Neuron-specific characteristics of miR135b- and miR137-mediated translational regulation

microRNAs are small, non-coding post-transcriptional regulators of gene expression that target specific mRNAs, typically effecting translational repression coupled to degradation. In contrast, previous work within the Luthi-Carter laboratory has shown that neuron-enriched microRNAs, including miR135b and miR137, co-exist with their targets, many of which encode proteins with important synaptic functions.

Neurons are highly polarised, interconnecting cells that form complex brain networks. The site of neurotransmission, the synapse, allows the directional flow of electrical and chemical signals via specialised neuronal subdomains, i.e. from the axon to dendritic fields. mRNA transport and local translation is one of several mechanisms modulating synaptic strength, in a phenomenon known as neuronal plasticity.

In this thesis, I test the hypothesis that neuronal microRNAs, miR135b and miR137, operate via non-degrading target interactions. In addition, I explore a related hypothesis and demonstrate that miR135b- and miR137-target interactions display characteristics consistent with a role in RNA transport to synapses, including their interaction with relevant RNA binding proteins.

My analyses provide strong evidence for non-degrading miR135b- and miR137-target interactions, by demonstrating that both microRNAs co-exist with, and in some cases positively modulate the levels of, their target mRNAs. Moreover, under the conditions of my experiments, overexpression of miR137 led to an increase, rather than a decrease, in the levels of its target proteins. Furthermore, I show that miR135b and miR137 and their targets coexist in MOV10-positive complexes, consistent with a role in translational repression during mRNA transport to pre- or postsynaptic fields. Although not expressly demonstrated, the association of miR135b and miR137 with MOV10 in this context strongly implicates these complexes in signal-dependent modulation of local protein translation to regulate synaptic and neuronal connectivity. The regulation of miR135b- and miR137-target interactions may therefore explain their previous associations with dysfunctional neuronal connectivity giving rise to human neurological and neuropsychiatric conditions.