The myelin sheaths wrapped around axons by oligodendrocytes are crucial for brain function. In ischaemia myelin is damaged in a Ca 2+ -dependent manner, abolishing action potential propagation 1,2 . This has been attributed to glutamate release activating Ca 2+ Ischaemia blocks action potential propagation through myelinated axons 1 . Electron microscopy 2 and imaging of dye-filled oligodendrocytes 3 show ischaemia-evoked Ca 2+ -dependent damage to the capacitancereducing myelin sheaths, which causes loss of action potential propagation. Glutamate receptor block reduces myelin damage and action potential loss 2-7 , and glutamate evokes a membrane current in oligodendrocytes mediated by AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)/kainate and NMDA receptors 2-4 . Thus, oligodendrocyte damage is thought to be excitotoxic: as for neurons in ischaemia, a rise of glutamate concentration 8 caused by reversal of glutamate transporters in oligodendrocytes and axons 9,10 activates receptors that raise 2 oligodendrocyte [Ca 2+ ] i , thus damaging the cells.However, although AMPA/kainate and NMDA receptors regulate oligodendrocyte precursor development 11,12 , these receptors are downregulated as the cells mature [13][14][15] . How can mature oligodendrocytes be damaged excitotoxically, if they express low levels of glutamate receptors? To investigate how oligodendrocyte [Ca 2+ ] i is raised in ischaemia, we characterized ischaemia-evoked membrane current and [Ca 2+ ] i changes in cerebellar white matter oligodendrocytes.Solution mimicking ischaemia (see Methods) evoked an increasing inward current in oligodendrocytes (Fig. 1a, b), often with a faster phase that was obscured when responses in many cells were averaged (Fig. 1c). When applied from before the ischaemia, NBQX and D-AP5 reduced the ischaemia-evoked current by 66% (Fig. 1c, d), while mGluR block had no effect (Extended Data Fig. 1a). Preloading for 30 min with the glutamate transport blocker PDC, to prevent ischaemia-evoked glutamate release by reversal of transporters in the white 9 and grey 16 matter, also reduced the inward current (by 68%, Fig. 1c, d), while blocking other candidate release mechanisms had no effect (Extended Data Fig. 1a). Thus, glutamate release by reversed uptake helps to trigger the ischaemiaevoked current. Notably, however, current flow through glutamate receptors generates only a small fraction of the sustained inward current evoked by ischaemia, since applying NBQX and D-AP5 from 200 s after ischaemia had started produced only a non-significant 21% suppression of the ischaemia-evoked inward current (Fig. 1d).In neurons, an ischaemia-evoked inward current triggered by glutamate release, but maintained by non-glutamatergic mechanisms, generates the 'extended neuronal depolarization' (END) that evokes neuronal death 17 . However, the ischaemia-evoked current in oligodendrocytes was not prevented by removing external Ca 2+ , nor by gadolinium, which both block the END 17 (Fig. 1d, e), implying a different mechanism maintai...

Myelin, made by oligodendrocytes, is essential for rapid information transfer in the central nervous system. Oligodendrocyte precursors (OPs) receive glutamatergic synaptic input from axons but how this affects their development is unclear. Murine OPs in white matter express AMPA receptor (AMPAR) subunits GluA2, GluA3 and GluA4. We generated mice in which OPs lack both GluA2 and GluA3, or all three subunits GluA2/3/4, which respectively reduced or abolished AMPAR-mediated input to OPs. In both double- and triple-knockouts OP proliferation and number were unchanged but ~25% fewer oligodendrocytes survived in the subcortical white matter during development. In triple knockouts, this shortfall persisted into adulthood. The oligodendrocyte deficit resulted in ~20% fewer myelin sheaths but the average length, number and thickness of myelin internodes made by individual oligodendrocytes appeared normal. Thus, AMPAR-mediated signalling from active axons stimulates myelin production in developing white matter by enhancing oligodendrocyte survival, without influencing myelin synthesis per se.DOI: http://dx.doi.org/10.7554/eLife.28080.001

SummaryNew myelin-forming oligodendrocytes (OLs) are generated in the mouse central nervous system during adulthood. These adult-born OLs might augment the existing population, contributing to neural plasticity, or else replace OLs that die in use (turnover). To distinguish between these alternatives, we induced genetic labeling of mature myelinating OLs in young adult mice and tracked their subsequent survival. OL survival rates were region dependent, being higher in corpus callosum (∼90% survival over 20 months) and motor cortex (∼70% survival) than in corticospinal tract or optic nerve (50%–60% survival). Survival rates over the first 8 months were 90%–100% in all regions except the optic nerve. In the corpus callosum, new OLs accumulate during young adulthood and are therefore likely to participate in adaptive myelination. We also found that the number of myelin internodes maintained by individual cortical OLs is stable for at least 8 months but declines ∼12% in the following year.

Adjusting the thickness and internodal length of the myelin sheath is a mechanism for tuning the conduction velocity of axons to match computational needs. Interactions between oligodendrocyte precursor cells (OPCs) and developing axons regulate the formation of myelin around axons. We now show, using organotypic cerebral cortex slices from mice expressing eGFP in Sox10‐positive oligodendrocytes, that endogenously released GABA, acting on GABAA receptors, greatly reduces the number of oligodendrocyte lineage cells. The decrease in oligodendrocyte number correlates with a reduction in the amount of myelination but also an increase in internode length, a parameter previously thought to be set by the axon diameter or to be a property intrinsic to oligodendrocytes. Importantly, while TTX block of neuronal activity had no effect on oligodendrocyte lineage cell number when applied alone, it was able to completely abolish the effect of blocking GABAA receptors, suggesting that control of myelination by endogenous GABA may require a permissive factor to be released from axons. In contrast, block of AMPA/KA receptors had no effect on oligodendrocyte lineage cell number or myelination. These results imply that, during development, GABA can act as a local environmental cue to control myelination and thus influence the conduction velocity of action potentials within the CNS. GLIA 2017;65:309–321