New red-fluorescent calcium indicators for optogenetics, photoactivation and multi-color imaging

Most chemical and, with only a few exceptions, all genetically encoded fluorimetric calcium (Ca 2+ ) indicators (GECIs) emit green fluorescence. Many of these probes are compatible with red-emitting cell- or organelle markers. But the bulk of available fluorescent-protein constructs and transgenic animals incorporate green or yellow fluorescent protein (GFP and YFP respectively). This is, in part, not only heritage from the tendency to aggregate of early-generation red-emitting FPs, and due to their complicated photochemistry, but also resulting from the compatibility of green-fluorescent probes with standard instrumentation readily available in most laboratories and core imaging facilities. Photochemical constraints like limited water solubility and low quantum yield have contributed to the relative paucity of red-emitting Ca 2+ probes compared to their green counterparts, too. The increasing use of GFP and GFP-based functional reporters, together with recent developments in optogenetics, photostimulation and super-resolution microscopies, has intensified the quest for red-emitting Ca 2+ probes. In response to this demand more red-emitting chemical and FP-based Ca 2+ -sensitive indicators have been developed since 2009 than in the thirty years before. In this topical review, we survey the physicochemical properties of these red-emitting Ca 2+ probes and discuss their utility for biological Ca 2+ imaging. Using the spectral separability index X ijk (Oheim M., 2010. Methods in Molecular Biology 591: 3–16) we evaluate their performance for multi-color excitation/emission experiments, involving the identification of morphological landmarks with GFP/YFP and detecting Ca 2+ -dependent fluorescence in the red spectral band. We also establish a catalog of criteria for evaluating Ca 2+ indicators that ideally should be made available for each probe. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau. For cell biological studies, there is an urgent need for long-wavelength Ca 2+ indicators since many molecular biology and transgenic tools extensively incorporate green-emitting fluorophores and optogenetics and photochemical uncaging make use of the blue end of the spectrum. In the present review, after a summary of the prerequisites for building a Ca 2+ sensor, we present the known good Ca 2+ chelators and the chemistry developed to insert them into red-emitting fluorescent dyes. We give a systematic critical appraisal of the different molecules presently available and also, the pros and cons of chemical small-molecules (fast kinetics) vs. genetically encoded red-emitting Ca 2+ probes (unequalled intensity and reduced photobleaching). Finally, we suggest that cargo-constructs for locally up-concentrating Ca 2+ sensors in proximity of putative Ca 2+ signaling sites, e.g., near the mouth of open Ca 2+ channel, will improve our understanding of spatially and temporally confined Ca 2+ signals (‘microdomains’). • Choices among red-emitting Ca 2+ probes have expanded with new chemical indicators and the red-emitting GECIs. • A careful evaluation of which fluorophores work best together with EGFP is required. • Sequential dual-cube measurements allow more fluorophore-specific detection but trade speed against spectral separability. • Control of Ca 2+ localization and local indicator concentration and Ca 2+ buffer capacity will be critical.