Study Grasps How Certain Brain Diseases Arise

The study was published Jan. 12 in Proceedings of the National Academy of Sciences of the United States of America.

The research builds on almost two decades of work by Popescu, who studies the brain’s NMDA (N-methyl-D-aspartate) receptors. Mutations in this protein — which is critical to learning and memory — can result in neuropsychiatric diseases, from epilepsy to schizophrenia. Such mutations are rare and have only been discovered in the past 10 years.

Popescu’s work focuses on how subtle changes in these key receptors cause them to produce altered electrical signals, which in turn affect how well the brain functions.

The work describes for the first time an open form of the NMDA receptor and identifies a direct interaction between two amino acid residues. This interaction forms only in the open receptor and helps it stay open for longer, a finding that has clinical implications.

“Cycling repeatedly through its open and closed forms is the main business of NMDA receptors, and the amount of time that the receptors stay open or closed determines the strength and duration of the electrical signal they produce when stimulated,” Popescu says.

The electricity generated by the opening and closing of the receptors, in turn, determines a neuron’s level of excitability, which has direct clinical consequences. 

“Too much excitability can mean epilepsy, seizures or neurodegeneration, whereas too little can result in schizophrenia and other cognitive disorders,” Popescu says.

Until now, the structure of an open form of the NMDA receptor was unknown. To date, the literature has reported only atomic structures for juvenile NMDA receptors, present in young mammals or young neurons, and these are believed to represent a closed form of the receptor.

Popescu and Wenjun Zheng, PhD, professor of physics in the College of Arts and Sciences, are senior authors on the paper. First authors are Gary J. Iacobucci, PhD, a postdoctoral associate in Popescu’s lab, and Han Wen, a doctoral candidate in Zheng’s lab.

In previous work, Popescu collaborated with Wen and Zheng to develop a model of how the predominant NMDA receptor protein in the adult brain might look.

In the current paper, the two teams built upon that work and used molecular dynamics simulation to “force” the closed adult receptor to open. This was accomplished through UB’s Center for Computational Research and mathematical algorithms developed in the Zheng lab.

“This simulated open structure is the first clue to how the internal organization of these receptors may change when they open,” Popescu says.

When they compared the positions of atoms between the closed and open NMDA receptor structures, the researchers were able to identify several locations where two amino acid residues had moved closer together, suggesting that they were engaging in a new interaction.

“When people have receptors that cannot form this interaction, their receptors and synapses are more sluggish and not as active,” Popescu says. “The observations in this paper are consistent with symptoms observed in patients whose receptors lack this interaction due to spontaneous mutation of one of the residues we identified here as important.”

She noted that precision medicine for NMDA receptors is still in its infancy and the U.S. Food and Drug Administration has only approved a few drugs that work on these receptors.  

“Functional studies like this will help us better understand how the various mutations affect receptor function and which therapy to try,” Popescu says.

The researchers will continue to collaborate to better understand not just open and closed NMDA receptors, but also their intermediary conformations. Large genome sequencing studies will also be crucial in identifying the spectrum of mutations in people and revealing how specific mutations lead to characteristic symptoms.

“By marrying advances in structure determination with new discoveries on the clinical significance of mutations, we will be able to more easily accomplish what we did in this paper: explain how a single, subtle change in a protein changes its function,” Popescu concludes. “Based on this information, other experts can ask more directed questions as to what are the consequences of this protein dysfunction for cellular and brain physiology, and ultimately for human behaviors, and finally, what pharmacologic approaches can one take to restore function?”

Co-authors are Beiying Liu, PhD, research scientist in the Department of Biochemistry, and Matthew Helou, a UB undergraduate biochemistry major in Popescu’s lab.

The research was funded by the National Institutes of Health.