Associate editor: D. HoyerStructure and function of serotonin G protein-coupled receptors
Introduction
Serotonin or 5-hydroxytryptamine (5-HT) remains one of the most widely studied chemical messengers. Serotonin produces a myriad of physiological effects in humans, mediated through 14 distinct receptor subtypes, of which 13 are G protein-coupled receptors (GPCRs), and one ligand-gated cation channel (Hoyer et al., 1994, Berger et al., 2009). 5-HT receptors have evolved over the course of 700-800 million years (Peroutka and Howell, 1994, Kroeze and Roth, 1998). In the human central nervous system (CNS) alone, all the serotonin receptor subtypes, with the exception of 5-HT5b, are expressed, and they are involved in the modulation of sleep-wake cycles, emesis, appetite, mood, memory, breathing (Ray et al., 2011), cognition, and many other functions (Berger et al., 2009, Meltzer and Roth, 2013). Much of the serotonin in the body, however, is not found in the CNS, rather in the gastrointestinal (GI) tract, where it causes peristalsis through either smooth muscle contraction or enteric nerve depolarization (Gershon et al., 1990). Lesser known, but historically important, serotonin is found in the blood, isolated from platelets in the serum, where it is involved in blood coagulation and vasoconstriction, a function which led to its name “sero” (from serum) and “tonin” (to induce contraction)(Rapport et al., 1948b). Not surprisingly, more than 125,000 published articles as of 2014 (pubmed search) have been published on serotonin and its receptors.
Much research regarding serotonin has been in the area of neuropsychiatric drug discovery in treatment of affective disorders, where there continues to be an extreme interest in the design of more efficacious pharmaceuticals. Drugs either targeting serotonin receptors or serotonin itself represent a large share of the top selling pharmaceuticals in the past decade, with more being approved for future use. This is most evident with antidepressants such as SSRIs (serotonin-selective reuptake inhibitors) dominating the drug market, with over $11 billion in sales in 2008 alone, and Cymbalta (duloexetine), a dual serotonin-norepinephrine reuptake inhibitor (SNRI), being in the top 10 drugs sold in the US in 2012 (Lindsley, 2013). Additionally, newer antipsychotics, such as aripiprazole (Abilify) and quetiapine (Seroquel), which have partial agonist activity and/or antagonist activity at serotonin receptors, were also in the top 10 drugs sold in the US. Recently, newer SNRIs are being approved by the FDA with an indication to treat depression, including the approved levomilnacipran (Fetzima), and SSRIs with direct agonist activity at the 5-HT1A receptor have been approved by the FDA including vilazodone (Viibryd) and vortioxetine (Brintellix) for treatment of major depressive disorder (Celada et al., 2013). There is, however, a need for the discovery of more efficacious CNS pharmaceuticals with reduced side effects associated with off-target activity. For example, agents for Parkinson’s disease, such as pergolide and bromocriptine, were discontinued in the US because of their ability to induce cardiac valve hypertrophy, which has been linked to off-target agonist action at the 5-HT2B receptor (Rothman et al., 2000, Setola et al., 2003, Horvath et al., 2004, Roth, 2007).
In addition to drug selectivity, the concept of functional selectivity (Urban et al., 2007) in pharmacology has revolutionized the drug discovery process with recent findings highlighting that functionally selective β-arrestin biased drugs can show efficacy in preclinical models of schizophrenia (Allen et al., 2011), and β-arrestin biased angiotensin II type 1 compounds are currently in Phase 2 clinical trials for treatment of congestive heart failure (Soergel et al., 2013). Thus, rational drug design of newer serotonin receptor drugs incorporating ligand bias appears to be a fruitful area of drug development that potentially could be facilitated by insights into the structure and function of their respective targets.
Recently, there has been a GPCR structural “renaissance” with crystal structures from several representative receptor classes being solved, including rhodopsin (Palczewski et al., 2000), β2 adrenergic (Cherezov et al., 2007), β1 adrenergic (Warne et al., 2008), A2A adenosine (Jaakola et al., 2008), H1 histamine (Shimamura et al., 2011), D3 dopamine (Chien et al., 2010), smoothened (Wang et al., 2013b), CXCR4 (Wu et al., 2010), sphingosine 1-phosphate (Hanson et al., 2012), protease-activated receptor 1(Zhang et al., 2012), M2 muscarinic (Haga et al., 2012), M3 muscarinic (Kruse et al., 2012), mu opiate (Manglik et al., 2012), delta opiate (Granier et al., 2012), kappa opiate (Wu et al., 2012), nociceptin/orphanin (Thompson et al., 2012), and recently the 5-HT1B and 5-HT2B serotonin receptors (Wacker et al., 2013, Wang, Jiang, et al., 2013). These receptor crystal structures provide a wealth of structural information for the design of more receptor subtype selective agents, and also provide insights into the chemo-mechanical processes involved in GPCR activatation (Mustafi & Palczewski, 2009). Thus, GPCR structures likely will serve medicinal chemists and pharmacologists to design new functionally selective drugs as potential therapies or pharmacological tools (Kenakin & Christopoulos, 2013).
The recent publications of two serotonin receptor crystal structures in complex with ergotamine (ERG), a former antimigraine agent, has opened up two major discussions relevant to the serotonin field, which concern the molecular basis for 1) serotonin receptor subtype recognition and 2) functional selectivity. The 5-HT1B crystal structure illustrates the molecular basis for the selectivity of antimigraine triptan drugs that are devoid of 5-HT2B receptor off-target activity, and the 5-HT2B crystal structure provides clues into the functional mechanics of a receptor in a β-arrestin biased state. These crystal structures not only further our understanding into serotonin’s therapeutic and off-target effects in the current drug discovery process, but also serve to refine our understanding of GPCRs in general.
This review will be divided into two major sections. The first part will provide a background and relevance of serotonin as it relates to our current understanding of its actions in the body and in medicine, with a particular emphasis on the 5-HT1B and 5-HT2B receptors. The second part will focus on the structural importance of the recently published serotonin receptor crystal structures, especially as it relates to our current understanding of GPCR structure and function. The final section will suggest future areas for the development of our understanding of functional selectivity and biased agonism.
Section snippets
Discovery of 5-HT and 5-HT receptors
Serotonin was actually discovered independently by two laboratories, Vittorio Erspamer’s lab in Rome, Italy (Erspamer & Boretti, 1950) and Irvine Page’s lab at the Cleveland Clinic (Rapport et al., 1948a, Rapport et al., 1948b). Although the action of serotonin is usually associated with CNS function, the majority of serotonin actually resides in the gastrointestinal (GI) tract, produced by enterochromaffin cells lining the lumen of the gut, where it causes increased peristaltic activity (
Structural features of serotonin 5-HT receptors
Since the cloning and annotation of the human genome, over 5% of the genome encodes for receptors, which more than 800 are GPCRs (Kroeze et al., 2003, Stevens et al., 2013). GPCRs are usually divided into five classes and are named according to a representative prototypical receptor: rhodopsin (Class A), secretin (Class B), metabotropic glutamate (Class C), adhesion and smoothened/frizzled/taste (Fredriksson et al., 2003). Within these families, they share considerable sequence homology, and
Conclusions and future directions
Crystal structures provide a rather static template for the design of novel drugs aimed at selectivity among the serotonin receptors. Currently, the 5-HT1B and 5-HT2B crystal structures have guided the development of novel compounds selective for the 5-HT1B receptor over the 5-HT2B receptor (Rodriguez et al., 2014). In fact, with respect to the design of functionally selective ligands, efforts are already underway to understand and exploit the extended binding site with molecular modeling, in
Methods used for generation of GPCR alignments
Figures utilized the 5-HT1B and 5-HT2B crystal structures with ERG bound (Wang, Jiang, et al., 2013, Wacker et al., 2013), and also the muscarinic M2 receptor crystal structure with iperoxo and LY2119620 bound (Kruse et al., 2013). All structures were downloaded from the Protein Data Bank (5-HT1B PDB code 4IAR; 5-HT2B PDB code 4IB4; M2 PDB code 4MQT). Using only the GPCR and complexed ligand, structures were aligned using the PyMOL Molecular Graphics System, Version 1.1 Schrödinger, LLC.
Conflict of interest statement
The authors declare that there are no conflicts of interest.
References (183)
- et al.
Mapping the binding site pocket of the serotonin 5-hydroxytryptamine2a receptor. Ser3.36(159) provides a second interaction site for the protonated amine of serotonin but not of lysergic acid diethylamide or bufotenin
J Biol Chem
(1996) - et al.
Mining the receptorome
J Biol Chem
(2005) - et al.
Integrated methods for the construction of three-dimensional models and computational probing of structure-function relations in G protein-coupled receptors
- et al.
5-HT1B receptors Are negatively coupled with adenylate cyclase in rat substantia nigra
Eur J Pharmacol
(1988) - et al.
Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine
Neuropharmacology
(1986) - et al.
Synthesis and pharmacological properties of a New hydrophilic and orally bioavailable 5-HT4 antagonist
Eur J Med Chem
(2013) - et al.
The human serotonin 5-HT2B receptor: Pharmacological link between 5-HT2 and 5-HT1D receptors
FEBS Lett
(1994) - et al.
A single mutation in the 5-HT4 receptor (5-HT4-R D100(3.32)a) generates a Gs-coupled receptor activated exclusively by synthetic ligands (RASSL)
J Biol Chem
(2003) Relevance of rhodopsin studies for GPCR activation
Biochim Biophys Acta
(2014)- et al.
Evidence for expression of the 5-hydroxytryptamine-2B receptor protein in the rat central nervous system
Neuroscience
(1997)
Role of 5-HT5A receptors in the consolidation of memory
Behav Brain Res
Molecular cloning and functional characterization of a human 5-HT1B serotonin receptor: A homologue of the Rat 5-HT1B receptor with 5-HT1D-like pharmacological specificity
Biochem Biophys Res Commun
Species differences in the pharmacology of terminal 5-HT autoreceptors in mammalian brain
Trends Pharmacol Sci
Mutations of transmembrane IV and V serines indicate that All tryptamines Do Not bind to the Rat 5-HT2A receptor in the same manner
Brain Res Mol Brain Res
The function of a highly-conserved arginine residue in activation of the muscarinic M1 receptor
Eur J Pharmacol
5-HT4 receptor enhances the propulsive power of the peristaltic reflex in the rat distal colon
Auton Neurosci
Allosteric sodium in class a GPCR signaling
Trends Biochem Sci
The molecular biology of serotonin receptors: Therapeutic implications for the interface of mood and psychosis
Biol Psychiatry
Arrestin/clathrin interaction. Localization of the clathrin binding domain of nonvisual arrestins to the carboxy terminus
J Biol Chem
Molecular cloning of a human gene (S31) encoding a novel serotonin receptor mediating inhibition of adenylyl cyclase
FEBS Lett
Amisulpride is a potent 5-HT7 antagonist: Relevance for antidepressant actions in vivo
Psychopharmacology (Berl)
The rat 5-hydroxytryptamine1b receptor is the species homologue of the human 5-hydroxytryptamine1d beta receptor
Mol Pharmacol
Strategies to discover unexpected targets for drugs active at G protein-coupled receptors
Annu Rev Pharmacol Toxicol
Discovery of beta-arrestin-biased dopamine D2 ligands for probing signal transduction pathways essential for antipsychotic efficacy
Proc Natl Acad Sci U S A
Contribution of a helix 5 locus to selectivity of hallucinogenic and nonhallucinogenic ligands for the human 5-Hydroxytryptamine2A and 5-Hydroxytryptamine2C receptors: Direct and indirect effects on ligand affinity mediated by the same locus
Mol Pharmacol
Rethinking 5-HT1A receptors: Emerging modes of inhibitory feedback of relevance to emotion-related behavior
ACS Chem Neurosci
The probable arrangement of the helices in G protein-coupled receptors
EMBO J
Serotonin-induced increases in adult cell proliferation and neurogenesis are mediated through different and common 5-HT receptor subtypes in the dentate gyrus and the subventricular zone
Neuropsychopharmacology
The effect of cooling and of 5-hydroxytryptamine on the peristaltic reflex of the isolated guinea-Pig ileum
Br J Pharmacol Chemother
The expanded biology of serotonin
Annu Rev Med
Piperazine derivatives including the putative anxiolytic drugs, buspirone and ipsapirone, are agonists at 5-HT1A receptors negatively coupled with adenylate cyclase in hippocampal neurons
Naunyn Schmiedebergs Arch Pharmacol
Pharmacological characterization of 5-Hydroxytryptamine4(5-HT4) receptors positively coupled to adenylate cyclase in adult guinea Pig hippocampal membranes: Effect of substituted benzamide derivatives
Mol Pharmacol
Mapping of serotonin 5-HT(4) receptor MRNA and ligand binding sites in the post-mortem human brain
Synapse
The pharmacology and distribution of human 5-hydroxytryptamine2b (5-HT2B) receptor gene products: Comparison with 5-HT2A and 5-HT2C receptors
Br J Pharmacol
Assessment of the roles of serines 5.43(239) and 5.46(242) for binding and potency of agonist ligands at the human serotonin 5-HT2A receptor
Mol Pharmacol
Primary pulmonary hypertension and fenfluramine use
Br Heart J
Serotonin 5-HT1A receptors as targets for agents to treat psychiatric disorders: Rationale and current status of research
CNS Drugs
High-resolution crystal structure of an engineered human beta2-adrenergic G protein-coupled receptor
Science
Structure of the human dopamine D3 receptor in complex with a D2/D3 selective antagonist
Science
A single point mutation (Phe340– > Leu340) of a conserved phenylalanine abolishes 4-[125I]iodo-(2,5-dimethoxy)phenylisopropylamine and [3H]mesulergine but Not [3H]ketanserin binding to 5-Hydroxytryptamine2 receptors
Mol Pharmacol
Differential ergoline and ergopeptine binding to 5-Hydroxytryptamine2A receptors: Ergolines require an aromatic residue at position 340 for high affinity binding
Mol Pharmacol
Attenuated response to stress and novelty and hypersensitivity to seizures in 5-HT4 receptor knock-out mice
J Neurosci
Engineering GPCR signaling pathways with RASSLs
Nat Methods
Valvular heart disease associated with fenfluramine-phentermine
N Engl J Med
5-HT2B receptor signaling in the Rat stomach fundus: Dependence on calcium influx, calcium release and protein kinase C
Behav Brain Res
The 5-HT5A receptor regulates excitability in the auditory startle circuit: Functional implications for sensorimotor gating
J Neurosci
5-HT3 receptors Are membrane ion channels
Nature
Preferential modulation of mesolimbic Vs. Nigrostriatal dopaminergic function by serotonin(2C/2B) receptor agonists: A combined in vivo electrophysiological and microdialysis study
Synapse
G-protein-coupled receptor structure, ligand binding and activation as studied by solid-state NMR spectroscopy
Biochem J
Activation mechanism of the beta2-adrenergic receptor
Proc Natl Acad Sci U S A
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