Real-time monitoring viscosity changes in lipid droplets in response to ER stress via a viscosity-sensitive ruthenium (II) fluorescent probe

Endoplasmic reticulum (ER) is the origin of lipid droplets (LD). Excessive lipid exposure results in ER stress which associates with many diseases. However, the change of the lipid droplet viscosity under ER stress is still unrevealed. Hence, it is important to develop a viable tool to investigate the lipid droplet viscosity under ER stress. In this study, a novel viscosity-sensitive fluorescent probe Ru(phen-CA)(3)Cl-2 with long-lived phosphorescence, good spectral stability and large stokes shifts was developed. The probe is prominently located at lipid droplet organelles rather than at lysosome and mitochondria, showing highly sensitive to viscosity. The fluorescence intensity is obviously increased with time under the treatment of dexamethasone and rapamycin. Invitro and in vivo experiments further confirm that this probe can be used for real-time monitoring the changes of lipid droplet viscosity in response to ER stress. This novel probe provides an effective approach to elucidate the viscosity change in lipid droplets under ER stress.