Regulation of neurofilament interactions in vitro by natural and synthetic polypeptides sharing Lys-Ser-Pro sequences with the heavy neurofilament subunit NF-H: neurofilament crossbridging by antiparallel sidearm overlapping

Neurofilaments are organised into parallel bundles in axons through crossbridges formed by lateral projections of neurofilament subunits. Pure neurofilaments form gels in vitro, consisting of interconnected parallel arrays of filaments regulated by the phosphorylation level of neurofilament subunits. Neurofilament-associated polypeptides sharing phosphorylated epitopes with the repetitive lysine-serine-proline (Lys-Ser-Pro) motifs of the neurofilament heavy subunit sidearm are characterised: they regulate in vitro the neurofilament gelation kinetics in a concentration- and phosphorylation-dependent manner. Studies with synthetic peptides show that interactions between neurofilaments involve both acid and base amino acid residues of neurofilament sidearms and demonstrate the opposite effects of peptides containing either one (inhibition) or two (activation) Lys-Ser-Pro motifs. Electron microscopy reveals an organised network of native neurofilament sidearms, regulated by the phosphorylation level of neurofilament subunits, suggesting a structural transition between intra- and inter-neurofilament sidearm interactions. These results favour the hypothesis of a mechanism of neurofilament crossbridging through the variable antiparallel overlapping of the phosphorylable Lys-Ser-Pro domains of neurofilament sidearms from adjacent filaments, following an equilibrium regulated by neurofilament-associated proteins, bivalent cations and the phosphorylation level of Lys-Ser-Pro motifs from both neurofilament sidearms and neurofilament-associated proteins.

Spinal motor neuron neuroaxonal spheroids in chronic aluminum neurotoxicity contain phosphatase-resistant high molecular weight neurofilament (NFH)

It has previously been shown that a single intracisternal inoculum of AlCl3 in young adult New Zealand white rabbits will induce a dose-dependent phosphatase resistance of high molecular weight neurofilament protein (NFH) that is proportionate to the extent of neurofilamentous inclusion formation (Strong and Jakowec, 1994). To determine if the potential for dissolution of aluminum-induced neurofilamentous inclusions was dependent on the degree of NFH phosphatase resistance, we have examined NFH phosphatase sensitivity in a reversible chronic model of aluminum neurotoxicity. Rabbits receiving repeated intracisternal inoculums of 100 microgram AlCl3 at 28 day intervals until day 267 develop spinal motor neuron perikaryal and neuroaxonal neurofilamentous aggregates in a stereotypic, dose-dependent fashion. In the rabbits receiving inoculums until day 156 with survival until day 267 without further aluminum exposure, neuroaxonal spheroids remained prominent while perikaryal inclusions largely resolved. Immunoreactivity to a monoclonal antibody recognizing phosphorylated NFH (SMI 31) was abolished in perikaryal aggregates at each time interval by dephosphorylation with bovine alkaline phosphatase. However, neuroaxonal spheroids maintained their immunoreactivity. Using time-course dephosphorylation studies of spinal cord homogenates, we observed a significant reduction in the rate of dephosphorylation of NFH following 267 days of AlCl3 exposure (P < 0.05). These observations suggest that neuroaxonal spheroids contain phosphatase-resistant NFH isoforms and that the potential for resolution of intraneuronal neurofilamentous inclusions correlates with the susceptibility of NF within these inclusions to enzymatic dephosphorylation.

The organization of neurofilaments accumulated in perikaryon following aluminum administration: relationship between structure and phosphorylation of neurofilaments

Neurofilaments accumulated in perikarya and dendrites of anterior horn cells and Purkinje cells of rabbit treated by aluminum chloride were analysed with a variety of techniques. Four different monoclonal antibodies against phosphorylated and nonphosphorylated epitopes on neurofilament H subunit were used to compare phosphorylation state of these accumulated neurofilaments with that of axonal neurofilaments. Although immunoblotting revealed no significant difference in phosphorylation between control and aluminum-treated brains, accumulated neurofilaments were immunocytochemically more phosphorylated than control perikaryal or dendritic neurofilaments. With detailed analysis of cryothin-section immunogold labeling, accumulated neurofilaments were, however, significantly less phosphorylated than axonal neurofilaments. With quick-freeze deep etching, core filaments of accumulated neurofilaments are as dense as axonal neurofilaments but much less regularly aligned. Cross-bridges of accumulated neurofilaments were less frequent and more branched than those of axonal neurofilaments, and when examined with combined immunocytochemistry and deep etching, were less phosphorylated. These results suggest that there is a relationship between the phosphorylation and the structural organization of neurofilaments. The phosphorylation of neurofilament H subunit may be necessary for formation of frequent and straight cross-bridges and resulting regular alignment of core filaments.

Multiple interactions of aluminum with neurofilament subunits: regulation by phosphate-dependent interactions between C-terminal extensions of the high and middle molecular weight subunits

Exposure of individual purified neurofilament (NF) proteins to AlCl3 alters their electrophoretic properties in a time- and concentration-dependent manner, as visualized by their failure to migrate into SDS gels. Co-incubation of purified high (NF-H) and middle (NF-M) but not low (NF-L) molecular weight NF subunits prevents this AlCl3-induced alteration in electrophoretic migration. This latter finding suggested that specific interactions between NF-H and NF-M other than filament formation influenced their interaction with AlCl3. Co-incubation of the 160 kDa alpha-chymotryptic cleavage product of NF-H (corresponding to the highly phosphorylated C-terminal sidearm domain) with native NF-M prevented alteration in subunit electrophoretic migration by AlCl3. By contrast, intact, dephosphorylated NF-H subunits were unable to prevent AlCl3-induced alteration of native NF-M electrophoretic migration. Taken together, these findings suggest that phosphate-dependent interactions between the sidearm extensions of NF-H and NF-M diminish the ability of AlCl3 to associate with either subunit in a manner that alters their electrophoretic migration. This interaction of NF-H and NF-M sidearms is SDS-sensitive, while AlCl3-induced alteration in electrophoretic migration of individual subunits is SDS-resistant. Addition of SDS to mixtures of NF-H and NF-M subunits disrupted the protective effect, and promoted AlCl3-induced alterations in subunit electrophoretic migration. These findings support and extend the current hypothesis that the ability of aluminum to interact with NF subunits is a function of subunit phosphorylation, assembly, and extent of neurofilament-neurofilament cross-linking.