Cytoskeletal proteins as targets for organophosphorus compound and aliphatic hexacarbon-induced neurotoxicity

Concurrent exposures to organophosphorus insecticide leptophos and the industrial solvents n-hexane and toluene were implicated in causing an outbreak of neuropathy in workers. Although both leptophos and n-hexane produce central-peripheral distal axonopathy, the morphology and distribution of neuropathic lesions are distinct, reflecting different modes of action. The molecular mechanisms of organophosphorus compound-induced delayed neurotoxicity (OPIDN) and aliphatic hexacarbon-induced neurotoxicity have been investigated utilizing various biochemical techniques, (i.e. one- and two-dimensional gel electrophoresis, immunoblotting, peptide mapping). Oral administration of tri-o-cresyl phosphate (TOCP) produced delayed neurotoxicity and increased in vitro Ca2+ and calmodulin-dependent kinase protein phosphorylation of cytoskeletal proteins in brain, spinal cord, and sciatic nerve of chickens. This enhanced protein phosphorylation correlated well with the following characteristics of OPIDN: test chemical, whether an OPIDN-producing or not; dose-dependence and time course of the effect; and the animal sex sensitivity, age selectivity, and species susceptibility. The proteins that showed an increased phosphorylation were identified to be; alpha- and beta-tubulin, microtubule-associated protein-2 (MAP-2), and the 3 neurofilament proteins 70 kDa, 160 kDa, and 210 kDa. Further studies suggested that the increased protein phosphorylation is not related to an effect on protein phosphatase or ATPase activity, but rather to altered Ca2+-calmodulin kinase II activity. Aliphatic hexacarbon-induced neurotoxicity is characterized by an accumulation of 10 nm neurofilaments above the nodes of Ranvier in the spinal cord and peripheral nerve. Treatment of rats with 2,5-hexanedione, the active neurotoxic metabolite of n-hexane, produced protein crosslinking in a dose-dependent manner. This treatment also decreased protein phosphorylation of neurofilament proteins as well as MAP-2. These studies demonstrate the involvement of cytoskeletal proteins in the molecular pathogenesis of chemical-induced neurotoxicity.