New insights into insect antimicrobials point the way to novel antibiotics

The emergence of antibiotic-resistant strains of bacteria has become a serious public-health concern, and, accordingly, scientists are investigating new classes of antimicrobials for their efficacy against disease-causing bacteria. One developing area of study involves antimicrobial peptides derived from insects. Recent studies have identified the protein target in bacteria of these antimicrobial peptides and suggested that the peptides are not toxic to mammalian cells including those of humans, raising the possibility that they could someday be used to develop new antibiotic drugs.

Now, in a new study of an insect-derived antimicrobial peptide called pyrrhocoricin, scientists at The Wistar Institute have identified which segments of the peptide are necessary for the killing of bacteria and which segments are involved in bacterial and mammalian cell entry. The Wistar scientists further confirmed that this antimicrobial peptide must bind to the previously identified intracellular bacterial protein target in order to kill bacteria. The research team also identified a possible binding site for the antimicrobial peptide on the target bacterial protein for the first time.

Because the stretches of the peptide that are responsible for cell entry are separate from the segments responsible for bacteria killing, the research team says that it might be possible to use an altered version of the peptide as a delivery vehicle for a variety of drugs into human cells, rather than solely as an antimicrobial. The results are published online today in the European Journal of Biochemistry.

“This study lays the groundwork for the design of a novel family of antimicrobials,” says Laszlo Otvos Jr., Ph.D., associate professor at The Wistar Institute and senior author of the study. “It also suggests that these peptides could be used as a universal drug delivery vehicle, whether for new drugs or to improve the delivery of existing peptide-based drugs.”

The antimicrobial peptide kills bacteria by binding to a protein target called DnaK. DnaK is a special type of protein called a heat-shock protein, responsible for correcting misshapen proteins. When the antimicrobial peptide binds to DnaK, it prevents DnaK from doing its protein-repair work, killing the bacteria.The Wistar research team studied the binding of engineered analogs of pyrrhocoricin to a series of bacterial strains. As they anticipated based on their previous investigations, they found a complete correlation between the peptide binding to a small fragment of bacterial DnaK and bacteria killing. The researchers also confirmed that the peptide does not bind to the mouse or human protein equivalents to DnaK, further suggesting that the peptide would not be toxic to mammals.

The investigators identified a possible binding surface for the antimicrobial peptide on an E. coli DnaK fragment. Knowledge of this binding site could lead to the development of new drugs tailored to combat E. coli. It may also be possible to develop drugs that would kill bacteria that are unresponsive to native pyrrhocoricin, but for which the DnaK structure is known.

In related ongoing studies, Otvos and his team have shown that analogs of pyrrhocoricin are able to kill clinical strains of resistant bacteria that cause urinary, gastrointestinal and respiratory-tract infections. In a mouse H. influenzae lung infection model, the researchers have shown that a pyrrhocoricin analog can dramatically reduce bacterial counts in the lungs and be administered in a non-invasive way. These studies are demonstrating that engineered antibacterial peptides can be used in a clinical setting against bacteria with resistance to existing antibiotics.

In addition to senior author Otvos, the lead author of the study is Goran Kragol, Ph.D., and co-authors are Michael A. Chattergoon, B.S., Mare Cudic, Ph.D., Barry A. Condie, B.S., and associate professor Luis J. Montaner, D.V.M., D.Phil., all of The Wistar Institute. Additional co-authors are Ralf Hoffmann, Ph.D., of Heinrich-Heine-Universität, Sandor Lovas, Ph.D., of Creighton University, Philippe Bulet, Ph.D., of Institut de Biologie Moleculaire et Cellulaire, and K. Johan Rosengren, Ph.D., of the University of Queensland.

The Wistar Institute is an independent nonprofit biomedical research institution dedicated to discovering the causes and cures for major diseases, including cancer, cardiovascular disease, autoimmune disorders, and infectious diseases. Founded in 1892 as the first institution of its kind in the nation, The Wistar Institute today is a National Cancer Institute-designated Cancer Cente- one of only eight focused on basic research. Discoveries at Wistar have led to the development of vaccines for such diseases as rabies and rubella, the identification of genes associated with breast, lung, and prostate cancer, and the development of monoclonal antibodies and other significant research technologies and tools.