Inhibition of cytosolic phospholipase A(2) alpha protects against focal ischemic brain damage in mice

TREM1 promotes neuroinflammation after traumatic brain injury in rats: Possible involvement of ERK/cPLA2 signalling pathway

The neuroinflammatory response promotes secondary brain injury after traumatic brain injury (TBI). Triggering receptor expressed on myeloid cells 1 (TREM1) is a key regulator of inflammation. However, the role of TREM1 in TBI is poorly studied. The purpose of this study was to investigate the role of TREM1 in TBI and the possible underlying mechanism. We found that the protein expression of TREM1 significantly increased after TBI in rats, and the TREM1 protein localized to microglia. Inhibition of the TREM1 protein with LP17 significantly blocked ERK phosphorylation and reduced cytoplasmic phospholipase A2 (cPLA2) protein expression and phosphorylation. In addition, LP17-mediated TREM1 inhibition significantly reduced the protein expression of iNOS and increased the protein expression of Arg1. Moreover, after TREM1 was inhibited, the secretion of the proinflammatory factors TNF-α and IL-1β was significantly reduced, while the secretion of the anti-inflammatory factors IL-4 and IL-10 was significantly increased. Additionally, inhibition of TREM1 by LP17 significantly reduced neuronal apoptosis and ameliorated nerve dysfunction in TBI model rats. In conclusion, our findings suggest that TREM1 enhances neuroinflammation and promotes neuronal apoptosis after TBI, and these effects may be partly mediated via the ERK/cPLA2 signalling pathway.

Interactions Between Ferroptosis and Oxidative Stress in Ischemic Stroke

Ischemic stroke is a devastating condition that occurs due to the interruption of blood flow to the brain, resulting in a range of cellular and molecular changes. In recent years, there has been growing interest in the role of ferroptosis, a newly identified form of regulated cell death, in ischemic stroke. Ferroptosis is driven by the accumulation of lipid peroxides and is characterized by the loss of membrane integrity. Additionally, oxidative stress, which refers to an imbalance between prooxidants and antioxidants, is a hallmark of ischemic stroke and significantly contributes to the pathogenesis of the disease. In this review, we explore the interactions between ferroptosis and oxidative stress in ischemic stroke. We examine the underlying mechanisms through which oxidative stress induces ferroptosis and how ferroptosis, in turn, exacerbates oxidative stress. Furthermore, we discuss potential therapeutic strategies that target both ferroptosis and oxidative stress in the treatment of ischemic stroke. Overall, this review highlights the complex interplay between ferroptosis and oxidative stress in ischemic stroke and underscores the need for further research to identify novel therapeutic targets for this condition.

Thrombin induces ACSL4-dependent ferroptosis during cerebral ischemia/reperfusion

Ischemic stroke represents a significant danger to human beings, especially the elderly. Interventions are only available to remove the clot, and the mechanism of neuronal death during ischemic stroke is still in debate. Ferroptosis is increasingly appreciated as a mechanism of cell death after ischemia in various organs. Here we report that the serine protease, thrombin, instigates ferroptotic signaling by promoting arachidonic acid mobilization and subsequent esterification by the ferroptotic gene, acyl-CoA synthetase long-chain family member 4 (ACSL4). An unbiased multi-omics approach identified thrombin and ACSL4 genes/proteins, and their pro-ferroptotic phosphatidylethanolamine lipid products, as prominently altered upon the middle cerebral artery occlusion in rodents. Genetically or pharmacologically inhibiting multiple points in this pathway attenuated outcomes of models of ischemia in vitro and in vivo. Therefore, the thrombin-ACSL4 axis may be a key therapeutic target to ameliorate ferroptotic neuronal injury during ischemic stroke.

The role of lipid-based signalling in wound healing and senescence

Lipid-based signalling modulates several cellular processes and intercellular communication during wound healing and tissue regeneration. Bioactive lipids include but are not limited to the diverse group of eicosanoids, phospholipids, and extracellular vesicles and mediate the attraction of immune cells, initiation of inflammatory responses, and their resolution. In aged individuals, wound healing and tissue regeneration are greatly impaired, resulting in a delayed healing process and non-healing wounds. Senescent cells accumulate with age in vivo, preferably at sites implicated in age-associated pathologies and their elimination was shown to alleviate many age-associated diseases and disorders. In contrast to these findings, the transient presence of senescent cells in the process of wound healing exerts beneficial effects and limits fibrosis. Hence, clearance of senescent cells during wound healing was repeatedly shown to delay wound closure in vivo. Recent findings established a dysregulated synthesis of eicosanoids, phospholipids and extracellular vesicles as part of the senescent phenotype. This intriguing connection between cellular senescence, lipid-based signalling, and the process of wound healing and tissue regeneration prompts us to compile the current knowledge in this review and propose future directions for investigation.

Mechanisms of neuronal cell death in ischemic stroke and their therapeutic implications

Ischemic stroke caused by arterial occlusion is the most common type of stroke, which is among the most frequent causes of disability and death worldwide. Current treatment approaches involve achieving rapid reperfusion either pharmacologically or surgically, both of which are time-sensitive; moreover, blood flow recanalization often causes ischemia/reperfusion injury. However, even though neuroprotective intervention is urgently needed in the event of stroke, the exact mechanisms of neuronal death during ischemic stroke are still unclear, and consequently, the capacity for drug development has remained limited. Multiple cell death pathways are implicated in the pathogenesis of ischemic stroke. Here, we have reviewed these potential neuronal death pathways, including intrinsic and extrinsic apoptosis, necroptosis, autophagy, ferroptosis, parthanatos, phagoptosis, and pyroptosis. We have also reviewed the latest results of pharmacological studies on ischemic stroke and summarized emerging drug targets with a focus on clinical trials. These observations may help to further understand the pathological events in ischemic stroke and bridge the gap between basic and translational research to reveal novel neuroprotective interventions.

S1PR2 antagonist alleviates oxidative stress-enhanced brain endothelial permeability by attenuating p38 and Erk1/2-dependent cPLA2 phosphorylation

Both sphingosine-1-phosphate receptor-2 (S1PR2) and cytosolic phospholipase A₂ (cPLA₂) are implicated in the disruption of cerebrovascular integrity in experimental stroke. However, the role of S1PR2 in induction of cPLA₂ phosphorylation during cerebral ischemia-induced endothelial dysfunction remains unknown. This study investigated the effect of S1PR2 blockade on oxidative stress-induced cerebrovascular endothelial barrier impairment and explored the possible mechanisms. In bEnd3 cells, cPLA₂ inhibitor CAY10502 as well as S1PR2 antagonist JTE013 profoundly suppressed hydrogen peroxide (H₂O₂)-induced changes of paracellular permeability and ZO-1 localization. Besides p38, extracellular signal-regulated kinase (Erk) 1/2 is required for H₂O₂-increased cPLA₂ phosphorylation and endothelial permeability. Pharmacological and genetic inhibition of S1PR2 significantly suppressed their phosphorylation in response to H₂O₂. Especially lentivirus-mediated knockdown of S1PR2 inhibited H₂O₂-induced ZO-1 redistribution and paracellular hyperpermeability. Using the permanent middle cerebral artery occlusion (pMCAO) mouse model, we found JTE013 pretreatment markedly reduced Evans blue dye (EBD) extravasation and reversed the decrease in VE-cadherin, occludin, claudin-5 and CD31 expression in infarcted hemisphere. Lentivirus-mediated S1PR2 knockdown also attenuated EBD extravasation. Furthermore, JTE013 pretreatment attenuated neurological deficit, brain edema and infarction volume. Therefore, our findings suggest the protective effect of JTE013 on brain endothelial barrier integrity is likely mediated by suppressing p38 and Erk1/2-dependent cPLA₂ phosphorylation under oxidative stress.

Adenoviruses-mediated RNA interference targeting cytosolic phospholipase A2α attenuates focal ischemic brain damage in mice

Cerebral ischemia injury is a clinical, frequently occurring disease, which causes a heavy burden on society and families. It has been demonstrated that cytosolic phospholipase A2α (cPLA2α) is significant in neurological injury caused by ischemic brain injury, and inhibition of cPLA2α may reduce stroke injury. In the present study, the role of cPLA2α was investigated in a mouse model of middle cerebral artery occlusion and/or reperfusion (MCAO/R) using an effective cPLA2α inhibitor and adenoviruses-mediated RNA interference. The most effective recombinant adenovirus encoding cPLA2α small interfering RNA (pAd-siRNA-cPLA2α) was constructed and selected. MCAO/R surgery is used to construct the model of focal ischemic brain damage in mice. Adenoviruses-mediated RNA interference targeting cPLA2α was administered by stereotactic surgery 2 h before the MCAO/R. The expression/activity of cPLA2α and cPLA2α-derived injurious lipid mediators was assessed. pAd-siRNA-cPLA2α-treated animals (RNA interference; RNAi group) were compared with pAd-siRNA-control-treated animals (negative group) with regard to neurological deficit, motor function, pathological changes, apoptosis, and infarct volume. The RNAi group animals reduced the expression level of cPLA2α, as determined by western blotting and reverse transcription-quantitative polymerase chain reaction, the improvement of locomotor function was evaluated by rotarod test, and the decrease of apoptosis was evaluated by terminal deoxynucleotidyl transferase dUTP nick end-labeling staining. The decreased infarct areas were evaluated by 2,3,5-triphenyltetrazolium chloride staining. The expression levels of prostaglandin E2, leukotrienes B4, lysophosphatidylcholine and free fatty acids were reduced in the RNAi group when compared with the negative control group. Thus, the data indicates that the expression level of cPLA2α was effectively controlled by pAd-siRNA-cPLA2α treatment. pAd-siRNA-cPLA2α treatment, in reducing the levels of inflammatory factors, neurological deficit and tissue damage, represents an effective potential therapeutic strategy. pAd-siRNA-cPLA2α reduces cPLA2α expression levels with long-term efficacy, thereby improving functional deficits and effectively attenuating ischemic brain damage. Thus, pAd-siRNA-cPLA2α shows potential value for therapeutic evaluation in ischemic brain damage.

Botanical Polyphenols Mitigate Microglial Activation and Microglia-Induced Neurotoxicity: Role of Cytosolic Phospholipase A2

Microglia play a significant role in the generation and propagation of oxidative/nitrosative stress, and are the basis of neuroinflammatory responses in the central nervous system. Upon stimulation by endotoxins such as lipopolysaccharides (LPS), these cells release pro-inflammatory factors which can exert harmful effects on surrounding neurons, leading to secondary neuronal damage and cell death. Our previous studies demonstrated the effects of botanical polyphenols to mitigate inflammatory responses induced by LPS, and highlighted an important role for cytosolic phospholipase A2 (cPLA2) upstream of the pro-inflammatory pathways (Chuang et al. in J Neuroinflammation 12(1):199, 2015. doi: 10.1186/s12974-015-0419-0 ). In this study, we investigate the action of botanical compounds and assess whether suppression of cPLA2 in microglia is involved in the neurotoxic effects on neurons. Differentiated SH-SY5Y neuroblastoma cells were used to test the neurotoxicity of conditioned medium from stimulated microglial cells, and WST-1 assay was used to assess for the cell viability of SH-SY5Y cells. Botanicals such as quercetin and honokiol (but not cyanidin-3-O-glucoside, 3CG) were effective in inhibiting LPS-induced nitric oxide (NO) production and phosphorylation of cPLA2. Conditioned medium from BV-2 cells stimulated with LPS or IFNγ caused neurotoxicity to SH-SY5Y cells. Decrease in cell viability could be ameliorated by pharmacological inhibitors for cPLA2 as well as by down-regulating cPLA2 with siRNA. Botanicals effective in inhibition of LPS-induced NO and cPLA2 phosphorylation were also effective in ameliorating microglial-induced neurotoxicity. Results demonstrated cytotoxic factors from activated microglial cells to cause damaging effects to neurons and potential use of botanical polyphenols to ameliorate the neurotoxic effects.

Cytosolic phospholipase A2 plays a crucial role in ROS/NO signaling during microglial activation through the lipoxygenase pathway

Background

Oxidative stress and inflammation are important factors contributing to the pathophysiology of numerous neurological disorders, including Alzheimer’s disease, Parkinson’s disease, acute stroke, and infections of the brain. There is well-established evidence that proinflammatory cytokines and glutamate, as well as reactive oxygen species (ROS) and nitric oxide (NO), are produced upon microglia activation, and these are important factors contributing to inflammatory responses and cytotoxic damage to surrounding neurons and neighboring cells. Microglial cells express relatively high levels of cytosolic phospholipase A₂ (cPLA₂), an enzyme known to regulate membrane phospholipid homeostasis and release of arachidonic acid (AA) for synthesis of eicosanoids. The goal for this study is to elucidate the role of cPLA₂IV in mediating the oxidative and inflammatory responses in microglial cells.

Methods

Experiments involved primary microglia cells isolated from transgenic mice deficient in cPLA₂α or iPLA₂β, as well as murine immortalized BV-2 microglial cells. Inhibitors of cPLA₂/iPLA₂/cyclooxygenase (COX)/lipoxygenase (LOX) were used in BV-2 microglial cell line. siRNA transfection was employed to knockdown cPLA₂ expression in BV-2 cells. Griess reaction protocol was used to determine NO concentration, and CM-H2DCF-DA was used to detect ROS production in primary microglia and BV-2 cells. WST-1 assay was used to assess cell viability. Western blotting was used to assess protein expression levels. Immunocytochemical staining for phalloidin against F-actin was used to demonstrate cell morphology.

Results

In both primary and BV-2 microglial cells, stimulation with lipopolysaccharide (LPS) or interferon gamma (IFNγ) resulted in a time-dependent increase in phosphorylation of cPLA₂ together with ERK1/2. In BV-2 cells, LPS- and IFNγ-induced ROS and NO production was inhibited by arachidonyl trifluoromethyl ketone (AACOCF3) and pyrrophenone as well as RNA interference, but not BEL, suggesting a link between cPLA₂, and not iPLA₂, on LPS/IFNγ-induced nitrosative and oxidative stress in microglial cells. Primary microglial cells isolated from cPLA₂α-deficient mice generated significantly less NO and ROS as compared with the wild-type mice. Microglia isolated from iPLA₂β-deficient mice did not show a decrease in LPS-induced NO and ROS production. LPS/IFNγ induced morphological changes in primary microglia, and these changes were mitigated by AACOCF3. Interestingly, despite that LPS and IFNγ induced an increase in phospho-cPLA₂ and prostaglandin E2 (PGE2) release, LPS- and IFNγ-induced NO and ROS production were not altered by the COX-1/2 inhibitor but were suppressed by the LOX-12 and LOX-15 inhibitors instead.

Conclusions

In summary, the results in this study demonstrated the role of cPLA₂ in microglial activation with metabolic links to oxidative and inflammatory responses, and this was in part regulated by the AA metabolic pathways, namely the LOXs. Further studies with targeted inhibition of cPLA₂/LOX in microglia during neuroinflammatory conditions can be valuable to investigate the therapeutic potential in ameliorating neurological disease pathology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0419-0) contains supplementary material, which is available to authorized users.

Synthetic and natural inhibitors of phospholipases A2: their importance for understanding and treatment of neurological disorders

Phospholipases A2 (PLA2) are a diverse group of enzymes that hydrolyze membrane phospholipids into arachidonic acid and lysophospholipids. Arachidonic acid is metabolized to eicosanoids (prostaglandins, leukotrienes, thromboxanes), and lysophospholipids are converted to platelet-activating factors. These lipid mediators play critical roles in the initiation, maintenance, and modulation of neuroinflammation and oxidative stress. Neurological disorders including excitotoxicity; traumatic nerve and brain injury; cerebral ischemia; Alzheimer's disease; Parkinson's disease; multiple sclerosis; experimental allergic encephalitis; pain; depression; bipolar disorder; schizophrenia; and autism are characterized by oxidative stress, inflammatory reactions, alterations in phospholipid metabolism, accumulation of lipid peroxides, and increased activities of brain phospholipase A2 isoforms. Several old and new synthetic inhibitors of PLA2, including fatty acid trifluoromethyl ketones; methyl arachidonyl fluorophosphonate; bromoenol lactone; indole-based inhibitors; pyrrolidine-based inhibitors; amide inhibitors, 2-oxoamides; 1,3-disubstituted propan-2-ones and polyfluoroalkyl ketones as well as phytochemical based PLA2 inhibitors including curcumin, Ginkgo biloba and Centella asiatica extracts have been discovered and used for the treatment of neurological disorders in cell culture and animal model systems. The purpose of this review is to summarize information on selective and potent synthetic inhibitors of PLA2 as well as several PLA2 inhibitors from plants, for treatment of oxidative stress and neuroinflammation associated with the pathogenesis of neurological disorders.

Oral administration of cytosolic PLA2 inhibitor arachidonyl trifluoromethyl ketone ameliorates cauda equina compression injury in rats

BACKGROUND

Phospholipase A2 (PLA2)-derived proinflammatory lipid mediators such as prostaglandin E2 (PGE2), leukotrienes B4 (LTB4), lysophosphatidylcholine (LPC), and free fatty acids (FFA) are implicated in spinal cord injury (SCI) pathologies. Reducing the levels of these injurious bioactive lipid mediators is reported to ameliorate SCI. However, the specific role of the group IVA isoform of PLA2 cytosolic PLA2 (cPLA2) in lumbar spinal canal stenosis (LSS) due to cauda equina compression (CEC) injury is not clear. In this study, we investigated the role of cPLA2 in a rat model of CEC using a non-toxic cPLA2-preferential inhibitor, arachidonyl trifluoromethyl ketone (ATK).

METHODS

LSS was induced in adult female rats by CEC procedure using silicone blocks within the epidural spaces of L4 to L6 vertebrae. cPLA2 inhibitor ATK (7.5 mg/kg) was administered by oral gavage at 2 h following the CEC. cPLA2-derived injurious lipid mediators and the expression/activity of cPLA2, 5-lipoxygenase (5-LOX), and cyclooxygenase-2 (COX-2) were assessed. ATK-treated (CEC + ATK) were compared with vehicle-treated (CEC + VEH) animals in terms of myelin levels, pain threshold, and motor function.

RESULTS

ATK treatment of CEC animals reduced the phosphorylation of cPLA2 (pcPLA2) determined by Western blot, improved locomotor function evaluated by rotarod task, and reduced pain threshold evaluated by mechanical hyperalgesia method. Levels of FFA and LPC, along with PGE2 and LTB4, were reduced in CEC + ATK compared with CEC + VEH group. However, ATK treatment reduced neither the activity/expression of 5-LOX nor the expression of COX-2 in CEC + VEH animals. Increased cPLA2 activity in the spinal cord from CEC + VEH animals correlated well with decreased spinal cord as well as cauda equina fiber myelin levels, which were restored after ATK treatment.

CONCLUSION

The data indicate that cPLA2 activity plays a significant role in tissue injury and pain after LSS. Reducing the levels of proinflammatory and tissue damaging eicosanoids and the deleterious lipid mediator LPC shows therapeutic potential. ATK inhibits cPLA2 activity, thereby decreasing the levels of injurious lipid mediators, reducing pain, improving functional deficits, and conferring protection against LSS injury. Thus, it shows potential for preclinical evaluation in LSS.

NADPH oxidase-2: linking glucose, acidosis, and excitotoxicity in stroke

SIGNIFICANCE

Neuronal superoxide production contributes to cell death in both glutamate excitotoxicity and brain ischemia (stroke). NADPH oxidase-2 (NOX2) is the major source of neuronal superoxide production in these settings, and regulation of NOX2 activity can thereby influence outcome in stroke.

RECENT ADVANCES

Reduced NOX2 activity can rescue cells from oxidative stress and cell death that otherwise occur in excitotoxicity and ischemia. NOX2 activity is regulated by several factors previously shown to affect outcome in stroke, including glucose availability, intracellular pH, protein kinase ζ/δ, casein kinase 2, phosphoinositide-3-kinase, Rac1/2, and phospholipase A2. The newly identified functions of these factors as regulators of NOX2 activity suggest alternative mechanisms for their effects on ischemic brain injury.

CRITICAL ISSUES

Key aspects of these regulatory influences remain unresolved, including the mechanisms by which rac1 and phospholipase activities are coupled to N-methyl-D-aspartate (NMDA) receptors, and whether superoxide production by NOX2 triggers subsequent superoxide production by mitochondria.

FUTURE DIRECTIONS

It will be important to establish whether interventions targeting the signaling pathways linking NMDA receptors to NOX2 in brain ischemia can provide a greater neuroprotective efficacy or a longer time window to treatment than provided by NMDA receptor blockade alone. It will likewise be important to determine whether dissociating superoxide production from the other signaling events initiated by NMDA receptors can mitigate the deleterious effects of NMDA receptor blockade.

Role of cytosolic phospholipase A2 in oxidative and inflammatory signaling pathways in different cell types in the central nervous system

Phospholipases A(2) (PLA(2)s) are important enzymes for the metabolism of fatty acids in membrane phospholipids. Among the three major classes of PLA(2)s in the mammalian system, the group IV calcium-dependent cytosolic PLA(2) alpha (cPLA(2)α) has received the most attention because it is widely expressed in nearly all mammalian cells and its active participation in cell metabolism. Besides Ca(2+) binding to its C2 domain, this enzyme can undergo a number of cell-specific post-translational modifications, including phosphorylation by protein kinases, S-nitrosylation through interaction with nitric oxide (NO), as well as interaction with other proteins and lipid molecules. Hydrolysis of phospholipids by cPLA(2) yields two important lipid mediators, arachidonic acid (AA) and lysophospholipids. While AA is known to serve as a substrate for cyclooxygenases and lipoxygenases, which are enzymes for the synthesis of eicosanoids and leukotrienes, lysophospholipids are known to possess detergent-like properties capable of altering microdomains of cell membranes. An important feature of cPLA(2) is its link to cell surface receptors that stimulate signaling pathways associated with activation of protein kinases and production of reactive oxygen species (ROS). In the central nervous system (CNS), cPLA(2) activation has been implicated in neuronal excitation, synaptic secretion, apoptosis, cell-cell interaction, cognitive and behavioral function, oxidative-nitrosative stress, and inflammatory responses that underline the pathogenesis of a number of neurodegenerative diseases. However, the types of extracellular agonists that target intracellular signaling pathways leading to cPLA(2) activation among different cell types and under different physiological and pathological conditions have not been investigated in detail. In this review, special emphasis is given to metabolic events linking cPLA(2) to activation in neurons, astrocytes, microglial cells, and cerebrovascular cells. Understanding the molecular mechanism(s) for regulation of this enzyme is deemed important in the development of new therapeutic targets for the treatment and prevention of neurodegenerative diseases.

Force spectroscopy measurements show that cortical neurons exposed to excitotoxic agonists stiffen before showing evidence of bleb damage

In ischemic and traumatic brain injury, hyperactivated glutamate (N-methyl-D-aspartic acid, NMDA) and sodium (Nav) channels trigger excitotoxic neuron death. Na⁺, Ca⁺⁺ and H₂O influx into affected neurons elicits swelling (increased cell volume) and pathological blebbing (disassociation of the plasma membrane’s bilayer from its spectrin-actomyosin matrix). Though usually conflated in injured tissue, cell swelling and blebbing are distinct processes. Around an injury core, salvageable neurons could be mildly swollen without yet having suffered the bleb-type membrane damage that, by rendering channels leaky and pumps dysfunctional, exacerbates the excitotoxic positive feedback spiral. Recognizing when neuronal inflation signifies non-lethal osmotic swelling versus blebbing should further efforts to salvage injury-penumbra neurons. To assess whether the mechanical properties of osmotically-swollen versus excitotoxically-blebbing neurons might be cytomechanically distinguishable, we measured cortical neuron elasticity (gauged via atomic force microscopy (AFM)-based force spectroscopy) upon brief exposure to hypotonicity or to excitotoxic agonists (glutamate and Nav channel activators, NMDA and veratridine). Though unperturbed by solution exchange per se, elasticity increased abruptly with hypotonicity, with NMDA and with veratridine. Neurons then invariably softened towards or below the pre-treatment level, sometimes starting before the washout. The initial channel-mediated stiffening bespeaks an abrupt elevation of hydrostatic pressure linked to NMDA or Nav channel-mediated ion/H₂O fluxes, together with increased [Ca⁺⁺]_int-mediated submembrane actomyosin contractility. The subsequent softening to below-control levels is consistent with the onset of a lethal level of bleb damage. These findings indicate that dissection/identification of molecular events during the excitotoxic transition from stiff/swollen to soft/blebbing is warranted and should be feasible.