A New Robust Fluorescent Calcium Indicator for Ca2+ Flux Assays in Living Cells

Introduction

Fluorescent Ca²⁺ indicators (e.g. Indo-1, Fura-2, Fluo-3 and Fluo-4) are commonly used to monitor Ca²⁺ response in a variety of applications including HTS, fluorescent imaging and flow cytometry. However, these dyes require the presence of an organic anion transporter inhibitor (e.g. probenecid) to prevent the leakage of the indicators in many cell types (e.g. CHO and Hela). The organic anion transporter inhibitors are toxic to cells and some of them are known to be the inhibitors of certain GPCRs (e.g. chemokine receptors, bitter taste receptors). Because of those reasons, the usages of these dyes are limited.

We have recently developed the first-in-class ultra bright probenecid-free fluorescent Ca²⁺ indicator, Calbryte™ 520 AM. Calbryte™ 520 AM is cell permeable and non-fluorescent. Once inside the cells, Calbryte™ 520 AM is processed into Calbryte™ 520 by esterases. When binding to Ca²⁺, Calbryte™ 520 produces exceptionally bright fluorescent signal with very high signal to background (S/B) ratio.

Figure 1. Principle of Calbryte™ 520 Am fluorescent calcium indicator.

Table 1. Calbryte™ 520 AM specifications.

Method

1. 100 µL/well cells (40K/well for adherent cells) were seeded in a 96-well black wall/clear bottom Costar plate at 37°C incubator overnight.
2. The cells were incubated with 100 µL dye loading buffers that contain Ca²⁺ indicators (5 µg/mL) at 37°C for 30 minutes to 1 hour.
3. Cells were then incubated with or without compounds of interest for additional 15 min at 37°C.
4. The dye loading buffer was either removed and replace with 200 µL HHBS (wash condition) or was left in wells (no-wash condition).
5. 50 µL/well of Ca²⁺ flux stimulant was added and the Ca²⁺ flux was monitored by Fluorescence Microscope (Keyence), FlexStation (Molecular Devices) or Flow Cytometer (Novocyte 3000).

Results

ATP/Carbachol Induced Ca²⁺ Flux in CHO-K1 Cells with Probenecid

Figure 2. ATP induced endogenous P2Y response in CHO-K1 cells. Calbryte™520 AM or Fluo-4 AM in Pluronic F127 containing buffer with 2.5 mM probenecid was incubated with the cells at 37°C for 45 min. 10 µM ATP (final conc.) was added, and images were taken by a fluorescence microscope using FITC channel.

ATP/Carbachol Induced Ca²⁺ Flux in CHO-M1 Cells without Probenecid

Figure 3. ATP induced endogenous P2Y response in CHO-K1 cells. Calbryte™520 AM or Fluo-4 AM in Pluronic F127 containing buffer with 2.5 mM probenecid was incubated with the cells at 37°C for 45min. 10 µM ATP (final conc.) was added, and images were taken by a fluorescence microscope using FITC channel.

RANTE Induced Ca²⁺ Flux in CHO-CCR5 Cells

Atropine Inhibited Ca²⁺ Flux in CHO-M1 Cells

	Calbryte™ 520	Fluo-4
IC50 (nM)	0.48	>100

	Calbryte™ 520	Fluo-4
IC50 (nM)	1.54	2.01

	Calbryte™ 520	Fluo-4
IC50 (nM)	1.1	2.12

Concanavalin A Induced CCE in Jurkat Cells

Figure 6. 250K Jurkat cells were suspended in calcium-free HHBS buffer per well, cells were incubated with Calbryte™520 AM or Fluo-4 AM in HHBS with Pluronic F127 Plus for 1 hr at 37°C. 3 µg/ml of channel opener concanavalin A (final conc.) with or without channel inhibitor capsaicin were incubated with the cells for 10 min at 37°C. The capacitive Ca²⁺ entry (CCE) was monitored before and after the addition of 5 mM CaCl₂ via FlexStation for 100s.

ATP Induced Ca²⁺ Flux by Flow Cytometry

B

Conclusion

1. Calbryte™520 AM generated much brighter signal (3-4 fold more) and much higher S/B ratio (3-4 folds more) in monitoring Ca²⁺ flux in CHO-K1, CHO-M1, CHO-CCR5 and Jurkat cells than Fluo-4 AM using fluorescent microscope, FlexStation and Flow Cytometer.
2. Without probenecid, Fluo-4 AM was unable to detect Ca²⁺ flux in CHO cells, however, Calbryte™520 AM still achieved excellent cellular retention and S/B ratio.
3. The calculated EC₅₀ of Ca²⁺ flux stimulants or inhibitors were comparable between Calbryte™520 AM and Fluo-4 AM incubated cells.

Ordering Information