Beta-adrenergic modulation of K+ current in human T lymphocytes

Modulation of Kv1.3 channels by protein kinase A I in T lymphocytes is mediated by the disc large 1-tyrosine kinase Lck complex

The cAMP/PKA signaling system constitutes an inhibitory pathway in T cells and, although its biochemistry has been thoroughly investigated, its possible effects on ion channels are still not fully understood. K(V)1.3 channels play an important role in T-cell activation, and their inhibition suppresses T-cell function. It has been reported that PKA modulates K(V)1.3 activity. Two PKA isoforms are expressed in human T cells: PKAI and PKAII. PKAI has been shown to inhibit T-cell activation via suppression of the tyrosine kinase Lck. The aim of this study was to determine the PKA isoform modulating K(V)1.3 and the signaling pathway underneath. 8-Bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), a nonselective activator of PKA, inhibited K(V)1.3 currents both in primary human T and in Jurkat cells. This inhibition was prevented by the PKA blocker PKI(6-22). Selective knockdown of PKAI, but not PKAII, with siRNAs abolished the response to 8-BrcAMP. Additional studies were performed to determine the signaling pathway mediating PKAI effect on K(V)1.3. Overexpression of a constitutively active mutant of Lck reduced the response of K(V)1.3 to 8-Br-cAMP. Moreover, knockdown of the scaffolding protein disc large 1 (Dlg1), which binds K(V)1.3 to Lck, abolished PKA modulation of K(V)1.3 channels. Immunohistochemistry studies showed that PKAI, but not PKAII, colocalizes with K(V)1.3 and Dlg1 indicating a close proximity between these proteins. These results indicate that PKAI selectively regulates K(V)1.3 channels in human T lymphocytes. This effect is mediated by Lck and Dlg1. We thus propose that the K(V)1.3/Dlg1/Lck complex is part of the membrane pathway that cAMP utilizes to regulate T-cell function.

The neurology of the immune system: neural reflexes regulate immunity

Parallel advances in neuroscience and immunology established the anatomical and cellular basis for bidirectional interactions between the nervous and immune systems. Like other physiological systems, the immune system--and the development of immunity--is modulated by neural reflexes. A prototypical example is the inflammatory reflex, comprised of an afferent arm that senses inflammation and an efferent arm, the cholinergic anti-inflammatory pathway, that inhibits innate immune responses. This mechanism is dependent on the alpha7 subunit of the nicotinic acetylcholine receptor, which inhibits NF-kappaB nuclear translocation and suppresses cytokine release by monocytes and macrophages. Here we summarize evidence showing that innate immunity is reflexive. Future advances will come from applying an integrative physiology approach that utilizes methods adapted from neuroscience and immunology.

Dopamine exerts no acute effects on Kv1.3 in activated encephalitogenic T cells

Apart from a central function in the extrapyramidal motor system, dopamine has been suggested to play a role in neuroimmune interactions. Particularly in diseases of the central nervous system, such as multiple sclerosis, alterations in dopamine homeostasis might have immunological consequences. We investigated potential effects of dopamine stabilized by ascorbic acid on specifically activated encephalitogenic T cells at the peak of activation. Those cells exhibited an upregulation of voltage-sensitive K+ channels which play a role in many neurotransmitter responses of lymphocytes and fulfilled a prerequisite to respond to dopamine, i.e. stable expression of mRNA for dopamine receptors DRD1, DRD2 and DRD3. However, whole-cell and perforated whole-cell recordings revealed no change in voltage-sensitive K+ currents. Moreover, T cell proliferation was not changed in the presence of dopamine. Previously reported dopamine effects on T cells may be explained by a comparatively lower activation of the cells under investigation, suggesting an activation dependence of dopamine effects that may not be mediated by K+ channels. Alternatively, the occurrence of dopamine degradation products under unprotected conditions may account for the changes reported. Nevertheless, care should be taken when using the dopamine-protecting anti-oxidant ascorbic acid, since we found that it markedly inhibited both K+ currents and lymphocyte proliferation at higher concentrations.

Antagonism of rat beta-cell voltage-dependent K+ currents by exendin 4 requires dual activation of the cAMP/protein kinase A and phosphatidylinositol 3-kinase signaling pathways

Antagonism of voltage-dependent K+ (Kv) currents in pancreatic beta-cells may contribute to the ability of glucagon-like peptide-1 (GLP-1) to stimulate insulin secretion. The mechanism and signaling pathway regulating these currents in rat beta-cells were investigated using the GLP-1 receptor agonist exendin 4. Inhibition of Kv currents resulted from a 20-mV leftward shift in the voltage dependence of steady-state inactivation. Blocking cAMP or protein kinase A (PKA) signaling (Rp-cAMP and H-89, respectively) prevented the inhibition of currents by exendin 4. However, direct activation of this pathway alone by intracellular dialysis of cAMP or the PKA catalytic subunit (cPKA) could not inhibit currents, implicating a role for alternative signaling pathways. A number of phosphorylation sites associated with phosphatidylinositol 3 (PI3)-kinase activation were up-regulated in GLP-1-treated MIN6 insulinoma cells, and the PI3 kinase inhibitor wortmannin could prevent antagonism of beta-cell currents by exendin 4. Antagonists of Src family kinases (PP1) and the epidermal growth factor (EGF) receptor (AG1478) also prevented current inhibition by exendin 4, demonstrating a role for Src kinase-mediated trans-activation of the EGF tyrosine kinase receptor. Accordingly, the EGF receptor agonist betacellulin could replicate the effects of exendin 4 in the presence of elevated intracellular cAMP. Downstream, the PKCzeta pseudosubstrate inhibitor could prevent current inhibition by exendin 4. Therefore, antagonism of beta-cell Kv currents by GLP-1 receptor activation requires both cAMP/PKA and PI3 kinase/PKCzeta signaling via trans-activation of the EGF receptor. This represents a novel dual pathway for the control of Kv currents by G protein-coupled receptors.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Common components of patch-clamp internal recording solutions can significantly affect protein kinase A activity

Common components of whole-cell internal recording solutions were tested both in vitro and in patch-clamp experiments for their effects on the activity of cAMP-dependent protein kinase. Potassium fluoride (KF), 440 mM trimethylamine chloride and exclusion of bovine serum albumin (BSA) decreased the activity of the enzyme, while ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) and the potassium salts of aspartate, gluconate, methylsulfate and monobasic phosphate increased its activity. Addition of KF to the internal solution produced a hyperpolarizing shift in the V1/2 of Ih channel activation, consistent with the KF-induced reduction of protein kinase A activity. Therefore, consideration of the composition of internal solutions is warranted when studying channel physiology by patch-clamp techniques.

Role of potassium channels in the antinociception induced by agonists of alpha2-adrenoceptors

1. The effect of the administration of pertussis toxin (PTX) as well as modulators of different subtypes of K+ channels on the antinociception induced by clonidine and guanabenz was evaluated in the mouse hot plate test. 2. Pretreatment with pertussis toxin (0.25 microg per mouse i.c.v.) 7 days before the hot-plate test, prevented the antinociception induced by both clonidine (0.08-0.2 mg kg(-1), s.c.) and guanabenz (0.1-0.5 mg kg(-1), s.c.). 3. The administration of the K(ATP) channel openers minoxidil (10 microg per mouse, i.c.v.), pinacidil (25 microg per mouse, i.c.v.) and diazoxide (100 mg kg(-1), p.o.) potentiated the antinociception produced by clonidine and guanabenz whereas the K(ATP) channel blocker gliquidone (6 microg per mouse, i.c.v.) prevented the alpha2 adrenoceptor agonist-induced analgesia. 4. Pretreatment with an antisense oligonucleotide (aODN) to mKv1.1, a voltage-gated K+ channel, at the dose of 2.0 nmol per single i.c.v. injection, prevented the antinociception induced by both clonidine and guanabenz in comparison with degenerate oligonucleotide (dODN)-treated mice. 5. The administration of the Ca2+-gated K+ channel blocker apamin (0.5-2.0 ng per mouse, i.c.v.) never modified clonidine and guanabenz analgesia. 6. At the highest effective doses, none of the drugs used modified animals' gross behaviour nor impaired motor coordination, as revealed by the rota-rod test. 7. The present data demonstrate that both K(ATP) and mKv1.1 K+ channels represent an important step in the transduction mechanism underlying central antinociception induced by activation of alpha2 adrenoceptors.

Regulation of native Kv1.3 channels by cAMP-dependent protein phosphorylation

We present evidence that activity of native Kv1.3 channels in human T lymphocytes can be increased by inhibiting phosphatases [using okadaic acid (OA)] or by activating protein kinase A (PKA). OA increased the maximal conductance (Gmax) by 40% and shifted the voltage dependence of activation and inactivation, resulting in a significant increase in window current around the normal membrane potential. PKA inhibition [using the PKA inhibitor peptide PKI-(5-24)] decreased Gmax by 43%, whereas PKA activation [by the Sp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Sp-cAMPS)] increased Gmax by 60% and shifted the inactivation curve, producing an increase in the window current. These results are consistent with our previously published work using cell-attached patches but differ from some studies of Kv1.3. Because we previously reported a similar upregulation by protein kinase C (PKC) activation in these cells, we tested whether the PKA and PKC effects were additive. Our results suggest that PKC-dependent phosphorylation acts as a master switch, inasmuch as calphostin C greatly inhibited the current even after Sp-cAMPS, OA, or PKC activation was used to increase protein phosphorylation. Inasmuch as phosphorylation by both kinases (phorbol ester followed by Sp-cAMPS) abrogated the effects of either kinase alone, our results support the view that Kv1.3 is regulated in a complex manner by serine/threonine phosphorylation.

Blockade of clomipramine and amitriptyline analgesia by an antisense oligonucleotide to mKv1.1, a mouse Shaker-like K+ channel

The effect of an antisense oligonucleotide to the K+ channel coding mKv1.1 mRNA on antinociception induced by the tricyclic antidepressants, clomipramine (20-35 mg kg(-1) s.c.) and amitriptyline (10-25 mg kg(-1) s.c.), was investigated in the mouse hot-plate test. Antisense oligonucleotide (0.5-1.0-2.0-3.0 nmol per i.c.v. injection) produced a dose-dependent inhibition of clomipramine and amitriptyline antinociception 72 h after the last i.c.v. injection. The sensitivity to both analgesic drugs returned 7 days after antisense oligonucleotide injection, indicating the absence of irreversible damage or toxicity. Treatment with a degenerated oligonucleotide did not modify the clomipramine- and amitriptyline-induced antinociception in comparison with that in naive (unpretreated controls), vector and saline i.c.v.-injected mice. A quantitative reverse transcription-polymerase chain reaction (RT-PCR) study demonstrated a reduction in mRNA levels only in the antisense oligonucleotide treated group. Antisense oligonucleotide, degenerated oligonucleotide or vector pretreatment, in the range of doses used, did not produce any behavioural impairment as revealed by the mouse rotarod and hole-board tests. The present results indicate that modulation of the mKv1.1 K+ channel plays an important role in the central analgesia induced by the tricyclic antidepressants, clomipramine and amitriptyline.

Analysis of the modulation by serotonin of a voltage-dependent potassium current in sensory neurons of Aplysia

Potassium currents in pleural sensory neurons of Aplysia were studied under control conditions and in the presence of serotonin (5-HT). Using pharmacological techniques we isolated a current that we refer to as IK,V. Although it is not known whether IK,V represents a distinct type of membrane channel, we described its properties using a Hodgkin-Huxley type model. The effects of 5-HT on IK,V were complex. 5-HT decreased by 50% the steady-state magnitude (Iss) of IK,V in response to a voltage-clamp pulse from -50 mV to +20 mV. In addition, 5-HT significantly slowed both activation kinetics (the time constant of activation was increased by 29% at +20 mV) and inactivation kinetics (the time constant of inactivation was increased by 518% at +20 mV). Mathematical descriptions of IK,V in control conditions and in the presence of 5-HT were used to estimate the relative contribution of serotonergic modulation of IK,V to the total 5-HT-induced modulation of membrane currents. Effects of 5-HT on IK,V account for more than 87% of the 5-HT-induced reduction in outward current during the first 20 ms of a voltage-clamp pulse to +20 mV. This result implies that 5-HT exerts many of its effects on spike width in sensory neurons via modulation of IK,V. Effects of 5-HT on IK,V are consistent with a model in which the maximal conductance underlying the current is decreased by 50%, and the rate constants between open and closed states of both the activation and inactivation processes are diminished in magnitude across all membrane potentials.

Full-length and truncated Kv1.3 K+ channels are modulated by 5-HT1c receptor activation and independently by PKC

In T-cells, the Shaker-related gene, Kv1.3 encodes the type n K+ channel, whereas the type l channel is a product of the Shaw. subfamily gene, Kv3.1. Both these genes are also expressed in the brain. We have used the Xenopus oocyte heterologous expression system to study the modulatory effects of serotonin (5-hydroxytryptamine, 5-HT) on both these cloned channels. In oocytes coexpressing the mouse 5-HT1c receptor and mouse Kv1.3 channel, addition of 100 nM 5-HT causes a complete and sustained suppression of Kv1.3 currents in approximately 20 min. In contrast, 5-HT has no effect on mouse Kv3.1 currents when coexpressed with 5-HT1c receptor. The 5-HT-mediated suppression of Kv1.3 currents proceeds via activation of a pertussis toxin-sensitive G protein and a subsequent rise in intracellular Ca2+, but Ca2+ does not directly block the channel. Protein kinase (PK) C activation is not part of the pathway linking 5-HT1c receptor to Kv1.3 channels. However, phorbol esters independently suppress Kv1.3 currents. Deletion of the first 146 amino acids from the NH2-terminal, containing putative tyrosine kinase and PKA phosphorylation sites, does not alter the time course of 5-HT-mediated suppression of Kv1.3 currents, indicating that these residues are not necessary for modulation. Treatment of oocytes with calmodulin or phosphatase inhibitors does not alter 5-HT-mediated modulation. Collectively, these experiments indicate that the mouse Kv1.3 channel is capable of being modulated by 5-HT via 5-HT1c receptor in a G protein and Ca(2+)-dependent manner, but the subsequent steps in the pathway remain elusive.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of a major cloned voltage-gated K+ channel from human T lymphocytes

When expressed into Xenopus oocytes, HLK3 K+ channel (Kv1-3) induced a slowly inactivating voltage-dependent K+ current. We have studied the modulation of this K+ current by co-expressing a cloned 5-HT2 receptor together with HLK3 K+ channel protein. Application of 5-HT caused a long-lasting inhibition of the voltage-gated K+ current. This inhibitory modulation was mimicked by intracellular injection of inositol triphosphate or Ca2+, as well as by incubation with phorbol esters or diacylglycerol analogs. Oocytes pretreatment with staurosporine and EGTA fully prevented 5-HT inhibitory action. Elevation of cAMP and cGMP levels into oocytes did not produce any detectable effect on the current recorded in the absence or the presence of 5-HT. These data suggest that the second messengers generated by phospholipase C activation may be important modulators of HLK3 K+ channels in the immune and the central nervous systems.

Characterization of K+ currents in rat malignant lymphocytes (Nb2 cells)

Membrane K+ currents of malignant lymphocytes (Nb2 cells) were studied with the whole-cell patch-clamp method. Upon depolarization, K+ currents activate with a delay and follow a sigmoid time course, resembling other delayed rectifier K+ currents present in nerve and muscle cells. The activation time constant of these currents is voltage dependent, increasing from 1 msec at +90 mV to approximately 37 msec at -30 mV. The fractional number of open channels has a sigmoid voltage dependence with a midpoint near -25 mV. Deactivation of K+ currents in Nb2 cells is voltage dependent and follows a simple exponential time course. Time constant of this process increases from 5 msec at -115 mV to almost 80 msec at -40 mV. The relative permeability of K+ channels to different monovalent cations follows the sequence: K+ (1) greater than Rb+ (0.75) greater than NH4+ (0.11) greater than Cs+ (0.07) greater than Na+ (0.05). Inactivation of K+ currents is a biexponential process with time constants of approximately 600 and 7,000 msec. Inactivation of K+ currents in Nb2 cells is not a voltage-dependent process. The steady-state inactivation curve of K+ currents has a midpoint near -40 mV. Following a 500-msec voltage pulse, inactivation of K+ currents recovers with a simple exponential process with a time constant of 9 sec. Short duration (approximately 50 msec) voltage-clamp pulses do not induce significant inactivation of these currents. K+ currents in malignant lymphocytes do not display the phenomenon of cumulative inactivation as described for other delayed rectifier-type K+ channels. Application of a train of voltage pulses to positive potentials at different frequencies induces a moderate decrease in peak outward currents. The use of substances (N-bromoacetamide, trypsin, chloramine-T, and papain) that remove the inactivation of Na+ and K+ currents in other cells are not effective in removing the inactivation of K+ currents present in this lymphoma cell line. Significant differences were found between the characteristics of K+ currents in this malignant cell line and those present in normal lymphocytes. Possible physiological implications for these differences and for the role of K+ currents in the proliferation of normal and malignant lymphocytes are discussed.