Synergistic apoptosis induced by bacterial endotoxin lipopolysaccharide and high glucose in rat microglia

Effects of diabetes on microglial physiology: a systematic review of in vitro, preclinical and clinical studies

Diabetes mellitus is a heterogeneous chronic metabolic disorder characterized by the presence of hyperglycemia, commonly preceded by a prediabetic state. The excess of blood glucose can damage multiple organs, including the brain. In fact, cognitive decline and dementia are increasingly being recognized as important comorbidities of diabetes. Despite the largely consistent link between diabetes and dementia, the underlying causes of neurodegeneration in diabetic patients remain to be elucidated. A common factor for almost all neurological disorders is neuroinflammation, a complex inflammatory process in the central nervous system for the most part orchestrated by microglial cells, the main representatives of the immune system in the brain. In this context, our research question aimed to understand how diabetes affects brain and/or retinal microglia physiology. We conducted a systematic search in PubMed and Web of Science to identify research items addressing the effects of diabetes on microglial phenotypic modulation, including critical neuroinflammatory mediators and their pathways. The literature search yielded 1327 records, including 18 patents. Based on the title and abstracts, 830 papers were screened from which 250 primary research papers met the eligibility criteria (original research articles with patients or with a strict diabetes model without comorbidities, that included direct data about microglia in the brain or retina), and 17 additional research papers were included through forward and backward citations, resulting in a total of 267 primary research articles included in the scoping systematic review. We reviewed all primary publications investigating the effects of diabetes and/or its main pathophysiological traits on microglia, including in vitro studies, preclinical models of diabetes and clinical studies on diabetic patients. Although a strict classification of microglia remains elusive given their capacity to adapt to the environment and their morphological, ultrastructural and molecular dynamism, diabetes modulates microglial phenotypic states, triggering specific responses that include upregulation of activity markers (such as Iba1, CD11b, CD68, MHC-II and F4/80), morphological shift to amoeboid shape, secretion of a wide variety of cytokines and chemokines, metabolic reprogramming and generalized increase of oxidative stress. Pathways commonly activated by diabetes-related conditions include NF-κB, NLRP3 inflammasome, fractalkine/CX3CR1, MAPKs, AGEs/RAGE and Akt/mTOR. Altogether, the detailed portrait of complex interactions between diabetes and microglia physiology presented here can be regarded as an important starting point for future research focused on the microglia-metabolism interface.

TREM2 Regulates High Glucose-Induced Microglial Inflammation via the NLRP3 Signaling Pathway

Background: TREM2 expressed on microglia plays an important role in modulating inflammation in neurodegenerative diseases. It remains unknown whether TREM2 modulates hyperglycemia-induced microglial inflammation. Methods: We investigated the molecular function of TREM2 in high glucose-induced microglial inflammation using western blotting, qPCR, ELISA, pulldown, and co-IP methods. Results: Our data showed that in high glucose-induced BV2 cells, TREM2 was increased, and the proinflammatory cytokine IL-1β was increased. TREM2 knockout (KO) attenuated the proinflammatory cytokine IL-1β; conversely, TREM2 overexpression (OE) exacerbated IL-1β expression. Furthermore, we found that high glucose promoted the interaction of TREM2 with NLRP3. TREM2 KO abolished the interaction of TREM2 with NLRP3, while TREM2 OE enhanced the interaction. Moreover, TREM2 KO reduced high glucose-induced NLRP3 inflammasome activation, and TREM2 OE augmented high glucose-induced NLRP3 inflammasome activation, indicating that high glucose enhances the expression of TREM2, which activates the NLRP3 inflammasome. To further clarify whether the NLRP3 signaling pathway mediates the TREM2-regulated inflammatory response, we blocked the NLRP3 inflammasome by knocking out NLRP3 and treating cells with a caspase1 inhibitor, which decreased the levels of the IL-1β proinflammatory cytokine but did not affect the high glucose-induced expression of TREM2. Conclusions: TREM2 modulates high glucose-induced microglial inflammation via the NLRP3 signaling pathway.

Hyperglycaemia augments lipopolysaccharide-induced reduction in rat and human macrophage phagocytosis via the endoplasmic stress-C/EBP homologous protein pathway

BACKGROUND

Macrophage phagocytosis constitutes an essential part of the host defence against microbes and the resolution of inflammation. Hyperglycaemia during sepsis is reported to reduce macrophage function, and thus, potentiate inflammatory deterioration. We investigated whether high-glucose concentrations augment lipopolysaccharide-induced reduction in macrophage phagocytosis via the endoplasmic stress-C/EBP homologous protein (CHOP) pathway using animal and laboratory investigations.

METHODS

Peritoneal macrophages of artificially ventilated male Wistar rats, divided into four groups based on target blood glucose concentrations achieved by glucose administration with or without lipopolysaccharide, were obtained after 24 h. Human macrophages were also cultured in normal or high glucose with or without lipopolysaccharide exposure for 72 h. Changes in the phagocytic activity, intranuclear CHOP expression, and intracellular Akt phosphorylation status of macrophages were evaluated. These changes were also evaluated in human macrophages after genetic knock-down of CHOP by specific siRNA transfection or resolvin D2 treatment.

RESULTS

Lipopolysaccharide impaired phagocytosis, increased intranuclear expression of CHOP, and inhibited Akt phosphorylation in both rat peritoneal and human macrophages. Hyperglycaemic glucose concentrations augmented these changes. Genetic knock-down of CHOP restored phagocytic ability and Akt phosphorylation in human macrophages. Furthermore, resolvin D2 co-incubation restored the inhibited phagocytosis and Akt phosphorylation along with the inhibition of intranuclear CHOP expression in human macrophages.

CONCLUSIONS

These findings imply that controlling endoplasmic reticulum stress might provide new strategies for restoring reduced macrophage phagocytosis in sepsis-induced hyperglycaemia.

Impact of Intraoperative Hyperglycemia on Brain Structures and Volumes

BACKGROUND AND PURPOSE

In hyperglycemic patients, who succumbed to septic shock, an increased rate of apoptosis of microglial cells and damaged neurons of the hippocampus were found. However, the influence of perioperative glucose levels on hippocampal brain structures has not yet been investigated.

METHODS

As part of the ongoing BIOCOG project, a subgroup of N = 65 elderly nondemented patients were analyzed who underwent elective surgery of ≥60 minutes. In these patients, at least one intraoperative blood glucose (BG) measurement was available from the medical charts. Intraoperative glucose maximum was determined in each patient. Preoperatively and at 3 months follow-up, structural neuroimaging was performed with T1-weighted magnetization prepared rapid gradient-echo sequence (MP-Rage) and a dedicated high-resolution hippocampus magnetic resonance imaging (MRI). The MRI scans were analyzed to assess pre- or postoperative volume changes of the hippocampus as a whole and hippocampal subfields. We also assessed changes of frontal lobe volume and cortical thickness.

RESULTS

Overall, 173 intraoperative BG levels were obtained in 65 patients (median 2 per patient). A total of 18 patients showed intraoperative hyperglycemia (glucose maximum ≥150 mg/dL). Controlling for age and diabetes status, no significant impact of intraoperative hyperglycemia was found on the pre-post volume change of the hippocampus as a whole, hippocampal subfields, frontal lobe, and frontal cortical thickness.

CONCLUSIONS

This study found no effect of intraoperative hyperglycemia on postoperative brain structures and volumes including volumes of hippocampus and hippocampal subfields, frontal lobe, and frontal cortical thickness. Further studies investigating the impact of intraoperatively elevated glucose levels should consider a tighter or even continuous glycemic measurement and the determination of central microglial activation.

ASK1 modulates the expression of microRNA Let7A in microglia under high glucose in vitro condition

Hyperglycemia results in oxidative stress and leads to neuronal apoptosis in the brain. Diabetes studies show that microglia participate in the progression of neuropathogenesis through their involvement in inflammation in vivo and in vitro. In high-glucose-induced inflammation, apoptosis signal regulating kinase 1 (ASK1) triggers the release of apoptosis cytokines and apoptotic gene expression. MicroRNA-Let7A (miR-Let7A) is reported to be a regulator of inflammation. In the present study, we investigated whether miR-Let7A regulates the function of microglia by controlling ASK1 in response to high-glucose-induced oxidative stress. We performed reverse transcription (RT) polymerase chain reaction, Taqman assay, real-time polymerase chain reaction, and immunocytochemistry to confirm the alteration of microglia function. Our results show that miR-Let7A is associated with the activation of ASK1 and the expression of anti-inflammatory cytokine (interleukin (IL)-10) and Mycs (c-Myc and N-Myc). Thus, the relationship between Let-7A and ASK1 could be a novel target for enhancing the beneficial function of microglia in central nervous system (CNS) disorders.

Microglial activation in neuroinflammation: implications for the etiology of neurodegeneration

BACKGROUND

Activated microglia secrete inflammatory cytokines and may play roles in the progression of neurodegenerative diseases. However, the mechanism underlying microglial activation remains unclear.

OBJECTIVE

Our aim was to examine the regulation of activated microglia through their cell death and survival pathways.

METHODS

We used mouse primary-cultured microglia, which are destined to die within a few days under ordinary culture conditions. The microglia live for longer than 1 month, without any measurable increase in apoptotic or necrotic cell death, when kept activated by sublethal concentrations of lipopolysaccharide (LPS).

RESULTS

LPS-treated microglia showed changes in shape. LPS treatment had no effect on the level of the proapoptotic Bcl-2-associated X protein but increased the level of the antiapoptotic protein Bcl-xL at day 1. Furthermore, the level of microtubule-associated light chain 3-II, a marker protein for autophagy, was decreased 3 h after exposure to LPS.

CONCLUSION

An increase in Bcl-xL seems to inhibit both apoptosis and autophagy. Our results suggest that long-lived microglia resulting from exposure to the optimal dose of LPS may play critical roles in the progression of neurodegeneration.

Hyperglycaemia and apoptosis of microglial cells in human septic shock

Introduction

The effect of hyperglycaemia on the brain cells of septic shock patients is unknown. The objective of this study was to evaluate the relationship between hyperglycaemia and apoptosis in the brains of septic shock patients.

Methods

In a prospective study of 17 patients who died from septic shock, hippocampal tissue was assessed for neuronal ischaemia, neuronal and microglial apoptosis, neuronal Glucose Transporter (GLUT) 4, endothelial inducible Nitric Oxide Synthase (iNOS), microglial GLUT5 expression, microglial and astrocyte activation. Blood glucose (BG) was recorded five times a day from ICU admission to death. Hyperglycaemia was defined as a BG 200 mg/dL g/l and the area under the BG curve (AUBGC) > 2 g/l was assessed.

Results

Median BG over ICU stay was 2.2 g/l. Neuronal apoptosis was correlated with endothelial iNOS expression (rho = 0.68, P = 0.04), while microglial apoptosis was associated with AUBGC > 2 g/l (rho = 0.70; P = 0.002). Neuronal and microglial apoptosis correlated with each other (rho = 0.69, P = 0.006), but neither correlated with the duration of septic shock, nor with GLUT4 and 5 expression. Neuronal apoptosis and ischaemia tended to correlate with duration of hypotension.

Conclusions

In patients with septic shock, neuronal apoptosis is rather associated with iNOS expression and microglial apoptosis with hyperglycaemia, possibly because GLUT5 is not downregulated. These data provide a mechanistic basis for understanding the neuroprotective effects of glycemic control.

Sepsis-associated encephalopathy and its differential diagnosis

Sepsis-associated encephalopathy (SAE) is defined as a diffuse cerebral dysfunction resulting from the systemic inflammatory response to an infection without direct infestation of the CNS. Although the pathophysiology of SAE is as yet unknown, some mechanisms have been suggested that involve BBB disruption as a consequence of proinflammatory mediators’ effects on endothelial cells. This leads to an increased passage of neurotoxic and proinflammatory mediators into the brain parenchyma, as well as an impairment of the movements of oxygen and metabolites through the BBB. Both neurons and glial cells are affected, resulting in neural functioning and neurotransmission impairment. The clinical translation of this process is an alteration of consciousness and awareness. SAE is a frequent condition in septic patients. Despite being considered reversible, SAE appears to be associated with long-term cognitive impairment. Detection and diagnosis can be challenging; it requires daily neurological assessment with the assistance of clinical scores. Use of biomarkers and neurophysiological testing is discussed. The aim of this article is to provide practical tools for detection of SAE, as well as an updated overview of its pathophysiology and therapeutic perspectives.

Sepsis-associated encephalopathy and its differential diagnosis

Sepsis is often complicated by an acute and reversible deterioration of mental status, which is associated with increased mortality and is consistent with delirium but can also be revealed by a focal neurologic sign. Sepsis-associated encephalopathy is accompanied by abnormalities of electroencephalogram and somatosensory-evoked potentials, increased in biomarkers of brain injury (i.e., neuron-specific enolase, S-100 beta-protein) and, frequently, by neuroradiological abnormalities, notably leukoencephalopathy. Its mechanism is highly complex, resulting from both inflammatory and noninflammatory processes that affect all brain cells and induce blood-brain barrier breakdown, dysfunction of intracellular metabolism, brain cell death, and brain injuries. Its diagnosis relies essentially on neurologic examination that can lead one to perform specific neurologic tests. Electroencephalography is required in the presence of seizure; neuroimaging in the presence of seizure, focal neurologic signs or suspicion of cerebral infection; and both when encephalopathy remains unexplained. In practice, cerebrospinal fluid analysis should be performed if there is any doubt of meningitis. Hepatic, uremic, or respiratory encephalopathy, metabolic disturbances, drug overdose, withdrawal of sedatives or opioids, alcohol withdrawal delirium, and Wernicke's encephalopathy are the main differential diagnoses of sepsis-associated encephalopathy. Patient management is based mainly on controlling infection, organ system failure, and metabolic homeostasis, at the same time avoiding neurotoxic drugs.

Psychoneuroimmune implications of type 2 diabetes: redux

A sizable body of knowledge has arisen demonstrating that type 2 diabetes (T2D) is associated with alterations in the innate immune system. The resulting proinflammatory-leaning imbalance is implicated in the development of secondary disease complications and comorbidities, such as delayed wound healing, accelerated progress of atherosclerosis, and retinopathy, in people who have T2D. New experimental data and the results of recently published health-related quality-of-life surveys indicate that individuals who have T2D experience diminished feelings of happiness, well being, and satisfaction with life. These emotional and psychological consequences of T2D point to altered neuroimmunity as a previously unappreciated complication of T2D. This article discusses recent data detailing the impact of T2D on a person's PNI response.

Psychoneuroimmune Implications of Type 2 Diabetes

The idea that type 2 diabetes is associated with augmented innate immune function characterized by increased circulating levels of acute phase reactants and altered macrophage biology is fairly well established, even though the mechanisms involved in this complex interaction still are not entirely clear. To date, the majority of studies investigating innate immune function in type 2 diabetes are limited to the context of wound healing, atherosclerosis, stroke, and other commonly identified comorbidities. Several important recurring themes come out of these data. First, type 2 diabetes is associated with a state of chronic, subclinical inflammation. Second, in macrophages, type 2 diabetic conditions enhance proinflammatory reactions and impair anti-inflammatory responses. Third, after acute activation of the innate immune system in type 2 diabetes, recovery or resolution of inflammation is impaired. The consequences of type 2 diabetes-associated inflammatory alterations on PNI processes have been recognized only recently. Given the impact of diminished emotional well-being on the quality of life in patients who have type 2 diabetes, diabetes-induced exacerbation of PNI responses should be considered a serious complication of type 2 diabetes that warrants further clinical attention.

Responses of microglia in vitro to the gram-positive bacterial component, lipoteichoic acid

An increase in incidence and severity of gram-positive infections has emerged in the past decade. In this regard, attention has been focused recently on immune responses of microglial cells in the central nervous system to gram-positive bacteria. The underlying immunological and cellular events in microglial activation induced by specific bacterial toxin of gram-positive bacteria, however, have not yet been clarified fully. This study reports that a simple cell wall product, lipoteichoic acid (LTA), derived from gram-positive bacteria (Staphylococcus aureus) could trigger microglial activation in vitro. Microglia challenged with LTA showed intense ruffling of plasma membrane in the form of lamellipodia or rounded up forming cell aggregates. MTT assay and Western blot analysis with anti-proliferating cell nuclear antigen antibody showed a significant microglial proliferation that may be induced at the later phases of LTA treatment with low doses but at the early period with a high dose. Concentrated LTA also caused apoptotic death of cultured microglia showing fragmented nuclei and increased expression of annexin V or caspase 3. In response to LTA, isolated microglia increased the expression of inducible nitric oxide synthase and major histocompatibility complex class II antigen. Microglial LTA receptors such as CD14 molecule, complement receptor type 3, and macrophage scavenger receptor were upregulated concurrently. In conclusion, staphylococcal LTA can exert an immunomodulatory effect on microglial morphology, cell cycle, and immunomolecules, including its receptors.

IL-1beta-mediated innate immunity is amplified in the db/db mouse model of type 2 diabetes

Chronic inflammation appears to play a critical role in type 2 diabetes and its complications. Here we tested the hypothesis that this inflammatory dysregulation affects the IL-1beta system and has functional consequences in the brain. Diabetic, db/db, and nondiabetic, db/+, mice were administered i.p. LPS, a potent cytokine inducer, at a dose of 100 microg/kg/mouse. db/db mouse innate immune-associated sickness behavior was 14.8, 33, 44.7, and 34% greater than that of db/+ mice at 2, 4, 8, and 12 h, respectively. When a fixed dose of LPS was used (5 microg/mouse), db/db mouse sickness was again enhanced 18.4, 22.2, and 14.5% at 4, 8, and 12 h as compared with db/+ mice. In diabetic mice, peritoneal macrophages produced more IL-1beta in response to LPS, and peritoneal levels of IL-1beta induced by LPS were increased. Importantly, IL-1R antagonist and type 2 IL-1 receptor (IL-1R2) failed to up-regulate in response to LPS in db/db mice. Finally, both peripheral and central administration of IL-1beta, itself, induced sickness in db/db mice that mimicked the effects of peripheral LPS and was significantly greater than that seen in db/+ mice. Taken together, these results indicate that IL-1beta-mediated innate immunity is augmented in db/db mice both at the periphery and in the brain, and the mechanism is due to diabetes-associated loss of IL-1beta counterregulation.

Lipopolysaccharide-induced increase in signalling in hippocampus is abrogated by IL-10--a role for IL-1 beta?

Parenterally administered lipopolysaccharide (LPS) increases the concentration of the pro-inflammatory cytokine interleukin-1beta (IL-1beta) in the rat hippocampus and evidence suggests that this effect plays a significant role in inhibiting long-term potentiation (LTP). The anti-inflammatory cytokine IL-10, antagonizes certain effects of IL-1beta, so if the effects of LPS are mediated through an increase in IL-1beta, it might be predicted that IL-10 would also abrogate the effect of LPS. Here, we report that IL-10 reversed the inhibitory effect of LPS on LTP and the data couple this with an inhibitory effect on the LPS-induced increase in IL-1beta. LPS treatment increased hippocampal expression of IL-1 receptor Type I protein. Consistent with the LPS-induced increases in IL-1beta concentration and receptor expression, were downstream changes which included enhanced phosphorylation of IRAK and the stress-activated kinases, JNK and p38; these LPS-induced changes were reversed by IL-10, which concurs with the idea that these events are triggered by increased activation of IL-1RI by IL-1beta. We provide evidence which indicates that LPS treatment leads to evidence of cell death and this was reversed in hippocampus prepared from LPS-treated rats which received IL-10. The evidence is therefore consistent with the idea that IL-10 acts to protect neuronal tissue from the detrimental effects induced by LPS.

Cytotoxic effects of the lipopolysaccharide from Pseudomonas fluorescens on neurons and glial cells

Pseudomonas fluorescens is an emerging pathogen closely related to Pseudomonas aeruginosa. In the present study, the effect of the lipopolysaccharide (LPS) from P. fluorescens MF37 was investigated using indicators of apoptosis and necrosis and was compared to the effect of the LPS from P. aeruginosa PAO1. Capillary electrophoresis analysis of the LPS from P. fluorescens MF37 revealed the existence of three forms of the endotoxin and the absence of homology with the LPS from P. aeruginosa. In neurons and glial cells the LPS from P. fluorescens induced major morphological changes including a condensation of the cytoplasmic proteins, a leakage of the cytoplasmic content, the formation of blebs on the nuclear membrane and a marked reorganization of the cytoskeleton. In glial cells, the LPS from P. fluorescens provoked the migration of phosphatidylserine at the surface of the cytoplasmic membrane, a sign of apoptosis, but this reaction was associated to an increase in the permeability to propidium iodide characteristic of necrosis. Biochemical studies revealed an important activation of an inducible nitric oxide synthase and a release of lactate dehydrogenase, a stable cytosolic enzyme. These results demonstrate that the LPS from P. fluorescens induces apoptosis and a concomitant and limited necrosis, reveal the unexpected cytotoxicity of this endotoxin and provide the first demonstration of the apoptotic effect of a non-aeruginosa Pseudomonas on nerve cells.