The transmembrane protein 16A Ca(2+)-activated Cl- channel in airway smooth muscle contributes to airway hyperresponsiveness

Modulating Ca²⁺ signals: a common theme for TMEM16, Ist2, and TMC

Since the discovery of TMEM16A (anoctamin 1, ANO1) as Ca(2+)-activated Cl(-) channel, the protein was found to serve different physiological functions, depending on the type of tissue. Subsequent reports on other members of the anoctamin family demonstrated a broad range of yet poorly understood properties. Compromised anoctamin function is causing a wide range of diseases, such as hearing loss (ANO2), bleeding disorder (ANO6), ataxia and dystonia (ANO3, 10), persistent borrelia and mycobacteria infection (ANO10), skeletal syndromes like gnathodiaphyseal dysplasia and limb girdle muscle dystrophy (ANO5), and cancer (ANO1, 6, 7). Animal models demonstrate CF-like airway disease, asthma, and intestinal hyposecretion (ANO1). Although present data indicate that ANO1 is a Ca(2+)-activated Cl(-) channel, it remains unclear whether all anoctamins form plasma membrane-localized or intracellular chloride channels. We find Ca(2+)-activated Cl(-) currents appearing by expression of most anoctamin paralogs, including the Nectria haematococca homologue nhTMEM16 and the yeast homologue Ist2. As recent studies show a role of anoctamins, Ist2, and the related transmembrane channel-like (TMC) proteins for intracellular Ca(2+) signaling, we will discuss the role of these proteins in generating compartmentalized Ca(2+) signals, which may give a hint as to the broad range of cellular functions of anoctamins.

Antagonists of the TMEM16A calcium-activated chloride channel modulate airway smooth muscle tone and intracellular calcium

BACKGROUND

Perioperative bronchospasm refractory to β agonists continues to challenge anesthesiologists and intensivists. The TMEM16A calcium-activated chloride channel modulates airway smooth muscle (ASM) contraction. The authors hypothesized that TMEM16A antagonists would relax ASM contraction by modulating membrane potential and calcium flux.

METHODS

Human ASM, guinea pig tracheal rings, or mouse peripheral airways were contracted with acetylcholine or leukotriene D4 and then treated with the TMEM16A antagonists: benzbromarone, T16Ainh-A01, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid, or B25. In separate studies, guinea pig tracheal rings were contracted with acetylcholine and then exposed to increasing concentrations of isoproterenol (0.01 nM to 10 μM) ± benzbromarone. Plasma membrane potential and intracellular calcium concentrations were measured in human ASM cells.

RESULTS

Benzbromarone was the most potent TMEM16A antagonist tested for relaxing an acetylcholine -induced contraction in guinea pig tracheal rings (n = 6). Further studies were carried out to investigate the clinical utility of benzbromarone. In human ASM, benzbromarone relaxed either an acetylcholine- or a leukotriene D4-induced contraction (n = 8). Benzbromarone was also effective in relaxing peripheral airways (n = 9) and potentiating relaxation by β agonists (n = 5 to 10). In cellular mechanistic studies, benzbromarone hyperpolarized human ASM cells (n = 9 to 12) and attenuated intracellular calcium flux from both the plasma membrane and the sarcoplasmic reticulum (n = 6 to 12).

CONCLUSION

TMEM16A antagonists work synergistically with β agonists and through a novel pathway of interrupting ion flux at both the plasma membrane and sarcoplasmic reticulum to acutely relax human ASM.

TMEM16A and myocardin form a positive feedback loop that is disrupted by KLF5 during Ang II-induced vascular remodeling

The TMEM16A protein is an important component of Ca(2+)-dependent Cl(-) channels (CaCCs) in vascular smooth muscle cells. A recent study showed that TMEM16A inhibits angiotensin II-induced proliferation in rat basilar smooth muscle cells. However, whether and how TMEM16A is involved in vascular remodeling characterized by vascular smooth muscle cell proliferation remains largely unclear. In this study, luciferase reporter, Western blotting, and qRT-PCR assays were performed. The results suggested that myocardin promotes TMEM16A expression by forming a complex with serum response factor (SRF) on the TMEM16A promoter in human aortic smooth muscle cells (HASMCs). In turn, upregulated TMEM16A promotes expression of myocardin and vascular smooth muscle cell marker genes, thus forming a positive feedback loop that induces cell differentiation and inhibits cell proliferation. Angiotensin II inhibits TMEM16A expression via Krüppel-like factor 5 (KLF5) in cultured HASMCs. Moreover, in vivo experiments show that infusion of angiotensin II into mice causes a marked reduction in TMEM16A expression and vascular remodeling, and angiotensin II-induced effects are largely reversed in KLF5 null (KLF5(-/-)) mice. KLF5 competes with SRF to interact with myocardin, thereby limiting myocardin binding to SRF and the synergistic activation of the TMEM16A promoter by myocardin and SRF. Our studies demonstrated that angiotensin II induces KLF5 expression and facilitates KLF5 association with myocardin to disrupt the myocardin-SRF complex, subsequently leading to inhibition of TMEM16A transcription. Blocking the positive feedback loop between myocardin and TMEM16A may be a novel therapeutic approach for vascular remodeling.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

Cl⁻ channels in smooth muscle cells

In smooth muscle cells (SMCs), the intracellular chloride ion (Cl−) concentration is high due to accumulation by Cl−/HCO3− exchange and Na+–K+–Cl− cotransportation. The equilibrium potential for Cl− (ECl) is more positive than physiological membrane potentials (Em), with Cl− efflux inducing membrane depolarization. Early studies used electrophysiology and nonspecific antagonists to study the physiological relevance of Cl− channels in SMCs. More recent reports have incorporated molecular biological approaches to identify and determine the functional significance of several different Cl− channels. Both "classic" and cGMP-dependent calcium (Ca2+)-activated (ClCa) channels and volume-sensitive Cl− channels are present, with TMEM16A/ANO1, bestrophins, and ClC-3, respectively, proposed as molecular candidates for these channels. The cystic fibrosis transmembrane conductance regulator (CFTR) has also been described in SMCs. This review will focus on discussing recent progress made in identifying each of these Cl− channels in SMCs, their physiological functions, and contribution to diseases that modify contraction, apoptosis, and cell proliferation.

An integrated assessment of histopathological changes of the enteric neuromuscular compartment in experimental colitis

Bowel inflammatory fibrosis has been largely investigated, but an integrated assessment of remodelling in inflamed colon is lacking. This study evaluated tissue and cellular changes occurring in colonic wall upon induction of colitis, with a focus on neuromuscular compartment. Colitis was elicited in rats by 2,4-dinitrobenzenesulfonic acid (DNBS). After 6 and 21 days, the following parameters were assessed on paraffin sections from colonic samples: tissue injury and inflammatory infiltration by histology; collagen and elastic fibres by histochemistry; HuC/D, glial fibrillar acidic protein (GFAP), proliferating cell nuclear antigen (PCNA), nestin, substance P (SP), von Willebrand factor, c-Kit and transmembrane 16A/Anoctamin1 (TMEM16A/ANO1) by immunohistochemistry. TMEM16A/ANO1 was also examined in isolated colonic smooth muscle cells (ICSMCs). On day 6, inflammatory alterations and fibrosis were present in DNBS-treated rats; colonic wall thickening and fibrotic remodelling were evident on day 21. Colitis was associated with both an increase in collagen fibres and a decrease in elastic fibres. Moreover, the neuromuscular compartment of inflamed colon displayed a significant decrease in neuron density and increase in GFAP/PCNA-positive glia of myenteric ganglia, enhanced expression of neural SP, blood vessel remodelling, reduced c-Kit- and TMEM16A/ANO1-positive interstitial cells of Cajal (ICCs), as well as an increase in TMEM16A/ANO1 expression in muscle tissues and ICSMCs. The present findings provide an integrated view of the inflammatory and fibrotic processes occurring in the colonic neuromuscular compartment of rats with DNBS-induced colitis. These morphological alterations may represent a suitable basis for understanding early pathophysiological events related to bowel inflammatory fibrosis.

Activation of Ca(2+) -activated Cl(-) channel ANO1 by localized Ca(2+) signals

Ca²⁺‐activated chloride channels (CaCCs) regulate numerous physiological processes including epithelial transport, smooth muscle contraction and sensory processing. Anoctamin‐1 (ANO1, TMEM16A) is a principal CaCC subunit in many cell types, yet our understanding of the mechanisms of ANO1 activation and regulation are only beginning to emerge. Ca²⁺ sensitivity of ANO1 is rather low and at negative membrane potentials the channel requires several micromoles of intracellular Ca²⁺ for activation. However, global Ca²⁺ levels in cells rarely reach such levels and, therefore, there must be mechanisms that focus intracellular Ca²⁺ transients towards the ANO1 channels. Recent findings indeed indicate that ANO1 channels often co‐localize with sources of intracellular Ca²⁺ signals. Interestingly, it appears that in many cell types ANO1 is particularly tightly coupled to the Ca²⁺ release sites of the intracellular Ca²⁺ stores. Such preferential coupling may represent a general mechanism of ANO1 activation in native tissues.

Variomics screen identifies the re-entrant loop of the calcium-activated chloride channel ANO1 that facilitates channel activation

The calcium-activated chloride channel ANO1 regulates multiple physiological processes. However, little is known about the mechanism of channel gating and regulation of ANO1 activity. Using a high-throughput, random mutagenesis-based variomics screen, we generated and functionally characterized ∼6000 ANO1 mutants and identified novel mutations that affected channel activity, intracellular trafficking, or localization of ANO1. Mutations such as S741T increased ANO1 calcium sensitivity and rendered ANO1 calcium gating voltage-independent, demonstrating a critical role of the re-entrant loop in coupling calcium and voltage sensitivity of ANO1 and hence in regulating ANO1 activation. Our data present the first unbiased and comprehensive study of the structure-function relationship of ANO1. The novel ANO1 mutants reported have diverse functional characteristics, providing new tools to study ANO1 function in biological systems, paving the path for a better understanding of the function of ANO1 and its role in health and diseases.

Calcineurin upregulates local Ca(2+) signaling through ryanodine receptor-1 in airway smooth muscle cells

Local Ca(2+) signals (Ca(2+) sparks) play an important role in multiple cellular functions in airway smooth muscle cells (ASMCs). Protein kinase Cϵ is known to downregulate ASMC Ca(2+) sparks and contraction; however, no complementary phosphatase has been shown to produce opposite effects. Here, we for the first time report that treatment with a specific calcineurin (CaN) autoinhibitory peptide (CAIP) to block CaN activity decreases, whereas application of nickel to activate CaN increases, Ca(2+) sparks in both the presence and absence of extracellular Ca(2+). Treatment with xestospogin-C to eliminate functional inositol 1,4,5-trisphosphate receptors does not prevent CAIP from inhibiting local Ca(2+) signaling. However, high ryanodine treatment almost completely blocks spark formation and prevents the nickel-mediated increase in sparks. Unlike CAIP, the protein phosphatase 2A inhibitor endothall has no effect. Local Ca(2+) signaling is lower in CaN catalytic subunit Aα gene knockout (CaN-Aα(-/-)) mouse ASMCs. The effects of CAIP and nickel are completely lost in CaN-Aα(-/-) ASMCs. Neither CAIP nor nickel produces an effect on Ca(2+) sparks in type 1 ryanodine receptor heterozygous knockout (RyR1(-/+)) mouse ASMCs. However, their effects are not altered in RyR2(-/+) or RyR3(-/-) mouse ASMCs. CaN inhibition decreases methacholine-induced contraction in isolated RyR1(+/+) but not RyR1(-/+) mouse tracheal rings. Supportively, muscarinic contractile responses are also reduced in CaN-Aα(-/+) mouse tracheal rings. Taken together, these results provide novel evidence that CaN regulates ASMC Ca(2+) sparks specifically through RyR1, which plays an important role in the control of Ca(2+) signaling and contraction in ASMCs.

Chloride channel blockade relaxes airway smooth muscle and potentiates relaxation by β-agonists

Severe bronchospasm refractory to β-agonists continues to cause significant morbidity and mortality in asthmatic patients. We questioned whether chloride channels/transporters are novel targets for the relaxation of airway smooth muscle (ASM). We have screened a library of compounds, derivatives of anthranilic and indanyloxyacetic acid, that were originally developed to antagonize chloride channels in the kidney. We hypothesized that members of this library would be novel calcium-activated chloride channel blockers for the airway. The initial screen of this compound library identified 4 of 20 compounds that relaxed a tetraethylammonium chloride-induced contraction in guinea pig tracheal rings. The two most effective compounds, compounds 1 and 13, were further studied for their potential to either prevent the initiation of or relax the maintenance phase of an acetylcholine (ACh)-induced contraction or to potentiate β-agonist-mediated relaxation. Both relaxed an established ACh-induced contraction in human and guinea pig ex vivo ASM. In contrast, the prevention of an ACh-induced contraction required copretreatment with the sodium-potassium-chloride cotransporter blocker bumetanide. The combination of compound 13 and bumetanide also potentiated relaxation by the β-agonist isoproterenol in guinea pig tracheal rings. Compounds 1 and 13 hyperpolarized the plasma cell membrane of human ASM cells and blocked spontaneous transient inward currents, a measure of chloride currents in these cells. These functional and electrophysiological data suggest that modulating ASM chloride flux is a novel therapeutic target in asthma and other bronchoconstrictive diseases.

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Small molecule-facilitated degradation of ANO1 protein: a new targeting approach for anticancer therapeutics

ANO1, a calcium-activated chloride channel, is highly expressed and amplified in human cancers and is a critical survival factor in these cancers. The ANO1 inhibitor CaCCinh-A01 decreases proliferation of ANO1-amplified cell lines; however, the mechanism of action remains elusive. We explored the mechanism behind the inhibitory effect of CaCCinh-A01 on cell proliferation using a combined experimental and in silico approach. We show that inhibition of ANO1 function is not sufficient to diminish proliferation of ANO1-dependent cancer cells. We report that CaCCinh-A01 reduces ANO1 protein levels by facilitating endoplasmic reticulum-associated, proteasomal turnover of ANO1. Washout of CaCCinh-A01 rescued ANO1 protein levels and resumed cell proliferation. Proliferation of newly derived CaCCinh-A01-resistant cell pools was not affected by CaCCinh-A01 as compared with the parental cells. Consistently, CaCCinh-A01 failed to reduce ANO1 protein levels in these cells, whereas ANO1 currents were still inhibited by CaCCinh-A01, indicating that CaCCinh-A01 inhibits cell proliferation by reducing ANO1 protein levels. Furthermore, we employed in silico methods to elucidate novel biological functions of ANO1 inhibitors. Specifically, we derived a pharmacophore model to describe inhibitors capable of promoting ANO1 degradation and report new inhibitors of ANO1-dependent cell proliferation. In summary, our data demonstrate that inhibition of the channel activity of ANO1 is not sufficient to inhibit ANO1-dependent cell proliferation, indicating that the role of ANO1 in cancer only partially depends on its function as a channel. Our results provide an impetus for gaining a deeper understanding of ANO1 modulation in cells and introduce a new targeting approach for antitumor therapy in ANO1-amplified cancers.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

Low concentrations of niflumic acid enhance basal spontaneous and carbachol-induced contractions of the detrusor

PURPOSE

The urinary bladder expresses Ca(2+)-activated Cl(-) channels (CACC), but its physiological role in governing contractility remains to be defined. The CACC modulator niflumic acid (NFA) is widely used despite the variable results arisen from different drug concentrations used. This study was designed to examine the effects of NFA at low concentrations on detrusor strip contractility.

METHODS

Rat detrusor strips with mucosa-intact (+MU) and mucosa-denuded (-MU) were prepared in transverse (Tr) and longitudinal (Lg) with respect to the bladder orientation. Isometric force measurements were made at baseline (for spontaneous phasic contractile activity) and during drug stimulation (by carbachol, CCh) with and without NFA.

RESULTS

NFA (1 and 10 μmol/L) pretreatment enhanced CCh-induced contractions more in +MU than -MU strips with no selectivity on contractile direction. For spontaneous phasic contractions, NFA-treated strips in the Tr direction showed increased phasic amplitude, while phasic frequency was unchanged.

CONCLUSIONS

The findings suggest low concentrations of NFA having a potentiating effect on detrusor contractions that was sensitive to the MU and contractile direction.