Calpastatin exon 1B-derived peptide, a selective inhibitor of calpain: Enhancing cell permeability by conjugation with penetratin

The ubiquitous calpains, and mcalpain, have been implicated in essential physiological processes and various pathologies. Cellpermeable specific inhibitors are important tools to elucidate the roles of calpains in cultivated cells and animal models. The synthetic Nacetylated 27-mer peptide derived from exon B of the inhibitory domain 1 of human calpastatin (CP1B) is unique as a potent and highly selective reversible calpain inhibitor, but is poorly cellpermeant. By addition of Nterminal cysteine residues we have generated a disulfideconjugated CP1B with the cellpenetrating 16-mer peptide penetratin derived from the third helix of the Antennapedia homeodomain protein. The inhibitory potency and selectivity of CP1B for calpain versus cathepsin B and L, caspase 3 and the proteasome was not affected by the conjugation with penetratin. The conjugate was shown to efficiently penetrate into living LCLC 103H cells, since it prevents ionomycininduced calpain activation at 200-fold lower concentration than the nonconjugated inhibitor and is able to reduce calpain triggered apoptosis of these cells. Penetratinconjugated CP1B seems to be a promising alternative to the widely used cellpermeable peptide aldehydes (e.g. calpain inhibitor I) which inhibit the lysosomal cathepsins and partially the proteasome as well or even better than the calpains.