Synaptosomal versus cytosolic microRNAs and synapse function in Alzheimer's disease.

Abstract

The current research aims to discuss novel findings in Alzheimer's disease (AD) regarding the identification and characterization of synapse-enriched microRNAs (miRNAs). In AD, it is unclear if any specific miRNA is localized at the synapse. If so, are synapse miRNAs expressed differently in AD than in a healthy state? Furthermore, synaptosomal miRNAs are expressed differentially in the cytosol? What function do synaptosomal miRNAs play in synaptic activity and neurotransmission in AD? To address these intriguing questions, synaptosomal and cytosolic fractions were separated from the AD and healthy control postmortem brains. Separation and purity of synaptosomal fractions were confirmed by transmission electron microscopy of synapse assembly, qRT-PCR of synaptic genes and immunoblotting of synapse associated proteins. Next, we performed the global affymetrix miRNA microarray analysis of synaptosomal and cytosolic miRNAs in both AD patients and healthy controls. Further, we determined the biological process, cellular components and molecular pathways of potential miRNAs using Ingenuity Pathway Analysis software. Transcriptome Analysis Console V.4 was used to compare miRNAs data in four different ways: 1) AD synaptosome vs AD cytosol, 2) healthy control (HC) synaptosome vs HC cytosol, 3) AD cytosol vs HC cytosol, and 4) AD synaptosome vs HC synaptosome. In different comparisons, some miRNAs tended to display a very significant (P<0.0001) and high fold changes variance (+/- 200-fold) in their expression levels. Validation analysis on a large number of postmortem brain samples revealed that certain possible miRNAs were consistent across all samples in various comparisons. MiRNAs were categorized as synaptosomal or cytosolic based on their existence and/or expression levels. In comparison to healthy controls, synaptosomal and cytosolic miRNAs were found to be dysregulated in AD patients. Further, Ingenuity Pathway Analysis of synaptosomal miRNAs showed the involvement of these miRNAs in nervous system development, cell junction organization and synapse assembly formation. For the first time, we distinguished synapse miRNA candidates from cytosolic miRNAs based on their localization and expression levels, and we investigated some previously unknown details about synaptosomal miRNAs, cytosolic miRNAs, and synapse functions in AD.