# Probing intracellular potassium dynamics in neurons with the genetically encoded sensor lc-LysM GEPII 1.0 in vitro and in vivo

## TL;DR

This study shows that the sensor lc-LysM GEPII 1.0 can detect large changes in intracellular potassium levels in neurons in both lab and living mouse models.

## Contribution

The study demonstrates the first in vivo use of lc-LysM GEPII 1.0 to measure intracellular potassium dynamics in neurons.

## Key findings

- lc-LysM GEPII 1.0 can detect intracellular potassium decreases during intense neuronal activity in vitro.
- In vivo experiments showed increased fluorescence lifetime during periinfarct depolarizations, indicating potassium loss.
- The sensor cannot detect potassium changes during single action potentials and needs optimization for smaller changes.

## Abstract

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.