Ca2+-induced Cl- efflux at rat distal colonic epithelium

The anoctamins: Structure and function

When activated by increase in intracellular Ca2+, anoctamins (TMEM16 proteins) operate as phospholipid scramblases and as ion channels. Anoctamin 1 (ANO1) is the Ca2+-activated epithelial anion-selective channel that is coexpressed together with the abundant scramblase ANO6 and additional intracellular anoctamins. In salivary and pancreatic glands, ANO1 is tightly packed in the apical membrane and secretes Cl-. Epithelia of airways and gut use cystic fibrosis transmembrane conductance regulator (CFTR) as an apical Cl- exit pathway while ANO1 supports Cl- secretion mainly by facilitating activation of luminal CFTR and basolateral K+ channels. Under healthy conditions ANO1 modulates intracellular Ca2+ signals by tethering the endoplasmic reticulum, and except of glands its direct secretory contribution as Cl- channel might be small, compared to CFTR. In the kidneys ANO1 supports proximal tubular acid secretion and protein reabsorption and probably helps to excrete HCO3-in the collecting duct epithelium. However, under pathological conditions as in polycystic kidney disease, ANO1 is strongly upregulated and may cause enhanced proliferation and cyst growth. Under pathological condition, ANO1 and ANO6 are upregulated and operate as secretory channel/phospholipid scramblases, partly by supporting Ca2+-dependent processes. Much less is known about the role of other epithelial anoctamins whose potential functions are discussed in this review.

Segmental differences in ion transport in rat cecum

Ion-transport properties of the epithelium of the cecum, the biggest fermental chamber in non-ruminant species, are largely unknown. Recently, in Ussing chamber experiments, segmental differences in basal short-circuit current (I_sc) in rat corpus ceci were observed. The oral segment usually exhibited a much lower or even negative basal I_sc in comparison with the aboral segment. The aim of the present study was the closer characterization of these differences. Basal I_sc was inhibited by bumetanide and tetrodotoxin in both segments, whereas indomethacin reduced basal I_sc only in the aboral corpus. Amiloride did not inhibit basal I_sc suggesting that spontaneous anion secretion (but not electrogenic Na⁺ absorption via ENaC) contributes to the baseline current. In both segments, mucosally applied K⁺ channel blockers increased I_sc indicating a spontaneous K⁺ secretion. Basolateral depolarization was used to characterize the ion conductances in the apical membrane. When a Cl^- gradient was applied, apical Cl^- conductance stimulated by carbachol and by forskolin was revealed. When the Cl^- gradient was omitted and instead a K⁺ gradient was used to drive currents across apical K⁺ channels, a Ba²⁺-sensititve K⁺ conductance was observed in both segments, and carbachol stimulated this conductance leading to a negative I_sc. Conversely, forskolin induced a positive I_sc under these conditions which was dependent on the presence of mucosal Na⁺ consistent with electrogenic Na⁺ absorption. This current was reduced by amiloride and several blockers of members of the TRP channel superfamily. These results indicate that similar transport mechanisms are involved in electrogenic ion transport across cecal oral and aboral segments, but with a higher spontaneous prostaglandin production in the aboral segment responsible for higher basal transport rates of both anions and cations.

Substituent Effects on the Inclusion of 1-Alkyl-6-alkoxy-quinolinium in 4-Sulfonatocalix[n]arenes

The effect of the chain length of the alkyl and alkoxy substituents on the binding characteristics of 1-alkyl-6-alkoxy-quinolinium cations was studied using 4-sulfonatocalix[4]arene (SCX4) and 4-sulfonatocalix[6]arene (SCX6) in neutral aqueous solutions at 298 K. Isothermal calorimetric titrations showed enthalpy-controlled inclusion with 1:1 stoichiometry. The equilibrium constants of complexation were always larger for the confinement in SCX4 than in its SCX6 homologue because the better matching between the host and guest sizes allowed more exothermic interaction. The binding affinity diminished with the lengthening of the aliphatic chain of the guests in the case of the association with SCX4, but insignificant change was found for SCX6 complexes. The most substantial change in the enthalpic and entropic contributions to the driving force of complex production occurred when the alkyl chain was linked to the heterocyclic nitrogen and the number of its carbon atoms varied between 1 and 4. ¹H NMR spectra evidenced that in SCX6, the 1-alkyl-6-alkoxy-quinolinium cations could be included within the macrocycle cavity. In the case of SCX4, the quinolinium ring is always inside the host, but the alkyl chain is included within SCX4 only for a short chain length (n up to 4). In contrast, the alkoxy chain displays a very weak interaction with the cavity irrespective of the length. Because of the outward orientation from the host, the lengthening of the alkoxy substituent of the quinolinium moiety barely influenced the thermodynamics of inclusion in SCX4. Distinct linear enthalpy-entropy correlations were found for the encapsulation in SCX4 and SCX6.

Stimulation of Na+ -K+ -pump currents by epithelial nicotinic receptors in rat colon

BACKGROUND AND PURPOSE

Acetylcholine-induced epithelial Cl^- secretion is generally thought to be mediated by epithelial muscarinic receptors and nicotinic receptors on secretomotor neurons. However, recent data have shown expression of nicotinic receptors by intestinal epithelium and the stimulation of Cl^- secretion by nicotine, in the presence of the neurotoxin, tetrodotoxin. Here, we aimed to identify the transporters activated by epithelial nicotinic receptors and to clarify their role in cholinergic regulation of intestinal ion transport.

EXPERIMENTAL APPROACH

Ussing chamber experiments were performed, using rat distal colon with intact epithelia. Epithelia were basolaterally depolarized to measure currents across the apical membrane. Apically permeabilized tissue was also used to measure currents across the basolateral membrane in the presence of tetrodotoxin.

KEY RESULTS

Nicotine had no effect on currents through Cl^- channels in the apical membrane or on currents through K⁺ channels in the apical or the basolateral membrane. Instead, nicotine stimulated the Na⁺ -K⁺ -pump as indicated by Na⁺ -dependency and sensitivity of the nicotine-induced current across the basolateral membrane to cardiac steroids. Effects of nicotine were inhibited by nicotinic receptor antagonists such as hexamethonium and mimicked by dimethyl-4-phenylpiperazinium, a chemically different nicotinic agonist. Simultaneous stimulation of epithelial muscarinic and nicotinic receptors led to a strong potentiation of transepithelial Cl^- secretion.

CONCLUSIONS AND IMPLICATIONS

These results suggest a novel concept for the cholinergic regulation of transepithelial ion transport by costimulation of muscarinic and nicotinic epithelial receptors and a unique role of nicotinic receptors controlling the activity of the Na⁺ -K⁺ -ATPase.

Epithelial propionyl- and butyrylcholine as novel regulators of colonic ion transport

BACKGROUND AND PURPOSE

The colonic surface epithelium produces acetylcholine, released after the binding of propionate to GPCRs for this short-chain fatty acid (SCFA). This epithelial acetylcholine then induces anion secretion via stimulation of acetylcholine receptors. The key enzyme responsible for acetylcholine synthesis, choline acetyltransferase, is known to be unselective as regards the fatty acid used for esterification of choline. As the colonic epithelium is permanently exposed to high concentrations of different SCFAs produced by bacterial fermentation, we investigated whether choline esters other than acetylcholine, propionylcholine and butyrylcholine, are produced by the colonic epithelium, too, and whether these 'atypical' esters are able to stimulate the acetylcholine receptors involved in the regulation of colonic ion transport.

EXPERIMENTAL APPROACH

Desorption electrospray ionization mass spectroscopy (DESI-MS), Ussing chamber and Ca(2+) -imaging experiments were performed on rat distal colon.

KEY RESULTS

DESI-MS analyses revealed the production of acetylcholine, propionylcholine and butyrylcholine in the surface epithelium. Relative expression rates were 2-3% in comparison with acetylcholine. In Ussing chamber experiments, both atypical choline esters caused a concentration-dependent increase in short-circuit current, that is, stimulated anion secretion. Inhibitor experiments in the absence and presence of the submucosal plexus revealed the involvement of neuronal and epithelial acetylcholine receptors. While butyrylcholine obviously stimulated both nicotinic and muscarinic receptors, propionylcholine predominantly acted on muscarinic receptors.

CONCLUSIONS AND IMPLICATIONS

These results suggest a novel pathway for communication between intestinal microbes producing SCFA and the host via modification of epithelial production of choline esters involved in the paracrine regulation of the colonic epithelium.

Hypoxia/Reoxygenation Effects on Ion Transport across Rat Colonic Epithelium

Ischemia causes severe damage in the gastrointestinal tract. Therefore, it is interesting to study how the barrier and transport functions of intestinal epithelium change under hypoxia and subsequent reoxygenation. For this purpose we simulated hypoxia and reoxygenation on mucosa-submucosa preparations from rat distal colon in Ussing chambers and on isolated crypts. Hypoxia (N₂ gassing for 15 min) induced a triphasic change in short-circuit current (I_sc): a transient decrease, an increase and finally a long-lasting fall below the initial baseline. During the subsequent reoxygenation phase, I_sc slightly rose to values above the initial baseline. Tissue conductance (G_t) showed a biphasic increase during both the hypoxia and the reoxygenation phases. Omission of Cl⁻ or preincubation of the tissue with transport inhibitors revealed that the observed changes in I_sc represented changes in Cl⁻ secretion. The radical scavenger trolox C reduced the I_sc response during hypoxia, but failed to prevent the rise of I_sc during reoxygenation. All changes in I_sc were Ca²⁺-dependent. Fura-2 experiments at loaded isolated colonic crypts revealed a slow increase of the cytosolic Ca²⁺ concentration during hypoxia and the reoxygenation phase, mainly caused by an influx of extracellular Ca²⁺. Surprisingly, no changes could be detected in the fluorescence of the superoxide anion-sensitive dye mitosox or the thiol-sensitive dye thiol tracker, suggesting a relative high capacity of the colonic epithelium (with its low O₂ partial pressure even under physiological conditions) to deal with enhanced radical production during hypoxia/reoxygenation.

"Store-operated" cAMP signaling contributes to Ca2+-activated Cl- secretion in T84 colonic cells

Apical cAMP-dependent CFTR Cl(-) channels are essential for efficient vectorial movement of ions and fluid into the lumen of the colon. It is well known that Ca(2+)-mobilizing agonists also stimulate colonic anion secretion. However, CFTR is apparently not activated directly by Ca(2+), and the existence of apical Ca(2+)-dependent Cl(-) channels in the native colonic epithelium is controversial, leaving the identity of the Ca(2+)-activated component unresolved. We recently showed that decreasing free Ca(2+) concentration ([Ca(2+)]) within the endoplasmic reticulum (ER) lumen elicits a rise in intracellular cAMP. This process, which we termed "store-operated cAMP signaling" (SOcAMPS), requires the luminal ER Ca(2+) sensor STIM1 and does not depend on changes in cytosolic Ca(2+). Here we assessed the degree to which SOcAMPS participates in Ca(2+)-activated Cl(-) transport as measured by transepithelial short-circuit current (Isc) in polarized T84 monolayers in parallel with imaging of cAMP and PKA activity using fluorescence resonance energy transfer (FRET)-based reporters in single cells. In Ca(2+)-free conditions, the Ca(2+)-releasing agonist carbachol and Ca(2+) ionophore increased Isc, cAMP, and PKA activity. These responses persisted in cells loaded with the Ca(2+) chelator BAPTA-AM. The effect on Isc was enhanced in the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), inhibited by the CFTR inhibitor CFTRinh-172 and the PKA inhibitor H-89, and unaffected by Ba(2+) or flufenamic acid. We propose that a discrete component of the "Ca(2+)-dependent" secretory activity in the colon derives from cAMP generated through SOcAMPS. This alternative mode of cAMP production could contribute to the actions of diverse xenobiotic agents that disrupt ER Ca(2+) homeostasis, leading to diarrhea.

Anoctamins support calcium-dependent chloride secretion by facilitating calcium signaling in adult mouse intestine

Intestinal epithelial electrolyte secretion is activated by increase in intracellular cAMP or Ca(2+) and opening of apical Cl(-) channels. In infants and young animals, but not in adults, Ca(2+)-activated chloride channels may cause secretory diarrhea during rotavirus infection. While detailed knowledge exists concerning the contribution of cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) channels, analysis of the role of Ca(2+)-dependent Cl(-) channels became possible through identification of the anoctamin (TMEM16) family of proteins. We demonstrate expression of several anoctamin paralogues in mouse small and large intestines. Using intestinal-specific mouse knockout models for anoctamin 1 (Ano1) and anoctamin 10 (Ano10) and a conventional knockout model for anoctamin 6 (Ano6), we demonstrate the role of anoctamins for Ca(2+)-dependent Cl(-) secretion induced by the muscarinic agonist carbachol (CCH). Ano1 is preferentially expressed in the ileum and large intestine, where it supports Ca(2+)-activated Cl(-) secretion. In contrast, Ano10 is essential for Ca(2+)-dependent Cl(-) secretion in jejunum, where expression of Ano1 was not detected. Although broadly expressed, Ano6 has no role in intestinal cholinergic Cl(-) secretion. Ano1 is located in a basolateral compartment/membrane rather than in the apical membrane, where it supports CCH-induced Ca(2+) increase, while the essential and possibly only apical Cl(-) channel is CFTR. These results define a new role of Ano1 for intestinal Ca(2+)-dependent Cl(-) secretion and demonstrate for the first time a contribution of Ano10 to intestinal transport.

Characterization of the motor inhibitory role of colonic mucosa under chemical stimulation in mice

The main roles of the colonic mucosa are the absorption of water and electrolytes and the barrier function that preserves the integrity of the colonic wall. The mediators and mechanisms to accomplish these functions are under continuous investigation, but little attention has been paid to a possible control of colonic motility by the mucosa that would fine tune the relationship between absorption and motility. The purpose of this study was to establish the role of the mucosa in the control of induced colonic contractility. Young ICR-CD1 mice (3-5 mo old) were studied. Isometric tension transducers were used to record contractility in full-thickness (FT) and mucosa-free (MF) strips from proximal colon. Proximal FT strips showed lower KCl- and bethanechol-induced responses than MF strips. The difference was not due to mechanical artefacts since the contractile response of FT strips to electrical field stimulation was around 50% lower than in MF. The inhibitory effects of the mucosa on FT strips were mimicked by immersion of separate strips of mucosa in the organ bath but not by addition of mucosal extract, suggesting gaseous molecules as mediators of this effect. Incubation of MF strips with synthase inhibitors of nitric oxide, carbon monoxide, and hydrogen sulfide abolished the inhibition caused by addition of the mucosal strip, indicating that mucosal gasotransmitters are the mediators of these effects. This suggests that the control of colonic motility exerted by the mucosa could fine tune the balance between transit and absorption.

STIM1-regulated Ca2+ influx across the apical and the basolateral membrane in colonic epithelium

In nonexcitable cells, store-operated Ca(2+) entry is the most important pathway for influx of extracellular Ca(2+) serving as a second messenger in the cytoplasm. The present study investigated the expression, localization and polar distribution of two key components of store-operated Ca(2+) entry identified, e.g., in lymphocytes or epithelial cell lines-STIM1 (stromal interacting molecule 1), working as a Ca(2+) sensor in the endoplasmic reticulum, and Orai1, working as the (or part of the) store-operated Ca(2+) channel in the plasma membrane-in a native intestinal epithelium, i.e., rat colon. Immunohistochemical investigations revealed expression of STIM1 and Orai1 in the rat colonic epithelium. Ca(2+) store depletion led to a translocation of STIM1 both to the basolateral as well as to the apical cell pole as observed by confocal microscopy. A Ca(2+) depletion/repletion protocol was used in Ussing chamber experiments to investigate the contribution of basolateral and apical store-operated Ca(2+) entry to the induction of anion secretion. These experiments revealed that Ca(2+)-dependent anion secretion was induced not only by basolateral Ca(2+) repletion but also, to a lesser extent, by apical Ca(2+) repletion. Both responses were suppressed by La(3+). The effect of basolateral Ca(2+) repletion was significantly inhibited by brefeldin A, a blocker of vesicular transport from the endoplasmic reticulum to the Golgi apparatus. In a final series of experiments, fura-2-loaded HT29/B6 cells were used. A carbachol-induced increase in the cytosolic Ca(2+) concentration was significantly reduced when cells were pretreated with siRNA against STIM1. In conclusion, these results demonstrate that STIM1 as a key component of intracellular Ca(2+) signaling is expressed by rat colonic epithelium and is involved in the regulation not only of basolateral but also of apical Ca(2+) influx.

ATP-sensitive K(+) channels in rat colonic epithelium

ATP-sensitive K(+) (KATP) channels couple the metabolic state of a cell to its electrical activity. They consist of a hetero-octameric complex with pore-forming Kir6.x (Kir6.1, Kir6.2) and regulatory sulfonylurea receptor (SUR) subunits. Functional data indicate that KATP channels contribute to epithelial K(+) currents at colonic epithelia. However, their molecular identity and their properties are largely unknown. Therefore, changes in short-circuit current (I sc) induced by the KATP channel opener pinacidil (5 10(-4) mol l(-1)) were measured in Ussing chambers under control conditions and in the presence of different blockers of KATP channels. The channel subunits expressed by the colonic epithelium were identified by immunohistochemistry and by RT-PCR. The K(+) channel opener, when administered at the serosal side, induced an increase in I sc consistent with the induction of transepithelial Cl(-) secretion after activation of basolateral K(+) channels, whereas mucosal administration of pinacidil resulted in a negative I sc. The increase in I sc evoked by serosal pinacidil was inhibited by serosal administration of glibenclamide (5 10(-4) mol l(-1)) and gliclazide (10(-6) mol l(-1)), but was resistant even against a high concentration (10(-2) mol l(-1)) of tolbutamide. In contrast, none of these inhibitors (administered at the mucosal side) reduced significantly the negative I sc induced by mucosal pinacidil. Instead, pinacidil inhibited Cl(-) currents across apical Cl(-) channels in basolaterally depolarized epithelia indicating that the negative I sc induced by mucosal pinacidil is due to a transient inhibition of Cl(-) secretion. In mRNA prepared from isolated colonic crypts, messenger RNA for both pore-forming subunits, Kir6.1 and Kir6.2, and two regulatory subunits (SUR1 and SUR2B) was found. Expression within the colonic epithelium was confirmed for these subunits by immunohistochemistry. In consequence, KATP channels are present in the basolateral membrane of the colonic epithelium; their exact subunit composition, however, has still to be revealed.

Epidermal growth factor chronically upregulates Ca(2+)-dependent Cl(-) conductance and TMEM16A expression in intestinal epithelial cells

Dysregulated epithelial fluid and electrolyte transport is a common feature of many intestinal disorders. However, molecular mechanisms that regulate epithelial transport processes are still poorly understood, thereby limiting development of new therapeutics. Previously, we showed that epidermal growth factor (EGF) chronically enhances intestinal epithelial secretory function. Here, we investigated a potential role for altered expression or activity of apical Cl(−) channels in mediating the effects of EGF. Cl(−) secretion across monolayers of T(84) colonic epithelia was measured as changes in short-circuit current. Protein expression/phosphorylation was measured by RT-PCR and Western blotting. Under conditions that specifically isolate apical Ca(2+)-activated Cl(−) channel (CaCC) currents, EGF pretreatment (100 ng ml(−1) for 15 min) potentiated carbachol (CCh)-induced responses to 173 ± 25% of those in control cells, when measured 24 h later (n = 26; P < 0.01). EGF-induced increases in CaCC currents were abolished by the transmembrane protein 16A (TMEM16A) inhibitor, T16A(inh)-A01 (10 μm). Furthermore, TMEM16A mRNA and protein expression was increased by EGF to 256 ± 38% (n = 7; P < 0.01) and 297 ± 46% (n = 9, P < 0.001) of control levels, respectively. In contrast, EGF did not alter CFTR expression or activity. EGF-induced increases in Cl(−) secretion, CaCC currents and TMEM16A expression were attenuated by a PKCδ inhibitor, rottlerin (20 μm), and a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY290042 (25 μm). Finally, LY290042 inhibited EGF-induced phosphorylation of PKCδ. We conclude that EGF chronically upregulates Ca(2+)-dependent Cl(−) conductances and TMEM16A expression in intestinal epithelia by a mechanism involving sequential activation of PI3K and PKCδ. Therapeutic targeting of EGF receptor-dependent signalling pathways may provide new approaches for treatment of epithelial transport disorders.

Modulation of electrogenic transport processes in the porcine proximal colon by enteric neurotransmitters

The aim of our study was to evaluate the involvement of essential pro- and antisecretory neurotransmitters in regulation of secretion in porcine proximal colon. Choline acetyltransferase (ChAT), nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), substance P (SP), somatostatin (SOM) and neuropeptide Y (NPY) were located immunohistochemically in the epithelium and subepithelial layer. Modulation of epithelial secretion was studied in Ussing chambers. Application of carbachol (CA), sodium nitroprussid (SNP), VIP and SP but not of NPY or SOM resulted in a chloride dependent increase in short circuit current (I(sc) ). I(sc) increase induced by CA, VIP or SNP was not altered by preincubation with tetrodotoxin or indomethacin. In contrast, SP-induced I(sc) increase was diminished by preincubation with tetrodotoxin, indomethacin, L-nitro-arginin-methyl-ester, and atropine but not hexamethonium. Simultaneous application of CA and VIP, or CA and SNP increased the I(sc) stronger as expected. Applying SP/CA led to a smaller increase in I(sc) as calculated. It is concluded that mainly prosecretory neurotransmitters are involved in regulation of colonic secretion. Cross-potentiations of acetylcholine and nitric oxide and acetylcholine and VIP suggest activation of different intracellular cascades. Similar intracellular pathways may be stimulated by acetylcholine and SP, thus preventing an additive effect of the transmitters.

Effects of carbon monoxide on ion transport across rat distal colon

The aim of the present study was to investigate whether carbon monoxide (CO) induces changes in ion transport across the distal colon of rats and to study the mechanisms involved. In Ussing chamber experiments, tricarbonyldichlororuthenium(II) dimer (CORM-2), a CO donor, evoked a concentration-dependent increase in short-circuit current (I(sc)). A maximal response was achieved at a concentration of 2.5·10(-4) mol/l. Repeated application of CORM-2 resulted in a pronounced desensitization of the tissue. Anion substitution experiments suggest that a secretion of Cl(-) and HCO(3)(-) underlie the CORM-2-induced current. Glibenclamide, a blocker of the apical cystic fibrosis transmembrane regulator channel, inhibited the I(sc) induced by the CO donor. Similarly, bumetanide, a blocker of the basolateral Na(+)-K(+)-2Cl(-) cotransporter, combined with 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid sodium salt, an inhibitor of the basolateral Cl(-)/HCO(3)(-) exchanger, inhibited the CORM-2-induced I(sc). Membrane permeabilization experiments indicated an activation of basolateral K(+) and apical Cl(-) channels by CORM-2. A partial inhibition by the neurotoxin, tetrodotoxin, suggests the involvement of secretomotor neurons in this response. In imaging experiments at fura-2-loaded colonic crypts, CORM-2 induced an increase of the cytosolic Ca(2+) concentration. This increase depended on the influx of extracellular Ca(2+), but not on the release of Ca(2+) from intracellular stores. Both enzymes for CO production, heme oxygenase I and II, are expressed in the colon as observed immunohistochemically and by RT-PCR. Consequently, endogenous CO might be a physiological modulator of colonic ion transport.

Role of the Ca2+-activated Cl- channels bestrophin and anoctamin in epithelial cells

Two families of proteins, the bestrophins (Best) and the recently cloned TMEM16 proteins (anoctamin, Ano), recapitulate properties of Ca2+-activated Cl- currents. Best1 is strongly expressed in the retinal pigment epithelium and could have a function as a Ca2+-activated Cl- channel as well as a regulator of Ca2+ signaling. It is also present at much lower levels in other cell types including epithelial cells, where it regulates plasma membrane localized Cl- channels by controlling intracellular Ca2+ levels. Best1 interacts with important Ca2+-signaling proteins such as STIM1 and can interact directly with other Ca2+-activated Cl- channels such as TMEM16A. Best1 is detected in the endoplasmic reticulum (ER) where it shapes the dynamic ER structure and regulates cell proliferation, which could be important for renal cystogenesis. Ca2+-activated Cl- channels of the anoctamin family (TMEM16A) show biophysical and pharmacological properties that are typical for endogenous Ca2+-dependent Cl- channels. TMEM16 proteins are abundantly expressed and many reports demonstrate their physiological importance in epithelial as well as non-epithelial cells. These channels are also activated by cell swelling and can therefore control cell volume, proliferation and apoptosis. To fully understand the function and regulation of Ca2+-activated Cl- currents, it is necessary to appreciate that Best1 and TMEM16A are embedded in a protein network and that they probably operate in functional microdomains.

Bestrophin-2 mediates bicarbonate transport by goblet cells in mouse colon

Anion transport by the colonic mucosa maintains the hydration and pH of the colonic lumen, and its disruption causes a variety of diarrheal diseases. Cholinergic agonists raise cytosolic Ca2+ levels and stimulate anion secretion, but the mechanisms underlying this effect remain unclear. Cholinergic stimulation of anion secretion may occur via activation of Ca2+-activated Cl- channels (CaCCs) or an increase in the Cl- driving force through CFTR after activation of Ca2+-dependent K+ channels. Here we investigated the role of a candidate CaCC protein, bestrophin-2 (Best2), using Best2-/- mice. Cholinergic stimulation of anion current was greatly reduced in Best2-/- mice, consistent with our proposed role for Best2 as a CaCC. However, immunostaining revealed Best2 localized to the basolateral membrane of mucin-secreting colonic goblet cells, not the apical membrane of Cl--secreting enterocytes. In addition, in the absence of HCO3-, cholinergic-activated current was identical in control and Best2-/- tissue preparations, which suggests that most of the Best2 current was carried by HCO3-. These data delineate an alternative model of cholinergic regulation of colonic anion secretion in which goblet cells play a critical role in HCO3- homeostasis. We therefore propose that Best2 is a HCO3- channel that works in concert with a Cl:HCO3- exchanger in the apical membrane to affect transcellular HCO3- transport. Furthermore, previous models implicating CFTR in cholinergic Cl- secretion may be explained by substantial downregulation of Best2 in Cftr-/- mice.

Bestrophin and TMEM16-Ca(2+) activated Cl(-) channels with different functions

In the past, a number of candidates have been proposed to form Ca(2+) activated Cl(-) currents, but it is only recently that two families of proteins, the bestrophins and the TMEM16-proteins, recapitulate reliably the properties of Ca(2+) activated Cl(-) currents. Bestrophin 1 is strongly expressed in the retinal pigment epithelium, but also at lower levels in other cell types. Bestrophin 1 may form Ca(2+) activated chloride channels and, at the same time, affect intracellular Ca(2+) signaling. In epithelial cells, bestrophin 1 probably controls receptor mediated Ca(2+) signaling. It may do so by facilitating Ca(2+) release from the endoplasmic reticulum, thereby indirectly activating membrane localized Ca(2+)-dependent Cl(-) channels. In contrast to bestrophin 1, the Ca(2+) activated Cl(-) channel TMEM16A (anoctamin 1, ANO1) shows most of the biophysical and pharmacological properties that have been attributed to Ca(2+)-dependent Cl(-) channels in various tissues. TMEM16A is broadly expressed in both mouse and human tissues and is of particular importance in epithelial cells. Thus exocrine gland secretion as well as electrolyte transport by both respiratory and intestinal epithelia requires TMEM16A. Because of its role for Ca(2+)-dependent Cl(-) secretion in human airways, it is likely to become a prime target for the therapy of cystic fibrosis lung disease, caused by defective cAMP-dependent Cl(-) secretion. It will be very exciting to learn, how TMEM16A and other TMEM16-proteins are activated upon increase in intracellular Ca(2+), and whether the other nine members of the TMEM16 family also form Cl(-) channels with properties similar to TMEM16A.

TMEM16 proteins: the long awaited calcium-activated chloride channels?

Currents mediated by calcium-activated chloride channels (CaCCs), observed for the first time in Xenopus oocytes, have been recorded in many cells and tissues ranging from different types of neurons to epithelial and muscle cells. CaCCs play a role in the regulation of excitability in neurons including sensory receptors. In addition, they are crucial mediators of chloride movements in epithelial cells where their activity regulates electrolyte and fluid transport. The roles of CaCCs, particularly in epithelia, are briefly reviewed with emphasis on their function in secretory epithelia. The recent identification by three independent groups, using different strategies, of TMEM16A as the molecular counterpart of the CaCC is discussed. TMEM16A is part of a family that has 10 other members in mice. The discovery of the potential TMEM16 anion channel activity opens the way for the molecular investigation of the role of these anion channels in specific cells and in organ physiology and pathophysiology. The identification of TMEM16A protein as a CaCC chloride channel molecule represents a great triumph of scientific perseverance and ingenuity. The varied approaches used by the three independent research groups also augur well for the solidity of the discovery.

Sites of action of hydrogen peroxide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was the identification of the mechanism of oxidant-induced intestinal secretion.

EXPERIMENTAL APPROACH

The action of H2O2 on ion transport across rat distal colon was evaluated in Ussing chambers. Changes in cytosolic Ca2+ concentration were measured using fura-2.

KEY RESULTS

H2O2 concentration-dependently induced an increase in short-circuit current (Isc), which was due to a stimulation of Cl(-) secretion. The effect of H2O2 was dependent on the presence of serosal Ca2+. It was inhibited after emptying of intracellular Ca2+ stores by cyclopiazonic acid or blockade of ryanodine receptors by ruthenium red, whereas a blocker of inositol 1,4,5-trisphosphate receptors was less effective. Fura 2-experiments confirmed an increase in the cytosolic Ca2+ concentration in the presence of H2O2. Measurements of Cl- currents across the apical membrane at basolaterally depolarized epithelia revealed the activation of a glibenclamide-sensitive, SITS-resistant Cl- conductance by the oxidant. The activation of this conductance was inhibited after blockade of protein kinases with staurosporine. When the apical membrane was permeabilized with nystatin, two sites of action of H2O2 were identified at the basolateral membrane. The oxidant stimulated a basolateral tetrapentylammonium-sensitive K+ conductance and increased the current generated by the Na+-K+ pump. Pretreatment of the tissues with H2O2 reduced the action of subsequently administered Ca2+-, cAMP- and cGMP-dependent secretagogues demonstrating a long-term downregulation after the initial secretory response evoked by the oxidant.

CONCLUSIONS AND IMPLICATIONS

H2O2 affects colonic anion secretion by action sites at both the apical, as well as the basolateral membrane.