A molecular and chemical perspective in defining melatonin receptor subtype selectivity

Abstract

Melatonin is primarily synthesized and secreted by the pineal gland during darkness in a normal diurnal cycle. In addition to its intrinsic antioxidant property, the neurohormone has renowned regulatory roles in the control of circadian rhythm and exerts its physiological actions primarily by interacting with the G protein-coupled MT1 and MT2 transmembrane receptors. The two melatonin receptor subtypes display identical ligand binding characteristics and mediate a myriad of signaling pathways, including adenylyl cyclase inhibition, phospholipase C stimulation and the regulation of other effector molecules. Both MT1 and MT2 receptors are widely expressed in the central nervous system as well as many peripheral tissues, but each receptor subtype can be linked to specific functional responses at the target tissue. Given the broad therapeutic implications of melatonin receptors in chronobiology, immunomodulation, endocrine regulation, reproductive functions and cancer development, drug discovery and development programs have been directed at identifying chemical molecules that bind to the two melatonin receptor subtypes. However, all of the melatoninergics in the market act on both subtypes of melatonin receptors without significant selectivity. To facilitate the design and development of novel therapeutic agents, it is necessary to understand the intrinsic differences between MT1 and MT2 that determine ligand binding, functional efficacy, and signaling specificity. This review summarizes our current knowledge in differentiating MT1 and MT2 receptors and their signaling capacities. The use of homology modeling in the mapping of the ligand-binding pocket will be described. Identification of conserved and distinct residues will be tremendously useful in the design of highly selective ligands.

Figure 1

Structures of melatonin ligands. (A) 2-iodomelatonin, 2-phenylmelatonin and TIK-301; (B) Ramelateon, Agomelatine and Tasimelteon; (C) MT₂ selective ligands, UCM765; (D) MT₁ selective ligands, agomelatine dimmers; (E) Substituted N-[3-(3-methoxyphenyl) propyl] amide (N-(3-{5-Methoxy-2-[2-(3-methoxy-phenyl)-methyleneoxy]-phenyl}-propyl)-propionamide); (F) Substituted isoquinolone (7-Methoxy-6-(3-methoxy-benzyloxy)-2-methylisoquinolin- 1(2H)-one).

Figure 1

Structures of melatonin ligands. (A) 2-iodomelatonin, 2-phenylmelatonin and TIK-301; (B) Ramelateon, Agomelatine and Tasimelteon; (C) MT₂ selective ligands, UCM765; (D) MT₁ selective ligands, agomelatine dimmers; (E) Substituted N-[3-(3-methoxyphenyl) propyl] amide (N-(3-{5-Methoxy-2-[2-(3-methoxy-phenyl)-methyleneoxy]-phenyl}-propyl)-propionamide); (F) Substituted isoquinolone (7-Methoxy-6-(3-methoxy-benzyloxy)-2-methylisoquinolin- 1(2H)-one).

Figure 2

Ligand binding pocket of phenylpropylamide-bound MT₂ receptor structure. (A) Superposition of human MT₁ and MT₂ receptor sequences. Residues corresponding to the transmembrane regions are overtyped in yellow. Identical or conserved residues are in black, conserved substitutions are underlined. Residues differing from one subtype to the other are in red. An asterisk (*) indicates a previously characterized single point mutation to the hMT₂ melatonin receptor; (B) Extracellular view of the model of the MT₂ receptor. Only the proposed amino acid residues participated in the binding of phenylpropylamide (Met120, Ile125, Val204, His208, Trp264, Asn268, Phe290, Tyr294 and Tyr298) are shown. The model of the MT₂ receptor was created by using homology modeling from the published crystal structure of the human β2 adrenergic receptor solved at 2.4 Å resolution (PDB access code 2RH1) [111]. Figure was generated using DeepView/Swiss-PdbViewer v3.7; (C) Schematic representation of the interactions between MT₂ receptor and the ligand phenylpropylamide. Mutation of amino acid in black box indicates potential polar interactions. Mutations of amino acids in white boxes indicate potential hydrophobic interactions, and grey boxes indicate both potential polar interactions and hydrophobic interactions.

Figure 2

Ligand binding pocket of phenylpropylamide-bound MT₂ receptor structure. (A) Superposition of human MT₁ and MT₂ receptor sequences. Residues corresponding to the transmembrane regions are overtyped in yellow. Identical or conserved residues are in black, conserved substitutions are underlined. Residues differing from one subtype to the other are in red. An asterisk (*) indicates a previously characterized single point mutation to the hMT₂ melatonin receptor; (B) Extracellular view of the model of the MT₂ receptor. Only the proposed amino acid residues participated in the binding of phenylpropylamide (Met120, Ile125, Val204, His208, Trp264, Asn268, Phe290, Tyr294 and Tyr298) are shown. The model of the MT₂ receptor was created by using homology modeling from the published crystal structure of the human β2 adrenergic receptor solved at 2.4 Å resolution (PDB access code 2RH1) [111]. Figure was generated using DeepView/Swiss-PdbViewer v3.7; (C) Schematic representation of the interactions between MT₂ receptor and the ligand phenylpropylamide. Mutation of amino acid in black box indicates potential polar interactions. Mutations of amino acids in white boxes indicate potential hydrophobic interactions, and grey boxes indicate both potential polar interactions and hydrophobic interactions.