Microglial phagocytosis of neurons in neurodegeneration, and its regulation

Near-infrared light reduces β-amyloid-stimulated microglial toxicity and enhances survival of neurons: mechanisms of light therapy for Alzheimer's disease

BACKGROUND

Low-intensity light can decelerate neurodegenerative disease progression and reduce amyloid β (Aβ) levels in the cortex, though the cellular and molecular mechanisms by which photobiomodulation (PBM) protects against neurodegeneration are still in the early stages. Microglia cells play a key role in the pathology of Alzheimer's disease by causing chronic inflammation. We present new results concerning the PBM of both oxidative stress and microglia metabolism associated with the activation of metabolic processes by 808 nm near-infrared light.

METHODS

The studies were carried out using healthy male mice to obtain the microglial cell suspension from the hippocampus. Oligomeric β-amyloid (1-42) was prepared and used to treat microglia cells. Light irradiation of cells was performed using diode lasers emitting at 808 nm (30 mW/cm² for 5 min, resulting in a dose of 10 J/cm²). Mitochondrial membrane potential, ROS level studies, cell viability, apoptosis, and necrosis assays were performed using epifluorescence microscopy. Phagocytosis, nitric oxide and H₂O₂ production, arginase, and glucose 6-phosphate dehydrogenase activities were measured using standard assays. Cytokines, glucose, lactate, and ATP were measurements with ELISA. As our data were normally distributed, two-way ANOVA test was used.

RESULTS

The light induces a metabolic shift from glycolysis to mitochondrial activity in pro-inflammatory microglia affected by oligomeric Aβ. Thereby, the level of anti-inflammatory microglia increases. This process is accompanied by a decrease in pro-inflammatory cytokines and an activation of phagocytosis. Light exposure decreases the Aβ-induced activity of glucose-6-phosphate dehydrogenase, an enzyme that regulates the rate of the pentose phosphate pathway, which activates nicotinamide adenine dinucleotide phosphate oxidases to further produce ROS. During co-cultivation of neurons with microglia, light prevents the death of neurons, which is caused by ROS produced by Aβ-altered microglia.

CONCLUSIONS

These original data clarify reasons for how PBM protects against neurodegeneration and support the use of light for therapeutic research in the treatment of Alzheimer's disease.

Opioid receptor activation suppresses the neuroinflammatory response by promoting microglial M2 polarization

Activation of microglia is considered the most important component of neuroinflammation. Microglia can adopt a pro-inflammatory (M1) or anti-inflammatory (M2) phenotype. Opioid receptors (ORs) have been shown to control neurotransmission of various peptidergic neurons, but their potential role in regulating microglial function is largely unknown. Here, we aimed to investigate the effect of the OR agonists DAMGO, DADLE and U-50488 on the polarization of C8-B4 microglial cells. We observed that opioids suppressed lipopolysaccharide (LPS)-triggered M1 polarization and promoted M2 polarization. This was reflected in lower phagocytic activity, lower production of NO, lower expression of TNF-α, IL-1β, IL-6, IL-86 and IL-12 beta p40 together with higher migration rate, and increased expression of IL-4, IL-10, arginase 1 and CD 206 in microglia, compared to cells affected by LPS. We demonstrated that the effect of opioids on microglial polarization is mediated by the TREM2/NF-κB signaling pathway. These results provide new insights into the anti-inflammatory and neuroprotective effects of opioids and highlight their potential in combating neurodegenerative diseases.

Galectin-3 Promotes Müller Glia Clearance Phagocytosis via MERTK and Reduces Harmful Müller Glia Activation in Inherited and Induced Retinal Degeneration

Clearance phagocytosis is a documented function of Müller glia in the retina. However, the molecular mechanisms of Müller glia phagocytosis remain largely undefined. Here, we show that extracellular galectin-3 and protein S promote clearance phagocytosis by immortalized human MIO-M1 Müller cells in an additive, saturable manner. Galectin-3 promotes phagocytosis by primary Müller glia from wild-type (WT) mice but not from mice that lack the engulfment receptor MERTK and therefore develop postnatal photoreceptor degeneration. Probing a possible functional link between Müller galectin-3 and MERTK, we discovered that mertk ^-/- Müller glia in situ show excess galectin-3 at postnatal day 20 (P20), an age prior to detectable photoreceptor degeneration. Moreover, double knockout (DKO) mice lacking both galectin-3 and MERTK show increased activation of Müller cells (but not of microglia) at P20 and more pronounced photoreceptor loss at P35 compared to mice lacking MERTK alone. Exploring the well-established sodium iodate injury model, we also found more severe activation specifically of Müller glia, and worse retinal damage in mice lacking galectin-3 compared to WT mice. Indeed, galectin-3 deficiency significantly increased sensitivity to injury, yielding Müller activation and retinal damage at a sodium iodate concentration that had no effect on the WT retina. Altogether, our results from both inherited and acutely induced models of retinal degeneration agree that eliminating galectin-3 exacerbates Müller cell activation and retinal degeneration. These data identify an important protective role for the MERTK ligand galectin-3 in the retina in restraining Müller glia activation.

Neu1 Is Released From Activated Microglia, Stimulating Microglial Phagocytosis and Sensitizing Neurons to Glutamate

Neuraminidase 1 (Neu1) hydrolyses terminal sialic acid residues from glycoproteins and glycolipids, and is normally located in lysosomes, but can be released onto the surface of activated myeloid cells and microglia. We report that endotoxin/lipopolysaccharide-activated microglia released Neu1 into culture medium, and knockdown of Neu1 in microglia reduced both Neu1 protein and neuraminidase activity in the culture medium. Release of Neu1 was reduced by inhibitors of lysosomal exocytosis, and accompanied by other lysosomal proteins, including protective protein/cathepsin A, known to keep Neu1 active. Extracellular neuraminidase or over-expression of Neu1 increased microglial phagocytosis, while knockdown of Neu1 decreased phagocytosis. Microglial activation caused desialylation of microglial phagocytic receptors Trem2 and MerTK, and increased binding to Trem2 ligand galectin-3. Culture media from activated microglia contained Neu1, and when incubated with neurons induced their desialylation, and increased the neuronal death induced by low levels of glutamate. Direct desialylation of neurons by adding sialidase or inhibiting sialyltransferases also increased glutamate-induced neuronal death. We conclude that activated microglia can release active Neu1, possibly by lysosomal exocytosis, and this can both increase microglial phagocytosis and sensitize neurons to glutamate, thus potentiating neuronal death.

The Pathobiology of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: The Case for Neuroglial Failure

Although myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) has a specific and distinctive profile of clinical features, the disease remains an enigma because causal explanation of the pathobiological matrix is lacking. Several potential disease mechanisms have been identified, including immune abnormalities, inflammatory activation, mitochondrial alterations, endothelial and muscular disturbances, cardiovascular anomalies, and dysfunction of the peripheral and central nervous systems. Yet, it remains unclear whether and how these pathways may be related and orchestrated. Here we explore the hypothesis that a common denominator of the pathobiological processes in ME/CFS may be central nervous system dysfunction due to impaired or pathologically reactive neuroglia (astrocytes, microglia and oligodendrocytes). We will test this hypothesis by reviewing, in reference to the current literature, the two most salient and widely accepted features of ME/CFS, and by investigating how these might be linked to dysfunctional neuroglia. From this review we conclude that the multifaceted pathobiology of ME/CFS may be attributable in a unifying manner to neuroglial dysfunction. Because the two key features - post exertional malaise and decreased cerebral blood flow - are also recognized in a subset of patients with post-acute sequelae COVID, we suggest that our findings may also be pertinent to this entity.

Editorial: Exciting developments in neurochemistry research and publishing

In this editorial, we are happy to connect with our community to explain the changes introduced to the Journal of Neurochemistry over the last year and provide some insights into new developments and exciting opportunities. We anticipate these developments, which are strongly guided to increase transparency and support early career researchers, will increase the value of the Journal of Neurochemistry for the authors and readers. Ultimately, we hope to improve the author experience with the Journal of Neurochemistry and continue to be the leading venue for fast dissemination of exciting new research focusing on how molecules, cells and circuits regulate the nervous system in health and disease.

Imeglimin Is Neuroprotective Against Ischemic Brain Injury in Rats-a Study Evaluating Neuroinflammation and Mitochondrial Functions

Imeglimin is a novel oral antidiabetic drug modulating mitochondrial functions. However, neuroprotective effects of this drug have not been investigated. The aim of this study was to investigate effects of imeglimin against ischemia-induced brain damage and neurological deficits and whether it acted via inhibition of mitochondrial permeability transition pore (mPTP) and suppression of microglial activation. Ischemia in rats was induced by permanent middle cerebral artery occlusion (pMCAO) for 48 h. Imeglimin (135 μg/kg/day) was injected intraperitoneally immediately after pMCAO and repeated after 24 h. Immunohistochemical staining was used to evaluate total numbers of neurons, astrocytes, and microglia as well as interleukin-10 (IL-10) producing cells in brain slices. Respiration of isolated brain mitochondria was assessed using high-resolution respirometry. Assessment of ionomycin-induced mPTP opening in intact cultured primary rat neuronal, astrocytic, and microglial cells was performed using fluorescence microscopy. Treatment with imeglimin significantly decreased infarct size, brain edema, and neurological deficits after pMCAO. Moreover, imeglimin protected against pMCAO-induced neuronal loss as well as microglial proliferation and activation, and increased the number of astrocytes and the number of cells producing anti-inflammatory cytokine IL-10 in the ischemic hemisphere. Imeglimin in vitro acutely prevented mPTP opening in cultured neurons and astrocytes but not in microglial cells; however, treatment with imeglimin did not prevent ischemia-induced mitochondrial respiratory dysfunction after pMCAO. This study demonstrates that post-stroke treatment with imeglimin exerts neuroprotective effects by reducing infarct size and neuronal loss possibly via the resolution of neuroinflammation and partly via inhibition of mPTP opening in neurons and astrocytes.

Compensatory engulfment and Müller glia reactivity in the absence of microglia

Microglia are known for important phagocytic functions in the vertebrate retina. Reports also suggest that Müller glia have phagocytic capacity, though the relative levels and contexts in which this occurs remain to be thoroughly examined. Here, we investigate Müller glial engulfment of dying cells in the developing zebrafish retina in the presence and absence of microglia, using a genetic mutant in which microglia do not develop. We show that in normal conditions clearance of dying cells is dominated by microglia; however, Müller glia do have a limited clearance role. In retinas lacking intact microglial populations, we found a striking increase in the engulfment load assumed by the Müller glia, which displayed prominent cellular compartments containing apoptotic cells, several of which localized with the early phagosome/endosome marker Rab5. Consistent with increased engulfment, lysosomal staining was also increased in Müller glia in the absence of microglia. Increased engulfment load led to evidence of Müller glia reactivity including upregulation of gfap but did not trigger cell cycle re-entry by differentiated Müller glia. Our work provides important insight into the phagocytic capacity of Müller glia and the ability for compensatory functions and downstream effects. Therefore, effects of microglial deficiency or depletion on other glial cell types should be well-considered in experimental manipulations, in neurodegenerative disease, and in therapeutic approaches that target microglia. Our findings further justify future work to understand differential mechanisms and contexts of phagocytosis by glial cells in the central nervous system, and the significance of these mechanisms in health and disease.

The Role of Immune Checkpoint Molecules on Macrophages in Cancer, Infection, and Autoimmune Pathologies

Immune checkpoint inhibitors have revolutionized immunotherapy against various cancers over the last decade. The use of checkpoint inhibitors results in remarkable re-activation of patients’ immune system, but is also associated with significant adverse events. In this review, we emphasize the importance of cell-type specificity in the context of immune checkpoint-based interventions and particularly focus on the relevance of macrophages. Immune checkpoint blockade alters the dynamic macrophage phenotypes and thereby substantially manipulates therapeutical outcome. Considering the macrophage-specific immune checkpoint biology, it seems feasible to ameliorate the situation of patients with severe side effects and even increase the probability of survival for non-responders to checkpoint inhibition. Apart from malignancies, investigating immune checkpoint molecules on macrophages has stimulated their fundamental characterization and use in other diseases as well, such as acute and chronic infections and autoimmune pathologies. Although the macrophage-specific effect of checkpoint molecules has been less studied so far, the current literature shows that a macrophage-centered blockade of immune checkpoints as well as a stimulation of their expression represents promising therapeutic avenues. Ultimately, the therapeutic potential of a macrophage-focused checkpoint therapy might be maximized by diagnostically assessing individual checkpoint expression levels on macrophages, thereby personalizing an effective treatment approach for each patient having cancer, infection, or autoimmune diseases.

Knockout of the P2Y6 Receptor Prevents Peri-Infarct Neuronal Loss after Transient, Focal Ischemia in Mouse Brain

After stroke, there is a delayed neuronal loss in brain areas surrounding the infarct, which may in part be mediated by microglial phagocytosis of stressed neurons. Microglial phagocytosis of stressed or damaged neurons can be mediated by UDP released from stressed neurons activating the P2Y₆ receptor on microglia, inducing microglial phagocytosis of such neurons. We show evidence here from a small trial that the knockout of the P2Y₆ receptor, required for microglial phagocytosis of neurons, prevents the delayed neuronal loss after transient, focal brain ischemia induced by endothelin-1 injection in mice. Wild-type mice had neuronal loss and neuronal nuclear material within microglia in peri-infarct areas. P2Y₆ receptor knockout mice had no significant neuronal loss in peri-infarct brain areas seven days after brain ischemia. Thus, delayed neuronal loss after stroke may in part be mediated by microglial phagocytosis of stressed neurons, and the P2Y₆ receptor is a potential treatment target to prevent peri-infarct neuronal loss.

Long-term high-fat diet consumption by mice throughout adulthood induces neurobehavioral alterations and hippocampal neuronal remodeling accompanied by augmented microglial lipid accumulation

High-fat diet (HFD) consumption is generally associated with an increased risk of cognitive and emotional dysfunctions that constitute a sizeable worldwide health burden with profound social and economic consequences. Middle age is a critical time period that affects one's health later in life; pertinently, the prevalence of HFD consumption is increasing among mature adults. Given the growing health-related economic burden imposed globally by increasing rates of noncommunicable diseases in rapidly aging populations, along with the pervasive but insidious health impairments associated with HFD consumption, it is critically important to understand the effects of long-term HFD consumption on brain function and to gain insights into their potential underlying mechanisms. In the present study, adult male C57BL/6J mice were randomly assigned a control diet (CD, 10 kJ% from fat) or an HFD (60 kJ% from fat) for 6 months (6 M) or 9 months (9 M) followed by behavioral tests, serum biochemical analysis, and histological examinations of both the dorsal and ventral regions of the hippocampus. In both the 6 M and 9 M cohorts, mice that consumed an HFD exhibited poorer memory performance in the Morris water maze test (MWM) and greater depression- and anxiety-like behavior during the open field test (OFT), sucrose preference test (SPT) and forced swim test (FST) than control mice. Compared with age-matched mice in the CD group, mice in the HFD group showed abnormal hippocampal neuronal morphology, which was particularly evident in the ventral hippocampus. Hippocampal microglia in mice in the HFD group generally had a more activated phenotype evidenced by a smaller microglial territory area and increased cluster of differentiation 68 (CD68, a marker of phagocytic activity) immunoreactivity, while the microglial density in the dentate gyrus (DG) was decreased, indicating microglial decline. The engulfment of postsynaptic density 95 (PSD95, a general postsynaptic marker) puncta by microglia was increased in the HFD groups. Histological analysis of neutral lipids using a fluorescent probe (BODIPY) revealed that the total neutral lipid content in regions of interests (ROIs) and the lipid load in microglia were increased in the HFD group relative to the age-matched CD group. In summary, our results demonstrated that chronic HFD consumption from young adulthood to middle age induced anxiety- and depression-like behavior as well as memory impairment. The negative influence of chronic HFD consumption on behavioral and hippocampal neuroplasticity appears to be linked to a change in microglial phenotype that is accompanied by a remarkable increase in cellular lipid accumulation. These observations highlighting the potential to target lipid metabolism deficits to reduce the risk of HFD-associated emotional dysfunctions.

Oral Bacteriotherapy Reduces the Occurrence of Chronic Fatigue in COVID-19 Patients

Long COVID refers to patients with symptoms as fatigue, “brain fog,” pain, suggesting the chronic involvement of the central nervous system (CNS) in COVID-19. The supplementation with probiotic (OB) would have a positive effect on metabolic homeostasis, negatively impacting the occurrence of symptoms related to the CNS after hospital discharge. On a total of 58 patients hospitalized for COVID-19, 24 (41.4%) received OB during hospitalization (OB+) while 34 (58.6%) taken only the standard treatment (OB–). Serum metabolomic profiling of patients has been performed at both hospital acceptance (T0) and discharge (T1). Six months after discharge, fatigue perceived by participants was assessed by administrating the Fatigue Assessment Scale. 70.7% of participants reported fatigue while 29.3% were negative for such condition. The OB+ group showed a significantly lower proportion of subjects reporting fatigue than the OB– one (p < 0.01). Furthermore, OB+ subjects were characterized by significantly increased concentrations of serum Arginine, Asparagine, Lactate opposite to lower levels of 3-Hydroxyisobutirate than those not treated with probiotics. Our results strongly suggest that in COVID-19, the administration of probiotics during hospitalization may prevent the development of chronic fatigue by impacting key metabolites involved in the utilization of glucose as well as in energy pathways.

Targeting Systemic Innate Immune Cells as a Therapeutic Avenue for Alzheimer Disease

Alzheimer disease (AD) is the first progressive neurodegenerative disease worldwide, and the disease is characterized by an accumulation of amyloid in the brain and neurovasculature that triggers cognitive decline and neuroinflammation. The innate immune system has a preponderant role in AD. The last decade, scientists focused their efforts on therapies aiming to modulate innate immunity. The latter is of great interest, since they participate to the inflammation and phagocytose the amyloid in the brain and blood vessels. We and others have developed pharmacological approaches to stimulate these cells using various ligands. These include toll-like receptor 4, macrophage colony stimulating factor, and more recently nucleotide-binding oligomerization domain-containing 2 receptors. This review will discuss the great potential to take advantage of the innate immune system to fight naturally against amyloid β accumulation and prevent its detrimental consequence on brain functions and its vascular system. SIGNIFICANCE STATEMENT: The focus on amyloid β removal from the perivascular space rather than targeting CNS plaque formation and clearance represents a new direction with a great potential. Small molecules able to act at the level of peripheral immunity would constitute a novel approach for tackling aberrant central nervous system biology, one of which we believe would have the potential of generating a lot of interest.

Botulinum Toxin A Ameliorates Neuroinflammation in the MPTP and 6-OHDA-Induced Parkinson's Disease Models

Recently, increasing evidence suggests that neuroinflammation may be a critical factor in the development of Parkinson's disease (PD) in addition to the ratio of acetylcholine/dopamine because dopaminergic neurons are particularly vulnerable to inflammatory attack. In this study, we investigated whether botulinum neurotoxin A (BoNT-A) was effective for the treatment of PD through its anti-neuroinflammatory effects and the modulation of acetylcholine and dopamine release. We found that BoNT-A ameliorated MPTP and 6-OHDA-induced PD progression, reduced acetylcholine release, levels of IL-1β, IL-6 and TNF-α as well as GFAP expression, but enhanced dopamine release and tyrosine hydroxylase expression. These results indicated that BoNT-A had beneficial effects on MPTP or 6-OHDA-induced PD-like behavior impairments via its anti-neuroinflammation properties, recovering dopamine, and reducing acetylcholine release.

The microglial P2Y6 receptor mediates neuronal loss and memory deficits in neurodegeneration

Microglia are implicated in neurodegeneration, potentially by phagocytosing neurons, but it is unclear how to block the detrimental effects of microglia while preserving their beneficial roles. The microglial P2Y₆ receptor (P2Y₆R) - activated by extracellular UDP released by stressed neurons - is required for microglial phagocytosis of neurons. We show here that injection of amyloid beta (Aβ) into mouse brain induces microglial phagocytosis of neurons, followed by neuronal and memory loss, and this is all prevented by knockout of P2Y₆R. In a chronic tau model of neurodegeneration (P301S TAU mice), P2Y₆R knockout prevented TAU-induced neuronal and memory loss. In vitro, P2Y₆R knockout blocked microglial phagocytosis of live but not dead targets and reduced tau-, Aβ-, and UDP-induced neuronal loss in glial-neuronal cultures. Thus, the P2Y₆ receptor appears to mediate Aβ- and tau-induced neuronal and memory loss via microglial phagocytosis of neurons, suggesting that blocking this receptor may be beneficial in the treatment of neurodegenerative diseases.

Neuronal Loss after Stroke Due to Microglial Phagocytosis of Stressed Neurons

After stroke, there is a rapid necrosis of all cells in the infarct, followed by a delayed loss of neurons both in brain areas surrounding the infarct, known as 'selective neuronal loss', and in brain areas remote from, but connected to, the infarct, known as 'secondary neurodegeneration'. Here we review evidence indicating that this delayed loss of neurons after stroke is mediated by the microglial phagocytosis of stressed neurons. After a stroke, neurons are stressed by ongoing ischemia, excitotoxicity and/or inflammation and are known to: (i) release "find-me" signals such as ATP, (ii) expose "eat-me" signals such as phosphatidylserine, and (iii) bind to opsonins, such as complement components C1q and C3b, inducing microglia to phagocytose such neurons. Blocking these factors on neurons, or their phagocytic receptors on microglia, can prevent delayed neuronal loss and behavioral deficits in rodent models of ischemic stroke. Phagocytic receptors on microglia may be attractive treatment targets to prevent delayed neuronal loss after stroke due to the microglial phagocytosis of stressed neurons.

Tau aggregation and its relation to selected forms of neuronal cell death

How neurons die in neurodegenerative diseases is still unknown. The distinction between apoptosis as a genetically controlled mechanism, and necrosis, which was viewed as an unregulated process, has blurred with the ever-increasing number of necrotic-like death subroutines underpinned by genetically defined pathways. It is therefore pertinent to ask whether any of them apply to neuronal cell death in tauopathies. Although Alzheimer's disease (AD) is the most prevalent tauopathy, tauopathies comprise an array of over 30 diseases in which the cytoplasmic protein tau aggregates in neurons, and also, in some diseases, in glia. Animal models have sought to distil the contribution of tau aggregation to the cell death process but despite intensive research, no one mechanism of cell death has been unequivocally defined. The process of tau aggregation, and the fibrillar structures that form, touch on so many cellular functions that there is unlikely to be a simple linear pathway of death; as one is blocked another is likely to take the lead. It is timely to ask how far we have advanced into defining whether any of the molecular players in the new death subroutines participate in the death process. Here we briefly review the currently known cell death routines and explore what is known about their participation in tau aggregation-related cell death. We highlight the involvement of cell autonomous and the more recent non-cell autonomous pathways that may enhance tau-aggregate toxicity, and discuss recent findings that implicate microglial phagocytosis of live neurons with tau aggregates as a mechanism of death.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

Impact of Fetal Growth Restriction on the Neonatal Microglial Proteome in the Rat

Microglial activation is a key modulator of brain vulnerability in response to intra-uterine growth restriction (IUGR). However, the consequences of IUGR on microglial development and the microglial proteome are still unknown. We used a model of IUGR induced by a gestational low-protein diet (LPD) in rats. Microglia, isolated from control and growth-restricted animals at P1 and P4, showed significant changes in the proteome between the two groups. The expression of protein sets associated with fetal growth, inflammation, and the immune response were significantly enriched in LPD microglia at P1 and P4. Interestingly, upregulation of protein sets associated with the oxidative stress response and reactive oxygen species production was observed at P4 but not P1. During development, inflammation-associated proteins were upregulated between P1 and P4 in both control and LPD microglia. By contrast, proteins associated with DNA repair and senescence pathways were upregulated in only LPD microglia. Similarly, protein sets involved in protein retrograde transport were significantly downregulated in only LPD microglia. Overall, these data demonstrate significant and multiple effects of LPD-induced IUGR on the developmental program of microglial cells, leading to an abnormal proteome within the first postnatal days.

Inflammatory neuronal loss in the substantia nigra induced by systemic lipopolysaccharide is prevented by knockout of the P2Y6 receptor in mice

Inflammation may contribute to multiple brain pathologies. One cause of inflammation is lipopolysaccharide/endotoxin (LPS), the levels of which are elevated in blood and/or brain during bacterial infections, gut dysfunction and neurodegenerative diseases, such as Parkinson’s disease. How inflammation causes neuronal loss is unclear, but one potential mechanism is microglial phagocytosis of neurons, which is dependent on the microglial P2Y₆ receptor. We investigated here whether the P2Y₆ receptor was required for inflammatory neuronal loss. Intraperitoneal injection of LPS on 4 successive days resulted in specific loss of dopaminergic neurons (measured as cells staining with tyrosine hydroxylase or NeuN) in the substantia nigra of wild-type mice, but no neuronal loss in cortex or hippocampus. This supports the hypothesis that neuronal loss in Parkinson’s disease may be driven by peripheral LPS. By contrast, there was no LPS-induced neuronal loss in P2Y₆ receptor knockout mice. In vitro, LPS-induced microglial phagocytosis of cells was prevented by inhibition of the P2Y₆ receptor, and LPS-induced neuronal loss was reduced in mixed glial–neuronal cultures from P2Y₆ receptor knockout mice. This supports the hypothesis that microglial phagocytosis contributes to inflammatory neuronal loss, and can be prevented by blocking the P2Y₆ receptor, suggesting that P2Y₆ receptor antagonists might be used to prevent inflammatory neuronal loss in Parkinson’s disease and other brain pathologies involving inflammatory neuronal loss.

Acid-Sensing Ion Channels: Expression and Function in Resident and Infiltrating Immune Cells in the Central Nervous System

Peripheral and central immune cells are critical for fighting disease, but they can also play a pivotal role in the onset and/or progression of a variety of neurological conditions that affect the central nervous system (CNS). Tissue acidosis is often present in CNS pathologies such as multiple sclerosis, epileptic seizures, and depression, and local pH is also reduced during periods of ischemia following stroke, traumatic brain injury, and spinal cord injury. These pathological increases in extracellular acidity can activate a class of proton-gated channels known as acid-sensing ion channels (ASICs). ASICs have been primarily studied due to their ubiquitous expression throughout the nervous system, but it is less well recognized that they are also found in various types of immune cells. In this review, we explore what is currently known about the expression of ASICs in both peripheral and CNS-resident immune cells, and how channel activation during pathological tissue acidosis may lead to altered immune cell function that in turn modulates inflammatory pathology in the CNS. We identify gaps in the literature where ASICs and immune cell function has not been characterized, such as neurotrauma. Knowledge of the contribution of ASICs to immune cell function in neuropathology will be critical for determining whether the therapeutic benefits of ASIC inhibition might be due in part to an effect on immune cells.

Celiac Disease Defined by Over-Sensitivity to Gliadin Activation and Superior Antigen Presentation of Dendritic Cells

The autoimmune condition, Celiac Disease (CeD), displays broad clinical symptoms due to gluten exposure. Its genetic association with DQ variants in the human leukocyte antigen (HLA) system has been recognised. Monocyte-derived mature dendritic cells (MoDCs) present gluten peptides through HLA-DQ and co-stimulatory molecules to T lymphocytes, eliciting a cytokine-rich microenvironment. Having access to CeD associated families prevalent in the Czech Republic, this study utilised an in vitro model to investigate their differential monocyte profile. The higher monocyte yields isolated from PBMCs of CeD patients versus control individuals also reflected the greater proportion of dendritic cells derived from these sources following lipopolysaccharide (LPS)/ peptic-tryptic-gliadin (PTG) fragment stimulation. Cell surface markers of CeD monocytes and MoDCs were subsequently profiled. This foremost study identified a novel bio-profile characterised by elevated CD64 and reduced CD33 levels, unique to CD14++ monocytes of CeD patients. Normalisation to LPS stimulation revealed the increased sensitivity of CeD-MoDCs to PTG, as shown by CD86 and HLA-DQ flow cytometric readouts. Enhanced CD86 and HLA-DQ expression in CeD-MoDCs were revealed by confocal microscopy. Analysis highlighted their dominance at the CeD-MoDC membrane in comparison to controls, reflective of superior antigen presentation ability. In conclusion, this investigative study deciphered the monocytes and MoDCs of CeD patients with the identification of a novel bio-profile marker of potential diagnostic value for clinical interpretation. Herein, the characterisation of CD86 and HLA-DQ as activators to stimulants, along with robust membrane assembly reflective of efficient antigen presentation, offers CeD targeted therapeutic avenues worth further exploration.

Oxygen Sensing and Signaling in Alzheimer's Disease: A Breathtaking Story!

Oxygen sensing and homeostasis is indispensable for the maintenance of brain structural and functional integrity. Under low-oxygen tension, the non-diseased brain has the ability to cope with hypoxia by triggering a homeostatic response governed by the highly conserved hypoxia-inducible family (HIF) of transcription factors. With the advent of advanced neuroimaging tools, it is now recognized that cerebral hypoperfusion, and consequently hypoxia, is a consistent feature along the Alzheimer's disease (AD) continuum. Of note, the reduction in cerebral blood flow and tissue oxygenation detected during the prodromal phases of AD, drastically aggravates as disease progresses. Within this scenario a fundamental question arises: How HIF-driven homeostatic brain response to hypoxia "behaves" during the AD continuum? In this sense, the present review is aimed to critically discuss and summarize the current knowledge regarding the involvement of hypoxia and HIF signaling in the onset and progression of AD pathology. Importantly, the promises and challenges of non-pharmacological and pharmacological strategies aimed to target hypoxia will be discussed as a new "hope" to prevent and/or postpone the neurodegenerative events that occur in the AD brain.

Microglia as the Critical Regulators of Neuroprotection and Functional Recovery in Cerebral Ischemia

Microglial activation is considered as the critical pathogenic event in diverse central nervous system disorders including cerebral ischemia. Proinflammatory responses of activated microglia have been well reported in the ischemic brain and neuroinflammatory responses of activated microglia have been believed to be the potential therapeutic strategy. However, despite having proinflammatory roles, microglia can have significant anti-inflammatory roles and they are associated with the production of growth factors which are responsible for neuroprotection and recovery after ischemic injury. Microglia can directly promote neuroprotection by preventing ischemic infarct expansion and promoting functional outcomes. Indirectly, microglia are involved in promoting anti-inflammatory responses, neurogenesis, and angiogenesis in the ischemic brain which are crucial pathophysiological events for ischemic recovery. In fact, anti-inflammatory cytokines and growth factors produced by microglia can promote neuroprotection and attenuate neurobehavioral deficits. In addition, microglia regulate phagocytosis, axonal regeneration, blood-brain barrier protection, white matter integrity, and synaptic remodeling, which are essential for ischemic recovery. Microglia can also regulate crosstalk with neurons and other cell types to promote neuroprotection and ischemic recovery. This review mainly focuses on the roles of microglia in neuroprotection and recovery following ischemic injury. Furthermore, this review also sheds the light on the therapeutic potential of microglia in stroke patients.

Ganglioside GD3 is up-regulated in microglia and regulates phagocytosis following global cerebral ischemia

Gangliosides, the major sialic-acid containing glycosphingolipids in the mammalian brain, play important roles in brain development and neural functions. Here, we show that the b-series ganglioside GD3 and its biosynthetic enzyme, GD3-synthase (GD3S), were up-regulated predominantly in the microglia of mouse hippocampus from 2 to 7 days following global cerebral ischemia (GCI). Interestingly, GD3S knockout (GD3S-KO) mice exhibited decreased hippocampal neuronal loss following GCI, as compared to wild-type (WT) mice. While comparable levels of astrogliosis and microglial proliferation were observed between WT and GD3S-KO mice, the phagocytic capacity of the GD3S-KO microglia was significantly compromised after GCI. At 2 and 4 days following GCI, the GD3S-KO microglia demonstrated decreased amoebic morphology, reduced neuronal material engulfment, and lower expression of the phagolysosome marker CD68, as compared to the WT microglia. Finally, by using a microglia-primary neuron co-culture model, we demonstrated that the GD3S-KO microglia isolated from mouse brains at 2 days after GCI are less neurotoxic to co-cultured hippocampal neurons than the WT-GCI microglia. Moreover, the percentage of microglia with engulfed neuronal elements in the co-cultured wells was also significantly decreased in the GD3S-KO mice after GCI. Interestingly, the impaired phagocytic capacity of GD3S-KO microglia could be partially restored by pre-treatment with exogenous ganglioside GD3. Altogether, this study provides functional evidence that ganglioside GD3 regulates phagocytosis by microglia in an ischemic stroke model. Our data also suggest that the GD3-linked microglial phagocytosis may contribute to the mechanism of delayed neuronal death following ischemic brain injury.

Functional Role of Non-Muscle Myosin II in Microglia: An Updated Review

Myosins are a remarkable superfamily of actin-based motor proteins that use the energy derived from ATP hydrolysis to translocate actin filaments and to produce force. Myosins are abundant in different types of tissues and involved in a large variety of cellular functions. Several classes of the myosin superfamily are expressed in the nervous system; among them, non-muscle myosin II (NM II) is expressed in both neurons and non-neuronal brain cells, such as astrocytes, oligodendrocytes, endothelial cells, and microglia. In the nervous system, NM II modulates a variety of functions, such as vesicle transport, phagocytosis, cell migration, cell adhesion and morphology, secretion, transcription, and cytokinesis, as well as playing key roles during brain development, inflammation, repair, and myelination functions. In this review, we will provide a brief overview of recent emerging roles of NM II in resting and activated microglia cells, the principal regulators of immune processes in the central nervous system (CNS) in both physiological and pathological conditions. When stimulated, microglial cells react and produce a number of mediators, such as pro-inflammatory cytokines, free radicals, and nitric oxide, that enhance inflammation and contribute to neurodegenerative diseases. Inhibition of NM II could be a new therapeutic target to treat or to prevent CNS diseases.

The Phagocytic Code Regulating Phagocytosis of Mammalian Cells

Mammalian phagocytes can phagocytose (i.e. eat) other mammalian cells in the body if they display certain signals, and this phagocytosis plays fundamental roles in development, cell turnover, tissue homeostasis and disease prevention. To phagocytose the correct cells, phagocytes must discriminate which cells to eat using a 'phagocytic code' - a set of over 50 known phagocytic signals determining whether a cell is eaten or not - comprising find-me signals, eat-me signals, don't-eat-me signals and opsonins. Most opsonins require binding to eat-me signals - for example, the opsonins galectin-3, calreticulin and C1q bind asialoglycan eat-me signals on target cells - to induce phagocytosis. Some proteins act as 'self-opsonins', while others are 'negative opsonins' or 'phagocyte suppressants', inhibiting phagocytosis. We review known phagocytic signals here, both established and novel, and how they integrate to regulate phagocytosis of several mammalian targets - including excess cells in development, senescent and aged cells, infected cells, cancer cells, dead or dying cells, cell debris and neuronal synapses. Understanding the phagocytic code, and how it goes wrong, may enable novel therapies for multiple pathologies with too much or too little phagocytosis, such as: infectious disease, cancer, neurodegeneration, psychiatric disease, cardiovascular disease, ageing and auto-immune disease.

New Insights Regarding Diagnosis and Medication for Schizophrenia Based on Neuronal Synapse-Microglia Interaction

Schizophrenia is a common psychiatric disorder that usually develops during adolescence and young adulthood. Since genetic and environmental factors are involved in the disease, the molecular status of the pathology of schizophrenia differs across patients. Recent genetic studies have focused on the association between schizophrenia and the immune system, especially microglia–synapse interactions. Microglia physiologically eliminate unnecessary synapses during the developmental period. The overactivation of synaptic pruning by microglia is involved in the pathology of brain disease. This paper focuses on the synaptic pruning function and its molecular machinery and introduces the hypothesis that excessive synaptic pruning plays a role in the development of schizophrenia. Finally, we suggest a strategy for diagnosis and medication based on modulation of the interaction between microglia and synapses. This review provides updated information on the involvement of the immune system in schizophrenia and proposes novel insights regarding diagnostic and therapeutic strategies for this disease.