Regulation Of Switching Of Membrane Organelles Between Cytoskeletal Transport Systems In Melanophores

Intracellular transport is driven by organelle-bound molecular motors that move cargo organelles along microtubules (MTs; motors of kinesin and dynein families) or actin filaments (AFs; myosin family motors). While transport along each cytoskeletal track type is well characterized, switching between the two types of transport is poorly understood. Here we used a combination of particle tracking and computational modeling approaches to measure parameters that determine how fast membrane organelles switch back and forth between MTs and AFs, and compare these parameters in different signaling states. As a model system we used melanophores, which aggregate thousands of membrane-bounded melanosomes in the cell center or redisperse them throughout the cytoplasm. Dispersion involves successive transport of melanosomes along the radial MTs and randomly arranged AFs. For aggregation, melanosomes that are transported along AFs transfer back onto MTs for movement to the MT minus ends clustered in the cell center. We performed tracking of individual pigment granules moving along MTs or AFs, determined major movement parameters (velocities and durations of runs to the plus or minus ends of MTs, and along AFs) using the Multiscale Trend Analysis, and incorporated them into a computational model for pigment transport. Comparison of the results of computational simulations of pigment distribution along the cell radius with experimentally obtained changes of pigment levels showed that regulation involves a single parameter: the transferring rate from AFs to MTs. This result suggests that MT transport is the defining factor whose regulation determines the choice of the cytoskeletal tracks during the transport of membrane organelles.