Fungused frogs fail to fix fast fluid flux

Amphibian populations are facing a crisis – the chytrid fungus Batrachochytrium dendrobatidis is responsible for the decline of more than 500 species around the world. This fungal infection targets the skin of amphibians, making it leaky and susceptible to water loss. To fight back, amphibians shed their outer layer of skin – called sloughing – hopefully removing the fungus in the process. However, skin sloughing itself can also interfere with water balance, as water moves easily through the highly permeable fresh skin. In healthy amphibians, a compensatory mechanism seems to exist, but chytrid-infected amphibians are not so lucky, and perhaps skin sloughing is to blame.A new study, led by Nicholas Wu at the University of Queensland, Australia, aimed to discover the molecular mechanisms used to regulate water balance during amphibian skin sloughing and asked whether chytrid interfered with this process. First, the authors compared how fast thirsty green tree frogs absorbed water through their skin, and found that the rate depended on the degree of chytrid infection. The more infected the frog, the slower the rate of water absorption, suggesting that the skin was impaired in some way.Next, the authors collected samples of frog skin and used radioactive water to measure the flow rate across the skin in each direction. Chytrid infection had no effect on inward water flow, but almost doubled the flow of water out of the frogs. To understand why outward water flow in infected frogs was so much faster, Wu and colleagues zeroed in on a group of protein water channels known as aquaporins. Salts constantly leak out of frogs across the skin into the environment, and this movement of salt tends to pull water along with it; aquaporins are thought to fight back against this process to help maintain water balance in the frogs. The team used mercury chloride to disrupt the aquaporin proteins in the skins of infected frogs and found that the rate of water loss across the skin increased, consistent with the idea that aquaporins are key to reducing outward water flow. Intrigued, the authors next measured the expression levels of genes that encode aquaporin proteins in healthy and infected frogs before and immediately after skin sloughing. In healthy frogs, the aquaporin gene expression levels increased as much as 45-fold after sloughing, suggesting that this is a key mechanism that maintains water balance during the sensitive period. However, the increase in aquaporin gene expression was completely absent in the infected frogs, which may explain why they lose water so much faster.The team also knew that water can move through the tiny gaps between skin cells and animals can slow this movement by tightening those inter-cellular gaps using junction proteins. To examine whether frogs use this tightening to conserve water after skin sloughing, the authors again measured gene expression levels. In healthy frogs, the junction protein gene expression levels increased after sloughing, suggesting that it usually works alongside increased aquaporin levels to protect the animals’ water balance. However, the junction protein gene expression levels were even higher in chytrid-infected frogs, even when they were not sloughing their skin. This is likely an attempt to compensate for the high rates of water loss after infection, but, unfortunately for the frogs, it is only a partial solution.This is the first evidence of chytrid infection directly interfering with the molecular machinery responsible for water balance in amphibian skin, but we need to examine more species to be certain. However, it appears that in green tree frogs at least, infection poses a ‘damned-if-you-do, damned-if-you-don't’ scenario. If frogs try to clear the fungus by sloughing their skins, impaired water uptake across the aquaporin-deficient fresh skin makes water balance problematic, but without sloughing, the frogs succumb to direct damage from the fungus.

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