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The Structure of Bias at Serotonin Receptors

26 March 2013. Understanding the targets of antipsychotic drugs, particularly G protein-coupled receptors (GPCRs), may be the key to designing better medicines for schizophrenia and other psychiatric disorders. In the world of GPCRs, two paths lead away from receptor activation: one involves the eponymous G protein second messenger cascade, and another “non-canonical” path activates β-arrestin, a protein that spurs a separate chain of command. But ligands for GPCR do not always stimulate both paths equally, and this may depend on slight differences in GPCR structure induced by a ligand, according to a paper published March 21 in Science.

Led by Raymond Stevens of the University of California in San Diego, and Bryan Roth at the University of North Carolina in Chapel Hill, the study reports finding a biased ligand, called ergotamine, for the serotonin 2B (5HT2B) receptor, and capturing the receptor’s crystal structure while bound to ergotamine. A comparison of this structure with that of the 5HT2A receptor published in a companion paper reveals key features likely to contribute to biased signaling.

There has already been significant work on decoding the structural sources of biased signaling, also called “functional selectivity.” In 2008, Marc Caron and colleagues reported that atypical antipsychotics, which block the dopamine 2 (D2) receptor, interfered with both pathways (see SRF related news story). Interestingly, their effects on the G protein pathway were more variable than on the β-arrestin pathway, which suggested that tamping down β-arrestin signaling might be the more important feature behind why they work. In 2011, Bryan Roth and colleagues developed new ligands for D2 receptors that selectively activated the β-arrestin pathway (see SRF related conference story). These had effects in rodents that matched those of antipsychotic drugs, without motor side effects.

The new findings lay bare for the first time the structures contributing to functional selectivity. Though atypical antipsychotics like clozapine target 5HT2A and 5HT1A receptors (Meltzer et al., 2011), and a biased ligand for a 5HT2A receptor has been designed (see SRF related news story), the 5HT2B receptor featured in the new study does not contribute to antipsychotic drug action. The lessons from it, however, could translate to other GPCRs.

Biased toward β-arrestin
First author Daniel Wacker and colleagues began by expressing cloned human 5HT2B and 5HT2A receptors in HEK293 cells in culture. Tracking cAMP production and phospholipase C activation gave a readout of the G protein pathway stimulation of 5HT1B and 5HT2B receptors, respectively, whereas monitoring interactions between β-arrestin and the 5HT receptors measured stimulation of the β-arrestin pathway. The researchers found that when ergotamine bound to 5HT2B receptors, it selectively activated the β-arrestin pathway; however, when bound to 5HT1B receptors, it lost this bias, activating both pathways fairly equally. This bias held true for related molecules, including lysergic acid diethylamide (LSD), which is derived from ergotamine.

This difference gave the researchers an opportunity to dissect the structural basis of functional selectivity. Crystallizing the 5HT2B receptor while bound to ergotamine, and determining its fine structure down to 2.7 Å, revealed a receptor looking very much like the GPCR it is, composed of seven helices spanning the membrane, bundled together. But a careful comparison of it with the 5HT1B structure solved in the companion paper (Wang et al., 2013), also while bound to ergotamine, revealed some key differences. One difference emerged along the base of the ligand binding pocket, where three amino acids form a kind of trigger, so that agonist binding changes their positions and triggers shifts in the positions of the helices. Compared to the 5HT2A receptor, the 5HT2B receptor shared the same position for two of these residues, but diverged for the third.

On the cytoplasmic side of the receptors, the researchers found a difference in the positions of two of the helices—something that alters the accessibility of binding sites for G proteins or β-arrestin. For the 5HT2B receptor, one of these helices held a position associated with β-arrestin signaling, but the other helix only partially conformed to the position associated with G protein signaling. In another part of the protein, a difference in the side chains in “micro-switch” domains was also discovered, with the 5HT2B receptor showing an intact salt bridge between two side chains, and the 5HT1B receptor missing this salt bridge, due to the non-permissive arrangement of its side chains.

Loops and leashes
The researchers noticed a possible explanation for these differences along an extracellular loop connecting two helices in the bundle. For the 5HT2B receptor, a sharp kink was formed at the connection between the extracellular loop and the top of one of the helices. This shortened the distance spanned by the loop, which could essentially put a leash on the range of movement of these helices. Thus, ergotamine binding would seem to stabilize a more restricted conformation of the receptor, and produce a distinct shape associated with β-arrestin signaling.

The findings illustrate the importance of examining the different structures induced by ligand binding, especially between two receptors with different responses to the same ligand. By tracking the microscopic shifts, rotations, and rearrangements induced by a ligand, researchers will usher in more rational drug design.—Michele Solis.

Reference:
Wacker D, Wang C, Katritch V, Han GW, Huang XP, Vardy E, McCorvy JD, Jiang Y, Chu M, Siu FY, Liu W, Xu HE, Cherezov V, Roth BL, Stevens RC. Structural Features for Functional Selectivity at Serotonin Receptors. Science. 2013 Mar 21. Abstract

Comments on Related News


Related News: Hidden Complexity Seen in Serotonin Signaling

Comment by:  Patricia Estani
Submitted 23 February 2008
Posted 26 February 2008
  I recommend the Primary Papers

Related News: Hidden Complexity Seen in Serotonin Signaling

Comment by:  Atheir Abbas
Submitted 25 February 2008
Posted 27 February 2008
  I recommend the Primary Papers

Implicit in the findings of Schmid et al. is the idea that the relationship among ligand, receptor signaling, and cellular context is an extremely complex one that will take a great deal more work to tease out. Thus, Dr. Bryan Roth has proposed on a number of occasions (see, for example, Gray and Roth, 2007; Abbas and Roth, 2005) that novel approaches for drug discovery may prove more effective in producing schizophrenia drugs that have greater therapeutic efficacy with lower side effect liability. Since it will likely be many years before the field has a detailed understanding of the "nitty-gritty" of the receptor signaling and trafficking relevant to schizophrenia and its treatment, we have suggested a number of approaches that are less reliant on such information.

For example, approaches based on screening for drugs that either mimic the gene expression profiles of gold standard drugs such as clozapine or normalize schizophrenia-associated changes in gene expression are being explored. Another approach is behavior-based screening, in which targeted screens are performed with drugs to find those that have efficacy in animal disease models. A further related approach, exemplified by Psychogenics' Smartcube(TM) (the associated database is called Smartbase[TM]) involves injecting drugs and monitoring the resulting behavior using computer-based machine learning to generate a multidimensional behavioral signature for gold standard drugs. Drugs can then be screened to look for those that mimic gold standard drugs in terms of their signatures. Though Psychogenics does not appear to have done much (at least publicly) with this approach, it represents the sort of innovative thinking that may prove fruitful in future behavior-based drug discovery efforts since it is not dependent on knowing anything about the mechanism. In the end, at least in the near future, we believe such approaches may prove extremely useful in drug discovery efforts since they do not rely on extensive mechanistic knowledge of the processes underlying schizophrenia.

References:

Gray JA, Roth BL. The pipeline and future of drug development in schizophrenia. Mol Psychiatry. 2007 Oct ;12(10):904-22. Abstract

Abbas A, Roth B. Progress towards better understanding and treatment of major psychiatric illnesses. Drug Discov Today. 2005 Jul 15;10(14):960-2. Abstract

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Related News: An Arrestin Development: Antipsychotic Drugs Hit Dopamine Signaling in New Way

Comment by:  Zachary Z. FreybergEneko UrizarHolly MooreJeffrey Lieberman (SRF Advisor)Jonathan Javitch
Submitted 30 December 2008
Posted 30 December 2008

Reevaluation of the dopamine D2 receptor in the treatment of schizophrenia: Novel intracellular mechanisms as predictors of antipsychotic efficacy
Since the advent of antipsychotic medications, there have been many speculations about their precise mechanisms of therapeutic action. Although it is apparent that blockade of dopamine D2 receptors (D2R) is crucial to the efficacy of all current antipsychotic medications, it is not clear which signaling events downstream of the D2R must be blocked for the therapeutic actions of antipsychotics and which events, when blocked, lead instead to side effects.

The best characterized D2R-mediated signaling pathways involve coupling of the receptor to pertussis toxin-sensitive G proteins of the Gi and Go subfamilies (Sidhu and Niznik, 2000), through which D2R activation results in a decrease in cyclic AMP (cAMP). D2R activation can also have a number of other effects, including enhancement of specific potassium currents, inhibition of L-type calcium currents, mediation of extracellular signal-regulated kinase 1 (ERK1) and potentiation of arachidonic acid release (Beom et al., 2004; Missale et al., 1998; Perez et al., 2006; Hernández-López et al., 2000). There is growing evidence that D2Rs can interact with a number of membrane-bound or intracellular proteins, which may further modulate signaling specificity (reviewed in Terrillon and Bouvier, 2004; Ferré et al., 2007a). In particular, D2R heteromerization may result in a switch from Gi/o coupling to Gs (i.e., through D2R and cannabinoid 1 receptor interaction) (Kearn et al., 2005) or to coupling with Gq (as suggested in D2R and D1R interactions) (Rashid et al., 2007). Moreover, heteromerization between D2R and other receptors such as the adenosine A2A receptor may allow for reciprocal modulation of D2R function (Ferré et al., 2007a; Ferré et al., 2007b). It also has been suggested that calcium signaling mechanisms may modulate D2R’s signaling efficacy; interaction between D2R and calcium-binding protein S100B results in enhanced D2R intracellular signaling (Liu et al., 2008; Stanwood, 2008).

The interaction between D2R and arrestin has received increasing attention. Following D2R activation, D2R signaling is attenuated by recruitment of arrestin 3 to the cell surface where it binds to the receptor (Klewe et al., 2008; Lan et al., 2008a ; Lan et al., 2008b), leading to inactivation and internalization of the D2R. Arrestin 3 also binds Akt—a serine/threonine kinase involved in multiple cellular functions and implicated clinically in schizophrenia (Arguello and Gogos; 2008; Beaulieu et al., 2005; Brazil and Hemmings, 2001; Brazil et al., 2004; Emamian et al., 2004; Kalkman, 2006). Following D2R activation by dopamine, the signaling scaffold formed by arrestin 3, while facilitating receptor desensitization and internalization, also recruits Akt into a complex with the phosphatase PP2A, which dephosphorylates and consequently inactivates Akt (Beaulieu et al., 2007a ). Thus, D2R activation inhibits Akt activity through an arrestin-dependent but G protein-independent pathway (Beaulieu et al., 2007a ; Beaulieu et al., 2007b). Curiously, the mood stabilizer, lithium, has been shown to disrupt the arrestin 3-Akt-PP2A complex, thereby preventing dopamine-induced dephosphorylation of Akt and blocking amphetamine-induced locomotion (Beaulieu et al., 2008). Moreover, amphetamine-induced locomotion is greatly diminished in arrestin 3 knockout mice, suggesting that this pathway is critical to at least some psychostimulant effects (Beaulieu et al., 2005).

Using newly developed BRET (bioluminescent resonance energy transfer) biosensors in assays that measure direct protein-protein interactions within the living cell, recent studies have demonstrated that antipsychotic medications prevent arrestin 3 recruitment by blocking D2R activation (Klewe et al., 2008; Masri et al., 2008). Masri et al. (2008) hypothesized that antipsychotic drugs achieve their therapeutic effect through a common mechanism involving blockade of arrestin-mediated signaling (Masri et al., 2008). Masri et al. (2008) also demonstrated that nearly all antipsychotics tested (including haloperidol, clozapine, olanzapine, desmethylclozapine, chlorpromazine, quetiapine, risperidone and ziprasidone) behave as inverse agonists to decrease constitutive G protein signaling as well as to prevent the agonist quinpirole from inhibiting cAMP synthesis (via D2R-mediated Gi/o signaling). The lone exception, aripiprazole, behaved as a partial agonist instead of as an inverse agonist of the G protein mediated effects. The latter finding is consistent with previous studies highlighting aripiprazole’s ability to differentially modulate various G protein-mediated effector pathways, a property termed “functional selectivity” (Mailman, 2007; Urban et al., 2007). Using the BRET assay, Klewe et al. (2008) and more recently Masri et al. (2008) demonstrated that all antipsychotics, including aripiprazole, block arrestin 3 recruitment. This finding has led Masri et al. (2008) to suggest that blockade of arrestin 3 recruitment to the D2R, and not modulation of G-protein-mediated pathways, is a common and specific property of all current antipsychotics and may be used to predict the antipsychotic efficacy of drugs in development (Masri et al., 2008). This hypothesis remains to be tested and at present appears to lean heavily on the evidence for aripiprazole’s atypical effects on constitutive (non-agonist-dependent) D2R-mediated G-protein signaling. Indeed, the fact that lithium acts to prevent arrestin-mediated signaling in response to amphetamine but is not an effective antipsychotic in monotherapy suggests that antipsychotic action may be more complex than simple blockade of D2R-mediated arrestin signaling. In addition, the ability of antipsychotics, including aripiprazole, to block agonist binding to the D2R and thus activation of the receptor, makes it likely that agonist-induced activity in multiple signaling pathways will also be blocked by these drugs.

Despite the paucity of direct evidence for D2R-arrestin coupling as the mechanism underlying the antipsychotic effects of drugs, the hypothesis remains quite intriguing Given that Akt and its downstream target GSK-3 (glycogen synthase kinase-3) have been implicated in schizophrenia in a number of genetic and postmortem studies, and the Akt/GSK-3 pathway might represent an opening into alternative therapeutics of schizophrenia. Akt is a serine/threonine kinase that may have significant roles in synaptic physiology and neurodegeneration (Brazil et al., 2004). Recruited to the cell surface by binding to phosphatidylinositol 3,4,5 trisphosphate, Akt is activated via phosphorylation of 3-phosphoinoitide-dependent protein kinase 1 (PDK1) and the rictor-mTOR complex (Brazil and Hemmings, 2001; Sarbassov et al., 2005). Once active, Akt phosphorylates GSK-3, thereby inactivating it. Since D2R activation leads to inactivation of Akt, this also results in increased GSK-3 activity (Beaulieu et al., 2004; Lovestone et al., 2007). GSK-3 activity also plays an important role in modulating the dopaminergic response to amphetamine. Amphetamine’s stimulation of DAT-mediated dopamine efflux and subsequent D2R stimulation likely results in Akt inactivation and increased GSK-3 activity. Rats treated with the specific GSK-3 inhibitor, AR-A014418, failed to display amphetamine-induced hyperactivity (Gould et al., 2004). Similarly, heterozygous GSK-3β knockout mice (expressing approximately half of wildtype levels of GSK-3β) displayed significantly reduced levels of locomotor activity following amphetamine treatment (Beaulieu et al., 2004). Additionally, treatment of dopamine transporter (DAT) knockout mice with multiple GSK-3 inhibitory drugs inhibited the ordinarily hyperactive behavior of the non-treated DAT knockout mice (Beaulieu et al., 2004).

In a mouse model, acute and chronic haloperidol treatment was shown to increase levels of active, phosphorylated Akt isoform Akt1 and increased phosphorylation and inactivation of GSK-3β (Emamian et al., 2004). Thus, it was suggested that haloperidol treatment may compensate for the decreased levels of endogenous Akt1 in the frontal cortex of people with schizophrenia (Emamian et al., 2004). Atypical antipsychotics also impact on the regulation of Akt and GSK-3β activities. For example, treatment with clozapine results in increased levels of phosphorylated GSK-3β (Kang et al., 2004; Sutton et al., 2007). Interestingly, however, differences between haloperidol and atypical antipsychotics have emerged in the kinetics of Akt/GSK-3 phosphorylation, the levels of proteins expressed following drug exposure, and the signaling pathways that are preferentially activated (Roh et al., 2007).

The abilities of antipsychotic drugs to activate distinct signaling pathways to mediate their ostensible differential pharmacologic effects would suggest clinical variation in their therapeutic effects. However, meaningful differences in the clinical effects of these compounds have not been clearly or consistently evident. The initial reports of superior efficacy of the so-called second generation or atypical antipsychotics on measures of psychosis (Kane et al., 1988), negative symptoms (Tollefson et al., 1997), cognitive deficits (Keefe et al., 1999), relapse prevention (Csernansky et al., 2002), adherence (Wahlbeck et al., 2001) and illness progression (Lieberman et al., 2005a), have not been borne out by more recent studies (Geddes et al., 2000; Lieberman et al., 2005b; Jones et al., 2006; Leucht et al., 2008). Indeed, the differences between antipsychotic drugs are most evident in the types, frequency and severity of side effects rather than in their therapeutic actions (Leucht et al., 1999; Allison et al., 1999; Henderson et al., 2005). In this regard the emerging pattern of variation in the molecular mechanisms of antipsychotic drugs in the face of their common clinical profiles resembles what was previously observed with the variability in neuroreceptor binding profiles (Kinon and Lieberman, 1996). The marked differences in the affinities and selectivity of the various antipsychotics for the receptors of different neurotransmitters were thought to underlie a rich pattern of clinical variation in the therapeutic actions of this group of drugs (Miyamoto et al., 2005). However, this hypothesis has not been supported by clinical studies (Lieberman, 2006; Lewis and Lieberman, 2008).

Nevertheless, there is reason to be hopeful that through functional selectivity, or other potential actions, the abilities of drugs to engage different signaling pathways will confer novel therapeutic effects that will improve the efficacy of treatments. In this context, the studies of Masri et al. (2008) and Klewe et al. (2008) highlight the plausibility that D2R/arrestin 3 modulation of Akt and GSK-3 activity is an important mechanism underlying psychosis and a potential target for future antipsychotic drugs. Further study of this pathway, including studies designed to reverse the effects of D2R antagonists or partial agonists (antipsychotic drugs) with systematic differential manipulation of the signaling pathways induced by D2R activation, is likely to be a fruitful path toward the development of novel treatments for schizophrenia-related disorders.

Acknowledgements: The authors would like to acknowledge the generous support of the Lieber Center for Schizophrenia Research at Columbia University

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Related News: SfN 2013—New Tools for Rational Drug Design

Comment by:  Hugo Geerts
Submitted 29 January 2014
Posted 5 February 2014

Multi-target drug discovery has typically been neglected in the world of genetics and high-throughput screening because of the difficulty of rationally defining a pharmacological profile, but it has major advantages for treating complex disorders such as schizophrenia. It is no wonder that the currently approved antipsychotics do have a rich pharmacology and substantially improve the clinical phenotype. With so many different genotypes defining individual patients, focusing on only one target is likely to have small effects that might disappear in clinical trials with larger patient populations. Even over all indications (not only CNS), more than half of the first-in-class medicines approved in the last decade have been found by using phenotypic assays and have typically multi-target pharmacology (Swinney and Anthony, 2011).

The approach presented here suggests a rational way to identify 1) a set of targets and 2) chemical structures that might serve as hits for further medical chemistry development. It might therefore alleviate the concerns of many medical chemistry departments in pharmaceutical companies.

Changing the mindset from developing the next extremely specific and potent inhibitor to pursuing multi-target pharmacology is urgently needed to break the deadlock of unsuccessful new drug development in schizophrenia.

References:

Swinney DC, Anthony J. How were new medicines discovered? Nat Rev Drug Discov . 2011 Jul ; 10(7):507-19. Abstract

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