Analysis of cellular functions by multipoint fluorescence correlation spectroscopy.

The biophysical investigation of living cells is currently possible by single molecular detection methods such as fluorescence correlation spectroscopy (FCS). FCS is applied for measuring the dynamic mobility of target molecules in living cells; however, the conventional FCS systems still lack quantitative analysis for many regions of interests (ROI) in real time. To improve this situation, we have developed a novel multipoint FCS system (M-FCS) that can measure multipoint correlation functions in the cell simultaneously. To evaluate its performance, we measured correlation functions for rhodamine 6G (Rh6G) in homogeneous conditions and for green fluorescence protein (GFP) in HeLa cells. We conclude that M-FCS possesses reliable performance. As a pharmacological application, glucocorticoid receptor protein fused GFP (GR-GFP) was transfected in HeLa cells and FCS measurements were carried out in the cytoplasm and the nucleus simultaneously. The translocation of GR-GFP from the cytoplasm to the nucleus by ligand stimulation was observed with laser scanning microscopy (LSM) and M-FCS. Particularly in the nucleus, the slower diffusion of GR-GFP suggested molecular interactions after the translocation. These data imply that M-FCS can be applied for quantitative analysis of kinetic processes in living cells.