Brain arachidonic acid metabolism in primary demyelination and remyelination

National Institute On Aging

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Abstract

Changes in arachidonic acid (AA) metabolism have been reported in multiple sclerosis (MS). We investigated whether the expression of genes involved in AA metabolism was altered in mice fed with 0.2% cuprizone, a model of progressive, but reversible, demyelination. Histochemical analysis demonstrated substantial demyelination of the corpus callosum between week four and six of cuprizone treatment. However, a decrease in the mRNA expression of myelin and oligodendrocyte markers was already detected at week one, and was accompanied by increased mRNA expression of glial cells markers. Coincident with these early changes, was an increase in the expression of cyclooxygenase (COX)-2 and 15-lipoxygenase (15-LO), suggesting that these genes are either involved in or respond to the earliest sign of demyelination. Expression of cytosolic and secretory phospholipases A2 (cPLA2 and sPLA2), 5-LO, 15-LO and (COX)-2 was robustly increased at the height of frank demyelination. A consistent increase in COX-1 protein expression was also observed. COX-1 and 15-LO immunostaining colocalized with microglia/macrophage, while COX-2 and 5-LO were expressed by astrocytes. 15-LO and COX-1 immunostaining colocalized with astrocytes only during the demyelination phase. COX-2, 15-LO and 5-LO gene expression was also upregulated during remyelination, and 12-LO gene expression was selectively upregulated only in the remyelinating phase, suggesting a role for these isoforms in remyelination and repair. Supporting this concept, 5-LO and 12-LO were found to be expressed by oligodendrocytes precursors (OPCs) and by mature oligodendrocytes. Our study is the first to demonstrate that multiple enzymes involved in AA metabolism are dynamically altered during cuprizone-induced demyelination and subsequent remyelination. COX-2 and 15-LO, which were upregulated before the onset of demyelination represent potential early biomarkers of the disease. To test whether the increase in 5-LO expression plays a causative role in the demyelination process, we administered MK886 (a 5-LO inhibitor) to cuprizone-exposed mice. We demonstrated that although MK886 did not attenuate cuprizone-induced demyelination in the corpus callosum and cortex, it reduced microglial activation, IL-6 production and motor dysfunction. These data suggest that the 5-LO pathway plays a key role in microglial activation and neuroinflammation independently of the demyelination process. We also demonstrated that 5-LO inhibition improved motor dysfunction during cuprizone-induced demyelination. These results suggest that pharmacological inhibition of 5-LO is a valuable anti-inflammatory treatment for brain inflammation and could provide symptomatic amelioration of MS symptoms.