Cephalic Neuronal Vesicle Formation is Developmentally Dependent and Modified by Methylmercury and sti-1 in Caenorhabditis elegans.

Methylmercury (MeHg) is a potent neurotoxicant. The mechanisms underlying MeHg-induced neurotoxicity are not fully understood. Several studies have shown that protein chaperones are involved in MeHg toxicity. The protein co-chaperone, stress inducible protein 1 (STI-1), has important functions in protein quality control of the chaperone pathway. In the current study, dopaminergic (DAergic) cephalic (CEP) neuronal morphology was evaluated in the Caenorhabditis elegans (C. elegans) sti-1 knockout strain. In the control OH7193 strain (dat-1::mCherry + ttx-3::mCherry), we characterized the morphology of CEP neurons by checking the presence of attached vesicles and unattached vesicles to the CEP dendrites. We showed that the attached vesicles were only present in adult stage worms; whereas they were absent in the younger L3 stage worms. In the sti-1 knockout strain, MeHg treatment significantly altered the structures of CEP dendrites with discontinuation of mCherry fluorescence and shrinkage of CEP soma, as compared to the control. 12 h post treatment on MeHg-free OP50-seeded plates, the discontinuation of mCherry fluorescence of CEP dendrites in worms treated with 0.05 or 0.5 µM MeHg returned to levels statistically indistinguishable from control, while in worms treated with 5 µM MeHg a higher percentage of discontinuation of mCherry fluorescence persisted. Despite this strong effect by 5 µM MeHg, CEP attached vesicles were increased upon 0.05 or 0.5 µM MeHg treatment, yet unaffected by 5 µM MeHg. The CEP attached vesicles of sti-1 knockout strain were significantly increased shortly after MeHg treatment, but were unaffected 48 h post treatment. In addition, there was a significant interactive effect of MeHg and sti-1 on the number of attached vesicles. Knock down sti-1 via RNAi did not alter the number of CEP attached vesicles. Taking together, our data suggests that the increased occurrence of attached vesicles in adult stage worms could initiate a substantial loss of membrane components of CEP dendrites following release of vesicles, leading to the discontinuation of mCherry fluorescence, and the formation of CEP attached vesicles could be regulated by sti-1 to remove cellular debris for detoxification.