A novel linear amphipathic beta-sheet cationic antimicrobial peptide with enhanced selectivity for bacterial lipids.

All known naturally occurring linear cationic peptides adopt an amphipathic alpha-helical conformation upon binding to lipids as an initial step in the induction of cell leakage. We designed an 18-residue peptide, (KIGAKI)3-NH2, that has no amphipathic character as an alpha-helix but can form a highly amphipathic beta-sheet. When bound to lipids, (KIGAKI)3-NH2 did indeed form a beta-sheet structure as evidenced by Fourier transform infrared and circular dichroism spectroscopy. The antimicrobial activity of this peptide was compared with that of (KIAGKIA)3-NH2, and it was better than that of GMASKAGAIAGKIAKVALKAL-NH2 (PGLa) and (KLAGLAK)3-NH2, all of which form amphipathic alpha-helices when bound to membranes. (KIGAKI)3-NH2 was much less effective at inducing leakage in lipid vesicles composed of mixtures of the acidic lipid, phosphatidylglycerol, and the neutral lipid, phosphatidylcholine, as compared with the other peptides. However, when phosphatidylethanolamine replaced phosphatidylcholine, the lytic potency of PGLa and the alpha-helical model peptides was reduced, whereas that of (KIGAKI)3-NH2 was improved. Fluorescence experiments using analogs containing a single tryptophan residue showed significant differences between (KIGAKI)3-NH2 and the alpha-helical peptides in their interactions with lipid vesicles. Because the data suggest enhanced selectivity between bacterial and mammalian lipids, linear amphipathic beta-sheet peptides such as (KIGAKI)3-NH2 warrant further investigation as potential antimicrobial agents.

Human beta-defensin-1 is a salt-sensitive antibiotic in lung that is inactivated in cystic fibrosis.

A human bronchial xenograft model was used to characterize the molecular basis for the previously described defect in bacterial killing that is present in the cystic fibrosis (CF) lung. Airway surface fluid from CF grafts contained abnormally high NaCl and failed to kill bacteria, defects that were corrected with adenoviral vectors. A full-length clone for the only known human beta-defensin (i.e., hBD-1) was isolated. This gene is expressed throughout the respiratory epithelia of non-CF and CF lungs, and its protein product shows salt-dependent antimicrobial activity to P. aeruginosa. Antisense oligonucleotides to hBD-1 ablated the antimicrobial activity in airway surface fluid from non-CF grafts. These data suggest that hBD-1 plays an important role in innate immunity that is compromised in CF by its salt-dependent inactivation.

Structure-activity studies on magainins and other host defense peptides.

Host defense peptides are widely distributed in nature, being found in species from bacteria to humans. The structures of these peptides from insects, horseshoe crabs, frogs, and mammals are known to have the common features of a net cationic charge due to the presence of multiple Arg and Lys residues and in most cases the ability to form amphipathic structures. These properties are important for the mechanism of action that is thought to be a nonreceptor-mediated interaction with the anionic phospholipids of the target cell followed by incorporation into the membrane and disruption of the membrane structure. Host defense peptides have been shown to have broad spectrum antimicrobial activity, able to kill most strains of bacteria as well as some fungi, protozoa, and in addition, many types of tumor cells. Specificity for pathogenic cells over host cells is thought to be due to the composition of the cell membranes, with an increased proportion of anionic phospholipids making the pathogen more susceptible and the presence of cholesterol making the host membranes more resistant. Structure-activity relationship studies have been performed on insect cecropins and apidaecins, horseshoe crab tachyplesins and polyphemusins, and the frog magainins, CPFs (caerulein precursor fragments) and PGLa. In general, changes that increased the basicity and stabilized the amphipathic structure have increased the antimicrobial activity; however, as the peptides become more hydrophobic the degree of specificity decreases. One magainin-2 analogue, MSI-78, has been developed by Magainin Pharmaceuticals as a topical antiinfective and is presently in clinical trials for the treatment of infected diabetic foot ulcers.