Role Of PKC-MAPK Pathway And PARP In Neurodevelopmental Toxicity By Prolonged Low Level Exposure Of MMC

The worldwide environmental contamination of methylmercury(MeHg) has being a focus from 50th of last centrury. The developing brainappears particularly sensitive to MeHg. Exposure levels in pregnantexperimental animals that do not result in detectable signs or symptomsin the mother can adversely affect the offspring's development. Studies ofhuman poisonings suggest that may also occur in humans. Humanexposure to MeHg is primarily dietary through the consumption of fish:MeHg is present in all fresh and saltwater fish. Populations that dependon fish as a major source of dietary protein may achieve MeHg exposurelevels hypothesized to adversely affect brain development. Increasingmercury levels in the environment have heightened concerns aboutdietary exposure and a possible role for MeHg in developmentaldisabilities. But the Mechanism in MeHg neurotoxicity is underunclear.Researches showed that cellular pathway play an important rolein neurodevelopment. So we assumed that PKC-RAS-MAPKs signalpathway maybe involved in MeHg neurotoxicity.Recently, the mechanisms about the activation PKC-Raf-MAPKspathway in neurotocxity were emphasized.Protein kinase C is a kind ofkinase that distributes widely in mammals cells, is a family that consistedof many kinds of isoenzymes, they have similar structures, but diferentbiochemical characteristics and physiological function.PKC is secondmessenger related to signal transduction from extracellular stimulation.MAPKs is a family of serine/threonine(Ser/Thr) kinases,which canbe activated by many factors, such as cytokines,hormones,neurotansm-itters and cell stress. MAPKs play a key role not only in cellularproliferation, but also in cellular differentiation and apoptosis. Inmammals,MAPK include 3 subtypes;ERK,JNK,p38. Signal pathways ofthese kinases are highly conserved.All these key kinases modulatecelular function through phosphorylated transcribe factors throughtransportation extracellular signal to nucleus. Diferent kinases act ondiferent substance to modulate diferent celular efects by relatedindependent signal pathway.ERK is one important family ofMAPKs,which can be activated by extracelular stimulation via G proteinreceptor, growth factor receptor and tyrosine protein kinase receptor andRaf-MEK-ERK series signal,then modulate transcribe factors and somekinases activites, promote protein synthesize, and cell proliferation.Apoptosis is relatedto neurotoxicity by prolonged low level exposure ofMeHg.Poly (ADP-ribose) polymerase (PARP) is involved in variouscellular functions, including DNA repair, the cell cycle and celldeath.While PARP activation could play a critical role in repairing braindamage, PARP inactivation caused by caspase 3-cleavage may also beimportant for apoptotic execution.PKC,MAPK and PARP are intracellularinteractional signal transfer.The aim of the present study was to investigate the role ofPKC-RAS-MAPKs signal pathway in mechanism of MeHgneurodevelopmental toxicity.For this purpose, PC12 cells were preparedand treated by low level MeHg, the cellular survival was determined withMTT assay,and apoptosis of PC12 cells was detected with flow cytometry(FCM).The effects of MeHg on the expression of PARP and PKCαandthe phosphorylation of ERK and JNK in PC12 cells and hippocampalneurons were detected by immunohistochemistry and western blot.Theresults show that low level exposure of MeHg affected the differentiationand survival of PC12 cells,and apoptosis of PC12 cells was alsostimulated.0.010-0.015μM MeHg increased the expression of PKCα andphosphorylation of ERK,however the expression of PKC α andphosphorylation of ERK were decreased and the expression of PARP andphosphorylation of JNK were increased by 0.020-0.030 μ M MeHg.Conclusion: The differentiation and survival can be affected by low levelexposure of MMC.The expression of PKCαand phosphorylation of ERKwere related to the alteration of PC12 cells' differentiation.Thephosphorylation of ERK in PC12 cells and hippocampal neurons by MeHgwas regulated by PKC-Raf-ERK pthway. The expression of PKCα,PARPand the phosphorylation of ERK,JNK in mice hippocampal neurons werealtered by low level exposure of MMC,and the learning and memory abilityof mice was also decreased with the alteration in ultrastructure ofhippocampal neurons.The present study would be the roretical basis for theunderstanding in mechanism of neurotoxicity by MeHg.