Depression and anxiety disorders affect a great many people and though we have antidepressants to treat symptoms, we actually know little about how the chemistry behind them works.
For example, we have known about an enzyme called Rines since 2002, but have only just discovered what the unusual behaviour a deficiency of it leads to in mice tells us about the role it could play in affecting our mood and emotions, and how that might open up new ways to treat conditions such as depression.
We now know that Rines regulates MAO-A, a brain protein that plays a major part in how we feel. MAO-A works by breaking down serotonin, norephinephrine and dopamine, a group of chemical neurotransmitters well known for their influence on emotion and mood.
Antidepressant drugs also work by altering the levels of these neurotransmitters. Selective serotonin reuptake inhibitors (SSRIs), which include brands such as Prozac, work by boosting serotonin in the brain and are the most widely prescribed. Monoamine oxidase inhibitors (MAOIs), an older class of antidepressants used to treat conditions such as panic disorders and social phobia, inhibit the activity of MAO-A, which prevents serotonin and other neurotransmitters from being broken down in the brain.
MAOIs are generally now prescribed as a last resort because of their side effects and restrictions on diet. But MAO-A and the mechanisms behind it still hold much promise for researchers. The gene that programmes MAO-A and nicknamed the “warrior gene”, for example, has a variant that has been associated with a higher risk of violent and anti-social behaviour.
Too much or too little?
In a 1993 paper, Dutch scientists reported on a family with a complete deficiency of MAO-A activity that exhibited abnormal behaviours: impulsive aggression, arson, attempted rape, and exhibitionism. On the other hand, increased MAO-A protein levels were observed in patients suffering from major depression.
However, while evidence points to a link between MAO-A levels and various emotional patterns, we still didn’t know anything about the mechanism that controls levels of it in the brain.
Our discovery of the role of the Rines enzyme, published in The Journal of Neuroscience, uncovers one of the mysteries of how MAO-A works.
We have known how Rines, or Ring finger-type E3 ubiquitin ligase, is distributed across the brain for more than a decade. But its physiological role was not well understood.
But using mice engineered without this enzyme threw up some interesting findings. These mice develop normally, with no abnormalities, and look just like any other normal mice. In behavioural tests, the mice also showed no clear deficits in motor abilities, pain sensation, or spatial learning.
But the mice did show abnormal behaviour in situations involving emotional responses. For example, we observed that Rines-deficient mice had altered social behaviour. They favoured being close to mice they had never seen before, but didn’t attack them as you might normally expect.
In an elevated maze test, a typical behavioural test where the mice are given the choice to go through a walled-off corridor or a completely open one, 60cm above the floor, the mice lacking Rines preferred the corridor surrounded by walls.
Taken together with other results, we interpreted this behaviour - the need to stay close to others and avoid open spaces - as indicating an elevated level of anxiety. Our mice also showed altered emotional behaviour in their lack of response to repetitive, unpleasant stimuli such as mild foot shocks or forced swimming.
These abnormal behaviours in Rines-deficient mice led the team to take a closer look at the neurotransmitters in their brain. We found that the levels of norepinephrine and serotonin were altered in the mice experiencing unpleasant stimuli. The alteration was particularly clear in the locus ceruleus, a region of the brain known to be a principal source of norepinephrine.
We also found MAO-A protein levels were high in this same region of the brain by examining its activity as an enzyme and quantifying this using antibodies that bind to MAO-A.
At this point, we thought that Rines may actually enhance the degradation of MAO-A, thus increasing levels of serotonin, dopamine and norepinephrine in the brain. We subsequently proved this by analysing how MAO-A and Rines interact, both in a petri dish and in fresh brain tissue. It turns out that Rines works by “flagging” MAO-A with a ubiquitin (another regulatory protein) tag that speeds up its breakdown.
Our team then tested the effects of MAOI drugs on Rines-deficient mice. As expected, the MAO inhibitors reversed some abnormal behavior in the mice, but they did not affect the same behaviours in Rines-deficient mice and normal mice.
Taken together, these results suggest that the Rines enzyme controls levels of MAO-A protein in the brain by regulating its degradation, and that a disrupted regulation could underlie the disturbed emotional responses found in patients with depression or anxiety disorder.
It opens up a promising new avenue for looking at the role MAO-A plays in how our brains function and how we might travel towards new therapies.