Opinion
Polymorphisms determine β-adrenoceptor conformation: implications for cardiovascular disease and therapy

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β1- and β2-adrenoceptors are crucial regulators of cardiovascular function. Agonists and antagonists at these receptor subtypes are cornerstones in the treatment of cardiovascular disease. In humans, both of the genes encoding the β1- and β2-adrenoceptors carry frequent polymorphisms resulting in different variants of the receptor proteins. Whether the polymorphic nature of the receptors causes the clinically observed differences with respect to the response of the patients to therapeutic drugs is currently a matter of intense discussion. Here, we discuss recent progress regarding the determination of β-adrenoceptor conformational changes and how these can help to clarify this issue. Specifically, novel optical methods enable us to directly assess the functional importance of β-adrenoceptor polymorphisms on ligand-induced changes of receptor conformation. The ability to determine polymorphism-dependent differences in drug efficacy directly on the receptor level might develop into an important approach to establish individualized drug therapies based on the genetic determinants of the patients.

Introduction

As primary target receptors mediating the effects of the endogenous catecholamines noradrenaline and adrenaline, β-adrenoceptors have a central role in the response of various organs to sympathetic stimulation. Consistent with their important function in physiology, agonists and antagonists at β1- and β2-adrenoceptors have evolved as effective and widely used remedies in various diseases (Table 1).

However, the therapeutic efficacy of β-adrenoceptor agonists and antagonists displays considerable variation among patients. An increasing body of evidence indicates that these interindividual differences in drug response might be due to the highly polymorphic nature of both receptor subtypes. Although consensus seems to form with regard to the role of polymorphisms in drug response, contradictory results have been reported with respect to the relevance of these polymorphisms in disease causation and progression, particularly in complex syndromes such as asthma and heart failure. These discrepancies might be attributed to the small sizes of the cohorts studied and the complex and highly regulated physiological parameters that were analyzed. These problems might partially be circumvented with experimental techniques that more directly assess receptor conformation and drug-induced receptor conformational changes. Here, we summarize the recent progress in the field with regard to our knowledge about structure and ligand-induced conformational changes of the β-adrenoceptors, their determination through receptor polymorphisms and possible consequences for drug therapy.

Section snippets

Functional relevance of β-adrenoceptor polymorphisms

β-adrenoceptors are members of the superfamily of seven-transmembrane-domain G-protein-coupled receptors (GPCRs) with an extracellular N terminus and an intracellular C terminus. Agonist binding to a β-adrenoceptor induces a change in receptor conformation that permits the receptor to interact with a membrane-bound G protein (Gs and under some conditions Gi). The interaction results in the dissociation of the heterotrimeric G protein into Gα and Gβγ subunits, and either of these modulates

Implications of polymorphisms for cardiovascular disease and therapy

Owing to their central role in cardiovascular homeostasis, numerous clinical studies in ethnically different cohorts have been conducted to assess the relevance of β-adrenoceptor polymorphisms in disease causation, progression and therapy.

Several studies have analyzed the relevance of the Ser49Gly and Gly389Arg β1-adrenoceptor polymorphisms with regard to the promotion of hypertension. Although initial evidence indicated a role for β1-adrenoceptor polymorphisms in blood-pressure regulation, no

Structural insights into β-adrenoceptor activation

Both the sites of interaction between ligands and the β-adrenoceptor and their putative G-protein-coupling regions have been characterized by mutagenesis studies 42, 43, 44, 45, 46. Apart from these analyses that necessarily focused on specific parts of the receptor, our understanding of GPCR structures has, until recently, been based largely on the high-resolution crystal structures of the inactive state and the photoactivated deprotonated state of rhodopsin 47, 48. The structural analysis of

Insights into β-adrenoceptor activation

Several kinetic models have been developed to explain GPCR activation using information derived from indirect measures of receptor conformation, such as changes in ligand-binding affinity and the activation of G proteins or effector enzymes. The first direct evidence for ligand-specific conformational changes occurring in a β-adrenoceptor has been provided by fluorescence spectroscopy assays of the β2-adrenoceptor. To this end, the purified receptors were labelled with conformationally

Impact of polymorphisms on receptor-activation kinetics

Cyan- and yellow-emitting variants of the green fluorescent protein (CFP, or its mutant variant Cerulean, and YFP, respectively) permitted the use of fluorescence resonance energy transfer (FRET) 61, 62 to directly study conformational changes within the β-adrenoceptor. Placement of YFP and CFP in the third intracellular loop and at the C terminus, respectively, enabled the generation of a β1-adrenoceptor construct that allowed the monitoring of receptor conformational changes in real time and

Conclusion

Taken together, both the recent FRET-based analyses of receptor conformation and several clinical studies point towards a functional importance of the polymorphic sites with respect to the therapeutic efficacy of different β-blockers. Real-time receptor-activation analysis in combination with structural data and clinical studies in large, well-characterized patient collectives might aid the choice from existing therapeutics for individualized, more effective pharmacological treatment in

Acknowledgements

We thank Hermann Schindelin, University of Wuerzburg, for helpful discussions of receptor structural data.

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