R-CaMP1.07, an improved genetically encoded red fluorescent Ca2+ indicator

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.

Simultaneous monitoring and manipulation of neuronal activity by co-expression of R-CaMP1.07 and ChR2.
A, A projection image of a CA3 pyramidal cell expressing R-CaMP1.07 and ChR2. The cell was whole-cell recorded to measure the number of APs. B, Imaging of ChR2-triggered-APs by changes in the R-CaMP1.07 fluorescence (top). APs were evoked by a pulse of 470-nm light with a duration of 300 to 3000 ms (blue region indicates time of photostimulation). Putative Ca2+ increases during the photostimulation are represented by broken lines (Bottom). The number of APs was recorded using the current-clamp mode. C, ΔF/F amplitude of the R-CaMP1.07 responses as a function of the number of APs induced by photostimulation. The peak ΔF/F amplitudes were calculated from the first frames after termination of the photostimulation. Individual data are plotted as black dots and their averages are shown in red. Error bars, s.e.m. (n = 4 cells). From Ohkura et al., 2012.

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