Receptors for interleukin-1 (alpha and beta) in mouse brain: mapping and neuronal localization in hippocampus

Localization of brain neuronal IL-1R1 reveals specific neural circuitries responsive to immune signaling

Interleukin-1 (IL-1) is a pro-inflammatory cytokine that exerts a wide range of neurological and immunological effects throughout the central nervous system (CNS) and is associated with the etiology of affective and cognitive disorders. The cognate receptor for IL-1, Interleukin-1 Receptor Type 1 (IL-1R1), is primarily expressed on non-neuronal cells (e.g., endothelial cells, choroidal cells, ventricular ependymal cells, astrocytes, etc.) throughout the brain. However, the presence and distribution of neuronal IL-1R1 (nIL-1R1) has been controversial. Here, for the first time, a novel genetic mouse line that allows for the visualization of IL-1R1 mRNA and protein expression (Il1r1GR/GR) was used to map all brain nuclei and determine the neurotransmitter systems which express nIL-1R1 in adult male mice. The direct responsiveness of nIL-1R1-expressing neurons to both inflammatory and physiological levels of IL-1β in vivo was tested. Neuronal IL-1R1 expression across the brain was found in discrete glutamatergic and serotonergic neuronal populations in the somatosensory cortex, piriform cortex, dentate gyrus, and dorsal raphe nucleus. Glutamatergic nIL-1R1 comprises most of the nIL-1R1 expression and, using Vglut2-Cre-Il1r1r/r mice, which restrict IL-1R1 expression to only glutamatergic neurons, an atlas of glutamatergic nIL-1R1 expression across the brain was generated. Analysis of functional outputs of these nIL-1R1-expressing nuclei, in both Il1r1GR/GR and Vglut2-Cre-Il1r1r/r mice, reveals IL-1R1+ nuclei primarily relate to sensory detection, processing, and relay pathways, mood regulation, and spatial/cognitive processing centers. Intracerebroventricular (i.c.v.) injections of IL-1 (20 ng) induces NFκB signaling in IL-1R1+ non-neuronal cells but not in IL-1R1+ neurons, and in Vglut2-Cre-Il1r1r/r mice IL-1 did not change gene expression in the dentate gyrus of the hippocampus (DG). GO pathway analysis of spatial RNA sequencing 1mo following restoration of nIL-1R1 in the DG neurons reveals IL-1R1 expression downregulates genes related to both synaptic function and mRNA binding while increasing select complement markers (C1ra, C1qb). Further, DG neurons exclusively express an alternatively spliced IL-1R Accessory protein isoform (IL-1RAcPb), a known synaptic adhesion molecule. Altogether, this study reveals a unique network of neurons that can respond directly to IL-1 via nIL-1R1 through non-autonomous transcriptional pathways; earmarking these circuits as potential neural substrates for immune signaling-triggered sensory, affective, and cognitive disorders.

Dopamine Transmission Imbalance in Neuroinflammation: Perspectives on Long-Term COVID-19

Dopamine (DA) is a key neurotransmitter in the basal ganglia, implicated in the control of movement and motivation. Alteration of DA levels is central in Parkinson’s disease (PD), a common neurodegenerative disorder characterized by motor and non-motor manifestations and deposition of alpha-synuclein (α-syn) aggregates. Previous studies have hypothesized a link between PD and viral infections. Indeed, different cases of parkinsonism have been reported following COVID-19. However, whether SARS-CoV-2 may trigger a neurodegenerative process is still a matter of debate. Interestingly, evidence of brain inflammation has been described in postmortem samples of patients infected by SARS-CoV-2, which suggests immune-mediated mechanisms triggering the neurological sequelae. In this review, we discuss the role of proinflammatory molecules such as cytokines, chemokines, and oxygen reactive species in modulating DA homeostasis. Moreover, we review the existing literature on the possible mechanistic interplay between SARS-CoV-2-mediated neuroinflammation and nigrostriatal DAergic impairment, and the cross-talk with aberrant α-syn metabolism.

Role of neuroinflammation mediated potential alterations in adult neurogenesis as a factor for neuropsychiatric symptoms in Post-Acute COVID-19 syndrome-A narrative review

Persistence of symptoms beyond the initial 3 to 4 weeks after infection is defined as post-acute COVID-19 syndrome (PACS). A wide range of neuropsychiatric symptoms like anxiety, depression, post-traumatic stress disorder, sleep disorders and cognitive disturbances have been observed in PACS. The review was conducted based on PRISMA-S guidelines for literature search strategy for systematic reviews. A cytokine storm in COVID-19 may cause a breach in the blood brain barrier leading to cytokine and SARS-CoV-2 entry into the brain. This triggers an immune response in the brain by activating microglia, astrocytes, and other immune cells leading to neuroinflammation. Various inflammatory biomarkers like inflammatory cytokines, chemokines, acute phase proteins and adhesion molecules have been implicated in psychiatric disorders and play a major role in the precipitation of neuropsychiatric symptoms. Impaired adult neurogenesis has been linked with a variety of disorders like depression, anxiety, cognitive decline, and dementia. Persistence of neuroinflammation was observed in COVID-19 survivors 3 months after recovery. Chronic neuroinflammation alters adult neurogenesis with pro-inflammatory cytokines supressing anti-inflammatory cytokines and chemokines favouring adult neurogenesis. Based on the prevalence of neuropsychiatric symptoms/disorders in PACS, there is more possibility for a potential impairment in adult neurogenesis in COVID-19 survivors. This narrative review aims to discuss the various neuroinflammatory processes during PACS and its effect on adult neurogenesis.

Neuroinflammation in Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a well-known mental illness, which is caused by various stressors, including memories of past physical assaults and psychological pressure. It is diagnosed as a mental and behavioral disorder, but increasing evidence is linking it to the immune system and inflammatory response. Studies on the relationship between inflammation and PTSD revealed that patients with PTSD had increased levels of inflammatory cytokine biomarkers, such as interleukin-1, interleukin-6, tumor necrosis factor-α, nuclear factor-κB, and C-reactive protein, compared with healthy controls. In addition, animal model experiments imitating PTSD patients suggested the role of inflammation in the pathogenesis and pathophysiology of PTSD. In this review, we summarize the definition of PTSD and its association with increased inflammation, its mechanisms, and future predictable diseases and treatment possibilities. We also discuss anti-inflammatory treatments to address inflammation in PTSD.

Transduction of inflammation from peripheral immune cells to the hippocampus induces neuronal hyperexcitability mediated by Caspase-1 activation

BACKGROUND

Recent studies report infiltration of peripheral blood mononuclear cells (PBMCs) into the central nervous system (CNS) in epileptic disorders, suggestive of a potential contribution of PBMC extravasation to the generation of seizures. Nevertheless, the underlying mechanisms involved in PBMC infiltrates promoting neuronal predisposition to ictogenesis remain unclear. Therefore, we developed an in vitro model mimicking infiltration of activated PBMCs into the brain in order to investigate potential transduction of inflammatory signals from PBMCs to the CNS.

METHODS

To establish our model, we first extracted PBMCs from rat spleen, then, immunologically primed PBMCs with lipopolysaccharide (LPS), followed by further activation with nigericin. Thereafter, we co-cultured these activated PBMCs with organotypic cortico-hippocampal brain slice cultures (OCHSCs) derived from the same rat, and compared PBMC-OCHSC co-cultures to OCHSCs exposed to PBMCs in the culture media. We further targeted a potential molecular pathway underlying transduction of peripheral inflammation to OCHSCs by incubating OCHSCs with the Caspase-1 inhibitor VX-765 prior to co-culturing PBMCs with OCHSCs. After 24 h, we analyzed inflammation markers in the cortex and the hippocampus using semiquantitative immunofluorescence. In addition, we analyzed neuronal activity by whole-cell patch-clamp recordings in cortical layer II/III and hippocampal CA1 pyramidal neurons.

RESULTS

In the cortex, co-culturing immunoreactive PBMCs treated with LPS + nigericin on top of OCHSCs upregulated inflammatory markers and enhanced neuronal excitation. In contrast, no excitability changes were detected after adding primed PBMCs (i.e. treated with LPS only), to OCHSCs. Strikingly, in the hippocampus, both immunoreactive and primed PBMCs elicited similar pro-inflammatory and pro-excitatory effects. However, when immunoreactive and primed PBMCs were cultured in the media separately from OCHSCs, only immunoreactive PBMCs gave rise to neuroinflammation and hyperexcitability in the hippocampus, whereas primed PBMCs failed to produce any significant changes. Finally, VX-765 application to OCHSCs, co-cultured with either immunoreactive or primed PBMCs, prevented neuroinflammation and hippocampal hyperexcitability in OCHSCs.

CONCLUSIONS

Our study shows a higher susceptibility of the hippocampus to peripheral inflammation as compared to the cortex, mediated via Caspase-1-dependent signaling pathways. Thus, our findings suggest that Caspase-1 inhibition may potentially provide therapeutic benefits during hippocampal neuroinflammation and hyperexcitability secondary to peripheral innate immunity.

The Perfect Cytokine Storm: How Peripheral Immune Challenges Impact Brain Plasticity & Memory Function in Aging

Precipitous declines in cognitive function can occur in older individuals following a variety of peripheral immune insults, such as surgery, infection, injury, and unhealthy diet. Aging is associated with numerous changes to the immune system that shed some light on why this abrupt cognitive deterioration may occur. Normally, peripheral-to-brain immune signaling is tightly regulated and advantageous; communication between the two systems is bi-directional, via either humoral or neural routes. Following an immune challenge, production, secretion, and translocation of cytokines into the brain is critical to the development of adaptive sickness behaviors. However, aging is normally associated with neuroinflammatory priming, notably microglial sensitization. Microglia are the brain's innate immune cells and become sensitized with advanced age, such that upon immune stimulation they will mount more exaggerated neuroimmune responses. The resultant elevation of pro-inflammatory cytokine expression, namely IL-1β, has profound effects on synaptic plasticity and, consequentially, cognition. In this review, we (1) investigate the processes which lead to aberrantly elevated inflammatory cytokine expression in the aged brain and (2) examine the impact of the pro-inflammatory cytokine IL-1β on brain plasticity mechanisms, including its effects on BDNF, AMPA and NMDA receptor-mediated long-term potentiation.

Cerebrospinal Fluid Inflammatory Markers in Alzheimer's Disease: Influence of Comorbidities

BACKGROUND

Alzheimer's disease (AD) develops into dementia after several years, and subjective cognitive impairment (SCI) and mild cognitive impairment (MCI) are used as intermediary diagnoses of increasing severity. Inflammation is an important part of AD pathology and provides potential novel biomarkers and treatment targets.

OBJECTIVE

To identify novel potential biomarkers of AD in cerebrospinal fluid (CSF) and create a molecular pattern of inflammatory factors providing differentiation between AD and SCI.

METHODS

We analyzed 43 inflammatory-related mediators in CSF samples from a cohort of SCI and AD cases vetted for confounding factors (Training cohort). Using multivariate analysis (MVA), a model for discrimination between SCI and AD was produced, which we then applied to a larger nonvetted cohort (named Test cohort). The data were analyzed for factors showing differences between diagnostic groups and factors that differed between the vetted and non-vetted cohorts. The relationship of the factors to the agreement between model and clinical diagnosis was investigated.

RESULTS

A good MVA model able to discriminate AD from SCI without including tangle and plaque biomarkers was produced from the Training cohort. The model showed 50% agreement with clinical diagnosis in the Test cohort. Comparison of the cohorts indicated different patterns of factors distinguishing SCI from AD. As an example, soluble interleukin (IL)-6Rα showed lower levels in AD cases in the Training cohort, whereas placental growth factor (PlGF) and serum amyloid A (SAA) levels were higher in AD cases of the Test cohort. The levels of p-tau were also higher in the Training cohort.

CONCLUSION

This study provides new knowledge regarding the involvement of inflammation in AD by indicating different patterns of factors in CSF depending on whether potential confounding comorbidities are present or not, and presents sIL-6Rα as a potential new biomarker for improved diagnosis of AD.

Neuroimmune Response Mediated by Cytokines in Natural Scrapie after Chronic Dexamethasone Treatment

The actual role of prion protein-induced glial activation and subsequent cytokine secretion during prion diseases is still incompletely understood. The overall aim of this study is to assess the effect of an anti-inflammatory treatment with dexamethasone on different cytokines released by neuroglial cells that are potentially related to neuroinflammation in natural scrapie. This study emphasizes the complex interactions existent among several pleiotropic neuromodulator peptides and provides a global approach to clarify neuroinflammatory processes in prion diseases. Additionally, an impairment of communication between microglial and astroglial populations mediated by cytokines, mainly IL-1, is suggested. The main novelty of this study is that it is the first one assessing in situ neuroinflammatory activity in relation to chronic anti-inflammatory therapy, gaining relevance because it is based on a natural model. The cytokine profile data would suggest the activation of some neurotoxicity-associated route. Consequently, targeting such a pathway might be a new approach to modify the damaging effects of neuroinflammation.

Low glucocorticoids in stress-related disorders: the role of inflammation

There is evidence that plasma cortisol concentration can be either increased or decreased in patients with depression and related anxiety and stress-related disorders; the exact pathophysiological mechanisms of this state are not almost clear. Several distinct theories were proposed and mechanisms, which could lead to decreased glucocorticoid signaling and/or levels, were described. However, there is a possible drawback in almost all the theories proposed: insufficient attention to the inflammatory process, which is undoubtedly present in several stress-related disorders, including post-traumatic stress disorder (PTSD). Previous studies only briefly mentioned the presence of an inflammatory reaction's signs in PTSD, without giving it due importance, although recognizing that it can affect the course of the disease. With that, the state of biochemical changes, characterized by the low glucocorticoids, glucocorticoid receptor's resistance and the signs of the persistent inflammation (with the high levels of circulating cytokines) might be observed not only in PTSD but in coronary heart diseases and systemic chronic inflammatory diseases (rheumatoid arthritis) as well. That is why the present review aims to depict the pathophysiological mechanisms, which lead to a decrease in glucocorticoids in PTSD due to the action of inflammatory stimuli. We described changes in the glucocorticoid system and inflammatory reaction as parts of an integral system, where glucocorticoids and the glucocorticoid receptor reside at the apex of a regulatory network that blocks several inflammatory pathways, while decreased glucocorticoid signaling and/or level leads to unchecked inflammatory reactions to promote pathologies such as PTSD. LAY SUMMARY This review emphasizes the importance of inflammatory reaction in the development of puzzling conditions sometimes observed in severe diseases including post-traumatic stress disorder - the decreased levels of glucocorticoids in the blood. Following the classical concepts, one would expect an increase in glucocorticoid hormones, since they are part of the feedback mechanism in the immune system, which reduces stress and inflammation. However, low levels of glucocorticoid hormones are also observed. Thus, this review describes potential mechanisms, which can lead to the development of such a state.

Enduring effects of an unhealthy diet during adolescence on systemic but not neurobehavioural measures in adult rats

Introduction: Adolescence is an important stage of maturation for various brain structures. It is during this time therefore that the brain may be more vulnerable to environmental factors such as diet that may influence mood and memory. Diets high in fat and sugar (termed a cafeteria diet) during adolescence have been shown to negatively impact upon cognitive performance, which may be reversed by switching to a standard diet during adulthood. Consumption of a cafeteria diet increases both peripheral and central levels of interleukin-1β (IL-1β), a pro-inflammatory cytokine which is also implicated in cognitive impairment during the ageing process. It is unknown whether adolescent exposure to a cafeteria diet potentiates the negative effects of IL-1β on cognitive function during adulthood.Methods: Male Sprague-Dawley rats consumed a cafeteria diet during adolescence after which time they received a lentivirus injection in the hippocampus to induce chronic low-grade overexpression of IL-1β. After viral integration, metabolic parameters, circulating and central pro-inflammatory cytokine levels, and cognitive behaviours were assessed.Results: Our data demonstrate that rats fed the cafeteria diet exhibit metabolic dysregulations in adulthood, which were concomitant with low-grade peripheral and central inflammation. Overexpression of hippocampal IL-1β in adulthood impaired spatial working memory. However, adolescent exposure to a cafeteria diet, combined with or without hippocampal IL-1β in adulthood did not induce any lasting cognitive deficits when the diet was replaced with a standard diet in adulthood. Discussion: These data demonstrate that cafeteria diet consumption during adolescence induces metabolic and inflammatory changes, but not behavioural changes in adulthood.

Hypercapnia promotes microglial pyroptosis via inhibiting mitophagy in hypoxemic adult rats

Background

Hypoxemia is a typical symptom of acute respiratory distress syndrome. To avoid pulmonary morbidity, low tidal volume ventilation is often applied. The ventilation strategy will certainly cause hypercapnia. This study aimed to explore whether hypercapnia would promote microglial pyroptosis via inhibiting mitophagy in adult rats with hypoxemia.

Methods

The cerebral oxygen extraction ratio (CERO₂) and partial pressure of brain tissue oxygen (PbtO₂) in a rat model of hypercapnia/hypoxemia were assessed. The reactive oxygen species (ROS) production and the expression of LC3‐II/I, p62, caspase‐1, gasdermin D‐N domains (GSDMD‐N), IL‐1β, and IL‐18 in microglial cells were detected.

Results

Hypercapnia decreased the PbtO₂ levels of the hypoxic rats, which was further evidenced by the increased levels of CERO₂. Expression levels of LC3‐II were reduced, while p62 expression was increased by hypercapnia in hypoxic microglia. Hypercapnia increased the production of ROS and the expression of caspase‐1, GSDMD‐N, IL‐1β, and IL‐18 in hypoxia‐activated microglia. Scavenging ROS inhibited microglial pyroptosis and expression of IL‐1β and IL‐18.

Conclusions

These results suggest that hypercapnia‐induced mitophagy inhibition may promote pyroptosis and enhance IL‐1β and IL‐18 release in hypoxia‐activated microglia.

Contributions of Interleukin-1 Receptor Signaling in Traumatic Brain Injury

Traumatic brain injury (TBI) in various forms affects millions in the United States annually. There are currently no FDA-approved therapies for acute injury or the chronic comorbidities associated with TBI. Acute phases of TBI are characterized by profound neuroinflammation, a process that stimulates the generation and release of proinflammatory cytokines including interleukin-1α (IL-1α) and IL-1β. Both forms of IL-1 initiate signaling by binding with IL-1 receptor type 1 (IL-1R1), a receptor with a natural, endogenous antagonist dubbed IL-1 receptor antagonist (IL-1Ra). The recombinant form of IL-1Ra has gained FDA approval for inflammatory conditions such as rheumatoid arthritis, prompting interest in repurposing these pharmacotherapies for other inflammatory diseases/injury states including TBI. This review summarizes the currently available preclinical and clinical literature regarding the therapeutic potential of inhibiting IL-1-mediated signaling in the context of TBI. Additionally, we propose specific research areas that would provide a greater understanding of the role of IL-1 signaling in TBI and how these data may be beneficial for the development of IL-1-targeted therapies, ushering in the first FDA-approved pharmacotherapy for acute TBI.

A genome-wide association study-derived polygenic score for interleukin-1β is associated with hippocampal volume in two samples

Accumulating research suggests that the pro-inflammatory cytokine interleukin-1β (IL-1β) has a modulatory effect on the hippocampus, a brain structure important for learning and memory as well as linked with both psychiatric and neurodegenerative disorders. Here, we used an imaging genetics strategy to test an association between an IL-1β polygenic score and hippocampal volume in two independent samples. Our polygenic score was derived using summary statistics from a recent genome-wide association study of circulating cytokines that included IL-1β (N = 3,309). In the first sample of 512 non-Hispanic Caucasian university students (274 women, mean age 19.78 ± 1.24 years) from the Duke Neurogenetics Study, we identified a significant positive correlation between IL-1β polygenic scores and hippocampal volume. This positive association was successfully replicated in a second sample of 7,960 white British volunteers (4,158 women, mean age 62.63 ± 7.45 years) from the UK Biobank. Our results lend further support in humans, to the link between IL-1β and the structure of the hippocampus.

Chronic interleukin-1β in the dorsal hippocampus impairs behavioural pattern separation

Understanding the long-term consequences of chronic inflammation in the hippocampus may help to develop therapeutic targets for the treatment of cognitive disorders related to stress, ageing and neurodegeneration. The hippocampus is particularly vulnerable to increases in the pro-inflammatory cytokine interleukin-1β (IL-1β), a mediator of neuroinflammation, with elevated levels implicated in the aetiology of neurodegenerative diseases such as Alzheimer's and Parkinson's, and in stress-related disorders such as depression. Acute increases in hippocampal IL-1β have been shown to impair cognition and reduce adult hippocampal neurogenesis, the birth of new neurons. However, the impact of prolonged increases in IL-1β, as evident in clinical conditions, on cognition has not been fully explored. Therefore, the present study utilized a lentiviral approach to induce long-term overexpression of IL-1β in the dorsal hippocampus of adult male Sprague Dawley rats and examine its impact on cognition. Following three weeks of viral integration, pattern separation, a process involving hippocampal neurogenesis, was impaired in IL-1β-treated rats in both object-location and touchscreen operant paradigms. This was coupled with a decrease in the number and neurite complexity of immature neurons in the hippocampus. Conversely, tasks involving the hippocampus, but not sensitive to disruption of hippocampal neurogenesis, including spontaneous alternation, novel object and location recognition were unaffected. Touchscreen operant visual discrimination, a cognitive task involving the prefrontal cortex, was largely unaffected by IL-1β overexpression. In conclusion, these findings suggest that chronically elevated IL-1β in the hippocampus selectively impairs pattern separation. Inflammatory-mediated disruption of adult hippocampal neurogenesis may contribute to the cognitive decline associated with neurodegenerative and stress-related disorders.

Aging and an Immune Challenge Interact to Produce Prolonged, but Not Permanent, Reductions in Hippocampal L-LTP and mBDNF in a Rodent Model with Features of Delirium

Aging increases the risk of abrupt declines in cognitive function after an event that triggers immune system activation (e.g. surgery, infection, or injury). This phenomenon is poorly understood, but rodent models may provide clues. We have previously shown that aging (24-mo-old) F344xBN rats generally do not show significant physical or cognitive impairments. However, their brains mount an exaggerated inflammatory response to signals triggered by a peripheral immune challenge (an intraperitoneal injection of Escherichia coli or laparotomy). Their hippocampal levels of the proinflammatory cytokine IL-1β are significantly elevated for at least 8 d, but generally less than 14 d, after infection or surgery. This IL-1β elevation is mirrored by prolonged deficits in a hippocampus-dependent long-term memory task. In contrast, young (3-mo-old) counterparts exhibit only transient elevations in IL-1β that drop to near baseline levels within 24 h. We previously demonstrated that theta burst–evoked late-phase long-term potentiation (L-LTP)—a BDNF-dependent form of synaptic plasticity—is impaired in hippocampal area CA1 of aged animals 4 d after infection. Also, levels of mature brain-derived neurotrophic factor (mBDNF)—the protein isoform required for stabilization of L-LTP—are reduced in hippocampal synaptoneurosomes of aged animals at the same time point. In this study, we investigated whether the deficits in L-LTP and mBDNF persist in parallel with the elevation in IL-1β and impairment in memory. This was the case, consistent with the idea that an exaggerated brain inflammatory response may compromise memory consolidation in part by altering availability of mBDNF to stabilize memory-related synaptic plasticity.

Hypercapnia induces IL-1β overproduction via activation of NLRP3 inflammasome: implication in cognitive impairment in hypoxemic adult rats

Background

Cognitive impairment is one of common complications of acute respiratory distress syndrome (ARDS). Increasing evidence suggests that interleukin-1 beta (IL-1β) plays a role in inducing neuronal apoptosis in cognitive dysfunction. The lung protective ventilatory strategies, which serve to reduce pulmonary morbidity for ARDS patients, almost always lead to hypercapnia. Some studies have reported that hypercapnia contributes to the risk of cognitive impairment and IL-1β secretion outside the central nervous system (CNS). However, the underlying mechanism of hypercapnia aggravating cognitive impairment under hypoxia has remained uncertain. This study was aimed to explore whether hypercapnia would partake in increasing IL-1β secretion via activating the NLRP3 (NLR family, pyrin domain-containing 3) inflammasome in the hypoxic CNS and in aggravating cognitive impairment.

Methods

The Sprague-Dawley (SD) rats that underwent hypercapnia/hypoxemia were used for assessment of NLRP3, caspase-1, IL-1β, Bcl-2, Bax, and caspase-3 expression by Western blotting or double immunofluorescence, and the model was also used for Morris water maze test. In addition, Z-YVAD-FMK, a caspase-1 inhibitor, was used to treat BV-2 microglia to determine whether activation of NLRP3 inflammasome was required for the enhancing effect of hypercapnia on expressing IL-1β by Western blotting or double immunofluorescence. The interaction effects were analyzed by factorial ANOVA. Simple effects analyses were performed when an interaction was observed.

Results

There were interaction effects on cognitive impairment, apoptosis of hippocampal neurons, activation of NLRP3 inflammasome, and upregulation of IL-1β between hypercapnia treatment and hypoxia treatment. Hypercapnia + hypoxia treatment caused more serious damage to the learning and memory of rats than those subjected to hypoxia treatment alone. Expression levels of Bcl-2 were reduced, while that of Bax and caspase-3 were increased by hypercapnia in hypoxic hippocampus. Hypercapnia markedly increased the expression of NLRP3, caspase-1, and IL-1β in hypoxia-activated microglia both in vivo and in vitro. Pharmacological inhibition of NLRP3 inflammasome activation and release of IL-1β might ameliorate apoptosis of neurons.

Conclusions

The present results suggest that hypercapnia-induced IL-1β overproduction via activating the NLRP3 inflammasome by hypoxia-activated microglia may augment neuroinflammation, increase neuronal cell death, and contribute to the pathogenesis of cognitive impairments.

Proinsulin protects against age-related cognitive loss through anti-inflammatory convergent pathways

Brain inflammaging is increasingly considered as contributing to age-related cognitive loss and neurodegeneration. Despite intensive research in multiple models, no clinically effective pharmacological treatment has been found yet. Here, in the mouse model of brain senescence SAMP8, we tested the effects of proinsulin, a promising neuroprotective agent that was previously proven to be effective in mouse models of retinal neurodegeneration. Proinsulin is the precursor of the hormone insulin but also upholds developmental physiological effects, particularly as a survival factor for neural cells. Adeno-associated viral vectors of serotype 1 bearing the human proinsulin gene were administered intramuscularly to obtain a sustained release of proinsulin into the blood stream, which was able to reach the target area of the hippocampus. SAMP8 mice and the control strain SAMR1 were treated at 1 month of age. At 6 months, behavioral testing exhibited cognitive loss in SAMP8 mice treated with the null vector. Remarkably, the cognitive performance achieved in spatial and recognition tasks by SAMP8 mice treated with proinsulin was similar to that of SAMR1 mice. In the hippocampus, proinsulin induced the activation of neuroprotective pathways and the downstream signaling cascade, leading to the decrease of neuroinflammatory markers. Furthermore, the decrease of astrocyte reactivity was a central effect, as demonstrated in the connectome network of changes induced by proinsulin. Therefore, the neuroprotective effects of human proinsulin unveil a new pharmacological potential therapy in the fight against cognitive loss in the elderly.

Regulation of Physical Microglia-Neuron Interactions by Fractalkine Signaling after Status Epilepticus

Microglia, the resident immune cells of the brain, perform elaborate surveillance in which they physically interact with neuronal elements. A novel form of microglia–neuron interaction named microglial process convergence (MPC) toward neuronal axons and dendrites has recently been described. However, the molecular regulators and pathological relevance of MPC have not been explored. Here, using high-resolution two-photon imaging in vivo and ex vivo, we observed a dramatic increase in MPCs after kainic acid– or pilocarpine-induced experimental seizures that was reconstituted after glutamate treatment in slices from mice. Interestingly, a deficiency of the fractalkine receptor (CX3CR1) decreased MPCs, whereas fractalkine (CX3CL1) treatment increased MPCs, suggesting that fractalkine signaling is a critical regulator of these microglia–neuron interactions. Furthermore, we found that interleukin-1β was necessary and sufficient to trigger CX3CR1-dependent MPCs. Finally, we show that a deficiency in fractalkine signaling corresponds with increased seizure phenotypes. Together, our results identify the neuroglial CX3CL1–CX3CR1 communication axis as a modulator of potentially neuroprotective microglia–neuron physical interactions during conditions of neuronal hyperactivity.

Neuroinflammatory challenges compromise neuronal function in the aging brain: Postoperative cognitive delirium and Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that targets memory and cognition, and is the most common form of dementia among the elderly. Although AD itself has been extensively studied, very little is known about early-stage preclinical events and/or mechanisms that may underlie AD pathogenesis. Since the majority of AD cases are sporadic in nature, advancing age remains the greatest known risk factor for AD. However, additional environmental and epigenetic factors are thought to accompany increasing age to play a significant role in the pathogenesis of AD. Postoperative cognitive delirium (POD) is a behavioral syndrome that primarily occurs in elderly patients following a surgical procedure or injury and is characterized by disruptions in cognition. Individuals that experience POD are at an increased risk for developing dementia and AD compared to normal aging individuals. One way in which cognitive function is affected in cases of POD is through activation of the inflammatory cascade following surgery or injury. There is compelling evidence that immune challenges (surgery and/or injury) associated with POD trigger the release of pro-inflammatory cytokines into both the periphery and central nervous system. Thus, it is possible that cognitive impairments following an inflammatory episode may lead to more severe forms of dementia and AD pathogenesis. Here we will discuss the inflammation associated with POD, and highlight the advantages of using POD as a model to study inflammation-evoked cognitive impairment. We will explore the possibility that advancing age and immune challenges may provide mechanistic evidence correlating early life POD with AD. We will review and propose neural mechanisms by which cognitive impairments occur in cases of POD, and discuss how POD may augment the onset of AD.

Are the emergence of affective disturbances in neuropathic pain states contingent on supraspinal neuroinflammation?

Neuro-immune interactions contribute to the pathogenesis of neuropathic pain due to peripheral nerve injury. A large body of preclinical evidence supports the idea that the immune system acts to modulate the sensory symptoms of neuropathy at both peripheral and central nervous system sites. The potential involvement of neuro-immune interactions in the highly debilitating affective disturbances of neuropathic pain, such as depression, anhedonia, impaired cognition and reduced motivation has received little attention. This is surprising given the widely accepted view that sickness behaviour, depression, cognitive impairment and other neuropsychiatric conditions can arise from inflammatory mechanisms. Moreover, there is a set of well-described immune-to-brain transmission mechanisms that explain how peripheral inflammation can lead to supraspinal neuroinflammation. In the last 5years increasing evidence has emerged that peripheral nerve injury induces supraspinal changes in cytokine or chemokine expression and alters glial cell activity. In this systematic review, based on strong preclinical evidence, we advance the argument that the emergence of affective disturbances in neuropathic pain states are contingent on pro-inflammatory mediators in the interconnected hippocampal-medial prefrontal circuitry that subserve affective behaviours. We explore how dysregulation of inflammatory mediators in these networks may result in affective disturbances through a wide variety of neuromodulatory mechanisms. There are also promising results from clinical trials showing that anti-inflammatory agents have efficacy in the treatment of a variety of neuropsychiatric conditions including depression and appear suited to sub-groups of patients with elevated pro-inflammatory profiles. Thus, although further research is required, aggressively targeting supraspinal pro-inflammatory mediators at critical time-points in appropriate clinical populations is likely to be a novel avenue to treat debilitating affective disturbances in neuropathic conditions.

Apoptosis of mouse hippocampal cells induced by Taenia crassiceps metacestode factor

Seizures, headache, depression and neurological deficits are the signs and symptoms most frequently reported in human neurocysticercosis. However, the cause of the associated learning and memory deficits is unknown. Here, we used Taenia crassiceps infection in mice as a model of human cysticercosis. The effects of T. crassiceps metacestode infection or T. crassiceps metacestode factor (MF) treatment on mouse hippocampal cells were studied; control mice were included. At 45 days after infection or treatment of the mice with MF, all mice were anaesthetized and perfused transcardially with saline followed by phosphate-buffered 10% formalin. Then the brains were carefully removed. Coronal sections stained using several techniques were analysed. Extensive and significant apoptosis was found in the experimental animals, mainly in the dentate gyrus, CA1, CA2, CA3 and neighbouring regions, in comparison with the apparently intact cells from control mice (P < 0.01). These results suggest that neurological deficits, especially the learning and memory deficits, may be generated by extensive apoptosis of hippocampal cells.

A Multiplex Protein Panel Applied to Cerebrospinal Fluid Reveals Three New Biomarker Candidates in ALS but None in Neuropathic Pain Patients

The objective of this study was to develop and apply a novel multiplex panel of solid-phase proximity ligation assays (SP-PLA) requiring only 20 μL of samples, as a tool for discovering protein biomarkers for neurological disease and treatment thereof in cerebrospinal fluid (CSF). We applied the SP-PLA to samples from two sets of patients with poorly understood nervous system pathologies amyotrophic lateral sclerosis (ALS) and neuropathic pain, where patients were treated with spinal cord stimulation (SCS). Forty-seven inflammatory and neurotrophic proteins were measured in samples from 20 ALS patients and 15 neuropathic pain patients, and compared to normal concentrations in CSF from control individuals. Nineteen of the 47 proteins were detectable in more than 95% of the 72 controls. None of the 21 proteins detectable in CSF from neuropathic pain patients were significantly altered by SCS. The levels of the three proteins, follistatin, interleukin-1 alpha, and kallikrein-5 were all significantly reduced in the ALS group compared to age-matched controls. These results demonstrate the utility of purpose designed multiplex SP-PLA panels in CSF biomarker research for understanding neuropathological and neurotherapeutic mechanisms. The protein changes found in the CSF of ALS patients may be of diagnostic interest.

The Effects of Ellagic Acid upon Brain Cells: A Mechanistic View and Future Directions

Ellagic acid (EA, 2,3,7,8-tetrahydroxy-chromeno; C14H6O8) is a polyphenol derived from fruits (pomegranates, berries) and nuts. EA exhibits antioxidant capacity and induces anti-inflammatory actions in several mammalian tissues. EA has been characterized as a possible neuroprotective agent, but the number of reports is still limited to conclude whether and how EA exerts neuroprotection in humans. In this regard, performing additional studies considering the potential beneficial and/or toxicological roles for EA on brain cells would be an important step towards fully understanding of when and how EA may be securely utilized by humans as a neuroprotective agent. The aim of the present work is to discuss data related to the neuronal and glial effects of EA and the mechanisms underlying such events. Moreover, future directions are suggested as a potential guide to be utilized by researchers interested in investigating the neuronal and glial actions of EA hereafter.

Cytokine levels contribute to the pathogenesis of minimal hepatic encephalopathy in patients with hepatocellular carcinoma via STAT3 activation

Hepatocellular carcinoma (HCC) patients were grouped according to the degree of encephalopathy, with healthy volunteers as controls. We investigated clinical presentation, protein and mRNA expression of 14 cytokines, and activation of six STAT proteins, the downstream signaling mediators. Levels of all 14 cytokines were significantly elevated in HCC patients with clinical hepatic encephalopathy. Statistical analysis showed that levels of IL-1β, IL-6, IFNγ, IL-17α, IFNλ2 and IFNλ3 were correlated with minimal hepatic encephalopathy (MHE). Multivariate regression analysis identified serum IL-6, IFNλ3 and IL-17α as independent risk factors for MHE. Increased mRNA levels of IL-6 and IFNγ were associated with MHE. Among the STAT proteins examined, only STAT3 was elevated in MHE. Treatment with a STAT3 inhibitor protected neurons from cytokine-induced apoptosis in vitro. In conclusion, this study identified potential biomarkers for MHE in HCC. The cytokines investigated may induce neural apoptosis via STAT3 in the pathogenesis of MHE in HCC.

A postnatal peak in microglial development in the mouse hippocampus is correlated with heightened sensitivity to seizure triggers

BACKGROUND

Explosive synaptogenesis and synaptic pruning occur in the hippocampus during the first two weeks of postnatal life, coincident with a heightened susceptibility to seizures in rodents. To determine the temporal correlation between microglial development and age-dependent susceptibility and response to seizures, we quantified developmental changes in basal microglia levels and seizure-induced microglial activation in the hippocampus of Cx3Cr1(GFP /+) transgenic mice.

METHODS

Basal levels of microglia were quantified in the hippocampi of Cx3Cr1(GFP /+) mice at P0, P5, P10, P15, P20, P25, P30, P40, and P60. Seizure susceptibility and seizure-induced microglial activation were assessed in response to febrile seizures (lipopolysaccharide followed by hyperthermia) and kainic acid-induced status epilepticus.

RESULTS

The density of microglia within the hippocampus increased rapidly after birth, reaching a peak during the second week of life - the age at which the animals became most vulnerable to seizure triggers. In addition, this peak of microglial development and seizure vulnerability during the second postnatal week represented the time of maximal seizure-induced microglia activation.

CONCLUSIONS

Overreactive innate immunity mediated by activated microglia may exacerbate acute injury to neuronal synapses and contribute to the long-term epileptogenic effects of early-life seizures. Anti-inflammatory therapy targeting excessive production of inflammatory mediators by activated microglia, therefore, may be an effective age-specific therapeutic strategy to minimize neuronal dysfunction and prevent increases in susceptibility to subsequent seizures in developing animals.

Interictal cytokine levels were correlated to seizure severity of epileptic patients: a retrospective study on 1218 epileptic patients

BACKGROUND

Many aspects on the correlation between epilepsy and cytokine levels were unclear. This study aims to investigate the correlations between cytokine levels and severe epilepsy.

METHODS

Totally 1218 epileptic patients were grouped by types of epilepsy: TLE (temporal lobe epilepsy, n = 409), XLE (extra-temporal lobe epilepsy, n = 290) and IGE (idiopathic generalized epilepsy, n = 519). Two hundred healthy volunteers were as controls. Clinical findings and levels of 14 serum and CSF cytokines and 6 STAT members were collected, measured and analyzed.

RESULTS

Analysis showed no differences in interictal cytokine levels among patients from TLE, XLE and IGE groups. Interictal serum levels of IL-1b, IL-1Ra, IL-6, IL-8, IFNγ, IFNλ3 and IL-17a were associated with seizure severity of epileptic patients, measured by seizure frequency, VA score or NHS3. Multivariate regression analysis indicated that interictal concentrations of serum IL-6, IFNγ, IL-17a, IFNλ3, and CSF IL-6, IL-17a, IFNλ3 were significant biomarkers for patients with severe epilepsy. mRNA levels of IL-6, IFNγ, IL-17a, and IFNλ3 were elevated in different types of epilepsy. Activation of all STATs was elevated in epilepsy, and STAT3 was activated 9-fold in average, which was the highest among all STATs.

CONCLUSIONS

Interictal serum IL-6, IFNγ, IL-17a, IFNλ3, and CSF IL-6, IL-17a, IFNλ3 could be used as potential biomarkers for severe epilepsy. Activation of STATs, especially STAT3, was important in epilepsy. Our findings pointed out crucial roles of cytokine levels in epilepsy.

Neurotoxicity induced by zinc oxide nanoparticles: age-related differences and interaction

This study mainly investigated the neurotoxicity induced by zinc oxide nanoparticle (ZnO NP) in different-aged mice and the interaction between age and ZnO NP exposure. Sixty adult and old male C57BL/6J mice were assigned to four groups based on a two-factor (age and ZnO NP exposure) design. Results showed that ZnO NPs (5.6 mg/kg, intraperitoneal) induced increased production of pro-inflammatory cytokines in the serum and the brain of mice. A synergistic reaction between aging and ZnO NP exposure occurred regarding serum interleukin 1 (IL-1) and interleukin 6 (IL-6). In the brain, increased oxidative stress level, impaired learning and memory abilities, and hippocampal pathological changes were identified, especially in old mice, following ZnO NP exposure. Then, a potential mechanism of cognitive impairment was examined. The contents of hippocampal cAMP response element binding protein (CREB), phosphorylated CREB, synapsin I, and cAMP were decreased in an age-dependent manner, and the most substantial decrease occurred in old mice treated with ZnO NPs. These findings demonstrated for the first time that aging and ZnO NP exposure synergistically influenced systemic inflammation, and indicated old individuals were more susceptible to ZnO NP-induced neurotoxicity. One of the mechanisms might due to the supression of cAMP/CREB signaling.

Interferon gamma, interleukin-6, and -17a levels were correlated with minimal hepatic encephalopathy in HBV patients

BACKGROUND AND OBJECTIVE

Cytokines have been reported to be involved in the cirrhosis and hepatic encephalopathy (HE). Many aspects on the correlation between minimal HE (MHE) and cytokine levels were still unclear.

METHODS

Two hundred eighty-nine HBV-infected cirrhotic patients were grouped: non MHE (n = 156), MHE (n = 98) and clinical HE (CHE, n = 213). Another 88 healthy volunteers were included as controls. Clinical and laboratory findings and levels of ten serum cytokines were analyzed.

RESULTS

All tested cytokines were significantly elevated in cirrhotic patients and patients with CHE compared with controls. Statistical analysis showed only IL-6, IFNγ and IL-17a were correlated MHE (all three p < 0.001). Multivariate regression analysis indicated that serum IL-6 and IL-17a levels were independent risk factors for MHE. Moreover, all patients with MHE had IL-17a levels higher than 49 pg/mL, whereas those without MHE had IL-17a levels lower than 49 pg/mL.

CONCLUSIONS

IL-6, IFNγ, and IL-17a were correlated with MHE in HBV-infected patients. Two independent risk factors (IL-6, IL-17a) for MHE were identified. Our findings pointed out the crucial roles of cytokines in MHE in HBV-infected patients.

Neuroinflammation in the normal aging hippocampus

A consequence of normal aging is a greater susceptibility to memory impairments following an immune challenge such as infection, surgery, or traumatic brain injury. The neuroinflammatory response, produced by these challenges results in increased and prolonged production of pro-inflammatory cytokines in the otherwise healthy aged brain. Here we discuss the mechanisms by which long-lasting elevations in pro-inflammatory cytokines in the hippocampus produce memory impairments. Sensitized microglia are a primary source of this exaggerated neuroinflammatory response and appear to be a hallmark of the normal aging brain. We review the current understanding of the causes and effects of normal aging-induced microglial sensitization, including dysregulations of the neuroendocrine system, potentiation of neuroinflammatory responses following an immune challenge, and the impairment of memories. We end with a discussion of therapeutic approaches to prevent these deleterious effects.

Ellagic acid prevents cognitive and hippocampal long-term potentiation deficits and brain inflammation in rat with traumatic brain injury

AIMS

Traumatic brain injury (TBI) remains one of the main clinical problems globally and is a common cause of death among youth. Cognitive defects such as thinking, memory and behavior or mental health disorders are considered as the most frequent effects of severe and moderate TBI. It has been reported that ellagic acid (EA), a natural polyphenol, exhibits protective effects against oxidative damage. This study was performed to examine the EA preventive effects on cognitive impairments, long-term potentiation (LTP) deficits in hippocampus and brain inflammation induced by diffuse TBI in rat.

MAIN METHODS

Subchronic oral administration of 100 mg/kg EA, 7 consecutive days before induction of trauma (once daily) was used to elucidate the EA effects on passive avoidance memory and hippocampal LTP following TBI. To illustrate the possible mechanisms related to the preventive effects of EA on brain function following TBI, brain content of IL-1β, IL-6 and blood-brain barrier (BBB) permeability were determined.

KEY FINDINGS

EA pretreatment significantly (P<0.001) prevented TBI-induced memory and hippocampal LTP impairments in rat. Furthermore TBI induced elevation in brain content of IL-1β, IL-6 and BBB permeability were decreased significantly (P<0.001) due to EA pre-treatment.

SIGNIFICANCE

Our findings suggest that EA can prevent cognitive and LTP deficits and also prevent brain inflammation following TBI.

Immune dysregulation and cognitive vulnerability in the aging brain: Interactions of microglia, IL-1β, BDNF and synaptic plasticity

Older individuals often experience declines in cognitive function after events (e.g. infection, or injury) that trigger activation of the immune system. This occurs at least in part because aging sensitizes the response of microglia (the brain's resident immune cells) to signals triggered by an immune challenge. In the aging brain, microglia respond to these signals by producing more pro-inflammatory cytokines (e.g. interleukin-1beta or IL-1β) and producing them for longer than microglia in younger brains. This exaggerated inflammatory response can compromise processes critical for optimal cognitive functioning. Interleukin-1β is central to the inflammatory response and is a key mediator and modulator of an array of associated biological functions; thus its production and release is usually very tightly regulated. This review will focus on the impact of dysregulated production of IL-1β on hippocampus dependent-memory systems and associated synaptic plasticity processes. The neurotrophin brain-derived neurotrophic factor (BNDF) helps to protect neurons from damage caused by infection or injury, and it plays a critical role in many of the same memory and hippocampal plasticity processes compromised by dysregulated production of IL-1β. This suggests that an exaggerated brain inflammatory response, arising from aging and a secondary immune challenge, may erode the capacity to provide the BDNF needed for memory-related plasticity processes at hippocampal synapses. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Immune activation promotes depression 1 month after diffuse brain injury: a role for primed microglia

BACKGROUND

Traumatic brain injury (TBI) is associated with a higher incidence of depression. The majority of individuals who suffer a TBI are juveniles and young adults, and thus, the risk of a lifetime of depressive complications is a significant concern. The etiology of increased TBI-associated depression is unclear but may be inflammatory-related with increased brain sensitivity to secondary inflammatory challenges (e.g., stressors, infection, and injury).

METHODS

Adult male BALB/c mice received a sham (n = 52) or midline fluid percussion injury (TBI; n = 57). Neuroinflammation, motor coordination (rotarod), and depressive behaviors (social withdrawal, immobility in the tail suspension test, and anhedonia) were assessed 4 hours, 24 hours, 72 hours, 7 days, or 30 days later. Moreover, 30 days after surgery, sham and TBI mice received a peripheral injection of saline or lipopolysaccharide (LPS) and microglia activation and behavior were determined.

RESULTS

Diffuse TBI caused inflammation, peripheral cell recruitment, and microglia activation immediately after injury coinciding with motor coordination deficits. These transient events resolved within 7 days. Nonetheless, 30 days post-TBI a population of deramified and major histocompatibility complex II(+) (primed) microglia were detected. After a peripheral LPS challenge, the inflammatory cytokine response in primed microglia of TBI mice was exaggerated compared with microglia of controls. Furthermore, this LPS-induced microglia reactivity 30 days after TBI was associated with the onset of depressive-like behavior.

CONCLUSIONS

These results implicate a primed and immune-reactive microglial population as a possible triggering mechanism for the development of depressive complications after TBI.

Febrile seizures

Febrile seizure (FS) is the most common seizure disorder of childhood, and occurs in an age-related manner. FS are classified into simple and complex. FS has a multifactorial inheritance, suggesting that both genetic and environmental factors are causative. Various animal models have elucidated the pathophysiological mechanisms of FS. Risk factors for a first FS are a family history of the disorder and a developmental delay. Risk factors for recurrent FS are a family history, age below 18 months at seizure onset, maximum temperature, and duration of fever. Risk factors for subsequent development of epilepsy are neurodevelopmental abnormality and complex FS. Clinicians evaluating children after a simple FS should concentrate on identifying the cause of the child's fever. Meningitis should be considered in the differential diagnosis for any febrile child. A simple FS does not usually require further evaluation such as ordering electroencephalography, neuroimaging, or other studies. Treatment is acute rescue therapy for prolonged FS. Antipyretics are not proven to reduce the recurrence risk for FS. Some evidence shows that both intermittent therapy with oral/rectal diazepam and continuous prophylaxis with oral phenobarbital or valproate are effective in reducing the risk of recurrence, but there is no evidence that these medications reduce the risk of subsequent epilepsy. Vaccine-induced FS is a rare event that does not lead to deleterious outcomes, but could affect patient and physician attitudes toward the safety of vaccination.

Nervous and immune systems signals and connections: cytokines in hippocampus physiology and pathology

Signaling through secretion of small molecules is a hallmark of both nervous and immune systems. The scope and influence of the intense message exchange between these two complex systems are only now becoming objects of scientific inquiry. Both neurotransmitters and cytokines affect their target cells through surface receptors and also by other molecular mechanisms. Cytokine receptors are present in neurons and glial cell populations in discrete brain regions. This review firstly focuses on the role of cytokines in hippocampal physiological processes, such as memory and learning, and secondly on the pathological involvement of cytokines in diseases like depression and epilepsy. Interleukin-1β is necessary for long-term potentiation (LTP) maintenance in the hippocampus. On the other hand, interleukin-6 has a negative regulatory role in long-term memory acquisition. Astrocyte-secreted tumor necrosis factor plays a role in synaptic strength by increasing surface translocation of glutamate AMPA receptors, and the chemokine CXCL12 can silence the tonic activity of Cajal-Retzius neurons in the hippocampus. Manifold increased concentrations of interleukin-10, interferon-γ, ICAM1, CCL2, and CCL4 are observed in the hippocampi of patients with temporal lobe epilepsy. A contemporary view of the role of cytokines as neuromodulators is emerging from studies in humans and manipulations of experimental animals. Despite the accumulating evidence of the role of cytokines on nervous system physiology and pathology, it is important not to exaggerate its relevance.

Volume of the hippocampal subfields in healthy adults: differential associations with age and a pro-inflammatory genetic variant

The hippocampus is one of the most age-sensitive brain regions, yet the mechanisms of hippocampal shrinkage remain unclear. Recent studies suggest that hippocampal subfields are differentially vulnerable to aging and differentially sensitive to vascular risk. Promoters of inflammation are frequently proposed as major contributors to brain aging and vascular disease but their effects on hippocampal subfields are unknown. We examined the associations of hippocampal subfield volumes with age, a vascular risk factor (hypertension), and genetic polymorphisms associated with variation in pro-inflammatory cytokines levels (IL-1β C-511T and IL-6 C-174G) and risk for Alzheimer's disease (APOEε4) in healthy adult volunteers (N = 80; age = 22-82 years). Volumes of three hippocampal subfields, cornu ammonis (CA) 1-2, CA3-dentate gyrus, and the subiculum were manually measured on high-resolution magnetic resonance images. Advanced age was differentially associated with smaller volume of CA1-2, whereas carriers of the T allele of IL-1β C-511T polymorphism had smaller volume of all hippocampal subfields than CC homozygotes did. Neither of the other genetic variants, nor diagnosis of hypertension, was associated with any of the measured volumes. The results support the notion that volumes of age-sensitive brain regions may be affected by pro-inflammatory factors that may be targeted by therapeutic interventions.

The role of interleukin-1β in the pentylenetetrazole-induced kindling of seizures, in the rat hippocampus

Because the contribution of inflammatory mediators to seizure disorders is unclear, we investigated the changes in the expression of interleukin-1β (IL-β) and its receptor - IL-1 receptor type 1 (IL-1R1), tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in the rat hippocampus at different stages of pentylenetetrazole (PTZ)-induced kindling. The occurrence and progressive development of seizures were induced by repeated systemic administration of PTZ, a non-competitive antagonist of the γ-aminobutyric acid type A (GABAA) receptor at a subconvulsive dose of 30 mg/kg. We also examined the effects of continuous intracerebroventricular administration of IL-1β and lipopolysaccharide (LPS) in this model of epilepsy using subcutaneously implanted osmotic mini-pumps. We observed enhanced IL-1R1 expression in the dentate gyrus (DG) at different stages of kindling, whereas the elevated IL-1β level was distinctive to fully kindled seizures. We did not detect significant changes in the concentration of IL-6 or TNF-α throughout the kindling process. LPS accelerated transiently the process of kindling, while IL-1β showed a predisposition to delay kindling acquisition. Our study supports the concept of seizure-related modifications in brain cytokine production during epileptogenesis. Although some evidence indicates a proconvulsant property of IL-1β activity, it cannot be ruled out that the alterations in IL-1 system reflect the activation of endogenous protective mechanisms with respect to the kindling of seizures.

Immune mechanisms in epileptogenesis

Epilepsy is a chronic brain disorder that affects 1% of the human population worldwide. Immune responses are implicated in seizure induction and the development of epilepsy. Pre-clinical and clinical evidence have accumulated to suggest a positive feedback cycle between brain inflammation and epileptogenesis. Prolonged or recurrent seizures and brain injuries lead to upregulation of proinflammatory cytokines and activated immune responses to further increase seizure susceptibility, promote neuronal excitability, and induce blood-brain barrier breakdown. This review focuses on the potential role of innate and adaptive immune responses in the pathogenesis of epilepsy. Both human studies and animal models that help delineate the contributions of brain inflammation in epileptogenesis will be discussed. We highlight the critical role of brain-resident immune mediators and emphasize the contribution of brain-infiltrating peripheral leukocytes. Additionally, we propose possible immune mechanisms that underlie epileptogenesis. Several proinflammatory pathways are discussed, including the interleukin-1 receptor/toll-like receptor signaling cascade, the pathways activated by damage-associated molecular patterns, and the cyclooxygenase-2/prostaglandin pathway. Finally, development of better therapies that target the key constituents and processes identified in these mechanisms are considered, for instance, engineering antagonizing agents that effectively block these pathways in an antigen-specific manner.

The role of inflammation in epileptogenesis

One compelling challenge in the therapy of epilepsy is to develop anti-epileptogenic drugs with an impact on the disease progression. The search for novel targets has focused recently on brain inflammation since this phenomenon appears to be an integral part of the diseased hyperexcitable brain tissue from which spontaneous and recurrent seizures originate. Although the contribution of specific proinflammatory pathways to the mechanism of ictogenesis in epileptic tissue has been demonstrated in experimental models, the role of these pathways in epileptogenesis is still under evaluation. We review the evidence conceptually supporting the involvement of brain inflammation and the associated blood-brain barrier damage in epileptogenesis, and describe the available pharmacological evidence where post-injury intervention with anti-inflammatory drugs has been attempted. Our review will focus on three main inflammatory pathways, namely the IL-1 receptor/Toll-like receptor signaling, COX-2 and the TGF-β signaling. The mechanisms underlying neuronal-glia network dysfunctions induced by brain inflammation are also discussed, highlighting novel neuromodulatory effects of classical inflammatory mediators such as cytokines and prostaglandins. The increase in knowledge about a role of inflammation in disease progression, may prompt the use of specific anti-inflammatory drugs for developing disease-modifying treatments. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

IL-1 receptor/Toll-like receptor signaling in infection, inflammation, stress and neurodegeneration couples hyperexcitability and seizures

Increasing evidence supports the involvement of immune and inflammatory processes in the etiopathogenesis of seizures. In particular, activation of innate immune mechanisms and the subsequent inflammatory responses, that are induced in the brain by infection, febrile seizures, neurotrauma, stroke are well documented conditions associated with acute symptomatic seizures and with a high risk of developing epilepsy. A decade ago, pharmacological experiments showed that elevated brain levels of the anti-inflammatory molecule IL-1 receptor antagonist reduced seizures in epilepsy models. This observation, together with the evidence of in situ induction of inflammatory mediators and their receptors in experimental and human epileptogenic brain tissue, established the proof-of-concept evidence that the activation of innate immunity and inflammation in the brain are intrinsic features of the pathologic hyperexcitable tissue. Recent breakthroughs in understanding the molecular organization of the innate immune system first in macrophages, then in the different cell types of the CNS, together with pharmacological and genetic studies in epilepsy models, showed that the activation of IL-1 receptor/Toll-like receptor (IL-1R/TLR) signaling significantly contributes to seizures. IL-1R/TLR mediated pro-excitatory actions are elicited in the brain either by mimicking bacterial or viral infections and inflammatory responses, or via the action of endogenous ligands. These ligands include proinflammatory cytokines, such as IL-1beta, or danger signals, such as HMGB1, released from activated or injured cells. The IL-1R/TLR signaling mediates rapid post-translational changes in voltage- and ligand-gated ion channels that increase excitability, and transcriptional changes in genes involved in neurotransmission and synaptic plasticity that contribute to lower seizure thresholds chronically. The anticonvulsant effects of inhibitors of the IL-1R/TLR signaling in various seizures models suggest that this system could be targeted to inhibit seizures in presently pharmaco-resistant epilepsies.

Alcoholism and inflammation: neuroimmunology of behavioral and mood disorders

Alcohol abuse changes behavior and can induce major mood disorders such as depression. Recent evidence in pre-clinical rodent models and humans now supports the conclusion that the innate immune system is an important physiological link between alcoholism and major depressive disorders. Deficiency of toll-like receptor 4 (TLR4), a protein that has been known to immunologists for 50 years, not only prevents lipopolysaccharide (LPS)-induced sickness behavior but recently has been demonstrated to induce resistance to chronic alcohol ingestion. Activation of the immune system by acute administration of LPS, a TLR4 agonist, as well as chronic infection with Bacille Calmette-Guérin (BCG), causes development of depressive-like behaviors in pre-clinical rodent models. Induction of an enzyme expressed primarily in macrophages and microglia, 2,3 indoleamine dioxygenase, shunts tryptophan catabolism to form kynurenine metabolites. This enzyme is both necessary and sufficient for expression of LPS and BCG-induced depressive-like behaviors in mice. New findings have extended these concepts to humans by showing that tryptophan catabolites of 2,3 indoleamine dioxygenase are elevated in the cerebrospinal fluid of hepatitis patients treated with the recombinant cytokine interferon-α. The remarkable conservation from mice to humans of the impact of inflammation on mood emphasizes the ever-expanding role for cross-talk among diverse physiological symptoms that are likely to be involved in the pathogenesis of alcohol abuse. These findings present new and challenging opportunities for scientists who are engaged in brain, behavior and immunity research.

Endothelial IL-1R1 is a critical mediator of EAE pathogenesis

Interleukin-1 (IL-1) has been implicated in the disease progression of multiple sclerosis (MS). In the animal model of MS, experimental autoimmune encephalomyelitis (EAE), the induction of disease is significantly attenuated in mice lacking the type I IL-1 receptor (IL-1R1). In this study, we created a transgenic mouse (eIL-1R1 kd) in which IL-1R1 expression is knocked down specifically in endothelial cells. Induction of EAE in eIL-1R1 kd mice results in a decrease in incidence, severity and delayed onset of EAE. In addition, eIL-1R1 kd mice show significant decrease in VCAM-1 expression and diminished CD45(+) and CD3(+) infiltrating leukocytes in the spinal cord in animals challenged with EAE. Further, IL-1 and IL-23 stimulate IL-17 production by splenocytes from both wild type and the eIL-1R1 kd animals. Similarly, IL-1 and IL-23 synergistically stimulate splenocytes proliferation in these two strains of animals. After immunization with MOG(79-96), although eIL-1R1 kd mice displayed greatly reduced clinical scores, their splenocytes produced IL-17 and proliferated in response to a second MOG challenge, similar to wild type animals. These findings indicate a critical role for endothelial IL-1R1 in mediating the pathogenesis of EAE, and describe a new model that can be used to study endothelial IL-1R1.

Morphological and behavioral changes induced by transgenic overexpression of interleukin-1ra in the brain

Interleukin-1 (IL-1) has a peak of expression in the brain in a period of maximal network reorganization and then virtually disappears from the normal adult brain. The aim of our study was to identify phenotypical alterations induced by chronically blocking IL-1 signalling. We used homozygous transgenic mice overexpressing human soluble IL-1ra and age-matched wild-type mice. We used littermates from litters obtained by mating heterozygous transgenic progenitors, and animals with predetermined genotype (nonlittermates). In littermates, the genotype was identified after the experiments had been completed. The mice were tested at the ages of 6 and 12 months with a battery of tests, including dark-light preference, footprint/gait analysis, and analysis of motor performance during swimming. MR imaging was performed on formalin-fixed brains; total and relative volumes of cortical and subcortical structures were estimated stereologically on the acquired images. Multivariate data analysis (PLS-DA) of the behavioral data showed separation between nonlittermate wild-type and transgenic mice at both 6 and 12 months, whereas the littermates displayed a more homogenous behavioral profile. The PLS-DA model for brain morphology showed a clear separation between wild-type and transgenic mice as well as between transgenic littermates and nonlittermates. Regression analysis by means of partial least squares (PLS) showed that the brain morphology accounts for the behavioral profile in a significant proportion (16.9%). In conclusion, we show that IL-1 signalling is important for normal development of the brain, and the initial alteration resulting from prenatal exposure to IL-1ra can be recovered provided that the IL-1 signalling pathway is intact.

Behavioral assessment of cognitive function using a translational neonatal piglet model

Pigs are used in myriad research disciplines related to human health, but no studies have employed the piglet to directly assess cognitive function during the neonatal period. Our objective was to develop a behavioral assay for neonatal piglets to assess learning and memory. At 2-wk of age, piglets were trained to locate a milk reward in an 8-arm radial maze using colored intra-maze cues (acquisition phase, 60-s trials with 8 trials per d for 4d). Cue colors were then reversed and pigs re-tested to assess learning and working memory (reversal phase). Piglets quickly learned the simple associative acquisition task, and proficiency greatly improved throughout reversal testing. To further assess the behavioral assay, piglets received an i.p. injection of saline or polyinosinic:polycytidylic acid (poly I:C; 5mg/kg body weight) immediately preceding reversal testing. Poly I:C-treated piglets exhibited acute sickness behaviors, but observationally, were asymptomatic 12-h post-injection. Pro-inflammatory cytokine mRNA expression was elevated 4-h post-injection in both peripheral and central compartments, and plasma cytokine protein levels were concurrently elevated. Specifically, poly I:C elicited the largest increases in interleukin (IL)-1β mRNA in the liver, spleen, and hippocampus. At 24-, 48-, and 72-h post-injection (i.e., after acute sickness), poly I:C-treated piglets committed more incorrect arm entries, required more time to complete the reversal task, and moved a greater distance in the maze compared with control piglets. Collectively, these data demonstrate that neonatal piglets are capable of being trained in traditional learning and memory tests, and peripheral immune activation elicits alterations in cognitive processing in the neonate.

Neurobiology of inflammation-associated anorexia

Compelling data demonstrate that inflammation-associated anorexia directly results from the action of pro-inflammatory factors, primarily cytokines and prostaglandins E2, on the nervous system. For instance, the aforementioned pro-inflammatory factors can stimulate the activity of peripheral sensory neurons, and induce their own de novo synthesis and release into the brain parenchyma and cerebrospinal fluid. Ultimately, it results in the mobilization of a specific neural circuit that shuts down appetite. The present article describes the different cell groups and neurotransmitters involved in inflammation-associated anorexia and examines how they interact with neural systems regulating feeding such as the melanocortin system. A better understanding of the neurobiological mechanisms underlying inflammation-associated anorexia will help to develop appetite stimulants for cancer and AIDS patients.

The more you have, the less you get: the functional role of inflammation on neuronal differentiation of endogenous and transplanted neural stem cells in the adult brain

The differentiation of neural stem cells toward a neuronal phenotype is determined by the extracellular and intracellular factors that form the neurogenic niche. In this review, we discuss the available data on the functional role of inflammation and in particular, pro- and anti-inflammatory cytokines, on neuronal differentiation from endogenous and transplanted neural stem/progenitor cells. In addition, we discuss the role of microglial cell activation on these processes and the fact that microglial cell activation is not univocally associated with a pro-inflammatory milieu. We conclude that brain cytokines could be regarded as part of the endogenous neurogenic niche. In addition, we propose that accumulating evidence suggests that pro-inflammatory cytokines have a negative effect on neuronal differentiation, while anti-inflammatory cytokines exert an opposite effect. The clarification of the functional role of cytokines on neuronal differentiation will be relevant not only to better understand adult neurogenesis, but also to envisage complementary treatments to modulate cytokine action that could increase the therapeutic benefit of future progenitor/stem cell-based therapies.

Peripheral lipopolysaccharide administration induces cytokine mRNA expression in the viscera and brain of fever-refractory mice lacking microsomal prostaglandin E synthase-1

We examined the expression of interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF) alpha in mice lacking microsomal prostaglandin E synthase-1 (mPGES-1), which neither produce prostaglandin E(2), nor mount a febrile response upon immune challenge. Intraperitoneal lipopolysaccharide (LPS) injection resulted in a strongly induced expression of all three cytokines in the brain and viscera, similar to wild-type animals. Several brain regions additionally showed modest induction of receptors for these cytokines in both genotypes. Telemetric recordings of body temperature showed that the mPGES-1 deficient mice remained afebrile upon LPS challenge, in contrast to the prominent fever displayed by the wild-type mice. These data demonstrate that LPS-induced cytokine expression occurs independently of prostaglandin E(2), and imply that endogenously expressed IL-1beta, IL-6, and TNFalpha are not pyrogenic per se, supporting the role of prostaglandin E(2) as the final and obligatory mediator of LPS-induced fever.