Study Of The Role Of MicroRNA-34a In Alzheimer's Disease

AD is a kind of neurodegenerative disease which is characterized by deposition of senile plaque,neurofibrillary tangles inside neurons and loss of neurons in hippocampus and cerebral cortex.About 50%-60%dementia cases are caused by this disease[1].It is reported that there are 2-4 million of AD patients in America and 17-25 million of AD patients all over the world.In the western countries,AD is the fourth cause that leads to death following the heart diseases,tumors and strok.The mechanisms of AD is not clear.The studies in present showed that APP,Tau,PS and ApoE may play an important role in the pathogenesis of AD.However,to our regret,the treatments target APP,Tau,,PS and ApoE don't acquire the exciting effects.So it is necessary for us to understand the pathogenesis of AD from a new field.MicroRNAs(miRNAs) are endogenously expressed small ssRNA sequences of about 22 nucleotides in length which naturally direct gene silencing by suppressing the translation and/or promoting the degradation of target messenger RNAs(mRNAs) by binding to their 3'-untranslated regions(3'-UTR) [2,3].microRNAs are abundant in the brain and play an important role in neurodevelopment and synaptic plasticity[4].The results of some experiments imply that the alteration of microRNAs expression may be involved in the pathogenesis of AD[5,6]. However,the mechanism by which miRNAs affect the occurrence and development of AD is unclear.mir-34a belongs to mir-34a family and this family consists of three highly related miRNAs expressed from two separate loci:miR-34a from chromosome lp36 and the miR-34b/miR-34c cluster from chromosome 11q23.Various studies have shown that it is a potential tumor suppressor by inducing a G₁ cell cycle arrest,senescence and apoptosis[7,8,9,10].Furthermore,miR-34a is located on lp36,a region of frequent hemizygous deletion in human neuroblastomas and a variety of other cancers.However, the relationship between mir-34a and AD is not shown anywhere.Bcl-2 is an anti-apoptotic protein primarily expressed in mitochondria and the outer nuclear membrane,which prevents caspase-9 activation through an interaction with Apaf-1[11].Some research showed that bcl2 is neuroprotective against apoptotic cell death in Alzheimer's disease[12].Caspases are key mediators in the regulation and execution of apoptosis.Caspase-9 is activated during the intrinsic pathway downstream of mitochondria while caspase-3 is an effector caspase that initiates degradation of the cell in the final stages of apoptosis.Active caspase-9 can cleave and activates caspase-3[13],thereby setting in motion the events that lead to DNA fragmentation and cell death.Some research also showed caspase-9 and caspase-3 promoted the processing of APP(amyloid precursor protein) and tau and increased the number of NFTs and extracellular deposits of Aβin a mouse medel of AD.In our study we used the APPswe/PSΔE9 mice as a model for Alzheimer's disease (AD).We detected the expression level of mir-34a by microRNAs microarray and real time RT PCR and found that mir-34a is over-expressed in APPswe/PSΔE9 mice compared with age-matched controls and the expression of mir-34a is inversely correlated with the protein level of bcl2.We recognize the target site for mir-34a in the 3'-untranslated region(3'UTR) of bcl2 mRNA,and we show that mir-34a ectopic overexpression directly results in bcl2 down-regulation in SH-SY5Y cells.We also detected the protein level of active caspase-9 and caspase-3,the results showed that both of the two proteins expressed at higher levels in APPswe/PSΔE9 mice and stable transfection cell line of mir-34a compared with controls.Consistently,mir-34a knock-down through antisense LNA oligonucleotides increased the level of bcl2 protein in SH-SY5Y cells,which results in the down-regulation of active caspase-9 and caspase-3.Our results suggested that bcl2 is an important functional target for mir-34a in AD,and that the over-expression of mir-34a may contribute to the pathogenesis of AD,at least in part by affecting the expression of bcl2.