New York, NY (January 27, 2003) -- Researchers at Weill Cornell Medical College have revealed a missing link in the fundamental yet little understood process of neurotransmission, the process by which signals are carried from one brain cell to another. The new finding, which is detailed in the current issue of "Nature", highlights a crucial mechanism that may be common to many signaling processes in cells.

Process of Neurotransmission

One of the mysteries of life is the way in which the brain interprets information from the senses and responds with thought, emotion, or movement. The process is enabled by substances called neurotransmitters, which are chemicals that act as "go" or "no-go" signals between brain cells called neurons. The neurotransmitters act very quickly, within thousandths of a second, linking up with molecules called receptors, which sit on the outside of the neurons, waiting to catch transmitter substances. The receptors, in turn, act to convert chemical information into electrical signals, which tell the recipient cell to "go" or "no-go" -- to become activated or to remain silent. In order to translate the chemical signals, the receptors must change shape incredibly rapidly in response to the incoming neurotransmitter. The way in which the receptors "morph" is unknown, but the new research by the Weill Cornell team has uncovered a key part of this process.

GABA Neurotransmitter and Receptorp> The team of scientists was led by Dr. Neil Harrison, professor of neuroscience and pharmacology at Weill Cornell Medical College and Graduate School of Medical Sciences. The Weill Cornell researchers studied one neurotransmitter, GABA, that functions as a "no-go" or inhibitor signal in the brain, preventing excess activity and protecting the brain against epilepsy and damage to neurons. The discovery focuses on the way in which GABA, after binding in a small groove on its receptor, induces morphing of the receptor protein, which is firmly embedded in the cell membrane. "The morphing results in the opening of a small hole within the protein that allows chloride ions to flow unimpeded into the cell," explains Dr. Harrison, "thereby converting the chemical signal, GABA, into an electrical 'no-go' signal." The morphing process that results in the appearance of this hole or "channel" is commonly referred to as "gating" -- because it is as though a gate were opened to allow ions through.

The curious puzzle is that the binding groove for GABA lies toward the outer or opposite end of its receptor protein, far from the location of the ion gate, which lies deep within the cell membrane of the neuron. The researchers asked the question: How is the gate unlocked by GABA? In order for this to occur, the receptor protein must somehow wiggle or twist all the way from the GABA groove down to the gate. Gating must therefore involve a "wave" of shape change that extends between these relatively distant points. The way in which this shape change occurs is unknown, but must involve some contacts or "handshaking" between the amino acids that are the building blocks of the receptor protein molecule itself.

The Structure Unraveled: The Loop-Linker Interaction

At this point, the researchers cleverly employed the x-ray structure of a snail protein, a distant relative of the GABA receptor molecule but with a structure that is already well known, to determine how exactly this shape change occurred -- that is, to predict the contacts involved in the molecular "handshake." First, they focused on a part of the receptor known as the 2-3 linker, which sits right above the cell membrane. Rare inherited mutations in this linker are known to cause neural diseases such as epilepsy, indicating its critical importance in neurotransmission. Dr. Harrison's previous research had already showed that this linker was important for gating. Now he could ask: "Where exactly are the linker's partners located in the handshake?"

The solution, first suggested by Dr. Harrison's colleagues -- Weill Cornell instructor Dr. Andrew Jenkins and graduate student Tom Kash -- lay in the snail protein structure, recently solved by a group in the Netherlands. In this structure, there are two loops (loops 2 and 7) that hang down almost from the binding groove region to the bottom of the molecule. The scientists argued that the GABA receptor must have similar loops and began to look for interactions between loops 2 and 7 and the 2-3 linker. By changing individual amino acids in a process known as mutagenesis, and by using chemical agents to "cross-link" parts of the protein, Kash and research technician Jill Kelley were able to prove that two negatively charged amino acids in the loops were involved in a "handshake" with one positively charged amino acid in the linker region. When a GABA molecule changed the shape of the outer part of the receptor, pushing the loops close to the linker, the charges on the amino acids caused the linker to move as well, transmitting the shape change to the inner part of the receptor molecule and to the gate.

Computer modeling by Dr. Jim Trudell at Stanford University confirmed the likely proximity of the loops and linker regions, and showed how well these could twist and slide together. Of course, this work identifies only some of the moving parts involved in the shape-change wave. The way in which the gate is opened remains unknown -- in part, because the structure of the channel region of these molecules is as yet unsolved.

The Wider Significance

The solution by the Weill Cornell team is beautifully elegant in its own right, but it also points to a wider significance and a future research direction. As Dr. Harrison puts it, "the generality and generalizability of the research in many ways really lie in the electrostatic interactions that act as a sort of 'intra-molecular Velcro,' and this rapid zipping and unzipping probably occurs in lots of signaling molecules that undergo critical conformational changes, as in the case of this receptor for GABA."

The Weill Cornell Harrison Laboratory

The work was carried out at Weill Cornell by Dr. Harrison's team in the C.V. Starr Laboratory, including Kash, Jenkins, and Kelley, and at Stanford, by Dr. Trudell. Drs. Harrison and Jenkins moved to New York in 1999 from the University of Chicago. Dr. Harrison joined Weill Cornell as a Strategic Research Plan Recruit for Neuroscience, with the assistance of the Starr Foundation. He serves as Director of Weill Cornell's Graduate Program in Neuroscience.

The Harrison Laboratory is interested in synaptic transmission generally, especially at inhibitory synapses, which are necessary for the normal processing of information in the mammalian brain. Failure of synaptic inhibition leads to epilepsy, while enhancement of synaptic inhibition is associated with reduced anxiety, muscle relaxation, sedation, hypnosis, and anesthesia. The lab studies the details of inhibitory synaptic function, its modulation and plasticity, using a variety of modern electrophysiological and molecular biological techniques. Projects within the lab study these synapses at several different levels of organization, including brain slice, single cell, and subcellular preparations. A major focus of the lab is on the key GABA-a receptor, the principal receptor protein at inhibitory synapses in the brain. The lab personnel include physiologists, biophysicists, molecular biologists, and pharmacologists. The Harrison laboratory is funded by, the National Institute of General Medical Services (NIGMS) and the National Institute on Alcohol Abuse and Alcoholism (NIAAA).