Distinct origins, gene expression and function of microglia and monocyte-derived macrophages in CNS myelin injury and regeneration

Monocyte-secreted Wnt reduces the efficiency of central nervous system remyelination

The regeneration of myelin in the central nervous system (CNS) reinstates nerve health and function, yet its decreased efficiency with aging and progression of neurodegenerative disease contributes to axonal loss and/or degeneration. Although CNS myeloid cells have been implicated in regulating the efficiency of remyelination, the distinct contribution of blood monocytes versus that of resident microglia is unclear. Here, we reveal that monocytes have non-redundant functions compared to microglia in regulating remyelination. Using a transgenic mouse in which classical monocytes have reduced egress from bone marrow (Ccr2-/-), we demonstrate that monocytes drive the timely onset of oligodendrocyte differentiation and myelin protein expression, yet impede myelin production. Ribonucleic acid sequencing revealed a Wnt signature in wild-type mouse lesion monocytes, which was confirmed in monocytes from multiple sclerosis white matter lesions and blood. Genetic or pharmacological inhibition of Wnt release by monocytes increased remyelination. Our findings reveal monocytes to be critical regulators of remyelination and identify monocytic Wnt signaling as a promising therapeutic target to inhibit for increased efficiency of CNS regeneration.

GPR37 and its neuroprotective mechanisms: bridging osteocalcin signaling and brain function

Osteocalcin (OCN) is a hormone secreted by osteoblasts and has attracted widespread attention for its role in regulating brain function. Clinical studies indicate a positive correlation between levels of circulating OCN and cognitive performance. Indeed, lower circulating OCN has been detected in various neurodegenerative diseases (NDs), while OCN supplementation under certain conditions may improve cognitive function. GPR37, a G protein-coupled receptor, has recently been identified as a receptor for OCN. It exhibits distinct expression patterns across various brain regions and cell types, potentially influencing its functional roles within the brain. Research indicates that GPR37 regulates neuronal migration, cell proliferation, differentiation, and myelination. Furthermore, GPR37 has been shown to mitigate inflammation and apoptosis through various mechanisms, exerting neuroprotective effects. However, its regulatory influence on brain function exhibits inconsistency, highlighting a duality in its actions. Therefore, this review thoroughly summarizes the roles and mechanisms of GPR37 in modulating cellular physiological activities and its involvement in immune responses, stress reactions, and neuroprotection. It aims to enhance the understanding of how GPR37 modulates brain function and facilitate the identification of novel therapeutic targets or strategies for related diseases.

Microglia in the context of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory and neurodegenerative disease that commonly results in nontraumatic disability in young adults. The characteristic pathological hallmark of MS is damage to myelin, oligodendrocytes, and axons. Microglia provide continuous surveillance in the CNS microenvironment and initiate defensive mechanisms to protect CNS tissue. Additionally, microglia participate in neurogenesis, synaptic refinement, and myelin pruning through the expression and release of different signaling factors. Continuous activation of microglia has been implicated in neurodegenerative disorders. We first review the lifetime of microglia, including the origin, differentiation, development, and function of microglia. We then discuss microglia participate in the whole processes of remyelination and demyelination, microglial phenotypes in MS, and the NF-κB/PI3K-AKT signaling pathway in microglia. The damage to regulatory signaling pathways may change the homeostasis of microglia, which would accelerate the progression of MS.

The Neuroimmunology of Multiple Sclerosis: Fictions and Facts

There have been tremendous advances in the neuroimmunology of multiple sclerosis over the past five decades, which have led to improved diagnosis and therapy in the clinic. However, further advances must take into account an understanding of some of the complex issues in the field, particularly an appreciation of "facts" and "fiction." Not surprisingly given the incredible complexity of both the nervous and immune systems, our understanding of the basic biology of the disease is very incomplete. This lack of understanding has led to many controversies in the field. This review identifies some of these controversies and facts/fictions with relation to the basic neuroimmunology of the disease (cells and molecules), and important clinical issues. Fortunately, the field is in a healthy transition from excessive reliance on animal models to a broader understanding of the disease in humans, which will likely lead to many improved treatments especially of the neurodegeneration in multiple sclerosis (MS).

Diversity of transcriptomic microglial phenotypes in aging and Alzheimer's disease

The morphological plasticity of microglia has fascinated neuroscientists for 100 years. Attempts to classify functional phenotypes are hampered by similarities between endogenous brain microglia and peripheral myeloid cells that can enter the brain under pathological conditions. Recent advances in single-cell -omic methodologies have led to an explosion of data regarding gene expression in microglia. Herein, we review the diversity of microglial phenotypes in healthy brains, aging, and Alzheimer's disease (AD); identify knowledge gaps in the body of evidence; and suggest areas in which new knowledge would be useful. Data from human samples and mouse models are compared and contrasted. Understanding the molecular complexity of the microglial response repertoire will suggest new avenues for therapeutic treatments in AD.

Monocytes in central nervous system remyelination

Remyelination failure with aging and progression of neurodegenerative disorders contributes to axonal dysfunction, highlighting the importance of understanding the mechanisms underpinning this process to develop regenerative therapies. Central nervous system (CNS) macrophages, encompassing both resident microglia and blood monocyte-derived cells, play a crucial role in driving successful remyelination. Although there has been a focus on the critical roles of microglia in remyelination, the specific contribution of monocyte-derived macrophages is still not fully understood. Until recently, the lack of tools enabling distinction between CNS macrophage populations has hindered our understanding of monocyte influence on remyelination. Recent advances have allowed for identification and characterization of monocyte populations in health, aging and in neurodegenerative conditions like multiple sclerosis, indicating heterogeneity of monocyte subsets impacted by both intrinsic and extrinsic factors. Here, we discuss the new tools enabling distinction between macrophage populations and advancements in understanding the importance of monocytes in remyelination, and reflect on the potential for therapeutic targeting of monocytes to promote remyelination.

Fascin-1 is Highly Expressed Specifically in Microglia After Spinal Cord Injury and Regulates Microglial Migration

Recent research indicates that after spinal cord injury (SCI), microglia accumulate at the borders of lesions between astrocytic and fibrotic scars and perform inflammation-limiting and neuroprotective functions, however, the mechanism of microglial migration remains unclear. Fascin-1 is a key actin-bundling protein that regulates cell migration, invasion and adhesion, but its role during SCI has not been reported. Here, we found that at 7–14 days after SCI in mice, Fascin-1 is significantly upregulated, mainly distributed around the lesion, and specifically expressed in CX3CR1-positive microglia. However, Fascin-1 is not expressed in GFAP-positive astrocytes, NeuN-positive neurons, NG2-positive cells, PDGFRβ-positive cells, or blood-derived Mac2-positive macrophages infiltrating into the lesion core. The expression of Fascin-1 is correspondingly decreased after microglia are specifically depleted in the injured spinal cord by the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. The upregulation of Fascin-1 expression is observed when microglia are activated by myelin debris in vitro, and microglial migration is prominently increased. The inhibition of Fascin-1 expression using small interfering RNA (siRNA) markedly suppresses the migration of microglia, but this effect can be reversed by treatment with myelin. The M1/M2-like polarization of microglia does not affect the expression of Fascin-1. Together, our results suggest that Fascin-1 is highly expressed specifically in microglia after SCI and can play an important role in the migration of microglia and the formation of microglial scars. Hence, the elucidation of this mechanism will provide novel therapeutic targets for the treatment of SCI.

Increased Expression of Ephrins on Immune Cells of Patients with Relapsing Remitting Multiple Sclerosis Affects Oligodendrocyte Differentiation

The effect of the inflammatory response on regenerative processes in the brain is complex. This complexity is even greater when the cause of the tissue damage is an autoimmune response. Multiple sclerosis (MS) is an immune-mediated disease in which demyelination foci are formed in the central nervous system. The degree of repair through oligodendrocyte regeneration and remyelination is insufficient. Ephrins are membrane-bound ligands activating tyrosine kinase signaling proteins that are known to have an inhibitory effect on oligodendrocyte regeneration. In this study, we examined the expression of ephrins on immune cells of 43 patients with relapsing-remitting (RR) MS compared to 27 matched healthy controls (HC). We found an increased expression of ephrin-A2, -A3 and -B3, especially on T cell subpopulations. We also showed overexpression of ephrins on immune cells of patients with RR-MS that increases the forward signaling pathway and that expression of ephrins on immune cells has an inhibitory effect on the differentiation of oligodendrocyte precursor cells (OPCs) in vitro. Our study findings support the concept that the immune activity of T cells in patients with RR-MS has an inhibitory effect on the differentiation capacity of OPCs through the expression and forward signaling of ephrins.

Microglial vesicles improve post-stroke recovery by preventing immune cell senescence and favoring oligodendrogenesis

Contrasting myelin damage through the generation of new myelinating oligodendrocytes represents a promising approach to promote functional recovery after stroke. Here, we asked whether activation of microglia and monocyte-derived macrophages affects the regenerative process sustained by G protein-coupled receptor 17 (GPR17)-expressing oligodendrocyte precursor cells (OPCs), a subpopulation of OPCs specifically reacting to ischemic injury. GPR17-iCreER^T2:CAG-eGFP reporter mice were employed to trace the fate of GPR17-expressing OPCs, labeled by the green fluorescent protein (GFP), after permanent middle cerebral artery occlusion. By microglia/macrophages pharmacological depletion studies, we show that innate immune cells favor GFP⁺ OPC reaction and limit myelin damage early after injury, whereas they lose their pro-resolving capacity and acquire a dystrophic "senescent-like" phenotype at later stages. Intracerebral infusion of regenerative microglia-derived extracellular vesicles (EVs) restores protective microglia/macrophages functions, limiting their senescence during the post-stroke phase, and enhances the maturation of GFP⁺ OPCs at lesion borders, resulting in ameliorated neurological functionality. In vitro experiments show that EV-carried transmembrane tumor necrosis factor (tmTNF) mediates the pro-differentiating effects on OPCs, with future implications for regenerative therapies.

Quercetin prevents necroptosis of oligodendrocytes by inhibiting macrophages/microglia polarization to M1 phenotype after spinal cord injury in rats

BACKGROUND

Oligodendrocytes (OLs) death after spinal cord injury (SCI) contributes to demyelination, even leading to a permanent neurological deficit. Besides apoptosis, our previous study demonstrated that OLs underwent receptor-interacting serine-threonine kinase 3(RIP3)/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis. Considering that necroptosis is always accompanied with pro-inflammatory response and quercetin has long been used as anti-inflammatory agent, in the present study we investigated whether quercetin could inhibit necroptosis of OLs and suppress the M1 macrophages/microglia-mediated immune response after SCI as well as the possible mechanism.

METHODS

In this study, we applied quercetin, an important flavonoid component of various herbs, to treat rats with SCI and rats injected with saline were employed as the control group. Locomotor functional recovery was evaluated using Basso-Beattie-Bresnahan (BBB) scoring and rump-height Index (RHI) assay. In vivo, the necroptosis, apoptosis, and regeneration of OLs were detected by immunohistochemistry, 5'-bromo-2'-deoxyuridine (BrdU) incorporation. The loss of myelin and axons after SCI were evaluated by Luxol fast blue (LFB) staining, immunohistochemistry, and electron microscopic study. The polarization of macrophages/microglia after SCI and the underlying mechanisms were detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and immunohistochemistry. In vitro, the ATP and reactive oxygen species (ROS) level examination, propidium iodide (PI) labeling, and Western blotting were used to analyze the necroptosis of cultured OLs, while the signaling pathways-mediated polarization of cultured macrophages/microglia was detected by qRT-PCR and Western blotting.

RESULTS

We demonstrated that quercetin treatment improved functional recovery in rats after SCI. We then found that quercetin significantly reduced necroptosis of OLs after SCI without influencing apoptosis and regeneration of OLs. Meanwhile, myelin loss and axon loss were also significantly reduced in quercetin-treated rats, as compared to SCI + saline control. Further, we revealed that quercetin could suppress macrophages/microglia polarized to M1 phenotype through inhibition of STAT1 and NF-κB pathway in vivo and in vitro, which contributes to the decreased necroptosis of OLs.

CONCLUSIONS

Quercetin treatment alleviated necroptosis of OLs partially by inhibiting M1 macrophages/microglia polarization after SCI. Our findings suggest that necroptosis of OLs may be a potential therapeutic target for clinical SCI.

Galectin-4, a Negative Regulator of Oligodendrocyte Differentiation, Is Persistently Present in Axons and Microglia/Macrophages in Multiple Sclerosis Lesions

Neuron-derived molecules are potent regulators of oligodendrocyte differentiation and myelination during brain development and upon demyelination. Their analysis will thus contribute to understanding remyelination failure in demyelinating diseases, such as multiple sclerosis (MS). Previously, we have identified neuronal galectin-4 as a novel negative soluble regulator in the timing of developmental myelination. Here, we investigated whether galectin-4 is re-expressed in axons upon demyelination to regulate the timing of remyelination. Our findings revealed that galectin-4 is transiently localized to axons in demyelinated areas upon cuprizone-induced demyelination. In contrast, in chronic demyelinated MS lesions, where remyelination fails, galectin-4 is permanently present on axons. Remarkably, microglia/macrophages in cuprizone-demyelinated areas also harbor galectin-4, as also observed in activated microglia/macrophages that are present in active MS lesions and in inflammatory infiltrates in chronic-relapsing experimental autoimmune encephalomyelitis. In vitro analysis showed that galectin-4 is effectively endocytosed by macrophages, and may scavenge galectin-4 from oligodendrocytes, and that endogenous galectin-4 levels are increased in alternatively interleukin-4-activated macrophages and microglia. Hence, similar to developmental myelination, the (re)expressed galectin-4 upon demyelination may act as factor in the timing of oligodendrocyte differentiation, while the persistent presence of galectin-4 on demyelinated axons may disrupt this fine-tuning of remyelination.

Central Nervous System Remyelination: Roles of Glia and Innate Immune Cells

In diseases such as multiple sclerosis (MS), inflammation can injure the myelin sheath that surrounds axons, a process known as demyelination. The spontaneous regeneration of myelin, called remyelination, is associated with restoration of function and prevention of axonal degeneration. Boosting remyelination with therapeutic intervention is a promising new approach that is currently being tested in several clinical trials. The endogenous regulation of remyelination is highly dependent on the immune response. In this review article, we highlight the cell biology of remyelination and its regulation by innate immune cells. For the purpose of this review, we discuss the roles of microglia, and also astrocytes and oligodendrocyte progenitor cells (OPCs) as they are being increasingly recognized to have immune cell functions.

Transferrin Enhances Microglial Phagocytic Capacity

Transferrin (Tf) is a glycoprotein playing a critical role in iron homeostasis and transport and distribution throughout the body and within tissues and cells. This molecule has been shown to accelerate the process of myelination and remyelination in the central nervous system (CNS) in vivo and induce oligodendroglial cell maturation in vitro. While the mechanisms involved in oligodendroglial precursor cell (OPC) differentiation have not been fully elucidated yet, our group has previously described the first molecular events taking place in OPC in response to extracellular Tf. Here, we show the effect of Tf on the different glial cell populations. We demonstrate that, after a CNS demyelinating injury, Tf can be incorporated by all glial cells-i.e., microglia, astrocytes, and OPC-and that, acting on microglial cells in vitro, Tf increases microglial proliferation rates and phagocytic capacity. It may be then speculated that the in vivo correlation of this process could generate a favorable microenvironment for OPC maturation and remyelination.

Interferon β-Mediated Protective Functions of Microglia in Central Nervous System Autoimmunity

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) leading to demyelination and axonal damage. It often affects young adults and can lead to neurological disability. Interferon β (IFNβ) preparations represent widely used treatment regimens for patients with relapsing-remitting MS (RRMS) with therapeutic efficacy in reducing disease progression and frequency of acute exacerbations. In mice, IFNβ therapy has been shown to ameliorate experimental autoimmune encephalomyelitis (EAE), an animal model of MS while genetic deletion of IFNβ or its receptor augments clinical severity of disease. However, the complex mechanism of action of IFNβ in CNS autoimmunity has not been fully elucidated. Here, we review our current understanding of the origin, phenotype, and function of microglia and CNS immigrating macrophages in the pathogenesis of MS and EAE. In addition, we highlight the emerging roles of microglia as IFNβ-producing cells and vice versa the impact of IFNβ on microglia in CNS autoimmunity. We finally discuss recent progress in unraveling the underlying molecular mechanisms of IFNβ-mediated effects in EAE.

Regenerating CNS myelin - from mechanisms to experimental medicines

Although the core concept of remyelination - based on the activation, migration, proliferation and differentiation of CNS progenitors - has not changed over the past 20 years, our understanding of the detailed mechanisms that underlie this process has developed considerably. We can now decorate the central events of remyelination with a host of pathways, molecules, mediators and cells, revealing a complex and precisely orchestrated process. These advances have led to recent drug-based and cell-based clinical trials for myelin diseases and have opened up hitherto unrecognized opportunities for drug-based approaches to therapeutically enhance remyelination.

Splitting the "Unsplittable": Dissecting Resident and Infiltrating Macrophages in Experimental Autoimmune Encephalomyelitis

Macrophages predominate the inflammatory landscape within multiple sclerosis (MS) lesions, not only regarding cellularity but also with respect to the diverse functions this cell fraction provides during disease progression and remission. Researchers have been well aware of the fact that the macrophage pool during central nervous system (CNS) autoimmunity consists of a mixture of myeloid cells. Yet, separating these populations to define their unique contribution to disease pathology has long been challenging due to their similar marker expression. Sophisticated lineage tracing approaches as well as comprehensive transcriptome analysis have elevated our insight into macrophage biology to a new level enabling scientists to dissect the roles of resident (microglia and non-parenchymal macrophages) and infiltrating macrophages with unprecedented precision. To do so in an accurate way, researchers have to know their toolbox, which has been filled with diverse, discriminating approaches from decades of studying neuroinflammation in animal models. Every method has its own strengths and weaknesses, which will be addressed in this review. The focus will be on tools to manipulate and/or identify different macrophage subgroups within the injured murine CNS.

Beyond immunomodulation: The regenerative role for regulatory T cells in central nervous system remyelination

Central nervous system regeneration after injury can occur in the form of remyelination, the reinstatement of myelin around axons which restores axon health and function. However, remyelination often fails in chronic neurological diseases, such as progressive multiple sclerosis. The lack of currently approved pro-remyelination therapies highlights the need to elucidate the cellular and molecular mechanisms underpinning this regenerative process. Whereas some T lymphocyte subsets such as Th1 and Th17 are implicated in inducing myelin injury, a recent study by Dombrowski et al. reveals a novel role for regulatory T cells (T_regs) in directly driving remyelination, independent of immunomodulation (Nat Neurosci doi:10.1038/nn.4528 2017)(Dombrowski et al., 2017). This study is summarized in this Bits and Bytes.