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NEUROBIOLOGY Worm Used to Hook New Serotonin ReceptorIf Human Homologue Netted, Drug Therapy Menu May Grow Serotonin is a mighty molecule to conjure with, especially considering recent "wonder" drugs that have revolutionized the treatment of disorders from depression to migraine by using antagonists, receptor antagonists, and re-uptake inhibitors to affect its function. Nearly all of these drugs target a single family of serotonin receptors—slow ones. Now comes word in the Nov. 23 Nature of the discovery by Stephen Cannon, HMS associate professor of neurobiology at Massachusetts General Hospital, and two MIT collaborators of a new serotonin receptor—a very fast one. Controlled by a gene named mod-1 (for modulation of locomotion defective), this new serotonin-gated chloride channel receptor mediates fast inhibitory neurotransmission, at least in the worm C. elegans. If a homologue can be found in mammals, mod-1 will expand the possibilities surrounding this second branch of serotonin receptors, including new drugs to affect them.
After identifying a worm serotonin receptor, Stephen Cannon is searching for a human homologue. Photo by Graham Ramsay That could be big news in serotonin biology, said Cannon. With only one exception, all the current serotonin drugs—from fluoxetine in mood-elevating Prozac to sumatriptan in migraine-blocking Imitrex—are thought to act on "slow" modulatory, or metabotropic, receptors. Until now, the fast class of ligand-activated, or ionotropic, serotonin receptors had but one known member. The characterization of the mod-1 receptor by Cannon and MIT graduate student Rajesh Ranganathan and Howard Hughes Medical Institute investigator Robert Horvitz makes two. And the two fast receptors seem to have opposite effects, one inhibitory and the other excitatory. Slow Cascade"There are many different types of receptors in the brain and in neurons that respond to serotonin," Cannon explained. "They have been divided into two subtypes. The largest group is the slow modulatory ones, in terms of numbers of receptor types that have been cloned. Binding of serotonin to receptors on the cell surface triggers a whole cascade of intracellular signaling reactions."Slow receptors are also called G protein–coupled receptors for the guanine-binding protein that switches on the intracellular signaling cascade. Slow is relative, especially in neurons. A "slow" modulatory receptor can take as long as 100 milliseconds, said Cannon, but once the cascade begins, the cell can be affected for hours. This makes slow receptors ideal drug targets for influencing states like mood, sleep–wake cycles, or satiety. An ionotropic receptor is faster but its impact more fleeting. "The principle here is that the same molecule that is the receptor and binds to the transmitter also forms a channel," he said. "When the receptor is bound, the channel tends to open and this occurs very quickly. The response is transient, on the order of milliseconds, because the channel either desensitizes or the neurotransmitter is degraded or taken up. These are called ionotropic because they make an ionophore, an ion channel in the membrane." The one previously known ionotropic serotonin receptor, 5HT3a ("5-HT" for 5-hydroxytryptamine, "3a" for the receptor), opens a nonselective channel for positively charged potassium and sodium cations that depolarize the neuron and make it more likely to fire. Thus it's excitatory. The researchers' new mod-1 channel allows the passage of only negatively charged chloride ions. Consequently, it's inhibitory.
A new inhibitory serotonin-gated channel receptor (left) identified by Stephen Cannon and colleagues at MIT joins just one other (middle) in the family of ionotropic, or fast, serotonin receptors. Members of the far larger family of metabotropic, or G protein–coupled, receptors lead to an intracellular signaling cascade that may affect the cell for hours. The fast receptors work more rapidly than these slow, metabotropic receptors, but their impact is fleeting. Adapted from original by Stephen Cannon Suspicious BehaviorMeantime, Cannon gives full credit for the discovery of mod-1 to the MIT team that first isolated a mutant C. elegans that exhibited a strange behavioral deficit: it failed to stop at food when it was hungry. Deprived of food for 30 minutes and then dumped onto an annular lawn of tasty bacteria, the mutant C. elegans raced to the edge without pausing to eat. The failure of the worm's "enhanced slowing response" suggested a serotonin-signaling problem. Working in the Horvitz lab, Ranganathan managed to locate the deviant gene on chromosome V and eventually clone mod-1."From looking at the DNA sequence, it was obvious that the gene he had stumbled upon was some kind of membrane receptor," said Cannon. "It was clear that it was a member of the fast ligand-activated channels. But just looking at the sequence, you cannot say explicitly that this will necessarily be a serotonin receptor." The MIT researchers knew they had to put mod-1 to functional tests to see if its protein was an ion channel receptor. "And that's where we came in," Cannon continued. "They are very good at worm genetics and biology, but the Horvitz lab doesn't explicitly assay channel function. Our lab here at MGH is interested in how ion channels regulate the electrical excitability of cells." Ranganathan came calling with mod-1. Cannon remembers his first response—a very firm no. Running a renowned "ion channel" lab, Cannon is constantly being asked to help out on interesting problems that are unfortunately far afield from his core concentration, how mutations in ion channel genes cause human diseases, particularly in skeletal muscle. "I sent him to Brandeis and to other people around the campus," Cannon recalled. "No one would take him up on it. Several weeks later, Rajesh came back and said, 'Could we at least try a pilot experiment?' He would make the RNA, and we would inject it into a frog egg and just see if there was any electrical response after applying serotonin. And the very first time, it worked beautifully. Amazing." The initial results were quickly replicated and thus began the work to fully characterize mod-1 as a new fast serotonin receptor that was specific for chloride ions and was not blocked by calcium ions or antagonists of the other fast-type receptor. It was, in turn, inhibited by the serotonin receptor antagonists mianserin and methiothepin. Mutant worms defective in mod-1 were also resistant to exogenous serotonin. That demonstrated, the search is now on for a mammalian version of mod-1. "We're optimistic that a homologous channel exists in humans, and we're searching for it now," Cannon said. Serotonin followers everywhere, stay tuned. —John Fleischman |
