Allosteric Modulation of GPCRs of Class A by Cholesterol

Abstract

G-protein coupled receptors (GPCRs) are membrane proteins that convey extracellular signals to the cellular milieu. They represent a target for more than 30% of currently marketed drugs. Here we review the effects of membrane cholesterol on the function of GPCRs of Class A. We review both the specific effects of cholesterol mediated via its direct high-affinity binding to the receptor and non-specific effects mediated by cholesterol-induced changes in the properties of the membrane. Cholesterol binds to many GPCRs at both canonical and non-canonical binding sites. It allosterically affects ligand binding to and activation of GPCRs. Additionally, it changes the oligomerization state of GPCRs. In this review, we consider a perspective of the potential for the development of new therapies that are targeted at manipulating the level of membrane cholesterol or modulating cholesterol binding sites on to GPCRs.

Keywords: GPCRs; allosteric modulation; cholesterol.

Figure 1

(A), a schematic representation of structural features of G-protein coupled receptors (GPCRs) of class A; (B) 3D representation of GPCR of class A. Yellow—the orthosteric binding site; gold—cholesterol dimer bound to cholesterol consensus motif (CCM); purple—palmitic acid covalently bound to cysteine in Helix 8.

Figure 2

Cholesterol (CLR) dimers. Three types of CLR dimers: (A), face-to-face dimer; (B), dimer stabilized by a hydrogen bond (yellow dashed line); (C) tail-to-tail dimer.

Figure 3

CLR binding sites. Orientation—extracellular side up, N-terminus—red, C-terminus— blue, cholesterol—gold, palmitic acid—purple. (A), CLR dimer binding to A_2A-adenosine receptor (5IU4) at TM2, TM3 and TM4. (B), CLR monomer binding to oxytocin receptor (6TPK) at TM4 and TM5. (C) CLR dimer binding to the A_2A-adenosine receptor (5IU4) at TM6. (D), CLR monomer binding to α_2C adrenergic receptor (6KUW) at TM1 and TM7. (E) CLR dimer binding to the β₂ adrenergic receptor (3D4S) at TM2, TM3, and TM4. (F), CLR monomer binding to the κ-opioid receptor (6VI4) at TM4 and TM5. (G), CLR monomer binding to the CCR9 chemokine receptor (5LWE) at TM6. (H), CLR monomer binding to the 5-HT_2B receptor (4IB4) at TM1 and helix 8.

Figure 4

Structure of cholesterol consensus motif (CCM). CCM in the structure of β₂-adrenergic receptor (3D4S) as viewed with TM2 (A) or TM4 (B) in front. Orientation, extracellular side up. Principal residues of CCM are coloured. Green—Y70 in TM2, blue R151, yellow—W158 and pink—I154 in TM4.

Figure 5

Docking of CLR to 5CXV structure of M₁ muscarinic receptor. Two molecules of CLR docked to the structure of M₁ muscarinic receptor (5CXV) as viewed with TM6 (A) or TM4 (B) in front. Orientation, extracellular side up. Principal residues in TM6 interacting with CLR are coloured. Blue—R365, orange—L372, pink—L376.

Figure 6

Dimer of the α_2C-adrenergic receptor. The structure of α_2C-adrenergic receptor dimer (6KUW) as viewed from the membrane side (A) and extracellular side (B). Blue—intracellular edge of the membrane; Red—extracellular edge of the membrane; Cyan—receptor; Gold—cholesterol.

Figure 7

Model of the β₂-adrenergic receptor dimer. Model of a dimer of β₂-adrenergic receptor based on the crystal structure 2RH1 [16] as viewed from the membrane side (A) and extracellular side (B). Cyan—receptor; Gold—cholesterol; Purple—palmitic acid; Red—extracellular edge of the membrane; Blue—intracellular edge of the membrane.