By David N. Leff
Like two men who suddenly discover they're married to the same woman, two widely dissimilar viruses turn out to share a common receptor in the body.
Adenoviruses (AV) are in vogue as possible — some say, impossible — viral vectors for gene therapy. But their day job consists of inflicting a panoply of infections on small children. These range from acute respiratory disease and deep-throat soreness to pinkeye to gastrointestinal distress.
In fact, adenovirus is shorthand for the pathogen's full descriptive name: adenoidal-pharyngeal-conjunctival virus.
AVs are double-stranded DNA viruses that develop in the nuclei of cells. Their virions, 70 to 90 nanometers in diameter, are studded with a dozen long projections, or fibers, tipped with bulbous knobs. These are the sensor-like objects that dock with the viral receptors.
RNA Coxsackie viruses are something else again.
They were first found about half a century ago in the upstate New York town of Coxsackie, 20 miles or so south of Albany. They came to light during the great search for field isolates of the poliovirus, for which a vaccine was urgently sought.
In Coxsackie, a child with polio yielded the viral hunter-gatherers a different virus, one that didn't cause polio in their test monkeys. So this novel species -- like most new viruses -- acquired its name from the place of its discovery. It's since been found lurking all over the world.
At 28 nanometers in diameter, Coxsackie viruses are a lot tinier than AVs, but can be even more virulent. They infect the brain, the throat, the chest and particularly heart muscle, to name but a few of their target tissues.
Years ago, Coxsackie virus was isolated from the pancreas of a child with Type 1 diabetes, and the suspicion has lingered ever since that it is a trigger of this disease. Witness to this allegation is the fact that Coxsackie, like its close genetic relative, poliovirus, flares up during the summer months. So does Type 1 diabetes.
Two Vicious Viruses; One Dock In Common
With so much misery to answer for, Coxsackie is a long-standing object of infectious-disease research. Thus in 1975, microbiologist and immunologist Richard Crowell, of Hahnemann University Medical College, in Philadelphia, reported finding that Coxsackie virus and adenovirus competed with each other in attaching to cells.
In the Feb. 28, 1997 Science, Crowell appears as co-author of a paper titled: "Isolation of a common receptor for Coxsackie B viruses and Adenoviruses 2 and 5." Its first author is pediatric infectious-disease specialist Jeffrey Bergelson, of Dana-Farber Cancer Institute, in Boston.
"The adenoviral receptor," Bergelson told BioWorld Today, "is something people have been trying to identify for a long time, but no one has ever managed to do it.
"So when Crowell sent us his antibody, which we thought recognized the Coxsackie receptor, we were hoping that we could isolate it. And that if it would also be the adenovirus receptor, we would get two for one."
He added: "And AV, because of the interest in gene therapy, we thought, would be exciting at this time."
Having confirmed, as reported in Science, that AV and Coxsackie do indeed mate to the same receptor, Bergelson said, "We're now setting up collaborations with gene therapists, such as James Wilson, in Philadelphia, and Ron Crystal, in New York, to see where in the body that receptor protein and its messenger RNA are expressed.
"We hope that information will be helpful in guiding transgenes to their proper target tissues," he added.
Bergelson and his collaborators have recently cloned a murine version of the two-fer receptor. It has 91-percent sequence homology with the human receptor.
Questions To Answer; Drugs To Design
"We want to know," he said, "if the reason it infects those tissues is because the receptor is there. We can conceivably make transgenic mice that don't have the receptor in their hearts. Will they still get sick if we put it in a new place? Will they get a different kind of illness?
"With my interest in Coxsackie virus as a cause of heart disease," Bergelson observed, "we're struck by the fact that there's a lot more of the receptor's mRNA in the heart than in other tissues. So we wonder if that partly explains why Coxsackie preferentially affects the heart."
Another aspect he and his co-authors are beginning to explore is how that receptor latches on to its two competing viruses. X-ray crystallography of those AV knobs easing into their receptor should help solve that structural question.
Coxsackie virions, on the other hand, have surface indentations instead of knob-tipped fibers, so their attachment structure calls for separate elucidation.
"One possibility for limiting these viral diseases," Bergelson suggested, "is to make drugs that interfere with attachment of the virus to that receptor. Knowing the crystal structure of the AV knob, one could potentially crystallize it together with the receptor and see exactly how they fit each other, then make compounds that fit in between."
Other compounds in development, he went on, "distort the Coxsackie surface indentations, and actually get in there and change their shape. Some keep the receptors from binding; others block later stages of infection."
Two reasons the Dana-Farber group succeeded in purifying the double-duty receptor, where Crowell had failed two decades earlier, Bergelson explained, "were improved sequence technology, even in the last year or two, and because we contracted with Cellex Biosciences Inc., of Minneapolis, to grow huge amounts of the HeLa cells we needed." (Cellex operates a national animal cell culture facility for the National Institutes of Health.)
"We purified a small amount of the receptor protein," Bergelson recalled, "then used its sequence to find the cDNA that encodes the protein."
They put that gene sequence into a hamster cell that normally doesn't express the protein, and isn't infected by Coxsackie virus. With the transfected cDNA, it was able to express the receptor, as recognized by Crowell's antibody, and could be infected by the virus.
When they put AVs into those cells, it too bound to them.
"AV and Coxsackie are totally different, structurally and genetically," Bergelson concluded. "It's funny they should use the same receptor." *