Diversity and Function of Glial Cell Types in Multiple Sclerosis

Abstract

Glial subtype diversity is an emerging topic in neurobiology and immune-mediated neurological diseases such as multiple sclerosis (MS). We discuss recent conceptual and technological advances that allow a better understanding of the transcriptomic and functional heterogeneity of oligodendrocytes (OLs), astrocytes, and microglial cells under inflammatory-demyelinating conditions. Recent single cell transcriptomic studies suggest the occurrence of novel homeostatic and reactive glial subtypes and provide insight into the molecular events during disease progression. Multiplexed RNA in situ hybridization has enabled 'mapping back' dysregulated gene expression to glial subtypes within the MS lesion microenvironment. These findings suggest novel homeostatic and reactive glial-cell-type functions both in immune-related processes and neuroprotection relevant to understanding the pathology of MS.

Keywords: astrocytes; microglia; multiple sclerosis; neuroinflammation; oligodendrocytes; transcriptomics.

Figure 1.. Tools to Perform Glial Cell Type-Specific Transcriptomic Profiling.

Upper panel illustrates high-throughput single-cell/nucleus RNA-sequencing (sc/snRNA-seq) discovery platform starting from glial cells/nuclei isolated from human multiple sclerosis (MS) or mouse experimental autoimmune encephalomyelitis (EAE) brain tissues with the presence of focal inflammatory–demyelinating lesions. Lower panel depicts bioinformatics analysis platform to analyze sc/snRNA-seq data featuring glial subtype clustering, differential (e.g., MS/EAE reactive versus healthy homeostatic subtypes) and dynamic (pseudotime/progressive) trajectory gene expression analysis followed by multiplex in situ RNA hybridization to visualize cell-type-specific transcriptomic changes in the inflamed lesion microenvironment. This figure was created using BioRender (https://biorender.com/).

Figure 2. Key Figure. Homeostatic and Reactive Glial Cell Type Diversity in Inflammatory Demyelination

Main cartoon highlights dynamic transformative states of glial cell types (left center) in response to a focal inflammatory–demyelinating lesion as regularly seen in multiple sclerosis (MS) with blood–brain barrier breakdown and immune cell infiltration into the surrounding parenchyma (right). Note a gradual morphological transformation of glial subtypes from a homeostatic (left) physiological to a reactive pathological state (center) relative to the inflamed lesion area. Under inflammatory–demyelinating conditions, astrocytes, and myeloid cells such as microglia regularly transform into reactive subtypes with enlarged cell bodies and retracted processes. Conversely, oligodendrocyte (OL) lineage cells including OL precursor cells (OPCs) and myelinating cells (OLs) become reactive and exhibit high metabolic stress. Note that most of them undergo cell death close to the demyelinating lesion rim. Lower panel illustrates increased subtype diversity in reactive glial cell populations isolated from inflamed MS brains compared with homeostatic subtype diversity in healthy brains by means of hierarchical clustering with respect to physiological versus pathological subtype functions. This figure was created using BioRender (https://biorender.com/).

Figure 3.. Human Transcriptomic Marker Genes Characterizing Homeostatic and Reactive Glial Subtypes.

Cartoon lists homeostatic (left) versus reactive (right) glial subtype signature genes by means of volcano plot visualization. Marker genes were selected by their specific enrichment in microglia, oligodendrocyte (OL) precursor, and myelinating cells, as well as astrocytes based on recent single-cell/nucleus RNA-sequencing (sc/snRNA-seq) studies from human control and multiple sclerosis (MS) tissues. See Boxes 2-4 in the main text for details about signature genes and their functional relevance. Note color spectrum on the bottom illustrates dynamic changes in gene expression between homeostatic and reactive glial subtypes according to cell type color code. This figure was created using BioRender (https://biorender.com/).

Figure 4.. Morphological and Functional Changes in Homeostatic and Reactive Glial Subtypes Relative to Neuron Damage in Multiple Sclerosis (MS) Lesions.

Main cartoon illustrates the sequence of tissue damage in leukocortical (affecting both cortical gray matter and subcortical white matter) MS lesion with presence of infiltrating immune cell types. Note gradual neuronal degeneration from left (healthy state) to right (degenerated state) with axonal degeneration and synapse loss followed by retrograde injury and damage to neuronal cell bodies. Neuron degeneration is paralleled by morphological and functional changes in glial cell types including complement secretion and activation, antigen presentation via MHC class I and II, cytokine production, and complement factor release. Other factors are meningeal inflammation and tissue infiltration of monocyte and lymphocyte subtypes producing antibodies and cytokines, thus further corroborating tissue damage in MS lesion areas. Lower panel demonstrates dynamic cycling of glial subtype states between homeostatic and reactive conditions relative to the inflammatory stage of MS lesions. Note that different glial cells can transform from healthy/physiological subtypes into disease/effector subtypes. This figure was created using BioRender (https://biorender.com/). Abbreviation: OL, oligodendrocyte.