Synaptic control of NG2 cell fate during myelination and myelin repair – SYNAMYEL

We are presently combining electrophysiological, molecular, cellular and genetic approaches to elucidate functional roles of neuronal-NG2 cell synapses in oligodendrocyte proliferation and differentiation, during the (re)myelination process. We have been using different transgenic mouse models to correlate cellular, morphological and electrophysiological changes in synaptic activity of NG2 cells with developmental myelination. For functional studies, we are creating genetic tools to to impair specifically glutamate receptor activity at the neuronal-NG2 cells synapses, leaving synapses between neurons intact. This will allow us to evaluate how these synapses affects the proliferative and differentiation capacity of NG2 cells, the generation of myelinating oligodendrocytes during myelination and myelin repair in experimental models of MS. Our next step will be to use optogenetics to analyze in details the interactions between neuronal fibers and NG2 cells during myelin regeneration in demyelinated lesions. Optogenetics will allow for a selective, simple and rapid activation of these fibers with light, i.e. a flexible and non-invasive tool, while recording NG2 cells.

To decipher the role of synapses between neurons and oligodendrocytes, we analyzed the profile of innervations of OPCs/NG2 cells during developmental myelination and after demyelinated lesions of the adult mouse brain, as well as in multiple sclerosis (MS) lesions. We first quantified the density of putative synapses during the myelination process of white matter fibres. We showed that the density of synaptic contacts on OPCs increased during the active phase of myelination. We next analysed the morphological features of neurons to OPCs/NG2 cells synapses after experimental demyelination of white matter fibres in the adulthood. Using histological methods, we demonstrated that OPCs receive synaptic contacts in demyelinations lesions, with a higher density during myelin repair of the lesions. Moreover, our data indicate that OPCs establish synaptic contacts with glutamatergic neurons in MS lesions. Intriguingly, we demonstrated that the density of innervations of the OPCs was significantly increased in active lesions of MS as compared to chronic silent lesions and normal appearing white matter. These histological analyses suggest a potential role of the neuronal activity on oligodendrocyte regeneration and repair.

To assess the electrical properties of neurons-OPCs synapses, we recorded electrical currents on brain slices of reporter transgenic mice, during developmental myelination and after a demyelinated lesion. Our data indicate that these neuronal inputs into OPCs are functional at the onset of brain myelination and remyelination of demyelinated lesions. Altogether our study suggests that neuronal activity play a critical role in the controal myelination during normal development and in myelin disorders.

This innovative project attempts to decipher the role of the synapses between neurons and oligodendrocyte precursor cells in demyelinating diseases. Our project should provide better insights on the cellular and molecular mechanisms that regulate oligodendrocyte development and regeneration. Furthermore, it may have profound consequences for the design of innovative therapies promoting myelin repair in demyelinating diseases, such as multiple sclerosis.

1. Vélez-Fort, M, Audinat, E, Angulo, MC (2011) The central role of GABA in neuron–glia interactions. The Neuroscientist 18(3):237-50 (Invited Review)

2. Maldonado, PP, Vélez-Fort, M, Angulo, MC (2011) Is neuronal communication with NG2 cells synaptic or extrasynaptic ? Journal of Anatomy 219(1):8-17) (Invited Review)

3. Sahel A, Ortiz-Cisternas F, Kerninon C, Maldonado PP, Angulo MC, Nait Oumesmar B (2012) Influences of neuronal-NG2 synapses on myelination and myelin repair (In preparation for Brain).

Myelinating oligodendrocytes are derived from oligodendrocyte precursor cells expressing the proteoglycan NG2 (NG2 cells) during development of the central nervous system (CNS). NG2 cells also persist in the adult CNS where they represent the main proliferating cell type that serve as a major source of remyelinating oligodendrocytes in demyelinating diseases, such as multiple sclerosis (MS). NG2 cells constitute an especially attractive target for developing therapeutic strategies aiming at increasing remyelination and thus, repairing effectively damaged tissue. For this, it is crucial to understand how proliferation and differentiation of these cells is controlled during physiological myelination and in white matter lesions.

In the last ten years, the existence of functional and morphological synaptic contacts between neurons and NG2 cells revolutionize the field of neuron-glia interactions. The role of these synaptic inputs, however, is still elusive and has raised questions of: i) which are the specific mechanisms of synaptic transmission between neurons and NG2 cells, and ii) which roles are played by the neuronal-NG2 cell synapses in oligodendrocyte proliferation, differentiation and myelination, under physiological and pathological conditions?

The main goal of the present project is to decipher whether neuronal synaptic inputs control NG2 cell fate in an activity-dependent manner during developmental myelination and after chemically-induced demyelinated lesions of the corpus callosum. To reach this goal, we will combine complementary expertises of cellular biologists (Brahim Nait Oumesmar’s, Partner 1) and physiologists (Maria Cecilia Angulo’s Partner 2) studying the function of NG2 cells/OPCs. In this application, we will address the following specific aims:

1) Determine the functional and morphological synaptic innervations of NG2 cells, pre-oligodendrocytes and mature oligodendrocytes during developmental myelination.
By means of electrophysiological and morphological techniques, we will correlate the myelination process and the properties of functional synaptic inputs of NG2 cells and other cells of the oligodendrocyte lineage. We will attempt to demonstrate the participation of NG2 cell synaptic inputs in myelination by using different transgenic mouse models.

2) Assess the role of synaptic inputs onto NG2 cells in their proliferation and differentiation. To reach this goal, the most straightforward strategy is to impair selectively glutamatergic transmission in NG2 cells and analyse the effect of this synaptic loss-of-function on oligodendrocyte proliferation, differentiation and myelination.

3) Evaluate the functional meaning of NG2 cell 'synaptic' activity in remyelination of lysolecithin-induced demyelinated lesions of mouse corpus callosum.
In order to test whether synaptic activity of NG2 cells play a role in remyelination, we will perform here a compilation of experiments proposed in the two previous aims, but in animals showing demyelinating lesions.