The eukaryotic plasma membrane represents a significant obstacle for the development of both drugs and biological research tools. Fortunately, cell penetrating peptides (CPPs) are a versatile technology that can overcome such practical limitations as relatively inert delivery vectors. A majority of CPPs are relatively short sequences (<30 amino acids) that bear a net positive charge contributed by the side chains of cationic residues of which arginine is often the predominant species. Bioportides, CPPs that possess the dual features of both cellular penetration and biological activity, can additionally discretely modulate the activities of intracellular proteins by a dominant-negative mode of action.
Despite substantial efforts, the precise mechanisms by which CPPs and bioportides gain access to the intracellular environment remain relatively ill-defined. Subsequently, a second question arises as to whether eukaryotic cells maintain a significant cytoplasmic distribution of free peptide? Indeed, our many studies with fluorescent CPPs and bioportides indicate that such peptides access discrete intracellular environments where they are differentially sequestered in a temporal manner. Homing motifs can also be chimerically applied to radically manipulate the intracellular distribution of some bioportides. Hence, we propose that the general utility of both CPP and bioportide technologies is mostly determined not by a cytoplasmic pool of material but is a consequence of the intracellular localisation of peptides at specific sites. Moreover, the intracellular targets where CPPs and bioportides accumulate include both organelles (nucleus and mitochondrion) and macromolecular protein complexes.
Most recently, we have identified inert CPPs able to access discrete pools of secretory mediators in mast cells (MCs) as the basis of an innovative cell-mediated therapy.1 Rationally-designed bioportides, including proteomimetic sequences within Stromal Interaction Molecule 1 (STIM1) and Phosphoprotein Phosphatase 1 (PPP1γ2), can respectively modulate calcium transients and abolish motility in human spermatozoa, a highly differentiated cell type that lacks endocytic machinery.2 There is, therefore, almost unlimited potential to further exploit both CPP and bioportide technologies even in MCs, which are usually sensitive to polycationic peptide secretagogues, or spermatozoa that are relatively impermeable to a majority of biochemical agents.