Microglia states and nomenclature: A field at its crossroads

Microglial research has advanced considerably in recent decades yet has been constrained by a rolling series of dichotomies such as "resting versus activated" and "M1 versus M2." This dualistic classification of good or bad microglia is inconsistent with the wide repertoire of microglial states and functions in development, plasticity, aging, and diseases that were elucidated in recent years. New designations continuously arising in an attempt to describe the different microglial states, notably defined using transcriptomics and proteomics, may easily lead to a misleading, although unintentional, coupling of categories and functions. To address these issues, we assembled a group of multidisciplinary experts to discuss our current understanding of microglial states as a dynamic concept and the importance of addressing microglial function. Here, we provide a conceptual framework and recommendations on the use of microglial nomenclature for researchers, reviewers, and editors, which will serve as the foundations for a future white paper.

Inhibiting Microglia Expansion Prevents Diet-Induced Hypothalamic and Peripheral Inflammation

Cell proliferation and neuroinflammation in the adult hypothalamus may contribute to the pathogenesis of obesity. We tested whether the intertwining of these two processes plays a role in the metabolic changes caused by 3 weeks of a high-saturated fat diet (HFD) consumption. Compared with chow-fed mice, HFD-fed mice had a rapid increase in body weight and fat mass and specifically showed an increased number of microglia in the arcuate nucleus (ARC) of the hypothalamus. Microglia expansion required the adequate presence of fats and carbohydrates in the diet because feeding mice a very high-fat, very low-carbohydrate diet did not affect cell proliferation. Blocking HFD-induced cell proliferation by central delivery of the antimitotic drug arabinofuranosyl cytidine (AraC) blunted food intake, body weight gain, and adiposity. AraC treatment completely prevented the increase in number of activated microglia in the ARC, the expression of the proinflammatory cytokine tumor necrosis factor-α in microglia, and the recruitment of the nuclear factor-κB pathway while restoring hypothalamic leptin sensitivity. Central blockade of cell proliferation also normalized circulating levels of the cytokines leptin and interleukin 1β and decreased peritoneal proinflammatory CD86 immunoreactive macrophage number. These findings suggest that inhibition of diet-dependent microglia expansion hinders body weight gain while preventing central and peripheral inflammatory responses due to caloric overload.

Neuronal Hyperactivity Disturbs ATP Microgradients, Impairs Microglial Motility, and Reduces Phagocytic Receptor Expression Triggering Apoptosis/Microglial Phagocytosis Uncoupling

Phagocytosis is essential to maintain tissue homeostasis in a large number of inflammatory and autoimmune diseases, but its role in the diseased brain is poorly explored. Recent findings suggest that in the adult hippocampal neurogenic niche, where the excess of newborn cells undergo apoptosis in physiological conditions, phagocytosis is efficiently executed by surveillant, ramified microglia. To test whether microglia are efficient phagocytes in the diseased brain as well, we confronted them with a series of apoptotic challenges and discovered a generalized response. When challenged with excitotoxicity in vitro (via the glutamate agonist NMDA) or inflammation in vivo (via systemic administration of bacterial lipopolysaccharides or by omega 3 fatty acid deficient diets), microglia resorted to different strategies to boost their phagocytic efficiency and compensate for the increased number of apoptotic cells, thus maintaining phagocytosis and apoptosis tightly coupled. Unexpectedly, this coupling was chronically lost in a mouse model of mesial temporal lobe epilepsy (MTLE) as well as in hippocampal tissue resected from individuals with MTLE, a major neurological disorder characterized by seizures, excitotoxicity, and inflammation. Importantly, the loss of phagocytosis/apoptosis coupling correlated with the expression of microglial proinflammatory, epileptogenic cytokines, suggesting its contribution to the pathophysiology of epilepsy. The phagocytic blockade resulted from reduced microglial surveillance and apoptotic cell recognition receptor expression and was not directly mediated by signaling through microglial glutamate receptors. Instead, it was related to the disruption of local ATP microgradients caused by the hyperactivity of the hippocampal network, at least in the acute phase of epilepsy. Finally, the uncoupling led to an accumulation of apoptotic newborn cells in the neurogenic niche that was due not to decreased survival but to delayed cell clearance after seizures. These results demonstrate that the efficiency of microglial phagocytosis critically affects the dynamics of apoptosis and urge to routinely assess the microglial phagocytic efficiency in neurodegenerative disorders.

Phagocytosis by microglia is tightly coupled to apoptosis, swiftly removing apoptotic cells and actively maintaining tissue homeostasis, but the neuronal hyperactivity associated with epilepsy disrupts the ATP gradients that drive phagocytosis, leading to the accumulation of apoptotic cells and inflammation.

Phagocytosis, the engulfment and digestion of cellular debris, is at the core of the regenerative response of the damaged tissue, because it prevents the spillover of toxic intracellular contents and is actively anti-inflammatory. In the brain, the professional phagocytes are microglia, whose dynamic processes rapidly engulf and degrade cells undergoing apoptosis—programmed cell death—in physiological conditions. Thus, microglia hold the key to brain regeneration, but their efficiency as phagocytes in the diseased brain is only presumed. Here, we have discovered a generalized response of microglia to apoptotic challenge induced by excitotoxicity and inflammation, in which they boost their phagocytic efficiency to account for the increase in apoptosis. To our surprise, this apoptosis/microglial phagocytosis coupling was lost in the hippocampus from human and experimental mesial temporal lobe epilepsy (MTLE), a major neurodegenerative disorder characterized by excitotoxicity, inflammation, and seizures. This uncoupling was due to widespread ATP release during neuronal hyperactivity, which “blinded” microglia to the ATP microgradients released by apoptotic cells as “find-me” signals. The impairment of phagocytosis led to the accumulation of apoptotic cells and the build-up of a detrimental inflammatory reaction. Our data advocates for systematic assessment of the efficiency of microglial phagocytosis in brain disorders.

Resolvin D1 and E1 promote resolution of inflammation in microglial cells in vitro

Sustained inflammation in the brain together with microglia activation can lead to neuronal damage. Hence limiting brain inflammation and activation of microglia is a real therapeutic strategy for inflammatory disease. Resolvin D1 (RvD1) and resolvin E1 (RvE1) derived from n-3 long chain polyunsaturated fatty acids are promising therapeutic compounds since they actively turn off the systemic inflammatory response. We thus evaluated the anti-inflammatory activities of RvD1 and RvE1 in microglia cells in vitro. BV2 cells were pre-incubated with RvD1 or RvE1 before lipopolysaccharide (LPS) treatment. RvD1 and RvE1 both decreased LPS-induced proinflammatory cytokines (TNF-α, IL-6 and IL-1β) gene expression, suggesting their proresolutive activity in microglia. However, the mechanisms involved are distinct as RvE1 regulates NFκB signaling pathway and RvD1 regulates miRNAs expression. Overall, our findings support that pro-resolving lipids are involved in the resolution of brain inflammation and can be considered as promising therapeutic agents for brain inflammation.

RGS9-2 negatively modulates L-3,4-dihydroxyphenylalanine-induced dyskinesia in experimental Parkinson's disease

Chronic L-dopa treatment of Parkinson's disease (PD) often leads to debilitating involuntary movements, termed L-dopa-induced dyskinesia (LID), mediated by dopamine (DA) receptors. RGS9-2 is a GTPase accelerating protein that inhibits DA D2 receptor-activated G proteins. Herein, we assess the functional role of RGS9-2 on LID. In monkeys, Western blot analysis of striatal extracts shows that RGS9-2 levels are not altered by MPTP-induced DA denervation and/or chronic L-dopa administration. In MPTP monkeys with LID, striatal RGS9-2 overexpression--achieved by viral vector injection into the striatum--diminishes the involuntary movement intensity without lessening the anti-parkinsonian effects of the D1/D2 receptor agonist L-dopa. In contrasts, in these animals, striatal RGS9-2 overexpression diminishes both the involuntary movement intensity and the anti-parkinsonian effects of the D2/D3 receptor agonist ropinirole. In unilaterally 6-OHDA-lesioned rats with LID, we show that the time course of viral vector-mediated striatal RGS9-2 overexpression parallels the time course of improvement of L-dopa-induced involuntary movements. We also find that unilateral 6-OHDA-lesioned RGS9-/- mice are more susceptible to L-dopa-induced involuntary movements than unilateral 6-OHDA-lesioned RGS9+/+ mice, albeit the rotational behavior--taken as an index of the anti-parkinsonian response--is similar between the two groups of mice. Together, these findings suggest that RGS9-2 plays a pivotal role in LID pathophysiology. However, the findings also suggest that increasing RGS9-2 expression and/or function in PD patients may only be a suitable therapeutic strategy to control involuntary movements induced by nonselective DA agonist such as L-dopa.

Essential omega-3 fatty acids tune microglial phagocytosis of synaptic elements in the mouse developing brain

Omega-3 fatty acids (n-3 PUFAs) are essential for the functional maturation of the brain. Westernization of dietary habits in both developed and developing countries is accompanied by a progressive reduction in dietary intake of n-3 PUFAs. Low maternal intake of n-3 PUFAs has been linked to neurodevelopmental diseases in Humans. However, the n-3 PUFAs deficiency-mediated mechanisms affecting the development of the central nervous system are poorly understood. Active microglial engulfment of synapses regulates brain development. Impaired synaptic pruning is associated with several neurodevelopmental disorders. Here, we identify a molecular mechanism for detrimental effects of low maternal n-3 PUFA intake on hippocampal development in mice. Our results show that maternal dietary n-3 PUFA deficiency increases microglia-mediated phagocytosis of synaptic elements in the rodent developing hippocampus, partly through the activation of 12/15-lipoxygenase (LOX)/12-HETE signaling, altering neuronal morphology and affecting cognitive performance of the offspring. These findings provide a mechanistic insight into neurodevelopmental defects caused by maternal n-3 PUFAs dietary deficiency.

Shaping of motor responses by incentive values through the basal ganglia

The striatum is a key neural interface for cognitive and motor information processing in which associations between reward value and visual stimulus can be used to modify motor commands. It can guide action-selection processes that occur farther downstream in the basal ganglia (BG) circuit, by encoding the reward value of an action. Here, we report on the study of simultaneously recorded neurons in the dorsal striatum (input stage of the BG) and the internal pallidum (output stage of the BG) in two monkeys performing a center-out motor task in which the visual targets were associated with different reward probabilities. We show that the tuning curves of motor-related neurons in both structures are modulated by the value of the action before movement initiation and during its execution. The representations of values associated with different actions change dynamically during the task in the internal globus pallidus, with a significant increase in the number of encoding neurons for the chosen target at the onset of movement. This report sheds additional light on the functional differences between the input and output structures of the BG and supports the assertion that the dorsal basal ganglia are involved in movement-related decision-making processes based on incentive values.

Microglia in neuronal plasticity: Influence of stress

The central nervous system (CNS) has previously been regarded as an immune-privileged site with the absence of immune cell responses but this dogma was not entirely true. Microglia are the brain innate immune cells and recent findings indicate that they participate both in CNS disease and infection as well as facilitate normal CNS function. Microglia are highly plastic and play integral roles in sculpting the structure of the CNS, refining neuronal circuitry and connectivity, and contribute actively to neuronal plasticity in the healthy brain. Interestingly, psychological stress can perturb the function of microglia in association with an impaired neuronal plasticity and the development of emotional behavior alterations. As a result it seemed important to describe in this review some findings indicating that the stress-induced microglia dysfunction may underlie neuroplasticity deficits associated to many mood disorders. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

n-3 LCPUFA improves cognition: the young, the old and the sick

Due to the implication of docosahexaenoic acid (DHA) in neurogenesis, synaptogenesis, neurite outgrowth and to its high incorporation into the brain, this n-3 long chain polyunsaturated fatty acid (LCPUFA) is considered as crucial in the development and maintenance of the learning memory performance throughout life. In the present chapter we aimed at reviewing data investigating the relation between DHA and cognition during the perinatal period, young adult- and adulthood and neurodegenerative diseases such as Alzheimer disease (AD). In Humans, dietary DHA supplementation from the perinatal period to adulthood does not reveal a clear and consistent memory improvement whereas it is the case in animal studies. The positive effects observed in animal models may have been enhanced by using n-3 PUFA deficient animal models as controls. In animal models of AD, a general consensus on the beneficial effects of n-3 LCPUFA in attenuating cognitive impairment was established. These studies make DHA a potential suitable micronutrient for the maintenance of cognitive performance at all periods of life.

Dietary omega-3 deficiency exacerbates inflammation and reveals spatial memory deficits in mice exposed to lipopolysaccharide during gestation

Maternal immune activation (MIA) is a common environmental insult on the developing brain and represents a risk factor for neurodevelopmental disorders. Animal models of in utero inflammation further revealed a causal link between maternal inflammatory activation during pregnancy and behavioural impairment relevant to neurodevelopmental disorders in the offspring. Accumulating evidence point out that proinflammatory cytokines produced both in the maternal and fetal compartments are responsible for social, cognitive and emotional behavioral deficits in the offspring. Polyunsaturated fatty acids (PUFAs) are essential fatty acids with potent immunomodulatory activities. PUFAs and their bioactive derivatives can promote or inhibit many aspects of the immune and inflammatory response. PUFAs of the n-3 series ('n-3 PUFAs', also known as omega-3) exhibit anti-inflammatory/pro-resolution properties and promote immune functions, while PUFAs of the n-6 series ('n-6 PUFAs' or omega-6) favor pro-inflammatory responses. The present study aimed at providing insight into the effects of n-3 PUFAs on the consequences of MIA on brain development. We hypothesized that a reduction in n-3 PUFAs exacerbates both maternal and fetal inflammatory responses to MIA and later-life defects in memory in the offspring. Based on a lipopolysaccharide (LPS) model of MIA (LPS injection at embryonic day 17), we showed that n-3 PUFA deficiency 1) alters fatty acid composition of the fetal and adult offspring brain; 2) exacerbates maternal and fetal inflammatory processes with no significant alteration of microglia phenotype, and 3) induces spatial memory deficits in the adult offspring. We also showed a strong negative correlation between brain content in n-3 PUFA and cytokine production in MIA-exposed fetuses. Overall, our study is the first to address the deleterious effects of n-3 PUFA deficiency on brain lipid composition, inflammation and memory performances in MIA-exposed animals and indicates that it should be considered as a potent environmental risk factor for the apparition of neurodevelopmental disorders.

Phenotype of striatofugal medium spiny neurons in parkinsonian and dyskinetic nonhuman primates: a call for a reappraisal of the functional organization of the basal ganglia

The classic view of anatomofunctional organization of the basal ganglia is that striatopallidal neurons of the "indirect" pathway express D2 dopamine receptors and corelease enkephalin with GABA, whereas striatopallidal neurons of the "direct" pathway bear D1 dopamine receptors and corelease dynorphin and substance P with GABA. Although many studies have investigated the pathophysiology of the basal ganglia after dopamine denervation and subsequent chronic levodopa (L-dopa) treatment, none has ever considered the possibility of plastic changes leading to profound reorganization and/or biochemical phenotype modifications of medium spiny neurons. Therefore, we studied the phenotype of striatal neurons in four groups of nonhuman primates, including the following: normal, parkinsonian, parkinsonian chronically treated with L-dopa without exhibiting dyskinesia, and parkinsonian chronically treated with L-dopa exhibiting overt dyskinesia. To identify striatal cells projecting to external (indirect) or internal (direct) segments of the globus pallidus, the retrograde tracer cholera toxin subunit B (CTb) was injected stereotaxically into the terminal areas. Using immunohistochemistry techniques, brain sections were double labeled for CTb and dopamine receptors, opioid peptides, or the substance P receptor (NK1). We also used HPLC-RIA to assess opioid levels throughout structures of the basal ganglia. Our results suggest that medium spiny neurons retain their phenotype because no variations were observed in any experimental condition. Therefore, it appears unlikely that dyskinesia is related to a phenotype modification of the striatal neurons. However, this study supports the concept of axonal collateralization of striatofugal cells that project to both globus pallidus pars externa and globus pallidus pars interna. Striatofugal pathways are not as segregated in the primate as previously considered.

Astrocyte-derived adenosine modulates increased sleep pressure during inflammatory response

Activation of the immune system elicits several behavioral changes collectively called sickness. Among the behavioral changes, systemic infections induce an increase in time spent in nonrapid-eye-movement (NREM) sleep and an increase of slow wave activity (or "sleep pressure"). Using an inducible, astrocyte-specific transgenic dominant negative SNARE (dnSNARE) mouse line we recently demonstrated that gliotransmission plays an important role in sleep homeostasis through an adenosine receptor 1 (A1R)-sensitive pathway. It has been hypothesized that systemic infection, mimicked by peripheral administration of lipopolysaccharide (LPS), increases sleeping behavior in part through upregulation of central adenosine levels. Moreover, as a source of immunologically relevant factors, astrocytes play a pivotal role in the central nervous system immune and inflammatory responses. However, little is known about the role of astrocytes in the CNS response to a peripheral immune challenge. We hypothesize that LPS impacts sleep homeostasis through the modulation of astrocyte-derived adenosine accumulation. We therefore used dnSNARE mice to determine whether astrocytes contribute to the increased sleep pressure under immune challenge and whether this is a result of changes in adenosine signaling. We demonstrate that dnSNARE-mediated gliotransmission is required for the ability of LPS to elevate sleep pressure as measured by the power of slow wave activity during NREM sleep. Moreover, in agreement with a role of astrocyte-derived adenosine in modulating sleep homeostasis, we find that intracerebroventricular infusion of the A1R antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) mimics this dnSNARE phenotype. Taken together, our data demonstrate that astrocytic adenosine acting through A1 receptors contributes to the modulation of sleep pressure by LPS.

Early morphofunctional plasticity of microglia in response to acute lipopolysaccharide

Within the central nervous system (CNS) the traditional role of microglia has been in brain infection and disease, phagocytosing debris and secreting factors to modify disease progression. This led to the concept of "resting" versus "activated" microglia. However, this is misleading because multiple phenotypic and morphological stages of microglia can influence neuronal structure and function in any condition and recent evidence extends their role to healthy brain homeostasis. The present work was thus aimed at reappraising the concept of morphofunctional activity of microglia in a context of peripheral acute immune challenge, where microglial activity is known to be modified, using the new state-of-the-art techniques available. To do so, mice were injected peripherally with lipopolysaccharide, a potent inducer of cerebral inflammation, and we assessed early cytokines production, phenotype, motility and morphology of microglial cells. Our results showed that LPS induced a widespread inflammatory response both peripherally and centrally, as revealed by the quantification of cytokines levels. We also found an alteration of microglial motility that was confirmed by in vivo studies showing an overall reduction of microglial processes length in the hippocampus of LPS-treated animals. Finally, analysis of various surface receptors expression revealed that LPS did not significantly impact microglial phenotype 2h after the injection but rather induced an increase of CD11b(+)/CD45(high) cells. These latter may be at the vasculature, at the CNS vicinity, or may have invaded the CNS.

Dietary n-3 long chain PUFA supplementation promotes a pro-resolving oxylipin profile in the brain

The brain is highly enriched in long chain polyunsaturated fatty acids (LC-PUFAs) that display immunomodulatory properties in the brain. At the periphery, the modulation of inflammation by LC-PUFAs occurs through lipid mediators called oxylipins which have anti-inflammatory and pro-resolving activities when derived from n-3 LC-PUFAs and pro-inflammatory activities when derived from n-6 LC-PUFAs. However, whether a diet rich in LC-PUFAs modulates oxylipins and neuroinflammation in the brain has been poorly investigated. In this study, the effect of a dietary n-3 LC-PUFA supplementation on oxylipin profile and neuroinflammation in the brain was analyzed. Mice were given diets deficient or supplemented in n-3 LC-PUFAs for a 2-month period starting at post-natal day 21, followed by a peripheral administration of lipopolysaccharide (LPS) at adulthood. We first showed that dietary n-3 LC-PUFA supplementation induced n-3 LC-PUFA enrichment in the hippocampus and subsequently an increase in n-3 PUFA-derived oxylipins and a decrease in n-6 PUFA-derived oxylipins. In response to LPS, n-3 LC-PUFA deficient mice presented a pro-inflammatory oxylipin profile whereas n-3 LC-PUFA supplemented mice displayed an anti-inflammatory oxylipin profile in the hippocampus. Accordingly, the expression of cyclooxygenase-2 and 5-lipoxygenase, the enzymes implicated in pro- and anti-inflammatory oxylipin synthesis, was induced by LPS in both diets. In addition, LPS-induced pro-inflammatory cytokine increase was reduced by dietary n-3 LC-PUFA supplementation. These results indicate that brain n-3 LC-PUFAs increase by dietary means and promote the synthesis of anti-inflammatory derived bioactive oxylipins. As neuroinflammation plays a key role in all brain injuries and many neurodegenerative disorders, the present data suggest that dietary habits may be an important regulator of brain cytokine production in these contexts.

Roles of Microglial Phagocytosis and Inflammatory Mediators in the Pathophysiology of Sleep Disorders

Sleep serves crucial learning and memory functions in both nervous and immune systems. Microglia are brain immune cells that actively maintain health through their crucial physiological roles exerted across the lifespan, including phagocytosis of cellular debris and orchestration of neuroinflammation. The past decade has witnessed an explosive growth of microglial research. Considering the recent developments in the field of microglia and sleep, we examine their possible impact on various pathological conditions associated with a gain, disruption, or loss of sleep in this focused mini-review. While there are extensive studies of microglial implication in a variety of neuropsychiatric and neurodegenerative diseases, less is known regarding their roles in sleep disorders. It is timely to stimulate new research in this emergent and rapidly growing field of investigation.

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.

Complement C3 mediates early hippocampal neurodegeneration and memory impairment in experimental multiple sclerosis

Memory impairment is one of the disabling manifestations of multiple sclerosis (MS) possibly present from the early stages of the disease and for which there is no specific treatment. Hippocampal synaptic dysfunction and dendritic loss, associated with microglial activation, can underlie memory deficits, yet the molecular mechanisms driving such hippocampal neurodegeneration need to be elucidated. In early-stage experimental autoimmune encephalomyelitis (EAE) female mice, we assessed the expression level of molecules involved in microglia-neuron interactions within the dentate gyrus and found overexpression of genes of the complement pathway. Compared to sham immunized mice, the central element of the complement cascade, C3, showed the strongest and 10-fold upregulation, while there was no increase of downstream factors such as the terminal component C5. The combination of in situ hybridization with immunofluorescence showed that C3 transcripts were essentially produced by activated microglia. Pharmacological inhibition of C3 activity, by daily administration of rosmarinic acid, was sufficient to prevent early dendritic loss, microglia-mediated phagocytosis of synapses in the dentate gyrus, and memory impairment in EAE mice, while morphological markers of microglial activation were still observed. In line, when EAE was induced in C3 deficient mice (C3KO), dendrites and spines of the dentate gyrus as well as memory abilities were preserved. Altogether, these data highlight the central role of microglial C3 in early hippocampal neurodegeneration and memory impairment in EAE and, therefore, pave the way toward new neuroprotective strategies in MS to prevent cognitive deficit using complement inhibitors.

Brain eicosapentaenoic acid metabolism as a lead for novel therapeutics in major depression

The results of several meta-analyses suggest that eicosapentaenoic acid (EPA) supplementation is therapeutic in managing the symptoms of major depression. It was previously assumed that because EPA is extremely low in the brain it did not cross the blood-brain barrier and any therapeutic effects it exerted would be via the periphery. However, more recent studies have established that EPA does enter the brain, but is rapidly metabolised following entry. While EPA does not accumulate within the brain, it is present in microglia and homeostatic mechanisms may regulate its esterification to phospholipids that serve important roles in cell signaling. Furthermore, a variety of signaling molecules from EPA have been described in the periphery and they have the potential to exert effects within the brain. If EPA is confirmed to be therapeutic in major depression as a result of adequately powered randomized clinical trials, future research on brain EPA metabolism could lead to the discovery of novel targets for treating or preventing major depression.

Signaling pathways of interleukin-1 actions in the brain: anatomical distribution of phospho-ERK1/2 in the brain of rat treated systemically with interleukin-1beta

Interleukin-1beta is released at the periphery during infection and acts on the nervous system to induce fever, neuroendocrine activation, and behavioral changes. These effects are mediated by brain type I IL-1 receptors. In vitro studies have shown the ability of interleukin-1beta to activate mitogen-activated protein kinase signaling pathways including p38, c-Jun N-terminal kinase and extracellular signal-regulated protein kinase 1 and 2 (ERK1/2). In contrast to other mitogen-activated protein kinases, little is known about ERK1/2 activation in the rat brain in response to interleukin-1beta. The aim of the present study was therefore to investigate spatial and temporal activation of ERK1/2 in the rat brain after peripheral administration of interleukin-1beta using immunohistochemistry to detect the phosphorylated form of the kinase. In non-stimulated conditions, phosphorylated ERK1/2 immunoreactivity was observed in neurons throughout the brain. Administration of interleukin-1beta (60 microg/kg, i.p.) induced the phosphorylation of ERK1/2 in areas at the interface between brain and blood or cerebrospinal fluid: meninges, circumventricular organs, endothelial like cells of the blood vessels, and in brain nuclei involved in behavioral depression, fever and neuroendocrine activation: paraventricular nucleus of the hypothalamus, supraoptic nucleus, central amygdala and arcuate nucleus. Double labeling of phosphorylated ERK1/2 and cell markers revealed the expression of phosphorylated ERK1/2 in neurons, astrocytes and microglia. Since phosphorylated ERK1/2 was found in structures in which type I IL-1 receptor has already been identified as well as in structures lacking this receptor, activation of ERK1/2 is likely to occur in response to both direct and indirect action of interleukin-1beta on its target cells.

Direct and indirect effects of lipids on microglia function

Microglia are key players in brain function by maintaining brain homeostasis across lifetime. They participate to brain development and maturation through their ability to release neurotrophic factors, to remove immature synapses or unnecessary neural progenitors. They modulate neuronal activity in healthy adult brains and they also orchestrate the neuroinflammatory response in various pathophysiological contexts such as aging and neurodegenerative diseases. One of the main features of microglia is their high sensitivity to environmental factors, partly via the expression of a wide range of receptors. Recent data pinpoint that dietary fatty acids modulate microglia function. Both the quantity and the type of fatty acid are potent modulators of microglia physiology. The present review aims at dissecting the current knowledge on the direct and indirect mechanisms (focus on gut microbiota and hormones) through which fatty acids influence microglial physiology. We summarize main discoveries from in vitro and in vivo models on fatty acid-mediated microglial modulation. All these studies represent a promising field of research that could promote using nutrition as a novel therapeutic or preventive tool in diseases involving microglia dysfunctions.

Bioactive lipids as new class of microglial modulators: When nutrition meets neuroimunology

Within the central nervous system the traditional role of microglia has been in brain infection and disease, phagocytosing debris and secreting factors to modify disease progression. More recently, microglia have been found to be important for normal brain development, circuit refinement, and synaptic plasticity in ways that were previously unsuspected. Hence, the brain innate immune system appears to be key in all situations, ranging from physiology to pathology. This unique feature of microglia is established by the wide array of receptors it is equipped with to sense molecular patterns. This includes receptors to most if not all neurotransmitters, neuromodulators and purines. We here review novel, yet extensive literature on a new class of microglia modulators, namely bioactive fatty acids. These lipids are issued from metabolism of nutrients and can cross the blood brain barrier to reach the CNS. They appear to be direct modulators of microglial activity, triggering/inhibiting inflammatory processes or enhancing/inhibiting the ability of these cells to respond to hazardous agents.

Enriched dairy fat matrix diet prevents early life lipopolysaccharide-induced spatial memory impairment at adulthood

Polyunsaturated fatty acids (PUFAs) are essential fatty acids, which are critical for brain development and later life cognitive functions. The main brain PUFAs are docosahexaenoic acid (DHA) for the n-3 family and arachidonic acid (ARA) for the n-6 family, which are provided to the post-natal brain by breast milk or infant formula. Recently, the use of dairy lipids (DL) in replacement of vegetable lipids (VL) was revealed to potently promote the accretion of DHA in the developing brain. Brain DHA, in addition to be a key component of brain development, display potent anti-inflammatory activities, which protect the brain from adverse inflammatory events. In this work, we evaluated the protective effect of partial replacement of VL by DL, supplemented or not with DHA and ARA, on post-natal inflammation and its consequence on memory. Mice were fed with diets poor in vegetal n-3 PUFA (Def VL), balanced in vegetal n-3/n-6 PUFA (Bal VL), balanced in dairy lipids (Bal DL) or enriched in DHA and ARA (Supp VL; Supp DL) from the first day of gestation until adulthood. At post-natal day 14 (PND14), pups received a single administration of the endotoxin lipopolysaccharide (LPS) and brain cytokine expression, microglia phenotype and neurogenesis were measured. In a second set of experiments, memory and neurogenesis were measured at adulthood. Overall, our data showed that lipid quality of the diet modulates early life LPS effect on microglia phenotype, brain cytokine expression and neurogenesis at PND14 and memory at adulthood. In particular, Bal DL diet protects from the adverse effect of early life LPS exposure on PND14 neurogenesis and adult spatial memory.

N-3 PUFA deficiency disrupts oligodendrocyte maturation and myelin integrity during brain development

Westernization of dietary habits has led to a progressive reduction in dietary intake of n-3 polyunsaturated fatty acids (n-3 PUFAs). Low maternal intake of n-3 PUFAs has been linked to neurodevelopmental disorders, conditions in which myelination processes are abnormal, leading to defects in brain functional connectivity. Only little is known about the role of n-3 PUFAs in oligodendrocyte physiology and white matter development. Here, we show that lifelong n-3 PUFA deficiency disrupts oligodendrocytes maturation and myelination processes during the postnatal period in mice. This has long-term deleterious consequences on white matter organization and hippocampus-prefrontal functional connectivity in adults, associated with cognitive and emotional disorders. Promoting developmental myelination with clemastine, a first-generation histamine antagonist and enhancer of oligodendrocyte precursor cell differentiation, rescues memory deficits in n-3 PUFA deficient animals. Our findings identify a novel mechanism through which n-3 PUFA deficiency alters brain functions by disrupting oligodendrocyte maturation and brain myelination during the neurodevelopmental period.

Maternal dietary omega-3 deficiency worsens the deleterious effects of prenatal inflammation on the gut-brain axis in the offspring across lifetime

Maternal immune activation (MIA) and poor maternal nutritional habits are risk factors for the occurrence of neurodevelopmental disorders (NDD). Human studies show the deleterious impact of prenatal inflammation and low n-3 polyunsaturated fatty acid (PUFA) intake on neurodevelopment with long-lasting consequences on behavior. However, the mechanisms linking maternal nutritional status to MIA are still unclear, despite their relevance to the etiology of NDD. We demonstrate here that low maternal n-3 PUFA intake worsens MIA-induced early gut dysfunction, including modification of gut microbiota composition and higher local inflammatory reactivity. These deficits correlate with alterations of microglia-neuron crosstalk pathways and have long-lasting effects, both at transcriptional and behavioral levels. This work highlights the perinatal period as a critical time window, especially regarding the role of the gut-brain axis in neurodevelopment, elucidating the link between MIA, poor nutritional habits, and NDD.