NMDARs in oligodendrocytes are activated during CNS ischemia, leading to intracellular Ca2+-dependent injuries to myelin and oligodendroglia (Stys and Lipton, 2007
; Alix and Fern, 2009
), and to impaired axonal function (Bakiri et al., 2008
), although some studies have failed to demonstrate protective roles of NMDAR antagonists against oligodendroglial death and axonal damage (Tekkok et al., 2007
; Basso et al., 2008
; Baltan, 2009
). Since CNS extracellular glutamate homeostasis is impaired in MS and in EAE (Bolton and Paul, 2006
), blocking oligodendroglial NMDARs might ameliorate disease severity and axonal damage in these autoimmune demyelinative disorders; the amelioration of EAE obtained with the NMDAR antagonist memantine (Wallstrom et al, 1996
) seemed to support this hypothesis. Unexpectedly, however, we found that specific genetic disruption of NR1 in oligodendrocytes did not alter the clinical or pathological severity of EAE.
Inclusion of NR3A in the NMDARs of neonatal cortical neurons protects them against Ca2+
-mediated excitotoxicity (Nakanishi et al., 2009
). Adult oligodendrocytes, unlike most adult neurons, express NR3A-containing NMDARs (Stys and Lipton, 2007
). We found, however, that there was no difference in EAE severity between wild-type mice and littermate controls in which one or both copies of NR3A gene was constitutively disrupted (), thus further weakening support for a role of oligodendroglial NMDARs (composed of NR1, NR2 and NR3A subunits) in WM injury in demyelinating disease.
Previous studies showed that NMDAR antagonists alleviated the severity of PVL and EAE (Wallstrom et al., 1996
; Manning et al., 2008
), but these ameliorative effects might have been due to their effects on neuronal (Stys and Lipton, 2007
) or microglial (Murugan et al., 2011
) NMDARs. Furthermore, intravitreal injection of NMDA caused axonal loss without myelin alterations in adult optic nerves (Kuribayashi et al., 2010
) suggesting that NMDARs in neuronal somas or axons, rather than in oligodendrocytes, were responsible for this axonopathy. Neither genetic ablation of oligodendroglial lineage NR1, a subunit essential for NMDAR function, nor constitutive ablation of NR3A, an NMDAR modulatory subunit in oligodendroglia, altered susceptibility to EAE. These results do not support a role for oligodendroglial NMDARs in the EAE model of multiple sclerosis.
NMDA-evoked currents have been detected in a subpopulation of OPCs (Ziskin et al., 2007
); current amplitude was significantly down-regulated after OPC differentiation (De Biase et al., 2010
; Kukley et al., 2010
). We employed two color non-radioactive ISH to visualize NR1 transcripts in Plp+ oligodendrocytes (). Our results, which were consistent with prior reports using 35
S-labeled and 33
P-labeled (South et al., 2003
) NR1 probes, showed that NR1 transcript signals were barely detectable in WM tracts, where many Plp+ oligodendrocytes were present (). Our NR1 immunostaining results were consistent with the barely detectable ISH transcript expression (), supporting previous microarray observations (Cahoy et al., 2008
; De Biase et al., 2010
). It is possible that the extremely low-level expression of oligodendroglial NR1, previously estimated by qRT/PCR to be only 1–2% of the abundance in neurons (Salter and Fern, 2005
), is still sufficient to generate physiologically relevant NMDA-evoked currents in the oligodendroglial lineage (Burzomato et al., 2010
), and sufficient Ca2+
influx to be pathologically significant (Karadottir et al., 2005
). Thus, our data do not negate the concept that oligodendroglial lineage NMDARs contribute to CNS WM ischemic injury. However, our findings do strongly suggest that oligodendroglial NMDARs are not major players in EAE, and likely also in multiple sclerosis, and also argue against a major role for OPC NMDARs in regulating OPC maturation and differentiation.