Live neuron from the brain of a rat with excitatory synapses labelled in green and inhibitory synapses in magenta. Image Credit: Aida Bareghamyan (CC BY 4.0)
Within the brain are circuits of neurons that communicate with one another via junctions known as synapses. The pre-synaptic neuron, which sends the signal, releases proteins known as neurotransmitters into the synapse which then bind to receptors on the receiving, or postsynaptic, neuron.
If the receptors at the synapse are excitatory, they increase the chances of the postsynaptic neuron ‘firing’ and propagating the signal. In contrast, if the receptors are inhibitory, this reduces the likelihood of the neuron firing, dampening communication across the circuit.
In a previous study, a tool known as GFE3 was developed that can specifically eliminate inhibitory synapses by binding to scaffolding proteins called gephyrins that anchor inhibitory receptors in place. Attached to GFE3 is an enzyme that triggers gephyrin degradation, leading to the dismantling of the synapse and blocking of the inhibitory signal. The tool has been used to study various behaviors, including how mice control their rhythmic whisker movements and vocalization patterns.
Bareghamyan et al. – who are part of the research group that carried out the previous work – have now refined GFE3 so it can be activated more precisely and controllably. The team produced two new versions: one that is activated by light, and another that is activated by a cell-permeable chemical compound known as trimethoprim-Halotag ligand.
In addition, Bareghamyan et al. engineered a new tool called PFE3 which is designed to eliminate excitatory synapses. PFE3 targets PSD-95, the predominant scaffold protein found in excitatory synapses. It also contains two enzymes that work together to degrade PSD-95, leading to a loss of excitatory receptors. Further experiments showed that PFE3 efficiently reduced the number of excitatory synapses in rat neurons cultured in a dish as well as in neurons found in the retinas of mice.
The synapse ablators developed by Bareghamyan et al. offer a fast, efficient and reversible approach for eliminating both excitatory and inhibitory synapses. These tools will make it easier for neuroscientists to silence specific postsynaptic neurons and expand the toolkit available for manipulating and studying neuronal circuits.
