Involvement of Chaperone Sigma1R in the Anxiolytic Effect of Fabomotizole

Abstract

Sigma-1 receptor (chaperone Sigma1R) is an intracellular protein with chaperone functions, which is expressed in various organs, including the brain. Sigma1R participates in the regulation of physiological mechanisms of anxiety (Su, T. P. et al., 2016) and reactions to emotional stress (Hayashi, T., 2015). In 2006, fabomotizole (ethoxy-2-[2-(morpholino)-ethylthio]benzimidazole dihydrochloride) was registered in Russia as an anxiolytic (Seredenin S. and Voronin M., 2009). The molecular targets of fabomotizole are Sigma1R, NRH: quinone reductase 2 (NQO2), and monoamine oxidase A (MAO-A) (Seredenin S. and Voronin M., 2009). The current study aimed to clarify the dependence of fabomotizole anxiolytic action on its interaction with Sigma1R and perform a docking analysis of fabomotizole interaction with Sigma1R. An elevated plus maze (EPM) test revealed that the anxiolytic-like effect of fabomotizole (2.5 mg/kg i.p.) administered to male BALB/c mice 30 min prior EPM exposition was blocked by Sigma1R antagonists BD-1047 (1.0 mg/kg i.p.) and NE-100 (1.0 mg/kg i.p.) pretreatment. Results of initial in silico study showed that fabomotizole locates in the active center of Sigma1R, reproducing the interactions with the site's amino acids common for established Sigma1R ligands, with the ΔG_bind value closer to that of agonist (+)-pentazocine in the 6DK1 binding site.

Keywords: (+)-pentazocine; BD-1047; NE-100; Sigma1R ligands; anxiolytic; chaperone Sigma1R; docking; elevated plus maze; fabomotizole.

Figure 5

A schematic diagram of pharmacophore model for Sigma1R ligand based on R. Glennon et al. [87]. Shared pharmacophore features of NE-100, (+)-pentazocine and fabomotizole and their locations in the reference crystallographic structures of Sigma1R (PDB IDs: 6DK0 and 6DK1). Large hydrophobic region—green, small hydrophobic region—purple, ‘N-zone’—blue.

Figure 1

The influence of Sigma1R antagonists on fabomotizole anxiolytic dose effect expressed as the number of entries into the elevated plus-maze open arms. (a) Number of entries into the open arms (N open); (b) percentage of open arm entries (%N open). Experimental groups were divided by drug administration: intact BALB/c mice (Intact), vehicle 1 + vehicle 2 (Veh1 + Veh2), BD-1047 1.0 mg/kg + vehicle 2 (BD-1047 1.0 + Veh2), NE-100 1.0 mg/kg + vehicle 2 (NE-100 1.0 + Veh2), vehicle 1 + fabomotizole 2.5 mg/kg (Veh1 + Fab 2.5), BD-1047 1.0 mg/kg + fabomotizole 2.5 mg/kg (BD-1047 1.0 + Fab 2.5), NE-100 1.0 mg/kg + fabomotizole 2.5 mg/kg (NE-100 1.0 + Fab 2.5). Data are presented as median with 95% CI. Statistically significant differences according to the Kruskal–Wallis test and the post hoc Dunn’s multiple comparisons test: ns—not significant; * p < 0.05; ** p < 0.01; *** p < 0.001.

Figure 2

The influence of Sigma1R antagonists on fabomotizole anxiolytic dose effect expressed as time spent in the elevated plus-maze open arms. (a) time spent in open arms (T open in seconds); (b) percentage of time spent in open arms (%T open). Experimental groups were divided by drug administration: intact BALB/c mice (Intact), vehicle 1 + vehicle 2 (Veh1 + Veh2), BD-1047 1.0 mg/kg + vehicle 2 (BD-1047 1.0 + Veh2), NE-100 1.0 mg/kg + vehicle 2 (NE-100 1.0 + Veh2), vehicle 1 + fabomotizole 2.5 mg/kg (Veh1 + Fab 2.5), BD-1047 1.0 mg/kg + fabomotizole 2.5 mg/kg (BD-1047 1.0 + Fab 2.5), NE-100 1.0 mg/kg + fabomotizole 2.5 mg/kg (NE-100 1.0 + Fab 2.5). Data are presented as median with 95% CI. Statistically significant differences according to the Kruskal–Wallis test and the post hoc Dunn’s multiple comparisons test: ns—not significant; * p < 0.05; ** p < 0.01; *** p < 0.001.

Figure 3

Top-scored docking poses for NE-100 and fabomotizole bound to Sigma1R binding site (PDB ID: 6DK0). (a) NE-100; (b) fabomotizole; (c) superposition of NE-100 and fabomotizole. Dotted green line—‘π-cationic’ interactions, dotted yellow lines—H-bonds, dotted purple lines—‘salt-bridges’, dotted blue lines—‘CH-π’ interaction.

Figure 4

Top-scored docking poses for (+)-pentazocine and fabomotizole bound to Sigma1R binding site (PDB ID: 6DK1). (a) (+)-pentazocine; (b) fabomotizole; (c) superposition of (+)-pentazocine and fabomotizole. Dotted green line—‘π-cationic’ interactions, dotted yellow lines—H-bonds, dotted purple lines—‘salt-bridges’.