ATP-dependent regulation of SK4/IK1-like currents in rat submandibular acinar cells: possible role of cAMP-dependent protein kinase

Small and Intermediate Calcium Activated Potassium Channels in the Heart: Role and Strategies in the Treatment of Cardiovascular Diseases

Calcium-activated potassium channels are a heterogeneous family of channels that, despite their different biophysical characteristics, structures, and pharmacological signatures, play a role of transducer between the ubiquitous intracellular calcium signaling and the electric variations of the membrane. Although this family of channels was extensively described in various excitable and non-excitable tissues, an increasing amount of evidences shows their functional role in the heart. This review aims to focus on the physiological role and the contribution of the small and intermediate calcium-activated potassium channels in cardiac pathologies.

Structure, Gating and Basic Functions of the Ca2+-activated K Channel of Intermediate Conductance

BACKGROUND

The KCa3.1 channel is the intermediate-conductance member of the Ca2+- activated K channel superfamily. It is widely expressed in excitable and non-excitable cells, where it plays a major role in a number of cell functions. This paper aims at illustrating the main structural, biophysical and modulatory properties of the KCa3.1 channel, and providing an account of experimental data on its role in volume regulation and Ca2+ signals.

METHODS

Research and online content related to the structure, structure/function relationship, and physiological role of the KCa3.1 channel are reviewed.

RESULTS

Expressed in excitable and non-excitable cells, the KCa3.1 channel is voltage independent, its opening being exclusively gated by the binding of intracellular Ca2+ to calmodulin, a Ca2+- binding protein constitutively associated with the C-terminus of each KCa3.1 channel α subunit. The KCa3.1 channel activates upon high affinity Ca2+ binding, and in highly coordinated fashion giving steep Hill functions and relatively low EC50 values (100-350 nM). This high Ca2+ sensitivity is physiologically modulated by closely associated kinases and phosphatases. The KCa3.1 channel is normally activated by global Ca2+ signals as resulting from Ca2+ released from intracellular stores, or by the refilling influx through store operated Ca2+ channels, but cases of strict functional coupling with Ca2+-selective channels are also found. KCa3.1 channels are highly expressed in many types of cells, where they play major roles in cell migration and death. The control of these complex cellular processes is achieved by KCa3.1 channel regulation of the driving force for Ca2+ entry from the extracellular medium, and by mediating the K+ efflux required for cell volume control.

CONCLUSION

Much work remains to be done to fully understand the structure/function relationship of the KCa3.1 channels. Hopefully, this effort will provide the basis for a beneficial modulation of channel activity under pathological conditions.

Extracellular Calcium-Dependent Modulation of Endothelium Relaxation in Rat Mesenteric Small Artery: The Role of Potassium Signaling

The nature of NO- and COX-independent endothelial hyperpolarization (EDH) is not fully understood but activation of small- and intermittent-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) is important. Previous studies have suggested that the significance of IK_Ca depends on [Ca²⁺]_out. Also it has been suggested that K⁺ is important through localized [K⁺]_out signaling causing activation of the Na⁺,K⁺-ATPase and inward-rectifying K⁺ channels (K_ir). Here we tested the hypothesis that the modulating effect of [Ca²⁺]_out on the EDH-like response depends on [K⁺]_out. We addressed this possibility using isometric myography of rat mesenteric small arteries. When [K⁺]_out was 4.2 mM, relaxation to acetylcholine (ACh) was stronger at 2.5 mM [Ca²⁺]_out than at 1 mM [Ca²⁺]_out. Inhibition of IK_Ca with TRAM34 suppressed the relaxations but did not change the relation between the relaxations at the low and high [Ca²⁺]_out. This [Ca²⁺]_out-dependence disappeared at 5.9 mM [K⁺]_out and in the presence of ouabain or BaCl₂. Our results suggest that IK_Ca are involved in the localized [K⁺]_out signaling which acts through the Na⁺,K⁺-ATPase and K_ir channels and that the significance of this endothelium-dependent pathway is modulated by [Ca²⁺]_out.

PKA reduces the rat and human KCa3.1 current, CaM binding, and Ca2+ signaling, which requires Ser332/334 in the CaM-binding C terminus

The Ca(2+)-dependent K(+) channel, KCa3.1 (KCNN4/IK/SK4), is widely expressed and contributes to cell functions that include volume regulation, migration, membrane potential, and excitability. KCa3.1 is now considered a therapeutic target for several diseases, including CNS disorders involving microglial activation; thus, we need to understand how KCa3.1 function is regulated. KCa3.1 gating and trafficking require calmodulin binding to the two ends of the CaM-binding domain (CaMBD), which also contains three conserved sites for Ser/Thr kinases. Although cAMP protein kinase (PKA) signaling is important in many cells that use KCa3.1, reports of channel regulation by PKA are inconsistent. We first compared regulation by PKA of native rat KCa3.1 channels in microglia (and the microglia cell line, MLS-9) with human KCa3.1 expressed in HEK293 cells. In all three cells, PKA activation with Sp-8-Br-cAMPS decreased the current, and this was prevented by the PKA inhibitor, PKI14-22. Inhibiting PKA with Rp-8-Br-cAMPS increased the current in microglia. Mutating the single PKA site (S334A) in human KCa3.1 abolished the PKA-dependent regulation. CaM-affinity chromatography showed that CaM binding to KCa3.1 was decreased by PKA-dependent phosphorylation of S334, and this regulation was absent in the S334A mutant. Single-channel analysis showed that PKA decreased the open probability in wild-type but not S334A mutant channels. The same decrease in current for native and wild-type expressed KCa3.1 channels (but not S334A) occurred when PKA was activated through the adenosine A2a receptor. Finally, by decreasing the KCa3.1 current, PKA activation reduced Ca(2+)-release-activated Ca(2+) entry following activation of metabotropic purinergic receptors in microglia.

cAMP-PKA inhibition of SK3 channel reduced both Ca2+ entry and cancer cell migration by regulation of SK3-Orai1 complex

SK3 channel mediates the migration of various cancer cells. When expressed in breast cancer cells, SK3 channel forms a complex with Orai1, a voltage-independent Ca(2+) channel. This SK3-Orai1 complex associates within lipid rafts where it controls a constitutive Ca(2+) entry leading to cancer cell migration and bone metastases development. Since cAMP was found to modulate breast cancer cell migration, we hypothesized that this could be explained by a modulation of SK3 channel activity. Herein, we study the regulation of SK3 channel by the cAMP-PKA pathway and the consequences for SK3-dependent Ca(2+) entry and cancer cell migration. We established that the beta-adrenergic receptor agonist, isoprenaline, or the direct adenylyl cyclase activator forskolin alone or in combination with the PDE4 inhibitor, CI-1044, decreased SK3 channel activity without modifying the expression of SK3 protein at the plasma membrane. Forskolin and CI-1044 reduced the SK3-dependent constitutive Ca(2+) entry and the SK3-dependent migration of MDA-MB-435s cells. PKA inhibition with KT 5720 reduced: (1) the effect of forskolin and CI-1044 by 50 % on Ca(2+) entry and (2) SK3 activity by inhibiting the serine phosphorylation of SK3. These cAMP-elevating agents displaced Orai1 protein outside lipid rafts in contrast to SK3, which remained in the lipid rafts fractions. All together, these results show that activation of the cAMP-PKA pathway decreases SK3 channel and SK3-Orai1 complex activities, leading to a decrease in both Ca(2+) entry and cancer cell migration. This work supports the potential use of cAMP-elevating agents to reduce cancer cell migration and may provide novel opportunities to address/prevent bone metastasis.

Selective inhibition of KCa3.1 channels mediates adenosine regulation of the motility of human T cells

Adenosine, a purine nucleoside, is present at high concentrations in tumors, where it contributes to the failure of immune cells to eliminate cancer cells. The mechanisms responsible for the immunosuppressive properties of adenosine are not fully understood. We tested the hypothesis that adenosine's immunosuppressive functions in human T lymphocytes are in part mediated via modulation of ion channels. The activity of T lymphocytes relies on ion channels. KCa3.1 and Kv1.3 channels control cytokine release and, together with TRPM7, regulate T cell motility. Adenosine selectively inhibited KCa3.1, but not Kv1.3 and TRPM7, in activated human T cells. This effect of adenosine was mainly mediated by A2A receptors, as KCa3.1 inhibition was reversed by SCH58261 (selective A2A receptor antagonist), but not by MRS1754 (A2B receptor antagonist), and it was mimicked by the A2A receptor agonist CGS21680. Furthermore, it was mediated by the cAMP/protein kinase A isoform (PKAI) signaling pathway, as adenylyl-cyclase and PKAI inhibition prevented adenosine effect on KCa3.1. The functional implication of the effect of adenosine on KCa3.1 was determined by measuring T cell motility on ICAM-1 surfaces. Adenosine and CGS21680 inhibited T cell migration. Comparable effects were obtained by KCa3.1 blockade with TRAM-34. Furthermore, the effect of adenosine on cell migration was abolished by pre-exposure to TRAM-34. Additionally, adenosine suppresses IL-2 secretion via KCa3.1 inhibition. Our data indicate that adenosine inhibits KCa3.1 in human T cells via A2A receptor and PKAI, thereby resulting in decreased T cell motility and cytokine release. This mechanism is likely to contribute to decreased immune surveillance in solid tumors.

Molecular basis of potassium channels in pancreatic duct epithelial cells

Potassium channels regulate excitability, epithelial ion transport, proliferation, and apoptosis. In pancreatic ducts, K⁺ channels hyperpolarize the membrane potential and provide the driving force for anion secretion. This review focuses on the molecular candidates of functional K⁺ channels in pancreatic duct cells, including KCNN4 (K_Ca3.1), KCNMA1 (K_Ca1.1), KCNQ1 (K_v7.1), KCNH2 (K_v11.1), KCNH5 (K_v10.2), KCNT1 (K_Ca4.1), KCNT2 (K_Ca4.2), and KCNK5 (K_2P5.1). We will give an overview of K⁺ channels with respect to their electrophysiological and pharmacological characteristics and regulation, which we know from other cell types, preferably in epithelia, and, where known, their identification and functions in pancreatic ducts and in adenocarcinoma cells. We conclude by pointing out some outstanding questions and future directions in pancreatic K⁺ channel research with respect to the physiology of secretion and pancreatic pathologies, including pancreatitis, cystic fibrosis, and cancer, in which the dysregulation or altered expression of K⁺ channels may be of importance.

Selective activation of KCa3.1 and CRAC channels by P2Y2 receptors promotes Ca(2+) signaling, store refilling and migration of rat microglial cells

Microglial activation involves Ca(2+) signaling, and numerous receptors can evoke elevation of intracellular Ca(2+). ATP released from damaged brain cells can activate ionotropic and metabotropic purinergic receptors, and act as a chemoattractant for microglia. Metabotropic P2Y receptors evoke a Ca(2+) rise through release from intracellular Ca(2+) stores and store-operated Ca(2+) entry, and some have been implicated in microglial migration. This Ca(2+) rise is expected to activate small-conductance Ca(2+)-dependent K(+) (SK) channels, if present. We previously found that SK3 (KCa2.3) and KCa3.1 (SK4/IK1) are expressed in rat microglia and contribute to LPS-mediated activation and neurotoxicity. However, neither current has been studied by elevating Ca(2+) during whole-cell recordings. We hypothesized that, rather than responding only to Ca(2+), each channel type might be coupled to different receptor-mediated pathways. Here, our objective was to determine whether the channels are differentially activated by P2Y receptors, and, if so, whether they play differing roles. We used primary rat microglia and a rat microglial cell line (MLS-9) in which riluzole robustly activates both SK3 and KCa3.1 currents. Using electrophysiological, Ca(2+) imaging and pharmacological approaches, we show selective functional coupling of KCa3.1 to UTP-mediated P2Y2 receptor activation. KCa3.1 current is activated by Ca(2+) entry through Ca(2+)-release-activated Ca(2+) (CRAC/Orai1) channels, and both CRAC/Orai1 and KCa3.1 channels facilitate refilling of Ca(2+) stores. The Ca(2+) dependence of KCa3.1 channel activation was skewed to abnormally high concentrations, and we present evidence for a close physical association of the two channel types. Finally, migration of primary rat microglia was stimulated by UTP and inhibited by blocking either KCa3.1 or CRAC/Orai1 channels. This is the first report of selective coupling of one type of SK channel to purinergic stimulation of microglia, transactivation of KCa3.1 channels by CRAC/Orai1, and coordinated roles for both channels in store refilling, Ca(2+) signaling and microglial migration.

Transepithelial bicarbonate secretion: lessons from the pancreas

Many cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelia secrete bicarbonate (HCO(3)(-))-containing fluids. Recent evidence suggests that defects in epithelial bicarbonate secretion are directly involved in the pathogenesis of cystic fibrosis, in particular by building up hyperviscous mucus in the ductal structures of the lung and pancreas. Pancreatic juice is one of the representative fluids that contain a very high concentration of bicarbonate among bodily fluids that are secreted from CFTR-expressing epithelia. We introduce up-to-date knowledge on the basic principles of transepithelial bicarbonate transport by showing the mechanisms involved in pancreatic bicarbonate secretion. The model of pancreatic bicarbonate secretion described herein may also apply to other exocrine epithelia. As a central regulator of bicarbonate transport at the apical membrane, CFTR plays an essential role in both direct and indirect bicarbonate secretion. The major role of CFTR in bicarbonate secretion would be variable depending on the tissue and cell type. For example, in epithelial cells that produce a low concentration of bicarbonate-containing fluid (up to 80 mm), either CFTR-dependent Cl(-)/HCO(3)(-) exchange or CFTR anion channel with low bicarbonate permeability would be sufficient to generate such fluid. However, in cells that secrete high-bicarbonate-containing fluids, a highly selective CFTR bicarbonate channel activity is required. Therefore, understanding the molecular mechanism of transepithelial bicarbonate transport and the role of CFTR in each specific epithelium will provide therapeutic strategies to recover from epithelial defects induced by hyposecretion of bicarbonate in cystic fibrosis.

An intermediate-conductance Ca2+-activated K+ channel is important for secretion in pancreatic duct cells

Potassium channels play a vital role in maintaining the membrane potential and the driving force for anion secretion in epithelia. In pancreatic ducts, which secrete bicarbonate-rich fluid, the identity of K(+) channels has not been extensively investigated. In this study, we investigated the molecular basis of functional K(+) channels in rodent and human pancreatic ducts (Capan-1, PANC-1, and CFPAC-1) using molecular and electrophysiological techniques. RT-PCR analysis revealed mRNAs for KCNQ1, KCNH2, KCNH5, KCNT1, and KCNT2, as well as KCNN4 coding for the following channels: KVLQT1; HERG; EAG2; Slack; Slick; and an intermediate-conductance Ca(2+)-activated K(+) (IK) channel (K(Ca)3.1). The following functional studies were focused on the IK channel. 5,6-Dichloro-1-ethyl-1,3-dihydro-2H-benzimidazole-2-one (DC-EBIO), an activator of IK channel, increased equivalent short-circuit current (I(sc)) in Capan-1 monolayer, consistent with a secretory response. Clotrimazole, a blocker of IK channel, inhibited I(sc). IK channel blockers depolarized the membrane potential of cells in microperfused ducts dissected from rodent pancreas. Cell-attached patch-clamp single-channel recordings revealed IK channels with an average conductance of 80 pS in freshly isolated rodent duct cells. These results indicated that the IK channels may, at least in part, be involved in setting the resting membrane potential. Furthermore, the IK channels are involved in anion and potassium transport in stimulated pancreatic ducts.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Calcium-activated potassium channels BK and IK1 are functionally expressed in human gliomas but do not regulate cell proliferation

Gliomas are morbid brain tumors that are extremely resistant to available chemotherapy and radiology treatments. Some studies have suggested that calcium-activated potassium channels contribute to the high proliferative potential of tumor cells, including gliomas. However, other publications demonstrated no role for these channels or even assigned them antitumorogenic properties. In this work we characterized the expression and functional contribution to proliferation of Ca²⁺-activated K⁺ channels in human glioblastoma cells. Quantitative RT-PCR detected transcripts for the big conductance (BK), intermediate conductance (IK1), and small conductance (SK2) K⁺ channels in two glioblastoma-derived cell lines and a surgical sample of glioblastoma multiforme. Functional expression of BK and IK1 in U251 and U87 glioma cell lines and primary glioma cultures was verified using whole-cell electrophysiological recordings. Inhibitors of BK (paxilline and penitrem A) and IK1 channels (clotrimazole and TRAM-34) reduced U251 and U87 proliferation in an additive fashion, while the selective blocker of SK channels UCL1848 had no effect. However, the antiproliferative properties of BK and IK1 inhibitors were seen at concentrations that were higher than those necessary to inhibit channel activity. To verify specificity of pharmacological agents, we downregulated BK and IK1 channels in U251 cells using gene-specific siRNAs. Although siRNA knockdowns caused strong reductions in the BK and IK1 current densities, neither single nor double gene silencing significantly affected rates of proliferation. Taken together, these results suggest that Ca²⁺-activated K⁺ channels do not play a critical role in proliferation of glioma cells and that the effects of pharmacological inhibitors occur through their off-target actions.

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Activation of large-conductance Ca(2+)-activated K(+) channels depresses basal synaptic transmission in the hippocampal CA1 area in APP (swe/ind) TgCRND8 mice

Large-conductance Ca(2+)-activated K(+) (BK) channels regulate synaptic transmission by contributing to the repolarization phase of the action potential that invades the presynaptic terminal. BK channels are prone to activation under pathological conditions, such as brain ischemia and epilepsy. It is unclear if activation of these channels contributes to the depression of synaptic transmission observed in the early stage of Alzheimer's disease (AD). In this study, we recorded the field excitatory postsynaptic potentials (fEPSPs) in the hippocampus CA1 region of brain slices from 6 to 9 weeks (pre-plaque) TgCRND8 mice, a mouse model of Alzheimer's disease that harbors a double amyloid precursor mutation (KM670N/671L "Swedish" and V717F "Indiana"). Compared to age-matched controls, the fEPSPs in these animals are significantly depressed. This depression is largely mediated by the activation of presynaptic BK channels in the CA1 area. Both BK channel blockers (charybdotoxin and paxilline), and the fast binding calcium chelator, BAPTA-AM, enhance the fEPSP by deactivating the BK channels. Repetitive stimulation to the afferent pathway enhances fEPSP. This enhancement is more prominent when BK channel blockers are added in Tg slices, suggesting that repetitive stimulation further promotes BK channel activation in Tg slices. The potential candidates that mediate the activation of BK channels in these pre-plaque Alzheimer's disease model mice might involve impaired calcium homeostasis and AD related over-generation of reactive oxygen species.

Regulation of contractility by Hsp27 and Hic-5 in rat mesenteric small arteries

The regulation of small artery contractility by vasoconstrictors is important for vascular function, and actin cytoskeleton remodeling is required for contraction. p38 MAPK and tyrosine kinases are implicated in actin polymerization and contraction through heat shock protein 27 (Hsp27) and the cytoskeletal protein paxillin, respectively. We evaluated the roles of downstream targets of p38 MAPK and tyrosine kinases in cytoskeletal reorganization and contraction and whether the two signaling pathways regulate contraction independent of each other. We identified the expression of the paxillin homologue hydrogen peroxide-inducible clone-5 (Hic-5) and showed its activation by norepinephrine (NE) in a Src-dependent manner. Furthermore, we demonstrated a NE-induced interaction of proline-rich tyrosine kinase-2 (PYK2) but not Src or p125 focal adhesion kinase with Hic-5. This interaction was Src dependent, suggesting that Hic-5 was a substrate for PYK2 downstream from Src. The activation of Hic-5 induced its relocalization to the cytosol. The parallel activation of Hsp27 by NE was p38 MAPK dependent and led to its dissociation from actin filaments and translocation from membrane to cytosol and increased actin polymerization. Both Hsp27 and Hic-5 activation resulted in their association within the same time frame as NE-induced contraction, and the inhibition of either p38 MAPK or Src inhibited the interaction between Hsp27 and Hic-5 and the contractile response. Furthermore, combined p38 MAPK and Src inhibition had no greater effect on contraction than individual inhibition, suggesting that the two pathways act through a common mechanism. These data show that NE-induced activation and the association of Hsp27 and Hic-5 are required for the reorganization of the actin cytoskeleton and force development in small arteries.

Ectopic expression of caveolin-1 restores physiological contractile response of aged colonic smooth muscle

Reduced colonic motility has been observed in aged rats with a parallel reduction in acetylcholine (ACh)-induced myosin light chain (MLC(20)) phosphorylation. MLC(20) phosphorylation during smooth muscle contraction is maintained by a coordinated signal transduction cascade requiring both PKC-alpha and RhoA. Caveolae are membrane microdomains that permit rapid and efficient coordination of different signal transduction cascades leading to sustained smooth muscle contraction of the colon. Here, we show that normal physiological contraction can be reinstated in aged colonic smooth muscle cells (CSMCs) upon transfection with wild-type caveolin-1 through the activation of both the RhoA/Rho kinase and PKC pathways. Our data demonstrate that impaired contraction in aging is an outcome of altered membrane translocation of PKC-alpha and RhoA with a concomitant reduction in the association of these molecules with the caveolae-specific protein caveolin-1, resulting in a parallel decrease in the myosin phosphatase-targeting subunit (MYPT) and CPI-17 phosphorylation. Decreased MYPT and CPI-17 phosphorylation activates MLC phosphatase activity, resulting in MLC(20) dephosphorylation, which may be responsible for decreased colonic motility in aged rats. Importantly, transfection of CSMCs from aged rats with wild-type yellow fluorescent protein-caveolin-1 cDNA restored translocation of RhoA and PKC-alpha and phosphorylation of MYPT, CPI-17, and MLC(20), thereby restoring the contractile response to levels comparable with young adult rats. Thus, we propose that caveolin-1 gene transfer may represent a promising therapeutic treatment to correct the age-related decline in colonic smooth muscle motility.

Regulation of membrane potential and fluid secretion by Ca2+-activated K+ channels in mouse submandibular glands

We have recently shown that the IK1 and maxi-K channels in parotid salivary gland acinar cells are encoded by the K(Ca)3.1 and K(Ca)1.1 genes, respectively, and in vivo stimulated parotid secretion is severely reduced in double-null mice. The current study tested whether submandibular acinar cell function also relies on these channels. We found that the K(+) currents in submandibular acinar cells have the biophysical and pharmacological footprints of IK1 and maxi-K channels and their molecular identities were confirmed by the loss of these currents in K(Ca)3.1- and K(Ca)1.1-null mice. Unexpectedly, the pilocarpine-stimulated in vivo fluid secretion from submandibular glands was essentially normal in double-null mice. This result and the possibility of side-effects of pilocarpine on the nervous system, led us to develop an ex vivo fluid secretion assay. Fluid secretion from the ex vivo assay was substantially (about 75%) reduced in animals with both K(+) channel genes ablated - strongly suggesting systemic complications with the in vivo assay. Additional experiments focusing on the membrane potential in isolated submandibular acinar cells revealed mechanistic details underlying fluid secretion in K(+) channel-deficient mice. The membrane potential of submandibular acinar cells from wild-type mice remained strongly hyperpolarized (-55 +/- 2 mV) relative to the Cl(-) equilibrium potential (-24 mV) during muscarinic stimulation. Similar hyperpolarizations were observed in K(Ca)3.1- and K(Ca)1.1-null mice (-51 +/- 3 and -48 +/- 3 mV, respectively), consistent with the normal fluid secretion produced ex vivo. In contrast, acinar cells from double K(Ca)3.1/K(Ca)1.1-null mice were only slightly hyperpolarized (-35 +/- 2 mV) also consistent with the ex vivo (but not in vivo) results. Finally, we found that the modest hyperpolarization of cells from the double-null mice was maintained by the electrogenic Na(+),K(+)-ATPase.

Ca2+-activated IK1 channels associate with lipid rafts upon cell swelling and mediate volume recovery

Restoration of cell volume in the continued presence of osmotic stimuli is essential, particularly in hepatocytes, which swell upon nutrient uptake. Responses to swelling involve the Ca2+-dependent activation of K+ channels, which promote fluid efflux to drive volume recovery; however, the channels involved in hepatocellular volume regulation have not been identified. We found that hypotonic exposure of HTC hepatoma cells evoked the opening of 50 pS K+-permeable channels, consistent with intermediate conductance (IK) channels. We isolated from rat liver and HTC cells a cDNA with sequence identity to the coding region of IK1. Swelling-activated currents were inhibited by transfection with a dominant interfering IK1 mutant. The IK channel blockers clotrimazole and TRAM-34 inhibited whole cell swelling-activated K+ currents and volume recovery. To determine whether IK1 underwent volume-sensitive localization, we expressed a green fluorescent protein fusion of IK1 in HTC cells. The localization of IK1 was suggestive of distribution in lipid rafts. Consistent with this, there was a time-dependent increase in colocalization between IK1 and the lipid raft ganglioside GM1 on the plasma membrane, which subsequently decreased with volume recovery. Pharmacological disruption of lipid rafts altered the plasma membrane distribution of IK1 and inhibited volume recovery after hypotonic exposure. Collectively, these findings support the hypothesis that IK1 regulates compensatory responses to hepatocellular swelling and suggest that regulation of cell volume involves coordination of signaling from lipid rafts with IK1 function.

Phosphorylated HSP27 modulates the association of phosphorylated caldesmon with tropomyosin in colonic smooth muscle

Thin-filament regulation of smooth muscle contraction involves phosphorylation, association, and dissociation of contractile proteins in response to agonist stimulation. Phosphorylation of caldesmon weakens its association with actin leading to actomyosin interaction and contraction. Present data from colonic smooth muscle cells indicate that acetylcholine induced a significant association of caldesmon with PKCalpha and sustained phosphorylation of caldesmon at ser789. Furthermore, acetylcholine induced significant and sustained increase in the association of phospho-caldesmon with heat-shock protein (HSP)27 with concomitant increase in the dissociation of phospho-caldesmon from tropomyosin. At the thin filament level, HSP27 plays a crucial role in acetylcholine-induced association of contractile proteins. Present data from colonic smooth muscle cells transfected with non-phospho-HSP27 mutant cDNA indicate that the absence of phospho-HSP27 inhibits acetylcholine-induced caldesmon phosphorylation. Our results further indicate that the presence of phospho-HSP27 significantly enhances acetylcholine-induced sustained association of phospho-caldesmon with HSP27 with a concomitant increase in acetylcholine-induced dissociation of phospho-caldesmon from tropomyosin. We thus propose a model whereby upon acetylcholine-induced phosphorylation of caldesmon at ser789, the association of phospho-caldesmon (ser789) with phospho-HSP27 results in an essential conformational change leading to dissociation of phospho-caldesmon from tropomyosin. This leads to the sliding of tropomyosin on actin thus exposing the myosin binding sites on actin for actomyosin interaction.

Regulation of airway smooth muscle alpha-actin expression by glucocorticoids

Airway smooth muscle hypertrophy appears to be present in severe asthma. However, the effect of corticosteroids on airway smooth muscle cell size or contractile protein expression has not been studied. We examined the effects of dexamethasone, fluticasone, and salmeterol on contractile protein expression in transforming growth factor (TGF)-beta-treated primary bronchial smooth muscle cells. Dexamethasone and fluticasone, but not salmeterol, each reduced expression of alpha-smooth muscle actin and the short isoform of myosin light chain kinase. Steady-state alpha-actin mRNA level and stability were unchanged, consistent with posttranscriptional control. Fluticasone significantly decreased alpha-actin protein synthesis following treatment with the transcriptional inhibitor actinomycin D, indicative of an inhibitory effect on mRNA translation. Fluticasone also significantly increased alpha-actin protein turnover. Finally, fluticasone reduced TGF-beta-induced incorporation of alpha-actin into filamentous actin, cell length, and cell shortening in response to ACh and KCl. We conclude that glucocorticoids reduce human airway smooth muscle alpha-smooth muscle actin expression and incorporation into contractile filaments, as well as contractile function, in part by attenuation of mRNA translation and enhancement of protein degradation.

The distribution of intermediate-conductance, calcium-activated, potassium (IK) channels in epithelial cells

Intermediate-conductance, calcium-activated, potassium (IK) channels were first identified by their roles in cell volume regulation, and were later shown to be involved in control of proliferation of lymphocytes and to provide a K+ current for epithelial secretory activity. Until now, there has been no systematic investigation of IK channel localization within different epithelia. IK channel immunoreactivity was present in most epithelia, where it occurred in surface membranes of epithelial cells. It was found in all stratified epithelia, including skin, cornea, oral mucosa, vaginal mucosa, urothelium and the oesophageal lining. It occurred in the ducts of fluid-secreting glands, the salivary glands, lacrimal glands and pancreas, and in the respiratory epithelium. A low level of expression was seen in serous acinar cells. It was also found in other epithelia with fluid-exchange properties, the choroid plexus epithelium, the ependyma, visceral pleura and peritoneum, bile ducts and intestinal lining epithelium. However, there was little or no expression in vascular endothelial cells, kidney tubules or collecting ducts, lung alveoli, or in sebaceous glands. It is concluded that the channel is present in surface epithelia (e.g. skin) where it has a cell-protective role against osmotic challenge, and in epithelia where there is anion secretion that is facilitated by a K+ current-dependent hyperpolarization. It was also in some epithelial cells where its roles are as yet unknown.

Regulation of the slow afterhyperpolarization in enteric neurons by protein kinase A

The slow after-hyperpolarization (sAHP) following the action potential is an important determinant of the firing patterns of enteric neurons. The channel responsible for the sAHP thus serves as a critical control point at which neurotransmitters and inflammatory mediators modulate gut motility. Many of these receptor-evoked pathways are known to inhibit the sAHP and, thus, excite enteric neurons. They act through protein kinase A (PKA) which is a strong inhibitor of the sAHP current while protein phosphatases enhance the current. Increasing evidence suggests that the sAHP is mediated by the opening of intermediate-conductance Ca-activated potassium (IK) channels. This neuronal IK channel, previously known to be expressed in a variety of non-excitable cells, is strongly influenced by protein kinases. Investigation of the molecular basis for the modulation of IK channels by protein phosphorylation indicates that there are multiple mechanisms of channel control. Inhibition of channel activity by PKA involves phosphorylation sites located within the calmodulin-binding domain of the channel. The localization of these sites within the region involved in Ca2+ activation suggests that PKA-mediated phosphorylation of the channel opposes the conformational changes caused by binding of Ca/calmodulin, which would otherwise lead to opening of the channel. We suggest that the channel exists as a macromolecular complex involving calmodulin, protein kinases, protein phosphatase and possibly other proteins. The regulation of the channel through kinases and phophatases results in exquisite control of neuronal firing and subsequent modulation of enteric reflexes.

Calcium-activated potassium channels and the regulation of vascular tone

Different calcium signals in the endothelium and smooth muscle target different types of Ca2+-sensitive K+ channels to modulate vascular function. These differential calcium signals and targets represent multilayered opportunities for prevention and/or treatment of vascular dysfunctions.

RhoA- and PKC-alpha-mediated phosphorylation of MYPT and its association with HSP27 in colonic smooth muscle cells

Agonist-induced activation of the RhoA/Rho kinase (ROCK) pathway results in inhibition of myosin phosphatase and maintenance of myosin light chain (MLC20) phosphorylation. We have shown that RhoA/ROCKII translocates and associates with heat shock protein (HSP)27 in the particulate fraction. We hypothesize that inhibition of the 130-kDa regulatory myosin-binding subunit (MYPT) requires its association with HSP27 in the particulate fraction. Furthermore, it is not certain whether regulation of MYPT by CPI-17 or by ROCKII is due to cross talk between RhoA and PKC-alpha. Presently, we examined the cross talk between RhoA and PKC-alpha in the regulation of MYPT phosphorylation in rabbit colon smooth muscle cells. Acetylcholine induced 1) sustained phosphorylation of PKC-alpha, CPI-17, and MYPT; 2) an increase in the association of phospho-MYPT with HSP27 in the particulate fraction; 3) a decrease in myosin phosphatase activity (66.21+/-3.52 and 42.19+/-3.85% nM/ml lysate at 30 s and 4 min); and 4) an increase in PKC activity (298.12+/-46.60% and 290.59+/-22.07% at 30 s and 4 min). Inhibition of RhoA/ROCKII by Y-27632 inhibited phosphorylation of MYPT and its association with HSP27. Both Y27632 and a negative dominant construct of RhoA inhibited phosphorylation of MYPT and CPI-17. Inhibition of PKCs or calphostin C or selective inhibition of PKC-alpha by negative dominant constructs inhibited phosphorylation of MYPT and CPI-17. The results suggest that 1) acetylcholine induces activation of both RhoA and/or PKC-alpha pathways, suggesting cross talk between RhoA and PKC-alpha resulting in phosphorylation of MYPT, inhibition of myosin phosphatase activity, and maintenance of MLC phosphorylation; and 2) phosphorylated MYPT is associated with HSP27 and translocated to the particulate fraction, suggesting a scaffolding role for HSP27 in mediating the association of the complex MYPT/RhoA-ROCKII. Thus both pathways (PKC and RhoA) converge on the regulation of myosin phosphatase activities and modulate sustained phosphorylation of MLC20.

Direct association of RhoA with specific domains of PKC-α

Previous studies performed at our laboratory have shown that agonist-induced contraction of smooth muscle is associated with translocation of protein kinase C (PKC)-α and RhoA to the membrane and that this interaction is due to a direct protein-protein interaction. To determine the domains of PKC-α involved in direct interaction with RhoA, His-tagged PKC-α proteins of individual domains and different combinations of PKC-α domains were used to perform in vitro binding assays with the fusion protein glutathione- S-transferase (GST)-RhoA. Coimmunoprecipitation was also performed using smooth muscle cells transfected with truncated forms of PKC-α in this study. The data indicate that RhoA directly bound to full-length PKC-α, both in vitro (82.57 ± 15.26% above control) and in transfected cells. RhoA bound in vitro to the C1 domain of PKC-α [PKC-α (C1)] (70.48 ± 20.78% above control), PKC-α (C2) (72.26 ± 29.96% above control), and PKC-α (C4) (90.58 ± 26.79% above control), but not to PKC-α (C3) (0.64 ± 5.18% above control). RhoA bound in vitro and in transfected cells to truncated forms of PKC-α, PKC-α (C2, C3, and C4), and PKC-α (C3 and C4) (94.09 ± 12.13% and 85.10 ± 16.16% above control, respectively), but not to PKC-α (C1, C2, and C3) or to PKC-α (C2 and C3) (0.47 ± 1.26% and 7.45 ± 10.76% above control, respectively). RhoA bound to PKC-α (C1 and C2) (60.78 ± 13.78% above control) only in vitro, but not in transfected cells, and PKC-α (C2, C3, and C4) and PKC-α (C3 and C4) bound well to RhoA. These data suggest that RhoA bound to fragments that may mimic the active form of PKC-α. The studies using cells transfected with truncated forms of PKC-α indicate that PKC-α (C1 and C2), PKC-α (C1, C2, and C3), and PKC-α (C2 and C3) did not associate with RhoA. Only full-length PKC-α, PKC-α (C2, C3, and C4), and PKC-α (C3 and C4) associated with RhoA. The association increased upon stimulation with acetylcholine. These results suggest that the functional association of PKC-α with RhoA may require the C4 domain.

Signal-transduction pathways that regulate visceral smooth muscle function. III. Coupling of muscarinic receptors to signaling kinases and effector proteins in gastrointestinal smooth muscles

Stimulation of muscarinic M3 and M2 receptors on gastrointestinal smooth muscle elicits contraction via activation of G proteins that are coupled to a diverse set of downstream signaling pathways and effector proteins. Many studies suggest a canonical excitation-contraction coupling pathway that includes activation of phospholipases, production of inositol 1,4,5-trisphosphate and diacylglycerol, release of calcium from the sarcoplasmic reticulum, activation of L-type calcium channels, and activation of nonselective cation channels. These events lead to elevated intracellular calcium concentration, which activates myosin light chain kinase to phosphorylate and activate myosin II thus causing contraction. In addition, muscarinic receptors are coupled to signaling pathways that modulate the effect of activator calcium. The Rho/Rho kinase pathway inhibits myosin light chain phosphatase, one of the key steps in sensitization of the contractile proteins to calcium. Phosphatidylinositol 3-kinases and Src family tyrosine kinases are also activated by muscarinic agonists. Src family tyrosine kinases regulate L-type calcium and nonselective cation channels. Src activation also leads to activation of ERK and p38 MAPKs. ERK MAPKs phosphorylate caldesmon, an actin filament binding protein. P38 MAPKs activate phospholipases and MAPKAP kinase 2/3, which phosphorylate HSP27. HSP27 may regulate cross-bridge function, actin filament formation, and actin filament attachment to the cell membrane. In addition to the well-known role of M3 muscarinic receptors to regulate myoplasmic calcium levels, the integrated effect of muscarinic activation probably also includes signaling pathways that modulate phospholipases, cyclic nucleotides, contractile protein function, and cytoskeletal protein function.

Beta2-adrenoceptor regulation of the K+ channel iKCa1 in human mast cells

Human mast cells express the intermediate conductance Ca2+-activated K+ channel iKCa1, which opens following IgE-dependent activation. This results in cell membrane hyperpolarization and potentiation of both Ca2+ influx and degranulation. Mast cell activation is attenuated following exposure to beta2-adrenoceptor agonists such as salbutamol, an effect postulated to operate via intracellular cyclic AMP. In this study, we show that salbutamol closes iKCa1 in mast cells derived from human lung and peripheral blood. Salbutamol (1-10 microM) inhibited iKCa1 currents following activation with both anti-IgE and the iKCa1 opener 1-EBIO, and was reversed by removing salbutamol or by the addition of the selective beta2-adrenoceptor antagonist and inverse agonist ICI 118551. Interestingly, ICI 118551 consistently opened iKCa1 in quiescent cells, suggesting that constitutive beta2-receptor signaling suppresses channel activity. Manipulation of intracellular cAMP, Galphai, and Galphas demonstrates that the beta2-adrenergic effects are consistent with a membrane-delimited mechanism involving Galphas. This is the first demonstration that gating of the iKCa1 channel is regulated by a G protein-coupled receptor and provides a clearly defined mechanism for the mast cell "stabilizing" effect of beta2-agonists. Furthermore, the degree of constitutive beta2-receptor "tone" may control the threshold for human mast cell activation through the regulation of iKCa1.

Agonist-induced association of tropomyosin with protein kinase Calpha in colonic smooth muscle

Smooth muscle contraction regulated by myosin light chain phosphorylation is also regulated at the thin-filament level. Tropomyosin, a thin-filament regulatory protein, regulates contraction by modulating actin-myosin interactions. Present investigation shows that acetylcholine induces PKC-mediated and calcium-dependent phosphorylation of tropomyosin in colonic smooth muscle cells. Our data also shows that acetylcholine induces a significant and sustained increase in PKC-mediated association of tropomyosin with PKCalpha in the particulate fraction of colonic smooth muscle cells. Immunoblotting studies revealed that in colonic smooth muscle cells, there is no significant change in the amount of tropomyosin or actin in particulate fraction in response to acetylcholine, indicating that the increased association of tropomyosin with PKCalpha in the particulate fraction may be due to acetylcholine-induced translocation of PKCalpha to the particulate fraction. To investigate whether the association of PKCalpha with tropomyosin was due to a direct interaction, we performed in vitro direct binding assay. Tropomyosin cDNA amplified from colonic smooth muscle mRNA was expressed as GST-tropomyosin fusion protein. In vitro binding experiments using GST-tropomyosin and recombinant PKCalpha indicated direct interaction of tropomyosin with PKCalpha. PKC-mediated phosphorylation of tropomyosin and direct interaction of PKCalpha with tropomyosin suggest that tropomyosin could be a substrate for PKC. Phosphorylation of tropomyosin may aid in holding the slided tropomyosin away from myosin binding sites on actin, resulting in actomyosin interaction and sustained contraction.

Protein kinase A inhibits intermediate conductance Ca2+-activated K+ channels expressed in Xenopus oocytes

Intermediate-conductance (IK) Ca(2+)-activated K(+) channels are expressed in many different cell types where they perform a variety of functions including cell volume regulation, transepithelial secretion, lymphocyte activation and cell cycle progression. IK channels are thought to be regulated by phosphorylation; however, whether kinases act directly on the channel is unclear. Using IK channels heterologously expressed in Xenopus oocytes, we demonstrate that IK channels are potently inhibited (60%) by the catalytic subunit of protein kinase A (PKA). Inhibition of IK channel current by PKA is abolished by mutation of four phosphorylation residues (S312, T327, S332, and T348) in the putative calmodulin-binding region of the channel. Evidence for direct modulation of the IK channel by PKA was further demonstrated using GST fusion proteins. The major site of phosphorylation was found to be serine 332; however, other residues were also phosphorylated. We conclude that IK channels can be directly regulated by the cAMP second-messenger system. The mechanism appears to involve direct phosphorylation by PKA of a modulatory locus in the cytoplasmic region of the channel, the site at which calmodulin is thought to interact. Modulation of IK channels by protein kinases may be an important mechanism regulating cell function.

Direct association and translocation of PKC-alpha with calponin

Calponin has been implicated in the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated MgATPase activity of phosphorylated myosin. Calponin has also been shown to interact with PKC. We have studied the interaction of calponin with PKC-alpha and with the low molecular weight heat-shock protein (HSP)27 in contraction of colonic smooth muscle cells. Particulate fractions from isolated smooth muscle cells were immunoprecipitated with antibodies to calponin and Western blot analyzed with antibodies to HSP27 and to PKC-alpha. Acetylcholine induced a sustained increase in the immunocomplexing of calponin with HSP27 and of calponin with PKC-alpha in the particulate fraction, indicating an association of the translocated proteins in the membrane. To examine whether the observed interaction in vivo is due to a direct interaction of calponin with PKC-alpha, a cDNA of 1.3 kb of human calponin gene was PCR amplified. PCR product encoding 622 nt of calponin cDNA (nt 351-972 corresponding to amino acids 92-229) was expressed as fusion glutathione S-transferase (GST) protein in the vector pGEX-KT. We have studied the direct association of GST-calponin fusion protein with recombinant PKC-alpha in vitro. Western blot analysis of the fractions collected after elution with reduced glutathione buffer (pH 8.0) show a coelution of GST-calponin with PKC-alpha, indicating a direct association of GST-calponin with PKC-alpha. These data suggest that there is a direct association of translocated calponin and PKC-alpha in the membrane and a role for the complex calponin-PKC-alpha-HSP27, in contraction of colonic smooth muscle cells.