Insight into the Structural Features of TSPO: Implications for Drug Development

Abstract

The translocator protein (TSPO), an 18-kDa transmembrane protein primarily found in the outer mitochondrial membrane, is evolutionarily conserved and widely distributed across species. In mammals, TSPO has been described as a key member of a multiprotein complex involved in many putative functions and, over the years, several classes of ligand have been developed to modulate these functions. In this review, we consider the currently available atomic structures of mouse and bacterial TSPO and propose a rationale for the development of new ligands for the protein. We provide a review of TSPO monomeric and oligomeric states and their conformational flexibility, together with ligand-binding site and interaction mechanisms. These data are expected to help considerably the development of high-affinity ligands for TSPO-based therapies or diagnostics.

Keywords: X-ray crystallography; nuclear magnetic resonance; positron emission tomography; protein flexibility; protein ligand interactions.

Figure 1.. Overlap of molecules in the cavity of TSPO.

(A) Scheme of PPIX ligand structure. (B) PPIX (carbons in green, oxygens in red) in RsTSPO (PDB entry 4UC1). (C) PK 11195 (carbons in magenta, hydrogens in white, nitrogens in blue, chlorine in green) in BcTSPO (PDB entry 4RYI). (D) PK 11195 (carbons in magenta, nitrogens in blue, chlorine in green) in mTSPO (PDB entry 2MGY). (E) Water (doted red spheres) in BcTSPO (PDB entry 4RYQ). Dark red spheres correspond to water molecules in the selected slice whereas shadowed red spheres correspond to water molecules located underneath. (F) Water (doted red spheres) in RsTSPO (PDB entry 4UC1). (G) DMSO (carbons in red, hydrogens in white and sulfur in yellow) in BcTSPO (PDB entry 4RYO). (H) Iodine (magenta spheres) in BcTSPO (PDB entry 4RYM) In each case, the TSPO atomic structure is shown as rainbow cartoon colored as follows: TM1, blue; TM2, green; TM3, light green; TM4, yellow; TM5, red using PyMol (https://pymol.org/2/) [93].

Box 1, Figure I.. Interaction of PK 11195 with mTSPO and BcTSPO

(A) Structure of (R) PK 11195 ligand. Φ1, Φ2, Φ3 and Φ4 are the respective dihedral angles for CH3-N-C=O, CH3-CH-N-CH3, N-CH-C=O and chlorophenyl-isoquinoline ring, respectively. Red cross shows the asymmetric carbon and the red arrows show the rotation of the bond corresponding to the various Φ angles. (B) Overlay of aligned (R) PK 11195 bound in the binding cavity of mTSPO (yellow) and BcTSPO (red) (PDB ID-2MGY and 4RYI respectively). The isoquinoline plane is shown as dotted parallelogram. (C) and (D) General structures of ER 176 and Nebiquinide, respectively. Red circles emphasize the chemical substitutions introduced compared to PK 11195 (A).