Electrotonic coupling of excitable and nonexcitable cells in the heart revealed by optogenetics

Significance

Heart pumping is triggered and coordinated by action potentials (APs) originating in and spreading among electrically excitable heart muscle cells (myocytes) via electrotonic coupling. Cardiac nonmyocytes are thought not to participate in AP conduction in situ, although heterocellular electrotonic coupling is common in cell culture. We used optogenetic tools involving cell-specific expression of a voltage-reporting fluorescent protein to monitor electrical activity in myocytes or nonmyocytes of mouse hearts. We confirm the suitability of this technique for measuring cell type-specific voltage signals and show that, when expressed in nonmyocytes, myocyte AP-like signals can be recorded in cryoinjured scar border tissue. This direct evidence of heterocellular electrotonic coupling in the whole heart necessitates a review of current concepts on cardiac electrical connectivity.

Abstract

Electrophysiological studies of excitable organs usually focus on action potential (AP)-generating cells, whereas nonexcitable cells are generally considered as barriers to electrical conduction. Whether nonexcitable cells may modulate excitable cell function or even contribute to AP conduction via direct electrotonic coupling to AP-generating cells is unresolved in the heart: such coupling is present in vitro, but conclusive evidence in situ is lacking. We used genetically encoded voltage-sensitive fluorescent protein 2.3 (VSFP2.3) to monitor transmembrane potential in either myocytes or nonmyocytes of murine hearts. We confirm that VSFP2.3 allows measurement of cell type-specific electrical activity. We show that VSFP2.3, expressed solely in nonmyocytes, can report cardiomyocyte AP-like signals at the border of healed cryoinjuries. Using EM-based tomographic reconstruction, we further discovered tunneling nanotube connections between myocytes and nonmyocytes in cardiac scar border tissue. Our results provide direct electrophysiological evidence of heterocellular electrotonic coupling in native myocardium and identify tunneling nanotubes as a possible substrate for electrical cell coupling that may be in addition to previously discovered connexins at sites of myocyte–nonmyocyte contact in the heart. These findings call for reevaluation of cardiac nonmyocyte roles in electrical connectivity of the heterocellular heart.