Mutation increases enzyme in mouse brains linked to schizophrenia behaviors

The mutation increases levels of glycine decarboxylase, or GLDC, an enzyme responsible for regulating glycine in the brain. Glycine activates receptors for the neurotransmitter glutamate, called NDMA receptors.

"The genetics of schizophrenia is very complex, and it is rare that mutations found in patients can be linked directly to the disease," said study leader Uwe Rudolph, a professor of comparative biosciences at Illinois. "Schizophrenia is not yet diagnosed by any kind of lab test or imaging; it's still a clinical diagnosis based on symptoms. The hope is that these kinds of rare mutations could lead us to the biochemical and physiological pathways that are important to study."

The study began when a genetic mutation was found in two patients with schizophrenia at McLean Hospital in Belmont, Mass. They had multiple copies of a section of DNA that included the gene for GLDC. Curious as to whether the mutation contributed to their symptoms, the McLean team studying the patients reached out to Rudolph's lab to create a line of mice with the same mutation.

The mice with mutations analogous to those in the human patients also showed schizophrenia-associated behaviors. To further narrow down the genetic link, the researchers next developed lines of mice with multiple copies of only a few genes contained within the larger chromosome segment repeated in the patients, then a single gene: GLDC.

"We found that extra copies of the GLDC gene alone were sufficient to render the schizophrenia-like behaviors we had observed," said Rudolph, who also is affiliated with the Neuroscience Program and the Carl R. Woese Institute for Genomic Biology at Illinois.

To understand why multiple copies of the GLDC gene would be solely responsible for the behavioral symptoms, the researchers took a closer look at what was happening in the brains of the mice, specifically to the levels of glycine and the function of NDMA receptors.

"We hypothesized that extra copies of GLDC would result in a lower level of glycine in the brain, since it degrades glycine. Then there would not be enough glycine to help activate the NDMA receptors," said Illinois postdoctoral researcher Maltesh Kambali, the first author of the paper. "We measured an increase in activity of the GLDC enzyme in the brains of our mice, which would point to that as well."

However, when the researchers measured glycine levels in the brains of their mice, there did not seem to be a significant difference between those with the extra GLDC and healthy mice. So Rudolph's team turned to colleagues in Germany who had developed sophisticated methods of tracking glycine in the brain.

The German team found that, while the overall amount of glycine in the whole brain was similar, the amount of glycine outside of the nerve cells and available to help activate NDMA receptors was significantly lower in a subregion of the hippocampus in the mice with multiple copies of GLDC.

To see why this region of the brain was so affected, Rudolph's team then worked with researchers at Harvard Medical School to perform functional studies on the affected brain subregion, called the dentate gyrus. They identified decreased activity in the neural synapses, the active junctions that send signals between neurons. They pinpointed differences in long-term potentiation, a sustained strengthening of activity in the synapse during the learning process.

"We saw how glycine measurements and the long-term potentiation measurements were showing converging changes in this dentate gyrus region, but not in other regions of the hippocampus. It's interesting because one theory links the development of psychosis to the activity in the dentate gyrus. So our findings fit with that theory," Kambali said.

The Illinois team then conducted a biochemical analysis of the dentate gyrus in the mice with extra copies of GLDC. They found that some pathways previously implicated in schizophrenia had reduce activity, indicating that the increase of GLDC and associated decrease of glycine was indeed sufficient to inhibit the NMDA receptors' function and was involved in the schizophrenia symptoms they had observed.

The researchers published their findings in the journal Molecular Psychiatry.

"This study demonstrated at multiple levels how GLDC functions as a novel regulator of NMDA receptors," Rudolph said. "Dysfunction of NMDA receptors has been shown to be important in the pathophysiology of schizophrenia. Yet this finding also is relevant independently of disease, as the NMDA receptor is essential for many brain functions, including learning and memory."

This work was supported by the National Institutes of Health, the Shervert Frazier Research Institute at McLean Hospital, the Stanley Center for Psychiatric Research at the Broad Institute of Harvard and MIT, and a Harvard Brain Science Initiative Bipolar Disorder Seed Grant.