Targeting the receptor-based interactome of the dopamine D1 receptor: looking for heteromer-selective drugs

Unmasking allosteric binding sites: Novel targets for GPCR drug discovery

INTRODUCTION

Unexpected non-apparent and hidden allosteric binding sites are non-classical and non-apparent allosteric centers in 3-D X-ray protein structures until orthosteric or allosteric ligands bind to them. The orthosteric center of one protomer that modulates binding centers of the other protomers within an oligomer is also an unexpected allosteric site. Furthermore, another partner protein can also produce these effects, acting as an unexpected allosteric modulator.

AREAS COVERED

This review summarizes both classical and non-classical allosterism. The authors focus on G protein-coupled receptor (GPCR) oligomers as a paradigm of allosteric molecules. Moreover, they show several examples of unexpected allosteric sites such as hidden allosteric sites in a protomer that appear after the interaction with other molecules and the allosterism exerted between orthosteric sites within GPCR oligomer, emphasizing on the allosteric modulations that can occur between binding sites.

EXPERT OPINION

The study of these new non-classical allosteric sites will expand the diversity of allosteric control on the function of orthosteric sites within proteins, whether GPCRs or other receptors, enzymes or transporters. Moreover, the design of new drugs targeting these hidden allosteric sites or already known orthosteric sites acting as allosteric sites in protein homo- or hetero-oligomers will increase the therapeutic potential of allosterism.

Heterobivalent Ligand for the Adenosine A2A-Dopamine D2 Receptor Heteromer

A G protein-coupled receptor heteromer that fulfills the established criteria for its existence in vivo is the complex between adenosine A_2A (A_2AR) and dopamine D₂ (D₂R) receptors. Here, we have designed and synthesized heterobivalent ligands for the A_2AR-D₂R heteromer with various spacer lengths. The indispensable simultaneous binding of these ligands to the two different orthosteric sites of the heteromer has been evaluated by radioligand competition-binding assays in the absence and presence of specific peptides that disrupt the formation of the heteromer, label-free dynamic mass redistribution assays in living cells, and molecular dynamic simulations. This combination of techniques has permitted us to identify compound 26 [K_DB1 (A_2AR) = 2.1 nM, K_DB1 (D₂R) = 0.13 nM], with a spacer length of 43-atoms, as a true bivalent ligand that simultaneously binds to the two different orthosteric sites. Moreover, bioluminescence resonance energy transfer experiments indicate that 26 favors the stabilization of the A_2AR-D₂R heteromer.

Heteromerization between α2A adrenoceptors and different polymorphic variants of the dopamine D4 receptor determines pharmacological and functional differences. Implications for impulsive-control disorders

Polymorphic alleles of the human dopamine D₄ receptor gene (DRD4) have been consistently associated with individual differences in personality traits and neuropsychiatric disorders, particularly between the gene encoding dopamine D_4.7 receptor variant and attention deficit hyperactivity disorder (ADHD). The α_2A adrenoceptor gene has also been associated with ADHD. In fact, drugs targeting the α_2A adrenoceptor (α_2AR), such as guanfacine, are commonly used in ADHD treatment. In view of the involvement of dopamine D₄ receptor (D₄R) and α_2AR in ADHD and impulsivity, their concurrent localization in cortical pyramidal neurons and the demonstrated ability of D₄R to form functional heteromers with other G protein-coupled receptors, in this study we evaluate whether the α_2AR forms functional heteromers with D₄R and weather these heteromers show different properties depending on the D₄R variant involved. Using cortical brain slices from hD_4.7R knock-in and wild-type mice, here, we demonstrate that α_2AR and D₄R heteromerize and constitute a significant functional population of cortical α_2AR and D₄R. Moreover, in cortical slices from wild-type mice and in cells transfected with α_2AR and D_4.4R, we detect a negative crosstalk within the heteromer. This negative crosstalk is lost in cortex from hD_4.7R knock-in mice and in cells expressing the D_4.7R polymorphic variant. We also show a lack of efficacy of D₄R ligands to promote G protein activation and signaling only within the α_2AR-D_4.7R heteromer. Taken together, our results suggest that α_2AR-D₄R heteromers play a pivotal role in catecholaminergic signaling in the brain cortex and are likely targets for ADHD pharmacotherapy.

l-DOPA promotes striatal dopamine release through D1 receptors and reversal of dopamine transporter

Previous studies have pointed out that l-DOPA can interact with D1 or D2 receptors independent of its conversion to endogenous dopamine. The present study was set to investigate whether l-DOPA modulates dopamine release from striatal nerve terminals, using a preparation of synaptosomes preloaded with [³H]DA. Levodopa (1 µM) doubled the K⁺-induced [³H]DA release whereas the D2/D3 receptor agonist pramipexole (100 nM) inhibited it. The l-DOPA-evoked facilitation was mimicked by the D1 receptor agonist SKF38393 (30-300 nM) and prevented by the D1/D5 antagonist SCH23390 (100 nM) but not the DA transporter inhibitor GBR12783 (300 nM) or the aromatic l-amino acid decarboxylase inhibitor benserazide (1 µM). Higher l-DOPA concentrations (10 and 100 µM) elevated spontaneous [³H]DA efflux. This effect was counteracted by GBR12783 but not SCH23390. Binding of [³H]SCH23390 in synaptosomes (in test tubes) revealed a dense population of D1 receptors (2105 fmol/mg protein). Both SCH23390 and SKF38393 fully inhibited [³H]SCH23390 binding (Ki 0.42 nM and 29 nM, respectively). l-DOPA displaced [³H]SCH23390 binding maximally by 44% at 1 mM. This effect was halved by addition of GBR12935 and benserazide. We conclude that l-DOPA facilitates exocytotic [³H]DA release through SCH23390-sensitive D1 receptors, independent of its conversion to DA. It also promotes non-exocytotic [³H]DA release, possibly via conversion to DA and reversal of DA transporter. These data confirm that l-DOPA can directly interact with dopamine D1 receptors and might extend our knowledge of the neurobiological mechanisms underlying l-DOPA clinical effects.

Orally Active Peptide Vector Allows Using Cannabis to Fight Pain While Avoiding Side Effects

The activation of cannabinoid CB₁ receptors (CB₁R) by Δ⁹-tetrahydrocannabinol (THC), the main component of Cannabis sativa, induces analgesia. CB₁R activation, however, also causes cognitive impairment via the serotonin 5HT_2A receptor (5HT_2AR), a component of a CB₁R–5HT_2AR heteromer, posing a serious drawback for cannabinoid therapeutic use. We have shown that peptides reproducing CB₁R transmembrane (TM) helices 5 and 6, fused to a cell-penetrating sequence (CPP), can alter the structure of the CB₁R–5HT_2AR heteromer and avert THC cognitive impairment while preserving analgesia. Here, we report the optimization of these prototypes into drug-like leads by (i) shortening the TM5, TM6, and CPP sequences, without losing the ability to disturb the CB₁R–5HT_2AR heteromer, and (ii) extensive sequence remodeling to achieve protease resistance and blood–brain barrier penetration. Our efforts have culminated in the identification of an ideal candidate for cannabis-based pain management, an orally active 16-residue peptide preserving THC-induced analgesia.

Chemical biology of noncanonical G protein-coupled receptor signaling: Toward advanced therapeutics

G protein-coupled receptors (GPCRs), the largest family of signaling membrane proteins, are the target of more than 30% of the drugs on the market. Recently, it has become clear that GPCR functions are far more multidimensional than previously thought, with multiple noncanonical aspects coming to light, including biased, oligomeric, and compartmentalized signaling. These additional layers of functional selectivity greatly expand opportunities for advanced therapeutic interventions, but the development of new chemical biology tools is absolutely required to improve our understanding of noncanonical GPCR regulation and pave the way for future drugs. In this opinion, we highlight the most notable examples of chemical and chemogenetic tools addressing new paradigms in GPCR signaling, discuss their promises and limitations, and explore future directions.