The Ca2+-activated chloride channel ANO1/TMEM16A: An emerging therapeutic target for epithelium-originated diseases?

Abstract

Anoctamin 1 (ANO1) or TMEM16A gene encodes a member of Ca²⁺ activated Cl^- channels (CaCCs) that are critical for physiological functions, such as epithelial secretion, smooth muscle contraction and sensory signal transduction. The attraction and interest in ANO1/TMEM16A arise from a decade long investigations that abnormal expression or dysfunction of ANO1 is involved in many pathological phenotypes and diseases, including asthma, neuropathic pain, hypertension and cancer. However, the lack of specific modulators of ANO1 has impeded the efforts to validate ANO1 as a therapeutic target. This review focuses on the recent progress made in understanding of the pathophysiological functions of CaCC ANO1 and the current modulators used as pharmacological tools, hopefully illustrating a broad spectrum of ANO1 channelopathy and a path forward for this target validation.

Keywords: ANO1; ANO1, anoctamin-1; ASM, airway smooth muscle; Ang II, angiotensin II; BBB, blood–brain barrier; CAMK, Ca2+/calmodulin-dependent protein kinase; CF, cystic fibrosis; CFTR, cystic fibrosis transmembrane conductance regulator; Ca2+-activated Cl– channels (CaCCs); CaCCinh-A01; CaCCs, Ca2+ activated chloride channels; Cancer; Cystic fibrosis; DRG, dorsal root ganglion; Drug target; EGFR, epidermal growth factor receptor; ENaC, epithelial sodium channels; ER, endoplasmic reticulum; ESCC, esophageal squamous cell carcinoma; FRT, fisher rat thyroid; GI, gastrointestinal; GIST, gastrointestinal stromal tumor; GPCR, G-protein coupled receptor; HNSCC, head and neck squamous cell carcinoma; HTS, high-throughput screening; ICC, interstitial cells of Cajal; IPAH, idiopathic pulmonary arterial hypertension; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor κB; PAH, pulmonary arterial hypertension; PAR2, protease activated receptor 2; PASMC, pulmonary artery smooth muscle cells; PIP2, phosphatidylinositol 4,5-bisphosphate; PKD, polycystic kidney disease; T16Ainh-A01; TGF-β, transforming growth factor-β; TMEM16A; VGCC, voltage gated calcium channel; VRAC, volume regulated anion channel; VSMC, vascular smooth muscle cells; YFP, yellow fluorescent protein.

Graphical abstract

Figure 1

The molecular structure of ANO1/TMEM16A channel. (A) The Ca²⁺-bound structure of mANO1 channel in dimer (chains A and B), and 2 yellow filled circles for Ca²⁺ in each monomer containing 10 transmembrane α helices. (B) Ca²⁺ binding sites formed by residues N650, N651, E654 from α6, E702, E705 from α7, and E734, D738 from α8^,. (C) Residues critical for ion selectivity including R515 from α3, N546, D554 from α4, N591, V599 from α5, K603, R621 from α5–6 linker, S639 from α6, and Q709, F716 from α7^,. (D) Putative binding sites, R515 from α3, K603, R621 from α5–6 linker, and R788 from α8, for ANO1 inhibitors NTTP and 1PBC; and N650 from α6, A697, E705 from α7, and L746 from α8 for ANO1 activator GRb1. The structure is regenerated based on the cryo-EM structure of ANO1 channel (PDB 5OYB). The residue number labeling is based on the sequence of mTMEM16A (ac) isoform (UniProt Q8BHY3.2).

Figure 2

Distribution of ANO1/TMEM16A in different tissues and its role in diseases. (A) In epithelial cells, ANO1 activation contributes to electrolyte and music secretion. Activation of GPCRs causes an increase in intracellular cAMP and Ca²⁺, further inducing Cl⁻ secretion through the activation of CFTR and ANO1. A crosstalk between CFTR and ANO1-dependent secretions also occurs for regulation of secretory signaling in the airway epithelia. ANO1 high expression or hyperactivity can cause inflammatory diseases such as asthma and diarrhea. (B) Outward flow of Cl⁻ through the activation of ANO1 in smooth muscle cells causes depolarization and smooth muscle contraction. ANO1 high expression or hyperactivity is responsible for asthma diarrhea, and hypertension. (C) In DRG sensory neurons, activation of ANO1 by intracellular Ca²⁺ or heat causes Cl⁻ efflux and increases neuronal excitability for induction of neuropathic pain. The functional coupling between TRPV1 and ANO1 is also involved in nociception. (D) In cancer cells, ANO1 upregulation promotes cell proliferation and migration, whereas ANO1 downregulation induces apoptosis through multiple signaling pathways, including EGFR/MAPK signaling pathway, CaMKII/MAPK signaling pathway, TGF-β signaling pathway and NF-κB signaling pathway. Pharmacological activation of ANO1 by activators or potentiators may serve as a therapeutic strategy for treatment of CF, dry mouth and dry eye syndromes, and inhibition of ANO1 by inhibitors may be beneficial for ANO1 related channelopathies including asthma, diarrhea, hypertension, neuropathic pain and cancers.