Reduced graphene oxide quenched peptide probe for caspase-8 activity detection and cellular imaging

Cysteinyl aspartate-specific protease 8 (caspase-8) plays a key role in various biological processes by regulating apoptosis. Therefore, this makes accurate detection and intracellular imaging of caspase-8 of great importance for drug screening, disease diagnosis, and prognostication. Here, by designing a reduced graphene oxide (rGO) quenched peptide probe, we constructed a new biosensing system for monitoring in vitro and intracellular caspase-8 activity. In this system, a fluorophore-labeled peptide and rGO were used as the substrate of caspase-8 and the fluorophore quencher, respectively. The hydrolysis of caspase-8 on the polypeptide probe substrate can generate two fragments with different lengths. The release of the short fragment labeled with the fluorophore causes recovery of the fluorescence signal (Ex/Em = 520/576 nm). Under the optimized conditions, the proposed fluorescence method exhibited a linear response range of 0.2 to 5 U·mL^-1 for caspase-8 with a limit of detection (LOD) of 0.2 U·mL^-1 in vitro. Furthermore, this method has been successfully applied to monitoring the upregulation of intracellular caspase-8 activity caused by tert-butyl hydroperoxide (TBHP) and fluorouracil. Flow cytometry assay indicated the positive relation between the upregulation of intracellular caspase-8 activity and cell apoptosis rate. In summary, the above results demonstrated the practical application of this method for apoptosis-related cell imaging.