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We have used protein engineering to expand the palette of genetically encoded calcium ion (Ca(2+)) indicators to include orange and improved red fluorescent variants, and validated the latter for combined use with optogenetic activation by channelrhodopsin-2 (ChR2). These indicators feature intensiometric signal changes that are 1.7- to 9.7-fold improved relatively to the progenitor Ca(2+) indicator, R-GECO1. In the course of this work, we discovered a photoactivation phenomenon in red fluorescent Ca(2+) indicators that, if not appreciated and accounted for, can cause false-positive artifacts in Ca(2+) imaging traces during optogenetic activation with ChR2. We demonstrate, in both a beta cell line and slice culture of developing mouse neocortex, that these artifacts can be avoided by using an appropriately low intensity of blue light for ChR2 activation.
Wu et al. (2013) studied this question.