Normal-appearing white matter in multiple sclerosis is in a subtle balance between inflammation and neuroprotection

Abstract

Multiple sclerosis is a chronic inflammatory disease of the CNS. Although progressive axonal injury and diffuse inflammatory damage has been shown in the chronic phase of the disease, little is known about the molecular mechanisms underlying these pathological processes. In order to identify these mechanisms, we have studied the gene expression profile in non-lesion containing tissue, the so-called normal-appearing white matter (NAWM). We performed differential gene expression analysis and quantitative RT-PCR on subcortical white matter from 11 multiple sclerosis and 8 control cases. Differentially expressed genes were further analysed in detail by in situ hybridization and immunofluorescence studies. We show that genes known to be involved in anti-inflammatory and protective mechanisms such as STAT6, JAK1, IL-4R, IL-10, Chromogranin C and Hif-1alpha are consistently upregulated in the multiple sclerosis NAWM. On the other hand, genes involved in pro-inflammatory mechanisms, such as STAT4, IL-1beta and MCSF, were also upregulated but less regularly. Immunofluorescence colocalization analysis revealed expression of STAT6, JAK1, IL-4R and IL-13R mainly in oligodendrocytes, whereas STAT4 expression was detected predominantly in microglia. In line with these data, in situ hybridization analysis showed an increased expression in multiple sclerosis NAWM of HIF-1alpha in oligodendrocytes and HLA-DRalpha in microglia cells. The consistency of the expression levels of STAT6, JAK1, JAK3 and IL-4R between the multiple sclerosis cases suggests an overall activation of the STAT6-signalling pathway in oligodendrocytes, whereas the expression of STAT4 and HLA-DRalpha indicates the activation of pro-inflammatory pathways in microglia. The upregulation of genes involved in anti-inflammatory mechanisms driven by oligodendrocytes may protect the CNS environment and thus limit lesion formation, whereas the activation of pro-inflammatory mechanisms in microglia may favour disease progression. Altogether, our data suggests an endogenous inflammatory reaction throughout the whole white matter of multiple sclerosis brain, in which oligodendrocytes actively participate. This reaction might further influence and to some extent facilitate lesion formation.

Gene expression profiling of the astrocyte transcriptome in multiple sclerosis normal appearing white matter reveals a neuroprotective role

“…Our analysis also revealed differential expression of genes associated with receptor mediated signalling pathways in NAWM, including MAPK and calcium signalling, confirming previous reports (Graumann et al, 2003, Zeis et al, 2008. The current study identified significant upregulation of MAPKAPK2 and MAPK4, both of which are linked to ischaemic preconditioning and oxidative stress in MS (Graumann et al, 2003).…”

“…Several papers have discussed the complexity of astrocytes in MS, with emphasis on their dual function: on the one hand astrocytes are neurotoxic, through a deleterious immune response, increasing BBB permeability and inhibiting remyelination; whilst on the other these glial cells are neuroprotective, supporting oligodendrocyte remyelination and axonal regeneration (Williams et al, 2007, Nair et al, 2008, Brosnan and Raine, 2013. To date five studies have performed RNA profiling of the NAWM in MS compared to control WM, one recent study characterising the microRNA profile (Guerau-de-Arellano et al, 2015) and four studies have reported mRNA expression changes (Graumann et al, 2003, Lindberg et al, 2004, Zeis et al, 2008, Mycko et al, 2012. All of these studies were carried out using RNA from whole tissue extracts, which thus comprised a heterogeneous cell population within the NAWM.…”

“…Yet despite this, to date only four microarray studies have investigated the gene expression profile of NAWM in MS (Graumann et al, 2003, Lindberg et al, 2004, Zeis et al, 2008, Mycko et al, 2012 with transcriptomic changes identified in whole tissue, constituting a heterogeneous cell population.…”

Use of ultrasmall superparamagnetic particles of iron oxide (USPIO)‐enhanced MRI to demonstrate diffuse inflammation in the normal‐appearing white matter (NAWM) of multiple sclerosis (MS) patients: An exploratory study

“…This inflammatory aspect of NAWM damage was also shown in gene expression studies (20), showing upregulation of proinflammatory cytokines produced by activated microglia and infiltrating monocytes. Unfortunately, in vivo MRI has so far lacked the specificity to demonstrate the cellular aspect of inflammation within the NAWM in MS.…”

Cerebral Arterial Bolus Arrival Time is Prolonged in Multiple Sclerosis and Associated with Disability

“…1 While inflammation within the lesions is the most visible aspect of the neuropathology of MS, it is increasingly recognized that a more widespread and subtle form of inflammation also occurs within the normal-appearing white and gray matter, characterized pathologically by infiltration of lymphocytes, perivascular inflammation, and clusters of activated microglia. 1,2 These widespread inflammatory changes may have clinical relevance, as they correlate with markers of neuroaxonal damage and loss, 3,4 which in turn is associated with the accrual of fixed disability. 1,3 Ongoing inflammatory processes in normal-appearing white and gray matter have been challenging to study using magnetic resonance imaging (MRI), as they are not associated with a frank breach of the blood-brain barrier that can be detected as a region of gadolinium contrast enhancement.…”