Regional modulation of neurofilament organization by myelination in normal axons

Previous studies in the hypomyelinating mouse mutant Trembler have suggested that demyelinating axons are smaller in caliber compared to normal axons, and that there are differences in the organization of axonal neurofilaments. In the normal PNS, however, the relationship between neurofilament organization and myelination has not been investigated extensively. In normal axons, only the initial segments, the nodes of Ranvier (~1 μm), and the terminals are not covered by myelin. We took advantage of an unusual feature of the primary sensory neurons in the dorsal root ganglion, the relatively long nonmyelinated stem process (up to several hundred micrometers), to determine if the presence of myelination correlates with differences in cytoskeletal organization and neurofilament phosphorylation. Axonal caliber and neurofilament numbers were substantially greater in the myelinated internodes than in the stem process or nodes of Ranvier. Neurofilament spacing, assessed by measuring the nearest-neighbor neurofilament distance, was 25-50% less in the stem processes and nodes of Ranvier than in the myelinated internodes. In the myelinated internodes, neurofilaments had greater immunoreactivity for phosphorylated epitopes than those in the stem process. These findings indicate that interactions with Schwann cells modulate neurofilament phosphorylation within the ensheathed axonal segments, and that increased phosphorylation within myelinated internodes leads to greater interfilament spacing. Lastly, the myelinated internodes had three fold more neurofilaments, but the same number of microtubules. Both the increased neurofilament spacing and the increase in neurofilament numbers in myelinated internodes contribute to a greater axonal caliber in the myelinated internodes.