δ-Opioid receptor activation ameliorates lipopolysaccharide-induced inflammation and apoptosis by inhibiting the MAPK/caspase-3 pathway in BV2 microglial cells

Delta-opioid receptor signaling alleviates neuropathology and cognitive impairment in the mouse model of Alzheimer's disease by regulating microglia homeostasis and inhibiting HMGB1 pathway

BACKGROUND

Recent studies suggest that opioid receptor signaling may differentially affect Alzheimer's disease (AD) pathology and the relevant behavioral dysfunctions. However, the precise roles and mechanisms of opioid receptor subtypes in AD pathologies are still unclear with major controversies.

METHODS

We compared the delta-opioid receptor (DOR)- and mu-opioid receptor (MOR)-mediated effects on AD-associated cognitive deficits, pathologies, neuroinflammations, cell death using transgenic APP/PS1 mouse model and BV2 cell line at behavioral, molecular, and cellular levels. Unpaired t-test and one/two way analysis for variance (ANOVA) were used to analyze statistical significance of the data.

RESULTS

We show a distinct role of DOR and its major difference with MOR in AD injury in an APP/PS1 mouse model. DOR activation by UFP-512, but not MOR activation by DAMGO, attenuated cognitive impairment, reduced beta-amyloid (Aβ) production and aggregation, as well as protected the neurons from apoptosis in APP/PS1 mice. DOR and MOR also differentially modulated microglia in APP/PS1 mice and in vitro AD cell model with a DOR-mediated inhibition on the excessive activation of microglia and the release of pro-inflammatory cytokines in AD pathologies. Gene expression profiling further revealed that the alternations in DOR/MOR are closely associated with microglial homeostatic signatures and high mobility group protein B1 (HMGB1) in AD. DOR activation inhibited HMGB1 secretion and its translocation from nuclear to cytoplasm. Our in-vitro studies further confirmed that DOR overexpression mitigated microglial inflammatory response and rescued neurons from AD injury via HMGB1-NF-κB signaling pathway.

CONCLUSIONS

These novel findings uncover previously unappreciated roles of DOR in neuroprotection against AD injury via modulating microglia-related inflammatory responses.

Automatic wound closure system: closure of large wounds by stretching the skin around the wound

Large-wound treatment often requires autologous skin grafting or skin flap transfer, causing iatrogenic secondary injuries. Thus, we have developed an automatic wound closure system that consists of a stretch module, microcontroller, and touch screen. Full-thickness wounds (8 × 14 cm) on Bama miniature pigs were manually closed by direct suture in control animals and with three different tension levels performed by the automatic device in the experimental animals. Wound-closure conditions, post-closure healing, and scars were evaluated. Post-operative microscopic changes in collagen fibers, local cell apoptosis, and changes in vascular density were compared between the two wound-closure techniques. In the control group and the first experimental group, which used a traction force of 15 N, primary wound closure could not be achieved. The other two experimental groups used a traction force of 30 N and 60 N and all wounds achieved primary closure. Collagen-fiber stretching was observed histologically in all groups and collagen-fiber breakdown occurred in some wounds when the traction force was 60 N. Scar hyperplasia was significantly reduced in the automatic wound closure system groups. The collagen content decreased, cell apoptosis increased, and vascular density decreased in local tissues in the early post-closure stage, but eventually recovered to normal-skin levels. In summary, we developed an automatic wound closure system that effectively and safely stretches dermal-collagen fibers under an appropriate traction force (30 N) and stretch wound-peripheral skin to cover the wound, achieve primary closure, and reduce scar hyperplasia.

Neuroprotective effects of miRNA-326 knockout in neonatal hypoxic-ischemic brain damage mice via the δ-opioid receptor

Hypoxic-ischemic brain damage (HIBD) in the perinatal period is an important cause of cerebral damage and long-term neurological sequelae, and can place much pressure on families and society. Our previous study demonstrated that miRNA-326 reduces neuronal apoptosis by up-regulating the δ-opioid receptor (DOR) under oxygen-glucose deprivation in vitro. In the present study, we aimed to explore the neuroprotective effects of the miRNA-326/DOR axis by inhibiting apoptosis in HIBD using neonatal miRNA-326 knockout mice. Neonatal C57BL/6 mice, neonatal miRNA-326 knockout mice, and neonatal miRNA-326 knockout mice intraperitoneally injected with the DOR inhibitor naltrindole were treated with hypoxic-ischemia (HI). Neurological deficit scores, magnetic resonance imaging, terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling, and Caspase-3, Bax, and B-cell lymphoma 2 (Bcl-2) expression were evaluated on day 2 after HI. Neurobehavioral analyses were performed on days 2 and 28 after HI. Additionally, the Morris water maze test was conducted on days 28. Compared with HI-treated neonatal C57BL/6 mice, HI-treated neonatal miRNA-326 knockout mice had higher neurological deficit scores, smaller cerebral infarction areas, and improved motor function, reaction ability, and long-term spatial learning and memory. These effects were likely the result of inhibiting apoptosis; the DOR inhibitor reversed these neuroprotective effects. Our findings indicate that miRNA-326 knockout plays a neuroprotective effect in neonatal HIBD by inhibiting apoptosis via the target gene DOR.

Research progress on mechanisms of ischemic stroke: Regulatory pathways involving Microglia

Microglia, as the intrinsic immune cells in the brain, are activated following ischemic stroke. Activated microglia participate in the pathological processes after stroke through polarization, autophagy, phagocytosis, pyroptosis, ferroptosis, apoptosis, and necrosis, thereby influencing the injury and repair following stroke. It has been established that polarized M1 and M2 microglia exhibit pro-inflammatory and anti-inflammatory effects, respectively. Autophagy and phagocytosis in microglia following ischemia are dynamic processes, where moderate levels promote cell survival, while excessive responses may exacerbate neurofunctional deficits following stroke. Additionally, pyroptosis and ferroptosis in microglia after ischemic stroke contribute to the release of harmful cytokines, further aggravating the damage to brain tissue due to ischemia. This article discusses the different functional states of microglia in ischemic stroke research, highlighting current research trends and gaps, and provides insights and guidance for further study of ischemic stroke.

KNT-127, a selective delta opioid receptor agonist, shows beneficial effects in the hippocampal dentate gyrus of a chronic vicarious social defeat stress mouse model

Delta opioid receptors (DOPs) play an important role in depression and other mood disorders. However, little is known about the underlying physiological mechanisms. The hypothalamic-pituitary-adrenal axis, adult hippocampal neurogenesis, and neuroinflammation are regarded as key pathophysiological factors in depression. In this study, we investigated the influence of DOP activation on those factors in a valid animal model of depression, chronic vicarious social defeat stress (cVSDS) mice. cVSDS mice (male C57BL/6J mice) were produced following a 10-day exposure to witness of social defeat stress, and each evaluation was performed more than 28 days after the stress period. Repeated administrations to cVSDS mice with a selective DOP agonist, KNT-127, both during (10 days) and after (28 days) the stress period respectively improved their decreased social interaction behaviors and increased serum corticosterone levels. When administered during the stress period, KNT-127 suppressed decreases in the hippocampal newborn neuron survival rate in cVSDS mice. Moreover, in both administration paradigms, KNT-127 reduced the number of Iba-1- and CD11b-positive cells in the subgranular zone and the granule cell layer of the hippocampal dentate gyrus, indicating a suppression of cVSDS-induced microglial overactivation. These results suggest that KNT-127 acts over the hypothalamic-pituitary-adrenal axis and regulates neurogenesis and neuroinflammation resulting in anti-stress effects, and the antidepressant-like effects of the DOP agonist are implicated in the suppression of the neuroinflammation. This study presents a new finding on the effects of repeated DOP activations on the pathophysiological states of depression.

Activation of Delta-Opioid Receptor Protects ARPE19 Cells against Oxygen-Glucose Deprivation/Reoxygenation-Induced Necroptosis and Apoptosis by Inhibiting the Release of TNF-α

PURPOSE

Retinal ischemia-reperfusion injury (RIRI) is the basis of the pathology that leads to many retinal diseases and induces necroptosis and apoptosis. Tumor necrosis factor-α (TNF-α) is critically involved in necroptosis and apoptosis. Delta-opioid receptor (DOR) activation inhibits TNF-α release in our previous studies, it might prevent necroptosis and apoptosis by inhibiting the release of TNF-α. However, the role of TNF-α and DOR in necroptosis and apoptosis of retinal pigment epithelial (RPE) cells remains largely unknown. Here, we explored the mechanisms of TNF-α and DOR in necroptosis and apoptosis using an oxygen-glucose deprivation/reoxygenation (OGD/R) model of adult retinal pigment epithelial cell line-19 (ARPE19) cells.

MATERIALS AND METHODS

ARPE19 cells were exposed to OGD/R conditions to mimic RIRI in vitro. Cell viability was quantified using the Cell Counting Kit-8 (CCK-8) assay. Morphological changes were observed by inverted microscopy. TNF-α protein levels in cell lysates were measured by enzyme-linked immunosorbent assay (ELISA). The DOR agonist TAN-67 and antagonist naltrindole (NTI) were used to pretreat cells for 1 or 2 hours before OGD24/R36 administration. Calcein acetoxymethylester/propidium iodide (Calcein-AM/PI) and Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining were used to detect necroptotic and apoptotic ARPE19 cells, respectively. The protein expression of DOR, p-RIP1 (RIP1), p-RIP3 (RIP3), p-MLKL (MLKL), and cleaved Caspase3 (Caspase3) was measured by western blotting.

RESULTS

OGD severely damaged ARPE19 cells. Prolonged reoxygenation significantly increased TNF-α level and decreased DOR expression in ARPE19 cells. Pretreatment with the DOR agonist TAN-67 (10 µM) significantly improved ARPE19 cell viability after OGD24/R36 by reducing the number of necroptotic and apoptotic cells. Furthermore, DOR activation significantly inhibited TNF-α release and suppressed the expression of proteins related to necroptosis and apoptosis, including p-RIP1, p-RIP3, p-MLKL, and cleaved Caspase3, after OGD24/R36. This effect was reversed by the DOR antagonist NTI.

CONCLUSION

These results strongly suggest that DOR activation inhibits necroptosis and apoptosis by decreasing TNF-α release, leading to the prevention of OGD/R-induced injury in ARPE19 cells. This study provides an innovative idea for clinical treatment strategies for retinal damage and vision loss due to RIRI.

Necrotic-like BV-2 microglial cell death due to methylmercury exposure

Methylmercury (MeHg) is a dangerous environmental contaminant with strong bioaccumulation in the food chain and neurotoxic properties. In the nervous system, MeHg may cause neurodevelopment impairment and potentially interfere with immune response, compromising proper control of neuroinflammation and aggravating neurodegeneration. Human populations are exposed to environmental contamination with MeHg, especially in areas with strong mining or industrial activity, raising public health concerns. Taking this into consideration, this work aims to clarify pathways leading to acute toxic effects caused by MeHg exposure in microglial cells. BV-2 mouse microglial cells were incubated with MeHg at different concentrations (0.01, 0.1, 1 and 10 µM) for 1 h prior to continuous Lipopolysaccharide (LPS, 0.5 μg/ml) exposure for 6 or 24 h. After cell exposure, reactive oxygen species (ROS), IL-6 and TNF-α cytokines production, inducible nitric oxide synthase (iNOS) expression, nitric oxide (NO) release, metabolic activity, propidium iodide (PI) uptake, caspase-3 and -9 activities and phagocytic activity were assessed. MeHg 10 µM decreased ROS formation, the production and secretion of pro-inflammatory cytokines IL-6, TNF-α, iNOS immunoreactivity, the release of NO in BV-2 cells. Furthermore, MeHg 10 µM decreased the metabolic activity of BV-2 and increased the number of PI-positive cells (necrotic-like cell death) when compared to the respective control group. Besides, MeHg did not interfere with caspase activity or the phagocytic profile of cells. The short-term effects of a high concentration of MeHg on BV-2 microglial cells lead to impaired production of several pro-inflammatory mediators, as well as a higher microglial cell death via necrosis, compromising their neuroinflammatory response. Clarifying the mechanisms underlying MeHg-induced neurotoxicity and neurodegeneration in brain cells is relevant to better understand acute and long-term chronic neuroinflammatory responses following MeHg exposure.

A Critical Role of δ-Opioid Receptor in Anti-microglial Activation Under Stress

Microglia are involved in the regulation of cerebral homeostasis and pathogen confrontation. There is, however, evidence showing that excessive microglia activation is implicated in various age-related cerebral diseases. On the other hand, microglia may experience complex changes of polarization in pathological insults, i.e., from a proinflammatory M1 to an anti-inflammatory M2 phenotype, which differentially contribute to the exacerbation or alleviation of cellular injury. Remolding the phenotype of microglia or inhibiting the excessive activation of microglia seems to be a promising approach against neurodegenerative pathologies. Since δ-opioid receptor (DOR) activation exhibits a strong protective capacity against various neuronal injuries, especially the hypoxic/ischemic injury, we asked if the DOR-induced neuroprotection is associated with its effect on microglia. We explored this fundamental issue by using pharmacological and genetic approaches in the BV2 cell line, a general type of microglial cells. The results showed that DOR expression significantly increased in the activated microglial M2 phenotype, but slightly decreased in the microglial M1 phenotype. Hypoxia induced dual polarizations of BV2 cells with an increase in DOR expression. Administration of a specific DOR agonist, UFP-512, largely inhibited lipopolysaccharide (LPS) or hypoxia-induced microglial M1 activation and inflammatory activity with high concentrations of UFP-512 being effective to reverse the interleukin-4 (IL4)-induced microglial activation. Consistent with these observations, inhibiting DOR or knocking-down DOR promoted the excessive activation of BV2 cells in both M1 and M2 directions, while DOR overexpression did the opposite. Furthermore, the PC12 cells exposed to the conditioned medium of BV2 cells treated by UFP-512 grew better than those treated directly with UFP-512 under LPS or hypoxic insults. DOR inhibitor naltrindole could block all the effects of DOR activation. The medium from the BV2 cells with DOR knock-down decreased the viability of PC12 cell, while the medium from the BV2 cells with DOR overexpression largely attenuated LPS or hypoxic injury in the PC12 cells. These first data suggest a close linkage between DOR expression/function and microglial polarization and a critical role of DOR in negative controlling microglial activation. Our work provides a novel clue for new protective strategies against neurodegenerative pathophysiology through DOR-mediated regulation of microglia.

Discernible effects of tDCS over the primary motor and posterior parietal cortex on different stages of motor learning

Implicit motor learning and memory involve complex cortical and subcortical networks. The induction of plasticity in these network components via non-invasive brain stimulation, including transcranial direct current stimulation (tDCS), has shown to improve motor learning. However, studies showing these effects are mostly restricted to stimulation of the primary motor cortex (M1) during the early stage of learning. Because of this, we aimed to explore the efficacy of anodal tDCS applied over the posterior parietal cortex (PPC), which is involved in memory processes, on serial reaction time task (SRTT) performance. Specifically, to evaluate the involvement of both motor learning network components, we compared the effects of tDCS applied over regions corresponding to M1 and PPC during the early and late stages of learning. The results revealed a selective improvement of reaction time (RT) during anodal stimulation over the PPC in the late stage of learning. These findings support the assumption that the PPC is relevant during specific phases of learning, at least for SRTT performance. The results also indicate that not only the target area (i.e., PPC), but also timing is crucial for achieving the effects of stimulation on motor learning.

δ-opioid Receptor, Microglia and Neuroinflammation

Neuroinflammation underlies the pathophysiology of multiple age-related neurological disorders. Microglia, the resident immune cells of the central nervous system, are critically involved in neuroinflammatory regulation and neural survival. Modulating microglial activation is thus a promising approach to alleviate neuronal injury. Our serial studies have revealed a neuroprotective role of the δ-opioid receptor (DOR) in several acute and chronic cerebral injuries by regulating neuroinflammation and cellular oxidative stress. More recently, we found an endogenous mechanism for the inhibition of neuroinflammation is closely related to DOR's modulation of microglia. Our recent studies showed that DOR activation could strongly protect neurons from hypoxia- and lipopolysaccharide (LPS)-induced injury by inhibiting microglial pro-inflammatory transformation, while knocking-down DOR or restraining DOR activity promoted microglia activation and the relevant inflammatory events with an aggravation of cell injury. This novel finding highlights a therapeutic potential of DOR in numerous age-related neurological disorders through the modulation of neuroinflammation by targeting microglia. This review summarized the current data regarding the role of microglia in neuroinflammation, oxidative stress, and age-related neurological diseases focusing on the pharmacological effects and signaling transduction of DOR in microglia.

Interaction of Opioids with TLR4-Mechanisms and Ramifications

Simple Summary

Recent evidence indicates that opioids can be active at a receptor that is abundantly expressed on innate immune cells as well as cancer cells: the receptor is termed toll-like receptor 4 (TLR4). TLR4 is increasingly recognised as playing key roles in tumour biology and anticancer defences. However, the issue of whether TLR4 mediates some of the effects of opioids on tumour growth and metastasis is entirely unknown. We review existing evidence, mechanisms, and functional consequences of the action of opioids at TLR4. This opens new avenues of research on the role of opioids in cancer.

Abstract

The innate immune receptor toll-like receptor 4 (TLR4) is known as a sensor for the gram-negative bacterial cell wall component lipopolysaccharide (LPS). TLR4 activation leads to a strong pro-inflammatory response in macrophages; however, it is also recognised to play a key role in cancer. Recent studies of the opioid receptor (OR)-independent actions of opioids have identified that TLR4 can respond to opioids. Opioids are reported to weakly activate TLR4, but to significantly inhibit LPS-induced TLR4 activation. The action of opioids at TLR4 is suggested to be non-stereoselective, this is because OR-inactive (+)-isomers of opioids have been shown to activate or to inhibit TLR4 signalling, although there is some controversy in the literature. While some opioids can bind to the lipopolysaccharide (LPS)-binding cleft of the Myeloid Differentiation factor 2 (MD-2) co-receptor, pharmacological characterisation of the inhibition of opioids on LPS activation of TLR4 indicates a noncompetitive mechanism. In addition to a direct interaction at the receptor, opioids affect NF-κB activation downstream of both TLR4 and opioid receptors and modulate TLR4 expression, leading to a range of in vivo outcomes. Here, we review the literature reporting the activity of opioids at TLR4, its proposed mechanism(s), and the complex functional consequences of this interaction.