Analysis of the Impact of CD200 on Phagocytosis.

One factor that impacts on microglial activation is the interaction between the ubiquitously expressed CD200 and CD200R, which is expressed only on microglia in the brain. Decreased signalling through CD200R, when CD200 expression is reduced, results in microglial activation and may, at least in part, explain the increased cell activity that is observed with age, in models of Alzheimer's and Parkinson's disease as well as in the human diseases. There is evidence of increased microglial activation in CD200-deficient mice, and isolated microglia prepared from these mice are more reactive to inflammatory stimuli like Toll-like receptor 2 and 4 agonists, and interferon-γ. Here, we examined the impact of CD200 deficiency on amyloid-ß (Aß)-induced changes in microglia and report, perhaps unexpectedly, that the effect of Aß was attenuated in microglia prepared from CD200-deficient mice. The evidence indicates that this is a consequence of increased phagocytosis, associated with increased lysosomal activity in CD200-deficient microglia. The data suggest that mTOR-related signalling is decreased in these cells and that inhibiting mTOR by rapamycin increases phagocytosis. Thus, while the findings to date have emphasized the anti-inflammatory effects of CD200-CD200R interaction, the present evidence indicates a previously unreported impact on lysosomal function.

Inhibition of JAK2 attenuates the increase in inflammatory markers in microglia from APP/PS1 mice.

There is a wealth of evidence indicating that macrophages adopt distinct phenotypes when exposed to specific stimuli and, in the past few years, accumulating data suggest that microglia behave somewhat similarly. Therefore, microglia can adopt the so-called M1 or M2 phenotypes in response to interferon-γ (IFNγ) and interleukin-4, respectively. Although it has yet to be unequivocally proven in the context of microglia, acutely activated M1 cells are probably protective, although a persistent M1 state is likely to be damaging, whereas M2 cells may be reparative and restorative. In this case, particularly because the current evidence suggests the development of a predominantly M1 state with age and in neurodegenerative diseases, it is important to identify mechanisms by which polarization of microglia can be modulated. The present findings indicate that exposure of cultured microglia to IFNγ increased expressions of the archetypal markers of the M1 phenotype, tumour necrosis factor-α, and inducible nitric oxide synthase, and preexposure of cells to amyloid-ß (Aß) sensitized microglia to subsequent stimulation with IFNγ. Importantly, this synergy was also evident in microglia prepared from the brains of transgenic mice that overexpress amyloid precursor protein (APP) and presenilin 1 (PS1, APP/PS1 mice) and are exposed to a combination of increasing concentrations of endogenous Aß from 4 or 5 months of age and an age-related increase in IFNγ. Significantly, the JAK2 inhibitor, TG101209, attenuated the IFNγ-induced changes in cultured microglia and in isolated microglia prepared from APP/PS1 mice. These findings suggest that targeting JAK2 may be a potential strategy for reducing neuroinflammation in Alzheimer's disease.

Bone marrow-derived macrophages from AßPP/PS1 mice are sensitized to the effects of inflammatory stimuli.

Macrophages are key cells in tissue defense in the periphery and, under certain circumstances, infiltrate the central nervous system, where they may play a similar role in the brain, perhaps supporting the function of microglia. Macrophages have been shown to adopt different activation states in response to various stimuli. Specifically, when exposed to inflammatory stimuli such as interferon (IFN)γ, the cells adopt the M1 phenotype, whereas when exposed to anti-inflammatory cytokines such as interleukin (IL)-4 or IL-13, the M2 phenotype is adopted. While M1 macrophages are associated with tissue defense and destruction of invading pathogens, M2 macrophages are involved in tissue repair and in terminating inflammation. It is well known that an inflammatory microenvironment exists in the brain of aged animals and also in the brain of mice that overexpress amyloid-ß protein precursor (AßPP) and presenilin 1 (PS1; AßPP/PS1 mice), a commonly-used model of Alzheimer's disease (AD). Recent studies have revealed that immune cells, including macrophages, infiltrate the brain in both circumstances raising the possibility that these cells adopt the M1 activation state and contribute to the already-existing neuroinflammation. We set out to examine the responses of bone marrow-derived macrophages prepared from wildtype and AßPP/PS1 mice and demonstrate that cells from AßPP/PS1 mice, even after several days in culture, respond more profoundly to IFNγ than those from wildtype mice. We suggest that this propensity to respond to M1-polarizing stimuli, together with the described changes in the brain of AßPP/PS1 mice, contribute to the development of chronic neuroinflammation.

How dependent is synaptic plasticity on microglial phenotype?

Microglia are particularly plastic cells which can be shifted from their resting state by numerous factors and adopt distinct phenotypes. The cells are multifunctional, though their main role is probably maintenance of homoeostasis. Resting cells are responsible for surveillance, whereas activation induces the cells to adopt neuroprotective or neurodetrimental roles, which are anti-inflammatory or pro-inflammatory respectively. The evidence indicates that activated cells with a pro-inflammatory phenotype predominate in neurodegenerative diseases and models of neurodegeneration and that this may significantly contribute to the deteriorating neuronal function. This question is considered in this review, in particular in the context of animal models of Alzheimer's disease (AD). This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Age-associated dysregulation of microglial activation is coupled with enhanced blood-brain barrier permeability and pathology in APP/PS1 mice.

Aging adversely affects inflammatory processes in the brain, which has important implications in the context of disease progression. It has been proposed that microglia become dysfunctional with age and may lose their neuroprotective properties leading to chronic neurodegeneration. Here, we sought to characterize inflammatory changes in a mouse model of Alzheimer's disease and to delineate differences between normal aging and those associated with disease pathology. A proinflammatory profile, characterized by the upregulation of markers of classical activation, was evident in APPswe/PS1dE9 mice, associated with increased interferon-γ (IFNγ) concentration and dysregulation of mechanisms designed to limit the proinflammatory response. The data indicate that microglia are not less active with age but alter their phenotype; indeed, changes observed in the deactivation state appear to relate to aging rather than disease pathology. We hypothesize that disruption of the blood-brain barrier, in tandem with an enhanced chemokine profile, permits the infiltration of immune cells serving to reinforce classical activation of microglia through their enhanced responsiveness to IFNγ.

Glial Activation in AßPP/PS1 Mice is Associated with Infiltration of IFNγ-Producing Cells.

Whereas the classical histological hallmarks of Alzheimer's disease (AD) are deposition of amyloid-containing plaques and development of neurofibrillary tangles, there is also clear evidence of inflammatory changes accompanied by the presence of activated microglia and astrocytosis. However, at this time, it remains uncertain whether inflammatory changes contribute to pathogenesis of the disease or if they are secondary to deposition of amyloid-ß or other pathological changes. A greater understanding of the sequence of events would clearly improve development of strategies to delay progression of the disease. There is a realistic expectation that advances in imaging technology may provide the key to uncovering this sequence. In this study, we employed non-invasive imaging techniques to examine changes in tissue state in hippocampus and cortex of transgenic mice which overexpress amyloid-ß protein precursor and presenilin 1 and show that the observed increase in T1 relaxation time was associated with astrogliosis while the decrease in T2 relaxation time was associated with microglial activation. We explored the possibility that interferon-γ might trigger glial activation and demonstrate a genotype-related infiltration of macrophages and natural killer cells, which release interferon-γ. The evidence suggests that IFNγ triggers glial activation and expression of proinflammatory cytokines, and these changes, in turn, contribute to the decrease in long-term potentiation.

Identifying early inflammatory changes in monocyte-derived macrophages from a population with IQ-discrepant episodic memory.

BACKGROUND: Cells of the innate immune system including monocytes and macrophages are the first line of defence against infections and are critical regulators of the inflammatory response. These cells express toll-like receptors (TLRs), innate immune receptors which govern tailored inflammatory gene expression patterns. Monocytes, which produce pro-inflammatory mediators, are readily recruited to the central nervous system (CNS) in neurodegenerative diseases. METHODS: This study explored the expression of receptors (CD11b, TLR2 and TLR4) on circulating monocyte-derived macrophages (MDMs) and peripheral blood mononuclear cells (PBMCs) isolated from healthy elderly adults who we classified as either IQ memory-consistent (high-performing, HP) or IQ memory-discrepant (low-performing, LP). RESULTS: The expression of CD11b, TLR4 and TLR2 was increased in MDMs from the LP group when compared to HP cohort. MDMs from both groups responded robustly to treatment with the TLR4 activator, lipopolysaccharide (LPS), in terms of cytokine production. Significantly, MDMs from the LP group displayed hypersensitivity to LPS exposure. INTERPRETATION: Overall these findings define differential receptor expression and cytokine profiles that occur in MDMs derived from a cohort of IQ memory-discrepant individuals. These changes are indicative of inflammation and may be involved in the prodromal processes leading to the development of neurodegenerative disease.

Amyloid-ß-induced astrocytic phagocytosis is mediated by CD36, CD47 and RAGE.

Astrocytes, the most numerous glial cell in the brain, have multiple functions and are key to maintenance of homeostasis in the central nervous system. Microglia are the resident immunocompetent cells in the brain and share several functions with macrophages, including their phagocytic ability. Indeed microglia are the resident phagocytes in the brain and express numerous cell surface proteins which act to enable receptor-mediated phagocytosis. However recent evidence suggests that astrocytes express some genes which permit phagocytosis of phosphatidylserine-decorated cells and this probably explains sporadic reports in the literature which suggest that astrocytes become phagocytic following brain trauma. Here we examined the potential of astrocytes to phagocytose fluorescently-labelled latex beads and amyloid-ß (Aß) and report that they competently engulf both in a manner that relies on actin polymerization since it was inhibited by cytochalasin D. The data indicate that incubation of cultured astrocytes or microglia with Aß increased phagocytosis and markers of activation of both cell types. Aß was found to markedly increase expression of the putative Aß-binding receptors CD36 and CD47 in astrocytes, while it decreased expression of the receptor for advanced glycation endproducts (RAGE). It is demonstrated that blocking these receptors using a neutralizing antibody attenuated Aß-induced phagocytosis of latex beads by astrocytes. Interestingly blocking these receptors also decreased uptake of beads even in the absence of Aß. Here we demonstrate that astrocytes are competent phagocytes and are capable of engulfing Aß.

Rosiglitazone attenuates the age-related changes in astrocytosis and the deficit in LTP.

Neuroinflammation is a significant and consistent feature of many neurodegenerative disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD). The greatest risk factor for neurodegenerative disorders is age and a proinflammatory phenotype in the aged brain is believed to contribute to these neurodegenerative conditions. In animal models, neuroinflammatory changes, characterized by increased microglial activation, have been associated with a loss of synaptic plasticity and here we show that treatment of aged rats with the PPARγ agonist, rosiglitazone, modulates the inflammatory changes and restores synaptic function. The evidence presented highlights an important role for astrocytes in inducing inflammatory changes and suggests that the age-related astrogliosis and astrocytosis is responsible for the increase in the proinflammatory cytokine, tumor necrosis factor alpha (TNF-α). Magnetic resonance (MR) imaging revealed an age-related increase in T1 relaxation time and, importantly, treatment of aged rats with rosiglitazone reversed the age-related increases in astrogliosis and astrocytosis, TNF-α concentration and T1 relaxation time. The evidence indicates that the site of action for rosiglitazone is endothelial cells, and suggests that its effect on astrocytes is secondary to its effect on endothelial cells.