TMEM16A is a Ca 2+ activated Cl − channel with important functions in airways, intestine, and other epithelial organs. Activation of TMEM16A is proposed as a therapy in cystic fibrosis (CF) to reinstall airway Cl − secretion and to enhance airway surface liquid (ASL). This CFTR-agnostic approach is thought to improve mucociliary clearance and lung function in CF. This could indeed improve ASL, however, mucus release and airway contraction may also be induced by activators of TMEM16A, particularly in inflamed airways of patients with asthma, COPD, or CF. Currently, both activators and inhibitors of TMEM16A are developed and examined in different types of tissues. Here we compare activation and inhibition of endogenous and overexpressed TMEM16A and analyze potential off-target effects. The three well-known blockers benzbromarone, niclosamide, and Ani9 inhibited both TMEM16A and ATP-induced Ca 2+ increase by variable degrees, depending on the cell type. Niclosamide, while blocking Ca 2+ activated TMEM16A, also induced a subtle but significant Ca 2+ store release and inhibited store-operated Ca 2+ influx. Niclosamide, benzbromarone and Ani9 also affected TMEM16F whole cell currents, indicating limited specificity for these inhibitors. The compounds Eact, cinnamaldehyde, and melittin, as well as the phosphatidylinositol diC8-PIP 2 are the reported activators of TMEM16A. However, the compounds were unable to activate endogenous TMEM16A in HT 29 colonic epithelial cells. In contrast, TMEM16A overexpressed in HEK293 cells was potently stimulated by these activators. We speculate that overexpressed TMEM16A might have a better accessibility to intracellular Ca 2+ , which causes spontaneous activity even at basal intracellular Ca 2+ concentrations. Small molecules may therefore potentiate pre-stimulated TMEM16A currents, but may otherwise fail to activate silent endogenous TMEM16A.

No abstract

Previous analysis of CFTR-knockout (CFTR−/−) in piglets has provided important insights into the pathology of cystic fibrosis. However, controversies exist as to the true contribution of CFTR to the pH balance in airways and intestine. We therefore compared ion transport properties in newborn wild-type (CFTR+/+) and CFTR-knockout (CFTR−/− piglets). Tracheas of CFTR−/− piglets demonstrated typical cartilage malformations and muscle cell bundles. CFTR−/− airway epithelial cells showed enhanced lipid peroxidation, suggesting inflammation early in life. CFTR was mainly expressed in airway submucosal glands and was absent in lungs of CFTR−/− piglets, while expression of TMEM16A was uncompromised. mRNA levels for TMEM16A, TMEM16F, and αβγENaC were unchanged in CFTR−/− airways, while mRNA for SLC26A9 appeared reduced. CFTR was undetectable in epithelial cells of CFTR−/− airways and intestine. Small intestinal epithelium of CFTR−/− piglets showed mucus accumulation. Secretion of both electrolytes and mucus was activated by stimulation with prostaglandin E2 and ATP in the intestine of CFTR+/+, but not of CFTR−/− animals. pH was measured inside small bronchi using a pH microelectrode and revealed no difference between CFTR+/+ and CFTR−/− piglets. Intracellular pH in porcine airway epithelial cells revealed only a small contribution of CFTR to bicarbonate secretion, which was absent in cells from CFTR−/− piglets. In contrast to earlier reports, our data suggest a minor impact of CFTR on ASL pH. In contrast, enhanced amiloride-sensitive Na+ absorption may contribute to lung pathology in CFTR−/− piglets, along with a compromised CFTR- and TMEM16A-dependent Cl− transport.

Activation of the Ca2+ activated Cl- channel TMEM16A is proposed as a treatment in inflammatory airway disease. It is assumed that activation of TMEM16A will induce electrolyte secretion, and thus reduce airway mucus plugging and improve mucociliary clearance. A benefit of activation of TMEM16A was shown in vitro and in studies in sheep, but others reported an increase in mucus production and airway contraction by activation of TMEM16A. We analyzed expression of TMEM16A in healthy and inflamed human and mouse airways and examined the consequences of activation or inhibition of TMEM16A in asthmatic mice. TMEM16A was found to be upregulated in the lungs of patients with asthma or cystic fibrosis, as well as in the airways of asthmatic mice. Activation or potentiation of TMEM16A by the compounds Eact or brevenal, respectively, induced acute mucus release from airway goblet cells and induced bronchoconstriction in mice in vivo. In contrast, niclosamide, an inhibitor of TMEM16A, blocked mucus production and mucus secretion in vivo and in vitro. Treatment of airway epithelial cells with niclosamide strongly inhibited expression of the essential transcription factor of Th2-dependent inflammation and goblet cell differentiation, SAM pointed domain-containing ETS-like factor (SPDEF). Activation of TMEM16A in people with inflammatory airway diseases is likely to induce mucus secretion along with airway constriction. In contrast, inhibitors of TMEM16A may suppress pulmonary Th2 inflammation, goblet cell metaplasia, mucus production, and bronchoconstriction, partially by inhibiting expression of SPDEF.

TMEM16A, a Ca2+-activated chloride channel (CaCC), and its regulator, CLCA1, are associated with inflammatory airway disease and goblet cell metaplasia. CLCA1 is a secreted protein with protease activity that was demonstrated to enhance membrane expression of TMEM16A. Expression of CLCA1 is particularly enhanced in goblet cell metaplasia and is associated with various lung diseases. However, mice lacking expression of CLCA1 showed the same degree of mucous cell metaplasia and airway hyperreactivity as asthmatic wild-type mice. To gain more insight into the role of CLCA1, we applied secreted N-CLCA1, produced in vitro, to mice in vivo using intratracheal instillation. We observed no obvious upregulation of TMEM16A membrane expression by CLCA1 and no differences in ATP-induced short circuit currents (Iscs). However, intraluminal mucus accumulation was observed by treatment with N-CLCA1 that was not seen in control animals. The effects of N-CLCA1 were augmented in ovalbumin-sensitized mice. Mucus production induced by N-CLCA1 in polarized BCi-NS1 human airway epithelial cells was dependent on TMEM16A expression. IL-13 upregulated expression of CLCA1 and enhanced mucus production, however, without enhancing purinergic activation of Isc. In contrast to polarized airway epithelial cells and mouse airways, which express very low levels of TMEM16A, nonpolarized airway cells express large amounts of TMEM16A protein and show strong CaCC. The present data show an only limited contribution of TMEM16A to airway ion secretion but suggest a significant role of both CLCA1 and TMEM16A for airway mucus secretion.