Discovering the Hidden Sanctuary of Mercury in Mammalian Brains

Recent studies at Purdue University have discovered the presence of mercury in the brains of mongoose, hinting at a greater environmental issue and potential dangers to human health. This pioneering finding advances our knowledge of the neurotoxic effects of mercury on land-dwelling animals. Credit: SciTechDaily.com

Researchers detected mercury within the brains of mongoose, indicating a more extensive environmental threat and heightening fears about human contact with mercury.

Mercury (Hg) exposure is highly neurotoxic in a variety of chemical forms. Even those who study mercury compounds are at threat due to possible exposure to Hg. Well-known physicist Michael Faraday became ill from Hg poisoning due to extended exposure to Hg vapors, which led him to stop his research at 49 because of his deteriorating health. In another case, lab chemist Karen Wetterhahn died from dimethylmercury poisoning when a few droplets from a pipette spilled onto one of her latex-covered hands.

Many research studies have honed in on the exposure and impacts of Hg, primarily in sea and ocean-dwelling creatures. It's widely known that humans should restrict the intake of particular fish, such as tuna, because of the presence of mercury. However, it leads to the question: can mercury ions infiltrate the brains of land-dwelling animals? Initially skeptical, Dr. Yulia Pushkar who is a Professor of Physics and Astronomy at the College of Science, Purdue University, has been overseeing a brain imaging program at the university since 2008. Her team, proficient in sample preparation, data interpretation, and measurements, is sought after by researchers both nationally and internationally, including those from Japan and more recently Australia.

Mercury, represented by the symbol Hg, is a toxic chemical element. It can damage the kidneys, nervous system, immune system, and liver upon exposure. Extensive studies have been conducted on mercury concentrations in marine and sea life, however, it's still uncertain whether it can amass within the brains of terrestrial creatures.

The responsibility of inspecting the presence of Hg in the brains of mongooses collected from Okinawa Island was given to Pushkar's research team. Astonishingly, brain scans revealed mercury within these invasive animals. The team enhanced the scans, achieving a resolution of several tens of nanometers to examine the affected brain cells. The combined findings were recently released in the journal Environmental Chemistry Letters.

The method by which mercury enters the mongoose brain remains undetermined. Potential sources could be bird eggs they eat, the water they drink, mineral exposure, or possibly the air they breathe. Despite the mystery, one fact is evident: this is a highly concerning sign.

“Mercury is severely toxic even in low concentrations as it can bind and alter the function of crucial biomolecules,” explains Pushkar. “The efficiency of detoxification will rely on the binding constant within detected buildups and potential leakage if the brain cells decease. Currently, there is no known method to safely dissolve these clusters from tissue and there are no accounts of reversing mercury poisoning of the neural system. The primary approach all should adopt is avoiding any exposures, particularly chronic ones, like in the case of Faraday.

The researchers were doubtful whether any mercury could be detected. As Pushkar explains, “Typically, neurotoxic elements, even if they enter the brains are present in extremely low concentrations”. The brain specimens were taken to the Advanced Photon Source at Argonne National Laboratory where they were exposed to intense X-rays. Contrary to their skepticism, the mercury signal was detected.

The researchers began tracing brain areas that seemed to have a higher mercury content as they scanned across brain samples. After three years of research and five trips to two national synchrotron facilities (Advanced Photon Source at Argonne National Laboratory and NSLS-II at Brookhaven National Laboratory), the researchers can now declare that specific brain cells, notably cells of choroid plexus (which form the blood cerebrospinal fluid barrier) and astrocytes of the subventricular zone, contain mercury-rich puncta (approximately 0.5-2 microns in size). The team, led by Pushkar, hypothesizes that these cells assist in filtering mercury from the blood and brain tissue and store it with the aid of another element, Selenium (Se). The identification of the specific Se-containing biological molecules that bind mercury remains unknown.

The research team for this publication is composed of graduate students Pavani Devabathini and Gabriel Bury, and then-undergraduate student Darrell Fischer (now at Harvard Graduate School). The entire team collectively gathered data, which was then analyzed by Devabathini and Fischer. After the data was interpreted, the entire group contributed to the writing of the publication.

This discovery holds potential implications for environmental monitoring in land-dwelling animals and offers new methods for tracking mercury in brain cells, potentially affecting human health and safety.

“Human activities result in the emission of 2000 metric tons of mercury compounds annually and we do not fully understand where all this neurotoxic Hg ends up,” says Pushkar. “Most studies so far focused on marine biota (fish and whales) but apparently terrestrial species are also affected. We expect the human brain react to Hg in a similar fashion via interactions with cells of choroid plexus and astrocytes. However, we do not know if the human brain has enough Se-containing biomolecules to bind to Hg.”

Reference: “High-resolution imaging of Hg/Se aggregates in the brain of small Indian mongoose, a wild terrestrial species: insights into intracellular Hg detoxification” by Pavani Devabathini, Darrell L. Fischer, Si Chen, Ajith Pattammattel, Gabriel Bury, Olga Antipova, Xiaojing Huang, Yong S. Chu, Sawako Horai and Yulia Pushkar, 16 November 2023, Environmental Chemistry Letters. DOI: 10.1007/s10311-023-01666-3