, migration, and death. They are potently inhibited by RGK GTP-ases (Rem, Rem2, Rad, and Gem/Kir), which decrease Ca2+ channel membrane expression, as well as directly inhibit membrane-resident channels. The mechanisms of membrane-resident channel inhibition are difficult to study because RGK-overexpression causes complete or near complete channel inhibition. Using titrated levels of Gem expression in Xenopus oocytes to inhibit WT P/Q-type calcium channels by ∼50%, we show that inhibition is dependent on channel inactivation. Interestingly, fast-inactivating channels, including Familial Hemiplegic Migraine mutants, are more potently inhibited than WT channels, while slow-inactivating channels, such as those expressed with the Cavβ2a auxiliary subunit, are spared. We found similar results in L-type channels, and, remarkably, Timothy Syndrome mutant channels were insensitive to Gem inhibition. Further results suggest that RGKs slow channel recovery from inactivation and further implicate RGKs as likely modulating factors in channelopathies.

Molecular insights into the gating mechanisms of voltage-gated calcium channel CaV2.3

High-voltage-activated R-type Ca_V2.3 channel plays pivotal roles in many physiological activities and is implicated in epilepsy, convulsions, and other neurodevelopmental impairments. Here, we determine the high-resolution cryo-electron microscopy (cryo-EM) structure of human Ca_V2.3 in complex with the α2δ1 and β1 subunits. The VSD_II is stabilized in the resting state. Electrophysiological experiments elucidate that the VSD_II is not required for channel activation, whereas the other VSDs are essential for channel opening. The intracellular gate is blocked by the W-helix. A pre-W-helix adjacent to the W-helix can significantly regulate closed-state inactivation (CSI) by modulating the association and dissociation of the W-helix with the gate. Electrostatic interactions formed between the negatively charged domain on S6_II, which is exclusively conserved in the Ca_V2 family, and nearby regions at the alpha-interacting domain (AID) and S4-S5_II helix are identified. Further functional analyses indicate that these interactions are critical for the open-state inactivation (OSI) of Ca_V2 channels.

Voltage-gated calcium currents in human dorsal root ganglion neurons

ABSTRACT

Voltage-gated calcium channels in sensory neurons underlie processes ranging from neurotransmitter release to gene expression and remain a therapeutic target for the treatment of pain. Yet virtually all we know about voltage-gated calcium channels has been obtained through the study of rodent sensory neurons and heterologously expressed channels. To address this, high voltage-activated (HVA) Ca2+ currents in dissociated human and rat dorsal root ganglion neurons were characterized with whole-cell patch clamp techniques. The HVA currents from both species shared basic biophysical and pharmacological properties. However, HVA currents in human neurons differed from those in the rat in at least 3 potentially important ways: (1) Ca2+ current density was significantly smaller, (2) the proportion of nifedipine-sensitive currents was far greater, and (3) a subpopulation of human neurons displayed relatively large constitutive current inhibition. These results highlight the need to for the study of native proteins in their native environment before initiating costly clinical trials.

Divergent Ca2+/calmodulin feedback regulation of CaV1 and CaV2 voltage-gated calcium channels evolved in the common ancestor of Placozoa and Bilateria

Ca_V1 and Ca_V2 voltage-gated calcium channels evolved from an ancestral Ca_V1/2 channel via gene duplication somewhere near the stem animal lineage. The divergence of these channel types led to distinguishing functional properties that are conserved among vertebrates and bilaterian invertebrates and contribute to their unique cellular roles. One key difference pertains to their regulation by calmodulin (CaM), wherein bilaterian Ca_V1 channels are uniquely subject to pronounced, buffer-resistant Ca²⁺/CaM-dependent inactivation, permitting negative feedback regulation of calcium influx in response to local cytoplasmic Ca²⁺ rises. Early diverging, nonbilaterian invertebrates also possess Ca_V1 and Ca_V2 channels, but it is unclear whether they share these conserved functional features. The most divergent animals to possess both Ca_V1 and Ca_V2 channels are placozoans such as Trichoplax adhaerens, which separated from other animals over 600 million years ago shortly after their emergence. Hence, placozoans can provide important insights into the early evolution of Ca_V1 and Ca_V2 channels. Here, we build upon previous characterization of Trichoplax Ca_V channels by determining the cellular expression and ion-conducting properties of the Ca_V1 channel orthologue, TCa_V1. We show that TCa_V1 is expressed in neuroendocrine-like gland cells and contractile dorsal epithelial cells. In vitro, this channel conducts dihydropyridine-insensitive, high-voltage–activated Ca²⁺ currents with kinetics resembling those of rat Ca_V1.2 but with left-shifted voltage sensitivity for activation and inactivation. Interestingly, TCa_V1, but not TCa_V2, exhibits buffer-resistant Ca²⁺/CaM-dependent inactivation, indicating that this functional divergence evolved prior to the emergence of bilaterian animals and may have contributed to their unique adaptation for cytoplasmic Ca²⁺ signaling within various cellular contexts.

Closed-state inactivation and pore-blocker modulation mechanisms of human CaV2.2

N-type voltage-gated calcium (Ca_V) channels mediate Ca²⁺ influx at presynaptic terminals in response to action potentials and play vital roles in synaptogenesis, release of neurotransmitters, and nociceptive transmission. Here, we elucidate a cryo-electron microscopy (cryo-EM) structure of the human Ca_V2.2 complex in apo, ziconotide-bound, and two Ca_V2.2-specific pore blockers-bound states. The second voltage-sensing domain (VSD) is captured in a resting-state conformation, trapped by a phosphatidylinositol 4,5-bisphosphate (PIP₂) molecule, which is distinct from the other three VSDs of Ca_V2.2, as well as activated VSDs observed in previous structures of Ca_V channels. This structure reveals the molecular basis for the unique inactivation process of Ca_V2.2 channels, in which the intracellular gate formed by S6 helices is closed and a W-helix from the domain II-III linker stabilizes closed-state inactivation. The structures of this inactivated, drug-bound complex lay a solid foundation for developing new state-dependent blockers for treatment of chronic pain.

Closed and open state dependent block of potassium channels cause opposing effects on excitability - a computational approach

Block of voltage-gated potassium (Kv) channels has been demonstrated to affect neuronal activity described as increasing excitability. The effect has been associated with a closed-state dependent block. However, the block of Kv channels in e.g. local anesthetic and antiarrhythmics, is open state-dependent. Since the reduced excitability in this case mainly is due to sodium channel block, the role of the Kv channel block is concealed. The present investigation aims to analyse the specific role of state-dependent Kv channel block for excitability. Using a computational approach, with introduced blocked states in the Kv channel of the Frankenhaeuser-Huxley axon membrane model, we calculated the effects on threshold, firing and presynaptic Ca influx. The Ca influx was obtained from an N-type Cav channel model linked to the Frankenhaeuser-Huxley membrane. The results suggested that a selective block of open Kv channels decreased the rate of repetitive firing and the consequent Ca influx, thus challenging the traditional view. In contrast, presence of a closed-state block, increased the firing rate and the Ca influx. These findings propose that Kv channel block may either increase or decrease cellular excitability, thus highlighting the importance of further investigating the role of state-specific blocking mechanisms.

Two Forms of Synaptic Depression Produced by Differential Neuromodulation of Presynaptic Calcium Channels

Neuromodulators are important regulators of synaptic transmission throughout the brain. At the presynaptic terminal, neuromodulation of calcium channels (CaVs) can affect transmission not only by changing neurotransmitter release probability, but also by shaping short-term plasticity (STP). Indeed, changes in STP are often considered a requirement for defining a presynaptic site of action. Nevertheless, some synapses exhibit non-canonical forms of neuromodulation, where release probability is altered without a corresponding change in STP. Here, we identify biophysical mechanisms whereby both canonical and non-canonical presynaptic neuromodulation can occur at the same synapse. At a subset of glutamatergic terminals in prefrontal cortex, GABAB and D1/D5 dopamine receptors suppress release probability with and without canonical increases in short-term facilitation by modulating different aspects of presynaptic CaV function. These findings establish a framework whereby signaling from multiple neuromodulators can converge on presynaptic CaVs to differentially tune release dynamics at the same synapse.

14-3-3 Proteins in Glutamatergic Synapses

The 14-3-3 proteins are a family of proteins that are highly expressed in the brain and particularly enriched at synapses. Evidence accumulated in the last two decades has implicated 14-3-3 proteins as an important regulator of synaptic transmission and plasticity. Here, we will review previous and more recent research that has helped us understand the roles of 14-3-3 proteins at glutamatergic synapses. A key challenge for the future is to delineate the 14-3-3-dependent molecular pathways involved in regulating synaptic functions.

Molecular moieties masking Ca2+-dependent facilitation of voltage-gated Cav2.2 Ca2+ channels

Ca²⁺-dependent facilitation is a positive feedback mechanism that regulates Ca_v2.1 P/Q-type channels but not closely related Ca_v2.2 N-type channels. Thomas et al. identify the molecular determinants that distinguish the ability of Ca_v2.1 and Ca_v2.2 to undergo Ca²⁺-dependent facilitation.

Voltage-gated Ca_v2.1 (P/Q-type) Ca²⁺ channels undergo Ca²⁺-dependent inactivation (CDI) and facilitation (CDF), both of which contribute to short-term synaptic plasticity. Both CDI and CDF are mediated by calmodulin (CaM) binding to sites in the C-terminal domain of the Ca_v2.1 α₁ subunit, most notably to a consensus CaM-binding IQ-like (IQ) domain. Closely related Ca_v2.2 (N-type) channels display CDI but not CDF, despite overall conservation of the IQ and additional sites (pre-IQ, EF-hand–like [EF] domain, and CaM-binding domain) that regulate CDF of Ca_v2.1. Here we investigate the molecular determinants that prevent Ca_v2.2 channels from undergoing CDF. Although alternative splicing of C-terminal exons regulates CDF of Ca_v2.1, the splicing of analogous exons in Ca_v2.2 does not reveal CDF. Transfer of sequences encoding the Ca_v2.1 EF, pre-IQ, and IQ together (EF-pre-IQ-IQ), but not individually, are sufficient to support CDF in chimeric Ca_v2.2 channels; Ca_v2.1 chimeras containing the corresponding domains of Ca_v2.2, either alone or together, fail to undergo CDF. In contrast to the weak binding of CaM to just the pre-IQ and IQ of Ca_v2.2, CaM binds to the EF-pre-IQ-IQ of Ca_v2.2 as well as to the corresponding domains of Ca_v2.1. Therefore, the lack of CDF in Ca_v2.2 likely arises from an inability of its EF-pre-IQ-IQ to transduce the effects of CaM rather than weak binding to CaM per se. Our results reveal a functional divergence in the CDF regulatory domains of Ca_v2 channels, which may help to diversify the modes by which Ca_v2.1 and Ca_v2.2 can modify synaptic transmission.

Lower Affinity of Isradipine for L-Type Ca2+ Channels during Substantia Nigra Dopamine Neuron-Like Activity: Implications for Neuroprotection in Parkinson's Disease

Ca2+-influx through L-type Ca2+-channels (LTCCs) is associated with activity-related stressful oscillations of Ca2+ levels within dopaminergic (DA) neurons in the substantia nigra (SN), which may contribute to their selective degeneration in Parkinson's disease (PD). LTCC blockers were neuroprotective in mouse neurotoxin models of PD, and isradipine is currently undergoing testing in a phase III clinical trial in early PD. We report no evidence for neuroprotection by in vivo pretreatment with therapeutically relevant isradipine plasma levels, or Cav1.3 LTCC deficiency in 6-OHDA-treated male mice. To explain this finding, we investigated the pharmacological properties of human LTCCs during SN DA-like and arterial smooth muscle (aSM)-like activity patterns using whole-cell patch-clamp recordings in HEK293 cells (Cav1.2 α1-subunit, long and short Cav1.3 α1-subunit splice variants; β3/α2δ1). During SN DA-like pacemaking, only Cav1.3 variants conducted Ca2+ current (ICa) at subthreshold potentials between action potentials. SN DA-like burst activity increased integrated ICa during (Cav1.2 plus Cav1.3) and after (Cav1.3) the burst. Isradipine inhibition was splice variant and isoform dependent, with a 5- to 11-fold lower sensitivity to Cav1.3 variants during SN DA-like pacemaking compared with Cav1.2 during aSM-like activity. Supratherapeutic isradipine concentrations reduced the pacemaker precision of adult mouse SN DA neurons but did not affect their somatic Ca2+ oscillations. Our data predict that Cav1.2 and Cav1.3 splice variants contribute differentially to Ca2+ load in SN DA neurons, with prominent Cav1.3-mediated ICa between action potentials and after bursts. The failure of therapeutically relevant isradipine levels to protect SN DA neurons can be explained by weaker state-dependent inhibition of SN DA LTCCs compared with aSM Cav1.2.SIGNIFICANCE STATEMENT The high vulnerability of dopamine (DA) neurons in the substantia nigra (SN) to neurodegenerative stressors causes Parkinson's disease (PD). Ca2+ influx through voltage-gated L-type Ca2+ channels (LTCCs), in particular Cav1.3, appears to contribute to this vulnerability, and the LTCC inhibitor isradipine is currently being tested as a neuroprotective agent for PD in a phase III clinical trial. However, in our study isradipine plasma concentrations approved for therapy were not neuroprotective in a PD mouse model. We provide an explanation for this observation by demonstrating that during SN DA-like neuronal activity LTCCs are less sensitive to isradipine than Cav1.2 LTCCs in resistance blood vessels (mediating dose-limiting vasodilating effects) and even at supratherapeutic concentrations isradipine fails to reduce somatic Ca2+ oscillations of SN DA neurons.

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

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

Analogue modulation of back-propagating action potentials enables dendritic hybrid signalling

We report that back-propagating action potentials (bAPs) are not simply digital feedback signals in dendrites but also carry analogue information about the overall state of neurons. Analogue information about the somatic membrane potential within a physiological range (from -78 to -64 mV) is retained by bAPs of dentate gyrus granule cells as different repolarization speeds in proximal dendrites and as different peak amplitudes in distal regions. These location-dependent waveform changes are reflected by local calcium influx, leading to proximal enhancement and distal attenuation during somatic hyperpolarization. The functional link between these retention and readout mechanisms of the analogue content of bAPs critically depends on high-voltage-activated, inactivating calcium channels. The hybrid bAP and calcium mechanisms report the phase of physiological somatic voltage fluctuations and modulate long-term synaptic plasticity in distal dendrites. Thus, bAPs are hybrid signals that relay somatic analogue information, which is detected by the dendrites in a location-dependent manner.

Measuring Ca2+-Dependent Modulation of Voltage-Gated Ca2+ Channels in HEK-293T Cells

Voltage-gated Ca(2+) (Cav) channels regulate a variety of biological processes, such as muscle contraction, gene expression, and neurotransmitter release. Cav channels are subject to diverse forms of regulation, including those involving the Ca(2+) ions that permeate the pore. High voltage-activated Cav channels undergo Ca(2+)-dependent inactivation (CDI) and facilitation (CDF), which can regulate processes such as cardiac rhythm and synaptic plasticity. CDI and CDF differ slightly between Cav1 (L-type) and Cav2 (P/Q-, N-, and R-type) channels. Human embryonic kidney cells transformed with SV40 large T-antigen (HEK-293T) are advantageous for studying CDI and CDF of a particular type of Cav channel. HEK-293T cells do not express endogenous Cav channels, but Cav channels can be expressed exogenously at high levels in these cells by transient transfection. This protocol describes how to characterize and analyze Ca(2+)-dependent modulation of recombinant Cav channels in HEK-293T cells.

Synaptic Vesicles: Turning Reluctance Into Action

Vesicle availability partly determines the efficacy of synaptic communication in the CNS. The authors recently found that some hippocampal glutamate vesicles exhibit reluctance to exocytose during short, high-frequency action potential trains. These same vesicles can be “coaxed” into exocytosis by increased Ca2+entry, by direct depolarization of synaptic terminals, or by challenge with hypertonic sucrose, a tool used to cause fusion of the population of release-ready synaptic vesicles. Interestingly, the authors did not find evidence of reluctance at hippocampal GABA synapses, suggesting that vesicle reluctance might be a negative feedback mechanism to prevent runaway excitation. It is also possible that synapses exhibit reluctance to retain a dormant population of readily accessible vesicles, ready to respond to triggers such as enhanced Ca2+ influx or neuromodulators. Recent work from the calyx of Held synapse suggests that reluctance might arise from inactivation of Ca2+ channels. The authors review this work, along with several other potential mechanisms of reluctance.

Large Ca²⁺-dependent facilitation of Ca(V)2.1 channels revealed by Ca²⁺ photo-uncaging

KEY POINTS

CaV 2.1 channels constitute a dominant Ca(2+) entry pathway into brain neurons, triggering downstream Ca(2+) -dependent processes such as neurotransmitter release. CaV 2.1 is itself modulated by Ca(2+) , resulting in activity-dependent enhancement of channel opening termed Ca(2+) -dependent facilitation (CDF). Real-time Ca(2+) imaging and Ca(2+) uncaging here reveal that CDF turns out to be strikingly faster, more Ca(2+) sensitive, and larger than anticipated on previous grounds. Robust resolution of the quantitative profile of CDF enables deduction of a realistic biophysical model for this process. These results suggest that CaV 2.1 CDF would figure most prominently in short-term synaptic plasticity and cerebellar Purkinje cell rhythmicity.

ABSTRACT

CaV 2.1 (P-type) voltage-gated Ca(2+) channels constitute a major source of neuronal Ca(2+) current, strongly influencing rhythmicity and triggering neurotransmitter release throughout the central nervous system. Fitting with such stature among Ca(2+) entry pathways, CaV 2.1 is itself feedback regulated by intracellular Ca(2+) , acting through calmodulin to facilitate channel opening. The precise neurophysiological role of this calcium-dependent facilitation (CDF) remains uncertain, however, in large measure because the very magnitude, Ca(2+) dependence and kinetics of CDF have resisted quantification by conventional means. Here, we utilize the photo-uncaging of Ca(2+) with CaV 2.1 channels fluxing Li(+) currents, so that voltage-dependent activation of channel gating is no longer conflated with Ca(2+) entry, and CDF is then driven solely by light-induced increases in Ca(2+) . By using this strategy, we now find that CDF can be unexpectedly large, enhancing currents by as much as twofold at physiological voltages. CDF is steeply Ca(2+) dependent, with a Hill coefficient of approximately two, a half-maximal effect reached by nearly 500 nm Ca(2+) , and Ca(2+) on/off kinetics in the order of milliseconds to tens of milliseconds. These properties were established for both native P-type currents in cerebellar Purkinje neurons, as well as their recombinant channel counterparts under heterologous expression. Such features suggest that CDF of CaV 2.1 channels may substantially enhance the regularity of rhythmic firing in cerebellar Purkinje neurons, where regularity is believed crucial for motor coordination. In addition, this degree of extensive CDF would be poised to exert large order-of-magnitude effects on short-term synaptic plasticity via rapid modulation of presynaptic Ca(2+) entry.

The tetramerization domain potentiates Kv4 channel function by suppressing closed-state inactivation

A-type Kv4 potassium channels undergo a conformational change toward a nonconductive state at negative membrane potentials, a dynamic process known as pre-open closed states or closed-state inactivation (CSI). CSI causes inhibition of channel activity without the prerequisite of channel opening, thus providing a dynamic regulation of neuronal excitability, dendritic signal integration, and synaptic plasticity at resting. However, the structural determinants underlying Kv4 CSI remain largely unknown. We recently showed that the auxiliary KChIP4a subunit contains an N-terminal Kv4 inhibitory domain (KID) that directly interacts with Kv4.3 channels to enhance CSI. In this study, we utilized the KChIP4a KID to probe key structural elements underlying Kv4 CSI. Using fluorescence resonance energy transfer two-hybrid mapping and bimolecular fluorescence complementation-based screening combined with electrophysiology, we identified the intracellular tetramerization (T1) domain that functions to suppress CSI and serves as a receptor for the binding of KID. Disrupting the Kv4.3 T1-T1 interaction interface by mutating C110A within the C3H1 motif of T1 domain facilitated CSI and ablated the KID-mediated enhancement of CSI. Furthermore, replacing the Kv4.3 T1 domain with the T1 domain from Kv1.4 (without the C3H1 motif) or Kv2.1 (with the C3H1 motif) resulted in channels functioning with enhanced or suppressed CSI, respectively. Taken together, our findings reveal a novel (to our knowledge) role of the T1 domain in suppressing Kv4 CSI, and that KChIP4a KID directly interacts with the T1 domain to facilitate Kv4.3 CSI, thus leading to inhibition of channel function.

Modest CaV1.342-selective inhibition by compound 8 is β-subunit dependent

Two voltage-gated calcium channel subtypes—Ca_V1.2 and Ca_V1.3—underlie the major L-type Ca²⁺ currents in the mammalian central nervous system. Owing to their high sequence homology, the two channel subtypes share similar pharmacological properties, and at high doses classic calcium channel blockers, such as dihydropyridines, phenylalkylamines and benzothiazepines, do not discriminate between the two channel subtypes. Recent progress in treating Parkinson’s disease (PD) was marked by the discovery of synthetic compound 8, which was reported to be a highly selective inhibitor of the Ca_V1.3 L-type calcium channels (LTCC). However, despite a previously reported IC₅₀ of ~24 μM, in our hands inhibition of the full-length Ca_V1.3₄₂ by compound 8 at 50 μM reaches a maximum of 45%. Moreover, we find that the selectivity of compound 8 towards Ca_V1.3 relative to Ca_V1.2_B15 channels is greatly influenced by the β-subunit type and its splice isoform variants.

Compound 8-1-(3-chlorophenethyl)-3-cyclopentylpyrimidine-2,4,6(1H,3H,5H)-trione was previously reported to be a selective inhibitor of the Ca_V1.3 calcium channel. Now, Huang et al. demonstrate that selectivity towards Ca_V1.3 relative to Ca_V1.2 is dependent on the type of β-subunit and Ca_V1.3 splice variant assayed.

Synaptic properties of SOM- and CCK-expressing cells in dentate gyrus interneuron networks

Hippocampal GABAergic cells are highly heterogeneous, but the functional significance of this diversity is not fully understood. By using paired recordings of synaptically connected interneurons in slice preparations of the rat and mouse dentate gyrus (DG), we show that morphologically identified interneurons form complex neuronal networks. Synaptic inhibitory interactions exist between cholecystokinin (CCK)-expressing hilar commissural associational path (HICAP) cells and among somatostatin (SOM)-containing hilar perforant path-associated (HIPP) interneurons. Moreover, both interneuron types inhibit parvalbumin (PV)-expressing perisomatic inhibitory basket cells (BCs), whereas BCs and HICAPs rarely target HIPP cells. HICAP and HIPP cells produce slow, weak, and unreliable inhibition onto postsynaptic interneurons. The time course of inhibitory signaling is defined by the identity of the presynaptic and postsynaptic cell. It is the slowest for HIPP-HIPP, intermediately slow for HICAP-HICAP, but fast for BC-BC synapses. GABA release at interneuron-interneuron synapses also shows cell type-specific short-term dynamics, ranging from multiple-pulse facilitation at HICAP-HICAP, biphasic modulation at HIPP-HIPP to depression at BC-BC synapses. Although dendritic inhibition at HICAP-BC and HIPP-BC synapses appears weak and slow, channelrhodopsin 2-mediated excitation of SOM terminals demonstrates that they effectively control the activity of target interneurons. They markedly reduce the discharge probability but sharpen the temporal precision of action potential generation. Thus, dendritic inhibition seems to play an important role in determining the activity pattern of GABAergic interneuron populations and thereby the flow of information through the DG circuitry.

Gene splicing of an invertebrate beta subunit (LCavβ) in the N-terminal and HOOK domains and its regulation of LCav1 and LCav2 calcium channels

The accessory beta subunit (Ca(v)β) of calcium channels first appear in the same genome as Ca(v)1 L-type calcium channels in single-celled coanoflagellates. The complexity of this relationship expanded in vertebrates to include four different possible Ca(v)β subunits (β1, β2, β3, β4) which associate with four Ca(v)1 channel isoforms (Ca(v)1.1 to Ca(v)1.4) and three Ca(v)2 channel isoforms (Ca(v)2.1 to Ca(v)2.3). Here we assess the fundamentally-shared features of the Ca(v)β subunit in an invertebrate model (pond snail Lymnaea stagnalis) that bears only three homologous genes: (LCa(v)1, LCa(v)2, and LCa(v)β). Invertebrate Ca(v)β subunits (in flatworms, snails, squid and honeybees) slow the inactivation kinetics of Ca(v)2 channels, and they do so with variable N-termini and lacking the canonical palmitoylation residues of the vertebrate β2a subunit. Alternative splicing of exon 7 of the HOOK domain is a primary determinant of a slow inactivation kinetics imparted by the invertebrate LCa(v)β subunit. LCa(v)β will also slow the inactivation kinetics of LCa(v)3 T-type channels, but this is likely not physiologically relevant in vivo. Variable N-termini have little influence on the voltage-dependent inactivation kinetics of differing invertebrate Ca(v)β subunits, but the expression pattern of N-terminal splice isoforms appears to be highly tissue specific. Molluscan LCa(v)β subunits have an N-terminal "A" isoform (coded by exons: 1a and 1b) that structurally resembles the muscle specific variant of vertebrate β1a subunit, and has a broad mRNA expression profile in brain, heart, muscle and glands. A more variable "B" N-terminus (exon 2) in the exon position of mammalian β3 and has a more brain-centric mRNA expression pattern. Lastly, we suggest that the facilitation of closed-state inactivation (e.g. observed in Ca(v)2.2 and Ca(v)β3 subunit combinations) is a specialization in vertebrates, because neither snail subunit (LCa(v)2 nor LCa(v)β) appears to be compatible with this observed property.

Impaired GABAergic inhibition in the prefrontal cortex of early postnatal phencyclidine (PCP)-treated rats

A compromised γ-aminobutyric acid (GABA)ergic system is hypothesized to be part of the underlying pathophysiology of schizophrenia. N-methyl-D-aspartate (NMDA) receptor hypofunction during neurodevelopment is proposed to disrupt maturation of interneurons causing an impaired GABAergic transmission in adulthood. The present study examines prefrontal GABAergic transmission in adult rats administered with the NMDA receptor channel blocker, phencyclidine (PCP), for 3 days during the second postnatal week. Whole-cell patch-clamp recordings from pyramidal cells in PCP-treated rats showed a 22% reduction in the frequency of miniature inhibitory postsynaptic currents in layer II/III, but not in layer V pyramidal neurons of the prefrontal cortex. Furthermore, early postnatal PCP treatment caused insensitivity toward effects of the GABA transporter 1 (GAT-1) inhibitor, 1,2,5,6-tetrahydro-1-[2-[[(diphenyl-methylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid, and also diminished currents passed by δ-subunit-containing GABAA receptors in layer II/III pyramidal neurons. The observed impairments in GABAergic function are compatible with the alteration of GABAergic markers as well as cognitive dysfunction observed in early postnatal PCP-treated rats and support the hypothesis that PCP administration during neurodevelopment affects the functionality of interneurons in later life.

Theoretical models of synaptic short term plasticity

Short term plasticity is a highly abundant form of rapid, activity-dependent modulation of synaptic efficacy. A shared set of mechanisms can cause both depression and enhancement of the postsynaptic response at different synapses, with important consequences for information processing. Mathematical models have been extensively used to study the mechanisms and roles of short term plasticity. This review provides an overview of existing models and their biological basis, and of their main properties. Special attention will be given to slow processes such as calcium channel inactivation and the effect of activation of presynaptic autoreceptors.

Modeling-independent elucidation of inactivation pathways in recombinant and native A-type Kv channels

A-type voltage-gated K⁺ (Kv) channels self-regulate their activity by inactivating directly from the open state (open-state inactivation [OSI]) or by inactivating before they open (closed-state inactivation [CSI]). To determine the inactivation pathways, it is often necessary to apply several pulse protocols, pore blockers, single-channel recording, and kinetic modeling. However, intrinsic hurdles may preclude the standardized application of these methods. Here, we implemented a simple method inspired by earlier studies of Na⁺ channels to analyze macroscopic inactivation and conclusively deduce the pathways of inactivation of recombinant and native A-type Kv channels. We investigated two distinct A-type Kv channels expressed heterologously (Kv3.4 and Kv4.2 with accessory subunits) and their native counterparts in dorsal root ganglion and cerebellar granule neurons. This approach applies two conventional pulse protocols to examine inactivation induced by (a) a simple step (single-pulse inactivation) and (b) a conditioning step (double-pulse inactivation). Consistent with OSI, the rate of Kv3.4 inactivation (i.e., the negative first derivative of double-pulse inactivation) precisely superimposes on the profile of the Kv3.4 current evoked by a single pulse because the channels must open to inactivate. In contrast, the rate of Kv4.2 inactivation is asynchronous, already changing at earlier times relative to the profile of the Kv4.2 current evoked by a single pulse. Thus, Kv4.2 inactivation occurs uncoupled from channel opening, indicating CSI. Furthermore, the inactivation time constant versus voltage relation of Kv3.4 decreases monotonically with depolarization and levels off, whereas that of Kv4.2 exhibits a J-shape profile. We also manipulated the inactivation phenotype by changing the subunit composition and show how CSI and CSI combined with OSI might affect spiking properties in a full computational model of the hippocampal CA1 neuron. This work unambiguously elucidates contrasting inactivation pathways in neuronal A-type Kv channels and demonstrates how distinct pathways might impact neurophysiological activity.

Shab K (+) channel slow inactivation: a test for U-type inactivation and a hypothesis regarding K (+) -facilitated inactivation mechanisms

Herein, we report the first characterization of Shab slow inactivation. Open Shab channels inactivate within seconds, with two voltage-independent time constants. Additionally, Shab presents significant closed-state inactivation. We found that with short depolarizing pulses, shorter than the slowest inactivation time constant, the resulting inactivation curve has a marked U-shape, but as pulse duration increases, approaching steady-state conditions, the U-shape vanishes, and the resulting inactivation curves converge to the classical Boltzmann h∞ curve. Regarding the mechanism of inactivation, we found that external K (+) and TEA facilitate both open- and closed-state inactivation, while the cavity blocker quinidine hinders inactivation. These results together with our previous observations regarding the K (+) -dependent stability of the K (+) conductance, suggest the novel hypothesis that inactivation of Shab channels, and possibly that of other Kv channels whose inactivation is facilitated by K (+) , does not involve a significant narrowing of the extracellular entry of the pore. Instead, we hypothesize that there is only a rearrangement of a more internal segment of the pore that affects the central cavity and halts K (+) conduction.

Mechanisms of conotoxin inhibition of N-type (Ca(v)2.2) calcium channels

N-type (Ca(v)2.2) voltage-gated calcium channels (VGCC) transduce electrical activity into other cellular functions, regulate calcium homeostasis and play a major role in processing pain information. Although the distribution and function of these channels vary widely among different classes of neurons, they are predominantly expressed in nerve terminals, where they control neurotransmitter release. To date, genetic and pharmacological studies have identified that high-threshold, N-type VGCCs are important for pain sensation in disease models. This suggests that N-type VGCC inhibitors or modulators could be developed into useful drugs to treat neuropathic pain. This review discusses the role of N-type (Ca(v)2.2) VGCCs in nociception and pain transmission through primary sensory dorsal root ganglion (DRG) neurons (nociceptors). It also outlines the potent and selective inhibition of N-type VGCCs by conotoxins, small disulfide-rich peptides isolated from the venom of marine cone snails. Of these conotoxins, ω-conotoxins are selective N-type VGCC antagonists that preferentially block nociception in inflammatory pain models, and allodynia and/or hyperalgesia in neuropathic pain models. Another conotoxin family, α-conotoxins, were initially proposed as competitive antagonists of muscle and neuronal nicotinic acetylcholine receptors (nAChR). Surprisingly, however, α-conotoxins Vc1.1 and RgIA, also potently inhibit N-type VGCC currents in the sensory DRG neurons of rodents and α9 nAChR knockout mice, via intracellular signaling mediated by G protein-coupled GABAB receptors. Understanding how conotoxins inhibit VGCCs is critical for developing these peptides into analgesics and may result in better pain management. This article is part of a Special Issue entitled: Calcium channels.

Ca²⁺-dependent regulation of Ca²⁺ currents in rat primary afferent neurons: role of CaMKII and the effect of injury

Currents through voltage-gated Ca²⁺ channels (I(Ca)) may be regulated by cytoplasmic Ca²⁺ levels ([Ca²⁺](c)), producing Ca²⁺-dependent inactivation (CDI) or facilitation (CDF). Since I(Ca) regulates sensory neuron excitability, altered CDI or CDF could contribute to pain generation after peripheral nerve injury. We explored this by manipulating [Ca²⁺](c) while recording I(Ca) in rat sensory neurons. In uninjured neurons, elevating [Ca²⁺](c) with a conditioning prepulse (-15 mV, 2 s) inactivated I(Ca) measured during subsequent test pulses (-15 mV, 5 ms). This inactivation was Ca²⁺-dependent (CDI), since it was decreased with elimination of Ca²⁺ influx by depolarization to above the I(Ca) reversal potential, with high intracellular Ca²⁺ buffering (EGTA 10 mm or BAPTA 20 mm), and with substitution of Ba²⁺ for extracellular Ca²⁺, revealing a residual voltage-dependent inactivation. At longer latencies after conditioning (>6 s), I(Ca) recovered beyond baseline. This facilitation also proved to be Ca²⁺-dependent (CDF) using the protocols limiting cytoplasmic Ca²⁺ elevation. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) blockers applied by bath (KN-93, myristoyl-AIP) or expressed selectively in the sensory neurons (AIP) reduced CDF, unlike their inactive analogues. Protein kinase C inhibition (chelerythrine) had no effect. Selective blockade of N-type Ca²⁺ channels eliminated CDF, whereas L-type channel blockade had no effect. Following nerve injury, CDI was unaffected, but CDF was eliminated in axotomized neurons. Excitability of sensory neurons in intact ganglia from control animals was diminished after a similar conditioning pulse, but this regulation was eliminated by injury. These findings indicate that I(Ca) in sensory neurons is subject to both CDI and CDF, and that hyperexcitability following injury-induced loss of CDF may result from diminished CaMKII activity.

Regulation of voltage-dependent calcium channels by RGK proteins

RGK proteins belong to the Ras superfamily of monomeric G-proteins, and currently include four members - Rad, Rem, Rem2, and Gem/Kir. RGK proteins are broadly expressed, and are the most potent known intracellular inhibitors of high-voltage-activated Ca²⁺ (Ca(V)1 and Ca(V)2) channels. Here, we review and discuss the evidence in the literature regarding the functional mechanisms, structural determinants, physiological role, and potential practical applications of RGK-mediated inhibition of Ca(V)1/Ca(V)2 channels. This article is part of a Special Issue entitled: Calcium channels.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

The role of a voltage-dependent Ca2+ channel intracellular linker: a structure-function analysis

Voltage-dependent calcium channels (VDCCs) allow the passage of Ca(2+) ions through cellular membranes in response to membrane depolarization. The channel pore-forming subunit, α1, and a regulatory subunit (Ca(V)β) form a high affinity complex where Ca(V)β binds to a α1 interacting domain in the intracellular linker between α1 membrane domains I and II (I-II linker). We determined crystal structures of Ca(V)β2 functional core in complex with the Ca(V)1.2 and Ca(V)2.2 I-II linkers to a resolution of 1.95 and 2.0 Å, respectively. Structural differences between the highly conserved linkers, important for coupling Ca(V)β to the channel pore, guided mechanistic functional studies. Electrophysiological measurements point to the importance of differing linker structure in both Ca(V)1 and 2 subtypes with mutations affecting both voltage- and calcium-dependent inactivation and voltage dependence of activation. These linker effects persist in the absence of Ca(V)β, pointing to the intrinsic role of the linker in VDCC function and suggesting that I-II linker structure can serve as a brake during inactivation.

Ginsenoside Rb1 selectively inhibits the activity of L-type voltage-gated calcium channels in cultured rat hippocampal neurons

AIM

To investigate the effect of ginsenoside Rb1 on voltage-gated calcium currents in cultured rat hippocampal neurons and the modulatory mechanism.

METHODS

Cultured hippocampal neurons were prepared from Sprague Dawley rat embryos. Whole-cell configuration of the patch-clamp technique was used to record the voltage-gated calcium currents (VGCCs) from the hippocampal neurons,and the effect of Rb1 was examined.

RESULTS

Rb1 (2-100 μmol/L) inhibited VGCCs in a concentration-dependent manner, and the current was mostly recovered upon wash-out. The specific L-type Ca(2+) channel inhibitor nifedipine (10 μmol/L) occluded Rb1-induced inhibition on VGCCs. Neither the selective N-type Ca(2+) channel blocker ω-conotoxin-GVIA (1 μmol/L), nor the selective P/Q-type Ca(2+) channel blocker ω-agatoxin IVA (30 nmol/L) diminished Rb1-sensitive VGCCs. Rb1 induced a leftward shift of the steady-state inactivation curve of I(Ca) to a negative potential without affecting its activation kinetics or reversal potential in the I-V curve. The inhibitory effect of Rb1 was neither abolished by the adenylyl cyclase activator forskolin (10 μmol/L), nor by the PKA inhibitor H-89 (10 μmol/L).

CONCLUSION

Ginsenoside Rb1 selectively inhibits the activity of L-type voltage-gated calcium channels, without affecting the N-type or P/Q-type Ca(2+) channels in hippocampal neurons. cAMP-PKA signaling pathway is not involved in this effect.

Similar intracellular Ca2+ requirements for inactivation and facilitation of voltage-gated Ca2+ channels in a glutamatergic mammalian nerve terminal

Voltage-gated Ca2+ channels (VGCCs) of the P/Q-type, which are expressed at a majority of mammalian nerve terminals, show two types of Ca2+-dependent feedback regulation-inactivation (CDI) and facilitation (CDF). Because of the nonlinear relationship between Ca2+ influx and transmitter release, CDI and CDF are powerful regulators of synaptic strength. To what extent VGCCs inactivate or facilitate during spike trains depends on the dynamics of free Ca2+ ([Ca2+]i) and the Ca2+ sensitivity of CDI and CDF, which has not been determined in nerve terminals. In this report, we took advantage of the large size of a rat auditory glutamatergic synapse--the calyx of Held--and combined voltage-clamp recordings of presynaptic Ca2+ currents (ICa(V)) with UV-light flash-induced Ca2+ uncaging and presynaptic Ca2+ imaging to study the Ca2+ requirements for CDI and CDF. We find that nearly half of the presynaptic VGCCs inactivate during 100 ms voltage steps and require several seconds to recover. This inactivation is caused neither by depletion of Ca2+ ions from the synaptic cleft nor by metabotropic feedback inhibition, because it is resistant to blockade of metabotropic and ionotropic glutamate receptors. Facilitation of ICa(V) induced by repetitive depolarizations or preconditioning voltage steps decays within tens of milliseconds. Since Ca2+ buffers only weakly affect CDI and CDF, we conclude that the Ca2+ sensors are closely associated with the channel. CDI and CDF can be induced by intracellular photo release of Ca2+ resulting in [Ca2+]i elevations in the low micromolar range, implying a surprisingly high affinity of the Ca2+ sensors.

Bile Acid Inhibition of N-type Calcium Channel Currents from Sympathetic Ganglion Neurons

Under some pathological conditions as bile flow obstruction or liver diseases with the enterohepatic circulation being disrupted, regurgitation of bile acids into the systemic circulation occurs and the plasma level of bile acids increases. Bile acids in circulation may affect the nervous system. We examined this possibility by studying the effects of bile acids on gating of neuronal (N)-type Ca²⁺ channel that is essential for neurotransmitter release at synapses of the peripheral and central nervous system. N-type Ca²⁺ channel currents were recorded from bullfrog sympathetic neuron under a cell-attached mode using 100 mM Ba²⁺ as a charge carrier. Cholic acid (CA, 10^-6 M) that is relatively hydrophilic thus less cytotoxic was included in the pipette solution. CA suppressed the open probability of N-type Ca²⁺ channel, which appeared to be due to an increase in null (no activity) sweeps. For example, the proportion of null sweep in the presence of CA was ~40% at +40 mV as compared with ~8% in the control recorded without CA. Other single channel properties including slope conductance, single channel current amplitude, open and shut times were not significantly affected by CA being present. The results suggest that CA could modulate N-type Ca²⁺ channel gating at a concentration as low as 10^-6 M. Bile acids have been shown to activate nonselective cation conductance and depolarize the cell membrane. Under pathological conditions with increased circulating bile acids, CA suppression of N-type Ca²⁺ channel function may be beneficial against overexcitation of the synapses.

Functional characterization of alternative splicing in the C terminus of L-type CaV1.3 channels

Ca(V)1.3 channels are unique among the high voltage-activated Ca(2+) channel family because they activate at the most negative potentials and display very rapid calcium-dependent inactivation. Both properties are of crucial importance in neurons of the suprachiasmatic nucleus and substantia nigra, where the influx of Ca(2+) ions at subthreshold membrane voltages supports pacemaking function. Previously, alternative splicing in the Ca(V)1.3 C terminus gives rise to a long (Ca(V)1.3(42)) and a short form (Ca(V)1.3(42A)), resulting in a pronounced activation at more negative voltages and faster inactivation in the latter. It was further shown that the C-terminal modulator in the Ca(V)1.3(42) isoforms modulates calmodulin binding to the IQ domain. Using splice variant-specific antibodies, we determined that protein localization of both splice variants in different brain regions were similar. Using the transcript-scanning method, we further identified alternative splicing at four loci in the C terminus of Ca(V)1.3 channels. Alternative splicing of exon 41 removes the IQ motif, resulting in a truncated Ca(V)1.3 protein with diminished inactivation. Splicing of exon 43 causes a frameshift and exhibits a robust inactivation of similar intensity to Ca(V)1.3(42A). Alternative splicing of exons 44 and 48 are in-frame, altering interaction of the distal modulator with the IQ domain and tapering inactivation slightly. Thus, alternative splicing in the C terminus of Ca(V)1.3 channels modulates its electrophysiological properties, which could in turn alter neuronal firing properties and functions.

Synaptically activated Ca2+ waves and NMDA spikes locally suppress voltage-dependent Ca2+ signalling in rat pyramidal cell dendrites

Postsynaptic [Ca(2+)](i) changes contribute to several kinds of plasticity in pyramidal neurons. We examined the effects of synaptically activated Ca(2+) waves and NMDA spikes on subsequent Ca(2+) signalling in CA1 pyramidal cell dendrites in hippocampal slices. Tetanic synaptic stimulation evoked a localized Ca(2+) wave in the primary apical dendrites. The [Ca(2+)](i) increase from a backpropagating action potential (bAP) or subthreshold depolarization was reduced if it was generated immediately after the wave. The suppression had a recovery time of 30-60 s. The suppression only occurred where the wave was generated and was not due to a change in bAP amplitude or shape. The suppression also could be generated by Ca(2+) waves evoked by uncaging IP(3), showing that other signalling pathways activated by the synaptic tetanus were not required. The suppression was proportional to the amplitude of the [Ca(2+)](i) change of the Ca(2+) wave and was not blocked by a spectrum of kinase or phosphatase inhibitors, consistent with suppression due to Ca(2+)-dependent inactivation of Ca(2+) channels. The waves also reduced the frequency and amplitude of spontaneous, localized Ca(2+) release events in the dendrites by a different mechanism, probably by depleting the stores at the site of wave generation. The same synaptic tetanus often evoked NMDA spike-mediated [Ca(2+)](i) increases in the oblique dendrites where Ca(2+) waves do not propagate. These NMDA spikes suppressed the [Ca(2+)](i) increase caused by bAPs in those regions. [Ca(2+)](i) increases by Ca(2+) entry through voltage-gated Ca(2+) channels also suppressed the [Ca(2+)](i) increases from subsequent bAPs in regions where the voltage-gated [Ca(2+)](i) increases were largest, showing that all ways of raising [Ca(2+)](i) could cause suppression.

Double mutant cycle analysis identified a critical leucine residue in the IIS4S5 linker for the activation of the Ca(V)2.3 calcium channel

Mutations in distal S6 were shown to significantly alter the stability of the open state of Ca(V)2.3 (Raybaud, A., Baspinar, E. E., Dionne, F., Dodier, Y., Sauvé, R., and Parent, L. (2007) J. Biol. Chem. 282, 27944-27952). By analogy with K(V) channels, we tested the hypothesis that channel activation involves electromechanical coupling between S6 and the S4S5 linker in Ca(V)2.3. Among the 11 positions tested in the S4S5 linker of domain II, mutations of the leucine residue at position 596 were found to destabilize significantly the closed state with a -50 mV shift in the activation potential and a -20 mV shift in its charge-voltage relationship as compared with Ca(V)2.3 wt. A double mutant cycle analysis was performed by introducing pairs of glycine residues between S4S5 and S6 of Domain II. Strong coupling energies (ΔΔG(interact) > 2 kcal mol(-1)) were measured for the activation gating of 12 of 39 pairs of mutants. Leu-596 (IIS4S5) was strongly coupled with distal residues in IIS6 from Leu-699 to Asp-704. In particular, the double mutant L596G/I701G showed strong cooperativity with a ΔΔG(interact) ≈6 kcal mol(-1) suggesting that both positions contribute to the activation gating of the channel. Altogether, our results highlight the role of a leucine residue in S4S5 and provide the first series of evidence that the IIS4S5 and IIS6 regions are energetically coupled during the activation of a voltage-gated Ca(V) channel.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Pregabalin modulation of neurotransmitter release is mediated by change in intrinsic activation/inactivation properties of ca(v)2.1 calcium channels

In this work, we studied the effects of the anticonvulsant and analgesic drug pregabalin (PGB) on excitatory postsynaptic currents (EPSCs) at principal neurons of the mouse medial nucleus of the trapezoid body and on presynaptic calcium currents at the calyx of Held. We found that the acute application of PGB reduced the amplitude of EPSCs in a dose-dependent manner with a maximal blocking effect of approximately 30%. A clinical high-concentration dose of PGB (e.g., 500 μM) blocked Ca(v)2.1 channel-mediated currents and decreased their facilitation during a 100-Hz train, without changing their voltage-dependent activation. Furthermore, PGB also removed the inactivation of Ca(v)2.1 channels at a clinically relevant low concentration of 100 μM. These results suggest novel modulatory mechanisms mediated by the acute administration of PGB on fast excitatory synaptic transmission and might contribute to better understanding PGB anticonvulsant/analgesic clinical effects.

Mechanisms of closed-state inactivation in voltage-gated ion channels

Inactivation of voltage-gated ion channels is an intrinsic auto-regulatory process necessary to govern the occurrence and shape of action potentials and establish firing patterns in excitable tissues. Inactivation may occur from the open state (open-state inactivation, OSI) at strongly depolarized membrane potentials, or from pre-open closed states (closed-state inactivation, CSI) at hyperpolarized and modestly depolarized membrane potentials. Voltage-gated Na(+), K(+), Ca(2+) and non-selective cationic channels utilize both OSI and CSI. Whereas there are detailed mechanistic descriptions of OSI, much less is known about the molecular basis of CSI. Here, we review evidence for CSI in voltage-gated cationic channels (VGCCs) and recent findings that shed light on the molecular mechanisms of CSI in voltage-gated K(+) (Kv) channels. Particularly, complementary observations suggest that the S4 voltage sensor, the S4S5 linker and the main S6 activation gate are instrumental in the installment of CSI in Kv4 channels. According to this hypothesis, the voltage sensor may adopt a distinct conformation to drive CSI and, depending on the stability of the interactions between the voltage sensor and the pore domain, a closed-inactivated state results from rearrangements in the selectivity filter or failure of the activation gate to open. Kv4 channel CSI may efficiently exploit the dynamics of the subthreshold membrane potential to regulate spiking properties in excitable tissues.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Contribution of calcium-dependent facilitation to synaptic plasticity revealed by migraine mutations in the P/Q-type calcium channel

The dynamics, computational power, and strength of neural circuits are essential for encoding and processing information in the CNS and rely on short and long forms of synaptic plasticity. In a model system, residual calcium (Ca(2+)) in presynaptic terminals can act through neuronal Ca(2+) sensor proteins to cause Ca(2+)-dependent facilitation (CDF) of P/Q-type channels and induce short-term synaptic facilitation. However, whether this is a general mechanism of plasticity at intact central synapses and whether mutations associated with human disease affect this process have not been described to our knowledge. In this report, we find that, in both exogenous and native preparations, gain-of-function missense mutations underlying Familial Hemiplegic Migraine type 1 (FHM-1) occlude CDF of P/Q-type Ca(2+) channels. In FHM-1 mutant mice, the alteration of P/Q-type channel CDF correlates with reduced short-term synaptic facilitation at cerebellar parallel fiber-to-Purkinje cell synapses. Two-photon imaging suggests that P/Q-type channels at parallel fiber terminals in FHM-1 mice are in a basally facilitated state. Overall, the results provide evidence that FHM-1 mutations directly affect both P/Q-type channel CDF and synaptic plasticity and that together likely contribute toward the pathophysiology underlying FHM-1. The findings also suggest that P/Q-type channel CDF is an important mechanism required for normal synaptic plasticity at a fast synapse in the mammalian CNS.