Calcium-Activated Cl− Channel: Insights on the Molecular Identity in Epithelial Tissues

Figures

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  Figure 1. Presence of Cystic Fibrosis transmembrane conductance regulator (CFTR) and non-CFTR mediated Cl− secretion in normal rat tracheal epithelium. Chloride secretion was measured as short-circuit current (ISC) in tracheal epithelia that were mounted under voltage clamp condition. (A) Effect of CFTR-172inh (CFTR inhibitor; 20 μM) on ATP stimulated Cl− secretion. Mucosal ATP (100 μM) addition stimulates Cl− secretion that consists of two components—the sharp initial increase in ISC immediately followed by a second, more rounded and prolonged current. Inhibition by mucosal CFTR-172inh (CFTR inhibitor) indicates that the ATP-stimulated sustained Cl− secretion is mediated by CFTR. The CFTR-172inh inhibition of ISC below baseline indicates that CFTR also accounts for a portion of basal Cl− secretion in normal trachea; (B) Effect of ATP on Cl− secretion in CFTR-172inh pre-incubated trachea. In CFTR-172inh pre-incubated trachea, mucosal ATP (which transiently increases intracellular Ca2+) transiently stimulates Cl− secretion with a minimal plateau phase of Cl− secretion. The transient Cl− secretion represents non-CFTR mediated Cl− secretion (i.e., through TMEM16A). The presence of a residual…

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  Figure 2. Schematic of TMEM16A channel protein with identified residues necessary for voltage and Ca2+-sensitivity. TMEM16A channel protein consists of 10 transmembrane domains (TMD). The intracellular loop between TMD-2 and TMD-3 contains the voltage-sensitive 444EEEE residues, as well as the 448EAVK residues that participate in Ca2+-sensitivity of the channel. The E702 and E705 residues located within the seventh TMD also participate in Ca2+-sensitivity of the channel. The third intracellular loop (shown in pink), and the residues E654, E730 and D734 have also been implicated to affect Ca2+-sensitivity of the channel.

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  Figure 3. Cellular model of Ca2+-activated Cl− secretion in tracheal epithelium. Either apical ATP or basolateral carbachol (CCH; a muscarinic receptor (M3R) agonist) administration transiently increases intracellular Ca2+ stores from endoplasmic reticulum (ER), leading to increased Ca2+-activated Cl− secretion through TMEM16A. G-protein (Gαq)-coupled receptors, including the apical P2YX purinergic receptor (UTP) and basolateral M3R mediate the release of internal Ca2+ stores. Either Ca2+-calmodulin (CAM) or the extracellular protease CLCA1 may modulate TMEM16A mediated Cl− secretion by activating the channel and/or increasing the plasma membrane expression. CLCA1—Cl− channel accessary-1; PLC—phospholipase C.