Environmental Magnetite in the Human Brain

In 1992, researchers discovered angular particles of magnetite in the human brain. Sixteen years later, a comparison of healthy and Alzheimer’s brains revealed that higher levels of magnetite correlated with the incidence of disease. Most recently, scientists studying rat neuronal cell cultures found that magnetite and amyloid-β peptides seemed to stabilize each other; the two particles together were more harmful to neurons in culture than amyloid-β peptides alone.

Maher had been studying the airborne particulate matter, including magnetite, along roadsides, and decided to examine whether that magnetite might enter the brain, where it could potentially have similar toxic effects. She and her colleagues examined the quantity and structure of the magnetite within frontal cortex samples of 37 human brains, which came from the Manchester Brain Bank in the U.K. and from people who had died in fatal accidents in Mexico City between 2004 and 2008.

Using high-resolution transmission electron microscopy, the researchers searched the brain slices. They identified endogenous, angular magnetite particles, which ranged in diameter from 50 to 150 nanometers, as well as spherical particles that ranged from less than 5 nanometers to more than 100 nanometers. The researchers believed these spherical compounds came from particulate matter in the air, Maher said.

Both in shape and in texture, these round magnetite particles resembled magnetite particles from roadside particulate matter, “and some of the particles have some very distinctive surface textures,” Maher said. Moreover, the magnetite particles “co-occur with other rather exotic metals, [which] are not metals that you would expect normally to find in the human brain,” she added. The researchers hypothesize that magnetite particles from the air are inhaled and enter the brain via the olfactory bulb, a neuronal gateway to the brain that does not have the same blood-brain barrier protection that other brain regions do, Maher said.

Dobson noted that a causal link between magnetite particles of any type and neurodegeneration has not been established. However “mechanisms proposed for a potential role in neurodegeneration would be the pretty much the same for both types of particles,” he added. 

Although researchers do not know if environmentally derived magnetite has the same effects as endogenous magnetite on neuron health, “it would be foolish to ignore the possibility that it could be creating an additional health hazard for humans,” Maher said.

The study “gives an explanation for the high abundance of magnetite in the brain and, in turn, points towards its connection with Alzheimer’s,” Jordi Soriano, a University of Barcelona biophysicist who has studied magnetite’s effects on neurons in cell culture and was not involved the present study, told The Scientist in an email.

He noted, however, that the researchers did not sample any brains from people who had lived in nonurban areas, which would be necessary “to prove the connection between pollution and magnetic nanoparticles accumulation and structure.” This type of epidemiological study correlating likely magnetite exposure and neurodegenerative disease is one area for future research, Maher said.

B. Maher et al., “Magnetite pollution nanoparticles in the human brain,” PNAS, doi:10.1073/pnas.1605941113, 2016.