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  The interior of all living cells, the cytoplasm, is surrounded by a thin molecular layer called the plasma membrane. This membrane serves to separate a cell from its surrounding environment protecting it from changes in the chemical and physical environment, and regulates the traffic of molecules into and out of the cell. Although flexible and exceedingly thin (~ 6 nm) the plasma membrane is very strong. It is made mostly from a bilayer (double layer) of phospholipid molecules. The membrane can be roughly thought as a wall with a hydrophobic interior that excludes water and hydrophilic molecules. The hydrophobic core of the phospholipid bilayer forms an extremely effective seal and only very few molecules can pass directly through the lipid bilayer to get from one side of the membrane to the other. Many substances that the cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose (a sugar that cells burn for energy), amino acids (the building blocks of proteins), and ions, such as sodium and potassium. Embedded within this membrane there are a variety of protein molecules that act as channels and pumps that move these vital molecules into and out of the cell. Therefore is said that the cell is selective permeable, able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival.


   Living organisms have developed different strategies to overcome this barrier for different purposes. For example, some viruses, like the HIV virus, use the own host cell receptors to insert into it and use the structure of the host cell for replication. Antimicrobial peptides (APs) are used by organisms to destroy bacteria by forming pores that permeabilize the plasma membrane destroying vital chemical gradients across the cell. These Antimicrobial peptides are composed of short sequences of a combination of hydrophilic and hydrophobic amino acids. It is believe that these peptides are able to interact favorably with the plasma membrane thanks to their high content of hydrophobic amino acids. It has been observed that these peptides are even able to cross the bacterial membrane on their own after forming these transient pores.


   Recently it has been discovered a family of short peptides highly hydrophilic that also seem to cross the plasma membrane on their own. These peptides are called “Cell-Penetrating Peptides” (CPPs), or the more pictorial name “Trojan Peptides”. The first discovered peptide of this family was the HIV-1 TAT peptide. More than ten years has passed since then and several other CPPs have been discovered, some naturally occurring and some synthetic. This finding has puzzled scientist since their discovery.


   A large difficulty for achieving therapeutic efficiency in many drugs lays in travelling trough the aqueous environment to reach the cell and then cross the hydrophobic barrier of cellular membranes. Since these peptides are extremely soluble and at the same time are able to cross these barriers, they are very attractive candidates for delivering therapeutic agents into the cell. This is being successfully applied to deliver a wide range of molecular compounds into the cell such as proteins, oligonucleotides, and even 200 nm large liposomes.


Authors: Niyada (E), Choi (S Y), ()

Keywords: , AIDS Vaccines/*therapeutic use, Amino Acid Sequence, Animals, Autoimmune Diseases/*prevention & control, Base Sequence, Tay-Sachs Disease/*genetics/metabolism/pathology/*therapy, beta-N-Acetylhexosaminidases/deficiency/genetics/metabolism

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