Straightening out serotonin receptors

LA JOLLA, Calif.—Serotonin is one of the best-knownchemicals in the human brain, mainly due to its role in the reward center. Butnow some of serotonin's receptors are getting some attention as well, thanks tonew findings on how certain serotonin receptors interact with and affect somedrugs.

This work was conducted by a team of researchers from TheScripps Research Institute (TSRI), the University of North Carolina (UNC) atChapel Hill and the Chinese Academy of Sciences, and focused on the analysis ofhigh-resolution atomic structures of two kinds of human serotonin receptors.

The laboratory of TSRI Prof. Raymond Stevens, a seniorinvestigator for the new research, pioneered the development of techniques forthe determination of the 3D atomic structures of cellular receptors,particularly those that fall into the class of G protein-coupled receptors(GPCRs). The Stevens laboratory has used X-ray crystallography to determine thestructures of 10 of the most important GPCRs in the human body, including theß2 adrenergic receptor, the A2a adenosine receptor, HIV-related CXCR4 receptor,the nociceptin receptor, S1P1 receptor, H1 histamine receptor and the D3domapine receptor. These receptors affect issues such as pain mediation,inflammatory disease, antihistamine medications and mood.

"Because G protein-coupled receptors are the targets ofnearly 50 percent of medicines, they are the focus of several major NationalInstitutes of Health (NIH) initiatives," Jean Chin of the NIH's NationalInstitute of General Medical Sciences (NIGMS) said in a press release. TheNIGMS partly funded this work through the Protein Structure Initiative. "Thesedetailed molecular structures of two serotonin receptor subfamilies bound toantimigraines, antipsychotics, antidepressants or appetite suppressants willhelp us understand how normal cellular signaling is affect by these drugs, andwill offer a valuable framework for designing safer and more effectivemedicines."

Serotonin, for its part, is linked not only to moodregulation and the brain's reward center, but also to consciousness and thebody's sleep/wake cycles, according to Daniel Wacker, lead author and TSRIgraduate student.

Bryan Roth, a professor of pharmacology at UNC and acollaborator on both studies, noted in a press release that serotonin receptors"also mediate a host of effects outside the brain, for example on bloodcoagulation, smooth muscle contraction and heart valve growth."

In the first study, Chong Wang, co-lead author and agraduate student in the Stevens lab, and colleagues determined the atomicstructure of serotonin receptor subtype 5-HT1B, and produced the receptor whileit was bound with either ergotamine or dihydroergotamine, anti-migraine drugsthat activate 5-HT1B receptors. A special fusion protein known as BRIL was usedto stabilize the structures and line them up in a regular crystal ordering.When x-ray crystallography was applied, it revealed the atomic structure of thereceptor with a main binding pocket as well as a separate, extended bindingpocket.

Wacker and colleagues took a similar approach in the secondstudy in determining the structure of the 5-HT2B receptor bound to ergotamine.The 5-HT2B receptor is a target most drug developers want to avoid due to theoff-target, and usually harmful, effects that can result. In 1997, fenfluramineand dexfenfluramine, two weight-loss drugs, were pulled from the U.S. marketafter being linked to heart valve disease, which Roth's lab later discoveredwas thanks to heart valve 5-HT2B receptors.

Wacker says they were "absolutely not" expecting the resultstheir studies presented.

"We knew that particular drugs hit both receptors, but we hadno idea how similar their interactions were, because if you were to model the receptorsbased on other structures, you would have never gotten to the results we'vegotten," he explains. "I think the biggest surprise for us was that the same compoundbinds in a very similar manner to two different receptors, but the receptors, in key areas of their structure,are very different. So exploiting these particular areas and improvingthe drugs will hopefully lead to making safer and more selective medications."

The labs of Profs. Eric Xu and Hualiang Jiang at theShanghai Institute of Materia Medica, part of the Chinese Academy of Sciences,then used the receptor structures to simulate the bindings of various drugs,demonstrating that anti-migraine drugs, known as triptans, should bind well tothe 5-HT1B receptors, but poorly to 5-HT2B receptors, while fenfluramine'sactive metabolite ought to bind tightly with the 5-HT2B receptor. Roth's labfound that ergotamine and LSD, an ergotamine-derived hallucinogen, favorß-arrestin signaling at the 5-HT2B receptor.

"I think the big breakthrough of this, the big insight fromthese studies, comes from providing selectivity information," says Wacker. "Alot of drugs that were designed to target specific serotonin receptors were foundto also target the serotonin receptor 5-HT2B. It was later found that activation of this receptor causes valvular heart disease and pulmonary hypertension. So looking at these structures and figuring out a way of how to avoid binding and activation the 5-HT2B receptor, or even to find drugs that would antagonize it, would be very beneficial."

The two ways to avoid triggering the 5-HT2B receptor, Wackernotes, would be to either modify a drug so that it is incapable of binding withthe receptor, or to assign an antagonist for the receptor. Moving forward, headds that looking at more of the serotonergic receptor class is important, as aclose relative of the 5-HT2B receptors, the 5HT-2C receptor, is a good targetfor appetite-suppressing anti-obesity drugs. The knowledge of both structures,he says, could allow for safer medications that would avoid fenfluramine anddexfenfluramine's harmful side effects.

The two studies are titled "Structural Basis for MolecularRecognition at Serotonin Receptors" and "Structural Features for FunctionalSelectivity at Serotonin Receptors," and appeared in two papers on March 21 inScience Express, the advance online version of Science.