Capturing a major signalling molecule with cryo-EM

In an effort to understand how brain cells exchange chemical messages, US scientists have used a highly specialised microscope to capture more precise details of how one of the most common signalling molecules — glutamate — opens a channel and allows a flood of charged particles to enter. Their findings, which have been published in the journal Nature, could advance the development of new drugs that block or open such signalling channels to treat conditions as varied as epilepsy and some intellectual disorders.

“Neurons are the cellular foundation of the brain, and the ability to experience our environment and learn depends on [chemical] communications between neurons,” said Assistant Professor Edward Twomey, from 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. 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 cryo-EM 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 before exposing them to glutamate. Immediately after this, the receptors were flash frozen and analysed with cryo-EM to get a snapshot of the AMPA receptors bound to glutamate.

After assembling more than a million images taken with cryo-EM, 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 said the new findings could be used to develop new drugs that bind to AMPA receptors in different ways that either open or close the signalling channels of brain cells.

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

Image caption: Illustration of an AMPA receptor and glutamate. Image credit: Ed Twomey, Johns Hopkins Medicine.