Roles of post-transcriptional gene silencing in the functional regulation of neuronal gene expression and plasticity

The phenomenon of synaptic plasticity in neurons is poorly understood, but is known to rely on appropriate temporo-spatial availability of mRNA. The complexity of neuronal cytoarchitecture necessitates an exquisite regulatory matrix that begins with the establishment of subcellular compartments during differentiation, however the molecular mechanisms that support trafficking and translational control are not well defined. The class of short, non-coding RNA molecules known as microRNA (miRNA) have well-established roles in neuronal differentiation and development, and growing evidence suggests that miRNA-mediated post-transcriptional gene silencing (PTGS) may be an important mediator of synaptic plasticity. To investigate this in a human genetic context, techniques were established for isolating distinct subcellular fractions of the SH-SY5Y neuroblastoma cell line and examining genome-wide miRNA and mRNA responses to neuronal cues such as differentiation and depolarisation. These studies identified a pattern of activity-associated miRNA expression changes unique to the neurites that was revealed to be connected to the release of exosomes from this compartment. Interestingly, some miRNA were found to be preferentially enriched in the nucleus. A motif detected within these sequences lead to the unexpected identification of putative transcription factor binding elements within their precursors, showing support for novel roles of miRNA outside PTGS. Connecting these findings was the unanticipated contribution of primate-specific miRNA, resulting in significant ontological enrichment of neuronal functionality. This demonstrates the importance and relevance of these cells as a vehicle for explicating the mechanisms underlying higher brain functions. Ultimately, substantial evidence was obtained to support a role for miRNA and the components of PTGS in the functional compartmentalisation of neurons and the response to activity, though further methodological developments are required to elaborate the novel mechanisms of miRNA function and investigate the direct contribution of miRNA-mediated PTGS to enabling real-time, activity-driven synaptic modification.