Ligand recognition of serine-cysteine amino acid exchanges in transmembrane domain 5 of alpha2-adrenergic receptors by UK 14,304

The signaling and selectivity of α-adrenoceptor agonists for the human α2A, α2B and α2C-adrenoceptors and comparison with human α1 and β-adrenoceptors

α2-adrenoceptors, (α2A, α2B and α2C-subtypes), are Gi-coupled receptors. Central activation of brain α2A and α2C-adrenoceptors is the main site for α2-agonist mediated clinical responses in hypertension, ADHD, muscle spasm and ITU management of sedation, reduction in opiate requirements, nausea and delirium. However, despite having the same Gi-potency in functional assays, some α2-agonists also stimulate Gs-responses whilst others do not. This was investigated. Agonist responses to 49 different α-agonists were studied (CRE-gene transcription, cAMP, ERK1/2-phosphorylation and binding affinity) in CHO cells stably expressing the human α2A, α2B or α2C-adrenoceptor, enabling ligand intrinsic efficacy to be determined (binding KD /Gi-IC50 ). Ligands with high intrinsic efficacy (e.g., brimonidine and moxonidine at α2A) stimulated biphasic (Gi-Gs) concentration responses, however for ligands with low intrinsic efficacy (e.g., naphazoline), responses were monophasic (Gi-only). ERK1/2-phosphorylation responses appeared to be Gi-mediated. For Gs-mediated responses to be observed, both a system with high receptor reserve and high agonist intrinsic efficacy were required. From the Gi-mediated efficacy ratio, the degree of Gs-coupling could be predicted. The clinical relevance and precise receptor conformational changes that occur, given the structural diversity of compounds with high intrinsic efficacy, remains to be determined. Comparison with α1 and β1/β2-adrenoceptors demonstrated subclass affinity selectivity for some compounds (e.g., α2:dexmedetomidine, α1:A61603) whilst e.g., oxymetazoline had high affinity for both α2A and α1A-subtypes, compared to all others. Some compounds had subclass selectivity due to selective intrinsic efficacy (e.g., α2:brimonidine, α1:methoxamine/etilefrine). A detailed knowledge of these agonist characteristics is vital for improving computer-based deep-learning and drug design.

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

Involvement of the first transmembrane segment of human α(2) -adrenoceptors in the subtype-selective binding of chlorpromazine, spiperone and spiroxatrine

BACKGROUND AND PURPOSE

Some large antagonist ligands (ARC239, chlorpromazine, prazosin, spiperone, spiroxatrine) bind to the human α(2A) -adrenoceptor with 10- to 100-fold lower affinity than to the α(2B)- and α(2C)-adrenoceptor subtypes. Previous mutagenesis studies have not explained this subtype selectivity.

EXPERIMENTAL APPROACH

The possible involvement of the extracellular amino terminus and transmembrane domain 1 (TM1) in subtype selectivity was elucidated with eight chimaeric receptors: six where TM1 and the N-terminus were exchanged between the α(2)-adrenoceptor subtypes and two where only TM1 was exchanged. Receptors were expressed in CHO cells and tested for ligand binding with nine chemically diverse antagonist ligands. For purposes of interpretation, molecular models of the three human α(2)-adrenoceptors were constructed based on the β(2)-adrenoceptor crystal structure.

KEY RESULTS

The affinities of three antagonists (spiperone, spiroxatrine and chlorpromazine) were significantly improved by TM1 substitutions of the α(2A)-adrenoceptor, but reciprocal effects were not seen for chimaeric receptors based on α(2B)- and α(2C)-adrenoceptors. Molecular docking of these ligands suggested that binding occurs in the orthosteric ligand binding pocket.

CONCLUSIONS AND IMPLICATIONS

TM1 is involved in determining the low affinity of some antagonist ligands at the human α(2A)-adrenoceptor. The exact mechanism is not known, but the position of TM1 at a large distance from the binding pocket indicates that TM1 does not participate in specific side-chain interactions with amino acids within the binding pocket of the receptor or with ligands bound therein. Instead, molecular models suggest that TM1 has indirect conformational effects related to the charge distribution or overall shape of the binding pocket.

The second extracellular loop of alpha2A-adrenoceptors contributes to the binding of yohimbine analogues

BACKGROUND AND PURPOSE

Rodent alpha(2A)-adrenoceptors bind the classical alpha(2)-antagonists yohimbine and rauwolscine with lower affinity than the human alpha(2A)-adrenoceptor. A serine-cysteine difference in the fifth transmembrane helix (TM; position 5.43) partially explains this, but all determinants of the interspecies binding selectivity are not known. Molecular models of alpha(2A)-adrenoceptors suggest that the second extracellular loop (XL2) folds above the binding cavity and may participate in antagonist binding.

EXPERIMENTAL APPROACH

Amino acids facing the binding cavity were identified using molecular models: side chains of residues 5.43 in TM5 and xl2.49 and xl2.51 in XL2 differ between the mouse and human receptors. Reciprocal mutations were made in mouse and human alpha(2A)-adrenoceptors at positions 5.43, xl2.49 and xl2.51, and tested with a set of thirteen chemically diverse ligands in competition binding assays.

KEY RESULTS

Reciprocal effects on the binding of yohimbine and rauwolscine in human and mouse alpha(2A)-adrenoceptors were observed for mutations at 5.43, xl2.49 and xl2.51. The binding profile of RS-79948-197 was reversed only by the XL2 substitutions.

CONCLUSIONS AND IMPLICATIONS

Positions 5.43, xl2.49 and xl2.51 are major determinants of the species preference for yohimbine and rauwolscine of the human versus mouse alpha(2A)-adrenoceptors. Residues at positions xl2.49 and xl2.51 determine the binding preference of RS-79948-197 for the human alpha(2A)-adrenoceptor. Thus, XL2 is involved in determining the species preferences of alpha(2A)-adrenoceptors of human and mouse for some antagonists.

Intracellularly truncated human alpha2B-adrenoceptors: stable and functional GPCRs for structural studies

All three alpha2-adrenoceptor subtypes have a long third intracellular loop (3i), which is conserved by overall size and charge-hydrophobic properties but not by amino acid sequence similarity. These properties must be relevant for function and structure, because they have been preserved during hundreds of millions of years of evolution. The contribution of different loop portions to agonist/antagonist binding properties and G protein coupling of the human alpha2B-adrenoceptor (alpha2B-AR) was investigated with a series of 3i truncated constructs (delta3i). We used a variety of agonists/antagonists in competition binding assays. We stimulated alpha2B-AR delta3i with various agonists and measured [35S]GTPgammaS binding in isolated cell membranes with or without antagonist inhibition. We also evaluated the ability of oligopeptides, analogous to the amino and carboxyl terminal parts of 3i, to promote G protein activation, monitored with the [35S]GTPgammaS assay. Our results reveal that the carboxyl end residues of 3i, R360(6.24) to V372(6.36), are important for Gi/Go protein activation. Deletions in regions from G206(5.72) to R245(5.110) altered the binding of some alpha2B-AR agonists, indicating that agonist binding is dependent on the conformation of the 3i domain, possibly through the involvement of G protein interactions. The truncated receptor constructs may be more stable on purification and thus be useful for structural characterization of alpha2B-AR.

Molecular mechanisms of ligand-receptor interactions in transmembrane domain V of the alpha2A-adrenoceptor

1. The structural determinants of catechol hydroxyl interactions with adrenergic receptors were examined using 12 alpha2-adrenergic agonists and a panel of mutated human alpha2A-adrenoceptors. The alpha2ASer201 mutant had a Cys --> Ser201 (position 5.43) amino-acid substitution, and alpha2ASer201Cys200 and alpha2ASer201Cys204 had Ser --> Cys200 (5.42) and Ser --> Cys204 (5.46) substitutions, respectively, in addition to the Cys --> Ser201 substitution. 2. Automated docking methods were used to predict the receptor interactions of the ligands. Radioligand-binding assays and functional [35S]GTPgammaS-binding assays were performed using transfected Chinese hamster ovary cells to experimentally corroborate the predicted binding modes. 3. The hydroxyl groups of phenethylamines were found to have different effects on ligand affinity towards the activated and resting forms of the wild-type alpha2A-adrenoceptor. Substitution of Ser200 or Ser204 with cysteine caused a deterioration in the capability of catecholamines to activate the alpha2A-adrenoceptor. The findings indicate that (i) Cys201 plays a significant role in the binding of catecholamine ligands and UK14,304 (for the latter, by a hydrophobic interaction), but Cys201 is not essential for receptor activation; (ii) Ser200 interacts with the meta-hydroxyl group of phenethylamine ligands, affecting both catecholamine binding and receptor activation; while (iii) substituting Ser204 with a cysteine interferes both with the binding of catecholamine ligands and with receptor activation, due to an interaction between Ser204 and the para-hydroxyl group of the catecholic ring.

Phenoxybenzamine binding reveals the helical orientation of the third transmembrane domain of adrenergic receptors

Phenoxybenzamine (PB), a classical alpha-adrenergic antagonist, binds irreversibly to the alpha-adrenergic receptors (ARs). Amino acid sequence alignments and the predicted helical arrangement of the seven transmembrane (TM) domains suggested an accessible cysteine residue in transmembrane 3 of the alpha(2)-ARs, in position C(3.36) (in subtypes A, B, and C corresponding to amino acid residue numbers 117/96/135, respectively), as a possible site for the PB interaction. Irreversible binding of PB to recombinant human alpha(2)-ARs (90 nm, 30 min) reduced the ligand binding capacity of alpha(2A)-, alpha(2B)-, and alpha(2C)-AR by 81, 96, and 77%. When the TM3 cysteine, Cys(117), of alpha(2A)-AR was mutated to valine (alpha(2A)-C117V), the receptor became resistant to PB (inactivation, 10%). The beta(2)-AR contains a valine in this position (V(3.36); position number 117) and a cysteine in the preceding position (Cys(116)) and was not inactivated by PB (10 microm, 30 min) (inactivation 26%). The helical orientation of TM3 was tested by exchanging the amino acids at positions 116 and 117 of the alpha(2A)-AR and beta(2)-AR. The alpha(2A)-F116C/C117V mutant was resistant to PB (inactivation, 7%), whereas beta(2)-V117C was irreversibly inactivated (inactivation, 93%), confirming that position 3.36 is exposed to receptor ligands, and position 3.35 is not exposed in the binding pocket.