Frog Skin Chemicals Block HIV in Lab Study
NASHVILLE, Tennessee, September 30, 2005 (ENS) - Compounds secreted by frog skin are potent blockers of HIV infection, scientists at Vanderbilt University Medical Center in Nashville have discovered. The findings could lead to topical treatments for preventing transmission of HIV, the virus that causes AIDS, and they underline the value of preserving the Earth's biodiversity.
"We need to protect these species long enough for us to understand their medicinal cabinet," said Louise Rollins-Smith, Ph.D., associate professor of microbiology and immunology, who has been studying the antimicrobial defenses of frogs for six years.
Frogs, she explained, have specialized granular glands in their skin that produce and store packets of peptides, small protein-like molecules.
In response to skin injury or alarm, the frog secretes large amounts of these antimicrobial peptides onto the surface of the skin to combat pathogens like bacteria, fungi and viruses.
Rollins-Smith and her colleagues creened 15 antimicrobial peptides from a variety of frog species for their ability to block HIV infection of T cells, which are immune system cells targeted by HIV. They found several that inhibited HIV infection without harming the T cells.
Colleague Derya Unutmaz, M.D., associate professor of microbiology and immunology at Vanderbilt, says the peptides appear to selectively kill the virus, perhaps by inserting themselves into the HIV outer membrane envelope and creating "holes" that cause the virus particle to fall apart.
Dendritic cells, he explained, are the sentinels of the immune system. They hang out in the mucosal surface tissues, scanning for invading pathogens.
"Their purpose in life is to capture the enemy, bring it to the lymph node - the command center - and present it to the general, the T cell, to activate a battle plan," Unutmaz said. "It's a very efficient system that has allowed us to survive many insults, pathogens and viruses."
But HIV is a survival expert. When it is picked up by a sentinel dendritic cell, it somehow evades destruction. Instead, it hides inside the dendritic cell, waiting to invade the T cell.
This mechanism, "may explain why after 20 years we don't have a vaccine for this virus," Unutmaz said.
To test the effectiveness of the frog peptides in preventing HIV transmission, postdoctoral fellow Scott VanCompernolle first allowed cultured dendritic cells to capture active HIV. He then incubated the HIV-harboring dendritic cells with antimicrobial peptides, washed the peptides away, and added T cells.
"Normally the dendritic cell passes the virus to the T cell, and we get very efficient infection of the T cell," Unutmaz said. "But when we treated the dendritic cells with peptides, the virus was gone, completely gone. This was a great surprise."
The finding was puzzling, he said, since the prevailing idea is that HIV captured by dendritic cells is hidden and protected. The investigators currently are using imaging technologies to test the hypothesis that HIV is actually cycling to the dendritic cell surface.
"We think maybe it's popping its head out, looking around for a T cell, and then going back inside to hide until it cycles out again," Unutmaz said. If peptide is present outside the cell, "it targets the virus that pops up and kills it." Preliminary experiments suggest that the hypothesis is correct.
"This is very exciting, as it suggests that these peptides could be very effective since the virus now has nowhere to hide," Unutmaz said. "And if this cycling is really happening, we may be able to generate a vaccine that will target virus captured by dendritic cells."
The investigators learned this week that the American Foundation for AIDS Research will fund their continuing quest to understand how the frog peptides kill HIV in dendritic cells.
Their plans include imaging how the peptides work, screening additional frog peptides for activity, and testing peptides on a mucosal cell system to study the feasibility of developing them as preventives against HIV infection.
"If we are able to learn the mechanisms these peptides are using to kill HIV, it might be possible to make small chemical molecules that achieve the same results," Unutmaz said. Such chemicals would be more practical as therapeutic microbicides, he said. "This study is a great example of how collaboration across disciplines leads to big discoveries," Unutmaz said.
Frogs around the world are in trouble. According to the IUCN Red List of Threatened Species, 1,856 of the 5,743 known amphibian species - almost one in three - are threatened with extinction. By comparison, one in eight birds face a similar level of threat, and one in four mammals.
Scientists said last week that 122 species, perhaps many more, appear to have gone extinct since 1980. Further research may increase this number, since 23 percent of all species were classified as data deficient.
And as frog species are lost, so are their biological treasures.
"Amphibian skin has long been favored in folklore for its medicinal properties," said Rollins-Smith, who is principal investigator for the new grant. "Frogs are a rich source of potentially useful molecules that might work against human pathogens."
With the new grant, Rollins-Smith and her team will be investigating the antimicrobial defenses of declining frog populations that are facing a particular skin fungus. Postdoctoral fellow Douglas Woodhams will be traveling to sites in Panama and in California to collect samples of the skin peptides from affected frogs.
"Our goal is to study frog populations that are ahead of an epidemic of this fungus, and those that are behind an epidemic to see if the ones that have survived have beneficial protective peptides," Rollins-Smith said.
The Mass Spectrometry Research Center at Vanderbilt is particularly valuable to the team's studies. Using mass spectrometry, it is possible to characterize the array of peptides in the samples and rapidly focus on and sequence those that might be antimicrobial.
"We hope to figure out which species are most vulnerable to this fungal pathogen so that they can be the focus of greater conservation efforts," Rollins-Smith said.
The studies may also reveal new antimicrobial peptides which could be useful blockers of HIV and other human diseases.