Cell-penetrating peptide

Cell-penetrating peptides (CPPs) are short peptides that facilitate cellular intake/uptake of various molecular equipment (from nanosize particles to small chemical molecules and large fragments of DNA). The 'cargo' is associated with the peptides either through chemical linkage via covalent bonds or through non-covalent interactions. The function of the CPPs are to deliver the cargo into cells, a process that commonly occurs through endocytosis with the cargo delivered to delivery vectors for use in research and medicine. Current use is limited by a lack of cell specificity in CPP-mediated cargo delivery and insufficient understanding of the modes of their uptake, that is why other delivery mechanisms have been developed like CellSqueeze and electroporation. Cell-penetrating peptides (CPPs) are short peptides that facilitate cellular intake/uptake of various molecular equipment (from nanosize particles to small chemical molecules and large fragments of DNA). The 'cargo' is associated with the peptides either through chemical linkage via covalent bonds or through non-covalent interactions. The function of the CPPs are to deliver the cargo into cells, a process that commonly occurs through endocytosis with the cargo delivered to delivery vectors for use in research and medicine. Current use is limited by a lack of cell specificity in CPP-mediated cargo delivery and insufficient understanding of the modes of their uptake, that is why other delivery mechanisms have been developed like CellSqueeze and electroporation. CPPs typically have an amino acid composition that either contains a high relative abundance of positively charged amino acids such as lysine or arginine or has sequences that contain an alternating pattern of polar/charged amino acids and non-polar, hydrophobic amino acids. These two types of structures are referred to as polycationic or amphipathic, respectively. A third class of CPPs are the hydrophobic peptides, containing only apolar residues, with low net chargeor have hydrophobic amino acid groups that are crucial for cellular uptake. The first CPP was discovered independently by two laboratories in 1988, when it was found that the trans-activating transcriptional activator (TAT) from human immunodeficiency virus 1 (HIV-1) could be efficiently taken up from the surrounding media by numerous cell types in culture. Since then, the number of known CPPs has expanded considerably and small molecule synthetic analogues with more effective protein transduction properties have been generated. A recent discovery found that Papillomaviridae such as the Human Papillomavirus use CPPs to penetrate the intracellular membrane in order to trigger retrograde trafficking of the viral unit to the nucleus. Cell-penetrating peptides are of different sizes, amino acid sequences, and charges but all CPPs have one distinct characteristic, which is the ability to translocate the plasma membrane and facilitate the delivery of various molecular cargoes to the cytoplasm or an organelle. There has been no real consensus as to the mechanism of CPP translocation, but the theories of CPP translocation can be classified into three main entry mechanisms: direct penetration in the membrane, endocytosis-mediated entry, and translocation through the formation of a transitory structure. CPP transduction is an area of ongoing research. Cell-penetrating peptides (CPP) are able to transport different types of cargo molecules across plasma membrane; thus, they act as molecular delivery vehicles. They have numerous applications in medicine as drug delivery agents in the treatment of different diseases including cancer and virus inhibitors, as well as contrast agents for cell labeling. Examples of the latter include acting as a carrier for GFP, MRI contrast agents, or quantum dots. The majority of early research suggested that the translocation of polycationic CPPs across biological membranes occurred via an energy-independent cellular process. It was believed that translocation could progress at 4oC and most likely involved a direct electrostatic interaction with negatively charged phospholipids. Researchers proposed several models in attempts to elucidate the biophysical mechanism of this energy-independent process. Although CPPs promote direct effects on the biophysical properties of pure membrane systems, the identification of fixation artifacts when using fluorescent labeled probe CPPs caused a reevaluation of CPP-import mechanisms. These studies promoted endocytosis as the translocation pathway. An example of direct penetration has been proposed for TAT. The first step in this proposed model is an interaction with the unfolded fusion protein (TAT) and the membrane through electrostatic interactions, which disrupt the membrane enough to allow the fusion protein to cross the membrane. After internalization, the fusion protein refolds due the chaperone system. This mechanism was not agreed upon, and other mechanisms involving clathrin-dependent endocytosis have been suggested. Many more detailed methods of CPP uptake have been proposed including transient pore formation. This mechanism involves strong interactions between cell-penetrating peptides and the phosphate groups on both sides of the lipid bilayer, the insertion of positively charged arginine side-chains that nucleate the formation of a transient pore, followed by the translocation of cell-penetrating peptides by diffusing on the pore surface. This mechanism explains how key ingredients, such as the cooperation among the peptides, the large positive charge, and specifically the guanidinium groups, contribute to the uptake. The proposed mechanism also illustrates the importance of membrane fluctuations. Indeed, mechanisms that involve large fluctuations of the membrane structure, such as transient pores and the insertion of charged amino acid side-chains, may be common and perhaps central to the functions of many membrane protein functions. Endocytosis is the second mechanism liable for cellular internalization. Endocytosis is the process of cellular ingestion by which the plasma membrane folds inward to bring substances into the cell. During this process cells absorb material from the outside of the cell by imbibing it with their cell membrane. The classification of cellular localization using fluorescence or by endocytosis inhibitors is the basis of most examination. However, the procedure used during preparation of these samples creates questionable information regarding endocytosis. Moreover, studies show that cellular entry of penetratin by endocytosis is an energy-dependent process. This process is initiated by polyarginines interacting with heparan sulphates that promote endocytosis. Research has shown that TAT is internalized through a form of endocytosis called macropinocytosis.