Binding Affinity of Some Endogenous and Synthetic TSPO Ligands Regarding the rs6971 Polymorphism.

Neydher Berroterán-Infante,Markus Mitterhauser,Wolfgang Wadsak,Marcus Hacker,Monika Tadić

doi:10.3390/ijms20030563

Abstract

An intriguing target involved in several pathophysiological processes is the 18 kDa translocator protein (TSPO), of which exact functions remained elusive until now. A single nucleotide polymorphism in the TSPO gene influences the binding affinity of endogenous and synthetic TSPO ligands by facilitating a lower-affinity conformation, which modifies a potential ligand binding site, ultimately leading to a binding profile classification according to each genotype. For instance, some clinical effects of the distinctive binding affinity profile of cholesterol toward the TSPO of individuals with different genotypes have been extensively discussed. Therefore, we conducted an investigation based on a radioligand binding assay, to determine the inhibition constants of some reported endogenous TSPO ligands (diazepam binding inhibitor and protoporphyrin IX), as well as synthetic ligands (disulfiram and derivatives). We observed no dependency of the polymorphism on the binding affinity of the evaluated endogenous ligands, whereas a high dependency on the binding affinity of the tested synthetic ligands was evident.

Highlights

First described as a peripheral binding site of some benzodiazepines [1,2], the 18 kDa translocator protein (TSPO) has been identified as a key component of the outer mitochondrial membrane (OMM), where it contributes to important mitochondrial tasks
Since the rs6971 polymorphism plays a pivotal role in the molecular characterization of TSPO and in the development of new TSPO ligands, we aimed to conduct a study to determine the influence of the rs6971 polymorphism on the binding affinity of two endogenous TSPO ligands: protoporphyrin IX (PPIX) and diazepam binding inhibitor (DBI)
This selection was based on a recent report by Yang et al, in which the disulfiram metabolite, dietyldithiocarbamate (DDC), was radiolabeled with the positron emission tomography (PET) radionuclide copper-64 {[64Cu]cupric diethyldithiocarbamate (Cu-DDC)} and proposed as a TSPO PET ligand without any information regarding its sensitivity towards the rs6971 polymorphism [24]

Summary

Introduction

First described as a peripheral binding site of some benzodiazepines [1,2], the 18 kDa translocator protein (TSPO) has been identified as a key component of the outer mitochondrial membrane (OMM), where it contributes to important mitochondrial tasks. Several benzodiazepines, isoquinolinecarboxamides, aryolxyanilides, and other classes of compounds are known as TSPO-targeting ligands [9,10,11] Of their high or low binding affinity, TSPO ligands have demonstrated their ability to modulate, in different grades, several TSPO functions, and even have been proposed as therapeutic agents for TSPO-mediated diseases [12,13,14,15]. Three affinity patterns related to this SNP can be distinguished as follows: High-affinity-binders (HAB, homozygotes), mixed-affinity-binders (MAB, heterozygotes), and low-affinity-binders (LAB, homozygotes) [16,17] The nature of this distinctive binding affinity has been explained based on allosteric modulations observed in LAB [18]. This selection was based on a recent report by Yang et al, in which the disulfiram metabolite, dietyldithiocarbamate (DDC), was radiolabeled with the PET radionuclide copper-64 {[64Cu]Cu-DDC} and proposed as a TSPO PET ligand without any information regarding its sensitivity towards the rs6971 polymorphism [24]

Objectives

Methods

Results

Discussion

Conclusion