MCP-1 and murine prion disease: separation of early behavioural dysfunction from overt clinical disease

The CXCL12/CXCR4/ACKR3 Response Axis in Chronic Neurodegenerative Disorders of the Central Nervous System: Therapeutic Target and Biomarker

There has been an increase in the incidence of chronic neurodegenerative disorders of the central nervous system, including Alzheimer's and Parkinson's diseases, over the recent years mostly due to the rise in the number of elderly individuals. In addition, various neurodegenerative disorders are related to imbalances in the CXCL12/CXCR4/ACKR3 response axis. Notably, the CXC Chemokine Ligand 12 (CXCL12) is essential for the development of the central nervous system. Moreover, the expression and distribution of CXCL12 and its receptors are associated with the aggravation or alleviation of symptoms of neurodegenerative disorders. Therefore, the current review sought to highlight the specific functions of CXCL12 and its receptors in various neurodegenerative disorders, in order to provide new insights for future research.

Neuroinflammatory Markers: Key Indicators in the Pathology of Neurodegenerative Diseases

Neuroinflammation, a protective response of the central nervous system (CNS), is associated with the pathogenesis of neurodegenerative diseases. The CNS is composed of neurons and glial cells consisting of microglia, oligodendrocytes, and astrocytes. Entry of any foreign pathogen activates the glial cells (astrocytes and microglia) and overactivation of these cells triggers the release of various neuroinflammatory markers (NMs), such as the tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-1β (IL-10), nitric oxide (NO), and cyclooxygenase-2 (COX-2), among others. Various studies have shown the role of neuroinflammatory markers in the occurrence, diagnosis, and treatment of neurodegenerative diseases. These markers also trigger the formation of various other factors responsible for causing several neuronal diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), multiple sclerosis (MS), ischemia, and several others. This comprehensive review aims to reveal the mechanism of neuroinflammatory markers (NMs), which could cause different neurodegenerative disorders. Important NMs may represent pathophysiologic processes leading to the generation of neurodegenerative diseases. In addition, various molecular alterations related to neurodegenerative diseases are discussed. Identifying these NMs may assist in the early diagnosis and detection of therapeutic targets for treating various neurodegenerative diseases.

Treatment of microglia with Anti-PrP monoclonal antibodies induces neuronal apoptosis in vitro

Previous reports highlighted the neurotoxic effects caused by some motif-specific anti-PrP^C antibodies in vivo and in vitro. In the current study, we investigated the detailed alterations of the proteome with liquid chromatography–mass spectrometry following direct application of anti-PrP^C antibodies on mouse neuroblastoma cells (N2a) and mouse primary neuronal (MPN) cells or by cross-linking microglial PrP^C with anti-PrP^C antibodies prior to co-culture with the N2a/MPN cells. Here, we identified 4 (3 upregulated and 1 downregulated) and 17 (11 upregulated and 6 downregulated) neuronal apoptosis-related proteins following treatment of the N2a and N11 cell lines respectively when compared with untreated cells. In contrast, we identified 1 (upregulated) and 4 (2 upregulated and 2 downregulated) neuronal apoptosis-related proteins following treatment of MPN cells and N11 when compared with untreated cells. Furthermore, we also identified 3 (2 upregulated and 1 downregulated) and 2 (1 upregulated and 1 downregulated) neuronal apoptosis-related related proteins following treatment of MPN cells and N11 when compared to treatment with an anti-PrP antibody that lacks binding specificity for mouse PrP. The apoptotic effect of the anti-PrP antibodies was confirmed with flow cytometry following labelling of Annexin V-FITC. The toxic effects of the anti-PrP antibodies was more intense when antibody-treated N11 were co-cultured with the N2a and the identified apoptosis proteome was shown to be part of the PrP^C-interactome. Our observations provide a new insight into the prominent role played by microglia in causing neurotoxic effects following treatment with anti-PrP^C antibodies and might be relevant to explain the antibody mediated toxicity observed in other related neurodegenerative diseases such as Alzheimer.

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Antibody cross-linking neuronal PrPC induces apoptosis.

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Antibody cross-linking microglial PrPC induces neuronal apoptosis.

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Different apoptotic pathways were triggered by specific anti-PrP antibody treatments.

Neuroinflammation in Prion Disease

Neuroinflammation, typically manifest as microglial activation and astrogliosis accompanied by transcriptomic alterations, represents a common hallmark of various neurodegenerative conditions including prion diseases. Microglia play an overall neuroprotective role in prion disease, whereas reactive astrocytes with aberrant phenotypes propagate prions and contribute to prion-induced neurodegeneration. The existence of heterogeneous subpopulations and dual functions of microglia and astrocytes in prion disease make them potential targets for therapeutic intervention. A variety of neuroinflammation-related molecules are involved in prion pathogenesis. Therapeutics targeting neuroinflammation represents a novel approach to combat prion disease. Deciphering neuroinflammation in prion disease will deepen our understanding of pathogenesis of other neurodegenerative disorders.

A CRHR1 antagonist prevents synaptic loss and memory deficits in a trauma-induced delirium-like syndrome

Older patients with severe physical trauma are at high risk of developing neuropsychiatric syndromes with global impairment of cognition, attention, and consciousness. We employed a thoracic trauma (TxT) mouse model and thoroughly analyzed age-dependent spatial and temporal posttraumatic alterations in the central nervous system. Up to 5 days after trauma, we observed a transient 50% decrease in the number of excitatory synapses specifically in hippocampal pyramidal neurons accompanied by alterations in attention and motor activity and disruption of contextual memory consolidation. In parallel, hippocampal corticotropin-releasing hormone (CRH) expression was highly upregulated, and brain-derived neurotrophic factor (BDNF) levels were significantly reduced. In vitro experiments revealed that CRH application induced neuronal autophagy with rapid lysosomal degradation of BDNF via the NF-κB pathway. The subsequent synaptic loss was rescued by BDNF as well as by specific NF-κB and CRH receptor 1 (CRHR1) antagonists. In vivo, the chronic application of a CRHR1 antagonist after TxT resulted in reversal of the observed histological, molecular, and behavioral alterations. The data suggest that neuropsychiatric syndromes (i.e., delirium) after peripheral trauma might be at least in part due to the activation of the hippocampal CRH/NF-κB/BDNF pathway, which results in a dramatic loss of synaptic contacts. The successful rescue by stress hormone receptor antagonists should encourage clinical trials focusing on trauma-induced delirium and/or other posttraumatic syndromes.

IP10, KC and M-CSF Are Remarkably Increased in the Brains from the Various Strains of Experimental Mice Infected with Different Scrapie Agents

Activation of inflammatory cells and upregulations of a number of cytokines in the central nervous system (CNS) of patients with prion diseases are frequently observed. To evaluate the potential changes of some brain cytokines that were rarely addressed during prion infection, the levels of 17 different cytokines in the brain homogenates of mice infected with different scrapie mouse-adapted agents were firstly screened with Luminex assay. Significant upregulations of interferon gamma-induced protein 10 (IP10), keratinocyte chemoattractant (KC) and macrophage colony stimulating factor (M-CSF) were frequently detected in the brain lysates of many strains of scrapie infected mice. The upregulations of those three cytokines in the brains of scrapie infected mice were further validated by the individual specific ELISA and immunohistochemical assay. Increased specific mRNAs of IP10, M-CSF and KC in the brains of scrapie infected mice were also detected by the individual specific qRT-PCRs and IP10-specific digital PCR. Dynamic analyses of the brain samples collected at different time points post infection revealed the time-dependent increases of those three cytokines, particularly IP10 during the incubation period of scrapie infection. In addition, we also found that the levels of IP10 in cerebral spinal fluid (CSF) of 45 sporadic Creutzfeldt-Jakob disease (sCJD) patients were slightly but significantly higher than those of the cases who were excluded the diagnosis of prion diseases. These data give us a better understanding of inflammatory reaction during prion infection and progression of prion disease.

Grading animal distress and side effects of therapies

In order to combine high-quality research with minimal harm to animals, a prospective severity assessment for animal experiments is legally required in many countries. In addition, an assessment of the evidence-based severity level might allow realistic harm-benefit analysis and the appraisal of refinement methods. However, only a few examples describe the distress of animals by simple, cost-efficient, and noninvasive methods. We, therefore, evaluated the severity of an orthotopic mouse model for pancreatic cancer using C57BL/6J mice when pursuing two different chemotherapies. We assessed fecal corticosterone metabolites, body weight, distress score, and burrowing, as well as nesting activity. Moreover, we established a multifactorial model using multivariate logistic regression to describe animal distress. This multifactorial analysis revealed that metformin + galloflavin treatment caused higher distress than metformin + α-cyano-4-hydroxycinnamate therapy. Similar results were obtained by using the best cutoff calculated by Youden's J index when using only single parameters, such as burrowing activity or fecal corticosterone metabolite concentration. Thus, the present study revealed that single readout parameters, as well as multivariate analysis, can help to assess the severity of animal experiments and detect side effects of therapies.

Genetic Factors in Mammalian Prion Diseases

Mammalian prion diseases are a group of neurodegenerative conditions caused by infection of the central nervous system with proteinaceous agents called prions, including sporadic, variant, and iatrogenic Creutzfeldt-Jakob disease; kuru; inherited prion disease; sheep scrapie; bovine spongiform encephalopathy; and chronic wasting disease. Prions are composed of misfolded and multimeric forms of the normal cellular prion protein (PrP). Prion diseases require host expression of the prion protein gene (PRNP) and a range of other cellular functions to support their propagation and toxicity. Inherited forms of prion disease are caused by mutation of PRNP, whereas acquired and sporadically occurring mammalian prion diseases are controlled by powerful genetic risk and modifying factors. Whereas some PrP amino acid variants cause the disease, others confer protection, dramatically altered incubation times, or changes in the clinical phenotype. Multiple mechanisms, including interference with homotypic protein interactions and the selection of the permissible prion strains in a host, play a role. Several non-PRNP factors have now been uncovered that provide insights into pathways of disease susceptibility or neurotoxicity.

Benefits of non-invasive methods compared to telemetry for distress analysis in a murine model of pancreatic cancer

Prospective severity assessment is legally required in many countries to ensure high-quality research along with high welfare standards for laboratory animals. Mice and rats, the most common laboratory species, are prey animals that usually suppress signs of pain and suffering. Therefore, highly sensitive readout parameters are necessary to adequately quantify distress. The present study compared the performance of different non-invasive methods in determining animal distress, such as measuring body weight, distress score, faecal corticosterone metabolites, burrowing, and nesting behaviour, with continuous monitoring of heart rate, body temperature and activity by telemetry. The distress caused by two surgical interventions was compared and the burden caused by tumour growth was described. Transmitter implantation caused higher distress than laparotomy plus carcinoma cell injection into the pancreas. Surprisingly, no significant increase in distress was observed during tumour growth. The receiver operating characteristic curve analysis revealed that some non-invasive distress-parameters, i.e., distress-score and burrowing activity, exhibited slightly better performance to quantify distress than the most suitable parameters measured by telemetry. Due to the high burden caused by the implantation of the telemetric device, the use of non-invasive methods to assess distress in laboratory animals after surgical interventions should be favoured in future studies.

Chronic neurodegeneration induces type I interferon synthesis via STING, shaping microglial phenotype and accelerating disease progression

Type I interferons (IFN‐I) are the principal antiviral molecules of the innate immune system and can be made by most cell types, including central nervous system cells. IFN‐I has been implicated in neuroinflammation during neurodegeneration, but its mechanism of induction and its consequences remain unclear. In the current study, we assessed expression of IFN‐I in murine prion disease (ME7) and examined the contribution of the IFN‐I receptor IFNAR1 to disease progression. The data indicate a robust IFNβ response, specifically in microglia, with evidence of IFN‐dependent genes in both microglia and astrocytes. This IFN‐I response was absent in stimulator of interferon genes (STING^−/−) mice. Microglia showed increased numbers and activated morphology independent of genotype, but transcriptional signatures indicated an IFNAR1‐dependent neuroinflammatory phenotype. Isolation of microglia and astrocytes demonstrated disease‐associated microglial induction of Tnfα, Tgfb1, and of phagolysosomal system transcripts including those for cathepsins, Cd68, C1qa, C3, and Trem2, which were diminished in IFNAR1 and STING deficient mice. Microglial increases in activated cathepsin D, and CD68 were significantly reduced in IFNAR1^−/− mice, particularly in white matter, and increases in COX‐1 expression, and prostaglandin synthesis were significantly mitigated. Disease progressed more slowly in IFNAR1^−/− mice, with diminished synaptic and neuronal loss and delayed onset of neurological signs and death but without effect on proteinase K‐resistant PrP levels. Therefore, STING‐dependent IFN‐I influences microglial phenotype and influences neurodegenerative progression despite occurring secondary to initial degenerative changes. These data expand our mechanistic understanding of IFN‐I induction and its impact on microglial function during chronic neurodegeneration.

Neuroinflammation, Microglia, and Cell-Association during Prion Disease

Prion disorders are transmissible diseases caused by a proteinaceous infectious agent that can infect the lymphatic and nervous systems. The clinical features of prion diseases can vary, but common hallmarks in the central nervous system (CNS) are deposition of abnormally folded protease-resistant prion protein (PrPres or PrPSc), astrogliosis, microgliosis, and neurodegeneration. Numerous proinflammatory effectors expressed by astrocytes and microglia are increased in the brain during prion infection, with many of them potentially damaging to neurons when chronically upregulated. Microglia are important first responders to foreign agents and damaged cells in the CNS, but these immune-like cells also serve many essential functions in the healthy CNS. Our current understanding is that microglia are beneficial during prion infection and critical to host defense against prion disease. Studies indicate that reduction of the microglial population accelerates disease and increases PrPSc burden in the CNS. Thus, microglia are unlikely to be a foci of prion propagation in the brain. In contrast, neurons and astrocytes are known to be involved in prion replication and spread. Moreover, certain astrocytes, such as A1 reactive astrocytes, have proven neurotoxic in other neurodegenerative diseases, and thus might also influence the progression of prion-associated neurodegeneration.

Systematic Assessment of Well-Being in Mice for Procedures Using General Anesthesia

In keeping with the 3R Principle (Replacement, Reduction, Refinement) developed by Russel and Burch, scientific research should use alternatives to animal experimentation whenever possible. When there is no alternative to animal experimentation, the total number of laboratory animals used should be the minimum needed to obtain valuable data. Moreover, appropriate refinement measures should be applied to minimize pain, suffering, and distress accompanying the experimental procedure. The categories used to classify the degree of pain, suffering, and distress are non-recovery, mild, moderate, or severe (EU Directive 2010/63). To determine which categories apply in individual cases, it is crucial to use scientifically sound tools. The well-being-assessment protocol presented here is designed for procedures during which general anesthesia is used. The protocol focuses on home cage activity, the Mouse Grimace Scale, and luxury behaviors such as burrowing and nest building behavior as indicators of well-being. It also uses the free exploratory paradigm for trait anxiety-related behavior. Fecal corticosterone metabolites as indicators of acute stress are measured over the 24-h post-anesthetic period. The protocol provides scientifically solid information on the well-being of mice following general anesthesia. Due to its simplicity, the protocol can easily be adapted and integrated in a planned study. Although it does not provide a scale to classify distress in categories according to the EU Directive 2010/63, it can help researchers estimate the degree of severity of a procedure using scientifically sound data. It provides a way to improve the assessment of well-being in a scientific, animal-centered manner.

Microglia in prion diseases

Prion diseases are a group of progressive and fatal neurodegenerative disorders characterized by deposition of scrapie prion protein (PrPSc) in the CNS. This deposition is accompanied by neuronal loss, spongiform change, astrogliosis, and conspicuous microglial activation. Here, we argue that microglia play an overall neuroprotective role in prion pathogenesis. Several microglia-related molecules, such as Toll-like receptors (TLRs), the complement system, cytokines, chemokines, inflammatory regulators, and phagocytosis mediators, are involved in prion pathogenesis. However, the molecular mechanisms underlying the microglial response to prion infection are largely unknown. Consequently, we lack a comprehensive understanding of the regulatory network of microglial activation. On the positive side, recent findings suggest that therapeutic strategies modulating microglial activation and function may have merit in prion disease. Moreover, studies on the role of microglia in prion disease could deepen our understanding of neuroinflammation in a broad range of neurodegenerative disorders.

The Role of Microglia in Prion Diseases: A Paradigm of Functional Diversity

Inflammation is a major component of neurodegenerative diseases. Microglia are the innate immune cells in the central nervous system (CNS). In the healthy brain, microglia contribute to tissue homeostasis and regulation of synaptic plasticity. Under disease conditions, they play a key role in the development and maintenance of the neuroinflammatory response, by showing enhanced proliferation and activation. Prion diseases are progressive chronic neurodegenerative disorders associated with the accumulation of the scrapie prion protein PrP^Sc, a misfolded conformer of the cellular prion protein PrP^C. This review article provides the current knowledge on the role of microglia in the pathogenesis of prion disease. A large body of evidence shows that microglia can trigger neurotoxic pathways contributing to progressive degeneration. Yet, microglia are also crucial for controlling inflammatory, repair and regenerative processes. This dual role of microglia is regulated by multiple pathways and evidences the ability of these cells to polarize into distinct phenotypes with characteristic functions. The awareness that the neuroinflammatory response is inextricably involved in producing tissue damage as well as repair in neurodegenerative disorders, opens new perspectives for the modulation of the immune system. A better understanding of this complex process will be essential for developing effective therapies for neurodegenerative diseases, in order to improve the quality of life of patients and mitigating the personal, economic and social consequences derived from these diseases.

Prion Diseases

Prion diseases are a group of invariably fatal and transmissible neurodegenerative disorders that are associated with the misfolding of the normal cellular prion protein, with the misfolded conformers constituting an infectious unit referred to as a "prion". Prions can spread within an affected organism by directly propagating this misfolding within and between cells and can transmit disease between animals of the same and different species. Prion diseases have a range of clinical phenotypes in humans and animals, with a principle determinant of this attributed to different conformations of the misfolded protein, referred to as prion strains. This chapter will describe the different clinical manifestations of prion diseases, the evidence that these diseases can be transmitted by an infectious protein and how the misfolding of this protein causes disease.

Distribution of Misfolded Prion Protein Seeding Activity Alone Does Not Predict Regions of Neurodegeneration

Protein misfolding is common across many neurodegenerative diseases, with misfolded proteins acting as seeds for "prion-like" conversion of normally folded protein to abnormal conformations. A central hypothesis is that misfolded protein accumulation, spread, and distribution are restricted to specific neuronal populations of the central nervous system and thus predict regions of neurodegeneration. We examined this hypothesis using a highly sensitive assay system for detection of misfolded protein seeds in a murine model of prion disease. Misfolded prion protein (PrP) seeds were observed widespread throughout the brain, accumulating in all brain regions examined irrespective of neurodegeneration. Importantly, neither time of exposure nor amount of misfolded protein seeds present determined regions of neurodegeneration. We further demonstrate two distinct microglia responses in prion-infected brains: a novel homeostatic response in all regions and an innate immune response restricted to sites of neurodegeneration. Therefore, accumulation of misfolded prion protein alone does not define targeting of neurodegeneration, which instead results only when misfolded prion protein accompanies a specific innate immune response.

The distribution of misfolded prion protein seeding activity alone does not predict regions of neurodegeneration in prion disease; rather, a complex microglial response appears to determine selective vulnerability and provides new strategies for therapy.

Normal brain function requires tight regulation of protein folding; when this goes wrong, proteins can fold into abnormal conformations, which have severe impacts on brain performance, leading to conditions like dementia. Previous studies show that abnormally folded proteins are found in restricted parts of the brain, and neuronal cells in these specific brain regions have been shown to undergo degeneration. Recent technological advances have enhanced the detection of abnormally folded prion protein (PrP) during disease; we used these technologies to test whether distribution of abnormally folded proteins is indeed restricted to regions of the brain undergoing degeneration. Surprisingly, we observed abnormally folded proteins throughout the brain, demonstrating that these proteins can accumulate in parts of the brain that do not show degeneration. Thus, the distribution of abnormally folded protein, by itself, is not sufficient for neuronal degeneration. In addition, we found that microglia (one of the nonneuronal cell types in the brain) change their response during prion disease in two different ways; one response is associated with resilient brain regions, and the second, an inflammatory response is associated with regions susceptible to degeneration. Thus, the microglial response appears to be important in determining the outcome of disease.

Insights into Mechanisms of Chronic Neurodegeneration

Chronic neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and prion diseases are characterised by the accumulation of abnormal conformers of a host encoded protein in the central nervous system. The process leading to neurodegeneration is still poorly defined and thus development of early intervention strategies is challenging. Unique amongst these diseases are Transmissible Spongiform Encephalopathies (TSEs) or prion diseases, which have the ability to transmit between individuals. The infectious nature of these diseases has permitted in vivo and in vitro modelling of the time course of the disease process in a highly reproducible manner, thus early events can be defined. Recent evidence has demonstrated that the cell-to-cell spread of protein aggregates by a “prion-like mechanism” is common among the protein misfolding diseases. Thus, the TSE models may provide insights into disease mechanisms and testable hypotheses for disease intervention, applicable to a number of these chronic neurodegenerative diseases.

Amyloid-β Activates Microglia and Regulates Protein Expression in a Manner Similar to Prions

Prions are the only convincingly demonstrated proteinaceous infectious particle, yet recent studies find that amyloid-β peptide (Aβ) aggregates are capable of self-propagation, which induces amyloidosis pathology in Alzheimer's disease (AD) model mice that is similar to the self-propagation phenomenon of prions in neurons. Gliosis is a common hallmark of AD and prion diseases, in which activated microglia accumulate around abnormal protein deposits. Analyses of the characteristics of activated microglia induced by Aβ in comparison with those induced by prions will provide new insight into the pathogenesis of AD. Therefore, we compared the characteristics of BV-2 cells (model microglia) activated by Aβ fibrillar peptides (Aβ1-42) and prions (PrP106-126). Aβ1-42 and PrP106-126, as well as the supernatants of the media collected from BV-2 cells cocultured with Aβ1-42 and PrP106-126, were potent activators of BV-2 microglial activity, but the chemotaxis index (CI) induced by Aβ1-42 was significantly higher than that induced by PrP106-126 at each concentration. Aβ1-42 and PrP106-126 increased the proliferation index (PI) and upregulated monocyte chemoattractant protein-1 (MCP-1) and transforming growth factor beta 1 (TGF-β1) expression after 12 h of exposure. Our results show that Aβ activates microglia and regulates microglial protein expression in a manner similar to prions and, thus, provide new insight into the pathogenesis of AD.

The ischemic environment drives microglia and macrophage function

Cells of myeloid origin, such as microglia and macrophages, act at the crossroads of several inflammatory mechanisms during pathophysiology. Besides pro-inflammatory activity (M1 polarization), myeloid cells acquire protective functions (M2) and participate in the neuroprotective innate mechanisms after brain injury. Experimental research is making considerable efforts to understand the rules that regulate the balance between toxic and protective brain innate immunity. Environmental changes affect microglia/macrophage functions. Hypoxia can affect myeloid cell distribution, activity, and phenotype. With their intrinsic differences, microglia and macrophages respond differently to hypoxia, the former depending on ATP to activate and the latter switching to anaerobic metabolism and adapting to hypoxia. Myeloid cell functions include homeostasis control, damage-sensing activity, chemotaxis, and phagocytosis, all distinctive features of these cells. Specific markers and morphologies enable to recognize each functional state. To ensure homeostasis and activate when needed, microglia/macrophage physiology is finely tuned. Microglia are controlled by several neuron-derived components, including contact-dependent inhibitory signals and soluble molecules. Changes in this control can cause chronic activation or priming with specific functional consequences. Strategies, such as stem cell treatment, may enhance microglia protective polarization. This review presents data from the literature that has greatly advanced our understanding of myeloid cell action in brain injury. We discuss the selective responses of microglia and macrophages to hypoxia after stroke and review relevant markers with the aim of defining the different subpopulations of myeloid cells that are recruited to the injured site. We also cover the functional consequences of chronically active microglia and review pivotal works on microglia regulation that offer new therapeutic possibilities for acute brain injury.

Differential role of CCR2 in the dynamics of microglia and perivascular macrophages during prion disease

The expansion of the microglial population is one of the hallmarks of numerous brain disorders. The addition of circulating progenitors to the pool of brain macrophages can contribute to the progression of brain disease and needs to be precisely defined to better understand the evolution of the glial and inflammatory reactions in the brain. We have analyzed the degree of infiltration/recruitment of circulating monocytes to the microglial pool, in a prion disease model of chronic neurodegeneration. Our results indicate a minimal/absent level of CCR2-dependent recruitment of circulating monocytes, local proliferation of microglia is the main driving force maintaining the amplification of the population. A deficiency in CCR2, and thus the absence of recruitment of circulating monocytes, does not impact microglial dynamics, the inflammatory profile or the temporal behavioral course of prion disease. However, the lack of CCR2 has unexpected effects including the failure to recruit perivascular macrophages in diseased but not healthy CNS and a small reduction in microglia proliferation. These data define the composition of the CNS-resident macrophage populations in prion disease and will help to understand the dynamics of the CNS innate immune response during chronic neurodegeneration.

Burrowing and nest building behavior as indicators of well-being in mice

The assessment of pain, distress and suffering, as well as evaluation of the efficacy of stress-reduction strategies, is crucial in animal experimentation but can be challenging in laboratory mice. Nest building and burrowing performance, observed in the home cage, have proved to be valuable and easy-to-use tools to assess brain damage or malfunction as well as neurodegenerative diseases. Both behaviors are used as parameters in models of psychiatric disorders or to monitor sickness behavior following infection. Their use has been proposed in more realistic and clinically relevant preclinical models of disease, and reduction of these behaviors seems to be especially useful as an early sign of dysfunction and to monitor disease progression. Finally, both behaviors are reduced by pain and stress. Therefore, in combination with specific disease markers, changes in nest building and burrowing performance may help provide a global picture of a mouse's state, and thus aid monitoring to ensure well-being in animal experimentation.

The immunobiology of prion diseases

Individuals infected with prions succumb to brain damage, and prion infections continue to be inexorably lethal. However, many crucial steps in prion pathogenesis occur in lymphatic organs and precede invasion of the central nervous system. In the past two decades, a great deal has been learnt concerning the cellular and molecular mechanisms of prion lymphoinvasion. These properties are diagnostically useful and have, for example, facilitated preclinical diagnosis of variant Creutzfeldt-Jakob disease in the tonsils. Moreover, the early colonization of lymphoid organs can be exploited for post-exposure prophylaxis of prion infections. As stromal cells of lymphoid organs are crucial for peripheral prion infection, the dedifferentiation of these cells offers a powerful means of hindering prion spread in infected individuals. In this Review, we discuss the current knowledge of the immunobiology of prions with an emphasis on how basic discoveries might enable translational strategies.

Ceramide 1-phosphate induces macrophage chemoattractant protein-1 release: involvement in ceramide 1-phosphate-stimulated cell migration

The bioactive sphingolipid ceramide 1-phosphate (C1P) is implicated in inflammatory responses and was recently shown to promote cell migration. However, the mechanisms involved in these actions are poorly described. Using J774A.1 macrophages, we have now discovered a new biological activity of C1P: stimulation of monocyte chemoattractant protein-1 (MCP-1) release. This novel effect of C1P was pertussis toxin (PTX) sensitive, suggesting the intervention of Gi protein-coupled receptors. Treatment of the macrophages with C1P caused activation of the phosphatidylinositol 3-kinase (PI3K)/Akt, mitogen-activated protein kinase kinase (MEK)/extracellularly regulated kinases (ERK), and p38 pathways. Inhibition of these kinases using selective inhibitors or specific siRNA blocked the stimulation of MCP-1 release by C1P. C1P stimulated nuclear factor-κB activity, and blockade of this transcription factor also resulted in complete inhibition of MCP-1 release. Also, C1P stimulated MCP-1 release and cell migration in human THP-1 monocytes and 3T3-L1 preadipocytes. A key observation was that sequestration of MCP-1 with a neutralizing antibody or treatment with MCP-1 siRNA abolished C1P-stimulated cell migration. Also, inhibition of the pathways involved in C1P-stimulated MCP-1 release completely blocked the stimulation of cell migration by C1P. It can be concluded that C1P promotes MCP-1 release in different cell types and that this chemokine is a major mediator of C1P-stimulated cell migration. The PI3K/Akt, MEK/ERK, and p38 pathways are important downstream effectors in this action.

Prion disease and the innate immune system

Prion diseases or transmissible spongiform encephalopathies are a unique category of infectious protein-misfolding neurodegenerative disorders. Hypothesized to be caused by misfolding of the cellular prion protein these disorders possess an infectious quality that thrives in immune-competent hosts. While much has been discovered about the routing and critical components involved in the peripheral pathogenesis of these agents there are still many aspects to be discovered. Research into this area has been extensive as it represents a major target for therapeutic intervention within this group of diseases. The main focus of pathological damage in these diseases occurs within the central nervous system. Cells of the innate immune system have been proven to be critical players in the initial pathogenesis of prion disease, and may have a role in the pathological progression of disease. Understanding how prions interact with the host innate immune system may provide us with natural pathways and mechanisms to combat these diseases prior to their neuroinvasive stage. We present here a review of the current knowledge regarding the role of the innate immune system in prion pathogenesis.

Early cytokine elevation, PrPres deposition, and gliosis in mouse scrapie: no effect on disease by deletion of cytokine genes IL-12p40 and IL-12p35

Neurodegenerative diseases are typically associated with an activation of glia and an increased level of cytokines. In our previous studies of prion disease, the cytokine response in the brains of clinically sick scrapie-infected mice was restricted to a small group of cytokines, of which IL-12p40, CCL2, and CXCL10 were present at the highest levels. The goal of our current research was to determine the relationship between cytokine responses, gliosis, and neuropathology during prion disease. Here, in time course studies of C57BL/10 mice intracerebrally inoculated with 22L scrapie, abnormal protease-resistant prion protein (PrPres), astrogliosis, and microgliosis were first detected at 40 days after intracerebral scrapie inoculation. In cytokine studies, IL-12p40 was first elevated by 60 days; CCL3, IL-1β, and CXCL1 were elevated by 80 days; and CCL2 and CCL5 were elevated by 115 days. IL-12p40 showed the most extensive increase throughout disease and was 30-fold above control levels at the terminal stage. Because of the early onset and dramatic elevation of IL-12p40 during scrapie, we investigated whether IL-12p40 contributed to the development of prion disease neuropathogenesis by using three different scrapie strains (22L, RML, 79A) to infect knockout mice in which the gene encoding IL-12p40 was deleted. We also studied knockout mice lacking IL-12p35, which combines with IL-12p40 to form active IL-12 heterodimers. In all instances, knockout mice did not differ from control mice in survival time, clinical tempo, or levels of spongiosis, gliosis, or PrPres in the brain. Thus, neither IL-12p40 nor IL-12p35 molecules were required for prion disease-associated neurodegeneration or neuroinflammation.

Microglia and neurodegeneration: the role of systemic inflammation

It is well accepted that CNS inflammation has a role in the progression of chronic neurodegenerative disease, although the mechanisms through which this occurs are still unclear. The inflammatory response during most chronic neurodegenerative disease is dominated by the microglia and mechanisms by which these cells contribute to neuronal damage and degeneration are the subject of intense study. More recently it has emerged that systemic inflammation has a significant role to play in the progression of these diseases. Well-described adaptive pathways exist to transduce systemic inflammatory signals to the brain, but activation of these pathways appears to be deleterious to the brain if the acute insult is sufficiently robust, as in severe sepsis, or sufficiently prolonged, as in repeated stimulation with robust doses of inflammogens such as lipopolysaccharide (LPS). Significantly, moderate doses of inflammogens produce new pathology in the brain and exacerbate or accelerate features of disease when superimposed upon existing pathology or in the context of genetic predisposition. It is now apparent in multiple chronic disease states, and in ageing, that microglia are primed by prior pathology, or by genetic predisposition, to respond more vigorously to subsequent inflammatory stimulation, thus transforming an adaptive CNS inflammatory response to systemic inflammation, into one that has deleterious consequences for the individual. In this review, the preclinical and clinical evidence supporting a significant role for systemic inflammation in chronic neurodegenerative diseases will be discussed. Mechanisms by which microglia might effect neuronal damage and dysfunction, as a consequence of systemic stimulation, will be highlighted.

5, 8, 11, 14-eicosatetraynoic acid suppresses CCL2/MCP-1 expression in IFN-γ-stimulated astrocytes by increasing MAPK phosphatase-1 mRNA stability

Background

The peroxisome proliferator-activated receptor (PPAR)-α activator, 5,8,11,14-eicosatetraynoic acid (ETYA), is an arachidonic acid analog. It is reported to inhibit up-regulation of pro-inflammatory genes; however, its underlying mechanism of action is largely unknown. In the present study, we focused on the inhibitory action of ETYA on the expression of the chemokine, CCL2/MCP-1, which plays a key role in the initiation and progression of inflammation.

Methods

To determine the effect of ETYA, primary cultured rat astrocytes and microglia were stimulated with IFN-γ in the presence of ETYA and then, expression of CCL2/MCP-1 and MAPK phosphatase (MKP-1) were determined using RT-PCR and ELISA. MKP-1 mRNA stability was evaluated by treating actinomycin D. The effect of MKP-1 and human antigen R (HuR) was analyzed by using specific siRNA transfection system. The localization of HuR was analyzed by immunocytochemistry and subcellular fractionation experiment.

Results

We found that ETYA suppressed CCL2/MCP-1 transcription and secretion of CCL2/MCP-1 protein through up-regulation of MKP-1mRNA levels, resulting in suppression of c-Jun N-terminal kinase (JNK) phosphorylation and activator protein 1 (AP1) activity in IFN-γ-stimulated brain glial cells. Moreover, these effects of ETYA were independent of PPAR-α. Experiments using actinomycin D revealed that the ETYA-induced increase in MKP-1 mRNA levels reflected an increase in transcript stability. Knockdown experiments using small interfering RNA demonstrated that this increase in MKP-1 mRNA stability depended on HuR, an RNA-binding protein known to promote enhanced mRNA stability. Furthermore, ETYA-induced, HuR-mediated mRNA stabilization resulted from HuR-MKP-1 nucleocytoplasmic translocation, which served to protect MKP-1 mRNA from the mRNA degradation machinery.

Conclusion

ETYA induces MKP-1 through HuR at the post-transcriptional level in a receptor-independent manner. The mechanism revealed here suggests eicosanoids as potential therapeutic modulators of inflammation that act through a novel target.

Early behavioral changes and quantitative analysis of neuropathological features in murine prion disease: stereological analysis in the albino Swiss mice model

Behavioral and neuropathological changes have been widely investigated in murine prion disease but stereological based unbiased estimates of key neuropathological features have not been carried out. After injections of ME7 infected (ME7) or normal brain homogenates (NBH) into dorsal CA1 of albino Swiss mice and C57BL6, we assessed behavioral changes on hippocampal-dependent tasks. We also estimated by optical fractionator at 15 and 18 weeks post-injections (w.p.i.) the total number of neurons, reactive astrocytes, activated microglia and perineuronal nets (PN) in the polymorphic layer of dentate gyrus (PolDG), CA1 and septum in albino Swiss mice. On average, early behavioral changes in albino Swiss mice start four weeks later than in C57BL6. Cluster and discriminant analysis of behavioral data in albino Swiss mice revealed that four of nine subjects start to change their behavior at 12 w.p.i. and reach terminal stage at 22 w.p.i and the remaining subjects start at 22 w.p.i. and reach terminal stage at 26 w.p.i. Biotinylated dextran-amine BDA-tracer experiments in mossy fiber pathway confirmed axonal degeneration, and stereological data showed that early astrocytosis, microgliosis and reduction in the perineuronal nets are independent of a change in the number of neuronal cell bodies. Statistical analysis revealed that the septal region had greater levels of neuroinflammation and extracellular matrix damage than CA1. This stereological and multivariate analysis at early stages of disease in an outbred model of prion disease provided new insights connecting behavioral changes and neuroinflammation and seems to be important to understand the mechanisms of prion disease progression.

Exacerbation of CNS inflammation and neurodegeneration by systemic LPS treatment is independent of circulating IL-1β and IL-6

Background

Chronic neurodegeneration comprises an inflammatory response but its contribution to the progression of disease remains unclear. We have previously shown that microglial cells are primed by chronic neurodegeneration, induced by the ME7 strain of prion disease, to synthesize limited pro-inflammatory cytokines but to produce exaggerated responses to subsequent systemic inflammatory insults. The consequences of this primed response include exaggerated hypothermic and sickness behavioural responses, acute neuronal death and accelerated progression of disease. Here we investigated whether inhibition of systemic cytokine synthesis using the anti-inflammatory steroid dexamethasone-21-phosphate was sufficient to block any or all of these responses.

Methods

ME7 animals, at 18-19 weeks post-inoculation, were challenged with LPS (500 μg/kg) in the presence or absence of dexamethasone-21-phosphate (2 mg/kg) and effects on core-body temperature and systemic and CNS cytokine production and apoptosis were examined.

Results

LPS induced hypothermia and decreased exploratory activity. Dexamethasone-21-phosphate prevented this hypothermia, markedly suppressed systemic IL-1β and IL-6 secretion but did not prevent decreased exploration. Furthermore, robust transcription of cytokine mRNA occurred in the hippocampus of both ME7 and NBH (normal brain homogenate) control animals despite the effective blocking of systemic cytokine synthesis. Microglia primed by neurodegeneration were not blocked from the robust synthesis of IL-1β protein and endothelial COX-2 was also robustly synthesized. We injected biotinylated LPS at 100 μg/kg and even at this lower dose this could be detected in blood plasma. Apoptosis was acutely induced by LPS, despite the inhibition of the systemic cytokine response.

Conclusions

These data suggest that LPS can directly activate the brain endothelium even at relatively low doses, obviating the need for systemic cytokine stimulation to transduce systemic inflammatory signals into the brain or to exacerbate existing pathology.

Genetics of prion disease

Prion diseases or transmissible spongiform encephalopathies (TSEs) are neurodegenerative disorders of humans and animals for which there are no effective treatments or cure. They include Creutzfeldt-Jakob disease (CJD) in humans and sheep scrapie, bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD) in cervids. The prion protein (PrP) is central to the disease process. An abnormal form of PrP is generally considered to be the sole or principal component of the infectious agent and a multimeric isomer (PrP(Sc)) is deposited in affected brains. Inherited prion diseases are caused by over 30 mutations in the prion protein gene (PRNP) and common polymorphisms can have a considerable affect on susceptibility and phenotype. Susceptibility and incubation time are also partly determined by other (non-PRNP) genetic modifiers. Understanding how these other genes modify prion diseases may lead to insights into biological mechanisms. Several approaches including human genome wide association studies (GWAS), mouse mapping and differential expression studies are now revealing some of these genes which include RARB (retinoic acid receptor beta), the E3 ubiquitin ligase HECTD2 and SPRN (Shadoo, shadow of prion protein gene).

CCL2 modulates cytokine production in cultured mouse astrocytes

Background

The chemokine CCL2 (also known as monocyte chemoattractant protein-1, or MCP-1) is upregulated in patients and rodent models of traumatic brain injury (TBI), contributing to post-traumatic neuroinflammation and degeneration by directing the infiltration of blood-derived macrophages into the injured brain. Our laboratory has previously reported that Ccl2-/- mice show reduced macrophage accumulation and tissue damage, corresponding to improved motor recovery, following experimental TBI. Surprisingly, Ccl2-deficient mice also exhibited delayed but exacerbated secretion of key proinflammatory cytokines in the injured cortex. Thus we sought to further characterise CCL2's potential ability to modulate immunoactivation of astrocytes in vitro.

Methods

Primary astrocytes were isolated from neonatal wild-type and Ccl2-deficient mice. Established astrocyte cultures were stimulated with various concentrations of lipopolysaccharide (LPS) and interleukin (IL)-1β for up to 24 hours. Separate experiments involved pre-incubation with mouse recombinant (r)CCL2 prior to IL-1β stimulation in wild-type cells. Following stimulation, cytokine secretion was measured in culture supernatant by immunoassays, whilst cytokine gene expression was quantified by real-time reverse transcriptase polymerase chain reaction.

Results

LPS (0.1-100 μg/ml; 8 h) induced the significantly greater secretion of five key cytokines and chemokines in Ccl2-/- astrocytes compared to wild-type cells. Consistently, IL-6 mRNA levels were 2-fold higher in Ccl2-deficient cells. IL-1β (10 and 50 ng/ml; 2-24 h) also resulted in exacerbated IL-6 production from Ccl2-/- cultures. Despite this, treatment of wild-type cultures with rCCL2 alone (50-500 ng/ml) did not induce cytokine/chemokine production by astrocytes. However, pre-incubation of wild-type astrocytes with rCCL2 (250 ng/ml, 12 h) prior to stimulation with IL-1β (10 ng/ml, 8 h) significantly reduced IL-6 protein and gene expression.

Conclusions

Our data indicate that astrocytes are likely responsible for the exacerbated cytokine response seen in vivo post-injury in the absence of CCL2. Furthermore, evidence that CCL2 inhibits cytokine production by astrocytes following IL-1β stimulation, suggests a novel, immunomodulatory role for this chemokine in acute neuroinflammation. Further investigation is required to determine the physiological relevance of this phenomenon, which may have implications for therapeutics targeting CCL2-mediated leukocyte infiltration following TBI.

Contribution of systemic inflammation to chronic neurodegeneration

Systemic infection or inflammation gives rise to signals that communicate with the brain and leads to changes in metabolism and behaviour collectively known as sickness behaviour. In healthy young individuals, these changes are normally transient with no long-term consequences. The microglia are involved in the immune to brain signalling pathways. In the aged or diseased brain, the microglia have a primed phenotype as a consequence of changes in their local microenvironment. Systemic inflammation impacts on these primed microglia and switches them from a relatively benign to an aggressive phenotype with the enhanced synthesis of pro-inflammatory mediators. Recent evidence suggests that systemic inflammation contributes to the exacerbation of acute symptoms of chronic neurodegenerative disease and may accelerate disease progression. The normal homeostatic role that microglia play in signalling about systemic infections and inflammation becomes maladaptive in the aged and diseased brain and this offers a route to therapeutic intervention. Prompt treatment of systemic inflammation or blockade of signalling pathways from the periphery to the brain may help to slow neurodegeneration and improve the quality of life for individuals suffering from chronic neurodegenerative disease.

Neonatal bacterial infection alters fever to live and simulated infections in adulthood

Fever is a critical component of the host immune response to infection. An emerging literature demonstrates that experience with infectious organisms early in life, during the perinatal period, may permanently program immune responses later in life, including fever. We explored the influence of neonatal infection with Escherichia coli on fever responses to lipopolysaccharide (LPS) and E. coli in adulthood. Fever to a low dose of LPS in adulthood did not significantly differ as a consequence of early-life infection. Eight days after the LPS injection, the same group of rats received a high dose of live E. coli. This time, neonatally infected rats exhibited a markedly longer fever than controls. In a subsequent experiment, fever to a single high dose of E. coli without prior LPS in adulthood did not differ by group, suggesting that the previous difference was a lack of tolerance to the dual challenges in early-infected rats. Finally, both groups exhibited decreased tumor necrosis factor (TNF)-alpha and toll-like-receptor (TLR) 4 production to dual LPS challenges in isolated splenocytes, whereas only rats infected as neonates exhibited increased cyclooxygenase-2 within the hypothalamus in response to adult infection, suggesting that early infection-induced changes in fever regulation may involve a change in central mechanisms. Taken together, these data indicate that early-life infection is associated with marked changes in host temperature regulation in adulthood.

The effect of non-steroidal anti-inflammatory agents on behavioural changes and cytokine production following systemic inflammation: Implications for a role of COX-1

Systemic inflammation gives rise to metabolic and behavioural changes, largely mediated by pro-inflammatory cytokines and prostaglandin production (PGE(2)) at the blood-brain barrier. Despite numerous studies, the exact biological pathways that give rise to these changes remains elusive. This study investigated the mechanisms underlying immune-to-brain communication following systemic inflammation using various anti-inflammatory agents. Mice were pre-treated with selective cyclo-oxygenase (COX) inhibitors, thromboxane synthase inhibitors or dexamethasone, followed by intra-peritoneal injection of lipopolysaccharide (LPS). Changes in body temperature, open-field activity, and burrowing were assessed and mRNA and/or protein levels of inflammatory mediators measured in serum and brain. LPS-induced systemic inflammation resulted in behavioural changes and increased production of IL-6, IL-1beta and TNF-alpha, as well as PGE(2) in serum and brain. Indomethacin and ibuprofen reversed the effect of LPS on behaviour without changing peripheral or central IL-6, IL-1beta and TNF-alpha mRNA levels. In contrast, dexamethasone did not alter LPS-induced behavioural changes, despite complete inhibition of cytokine production. A selective COX-1 inhibitor, piroxicam, but not the selective COX-2 inhibitor, nimesulide, reversed the LPS-induced behavioural changes without affecting IL-6, IL-1beta and TNF-alpha protein expression levels in the periphery or mRNA levels in the hippocampus. Our results suggest that the acute LPS-induced changes in burrowing and open-field activity depend on COX-1. We further show that COX-1 is not responsible for the induction of brain IL-6, IL-1beta and TNF-alpha synthesis or LPS-induced hypothermia. Our results may have implications for novel therapeutic strategies to treat or prevent neurological diseases with an inflammatory component.

Context dependent neuroprotective properties of prion protein (PrP)

Although it has been known for more than twenty years that an aberrant conformation of the prion protein (PrP) is the causative agent in prion diseases, the role of PrP in normal biology is undetermined. Numerous studies have suggested a protective function for PrP, including protection from ischemic and excitotoxic lesions and several apoptotic insults. On the other hand, many observations have suggested the contrary, linking changes in PrP localization or domain structure--independent of infectious prion conformation--to severe neuronal damage. Surprisingly, a recent report suggests that PrP is a receptor for toxic oligomeric species of a-beta, a pathogenic fragment of the amyloid precursor protein, and likely contributes to disease pathogenesis of Alzheimer disease. We sought to access the role of PrP in diverse neurological disorders. First, we confirmed that PrP confers protection against ischemic damage using an acute stroke model, a well characterized association. After ischemic insult, PrP knockouts had dramatically increased infarct volumes and decreased behavioral performance compared to controls. To examine the potential of PrP's neuroprotective or neurotoxic properties in the context of other pathologies, we deleted PrP from several transgenic models of neurodegenerative disease. Deletion of PrP did not substantially alter the disease phenotypes of mouse models of Parkinson disease or tauopathy. Deletion of PrP in one of two Huntington disease models tested, R6/2, modestly slowed motor deterioration as measured on an accelerating rotarod but otherwise did not alter other major features of the disease. Finally, transgenic overexpression of PrP did not exacerbate the Huntington motor phenotype. These results suggest that PrP has a context-dependent neuroprotective function and does not broadly contribute to the disease models tested herein.

Analysis of protein levels of 24 cytokines in scrapie agent-infected brain and glial cell cultures from mice differing in prion protein expression levels

Activation of microglia and astroglia is seen in many neurodegenerative diseases including prion diseases. Activated glial cells produce cytokines as a protective response against certain pathogens and as part of the host inflammatory response to brain damage. In addition, cytokines might also exacerbate tissue damage initiated by other processes. In the present work using multiplex assays to analyze protein levels of 24 cytokines in scrapie agent-infected C57BL/10 mouse brains, we observed elevation of CCL2, CCL5, CXCL1, CXCL10, granulocyte-macrophage colony-stimulating factor (GM-CSF), gamma interferon (IFN-gamma), interleukin 1alpha (IL-1alpha), IL-1beta, IL-6, and IL-12p40. Scrapie agent-infected wild-type mice and transgenic mice expressing anchorless prion protein (PrP) had similar cytokine responses in spite of extensive differences in neuropathology. Therefore, these responses may be primarily a reaction to brain damage induced by prion infection rather than specific inducers of a particular type of pathology. To study the roles of astroglia and microglia in these cytokine responses, primary glial cultures were exposed to scrapie agent-infected brain homogenates. Microglia produced only IL-12p40 and CXCL10, whereas astroglia produced these cytokines plus CCL2, CCL3, CCL5, CXCL1, G-CSF, IL-1beta, IL-6, IL-12p70, and IL-13. Glial cytokine responses from wild-type mice and transgenic mice expressing anchorless PrP differed only slightly, but glia from PrP-null mice produced only IL-12p40, indicating that PrP expression was required for scrapie agent induction of other cytokines detected. The difference in cytokine response between microglia and astroglia correlated with 20-fold-higher levels of PrP expression in astroglia versus microglia, suggesting that high-level PrP expression on astroglia might be important for induction of certain cytokines.

The multifaceted profile of activated microglia

Although relatively neglected previously, research efforts in the past decade or so have identified a pivotal role for glial cells in regulating neuronal function. Particular emphasis has been placed on increasing our understanding of the function of microglia because a change from the ramified "resting" state of these cells has been associated with the pathogenesis of several neurodegenerative diseases, notably Alzheimer's disease. However, it is not clear whether activation of microglia and the associated inflammatory changes play a part in triggering disease processes or whether cell activation is a response to the early changes associated with the disease. In either case, the possibility exists that modulation of microglial activation may be beneficial in some circumstances, underlying the need to pursue research in this area. The original morphological categorization of microglia by Del Rio Hortega into ameboid, ramified, and intermediate forms, must now be elaborated to encompass a functional description. The evidence which has been generated recently suggests that microglia are probably never in a "resting" state and that several intermediate transitional states, based on function and morphology, probably exist. A more complete understanding of these states and the triggers which lead to a change from one to another state, and the factors which modulate the molecular switch that determines the persistence of the "activated" state remain to be identified.

Systemic inflammation induces acute behavioral and cognitive changes and accelerates neurodegenerative disease

BACKGROUND

Chronic neurodegeneration results in microglial activation, but the contribution of inflammation to the progress of neurodegeneration remains unclear. We have shown that microglia express low levels of proinflammatory cytokines during chronic neurodegeneration but are "primed" to produce a more proinflammatory profile after systemic challenge with bacterial endotoxin (lipopolysaccharide [LPS]).

METHODS

Here, we investigated whether intraperitoneal (IP) challenge with LPS, to mimic systemic infection, in the early stages of prion disease can 1) produce exaggerated acute behavioral (n = 9) and central nervous system (CNS) inflammatory (n = 4) responses in diseased animals compared with control animals, and 2) whether a single LPS challenge can accelerate disease progression (n = 34-35).

RESULTS

Injection of LPS (100 microg/kg), at 12 weeks postinoculation (PI), resulted in heightened CNS interleukin-1 beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), and interferon-beta (IFN-beta) transcription and microglial IL-1beta translation in prion-diseased animals relative to control animals. This inflammation caused exaggerated impairments in burrowing and locomotor activity, and induced hypothermia and cognitive changes in prion-diseased animals that were absent in LPS-treated control animals. At 15 weeks PI, LPS (500 microg/kg) acutely impaired motor coordination and muscle strength in prion-diseased but not in control animals. After recovery, these animals also showed earlier onset of disease-associated impairments on these parameters.

CONCLUSIONS

These data demonstrate that transient systemic inflammation superimposed on neurodegenerative disease acutely exacerbates cognitive and motor symptoms of disease and accelerates disease progression. These deleterious effects of systemic inflammation have implications for the treatment of chronic neurodegeneration and associated delirium.

Accelerated prion replication in, but prolonged survival times of, prion-infected CXCR3-/- mice

Prion diseases have a significant inflammatory component. Glia activation, which is associated with increased production of cytokines and chemokines, may play an important role in disease development. Among the chemokines upregulated highly and early upregulated during scrapie infections are ligands of CXCR3. To gain more insight into the role of CXCR3 in a prion model, CXCR3-deficient (CXCR3(-/-)) mice were infected intracerebrally with scrapie strain 139A and characterized in comparison to similarly infected wild-type controls. CXCR3(-/-) mice showed significantly prolonged survival times of up to 30 days on average. Surprisingly, however, they displayed accelerated accumulation of misfolded proteinase K-resistant prion protein PrP(Sc) and 20 times higher infectious prion titers than wild-type mice at the asymptomatic stage of the disease, indicating that these PrP isoforms may not be critical determinants of survival times. As demonstrated by immunohistochemistry, Western blotting, and gene expression analysis, CXCR3-deficient animals develop an excessive astrocytosis. However, microglia activation is reduced. Quantitative analysis of gliosis-associated gene expression alterations demonstrated reduced mRNA levels for a number of proinflammatory factors in CXCR3(-/-) compared to wild-type mice, indicating a weaker inflammatory response in the knockout mice. Taken together, this murine prion model identifies CXCR3 as disease-modifying host factor and indicates that inflammatory glial responses may act in concert with PrP(Sc) in disease development. Moreover, the results indicate that targeting CXCR3 for treatment of prion infections could prolong survival times, but the results also raise the concern that impairment of microglial migration by ablation or inhibition of CXCR3 could result in increased accumulation of misfolded PrP(Sc).

Immune-to-brain signalling: the role of cerebral CD163-positive macrophages

Systemic inflammation induces cytokine synthesis within the central nervous system. This results in sickness behaviour and may exacerbate ongoing neuroinflammatory disease. The precise mechanisms underlying the relay of signal from the periphery to the central nervous system are not entirely understood. CD163-positive macrophages occupy a unique position at the blood-brain barrier and upregulate prostaglandin-synthesizing enzymes in response to systemic inflammation. This finding suggests that they might play a role in signalling inflammation to the central nervous system. However, here we demonstrate that de novo brain cytokine transcription during systemic endotoxaemia may be prostaglandin-independent. We therefore set out to interrogate more directly the role of CD163-positive macrophages in immune-to-brain signalling. Intracerebroventricular injections of clodronate liposomes were used to selectively deplete CD163-positive macrophages. We show that de novo brain cytokine synthesis during systemic endotoxaemia persists in the absence of CD163-positive macrophages. Cerebral endothelial cells outnumber CD163-positive macrophages and are arguably better situated to signal circulating inflammatory stimuli to the brain.

Investigation of mcp1 as a quantitative trait gene for prion disease incubation time in mouse

The genetic basis of prion disease incubation time is principally determined by polymorphisms in the prion protein gene, Prnp. However, it is now known that other genetic factors are important. Several quantitative trait loci (QTL) have been identified across the genome including a broad region of linkage on Mmu11. Monocyte chemoattractant protein 1 (MCP-1) maps to this region and has been associated with microglial activation and reduced survival in the ME7 mouse scrapie model of prion disease. We have identified 10 polymorphisms, 3 of which are nonsynonomous, in Mcp1 between "long" (CAST) and "short" (SJL or NZW) incubation-time mouse strains. Crosses between these strains and Mcp1(-/-) mice inoculated with the Chandler/RML mouse scrapie prion strain formed the basis of a quantitative complementation test. In these models loss of Mcp1 did not show an increase in incubation time suggesting that the effects of Mcp1 may be specific to the ME7 prion strain and that Mcp1 does not contribute to the QTL described on Mmu11.

Genes contributing to prion pathogenesis

Prion diseases are caused by conversion of a normally folded, non-pathogenic isoform of the prion protein (PrP(C)) to a misfolded, pathogenic isoform (PrP(Sc)). Prion inoculation experiments in mice expressing homologous PrP(C) molecules on different genetic backgrounds displayed different incubation times, indicating that the conversion reaction may be influenced by other gene products. To identify genes that contribute to prion pathogenesis, we analysed incubation times of prions in mice in which the gene product was inactivated, knocked out or overexpressed. We tested 20 candidate genes, because their products either colocalize with PrP, are associated with Alzheimer's disease, are elevated during prion disease, or function in PrP-mediated signalling, PrP glycosylation, or protein maintenance. Whereas some of the candidates tested may have a role in the normal function of PrP(C), our data show that many genes previously implicated in prion replication have no discernible effect on the pathogenesis of prion disease. While most genes tested did not significantly affect survival times, ablation of the amyloid beta (A4) precursor protein (App) or interleukin-1 receptor, type I (Il1r1), and transgenic overexpression of human superoxide dismutase 1 (SOD1) prolonged incubation times by 13, 16 and 19 %, respectively.

ERK1/2 and p38 MAP kinases control prion protein fragment 90-231-induced astrocyte proliferation and microglia activation

Astrogliosis and microglial activation are a common feature during prion diseases, causing the release of chemoattractant and proinflammatory factors as well as reactive free radicals, involved in neuronal degeneration. The recombinant protease-resistant domain of the prion protein (PrP90-231) displays in vitro neurotoxic properties when refolded in a beta-sheet-rich conformer. Here, we report that PrP90-231 induces the secretion of several cytokines, chemokines, and nitric oxide (NO) release, in both type I astrocytes and microglial cells. PrP90-231 elicited in both cell types the activation of ERK1/2 MAP kinase that displays, in astrocytes, a rapid kinetics and a proliferative response. Conversely, in microglia, PrP90-231-dependent MAP kinase activation was delayed and long lasting, inducing functional activation and growth arrest. In microglial cells, NO release, dependent on the expression of the inducible NO synthase (iNOS), and the secretion of the chemokine CCL5 were Ca(2+) dependent and under the control of the MAP kinases ERK1/2 and p38: ERK1/2 inhibition, using PD98059, reduced iNOS expression, while p38 blockade by PD169316 inhibited CCL5 release. In summary, we demonstrate that glial cells are activated by extracellular misfolded PrP90-231 resulting in a proliferative/secretive response of astrocytes and functional activation of microglia, both dependent on MAP kinase activation. In particular, in microglia, PrP90-231 activated a complex signalling cascade involved in the regulation of NO and chemokine release. These data argue in favor of a causal role for misfolded prion protein in sustaining glial activation and, possibly, glia-mediated neuronal death.

Classical sheep transmissible spongiform encephalopathies: pathogenesis, pathological phenotypes and clinical disease

Scrapie is a prion disease or transmissible spongiform encephalopathy (TSE) of sheep, goats and moufflon. As with its human counterparts, pathology consists of vacuolation, gliosis and accumulations of abnormal forms of a host prion protein (PrPd) in the brain of affected individuals. Immunohistochemical methods can be used to identify both the intracellular truncation sites of PrPd in different cell types (PrPd epitope mapping) and the different morphological patterns of accumulation (PrPd profiling). Differences in the inferred truncation sites of PrPd are found for different strains of sheep TSEs and for different infected cell types within individual strains. Immunochemical methods of characterizing strains broadly correspond to PrPd mapping discriminatory results, but distinct PrPd profiles, which provide strain- and source-specific information on both the cell types which sustain infection (cellular tropisms) and the cellular processing of PrPd, have no immunoblotting counterparts. The cause of neurological dysfunction in human is commonly considered to be neuronal loss secondary to a direct or indirect effect of the accumulation of PrPd. However, in sheep scrapie there is no significant neuronal loss, and relationships between different magnitudes, topographical and cytological forms of PrPd accumulation and clinical signs are not evident. PrPd accumulation also occurs in lymphoid tissues, for which there is indirect evidence of a pathological effect, in the peripheral nervous system and in other tissues. It is generally assumed that neuroinvasion results from infection of the enteric nervous system neurones subsequent to amplification of infectivity in lymphoid tissues and later spread via sympathetic and parasympathetic pathways. The evidence for this is, however, circumstantial. Accumulation of PrPd and presence of infectivity in tissues other than the nervous and lymphoreticular systems gives insights on the ways of transmission of infection and on food safety.

Systemic infections and inflammation affect chronic neurodegeneration

It is well known that systemic infections cause flare-ups of disease in individuals with asthma and rheumatoid arthritis, and that relapses in multiple sclerosis can often be associated with upper respiratory-tract infections. Here we review evidence to support our hypothesis that in chronic neurodegenerative diseases such as Alzheimer's disease, with an ongoing innate immune response in the brain, systemic infections and inflammation can cause acute exacerbations of symptoms and drive the progression of neurodegeneration.

Simvastatin prolongs survival times in prion infections of the central nervous system

Prion diseases are fatal and at present there are neither cures nor palliative therapies known/available, which delay disease onset or progression. Cholesterol-lowering drugs have been reported to inhibit prion replication in infected cell cultures and to modulate inflammatory reactions. We aimed to determine whether simvastatin-treatment could delay disease onset in a murine prion model. Groups of mice were intracerebrally infected with two doses of scrapie strain 139A. Simvastatin-treatment commenced 100 days postinfection. The treatment did not affect deposition of misfolded prion protein PrP(res). However, expression of marker proteins for glia activation like major histocompatibility class II and galectin-3 was found to be affected. Analysis of brain cholesterol synthesis and metabolism revealed a mild reduction in cholesterol precursor levels, whereas levels of cholesterol and cholesterol metabolites were unchanged. Simvastatin-treatment significantly delayed disease progression and prolonged survival times in established prion infection of the CNS (p < or = 0.0003). The results suggest that modulation of glial responses and the therapeutic benefit observed in our murine prion model of simvastatin is not due to the cholesterol-lowering effect of this drug.

Brain cytokine synthesis induced by an intraparenchymal injection of LPS is reduced in MCP-1-deficient mice prior to leucocyte recruitment

We have previously shown that ischaemic lesions are smaller in monocyte chemoattractant protein-1-deficient (MCP-1(-/-)) mice than in wild-type (wt) controls. In addition to its role as a monocyte chemoattractant, monocyte chemoattractant protein-1 (MCP-1) has been proposed to contribute to lesion progression after focal ischaemia by driving local cytokine synthesis by resident glia. To investigate this hypothesis we injected lipopolysaccharide (LPS) into the brain parenchyma of MCP-1(-/-) mice and compared the resulting inflammatory response and production of proinflammatory cytokines to those in wt mice. Microglial and astrocyte morphological activation was the same in the two strains, but MCP-1(-/-) mice showed significantly lower levels of proinflammatory cytokine synthesis; interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha) levels were up to 50% lower than in wt controls after 6 h. This reduced synthesis of proinflammatory cytokines occurred well before leucocyte recruitment to the central nervous system (CNS) is observed in this model of acute inflammation and thus cannot be attributed to lower numbers of recruited monocytes at the site of injury. We propose that MCP-1 contributes to acute CNS inflammation by pleiotropic mechanisms. In addition to being a potent chemoattractant for monocytes, we provide evidence here that MCP-1 can modify the responsiveness of CNS glia to acute inflammatory stimuli prior to leucocyte recruitment, thereby acting as a priming stimulus for cytokine synthesis in cells such as microglia.