Development of a Novel Blue Fluorescent Gene-encoded Calcium Indicator Modified from GCaMP3

Intracellular calcium can be monitored by various calcium-specific fluorescent dyes including gene-encoded calcium indicators (GECI). GCaMP is a widely-used GECI that emits green fluorescence proportional to the level of intracellular calcium. However, since many tagging proteins also emit green fluorescence, GCaMP cannot be used with another green fluorescent protein. Therefore, it would be ideal to develop a GECI that has a distinct color profile other than green. In this regard, we developed a novel blue fluorescent calcium indicator modified from GCaMP called Ser²²²-Ala²²⁹-Cys³³⁰-BCaMP3. Specifically, a simple threonine to histidine substitution to a green fluorescent Cys³³⁰-GCaMP3 successfully changed its fluorescence to blue (Cys³³⁰-BCaMP3, B for blue). Furthermore, a couple of additional amino acid substitutions resulted in more enhanced blue fluorescence intensity. Among other Cys³³⁰-BCaMP3 variants, it was found that Ser²²²-Ala²²⁹-Cys³³⁰-BCaMP3 exhibited the strongest blue fluorescence intensity. When Ser²²²-Ala²²⁹-Cys³³⁰-BCaMP3 was co-expressed with TRPA1 - a non-selective cation channel - in HEK293T cells, it showed moderate blue fluorescence. One of the drawbacks of Ser²²²-Ala²²⁹-Cys³³⁰-BCaMP3 was that the fluorescence intensity was not enough when cells were cultured under 37°C. However, this limitation was circumvented by lowering cell culture temperature to 28°C, allowing much more enhanced blue fluorescence. Although Ser²²²-Ala²²⁹-Cys³³⁰-BCaMP3 mandates further optimization, the present study has found a promising blue fluorescent GECI that is derived from GCaMP3.