Elevation of intracellular Ca2+ modulates A-currents in rat cerebellar granule neurons

R-type voltage-gated Ca2+ channels mediate A-type K+ current regulation of synaptic input in hippocampal dendrites

The subthreshold voltage-gated transient K+ current (IA) carried by pore-forming Kv4.2 subunits regulates the propagation of synaptic input, dendritic excitability, and synaptic plasticity in CA1 pyramidal neuron dendrites of the hippocampus. We report that the Ca2+ channel subunit Cav2.3 regulates IA in this cell type. We initially identified Cav2.3 as a Kv4.2-interacting protein in a proteomic screen and we confirmed Cav2.3-Kv4.2 complex association using multiple techniques. Functionally, Cav2.3 Ca2+-entry increases Kv4.2-mediated whole-cell current due to an increase in Kv4.2 surface expression. Using pharmacology and Cav2.3 knockout mice, we show that Cav2.3 regulates the dendritic gradient of IA. Furthermore, the loss of Cav2.3 function leads to the enhancement of AMPA receptor-mediated synaptic currents and NMDA receptor-mediated spine Ca2+ influx. These results propose that Cav2.3 and Kv4.2 are integral constituents of an ion channel complex that affects synaptic function in the hippocampus.

Kv channel-interacting proteins as neuronal and non-neuronal calcium sensors

Kv channel-interacting proteins (KChIPs) belong to the neuronal calcium sensor (NCS) family of Ca²⁺-binding EF-hand proteins. KChIPs constitute a group of specific auxiliary β-subunits for Kv4 channels, the molecular substrate of transient potassium currents in both neuronal and non-neuronal tissues. Moreover, KChIPs can interact with presenilins to control ER calcium signaling and apoptosis, and with DNA to control gene transcription. Ca²⁺ binding via their EF-hands, with the consequence of conformational changes, is well documented for KChIPs. Moreover, the Ca²⁺ dependence of the presenilin/KChIP complex may be related to Alzheimer’s disease and the Ca²⁺ dependence of the DNA/KChIP complex to pain sensing. However, only in few cases could the Ca²⁺ binding to KChIPs be directly linked to the control of excitability in nerve and muscle cells known to express Kv4/KChIP channel complexes. This review summarizes current knowledge about the Ca²⁺ binding properties of KChIPs and the Ca²⁺ dependencies of macromolecular complexes containing KChIPs, including those with presenilins, DNA and especially Kv4 channels. The respective physiological or pathophysiolgical roles of Ca²⁺ binding to KChIPs are discussed.

Modulation of human Kv4.3/KChIP2 channel inactivation kinetics by cytoplasmic Ca2

The transient outward current (I _to) in the human heart is mediated by Kv4.3 channels complexed with Kv channel interacting protein (KChIP) 2, a cytoplasmic Ca²⁺-binding EF-hand protein known to modulate Kv4.3 inactivation gating upon heterologous co-expression. We studied Kv4.3 channels co-expressed with wild-type (wt) or EF-hand-mutated (ΔEF) KChIP2 in human embryonic kidney (HEK) 293 cells. Co-expression took place in the absence or presence of BAPTA-AM, and macroscopic currents were recorded in the whole-cell patch-clamp configuration with different free Ca²⁺ concentrations in the patch-pipette. Our data indicate that Ca²⁺ is not necessary for Kv4.3/KChIP2 complex formation. The Kv4.3/KChIP2-mediated current decay was faster and the recovery of Kv4.3/KChIP2 channels from inactivation slower with 50 μM Ca²⁺ than with BAPTA (nominal Ca²⁺-free) in the patch-pipette. The apparent Ca²⁺-mediated slowing of recovery kinetics was still observed when EF-hand 4 of KChIP2 was mutated (ΔEF4) but not when EF-hand 2 (ΔEF2) was mutated, and turned into a Ca²⁺-mediated acceleration of recovery kinetics when EF-hand 3 (ΔEF3) was mutated. In the presence of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 cytoplasmic Ca²⁺ (50 μM) induced an acceleration of Kv4.3/KChIP2 recovery kinetics, which was still observed when EF-hand 2 was mutated (ΔEF2) but not when EF-hand 3 (ΔEF3) or EF-hand 4 (ΔEF4) was mutated. Our results support the notion that binding of Ca²⁺ to KChIP2 EF-hands can acutely modulate Kv4.3/KChIP2 channel inactivation gating, but the Ca²⁺-dependent gating modulation depends on CaMKII action. Our findings speak for an acute modulation of I _to kinetics and frequency-dependent I _to availability in cardiomyocytes under conditions with elevated Ca²⁺ levels and CaMKII activity.

Synergistic neurotoxicity of oxygen-glucose deprivation and tetrabromobisphenol A in vitro: role of oxidative stress

BACKGROUND

Tetrabromobisphenol A (TBBPA) is a toxic brominated flame retardant. Previous studies have demonstrated that exposure of primary cultures of rat cerebellar granule cells (CGC) to ≥ 10 μM TBBPA induces toxicity and excitotoxicity, and the underlying mechanism may involve calcium imbalance and oxidative stress. Here we examined whether the application of TBBPA at subtoxic concentrations may exacerbate acute damage of CGC challenged with oxygen-glucose deprivation (OGD), and evaluated with fluorescent indicators the involvement of calcium imbalance, mitochondrial depolarization and oxidative stress.

METHODS

Survival of CGC was assessed 24 h after OGD/TBBPA using fluorescent dyes. An OGD challenge lasting for 45, 60 or 75 min induced a duration-dependent injury to the neurons.

RESULTS

Application of 2.5, 5 or 7.5 μM TBBPA for 45 min to normoxic and glucose-containing incubation medium did not reduce the viability of cultured CGC, but this compound exacerbated the toxic effects of OGD in a concentration-dependent way. Moreover, TBBPA had a slight effect on calcium homeostasis and mitochondrial membrane potential, but significantly activated the production of reactive oxygen species in CGC. The application of H(2)O(2) at 5, 10 and 25 μM mimicked the effects of TBBPA on OGD toxicity, while 0.1 mM ascorbic acid or 1 mM glutathione ameliorated this toxicity.

CONCLUSION

These results suggest the involvement of oxidative stress in the synergistic neurotoxic effects of TBBPA and OGD.

Incorporation of DPP6a and DPP6K variants in ternary Kv4 channel complex reconstitutes properties of A-type K current in rat cerebellar granule cells

Dipeptidyl peptidase-like protein 6 (DPP6) proteins co-assemble with Kv4 channel α-subunits and Kv channel-interacting proteins (KChIPs) to form channel protein complexes underlying neuronal somatodendritic A-type potassium current (I_SA). DPP6 proteins are expressed as N-terminal variants (DPP6a, DPP6K, DPP6S, DPP6L) that result from alternative mRNA initiation and exhibit overlapping expression patterns. Here, we study the role DPP6 variants play in shaping the functional properties of I_SA found in cerebellar granule (CG) cells using quantitative RT-PCR and voltage-clamp recordings of whole-cell currents from reconstituted channel complexes and native I_SA channels. Differential expression of DPP6 variants was detected in rat CG cells, with DPP6K (41±3%)>DPP6a (33±3%)>>DPP6S (18±2%)>DPP6L (8±3%). To better understand how DPP6 variants shape native neuronal I_SA, we focused on studying interactions between the two dominant variants, DPP6K and DPP6a. Although previous studies did not identify unique functional effects of DPP6K, we find that the unique N-terminus of DPP6K modulates the effects of KChIP proteins, slowing recovery and producing a negative shift in the steady-state inactivation curve. By contrast, DPP6a uses its distinct N-terminus to directly confer rapid N-type inactivation independently of KChIP3a. When DPP6a and DPP6K are co-expressed in ratios similar to those found in CG cells, their distinct effects compete in modulating channel function. The more rapid inactivation from DPP6a dominates during strong depolarization; however, DPP6K produces a negative shift in the steady-state inactivation curve and introduces a slow phase of recovery from inactivation. A direct comparison to the native CG cell I_SA shows that these mixed effects are present in the native channels. Our results support the hypothesis that the precise expression and co-assembly of different auxiliary subunit variants are important factors in shaping the I_SA functional properties in specific neuronal populations.

Signaling complexes of voltage-gated calcium channels

Voltage gated calcium channels are key mediators of depolarization induced calcium entry into electrically excitable cells. There is increasing evidence that voltage gated calcium channels, like many other types of ionic channels, do not operate in isolation, but instead forms signaling complexes with signaling molecules, G protein coupled receptors, and other types of ion channels. Furthermore, there appears to be bidirectional signaling within these protein complexes, thus allowing not only for efficient translation of calcium signals into cellular responses, but also for tight control of calcium entry per se. In this review, we will focus predominantly on signaling complexes between G protein-coupled receptors and high voltage activated calcium channels, and on complexes of voltage-gated calcium channels and members of the potassium channel superfamily.

Regulation of neuronal activity by Cav3-Kv4 channel signaling complexes

Kv4 low voltage-activated A-type potassium channels are widely expressed in excitable cells, where they control action potential firing, dendritic activity and synaptic integration. Kv4 channels exist as a complex that includes K(+) channel-interacting proteins (KChIPs), which contain calcium-binding domains and therefore have the potential to confer calcium dependence on the Kv4 channel. We found that T-type calcium channels and Kv4 channels form a signaling complex in rat that efficiently couples calcium influx to KChIP3 to modulate Kv4 function. This interaction was critical for allowing Kv4 channels to function in the subthreshold membrane potential range to regulate neuronal firing properties. The widespread expression of these channels and accessory proteins indicates that the Cav3-Kv4 signaling complex is important for the function of a wide range of electrically excitable cells.

Chemical sympathetic denervation, suppression of myocardial transient outward potassium current, and ventricular fibrillation in the rat

Sympathetic denervation is frequently observed in heart disease. To investigate the linkage of sympathetic denervation and cardiac arrhythmia, we developed a rat model of chemical sympathectomy by subcutaneous injections of 6-hydroxydopamine (6-OHDA). Cardiac sympathetic innervation was visualized by means of a glyoxylic catecholaminergic histofluorescence method. Transient outward current (Ito) of ventricular myocytes was recorded with the whole-cell configuration of the patch clamp technique. We observed that sympathectomy (i) decreased cardiac sympathetic nerve density and norepinephrine level, (ii) reduced the protein expression of Kv4.2, Kv1.4, and Kv channel-interacting protein 2 (KChIP2), (iii) decreased current densities and delayed activation of Ito channels, (iv) reduced the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP response element-binding protein (CREB), and (v) increased the severity of ventricular fibrillation induced by rapid pacing. Three weeks after 6-OHDA injections, which allowed time for sympathetic regeneration, we found cardiac sympathetic nerve density, norepinephrine levels, expression levels of Kv4.2 and KChIP2 proteins, and I(to) densities were partially normalized and ventricular fibrillation severity was decreased. We conclude that chemical sympathectomy downregulates the expression of selective Kv channel subunits and decreases myocardial I(to) channel activities, contributing to the elevated susceptibility to ventricular fibrillation.

Mefenamic acid bi-directionally modulates the transient outward K+ current in rat cerebellar granule cells

The effect of non-steroidal anti-inflammatory drugs (NSAIDs) on ion channels has been widely studied in several cell models, but less is known about their modulatory mechanisms. In this report, the effect of mefenamic acid on voltage-activated transient outward K(+) current (I(A)) in cultured rat cerebellar granule cells was investigated. At a concentration of 5 microM to 100 microM, mefenamic acid reversibly inhibited I(A) in a dose-dependent manner. However, mefenamic acid at a concentration of 1 microM significantly increased the amplitude of I(A) to 113+/-1.5% of the control. At more than 10 microM, mefenamic acid inhibited the amplitude of I(A) without any effect on activation or inactivation. In addition, a higher concentration of mefenamic acid induced a significant acceleration of recovery from inactivation with an increase of the peak amplitude elicited by the second test pulse. Intracellular application of mefenamic acid could significantly increase the amplitude of I(A), but had no effect on the inhibition induced by extracellular mefenamic acid, implying that mefenamic acid may exert its effect from both inside and outside the ion channel. Furthermore, the activation of current induced by intracellular application of mefenamic acid was mimicked by other cyclooxygenase inhibitors and arachidonic acid. Our data demonstrate that mefenamic acid is able to bi-directionally modulate I(A) channels in neurons at different concentrations and by different methods of application, and two different mechanisms may be involved.

Flufenamic acid bi-directionally modulates the transient outward K(+) current in rat cerebellar granule cells

In this report, the effect of flufenamic acid on voltage-activated transient outward K(+) current (I(A)) in cultured rat cerebellar granule cells was investigated. At a concentration of 20 microM to 1 mM, flufenamic acid reversibly inhibited I(A) in a dose-dependent manner. However, flufenamic acid at a concentration of 0.1 to 10 microM significantly increased the current amplitude of I(A). In addition to the current amplitude of I(A), a higher concentration of flufenamic acid had a significant effect on the kinetic parameters of the steady-state activation and inactivation process, suggesting that the binding affinity of flufenamic acid to I(A) channels may be state-dependent. Silencing the K(v)4.2, K(v)4.3, and K(v)1.1 genes of I(A) channels using small interfering RNA did not change the inhibitory effect of flufenamic on I(A), indicating that flufenamic acid did not act specifically on any of the subunits of the I(A)-channel protein. Intracellular application of flufenamic acid could significantly increase the I(A) amplitude but did not alter the inhibited effect induced by extracellular application of flufenamic acid, implying that flufenamic acid may exert its effect from both the inside and outside sites of the channel. Furthermore, the activation of current induced by intracellular application of flufenamic acid could mimic other cyclooxygenase inhibitors and arachidonic acid. Our data are the first that demonstrate how flufenamic acid is able to bidirectionally modulate I(A) channels in neurons at different concentrations and by different methods of application and that two different mechanisms may be involved.