Galectin-3 controls the response of microglial cells to limit cuprizone-induced demyelination

Emerging role of galectin 3 in neuroinflammation and neurodegeneration

Neuroinflammation and neurodegeneration are key processes that mediate the development and progression of neurological diseases. However, the mechanisms modulating these processes in different diseases remain incompletely understood. Advances in single cell based multi-omic analyses have helped to identify distinct molecular signatures such as Lgals3 that is associated with neuroinflammation and neurodegeneration in the central nervous system (CNS). Lgals3 encodes galectin-3 (Gal3), a β-galactoside and glycan binding glycoprotein that is frequently upregulated by reactive microglia/macrophages in the CNS during various neurological diseases. While Gal3 has previously been associated with non-CNS inflammatory and fibrotic diseases, recent studies highlight Gal3 as a prominent regulator of inflammation and neuroaxonal damage in the CNS during diseases such as multiple sclerosis, Alzheimer's disease, and Parkinson's disease. In this review, we summarize the pleiotropic functions of Gal3 and discuss evidence that demonstrates its detrimental role in neuroinflammation and neurodegeneration during different neurological diseases. We also consider the challenges of translating preclinical observations into targeting Gal3 in the human CNS.

Monomeric Amyloid Peptide-induced Toxicity in Human Oligodendrocyte Cell Line and Mouse Brain Primary Mixed-glial Cell Cultures: Evidence for a Neuroprotective Effect of Neurosteroid 3α-O-allyl-allopregnanolone

Amyloid-peptide (Aβ) monomeric forms (ABM) occurring in presymptomatic Alzheimer's disease (AD) brain are thought to be devoid of neurotoxicity while the transition/aggregation of ABM into oligomers is determinant for Aβ-induced toxicity since Aβ is predominantly monomeric up to 3 µM and aggregates over this concentration. However, recent imaging and/or histopathological investigations revealed alterations of myelin in prodromal AD brain in absence of aggregated Aβ oligomers, suggesting that ABM may induce toxicity in myelin-producing cells in early AD-stages. To check this hypothesis, here we studied ABM effects on the viability of the Human oligodendrocyte cell line (HOG), a reliable oligodendrocyte model producing myelin proteins. Furthermore, to mimic closely interactions between oligodendrocytes and other glial cells regulating myelination, we investigated also ABM effects on mouse brain primary mixed-glial cell cultures. Various methods were combined to show that ABM concentrations (600 nM-1 µM), extremely lower than 3 µM, significantly decreased HOG cell and mouse brain primary mixed-glial cell survival. Interestingly, flow-cytometry studies using specific cell-type markers demonstrated that oligodendrocytes represent the most vulnerable glial cell population affected by ABM toxicity. Our work also shows that the neurosteroid 3α-O-allyl-allopregnanolone BR351 (250 and 500 nM) efficiently prevented ABM-induced HOG and brain primary glial cell toxicity. Bicuculline (50-100 nM), the GABA-A-receptor antagonist, was unable to block/reduce BR351 effect against ABM-induced HOG and primary glial cell toxicity, suggesting that BR351-evoked neuroprotection of these cells may not depend on GABA-A-receptor allosterically modulated by neurosteroids. Altogether, our results suggest that further exploration of BR351 therapeutic potential may offer interesting perspectives to develop effective neuroprotective strategies.

The emerging neuroimmune hypothesis of bipolar disorder: An updated overview of neuroimmune and microglial findings

Bipolar disorder (BD) is a severe and multifactorial disease, with onset usually in young adulthood, which follows a progressive course throughout life. Replicated epidemiological studies have suggested inflammatory mechanisms and neuroimmune risk factors as primary contributors to the onset and development of BD. While not all patients display overt markers of inflammation, significant evidence suggests that aberrant immune signaling contributes to all stages of the disease and seems to be mood phase dependent, likely explaining the heterogeneity of findings observed in this population. As the brain's immune cells, microglia orchestrate the brain's immune response and play a critical role in maintaining the brain's health across the lifespan. Microglia are also highly sensitive to environmental changes and respond to physiological and pathological events by adapting their functions, structure, and molecular expression. Recently, it has been highlighted that instead of a single population of cells, microglia comprise a heterogeneous community with specialized states adjusted according to the local molecular cues and intercellular interactions. Early evidence has highlighted the contribution of microglia to BD neuropathology, notably for severe outcomes, such as suicidality. However, the roles and diversity of microglial states in this disease are still largely undermined. This review brings an updated overview of current literature on the contribution of neuroimmune risk factors for the onset and progression of BD, the most prominent neuroimmune abnormalities (including biomarker, neuroimaging, ex vivo studies) and the most recent findings of microglial involvement in BD neuropathology. Combining these different shreds of evidence, we aim to propose a unifying hypothesis for BD pathophysiology centered on neuroimmune abnormalities and microglia. Also, we highlight the urgent need to apply novel multi-system biology approaches to characterize the diversity of microglial states and functions involved in this enigmatic disorder, which can open bright perspectives for novel biomarkers and therapeutic discoveries.

TMEM106B regulates microglial proliferation and survival in response to demyelination

TMEM106B, a lysosomal transmembrane protein, has been closely associated with brain health. Recently, an intriguing link between TMEM106B and brain inflammation has been discovered, but how TMEM106B regulates inflammation is unknown. Here, we report that TMEM106B deficiency in mice leads to reduced microglia proliferation and activation and increased microglial apoptosis in response to demyelination. We also found an increase in lysosomal pH and a decrease in lysosomal enzyme activities in TMEM106B-deficient microglia. Furthermore, TMEM106B loss results in a significant decrease in the protein levels of TREM2, an innate immune receptor essential for microglia survival and activation. Specific ablation of TMEM106B in microglia results in similar microglial phenotypes and myelination defects in mice, supporting the idea that microglial TMEM106B is critical for proper microglial activities and myelination. Moreover, the TMEM106B risk allele is associated with myelin loss and decreased microglial numbers in humans. Collectively, our study unveils a previously unknown role of TMEM106B in promoting microglial functionality during demyelination.

Progranulin deficiency results in sex-dependent alterations in microglia in response to demyelination

Heterozygous mutations in the granulin (GRN) gene, resulting in the haploinsufficiency of the progranulin (PGRN) protein, is a leading cause of frontotemporal lobar degeneration (FTLD). Complete loss of the PGRN protein causes neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disorder. Polymorphisms in the GRN gene have also been associated with several other neurodegenerative diseases, including Alzheimer's disease (AD), and Parkinson's disease (PD). PGRN deficiency has been shown to cause myelination defects previously, but how PGRN regulates myelination is unknown. Here, we report that PGRN deficiency leads to a sex-dependent myelination defect with male mice showing more severe demyelination in response to cuprizone treatment. This is accompanied by exacerbated microglial proliferation and activation in the male PGRN-deficient mice. Interestingly, both male and female PGRN-deficient mice show sustained microglial activation after cuprizone removal and a defect in remyelination. Specific ablation of PGRN in microglia results in similar sex-dependent phenotypes, confirming a microglial function of PGRN. Lipid droplets accumulate in microglia specifically in male PGRN-deficient mice. RNA-seq analysis and mitochondrial function assays reveal key differences in oxidative phosphorylation in male versus female microglia under PGRN deficiency. A significant decrease in myelination and accumulation of myelin debris and lipid droplets in microglia were found in the corpus callosum regions of FTLD patients with GRN mutations. Taken together, our data support that PGRN deficiency leads to sex-dependent alterations in microglia with subsequent myelination defects.

Deciphering the Genetic Crosstalk between Microglia and Oligodendrocyte Precursor Cells during Demyelination and Remyelination Using Transcriptomic Data

Demyelinating disorders show impaired remyelination due to failure in the differentiation of oligodendrocyte progenitor cells (OPCs) into mature myelin-forming oligodendrocytes, a process driven by microglia-OPC crosstalk. Through conducting a transcriptomic analysis of microarray studies on the demyelination-remyelination cuprizone model and using human samples of multiple sclerosis (MS), we identified molecules involved in this crosstalk. Differentially expressed genes (DEGs) of specific regions/cell types were detected in GEO transcriptomic raw data after cuprizone treatment and in MS samples, followed by functional analysis with GO terms and WikiPathways. Additionally, microglia-OPC crosstalk between microglia ligands, OPC receptors and target genes was examined with the NicheNet model. We identified 108 and 166 DEGs in the demyelinated corpus callosum (CC) at 2 and 4 weeks of cuprizone treatment; 427 and 355 DEGs in the remyelinated (4 weeks of cuprizone treatment + 14 days of normal diet) compared to 2- and 4-week demyelinated CC; 252 DEGs in MS samples and 2730 and 12 DEGs in OPC and microglia of 4-week demyelinated CC. At this time point, we found 95 common DEGs in the CC and OPCs, and one common DEG in microglia and OPCs, mostly associated with myelin and lipid metabolism. Crosstalk analysis identified 47 microglia ligands, 43 OPC receptors and 115 OPC target genes, all differentially expressed in cuprizone-treated samples and associated with myelination. Our differential expression pipeline identified demyelination/remyelination transcriptomic biomarkers in studies using diverse platforms and cell types/tissues. Cellular crosstalk analysis yielded novel markers of microglia ligands, OPC receptors and target genes.

Galectin-3 administration drives remyelination after hypoxic-ischemic induced perinatal white matter injury

Hypoxic-ischemic (HI) induced perinatal white matter injury (PWMI) is a major cause of neurologic disabilities characterized by selective oligodendroglial death and myelin disruption. Galectin-3 (Gal-3) modulates postnatal subventricular zone gliogenesis and attenuates ischemic injury. However, the association between Gal-3 and myelin formation still remains unclear. In this study, we first perform Gal-3 knockdown (KD) to identify the importance of Gal-3 on myelin formation. Our results show impeded myelin formation, manifested by Olig2/CC1 (+) mature oligodendrocytes number, expression of oligodendroglial maturation-associated markers (MBP and CNPase), and myelin thickness and integrity. Then we perform recombinant Gal-3 (rGal-3) administration by intracerebroventricular injection. Notably, although rGal-3 administration shows no beneficial effect on oligodendrogenesis and myelin formation under normal condition, our results show that rGal-3 administration attenuates cognitive deficits and drives remyelination after PWMI, which are coupled to signs of enhanced myelin resiliency and cognition. Also, our results indicates that the significant increases in substrates for remyelination of rGal-3 administration are accompanied by enhanced Iba-1 (microglia marker)/ Mrc1 (M2 marker) (+) microglia and decreased Iba-1/ iNOS (M1 marker) (+) microglia. Altogether, our data in this research confirm the association between Gal-3 and myelin formation, underscore its position for the capacity for remyelination and restoration of function, and unveils the efficacy of rGal-3 administration with anti-inflammatory phenotype microglia (M2 microglia) activation. Thus, the findings suggest that Gal-3 plays a significant role in myelin formation and remyelination restoration.

NG2-glia crosstalk with microglia in health and disease

Neurodegenerative diseases are increasingly becoming a global problem. However, the pathological mechanisms underlying neurodegenerative diseases are not fully understood. NG2‐glia abnormalities and microglia activation are involved in the development and/or progression of neurodegenerative disorders, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, and cerebrovascular diseases. In this review, we summarize the present understanding of the interaction between NG2‐glia and microglia in physiological and pathological states and discuss unsolved questions concerning their fate and potential fate. First, we introduce the NG2‐glia and microglia in health and disease. Second, we formulate the interaction between NG2‐glia and microglia. NG2‐glia proliferation, migration, differentiation, and apoptosis are influenced by factors released from the microglia. On the other hand, NG2‐glia also regulate microglia actions. We conclude that NG2‐glia and microglia are important immunomodulatory cells in the brain. Understanding the interaction between NG2‐glia and microglia will help provide a novel method to modulate myelination and treat neurodegenerative disorders.

Galectin-3, a rising star in modulating microglia activation under conditions of neurodegeneration

The advent of high-throughput single-cell transcriptomic analysis of microglia has revealed different phenotypes that are inherently associated with disease conditions. A common feature of some of these activated phenotypes is the upregulation of galectin-3. Representative examples of these phenotypes include disease-associated microglia (DAM) and white-associated microglia (WAM), whose role(s) in neuroprotection/neurotoxicity is a matter of high interest in the microglia community. In this review, we summarise the main findings that demonstrate the ability of galectin-3 to interact with key pattern recognition receptors, including, among others, TLR4 and TREM2 and the importance of galectin-3 in the regulation of microglia activation. Finally, we discuss increasing evidence supporting the involvement of this lectin in the main neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, traumatic brain injury, and stroke.

Dynamics of Microglia Activation in the Ischemic Brain: Implications for Myelin Repair and Functional Recovery

Ischemic stroke is a neurological disorder representing a leading cause of death and permanent disability world-wide, for which effective regenerative treatments are missing. Oligodendrocyte degeneration and consequent myelin disruption are considered major contributing factors to stroke-associated neurological deficits. Therefore, fostering myelin reconstruction by oligodendrocyte precursor cells (OPCs) has emerged as a promising therapeutic approach to enhance functional recovery in stroke patients. A pivotal role in regulating remyelination is played by microglia, the resident immune cells of the brain. Early after stroke, microglial cells exert beneficial functions, promoting OPC recruitment toward the ischemic lesion and preserving myelin integrity. However, the protective features of microglia are lost during disease progression, contributing to remyelination failure. Unveiling the mechanisms driving the pro-remyelination properties of microglia may provide important opportunities for both reducing myelin damage and promoting its regeneration. Here, we summarize recent evidence describing microglia activation kinetics in experimental models of ischemic injury, focusing on the contribution of these innate immune cells to myelin damage and repair. Some molecular signals regulating the pro-regenerative functions of microglia after stroke have been highlighted to provide new possible therapeutic targets involved in the protective functions of these cells. Finally, we analyzed the impact of microglia-to-OPCs communication via extracellular vesicles on post-stroke remyelination and functional recovery. The results collected in this review underline the importance of supporting the pro-remyelination functions of microglial cells after stroke.

The Inflammatory Response after Moderate Contusion Spinal Cord Injury: A Time Study

Simple Summary

The neuroinflammatory response is a rather complex event in spinal cord injury (SCI) and has the capacity to exacerbate cell damage but also to contribute to the repair of the injury. This complexity is thought to depend on a variety of inflammatory mediators, of which tumor necrosis factor (TNF) plays a key role. Evidence indicates that TNF can be both protective and detrimental in SCI. In the present study, we studied the temporal and cellular expression of TNF and its receptors after SCI in mice. We found TNF to be significantly increased in both the acute and the delayed phases after SCI, alongside a robust neuroinflammatory response. As we could verify some of our results in human postmortem tissue, our results imply that diminishing the detrimental immune signaling after SCI could also enhance recovery in humans.

Abstract

Spinal cord injury (SCI) initiates detrimental cellular and molecular events that lead to acute and delayed neuroinflammation. Understanding the role of the inflammatory response in SCI requires insight into the temporal and cellular synthesis of inflammatory mediators. We subjected C57BL/6J mice to SCI and investigated inflammatory reactions. We examined activation, recruitment, and polarization of microglia and infiltrating immune cells, focusing specifically on tumor necrosis factor (TNF) and its receptors TNFR1 and TNFR2. In the acute phase, TNF expression increased in glial cells and neuron-like cells, followed by infiltrating immune cells. TNFR1 and TNFR2 levels increased in the delayed phase and were found preferentially on neurons and glial cells, respectively. The acute phase was dominated by the infiltration of granulocytes and macrophages. Microglial/macrophage expression of Arg1 increased from 1–7 days after SCI, followed by an increase in Itgam, Cx3cr1, and P2ry12, which remained elevated throughout the study. By 21 and 28 days after SCI, the lesion core was populated by galectin-3⁺, CD68⁺, and CD11b⁺ microglia/macrophages, surrounded by a glial scar consisting of GFAP⁺ astrocytes. Findings were verified in postmortem tissue from individuals with SCI. Our findings support the consensus that future neuroprotective immunotherapies should aim to selectively neutralize detrimental immune signaling while sustaining pro-regenerative processes.

Different Methods for Evaluating Microglial Activation Using Anti-Ionized Calcium-Binding Adaptor Protein-1 Immunohistochemistry in the Cuprizone Model

Microglia play an important role in the pathology of various central nervous system disorders, including multiple sclerosis (MS). While different methods exist to evaluate the extent of microglia activation, comparative studies investigating the sensitivity of these methods are missing for most models. In this study, we systematically evaluated which of the three commonly used histological methods (id est, quantification of microglia density, densitometrically evaluated staining intensity, or cellular morphology based on the determination of a ramification index, all measured in anti-ionized calcium-binding adaptor protein-1 (IBA1) immunohistochemical stains) is the most sensitive method to detect subtle changes in the microglia activation status in the context of MS. To this end, we used the toxin-induced cuprizone model which allows the experimental induction of a highly reproducible demyelination in several central nervous system regions, paralleled by early microglia activation. In this study, we showed that after 3 weeks of cuprizone intoxication, all methods reveal a significant microglia activation in the white matter corpus callosum. In contrast, in the affected neocortical grey matter, the evaluation of anti-IBA1 cell morphologies was the most sensitive method to detect subtle changes of microglial activation. The results of this study provide a useful guide for future immunohistochemical evaluations in the cuprizone and other neurodegenerative models.

Galectins as Emerging Glyco-Checkpoints and Therapeutic Targets in Glioblastoma

Despite recent advances in diagnosis and treatment, glioblastoma (GBM) represents the most common and aggressive brain tumor in the adult population, urging identification of new rational therapeutic targets. Galectins, a family of glycan-binding proteins, are highly expressed in the tumor microenvironment (TME) and delineate prognosis and clinical outcome in patients with GBM. These endogenous lectins play key roles in different hallmarks of cancer by modulating tumor cell proliferation, oncogenic signaling, migration, vascularization and immunity. Additionally, they have emerged as mediators of resistance to different anticancer treatments, including chemotherapy, radiotherapy, immunotherapy, and antiangiogenic therapy. Particularly in GBM, galectins control tumor cell transformation and proliferation, reprogram tumor cell migration and invasion, promote vascularization, modulate cell death pathways, and shape the tumor-immune landscape by targeting myeloid, natural killer (NK), and CD8⁺ T cell compartments. Here, we discuss the role of galectins, particularly galectin-1, -3, -8, and -9, as emerging glyco-checkpoints that control different mechanisms associated with GBM progression, and discuss possible therapeutic opportunities based on inhibition of galectin-driven circuits, either alone or in combination with other treatment modalities.

Colony-stimulating factor-1 receptor inhibition attenuates microgliosis and myelin loss but exacerbates neurodegeneration in the chronic cuprizone model

Multiple sclerosis (MS), especially in its progressive phase, involves early axonal and neuronal damage resulting from a combination of inflammatory mediators, demyelination, and loss of trophic support. During progressive disease stages, a microenvironment is created within the central nervous system (CNS) favoring the arrival and retention of inflammatory cells. Active demyelination and neurodegeneration have also been linked to microglia (MG) and astrocyte (AST)-activation in early lesions. While reactive MG can damage tissue, exacerbate deleterious effects, and contribute to neurodegeneration, it should be noted that activated MG possess neuroprotective functions as well, including debris phagocytosis and growth factor secretion. The progressive form of MS can be modelled by the prolonged administration to cuprizone (CPZ) in adult mice, as CPZ induces highly reproducible demyelination of different brain regions through oligodendrocyte (OLG) apoptosis, accompanied by MG and AST activation and axonal damage. Therefore, our goal was to evaluate the effects of a reduction in microglial activation through orally administered brain-penetrant colony-stimulating factor-1 receptor (CSF-1R) inhibitor BLZ945 (BLZ) on neurodegeneration and its correlation with demyelination, astroglial activation and behavior in a chronic CPZ-induced demyelination model. Our results show that BLZ treatment successfully reduced the microglial population and myelin loss. However, no correlation was found between myelin preservation and neurodegeneration, as axonal degeneration was more prominent upon BLZ treatment. Concomitantly, BLZ failed to significantly offset CPZ-induced astroglial activation and behavioral alterations. These results should be taken into account when proposing the modulation of microglial activation in the design of therapies relevant for demyelinating diseases.

Transgenic Overexpression of Galectin-3 in Pancreatic β Cells Attenuates Hyperglycemia in Mice: Synergistic Antidiabetic Effect With Exogenous IL-33

Galectin-3 (Gal-3) has diverse roles in inflammatory and autoimmune diseases. There is evidence that Gal-3 plays a role in both type 1 and type 2 diabetes. While the role of Gal-3 expression in immune cells invading the pancreatic islets in the experimental model of type 1 diabetes mellitus has been already studied, the importance of the overexpression of Gal-3 in the target β cells is not defined. Therefore, we used multiple low doses of streptozotocin (MLD–STZ)–induced diabetes in C57Bl/6 mice to analyze the effect of transgenic (TG) overexpression of Gal-3 in β cells. Our results demonstrated that the overexpression of Gal-3 protected β cells from apoptosis and attenuated MLD–STZ–induced hyperglycemia, glycosuria, and ketonuria. The cellular analysis of pancreata and draining lymph nodes showed that Gal-3 overexpression significantly decreased the number of pro-inflammatory cells without affecting the presence of T-regulatory cells. As the application of exogenous interleukin 33 (IL-33) given from the beginning of MLD–STZ diabetes induction attenuates the development of disease, by increasing the presence of regulatory FoxP3⁺ ST2⁺ cells, we evaluated the potential synergistic effect of the exogenous IL-33 and TG overexpression of Gal-3 in β cells at the later stage of diabetogenesis. The addition of IL-33 potentiated the survival of β cells and attenuated diabetes even when administered later, after the onset of hyperglycemia (12–18 days), suggesting that protection from apoptosis and immunoregulation by IL-33 may attenuate type 1 diabetes.

Novel Galectin-3 Roles in Neurogenesis, Inflammation and Neurological Diseases

Galectin-3 (Gal-3) is an evolutionarily conserved and multifunctional protein that drives inflammation in disease. Gal-3's role in the central nervous system has been less studied than in the immune system. However, recent studies show it exacerbates Alzheimer's disease and is upregulated in a large variety of brain injuries, while loss of Gal-3 function can diminish symptoms of neurodegenerative diseases such as Alzheimer's. Several novel molecular pathways for Gal-3 were recently uncovered. It is a natural ligand for TREM2 (triggering receptor expressed on myeloid cells), TLR4 (Toll-like receptor 4), and IR (insulin receptor). Gal-3 regulates a number of pathways including stimulation of bone morphogenetic protein (BMP) signaling and modulating Wnt signalling in a context-dependent manner. Gal-3 typically acts in pathology but is now known to affect subventricular zone (SVZ) neurogenesis and gliogenesis in the healthy brain. Despite its myriad interactors, Gal-3 has surprisingly specific and important functions in regulating SVZ neurogenesis in disease. Gal-1, a similar lectin often co-expressed with Gal-3, also has profound effects on brain pathology and adult neurogenesis. Remarkably, Gal-3's carbohydrate recognition domain bears structural similarity to the SARS-CoV-2 virus spike protein necessary for cell entry. Gal-3 can be targeted pharmacologically and is a valid target for several diseases involving brain inflammation. The wealth of molecular pathways now known further suggest its modulation could be therapeutically useful.

Specific microglial phagocytic phenotype and decrease of lipid oxidation in white matter areas during aging: Implications of different microenvironments

Physiological aging is characterized by an imbalance of pro-inflammatory and anti-inflammatory mediators leading to neuroinflammation. Microglial cells, which are highly regulated by the local microenvironment, undergo specific changes depending upon the brain area during aging. The aim of this study was to evaluate the influence of age over microglial cells along different brain areas and microenvironments. For this purpose, transgenic mice with overproduction of either the anti-inflammatory IL-10 cytokine or the pro-inflammatory IL-6 cytokine were used. Our results show that, during aging, microglial cells located in white matter (WM) areas maintain their phagocytic capacity but present a specific phagocytic phenotype with receptors involved in myelin recognition, arguing for aging-derived myelin damage. Whereas IL-10 overproduction anticipates the age-related microglial phagocytic phenotype, maintaining it over time, IL-6 overproduction exacerbates this phenotype in aging. These modifications were linked with a higher efficiency of myelin engulfment by microglia in aged transgenic animals. Moreover, we show, in a novel way, lower lipid oxidation during aging in WM areas, regardless of the genotype. The novelty of the insights presented in this study open a window to deeply investigate myelin lipid oxidation and the role of microglial cells in its regulation during physiological aging.

Oligodendrocytes and Microglia: Key Players in Myelin Development, Damage and Repair

Oligodendrocytes, the myelin-making cells of the CNS, regulate the complex process of myelination under physiological and pathological conditions, significantly aided by other glial cell types such as microglia, the brain-resident, macrophage-like innate immune cells. In this review, we summarize how oligodendrocytes orchestrate myelination, and especially myelin repair after damage, and present novel aspects of oligodendroglial functions. We emphasize the contribution of microglia in the generation and regeneration of myelin by discussing their beneficial and detrimental roles, especially in remyelination, underlining the cellular and molecular components involved. Finally, we present recent findings towards human stem cell-derived preclinical models for the study of microglia in human pathologies and on the role of microbiome on glial cell functions.

Connecting Neuroinflammation and Neurodegeneration in Multiple Sclerosis: Are Oligodendrocyte Precursor Cells a Nexus of Disease?

The pathology in neurodegenerative diseases is often accompanied by inflammation. It is well-known that many cells within the central nervous system (CNS) also contribute to ongoing neuroinflammation, which can promote neurodegeneration. Multiple sclerosis (MS) is both an inflammatory and neurodegenerative disease in which there is a complex interplay between resident CNS cells to mediate myelin and axonal damage, and this communication network can vary depending on the subtype and chronicity of disease. Oligodendrocytes, the myelinating cell of the CNS, and their precursors, oligodendrocyte precursor cells (OPCs), are often thought of as the targets of autoimmune pathology during MS and in several animal models of MS; however, there is emerging evidence that OPCs actively contribute to inflammation that directly and indirectly contributes to neurodegeneration. Here we discuss several contributors to MS disease progression starting with lesion pathology and murine models amenable to studying particular aspects of disease. We then review how OPCs themselves can play an active role in promoting neuroinflammation and neurodegeneration, and how other resident CNS cells including microglia, astrocytes, and neurons can impact OPC function. Further, we outline the very complex and pleiotropic role(s) of several inflammatory cytokines and other secreted factors classically described as solely deleterious during MS and its animal models, but in fact, have many neuroprotective functions and promote a return to homeostasis, in part via modulation of OPC function. Finally, since MS affects patients from the onset of disease throughout their lifespan, we discuss the impact of aging on OPC function and CNS recovery. It is becoming clear that OPCs are not simply a bystander during MS progression and uncovering the active roles they play during different stages of disease will help uncover potential new avenues for therapeutic intervention.

Behavioral and neurobiological changes in a novel mouse model of schizophrenia induced by the combination of cuprizone and MK-801

Schizophrenia is a mental illness characterized by episodes of psychosis, apathy, social withdrawal, and cognitive impairment. White matter lesions and glutamatergic hypofunction are reported to be the key pathogeneses underlying the multiple clinical symptoms of schizophrenia. Cuprizone (CPZ) is a copper chelator that selectively injures oligodendrocytes, and MK-801 is an antagonist of the N-methyl d-aspartate (NMDA) receptor. To better mimic the psychosis and complicated pathogenesis of schizophrenia, a novel possible mouse model was established by the combination of CPZ and MK-801. After exposure to CPZ for 5 weeks, the mice received a daily intraperitoneal injection of MK-801 for 2-weeks. Behavioral changes in the mouse model were evaluated using Y-maze, object recognition, and open field tests. Pathological changes were observed by transmission electron microscopy, oil red O staining, immunohistochemistry, and western blotting. The results showed that the novel mouse model induced by CPZ plus MK-801 exhibited severe spatial and recognition memory deficits, hyperactivity, and anxiety disorder. Moreover, the mice showed obvious demyelination and white matter damage and decreased expression levels of myelin basic protein (MBP) and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) in the corpus callosum. Furthermore, the phosphorylation levels of Fyn and NMDA receptor 2B in the corpus callosum and NMDA receptor 1 in the cerebral cortex were noticeably decreased. Taken together, the novel mouse model induced by the combination of cuprizone and MK-801 showed comprehensive behavioral and neurobiological changes, which might make it a suitable animal model for schizophrenia.

Post-stroke Delivery of Valproic Acid Promotes Functional Recovery and Differentially Modifies Responses of Peri-Infarct Microglia

The specific role of peri-infarct microglia and the timing of its morphological changes following ischemic stroke are not well understood. Valproic acid (VPA) can protect against ischemic damage and promote recovery. In this study, we first determined whether a single dose of VPA after stroke could decrease infarction area or improve functional recovery. Next, we investigated the number and morphological characteristic of peri-infarct microglia at different time points and elucidated the mechanism of microglial response by VPA treatment. Male Sprague-Dawley rats were subjected to distal middle cerebral artery occlusion (dMCAo) for 90 min, followed by reperfusion. Some received a single injection of VPA (200 mg/kg) 90 min after the induction of ischemia, while vehicle-treated animals underwent the same procedure with physiological saline. Infarction volume was calculated at 48 h after reperfusion, and neurological symptoms were evaluated. VPA didn’t significantly reduce infarct volume but did ameliorate neurological deficit at least partially compared with vehicle. Meanwhile, VPA reduced dMCAo-induced elevation of IL-6 at 24 h post-stroke and significantly decreased the number of CD11b-positive microglia within peri-infarct cortex at 7 days. Morphological analysis revealed that VPA therapy leads to higher fractal dimensions, smaller soma size and lower circularity index of CD11b-positive cells within peri-infarct cortex at both 2 and 7 days, suggesting that VPA has core effects on microglial morphology. The modulation of microglia morphology caused by VPA might involve HDAC inhibition-mediated suppression of galectin-3 production. Furthermore, qPCR analysis of CD11b-positive cells at 3 days post-stroke suggested that VPA could partially enhance M2 subset polarization of microglia in peri-infarct cortex. Analysis of VPA-induced changes to gene expressions at 3 days post-stroke implies that these alternations of the biomarkers and microglial responses are implicated in the upregulation of wound healing, collagen trimmer, and extracellular matrix genes within peri-infarct cortex. Our results are the first to show that a low dose of VPA promotes short-term functional recovery but does not alter infarct volume. The decreases in the expression of both IL-6 and galectin-3 might influence the morphological characteristics and transcriptional profiles of microglia and extracellular matrix remodeling, which could contribute to the improved recovery.

Patient-derived glial enriched progenitors repair functional deficits due to white matter stroke and vascular dementia in rodents

Subcortical white matter stroke (WMS) accounts for up to 30% of all stroke events. WMS damages primarily astrocytes, axons, oligodendrocytes, and myelin. We hypothesized that a therapeutic intervention targeting astrocytes would be ideally suited for brain repair after WMS. We characterize the cellular properties and in vivo tissue repair activity of glial enriched progenitor (GEP) cells differentiated from human-induced pluripotent stem cells, termed hiPSC-derived GEPs (hiPSC-GEPs). hiPSC-GEPs are derived from hiPSC-neural progenitor cells via an experimental manipulation of hypoxia inducible factor activity by brief treatment with a prolyl hydroxylase inhibitor, deferoxamine. This treatment permanently biases these cells to further differentiate toward an astrocyte fate. hiPSC-GEPs transplanted into the brain in the subacute period after WMS in mice migrated widely, matured into astrocytes with a prorepair phenotype, induced endogenous oligodendrocyte precursor proliferation and remyelination, and promoted axonal sprouting. hiPSC-GEPs enhanced motor and cognitive recovery compared to other hiPSC-differentiated cell types. This approach establishes an hiPSC-derived product with easy scale-up capabilities that might be effective for treating WMS.

18β-Glycyrrhetinic acid alleviates demyelination by modulating the microglial M1/M2 phenotype in a mouse model of cuprizone-induced demyelination

This research aimed to examine the nutritious supplementary function of 18β-Glycyrrhetinic acid (18β-GA) in moderating the myelin sheath destruction and behavioral impairments observed in the cuprizone model of demyelination. Mice were fed daily on food containing cuprizone (0.3 %) and given doses of 18β-GA (5 or 1 mg/kg) for a period of five weeks. The groups treated with 18β-GA exhibited improvements in exploratory behavior, locomotive activity, and weight. As assessed using luxol-fast blue and myelin basic protein (MBP) staining, which were used to detect demyelination in the brain, 18β-GA both reduced and prevented instances of cuprizone-induced demyelinating lesions; treatment with 18β-GA also caused the MBP level in the corpus callosum to increase. Furthermore, alongside these positive results following 18β-GA treatment, microglial polarisation was also observed to shift towards the beneficial M2 phenotype. The results of this research thus indicate the potential clinical application of 18β-GA for the prevention of myelin damage and behavioral dysfunction.

Cuprizone and EAE mouse frontal cortex proteomics revealed proteins altered in multiple sclerosis

Two pathophysiological different experimental models for multiple sclerosis were analyzed in parallel using quantitative proteomics in attempts to discover protein alterations applicable as diagnostic-, prognostic-, or treatment targets in human disease. The cuprizone model reflects de- and remyelination in multiple sclerosis, and the experimental autoimmune encephalomyelitis (EAE, MOG1-125) immune-mediated events. The frontal cortex, peripheral to severely inflicted areas in the CNS, was dissected and analyzed. The frontal cortex had previously not been characterized by proteomics at different disease stages, and novel protein alterations involved in protecting healthy tissue and assisting repair of inflicted areas might be discovered. Using TMT-labelling and mass spectrometry, 1871 of the proteins quantified overlapped between the two experimental models, and the fold change compared to controls was verified using label-free proteomics. Few similarities in frontal cortex between the two disease models were observed when regulated proteins and signaling pathways were compared. Legumain and C1Q complement proteins were among the most upregulated proteins in cuprizone and hemopexin in the EAE model. Immunohistochemistry showed that legumain expression in post-mortem multiple sclerosis brain tissue (n = 19) was significantly higher in the center and at the edge of white matter active and chronic active lesions. Legumain was associated with increased lesion activity and might be valuable as a drug target using specific inhibitors as already suggested for Parkinson's and Alzheimer's disease. Cerebrospinal fluid levels of legumain, C1q and hemopexin were not significantly different between multiple sclerosis patients, other neurological diseases, or healthy controls.

Microglial Phagocytosis-Rational but Challenging Therapeutic Target in Multiple Sclerosis

Multiple sclerosis (MS) is the most common autoimmune and demyelinating disease of the central nervous system (CNS), characterized, in the majority of cases, by initial relapses that later evolve into progressive neurodegeneration, severely impacting patients’ motor and cognitive functions. Despite the availability of immunomodulatory therapies effective to reduce relapse rate and slow disease progression, they all failed to restore CNS myelin that is necessary for MS full recovery. Microglia are the primary inflammatory cells present in MS lesions, therefore strongly contributing to demyelination and lesion extension. Thus, many microglial-based therapeutic strategies have been focused on the suppression of microglial pro-inflammatory phenotype and neurodegenerative state to reduce disease severity. On the other hand, the contribution of myelin phagocytosis advocating the neuroprotective role of microglia in MS has been less explored. Indeed, despite the presence of functional oligodendrocyte precursor cells (OPCs), within lesioned areas, MS plaques fail to remyelinate as a result of the over-accumulation of myelin-toxic debris that must be cleared away by microglia. Dysregulation of this process has been associated with the impaired neuronal recovery and deficient remyelination. In line with this, here we provide a comprehensive review of microglial myelin phagocytosis and its involvement in MS development and repair. Alongside, we discuss the potential of phagocytic-mediated therapeutic approaches and encourage their modulation as a novel and rational approach to ameliorate MS-associated pathology.

The emerging role of galectins in (re)myelination and its potential for developing new approaches to treat multiple sclerosis

Multiple sclerosis (MS) is an inflammatory, demyelinating and neurodegenerative disease of the central nervous system with unknown etiology. Currently approved disease-modifying treatment modalities are immunomodulatory or immunosuppressive. While the applied drugs reduce the frequency and severity of the attacks, their efficacy to regenerate myelin membranes and to halt disease progression is limited. To achieve such therapeutic aims, understanding biological mechanisms of remyelination and identifying factors that interfere with remyelination in MS can give respective directions. Such a perspective is given by the emerging functional profile of galectins. They form a family of tissue lectins, which are potent effectors in processes as diverse as adhesion, apoptosis, immune mediator release or migration. This review focuses on endogenous and exogenous roles of galectins in glial cells such as oligodendrocytes, astrocytes and microglia in the context of de- and (re)myelination and its dysregulation in MS. Evidence is arising for a cooperation among family members so that timed expression and/or secretion of galectins-1, -3 and -4 result in modifying developmental myelination, (neuro)inflammatory processes, de- and remyelination. Dissecting the mechanisms that underlie the distinct activities of galectins and identifying galectins as target or tool to modulate remyelination have the potential to contribute to the development of novel therapeutic strategies for MS.

Oligodendrocytes Death Induced Sensorimotor and Cognitive Deficit in N-nitro-L-arginine methyl Rat Model of Pre-eclampsia

Pre-eclampsia (PE) is a pregnancy complicated syndrome that affects multiple organs including the brain that continue post- delivery in both mother and the offspring. We evaluated the expression of oligodendrocytes in the brain of PE rat model through development as well as the cognitive changes and other behavioural modifications that may occur later in the life of offspring of PE-like rat model. Pregnant rats divided into early-onset and late-onset groups were administered with N-nitro- L-arginine methyl (L-NAME) through drinking water at gestational days (GD) 8-17. Rats were allowed free access to water throughout the pregnancy. At GD 19, post-natal day (PND) 1 and 60, rats were sacrificed and brain excised for further analysis. The offspring were subjected to behavioural studies for cognitive and sensorimotor impairments before sacrificed at PND 60. Results showed significant down-regulation in the expression of OLIG2 in PE at GD 19 brain which persists till PND 60. Likewise, there was a significant increase in the latency to locate the platform in Morris water maze, time to traverse the balance beam and reduced hanging time on the wire test between the control and the PE treated. PE could lead to impaired neuronal signalling through demyelination which may contributes significantly to long-term sensorimotor and cognitive deficit.

Galectin-3 modulates postnatal subventricular zone gliogenesis

Postnatal subventricular zone (SVZ) neural stem cells generate forebrain glia, namely astrocytes and oligodendrocytes. The cues necessary for this process are unclear, despite this phase of brain development being pivotal in forebrain gliogenesis. Galectin‐3 (Gal‐3) is increased in multiple brain pathologies and thereby regulates astrocyte proliferation and inflammation in injury. To study the function of Gal‐3 in inflammation and gliogenesis, we carried out functional studies in mouse. We overexpressed Gal‐3 with electroporation and using immunohistochemistry surprisingly found no inflammation in the healthy postnatal SVZ. This allowed investigation of inflammation‐independent effects of Gal‐3 on gliogenesis. Loss of Gal‐3 function via knockdown or conditional knockout reduced gliogenesis, whereas Gal‐3 overexpression increased it. Gal‐3 overexpression also increased the percentage of striatal astrocytes generated by the SVZ but decreased the percentage of oligodendrocytes. These novel findings were further elaborated with multiple analyses demonstrating that Gal‐3 binds to the bone morphogenetic protein receptor one alpha (BMPR1α) and increases bone morphogenetic protein (BMP) signaling. Conditional knockout of BMPR1α abolished the effect of Gal‐3 overexpression on gliogenesis. Gain‐of‐function of Gal‐3 is relevant in pathological conditions involving the human forebrain, which is particularly vulnerable to hypoxia/ischemia during perinatal gliogenesis. Hypoxic/ischemic injury induces astrogliosis, inflammation and cell death. We show that Gal‐3 immunoreactivity was increased in the perinatal human SVZ and striatum after hypoxia/ischemia. Our findings thus show a novel inflammation‐independent function for Gal‐3; it is necessary for gliogenesis and when increased in expression can induce astrogenesis via BMP signaling.

Glial Plasticity in the Trigeminal Root Entry Zone of a Rat Trigeminal Neuralgia Animal Model

The trigeminal root entry zone (TREZ) is the transitional zone of central and peripheral tissue compartments in the trigeminal root. Microvascular compression on the TREZ is the main etiology of most idiopathic trigeminal neuralgia (TN) patients. However, the pathogenesis of TN is still uncertain. To investigate the glial plasticity changes in oligodendrocytes, Schwann cells, astrocytes and microglia/macrophages in the TREZ in TN, immunohistochemical staining and Western blot methods were performed in rats with TN induced by compression injury. The results showed that mechanical compression injury in the trigeminal nerve of the TN rats induced glial plasticity in the TREZ, which dynamically changed the glial interface of the CNS-PNS transitional zone. Additionally, glial fibrillary acidic protein (GFAP)-immunoreactive astrocyte processes significantly proliferated and extended distally from the central region to the peripheral side of the TREZ after nerve compression injury in the TN group. Moreover, the expression of p75 in Schwann cells was upregulated on the peripheral side of the TREZ, and activated Iba-1-immunoreactive microglia/macrophages were observed on both sides of the TREZ. A significantly higher number of Schwann cells, astrocytes and microglia/macrophages were found in the TN group than in the sham operation group (p < 0.05). In conclusion, mechanical compression injury in the TN rats activated various glial cells, including oligodendrocytes, astrocytes, Schwann cells and microglia/macrophages, in the CNS-PNS transitional zone of TREZ. Changes in glial cell plasticity in the TREZ after compression injury might be involved in TN pathogenesis.

The role of galectin-3 in modulation of anxiety state level in mice

Galectin-3 (Gal-3), a member of lectin family that binds to oligosaccharides, is involved in several biological processes, including maturation and function of nervous system. It had been reported that Gal-3 regulates oligodendrocytes differentiation and that Gal-3/Toll-like receptor-4 (TLR4) axis is involved in neuroinflammation. As both, central nervous system (CNS) maturation and neuroinflammation may affect behavior, the principle aim of this study was to examine the effects of Gal-3 gene deletion on behavior. Here we provide the evidence that Gal-3 deficiency shows clear anxiogenic effect in mature untreated animals (basal conditions). This was accompanied with lower interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) relative gene expression and hippocampal content, with no effect on TLR4 expression. Gal-3 deficiency was also accompanied with lower brain-derived neurotrophic factor (BDNF) relative gene expression and immunoreactivity in hippocampus (predominantly in CA1 region). Besides, the Gal-3 gene deletion resulted in attenuation of the hippocampal relative gene expression of GABA-A receptor subunits 2 and 5 (GABA-AR2S and GABA-AR5S), On the other hand, Gal-3 deficiency attenuates LPS-induced neuroinflammation. The anxiogenic effect of acute neuroinflammation was accompanied with increased hippocampal IL-6, TNF-α and TLR4 gene expression, as well as decreased gene and immunohistochemical BDNF expression in hippocampus, with significant decline in GABA-AR2S in wild type (WT) mice in comparison to basal conditions. Gal-3 gene deletion prevented the increase in IL-6, the decline in BDNF gene expression and immunoreactivity, and reduction in hippocampal GABA-AR2S, and therefore attenuated the anxiogenic effect of neuroinflammation. In summary, our data demonstrate that apparently opposite effects of Gal-3 deficiency on anxiety levels (anxiogenic effect under basal conditions and anxiolytic action during neuroinflammation) seem to be related to the shift in IL-6, TNF-α and hippocampal BDNF.

Delayed Galectin-3-Mediated Reprogramming of Microglia After Stroke is Protective

Galectin-3 (Gal-3), a β-galactoside-binding lectin, has recently emerged as a molecule with immunoregulatory functions. We investigated the effects of Gal-3 on microglia morphology, migration, and secretory profile under physiological conditions and in the context of ischemic injury. We show that in the control conditions, exposure to recombinant Gal-3 increases microglial ramification and motility in vitro and in vivo via an IL-4-dependent mechanism. Importantly, after stroke, Gal-3 exerted marked immune-modulatory properties. Delivery of Gal-3 at 24 h after middle cerebral artery occlusion (MCAO) was associated with an increase in Ym1-positive microglia and decrease in iNOS. Analysis of cytokine profiles at the protein level revealed downregulation of pro-inflammatory cytokines and a marked upregulation of the anti-inflammatory cytokine, IL-4, 24 h after i.c.v. injection of Gal-3. Importantly, the observed shift in cytokines in microglia was associated with a significant decrease in the infarct size. Taken together, our results suggest that when delivered well after ischemic injury, Gal-3 might fine tune innate immunity and induce a therapeutic shift in microglia polarization.

High cholesterol triggers white matter alterations and cognitive deficits in a mouse model of cerebrovascular disease: benefits of simvastatin

Transgenic mice overexpressing transforming growth factor-β1 (TGF mice) display impaired cerebrovascular reactivity, cerebral hypoperfusion and neurovascular uncoupling, but no overt cognitive deficits until old age. Cardiovascular diseases are a major risk factor for vascular cognitive impairment and dementia (VCID). We investigated the impact of a high cholesterol diet (HCD) on cerebrovascular and cognitive function in adult (6 months) and aged (12 months) TGF mice, together with the potential benefit of simvastatin (SV), an anti-cholesterol drug with pleiotropic effects, in adult mice. HCD increased blood, but not brain, cholesterol levels in treated mice, which SV did not reduce. In WT mice, HCD induced small, albeit significant, impairment in endothelium-dependent dilatory function. In TGF mice, HCD worsened the established brain vessel dilatory dysfunction in an age-dependent manner and increased the number of string vessels in the white matter (WM), alterations respectively normalized and significantly countered by SV. HCD triggered cognitive decline only in TGF mice at both ages, a deficit prevented by SV. Concurrently, HCD upregulated galectin−3 immunoreactivity in WM microglial cells, a response significantly reduced in SV-treated TGF mice. Grey matter astrogliosis and microgliosis were not affected by HCD or SV. In the subventricular zone of adult HCD-treated TGF mice, SV promoted oligogenesis and migration of oligodendrocyte progenitor cells. The results demonstrate that an underlying cerebrovascular pathology increases vulnerability to cognitive failure when combined to another risk factor for dementia, and that WM alterations are associated with this loss of function. The results further indicate that myelin repair mechanisms, as triggered by SV, may bear promise in preventing or delaying cognitive decline related to VCID.

Carbon monoxide poisoning-induced delayed encephalopathy accompanies decreased microglial cell numbers: Distinctive pathophysiological features from hypoxemia-induced brain damage

Carbon monoxide (CO) causes not only acute fatal poisoning but also may cause a delayed neurologic syndrome called delayed encephalopathy (DE), which occasionally occurs after an interval of several days to several weeks post-exposure. However, the mechanisms of DE have not been fully elucidated. This study aimed to clarify the pathophysiology of CO-induced DE and its distinctive features compared with hypoxemic hypoxia. Rats were randomly assigned to three groups; the air group, the CO group (exposed to CO), and the low O₂ group (exposed to low concentration of O₂). Impairment of memory function was observed only in the CO group. The hippocampus tissues were collected and analyzed for assessment of CO-induced changes and microglial reaction. Demyelination was observed only in the CO group and it was more severe and persisted longer than that observed in the low O₂ group. Moreover, in the CO group, decreased in microglial cell numbers were observed using flow cytometry, and microglia with detached branches were observed were observed using immunohistochemistry. Conversely, microglial cells with shortened branches and enlarged somata were observed in the low O₂ group. Furthermore, mRNAs encoding several neurotrophic factors expressed by microglia were decreased in the CO group but were increased in the low O₂ group. Thus, CO-induced DE displayed distinctive pathological features from those of simple hypoxic insults: prolonged demyelination accompanying a significant decrease in microglial cells. Decreased neurotrophic factor expression by microglial cells may be one of the causes of CO-induced DE.

Galectin-3-Mediated Glial Crosstalk Drives Oligodendrocyte Differentiation and (Re)myelination

Galectin-3 (Gal-3) is the only chimeric protein in the galectin family. Gal-3 structure comprises unusual tandem repeats of proline and glycine-rich short stretches bound to a carbohydrate-recognition domain (CRD). The present review summarizes Gal-3 functions in the extracellular and intracellular space, its regulation and its internalization and secretion, with a focus on the current knowledge of Gal-3 role in central nervous system (CNS) health and disease, particularly oligodendrocyte (OLG) differentiation, myelination and remyelination in experimental models of multiple sclerosis (MS). During myelination, microglia-expressed Gal-3 promotes OLG differentiation by binding glycoconjugates present only on the cell surface of OLG precursor cells (OPC). During remyelination, microglia-expressed Gal-3 favors an M2 microglial phenotype, hence fostering myelin debris phagocytosis through TREM-2b phagocytic receptor and OLG differentiation. Gal-3 is necessary for myelin integrity and function, as evidenced by myelin ultrastructural and behavioral studies from LGALS3^-/- mice. Mechanistically, Gal-3 enhances actin assembly and reduces Erk 1/2 activation, leading to early OLG branching. Gal-3 later induces Akt activation and increases MBP expression, promoting gelsolin release and actin disassembly and thus regulating OLG final differentiation. Altogether, findings indicate that Gal-3 mediates the glial crosstalk driving OLG differentiation and (re)myelination and may be regarded as a target in the design of future therapies for a variety of demyelinating diseases.

Microglial Lectins in Health and Neurological Diseases

Microglia are the innate sentinels of the central nervous system (CNS) and are responsible for the homeostasis and immune defense of the CNS. Under the influence of the local environment and cell-cell interaction, microglia exhibit a multidimensional and context-dependent phenotypes that can be cytotoxic and neuroprotective. Recent studies suggest that microglia express multitudinous types of lectins, including galectins, Siglecs, mannose-binding lectins (MBLs) and other glycan binding proteins. Because most studies that examine lectins focus on the peripheral system, the functions of lectins have not been critically investigated in the CNS. In addition, the types of brain cells that contribute to the altered levels of lectins present in diseases are often unclear. In this review, we will discuss how galectins, Siglecs, selectins and MBLs contribute to the dynamic functions of microglia. The interacting ligands of these lectins are complex glycoconjugates, which consist of glycoproteins and glycolipids that are expressed on microglia or surrounding cells. The current understanding of the heterogeneity and functions of glycans in the brain is limited. Galectins are a group of pleotropic proteins that recognize both β-galactoside-containing glycans and non- β-galactoside-containing proteins. The function and regulation of galectins have been implicated in immunomodulation, neuroinflammation, apoptosis, phagocytosis and oxidative bursts. Most Siglecs are expressed at a low level on the plasma membrane and bind to sialic acid residues for immunosurveillance and cell-cell communication. Siglecs are classified based on their inhibitory and activatory downstream signaling properties. Inhibitory Siglecs negatively regulate microglia activation upon recognizing the intact sialic acid patterns and vice versa. MBLs are expressed upon infection in cytoplasm and can be secreted in order to recognize molecules containing terminal mannose as an innate immune defense machinery. Most importantly, multiple studies have reported dysregulation of lectins in neurological disorders. Here, we reviewed recent studies on microglial lectins and their functions in CNS health and disease, and suggest that these lectin families are novel, potent therapeutic targets for neurological diseases.

Microglia: Senescence Impairs Clearance of Myelin Debris

Growing evidence highlights the crucial physiological functions of microglia that rely on their phagocytic activities, which can be compromised with age. A new study reports the impaired clearance of myelin debris by microglia in the brain, leading to insoluble lysosomal inclusions and contributing to the immune dysfunction and senescence of these cells.

Neuropharmacological characterization of frutalin in mice: Evidence of an antidepressant-like effect mediated by the NMDA receptor/NO/cGMP pathway

In this study we evaluated the effect of frutalin (FTL) on mouse behavior. Mice (n=6/group) were treated (i.p.) with FTL (0.25; 0.5 or 1mg/kg) or vehicle and submitted to several tests (hole-board/HBT, elevated plus maze/PMT, open field/OFT, tail suspension/TST, or forced swimming/FST). Yohimbine, ketamine, l-NAME, aminoguanidine, 7-NI, methylene blue, l-arginine or dl-serine was administered 30min before FTL (0.5mg/kg). To evaluate the subchronic effect, animals were injected with FTL or vehicle for 7days and submitted to the FST. Molecular docking was simulated using FTL against NOS and the NMDA receptor. No changes were observed in the HBT or the OFT. FTL (0.25mg/kg) increased the number of entries into enclosed arms in the PMT. FTL reduced immobility in the TST (0.25 and 0.5mg/kg) and the FST (0.25mg/kg; 0.5mg/kg). The effect of FTL was dependent on carbohydrate interaction and protein structure integrity and was reduced by ketamine, l-NAME, aminoguanidine, 7-NI and methylene blue, but not by l-arginine, yohimbine or dl-serine. The antidepressant-like effect remained after subchronic treatment. The molecular docking study revealed a strong interaction between FTL and NOS and NMDA. FTL was found to have an antidepressant-like effect mediated by the NMDA receptor/NO/cGMP pathway.

Galectin-3: mediator of microglia responses in injured brain

Galectin-3 is a pleiotropic protein involved in cell activation, proliferation and migration and plays a pivotal part as an inflammatory mediator in neurodegeneration. Galectin-3 is associated with microglial activation and proliferation after ischemia. Given its putative role as a dynamic fine-tuner of microglia, activation of Galectin-3 provides molecular cues in design of new immunomodulatory strategies for stroke management. This review summarizes recent evidence on the role of Galectin-3 as a mediator of immune responses in damaged brain and mechanisms employed by Galectin-3 to affect microglial function.

A novel flavanone derivative ameliorates cuprizone-induced behavioral changes and white matter pathology in the brain of mice

Recent studies have shown that white matter lesions play an important role in the pathogenesis of schizophrenia. DHF-6 is a novel flavanone derivative synthesized in our laboratory. The purpose of the present study was to investigate the effects of DHF-6 on behavioral changes and white matter pathology in a 0.2% cuprizone-fed C57BL/6 mice model. The results showed that cuprizone induced a decrease in spontaneous alternations in the Y-maze test, an increase in locomotor activity in the open field test, demyelination determined by electron microscopy, a decline in the expression of myelin basic protein (MBP), a decrease in the differentiation of oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes (OLs), and an activation of microglia and astrocytes in the corpus callosum measured by western blot and/or immunocytochemical analyses. Intragastric administration of DHF-6 (25 and 50mg/kg) for 5-weeks increased the spontaneous alternations, reduced locomotor activity, reversed demyelination and MBP decrease, promoted OPCs differentiation into mature OLs, and inhibited the activation of microglia and astrocytes. These results suggest that DHF-6 may improve cognitive impairment and the positive symptoms of schizophrenia by alleviating white matter lesions via facilitating remyelination and inhibiting neuroinflammation, thus may be beneficial in the treatment of schizophrenia.

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies.

Toll-Like Receptor 4 (TLR4) and Triggering Receptor Expressed on Myeloid Cells-2 (TREM-2) Activation Balance Astrocyte Polarization into a Proinflammatory Phenotype

Astrocytes react to brain injury with a generic response known as reactive gliosis, which involves activation of multiple intracellular pathways including several that may be beneficial for neuronal survival. However, by unknown mechanisms, reactive astrocytes can polarize into a proinflammatory phenotype that induces neurodegeneration. In order to study reactive gliosis and astroglial polarization into a proinflammatory phenotype, we used cortical devascularization-induced brain ischemia in Wistar rats and primary astroglial cell cultures exposed to oxygen-glucose deprivation (OGD). We analyzed the profile of TLR4 expression and the consequences of its activation by gain- and loss-of-function studies, and the effects produced by the activation of triggering receptor expressed on myeloid cells-2 (TREM-2), a negative regulator of TLR4 signaling. Both OGD exposure on primary astroglial cell cultures and cortical devascularization brain ischemia in rats induced TLR4 expression in astrocytes. In vivo, astroglial TLR4 expression was specifically observed in the ischemic penumbra surrounding necrotic core. Functional studies showed that OGD increased the astroglial response to the TLR4 agonist lipopolysaccharide (LPS), and conversely, TLR4 knockout primary astrocytes had impaired nuclear factor kappa-B (NF-κB) activation when exposed to LPS. In gain-of-function studies, plasmid-mediated TLR4 over-expression exacerbated astroglial response to LPS as shown by sustained NF-κB activation and increased expression of proinflammatory cytokines IL-1β and TNFα. TREM-2 expression, although present in naïve primary astrocytes, was induced by OGD, LPS, or high-mobility group box 1 protein (HMGB-1) exposure. TREM-2 activation by antibody cross-linking or the overexpression of TREM-2 intracellular adaptor, DAP12, partially suppressed LPS-induced NF-κB activation in purified astrocytic cultures. In vivo, TREM-2 expression was observed in macrophages and astrocytes located in the ischemic penumbra. While TREM-2+ macrophages were abundant at 3 days post-lesion (DPL) in the ischemic core, TREM-2+ astrocytes persisted in the penumbra until 14DPL. This study demonstrates that TLR4 expression increases astroglial sensitivity to ligands facilitating astrocyte conversion towards a proinflammatory phenotype, and that astroglial TREM-2 modulates this response reducing the downstream NF-κB activation. Therefore, the availability of TLR4 and TREM-2 ligands in the ischemic environment may control proinflammatory astroglial conversion to the neurodegenerative phenotype.

Microglia activation and phagocytosis: relationship with aging and cognitive impairment in the rhesus monkey

While cognitive decline is observed in the normal aging monkey, neurons are not lost with age. Instead, frontal white matter is lost as myelin degenerates and both correlate with age-related cognitive decline. As age-related myelin damage increases, there should be an increase in clearance of damaged myelin by microglial phagocytosis. In this study, brains of behaviorally tested rhesus monkeys were assessed using unbiased stereology to quantify the density of activated microglia (LN3 antibody positive) and phagocytic microglia (galectin-3 (Gal-3) antibody positive) in three white matter regions: the corpus callosum, cingulum bundle (CGB), and frontal white matter (FWM). LN3 cell density was significantly increased in the CGB, whereas Gal-3 cell density was significantly increased in all regions. Increases in Gal-3 cell density in the FWM were associated with cognitive impairment. In the FWM of old animals, Gal-3-positive microglia were classified by morphological subtype as ramified, hypertrophic, or amoeboid. The densities of hypertrophic and amoeboid microglia significantly correlated with cognitive impairment. Finally, microglia were double-labeled with LN3 and Gal-3 showing that 91% of Gal-3 cells were also LN3 positive, thus expressing an "activated" phenotype. Furthermore, 15% of all double-labeled cells formed phagocytic cups. Overall, these results suggest that microglia become activated in white matter with age where the majority express a phagocytic phenotype. We hypothesize that age-related phagocytic activation of microglia is a response to accumulating myelin pathology. The association of Gal-3 in the FWM with cognitive impairment may reflect regional differences in damage or dysfunction of normal clearance mechanisms.

Galectin-1 circumvents lysolecithin-induced demyelination through the modulation of microglial polarization/phagocytosis and oligodendroglial differentiation

Galectin-1 (Gal-1), a member of a highly conserved family of animal lectins, binds to the common disaccharide [Galβ(1-4)-GlcNAc] on both N- and O-glycans decorating cell surface glycoconjugates. Current evidence supports a role for Gal-1 in the pathophysiology of multiple sclerosis (MS), one of the most prevalent chronic inflammatory diseases. Previous studies showed that Gal-1 exerts neuroprotective effects by promoting microglial deactivation in a model of autoimmune neuroinflammation and induces axonal regeneration in spinal cord injury. Seeking a model that could link demyelination, oligodendrocyte (OLG) responses and microglial activation, here we used a lysolecithin (LPC)-induced demyelination model to evaluate the ability of Gal-1 to preserve myelin without taking part in T-cell modulation. Gal-1 treatment after LPC-induced demyelination promoted a significant decrease in the demyelinated area and fostered more efficient remyelination, concomitantly with an attenuated oligodendroglial progenitor response reflecting less severe myelination damage. These results were accompanied by a decrease in the area of microglial activation with a shift toward an M2-polarized microglial phenotype and diminished astroglial activation. In vitro studies further showed that, mechanistically, Gal-1 targets activated microglia, promoting an increase in their myelin phagocytic capacity and their shift toward an M2 phenotype, and leads to oligodendroglial differentiation. Therefore, this study supports the use of Gal-1 as a potential treatment for demyelinating diseases such as MS.

Cuprizone demyelination induces a unique inflammatory response in the subventricular zone

Background

Cuprizone leads to demyelination of the corpus callosum (CC) and activates progenitor cells in the adjacent subventricular zone (SVZ), a stem cell niche which contributes to remyelination. The healthy SVZ contains semi-activated microglia and constitutively expresses the pro-inflammatory molecule galectin-3 (Gal-3) suggesting the niche uniquely regulates inflammation.

Methods

We studied the inflammatory response to cuprizone in the SVZ and CC in Gal-3 knockout mice using immunohistochemistry and with the in vitro neurosphere assay.

Results

Cuprizone caused loss of myelin basic protein (MBP) immunofluorescence in the CC suggesting demyelination. Cuprizone increased the density of CD45+/Iba1+ microglial cells and also increased Gal-3 expression in the CC. Surprisingly, the number of Gal-3+ and CD45+ cells decreased in the SVZ after cuprizone, suggesting inflammation was selectively reduced therein. Inflammation can regulate SVZ proliferation and indeed the number of phosphohistone H3+ (PHi3+) cells decreased in the SVZ but increased in the CC in both genotypes after cuprizone treatment. BrdU+ SVZ cell numbers also decreased in the SVZ after cuprizone, and this effect was significantly greater at 3 weeks in Gal-3^−/− mice compared to WT, suggesting Gal-3 normally limits SVZ cell emigration following cuprizone treatment.

Conclusions

This study reveals a uniquely regulated inflammatory response in the SVZ and shows that Gal-3 participates in remyelination in the cuprizone model. This contrasts with more severe models of demyelination which induce SVZ inflammation and suggests the extent of demyelination affects the SVZ neurogenic response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0651-2) contains supplementary material, which is available to authorized users.

Oligodendrocyte, Astrocyte, and Microglia Crosstalk in Myelin Development, Damage, and Repair

Oligodendrocytes are the myelinating glia of the central nervous system. Myelination of axons allows rapid saltatory conduction of nerve impulses and contributes to axonal integrity. Devastating neurological deficits caused by demyelinating diseases, such as multiple sclerosis, illustrate well the importance of the process. In this review, we focus on the positive and negative interactions between oligodendrocytes, astrocytes, and microglia during developmental myelination and remyelination. Even though many lines of evidence support a crucial role for glia crosstalk during these processes, the nature of such interactions is often neglected when designing therapeutics for repair of demyelinated lesions. Understanding the cellular and molecular mechanisms underlying glial cell communication and how they influence oligodendrocyte differentiation and myelination is fundamental to uncover novel therapeutic strategies for myelin repair.

Age-related myelin degradation burdens the clearance function of microglia during aging

Myelin is synthesized as a multilamellar membrane, but the mechanisms of membrane turnover are unknown. We found that myelin pieces were gradually released from aging myelin sheaths and were subsequently cleared by microglia. Myelin fragmentation increased with age and led to the formation of insoluble, lipofuscin-like lysosomal inclusions in microglia. Thus, age-related myelin fragmentation is substantial, leading to lysosomal storage and contributing to microglial senescence and immune dysfunction in aging.