Confocal Imaging of Subcellular Ca2+ Concentrations Using a Dual-Excitation Ratiometric Indicator Based on Green Fluorescent Protein

Science's STKE  26 Mar 2002:
Vol. 2002, Issue 125, pp. pl4
DOI: 10.1126/stke.2002.125.pl4

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Dual-excitation ratiometric dyes are excited alternately at two different wavelengths, but the emission is collected at a single fixed wavelength. Therefore, the pair of intensity measurements must be collected sequentially. Ratiometric-pericam is a fluorescent Ca2+ indicator based on a chimeric fusion protein of circularly permuted green fluorescent protein and calmodulin. Upon binding to calcium, its excitation peak shifts from 415 nm to 494 nm. Ca2+ imaging using ratiometric-pericam was thought to be inadequate to follow very fast Ca2+ dynamics or Ca2+ changes in highly motile cell samples; however, we describe a technique that allows high spatial and time resolution of images acquired with ratiometric-pericam. To obtain confocal images of Ca2+ using ratiometric-pericam, we established a system in which two laser beams (excitation 408 nm and 488 nm) are alternated on every scanning line under the control of two acousto-optic tunable filters. This system increases the rate at which ratio measurements are done to 200 Hz, and provides confocal images at 1 to 10 Hz depending on the image size. The ratio images are free from noise caused by the fluctuation of laser power, because the system is equipped with a violet laser diode (408 nm) and a diode-pumped solid-state laser (488 nm), both of which are stable. We visualized the dynamic propagation of Ca2+ waves from the cytosol to the nucleus and changes in Ca2+ concentrations in motile mitochondria of HeLa cells. We demonstrate that this new confocal imaging system expands the range of potential applications of ratiometric-pericam and other dual-excitation ratiometric indicators.

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