Evaluation strategy to determine reliable demyelination in the cuprizone model

Sex Chromosomes and Gonads Shape the Sex-Biased Transcriptomic Landscape in Tlr7-Mediated Demyelination During Aging

Demyelination occurs in aging and associated diseases, including Alzheimer's disease. Several of these diseases exhibit sex differences in prevalence and severity. Biological sex primarily stems from sex chromosomes and gonads releasing sex hormones. To dissect mechanisms underlying sex differences in demyelination of aging brains, we constructed a transcriptomic atlas of cell type-specific responses to illustrate how sex chromosomes, gonads, and their interaction shape responses to demyelination. We found that sex-biased oligodendrocyte and microglial responses are driven by interaction of sex chromosomes and gonads prior to myelin loss. Post demyelination, sex chromosomes mainly guide microglial responses, while gonadal composition influences oligodendrocyte signaling. Significantly, ablation of the X-linked gene Toll-like receptor 7 (Tlr7), which exhibited sex-biased expression during demyelination, abolished the sex-biased responses and protected against demyelination.

Animal models to investigate the effects of inflammation on remyelination in multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating, and neurodegenerative disease of the central nervous system (CNS). In people with MS, impaired remyelination and axonal loss lead to debilitating long-term neurologic deficits. Current MS disease-modifying drugs mainly target peripheral immune cells and have demonstrated little efficacy for neuroprotection or promoting repair. To elucidate the pathological mechanisms and test therapeutic interventions, multiple animal models have been developed to recapitulate specific aspects of MS pathology, particularly the acute inflammatory stage. However, there are few animal models that facilitate the study of remyelination in the presence of inflammation, and none fully replicate the biology of chronic demyelination in MS. In this review, we describe the animal models that have provided insight into the mechanisms underlying demyelination, myelin repair, and potential therapeutic targets for remyelination. We highlight the limitations of studying remyelination in toxin-based demyelination models and discuss the combinatorial models that recapitulate the inflammatory microenvironment, which is now recognized to be a major inhibitor of remyelination mechanisms. These models may be useful in identifying novel therapeutics that promote CNS remyelination in inflammatory diseases such as MS.

Endogenous neural stem cells modulate microglia and protect against demyelination

In response to corpus callosum (CC) demyelination, subventricular zone-derived neural progenitors (SVZdNPs) are mobilized and generate new myelinating oligodendrocytes (OLG). Here, we examine the putative immunomodulatory properties of endogenous SVZdNPs during demyelination in the cuprizone model. SVZdNP density was higher in the lateral and rostral CC regions, and demyelination was inversely correlated with activated microglial density and pro-inflammatory cytokine levels. Single-cell RNA sequencing showed that CC areas with high levels of SVZdNP mobilization were enriched in a microglial cell subpopulation with an immunomodulatory signature. We propose MFGE8 (milk fat globule-epidermal growth factor-8) and β3 integrin as a ligand/receptor pair involved in dialogue between SVZdNPs and microglia. Immature SVZdNPs mobilized to the demyelinated CC were found highly enriched in MFGE8, which promoted the phagocytosis of myelin debris in vitro. Overall, these results demonstrate that, in addition to their cell replacement capacity, endogenous progenitors have immunomodulatory properties, highlighting a new role for endogenous SVZdNPs in myelin regeneration.

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Demyelination is limited in corpus callosum areas rich in subventricular zone–derived progenitors

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In these areas microglial cells adopt an immunomodulatory phenotype

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Mobilized SVZ progenitors secrete MFGE8, which promotes myelin debris phagocytosis

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SVZ-derived progenitors minimize demyelination by modulating microglial activity

Focal white matter lesions induce long-lasting axonal degeneration, neuroinflammation and behavioral deficits

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) with episodes of inflammatory demyelination and remyelination. While remyelination has been linked with functional recovery in MS patients, there is evidence of ongoing tissue damage despite complete myelin repair. In this study, we investigated the long-term consequences of an acute demyelinating white matter CNS lesion. For this purpose, acute demyelination was induced by 5-week-cuprizone intoxication in male C57BL/6 J mice, and the tissues were examined after a 7-month recovery period. While myelination and oligodendrocyte densities appeared normal, ongoing axonal degeneration and glia cell activation were found in the remyelinated corpus callosum. Neuropathologies were paralleled by subtle gait abnormalities evaluated using DigiGait™ high speed ventral plane videography. Gene array analyses revealed increased expression levels of various inflammation related genes, among protein kinase c delta (PRKCD). Immunofluorescence stains revealed predominant microglia/macrophages PRKCD expression in both, cuprizone tissues and post-mortem MS lesions. These results support the hypothesis that chronic microglia/macrophages driven tissue injury represents a key aspect of progressive neurodegeneration and functional decline in MS.

CD44 expression in the cuprizone model

Numerous studies report that changes in extracellular matrix components and receptors, such as CD44, contribute to immune cell recruitment and thus lesion formation in multiple sclerosis (MS). In the present study, we used the cuprizone model to elucidate the expression pattern of CD44 in a toxin-induced MS model. Therefore, tissues of cuprizone-intoxicated mice were analyzed by real-time qRT-PCR and immunohistochemical staining against CD44. Co-localization analyses of CD44-positive cells with glial cell markers were performed by immunofluorescence labeling and in-situ hybridization. To investigate the functional importance of CD44 expression for myelination and glial cell activation, Cd44-deficient mice were used. In this study we demonstrate that CD44 expression is induced in a time-dependent manner in an autoimmune-independent model of MS. Up-regulation of CD44 expression was primarily associated to the superficial and perivascular glia limitans and demyelinated white matter structures, particularly the corpus callosum. In the demyelinated corpus callosum, CD44 was localized on GFAP⁺ astrocytes and IBA1⁺ microglial cells. Despite a robust expression induction, Cd44-deficiency did not ameliorate cuprizone-induced pathology. Although further studies will be needed to examine the functional relevance of CD44 in the cuprizone model, the spatial and temporal expression pattern of CD44 will pave the way to evaluate its precise role in different (immune and non-immune) pathological conditions.

The Cuprizone Model: Dos and Do Nots

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system. Various pre-clinical models with different specific features of the disease are available to study MS pathogenesis and to develop new therapeutic options. During the last decade, the model of toxic demyelination induced by cuprizone has become more and more popular, and it has contributed substantially to our understanding of distinct yet important aspects of the MS pathology. Here, we aim to provide a practical guide on how to use the cuprizone model and which pitfalls should be avoided.

Stereological Investigation of Regional Brain Volumes after Acute and Chronic Cuprizone-Induced Demyelination

Brain volume measurement is one of the most frequently used biomarkers to establish neuroprotective effects during pre-clinical multiple sclerosis (MS) studies. Furthermore, whole-brain atrophy estimates in MS correlate more robustly with clinical disability than traditional, lesion-based metrics. However, the underlying mechanisms leading to brain atrophy are poorly understood, partly due to the lack of appropriate animal models to study this aspect of the disease. The purpose of this study was to assess brain volumes and neuro-axonal degeneration after acute and chronic cuprizone-induced demyelination. C57BL/6 male mice were intoxicated with cuprizone for up to 12 weeks. Brain volume, as well as total numbers and densities of neurons, were determined using design-based stereology. After five weeks of cuprizone intoxication, despite severe demyelination, brain volumes were not altered at this time point. After 12 weeks of cuprizone intoxication, a significant volume reduction was found in the corpus callosum and diverse subcortical areas, particularly the internal capsule and the thalamus. Thalamic volume loss was accompanied by glucose hypermetabolism, analyzed by [¹⁸F]-fluoro-2-deoxy-d-glucose (18F-FDG) positron-emission tomography. This study demonstrates region-specific brain atrophy of different subcortical brain regions after chronic cuprizone-induced demyelination. The chronic cuprizone demyelination model in male mice is, thus, a useful tool to study the underlying mechanisms of subcortical brain atrophy and to investigate the effectiveness of therapeutic interventions.

Animal Weight Is an Important Variable for Reliable Cuprizone-Induced Demyelination

An elegant model to study mechanisms operant during oligodendrocyte degeneration and subsequent demyelination is the cuprizone model. In that model, mice are intoxicated with the copper chelation agent cuprizone which results in early oligodendrocyte stress, oligodendrocyte apoptosis, and, finally, demyelination. Here, we systematically investigated to what extent the animals' weight at the beginning of the cuprizone intoxication period is critical for the reproducibility of the cuprizone-induced pathology. We can demonstrate that a negative correlation exists between the two variables "extent of cuprizone-induced demyelination" and "starting weight." Demyelination and microglia activation were more severe in low weight compared to heavy weight mice. These findings are highly relevant for the experimental design using the cuprizone model.