A-type Channels Research Articles

Neurocardiac pathologies associated with potassium channelopathies.

Voltage-gated potassium channels are expressed throughout the human body and are essential for physiological functions. These include delayed rectifiers, A-type channels, outward rectifiers, and inward rectifiers. They impact electrical function in the heart (repolarization) and brain (repolarization and stabilization of the resting membrane potential). KCNQx and KCNHx encode Kv7.x and Kv11.x proteins, which form delayed rectifier potassium channels. KCNQx and KCNHx channelopathies are associated with both cardiac and neuronal pathologies. These include electrocardiographic abnormalities, cardiac arrhythmias, sudden cardiac death (SCD), epileptiform discharges, seizures, bipolar disorder, and sudden unexpected death in epilepsy (SUDEP). Due to the ubiquitous expression of KCNQx and KCNHx channels, abnormalities in their function can be particularly harmful, increasing the risk of sudden death. For example, KCNH2 variants have a dual role in both cardiac and neuronal pathologies, whereas KCNQ2 and KCNQ3 variants are associated with severe and refractory epilepsy. Recurrent and uncontrolled seizures lead to secondary abnormalities, which include autonomics, cardiac electrical function, respiratory drive, and neuronal electrical activity. Even with a wide array of anti-seizure therapies available on the market, one-third of the more than 70 million people worldwide with epilepsy have uncontrolled seizures (i.e., intractable/drug-resistant epilepsy), which negatively impact neurodevelopment and quality of life. To capture the current state of the field, this review examines KCNQx and KCNHx expression patterns and electrical function in the brain and heart. In addition, it discusses several KCNQx and KCNHx variants that have been clinically and electrophysiologically characterized. Because these channel variants are associated with multi-system pathologies, such as epileptogenesis, Kv7 channel modulators provide a potential anti-seizure therapy, particularly for people with intractable epilepsy. Ultimately an increased understanding of the role of Kv channels throughout the body will fuel the development of innovative, safe, and effective therapies for people at a high risk of sudden death (SCD and SUDEP).

High-throughput computational screening of adsorbents and membrane materials for acetylene capture

Adsorption-based and membrane-based separation technologies have attracted much attention in acetylene purification due to their low energy consumption and environmental friendliness. Herein, through high-throughput Monte Carlo simulations, we sweep through a total of 5039 materials available in crystal database and find that as many as 165 of them are potential acetylene-type adsorbents. Based on the first-principles force field, three structures with open metal sites were determined to have high adsorption capacity, surpassing that of CuBTC. An adsorption mode map of the trapped acetylene is depicted. Then, high-throughput molecular dynamics simulations were carried out to calculate the diffusion behavior of selected membrane materials and the Robeson Upper Bound for C2H2/CO2 was plotted. The diffusion path reveals the transition of the confined space of acetylene molecule from anion pillar to A-type channel and then to B-type channel in nanopores in a simple cubic topology. Moreover, data-driven exploration reveals that the H, O, and F atoms in the pores are preferential adsorption sites. As demonstration, the extreme gradient boosting model can accurately predict the loading of C2H2, and the decision tree model was used to show an optimal selection path (RE/T > 0.118, PLD ≤6.831 Å, GSA > 2724.7 m2/g) for the permeability of membrane materials.

Characterization of the A-type potassium current in murine gastric fundus smooth muscles.

Transient outward, or "A-type," currents are rapidly inactivating voltage-gated potassium currents that operate at negative membrane potentials. A-type currents have not been reported in the gastric fundus, a tonic smooth muscle. We used whole cell voltage clamp to identify and characterize A-type currents in smooth muscle cells (SMCs) isolated from murine fundus. A-type currents were robust in these cells with peak amplitudes averaging 1.5 nA at 0 mV. Inactivation was rapid with a time constant of 71 ms at 0 mV; recovery from inactivation at -80 mV was similarly rapid with a time constant of 75 ms. A-type currents in fundus were blocked by 4-aminopyridine (4-AP), flecainide, and phrixotoxin-1 (PaTX1). Remaining currents after 4-AP and PaTX1 displayed half-activation potentials that were shifted to more positive potentials and showed incomplete inactivation. Currents after tetraethylammonium (TEA) displayed half inactivation at -48.1 ± 1.0 mV. Conventional microelectrode and contractile experiments on intact fundus muscles showed that 4-AP depolarized membrane potential and increased tone under conditions in which enteric neurotransmission was blocked. These data suggest that A-type K+ channels in fundus SMCs are likely active at physiological membrane potentials, and sustained activation of A-type channels contributes to the negative membrane potentials of this tonic smooth muscle. Quantitative analysis of Kv4 expression showed that Kcnd3 was dominantly expressed in fundus SMCs. These data were confirmed by immunohistochemistry, which revealed Kv4.3-like immunoreactivity within the tunica muscularis. These observations indicate that Kv4 channels likely form the A-type current in murine fundus SMCs.

The effects of gastrin-releasing peptide on the voltage-gated channels in rat hippocampal neurons

Gastrin-releasing peptide (GRP) has been implicated in several aspects of physiology and behavior including digestion, cancer, lung development, and memory process. Increasing evidence in rodents shows that GRP may contribute to hippocampal circuit function. Though the central role of GRP in the brain has been established, the cellular and molecular mechanisms of its actions have not been well defined. Thus in this study, we verified the expression of GRPR in the rat hippocampal CA1 region. Then we examined the mechanisms closely related to neuronal excitability, the effects of GRP on voltage-gated ion channels in CA1 neurons using patch-clamp. The results showed that GRP could decrease voltage-gated sodium currents mainly by affecting the kinetics of recovery from the inactivated state. However, GRP enhanced both kinds of voltage-gated potassium channels, the A-type channels were more sensitive to GRP than K-type channels. In conclusion, we found that GRP could alter the voltage-gated Na+ and K+ ion channel characteristics which might be the ionic mechanisms of the physiological function of GRP in the brain.

Modulation of Kv4.2/KChIP3 interaction by the ceroid lipofuscinosis neuronal 3 protein CLN3

Voltage-gated potassium (Kv) channels of the Kv4 subfamily associate with Kv channel-interacting proteins (KChIPs), which leads to enhanced surface expression and shapes the inactivation gating of these channels. KChIP3 has been reported to also interact with the late endosomal/lysosomal membrane glycoprotein CLN3 (ceroid lipofuscinosis neuronal 3), which is modified because of gene mutation in juvenile neuronal ceroid lipofuscinosis (JNCL). The present study was undertaken to find out whether and how CLN3, by its interaction with KChIP3, may indirectly modulate Kv4.2 channel expression and function. To this end, we expressed KChIP3 and CLN3, either individually or simultaneously, together with Kv4.2 in HEK 293 cells. We performed co-immunoprecipitation experiments and found a lower amount of KChIP3 bound to Kv4.2 in the presence of CLN3. In whole-cell patch-clamp experiments, we examined the effects of CLN3 co-expression on the KChIP3-mediated modulation of Kv4.2 channels. Simultaneous co-expression of CLN3 and KChIP3 with Kv4.2 resulted in a suppression of the typical KChIP3-mediated modulation; i.e. we observed less increase in current density, less slowing of macroscopic current decay, less acceleration of recovery from inactivation, and a less positively shifted voltage dependence of steady-state inactivation. The suppression of the KChIP3-mediated modulation of Kv4.2 channels was weaker for the JNCL-related missense mutant CLN3R334C and for a JNCL-related C-terminal deletion mutant (CLN3ΔC). Our data support the notion that CLN3 is involved in Kv4.2/KChIP3 somatodendritic A-type channel formation, trafficking, and function, a feature that may be lost in JNCL.

MicroRNA inhibition upregulates hippocampal A-type potassium current and reduces seizure frequency in a mouse model of epilepsy

Epilepsy is often associated with altered expression or function of ion channels. One example of such a channelopathy is the reduction of A-type potassium currents in the hippocampal CA1 region. The underlying mechanisms of reduced A-type channel function in epilepsy are unclear. Here, we show that inhibiting a single microRNA, miR-324-5p, which targets the pore-forming A-type potassium channel subunit Kv4.2, selectively increased A-type potassium currents in hippocampal CA1 pyramidal neurons in mice. Resting membrane potential, input resistance and other potassium currents were not altered. In a mouse model of acquired chronic epilepsy, inhibition of miR-324-5p reduced the frequency of spontaneous seizures and interictal epileptiform spikes supporting the physiological relevance of miR-324-5p-mediated control of A-type currents in regulating neuronal excitability. Mechanistic analyses demonstrated that microRNA-induced silencing of Kv4.2 mRNA is increased in epileptic mice leading to reduced Kv4.2 protein levels, which is mitigated by miR-324-5p inhibition. By contrast, other targets of miR-324-5p were unchanged. These results suggest a selective miR-324-5p-dependent mechanism in epilepsy regulating potassium channel function, hyperexcitability and seizures.

Tuning excitability of the hypothalamus via glutamate and potassium channel coupling

Abstract Heart failure and hypertension are both prevalent and serious health concerns, and progress toward understanding their underlying mechanisms could significantly impact development of therapeutic approaches This article is protected by copyright. All rights reserved

The Experimental Characterization and Numerical Simulation of A-Segregates in 27SiMn Steel

A 500-kg 27SiMn steel ingot has been produced via low-oxygen purifying technology and then fully dissected in the longitudinal center plane to investigate the typical features and formation mechanism of A-segregates. The macrostructure, microstructure, inclusions, cavity and solute segregation surrounding the A-segregates were characterized and analyzed in detail. It shows that the main solute components such as C, Si, and Mn, and even some large cavities accumulate obviously within the A-type channels. Owing to the low content of oxygen and other impurities, there is no apparent enrichment of the inclusions. Based on the classical theory of macrosegregation, driven by thermo-solutal buoyancy, A-segregates in the body of a 27SiMn steel ingot have been simulated successfully by using the multi-component continuum model. They disappear in a referred 1045 steel ingot. By comparing the evolution processes during solidification of both steels, it is demonstrated that the strong thermo-solutal convection originated from the high content of Si element in 27SiMn steel destabilizes the mushy zone and therefore induces the A-segregates. In terms of the occurrence zones of A-segregates and corresponding distributions of Rayleigh number, its critical value to predict the A-segregates in industrial steel ingots was proposed and then applied successfully.

NG2 glial cells integrate synaptic input in global and dendritic calcium signals.

Synaptic signaling to NG2-expressing oligodendrocyte precursor cells (NG2 cells) could be key to rendering myelination of axons dependent on neuronal activity, but it has remained unclear whether NG2 glial cells integrate and respond to synaptic input. Here we show that NG2 cells perform linear integration of glutamatergic synaptic inputs and respond with increasing dendritic calcium elevations. Synaptic activity induces rapid Ca2+ signals mediated by low-voltage activated Ca2+ channels under strict inhibitory control of voltage-gated A-type K+ channels. Ca2+ signals can be global and originate throughout the cell. However, voltage-gated channels are also found in thin dendrites which act as compartmentalized processing units and generate local calcium transients. Taken together, the activity-dependent control of Ca2+ signals by A-type channels and the global versus local signaling domains make intracellular Ca2+ in NG2 cells a prime signaling molecule to transform neurotransmitter release into activity-dependent myelination.

The pH sensitivity of Aqp0 channels in tetraploid and diploid teleosts.

Water homeostasis and the structural integrity of the vertebrate lens is partially mediated by AQP0 channels. Emerging evidence indicates that external pH may be involved in channel gating. Here we show that a tetraploid teleost, the Atlantic salmon, retains 4 aqp0 genes (aqp0a1, -0a2, -0b1, and -0b2), which are highly, but not exclusively, expressed in the lens. Functional characterization reveals that, although each paralog permeates water efficiently, the permeability is respectively shifted to the neutral, alkaline, or acidic pH in Aqp0a1, -0a2, and -0b1, whereas that of Aqp0b2 is not regulated by external pH. Mutagenesis studies demonstrate that Ser38, His39, and His40 residues in the extracellular transmembrane domain of α-helix 2 facing the water pore are critical for the pH modulation of water transport. To validate these findings, we show that both zebrafish Aqp0a and -0b are functional water channels with respective pH sensitivities toward alkaline or acid pH ranges and that an N-terminal allelic variant (Ser19) of Aqp0b exists that abolishes water transport in Xenopus laevis oocytes. The data suggest that the alkaline pH sensitivity is a conserved trait in teleost Aqp0 a-type channels, whereas mammalian AQP0 and some teleost Aqp0 b-type channels display an acidic pH permeation preference.—Chauvigné, F., Zapater, C., Stavang, J. A., Taranger, G. L., Cerdà, J., Finn, R. N. The pH sensitivity of Aqp0 channels in tetraploid and diploid teleosts.

Structural and thermal transformations on high energy milling of natural apatite

In this work we investigated the isomorphic substitution and thermal decomposition of sedimentary fluor apatite (FAp) (with Ca/P ratio > 1.67) from Tunisia after high-energy-milling (HEM) activation at different times from 10 to 600 min. The chemical composition of the material includes: 29.6 % P2O5total and 46.5 % CaO (main components) and 3.5 % F; 0.55 % R2O3 (R = Al, Fe); 1.1 % SO3; 1.9 % SiO2 (a low content in a comparison with other natural apatites from North Africa or Asia); 0.35 % MgO; 0.05 % Cl; 6.6 % CO2 are impurities. HEM is a well-known approach for preparing various solid materials and for increasing their reactivity. The solid-state transformation of the initial and HEM-activated apatite samples was examined by chemical analysis, BET, powder XRD, FTIR spectroscopy, and thermal analysis. The structure of natural apatite allows isomorphic substitutions of carbonate, hydroxyl, and metal ions by PO43−, Ca2+, and F−. The obtained powder XRD data indicate an increased defectiveness of the apatite structure in the course of the HEM. The solid-state transformations of the initial and HEM-activated apatite are examined by TG–DTA analyses. It is found that the thermal stability of the activated samples decreases as compared to the initial sample. This is related to the increased defectivity of the apatite structure during the high-energy milling shown by the XRD data. The thermal analysis allows the differentiation of the structurally bonded A, B, and A-B types carbonate ions from these originating from the calcite and dolomite admixtures. The results obtained demonstrate that the mechanical distortion and the structural changes related to the migration of the carbonate ions from B type to A-type channel positions are the main factors responsible for the enhanced solubility of the high-energy activated FAp.

Transforming growth factor‐β1 primes proliferating adult neural progenitor cells to electrophysiological functionality

The differentiation of adult neural progenitors (NPCs) into functional neurons is still a limiting factor in the neural stem cell field but mandatory for the potential use of NPCs in therapeutic approaches. Neuronal function requires the appropriate electrophysiological properties. Here, we demonstrate that priming of NPCs using transforming growth factor (TGF)-β1 under conditions that usually favor NPCs' proliferation induces electrophysiological neuronal properties in adult NPCs. Gene chip array analyses revealed upregulation of voltage-dependent ion channel subunits (Kcnd3, Scn1b, Cacng4, and Accn1), neurotransmitters, and synaptic proteins (Cadps, Snap25, Grik4, Gria3, Syngr3, and Gria4) as well as other neuronal proteins (doublecortin [DCX], Nrxn1, Sept8, and Als2cr3). Patch-clamp analysis demonstrated that control-treated cells expressed only voltage-dependent K(+) -channels of the delayed-rectifier type and the A-type channels. TGF-β1-treated cells possessed more negative resting potentials than nontreated cells owing to the presence of delayed-rectifier and inward-rectifier channels. Furthermore, TGF-β1-treated cells expressed voltage-dependent, TTX-sensitive Na(+) channels, which showed increasing current density with TGF-β1 treatment duration and voltage-dependent (+)BayK8644-sensitive L-Type Ca(2+) channels. In contrast to nontreated cells, TGF-β1-treated cells responded to current injections with action-potentials in the current-clamp mode. Furthermore, TGF-β1-treated cells responded to application of GABA with an increase in membrane conductance and showed spontaneous synaptic currents that were blocked by the GABA-receptor antagonist picrotoxine. Only NPCs, which were treated with TGF-β1, showed Na(+) channel currents, action potentials, and GABAergic currents. In summary, stimulation of NPCs by TGF-β1 fosters a functional neuronal phenotype, which will be of relevance for future cell replacement strategies in neurodegenerative diseases or acute CNS lesions.

The Auxiliary Subunit KChIP2 Is an Essential Regulator of Homeostatic Excitability

The necessity for, or redundancy of, distinctive KChIP proteins is not known. Deletion of KChIP2 leads to increased susceptibility to epilepsy and to a reduction in IA and increased excitability in pyramidal hippocampal neurons. KChIP2 is essential for homeostasis in hippocampal neurons. Mutations in K(A) channel auxiliary subunits may be loci for epilepsy. The somatodendritic IA (A-type) K(+) current underlies neuronal excitability, and loss of IA has been associated with the development of epilepsy. Whether any one of the four auxiliary potassium channel interacting proteins (KChIPs), KChIP1-KChIP4, in specific neuronal populations is critical for IA is not known. Here we show that KChIP2, which is abundantly expressed in hippocampal pyramidal cells, is essential for IA regulation in hippocampal neurons and that deletion of Kchip2 affects susceptibility to limbic seizures. The specific effects of Kchip2 deletion on IA recorded from isolated hippocampal pyramidal neurons were a reduction in amplitude and shift in the V½ for steady-state inactivation to hyperpolarized potentials when compared with WT neurons. Consistent with the relative loss of IA, hippocampal neurons from Kchip2(-/-) mice showed increased excitability. WT cultured neurons fired only occasional single action potentials, but the average spontaneous firing rate (spikes/s) was almost 10-fold greater in Kchip2(-/-) neurons. In slice preparations, spontaneous firing was detected in CA1 pyramidal neurons from Kchip2(-/-) mice but not from WT. Additionally, when seizures were induced by kindling, the number of stimulations required to evoke an initial class 4 or 5 seizure was decreased, and the average duration of electrographic seizures was longer in Kchip2(-/-) mice compared with WT controls. Together, these data demonstrate that the KChIP2 is essential for physiologic IA modulation and homeostatic stability and that there is a lack of functional redundancy among the different KChIPs in hippocampal neurons.

Hippocampal A-type current and Kv4.2 channel modulation by the sulfonylurea compound NS5806

We examined the effects of the sulfonylurea compound NS5806 on neuronal A-type channel function. Using whole-cell patch-clamp we studied the effects of NS5806 on the somatodendritic A-type current (ISA) in cultured hippocampal neurons and the currents mediated by Kv4.2 channels coexpressed with different auxiliary β-subunits, including both Kv channel interacting proteins (KChIPs) and dipeptidyl aminopeptidase-related proteins (DPPs), in HEK 293 cells. The amplitude of the ISA component in hippocampal neurons was reduced in the presence of 20 μM NS5806. ISA decay kinetics were slowed and the recovery kinetics accelerated, but the voltage dependence of steady-state inactivation was shifted to more negative potentials by NS5806. The peak amplitudes of currents mediated by ternary Kv4.2 channel complexes, associated with DPP6-S (short splice-variant) and either KChIP2, KChIP3 or KChIP4, were potentiated and their macroscopic inactivation slowed by NS5806, whereas the currents mediated by binary Kv4.2 channels, associated only with DPP6-S, were suppressed, and the NS5806-mediated slowing of macroscopic inactivation was less pronounced. Neither potentiation nor suppression and no effect on current decay kinetics in the presence of NS5806 were observed for Kv4.2 channels associated with KChIP3 and the N-type inactivation-conferring DPP6a splice-variant. For all recombinant channel complexes, NS5806 slowed the recovery from inactivation and shifted the voltage dependence of steady-state inactivation to more negative potentials. Our results demonstrate the activity of NS5806 on native ISA and possible molecular correlates in the form of recombinant Kv4.2 channels complexed with different KChIPs and DPPs, and they shed some light on the mechanism of NS5806 action.

What does type A mean to a pyramidal cell?

Potassium channels powerfully control neuronal excitability and exhibit great molecular diversity. Despite this diversity, classification of voltage-gated potassium currents is often simplified into a distinction between a rapidly activating, transient A-type current and a slowly activating and inactivating delayed rectifier (DR) current. However, individual neurons express multiple potassium channel subunits from several families and knowing the functional roles of each of these subunits is essential to understanding neural computations and pathophysiology. As an example, components of the A-type current in neocortical pyramidal cells are generated by Kv4.2, Kv4.3 and Kv1.4 α subunits (Norris & Nerbonne, 2010). In this issue of The Journal of Physiology, Carrasquillo et al. (2012) address the functional roles of each of these subunit types with slice recordings from visual cortex of mice where Kv4.2 (Kv4.2−/−), Kv4.3 (Kv4.3−/−), or Kv1.4 (Kv1.4−/−) subunit protein was eliminated. The consequences of null mutations differed between the three subunits, suggesting distinct roles in regulating membrane properties (Fig. 1). Figure 1 Functional roles of Kv4.2, Kv4.3 and Kv1.4 subunits in neocortical pyramidal neurons (determined by Carrasquillo et al. 2012) In pyramidal neurons from Kv4.2−/− mice, input resistance was increased, the current threshold for action potential (AP) generation was reduced, APs were broader and rebound firing in response to prolonged hyperpolarization was increased. Cells from Kv4.3−/− mice also exhibited increased spike width, but input resistance, current threshold and rebound firing were unchanged. These data suggest greater importance of Kv4.2 at subthreshold voltages, perhaps due to a more hyperpolarized activation range. Elimination of either Kv4.2 or Kv4.3 resulted in faster firing than wild-type neurons but changes in the Kv4.3−/− cells were not significant until larger current injections. In contrast, the Kv4.2−/− cells exhibited greater effects to small current injections than larger ones. The reduced effect of Kv4.2−/− on firing in response to large stimuli may reflect slower recovery from inactivation of Kv4.2 channels. The effects of removing Kv4.2 and Kv4.3 were generally additive, although spike doublets were observed in some of the double cells lacking both Kv4.2 and Kv4.3. These findings allow consideration of whether Kv4 channels normally exist as homomeric or heteromeric channels in pyramidal cells. Previous studies suggested that Kv4.2 and Kv4.3 α subunits form heteromultimers but the present results show differential effects of removal of Kv4.2 and Kv4.3, suggesting that not all channels contain both subunits or that the heteromeric channels have different properties than homomeric channels. It remains to be determined whether the differential effects of subunit removal reflect the elimination of homomeric channels with different biophysical properties, replacement of heteromeric Kv4.2/Kv4.3 channels with homomers, or changes in channel targeting as a result of subunit removal. Carrasquillo et al. (2012) found no significant differences in input resistance, current thresholds, or firing rate in the Kv1.4 −/− mouse; however, pharmacological block of Kv4-mediated A currents with Ba2+ unmasked contributions of Kv1.4 to input resistance, AP current threshold, and repetitive firing. Interestingly, APs were actually narrower in Kv1.4−/− cells, apparently due to a compensatory increase in Kv4.2 channels. It is unclear if upregulation of Kv4.3 or Kv1.4 subunits also occurs with Kv4.2 elimination, but previous studies suggest functional compensation by the DR current. For example, Yuan et al. (2005) reported the absence of A-type current and slower AP repolarization with acute reduction of Kv4.2 channels. In contrast, in most cells from animals lacking Kv4.2 from birth the A-type current was absent yet AP repolarization was normal (Nerbonne et al. 2008). An increased DR current was observed in cells from the Kv4.2−/− animals and this was apparently able to fully compensate for AP repolarization. It was subsequently observed that partial block of the DR with TEA in Kv4.2−/− cells unmasked A-type currents mediated by Kv4.3 and Kv1.4 subunits (confirmed with Kv4.3−/− and Kv1.4−/− animals: Norris & Nerbonne, 2010). Collectively, these observations suggest that long-term elimination of an A-type current component may lead to compensatory changes in other A-type channels or other types of potassium channel. This obviously complicates the interpretation of findings in –/– animals and underscores the need to understand the rules by which such compensatory changes may occur. While the present results concentrate on somatodendritic channels, functional roles of A-type channels depend upon the precise location of their expression. It is not clear to what degree channel localization is altered in the –/– animals. The effectiveness of functional compensation may also vary in different cellular compartments. Still to be resolved is how much natural variation exists in the expression of Kv1.4, Kv4.2, and Kv4.3 and how this relates to known differences in active properties of pyramidal cell populations defined by layer, gene expression, or projection targets (cf. Hattox & Nelson, 2007). With this comprehensive series of papers, Nerbonne and colleagues show us just how complex the molecular basis and functional consequences can be for the seemingly simple type A current.

Distinct cellular distributions of Kv4 pore-forming and auxiliary subunits in rat dorsal root ganglion neurons

AimsDorsal root ganglia contain heterogeneous populations of primary afferent neurons that transmit various sensory stimuli. This functional diversity may be correlated with differential expression of voltage-gated K+ (Kv) channels. Here, we examine cellular distributions of Kv4 pore-forming and ancillary subunits that are responsible for fast-inactivating A-type K+ current. Main methodsExpression pattern of Kv α-subunit, β-subunit and auxiliary subunit was investigated using immunohistochemistry, in situ hybridization and RT-PCR technique. Key findingsThe two pore-forming subunits Kv4.1 and Kv4.3 show distinct cellular distributions: Kv4.3 is predominantly in small-sized C-fiber neurons, whereas Kv4.1 is seen in DRG neurons in various sizes. Furthermore, the two classes of Kv4 channel auxiliary subunits are also distributed in different-sized cells. KChIP3 is the only significantly expressed Ca2+-binding cytosolic ancillary subunit in DRGs and present in medium to large-sized neurons. The membrane-spanning auxiliary subunit DPP6 is seen in a large number of DRG neurons in various sizes, whereas DPP10 is restricted in small-sized neurons. SignificanceDistinct combinations of Kv4 pore-forming and auxiliary subunits may constitute A-type channels in DRG neurons with different physiological roles. Kv4.1 subunit, in combination with KChIP3 and/or DPP6, form A-type K+ channels in medium to large-sized A-fiber DRG neurons. In contrast, Kv4.3 and DPP10 may contribute to A-type K+ current in non-peptidergic, C-fiber somatic afferent neurons.

Differential contribution of A-type potassium currents in shaping neuronal responses to synaptic input

Neurons receive input from thousands of excitatory and inhibitory synapses, but they are not merely passive receivers and transmitters of information. The intrinsic properties of neurons, and more specifically the collection of ion channels expressed, influence a cell's excitability and integration processes. The role of different voltage-dependent transient (A-type) and persistent (delayed rectifier) potassium currents in shaping the activity of neurons in response to synaptic input is not well understood. We developed a biophysical model incorporating realistic synaptic input [1] to investigate the specific roles played by distinct A-type potassium channels encoded by different genes, namely Shaker/Kv1 and Shal/Kv4. We find that these two channel types, which are functionally distinguished by their inactivation properties, exert different control mechanisms on the transitions between rest and repetitive spiking in neurons. Bifurcation analysis is used to characterize the influence of each channel and the overall behavior of the membrane as a function of the relative presence of the two A-type channels. We found that the influence of each channel type depends on the resting potential of the neuron, suggesting that modulation and/or network states may shift their relative contribution. For example, at a particular membrane potential, Shal/Kv4 channels are more likely to mediate delays to first spike in response to excitatory input. Overall, this work increases our understanding of the interaction between intrinsic properties and synaptic input in creating neuronal response profiles. Further, we provide a link between relative gene expression and bifurcation structures in dynamical systems modeling neuronal membranes.

Dendritic mechanisms controlling the threshold and timing requirement of synaptic plasticity

Active conductances located and operating on neuronal dendrites are expected to regulate synaptic integration and plasticity. We investigate how Kv4.2-mediated A-type K(+) channels and Ca(2+) -activated K(+) channels are involved in the induction process of Hebbian-type plasticity that requires correlated pre- and postsynaptic activities. In CA1 pyramidal neurons, robust long-term potentiation (LTP) induced by a theta burst pairing protocol usually occurred within a narrow window during which incoming synaptic potentials coincided with postsynaptic depolarization. Elimination of dendritic A-type K(+) currents in Kv4.2(-/-) mice, however, resulted in an expanded time window, making the induction of synaptic potentiation less dependent on the temporal relation of pre- and postsynaptic activity. For the other type of synaptic plasticity, long-term depression, the threshold was significantly increased in Kv4.2(-/-) mice. This shift in depression threshold was restored to normal when the appropriate amount of internal free calcium was chelated during induction. In concert with A-type channels, Ca(2+) -activated K(+) channels also exerted a sliding effect on synaptic plasticity. Blocking these channels in Kv4.2(-/-) mice resulted in an even larger potentiation while by contrast, the depression threshold was shifted further. In conclusion, dendritic A-type and Ca(2+) -activated K(+) channels dually regulate the timing-dependence and thresholds of synaptic plasticity in an additive way.

Differential contribution of voltage-dependent potassium currents to neuronal excitability

The excitability of a neuron depends on the different inward and outward currents that flow across its membrane. The specific role of A-type and persistent K-currents in shaping neuronal excitability remains partially unexplained by electrophysiological data. Drosophila motor neurons provide a model system to study the differential contributions of voltage-dependent K-currents to the dynamics of the membrane potential. In this work, the theoretical plausibility of existing hypotheses about the differential involvement of A-type currents in delaying spiking activity is examined through a mathematical model constructed using known macroscopic biophysical properties of voltage-dependent, slowly inactivating and fast inactivating A-type Drosophila channels. The model is constrained first by electrophysiological data, and an analysis of the membrane dynamics is performed through systematic variation of the ratios of the maximal whole-membrane currents. Different ratios among the numbers of the different channels in the model capture the basic features of responses to square pulse stimulation previously observed in Drosophila motor neurons for embryo, larvae and adult motor neurons, Kenyon cells, and giant cultured cells. The model supports the notion that slowly inactivating potassium currents are necessary for sustained spiking activity. The model also supports the hypothesis that early inactivating A-type K+ (Shal) channels are responsible for experimentally observed delays in the onset of spiking. In contrast, Shaker A-type channels with more depolarized steady state inactivation also contribute to the delay to first spike, but less than Shal. Instead, Shaker channels gate single and repetitive spiking. Furthermore, the model elucidates a biophysical mechanism that allows neurons to diversify their function, in this case by combining additive and resonant properties. Our modeling results are consistent with experimental results from different preparations including Drosophilaand lobster.

Dipeptidyl Peptidase-Like Protein 6 Is Required for Normal Electrophysiological Properties of Cerebellar Granule Cells

In cerebellar granule (CG) cells and many other neurons, A-type potassium currents play an important role in regulating neuronal excitability, firing patterns, and activity-dependent plasticity. Protein biochemistry has identified dipeptidyl peptidase-like protein 6 (DPP6) as an auxiliary subunit of Kv4-based A-type channels and thus a potentially important regulator of neuronal excitability. In this study, we used an RNA interference (RNAi) strategy to examine the role DPP6 plays in forming and shaping the electrophysiological properties of CG cells. DPP6 RNAi delivered by lentiviral vectors effectively disrupts DPP6 protein expression in CG cells. In response to the loss of DPP6, I(SA) peak conductance amplitude is reduced by >85% in parallel with a dramatic reduction in the level of I(SA) channel protein complex found in CG cells. The I(SA) channels remaining in CG cells after suppression of DPP6 show alterations in gating similar to Kv4 channels expressed in heterologous systems without DPP6. In addition to these effects on A-type current, we find that loss of DPP6 has additional effects on input resistance and Na(+) channel conductance that combine with the effects on I(SA) to produce a global change in excitability. Overall, DPP6 expression seems to be critical for the expression of a high-frequency electrophysiological phenotype in CG cells by increasing leak conductance, A-type current levels and kinetics, and Na(+) current amplitude.