Scientists know many of the proteins that make neurotransmission possible, but they don’t have a handle on how all the pieces work together. “As someone who has studied the synapse for a long time, I still find it frustrating that we don’t really understand how synapses work,” says geneticist Jeremy Dittman of Weill Cornell Medical College.
Now, in two papers published in Neuron, scientists report new insights into one key player, the protein Unc13. The work offers potentially important molecular clues as to not only how neurotransmission works but also how it may go awry.
Before a neuron fires, chemical messengers called neurotransmitters wait inside vesicles. Proteins known as SNAREs join together to hold many of these vesicles tight against the neuron’s plasma membrane. When a neuron changes voltage, calcium from outside the neuron pours in, triggering the SNARE complexes to force vesicles to fuse with the cellular membrane and spill neurotransmitters into the synapse. But if the SNARE proteins come together in the wrong configuration, neurotransmission slows.
Scientists already knew that Unc13 plays a key role by prying the protein Unc18 from another called syntaxin, so that syntaxin is then free to become part of the SNARE complex. Biophysicist Axel Brunger, a Howard Hughes Medical Institute investigator at Stanford University, and colleagues report another key role of Unc13.
His team used florescent markers in a technique known as smFRET to track individual proteins in a test tube as the SNARE complex formed. They watched SNARE proteins assemble without Unc13, without Unc18, and without both of the proteins. It turns out that Unc13 is responsible for chaperoning one component of the SNARE complex into position while also working with Unc18 to orient another component. When both Unc13 and Unc18 were absent, the SNARE complex only formed in the correct orientation half of the time.
The team then tested this finding in a mouse model whose Unc13 proteins were completely inactivated, which severely limited neurotransmission. Introducing a genetic mutation in syntaxin caused Unc18 to release syntaxin without the help of Unc13. The team only saw a very partial return to normal function, suggesting that Unc13 plays a role beyond prying Unc18 from syntaxin.
Dittman’s team reports another insight into Unc13’s function. To understand how the protein works, they first tried to break it down, deleting an entire section of Unc13 in the worm model C. elegans. “We completely thought we were going to destroy the protein’s function,” says Dittman. “Instead, the synapses were releasing more neurotransmitters.” It turned out that the section they deleted known as the C2B domain actually inhibits Unc13’s function in neurotransmission. They then found that when calcium binds to the C2B domain, Unc13’s function ramps up, increasing the probability that a vesicle will fuse with the cell wall. So Unc13 works as a check to limit neurotransmission until the optimal moment.
To fully understand brain function, we must understand the multiple roles of Unc13, says neuroscientist Mark Palfreyman, a postdoctoral fellow at the University of Utah who coauthored an analysis of Brunger’s and Dittman’s work. “Brunger addresses how [Unc13] might function in regulation of the fusion machinery, and the Dittman paper suggests how Unc13 itself might be regulated,” he says.
“We’re still fascinated by this protein because we’ve only figured out a little clue,” says Dittman. He’s now searching for additional molecules that it may interact with. Brunger will soon publish a new crystal structure of the primed SNARE complex, and he’d like to capture images of the structure changing form as calcium triggers fusion.
Ultimately, both teams hope that by detailing the inner workings of the neurotransmission machinery, they may contribute to future therapies. “How does this machinery get affected by neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease?” questions Brunger. “Understanding at a molecular level might also open the door to having treatments that for some period may reduce the symptoms of these diseases.”