Real-Time Imaging of Lithium ‘Hot-Spots’: An Analysis of Ion Conductance in Aquaporin-1 using Novel Photo-Switchable Sensor

Abstract

Aquaporin-1 (AQP1) belongs to the aquaporin (AQP) protein family that facilitate both the flux of water and other solutes across membranes. In each of the four channel subunits, AQP1 has pores for water conductance, while the central pore of the tetrameric channel contributes the monovalent cation conductance. Among all 13 AQPs, AQP1 is the one that plays a crucial role in cell migration. In various studies, cells with genetic knockdown of AQP1 expression have demonstrated significant impaired cell motility, whereas reintroduction of AQP1 restored cell migration. The mechanism of AQP1 action on cell migration remains unknown, it seems likely to involve ion fluxes. Our previous works have also reported impaired migration of HT29 cancer cells when treated with 2 different AQP1 ion channel antagonists (AqB011 and Bacopaside I). To understating the role ion channel function of AQP1 in rapid cell motility would advance our knowledge of the physiological relevance of aquaporin ion channels. Here we report a newly designed ion sensor named ‘SHL’ that is Li+-selective and photo-switchable. SHL has been used to monitor AQP1 ion channel activity in living colon cancer cells to at real time by the appearance of lithium hot spots imaged using confocal microscopy. In HT29 colon cancer cells, which has higher AQP1-expressing, the Li+ hot spots are clustered in the lamellipodial leading edges. The hot-spots are blocked by the AQP1-ion channel antagonist AqB011. Lithium hot spots are not observable in cell line SW620 that lacks comparable membrane expression of AQP1. The correlation between the subcellular distribution and the visualized ion channel activity of AQP1 channels in cancer cells supports the proposed role of the AQP1 ion channel in cell migration which makes it a novel candidate for therapeutics.