Quantitative morphometric analyses have demonstrated that axon
atrophy is the primary neuropathic feature in the CNS and PNS of rats intoxicated with
2,5-hexanedione (HD). Axon caliber is maintained by the exchange of mobile neurofilament (NF) subunits with the stationary
polymer and, therefore, HD might produce
atrophy by disrupting cytoskeletal turnover. To evaluate this possibility, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and prevalent axon
atrophy in spinal cord white matter tracts. Lumbar spinal cord regions from HD-intoxicated rats and their age-matched controls were Triton-extracted and separated by differential fractionation into a low-speed, insoluble pellet (P1) of NF
polymer and a high-speed supernatant fraction (S2), which presumably contained mobile monomer. Cytoskeletal
protein contents (NF-L, -M, -H, and
beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, the NF
polymer in P1 remained unaffected, although soluble monomer in the S2 fraction was depleted significantly (60-80% reduction). Fractional
beta-tubulin contents were inconsistently affected and abnormal higher-molecular-weight NF
proteins were detected in the P1 fraction only. Studies with
antibodies directed against phosphorylated (RT97) and nonphosphorylated (SMI32)
epitopes on NF-H and measurements of corresponding isoelectric range suggested that alterations in phosphorylation were not involved. The selective depletion of Triton-soluble
protein suggested that HD adduction of NFs interfered with the dynamic interactions of the polymeric and mobile monomeric pools.