Caspase-activated cell-penetrating peptides reveal temporal coupling between endosomal release and apoptosis in an RGC-5 cell model.

Caspase-activatable cell-penetrating peptide (CPP) probes, designed for efficient cell uptake and specificity via cleavable intramolecular quenched-fluorophore strategies, show promise for identifying and imaging retinal ganglion cell apoptosis in vivo. However, initial cell uptake and trafficking events cannot be visualized because the probes are designed to be optically quenched in the intact state. To visualize subcellular activation events in real-time during apoptosis, a new series of matched quenched and nonquenched CPP probes were synthesized. In both native and staurosporine-differentiated RGC-5 cells, probe uptake was time- and concentration-dependent through clathrine-, caveolin-, and pinocytosis-mediated endocytic mechanisms. During apoptosis, KcapTR488, a novel dual fluorophore CPP probe, revealed by multispectral imaging a temporal coupling of endosomal release and effector caspase activation in RGC-5 cells. The novel CPPs described herein provide new tools to study spatial and temporal regulation of endosomal permeability during apoptosis.