Myelin-forming oligodendrocytes (OLs) are formed continuously in the healthy adult brain. In this work, we study the function of these late-forming cells and the myelin they produce. Learning a new motor skill (such as juggling) alters the structure of the brain's white matter, which contains many OLs, suggesting that late-born OLs might contribute to motor learning. Consistent with this idea, we show that production of newly formed OLs is briefly accelerated in mice that learn a new skill (running on a "complex wheel" with irregularly spaced rungs). By genetically manipulating the transcription factor myelin regulatory factor in OL precursors, we blocked production of new OLs during adulthood without affecting preexisting OLs or myelin. This prevented the mice from mastering the complex wheel. Thus, generation of new OLs and myelin is important for learning motor skills.

SummaryOligodendrocyte precursors (OPs) continue to proliferate and generate myelinating oligodendrocytes (OLs) well into adulthood. It is not known whether adult-born OLs ensheath previously unmyelinated axons or remodel existing myelin. We quantified OP division and OL production in different regions of the adult mouse CNS including the 4-month-old optic nerve, in which practically all axons are already myelinated. Even there, all OPs were dividing and generating new OLs and myelin at a rate higher than can be explained by first-time myelination of naked axons. We conclude that adult-born OLs in the optic nerve are engaged in myelin remodeling, either replacing OLs that die in service or intercalating among existing myelin sheaths. The latter would predict that average internode length should decrease with age. Consistent with that, we found that adult-born OLs elaborated much shorter but many more internodes than OLs generated during early postnatal life.

We carried out an investigation to identify neuromelanin-containing noradrenergic and dopaminergic neurons in the locus ceruleus and substantia nigra pars compacta of healthy volunteers and patients with Parkinson's disease using a newly developed magnetic resonance imaging technique that can demonstrate neuromelanin-related contrast. The high-resolution neuromelanin images obtained by a 3-T scanner revealed high signal areas in the brain stem and these corresponded well with the location of the locus ceruleus and substantia nigra pars compacta in gross specimens. In Parkinson's disease patients, the signal intensity in the locus ceruleus and substantia nigra pars compacta was greatly reduced, suggesting depletion of neuromelanin-containing neurons. We conclude that neuromelanin magnetic resonance imaging can be used for direct visualization of the locus ceruleus and substantia nigra pars compacta, and may help in detecting pathological changes in Parkinson's disease and related disorders.

Myelination speeds conduction of the nerve impulse, enhancing cognitive power. Changes of white matter structure contribute to learning, and are often assumed to reflect an altered number of myelin wraps. We now show that, in rat optic nerve and cerebral cortical axons, the node of Ranvier length varies over a 4.4-fold and 8.7-fold range respectively and that variation of the node length is much less along axons than between axons. Modelling predicts that these node length differences will alter conduction speed by ~20%, similar to the changes produced by altering the number of myelin wraps or the internode length. For a given change of conduction speed, the membrane area change needed at the node is >270-fold less than that needed in the myelin sheath. Thus, axon-specific adjustment of node of Ranvier length is potentially an energy-efficient and rapid mechanism for tuning the arrival time of information in the CNS.DOI: http://dx.doi.org/10.7554/eLife.23329.001

Microglia may contribute to cell death in neurodegenerative diseases. We studied the activation of microglia in affected regions of Huntington disease (HD) brain by localizing thymosin beta-4 (Tbeta4), which is increased in reactive microglia. Activated microglia appeared in the neostriatum, cortex, and globus pallidus and the adjoining white matter of the HD brain, but not in control brain. In the striatum and cortex, reactive microglia occurred in all grades of pathology, accumulated with increasing grade, and grew in density in relation to degree of neuronal loss. The predominant morphology of activated microglia differed in the striatum and cortex. Processes of reactive microglia were conspicuous in low-grade HD, suggesting an early microglia response to changes in neuropil and axons and in the grade 2 and grade 3 cortex, were aligned with the apical dendrites of pyramidal neurons. Some reactive microglia contacted pyramidal neurons with huntingtin-positive nuclear inclusions. The early and proximate association of activated microglia with degenerating neurons in the HD brain implicates a role for activated microglia in HD pathogenesis.

Voltage-gated Naϩ (Na v ) channels are highly concentrated at nodes of Ranvier in myelinated axons and facilitate rapid action potential conduction. Autoantibodies to gangliosides such as GM1 have been proposed to disrupt nodal Nav channels and lead to Guillain-Barré syndrome, an autoimmune neuropathy characterized by acute limb weakness. To test this hypothesis, we examined the molecular organization of nodes in a disease model caused by immunization with gangliosides. At the acute phase with progressing limb weakness, Na v channel clusters were disrupted or disappeared at abnormally lengthened nodes concomitant with deposition of IgG and complement products. Paranodal axoglial junctions, the nodal cytoskeleton, and Schwann cell microvilli, all of which stabilize Na v channel clusters, were also disrupted. The nodal molecules disappeared in lesions with complement deposition but no localization of macrophages. During recovery, complement deposition at nodes decreased, and Na v channels redistributed on both sides of affected nodes. These results suggest that Na v channel alterations occur as a consequence of complement-mediated disruption of interactions between axons and Schwann cells. Our findings support the idea that acute motor axonal neuropathy is a disease that specifically disrupts the nodes of Ranvier.

Electrophilic neurite outgrowth-promoting prostaglandin (NEPP) compounds protect neurons from oxidative insults. At least part of the neuroprotective action of NEPPs lies in induction of hemeoxygenase-1 (HO-1), which, along with other phase II enzymes, serve as a defense system against oxidative stress. Here, we found that, by using fluorescent tags and immunoprecipitation assays, NEPPs are taken up preferentially into neurons and bind in a thiol-dependent manner to Keap1, a negative regulator of the transcription factor Nrf2. By binding to Keap1, NEPPs prevent Keap1-mediated inactivation of Nrf2 and, thus, enhance Nrf2 translocation into the nucleus of cultured neuronal cells. In turn, Nrf2 binds to antioxidant͞ electrophile-responsive elements of the HO-1 promoter to induce HO-1 expression. Consistent with this notion, NEPP induction of an HO-1 reporter construct is prevented if the antioxidant-responsive elements are mutated. We show that NEPPs are neuroprotective both in vitro from glutamate-related excitotoxicity and in vivo in a model of cerebral ischemia͞reperfusion injury (stroke). Our results suggest that NEPPs prevent excitotoxicity by activating the Keap1͞ Nrf2͞HO-1 pathway. Because NEPPs accumulate preferentially in neurons, they may provide a category of neuroprotective compounds, distinct from other electrophilic compounds such as tertbutylhydroquinone, which activates the antioxidant-responsive element in astrocytes. NEPPs thus represent a therapeutic approach for stroke and neurodegenerative disorders.hemeoxygenase-1 ͉ middle cerebral artery occlusion ͉ neurite outgrowth-promoting prostaglandin ͉ stroke ͉ neurodegenerative diseases

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