Neurodegenerative disease-associated microRNAs acting as signaling molecules modulate CNS neuron structure and viability.

BACKGROUND: Dysregulation of microRNA (miRNA) expression in the brain is a common feature of neurodegenerative diseases. Beyond their conventional role in regulating gene expression at the post-transcriptional level, certain miRNAs can act extracellularly as signaling molecules. Our study elucidates the identity of such miRNA species serving as ligands for membrane receptors expressed in central nervous system (CNS) neurons and the impact of such miRNAs on neurons in the context of neurodegenerative disease. METHODS: We combined a machine learning approach with the analysis of disease-associated miRNA databases to predict Alzheimer's disease (AD)-associated miRNAs as potential signaling molecules for single-stranded RNA-sensing Toll-like receptors (TLRs) 7 and 8. TLR-expressing HEK-Blue reporter cells, primary murine microglia, and human THP-1 macrophages were used to validate the AD miRNAs as ligands for human and mouse TLR7 and/or TLR8. Interaction between mouse cortical neurons and extracellularly applied AD miRNAs was analyzed by live cell imaging and confocal microscopy. Transcriptome changes in cortical neurons exposed to AD miRNAs were assessed by RNAseq and RT-qPCR. The extracellular AD miRNAs' effects on CNS neuron structure were investigated in cell cultures of murine primary cortical neurons and iPSC-derived human cortical neurons by immunocytochemistry. We employed a mouse model of intrathecal injection to assess effects of AD miRNAs acting as signaling molecules on neurons in vivo. RESULTS: We identified the AD-associated miRNAs miR-124-5p, miR-92a-1-5p, miR-9-5p, and miR-501-3p as novel endogenous ligands for TLR7 and/or TLR8. These miRNAs being extracellularly stable and active were taken up by murine cortical neurons via endocytosis and induced changes in neuronal inflammation-, proliferation-, and apoptosis-related gene expression. Exposure of both murine and human cortical neurons to the AD-associated miRNAs led to alterations of dendrite and axon structure, synapse protein expression, and cell viability in a sequence-dependent fashion. Extracellular introduction of the AD miRNAs into the cerebrospinal fluid of mice resulted in both changes in neuronal structure and synapses, and neuronal loss in the cerebral cortex. Most of the observed extracellular miRNA-induced effects on cortical neurons involved TLR7/8 signaling. CONCLUSION: Neurodegenerative disease-associated miRNAs in extracellular form act as signaling molecules for CNS neurons including human cortical neurons, thereby modulating their structure and viability.