Atomic-Resolution Details of Neurotransmitters Discovered

Neuroscience  

Researchers have for the first time determined, at atomic-scale resolution, the 3D structure of a protein complex that provides the rapid trigger for chemicals messages sent between nerve cells in our brains. The discovery, which provides a new understanding of the molecular machinery driving brain function, builds on decades of research.

Researchers have uncovered never-before-seen details of how our brains send rapid-fire messages between neurons. They mapped the atomic structure of a two-part protein complex that controls the release of signaling chemicals, called neurotransmitters in three dimensions, from brain cells. It is believed that understanding how cells release those signals in less than one-thousandth of a second could help launch a new wave of research on drugs for treating brain disorders.

The experiments, at the Linac Coherent Light Source (LCLS) X-ray laser at the Department of Energy's SLAC National Accelerator Laboratory, build upon decades of previous research at Stanford University, Stanford School of Medicine and SLAC.

The scientists presented their latest findings in the journal Nature.

"This is a very important, exciting advance that may open up possibilities for targeting new drugs to control neurotransmitter release. Many mental disorders, including depression, schizophrenia and anxiety, affect neurotransmitter systems," said Axel Brunger, the study's principal investigator. He is a professor at Stanford School of Medicine and SLAC and a Howard Hughes Medical Institute investigator.

"Both parts of this protein complex are essential," Brunger said, "but until now it was unclear how its two pieces fit and work together."

The two protein parts are known as neuronal SNAREs and synaptotagmin-1.

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Earlier X-ray studies, including experiments at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) nearly two decades ago, shed light on the structure of the SNARE complex, a helical protein bundle found in yeasts and mammals.

SNAREs play a key role in the brain's chemical signaling by joining, or "fusing," little packets of neurotransmitters to the outer edges of neurons, where they are released and then dock with chemical receptors in another neuron to trigger a response.

In this latest research, the scientists found that when the SNAREs and synaptotagmin-1 interact, they act as an amplifier for a slight increase in calcium concentration, triggering a rapid release of neurotransmitters from one neuron to another. They also discovered that the proteins join together before they arrive at a neuron's membrane, which helps to explain how they trigger brain signals.

The researchers speculate that several of the joined protein complexes may group together and simultaneously interact with the same vesicle to efficiently trigger neurotransmitter release, an exciting area for further studies.

"The structure of the SNARE-synaptotagmin-1 complex is a milestone that the field has awaited for a long time, and it sets the framework for a better understanding of the system," said James Rothman, a professor at Yale University who discovered the SNARE proteins and shared the 2013 Nobel Prize in Physiology or Medicine.

"The structure of the SNARE-synaptotagmin-1 complex is a milestone that the field has awaited for a long time, and it sets the framework for a better understanding of the system."

Thomas C. Südhof, a professor at the Stanford School of Medicine and Howard Hughes Medical Institute investigator who shared that 2013 Nobel Prize with Rothman, discovered synaptotagmin-1 and showed that it plays an important role as a calcium sensor and calcium-dependent trigger for neurotransmitter release.

"The new structure has identified unanticipated interfaces between synaptotagmin-1 and the neuronal SNARE complex that change how we think about their interaction by revealing, in atomic detail, exactly where they bind together," Südhof said. "This is a new concept that goes much beyond previous general models of how synaptotagmin-1 functions.

To study the joined protein structure, researchers in Brunger's laboratory at the Stanford School of Medicine found a way to grow crystals of the complex. They used a robotic system developed at SSRL to study the crystals at SLAC's LCLS, an X-ray laser that is one of the brightest sources of X-rays on the planet. SSRL and LCLS are DOE Office of Science User Facilities.

The researchers combined and analyzed hundreds of X-ray images from about 150 protein crystals to reveal the atomic-scale details of the joined structure.

Brunger said future studies will explore other protein interactions relevant to neurotransmitter release. "What we studied is only a subset," he said. "There are many other factors interacting with this system and we want to know what these look like. This by no means is the end of the story."

SOURCE  SLAC

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