Linearly polarized excitation enhances signals from fluorescent voltage indicators

Voltage imaging in cells requires high-speed recording of small fluorescent signals, often leading to low signal-to-noise ratios. Because voltage indicators are membrane-bound, their orientations are partially constrained by the plane of the membrane. We explored whether tuning the linear polarization of excitation light could enhance voltage indicator fluorescence. We tested a panel of dye and protein-based voltage indicators in mammalian cells. The dye BeRST1 showed a 73% increase in brightness between the least and most favorable polarizations. The protein-based reporter ASAP1 showed a 22% change in brightness, and QuasAr3 showed a 14% change in brightness. In very thin neurites expressing QuasAr3, improvements were anomalously large, with a 170% increase in brightness between polarization parallel vs perpendicular to the dendrite. Signal-to-noise ratios of optically recorded action potentials were increased by up to 50% in neurites expressing QuasAr3. These results demonstrate that polarization control can be a facile means to enhance signals from fluorescent voltage indicators, particularly in thin neurites or in high-background environments.