UW Lab Lifts Veil From 50-Year Mystery of How Neurons Communicate
Madison, Wisconsin - Nearly 45 years after Sir Bernard Katz won a Nobel Prize for discovering that neurotransmitters are released into a synapse in countable packets known as quanta, scientists still don’t know how that release works.
But what started as a simple structure-function paper from a University of Wisconsin-Madison lab has unveiled a big part of what Katz witnessed back in the 1950s.
According to the study published in Nature Neuroscience, one special protein may act as a master regulator, playing a role from beginning to end in the process of how - and when - a vesicle, a package inside a cell that contains cargo like neurotransmitters, is released.
Excitation-secretion coupling, a big phrase for the process of vesicles fusing with a cell membrane and releasing their cargo into the synapse when an electrical charge allows calcium ions to enter the cell, seems to be controlled by a protein called Synaptotagmin 1 (Syt1).
The protein regulates the basic machinery of a nerve cell once it binds with that calcium, ultimately causing the cell to communicate with its neighbors by secreting vesicle contents.
“In the course of trying to uncover a step in the process of excitation-secretion coupling that Katz worked on, we made a whole bunch of interesting discoveries,” said Ed Chapman, professor of neuroscience at the UW School of Medicine and Public Health and lead author.
Vital Findings from Study
According to the study, Syt1 prevents vesicles from fusing to the cell membrane - and eventually releasing their cargo - in the absence of a calcium ion. When calcium is present, Syt1 binds with the calcium and accelerates the rate at which vesicles release their cargo.
These findings are vital because it takes a synchronized effort on the part of a nerve cell to release many vesicles at once. The seemingly random release of one vesicle at a time doesn’t provide enough of a signal to transmit messages down a chain of nerves, like the chain connecting your eyes to your brain that helps you read this sentence. With approximately 1015 synapses in the human body, these events are happening constantly and with incredible speed.
Another molecular machine that assists in the fusion process involves a set of SNARE proteins. SNAREs, one found on the vesicle and one on the target membrane, intertwine and pull the vesicle and cell membranes together and start fusing the two membranes, but their function isn’t regulated by calcium, which drives the synchronized release.
“We thought that the ‘work’ that Syt1 does on SNAREs was going to be important for vesicle fusion and cargo release, but we didn’t see a correlation,” said Chapman. Instead, Syt1 mediated release via its ability to do ‘work’ on membranes.
Results Derived from Earlier Experiments
What’s more surprising is that all these results in the new study were derived from a set of experiments that manipulate a small chain of amino acids only a nanometer long. The experiments were performed by Chapman lab members Huisheng Liu, associate research scientist at the UW Waisman Center, and Hua Bai, a UW graduate student in the physiology program.
Each Syt1 molecule is anchored to the vesicle membrane, extending outward like two arms prepared to grab the cell membrane to begin the fusion process. At the end of each protein lie two calcium sensors that look like pearls on a string. In the presence of calcium, those pearls penetrate the cell membrane, but whether this penetration triggered fusion remained unknown until this new study.
The two sensors are connected by a tiny linker that is only nine amino acids long. By testing combinations of the mutated linker of varying length and stiffness, correlations between in vitro and in vivo experiments fell too closely inline to be a coincidence.
“The beauty of Huisheng’s work is that, when we manipulated the linker, the synaptic transmission is different for all the different constructs, so we’re able to get a spread of different responses,” said Chapman.
“Years later when Hua did the membrane penetration experiments, he saw a spread of different responses, and the shocker was when we plotted them against each other, they were really tightly correlated. People have worked on excitation secretion coupling for 50 years, and we’ve just peered into a process that Katz described.”
Katz collaborated with Drs. Paul Fatt, José del Castillo and Ricardo Miledi. Chapman first met Miledi at a conference, and Miledi showed interest in the work he was doing on Syt1.
“He befriended me, so this is sort of personal for me,” said Chapman. “These guys (Katz, Fatt, Castillo and Miledi) did all this work showing calcium’s role, I uncover this little penetration step here as a snot-nosed assistant professor back in 1998, and Hua and Huisheng, 16 years later, do the physiology and the biophysics and publish this paper explaining an important part of excitation secretion coupling.”
Date Published: 04/23/2014