Developing Activity Localization Fluorescence Peptide Probe Using Thiol-Ene Click Reaction for Spatially Resolved Imaging of Caspase-8 in Live Cells

Small molecule probes suitable for high resolution fluorescence imaging of enzyme activity pose a challenge in chemical biology. We developed a novel design of activity localization fluorescence (ALF) peptide probe, which enables spatially resolved, highly sensitive imaging of peptidase in live cells. The ALF probe was synthesized by a facile thiol-ene click reaction of a cysteine-appended peptide with an acryloylated fluorophore. Upon cleavage by peptidase, the probe undergoes a seven-membered intramolecular cyclization and releases the fluorophore with the excited-state intramolecular photon transfer (ESIPT) effect. A highly fluorescent, insoluble aggregate was formed around the, enzyme, which facilitates high-sensitivity and high-resolution imaging. This design is demonstrated for detection of caspase-8 activation. The results show that our design allows easy, high-yield synthesis of the.:probe, and the probe affords high sensitivity for caspase-8 detection. Live cell imaging reveals that the probe is able to render highly localized and high-contrast fluorescence Signal for caspase-8. Our design holds the potential as a generally applicable strategy for developing high-sensitivity and high-resolution imaging peptide probes in cell biology and diagnostics.