A fluorogenic, peptide-based probe for the detection of Cathepsin D in macrophages

Cathepsin D is a protease that is an effector in the immune response of macrophages, yet to date, only a limited number of probes have been developed for its detection. Herein, we report a water soluble, highly sensitive, pH insensitive fluorescent probe for the detection of Cathepsin D activity that provides a strong OFF/ON signal upon activation and with bright emission at 515 nm. The probe was synthesised using a combination of solid and solution-phase chemistries, with probe optimisation to increase its water solubility and activation kinetics by addition of a long PEG chain (5 kDa) at the C-terminus. A BODIPY fluorophore allowed detection of Cathepsin D across a wide pH range, important as the protease is active both at the low pH found in lysosomes and also in higher pH phagolysosomes, and in the cytosol. The probe was successfully used to detect Cathepsin D activity in macrophages challenged by exposure to bacteria.