GM1 induces p38 and microtubule dependent ramification of rat primary microglia in vitro

Downregulation of Fidgetin-Like 2 Increases Microglial Function: The Relationship Between Microtubules, Morphology, and Activity

The microtubule cytoskeleton regulates microglial morphology, motility, and effector functions. The microtubule-severing enzyme, fidgetin-like 2 (FL2), negatively regulates cell motility and nerve regeneration, making it a promising therapeutic target for central nervous system injury. Microglia perform important functions in response to inflammation and injury, but how FL2 affects microglia is unclear. In this study, we investigated the role of FL2 in microglial morphology and injury responses in vitro. We first determined that the pro-inflammatory stimulus, lipopolysaccharide (LPS), induced a dose- and time-dependent reduction in FL2 expression associated with reduced microglial ramification. We then administered nanoparticle-encapuslated FL2 siRNA to knockdown FL2 and assess microglial functions compared to negative control siRNA and vehicle controls. Time-lapse live-cell microscopy showed that FL2 knockdown increased the velocity of microglial motility. After incubation with fluorescently labeled IgG-opsonized beads, FL2 knockdown increased phagocytosis. Microglia were exposed to low-dose LPS after nanoparticle treatment to model injury-induced cytokine secretion. FL2 knockdown enhanced LPS-induced cytokine secretion of IL-1α, IL-1β, and TNFα. These results identify FL2 as a regulator of microglial morphology and suggest that FL2 can be targeted to increase or accelerate microglial injury responses.

Early-life stress lastingly impacts microglial transcriptome and function under basal and immune-challenged conditions

Early-life stress (ELS) leads to increased vulnerability to psychiatric disorders including depression later in life. Neuroinflammatory processes have been implicated in ELS-induced negative health outcomes, but how ELS impacts microglia, the main tissue-resident macrophages of the central nervous system, is unknown. Here, we determined the effects of ELS-induced by limited bedding and nesting material during the first week of life (postnatal days [P]2-9) on microglial (i) morphology; (ii) hippocampal gene expression; and (iii) synaptosome phagocytic capacity in male pups (P9) and adult (P200) mice. The hippocampus of ELS-exposed adult mice displayed altered proportions of morphological subtypes of microglia, as well as microglial transcriptomic changes related to the tumor necrosis factor response and protein ubiquitination. ELS exposure leads to distinct gene expression profiles during microglial development from P9 to P200 and in response to an LPS challenge at P200. Functionally, synaptosomes from ELS-exposed mice were phagocytosed less by age-matched microglia. At P200, but not P9, ELS microglia showed reduced synaptosome phagocytic capacity when compared to control microglia. Lastly, we confirmed the ELS-induced increased expression of the phagocytosis-related gene GAS6 that we observed in mice, in the dentate gyrus of individuals with a history of child abuse using in situ hybridization. These findings reveal persistent effects of ELS on microglial function and suggest that altered microglial phagocytic capacity is a key contributor to ELS-induced phenotypes.

Anti-inflammatory role of GM1 and other gangliosides on microglia

Background

Gangliosides are glycosphingolipids highly enriched in the brain, with important roles in cell signaling, cell-to-cell communication, and immunomodulation. Genetic defects in the ganglioside biosynthetic pathway result in severe neurodegenerative diseases, while a partial decrease in the levels of specific gangliosides was reported in Parkinson’s disease and Huntington’s disease. In models of both diseases and other conditions, administration of GM1—one of the most abundant gangliosides in the brain—provides neuroprotection. Most studies have focused on the direct neuroprotective effects of gangliosides on neurons, but their role in other brain cells, in particular microglia, is not known. In this study we investigated the effects of exogenous ganglioside administration and modulation of endogenous ganglioside levels on the response of microglia to inflammatory stimuli, which often contributes to initiation or exacerbation of neurodegeneration.

Methods

In vitro studies were performed using BV2 cells, mouse, rat, and human primary microglia cultures. Modulation of microglial ganglioside levels was achieved by administration of exogenous gangliosides, or by treatment with GENZ-123346 and L–t-PDMP, an inhibitor and an activator of glycolipid biosynthesis, respectively. Response of microglia to inflammatory stimuli (LPS, IL-1β, phagocytosis of latex beads) was measured by analysis of gene expression and/or secretion of pro-inflammatory cytokines. The effects of GM1 administration on microglia activation were also assessed in vivo in C57Bl/6 mice, following intraperitoneal injection of LPS.

Results

GM1 decreased inflammatory microglia responses in vitro and in vivo, even when administered after microglia activation. These anti-inflammatory effects depended on the presence of the sialic acid residue in the GM1 glycan headgroup and the presence of a lipid tail. Other gangliosides shared similar anti-inflammatory effects in in vitro models, including GD3, GD1a, GD1b, and GT1b. Conversely, GM3 and GQ1b displayed pro-inflammatory activity. The anti-inflammatory effects of GM1 and other gangliosides were partially reproduced by increasing endogenous ganglioside levels with L–t-PDMP, whereas inhibition of glycolipid biosynthesis exacerbated microglial activation in response to LPS stimulation.

Conclusions

Our data suggest that gangliosides are important modulators of microglia inflammatory responses and reveal that administration of GM1 and other complex gangliosides exerts anti-inflammatory effects on microglia that could be exploited therapeutically.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02374-x.

Platinum nanoparticles Protect Against Lipopolysaccharide-Induced Inflammation in Microglial BV-2 Cells via Decreased Oxidative Damage and Increased Phagocytosis

Neuroinflammation and oxidative stress cooperate to compromise the function of the central nervous system (CNS). Colloidal platinum nanoparticles (Pt NPs) are ideal candidates for reducing the deleterious effects of neuroinflammation since they act as free radical scavengers. Here we evaluated the effects of Pt NPs on several markers of lipopolysaccharide (LPS)-induced inflammation in cultured BV-2 microglial cells. BV-2 cells were treated with increased dilutions (1-100 ppm) of Colloidal Pt and/or LPS (1-10 µg/mL) at different exposure times. Three different protocols of exposure were used combining Pt NPs and LPS: (a) conditioning-protective effect (pre-post-treat), (b) therapeutic effect (co-treat) and (c) conditioning-therapeutic effect (pre-co-treat). After exposure to LPS for 24 h, cells were used for assessment of cell viability, reactive oxygen species (ROS) generation, lactate dehydrogenase (LDH) activity, apoptosis and caspase-3 levels, cell proliferation, mitochondrial membrane potential, inducible nitric oxide (iNOS) activity, pro-inflammatory cytokine (IL-1β, TNF-α and IL-6) levels, and phagocytic activity. Low concentrations (below or equal to 10 ppm) of Colloidal Pt prevented or ameliorated the LPS-induced increase in ROS formation, loss of mitochondrial membrane potential, induction of apoptosis, increase in LDH release, increase in pro-inflammatory cytokines and iNOS, inhibition of phagocytosis linked to microglial persistence in the M1 phase phenotype, loss of cell adhesion, differentiation and/or proliferation, as well as loss of cell viability. These protective effects were evident when cells were preconditioned with Pt NPs prior to LPS treatment. Collectively, the findings demonstrate that at low concentrations, Pt NPs can regulate the function and phenotype of BV-2 cells, activating protective mechanisms to maintain the microglial homeostasis and reduce inflammatory events triggered by the inflammatory insults induced by LPS. These preventive/protective effects on the LPS pro-inflammatory model are linked to the antioxidant properties and phagocytic activity of these NPs.

Regulation of microglial process elongation, a featured characteristic of microglial plasticity

Microglia, a type of glia within the brain characterized by a ramified morphology, are essential for removing neuronal debris and restricting the expansion of a lesion site. Upon moderate activation, they undergo a transformation in morphology inducing beneficial responses. However, upon strong stimulation, they mediate neuronal damage via production of pro-inflammatory cytokines. The inhibition of this cascade is considered an effective strategy for neuroinflammation-associated disorder therapy. During this pathological activation microglia also undergo a shortening of process length which contributes to the pathogenesis of such disorders. Thus, microglial plasticity should be considered to have two components: one is the production of inflammatory mediators, and the other is the dynamic changes in their processes. The former role has been well-documented in previous studies, while the latter one remains largely unknown. Recently, we and others have reported that the elongation of microglial process is associated with the transformation of microglia from a pro-inflammatory to an anti-inflammatory state, suggesting that the shortening of process length would make the microglia lose their ability to restrict pathological injury, while the elongation of microglial process would help attenuate neuroinflammation. Compared with the traditional anti-neuroinflammatory strategy, stimulating elongation of microglial process not only reduces the production of pro-inflammatory cytokines, but restores the ability of microglia to scan their surrounding environments, thus rendering their homeostasis regulation more effective. In this review, we provide a discussion of the factors that regulate microglial process elongation in vitro and in vivo, aiming to further drive the understanding of microglial process plasticity.

The ketone body metabolite β-hydroxybutyrate induces an antidepression-associated ramification of microglia via HDACs inhibition-triggered Akt-small RhoGTPase activation

Direct induction of macrophage ramification has been shown to promote an alternative (M2) polarization, suggesting that the ramified morphology may determine the function of immune cells. The ketone body metabolite β-hydroxybutyrate (BHB) elevated in conditions including fasting and low-carbohydrate ketogenic diet (KD) can reduce neuroinflammation. However, how exactly BHB impacts microglia remains unclear. We report that BHB as well as its producing stimuli fasting and KD induced obvious ramifications of murine microglia in basal and inflammatory conditions in a reversible manner, and these ramifications were accompanied with microglial profile toward M2 polarization and phagocytosis. The protein kinase B (Akt)-small RhoGTPase axis was found to mediate the effect of BHB on microglial shape change, as (i) BHB activated the microglial small RhoGTPase (Rac1, Cdc42) and Akt; (ii) Akt and Rac1-Cdc42 inhibition abolished the pro-ramification effect of BHB; (iii) Akt inhibition prevented the activation of Rac1-Cdc42 induced by BHB treatment. Incubation of microglia with other classical histone deacetylases (HDACs) inhibitors, but not G protein-coupled receptor 109a (GPR109a) activators, also induced microglial ramification and Akt activation, suggesting that the BHB-induced ramification of microglia may be triggered by HDACs inhibition. Functionally, Akt inhibition was found to abrogate the effects of BHB on microglial polarization and phagocytosis. In neuroinflammatory models induced by lipopolysaccharide (LPS) or chronic unpredictable stress (CUS), BHB prevented the microglial process retraction and depressive-like behaviors, and these effects were abolished by Akt inhibition. Our findings for the first time showed that BHB exerts anti-inflammatory actions via promotion of microglial ramification.

Brain inflammation and microglia: facts and misconceptions

THE INFLAMMATION THAT ACCOMPANIES ACUTE INJURY HAS DUAL FUNCTIONS: bactericidal action and repair. Bactericidal functions protect damaged tissue from infection, and repair functions are initiated to aid in the recovery of damaged tissue. Brain injury is somewhat different from injuries in other tissues in two respects. First, many cases of brain injury are not accompanied by infection: there is no chance of pathogens to enter in ischemia or even in traumatic injury if the skull is intact. Second, neurons are rarely regenerated once damaged. This raises the question of whether bactericidal inflammation really occurs in the injured brain; if so, how is this type of inflammation controlled? Many brain inflammation studies have been conducted using cultured microglia (brain macrophages). Even where animal models have been used, the behavior of microglia and neurons has typically been analyzed at or after the time of neuronal death, a time window that excludes the inflammatory response, which begins immediately after the injury. Therefore, to understand the patterns and roles of brain inflammation in the injured brain, it is necessary to analyze the behavior of all cell types in the injured brain immediately after the onset of injury. Based on our experience with both in vitro and in vivo experimental models of brain inflammation, we concluded that not only microglia, but also astrocytes, blood inflammatory cells, and even neurons participate and/or regulate brain inflammation in the injured brain. Furthermore, brain inflammation played by these cells protects neurons and repairs damaged microenvironment but not induces neuronal damage.

Chronic stress induced remodeling of the prefrontal cortex: structural re-organization of microglia and the inhibitory effect of minocycline

Recently, it has been discovered that the working memory deficits induced by exposure to chronic stress can be prevented by treating stressed animals with minocycline, a putative inhibitor of microglial activity. One of the pressing issues that now requires clarification is exactly how exposure to chronic stress modifies microglial morphology, this being a significant issue as microglial morphology is tightly coupled with their function. To examine how chronic stress alters microglial morphology, we digitally reconstructed microglia within the rat medial prefrontal cortex. Our analysis revealed that stress increased the internal complexity of microglia, enhancing ramification (i.e. branching) without altering the overall area occupied by the cell and that this effect was more pronounced in larger cells. We subsequently determined that minocycline treatment largely abolished the pro-ramifying effects of stress. With respect to mechanisms, we could not find any evidence of increased inflammation or neurodegeneration (interleukin-1β, MHC-II, CD68, terminal deoxynucleotidyl transferase dUTP nick end labeling, and activated caspase-3). We did, however, find that chronic stress markedly increased the expression of β1-integrin (CD29), a protein previously implicated in microglial ramification. Together, these findings highlight that increased ramification of microglia may represent an important neurobiological mechanism through which microglia mediate the behavioral effects of chronic psychological stress.

Cerebellar alterations and gait defects as therapeutic outcome measures for enzyme replacement therapy in α-mannosidosis

α-Mannosidosis is a rare lysosomal storage disease with accumulation of undegraded mannosyl-linked oligosaccharides in cells throughout the body, most notably in the CNS. This leads to a broad spectrum of neurological manifestations, including progressive intellectual impairment, disturbed motor functions, and cerebellar atrophy. To develop therapeutic outcome measures for enzyme replacement therapy that could be used for human patients, a gene knockout model of α-mannosidosis in mice was analyzed for CNS pathology and motor deficits. In the cerebellar molecular layer, α-mannosidosis mice display clusters of activated Bergman glia, infiltration of phagocytic macrophages, and accumulation of free cholesterol and gangliosides (GM1), notably in regions lacking Purkinje cells. α-Mannosidosis brain lysates also displayed increased expression of Lamp1 and hyperglycosylation of the cholesterol binding protein NPC2. Detailed assessment of motor function revealed age-dependent gait defects in the mice that resemble the disturbed motor function in human patients. Short-term enzyme replacement therapy partially reversed the observed cerebellar pathology with fewer activated macrophages and astrocytes but unchanged levels of hyperglycosylated NPC2, gangliosides, and cholesterol. The present study demonstrates cerebellar alterations in α-mannosidosis mice that relate to the motor deficits and pathological changes seen in human patients and can be used as therapeutic outcome measures.

The influences of sphingolipid metabolites on gentamicin-induced hair cell loss of the rat cochlea

Sphingolipid metabolites inducing ceramide, sphingosine, and sphingosine-1-phosphate (S1P) play important roles in the regulation of cell proliferation, survival, and death. Aminoglycoside antibiotics including gentamicin induce inner ear hair cell loss and sensorineural hearing loss. Apoptotic cell death is considered to play a key role in this injury. The present study was designed to investigate the possible involvement of ceramide and S1P in hair cell death due to gentamicin. In addition, the effects of other metabolites of ceramide, gangliosides GM1 (GM1) and GM3 (GM3), on gentamicin ototoxicity were also investigated. Basal turn organ of Corti explants from p3 to p5 rats were maintained in tissue culture and exposed to 20 or 35μM gentamicin for 48h. The effects of ceramide, S1P, GM1, and GM3 on gentamicin-induced hair cell loss were examined. Gentamicin-induced hair cell loss was increased by ceramide but was decreased by S1P. GM1 and GM3 exhibited protective effects against gentamicin-induced hair cell death at the limited concentrations. These results indicate that ceramide enhances gentamicin ototoxicity by promoting apoptotic hair cell death, and that S1P, GM1, and GM3 act as cochlear protectants. In conclusion, sphingolipid metabolites influence the apoptotic reaction of hair cells to gentamicin ototoxicity.

CD14 and toll-like receptors 2 and 4 are required for fibrillar A{beta}-stimulated microglial activation

Microglia are the brain's tissue macrophages and are found in an activated state surrounding beta-amyloid plaques in the Alzheimer's disease brain. Microglia interact with fibrillar beta-amyloid (fAbeta) through an ensemble of surface receptors composed of the alpha(6)beta(1) integrin, CD36, CD47, and the class A scavenger receptor. These receptors act in concert to initiate intracellular signaling cascades and phenotypic activation of these cells. However, it is unclear how engagement of this receptor complex is linked to the induction of an activated microglial phenotype. We report that the response of microglial cells to fibrillar forms of Abeta requires the participation of Toll-like receptors (TLRs) and the coreceptor CD14. The response of microglia to fAbeta is reliant upon CD14, which act together with TLR4 and TLR2 to bind fAbeta and to activate intracellular signaling. We find that cells lacking these receptors could not initiate a Src-Vav-Rac signaling cascade leading to reactive oxygen species production and phagocytosis. The fAbeta-mediated activation of p38 MAPK also required CD14, TLR4, and TLR2. Inhibition of p38 abrogated fAbeta-induced reactive oxygen species production and attenuated the induction of phagocytosis. Microglia lacking CD14, TLR4, and TLR2 showed no induction of phosphorylated IkappaBalpha following fAbeta. These data indicate these innate immune receptors function as members of the microglial fAbeta receptor complex and identify the signaling mechanisms whereby they contribute to microglial activation.

Gangliosides are important for the preservation of the structure and organization of RBL-2H3 mast cells

Gangliosides are known to be important in many biological processes. However, details concerning the exact function of these glycosphingolipids in cell physiology are poorly understood. In this study, the role of gangliosides present on the surface of rodent mast cells in maintaining cell structure was examined using RBL-2H3 mast cells and two mutant cell lines (E5 and D1) deficient in the gangliosides, GM(1) and the alpha-galactosyl derivatives of the ganglioside GD(1b). The two deficient cell lines were morphologically different from each other as well as from the parental RBL-2H3 cells. Actin filaments in RBL-2H3 and E5 cells were under the plasma membrane following the spindle shape of the cells, whereas in D1 cells, they were concentrated in large membrane ruffles. Microtubules in RBL-2H3 and E5 cells radiated from the centrosome and were organized into long, straight bundles. The bundles in D1 cells were thicker and organized circumferentially under the plasma membrane. The endoplasmic reticulum, the Golgi complex, and the secretory granule matrix were also altered in the mutant cell lines. These results suggest that the mast cell-specific alpha-galactosyl derivatives of ganglioside GD(1b) and GM(1) are important in maintaining normal cell morphology.