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Investigating the Fungus Killing the World’s Frogs

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Starting in the late 1990s, field biologists began noticing a disturbing and ongoing trend: The world’s frogs were disappearing and dying en masse. Then in 1998, during a meeting put on by the National Science Foundation to tackle the problem, researchers began to suspect an infectious disease was at play. Later, a key suspect emerged; a pathogenic fungus called Batrachochytrium dendrobatidis (Bd). For AAAS Fellow Louise Rollins-Smith, this just raised more questions. 

“For a long time it was really a puzzle why frogs were having so much trouble dealing with a fungal pathogen,” said Rollins-Smith. “Frogs have very adequate immune systems, as complex as any higher vertebrate, man and mouse included.”

A biologist and associate professor at Vanderbilt University, Rollins-Smith has spent the nearly two decades since that NSF meeting investigating the biological mechanisms that have made Bd so detrimental to the world’s frogs. Her credits include being subject editor of the journal Diseases of Aquatic Organisms, and academic editor for PLOS ONE. Her work has laid the foundation for much of what is now known about Bd, including how the fungus has managed to confuse and bypass the amphibian immune system, as well as what natural amphibian defenses could potentially be modified in the future to help stave off the pathogen’s impact.

Raised on a dairy farm not far from Rochester, Minnesota, home of the Mayo Clinic, 67-year-old Rollins-Smith says it was the beauty of the natural world that first attracted her to biology. This love was a little hard to pin down at first and Rollins-Smith gladly studied broadly, from vertebrates to invertebrates. It wasn’t until later, while she was working on her Ph.D. in zoology at the University of Minnesota, that she finally put down her career roots. The lab she was working in was studying the development of frog embryos and how that development was able to reverse the growth of cancerous tumors. She was hooked.

“That was a big deal at the time. It was generally thought that evolved nuclei, a tumor, could never go backwards and be reverted to a more normal phenotype,” said Rollins-Smith.

As if this new finding wasn’t interesting enough for the early 1980s, the cancer Rollins-Smith was studying was caused by a virus, a phenomenon that was just beginning to be examined.

This work at the crossroads of embryology and pathology led Rollins-Smith to study how frogs’ immune systems undergo major transformations as they develop from tadpoles to adults. What happens, in effect, is that tadpoles' systems are overrun with cortical steroids, which kicks off a process that destroys their immune systems by metabolizing their white blood cells, or lymphocytes. Tadpoles do this, as the working theory goes, to conserve energy and not accidentally attack their own developing adult cells. This slash-and-burn destruction of the amphibian defenses makes them vulnerable to infection, a fact, says Rollins-Smith, that has become glaringly apparent in the Bd outbreak, which seems to be hitting juvenile frogs especially hard. But this isn’t the whole story behind Bd’s virulence.

A generalist pathogen, Bd’s origin is still unknown. What is clear is that the outbreak is tied to global trade (specifically the pet trade and frogs raised for human consumption), an assertion backed by the fact that while the pathogen is now found the world over, from parts of Central America to Australia, only a handful of species appear to have evolved immunities to the fungus.

“It’s gone into areas where frogs have no good defense against it,” said Rollins-Smith.

Bd is spread as a zoospore that attacks its host’s skin. For frogs and other amphibians, the skin acts as a kind of second kidney that helps to regulate the body’s internal chemistry. Bd cuts off the flow of the essential ions found in skin, namely sodium and potassium, a process that eventually leads to cardiac arrest. That Bd does this while pretty much unobstructed by the frog’s immune system is the pathogen’s biggest mystery. A possible answer found by Rollins-Smith and her lab is that the fungus itself seems to be producing a factor that is inhibiting the frog’s immune system.

Publishing their findings in Science in 2013, Rollins-Smith and her colleagues outlined research that suggests Bd is releasing some kind of compound from its own cell walls that appears to not only be unrecognized by the immune system’s sentinel cells (macrophages and neutrophils), but also appears to bind directly to receptors on the lymphocytes themselves, effectively inducing them to commit suicide.

“That was kind of unexpected that the fungus was making some kind of factor that was directly targeting lymphocytes,” said Rollins-Smith.

To date, the exact compound that Bd is releasing from its cell walls remains elusive to Rollins-Smith and other researchers. She and her team are currently investigating new potential compounds.

Another essential piece of Rollins-Smith’s Bd research has revolved around how the pathogen manages to get from the skin’s surface to inside the skin’s cell walls. Amphibian skin contains a thin mucous layer that’s packed with antimicrobial peptides, small chains of amino acids that attack microbial invaders. In her lab, Rollins-Smith and her colleagues conducted some of the first research examining how these peptides might interact with Bd.

Here too, the research hasn’t produced an answer to the Bd outbreak, but has produced some interesting crossover research, including that amphibian anti-microbial peptides can kill human immunodeficiency virus (HIV). Frogs have another defense mechanism as well: native microorganisms that kill invading microbes. Rollins-Smith is currently examining how the power of these beneficial microbes might be harnessed in the fight against Bd. In the meantime, she says, she will continue to investigate the outbreak, piece by piece.

“When you have a puzzle and you can do enough experiments so that you start to get some real answers, that’s very satisfying. That’s what brings us to work every day,” she said.

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Nathan Gilles