A study on roles of microRNA-188 in dendritic plasticity and its pathophysiological significance in Alzheimer's disease

Abstract

Introduction: In the central nervous system (CNS), miRNAs have been shown to regulate development, survival, function and plasticity. Despite considerable evidence for the regulatory functions of miRNAs, the identities of the miRNA species that are involved in the regulation of synaptic transmission and plasticity and the mechanisms by which these miRNAs exert their functional roles remain largely unknown. Moreover, neurodegenerative diseases, such as Alzheimers disease (AD), may be primarily a disorder derived from synaptic failure through dysfunction of dendritic spine and synaptic transmission, finally resulting in cognitive dysfunction. The goal of this study is to investigate the changes of miRNAs during long-term potentiation (LTP) induction and regulatory mechanisms or miRNAs in terms of dendritic plasticity in rat hippocampus. In addition, I also tried to examine pathophysiological roles of miR-188, one of miRNAs of which expression has been altered during LTP induction, in AD. Methods: To investigate the changes in expression profiles of miRNAs during LTP in rat hippocampal acute slice, miRNAs microarray and RT-qPCR were performed. Based on the profiled miRNAs microarray data, potential targets for miRNAs, of which expression were altered by LTP, were sought using bioinformatic programs. I examined the effects of miR-188, whose expression was upregulated by LTP and targets neuropilin-2 (Nrp-2), on dendritic spine density in rat primary hippocampal neuron cultures. Next, to investigate the pathophysiological roles of miR-188 in cognitive dysfunction in AD, first the levels of expression of miR-188 in the brains from human AD patients and 5x familial AD (5 x FAD) animal model were determined. I also examined whether oligomeric beta amyloid 1-42 (oAβ1-42) affected the expression of miR-188 and whether miR-188 rescues the reduction in dendritic spine density induced by oAβ1-42 in rat primary hippocampal neuron cultures. In addition, T-maze and contextual fear conditioning tests were performed with 5 x FAD AD animal model 3 weeks after viral-mediated expression of miR-188 in hippocampal CA1 region. Results: The expression of miR-188 was found to be upregulated by LTP induction. The protein level of Nrp-2, one of possible molecular targets for miR-188, was decreased during LTP induction. I also confirmed that the luciferase activity of the 3-untranslated region (UTR) of Nrp-2 was diminished by treatment with a miR-188 oligonucleotide but not with a scrambled miRNA oligonucleotide. Nrp-2 serves as a receptor for semaphorin 3F (Sema3F), which is a negative regulator of spine development and synaptic structure. In addition, miR-188 specifically rescued the reduction in dendritic spine density induced by Nrp-2 expression in rat primary hippocampal neuron cultures. miR-188 was significantly downregulated in the cerebral cortices (medial frontal gyrus) and hippocampi from AD patients, compared to those from age-matched control subjects. In addition, immunoreactivity against Nrp-2 was highly upregulated in the brain from AD patients, compared to those from age-matched control subjects. I also demonstrated that the treatment with oAβ1-42 significantly diminisheed the expression of miR-188 whereas the treatment of brain-derived neurotrophic factor significantly upregulated the expression of miR-188 in primary hippocampal neuron cultures. The treatment with miR-188 rescued the reduction in dendritic spine induced by oAβ1-42 in rat primary hippocampal neuron cultures. In addition, I found that viral-mediated expression of miR-188 into hippocampal CA1 ameliorated the cognitive impairment in 5 x FAD mice. Conclusions: I showed that an activity-dependent miRNA, miR-188, regulates dendritic plasticity by downregulating Nrp-2, which was known to be a negative regulator of dendritic spine formation and synaptic transmission. In addition, I demonstrated that the reduction in the expression of miR-188 in the brains from AD patients and AD animal model may contribute to the cognitive dysfunction observed in the disease. Taken together, this study provides that miR-188 act as an important regulator contributing to dendritic plasticity by downregulating Nrp-2 in synaptic functions. In addition, deficiency in miR-188 expression results in the reduction in dendritic spine numbers and accelerates cognitive impairment in AD. Therefore, I suggest that miR-188 has a therapeutic potential for AD, and could be developed as a diagnostic biomarker for the disease.