Researchers at the University of Chicago have uncovered a link between a metabolic byproduct and brain activity that could result in new treatments for anxiety and other psychiatric disorders.
While testing the role of a gene called Glo1 in anxiety, scientists uncovered a new inhibitory factor in the brain: the metabolic byproduct methylglyoxal (MG). The study, published in the Journal of Clinical Investigation, found that animals with multiple copies of the Glo1 gene were more likely to exhibit anxiety-like behavior in laboratory tests, said Abraham Palmer, Ph.D., assistant professor of human genetics at the University of Chicago Medicine and senior author of the study.
“We showed that Glo1 was causally related to anxiety-like behavior, rather than merely correlated,” he said.
Further experiments showed that Glo1 increased anxiety-like behavior by lowering levels of MG. When researchers inhibited Glo1 or raised MG levels, it reduced anxiety behaviors.
In 2005, a comparison of different mouse strains found a link between anxiety-like behaviors and Glo1, the gene encoding the metabolic enzyme glyoxylase 1. However, subsequent studies questioned the link, and the lack of an obvious connection between glyoxylase 1 and brain function or behavior made some scientists skeptical, according to the researcher.
“When people discover a gene, they’re always most comfortable when they discover something they already knew,” Palmer said. “The alarming thing here was there was a discovery of something that nobody knew, and therefore it seemed less likely to actually be correct.”
A 2009 study from Palmer’s laboratory suggested that differences in Glo1 expression between mouse strains were due to copy number variants, where the segment of the genome containing the gene is repeated multiple times. To test this hypothesis, lead author Margaret Distler inserted two, eight or 10 copies of the Glo1 gene into mouse lines. She then ran experiments, such as the open field test, in which researchers measure how much time a mouse spends in the center of an arena versus along the walls, to detect changes in anxiety.
The results confirmed that mice with more copies of the Glo1 gene exhibited higher anxiety-like behavior, the researchers said.
“It’s the first study to show that it’s the copy number variant that has the potential to change Glo1 expression and behavior,” said Distler, an M.D./Ph.D. student in the Pritzker School of Medicine’s Medical Scientist Training Program.
The researchers then set about solving the mystery of how Glo1 influences anxiety-like behaviors. The primary function of Glo1 is to metabolize and lower cellular levels of MG, a waste product of glycolysis. Distler produced the opposite effect by injecting MG to artificially increase its levels in the brain, finding that raising MG levels quickly reduced anxiety symptoms in mice.
“Methylglyoxal changed behavior within 10 minutes of administration, which means it’s a rapid onset. It’s not changing gene expression, and it’s not having long-term downstream effects,” Distler said. “That was our first breakthrough.”
The short time suggested that MG might have a direct effect on neuronal activity, she said. MG also demonstrated sedative effects at high doses, a hallmark of drugs that activate inhibitory GABA receptors on neurons, the researchers add. In collaboration with Leigh Plant, Ph.D., now at Brandeis University, the researchers demonstrated that MG activated GABA-A receptors on neurons, a previously unknown inhibitory mechanism.
“It’s a completely different system that is tying neuronal inhibitory tone into metabolic activity,” Palmer said. “It turns out now that methylglyoxal, which has been around ever since glycolysis evolved, was also acting at these receptors, and nobody knew that.”
Anxiety is usually treated with drugs that activate the GABA-A receptor, such as benzodiazepines and barbiturates, which are prone to abuse and dangerous side effects. The researchers theorized that targeting the Glo1/MG interaction could provide a more selective strategy for reducing anxiety by subtly influencing inhibitory tone.
“The GABA-A receptor agents already out there have a lot of side effects, such as sedation and hypothermia, as well as a high abuse liability,” Distler said. “It’s possible that taking a Glo1 inhibitor will increase only MG levels to a certain maximum. You could have the potential for more specificity, given that you’re activating a system that’s already in place, not just dumping methylglyoxal or some other GABA-A receptor agent throughout the brain.”
Preliminary experiments with a small molecule inhibitor of Glo1 supported the theory. Injections of the inhibitor, developed by John Termini at the Beckman Research Institute of the City of Hope, reduced anxiety-like symptoms in mice.
“It’s a different way of hitting these GABA-A receptors,” Palmer said. “We have yet to determine if that’s a better way of doing it, but it’s certainly different, and it gives us a unique angle of attack on this system and potential advantages that we have yet to evaluate.”
Such a drug may also be useful in treating epilepsy and sleep disorders, where GABA-A drugs have shown success, he added.
While the therapeutic potential is yet to be determined, the research clears the fog around the role of Glo1 in anxiety by adding behavioral and cellular evidence, the researchers note.
“What’s neat is that we started with exploratory, open-ended genetic studies in mice, and we’ve now gotten into some fundamental new physiology that nobody had appreciated or put together before,” Palmer said. “Now we’re starting to reap some of the fruit from those types of genetic studies to enrich our understanding of more classical aspects of biology.”