Age-dependent responses of glial cells and leptomeninges during systemic inflammation

Age and sex differences in primary microglia culture: A comparative study

BACKGROUND

Microglia play a central role in neuroinflammation in various CNS diseases.Neonatal microglial culture has been extensively used to in vitro study microglial activation; however, as many neuroinflammatory diseases occur in the elderly, the neonatal microglial culture may not fully replicate the aged microglial activity seen in these diseases.

NEW METHOD

Primary microglia from both 18-24-month-old and P0-P4 C57BL/6 mice were cultured simultaneously. Morphology and activation profiles of the two age groups of microglia were examined following ischemic stimulation, by ELISA, RT-PCR, live microscopy, immunocytochemistry, and Western blotting.

RESULTS

We showed that aged microglia had larger cell bodies, more cytoplasmic inclusions, and enhanced phagocytosis than neonatal microglia. Cytokine production in these cells exhibited heterogeneity either after or before ischemic stimulation. The baseline expression of microglial marker CD11b was significantly higher in aged vs. neonatal cells; ischemic stimulation increased the expression in neonatal vs. aged microglia only in males but not in females.

COMPARISON WITH EXISTING METHODS

Previous primary microglia cultures have been limited to using neonatal/adult cells. This method is complementary to exiting methods and works for aged microglia, and does not suffer from potential limitations due to filtering artifacts. The protocol renders microglial culture no need for meningeal/hippocampal removal prior to brain tissue dissociation, and compares microglia between males vs. females, and between the aged vs. neonates.

CONCLUSIONS

We concluded that neonatal microglial culture is not appropriate for those in vitro studies that mimic the neuroinflammatory central nervous system disorders occurring in the elderly, in which case the aged microglial culture should be applied, and sex differences should be considered.

Porphyromonas Gingivalis Infection Induces Synaptic Failure via Increased IL-1β Production in Leptomeningeal Cells

BACKGROUND

Studies have reported that synaptic failure occurs before the Alzheimer's disease (AD) onset. The systemic Porphyromonas gingivalis (P. gingivalis) infection is involved in memory decline. We previously showed that leptomeningeal cells, covering the brain, activate glial cells by releasing IL-1β in response to systemic inflammation.

OBJECTIVE

In the present study, we focused on the impact of leptomeningeal cells on neurons during systemic P. gingivalis infection.

METHODS

The responses of leptomeningeal cells and cortical neurons to systemic P. gingivalis infection were examined in 15-month-old mice. The mechanism of IL-1β production by P. gingivalis infected leptomeningeal cells was examined, and primary cortical neurons were treated with P. gingivalis infected leptomeningeal cells condition medium (Pg LCM).

RESULTS

Systemic P. gingivalis infection increased the expression of IL-1β in leptomeninges and reduced the synaptophysin (SYP) expression in leptomeninges proximity cortex in mice. Leptomeningeal cells phagocytosed P. gingivalis resulting in lysosomal rupture and Cathepsin B (CatB) leakage. Leaked CatB mediated NLRP3 inflammasome activation inducing IL-1β secretion in leptomeningeal cells. Pg LCM decreased the expression of synaptic molecules, including SYP, which was inhibited by an IL-1 receptor antagonist pre-treatment.

CONCLUSION

These observations demonstrate that P. gingivalis infection is involved in synaptic failure by inducing CatB/NLRP3 inflammasome-mediated IL-1β production in leptomeningeal cells. The periodontal bacteria-induced synaptic damage may accelerate the onset and cognitive decline of AD.

Inflammation Spreading: Negative Spiral Linking Systemic Inflammatory Disorders and Alzheimer's Disease

As a physiological response to injury in the internal body organs, inflammation is responsible for removing dangerous stimuli and initiating healing. However, persistent and exaggerative chronic inflammation causes undesirable negative effects in the organs. Inflammation occurring in the brain and spinal cord is known as neuroinflammation, with microglia acting as the central cellular player. There is increasing evidence suggesting that chronic neuroinflammation is the most relevant pathological feature of Alzheimer’s disease (AD), regulating other pathological features, such as the accumulation of amyloid-β (Aβ) and hyperphosphorylation of Tau. Systemic inflammatory signals caused by systemic disorders are known to strongly influence neuroinflammation as a consequence of microglial activation, inflammatory mediator production, and the recruitment of peripheral immune cells to the brain, resulting in neuronal dysfunction. However, the neuroinflammation-accelerated neuronal dysfunction in AD also influences the functions of peripheral organs. In the present review, we highlight the link between systemic inflammatory disorders and AD, with inflammation serving as the common explosion. We discuss the molecular mechanisms that govern the crosstalk between systemic inflammation and neuroinflammation. In our view, inflammation spreading indicates a negative spiral between systemic diseases and AD. Therefore, “dampening inflammation” through the inhibition of cathepsin (Cat)B or CatS may be a novel therapeutic approach for delaying the onset of and enacting early intervention for AD.

A novel cyclic peptide (Naturido) modulates glia-neuron interactions in vitro and reverses ageing-related deficits in senescence-accelerated mice

The use of agents that target both glia and neurons may represent a new strategy for the treatment of ageing disorders. Here, we confirmed the presence of the novel cyclic peptide Naturido that originates from a medicinal fungus (Isaria japonica) grown on domestic silkworm (Bombyx mori). We found that Naturido significantly enhanced astrocyte proliferation and activated the single copy gene encoding the neuropeptide VGF and the neuron-derived NGF gene. The addition of the peptide to the culture medium of primary hippocampal neurons increased dendrite length, dendrite number and axon length. Furthermore, the addition of the peptide to primary microglial cultures shifted CGA-activated microglia towards anti-inflammatory and neuroprotective phenotypes. These findings of in vitro glia–neuron interactions led us to evaluate the effects of oral administration of the peptide on brain function and hair ageing in senescence-accelerated mice (SAMP8). In vivo analyses revealed that spatial learning ability and hair quality were improved in Naturido-treated mice compared with untreated mice, to the same level observed in the normal ageing control (SAMR1). These data suggest that Naturido may be a promising glia–neuron modulator for the treatment of not only senescence, but also Alzheimer’s disease and other neurodegenerative diseases.

Glia-Driven Neuroinflammation and Systemic Inflammation in Alzheimer's Disease

The neuroinflammatory hypothesis of Alzheimer's disease (AD) was proposed more than 30 years ago. The involvement of the two main types of glial cells microglia and astrocytes, in neuroinflammation, was suggested early on. In this review, we highlight that the exact contributions of reactive glia to AD pathogenesis remain difficult to define, likely due to the heterogeneity of glia populations and alterations in their activation states through the stages of AD progression. In the case of microglia, it is becoming apparent that both beneficially and adversely activated cell populations can be identified at various stages of AD, which could be selectively targeted to either limit their damaging actions or enhance beneficial functions. In the case of astrocytes, less information is available about potential subpopulations of reactive cells; it also remains elusive whether astrocytes contribute to the neuropathology of AD by mainly gaining neurotoxic functions or losing their ability to support neurons due to astrocyte damage. We identify L-type calcium channel blocker, nimodipine, as a candidate drug for AD, which potentially targets both astrocytes and microglia. It has already shown consistent beneficial effects in basic experimental and clinical studies. We also highlight the recent evidence linking peripheral inflammation and neuroinflammation. Several chronic systemic inflammatory diseases, such as obesity, type 2 diabetes mellitus, and periodontitis, can cause immune priming or adverse activation of glia, thus exacerbating neuroinflammation and increasing risk or facilitating the progression of AD. Therefore, reducing peripheral inflammation is a potentially effective strategy for lowering AD prevalence.

U.S. state correlations between oral health metrics and Alzheimer's disease mortality, prevalence and subjective cognitive decline prevalence

Given the association between periodontal disease (PdD) and Alzheimer’s disease (AD), we examined correlations between states’ age-adjusted AD mortality rates, AD prevalence, subjective cognitive decline (SCD) prevalence, and oral health data. Data sources include the Centers for Disease Control and Prevention, scientific literature, and oral health rankings formulated by WalletHub.com and Toothbrush.org. Pearson (r) or Spearman (r_s) correlation coefficients were generated and evaluated. AD mortality rates correlate with dental visits (r = − 0.50, p = 0.0003), partial (r = 0.39, p = 0.005) or total (r = 0.44, p = 0.001) edentulism, WalletHub.com (r_s = 0.30, p = 0.03) and Toothbrush.org (r_s = 0.35, p = 0.01) rankings. AD prevalence correlates with dental visits (r = − 0.30, p = 0.03), partial (r = 0.55, p = 0.00003) or total (r = 0.46, p = 0.0009) edentulism, prevalence of any (r = 0.38, p = 0.006) or severe-stage (r = 0.46, p = 0.0009) PdD, and WalletHub.com (r_s = 0.38, p = 0.006) rankings. SCD prevalence in adults aged ≥ 45 years correlates with dental visits (r = − 0.69, p < 0.00001), partial (r = 0.33, p = 0.02) or total (r = 0.37, p = 0.008) edentulism, prevalence of any (r = 0.53, p = 0.0001) or severe-stage (r = 0.57, p = 0.00002) PdD, WalletHub.com (r_s = 0.53, p = 0.00008) and Toothbrush.org (r_s = 0.60, p < 0.00001) rankings. State metrics indicative of compromised oral health correlate with AD mortality rates, AD prevalence and SCD prevalence.

Microglia Demonstrate Local Mixed Inflammation and a Defined Morphological Shift in an APP/PS1 Mouse Model

Background: Microglia are traditionally described as the immune cells of the brain and have an inflammatory role in Alzheimer’s disease (AD). Microglial morphological and phenotypic shifts in AD have not been fully characterized; however, microglia are often described as either pro- or anti-inflammatory. Objective: To determine microglial if microglial morphology and phenotype changes with disease status. Methods: This study observed morphology through Iba1 immunohistochemistry on tissue sections encompassing the primary motor cortex and somatosensory barrel fields. Immunohistochemistry for pro-inflammatory markers: CD14 and CD40; and anti-inflammatory markers: CD16 and TREM2, was performed at 3, 6, and 12 months of age which correlated with pre-plaque, onset, and significant plaque load in APP/PS1 brains (n = 6) and compared to age-matched littermate controls (n = 6). Results: Microglia demonstrated a defined morphological shift with time. Deramified morphologies increased in the APP/PS1, at both 6 months (p < 0.0001) and 12 months (p < 0.0001). At 12 months, there were significantly lower numbers of ramified microglia (p < 0.001). Results indicated that microglia have a heterogenic marker immunoreactivity as CD16, TREM2, and CD40 were associated with an activated morphology at the same time points. All inflammatory markers were significantly upregulated at 12 months in the APP/PS1 mice (TREM2 (F (2,30)  = 10.75, p = 0.0003), CD40 (F (2,30)  = 15.86, p < 0.0001), CD14 (F (2,30)  = 6.84, p = 0.0036), and CD16 (F (2,30)  = 3.026, p = 0.0635)). Conclusion: Our data indicate that pro- and anti-inflammatory factors of microglia occur in APP/PS1 mice.

Interaction between genetic factors, Porphyromonas gingivalis and microglia to promote Alzheimer's disease

In late-onset Alzheimer disease (AD) pathogenesis, genes, infections and immunity could be significant factors. We have reviewed if the keystone periodontal pathogen Porphyromonas gingivalis may affect genes and microglia (primary immune cells in the brain) to promote AD development. Genes for apolipoprotein, clusterin, CD33, triggering receptor expressed on myeloid cells-2 (TREM-2), tyrosine kinase binding protein (TYR-OBP), and complement receptors can affect microglia. Most of these genes can also be affected by P. gingivalis via its mastering of immune suppression. Besides, P. gingivalis can affect microglia directly in several ways. Taken together, genetic predisposition, P. gingivalis infection and microglia could promote neurodegeneration typical of that reported for AD.

Periodontal disease as a possible cause for Alzheimer's disease

Approximately 47 million people worldwide have been diagnosed with dementia, 60%-80% of whom have dementia of the Alzheimer's disease type. Unfortunately, there is no cure in sight. Defining modifiable risk factors for Alzheimer's disease may have a significant impact on its prevalence. An increasing body of evidence suggests that chronic inflammation and microbial dysbiosis are risk factors for Alzheimer's disease. Periodontal disease is a chronic inflammatory disease that develops in response to response to microbial dysbiosis. Many studies have shown an association between periodontal disease and Alzheimer's disease. The intent of this paper was to review the existing literature and determine, using the Bradford Hill criteria, whether periodontal disease is causally related to Alzheimer's disease.

Systemic Exposure to Lipopolysaccharide from Porphyromonas gingivalis Induces Bone Loss-Correlated Alzheimer's Disease-Like Pathologies in Middle-Aged Mice

BACKGROUND

Alzheimer's disease (AD) and bone loss are clinically exacerbated. However, the mechanism of exacerbation remains understood.

OBJECTIVE

We tested our hypothesis that periodontitis is involved in the exacerbation, contributing to AD pathologies.

METHODS

The bone, memory, and inflammation in bone and brain were examined in 12-month-old mice after systemic exposure to lipopolysaccharide from Porphyromonas gingivalis (P gLPS) for 3 consecutive weeks.

RESULTS

Compared with control mice, bone loss in tibia (26% decrease) and memory decline (47% decrease) were induced in mice with a positive correlation after exposure to P gLPS (r = 0.7378, p = 0.0011). The IL-6 and IL-17 expression in tibia was negatively correlated with the bone volume/total tissue volume (r = -0.6619, p = 0.0052; r = -0.7129, p = 0.0019), while that in the cortex was negatively correlated with the memory test latency (r = -0.7198, p = 0.0017; p = 0.0351, r = -0.5291). Furthermore, the IL-17 expression in microglia was positively correlated with Aβ42 accumulation in neurons (r = 0.8635, p < 0.0001). In cultured MG6 microglia, the P gLPS-increased IL-6 expression was inhibited by a PI3K-specific inhibitor (68% decrease), and that of IL-17 was inhibited by IL-6 antibody (41% decrease). In cultured N2a neurons, conditioned medium from P gLPS-stimulated microglia (MCM) but not P gLPS increased the productions of AβPP, CatB, and Aβ42, which were significantly inhibited by pre-treatment with IL-17 antibody (67%, 51%, and 41% decrease).

CONCLUSION

These findings demonstrated that chronic systemic exposure to P gLPS simultaneously induces inflammation-dependent bone loss and AD-like pathologies by elevating IL-6 and IL-17 from middle age, suggesting that periodontal bacteria induce exacerbation of bone loss and memory decline, resulting in AD progression.

Evaluation of the presence and distribution of leptomeningeal inflammation in SIDS/SUDI cases and comparison with a hospital-based cohort

INTRODUCTION

Prior research demonstrates that leptomeninges of infants and late-term fetuses derived from a non-traumatic, hospital-based cohort contain a surprisingly large number of inflammatory cells and stainable iron. These were present irrespective of the findings from the general autopsy, the neuropathologic examination, and the mode of delivery.

MATERIALS AND METHODS

We applied a similar methodology to a sudden infant death syndrome/sudden unexpected death in infancy (SIDS/SUDI) cohort. Forty-two SIDS/SUDI cases autopsied between 2006 and 2014 by the San Diego County Medical Examiner's Office were identified. An interpretable amount of leptomeninges from at least two areas of the brain (cerebral cortex, brain stem, cerebellum) were present in each case. Immunoperoxidase (IPOX) staining with CD45 and CD68 was performed and Perl's method was used to detect the presence of iron. The number of immunoreactive cells per IPOX stain within the leptomeninges in each slide was manually tabulated and the density subsequently quantified. The presence or absence of stainable iron was noted.

RESULTS

This cohort represented 22 males and 20 females ranging in age from 2 to 311 days, with relatively evenly divided modes of delivery. The examined brain sections included 32 of the cerebral cortex, 18 of the brain stem, and 36 of the cerebellum. The lengths of the examined leptomeninges ranged from 2 to 40 mm. The ranges of the number of cells per millimeter, and the standard deviations of the means were wide and varied. Overall, there was no significant difference in the number of CD45 or CD68 immunoreactive cells/millimeter between the three brain sites. Comparing this cohort to a subpopulation of hospitalized infants in our prior study, there were no significant differences between the density of inflammatory cells in the sections from the cerebral cortex and brain stem. There were differences in the CD68 densities, particularly in the cerebellar sections which may be attributable to methodological differences. Iron was identified in only a single section in this cohort but was present in most of the cases in the hospital-based cohort.

CONCLUSION

This study further elucidates the relevance of the presence of inflammatory cells and iron in the leptomeninges. Whether in a hospital-based or more forensically relevant population, the presence of inflammatory cells in the leptomeninges (even in great abundance) is common.

Brazilian Green Propolis Prevents Cognitive Decline into Mild Cognitive Impairment in Elderly People Living at High Altitude

BACKGROUND

Systemic inflammation is known as a risk factor of cognitive decline.

OBJECTIVE

To investigate the effects of propolis on cognitive decline and systemic inflammation in elderly people living at high altitude.

METHODS

Sixty participants (average 72.8 years) living at altitude (2,260 meters) were randomized to receive propolis (0.83 g, n = 30) or placebo (n = 30) for 24 months. Cognitive outcomes were assessed using MMSE and serum cytokine levels were measured for 24 months in a double-blind study.

RESULTS

MMSE scores were 26.17 at baseline and 23.87 at 24 months in placebo group. Compared to placebo group, improvements of MMSE scores were significant in propolis-treated subjects (p = 0.007) with a response emerging over time (time points×group interaction, p = 0.016). In addition, the serum IL-1β and IL-6 levels were significantly different across treatments (p < 0.0001) showing upward and downward trends in placebo- and propolis-treated subjects, respectively (p < 0.0001). Serum levels of TNF-α were not significantly different across treatment (p = 0.0528) but with a response emerging over time (time points×group interaction, p = 0.016). In contrast, serum levels of TGFβ1 were significantly different across treatments (p < 0.0001) showing downward and upward trends in placebo- and propolis-treated subjects, respectively. Serum levels of IL-10 were significant for the effect of groups (p = 0.0411). Furthermore, MMSE scores correlated with the decrease in IL-1β and the increase in TGFβ1 in serum.

CONCLUSION

Elderly people living at high altitude developed to MCI in 24 months with exacerbation of systemic inflammation. Ingestion of propolis (>12 months) protected against cognitive decline after systemic inflammation was reduced.

Resolution of inflammation-induced depression requires T lymphocytes and endogenous brain interleukin-10 signaling

In humans, depression is often associated with low-grade inflammation, activation of the tryptophan/kynurenine pathway, and mild lymphopenia. Preclinical research confirms that inflammation induces depression-like behavior through activation of the tryptophan/kynurenine pathway. However, the mechanisms governing recovery from depression are unknown. Understanding the pathways leading to resolution of depression will likely lead to identification of novel targets for treatment. We investigated the contribution of T lymphocytes to the resolution of lipopolysaccharide-induced depression-like behavior. Duration of depression-like behavior was markedly prolonged in mice without mature T or B lymphocytes (Rag1^-/- mice). This prolonged depression-like behavior was associated with persistent upregulation of the tryptophan-metabolizing enzyme indoleamine-2,3-dioxygenase (Ido)1 in the prefrontal cortex (PFC). Reconstitution of Rag1^-/- mice with T lymphocytes normalized resolution of depression-like behavior and expression of Ido1 in the PFC. During resolution of inflammation-induced depression-like behavior, T lymphocytes accumulated in the meninges and were required for induction of interleukin (IL)-10 in the meninges and the PFC. Inhibition of IL-10 signaling by nasal administration of neutralizing anti-IL-10 antibody to WT mice led to persistent upregulation of Ido1 in the PFC and prolonged depression-like behavior. Conversely, nasal administration of recombinant IL-10 in Rag1^-/- mice normalized Ido1 expression and resolution of depression-like behavior. In conclusion, the present data show for the first time that resolution of inflammation-induced depression is an active process requiring T lymphocytes acting via an IL-10-dependent pathway to decrease Ido1 expression in the brain. We propose that targeting the T lymphocyte/IL-10 resolution pathway could represent a novel approach to promote recovery from major depressive disorder.

Porphyromonas gingivalis, a periodontitis causing bacterium, induces memory impairment and age-dependent neuroinflammation in mice

Background

A possible relationship between periodontitis and Alzheimer's disease (AD) has been reported. However, there is limited information on the association between the Porphyromonas gingivalis (P. gingivalis) periodontal infection and the pathological features of AD. The hypothesis that P. gingivalis periodontal infection may cause cognitive impairment via age-dependent neuroinflammation was tested.

Results

Thirty 4-week-old (young) female C57BL/6 J mice were randomly divided into two groups, the control group and the experimental group. Thirty 12-month-old (middle-aged) were grouped as above. The mouth of the mice in the experimental group was infected with P. gingivalis. Morris water maze(MWM) was performed to assess the learning and memory ability of mice after 6 weeks. Moreover, the expression levels of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the mice brain tissues were determined by Quantitative real-time polymerase chain reaction (qRT-PCR), Enzyme Linked Immunosorbent Assay(ELISA) and immunohistochemistry. Our results showed that the learning and memory abilities of the middle-aged P. gingivalis infected mice were impaired. Moreover, the expression levels of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the brain tissues of the middle-aged P. gingivalis infected mice were increased.

Conclusions

These results suggest that P. gingivalis periodontal infection may cause cognitive impairment via the release of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the brain tissues of middle-aged mice.

Cathepsin B plays a critical role in inducing Alzheimer's disease-like phenotypes following chronic systemic exposure to lipopolysaccharide from Porphyromonas gingivalis in mice

A number of clinical and experimental studies have revealed a strong association between periodontitis and accelerated cognitive decline in Alzheimer's disease (AD); however, the mechanism of the association is unknown. In the present study, we tested the hypothesis that cathepsin (Cat) B plays a critical role in the initiation of neuroinflammation and neural dysfunction following chronic systemic exposure to lipopolysaccharide from Porphyromonas gingivalis (PgLPS) in mice (1mg/kg, daily, intraperitoneally). Young (2months old) and middle-aged (12months old) wild-type (WT; C57BL/6N) or CatB-deficient (CatB^-/-) mice were exposed to PgLPS daily for 5 consecutive weeks. The learning and memory function were assessed using the passive avoidance test, and the expression of amyloid precursor protein (APP), CatB, TLR2 and IL-1β was analyzed in brain tissues by immunohistochemistry and Western blotting. We found that chronic systemic exposure to PgLPS for five consecutive weeks induced learning and memory deficits with the intracellular accumulation of Aβ in neurons in the middle-aged WT mice, but not in young WT or middle-aged CatB^-/- mice. PgLPS significantly increased the expression of CatB in both microglia and neurons in middle-aged WT mice, while increased expression of mature IL-1β and TLR2 was restricted to microglia in the hippocampus of middle-aged WT mice, but not in that of the middle-aged CatB^-/- ones. In in vitro studies, PgLPS (1µg/ml) stimulation upregulated the mean mRNA expression of IL-1β, TLR2 and downregulated the protein levels of IκBα in the cultured MG6 microglia as well as in the primary microglia from WT mice, which were significantly inhibited by the CatB-specific inhibitor CA-074Me as well as by the primary microglia from CatB^-/- mice. Furthermore, the mean mRNA expression of APP and CatB were significantly increased in the primary cultured hippocampal neurons after treatment with conditioned medium from PgLPS-treated WT primary microglia, but not after treatment with conditioned medium neutralized with anti-IL-1beta, and not after treatment with conditioned medium from PgLPS-treated CatB^-/- primary microglia or with PgLPS directly. Taken together, these findings indicate that chronic systemic exposure to PgLPS induces AD-like phenotypes, including microglia-mediated neuroinflammation, intracellular Aβ accumulation in neurons and impairment of the learning and memory functions in the middle-aged mice in a CatB-dependent manner. We propose that CatB may be a therapeutic target for preventing periodontitis-associated cognitive decline in AD.

Cathepsin S Is Involved in Th17 Differentiation Through the Upregulation of IL-6 by Activating PAR-2 after Systemic Exposure to Lipopolysaccharide from Porphyromonas gingivalis

Positive links have been found between periodontitis and numerous diseases in humans via persistent inflammation throughout the body. However, the main factors responsible for maintaining this pro-inflammatory condition are poorly understood. The spleen, the largest secondary immune organ, is a central hub regulating the immune response/inflammation due to the dendritic cell (DC) response to CD4⁺ T cell subtype differentiation, and lysosomal proteinase cathepsin S (CatS) is known to be involved in DC functions. In the present study, we found that CatS-induced IL-6 production by splenic DCs subsequently promotes Th17 differentiation, in response to systemic exposure to lipopolysaccharide derived from Porphyromonas gingivalis (PgLPS). The population of CD11c⁺ DCs was significantly increased in the splenic marginal zone (MZ) locally of wild-type (DBA/2) mice with splenomegaly but not in that of CatS deficient (CatS^-/-) mice after systemic exposure to PgLPS for 7 consecutive days (5 mg/kg/day, intraperitoneal). Similarly, the population of Th17⁺CD4⁺ T cells was also significantly increased in the splenic MZ of wild-type mice but not in that of CatS^-/- mice after PgLPS exposure. Furthermore, the increase in the Th17⁺ CD4⁺ T cell population paralleled increases in the levels of CatS and IL-6 in CD11c⁺ cells in the splenic MZ. In isolated primary splenic CD11c⁺ cells, the mRNA expression and the production of IL-6 was dramatically increased in wild-type mice but not in CatS^-/- mice after direct stimulation with PgLPS (1 μg/ml), and this PgLPS-induced increase in the IL-6 expression was completely abolished by pre-treatment with Z-Phe-Leu-COCHO (Z-FL), the specific inhibitor of CatS. The PgLPS activated protease-activated receptor (PAR) 2 in the isolated splenic CD11c⁺ cells was also significantly inhibited by CatS deficiently. In addition, the PgLPS-induced increase in the IL-6 production by splenic CD11c⁺ cells was completely abolished by pre-treatment with FSLLRY-NH₂, a PAR2 antagonist, as well as Akti, a specific inhibitor of Akt. These findings indicate that CatS plays a critical role in driving splenic DC-dependent Th17 differentiation through the upregulation of IL-6 by activating PAR2 after exposure to components of periodontal bacteria. Therefore, CatS-specific inhibitors may be effective in alleviating periodontitis-related immune/inflammation.

Diurnal dynamic behavior of microglia in response to infected bacteria through the UDP-P2Y6 receptor system

It has long been believed that microglia morphologically transform into the activated state by retracting their long processes and consuming pathogens when bacteria infect into the brain parenchyma. In the present study, however, we showed for the first time that murine cortical microglia extend their processes towards focally injected Porphyromonas gingivalis. This P. gingivalis-induced microglial process extension was significantly increased during the light (sleeping) phase than the dark (waking) phase. In contrast, focally injected ATP-induced microglial process extension was significantly increased during the dark phase than the light phase. Furthermore, in contrast to the P2Y₁₂ receptor-mediated mechanism of ATP-induced microglial process extension, the P. gingivalis-mediated microglial process extension was mediated by P2Y₆ receptors. The infection of bacteria such as P. gingivalis to the brain parenchyma may induce the secretion of UDP from microglia at the site of infection, which in turn induces the process extension of the neighboring microglia.

Nutrients, Microglia Aging, and Brain Aging

As the life expectancy continues to increase, the cognitive decline associated with Alzheimer's disease (AD) becomes a big major issue in the world. After cellular activation upon systemic inflammation, microglia, the resident immune cells in the brain, start to release proinflammatory mediators to trigger neuroinflammation. We have found that chronic systemic inflammatory challenges induce differential age-dependent microglial responses, which are in line with the impairment of learning and memory, even in middle-aged animals. We thus raise the concept of “microglia aging.” This concept is based on the fact that microglia are the key contributor to the acceleration of cognitive decline, which is the major sign of brain aging. On the other hand, inflammation induces oxidative stress and DNA damage, which leads to the overproduction of reactive oxygen species by the numerous types of cells, including macrophages and microglia. Oxidative stress-damaged cells successively produce larger amounts of inflammatory mediators to promote microglia aging. Nutrients are necessary for maintaining general health, including the health of brain. The intake of antioxidant nutrients reduces both systemic inflammation and neuroinflammation and thus reduces cognitive decline during aging. We herein review our microglia aging concept and discuss systemic inflammation and microglia aging. We propose that a nutritional approach to controlling microglia aging will open a new window for healthy brain aging.

Role of TGFβ signaling in the pathogenesis of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); being associated with conspicuous changes on microglia activation. Aged microglia exhibit an increased expression of cytokines, exacerbated reactivity to various stimuli, oxidative stress, and reduced phagocytosis of β-amyloid (Aβ). Whereas normal inflammation is protective, it becomes dysregulated in the presence of a persistent stimulus, or in the context of an inflammatory environment, as observed in aging. Thus, neuroinflammation can be a self-perpetuating deleterious response, becoming a source of additional injury to host cells in neurodegenerative diseases. In aged individuals, although transforming growth factor β (TGFβ) is upregulated, its canonical Smad3 signaling is greatly reduced and neuroinflammation persists. This age-related Smad3 impairment reduces protective activation while facilitating cytotoxic activation of microglia through several cellular mechanisms, potentiating microglia-mediated neurodegeneration. Here, we critically discuss the role of TGFβ-Smad signaling on the cytotoxic activation of microglia and its relevance in the pathogenesis of AD. Other protective functions, such as phagocytosis, although observed in aged animals, are not further induced by inflammatory stimuli and TGFβ1. Analysis in silico revealed that increased expression of receptor scavenger receptor (SR)-A, involved in Aβ uptake and cell activation, by microglia exposed to TGFβ, through a Smad3-dependent mechanism could be mediated by transcriptional co-factors Smad2/3 over the MSR1 gene. We discuss that changes of TGFβ-mediated regulation could at least partially mediate age-associated microglia changes, and, together with other changes on inflammatory response, could result in the reduction of protective activation and the potentiation of cytotoxicity of microglia, resulting in the promotion of neurodegenerative diseases.

Lessons from Microglia Aging for the Link between Inflammatory Bone Disorders and Alzheimer's Disease

Bone is sensitive to overactive immune responses, which initiate the onset of inflammatory bone disorders, such as rheumatoid arthritis and periodontitis, resulting in a significant systemic inflammatory response. On the other hand, neuroinflammation is strongly implicated in Alzheimer's disease (AD), which can be enhanced by systemic inflammation, such as that due to cardiovascular disease and diabetes. There is growing clinical evidence supporting the concept that rheumatoid arthritis and periodontitis are positively linked to AD, suggesting that inflammatory bone disorders are risk factors for this condition. Recent studies have suggested that leptomeningeal cells play an important role in transducing systemic inflammatory signals to brain-resident microglia. More importantly, senescent-type, but not juvenile-type, microglia provoke neuroinflammation in response to systemic inflammation. Because the prevalence of rheumatoid arthritis and periodontitis increases with age, inflammatory bone disorders may be significant sources of covert systemic inflammation among elderly people. The present review article highlights our current understanding of the link between inflammatory bone disorders and AD with a special focus on microglia aging.

Interleukin-19 acts as a negative autocrine regulator of activated microglia

Activated microglia can exert either neurotoxic or neuroprotective effects, and they play pivotal roles in the pathogenesis and progression of various neurological diseases. In this study, we used cDNA microarrays to show that interleukin-19 (IL-19), an IL-10 family cytokine, is markedly upregulated in activated microglia. Furthermore, we found that microglia are the only cells in the nervous system that express the IL-19 receptor, a heterodimer of the IL-20Rα and IL-20Rβ subunits. IL-19 deficiency increased the production of such pro-inflammatory cytokines as IL-6 and tumor necrosis factor-α in activated microglia, and IL-19 treatment suppressed this effect. Moreover, in a mouse model of Alzheimer's disease, we observed upregulation of IL-19 in affected areas in association with disease progression. Our findings demonstrate that IL-19 is an anti-inflammatory cytokine, produced by activated microglia, that acts negatively on microglia in an autocrine manner. Thus, microglia may self-limit their inflammatory response by producing the negative regulator IL-19.

The activation of NG2 expressing cells is downstream to microglial reaction and mediated by the transforming growth factor beta 1

In the present study, we investigated the mechanism of activation of NG2 expressing cells. Application of microglial inhibitors not only attenuated morphological changes but also significantly retarded increase in the number of NG2 expressing cells. Intracerebral injection of TGF-β1 led to a profound activation of NG2 glia as well as an earlier accumulation of NG2(+)-microglia, whilst inhibition of TGF-β1 Smad2/3 signalling pathway eventually attenuated their active responses. We conclude that the activation of NG2 expressing cells is an event downstream to microglial reaction and TGF-β1 secreted from microglia might play an important role in modulation of the function of NG2 expressing cells.

Cathepsin D deficiency induces oxidative damage in brain pericytes and impairs the blood-brain barrier

Recent evidence suggests that peripheral blood mononuclear cells (PBMCs) contribute to the pathogenesis of neuropathological changes in patients with neuronal ceroid lipofuscinosis (NCL) and lysosomal storage diseases. In order to examine the possible increase in the permeability of the blood-brain-barrier (BBB) and resultant infiltration of PBMCs due to cathepsin D (CatD) deficiency, a process underlying the onset of congenital NCL, we examined structural changes in brain vessels in CatD-/- mice. Consequently, the mean diameter of the brain vessels in the cerebral cortex on postnatal day 24 (P24) was significantly larger in CatD-/- mice than in wild-type mice. Furthermore, the mean number of brain pericytes in CatD-/- mice began to decline significantly on P16 and almost disappeared on P24, and oxidative DNA damage was first detected in brain pericytes on P12. Examinations with electron microscopy revealed that brain pericytes were laden with dense granular bodies, cytoplasmic vacuoles and lipid droplets. The infiltration of PBMCs characterized by segmented nucleus laden with dense granular bodies was also noted in the cerebral cortex of CatD-/- mice. When primary cultured microglia prepared from enhanced green fluorescent protein (GFP)-expressing transgenic rats were injected into the common carotid artery, GFP-positive microglia were detected in the brain parenchyma of CatD-/-, but not wild-type, mice. Moreover, pepstatin A, a specific aspartic protease inhibitor, induced mitochondria-derived reactive oxygen species (ROS) production in the isolated brain pericytes, which decreased the cell viability. These observations suggest that increased lysosomal storage due to CatD deficiency causes oxidative damage in brain pericytes, subsequently resulting in an increased vessel diameter, enhanced permeability of the BBB and the infiltration of PBMCs. Therefore, protecting brain pericytes against lysosomal storage-induced oxidative stress may represent an alternative treatment strategy for congenital NCL.

Gap junctions and hemichannels composed of connexins: potential therapeutic targets for neurodegenerative diseases

Microglia are macrophage-like resident immune cells that contribute to the maintenance of homeostasis in the central nervous system (CNS). Abnormal activation of microglia can cause damage in the CNS, and accumulation of activated microglia is a characteristic pathological observation in neurologic conditions such as trauma, stroke, inflammation, epilepsy, and neurodegenerative diseases. Activated microglia secrete high levels of glutamate, which damages CNS cells and has been implicated as a major cause of neurodegeneration in these conditions. Glutamate-receptor blockers and microglia inhibitors (e.g., minocycline) have been examined as therapeutic candidates for several neurodegenerative diseases; however, these compounds exerted little therapeutic benefit because they either perturbed physiological glutamate signals or suppressed the actions of protective microglia. The ideal therapeutic approach would hamper the deleterious roles of activated microglia without diminishing their protective effects. We recently found that abnormally activated microglia secrete glutamate via gap-junction hemichannels on the cell surface. Moreover, administration of gap-junction inhibitors significantly suppressed excessive microglial glutamate release and improved disease symptoms in animal models of neurologic conditions such as stroke, multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease. Recent evidence also suggests that neuronal and glial communication via gap junctions amplifies neuroinflammation and neurodegeneration. Elucidation of the precise pathologic roles of gap junctions and hemichannels may lead to a novel therapeutic strategies that can slow and halt the progression of neurodegenerative diseases.

Connection between periodontitis and Alzheimer's disease: possible roles of microglia and leptomeningeal cells

Neuroinflammation, inflammation of the brain, is strongly implicated in Alzheimer's disease (AD), which can be enhanced by systemic inflammation. Therefore, the initiation and progression of AD are affected by systemic diseases such as cardiovascular disease and diabetes. This concept suggests a possible link between periodontitis and AD because periodontitis is a peripheral, chronic infection that elicits a significant systemic inflammatory response. There is now growing clinical evidence that chronic periodontitis is closely linked to the initiation and progression of AD. Recent studies have suggested that leptomeningeal cells play an important role in transducing systemic inflammatory signals to the brain-resident microglia, which in turn initiate neuroinflammation. Furthermore, it is apparent that senescent-type microglia respond in an exaggerated manner to systemic inflammation. It is estimated that a high percentage of adults are suffering from periodontitis, and the prevalence of periodontitis increases with age. Therefore, chronic periodontitis can be a significant source of covert systemic inflammation within the general population. The present review article highlights our current understanding of the link between periodontitis and AD.

Leptomeningeal cells transduce peripheral macrophages inflammatory signal to microglia in reponse to Porphyromonas gingivalis LPS

We report here that the leptomeningeal cells transduce inflammatory signals from peripheral macrophages to brain-resident microglia in response to Porphyromonas gingivalis (P.g.) LPS. The expression of Toll-like receptor 2 (TLR2), TLR4, TNF-α, and inducible NO synthase was mainly detected in the gingival macrophages of chronic periodontitis patients. In in vitro studies, P.g. LPS induced the secretion of TNF-α and IL-1β from THP-1 human monocyte-like cell line and RAW264.7 mouse macrophages. Surprisingly, the mean mRNA levels of TNF-α and IL-1β in leptomeningeal cells after treatment with the conditioned medium from P.g. LPS-stimulated RAW264.7 macrophages were significantly higher than those after treatment with P.g. LPS alone. Furthermore, the mean mRNA levels of TNF-α and IL-1β in microglia after treatment with the conditioned medium from P.g. LPS-stimulated leptomeningeal cells were significantly higher than those after P.g. LPS alone. These observations suggest that leptomeninges serve as an important route for transducing inflammatory signals from macrophages to microglia by secretion of proinflammatory mediators during chronic periodontitis. Moreover, propolis significantly reduced the P.g. LPS-induced TNF-α and IL-1 β production by leptomeningeal cells through inhibiting the nuclear factor-κB signaling pathway. Together with the inhibitory effect on microglial activation, propolis may be beneficial in preventing neuroinflammation during chronic periodontitis.

Distinct activation profiles in microglia of different ages: a systematic study in isolated embryonic to aged microglial cultures

Microglia have been implicated in disease progression for several age-related brain disorders. However, while microglia's contribution to the progression of these disorders is accepted, the effect of aging on their endogenous cellular characteristics has received limited attention. In fact, a comprehensive study of how the structure and function of microglia changes as a function of developmental age has yet to be performed. Here, we describe the functional response characteristics of primary microglial cultures prepared from embryonic, neonatal (Neo), 2-3month-old, 6-8month-old, 9-11month-old, and 13-15month-old rats. Microglial morphology, glutamate (GLU) uptake, and release of trophic and inflammatory factors were assessed under basal conditions and in microglia activated with adenosine 5'-triphosphate (ATP) or lipopolysaccharide. We found that microglia from different age groups were both morphologically and functionally distinct. Upon activation by ATP, Neo microglia were the most reactive, upregulating nitric oxide, tumor necrosis factor-α, and brain-derived neurotrophic factor release as well as GLU uptake. This upregulation translated into neurotoxicity in microglia-neuron co-cultures that were not observed with microglia of different developmental ages. Interestingly, 13-15month-old microglia exhibited similar activation profiles to Neo microglia, whereas microglia from younger adults and embryos were activated less by ATP. Our data also identify age-dependent differences in purinergic receptor subtype expression that contribute to the regulation of neuronal survival. Combined, our data demonstrate that microglial activation and purinergic receptor profiles vary non-linearly with developmental age, a potentially important finding for studies examining the role of microglia in neurodegenerative disorders.

Brazilian green propolis suppresses the hypoxia-induced neuroinflammatory responses by inhibiting NF-κB activation in microglia

Hypoxia has been recently proposed as a neuroinflammatogen, which drives microglia to produce proinflammatory cytokines, including interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and IL-6. Considering the fact that propolis has hepatoprotective, antitumor, antioxidative, and anti-inflammatory effects, propolis may have protective effects against the hypoxia-induced neuroinflammatory responses. In this study, propolis (50 μg/mL) was found to significantly inhibit the hypoxia-induced cytotoxicity and the release of proinflammatory cytokines, including IL-1β, TNF-α, and IL-6, by MG6 microglia following hypoxic exposure (1% O2, 24 h). Furthermore, propolis significantly inhibited the hypoxia-induced generation of reactive oxygen species (ROS) from mitochondria and the activation of nuclear factor-κB (NF-κB) in microglia. Moreover, systemic treatment with propolis (8.33 mg/kg, 2 times/day, i.p.) for 7 days significantly suppressed the microglial expression of IL-1β, TNF-α, IL-6, and 8-oxo-deoxyguanosine, a biomarker for oxidative damaged DNA, in the somatosensory cortex of mice subjected to hypoxia exposure (10% O2, 4 h). These observations indicate that propolis suppresses the hypoxia-induced neuroinflammatory responses through inhibition of the NF-κB activation in microglia. Furthermore, increased generation of ROS from the mitochondria is responsible for the NF-κB activation. Therefore, propolis may be beneficial in preventing hypoxia-induced neuroinflammation.

Differential pathways for interleukin-1β production activated by chromogranin A and amyloid β in microglia

Although chromogranin A (CGA) is frequently present in Alzheimer's disease (AD), senile plaques associated with microglial activation, little is known about basic difference between CGA and fibrillar amyloid-β (fAβ) as neuroinflammatory factors. Here we have compared the interleukin-1β (IL-1β) production pathways by CGA and fAβ in microglia. In cultured microglia, production of IL-1β was induced by CGA, but not by fAβ. CGA activated both nuclear factor-κB (NF-κB) and pro-caspase-1, whereas fAβ activated pro-caspase-1 only. For the activation of pro-caspase-1, both CGA and fAβ needed the enzymatic activity of cathepsin B (CatB), but only fAβ required cytosolic leakage of CatB and the NLRP3 inflammasome activation. In contrast, fAβ induced the IL-1β secretion from microglia isolated from the aged mouse brain. In AD brain, highly activated microglia, which showed intense immunoreactivity for CatB and IL-1β, surrounded CGA-positive plaques more frequently than Aβ-positive plaques. These observations indicate differential pathways for the microglial IL-1β production by CGA and fAβ, which may aid in better understanding of the pathological significance of neuroinflammation in AD.

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

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

Fingolimod phosphate promotes the neuroprotective effects of microglia

Fingolimod phosphate (FTY720) is a sphingosine 1-phosphate (S1P) receptor agonist that is being used as a new oral drug for multiple sclerosis. FTY720 prevents lymphocytes from moving out of the lymphoid organs and inhibits autoreactive lymphocytes from infiltrating the central nervous system. Whether FTY720 directly affects microglia-the innate immune cells of the central nervous system-is unclear. Here we show that FTY720 binds S1P1 receptors to downregulate activated microglial production of such pro-inflammatory cytokines as tumor necrosis factor-α, interleukin-1β, and interleukin-6. FTY720 also upregulates microglial production of brain-derived neurotrophic factor and glial cell-derived neurotrophic factor. These results suggested that FTY720 directly promotes the neuroprotective effects of microglia. Therefore, FTY720 may be a potent therapeutic agent for not only multiple sclerosis but also other neurologic diseases associated with microglial activation.

Innate inflammation in Parkinson's disease

The resident macrophages of the brain--the microglia--are morphologically activated during the progression of Parkinson's disease. Observational studies in human postmortem material and studies in animal models seek to define the contribution that this innate immune response might make to the pathogenesis and rate of progression of Parkinson's disease. We review here some of the key conceptual issues that need to be considered when performing these studies. We highlight the fact that most postmortem studies have not given due consideration to common comorbidities present in patients with Parkinson's disease and also the limitations of attempting to extrapolate from animal models to a chronic progressive neurodegenerative disease in humans that lasts for many years.

The neuroprotective functions of transforming growth factor beta proteins

Transforming growth factor beta (TGF-β) proteins are multifunctional cytokines whose neural functions are increasingly recognized. The machinery of TGF-β signaling, including the serine kinase type transmembrane receptors, is present in the central nervous system. However, the 3 mammalian TGF-β subtypes have distinct distributions in the brain suggesting different neural functions. Evidence of their involvement in the development and plasticity of the nervous system as well as their functions in peripheral organs suggested that they also exhibit neuroprotective functions. Indeed, TGF-β expression is induced following a variety of types of brain tissue injury. The neuroprotective function of TGF-βs is most established following brain ischemia. Damage in experimental animal models of global and focal ischemia was shown to be attenuated by TGF-βs. In addition, support for their neuroprotective actions following trauma, sclerosis multiplex, neurodegenerative diseases, infections, and brain tumors is also accumulating. The review will also describe the potential mechanisms of neuroprotection exerted by TGF-βs including anti-inflammatory, -apoptotic, -excitotoxic actions as well as the promotion of scar formation, angiogenesis, and neuroregeneration. The participation of these mechanisms in the neuroprotective effects of TGF-βs during different brain lesions will also be discussed.

Age-dependent neuroinflammatory responses and deficits in long-term potentiation in the hippocampus during systemic inflammation

Chronic systemic inflammation induces age-dependent differential phenotypic changes in microglia and astrocytes, yielding an anti-inflammatory cell phenotype in young rats and a proinflammatory cell phenotype in middle-aged rats. These observations prompted further investigation of the functional outcomes of the resultant differential microglial phenotypic changes. The present study examined the effects of age-dependent differential microglial phenotypic changes following chronic systemic inflammation on the formation of the post-tetanic potentiation (PTP) and long-term potentiation (LTP) in the hippocampus. Microglia formed a proinflammatory cell phenotype to express ED1 and interleukin-1β (IL-1β) in the hippocampal CA1 region of middle-aged rats, but not in young rats following the establishment of adjuvant arthritis (AA). Furthermore, AA induced deficits in the formation of LTP in the Schaffer collateral-CA1 synapses of middle-aged rats, but not in young rats. On the other hand, the formation of PTP was impaired in both young and middle-aged AA rats. Minocycline, a known inhibitor of microglial activation, was systemically administered to middle-aged AA rats significantly restoring the mean magnitudes of both PTP and LTP. The mean expression levels of ED1 and IL-1β were significantly suppressed. These observations strongly suggest that chronic systemic inflammation induces deficits in the hippocampal LTP in middle-aged rats through neuroinflammation mainly induced by microglia.

Microglia in the normally aged hippocampus

The hippocampus plays important roles in the regulation and combination of short and long term memory and spatial navigation with other brain centers. Aging is accompanied by a functional decline of the hippocampus and degenerative disease. Microglia are major immune cells in the central nervous system and response to degenerative changes in the aged brain. In this respect, functional and morphological changes of the hippocampus have been closely related to microglial changes during normal aging with or without disease. Therefore, in this review, we discuss morphological and functional changes of the hippocampus and microglia in the aging brain.

Distribution of mRNAs encoding transforming growth factors-beta1, -2, and -3 in the intact rat brain and after experimentally induced focal ischemia

Transforming growth factors-beta1 (TGF-beta1), -2, and -3 form a small group of related proteins involved in the regulation of proliferation, differentiation, and survival of various cell types. Recently, TGF-betas were also demonstrated to be neuroprotective. In the present study, we investigated their distribution in the rat brain as well as their expression following middle cerebral artery occlusion. Probes were produced for all types of TGF-betas, and in situ hybridization was performed. We demonstrated high TGF-beta1 expression in cerebral cortex, hippocampus, central amygdaloid nucleus, medial preoptic area, hypothalamic paraventricular nucleus, substantia nigra, brainstem reticular formation and motoneurons, and area postrema. In contrast, TGF-beta2 was abundantly expressed in deep cortical layers, dentate gyrus, midline thalamic nuclei, posterior hypothalamic area and mamillary body, superior olive, areas of monoaminergic neurons, spinal trigeminal nucleus, dorsal vagal complex, cerebellum, and choroid plexus, and a high level of TGF-beta3 mRNA was found in cerebral cortex, hippocampus, basal amygdaloid nuclei, lateral septal nucleus, several thalamic nuclei, arcuate and supramamillary nuclei, superior colliculus, superior olive, brainstem reticular formation and motoneurons, area postrema, and inferior olive. Focal brain ischemia induced TGF-betas with markedly different expression patterns. TGF-beta1 was induced in the penumbral region of cortex and striatum, whereas TGF-beta2 and -beta3 were induced in different layers of the ipsilateral cortex. The expression of the subtypes of TGF-betas in different brain regions suggests that they are involved in the regulation of different neurons and bind to different latent TGF-beta binding proteins. Furthermore, they might have subtype-specific functions following ischemic attack.

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.

Traumatic brain injury and intestinal dysfunction: uncovering the neuro-enteric axis

Traumatic brain injury (TBI) can lead to several physiologic complications including gastrointestinal dysfunction. Specifically, TBI can induce an increase in intestinal permeability, which may lead to bacterial translocation, sepsis, and eventually multi-system organ failure. However, the exact mechanism of increased intestinal permeability following TBI is unknown. We hypothesized that expression of tight junction protein ZO-1 and occludin, responsible for intestinal architectural and functional integrity, will decrease following TBI and increase intestinal permeability. BALB/c mice underwent a weight drop TBI model following anesthesia. Brain injury was confirmed by a neurologic assessment and gross brain pathology. Six hours following injury, FITC-dextran (25 mg 4.4 kDa FITC-dextran) was injected into the intact lumen of the isolated ileum. Intestinal permeability was measured in plasma 30 min following injection, by using spectrophotometry to determine plasma FITC-dextran concentrations. Whole ileum extracts were used to measure expression of tight junction proteins ZO-1 and occludin by Western blot. TBI caused a significant increase in intestinal permeability (110.0 microg/mL +/-22.2) compared to sham animals (29.4 microg/mL +/- 9.7) 6 h after injury (p = 0.016). Expression of ZO-1 was decreased by 49% relative to sham animals (p < 0.02), whereas expression of occludin was decreased by 73% relative to sham animals (p < 0.001). An increase in intestinal permeability corresponds with decreased expression of tight junction proteins ZO-1 and occludin following TBI. Expression of intestinal tight junction proteins may be an important factor in gastrointestinal dysfunction following brain injury.

Traumatic brain injury and intestinal dysfunction: uncovering the neuro-enteric axis

Traumatic brain injury (TBI) can lead to several physiologic complications including gastrointestinal dysfunction. Specifically, TBI can induce an increase in intestinal permeability, which may lead to bacterial translocation, sepsis, and eventually multi-system organ failure. However, the exact mechanism of increased intestinal permeability following TBI is unknown. We hypothesized that expression of tight junction protein ZO-1 and occludin, responsible for intestinal architectural and functional integrity, will decrease following TBI and increase intestinal permeability. BALB/c mice underwent a weight drop TBI model following anesthesia. Brain injury was confirmed by a neurologic assessment and gross brain pathology. Six hours following injury, FITC-dextran (25 mg 4.4 kDa FITC-dextran) was injected into the intact lumen of the isolated ileum. Intestinal permeability was measured in plasma 30 min following injection, by using spectrophotometry to determine plasma FITC-dextran concentrations. Whole ileum extracts were used to measure expression of tight junction proteins ZO-1 and occludin by Western blot. TBI caused a significant increase in intestinal permeability (110.0 microg/mL +/-22.2) compared to sham animals (29.4 microg/mL +/- 9.7) 6 h after injury (p = 0.016). Expression of ZO-1 was decreased by 49% relative to sham animals (p < 0.02), whereas expression of occludin was decreased by 73% relative to sham animals (p < 0.001). An increase in intestinal permeability corresponds with decreased expression of tight junction proteins ZO-1 and occludin following TBI. Expression of intestinal tight junction proteins may be an important factor in gastrointestinal dysfunction following brain injury.

Heterogeneity of microglial activation in the innate immune response in the brain

The immune response in the brain has been widely investigated and while many studies have focused on the proinflammatory cytotoxic response, the brain's innate immune system demonstrates significant heterogeneity. Microglia, like other tissue macrophages, participate in repair and resolution processes after infection or injury to restore normal tissue homeostasis. This review examines the mechanisms that lead to reduction of self-toxicity and to repair and restructuring of the damaged extracellular matrix in the brain. Part of the resolution process involves switching macrophage functional activation to include reduction of proinflammatory mediators, increased production and release of anti-inflammatory cytokines, and production of cytoactive factors involved in repair and reconstruction of the damaged brain. Two partially overlapping and complimentary functional macrophage states have been identified and are called alternative activation and acquired deactivation. The immunosuppressive and repair processes of each of these states and how alternative activation and acquired deactivation participate in chronic neuroinflammation in the brain are discussed.