Aluminum induces neurofilament aggregation by stabilizing cross-bridging of phosphorylated c-terminal sidearms

Abstract

Exposure to neurotoxin aluminum neurotoxicity is accompanied by the perikaryal accumulation of tangles of phosphorylated neurofilaments (NFs). We examined their formation and reversibility under cell-free conditions. AlCl 3 induced dose-dependent formation of NF aggregates, ultimately incorporating 100% of detectable NFs. The same concentration of CaCl 2 induced approximately 25% of NFs to form longitudinal dimers and did not induce aggregation. AlCl 3 induced similar percentages of aggregates in the presence or absence of CaCl 2, and CaCl 2 could not reduce pre-formed aggregates. CaCl 2-induced dimers and AlCl 3-induced aggregates were prevented by prior NF dephosphorylation. While CaCl 2-induced dimers were dissociated by phosphatase treatment, AlCl 3-induced aggregates were only reduced by approximately 50%, suggesting that aggregates may sequester phosphorylation sites. Since phosphatases regulate NF phosphorylation within perikarya, inhibition of NF dephosphorylation by aluminum would promote perikaryal NF phosphorylation and foster precocious phospho-dependent NF–NF associations. These findings are consistent with the notion that prolonged interactions induced among phospho-NFs by the trivalent aluminum impairs axonal transport and promotes perikaryal aggregation.