Far-red fluorescent genetically encoded calcium ion indicators.

Dalangin, Rochelin; Jia, Bill Z; Qi, Yitong; Aggarwal, Abhi; Sakoi, Kenryo; Drobizhev, Mikhail; Molina, Rosana S; Patel, Ronak et al. · Nat Commun · 2025

basic_science · Level V

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Abstract

Genetically encoded calcium ion (Ca<sup>2+</sup>) indicators (GECIs) are widely-used molecular tools for functional imaging of Ca<sup>2+</sup> dynamics and neuronal activities with single-cell resolution. Here we report the design and development of two far-red fluorescent GECIs, FR-GECO1a and FR-GECO1c, based on the monomeric far-red fluorescent proteins mKelly1 and mKelly2. FR-GECOs have excitation and emission maxima at ~596 nm and ~644 nm, respectively, display large responses to Ca<sup>2+</sup> in vitro (ΔF/F<sub>0</sub> = 6 for FR-GECO1a, 18 for FR-GECO1c), are bright under both one-photon and two-photon illumination, and have high affinities (apparent K<sub>d</sub> = 29 nM for FR-GECO1a, 83 nM for FR-GECO1c) for Ca<sup>2+</sup>. FR-GECOs offer sensitive and fast detection of single action potentials in neurons, and enable in vivo all-optical manipulation and measurement of cellular activities in combination with optogenetic actuators.

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