Endotoxin induces a delayed loss of TH-IR neurons in substantia nigra and motor behavioral deficits

Suppression of inflammation with conditional deletion of the prostaglandin E2 EP2 receptor in macrophages and brain microglia

Prostaglandin E2 (PGE2), a potent lipid signaling molecule, modulates inflammatory responses through activation of downstream G-protein coupled EP(1-4) receptors. Here, we investigated the cell-specific in vivo function of PGE2 signaling through its E-prostanoid 2 (EP2) receptor in murine innate immune responses systemically and in the CNS. In vivo, systemic administration of lipopolysaccharide (LPS) resulted in a broad induction of cytokines and chemokines in plasma that was significantly attenuated in EP2-deficient mice. Ex vivo stimulation of peritoneal macrophages with LPS elicited proinflammatory responses that were dependent on EP2 signaling and that overlapped with in vivo plasma findings, suggesting that myeloid-lineage EP2 signaling is a major effector of innate immune responses. Conditional deletion of the EP2 receptor in myeloid lineage cells in Cd11bCre;EP2(lox/lox) mice attenuated plasma inflammatory responses and transmission of systemic inflammation to the brain was inhibited, with decreased hippocampal inflammatory gene expression and cerebral cortical levels of IL-6. Conditional deletion of EP2 significantly blunted microglial and astrocytic inflammatory responses to the neurotoxin MPTP and reduced striatal dopamine turnover. Suppression of microglial EP2 signaling also increased numbers of dopaminergic (DA) neurons in the substantia nigra independent of MPTP treatment, suggesting that microglial EP2 may influence development or survival of DA neurons. Unbiased microarray analysis of microglia isolated from adult Cd11bCre;EP2(lox/lox) and control mice demonstrated a broad downregulation of inflammatory pathways with ablation of microglial EP2 receptor. Together, these data identify a cell-specific proinflammatory role for macrophage/microglial EP2 signaling in innate immune responses systemically and in brain.

Progression of intestinal permeability changes and alpha-synuclein expression in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a multifocal degenerative disorder for which there is no cure. The majority of cases are sporadic with unknown etiology. Recent data indicate that untreated patients with de novo PD have increased colonic permeability and that both de novo and premotor patients have pathological expression of α-synuclein (α-syn) in their colon. Both endpoints potentially can serve as disease biomarkers and even may initiate PD events through gut-derived, lipopolysaccharide (LPS)-induced neuronal injury. Animal models could be ideal for interrogating the potential role of the intestines in the pathogenesis of PD; however, few current animal models of PD encompass these nonmotor features. We sought to establish a progressive model of PD that includes the gastrointestinal (GI) dysfunction present in human patients. C57/BL6 mice were systemically administered one dose of either LPS (2.5 mg/kg) or saline and were sacrificed in monthly intervals (n = 5 mice for 5 months) to create a time-course. Small and large intestinal permeability was assessed by analyzing the urinary output of orally ingested sugar probes through capillary column gas chromatography. α-Syn expression was assessed by counting the number of mildly, moderately, and severely affected myenteric ganglia neurons throughout the GI tract, and the counts were validated by quantitative optical density measurements. Nigrostriatal integrity was assessed by tyrosine hydroxylase immunohistochemistry stereology and densitometry. LPS caused an immediate and progressive increase in α-syn expression in the large intestine but not in the small intestine. Intestinal permeability of the whole gut (large and small intestines) progressively increased between months 2 and 4 after LPS administration but returned to baseline levels at month 5. Selective measurements demonstrated that intestinal permeability in the small intestine remained largely intact, suggesting that gut leakiness was predominately in the large intestine. Phosphorylated serine 129-α-syn was identified in a subset of colonic myenteric neurons at months 4 and 5. Although these changes were observed in the absence of nigrostriatal degeneration, an abrupt but insignificant increase in brainstem α-syn was observed that paralleled the restoration of permeability. No changes were observed over time in controls. LPS, an endotoxin used to model PD, causes sequential increases in α-syn immunoreactivity, intestinal permeability, and pathological α-syn accumulation in the colon in a manner similar to that observed in patients with PD. These features are observed without nigrostriatal degeneration and incorporate PD features before the motor syndrome. This allows for the potential use of this model in testing neuroprotective and disease-modifying therapies, including intestinal-directed therapies to fortify intestinal barrier integrity.

NADPH oxidase and aging drive microglial activation, oxidative stress, and dopaminergic neurodegeneration following systemic LPS administration

Parkinson's disease is characterized by a progressive degeneration of substantia nigra (SN) dopaminergic neurons with age. We previously found that a single systemic lipopolysaccharide (LPS, 5 mg/kg, i.p.) injection caused a slow progressive loss of tyrosine hydroxylase immunoreactive (TH+IR) neurons in SN associated with increasing motor dysfunction. In this study, we investigated the role of NADPH oxidase (NOX) in inflammation-mediated SN neurotoxicity. A comparison of control (NOX2(+/+) ) mice with NOX subunit gp91(phox) -deficient (NOX2(-/-) ) mice 10 months after LPS administration (5 mg/kg, i.p.) resulted in a 39% (P < 0.01) loss of TH+IR neurons in NOX2(+/+) mice, whereas NOX2(-/-) mice did not show a significant decrease. Microglia (Iba1+IR) showed morphological activation in NOX2(+/+) mice, but not in NOX2(-/-) mice at 1 hr. Treatment of NOX2(+/+) mice with LPS resulted in a 12-fold increase in NOX2 mRNA in midbrain and 5.5-6.5-fold increases in NOX2 protein (+IR) in SN compared with the saline controls. Brain reactive oxygen species (ROS), determined using diphenyliodonium histochemistry, was increased by LPS in SN between 1 hr and 20 months. Diphenyliodonium (DPI), an NOX inhibitor, blocked LPS-induced activation of microglia and production of ROS, TNFα, IL-1β, and MCP-1. Although LPS increased microglial activation and ROS at all ages studied, saline control NOX2(+/+) mice showed age-related increases in microglial activation, NOX, and ROS levels at 12 and 22 months of age. Together, these results suggest that NOX contributes to persistent microglial activation, ROS production, and dopaminergic neurodegeneration that persist and continue to increase with age.

Cognitive dysfunction and depression in Parkinson's disease: what can be learned from rodent models?

Parkinson's disease (PD) has for decades been considered a pure motor disorder and its cardinal motor symptoms have been attributed to the loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta and to nigral Lewy body pathology. However, there has more recently been a shift in the conceptualization of the disease, and its pathological features have now been recognized as involving several other areas of the brain and indeed even outside the central nervous system. There are a corresponding variety of intrinsic non-motor symptoms such as autonomic dysfunction, cognitive impairment, sleep disturbances and neuropsychiatric problems, which cannot be explained exclusively by nigral pathology. In this review, we will focus on cognitive impairment and affective symptoms in PD, and we will consider whether, and how, these deficits can best be modelled in rodent models of the disorder. As only a few of the non-motor symptoms respond to standard DA replacement therapies, the quest for a broader therapeutic approach remains a major research effort, and success in this area in particular will be strongly dependent on appropriate rodent models. In addition, better understanding of the different models, as well as the advantages and disadvantages of the available behavioural tasks, will result in better tools for evaluating new treatment strategies for PD patients suffering from these neuropsychological symptoms.

Chronic ethanol increases systemic TLR3 agonist-induced neuroinflammation and neurodegeneration

Background

Increasing evidence links systemic inflammation to neuroinflammation and neurodegeneration. We previously found that systemic endotoxin, a TLR4 agonist or TNFα, increased blood TNFα that entered the brain activating microglia and persistent neuroinflammation. Further, we found that models of ethanol binge drinking sensitized blood and brain proinflammatory responses. We hypothesized that blood cytokines contribute to the magnitude of neuroinflammation and that ethanol primes proinflammatory responses. Here, we investigate the effects of chronic ethanol on neuroinflammation and neurodegeneration triggered by toll-like receptor 3 (TLR3) agonist poly I:C.

Methods

Polyinosine-polycytidylic acid (poly I:C) was used to induce inflammatory responses when sensitized with D-galactosamine (D-GalN). Male C57BL/6 mice were treated with water or ethanol (5 g/kg/day, i.g., 10 days) or poly I:C (250 μg/kg, i.p.) alone or sequentially 24 hours after ethanol exposure. Cytokines, chemokines, microglial morphology, NADPH oxidase (NOX), reactive oxygen species (ROS), high-mobility group box 1 (HMGB1), TLR3 and cell death markers were examined using real-time PCR, ELISA, immunohistochemistry and hydroethidine histochemistry.

Results

Poly I:C increased blood and brain TNFα that peaked at three hours. Blood levels returned within one day, whereas brain levels remained elevated for at least three days. Escalating blood and brain proinflammatory responses were found with ethanol, poly I:C, and ethanol-poly I:C treatment. Ethanol pretreatment potentiated poly I:C-induced brain TNFα (345%), IL-1β (331%), IL-6 (255%), and MCP-1(190%). Increased levels of brain cytokines coincided with increased microglial activation, NOX gp91^phox, superoxide and markers of neurodegeneration (activated caspase-3 and Fluoro-Jade B). Ethanol potentiation of poly I:C was associated with ethanol-increased expression of TLR3 and endogenous agonist HMGB1 in the brain. Minocycline and naltrexone blocked microglial activation and neurodegeneration.

Conclusions

Chronic ethanol potentiates poly I:C blood and brain proinflammatory responses. Poly I:C neuroinflammation persists after systemic responses subside. Increases in blood TNFα, IL-1β, IL-6, and MCP-1 parallel brain responses consistent with blood cytokines contributing to the magnitude of neuroinflammation. Ethanol potentiation of TLR3 agonist responses is consistent with priming microglia-monocytes and increased NOX, ROS, HMGB1-TLR3 and markers of neurodegeneration. These studies indicate that TLR3 agonists increase blood cytokines that contribute to neurodegeneration and that ethanol binge drinking potentiates these responses.

β2-adrenergic receptor activation prevents rodent dopaminergic neurotoxicity by inhibiting microglia via a novel signaling pathway

The role of the β2 adrenergic receptor (β2AR) in the regulation of chronic neurodegenerative inflammation within the CNS is poorly understood. The purpose of this study was to determine neuroprotective effects of long-acting β2AR agonists such as salmeterol in rodent models of Parkinson's disease. Results showed salmeterol exerted potent neuroprotection against both LPS and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/1-methyl-4-phenylpyridinium-induced dopaminergic neurotoxicity both in primary neuron-glia cultures (at subnanomolar concentrations) and in mice (1-10 μg/kg/day doses). Further studies demonstrated that salmeterol-mediated neuroprotection is not a direct effect on neurons; instead, it is mediated through the inhibition of LPS-induced microglial activation. Salmeterol significantly inhibited LPS-induced production of microglial proinflammatory neurotoxic mediators, such as TNF-α, superoxide, and NO, as well as the inhibition of TAK1-mediated phosphorylation of MAPK and p65 NF-κB. The anti-inflammatory effects of salmeterol required β2AR expression in microglia but were not mediated through the conventional G protein-coupled receptor/cAMP pathway. Rather, salmeterol failed to induce microglial cAMP production, could not be reversed by either protein kinase A inhibitors or an exchange protein directly activated by cAMP agonist, and was dependent on β-arrestin2 expression. Taken together, our results demonstrate that administration of extremely low doses of salmeterol exhibit potent neuroprotective effects by inhibiting microglial cell activation through a β2AR/β-arrestin2-dependent but cAMP/protein kinase A-independent pathway.

Prenatal LPS increases inflammation in the substantia nigra of Gdnf heterozygous mice

Prenatal systemic inflammation has been implicated in neurological diseases, but optimal animal models have not been developed. We investigated whether a partial genetic deletion of glial cell line-derived neurotrophic factor (Gdnf(+/-)) increased vulnerability of dopamine (DA) neurons to prenatal lipopolysaccharide (LPS). LPS [0.01 mg/kg intraperitoneal (i.p.)] or saline was administered to wild-type (WT) or Gdnf(+/-) pregnant mice on gestational day 9.5. Male offspring were examined at 3 weeks, 3 and 12 months of age. There was a progressive degeneration of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra (SN) with age in Gdnf(+/-) but not in WT mice, with no observed effects on locus coeruleus (LC) noradrenergic neurons or DA neurons of the ventral tegmental area. Inflammatory markers were elevated in SN of LPS treated offspring, with exacerbation in Gdnf(+/-) mice. Intracellular accumulation of α-synuclein (α-syn) immunoreactivity in DA neurons of SN was observed in all groups of Gdnf(+/-) and in WT mice with prenatal LPS, with altered distribution between pars reticulata (pr) and pars compacta (pc). The findings suggest that prenatal LPS leads to accelerated neuropathology in the SN with age, and that a partial loss of GDNF exacerbates these effects, providing a novel model for age-related neuropathology of the nigrostriatal DA system.

Mouse brain plasmalogens are targets for hypochlorous acid-mediated modification in vitro and in vivo

Plasmalogens, 1-O-alk-1'-enyl-2-acyl-sn-glycerophospholipids, are significant constituents of cellular membranes and are essential for normal brain development. Plasmalogens, which contain a vinyl ether bond at the sn-1 position, are preferential targets for hypochlorous acid (HOCl), generated by myeloperoxidase (MPO) from H(2)O(2) and chloride ions. Because MPO is implicated in neurodegeneration, this study pursued two aims: (i) to investigate the reactivity of mouse brain plasmalogens toward HOCl in vitro and (ii) to obtain in vivo evidence for MPO-mediated brain plasmalogen modification. Liquid chromatography coupled to hybrid linear ion trap-Fourier transform-ion cyclotron resonance mass spectrometry revealed plasmalogen modification in mouse brain lipid extracts at lower HOCl concentrations as observed for diacylphospholipids, resulting in the generation of 2-chloro fatty aldehydes and lysophospholipids. Lysophosphatidylethanolamine accumulation was transient, whereas lysophosphatidylcholine species containing saturated acyl residues remained stable. In vivo, a single, systemic endotoxin injection resulted in upregulation of cerebral MPO mRNA levels to a range comparable to that observed for tumor necrosis factor-α and cyclooxygenase-2. This inflammatory response was accompanied by a significant decrease in several brain plasmalogen species and concomitant in vivo generation of 2-chlorohexadecanal. The present findings demonstrate that activation of the MPO-H(2)O(2)-chloride system under neuroinflammatory conditions results in oxidative attack of the total cerebral plasmalogen pool. As this lipid class is indispensable for normal neuronal function, HOCl-mediated plasmalogen modification is likely to compromise normal synaptic transmission.

Recurrent systemic infections with Streptococcus pneumoniae do not aggravate the course of experimental neurodegenerative diseases

Neurological symptoms of patients suffering from neurodegenerative diseases such as Alzheimer's dementia (AD), Parkinson's disease (PD), or amyotrophic lateral sclerosis (ALS) often worsen during infections. We assessed the disease-modulating effects of recurrent systemic infections with the most frequent respiratory pathogen, Streptococcus pneumoniae, on the course of AD, PD, and ALS in mouse models of these neurodegenerative diseases [transgenic Tg2576 mice, (Thy1)-[A30P]alpha SYN mice, and Tg(SOD1-G93A) mice]. Mice were repeatedly challenged intraperitoneally with live S. pneumoniae type 3 and treated with ceftriaxone for 3 days. Infection caused an increase of interleukin-6 concentrations in brain homogenates. The clinical status of (Thy1)-[A30P]alpha SYN mice and Tg(SOD1-G93A) mice was monitored by repeated assessment with a clinical score. Motor performance was controlled by the tightrope test and the rotarod test. In Tg2576 mice, spatial memory and learning deficits were assessed in the Morris water maze. In none of the three mouse models onset or course of the disease as evaluated by the clinical tests was affected by the recurrent systemic infections performed. Levels of alpha-synuclein in brains of (Thy1)-[A30P]alpha SYN mice did not differ between infected animals and control animals. Plaque sizes and concentrations of A beta 1-40 and A beta 1-42 were not significantly different in brains of infected and uninfected Tg2576 mice. In conclusion, onset and course of disease in mouse models of three common neurodegenerative disorders were not influenced by repeated systemic infections with S. pneumoniae, indicating that the effect of moderately severe acute infections on the course of neurodegenerative diseases may be less pronounced than suspected.

The prostaglandin E2 E-prostanoid 4 receptor exerts anti-inflammatory effects in brain innate immunity

Peripheral inflammation leads to immune responses in brain characterized by microglial activation, elaboration of proinflammatory cytokines and reactive oxygen species, and secondary neuronal injury. The inducible cyclooxygenase (COX), COX-2, mediates a significant component of this response in brain via downstream proinflammatory PG signaling. In this study, we investigated the function of the PGE2 E-prostanoid (EP) 4 receptor in the CNS innate immune response to the bacterial endotoxin LPS. We report that PGE2 EP4 signaling mediates an anti-inflammatory effect in brain by blocking LPS-induced proinflammatory gene expression in mice. This was associated in cultured murine microglial cells with decreased Akt and I-kappaB kinase phosphorylation and decreased nuclear translocation of p65 and p50 NF-kappaB subunits. In vivo, conditional deletion of EP4 in macrophages and microglia increased lipid peroxidation and proinflammatory gene expression in brain and in isolated adult microglia following peripheral LPS administration. Conversely, EP4 selective agonist decreased LPS-induced proinflammatory gene expression in hippocampus and in isolated adult microglia. In plasma, EP4 agonist significantly reduced levels of proinflammatory cytokines and chemokines, indicating that peripheral EP4 activation protects the brain from systemic inflammation. The innate immune response is an important component of disease progression in a number of neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In addition, recent studies demonstrated adverse vascular effects with chronic administration of COX-2 inhibitors, indicating that specific PG signaling pathways may be protective in vascular function. This study supports an analogous and beneficial effect of PGE2 EP4 receptor signaling in suppressing brain inflammation.

Neuroinflammation in Parkinson's disease

During the last two decades, a wealth of animal and human studies has implicated inflammation-derived oxidative stress and cytokine-dependent neurotoxicity in the progressive degeneration of the dopaminergic nigrostriatal pathway, the hallmark of Parkinson's disease (PD). In this review, we discuss the various hypotheses regarding the role of microglia and other immune cells in PD pathogenesis and progression, the inflammatory mechanisms implicated in disease progression from pre-clinical and clinical studies, the recent evidence that systemic inflammation can trigger microglia activation in PD-relevant central nervous system regions, the synergism between gene products linked to parkinsonian phenotypes (alpha-synuclein, parkin, Nurr1, and regulator of G-protein signaling-10) and neuroinflammation in promoting neurodegeneration of the nigrostriatal pathway, and the latest update on meta-analysis of epidemiological studies on the risk-lowering effects of anti-inflammatory drug regimens.

Modulation of sickness behavior by sleep: the role of neurochemical and neuroinflammatory pathways in mice

Activation of the immune system elicits several behavioral changes that are collectively called sickness behavior and consists in a strategy to overcome infection. Sleep deprivation can increase susceptibility to pathogens and to behavioral alterations. Thus, the present study aimed to determine how paradoxical sleep deprivation (PSD) affects the behavioral and neurochemical responses to lipopolysaccharide (LPS, potent activator of the immune response). Adult inbred mice were paradoxical sleep deprived (72 h), whereas the control group was kept in their home cages. Both groups received either an injection of saline or LPS (5, 10 or 20 microg/animal ip) before behavioral tasks and tissue collection. During the recovery sleep period, LPS provoked a strong inhibition of sleep rebound due to a suppression of paradoxical sleep. PSD increased the susceptibility of mice to LPS-induced immobility in the open field, which was capable of affecting the anxiety-like behavior also. These altered behavioral responses to LPS were accompanied by reduction in dopamine turnover within the striatum and increased expression of cyclooxygenase-2 in the cortex. The study provides some insights into how the sleep-wake cycle affects the expression of sickness behavior induced by LPS.

Lipopolysaccharide alters the blood-brain barrier transport of amyloid beta protein: a mechanism for inflammation in the progression of Alzheimer's disease

Alzheimer's disease (AD) brains are characterized by accumulation of amyloid beta protein (Abeta) and neuroinflammation. Increased blood-to-brain influx and decreased brain-to-blood efflux across the blood-brain barrier (BBB) have been proposed as mechanisms for Abeta accumulation. Epidemiological studies suggest that the nonsteroidal anti-inflammatory drug (NSAID) indomethacin slows the progression of AD. We hypothesized that inflammation alters BBB handling of Abeta. Mice treated with lipopolysaccharide (LPS) had increased brain influx and decreased brain efflux of Abeta, recapitulating the findings in AD. Neither influx nor efflux was mediated by LPS acting directly on BBB cells. Increased influx was mediated by a blood-borne factor, indomethacin-independent, blocked by the triglyceride triolein, and not related to expression of the blood-to-brain transporter of Abeta, RAGE. Serum levels of IL-6, IL-10, IL-13, and MCP-1 mirrored changes in Abeta influx. Decreased efflux was blocked by indomethacin and accompanied by decreased protein expression of the brain-to-blood transporter of Abeta, LRP-1. LPS paradoxically increased expression of neuronal LRP-1, a major source of Abeta. Thus, inflammation potentially increases brain levels of Abeta by three mechanisms: increased influx, decreased efflux, and increased neuronal production.

Dopaminergic and serotoninergic deficiencies in young adult rats prenatally exposed to the bacterial lipopolysaccharide

We have reported previously that prenatal bacterial lipopolysaccharide (LPS) exposure at the gestation window of vulnerability could consistently lead to dopamine (DA) neuron loss in the substantia nigra (SN). Thus, we suggested that prenatal LPS exposure might represent as a risk factor for the development of Parkinson's disease (PD). Here, we report that the same exposure could lead to tryptophan hydroxylase (TPH, a serotonin neuron marker) immunoreactive cell loss in the dorsal raphe nucleus (DRN). Twenty two pups born to saline or LPS-injected gravid female rats at E10.5 were used in the current study. Twelve male pups at age of 4 months (6 from each of two prenatal groups) were used for the tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) immunochemistry studies. The other 10 (5 from each of two prenatal groups) males were used in the biochemistry studies. A 29% THir neuron loss in the substantia nigra (F(1,11)=17.573, P=0.002) and a 31% TPHir neuron loss (F(1,11)=44.005, P<0.001) in the DRN were seen. Significant DA and 5-hydroxytryptamine (5-HT) reductions (P<0.05) were found in the frontal cortex, nucleus accumbens, striatum, amygdala, hippocampus, and hypothalamus. The losses of DA and 5-HT were accompanied by the significant increases in homovanillic acid over DA and 5-hydroxyindoleacetic acid over 5-HT ratios in the most areas tested. These data further validate prenatal LPS exposure as a model of PD since DA and 5-HT changes similar to those seen in PD patients. They also suggest that prenatal LPS might be a risk factor for other diseases including mood disorders.