A broad spectrum of diseases is characterized by myelin abnormalities and/or oligodendrocyte pathology. In most, if not all, of these diseases, early activation of microglia occurs. Our knowledge regarding the factors triggering early microglia activation is, however, incomplete. In this study, we used the cuprizone model to investigate the temporal and causal relationship of oligodendrocyte apoptosis and early microglia activation. Genome-wide gene expression studies revealed the induction of distinct chemokines, among them Cxcl10, Ccl2, and Ccl3 in cuprizone-mediated oligodendrocyte apoptosis. Early microglia activation was unchanged in CCL2- and CCL3-deficient knockouts, but was significantly reduced in CXCL10-deficient mice, resulting in an amelioration of cuprizone toxicity at later time points. Subsequent in vitro experiments revealed that recombinant CXCL10 induced migration and a proinflammatory phenotype in cultured microglia, without affecting their phagocytic activity or proliferation. In situ hybridization analyses suggest that Cxcl10 mRNA is mainly expressed by astrocytes, but also oligodendrocytes, in short-term cuprizone-exposed mice. Our results show that CXCL10 actively participates in the initiation of microglial activation. These findings have implications for the role of CXCL10 as an important mediator during the initiation of neuroinflammatory processes associated with oligodendrocyte pathology.

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Proton magnetic resonance spectroscopy (1H-MRS) is a quantitative MR imaging technique often used to complement conventional MR imaging with specific metabolic information. A key metabolite is the amino acid derivative N-Acetylaspartate (NAA) which is an accepted marker to measure the extent of neurodegeneration in multiple sclerosis (MS) patients. NAA is catabolized by the enzyme aspartoacylase (ASPA) which is predominantly expressed in oligodendrocytes. Since the formation of MS lesions is paralleled by oligodendrocyte loss, NAA might accumulate in the brain, and therefore, the extent of neurodegeneration might be underestimated. In the present study, we used the well-characterized cuprizone model. There, the loss of oligodendrocytes is paralleled by a reduction in ASPA expression and activity as demonstrated by genome-wide gene expression analysis and enzymatic activity assays. Notably, brain levels of NAA were not increased as determined by gas chromatography-mass spectrometry and 1H-MRS. These important findings underpin the reliability of NAA quantification as a valid marker for the paraclinical determination of the extent of neurodegeneration, even under conditions of oligodendrocyte loss in which impaired metabolization of NAA is expected. Future studies have to reveal whether other enzymes are able to metabolize NAA or whether an excess of NAA is cleared by other mechanisms rather than enzymatic metabolism.

Objective and designIt has been demonstrated that changes in the normal-appearing white matter (NAWM) in multiple sclerosis precede the appearance of classical lesions. The understanding of NAWM biology in an established disease model might help to clarify why some of them progress to active demyelinating lesions.Material or subjectsC57BL6 male mice (19–21 g) were used in this study.TreatmentDemyelination was induced by feeding mice a diet containing 0.2% cuprizone for up to 5 weeks.MethodsRoutine stainings (luxol fast blue, and hematoxylin and eosin) and immunohistochemistry were performed to assess myelin status and the inflammatory infiltrate.ResultsWe demonstrated that, in the toxic demyelination cuprizone model, the corpus callosum is severely demyelinated after a 5-week cuprizone challenge (acute demyelination) whereas the fimbria of the hippocampus appear normal in routine myelin stainings. Microgliosis but not astrogliosis is evident after acute demyelination in the fimbria. Interestingly, both regions, the fimbria and the corpus callosum, demonstrated early oligodendrocyte apoptosis as well as intense microglia accumulation and activation. However, only the corpus callosum progresses to actively demyelination lesions whereas the fimbria does not.ConclusionsThe applied model appears suitable for elucidating pathways which promote progression of affected tissue to an active lesion.