Lithium 'Hot-spots': Real-time Analysis of non-selective cation channel activity in migrating cancer cells.

Work here is the first to use a newly designed Li+ -selective photoswitchable probe ('SHL') in living colon cancer cells to non-invasively monitor cation channel activity in real time by the appearance of lithium hot spots, detected by confocal microscopy. Punctate Li+ hot spots are clustered in the lamellipodial leading edges of HT29 human colon cancer cells, co-localized with aquaporin-1 (AQP1) channels. AQP1 is a dual water and cyclic-nucleotide-gated cation channel, located in lamellipodia and essential for rapid cell migration in a subset of aggressive cancers. Both the Li+ hot spots and cell migration are blocked in HT29 cells by the AQP1 ion channel antagonist AqB011. In contrast, Li+ hot spots are not evident in a poorly-migrating colon cancer cell line SW620, which lacks comparable membrane expression of AQP1. Knockdown of AQP1 by RNAi in HT29 cells significantly impairs Li+ hot spot activity. The SHL probe loaded in living cells shows signature chemical properties of ionic selectivity and reversibility. Dynamic properties of the Li+ hot spots, turning on and off, are confirmed by time-lapse imaging. SHL is a powerful tool for evaluating cation channel function in living cells in real time, with particular promise for studies of motile cells or interlinked networks not easily analyzed by electrophysiological methods. The ability to reset SHL by photoswitching allows monitoring of dynamic signals over time. Future applications of the Li+ probe could include high-throughput optical screening for discovering new classes of channels, or finding new pharmacological modulators for non-selective cation channels.