Switching between microtubule- and actin-based transport systems in melanophores is controlled by cAMP levels

Background: Intracellular transport involves the movement of organelles along microtubules (MTs) or actin filaments (AFs) by means of opposite-polarity MT motors or actin-dependent motors of the myosin family. The correct delivery of organelles to their different destinations involves a precise coordination of the two transport systems. Such coordination could occur through regulation of the densities of the two cytoskeletal systems or through regulation of the activities of the cytoskeletal motors by signaling mechanisms. Results: To investigate the mechanisms of switching between MT and AF-dependent transport, we examine the influence of the densities of the MT and AF network on pigment transport in fish melanophores. We also change signaling by using activators and inhibitors of Protein Kinase A (PKA). We find that the key parameters characterizing pigment granule transport along MTs do not depend on MT density and are not significantly altered by complete disruption of AFs. In contrast, the kinetics of changes in these parameters correlate with the kinetics of changes in the intracellular levels of CAMP and are affected by the inhibitors of PKA, suggesting the regulation of MT- and AF-dependent motors by cAMP-induced signaling. Furthermore, perturbation of cAMP levels prevents the transfer of pigment granules from MTs onto AFs. Conclusions: We conclude that the switching of pigment granules between the two major cytoskeletal systems is independent of the densities of MT or AF but is tightly controlled by signaling events.