Researchers use FRET to make activation of AMPA receptors optically visible

Our brain is a high-performance computer. One of the key players in this complex system is the AMPA-type glutamate receptor. It ensures that neurotransmission proceeds at a breakneck pace from cell to cell. Researchers from the Leibniz Institute for Molecular Pharmacology (FMP) in Berlin have now succeeded, by fluorescence resonance energy transfer (FRET), to watch activated receptors at work. The scientists detected movements of unexpected parts of the receptor. Never before was activation of AMPARs measured optically and electrically at the same time. This technological breakthrough was recently published in the journal "PNAS", and suggests new ways to see into the brain, including in pathological situations.

The human brain would crumble without AMPA-type glutamate receptor (AMPAR; α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor). AMPARs ensure that neurotransmitter messages are transmitted across synapses with extreme speed. This lightning-fast synaptic transmission is essential for everything we do, think and remember. AMPARs are the most common excitatory receptors in the brain and essential for our survival.

Although the binding of neurotransmitters to AMPARs is quite well understood, its activation mechanisms have not been fully unraveled. Researchers from the Leibniz Institute for Molecular Pharmacology (FMP) in Berlin have literally brought light into this darkness. Using fluorescence resonance energy transfer (FRET), Dr. Andrew Plested and PhD student Ljudmila Katchan made the activation of AMPARs optically visible. They found that neurotransmitters drive conformational changes in two intracellular regions of the receptor, as well as at extracellular location where the glutamate binds to the receptor. Even more, the researchers were able to watch the receptor activity in real time. The motivation for this research is that given a receptor that has big enough changes in it's light output, they could also watch synapses.

Research group leader Plested explained that the main novelty of the work. "was to simultaneously measure activation optically and electronically". Coauthors on the paper from Anders Kristensen's team in Denmark, first saw definite optical signals with fluorescence-lifetime imaging microscopy (FLIM), but this method is too slow to see activation. "We have provided the proof of principle, opening the door to use this technique for research into the functional properties of other signalling molecules in the brain," says biophysicist Plested.

The two members of the FMP research group "Molecular Neuroscience and Biophysics" needed four years to complete the research . Now the results were published in the prestigious journal "PNAS". The current study will now be followed by further experiments. Here, the researchers want to investigate other molecules essential for neurotransmission. This work should be done first in cell cultures and later in brain.

Better understand the thinking and neurodegenerative diseases

Even if all the billions of synapses in the brain cannot be visualized at once with the new method, following just a handful of them could give new insights into thinking. Moreover, these approaches could one day help to gain a better understanding about neurodegenerative diseases. In these diseases, loss of synapses seems to be a major problem, but we have no idea about the activity or function of the lost synapses and the ones that remain. It would be strange, but we can imagine that in Alzheimer's or Parkinson only inactive synapses disappear, and the active ones are spared, says Plested. But it could just as easily be the other way round. Right now, researchers can't say. "We hope to be able to see synapses at work," says the biophysicist, "both in healthy and in the diseased brain."