Regulation of brain interleukin-1 beta (IL-1 beta) system mRNAs in response to pathophysiological concentrations of IL-1 beta in the cerebrospinal fluid

Interleukin-1β signaling in fenestrated capillaries is sufficient to trigger sickness responses in mice

BACKGROUND

The physiological and behavioral symptoms of sickness, including fever, anorexia, behavioral depression, and weight loss can be both beneficial and detrimental. These sickness responses are triggered by pro-inflammatory cytokines acting on cells within the brain. Previous research demonstrates that the febrile response to peripheral insults depends upon prostaglandin production by vascular endothelial cells, but the mechanisms and specific cell type(s) responsible for other sickness responses remain unknown. The purpose of the present study was to identify which cells within the brain are required for sickness responses triggered by central nervous system inflammation.

METHODS

Intracerebroventricular (ICV) administration of 10 ng of the potent pro-inflammatory cytokine interleukin-1β (IL-1β) was used as an experimental model of central nervous system cytokine production. We examined which cells respond to IL-1β in vivo via fluorescent immunohistochemistry. Using multiple transgenic mouse lines expressing Cre recombinase under the control of cell-specific promoters, we eliminated IL-1β signaling from different populations of cells. Food consumption, body weight, movement, and temperature were recorded in adult male mice and analyzed by two-factor ANOVA to determine where IL-1β signaling is essential for sickness responses.

RESULTS

Endothelial cells, microglia, ependymal cells, and astrocytes exhibit nuclear translocation of NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) in response to IL-1β. Interfering with IL-1β signaling in microglia, endothelial cells within the parenchyma of the brain, or both did not affect sickness responses. Only mice that lacked IL-1β signaling in all endothelium including fenestrated capillaries lacked sickness responses.

CONCLUSIONS

These experiments show that IL-1β-induced sickness responses depend on intact IL-1β signaling in blood vessels and suggest that fenestrated capillaries act as a critical signaling relay between the immune and nervous systems.

TRIAL REGISTRATION

Not applicable.

Brain cytokines and disease

Cytokines (e.g. various interleukins and subfamily members, tumor necrosis factors, interferons, chemokines and growth factors) act in the brain as immunoregulators and neuromodulators. Over a decade ago, the integrative article 'Immunoregulators in the Nervous System' (Neurosci Biobehav Rev 1991; 15: 185-215) provided a comprehensive framework of pivotal issues on cytokines and the nervous system that recently have been extensively studied. Cytokine profiles in the brain, including cytokine generation and action, have been studied in multiple models associated with neuropathophysiological conditions. These include: (1) acute conditions and disorders such as stroke (cerebral ischemia or infarction and intracranial hemorrhage), traumatic brain injury, spinal cord injury and acute neuropathies; (2) chronic neurodegenerative disorders and chronic conditions, including Alzheimer's disease, Parkinson's disease, neuropathic pain, epilepsy and chronic neuropathies; (3) brain infections, including bacterial meningitis and encephalitis; (4) brain tumors; (5) neuroimmunological disorders per se, such as multiple sclerosis; (5) psychiatric disorders, including schizophrenia and depression; (6) neurological and neuropsychiatric manifestations associated with non- central nervous system (CNS) disorders such as peripheral cancer, liver, kidney and metabolic compromise, and peripheral infectious and inflammatory conditions; and (7) cytokine immunotherapy, which can be accompanied by neuropsychiatric manifestations when administered either via peripheral or brain routes. Cytokine profiles have also been studied in multiple animal models challenged with inflammatory, infectious, chemical, malignant and stressor insults. Essentially data show that cytokines play a pivotal role in multiple neuropathophysiological processes associated with different types of disorders and insults. Cytokine expression and action in the brain shows a different profile across conditions, but some similarities exist. Under a defined temporal sequence, cytokine involvement in neuroprotection or the induction of a deleterious pathophysiological cascade and in resolution/healing is proposed depending on the type of cytokine. In the brain, functional interactions among cytokines, balance between pro-inflammatory and anti-inflammatory cytokines and functional interactions with neurotransmitters and neuropeptides play a pivotal role in the overall cytokine profile, pattern of neuropathophysiological cascades, and quality and magnitude of neuropsychiatric manifestations. In this brief review various selected cytokine-related issues with relevance to the brain are discussed.

How the immune and nervous systems interact during disease-associated anorexia

Anorexia is one of the most common symptoms associated with illness and constitutes an adaptive strategy in fighting acute infectious diseases. However, prolonged reduction in food intake and an increase in metabolic rate, as seen in the anorexia-cachexia syndrome, lead to depletion of body fat and protein reserves, thus worsening the organism's condition. Because the central nervous system controls many aspects of food intake, soluble factors known as cytokines that are secreted by immune cells might act on the brain to induce anorexia during disease. This review focuses on the communication pathways from the immune system to the brain that might mediate anorexia during disease. The vagus nerve is a rapid route of communication from the immune system to the brain, as subdiaphragmatic vagotomy attenuates the decrease in food-motivated behavior and c-Fos expression in the central nervous system in response to peripheral administration of the proinflammatory cytokine, interleukin-1beta, or bacterial lipopolysaccharide. At later time points after peripheral lipopolysaccharide administration, interleukin-1 itself acts in the brain to mediate anorexia and is found in the arcuate nucleus of the hypothalamus. The mechanisms by which interleukin-1beta gains access to the brain and the potential role of neuropeptide-Y-containing neurons in the arcuate hypothalamus in mediating anorexia during disease are discussed.

Pro-inflammatory and anti-inflammatory cytokine mRNA induction in the periphery and brain following intraperitoneal administration of bacterial lipopolysaccharide

Gram-negative bacteria-derived lipopolysaccharide (LPS or endotoxin) is known to play an important role in immune and neurological manifestations during bacterial infections. LPS exerts its effects through cytokines, and peripheral or brain administration of LPS activates cytokine production in the brain. In this study, we investigated cytokine and neuropeptide mRNA profiles in specific brain regions and peripheral organs, as well as serum tumor necrosis factor (TNF)-alpha protein levels, in response to the intraperitoneal administration of LPS. For the first time, the simultaneous analysis of interleukin (IL)-1beta system components (ligand, signaling receptor, receptor accessory proteins, receptor antagonist), TNF-alpha, transforming growth factor (TGF)-beta1, glycoprotein 130 (IL-6 receptor signal transducer), OB protein (leptin) receptor, neuropeptide Y, and pro-opiomelanocortin (opioid peptide precursor) mRNAs was done in samples from specific brain regions in response to peripherally administered LPS. The same brain region/organ sample was assayed for all cytokine mRNA components. Peripherally administered LPS up-regulated pro-inflammatory cytokine (IL-1beta and/or TNF-alpha) mRNAs within the cerebral cortex, cerebellum, hippocampus, spleen, liver, and adipose tissue. LPS also increased plasma levels of TNF-alpha protein. LPS did not up-regulate inhibitory (anti-inflammatory) cytokine (IL-1 receptor antagonist and TGF-beta1) mRNAs in most brain regions (except for IL-1 receptor antagonist in the cerebral cortex and for TGF-beta1 in the hippocampus), while they were increased in the liver, and IL-1 receptor antagonist was up-regulated in the spleen and adipose tissue. Overall, peripherally administered LPS modulated the levels of IL-1beta system components within the brain and periphery, but did not affect the neuropeptide-related components studied. The data suggest specificity of transcriptional changes induced by LPS and that cytokine component up-regulation in specific brain regions is relevant to the neurological and neuropsychiatric manifestations associated with peripheral LPS challenge.

Spatiotemporal induction patterns of cytokine and related immune signal molecule mRNAs in response to intrastriatal injection of lipopolysaccharide

The brain's response to a direct immune challenge was examined by in situ hybridization histochemistry. Lipopolysaccharide (bacterial endotoxin) injected acutely into rat striatum induced mRNA expression for inhibitory factor kappaBalpha, interleukin (IL)-1beta, tumor necrosis factor-alpha, IL-6, IL-12 p35, inducible nitric oxide synthase, IL-1 receptor antagonist, and the type 1 IL-1 receptor. Expression patterns were evaluated at select time points ranging from 15 min to 3 days post-injection. Rats injected with vehicle alone were used to control for mechanical effects. Following lipopolysaccharide administration, a wave of mRNA induction within brain parenchyma radiated outward from the injection site, generally peaking in intensity at the 16-h time point. The individual profiles of cytokine mRNA induction patterns reveal that the brain's immune response to local inflammatory stimulation is quite elaborate and in many ways resembles the progression of cytokine induction customary of localized inflammation in peripheral tissues.

Spatiotemporal induction patterns of cytokine and related immune signal molecule mRNAs in response to intrastriatal injection of lipopolysaccharide

The brain's response to a direct immune challenge was examined by in situ hybridization histochemistry. Lipopolysaccharide (bacterial endotoxin) injected acutely into rat striatum induced mRNA expression for inhibitory factor kappaBalpha, interleukin (IL)-1beta, tumor necrosis factor-alpha, IL-6, IL-12 p35, inducible nitric oxide synthase, IL-1 receptor antagonist, and the type 1 IL-1 receptor. Expression patterns were evaluated at select time points ranging from 15 min to 3 days post-injection. Rats injected with vehicle alone were used to control for mechanical effects. Following lipopolysaccharide administration, a wave of mRNA induction within brain parenchyma radiated outward from the injection site, generally peaking in intensity at the 16-h time point. The individual profiles of cytokine mRNA induction patterns reveal that the brain's immune response to local inflammatory stimulation is quite elaborate and in many ways resembles the progression of cytokine induction customary of localized inflammation in peripheral tissues.

Cytokine-cytokine interactions and the brain

Cytokine-cytokine interactions play a role in health and are crucial during immunological and inflammatory responses in disease. Cytokine interactions can result in additive, antagonist, or synergistic activities in maintaining physiological functions such as feeding, body temperature, and sleep, as well as in anorectic, pyrogenic, and somnogenic neurological manifestations of acute and chronic disease. These interactions involve signaling homology, convergence of signaling pathways, and/or positive or negative feedbacks within and among cytokine systems. The interplay of cytokines with neurotransmitters, peptides/neuropeptides, and hormones also influence cytokine action in the brain. Interactive chemical cascades involving cytokines are consistent with the homeostatic physiological mechanisms and with the multi-humoral, pleiotropic, and redundant processes that occur during acute and chronic disease.

Anorexia in response to acute illness

Anorexia associated with acute illness remains one of the most common, challenging, and difficult symptoms to treat. Surprisingly, little attention has been devoted to development of interventions to reverse this form of anorexia. Although incomplete, current understanding of the mechanisms responsible for illness-induced anorexia is sufficient to suggest therapeutic approaches. In this article, the major physiologic mechanisms underlying illness-induced anorexia are described. In addition, potential moderating effects of social, psychologic, and environmental factors are discussed. This information was used to develop recommendations for the treatment of anorexia. A majority of these interventions, however, are not research based. Further advances in the treatment of illness-induced anorexia will require greater understanding of the complex, interactive effects of psychologic, environmental, and biologic factors on eating behavior during illness. Therefore, areas requiring continued investigation are also outlined.

Dynamic regulation of the proinflammatory cytokine, interleukin-1beta: molecular biology for non-molecular biologists

Interleukin-1beta (IL-1beta) is a key mediator and modulator of a wide array of physiological responses important for survival. It is created by a variety of cell types, including immune cells, glia, and neurons. It is a very potent biological molecule, acting both at the periphery as well as within the central nervous system. The production and release of IL-1beta is tightly regulated by far more complex processes than previously thought. An appreciation of this complexity is necessary for proper interpretation of apparent contradictions in the literature where different aspects of IL-1beta expression are measured. Given that many researchers are not molecular biologists by training, yet need an appreciation of the controls that regulate the function of key proteins such as IL-1beta, this review is aimed at both: (a) clarifying the multiple levels at which IL-1beta production is modulated and (b) using IL-1beta regulation to explain the dynamics of gene regulation to non-molecular biologists. Three major topics will be discussed. First, regulation of IL-1beta production will be examined at every level from extracellular signals that trigger gene activation through release of active protein into the extracellular fluid. Second, regulation of IL-1beta bioavailability and bioactivity will be discussed. This section examines the fact that even after IL-1beta is released, it may or may not be able to exert a biological action due to multiple modulatory factors. Last is the introduction of the idea that IL-1beta regulation is, at times, beyond the direct control of host; that is, when IL-1beta production becomes dysregulated by pathogens.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

1998 Curt P. Richter Award. Brain mechanisms in cytokine-induced anorexia

Our research focuses on the mechanisms underlying cytokine action in the central nervous system (CNS) using an integrative and multidisciplinary strategy organized through supracellular (behavioral analysis by computerized monitoring systems), cellular (extracellular and intracellular neurophysiological recording), and molecular (patch-clamp recording, and DNA, RNA and protein analyses) approaches. An integrative strategy that combines computerized meal pattern analyses with cellular and molecular biology approaches allows the study of underlying brain mechanisms in cytokine- and disease-associated anorexia. This paper presents a comprehensive discussion of our laboratory's previously published data on brain mechanisms involved in cytokine-induced anorexia including the relevance of meal pattern analysis (meal size, meal duration, meal frequency, intermeal intervals), modulation of hypothalamic neuronal activity, molecular processes involving ionic conductances, cytokine-cytokine and cytokine-peptide interactions, and modulation of cytokine and peptide/neuropeptide system components (ligands, endogenous inhibitors, receptor subtypes, signal transduction molecules, intracellular mediators) and cytokine feedback systems.

Cytokine-induced anorexia. Behavioral, cellular, and molecular mechanisms

Cytokines induce anorexia. Recent issues concerning mechanistic aspects are: (1) Cytokines induce anorexia through different modes of behavioral action, that is, by affecting meal size, meal duration, and meal frequency differentially. Profiles also depend on the concentration or dosage. (2) The interface between the periphery and brain. Specific cytokines may be transported from the periphery to the brain. Cytokines generate mediators that can act on peripheral and/or brain target sites. Cerebrovasculature endothelium can also generate signals to modulate neural activities. Evidence indicates that the proposed vagal afferent signaling requires reassessment. Because of paracrine and autocrine actions, local cytokine production within the brain can induce anorexia. (3) Cytokines act directly on hypothalamic neurons proposed to participate in feeding. (4) Cytokine<-->cytokine and cytokine<-->peptide/neurotransmitter interactions are critical; for example, cytokines interact to induce anorexia synergistically, neuropeptide Y<-->cytokine interactions are antagonist, and cytokine<-->neurotransmitter and cytokine<-->leptin<-->neuropeptide Y<-->CRH-glucocorticoid and other endocrine interactions are important. A leptin receptor is related to gp 130, a signal transducer among interleukin (IL)-6 subfamily receptors; gp 130 and related molecules may be an interface for feeding control in health and disease. Various cytokines upregulate leptin and gp 130. An integrative approach combining computerized meal pattern analyses with cellular and molecular approaches is being used to characterize mechanisms (ligands, receptors, transducing molecules, and intracellular mediators) involved in cytokine-induced anorexia.

Brain cytokine mRNAs in anorectic rats bearing prostate adenocarcinoma tumor cells

Cancer is consistently associated with anorexia. The Lobund-Wistar rat model of prostate cancer exhibits clinical manifestations (including anorexia) that resemble many aspects of the human disease. Cytokines are proposed to be involved in cancer-associated anorexia. Here we investigated mRNA profiles of feeding-modulatory cytokines and neuropeptides in specific brain regions of anorectic Lobund-Wistar rats bearing prostate adenocarcinoma tumor cells. Interleukin (IL)-1beta system components (ligand, signaling receptor, receptor accessory proteins, receptor antagonist), tumor necrosis factor-alpha, transforming growth factor-beta1, glycoprotein 130 (IL-6 receptor signal transducer), proopiomelanocortin (POMC, opioid peptide precursor), and neuropeptide Y (NPY) mRNAs were analyzed with sensitive and specific RNase protection assays. The same brain region sample was assayed for all components. The data show that early anorexia in tumor-bearing rats was associated with an upregulation of IL-1beta mRNA in the brain regions examined (cerebellum, cortex, and hypothalamus). IL-1 receptor antagonist (IL-1Ra) mRNA and IL-1 receptor type I mRNA levels were also significantly increased in the cortex and hypothalamus. All other cytokine components, POMC, or NPY mRNA levels were not significantly different between tumor-bearing and pair-fed (control) rats. IL-1beta mRNA and IL-1Ra mRNA were also significantly upregulated in the spleen of tumor-bearing rats. These data suggest that 1) IL-1beta mRNA upregulation in the brain may be relevant to the anorexia exhibited by the tumor-bearing Lobund-Wistar rat and 2) in vivo characterization of cytokine components in discrete brain regions during cancer is necessary to understand underlying molecular mechanisms responsible for cancer-associated neurological manifestations.

Interleukin-1beta (IL-1beta)-induced modulation of the hypothalamic IL-1beta system, tumor necrosis factor-alpha, and transforming growth factor-beta1 mRNAs in obese (fa/fa) and lean (Fa/Fa) Zucker rats: implications to IL-1beta feedback systems and cytokine-cytokine interactions

Interleukin-1beta (IL-1beta) induces anorexia, fever, sleep changes, and neuroendocrine alterations when administered into the brain. Here, we investigated the regulation of the IL-1beta system (ligand, receptors, receptor accessory protein, and receptor antagonist), tumor necrosis factor-alpoha (TNF-alpha), transforming growth factor (TGF)-beta1, and TGF-alpha mRNAs in the hypothalamus of obese (fa/fa) and lean (Fa/Fa) Zucker rats in response to the intracerebroventricular microinfusion of IL-1beta (8.0 ng/24 hr for 72 hr, a dose that yields estimated pathophysiological concentrations in the cerebrospinal fluid). IL-1beta increased IL-1beta, IL-1 receptor types I and II (IL-1RI and IL-1RII), IL-1 receptor accessory protein soluble form (IL-1R AcP II), IL-1 receptor antagonist (IL-1Ra), TNF-alpha, and TGF-beta1 mRNAs in the hypothalamus from obese and lean rats. IL-1beta-induced IL-1beta system and ligand (IL-1beta, TNF-alpha, and TGF-beta1) mRNA profiles were highly intercorrelated in the same samples. Levels of membrane-bound IL-1R AcP and TGF-alpha mRNAs did not change. Heat-inactivated IL-1beta had no effect. The data suggest 1) the operation of an IL-1beta feedback system (IL-1beta/IL-1RI/IL-1R Acp II/IL-1RII/IL-1Ra) and 2) potential cytokine-cytokine interactions with positive (IL-1beta <--> TNF-alpha) and negative (TGF-beta1 --> IL-1beta/TNF-alpha) feedback. Dysregulation of the IL-1beta feedback system and the TGF-beta1/IL-1beta-TNF-alpha balance may have implications for neurological disorders associated with high levels of IL-1beta in the brain.

HIV-1 gp120 modulates hypothalamic cytokine mRNAs in vivo: implications to cytokine feedback systems

HIV-1-derived envelope glycoprotein 120 (gp120) may play an important role in HIV-1 neuropathology. Gp120 may act through mediators including proinflammatory cytokines. Here, we investigated the regulation of the IL-1 beta system [IL-1 beta, IL-1 receptor type I (IL-1RI), IL-1 receptor antagonist (IL-1Ra), IL-1 receptor accessory proteins (IL-1R AcP I and II)], TNF-alpha, TGF-alpha, and TGF-beta 1 mRNAs in the hypothalamus of Wistar rats in response to the chronic intracerebroventricular (ICV) microinfusion (via osmotic minipumps) of HIV-1 gp120 (100, 500, and 1000 ng/24 h for 72 h). Gp120 increased IL-1 beta, IL-1Ra, TNF-alpha, and TGF-beta 1 mRNAs. Gp120-induced cytokine mRNA profiles were highly intercorrelated in the same samples. Levels of IL-1RI, IL-1R AcP I and II, and TGF-alpha did not change significantly, and levels of GAPDH mRNA were constant. The data suggest potential cytokine-cytokine interactions with positive (IL-1 beta<-->TNF-alpha) and negative (IL-1Ra-->IL-1 beta; TGF-beta 1-->IL-1 beta/TNF-alpha) feedback in gp120 action. A dysregulation of the balance between stimulatory and inhibitory cytokine mechanisms may participate in the initiation, propagation, and/or aggravation of HIV-1 neuropathology.

HIV-1 envelope glycoprotein 120 regulates brain IL-1beta system and TNF-alpha mRNAs in vivo

Human immunodeficiency virus type I (HIV-1)-derived envelope glycoprotein 120 (gp120) is proposed to play an important role in HIV-1 neuropathology. Gp120 may act through mediators including proinflammatory cytokines. Here, we investigated the regulation of the IL-1beta system [IL-1beta, IL-1 receptor type I (IL-1RI), IL-1 receptor antagonist (IL-1Ra)], TNF-alpha and TGF-alpha mRNAs in the rat central nervous system (CNS) in response to the constant intracerebroventricular (ICV) microinfusion of HIV-1 gp120 for 72 h and 144 h. The results show that gp120: (1) increased IL-1beta and IL-1Ra mRNAs levels in the same samples from the cerebellum, hypothalamus and midbrain, with the largest increase in the hypothalamus; (2) induced profiles of IL-1beta mRNA and IL-1Ra mRNA that were highly intercorrelated; (3) increased the hypothalamic TNF-alpha mRNA levels; and (4) did not affect the IL-1RI mRNA and TGF-alpha mRNA levels in any brain region. A dysregulation in the IL-1beta/IL-1Ra CNS balance and a mutual induction and synergistic activity of IL-1beta and TNF-alpha could result in a deleterious amplification cycle of cellular activation and cytotoxicity with implications to HIV-1-associated encephalitis, encephalopathy, and neurological manifestations.

Interleukin-1 receptor type I mRNA levels in brain regions from male and female rats

Interleukin-1 (IL-1) induces a variety of neurological manifestations by direct action in the central nervous system (CNS). The IL-1 receptor type I (IL-1RI) mediates IL-1 signalling. In the present study, the steady-state content of IL-1RI mRNA was determined by a sensitive RNase protection assay in brain regions obtained from normal male and nonestrous female Wistar rats. The results show that brain regions differ in IL-1RI mRNA content. Highest levels of IL-1RI mRNA were detected in the male and female cerebral cortex. High levels of IL-1RI mRNA were also observed in the brain stem and its structures, and the cerebellum. Male and female rats exhibited similar differential profile of IL-1RI mRNA levels in the frontal cortex, hippocampus, hypothalamus, and cerebellum. The present data on brain distribution of IL-1RI mRNA levels suggest that distinct brain regions may depend differentially on the IL-1 system.