Jean Shih with USC molecular pharmacologist Kevin Chen Photo by Joe Pugliese

Issue: Spring 2005

MAO’s Revolutionary

Researcher Jean Shih has followed mice and men down a winding road to understanding a pair of crucial brain enzymes.

The riddle Jean Shih has been unraveling through her entire professional career is posed by the singular record of an unfortunate family in Holland.

The males consistently displayedaggressive behavior of an exaggerated and pathological kind – so consistent that scientists suspected it had a genetic root.

Careful testing found a specific neurochemical deficit among the men of this family: their brains were deficient in producing an enzyme called monoamine monoxidase (MAO).

But how can the lack of an enzyme produce aggression? Science still does not have the entire answer, but the work of Jean Shih has led the way in explaining what is going on, and is helping to produce drugs and treatments that can change the equation.

We now know that MAO breaks down a number of neurotransmitters – substances that mediate the exchange of electrical messages between brain cells. In its absence, these neurotransmitters accumulate to far-higher-than-normal levels, overexciting the system.

If too much is present, the opposite effect occurs: The brain works sluggishly, like an electric device with a weak battery.

MAO was first isolated in 1942; in 1962 it was shown to come in two forms, MAO A and MAO B. MAO A deals primarily with serotonin, dopamine and norepinephrine, chemicals essential to mood, aggression, anxiety and the sleep-waking cycle. MAO B mainly breaks down phenylethylamine, another important mood regulator.

We now know, thanks to Shih, that the gene controlling MAO production lies on the X chromosome. This location provides a neat explanation for the male-only distribution of aggression in the Dutch family: Women have two copies of the X chromosome, giving them a back-up in case of genetic damage to one.

Shih and her collaborators – USC molecular pharmacologist Kevin Chen has been with her for most of the journey – are the first to have modeled the molecule’s three-dimensional structure and the first to have worked out the key amino acids involved in MAO’s active sites, which they reported in three landmark papers in 2001.

These discoveries were major steps toward understanding MAO well enough to start designing drugs that specifically modify its action – a goal dear to psychopharmacologists, who expect it will have several important applications: better antidepressants, a deeper understanding of anxiety and aggression, treatment for Parkinson’s disease and perhaps the ability to extend cell life spans.

Scientists who study the brain often employ bioengineered mouse models as a way to observe neurotransmitter pathways in a living system. Here too Shih pioneered.

Working with collaborators, she created several strains of “knockout” mice – that is, mice lacking one or more of the MAO genes. She has shown that mice in which the MAO A enzyme is inactivated tend to become unusually aggressive – precisely as did the males in the Dutch family.

She also took advantage of a spontaneous mutation occurring in her mouse gene-lines when she discovered animals missing both the MAO A and MAO B genes. The behavior of these “double knockout” mice is intriguing because it showed both aggression and pronounced anxiety.

“This tells us that anxiety and aggression are regulated by similar neurotransmitter pathways,” says Shih.

She also has bred mice that lack just the MAO B gene, and these offer a window into still another mechanism: the one producing Parkinson’s disease. A progressive, dopamine-related brain disorder, Parkinson’s degrades motor faculties and leads to neurodegeneration.

The brains of MAO B knockout mice metabolize certain neurotoxins differently than normal mice, Shih found – a difference that protects them from Parkinson’s.

“The availability of three different MAO knockout mice provides a unique opportunity to further examine the molecular details of the monoamine neurochemical systems associated with specific behavior or psychological states,” Shih says. “It will also provide new insights for developing selective pharmacological interventions for depression, anxiety disorder and Parkinson’s disease.”

Shih recently received a $2 million grant from the National Institute of Mental Health to study how precise defects in DNA coding – a special sequence called the “MAO promoter” – affects MAO production.

She and her team have discovered a novel protein called R1 that regulates the production of MAO A. Studying this protein, they will be able to come up with new ways to inhibit the production of MAO A at the transcription level.

“This will allow scientists to develop more efficient medications for mood and anxiety disorders,” she says.

Despite decades spent unraveling the action of a single mechanism, Shih sees no shortage of exciting challenges ahead.

“This enzyme has many different functions and acts on so many different pathways,” she says. “It was recently found that MAO A expression increases significantly in prostate cancer cell lines. It was recently shown to play a role in cell death, and that inhibiting MAO can inhibit cell death. So that’s a whole new direction for MAO research. Actually, that’s why I choose to work with MAO; it’s very distinctive and unusual. It’s interesting that one enzyme can take up many scientists’ whole lifetimes and still bring surprises.”

Shih’s example provides a model of doggedness and scientific ingenuity, according to USC dean of pharmacy Timothy Chan.

Her search points out her strengths as a scientist, according to Chan. In order to follow the winding pathway presented by MAO, she and her team have had to learn a whole gamut of new techniques and tools – the use of knockout mice, for example.

The three-dimensional structure of MAO. Shih and her collaborators were the first to model the important brain enzyme.

It has also, strikingly, brought new ideas and resources to the school. When Shih began her investigations, the school was oriented almost exclusively toward biochemistry – the long, traditional approach to developing new drugs. It is now focused equally or even more so on molecular biology.

“She has a nose for breakthroughs, whether planned or serendipitous,” Chan says. “The way her work progressed really required a lot of innovation. She had to be able to spot new indicators and find new ways to pursue them. And she did.”

– Eric Mankin and Alexis Bergen