Lysophosphatidic acid receptor activation affects the C13NJ microglia cell line proteome leading to alterations in glycolysis, motility, and cytoskeletal architecture

Complement C3aR signaling: Immune and metabolic modulation and its impact on Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia among the elderly population. Despite its widespread prevalence, our comprehension of the intricate mechanisms governing the pathogenesis of the disease remains incomplete, posing a challenge for the development of efficient therapies. Pathologically characterized by the presence of amyloid β plaques and neurofibrillary tau tangles, AD is also accompanied by the hyperactivation of glial cells and the immune system. The complement cascade, the evolutionarily conserved innate immune pathway, has emerged as a significant contributor to AD. This review focuses on one of the complement components, the C3a receptor (C3aR), covering its structure, ligand-receptor interaction, intracellular signaling and its functional consequences. Drawing insights from cellular and AD mouse model studies, we present the multifaceted role of complement C3aR signaling in AD and attempt to convey to the readers that C3aR acts as a crucial immune and metabolic modulator to influence AD pathogenesis. Building on this framework, the objective of this review is to inform future research endeavors and facilitate the development of therapeutic strategies for this challenging condition.

The Roles of RhoA/ROCK/NF-κB Pathway in Microglia Polarization Following Ischemic Stroke

Ischemic stroke is the leading cause of death and disability worldwide. Nevertheless, there still lacks the effective therapies for ischemic stroke. Microglia are resident macrophages of the central nervous system (CNS) and can initiate immune responses and monitor the microenvironment. Microglia are activated and polarize into proinflammatory or anti‑inflammatory phenotype in response to various brain injuries, including ischemic stroke. Proinflammatory microglia could generate immunomodulatory mediators, containing cytokines and chemokines, these mediators are closely associated with secondary brain damage following ischemic stroke. On the contrary, anti-inflammatory microglia facilitate recovery following stroke. Regulating the activation and the function of microglia is crucial in exploring the novel treatments for ischemic stroke patients. Accumulating studies have revealed that RhoA/ROCK pathway and NF-κB are famous modulators in the process of microglia activation and polarization. Inhibiting these key modulators can promote the polarization of microglia to anti-inflammatory phenotype. In this review, we aimed to provide a comprehensive overview on the role of RhoA/ROCK pathway and NF-κB in the microglia activation and polarization, reveal the relationship between RhoA/ROCK pathway and NF-κB in the pathological process of ischemic stroke. In addition, we likewise discussed the drug modulators targeting microglia polarization.

H2S-RhoA/ROCK Pathway and Glial Cells in Axonal Remyelination After Ischemic Stroke

Ischemic stroke is one of the main reasons of disability and death. Stroke-induced functional deficits are mainly due to the secondary degeneration of the white matter characterized by axonal demyelination and injury of axon-glial integrity. Enhancement of the axonal regeneration and remyelination could promote the neural functional recovery. However, cerebral ischemia-induced activation of RhoA/Rho kinase (ROCK) pathway plays a crucial and harmful role in the process of axonal recovery and regeneration. Inhibition of this pathway could promote the axonal regeneration and remyelination. In addition, hydrogen sulfide (H2S) has the significant neuroprotective role during the recovery of ischemic stroke via inhibiting the inflammatory response and oxidative stress, regulating astrocyte function, promoting the differentiation of endogenous oligodendrocyte precursor cells (OPCs) to mature oligodendrocyte. Among all of these effects, promoting the formation of mature oligodendrocyte is a crucial part of axonal regeneration and remyelination. Furthermore, numerous studies have uncovered the crosstalk between astrocytes and oligodendrocyte, microglial cells and oligodendrocyte in the axonal remyelination following ischemic stroke. The purpose of this review was to discuss the relationship among H2S, RhoA/ROCK pathway, astrocytes, and microglial cells in the axonal remyelination following ischemic stroke to reveal new strategies for preventing and treating this devastating disease.

Microglial hexokinase 2 deficiency increases ATP generation through lipid metabolism leading to β-amyloid clearance

Microglial cells consume adenosine triphosphate (ATP) during phagocytosis to clear neurotoxic β-amyloid in Alzheimer's disease (AD). However, the contribution of energy metabolism to microglial function in AD remains unclear. Here, we demonstrate that hexokinase 2 (HK2) is elevated in microglia from an AD mouse model (5xFAD) and AD patients. Genetic deletion or pharmacological inhibition of HK2 significantly promotes microglial phagocytosis, lowers the amyloid plaque burden and attenuates cognitive impairment in male AD mice. Notably, the ATP level is dramatically increased in HK2-deficient or inactive microglia, which can be attributed to a marked upregulation in lipoprotein lipase (LPL) expression and subsequent increase in lipid metabolism. We further show that two downstream metabolites of HK2, glucose-6-phosphate and fructose-6-phosphate, can reverse HK2-deficiency-induced upregulation of LPL, thus supporting ATP production and microglial phagocytosis. Our findings uncover a crucial role for HK2 in phagocytosis through regulation of microglial energy metabolism, suggesting a potential therapeutic strategy for AD by targeting HK2.

Salvianolic acid C improves cerebral ischemia reperfusion injury through suppressing microglial cell M1 polarization and promoting cerebral angiogenesis

This study aimed to investigate the mechanism of salvianolic acid C (SAC), the active ingredient in Salvia miltiorrhiza, in improving cerebral ischemia injury. The mouse microglial cells BV2 and mouse endothelial cells bEnd.3 were used as the objects of study. LPS/IFN-γ was applied to simulate the BV2 polarization, and bEnd.3 cells were treated under hypoxic condition. The BV2 cell polarization level was measured through flow cytometry (FCM), the TLR4 and MyD88 expression levels were detected by fluorescence staining, whereas the expression of inflammatory factors TNF-α, IL-6 and IL-1β was analyzed through ELISA. Tubule formation assay was also conducted to observe the tubule formation ability of bEnd.3 cells in vitro, and the level of VEGFR2 was detected by fluorescence staining. Cells were treated with the PKM2 inhibitor IN3, aiming to observe the influence of SAC on glycolysis of BV2 cells. In addition, the mouse model of cerebral ischemia was constructed through the middle cerebral artery occlusion (MCAO) method, and the pathological changes in brain tissues were detected after SAC intervention. Meanwhile, the levels of IBA-1, CD31 and ZO-1 were determined through histochemical staining. Nissl staining to detect nerve cell damage. In BV2 cell experiment, SAC suppressed the M1 polarization of BV2 cells, reduced the inflammatory factor levels, and inhibited the activation of TLR4 signal through suppressing glycolysis. When PKM2 was suppressed, the effects of SAC were antagonized. In the bEnd.3 model, SAC promoted tubule formation in bEnd.3 cells under hypoxic condition, and increased the expression of VEGFR2 and Notch1. In the mouse model, SAC improved the neurological function in MCAO mice, and inhibited the activation of microglial cells and the expression of inflammatory factors. At the same time, SAC up-regulated the expression of ZO-1 and CD31, and maintained the blood-brain barrier (BBB) function. As a major component of Salvia miltiorrhiza, SAC can suppress microglial cell polarization and promote tubule formation in endothelial cells to exert the neurological repair function in cerebral ischemia. SAC is a multi-functional neuroprotective small molecule.

Recent Advances in Microglia Modelling to Address Translational Outcomes in Neurodegenerative Diseases

Neurodegenerative diseases are deteriorating conditions of the nervous system that are rapidly increasing in the aging population. Increasing evidence suggests that neuroinflammation, largely mediated by microglia, the resident immune cells of the brain, contributes to the onset and progression of neurodegenerative diseases. Hence, microglia are considered a major therapeutic target that could potentially yield effective disease-modifying treatments for neurodegenerative diseases. Despite the interest in studying microglia as drug targets, the availability of cost-effective, flexible, and patient-specific microglia cellular models is limited. Importantly, the current model systems do not accurately recapitulate important pathological features or disease processes, leading to the failure of many therapeutic drugs. Here, we review the key roles of microglia in neurodegenerative diseases and provide an update on the current microglia platforms utilised in neurodegenerative diseases, with a focus on human microglia-like cells derived from peripheral blood mononuclear cells as well as human-induced pluripotent stem cells. The described microglial platforms can serve as tools for investigating disease biomarkers and improving the clinical translatability of the drug development process in neurodegenerative diseases.

Sex differences in the fecal microbiome and hippocampal glial morphology following diet and antibiotic treatment

Rising obesity rates have become a major public health concern within the United States. Understanding the systemic and neural effects of obesity is crucial in designing preventive and therapeutic measures. In previous studies, administration of a high fat diet has induced significant weight gain for mouse models of obesity. Interestingly, sex differences in high-fat diet-induced weight gain have been observed, with female mice gaining significantly less weight compared to male mice on the same high-fat diet. It has also been observed that consumption of a high-fat diet can increase neurogliosis, but the mechanism by which this occurs is still not fully understood. Recent research has suggested that the gut microbiome may mediate diet-induced glial activation. The current study aimed to (1) analyze changes to the gut microbiome following consumption of a high fat (HF) diet as well as antibiotic treatment, (2) evaluate hippocampal microgliosis and astrogliosis, and (3) identify sex differences within these responses. We administered a low fat (Research Diets D12450 K) or high fat diet (Research Diets D12451) to male and female C57Bl/6 mice for sixteen weeks. Mice received an antibiotic cocktail containing 0.5g/L of vancomycin, 1.0 g/L ampicillin, 1.0 g/L neomycin, and 1.0 g/L metronidazole in their drinking water during the last six weeks of the study and were compared to control mice receiving normal drinking water throughout the study. We observed a significant reduction in gut microbiome diversity for groups that received the antibiotic cocktail, as determined by Illumina next-generation sequencing. Male mice fed the HF diet (± antibiotics) had significantly greater body weights compared to all other groups. And, female mice fed the low fat (LF) diet and administered antibiotics revealed significantly decreased microgliosis and astrogliosis in the hippocampus compared to LF-fed females without antibiotics. Interestingly, male mice fed the LF diet and administered antibiotics revealed significantly increased microgliosis, but decreased astrogliosis, compared to LF-fed males without antibiotics. The observed sex differences in LF-fed mice given antibiotics brings forward questions about sex differences in nutrient metabolism, gut microbiome composition, and response to antibiotics.

In Vitro Methodologies to Study the Role of Advanced Glycation End Products (AGEs) in Neurodegeneration

Advanced glycation end products (AGEs) can be present in food or be endogenously produced in biological systems. Their formation has been associated with chronic neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis. The implication of AGEs in neurodegeneration is related to their ability to bind to AGE-specific receptors and the ability of their precursors to induce the so-called “dicarbonyl stress”, resulting in cross-linking and protein damage. However, the mode of action underlying their role in neurodegeneration remains unclear. While some research has been carried out in observational clinical studies, further in vitro studies may help elucidate these underlying modes of action. This review presents and discusses in vitro methodologies used in research on the potential role of AGEs in neuroinflammation and neurodegeneration. The overview reveals the main concepts linking AGEs to neurodegeneration, the current findings, and the available and advisable in vitro models to study their role. Moreover, the major questions regarding the role of AGEs in neurodegenerative diseases and the challenges and discrepancies in the research field are discussed.

Cytokine Signalling at the Microglial Penta-Partite Synapse

Microglial cell processes form part of a subset of synaptic contacts that have been dubbed microglial tetra-partite or quad-partite synapses. Since tetrapartite may also refer to the presence of extracellular matrix components, we propose the more precise term microglial penta-partite synapse for synapses that show a microglial cell process in close physical proximity to neuronal and astrocytic synaptic constituents. Microglial cells are now recognised as key players in central nervous system (CNS) synaptic changes. When synaptic plasticity involving microglial penta-partite synapses occurs, microglia may utilise their cytokine arsenal to facilitate the generation of new synapses, eliminate those that are not needed anymore, or modify the molecular and structural properties of the remaining synaptic contacts. In addition, microglia–synapse contacts may develop de novo under pathological conditions. Microglial penta-partite synapses have received comparatively little attention as unique sites in the CNS where microglial cells, cytokines and other factors they release have a direct influence on the connections between neurons and their function. It concerns our understanding of the penta-partite synapse where the confusion created by the term “neuroinflammation” is most counterproductive. The mere presence of activated microglia or the release of their cytokines may occur independent of inflammation, and penta-partite synapses are not usually active in a neuroimmunological sense. Clarification of these details is the main purpose of this review, specifically highlighting the relationship between microglia, synapses, and the cytokines that can be released by microglial cells in health and disease.

Biologically active lipids in the regulation of lymphangiogenesis in disease states

Lymphatic vessels have crucial roles in the regulation of interstitial fluids, immune surveillance, and the absorption of dietary fat in the intestine. Lymphatic function is also closely related to the pathogenesis of various disease states such as inflammation, lymphedema, endometriosis, liver dysfunction, and tumor metastasis. Lymphangiogenesis, the formation of new lymphatic vessels from pre-existing lymphatic vessels, is a critical determinant in the above conditions. Although the effect of growth factors on lymphangiogenesis is well-characterized, and biologically active lipids are known to affect smooth muscle contractility and vasoaction, there is accumulating evidence that biologically active lipids are also important inducers of growth factors and cytokines that regulate lymphangiogenesis. This review discusses recent advances in our understanding of biologically active lipids, including arachidonic acid metabolites, sphingosine 1-phosphate, and lysophosphatidic acid, as regulators of lymphangiogenesis, and the emerging importance of the lymphangiogenesis as a therapeutic target.

The role of the immune system in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder where the accumulation of amyloid plaques and the formation of tau tangles are the prominent pathological hallmarks. Increasing preclinical and clinical studies have revealed that different components of the immune system may act as important contributors to AD etiology and pathogenesis. The recognition of misfolded Aβ and tau by immune cells can trigger a series of complex immune responses in AD, and then lead to neuroinflammation and neurodegeneration. In parallel, genome-wide association studies have also identified several immune related loci associated with increased - risk of AD by interfering with the function of immune cells. Other immune related factors, such as impaired immunometabolism, defective meningeal lymphatic vessels and autoimmunity might also be involved in the pathogenesis of AD. Here, we review the data showing the alterations of immune cells in the AD trajectory and seek to demonstrate the crosstalk between the immune cell dysfunction and AD pathology. We then discuss the most relevant research findings in regards to the influences of gene susceptibility of immune cells for AD. We also consider impaired meningeal lymphatics, immunometabolism and autoimmune mechanisms in AD. In addition, immune related biomarkers and immunotherapies for AD are also mentioned in order to offer novel insights for future research.

The Roles of Lpar1 in Central Nervous System Disorders and Diseases

Lysophosphatidic acid receptor 1 (Lpar1), which is found in almost all human tissues but is most abundant in the brain, can couple to G protein-coupled receptors (GPCRs) and participate in regulating cell proliferation, migration, survival, and apoptosis. Endothelial differentiation gene-2 receptor (Edg2), the protein encoded by the Lpar1 gene, is present on various cell types in the central nervous system (CNS), such as neural stem cells (NSCs), oligodendrocytes, neurons, astrocytes, and microglia. Lpar1 deletion causes neurodevelopmental disorders and CNS diseases, such as brain cancer, neuropsychiatric disorders, demyelination diseases, and neuropathic pain. Here, we summarize the possible roles and mechanisms of Lpar1/Edg2 in CNS disorders and diseases and propose that Lpar1/Edg2 might be a potential therapeutic target for CNS disorders and diseases.

Critical Roles of Lysophospholipid Receptors in Activation of Neuroglia and Their Neuroinflammatory Responses

Activation of microglia and/or astrocytes often releases proinflammatory molecules as critical pathogenic mediators that can promote neuroinflammation and secondary brain damages in diverse diseases of the central nervous system (CNS). Therefore, controlling the activation of glial cells and their neuroinflammatory responses has been considered as a potential therapeutic strategy for treating neuroinflammatory diseases. Recently, receptor-mediated lysophospholipid signaling, sphingosine 1-phosphate (S1P) receptor- and lysophosphatidic acid (LPA) receptor-mediated signaling in particular, has drawn scientific interest because of its critical roles in pathogenies of diverse neurological diseases such as neuropathic pain, systemic sclerosis, spinal cord injury, multiple sclerosis, cerebral ischemia, traumatic brain injury, hypoxia, hydrocephalus, and neuropsychiatric disorders. Activation of microglia and/or astrocytes is a common pathogenic event shared by most of these CNS disorders, indicating that lysophospholipid receptors could influence glial activation. In fact, many studies have reported that several S1P and LPA receptors can influence glial activation during the pathogenesis of cerebral ischemia and multiple sclerosis. This review aims to provide a comprehensive framework about the roles of S1P and LPA receptors in the activation of microglia and/or astrocytes and their neuroinflammatory responses in CNS diseases.

Gintonin mitigates experimental autoimmune encephalomyelitis by stabilization of Nrf2 signaling via stimulation of lysophosphatidic acid receptors

Gintonin (GT), a glycolipoprotein fraction isolated from ginseng, exerts neuroprotective effects in models of neurodegenerative diseases such as Alzheimer's disease. However, the in vivo role of GT in multiple sclerosis (MS) has not been clearly resolved. We investigated the effect of GT in myelin oligodendrocyte glycoprotein (MOG_35-55)-induced experimental autoimmune encephalomyelitis (EAE), an animal model of MS. GT alleviated behavioral symptoms of EAE associated with reduced demyelination, diminished infiltration and activation of immune cells (microglia and macrophage), and decreased expression of inflammatory mediators in the spinal cord of the EAE group compared to that of the sham group. GT reduced the percentages of CD4⁺/IFN-γ⁺ (Th1) and CD4⁺/IL-17⁺ (Th17) cells but increased the population of CD4⁺/CD25⁺/Foxp3⁺ (Treg) cells in the spinal cord, in agreement with altered mRNA expression of IFN-γ, IL-17, and TGF-ß in the spinal cord in concordance with mitigated blood-brain barrier disruption. The underlying mechanism is related to inhibition of the ERK and p38 mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB) pathways and the stabilization of nuclear factor erythroid 2-related factor 2 (Nrf2) via increased expression of lysophosphatidic acid receptor (LPAR) 1-3. Impressively, these beneficial effects of GT were completely neutralized by inhibiting LPARs with Ki16425, a LPAR1/3 antagonist. Our results strongly suggest that GT may be able to alleviate EAE due to its anti-inflammatory and antioxidant activities through LPARs. Therefore, GT is a potential therapeutic option for treating autoimmune disorders including MS.

Lysophosphatidic Acid Induces Aerobic Glycolysis, Lipogenesis, and Increased Amino Acid Uptake in BV-2 Microglia

Lysophosphatidic acid (LPA) species are a family of bioactive lipids that transmit signals via six cognate G protein-coupled receptors, which are required for brain development and function of the nervous system. LPA affects the function of all cell types in the brain and can display beneficial or detrimental effects on microglia function. During earlier studies we reported that LPA treatment of microglia induces polarization towards a neurotoxic phenotype. In the present study we investigated whether these alterations are accompanied by the induction of a specific immunometabolic phenotype in LPA-treated BV-2 microglia. In response to LPA (1 µM) we observed slightly decreased mitochondrial respiration, increased lactate secretion and reduced ATP/ADP ratios indicating a switch towards aerobic glycolysis. Pathway analyses demonstrated induction of the Akt-mTOR-Hif1α axis under normoxic conditions. LPA treatment resulted in dephosphorylation of AMP-activated kinase, de-repression of acetyl-CoA-carboxylase and increased fatty acid content in the phospholipid and triacylglycerol fraction of BV-2 microglia lipid extracts, indicating de novo lipogenesis. LPA led to increased intracellular amino acid content at one or more time points. Finally, we observed LPA-dependent generation of reactive oxygen species (ROS), phosphorylation of nuclear factor erythroid 2–related factor 2 (Nrf2), upregulated protein expression of the Nrf2 target regulatory subunit of glutamate-cysteine ligase and increased glutathione synthesis. Our observations suggest that LPA, as a bioactive lipid, induces subtle alterations of the immunometabolic program in BV-2 microglia.

Activation of Macrophages by Lysophosphatidic Acid through the Lysophosphatidic Acid Receptor 1 as a Novel Mechanism in Multiple Sclerosis Pathogenesis

Multiple sclerosis (MS) is a neuroinflammatory disease whose pathogenesis remains unclear. Lysophosphatidic acid (LPA) is an endogenous phospholipid involved in multiple immune cell functions and dysregulated in MS. Its receptor LPA1 is expressed in macrophages and regulates their activation, which is of interest due to the role of macrophage activation in MS in both destruction and repair. In this study, we studied the genetic deletion and pharmaceutical inhibition of LPA1 in the mouse MS model, experimental autoimmune encephalomyelitis (EAE). LPA1 expression was analyzed in EAE mice and MS patient immune cells. The effect of LPA and LPA1 on macrophage activation was studied in human monocyte-derived macrophages. We show that lack of LPA1 activity induces milder clinical EAE course and that Lpar1 expression in peripheral blood mononuclear cells (PBMC) correlates with onset of relapses and severity in EAE. We see the same over-expression in PBMC from MS patients during relapse compared with progressive forms of the disease and in stimulated monocyte-derived macrophages. LPA induced a proinflammatory-like response in macrophages through LPA1, providing a plausible way in which LPA and LPA1 dysregulation can lead to the inflammation in MS. These data show a new mechanism of LPA signaling in the MS pathogenesis, prompting further research into its use as a therapeutic target biomarker.

Microglial Immunometabolism in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by amyloid-β (Aβ) plaques and the formation of neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. In response to Aβ and tau aggregates, microglia, the primary innate immune cells of the central nervous system (CNS), facilitate Aβ and tau clearance and contribute to neuroinflammation that damages neurons. Microglia also perform a wide range of other functions, e.g., synaptic pruning, within the CNS that require a large amount of energy. Glucose appears to be the primary energy source, but microglia can utilize several other substrates for energy production including other sugars and ketone bodies. Recent studies have demonstrated that changes in the metabolic profiles of immune cells, including macrophages, are important in controlling their activation and effector functions. Additional studies have focused on the role of metabolism in neuron and astrocyte function while until recently microglia metabolism has been considerably less well understood. Considering many neurological disorders, such as neurodegeneration associated with AD, are associated with chronic inflammation and alterations in brain energy metabolism, it is hypothesized that microglial metabolism plays a significant role in the inflammatory responses of microglia during neurodegeneration. Here, we review the role of microglial immunometabolism in AD.

An association between mitochondria and microglia effector function. What do we think we know?

While resident innate immune cells of the central nervous system, the microglia, represent a cell population unique in origin, microenvironment, and longevity, they assume many properties displayed by peripheral macrophages. One prominent shared property is the ability to undergo a metabolic switch towards glycolysis and away from oxidative phosphorylation (OXPHOS) upon activation by the pro-inflammatory stimuli lipopolysaccharide. This shift serves to meet specific cellular demands and allows for cell survival, similar to the Warburg effect demonstrated in cancer cells. In contrast, normal survelliance phenotype or stimulation to a non-proinflammatory phenotype relies primarily on OXPHOS and fatty acid oxidation. Thus, mitochondria appear to function as a pivotal signaling platform linking energy metabolism and macrophage polarization upon activation. These unique shifts in cell bioenergetics in response to different stimuli are essential for proper effector responses at sites of infection, inflammation, or injury. Here we present a summary of recent developments as to how these dynamics characterized in peripheral macrophages are displayed in microglia. The new insights provided by an increased understanding of metabolic reprogramming in macrophages may allow for translation to the CNS and a better understanding of microglia heterogeneity, regulation, and function.

MAPK signaling determines lysophosphatidic acid (LPA)-induced inflammation in microglia

Background

In the extracellular environment, lysophosphatidic acid (LPA) species are generated via autotaxin (ATX)-mediated hydrolysis of lysophospholipid precursors. Members of the LPA family are potent lipid mediators transmitting signals via six different G protein-coupled LPA receptors (LPAR1-6). The LPA signaling axis is indispensable for brain development and function of the nervous system; however, during damage of the central nervous system, LPA levels can increase and aberrant signaling events counteract brain function. Here, we investigated regulation of the ATX/LPA/LPAR axis in response to lipopolysaccharide-induced systemic inflammation in mice and potential neurotoxic polarization programs in LPA-activated primary murine microglia.

Methods

In vivo, LPAR1-6 expression was established by qPCR in whole murine brain homogenates and in FACS-sorted microglia. ELISAs were used to quantitate LPA concentrations in the brain and cyto-/chemokine secretion from primary microglia in vitro. Transcription factor phosphorylation was analyzed by immunoblotting, and plasma membrane markers were analyzed by flow cytometry. We used MAPK inhibitors to study signal integration by the JNK, p38, and ERK1/2 branches in response to LPA-mediated activation of primary microglia.

Results

Under acute and chronic inflammatory conditions, we observed a significant increase in LPA concentrations and differential regulation of LPAR, ATX (encoded by ENPP2), and cytosolic phospholipase A2 (encoded by PLA2G4A) gene expression in the brain and FACS-sorted microglia. During pathway analyses in vitro, the use of specific MAPK antagonists (SP600125, SB203580, and PD98059) revealed that JNK and p38 inhibition most efficiently attenuated LPA-induced phosphorylation of proinflammatory transcription factors (STAT1 and -3, p65, and c-Jun) and secretion of IL-6 and TNFα. All three inhibitors decreased LPA-mediated secretion of IL-1β, CXCL10, CXCL2, and CCL5. The plasma membrane marker CD40 was solely inhibited by SP600125 while all three inhibitors affected expression of CD86 and CD206. All MAPK antagonists reduced intracellular COX-2 and Arg1 as well as ROS and NO formation, and neurotoxicity of microglia-conditioned media.

Conclusion

In the present study, we show that systemic inflammation induces aberrant ATX/LPA/LPAR homeostasis in the murine brain. LPA-mediated polarization of primary microglia via MAPK-dependent pathways induces features reminiscent of a neurotoxic phenotype.

Metabolic Reprograming of Microglia in the Regulation of the Innate Inflammatory Response

Microglia sustain normal brain functions continuously monitoring cerebral parenchyma to detect neuronal activities and alteration of homeostatic processes. The metabolic pathways involved in microglia activity adapt at and contribute to cell phenotypes. While the mitochondrial oxidative phosphorylation is highly efficient in ATP production, glycolysis enables microglia with a faster rate of ATP production, with the generation of intermediates for cell growth and cytokine production. In macrophages, pro-inflammatory stimuli induce a metabolic switch from oxidative phosphorylation to glycolysis, a phenomenon similar to the Warburg effect well characterized in tumor cells. Modification of metabolic functions allows macrophages to properly respond to a changing environment and many evidence suggest that, similarly to macrophages, microglial cells are capable of a plastic use of energy substrates. Neuroinflammation is a common condition in many neurodegenerative diseases and the metabolic reprograming of microglia has been reported in neurodegeneration. Here we review the existing data on microglia metabolism and the connections with neuroinflammatory diseases, highlighting how metabolic changes contribute to module the homeostatic functions of microglia.

mTOR-mediated metabolic reprogramming shapes distinct microglia functions in response to lipopolysaccharide and ATP

Microglia constantly survey the brain microenvironment and rapidly adopt different phenotypes in response to environmental stimuli. Such dynamic functions require a unique metabolism and bioenergetics. However, little is known about the basic metabolism of microglia and how metabolic changes regulate microglia function. Here, we uncover that microglia activation is accompanied by extensive transcriptional changes in glucose and lipid metabolism-related genes. Using metabolic flux assays, we found that LPS, a prototype of the pathogen-associated molecular patterns (PAMPs), significantly enhanced glycolysis but suppressed oxidative phosphorylation (OXPHOS) in primary cultured microglia. By contrast, ATP, a known damage-associated molecular pattern (DAMPs) that triggers sterile activation of microglia, boosted both glycolysis and OXPHOS. Importantly, both LPS and ATP activated the mechanistic target of rapamycin (mTOR) pathway and enhanced the intracellular reactive oxygen species (ROS). Inhibition of mTOR activity suppressed glycolysis and ROS production in both conditions but exerted different effects on OXPHOS: it attenuated the ATP-induced elevation of OXPHOS, yet had no impact on the LPS-induced suppression of OXPHOS. Further, inhibition of mTOR or glycolysis decreased production of LPS-induced proinflammatory cytokines and ATP-induced tumor necrosis factor-α (TNF-α) and brain derived neurotrophic factor (BDNF) in microglia. Our study reveals a critical role for mTOR in the regulation of metabolic programming of microglia to shape their distinct functions under different states and shed light on the potential application of targeting metabolism to interfere with microglia-mediated neuroinflammation in multiple disorders.

Small-Molecule Lysophosphatidic Acid Receptor 5 (LPAR5) Antagonists: Versatile Pharmacological Tools to Regulate Inflammatory Signaling in BV-2 Microglia Cells

Lysophosphatidic acid (LPA) species in the extracellular environment induce downstream signaling via six different G protein-coupled receptors (LPAR1–6). These signaling cascades are essential for normal brain development and function of the nervous system. However, in response to acute or chronic central nervous system (CNS) damage, LPA levels increase and aberrant signaling events can counteract brain function. Under neuro-inflammatory conditions signaling along the LPA/LPAR5 axis induces a potentially neurotoxic microglia phenotype indicating the need for new pharmacological intervention strategies. Therefore, we compared the effects of two novel small-molecule LPAR5 antagonists on LPA-induced polarization parameters of the BV-2 microglia cell line. AS2717638 is a selective piperidine-based LPAR5 antagonist (IC₅₀ 0.038 μM) while compound 3 is a diphenylpyrazole derivative with an IC₅₀ concentration of 0.7 μM in BV-2 cells. Both antagonists compromised cell viability, however, at concentrations above their IC₅₀ concentrations. Both inhibitors blunted LPA-induced phosphorylation of STAT1 and STAT3, p65, and c-Jun and consequently reduced the secretion of pro-inflammatory cyto-/chemokines (IL-6, TNFα, IL-1β, CXCL10, CXCL2, and CCL5) at non-toxic concentrations. Both compounds modulated the expression of intracellular (COX-2 and Arg1) and plasma membrane-located (CD40, CD86, and CD206) polarization markers yet only AS2717638 attenuated the neurotoxic potential of LPA-activated BV-2 cell-conditioned medium towards CATH.a neurons. Our findings from the present in vitro study suggest that the two LPAR5 antagonists represent valuable pharmacological tools to interfere with LPA-induced pro-inflammatory signaling cascades in microglia.

Can the emerging field of immunometabolism provide insights into neuroinflammation?

In the past few years it has become increasingly clear that an understanding of the interaction between metabolism and immune function can provide an insight into cellular responses to challenges. Significant progress has been made in terms of how macrophages are metabolically re-programmed in response to inflammatory stimuli but, to date, little emphasis has been placed on evaluating equivalent changes in microglia. The need to make progress is driven by the fact that, while microglial activation and the cell's ability to adopt an inflammatory phenotype is necessary to fulfil the neuroprotective function of the cell, persistent activation of microglia and the associated neuroinflammation is at the heart of several neurodegenerative diseases. Understanding the metabolic changes that accompany microglial responses may broaden our perspective on how dysfunction might arise and be tempered. This review will evaluate the current literature that addresses the interplay between inflammation and metabolic reprogramming in microglia, reflecting on the parallels that exist with macrophages. It will consider the changes that take place with age including those that have been reported in neurons and astrocytes with the development of non-invasive imaging techniques, and reflect on the literature that is currently available relating to metabolic reprogramming of microglia with age and in neurodegeneration. Finally it will consider the possibility that manipulating microglial metabolism may provide a valuable approach to modulating neuroinflammation.

The energetic brain - A review from students to students

The past 20 years have resulted in unprecedented progress in understanding brain energy metabolism and its role in health and disease. In this review, which was initiated at the 14th International Society for Neurochemistry Advanced School, we address the basic concepts of brain energy metabolism and approach the question of why the brain has high energy expenditure. Our review illustrates that the vertebrate brain has a high need for energy because of the high number of neurons and the need to maintain a delicate interplay between energy metabolism, neurotransmission, and plasticity. Disturbances to the energetic balance, to mitochondria quality control or to glia-neuron metabolic interaction may lead to brain circuit malfunction or even severe disorders of the CNS. We cover neuronal energy consumption in neural transmission and basic ('housekeeping') cellular processes. Additionally, we describe the most common (glucose) and alternative sources of energy namely glutamate, lactate, ketone bodies, and medium chain fatty acids. We discuss the multifaceted role of non-neuronal cells in the transport of energy substrates from circulation (pericytes and astrocytes) and in the supply (astrocytes and microglia) and usage of different energy fuels. Finally, we address pathological consequences of disrupted energy homeostasis in the CNS.

Lysophosphatidic acid receptor 1 (LPA1) plays critical roles in microglial activation and brain damage after transient focal cerebral ischemia

Background

Lysophosphatidic acid receptor 1 (LPA₁) is in the spotlight because its synthetic antagonist has been under clinical trials for lung fibrosis and psoriasis. Targeting LPA₁ might also be a therapeutic strategy for cerebral ischemia because LPA₁ triggers microglial activation, a core pathogenesis in cerebral ischemia. Here, we addressed this possibility using a mouse model of transient middle cerebral artery occlusion (tMCAO).

Methods

To address the role of LPA₁ in the ischemic brain damage, we used AM095, a selective LPA₁ antagonist, as a pharmacological tool and lentivirus bearing a specific LPA₁ shRNA as a genetic tool. Brain injury after tMCAO challenge was accessed by determining brain infarction and neurological deficit score. Role of LPA₁ in tMCAO-induced microglial activation was ascertained by immunohistochemical analysis. Proinflammatory responses in the ischemic brain were determined by qRT-PCR and immunohistochemical analyses, which were validated in vitro using mouse primary microglia. Activation of MAPKs and PI3K/Akt was determined by Western blot analysis.

Results

AM095 administration immediately after reperfusion attenuated brain damage such as brain infarction and neurological deficit at 1 day after tMCAO, which was reaffirmed by LPA₁ shRNA lentivirus. AM095 administration also attenuated brain infarction and neurological deficit at 3 days after tMCAO. LPA₁ antagonism attenuated microglial activation; it reduced numbers and soma size of activated microglia, reversed their morphology into less toxic one, and reduced microglial proliferation. Additionally, LPA₁ antagonism reduced mRNA expression levels of proinflammatory cytokines and suppressed NF-κB activation, demonstrating its regulatory role of proinflammatory responses in the ischemic brain. Particularly, these LPA₁-driven proinflammatory responses appeared to occur in activated microglia because NF-κB activation occurred mainly in activated microglia in the ischemic brain. Regulatory role of LPA₁ in proinflammatory responses of microglia was further supported by in vitro findings using lipopolysaccharide-stimulated cultured microglia, showing that suppressing LPA₁ activity reduced mRNA expression levels of proinflammatory cytokines. In the ischemic brain, LPA₁ influenced PI3K/Akt and MAPKs; suppressing LPA₁ activity decreased MAPK activation and increased Akt phosphorylation.

Conclusion

This study demonstrates that LPA₁ is a new etiological factor for cerebral ischemia, strongly indicating that its modulation can be a potential strategy to reduce ischemic brain damage.

Electronic supplementary material

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Gintonin, a ginseng-derived ingredient, as a novel therapeutic strategy for Huntington's disease: Activation of the Nrf2 pathway through lysophosphatidic acid receptors

Gintonin (GT), a ginseng-derived lysophosphatidic acid receptor ligand, regulates various cellular effects and represses inflammation. However, little is known about the potential value of GT regarding inflammation in the neurodegenerative diseases, such as Huntington's disease (HD). In this study, we investigated whether GT could ameliorate the neurological impairment and striatal toxicity in cellular or animal model of HD. Pre-, co-, and onset-treatment with GT (25, 50, or 100 mg/kg/day, p.o.) alleviated the severity of neurological impairment and lethality following 3-nitropropionic acid (3-NPA). Pretreatment with GT also attenuated mitochondrial dysfunction i.e. succinate dehydrogenase and MitoSOX activities, apoptosis, microglial activation, and mRNA expression of inflammatory mediators i.e. IL-1β, IL-6, TNF-α, COX-2, and iNOS in the striatum after 3-NPA-intoxication. Its action mechanism was associated with lysophosphatidic acid receptors (LPARs) and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway activations and the inhibition of mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB) signaling pathways. These beneficial effects of GT were neutralized by pre-inhibiting LPARs with Ki16425 (a LPAR1/3 antagonist). Interestingly, GT reduced cell death and mutant huntingtin (HTT) aggregates in STHdh cells. It also mitigated neurological impairment in mice with adeno-associated viral (AAV) vector serotype DJ-mediated overexpression of N171-82Q-mutant HTT in the striatum. Taken together, our findings firstly suggested that GT has beneficial effects with a wide therapeutic time-window in 3-NPA-induced striatal toxicity by antioxidant and anti-inflammatory activities through LPA. In addition, GT exerts neuroprotective effects in STHdh cells and AAV vector-infected model of HD. Thus GT might be an innovative therapeutic candidate to treat HD-like syndromes.

Microglia-Specific Metabolic Changes in Neurodegeneration

The high energetic demand of the brain deems this organ rather sensitive to changes in energy supply. Therefore, even minor alterations in energy metabolism may underlie detrimental disturbances in brain function, contributing to the generation and progression of neurodegenerative diseases. Considerable evidence supports the key role of deficits in cerebral energy metabolism, particularly hypometabolism of glucose and mitochondrial dysfunction, in the pathophysiology of brain disorders. Major breakthroughs in the field of bioenergetics and neurodegeneration have been achieved through the use of in vitro and in vivo models of disease as well as sophisticated neuroimaging techniques in patients, yet these have been mainly focused on neuron and astrocyte function. Remarkably, the subcellular metabolic mechanisms linked to neurodegeneration that operate in other crucial brain cell types such as microglia have remain obscured, although they are beginning to be unraveled. Microglia, the brain-resident immune sentinels, perform a diverse range of functions that require a high-energy expenditure, namely, their role in brain development, maintenance of the neural environment, response to injury and infection, and activation of repair programs. Interestingly, another key mechanism underlying several neurodegenerative diseases is neuroinflammation, which can be associated with chronic microglia activation. Considering that many brain disorders are accompanied by changes in brain energy metabolism and sustained inflammation, and that energy metabolism has a strong influence on the inflammatory responses of microglia, the emerging significance of microglial energy metabolism in neurodegeneration is highlighted in this review.

Microglia metabolism in health and disease

In the last decade tremendous progress has been made in understanding how the immune system reacts to insults. During this progress it became obvious that those immune responses are tightly regulated and cross-linked with distinct metabolic changes in immune cells. Extensive research has been conducted mainly on subtypes of T cells, which use different metabolic pathways during differentiation processes and activation states. In addition, it has also been established later, that the innate immune cell lineage of myeloid cells includes a variety of different subsets of bone marrow-derived as well as tissue-specific macrophages, which elicit much more functions than simply killing bacteria. To execute this high variety of functions, also macrophages use different metabolic pathways and are tightly regulated by key metabolic regulators, such as the mechanistic target of rapamycin (mTOR). Upon activation, metabolic changes within the cell occur to meet the requirements of the phenotypic switch. In addition, metabolic changes correlate with the ability of innate immune cells to show hallmarks of adaptive immune responses. Little is known about specific metabolic changes of myeloid cells and specifically microglia in vivo. Microglia are key players in neurodegenerative and neuroinflammatory diseases and have become a major target of medical research. Here, we review the existing data on microglia metabolism and the connection of microglia phenotypes with neuroinflammatory and neurodegenerative diseases. Lastly, we will discuss how our knowledge about the cellular metabolism might be used to develop new treatment options for neurological diseases.

Mechanisms of Lysophosphatidic Acid-Mediated Lymphangiogenesis in Prostate Cancer

Prostate cancer (PCa) is the most common noncutaneous cancer in men worldwide. One of its major treatments is androgen deprivation therapy, but PCa frequently relapses as aggressive castration resistant local tumors and distal metastases. Hence, the development of novel agents or treatment modalities for advanced PCa is crucial. Many tumors, including PCa, first metastasize to regional lymph nodes via lymphatic vessels. Recent findings demonstrate that the bioactive lipid lysophosphatidic acid (LPA) promotes PCa progression by regulating vascular endothelial growth factor-C (VEGF-C), a critical mediator of tumor lymphangiogenesis and lymphatic metastasis. Many of the underlying molecular mechanisms of the LPA–VEGF-C axis have been described, revealing potential biomarkers and therapeutic targets that may aid in the diagnosis and treatment of advanced PCa. Herein, we review the literature that illustrates a functional role for LPA signaling in PCa progression. These discoveries may be especially applicable to anti-lymphangiogenic strategies for the prevention and therapy of metastatic PCa.

Role of metabolic programming in the modulation of microglia phagocytosis by lipids

Microglia phagocytosis is an essential process to maintain lifelong brain homeostasis and clear potential toxic factors from the neuropil. Microglia can engulf cells or part of cells through the expression of specific receptors at their surface and activation of downstream signaling pathways to engulf material. Microglia phagocytosis is finely regulated and is under the dependence of many factors, including environmental cues such as dietary lipids. Yet, the molecular mechanisms implicated are still largely unknown. The present publication is a 'hypothesis review', assessing the possibility that lipid-mediated modulation of phagocytosis occurs by affecting bioenergetic pathways within microglia. I assess our present knowledge and the elements that allow drawing such hypothesis. I also list some of the important gaps in the literature that need to be filled in. I also consider opportunities for future therapeutic target including nutritional interventions.

Identifying lysophosphatidic acid receptor subtype 1 (LPA1) as a novel factor to modulate microglial activation and their TNF-α production by activating ERK1/2

Microglia regulate immune responses in the brain, and their activation is key to the pathogenesis of diverse neurological diseases. Receptor-mediated lysophosphatidic acid (LPA) signaling has been known to regulate microglial biology, but it is still unclear which receptor subtypes guide the biology, particularly, microglial activation. Here, we investigated the pathogenic aspects of LPA receptor subtype 1 (LPA₁) in microglial activation using a systemic lipopolysaccharide (LPS) administration-induced septic mouse model in vivo and LPS-stimulated rat primary microglia in vitro. LPA₁ knockdown in the brain with its specific shRNA lentivirus attenuated the sepsis-induced microglia activation, morphological transformation, and proliferation. LPA₁ knockdown also resulted in the downregulation of TNF-α, at both mRNA and protein levels in septic brains, but not IL-1β or IL-6. In rat primary microglia, genetic or pharmacological blockade of LPA₁ attenuated gene upregulation and secretion of TNF-α in LPS-stimulated cells. In particular, the latter was associated with the suppressed TNF-α converting enzyme (TACE) activity. We reaffirmed these biological aspects using a BV2 microglial cell line in which LPA₁ expression was negligible. LPA₁ overexpression in BV2 cells led to significant increments in TNF-α production upon stimulation with LPS, whereas inhibiting LPA₁ reversed the production. We further identified ERK1/2, but not p38 MAPK or Akt, as the underlying effector pathway after LPA₁ activation in both septic brains and stimulated microglia. The current findings of the novel role of LPA₁ in microglial activation along with its mechanistic aspects could be applied to understanding the pathogenesis of diverse neurological diseases that involve microglial activation.

Autotaxin⁻Lysophosphatidic Acid Signaling in Alzheimer's Disease

The brain contains various forms of lipids that are important for maintaining its structural integrity and regulating various signaling cascades. Autotaxin (ATX) is an ecto-nucleotide pyrophosphatase/phosphodiesterase-2 enzyme that hydrolyzes extracellular lysophospholipids into the lipid mediator lysophosphatidic acid (LPA). LPA is a major bioactive lipid which acts through G protein-coupled receptors (GPCRs) and plays an important role in mediating cellular signaling processes. The majority of synthesized LPA is derived from membrane phospholipids through the action of the secreted enzyme ATX. Both ATX and LPA are highly expressed in the central nervous system. Dysfunctional expression and activity of ATX with associated changes in LPA signaling have recently been implicated in the pathogenesis of Alzheimer’s disease (AD). This review focuses on the current understanding of LPA signaling, with emphasis on the importance of the autotaxin–lysophosphatidic acid (ATX–LPA) pathway and its alterations in AD and a brief note on future therapeutic applications based on ATX–LPA signaling.

Stress, Depression, Resilience and Ageing: A Role for the LPA-LPA1 Pathway

BACKGROUND

Chronic stress affects health and the quality of life, with its effects being particularly relevant in ageing due to the psychobiological characteristics of this population. However, while some people develop psychiatric disorders, especially depression, others seem very capable of dealing with adversity. There is no doubt that along with the identification of neurobiological mechanisms involved in developing depression, discovering which factors are involved in positive adaptation under circumstances of extreme difficulty will be crucial for promoting resilience.

METHODS

Here, we review recent work in our laboratory, using an animal model lacking the LPA1 receptor, together with pharmacological studies and clinical evidence for the possible participation of the LPA1 receptor in mood and resilience to stress.

RESULTS

Substantial evidence has shown that the LPA1 receptor is involved in emotional regulation and in coping responses to chronic stress, which, if dysfunctional, may induce vulnerability to stress and predisposition to the development of depression. Given that there is commonality of mechanisms between those involved in negative consequences of stress and in ageing, this is not surprising, considering that the LPA1 receptor may be involved in coping with adversity during ageing.

CONCLUSION

Alterations in this receptor may be a susceptibility factor for the presence of depression and cognitive deficits in the elderly population. However, because this is only a promising hypothesis based on previous data, future studies should focus on the involvement of the LPA-LPA1 pathway in coping with stress and resilience in ageing.

The Rho-associated kinase inhibitors Y27632 and fasudil promote microglial migration in the spinal cord via the ERK signaling pathway

Rho-associated kinase (ROCK) is a key regulatory protein involved in inflammatory secretion in microglia in the central nervous system. Our previous studies showed that ROCK inhibition enhances phagocytic activity in microglia through the extracellular signal-regulated kinase (ERK) signaling pathway, but its effect on microglial migration was unknown. Therefore, in this study, we investigated the effects of the ROCK inhibitors Y27632 and fasudil on the migratory activity of primary cultured microglia isolated from the spinal cord, and we examined the underlying mechanisms. The microglia were treated with Y27632, fasudil and/or the ERK inhibitor U0126. Cellular morphology was observed by immunofluorescence. Transwell chambers were used to assess cell migration. ERK levels were measured by in-cell western blot assay. Y27632 and fasudil increased microglial migration, and the microglia were irregularly shaped and had many small processes. These inhibitors also upregulated the levels of phosphorylated ERK protein. The ERK inhibitor U0126 suppressed these effects of Y27632 and fasudil. These findings suggest that the ROCK inhibitors Y27632 and fasudil promote microglial migration in the spinal cord through the ERK signaling pathway.

Lysophosphatidic acid via LPA-receptor 5/protein kinase D-dependent pathways induces a motile and pro-inflammatory microglial phenotype

BACKGROUND

Extracellular lysophosphatidic acid (LPA) species transmit signals via six different G protein-coupled receptors (LPAR1-6) and are indispensible for brain development and function of the nervous system. However, under neuroinflammatory conditions or brain damage, LPA levels increase, thereby inducing signaling cascades that counteract brain function. We describe a critical role for 1-oleyl-2-hydroxy-sn-glycero-3-phosphate (termed "LPA" throughout our study) in mediating a motile and pro-inflammatory microglial phenotype via LPAR5 that couples to protein kinase D (PKD)-mediated pathways.

METHODS

Using the xCELLigence system and time-lapse microscopy, we investigated the migrational response of microglial cells. Different M1 and M2 markers were analyzed by confocal microscopy, flow cytometry, and immunoblotting. Using qPCR and ELISA, we studied the expression of migratory genes and quantitated the secretion of pro-inflammatory cytokines and chemokines, respectively. Different transcription factors that promote the regulation of pro-inflammatory genes were analyzed by western blot. Reactive oxygen species (ROS) and nitric oxide (NO) production, phagocytosis, and microglial cytotoxicity were determined using commercially available assay kits.

RESULTS

LPA induces MAPK family and AKT activation and pro-inflammatory transcription factors' phosphorylation (NF-κB, c-Jun, STAT1, and STAT3) that were inhibited by both LPAR5 and PKD family antagonists. LPA increases migratory capacity, induces secretion of pro-inflammatory cytokines and chemokines and expression of M1 markers, enhances production of ROS and NO by microglia, and augments cytotoxicity of microglial cell-conditioned medium towards neurons. The PKD family inhibitor blunted all of these effects. We propose that interference with this signaling axis could aid in the development of new therapeutic approaches to control neuroinflammation under conditions of overshooting LPA production.

CONCLUSIONS

In the present study, we show that inflammatory LPA levels increased the migratory response of microglia and promoted a pro-inflammatory phenotype via the LPAR5/PKD axis. Interference with this signaling axis reduced microglial migration, blunted microglial cytotoxicity, and abrogated the expression and secretion of pro-inflammatory mediators.

Bioenergetic regulation of microglia

Microglia have diverse actions, ranging from synapse pruning in development to cytotoxic effects in disease. Brain energy metabolism and substrate availability vary under normal and disease states, but how these variations influence microglial function is relatively unknown. Microglia, like most other cell types, express the full complement of gene products required for both glycolytic and oxidative metabolism. Evidence suggests that microglia increase aerobic glycolysis and decrease respiration when activated by various stimuli. Mitochondrial function, glucose availability, and glycolytic rate influence pro-inflammatory gene expression at both transcriptional and post-translational levels. These effects are mediated through CtBP, an NADH-sensitive transcriptional co-repressor; through effects on NLRP3 inflammasome assembly and caspase-1 activation; through formation of advanced glycation end-products; and by less well-defined mechanisms. In addition to these transcriptional effects, microglial glucose metabolism is also required for superoxide production by NADPH oxidase, as glucose is the obligate substrate for regenerating NADPH in the hexose monophosphate shunt. Microglia also metabolize acetoacetate and β-hydroxybutyrate, which are generated during fasting or ketogenic diet, and respond to these ketones as metabolic signals. β-Hydroxybutyrate inhibits histone de-acetylases and activates microglial GRP109A receptors. These actions suppress microglia activation after brain injury and promote neuroprotective microglia phenotypes. As our understanding of microglial activation matures, additional links between energy metabolism and microglial function are likely to be identified.

Brain Renin-Angiotensin System and Microglial Polarization: Implications for Aging and Neurodegeneration

Microglia can transform into proinflammatory/classically activated (M1) or anti-inflammatory/alternatively activated (M2) phenotypes following environmental signals related to physiological conditions or brain lesions. An adequate transition from the M1 (proinflammatory) to M2 (immunoregulatory) phenotype is necessary to counteract brain damage. Several factors involved in microglial polarization have already been identified. However, the effects of the brain renin-angiotensin system (RAS) on microglial polarization are less known. It is well known that there is a “classical” circulating RAS; however, a second RAS (local or tissue RAS) has been observed in many tissues, including brain. The locally formed angiotensin is involved in local pathological changes of these tissues and modulates immune cells, which are equipped with all the components of the RAS. There are also recent data showing that brain RAS plays a major role in microglial polarization. Level of microglial NADPH-oxidase (Nox) activation is a major regulator of the shift between M1/proinflammatory and M2/immunoregulatory microglial phenotypes so that Nox activation promotes the proinflammatory and inhibits the immunoregulatory phenotype. Angiotensin II (Ang II), via its type 1 receptor (AT1), is a major activator of the NADPH-oxidase complex, leading to pro-oxidative and pro-inflammatory effects. However, these effects are counteracted by a RAS opposite arm constituted by Angiotensin II/AT2 receptor signaling and Angiotensin 1–7/Mas receptor (MasR) signaling. In addition, activation of prorenin-renin receptors may contribute to activation of the proinflammatory phenotype. Aged brains showed upregulation of AT1 and downregulation of AT2 receptor expression, which may contribute to a pro-oxidative pro-inflammatory state and the increase in neuron vulnerability. Several recent studies have shown interactions between the brain RAS and different factors involved in microglial polarization, such as estrogens, Rho kinase (ROCK), insulin-like growth factor-1 (IGF-1), tumor necrosis factor α (TNF)-α, iron, peroxisome proliferator-activated receptor gamma, and toll-like receptors (TLRs). Metabolic reprogramming has recently been involved in the regulation of the neuroinflammatory response. Interestingly, we have recently observed a mitochondrial RAS, which is altered in aged brains. In conclusion, dysregulation of brain RAS plays a major role in aging-related changes and neurodegeneration by exacerbation of oxidative stress (OS) and neuroinflammation, which may be attenuated by pharmacological manipulation of RAS components.

Menopause and Parkinson's disease. Interaction between estrogens and brain renin-angiotensin system in dopaminergic degeneration

The neuroprotective effects of menopausal hormonal therapy in Parkinson's disease (PD) have not yet been clarified, and it is controversial whether there is a critical period for neuroprotection. Studies in animal models and clinical and epidemiological studies indicate that estrogens induce dopaminergic neuroprotection. Recent studies suggest that inhibition of the brain renin-angiotensin system (RAS) mediates the effects of estrogens in PD models. In the substantia nigra, ovariectomy induces a decrease in levels of estrogen receptor-α (ER-α) and increases angiotensin activity, NADPH-oxidase activity and expression of neuroinflammatory markers, which are regulated by estrogen replacement therapy. There is a critical period for the neuroprotective effect of estrogen replacement therapy, and local ER-α and RAS play a major role. Astrocytes play a major role in ER-α-induced regulation of local RAS, but neurons and microglia are also involved. Interestingly, treatment with angiotensin receptor antagonists after the critical period induced neuroprotection.

1-Oleyl-lysophosphatidic acid (LPA) promotes polarization of BV-2 and primary murine microglia towards an M1-like phenotype

Background

Microglia, the immunocompetent cells of the CNS, rapidly respond to brain injury and disease by altering their morphology and phenotype to adopt an activated state. Microglia can exist broadly between two different states, namely the classical (M1) and the alternative (M2) phenotype. The first is characterized by the production of pro-inflammatory cytokines/chemokines and reactive oxygen and/or nitrogen species. In contrast, alternatively activated microglia are typified by an anti-inflammatory phenotype supporting wound healing and debris clearance. The objective of the present study was to determine the outcome of lysophosphatidic acid (LPA)-mediated signaling events on microglia polarization.

Methods

LPA receptor expression and cyto-/chemokine mRNA levels in BV-2 and primary murine microglia (PMM) were determined by qPCR. M1/M2 marker expression was analyzed by Western blotting, immunofluorescence microscopy, or flow cytometry. Cyto-/chemokine secretion was quantitated by ELISA.

Results

BV-2 cells express LPA receptor 2 (LPA2), 3, 5, and 6, whereas PMM express LPA1, 2, 4, 5, and 6. We show that LPA treatment of BV-2 and PMM leads to a shift towards a pro-inflammatory M1-like phenotype. LPA treatment increased CD40 and CD86 (M1 markers) and reduced CD206 (M2 marker) expression. LPA increased inducible nitric oxide synthase (iNOS) and COX-2 levels (both M1), while the M2 marker Arginase-1 was suppressed in BV-2 cells. Immunofluorescence studies (iNOS, COX-2, Arginase-1, and RELMα) extended these findings to PMM. Upregulation of M1 markers in BV-2 and PMM was accompanied by increased cyto-/chemokine transcription and secretion (IL-1β, TNFα, IL-6, CCL5, and CXCL2). The pharmacological LPA5 antagonist TCLPA5 blunted most of these pro-inflammatory responses.

Conclusions

LPA drives BV-2 and PMM towards a pro-inflammatory M1-like phenotype. Suppression by TCLPA5 indicates that the LPA/LPA5 signaling axis could represent a potential pharmacological target to interfere with microglia polarization in disease.

Lysophosphatidic Acid Up-Regulates Hexokinase II and Glycolysis to Promote Proliferation of Ovarian Cancer Cells

Lysophosphatidic acid (LPA), a blood-borne lipid mediator, is present in elevated concentrations in ascites of ovarian cancer patients and other malignant effusions. LPA is a potent mitogen in cancer cells. The mechanism linking LPA signal to cancer cell proliferation is not well understood. Little is known about whether LPA affects glucose metabolism to accommodate rapid proliferation of cancer cells. Here we describe that in ovarian cancer cells, LPA enhances glycolytic rate and lactate efflux. A real time PCR-based miniarray showed that hexokinase II (HK2) was the most dramatically induced glycolytic gene to promote glycolysis in LPA-treated cells. Analysis of the human HK2 gene promoter identified the sterol regulatory element-binding protein as the primary mediator of LPA-induced HK2 transcription. The effects of LPA on HK2 and glycolysis rely on LPA₂, an LPA receptor subtype overexpressed in ovarian cancer and many other malignancies. We further examined the general role of growth factor-induced glycolysis in cell proliferation. Like LPA, epidermal growth factor (EGF) elicited robust glycolytic and proliferative responses in ovarian cancer cells. Insulin-like growth factor 1 (IGF-1) and insulin, however, potently stimulated cell proliferation but only modestly induced glycolysis. Consistent with their differential effects on glycolysis, LPA and EGF-dependent cell proliferation was highly sensitive to glycolytic inhibition while the growth-promoting effect of IGF-1 or insulin was more resistant. These results indicate that LPA- and EGF-induced cell proliferation selectively involves up-regulation of HK2 and glycolytic metabolism. The work is the first to implicate LPA signaling in promotion of glucose metabolism in cancer cells.

Microglial brain region-dependent diversity and selective regional sensitivities to aging

Microglia have critical roles in neural development, homeostasis and neuroinflammation and are increasingly implicated in age-related neurological dysfunction. Neurodegeneration often occurs in disease-specific, spatially restricted patterns, the origins of which are unknown. We performed to our knowledge the first genome-wide analysis of microglia from discrete brain regions across the adult lifespan of the mouse, and found that microglia have distinct region-dependent transcriptional identities and age in a regionally variable manner. In the young adult brain, differences in bioenergetic and immunoregulatory pathways were the major sources of heterogeneity and suggested that cerebellar and hippocampal microglia exist in a more immune-vigilant state. Immune function correlated with regional transcriptional patterns. Augmentation of the distinct cerebellar immunophenotype and a contrasting loss in distinction of the hippocampal phenotype among forebrain regions were key features during aging. Microglial diversity may enable regionally localized homeostatic functions but could also underlie region-specific sensitivities to microglial dysregulation and involvement in age-related neurodegeneration.

Microglial M1/M2 polarization and metabolic states

Microglia are critical nervous system-specific immune cells serving as tissue-resident macrophages influencing brain development, maintenance of the neural environment, response to injury and repair. As influenced by their environment, microglia assume a diversity of phenotypes and retain the capability to shift functions to maintain tissue homeostasis. In comparison with peripheral macrophages, microglia demonstrate similar and unique features with regards to phenotype polarization, allowing for innate immunological functions. Microglia can be stimulated by LPS or IFN-γ to an M1 phenotype for expression of pro-inflammatory cytokines or by IL-4/IL-13 to an M2 phenotype for resolution of inflammation and tissue repair. Increasing evidence suggests a role of metabolic reprogramming in the regulation of the innate inflammatory response. Studies using peripheral immune cells demonstrate that polarization to an M1 phenotype is often accompanied by a shift in cells from oxidative phosphorylation to aerobic glycolysis for energy production. More recently, the link between polarization and mitochondrial energy metabolism has been considered in microglia. Under these conditions, energy demands would be associated with functional activities and cell survival and thus, may serve to influence the contribution of microglia activation to various neurodegenerative conditions. This review examines the polarization states of microglia and their relationship to mitochondrial metabolism. Additional supporting experimental data are provided to demonstrate mitochondrial metabolic shifts in primary microglia and the BV-2 microglia cell line induced under LPS (M1) and IL-4/IL-13 (M2) polarization.

Rho-Associated Kinase Inhibitors Promote Microglial Uptake Via the ERK Signaling Pathway

Microglia are immunocompetent cells in the central nervous system that take up tissue debris and pathogens. Rho-associated kinase (ROCK) has been identified as an important regulator of uptake, proliferation, secretion, and differentiation in a number of cell types. Although ROCK plays critical roles in the microglial secretion of inflammatory factors, migration, and morphology, its effects on microglial uptake activity have not been well characterized. In the present study, we found that treatment of BV2 microglia and primary microglia with the ROCK inhibitors Y27632 and fasudil increased uptake activity and was associated with morphological changes. Furthermore, western blots showed that this increase in uptake activity was mediated through the extracellular-signal-regulated kinase (ERK) signaling cascade, indicating the importance of ROCK in regulating microglial uptake activity.

Chatting with the neighbors: crosstalk between Rho-kinase (ROCK) and other signaling pathways for treatment of neurological disorders

ROCK inhibition has been largely applied as a strategy to treat neurodegenerative diseases (NDDs) and promising results have been obtained in the recent years. However, the underlying molecular and cellular mechanisms are not fully understood and different models have been proposed for neurodegenerative disorders. Here, we aim to review the current knowledge obtained for NDDs identifying common mechanisms as well as disease-specific models. In addition to the role of ROCK in different cell types such as neurons and microglia, we focus on the molecular signaling-pathways which mediate the beneficial effects of ROCK. Besides canonical ROCK signaling, modulation of neighboring pathways by non-canonical ROCK-crosstalk is a recurrent pattern in many NDD-model systems and has been suggested to mediate beneficial effects of ROCK-inhibition.

Lysophosphatidic Acid signaling in the nervous system

The brain is composed of many lipids with varied forms that serve not only as structural components but also as essential signaling molecules. Lysophosphatidic acid (LPA) is an important bioactive lipid species that is part of the lysophospholipid (LP) family. LPA is primarily derived from membrane phospholipids and signals through six cognate G protein-coupled receptors (GPCRs), LPA1-6. These receptors are expressed on most cell types within central and peripheral nervous tissues and have been functionally linked to many neural processes and pathways. This Review covers a current understanding of LPA signaling in the nervous system, with particular focus on the relevance of LPA to both physiological and diseased states.

Rho Kinase and Dopaminergic Degeneration

The small GTP-binding protein Rho plays an important role in several cellular functions. RhoA, which is a member of the Rho family, initiates cellular processes that act on its direct downstream effector Rho-associated kinase (ROCK). ROCK inhibition protects against dopaminergic cell death induced by dopaminergic neurotoxins. It has been suggested that ROCK inhibition activates neuroprotective survival cascades in dopaminergic neurons. Axon-stabilizing effects in damaged neurons may represent another mechanism of neuroprotection of dopaminergic neurons by ROCK inhibition. However, it has been shown that microglial cells play a crucial role in neuroprotection by ROCK inhibition and that activation of microglial ROCK mediates major components of the microglial inflammatory response. Additional mechanisms such as interaction with autophagy may also contribute to the neuroprotective effects of ROCK inhibition. Interestingly, ROCK interacts with several brain factors that play a major role in dopaminergic neuron vulnerability such as NADPH-oxidase, angiotensin, and estrogen. ROCK inhibition may provide a new neuroprotective strategy for Parkinson’s disease. This is of particular interest because ROCK inhibitors are currently used against vascular diseases in clinical practice. However, it is necessary to develop more potent and selective ROCK inhibitors to reduce side effects and enhance the efficacy.

Microglial TNF-α mediates enhancement of dopaminergic degeneration by brain angiotensin

In vitro and in vivo models of Parkinson's disease were used to investigate whether TNF-α plays a major role in the enhancement of the microglial response and dopaminergic degeneration induced by brain angiotensin hyperactivity. Treatment of primary mesencephalic cultures with low doses of the neurotoxin MPP(+) induced a significant loss of dopaminergic neurons, which was enhanced by cotreatment with angiotensin II and inhibited by TNF-α inhibitors. Treatment of primary cultures with angiotensin induced a marked increase in levels of TNF-α, which was inhibited by treatment with angiotensin type-1-receptor antagonists, NADPH-oxidase inhibitors and NFK-β inhibitors. However, TNF-α levels were not significantly affected by treatment with angiotensin in the absence of microglia. The microglial origin of the angiotensin-induced increase in TNF-α levels was confirmed using dopaminergic (MES 23.5) and microglial (N9) cell lines. Inhibition of the microglial Rho-kinase activity also blocked the AII-induced increase in TNF-α levels. Treatment of the dopaminergic cell line with TNF-α revealed that NFK-β activation mediates the deleterious effect of microglial TNF-α on dopaminergic neurons. Treatment of mice with MPTP also induced significant increases in striatal and nigral TNF-α levels, which were inhibited by angiotensin type-1-receptor antagonists or NFK-β inhibitors. The present results show that microglial TNF-α plays a major role in angiotensin-induced dopaminergic cell death and that the microglial release of TNF-α is mediated by activation of angiotensin type-1 receptors, NADPH-oxidase, Rho-kinase and NFK-β.

Inhibition of the microglial response is essential for the neuroprotective effects of Rho-kinase inhibitors on MPTP-induced dopaminergic cell death

Several recent studies have shown that activation of the RhoA/Rho-associated kinase (ROCK) pathway is involved in the MPTP-induced dopaminergic cell degeneration and possibly in Parkinson's disease. ROCK inhibitors have been suggested as candidate neuroprotective drugs for Parkinson's disease. However, the mechanism responsible for the increased survival of dopaminergic neurons after treatment with ROCK inhibitors is not clear. We exposed primary (neuron-glia) mesencephalic cultures, cultures of the MES 23.5 dopaminergic neuron cell line and primary mesencephalic cultures lacking microglial cells to the dopaminergic neurotoxin MPP+ and the ROCK inhibitor Y-27632 in order to study the effects of ROCK inhibition on dopaminergic cell loss and the length of neurites of surviving dopaminergic neurons. In primary (neuron-glia) cultures, simultaneous treatment with MPP+ and the ROCK inhibitor significantly reduced the loss of dopaminergic neurons. In the absence of microglia, treatment with the ROCK inhibitor did not induce a significant reduction in the dopaminergic cell loss. Treatment with the ROCK inhibitor induced a significant decrease in axonal retraction in primary cultures with and without microglia and in cultures of the MES 23.5 neuron cell line. In conclusion, inhibition of microglial ROCK is essential for the neuroprotective effects of ROCK inhibitors against cell death induced by the dopaminergic neurotoxin MPP+. In addition, ROCK inhibition induced a direct effect against axonal retraction in surviving neurons. However, the latter effect was not sufficient to cause a significant increase in the survival of dopaminergic neurons after treatment with MPP+.