Can Exploring Oligodendroglial Dynamics at the Single-Cell Level Further Our Understanding of Multiple Sclerosis?

Researchers led by Gonçalo Castelo-Branco (Karolinska Institutet) previously applied independent single-cell transposase-accessible chromatin using sequencing (scATAC-seq) and scRNA-sequencing to oligodendroglia isolated from a mouse model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE) (Meijer et al.). This approach allowed them to identify a cohort of immune-associated genes with elevated chromatin accessibility that exhibited upregulated expression under a single timepoint (designated as the “peak” disease condition) (Meijer et al.).

While many previous studies had concentrated on the impact of T and B immune cells (van Langelaar et al.) as the drivers of this chronic inflammatory autoimmune disease of the central nervous system (Olek, 2021), the Castelo-Branco lab instead focused on oligodendroglia due to studies that had highlighted the importance of immunomodulation by distinct types of heterogeneous oligodendroglial cells in multiple sclerosis (Falcão et al. and Kirby et al.), Alzheimer’s disease (Morabito et al. and Pandey et al.), and aging (Kaya et al.). Of note, the main function of oligodendroglia involves the provision of the myelin sheath around neuronal axons in the central nervous system, and multiple sclerosis targets myelinating mature oligodendrocytes (MOLs) and their myelin.

Following this previous publication, the authors set out to explore disease-associated oligodendroglial dynamics by investigating the epigenomic and transcriptional profiles of oligodendrocyte precursor cells (OPCs) and MOLs using single-cell ATAC-seq and RNA-seq on cells isolated from EAE mice from early stages (associated with disease onset), the peak disease stage analyzed in previous studies, and later, chronic disease stages. Their new study, published in Nature Neuroscience, provides a single-cell multiomic resource for basic research and highlights dynamic, subtype-specific responses of oligodendroglia in a mouse model of multiple sclerosis, which may help to provide therapeutic targets to boost immune modulation and myelin regeneration (Zheng et al.).

This study employed single-cell ATAC-seq and RNA-seq analyses, but could the simultaneous analysis of histone modification patterns and transcriptomics in single cells have provided further insight into the control of dynamic disease-associated gene expression programs? Paired-Tag from Epigenome Technologies, which generates joint epigenetic and transcriptomic profiles at single-cell resolution and detects histone modifications and RNA transcripts in individual nuclei with efficiency comparable to single-nucleus RNA-seq/ChIP-seq assays, could represent an efficient means of exploring this exciting concept.

Single-Cell Epigenetic and Transcriptomic Analyses Reveal Novel Disease-associated Dynamics in Oligodendroglia

Following the induction of EAE in male and female mice, the authors of this new study collected spinal cord tissues at early, peak, and late/chronic stages of disease development and evaluated single-cell epigenetic and transcriptional profiles in oligodendroglia. Interestingly, this initial analysis revealed that OPCs and MOLs transitioned to an immune-like state characterized by elevated chromatin accessibility at expressed immune-associated gene loci during early-stage disease (before lesions reach a fully developed state; Kukanja et al.). More importantly, they observed the persistence of these immune-gene-associated epigenetic and transcriptional alterations into late-stage disease, despite the resolution of inflammation. The authors suggest that infiltrating immune cells, additional disease-associated glia, and inflammatory environmental factors all contribute to the induction of these disease-associated immune-like oligodendroglia. They also hypothesized that the persistence of elevated chromatin accessibility at immune-associated gene loci in oligodendroglia into late-stage disease (in the presence of decreased inflammation) suggested the existence of an epigenetic memory of the peak immune-like state. As such, subsequent in vitro analyses using cultured neonatal OPCs (as MOLs undergo cell death after prolonged culture) provided further evidence for an epigenetic memory of prior neuroinflammation at the level of chromatin accessibility, which may place oligodendroglia in a state prone to transcriptionally reactivating an immunological profile.

Overall, these findings agree with previous analyses that provided evidence for epigenetic memory in disease-associated astrocytes following neuroinflammation (Lee et al.) and adipocytes in response to obesity (associated with inflammatory signaling) (Hinte et al.). Significantly, multiple sclerosis patients suffer multiple remissions and relapses, and the worsening of symptoms after each relapse (Zimmet et al.); therefore, the epigenetic memory of immune-related gene expression in oligodendroglia could heighten immune-associated gene expression and, as such, any response during the relapse stage in affected patients and contribute to disease chronicity and the therapeutic difficulties frequently encountered.

The second half of this fascinating paper evaluated oligodendroglial heterogeneity based on distinct gene expression profiles; fascinatingly, the authors found that the white matter-enriched MOL2 subpopulation (white matter: where immune infiltrates and lesions display the most significant prominence in EAE; (Guerra-Cantera et al. and Schindelin et al.) exhibited stronger immune responses to the neuroinflammatory environment (compared to the grey-matter-enriched MOL5/MOL6 subpopulation) and played a more significant role in EAE progression. Importantly, they linked these findings to increased chromatin accessibility at immune-associated gene loci, while chromatin accessibility data also suggested the involvement of the MOL5/MOL6 subpopulation in nervous system repair and remyelination and promotion during late disease stages. Overall, these data provide evidence that subpopulation-specific therapeutic strategies targeting MOLs may be required for effective disease management.

Paired-Tag: Extending Our Epigenetic Understanding of Single-Cell Oligodendroglial Dynamics in Multiple Sclerosis?

Overall, this exciting single-cell epigenetic and transcriptomic study helped to define the dynamic role of oligodendroglia in an animal model of multiple sclerosis and reveal the distinct roles of MOL subpopulations during disease progression. Of note, the team behind this research suggest additional means to improve our burgeoning understanding in this arena, which include the integration of spatial analyses, the use of intermediate disease-associated time points, an expanded analysis of the basis of epigenetic memory, an exploration of MOL-associated remyelination in multiple sclerosis, and a deeper analysis of the single-cell epigenetic landscape at a single-cell level by examining histone modifications and DNA methylation to elucidate mechanisms that activate gene programs and promote remyelination.

Could the Paired-Tag technology from Epigenome Technologies help this latter ambitious aim? Our technology generates joint epigenetic and transcriptomic profiles at single-cell resolution and detects histone modifications and RNA transcripts in individual nuclei with an efficiency comparable to that of single-nucleus RNA-seq/ChIP-seq assays.