In a recent paper published in PLoS One, Ohkura et al. introduce R-CaMP1.07, an improved variant of a recently developed red fluorescent Ca2+ indicator protein called R-GECO1. While the sensitivity of R-CaMP1.07 is similar to that of R-GECO1 (Kd for Ca2+ is around 200 nM), R-CaMP1.07 shows 1.5–2-fold greater signals than R-GECO1 due to its enhanced dynamic range (Fmax/Fmin is near 30). The greatest advantage of R-CaMP1.07 is that its excitation wavelength ranges from 500 to 580 nm, which rarely overlaps with the photo-stimulation range of channelrhodopsin-2 (ChR2). Taking this merit of R-CaMP1.07, the authors demonstrate an application example of this indicator in hippocampal pyramidal neurons expressing ChR2: the successful detection of Ca2+ signals in response to action potentials evoked by the photo-stimulation of ChR2. Needless to say, this red fluorescent R-CaMP1.07 can be used for Ca2+ imaging of cells expressing blue, cyan or green fluorescent proteins.
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