Small Molecule-Based Synthetic Ion Channels Modulate Smooth Muscle Contraction and Epithelial Ion Transport

This dissertation, "Small Molecule-based Synthetic Ion Channels Modulate Smooth Muscle Contraction and Epithelial Ion Transport" by Kwok-hei, Yau, 邱國禧, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: In living systems, ion channels are membrane transport proteins that provide pathways for the passive diffusion of ions through lipid membranes. The flow of ions across membranes is the basis of many important physiological processes, including but not limited to the regulation of membrane potential, transepithelial transport and cell volume. While many efforts have been made to understand the biological roles of natural ion channels, the biological activities of artificial ion channels remain largely unknown. Recently, it was reported that a small molecule 1, which forms synthetic chloride (Cl-) channels in membranes via self-assembly, is capable of modulating vascular functions. In this thesis, novel small molecules that are structurally similar to 1 are shown to form artificial ion channels in membranes. Together with 1, the effects of these small molecules on the contractile activities of smooth muscles and epithelial ion transport are explored. The therapeutic implications of the findings are also discussed. A collection of small molecules was screened using liposome-based fluorescence assays. In these assays, the ability of the synthetic compounds to modulate membrane potential was monitored. The screening yielded compound 3 that formed synthetic potassium (K+) channels in liposomal membranes, although the liposome-based fluorescence experiments suggested that 3 also transported Cl-. Two derivatives of 3, namely, compounds 2 and 4 were also examined. Single-channel recording experiments suggested that 2 forms synthetic Cl- channels in liposomal membranes. The effects of compounds 2 and 3 on the functions of the vascular smooth muscle are explored. Using confocal imaging, it was shown that both 2 and 3 counteracted the effects of high-K+ depolarizing solution on membrane potential and intracellular Ca2+ concentration ([Ca2]]i) in cultured vascular smooth muscle cells. 2 and 3 also relaxed mice aortic rings pre-contracted with high-K+ solution. These observations can be explained in terms of the Cl- transporting functions of 2 and 3. To determine the potential for developing the compounds into bronchodilators, the effects of compounds 1 and 3 on the contractile activities of the airway smooth muscle (ASM) were explored using organ bath technique. The contractile activities of the trachea isolated from Sprague-Dawley (SD) rats were first characterized. Among the contractile agents used, only potassium chloride (KCl), cholinergic agonists, serotonin and endothelin-1 were contractile to the SD rat trachea. 1 and 3 relaxed the ASM pre-contracted with KCl, whereas the contractions induced by other agonists were not affected. The ability of compounds 2, 3 and 4 to modulate ion transport across cultured epithelia was tested by the short-circuit current measurement technique. It was shown that the compounds were capable of inducing Cl- secretion when applied to the apical side of airway and colonic epithelia. Importantly, the synthetic compounds induced apical Cl- secretion in immortalized cystic fibrosis (CF) bronchial epithelia. This suggests that the synthetic compounds may be used to correct the anion transport defect in CF epithelia. In summary, the small-molecule based synthetic ion channels demonstrated two important general functions of natural ion channels, namely, the regulation