Role of the BK channel (KCa1.1) during activation of electrogenic K+ secretion in guinea pig distal colon

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Intestinal distension orchestrates neuronal activity in the enteric nervous system of adult mice

The enteric nervous system (ENS) regulates the motor, secretory and defensive functions of the gastrointestinal tract. Enteric neurons integrate mechanical and chemical inputs from the gut lumen to generate complex motor outputs. How intact enteric neural circuits respond to changes in the gut lumen is not well understood. We recorded intracellular calcium in live-cell confocal recordings in neurons from intact segments of mouse intestine in order to investigate neuronal response to luminal mechanical and chemical stimuli. Wnt1-, ChAT- and Calb1-GCaMP6 mice were used to record neurons from the jejunum and colon. We measured neuronal calcium response to KCl (75 mM), veratridine (10 μM), 1,1-dimethyl-4-phenylpiperazinium (DMPP; 100 μM) or luminal nutrients (Ensure®), in the presence or absence of intraluminal distension. In the jejunum and colon, distension generated by the presence of luminal content (chyme and faecal pellets, respectively) renders the underlying enteric circuit unresponsive to depolarizing stimuli. In the distal colon, high levels of distension inhibit neuronal response to KCl, while intermediate levels of distension reorganize Ca2+ response in circumferentially propagating slow waves. Mechanosensitive channel inhibition suppresses distension-induced Ca2+ elevations, and calcium-activated potassium channel inhibition restores neuronal response to KCl, but not DMPP in the distended colon. In the jejunum, distension prevents a previously unknown tetrodotoxin-resistant neuronal response to luminal nutrient stimulation. Our results demonstrate that intestinal distension regulates the excitability of ENS circuits via mechanosensitive channels. Physiological levels of distension locally silence or synchronize neurons, dynamically regulating the excitability of enteric neural circuits based on the content of the intestinal lumen. KEY POINTS: How the enteric nervous system of the gastrointestinal tract responds to luminal distension remains to be fully elucidated. Here it is shown that intestinal distension modifies intracellular calcium levels in the underlying enteric neuronal network, locally and reversibly silencing neurons in the distended regions. In the distal colon, luminal distension is integrated by specific mechanosensitive channels and coordinates the dynamics of neuronal activation within the enteric network. In the jejunum, distension suppresses the neuronal calcium responses induced by luminal nutrients. Physiological levels of distension dynamically regulate the excitability of enteric neuronal circuits.

Resveratrol Relaxes Human Gastric Smooth Muscles Through High Conductance Calcium-Activated Potassium Channel in a Nitric Oxide-independent Manner

Resveratrol, as a polyphenolic compound that can be isolated from plants, and also a component of red wine has broad beneficial pharmacological properties. The aim was to investigate the role of nitric oxide and potassium channels in resveratrol-induced relaxation of human gastric smooth muscle. Gastric tissues were obtained from patients who underwent sleeve gastrectomy for severe obesity (n = 10 aged 21–48; BMI 48.21 ± 1.14). The mechanical activity from the muscle strips was detected under isometric conditions as the response to increasing concentrations of resveratrol before and after different pharmacological treatments. Resveratrol caused an observable, dose-dependent gastric muscle relaxation. The maximal response caused by the highest concentration of resveratrol was 83.49 ± 2.85% (p < 0.0001) of the control. Preincubation with L-NNA, L-NAME, or ODQ did not prevent the resveratrol-induced relaxation. Apamin, glibenclamide, 4AP or tamoxifen, did not inhibit the relaxing effect of resveratrol, as well. In turn, blocking BK_Ca by TEA, iberiotoxin, or charybdotoxin resulted in inhibition of resveratrol-induced relaxation (91.08 ± 2.07, p < 0.05; 95.60 ± 1.52, p < 0.01 and 89.58 ± 1.98, p < 0.05, respectively). This study provides the first observation that the relaxant effects of resveratrol in human gastric muscle strips occur directly through BK_Ca channels and independently of nitric oxide signaling pathways. Furthermore, there is considerable potential for further extensive clinical studies with resveratrol as an effective new drug or health supplement to treat gastrointestinal dyspepsia and other gastric hypermotility disorders.

Goblet cell LRRC26 regulates BK channel activation and protects against colitis in mice

Goblet cells (GCs) are specialized cells of the intestinal epithelium contributing critically to mucosal homeostasis. One of the functions of GCs is to produce and secrete MUC2, the mucin that forms the scaffold of the intestinal mucus layer coating the epithelium and separates the luminal pathogens and commensal microbiota from the host tissues. Although a variety of ion channels and transporters are thought to impact on MUC2 secretion, the specific cellular mechanisms that regulate GC function remain incompletely understood. Previously, we demonstrated that leucine-rich repeat-containing protein 26 (LRRC26), a known regulatory subunit of the Ca²⁺-and voltage-activated K⁺ channel (BK channel), localizes specifically to secretory cells within the intestinal tract. Here, utilizing a mouse model in which MUC2 is fluorescently tagged, thereby allowing visualization of single GCs in intact colonic crypts, we show that murine colonic GCs have functional LRRC26-associated BK channels. In the absence of LRRC26, BK channels are present in GCs, but are not activated at physiological conditions. In contrast, all tested MUC2^- cells completely lacked BK channels. Moreover, LRRC26-associated BK channels underlie the BK channel contribution to the resting transepithelial current across mouse distal colonic mucosa. Genetic ablation of either LRRC26 or BK pore-forming α-subunit in mice results in a dramatically enhanced susceptibility to colitis induced by dextran sodium sulfate. These results demonstrate that normal potassium flux through LRRC26-associated BK channels in GCs has protective effects against colitis in mice.

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

Parallel intermediate conductance K+ and Cl- channel activity mediates electroneutral K+ exit across basolateral membranes in rat distal colon

Transepithelial K⁺ absorption requires apical K⁺ uptake and basolateral K⁺ exit. In the colon, apical H⁺-K⁺-ATPase mediates cellular K⁺ uptake, and it has been suggested that electroneutral basolateral K⁺ exit reflects K⁺-Cl^- cotransporter-1 (KCC1) operating in parallel with K⁺ and Cl^- channels. The present study was designed to identify basolateral transporter(s) responsible for K⁺ exit in rat distal colon. Active K⁺ absorption was determined by measuring ⁸⁶Rb⁺ (K⁺ surrogate) fluxes across colonic epithelia under voltage-clamp conditions. With zero Cl^- in the mucosal solution, net K⁺ absorption was reduced by 38%, indicating that K⁺ absorption was partially Cl^--dependent. Serosal addition of DIOA (KCC1 inhibitor) or Ba²⁺ (nonspecific K⁺ channel blocker) inhibited net K⁺ absorption by 21% or 61%, respectively, suggesting that both KCC1 and K⁺ channels contribute to basolateral K⁺ exit. Clotrimazole and TRAM34 (IK channel blockers) added serosally inhibited net K⁺ absorption, pointing to the involvement of IK channels in basolateral K⁺ exit. GaTx2 (CLC2 blocker) added serosally also inhibited net K⁺ absorption, suggesting that CLC2-mediated Cl^- exit accompanies IK channel-mediated K⁺ exit across the basolateral membrane. Net K⁺ absorption was not inhibited by serosal addition of either IbTX (BK channel blocker), apamin (SK channel blocker), chromanol 293B (K_V7 channel blocker), or CFTR_inh172 (CFTR blocker). Immunofluorescence studies confirmed basolateral membrane colocalization of CLC2-like proteins and Na⁺-K⁺-ATPase α-subunits. We conclude that active K⁺ absorption in rat distal colon involves electroneutral basolateral K⁺ exit, which may reflect IK and CLC2 channels operating in parallel.NEW & NOTEWORTHY This study demonstrates that during active electroneutral K⁺ absorption in rat distal colon, K⁺ exit across the basolateral membrane mainly reflects intermediate conductance K⁺ channels operating in conjunction with chloride channel 2, with a smaller, but significant, contribution from K⁺-Cl^- cotransporter-1 (KCC1) activity.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Estradiol mediates relaxation of porcine lower esophageal sphincter

Most pregnant women have symptoms of gastroesophageal reflux disease (GERD) during pregnancy. Postmenopausal hormone replacement therapy is associated with GERD. The effects of estradiol on lower esophageal sphincter (LES) motility and GERD are not clearly known. The purpose of this study is to investigate the effects of estradiol on the motility of the porcine LES. Relaxations of clasp and sling strips of porcine LES caused by estradiol were measured using isometric transducers. We investigated the mechanism of estradiol-induced relaxation of the porcine LES using tetraethylammonium, apamine, iberiotoxin, glibenclamide, KT5720, KT5823, NG-nitro-l-arginine, tetrodotoxin, and ω-conotoxin GVIA. Reverse transcription polymerase chain reaction (PCR) analysis and immunohistochemistry (IHC) were performed to determine the existence of the G protein-coupled estrogen receptor (GPER) in the porcine LES. In endothelin-1-precontracted porcine LES strips, estradiol caused marked relaxations in a concentration-dependent manner. The mechanism of estradiol-induced relaxation on the porcine LES was associated with the potassium channel. Reverse transcription PCR analysis and IHC revealed that GPER was expressed in the sling and clasp fibers of the porcine LES. This finding suggests that GPER mediates the relaxation of the porcine LES. Estradiol may play a role in LES motility.

Dextran sulfate sodium-induced chronic colitis attenuates Ca2+-activated Cl- secretion in murine colon by downregulating TMEM16A

Attenuated Ca2+-activated Cl- secretion has previously been observed in the model of dextran sulfate sodium (DSS)-induced colitis. Prior studies have implicated dysfunctional muscarinic signaling from basolateral membranes as the potential perpetrator leading to decreased Ca2+-activated Cl- secretion. However, in our chronic model of DSS-colitis, cholinergic receptor muscarinic 3 ( Chrm3) transcript (1.028 ± 0.12 vs. 1.029 ± 0.27, P > 0.05) and CHRM3 protein expression (1.021 ± 0.24 vs. 0.928 ± 0.09, P > 0.05) were unchanged. Therefore, we hypothesized that decreased carbachol (CCH)-stimulated Cl- secretion in DSS-induced colitis could be attributed to a loss of Ca2+-activated Cl- channels (CaCC) in apical membranes of colonic epithelium. To establish this chemically-induced colitis, Balb/C mice were exposed to 4% DSS for five alternating weeks to stimulate a more moderate, chronic colitis. Upon completion of the protocol, whole thickness sections of colon were mounted in an Ussing chamber under voltage-clamp conditions. DSS-induced colitis demonstrated a complete inhibition of basolateral administration of CCH-stimulated Cl- secretion that actually displayed a reversal in polarity (15.40 ± 2.22 μA/cm2 vs. -2.47 ± 0.25 μA/cm2). Western blotting of potential CaCCs, quantified by densitometric analysis, demonstrated no change in bestrophin-2 and cystic fibrosis transmembrane regulator, whereas anoctamin-1 [ANO1, transmembrane protein 16A (TMEM16A)] was significantly downregulated (1.001 ± 0.13 vs. 0.510 ± 0.12, P < 0.05). Our findings indicate that decreased expression of TMEM16A in DSS-induced colitis contributes to the decreased Ca2+-activated Cl- secretion in murine colon.

Mechanism of resveratrol-induced relaxation of the guinea pig fundus

BACKGROUND

Resveratrol is a polyphenolic compound that can be isolated from plants and also is a constituent of red wine. Resveratrol induces relaxation of vascular smooth muscle and may prevent cardiovascular diseases.

PURPOSE

Impaired gastric accommodation plays an important role in functional dyspepsia and fundic relaxation and is a therapeutic target of functional dyspepsia. Although drugs for fundic relaxation have been developed, these types of drugs are still rare. The purpose of this study was to investigate the relaxant effects of resveratrol in the guinea pig fundus.

STUDY DESIGN

We studied the relaxant effects of resveratrol in the guinea pig fundus. In addition, we investigated the mechanism of resveratrol-induced relaxation on the guinea pig fundus by using tetraethylammonium (a non-selective potassium channel blocker), apamine (a selective inhibitor of the small conductance calcium-activated potassium channel), iberiotoxin (an inhibitor of large conductance calcium-activated potassium channels), glibenclamide (an ATP-sensitive potassium channel blocker), KT 5720 (a cAMP-dependent protein kinase A inhibitor), KT 5823 (a cGMP-dependent protein kinase G inhibitor), NG-nitro-L-arginine (a competitive inhibitor of nitric oxide synthase), tetrodotoxin (a selective neuronal Na⁺ channel blocker), ω-conotoxin GVIA (a selective neuronal Ca²⁺ channel blocker) and G-15 (a G-protein coupled estrogen receptor antagonist).

RESULTS

The results of this study showed that resveratrol has potent and dose-dependent relaxant effects on the guinea pig fundic muscle. In addition, the results showed that resveratrol-induced relaxation of the guinea pig fundus occurs through nitric oxide and ATP-sensitive potassium channels.

CONCLUSION

This study provides the first evidence concerning the relaxant effects of resveratrol in the guinea pig fundic muscle strips. Furthermore, resveratrol may be a potential drug to relieve gastrointestinal dyspepsia.

Mechanism of resveratrol-induced relaxation in the human gallbladder

BACKGROUND

Resveratrol is a polyphenolic compound extracted from plants and is also a constituent of red wine. Resveratrol produces relaxation of vascular smooth muscle and may prevent cardiovascular diseases. Although resveratrol has been reported to cause relaxation of the guinea pig gallbladder, limited data are available about the effect of resveratrol on the gallbladder smooth muscle in humans. The purpose of this study was to investigate the relaxation effects of resveratrol in human gallbladder muscle strips.

METHODS

We studied the relaxant effects of resveratrol in human gallbladder. In addition, we also investigated mechanism of resveratrol-induced relaxation in human gallbladder by tetraethylammonium (a non-selective potassium channels blocker), iberiotoxin (an inhibitor of large conductance calcium-activated potassium channel), glibenclamide (an ATP-sensitive potassium channel blocker), charybdotoxin (an inhibitor of large conductance calcium-activated potassium channels and slowly inactivating voltage-gated potassium channels), apamine (a selective inhibitor of the small conductance calcium-activated potassium channel), KT 5720 (a cAMP-dependent protein kinase A inhibitor), KT 5823 (a cGMP-dependent protein kinase G inhibitor), NG-Nitro-L-arginine (a competitive inhibitor of nitric oxide synthase), tetrodotoxin (a selective neuronal Na⁺ channel blocker), and ω-conotoxin GVIA (a selective neuronal Ca²⁺ channel blocker).

RESULTS

The present study showed that resveratrol has relaxant effects in human gallbladder muscle strips. In addition, we found that resveratrol-induced relaxation in human gallbladder is associated with nitric oxide, ATP-sensitive potassium channel, and large conductance calcium-activated potassium channel pathways.

CONCLUSIONS

This study provides the first evidence concerning the relaxant effects of resveratrol in human gallbladder muscle strips. Furthermore, these results demonstrate that resveratrol is a potential new drug or health supplement in the treatment of biliary colic.

Intact colonic KCa1.1 channel activity in KCNMB2 knockout mice

Mammalian potassium homeostasis results from tightly regulated renal and colonic excretion, which balances the unregulated dietary K⁺ intake. Colonic K⁺ secretion follows the pump-leak model, in which the large conductance Ca²⁺-activated K⁺ channel (K_Ca1.1) is well established as the sole, but highly regulated apical K⁺ conductance. The physiological importance of auxiliary β and γ subunits of the pore-forming α-subunit of the K_Ca1.1 channel is not yet fully established. This study investigates colonic K⁺ secretion in a global knockout mouse of the K_Ca1.1-β2-subunit (KCNMB2^-/-), which apparently is the only β-subunit of the colonic enterocyte K_Ca1.1 channel. We can report that: (1) Neither K_Ca1.1 α- nor the remaining β-subunits were compensatory transcriptional regulated in colonic epithelia of KCNMB2^-/- mice. (2) Colonic epithelia from KCNMB2^-/- mice displayed equal basal and ATP-induced K_Ca1.1-mediated K⁺ conductance as compared to KCNMB2^+/+ (3) K⁺ secretion was increased in KCNMB2^-/- epithelia compared to wild-type epithelia from animals fed an aldosterone-inducing diet. (4) Importantly, the apical K⁺ conductance was abolished by the specific blocker of K_Ca1.1 channel iberiotoxin in both KCNMB2^+/+ and KCNMB2^-/- mice. Recently a novel family of auxiliary γ-subunits of the K_Ca1.1 channel has been described. (5) We detected the γ1-subunit (LRRC26) mRNA in colonic epithelia. To investigate the physiological role of the γ1-subunit of K_Ca1.1 channels in colonic K⁺ secretion, we acquired an LRRC26 knockout mouse. (6) Unexpectedly, LRRC26 mice had a perinatal lethal phenotype, thus preventing functional measurements. On this basis we conclude that colonic K⁺ secretion is intact or even increased in mice lacking the β2-subunit of K_Ca1.1 channel complex despite no additional compensatory induction of K_Ca1.1 β-subunits.

Peptide toxins and small-molecule blockers of BK channels

Large conductance, Ca(2+)-activated potassium (BK) channels play important roles in the regulation of neuronal excitability and the control of smooth muscle contractions. BK channels can be activated by changes in both the membrane potential and intracellular Ca(2+) concentrations. Here, we provide an overview of the structural and pharmacological properties of BK channel blockers. First, the properties of different venom peptide toxins from scorpions and snakes are described, with a focus on their characteristic structural motifs, including their disulfide bond formation pattern, the binding interface between the toxin and BK channel, and the functional consequence of the blockage of BK channels by these toxins. Then, some representative non-peptide blockers of BK channels are also described, including their molecular formula and pharmacological effects on BK channels. The detailed categorization and descriptions of these BK channel blockers will provide mechanistic insights into the blockade of BK channels. The structures of peptide toxins and non-peptide compounds could provide templates for the design of new channel blockers, and facilitate the optimization of lead compounds for further therapeutic applications in neurological disorders or cardiovascular diseases.

A whole-genome RNAi screen uncovers a novel role for human potassium channels in cell killing by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica kills human cells resulting in ulceration, inflammation and invasion of the colonic epithelium. We used the cytotoxic properties of ameba to select a genome-wide RNAi library to reveal novel host factors that control susceptibility to amebic killing. We identified 281 candidate susceptibility genes and bioinformatics analyses revealed that ion transporters were significantly enriched among susceptibility genes. Potassium (K⁺) channels were the most common transporter identified. Their importance was further supported by colon biopsy of humans with amebiasis that demonstrated suppressed K⁺ channel expression. Inhibition of human K⁺ channels by genetic silencing, pharmacologic inhibitors and with excess K⁺ protected diverse cell types from E. histolytica-induced death. Contact with E. histolytica parasites triggered K⁺ channel activation and K⁺ efflux by intestinal epithelial cells, which preceded cell killing. Specific inhibition of Ca²⁺-dependent K⁺ channels was highly effective in preventing amebic cytotoxicity in intestinal epithelial cells and macrophages. Blockade of K⁺ efflux also inhibited caspase-1 activation, IL-1β secretion and pyroptotic death in THP-1 macrophages. We concluded that K⁺ channels are host mediators of amebic cytotoxicity in multiple cells types and of inflammasome activation in macrophages.

Evidence of K+ channel function in epithelial cell migration, proliferation, and repair

Efficient repair of epithelial tissue, which is frequently exposed to insults, is necessary to maintain its functional integrity. It is therefore necessary to better understand the biological and molecular determinants of tissue regeneration and to develop new strategies to promote epithelial repair. Interestingly, a growing body of evidence indicates that many members of the large and widely expressed family of K(+) channels are involved in regulation of cell migration and proliferation, key processes of epithelial repair. First, we briefly summarize the complex mechanisms, including cell migration, proliferation, and differentiation, engaged after epithelial injury. We then present evidence implicating K(+) channels in the regulation of these key repair processes. We also describe the mechanisms whereby K(+) channels may control epithelial repair processes. In particular, changes in membrane potential, K(+) concentration, cell volume, intracellular Ca(2+), and signaling pathways following modulation of K(+) channel activity, as well as physical interaction of K(+) channels with the cytoskeleton or integrins are presented. Finally, we discuss the challenges to efficient, specific, and safe targeting of K(+) channels for therapeutic applications to improve epithelial repair in vivo.

Enhanced K(+) secretion in dextran sulfate-induced colitis reflects upregulation of large conductance apical K(+) channels (BK; Kcnma1)

Defective colonic Na(+) and Cl(-) absorption is a feature of active ulcerative colitis (UC), but little is known about changes in colonic K(+) transport. We therefore investigated colonic K(+) transport in a rat model of dextran sulfate-induced colitis. Colitis was induced in rat distal colon using 5% dextran sulfate sodium (DSS). Short-circuit current (Isc, indicating electrogenic ion transport) and (86)Rb (K(+) surrogate) fluxes were measured in colonic mucosa mounted in Ussing chambers under voltage-clamp conditions in the presence of mucosal orthovanadate (a P-type ATPase inhibitor). Serum aldosterone was measured by immunoassay. Control animals exhibited zero net K(+) flux. By contrast, DSS-treated animals exhibited active K(+) secretion, which was inhibited by 98, 76, and 22% by Ba(2+) (nonspecific K(+) channel blocker), iberiotoxin (IbTX; BK channel blocker), and TRAM-34 (IK channel blocker), respectively. Apical BK channel α-subunit mRNA abundance and protein expression, and serum aldosterone levels in DSS-treated animals, were enhanced 6-, 3-, and 6-fold respectively, compared with controls. Increasing intracellular Ca(2+) with carbachol (CCH), or intracellular cAMP with forskolin (FSK), stimulated both active Cl(-) secretion and active K(+) secretion in controls but had no or little effect in DSS-treated animals. In DSS-induced colitis, active K(+) secretion involves upregulation of apical BK channel expression, which may be aldosterone-dependent, whereas Cl(-) secretion is diminished. Since similar ion transport abnormalities occur in patients with UC, diarrhea in this disease may reflect increased colonic K(+) secretion (rather than increased Cl(-) secretion), as well as defective Na(+) and Cl(-) absorption.