Microtubule-associated proteins regulate microtubule function as the track for intracellular membrane organelle transports.

Axonal microtubules have two essential roles: providing the track for organelle transport and forming the cytoskeletal framework to maintain axonal morphology. Microtubule-associated proteins (MAPs) are essential for the formation of cytoskeletal architecture. However, they may have additional roles on the regulation of organelle transport by their interaction with motor proteins on the microtubules. We first examined the effects of axonal MAPs on the organelle movement along microtubules in a heterologous system using COS fibroblasts, which express no axonal MAPs, such as tau or MAP2C. Transfection of tau or MAP2C gene suppressed organelle movement almost completely in this cell type, hence interaction of axonal MAPs with microtubules interferes with organelle transports. It is known that the phosphorylation of MAPs reduces their interaction with microtubules. In this sense, phosphorylation of MAPs can be a good candidate for the molecular switch to regulate the organelle transport. As a second set of experiments, we investigated the effects of modulating cAMP dependent protein kinase pathway on organelle transports in primary sensory neurons, where high-molecular-weight tau protein is the major MAP. We found that the application of dibutyryl cAMP enhanced transports of large organelles in the axon. Furthermore, this drug treatment phosphorylated endogenous tau protein and thus reduced the affinity of tau to microtubules. These results indicate that axonal MAPs can work as a phosphorylation-dependent regulator of organelle transport. Local activation of protein kinase pathways in the axon might play an important role on the segregation of microtubules serving for either organelle transport or cytoskeletal architecture.