Calcium ion sensor Case12

- High dynamic range calcium indicator
- Direct expression in cells
- High selectivity and sensitivity
- High brightness of fluorescent response
- Fast maturation at 37°C
- Relatively high pH stability
- Recommended for monitoring change of calcium concentration inside living cells

Protein description

Case12 is a high dynamic range genetically encoded fluorescent sensor for detection of intracellular Ca2+ changes [Souslova et al., 2007]. The sensor allows direct measurement of changes of calcium concentration in a physiological range from hundred nanomoles to micromoles with a high signal-to-noise ratio. Binding of Ca2+ is fast and reversible, allowing monitoring high-frequency-Ca2+ oscillations. In response to Ca2+ concentration rise, Case12 shows up to 12-fold increase of fluorescence brightness. Fluorescence of Case12 is characterized by single excitation/emission maxima peaked at 491/516 nm. Case12 is recommended for monitoring change of calcium concentration inside living cells during various physiological and pathological conditions.

Main properties

Normalized excitation (thin line) and emission (thick line) spectra.	CHARACTERISTIC   Emission maximum, nm 516 Excitation maximum, nm 491 Fluorescence color green Polypeptide length, aa 415 Molecular weight, kDa 46.4 Specificity Ca2+ Kd for Ca2+ 1 μM pKa 7.2 Structure monomer Aggregation no Maturation rate at 37°C fast

	Ca2+ titration curves. The apparent Kd for calcium ions binding was found to be 1 μM, which lies within the physiological range of calcium ions concentrations. Image from Souslova et al., 2007.

Compatibility with existing filter sets

We recommend standard GFP filter sets. Appropriate Omega Optical filter sets for Case12 are QMAX-Green, XF100-2 and XF100-3. It can also be detected using Chroma Technology Corp. filter sets 41001, 41017, 41020, 41025 or similar.

Performance and use

The common weak point of conventional calcium sensors is their low pH stability. For example, pKa (meaning of pH at which fluorescence brightness is 50% of maximum) for Pericams reaches as high as 8.0. Therefore, at physiological pH (7.2-7.5) such sensors exhibit low brightness and dynamic range [Nagai et al., 2001]. In contrast, the pKa of Case12 is 7.2 (in the presence of 10 μM Ca²⁺) close to that reported for G-CaMP [Nakai et al., 2001]. This relatively high pH stability makes Case12 well suitable for in vivo use. Image from Souslova et al., 2007.

	Dependence of Case12 fluorescence on pH in the presence (thick line) and in the absence (thin line) of Ca2+.

Case12 is characterized by fast maturation at 37°C and bright fluorescent response to Ca²⁺. It can be directly expressed by target cells, both individually and in fusion with a specific localization signal. No aggregation is observed upon long-term (5 days) expression of Case12 in transiently transfected cells, indicating that Case12 is suitable for the generation of stable cell lines and transgenic animals. Case12 can be used for monitoring intracellular Ca²⁺ changes in various physiological and pathological conditions.

Monitoring changes in green emission of Case12 in response to intracellular changes of Ca²⁺ concentration should be carried out by excitation by blue light (488 nm laser line or standard GFP filter set). Emission can be collected at approximately 500-540 nm. Intensity of excitation light should be individually determined for particular biological system and instrumentation. In general, we recommend that you minimize excitation light intensity and duration.

Note: Yellow fluorescent core of Case12 undergoes partial photoconversion to a dark state upon irradiation with blue light. It means that an apparent "bleaching" effect occurs at the beginning of time series imaging of cells expressing Case12 protein. Unlike the real bleaching, in the case of Case12, signal drops to the level of dynamic equilibrium between fluorescent and dark state of the chromophore, and then remains stable.

Maximum dynamic range in HeLa cells: HeLa cells transfected with Case12 showed relatively weak green fluorescence, which was detected with a Leica microscope DM IRE2, confocal TCS-SP2, objective HCX-PL-APO-63x/1.40-0.60/OIL. Addition of 20 μM calcium ionophore A23187, allowing calcium to enter cells (2 mM Ca²⁺ in the medium), resulted in 5-6-fold increase in green fluorescence brightness. Subsequent addition of 20 mM EGTA removed Ca²⁺ and decreased the fluorescence signal close to baseline level, with the final contrast of 11-12 fold.

		Testing Case12 in living cells. (A) Typical response of HeLa cells expressing Case12 to calcium ionophore A23187. (B, C) HeLa cells expressing Case12 shown before (B) and after (C) ionophore addition. Image from (Souslova et al., 2007).

Monitoring of Ca²⁺ changes under physiological conditions: Mammalian cells expressing Case12 displayed a nice high dynamic range response upon addition of ATP at a final concentration of 100 μM. This experiment clearly showed that Case12 fluorescence response to Ca²⁺ oscillations is fast and reversible. It also demonstrated that the sensor responds to changes in Ca²⁺ concentration in living cells in the nanomolar range.

	Fluorescence changes of human melanoma-derived M21 cells expressing Case12 in response to 100 μM ATP. Images were captured every 0.294 sec on the confocal microscope.

Available variants and fusions

Case12 codon usage is optimized for high expression in mammalian cells [Haas et al., 1996], but it can be successfully expressed in other heterological systems.

Case12-Cyto variant is localized in cell cytosol.