|John F. Blazyk
277 Clippinger Laboratories
Since most living organisms are very similar at the molecular level, it is difficult to find substances which are lethal to certain organisms without being harmful to others. Antibiotics are drugs that attack bacteria or viruses but not host cells. One example is penicillin, which blocks cell wall synthesis in susceptible bacteria. Since human cells do not possess a cell wall and bacterial cell wall structure is unique, penicillin is a clinically useful antibiotic. Unfortunately, many bacteria are resistant to penicillin and other antibiotics. The widespread use of common antibiotics increases the number of resistant organisms, posing a health risk and creating a challenge to develop novel agents to thwart virulent organisms.
The discovery of a variety of naturally occurring antimicrobial peptides opens a new dimension for antibiotic development. Magainins are small cationic peptides recently isolated from frog skin which possess antimicrobial activity by virtue of their ability to selectively disrupt bacterial membranes. PGLa, also produced in frog skin, has greater antimicrobial activity than magainins. These peptides can kill a wide range of microorganisms, including Gram-positive and Gram-negative bacteria, fungi, parasites, and enveloped viruses, without harming mammalian cells. In addition, some can selectively destroy tumor cells. These naturally occurring defense molecules are among the simplest and smallest found in vertebrates. A critical factor to be elucidated is the molecular basis for selectivity between (1) bacterial and mammalian cells and (2) tumor and normal cells. Our hypothesis is that differences in lipid distribution and cholesterol content in the plasma membranes of these cells determines whether or not these peptides can permeabilize the cell. Understanding the mode of action of magainins and related peptides will lead to the design and synthesis of new therapeutic agents for the treatment of infection and cancer.
A common feature of these peptides is their capacity to form an amphipathic alpha-helix (with polar and nonpolar groups on opposite faces of the helix), a structural feature believed to be important in their function as antimicrobial agents. Numerous analogues with sequences derived from these peptides have been prepared and examined. In nearly all cases, the strategy employed in enhancing activity involved increasing the amphipathic alpha-helical character of the peptide. Recently, we designed a peptide that has no potential to form an amphipathic alpha-helix, but it can form a highly amphipathic beta-sheet. This peptide has antimicrobial activity equivalent to that of alpha-helix-forming peptides. We now are testing ways by which we can enhance the potency and facilitate the delivery of these peptides.
To date, no one has attempted a comprehensive study of the structure-function relationships of a family of closely related linear peptides with simple sequences designed to adopt different secondary structures. This approach will reveal the importance of different secondary structures with varying levels of amphipathic character in determining antimicrobial activity and selectivity between bacterial and mammalian membranes. Using two families of peptides with varying capacity to form amphipathic alpha-helical and beta-sheet structures, we will investigate the following areas: 1) the relationship of secondary structure and amphipathic character of the peptides to antimicrobial activity; 2) the amount of peptide that must bind to the membrane in order to increase permeability; and 3) the role of the membrane lipid composition in determining susceptibility to peptide-induced increase in permeability. Our ultimate goal is to design more effective antimicrobial peptides that will augment the arsenal of available antibiotics in order to keep pace with the ever increasing resistance of bacteria to the drugs in current use.
A new monofluorinated phosphatidylcholine forms interdigitated bilayers, D.J. Hirsh, N. Lazaro, L.R. Wright, J.M. Boggs, T.J. McIntosh, J. Schaefer, and J. Blazyk, Biophys. J., 75, 1858 (1998).
Insertion of magainin into the lipid bilayer detected using lipid photolabels, E. Jo, J. Blazyk, and J.M. Boggs, Biochemistry, 37, 13791 (1998).
Conformation and activity of antimicrobial peptides related to PGLa, J. Blazyk, A. Kearns, J. Hammer, L. Maloy, and U. P. Kari, in Peptides: Frontiers of Peptide Science, J.P. Tam and P.T.P. Kaumaya, eds., Kluwer Academic Publishers, Dordrecht, 385 (1999).
The phase behavior of monofluorinated and difluorinated phosphatidylcholines: increased thermal stability via interdigitation? N. A. Cartwright and J. Blazyk, Biophys. J., 78, 489A (2000).
Hyperactive cationic antimicrobial peptides related to PGLa: how important is secondary structure? J. Blazyk, R. Wiegand, J. Klein, W. L. Maloy, and U. P. Kari, Biophys. J., 78, 321A (2000).