An endoplasmic reticulum-targeted near-infrared fluorescent probe for monitoring viscosity in fatty liver diseases.

Alterations in endoplasmic reticulum (ER) luminal viscosity perturb molecular diffusion and protein-folding kinetics, underlying ER stress and the pathogenesis of metabolic liver disorders such as nonalcoholic fatty liver disease. Herein, we developed ER-XCC, a NIR fluorescent probe that selectively targets the ER for real-time viscosity monitoring in fatty liver. ER-XCC conjugates xanthene and coumarin moieties to expand the conjugated π-system, producing NIR emission at 806 nm. The probe displays viscosity-dependent fluorescence: low viscosity enables twisted intramolecular charge transfer (TICT) and fluorescence quenching, while high viscosity suppresses TICT to enhance fluorescence. Additionally, ER-XCC incorporates a toluenesulfonamide moiety that facilitates its selective accumulation within the ER, enabling real-time visualization of microenvironmental viscosity fluctuations during ER stress. Furthermore, in vivo investigations demonstrated that ER-XCC successfully detected elevated hepatic viscosity in a murine model of fatty liver, a phenotype that was mitigated by pharmacological intervention. Ex vivo imaging and histopathological staining confirmed the correlation between ER viscosity and fatty liver progression. The results establish ER viscosity as a functional biomarker in fatty liver disease and ER-XCC as a powerful tool for mechanistic studies, early diagnosis, and therapeutic evaluation of metabolic liver disease.