A Kinetic Model to Account for Selective Trapping of Weak Bases Inside Acidic Intra-Cellular Vesicles

Abstract

Recent work from our group has characterized selective trapping of nicotinic acetylcholine receptor (nAChR) ligands within neurons in the brain (Govind et al. eLife 2017). The ligands are weak bases that bind with high-affinity to α4β2-type nAChRs (α4β2Rs), such as the anti-smoking drug varenicline (Chantix) and epibatidine. Selective trapping occurs within α4β2s-containing acidic vesicles of cells and neurons. Slow release of trapped varenicline reduces effects of long-term nicotine exposure. Selective trapping is further regulated by nicotine exposure, which increases the number of α4β2s-containing acidic vesicles. Nicotine, also a weak base, is not trapped due to its lower pKa and lower affinity for α4β2Rs. Here, we present a kinetic model that accounts for the biphasic behavior of these weak bases leaving the cell after accumulating in the acidic vesicles and binding to the nAChRs on the surface of the vesicles, within intracellular membranes, and on the exterior of the cell. Intracellular accumulation and release of molecules by diffusion into and out of cytosol and vesicles is calculated using Fick's Law of diffusion (neutral molecule) and the Nernst-Planck equation for ions (Trapp and Horobin 2005). Ligand binding to α4β2Rs is modeled using empirical receptor-ligand kinetics. Solving our coupled differential equations numerically has accurately approximated experimental data for epibatidine accumulation and release in vesicles. The model is mainly limited by inclusion of pKa and binding affinity as the only parameters directly causing the trapping. Our model also suggests that the vesicle membrane receptor concentration is an important factor, a notion that we hope to verify experimentally. Using this model, we have been able to model kinetic trapping of the weak-base α4β2 ligands Nifene and 2-F-A8538, and to explain differences in their kinetics during positron emission tomography.