Noninvasive determination of cell nucleoplasmic viscosity by fluorescence correlation spectroscopy.

Noninvasive and reliable quantification of rheological characteristics in the nucleus is extremely useful for fundamental research and practical applications in medicine and biology. This study examines the use of fluorescence correlation spectroscopy (FCS) to noninvasively determine nucleoplasmic viscosity (eta(nu)), an important parameter of nucleoplasmic rheology. Our FCS analyses show that eta(nu) of lung adenocarcinoma (ASTC-a-1) and HeLa cells are 1.77+/-0.42 cP and 1.40+/-0.27 cP, respectively, about three to four times larger than the water viscosity at 37 degrees C. eta(nu) was reduced by 31 to 36% upon hypotonic exposure and increased by 28 to 52% from 37 to 24 degrees C. In addition, we found that eta(nu) of HeLa cells reached the lowest value in the S phase and that there was no significant difference of eta(nu) between in the G1 and G2 phases. Last, nucleoplasmic viscosity was found to be larger than cytoplasmic viscosity in both HeLa and ASTC-a-1 cells. These results indicate that FCS can be used as a noninvasive tool to investigate the microenvironment of living cells. This is the first report on the measurement of eta(nu) in living cells synchronized in the G1, S, and G2 phases.