Cell-penetrating Peptide - Applications of CPPs - CPP As Contrast Agents Transporters

CPP As Contrast Agents Transporters

CPP found applications as transporters of contrast agents across plasma membrane. This contrast agents are able to label the tumor cells, making this compounds important tools in cancer diagnosis; they are also used in vivo and vitro experiments cellular experiments. The most important classes of CPP are isolated from viruses, such as TAT (Transactivated-transcription) derived from HIV-1, penetratin, transportan. The most widely used CPPs are based on TAT derivatives. TAT, is an arginine-rich CPP. Several improvements for this substrate includes the usage of unnatural β or γ amino acids. This strategy offers multiple advantages such resistance to proteolytic degradation a natural degradation process by which peptide bond is hydrolyzed to amino acid. Unnatural acid insertion in the peptide chain has multiple advantages. It facilitate the formation of stable foldamers with distinct secondary structure. β-Peptides are conformationally more stable in aqueous solution than naturally occurring peptides, especially for small chains. The secondary structure is reinforced by the presence of a rigid β-amino acid, which contains cyclohexane or cyclopentane fragments. These fragments generate a more rigid structure and influence the opening angle of the foldamer. These features are very important for new peptide design. Helical β-peptides mimic antimicrobial activities of host defense peptides. This feature requires the orientation of cationic –hydrophilic on one side, and hydrophobic residues on the other side of the helix. The attachment of fluorescent group on one head of the molecule confers contrast properties. A new strategy to enhance the cellular up-take capacity of CPP is based on association of polycationic and polyanionic domains that are separated by a linker. Cellular association of polycationic residues (polyarginine) with negatively charged membrane cells is effectively blocked by the presence of polyanionic residue (poly-glutamic acid) and the linker, which confer the proper distance between these two charged residues in order to maximize their interaction. These peptides adopt hair spin structure, confirmed by overhauser effect correlation for proton-proton proximities of the two charged moieties. At this stage only the linker is exposed to protease hydrolysis in vivo applications. The linker hydrolysis occur and the two charged fragments experience more conformational freedom. In the absence of linker, the cationic peptide can interact more efficient with the target cell and cellular uptake occurs before proteolysis. This strategy found applications in labeling tumor cells in vivo. Tumor cells were marked in minutes. Linker degradation can be predicted by the amount of D-aminoacids (the unnatural isomer) incorporated in the peptide chain, this restricts in vivo proteolysis to the central linker.