Study Of The Role Of MicroRNA-106b In Alzheimer's Disease

Alzheimer's disease (AD) is a kind of neurodegenerative disease. It is the main reason leading to dementia in old people. It is reported that there are more than 20 million AD patients all over the world, and AD patients increase 460 million per year. AD seriously affect the life quality of older persons and bring heavy burden to family and social, becoming a major health and socio-economic problems that many countries is facing or will face to. 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. Therefore, it is necessary to study the pathogenesis of AD and explore new treatment for AD from a new field. MicroRNAs (miRNAs) are recently discovered small non-coding regulatory RNA of 18-25 nucleotides in length and regulate the expression of target gene at post-transcriptional levels. miRNAs is abundant in brain tissue and play an important role in nervous system development and nerve cell differentiation. Recently, more and more studies show that microRNA plays an important role in pathogenesis of AD. MiR-107, miR-29a/b, miR-298, miR-328 are all implicated in pathogenesis of AD by regulating beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1). However, the role of miRNAs in the AD mechanism is still in its infancy.TGF-βs including TGF-β1, TGF-β2, TGF-β3, are multifunctional cytokine that has important neurotrophic and neuroprotective role in growth and survival of neurons. TGF-βtypeⅡreceptor (TGFBR2) is a serine/threonine receptor kinase and is a high affinity transmembrane receptor for TGF-Ps. TGF-βs bind to TGFBR2, which leads TGFBR2 forms a heterodimeric complex with TGFBR1. This receptor/ligand complex phosphorylates receptor-regulated Smads (R-Smads) 2 and 3. Then, pSmad2 and 3 bind to the common Smad (Co-smad) 4 and forms a heterodimeric complex. This complex then enters the cell nucleus to regulate the expression of target gene. Smad 6 and 7 belong inhibitory Smads (I-Smads). Their expression is inversely correlated with the activity of TGF-βsignaling. Levels of TGF-β1 are increased in human AD brain tissue and cerebrospinal fluid (CSF), but decreased in human AD serum. Furthermore, some researchers found the expression of TGFBR2 is reduced in the brain of AD patient.In our study we used the APPSWC/PSAE9 mice as a model for AD.We detected that the expression level of miR-106b by microRNAs microarray and real time RT-PCR and found that miR-106b is over-expressed in 3-month-old and 6-month-old AD mice and low-expressed in 9-month-old AD mice compared with age-matched control. We found there are two putative binding sites between TGFBR2 3'UTR and miR-106b through TargetScanS, miRanda and Pictar database. The expression of miR-106b is inversely correlated with the protein level of TGFBR2 in vitro. We further confirmed the direct regulatory relationship between miR-106b and TGFBR2 through luciferase assay and we find that only the first putative bindng site is useful for regulating the expression of TGFBR2. We induced miR-106b stable transfecant cells with retinoic acid to observe the influence of miR-106b on neuron growth and differentiation and found miR-106b stable transfecant cells displayed obvious degenerative change compared with control. Then, we detected the protein level of Smad2/3, pSmad2/3 and Smad6/7, the results showed that tatal Smad2/3 protein is unchanged, pSmad2/3 is down-regulated and Smad6/7 is up-regulated in stable transfection cell line of miR-106b compared with controls. Furthermore, we treat SH-SY5Y cells with Aβ42 oligomers found that the expression of miR-106b is increased 12,24,36 hours after treatment, but decreased 48 hours after treatment. Our results in vitro suggested that Aβ42 oligomers induced-miR-106b results in deficiency of TGF-βsignaling trough TβRⅡ, which led to retinoic acid induced-neurodegeneration in SH-SY5Y cells.We also detected the expression of TGFBR2 in APPSWC/PSΔE9 mice and found the level of TGFBR2 is downregulated. Moreover, we constructed miR-106b transgenic mice to study the regulation of miR-106b in vivo. The expression of TGFBR2 is upregulated in miR-106b transgenic mice compared with control.Our results suggested that TGFBR2 is an important functional target for miR-106b in AD, and that miR-106b may contribute to the pathogenesis of AD by affecting the expression of TGFBR2.