Johns Hopkins biophysicists have discovered that full activation of a protein ensemble essential for communication between nerve cells in the brain and spinal cord requires a lot of organized back-and-forth motion of some of the ensemble's segments. Their research, they say, may reveal multiple sites within the protein ensemble that could be used as drug targets to normalize its activity in such neurological disorders as epilepsy, schizophrenia, Parkinson's and Alzheimer's disease.
A summary of the results, published online in the journal Neuron on Aug. 7, shows that full activation of so-called ionotropic glutamate receptors is more complex than previously envisioned. In addition to the expected shape changes that occur when the receptor "receives" and clamps down on glutamate messenger molecules, the four segments of the protein ensemble also rock back and forth in relation to each other when fewer than four glutamates are bound.
"We believe that our study is the first to show the molecular architecture and behavior of a prominent neural receptor protein ensemble in a state of partial activation," says Albert Lau, Ph.D., assistant professor of biophysics and biophysical chemistry at the Johns Hopkins University School of Medicine.
Glutamate receptors reside in the outer envelope of every nerve cell in the brain and spinal cord, Lau notes, and are responsible for changing chemical information—the release of glutamate molecules from a neighboring nerve cell—into electrical information, the flow of charged particles into the receiving nerve cell. There would be sharply reduced communication between nerve cells in our brains if these receptors were disabled, he added, and thought and normal brain function in general would be severely compromised. Malfunctioning receptors, says Lau, have been linked with numerous neurological disorders and are therefore potential targets for drug therapies.
Lau explained that each glutamate receptor is a united group of four protein segments that has a pocket for clamping down on glutamate like a Venus fly trap snaring a bug. Below the glutamate-binding segments are four other segments embedded in the cell's outer envelope to form a channel for charged particles to flow through. When no glutamates are bound to the receptor, the channel is closed; full activation of the receptor and full opening of the channel occur when four glutamates are bound, each to a difference pocket.
Previously, Lau says, investigators thought that the level of receptor activation simply corresponded to the degree to which each glutamate-binding segment changed shape during the glutamate-binding process. Using a combination of computer modeling, biophysical "imaging" of molecular structure, biochemical analysis and electrical monitoring of individual cells, the researchers teased apart some of the steps in between zero activation and full activation. They were able to show that the four glutamate-binding segments, in addition to clamping down on glutamate, also rock back and forth in pairs when fewer than four glutamates are bound.
"It isn't clear yet how this rocking motion affects receptor function, but we now know that activation depends on more than how much each glutamate-binding segment clamps down," says Lau. Previous development of drugs targeting the receptor focused on the four glutamate-binding pockets. "Our discovery of this molecular motion could aid the development of drugs by revealing additional drug-binding sites on the receptor," he adds.