Microglia-mediated Demyelination as a Protective Mechanism in CNS Neurodegeneration

A recent study by Groh et al., published in Nature Communications, explores the relationship between adaptive immunity and neurodegeneration in the central nervous system (CNS). Their findings suggest a previously unknown inverse relationship between demyelination and axonal degeneration, where microglia-mediated demyelination may actually protect axons from cytotoxic T-cell-dependent degradation. By using genetically modified mice with defects in proteolipid protein (PLP), the primary protein in myelin sheaths, the researchers identified mechanisms that contribute to axonal degeneration, including cytoskeletal changes in myelinating oligodendrocytes and paranodal constrictions, which may explain the formation of axonal spheroids—a hallmark of neurodegeneration.

PLP-Deficient Mice as a Model for Axonal Degeneration

The study compared two genetically modified mouse models with altered PLP expression: one overexpression model (PLPgt) and one with a point mutation causing defective PLP (PLPmut). Both models exhibited axonal degeneration, particularly the formation of axonal spheroids and heightened cytotoxic T-cell activity in surrounding white matter. However, they presented distinct clinical manifestations: PLPmut mice showed significant motor coordination deficits, while PLPgt mice did not. Similarly, when assessing visual acuity using Striatech’s OptoDrum system, PLPmut mice, but not PLPgt mice, exhibited a notable decline. This was accompanied by a pronounced loss of retinal ganglion cells and thinning of the inner retinal layer. Electron microscopy further revealed severe axonal loss in the optic nerve of PLPmut mice, reinforcing the notion that these mice experience more pronounced neuroaxonal degeneration.

Axonal spheroids formed at the nodes of Ranvier in both PLPmut and PLPgt mice, but with key differences. PLPgt mice exhibited smaller spheroids that were more frequently demyelinated, particularly as the disease progressed. Surprisingly, despite stronger demyelination, PLPgt mice exhibited less axonal loss in the optic nerve compared to PLPmut mice. This raised a critical question: why does stronger demyelination correlate with reduced axonal degeneration?

Microglial and Oligodendrocyte Interactions Drive Demyelination

To understand the role of microglia in PLP-mediated disease progression, the researchers performed single-cell RNA sequencing. They identified populations of activated microglia (AMG) in both mouse models, characterized by upregulated cellular stress and pro-inflammatory signaling genes. However, PLPgt mice exhibited an additional microglial subtype, AMG2, which displayed strong phagocytic properties. This suggested that in PLPgt mice, microglia were actively removing myelin, potentially explaining the increased demyelination. But how does this connect to cytotoxic T-cell activity?

Microglia-Mediated Demyelination Protects Axons from Cytotoxic T-Cell Damage

To determine whether microglial demyelination protects axons from degeneration, the researchers conducted two key experiments. First, they enhanced microglial activity in PLPmut mice using a cuprizone-based toxin, which led to increased demyelination. Additionally, this also resulted in less axonal loss and a greater presence of phagocytic microglia (AMG2). Second, they depleted microglia in PLPgt mice using PLX5622, which reduced demyelination but caused greater axonal damage. These findings suggest that microglia play a protective role by demyelinating axons, thereby reducing targets for cytotoxic T-cell attacks.

Paranodal Constriction as a Source of Axonal Damage

Previous research suggested that juxtaparanodal domains—regions adjacent to the nodes of Ranvier—are hotspots for axonal spheroid formation, often accumulating cellular debris. This study uncovered a new mechanism: paranodal constriction correlates with the severity of axonal spheroids. In PLPmut mice, greater paranodal constriction was directly linked to axonal swelling and debris accumulation, possibly disrupting axonal transport and organelle distribution.

To confirm that paranodal constriction was driven by adaptive immunity, the researchers inhibited adaptive immune responses, resulting in reduced constriction and axonal degeneration. This suggests that cytotoxic T-cell attacks on myelinated fibers induce paranodal narrowing, impairing axonal function.

Cytoskeletal Remodeling in Oligodendrocytes upon Immune Activation

Transcriptomic analysis of oligodendrocytes revealed upregulation of genes associated with cell repair and cytoskeletal dynamics. Further investigation showed increased levels of filamentous actin (F-actin) and phosphorylated myosin light chain (pMLC) at paranodal myelination sites, which were enhanced in an immune-dependent manner. Stimulating isolated oligodendrocyte progenitors with cytotoxic molecules similarly increased F-actin and pMLC levels, indicating that paranodal constriction results from cytoskeletal remodeling and actomyosin contractility within oligodendrocyte processes.

This mechanism is reminiscent of Schwann cells in the peripheral nervous system, which constrict axons upon damage to facilitate fragmentation and regeneration.

Axonal Spheroids Disrupt Axonal Transport

Finally, the researchers assessed whether axonal spheroids impair axonal transport, as suggested in previous studies, and how that relates to oligodendrocyte cytoskeletal activity. They found that PLPmut mice exhibited reduced axonal transport rates, particularly in neurons with larger spheroids. This impairment was linked to cytoskeletal alterations. Treating these mice with Fasudil, a Rho-associated kinase (ROCK) inhibitor, reduced axonal spheroid formation without affecting cytotoxic T-cell activity in white matter. This suggests that Fasudil blocks T-cell-induced actomyosin contraction at paranodal regions, thereby preserving axonal transport.

Implications for Neurodegeneration and Therapeutic Approaches

This study establishes an inverse correlation between neurodegeneration and demyelination, where microglia actively protect axons by removing dysfunctional myelin before cytotoxic T-cells can target it. The researchers propose that oligodendrocytes respond to T-cell-mediated damage by inducing cytoskeletal changes that constrict paranodal regions, ultimately impairing axonal transport. This mechanism may normally serve as a protective strategy, akin to Schwann cell-mediated axonal remodeling in the peripheral nervous system.

Importantly, the study identifies promising therapeutic targets for neurodegenerative diseases. Fasudil, by preventing actomyosin contraction, offers a potential strategy to mitigate axonal degeneration. Further research into microglial modulation and cytoskeletal interventions could open new avenues for neuroprotection in diseases such as multiple sclerosis and other demyelinating disorders.

Groh et al’s findings challenge the conventional view of demyelination as purely detrimental and suggest a more nuanced role of microglia in CNS neurodegeneration. By shedding light on the interplay between adaptive immunity, oligodendrocytes, and microglia, this study paves the way for novel therapeutic strategies that could slow or prevent axonal damage in neurodegenerative diseases.

Blog author: Emilia Kawecka, Technical University of Munich, Student Assistant at Striatech