Structural diversity of leukotriene G-protein coupled receptors

Abstract

Dihydroxy acid leukotriene (LTB₄) and cysteinyl leukotrienes (LTC₄, LTD₄, and LTE₄) are inflammatory mediators derived from arachidonic acid via the 5-lipoxygenase pathway. While structurally similar, these two types of leukotrienes (LTs) exert their functions through interactions with two distinct G protein-coupled receptor (GPCR) families, BLT and CysLT receptors, which share low sequence similarity and belong to phylogenetically divergent GPCR groups. Selective antagonism of LT receptors has been proposed as a promising strategy for the treatment of many inflammation-related diseases including asthma and chronic obstructive pulmonary disease, rheumatoid arthritis, cystic fibrosis, diabetes, and several types of cancer. Selective CysLT₁R antagonists are currently used as antiasthmatic drugs, however, there are no approved drugs targeting CysLT₂ and BLT receptors. In this review, we highlight recently published structures of BLT₁R and CysLTRs revealing unique structural features of the two receptor families. X-ray and cryo-EM data shed light on their overall conformations, differences in functional motifs involved in receptor activation, and details of the ligand-binding pockets. An unexpected binding mode of the selective antagonist BIIL260 in the BLT₁R structure makes it the first example of a compound targeting the sodium-binding site of GPCRs and suggests a novel strategy for the receptor activity modulation. Taken together, these recent structural data reveal dramatic differences in the molecular architecture of the two LT receptor families and pave the way to new therapeutic strategies of selective targeting individual receptors with novel tool compounds obtained by the structure-based drug design approach.

Keywords: GPCR; activation pathway; leukotriene receptors; ligand binding pocket; structural biology.

Figure 1

Diversity of leukotrienes and their receptors. A, biosynthetic pathways of dihydroxy and cysteinyl leukotrienes. The ovals around ligands indicate the ability of color-coded receptors to bind the marked ligand(s) (green stands for BLT₁R, red–BLT₂R, yellow–CysLT₁R, and blue–CysLT₂R). The enzymes for substance interconversion are labeled above their corresponding arrows. B, GPCR sequence homology tree (33) with leukotriene receptor positions marked. С, amino acid sequence identity and similarity between each leukotriene receptor and other class A GPCRs (the ranking is based on the GPCRdb (38) sequence similarity search tool). GPCR, G protein–coupled receptor.

Figure 2

Structural comparison of leukotriene receptors. A, superposition of CysLT₁R (yellow), CysLT₂R (blue), and BLT₁R (inactive–brown, active–green) structures. B, D, and E conserved motifs N(D)PxxY (B), P-I-F (D), and DRY (E) of BLT₁R (hBLT₁R-MK-D-046–PDB ID 7K15 in panels A–D, gpBLT₁R- BIIL260–PDB ID 5X33 in panel E in brown, hBLT₁R-LTB₄–PDB ID 7VKT in green) and CysLT_1-2R (CysLT₁R-pranlukast–PDB ID 6RZ4 in panels A–D, CysLT₁R-zafirlukast–PDB ID 6RZ5 in panel E in yellow, and CysLT₂R-ONO-2570366–PDB ID 6RZ6 in blue). C, helical positions of the abovementioned receptors viewed from the intracellular side. Key hydrogen bonds and salt bridges are shown as dashed lines. PDB, Protein Data Bank.

Figure 3

Insights into the atomic architecture of the sodium-binding site in leukotriene receptors.A, comparison of sodium-binding sites in CysLT₁R and A_2AAR. Residues and ions are shown in the same color as the corresponding receptor (CysLT₁R–yellow (PDB ID 6RZ5), A_2AAR–magenta (PDB ID 4EIY)). B, Na⁺-binding pocket residue alignment of human BLTRs and CysLTRs together with canonical human A_2AAR. The coloring scheme is adapted from GPCRdb.org (38), and indicates hydrogen bonding (purple), hydrophobic aliphatic (yellow), charged (red), aromatic (green), and proline, forming an α-helix kink (pink). C, Na⁺-binding site of gpBLT₁R (PDB ID 5X33, green) aligned with the Na⁺-binding site of CysLT₁R (PDB ID 6RZ5, yellow). Na⁺ in the CysLT₁R structure is shown as a purple sphere. gpBLT₁R antagonist BIIL260 is shown in pink. Key hydrogen bonds and salt bridges are shown as dashed lines. GPCR, G protein–coupled receptor; PDB, Protein Data Bank.

Figure 4

Ligand poses and corresponding ligand-binding pockets in leukotriene receptors. Structures of CysLT₁R-LTD₄ (a model based on the CysLT₁R-pranlukast complex) (A and D) (28); CysLT₁R-pranlukast–PDB ID 6RZ4 (B and E); CysLT₂R-ONO2570366–PDB ID 6RZ6 (C and F); hBLT₁R-LTB₄–PDB ID 7VKT (G and J); hBLT₁R-MK-D-046–PDB ID 7K15 (H and K); gpBLT₁R-BIIL260–PDB ID 5X33 (I and L). Key hydrogen bonds and salt bridges are shown as dashed lines. Red spheres indicate water molecules. EC, extracellular side; IC, intracellular side; PDB, Protein Data Bank; TM, transmembrane domain.

Figure 5

Structure of the hBLT₁R-G_icomplex.A, overview of the complex (PDB ID 7VKT). The membrane is shown as a gray slab. EC, extracellular side; IC, intracellular side. B, close-up of the interaction interface. Key hydrogen bonds and salt bridges are shown as dashed lines. PDB, Protein Data Bank; TM, transmembrane domain.

Figure 6

Structural mapping of single-nucleotide variants (SNVs) in leukotriene receptors obtained from GPCRdb. SNVs are colored according to their location: ligand-binding pocket (red), microswitches or sodium site (dark blue), and G protein/β-arrestin-binding interface (magenta); other SNVs are colored gray. Amino acids associated with disease-related mutations L129^3.43Q, M201^5.38V in CysLT₂R, and G300^8.48S in CysLT₁R are labeled and shown as spheres. The ligands are shown as semitransparent spheres, colored dark green for MK-D-046 in BLT₁R (PDB ID 7K15), brown for pranlukast in CysLT₁R (PDB ID 6RZ4), and dark blue for ONO257036 in CysLT₂R (PDB ID 6RZ6). The membrane is shown as a gray rectangle based on OPM (orientations of proteins in membranes (95)) database predictions. EC, extracellular side; GPCR, G protein–coupled receptor; IC, intracellular side; PDB, Protein Data Bank; SNVs, single-nucleotide variants.