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Neuronal activity is accompanied by a net outflow of potassium ions (K + ) from the intra- to the extracellular space. While extracellular [K + ] changes during neuronal activity are well characterized, intracellular dynamics have been less well investigated due to lack of respective probes. In the current study we characterized the FRET-based K + biosensor lc-LysM GEPII 1.0 for its capacity to measure intracellular [K + ] changes in primary cultured neurons and in mouse cortical neurons in vivo. We found that lc-LysM GEPII 1.0 can resolve neuronal [K + ] decreases in vitro during seizure-like and intense optogenetically evoked activity. [K + ] changes during single action potentials could not be recorded. We confirmed these findings in vivo by expressing lc-LysM GEPII 1.0 in mouse cortical neurons and performing 2-photon fluorescence lifetime imaging. We observed an increase in the fluorescence lifetime of lc-LysM GEPII 1.0 during periinfarct depolarizations, which indicates a decrease in intracellular neuronal [K + ]. Our findings suggest that lc-LysM GEPII 1.0 can be used to measure large changes in [K + ] in neurons in vitro and in vivo but requires optimization to resolve smaller changes as observed during single action potentials., (© 2024. The Author(s).)

Given the importance of ion gradients and fluxes in biology, monitoring ions locally at the exterior of the plasma membrane of intact cells in a noninvasive manner is highly desirable but challenging. Classical targeting of genetically encoded biosensors at the exterior of cell surfaces would be a suitable approach; however, it often leads to intracellular accumulation of the tools in vesicular structures and adverse modifications, possibly impairing sensor functionality. To tackle these issues, we generated recombinant fluorescent ion biosensors fused to traptavidin (TAv) specifically coupled to a biotinylated AviTag expressed on the outer cell surface of cells. We show that purified chimeras of TAv and pH-Lemon or GEPII 1.0, Förster resonance energy transfer-based pH and K + biosensors, can be immobilized directly and specifically on biotinylated surfaces including glass platelets and intact cells, thereby remaining fully functional for imaging of ion dynamics. The immobilization of recombinant TAv-GEPII 1.0 on the extracellular cell surface of primary cortical rat neurons allowed imaging of excitotoxic glutamate-induced K + efflux in vitro. We also performed micropatterning of purified TAv biosensors using a microperfusion system to generate spatially separated TAv-pH-Lemon and TAv-GEPII 1.0 spots for simultaneous pH and K + measurements on cell surfaces. Our results suggest that the approach can be greatly expanded by immobilizing various biosensors on extracellular surfaces to quantitatively visualize microenvironmental transport and signaling processes in different cell culture models and other experimental settings.