Neurotoxicity induced in differentiated SK-N-SH-SY5Y human neuroblastoma cells by organophosphorus compounds

Organophosphorus (OP) compounds used as insecticides and chemical warfare agents are known to cause potent neurotoxic effects in humans and animals. Organophosphorus-induced delayed neuropathy (OPIDN) is currently thought to result from inhibition of neurotoxic esterase (NTE), but the actual molecular and cellular events leading to the development of OPIDN have not been characterized. This investigation examined the effects of OP compounds on the SY5Y human neuroblastoma cells at the cellular level to further characterize cellular targets of OP neurotoxicity. Mipafox and paraoxon were used as OP models that respectively do and do not induce OPIDN. Mipafox (0.05 mM) significantly decreased neurite length in SY5Y cells differentiated with nerve growth factor (NGF) while paraoxon at the same concentration had no effect when evaluated after each of three 4-day developmental windows during which cells were treated daily with OP or vehicle. In contrast, paraoxon but not mipafox altered intracellular calcium ion levels ([Ca 2+] i), as seen in three types of experiments. First, immediately following the addition of a single high concentration of OP to the culture, paraoxon caused a transient increase in [Ca 2+] i, while mipafox up to 2 mM had no effect. Paraoxon hydrolysis products could also increase intracellular Ca 2+ levels, although the pattern of rise was different than it appeared immediately after paraoxon administration. Second, repeated low-level paraoxon treatment (0.05 mM/day for 4 days) decreased basal [Ca 2+] i in NGF-differentiated cells, though mipafox had no effect. Third, carbachol, a muscarinic acetylcholine receptor agonist, transiently increased [Ca 2+] i in differentiated cells, an affect attenuated by 4-day pretreatment with paraoxon (0.05 mM/day), but not by pretreatment with mipafox. These results indicate that the decrease in neurite extension that resulted from mipafox treatment was not caused by a disruption of Ca 2+ homeostasis. The effects of OPs that cause or do not cause OPIDN were clearly distinguishable, not only by their effects on neurite length, but also by their effects on Ca 2+ homeostasis in differentiated SY5Y cells.