Increasing the activity of two enzymes better known for their role in oxidative stress metabolism turns normally relaxed mice into "Nervous Nellies," according to research conducted at the Salk Institute for Biological Studies and reported in the early online edition of Nature.
Locally overexpressing either glyoxalase 1 or glutathione reductase 1 in mouse brains significantly increased anxiety in usually relaxed mice and made already jittery mice even more anxiety-ridden. Inhibition of glyoxalase 1 had the opposite effect.
"Currently, very little is known about the genes that predispose to psychiatric disease," says first author Iiris Hovatta, who was a postdoctoral research in Salk's Laboratory of Genetics when the research was conducted. "All of the 17 genes that we identified are very good candidates for human anxiety disorders and most of them have never been associated with anxiety-related behavior before," she adds.
"This is a very exciting study where we can genetically interfere with the behavior outcome, emphasizing the genetic hard wiring of certain traits,'' says Inder Verma, professor in the Laboratory for Genetics at the Salk Institute.
Out of the 17 candidates, the researchers focused on the most promising ones, glyoxalase 1 and glutathione reductase 1, since both enzymes belonged to the same metabolic pathway. In addition, a study by Turkish scientists had found elevated levels of oxidative stress markers in patients with severe anxiety disorders. "It might be that oxidative stress metabolism and anxiety levels are linked, although we do not know the exact mechanism at the moment," says Hovatta.
Like other complex psychiatric traits, fear and anxiety are influenced by many genes. There is no such thing as a single "fear" gene that lets anxiety spiral out of control when the gene's regulation is disturbed, making it difficult to identify the genetic roots of anxiety disorders.
For their study, the scientists relied on inbred mouse strains that differ considerably in their natural anxiety levels. Just like in humans suffering from anxiety disorders, the sights and sounds of unfamiliar environments can trigger panic in mice with anxious dispositions, causing them to freeze in place. Unlike their more relaxed contemporaries, naturally nervous mice are not explorers and may seem wary of open spaces.
Instead of studying individual genes the researchers simultaneously assessed the activity patterns of about 10,000 genes in specific brain regions with the help of microarrays. This extensive scan allowed the researchers to pinpoint multiple genes whose expression levels differed in relaxed and anxiety-prone mice.
To increase the specificity of their microarray analysis, they looked in only specific brain areas that have been shown to play a role in anxiety and fear (the amygdala, bed nucleus of the stria terminalis, cingulate cortex, hippocampus, hypothalamus, central peri-aqueductal grey and pituitary gland).
"We were incredibly surprised since out of the entire genome only 17 genes were robustly correlated with anxiety levels across many different strains," says Carrolee Barlow, lead author of the study and an adjunct professor in the Laboratory of Genetics. "Almost half of them were enzymes and not neurotransmitters as one might expect."
In the past, scientists tried to correlate complex psychiatric diseases with different forms of the genes controlling neurotransmitters, the chemical messengers that brain cells use to shuttle outgoing signals to neighboring cells, and their receptors, albeit with limited success. "That's why we chose an unbiased approach that didn't limit us to neurotransmitters," explains Barlow.
Now, Hovatta wants to find out what relevance, if any, the identified genes have to human anxiety disorders. "It is really exciting to study neurobiology of anxiety in mice and to understand the molecular mechanisms behind the regulation of behavior, but I am mostly interested in trying to find genes that predispose humans to anxiety disorders and to perhaps in the future try to develop better treatment practices. We are still far away from that," she cautions, "but it is the long term goal of the project."
Researchers contributing to the study include first author Iiris Hovatta, formerly at the Salk Institute, now at National Public Health Institute in Helsinki Finland, research assistants Richard S. Tennant and Robert Helton, research fellows Robert A. Marr and Oded Singer, both in the Laboratory of Genetics at the Salk Institute, Jeffrey M. Redwine at Neurome Inc., Julie A. Ellison, formerly at the Salk, now at Helix Medical Communications, Eric E. Schadt at Rosetta Inpharmatics LLC, Inder Verma, professor in the Laboratory for Genetics at the Salk Institute, David J. Lockhart, co-principal investigator and visiting scholar at the Salk Institute, and Carrolee Barlow, at Braincells Inc.