Abstract

Oligodendrocytes are glial cells that mediate myelination of neurons, a process that allows efficient electrical impulse transmission in the central nervous system (CNS). An autoimmune response against myelin triggers demyelination in multiple sclerosis (MS). Oligodendrocyte precursor cells (OPCs) can initially differentiate and promote remyelination in MS, but this process eventually fails in progressive MS. In order to clearly define transcriptional of oligodendrocyte lineage cells in multiple sclerosis, we performed single-cell RNA-Seq in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS and in different kind of CNS lesions from MS patients. We identified disease-specific of oligodendrocytes and OPC populations in EAE, and altered heterogeneity of the oligodendrocyte lineage in MS patients. One of the populations expressed genes involved in antigen processing and presentation and immunoprotection, and presented immunomodulatory properties. We also performed single-cell ATAC-Seq in oligodendroglia from the EAE mouse model of MS and observed transitions in chromatin accessibility correlating with the single-cell transcriptomics data. Thus, our single cell transcriptomics and epigenomics analysis unveiled a transcriptional overhaul during chronic inflammatory demyelination in multiple sclerosis.

Abstract

Astrocytes fulfill multiple homeostatic and regulatory functions in healthy CNS. In MS, reactive astrocytes can enhance the pathological process by impairing blood brain barrier function, recruiting and activating lymphocytes, decreasing homeostatic functions and by adopting a neurotoxic phenotype.

We have shown that a genetic risk variant for MS susceptibility dysregulates astrocyte functions, leading to increased astrocyte-mediated neurotoxicity and more lymphocyte infiltration in MS lesion tissue. This suggests that genetic risk for MS is mediated not only by peripheral immune cells but in part also by astrocytes.

In addition, we have shown that MS progression is associated with increased adenosine A_2A receptor expression in perilesional white matter, as measured by PET imaging. Moreover, we demonstrated that A_2AR is expressed by two astrocyte phenotypes in MS lesion, characterized by pan-activation and by high oxidative damage. Finally, in human astrocyte cultures, A_2AR signaling enhances oxidative damage in reactive human astrocytes. These findings indicate that A_2AR signaling in specific astrocyte populations may drive oxidative damage which contributes to MS progression.

In addition to genetic risk variants, astrocyte function has been shown to be modified by environmental toxins and gut bacterial metabolites. Astrocyte-intrinsic pathways that are affected by modifiers may provide targets for treatment of relapsing-remitting and progressive MS.

Abstract

Neuron damage and cortical pathology are critical determinants of irreversible clinical progression in MS patients. Cutting across different anatomical areas, damage and loss of neurons are key features in both acute and chronic MS lesions. Recent cell-type specific transcriptomic studies showed a selective change of gene expression in specific neuron subtypes supporting the idea of a gradual, yet cell-type specific level of vulnerability and resilience towards the MS tissue microenvironment. For example, a selective vulnerability and loss of excitatory projection neurons in upper cortical layers underlying meningeal inflammation has been demonstrated. Such MS neuron populations showed a substantial upregulation of pathway genes related to cell stress, protein accumulation, energy metabolism and oxidative stress, while genes linked to energy consumption, glutamate signaling, ion homeostasis and axon guidance were downregulated. Hence, understanding neuron subtype-specific pathologies and their precise spatial positioning in MS lesion and non-lesion areas will be key to develop novel cell-type specific therapies and biomarkers for progressive MS.

Background

The brain’s endogenous capacity to restore damaged myelin deteriorates during the course of demyelinating disorders including multiple sclerosis (MS). Currently, none of the available treatment options for MS directly establishes remyelination. We have previously shown that cytokines of the IL-6 family are upregulated in post mortem MS lesions and have regenerative potential in animal models of MS.

Objectives

Here we investigated how oncostatin M (OSM), a member of the IL-6 family of cytokines, contributes to the remyelination process using animal model systems and aimed at identifying the underlying mode of action related to that.

Methods

Loss and gain of function experiments were performed using either knock out (KO) strains or CNS directed stereotactic application of lentiviral vectors (LV) that overexpress the transgene. The acute and chronic cuprizone induced demyelination model was used as an in vivo to investigate de- and remyelination. Primary oligodendrocyte and astrocyte cultures were used for in vitro experiments. Genes and proteins were identified by qPCR and/or western blotting. Pathological analyses were based on immunohistochemistry and electron microscopy.

Results

OSM and its downstream mediator tissue inhibitor of metalloproteinases-1 (TIMP-1) were identified as two promising therapeutic targets. While remyelination was completely abrogated in OSM receptor (OSMRβ) KO mice, LV mediated OSM overexpression in the chronically demyelinated CNS established remyelination. Astrocytic TIMP-1 was demonstrated to play a pivotal role in OSM-mediated remyelination. Astrocyte-derived TIMP-1 drove differentiation of oligodendrocyte precursor cells into mature oligodendrocytes in vitro. In vivo, TIMP-1 deficiency completely abolished spontaneous remyelination, phenocopying OSMRβ KO mice. Finally, TIMP-1 was expressed by human astrocytes in demyelinated multiple sclerosis lesions, confirming the human value of our findings.

Conclusions

Taken together, OSM and its downstream mediator TIMP-1 have the therapeutic potential to boost remyelination in demyelinating disorders.

Background

Remyelination is observed in the white and grey matter of patients with multiple sclerosis (MS). However, whether remyelinating oligodendrocytes arise from newly formed oligodendrocyte progenitor cells (OPCs) or from preexisting OPCs generated during development is not known.

Objectives

The aim of the study is to investigate the origin of remyelinating oligodendrocytes in MS. Therefore, we intensively studied oligodendrocytes, oligodendrocyte-proliferation and differentiation in MS and demyelinating models of MS.

Methods

We performed immunohistochemistry in white and grey matter lesions of patients with early MS obtained at biopsy and late cortical lesions of patients with progressive MS obtained at autopsy. In comparison, we analyzed experimental models of MS, targeting the cortex or the white matter.

Results

In experimental demyelination as well as MS we found very little proliferation of oligodendrocytes and only single mature oligodendrocytes with BrdU incorporation indicating that newly formed oligodendrocytes did not participate in remyelination. Actively remyelinating, breast carcinoma amplified sequence 1 (BCAS1)-positive oligodendrocytes were to identify active remyelination and find that BCAS1 positive myelinating oligodendrocytes were generated immediately after experimental lesion induction and in early active MS lesions. In MS, the total number of oligodendrocytes in the cerebral cortex of early active MS lesions remained unchanged, but decreased with disease duration. This was associated with increased numbers of TPPP/p25 mature oligodendrocytes, suggesting oligodendrocyte differentiation. Furthermore, we identified that a fraction of BCAS1 positive oligodendrocytes in the cerebral cortex are perineuronal satellite cells. These cells obtain a myelinating morphology and express MAG after toxic demyelination and in early MS indicating that these cells participate in remyelination. In progressive MS and at late remyelination time-points in animal models we observed increased numbers of TPPP/p25 positive satellite oligodendrocytes.

Conclusions

Our data show that remyelination is predominantly performed by preexisting oligodendrocytes and is most efficient immediately after demyelination. Furthermore, we were able to demonstrate for the first time that BCAS1 positive perineuronal satellite cells in the cerebral cortex serve as a pool to generate remyelinating oligodendrocytes after experimental demyelination and in MS.