Temporal and spatial variations in slow axonal transport velocity along peripheral motoneuron axons

Neuronal axons are cellular extensions that can reach more than a meter in length. To maintain such a structure, macromolecules synthesized in cell bodies must be transported to the distal axons. Proteins associated with membranous organelles are generally transported in several fast transported groups, while cytoplasmic proteins, mostly composed of cytoskeletal proteins, are transported in slowly transported groups. Neurofilaments are a main component in the slowly transported group. Composed of three polypeptide subunits (NF-H, NF-M and NF-L), they are the most abundant cytoskeletal element in large myelinated axons. In various neurological or neurotoxic disorders, selective accumulation of neurofilaments was observed in different compartments of a neuron (cell bodies, proximal or distal axons). The underlying mechanism for this regional selectivity has been unclear. Using the classical pulse labeling method, we examined the changes in neurofilament transport velocity in transgenic mice that overexpress different neurofilament subunits. We present evidence that at least three velocities of neurofilament transport exist along peripheral motor axons. Each of these velocities was altered differently depending on which neurofilament subunit was overexpressed. We suggest that neurofilament transport in motor axons consists of multiple successive stages and that each of these stages is carried out by different transport mechanisms. These differences provide a basis for the regional deficiencies in axonal transport associated with several neurological disorders.