Modeling bidirectional transport of quantum dot nanoparticles in membrane nanotubes

This paper develops a model of transport of quantum dot (QD) nanoparticles in membrane nanotubes (MNTs). It is assumed that QDs are transported inside intracellular organelles (called here nanoparticle-loaded vesicles, NLVs) that are propelled by either kinesin or dynein molecular motors while moving on microtubules (MTs). A vesicle may have both types of motors attached to it, but the motors are assumed to work in a cooperative fashion, meaning that at a given time the vesicle is moved by either kinesin or dynein motors. The motors are assumed not to work against each other, when one type of motors is pulling the vesicle, the other type is inactive. From time to time the motors may switch their roles: passive motors can become active motors and vice versa, resulting in the change of the vesicle's direction of motion. It is further assumed that QDs can escape NLVs and become free QDs, which are then transported by diffusion. Free QDs can be internalized by NLVs. The effects of two possible types of MT orientation in MNTs are investigated: when all MTs have a uniform polarity orientation, with their plus-ends directed toward one of the cells connected by an MNT, and when MTs have a mixed polarity orientation, with half of MTs having their plus-ends directed toward one of the cells and the other half having their plus-ends directed toward the other cell. Computational results are presented for three cases. The first case is when organelles are as likely to be transported by kinesin motors as by dynein motors. The second case is when organelles are more likely to be transported by kinesin motors than by dynein motors, and the third case is when NLVs do not associate with dynein motors at all.