Regulation Of Voltage-gated Ion Channels Research Articles

The role of astrocytes from synaptic to non-synaptic plasticity.

Information storage and transfer in the brain require a high computational power. Neuronal network display various local or global mechanisms to allow information storage and transfer in the brain. From synaptic to intrinsic plasticity, the rules of input-output function modulation have been well characterized in neurons. In the past years, astrocytes have been suggested to increase the computational power of the brain and we are only just starting to uncover their role in information processing. Astrocytes maintain a close bidirectional communication with neurons to modify neuronal network excitability, transmission, axonal conduction, and plasticity through various mechanisms including the release of gliotransmitters or local ion homeostasis. Astrocytes have been significantly studied in the context of long-term or short-term synaptic plasticity, but this is not the only mechanism involved in memory formation. Plasticity of intrinsic neuronal excitability also participates in memory storage through regulation of voltage-gated ion channels or axonal morphological changes. Yet, the contribution of astrocytes to these other forms of non-synaptic plasticity remains to be investigated. In this review, we summarized the recent advances on the role of astrocytes in different forms of plasticity and discuss new directions and ideas to be explored regarding astrocytes-neuronal communication and regulation of plasticity.

Maintenance of CaV2.2 channel-current by PIP2 unveiled by neomycin in sympathetic neurons of the rat

Membrane lipids are key determinants in the regulation of voltage-gated ion channels. Phosphatidylinositol 4,5-bisphosphate (PIP2), a native membrane phospholipid, has been involved in the maintenance of the current amplitude and in the voltage-independent regulation of voltage-gated calcium channels (VGCC). However, the nature of the PIP2 regulation on VGCC has not been fully elucidated. This work aimed to investigate whether the interacting PIP2 electrostatic charges may account for maintaining the current amplitude of CaV2.2 channels. Furthermore, we tested whether charge shielding of PIP2 mimics the voltage-independent inhibition induced by M1 muscarinic acetylcholine receptor (M1R) activation. Therefore, neomycin, a polycation that has been shown to block electrostatic interactions of PIP2, was intracellularly dialyzed in superior cervical ganglion (SCG) neurons of the rat. Consistently, neomycin time-dependently diminished the calcium current amplitude letting the channel exhibit the hallmarks of the voltage-independent regulation. These results support that interacting PIP2 charges not only underly the maintenance of the channel-current but also that charge screening of PIP2 by itself unveils the voltage-independent features of CaV2.2 channels in SCG neurons.

Cerebellar learning modulates surface expression of a voltage-gated ion channel in cerebellar cortex

Numerous experiments using ex vivo electrophysiology suggest that mammalian learning and memory involves regulation of voltage-gated ion channels in terms of changes in function. Yet, little is known about learning-related regulation of voltage-gated ion channels in terms of changes in expression. In two experiments, we examined changes in cell surface expression of the voltage-gated potassium channel alpha-subunit Kv1.2 in a discrete region of cerebellar cortex after eyeblink conditioning (EBC), a well-studied form of cerebellar-dependent learning. Kv1.2 in cerebellar cortex is expressed almost entirely in basket cells, primarily in the axon terminal pinceaux (PCX) region, and Purkinje cells, primarily in dendrites. Cell surface expression of Kv1.2 was measured using both multiphoton microscopy, which allowed measurement confined to the PCX region, and biotinylation/western blot, which measured total cell surface expression. In the first experiment, rats underwent three sessions of EBC, explicitly unpaired stimulus exposure, or context-only exposure and the results revealed a decrease in Kv1.2 cell surface expression in the unpaired group as measured with microscopy but no change as measured with western blot. In the second experiment, the same three training groups underwent only one half of a session of training, and the results revealed an increase in Kv1.2 cell surface expression in the unpaired group as measured with western blot but no change as measured with microscopy. In addition, rats in the EBC group that did not express conditioned responses (CRs) exhibited the same increase in Kv1.2 cell surface expression as the unpaired group. The overall pattern of results suggests that cell surface expression of Kv1.2 is changed with exposure to EBC stimuli in the absence, or prior to the emergence, of CRs.

Nedd4-2 regulation of voltage-gated ion channels: an update on structure–function relationships and the pathophysiological consequences of dysfunction

Neuronal excitability is mediated mainly by voltage-gated ion channels (VGICs), which include voltage-gated Na + , K + , and Cl − channels located along the axon and at neuronal synapses. Voltage-gated channels play pivotal roles in the proper functioning of the nervous system because they set the resting membrane potential, initiate and propagate action poten- tials, and regulate neurotransmitter release. The abnormal activity or misregulation of VGICs caused by mutations has been directly linked to neurological and cardiac diseases. Among other posttranslational modifications, the ubiquitination of VGICs is a key to the regulation of the number of channels in the cell surface, and hence, neuronal excitability. Nedd4-2 is an E3 ubiquitin ligase that ubiquitinates several proteins, including different VGICs. Accordingly, understanding the molecular mechanisms underlying channel regulation will provide insights to design drugs to treat illnesses. The focus of the present review is to provide an update about the regulation of VGICs upon ubiquitination by Nedd4-2 and the relevance of such regulation in the pathophysiological consequences of dysfunction.

Spike-Time Precision and Network Synchrony Are Controlled by the Homeostatic Regulation of the D-Type Potassium Current

Homeostatic plasticity of neuronal intrinsic excitability (HPIE) operates to maintain networks within physiological bounds in response to chronic changes in activity. Classically, this form of plasticity adjusts the output firing level of the neuron through the regulation of voltage-gated ion channels. Ion channels also determine spike timing in individual neurons by shaping subthreshold synaptic and intrinsic potentials. Thus, an intriguing hypothesis is that HPIE can also regulate network synchronization. We show here that the dendrotoxin-sensitive D-type K+ current (ID) disrupts the precision of AP generation in CA3 pyramidal neurons and may, in turn, limit network synchronization. The reduced precision is mediated by the sequence of outward ID followed by inward Na+ current. The homeostatic downregulation of ID increases both spike-time precision and the propensity for synchronization in iteratively constructed networks in vitro. Thus, network synchronization is adjusted in area CA3 through activity-dependent remodeling of ID.

Spike-timing dependent plasticity beyond synapse - pre- and post-synaptic plasticity of intrinsic neuronal excitability

Long-lasting plasticity of synaptic transmission is classically thought to be the cellular substrate for information storage in the brain. Recent data indicate however that it is not the whole story and persistent changes in the intrinsic neuronal excitability have been shown to occur in parallel to the induction of long-term synaptic modifications. This form of plasticity depends on the regulation of voltage-gated ion channels. Here we review the experimental evidence for plasticity of neuronal excitability induced at pre- or postsynaptic sites when long-term plasticity of synaptic transmission is induced with Spike-Timing Dependent Plasticity (STDP) protocols. We describe the induction and expression mechanisms of the induced changes in excitability. Finally, the functional synergy between synaptic and non-synaptic plasticity and their spatial extent are discussed.

Plasticity of neuronal excitability in vivo

Plasticity of neuronal excitability <i>in vivo</i>

Supramolecular Assemblies and Localized Regulation of Voltage-Gated Ion Channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

Inhibition of Conditioned Stimulus Pathway Phosphoprotein 24 Expression Blocks the Reduction in A-Type Transient K+Current Produced by One-TrialIn VitroConditioning ofHermissenda

Long-term intrinsic enhanced excitability is a characteristic of cellular plasticity and learning-dependent modifications in the activity of neural networks. The regulation of voltage-dependent K+ channels by phosphorylation/dephosphorylation and their localization is proposed to be important in the control of cellular plasticity. One-trial conditioning in Hermissenda results in enhanced excitability in sensory neurons, type B photoreceptors, of the conditioned stimulus pathway. Conditioning also regulates the phosphorylation of conditioned stimulus pathway phosphoprotein 24 (Csp24), a cytoskeletal-related protein containing multiple beta-thymosin-like domains. Recently, it was shown that the downregulation of Csp24 expression mediated by an antisense oligonucleotide blocked the development of enhanced excitability in identified type B photoreceptors after one-trial conditioning without affecting short-term excitability. Here, we show using whole-cell patch recordings that one-trial in vitro conditioning applied to isolated photoreceptors produces a significant reduction in the amplitude of the A-type transient K+ current (I(A)) detected 1.5-16 h after conditioning. One-trial conditioning produced a depolarized shift in the steady-state activation curve of I(A) without altering the inactivation curve. The conditioning-dependent reduction in I(A) was blocked by preincubation of the photoreceptors with Csp antisense oligonucleotide. These results provide an important link between Csp24, a cytoskeletal protein, and regulation of voltage-gated ion channels associated with intrinsic enhanced excitability underlying pavlovian conditioning.

Actin cytoskeleton regulates ion channel activity in retinal neurons

The actin cytoskeleton is an important contributor to the integrity of cellular shape and responses in neurons. However, the molecular mechanisms associated with functional interactions between the actin cytoskeleton and neuronal ion channels are largely unknown. Whole-cell and single channel recording techniques were thus applied to identified retinal bipolar neurons of the tiger salamander (Ambystoma tigrinum) to assess the role of acute changes in actin-based cytoskeleton dynamics in the regulation of voltage-gated ion channels. Disruption of endogenous actin filaments after brief treatment (20-30 min) with cytochalasin D (CD) activated voltage-gated K+ currents in bipolar cells, which were largely prevented by intracellular perfusion with the actin filament-stabilizer agent, phalloidin. Either CD treatment under cell-attached conditions or direct addition of actin to excised, inside-out patches of bipolar cells activated and/or increased single K+ channels. Thus, acute changes in actin-based cytoskeleton dynamics regulate voltage-gated ion channel activity in bipolar cells.

Regulation of Voltage-Gated Ion Channels during Ascidian jpHI Embryogenesis

In the proto-chordate Halocynthia roretzi, a voltage-activated sodium current undergoes a change in kinetics within 48 h of fertilization. Molecular cloning and microinjection of antisense DNA into single cells suggest that the kinetic changes are due to the increased expression of a neural-specific sodium channel gene, TuNa I. TuNa I gene transcription is first induced in late-stage gastrulae, preceding the appearance of the rapidly inactivating sodium current unique to alpha 4-2 derived neural cells. In cleavage-arrested and intact embryos, cell interactions between specific animal and vegetal blastomeres are required for induction of TuNa I gene expression. By contrast, another sodium channel gene, TuNa II, is mainly expressed in neural cells of the alpha 4-1 lineage. In this lineage, cells of endoderm, notochord and muscle develop. TuNa II gene transcription occurs in isolated alpha 4-1 blastomeres without interacting with other lineage cells. Thus, there are two distinct neural cell lineages in the ascidian tadpole: one is similar to the ectoderm-derived neural tissue controlled by a mechanism reminiscent of the neural induction in the vertebrate, the other is the lineage of posterior neural tube in which motor neurons develop in close association with the notochord or muscle lineage.

Regulation of voltage-gated ion channels by NGF and ciliary neurotrophic factor in SK-N-SH neuroblastoma cells

Neurotrophic factors have powerful effects on neuronal differentiation and the maintenance of neuronal phenotype, but understanding of their regulation of one important aspect of neuronal function, excitability, remains limited. We have examined the regulation of voltage-gated ion channels by two unrelated neurotrophic factors, NGF and ciliary neurotrophic factor (CNTF), in the SK-N-SH neuroblastoma cell line that is responsive to both factors. NGF and CNTF have strikingly different neuronal specificities and distributions in the nervous system, and might be expected to have significantly different effects on neuronal function. Using whole-cell, perforated-patch, and single-channel recording, we found that treatment with NGF increased levels of voltage-gated sodium, calcium, and potassium currents. In contrast, CNTF treatment increased levels of potassium currents only. NGF and CNTF appeared to regulate the same delayed-rectifier potassium current; in addition, NGF treatment resulted in increased levels of a second potassium current component. Such differential effects of neurotrophic factors on the expression of voltage-gated ion channels would have profound effects on the excitability of target neurons in vivo.

Beta-adrenergic modulation of calcium channels in frog ventricular heart cells.

Adrenergic modulation of calcium channels profoundly influences cardiac function, and has served as a prime example of neurohormonal regulation of voltage-gated ion channels. Channel modulation and increased Ca influx are mediated by elevation of intracellular cyclic AMP and protein phosphorylation. The molecular mechanism of the augmented membrane Ca conductance has attracted considerable interest. An increase in the density of functional channels has often been proposed, but there has previously been no direct evidence. Single-channel recordings show that isoprenaline or 8-bromocyclic AMP increase the proportion of time individual channels spend open by prolonging openings and shortening the closed periods between openings. To look for an additional contribution of changes in the number of functional channels, we applied ensemble fluctuation analysis to whole-cell recordings of cardiac Ca channel activity. Here we present evidence that in frog ventricular heart cells beta-adrenergic stimulation increases NF, the average number of functional Ca channels per cell. We also find that isoprenaline slows the time course of both activation and inactivation, and that the enhancement of peak current decreases gradually with greater membrane depolarization.