SUMMARY Collapse of membrane lipid asymmetry is a hallmark of blood coagulation. TMEM16F of the TMEM16 family that includes TMEM16A/B Ca2+-activated Cl− channels (CaCCs) is linked to Scott syndrome with deficient Ca2+-dependent lipid scrambling. We generated TMEM16F knockout mice that exhibit bleeding defects and protection in an arterial thrombosis model associated with platelet deficiency in Ca2+-dependent phosphatidylserine exposure and procoagulant activity and lack a Ca2+-activated cation current in the platelet precursor megakaryocytes. Heterologous expression of TMEM16F generates a small-conductance Ca2+-activated nonselective cation (SCAN) current with subpicosiemens single-channel conductance rather than a CaCC. TMEM16F-SCAN channels permeate both monovalent and divalent cations, including Ca2+, and exhibit synergistic gating by Ca2+ and voltage. We further pinpointed a residue in the putative pore region important for the cation versus anion selectivity of TMEM16F-SCAN and TMEM16A-CaCC channels. This study thus identifies a Ca2+-activated channel permeable to Ca2+ and critical for Ca2+-dependent scramblase activity during blood coagulation.

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Mucous cell hyperplasia and airway smooth muscle (ASM) hyperresponsiveness are hallmark features of inflammatory airway diseases, including asthma. Here, we show that the recently identified calciumactivated chloride channel (CaCC) TMEM16A is expressed in the adult airway surface epithelium and ASM. The epithelial expression is increased in asthmatics, particularly in secretory cells. Based on this and the proposed functions of CaCC, we hypothesized that TMEM16A inhibitors would negatively regulate both epithelial mucin secretion and ASM contraction. We used a high-throughput screen to identify small-molecule blockers of TMEM16A-CaCC channels. We show that inhibition of TMEM16A-CaCC significantly impairs mucus secretion in primary human airway surface epithelial cells. Furthermore, inhibition of TMEM16A-CaCC significantly reduces mouse and human ASM contraction in response to cholinergic agonists. TMEM16A-CaCC blockers, including those identified here, may positively impact multiple causes of asthma symptoms.A sthma is a significant cause of morbidity and mortality worldwide, and the prevalence of this disease is increasing among all age, sex, and racial groups. Characteristic features of asthma include inflammation, subepithelial fibrosis, hyperplasia of mucus-producing cells, accumulation of mucus within airway lumens, hyperplasia of airway smooth muscle (ASM), and ASM hyperresponsiveness. Together, these symptoms impair lung function by limiting the flow of gases to and from the alveoli in the distal lung.The current standard of care for asthma involves inhaled corticosteroids for the management of inflammation combined with long-acting agonists of β2-adrenergic receptors. Despite this treatment, lung function is not improved in 30-45% of asthmatic patients, and exacerbations continue to be a major problem (reviewed in ref. 1). Asthma can be divided into at least two distinct molecular phenotypes defined by the degree of Th2 inflammation (2, 3). Cytokines, including IL-4 and IL-13, promote airway epithelial mucous cell metaplasia, subepithelial fibrosis, and hyperplasia of smooth muscle in Th2-high asthmatics, and these patients generally show improved lung function with inhaled corticosteroid therapy. A greater understanding of this heterogeneity and the molecular and physiological events that lead to airway remodeling might lead to improved diagnosis and treatment.Calcium-activated chloride channels (CaCCs) have been ascribed numerous cellular functions (reviewed in refs. 4 and 5), among these are epithelial fluid secretion and smooth muscle contraction, both of which contribute to the progression and severity of asthma. Moreover, calcium-activated chloride currents in the airway epithelium are enhanced by the Th2 cytokines IL-4 and IL-13, as well as IFN-γ (6). For these reasons, CaCC is an attractive potential therapeutic target for asthma (7). However, the study of CaCC was impeded by lack of information about the gene(s) encoding this channel. It was only relatively recently that TMEM16A (transmembrane p...