Deciphering the role of microRNAs in neuronal maturation using live imaging

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The optimal functioning of the brain depends, among other things, on a delicate balance between the excitatory and inhibitory activity of neurons. GABA, a neurotransmitter or chemical messenger involved in cellular communication, is the major player of inhibitory activity in the brain. Indeed, normally, when GABA binds to a neuron, the activity of this neuron is inhibited. However, it is interesting to note that for immature neurons, this neurotransmitter rather plays an opposite role and has an excitatory function. During the neuronal maturation, the action of GABA will gradually change from excitatory to inhibitory. This critical period is believed to play an important role in development and its timing can modify the neurodevelopmental trajectory of an individual and indeed have lasting effects on the brain functions. Whereas the proteins responsible for this change are well known, the mechanisms that regulate them are much less understood. RNAs are known to be essential cellular building blocks that carry crucial information for all functions, including protein expression. In the early 1990's, researchers discovered a new class of small RNA molecules, called microRNA. Research on this subject has since shown that these molecules are essential for neuron survival and function. MicroRNAs operate by finely regulating the protein expression in cells. In humans, over two thousand different microRNAs have been identified, each controlling a few dozen proteins with diverse functions. Our hypothesis is that some of these microRNAs are important in the fine-tuning of the excitatory to inhibitory switch of GABA action in neurons. In order to study the role of microRNAs in this process, we need to be able to monitor the effects of GABA on the activity of living neurons while varying the amount of individual microRNAs in the cells. For this research program, we will use mouse neuronal primary culture to test the developmental maturation of single cells as a function of the number of individual microRNAs. Immature neurons are usually excited by the action of GABA while mature neurons are not. We have separated this project into three distinct objectives. Firstly, we will identify which microRNAs play a significant role in neuronal maturation. Secondly, we will evaluate the effect of modulating the number of these microRNAs on neuronal activity. Finally, we will develop a new methodology, combining state-of-the art RNA sequencing approach and multiple optical microscopy techniques, to relate the molecular content of single neurons to their activity. This program will allow a better understanding of the role of microRNAs in the fine-tuning of neuronal maturation. This multidisciplinary effort will require the development of innovative strategies to study the action of microRNAs at the single cell resolution and will ultimately shed light on neurodevelopmental disorders in a multitude of contexts.