Effects of neuropathic and non-neuropathic isomers of tricresyl phosphate and their microsomal activation on the production of axon-like processes bydifferentiating mouse N2a neuroblastoma cells

The aim of this work was to investigate the sublethal neuropathic effects o f tricresyl phosphate (TCP: mixed isomers), triorthocresyl phosphate (TOCP) and triparacresyl phosphate (TPCP) on differentiating mouse N2a neuroblast oma cells. This was achieved by a combination of measurements of cell viabi lity, axon outgrowth and the levels of cytoskeletal proteins detectable on western blots of extracts from cells induced to differentiate in the presen ce and absence of the compounds. In a time-course experiment TCP inhibited the outgrowth of axon-like processes following exposure times of 24 h or lo nger. Dose-response experiments indicated that TCP and TOCP exhibited simil ar sustained levels of toxicity following both 24 and 48h exposure, with no significant difference between their respective IC50 values. By contrast, TPCP demonstrated a transient effect on the outgrowth of axon-like processe s, which was detectable after 24 but not 48 h of exposure. Isomer-specific patterns of toxicity were also evident at earlier time-points, with only th e ortho isomer showing significant levels of inhibition of axon outgrowth f ollowing 4-8h exposure. Probing of western blots with antibodies against cy toskeletal proteins indicated that the inhibition of axon outgrowth by thes e compounds was associated with a sustained reduction in the levels of phos phorylated neurofilament heavy chain. The inhibitory effect on axon outgrow th of TOCP but not TPCP was enhanced in the presence of a microsomal activa tion system. Since TOCP is the most neuropathic of the isomers of TCP in vi vo, differentiating N2a cells provide a useful cellular system for mechanis tic studies of the neurodegenerative effects of this organophosphate.