More than one billion people worldwide rely on fish as an important source of animal protein, states the United Nations Food and Agriculture Organization. And while fish provide slightly over 7 per cent of animal protein in North America, in Asia they represent about 23 per cent of consumption.
Humans consume low levels of methylmercury by eating fish and seafood. Methylmercury compounds specifically target the central nervous system, and among the many effects of their exposure are visual disturbances, which were previously thought to be solely due to methylmercury-induced damage to the brain visual cortex. However, after combining powerful synchrotron X-rays and methylmercury-poisoned zebrafish larvae, scientists have found that methylmercury may also directly affect vision by accumulating in the retinal photoreceptors, i.e. the cells that respond to light in our eyes.
Dr. Gosia Korbas, BioXAS staff scientist at the Canadian Light Source (CLS), says the results of this experiment show quite clearly that methylmercury localizes in the part of the photoreceptor cell called the outer segment, where the visual pigments that absorb light reside.
"There are many reports of people affected by methylmercury claiming a constricted field of vision or abnormal colour vision," said Korbas. "Now we know that one of the reasons for their symptoms may be that methylmercury directly targets photoreceptors in the retina."
Korbas and the team of researchers from the University of Saskatchewan including Profs. Graham George, Patrick Krone and Ingrid Pickering conducted their experiments using three X-ray fluorescence imaging beamlines (2-ID-D, 2-ID-E and 20-ID-B) at the Advanced Photon Source, Argonne National Laboratory near Chicago, US, as well as the scanning X-ray transmission beamline (STXM) at the Canadian Light Source in Saskatoon, Canada.
After exposing zebrafish larvae to methylmercury chloride in water, the team was able to obtain high-resolution maps of elemental distributions, and pinpoint the localization of mercury in the outer segments of photoreceptor cells in both the retina and pineal gland of zebrafish specimens. The results of the research were published in ACS Chemical Biology under the title "Methylmercury Targets Photoreceptor Outer Segments".
Korbas said zebrafish are an excellent model for investigating the mechanisms of heavy metal toxicity in developing vertebrates. One of the reasons for that is their high degree of correlation with mammals. Recent studies have demonstrated that about 70 per cent of protein-coding human genes have their counterparts in zebrafish, and 84 per cent of genes linked to human diseases can be found in zebrafish.
"Researchers are studying the potential effects of low level chronic exposure to methylmercury, which is of global concern due to methylmercury presence in fish, but the message that I want to get across is that such exposures may negatively affect vision. Our study clearly shows that we need more research into the direct effects of methylmercury on the eye," Korbas concluded.
Acknowledgments: This work was supported by the Canadian Institutes of Health Research, the Saskatchewan Health Research Foundation and the Natural Sciences and Engineering Research Council of Canada.