Improved detection of electrical activity with a voltage probe based on a voltage‐sensing phosphatase

Key points The use of genetically encoded voltage probes has been expected to enable sensitive detection of spatiotemporal electrical activities in excitable cells. However, existing probes suffer from low signal amplitude and/or kinetics too slow to detect fast electrical activity. We have developed an improved voltage probe named Mermaid2. Mermaid2 provides ratiometric readouts of electrical activity with fast kinetics and great sensitivity, and was able to detect single‐event electrical activity both in vitro and in vivo. Mermaid2 will expand our chances to analyse electrical events that have been less accessible by using other techniques. Abstract One of the most awaited techniques in modern physiology is the sensitive detection of spatiotemporal electrical activity in a complex network of excitable cells. The use of genetically encoded voltage probes has been expected to enable such analysis. However, in spite of recent progress, existing probes still suffer from low signal amplitude and/or kinetics too slow to detect fast electrical activity. Here, we have developed an improved voltage probe named Mermaid2, which is based on the voltage‐sensor domain of the voltage‐sensing phosphatase from Ciona intestinalis and Förster energy transfer between a pair of fluorescent proteins. In mammalian cells, Mermaid2 permits ratiometric readouts of fractional changes of more than 50% over a physiologically relevant voltage range with fast kinetics, and it was used to follow a train of action potentials at frequencies of up to 150 Hz. Mermaid2 was also able to detect single action potentials and subthreshold voltage responses in hippocampal neurons in vitro, in addition to cortical electrical activity evoked by sound stimuli in single trials in living mice.