MicroRNA-16 Targets MRNA Involved in Neurite Extension and Branching in Hippocampal Neurons During Presymptomatic Prion Disease

The mechanisms that lead to neuronal death in neurodegenerative diseases are poorly understood. Prion diseases, like many more common disorders such as Alzheimer's and Parkinson's diseases, are characterized by the progressive accumulation of misfolded disease-specific proteins. The earliest changes observed in brain tissue include a reduction in synaptic number and retraction of dendritic spines, followed by reduced length and branching of neurites. These pathologies are observable during presymptomatic stages of disease and are accompanied by altered expression of transcripts that include miRNAs. Here we report that miR-16 localized within hippocampal CA1 neurons is increased during early prion disease. Modulating miR-16 expression in mature murine hippocampal neurons by expression from a lentivirus, thus mimicking the modest increase seen in vivo, was found to induce neurodegeneration. This was characterized by retraction of neurites and reduced branching. We performed immunoprecipitation of the miR-16 enriched RISC complex, and identified associated transcripts from the co-immunoprecipitated RNA (Ago2 RIP-Chip). These transcripts were enriched with predicted binding sites for miR-16, including the validated miR-16 targets APP and BCL2, as well as numerous novel targets. In particular, genes within the neurotrophin receptor mediated MAPK/ERK pathway were potentially regulated by miR-16; including TrkB (NTRK2), MEK1 (MAP2K1) and c-Raf (RAF). Increased miR-16 expression in neurons during presymptomatic prion disease and reduction in proteins involved in MAPK/ERK signaling represents a possible mechanism by which neurite length and branching are decreased during early stages of disease.