In Silico Characterization of Ligand Binding Modes in the Human Histamine H₄ Receptor and their Impact on Receptor Activation

Abstract

The human histamine H 4 receptor (hH 4 R) is the most recently discovered member of the histamine receptor family. Like the other three histamine receptor subtypes, it belongs to the big family of class A (rhodopsin-like) G protein-coupled receptors (GPCR). Its effects are mainly mediated by an inhibitory/olfactory G alpha subunit (Ga i/o ). Predominantly expressed peripherally on hematopoietic cells, the hH 4 R is functionally involved in physiological and pathophysiological processes in the immune system. The high constitutive activity hints at a modulating function in inflammatory signaling, which could enable a negative feedback mechanism. The hH 4 R is a topical target in the development of drug candidates that potentially find their application in the therapy of chronic inflammatory diseases and immune disorders. [1] Because of the high constitutive activity, pronounced differences in species, and distinct expression pattern, it is important to extensively characterize receptor-ligand interactions before starting the in vivo evaluation of H 4 R ligands. Several compounds, including the first reported antagonist JNJ-7777120, [2] have shown distinct effects in various in silico, in vitro, and in vivo models. Aminopyrimidines, as the most advanced class of hH 4 R ligands, exhibit remarkable differences in structure-affinity/efficacy relationships.[3] We recently reported a series of 2,4-diaminopyrimidines that showed differing functionalities, ranging from partial agonism to inverse agonism, depending on substitution pattern and degree of conformational constriction.[4] Aiming to clarify the binding mode within this class of structurally highly related compounds and the resulting receptor activation mechanisms we performed molecular dynamics (MD) simulations with the partial agonist ST-989 (maximal efficacy, E max = 24 % related to maximal response of histamine as full agonist) and the inverse agonist ST-1012 (E max = À111 % related to maximal inhibition of constitutive activity by thioperamide as a reference inverse agonist; Scheme 1). Both ligands displayed potencies in the same concentration range (pEC 50~7 .5). [4] GPCRs are allosteric proteins that respond to small molecules by conformational changes, thereby affecting large protein-protein interactions. Basically, two receptor conformations, the inactive (R) and the active (R*) state, are distinguished. The equilibrium between these two states is defined by an allosteric constant (two-state model) [5] that is directly influenced by ligand binding and/or G protein-coupling. Ternary complex models include more than two receptor conformations.[6] Usually, agonists stabilize R*, whereas inverse agonists stabilize the R conformation. Partial agonists stabilize R* less efficiently than full agonists, or they evoke a different active state. In both active and inactive states, a receptor can spontaneously interact with G proteins in the absence of ligands (constitutive activity). In the case of the constitutively active hH 4 R, the equilibrium is shifted towards...

A structural chemogenomics analysis of aminergic GPCRs: lessons for histamine receptor ligand design

“…The conserved fold of GPCRs has enabled the construction of protein homology models including histamine receptor models (Wieland et al ., ; Kelley et al ., ; Jongejan et al ., 2005; 2008; Schlegel et al ., ; Jojart et al ., ; Kiss et al ., 2008b; Igel et al ., ; Strasser et al ., ; Lim et al ., ; Werner et al ., ; Istyastono et al ., 2011b; Schultes et al ., ; Sirci et al ., ; Seifert et al ., ) to predict GPCR‐ligand interactions (de Graaf and Rognan, ; Kooistra et al ., ). GPCR homology models have furthermore been successfully used to discover new ligands by structure‐based virtual screening (de Graaf and Rognan, ; Kooistra et al ., ), as demonstrated for H 3 R (Schlegel et al ., ; Sirci et al ., ) and H 4 R (Kiss et al ., 2008a; Istyastono, ).…”

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

“…Yet, the H 4 receptor contains an alanine residue at position 6.30, and mutation of this residue (e.g., into glutamate) does not really alter the constitutive signaling (Schneider et al, 2010a). A Histamine Receptors pseudoionic lock has been claimed for differentiating (partial) agonist from antagonist binding modes (Werner et al, 2010).…”

“…Subsequently, further 2-aminopyrimidine H 4 receptor antagonists were published by Johnson & Johnson (66) (Cai et al, 2008), Pfizer (67) (Bell et al, 2007), Palau (Palau-solità i Plegamans, Barcelona, Spain) (68) (Carceller Gonzalez et al, 2007), UCB (Brussels, Belgium) (69) (Raphy et al, 2008), Abbott (70) Cowart et al, 2008), Incyte (Wilmington, DE) (74), and academic institutions Werner et al, 2010). ZPL-3893787 or formerly PF-3893787 (67) has completed phase I studies and is being developed by Ziarco Pharma (Sandwich, UK) as an oral treatment of atopic dermatitis (Liu, 2014).…”

DOGS: Reaction-Driven de novo Design of Bioactive Compounds

“…A reason for the weak affinity of 7 might be a missing hydrogen-bond donor in the central part, which has been suggested to play a role in the interaction of some known H 4 ligands with the receptor [39], [44]. In fact, the nitrogen atom of the indole moiety of reference compound JNJ7777120 can act as a hydrogen-bond donor.…”