Learning and Thinking: In an effort to understand how brain cells exchange chemical messages, scientists say they have successfully used a highly specialized microscope to capture more precise details of how one of the most common signaling molecules, glutamate, opens a channel and allows a flood of charged particles to enter. The finding, which resulted from a study led by Johns Hopkins Medicine researchers, could advance the development of new drugs that block or open such signaling channels to treat conditions as varied as epilepsy and some intellectual disorders. 

A report on the experiments, funded by the National Institutes of Health and in collaboration with scientists at UTHealth Houston, was published March 26 in the journal Nature. 

“Neurons are the cellular foundation of the brain, and the ability to experience our environment and learn depends on [chemical] communications between neurons,” says Edward Twomey, Ph.D., assistant professor of biophysics and biophysical chemistry at the Johns Hopkins University School of Medicine.  

Scientists have long known that a major molecule responsible for neuron-to-neuron communications is the neurotransmitter glutamate, a molecule abundant in the spaces between neurons. Its landing place on neurons is a channel called an AMPA receptor, which interacts with glutamate, and then acts like a pore that takes in charged particles. The ebb and flow of charged particles creates electrical signals that form communications between neurons. 

To figure out details of the miniscule movements of AMPA receptors (at the level of single atoms), researchers used a very high-powered microscope to image these channels during specific steps in the communications processes. For the study, the scientists used a cryo-electron microscope (cryo-EM) in a facility at the Johns Hopkins University School of Medicine. 

Typically, scientists find it easier to study cell samples that are chilled, a state that provides a stable environment. But at normal body temperature, Twomey’s team found that the AMPA receptors and glutamate activity increased, providing more opportunities to capture this process in cryoEM images. 

To that end, the scientists purified AMPA receptors, taken from lab-grown human embryonic cells that are used widely in neuroscience research to produce such proteins. Then, they heated the receptors to body temperature (37 degrees Celsius or 98.6 degrees Fahrenheit) before exposing them to glutamate. Immediately after this, the receptors were flash frozen and analyzed with cryoEM to get a snapshot of the AMPA receptors bound to the major signaling molecule, glutamate. 

After assembling more than a million images taken with cryoEM, the team found that glutamate molecules act like a key that unlocks the door to the channel, enabling it to open more widely. This occurs by the clamshell-like structure of the AMPA receptor closing around glutamate, an action that pulls open the channel below. 

Twomey’s previous research has shown that drugs such as perampanel, used to treat epilepsy, act as a door stopper around the AMPA receptor to limit the channel from opening and reducing the abundance of activity known to happen in brain cells of people with epilepsy. 

Twomey says the findings could be used to develop new drugs that bind to AMPA receptors in different ways that either open or close the signaling channels of brain cells. 

“With each new finding, we are figuring out each of the building blocks that enable our brains to function,” says Twomey. 

Additional scientists who contributed to the work are Anish Kumar Mondal from Johns Hopkins and Elisa Carrillo and Vasanthi Jayaraman from UTHealth Houston.