The versatility of the nervous system comes not only from the diversity of the ways neurons communicate in circuits, but also from their “plasticity” or their ability to alter these connections when new information needs to be memorized, when their circuit partners change, or other conditions emerge. A new study by neuroscientists at MIT’s Picower Institute for Learning and Memory shows how a single protein at the frontline of neural connections, or synapses, can profoundly change the way certain neurons communicate and implement plasticity.
The team found that expression of the tomosyn protein is a major determinant of whether the “presynaptic” neurons that send signals to control muscle contraction will be “phasic,” meaning that they quickly release a large amount of muscle contraction. part of the neurotransmitter glutamate through synapses to conduct communication, or will be “tonic,” meaning that they will distribute glutamate in measured doses, keeping some in reserve. The study shows that because tonic neurons have these reserves, they can increase the release of glutamate when receptors in the synapse start to weaken, a plasticity known as presynaptic homeostatic potentiation (PHP). Phasic neurons, with little or no tomosyne-mediated reserve, cannot respond in the same way.
“If you break the synapse on the postsynaptic side, the presynaptic neuron will recognize it and generate more output to keep the overall synaptic response the same. This critical type of adaptive plasticity requires tomosyn, ”says Troy Littleton, lead author of the new study in eLife and Menicon Professor of Neuroscience at the Picower Institute and the Departments of Biology and Brain Sciences and Cognitive Sciences at MIT. “The diversity in the ability of different neurons to express this form of plasticity depends on whether they normally express the protein or not.”
Understanding the role of Tomosyn in neurons is important not only for defining the fundamental functioning of synapses and plasticity mechanisms, a long-term goal of Littleton’s lab, but also because, like flies, humans make tomosyn proteins. and have classes of tonic and phasic neurons.
A diversionary lure
Prior to the study, tomosyn was known to be involved in the “SNARE” molecular machinery of presynaptic neurons. SNARE proteins dock bundles, or vesicles, of neurotransmitters such as glutamate to the membrane of neurons so that they can be released across the synapse. Tomosyn was also believed to be the target of an enzyme believed to be important for learning, memory and plasticity, Littleton said.
Picower scholar and former graduate student Chad Sauvola led the new study in the Littleton lab to determine exactly what tomosyn is doing. He took over the work started by co-author Nicole Aponte-Santiago PhD ’20, who had performed (but not yet tested) mutations in the tomosyn gene in her research on tonic and phasic plasticity of neurons.
When Sauvola began recording the synaptic transmission of neurons with tomosyn mutations, which were designed to turn off the protein, he saw that the synapses engaged in much greater transmission of glutamate, with the muscles having much larger responses. than normal. The loss of normal tomosyne apparently slowed down the release of glutamate. Notably, it could repair the effects of the mutation by exchanging the human tomosyn protein, suggesting the retention of protein property across species.
To learn how tomosyn works, Sauvola studied its structure and found that the protein prevented synaptic vesicles from attaching to the membrane by acting as a decoy to sequester SNARE proteins on the plasma membrane. He confirmed this by electron microscopy of neurons, with tomosyn-lacking synapses showing 50 percent more vesicles at the membrane level than those with tomosyn present. He also deliberately stimulated synapses to encourage the release of glutamate and found that although normal tomosyn normally kept a lid on activity in wild-type animals, mutants could not properly curb the amount of synaptic transmission. .
A glaring difference
Considering the difference in glutamate release behavior between tonic and phasic neurons, Sauvola decided to examine the levels of tomosyn in these cell types. Weaker tonic neurons were found to have more than twice as much tomosyne as stronger phasic neurons, suggesting that tomosyne levels might explain the difference in glutamate release style.
To determine if tomosyn had such a central role, Sauvola did more stimulation experiments in both types of neurons. After stimulation in normal animals, phasic neurons emitted significantly more glutamate than tonic neurons, as expected. However, in tomosyn mutants, the two neuronal classes behaved similarly, with tonic neurons releasing more similarly to their phasic neuronal counterparts.
If tomosyn retained the release of glutamate vesicles specifically in tonic neurons, this could explain why only tonic neurons are capable of exhibiting PHP plasticity. Sure enough, when Sauvola disrupted glutamate receptors in muscle cells to induce the PHP response, he found that tonic neurons lacking tomosyn, just like controlling phasic neurons, couldn’t trigger this form of plasticity. But when he examined the response of normal tonic neurons, he found that synapse by synapse, there were significant increases in glutamate release – even synapses that previously showed very low propensity seemed to acquire a substantial ability to release. synaptic signals.
“This is truly an incredible discovery that I hadn’t anticipated,” says Littleton. “It is very surprising to see that these weak synapses could act much more mature on a very fast time scale.”
One of the next steps for the lab will be to determine what molecular interaction causes tomosyn to release the brakes when PHP is needed, Littleton says. Another future direction will be to examine other types of neurons, particularly in the brain, to see how tomosyn levels vary and how this affects their synaptic output.
But the new results definitely show that tomosyn’s ability to inhibit SNARE binding of vesicles and the resulting glutamate release makes a dramatic difference in the style of neural communication between tonic and phasic neurons.
Besides Sauvola, Littleton and Aponte-Santiago, the other authors of the article are Yulia Akbergenova and Karen Cunningham.
The National Institutes of Health and the JPB Foundation funded the research.