Microglia and neuroprotection: implications for Alzheimer's disease

Neuroprotective and neurotoxic properties of glial cells in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) affects more than 18 million people worldwide and is characterized by progressive memory deficits, cognitive impairment and personality changes. The main cause of AD is generally attributed to the increased production and accumulation of amyloid-β (Aβ), in association with neurofibrillary tangle (NFT) formation. Increased levels of pro-inflammatory factors such as cytokines and chemokines, and the activation of the complement cascade occurs in the brains of AD patients and contributes to the local inflammatory response triggered by senile plaque. The existence of an inflammatory component in AD is now well known on the basis of epidemiological findings showing a reduced prevalence of the disease upon long-term medication with anti-inflammatory drugs, and evidence from studies of clinical materials that shows an accumulation of activated glial cells, particularly microglia and astrocytes, in the same areas as amyloid plaques. Glial cells maintain brain plasticity and protect the brain for functional recovery from injuries. Dysfunction of glial cells may promote neurodegeneration and, eventually, the retraction of neuronal synapses, which leads to cognitive deficits. The focus of this review is on glial cells and their diversity properties in AD.

Complement component C1q inhibits beta-amyloid- and serum amyloid P-induced neurotoxicity via caspase- and calpain-independent mechanisms

Alzheimer's disease is a neurodegenerative disorder characterized by neuronal loss, beta-amyloid (Abeta) plaques, and neurofibrillary tangles. Complement protein C1q has been found associated with fibrillar Abeta deposits, however the exact contributions of C1q to Alzheimer's disease is still unknown. There is evidence that C1q, as an initiator of the inflammatory complement cascade, may accelerate disease progression. However, neuronal C1q synthesis is induced after injury/infection suggesting that it may be a beneficial response to injury. In this study, we report that C1q enhances the viability of neurons in culture and protects neurons against Abeta- and serum amyloid P (SAP)-induced neurotoxicity. Investigation of potential signaling pathways indicates that caspase and calpain are activated by Abeta, but C1q had no effect on either of these pathways. Interestingly, SAP did not induce caspase and calpain activation, suggesting that C1q neuroprotection is in distinct from caspase and calpain pathways. In contrast to Abeta- and SAP-induced neurotoxicity, neurotoxicity induced by etoposide or FCCP was unaffected by the addition of C1q, indicating pathway selectivity for C1q neuroprotection. These data support a neuroprotective role for C1q which should be further investigated to uncover mechanisms which may be therapeutically targeted to slow neurodegeneration via direct inhibition of neuronal loss.

Microglia as a pharmacological target in infectious and inflammatory diseases of the brain

Following an eclipse of scientific inquiry regarding the biology of microglia that lasted 50 years, recognition toward the end of the 20th century of their neuropathogenic role in HIV-associated dementia and in neuroinflammatory/neurodegenerative diseases fueled a renaissance of interest in these resident macrophages of the brain parenchyma. Results of a large number of in vitro studies, using isolated microglial cells or glial/neuronal cell cultures, and parallel findings emerging from animal models and clinical studies have demonstrated that activated microglia produce a myriad of inflammatory mediators that both serve important defense functions against invading neurotropic pathogens and have been implicated in brain damage in infectious as well as neuroinflammatory/neurodegenerative diseases, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. This review provides a brief background regarding the physiological and pathophysiological roles of microglia and highlights current pharmacological approaches that target activated microglia with the goal of ameliorating infectious and neuroinflammatory/neurodegenerative diseases of the brain. Although this aspect of the field of neuroimmunopharmacology is in its infancy, it holds great promise for developing new treatments and prevention of diseases that are, in many cases, epidemic throughout the world.

Spatiotemporal patterns of postsynaptic density (PSD)-95 expression after rat spinal cord injury

AIMS

Postsynaptic density (PSD)-95 is a scaffolding protein linking the N-methyl-D-aspartate receptor with neuronal nitric oxide synthase (nNOS), which contributes to many physiological and pathological actions. We here investigated whether PSD-95 was involved in the secondary response following spinal cord injury (SCI).

METHODS

Spinal cord contusion (SCC) and spinal cord transection (SCT) models at thoracic (T) segment 9 (T(9)) were established in adults rats. Real-time polymerase chain reaction, Western blot, immunohistochemistry and immunofluorescence were used to detect the temporal profile and spatial distribution of PSD-95 after SCI. The association between PSD-95 and nNOS in the injured cords was also assessed by coimmmunoprecipation and double immunofluorescent staining.

RESULTS

The mRNA and protein for PSD-95 expression were significantly increased at 2 h or 8 h, and then gradually declined to the baseline level, ultimately up-regulated again from 5 days to 7 days for its mRNA level and at 7 days or 14 days for its protein level after either SCC or SCT. PSD-95 immunoreactivity was found in neurones, oligodendrocytes and synaptic puncta of spinal cord tissues within 5 mm from the lesion site. Importantly, injury-induced expression of PSD-95 was colabelled by active caspase-3 (apoptotic marker), Tau-1 (the marker for pathological oligodendrocytes) and nNOS.

CONCLUSIONS

Accompanied by the spatio-temporal changes for PSD-95 expression, the association between PSD-95 and nNOS undergoes substantial alteration after SCI. These two molecules are likely to form a complex on apoptotic neurones and pathological oligodendrocytes, which may in turn be involved in the secondary response after SCI.

A rose by any other name? The potential consequences of microglial heterogeneity during CNS health and disease

Microglial activation and macrophage infiltration into the CNS are common features of CNS autoimmune disease and of chronic neurodegenerative diseases. Because these cells largely express an overlapping set of common macrophage markers, it has been difficult to separate their respective contributions to disease onset and progression. This problem is further confounded by the many types of macrophages that have been termed microglia. Several approaches, ranging from molecular profiling of isolated cells to the generation of irradiation chimeric rodent models, are now beginning to generate rudimentary definitions distinguishing the various types of microglia and macrophages found within the CNS and the potential roles that these cells may play in health and disease.

NGF promotes microglial migration through the activation of its high affinity receptor: modulation by TGF-beta

Activation and mobilization of microglia are early events in the majority of brain pathologies. Among the signalling molecules that can affect microglial behaviour, we investigated whether nerve growth factor (NGF) was able to influence microglial motility. We found that NGF induced chemotaxis of microglial cells through the activation of TrkA receptor. In addition, NGF chemotactic activity was increased in the presence of low concentrations (< or =0.2 ng/ml) of transforming growth factor-beta (TGF-beta), which at this concentration showed chemotactic activity per se. On the contrary, NGF-induced microglial migration was reduced in the presence of chemokinetic concentration of TGF-beta (> or =2 ng/ml). Finally, both basal and NGF-induced migratory activity of microglial cells was increased after a long-term exposure of primary mixed glial cultures to 2 ng/ml of TGF-beta. Our observations suggest that both NGF and TGF-beta contribute to microglial recruitment. The chemotactic activities of these two pleiotropic factors could be particularly relevant during chronic diseases in which recruited microglia remove apoptotic neurons in the absence of a typical inflammatory reaction.

The microglial networks of the brain and their role in neuronal network plasticity after lesion

Microglia are the resident inflammatory cells of the central nervous system (CNS) extending a network of processes in the CNS parenchyma. Following axon lesion to neurons, most extensively studied in motoneurons, there is a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Microglia have been attributed a main and active role in this process, although also an involvement of activated astrocytes has been suggested. The signaling pathways for this 'synaptic stripping' have so far been unknown, but recently some classical immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Since there is an expression of MHC class I in both neurons and glia, in particular microglia, as well as MHC class I related receptors in axons and microglia, there are good reasons to believe that classical immune recognition signaling between neurons and glia underlies part of the 'stripping' response. A role for microglia in an interplay with synapses based on this type of signaling is further exemplified by the fact that, in the absence of some MHC class I related receptors normally found on microglia during development, profound effects on synaptic function and biochemistry have been demonstrated. Such effects may be linked to the development of various disorders of the CNS, such as degenerative disease.

Cellular therapy using microglial cells

Recent insights into the function and dysfunction of microglia may inform future therapies to combat neurodegeneration. We hypothesise how different aspects of microglial activity including migration, activation, oxidative response, phagocytosis, proteolysis, and replenishment could be targeted by novel therapeutic approaches. A combined approach is suggested, encompassing opsonization and anti-inflammatory strategies in conjunction with an engineering of microglial precursors. Xenoproteases for bioremediation could be used to enhance intracellular and extracellular proteolytic capacity. The capacity of microglial precursors to cross the blood-brain barrier and to home in on sites of neural damage and inflammation might prove to be particularly useful for future therapeutic strategies.

Involvement of lysosomal storage-induced p38 MAP kinase activation in the overproduction of nitric oxide by microglia in cathepsin D-deficient mice

Nitric oxide (NO) and peroxynitrite, which are produced by activated microglia, are responsible for accelerated neurodegeneration in cathepsin D-deficient (CD-/-) mice. To elucidate the mechanisms by which microglia are initially activated in CD-/- mice, we analyzed the possible relationship between lysosomal storage and microglial activation. In CD-/- mice, the microglial NO-generating activity that was closely associated with the induction of inducible NO synthase and the cationic amino acid transporter-2 (CAT-2) coincided well with the lysosomal storage of subunit c of mitochondrial F0F1ATPase and the formation of ceroid/lipofuscin. Furthermore, activated microglia, which are often accumulating subunit c and ceroid/lipofuscin, showed proliferation activity and an activation of p38 mitogen-activated protein (MAP) kinase. In the primary cultured microglia, pepstatin A was found to enhance the generation of NO and superoxide anion radicals. In these pepstatin A-treated microglia, both an increased generation of the intracellular reactive oxygen species (ROS) and an activation of p38 MAP kinase were observed. These results suggest that the ceroid/lipofuscin which form in microglia activate the p38 MAP kinase cascade through the increased intracellular generation of ROS in CD-/- mice. The activated p38 MAP kinase cascade then promotes the expression of iNOS and CAT-2, thereby inducing the overproduction of NO.

Neuroprotective role of the innate immune system by microglia

Innate immunity is a rapid series of reactions to pathogens, cell injuries and toxic proteins. A key component of this natural response is the production of inflammatory mediators by resident microglia and infiltrating macrophages. There is accumulating evidence that inflammation contributes to acute injuries and more chronic CNS diseases, though other studies have shown that inhibition of microglia is, in contrast, associated with more damages or less repair. The controversies regarding the neuroprotective and neurodegenerative properties of microglia may depend on the experimental approaches. Neurotoxic substances are frequently used to produce animal models of acute injuries or diseases and they may activate microglia either directly or indirectly by their ability to cause neuronal death and demyelination. Whether microglia and the immune response play a direct role in such processes still remains an open question. On the other hand, there are data supporting the role of resident microglia and those derived from the bone marrow in the stimulation of myelin repair, removal of toxic proteins from the CNS and the prevention of neurodegeneration in chronic brain diseases. The ability of glucocorticoids to provide a negative feedback on nuclear factor kappa B pathways in microglia may be a determinant mechanism underlying the ultimate fate of the inflammatory response in the CNS. This review presents new concepts regarding the neuroprotective role of the innate immune response in the brain and how microglia can be directed to improve recovery after injuries and prevent/delay neurodegeneration.

Enhanced cell-to-cell contacts between activated microglia and pyramidal cell dendrites following kainic acid-induced neurotoxicity in the hippocampus

Microglia participate in immune responses in the brain. However, little is known about the contact-mediated interaction between microglia and neurons. We report here that the cell-to-cell contacts between microglial processes and dendrites of hippocampal CA1 neurons were dramatically increased in density and area following local injection of kainic acid (KA). A similar KA-induced increase in the degree of intercellular contacts was observed in mice lacking telencephalin (TLCN), a neuronal dendritic adhesion molecule of ICAM family. The results suggest that adhesive contacts independent of TLCN and contact-mediated interactions between microglia and dendrites were promoted by excitotoxic brain injury.

Pro-inflammatory conditions promote neuronal damage mediated by Amyloid Precursor Protein and decrease its phagocytosis and degradation by microglial cells in culture

Aberrant handling of Amyloid Precursor Protein (APP) and beta-amyloid (Abeta), glial activation and inflammation are key events in Alzheimer's disease. We set out to determine the role of inflammation on microglial reactivity against APP. We studied microglia-mediated neurotoxicity, uptake and degradation of a biotinylated APP construct (biotin-APP-C-244). APP, in contrast to Abeta, only induced mild activation of glial cells. However, under pro-inflammatory conditions, APP induced microglial-mediated cytotoxicity. Biotin-APP-C-244 or lipopolysaccharide and interferon-gamma (LPS+IFNgamma), administered separately, did not change reduction metabolism of microglia. However, biotin-APP-C-244+(LPS+IFNgamma) increased microglial reactivity and decreased reduction metabolism by 75% (P<0.001). Biotin-APP-C-244 was readily taken up by microglial cells; 80% was phagocytosed at 2 h. In the presence of LPS+IFNgamma, phagocytosis of biotin-APP-C-244 was reduced at 2 h; and cell damage was evident after 4 h. Our results support our hypothesis that, in neuroinflammation, microglial scavenger function is impaired and reactivity against APP enhanced as an initial step for neurodegeneration.

Proteolytic activation of monocyte chemoattractant protein-1 by plasmin underlies excitotoxic neurodegeneration in mice

Exposure of neurons to high concentrations of excitatory neurotransmitters causes them to undergo excitotoxic death via multiple synergistic injury mechanisms. One of these mechanisms involves actions undertaken locally by microglia, the CNS-resident macrophages. Mice deficient in the serine protease plasmin exhibit decreased microglial migration to the site of excitatory neurotransmitter release and are resistant to excitotoxic neurodegeneration. Microglial chemotaxis can be signaled by the chemokine monocyte chemoattractant protein-1 (MCP-1)/CCL2 (CC chemokine ligand 2). We show here that mice genetically deficient for MCP-1 phenocopy plasminogen deficiency by displaying decreased microglial recruitment and resisting excitotoxic neurodegeneration. Connecting these pathways, we demonstrate that MCP-1 undergoes a proteolytic processing step mediated by plasmin. The processing, which consists of removal of the C terminus of MCP-1, enhances the potency of MCP-1 in in vitro migration assays. Finally, we show that infusion of the cleaved form of MCP-1 into the CNS restores microglial recruitment and excitotoxicity in plasminogen-deficient mice. These findings identify MCP-1 as a key downstream effector in the excitotoxic pathway triggered by plasmin and identify plasmin as an extracellular chemokine activator. Finally, our results provide a mechanism that explains the resistance of plasminogen-deficient mice to excitotoxicity.

Evidence for synaptic stripping by cortical microglia

Recent studies have described significant demyelination and microglial activation in the cerebral cortex of brains from multiple sclerosis patients. To date, however, experimental models of cortical demyelination or cortical inflammation have not been extensively studied. In this report we describe focal cortical inflammation induced by stereotaxic injection of killed bacteria (BCG), followed 1 month later by subcutaneous injection of the same antigen, a protocol that overcomes the immune privilege of the cortex. Intracerebral BCG injection produced focal microglial activation at the injection site (termed acute lesion). Ten days after peripheral challenge (termed immune-mediated lesion), larger areas and higher densities of activated microglia were found near the injection site. In both paradigms, activated microglia and/or their processes closely apposed neuronal perikarya and apical dendrites. In the immune-mediated lesions, approximately 45% of the axosomatic synapses was displaced by activated microglia. Upon activation, therefore, cortical microglial migrate to and strip synapses from neuronal perikarya. Since neuronal pathology was not a feature of either the acute or immune-mediated lesion, synaptic stripping by activated microglia may have neuroprotective consequences.

Age-related changes of astorocytes, oligodendrocytes and microglia in the mouse hippocampal CA1 sector

We investigated the age-related alterations of astorocyte, oligodendrocyte and microglia in the mouse hippocampal CA1 sector under the same conditions using immunohistochemistry. Glial fibrillary acidic protein (GFAP), 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and isolectin B(4) immunoreactivity was measured in 2-, 8-, 18-, 40-42- and 50-59-week-old mice. Total number of GFAP-positive cells was unchanged in the hippocampal CA1 sector up to 40-42 weeks of birth. In 50-59-week-old mice, however, a significant increase in the number of GFAP-positive cells was observed in the hippocampal CA1 sector, exhibiting the morphology of reactive astrocytes. In contrast, the fibers of CNPase immunoreactivity were unchanged in the hippocampal CA1 sector up to 18 weeks of birth. In 40-42- and 50-59-week-old mice, however, a significant decrease in the densities of CNPase-positive fibers was observed in the hippocampal CA1 sector. On the other hand, total number of isolectin B(4)-positive cells was unchanged in the hippocampal CA1 sector up to 40-42 weeks of birth. In 50-59-week-old mice, however, a significant decrease in the number of isolectin B(4)-positive cells was observed in the hippocampal CA1 sector. Our results show that astrocytes proliferate and are activated in the hippocampal CA1 sector with advancing age. Furthermore, the present study demonstrates that the fibers of oligodendrocytes and total number of microglial cells in the hippocampal CA1 sector are decreased during ageing processes. These results suggest that age-related changes of astorocytes, oligodendrocytes and microglia had occurred in the mouse hippocampal CA1 sector.

Microglia activation in sepsis: a case-control study

Background

infection induces an acute phase response that is accompanied by non-specific symptoms collectively named sickness behavior. Recent observations suggest that microglial cells play a role in mediating behavioral changes in systemic infections. In animal models for sepsis it has been shown that after inducing lipopolysaccharide, LPS, microglia in the brain were activated. The aim of this study was to investigate whether activation of microglia can be detected in patients who died of sepsis.

Methods

in a case-control study brain tissue of 13 patients who died with sepsis was compared with that of 17 controls. Activated microglia were identified by expression of MHC-class II antigens and CD68. Microglia activation was analyzed by a semiquantitative score combining both the number of the immunoreactive cells and their morphology.

Results

in patients who died with sepsis there was a significant increase in activated microglia in the grey matter when stained with CD68 compared to controls. This effect was independent of the effect of age.

Conclusion

this study shows for the first time in human brain tissue an association between a systemic infection and activation of microglia in the brain. Activated microglia during sepsis could play a role in behavioral changes associated with systemic infection.

Inhibitory effects of blueberry extract on the production of inflammatory mediators in lipopolysaccharide-activated BV2 microglia

Sustained microglial activation in the central nervous system (CNS) has been extensively investigated in age-related neurodegenerative diseases and has been postulated to lead to neuronal cell loss in these conditions. Recent studies have shown that antiinflammatory drugs may suppress microglial activation and thus protect against microglial overactivation and subsequent cell loss. Research also suggests that fruits such as berries may contain both antioxidant and antiinflammatory polyphenols that may be important in this regard. Our previous research showed that blueberry extract was effective in preventing oxidant-induced calcium response deficits in M1 (muscarinic receptor)-transfected COS-7 cells. Extrapolating from these findings, the current study investigated the effect of blueberry extract on preventing inflammation-induced activation of microglia. Results indicated that treatments with blueberry extract inhibited the production of the inflammatory mediator nitric oxide (NO) as well as the cytokines interleukin-1beta and tumor necrosis factor-alpha, in cell conditioned media from lipopolysaccharide (LPS)-activated BV2 microglia. Also, mRNA and protein levels of inducible nitric oxide synthase and cyclooxygenase-2 in LPS-activated BV2 cells were significantly reduced by treatments with blueberry extract. The results suggest that blueberry polyphenols attenuate inflammatory responses of brain microglia and could be potentially useful in modulation of inflammatory conditions in the CNS.

Microglia derived from aging mice exhibit an altered inflammatory profile

Microglia play a critical role in neurodegenerative diseases and in the brain aging process. Yet, little is known about the functional dynamics of microglia during aging. Thus, using young and aging transgenic mice expressing enhanced-green fluorescent protein (EGFP) under the promoter of the c-fms gene for macrophage-colony stimulating factor receptor, we evaluated in vivo-induced inflammatory responses of EGFP-expressing microglia sorted by flow cytometry. Aging microglia were characterized by the presence of lipofuscin granules, decreased processes complexity, altered granularity, and increased mRNA expression of both pro-inflammatory (TNFalpha, IL-1beta, IL-6) and anti-inflammatory (IL-10, TGFbeta1) cytokines. Following lipopolysaccharide (LPS) challenge (1 mg/kg, 3 h), aging microglia exhibit increased basal expression of TNFalpha, IL-1beta, IL-6, and IL-10. Yet, the fold-over-basal LPS response remained constant across age, implying that the inflammatory machinery in aging microglia is functional and adjusted to the basal state. Gender differences were not overall observed across the treatments (age, LPS). The low but sustained production of pro-inflammatory cytokines by aging microglia may have a profound impact in the brain aging process.

The inflammatory bases of hepatic encephalopathy

A hypothesis about the inflammatory etiopathogeny mediated by astroglia of hepatic encephalopathy is being proposed. Three evolutive phases are considered in chronic hepatic encephalopathy: an immediate or nervous phase with ischemia-reperfusion, which is associated with reperfusion injury, edema and oxidative stress; an intermediate or immune phase with microglia hyperactivity, which produces cytotoxic cytokines and chemokines and is involved in enzyme hyperproduction and phagocytosis; and a late or endocrine phase, in which neuroglial remodeling, with an alteration of angiogenesis and neurogenesis, stands out. The increasingly complex trophic meaning that the metabolic alterations have in the successive phases making up this chronic inflammation could explain the metabolic regression produced in acute and acute-on-chronic hepatic encephalopathy. In these two types of hepatic encephalopathy, characterized by edema, neuronal nutrition by diffusion would guarantee an appropriate support of substrates, in accordance with the reduced metabolic needs of the cerebral tissue.

Fluoro-Jade B staining as useful tool to identify activated microglia and astrocytes in a mouse transgenic model of Alzheimer's disease

Fluoro-Jade B is known as a high affinity fluorescent marker for the localization of neuronal degeneration during acute neuronal distress. However, one study suggested that fluoro-Jade B stains reactive astroglia in the primate cerebral cortex. In this study, we analyzed the staining of fluoro-Jade B alone or combined with specific markers for detection of glial fibrillary acidic protein (GFAP) or activated CD68 microglia in the double APP(SL)/PS1 KI transgenic mice of Alzheimer's disease (AD), which display a massive neuronal loss in the CA1 region of the hippocampus. Our results showed that fluoro-Jade B did not stain normal and degenerating neurons in this double mouse transgenic model. Fluoro-Jade B was co-localized with Abeta in the core of amyloid deposits and in glia-like cells expressing Abeta. Furthermore, fluoro-Jade B was co-localized with CD68/macrosialin, a specific marker of activated microglia, and with GFAP for astrocytes in APP(SL)/PS1 KI transgenic mice of AD. Taken together, these findings showed that fluoro-Jade B can be used to label activated microglia and astrocytes which are abundant in the brain of these AD transgenic mice. It could stain degenerating neurons as a result of acute insult while it could label activated microglia and astrocytes during a chronic neuronal degenerative process such as AD for example.

The growth compromised HSV-2 mutant DeltaRR prevents kainic acid-induced apoptosis and loss of function in organotypic hippocampal cultures

We have previously shown that the HSV-2 anti-apoptotic protein ICP10PK is delivered by the replication incompetent virus mutant DeltaRR and prevents kainic acid (KA)-induced epileptiform seizures and neuronal cell loss in the mouse and rat models of temporal lobe epilepsy. The present studies used DeltaRR and the ICP10PK deleted virus mutant DeltaPK to examine the mechanism of neuroprotection. DeltaRR-infected neuronal cells expressed a chimeric protein in which ICP10PK is fused in frame to LacZ (p175) while retaining ICP10PK kinase activity. DeltaPK-infected neuronal cells expressed a mutant ICP10 protein that is deleted in the PK domain and is kinase negative (p95). p175 and p95 were expressed in CA3 (86+/-3%) and CA1 (69+/-7%) cells from DeltaRR or DeltaPK-infected organotypic hippocampal cultures (OHC) and 80-85% of the ICP10 positive cells co-stained with antibody to beta(III) Tubulin (neuronal marker). DeltaRR, but not DeltaPK, inhibited KA-induced cell death and caspase-3 activation in CA3 neurons, an inhibition seen whether DeltaRR was delivered 2 days before or 2 days after KA administration (95% neuroprotection). Neuroprotection was associated with ERK and Akt activation and was abrogated by simultaneous treatment with the MEK (U0126) and PI3-K (LY294002) inhibitors. DeltaRR-mediated neuroprotection was associated with increased expression of the anti-apoptotic protein Bag-1 and decreased expression of the pro-apoptotic protein Bad. The surviving neurons retained normal synaptic function potentially related to increased expression of the transcription factor CREB. The data indicate that DeltaRR is a promising platform for neuroprotection from excitotoxic injury.

Kainate induces rapid redistribution of the actin cytoskeleton in ameboid microglia

Microglia are key mediators of the immune response in the central nervous system (CNS). They are closely related to macrophages and undergo dramatic morphological and functional changes after CNS trauma or excitotoxic lesions. Microglia can be directly stimulated by excitatory neurotransmitters and are known to express many neurotransmitter receptors. The role of these receptors, however, is not clear. This study describes the microglial response to the glutamate receptor agonist kainate (KA) and shows via immunochemistry that the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptor subunit GluR1 is present on cultured microglia. In the presence of 100 microM or 1 mM KA, cultured microglia underwent dramatic morphological and cytoskeletal changes as observed by time-lapse photography and quantitative confocal analysis of phalloidin labeling. KA-stimulated microglia showed condensation of cytoplasmic actin filaments, rapid de- and repolymerization, and cytoplasmic redistribution of condensed actin bundles. Rearrangement of actin filaments-thought to be involved in locomotion and phagocytosis and to indicate an increased level of activation (for reviews see Greenberg [ 1995] Trends Cell Biol. 5:93-99; Imai and Kohsaka [ 2002] Glia 40:164-174)-was significantly increased in treated vs. control cultures. Morphological plasticity and membrane ruffling were also seen. These findings suggest direct microglial excitation via glutamate receptor pathways. Thus, neurotransmitter release after brain or spinal cord injury might directly modulate the inflammatory response.

Microglia and the control of autoreactive T cell responses

Microglial activation is one of the earliest and most prominent features of nearly all CNS neuropathologies often occurring prior to other indicators of overt neuropathology. Whether microglial activation in seemingly healthy CNS tissue during the early stages of several is a response to early stages of neuronal or glial distress or an early sign of microglial dysfunction causing subsequent neurodegeneration is unknown. Here we characterize and discuss how changes in the CNS microenvironment (neuronal activity/viability, glial activation) lead to specific forms of microglial activation. Specifically, we examine the potential role that TREM-2 expressing microglia may play in regulating the effector function of autoreactive T cell responses. Thus, we suggest that ubiquitous suppression of microglial activation during CNS inflammatory disorders rather than targeted manipulation of microglial activation, may in the end be maladaptive leading to incomplete remission of symptoms.

Microglia in the aging brain

The aging brain is characterized by a demonstrable decrease in weight and volume, particularly after the age of 50. This atrophy, which affects both grey and white matter, is presumed to result from a loss of neurons and myelinated axons. Glial cells, on the other hand, appear to increase in the aging brain, which exhibits greater immunoreactivity with both astrocytic and microglial markers. This review is focused on the morphologic and phenotypic changes that occur in microglial cells with normal aging. Although there is a consistent aging-related upregulation of microglial activation markers in experimental animals and humans that could be interpreted as aging-related neuroinflammation, it is generally difficult to show a direct correlation between ostensible microglial activation and neurodegeneration. This raises questions about whether aging-related microglial activation indeed represents reactive gliosis in the conventional sense. As an alternative, we discuss the possibility that structural and phenotypic changes that occur in microglia are a direct reflection of the aging process on microglia. Thus, microglia cells themselves may be subject to cellular senescence in the sense that they no longer function efficiently. The concept of microglial senescence offers a novel perspective on aging-related neurodegeneration, namely that neurodegeneration could also occur secondary to microglial degeneration.

Neuroprotective properties of the innate immune system and bone marrow stem cells in Alzheimer's disease

The role of innate immunity and microglia in the brain is currently a matter of great debate and controversy. While several studies have provided evidence that they contribute to neurodegeneration in various animal models of brain diseases and traumas, others have shown that their inhibition may in contrast be associated with more damages or less repair. We have recently reported the existence of two different types of microglia, the resident and the newly differentiated microglia that derive from the bone marrow stem cells. Of great interest is the fact that blood-derived microglial cells are associated with amyloid plaques and these cells are able to prevent the formation or eliminate the presence of amyloid deposits in mice that develop the major hallmark of Alzheimer's disease (AD). These newly recruited cells are specifically attracted to the beta-amyloid 40/42 isoforms in vivo and they participate in the elimination of these proteins by phagocytosis. This review presents the mechanisms involved in the control of the innate immune response by microglia and the beneficial properties of such a response in brain diseases, such as AD.

Involvement of environmental mercury and lead in the etiology of neurodegenerative diseases

The incidence of neurodegenerative disease like Parkinson's disease and Alzheimer's disease (AD) increases dramatically with age; only a small percentage is directly related to familial forms. The etiology of the most abundant, sporadic forms is complex and multifactorial, involving both genetic and environmental factors. Several environmental pollutants have been associated with neurodegenerative disorders. The present article focuses on results obtained in experimental neurotoxicology studies that indicate a potential pathogenic role of lead and mercury in the development of neurodegenerative diseases. Both heavy metals have been shown to interfere with a multitude of intracellular targets, thereby contributing to several pathogenic processes typical of neurodegenerative disorders, including mitochondrial dysfunction, oxidative stress, deregulation of protein turnover, and brain inflammation. Exposure to heavy metals early in development can precondition the brain for developing a neurodegenerative disease later in life. Alternatively, heavy metals can exert their adverse effects through acute neurotoxicity or through slow accumulation during prolonged periods of life. The pro-oxidant effects of heavy metals can exacerbate the age-related increase in oxidative stress that is related to the decline of the antioxidant defense systems. Brain inflammatory reactions also generate oxidative stress. Chronic inflammation can contribute to the formation of the senile plaques that are typical for AD. In accord with this view, nonsteroidal anti-inflammatory drugs and antioxidants suppress early pathogenic processes leading to Alzheimer's disease, thus decreasing the risk of developing the disease. The effects of lead and mercury were also tested in aggregating brain-cell cultures of fetal rat telencephalon, a three-dimensional brain-cell culture system. The continuous application for 10 to 50 days of non-cytotoxic concentrations of heavy metals resulted in their accumulation in brain cells and the occurrence of delayed toxic effects. When applied at non-toxic concentrations, methylmercury, the most common environmental form of mercury, becomes neurotoxic under pro-oxidant conditions. Furthermore, lead and mercury induce glial cell reactivity, a hallmark of brain inflammation. Both mercury and lead increase the expression of the amyloid precursor protein; mercury also stimulates the formation of insoluble beta-amyloid, which plays a crucial role in the pathogenesis of AD and causes oxidative stress and neurotoxicity in vitro. Taken together, a considerable body of evidence suggests that the heavy metals lead and mercury contribute to the etiology of neurodegenerative diseases and emphasizes the importance of taking preventive measures in this regard.

Microglial phenotype: is the commitment reversible?

Microglia, the standby cells for immune defense in the CNS, have a reputation for exacerbating the neural damage that occurs in neurodegenerative diseases. However, research over the past few years has established that microglia do not constitute a single, uniform cell population, but rather comprise a family of cells with diverse phenotypes--some that are beneficial and others that the CNS can barely tolerate and that are therefore destructive. This finding raised several questions. What instructs microglia to acquire a particular phenotype, and how do these phenotypes differ? How committed are microglia to a specific phenotype? Can destructive microglia become protective, and can protective microglia retain their beneficial phenotype even when they encounter a destructive environment? Here, we address these questions, and the background of research that elicited them.

A species-specific difference in the effects of harmaline on the rodent olivocerebellar system

The rodent model of harmaline-induced tremor has been widely used for experimental analysis of tremor. Activation of the olivocerebellar system plays a key role in tremor-generating mechanisms. One undetermined problem is whether there are species-specific differences in effects of harmaline. The present study investigated effects of harmaline on olivocerebellar systems of mice and rats. Systemic administration of harmaline, but not vehicle, produced generalized, high-frequency tremors in both types of rodents. Immunohistochemical studies revealed significant degeneration of Purkinje cells that was associated with activated microgliosis in the cerebellar cortex, following administration of harmaline in rats but not in mice. However, in mice but not rats, microgliosis was induced following administration of harmaline in the inferior olivary nucleus (ION), particularly in its caudal and medial subdivisions. Numbers of neurons in the mouse ION did not decrease, suggesting the possibility that microgliosis in ION might not be a simple neurotoxic effect. Presumably, differences in sensitivity of Purkinje cells between rats and mice may be related to differences in functional alterations in their respective olivocerebellar systems induced by harmaline. Recognition of these species-specific differences in the response of the olivocerebellar system to harmaline is an important consideration for experimental analysis of the rodent model of tremors.

Alpha-tocopherol (vitamin E) induces rapid, nonsustained proliferation in cultured rat microglia

Microglial cells undergo cell division in vitro, as well as in vivo after brain injury. Mitotic activity of microglia suggests that they have limited life spans and rely on self-renewal to replace senescent cells. In the current study we examined long-term effects of antioxidants vitamin E and alpha-lipoic acid on cultured rat microglia with respect to proliferative ability, telomere length, telomerase activity, and interleukin-1beta (IL-1beta) production. We report that vitamin E induces dramatic microglial proliferation, as measured by MTT assay and BrdU incorporation, surpassing that of the well-known microglial mitogen granulocyte macrophage-colony stimulating factor, and therefore establishing vitamin E as the most potent, known mitogen for microglia in vitro. The high rate of microglial proliferation resulted in a concomitant decrease in telomere length and telomerase activity. Production of IL-1beta was significantly decreased in vitamin E-treated microglia in vitro. Our findings provide an impetus to investigate potential benefits of vitamin E supplementation on microglial renewal capacity in vivo during aging or after brain injury.

Estrogen, neuroinflammation and neuroprotection in Parkinson's disease: glia dictates resistance versus vulnerability to neurodegeneration

Post-menopausal estrogen deficiency is recognized to play a pivotal role in the pathogenesis of a number of age-related diseases in women, such as osteoporosis, coronary heart disease and Alzheimer's disease. There are also sexual differences in the progression of diseases associated with the nigrostriatal dopaminergic system, such as Parkinson's disease, a chronic progressive degenerative disorder characterized by the selective degeneration of mesencephalic dopaminergic neurons in the substancia nigra pars compacta. The mechanism(s) responsible for dopaminergic neuron degeneration in Parkinson's disease are still unknown, but oxidative stress and neuroinflammation are believed to play a key role in nigrostriatal dopaminergic neuron demise. Estrogen neuroprotective effects have been widely reported in a number of neuronal cell systems including the nigrostriatal dopaminergic neurons, via both genomic and non-genomic effects, however, little is known on estrogen modulation of astrocyte and microglia function in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease. We here highlight estrogen modulation of glial neuroinflammatory reaction in the protection of mesencephalic dopaminergic neurons and emphasize the cardinal role of glia-neuron crosstalk in directing neuroprotection vs neurodegeneration. In particular, the specific role of astroglia and its pro-/anti-inflammatory mechanisms in estrogen neuroprotection are presented. This study shows that astrocyte and microglia response to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injury vary according to the estrogenic status with direct consequences for dopaminergic neuron survival, recovery and repair. These findings provide a new insight into the protective action of estrogen that may possibly contribute to the development of novel therapeutic treatment strategies for Parkinson's disease.

To be or not to be (inflamed) – is that the question in anti-inflammatory drug therapy of neurodegenerative disorders?

A sustained inflammatory reaction is present in acute (e.g. stroke) and chronic (e.g. Alzheimer's disease, Parkinson's disease and multiple sclerosis) neurodegenerative disorders. Inflammation, which is fostered by both residential glial cells and blood-circulating cells that infiltrate the diseased brain, probably starts as a time- and site-specific defense mechanism that could later evolve into a destructive and uncontrolled reaction. In this article, we review the crucial dichotomy of brain inflammation, where failure to resolve an acute beneficial response could lead to a vicious and anarchic state of chronic activation. The possible use of non-steroidal anti-inflammatory drugs for the management of neurodegenerative diseases is discussed in light of recent data demonstrating a neuroprotective role of local innate and adaptive immune responses. Novel therapeutic approaches must rely on potentiation of endogenous anti-inflammatory pathways, identification of early markers of neuronal deterioration and a combination treatment involving immune modulation and anti-inflammatory therapies.

Effect of aging on the microglial response to peripheral nerve injury

Microglial morphology and immunophenotype have been studied extensively in aging-related neurodegenerative diseases, but to a lesser extent in the normally aged CNS, and little is known about how aging affects the ability of microglia to respond to neuronal injury. The goal of the current study was to determine if aging affects the ability of microglia to divide during the early response to facial nerve axotomy. In addition, we investigated the incidence of microglial cell death during later post-axotomy time points to determine if aging had an effect on microglial turnover. We employed DNA labeling with 3H-thymidine, TUNEL and lectin histochemistry after facial nerve axotomy in young (3 months), middle-aged (15 months), and old (30 months) Fisher344-Brown Norway hybrid rats. Proliferation of microglia in old rats remained significantly higher than in young rats 4 days after injury, suggesting that regulation of microglial proliferation changes with aging. There was no aging-related difference in microglial TUNEL staining at 7, 14 or 21 days post-axotomy. Lectin histochemistry in the unoperated facial nucleus revealed aging-related morphological changes in resting microglia, including hypertrophy of the cytoplasm with dense perinuclear staining. Aging-related differences in activated microglia on the lesioned side were more subtle, although many activated microglia of aged animals continued to exhibit dense perinuclear lectin reactivity. We propose that aging-related changes in morphology in conjunction with a less regulated proliferative response in the aged facial nucleus may be a reflection of microglial senescence.