Microglia and neuroprotection: implications for Alzheimer's disease

Opposing Effects of NGF and proNGF on HIV Induced Macrophage Activation

Macrophage and microglial activation by HIV in the central nervous system (CNS) triggers the secretion of soluble factors which damage neurons. Therapeutic approaches designed to restore cognitive function by suppressing this inflammatory activity have not yet been successful. Recent studies have indicated that the phenotype of macrophages is differentially controlled by the mature and pro form of nerve growth factor. These cells therefore may be highly responsive to the imbalance in pro versus mature neurotrophins often associated with neurodegenerative diseases. In this study we evaluated the interactions between neurotrophins and HIV induced macrophage activation. HIV stimulation of macrophages induced a neurotoxic phenotype characterized by the expression of podosomes, suppression of calcium spiking and increased neurotoxin production. The secretome of the activated macrophages revealed a bias toward anti-angiogenic like activity and increased secretion of MMP-9. Co-stimulation with NGF and HIV suppressed neurotoxin secretion, increased calcium spiking, suppressed podosome expression and reversed 86% of the proteins secreted in response to HIV, including MMP-9 and many growth factors. In contrast, co-stimulation of macrophages with proNGF not only failed to reverse the effects of HIV but increased the neurotoxic phenotype. These differential effects of proNGF and NGF on HIV activation provide a potential novel therapeutic avenue for controlling macrophage activation in response to HIV.

(+)-Catechin Attenuates NF-κB Activation Through Regulation of Akt, MAPK, and AMPK Signaling Pathways in LPS-Induced BV-2 Microglial Cells

(+)-catechin is a flavanol that possesses various health and medicinal values, which include neuroprotection, anti-oxidation, antitumor and antihepatitis activities. This study investigated the modulatory effects of (+)-catechin on the lipopolysaccharides (LPS)-stimulated BV-2 cells. (+)-catechin attenuated LPS-induced inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) and inhibited microglial NO and ROS production. Additionally, (+)-catechin suppressed the production of tumor necrosis factor-α (TNF-α) and interleukin (IL)-6, while augmenting IL-4. (+)-catechin attenuated LPS-induced nuclear factor-κB (NF-κB) p65 nuclear translocation via the inhibition of IκB-α phosphorylation. Moreover, (+)-catechin blocked the activation of Akt and its inhibition was shown to play a crucial role in LPS-induced inflammation in BV-2 microglial cells. (+)-catechin also attenuated the LPS-induced phosphorylation of extracellular signal-regulated kinase (ERK1/2), and p-38 mitogen activated protein kinases (p38 MAPK) and specific inhibitors of ERK1/2 (UO126) and p38 MAPK (SB202190) subsequently down-regulated the expression of the proinflammatory mediators iNOS and COX-2. Further mechanistic study revealed that (+)-catechin acted through the amelioration of the LPS-induced suppression of adenosine monophosphate-activated protein kinase (AMPK) activity. Taken together, our data indicate that (+)-catechin exhibits anti-inflammatory effects in BV-2 cells by suppressing the production of proinflammatory mediators and mitigation of NF-κB through Akt, ERK, p38 MAPK, and AMPK pathways.

Neuroinflammatory signals in Alzheimer disease and APP/PS1 transgenic mice: correlations with plaques, tangles, and oligomeric species

To understand neuroinflammation-related gene regulation during normal aging and in sporadic Alzheimer disease (sAD), we performed functional genomics analysis and analyzed messenger RNA (mRNA) expression by quantitative reverse transcription-polymerase chain reaction of 22 genes involved in neuroinflammation-like responses in the cerebral cortex of wild-type and APP/PS1 transgenic mice. For direct comparisons, mRNA expression of 18 of the same genes was then analyzed in the entorhinal cortex, orbitofrontal cortex, and frontal cortex area 8 of middle-aged human subjects lacking Alzheimer disease-related pathology and in older subjects with sAD pathology covering Stages I-II/0(A), III-IV/A-B, and V-VI/C of Braak and Braak classification. Modifications of cytokine and immune mediator mRNA expression were found with normal aging in wild-type mice and in middle-aged individuals and patients with early stages of sAD-related pathology; these were accompanied by increased protein expression of certain mediators in ramified microglia. In APP/PS1 mice, inflammatory changes coincided with β-amyloid (Aβ) deposition; increased levels of soluble oligomers paralleled the modified mRNA expression of cytokines and mediators in wild-type mice. In patients with sAD, regulation was stage- and region-dependent and not merely acceleration and exacerbation of mRNA regulation with aging. Gene regulation at first stages of AD was not related to hyperphosphorylated tau deposition in neurofibrillary tangles, Aβ plaque burden, concentration of Aβ1-40 (Aβ40) and Aβ1-42 (Aβ42), or fibrillar Aβ linked to membranes but rather to increased levels of soluble oligomers. Thus, species differences and region- and stage-dependent inflammatory responses in sAD, particularly at the initial stages, indicate the need to identify new anti-inflammatory compounds with specific molecular therapeutic targets.

Glia: guardians, gluttons, or guides for the maintenance of neuronal connectivity?

An emerging aspect of neuronal-glial interactions is the connection glial cells have to synapses. Mounting research now suggests a far more intimate relationship than previously recognized. Moreover, the current evidence implicating synapse loss in neurodegenerative disease etiology is overwhelming, but the role of glia in the process of synaptic degeneration has only recently been considered in earnest. Each main class of glial cell, including astrocytes, oligodendrocytes, and microglia, performs crucial and multifaceted roles in the maintenance of synaptic function and excitability. As such, aging and/or neuronal stress from disease-related misfolded proteins may involve disruption of multiple non-cell-autonomous synaptic support systems that are mediated by neighboring glia. In addition, glial cell activation induced by injury, ischemia, or neurodegeneration is thought to greatly alter the behavior of glial cells toward neuronal synapses, suggesting that neuroinflammation potentially contributes to synapse loss primarily mediated by altered glial functions. This review discusses recent evidence highlighting novel roles for glial cells at neuronal synapses and in the maintenance of neuronal connectivity, focusing primarily on their implications for neurodegenerative disease research.

Inflammation and neurodegenerative disorders: is there still hope for therapeutic intervention?

PURPOSE OF REVIEW

This review aims to describe the current understanding of neuroinflammation in neurodegeneration and evaluate the value of various anti-inflammatory treatments.

RECENT FINDINGS

Inflammation plays important roles in common disease such as dementia and depression. Underlying mechanisms including the role of inflammasomes in these diseases have been recently described. Interventions using Ω-3 polyunsaturated fatty acids, NSAIDs and targeted antagonists (e.g., etanercept) show no convincing clinical efficacy in inflammation-associated depression, cognitive decline and dementia.

SUMMARY

Therapeutic targeting of inflammation appears to be relevant in brain conditions characterized by neuroinflammation and neurodegeneration, although published anti-inflammatory interventions have shown no relevant clinical efficacy. Newly described pharmacological targets in the neuroinflammation pathways may not only offer a more profound understanding of the underlying pathophysiology but also raise hope for the development of novel pharmacological agents.

Population-scale three-dimensional reconstruction and quantitative profiling of microglia arbors

MOTIVATION

The arbor morphologies of brain microglia are important indicators of cell activation. This article fills the need for accurate, robust, adaptive and scalable methods for reconstructing 3-D microglial arbors and quantitatively mapping microglia activation states over extended brain tissue regions.

RESULTS

Thick rat brain sections (100-300 µm) were multiplex immunolabeled for IBA1 and Hoechst, and imaged by step-and-image confocal microscopy with automated 3-D image mosaicing, producing seamless images of extended brain regions (e.g. 5903 × 9874 × 229 voxels). An over-complete dictionary-based model was learned for the image-specific local structure of microglial processes. The microglial arbors were reconstructed seamlessly using an automated and scalable algorithm that exploits microglia-specific constraints. This method detected 80.1 and 92.8% more centered arbor points, and 53.5 and 55.5% fewer spurious points than existing vesselness and LoG-based methods, respectively, and the traces were 13.1 and 15.5% more accurate based on the DIADEM metric. The arbor morphologies were quantified using Scorcioni's L-measure. Coifman's harmonic co-clustering revealed four morphologically distinct classes that concord with known microglia activation patterns. This enabled us to map spatial distributions of microglial activation and cell abundances.

AVAILABILITY AND IMPLEMENTATION

Experimental protocols, sample datasets, scalable open-source multi-threaded software implementation (C++, MATLAB) in the electronic supplement, and website (www.farsight-toolkit.org). http://www.farsight-toolkit.org/wiki/Population-scale_Three-dimensional_Reconstruction_and_Quanti-tative_Profiling_of_Microglia_Arbors

CONTACT

broysam@central.uh.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Selective PCAF inhibitor ameliorates cognitive and behavioral deficits by suppressing NF-κB-mediated neuroinflammation induced by Aβ in a model of Alzheimer's disease

Several recent studies have reported an association between neurodegeneration and histone modifications, such as acetylation, deacetylation and methylation. In addition, questions have been raised regarding a potential functional role for the histone acetylation enzymes in β-amyloid (Aβ)-mediated neurotoxicity, particularly the p300/CBP-associated factor (PCAF) enzyme. We recently reported the potential utility of a PCAF inhibitor in the suppression of Aβ-induced neuronal cell death, although the in vivo effectiveness of the PCAF inhibitor remained unclear. In this study, we modified the PCAF inhibitor by chemical derivatization and selected compound C-30-27 as the most potent PCAF inhibitor. We demonstrated that C-30-27 selectively inhibited acetylation-dependent nuclear factor-κB (NF-κB) at Lys-122 and suppressed the NF-κB-mediated inflammatory response induced by lipopolysaccharide (LPS) or Aβ in both BV2 and Neuro-2A (N2A) cells. Finally, we demonstrated that C-30-27 improved cognitive deficits, as well as the capacity for locomotion and the damaged cholinergic system in the Aβ-treated rats. In conclusion, our results demonstrate that this selective PCAF inhibitor has the potential to reduce the neuroinflammatory response induced by Aβ.

Cathepsin D deficiency induces oxidative damage in brain pericytes and impairs the blood-brain barrier

Recent evidence suggests that peripheral blood mononuclear cells (PBMCs) contribute to the pathogenesis of neuropathological changes in patients with neuronal ceroid lipofuscinosis (NCL) and lysosomal storage diseases. In order to examine the possible increase in the permeability of the blood-brain-barrier (BBB) and resultant infiltration of PBMCs due to cathepsin D (CatD) deficiency, a process underlying the onset of congenital NCL, we examined structural changes in brain vessels in CatD-/- mice. Consequently, the mean diameter of the brain vessels in the cerebral cortex on postnatal day 24 (P24) was significantly larger in CatD-/- mice than in wild-type mice. Furthermore, the mean number of brain pericytes in CatD-/- mice began to decline significantly on P16 and almost disappeared on P24, and oxidative DNA damage was first detected in brain pericytes on P12. Examinations with electron microscopy revealed that brain pericytes were laden with dense granular bodies, cytoplasmic vacuoles and lipid droplets. The infiltration of PBMCs characterized by segmented nucleus laden with dense granular bodies was also noted in the cerebral cortex of CatD-/- mice. When primary cultured microglia prepared from enhanced green fluorescent protein (GFP)-expressing transgenic rats were injected into the common carotid artery, GFP-positive microglia were detected in the brain parenchyma of CatD-/-, but not wild-type, mice. Moreover, pepstatin A, a specific aspartic protease inhibitor, induced mitochondria-derived reactive oxygen species (ROS) production in the isolated brain pericytes, which decreased the cell viability. These observations suggest that increased lysosomal storage due to CatD deficiency causes oxidative damage in brain pericytes, subsequently resulting in an increased vessel diameter, enhanced permeability of the BBB and the infiltration of PBMCs. Therefore, protecting brain pericytes against lysosomal storage-induced oxidative stress may represent an alternative treatment strategy for congenital NCL.

Redox control of microglial function: molecular mechanisms and functional significance

Neurodegenerative diseases are characterized by chronic microglial over-activation and oxidative stress. It is now beginning to be recognized that reactive oxygen species (ROS) produced by either microglia or the surrounding environment not only impact neurons but also modulate microglial activity. In this review, we first analyze the hallmarks of pro-inflammatory and anti-inflammatory phenotypes of microglia and their regulation by ROS. Then, we consider the production of reactive oxygen and nitrogen species by NADPH oxidases and nitric oxide synthases and the new findings that also indicate an essential role of glutathione (γ-glutamyl-l-cysteinylglycine) in redox homeostasis of microglia. The effect of oxidant modification of macromolecules on signaling is analyzed at the level of oxidized lipid by-products and sulfhydryl modification of microglial proteins. Redox signaling has a profound impact on two transcription factors that modulate microglial fate, nuclear factor kappa-light-chain-enhancer of activated B cells, and nuclear factor (erythroid-derived 2)-like 2, master regulators of the pro-inflammatory and antioxidant responses of microglia, respectively. The relevance of these proteins in the modulation of microglial activity and the interplay between them will be evaluated. Finally, the relevance of ROS in altering blood brain barrier permeability is discussed. Recent examples of the importance of these findings in the onset or progression of neurodegenerative diseases are also discussed. This review should provide a profound insight into the role of redox homeostasis in microglial activity and help in the identification of new promising targets to control neuroinflammation through redox control of the brain.

Inflammasomes in neuroinflammation and changes in brain function: a focused review

Recent literature has pointed to the existence of inflammasome-mediated inflammatory pathways in central nervous system (CNS) disorders and associated changes in behavior. Neuroinflammation, which is an innate immune response in the CNS against harmful and irritable stimuli such as pathogens and metabolic toxic waste, as well as to chronic mild stress, is mediated by protein complexes known as inflammasomes. Inflammasomes activate pro-inflammatory caspases 1 and 5, which then cleave the precursor forms of pro-inflammatory cytokines IL-1β, IL-18, and IL-33 into their active forms. These pro-inflammatory cytokines have been shown to promote a variety of innate immune processes associated with infection, inflammation, and autoimmunity, and thereby play an instrumental role in the instigation of neuroinflammation during old age and subsequent occurrence of neurodegenerative diseases, cognitive impairment, and dementia. In particular, NLRP inflammasomes may also have a role in the etiologies of depression, Alzheimer's disease (AD) and in metabolic disorders, such as Type II diabetes, obesity and cardiovascular diseases that have been shown to be co-morbid with psychiatric illnesses. It has been reported that while these inflammasomes may be activated through TNF-α dependent pathways, other cytokines, like IFN-γ, may assist in inhibiting their activation and thus delay disease progression. Furthermore, some other cytokines, including IL-6, may not have a direct role in inflammasome-mediated diseases. An array of recent research suggests that NLRP inflammasomes targeted therapies could be used for alleviating neuroinflammation and for treatment of associated psychiatric illnesses, although this still remains a challenge and necessitates further extensive research. This review examines the complex inflammatory signaling pathways involved in the activation of NLRP inflammasomes and the role they play in promoting neuroinflammation and subsequent behavioral changes.

Fractalkine over expression suppresses α-synuclein-mediated neurodegeneration

In Parkinson's disease, α-synuclein is known to activate microglia and this activation has been proposed as one of the mechanisms of neurodegeneration. There are several signals produced by neurons that have an anti-inflammatory action on microglia, including CX3CL1 (fractalkine). We have shown that a soluble form of CX3CL1 is required to reduce neuron loss in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice and that fractalkine agonism can reduce neuron loss in a 6-hydroxydopamine lesion model. Here, we show that fractalkine can reduce α-synuclein-mediated neurodegeneration in rats. Rats that received fractalkine showed abrogated loss of tyrosine hydroxylase and Neu-N staining. This was replicated in animals where we expressed fractalkine from astrocytes with the glial fibrillary acid protein (GFAP) promoter. Interestingly, we did not observe a reduction in MHCII expression suggesting that soluble fractalkine is likely altering the microglial state to a more neuroprotective one rather than reducing antigen presentation.

Fibrillar β-amyloid 1-42 alters cytokine secretion, cholinergic signalling and neuronal differentiation

Adult neurogenesis is impaired by inflammatory processes, which are linked to altered cholinergic signalling and cognitive decline in Alzheimer's disease. In this study, we investigated how amyloid beta (Aβ)-evoked inflammatory responses affect the generation of new neurons from human embryonic stem (hES) cells and the role of cholinergic signalling in regulating this process. The hES were cultured as neurospheres and exposed to fibrillar and oligomeric Aβ(1-42) (Aβf, AβO) or to conditioned medium from human primary microglia activated with either Aβ(1-42) or lipopolysaccharide. The neurospheres were differentiated for 29 days in vitro and the resulting neuronal or glial phenotypes were thereafter assessed. Secretion of cytokines and the enzymes acetylcholinesterase (AChE), butyrylcholinesterase (BuChE) and choline acetyltransferase (ChAT) involved in cholinergic signalling was measured in medium throughout the differentiation. We report that differentiating neurospheres released various cytokines, and exposure to Aβf, but not AβO, increased the secretion of IL-6, IL-1β and IL-2. Aβf also influenced the levels of AChE, BuChE and ChAT in favour of a low level of acetylcholine. These changes were linked to an altered secretion pattern of cytokines. A different pattern was observed in microglia activated by Aβf, demonstrating decreased secretion of TNF-α, IL-1β and IL-2 relative to untreated cells. Subsequent exposure of differentiating neurospheres to Aβf or to microglia-conditioned medium decreased neuronal differentiation and increased glial differentiation. We suggest that a basal physiological secretion of cytokines is involved in shaping the differentiation of neurospheres and that Aβf decreases neurogenesis by promoting a microenvironment favouring hypo-cholinergic signalling and gliogenesis.

An in vitro toxicity evaluation of gold-, PLLA- and PCL-coated silica nanoparticles in neuronal cells for nanoparticle-assisted laser-tissue soldering

The uptake of silica (Si) and gold (Au) nanoparticles (NPs) engineered for laser-tissue soldering in the brain was investigated using microglial cells and undifferentiated and differentiated SH-SY5Y cells. It is not known what effects NPs elicit once entering the brain. Cellular uptake, cytotoxicity, apoptosis, and the potential induction of oxidative stress by means of depletion of glutathione levels were determined after NP exposure at concentrations of 10(3) and 10(9)NPs/ml. Au-, silica poly (ε-caprolactone) (Si-PCL-) and silica poly-L-lactide (Si-PLLA)-NPs were taken up by all cells investigated. Aggregates and single NPs were found in membrane-surrounded vacuoles and the cytoplasm, but not in the nucleus. Both NP concentrations investigated did not result in cytotoxicity or apoptosis, but reduced glutathione (GSH) levels predominantly at 6 and 24h, but not after 12 h of NP exposure in the microglial cells. NP exposure-induced GSH depletion was concentration-dependent in both cell lines. Si-PCL-NPs induced the strongest effect of GSH depletion followed by Si-PLLA-NPs and Au-NPs. NP size seems to be an important characteristic for this effect. Overall, Au-NPs are most promising for laser-assisted vascular soldering in the brain. Further studies are necessary to further evaluate possible effects of these NPs in neuronal cells.

Correlated inflammatory responses and neurodegeneration in peptide-injected animal models of Alzheimer's disease

Animal models of Alzheimer's disease (AD) which emphasize activation of microglia may have particular utility in correlating proinflammatory activity with neurodegeneration. This paper reviews injection of amyloid- β (A β ) into rat brain as an alternative AD animal model to the use of transgenic animals. In particular, intrahippocampal injection of Aβ 1-42 peptide demonstrates prominent microglial mobilization and activation accompanied by a significant loss of granule cell neurons. Furthermore, pharmacological inhibition of inflammatory reactivity is demonstrated by a broad spectrum of drugs with a common endpoint in conferring neuroprotection in peptide-injected animals. Peptide-injection models provide a focus on glial cell responses to direct peptide injection in rat brain and offer advantages in the study of the mechanisms underlying neuroinflammation in AD brain.

Cellular and molecular mechanisms of immunomodulation in the brain through environmental enrichment

Recent studies on environmental enrichment (EE) have shown cytokines, cellular immune components [e.g., T lymphocytes, natural killer (NK) cells], and glial cells in causal relationship to EE in bringing out changes to neurobiology and behavior. The purpose of this review is to evaluate these neuroimmune mechanisms associated with neurobiological and behavioral changes in response to different EE methods. We systematically reviewed common research databases. After applying all inclusion and exclusion criteria, 328 articles remained for this review. Physical exercise (PE), a form of EE, elicits anti-inflammatory and neuromodulatory effects through interaction with several immune pathways including interleukin (IL)-6 secretion from muscle fibers, reduced expression of Toll-like receptors on monocytes and macrophages, reduced secretion of adipokines, modulation of hippocampal T cells, priming of microglia, and upregulation of mitogen-activated protein kinase phosphatase-1 in central nervous system. In contrast, immunomodulatory roles of other enrichment methods are not studied extensively. Nonetheless, studies showing reduction in the expression of IL-1β and tumor necrosis factor-α in response to enrichment with novel objects and accessories suggest anti-inflammatory effects of novel environment. Likewise, social enrichment, though considered a necessity for healthy behavior, results in immunosuppression in socially defeated animals. This has been attributed to reduction in T lymphocytes, NK cells and IL-10 in subordinate animals. EE through sensory stimuli has been investigated to a lesser extent and the effect on immune factors has not been evaluated yet. Discovery of this multidimensional relationship between immune system, brain functioning, and EE has paved a way toward formulating environ-immuno therapies for treating psychiatric illnesses with minimal use of pharmacotherapy. While the immunomodulatory role of PE has been evaluated extensively, more research is required to investigate neuroimmune changes associated with other enrichment methods.

Astrogliosis is a possible player in preventing delayed neuronal death

Mitigating secondary delayed neuronal injury has been a therapeutic strategy for minimizing neurological symptoms after several types of brain injury. Interestingly, secondary neuronal loss appeared to be closely related to functional loss and/or death of astrocytes. In the brain damage induced by agonists of two glutamate receptors, N-ethyl-D-aspartic acid (NMDA) and kainic acid (KA), NMDA induced neuronal death within 3 h, but did not increase further thereafter. However, in the KA-injected brain, neuronal death was not obviously detectable even at injection sites at 3 h, but extensively increased to encompass the entire hemisphere at 7 days. Brain inflammation, a possible cause of secondary neuronal damage, showed little differences between the two models. Importantly, however, astrocyte behavior was completely different. In the NMDA-injected cortex, the loss of glial fibrillary acidic protein-expressing (GFAP+) astrocytes was confined to the injection site until 7 days after the injection, and astrocytes around the damage sites showed extensive gliosis and appeared to isolate the damage sites. In contrast, in the KA-injected brain, GFAP+ astrocytes, like neurons, slowly, but progressively, disappeared across the entire hemisphere. Other markers of astrocytes, including S100β, glutamate transporter EAAT2, the potassium channel Kir4.1 and glutamine synthase, showed patterns similar to that of GFAP in both NMDA- and KA-injected cortexes. More importantly, astrocyte disappearance and/or functional loss preceded neuronal death in the KA-injected brain. Taken together, these results suggest that loss of astrocyte support to neurons may be a critical cause of delayed neuronal death in the injured brain.

Absence of Delayed Neuronal Death in ATP-Injected Brain: Possible Roles of Astrogliosis

Although secondary delayed neuronal death has been considered as a therapeutic target to minimize brain damage induced by several injuries, delayed neuronal death does not occur always. In this study, we investigated possible mechanisms that prevent delayed neuronal death in the ATP-injected substantia nigra (SN) and cortex, where delayed neuronal death does not occur. In both the SN and cortex, ATP rapidly induced death of the neurons and astrocytes in the injection core area within 3 h, and the astrocytes in the penumbra region became hypertropic and rapidly surrounded the damaged areas. It was observed that the neurons survived for up to 1-3 months in the area where the astrocytes became hypertropic. The damaged areas of astrocytes gradually reduced at 3 days, 7 days, and 1-3 months. Astrocyte proliferation was detectable at 3-7 days, and vimentin was expressed in astrocytes that surrounded and/or protruded into the damaged sites. The NeuN-positive cells also reappeared in the injury sites where astrocytes reappeared. Taken together, these results suggest that astroycte survival and/or gliosis in the injured brain may be critical for neuronal survival and may prevent delayed neuronal death in the injured brain.

Amyloid β peptide-mediated neurotoxicity is attenuated by the proliferating microglia more potently than by the quiescent phenotype

Background

The specific role of microglia on Aβ-mediated neurotoxicity is difficult to assign in vivo due to their complicated environment in the brain. Therefore, most of the current microglia-related studies employed the isolated microglia. However, the previous in vitro studies have suggested either beneficial or destructive function in microglia. Therefore, to investigate the phenotypes of the isolated microglia which exert activity of neuroprotective or destructive is required.

Results

The present study investigates the phenotypes of isolated microglia on protecting neuron against Aβ-mediated neurotoxicity. Primary microglia were isolated from the mixed glia culture, and were further cultured to distinct phenotypes, designated as proliferating amoeboid microglia (PAM) and differentiated process-bearing microglia (DPM). Their inflammatory phenotypes, response to amyloid β (Aβ), and the beneficial or destructive effects on neurons were investigated. DPM may induce both direct neurotoxicity without exogenous stimulation and indirect neurotoxicity after Aβ activation. On the other hand, PAM attenuates Aβ-mediated neurotoxicity through Aβ phagocytosis and/or Aβ degradation.

Conclusions

Our results suggest that the proliferating microglia, but not the differentiated microglia, protect neurons against Aβ-mediated neurotoxicity. This discovery may be helpful on the therapeutic investigation of Alzheimer’s disease.

Age-dependent effects of microglial inhibition in vivo on Alzheimer's disease neuropathology using bioactive-conjugated iron oxide nanoparticles

Background

Tau dysfunction is believed to be the primary cause of neurodegenerative disorders referred to as tauopathies, including Alzheimer’s disease, Pick’s disease, frontotemporal dementia and Parkinsonism. The role of microglial cells in the pathogenesis of tauopathies is still unclear. The activation of microglial cells has been correlated with neuroprotective effects through the release of neurotrophic factors and through clearance of cell debris and phagocytosis of cells with intracellular inclusions. In contrast, microglial activation has also been linked with chronic neuroinflammation contributing to the development of neurodegenerative diseases such as tauopathies. Microglial activation has been recently reported to precede tangle formation and the attenuation of tau pathology occurs after immunosuppression of transgenic mice.

Methods

Here we report the specific inhibition of microglial cells in rTg4510 tau-mutant mice by using fibrin γ^377-395 peptide conjugated to iron oxide (γ-Fe₂O₃) nanoparticles of 21 ± 3.5 nm diameter.

Results

Stabilization of the peptide by its covalent conjugation to the γ-Fe₂O₃ nanoparticles significantly decreased the number of the microglial cells compared to the same concentration of the free peptide. The specific microglial inhibition induces different effects on tau pathology in an age dependent manner. The reduction of activation of microglial cells at an early age increases the number of neurons with hyperphosphorylated tau in transgenic mice. In contrast, reduction of activation of microglial cells reduced the severity of the tau pathology in older mice. The number of neurons with hyperphosphorylated tau and the number of neurons with tangles are reduced than those in animals not receiving the fibrin γ^377-395 peptide-nanoparticle conjugate.

Conclusions

These results demonstrate a differential effect of microglial activity on tau pathology using the fibrin γ^377-395 peptide-nanoparticle conjugate, depending on age and/or stage of the neuropathological accumulation and aggregation.

Microglia: a new frontier for synaptic plasticity, learning and memory, and neurodegenerative disease research

We focus on emerging roles for microglia in synaptic plasticity, cognition and disease. We outline evidence that ramified microglia, traditionally thought to be functionally "resting" (i.e. quiescent) in the normal brain, in fact are highly dynamic and plastic. Ramified microglia continually and rapidly extend processes, contact synapses in an activity and experience dependent manner, and play a functionally dynamic role in synaptic plasticity, possibly through release of cytokines and growth factors. Ramified microglial also contribute to structural plasticity through the elimination of synapses via phagocytic mechanisms, which is necessary for normal cognition. Microglia have numerous mechanisms to monitor neuronal activity and numerous mechanisms also exist to prevent them transitioning to an activated state, which involves retraction of their surveying processes. Based on the evidence, we suggest that maintaining the ramified state of microglia is essential for normal synaptic and structural plasticity that supports cognition. Further, we propose that change of their ramified morphology and function, as occurs in inflammation associated with numerous neurological disorders such as Alzheimer's and Parkinson's disease, disrupts their intricate and essential synaptic functions. In turn altered microglia function could cause synaptic dysfunction and excess synapse loss early in disease, initiating a range of pathologies that follow. We conclude that the future of learning and memory research depends on an understanding of the role of non-neuronal cells and that this should include using sophisticated molecular, cellular, physiological and behavioural approaches combined with imaging to causally link the role of microglia to brain function and disease including Alzheimer's and Parkinson's disease and other neuropsychiatric disorders.

Targeting microglial K(ATP) channels to treat neurodegenerative diseases: a mitochondrial issue

Neurodegeneration is a complex process involving different cell types and neurotransmitters. A common characteristic of neurodegenerative disorders is the occurrence of a neuroinflammatory reaction in which cellular processes involving glial cells, mainly microglia and astrocytes, are activated in response to neuronal death. Microglia do not constitute a unique cell population but rather present a range of phenotypes closely related to the evolution of neurodegeneration. In a dynamic equilibrium with the lesion microenvironment, microglia phenotypes cover from a proinflammatory activation state to a neurotrophic one directly involved in cell repair and extracellular matrix remodeling. At each moment, the microglial phenotype is likely to depend on the diversity of signals from the environment and of its response capacity. As a consequence, microglia present a high energy demand, for which the mitochondria activity determines the microglia participation in the neurodegenerative process. As such, modulation of microglia activity by controlling microglia mitochondrial activity constitutes an innovative approach to interfere in the neurodegenerative process. In this review, we discuss the mitochondrial K_ATP channel as a new target to control microglia activity, avoid its toxic phenotype, and facilitate a positive disease outcome.

The endocannabinoid system in normal and pathological brain ageing

The role of endocannabinoids as inhibitory retrograde transmitters is now widely known and intensively studied. However, endocannabinoids also influence neuronal activity by exerting neuroprotective effects and regulating glial responses. This review centres around this less-studied area, focusing on the cellular and molecular mechanisms underlying the protective effect of the cannabinoid system in brain ageing. The progression of ageing is largely determined by the balance between detrimental, pro-ageing, largely stochastic processes, and the activity of the homeostatic defence system. Experimental evidence suggests that the cannabinoid system is part of the latter system. Cannabinoids as regulators of mitochondrial activity, as anti-oxidants and as modulators of clearance processes protect neurons on the molecular level. On the cellular level, the cannabinoid system regulates the expression of brain-derived neurotrophic factor and neurogenesis. Neuroinflammatory processes contributing to the progression of normal brain ageing and to the pathogenesis of neurodegenerative diseases are suppressed by cannabinoids, suggesting that they may also influence the ageing process on the system level. In good agreement with the hypothesized beneficial role of cannabinoid system activity against brain ageing, it was shown that animals lacking CB1 receptors show early onset of learning deficits associated with age-related histological and molecular changes. In preclinical models of neurodegenerative disorders, cannabinoids show beneficial effects, but the clinical evidence regarding their efficacy as therapeutic tools is either inconclusive or still missing.

Apoptosis of oligodendrocytes in the central nervous system results in rapid focal demyelination

OBJECTIVE

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that presents with variable pathologies that may reflect different disease-causing mechanisms. Existing animal models of MS induce pathology using either local injection of gliotoxins or stimulation of the immune system with myelin-related peptides. In none of these models is the primary cellular target well characterized, and although demyelination is a hallmark pathological feature in MS, it is unclear to what extent this reflects local oligodendrocyte loss. To unambiguously identify the effects of oligodendrocyte death in the absence of inflammatory stimulation, we developed a method for experimentally inducing programmed cell death selectively in mature oligodendrocytes and assessed the effects on demyelination, immunological stimulation, and gliosis. The resulting pathology is discussed relative to observed MS pathologies.

METHODS

Oligodendrocyte apoptosis was induced in the adult rat brain using a lentivirus to express experimentally inducible caspase 9 (iCP9) cDNA under transcriptional control of the promoter for myelin basic protein, which is oligodendrocyte-specific. Activation of iCP9 was achieved by distal injection of a small molecule dimerizer into the lateral ventricle resulting in localized, acute oligodendrocyte apoptosis.

RESULTS

Induced oligodendrocyte apoptosis resulted in rapid demyelination and robust, localized microglial activation in the absence of peripheral immune cell infiltration. Lesion borders showed layers of preserved and degraded myelin, whereas lesion cores were demyelinated but only partially cleared of myelin debris. This resulted in local proliferation and mobilization of the oligodendrocyte progenitor pool.

INTERPRETATION

This approach provides a novel model to understand the pathological changes that follow from localized apoptosis of myelinating oligodendrocytes. It provides the first direct proof that initiation of apoptosis in oligodendrocytes is sufficient to cause rapid demyelination, gliosis, and a microglial response that result in lesions sharing some pathological characteristics with a subset of MS lesions.

Sex and estradiol influence glial pro-inflammatory responses to lipopolysaccharide in rats

There is a greater prevalence of neuroinflammatory diseases in females than males. Microglia, the major immunocompetent cells of the central nervous system, play a key role in neuroinflammation. We aimed to determine if inherent differences in toll-like receptor 4 mediated pro-inflammatory response in glia could possibly contribute to the skewed female prevalence of neuroinflammatory disorders. In addition, in order to identify if estradiol (E2), the major female sex steroid contributes to a heightened pro-inflammatory response, estradiol was added both in vivo and in vitro. Microglia and astrocytes were isolated from neonatal pups and stimulated with lipopolysaccharide (LPS) in the presence and absence of E2. Hippocampal microglia were isolated from adult male and female rats and stimulated ex vivo with LPS. Male neonatal microglia and astrocytes produced greater IL-1β mRNA than females. However, when co-incubated with varying doses of estradiol (E2), the E2 produced anti-inflammatory effects in the male microglia but a pro-inflammatory effect in female microglia. LPS-induced IL-1β mRNA was attenuated by E2 in female but not male adult hippocampal microglia. However, females supplemented with E2 in vivo produced a potentiated IL-1β mRNA response. TLR4 mRNA was decreased by LPS in both microglia and astrocytes but was not affected by sex or E2. CD14 mRNA was increased by LPS and may be elevated more in females than males in microglia but not astrocytes. Therefore, sexual dimorphic differences do occur in both neonatal and adult microglia though maturity of the microglia at the time of isolation influences the pro-inflammatory response.

Granulocyte-colony stimulating factor improves MDX mouse response to peripheral nerve injury

BACKGROUND

G-CSF has been shown to increase neuronal survival, which may positively influence the spinal cord microenvironment during the course of muscular dystrophies.

METHODOLOGY/PRINCIPAL FINDINGS

Male MDX mice that were six weeks of age received a left sciatic nerve transection and were treated with intraperitoneal injections of 200 µg/kg/day of G-CSF 7 days before and 7 days after the transection. The axotomy was performed after the cycles of muscular degeneration/regeneration, consistent with previous descriptions of this model of muscular dystrophy. C57BL/10 mice were used as control subjects. Seven days after the surgery, the animals were sacrificed and their lumbar spinal cords were processed for immunohistochemistry (anti-MHC I, anti-Synaptophysin, anti-GFAP and anti-IBA-1) and transmission electron microscopy. MHC I expression increased in both strains of mice after the axotomy. Nevertheless, the MDX mice displayed a significantly smaller MHC I upregulation than the control mice. Regarding GFAP expression, the MDX mice showed a stronger astrogliosis compared with the C57BL/10 mice across all groups. Both groups that were treated with G-CSF demonstrated preservation of synaptophysin expression compared with the untreated and placebo groups. The quantitative analysis of the ultrastructural level showed a preservation of the synaptic covering for the both groups that were treated with G-CSF and the axotomized groups showed a smaller loss of synaptic contact in relation to the treated groups after the lesion.

CONCLUSIONS/SIGNIFICANCE

The reduction of active inputs to the alpha-motoneurons and increased astrogliosis in the axotomized and control groups may be associated with the cycles of muscle degeneration/regeneration that occur postnatally. The G-CSF treated group showed a preservation of the spinal cord microenvironment after the lesion. Moreover, the increase of MHC I expression in the MDX mice that were treated with G-CSF may indicate that this drug performs an active role in regenerative potential after lesions.

Etanercept, a widely used inhibitor of tumor necrosis factor-α (TNF-α), prevents retinal ganglion cell loss in a rat model of glaucoma

BACKGROUND

Visual loss in glaucoma is associated with pathological changes in retinal ganglion cell (RGC) axons and a slow decline in the RGC population. Age and elevated intraocular pressure (IOP) are the main risk factors for glaucomatous loss of vision. Several studies have implicated the proinflammatory cytokine tumor necrosis factor-α (TNF-α) as a link between elevated IOP and RGC death, but the cellular source of TNF-α and its causative role in RGC death remain uncertain. Here, using a rat model of glaucoma, we investigated the source of elevated TNF-α and examined whether Etanercept, a TNF-α blocker that is in common clinical use for other indications, is protective against RGC death.

METHODOLOGY/PRINCIPAL FINDINGS

Episcleral vein cauterization (EVC) caused intraocular pressure (IOP) to be elevated for at least 28 days. IOP elevation resulted in a dramatic increase in TNF-α levels within a few days, axonal degeneration, and a 38% loss of RGCs by 4 weeks. Immunostaining coupled with confocal microscopy showed that OHT induced robust induction of TNF-α in Iba-1-positive microglia around the optic nerve head (ONH). Despite persistent elevation of IOP, Etanercept reduced microglial activation, TNF-α levels, axon degeneration in the optic nerve, and the loss of RGCs.

CONCLUSIONS/SIGNIFICANCE

Ocular hypertension (OHT) triggers an inflammatory response characterized by the appearance of activated microglia around the ONH that express TNF-α. Blocking TNF-α activity with a clinically approved agent inhibits this microglial response and prevents axonal degeneration and loss of RGCs. These findings suggest a new treatment strategy for glaucoma using TNF-α antagonists or suppressors of inflammation.

Is autism a member of a family of diseases resulting from genetic/cultural mismatches? Implications for treatment and prevention

Several lines of evidence support the view that autism is a typical member of a large family of immune-related, noninfectious, chronic diseases associated with postindustrial society. This family of diseases includes a wide range of inflammatory, allergic, and autoimmune diseases and results from consequences of genetic/culture mismatches which profoundly destabilize the immune system. Principle among these consequences is depletion of important components, particularly helminths, from the ecosystem of the human body, the human biome. Autism shares a wide range of features in common with this family of diseases, including the contribution of genetics/epigenetics, the identification of disease-inducing triggers, the apparent role of immunity in pathogenesis, high prevalence, complex etiologies and manifestations, and potentially some aspects of epidemiology. Fortunately, using available resources and technology, modern medicine has the potential to effectively reconstitute the human biome, thus treating or even avoiding altogether the consequences of genetic/cultural mismatches which underpin this entire family of disease. Thus, if indeed autism is an epidemic of postindustrial society associated with immune hypersensitivity, we can expect that the disease is readily preventable.

Spatial and temporal correlation in progressive degeneration of neurons and astrocytes in contusion-induced spinal cord injury

BACKGROUND

Traumatic spinal cord injury (SCI) causes acute neuronal death followed by delayed secondary neuronal damage. However, little is known about how microenvironment regulating cells such as microglia, astrocytes, and blood inflammatory cells behave in early SCI states and how they contribute to delayed neuronal death.

METHODS

We analyzed the behavior of neurons and microenvironment regulating cells using a contusion-induced SCI model, examining early (3-6 h) to late times (14 d) after the injury.

RESULTS

At the penumbra region close to the damaged core (P1) neurons and astrocytes underwent death in a similar spatial and temporal pattern: both neurons and astrocytes died in the medial and ventral regions of the gray matter between 12 to 24 h after SCI. Furthermore, mRNA and protein levels of transporters of glutamate (GLT-1) and potassium (Kir4.1), functional markers of astrocytes, decreased at about the times that delayed neuronal death occurred. However, at P1 region, ramified Iba-1+ resident microglia died earlier (3 to 6 h) than neurons (12 to 24 h), and at the penumbra region farther from the damaged core (P2), neurons were healthy where microglia were morphologically activated. In addition, round Iba-1/CD45-double positive monocyte-like cells appeared after neurons had died, and expressed phagocytic markers, including mannose receptors, but rarely expressed proinflammatory mediators.

CONCLUSION

Loss of astrocyte function may be more critical for delayed neuronal death than microglial activation and monocyte infiltration.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.

ATP-dependent potassium channel blockade strengthens microglial neuroprotection after hypoxia-ischemia in rats

Stroke causes CNS injury associated with strong fast microglial activation as part of the inflammatory response. In rat models of stroke, sulphonylurea receptor blockade with glibenclamide reduced cerebral edema and infarct volume. We postulated that glibenclamide administered during the early stages of stroke might foster neuroprotective microglial activity through ATP-sensitive potassium (K(ATP)) channel blockade. We found in vitro that BV2 cell line showed upregulated expression of K(ATP) channel subunits in response to pro-inflammatory signals and that glibenclamide increases the reactive morphology of microglia, phagocytic capacity and TNFα release. Moreover, glibenclamide administered to rats 6, 12 and 24h after transient Middle Cerebral Artery occlusion improved neurological outcome and preserved neurons in the lesioned core three days after reperfusion. Immunohistochemistry with specific markers to neuron, astroglia, microglia and lymphocytes showed that resident amoeboid microglia are the main cell population in that necrotic zone. These reactive microglial cells express SUR1, SUR2B and Kir6.2 proteins that assemble in functional K(ATP) channels. These findings provide that evidence for the key role of K(ATP) channels in the control of microglial reactivity are consistent with a microglial effect of glibenclamide into the ischemic brain and suggest a neuroprotective role of microglia in the early stages of stroke.

The spider effect: morphological and orienting classification of microglia in response to stimuli in vivo

The different morphological stages of microglial activation have not yet been described in detail. We transected the olfactory bulb of rats and examined the activation of the microglial system histologically. Six stages of bidirectional microglial activation (A) and deactivation (R) were observed: from stage 1A to 6A, the cell body size increased, the cell process number decreased, and the cell processes retracted and thickened, orienting toward the direction of the injury site; until stage 6A, when all processes disappeared. In contrast, in deactivation stages 6R to 1R, the microglia returned to the original site exhibiting a stepwise retransformation to the original morphology. Thin highly branched processes re-formed in stage 1R, similar to those in stage 1A. This reverse transformation mirrored the forward transformation except in stages 6R to 1R: cells showed multiple nuclei which were slowly absorbed. Our findings support a morphologically defined stepwise activation and deactivation of microglia cells.

Sexual differentiation of the rodent brain: dogma and beyond

Steroid hormones of gonadal origin act on the neonatal brain to produce sex differences that underlie adult reproductive physiology and behavior. Neuronal sex differences occur on a variety of levels, including differences in regional volume and/or cell number, morphology, physiology, molecular signaling, and gene expression. In the rodent, many of these sex differences are determined by steroid hormones, particularly estradiol, and are established by diverse downstream effects. One brain region that is potently organized by estradiol is the preoptic area (POA), a region critically involved in many behaviors that show sex differences, including copulatory and maternal behaviors. This review focuses on the POA as a case study exemplifying the depth and breadth of our knowledge as well as the gaps in understanding the mechanisms through which gonadal hormones produce lasting neural and behavioral sex differences. In the POA, multiple cell types, including neurons, astrocytes, and microglia are masculinized by estradiol. Multiple downstream molecular mediators are involved, including prostaglandins, various glutamate receptors, protein kinase A, and several immune signaling molecules. Moreover, emerging evidence indicates epigenetic mechanisms maintain sex differences in the POA that are organized perinatally and thereby produce permanent behavioral changes. We also review emerging strategies to better elucidate the mechanisms through which genetics and epigenetics contribute to brain and behavioral sex differences.

Thioredoxin interacting protein is a novel mediator of retinal inflammation and neurotoxicity

BACKGROUND AND PURPOSE

Up-regulation of thioredoxin interacting protein (TXNIP), an endogenous inhibitor of thioredoxin (Trx), compromises cellular antioxidant and anti-apoptotic defences and stimulates pro-inflammatory cytokines expression, implying a role for TXNIP in apoptosis. Here we have examined the causal role of TXNIP expression in mediating retinal neurotoxicity and assessed the neuroprotective actions of verapamil, a calcium channel blocker and an inhibitor of TXNIP expression.

EXPERIMENTAL APPROACH

Retinal neurotoxicity was induced by intravitreal injection of NMDA in Sprague-Dawley rats, which received verapamil (10 mg·kg(-1), p.o.) or vehicle. Neurotoxicity was examined by terminal dUTP nick-end labelling assay and ganglion cell count. Expression of TXNIP, apoptosis signal-regulating kinase 1 (ASK-1), NF-κB, p38 MAPK, JNK, cleaved poly-ADP-ribose polymerase (PARP), caspase-3, nitrotyrosine and 4-hydroxy-nonenal were examined by Western and slot-blot analysis. Release of TNF-α and IL-1β was examined by elisa.

KEY RESULTS

NMDA injection enhanced TXNIP expression, decreased Trx activity, causing increased oxidative stress, glial activation and release of TNF-α and IL-1β. Enhanced TXNIP expression disrupted Trx/ASK-1 inhibitory complex leading to release of ASK-1 and activation of the pro-apoptotic p38 MAPK/JNK pathway, as indicated by cleaved PARP and caspase-3 expression. Treatment with verapamil blocked these effects.

CONCLUSION AND IMPLICATIONS

Elevated TXNIP expression contributed to retinal neurotoxicity by three different mechanisms, inducing release of inflammatory mediators such as TNF-α and IL-1β, altering antioxidant status and disrupting the Trx-ASK-1 inhibitory complex leading to activation of the p38 MAPK/JNK apoptotic pathway. Targeting TXNIP expression is a potential therapeutic target for retinal neurodegenerative disease.

Microglia and memory: modulation by early-life infection

The proinflammatory cytokine interleukin-1β (IL-1β) is critical for normal hippocampus (HP)-dependent cognition, whereas high levels can disrupt memory and are implicated in neurodegeneration. However, the cellular source of IL-1β during learning has not been shown, and little is known about the risk factors leading to cytokine dysregulation within the HP. We have reported that neonatal bacterial infection in rats leads to marked HP-dependent memory deficits in adulthood. However, deficits are only observed if unmasked by a subsequent immune challenge [lipopolysaccharide (LPS)] around the time of learning. These data implicate a long-term change within the immune system that, upon activation with the "second hit," LPS, acutely impacts the neural processes underlying memory. Indeed, inhibiting brain IL-1β before the LPS challenge prevents memory impairment in neonatally infected (NI) rats. We aimed to determine the cellular source of IL-1β during normal learning and thereby lend insight into the mechanism by which this cytokine is enduringly altered by early-life infection. We show for the first time that CD11b(+) enriched cells are the source of IL-1β during normal HP-dependent learning. CD11b(+) cells from NI rats are functionally sensitized within the adult HP and produce exaggerated IL-1β ex vivo compared with controls. However, an exaggerated IL-1β response in vivo requires LPS before learning. Moreover, preventing microglial activation during learning prevents memory impairment in NI rats, even following an LPS challenge. Thus, early-life events can significantly modulate normal learning-dependent cytokine activity within the HP, via a specific, enduring impact on brain microglial function.

Neuroinflammation in Alzheimer's disease wanes with age

BACKGROUND

Inflammation is a prominent feature in Alzheimer's disease (AD). It has been proposed that aging has an effect on the function of inflammation in the brain, thereby contributing to the development of age-related diseases like AD. However, the age-dependent relationship between inflammation and clinical phenotype of AD has never been investigated.

METHODS

In this study we have analysed features of the neuroinflammatory response in clinically and pathologically confirmed AD and control cases in relation to age (range 52-97 years). The mid-temporal cortex of 19 controls and 19 AD cases was assessed for the occurrence of microglia and astrocytes by immunohistochemistry using antibodies directed against CD68 (KP1), HLA class II (CR3/43) and glial fibrillary acidic protein (GFAP).

RESULTS

By measuring the area density of immunoreactivity we found significantly more microglia and astrocytes in AD cases younger than 80 years compared to older AD patients. In addition, the presence of KP1, CR3/43 and GFAP decreases significantly with increasing age in AD.

CONCLUSION

Our data suggest that the association between neuroinflammation and AD is stronger in relatively young patients than in the oldest patients. This age-dependent relationship between inflammation and clinical phenotype of AD has implications for the interpretation of biomarkers and treatment of the disease.

Emerging roles of p53 in glial cell function in health and disease

Emerging evidence suggests that p53, a tumor suppressor protein primarily involved in cancer biology, coordinates a wide range of novel functions in the CNS including the mediation of pathways underlying neurodegenerative disease pathogenesis. Moreover, an evolving concept in cell and molecular neuroscience is that glial cells are far more fundamental to disease progression than previously thought, which may occur via a noncell-autonomous mechanism that is heavily dependent on p53 activities. As a crucial hub connecting many intracellular control pathways, including cell-cycle control and apoptosis, p53 is ideally placed to coordinate the cellular response to a range of stresses. Although neurodegenerative diseases each display a distinct and diverse molecular pathology, apoptosis is a widespread hallmark feature and the multimodal capacity of the p53 system to orchestrate apoptosis and glial cell behavior highlights p53 as a potential unifying target for therapeutic intervention in neurodegeneration.

Luteolin reduces primary hippocampal neurons death induced by neuroinflammation

OBJECTIVES

This study examined whether luteolin may exert an anti-inflammatory effect in microglia and may be neuroprotective by regulating microglia activation.

METHODS

We treated BV2 microglia with 1.0 μg/ml lipopolysaccharide (LPS) after incubation with luteolin for 1 hour, the nitric oxide (NO) levels were determined by a Griess reaction, the inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), tumor necrosis factor-alpha (TNF-alpha), and interleukin 1beta (IL-1beta) mRNA expression were determined by real-time PCR analysis, the iNOS and COX-2 protein induction were determined by Western blot analysis, and the levels of prostaglandin E(2) (PGE(2)), TNF-alpha, and IL-1beta were determined by enzyme-linked immunosorbent assay (ELISA) kits. Rat primary hippocampal neurons were co-cultured with LPS-activated BV2 microglia with 20 μM luteolin for 24 hours, the hippocampal neurons viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and the number of apoptotic hippocampal neurons was determined by immunofluorescence detection.

RESULTS

Luteolin significantly inhibited the expression of iNOS and COX-2 in LPS-induced BV2 microglia. Moreover, the compound down-regulated the proinflammatory cytokines (TNF-alpha and IL-1beta) as well as the production of NO and PGE(2) in these cells. When hippocampal neurons were co-cultured with LPS-stimulated BV2 microglia, the administration of 20 μM luteolin increased the neurons viability and reduced the number of apoptotic neurons.

CONCLUSION

These data demonstrate that anti-inflammatory activity of luteolin in microglia contributes to its neuroprotective effect and suggest that it may have a potential therapeutic application in the treatment of neurodegenerative diseases.

Effects of methylmercury on the secretion of pro-inflammatory cytokines from primary microglial cells and astrocytes

Glial cells, including oligodendrocytes, astrocytes and microglia are important to proper central nervous system (CNS) function. Deregulation or changes to CNS populations of astrocytes and microglia in particular are expected to play a role in many neurodegenerative diseases, including Parkinson's disease, amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD). Previous studies have reported methylmercury (MeHg) induced changes in glial cell function; however, the effects of MeHg on these cells remains poorly understood. This study aims to examine the effect of MeHg on the secretion of pro-inflammatory cytokines from microglia and astrocytes. The impact of the microglia/astrocyte ratio on cytokine secretion was also examined. Microglia and astrocytes were cultured from the brains of neo-natal BALB/C mice and dosed with MeHg (0-1 μM) and stimulated with PAM(3)CSK(4) (PAM(3)), a toll-like receptor (TLR) ligand. After this, the secretion of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α) and interleukin-1-beta (IL-1β) was measured by ELISA. MeHg reduced the secretion of IL-6 in a dose dependant manner but did not effect the secretion of TNF-α. No change in IL-1β was observed in any treatments, indicating that PAM(3) cannot induce the secretion of this cytokine from glial cells. Additionally, the ratio of microglia/astrocyte had an effect on the secretion of IL-6 but not TNF-α. These results indicate that MeHg can modify the response of glial cells and the interactions with astrocytes can affect the response of the microglia cells in culture. These results are significant in understanding the potential relationship with MeHg and neurodegenerative diseases and for the interpretation of results of future in vitro studies using monoculture.

Targeting progressive neuroaxonal injury: lessons from multiple sclerosis

Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), are characterized by progressive neuroaxonal injury, suggesting a common pathophysiological pathway. Identification and development of neuroprotective therapies for such diseases has proven a major challenge, particularly because of an already substantial neuroaxonal compromise at the time of initial onset of clinical symptoms. Methods for early identification of neurodegeneration are therefore vital to ensure that neuroprotective therapies are applied as early as possible. Recent investigations have enhanced our understanding of the role of neuroaxonal injury in multiple sclerosis (MS). As MS generally manifests earlier in life and can be diagnosed much earlier in the course of the disease than the above-mentioned 'classic' neurodegenerative diseases, it is possible that MS could be used as a model disease to study degeneration and regeneration of the CNS. The mechanism of neuroaxonal injury in MS is believed to be inflammation-led neurodegeneration; however, the reverse may also be true (i.e. neuroaxonal degeneration may precede inflammation). Animal models of PD, AD and ALS have shown that it is likely that most cases of disease are due to initial inflammation, followed by a degenerative process, providing a parallel between MS and the classic neurodegenerative diseases. Other common factors between MS and the neurodegenerative diseases include iron and mitochondrial dysregulation, abnormalities in α-synuclein and tau protein, and a number of immune mediators. Conventional MRI techniques, using markers such as T2-weighted lesions, gadolinium-enhancing lesions and T1-weighted hypointensities, are readily available and routinely used in clinical practice; however, the utility of these MRI measures to predict disease progression in MS is limited. More recently, MRI techniques that provide more pathology-specific data have been applied in MS studies, including magnetic resonance spectroscopy, magnetization transfer ratio and myelin water imaging. Optical coherence tomography (OCT) is a non-MRI technique that quantifies optic nerve integrity and retinal ganglion cell loss as markers of neuroaxonal injury; more research is needed to evaluate whether information obtained from OCT is a reliable marker of axonal injury and long-term disability in MS. Using these advanced techniques, it may become possible to follow degeneration and regeneration longitudinally in patients with MS and to better differentiate the effects of drugs under investigation. Currently available immune-directed therapies that are approved by the US FDA for the first-line treatment of MS (interferon-β and glatiramer acetate) have been shown to decelerate the inflammatory process in MS; however, such therapy is less effective in preventing the progression of the disease and neuroaxonal injury. The use of MS as a clinical model to study modulation of neuroaxonal injury in the brain could have direct implications for the development of treatment strategies in neurodegenerative diseases such as AD, PD and ALS.

Luteolin inhibits microglial inflammation and improves neuron survival against inflammation

Microglia activation is one of the causative factors for neuroinflammation, which results in brain damage during neurodegenerative disease. Accumulating evidence has shown that the flavonoid luteolin (Lut) possesses potent anti-inflammatory properties; however, its effect on microglia inhibition is currently unknown. Moreover, it is not clear whether Lut also has indirect neuroprotective effects by reducing inflammatory mediators and suppressing microglia activation. In this study, we examined the effects of Lut on lipopolysaccharide (LPS)-induced proinflammatory mediator production and signaling pathways in murine BV2 microglia. In addition, we cocultured microglia and neurons to observe the indirect neuroprotective effects of Lut. Lut inhibited the LPS-stimulated expression of inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), tumor necrosis factor alpha (TNF-α), and interleukin-1β (IL-1β) as well as the production of nitric oxide (NO) and prostaglandin E(2) (PGE(2)). Moreover, Lut blocked LPS-induced nuclear factor kappa B (NF-κB) activation. Preincubation of microglia with Lut diminished the neurotoxic effects, owing to the direct anti-inflammatory effects of the compound. Taken together, our findings suggest that Lut may have a potential therapeutic application in the treatment of neuroinflammatory disorders.

Protein changes in CSF of HIV-infected patients: evidence for loss of neuroprotection

To begin to unravel the complexity of HIV-associated changes in the brain, broader, multifaceted analyses of cerebrospinal fluid (CSF) are needed that examine a wide range of proteins reflecting different functions. To provide the first broad profiles of protein changes in the CSF of HIV-infected patients, we used antibody arrays to measure 120 cytokines, chemokines, growth factors, and other proteins. CSF from HIV-infected patients with a range of cognitive deficits was compared to CSF from uninfected, cognitively normal patients to begin to identify protein changes associated with HIV infection and neurological disease progression. Uninfected patients showed relatively consistent patterns of protein expression. Highly expressed proteins in CSF included monocyte chemotactic protein-1, tissue inhibitors of metalloproteases, granulocyte colony-stimulating factor, adiponectin, soluble tumor necrosis factor receptor-1, urokinase-type plasminogen activator receptor, and insulin-like growth factor binding protein-2. Inflammatory and anti-inflammatory cytokines were expressed at low levels. HIV-infected patients showed increases in inflammatory proteins (interferon-gamma, tumor necrosis factor-alpha), anti-inflammatory proteins (IL-13), and chemokines but these correlated poorly with neurological status. The strongest correlation with increasing severity of neurological disease was a decline in growth factors, particularly, brain-derived neurotrophic factor and NT-3. These studies illustrate that HIV infection is associated with parallel changes in both inflammatory and neuroprotective proteins in the CSF. The inverse relationship between growth factors and neurological disease severity suggests that a loss of growth factor neuroprotection may contribute to the development of neural damage and may provide useful markers of disease progression.

Naloxone ameliorates retinal lesions in Ccl2/Cx3cr1 double-deficient mice via modulation of microglia

PURPOSE

The role of naloxone, an opioid receptor antagonist, on microglial inhibition and neuroprotective effects has been reported in lipopolysaccharide (LPS)-induced neurodegeneration and light-induced photoreceptor degeneration. The authors evaluated the effects of naloxone on Ccl2(-/-)/Cx3cr1(-/-) (DKO) mice, a murine model of age-related macular degeneration (AMD).

METHODS

Two-month-old DKO and wild-type controls were given daily intraperitoneal injections of naloxone or PBS for 2 months. Animals were examined monthly by funduscopy. Ocular tissue was analyzed histologically and in retinal flat mount preparations. Ocular A2E was measured using HPLC. Quantitative RT-PCR analyzed TNF-α, IL-1β, IL-10 and TLR4 transcripts in the DKO eyes and LPS activated culture microglial cells. Serum nitrite was measured using Griess colorimetric reaction.

RESULTS

Naloxone ameliorated the clinical progression and severity of retinal lesions in the DKO mice compared with those of untreated controls. Histopathology also showed less focal retinal degeneration in the treated DKO mice than in controls. The aggregation of microglia in the outer retina in DKO mice was significantly reduced in naloxone-treated animals compared with control untreated DKO. Ocular TNF-α, IL-1β, and TLR4 transcripts and A2E were significantly lower in naloxone-treated DKO animals and cultured microglial cells than in controls, as were serum nitrite levels.

CONCLUSIONS

Naloxone significantly reduces the progress of retinal lesions in DKO mice. Naloxone modulates microglia accumulation and activation at the site of retinal degeneration, which may be mediated by inhibition of the proinflammatory molecules of NO, TNF-α, and IL-β. The potential therapeutic effects of naloxone on retinal degeneration, including AMD, warrants further investigation.

Early microglia activation in a mouse model of chronic glaucoma

Changes in microglial cell activation and distribution are associated with neuronal decline in the central nervous system (CNS), particularly under pathological conditions. Activated microglia converge on the initial site of axonal degeneration in human glaucoma, yet their part in its pathophysiology remains unresolved. To begin with, it is unknown whether microglia activation precedes or is a late consequence of retinal ganglion cell (RGC) neurodegeneration. Here we address this critical element in DBA/2J (D2) mice, an established model of chronic inherited glaucoma, using as a control the congenic substrain DBA/2J Gpnmb(+/SjJ) (D2G), which is not affected by glaucoma. We analyzed the spatial distribution and timecourse of microglial changes in the retina, as well as within the proximal optic nerve prior to and throughout ages when neurodegeneration has been reported. Exclusively in D2 mice, we detected early microglia clustering in the inner central retina and unmyelinated optic nerve regions, with microglia activation peaking by 3 months of age. Between 5 and 8 months of age, activated microglia persisted and concentrated in the optic disc, but also localized to the retinal periphery. Collectively, our findings suggest microglia activation is an early alteration in the retina and optic nerve in D2 glaucoma, potentially contributing to disease onset or progression. Ultimately, detection of microglial activation may have value in early disease diagnosis, while modulation of microglial responses may alter disease progression.

Dental X-ray exposure and Alzheimer's disease: a hypothetical etiological association

Despite the fact that Alzheimer's disease was identified more than 100 years ago, its cause remains elusive. Although the chance of developing Alzheimer's disease increases with age, it is not a natural consequence of aging. This article proposes that dental X-rays can damage microglia telomeres - the structures at the end of chromosomes that determine how many times cells divide before they die - causing them to age prematurely. Degenerated microglia lose their neuroprotective properties, resulting in the formation of neurofibrillary tau tangles and consequently, the neuronal death that causes Alzheimer's dementia. The hypothesis that Alzheimer's is caused specifically by microglia telomere damage would explain the delay of one decade or longer between the presence of Alzheimer's brain pathology and symptoms; telomere damage would not cause any change in microglial function, it would just reset the countdown clock so that senescence and apoptosis occurred earlier than they would have without the environmental insult. Once microglia telomere damage causes premature aging and death, the adjacent neurons are deprived of the physical support, maintenance and nourishment they require to survive. This sequence of events would explain why therapies and vaccines that eliminate amyloid plaques have been unsuccessful in stopping dementia. Regardless of whether clearing plaques is beneficial or harmful - which remains a subject of debate - it does not address the failing microglia population. If microglia telomere damage is causing Alzheimer's disease, self-donated bone marrow or dental pulp stem cell transplants could give rise to new microglia populations that would maintain neuronal health while the original resident microglia population died.