Brain function depends on the proper maturation of neuronal signals and their integration across different brain regions. In this regard, the process of myelination plays a key role during development. Myelination of neuronal fibers speeds up neuronal transmission ensuring a successful development of neuronal circuits. Recent studies in vitro and in vivo suggest that neuronal activity influences myelination through vesicular release of neurotransmitters. However, the mechanisms by which myelination is fine-tuned by neuronal activity are poorly understood.
Oligodendrocyte precursor cells (OPCs) generate oligodendrocytes, the glial cells responsible for producing myelin in the central nervous system. In the last 15 years, it has been demonstrated the existence of functional synaptic inputs between neurons and OPCs throughout the brain. This surprising discovery of “neuron-glia synapses” has challenged the dogma that synapses are a unique feature of neurons in the brain. However, the role of synapses onto OPCs remains elusive. Since myelination is a process modulated by neuronal activity, it has been hypothesized that neuron-OPC synapses are implicated in the mechanisms controlling oligodendrogenesis and myelination. Previous studies of my host team have demonstrated that OPCs of the somatosensory cortex receive a transient, but major synaptic input from GABAergic interneurons which correlates with the onset of cortical oligodendrogenesis. These findings show that OPCs are privileged partners of GABAergic interneurons during development and suggest that interneurons control OPC function during a critical period for OPC differentiation onto oligodendrocytes.
The main goal of this project is to study the impact of interneuronal activity on OPC proliferation and differentiation into myelinating oligodendrocytes as well as on cortical myelination during development of the somatosensory cortex. We will use a multidisciplinary approach combining optogenetic tools with electrophysiology and immunostainings in different transgenic mice expressing the light-sensitive protein channelrhodopsin-2 (ChR2) in interneurons. We will address the following specific aims: 1) we will evaluate the best parameters for an efficient photostimulation of ChR2-expressing interneurons in different transgenic lines; 2) we will perform in vivo phostostimulation of ChR2-expressing interneurons in these lines and analyze the effect of photoactivation on OPC proliferation, differentiation and myelination, and 3) we will assess whether the effect of interneuronal activity depends on synaptic transmission between interneurons and OPCs by inactivating the GABAergic synaptic activity of OPCs using an inducible knockout mouse. During my Erasmus fellowship in Paris, I recorded ChR2-expressing interneurons using patch-clamp recordings in acute slices in a transgenic line for two types of interneurons, and found the parameters to efficiently photoactivate them. I also implemented a surgery at postnatal day 10 of the mouse to photostimulate interneurons in vivo and record the extracellular activity elicited by photoactivation. These preliminary results show the viability of the proposed experiments.
Using state-of-the-art techniques in Neurobiology, this project will address a new role of GABAergic interneurons in regulating non-neuronal functions during cortical development. We expect to clarify the properties and mechanisms allowing different interneuron subtypes to regulate OPC dynamics, oligodendrogenesis and myelination. This project could also help to understand how neuronal activity may promote myelination and thus open a new avenue for the design of innovative therapies promoting myelination and oligodendrocyte regeneration in developmental myelin disorders.