Brain activation of monocyte-lineage cells: involvement of interleukin-6

Microglia priming by interleukin-6 signaling is enhanced in aged mice

During peripheral infection, excessive production of pro-inflammatory cytokines in the aged brain from primed microglia induces exaggerated behavioral pathologies. While the pro-inflammatory cytokine IL-6 increases in the brain with age, its role in microglia priming is not known. This study examined the functional role of IL-6 signaling on microglia priming. Our hypothesis is that IL-6 signaling mediates primed states of microglia in the aged. An initial study assessed age-related alteration in IL-6 signaling molecules; sIL-6R and sgp130 were measured in cerebrospinal fluid of young and aged wild-type animals. Subsequent studies of isolated microglia from C57BL6/J (IL-6+/+) and IL-6 knock-out (IL-6-/-) mice showed significantly less MHC-II expression in aged IL-6-/- compared to IL-6+/+ counterparts. Additionally, adult and aged IL-6+/+ and IL-6-/- animals were administered lipopolysaccharide (LPS) to simulate a peripheral infection; sickness behaviors and hippocampal cytokine gene expression were measured over a 24 h period. Aged IL-6-/- animals were resilient to LPS-induced sickness behaviors and recovered more quickly than IL-6+/+ animals. The age-associated baseline increase of IL-1β gene expression was ablated in aged IL-6-/- mice, suggesting IL-6 is a key driver of cytokine activity from primed microglia in the aged brain. We employed in vitro studies to understand molecular mechanisms in priming factors. MHC-II and pro-inflammatory gene expression (IL-1β, IL-10, IL-6) were measured after treating BV.2 microglia with sIL-6R and IL-6 or IL-6 alone. sIL-6R enhanced expression of both pro-inflammatory genes and MHC-II. Taken together, these data suggest IL-6 expression throughout life is involved in microglia priming and increased amounts of IL-6 following peripheral LPS challenge are involved in exaggerated sickness behaviors in the aged.

Glial contributions to neurodegeneration in tauopathies

Tauopathies are a broad set of neurodegenerative dementias characterized by aggregation of the tau protein into filamentous inclusions that can be found in neurons and glial cells. Activated microglia, astrocytes and elevated levels of proinflammatory molecules are also pathological hallmarks that are found in brain regions affected by tau pathology. There has been abundant research in recent years to understand the role of gliosis and neuroinflammation in neurodegenerative diseases, particularly in Alzheimer's disease (AD) which is the most common form of dementia. AD is a tauopathy characterized by both extracellular amyloid-β plaques in addition to intracellular neurofibrillary tangles and neuropil threads containing aggregated tau protein. Accumulating evidence suggests that neuroinflammation offers a possible mechanistic link between these pathologies. Additionally, there appears to be a role for neuroinflammation in aggravating tau pathology and neurodegeneration in tauopathies featuring tau deposits as the predominant pathological signature. In this review, we survey the literature regarding inflammatory mechanisms that may impact neurodegeneration in AD and related tauopathies. We consider a physical role for microglia in the spread of tau pathology as well as the non-cell autonomous effects of secreted proinflammatory cytokines, specifically interleukin 1 beta, interleukin 6, tumor necrosis factor alpha and complement proteins. These molecules appear to have direct effects on tau pathophysiology and overall neuronal health. They also indirectly impact neuronal homeostasis by altering glial function. We conclude by proposing a complex role for gliosis and neuroinflammation in accelerating the progression of AD and other tauopathies.

Benfotiamine attenuates inflammatory response in LPS stimulated BV-2 microglia

Microglial cells are resident immune cells of the central nervous system (CNS), recognized as key elements in the regulation of neural homeostasis and the response to injury and repair. As excessive activation of microglia may lead to neurodegeneration, therapeutic strategies targeting its inhibition were shown to improve treatment of most neurodegenerative diseases. Benfotiamine is a synthetic vitamin B1 (thiamine) derivate exerting potentially anti-inflammatory effects. Despite the encouraging results regarding benfotiamine potential to alleviate diabetic microangiopathy, neuropathy and other oxidative stress-induced pathological conditions, its activities and cellular mechanisms during microglial activation have yet to be elucidated. In the present study, the anti-inflammatory effects of benfotiamine were investigated in lipopolysaccharide (LPS)-stimulated murine BV-2 microglia. We determined that benfotiamine remodels activated microglia to acquire the shape that is characteristic of non-stimulated BV-2 cells. In addition, benfotiamine significantly decreased production of pro-inflammatory mediators such as inducible form of nitric oxide synthase (iNOS) and NO; cyclooxygenase-2 (COX-2), heat-shock protein 70 (Hsp70), tumor necrosis factor alpha α (TNF-α), interleukin-6 (IL-6), whereas it increased anti-inflammatory interleukin-10 (IL-10) production in LPS stimulated BV-2 microglia. Moreover, benfotiamine suppressed the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinases (JNK) and protein kinase B Akt/PKB. Treatment with specific inhibitors revealed that benfotiamine-mediated suppression of NO production was via JNK1/2 and Akt pathway, while the cytokine suppression includes ERK1/2, JNK1/2 and Akt pathways. Finally, the potentially protective effect is mediated by the suppression of translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) in the nucleus. Therefore, benfotiamine may have therapeutic potential for neurodegenerative diseases by inhibiting inflammatory mediators and enhancing anti-inflammatory factor production in activated microglia.

Corticosterone suppresses the proliferation of BV2 microglia cells via glucocorticoid, but not mineralocorticoid receptor

AIMS

Corticosterone (CORT), which is often referred to as the stress hormone, is a well-known regulator of peripheral immune responses and also shows anti-inflammatory properties in the brain. Microglia play a key role in immune response and inflammation in the brain. However, it is still unclear how CORT affects microglia. In this study, we focused on the effects of CORT on the proliferation and survival of microglia using mouse microglia cell line BV2.

MAIN METHODS

We used WST-8 and LDH (lactate dehydrogenase) assays to check the effects of CORT for the proliferation and survival in BV2 microglia cells. We also analyzed the expression pattern of proteins which related to CORT signal cascades using western blotting analysis.

KEY FINDINGS

Under treatment with 0.1, 1 and 10μM CORT for 24h, the BV2 proliferation rate decreased to 83, 77 and 70% of that in the control. Moreover, this inhibition was blocked by treatment with mifepristone, a glucocorticoid receptor (GR) antagonist, but not by spironolactone, a mineralocorticoid receptor (MR) antagonist. Moreover, an LDH assay showed that CORT was dose-dependently cytotoxic toward BV2 microglia cells and this cytotoxicity was partially abolished by treatment with mifepristone. In addition, treatment with CORT resulted in the translocation of GR, but not MR, from the cytosol to the nucleus.

SIGNIFICANCE

Our findings suggested that CORT suppresses the proliferation of BV2 microglia cells accompanied with a cytotoxic effect that is induced by the formation of a CORT-GR complex.

Interleukin-6, a mental cytokine

Almost a quarter of a century ago, interleukin-6 (IL-6) was discovered as an inflammatory cytokine involved in B cell differentiation. Today, IL-6 is recognized to be a highly versatile cytokine, with pleiotropic actions not only in immune cells, but also in other cell types, such as cells of the central nervous system (CNS). The first evidence implicating IL-6 in brain-related processes originated from its dysregulated expression in several neurological disorders such as multiple sclerosis, Alzheimer's disease and Parkinson's disease. In addition, IL-6 was shown to be involved in multiple physiological CNS processes such as neuron homeostasis, astrogliogenesis and neuronal differentiation. The molecular mechanisms underlying IL-6 functions in the brain have only recently started to emerge. In this review, an overview of the latest discoveries concerning the actions of IL-6 in the nervous system is provided. The central position of IL-6 in the neuroinflammatory reaction pattern, and more specifically, the role of IL-6 in specific neurodegenerative processes, which accompany Alzheimer's disease, multiple sclerosis and excitotoxicity, are discussed. It is evident that IL-6 has a dichotomic action in the CNS, displaying neurotrophic properties on the one hand, and detrimental actions on the other. This is in agreement with its central role in neuroinflammation, which evolved as a beneficial process, aimed at maintaining tissue homeostasis, but which can become malignant when exaggerated. In this perspective, it is not surprising that 'well-meant' actions of IL-6 are often causing harm instead of leading to recovery.