Physiological Roles of the Intermediate Conductance, Ca2+-activated Potassium Channel Kcnn4

In vivo characterisation of the small-conductance KCa (SK) channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) as a potential anticonvulsant

Owing to their activation by increased intracellular Ca(2+) levels following burst firing, and the resultant hyperpolarisation and dampening of neuronal excitability, the small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels have been proposed as a potential target for novel antiepileptic drugs. Indeed, the channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) has been shown to reduce epileptiform activity in vitro. Accordingly, this study has investigated the therapeutic potential of 1-EBIO using a range of in vivo seizure models, and assessed the adverse effect liability with the rotarod and locomotor activity paradigms. To aid benchmarking of 1-EBIO's therapeutic and adverse effect potential, it was tested alongside two currently marketed antiepileptic drugs, phenytoin and levetiracetam. 1-EBIO was found to be effective at reducing seizure incidence in mice following maximal electroshock (ED(50) 36.0 mg/kg) as well as increasing the threshold to electrically- and pentylenetetrazole-induced seizures (TID(10)s 7.3 and 21.5 mg/kg, respectively). However, results from the mouse rotarod test revealed a strong adverse effect potential within the therapeutic dose range (ID(50) 35.6 mg/kg), implying a significantly inferior therapeutic index with respect to the comparator compounds. These results, therefore, support the in vitro data detailing 1-EBIO's reduction of epileptiform activity. However, the use of in vivo models has revealed a significant adverse effect potential within the therapeutic dose range. Nevertheless, given the multiplicity of SK(Ca) channel subunits and that 1-EBIO has been shown to enhance additional, non-SK(Ca) carried currents, these findings do not preclude the possibility that more selective enhancers of SK(Ca) function could prove to be effective as antiepileptic medications.

Expression of intermediate-conductance, Ca2+-activated K+ channel (KCNN4) in H441 human distal airway epithelial cells

Electrophysiological studies of H441 human distal airway epithelial cells showed that thapsigargin caused a Ca(2+)-dependent increase in membrane conductance (G(Tot)) and hyperpolarization of membrane potential (V(m)). These effects reflected a rapid rise in cellular K(+) conductance (G(K)) and a slow fall in amiloride-sensitive Na(+) conductance (G(Na)). The increase in G(Tot) was antagonized by Ba(2+), a nonselective K(+) channel blocker, and abolished by clotrimazole, a KCNN4 inhibitor, but unaffected by other selective K(+) channel blockers. Moreover, 1-ethyl-2-benzimidazolinone (1-EBIO), which is known to activate KCNN4, increased G(K) with no effect on G(Na). RT-PCR-based analyses confirmed expression of mRNA encoding KCNN4 and suggested that two related K(+) channels (KCNN1 and KCNMA1) were absent. Subsequent studies showed that 1-EBIO stimulates Na(+) transport in polarized monolayers without affecting intracellular Ca(2+) concentration ([Ca(2+)](i)), suggesting that the activity of KCNN4 might influence the rate of Na(+) absorption by contributing to G(K). Transient expression of KCNN4 cloned from H441 cells conferred a Ca(2+)- and 1-EBIO-sensitive K(+) conductance on Chinese hamster ovary cells, but this channel was inactive when [Ca(2+)](i) was <0.2 microM. Subsequent studies of amiloride-treated H441 cells showed that clotrimazole had no effect on V(m) despite clear depolarizations in response to increased extracellular K(+) concentration ([K(+)](o)). These findings thus indicate that KCNN4 does not contribute to V(m) in unstimulated cells. The present data thus establish that H441 cells express KCNN4 and highlight the importance of G(K) to the control of Na(+) absorption, but, because KCNN4 is quiescent in resting cells, this channel cannot contribute to resting G(K) or influence basal Na(+) absorption.

Distinct K+ conductive pathways are required for Cl- and K+ secretion across distal colonic epithelium

Secretion of Cl(-) and K(+) in the colonic epithelium operates through a cellular mechanism requiring K(+) channels in the basolateral and apical membranes. Transepithelial current [short-circuit current (I(sc))] and conductance (G(t)) were measured for isolated distal colonic mucosa during secretory activation by epinephrine (Epi) or PGE(2) and synergistically by PGE(2) and carbachol (PGE(2) + CCh). TRAM-34 at 0.5 microM, an inhibitor of K(Ca)3.1 (IK, Kcnn4) K(+) channels (H. Wulff, M. J. Miller, W. Hänsel, S. Grissmer, M. D. Cahalan, and K. G. Chandy. Proc Natl Acad Sci USA 97: 8151-8156, 2000), did not alter secretory I(sc) or G(t) in guinea pig or rat colon. The presence of K(Ca)3.1 in the mucosa was confirmed by immunoblot and immunofluorescence detection. At 100 microM, TRAM-34 inhibited I(sc) and G(t) activated by Epi ( approximately 4%), PGE(2) ( approximately 30%) and PGE(2) + CCh ( approximately 60%). The IC(50) of 4.0 microM implicated involvement of K(+) channels other than K(Ca)3.1. The secretory responses augmented by the K(+) channel opener 1-EBIO were inhibited only at a high concentration of TRAM-34, suggesting further that K(Ca)3.1 was not involved. Sensitivity of the synergistic response (PGE(2) + CCh) to a high concentration TRAM-34 supported a requirement for multiple K(+) conductive pathways in secretion. Clofilium (100 microM), a quaternary ammonium, inhibited Cl(-) secretory I(sc) and G(t) activated by PGE(2) ( approximately 20%) but not K(+) secretion activated by Epi. Thus Cl(-) secretion activated by physiological secretagogues occurred without apparent activity of K(Ca)3.1 channels but was dependent on other types of K(+) channels sensitive to high concentrations of TRAM-34 and/or clofilium.

Membrane-delimited inhibition of maxi-K channel activity by the intermediate conductance Ca2+-activated K channel

The complexity of mammalian physiology requires a diverse array of ion channel proteins. This diversity extends even to a single family of channels. For example, the family of Ca²⁺-activated K channels contains three structural subfamilies characterized by small, intermediate, and large single channel conductances. Many cells and tissues, including neurons, vascular smooth muscle, endothelial cells, macrophages, and salivary glands express more than a single class of these channels, raising questions about their specific physiological roles. We demonstrate here a novel interaction between two types of Ca²⁺-activated K channels: maxi-K channels, encoded by the K_Ca1.1 gene, and IK1 channels (K_Ca3.1). In both native parotid acinar cells and in a heterologous expression system, activation of IK1 channels inhibits maxi-K activity. This interaction was independent of the mode of activation of the IK1 channels: direct application of Ca²⁺, muscarinic receptor stimulation, or by direct chemical activation of the IK1 channels. The IK1-induced inhibition of maxi-K activity occurred in small, cell-free membrane patches and was due to a reduction in the maxi-K channel open probability and not to a change in the single channel current level. These data suggest that IK1 channels inhibit maxi-K channel activity via a direct, membrane-delimited interaction between the channel proteins. A quantitative analysis indicates that each maxi-K channel may be surrounded by four IK1 channels and will be inhibited if any one of these IK1 channels opens. This novel, regulated inhibition of maxi-K channels by activation of IK1 adds to the complexity of the properties of these Ca²⁺-activated K channels and likely contributes to the diversity of their functional roles.

Role of an S4-S5 linker lysine in the trafficking of the Ca(2+)-activated K(+) channels IK1 and SK3

We have investigated the role of the S4-S5 linker in the trafficking of the intermediate (human (h) IK1) and small (rat SK3) conductance K(+) channels using a combination of patch-clamp, protein biochemical, and immunofluorescence-based techniques. We demonstrate that a lysine residue (Lys(197)) located on the intracellular loop between the S4 and S5 domains is necessary for the correct trafficking of hIK1 to the plasma membrane. Mutation of this residue to either alanine or methionine precluded trafficking of the channel to the membrane, whereas the charge-conserving arginine mutation had no effect on channel localization or function. Immunofluorescence localization demonstrated that the K197A mutation resulted in a channel that was primarily retained in the endoplasmic reticulum, and this could not be rescued by incubation at 27 degrees C. Furthermore, immunoblot analysis revealed that the K197A mutation was overexpressed compared with wild-type hIK1 and that this was due to a greatly diminished rate of channel degradation. Co-immunoprecipitation studies demonstrated that the K197A mutation did not preclude multimer formation. Indeed, the K197A mutation dramatically suppressed expression of wild-type hIK1 at the cell surface. Finally, mutation of this conserved lysine in rat SK3 similarly resulted in a channel that failed to correctly traffic to the plasma membrane. These results are the first to demonstrate a critical role for the S4-S5 linker in the trafficking and/or function of IK and SK channels.

Mechanisms underlying regulation of a barium-sensitive K+ conductance by ATP in single proximal tubule cells isolated from frog kidney

K(+) channels play an important role in pump-leak coupling and volume regulation in the renal proximal tubule. Previous experiments have identified a barium-sensitive K(+) conductance (G(Ba)) in proximal tubule cells isolated from frog kidneys. In this paper we examine the regulation of G(Ba) by ATP. G(Ba) was measured in single cells isolated from frog kidney using the whole-cell patch-clamp technique. G(Ba) was activated by 2 mM: intracellular ATP. This activation was enhanced by inhibition of protein kinase C and attenuated by inhibition of protein kinase A, indicating reciprocal regulation by these kinases. Activation by ATP was reduced in the presence of a hypertonic bath solution, suggesting that cell swelling was required. However, after activation to steady-state, G(Ba )was not sensitive to cell-volume changes. Hypotonic shock-induced volume regulation was inhibited by barium and quinidine, inhibitors of G(Ba). The effect of maximal inhibitory concentrations of barium and quinidine on volume regulation was similar and addition of both blockers together did not augment the inhibitory response. G(Ba) was also activated by ADP, via a mechanism dependent on the presence of Mg(2+). However, the responses to ADP and ATP were not additive, suggesting that these nucleotides may share a common mechanism of activation. The regulation of G(Ba) by ATP was biphasic, with a half-maximal activating concentration of 0.89 mM and a half maximal inhibitory concentration of 6.71 mM. The sensitivity to nucleotides suggests that G(Ba) may be regulated by the metabolic state of the cell. Furthermore, the sensitivity to solution osmolality, coupled with the blocker profile of inhibition of volume regulation, suggests that G(Ba) could play a role in volume regulation.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

Blocking ion channel KCNN4 alleviates the symptoms of experimental autoimmune encephalomyelitis in mice

The KCNN4 potassium-ion channel has been reported to play an important role in regulating antigen-induced T cell effector functions in vitro. This study presents the first evidence that a selective KCNN4 blocker, TRAM-34, confers protection against experimental autoimmune encephalomyelitis (EAE) in the mouse model. Treatment with the KCNN4 blocker did not prevent infiltration of T cells in the spinal cord, but resulted in the reduction of both the protein and the message levels of TNF-alpha and IFN-gamma as well as the message levels of several other pro-inflammatory molecules in the spinal cord. Plasma concentrations of TRAM-34 within a 24-h period were between the in vitro IC(50) and IC(90) values for the KCNN4 channel. The effect of TRAM-34 was reversible, as indicated by the development of clinical EAE symptoms within 48 h after withdrawal of treatment. In summary, our data support the idea that KCNN4 channels play a critical role in the immune response during the development of MOG-induced EAE in C57BL/6 mice.

Agonist activation of arachidonate-regulated Ca2+-selective (ARC) channels in murine parotid and pancreatic acinar cells

ARC channels (arachidonate-regulated Ca(2+)-selective channels) are a novel type of highly Ca(2+)-selective channel that are specifically activated by low concentrations of agonist-induced arachidonic acid. This activation occurs in the absence of any depletion of internal Ca(2+) stores (i.e. they are 'non-capacitative'). Previous studies in HEK293 cells have shown that these channels provide the predominant pathway for the entry of Ca(2+) seen at low agonist concentrations where oscillatory [Ca(2+)](i) signals are typically produced. In contrast, activation of the more widely studied store-operated Ca(2+) channels (e.g. CRAC channels) is only seen at higher agonist concentrations where sustained 'plateau-type'[Ca(2+)](i) responses are observed. We have now demonstrated the presence of ARC channels in both parotid and pancreatic acinar cells and shown that, again, they are specifically activated by the low concentrations of appropriate agonists (carbachol in the parotid, and both carbachol and cholecystokinin in the pancreas) that are associated with oscillatory [Ca(2+)](i) signals in these cells. Uncoupling the receptor-mediated activation of cytosolic phospholipase A(2) (cPLA(2)) with isotetrandrine reduces the activation of the ARC channels by carbachol and, correspondingly, markedly inhibits the [Ca(2+)](i) signals induced by low carbachol concentrations, whilst those signals seen at high agonist concentrations are essentially unaffected. Interestingly, in the pancreatic acinar cells, activation by cholecystokinin induces a current through the ARC channels that is only approximately 60% of that seen with carbachol. This is consistent with previous reports indicating that carbachol-induced [Ca(2+)](i) signals in these cells are much more dependent on Ca(2+) entry than are the cholecystokinin-induced responses.