NEUROBIOLOGY:
Filling in the Blanks of The GABA_B Receptor

Valium and its copycat drugs soothe jangled nerves by augmenting the actions of the brain's own sedative, a neurotransmitter known as g-aminobutyric acid (GABA). They do this by binding to one of the cell-surface molecules through which GABA exerts its effects, the GABA_A receptor. But neurons have other GABA receptors that could also serve as drug targets for treating disorders ranging from epilepsy to pain. Now, four research teams have discovered a feature of this second class of GABA_B receptors that could open the way to more effective and subtle manipulations of the brain's GABA system.

The groups--one reporting its results in this issue of Science--have found that the GABA_B receptor is not a single molecule but instead consists of two different proteins, neither of which is effective on its own. This marriage of two disparate proteins to produce a functional receptor offers greater opportunities for drug design, as researchers can now target each protein separately as well as the receptor as a whole. And it has researchers speculating that the same kind of marriage, called a heterodimer, might also turn up in other members of the receptor class to which GABA_B belongs. These are known as G protein-coupled receptors for the kind of protein that relays their signal into the cell, and they number some 1000 in all.

"This is pretty wild," says neurobiologist Roger Nicoll of the University of California, San Francisco. "No one had ever shown that these [G protein-coupled] receptors can form heterodimers." Kenneth Jones at Synaptic Pharmaceutical Corp. in Paramus, New Jersey, whose group reported its findings in Nature, says the research "has major implications" both for understanding the workings of this large class of molecules, which also includes receptors for the neurotransmitter serotonin and for opiates, and for developing novel drugs to block or stimulate them.

The discovery solves a mystery that arose early in 1997 when molecular biologist Bernhard Bettler at the drug giant Novartis in Basel, Switzerland, and his colleagues cloned the first gene for a GABA_B component, a protein called GBR1. When inserted into cells, however, GBR1 could not perform a key function of natural GABA_B receptors: opening membrane channels that allow potassium ions to flow out of the cell. Now, Bettler's team and three others have found out why.

Aware that something seemed to be missing from the receptor cloned by the Bettler team, groups led by Hans-Christian Kornau of the biotech firm BASF-LYNX Bioscience AG in Heidelberg, Germany, and by Fiona Marshall at Glaxo Wellcome's Molecular Pharmacology unit in Stevenage, England, coaxed yeast cells to express the tail of GBR1. The tail was to serve as a bait for picking up any proteins that interact with it and might be needed for GABA_B function. Both teams turned up the same protein, which Kornau dubbed GBR2. Like GBR1, GBR2 turned out to contain seven hydrophobic regions that could thread through the lipid-rich cell membrane and two ends that could project inside and outside the cell. This structure suggested that GBR2 is also a receptor, and thus that two receptor molecules may operate as a duet in cells.

Meanwhile, the Novartis, Synaptic, and Glaxo teams were searching the GenBank human gene database for proteins resembling GBR1 in hopes of finding one that would do a better job of reproducing the GABA_B receptor's functions. Remarkably, they all picked out the same protein that had popped up in the yeast. But when the researchers coaxed cultured cells to express GBR2, along with the requisite potassium channels, this receptor also failed to produce robust potassium currents in response to GABA treatment.

Thinking they had missed the active part of the GABA_B receptor, the Synaptic team was about ready to give up when they looked at the expression patterns of both GBR1 and GBR2 in sections of rat brain, and noticed a striking overlap. This overlap, which the other scientific teams also saw, suggested that the two proteins may work together in individual neurons.

And that's what all the groups have now shown. When they expressed both GBR1 and GBR2 in cultured frog or human cells, the cells produced potassium currents. "It worked beautifully," says Jones. The Novartis group went one step further: Aided by specific antibodies, they demonstrated that the two proteins are closely associated on individual brain neurons. In addition, the Kornau group has mapped the site where the two proteins interact.

The BASF-LYNX group reports its findings on page 74; the other three papers appeared in the 17 December 1998 Nature. Together, the four papers suggest that GBR1 and GBR2 cooperate in at least two ways. First, the proteins are likely to help each other transmit GABA's signal within a neuron, allowing the neurotransmitter to activate the potassium channels. In addition, GBR2 may help shuttle GBR1 to its final location on the cell membrane, since the Glaxo team showed that GBR1 does not get to the membrane unless GBR2 is present.

Whatever the nature of the partnership, by providing a fully functional receptor, the discovery of GBR2 should help researchers design new drugs that work through GABA_B. Drugs that target GABA_B might, for example, provide a new range of therapies that help depress the excessive neuronal firing characteristic of epilepsy, pain, and anxiety, or perhaps help relieve neuronal inhibition to bolster memory or ameliorate depression. As yet, however, nobody can predict how successful such drug development efforts will be. "We're still far from a direct clinical application," says Kornau, "but knowing this receptor's structure is a significant step forward."