Neurofilaments consist of distinct populations that can be distinguished by C-terminal phosphorylation, bundling, and axonal transport rate in growing axonal neurites

We examined the steady-state distribution and axonal transport of neurofilament (NF) subunits within growing axonal neurites of NB2a/d1 cells. Ultrastructural analyses demonstrated a longitudinally oriented "bundle" of closely apposed NFs that was surrounded by more widely spaced individual NFs. NF bundles were recovered during fractionation and could be isolated from individual NFs by sedimentation through sucrose. Immunoreactivity toward the restrictive C-terminal phospho-dependent antibody RT97 was significantly more prominent on bundled than on individual NFs. Microinjected biotinylated NF subunits, GFP-tagged NF subunits expressed after transfection, and radiolabeled endogenous subunits all associated with individual NFs before they associated with bundled NFs. Biotinylated and GFP-tagged NF subunits did not accumulate uniformly along bundled NFs; they initially appeared within the proximal portion of the NF bundle and only subsequently were observed along the entire length of bundled NFs. These findings demonstrate that axonal NFs are not homogeneous but, rather, consist of distinct populations. One of these is characterized by less extensive C-terminal phosphorylation and a relative lack of NF-NF interactions. The other is characterized by more extensive C-terminal NF phosphorylation and increased NF-NF interactions and either undergoes markedly slower axonal transport or does not transport and undergoes turnover via subunit and/or filament exchange with individual NFs. Inhibition of phosphatase activities increased NF-NF interactions within living cells. These findings collectively suggest that C-terminal phosphorylation and NF-NF interactions are responsible for slowing NF axonal transport.

Nerve fiber size-related block of action currents by phenytoin in mammalian nerve

We investigated nerve fiber size-related actions of phenytoin (PHT) by applying the anticonvulsant on 2-mm-long stretches of desheathed whole nerves, excised from rat sural nerve. Compound action potentials (APs) were elicited by voltage pulses of increasing amplitude and recorded as monophasic action currents of the A alpha beta-type along the surface of the nerve. The area under the action current Q at supramaximal stimulation was reduced by 11 and 30% in solutions containing 10 and 100 microM PHT, respectively, similar to the reduction in peak action current. However, a greater reduction in Q induced by PHT was observed with smaller stimuli at both concentrations. This stimulus-dependent reduction was believed to originate from selective inhibition of the thicker nerve fibers. Using a mathematical model, we separated Q into contributions Q alpha of the alpha-fibers and Q beta of the beta-fibers. In solutions containing 10 microM PHT, Q alpha was reduced by 15% maximally, whereas Q beta was not affected. Both fiber types were reduced < or = 30% in the presence of 100 microM PHT, whereas the relations between Q alpha and Q beta, respectively, and stimulus voltage shifted along the voltage axis for 0.3 V, suggesting that the larger fibers in the A alpha beta-groups were more inhibited by PHT than the smaller ones. Abolition of the early phases of the compound action currents by PHT also indicated loss mainly of faster conducting nerve fibers. We conclude that primarily the larger fibers in the A alpha beta populations were inhibited by the anticonvulsant, strongly suggesting a differential mode of action by PHT on myelinated nerve fibers.

Staining with monoclonal antibodies to neurofilaments distinguishes between subpopulations of neurofibrillary tangles, between groups of axons and between groups of dendrites

A new monoclonal antibody (mab) against neurofilaments is described (mab 1215) and its reactions compared with previously characterized mabs (BF10; RT97). Mab 1215 recognizes an epitope on the heavy neurofilament polypeptide (NF-H). In Alzheimer's disease, mab 1215 recognizes only a subpopulation of neurofibrillary tangles and stains a proportion of tangles in the hippocampus but none of those in the olfactory bulb. However, mabs RT97 and BF10 stain the majority of tangles in both brain areas. Of the three antibodies, only mab BF10 recognizes, specifically, axons of granular cells in the dentate gyrus of the hippocampus. Mab 1215 recognizes more dendrites in the pyramidal layer than either mab BF10 or mab RT97. Our observations indicate that neurofilaments are not identical in all axons and that, contrary to previous reports, NF-H is present in dendrites. The dendritic form of NF-H appears to be different from NF-H in axons and this could be due to differences in the state of phosphorylation of NF-H. We suggest that the finding that distinct subpopulations of tangles exist indicates that tangles are not static lesions. Further investigations into this possibility may illuminate the pathophysiology of Alzheimer's disease.

Nucleus raphe dorsalis in parkinsonism-dementia complex of Guam

Recently, morphometric analysis has shown that the nucleus raphe dorsalis, which is presumed to project diffuse serotonergic fibers to the telencephalon, is affected in Alzheimer's disease. A similar study was conducted in this report in two patients with parkinsonism-dementia complex of Guam and a Guamanian control. It demonstrated a significant reduction in the number of large neurons, and the presence of abundant Alzheimer's neurofibrillary tangles in this nucleus.