By David N. Leff
If you go to Europe, you might run across an Old World insect called the European Sap-Sucking Bug. "This is a black bug with red dots, about twice the size of an American ladybug," said physical organic chemist Laszlo Otvos Jr., of the Wistar Institute, in Philadelphia. "In Hungary," he added, "they call it the Shoemaker Bug."
Otvos explained what a physical chemist is doing deep in the field of entomology: "We are working on a peptide that is secreted by the European Sap-Sucking Bug, Pyrrhocoris apterus. And we're also researching similar peptides from fruit flies, ants and honey bees.
"Like all insects," he related, "these have similar but not identical immune defenses against attack by bacteria and fungi. Their counterattack peptides are similar to the innate immune defenses in mammals, including humans.
"Mammals," Otvos said, "depend mainly on the so-called adaptive immunity - which means generating antibodies and T cells. But we also have so-called innate immunity, which is not dominant in mammals. Actually, this third immune defense arm in mammals is also based on antibiotic peptides, such as the defensins in your saliva. This means that the pathogen comes and turns on certain genes that express antibacterial and antifungal peptides. It's called innate because you already have the genes; they just need to be activated." (See BioWorld Today, Oct. 17, 2000.)
Otvos made the point that "insects rely 100 percent on innate immunity." He is senior author of a paper in the American Chemical Society's journal, Biochemistry, released electronically on Oct. 21, 2000. Its title: "Interaction between heat shock proteins and antimicrobial peptides."
"It shows for the first time," he told BioWorld Today, "that antibacterial peptides, originating from insects, bind to the bacterial heat shock protein, DnaK." Citing a new discovery he made subsequent to submitting his journal paper, Otvos added, "It makes a connection between bacterial killing and association with DnaK. That is, these insect peptides are the first molecules that would inhibit protein folding. It means that the bacteria will be unable to generate active enzymes and other proteins."
Compounds Not Toxic For Humans
"And since these insect-derived peptides do not bind to a human equivalent because there are major sequence dissimilarities between mammalian and bacterial heat shock proteins," he noted, "these insect peptides are not toxic for humans. This basically paves the road toward designing a new generation of antibacterial and antimicrobial compounds."
Otvas explained: "The antibacterials that we know act on the one hand on the bacterial membranes. Others in general have protein targets. We knew for a long time that some antibacterial insect peptides have protein targets in the pathogens that infect them. But none of these peptides ever reached the in vivo stage, where we could demonstrate that they actually could protect mammals from bacterial infection.
"The beauty of this work," he said, "is first of all that we now have a family of peptides that are active in vivo - in mice - and also that we have identified a target bacterial protein these peptides can inhibit. This means we can block a number of functions - most significantly, protein folding. That means it is specific to bacteria, a phenomenon unknown so far."
Heat shock genes express their proteins (DnaK in bacteria) to cool down sudden spikes of high temperature, such as infections cause. Excessive fevers distort all the cells in the body, and impair their function. Heat shock proteins correct the crisis, and restore the misshapen cells to functionality. The way Otvos's peptides kill bacteria is by wrecking the heat shock protein's repair system.
It worked in mice.
"We selected pyrrhocorisin, the native peptide from this sap-sucking bug," Otvas recounted, "and added another one which had some sequence modifications in it for enhanced stability. First we infected mice with Escherichia coli bacteria. After infection, we administered two shots of pyrrhocorisin or its analog, which protected the mice. They survived. Control animals, which got only the E. coli challenge, died or were clinically sick.
"This was a tremendous result," Otvas observed, "because it means these peptides have great potential to develop as a systemic antibiotic drug."
Can It Escape Antibiotics' Exploding Drug Resistance?
One of the co-authors' hopes is that their prototypical antibiotic will escape the drug resistance tide that currently threatens the control or cure of pathogenic infections worldwide. "Certainly, antibacterial resistance is dangerous everywhere," Otvas said. "In our peptides I see it as less of a danger because we are attacking a housekeeping protein of the bacterium. So to change the structure of that protein cannot be that easy for the pathogen.
"But the biggest problem or danger that these peptides - and all peptides - have is their proneness to proteolytic cleavage. The peptides are just deactivated by chopping them apart. And I know that some bacteria elevate the level of their proteolytic activity in the presence of antibacterial peptides - but not our peptides. That's why we modified their sequences to make them more resistant to proteolysis. So the bacteria cannot just increase the level of their heat shock protein and enzyme production, and deactivate the peptides. So I do not expect that much of a drug resistance as we know it now."
But he and Wistar are encountering a different kind of resistance - from industry.
"In my lab," Otvos observed, "we have a few industrial connections. But we have just one in vivo experiment on a single strain to show them. They want to see an application across the bacterial strain spectrum. So for that I need to repeat a number of potentially clinically relevant strains, to provide a deeper set of in vivo data.
"We have a number of pharmaceutical and biotech companies that have expressed interest in this work. Wistar has two patent applications pending - one on the ligands, one on the target proteins - and cannot yet offer to license our technology. We are trying to license this out and potentially get research funding." The companies are very sympathetic, he said, "but tell us, 'we can continue talking after you have additional data.'"