Fluorescent Visualization of Cellular Proton Fluxes

Proton fluxes through plasma membranes are essential for regulating intracellular and extracellular pH and mediating co-transport of metabolites and ions. Although conventional electrical measurements are highly sensitive and precise for proton current detection, they provide limited specificity and spatial information. My thesis focuses on developing optical approaches to visualize proton fluxes from ion channels and transporters. It has been demonstrated that channel-mediated acid extrusion causes proton depletion at the inner surface of the plasma membrane. Yet, proton dynamics at the extracellular microenvironment are still unclear. In Chapter II, we developed an optical approach to directly measure pH change in this nanodomain by covalently attaching small-molecule, fluorescent proton sensors to the cell’s glycocalyx using glyco-engineering and copper free ‘click’ chemistry. The extracellularly facing sensors enable real-time detection of proton accumulation and depletion at the plasma membrane, providing an indirect readout of channel and transporter activity that correlated with whole-cell proton current. Moreover, the proton wavefront emanating from one cell was readily visible as it crossed over nearby cells. The transport of monocarboxylates, such as lactate and pyruvate is critical for energy metabolism and is mainly mediated by proton-coupled monocarboxylate transporters (MCT1-MCT4). Although pH electrodes and intracellular fluorescent pH sensors have been widely used for measuring the transport of proton-coupled MCTs, they are unable to monitor the subcellular activities and may underestimate the transport rate due to cell’s volume and intracellular buffering. In Chapter III, we used the Chapter II approach to visualize proton-coupled transport by MCT1-transfected HEK293T cells and observed proton depletion followed by a recovery upon extracellular perfusion of L-lactate or pyruvate. In addition, we identified a putative MCT, CG11665/Hrm that is essential for autophagy during cell death in Drosophila. The results demonstrate that Hrm is a bona fide proton-coupled monocarboxylate transporter that transports pyruvate faster than lactate. Although the approach developed in Chapter II enables visualization of proton fluxes from ion channels and transporters, it’s not applicable in some cell types which cannot incorporate unnatural sialic acid precursors into their glycocalyx, such as INS-1 cells and cardiomyocytes. To address this, in Chapter IV we developed a pH-sensitive, fluorescent WGA conjugate, WGA-pHRho that binds to endogenous glycocalyx. Compared to the results in Chapter II and III, cell surface-attached WGA-pHRho has similar fluorescent signals in response to proton fluxes from proton channel Hv1, omega mutant Shaker-IR R362H and MCT1. With WGA-pHRho, we were able to label the plasma membrane of INS-cells and cardiomyocytes and visualized the transport activity of MCT1 in these cells. Taken together, these findings provide news insights into proton dynamics at the extracellular environment and provide new optical tools to visualize proton fluxes from ion channels and transporters. Moreover, the modularity of the approaches makes them adaptable to study any transport events at the plasma membrane in cells, tissues, and organisms