Immunological aspects of microglia: relevance to Alzheimer's disease

Critical role of microglial CD40 in the maintenance of mechanical hypersensitivity in a murine model of neuropathic pain

We recently demonstrated a contributing role of spinal cord infiltrating CD4+ T lymphocytes in the maintenance of mechanical hypersensitivity in a rodent model of neuropathic pain, spinal nerve L5 transection (L5Tx). It has been demonstrated that microglia play a role in the etiology of pain states. We hypothesized that infiltrating CD4+ T lymphocytes communicate with microglia via a CD40-CD154 interaction. Here, we investigated the role of CD40 in the development of mechanical hypersensitivity post-L5Tx. CD40 KO mice displayed significantly decreased mechanical sensitivity compared with WT mice starting from day 5 post-L5Tx. Using bone marrow chimeric mice, we further identified a pro-nociceptive role of CNS microglial CD40 rather than the peripheral leukocytic CD40. Flow cytometric analysis determined a significant increase of CD40+ microglia in the ipsilateral side of lumbar spinal cord post-L5Tx. Further, spinal cord proinflammatory cytokine (IL-1beta, IL-6, IL-12, and TNF-alpha) profiling demonstrated an induction of IL-6 in both WT and CD40 KO mice post-L5Tx prior to the increase of microglial CD40 expression, indicating a CD40-independent induction of IL-6 following L5Tx. These data establish a novel role of microglial CD40 in the maintenance of nerve injury-induced behavioral hypersensitivity, a behavioral sign of neuropathic pain.

Neuroglia in neurodegeneration

Neuroglial cells are fundamental for control of brain homeostasis and they represent the intrinsic brain defence system. All forms in neuropathology therefore inevitably involve glia. The neurodegenerative diseases disrupt connectivity within brain circuits affecting neuronal-neuronal, neuronal-glial and glial-glial contacts. In addition neurodegenerative processes trigger universal and conserved glial reactions represented by astrogliosis and microglial activation. The complex of recently acquired knowledge allows us to regard the neurodegenerative diseases as primarily gliodegenerative processes, in which glial cells determine the progression and outcome of neuropathological process.

IL-13-induced oxidative stress via microglial NADPH oxidase contributes to death of hippocampal neurons in vivo

In the present study, we investigated the effects of IL-13, a well-known anti-inflammatory cytokine, on the thrombin-treated hippocampus in vivo. NeuN immunohistochemistry and Nissl staining revealed significant loss of hippocampal CA1 neurons upon intrahippocampal injection of thrombin. This neurotoxicity was accompanied by substantial microglial activation, as evident from OX-42 immunohistochemistry results. In parallel, Western blot analysis and hydroethidine histochemistry disclosed activation of NADPH oxidase, generation of reactive oxygen species, and oxidative damage in the hippocampal CA1 area showing hippocampal neuron degeneration. Interestingly, immunohistochemical and biochemical experiments showed that intrahippocampal injection of thrombin increased IL-13 immunoreactivity and IL-13 levels as early as 8 h after thrombin, reaching a peak at 7 days, which was maintained up to 14 days. Moreover, double-label immunohistochemistry revealed IL-13 immunoreactivity exclusively in activated microglia. IL-13-neutralizing Abs significantly rescued CA1 hippocampal neurons from thrombin neurotoxicity. In parallel, neutralization of IL-13 inhibited activation of NADPH oxidase, reactive oxygen species production, and oxidative damage. Additionally, IL-13 neutralization suppressed the expression of inducible NO synthase and several proinflammatory cytokines. To our knowledge, the present study is the first to show that IL-13 triggers microglial NADPH oxidase-derived oxidative stress, leading to the degeneration of hippocampal neurons in vivo, as occurs in cases of Alzheimer's disease.

PK11195 labels activated microglia in Alzheimer's disease and in vivo in a mouse model using PET

Activated microglia may promote neurodegeneration in Alzheimer's disease (AD) and may also help in amyloid clearance in immunization therapies. In vivo imaging of activated microglia using positron emission tomography (PET) could assist in defining the role of activated microglia during AD progression and therapeutics. We hypothesized that PK11195, a ligand that binds activated microglia, could label these cells in postmortem AD tissues and in vivo in an animal model of AD using PET. [(3)H](R)-PK11195 binding was significantly higher in AD frontal cortex compared to controls and correlated mainly with the abundance of immunohistochemically labeled activated microglia. With age, the brains of APP/PS1 transgenic mice showed progressive increase in [(3)H](R)-PK11195 binding and [(11)C](R)-PK11195 retention in vivo assessed using microPET, which correlated with the histopathological abundance of activated microglia. These results suggest that PK11195 binding in AD postmortem tissue and transgenic mice in vivo correlates with the extent of microglial activation and may help define the role of activated microglia in the pathogenesis and treatment of AD.

Lipopolysaccharide extends the lifespan of mouse primary-cultured microglia

Microglial activation has been implicated in the recognition and phagocytic removal of degenerating neurons; however, this process must be tightly regulated in the central nervous system, because prolonged activation could damage normal neurons. We report that mouse primary-cultured microglia, which are destined to die within a few days under ordinary culture conditions, can live for more than 1 month when kept activated by lipopolysaccharide (LPS) treatment. Primary-cultured microglia treated with sublethal doses of LPS remained viable, without any measurable increase in apoptotic or necrotic cell death. LPS-treated microglia had an arborescent shape, with enlarged somata and thickened cell bodies. Although the amount of intracellular ATP in these microglia was reduced by 2 h after the start of LPS treatment, this had no effect on the viability of the cells. LPS treatment of microglia increased the antiapoptotic factor Bcl-xL protein level at day 1, although the level of the proapoptotic Bcl-associated X-protein was unaffected. Furthermore, the level of microtubule-associated light chain 3, a marker protein for autophagy, decreased at 3 h after exposure to LPS. These data show that the optimal dose of LPS suppresses the induction of both apoptosis and autophagy in primary-cultured microglia, allowing the cells to stay alive for more than 1 month. Because long-lived microglia may play critical roles in the exacerbation of neurodegeneration, our findings suggest that inducing a resting stage in active microglia could be a new and promising strategy to inhibit the deterioration of neurodegenerative disease.

IL-6 release by LPS-stimulated peripheral blood mononuclear cells as a potential biomarker in Alzheimer's disease

BACKGROUND:

In Alzheimer's disease (AD), microglia are activated by amyloid-beta (Aβ) to release IL6 among other cytokines, which in turn may be neurotoxic. Prior studies suggest that the brain inflammatory response to various antigens can be modeled by measuring the release of IL6 from peripheral blood mononuclear cells stimulated by lipopolysaccharide (LPS). We sought to replicate these results and extend to an AD-specific stimulus (Aβ).

METHOD:

PBMCs were purified from 5 AD and 5 age-gender matched cognitively healthy controls and exposed to LPS at two concentrations (20 and 100 ng/ml) and Aβ1-42(20 ug/ml). IL6 release was measured with standard ELISA kits, and the ratio of “IL6 release ” with and without LPS stimulation was reported as the “IL6 release ratio .” Correlations were performed with Pearson's r2.

RESULTS:

IL6 release ratios were increased in AD participants as compared to cognitively normal age-gender matched controls with LPS 100 ng/ml exposure at a trend level (p=.07). Aβ1-42 increased the IL6 release ratio at a trend level (P=.07) with LPS 20 ng/ml exposure. IL6 release ratio was significantly correlated with worse performance on a verbal category fluency test (p=.03, r2=.49)and higher scores on the Neuropsychiatric Inventory (p=.01, r2=.63). There were trend level correlations of Il6 release with worse ratings on the CDR and MMSE.

CONCLUSIONS:

The IL6 release ratio derived from peripheral blood has potential as a biomarker of AD disease severity, both for cognitive outcomes and neuropsychiatric symptoms of AD.

Interleukin-4 as a neuromodulatory cytokine: roles and signaling in the nervous system

Although interleukin (IL)-4 is described as a prototypical anti-inflammatory cytokine, in recent years its role as a neuromodulatory cytokine has been extensively discussed. This review highlights the pivotal contributions of IL-4 during the development and normal physiology of neural cells as well as IL-4 connections with the pathophysiology of degenerative or inflammatory processes observed in the central and peripheral nervous system.

Human misfolded truncated tau protein promotes activation of microglia and leukocyte infiltration in the transgenic rat model of tauopathy

It has been hypothesized that misfolded tau protein could be a mediator of the inflammatory response in human tauopathies. Here we show that neurodegenerative lesions caused by human truncated tau promote inflammatory response manifested by upregulation of immune-molecules (CD11a,b, CD18, CD4, CD45 and CD68) and morphological activation of microglial cells in a rat model of tauopathy. In parallel, the innate immune brain response promotes activation of MHC class II positive blood-borne leukocytes and their influx into the brain parenchyma. These findings have important consequences for the rationale drug development of effective inflammation-based therapeutic strategies for human tauopathies.

Differential regulation of toll-like receptor mRNAs in amyloid plaque-associated brain tissue of aged APP23 transgenic mice

Alzheimer's disease (AD) is characterized by the pathological deposition of amyloid-beta protein in the aged brain. Inefficient clearance of amyloid-beta from brain tissue is believed to play a major role in the pathogenesis of these deposits. Since amyloid-beta clearance likely involves activation of microglial cells via toll-like receptors and since these receptors and their signaling pathways are regarded as potential therapeutic targets, we have studied the expression of toll-like receptor (tlr) mRNAs in an animal model of AD (APP23 transgenic mice). Laser microdissection was used to harvest plaques, tissue surrounding plaques and plaque-free tissue from cortex of aged APP23 transgenic mice and age-matched controls. Real-time RT-PCR was employed to quantify expression levels of different tlr mRNAs in these tissues. This revealed a strong upregulation of tlr2, tlr4, tlr5, tlr7 and tlr9 mRNAs in plaque material compared to plaque-free tissue. In contrast, tlr3 was not significantly upregulated. Plaque-free tissue did not show an increased expression of any tlr mRNAs compared to age-matched control mice. Double-immunofluorescence for TLR2 and the microglial marker Iba1 was used to demonstrate localization of TLR2 on plaque-associated microglia. Taken together, these data show a strong upregulation of mRNAs encoding surface TLRs in plaque-associated brain tissue of aged APP23 transgenic mice. Since TLR-upregulation is restricted to plaques, modifying TLR-signaling may be a promising therapeutic strategy for plaque removal.

TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice

Alzheimer's disease (AD) is characterized by extracellular deposits of amyloid-beta protein which attract dense clusters of microglial cells. Here, we analyzed amyloid plaque-associated areas in aged APP23 transgenic mice, an animal model of AD, by combining laser microdissection with microarray analysis and quantitative RT-PCR (qPCR). By comparing gene expression profiles, we found that 538 genes (1.3% of a total of 41,234 analyzed genes) were differentially expressed in plaque-associated versus plaque-free tissue of aged APP23 transgenic mice. One of these genes is the microglia-associated triggering receptor expressed on myeloid cells (TREM2) which enhances phagocytosis, but abrogates cytokine production as well as TLR and Fc receptor-mediated induction of TNF secretion. Western Blot analysis demonstrated an upregulation of TREM2 protein in APP23 transgenic compared with nontransgenic mice. Confocal imaging studies, furthermore, confirmed colocalization of TREM2 protein with microglia. Thus, when TREM2 is induced on microglia in plaque-loaded brain areas the respective signaling may prevent inflammation-induced bystander damage of neurons. At the same time, TREM2 signaling may also account for the failure to sufficiently eliminate extracellular amyloid with the help of a systemic immune response.

Leptin: a novel therapeutic strategy for Alzheimer's disease

Adipocyte-derived leptin appears to regulate a number of features defining Alzheimer's disease (AD) at the molecular and physiological level. Leptin has been shown to reduce the amount of extracellular amyloid beta, both in cell culture and animal models, as well as to reduce tau phosphorylation in neuronal cells. Importantly, chronic administration of leptin resulted in a significant improvement in the cognitive performance of transgenic animal models. In AD, weight loss often precedes the onset of dementia and the level of circulating leptin is inversely proportional to the severity of cognitive decline. It is speculated that a deficiency in leptin levels or function may contribute to systemic and CNS abnormalities leading to disease progression. Furthermore, a leptin deficiency may aggravate insulin-controlled pathways, known to be aberrant in AD. These observations suggest that a leptin replacement therapy may be beneficial for these patients.

Macrophage antigen complex-1 mediates reactive microgliosis and progressive dopaminergic neurodegeneration in the MPTP model of Parkinson's disease

Neuronal death is known to trigger reactive microgliosis. However, little is known regarding the manner by which microglia are activated by injured neurons and how microgliosis participates in neurodegeneration. In this study we delineate the critical role of macrophage Ag complex-1 (MAC1), a member of the beta(2) integrin family, in mediating reactive microgliosis and promoting dopaminergic (DAergic) neurodegeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. MAC1 deficiency greatly attenuated the DAergic neurodegeneration induced by MPTP or 1-methyl-4-phenyl-pyridium iodide (MPP(+)) exposure both in vivo and in vitro, respectively. Reconstituted experiments created by adding microglia from MAC1(-/-) or MAC1(+/+) mice back to MAC1(+/+) neuron-enriched cultures showed that microglia with functional MAC1 expression was mandatory for microglia-enhanced neurotoxicity. Both in vivo and in vitro morphological and Western blot studies demonstrated that MPTP/MPP(+) produced less microglia activation in MAC1(-/-) mice than MAC1(+/+) mice. Further mechanistic studies revealed that a MPP(+)-mediated increase in superoxide production was reduced in MAC1(-/-) neuron-glia cultures compared with MAC1(+/+) cultures. The stunted production of superoxide in MAC1(-/-) microglia is likely linked to the lack of translocation of the cytosolic NADPH oxidase (PHOX) subunit (p47(phox)) to the membrane. In addition, the production of PGE(2) markedly decreased in neuron plus MAC1(-/-) microglia cocultures vs neuron plus MAC1(+/+) microglia cocultures. Taken together, these results demonstrate that MAC1 plays a critical role in MPTP/MPP(+)-induced reactive microgliosis and further support the hypothesis that reactive microgliosis is an essential step in the self-perpetuating cycle leading to progressive DAergic neurodegeneration observed in Parkinson's disease.

Microglia and myeloperoxidase: a deadly partnership in neurodegenerative disease

The role of inflammation in Alzheimer's disease, Parkinson's disease, and multiple sclerosis has recently come under increased scrutiny. Associated with these inflammatory responses are tumor necrosis factor-alpha (TNF-alpha) and reactive oxygen species (ROS), both believed to be derived from brain microglia. In addition to the above, the presence of myeloperoxidase (MPO) in these diseased brains has been reported by a number of investigators. However, the possible role of MPO and enzymatically inactive MPO (iMPO) as the "choreographers" of the destruction done by TNF-alpha and ROS is not generally recognized. Previously, our laboratory has reported that MPO/iMPO enhance macrophage generation of ROS and expression of proinflammatory cytokine genes as well as gene products. Recent studies in our laboratory indicate that the same response occurs with microglia. A paradigm is presented for the perpetuation of inflammation associated with neurodegenerative diseases. This model describes the unrecognized consequences of the stimulation of microglia by MPO or iMPO. Both MPO and iMPO and/or its receptor may represent new therapeutic targets for the treatment of these diseases.

The peroxisome proliferator-activated receptor gamma (PPAR-γ2) Pro12Ala polymorphism is associated with higher risk for Alzheimer's disease in octogenarians

Recent observations support the hypothesis that inflammatory processes at the brain level may contribute to the pathogenesis of Alzheimer's disease (AD). Peroxisome proliferator-activated receptor gamma (PPAR-gamma) is involved in such processes, so we thought it interesting to study the Pro12Ala polymorphism in exon 2 of the gene in a sample of late-onset AD patients. We found that Ala genotypes were significantly overrepresented among octogenarian patients compared to controls (p=0.034). Using logistic regression we observed that carrying the Ala allele significantly increased by nearly two-fold the risk of developing AD in subjects 80 years or older (OR=1.98; 95% CI 1.03-3.80, p=0.04). Though this difference was borderline significant after correction for multiple comparisons, our results suggest a possible involvement of the PPAR-gamma gene in susceptibility to late-onset AD in octogenarians.

Anti-inflammatory effect of melatonin on Aβ vaccination in mice

A beta vaccination as a therapeutic intervention of Alzheimer's has many challenges, key among them is the regulation of inflammatory processes concomitant with excessive generation of free radicals seen during such interventions. Here we report the beneficial effects of melatonin on inflammation associated with A beta vaccination in the central and peripheral nervous system of mice. Mice were divided into three groups (n=8 in each): control, inflammation (IA), and melatonin-treated (IAM). The brain, liver, and spleen samples were collected after 5 days for quantitative assessment of plasma lipid peroxides (LPO), an oxidative stress marker, and antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione peroxidase (Gpx). IA group mice have shown the elevated concentration of LPO significantly while there was a reduction at antioxidant enzyme levels. In addition, a significant (P<0.05) reduction in neurotransmitters like dopamine (DA), 5-hydroxytryptamine (5-HT), and norepinephrine (NE) was also observed in the IA group mice. Nevertheless, their metabolites, such as homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) increased significantly (P<0.05) as compared to control. Samples were further evaluated at microscopic level to examine the neuropathological changes by immunohistochemical methods. Melatonin treatment effectively reversed these above changes and normalized the LPO and antioxidant enzyme levels (P<0.05). Furthermore, melatonin salvaged the brain cells from inflammation. Our Immunohistochemical findings in the samples of melatonin-treated animals (IAM group) indicated diminished expression of glial fibrillary acidic protein (GFAP) and nuclear factor kappa B (Nf kappa B) than those observed in the IA group samples. Our results suggest that administration of melatonin protects inflammation associated with A beta vaccination, through its direct and indirect actions and it can be an effective adjuvant in the development of vaccination in immunotherapy for Alzheimer's disease (AD).

The age-related attenuation in long-term potentiation is associated with microglial activation

It is well established that inflammatory changes contribute to brain ageing, and an increased concentration of proinflammatory cytokine, interleukin-1beta (IL-1beta), has been reported in the aged brain associated with a deficit in long-term potentiation (LTP) in rat hippocampus. The precise age at which changes are initiated is unclear. In this study, we investigate parallel changes in markers of inflammation and LTP in 3-, 9- and 15-month-old rats. We report evidence of increased hippocampal concentrations of the proinflammatory cytokines IL-1alpha, IL-18 and interferon-gamma (IFNgamma), which are accompanied by deficits in LTP in the older rats. We also show an increase in expression of markers of microglial activation, CD86, CD40 and intercellular adhesion molecules (ICAM). Associated with these changes, we observed a significant impairment of hippocampal LTP in the same rats. The importance of microglial activation in the attenuation of long-term potentiation (LTP) was demonstrated using an inhibitor of microglial activation, minocycline; partial restoration of LTP in 15-month-old rats was observed following administration of minocycline. We propose that signs of neuroinflammation are observed in middle age and that these changes, which are characterized by microglial activation, may be triggered by IL-18.

Peripheral inflammatory mechanisms modulate microglial activation in response to mild impairment of oxidative metabolism

Thiamine deficiency (TD) models the selective neurodegeneration that accompanies the mild impairment of oxidative metabolism, which is observed in a variety of neurodegenerative diseases. Several markers of inflammation accompany neuronal death in TD and in these diseases. Studies in the submedial thalamic nucleus (SmTN), the region most sensitive to TD, have begun to define the temporal response of inflammation, immune response and neurodegeneration. Our previous studies show that the immune response is involved in TD-induced neurodegeneration. The current experiments tested the roles of other inflammatory cascades in TD-induced neuronal death. Deletion of genes for CD4, or CD8 (the co-receptors for T-cells), IFN-gamma (the cytokine produced by T-cell), or NADPH oxidase (the inflammation related oxidase) were tested. None protected against neuronal death in late stages of TD. On the other hand, deletion of the genes for CD4, CD8 and IFN-gamma increased the microglial activation, and deletion of the gene for NADPH oxidase decreased microglial activation when compared to control mice. In wild type mice, TD caused hypertrophy of CD68 positive microglia without increasing the number of microglia. However, TD induced hypertrophy and proliferation of CD68-positive microglia in the CD4 (97%), CD8 (57%) or IFN-gamma (96%) genetic knockout mice. In the genetic knockout mice for NADPH oxidase, the microglial activation was 65% less than the wild type mice. The results demonstrate that mice deficient in specific T cells (CD4-/-, CD8-/-) or activated T cell product, (IFN-gamma-/-) have increased microglia activation, but mice deficient in NADPH oxidase have decreased microglial activation. However, at the time point tested, the deletions were not neuroprotective. The results suggest that inflammatory responses play a role in TD-induced pathological changes in the brain, and the inflammation appears to be a late event that reflects a response to neuronal damage, which may spread the damage to other brain regions.

Neither sequence variation in the IL-10 gene promoter nor presence of IL-10 protein in the cerebral cortex is associated with Alzheimer's disease

Interleukin 10 (IL-10) is an important anti-inflammatory cytokine produced in response to neuroinflammation and might be involved in modulating the progression of Alzheimer's disease (AD) through inhibiting the action of pro-inflammatory cytokines. We have used immunohistochemistry, Western blotting, real time-PCR (RT-PCR) on frontal (BA 6/24) and temporal (BA 20-22) neocortex and hippocampus from AD and control brains as well as genetic association analysis to address the possible involvement of IL-10 in AD. Expression of IL-10 in AD and control brains at both protein and mRNA levels were detected. However, the level of expression, particularly of IL-10 protein, varied considerably in individual brains and we did not find a significant difference between AD and controls. Using direct sequencing we examined five single nucleotide polymorphisms (SNPs) (-3538, -1354, -1087, -824, -597) and two microsatellites (IL-10-G, IL-10-R) in the promoter region of the IL-10 gene. None of the identified SNPs were found to be associated with AD either individually or as haplotypes. Levels of IL-10 protein and gene expression examined also did not appear to be related to AD. Despite this being a relatively small sample, these data suggest that IL-10 does not play a major role in the development of AD.

The brain as a target of inflammation: common pathways link inflammatory and neurodegenerative diseases

Classical knowledge distinguishes between inflammatory and non-inflammatory diseases of the brain. Either the immune system acts on the CNS and initiates a damage cascade, as in autoimmune (e.g. multiple sclerosis) and infectious conditions, or the primary insult is not inflammation but ischemia or degeneration, as in stroke and Alzheimer's disease, respectively. However, as we review here, recent advances have blurred this distinction. On the one hand, the classical inflammatory diseases of the brain also exhibit profound and early neurodegenerative features - remarkably, it has been known for more than a century that neuronal damage is a key feature of multiple sclerosis pathology, yet this was neglected until very recently. On the other hand, immune mechanisms might set the pace of progressive CNS damage in primary neurodegeneration. Despite differing initial events, increasing evidence indicates that even in clinically heterogeneous diseases, there might be common immunological pathways that result in neurotoxicity and reveal targets for more efficient therapies.

Tomoregulin-2 is found extensively in plaques in Alzheimer's disease brain

Tomoregulin (TR)2 is a transmembrane protein predominantly expressed in brain. It has a unique extracellular domain, containing epidermal growth factor-like and follistatin-like modules. The ectodomain is released from the cell surface, and thought to function as a neurotrophic factor and dendritogenic agent. During CNS development and in the neuronal storage disease GM2 gangliosidosis, which is characterized by ectopic dendrites, the TR2 ectodomain is present in neuronal nuclei where it may function in dendrite initiation. Data presented here demonstrate that TR2 is found extensively in Alzheimer's disease (AD) plaques. Confocal microscopy shows that TR2 is present throughout plaques. Interestingly, TR2 is absent from plaques in the presenilin-1/amyloid precursor protein mouse model of AD. From these data, and what is known about TR2, it is hypothesized that TR2 may participate in amyloid plaque formation and contribute to the pathogenesis of AD. The human TR2 gene is located on chromosome 2q32.3, near a locus linked to Parkinson's disease. TR2 is reported to be a trophic factor for dopaminergic mesencephalic neurons.

Amyloid β peptide (25–35) activates protein kinase C leading to cyclooxygenase-2 induction and prostaglandin E2 release in primary midbrain astrocytes

Prostaglandins (PGs) are generated by the enzymatic activity of cyclooxygenase-1 and -2 (COX-1/2) and modulate several functions in the CNS such as the generation of fever, the sleep/wake cycle, and the perception of pain. Moreover, the induction of COX-2 and the generation of PGs has been linked to neuroinflammatory aspects of Alzheimer's disease (AD). Non-steroidal anti-inflammatory drugs (NSAIDs) that block COX enzymatic activity have been shown to reduce the incidence of AD in various epidemiological studies. While several reports investigated the expression of COX-2 in neurons and microglia, expression of COX-2 in astroglial cells has not been investigated in detail. Here we show that amyloid beta peptide 25-35 (Abeta(25-35)) induces COX-2 mRNA and protein synthesis and a subsequent release of prostaglandin E(2) (PGE(2)) in primary midbrain astrocytes. We further demonstrate that protein kinase C (PKC) is involved in Abeta(25-35)-induced COX-2/PGE(2) synthesis. PKC-inhibitors prevent Abeta(25-35)-induced COX-2 and PGE(2) synthesis. Furthermore Abeta(25-35) rapidly induces the phosphorylation and enzymatic activation of PKC in primary rat midbrain glial cells and in primary human astrocytes from post mortem tissue. Our data suggest that the PKC isoforms alpha and/or beta are most probably involved in Abeta(25-35)-induced expression of COX-2 in midbrain astrocytes. The potential role of astroglial cells in the phagocytosis of amyloid and the involvement of PGs in this process suggests that a modulation of PGs synthesis may be a putative target in the prevention of amyloid deposition.

Eicosapentaenoic acid confers neuroprotection in the amyloid-beta challenged aged hippocampus

Among the changes that occur in the hippocampus with age, is a deficit in long-term potentiation (LTP). This impairment is associated with inflammatory changes, which are typified by increased concentration of the pro-inflammatory cytokine interleukin-1beta (IL-1beta). Activated microglia are the most likely cell source of IL-1beta, but data demonstrating an age-related increase in microglial activation is equivocal. Here we demonstrate that the age-related deficit in LTP is accompanied by increased expression of cell surface markers of activated microglia (major histocompatibility complex II and CD40) and increased IL-1beta production, and that these changes may be stimulated by interferon-gamma. Treatment of aged rats with eicosapentaenoic acid (EPA) attenuates these changes and we suggest that IL-4 mediates the action of EPA. We demonstrate that aged rats exhibit an exaggerated response to intracerebroventricular injection of beta-amyloid peptide 1-40 (Abeta). Thus Abeta inhibited LTP in aged, but not young, rats and induced a further increase in hippocampal IL-1beta concentration. Of particular significance is the demonstration that EPA protects the aged brain so that the increased vulnerability to Abeta is ameliorated in EPA-treated rats.

Beta-amyloid-dependent expression of NOS2 in neurons: prevention by an alpha2-adrenergic antagonist

The neurotransmitter noradrenaline (NA) exerts important antiinflammatory effects on glial cells including suppression of the inducible form of nitric oxide synthase (NOS2). The authors examined the consequences of manipulating NA in vivo by treating adult rats with the neurotoxin DSP4, which selectively lesions noradrenergic neurons of the locus ceruleus (LC), and reduces cortical NA levels. Following LC lesion, intracortical injection of aggregated amyloid beta 1-42 (Abeta1-42) caused appearance of NOS2 within neurons, and increased neuronal damage assessed by staining for nonphosphorylated neurofilament proteins with antibody SMI-32. Co-treatment with a selective alpha2-adrenergic antagonist reduced neuronal NOS2 staining as well as SMI-32 staining. Neuronal damage was dependent on NOS2 expression since injection of Abeta1-42 into DSP4-treated NOS2-deficient mice did not result in neuronal damage. These results demonstrate that decrease of NA levels in vivo can exacerbate inflammatory responses and neuronal damage due to inflammatory stimuli such as Abeta. These findings suggest that alpha2-adrenergic antagonists could provide therapeutic benefit in neurological diseases such as AD or PD where LC loss is known to occur.

Gender differences in neurological disease: role of estrogens and cytokines

Increasing evidence suggests that inflammatory response may be a critical component of different brain pathologies. However, the role played by this reaction is not fully understood. The present findings suggest that neuroinflammtory mediators such as cytokines may be involved in a number of key steps in the pathological cascade of events leading to neuronal injury. This hypothesis is strongly supported by experimental and clinical observations indicating that inhibition of the inflammatory reaction correlates with less neuronal damage. Estrogens are thought to play a role in the sex difference observed in many neurological diseases with inflammatory components including stroke, Alzheimer's and Parkinson's diseases, multiple sclerosis, or amyotrophic lateral sclerosis. Clinical and experimental studies have established estrogen as a neuroprotective hormone in these diseases. However, the exact mechanisms involved in the neuroprotective effects of estrogens are still unclear. It is possible that the beneficial effects of these hormones may be dependent on their inhibitory activity on the inflammatory reaction associated with the above-mentioned brain pathologies. Here, we review the current clinical and experimental evidence with respect to the inflammation-modulating effects of estrogens as one potential explanatory factor for sexual dimorzphism in the prevalence of numerous neurological diseases.

Thrombin-induced oxidative stress contributes to the death of hippocampal neurons in vivo: role of microglial NADPH oxidase

The present study investigated whether thrombin, a potent microglial activator, can induce reactive oxygen species (ROS) generation through activation of microglial NADPH oxidase and if this may contribute to oxidative damage and consequent neurodegeneration. Seven days after intrahippocampal injection of thrombin, Nissl staining and immunohistochemistry using the neuronal-specific nuclear protein NeuN revealed a significant loss in hippocampal CA1 neurons. In parallel, thrombin-activated microglia, assessed by OX-42 and OX-6 immunohistochemistry, and ROS production, assessed by hydroethidine histochemistry, were observed in the hippocampal CA1 area in which degeneration of hippocampal neurons occurred. Reverse transcription-PCR at various time points after thrombin administration demonstrated an early and transient expression of inducible nitric oxide synthase (iNOS) and several proinflammatory cytokines. Western blot analysis and double-label immunohistochemistry showed an increase in the expression of and the localization of iNOS within microglia. Additional studies demonstrated that thrombin induced the upregulation of membrane (gp91(phox)) and cytosolic (p47(phox) and p67(phox)) components, translocation of cytosolic proteins (p47(phox), p67(phox), and Rac1) to the membrane, and p67(phox) expression of the NADPH oxidase in microglia in the hippocampus in vivo, indicating the activation of NADPH oxidase. The thrombin-induced oxidation of proteins and loss of hippocampal CA1 neurons were partially inhibited by an NADPH oxidase inhibitor and by an antioxidant. To our knowledge, the present study is the first to demonstrate that thrombin-induced neurotoxicity in the hippocampus in vivo is caused by microglial NADPH oxidase-mediated oxidative stress. This suggests that thrombin inhibition or enhancing antioxidants may be beneficial for the treatment of neurodegenerative diseases, such as Alzheimer's disease, that are associated with microglial-derived oxidative damage.

Cognitive disturbances in old dogs suffering from the canine counterpart of Alzheimer's disease

In geriatric dogs, Alzheimer-like behavior is frequently observed. This behavior has been classified by several authors using questionnaires and a correlation has been described between cognitive dysfunctions and Alzheimer-like pathology. In the present study, cognitive performance was correlated with brain pathology for 30 dogs of varying ages. Within these animals, two age-matched groups of old dogs with and without behavioral changes were compared. The behavioral changes were analyzed and scored with questionnaires and necropsy was performed to rule out any other cause for changed behavior. Measurements, (immuno)-histochemical staining and fluorescence microscopy were used to detect cortex atrophy, amyloid, rest-products of oxidative damage, demyelination and accumulations of macrophages in the brains of these dogs. Spearman rank correlation coefficients (r) were calculated and adjusted according to Bonferonni. In the whole group (young to very old dogs), the age of the animal showed a significant correlation with various behavioral changes (r = 0.7 to 0.9, P < 0.01). The dementia score correlated significantly (r = 0.6 to 0.8, P < 0.01) with all the brain lesions studied, except one, i.e. demyelination (r = -0.4, P > 0.05). These results suggest that a questionnaire can be used to diagnose Alzheimer-like changes in canine practice. Oxidative damage on a cellular and a nuclear level plays an important role in behavior changes.

The neuroprotective efficacy of alpha-crystallin against acute inflammation in mice

Acute inflammation activates macrophages or monocytes and subsequently releases several inflammatory cytokines and reactive oxygen and nitrogen species. These proinflammatory cytokines activate astrocytes and trigger neurodegenerative diseases. In this work, we chose to address the mechanistic aspects of alpha-crystallin's protective function in inflammation-triggered neurotoxicity in mice. Alpha-crystallin, a lens structural protein, comprising alpha-A and alpha-B subunits is an ubiquitous molecular chaperone, which have been shown to reduce reactive oxygen species (ROS) production and enhance cellular glutathione level in the acute inflammation-induced mice. Results show that the proinflammatory cytokines such as interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) were significantly high (P<0.05) in the plasma, liver, cortex and hippocampus of inflammation-induced mice when compared to control. Alpha-crystallin pretreatment prevents inflammation-induced cytokines and NO production. In addition, a significant (P<0.05) reduction of dopamine (DA), 5-hydroxytryptamine (5-HT) and norepinephrine (NE) was also observed in the inflammation-induced mice. Nevertheless, their metabolites, such as 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) increased significantly (P<0.05) as compared to control. The results indicate that alpha-crystallin pretreatment controls the inflammation-induced DA, 5-HT and NE catabolism and suggest that alpha-crystallin has the potential to act as an anti-inflammatory agent in the neuroprotective processes.

Clinical aspects of inflammation in Alzheimer's disease

In Alzheimer's disease (AD) there is increasing evidence that neurotoxicity is mediated by CNS inflammatory processes. These processes involve activation of microglia by amyloid-beta leading to release of pro-inflammatory cytokines including IL-1beta, IL-6, and TNF-alpha among others. Neurotoxic processes mediated by these cytokines may include direct neuronal death by enhancement of apoptosis, decreased synaptic function as evidence by inhibition of long-term potentiation, and inhibition of hippocampal neurogenesis. Central nervous system (CNS) inflammation may predate the development of senile plaques and neurofibrillary tangles in AD and may prove to be a more sensitive marker of prodromal AD. New developments in measuring CNS inflammation include measuring cytokine release by peripheral blood mononuclear cells and the development of PET markers of microglial activation. There is epidemiological evidence that circulating serum IL-6 is associated with poorer cognition. While epidemiological studies suggest a protective effect of NSAIDs against development of AD, controlled trials of NSAIDs to date have not shown any protective effect of drug. New anti-inflammatory agents for treating or preventing AD may include novel NSAIDs and opioid antagonists. These developments provide an alternative or potential adjunct to anti-amyloid therapies for AD.

Neuroprotective role of microglia expressing interleukin-4

Little is known about the underlying mechanisms responsible for the death of activated microglia and the functional consequences of the death of these cells, especially in vivo. We show here that intracortical injection of lipopolysaccharide (LPS) led to upregulation of interleukin-4 (IL-4) immunoreactivity, followed by a substantial loss of microglia 3 days later, as visualized by complement receptor type 3 (OX-42) immunostaining and tomato lectin staining. Cells positive for caspase-3 and terminal deoxynucleotidyl transferase mediated fluorescein-dUTP nick-end labeling (TUNEL) were also localized within LPS-activated microglia. IL-4 immunoreactivity was detected as early as 12 hr post-LPS, disappearing at 72 hr. Surprisingly, IL-4 immunoreactivity was detected exclusively in microglia, but not in astrocytes or neurons. In addition, IL-4-neutralizing antibodies markedly increased the survival of activated microglia at 3 days post-LPS. The expression of inducible nitric oxide synthase (iNOS) and tumor-necrosis factor (TNF)-alpha was sustained in parallel in activated microglia, consequently increasing neuronal cell death. To our knowledge, this study is the first to show the endogenous expression of IL-4 in LPS-activated microglia in vivo. Our findings suggest that IL-4 may regulate brain inflammation by inducing the death of activated microglia in vivo and increasing neuronal survival.

Cytokine production by a human microglial cell line: effects of beta-amyloid and alpha-melanocyte-stimulating hormone

Senile plaques in the Alzheimer's disease (AD) are formed by aggregation of beta-amyloid (Abeta) peptide. Abeta peptide has been shown to activate microglia and stimulate their production of inflammatory factors, such as cytokines. In the AD brain, the continued presence of amyloid plaques may keep microglia persistently activated, leading to chronic inflammation in the CNS. It is well established that alpha-melanocyte-stimulating hormone (alpha-MSH) gives rise to anti-inflammatory and anti-pyretic effects. The biological activities of alpha-MSH are mediated by one or more of the melanocortin receptor (MCR) subtypes, i.e. MCR1 - MCR5. The aim of the present study was to determine the effect of alpha-MSH alone and on Abeta-activated microglial cells with regard to the secretion of inflammatory cytokines, such as interleukin-6 (IL-6), and to determine which receptor subtype mediates the effects of alpha-MSH. The human microglial cell line, CHME3, was incubated for 24 h with freshly dissolved Abeta(1-40), interferon-gamma (IFN-gamma) and/or alpha-MSH. Freshly dissolved Abeta(1-40) (5-60 microM) resulted in a dose-dependent decrease in cell viability, along with a dose-dependent increase in IL-6 release. Neither IFN-gamma nor alpha-MSH affected the Abeta-induced secretion of IL-6, but resulted in a dose-dependent increase in basal IL-6 release. Agouti, the endogenous antagonist of MCR1 and 4, further increased the alpha-MSH-induced secretion of IL-6. RT-PCR showed the expression of MCR1, MCR3, MCR4 and MCR5 mRNA. The combined data suggest that the effect of alpha-MSH in increasing IL-6 release from the human microglial cell line is mediated by MCR3 or MCR5.

Beta-amyloid protein structure determines the nature of cytokine release from rat microglia

Activated microglia represent a major source of inflammatory factors in Alzheimer's disease and a possible source of cytotoxic factors. beta-Amyloid (Abeta) peptide, the predominant component in amyloid plaques, has been shown to activate microglia and stimulate their production of inflammatory factors. The present study was performed to analyze the responses of microglia to different forms of Abeta, with regard to release of the proinflammatory cytokines interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), IL-6, and interferon-gamma (IFN-gamma), as well as the IL-1 receptor antagonist (IL-1ra). Primary cultures of microglia from rat neonatal cerebral cortex were incubated with freshly dissolved Abeta1-40 or Abeta1-42, Abeta1-40 fibrils, Abeta1-40 betaamy balls, or vehicle. Abeta1-40 fibrils did not significantly stimulate any of these cytokines. Freshly dissolved Abeta1-40 resulted in a marked increase in the release of IL-1beta, and freshly dissolved Abeta1-42 significantly stimulated both IL-1alpha and IFN-gamma secretion. The Abeta1-40 betaamy balls stimulated the secretion of IL-1alpha and IL-1beta. Incubation with Abeta peptides did not affect the secretion of IL-1ra, IL-6, or TNF-alpha. In the case of IL-1beta, the response is correlated with the presence of Abeta peptide as monomers and oligomers.

Inflammation, the complement system and the diseases of aging

Inflammation is characteristic of neurodegenerative diseases of aging. Neuropathological evidence of activated microglia and activated astrocytes in lesioned areas, combined with epidemiological evidence of sparing of Alzheimer's disease (AD), Parkinson's disease (PD) and age-related macular degeneration (AMD) in long-term users of anti-inflammatory agents, indicates that inflammation is autodestructive of neurons. Locally produced autodestructive molecules include the membrane attack complex (MAC) of complement and oxygen-free radicals. Stimulation is provided by a variety of inflammatory cytokines. Agents which reduce the intensity of inflammation should have broad spectrum application in degenerative diseases of aging.

Hyperphosphorylated tau and amyloid precursor protein deposition is increased in the brains of young drug abusers

Drug abuse is a major problem worldwide. The incidence of drug-related deaths attributed to opiate abuse is increasing annually. Apart from routine examination, little is known of the neuropathology of drug abuse. We, and others, have shown previously that drug abuse is associated with microglial activation. We hypothesised that neuroinflammation might lead to premature neurodegeneration in drug abusers. We investigated the brains of young opiate abusers (n=34, all<40 years) for the presence of proteins associated with neurodegenerative diseases and compared them with the brains of age-matched, non-drug users (n=16) all of whom died suddenly. Detailed immunohistochemical analysis of the hippocampus, brainstem and basal ganglia for hyperphosphorylated tau, beta-amyloid, beta-amyloid precursor protein (betaAPP) and ubiquitin demonstrated an excess of AT 8-positive neurofibrillary tangles (NFT) in the drug abusers. These were not only more prevalent in the drug abusers than in controls (44%vs. 19%) but also involved more brain areas. In controls NFT were confined to the entorhinal cortex whereas in drug users they were also found in the subiculum, temporal neocortex, nucleus basalis of Meynert and the locus coeruleus. Virtually no amyloid plaques were present but betaAPP positivity was again much more common in drug abusers than controls (73%vs. 20% in the brainstem and 59%vs. 23% in the temporal lobe). There is no suggestion that these drug abusers had displayed major cognitive impairment although detailed neuropsychological assessment is difficult in this subject group. Likely causes of hyperphosphorylated tau deposition in drug abuse include hypoxic-ischaemic injury, microglial-associated cytokine release and possibly drug-associated neurotoxicity or hepatitis. Head injury, which is another major risk factor, does not appear to have contributed to our findings. Genetic factors also merit consideration. It is unclear at present how much of the hyperphosphorylated tau detected in these young drug abusers represents a transitory phenomenon.

Oxidative and antioxidative potential of brain microglial cells

Microglial cells are the resident immune cells of the central nervous system. These cells defend the central nervous system against invading microorganisms and clear the debris from damaged cells. Upon activation, microglial cells produce a large number of neuroactive substances that include cytokines, proteases, and prostanoids. In addition, activated microglial cells release radicals, such as superoxide and nitric oxide, that are products of the enzymes NADPH oxidase and inducible nitric oxide synthase, respectively. Microglia-derived radicals, as well as their reactive reaction products hydrogen peroxide and peroxynitrite, have the potential to harm cells and have been implicated in contributing to oxidative damage and neuronal cell death in neurological diseases. For self-protection against oxidative damage, microglial cells are equipped with efficient antioxidative defense mechanisms. These cells contain glutathione in high concentrations, substantial activities of the antioxidative enzymes superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, as well as NADPH-regenerating enzymes. Their good antioxidative potential protects microglial cells against oxidative damage that could impair important functions of these cells in defense and repair of the brain.

Deglycosylation of anti-beta amyloid antibodies inhibits microglia activation in BV-2 cellular model

Immunotherapy has become a strategy for treatment of Alzheimer's disease, by inducing antibody response to amyloid-beta peptide (AbetaP) or by passive administration of anti-AbetaP antibodies. Clearance of amyloid plaques involves interaction of immunoglobulin Fc receptor (FcR)-expressing microglia and antibodyopsonized Abeta deposits, stimulating phagocytosis but may promote neuroinflammation. Carbohydrate moiety of Fc of the immunoglobulin G molecule plays a significant role in modulating binding to FcR and its effector functions. Here, we enzymatically removed Fc glycan from monoclonal antibody 196 raised against AbetaP Antigen binding ability and in vitro stability of deglycosylated antibody were unaffected by deglycosylation. Moreover, the deglycosylated antibody exhibits low affinity to FcR on microglial BV-2 cells and has limited ability to mediate microglial chemotaxis and antibodydependent cytotoxicity compared to native antibody. These data suggest that deglycosylation of anti-Abeta antibodies before in vivo administration might prevent microglial overactivation, thus reducing the risk of neuroinflammatory response during passive immunization.

Pro- and anti-inflammatory cytokines regulate the ERK pathway: Implication of the timing for the activation of microglial cells

Pro-inflammatory molecules induce glial activation and the release of potentially detrimental factors capable of generating oxidative damage, such as nitric oxide (NO) and superoxide anion (O2.-). Activated glial cells (astrocytes and microglia) are associated to the inflammatory process in neurodegenerative diseases. A strong inflammatory response could escape endogenous control becoming toxic to neurons and contributing to the course of the disease. We evaluated in a hippocampal cells-microglia co-culture model, if the pro-inflammatory condition induced by lipopolysaccharide + interferon-gamma (LPS+IFN-gamma) promoted damage directly or if damage was secondary to glial activation. In addition, we explored the effect of the anti-inflammatory cytokine transforming growth factor-beta1 (TGF-beta1), and pro-inflammatory cytokines, interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) on the regulation of the inflammatory response of microglia. We found that LPS+IFN-gamma-induced damage on hippocampal cultures was dependent on the presence of microglial cells. In hippocampal cultures exposed to LPS+IFN-gamma, TGF-beta1 was induced whereas in microglial cell cultures LPS+IFN-gamma induced the secretion of IL-1beta. TGF-beta1 and IL-1beta but not TNF-alpha decreased the NO production by 70-90%. PD98059, an inhibitor of MAP kinase (MEK), reduced the IFN-gamma-induced NO production by 40%. TGF-beta and IL-1beta reduced the IFN-gamma induced phosphorylation of ERK1,2 by 60% and 40%, respectively. However, the effect of IL-1beta was observed at 30 min and that of TGF-beta1 only after 24 h of exposure. We propose that acting with different timing, TGF-beta1 and IL-1beta can modulate the extracellular signal-regulated kinase ERK1,2, as a common element for different transduction pathways, regulating the amplitude and duration of glial activation in response to LPS+IFN-gamma. Cross-talk among brain cells may be key for the understanding of inflammatory mechanisms involved in pathogenesis of neurodegenerative diseases.

CD40L deletion delays neuronal death in a model of neurodegeneration due to mild impairment of oxidative metabolism

Inflammatory/immune processes are important in the pathogenesis of neurodegenerative diseases. Thiamine deficiency (TD) models the region selective neuronal loss in brain that accompanies mild impairment of oxidative metabolism. TD induces well-defined alterations in neurons, microglia, astrocytes, and endothelial cells. To test the role of inflammatory/immune mechanisms in TD-induced neurodegeneration, the temporal profile of neurodegeneration was compared to the activation of CD68-positive microglia and ICAM-1-positive endothelial cells during TD in wild type mice and in CD40L-/- mice. CD40L-/- delayed the onset of TD-induced neuronal death as well as the activation of microglia and endothelial cells. The current results suggest that CD40L-mediated immune and inflammatory responses have a role in TD-induced neuronal death.

Excitotoxic and apoptotic neuronal death induce different patterns of glial activation in vitro

We have studied glial activation in rat cerebellar neuronal-glial cultures after inducing neuronal death using various stimuli. Cultures were exposed to 100 microm glutamate for 20 min, which induces excitotoxic neuronal death, or to potassium/serum deprivation, which induces apoptosis of granule neurons. We evaluated alterations in several parameters related to glial activation: nuclear factor-kappaB activation, nitric oxide and tumour necrosis factor-alpha production, which are associated with a pro-inflammatory response, glial proliferation and phagocytic activity. Although the two experimental models of neuronal damage resulted in the death of most neuronal cells within 24 h, differences were observed in the response of the various glial parameters evaluated. While nitric oxide production was not detected in any case, tumour necrosis factor-alpha production, nuclear factor-kappaB activation and glial proliferation were only induced in the presence of excitotoxic neuronal death. However, phagocytosis was induced in both cases, although earlier in the case of apoptotic neuronal death. These results show that glial cells respond to excitotoxic neuronal death with an inflammatory response associated with proliferation and phagocytosis. In contrast, whilst glial cells do not produce pro-inflammatory molecules in the presence of apoptotic neuronal death, phagocytic activity is rapidly induced.

Evidence that vitamin D3 reverses age-related inflammatory changes in the rat hippocampus

One of the major challenges in neuroscience is to identify the changes which accompany aging and which contribute to the well-documented age-related deterioration in cognitive function. This is a particular challenge in the light of the vast array of reported changes, which include morphological changes like synaptic and perhaps cell loss, alteration in membrane composition and the resultant changes in function of membrane proteins, modulation of the hypothalamo–pituitary axis, impaired calcium homoeostatic mechanisms, alteration in enzyme function and decreased neurotransmitter release. In the past few years, evidence suggesting that an aged brain exhibits signs of oxidative stress and inflammatory stress has been accumulating, and recent evidence using microarray analysis has added support to this view. In this paper, we provide evidence to suggest that vitamin D3 acts as an anti-inflammatory agent and reverses the age-related increase in microglial activation and the accompanying increase in IL-1β (interleukin-1β) concentration.

Peripheral injection of lipopolysaccharide enhances expression of inflammatory cytokines in murine locus coeruleus: possible role of increased norepinephrine turnover

Cytokines and catecholamines are known to constitute a significant portion of the regulatory neuroimmune networks involved in maintaining homeostasis in the central nervous system (CNS). Although we have already reported an increase in norepinephrine (NE) turnover within the locus coeruleus (LC) at 2 and 4 h after the intraperitoneal (i.p.) injection of lipopolysaccharide (LPS), the implication of this increase remains unclear. In view of evidence that norepinephrine (NE) acts in an anti-inflammatory manner by way of negatively regulating pro-inflammatory cytokine expression, we examined the inflammatory cytokine expression levels in the LC of C3H/HeN mice (male, 8 weeks old) after an i.p. LPS injection. The mRNA expression levels of the genes encoding IL-1beta and TNF-alpha within the LC increased during the first 2 h, and showed two peaks, the first at 4 h and the second lesser one at 15 h after the LPS injection. Microglia, which are one of the major cell types that produce pro-inflammatory cytokines in the CNS, were isolated from mouse neonate brains in order to clarify more precisely the relationship between the changes in NE content and the up-regulation of inflammatory cytokines in the LC. Simultaneous incubation of microglia with LPS and NE enhanced the expression of IL-1beta at both mRNA and protein levels, but reduced the mRNA and protein levels of TNF-alpha. These data support the hypothesis that NE negatively regulates the expression of pro-inflammatory cytokine expression, at least in the case of TNF-alpha, which action could contribute to the observed anti-inflammatory properties of NE. This report, based on the results of both in vivo and in vitro experiments, is the first to suggest a relationship between NE content and cytokine expression levels in the CNS.

Alzheimer's disease beta-secretase BACE1 is not a neuron-specific enzyme

The brains of Alzheimer's disease (AD) patients are morphologically characterized by neurofibrillar abnormalities and by parenchymal and cerebrovascular deposits of beta-amyloid peptides. The generation of beta-amyloid peptides by proteolytical processing of the amyloid precursor protein (APP) requires the enzymatic activity of the beta-site APP cleaving enzyme 1 (BACE1). The expression of this enzyme has been localized to the brain, in particular to neurons, indicating that neurons are the major source of beta-amyloid peptides in brain. Astrocytes, on the contrary, are known to be important for beta-amyloid clearance and degradation, for providing trophic support to neurons, and for forming a protective barrier between beta-amyloid deposits and neurons. However, under certain conditions related to chronic stress, the role of astrocytes may not be beneficial. Here we present evidence demonstrating that astrocytes are an alternative source of BACE1 and therefore may contribute to beta-amyloid plaque formation. While resting astroyctes in brain do not express BACE1 at detectable levels, cultured astrocytes display BACE1 promoter activity and express BACE1 mRNA and enzymatically active BACE1 protein. Additionally, in animal models of chronic gliosis and in brains of AD patients, there is BACE1 expression in reactive astrocytes. This would suggest that the mechanism for astrocyte activation plays a role in the development of AD and that therapeutic strategies that target astrocyte activation in brain may be beneficial for the treatment of AD. Also, there are differences in responses to chronic versus acute stress, suggesting that one consequence of chronic stress is an incremental shift to different phenotypic cellular states.

Modulation of human microglia and THP-1 cell toxicity by cytokines endogenous to the nervous system

Neuroinflammatory processes are thought to be a significant factor in the pathology of a number of degenerative neurological diseases. A variety of cytokines influence inflammatory levels. Here we show that a cooperative action of two or more cytokines is required to induce significantly human microglial and monocytic THP-1 cell toxicity towards SH-SY5Y neuroblastoma cells. Such toxicity was induced by the following combinations: interferon-gamma (IFN-gamma) with tumor necrosis factor-alpha (TNF-alpha); IFN-gamma with interleukin (IL) 1alpha or IL-1beta in the presence of TNF-alpha; and IL-6 with TNF-alpha. Toxicity induced by the various stimulatory combinations was not accompanied by an increased nitrite production. Of the potential inhibitors tested, IL-4 downregulated the toxic action of microglia when applied to THP-1 cells either before stimulation or 24 h after stimulation. Toxicity was not inhibited by IL-10, and was even enhanced by transforming growth factor-beta1 (TGF-beta1) and basic fibroblast growth factor (bFGF). These data suggest that antagonists of cytokine receptors, as well as inhibitors of their intracellular pathways may be effective anti-inflammatory agents.

Development of a rapid autopsy program for studies of brain immunity

Human glia are essential cellular models used for studies of neurodegenerative diseases. Fetal neuroglia are commonly used, as they can be recovered in large quantities and sustained for long periods in culture. However, fetal neuroglia may have limitations in reflecting adult diseases and additionally can pose ethical issues in translating products of abortion for research use. To address these concerns, we developed a rapid autopsy program to procure age- and disease-specific neuroglia from adult brain tissues within hours of death. The challenges in developing this initiative, reflecting experiences from 69 autopsies over 4 years, are presented.

Microglial cell population dynamics in the injured adult central nervous system

Reactive microgliosis is characteristic of trauma and stroke as well as inflammatory and chronic neurodegenerative disease. A conspicuous feature of the microglial reaction to acute neural injury is a massive expansion of the microglial cell population which peaks a few days following injury. New data based on the use of radiation bone marrow-chimeric mice suggest this expansion also involves recruitment of bone marrow-derived cells, which migrate into the neural parenchyma and differentiate into microglia. Here, we discuss the contribution of bone marrow-derived cells to the injury-induced expansion of the microglial cell population, seen in the dentate gyrus with ongoing anterograde axonal and terminal synaptic degeneration, subsequent to transection of the entorhino-dentate perforant path projection. In this paradigm of minor brain injury, the bone marrow-derived cells are grossly outnumbered by activated resident microglia, which express the stem cell antigen CD34 concurrent to a marked capacity for self-renewal. The observation of a mixed origin of lesion-reactive microglia, consisting of a smaller subpopulation of exogenous bone marrow-derived microglia, and a larger population of activated resident microglia, the majority of which express CD34 and undergo proliferation, suggests that lesion-reactive microglia consist of functionally distinct cell populations. The demonstration of an injury-enhanced recruitment of bone marrow-derived cells into the perforant path-denervated dentate gyrus, raises the possibility of using genetically manipulated cells as vectors for lesion-site-specific gene therapy even in minimally injured areas of the central nervous system.

Absence of C1q leads to less neuropathology in transgenic mouse models of Alzheimer's disease

C1q, the recognition component of the classical complement activation pathway, is a multifunctional protein known to be expressed in brain of Alzheimer's disease (AD) patients. To experimentally address the role of C1q in AD, a mouse model lacking C1q (APPQ-/-) was generated by crossing Tg2576 animals (APP) with C1q-deficient mice. The pathology of APPQ-/- was compared with that of APP mice and B6SJL controls at 3-16 months of age by immunohistochemistry and Western blot analysis. At younger ages (3-6 months), when no plaque pathology was present, no significant differences were seen in any of the neuronal or glial markers tested. At older ages (9-16 months), the APP and APPQ-/- mice developed comparable total amyloid and fibrillar beta-amyloid in frontal cortex and hippocampus; however, the level of activated glia surrounding the plaques was significantly lower in the APPQ-/- mice at 12 and 16 months. In addition, although Tg2576 mice showed a progressive decrease in synaptophysin and MAP2 in the CA3 area of hippocampus compared with control B6SJL at 9, 12, and 16 months, the APPQ-/- mice had significantly less of a decrease in these markers at 12 and 16 months. In a second murine model for AD containing transgenes for both APP and mutant presenilin 1 (APP/PS1), a similar reduction of pathology was seen in the APPPS1Q-/- mice. These data suggest that at ages when the fibrillar plaque pathology is present, C1q exerts a detrimental effect on neuronal integrity, most likely through the activation of the classical complement cascade and the enhancement of inflammation.

Oncostatin M: a pleiotropic cytokine in the central nervous system

Oncostatin M (OSM), a member of the interleukin-6 (IL-6) cytokine family, has yet to be well studied, especially in the context of the central nervous system (CNS). The biological functions of OSM are complex and variable, depending on the cellular microenvironment. Inflammatory responses and tumor development are among two of the major events that OSM is involved in. Although OSM levels remain low in the normal CNS, elevated expression occurs in pathological conditions. Therefore, it is crucial to understand the regulation of OSM to control its expression and/or its effects. Accumulating data demonstrate that OSM binds to specific receptor complexes, then activates two major signaling pathways: Janus Kinase-Signal Transducers and Activators of Transcription (JAK-STAT) and Mitogen-Activated Protein Kinase (MAPK), to regulate downstream events. In this review, we focus on the biological functions of OSM, the signaling pathways of OSM in the CNS, and OSM involvement in CNS diseases.