Novel human-derived cell-penetrating peptides for specific subcellular delivery of therapeutic biomolecules

Cellular delivery of small interfering RNA by a non-covalently attached cell-penetrating peptide: quantitative analysis of uptake and biological effect

Cell-penetrating peptides (CPPs) have evolved as promising new tools to deliver nucleic acids into cells. So far, the majority of these delivery systems require a covalent linkage between carrier and cargo. To exploit the higher flexibility of a non-covalent strategy, we focused on the characterisation of a novel carrier peptide termed MPGα, which spontaneously forms complexes with nucleic acids. Using a luciferase-targeted small interfering RNA (siRNA) as cargo, we optimised the conditions for MPGα-mediated transfection of mammalian cells. In this system, reporter gene activity could be inhibited up to 90% with an IC₅₀ value in the sub-nanomolar range. As a key issue, we addressed the cellular uptake mechanism of MPGα/siRNA complexes applying various approaches. First, transfection of HeLa cells with MPGα/siRNA complexes in the presence of several inhibitors of endocytosis showed a significant reduction of the RNA interference (RNAi) effect. Second, confocal laser microscopy revealed a punctual intracellular pattern rather than a diffuse distribution of fluorescently labelled RNA-cargo. These data provide strong evidence of an endocytotic pathway contributing significantly to the uptake of MPGα/siRNA complexes. Finally, we quantified the intracellular number of siRNA molecules after MPGα-mediated transfection. The amount of siRNA required to induce half maximal RNAi was 10 000 molecules per cell. Together, the combination of methods provided allows for a detailed side by side quantitative analysis of cargo internalisation and related biological effects. Thus, the overall efficiency of a given delivery technique as well as the mechanism of uptake can be assessed.

Cell penetration properties of maurocalcine, a natural venom peptide active on the intracellular ryanodine receptor

Maurocalcine (MCa) is a 33-amino acid residue peptide toxin initially isolated from the scorpion Scorpio maurus maurus. Its structural and functional features make it resembling many Cell Penetrating Peptides. In particular, MCa exhibits a characteristic positively charged face that may interact with membrane lipids. External application of MCa is known to produce Ca2+-release from intracellular stores within seconds. MCa binds directly to the skeletal muscle isoform of the ryanodine receptor, an intracellular channel target of the endoplasmic reticulum, and induces long-lasting channel openings in a mode of smaller conductance. The binding sites for MCa have been mapped within the cytoplasmic domain of the ryanodine receptor. In this manuscript, we further investigated how MCa proceeds to cross biological membranes in order to reach its target. A biotinylated derivative of MCa (MCab) was chemically synthesized, coupled to a fluorescent streptavidin indicator (Cy3 or Cy5) and the cell penetration of the entire complex followed by confocal microscopy and FACS analysis. The data provide evidence that MCa allows the penetration of the macro proteic complex and therefore may be used as a vector for the delivery of proteins in the cytoplasm as well as in the nucleus. Using both FACS and confocal analysis, we show that the cell penetration of the fluorescent complex is observed at concentrations as low as 10 nM, is sensitive to membrane potential and is partly inhibited by heparin. We also show that MCa interacts with the disialoganglioside GD3, the most abundant charged lipid in natural membranes. Despite its action on ryanodine receptor, MCa showed no sign of cell toxicity on HEK293 cells suggesting that it may have a wider application range. These data indicate that MCa may cross the plasma membrane directly by cell translocation and has a promising future as a carrier of various drugs and agents of therapeutic, diagnostic and technological value.

Mechanistic aspects of CPP-mediated intracellular drug delivery: relevance of CPP self-assembly

In recent years, cell-penetrating peptides have proven to be an efficient intracellular delivery system. The mechanism for CPP internalisation, which first involves interaction with the extracellular matrix, is followed in most cases by endocytosis and finally, depending on the type of endocytosis, an intracellular fate is reached. Delivery of cargo attached to a CPP requires endosomal release, for which different methods have recently been proposed. Positively charged amino acids, hydrophobicity and/or amphipathicity are common to CPPs. Moreover, some CPPs can self-assemble. Herein is discussed the role of self assembly in the cellular uptake of CPPs. Sweet Arrow Peptide (SAP) CPP has been shown to aggregate by CD and TEM (freeze-fixation/freeze-drying), although the internalised species have yet to be identified as either the monomer or an aggregate.