Imaging mitochondrial electric flickers in intact cells with a membrane-anchored indicator

Abstract Mitochondria are excitable organelles, and their electrophysiological activity across the inner mitochondrial membrane (IMM) plays a critical role in energy metabolism, cell signaling and cell fate regulation. While the transmembrane voltage of the IMM (Δψ m ) is traditionally measured with Nernstian dyes such as tetramethylrhodamine methyl ester (TMRM), here we introduce HB mito Crimson (HB mito ), a novel membrane-anchored voltage sensor to probe mitochondrial near-field potential (V nf ) in intact cells by capturing fluorescence signal variations through rapid optical imaging. V nf encompasses surface potential and the innermost portion of Δψ m . During mitochondrial action potential, HB mito -reported V nf -related signals show up to a 1.9-fold increase, concurrent with Δψ m changes reported by TMRM. At an imaging rate of 1000 frames per second, transient miniature V nf -related flickers were further revealed. And these events were three orders of magnitude smaller than action potentials and likely reflected the activity of individual mitochondrial ionic channels. These results provide the first optical imaging evidence of fundamental electrophysiological events in intact mitochondria, and highlight a novel approach for studying excitable membranes.