Efficient entry of cell-penetrating peptide nona-arginine into adherent cells involves a transient increase in intracellular calcium

Understanding the mechanism of entry of cationic peptides such as nona-arginine (R9) into cells remains an important challenge to their use as efficient drug-delivery vehicles. At nanomolar to low micromolar R9 concentrations and at physiological temperature, peptide entry involves endocytosis. In contrast, at a concentration ≥10 μM, R9 induces a very effective non-endocytic entry pathway specific for cationic peptides. We found that a similar entry pathway is induced at 1-2 μM concentrations of R9 if peptide application is accompanied by a rapid temperature drop to 15°C. Both at physiological and at sub-physiological temperatures, this entry mechanism was inhibited by depletion of the intracellular ATP pool. Intriguingly, we found that R9 at 10-20 μM and 37°C induces repetitive spikes in intracellular Ca(2+) concentration. This Ca(2+) signalling correlated with the efficiency of the peptide entry. Pre-loading cells with the Ca(2+) chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) inhibited both Ca(2+) spikes and peptide entry, suggesting that an increase in intracellular Ca(2+) precedes and is required for peptide entry. One of the hallmarks of Ca(2+) signalling is a transient cell-surface exposure of phosphatidylserine (PS), a lipid normally residing only in the inner leaflet of the plasma membrane. Blocking the accessible PS with the PS-binding domain of lactadherin strongly inhibited non-endocytic R9 entry, suggesting the importance of PS externalization in this process. To conclude, we uncovered a novel mechanistic link between calcium signalling and entry of cationic peptides. This finding will enhance our understanding of the properties of plasma membrane and guide development of future drug-delivery vehicles.