For twenty years, Daniel Choquet, CNRS research director and neurobiologist at the University of Bordeaux has been passionate regarding the question of the dynamics of protein organization in the synapse – the contact zone between two neurons – as well as on the impact of this dynamic on the different functions of the brain. On the surface of synapses, molecular receptors, allowing communication between neurons, are created and agitated. It was he, moreover, who, 20 years ago, had proven this permanent mobility of synaptic receptors (Borgdoff & Choquet, 2002). But what is the importance of this mobility in synaptic plasticity, in memory and learning? It was then that Yann Humeau, at the CNRS and also working at the University of Bordeaux brought its expertise on the side of integrated physiology, behavior and the study of memory. With their teams, they managed to visualize on a mouse model this mobility of synaptic receptors within the nervous tissue and its importance for certain forms of memory. The result of their work has just been published in the journal Sciences Advances
The human brain has regarding a hundred million billion synapses, each with regarding a hundred molecular receptors. Neurons are highly specialized, and to find out which types of neurons are involved in which behavior, you have to directly target the synapses that connect them. Until now, genetic tools immobilized receptors using antibodies, but these tools were not very specific: all receptors of all types of neurons were concerned. Moreover, they did not allow looking at the mobility of receptors in intact brain tissue.
Several innovations were necessary to make these tools specific, first of all the CRISPR genetic scissors, thanks to which the DNA sequences can be modified at will. ” The idea of this new tool is to target receptors by adding a small tag to them: a sequence of a few amino acids. Thanks to CRISPR technology, genetically modified mice have these tags on all their receptors. Subsequently, we manage to recognize these receptors thanks to the addition of a fluorescent ligand – a substance that binds to receptors – manufactured by chemists. Thus, we can mark the receptors in the neurons that we want, in the region of the brain that we choose, in the subtypes of neurons that we want” explains Daniel Choquet. What were the results ? « This confirmed my hypothesis on the mobility of receptors established 20 years ago in situ: we were thus able to show that receptors are also mobile in brain slices. continues the neurobiologist.
Moreover, in addition to following the receptors, these ligands also make it possible to immobilize and freeze the receptors, in order to see the impact of this non-mobility on the various functions of the brain. This new tool is therefore threefold: a genetically modified mouse, ligands which make it possible to mark the receptors in tissue slices, and a light sheet imaging technique making it possible to image brain slices in depth.
The possibilities opened up by this innovation are numerous, in particular with a view to finding ways of tackling certain pathologies. Indeed, a number of diseases trigger abnormal movement of receptors, resulting in problems with synaptic plasticity, memory, or mental retardation. Researchers therefore hope to regulate the mobility of these receptors on abnormally functioning synapses to restore normal functioning. We might thus control synaptic plasticity at a certain time and in a certain place in the brain, which would be useful to help manage post-traumatic stress, for example. The other potential gain would be to correct abnormally functioning synapses. ” Synaptic plasticity is a mechanism known for a long time, but its link with memorization is still mysterious. Why is there plasticity in certain synapses at certain times and in certain places rather than others? What importance can this have in the phenomenon of memorization? This new tool will certainly make it possible to better know, to better locate both spatially and temporally, where and when plasticity phenomena are triggered. Knowing is good, but so is acting. We would ultimately like to act on types of pathological memories, post-traumatic disorders, mental retardation, or even Alzheimer’s concludes Yann Humeau.
By Odyssey Piettre
Credit: Angela Getz, Mathieu Ducros, Daniel Choquet / IINS and BIC / CNRS-University of Bordeaux-Inserm
Image: Neuron in a slice of hippocampus from the mouse model developed by the team, in which the glutamate receptor GluA2 is “tagged” in such a way that an isolated neuron can be marked. Magenta: labeling of GluA2, on the dendrites of the neuron; green: soluble GFP which labels the cell body and the axon. The neuron is imaged with a light sheet microscope, a high-resolution imaging technology.
