Central nervous system myelin is a specialized structure produced by oligodendrocytes that ensheaths axons, allowing rapid and efficient saltatory conduction of action potentials. 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 (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptor antagonist NBQX 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-AP5), but not by more selective blockers of NR2A and NR2B subunit-containing receptors (NVP-AAM077 and ifenprodil). In vitro ischaemia causes ultrastructural damage to both axon cylinders and myelin. 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 glutamate, 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.

No abstract

It is widely recognized that myelination of axons greatly enhances the speed of signal transmission. An exciting new finding is the dynamic communication between axons and their myelin-forming oligodendrocytes, including activity-dependent signalling from axon to myelin. The oligodendrocyte-myelin complex may in turn respond by providing metabolic support or alter subtle myelin properties to modulate action potential propagation. In this Opinion, we discuss what is known regarding the molecular physiology of this novel, synapse-like communication and speculate on potential roles in disease states including multiple sclerosis, schizophrenia and Alzheimer disease. An emerging appreciation of the contribution of white-matter perturbations to neurological dysfunction identifies the axo-myelinic synapse as a potential novel therapeutic target.

No abstract

Here we describe a technique for measuring changes in Ca2+ in the cytosolic domain of mature compact myelin of live axons in the central nervous system (CNS). We label the myelin sheath of optic nerve and dorsal column axons by using the Ca2+ indicator X-rhod-1 coupled with DiOC6(3) to produce bright myelin counterstaining, thereby providing unambiguous identification of the myelin sheath for analysis of two-photon excited fluorescence. We present evidence for localization of the Ca2+ reporter to the cytosolic domain of myelin, obtained by using fluorescence lifetime, spectral measurements and Mn2+ quenching. Chemical ischemia increased myelinic X-rhod-1 fluorescence (approximately 50% after 30 min) in a manner dependent on extracellular Ca2+. Inhibiting Na+-dependent glutamate transporters (with TBOA) or glycine transporters (with sarcosine and ALX-1393) reduced the ischemia-induced increase in Ca2+. We show that myelinic N-methyl-D-aspartate (NMDA) receptors are activated by the two conventional coagonists glutamate and glycine, which are released by specific transporters under conditions of cellular Na+ loading and depolarization in injured white matter. This new technique facilitates detailed studies of living myelin, a vital component of the mammalian CNS.

While anti-VEGF drugs are commonly used to inhibit pathological retinal and choroidal neovascularization, not all patients respond in an optimal manner. Mechanisms underpinning resistance to anti‑VEGF therapy include the upregulation of other proangiogenic factors. Therefore, therapeutic strategies that simultaneously target multiple growth factor signaling pathways would have significant value. Here, we show that Ca 2+ /calmodulin-dependent kinase II (CAMKII) mediates the angiogenic actions of a range of growth factors in human retinal endothelial cells and that this kinase acts as a key nodal point for the activation of several signal transduction cascades that are known to play a critical role in growth factor–induced angiogenesis. We also demonstrate that endothelial CAMKIIγ and -δ isoforms differentially regulate the angiogenic effects of different growth factors and that genetic deletion of these isoforms suppresses pathological retinal and choroidal neovascularization in vivo. Our studies suggest that CAMKII could provide a novel and efficacious target to inhibit multiple angiogenic signaling pathways for the treatment of vasoproliferative diseases of the eye. CAMKIIγ represents a particularly promising target, as deletion of this isoform inhibited pathological neovascularization, while enhancing reparative angiogenesis in the ischemic retina.

No abstract

This corrects the article DOI: 10.1038/nrn.2017.128.