Selective contribution of interleukin-6 and leptin to brain inflammatory signals induced by systemic LPS injection in mice

Systemic Lipopolysaccharide Challenge Induces Inflammatory Changes in Rat Dorsal Root Ganglia: An Ex Vivo Study

Inflammatory processes within the peripheral nervous system (PNS) are associated with symptoms of hyperalgesia and allodynia. Pro-inflammatory mediators, such as cytokines or prostaglandins, modulate the excitability of nociceptive neurons, called peripheral sensitization. Here, we aimed to examine if previously reported effects of in vitro stimulation with lipopolysaccharide (LPS) on primary cell cultures of dorsal root ganglia (DRG) reflect changes in a model of LPS-induced systemic inflammation in vivo. Male rats were intraperitoneally injected with LPS (100 µg/kg) or saline. Effects of systemic inflammation on expression of inflammatory mediators, neuronal Ca2+ responses, and activation of inflammatory transcription factors in DRG were assessed. Systemic inflammation was accompanied by an enhanced expression of pro-inflammatory cytokines and cyclooxygenase-2 in lumbar DRG. In DRG primary cultures obtained from LPS-treated rats enhanced neuronal capsaicin-responses were detectable. Moreover, we found an increased activation of inflammatory transcription factors in cultured macrophages and neurons after an in vivo LPS challenge compared to saline controls. Overall, our study emphasizes the role of inflammatory processes in the PNS that may be involved in sickness-behavior-associated hyperalgesia induced by systemic LPS treatment. Moreover, we present DRG primary cultures as tools to study inflammatory processes on a cellular level, not only in vitro but also ex vivo.

Gabapentinoids Suppress Lipopolysaccharide-Induced Interleukin-6 Production in Primary Cell Cultures of the Rat Spinal Dorsal Horn

INTRODUCTION

Gabapentin and pregabalin are drugs to treat neuropathic pain. Several studies highlighted effects on presynaptic terminals of nociceptors. Via binding to α2δ subunits of voltage-gated calcium channels, gabapentinoids modulate the synaptic transmission of nociceptive information. However, recent studies revealed further properties of these substances. Treatment with gabapentin or pregabalin in animal models of neuropathic pain resulted not only in reduced symptoms of hyperalgesia but also in an attenuated activation of glial cells and decreased production of pro-inflammatory mediators in the spinal dorsal horn.

METHODS

In the present study, we aimed to investigate the impact of gabapentinoids on the inflammatory response of spinal dorsal horn cells, applying the established model of neuro-glial primary cell cultures of the superficial dorsal horn (SDH). We studied effects of gabapentin and pregabalin on lipopolysaccharide (LPS)-induced cytokine release (bioassays), expression of inflammatory marker genes (RT-qPCR), activation of transcription factors (immunocytochemistry), and Ca2+ responses of SDH neurons to stimulation with substance P and glutamate (Ca2+-imaging).

RESULTS

We detected an attenuated LPS-induced expression and release of interleukin-6 by SDH cultures in the presence of gabapentinoids. In addition, a significant main effect of drug treatment was observed for mRNA expression of microsomal prostaglandin E synthase 1 and the inhibitor of nuclear factor kappa B. Nuclear translocation of inflammatory transcription factors in glial cells was not significantly affected by gabapentinoid treatment. Moreover, both substances did not modulate neuronal responses upon stimulation with substance P or glutamate.

CONCLUSION

Our results provide evidence for anti-inflammatory capacities of gabapentinoids on the acute inflammatory response of SDH primary cultures upon LPS stimulation. Such effects may contribute to the pain-relieving effects of gabapentinoids.

SK-Channel Activation Alters Peripheral Metabolic Pathways in Mice, but Not Lipopolysaccharide-Induced Fever or Inflammation

Purpose

Previously, we have shown that CyPPA (cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine), a pharmacological small-conductance calcium-activated potassium (SK)–channel positive modulator, antagonizes lipopolysaccharide (LPS)-induced cytokine expression in microglial cells. Here, we aimed to test its therapeutic potential for brain-controlled sickness symptoms, brain inflammatory response during LPS-induced systemic inflammation, and peripheral metabolic pathways in mice.

Methods

Mice were pretreated with CyPPA (15 mg/kg IP) 24 hours before and simultaneously with LPS stimulation (2.5 mg/kg IP), and the sickness response was recorded by a telemetric system for 24 hours. A second cohort of mice were euthanized 2 hours after CyPPA or solvent treatment to assess underlying CyPPA-induced mechanisms. Brain, blood, and liver samples were analyzed for inflammatory mediators or nucleotide concentrations using immunohistochemistry, real-time PCR and Western blot, or HPLC. Moreover, we investigated CyPPA-induced changes of UCP1 expression in brown adipose tissue (BAT)–explant cultures.

Results

CyPPA treatment did not affect LPS-induced fever, anorexia, adipsia, or expression profiles of inflammatory mediators in the hypothalamus or plasma or microglial reactivity to LPS (CD11b staining and CD68 mRNA expression). However, CyPPA alone induced a rise in core body temperature linked to heat production via altered metabolic pathways like reduced levels of adenosine, increased protein content, and increased UCP1 expression in BAT-explant cultures, but no alteration in ATP/ADP concentrations in the liver. CyPPA treatment was accompanied by altered pathways, including NFκB signaling, in the hypothalamus and cortex, while circulating cytokines remained unaltered.

Conclusion

Overall, while CyPPA has promise as a treatment strategy, in particular according to results from in vitro experiments, we did not reveal anti-inflammatory effects during severe LPS-induced systemic inflammation. Interestingly, we found that CyPPA alters metabolic pathways inducing short hyperthermia, most likely due to increased energy turnover in the liver and heat production in BAT.

LPS Primes Brain Responsiveness to High Mobility Group Box-1 Protein

High mobility group box (HMGB)1 action contributes to late phases of sepsis, but the effects of increased endogenous plasma HMGB1 levels on brain cells during inflammation are unclear. Here, we aimed to further investigate the role of HMGB1 in the brain during septic-like lipopolysaccharide-induced inflammation in rats (LPS, 10 mg/kg, i.p.). HMGB-1 mRNA expression and release were measured in the periphery/brain by RT-PCR, immunohistochemistry and ELISA. In vitro experiments with disulfide-HMGB1 in primary neuro-glial cell cultures of the area postrema (AP), a circumventricular organ with a leaky blood–brain barrier and direct access to circulating mediators like HMGB1 and LPS, were performed to determine the direct influence of HMGB1 on this pivotal brain structure for immune-to-brain communication. Indeed, HMGB1 plasma levels stayed elevated after LPS injection. Immunohistochemistry of brains and AP cultures confirmed LPS-stimulated cytoplasmatic translocation of HMGB1 indicative of local HMGB1 release. Moreover, disulfide-HMGB1 stimulation induced nuclear factor (NF)-κB activation and a significant release of interleukin-6, but not tumor necrosis factor α, into AP culture supernatants. However, only a few AP cells directly responded to HMGB1 with increased intracellular calcium concentration. Interestingly, priming with LPS induced a seven-fold higher percentage of responsive cells to HMGB1. We conclude that, as a humoral and local mediator, HMGB1 enhances brain inflammatory responses, after LPS priming, linked to sustained sepsis symptoms.

TLR4 Signaling Selectively and Directly Promotes CGRP Release from Vagal Afferents in the Mouse

There has been a long-standing debate regarding the role of peripheral afferents in mediating rapid-onset anorexia among other responses elicited by peripheral inflammatory insults. Thus, the current study assessed the sufficiency of peripheral afferents expressing toll-like receptor 4 (TLR4) to the initiation of the anorexia caused by peripheral bacterial lipopolysaccharide (LPS). We generated a Tlr4 null (Tlr4^LoxTB) mouse in which Tlr4 expression is globally disrupted by a loxP-flanked transcription blocking (TB) cassette. This novel mouse model allowed us to restore the endogenous TLR4 expression in specific cell types. Using Zp3-Cre and Na_v1.8-Cre mice, we produced mice that express TLR4 in all cells (Tlr4^LoxTB X Zp3-Cre) and in peripheral afferents (Tlr4^LoxTB X Na_v1.8-Cre), respectively. We validated the Tlr4^LoxTB mice, which were phenotypically identical to previously reported global TLR4 knock-out mice. Contrary to our expectations, the administration of LPS did not cause rapid-onset anorexia in mice with Na_v1.8-restricted TLR4. The later result prompted us to identify Tlr4-expressing vagal afferents using in situ hybridization (ISH). In vivo, we found that Tlr4 mRNA was primarily enriched in vagal Na_v1.8 afferents located in the jugular ganglion that co-expressed calcitonin gene-related peptide (CGRP). In vitro, the application of LPS to cultured Na_v1.8-restricted TLR4 afferents was sufficient to stimulate the release of CGRP. In summary, we demonstrated using a new mouse model that vagally-expressed TLR4 is selectively involved in stimulating the release of CGRP but not in causing anorexia.

Age-Dependent Changes of Adipokine and Cytokine Secretion From Rat Adipose Tissue by Endogenous and Exogenous Toll-Like Receptor Agonists

White adipose tissue but recently also brown adipose tissue have emerged as endocrine organs. Age-associated obesity is accompanied by prolonged and elevated lipopolysaccharide (LPS)-induced sickness symptoms and increased cytokine and adipokine levels in the circulation partially originating from adipose tissue. In the present study, ex vivo fat explants were used to investigate how the exogenous pathogen-associated molecular pattern (PAMP) LPS or the endogenous danger-associated molecular patterns (DAMPs) high mobility group box-1 protein (HMGB1) and biglycan modulate the release of cytokines and adipokines/batokines and, thus, could influence systemic and/or local inflammation. The response of adipose tissue (epididymal, retroperitoneal, subcutaneous, and brown) was compared between young lean and old obese rats (2 vs. 24 months old). LPS induced a strong interleukin (IL)-6 and tumor necrosis factor (TNF) alpha release into the supernatant of all adipose tissue types investigated. HMGB1 (subcutaneous) and biglycan (retroperitoneal) led to an increased release of IL-6 and TNFalpha (HMGB1) and decreased visfatin and adiponectin (biglycan) secretion from epididymal adipose tissue (young rats). Visfatin was also decreased by HMGB1 in retroperitoneal adipose tissue of old rats. We found significantly higher leptin (all fat pads) and adiponectin (subcutaneous) levels in supernatants of adipose tissue from old compared to young rats, whereas visfatin secretion showed the opposite. The expression of the biglycan receptor Toll-like receptor (TLR) 2 as well as the LPS and HMGB1 receptors TLR4 and receptor for advanced glycation end products (RAGE) were reduced with age (TLR4/RAGE) and by stimulation with their ligands (subcutaneous). Overall, we revealed that adipokines/adipose-tissue released cytokines show some modulation of their release caused by mediators of septic (batokines) and sterile inflammation with potential implication for acute and chronic disease. Moreover, aging may increase or decrease the release of fat-derived mediators. These data show that DAMPS and LPS locally modulate cytokine secretion while only DAMPS but not LPS can locally alter adipokine secretion during inflammation.

ANGPTL4 exacerbates pancreatitis by augmenting acinar cell injury through upregulation of C5a

Pancreatitis is the inflammation of the pancreas. However, little is known about the genes associated with pancreatitis severity. Our microarray analysis of pancreatic tissues from mild and severe acute pancreatitis mice models identified angiopoietin-like 4 (ANGPTL4) as one of the most significantly upregulated genes. Clinically, ANGPTL4 expression was also increased in the serum and pancreatic tissues of pancreatitis patients. The deficiency in ANGPTL4 in mice, either by gene deletion or neutralizing antibody, mitigated pancreatitis-associated pathological outcomes. Conversely, exogenous ANGPTL4 exacerbated pancreatic injury with elevated cytokine levels and apoptotic cell death. High ANGPTL4 enhanced macrophage activation and infiltration into the pancreas, which increased complement component 5a (C5a) level through PI3K/AKT signaling. The activation of the C5a receptor led to hypercytokinemia that accelerated acinar cell damage and furthered pancreatitis. Indeed, C5a neutralizing antibody decreased inflammatory response in LPS-activated macrophages and alleviated pancreatitis severity. In agreement, there was a significant positive correlation between C5a and ANGPTL4 levels in pancreatitis patients. Taken together, our study suggests that targeting ANGPTL4 is a potential strategy for the treatment of pancreatitis.

Visualizing and Profiling Lipids in the OVLT of Fat-1 and Wild Type Mouse Brains during LPS-Induced Systemic Inflammation Using AP-SMALDI MSI

Lipids, including omega-3 polyunsaturated fatty acids (n-3-PUFAs), modulate brain-intrinsic inflammation during systemic inflammation. The vascular organ of the lamina terminalis (OVLT) is a brain structure important for immune-to-brain communication. We, therefore, aimed to profile the distribution of several lipids (e.g., phosphatidyl-choline/ethanolamine, PC/PE), including n-3-PUFA-carrying lipids (esterified in phospholipids), in the OVLT during systemic lipopolysaccharide(LPS)-induced inflammation. We injected wild type and endogenously n-3-PUFA producing fat-1 transgenic mice with LPS (i.p., 2.5 mg/kg) or PBS. Brain samples were analyzed using immunohistochemistry and high-resolution atmospheric-pressure scanning microprobe matrix-assisted laser desorption/ionization orbital trapping mass spectrometry imaging (AP-SMALDI-MSI) for spatial resolution of lipids. Depending on genotype and treatment, several distinct distribution patterns were observed for lipids [e.g., lyso(L)PC (16:0)/(18:0)] proposed to be involved in inflammation. The distribution patterns ranged from being homogeneously disseminated [LPC (18:1)], absent/reduced signaling within the OVLT relative to adjacent preoptic tissue [PE (38:6)], either treatment- and genotype-dependent or independent low signal intensities [LPC (18:0)], treatment- and genotype-dependent [PC 38:6)] or independent accumulation in the OVLT [PC (38:7)], and accumulation in commissures, e.g., nerve fibers like the optic nerve [LPE (18:1)]. Overall, screening of lipid distribution patterns revealed distinct inflammation-induced changes in the OVLT, highlighting the prominent role of lipid metabolism in brain inflammation. Moreover, known and novel candidates for brain inflammation and immune-to-brain communication were detected specifically within this pivotal brain structure, a window between the periphery and the brain. The biological significance of these newly identified lipids abundant in the OVLT and the adjacent preoptic area remains to be further analyzed.

The role of neutrophil granulocytes in immune-to-brain communication

Immune-to-brain communication has been studied in a variety of experimental models. Crucial insights into signalling and mechanisms were previously revealed in studies investigating fever induction pathways. The scientific community has primarily focused on neuronal and humoral pathways in the manifestation of this response. Emerging evidence has now shown that immune-to-brain signalling via immune cells is pivotal for normal brain function and brain pathology. The present manuscript aims to provide a brief overview on the current understanding of how immune cells signal to the brain. Insights are summarized on the potential physiological significance of some immune cells signalling from the periphery to the brain. A particular focus is laid on the role of neutrophil granulocytes. As such, IL-1β expressing neutrophil granulocytes have been shown to transfer inflammatory information to the brain and contribute to prolonged behavioural changes due to septic encephalopathy in rats during severe systemic inflammation induced by the bacterial component and TLR4 agonist lipopolysaccharide. Modulation of immune cell recruitment to the brain is discussed by various confounding factors including sleep, exercise, the nutritional status e.g. obesity, leptin and omega 3 fatty acids, and psychological or inflammatory stressors. The physiological significance of immune cell mediated communication between the immune system and the brain is highlighted by the fact that systemic inflammatory insults can exacerbate ongoing brain pathologies via immune cell trafficking. New insights into mechanisms and mediators of immune cell mediated immune-to-brain communication are important for the development of new therapeutic strategies and the better understanding of existing ones. Abbreviations: ACTH: adrenocorticotropic hormone; BBB: blood-brain barrier; BBI: blood-brain interface; CD: cluster of differentiation; CINC: cytokine-induced neutrophil chemoattractant; CRH: corticotropin releasing hormone; CVOs: circumventricular organs; CXCR: chemokine receptor; DAPI: 40:6-diamidino-2-phenylindole dilactate; DHA: docosahexaenoid acid; ICAM: intracellular adhesion molecule; IL: interleukin; i.p.: intraperitoneal; i.v.: intravenous; KC: keratinocytes-derived chemokine; LPS: lipopolysaccharide; MIP: macrophage inflammatory protein; MS: multiple sclerosis; NFκB: nuclear factor kappa B; NF-IL6: nuclear factor IL-6; PCTR: protectin conjugates in tissue regeneration; PG: prostaglandin; p.i.: post injection; PVN: paraventricular nucleus; ra: receptor antagonist; STAT3: signal transducer and activator of transcription 3; TIMP: tissue inhibitors of metalloproteinases; TLR: toll-like receptor; TNFα: tumor necrosis factor alpha.

Neural Mechanisms of Inflammation-Induced Fever

Fever is a common symptom of infectious and inflammatory disease. It is well-established that prostaglandin E2 is the final mediator of fever, which by binding to its EP3 receptor subtype in the preoptic hypothalamus initiates thermogenesis. Here, we review the different hypotheses on how the presence of peripherally released pyrogenic substances can be signaled to the brain to elicit fever. We conclude that there is unequivocal evidence for a humoral signaling pathway by which proinflammatory cytokines, through their binding to receptors on brain endothelial cells, evoke fever by eliciting prostaglandin E2 synthesis in these cells. The evidence for a role for other signaling routes for fever, such as signaling via circumventricular organs and peripheral nerves, as well as transfer into the brain of peripherally synthesized prostaglandin E2 are yet far from conclusive. We also review the efferent limb of the pyrogenic pathways. We conclude that it is well established that prostaglandin E2 binding in the preoptic hypothalamus produces fever by disinhibition of presympathetic neurons in the brain stem, but there is yet little understanding of the mechanisms by which factors such as nutritional status and ambient temperature shape the response to the peripheral immune challenge.

Age Dependent Hypothalamic and Pituitary Responses to Novel Environment Stress or Lipopolysaccharide in Rats

Previously, we have shown that the transcription factor nuclear factor interleukin (NF-IL)6 can be used as an activation marker for inflammatory lipopolysaccharide (LPS)-induced and psychological novel environment stress (NES) in the rat brain. Here, we aimed to investigate age dependent changes of hypothalamic and pituitary responses to NES (cage switch) or LPS (100 μg/kg) in 2 and 24 months old rats. Animals were sacrificed at specific time points, blood and brains withdrawn and analyzed using immunohistochemistry, RT-PCR and bioassays. In the old rats, telemetric recording revealed that NES-induced hyperthermia was enhanced and prolonged compared to the young group. Plasma IL-6 levels remained unchanged and hypothalamic IL-6 mRNA expression was increased in the old rats. Interestingly, this response was accompanied by a significant upregulation of corticotropin-releasing hormone mRNA expression only in young rats after NES and overall higher plasma corticosterone levels in all aged animals. Immunohistochemical analysis revealed a significant upregulation of NF-IL6-positive cells in the pituitary after NES or LPS-injection. In another important brain structure implicated in immune-to-brain communication, namely, in the median eminence (ME), NF-IL6-immunoreactivity was increased in aged animals, while the young group showed just minor activation after LPS-stimulation. Interestingly, we found a higher amount of NF-IL6-CD68-positive cells in the posterior pituitary of old rats compared to the young counterparts. Moreover, aging affected the regulation of cytokine interaction in the anterior pituitary lobe. LPS-treatment significantly enhanced the secretion of the cytokines IL-6 and TNFα into supernatants of primary cell cultures of the anterior pituitary. Furthermore, in the young rats, incubation with IL-6 and IL-10 antibodies before LPS-stimulation led to a robust decrease of IL-6 production and an increase of TNFα production by the pituitary cells. In the old rats, this specific cytokine interaction could not be detected. Overall, the present results revealed strong differences in the activation patterns and pathways between old and young rats after both stressors. The prolonged hyperthermic and inflammatory response seen in aged animals seems to be linked to dysregulated pituitary cytokine interactions and brain cell activation (NF-IL6) in the hypothalamus-pituitary-adrenal axis.

Leptin Receptor Expression in Mouse Intracranial Perivascular Cells

Past studies have suggested that non-neuronal brain cells express the leptin receptor. However, the identity and distribution of these leptin receptor-expressing non-neuronal brain cells remain debated. This study assessed the distribution of the long form of the leptin receptor (LepRb) in non-neuronal brain cells using a reporter mouse model in which LepRb-expressing cells are permanently marked by tdTomato fluorescent protein (LepRb-Cre^tdTomato). Double immunohistochemistry revealed that, in agreement with the literature, the vast majority of tdTomato-tagged cells across the mouse brain were neurons (i.e., based on immunoreactivity for NeuN). Non-neuronal structures also contained tdTomato-positive cells, including the choroid plexus and the perivascular space of the meninges and, to a lesser extent, the brain. Based on morphological criteria and immunohistochemistry, perivascular cells were deduced to be mainly pericytes. Notably, tdTomato-positive cells were immunoreactive for vitronectin and platelet derived growth factor receptor beta (PDGFBR). In situ hybridization studies confirmed that most tdTomato-tagged perivascular cells were enriched in leptin receptor mRNA (all isoforms). Using qPCR studies, we confirmed that the mouse meninges were enriched in Leprb and, to a greater extent, the short isoforms of the leptin receptor. Interestingly, qPCR studies further demonstrated significantly altered expression for Vtn and Pdgfrb in the meninges and hypothalamus of LepRb-deficient mice. Collectively, our data demonstrate that the only intracranial non-neuronal cells that express LepRb in the adult mouse are cells that form the blood-brain barrier, including, most notably, meningeal perivascular cells. Our data suggest that pericytic leptin signaling plays a role in the integrity of the intracranial perivascular space and, consequently, may provide a link between obesity and numerous brain diseases.

Multiple lipopolysaccharide (LPS) injections alter interleukin 6 (IL-6), IL-7, IL-10 and IL-6 and IL-7 receptor mRNA in CNS and spleen

Neuroinflammation is proposed to be an important component in the development of several central nervous system (CNS) disorders including depression, Alzheimer's disease, Parkinson's disease, and traumatic brain injury. However, exactly how neuroinflammation leads to, or contributes to, these central disorders is unclear. The objective of the study was to examine and compare the expression of mRNAs for interleukin-6 (IL-6), IL-7, IL-10 and the receptors for IL-6 (IL-6R) and IL-7 (IL-7R) using in situ hybridization in discrete brain regions and in the spleen after multiple injections of 3mg/kg lipopolysaccharide (LPS), a model of neuroinflammation. In the spleen, LPS significantly elevated IL-6 mRNA expression, then IL-10 mRNA, with no effect on IL-7 or IL-7R mRNA, while significantly decreasing IL-6R mRNA expression. In the CNS, LPS administration had the greatest effect on IL-6 and IL-6R mRNA. LPS increased IL-6 mRNA expression only in non-neuronal cells throughout the brain, but significantly elevated IL-6R mRNA in neuronal populations, where observed, except the cerebellum. LPS resulted in variable effects on IL-10 mRNA, and had no effect on IL-7 or IL-7R mRNA expression. These studies indicate that LPS-induced neuroinflammation has substantial but variable effects on the regional and cellular patterns of CNS IL-6, IL-7 and IL-10, and for IL-6R and IL-7R mRNA expression. It is apparent that administration of LPS can affect non-neuronal and neuronal cells in the brain. Further research is required to determine how CNS inflammatory changes associated with IL-6, IL-10 and IL-6R could in turn contribute to the development of CNS neurological disorders.

Inhibiting Microglia Expansion Prevents Diet-Induced Hypothalamic and Peripheral Inflammation

Cell proliferation and neuroinflammation in the adult hypothalamus may contribute to the pathogenesis of obesity. We tested whether the intertwining of these two processes plays a role in the metabolic changes caused by 3 weeks of a high-saturated fat diet (HFD) consumption. Compared with chow-fed mice, HFD-fed mice had a rapid increase in body weight and fat mass and specifically showed an increased number of microglia in the arcuate nucleus (ARC) of the hypothalamus. Microglia expansion required the adequate presence of fats and carbohydrates in the diet because feeding mice a very high-fat, very low-carbohydrate diet did not affect cell proliferation. Blocking HFD-induced cell proliferation by central delivery of the antimitotic drug arabinofuranosyl cytidine (AraC) blunted food intake, body weight gain, and adiposity. AraC treatment completely prevented the increase in number of activated microglia in the ARC, the expression of the proinflammatory cytokine tumor necrosis factor-α in microglia, and the recruitment of the nuclear factor-κB pathway while restoring hypothalamic leptin sensitivity. Central blockade of cell proliferation also normalized circulating levels of the cytokines leptin and interleukin 1β and decreased peritoneal proinflammatory CD86 immunoreactive macrophage number. These findings suggest that inhibition of diet-dependent microglia expansion hinders body weight gain while preventing central and peripheral inflammatory responses due to caloric overload.

Effectiveness of conservative interventions for sickness and pain behaviors induced by a high repetition high force upper extremity task

Background

Systemic inflammation is known to induce sickness behaviors, including decreased social interaction and pain. We have reported increased serum inflammatory cytokines in a rat model of repetitive strain injury (rats perform an upper extremity reaching task for prolonged periods). Here, we sought to determine if sickness behaviors are induced in this model and the effectiveness of conservative treatments.

Methods

Experimental rats underwent initial training to learn a high force reaching task (10 min/day, 5 days/week for 6 weeks), with or without ibuprofen treatment (TRHF vs. TRHF + IBU rats). Subsets of trained animals went on to perform a high repetition high force (HRHF) task for 6 or 12 weeks (2 h/day, 3 days/week) without treatment, or received two secondary interventions: ibuprofen (HRHF + IBU) or a move to a lower demand low repetition low force task (HRHF-to-LRLF), beginning in task week 5. Mixed-effects models with repeated measures assays were used to assay duration of social interaction, aggression, forepaw withdrawal thresholds and reach performance abilities. One-way and two-way ANOVAs were used to assay tissue responses. Corrections for multiple comparisons were made.

Results

TRHF + IBU rats did not develop behavioral declines or systemic increases in IL-1beta and IL-6, observed in untreated TRHF rats. Untreated HRHF rats showed social interaction declines, difficulties performing the operant task and forepaw mechanical allodynia. Untreated HRHF rats also had increased serum levels of several inflammatory cytokines and chemokines, neuroinflammatory responses (e.g., increased TNFalpha) in the brain, median nerve and spinal cord, and Substance P and neurokinin 1 immunoexpression in the spinal cord. HRHF + IBU and HRHF-to-LRLF rats showed improved social interaction and reduced inflammatory serum, nerve and brain changes. However, neither secondary treatment rescued HRHF-task induced forepaw allodynia, or completely attenuated task performance declines or spinal cord responses.

Conclusions

These results suggest that inflammatory mechanisms induced by prolonged performance of high physical demand tasks mediate the development of social interaction declines and aggression. However, persistent spinal cord sensitization was associated with persistent behavioral indices of discomfort, despite use of conservative secondary interventions indicating the need for prevention or more effective interventions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-017-0354-3) contains supplementary material, which is available to authorized users.

Inflammatory transcription factors as activation markers and functional readouts in immune-to-brain communication

Immune-to-brain communication pathways involve humoral mediators, including cytokines, central modulation by neuronal afferents and immune cell trafficking to the brain. During systemic inflammation these pathways contribute to mediating brain-controlled sickness symptoms including fever. Experimentally, activation of these signaling pathways can be mimicked and studied when injecting animals with pathogen associated molecular patterns (PAMPS). One central component of the brain inflammatory response, which leads, for example, to fever induction, is transcriptional activation of brain cells via cytokines and PAMPS. We and others have studied the spatiotemporal activation and the physiological significance of transcription factors for the induction of inflammation within the brain and the manifestation of fever. Evidence has revealed a role of nuclear factor (NF)κB in the initiation, signal transducer and activator of transcription (STAT)3 in the maintenance and NF-interleukin (IL)6 in the maintenance or even termination of brain-inflammation and fever. Moreover, psychological stressors, such as exposure to a novel environment, leads to increased body core temperature and genomic NF-IL6-activation, suggesting a potential use of NF-IL6-immunohistochemistry as a multimodal brain cell activation marker and a role for NF-IL6 for differential brain activity. In addition, the nutritional status, as reflected by circulating levels of the cytokine-like hormone leptin, influence immune-to-brain communication and age-dependent changes in LPS-induced fever. Overall, transcription factors remain therapeutically important targets for the treatment of brain-inflammation and fever induction during infectious/non-infectious inflammatory and psychological stress. However, the exact physiological role and significance of these transcription factors requires to be further investigated.

Obesity Impacts Fever and Sickness Behavior During Acute Systemic Inflammation

Obesity is reaching dramatic proportions in humans and is associated with a higher risk for cardiovascular disease, diabetes, and cognitive alterations, and a higher mortality during infection and inflammation. The focus of the present review is on the influence of obesity on the presentation of fever, sickness behavior, and inflammatory responses during acute systemic inflammation.

The transcription factor nuclear factor interleukin 6 mediates pro- and anti-inflammatory responses during LPS-induced systemic inflammation in mice

The transcription factor nuclear factor interleukin 6 (NF-IL6) plays a pivotal role in neuroinflammation and, as we previously suggested, hypothalamus-pituitary-adrenal-axis-activation. Here, we investigated its contribution to immune-to-brain communication and brain controlled sickness symptoms during lipopolysaccharide (LPS)-induced (50 or 2500 μg/kg i.p.) systemic inflammation in NF-IL6-deficient (KO) or wildtype mice (WT). In WT LPS induced a dose-dependent febrile response and reduction of locomotor activity. While KO developed a normal fever after low-dose LPS-injection the febrile response was almost abolished 3-7 h after a high LPS-dose. High-dose LPS-stimulation was accompanied by decreased (8 h) followed by enhanced (24 h) inflammation in KO compared to WT e.g. hypothalamic mRNA-expression including microsomal prostaglandin E synthase, inducible nitric oxide synthase and further inflammatory mediators, neutrophil recruitment to the brain as well as plasma levels of inflammatory markers such as IL-6 and IL-10. Interestingly, KO showed reduced locomotor activity even under basal conditions, but enhanced locomotor activity to novel environment stress. Hypothalamic-pituitary-adrenal-axis-activity of KO was intact, but tryptophan-metabolizing enzymes were shifted to enhanced serotonin production and reuptake. Overall, we showed for the first time that NF-IL6 plays a dual role for sickness response and immune-to-brain communication: acting pro-inflammatory at 8h but anti-inflammatory at 24 h after onset of the inflammatory response reflecting active natural programming of inflammation. Moreover, reduced locomotor activity observed in KO might be due to altered tryptophan metabolism and serotonin reuptake suggesting some role for NF-IL6 as therapeutic target for depressive disorders.

Obesity, adipokines and neuroinflammation

Global levels of obesity are reaching epidemic proportions, leading to a dramatic increase in incidence of secondary diseases and the significant economic burden associated with their treatment. These comorbidities include diabetes, cardiovascular disease, and some psychopathologies, which have been linked to a low-grade inflammatory state. Obese individuals exhibit an increase in circulating inflammatory mediators implicated as the underlying cause of these comorbidities. A number of these molecules are also manufactured and released by white adipose tissue (WAT), in direct proportion to tissue mass and are collectively known as adipokines. In the current review we focused on the role of two of the better-studied members of this family namely, leptin and adiponectin, with particular emphasis on their role in neuro-immune interactions, neuroinflammation and subsequent brain diseases. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Leptin modulates the late fever response to LPS in diet-induced obese animals

Leptin is an important modulator of both inflammation and energy homeostasis, making it a key interface between the inflammatory response to pathogenic stimuli and the energy status of the host. In previous studies we demonstrated that sickness responses to systemic immune challenge, including fever, are significantly exacerbated in diet induced obese animals. To investigate whether this exacerbation is functionally linked to the obesity associated increase in circulating levels of leptin, a species-specific leptin antiserum (LAS) was used to neutralize endogenous leptin in diet-induced obese adult male Wistar rats treated with a single intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) (100μg/kg). LAS significantly reduced the magnitude of the later phases of the fever response, and attenuated the circulating levels of IL-6, IL-1ra and bioactivity of leptin in the obese animals. In addition, the antiserum significantly attenuated the hypothalamic expression of IL-1ß, IκBα, COX2, SOCS3 and IL-6 in both lean and obese rats 10h after the LPS injection and NF-IL6 in the hypothalamus of obese rats only. The relatively late rise in brain IL-6 suggested a role in mediating the extended fever response in obese animals and we tested this by neutralizing brain IL-6 using an IL6-AS injected intracerebroventricularly (4μl, icv). The IL6-AS significantly but transiently (between 9h and 12h post LPS) reduced the late fever response of obese rats. These results demonstrate that leptin plays an important part in modulating the late portion of the fever response to LPS, likely through the induction of hypothalamic IL-6 in obese animals.

In-depth conversation: spectrum and kinetics of neuroimmune afferent pathways

Since my last review on neuroimmune communication afferents in 2008, this area has witnessed substantial growth. At a basic science level, numerous new and exciting phenomena have been described, adding both depth and complexity to the crosstalk between the immune system and the nervous system. At a translational level, accumulating evidence indicates neuroimmune interaction could be a contributing factor for many disease states, as well as an effective physiological mechanism that coordinates the activities of these two systems in healthy individuals or during tissue distress. Furthermore, new evidence suggests neuroimmune interactions are inherently dynamic: varying activities in either the nervous system or the immune system could impact interactions between them. In this review I will attempt to integrate multifarious, and sometimes disparate, findings into a modified conceptual framework that describes the concordance of neuroimmune communication through the cooperative connection between these two systems and the dysfunction that may arise when their inappropriate crosstalk occurs.

Molecular and cellular neuroinflammatory status of mouse brain after systemic lipopolysaccharide challenge: importance of CCR2/CCL2 signaling

Background

Genetic and environmental factors are critical elements influencing the etiology of major depression. It is now accepted that neuroinflammatory processes play a major role in neuropsychological disorders. Neuroinflammation results from the dysregulation of the synthesis and/or release of pro- and anti-inflammatory cytokines with central or peripheral origin after various insults. Systemic bacterial lipopolysaccharide (LPS) challenge is commonly used to study inflammation-induced depressive-like behaviors in rodents. In the present study, we investigated immune-to-brain communication in mice by examining the effects of peripheral LPS injection on neuroinflammation encompassing cytokine and chemokine production, microglia and central nervous system (CNS)-associated phagocyte activation, immune cell infiltration and serotonergic neuronal function.

Methods

LPS was administered to C57BL/6 J mice by intraperitoneal injection; brains were collected and pro-inflammatory cytokine mRNA and proteins were measured. To examine the relative contribution of the different populations of brain immune cells to the occurrence of neuroinflammation after acute systemic inflammation, we precisely characterized them by flow cytometry, studied changes in their proportions and level of activation, and measured the amount of cytokines they released by Cytometric Bead Array™ after cell sorting and ex vivo culture. Because of the central role that the chemokine CCL2 seems to play in our paradigm, we studied the effect of CCL2 on the activity of serotonergic neurons of the raphe nucleus using electrophysiological recordings.

Results

We report that systemic LPS administration in mice caused a marked increase in pro-inflammatory IL-1β, IL-6, TNFα and CCL2 (monocyte chemoattractant protein-1) mRNA and protein levels in the brain. Moreover, we found that LPS caused microglia and CNS-associated phagocyte activation characterized by upregulation of CCR2, TLR4/CD14, CD80 and IL-4Rα, associated with overproduction of pro-inflammatory cytokines and chemokines, especially CCL2. LPS also induced a marked and selective increase of CCR2⁺ inflammatory monocytes within the brain. Finally, we showed that CCL2 hyperpolarized serotonergic raphe neurons in mouse midbrain slices, thus probably reducing the serotonin tone in projection areas.

Conclusion

Together, we provide a detailed characterization of the molecular and cellular players involved in the establishment of neuroinflammation after systemic injection of LPS. This highlights the importance of the CCL2/CCR2 signaling and suggests a possible link with depressive disorders.

Role of brain transmigrating neutrophils in depression-like behavior during systemic infection

Peripheral inflammation induces transmigration of interleukin (IL)-1β-expressing neutrophils to the brain. We investigated the possibility that this presents a new route of immune-to-brain communication by assessing their role in sickness behaviors relevant for mood disorders. Mice treated with lipopolysaccharide (LPS) developed despair-like behavior, and administration of an anti-polymorphonuclear antibody abolished LPS-induced despair-like and asocial behaviors, which correlated with the levels of IL-1β expression in the brain. These behavioral changes were directly mediated by the energy-regulating hormone, leptin. Increasing the concentration of endogenous leptin during obesity exacerbated, whereas its neutralization using a specific antiserum attenuated sickness behaviors and importantly the neutrophil transmigrating process. Our results indicate a role for peripheral neutrophils in conveying inflammatory signals to the brain, which appears to be dependent on the energy status of the organism. This constitutes a novel mechanism of immune-to-brain communication relevant to mood disorders.

Activation of the inflammatory transcription factor nuclear factor interleukin-6 during inflammatory and psychological stress in the brain

BACKGROUND

The transcription factor nuclear factor interleukin 6 (NF-IL6) is known to be activated by various inflammatory stimuli in the brain. Interestingly, we recently detected NF-IL6-activation within the hypothalamus-pituitary-adrenal (HPA)-axis of rats after systemic lipopolysaccharide (LPS)-injection. Thus, the aim of the present study was to investigate whether NF-IL6 is activated during either, inflammatory, or psychological stress in the rat brain.

METHODS

Rats were challenged with either the inflammatory stimulus LPS (100 μg/kg, i.p.) or exposed to a novel environment. Core body temperature (Tb) and motor activity were monitored using telemetry, animals were killed at different time points, brains and blood removed, and primary cell cultures of the anterior pituitary lobe (AL) were investigated. Analyses were performed using immunohistochemistry, RT-PCR, and cytokine-specific bioassays.

RESULTS

Stress stimulation by a novel environment increased NF-IL6-immunoreactivity (IR) in the pituitary's perivascular macrophages and hypothalamic paraventricular cells and a rise in Tb lasting approximately 2 h. LPS stimulation lead to NF-IL6-IR in several additional cell types including ACTH-IR-positive corticotrope cells in vivo and in vitro. Two other proinflammatory transcription factors, namely signal transducer and activator of transcription (STAT)3 and NFκB, were significantly activated and partially colocalized with NF-IL6-IR in cells of the AL only after LPS-stimulation, but not following psychological stress. In vitro NF-IL6-activation was associated with induction and secretion of TNFα in folliculostellate cells, which could be antagonized by the JAK-STAT-inhibitor AG490.

CONCLUSIONS

We revealed, for the first time, that NF-IL6 activation occurs not only during inflammatory LPS stimulation, but also during psychological stress, that is, a novel environment. Both stressors were associated with time-dependent activation of NF-IL6 in different cell types of the brain and the pituitary. Moreover, while NF-IL6-IR was partially linked to STAT3 and NFκB activation, TNFα production, and ACTH-IR after LPS stimulation; this was not the case after exposure to a novel environment, suggesting distinct underlying signaling pathways. Overall, NF-IL6 can be used as a broad activation marker in the brain and might be of interest for therapeutic approaches not only during inflammatory but also psychological stress.

Role of Angptl4 in vascular permeability and inflammation

BACKGROUND

Angptl4 is a secreted protein involved in the regulation of vascular permeability, angiogenesis, and inflammatory responses in different kinds of tissues. Increases of vascular permeability and abnormality changes in angiogenesis contribute to the pathogenesis of tumor metastasis, ischemic-reperfusion injury. Inflammatory response associated with Angptl4 also leads to minimal change glomerulonephritis, wound healing. However, the role of Angptl4 in vascular permeability, angiogenesis, and inflammation is controversy. Hence, an underlying mechanism of Angptl4 in different kind of tissues needs to be further clarified.

METHODS

Keywords such as angptl4, vascular permeability, angiogenesis, inflammation, and endothelial cells were used in search tool of PUBMED, and then the literatures associated with Angptl4 were founded and read.

RESULTS

Data have established Angptl4 as the key modulator of both vascular permeability and angiogenesis; furthermore, it may also be related to the progression of metastatic tumors, cardiovascular events, and inflammatory diseases. This view focuses on the recent advances in our understanding of the role of Angptl4 in vascular permeability, angiogenesis, inflammatory signaling and the link between Angptl4 and multiple diseases such as cancer, cardiovascular diseases, diabetic retinopathy, and kidney diseases.

CONCLUSIONS

Taken together, Angptl4 modulates vascular permeability, angiogenesis, inflammatory signaling, and associated diseases. The use of Angptl4-modulating agents such as certain drugs, food constituents (such as fatty acids), nuclear factor (such as PPARα), and bacteria may treat associated diseases such as tumor metastasis, ischemic-reperfusion injury, inflammation, and chronic low-grade inflammation. However, the diverse physiological functions of Angptl4 in different tissues can lead to potentially deleterious side effects when used as a therapeutic target. In this regard, a better understanding of the underlying mechanisms for Angptl4 in different tissues is necessary.

Meningeal and choroid plexus cells--novel drug targets for CNS disorders

The meninges and choroid plexus perform many functions in the developing and adult human central nervous system (CNS) and are composed of a number of different cell types. In this article I focus on meningeal and choroid plexus cells as targets for the development of drugs to treat a range of traumatic, ischemic and chronic brain disorders. Meningeal cells are involved in cortical development (and their dysfunction may be involved in cortical dysplasia), fibrotic scar formation after traumatic brain injuries (TBI), brain inflammation following infections, and neurodegenerative disorders such as Multiple Sclerosis (MS) and Alzheimer's disease (AD) and other brain disorders. The choroid plexus regulates the composition of the cerebrospinal fluid (CSF) as well as brain entry of inflammatory cells under basal conditions and after injuries. The meninges and choroid plexus also link peripheral inflammation (occurring in the metabolic syndrome and after infections) to CNS inflammation which may contribute to the development and progression of a range of CNS neurological and psychiatric disorders. They respond to cytokines generated systemically and secrete cytokines and chemokines that have powerful effects on the brain. The meninges may also provide a stem cell niche in the adult brain which could be harnessed for brain repair. Targeting meningeal and choroid plexus cells with therapeutic agents may provide novel therapies for a range of human brain disorders.

Role of IL-6 in the etiology of hyperexcitable neuropsychiatric conditions: experimental evidence and therapeutic implications

Many neuropsychiatric conditions are primed or triggered by different types of stressors. The mechanisms through which stress induces neuropsychiatric disease are complex and incompletely understood. A ‘double hit’ hypothesis of neuropsychiatric disease postulates that stress induces maladaptive behavior in two phases separated by a dormant period. Recent research shows that the pleiotropic cytokine IL-6 is released centrally and peripherally following physical and psychological stress. In this article, we analyze evidence from clinics and animal models suggesting that stress-induced elevation in the levels of IL-6 may play a key role in the etiology of a heterogeneous family of hyperexcitable central conditions including epilepsy, schizophrenic psychoses, anxiety and disorders of the autistic spectrum. The cellular mechanism leading to hyperexcitable conditions might be a decrease in inhibitory/excitatory synaptic balance in either or both temporal phases of the conditions. Following these observations, we discuss how they may have important implications for optimal prophylactic and therapeutic pharmacological treatment.

Lipopolysaccharide-induced fever depends on prostaglandin E2 production specifically in brain endothelial cells

Immune-induced prostaglandin E2 (PGE2) synthesis is critical for fever and other centrally elicited disease symptoms. The production of PGE2 depends on cyclooxygenase-2 and microsomal prostaglandin E synthase-1 (mPGES-1), but the identity of the cells involved has been a matter of controversy. We generated mice expressing mPGES-1 either in cells of hematopoietic or nonhematopoietic origin. Mice lacking mPGES-1 in hematopoietic cells displayed an intact febrile response to lipopolysaccharide, associated with elevated levels of PGE2 in the cerebrospinal fluid. In contrast, mice that expressed mPGES-1 only in hematopoietic cells, although displaying elevated PGE2 levels in plasma but not in the cerebrospinal fluid, showed no febrile response to lipopolysaccharide, thus pointing to the critical role of brain-derived PGE2 for fever. Immunohistochemical stainings showed that induced cyclooxygenase-2 expression in the brain exclusively occurred in endothelial cells, and quantitative PCR analysis on brain cells isolated by flow cytometry demonstrated that mPGES-1 is induced in endothelial cells and not in vascular wall macrophages. Similar analysis on liver cells showed induced expression in macrophages and not in endothelial cells, pointing at the distinct role for brain endothelial cells in PGE2 synthesis. These results identify the brain endothelial cells as the PGE2-producing cells critical for immune-induced fever.

The immune theory of psychiatric diseases: a key role for activated microglia and circulating monocytes

This review describes a key role for mononuclear phagocytes in the pathogenesis of major psychiatric disorders. There is accumulating evidence for activation of microglia (histopathology and PET scans) and circulating monocytes (enhanced gene expression of immune genes, an overproduction of monocyte/macrophage-related cytokines) in patients with bipolar disorder, major depressive disorder, and schizophrenia. These data are strengthened by observations in animal models, such as the MIA models, the chronic stress models, and the NOD mouse model. In these animal models of depressive-, anxiety-, and schizophrenia-like behavior, similar activations of microglia and circulating monocytes can be found. These animal models also make in-depth pathogenic studies possible and show that microglia activation impacts neuronal development and function in brain areas congruent with the altered depressive and schizophrenia-like behaviors.

Angiopoietin-like protein 4: health effects, modulating agents and structure-function relationships

Angiopoietin-like protein 4 (ANGPTL4) has been identified as a multifunctional signal protein. It is produced by a variety of tissues, and is secreted into the bloodstream in glycosylated, oligomerized, native and cleaved isoforms to modulate physiological events such as angiogenesis, cell differentiation and the crosstalk between liver, brain, adipose and muscle tissue in lipid and glucose metabolism. In addition, the expression and isoform appearance of ANGPTL4 are modified by the intestinal microbiota. With an eye on an effective strategy to improve health using ANGPTL4, we will focus on: health issues associated with ANGPTL4 expression, including obesity, Type 2 diabetes, cardiovascular diseases and cancer; several modulators of ANGPTL4 of chemical, microbiological, food and host origin; and the correlation of the specific ANGPTL4 isoforms with these modulators and their health effects.

Unmyelinated nerve fibers in the human dental pulp express markers for myelinated fibers and show sodium channel accumulations

BACKGROUND

The dental pulp is a common source of pain and is used to study peripheral inflammatory pain mechanisms. Results show most fibers are unmyelinated, yet recent findings in experimental animals suggest many pulpal afferents originate from fibers that are myelinated at more proximal locations. Here we use the human dental pulp and confocal microscopy to examine the staining relationships of neurofilament heavy (NFH), a protein commonly expressed in myelinated afferents, with other markers to test the possibility that unmyelinated pulpal afferents originate from myelinated axons. Other staining relationships studied included myelin basic protein (MBP), protein gene product (PGP) 9.5 to identify all nerve fibers, tyrosine hydroxylase (TH) to identify sympathetic fibers, contactin-associated protein (caspr) to identify nodal sites, S-100 to identify Schwann cells and sodium channels (NaChs).

RESULTS

Results show NFH expression in most PGP9.5 fibers except those with TH and include the broad expression of NFH in axons lacking MBP. Fibers with NFH and MBP show NaCh clusters at nodal sites as expected, but surprisingly, NaCh accumulations are also seen in unmyelinated fibers with NFH, and in fibers with NFH that lack Schwann cell associations.

CONCLUSIONS

The expression of NFH in most axons suggests a myelinated origin for many pulpal afferents, while the presence of NaCh clusters in unmyelinated fibers suggests an inherent capacity for the unmyelinated segments of myelinated fibers to form NaCh accumulations. These findings have broad implications on the use of dental pulp to study pain mechanisms and suggest possible novel mechanisms responsible for NaCh cluster formation and neuronal excitability.

The γ-secretase blocker DAPT impairs recovery from lipopolysaccharide-induced inflammation in rat brain

γ-Secretase is an important contributing enzyme in Alzheimer's disease and is therefore an important therapeutic target. However, the impact of γ-secretase inhibition is not well studied in acute neuroinflammation induced by systemic infection. In this study the influence of γ-secretase on the expression of some proinflammatory markers was assessed in the acute phase as well as the subsiding phase of neuroinflammation. Cerebral γ-secretase cleavage activity was measured by a fluorometric assay after lipopolysaccharide (LPS) intraperitoneal administration. Time profiles of TNF-α and COX-II expression were then determined to detect the time points relevant to the maximal inflammatory responses and the subsequent recovery phase. γ-Secretase activity coincident with TNF-α protein expression returned to its basal level till 8-12 h after systemic challenge with low dose LPS while COX-II over expression lasted for 48-72 h later. Pharmacological inhibition of γ-secretase with local or systemic administration of DAPT (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester) was performed to indicate the results on the developmental and sinking phases of inflammatory responses in 6 and 72 h post LPS respectively. Our results demonstrate that both local and systemic modulation of γ-secretase hyper-activity with DAPT increase the duration of TNF-α, COX-II, and NFκB induction. We consistently found mild augmented apoptosis in animals treated with DAPT as determined by measuring cleaved caspase-3 expression and by TUNEL assay 72 h following LPS injection. These results suggest that γ-secretase modulation interferes with certain immune regulatory pathways which may restrict some inflammatory transcription factors such as NFκB.

Regulatory T cells suppress sickness behaviour development without altering liver injury in cholestatic mice

BACKGROUND & AIMS

Cholestatic liver diseases are commonly accompanied by debilitating symptoms, collectively termed sickness behaviours. Regulatory T cells (T(regs)) can suppress inflammation; however, a role for T(regs) in modulating sickness behaviours has not been evaluated.

METHODS

A mouse model of cholestatic liver injury due to bile duct ligation (BDL) was used to study the role of T(regs) in sickness behaviour development.

RESULTS

BDL mice developed reproducible sickness behaviours, as assessed in a social investigation paradigm, characterized by decreased social investigative behaviour and increased immobility. Depletion of peripheral T(regs) in BDL mice worsened BDL-associated sickness behaviours, whereas infusion of T(regs) improved these behaviours; however, liver injury severity was not altered by T(reg) manipulation. Hepatic IL-6 mRNA and circulating IL-6 levels were elevated in BDL vs. control mice, and were elevated further in T(reg)-depleted BDL mice, but were decreased after infusion of T(regs) in BDL mice. IL-6 knock out (KO) BDL mice exhibited a marked reduction in sickness behaviours, compared to wildtype BDL mice. Furthermore, IL-6 KO BDL mice injected with rmIL-6 displayed sickness behaviours similar to wildtype BDL mice, whereas saline injection did not alter behaviour in IL-6 KO BDL mice. BDL was associated with increased hippocampal cerebral endothelial cell p-STAT3 expression, which was significantly reduced in IL-6 KO BDL mice.

CONCLUSIONS

T(regs) modulate sickness behaviour development in the setting of cholestatic liver injury, driven mainly through T(reg) inhibition of circulating monocyte and hepatic IL-6 production, and subsequent signalling via circulating IL-6 acting at the level of the cerebral endothelium.

Fever, sickness behavior, and expression of inflammatory genes in the hypothalamus after systemic and localized subcutaneous stimulation of rats with the Toll-like receptor 7 agonist imiquimod

The Toll-like receptor 7 (TLR7) agonist imiquimod is used for topical treatment of skin cancers. We studied the consequences of injections of imiquimod into a subcutaneous (s.c.) air pouch or of intraperitoneal (i.p.) injections on the manifestation of fever, sickness behavior, and the peripheral and brain-intrinsic induction of a variety of inflammatory molecules. Rats were given imiqimod s.c. or i.p. (1 or 5 mg/kg). Body temperature, motor activity, and food and water intake were recorded by telemetric devices. Peripheral and brain-intrinsic induction of inflammatory mediators was analyzed by real-time polymerase chain reaction (RT-PCR), bioassays, enzyme-linked immunosorbent assays (ELISAs), and immunohistochemistry. Imiquimod is the first TLR-agonist to produce more potent effects with s.c. than i.p. administration. Peripheral induction of interferons (IFNs) and putative circulating pyrogens corresponded to the magnitude of the illness responses. In the brain, an expression of cytokines (TNFα, IL-1β, and IL-6) and inducible forms of enzymes for prostaglandin E2 synthesis (COX-2 and mPGES) occurred, which was accompanied by a moderate activation of the transcription factors NFκB and STAT3, and a strong activation of the transcription factor NF-IL6, in cells of specific areas with an open blood-brain barrier. These inflammatory responses noted within the brain were more marked after s.c. administration, than i.p. administration of imiquimod. At a dose of 5 mg/kg, imiquimod causes rather moderate brain-inflammatory responses, which are related to peripheral IFN-expression and possibly mediated by brain-intrinsic activation of NF-IL6 and induction of a proinflammatory cocktail. The lack of a septic-like state in imiquimod-treated rats reinforces the therapeutic use of this drug.

Fever induction by systemic stimulation with macrophage-activating lipopeptide-2 depends upon TLR2 but not CD36

This study was designed to test the responses of TLR2-knockout mice (TLR2-KO) and wild- type mice (C57/BL-6), and of CD36 deficient spontaneously hypertensive rats (SHR) and their genetic controls [Wistar Kyoto (WKY) rats] to systemic stimulations with the TLR2/6 agonist MALP-2 and the TLR4 agonist LPS. Fever and formation of TNF-α and IL-6 induced by intraperitoneal injections of MALP-2 (1000 µg/kg) were completely blunted in TLR2-KO, while LPS (100 µg/kg)-induced responses were not abolished in these animals. In SHR lacking CD36, a reduction of fever was observed in response to MALP-2 (100 µg/kg), but LPS-fever was even more attenuated in SHR when compared with WKY controls. Concentrations of circulating IL-6 tended to be lower in SHR after stimulation with both pyrogens. However, the IL-6-mediated activation of the transcription factor STAT3 in the brain was identical in both strains, indicating that the brain-controlled inflammatory response to MALP-2 (and LPS) is not impaired in the absence of CD36. In addition, stimulation of peritoneal macrophages with LPS and MALP-2 (10 µg/ml) caused the appearance of similar concentrations of bioactive cytokines in the supernatants from cells of both rat strains. These results demonstrate that TLR2 is essential for the manifestation of MALP-2, but not LPS-induced inflammatory responses. A moderate participation of CD36 in MALP-2-induced sickness- and cytokine-responses can not be ruled out but is unlikely as LPS-induced inflammatory responses were also attenuated in SHR.

Alterations in cognitive function and behavioral response to amphetamine induced by prenatal inflammation are dependent on the stage of pregnancy

Maternal infection during human pregnancy has been associated with the development of schizophrenia in the adult offspring. The stage of development and the maternal inflammatory response to infection, which undergoes quantitative and qualitative changes throughout gestation, are thought to determine critical windows of vulnerability for the developing brain. In order to investigate how these two factors may contribute to the outcome in the offspring, we studied the inflammatory response to turpentine (TURP) injection (100 μl/dam) and its consequences in the adult offspring, in pregnant rats at gestational day (GD) 15 or 18, which correspond to late first and early second trimester of human pregnancy, respectively. Maternal inflammatory response to TURP was different between the two GDs, with fever and circulating levels of the pro-inflammatory interleukin (IL)-6 significantly attenuated at GD 18, compared to GD 15. In the adult offspring, TURP challenge at GD 15 induced a significant decrease in pre-pulse inhibition (PPI) of acoustic startle, increased latency in the cued task of the Morris-water maze, prolonged conditioned fear response and enhanced locomotor effect of amphetamine. In contrast, the same immune challenge at GD 18 induced only a prolonged conditioned fear response. These results suggest a window of vulnerability at GD 15, at which TURP seems to affect several behaviors that are strongly modulated by dopamine. This was supported by increased tyrosine hydroxylase expression in the nucleus accumbens of the adult offspring of mothers treated at GD 15.

Spatiotemporal nuclear factor interleukin-6 expression in the rat brain during lipopolysaccharide-induced fever is linked to sustained hypothalamic inflammatory target gene induction

Rats injected with lipopolysaccharide (LPS) show brain-controlled sickness symptoms, including fever. In these animals, early genomic activation of brain cells was previously monitored by immunohistochemical detection of transcription factors such as nuclear factor (NF)-κB or signal transducer and activator of transcription (STAT)3 and was linked to the initiation or maintenance of the febrile response. To investigate whether NF-IL6 might be another important transcription factor implicated in this kind of immune-to-brain signaling, rats were injected with LPS (100 μg/kg, intraperitoneally) or phosphate-buffered saline, and brains were analyzed by immunohistochemistry, real-time PCR, or Western blot 4, 6, 8, and 10 hours later. Moderate to strong LPS-induced nuclear NF-IL6 immunoreactivity (IR) occurred in a time-dependent manner within circumventricular organs, namely, the vascular organ of the lamina terminalis, the subfornical organ, the area postrema, and the median eminence, brain structures with a leaky blood-brain barrier. Furthermore, nuclear NF-IL6-IR was observed in the pituitary gland, the choroid plexus, and the meninges as well as blood vessels throughout the entire brain. Endothelial, microglial, and ependymal cells, astrocytes, perivascular macrophages, and neurons exhibited LPS-induced nuclear NF-IL6-IR; mRNA levels of NF-IL6, responsive inflammatory genes, and NF-IL6 protein levels were significantly elevated. As opposed to observations on STAT3 or NFκB, the percentage of NF-IL6-reactive cells increased in parallel to late phases of the febrile response. In conclusion, these results suggest a potential role for NF-IL6 in the maintenance or possibly the termination of LPS-induced fever. Moreover, we propose NF-IL6 to be a delayed brain cell activation marker.

Systemic LPS administration induces brain inflammation but not dopaminergic neuronal death in the substantia nigra

It has been suggested that brain inflammation is important in aggravation of brain damage and/or that inflammation causes neurodegenerative diseases including Parkinson's disease (PD). Recently, systemic inflammation has also emerged as a risk factor for PD. In the present study, we evaluated how systemic inflammation induced by intravenous (iv) lipopolysaccharides (LPS) injection affected brain inflammation and neuronal damage in the rat. Interestingly, almost all brain inflammatory responses, including morphological activation of microglia, neutrophil infiltration, and mRNA/protein expression of inflammatory mediators, appeared within 4-8 h, and subsided within 1-3 days, in the substantia nigra (SN), where dopaminergic neurons are located. More importantly, however, dopaminergic neuronal loss was not detectable for up to 8 d after iv LPS injection. Together, these results indicate that acute induction of systemic inflammation causes brain inflammation, but this is not sufficiently toxic to induce neuronal injury.

Neonatal programming of innate immune function

The early life environment can be crucial in influencing the development of an animal's long-term physiology. There is now much evidence to suggest that perinatal challenges to an animal's immune system will result in changes in adult rat behavior, physiology, and molecular pathways following a single inflammatory event during development caused by the bacterial endotoxin lipopolysaccharide (LPS). In particular, it is now apparent that neonatal LPS administration can influence the adult neuroimmune response to a second LPS challenge through hypothalamic-pituitary-adrenal axis modifications, some of which are caused by alterations in peripheral prostaglandin synthesis. These pronounced changes are accompanied by a variety of alterations in a number of disparate aspects of endocrine physiology, with significant implications for the health and well-being of the adult animal. In this review, we discuss the newly elucidated mechanisms by which neonatal immune challenge can permanently alter an animal's endocrine and metabolic physiology and the implications this has for various disease states.

Leptin regulates leukocyte recruitment into the brain following systemic LPS-induced inflammation

The appetite suppressing hormone leptin has emerged as an important modulator of immune function and is now considered to be a critical link between energy balance and host defense responses to pathogens. These 'adaptive' responses can, in situations of severe and sustained systemic inflammation, lead to adverse effects including brain damage that is partly mediated by neutrophil recruitment into the brain. We examined the contribution of leptin to this process in leptin-deficient (ob/ob), -resistant (db/db) and wild-type (WT) mice injected intraperitoneally with a septic dose of lipopolysaccharide (LPS). This treatment induced a dramatic increase in the number of neutrophils entering the brain of WT mice, an effect that was almost totally abolished in the mutant mice and correlated with a significant reduction in the mRNA levels of interleukin-1beta, intracellular adhesion molecule-1 and neutrophil-specific chemokines. These effects were reversed with leptin replenishment in ob/ob mice leading to recovery of neutrophil recruitment into the brain. Moreover, 48 h food deprivation in WT mice, which decreased circulating leptin levels, attenuated the LPS-induced neutrophil recruitment as did a single injection of an anti-leptin antiserum 4 h before LPS treatment in WT mice. These results provide the first demonstration that leptin has a critical role in leukocyte recruitment to the brain following severe systemic inflammation with possible implications for individuals with altered leptin levels such as during obesity or starvation.

Leptin modulates cell morphology and cytokine release in microglia

The appetite suppressing hormone, leptin is now established as an important component of the immune response to pathogens partly via the induction of brain IL-1beta. We have previously demonstrated that this hormone acts on microglia to induce the release of IL-1beta through actions on its functional receptors. In the present study, we extended these findings by demonstrating that leptin's action on microglia is that of a modulator rather than a direct trigger of inflammation. Using primary microglia cultures prepared from rat brain we show that pre-incubation of these cells with leptin for 24h prior to treatment with LPS increased the IL-1beta output 2-fold. This effect was not limited to IL-1beta but was also true for another cytokine, TNF-alpha and chemokines such as CINC-1 and MIP-2. The role of leptin in potentiating the microglial response to LPS appeared to be linked to morphological changes rendering the microglia more reactive. These results suggest that leptin has an important role in microglial function in inflammation and given that its circulating levels fluctuate across a number of conditions, these findings can have important implications for an individual's ability to mount an efficient and complete response to invading pathogens.

Febrile seizures: current views and investigations

Febrile seizures (FSs) are seizures that occur during fever, usually at the time of a cold or flu, and represent the most common cause of seizures in the pediatric population. Up to 5% of children between the ages of six months and five years-of-age will experience a FS. Clinically these seizures are categorized as benign events with little impact on the growth and development of the child. However, studies have linked the occurrence of FSs to an increased risk of developing adult epileptic disorders. There are many unanswered questions about FSs, such as the mechanism of their generation, the long-term effects of these seizures, and their role in epileptogenesis. Answers are beginning to emerge based on results from animal studies. This review summarizes the current literature on animal models of FSs, mechanisms underlying the seizures, and functional, structural, and molecular changes that may result from them.

MyD88 plays a key role in LPS-induced Stat3 activation in the hypothalamus

Infection causes the production of proinflammatory cytokines, which act on the central nervous system (CNS) and can result in fever, sleep disorders, depression-like behavior, and even anorexia, although precisely how cytokines regulate the functions of the CNS remain unclear. In the present study, we investigated the regulatory-molecular mechanisms by which cytokines affect hypothalamic function in a state of infection. The intraperitoneal administration of lipopolysaccharide (LPS), a ligand of Toll-like receptor 4 (TLR4), time-dependently (2-24 h) increased signal transducer and activator of transcription 3 (STAT3) phosphorylation in the hypothalamus and liver, which corresponded with anorexia observed within 24 h. Interestingly, the pattern of phosphorylation in response to LPS differed between the hypothalamus and liver. In the hypothalamus, LPS increased STAT3 phosphorylation from 2 h, with a peak at 4 h and a decline thereafter, whereas, in the liver, the peak activation of STAT3 persisted from 2 to 8 h. The time course of the LPS-induced expression of suppressor of cytokine signaling 3 (SOCS3), a STAT3-induced negative regulator of the Janus kinase-STAT pathway, was similar to that of STAT3 phosphorylation. Using mice deficient in myeloid differentiation primary-response protein 88 (MyD88), an adapter protein of TLR4, we found that LPS-induced STAT3 phosphorylation and SOCS3 expression in the hypothalamus and liver were predominantly mediated through MyD88. Moreover, we observed that MyD88-deficient mice were resistant to LPS-induced anorexia. Taken together, our findings reveal a novel mechanism, i.e., MyD88 plays a key role in mediating STAT3 phosphorylation and anorexia in the CNS in a state of infection and inflammation.