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NMDA receptors mediate calcium accumulation in myelin during chemical ischaemia

Abstract

Central nervous system myelin is a specialized structure produced by oligodendrocytes that ensheaths axons, allowing rapid and efficient saltatory conduction of action potentials1. Many disorders promote damage to and eventual loss of the myelin sheath, which often results in significant neurological morbidity. However, little is known about the fundamental mechanisms that initiate myelin damage, with the assumption being that its fate follows that of the parent oligodendrocyte. Here we show that NMDA (N-methyl-d-aspartate) glutamate receptors mediate Ca2+ accumulation in central myelin in response to chemical ischaemia in vitro. Using two-photon microscopy, we imaged fluorescence of the Ca2+ indicator X-rhod-1 loaded into oligodendrocytes and the cytoplasmic compartment of the myelin sheath in adult rat optic nerves. The AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptor antagonist NBQX2 completely blocked the ischaemic Ca2+ increase in oligodendroglial cell bodies, but only modestly reduced the Ca2+ increase in myelin. In contrast, the Ca2+ increase in myelin was abolished by broad-spectrum NMDA receptor antagonists (MK-801, 7-chlorokynurenic acid, d-AP53,4), but not by more selective blockers of NR2A and NR2B subunit-containing receptors (NVP-AAM0775 and ifenprodil2,4). In vitro ischaemia causes ultrastructural damage to both axon cylinders and myelin6. NMDA receptor antagonism greatly reduced the damage to myelin. NR1, NR2 and NR3 subunits were detected in myelin by immunohistochemistry and immunoprecipitation, indicating that all necessary subunits are present for the formation of functional NMDA receptors. Our data show that the mature myelin sheath can respond independently to injurious stimuli. Given that axons are known to release glutamate7,8,9, our finding that the Ca2+ increase was mediated in large part by activation of myelinic NMDA receptors suggests a new mechanism of axo–myelinic signalling. Such a mechanism may represent a potentially important therapeutic target in disorders in which demyelination is a prominent feature, such as multiple sclerosis, neurotrauma, infections (for example, HIV encephalomyelopathy) and aspects of ischaemic brain injury.

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Figure 1: Ca 2+ imaging in optic nerve oligodendrocytes and myelin.
Figure 2: Effects of glutamate receptor antagonists on ischaemic Ca 2+ increase.
Figure 3: Immunodetection of NMDA receptor subunits in myelin.
Figure 4: NMDA receptor inhibition protects myelin against ischaemic injury.

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Acknowledgements

This work was supported by the NINDS, CIHR, Heart and Stroke Foundation of Ontario Center for Stroke Recovery, an HSFO Career Investigator Award, the Canadian Institute for Photonic Innovations, and the generosity of private donors to P.K.S. G.W.Z. is a Canada Research Chair and an AHFMR Senior Scholar. This work was additionally supported by the CIHR (G.W.Z., J.E.M.) and NINDS (B.D.T.). We thank Y. Auberson for the gift of NVP-AAM077; J. A. Wang and C. E. Morris for providing HEK293 cells for fluorescence-lifetime measurements; and M. Nikolaeva for assistance with fluo-4 Ca2+ imaging.

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Correspondence to P. K. Stys.

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This file contains Supplementary Figures 1–3 and Supplementary Legends 1–3, Supplementary Methods, Supplementary Notes (statistics, abbreviations, author contributions, additional references pertaining to supplementary methods) and Supplementary Methods.

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Micu, I., Jiang, Q., Coderre, E. et al. NMDA receptors mediate calcium accumulation in myelin during chemical ischaemia. Nature 439, 988–992 (2006). https://doi.org/10.1038/nature04474

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