Systemic inflammation switches the inflammatory cytokine profile in CNS Wallerian degeneration

Systemic immune challenge exacerbates neurodegeneration in a model of neurological lysosomal disease

Mucopolysaccharidosis type IIIA (MPS IIIA) is a rare paediatric lysosomal storage disorder, caused by the progressive accumulation of heparan sulphate, resulting in neurocognitive decline and behavioural abnormalities. Anecdotal reports from paediatricians indicate a more severe neurodegeneration in MPS IIIA patients, following infection, suggesting inflammation as a potential driver of neuropathology. To test this hypothesis, we performed acute studies in which WT and MPS IIIA mice were challenged with the TLR3-dependent viral mimetic poly(I:C). The challenge with an acute high poly(I:C) dose exacerbated systemic and brain cytokine expression, especially IL-1β in the hippocampus. This was accompanied by an increase in caspase-1 activity within the brain of MPS IIIA mice with concomitant loss of hippocampal GFAP and NeuN expression. Similar levels of cell damage, together with exacerbation of gliosis, were also observed in MPS IIIA mice following low chronic poly(I:C) dosing. While further investigation is warranted to fully understand the extent of IL-1β involvement in MPS IIIA exacerbated neurodegeneration, our data robustly reinforces our previous findings, indicating IL-1β as a pivotal catalyst for neuropathological processes in MPS IIIA.

Noteworthy perspectives on microglia in neuropsychiatric disorders

Microglia are so versatile that they not only provide immune surveillance for central nervous system, but participate in neural circuitry development, brain blood vessels formation, blood-brain barrier architecture, and intriguingly, the regulation of emotions and behaviors. Microglia have a profound impact on neuronal survival, brain wiring and synaptic plasticity. As professional phagocytic cells in the brain, they remove dead cell debris and neurotoxic agents via an elaborate mechanism. The functional profile of microglia varies considerately depending on age, gender, disease context and other internal or external environmental factors. Numerous studies have demonstrated a pivotal involvement of microglia in neuropsychiatric disorders, including negative affection, social deficit, compulsive behavior, fear memory, pain and other symptoms associated with major depression disorder, anxiety disorder, autism spectrum disorder and schizophrenia. In this review, we summarized the latest discoveries regarding microglial ontogeny, cell subtypes or state spectrum, biological functions and mechanistic underpinnings of emotional and behavioral disorders. Furthermore, we highlight the potential of microglia-targeted therapies of neuropsychiatric disorders, and propose outstanding questions to be addressed in future research of human microglia.

GSDMD in peripheral myeloid cells regulates microglial immune training and neuroinflammation in Parkinson's disease

Peripheral bacterial infections without impaired blood-brain barrier integrity have been attributed to the pathogenesis of Parkinson's disease (PD). Peripheral infection promotes innate immune training in microglia and exacerbates neuroinflammation. However, how changes in the peripheral environment mediate microglial training and exacerbation of infection-related PD is unknown. In this study, we demonstrate that GSDMD activation was enhanced in the spleen but not in the CNS of mice primed with low-dose LPS. GSDMD in peripheral myeloid cells promoted microglial immune training, thus exacerbating neuroinflammation and neurodegeneration during PD in an IL-1R-dependent manner. Furthermore, pharmacological inhibition of GSDMD alleviated the symptoms of PD in experimental PD models. Collectively, these findings demonstrate that GSDMD-induced pyroptosis in myeloid cells initiates neuroinflammation by regulating microglial training during infection-related PD. Based on these findings, GSDMD may serve as a therapeutic target for patients with PD.

Vitamin B3 Provides Neuroprotection via Antioxidative Stress in a Rat Model of Anterior Ischemic Optic Neuropathy

Supplementing with vitamin B3 has been reported to protect against retinal ganglion cell (RGC) damage events and exhibit multiple neuroprotective properties in a mouse model of optic nerve injury. In this study, a rat model of anterior ischemic optic neuropathy was used to assess the neuroprotective benefits of vitamin B3 (rAION). Vitamin B3 (500 mg/kg/day) or phosphate-buffered saline (PBS) was administered to the rAION-induced rats every day for 28 days. The vitamin B3-treated group had significantly higher first positive and second negative peak (P1-N2) amplitudes of flash visual-evoked potentials and RGC densities than the PBS-treated group (p < 0.05). A terminal deoxynucleotidyl transferase dUTP nick end labeling assay conducted on vitamin B3-treated rats revealed a significant reduction in apoptotic cells (p < 0.05). Superoxide dismutase and thiobarbituric acid reactive substance activity showed that vitamin B3 treatment decreased reactive oxygen species (p < 0.05). Therefore, vitamin B3 supplementation preserves vision in rAION-induced rats by reducing oxidative stress, neuroinflammation, and mitochondrial apoptosis.

Gonadal hormone trigger the dynamic microglial alterations through Traf6/TAK1 axis that correlate with depressive behaviors

Gonadal hormone deficiency is associated with the development of depression, but what mediates this association is unclear. To test the possibility that it reflects neuroimmune and neuroinflammatory processes, we analyzed how gonadal hormone deficiency and replacement affect microglial activation and inflammatory response during the development of depressive symptomatology in gonadectomized male mice. Testosterone level and the ratio of testosterone to estradiol in the serum and brain tissue of mice exposed to 3-35 days of chronic unpredictable stress were much lower than in control animals. Gonadal hormone sustained deficiency in gonadectomized mice and subsequent led to acute inflammation at day 7 following castration. Activating microglia in mice exposed to 7 days of castration subsequently suppressed the proliferation of microglia, such that their numbers in hippocampus and cortex were lower than the numbers in sham-operated mice after 30 days of castration. Here, we showed that gonadal hormone deficiency induces Traf6-mediated microglia activation, a type of inflammatory mediator. Microglia treated in this way for long time showed down-regulation of activation markers, abnormal morphology and depressive-like behaviors. Restoration and maintenance of a fixed ratio of testosterone to estradiol significantly suppressed microglial activation, neuronal necroptosis, dramatically inducing hippocampal neurogenesis and reducing depressive behaviors via the suppression of Traf6/TAK1 pathway. These findings suggest that activated or immunoreactive microglia contribute to gonadal hormone deficiency-induced depression, as well as testosterone and estradiol exert synergistic anti-depressant effects via suppressing microglial activaton in gonadectomized male mice, possibly through Traf6 signaling.

Traumatic Brain Injury and Risk of Neurodegenerative Disorder

Traumatic brain injury (TBI), particularly of greater severity (i.e., moderate to severe), has been identified as a risk factor for all-cause dementia and Parkinson's disease, with risk for specific dementia subtypes being more variable. Among the limited studies involving neuropathological (postmortem) confirmation, the association between TBI and risk for neurodegenerative disease increases in complexity, with polypathology often reported on examination. The heterogeneous clinical and neuropathological outcomes associated with TBI are likely reflective of the multifaceted postinjury acute and chronic processes that may contribute to neurodegeneration. Acutely in TBI, axonal injury and disrupted transport influences molecular mechanisms fundamental to the formation of pathological proteins, such as amyloid-β peptide and hyperphosphorylated tau. These protein deposits may develop into amyloid-β plaques, hyperphosphorylated tau-positive neurofibrillary tangles, and dystrophic neurites. These and other characteristic neurodegenerative disease pathologies may then spread across brain regions. The acute immune and neuroinflammatory response involves alteration of microglia, astrocytes, oligodendrocytes, and endothelial cells; release of downstream pro- and anti-inflammatory cytokines and chemokines; and recruitment of peripheral immune cells. Although thought to be neuroprotective and reparative initially, prolongation of these processes may promote neurodegeneration. We review the evidence for TBI as a risk factor for neurodegenerative disorders, including Alzheimer's dementia and Parkinson's disease, in clinical and neuropathological studies. Further, we describe the dynamic interactions between acute response to injury and chronic processes that may be involved in TBI-related pathogenesis and progression of neurodegeneration.

Developmental Stressors Induce Innate Immune Memory in Microglia and Contribute to Disease Risk

Many types of stressors have an impact on brain development, function, and disease susceptibility including immune stressors, psychosocial stressors, and exposure to drugs of abuse. We propose that these diverse developmental stressors may utilize a common mechanism that underlies impaired cognitive function and neurodevelopmental disorders such as schizophrenia, autism, and mood disorders that can develop in later life as a result of developmental stressors. While these stressors are directed at critical developmental windows, their impacts are long-lasting. Immune activation is a shared pathophysiology across several different developmental stressors and may thus be a targetable treatment to mitigate the later behavioral deficits. In this review, we explore different types of prenatal and perinatal stressors and their contribution to disease risk and underlying molecular mechanisms. We highlight the impact of developmental stressors on microglia biology because of their early infiltration into the brain, their critical role in brain development and function, and their long-lived status in the brain throughout life. Furthermore, we introduce innate immune memory as a potential underlying mechanism for developmental stressors' impact on disease. Finally, we highlight the molecular and epigenetic reprogramming that is known to underlie innate immune memory and explain how similar molecular mechanisms may be at work for cells to retain a long-term perturbation after exposure to developmental stressors.

Dysregulated phosphoinositide 3-kinase signaling in microglia: shaping chronic neuroinflammation

Microglia are integral mediators of innate immunity within the mammalian central nervous system. Typical microglial responses are transient, intending to restore homeostasis by orchestrating the removal of pathogens and debris and the regeneration of damaged neurons. However, prolonged and persistent microglial activation can drive chronic neuroinflammation and is associated with neurodegenerative disease. Recent evidence has revealed that abnormalities in microglial signaling pathways involving phosphatidylinositol 3-kinase (PI3K) and protein kinase B (AKT) may contribute to altered microglial activity and exacerbated neuroimmune responses. In this scoping review, the known and suspected roles of PI3K-AKT signaling in microglia, both during health and pathological states, will be examined, and the key microglial receptors that induce PI3K-AKT signaling in microglia will be described. Since aberrant signaling is correlated with neurodegenerative disease onset, the relationship between maladapted PI3K-AKT signaling and the development of neurodegenerative disease will also be explored. Finally, studies in which microglial PI3K-AKT signaling has been modulated will be highlighted, as this may prove to be a promising therapeutic approach for the future treatment of a range of neuroinflammatory conditions.

Macrophages in the cochlea; an immunological link between risk factors and progressive hearing loss

Macrophages are abundant in the cochlea; however, their role in hearing loss is not well understood. Insults to the cochlea, such as noise or insertion of a cochlear implant, cause an inflammatory response, which includes activation of tissue-resident macrophages. Activation is characterized by changes in macrophage morphology, mediator expression, and distribution. Evidence from other organs shows activated macrophages can become primed, whereby subsequent insults cause an elevated inflammatory response. Primed macrophages in brain pathologies respond to circulating inflammatory mediators by disproportionate synthesis of inflammatory mediators. This signaling occurs behind an intact blood-brain barrier, similar to the blood-labyrinth barrier in the cochlea. Local tissue damage can occur as the result of mediator release by activated macrophages. Damage is typically localized; however, if it is to structures with limited ability to repair, such as neurons or hair cells within the cochlea, it is feasible that this contributes to the progressive loss of function seen in hearing loss. We propose that macrophages in the cochlea link risk factors and hearing loss. Injury to the cochlea causes local macrophage activation that typically resolves. However, in susceptible individuals, some macrophages enter a primed state. Once primed, these macrophages can be further activated, as a consequence of circulating inflammatory molecules associated with common co-morbidities. Hypothetically, this would lead to further cochlear damage and loss of hearing. We review the evidence for the role of tissue-resident macrophages in the cochlea and propose that cochlear macrophages contribute to the trajectory of hearing loss and warrant further study.

The Effects of Two Nrf2 Activators, Bardoxolone Methyl and Omaveloxolone, on Retinal Ganglion Cell Survival during Ischemic Optic Neuropathy

Nonarteritic anterior ischemic optic neuropathy (NAION) is one of the most common acute optic neuropathies that affect the over 55-year-old population. NAION causes the loss of visual function, and it has no safe and effective therapy. Bardoxolone methyl (methyl 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate; CDDO-Me; RTA 402) is a semisynthetic triterpenoid with effects against antioxidative stress and inflammation in neurodegeneration and kidney disease that activates the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. Moreover, RTA 402 is an FDA-approved compound for the treatment of solid tumors, lymphoid malignancies, melanoma, and chronic kidney disease. Omaveloxolone (RTA 408) is an activator of Nrf2 and an inhibitor of NFκB, possessing antioxidative and anti-inflammatory activities in mitochondrial bioenergetics. RTA 408 is also under clinical investigation for Friedreich ataxia (FA). In this study, a rodent anterior ischemic optic neuropathy (rAION) model induced by photothrombosis was used to examine the therapeutic effects of RTA 402 and RTA 408. Treatment with RTA402 results in antiapoptotic, antioxidative stress, anti-inflammatory, and myelin-preserving effects on retinal ganglion cell (RGC) survival and visual function via regulation of NQO1 and HO-1, reduced IL-6 and Iba1 expression in macrophages, and promoted microglial expression of TGF-β and Ym1 + 2 in the retina and optic nerve. However, these effects were not observed after RTA 408 treatment. Our results provide explicit evidence that RTA 402 modulates the Nrf2 and NFκB signaling pathways to protect RGCs from apoptosis and maintain the visual function in an rAION model. These findings indicate that RTA 402 may a potential therapeutic agent for ischemic optic neuropathy.

Human in vivo neuroimaging to detect reprogramming of the cerebral immune response following repeated systemic inflammation

Despite increasing evidence that immune training within the brain may affect the clinical course of neuropsychiatric diseases, data on cerebral immune tolerance are scarce. This study in healthy volunteers examined the trajectory of the immune response systemically and within the brain following repeated lipopolysaccharide (LPS) challenges. Five young males underwent experimental human endotoxemia (intravenous administration of 2 ng/kg LPS) twice with a 7-day interval. The systemic immune response was assessed by measuring plasma cytokine levels. Four positron emission tomography (PET) examinations, using the translocator protein (TSPO) ligand ¹⁸F-DPA-714, were performed in each participant, to assess brain immune cell activation prior to and 5 hours after both LPS challenges. The first LPS challenge caused a profound systemic inflammatory response and resulted in a 53% [95%CI 36-71%] increase in global cerebral ¹⁸F-DPA-714 binding (p < 0.0001). Six days after the first challenge, ¹⁸F-DPA-714 binding had returned to baseline levels (p = 0.399). While the second LPS challenge resulted in a less pronounced systemic inflammatory response (i.e. 77 ± 14% decrease in IL-6 compared to the first challenge), cerebral inflammation was not attenuated, but decreased below baseline, illustrated by a diffuse reduction of cerebral ¹⁸F-DPA-714 binding (-38% [95%CI -47 to -28%], p < 0.0001). Our findings constitute evidence for in vivo immunological reprogramming in the brain following a second inflammatory insult in healthy volunteers, which could represent a neuroprotective mechanism. These results pave the way for further studies on immunotolerance in the brain in patients with systemic inflammation-induced cerebral dysfunction.

Oral Administration of Brain Protein Combined With Probiotics Induces Immune Tolerance Through the Tryptophan Pathway

Excessive inflammation leads to secondary immune damage after traumatic brain injury (TBI). The intestinal mucosa is a key component of immune tolerance due to gut-brain axis regulation, but the curative effect is not optimal. Intestinal dysfunction impairs the establishment of immune tolerance in patients with TBI. Therefore, we orally administered brain protein (BP) combined with probiotics to induce immune tolerance and explored the mechanism by which the homeostasis of the microbiota contributes to the enhancement of curative effects by BPs. Herein, we demonstrated that patients with TBI and surgical brain injury (SBI) models of rats had obvious dysbiosis. Notably, the intestinal barrier, proinflammatory cytokines, and activation of microglia demonstrated that excessive inflammatory damage was better controlled in the combined group (oral administration of BP combined with probiotics) than in the BP group (oral administration of BP). Fundamentally, tandem mass tag (TMT)-based quantitative proteomics analysis revealed that BP and probiotics preferentially affect Try-related pathways. A series of experiments further confirmed that Indoleamine 2,3 dioxygenase (IDO)/Kynurenine (Kyn)/Aryl hydrocarbon receptor (AhR) expression was high in the BP group, while Tryptophan hydroxylase 1(TpH1)/5-hydroxytryptamine (5-HT) only changed in the combined group. This study suggests that probiotics can enhance the efficacy of oral BP-induced immune tolerance through the Try pathway.

Mechanisms and repair strategies for white matter degeneration in CNS injury and diseases

White matter degeneration is an important pathophysiological event of the central nervous system that is collectively characterized by demyelination, oligodendrocyte loss, axonal degeneration and parenchymal changes that can result in sensory, motor, autonomic and cognitive impairments. White matter degeneration can occur due to a variety of causes including trauma, neurotoxic exposure, insufficient blood flow, neuroinflammation, and developmental and inherited neuropathies. Regardless of the etiology, the degeneration processes share similar pathologic features. In recent years, a plethora of cellular and molecular mechanisms have been identified for axon and oligodendrocyte degeneration including oxidative damage, calcium overload, neuroinflammatory events, activation of proteases, depletion of adenosine triphosphate and energy supply. Extensive efforts have been also made to develop neuroprotective and neuroregenerative approaches for white matter repair. However, less progress has been achieved in this area mainly due to the complexity and multifactorial nature of the degeneration processes. Here, we will provide a timely review on the current understanding of the cellular and molecular mechanisms of white matter degeneration and will also discuss recent pharmacological and cellular therapeutic approaches for white matter protection as well as axonal regeneration, oligodendrogenesis and remyelination.

Neurobiology of resilience in depression: immune and vascular insights from human and animal studies

Major depressive disorder (MDD) is a chronic and recurrent psychiatric condition characterized by depressed mood, social isolation and anhedonia. It will affect 20% of individuals with considerable economic impacts. Unfortunately, 30-50% of depressed individuals are resistant to current antidepressant treatments. MDD is twice as prevalent in women and associated symptoms are different. Depression's main environmental risk factor is chronic stress, and women report higher levels of stress in daily life. However, not every stressed individual becomes depressed, highlighting the need to identify biological determinants of stress vulnerability but also resilience. Based on a reverse translational approach, rodent models of depression were developed to study the mechanisms underlying susceptibility vs resilience. Indeed, a subpopulation of animals can display coping mechanisms and a set of biological alterations leading to stress resilience. The aetiology of MDD is multifactorial and involves several physiological systems. Exacerbation of endocrine and immune responses from both innate and adaptive systems are observed in depressed individuals and mice exhibiting depression-like behaviours. Increasing attention has been given to neurovascular health since higher prevalence of cardiovascular diseases is found in MDD patients and inflammatory conditions are associated with depression, treatment resistance and relapse. Here, we provide an overview of endocrine, immune and vascular changes associated with stress vulnerability vs. resilience in rodents and when available, in humans. Lack of treatment efficacy suggests that neuron-centric treatments do not address important causal biological factors and better understanding of stress-induced adaptations, including sex differences, could contribute to develop novel therapeutic strategies including personalized medicine approaches.

Microglial innate memory and epigenetic reprogramming in neurological disorders

Microglia are myeloid-derived cells recognized as brain-resident macrophages. They act as the first and main line of immune defense in the central nervous system (CNS). Microglia have high phenotypic plasticity and are essential for regulating healthy brain homeostasis, and their dysregulation underlies the onset and progression of several CNS pathologies through impaired inflammatory responses. Aberrant microglial activation, following an inflammatory insult, is associated with epigenetic dysregulation in various CNS pathologies. Emerging data suggest that certain stimuli to myeloid cells determine enhanced or attenuated responses to subsequent stimuli. These phenomena, generally termed innate immune memory (IIM), are highly dependent on epigenetic reprogramming. Microglial priming has been reported in several neurological diseases and corresponds to a state of increased permissiveness or exacerbated response, promoted by continuous exposure to a chronic pro-inflammatory environment. In this article, we provide extensive evidence of these epigenetic-mediated phenomena under neurological conditions and discuss their contribution to pathogenesis and their clinical implications, including those concerning potential novel therapeutic approaches.

Coupled Control of Distal Axon Integrity and Somal Responses to Axonal Damage by the Palmitoyl Acyltransferase ZDHHC17

After optic nerve crush (ONC), the cell bodies and distal axons of most retinal ganglion cells (RGCs) degenerate. RGC somal and distal axon degenerations were previously thought to be controlled by two parallel pathways, involving activation of the kinase dual leucine-zipper kinase (DLK) and loss of the axon survival factor nicotinamide mononucleotide adenylyltransferase-2 (NMNAT2), respectively. Here, we report that palmitoylation of both DLK and NMNAT2 by the palmitoyl acyltransferase ZDHHC17 couples these signals. ZDHHC17-dependent palmitoylation enables DLK-dependent somal degeneration after ONC and also ensures NMNAT-dependent distal axon integrity in healthy optic nerves. We provide evidence that ZDHHC17 also controls survival-versus-degeneration decisions in dorsal root ganglion (DRG) neurons, and we identify conserved motifs in NMNAT2 and DLK that govern their ZDHHC17-dependent regulation. These findings suggest that the control of somal and distal axon integrity should be considered as a single, holistic process, mediated by the concerted action of two palmitoylation-dependent pathways.

Accelerated onset of CNS prion disease in mice co-infected with a gastrointestinal helminth pathogen during the preclinical phase

Prion infections in the central nervous system (CNS) can cause extensive neurodegeneration. Systemic inflammation can affect the progression of some neurodegenerative disorders. Therefore, we used the gastrointestinal helminth pathogen Trichuris muris to test the hypothesis that a chronic systemic inflammatory response to a gastrointestinal infection would similarly affect CNS prion disease pathogenesis. Mice were injected with prions directly into the CNS and subsequently orally co-infected with T. muris before the onset of clinical signs. We show that co-infection with a low dose of T. muris that leads to the development of a chronic T helper cell type 1-polarized systemic immune response accelerated the onset of clinical prion disease. In contrast, co-infection with a high dose of T. muris that induces a T helper cell type 2-polarized immune response did not affect prion disease pathogenesis. The reduced survival times in mice co-infected with a low dose of T. muris on d 105 after CNS prion infection coincided with enhanced astrocyte activation in the brain during the preclinical phase. These data aid our understanding of how systemic inflammation may augment the progression of neurodegeneration in the CNS.

Integrin CD11b Deficiency Aggravates Retinal Microglial Activation and RGCs Degeneration After Acute Optic Nerve Injury

Neuroinflammation plays a vital role in the process of a variety of retinal ganglion cells (RGCs) degenerative diseases including traumatic optic neuropathy (TON). Retinal microglial activation is believed as a harbinger of TON, and robust microglial activation can aggravate trauma-induced RGCs degeneration, which ultimately leads to RGCs loss. Toll like receptor 4 (TLR4)-triggered inflammation is of great importance in retinal inflammatory response after optic nerve injury. CD11b on macrophage and brain microglia can inhibit TLR4-triggered inflammation. However, the functional role of CD11b in retinal microglia is not well understood. Here, using an optic nerve crush model and CD11b gene deficient mice, we found that CD11b protein expression was mainly on retinal microglia, significantly increased after optic nerve injury, and still maintained at a high level till at least 28 days post crush. Compared with wild type mice, following acute optic nerve injury, CD11b deficient retinae exhibited more exacerbated microglial activation, accelerated RGCs degeneration, less growth associated protein-43 expression, as well as more proinflammatory cytokines such as interleukin-6 and tumor necrosis factor α while less anti-inflammatory factors such as arginase-1 and interleukin-10 production. We conclude that CD11b is essential in regulating retinal microglial activation and neuroinflammatory responses after acute optic nerve injury, which is critical for subsequent RGCs degeneration and loss.

A single systemic inflammatory insult causes acute motor deficits and accelerates disease progression in a mouse model of human tauopathy

Introduction

Neuroinflammation, which contributes to neurodegeneration, is a consistent hallmark of dementia. Emerging evidence suggests that systemic inflammation also contributes to disease progression.

Methods

The ability of systemically administered lipopolysaccharide (LPS - 500 μg/kg) to effect acute and chronic behavioural changes in C57BL/6 and P301S tauopathy mice was assessed. Markers of pathology were assessed in the brain and spinal cord.

Results

P301S mice display regional microgliosis. Systemic LPS treatment induced exaggerated acute sickness behaviour and motor dysfunction in P301S mice compared with wild-type controls and advanced the onset and accelerated chronic decline. LPS treatment was associated with increased tau pathology 24 hours after LPS injection and spinal cord microgliosis at the end stage.

Discussion

This is the first demonstration that a single systemic inflammatory episode causes exaggerated acute functional impairments and accelerates the long-term trajectory of functional decline associated with neurodegeneration in a mouse model of human tauopathy. The findings have relevance to management of human dementias.

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P301S microgliosis is regional; activation occurs in the spinal cord but not in the cortex.

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Systemic LPS injection caused acute neurological deficits in P301S mice.

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This was associated with increased tau pathology 24 hours after LPS injection.

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This was independent of microglial priming as measured by IL-1β hyperexpression.

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LPS injection advanced the onset of chronic decline in P301S mice.

Its complicated: The relationship between alcohol and microglia in the search for novel pharmacotherapeutic targets for alcohol use disorders

Alcohol use disorder (AUD) is a chronic relapsing disorder with wide-ranging health consequences. Alcohol targets the central nervous system producing neurodegeneration and subsequent cognitive and behavioral deficits, but the mechanisms behind these effects remain unclear. Recently, evidence has been mounting for the role of neuroimmune activation in the pathogenesis of AUDs, but our nascent state of knowledge about the interaction of alcohol with the neuroimmune system supports that the relationship is complicated. As the resident macrophage of the central nervous system, microglia are a central focus. Human and animal research on the interplay between microglia and alcohol in AUDs has proven to be complex, and though early research focused on a pro-inflammatory phenotype of microglia, the anti-inflammatory and homeostatic roles of microglia must be considered. How these new roles for microglia should be incorporated into our thinking about the neuroimmune system in AUDs is discussed in the context of developing novel pharmacotherapies for AUDs.

Chronic neurodegeneration induces type I interferon synthesis via STING, shaping microglial phenotype and accelerating disease progression

Type I interferons (IFN‐I) are the principal antiviral molecules of the innate immune system and can be made by most cell types, including central nervous system cells. IFN‐I has been implicated in neuroinflammation during neurodegeneration, but its mechanism of induction and its consequences remain unclear. In the current study, we assessed expression of IFN‐I in murine prion disease (ME7) and examined the contribution of the IFN‐I receptor IFNAR1 to disease progression. The data indicate a robust IFNβ response, specifically in microglia, with evidence of IFN‐dependent genes in both microglia and astrocytes. This IFN‐I response was absent in stimulator of interferon genes (STING^−/−) mice. Microglia showed increased numbers and activated morphology independent of genotype, but transcriptional signatures indicated an IFNAR1‐dependent neuroinflammatory phenotype. Isolation of microglia and astrocytes demonstrated disease‐associated microglial induction of Tnfα, Tgfb1, and of phagolysosomal system transcripts including those for cathepsins, Cd68, C1qa, C3, and Trem2, which were diminished in IFNAR1 and STING deficient mice. Microglial increases in activated cathepsin D, and CD68 were significantly reduced in IFNAR1^−/− mice, particularly in white matter, and increases in COX‐1 expression, and prostaglandin synthesis were significantly mitigated. Disease progressed more slowly in IFNAR1^−/− mice, with diminished synaptic and neuronal loss and delayed onset of neurological signs and death but without effect on proteinase K‐resistant PrP levels. Therefore, STING‐dependent IFN‐I influences microglial phenotype and influences neurodegenerative progression despite occurring secondary to initial degenerative changes. These data expand our mechanistic understanding of IFN‐I induction and its impact on microglial function during chronic neurodegeneration.

Acute transient cognitive dysfunction and acute brain injury induced by systemic inflammation occur by dissociable IL-1-dependent mechanisms

Systemic inflammation can impair cognition with relevance to dementia, delirium and post-operative cognitive dysfunction. Episodes of delirium also contribute to rates of long-term cognitive decline, implying that these acute events induce injury. Whether systemic inflammation-induced acute dysfunction and acute brain injury occur by overlapping or discrete mechanisms remains unexplored. Here we show that systemic inflammation, induced by bacterial LPS, produces both working-memory deficits and acute brain injury in the degenerating brain and that these occur by dissociable IL-1-dependent processes. In normal C57BL/6 mice, LPS (100 µg/kg) did not affect working memory but impaired long-term memory consolidation. However prior hippocampal synaptic loss left mice selectively vulnerable to LPS-induced working memory deficits. Systemically administered IL-1 receptor antagonist (IL-1RA) was protective against, and systemic IL-1β replicated, these working memory deficits. Dexamethasone abolished systemic cytokine synthesis and was protective against working memory deficits, without blocking brain IL-1β synthesis. Direct application of IL-1β to ex vivo hippocampal slices induced non-synaptic depolarisation and irreversible loss of membrane potential in CA1 neurons from diseased animals and systemic LPS increased apoptosis in the degenerating brain, in an IL-1RI-dependent fashion. The data suggest that LPS induces working memory dysfunction via circulating IL-1β but direct hippocampal action of IL-1β causes neuronal dysfunction and may drive neuronal death. The data suggest that acute systemic inflammation produces both reversible cognitive deficits, resembling delirium, and acute brain injury contributing to long-term cognitive impairment but that these events are mechanistically dissociable. These data have significant implications for management of cognitive dysfunction during acute illness.

Unravelling the glial response in the pathogenesis of Alzheimer's disease

Alzheimer's disease is a progressive, incurable neurodegenerative disease targeting specific neuronal populations within the brain while neighboring neurons appear unaffected. The focus for defining mechanisms has therefore been on the pathogenesis in affected neuronal populations and developing intervention strategies to prevent their cell death. However, there is growing recognition of the importance of glial cells in the development of pathology. Determining exactly how glial cells are involved in the disease process and the susceptibility of the aging brain provides unprecedented challenges. The present review examines recent studies attempting to unravel the glial response during the course of disease and how this action may dictate the outcome of neurodegeneration. The importance of regional heterogeneity of glial cells within the CNS during healthy aging and disease is examined to understand how the glial cells may contribute to neuronal susceptibility or resilience during the neurodegenerative process.-Alibhai, J. D., Diack, A. B., Manson, J. C. Unravelling the glial response in the pathogenesis of Alzheimer's disease.

Macrophage Depletion Ameliorates Peripheral Neuropathy in Aging Mice

Aging is known as a major risk factor for the structure and function of the nervous system. There is urgent need to overcome such deleterious effects of age-related neurodegeneration. Here we show that peripheral nerves of 24-month-old aging C57BL/6 mice of either sex show similar pathological alterations as nerves from aging human individuals, whereas 12-month-old adult mice lack such alterations. Specifically, nerve fibers showed demyelination, remyelination and axonal lesion. Moreover, in the aging mice, neuromuscular junctions showed features typical for dying-back neuropathies, as revealed by a decline of presynaptic markers, associated with α-bungarotoxin-positive postsynapses. In line with these observations were reduced muscle strengths. These alterations were accompanied by elevated numbers of endoneurial macrophages, partially comprising the features of phagocytosing macrophages. Comparable profiles of macrophages could be identified in peripheral nerve biopsies of aging persons. To determine the pathological impact of macrophages in aging mice, we selectively targeted the cells by applying an orally administered CSF-1R specific kinase (c-FMS) inhibitor. The 6-month-lasting treatment started before development of degenerative changes at 18 months and reduced macrophage numbers in mice by ∼70%, without side effects. Strikingly, nerve structure was ameliorated and muscle strength preserved. We show, for the first time, that age-related degenerative changes in peripheral nerves are driven by macrophages. These findings may pave the way for treating degeneration in the aging peripheral nervous system by targeting macrophages, leading to reduced weakness, improved mobility, and eventually increased quality of life in the elderly.SIGNIFICANCE STATEMENT Aging is a major risk factor for the structure and function of the nervous system. Here we show that peripheral nerves of 24-month-old aging mice show similar degenerative alterations as nerves from aging human individuals. Both in mice and humans, these alterations were accompanied by endoneurial macrophages. To determine the pathological impact of macrophages in aging mice, we selectively targeted the cells by blocking a cytokine receptor, essential for macrophage survival. The treatment strongly reduced macrophage numbers and substantially improved nerve structure and muscle strength. We show, for the first time, that age-related degenerative changes in peripheral nerves are driven by macrophages. These findings may be helpful for treatment weakness and reduced mobility in the elderly.

The bidirectional gut-brain-microbiota axis as a potential nexus between traumatic brain injury, inflammation, and disease

As head injuries and their sequelae have become an increasingly salient matter of public health, experts in the field have made great progress elucidating the biological processes occurring within the brain at the moment of injury and throughout the recovery thereafter. Given the extraordinary rate at which our collective knowledge of neurotrauma has grown, new insights may be revealed by examining the existing literature across disciplines with a new perspective. This article will aim to expand the scope of this rapidly evolving field of research beyond the confines of the central nervous system (CNS). Specifically, we will examine the extent to which the bidirectional influence of the gut-brain axis modulates the complex biological processes occurring at the time of traumatic brain injury (TBI) and over the days, months, and years that follow. In addition to local enteric signals originating in the gut, it is well accepted that gastrointestinal (GI) physiology is highly regulated by innervation from the CNS. Conversely, emerging data suggests that the function and health of the CNS is modulated by the interaction between 1) neurotransmitters, immune signaling, hormones, and neuropeptides produced in the gut, 2) the composition of the gut microbiota, and 3) integrity of the intestinal wall serving as a barrier to the external environment. Specific to TBI, existing pre-clinical data indicates that head injuries can cause structural and functional damage to the GI tract, but research directly investigating the neuronal consequences of this intestinal damage is lacking. Despite this void, the proposed mechanisms emanating from a damaged gut are closely implicated in the inflammatory processes known to promote neuropathology in the brain following TBI, which suggests the gut-brain axis may be a therapeutic target to reduce the risk of Chronic Traumatic Encephalopathy and other neurodegenerative diseases following TBI. To better appreciate how various peripheral influences are implicated in the health of the CNS following TBI, this paper will also review the secondary biological injury mechanisms and the dynamic pathophysiological response to neurotrauma. Together, this review article will attempt to connect the dots to reveal novel insights into the bidirectional influence of the gut-brain axis and propose a conceptual model relevant to the recovery from TBI and subsequent risk for future neurological conditions.

Investigation of Sex Differences in the Microglial Response to Binge Ethanol and Exercise

The female brain appears selectively vulnerable to the neurotoxic effects of alcohol, but the reasons for this are unclear. One possibility is an exaggerated neuroimmune response in the female brain, such that alcohol increases microglia number and reactivity to subsequent stimuli, such as exercise. It is important to better characterize the interactive neural effects of alcohol and exercise, as exercise is increasingly being used in the treatment of alcohol use disorders. The present study compared the number of microglia and evidence of their activation in alcohol-vulnerable regions of the brain (medial prefrontal cortex and hippocampus) in male and female rats following binge alcohol and/or exercise. Binge alcohol increased microglia number and morphological characteristics consistent with their activation in the female brain but not the male, regardless of exercise. Binge alcohol followed by exercise did increase the number of MHC II+ (immunocompetent) microglia in females, although the vast majority of microglia did not express MHC II. These results indicate that binge alcohol exerts sex-specific effects on microglia that may result in enhanced reactivity to a subsequent challenge and in part underlie the apparent selective vulnerability of the female brain to alcohol.

Peripheral Inflammation and Demyelinating Diseases

In recent decades, several neurodegenerative diseases have been shown to be exacerbated by systemic inflammatory processes. There is a wide range of literature that demonstrates a clear but complex relationship between the central nervous system (CNS) and the immunological system, both under naïve or pathological conditions. In diseased brains, peripheral inflammation can transform "primed" microglia into an "active" state, which can trigger stronger pathological responses. Demyelinating diseases are a group of neurodegenerative diseases characterized by inflammatory lesions associated with demyelination, which in turn induces axonal damage, neurodegeneration, and progressive loss of function. Among them, the most important are multiple sclerosis (MS) and neuromyelitis optica (NMO). In this review, we will analyze the effect of specific peripheral inflammatory stimuli in the progression of demyelinating diseases and discuss their animal models. In most cases, peripheral immune stimuli are exacerbating.

Inflammatory responses in Multiple Sclerosis normal-appearing white matter and in non-immune mediated neurological conditions with wallerian axonal degeneration: A comparative study

Inflammatory-like changes in the white matter (WM) are commonly observed in conditions of axonal degeneration by different etiologies. This study is a systematic comparison of the principal features of the inflammatory-like changes in the WM in different pathological conditions characterized by axonal damage/degeneration, focusing in particular on Multiple Sclerosis (MS) normal-appearing white matter (NAWM) compared to non immune-mediated disorders. The study was performed on sections of NAWM from 15 MS cases, 11 cases of non immune-mediated disorders with wallerian axonal degeneration (stroke, trauma, amyotrophic lateral sclerosis), 3 cases of viral encephalitis, 6 control cases. Common features of the inflammatory-like changes observed in all of the conditions of WM pathology were diffuse endothelial expression of VCAM-1, microglial activation with expression of M2 markers, increased expression of sphingosine receptors. Inflammation in MS NAWM was characterized, compared to non immune-mediated conditions, by higher VCAM-1 expression, higher density of perivascular lymphocytes, focal perivascular inflammation with microglial expression of M1 markers, ongoing acute axonal damage correlating with VCAM-1 expression but not with microglia activation. Inflammatory changes in MS NAWM share all the main features observed in the WM in non immune-mediated conditions with wallerian axonal degeneration (with differences to a large extent more quantitative than qualitative), but with superimposition of disease-specific perivascular inflammation and ongoing acute axonal damage.

Oroxylin A promotes retinal ganglion cell survival in a rat optic nerve crush model

Purpose

To investigate the effect of oroxylin A on the survival of retinal ganglion cells (RGC) and the activation of microglial cells in a rat optic nerve (ON) crush model.

Methods

Oroxylin A (15mg/Kg in 0.2ml phosphate-buffered saline) or phosphate-buffered saline (PBS control) was immediately administered after ON crush once by subcutaneous injection. Rats were euthanized at 2 weeks after the crush injury. The density of RGC was counted by retrograde labeling with FluoroGold and immunostaining of retina flat mounts for Brn3a. Electrophysiological visual function was assessed by flash visual evoked potentials (FVEP). TUNEL assay, immunoblotting analysis of glial fibrillary acidic protein (GFAP), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in the retinas, and immunohistochemistry of GFAP in the retinas and ED1 in the ON were evaluated.

Results

Two weeks after the insult, the oroxylin A-treated group had significantly higher FG labeled cells and Brn3a+ cells suggesting preserved RGC density in the central and mid-peripheral retinas compared with those of the PBS-treated group. FVEP measurements showed a significantly better preserved latency of the P1 wave in the ON-crushed, oroxylin A-treated rats than the ON-crushed, PBS treated rats. TUNEL assays showed fewer TUNEL positive cells in the ON-crushed, oroxylin A-treated rats. The number of ED1 positive cells was reduced at the lesion site of the optic nerve in the ON-crushed, oroxylin A-treated group. Increased GFAP expression in the retina was reduced greatly in ON-crushed, oroxylin A-treated group. Furthermore, administration of oroxylin A significantly attenuated ON crush insult-induced iNOS and COX-2 expression in the retinas.

Conclusions

These results demonstrated that oroxylin A hasss neuroprotective effects on RGC survival with preserved visual function and a decrease in microglial infiltration in the ONs after ON crush injury.

Aging with a traumatic brain injury: Could behavioral morbidities and endocrine symptoms be influenced by microglial priming?

A myriad of factors influence the developmental and aging process and impact health and life span. Mounting evidence indicates that brain injury, even moderate injury, can lead to lifetime of physical and mental health symptoms. Therefore, the purpose of this mini-review is to discuss how recovery from traumatic brain injury (TBI) depends on age-at-injury and how aging with a TBI affects long-term recovery. TBI initiates pathophysiological processes that dismantle circuits in the brain. In response, reparative and restorative processes reorganize circuits to overcome the injury-induced damage. The extent of circuit dismantling and subsequent reorganization depends as much on the initial injury parameters as other contributing factors, such as genetics and age. Age-at-injury influences the way the brain is able to repair itself, as a result of developmental status, extent of cellular senescence, and injury-induced inflammation. Moreover, endocrine dysfunction can occur with TBI. Depending on the age of the individual at the time of injury, endocrine dysfunction may disrupt growth, puberty, influence social behaviors, and possibly alter the inflammatory response. In turn, activation of microglia, the brain's immune cells, after injury may continue to fuel endocrine dysfunction. With age, the immune system develops and microglia become primed to subsequent challenges. Sustained inflammation and microglial activation can continue for weeks to months post-injury. This prolonged inflammation can influence developmental processes, behavioral performance and age-related decline. Overall, brain injury may influence the aging process and expedite glial and neuronal alterations that impact mental health.

Complement C3 on microglial clusters in multiple sclerosis occur in chronic but not acute disease: Implication for disease pathogenesis

Microglial clusters with C3d deposits are observed in the periplaque of multiple sclerosis (MS) brains and were proposed as early stage of lesion formation. As such they should appear in the brain of MS donors with acute disease but thus far this has not been shown. Using postmortem brain tissue from acute (n = 10) and chronic (n = 15) MS cases we investigated whether C3d+ microglial clusters are part of an acute attack against myelinated axons, which could have implications for disease pathogenesis. The specificity of our findings to MS was tested in ischemic stroke cases (n = 8) with initial or advanced lesions and further analyzed in experimental traumatic brain injury (TBI, n = 26), as both conditions are primarily nondemyelinating but share essential features of neurodegeneration with MS lesions. C3d+ microglial clusters were found in chronic but not acute MS. They were not associated with antibody deposits or terminal complement activation. They were linked to slowly expanding lesions, localized on axons with impaired transport and associated with neuronal C3 production. C3d+ microglial clusters were not specific to MS as they were also found in stroke and experimental TBI. We conclude that C3d+ microglial clusters in MS are not part of an acute attack against myelinated axons. As such it is unlikely that they drive formation of new lesions but could represent a physiological mechanism to remove irreversibly damaged axons in chronic disease. GLIA 2017;65:264–277

Priming the inflammatory pump of the CNS after traumatic brain injury

Traumatic brain injury (TBI) can lead to secondary neuropsychiatric problems that develop and persist years after injury. Mounting evidence indicates that neuroinflammatory processes progress after the initial head injury and worsen with time. Microglia contribute to this inflammation by maintaining a primed profile long after the acute effects of the injury have dissipated. This may set the stage for glial dysfunction and hyperactivity to challenges including subsequent head injury, stress, or induction of a peripheral immune response. This review discusses the evidence that microglia become primed following TBI and how this corresponds with vulnerability to a 'second hit' and subsequent neuropsychiatric and neurodegenerative complications.

Laquinimod dampens hyperactive cytokine production in Huntington's disease patient myeloid cells

Huntington's disease (HD) is a neurodegenerative condition characterized by pathology in the brain and peripheral tissues. Hyperactivity of the innate immune system, due in part to NFκB pathway dysregulation, is an early and active component of HD. Evidence suggests targeting immune disruption may slow disease progression. Laquinimod is an orally active immunomodulator that down-regulates proinflammatory cytokine production in peripheral blood mononuclear cells, and in the brain down-regulates astrocytic and microglial activation by modulating NFκB signalling. Laquinimod had beneficial effects on inflammation, brain atrophy and disease progression in multiple sclerosis (MS) in two phase III clinical trials. This study investigated the effects of laquinimod on hyperactive proinflammatory cytokine release and NFκB signalling in HD patient myeloid cell cultures. Monocytes from manifest (manHD) and pre-manifest (preHD) HD gene carriers and healthy volunteers (HV) were treated with laquinimod and stimulated with lipopolysaccharide. After 24 h pre-treatment with 5 μM laquinimod, manHD monocytes released lower levels of IL-1β, IL-5, IL-8, IL-10, IL-13 and TNFα in response to stimulation. PreHD monocytes released lower levels of IL-8, IL-10 and IL-13, with no reduction observed in HV monocytes. The effects of laquinimod on dysfunctional NFκB signalling in HD was assessed by inhibitor of kappa B (IκB) degradation kinetics, nuclear translocation of NFκB and interactions between IκB kinase (IKK) and HTT, in HD myeloid cells. No differences were observed between laquinimod-treated and untreated conditions. These results provide evidence that laquinimod dampens hyper-reactive cytokine release from manHD and preHD monocytes, with a much reduced effect on HV monocytes. Evidence suggests targeting CNS and peripheral immune disruption may slow Huntington's disease (HD) neurodegenerative processes. The effects of laquinimod, an orally active immunomodulator, on hyperactive cytokine release and dysfunctional NFκB signalling in stimulated myeloid cell cultures from pre-manifest and manifest HD gene carriers and healthy volunteers were investigated. Laquinimod dampened cytokine release but did not impact NFκB signalling. Read the Editorial Highlight for this article on page 670.

Defining the Microglia Response during the Time Course of Chronic Neurodegeneration

Inflammation has been proposed as a major component of neurodegenerative diseases, although the precise role it plays has yet to be defined. We examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a prion disease model of chronic neurodegeneration. Initially, we performed an extensive reanalysis of a large study of prion disease, where the transcriptome of mouse brains had been monitored throughout the time course of disease. Our analysis has provided a detailed classification of the disease-associated genes based on cell type of origin and gene function. This revealed that the genes upregulated during disease, regardless of the strain of mouse or prion protein, are expressed predominantly by activated microglia. In order to study the microglia contribution more specifically, we established a mouse model of prion disease in which the 79A murine prion strain was introduced by an intraperitoneal route into BALB/cJ(Fms-EGFP/-) mice, which express enhanced green fluorescent protein under the control of the c-fms operon. Samples were taken at time points during disease progression, and histological analysis of the brain and transcriptional analysis of isolated microglia was carried out. The analysis of isolated microglia revealed a disease-specific, highly proinflammatory signature in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the importance of microglia within the prion disease process and identifies the nature of the response through gene expression analysis of isolated microglia.

IMPORTANCE

Inflammation has been proposed as a major component of neurodegenerative diseases. We have examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a murine prion disease model of chronic neurodegeneration. Our study demonstrates that genes upregulated throughout the disease process are expressed predominantly by microglia. A disease-specific, highly proinflammatory signature was observed in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the important contribution of microglia to a chronic neurodegenerative disease process.

Emerging roles of the acute phase protein pentraxin-3 during central nervous system disorders

Pentraxin-3 (PTX3) is an acute phase protein (APP) and a member of the long pentraxin family that is recognised for its role in peripheral immunity and vascular inflammation in response to injury, infection and diseases such as atherosclerosis, cancer and respiratory disease. Systemic levels of PTX3 are highly elevated in these conditions, and PTX3 is now recognised as a new biomarker of disease risk and progression. There is extensive evidence demonstrating that central nervous system (CNS) disorders are primarily characterised by central activation of innate immunity, as well as activation of a potent peripheral acute phase response (APR) that influences central inflammation and contributes to poor outcome. PTX3 has been recently recognised to play important roles in CNS disorders, having both detrimental and neuroprotective effects. The present review aims to give an up-to-date account of the emerging roles of PTX3 in CNS disorders, and to provide a critical comparison between peripheral and central actions of PTX3 in inflammatory diseases.

Pentraxin-3 is upregulated in the central nervous system during MS and EAE, but does not modulate experimental neurological disease

Pentraxin-3 (PTX3), an acute-phase protein released during inflammation, aids phagocytic clearance of pathogens and apoptotic cells, and plays diverse immunoregulatory roles in tissue injury. In neuroinflammatory diseases, like MS, resident microglia could become activated by endogenous agonists for Toll like receptors (TLRs). Previously we showed a strong TLR2-mediated induction of PTX3 in cultured human microglia and macrophages by HspB5, which accumulates in glia during MS. Given the anti-inflammatory effects of HspB5, we examined the contribution of PTX3 to these effects in MS and its animal model EAE. Our data indicate that TLR engagement effectively induces PTX3 expression in human microglia, and that such expression is readily detectable in MS lesions. Enhanced PTX3 expression is prominently expressed in microglia in preactive MS lesions, and in microglia/macrophages engaged in myelin phagocytosis in actively demyelinating lesions. Yet, we did not detect PTX3 in cerebrospinal fluid of MS patients. PTX3 expression is also elevated in spinal cords during chronic relapsing EAE in Biozzi ABH mice, but the EAE severity and time course in PTX3-deficient mice did not differ from WT mice. Moreover, systemic PTX3 administration did not alter the disease onset or severity. Our findings reveal local functions of PTX3 during neuroinflammation in facilitating myelin phagocytosis, but do not point to a role for PTX3 in controlling the development of autoimmune neuroinflammation.

Neuroprotective effect of 4-(Phenylsulfanyl)butan-2-one on optic nerve crush model in rats

This study is to investigate the effect of coral-related compound, 4-(phenylsulfanyl)butan-2-one (4-PSB-2) on optic nerves (ON) and retinal ganglion cells (RGC) in a rat model subjected to ON crush. The ONs of adult male Wistar rat (150-180 g) were crushed by a standardized method. The control eyes received a sham operation. 4-PSB-2 (5 mg/kg in 0.2 mL phosphate-buffered saline) or phosphate-buffered saline (PBS control) was immediately administered after ON crush once by subcutaneous injection. Rats were euthanized at 2 weeks after the crush injury. RGC density was counted by retrograde labeling with FluoroGold (FG) application to the superior colliculus, and visual function was assessed by flash visual evoked potentials (FVEP). TUNEL assay, immunoblotting analysis of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX2) in the retinas, and immunohistochemistry of ED1 in the ON were evaluated. Two weeks after the insult, the RGC densities in the central and mid-peripheral retinas in ON-crushed, 4-PSB-2-treated rats were significantly higher than that of the corresponding ON-crushed, PBS-treated rats FVEP measurements showed a significantly better preserved latency of the P1 wave in the ON-crushed, 4-PSB-2-treated rats than the ON-crushed, PBS treated rats. TUNEL assays showed fewer TUNEL positive cells in the ON-crushed, 4-PSB-2-treated rats. The number of ED1 positive cells was reduced at the lesion site of the optic nerve in the ON-crushed, 4-PSB-2-treated group. Furthermore, administration of 4-PSB-2 significantly attenuated ON crush insult-stimulated iNOS and COX2 expression in the retinas. These results demonstrated that 4-PSB-2 protects RGCs and helps preserve the visual function in the rat model of optic nerve crush. 4-PSB-2 may work by being anti-apoptotic and by attenuation of the inflammatory responses involving less ED1 positive cells infiltration in ON as well as suppression of iNOS/COX-2 signaling pathway in the retinas to rescue RGCs after ON crush injury.

Microglia-Induced Maladaptive Plasticity Can Be Modulated by Neuropeptides In Vivo

Microglia-induced maladaptive plasticity is being recognized as a major cause of deleterious self-sustaining pathological processes that occur in neurodegenerative and neuroinflammatory diseases. Microglia, the primary homeostatic guardian of the central nervous system, exert critical functions both during development, in neural circuit reshaping, and during adult life, in the brain physiological and pathological surveillance. This delicate critical role can be disrupted by neural, but also peripheral, noxious stimuli that can prime microglia to become overreactive to a second noxious stimulus or worsen underlying pathological processes. Among regulators of microglia, neuropeptides can play a major role. Their receptors are widely expressed in microglial cells and neuropeptide challenge can potently influence microglial activity in vitro. More relevantly, this regulator activity has been assessed also in vivo, in experimental models of brain diseases. Neuropeptide action in the central nervous system has been associated with beneficial effects in neurodegenerative and neuroinflammatory pathological experimental models. This review describes some of the mechanisms of the microglia maladaptive plasticity in vivo and how neuropeptide activity can represent a useful therapeutical target in a variety of human brain pathologies.

Immune dysfunction in Niemann-Pick disease type C

Lysosomal storage diseases are inherited monogenic disorders in which lysosome function is compromised. Although individually very rare, they occur at a collective frequency of approximately one in five thousand live births and usually have catastrophic consequences for health. The lysosomal storage diseases Niemann‐Pick disease type C (NPC) is caused by mutations predominantly in the lysosomal integral membrane protein NPC1 and clinically presents as a progressive neurodegenerative disorder. In this article we review data that demonstrate significant dysregulation of innate immunity in NPC, which occurs both in peripheral organs and the CNS. In particular pro‐inflammatory responses promote disease progression and anti‐inflammatory drugs provide benefit in animal models of the disease and are an attractive target for clinical intervention in this disorder.

Niemann‐Pick disease type C is a rare, devastating, inherited lysosomal storage disease with a unique cellular phenotype characterized by lysosomal accumulation of sphingosine, various glycosphingolipids and cholesterol and a reduction in lysosomal calcium. In this review we highlight the impact of the disease on innate immune activities in both the central nervous system (CNS) and peripheral tissues and discuss their contributions to pathology and the underlying mechanisms.

Life style, Perfusion deficits and Co-morbidities Precipitate Inflammation and Cerebrovascular Disorders in Aged Subjects

Cerebrovascular diseases represent 2nd leading cause of death worldwide. Understanding how genetic predispositions and their interaction with environmental factors affect cerebrovascular diseases is fundamental for prevention, diagnosis and for the development of safe and efficient therapies. Cerebrovascular diseases have not only a very high mortality rate, but also results in debilitating neurological impairments or permanent disability in survivors associated with huge economic losses. Among the women and men individuals with a low-risk lifestyle (smoking, exercising daily, consuming a prudent diet including moderate alcohol and having a healthy weight during mid-life) had a significantly lower risk of stroke than individuals without a low-risk lifestyle. Current review focuses on determining the relationship between diet, as an important component of ‘life style’, aging and cerebrovascular diseases.This review may help to unravel biological mechanisms linking lifestyle, diet-induced, metabolic inflammation, aging and cerebral hypoperfusion to development of cerebrovascular diseases, a prerequisite for development of science-based preventive strategies needed to combat the major public health challenges like obesity and stroke.

Co-morbidity and systemic inflammation as drivers of cognitive decline: new experimental models adopting a broader paradigm in dementia research

Dementia prevalence increases with age and Alzheimer’s disease (AD) accounts for up to 75% of cases. However, significant variability and overlap exists in the extent of amyloid-β and Tau pathology in AD and non-demented populations and it is clear that other factors must influence progression of cognitive decline, perhaps independent of effects on amyloid pathology. Coupled with the failure of amyloid-clearing strategies to provide benefits for AD patients, it seems necessary to broaden the paradigm in dementia research beyond amyloid deposition and clearance. Evidence has emerged from alternative animal model approaches as well as clinical and population epidemiological studies that co-morbidities contribute significantly to neurodegeneration/cognitive decline and systemic inflammation has been a strong common theme in these approaches. We hypothesise, and discuss in this review, that a disproportionate inflammatory response to infection, injury or chronic peripheral disease is a key determinant of cognitive decline. We propose that detailed study of alternative models, which encompass acute and chronic systemic inflammatory co-morbidities, is an important priority for the field and we examine the cognitive consequences of several of these alternative experimental approaches. Experimental models of severe sepsis in normal animals or moderate acute systemic inflammation in animals with existing neurodegenerative pathology have uncovered roles for inflammatory mediators interleukin-1β, tumour necrosis factor-α, inducible nitric oxide synthase, complement, prostaglandins and NADPH oxidase in inflammation-induced cognitive dysfunction and neuronal death. Moreover, microglia are primed by existing neurodegenerative pathology to produce exaggerated responses to subsequent stimulation with bacterial lipopolysaccharide or other inflammatory stimuli and these insults drive acute dysfunction and negatively affect disease trajectory. Chronic co-morbidities, such as arthritis, atherosclerosis, obesity and diabetes, are risk factors for subsequent dementia and those with high inflammatory status are particularly at risk. Models of chronic co-morbidities, and indeed low grade systemic inflammation in the absence of specific pathology, indicate that interleukin-1β, tumour necrosis factor-α and other inflammatory mediators drive insulin resistance, hypothalamic dysfunction, impaired neurogenesis and cognitive function and impact on functional decline. Detailed study of these pathways will uncover important mechanisms of peripheral inflammation-driven cognitive decline and are already driving clinical initiatives to mitigate AD progression through minimising systemic inflammation.

Microglial priming and enhanced reactivity to secondary insult in aging, and traumatic CNS injury, and neurodegenerative disease

Glia of the central nervous system (CNS) help to maintain homeostasis in the brain and support efficient neuronal function. Microglia are innate immune cells of the brain that mediate responses to pathogens and injury. They have key roles in phagocytic clearing, surveying the local microenvironment and propagating inflammatory signals. An interruption in homeostasis induces a cascade of conserved adaptive responses in glia. This response involves biochemical, physiological and morphological changes and is associated with the production of cytokines and secondary mediators that influence synaptic plasticity, cognition and behavior. This reorganization of host priorities represents a beneficial response that is normally adaptive but may become maladaptive when the profile of microglia is compromised. For instance, microglia can develop a primed or pro-inflammatory mRNA, protein and morphological profile with aging, traumatic brain injury and neurodegenerative disease. As a result, primed microglia exhibit an exaggerated inflammatory response to secondary and sub-threshold challenges. Consequences of exaggerated inflammatory responses by microglia include the development of cognitive deficits, impaired synaptic plasticity and accelerated neurodegeneration. Moreover, impairments in regulatory systems in these circumstances may make microglia more resistant to negative feedback and important functions of glia can become compromised and dysfunctional. Overall, the purpose of this review is to discuss key concepts of microglial priming and immune-reactivity in the context of aging, traumatic CNS injury and neurodegenerative disease. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Immune activation promotes depression 1 month after diffuse brain injury: a role for primed microglia

BACKGROUND

Traumatic brain injury (TBI) is associated with a higher incidence of depression. The majority of individuals who suffer a TBI are juveniles and young adults, and thus, the risk of a lifetime of depressive complications is a significant concern. The etiology of increased TBI-associated depression is unclear but may be inflammatory-related with increased brain sensitivity to secondary inflammatory challenges (e.g., stressors, infection, and injury).

METHODS

Adult male BALB/c mice received a sham (n = 52) or midline fluid percussion injury (TBI; n = 57). Neuroinflammation, motor coordination (rotarod), and depressive behaviors (social withdrawal, immobility in the tail suspension test, and anhedonia) were assessed 4 hours, 24 hours, 72 hours, 7 days, or 30 days later. Moreover, 30 days after surgery, sham and TBI mice received a peripheral injection of saline or lipopolysaccharide (LPS) and microglia activation and behavior were determined.

RESULTS

Diffuse TBI caused inflammation, peripheral cell recruitment, and microglia activation immediately after injury coinciding with motor coordination deficits. These transient events resolved within 7 days. Nonetheless, 30 days post-TBI a population of deramified and major histocompatibility complex II(+) (primed) microglia were detected. After a peripheral LPS challenge, the inflammatory cytokine response in primed microglia of TBI mice was exaggerated compared with microglia of controls. Furthermore, this LPS-induced microglia reactivity 30 days after TBI was associated with the onset of depressive-like behavior.

CONCLUSIONS

These results implicate a primed and immune-reactive microglial population as a possible triggering mechanism for the development of depressive complications after TBI.

Perfusion deficits, inflammation and aging precipitate depressive behaviour

Major depressive disorder (MDD) is a severe psychiatric illness that is associated with significant morbidity and mortality. Despite advances in the treatment of major depression, one-third of depressed patients fail to respond to conventional antidepressant medication. One pathophysiologic mechanism hypothesized to contribute to treatment resistance in depression is inflammation. Inflammation has been linked to depression by a number of putative mechanisms involving perfusion deficits that can trigger microglial activation and subsequent neuroinflammation in the elderly. However, the pathophysiological mechanisms remain to be further elucidated. This review focusses on recent studies addressing the complex relationships between depression, aging, inflammation and perfusion deficits in the elderly. We expect that a better understanding of neuroinflammatory mechanisms associated with age-related diseases may lead to the discovery of new biomarkers of MDD and development of new therapeutic interventions.

Mechanisms involved in dual vasopressin/apelin neuron dysfunction during aging

Normal aging is associated with vasopressin neuron adaptation, but little is known about its effects on the release of apelin, an aquaretic peptide colocalized with vasopressin. We found that plasma vasopressin concentrations were higher and plasma apelin concentrations lower in aged rats than in younger adults. The response of AVP/apelin neurons to osmotic challenge was impaired in aged rats. The overactivity of vasopressin neurons was sustained partly by the increased expression of Transient receptor potential vanilloid2 (Trpv2), because central Trpv blocker injection reversed the age-induced increase in plasma vasopressin concentration without modifying plasma apelin concentration. The morphofunctional plasticity of the supraoptic nucleus neuron-astrocyte network normally observed during chronic dehydration in adults appeared to be impaired in aged rats as well. IL-6 overproduction by astrocytes and low-grade microglial neuroinflammation may contribute to the modification of neuronal functioning during aging. Indeed, central treatment with antibodies against IL-6 decreased plasma vasopressin levels and increased plasma apelin concentration toward the values observed in younger adults. Conversely, minocycline treatment (inhibiting microglial metabolism) did not affect plasma vasopressin concentration, but increased plasma apelin concentration toward control values for younger adults. This study is the first to demonstrate dual vasopressin/apelin adaptation mediated by inflammatory molecules and neuronal Trpv2, during aging.

Inflammation in Alzheimer disease-a brief review of the basic science and clinical literature

Biochemical and neuropathological studies of brains from individuals with Alzheimer disease (AD) provide clear evidence for an activation of inflammatory pathways, and long-term use of anti-inflammatory drugs is linked with reduced risk to develop the disease. As cause and effect relationships between inflammation and AD are being worked out, there is a realization that some components of this complex molecular and cellular machinery are most likely promoting pathological processes leading to AD, whereas other components serve to do the opposite. The challenge will be to find ways of fine tuning inflammation to delay, prevent, or treat AD.

Developmental neuroinflammation and schizophrenia

There is increasing interest in and evidence for altered immune factors in the etiology and pathophysiology of schizophrenia. Stimulated by various epidemiological findings reporting elevated risk of schizophrenia following prenatal exposure to infection, one line of current research aims to explore the potential contribution of immune-mediated disruption of early brain development in the precipitation of long-term psychotic disease. Since the initial formulation of the "prenatal cytokine hypothesis" more than a decade ago, extensive epidemiological research and remarkable advances in modeling prenatal immune activation effects in animal models have provided strong support for this hypothesis by underscoring the critical role of cytokine-associated inflammatory events, together with downstream pathophysiological processes such as oxidative stress, hypoferremia and zinc deficiency, in mediating the short- and long-term neurodevelopmental effects of prenatal infection. Longitudinal studies in animal models further indicate that infection-induced developmental neuroinflammation may be pathologically relevant beyond the antenatal and neonatal periods, and may contribute to disease progression associated with the gradual development of full-blown schizophrenic disease. According to this scenario, exposure to prenatal immune challenge primes early pre- and postnatal alterations in peripheral and central inflammatory response systems, which in turn may disrupt the normal development and maturation of neuronal systems from juvenile to adult stages of life. Such developmental neuroinflammation may adversely affect processes that are pivotal for normal brain maturation, including myelination, synaptic pruning, and neuronal remodeling, all of which occur to a great extent during postnatal brain maturation. Undoubtedly, our understanding of the role of developmental neuroinflammation in progressive brain changes relevant to schizophrenia is still in infancy. Identification of these mechanisms would be highly warranted because they may represent a valuable target to attenuate or even prevent the emergence of full-blown brain and behavioral pathology, especially in individuals with a history of prenatal complications such as in-utero exposure to infection and/or inflammation.