An antimicrobial agent found in some shampoos and hand creams and widely used in industrial settings inhibits the development of particular neuron structures that are essential for transmitting signals between cells, according to a University of Pittsburgh study presented today at Cell Biology 2004, the 44th annual meeting of the American Society for Cell Biology.
Prolonged exposure to low levels of methylisothiazolinone (MIT) restricted growth of axons and dendrites of immature rat nerve cells in culture, apparently by disengaging the machinery of a key enzyme that is activated in response to cell-to-cell contact, and may have potentially damaging consequences to a developing nervous system, the researchers report.
Senior author Elias Aizenman, Ph.D., professor of neurobiology at the University of Pittsburgh School of Medicine, became interested in MIT as an offshoot to his primary area of research on the mechanisms of neuronal cell death. The first he heard of the chemical was when its name came up in a literature search for compounds with specific chemical properties that he thought would incite a particular cell death pathway he recently had identified. As it turned out, MIT activated a different, novel pathway, but Dr. Aizenman remained intrigued, in large part because of the considerable lack of scientific data about a compound that he came to realize was listed on numerous consumer product labels and was very widely used in industry.
As an antimicrobial agent, or biocide, MIT and related compounds kill harmful bacteria that like to grow near moisture or water. As such, they often are found in personal care products, as well as in water-cooling systems and at factories that require water for manufacturing. Since learning about MIT, Dr. Aizenman has not found any published neurotoxicity reports, or concrete data in any public documents filed with the Environmental Protection Agency.
The first set of studies he and his team published in 2002 in The Journal of Neuroscience involved acute exposure to mature rat neurons. They reported that 10-minute exposure at a high concentration – roughly 100 times the dose used in their current study – was lethal to these cells.
To understand what effect chronic exposure would have on immature, developing neurons, the researchers kept cells in a media solution containing low concentrations of MIT for 18 hours. In a standard culture, an immature neuron will in such time develop an axon, the extension from the cell body used for sending signals to other cells, and several dendrites, elaborate projections that receive incoming information. But after exposure to MIT, the cells had few, if any, axons and dendrites, with the inhibition of their growth being dose-dependent, reported Kai He, Ph.D., a postdoctoral fellow working with Dr. Aizenman who presented the data at Cell Biology 2004.
Additional studies revealed that MIT significantly hindered tyrosine phosphorylation, a process that initiates molecular events during cell-to-cell contact, and that a particular protein enzyme was its target. This enzyme, focal adhesion kinase (FAK), is known to be important for outgrowth of axons and dendrites as well as necessary for cell signaling. But to kick into action, FAK must undergo tyrosine phosphorylation, whereby collections of molecules called phosphate groups are added to FAK's sequence of amino acids. Like all proteins, FAK is comprised of a unique sequence of some 20 amino acids, including tyrosine, so any change, such as through phosphorylation, essentially changes its function. To pinpoint the exact site along FAK's sequence where tyrosine phosphorylation was being inhibited, the researchers had to determine which of its tyrosines were targeted by MIT. Digging further, they found that one tyrosine in particular (amino acid 576 in the protein sequence) was more substantially affected by MIT, an indication that this particular amino acid residue is important for axon and dendrite growth and development.
The authors, who also included Carl F. Lagenaur, Ph.D., associate professor of neurobiology at the University of Pittsburgh School of Medicine, plan additional research to further understand the molecular mechanisms underlying MIT's neurotoxic effect on cells, as well as studies involving whole animals. They are hopeful that their work will stimulate additional research by other groups as well as bring heightened awareness about the potential risks from human exposure.
"This chemical is being used more and more extensively, yet there have been no neurotoxicity studies in humans to indicate what kind and at what level exposure is safe. I realize it's a big leap to suggest there may be a parallel between environmental exposure and the noticeably higher rates of diagnosed childhood developmental disabilities, but I would caution that based on our data, there very well could be neurodevelopmental consequences from MIT. Clearly, more study is needed, with both scientists and government regulators equally engaged," added Dr. Aizenman.