Aged mice exhibit greater mortality concomitant to increased brain and plasma TNF-alpha levels following intracerebroventricular injection of lipopolysaccharide

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

Prolonged stress-induced depression-like behaviors in aged rats are mediated by endoplasmic reticulum stress and apoptosis in the hippocampus

Structural and functional recovery from stress-induced depression is impaired in the context of aging brain. Since investigating the molecular substrates that facilitate behavioral recovery may have important implications for understanding brain plasticity and resilience of individuals, we studied depressive-like behaviors in young and aged rats 6 weeks after chronic stress exposure as a recovery period and examined the levels of TNF-α and IL-6 inflammatory cytokines, NADH oxidase activity, NADPH oxidase, endoplasmic reticulum (ER) stress markers, and apoptosis in the hippocampus. Young (3 months old) and aged (22 months old) male Wistar rats were divided into four groups; young control (Young), depression model of young rats that received chronic stress procedure followed by a 6-week recovery period (Young+S), aged control (Aged), and depression model of aged rats that received chronic stress procedure followed by a 6-week recovery period (Aged+S). After the recovery period, aged but not young rats showed depression-like behaviors, evaluated by the sucrose preference test (SPT) and forced swimming test (FST), coincided with the altered levels of TNF-α, IL-6, NADH oxidase activity, NADPH oxidase, GRP78, CHOP, and cleaved caspase-12 in the hippocampus of these animals. These data suggested that oxidative and ER stress-induced apoptosis in the aging hippocampus may affect the recovery-related outcomes after the stress paradigm.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Exosomes: A missing link between chronic systemic inflammation and Alzheimer's disease?

Exosomes are potent mediators of physiological and pathological processes. In Alzheimer's disease and inflammatory disorders, due to exosomes' distinctive ability to cross the blood-brain barrier, a bidirectional communication between the periphery and the central nervous system exists. Since exosomes can carry various biochemical molecules, this review investigates the role of exosomes as possible mediators between chronic systemic inflammatory diseases and Alzheimer's disease. Exosomes carry pro-inflammatory molecules generated in the periphery, travel to the central nervous system, and target glial and neuronal cells. Microglia and astrocytes then become activated, initiating chronic neuroinflammation. As the aging brain is more susceptible to such changes, this state of neuroinflammation can stimulate neuropathologies, impair amyloid-beta clearance capabilities, and generate dysregulated microRNAs that alter the expression of genes critical in Alzheimer's disease pathology. These processes, individually and collectively, become significant risk factors for the development of Alzheimer's disease.

Molecular Signaling Pathway Targeted Therapeutic Potential of Thymoquinone in Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disease with rapid progression. Black cumin (Nigella sativa) is a nutraceutical that has been investigated as a prophylactic and therapeutic agent for this disease due to its ability to prevent or retard the progression of neurodegeneration. Thymoquinone (TQ) is the main bioactive compound isolated from the seeds of black cumin. Several reports have shown that it has promising potential in the prevention and treatment of AD due to its significant antioxidative, anti-inflammatory, and antiapoptotic properties along with several other mechanisms that target the altered signaling pathways due to the disease pathogenesis. In addition, it shows anticholinesterase activity and prevents α-synuclein induced synaptic damage. The aim of this review is to summarize the potential aspects and mechanisms by which TQ imparts its action in AD.

Alzheimer's disease (AD) therapeutics - 2: Beyond amyloid - Re-defining AD and its causality to discover effective therapeutics

Compounds targeted for the treatment of Alzheimer's Disease (AD) have consistently failed in clinical trials despite evidence for target engagement and pharmacodynamic activity. This questions the relevance of compounds acting at current AD drug targets - the majority of which reflect the seminal amyloid and, to a far lesser extent, tau hypotheses - and limitations in understanding AD causality as distinct from general dementia. The preeminence of amyloid and tau led to many alternative approaches to AD therapeutics being ignored or underfunded to the extent that their causal versus contributory role in AD remains unknown. These include: neuronal network dysfunction; cerebrovascular disease; chronic, local or systemic inflammation involving the innate immune system; infectious agents including herpes virus and prion proteins; neurotoxic protein accumulation associated with sleep deprivation, circadian rhythm and glymphatic/meningeal lymphatic system and blood-brain-barrier dysfunction; metabolic related diseases including diabetes, obesity hypertension and hypocholesterolemia; mitochondrial dysfunction and environmental factors. As AD has become increasingly recognized as a multifactorial syndrome, a single treatment paradigm is unlikely to work in all patients. However, the biomarkers required to diagnose patients and parse them into mechanism/disease-based sub-groups remain rudimentary and unvalidated as do non-amyloid, non-tau translational animal models. The social and economic impact of AD is also discussed in the context of new FDA regulatory draft guidance and a proposed biomarker-based Framework (re)-defining AD and its stages as part of the larger landscape of treating dementia via the 2013 G8 initiative to identify a disease-modifying therapy for dementia/AD by 2025.

The role of astrocytes in amyloid production and Alzheimer's disease

Alzheimer's disease (AD) is marked by the presence of extracellular amyloid beta (Aβ) plaques, intracellular neurofibrillary tangles (NFTs) and gliosis, activated glial cells, in the brain. It is thought that Aβ plaques trigger NFT formation, neuronal cell death, neuroinflammation and gliosis and, ultimately, cognitive impairment. There are increased numbers of reactive astrocytes in AD, which surround amyloid plaques and secrete proinflammatory factors and can phagocytize and break down Aβ. It was thought that neuronal cells were the major source of Aβ. However, mounting evidence suggests that astrocytes may play an additional role in AD by secreting significant quantities of Aβ and contributing to overall amyloid burden in the brain. Astrocytes are the most numerous cell type in the brain, and therefore even minor quantities of amyloid secretion from individual astrocytes could prove to be substantial when taken across the whole brain. Reactive astrocytes have increased levels of the three necessary components for Aβ production: amyloid precursor protein, β-secretase (BACE1) and γ-secretase. The identification of environmental factors, such as neuroinflammation, that promote astrocytic Aβ production, could redefine how we think about developing therapeutics for AD.

Peripheral immunomodulation with ginsenoside Rg1 ameliorates neuroinflammation-induced behavioral deficits in rats

Neuroinflammatory disturbances have been closely associated with depression and many other neuropsychiatric diseases. Although targeting neuroinflammatory mediators with centrally acting drugs has shown certain promise, its translation is faced with several challenges especially drug delivery and safety concerns. Here, we report that neuroinflammation-induced behavioral abnormality could be effectively attenuated with immunomodulatory agents that need not to gain brain penetration. In a rat model with intracerebral lipopolysaccharide (LPS) challenge, we validated that ginsenoside Rg1 (Rg1), a well-established anti-inflammatory agent, was unable to produce a direct action in the brain. Interestingly, peripherally restricted Rg1 could effectively attenuate the weight loss, anorexic- and depressive-like behavior as well as neurochemical disturbances associated with central LPS challenge. Biochemical assay of neuroimmune mediators in the periphery revealed that Rg1 could mitigate the deregulation of the hypothalamic-pituitary-adrenal axis and selectively blunt the increase in circulating interleukin-6 levels. Furthermore, these peripheral regulatory effects were accompanied by dampened microglial activation, mitigated expression of pro-inflammatory mediators and neurotoxic species in the central compartment. Taken together, our work suggested that targeting the peripheral immune system may serve as a novel therapeutic approach to neuroinflammation-induced neuropsychiatric disorders. Moreover, our findings provided the rationale for employing peripherally active agents like Rg1 to combat mental disturbances.

The influence of aging and estradiol to progesterone ratio on rat macrophage phenotypic profile and NO and TNF-α production

The phenotype and function of tissue macrophages substantially depend on the cellular milieu and biological effector molecules, such as steroid hormones, to which they are exposed. Furthermore, in female rats, aging is associated with the altered macrophage functioning and the increased estrogen level is followed by a decrease in that of progesterone. Therefore, the present study aimed to investigate the influence of estradiol/progesterone balance on rat macrophage function and phenotype throughout whole adult lifespan. We ovariectomized rats at the late prepubertal age or at the very end of reproductive lifespan, and examined the expression of ED2 (CD163, a marker of mature resident macrophages related to secretion of inflammatory mediators) on peritoneal macrophages and their ability to produce TNF-α and NO upon LPS-stimulation at different age points. In addition, to delineate direct and indirect effects of estrogen, we assessed the in vitro influence of different concentrations of 17β-estradiol on LPS-induced macrophage TNF-α and NO production. Results showed that: (a) the low frequency of ED2(high) cells amongst peritoneal macrophages of aged rats was accompanied with the reduced TNF-α, but not NO production; (b) estradiol level gradually increased following ovariectomy; (c) macrophage ED2 expression and TNF-α production were dependent on estradiol/progesterone balance and they changed in the same direction; (d) changes in estradiol/progesterone balance differentially affected macrophages TNF-α and NO production; and (e) estradiol exerted pro-inflammatory and anti-inflammatory effects on macrophages in vivo and in vitro, respectively. Overall, our study discloses that estradiol/progesterone balance contributes to the fine-tuning of rat macrophage secretory capacity, and adds to a better understanding of the ovarian steroid hormone role in the regulation of macrophage function, and its significance for the age-associated changes in innate immunity.

Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Interleukin-1β with learning and memory

Interleukin-1β (IL-1β) is one of the first cytokines ever described. It has long been recognized to play an important role in mediating inflammation and orchestrating the physiological and behavioral adjustments that occur during sickness. Recently, accumulating evidence has indicated that IL-1β also adversely affects cognitive function. Nevertheless, there are also some reports showing no effects or even beneficial effects of IL-1β on learning and memory. The relationship between IL-1β and cognitive impairment has not been clearly elucidated. Here we reviewed the evidence of both negative and positive effects of IL-1β on learning and memory, and the key factors that may affect the effects of IL-1β on learning and memory were discussed.

Glial transcripts and immune-challenged glia in the suprachiasmatic nucleus of young and aged mice

Biological rhythms are frequently disturbed with advancing age, and aging-related changes of glia in the hypothalamic suprachiasmatic nucleus (SCN), the master circadian pacemaker, require special attention. In particular, astrocytes contribute to SCN function, and aging is associated with increased inflammatory activity in the brain, in which microglia could be especially implicated. On this basis, we investigated in the SCN of young and old mice glial transcripts and cell features, and the glial cell response to a central inflammatory challenge. Quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to analyze the expression of mRNAs encoding the astrocytic glial fibrillary acidic protein and the microglial antigen CD11b. Both these transcripts, here investigated in the SCN for the first time, were significantly increased in the old SCN. Glial cell phenotyping with immunohistochemistry revealed hypertrophic and intensely stained astrocytes and microglia in the aged SCN. In both age groups, microglia were scattered throughout the SCN and astrocytes were prominent in the ventral portion, where retinal fibers are densest; in the aged SCN, astrocytes were also numerous in the dorsal portion. After intracerebroventricular injections of a mixture of interferon-gamma and tumor necrosis factor-alpha, or phosphate-buffered saline as control, immunolabeling was evaluated with stereological cell counts and confocal microscopy. Phenotypic features of astrocyte and microglia activation in response to cytokine injections were markedly enhanced in the aged SCN. Subregional variations in glial cell density were also documented in the aged compared to the young SCN. Altogether, the findings show increases in the expression of glial transcripts and hypertrophy of astrocytes and microglia in the aged SCN, as well as age-dependent variation in the responses of immune-challenged SCN glia. The data thus point out an involvement of glia in aging-related changes of the biological clock.

Regulation of cytokine signaling and T-cell recruitment in the aging mouse brain in response to central inflammatory challenge

Aging is often accompanied by increased levels of inflammatory molecules in the organism, but age-related changes in the brain response to inflammatory challenges still require clarification. We here investigated in mice whether cytokine signaling and T-cell neuroinvasion undergo age-related changes. We first analyzed the expression of molecules involved in T-cell infiltration and cytokine signaling regulation in the septum and hippocampus of 2-3 months and 20- to 24-month-old mice at 4h after intracerebroventricular injections of tumor necrosis factor (TNF)-alpha or interferon-gammaversus saline injections. Transcripts of the chemokine CXCL9, intercellular adhesion molecule (ICAM)-1 and suppressor of cytokine signaling molecules (SOCS) 1 and 3 were increased in both age groups after cytokine injection; microglia-derived matrix metalloproteinase (MMP) 12 mRNA was induced in old mice also after control saline injections. Age-related changes in ICAM-1 protein expression and T-cell infiltration were then analyzed in mice of 3-4, 8-9 and 15-16 months at 48h after TNF-alpha injections. ICAM-1 immunoreactivity, and Western blotting in striatum, septum, hippocampus and hypothalamus showed progressive age-related enhancement of TNF-alpha-elicited ICAM-1 upregulation. Double immunofluorescence revealed ICAM-1 expression in microglia and astrocytic processes. CD3(+), CD4(+) and CD8(+) T-cells exhibited progressive age-related increases in brain parenchyma and choroid plexus after cytokine exposure. The findings indicate that the brain responses to inflammatory challenges are not only preserved with advancing age, but also include gradual amplification of ICAM-1 expression and T-cell recruitment. The data highlight molecular and cellular correlates of age-related increase of brain sensitivity to inflammatory stimuli, which could be involved in altered brain vulnerability during aging.

Age and neuroinflammation: a lifetime of psychoneuroimmune consequences

Aging can impair functional interaction that occurs between the brain and the immune system. Recent findings indicate that microglia and astrocytes, innate immune cells of the brain, become more reactive during normal aging. This age-associated increase in innate immune reactivity sets the stage for an exaggerated inflammatory cytokine response in the brain after activation of the peripheral innate immune system. This elevated neuroinflammatory response may lead to more severe long-lasting behavioral and cognitive deficits. This article discusses new evidence that aging creates a brain environment that is permissive to the occurrence of mental health complications following innate immune activation.

Immune and behavioral consequences of microglial reactivity in the aged brain

Bidirectional communication between the immune system and the brain is essential for mounting the appropriate immunological, physiological, and behavioral responses to immune activation. Aging, however, may impair this important bi-directional interaction. In support of this notion, peripheral infection in the elderly is associated with an increased frequency of behavioral and cognitive complications. Recent findings in animal models of aging and neurodegenerative disease indicate that microglia, innate immune cells of the brain, become primed or reactive. Understanding age- and disease-associated alterations in microglia is important because glia (microglia and astrocytes) play an integral role in propagating inflammatory signals that are initiated in the periphery. In this capacity, brain glia produce inflammatory cytokines that target neuronal substrates and elicit a sickness-behavior syndrome that is normally beneficial to the host organism. Increased reactivity of microglia sets the stage for an exaggerated neuroinflammatory cytokine response following activation of the peripheral innate immune system, which may underlie subsequent long-lasting behavioral and cognitive deficits. In support of this premise, recent findings indicate that stimulation of the peripheral immune system in aged rodents causes exaggerated neuroinflammation that is paralleled by cognitive impairment, prolonged sickness, and depressive-like complications. Therefore, the purpose of this review is to discuss the new evidence that age-associated priming of microglia could play a pathophysiological role in exaggerated behavioral and cognitive sequelae to peripheral infection.

Role of neutral sphingomyelinases in aging and inflammation

Aging is characterized by changes in the organism's immune functions and stress response, which in the elderly leads to increased incidence of complications and mortality following inflammatory stress. Alterations in the neuro-endocrine axes and overall decline in the immune system play an essential role in this process. Overwhelming evidence however suggests that many cellular cytokine signaling pathways are also affected, thus underscoring the idea that both, "cellular" and "systemic" changes contribute to aging. IL-1beta for example, induces more potent cellular responses in hepatocytes isolated from aged animals then in hepatocytes from young rats. This phenomenon is referred to as IL-1b hyperresponsiveness and is linked to abnormal regulation of various acute phase proteins during aging.Evidence has consistently indicated that activation of neutral sphingomyelinase and the resulting accumulation of ceramide mediate cellular responses to LPS, IL-1beta, and TNFalpha in young animals. More recent studies identified the cytokine-inducible neutral sphingomyelinase with nSMase2 (smpd3) that is localized in the plasma membrane and mediates cellular responses to IL-1beta and TNFalpha. Intriguingly, constitutive up-regulation of nSMase2 occurs in aging and it underlies the hepatic IL-1b hyperresponsiveness. The increased activity of nSMases2 in aging is caused by a substantial decline in hepatic GSH content linking thereby oxidative stress to the onset of pro-inflammatory state in liver. nSMase2 apparently follows a pattern of regulation consisting with "developmental-aging" continuum, since in animal models of delayed aging, like calorie-restricted animals, the aging-associated changes in NSMase activity and function are reversed.

WITHDRAWN: Bcl-2 Upregulation is Significantly Enhanced in the Hippocampus of Normal Aging Mice After an Acute Challenge Elicited by Pro-inflammatory Cytokines Circulating in the Cerebrospinal Fluid

Ahead of Print article withdrawn by publisher

Differential response of apoptosis-regulatory Bcl-2 and Bax proteins to an inflammatory challenge in the cerebral cortex and hippocampus of aging mice

Apoptosis plays a key role in normal aging and neurodegeneration. It is now known that normal aging implies low-grade inflammation and increases susceptibility to neurodegenerative diseases, which, in turn, include a neuroinflammatory component. We here investigated, using mice of 2-3 months, 10-11 months, or 18-21 months of age, the expression of apoptosis-regulatory proteins in cortical brain regions in response to intracerebroventricular administration of pro-inflammatory cytokines. A mixture of interferon-gamma and tumor necrosis factor-alpha was injected, using vehicle (phosphate-buffered saline) as control. At 4 days, levels of the anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins in the cerebral cortex and hippocampus, examined with Western blotting, were markedly upregulated by cytokine exposure in mice of all age groups with respect to controls. Interestingly, cytokine-elicited Bcl-2 upregulation was aging-dependent, with significant enhancement paralleling the animals' age. Cytokine-elicited Bax expression did not exhibit instead significant aging-related variation. Using the same paradigm and 1 or 2 day survival, Bcl-2 immunoreactivity was observed mainly in neurons of cortex and hippocampus of both control and cytokine-treated mice of all age groups. Furthermore, immunohistochemistry confirmed the enhancement of cytokine-elicited Bcl-2 expression in the cerebral cortex and hippocampus of old mice, and showed that this finding was already evident in the second day after cytokine exposure. The data point out the novel finding that Bcl-2 and Bax expression in cortical brain regions is differentially regulated during senescence in response to an acute inflammatory challenge. Aging-related Bcl-2 increases in neurons after cytokine exposure could contribute to amplify neuroprotective mechanisms in the old brain.

Deletion of mu-opioid receptor in mice alters the development of acute neuroinflammation

The realization that the mu-opioid system plays a key role in the control of the process of neuroinflammation is a new concept that may lead to identification of novel therapies for this extremely widespread and intractable syndrome. Fever is the hallmark among the defense mechanisms evoked by the entry into the body of pathogens to initiate the innate immune responses. In an attempt to determine the possible involvement of mu-opioid receptors in the control of brain inflammation, we examined the effect of their deletion on the fever induced by i.c.v. injection of lipopolysaccharide (LPS). The first series of experiments examined the thermal consequence of the absence of mu-opioid receptors on circadian body temperature rhythm and basal body temperature. Mu-opioid receptor knockout mice (MOP-KO) showed a normal circadian body temperature rhythm and basal body temperature compared with the wild type (WT). The second series of experiments investigated i.c.v. administration of LPS on body temperature in WT and MOP-KO. In the WT, i.c.v. injection of 100 ng of LPS induced fever, but there was no increase in body temperature in the MOP-KO mice. Saline, given i.c.v., did not alter the body temperature, either in WT or MOP-KO. These results show that the mu-opioid system participates in the control of acute neuroinflammation, further reinforcing our earlier finding that the opioid system is involved in the pathogenesis of fever induced by bacterial LPS, and that mu-opioid receptors are the target for morphine-induced hyperthermia.

Exaggerated sickness behavior and brain proinflammatory cytokine expression in aged mice in response to intracerebroventricular lipopolysaccharide

Age-associated changes in glial reactivity may predispose individuals to exacerbated neuroinflammatory cytokine responses that are permissive to cognitive and behavioral complications. The purpose of this study was to determine if aging is associated with an exaggerated sickness response to central innate immune activation. Our results show that intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) caused a heightened proinflammatory cytokine response (IL-1beta, IL-6, and TNFalpha) in the cerebellum 2h post i.c.v. injection in aged mice compared to adults. This amplified inflammatory profile was consistent with a brain region-dependent increase in reactive glial markers (MHC class II, TLR2 and TLR4). Moreover, LPS caused a prolonged sickness behavior response in aged mice that was paralleled by a protracted expression of brain cytokines in the cerebellum and hippocampus. Finally, central LPS injection caused amplified and prolonged IL-6 levels at the periphery of aged mice. Collectively, these data establish that activation of the central innate immune system leads to exacerbated neuroinflammation and prolonged sickness behavior in aged as compared to adult mice.

Gene expression of cyclooxygenase-1 and Ca(2+)-independent phospholipase A(2) is altered in rat hippocampus during normal aging

Brain aging is associated with inflammatory changes. However, data on how the brain arachidonic acid (AA) metabolism is altered as a function of age are limited and discrepant. AA is released from membrane phospholipids by phospholipase A(2) (PLA(2)) and then further metabolized to bioactive prostaglandins and thromboxanes by cyclooxygenases (COX)-1 and -2. We examined the phospholipase A(2) (PLA(2))/COX-mediated AA metabolic pathway in the hippocampus and cerebral cortex of 4-, 12-, 24- and 30-month-old rats. A two-fold increase in brain thromboxane B(2) level in 24 and 30 months was accompanied by increased hippocampal COX-1 mRNA levels at 12, 24, and 30 months. COX-2 mRNA expression was significantly decreased only at 30 months. Hippocampal Ca(2+)-independent iPLA(2) mRNA levels were decreased at 24 and 30 months without any change in Ca(2+)-dependent PLA(2) expression. In the cerebral cortex, mRNA levels of COX and PLA(2) were not significantly changed. The specific changes in the AA cascade observed in the hippocampus may alter phospholipids homeostasis and possibly increase the susceptibility of the aging brain to neuroinflammation.

The dynamics of the LPS triggered inflammatory response of murine microglia under different culture and in vivo conditions

Overall, the inflammatory potential of lipopolysaccharide (LPS) in vitro and in vivo was investigated using different omics technologies. We investigated the hippocampal response to intracerebroventricular (i.c.v) LPS in vivo, at both the transcriptional and protein level. Here, a time course analysis of interleukin-6 (IL-6) and monocyte chemotactic protein-1 (MCP-1) showed a sharp peak at 4 h and a return to baseline at 16 h. The expression of inflammatory mediators was not temporally correlated with expression of the microglia marker F4/80, which did not peak until 2 days after LPS injection. Of 480 inflammation-related genes present on a microarray, 29 transcripts were robustly up-regulated and 90% of them were also detected in LPS stimulated primary microglia (PM) cultures. Further in vitro to in vivo comparison showed that the counter regulation response observed in vivo was less evident in vitro, as transcript levels in PM decreased relatively little over 16 h. This apparent deficiency of homeostatic control of the innate immune response in cultures may also explain why a group of genes comprising tnf receptor associated factor-1, endothelin-1 and schlafen-1 were regulated strongly in vitro, but not in vivo. When the overall LPS-induced transcriptional response of PM was examined on a large Affymetrix chip, chemokines and cytokines constituted the most strongly regulated and largest groups. Interesting new microglia markers included interferon-induced protein with tetratricopeptide repeat (ifit), immune responsive gene-1 (irg-1) and thymidylate kinase family LPS-inducible member (tyki). The regulation of the former two was confirmed on the protein level in a proteomics study. Furthermore, conspicuous regulation of several gene clusters was identified, for instance that of genes pertaining to the extra-cellular matrix and enzymatic regulation thereof. Although most inflammatory genes induced in vitro were transferable to our in vivo model, the observed discrepancy for some genes potentially represents regulatory factors present in the central nervous system (CNS) but not in vitro.

Age and Neuroinflammation: A Lifetime of Psychoneuroimmune Consequences

This article reviews the literature indicating that the innate immune cells of the brain become more reactive with age. Although it is unclear how glia reactivity increases, emerging evidence suggests these alterations allow exacerbated neuroinflammation and sickness behavior following peripheral immune activation. This amplified or prolonged exposure to inflammatory cytokines in the brain may impair neuronal plasticity and underlie a heightened neuroinflammatory response in the aged that also may lead to other neurobehavioral impairments such as delirium, depression, and, potentially, the onset of neurologic disease. Therefore pharmacologic strategies to decrease neuroinflammation associated with infection may be important for improving recovery from sickness and reducing neurobehavioral deficits in the elderly.

Cytokine-induced activation of glial cells in the mouse brain is enhanced at an advanced age

Numerous neurological diseases which include neuroinflammatory components exhibit an age-related prevalence. The aging process is characterized by an increase of inflammatory mediators both systemically and in the brain, which may prime glial cells. However, little information is available on age-related changes in the glial response of the healthy aging brain to an inflammatory challenge. This problem was here examined using a mixture of the proinflammatory cytokines interferon-gamma and tumor necrosis factor-alpha, which was injected intracerebroventricularly in young (2-3.5 months), middle-aged (10-11 months) and aged (18-21 months) mice. Vehicle (phosphate-buffered saline) was used as control. After a survival of 1 or 2 days (all age groups) or 4 days (young and middle-aged animals), immunohistochemically labeled astrocytes and microglia were investigated both qualitatively and quantitatively. In all age groups, astrocytes were markedly activated in periventricular as well as in deeper brain regions 2 days following cytokine treatment, whereas microglia activation was already evident at 24 h. Interestingly, cytokine-induced activation of both astrocytes and microglia was significantly more marked in the brain of aged animals, in which it included numerous ameboid microglia, than of younger age groups. Moderate astrocytic activation was also seen in the hippocampal CA1 field of vehicle-treated aged mice. FluoroJade B histochemistry and the terminal deoxynucleotidyl transferase-mediated UTP nick-end labeling technique, performed at 2 days after cytokine administration, did not reveal ongoing cell death phenomena in young or aged animals. This indicated that glial cell changes were not secondary to neuronal death. Altogether, the findings demonstrate for the first time enhanced activation of glial cells in the old brain, compared with young and middle-aged subjects, in response to cytokine exposure. Interestingly, the results also suggest that such enhancement does not develop gradually since youth, but appears characterized by relatively late onset.

Blueberry extract enhances survival of intraocular hippocampal transplants

Transplantation of neural tissue has been explored as a potential therapy to replace dead or dying cells in the brain, such as after brain injury or neurodegenerative disease. However, survival of transplanted tissue is poor, especially when the transplant recipient is of advanced age. Recent studies have demonstrated improvement of neuronal deficits in aged animals given a diet supplemented with blueberry extract. The present study focuses on the survival of fetal hippocampal transplants to young (4 months) or middle-aged (16 months) animals with or without dietary supplementation with blueberry extract. Results indicate that fetal hippocampus transplanted to middle-aged host animals exhibits poor survival characterized by reduced growth and compromised tissue organization. However, when middle-aged animals were maintained on a diet supplemented with 2% blueberry extract, hippocampal graft growth was significantly improved and cellular organization of grafts was comparable to that seen in tissue grafted to young host animals. Thus, the data suggest that factor(s) in blueberries may have significant effects on development and organization of this important brain region.

alpha-Tocopherol and selenium facilitate recovery from lipopolysaccharide-induced sickness in aged mice

The elderly suffer a decline in immune function that increases their vulnerability to infections. Because antioxidants improve some age-related deficits in immune and cognitive function, our goal was to determine whether dietary alpha-tocopherol (alpha-T) and selenium inhibit LPS-induced sickness behavior in aged mice. Male BALB/c mice were fed modified AIN93-M diets that were low, adequate, or high in both alpha-T (10, 75, or 500 mg/kg) and selenium (0.05, 0.15, or 2 mg/kg) from 18 to 21 mo of age. Sickness was quantified by measuring time in social exploration of a novel juvenile conspecific. The lipopolysaccharide treatment reduced social exploration by 74% at 2 h, regardless of diet. By 4 h, aged mice fed the low diet were 88% less social, whereas mice fed the adequate and high diets displayed only approximately 40% reductions due to LPS treatment. Mice fed the low diet had greater LPS-induced weight loss than mice fed the high diet. Plasma alpha-T concentration and glutathione peroxidase (GPX) activity increased with each increment in alpha-T and selenium 24 h post-LPS treatment. Brain alpha-T concentration and GPX activity were lower in mice fed the low diet than in those fed the adequate or high diet. Regardless of diet, interleukin (IL)-6, IL-1beta, and tumor necrosis factor (TNF)alpha mRNA levels were elevated by LPS approximately 3-fold in cortex, cerebellum, striatum, and hippocampus. Thus, antioxidants inhibit sickness behavior independently of IL-6, IL-1beta, and TNFalpha mRNA levels 2 h post-LPS in the brain regions analyzed. Taken together, these findings suggest that adequate intake of dietary alpha-T and selenium may help promote recovery from gram-negative bacterial infection in the aged.

Exaggerated neuroinflammation and sickness behavior in aged mice following activation of the peripheral innate immune system

Acute cognitive impairment (i.e., delirium) is common in elderly emergency department patients and frequently results from infections that are unrelated to the central nervous system. Since activation of the peripheral innate immune system induces brain microglia to produce inflammatory cytokines that are responsible for behavioral deficits, we investigated if aging exacerbated neuroinflammation and sickness behavior after peripheral injection of lipopolysaccharide (LPS). Microarray analysis revealed a transcriptional profile indicating the presence of primed or activated microglia and increased inflammation in the aged brain. Furthermore, aged mice had a unique gene expression profile in the brain after an intraperitoneal injection of LPS, and the LPS-induced elevation in the brain inflammatory cytokines and oxidative stress was both exaggerated and prolonged compared with adults. Aged mice were anorectic longer and lost more weight than adults after peripheral LPS administration. Moreover, reductions in both locomotor and social behavior remained 24 h later in aged mice, when adults had fully recovered, and the exaggerated neuroinflammatory response in aged mice was not reliably paralleled by increased circulating cytokines in the periphery. Taken together, these data establish that activation of the peripheral innate immune system leads to exacerbated neuroinflammation in the aged as compared with adult mice. This dysregulated link between the peripheral and central innate immune system is likely to be involved in the severe behavioral deficits that frequently occur in older adults with systemic infections.

Rotenone potentiates dopamine neuron loss in animals exposed to lipopolysaccharide prenatally

We previously demonstrated that treating gravid female rats with the bacteriotoxin lipopolysaccharide (LPS) led to the birth of offspring with fewer than normal dopamine (DA) neurons. This DA neuron loss was long-lived and associated with permanent increases in the pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha). Because of this pro-inflammatory state, we hypothesized that these animals would be more susceptible to subsequent exposure of DA neurotoxins. We tested this hypothesis by treating female Sprague-Dawley rats exposed to LPS or saline prenatally with a subtoxic dose of the DA neurotoxin rotenone (1.25 mg/kg per day) or vehicle for 14 days when they were 16 months old. After another 14 days, the animals were sacrificed. Tyrosine hydroxylase-immunoreactive (THir) cell counts were used as an index of DA neuron survival. Animals exposed to LPS prenatally or rotenone postnatally exhibited a 22% and 3%, respectively, decrease in THir cell counts relative to controls. The combined effects of prenatal LPS and postnatal rotenone exposure produced a synergistic 39% THir cell loss relative to controls. This loss was associated with decreased striatal DA and increased striatal DA activity ([HVA]/[DA]) and TNFalpha. Animals exposed to LPS prenatally exhibited a marked increase in the number of reactive microglia that was further increased by rotenone exposure. Prenatal LPS exposure also led to increased levels of oxidized proteins and the formation of alpha-Synuclein and eosin positive inclusions resembling Lewy bodies. These results suggest that exposure to low doses of an environmental neurotoxin like rotenone can produce synergistic DA neuron losses in animals with a preexisting pro-inflammatory state. This supports the notion that Parkinson's disease (PD) may be caused by multiple factors and the result of "multiple hits" from environmental toxins.

Cytokines and the aging brain – what we don't know might help us

Cognitive aspects of aging represent a grave challenge for our societal circumstances as members of the baby-boom generation spiral toward a collective 'senior moment'. In addition, age-related changes in the CNS can contribute to motor deficits and other somatic aberrations. Inflammation and its regulation by cytokines have been connected to many aspects of aging, and mechanisms addressed here provide a rationale for this. Nevertheless, a role for cytokines in normal aging of the human brain has not been confirmed, and it seems to be possible to ameliorate both cognitive decline and cytokine elevation via lifestyle choices. So ignorance of the brain should not prohibit development of successful strategies for delaying or avoiding neurological deficits.

Peripheral treatment with enoxaparin, a low molecular weight heparin, reduces plaques and beta-amyloid accumulation in a mouse model of Alzheimer's disease

We investigated the effect of long-term, peripheral treatment with enoxaparin, a low molecular weight heparin, in transgenic mice overexpressing human amyloid precursor protein(751). Enoxaparin (6 IU per mouse intraperitoneally, three times a week for 6 months) significantly lowered the number and the area occupied by cortical beta-amyloid deposits and the total beta-amyloid (1-40) cortical concentration. Immunocytochemical analysis of glial fibrillary acid protein-positive cells showed that enoxaparin markedly reduced the number of activated astrocytes surrounding beta-amyloid deposits. In vitro, the drug dose-dependently attenuated the toxic effect of beta-amyloid on neuronal cells. Enoxaparin dose-dependently reduced the ability of beta-amyloid to activate complement and contact systems, two powerful effectors of inflammatory response in AD brain. By reducing the beta-amyloid load and cytotoxicity and proinflammatory activity, enoxaparin offers promise as a tool for slowing the progression of Alzheimer's disease.

Neuroprotection by complement (C1) inhibitor in mouse transient brain ischemia

The authors investigated the effect of the C1 inhibitor (C1-INH), the only known inhibitor of complement C1, in a murine model of transient focal ischemia. Ischemia was induced by intraluminal occlusion of the middle cerebral artery. After 2 hours, reperfusion was produced by removing the nylon monofilament occluding the artery. The effect of 15 U C1-INH (intravenously) was evaluated in terms of general and focal neurologic deficits, ischemic volume, neutral red staining (to identify the brain areas subject to ischemic damage), and glial fibrillary acidic protein immunoreactivity (to show astrocytic response). Forty-eight hours after ischemia, C1-INH significantly improved general and focal deficits by 36% and 54%, respectively, and significantly reduced infarct volume (CI-INH, 6.69% +/- 2.93%; saline, 24.24% +/- 8.24%) of total brain. Neutral red staining further showed the strong protective effect of C1-INH in cortex, hippocampus, and striatum. Astrocyte activation induced by ischemia was not affected by C1-INH. These findings show that C1-INH displayed a potent neuroprotective action by effectively reducing ischemia-reperfusion injury.

Human apoE3 but not apoE4 rescues impaired astrocyte activation in apoE null mice

The allele E4 of apolipoprotein E (apoE) is an important risk factor for Alzheimer's disease (AD) and the chronic brain inflammation which is associated with AD is more pronounced in subjects who carry this allele. In the present study, we employed mice transgenic for the human apoE isoforms apoE3 or apoE4 on a null mouse apoE background and intracerebroventricular injection of LPS to investigate the possibility that the regulation of brain inflammation is affected by the apoE genotype. LPS treatment of control mice resulted in activation of brain astrocytes and microglia whose extent decreased with age. LPS treatment of 6-month-old apoE transgenic and control mice resulted in marked activation of brain astrocytes in the control and apoE3 transgenic mice but had no effect on astrogliosis of age-matched apoE-deficient and apoE4 transgenic mice. In contrast, there were no significant differences between the levels of activated microglia of the apoE3 and apoE4 transgenic mice following LPS treatment. Immunoblot assays revealed that the apoE4 and apoE3 transgenic mice had the same levels of brain apoE, which were similarly increased following LPS treatment. These results show that LPS-induced astrogliosis in apoE transgenic mice is regulated isoform-specifically by apoE3 and not by apoE4 and suggest that similar mechanisms may mediate the phenotypic expression of the apoE4 genotype in AD and in other neurodegenerative diseases.

Immune response gene expression increases in the aging murine hippocampus

Using GeneChips, basal and lipopolysaccharide (LPS)-induced gene expression was examined in the hippocampus of 3-, 12-, 18- and 24-month-old male C57BL/6 mice to identify genes whose altered expression could influence hippocampal function in advanced age. Gene elements that changed with age were selected with a t-statistic and specific expression patterns were confirmed with real-time quantitative PCR. Basal expression of 128 gene elements clearly changed with age in the hippocampus. Fourteen gene elements showed increased expression with age and these increases were validated after LPS stimulation. Major histocompatibility complex (MHC) TL region and thymic shared antigen (TSA-1) gene expression increased, suggesting T cell activation in the hippocampus with age. Cytokine (interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha) and chemokine (macrophage chemotactic protein-1) expression increased sharply in 24-month-old mice. These findings are in contrast to a decrease in the peripheral immune response, documented by decreased T cell proliferation and decreased ratios of naive to memory T cells. Age-related increases in inflammatory potential in the brain may contribute to neurodegenerative diseases of the aged.

Age- and concentration-dependent neuroprotection and toxicity by TNF in cortical neurons from beta-amyloid

The induction of an inflammatory response and release of cytokines such as TNF may be involved in the age-related etiology of Alzheimer disease (AD). In the brain, microglia have been shown to produce a wide variety of immune mediators, including the pro-inflammatory cytokine tumor necrosis factor (TNF). We hypothesize that with age there is increased ability of microglia to produce TNF or that age decreases the neuroprotective effect of TNF against beta-amyloid (Abeta) toxicity in neurons. We investigated the effects of Abeta(1-40) on TNF secretion from forebrain cultures of microglia from embryonic, middle-age (9-month) and old (36-month) rats. Over the first 12 hr of exposure to 10 microM Abeta (1-40), microglia from embryonic and old rats increase TNF secretion, although microglia from middle-age rats did not produce detectable levels of TNF. When low concentrations of TNF are added to neurons together with Abeta (1-40) in the absence of exogenous antioxidants, neuroprotection for old neurons is significantly less than neuroprotection for middle-age neurons. In neurons from old rats, high levels of TNF together with Abeta are more toxic than in neurons from middle-age or embryonic rats. These results are discussed in relation to neuroprotection and toxicity of the age-related pathology of AD.

Aging and proinflammatory cytokines

Aging is associated with increased inflammatory activity reflected by increased circulating levels of TNF-alpha, IL-6, cytokine antagonists and acute phase proteins in vivo. Epidemiologic studies suggest that chronic low-grade inflammation in aging promotes an atherogenic profile and is related to age-associated disorders (eg, Alzheimer disease, atherosclerosis, type 2 diabetes, etc.) and enhanced mortality risk. Accordingly, a dysregulated production of inflammatory cytokines has an important role in the process of aging. Studies of age-related differences in the production of proinflammatory cytokines in response to acute stimulations in vitro have yielded inconsistent results. However, in vivo infectious models show delayed termination of inflammatory activity and a prolonged fever response in elderly humans, suggesting that the acute phase response is altered in aging. However, a causal relation between the acute phase response and the increased mortality because of bacterial infections in older patients remains to be demonstrated.

In vivo neutralization of endogenous brain fractalkine increases hippocampal TNFalpha and 8-isoprostane production induced by intracerebroventricular injection of LPS

Fractalkine is a chemokine widely and constitutively expressed in the brain and, as suggested by in vitro studies, it is involved in brain inflammatory responses. In this study, we have investigated the in vivo anti-inflammatory potential of fractalkine in a model of neuroinflammation induced by intracerebroventricular injection of lipopolysaccharide (LPS) in rats. LPS induces a rapid and acute production of the pro-inflammatory cytokine, TNFalpha, in hippocampus and cerebrospinal fluid (CSF), and an increase of 8-isoprostane levels, a marker of oxidative stress, in hippocampus. Although intracerebroventricular injection of fractalkine has no effect on TNFalpha and 8-isoprostane production, neutralization of endogenous fractalkine within the brain with a specific anti-fractalkine antibody potentiates LPS effects. These data emphasize the involvement of constitutive brain fractalkine in the control of inflammatory reaction in CNS.

Central monoamine and plasma corticosterone changes induced by a bacterial endotoxin: sensitization and cross-sensitization effects

Low doses of lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), or exposure to a stressor (restraint) increased plasma corticosterone levels. In animals pretreated with lipopolysaccharide, a marked sensitization of the corticosterone response was evident upon subsequent exposure to lipopolysaccharide, TNF-alpha, or restraint, 1 day later. As well, the sickness-inducing effects of lipopolysaccharide, TNF-alpha and IL-1 beta were markedly increased in mice pretreated with lipopolysaccharide. The sensitization effects were marked when the second treatment was administered 1 day after lipopolysaccharide administration, but not when a 28-day interval elapsed. In a second experiment, TNF-alpha influenced monoamine functioning in the paraventricular nucleus of the hypothalamus and within extrahypothalamic regions, including the central amygdala, locus coeruleus, prefrontal cortex. Moreover, serotonin activity within the central amygdala, as well as dopamine activity within the prefrontal cortex, were subject to a sensitization effect in animals pretreated with lipopolysaccharide 1 day earlier. Macrophage depletion by a suspension of clodronate liposomes attenuated the plasma corticosterone changes induced by TNF-alpha, but did not affect the sensitization. In contrast, the acute effects of TNF-alpha on central neurotransmitters were unaffected by the liposome suspension, but this treatment prevented the sensitization. These data may be relevant to clinical situations in which individuals exposed to bacterial infections may be rendered more susceptible to the behavioural and neurochemical effects of subsequently encountered stressors and immunological challenges.

Depletion of systemic macrophages by liposome-encapsulated clodronate attenuates striatal macrophage invasion and neurodegeneration following local endotoxin infusion in gerbils

CNS-localized inflammation with microglial activation and macrophage infiltration contributes to the pathogenesis of a broad spectrum of neurologic diseases. A direct injection of lipopolysaccharide (LPS) into the striatum of gerbils induced lectin-positive macrophage parenchymal invasion, minimal local microglial staining but extensive neurodegeneration (cresyl violet and silver staining) when evaluated 4 days later. In mice, LPS activated microglia (increased lectin staining of morphologically identified cells) with substantially less macrophage invasion but no neurodegeneration was seen at 4 days post LPS infusion. To evaluate the role of infiltrating macrophages in the neurodegenerative response in gerbils, peripheral macrophages were depleted by an intravenous injection of liposome-encapsulated clodronate. This preparation depleted spleen and liver macrophages (>95%), decreased blood monocytes by 55% and attenuated striatal macrophage infiltration (32 to 73% in five representative sections). Notably, the liposome-encapsulated clodronate reduced the severity of LPS-induced neurodegeneration, as visualized by cresyl violet staining and quantified in 20 serially stained silver sections (total volume, 1.32+/-0.41 mm(3) in liposome-encapsulated clodronate-treated versus 3.04+/-0.72 mm(3) in saline-treated controls). These results indicate that a local LPS infusion in gerbil brain may be a useful model in which to investigate the role of invading macrophages and other inflammatory responses in neurodegeneration in inflammatory neurological disease.

Augmentation of mercury-induced nephrotoxicity by endotoxin in the mouse

Endotoxin (lipopolysaccharide; LPS) and mercury are compounds of food safety concern. Endotoxin is a product of cell walls of gram negative bacteria. Humans are constantly exposed to LPS through infection plus translocation into circulation from the gastrointestinal tract. Food is the major source of mercury in humans. The toxic interaction between LPS and mercury has not been well investigated. In a previous study, we demonstrated that LPS potentiated mercury-induced nephrotoxicity in the rat. Whether this observation was species specific was not clear. In this study we tested the hypothesis that LPS enhances mercuric chloride (HgCl(2))-induced nephrotoxicity in mice. In a 2x2 factorial design, mice received either Escherichia coli 0128:B12 endotoxin (2.0 mg/kg body weight) or 200 microliter of 0.9% sodium chloride (saline), and this was followed 4 h later by either mercury (1.75 mg mercuric chloride per kg body weight) or 200 microliter of saline. Mice were monitored for 48 h. Monitored end-points included body and renal weights, urine volume, renal histology and ultrastructural pathology, serum urea nitrogen and creatinine, selected serum and urine cytokines, and renal mercury concentrations. Endotoxin by itself was not nephrotoxic at the dose used in this study. Overall, mice given LPS plus mercury were the most severely affected. Mice given LPS and mercury also had significantly greater renal mercury concentration than those given mercury alone (P</=0.05). In conclusion, LPS potentiates mercury-induced nephrotoxicity in the mouse.