Frogs may help in fight against HIV
by Leigh MacMillan,
Vanderbilt University Medical Center
October 1, 2005
A new weapon in the battle against HIV may come from an unusual source — a small tropical frog.
Investigators at Vanderbilt University Medical Center reported this month in the Journal of Virology that compounds secreted by frog skin are potent blockers of HIV infection.
The findings could lead to topical treatments for preventing HIV transmission, and they reinforce 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 A. Rollins-Smith, Ph.D., associate professor of Microbiology & Immunology, who has been studying the antimicrobial defenses of frogs for about six years. Frogs, she explained, have specialized granular glands in the 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.
Derya Unutmaz, M.D., left, Louise Rollins-Smith, Ph.D., and Scott VanCompernolle, Ph.D., discovered that compounds made by frogs block HIV infection. photo by Dana Johnson
Rollins-Smith happens to have the laboratory next door to Derya Unutmaz, M.D., associate professor of Microbiology & Immunology. During a hallway chat one day, the two decided it would be interesting to investigate whether any frog peptides have activity against human viruses, specifically HIV, the focus of Unutmaz’s group.
Postdoctoral fellow Scott E. VanCompernolle, Ph.D., screened 15 antimicrobial peptides from a variety of frog species for their ability to block HIV infection of T cells, immune system cells targeted by HIV. He found several that inhibited HIV infection without harming the T cells.
Researchers study frog peptides as anti-microbial agents, including HIV blockers
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, Unutmaz said.
“We like to call these peptides WMDs — weapons of membrane destruction,” Unutmaz said. It is curious that the antimicrobial peptides do not harm the T cells at concentrations that are effective against the virus, he noted, since HIV’s outer membrane is derived from, and therefore essentially identical to, the cellular membrane. The investigators have proposed that the peptides act selectively on the virus in part because of its small size relative to cells.
The ability of the peptides to destroy HIV was enticing, but to be really effective as antimicrobial agents, they need to prevent transmission of HIV from dendritic cells to T cells, Unutmaz said.
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 wily foe. When it is picked up at the mucosal surface by a sentinel dendritic cell, it somehow evades destruction. Instead, it hides inside the cell, waiting to invade the T cell with a Trojan Horse-like mechanism. The ability of HIV to remain hidden in the dendritic cell, avoiding destruction by circulating antibodies and immune system cells, “may explain why after 20 years we don’t have a vaccine for this virus,” Unutmaz said.
Convergent Evolution of Poison Frogs and Ants September 20, 2005
Yesterday conservation scientists proposed a $404 million effort to preserve declining global amphibian poplations. The strategy would call for funding from governments, private institutions and individual donors to finance long-term research, protect critical habitats, reduce the trade in amphibians for food and pets, and establish captive breeding programs. Earlier this year, the Global Amphibian Assessment, a survey of the planet’s amphibian species, found that nearly a third (32%) of the world’s 5743 known amphibian species are threatened and 129 species have gone extinct since 1980. Scientists believe there may be around 10,000 amphibian species on the planet.
Rainforest plants have long been recognized for their potential to provide healing compounds. Indigenous peoples of the rainforest have used medicinal plants for treating a wide variety of health conditions while western pharmacologists have derived a number of drugs from such plants. However, as forests around the world continue to fall — the Amazon alone has lost more than 200,000 miles of forest since the 1970s — there is a real risk that pharmaceutically-useful plants will disappear before they are examined for their chemical properties. Increasingly, it is becoming a race against time to collect and screen plants before their native habitats are destroyed. One near miss occurred recently with a compound that has shown significant anti-HIV effects, Calanolide A.
Crocodile blood shows anti-HIV activity August 16, 2005
Scientists in Australia’s tropical north are collecting blood from crocodiles in the hope of developing a powerful antibiotic for humans, after tests showed that the reptile’s immune system kills the HIV virus. The crocodile’s immune system is much more powerful than that of humans, preventing life-threatening infections after savage territorial fights which often leave the animals with gaping wounds and missing limbs.
Convergent Evolution of Poison Frogs and Ants August 10, 2005
Some frog species in both Madagascar and the Neotropics secrete a variety of toxic skin chemicals, called alkaloids, for protection against predation. These “poison frogs” do not produce the alkaloids, however, but instead attain them from their insect-rich diet. While Neotropic frogs are well-studied, the alkaloid sources for Malagasy frogs are unknown. Valerie Clark and colleagues extracted alkaloid samples from both Malagasy frogs and their food sources, which were determined by examining the frogs’ stomach contents. The authors found that Malagasy frogs, like their New World counterparts, acquire their alkaloids from a diet rich in ants. Thirteen of the 16 Malagasy alkaloids detected are also known to exist in insects and frogs in the Americas. Neither the frogs nor the ants in these two regions are closely related, which suggests that the evolution of acquisition mechanisms for protective alkaloids in these ant species was likely responsible for the subsequent convergent evolution of the frogs that preyed on them. Additionally, the researchers found the well-known plant alkaloid nicotine in one Malagasy frog species, suggesting a possible plant-insect-frog toxin food chain.
Study discovers why poison dart frogs are toxic August 9, 2005
Poison poison dart frogs are small, colorful frogs found in the tropical forests of Central and South America. The brilliant coloration of these amphibians warns predators of their extraordinary toxicity — the golden poison frog (Phyllobates terribilis) of Colombia is said to be lethal if held in one’s hand. Scientists have long speculated on the origin of their toxins, but now, a new study published in the current issue of The Proceedings of the National Academy of Sciences reports that poison dart frogs, as well as the Mantella poison frogs of Madagascar, derive their toxicity from the ants they eat. Specifically, both groups are frogs are capable of storing ants’ toxic alkaloid molecules in their glands without being harmed. Ants either synthesize these alkaloids themselves or acquire them from the plants on which they feed.
To test the effectiveness of the frog peptides in preventing HIV transmission, 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 added, since the prevailing notion 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 frog peptides are an exceptional tool for probing “what the virus knows about the dendritic cell that we don’t know,” Unutmaz added. “How does HIV manage to survive and cycle back and forth to the cell membrane? If we can understand that, we’ll find the gaps, and that will open a whole new universe of targets for intervention.”
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 prophylactics 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.
Other members of the Department of Microbiology and Immunology assisted the investigators by providing viruses for testing. The team found that membrane-coated viruses were susceptible to destruction by the frog peptides, but non-coated viruses, such as reovirus and adenovirus, were not affected.
R. Jeffery Taylor, Ph.D., Kyra Oswald-Richter, Ph.D., Jiyang Jiang, Ph.D., Bryan E Youree, M.D., Christopher R. Aiken, Ph.D., and Terence S. Dermody, M.D., at Vanderbilt are co-authors of the study. The research was supported by the National Institutes of Health, the Elizabeth B. Lamb Center for Pediatric Research, and the National Science Foundation.
This is an adapted press release from the Vanderbilt University Medical Center. The original version appears at Frogs may aid in HIV fight: study.