C-terminal phosphorylation of the high molecular weight neurofilament subunit correlates with decreased neurofilament axonal transport velocity

We probed the relationship of NF axonal transport of neurofilaments (NFs) t o their phosphorylation state by comparing these parameters in two closely- aged groups of young adult mice - 2 and 5 months of age. This particular ti me interval was selected since prior studies demonstrate that optic axons h ave already completed axonal caliber expansion and attained adult NF levels by 2 months but, as shown herein, continue to increase NF-H C-terminal pho sphorylation. NF axonal transport was monitored by autoradiographic analysi s of the distribution of radiolabeled subunits immunoprecipitated from opti c axon segments at intervals following intravitreal injection of S-35-methi onine. Both the peak and front of radiolabeled NFs translocated Faster in 2 - vs. 5-month-old mice. This developmental decline in NF transport rate was not due to reduced incorporation of NFs into the cytoskeleton, nor to an o verall decline in slow axonal transport. By excluding or minimizing other f actors, these findings support previous conclusions that C-terminal NF phos phorylation regulates NF axonal transport. (C) 2000 Published by Elsevier S cience B.V. All rights reserved.