Methylmercury exposure downregulates the expression of Racl and leads to neuritic degeneration and ultimately apoptosis in cerebrocortical neurons.
Methylmercury (MeHg) has been recognized as a neurotoxicant targeted on the central nervous system including cerebellum and cerebral cortex. Some molecular targets of MeHg have been identified using cerebellar neuronal cells, but little is known in the cerebrocortical neuronal cells. In this study, the molecular mechanism underlying MeHg-induced cell death in cerebrocortical neurons was investigated using a primary culture of embryonic rat cortical neuronal cells. The cultured cells exhibited apoptosis 3 days after exposure to 100 nM MeHg, suggesting the involvement of caspase-dependent apoptotic pathways. We demonstrated for the first time that neuritic degeneration precedes MeHg-induced apoptotic death in neurons exposed to 100 nM MeHg. Immunocytochemical and ELISA analyses for neurite-specific proteins namely, tau and MAP2, showed that injury to tau-positive axons was first induced followed by damage to the dendrites and cellular bodies. To further investigate the factors responsible for neuronal death, we investigated the expression levels of Rho-family proteins (Rac1, Cdc42, and RhoA), which regulate neuritic functions and apoptosis in neurons. Western blot analysis demonstrated that MeHg downregulated the expression levels of Rac1 and Cdc42 but did not affect RhoA. The exposure concentration and time course studies confirmed that Rac1 is targeted during an early stage of MeHg-induced cytotoxicity. The results indicate that neuritic degeneration, in particular axonal degeneration triggered by the downregulation of Rac1 expression, contributes to MeHg-induced apoptotic cell death in cultured cerebrocortical neurons.
Fujimura
Masatake
M
Department of Basic Medical Sciences, National Institute for Minamata Disease, Kumamoto, Japan. fujimura@nimd.go.jp
Usuki
Fusako
F
Sawada
Masumi
M
Rostene
William
W
Godefroy
David
D
Takashima
Akihiko
A
D000818:Animals; D017209:Apoptosis; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D003712:Dendrites / Q000737:chemistry / Q000187:drug effects; D015536:Down-Regulation; D008767:Methylmercury Compounds / Q000633:toxicity*; D008869:Microtubule-Associated Proteins / Q000032:analysis; D016501:Neurites / Q000737:chemistry / Q000187:drug effects*; D009474:Neurons / Q000737:chemistry / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020764:cdc42 GTP-Binding Protein / Q000032:analysis; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000032:analysis; D016875:tau Proteins / Q000032:analysis
C489787:MAP2 protein, rat; C489131:Mapt protein, rat; D008767:Methylmercury Compounds; D008869:Microtubule-Associated Proteins; D016875:tau Proteins; D020764:cdc42 GTP-Binding Protein; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein; C004925:methylmercuric chloride