In the article “A Bright and Fast Red Fluorescent Protein Voltage Indicator That Reports Neuronal Activity in Organotypic Brain Slices” by Ahmed S. Abdelfattah, Samouil L. Farhi, Yongxin Zhao, Daan Brinks, Peng Zou, Araya Ruangkittisakul, Jelena Platisa, Vincent A. Pieribone, Klaus Ballanyi, Adam E. Cohen, and Robert E. Campbell, which appeared on pages 2458–2472 of the February 24, 2016 issue, the authors regrettably note that the units on the y-axes of Figure 3, D and E, were incorrectly labeled as ΔF/F0. The y-axes should instead be labeled as F/Fmin. Figure 3, D and E, has now been replaced with the correct image in the online PDF version and is displayed below. The figure legend has been revised to include definitions of ΔF/F0 and F/Fmin. This correction does not affect the conclusions of the article.
Characterization of FlicR1. A, Image of HEK293 cells expressing FlicR1 under the CMV promoter. Scale bar, 10 μm. B, Fluorescence response (top) to a triangle wave in membrane potential (bottom) from −100 mV to +50 mV. Fluorescence trace (acquired at 10 Hz) is filtered using a 15 point moving-average low-pass filter. ΔF/F0 is calculated as (F − F0)/F0, where F is the fluorescence at a particular time point and F0 is initial fluorescence. C, FlicR1 fluorescence response (top) from a representative cell to a square wave in membrane potential (bottom) from −70 mV to +30 mV. D, E, FlicR1 (D) and ArcLight Q239 (E) fluorescence as a function of membrane voltage in a representative HEK293 cell. Fluorescence (F/Fmin, where F is the fluorescence intensity at a particular membrane voltage and Fmin is the minimum fluorescence intensity over the range of −100 mV to +50 mV) is the mean of three ramp cycles from −100 mV to +50 mV and back. Fluorescence is plotted starting at −25 mV, depolarizing to +50 mV, hyperpolarizing to −100 mV, and then returning back up to −25 mV, as marked by the arrows. Fluorescence showed little hysteresis between increasing and decreasing voltage ramps. F, FlicR1 fluorescence response to a 100 mV step potential in HEK293 cells. Solid line shows fluorescence response at 34°C. Dotted line shows fluorescence response at 22°C. G, ArcLight Q239 fluorescence response to a 100 mV step potential in HEK293 cells at 22°C. Note the different time axis compared with F. H, Magnification of the “on” and “off” portions of 22°C fluorescence traces from FlicR (red) and ArcLight (black). I, Normalized bleaching curves for FlicR1, ArcLight, and ASAP1 in HEK239 cells. J, Time constants for photobleaching of FlicR1, ASAP1, and ArcLight Q239 in HEK293 cells using continuous 10 W/cm2 561 nm light illumination for FlicR1 and continuous 10 W/cm2 488 nm light illumination for ASAP1 and ArcLight Q239. Fluorescence was captured every 500 ms. Time constants are based on single exponential fits. Error bars indicate SEM for FlicR1 (n = 5 cells), ASAP1 (n = 5 cells), and ArcLight Q239 (n = 4 cells). K, Spectral characterization of FlicR1 in vitro. Shown are absorbance (solid black line), excitation (dotted red line), and emission (solid red line) of FlicR1. Fluorescence imaging for voltage sensitivity measurements was performed at 10 Hz. Step responses were recorded at 2 kHz for FlicR1 and 1 kHz for ArcLight Q239. Illumination intensities were 10 W/cm2.
