CNS White Matter Research Articles

Relationship between glial potassium regulation and axon excitability: a role for glial Kir4.1 channels.

Uptake of K(+) released by axons during action potential propagation is a major function of astrocytes. Here, we demonstrate the importance of glial inward rectifying potassium channels (Kir) in regulating extracellular K(+) ([K(+)](o)) and axonal electrical activity in CNS white matter of the mouse optic nerve. Increasing optic nerve stimulation frequency from 1 Hz to 10-35 Hz for 120 s resulted in a rise in [K(+)](o) and consequent decay in the compound action potential (CAP), a measure of reduced axonal activity. On cessation of high frequency stimulation, rapid K(+) clearance resulted in a poststimulus [K(+)](o) undershoot, followed by a slow recovery of [K(+)](o) and the CAP, which were more protracted with increasing stimulation frequency. Blockade of Kir (100 μM BaCl(2)) slowed poststimulus recovery of [K(+)](o) and the CAP at all stimulation frequencies, indicating a primary function of glial Kir was redistributing K(+) to the extracellular space to offset active removal by Na(+)-K(+) pumps. At higher levels of axonal activity, Kir blockade also increased [K(+)](o) accumulation, exacerbating the decline in the CAP and impeding its subsequent recovery. In the Kir4.1-/- mouse, astrocytes displayed a marked reduction of inward currents and were severely depolarized, resulting in retarded [K(+)](o) regulation and reduced CAP. The results demonstrate the importance of glial Kir in K(+) spatial buffering and sustaining axonal activity in the optic nerve. Glial Kir have increasing importance in K(+) clearance at higher levels of axonal activity, helping to maintain the physiological [K(+)](o) ceiling and ensure the fidelity of signaling between the retina and brain.

The Energetics of CNS White Matter

The energetics of CNS white matter are poorly understood. We derive a signaling energy budget for the white matter (based on data from the rodent optic nerve and corpus callosum) which can be compared with previous energy budgets for the gray matter regions of the brain, perform a cost-benefit analysis of the energetics of myelination, and assess mechanisms for energy production and glucose supply in myelinated axons. We show that white matter synapses consume ≤0.5% of the energy of gray matter synapses and that this, rather than more energy-efficient action potentials, is the main reason why CNS white matter uses less energy than gray matter. Surprisingly, while the energetic cost of building myelin could be repaid within months by the reduced ATP cost of neuronal action potentials, the energetic cost of maintaining the oligodendrocyte resting potential usually outweighs the saving on action potentials. Thus, although it dramatically speeds action potential propagation, myelination need not save energy. Finally, we show that mitochondria in optic nerve axons could sustain measured firing rates with a plausible density of glucose transporters in the nodal membrane, without the need for energy transfer from oligodendrocytes.

De- and remyelination in the CNS white and grey matter induced by cuprizone: the old, the new, and the unexpected.

The copper chelator cuprizone (bis-cyclohexanone oxaldihydrazone) was established as a neurotoxin in rodents in 1966 by Carlton. During the following years the usefulness of cuprizone feeding in mice to induce oligodendrocyte death with secondary demyelination of the superior cerebellar peduncles was described by Blakemore. In 1998 the cuprizone model experienced a renaissance as the group of Matsushima described the effects of cuprizone on the white matter of the cerebrum and focussed on demyelination in the corpus callosum, where the extent of demyelination could be scored more easily and consistently. Since then the toxic cuprizone model has been widely used to study experimental de- and remyelination in the corpus callosum. Recently, we and others have extended these studies and have shown several new aspects characteristic for this model. Many lessons can be learned from these recent findings that have implications for the basic understanding of remyelination and the design of remyelinating and neuroprotective strategies in demyelinating diseases of the CNS. Although the model is often mentioned in the context of multiple sclerosis, it must always be kept in mind that this model has a fundamentally different induction of demyelination. We highlight the important findings delineated from this model and critically discuss both the advantages and the shortcomings of cuprizone induced demyelination.

Dual-specific Phosphatase-6 (Dusp6) and ERK Mediate AMPA Receptor-induced Oligodendrocyte Death

Oligodendrocytes, the myelinating cells of the CNS, are highly vulnerable to glutamate excitotoxicity, a mechanism involved in tissue damage in multiple sclerosis. Thus, understanding oligodendrocyte death at the molecular level is important to develop new therapeutic approaches to treat the disease. Here, using microarray analysis and quantitative PCR, we observed that dual-specific phosphatase-6 (Dusp6), an extracellular regulated kinase-specific phosphatase, is up-regulated in oligodendrocyte cultures as well as in optic nerves after AMPA receptor activation. In turn, Dusp6 is overexpressed in optic nerves from multiple sclerosis patients before the appearance of evident damage in this structure. We further analyzed the role of Dusp6 and ERK signaling in excitotoxic oligodendrocyte death and observed that AMPA receptor activation induces a rapid increase in ERK1/2 phosphorylation. Blocking Dusp6 expression, which enhances ERK1/2 phosphorylation, significantly diminished AMPA receptor-induced oligodendrocyte death. In contrast, MAPK/ERK pathway inhibition with UO126 significantly potentiates excitotoxic oligodendrocyte death and increases cytochrome c release, mitochondrial depolarization, and mitochondrial calcium overload produced by AMPA receptor stimulation. Upstream analysis demonstrated that MAPK/ERK signaling alters AMPA receptor properties. Indeed, Dusp6 overexpression as well as incubation with UO126 produced an increase in AMPA receptor-induced inward currents and cytosolic calcium overload. Together, these data suggest that levels of phosphorylated ERK, controlled by Dusp6 phosphatase, regulate glutamate receptor permeability and oligodendroglial excitotoxicity. Therefore, targeting Dusp6 may be a useful strategy to prevent oligodendrocyte death in multiple sclerosis and other diseases involving CNS white matter.

Progressive multifocal leukoencephalopathy (PML)

Progressive multifocal leukoencephalopathy (PML) is caused by reactivation of latently infected JCV when hosts' immune system is impaired by HIV infection, hematologic diseases, collagen diseases, immunemodulatory therapy and so on. PML was rare but HIV infection and Natalizumab have made it much more common while the prognosis is much better than other PML. PML patients present with various signs and symptoms including hemiparesis, dementia, aphasia, visual disturbance, cranial nerve paresis, cerebellar signs and bladder bowel disturbance. Brain MRI reveals characteristic demyelinating lesions in the CNS white matter and CSF mild increase of protein with or without mild mononuclear pleocytosis. Detection of JCV genome from CSF is crucial for the clinical diagnosis of PML. PML was once thought to be fatal but some HIV infected PML patients showed halting progression or even recovery after introduction of HAART. In addition, anti-malarial drug mefloquine was found to be effective. Recovery of immunity may provoke some inflammatory responses known as immune reconstruction inflammatory syndrome (IRIS) which requires high dose corticosteroid. In Japan, we are providing free test of CSF-JCV genome and organized a unique system for surveillance and clinical research of PML. Using this system we hope to improve diagnosis and therapy of PML in Japan.

Visualization of cytoplasmic diffusion within living myelin sheaths of CNS white matter axons using microinjection of the fluorescent dye Lucifer Yellow

The compactness of myelin allows for efficient insulation defining rapid propagation of action potentials, but also raises questions about how cytoplasmic access to its membranes is achieved, which is critical for physiological activity. Understanding the organization of cytoplasmic (‘water’) spaces of myelin is also important for diffusion MRI studies of CNS white matter. Using longitudinal slices of mature rat spinal cord, we monitored the diffusion of the water-soluble fluorescent dye Lucifer Yellow injected into individual oligodendrocytes or internodal myelin. We show that living myelin sheaths on CNS axons are fenestrated by a network of diffusionally interconnected cytoplasmic ‘pockets’ (1.9 ± 0.2 pockets per 10 μm sheath length, n = 58) that included Schmidt–Lanterman clefts (SLCs) and numerous smaller compartments. 3-D reconstructions of these cytoplasmic networks show that the outer cytoplasmic layer of CNS myelin is cylindrically ‘encuffing’, which differs from EM studies using fixed tissue. SLCs were found in different ‘open states’ and remained stable within a 1–2 hour observation period. Unlike the peripheral nervous system, where similarly small (< 500 Da) molecules diffuse along the whole myelin segment within a few minutes, in mature CNS this takes more than one hour. The slower cytoplasmic diffusion in CNS myelin possibly contributes to its known vulnerability to injury and limited capacity for repair. Our findings point to an elaborate cytoplasmic access to compact CNS myelin. These results could be of relevance to MRI studies of CNS white matter and to CNS repair/regeneration strategies.

Cortical lesions in multiple sclerosis

Multiple sclerosis (MS) is typically considered to be a chronic inflammatory-demyelinating disease of CNS white matter. In the past decade, however, pathological and MRI studies have shown that lesions are often located in the gray matter, especially in the cerebral cortex. The histopathological characteristics of these cortical lesions differ substantially from lesions located in the white matter, which suggests location-dependent expression of the MS immunopathological process. Double inversion recovery imaging--an MRI technique that selectively images gray matter and lesions--has enabled researchers to image cortical lesions in vivo. Double inversion recovery studies have shown that cortical lesions can be detected at the earliest clinical stages of MS, and cortical lesion burden positively correlates with the severity of physical and cognitive impairments. These gray matter lesions are also independent predictors of subsequent disease evolution. This Review provides a summary of the main histopathological and MRI findings with regard to cortical lesions in MS, and indicates that increasing our understanding of cortical lesions has increased our knowledge of MS pathobiology.

Lugaro's Forgotten Legacy: The Hypothesis of Negative Neurotropism

Among the most fascinating riddles in neuroscience is the one concerning the poor regeneration capacity of the adult mammalian CNS. A central ætiological hypothesis for the lack of axonal regeneration in the CNS concerns the inhibitory role of myelin components of the white matter. The main exponent of this hypothesis in the 1980s and 1990s has been Martin Schwab, although before him Martin Berry and others had suggested an inhibitory role for CNS myelin. However, a hypothesis for a negative neurotropic action exerted by CNS white matter was formulated by Lugaro already in 1906. This article is a belated tribute to Lugaro's original contribution in the field of neuroregeneration.

MICROSTRUCTURAL WHITE MATTER CHANGES IN METABOLIC SYNDROME: A DIFFUSION TENSOR IMAGING STUDY

Segura et al.1 presented an elegant study on CNS white matter changes in metabolic syndrome. The syndrome is defined by inappropriate control of blood glucose so its metabolic consequences can be compared to diabetes. Diabetes is associated with peripheral polyneuropathy with loss of principal myelin protein species in Wallerian degeneration.2 This may be due to …

Laminin Alters Fyn Regulatory Mechanisms and Promotes Oligodendrocyte Development

Mutations in LAMA2, the gene for the extracellular matrix protein laminin-alpha2, cause a severe muscular dystrophy termed congenital muscular dystrophy type-1A (MDC1A). MDC1A patients have accompanying CNS neural dysplasias and white matter abnormalities for which the underlying mechanisms remain unknown. Here, we report that in laminin-deficient mice, oligodendrocyte development was delayed such that oligodendrocyte progenitors accumulated inappropriately in adult brains. Conversely, laminin substrates were found to promote the transition of oligodendrocyte progenitors to newly formed oligodendrocytes. Laminin-enhanced differentiation was Src family kinase-dependent and resulted in the activation of the Src family kinase Fyn. In laminin-deficient brains, however, increased Fyn repression was accompanied by elevated levels of the Src family kinase negative regulatory proteins, Csk (C-terminal Src kinase), and its transmembrane adaptor, Cbp (Csk-binding protein). These findings indicate that laminin deficiencies delay oligodendrocyte maturation by causing dysregulation of signaling pathways critical for oligodendrocyte development, and suggest that a normal role for CNS laminin is to promote the development of oligodendrocyte progenitors into myelin-forming oligodendrocytes via modulation of Fyn regulatory molecules.

Age-Dependent B Cell Autoimmunity to a Myelin Surface Antigen in Pediatric Multiple Sclerosis

Multiple sclerosis (MS) typically manifests in early to mid adulthood, but there is increasing recognition of pediatric-onset MS, aided by improvements in imaging techniques. The immunological mechanisms of disease are largely unexplored in pediatric-onset MS, in part because studies have historically focused on adult-onset disease. We investigated autoantibodies to myelin surface Ags in a large cohort of pediatric MS cases by flow cytometric labeling of transfectants that expressed different myelin proteins. Although Abs to native myelin oligodendrocyte glycoprotein (MOG) were uncommon among adult-onset patients, a subset of pediatric patients had serum Abs that brightly labeled the MOG transfectant. Abs to two other myelin surface Ags were largely absent. Affinity purification of MOG Abs as well as competition of binding with soluble MOG documented their binding specificity. Such affinity purified Abs labeled myelin and glial cells in human CNS white matter as well as myelinated axons in gray matter. The prevalence of such autoantibodies was highest among patients with a very early onset of MS: 38.7% of patients less than 10 years of age at disease onset had MOG Abs, compared with 14.7% of patients in the 10- to 18-year age group. B cell autoimmunity to this myelin surface Ag is therefore most common in patients with a very early onset of MS.

Association of genetic variants of methionine metabolism with methotrexate-induced CNS white matter changes in patients with primary CNS lymphoma

Methotrexate (MTX) is an important anticancer drug and the most efficient chemotherapy component in primary CNS lymphoma (PCNSL). A typical side effect of intravenous high-dose MTX is the occurrence of confluent CNS white matter changes (WMC). Because MTX directly interferes with methionine metabolism, we analyzed the impact of genetic variants of methionine metabolism on the occurrence of WMC as a model of MTX toxicity. In a retrospective analysis of 68 PCNSL patients treated with MTX-based polychemotherapy with (n = 42) or without (n = 26) intraventricular treatment, 10 genetic variants influencing methionine metabolism were analyzed. Pearson's chi(2) test and multinominal regression analysis were used to define the relevance of these genetic variants for the occurrence of WMC. In this patient sample, the occurrence of WMC was significantly predicted by the TT genotype of methylenetetrahydrofolate reductase c.677C>T (chi(2) = 8.67; p = 0.013; df = 2), the AA genotype of methylenetetrahydrofolate reductase c.1298A>C (chi(2) = 13.5; p = 0.001; df = 2), and the GG genotype of transcobalamin 2 c.776C>G (chi(2) = 19.73; p < 0.001), in addition to male gender (chi(2) = 11.95; p = 0.001). These data strengthen the hypothesis that MTX effects are influenced by methionine metabolism, which may offer new strategies to improve MTX-based therapies.

Magnesium concentration in brains from multiple sclerosis patients

Magnesium (Mg) concentrations were studied in the brains of 4 patients with definite multiple sclerosis (MS) and 5 controls. The magnesium contents were determined by inductively coupled plasma emission spectrometry in autopsy samples taken from 26 sites of central nervous system tissues, and visceral organs such as liver, spleen, kidney, heart and lung. The average Mg content in the CNS tissues, as well as visceral organs except for spleen, of MS patients showed a significantly lower value than that seen in control cases. The most marked reduction of Mg content was observed in CNS white matter including demyelinated plaques of MS samples. Whether or not these significantly lower Mg contents found in CNS and visceral organs of MS patients may play an essential role in the demyelinating process remain unclear, requiring further studies on MS pathogenesis from the point of metal metabolism.

Acute CNS white matter lesions in patients with inflammatory bowel disease

Neurological manifestations in patients with inflammatory bowel disease supposedly are rare, although the exact frequency is not known. Most previous reports involve cerebral venous thrombosis, central nervous system vasculitis, or peripheral nerve inflammation. Two cases of patients diagnosed with inflammatory bowel disease developing neurological symptoms with corresponding lesions in the white matter of the central nervous system led us to search a neurological database with clinical and radiological data for similar cases. We identified five patients who presented with acute neurological deficits preceding or following a diagnosis of inflammatory bowel disease with evidence of lesions in the central nervous system white matter on magnetic resonance imaging. Ancillary investigations did not provide evidence of systemic infetcion, coagulation disorders, or vasculitis. These cases, together with previous reports, suggest that white matter lesions may be another extraintestinal manifestation of inflammatory bowel disease.

Reflections

CNS white matter from higher vertebrates and cultured differentiated oligodendrocytes are nonpermissive substrates for neurite growth and fibroblast spreading. Membrane proteins of 35 kd and 250 kd with highly nonpermissive substrate properties could be extracted from CNS myelin fractions. Monoclonal antibodies were raised against these proteins: IN-1 and IN-2 bound both to the 35 kd and 250 kd inhibitors and to the surface to differentiated cultured oligodendrocytes. Adsorption of nonpermissive CNS myelin or nonpermissive oligodendrocytes with either antibody markedly improved their substrate properties. Optic nerve explants injected with IN-1 or IN-2 allowed axon ingrowth of cocultured sensory and sympathetic neurons. We conclude that the nonpermissive substrate properties of CNS white matter are due to these membrane proteins on the surface of differentiated oligodendrocytes and to their in vivo product, myelin.

Mevalonate kinase deficiency (MKD): long-term follow-up of clinical and biological features in 40 patients

MKD causes hyper-IgD syndrome and mevalonic aciduria. Disease severity and clinical phenotype has not yet been linked to specific mutations. In this study, we intended to review clinical presentation and outcome of pediatric and adult patients. 40 patients were identified through MVK mutation database in one central reference laboratory and physicians' networking. Medical charts were reviewed (21/40) or questionnaires were filled in by treating physician (19/40). Follow-up ranged from 2 years to 60 years. First symptoms occurred before age 2 in 90% of patients. Main clinical features were fever (100%), abdominal pain (92%), diarrhea (88%), enlarged lymph nodes (88%), skin rash (86%), arthralgias (85%), arthritis (43%), growth delay (33%), CNS white matter involvement (7,5%) and macrophage activation syndrome (5%). We recorded 3 disease-related deaths. IgD level ranged from normal (<100 IU/ml) to 2614 IU/ml and were persistently normal in 6 patients. IgA level was above upper limit in all but 2 patients. Mevalonate kinase activity was deeply impaired (0,7–7% of normal controls). The most prevalent mutation was V377I. In 3 independent kindreds, only one mutated allele was found in the coding sequence of the gene. Most patients received NSAIDs, steroids and colchicin. Etanercept and anakinra at high doses showed some efficacy. Some patients were refractory to all treatments. One patient succesfully received allogenic stem cell transplantation. MKD is a challenging condition, with a wide array of severity, potentially severe complications and a high burden on quality of life.

High-affinity choline uptake and acetylcholine-metabolizing enzymes in CNS white matter. A quantitative study

The presence of nicotinic and muscarinic receptors suggests the occurrence of cholinergic neurotransmission in white matter; however no quantitative information exists on acetylcholine formation and breakdown in white matter. We compared white structures of pig brain (fimbria, corpus callosum, pyramidal tracts, and occipital white matter) to gray structures (temporal, parietal and cerebellar cortices, hippocampus, and caudate) and found that sodium-dependent, high-affinity choline uptake in white structures was 25–31% of that in hippocampus. White matter choline acetyltransferase activity was 10–50% of the hippocampal value; the highest activity was found in fimbria. Acetylcholine esterase activity in white structures was 20–25% of that in hippocampus. The caudate, which is rich in cholinergic interneurons, gave values for all three parameters that were 2.8–4 times higher than in hippocampus. The results suggest a certain capacity for cholinergic neurotransmission in central nervous white matter. The white matter activity of pyruvate dehydrogenase, which provides acetyl-CoA for acetylcholine synthesis, ranged between 33 and 50% of the hippocampal activity; the activity in the caudate was similar to that in hippocampus and the other gray structures, which was true also for other enzymes of glucose metabolism: hexokinase, phosphoglucomutase, and glucose-6-phosphate dehydrogenase. Acetylcholine esterase activity in white matter was inhibited by the nerve agent soman, which may help explain the reported deleterious effect of soman on white matter. Further, this finding suggests that acetylcholine esterase inhibitors used in Alzheimer's disease may have an effect in white matter.

Clonally expanded plasma cells in the cerebrospinal fluid of MS patients produce myelin‐specific antibodies

Clonally expanded plasma cells (cePC) and their presumed products, oligoclonal immunoglobulin G bands (OCB), are characteristic findings in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS). While cePC and OCB strongly suggest an involvement of B cell-dependent immune mechanisms in the pathogenesis of MS, their actual pathological relevance and target antigens remain unknown. To further understand the potential role played by cePC, we generated a panel of monoclonal antibodies (MS-mAb) from CSF-derived cePC from four patients with early or definite MS. Single-cell RT-PCR of correctly paired heavy and light chain immunoglobulin genes from individual cePC ensured the subsequent resurrection of their original antigen specificity. Immunofluorescence stainings of MS lesion tissue with MS-mAb revealed myelin reactivity in the cePC repertoire of all four patients and intracellular filament reactivity in one patient. While myelin staining by MS-mAb was only rarely detectable in non-MS CNS white matter tissue, it was greatly enhanced at the edge of demyelinating lesions in MS brain tissue. Our findings provide conclusive evidence for the presence of an antigen-driven B cell response in the CSF of MS patients directed against epitopes present in areas of myelin degradation.

Functional polymorphic variants of methionine metabolism influence neurotoxicity and survival in primary central nervous system lymphoma (PCNSL) treated with high-dose methotrexate

8579 Background: Methotrexate (MTX) is the most efficient chemotherapeutic drug in primary central nervous system lymphoma (PCNSL); it directly interferes with methionine metabolism. Patients and Methods: Eight functional polymorphic variants influencing methionine metabolism were investigated in 65 PCNSL patients and were analyzed for their impact on the occurrence of confluent CNS white matter changes (WMC) (Pearson’s Chi-square test and multinominal regression analysis) and for their influence on survival (univariate Log Rank and multivariate COX regression analyses). Results: Twenty/65 (31%) patients developed WMC, which was significantly predicted by the genotype of methylenetetrahydrofolate reductase c.1298A>C (p.E429A; patients with WMC, AA/AC/CC: 0.80/0.10/0.10; patients without WMC: 0.40/0.47/0.13; nominal logistic regression: χ2=11.54; p=0.003). In addition, the transcobalamin 2 variant c.776C>G (p.P259R) influenced the occurrence of WMC (patients with WMC: 0.20/0.45/0.35; patients without WMC: ...

What’s So Exciting About Glia?

Káradóttir et al . present intriguing data suggesting that, contrary to long-accepted dogma, a class of glial cells can generate action potentials. Neurons and glia constitute the two classes of cell found in neural tissue, with inability to produce action potentials a defining feature that distinguishes glia from neurons. Káradóttir et al . identified oligodendrocyte precursor glial cells (OPCs) in the white matter of cerebellar slices from young rats on the basis of their morphology and expression of the proteoglycan NG2 and the oligodendrocyte transcription factor Olig2. These NG2- and Olig2-expressing cells did not express the neuronal marker NeuN, nor did they express the astrocyte marker S100β; about half were immunoreactive for Na + channels. Whole-cell patch clamp analysis revealed two electrophysiologically distinct classes of OPCs: One class exhibited currents mediated by voltage-gated Na + and K + channels and generated action potentials in response to depolarization, whereas the other (morphologically indistinguishable) class did not. The two classes of OPCs were also present in adult animals. Most of the excitable OPCs exhibited excitatory postsynaptic currents (EPSCs), inhibitory postsynaptic currents (IPSCs), or both, whereas the vast majority of the inexcitable cells did not. Pharmacological analysis indicated that EPSCs reflected glutamatergic input and that IPSCs were a response to GABA. Glutamate and the glutamate antagonist NMDA elicited larger currents in excitable OPCs than in OPCs that lacked Na + channels, and the former were also more susceptible to ischemic death (pertinent to conditions such as stroke or spinal cord injury). Otis and Sofroniew provide thoughtful commentary and discuss the possibility that some NG2-expressing glia may have functions other than acting as OPCs. R. Káradóttir, N. B. Hamilton, Y. Bakiri, D. Attwell, Spiking and nonspiking classes of oligodendrocyte precursor glia in CNS white matter. Nat. Neurosci . 11 , 450-456 (2008). [PubMed] T. S. Otis, M.V. Sofroniew, Glia get excited. Nat. Neurosci. 11 , 379-380 (2008). [PubMed]