Stress sounds the alarmin: The role of the danger-associated molecular pattern HMGB1 in stress-induced neuroinflammatory priming

Toll-Like Receptor 4 in Pain: Bridging Molecules-to-Cells-to-Systems

Pain impacts the lives of billions of people around the world - both directly and indirectly. It is complex and transcends beyond an unpleasant sensory experience to encompass emotional experiences. To date, there are no successful treatments for sufferers of chronic pain. Although opioids do not provide any benefit to chronic pain sufferers, they are still prescribed, often resulting in more complications such as hyperalgesia and dependence. In order to develop effective and safe medications to manage, and perhaps even treat pain, it is important to evaluate novel contributors to pain pathologies. As such, in this chapter we review the role of Toll-like receptor 4, a receptor of the innate immune system, that continues to gain substantial attention in the field of pain research. Positioned in the nexus of the neuro and immune systems, TLR4 may provide one of the missing pieces in understanding the complexities of pain. Here we consider how TLR4 enables a mechanistical understanding of pain as a multidimensional biopsychosocial state from molecules to cells to systems and back again.

Neuroinflammation and Mitochondrial Dysfunction Link Social Stress to Depression

Major depressive disorder is a debilitating mental illness and a leading cause of global disease burden. While many etiological factors have been identified, social stress is a highly prevalent causative factor for the onset of depression. Unfortunately, rates of depression continue to increase around the world, and the recent COVID-19 pandemic has further exacerbated this mental health crisis. Though several therapeutic strategies are available, nearly 50% of patients who receive treatment never reach remission. The exact mechanisms by which social stress exposure promotes the development of depression are unclear, making it challenging to develop novel and more effective therapeutics. However, accumulating evidence points to a role for stress-induced neuroinflammation, particularly in treatment-resistant patients. Moreover, recent evidence has expanded the concept of the pathogenesis of depression to mitochondrial dysfunction, suggesting that the combined effects of social stress on mitochondria and inflammation may synergize to facilitate stress-related depression. In this chapter, we review evidence for neuroinflammation and mitochondrial dysfunction in the pathogenesis of social stress-induced depression and discuss these in the context of novel therapeutic targets for the treatment of depression.

Effects of High-fat Diet and Chronic Mild Stress on Depression-like Behaviors and Levels of Inflammatory Cytokines in the Hippocampus and Prefrontal Cortex of Rats

Obesity and depression tend to co-occur, and obese patients with chronic low-grade inflammation have a higher risk of developing depression. However, mechanisms explaining these connections have not been fully elucidated. Here, an animal model of comorbid obesity and depression induced by high-fat diet (HFD) combined with chronic unpredictable mild stress (CUMS) was used, and sucrose preference, open field, elevated plus maze and Morris water maze tests were used to detected depression-and anxiety-like behaviors and spatial memory. The levels of inflammatory cytokines and NF-κB and microglial activation in the hippocampus and prefrontal cortex were examined in the study. Our results revealed that the comorbidity group exhibited the most severe depression-like behavior. Obesity but unstressed rats had the highest serum lipid levels among groups. The HFD and CUMS alone and combination of them increased levels of IL-1β, IL-6 and TNF-α in the hippocampus and prefrontal cortex, which was significantly related to depression-like behaviors. Further, NF-κB protein and mRNA levels and microglial activation in the hippocampus and prefrontal cortex significantly increased in stressed, obese and comorbid groups, with animals in comorbid group having the highest NF-κB mRNA levels in the hippocampus and level of NF-κB proteins in the prefrontal cortex, and the highest microglial activation in both brain areas. The study concluded that HFD and CUMS alone and combination induce depression-like symptoms, abnormal serum lipid levels, microglial activation and increased inflammatory cytokines in the brain, effects that are possibly mediated by TLR4-NF-κB signaling.

Post-Translational Modification of HMGB1 Disulfide Bonds in Stimulating and Inhibiting Inflammation

High mobility group box 1 protein (HMGB1), a highly conserved nuclear DNA-binding protein, is a “damage-associated molecular pattern” molecule (DAMP) implicated in both stimulating and inhibiting innate immunity. As reviewed here, HMGB1 is an oxidation-reduction sensitive DAMP bearing three cysteines, and the post-translational modification of these residues establishes its proinflammatory and anti-inflammatory activities by binding to different extracellular cell surface receptors. The redox-sensitive signaling mechanisms of HMGB1 also occupy an important niche in innate immunity because HMGB1 may carry other DAMPs and pathogen-associated molecular pattern molecules (PAMPs). HMGB1 with DAMP/PAMP cofactors bind to the receptor for advanced glycation end products (RAGE) which internalizes the HMGB1 complexes by endocytosis for incorporation in lysosomal compartments. Intra-lysosomal HMGB1 disrupts lysosomal membranes thereby releasing the HMGB1-transported molecules to stimulate cytosolic sensors that mediate inflammation. This HMGB1-DAMP/PAMP cofactor pathway slowed the development of HMGB1-binding antagonists for diagnostic or therapeutic use. However, recent discoveries that HMGB1 released from neurons mediates inflammation via the TLR4 receptor system, and that cancer cells express fully oxidized HMGB1 as an immunosuppressive mechanism, offer new paths to targeting HMGB1 for inflammation, pain, and cancer.

HMGB1 mediates cognitive impairment caused by the NLRP3 inflammasome in the late stage of traumatic brain injury

Background

Cognitive impairment in the late stage of traumatic brain injury (TBI) is associated with the NOD-, LRR and pyrin domain-containing protein 3 (NLRP3) inflammasome, which plays an important role in neuroinflammation. Although classical inflammatory pathways have been well-documented in the late stage of TBI (4–8 weeks post-injury), the mechanism by which the NLRP3 inflammasome impairs cognition is still unclear.

Methods

Mice lacking the gene encoding for NLRP3 (NLRP3-knockout mice) and their wild-type littermates were used in a controlled cortical impact model of TBI. Levels of NLRP3 inflammasome and inflammatory factors such as IL-1β and HMGB1 were detected in post-injury hippocampal tissue, as well as long-term potentiation. Behaviors were assessed by T-maze test, novel object recognition, and nesting tests. Glycyrrhizin was used to antagonize HMGB1. Calcium imaging were performed on primary neuronal cultures.

Results

By using the NLRP3-knockout TBI model, we found that the continuous activation of the NLRP3 inflammasome and high mobility group box 1 (HMGB1) release were closely related to cognitive impairment. We also found that inhibition of HMGB1 improved LTP reduction and cognitive function by increasing the phosphorylation level of the NMDAR1 subunit at serine 896 while reducing NLRP3 inflammasome activation.

Conclusion

NLRP3 inflammasome damages memory in the late stage of TBI primarily through HMGB1 upregulation and provides an explanation for the long-term progression of cognitive dysfunction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02274-0.

Role of inflammasomes in neuroinflammation after ischemic stroke

Ischemic stroke is a devastating disease for which there is no effective medical treatment. In the era of extensive reperfusion strategies, established neuroprotectant candidates and novel therapeutic drugs with better targets are promising for treatment of acute ischemic stroke. Such targets include the inflammasome pathway, which contributes significantly to the pathogenesis of ischemic stroke. Following ischemic stroke, damage-associated molecular patterns from damaged cells activate inflammasomes, incur inflammatory responses, and induce cell death. Therefore, inhibiting inflammasome pathways has great promise for treatment of ischemic stroke. However, the efficacy and safety of inflammasome inhibitors remain controversial, and better upstream targets are needed for effective modulation. Herein, the roles of the inflammasome in ischemic injury caused by stroke are reviewed and the potential of neuroprotectants targeting the inflammasome is discussed.

Microglia and BDNF at the crossroads of stressor related disorders: Towards a unique trophic phenotype

Stressors ranging from psychogenic/social to neurogenic/injury to systemic/microbial can impact microglial inflammatory processes, but less is known regarding their effects on trophic properties of microglia. Recent studies do suggest that microglia can modulate neuronal plasticity, possibly through brain derived neurotrophic factor (BDNF). This is particularly important given the link between BDNF and neuropsychiatric and neurodegenerative pathology. We posit that certain activated states of microglia play a role in maintaining the delicate balance of BDNF release onto neuronal synapses. This focused review will address how different "activators" influence the expression and release of microglial BDNF and address the question of tropomyosin receptor kinase B (TrkB) expression on microglia. We will then assess sex-based differences in microglial function and BDNF expression, and how microglia are involved in the stress response and related disorders such as depression. Drawing on research from a variety of other disorders, we will highlight challenges and opportunities for modulators that can shift microglia to a "trophic" phenotype with a view to potential therapeutics relevant for stressor-related disorders.

Neuroprotective Effects of Anti-high Mobility Group Box-1 Monoclonal Antibody Against Methamphetamine-Induced Dopaminergic Neurotoxicity

High mobility group box-1 (HMGB1) is a ubiquitous non-histone nuclear protein that plays a key role as a transcriptional activator, with its extracellular release provoking inflammation. Inflammatory responses are essential in methamphetamine (METH)-induced acute dopaminergic neurotoxicity. In the present study, we examined the effects of neutralizing anti-HMGB1 monoclonal antibody (mAb) on METH-induced dopaminergic neurotoxicity in mice. BALB/c mice received a single intravenous administration of anti-HMGB1 mAb prior to intraperitoneal injections of METH (4 mg/kg × 2, at 2-h intervals). METH injections induced hyperthermia, an increase in plasma HMGB1 concentration, degeneration of dopaminergic nerve terminals, accumulation of microglia, and extracellular release of neuronal HMGB1 in the striatum. These METH-induced changes were significantly inhibited by intravenous administration of anti-HMGB1 mAb. In contrast, blood-brain barrier disruption occurred by METH injections was not suppressed. Our findings demonstrated the neuroprotective effects of anti-HMGB1 mAb against METH-induced dopaminergic neurotoxicity, suggesting that HMGB1 could play an initially important role in METH toxicity.

[Prospects for pharmacological adaptation of neurovascular unit in conditions of neurotropic viral infection]

The article discusses the prospects for pharmacological conditioning as a method for adaptation of neurovascular unit in conditions of neurotropic viral infection. A step-by-step mechanism for development of preconditioning and postconditioning is presented with a detailed description of it's main stages (trigger, signal and effector). The role of neuroinflammation as the leading mechanism of damage and the possibility of influencing the brain neurotrophic factor are considered. It is shown that different medications including neurotrophic drugs (cerebrolysin) can serve as inducers of conditioning. Usage of neurotrophic drugs in different doses for preconditioning and postconditioning is pathogenetically justified.

Green tea and its active compound epigallocathechin-3-gallate (EGCG) inhibit neuronal apoptosis in a middle cerebral artery occlusion (MCAO) model

OBJECTIVES

To determine the effect of green tea with the active ingredient epigallocathechin-3-gallate (EGCG) on the inhibition of apoptosis in the middle cerebral artery occlusion (MCAO) model.

METHODS

Four month old male Rattus norvegicus rats with a body weight of 200-275 g was used for the MCAO model and divided into five groups, and the treatment was carried out for 7 days. Before being sacrificed, the subject had 1 cc of blood drawn for high mobility group box 1 (HMGB-1) examination using enzyme-linked immunosorbent assay (ELISA), and after being sacrificed, the brain tissue specimen was taken to examine caspase-3 and B-cell lymphoma 3 (BCL-3) using immunohistochemistry methods.

RESULTS

There was no significant difference in HMGB-1 results for the treatment group compared to the control group (P1: 384.20 ± 231.72 [p = 0.553]; P2: 379.11 ± 268.4 [p = 0.526]; P3: 284, 87 ± 276.19 [p = 0.140]; P4: 435.32 ± 279.95 [p = 0.912]). There is a significant increase in BCL-2 expression between the treatment group compared to the control group (P1: 2.58 ± 0.51 [p = 0.04]; P2: 3.36 ± 0.50 [p<0.001]; P3: 4.00 ± 0.42 [p<0.001]; P4: 3.60 ± 0.52 [p<0.001]). There was a significant difference in caspase-3 expression compared to the control group in the P3 group (P1: 4.33 ± 0.49 [p = 0.652]; P2: 4.09 ± 0.30 [p = 0.136]; P3: 3.58 ± 0.51 [p = 0.01]; P4: 3.89 ± 0.42 [p = 0.063]). There is no correlation between HMGB-1 and caspase-3 (r = -0.063; p = 0.613) or BCL-2 (r = -0.106; p = 0.396). There is significant negative correlation between caspase-3 and BCL-2 (r = -0.459; p = 0.000).

CONCLUSIONS

Green tea with the active ingredient EGCG can inhibit neuronal cell death through the apoptotic pathway and not through the activation of HMGB-1.

SARS-CoV-2 (COVID-19) as a Predictor of Neuroinflammation and Neurodegeneration: Potential Treatment Strategies

The SARS-CoV-2 (COVID-19) pandemic has attracted attention to the challenge of neuroinflammation as an unavoidable component of viral infections. Acute neuroinflammatory responses include activation of resident tissue macrophages in the CNS followed by release of a variety of cytokines and chemokines associated with activation of oxidative stress and delayed neuron damage. This makes the search for treatments with indirect anti-inflammatory properties relevant. From this point of view, attention is focused on further study of the treatment of patients with COVID-19 with dipyridamole (Curantil) which, having antiviral and anti-inflammatory effects, can inhibit acute inflammatory activity and progression of fibrosis, is a drug with potential, especially among patients with early increases in the D-dimer concentration and severe signs of microangiopathy.

LPS Primes Brain Responsiveness to High Mobility Group Box-1 Protein

High mobility group box (HMGB)1 action contributes to late phases of sepsis, but the effects of increased endogenous plasma HMGB1 levels on brain cells during inflammation are unclear. Here, we aimed to further investigate the role of HMGB1 in the brain during septic-like lipopolysaccharide-induced inflammation in rats (LPS, 10 mg/kg, i.p.). HMGB-1 mRNA expression and release were measured in the periphery/brain by RT-PCR, immunohistochemistry and ELISA. In vitro experiments with disulfide-HMGB1 in primary neuro-glial cell cultures of the area postrema (AP), a circumventricular organ with a leaky blood–brain barrier and direct access to circulating mediators like HMGB1 and LPS, were performed to determine the direct influence of HMGB1 on this pivotal brain structure for immune-to-brain communication. Indeed, HMGB1 plasma levels stayed elevated after LPS injection. Immunohistochemistry of brains and AP cultures confirmed LPS-stimulated cytoplasmatic translocation of HMGB1 indicative of local HMGB1 release. Moreover, disulfide-HMGB1 stimulation induced nuclear factor (NF)-κB activation and a significant release of interleukin-6, but not tumor necrosis factor α, into AP culture supernatants. However, only a few AP cells directly responded to HMGB1 with increased intracellular calcium concentration. Interestingly, priming with LPS induced a seven-fold higher percentage of responsive cells to HMGB1. We conclude that, as a humoral and local mediator, HMGB1 enhances brain inflammatory responses, after LPS priming, linked to sustained sepsis symptoms.

Increased Serum Concentrations of High Mobility Group Box 1 (HMGB1) Protein in Children with Autism Spectrum Disorder

High mobility group box 1 protein (HMGB1) has been suggested to be involved in the immune dysfunction and inflammation reported in autism spectrum disorder (ASD). We aimed to assess HMGB1 serum concentrations (SCs) in high-functioning ASD children compared to typically developing (TD) controls and to explore their associations with the autism spectrum quotient (AQ), the empathy quotient (EQ), and the systemizing quotient (SQ). The study involved 42 ASD children and 38 TD children, all-male, aged between 6.1 and 13.3 years old. HMGB1 SCs were measured by enzyme-linked immunosorbent assay (ELISA). Groups were comparable regarding age, general IQ, birth weight, and maternal age at birth. ASD children showed significantly higher HMGB1 SCs compared to TD children (1.25 ± 0.84 ng/mL versus 1.13 ± 0.79 ng/mL, respectively, p = 0.039). The Spearman’s rho revealed that HMGB1 SCs were positively correlated with the AQ attention to detail subscale (rs = 0.46, p = 0.045) and with the SQ total score (rs = 0.42, p = 0.04) in the ASD group. These results show that HMGB1 serum concentrations are altered in ASD children, and suggest that inflammatory processes mediated by HMGB1 may be associated with specific cognitive features observed in ASD.

An emerging role for microglia in stress‐effects on memory

Stressful experiences evoke, among others, a rapid increase in brain (nor)epinephrine (NE) levels and a slower increase in glucocorticoid hormones (GCs) in the brain. Microglia are key regulators of neuronal function and contain receptors for NE and GCs. These brain cells may therefore potentially be involved in modulating stress effects on neuronal function and learning and memory. In this review, we discuss that stress induces (1) an increase in microglial numbers as well as (2) a shift toward a pro‐inflammatory profile. These microglia have (3) impaired crosstalk with neurons and (4) disrupted glutamate signaling. Moreover, microglial immune responses after stress (5) alter the kynurenine pathway through metabolites that impair glutamatergic transmission. All these effects could be involved in the impairments in memory and in synaptic plasticity caused by (prolonged) stress, implicating microglia as a potential novel target in stress‐related memory impairments.

Transcriptomic biomarker pathways associated with death in HIV-infected patients with cryptococcal meningitis

BACKGROUND

Cryptococcal meningitis (CM) is a major cause of death in HIV-infected patients in sub-Saharan Africa. Many CM patients experience cryptococcosis-associated immune reconstitution inflammatory syndrome (C-IRIS), which is often fatal. We sought to identify transcriptomic biomarker pathways in peripheral blood that are associated with or predict the development of death or fatal C-IRIS among patients with CM who were enrolled in the Cryptococcal Optimal ART Timing Trial.

METHODS

We assessed peripheral blood gene expression using next-generation RNA sequencing in 4 groups of patients with CM: (1) no C-IRIS or Death; (2) C-IRIS survivors; (3) fatal C-IRIS; (4) Death without C-IRIS. Gene expression was assessed at the time of ART initiation, at 1, 4, and 8 weeks on ART, and at the time of C-IRIS events.

RESULTS

We identified 12 inflammatory and stress response pathways, including interferon type 1 signaling, that were upregulated at the time of ART initiation in patients with future fatal C-IRIS, as compared with survivors. The upregulation of transcripts involved in innate immunity (inflammasome, Toll-like receptor signaling), was observed at the time of fatal or nonfatal C-IRIS events. At the time of fatal C-IRIS events, numerous transcripts within fMLP, Rho family GTPases, HMGB1, and other acute phase response signaling pathways were upregulated, which reflects the severity of inflammation and systemic oxidative stress. Patients who died without recognized C-IRIS also had increased expression of pathways associated with oxidative stress and tissue damage.

CONCLUSIONS

Our results showed that overactivated innate immunity, involving Toll-like receptor/inflammasome pathways, and inflammation-induced oxidative stress, are associated with fatal outcomes. The results of this study provide insight into the molecular drivers of death and fatal C-IRIS to inform future diagnostic test development or guide targeted treatments.

Impact of Stress on Epilepsy: Focus on Neuroinflammation-A Mini Review

Epilepsy, one of the most common neurological disorders worldwide, is characterized by recurrent seizures and subsequent brain damage. Despite strong evidence supporting a deleterious impact on seizure occurrence and outcome severity, stress is an overlooked component in people with epilepsy. With regard to stressor duration and timing, acute stress can be protective in epileptogenesis, while chronic stress often promotes seizure occurrence in epilepsy patients. Preclinical research suggests that chronic stress promotes neuroinflammation and leads to a depressive state. Depression is the most common psychiatric comorbidity in people with epilepsy, resulting in a poor quality of life. Here, we summarize studies investigating acute and chronic stress as a seizure trigger and an important factor that worsens epilepsy outcomes and psychiatric comorbidities. Mechanistic insight into the impact of stress on epilepsy may create a window of opportunity for future interventions targeting neuroinflammation-related disorders.

Disulfide HMGB1 acts via TLR2/4 receptors to reduce the numbers of oligodendrocyte progenitor cells after traumatic injury in vitro

Traumatic brain injury (TBI) is associated with poor clinical outcomes; autopsy studies of TBI victims demonstrate significant oligodendrocyte progenitor cell (OPC) death post TBI; an observation, which may explain the lack of meaningful repair of injured axons. Whilst high-mobility group box-1 (HMGB1) and its key receptors TLR2/4 are identified as key initiators of neuroinflammation post-TBI, they have been identified as attractive targets for development of novel therapeutic approaches to improve post-TBI clinical outcomes. In this report we establish unequivocal evidence that HMGB1 released in vitro impairs OPC response to mechanical injury; an effect that is pharmacologically reversible. We show that needle scratch injury hyper-acutely induced microglial HMGB1 nucleus-to-cytoplasm translocation and subsequent release into culture medium. Application of injury-conditioned media resulted in significant decreases in OPC number through anti-proliferative effects. This effect was reversed by co-treatment with the TLR2/4 receptor antagonist BoxA. Furthermore, whilst injury conditioned medium drove OPCs towards an activated reactive morphology, this was also abolished after BoxA co-treatment. We conclude that HMGB1, through TLR2/4 dependant mechanisms, may be detrimental to OPC proliferation following injury in vitro, negatively affecting the potential for restoring a mature oligodendrocyte population, and subsequent axonal remyelination. Further study is required to assess how HMGB1-TLR signalling influences OPC maturation and myelination capacity.

Chronic stress promotes acute myeloid leukemia progression through HMGB1/NLRP3/IL-1β signaling pathway

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy with poor prognosis and overall survival. Clinical investigations show that chronic stress is commonly present in the course of AML and associated with adverse outcome. However, the underlying molecular mechanisms are elusive. In the present study, a chronic restraint stress mouse model was established to evaluate the effect of stress on AML. We found that mice under chronic stress exhibited significantly increased liver and spleen infiltration of leukemic cells and poorer overall survival. This was accompanied by elevated cellular NLR family pyrin domain containing 3 (NLRP3) and interleukin-1β (IL-1β) in the liver or bone marrow, and secreted IL-1β in the plasma, indicating the activation of inflammasomes under chronic restraint stress. High mobility group box 1 (HMGB1) expression was markedly increased in newly diagnosed AML patients, but reduced in complete remission AML patients. The expression level of HMGB1 was positively correlated with NLRP3 mRNA in AML patients. Knockdown of HMGB1 significantly decreased NLRP3 and IL-1β expression in AML cell lines, and secreted IL-1β in supernatant of AML cell culture, while HMGB1 stimulation caused contrary effects. These results implied that HMGB1 could be involved in the regulation of inflammasome activation in AML development. Mice model showed that chronic restraint stress-facilitated proliferation and infiltration of AML cells were largely abrogated by knocking down HMGB1. Knockdown of HMGB1 also ameliorated overall survival and remarkably neutralized NLRP3 and IL-1β expression under chronic restraint stress. These findings provide evidences that chronic stress promotes AML progression via HMGB1/NLRP3/IL-1β dependent mechanism, suggesting that HMGB1 is a potential therapeutic target for AML. KEY MESSAGES: • Chronic restraint stress promoted acute myeloid leukemia (AML) progression and mediated NLRP3 inflammasome activation in xenograft mice. • HMGB1 mediated NLRP3 inflammasome activation in AML cells. • Knockdown of HMGB1 inhibited AML progression under chronic stress in vivo.

Neuroendocrine, neuroinflammatory and pathological outcomes of chronic stress: A story of microglial remodeling

Microglia, the resident macrophage cells of the central nervous system (CNS), are involved in a myriad of processes required to maintain CNS homeostasis. These cells are dynamic and can adapt their phenotype and functions to the physiological needs of the organism. Microglia rapidly respond to changes occurring in their microenvironment, such as the ones taking place during stress. While stress can be beneficial for the organism to adapt to a situation, it can become highly detrimental when it turns chronic. Microglial response to prolonged stress may lead to an alteration of their beneficial physiological functions, becoming either maladaptive or pro-inflammatory. In this review, we aim to summarize the effects of chronic stress exerted on microglia through the neuroendocrine system and inflammation at adulthood. We also discuss how these effects of chronic stress could contribute to microglial involvement in neuropsychiatric and sleep disorders, as well as neurodegenerative diseases.

Inhibition of phosphodiesterase-4 suppresses HMGB1/RAGE signaling pathway and NLRP3 inflammasome activation in mice exposed to chronic unpredictable mild stress

Inhibition of phosphodiesterase-4 (PDE4) produces robust anti-inflammatory and antidepressant-like effects in multiple animal models. However, the detailed mechanisms have not been well studied. Receptor for advanced glycation endproducts (RAGE) and inflammasome activation are implicated in the etiology of depression. Here, we aimed to investigate the involvement of RAGE and nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome in the antidepressant-like effects of PDE4 inhibition in mice. We found that inhibition of PDE4 by roflupram (ROF, 0.5, and 1.0 mg/kg, i.g.) exerted antidepressant-like effects in mice subjected to chronic unpredictable mild stress (CUMS). Simultaneously, ROF inhibited CUMS-induced microglial activation and restored the morphology of microglial cells in the hippocampus, as evidenced by reduced total process length, area, volume, number of branching points, number of terminal points and total sholl intersections of microglia. ROF also decreased the expression of ionized calcium-binding adapter molecule-1 and the level of interleukin-1β. Western blot analysis showed that PDE4 inhibition suppressed the high-mobility group box 1 protein (HMGB1)/RAGE signaling pathway, as the levels of HMGB1, RAGE, toll-like receptor 4, phosphorylated p38 mitogen-activated protein kinase, and nuclear factor κ-B were decreased in both hippocampus and cortex in mice after treatment with ROF. Moreover, ROF also attenuated the protein levels of NLRP3, the apoptosis-associated speck-like protein containing (ASC), and cysteine-requiring aspartate protease-1 (Caspase-1), which are key proteins in the NLRP3-mediated inflammasome signaling pathway. In summary, these results demonstrate that the down-regulation of HMGB1/RAGE signaling pathway and inflammasome suppression possibly contribute to the antidepressant-like effects of PDE4 inhibitors. And, ROF has potential as a candidate drug in the treatment of depression.

Modulation of the HMGB1/TLR4/NF-κB signaling pathway in the CNS by matrine in experimental autoimmune encephalomyelitis

The inflammatory mediator high-mobility group box 1 (HMGB1)-induced signaling pathway has been shown to play an important role in the pathogenesis of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). Matrine (MAT), a quinolizidine alkaloid component derived from the root of Sophorae flavescens, has the capacity to effectively suppress EAE. However, the impact of MAT treatment on HMGB1-induced signaling is not known. In the present study, we show that MAT treatment alleviated disease severity of ongoing EAE, reduced inflammatory infiltration and demyelination, and reduced the production of inflammatory factors including TNF-α, IL-6, and IL-1β in the CNS. Moreover, MAT administration significantly reduced the protein and RNA expression of HMGB1 and TLR4 in the spinal cord, particularly in astrocytes and microglia/infiltrating macrophages. The expression of MyD88 and TRAF6, and the phosphorylation of NF-κB p65, was also down-regulated after MAT treatment. In contrast, the level of IκB-α, an inhibitory molecule for NF-κB activation, was significantly increased. Furthermore, the direct inhibitory effect of MAT on HMGB1/TLR4/NF-κB signaling in macrophages was further confirmed in vitro. Taken together, these findings demonstrate that MAT treatment alleviated CNS inflammatory demyelination and activation of astrocytes and microglia/macrophages in EAE rats, and that the mechanism underlying these effects may be closely related to modulation of HMGB1/TLR4/NF-κB signaling pathway.

Chinese Herbal Medicine for the Treatment of Depression: Effects on the Neuroendocrine-Immune Network

The neuroimmune and neuroendocrine systems are two critical biological systems in the pathogenesis of depression. Clinical and preclinical studies have demonstrated that the activation of the neuroinflammatory response of the immune system and hyperactivity of the hypothalamus–pituitary–adrenal (HPA) axis of the neuroendocrine system commonly coexist in patients with depression and that these two systems bidirectionally regulate one another through neural, immunological, and humoral intersystem interactions. The neuroendocrine-immune network poses difficulties associated with the development of antidepressant agents directed toward these biological systems for the effective treatment of depression. On the other hand, multidrug and multitarget Chinese Herbal Medicine (CHM) has great potential to assist in the development of novel medications for the systematic pharmacotherapy of depression. In this narrative essay, we conclusively analyze the mechanisms of action of CHM antidepressant constituents and formulas, specifically through the modulation of the neuroendocrine-immune network, by reviewing recent preclinical studies conducted using depressive animal models. Some CHM herbal constituents and formulas are highlighted as examples, and their mechanisms of action at both the molecular and systems levels are discussed. Furthermore, we discuss the crosstalk of these two biological systems and the systems pharmacology approach for understanding the system-wide mechanism of action of CHM on the neuroendocrine-immune network in depression treatment. The holistic, multidrug, and multitarget nature of CHM represents an excellent example of systems medicine in the effective treatment of depression.

Impact of COVID-19 in the Mental Health in Elderly: Psychological and Biological Updates

Since December 2019, the world has been experiencing the challenge of facing coronavirus disease-19 (COVID-19), a severe infectious disease caused by the new coronavirus, SARS-CoV-2. The individuals with the most severe symptoms and the highest risk of death are the elderly and those with chronic illness. Among chronic conditions, those with a certain degree of chronic inflammation may predispose to a more severe evolution of COVID-19. Elderly with psychiatric disorders can present a persistent inflammatory state, a characteristic of the age’s immunological senescence, but the disorder can accentuate that. Social isolation is still the safest way to avoid contamination. However, isolated older people may have or worsen mental health conditions due to isolation and health concerns. In this scenario, a SARS-CoV-2 infection may progress to more severe disease. Conversely, COVID-19 can predispose or aggravate psychiatric disorders, as it induces a cytokine storm, causing systemic hyper inflammation. It may damage the blood-brain barrier, resulting in inflammation in the central nervous system. Besides, SARS-CoV-2 is likely to reach and trigger an inflammatory process directly in the nervous system. This review makes an update about research on the mental health of the elderly during the pandemic. Also, it discusses the vulnerability of these individuals in the face of stress and in the case of contracting COVID-19, considering mainly the stress’s hormonal and inflammatory mechanisms. Finally, the review points out possible care and attention strategies and entertainment and activities that can reduce the damage to mental and physical health and improve the elderly’s quality of life.

Graphical abstract

Discovery and characterization of small-molecule inhibitors of NLRP3 and NLRC4 inflammasomes

Inflammasomes are macromolecular complexes involved in the host response to external and endogenous danger signals. Inflammasome-mediated sterile inflammation plays a central role in several human conditions such as autoimmune diseases, type-2 diabetes, and neurodegenerative disorders, indicating inflammasomes could be appealing therapeutic targets. Previous work has demonstrated that inhibiting the ATPase activity of the nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing protein 3 (NLRP3), disrupts inflammasome assembly and function. However, there is a necessity to find new potent compounds with therapeutic potential. Here we combine computational modeling of the target and virtual screening to discover a group of novel compounds predicted to inhibit NLRP3. We characterized the best compounds and determined their potency, specificity, and ability to inhibit processes downstream from NLRP3 activation. Moreover, we analyzed in mice the competence of a lead candidate to reduce lipopolysaccharide-induced inflammation. We also validated the active pharmacophore shared among all the NLRP3 inhibitors, and through computational docking, we clarify key structural features for compound positioning within the inflammasome ATP-binding site. Our study sets the basis for rational design and optimization of inflammasome-targeting probes and drugs.

Acute stress induces the rapid and transient induction of caspase-1, gasdermin D and release of constitutive IL-1β protein in dorsal hippocampus

The proinflammatory cytokine interleukin (IL)-1β plays a pivotal role in the behavioral manifestations (i.e., sickness) of the stress response. Indeed, exposure to acute and chronic stressors induces the expression of IL-1β in stress-sensitive brain regions. Thus, it is typically presumed that exposure to stressors induces the extra-cellular release of IL-1β in the brain parenchyma. However, this stress-evoked neuroimmune phenomenon has not been directly demonstrated nor has the cellular process of IL-1β release into the extracellular milieu been characterized in brain. This cellular process involves a form of inflammatory cell death, termed pyroptosis, which involves: 1) activation of caspase-1, 2) caspase-1 maturation of IL-1β, 3) caspase-1 cleavage of gasdermin D (GSDMD), and 4) GSDMD-induced permeability of the cell membrane through which IL-1β is released into the extracellular space. Thus, the present study examined whether stress induces the extra-cellular release of IL-1β and engages the above cellular process in mediating IL-1β release in the brain. Male Sprague-Dawley rats were exposed to inescapable tailshock (IS). IL-1β extra-cellular release, caspase-1 activity and cleavage of GSDMD were measured in dorsal hippocampus. We found that exposure to IS induced a transient increase in the release of IL-1β into the extracellular space immediately after termination of the stressor. IS also induced a transient increase in caspase-1 activity prior to IL-1β release, while activation of GSDMD was observed immediately after termination of the stressor. IS also increased mRNA and protein expression of the ESCRTIII protein CHMP4B, which is involved in cellular repair. The present results suggest that exposure to an acute stressor induces the hallmarks of pyroptosis in brain, which might serve as a key cellular process involved in the release of IL-1β into the extracellular milieu of the brain parenchyma.

Inhibiting HMGB1-RAGE axis prevents pro-inflammatory macrophages/microglia polarization and affords neuroprotection after spinal cord injury

Background

Spinal cord injury (SCI) favors a persistent pro-inflammatory macrophages/microglia-mediated response with only a transient appearance of anti-inflammatory phenotype of immune cells. However, the mechanisms controlling this special sterile inflammation after SCI are still not fully elucidated. It is known that damage-associated molecular patterns (DAMPs) released from necrotic cells after injury can trigger severe inflammation. High mobility group box 1(HMGB1), a ubiquitously expressed DNA binding protein, is an identified DAMP, and our previous study demonstrated that reactive astrocytes could undergo necroptosis and release HMGB1 after SCI in mice. The present study aimed to explore the effects and the possible mechanism of HMGB1on macrophages/microglia polarization, as well as the neuroprotective effects by HMGB1 inhibition after SCI.

Methods

In this study, the expression and the concentration of HMGB1 was determined by qRT-PCR, ELISA, and immunohistochemistry. Glycyrrhizin was applied to inhibit HMGB1, while FPS-ZM1 to suppress receptor for advanced glycation end products (RAGE). The polarization of macrophages/microglia in vitro and in vivo was detected by qRT-PCR, immunostaining, and western blot. The lesion area was detected by GFAP staining, while neuronal survival was examined by Nissl staining. Luxol fast blue (LFB) staining, DAB staining, and western blot were adopted to evaluate the myelin loss. Basso-Beattie-Bresnahan (BBB) scoring and rump-height Index (RHI) assay was applied to evaluate locomotor functional recovery.

Results

Our data showed that HMGB1 can be elevated and released from necroptotic astrocytes and HMGB1 could induce pro-inflammatory microglia through the RAGE-nuclear factor-kappa B (NF-κB) pathway. We further demonstrated that inhibiting HMGB1 or RAGE effectively decreased the numbers of detrimental pro-inflammatory macrophages/microglia while increased anti-inflammatory cells after SCI. Furthermore, our data showed that inhibiting HMGB1 or RAGE significantly decreased neuronal loss and demyelination, and improved functional recovery after SCI.

Conclusions

The data implicated that HMGB1-RAGE axis contributed to the dominant pro-inflammatory macrophages/microglia-mediated pro-inflammatory response, and inhibiting this pathway afforded neuroprotection for SCI. Thus, therapies designed to modulate immune microenvironment based on this cascade might be a prospective treatment for SCI.

Molecular insights into the therapeutic promise of targeting HMGB1 in depression

Depression is a common psychiatric disorder, the exact pathogenesis of which is still elusive. Studies have proposed that immunity disproportion and enhancement in proinflammatory cytokines might be linked with the development of depression. HMGB1 (High-mobility group box (1) protein has obtained more interest as an essential factor in inherent immune reactions and a regulating factor in various inflammation-related diseases. HMGB1 is a ubiquitous chromatin protein and is constitutively expressed in nucleated mammalian cells. HMGB1 is released by glial cells and neurons upon inflammasome activation and act as a pro-inflammatory cytokine. HMGB1 is a late mediator of inflammation and has been indicated as a major mediator in various neuroinflammatory diseases. Microglia, which is the brain immune cell, is stimulated by HMGB1 and released inflammatory mediators and induces chronic neurodegeneration in the CNS (central nervous system). In the current review, we aimed to investigate the role of HMGB1 in the pathogenesis of depression. The studies found that HMGB1 functions as proinflammatory cytokines primarily via binding receptors like RAGE (receptor for advanced glycation end product), TLR2 and TLR4 (Toll-like receptor 2 and 4). Further, HMGB1 added to the preparing impacts of stress-pretreatment and assumed a major function in neurodegenerative conditions through moderating neuroinflammation. Studies demonstrated that neuroinflammation played a major role in the development of depression. The patients of depression generally exhibited an elevated amount of proinflammatory cytokines in the serum, microglia activation and neuronal deficit in the CNS.

Acute stress induces chronic neuroinflammatory, microglial and behavioral priming: A role for potentiated NLRP3 inflammasome activation

Prior exposure to acute and chronic stressors potentiates the neuroinflammatory and microglial pro-inflammatory response to subsequent immune challenges suggesting that stressors sensitize or prime microglia. Stress-induced priming of the NLRP3 inflammasome has been implicated in this priming phenomenon, however the duration/persistence of these effects has not been investigated. In the present study, we examined whether exposure to a single acute stressor (inescapable tailshock) induced a protracted priming of the NLRP3 inflammasome as well as the neuroinflammatory, behavioral and microglial proinflammatory response to a subsequent immune challenge in hippocampus. In male Sprague-Dawley rats, acute stress potentiated the neuroinflammatory response (IL-1β, IL-6, and NFκBIα) to an immune challenge (lipopolysaccharide; LPS) administered 8 days after stressor exposure. Acute stress also potentiated the proinflammatory cytokine response (IL-1β, IL-6, TNF and NFκBIα) to LPS ex vivo. This stress-induced priming of microglia also was observed 28 days post-stress. Furthermore, challenge with LPS reduced juvenile social exploration, but not sucrose preference, in animals exposed to stress 8 days prior to immune challenge. Exposure to acute stress also increased basal mRNA levels of NLRP3 and potentiated LPS-induction of caspase-1 mRNA and protein activity 8 days after stress. The present findings suggest that acute stress produces a protracted vulnerability to the neuroinflammatory effects of subsequent immune challenges, thereby increasing risk for stress-related psychiatric disorders with an etiological inflammatory component. Further, these findings suggest the unique possibility that acute stress might induce innate immune memory in microglia.

Early Life Stress Exacerbates Outcome after Traumatic Brain Injury

The neurocognitive impairments associated with mild traumatic brain injury (TBI) often resolve within 1-2 weeks; however, a subset of people exhibit persistent cognitive dysfunction for weeks to months after injury. The factors that contribute to these persistent deficits are unknown. One potential risk factor for worsened outcome after TBI is a history of stress experienced by a person early in life. Early life stress (ELS) includes maltreatment such as neglect, and interferes with the normal construction of cortical and hippocampal circuits. We hypothesized that a history of ELS contributes to persistent learning and memory dysfunction following a TBI. To explore this interaction, we modeled ELS by separating Sprague Dawley pups from their nursing mothers from post-natal days 2-14 for 3 h daily. At 2 months of age, male rats received sham surgery or mild to moderate parasagittal fluid-percussion brain injury. We found that the combination of ELS with TBI in adulthood impaired hippocampal-dependent learning, as assessed with contextual fear conditioning, the water maze task, and spatial working memory. Cortical atrophy was significantly exacerbated in TBI animals exposed to ELS compared with normal-reared TBI animals. Changes in corticosterone in response to restraint stress were prolonged in TBI animals that received ELS compared with TBI animals that were normally reared or sham animals that received ELS. Our findings indicate that ELS is a risk factor for worsened outcome after TBI, and results in persistent learning and memory deficits, worsened cortical pathology, and an exacerbation of the hormonal stress response.

Alcohol Use Disorder, Neurodegeneration, Alzheimer's and Parkinson's Disease: Interplay Between Oxidative Stress, Neuroimmune Response and Excitotoxicity

Alcohol use disorder (AUD) has been associated with neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Prolonged excessive alcohol intake contributes to increased production of reactive oxygen species that triggers neuroimmune response and cellular apoptosis and necrosis via lipid peroxidation, mitochondrial, protein or DNA damage. Long term binge alcohol consumption also upregulates glutamate receptors, glucocorticoids and reduces reuptake of glutamate in the central nervous system, resulting in glutamate excitotoxicity, and eventually mitochondrial injury and cell death. In this review, we delineate the following principles in alcohol-induced neurodegeneration: (1) alcohol-induced oxidative stress, (2) neuroimmune response toward increased oxidants and lipopolysaccharide, (3) glutamate excitotoxicity and cell injury, and (4) interplay between oxidative stress, neuroimmune response and excitotoxicity leading to neurodegeneration and (5) potential chronic alcohol intake-induced development of neurodegenerative diseases, including Alzheimer's and Parkinson's disease.

Antimicrobial Peptides as Anticancer Agents: Functional Properties and Biological Activities

Antimicrobial peptides (AMPs), or host defense peptides, are small cationic or amphipathic molecules produced by prokaryotic and eukaryotic organisms that play a key role in the innate immune defense against viruses, bacteria and fungi. AMPs have either antimicrobial or anticancer activities. Indeed, cationic AMPs are able to disrupt microbial cell membranes by interacting with negatively charged phospholipids. Moreover, several peptides are capable to trigger cytotoxicity of human cancer cells by binding to negatively charged phosphatidylserine moieties which are selectively exposed on the outer surface of cancer cell plasma membranes. In addition, some AMPs, such as LTX-315, have shown to induce release of tumor antigens and potent damage associated molecular patterns by causing alterations in the intracellular organelles of cancer cells. Given the recognized medical need of novel anticancer drugs, AMPs could represent a potential source of effective therapeutic agents, either alone or in combination with other small molecules, in oncology. In this review we summarize and describe the properties and the mode of action of AMPs as well as the strategies to increase their selectivity toward specific cancer cells.

miR-505 inhibits proliferation of osteosarcoma via HMGB1

Osteosarcoma is a malignant bone tumor, and clinically detectable metastases can be detected in ~ 15–20% of patients when they seek medical advice; patients with metastatic disease have extremely poor prognosis. Here, we examined the involvement of the microRNA miR‐505 in osteosarcoma. Eighty‐four patients seeking treatment for osteosarcoma were included in the study group (SG), and 63 healthy subjects were allocated to the control group (CG). Normal human bone cells MG‐63 and U20S cells were transfected with miR‐505 mimics, miR‐NC, HMGB1 RNA for targeted inhibition (si‐HMGB1), and si‐NC to examine the effects on HMGB1 expression. Cell proliferation, invasion, and apoptosis were measured using CCK‐8, scratch assays, and flow cytometry (FCM), respectively, and the relationship between miR‐505 and HMGB1 was determined using the dual‐luciferase reporter assay. In patient tissues and serum, miR‐505 was expressed at a low level, and HMGB1 was expressed at a high level, with an area under curve of > 0.9. Furthermore, the expression of miR‐505 and HMGB1 in tissues had a positive association with that in the serum, whereas the expression of miR‐505 had a negative association with that of HMGB1 in tissues only. Overexpression of miR‐505 and silencing of HMGB1 suppressed the proliferation, migration, and invasion of osteosarcoma cells and increased the rate of apoptosis, whereas the co‐transfected miR‐505 mimics + si‐HMGB1 demonstrated a more significant inhibitory effect on the proliferation and invasion of osteosarcoma cells and a higher apoptosis rate. miR‐505 may inhibit the proliferation and invasion and promote apoptosis of osteosarcoma cells by targeting and suppressing HMGB1.

Extracellular HMGB1: a therapeutic target in severe pulmonary inflammation including COVID-19?

BACKGROUND

The 2019 novel coronavirus disease (COVID-19) causes for unresolved reasons acute respiratory distress syndrome in vulnerable individuals. There is a need to identify key pathogenic molecules in COVID-19-associated inflammation attainable to target with existing therapeutic compounds. The endogenous damage-associated molecular pattern (DAMP) molecule HMGB1 initiates inflammation via two separate pathways. Disulfide-HMGB1 triggers TLR4 receptors generating pro-inflammatory cytokine release. Extracellular HMGB1, released from dying cells or secreted by activated innate immunity cells, forms complexes with extracellular DNA, RNA and other DAMP or pathogen-associated molecular (DAMP) molecules released after lytic cell death. These complexes are endocytosed via RAGE, constitutively expressed at high levels in the lungs only, and transported to the endolysosomal system, which is disrupted by HMGB1 at high concentrations. Danger molecules thus get access to cytosolic proinflammatory receptors instigating inflammasome activation. It is conceivable that extracellular SARS-CoV-2 RNA may reach the cellular cytosol via HMGB1-assisted transfer combined with lysosome leakage. Extracellular HMGB1 generally exists in vivo bound to other molecules, including PAMPs and DAMPs. It is plausible that these complexes are specifically removed in the lungs revealed by a 40% reduction of HMGB1 plasma levels in arterial versus venous blood. Abundant pulmonary RAGE expression enables endocytosis of danger molecules to be destroyed in the lysosomes at physiological HMGB1 levels, but causing detrimental inflammasome activation at high levels. Stress induces apoptosis in pulmonary endothelial cells from females but necrosis in cells from males.

CONCLUSION

Based on these observations we propose extracellular HMGB1 to be considered as a therapeutic target for COVID-19.

Interleukin-1α in the ventral hippocampus increases stress vulnerability and inflammation-related processes

Mechanisms of stress vulnerability remain elusive. Previous research demonstrated that inflammation-related processes in the brain play a role in stress vulnerability. Our previous research showed that inflammatory processes in the ventral hippocampus (vHPC) induced a stress vulnerable phenotype. To further understand neuroinflammatory processes in the vHPC in stressed rats, we determined that protein levels of the pro-inflammatory cytokine interleukin-1-α (IL-1α), but not interleukin-1β (IL-1β), were increased in the vHPC of rats vulnerable to the effects of repeated social defeat compared to rats resilient to its effects. Injections of IL-1α into the vHPC increased stress vulnerability as characterized by increases in passive coping during defeat and subsequent decreased social interactions. Conversely, injections of recombinant IL-1 receptor antagonist (IL1-RA) increased latencies to social defeat and decreased anxiety-like behaviors during social interaction, suggesting an reduction in stress vulnerability. Protein analyses revealed increased FosB expression in the vHPC of IL-1α-injected rats, and increased HPA activation following a social encounter. Further analysis of vHPC of IL1-α-injected rats showed increased density of microglia, increased expression of the pro-inflammatory cytokine HMGB1, and increases in a marker for vascular remodeling. Taken together, these data show increasing IL-1α during stress exposure is sufficient to produce a stress vulnerable phenotype potentially by increasing inflammation-related processes in the vHPC.LAY SUMMARYOur previous research demonstrated that inflammation-related processes in the brain play a role in inducing vulnerability to the effects of repeated social stress in rats. Here we demonstrate that a pro-inflammatory cytokine interleukin-1-α (IL-1α) induces inflammatory processes in the vHPC and behavioral vulnerability in stressed rats, whereas blocking IL receptors produces the opposite effects on behavioral vulnerability. Together, these results identify a substrate in the vHPC that produces vulnerability to stress by increasing inflammation-related processes in the vHPC.

Corticosterone Induces HMGB1 Release in Primary Cultured Rat Cortical Astrocytes: Involvement of Pannexin-1 and P2X7 Receptor-Dependent Mechanisms

A major risk factor for major depressive disorder (MDD) is stress. Stress leads to the release of high-mobility group box-1 (HMGB1), which in turn leads to neuroinflammation, a potential pathophysiological basis of MDD. The mechanism underlying stress-induced HMGB1 release is not known, but stress-associated glucocorticoids could be involved. To test this, rat primary cultured cortical astrocytes, the most abundant cell type in the central nervous system (CNS), were treated with corticosterone and HMGB1 release was assessed by Western blotting and ELISA. Significant HMGB1 was released with treatment with either corticosterone or dexamethasone, a synthetic glucocorticoid. HMGB1 translocated from the nucleus to the cytoplasm following corticosterone treatment. HMGB1 release was significantly attenuated with glucocorticoid receptor blocking. In addition, inhibition of pannexin-1, and P2X7 receptors led to a significant decrease in corticosterone-induced HMGB1 release. Taken together, corticosterone stimulates astrocytic glucocorticoid receptors and triggers cytoplasmic translocation and extracellular release of nuclear HMGB1 through a mechanism involving pannexin-1 and P2X7 receptors. Thus, under conditions of stress, glucocorticoids induce astrocytic HMGB1 release, leading to a neuroinflammatory state that could mediate neurological disorders such as MDD.

Minocycline alleviates NLRP3 inflammasome-dependent pyroptosis in monosodium glutamate-induced depressive rats

BACKGROUND

Inflammasome activation and followed by the release of proinflammatory cytokines play a pivotal role in the development and progression of depression. However, the involvement of gasdermin D (GSDMD)-mediated pyroptosis in inflammasome-associated depression has not been studied. The present study aimed to determine the involvement of pyroptosis in the development of depression.

METHODS

The rat depressive model was established by the administration of monosodium glutamate (MSG) in postnatal rats. Minocycline (an anti-inflammatory agent) and VX-765 (a specific inhibitor of caspase-1) were given as intervention treatments when rats were two-month-old. Rat depressive behaviors were evaluated by behavioral tests, including open field test, sucrose preference test, and forced swim test. Rat hippocampi were collected for western blotting and immunofluorescence examination.

RESULTS

MSG administration induced depressive-like behavior in rats. MSG upregulated protein presences of caspase-1, GSDMD, interleukin-1β (IL-1β), interleukin-18 (IL-18), NLR pyrin domain-containing 3 (NLRP3), apoptosis-associated speck-like protein (ASC), high mobility group box 1 protein (HMGB1), and the receptor for advanced glycation end products (RAGE) in the hippocampus. Protein presences of HMGB1, NLRP3 and GSDMD were upregulated in Olig2+ oligodendrocytes in the hippocampus. The data suggest that both HMGB1/RAGE/NLRP3 signalings and GSDMD-dependent pyroptosis were activated. Both minocycline and VX-765 treatments improved depressive-like behaviors. Minocycline treatment significantly reduced both HMGB1/RAGE/NLRP3 inflammasome signalings and GSDMD-dependent pyroptosis. VX-765 downregulated GSDMD-dependent pyroptosis, but not HMGB1/RAGE signalings, indicating that GSDMD-dependent pyroptosis is a key player in the progress of depression.

CONCLUSION

In rats hippocampus, NLRP3 inflammasome activates GSDMD mediated-pyroptosis in the hippocampus of MSG-induced depressive rats.

HMGB1/RAGE axis mediates stress-induced RVLM neuroinflammation in mice via impairing mitophagy flux in microglia

Background

Microglial mediated neuroinflammation in the rostral ventrolateral medulla (RVLM) plays roles in the etiology of stress-induced hypertension (SIH). It was reported that autophagy influenced inflammation via immunophenotypic switching of microglia. High-mobility group box 1 (HMGB1) acts as a regulator of autophagy and initiates the production of proinflammatory cytokines (PICs), but the underlying mechanisms remain unclear.

Methods

The stressed mice were subjected to intermittent electric foot shocks plus noises administered for 2 h twice daily for 15 consecutive days. In mice, blood pressure (BP) and renal sympathetic nerve activity (RSNA) were monitored by noninvasive tail-cuff method and platinum-iridium electrodes placed respectively. Microinjection of siRNA-HMGB1 (siHMGB1) into the RVLM of mice to study the effect of HMGB1 on microglia M1 activation was done. mRFP-GFP-tandem fluorescent LC3 (tf-LC3) vectors were transfected into the RVLM to evaluate the process of autolysosome formation/autophagy flux. The expression of RAB7, lysosomal-associated membrane protein 1 (LAMP1), and lysosomal pH change were used to evaluate lysosomal function in microglia. Mitophagy was identified by transmission electron microscopic observation or by checking LC3 and MitoTracker colocalization under a confocal microscope.

Results

We showed chronic stress increased cytoplasmic translocations of HMGB1 and upregulation of its receptor RAGE expression in microglia. The mitochondria injury, oxidative stress, and M1 polarization were attenuated in the RVLM of stressed Cre-CX3CR1/RAGE^fl/fl mice. The HMGB1/RAGE axis increased at the early stage of stress-induced mitophagy flux while impairing the late stages of mitophagy flux in microglia, as revealed by decreased GFP fluorescence quenching of GFP-RFP-LC3-II puncta and decreased colocalization of lysosomes with mitochondria. The expressions of RAB7 and LAMP1 were decreased in the stressed microglia, while knockout of RAGE reversed these effects and caused an increase in acidity of lysosomes. siHMGB1 in the RVLM resulted in BP lowering and RSNA decreasing in SIH mice. When the autophagy inducer, rapamycin, is used to facilitate the mitophagy flux, this treatment results in attenuated NF-κB activation and reduced PIC release in exogenous disulfide HMGB1 (ds-HMGB1)-stimulated microglia.

Conclusions

Collectively, we demonstrated that inhibition of the HMGB1/RAGE axis activation led to increased stress-induced mitophagy flux, hence reducing the activity of microglia-mediated neuroinflammation and consequently reduced the sympathetic vasoconstriction drive in the RVLM.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1673-3) contains supplementary material, which is available to authorized users.

Biochemical Mechanisms and Translational Relevance of Hippocampal Vulnerability to Distant Focal Brain Injury: The Price of Stress Response

Focal brain injuries (in particular, stroke and traumatic brain injury) induce with high probability the development of delayed (months, years) cognitive and depressive disturbances which are frequently comorbid. The association of these complications with hippocampal alterations (in spite of the lack of a primary injury of this structure), as well as the lack of a clear dependence between the probability of depression and dementia development and primary damage severity and localization served as the basis for a new hypothesis on the distant hippocampal damage as a key link in the pathogenesis of cognitive and psychiatric disturbances. According to this hypothesis, the excess of corticosteroids secreted after a focal brain damage, in particular in patients with abnormal stress-response due to hypothalamic-pituitary-adrenal axis (HPAA) dysfunction, interacts with corticosteroid receptors in the hippocampus inducing signaling pathways which stimulate neuroinflammation and subsequent events including disturbances in neurogenesis and hippocampal neurodegeneration. In this article, the molecular and cellular mechanisms associated with the regulatory role of the HPAA and multiple functions of brain corticosteroid receptors in the hippocampus are analyzed. Functional and structural damage to the hippocampus, a brain region selectively vulnerable to external factors and responding to them by increased cytokine secretion, forms the basis for cognitive function disturbances and psychopathology development. This concept is confirmed by our own experimental data, results of other groups and by prospective clinical studies of post-stroke complications. Clinically relevant biochemical approaches to predict the risks and probability of post-stroke/post-trauma cognitive and depressive disturbances are suggested using the evaluation of biochemical markers of patients' individual stress-response. Pathogenetically justified ways for preventing these consequences of focal brain damage are proposed by targeting key molecular mechanisms underlying hippocampal dysfunction.

Connexins-Based Hemichannels/Channels and Their Relationship with Inflammation, Seizures and Epilepsy

Connexins (Cxs) are a family of 21 protein isoforms, eleven of which are expressed in the central nervous system, and they are found in neurons and glia. Cxs form hemichannels (connexons) and channels (gap junctions/electric synapses) that permit functional and metabolic coupling between neurons and astrocytes. Altered Cx expression and function is involved in inflammation and neurological diseases. Cxs-based hemichannels and channels have a relevance to seizures and epilepsy in two ways: First, this pathological condition increases the opening probability of hemichannels in glial cells to enable gliotransmitter release, sustaining the inflammatory process and exacerbating seizure generation and epileptogenesis, and second, the opening of channels favors excitability and synchronization through coupled neurons. These biological events highlight the global pathological mechanism of epilepsy, and the therapeutic potential of Cxs-based hemichannels and channels. Therefore, this review describes the role of Cxs in neuroinflammation and epilepsy and examines how the blocking of channels and hemichannels may be therapeutic targets of anti-convulsive and anti-epileptic treatments.

Activation of NLRP3 Inflammasome Promotes Foam Cell Formation in Vascular Smooth Muscle Cells and Atherogenesis Via HMGB1

Background

This study aimed at investigating whether NLRP3 (the Nod like receptor family, pyrin domain‐containing 3 protein) inflammasome activation induced HMGB1 (high mobility group box‐1 protein) secretion and foam cell formation in human vascular smooth muscle cells (VSMCs) and atherosclerosis in ApoE^−/− mice.

Methods and Results

VSMCs or ApoE^−/− mice were treated with lipopolysaccharides (LPS) and/or ATP or LPS and high‐fat diet to induce NLRP3 inflammasome activation. HMGB1 distribution and foam cell formation in VSMCs were characterized. Liver X receptor α and ATP‐binding cassette transporter expression were determined. The impact of NLRP3 or receptor for advanced glycation end product silencing, ZYVAD‐FMK (caspase‐1 inhibitor), glycyrrhizin (HMGB1 inhibitor) or receptor for advanced glycation end product antagonist peptide on HMGB1 secretion, foam cell formation, liver X receptor α and ATP‐binding cassette transporter expression was examined. Expression level of HMGB1 in human atherosclerosis obliterans arterial tissues was characterized. Our results found that NLRP3 inflammasome activation promoted foam cell formation and HMGB1 secretion in VSMCs. Extracellular HMGB1 was a key signal molecule in inflammasome activation‐mediated foam cell formation. Furthermore, inflammasome activation‐induced HMGB1 activity and foam cell formation were achieved by receptor for advanced glycation end product/liver X receptor α /ATP‐binding cassette transporter glycyrrhizin. Experiments in vivo found glycyrrhizin significantly attenuated the LPS/high‐fat diet‐induced atherosclerosis and serum HMGB1 levels in mice. Finally, levels of HMGB1 and NLRP3 were increased in tunica media adjacent to intima of atherosclerosis obliteran arteries.

Conclusions

Our results revealed that HMGB1 is a key downstream signal molecule of NLRP3 inflammasome activation and plays an important role in VSMCs foam cell formation and atherogenesis by downregulating liver X receptor α and ATP‐binding cassette transporter expression through receptor for advanced glycation end product.

A Jack of All Trades: Impact of Glucocorticoids on Cellular Cross-Talk in Osteoimmunology

Glucocorticoids (GCs) are known to have a strong impact on the immune system, metabolism, and bone homeostasis. While these functions have been long investigated separately in immunology, metabolism, or bone biology, the understanding of how GCs regulate the cellular cross-talk between innate immune cells, mesenchymal cells, and other stromal cells has been garnering attention rather recently. Here we review the recent findings of GC action in osteoporosis, inflammatory bone diseases (rheumatoid and osteoarthritis), and bone regeneration during fracture healing. We focus on studies of pre-clinical animal models that enable dissecting the role of GC actions in innate immune cells, stromal cells, and bone cells using conditional and function-selective mutant mice of the GC receptor (GR), or mice with impaired GC signaling. Importantly, GCs do not only directly affect cellular functions, but also influence the cross-talk between mesenchymal and immune cells, contributing to both beneficial and adverse effects of GCs. Given the importance of endogenous GCs as stress hormones and the wide prescription of pharmaceutical GCs, an improved understanding of GC action is decisive for tackling inflammatory bone diseases, osteoporosis, and aging.

Neural-respiratory inflammasome axis in traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of morbidity and mortality. Approximately 20-25% of TBI subjects develop Acute Lung Injury (ALI), but the pathomechanisms of TBI-induced ALI remain poorly defined. Currently, mechanical ventilation is the only therapeutic intervention for TBI-induced lung injury. Our recent studies have shown that the inflammasome plays an important role in the systemic inflammatory response leading to lung injury-post TBI. Here, we outline the role of the extracellular vesicle (EV)-mediated inflammasome signaling in the etiology of TBI-induced ALI. Furthermore, we evaluate the efficacy of a low molecular weight heparin (Enoxaparin, a blocker of EV uptake) and a monoclonal antibody against apoptosis speck-like staining protein containing a caspase recruitment domain (anti-ASC) as therapeutics for TBI-induced lung injury. We demonstate that activation of an EV-mediated Neural-Respiratory Inflammasome Axis plays an essential role in TBI-induced lung injury and disruption of this axis has therapeutic potential as a treatment strategy.

Microglia Activated by Excess Cortisol Induce HMGB1 Acetylation and Neuroinflammation in the Hippocampal DG Region of Mice Following Cold Exposure

Cold stress can induce neuroinflammation in the hippocampal dentate gyrus (DG), but the mechanism underlying neuronal apoptosis induced by cold stress is not well-understood. To address this issue, male and female C57BL/6 mice were exposed to a temperature of 4 °C for 3 h per day for 1 week, and glial cell activation, neuronal apoptosis, and neuroinflammation were evaluated by western blotting, immunofluorescence, terminal deoxynucleotidyl transferase 2’-deoxyuridine 5’-triphosphate (dUTP) nick end labeling, Nissl staining, and immunohistochemistry. Additionally, BV2 cells were treated with different concentrations of cortisol (CORT) for 3 h to mimic stress and molecular changes were assessed by western blotting, immunofluorescence, and co-immunoprecipitation. We found that excess CORT activated glial cells and increased neuroinflammation in the DG of mice exposed to cold temperatures, which was associated with increased acetylation and nuclear factor-κB signaling. These effects were mediated by the acetylation of lysine 9 of histone 3 and lysine 310 of p65, which resulted in increased mitogen-activated protein kinase phosphorylation, nuclear translocation of p65, microglia activation, and acetylation of high-mobility group box 1. Neuroinflammation was more severe in male compared to female mice. These findings provide new insight into the mechanisms of the cold stress response, which can inform the development of new strategies to combat the effects of hypothermia.

HMGB1 involved in stress-induced depression and its neuroinflammatory priming role: a systematic review

Background

Evidence from clinical and preclinical studies has demonstrated that stress can cause depressive-like symptoms including anhedonia and psychomotor retardation, namely, the manifestation of motivational deficits in depression. The proximate mediator of linking social-environmental stress with internal motivational deficits remains elusive, although substantial studies proposed neural endocrine mechanisms. As an endogenous danger-associated molecule, high mobility group box-1 (HMGB1) is necessary and sufficient for stress-induced sensitization of innate immune cells and subsequent (neuro)inflammation.

Aim

This review aims to provide evidence to unveil the potential mechanism of the relationship between motivational deficits and stress in depression.

Methods

We reviewed original case-control studies investigating the association between HMGB1-mediated inflammation and stress-induced depression. The literature search of Pubmed and Web of Science electronic database from inception up to March 28th, 2019 were conducted by two independent authors. We performed a qualitative systematic review approach to explore the correlation between HMGB1-mediated inflammation and anhedonia/psychomotor retardation in depression.

Results

A total of 69 studies based on search strategy were retrieved and seven eligible studies met the inclusion criteria. Studies showed that HMGB1 was implicated with depressive-like behaviors, which are similar with motivational deficits. Furthermore, HMGB1-mediated inflammation in depressive-like behaviors may be involved in Nod-like receptor family pyrin domain containing three (NLRP3) inflammasome and proinflammatory cytokines, abnormal kynurenine pathway and imbalance between neuroprotective and neurotoxic factors.

Conclusions

We found that stress-induced inflammation mediated by HMGB1 may affect motivational deficits through regulating dopamine pathway in corticostriatal neurocircuitry. The systematic review may shed light on the novel neurobiological underpinning for treatment of motivation deficits in depression.

The Stress Hormone Cortisol Enhances Interferon-υ-Mediated Proinflammatory Responses of Human Immune Cells

BACKGROUND

Cortisol is a prototypical human stress hormone essential for life, yet the precise role of cortisol in the human stress response to injury or infection is still uncertain. Glucocorticoids (GCs) such as cortisol are widely understood to suppress inflammation and immunity. However, recent research shows that GCs also induce delayed immune effects manifesting as immune stimulation. In this study, we show that cortisol enhances the immune-stimulating effects of a prototypical proinflammatory cytokine, interferon-υ (IFN-υ). We tested the hypothesis that cortisol enhances IFN-υ-mediated proinflammatory responses of human mononuclear phagocytes (monocyte/macrophages [MOs]) stimulated by bacterial endotoxin (lipopolysaccharide [LPS]).

METHODS

Human MOs were cultured for 18 hours with or without IFN-υ and/or cortisol before LPS stimulation. MO differentiation factors granulocyte-macrophage colony stimulating factor (GM-CSF) or M-CSF were added to separate cultures. We also compared the inflammatory response with an acute, 4-hour MO incubation with IFN-υ plus cortisol and LPS to a delayed 18-hour incubation with cortisol before LPS exposure. MO activation was assessed by interleukin-6 (IL-6) release and by multiplex analysis of pro- and anti-inflammatory soluble mediators.

RESULTS

After the 18-hour incubation, we observed that cortisol significantly increased LPS-stimulated IL-6 release from IFN-υ-treated undifferentiated MOs. In GM-CSF-pretreated MOs, cortisol increased IFN-υ-mediated IL-6 release by >4-fold and release of the immune stimulant IFN-α2 (IFN-α2) by >3-fold, while suppressing release of the anti-inflammatory mediator, IL-1 receptor antagonist to 15% of control. These results were reversed by either the GC receptor antagonist RU486 or by an IFN-υ receptor type 1 antibody antagonist. Cortisol alone increased expression of the IFN-υ receptor type 1 on undifferentiated and GM-CSF-treated MOs. In contrast, an acute 4-hour incubation of MOs with IFN-υ and cortisol showed classic suppression of the IL-6 response to LPS.

CONCLUSIONS

These results reveal a surprisingly robust proinflammatory interaction between the human stress response hormone cortisol and the immune activating cytokine IFN-υ. The results support an emerging physiological model with an adaptive role for cortisol, wherein acute release of cortisol suppresses early proinflammatory responses but also primes immune cells for an augmented response to a subsequent immune challenge. These findings have broad clinical implications and provide an experimental framework to examine individual differences, mechanisms, and translational implications of cortisol-enhanced immune responses in humans.

Neuroimmune mechanisms of psychostimulant and opioid use disorders

Substance use disorders are global health problems with few effective treatment options. Unfortunately, most potential pharmacological treatments are hindered by abuse potential of their own, limited efficacy, or adverse side effects. As a consequence, there is a pressing need for the development of addiction treatments with limited abuse potential and fewer off target effects. Given the difficulties in developing new pharmacotherapies for substance use disorders, there has been growing interest in medications that act on non-traditional targets. Recent evidence suggests a role for dysregulated immune signaling in the pathophysiology of multiple psychiatric diseases. While there is evidence that immune responses in the periphery and the central nervous system are altered by exposure to drugs of abuse, the contributions of neuroimmune interactions to addictive behaviors are just beginning to be appreciated. In this review, we discuss the data on immunological changes seen in clinical populations with substance use disorders, as well as in translational animal models of addiction. Importantly, we highlight those mechanistic findings showing causal roles for central or peripheral immune mediators in substance use disorder and appropriate animal models. Based on the literature reviewed here, it is clear that brain-immune system interactions in substance use disorders are much more complex and important than previously understood. While much work remains to be done, there are tremendous potential therapeutic implications for immunomodulatory treatments in substance use disorders.

SIRT1-regulated HMGB1 release is partially involved in TLR4 signal transduction: A possible anti-neuroinflammatory mechanism of resveratrol in neonatal hypoxic-ischemic brain injury

Neonatal hypoxic-ischemic brain injury (HIBI) is a knotty disease that lacks appropriate treatment. Inflammation is an important contributor to brain damage, and microglia are responsible for eliciting early and pronounced inflammatory reactions in the immature brain after hypoxic-ischemic (HI) insult. Acetylated HMGB1 can be released from immune cells into the extracellular space, where it acts as a danger-associated molecular pattern molecule to activate TLR4 signalling-mediated inflammatory responses. Resveratrol has neuroprotective and anti-inflammatory effects against HIBI, but whether these effects involve the regulation of the TLR4 signalling pathway and whether HMGB1 participates in this process is still unclear. We investigated the anti-inflammatory effects of resveratrol in HIBI and the molecular mechanisms potentially involved in the effect. The in vivo and in vitro results indicated that the level of cytoplasmic HMGB1 in microglia increased after insult and that treating experimental animals or mouse BV2 microglial cells with resveratrol attenuated HI insult-induced neuroinflammation, which was characterized by improved behavioural defects, reduced microglial activation and TLR4/MyD88/NF-κB signalling, and attenuated primary neuronal damage; this was accompanied by the inhibition of HMGB1 nucleoplasmic transfer and extracellular release. EX527 pretreatment reversed these effects. In addition, co-immunoprecipitation confirmed that SIRT1 was directly involved in the HMGB1 acetylation process in BV2 cells after oxygen glucose deprivation. These data demonstrate that resveratrol plays a neuroprotective role in neonatal HIBI by activating SIRT1 to inhibit HMGB1/TLR4/MyD88/NF-κB signalling and subsequent neuroinflammatory responses.