Alteration of synaptic connectivity of oligodendrocyte precursor cells following demyelination.

Aurã©Lia Sahel,Fernando C Ortiz,Christophe Kerninon,Marã­A Cecilia Angulo,Brahim Nait-Oumesmar,Paloma P Maldonado

doi:10.3389/fncel.2015.00077

Abstract

Oligodendrocyte precursor cells (OPCs) are a major source of remyelinating oligodendrocytes in demyelinating diseases such as Multiple Sclerosis (MS). While OPCs are innervated by unmyelinated axons in the normal brain, the fate of such synaptic contacts after demyelination is still unclear. By combining electrophysiology and immunostainings in different transgenic mice expressing fluorescent reporters, we studied the synaptic innervation of OPCs in the model of lysolecithin (LPC)-induced demyelination of corpus callosum. Synaptic innervation of reactivated OPCs in the lesion was revealed by the presence of AMPA receptor-mediated synaptic currents, VGluT1+ axon-OPC contacts in 3D confocal reconstructions and synaptic junctions observed by electron microscopy. Moreover, 3D confocal reconstructions of VGluT1 and NG2 immunolabeling showed the existence of glutamatergic axon-OPC contacts in post-mortem MS lesions. Interestingly, patch-clamp recordings in LPC-induced lesions demonstrated a drastic decrease in spontaneous synaptic activity of OPCs early after demyelination that was not caused by an impaired conduction of compound action potentials. A reduction in synaptic connectivity was confirmed by the lack of VGluT1+ axon-OPC contacts in virtually all rapidly proliferating OPCs stained with EdU (50-ethynyl-20-deoxyuridine). At the end of the massive proliferation phase in lesions, the proportion of innervated OPCs rapidly recovers, although the frequency of spontaneous synaptic currents did not reach control levels. In conclusion, our results demonstrate that newly-generated OPCs do not receive synaptic inputs during their active proliferation after demyelination, but gain synapses during the remyelination process. Hence, glutamatergic synaptic inputs may contribute to inhibit OPC proliferation and might have a physiopathological relevance in demyelinating disorders.

Highlights

Demyelination is defined by the loss of the myelin sheath insulating nerve fibers
The large majority of DsRed+/GFP+ cells in the lesion had the immunohistochemical phenotype of NG2+ Oligodendrocyte precursor cells (OPCs) (Figure 2A) while DsRed−/GFP+ cells were labeled for CC1, a specific marker of differentiated oligodendrocytes (Figure 2B)
As expected in the LPC model of demyelination (Watanabe et al, 2002), the population of DsRed+/GFP+ OPCs was largely increased at 7 dpi whereas a clear increase in differentiated DsRed−/GFP+/CC1+ oligodendrocytes was detected at 14 dpi (Figure 2D)

Summary

Introduction

Demyelination is defined by the loss of the myelin sheath insulating nerve fibers. The important consequences of demyelination are imposed on the axon in the form of disturbed conduction and compromised survival. OPC synaptic activity after demyelination system (CNS) has the capacity to restore myelin sheaths to demyelinated axons enabling them to recover conduction of action potentials and to provide effective neuroprotection (Franklin and Ffrench-Constant, 2008). This regenerative process is mainly mediated by the endogenous oligodendrocyte precursor cells (OPCs) expressing the proteoglycan NG2 (Nishiyama et al, 2009), that serve as a major source of remyelinating oligodendrocytes in demyelinating diseases, such as multiple sclerosis (MS). This is of significant clinical interest as it might open up perspectives for new remyelinating therapies

Methods

Results

Conclusion