Aging sensitizes mice to behavioral deficits induced by central HIV-1 gp120

Immune proteins C1q and CD47 may contribute to aberrant microglia-mediated synapse loss in the aging monkey brain that is associated with cognitive impairment

Cognitive impairment in learning, memory, and executive function occurs in normal aging even in the absence of Alzheimer's disease (AD). While neurons do not degenerate in humans or monkeys free of AD, there are structural changes including synapse loss and dendritic atrophy, especially in the dorsolateral prefrontal cortex (dlPFC), and these correlate with cognitive age-related impairment. Developmental studies revealed activity-dependent neuronal properties that lead to synapse remodeling by microglia. Microglia-mediated phagocytosis that may eliminate synapses is regulated by immune "eat me" and "don't eat me" signaling proteins in an activity-dependent manner, so that less active synapses are eliminated. Whether this process contributes to age-related synapse loss remains unknown. The present study used a rhesus monkey model of normal aging to investigate the balance between the "eat me" signal, complement component C1q, and the "don't eat me" signal, transmembrane glycoprotein CD47, relative to age-related synapse loss in dlPFC Area 46. Results showed an age-related elevation of C1q and reduction of CD47 at PSD95+ synapses that is associated with cognitive impairment. Additionally, reduced neuronal CD47 RNA expression was found, indicating that aged neurons were less able to produce the protective signal CD47. Interestingly, microglia do not show the hypertrophic morphology indicative of phagocytic activity. These findings suggest that in the aging brain, changes in the balance of immunologic proteins give microglia instructions favoring synapse elimination of less active synapses, but this may occur by a process other than classic phagocytosis such as trogocytosis.

Kynurenic acid blunts A1 astrocyte activation against neurodegeneration in HIV-associated neurocognitive disorders

Despite extensive astrocyte activation in patients suffering from HIV-associated neurocognitive disorders (HAND), little is known about the contribution of astrocytes to HAND neuropathology. Here, we report that the robust activation of neurotoxic astrocytes (A1 astrocytes) in the CNS promotes neuron damage and cognitive deficits in HIV-1 gp120 transgenic mice. Notably, knockout of α7 nicotinic acetylcholine receptors (α7nAChR) blunted A1 astrocyte responses, ultimately facilitating neuronal and cognitive improvement in the gp120tg mice. Furthermore, we provide evidence that Kynurenic acid (KYNA), a tryptophan metabolite with α7nAChR inhibitory properties, attenuates gp120-induced A1 astrocyte formation through the blockade of α7nAChR/JAK2/STAT3 signaling activation. Meanwhile, compared with gp120tg mice, mice fed with tryptophan showed dramatic improvement in cognitive performance, which was related to the inhibition of A1 astrocyte responses. These initial and determinant findings mark a turning point in our understanding of the role of α7nAChR in gp120-mediated A1 astrocyte activation, opening up new opportunities to control neurotoxic astrocyte generation through KYNA and tryptophan administration.

Postoperative cognitive dysfunction is made persistent with morphine treatment in aged rats

Postoperative cognitive dysfunction (POCD) is the collection of cognitive impairments, lasting days to months, experienced by individuals following surgery. Persistent POCD is most commonly experienced by older individuals and is associated with a greater vulnerability to developing Alzheimer's disease, but the underlying mechanisms are not known. It is known that laparotomy (exploratory abdominal surgery) in aged rats produces memory impairments for 4 days. Here we report that postsurgical treatment with morphine extends this deficit to at least 2 months while having no effects in the absence of surgery. Indeed, hippocampal-dependent long-term memory was impaired 2, 4, and 8 weeks postsurgery only in aged, morphine-treated rats. Short-term memory remained intact. Morphine is known to have analgesic effects via μ-opioid receptor activation and neuroinflammatory effects through Toll-like receptor 4 activation. Here we demonstrate that persistent memory deficits were mediated independently of the μ-opioid receptor, suggesting that they were evoked through a neuroinflammatory mechanism and unrelated to pain modulation. In support of this, aged, laparotomized, and morphine-treated rats exhibited increased gene expression of various proinflammatory markers (IL-1β, IL-6, TNFα, NLRP3, HMGB1, TLR2, and TLR4) in the hippocampus at the 2-week time point. Furthermore, central blockade of IL-1β signaling with the specific IL-1 receptor antagonist (IL-1RA), at the time of surgery, completely prevented the memory impairment. Finally, synaptophysin and PSD95 gene expression were significantly dysregulated in the hippocampus of aged, laparotomized, morphine-treated rats, suggesting that impaired synaptic structure and/or function may play a key role in this persistent deficit. This instance of long-term memory impairment following surgery closely mirrors the timeline of persistent POCD in humans and may be useful for future treatment discoveries.

Aging sensitizes male mice to cognitive dysfunction induced by central HIV-1 gp120

Although highly active antiretroviral therapy has led to improved prognosis and alleviation of some HIV-related disease complications, it has not provided complete protection against HIV-associated dementia. As the population of persons living with HIV grows older and aged persons represent a significant number of new infections, it is important to understand how HIV may affect the aged brain. In the current study, both adult and aged mice were treated with HIV gp120 and trained in a reference memory version of the water maze. Analysis of probe data revealed that aged animals treated with gp120 demonstrated profound decrements in water maze performance compared to gp120 treated young animals and saline treated aged or young animals. Additionally, we examined the neuroinflammatory responses in the aged and adult brain 4 h after treatment with gp120. Pro-inflammatory cytokines associated with neuroinflammation are known to be antagonistic to learning and memory processes and aged and adult animals treated with gp120 demonstrated similar increases in IL-1β and IL-6 in the hippocampus and cortex. Additionally, gp120 treatment was associated with an increase in MHCII gene expression, a marker of microglial activation, in the hippocampus. Although, the aged brain demonstrated a similar inflammatory profile at the time point measured, aged animals were more sensitive to cognitive dysfunction related to gp120 treatment. This finding supports the theory that aging may be a significant risk factor in the development of HIV-associated dementia.

Microglial NLRP3 inflammasome activation mediates IL-1β release and contributes to central sensitization in a recurrent nitroglycerin-induced migraine model

Background

Central sensitization is an important mechanism of chronic migraine (CM) and is related to the inflammatory response of microglia. The NOD-like receptor protein 3 (NLRP3) inflammasome may regulate the inflammatory process of microglia in several neurological diseases, but its role in CM is largely unknown. Therefore, the aim of this study was to identify the precise role of microglial NLRP3 in CM.

Methods

An experimental CM mouse model was established by repeated intraperitoneal (i.p) injection with nitroglycerin (NTG). We evaluated the expression levels of NLRP3 and its downstream interleukin (IL)-1β protein in the trigeminal nucleus caudalis (TNC; which is a central area relevant to migraine pain) at different time points. To further examine the effects of the NLRP3 inflammasome pathway on central sensitization of CM, we examined MCC950, an NLRP3 inflammasome-specific inhibitor, and IL-1ra, an IL-1β antagonist, whether altered NTG-induced mechanical hyperalgesia of the periorbital area and hind paw. The effect of MCC950 and IL-1ra on c-Fos, phosphorylated extracellular signal-regulated kinase (p-ERK) and calcitonin gene-related peptide (CGRP) expression in the TNC were also analyzed. The cell localization of NLRP3 and IL-1β in the TNC was evaluated by immunofluorescence staining.

Results

Repeated NTG administration induced acute and chronic mechanical hyperalgesia and increased expression of NLRP3 and IL-1β. Blockade of NLRP3 or IL-1β reduced NTG-induced hyperalgesia, and this effect was accompanied by a significant inhibition of the NTG-induced increase in p-ERK, c-Fos and CGRP levels in the TNC. Immunofluorescence staining revealed that NLRP3 and IL-1β were mainly expressed in microglia in the TNC, and the IL-1β receptor, IL-1R, was mainly expressed in neurons in the TNC.

Conclusions

These results indicate that NLRP3 activation in the TNC participates in the microglial-neuronal signal by mediating the inflammatory response. This process contributes to the central sensitization observed in CM.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1459-7) contains supplementary material, which is available to authorized users.

Forced turnover of aged microglia induces an intermediate phenotype but does not rebalance CNS environmental cues driving priming to immune challenge

Microglia are the resident innate immune cells of the central nervous system. Limited turnover throughout the lifespan leaves microglia susceptible to age-associated dysfunction. Indeed, we and others have reported microglia develop a pro-inflammatory or "primed" profile with age, characterized by increased expression of inflammatory mediators (e.g., MHC-II, CD68, IL-1β). Moreover, immune challenge with lipopolysaccharide (LPS) causes an exaggerated and prolonged neuroinflammatory response mediated by primed microglia in the aged brain. Recent studies show colony-stimulating factor 1 receptor (CSF1R) antagonism results in rapid depletion of microglia without significant complications. Therefore, we hypothesized that CSF1R antagonist-mediated depletion of microglia in the aged brain would result in repopulation with new and unprimed microglia. Here we provide novel evidence that microglia in the brain of adult (6-8 weeks old) and aged (16-18 months old) BALB/c mice were depleted following 3-week oral PLX5622 administration. When CSF1R antagonism was stopped, microglia repopulated equally in the adult and aged brain. Microglial depletion and repopulation reversed age-associated increases in microglial CD68+ lysosome enlargement and lipofuscin accumulation. Microglia-specific RNA sequencing revealed 511 differentially expressed genes with age. Of these, 117 genes were reversed by microglial repopulation (e.g., Apoe, Tgfb2, Socs3). Nevertheless, LPS challenge still induced an exaggerated microglial inflammatory response in the aged brain compared to adults. RNA sequencing of whole-brain tissue revealed an age-induced inflammatory signature, including reactive astrocytes, that was not restored by microglial depletion and repopulation. Furthermore, the microenvironment of the aged brain produced soluble factors that influenced developing microglia ex vivo and induced a profile primed to LPS challenge. Thus, the aged brain microenvironment promotes microglial priming despite repopulation of new microglia. Collectively, aged microglia proliferate and repopulate the brain, but these new cells still adopt a pro-inflammatory profile in the aged brain.

Polybrene induces neural degeneration by bidirectional Ca2+ influx-dependent mitochondrial and ER-mitochondrial dynamics

Hexadimethrine bromide (Polybrene) was once used clinically as a heparin neutralizer and has recently found use as a promoter in virus-mediated gene therapy trials and gene transfer in research. However, the potential for tissue-specific toxicity of polybrene at low doses has been ignored so far. Here, we found that after intracerebroventricular (ICV) polybrene injection, mice showed disability of movement accompanied neural death and gliosis in brain, and in human neurons, polybrene induces concentration-dependent neuritic beading and fragmentation. Mechanistically, polybrene induces a rapid voltage-dependent calcium channel (VDCC)-mediated influx of extracellular Ca²⁺. The elevated cytoplasmic Ca²⁺ activates DRP1, which leads to mitochondrial fragmentation and metabolic dysfunction. At the same time, Ca²⁺ influx induces endoplasmic reticulum (ER) fragmentation and tightened associations between ER and mitochondria, which makes mitochondria prone to Ca²⁺ overloading and ensuing permeability transition. These results reveal an unexpected neuronal toxicity of polybrene, wherein Ca²⁺ influx serves as a regulator for both mitochondrial dynamics and ER–mitochondrial remodeling.

Does the Interplay Between Aging and Neuroinflammation Modulate Alzheimer's Disease Clinical Phenotypes? A Clinico-Pathological Perspective

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and is the most common cause of dementia worldwide. Cumulative data suggests that neuroinflammation plays a prominent and early role in AD, and there is compelling data from different research groups of age-associated dysregulation of the neuroimmune system. From the clinical point of view, despite clinical resemblance and neuropathological findings, there are important differences between the group of patients with sporadic early-onset (<65 years old) and late-onset AD (>65 years old). Thus, it seems important to understand the age-dependent relationship between neuroinflammation and the underlying biology of AD in order to identify potential explanations for clinical heterogeneity, interpret biomarkers, and promote the best treatment to different clinical AD phenotypes. The study of the delicate balance between pro-inflammatory or anti-inflammatory sides of immune players in the different ages of onset of AD would be important to understand treatment efficacy in clinical trials and eventually, not only direct treatment to early disease stages, but also the possibility of establishing different treatment approaches depending on the age of the patient. In this review, we would like to summarize what is currently known about the interplay between "normal" age associated inflammatory changes and AD pathological mechanisms, and also the potential differences between early-onset and late-onset AD taking into account the age-related neuroimmune background at disease onset.

Dexmedetomidine reduces lipopolysaccharide induced neuroinflammation, sickness behavior, and anhedonia

Background

Peripheral innate immune response may induce sickness behavior through activating microglia, excessive cytokines production, and neuroinflammation. Dexmedetomidine (Dex) has anti-inflammatory effect. We investigated the effects of Dex on lipopolysaccharide (LPS)-induced neuroinflammation and sickness behavior in mice.

Materials and methods

BALB/c mice were intraperitoneally (i.p.) injected with Dex (50 ug/kg) or vehicle. One hour later, the mice were injected (i.p.) with Escherichia coli LPS (0.33 mg/kg) or saline (n = 6 in each group). We analyzed the food and water intake, body weight loss, and sucrose preference of the mice for 24h. We also determined microglia activation and cytokines expression in the brains of the mice. In vitro, we determine cytokines expression in LPS-treated BV-2 microglial cells with or without Dex treatment.

Results

In the Dex-pretreated mice, LPS-induced sickness behavior (anorexia, weight loss, and social withdrawal) were attenuated and microglial activation was lower than vehicle control. The mRNA expression of TNF-α, MCP-1, indoleamine 2, 3 dioxygenase (IDO), caspase-3, and iNOS were increased in the brain of LPS-challenged mice, which were reduced by Dex but not vehicle.

Conclusion

Dexmedetomidine diminished LPS-induced neuroinflammation in the mouse brain and modulated the cytokine-associated changes in sickness behavior.

Mouse Testing Methods in Psychoneuroimmunology 2.0: Measuring Behavioral Responses

The field of psychoneuroimmunology (PNI) aims to uncover the processes and consequences of nervous, immune, and endocrine system relationships. Behavior is a consequence of such interactions and manifests from a complex interweave of factors including immune-to-neural and neural-to-immune communication. Often the signaling molecules involved during a particular episode of neuroimmune activation are not known but behavioral response provides evidence that bioactives such as neurotransmitters and cytokines are perturbed. Immunobehavioral phenotyping is a first-line approach when examining the neuroimmune system and its reaction to immune stimulation or suppression. Behavioral response is significantly more sensitive than direct measurement of a single specific bioactive and can quickly and efficiently rule in or out relevance of a particular immune challenge or therapeutic to neuroimmunity. Classically, immunobehavioral research was focused on sickness symptoms related to bacterial infection but neuroimmune activation is now a recognized complication of diseases and disorders ranging from cancer to diabesity to Alzheimer's. Immunobehaviors include lethargy, loss of appetite, and disinterest in social activity/surrounding environment. In addition, neuroimmune activation can diminish physical activity, precipitate feelings of depression and anxiety, and impair cognitive and executive function. Provided is a detailed overview of behavioral tests frequently used to examine neuroimmune activation in mice with a special emphasis on pre-experimental conditions that can confound or prevent successful immunobehavioral experimentation.

High-fat diet and aging interact to produce neuroinflammation and impair hippocampal- and amygdalar-dependent memory

More Americans are consuming diets higher in saturated fats and refined sugars than ever before, and based on increasing obesity rates, this is a growing trend among older adults as well. While high saturated fat diet (HFD) consumption has been shown to sensitize the inflammatory response to a subsequent immune challenge in young adult rats, the inflammatory effect of HFD in the already-vulnerable aging brain has not yet been assessed. Here, we explored whether short-term (3 days) consumption of HFD would serve as a neuroinflammatory trigger in aging animals, leading to cognitive deficits. HFD impaired long-term contextual (hippocampal dependent) and auditory-cued fear (amygdalar dependent) memory in aged, but not young adult rats. Short-term memory performance for both tasks was intact, suggesting that HFD impairs memory consolidation processes. Microglial markers of activation Iba1 and cd11b were only increased in the aged rats, while MHCII was further amplified by HFD. Furthermore, these HFD-induced long-term memory impairments were accompanied by IL-1β protein increases in both the hippocampus and amygdala in aged rats. Central administration of IL-1RA in aged rats following conditioning mitigated both contextual and auditory-cued fear memory impairments caused by HFD, strongly suggesting that IL-1β plays a critical role in these effects. Voluntary wheel running, known to have anti-inflammatory effects in the hippocampus, rescued hippocampal-dependent but not amygdalar-dependent memory impairments caused by HFD. Together, these data suggest that short-term consumption of HFD can lead to memory deficits and significant brain inflammation in the aged animal, and strongly suggest that appropriate diet is crucial for cognitive health.

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.

Neuroinflammation: the devil is in the details

There is significant interest in understanding inflammatory responses within the brain and spinal cord. Inflammatory responses that are centralized within the brain and spinal cord are generally referred to as 'neuroinflammatory'. Aspects of neuroinflammation vary within the context of disease, injury, infection, or stress. The context, course, and duration of these inflammatory responses are all critical aspects in the understanding of these processes and their corresponding physiological, biochemical, and behavioral consequences. Microglia, innate immune cells of the CNS, play key roles in mediating these neuroinflammatory responses. Because the connotation of neuroinflammation is inherently negative and maladaptive, the majority of research focus is on the pathological aspects of neuroinflammation. There are, however, several degrees of neuroinflammatory responses, some of which are positive. In many circumstances including CNS injury, there is a balance of inflammatory and intrinsic repair processes that influences functional recovery. In addition, there are several other examples where communication between the brain and immune system involves neuroinflammatory processes that are beneficial and adaptive. The purpose of this review is to distinguish different variations of neuroinflammation in a context-specific manner and detail both positive and negative aspects of neuroinflammatory processes. In this review, we will use brain and spinal cord injury, stress, aging, and other inflammatory events to illustrate the potential harm and benefits inherent to neuroinflammation. Context, course, and duration of the inflammation are highly important to the interpretation of these events, and we aim to provide insight into this by detailing several commonly studied insults. This article is part of the 60th anniversary supplemental issue.

Sulforaphane induces Nrf2 target genes and attenuates inflammatory gene expression in microglia from brain of young adult and aged mice

Increased neuroinflammation and oxidative stress resulting from heightened microglial activation are associated with age-related cognitive impairment. The objectives of this study were to examine the effects of the bioactive sulforaphane (SFN) on the nuclear factor E2-related factor 2 (Nrf2) pathway in BV2 microglia and primary microglia, and to evaluate proinflammatory cytokine expression in lipopolysaccharide (LPS)-stimulated primary microglia from adult and aged mice. BV2 microglia and primary microglia isolated from young adult and aged mice were treated with SFN and LPS. Changes in Nrf2 activity, expression of Nrf2 target genes, and levels of proinflammatory markers were assessed by quantitative PCR and immunoassay. SFN increased Nrf2 DNA-binding activity and upregulated Nrf2 target genes in BV2 microglia, while reducing LPS-induced interleukin (IL-)1β, IL-6, and inducible nitric oxide synthase (iNOS). In primary microglia from adult and aged mice, SFN increased expression of Nrf2 target genes and attenuated IL-1β, IL-6, and iNOS induced by LPS. These data indicate that SFN is a potential beneficial supplement that may be useful for reducing microglial mediated neuroinflammation and oxidative stress associated with aging.

Interleukin17A Promotes Postoperative Cognitive Dysfunction by Triggering β-Amyloid Accumulation via the Transforming Growth Factor-β (TGFβ)/Smad Signaling Pathway

Although postoperative cognitive dysfunction (POCD) is relatively common in elderly patients who have undergone major surgery, the mechanisms underlying this postoperative complication are unclear. Previously, we have investigated the role of cytokine-mediated hippocampal inflammation in the development of POCD in a rat model. Here, we sought to determine in mice the role of cytokine interleukin17A (IL17A) in POCD and to characterize the associated signaling pathways. Old mice underwent hepatectomy surgery in the presence or absence of IL17A monoclonal antibody, and cognitive function, hippocampal neuroinflammation, and pathologic markers of Alzheimer’s disease (AD) were assessed. We found that the level of IL17A in the hippocampus was increased in hepatectomy mice and that cognitive impairment after surgery was associated with the appearance of certain pathological hallmarks of AD: activation of astrocytes, β-amyloid_1-42 (Aβ_1–42) production, upregulation of transforming growth factor-β (TGFβ), and increased phosphorylation of signaling mother against decapentaplegic peptide 3 (Smad3) protein in the hippocampus. Surgery-induced changes in cognitive dysfunction and changes in Aβ_1–42 and TGFβ/Smad signaling were prevented by the administration of IL17A monoclonal antibody. In addition, IL17A-stimulated TGFβ/Smad activation and Aβ_1–42 expression were reversed by IL17A receptor small interfering RNA and a TGFβ receptor inhibitor in cultured astrocytes. Our findings suggest that surgery can provoke IL17A-related hippocampal damage, as characterized by activation of astrocytes and TGFβ/Smad pathway dependent Aβ_1–42 accumulation in old subjects. These changes likely contribute to the cognitive decline seen in POCD.

Histone Deacetylase Inhibitor Trichostatin A Ameliorated Endotoxin-Induced Neuroinflammation and Cognitive Dysfunction

Excessive production of cytokines by microglia may cause cognitive dysfunction and long-lasting behavioral changes. Activating the peripheral innate immune system stimulates cytokine secretion in the central nervous system, which modulates cognitive function. Histone deacetylases (HDACs) modulate cytokine synthesis and release. Trichostatin A (TSA), an HDAC inhibitor, is documented to be anti-inflammatory and neuroprotective. We investigated whether TSA reduces lipopolysaccharide- (LPS-) induced neuroinflammation and cognitive dysfunction. ICR mice were first intraperitoneally (i.p.) injected with vehicle or TSA (0.3 mg/kg). One hour later, they were injected (i.p.) with saline or Escherichia coli LPS (1 mg/kg). We analyzed the food and water intake, body weight loss, and sucrose preference of the injected mice and then determined the microglia activation and inflammatory cytokine expression in the brains of LPS-treated mice and LPS-treated BV-2 microglial cells. In the TSA-pretreated mice, microglial activation was lower, anhedonia did not occur, and LPS-induced cognitive dysfunction (anorexia, weight loss, and social withdrawal) was attenuated. Moreover, mRNA expression of HDAC2, HDAC5, indoleamine 2,3-dioxygenase (IDO), TNF-α, MCP-1, and IL-1β in the brain of LPS-challenged mice and in the LPS-treated BV-2 microglial cells was lower. TSA diminished LPS-induced inflammatory responses in the mouse brain and modulated the cytokine-associated changes in cognitive function, which might be specifically related to reducing HDAC2 and HDAC5 expression.

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.

Feeding the beast: can microglia in the senescent brain be regulated by diet?

Microglial cells, resident macrophages in the central nervous system (CNS), are relatively quiescent but can respond to signals from the peripheral immune system and induce neuroinflammation. In aging, microglia tend to transition to the M1 pro-inflammatory state and become hypersensitive to messages emerging from immune-to-brain signaling pathways. Thus, whereas in younger individuals where microglia respond to signals from the peripheral immune system and induce a well-controlled neuroinflammatory response that is adaptive (e.g., when well controlled, fever and sickness behavior facilitate recovery from infection), in older individuals with an infection, microglia overreact and produce excessive levels of inflammatory cytokines causing behavioral pathology including cognitive dysfunction. Importantly, recent studies indicate a number of naturally occurring bioactive compounds present in certain foods have anti-inflammatory properties and are capable of mitigating brain microglial cells. These include, e.g., flavonoid and non-flavonoid compounds in fruits and vegetables, and n-3 polyunsaturated fatty acids (PUFA) in oily fish. Thus, dietary bioactives have potential to restore the population of microglial cells in the senescent brain to a more quiescent state. The pragmatic concept to constrain microglia through dietary intervention is significant because neuroinflammation and cognitive deficits are co-morbid factors in many chronic inflammatory diseases. Controlling microglial cell reactivity has important consequences for preserving adult neurogenesis, neuronal structure and function, and cognition.

Microglial priming and enhanced reactivity to secondary insult in aging, and traumatic CNS injury, and neurodegenerative disease

Glia of the central nervous system (CNS) help to maintain homeostasis in the brain and support efficient neuronal function. Microglia are innate immune cells of the brain that mediate responses to pathogens and injury. They have key roles in phagocytic clearing, surveying the local microenvironment and propagating inflammatory signals. An interruption in homeostasis induces a cascade of conserved adaptive responses in glia. This response involves biochemical, physiological and morphological changes and is associated with the production of cytokines and secondary mediators that influence synaptic plasticity, cognition and behavior. This reorganization of host priorities represents a beneficial response that is normally adaptive but may become maladaptive when the profile of microglia is compromised. For instance, microglia can develop a primed or pro-inflammatory mRNA, protein and morphological profile with aging, traumatic brain injury and neurodegenerative disease. As a result, primed microglia exhibit an exaggerated inflammatory response to secondary and sub-threshold challenges. Consequences of exaggerated inflammatory responses by microglia include the development of cognitive deficits, impaired synaptic plasticity and accelerated neurodegeneration. Moreover, impairments in regulatory systems in these circumstances may make microglia more resistant to negative feedback and important functions of glia can become compromised and dysfunctional. Overall, the purpose of this review is to discuss key concepts of microglial priming and immune-reactivity in the context of aging, traumatic CNS injury and neurodegenerative disease. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Review: microglia of the aged brain: primed to be activated and resistant to regulation

Innate immunity within the central nervous system (CNS) is primarily provided by resident microglia. Microglia are pivotal in immune surveillance and also facilitate the co-ordinated responses between the immune system and the brain. For example, microglia interpret and propagate inflammatory signals that are initiated in the periphery. This transient microglial activation helps mount the appropriate physiological and behavioural response following peripheral infection. With normal ageing, however, microglia develop a more inflammatory phenotype. For instance, in several models of ageing there are increased pro-inflammatory cytokines in the brain and increased expression of inflammatory receptors on microglia. This increased inflammatory status of microglia with ageing is referred to as primed, reactive or sensitized. A modest increase in the inflammatory profile of the CNS and altered microglial function in ageing has behavioural and cognitive consequences. Nonetheless, there are major differences in microglial biology between young and old age when the immune system is challenged and microglia are activated. In this context, microglial activation is amplified and prolonged in the aged brain compared with adults. The cause of this amplified microglial activation may be related to impairments in several key regulatory systems with age that make it more difficult to resolve microglial activation. The consequences of impaired regulation and microglial hyper-activation following immune challenge are exaggerated neuroinflammation, sickness behaviour, depressive-like behaviour and cognitive deficits. Therefore the purpose of this review is to discuss the current understanding of age-associated microglial priming, consequences of priming and reactivity, and the impairments in regulatory systems that may underlie these age-related deficits.

Central inhibition of interleukin-6 trans-signaling during peripheral infection reduced neuroinflammation and sickness in aged mice

During systemic infection, inflammatory cytokines such as interleukin (IL)-6 are produced in excess in the brain of aged mice and induce severe behavioral deficits. However, no studies have examined how pro-inflammatory IL-6 trans-signaling is involved in the exaggerated production of IL-6 in the aged brain, nor the extent to which IL-6 trans-signaling affects other markers of neuroinflammation, adhesion molecules, and behavior. Therefore, this study investigated in aged mice the presence of IL-6 signaling subunits in microglia; the central effects of soluble gp130 (sgp130)-a natural inhibitor of the IL-6 trans-signaling pathway-on IL-6 production in microglia; and the effects of sgp130 given intracerebroventricularly (ICV) on neuroinflammation and sickness behavior caused by i.p. injection of lipopolysaccharide (LPS). Here we show that microglia isolated from aged mice have higher expression of IL-6 receptor (IL-6R) compared to microglia from adults; and the level of mRNA for ADAM17, the enzyme responsible for shedding membrane-bound IL-6R in trans-signaling, is higher in the hippocampus of aged mice compared to adults. Additionally, we show in aged mice that peripheral LPS challenge elicits a hyperactive IL-6 response in microglia, and selective blockade of trans-signaling by ICV injection of sgp130 mitigates this. The sgp130-associated inhibition of IL-6 was paralleled by amelioration of exaggerated and protracted sickness behavior in aged mice. Taken together, the results show that microglia are important regulators of the IL-6 trans-signaling response in the aged brain and sgp130 exerts an anti-inflammatory effect by inhibiting the pro-inflammatory arm of IL-6 signaling.

Microglial aging in the healthy CNS: phenotypes, drivers, and rejuvenation

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and age-related macular degeneration (AMD), share two characteristics in common: (1) a disease prevalence that increases markedly with advancing age, and (2) neuroinflammatory changes in which microglia, the primary resident immune cell of the CNS, feature prominently. These characteristics have led to the hypothesis that pathogenic mechanisms underlying age-related neurodegenerative disease involve aging changes in microglia. If correct, targeting features of microglial senescence may constitute a feasible therapeutic strategy. This review explores this hypothesis and its implications by considering the current knowledge on how microglia undergo change during aging and how the emergence of these aging phenotypes relate to significant alterations in microglial function. Evidence and theories on cellular mechanisms implicated in driving senescence in microglia are reviewed, as are “rejuvenative” measures and strategies that aim to reverse or ameliorate the aging microglial phenotype. Understanding and controlling microglial aging may represent an opportunity for elucidating disease mechanisms and for formulating novel therapies.

Peripheral innate immune challenge exaggerated microglia activation, increased the number of inflammatory CNS macrophages, and prolonged social withdrawal in socially defeated mice

Repeated social defeat (RSD) activates neuroendocrine pathways that have a significant influence on immunity and behavior. Previous studies from our lab indicate that RSD enhances the inflammatory capacity of CD11b⁺ cells in the brain and promotes anxiety-like behavior in an interleukin (IL)-1 and β-adrenergic receptor-dependent manner. The purpose of this study was to determine the degree to which mice subjected to RSD were more responsive to a secondary immune challenge. Therefore, RSD or control (HCC) mice were injected with saline or lipopolysaccharide (LPS) and activation of brain CD11b⁺ cells and behavioral responses were determined. Peripheral LPS (0.5 mg/kg) injection caused an extended sickness response with exaggerated weight loss and prolonged social withdrawal in socially defeated mice. LPS injection also amplified mRNA expression of IL-1β, tumor necrosis factor (TNF)-α, inducible nitric oxide synthase (iNOS), and CD14 in enriched CD11b⁺ cells isolated from socially defeated mice. In addition, IL-1β mRNA levels in enriched CD11b⁺ cells remained elevated in socially defeated mice 24 h and 72 h after LPS. Moreover, microglia and CNS macrophages isolated from socially defeated mice had the highest CD14 expression after LPS injection. Both social defeat and LPS injection increased the percentage of CD11b⁺/CD45(high) macrophages in the brain and the number of inflammatory macrophages (CD11b⁺/CD45(high)/CCR2⁺) was highest in RSD-LPS mice. Anxiety-like behavior was increased by social defeat, but was not exacerbated by the LPS challenge. Nonetheless, reduced locomotor activity and increased social withdrawal were still present in socially defeated mice 72 h after LPS. Last, LPS-induced microglia activation was most evident in the hippocampus of socially defeated mice. Taken together, these findings demonstrate that repeated social defeat enhanced the neuroinflammatory response and caused prolonged sickness following innate immune challenge.

Aging-related changes in neuroimmune-endocrine function: implications for hippocampal-dependent cognition

Healthy aged individuals are more likely to suffer profound memory impairments following a challenging life event such as a severe bacterial infection, surgery, or an intense psychological stressor, than are younger adults. Importantly, these peripheral challenges are capable of producing a neuroinflammatory response, (e.g., increased pro-inflammatory cytokines). In this review we will present the literature demonstrating that in the healthy aged brain this response is exaggerated and prolonged. Normal aging primes or sensitizes microglia and this appears to be the source of this amplified response. We will review the growing literature suggesting that a dysregulated neuroendocrine response in the aged organism is skewed toward higher brain CORT levels, and that this may play a critical role in priming microglia. Among the outcomes of an exaggerated neuroinflammatory response are impairments in synaptic plasticity, and reductions in key downstream mediators such as Arc and BDNF. We will show that each of these mechanisms is important for long-term memory formation, and is compromised by elevated pro-inflammatory cytokines.

Interleukin-6 trans-signaling in the senescent mouse brain is involved in infection-related deficits in contextual fear conditioning

Excessive production of pro-inflammatory cytokines in the senescent brain in response to peripheral immune stimulation is thought to induce behavioral pathology, however, few studies have examined if the increase in pro-inflammatory cytokines is accompanied by an increase in cytokine signaling. Here, we focused on IL-6 as a prototypic pro-inflammatory cytokine and used phosphorylated STAT3 as a marker of IL-6 signaling. In an initial study, IL-6 mRNA and the magnitude and duration of STAT3 activation were increased in the hippocampus of senescent mice compared to adults after i.p. injection of LPS. The LPS-induced increase in STAT3 activity was ablated in aged IL-6(-/-) mice, suggesting IL-6 is a key driver of STAT3 activity in the aged brain. To determine if IL-6 activated the classical or trans-signaling pathway, before receiving LPS i.p., aged mice were injected ICV with sgp130, an antagonist of the trans-signaling pathway. Importantly, the LPS-induced increases in both IL-6 and STAT3 activity in the hippocampus were inhibited by sgp130. To assess hippocampal function, aged mice were injected ICV with sgp130 and i.p. with LPS immediately after the acquisition phase of contextual fear conditioning, and immobility was assessed in the retention phase 48h later. LPS reduced immobility in aged mice, indicating immune activation interfered with memory consolidation. However, sgp130 blocked the deficits in contextual fear conditioning caused by LPS. Taken together, the results suggest IL-6 trans-signaling is increased in the senescent brain following peripheral LPS challenge and that sgp130 may protect against infection-related neuroinflammation and cognitive dysfunction in the aged.

Lipopolysaccharide-induced interleukin (IL)-4 receptor-α expression and corresponding sensitivity to the M2 promoting effects of IL-4 are impaired in microglia of aged mice

In several models of aging, microglia become more inflammatory and reactive to immune challenges. For example, peripheral LPS injection causes exaggerated microglial activation associated with prolonged sickness and depressive-like behavior in aged BALB/c mice. Therefore, the purpose of this study was to determine the extent to which age-related amplified microglial activation was associated with reduced sensitivity to the anti-inflammatory and M2 promoting cytokines interleukin (IL)-10 and IL-4. In initial studies with adult mice, LPS induced a time-dependent increase in M1 and M2 mRNA profiles in microglia. Furthermore, peripheral LPS injection markedly increased surface expression of IL-4 receptor-alpha (IL-4Rα), but not IL-10 receptor-1 (IL-10R1) on microglia. In BV-2 cells, IL-4, but not IL-10, re-directed LPS-activated microglia towards an M2 phenotype. Based on these findings, comparisons of M1 and M2 activation profiles, induction of IL-4Rα, and sensitivity to IL-4 were determined in microglia from adult (3-4 mo) and aged (18-22 mo) mice. In aged microglia, LPS promoted an exaggerated and prolonged M1 and M2 profile compared to adults. Moreover, IL-4Rα protein was not increased on aged microglia following LPS injection. To determine the consequence of impaired IL-4Rα upregulation, adult and aged mice were injected with LPS and activated microglia were then isolated and treated ex vivo with IL-4. While ex vivo IL-4 induced an M2 profile in activated microglia from adult mice, activated microglia from aged mice retained a prominent M1 profile. These data indicate that activated microglia from aged mice are less sensitive to the anti-inflammatory and M2-promoting effects of IL-4.

Mouse testing methods in psychoneuroimmunology: an overview of how to measure sickness, depressive/anxietal, cognitive, and physical activity behaviors

The field of psychoneuroimmunology (PNI) aims to uncover the processes and consequences of nervous, immune, and endocrine system relationships. Behavior is a consequence of such interactions and manifests from a complex interweave of factors including immune-to-neural and neural-to-immune communication. Often the signaling molecules involved during a particular episode of neuroimmune activation are not known but behavioral response provides evidence that bioactives such as neurotransmitters and cytokines are perturbed. Immunobehavioral phenotyping is a first-line approach when examining the neuroimmune system and its reaction to immune stimulation or suppression. Behavioral response is significantly more sensitive than direct measurement of a single specific bioactive and can quickly and efficiently rule in or out relevance of a particular immune challenge or therapeutic to neuroimmunity. Classically, immunobehavioral research was focused on sickness symptoms related to bacterial infection but neuroimmune activation is now a recognized complication of diseases and disorders ranging from cancer to diabesity. Immunobehaviors include lethargy, loss of appetite, and disinterest in social activity and the surrounding environment. In addition, neuroimmune activation can precipitate feelings of depression and anxiety while negatively impacting cognitive function and physical activity. Provided is a detailed overview of behavioral tests frequently used to examine neuroimmune activation in mice with a special emphasis on preexperimental conditions that can confound or prevent successful immunobehavioral experimentation.

Intracerebroventricular administration of HIV-1 Tat induces brain cytokine and indoleamine 2,3-dioxygenase expression: a possible mechanism for AIDS comorbid depression

Human immunodeficiency virus (HIV) remains a major public health concern despite a large education effort during the past 25 years. A persistent problem with HIV infection is the high comorbity rate of clinical depression. We previously established that increasing proinflammatory cytokines within the brain of mice induces sickness that can culminate in depressive-like behavior. Here we investigated the role of the HIV transactivator of transcription (Tat) protein in activation of brain cytokine signaling and subsequent induction of depressive-like behavior in a murine model. Adult Balb/c mice were administered a single intracerebroventricular (ICV) injection of Tat (40 ng). Social investigation of a novel juvenile was measured at 2, 4, 8 and 24 h post-treatment. Mice treated with Tat did not display signs of sickness, as measured by either decreased social investigation or loss of body weight. At 24 h post-injection, mice were subjected to the forced swim test (FST). ICV administration of Tat to Balb/c mice increased immobility in the FST at 24 h post injection. A different strain of mice, C57BL/6J, responded similarly in the FST. Furthermore, adult C57BL/6J mice injected with Tat and tested in a two-bottle 1% sucrose preference test displayed reduced preference for sucrose during the 24 h post-injection period. Subsequently, brain tissues from Tat-treated and control C57BL/6J mice were collected at 4 and 24 h post injection. CNS tissue from Tat-treated mice had increased expression of IL-1β, TNF-α, IL-6, and IDO mRNAs at 4 h post injection. These data demonstrate that a single exposure to Tat in the brain is sufficient to induce brain cytokine signaling that culminates in depressive-like behavior. The results reveal a potential role for Tat in the development of comorbid depression in HIV-infected individuals.

Cognitive and behavioral consequences of impaired immunoregulation in aging

A hallmark of the aged immune system is impaired immunoregulation of the innate and adaptive immune system in the periphery and also in the central nervous system (CNS). Impaired immunoregulation may predispose older individuals to an increased frequency of peripheral infections with concomitant cognitive and behavioral complications. Thus, normal aging is hypothesized to alter the highly coordinated interactions between the immune system and the brain. In support of this notion, mounting evidence in rodent models indicate that the increased inflammatory status of the brain is associated with increased reactivity of microglia, the innate immune cells of the CNS. Understanding how immunity is affected with age is important because CNS immune cells play an integral role in propagating inflammatory signals that are initiated in the periphery. Increased reactivity of microglia sets the stage for an exaggerated inflammatory cytokine response following activation of the peripheral innate immune system that is paralleled by prolonged sickness, depressive-like complications and cognitive impairment. Moreover, amplified neuroinflammation negatively affects several aspects of neural plasticity (e.g., neurogenesis, long-term potentiation, and dendritic morphology) that can contribute to the severity of neurological complications. The purpose of this review is to discuss several key peripheral and central immune changes that impair the coordinated response between the immune system and the brain and result in behavioral and cognitive deficits.

CCAAT/enhancer binding protein β expression is increased in the brain during HIV-1-infection and contributes to regulation of astrocyte tissue inhibitor of metalloproteinase-1

Human immunodeficiency virus (HIV)-1-associated neurocognitive disorders (HAND) associated with infection and activation of mononuclear phagocytes (MP) in the brain, occur late in disease. Infected/activated MP initiate neuroinflammation activating glial cells and ultimately disrupting neuronal function. Astrocytes secrete tissue inhibitor of metalloproteinase (TIMP)-1 in response to neural injury. Altered TIMP-1 levels are implicated in several CNS diseases. CCAAT enhancer-binding protein β (C/EBPβ), a transcription factor, is expressed in rodent brains in response to neuroinflammation, implicating it in Alzheimer's, Parkinson's, and HAND. Here, we report that C/EBPβ mRNA levels are elevated and its isoforms differentially expressed in total brain tissue lysates of HIV-1-infected and HIV-1 encephalitis patients. In vitro, HAND-relevant stimuli additively induce C/EBPβ nuclear expression in human astrocytes through 7 days of treatment. Over-expression of C/EBPβ increases TIMP-1 promoter activity, mRNA, and protein levels in human astrocytes activated with interleukin-1β. Knockdown of C/EBPβ with siRNA decreases TIMP-1 mRNA and protein levels. These data suggest that C/EBPβ isoforms are involved in complex regulation of astrocyte TIMP-1 production during HIV-1 infection; however, further studies are required to completely understand their role during disease progression.

Inhibition of interleukin-6 trans-signaling in the brain facilitates recovery from lipopolysaccharide-induced sickness behavior

Background

Interleukin (IL)-6 is produced in the brain during peripheral infection and plays an important but poorly understood role in sickness behavior. Therefore, this study investigated the capacity of soluble gp130 (sgp130), a natural inhibitor of the IL-6 trans-signaling pathway to regulate IL-6 production in microglia and neurons in vitro and its effects on lipopolysaccharide (LPS)-induced sickness behavior in vivo.

Methods

A murine microglia (BV.2) and neuronal cell line (Neuro.2A) were used to study the effects of stimulating and inhibiting the IL-6 signaling pathway in vitro. In vivo, adult (3-6 mo) BALB/c mice received an intracerebroventricular (ICV) injection of sgp130 followed by an intraperitoneal (i.p.) injection of LPS, and sickness behavior and markers of neuroinflammation were measured.

Results

Soluble gp130 attenuated IL-6- and LPS-stimulated IL-6 receptor (IL-6R) activation along with IL-6 protein release in both microglial (BV.2) and neuronal (Neuro.2A) cell types in vitro. Moreover, in vivo experiments showed that sgp130 facilitated recovery from LPS-induced sickness, and this sgp130-associated recovery was paralleled by reduced IL-6 receptor signaling, mRNA, and protein levels of IL-6 in the hippocampus.

Conclusions

Taken together, the results show that sgp130 may exert an anti-inflammatory effect on microglia and neurons by inhibiting IL-6 binding. These data indicate that sgp130 inhibits the LPS-induced IL-6 trans-signal and show IL-6 and its receptor are involved in maintaining sickness behavior.

The chemokine receptor CCR5 in the central nervous system

The expression and the role of the chemokine receptor CCR5 have been mainly studied in the context of HIV infection. However, this protein is also expressed in the brain, where it can be crucial in determining the outcome in response to different insults. CCR5 expression can be deleterious or protective in controlling the progression of certain infections in the CNS, but it is also emerging that it could play a role in non-infectious diseases. In particular, it appears that, in addition to modulating immune responses, CCR5 can influence neuronal survival. Here, we summarize the present knowledge about the expression of CCR5 in the brain and highlight recent findings suggesting its possible involvement in neuroprotective mechanisms.

Postoperative cognitive deficits and neuroinflammation in the hippocampus triggered by surgical trauma are exacerbated in aged rats

Postoperative cognitive dysfunction (POCD) is characterized by the progressive deterioration of intellectual/cognitive function following surgery. It has been suggested that the senile brain, which characteristically expresses higher levels of central proinflammatory cytokines, interleukin (IL)-6, IL-1β, and tumor necrosis factor (TNF)-α, is more susceptible to additional insult following surgery. The authors of this study investigated the expression of central cytokines IL-1β, IL-6 and TNF-α and hippocampal glial cell activation in aged and adult rats following partial hepatectomy. Cognitive function was assessed in a reversal-learning version of the Morris water maze (MWM) before and after surgery. Hippocampal pro-inflammatory cytokines IL-1β, IL-6 and TNF-α and glial cell activation markers glial fibrillary acidic protein (GFAP) and S100β were measured at each time point; CD200 and CD200R were also measured to explore potential mechanisms of glial cell activation. Surgical trauma resulted in impairments in distance and latency only on postoperative day 1 (p<0.001, respectively) in adult rats. Aged rats exhibited impairments on day 1 (p<0.001) that persisted until postoperative day 3 (p=0.002 and p=0.001, respectively). All significant impairments paralleled upregulated cytokine IL-1β and IL-6 expression. Immunohistochemistry assay further showed more hippocampal glial cell activation in aged rats compared to that in adults. Overall, these findings suggest that surgical trauma, rather than anesthesia, resulted in cognitive function impairment potentiated by aging. Hippocampal pro-inflammatory cytokines and glial cell activation might mediate trauma-induced POCD.

Dysregulated neuronal-microglial cross-talk during aging, stress and inflammation

Communication between neurons and microglia is essential for maintaining homeostasis in the central nervous system (CNS) during both physiological and inflammatory conditions. While microglial activation is necessary and beneficial in response to injury or disease, excessive or prolonged activation can have deleterious effects on brain function and behavior. To prevent inflammation-associated damage, microglia reactivity is actively modulated by neurons in the healthy brain. Age or stress-induced disruption of normal neuronal-microglial communication could lead to an aberrant central immune response when additional stressors are applied. Recent work suggests that both aging and stress shift the CNS microenvironment to a pro-inflammatory state characterized by increased microglial reactivity and a reduction in anti-inflammatory and immunoregulatory factors. This review will discuss how heightened neuroinflammation associated with aging and stress may be compounded by the concomitant loss of neuronally derived factors that control microglial activation, leaving the brain vulnerable to excessive inflammation and neurobehavioral complications upon subsequent immune challenge.

Protracted downregulation of CX3CR1 on microglia of aged mice after lipopolysaccharide challenge

Fractalkine (CX(3)CL1) to fractalkine receptor (CX(3)CR1) interactions in the brain are involved in the modulation of microglial activation. Our recent findings indicate that there is microglial hyperactivity in the aged brain during an inflammatory challenge. The underlying cause of this amplified microglial response in the aged brain is unknown. Therefore, the purpose of this study was to determine the degree to which age-associated impairments of CX(3)CL1 and CX(3)CR1 in the brain contribute to exaggerated microglial activation after intraperitoneal (i.p.) injection of lipopolysaccharide (LPS). Here we show that CX(3)CL1 protein was reduced in the brain of aged (18-22 mo) BALB/c mice compared to adult (3-6 mo) controls. CX(3)CL1 protein, however, was unaltered by LPS injection. Next, CX(3)CR1 levels were determined in microglia (CD11b(+)/CD45(low)) isolated by Percoll density gradient separation at 4 and 24h after LPS injection. Flow cytometric and mRNA analyses of these microglia showed that LPS injection caused a marked decrease of CX(3)CR1 and a simultaneous increase of IL-1β at 4h after LPS injection. While surface expression of CX(3)CR1 was enhanced on microglia of adult mice by 24h, it was still significantly downregulated on a subset of microglia from aged mice. This protracted reduction of CX(3)CR1 corresponded with a delayed recovery from sickness behavior, prolonged IL-1β induction, and decreased TGFß expression in the aged brain. In the last set of studies BV2 microglia were used to determine effect of TGFß on CX(3)CR1. These results showed that TGFβ enhanced CX(3)CR1 expression and attenuated the LPS-induced increase in IL-1β expression.

Aging sensitizes rapidly isolated hippocampal microglia to LPS ex vivo

The present study tested whether aging sensitizes hippocampal microglia to a pro-inflammatory challenge ex vivo. Hippocampal microglia from 3 and 24 mo old male F344 x BN F1 rats were exposed to LPS (0, 0.1, 1, 10 and 100 ng/ml) ex vivo. 2 h post-LPS challenge, gene expression of microglial activation markers and cytokines were assessed. 24 mo old animals exhibited a potentiated pro-inflammatory cytokine (IL-1β and IL-6) response to LPS and increased levels of CD11b, Iba-1 and MHCII irrespective of LPS treatment. The present results demonstrate that aging sensitizes hippocampal microglia to pro-inflammatory challenges.

Regulation of learning and memory by meningeal immunity: a key role for IL-4

Proinflammatory cytokines have been shown to impair cognition; consequently, immune activity in the central nervous system was considered detrimental to cognitive function. Unexpectedly, however, T cells were recently shown to support learning and memory, though the underlying mechanism was unclear. We show that one of the steps in the cascade of T cell-based support of learning and memory takes place in the meningeal spaces. Performance of cognitive tasks led to accumulation of IL-4-producing T cells in the meninges. Depletion of T cells from meningeal spaces skewed meningeal myeloid cells toward a proinflammatory phenotype. T cell-derived IL-4 was critical, as IL-4(-/-) mice exhibited a skewed proinflammatory meningeal myeloid cell phenotype and cognitive deficits. Transplantation of IL-4(-/-) bone marrow into irradiated wild-type recipients also resulted in cognitive impairment and proinflammatory skew. Moreover, adoptive transfer of T cells from wild-type into IL-4(-/-) mice reversed cognitive impairment and attenuated the proinflammatory character of meningeal myeloid cells. Our results point to a critical role for T cell-derived IL-4 in the regulation of cognitive function through meningeal myeloid cell phenotype and brain-derived neurotrophic factor expression. These findings might lead to the development of new immune-based therapies for cognitive impairment associated with immune decline.

IL-1RA blocks E. coli-induced suppression of Arc and long-term memory in aged F344xBN F1 rats

In normal aging, a peripheral immune challenge induces a sensitized and protracted neuroinflammatory response in parallel with long-term memory (LTM) impairments. Pro-inflammatory mediators of neuroinflammation impair LTM, synaptic plasticity and LTP. The immediate early gene Arc is considered a critical protein regulating LTM and synaptic plasticity. The present investigation examined whether (1) a peripheral Escherichia coli infection suppresses hippocampal Arc expression, and (2) central pro-inflammatory cytokines (IL-1beta and IL-6) mediate the effects of peripheral E. coli infection on Arc and LTM. In 24 months F344xBN F1 rats, E. coli infection suppressed basal Arc gene expression as well as contextual fear conditioning-induced Arc expression. E. coli treatment failed to alter either basal or conditioning-induced c-Fos expression. At 24h post-infection, intra-cisterna magna (ICM) treatment with the anti-inflammatory cytokine IL-1RA blocked the E. coli-induced suppression of hippocampal Arc and increases in IL-6 protein. At 4-day post-infection, IL-1RA blocked the E. coli-induced LTM impairments and increases in IL-6 protein. The present results suggest that central pro-inflammatory cytokines play a salient role in the suppression of Arc and impairments of LTM by a peripheral immune challenge in older animals.

Age-related neuroinflammatory changes negatively impact on neuronal function

Neuroinflammatory changes, characterized by an increase in microglial activation and often accompanied by upregulation of inflammatory cytokines like interleukin-1β (IL-1β), are common to many, if not all, neurodegenerative diseases. Similar, though less dramatic neuroinflammatory changes, are also known to occur with age. Among the consequences of these changes is an impairment in synaptic function and the evidence suggests that inflammatory cytokines may be the primary contributory factor responsible for the deficits in synaptic plasticity which have been identified in aged rodents. Specifically a decrease in the ability of aged rats to sustain long-term potentiation (LTP) in perforant path-granule cells of the hippocampus is associated with increased microglial activation. This review considers the evidence which suggests a causal relationship between these changes and the factors which contribute to the age-related microglial activation, and reflects on data which demonstrate that agents which inhibit microglial activation also improve ability of rats to sustain LTP.

Regulation of cytokine signaling and T-cell recruitment in the aging mouse brain in response to central inflammatory challenge

Aging is often accompanied by increased levels of inflammatory molecules in the organism, but age-related changes in the brain response to inflammatory challenges still require clarification. We here investigated in mice whether cytokine signaling and T-cell neuroinvasion undergo age-related changes. We first analyzed the expression of molecules involved in T-cell infiltration and cytokine signaling regulation in the septum and hippocampus of 2-3 months and 20- to 24-month-old mice at 4h after intracerebroventricular injections of tumor necrosis factor (TNF)-alpha or interferon-gammaversus saline injections. Transcripts of the chemokine CXCL9, intercellular adhesion molecule (ICAM)-1 and suppressor of cytokine signaling molecules (SOCS) 1 and 3 were increased in both age groups after cytokine injection; microglia-derived matrix metalloproteinase (MMP) 12 mRNA was induced in old mice also after control saline injections. Age-related changes in ICAM-1 protein expression and T-cell infiltration were then analyzed in mice of 3-4, 8-9 and 15-16 months at 48h after TNF-alpha injections. ICAM-1 immunoreactivity, and Western blotting in striatum, septum, hippocampus and hypothalamus showed progressive age-related enhancement of TNF-alpha-elicited ICAM-1 upregulation. Double immunofluorescence revealed ICAM-1 expression in microglia and astrocytic processes. CD3(+), CD4(+) and CD8(+) T-cells exhibited progressive age-related increases in brain parenchyma and choroid plexus after cytokine exposure. The findings indicate that the brain responses to inflammatory challenges are not only preserved with advancing age, but also include gradual amplification of ICAM-1 expression and T-cell recruitment. The data highlight molecular and cellular correlates of age-related increase of brain sensitivity to inflammatory stimuli, which could be involved in altered brain vulnerability during aging.

Memory impairments in healthy aging: Role of aging-induced microglial sensitization

Healthy aged individuals are more likely to suffer profound memory impairments following a challenging life event such as a severe bacterial infection, surgery, or an intense psychological stressor, than are younger adults. These peripheral challenges are capable of producing a neuroinflammatory response, (e.g., increased pro-inflammatory cytokines), and in the healthy aged brain this response is exaggerated and prolonged. Normal aging primes or sensitizes microglia and this appears to be the source of this amplified response. Among the outcomes of this exaggerated neuroinflammatory response is an impairment in synaptic plasticity, and a reduction in key downstream mediators such as Arc and BDNF. Each of these mechanisms is important for long-term memory formation, and is compromised by elevated pro-inflammatory cytokines. Pharmacological, dietary and physical interventions are discussed as potential therapies to abrogate the challenge-induced neuroinflammatory response, thereby preventing or reducing memory deficits in aged subjects.

Immune and behavioral consequences of microglial reactivity in the aged brain

Bidirectional communication between the immune system and the brain is essential for mounting the appropriate immunological, physiological, and behavioral responses to immune activation. Aging, however, may impair this important bi-directional interaction. In support of this notion, peripheral infection in the elderly is associated with an increased frequency of behavioral and cognitive complications. Recent findings in animal models of aging and neurodegenerative disease indicate that microglia, innate immune cells of the brain, become primed or reactive. Understanding age- and disease-associated alterations in microglia is important because glia (microglia and astrocytes) play an integral role in propagating inflammatory signals that are initiated in the periphery. In this capacity, brain glia produce inflammatory cytokines that target neuronal substrates and elicit a sickness-behavior syndrome that is normally beneficial to the host organism. Increased reactivity of microglia sets the stage for an exaggerated neuroinflammatory cytokine response following activation of the peripheral innate immune system, which may underlie subsequent long-lasting behavioral and cognitive deficits. In support of this premise, recent findings indicate that stimulation of the peripheral immune system in aged rodents causes exaggerated neuroinflammation that is paralleled by cognitive impairment, prolonged sickness, and depressive-like complications. Therefore, the purpose of this review is to discuss the new evidence that age-associated priming of microglia could play a pathophysiological role in exaggerated behavioral and cognitive sequelae to peripheral infection.

Central inhibition of interleukin-1beta ameliorates sickness behavior in aged mice

In elderly individuals high levels of interleukin-1beta (IL-1beta) in the brain have been implicated in infection-related behavioral pathologies but this has not been directly tested. Therefore, the current study investigated if sickness behavior in aged animals elicited by peripheral injection of lipopolysaccharide (LPS) is mediated through central IL-1beta. Adult and aged mice were injected intracerebroventricularly with either saline or IL-1ra (4mug) immediately prior to intraperitoneal administration of saline or LPS (10mug) and locomotor and social behaviors were assessed. As anticipated, LPS depressed locomotor activity and social behavior in both adult and aged mice but the behavioral deficits were markedly greater in the aged at 24h. Pretreatment with IL-1ra did not affect LPS-induced sickness behavior in adults; however, in aged mice IL-1ra attenuated LPS-induced sickness behavior, restoring it to the level exhibited by young adults. Twenty-four hours post-injection hippocampal and hypothalamic tissues were collected to determine IL-1beta mRNA expression. Neither LPS nor IL-1ra affected IL-1beta mRNA levels in adults, presumably because any effect of LPS had dissipated by 24h. In contrast, IL-1beta mRNA was markedly higher in aged mice 24h after LPS, and prior treatment with IL-1ra either blocked or attenuated this effect in the hippocampus and hypothalamus, respectively. Taken together these data provide the first direct evidence that central IL-1beta is responsible for the severe sickness behavior observed in aged animals after LPS treatment. Thus, inhibiting the central actions of IL-1beta may be useful for minimizing behavioral complications in older individuals with an infection.

Peripheral lipopolysaccharide (LPS) challenge promotes microglial hyperactivity in aged mice that is associated with exaggerated induction of both pro-inflammatory IL-1beta and anti-inflammatory IL-10 cytokines

In the elderly, systemic infection is associated with an increased frequency of behavioral and cognitive complications. We have reported that peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes an exaggerated neuroinflammatory response and prolonged sickness/depressive-like behaviors in aged BALB/c mice. Therefore, the purpose of this study was to determine the degree to which LPS-induced neuroinflammation was associated with microglia-specific induction of neuroinflammatory mediators. Here, we show that peripheral LPS challenge caused a hyperactive microglial response in the aged brain associated with higher induction of inflammatory IL-1beta and anti-inflammatory IL-10. LPS injection caused a marked induction of mRNA expression of both IL-1beta and IL-10 in the cortex of aged mice compared to adults. In the next set of studies, microglia (CD11b(+)/CD45(low)) were isolated from the brain of adult and aged mice following experimental treatments. An age-dependent increase in major histocompatibility complex (MHC) class II mRNA and protein expression was detected in microglia. Moreover, peripheral LPS injection caused a more pronounced increase in IL-1beta, IL-10, Toll-like receptor (TLR)-2, and indoleamine 2,3-dioxygenase (IDO) mRNA levels in microglia isolated from aged mice than adults. Intracellular cytokine protein detection confirmed that peripheral LPS caused the highest increase in IL-1beta and IL-10 levels in microglia of aged mice. Finally, the most prominent induction of IL-1beta was detected in MHC II(+) microglia from aged mice. Taken together, these findings provide novel evidence that age-associated priming of microglia plays a central role in exaggerated neuroinflammation induced by activation of the peripheral innate immune system.

Neuroinflammatory changes increase the impact of stressors on neuronal function

In the last few years, several research groups have reported that neuroinflammation is one feature common to several neurodegenerative diseases and that similar, although perhaps less profound, neuroinflammatory changes also occur with age. Age is the greatest risk factor in many neurodegenerative diseases, and the possibility exists that the underlying age-related neuroinflammation may contribute to this increased risk. Several animal models have been used to examine this possibility, and it is now accepted that, under experimental conditions in which microglial activation is up-regulated, responses to stressors are exacerbated. In the present article, these findings are discussed and data are presented from in vitro and in vivo experiments which reveal that responses to Abeta (amyloid beta-peptide) are markedly up-regulated in the presence of LPS (lipopolysaccharide). These, and previous findings, point to a vulnerability associated with inflammation and suggest that, even though inflammation may not be the primary cause of neurodegenerative disease, its treatment may decelerate disease progression.

Time course of hippocampal IL-1 beta and memory consolidation impairments in aging rats following peripheral infection

We previously reported that aging F344XBN rats are more vulnerable to disruptions of memory consolidation processes following an injection of Escherichia coli than are young rats. Furthermore, this disruption was specific to hippocampal-dependent memory. In the present study we examined the time course of the proinflammatory cytokine IL-1 beta in young and old rats following a peripheral injection of E. coli. Compared to young rats, aging rats treated with E. coli showed an exaggerated and prolonged up-regulation of IL-1 beta protein in the hippocampus, but not in hypothalamus, parietal cortex, prefrontal cortex, serum or spleen. Aging rats showed greater hippocampal IL-1 beta protein levels than their young counterparts 4h after E. coli, and these levels remained significantly elevated for 8 but not 14 days after E. coli. In a second experiment, aging rats exhibited anterograde memory consolidation impairments 4 and 8 days after an E. coli injection, but not after 14 days. A third experiment revealed that following an E. coli injection, bacterial clearance from the spleen and peritoneum was not impaired in aged rats, suggesting that elevations in hippocampal IL-1 beta were not mediated by impaired clearance in the periphery in aging rats. These data suggest that the exaggerated and prolonged elevation of IL-1 beta, specifically in the hippocampus, may be responsible for hippocampal-dependent memory impairments observed in aging rats following a bacterial infection.

Architectural changes to CA1 pyramidal neurons in adult and aged mice after peripheral immune stimulation

The expression of several inflammatory cytokines that inhibit synaptic plasticity and hippocampal-dependent learning and memory is higher in the brains of aged mice compared to young adults after peripheral injection of lipopolysaccharide (LPS). In this study we investigated whether the exaggerated inflammatory cytokine response in the hippocampus of aged mice after IP injection of LPS is associated with architectural changes to dendrites of pyramidal neurons in the dorsal CA1 hippocampus. Compared to young adults, aged mice had higher basal expression of MHC class II, lower basal expression of two neurotrophins, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), and a decrease in total dendritic length in both the basal and apical tree. After IP LPS administration, expression of IL-1beta, IL-6, and TNFalpha mRNA was higher in hippocampus of aged mice compared to young adults whereas NGF and BDNF mRNA was reduced similarly in both age groups. The basal dendritic tree was not affected by LPS in either adult or aged mice 72h after treatment; however, length and branching of the apical tree was reduced by LPS in aged but not adult mice. The present findings indicate that a peripheral infection in the aged can cause a heightened inflammatory cytokine response in the hippocampus and atrophy of hippocampal neurons. Architectural changes to dorsal CA1 hippocampal neurons may contribute to cognitive disorders evident in elderly patients with an infection.

Immunity and cognition: what do age-related dementia, HIV-dementia and 'chemo-brain' have in common?

Until recently, dogma dictated that the immune system and the central nervous system (CNS) live mostly separate, parallel lives, and any interactions between the two were assumed to be limited to extreme cases of pathological insult. It was only a decade ago that T cells in the injured brain were shown to play a protective rather than a destructive role. In this article, we explore the role of the immune system in the healthy brain, focusing on the key function that T lymphocytes have in the regulation of cognition. We discuss candidate mechanisms underlying T cell-mediated control of cognitive function in human cognitive diseases associated with immune decline, such as age- and HIV-related dementias, 'chemo-brain' and others.