Frogs get the jump on microbes.

Frogs get the jump on microbes

While staring at a frog recovering from a surgical procedure last summer, microbiologist Michael Zasloff took a leap of thought that led to the discovery of a fastacting family of microbe-killing perptides. The frogs used in Zasloff's genetics studies at the National Institute of Child Health and Human Development swim in water teeming with bacteria, fungi and parasites. Yet in spite of the incisions and sutures giving the microbes a perfect entree into the frogs' bodies, the invaders appear to be thwarted because the frogs heal normally, without a trace of infection.

As described in the August PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 84, No. 15), Zasloff first posited and then showed that something in the frogs' skin staves off microbial attack. The something turned out to be two 23-amino-acid peptides that Zasloff says are functionally distinct from any substance so far described in animals. Moreover, their discovery in frogs represents the first time a chemical defense system outside of the immune system has been identified in vertebrates. Zasloff suspects that similar peptides--which he has dubbed "magainins' after the Hebrew word magain, or shield--will be found in humans. If so, they may help scientists understand and treat a variety of diseases and conditions.

In low concentrations, the frog magainins kill various (though not all) types of invading bacteria, fungi and protozoans, but they leave human red blood cells intact. Zasloff says he doesn't know how the magainins are able to distinguish between the invaders and the "good' cells indigenous to the body. But he does have some clues to how they attack the infecting microbes.

The peptides are amphiphilic: Half the molecule is oily and half is water-loving. This enables them to coil into structures that are ideal for interacting with membranes. When Zasloff exposed a paramecium to magainins, it swelled and burst within seconds.

"What exactly [the magainins] are doing to membranes we don't know,' he says. "But in [the paramecium] they are dramatically affecting water flux.' His laboratory is now testing magainin potency against viruses and cancer cells.

Zasloff suspects that magainins may be the vertebrate counterpart of cecropins, 37-amino-acid-long antibacterial peptides found circulating in insects such as the Cecropia moth, which lacks both lymphocytes and antibodies. "This is almost certainly part of the invertebrate defense system generally,' says Zasloff. "It was thought that things like this would not be found in vertebrates. But [magainins] are a major component of the frog's skin and [probably] circulate throughout its body.'

Zasloff believes that magainins in the frog represent a first-line defense system developed very early in evolution. How, he asks, did simple metazoan creatures, devoid of white cells or other immune system elements, survive the microbial soup in which they lived? "Why weren't they seen as pieces of meat [by the microbes]?' Like these creatures, frogs, which survived millions of years in water and which now have immune systems similar to ours, must also have had some primitive way of protecting themselves or they wouldn't have survived, he adds.

If frogs possess magainins, why not humans? Zasloff notes that the frog magainins are made throughout the animal's skin by the granular gland, which also produces neurotransmitters and hormones. Many proteins isolated from this gland, he says, have been found to have analogs in the gut and nervous systems of mammals. There's a good chance, he says, that "we're going to find the same antimicrobial peptides in those sites in mammals, including humans.'

Zasloff has cloned the genetic message of the frog magainins and is using these clones as probes to search for similar peptides in humans. If they are found, they might explain how corneas and other body parts are able to heal without the inflammation associated with white blood cells.

The discovery of magainins in frogs may explain the frog's popularity in folk medicine. Zasloff says he's been told, for example, that people living in Argentine villages commonly strap a frog onto wounds to help them heal. Perhaps this practice induces the frog to release magainins in the same way Zasloff has found that a shot of adrenaline will cause frogs in his laboratory to release the contents of their granular glands. The finding also suggests that synthesized magainins may have medicinal uses such as the treatment of burns.

But what excites Zasloff most about his discovery is that it might offer a long-sought explanation of cystic fibrosis. From about the time of birth, the lungs of children with this inherited disease become colonized by bacteria, but for some reason the bacteria do not infect the rest of the body. Moreover, mucus-secreting cells in the children's airways (and other secreting cells elsewhere) have difficulty transporting water and ions across their membranes. If magainins, with their antibacterial and membrane-damaging abilities, and found in humans and especially in the respiratory tract, he says, "my gut sense is that a defect or inteference in [the magainin system] could underlie some of the symptomatology of cystic fibrosis.'

As a pediatrician as well as a microbiologist, Zasloff says he has been plagued by the cystic fibrosis question for a decade. With the frog magainins, he thinks he has at last found an experimental entry into the study of the disease.