Roles of phospholipases A2 in brain cell and tissue injury associated with ischemia and excitotoxicity

Underlying Mechanism of Lysosomal Membrane Permeabilization in CNS Injury: A Literature Review

Lysosomes play a crucial role in various intracellular pathways as their final destination. Various stressors, whether mild or severe, can induce lysosomal membrane permeabilization (LMP), resulting in the release of lysosomal enzymes into the cytoplasm. LMP not only plays a pivotal role in various cellular events but also significantly contributes to programmed cell death (PCD). Previous research has demonstrated the participation of LMP in central nervous system (CNS) injuries, including traumatic brain injury (TBI), spinal cord injury (SCI), subarachnoid hemorrhage (SAH), and hypoxic-ischemic encephalopathy (HIE). However, the mechanisms underlying LMP in CNS injuries are poorly understood. The occurrence of LMP leads to the activation of inflammatory pathways, increased levels of oxidative stress, and PCD. Herein, we present a comprehensive overview of the latest findings regarding LMP and highlight its functions in cellular events and PCDs (lysosome-dependent cell death, apoptosis, pyroptosis, ferroptosis, and autophagy). In addition, we consolidate the most recent insights into LMP in CNS injury by summarizing and exploring the latest advances. We also review potential therapeutic strategies that aim to preserve LMP or inhibit the release of enzymes from lysosomes to alleviate the consequences of LMP in CNS injury. A better understanding of the role that LMP plays in CNS injury may facilitate the development of strategic treatment options for CNS injury.

BACE1 and SCD1 are associated with neurodegeneration

Introduction

Proteolytic processing of amyloid protein precursor by β-site secretase enzyme (BACE1) is dependent on the cellular lipid composition and is affected by endomembrane trafficking in dementia and Alzheimer's disease (AD). Stearoyl-CoA desaturase 1 (SCD1) is responsible for the synthesis of fatty acid monounsaturation (MUFAs), whose accumulation is strongly associated with cognitive dysfunction.

Methods

In this study, we analyzed the relationship between BACE1 and SCD1 in vivo and in vitro neurodegenerative models and their association in familial AD (FAD), sporadic AD (SAD), and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) using microscopy, biochemical, and mass SPECT approach.

Results

Our findings showed that BACE1 and SCD1 immunoreactivities were increased and colocalized in astrocytes of the hippocampus in a rat model of global cerebral ischemia (2-VO). A synergistic effect of double BACE1/SCD1 silencing on the recovery of motor and cognitive functions was obtained. This neuroprotective regulation involved the segregation of phospholipids (PLs) associated with polyunsaturated fatty acids in the hippocampus, cerebrospinal fluid, and serum. The double silencing in the sham and ischemic groups was stronger in the serum, inducing an inverse ratio between total phosphatydilcholine (PC) and lysophosphatidylcholine (LPC), represented mainly by the reduction of PC 38:4 and PC 36:4 and an increase in LPC 16:0 and LPC 18:0. Furthermore, PC 38:4 and PC:36:4 levels augmented in pathological conditions in in vitro AD models. BACE1 and SCD1 increases were confirmed in the hippocampus of FAD, SAD, and CADASIL.

Conclusion

Therefore, the findings suggest a novel convergence of BACE-1 and SCD1 in neurodegeneration, related to pro-inflammatory phospholipids.

Inhibition of Cytosolic Phospholipase A2 Has Neuroprotective Effects on Motoneuron and Muscle Atrophy after Spinal Cord Injury

Surviving motoneurons undergo dendritic atrophy after spinal cord injury (SCI), suggesting an important therapeutic target for neuroprotective strategies to improve recovery of function after SCI. Our previous studies showed that cytosolic phospholipase A₂ (PLA₂) may play an important role in the pathogenesis of SCI. In the present study, we investigated whether blocking cytosolic PLA₂ (cPLA₂) pharmacologically with arachidonyl trifluoromethyl ketone (ATK) or genetically using cPLA₂ knockout (KO) mice attenuates motoneuron atrophy after SCI. C57BL/6 mice received either sham or contusive SCI at the T10 level. At 30 min after SCI, mice were treated with ATK or vehicle. Four weeks later, motoneurons innervating the vastus lateralis muscle of the quadriceps were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Soma volume, motoneuron number, lesion volume, and tissue sparing were also assessed, as were muscle weight, fiber cross-sectional area, and motor endplate size and density. ATK administration reduced percent lesion volume and increased percent volume of spared white matter, compared to the vehicle-treated control animals. SCI with or without ATK treatment had no effect on the number or soma volume of quadriceps motoneurons. However, SCI resulted in a decrease in dendritic length of quadriceps motoneurons in untreated animals, and this decrease was completely prevented by treatment with ATK. Similarly, vastus lateralis muscle weights of untreated SCI animals were smaller than those of sham surgery controls, and these reductions were prevented by ATK treatment. No effects on fiber cross-sectional areas, motor endplate area, or density were observed across treatment groups. Remarkably, genetically deleting cPLA₂ in cPLA₂ KO mice attenuated dendritic atrophy after SCI. These findings suggest that, after SCI, cord tissue damage and regressive changes in motoneuron and muscle morphology can be reduced by inhibition of cPLA₂, further supporting a role for cPLA₂ as a neurotherapeutic target for SCI treatment.

Translationally Controlled Tumor Protein Stimulates Dopamine Release from PC12 Cells via Ca2+-Independent Phospholipase A₂ Pathways

The translationally controlled tumor protein (TCTP), initially identified as a tumor- and growth-related protein, is also known as a histamine-releasing factor (HRF). TCTP is widely distributed in the neuronal systems, but its function is largely uncharacterized. Here, we report a novel function of TCTP in the neurotransmitter release from a neurosecretory, pheochromocytoma (PC12) cells. Treatment with recombinant TCTP (rTCTP) enhanced both basal and depolarization (50 mM KCl)-evoked [³H]dopamine release in concentration- and time-dependent manners. Interestingly, even though rTCTP induced the increase in intracellular calcium levels ([Ca²⁺]_i), the rTCTP-driven effect on dopamine release was mediated by a Ca²⁺-independent pathway, as evidenced by the fact that Ca²⁺-modulating agents such as Ca²⁺ chelators and a voltage-gated L-type Ca²⁺-channel blocker did not produce any changes in rTCTP-evoked dopamine release. In a study to investigate the involvement of phospholipase A₂ (PLA₂) in rTCTP-induced dopamine release, the inhibitor for Ca²⁺-independent PLA₂ (iPLA₂) produced a significant inhibitory effect on rTCTP-induced dopamine release, whereas this release was not significantly inhibited by Ca²⁺-dependent cytosolic PLA₂ (cPLA₂) and secretory PLA₂ (sPLA₂) inhibitors. We found that rTCTP-induced dopamine release from neuronal PC12 cells was modulated by a Ca²⁺-independent mechanism that involved PLA₂ in the process, suggesting the regulatory role of TCTP in the neuronal functions.

Brain Cytosolic Phospholipase A2: Localization, Role, and Involvement in Neurological Diseases

Cytosolic phospholipase A2 (cPLA2) hydrolyzes the arachidonoyl group from the sn-2 position of glycerophospholipids generating arachidonic acid and lysophospholipids. The products of the cPLA2-catalyzed reaction act as second messengers themselves or further metabolize to eicosanoids, platelet activating factor, and lysophosphatidic acid. cPLA2 has not been purified from brain tissue. Immunocytochemical studies have indicated that cPLA2 is expressed in neurons and astrocytes. The hindbrain and spinal cord contain dense immunoreactivity for cPLA2. Activity and immunoreactivity of cPLA2 are markedly increased in ischemia, Alzheimer’s disease, and kainic acid neurotoxicity. This increase in cPLA2 activity and immunoreactivity is accompanied by marked alterations in neural membrane phospholipid composition and the accumulation of lipid peroxides and eicosanoids. At present, it is not known whether the increased activity and immunoreactivity of cPLA2 in neural trauma (e.g., in ischemia) and neurodegenerative disease (Alzheimer’s disease) is the cause or effect of neurodegeneration. Recent studies on the role of this enzyme in brain tissue suggest that cPLA2 may be involved in synaptic plasticity, generation of second messengers, axon regeneration, and neurodegeneration.

Cytosolic phospholipase A2 protein as a novel therapeutic target for spinal cord injury

Objective

The objective of this study was to investigate whether cytosolic phospholipase A₂ (cPLA₂), an important isoform of PLA₂ that mediates the release of arachidonic acid, plays a role in the pathogenesis of spinal cord injury (SCI).

Methods

A combination of molecular, histological, immunohistochemical, and behavioral assessments were used to test whether blocking cPLA₂ activation pharmacologically or genetically reduced cell death, protected spinal cord tissue, and improved behavioral recovery after a contusive SCI performed at the 10th thoracic level in adult mice.

Results

SCI significantly increased cPLA₂ expression and activation. Activated cPLA₂ was localized mainly in neurons and oligodendrocytes. Notably, the SCI-induced cPLA₂ activation was mediated by the extracellular signal-regulated kinase signaling pathway. In vitro, activation of cPLA₂ by ceramide-1-phosphate or A23187 induced spinal neuronal death, which was substantially reversed by arachidonyl trifluoromethyl ketone, a cPLA₂ inhibitor. Remarkably, blocking cPLA₂ pharmacologically at 30 minutes postinjury or genetically deleting cPLA₂ in mice ameliorated motor deficits, and reduced cell loss and tissue damage after SCI.

Interpretation

cPLA₂ may play a key role in the pathogenesis of SCI, at least in the C57BL/6 mouse, and as such could be an attractive therapeutic target for ameliorating secondary tissue damage and promoting recovery of function after SCI.

Characterizing phospholipase A2-induced spinal cord injury-a comparison with contusive spinal cord injury in adult rats

To assess whether phospholipase A2 (PLA2) plays a role in the pathogenesis of spinal cord injury (SCI), we compared lesions either induced by PLA2 alone or by a contusive SCI. At 24-h post-injury, both methods induced a focal hemorrhagic pathology. The PLA2 injury was mainly confined within the ventrolateral white matter, whereas the contusion injury widely affected both the gray and white matter. A prominent difference between the two models was that PLA2 induced a massive demyelination with axons remaining in the lesion area, whereas the contusion injury induced axonal damage and myelin breakdown. At 4 weeks, no cavitation was found within the PLA2 lesion, and numerous axons were myelinated by host-migrated Schwann cells. Among them, 45% of animals had early transcranial magnetic motor-evoked potential (tcMMEP) responses. In contrast, the contusive SCI induced a typical centralized cavity with reactive astrocytes forming a glial border. Only 15% of rats had early tcMMEP responses after the contusion. BBB scores were similarly reduced in both models. Our study indicates that PLA2 may play a unique role in mediating secondary SCI likely by targeting glial cells, particularly those of oligodendrocytes. This lesion model could also be used for studying demyelination and remyelination in the injured spinal cord associated with PLA2-mediated secondary SCI.

Anti-neuroinflammatory and neurotrophic effects of combined therapy with annexin II and Reg-2 on injured spinal cord

The present study was designed to investigate the neuroprotective effects of Ca(2+)-dependent phospholipid-binding protein annexin II and a secreted protein Reg-2 (regeneration gene protein 2) in spinal cord injury (SCI) model produced by contusion SCI at T(9) using the weight drop method. The agents were delivered intrathecally with Alzet miniosmotic pumps. We found that annexin II and Reg-2 remarkably reduced neuronal death, attenuated tissue damage and alleviated detrimental inflammation in vivo; meanwhile, a significant increase in white matter sparing and myelination area was observed. The propriospinal axons and long-distance supraspinal pathways were protected by the treatments as revealed by retrograde tracing. Basso Beattie Bresnahan locomotor rating scores also revealed a measurable behavioral improvement. However, no evident behavioral improvements in locomotor performance were achieved by the combined treatment with annexin II and Reg-2, compared with the separate treatment with annexin II and Reg-2.

Lipid metabolites in the pathogenesis and treatment of neovascular eye disease

Lipids and lipid metabolites have long been known to play biological roles that go beyond energy storage and membrane structure. In age-related macular degeneration and diabetes, for example, dysregulation of lipid metabolism is closely associated with disease onset and progression. At the same time, some lipids and their metabolites can exert beneficial effects in the same disorders. This review summarises our current knowledge of the contributions of lipids to both the pathogenesis and treatment of neovascular eye disease. The clinical entities covered are exudative age-related macular degeneration, diabetic retinopathy and retinopathy of prematurity, with a special emphasis on the potential therapeutic effects of ω3- (also known as n-3) polyunsaturated fatty acids.

Phospholipase A2 and its molecular mechanism after spinal cord injury

Phospholipases A(2) (PLA(2)s) are a diverse family of lipolytic enzymes which hydrolyze the acyl bond at the sn-2 position of glycerophospholipids to produce free fatty acids and lysophospholipids. These products are precursors of bioactive eicosanoids and platelet-activating factor which have been implicated in pathological states of numerous acute and chronic neurological disorders. To date, more than 27 isoforms of PLA(2) have been found in the mammalian system which can be classified into four major categories: secretory PLA(2), cytosolic PLA(2), Ca(2+)-independent PLA(2), and platelet-activating factor acetylhydrolases. Multiple isoforms of PLA(2) are found in the mammalian spinal cord. Under physiological conditions, PLA(2)s are involved in diverse cellular responses, including phospholipid digestion and metabolism, host defense, and signal transduction. However, under pathological situations, increased PLA(2) activity, excessive production of free fatty acids and their metabolites may lead to the loss of membrane integrity, inflammation, oxidative stress, and subsequent neuronal injury. There is emerging evidence that PLA(2) plays a key role in the secondary injury process after traumatic spinal cord injury. This review outlines the current knowledge of the PLA(2) in the spinal cord with an emphasis being placed on the possible roles of PLA(2) in mediating the secondary SCI.

Role of secretory phospholipase a(2) in CNS inflammation: implications in traumatic spinal cord injury

Secretory phospholipases A(2) (sPLA(2)s) are a subfamily of lipolytic enzymes which hydrolyze the acyl bond at the sn-2 position of glycerophospholipids to produce free fatty acids and lysophospholipids. These products are precursors of bioactive eicosanoids and platelet-activating factor (PAF). The hydrolysis of membrane phospholipids by PLA(2) is a rate-limiting step for generation of eicosanoids and PAF. To date, more than 10 isozymes of sPLA(2) have been found in the mammalian central nervous system (CNS). Under physiological conditions, sPLA(2)s are involved in diverse cellular responses, including host defense, phospholipid digestion and metabolism. However, under pathological situations, increased sPLA(2) activity and excessive production of free fatty acids and their metabolites may lead to inflammation, loss of membrane integrity, oxidative stress, and subsequent tissue injury. Emerging evidence suggests that sPLA(2) plays a role in the secondary injury process after traumatic or ischemic injuries in the brain and spinal cord. Importantly, sPLA(2) may act as a convergence molecule that mediates multiple key mechanisms involved in the secondary injury since it can be induced by multiple toxic factors such as inflammatory cytokines, free radicals, and excitatory amino acids, and its activation and metabolites can exacerbate the secondary injury. Blocking sPLA(2) action may represent a novel and efficient strategy to block multiple injury pathways associated with the CNS secondary injury. This review outlines the current knowledge of sPLA(2) in the CNS with emphasis placed on the possible roles of sPLA(2) in mediating CNS injuries, particularly the traumatic and ischemic injuries in the brain and spinal cord.

Role of Ca2+-independent phospholipase A2 in cell growth and signaling

Phospholipase A(2) (PLA(2)) are esterases that cleave glycerophospholipids to release fatty acids and lysophospholipids. Several studies demonstrate that PLA(2) regulate growth and signaling in several cell types. However, few of these studies have focused on Ca2+-independent phospholipase A(2) (iPLA(2) or Group VI PLA(2)). This class of PLA(2) was originally suggested to mediate phospholipid remodeling in several cell types including macrophages. As such, it was labeled as a housekeeping protein and thought not to play as significant of roles in cell growth as its older counterparts cytosolic PLA(2) (cPLA(2) or Group IV PLA(2)) and secretory PLA(2) (sPLA(2) or Groups I-III, V and IX-XIV PLA(2)). However, several recent studies demonstrate that iPLA(2) mediate cell growth, and do so by participating in signal transduction pathways that include epidermal growth factor receptors (EGFR), mitogen activated protein kinases (MAPK), mdm2, and even the tumor suppressor protein p53 and the cell cycle regulator p21. The exact mechanism by which iPLA(2) mediates these pathways are not known, but likely involve the generation of lipid signals such as arachidonic acid, lysophosphatidic acid (LPA) and lysophosphocholines (LPC). This review discusses the role of iPLA(2) in cell growth with special emphasis placed on their role in cell signaling. The putative lipid signals involved are also discussed.

Annexin A1 reduces inflammatory reaction and tissue damage through inhibition of phospholipase A2 activation in adult rats following spinal cord injury

Annexin A1 (ANXA1) has been suggested to be a mediator of the anti-inflammatory actions of glucocorticoids and more recently an endogenous neuroprotective agent. In the present study, we investigated the anti-inflammatory and neuroprotective effects of ANXA1 in a model of contusive spinal cord injury (SCI). Here we report that injections of ANXA1 (Ac 2-26) into the acutely injured spinal cord at 2 concentrations (5 and 20 microg) inhibited SCI-induced increases in phospholipase A2 and myeloperoxidase activities. In addition, ANXA1 administration reduced the expression of interleukin-1beta and activated caspase-3 at 24 hours, and glial fibrillary acidic protein at 4 weeks postinjury. Furthermore, ANXA1 administration significantly reversed phospholipase A2-induced spinal cord neuronal death in vitro and reduced tissue damage and increased white matter sparing in vivo, compared to the vehicle-treated controls. Fluorogold retrograde tracing showed that ANXA1 administration protected axons of long descending pathways at 6 weeks post-SCI. ANXA1 administration also significantly increased the number of animals that responded to transcranial magnetic motor-evoked potentials. However, no measurable behavioral improvement was found after these treatments. These results, particularly the improvements obtained in tissue sparing and electrophysiologic measures, suggest a neuroprotective effect of ANXA1.

Phospholipase A2 as targets for anti-cancer drugs

Phospholipase A(2) (PLA(2)) are esterases that cleave glycerophospholipids to release fatty acids and lysophospholipids. Inhibition of PLA(2) alters cancer cell growth and death in vitro and PLA(2) expression is increased in breast, lung, and prostate cancers compared to control tissues. Thus, PLA(2) may be novel targets for chemotherapeutics. However, PLA(2) are a diverse family of enzymes, encompassing 19 members. The selectivity of these individual PLA(2) for phospholipids varies, as does their location within the cell, and tissue expression. Thus, their role in cancer may also vary. This review summarizes the expression of individual PLA(2) in cancers, focuses on the potential mechanisms by which these esterases mediate carcinogenesis, and suggests that select PLA(2) isoforms may be targets for anti-cancer drugs.

A novel role of phospholipase A2in mediating spinal cord secondary injury

OBJECTIVE

To investigate whether phospholipase A2 (PLA2) plays a role in the pathogenesis of spinal cord injury (SCI).

METHODS

Biochemical, Western blot, histological, immunohistochemical, electron microscopic, electrophysiological, and behavior assessments were performed to investigate (1) SCI-induced PLA2 activity, expression, and cellular localization after a contusive SCI; and (2) the effects of exogenous PLA2 on spinal cord neuronal death in vitro and tissue damage, inflammation, and function in vivo.

RESULTS

After SCI, both PLA2 activity and cytosolic PLA2 expression increased significantly, with cytosolic PLA2 expression being localized mainly in neurons and oligodendrocytes. Both PLA2 and melittin, an activator of endogenous PLA2, induced spinal neuronal death in vitro, which was substantially reversed by mepacrine, a PLA2 inhibitor. When PLA2 or melittin was microinjected into the normal spinal cord, the former induced confined demyelination and latter diffuse tissue necrosis. Both injections induced inflammation, oxidation, and tissue damage, resulting in corresponding electrophysiological and behavioral impairments. Importantly, the PLA2-induced demyelination was significantly reversed by mepacrine.

INTERPRETATION

PLA2, increased significantly after SCI, may play a key role in mediating neuronal death and oligodendrocyte demyelination following SCI. Blocking PLA2 action may represent a novel repair strategy to reduce tissue damage and increase function after SCI.

Nitric oxide alters arachidonic acid turnover in brain cortex synaptoneurosomes

Nitric oxide (NO) and arachidonic acid (AA) and also its metabolites are very important inter- and intracellular second messengers. They are involved in mechanisms of learning and memory. However, liberated in excessive amount in brain ischemia, Parkinson and Alzheimer diseases they are responsible for cell degeneration and death. Previously, we could show that Alzheimer disease's amyloid-beta protein enhanced nitric oxide liberation. The role of NO in AA metabolism is till now not well understood. Therefore, the aim of the present study was to investigate the mechanisms of NO-evoked activation of AA release and inhibition of AA incorporation into phospholipids of cortical rat brain synaptoneurosomes. The studies were carried out using NO donors, butyryl-cGMP (b-cGMP) and H2O2. All these compounds enhanced AA liberation from phosphatydilinositol (PI) and phosphatidylcholine (PC). Protein kinase ERK1/2, protein kinase C (PKC), cGMP-dependent protein kinase G (PKG) were involved in basal and NO-induced cytosolic phospholipase A2 (cPLA2) activation. Moreover, NO donors, b-cGMP and hydrogen peroxide (H2O2) exerted inhibitory effect on AA incorporation into PI and PC influencing arachidonyl-CoA transferase (AA-CoA-T) activity. AA-CoA synthase (AA-CoA-S) activity did not change. Specific inhibitors of protein kinase ERK1/2 (UO126), PKC (GF109203X), PKG (KT5823) had no effect on NO-mediated lowering of AA incorporation into PI and PC but inhibited the basal AA-CoA-S activity. Our data indicated that AA (10 microM) itself markedly decreased AA incorporation by about 50% into phospholipids of synaptoneurosomes membranes. Increasing release of AA and its metabolites causes the lowering of AA incorporation evoked by NO, b-cGMP and H2O2. Antioxidant, Resveratrol (100 microM) prevented NO- and cGMP-evoked inhibition of AA incorporation. These results suggest that NO affects the intracellular level of AA through alteration of cPLA2 and AA-CoA acyltransferase activities and may have an important implication in alterations of nerve endings properties and function.

New insights into the retinal circulation: inflammatory lipid mediators in ischemic retinopathy

Ischemic proliferative retinopathy develops in various retinal disorders, including retinal vein occlusion, diabetic retinopathy and retinopathy of prematurity. Ischemic retinopathy remains a common cause of visual impairment and blindness in the industrialized world due to relatively ineffective treatment. Oxygen-induced retinopathy (OIR) is an established model of retinopathy of prematurity associated with vascular cell injury culminating in microvascular degeneration, which precedes an abnormal neovascularization. The retina is a tissue particularly rich in polyunsaturated fatty acids and the ischemic retina becomes highly sensitive to lipid peroxidation initiated by oxygenated free radicals. Consequently, the retina constitutes an excellent model for testing the functional consequences of membrane lipid peroxidation. Retinal tissue responds to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. Activation of phospholipase A(2) is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and a membrane-derived phospholipid mediator platelet-activating factor (PAF). These lipid mediators accumulate in the retina in response to injury and a physiologic role of these metabolites in retinal vasculature remains for the most part to be determined; albeit proposed roles have been suggested for some. The eicosanoids, in particular the prostanoids, thromboxane (TXA2) and PAF are abundantly generated following an oxidant stress and contribute to neurovascular injury. TXA2 and PAF play an important role in the retinal microvacular degeneration of OIR by directly inducing endothelial cell death and potentially could contribute to the pathogenesis of ischemic retinopathies. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. This review focuses on mechanisms that precede the development of neovascularization, most notably regarding the role of lipid mediators that partake in microvascular degeneration.

Rat Brain Cortex Mitochondria Release Group II Secretory Phospholipase A2 under Reduced Membrane Potential

Activation of brain mitochondrial phospholipase(s) A(2) (PLA(2)) might contribute to cell damage and be involved in neurodegeneration. Despite the potential importance of the phenomenon, the number, identities, and properties of these enzymes are still unknown. Here, we demonstrate that isolated mitochondria from rat brain cortex, incubated in the absence of respiratory substrates, release a Ca(2+)-dependent PLA(2) having biochemical properties characteristic to secreted PLA(2) (sPLA(2)) and immunoreacting with the antibody raised against recombinant type IIA sPLA(2) (sPLA(2)-IIA). Under identical conditions, no release of fumarase in the extramitochondrial medium was observed. The release of sPLA(2) from mitochondria decreases when mitochondria are incubated in the presence of respiratory substrates such as ADP, malate, and pyruvate, which causes an increase of transmembrane potential determined by cytofluorimetric analysis using DiOC(6)(3) as a probe. The treatment of mitochondria with the uncoupler carbonyl cyanide 3-chlorophenylhydrazone slightly enhances sPLA(2) release. The increase of sPLA(2) specific activity after removal of mitochondrial outer membrane indicates that the enzyme is associated with mitoplasts. The mitochondrial localization of the enzyme has been confirmed by electron microscopy in U-251 astrocytoma cells and by confocal laser microscopy in the same cells and in PC-12 cells, where the structurally similar isoform type V-sPLA(2) has mainly nuclear localization. In addition to sPLA(2), mitochondria contain another phospholipase A(2) that is Ca(2+)-independent and sensitive to bromoenol lactone, associated with the outer mitochondrial membrane. We hypothesize that, under reduced respiratory rate, brain mitochondria release sPLA(2)-IIA that might contribute to cell damage.

Upregulation of annexins I, II, and V after traumatic spinal cord injury in adult rats

The posttraumatic inflammatory reaction contributes to progressive tissue damage after spinal cord injury (SCI). Annexins, a family of structurally related calcium- and phospholipid-binding proteins, have potent anti-inflammatory effects by inhibiting the activity of phospholipase A(2) (PLA(2)), a key enzyme responsible for inflammation and cytotoxicity. We investigated spatiotemporal expression of annexins I, II, and V after a contusive SCI using the New York University impact device (a 10-g rod, height 12.5 mm) in adult rats. Western blot analysis revealed that annexin I expression increased at 3 days after injury, peaked at 7 days (1.75-fold above the baseline level; P < 0.01), started to decline at 14 days, and returned to the baseline level at and beyond 28 days post-injury. The expression of annexin II started to increase at 3 days, reached its maximal level at 14 days (2.73-fold; P < 0.01), remained at a high level up to 28 days, and then declined to the basal level by 56 days after injury. Annexin V expression started at 3 days, reached its maximal level at 7 days (1.61-fold; P < 0.05) and remained at this level until 56 days after injury. RT-PCR results confirmed expression of all three annexins at the mRNA level after SCI. Immunohistochemistry and immunofluorescence double-labeling analyses revealed that increased annexins I, II, and V were localized in neurons and glial cells. The present study thus revealed increased expression of the three annexin isoforms after moderate contusive SCI. The precise role of annexins in posttraumatic inflammation and neuroprotection after SCI remains to be determined.

Naproxen reduces excitotoxic neurodegeneration in vivo with an extended therapeutic window

The purpose of this study was to examine the optimal dose and therapeutic window of opportunity of the nonsteroidal anti-inflammatory drug naproxen in an animal model of excitotoxic neuronal injury. Injection of N-methyl-D-aspartate (NMDA; 18-20 nmol) into the CA1 region of the left hippocampus resulted in significant brain edema as measured by the percentage of total forebrain water content that occurred 24 h after intrahippocampal microinjection of NMDA with approximately 50% loss of CA1 neurons assessed 72 h later. Naproxen pretreatment (20 mg/kg) resulted in significantly less brain edema. Ten, 15, or 20 mg/kg naproxen, administered systemically 1 day (b.i.d.) before and for 3 days after (b.i.d.) NMDA injection, attenuated the neuronal damage by 27.2 +/- 7.8, 39.6 +/- 11.1, and 57.0 +/- 5.2%, respectively. By comparison, a single dose of MK-801 (2 mg/kg i.p.) given 20 min before NMDA injection inhibited subsequent hippocampal injury by 65.6 +/- 8.8%. Most importantly, neuroprotection was still evident when naproxen treatment (20 mg/kg i.p.) was initiated 6 h after NMDA microinjection. Protection was lost if administration of naproxen was delayed for 20 h. These findings demonstrate that naproxen can prevent excitotoxic neuronal injury in vivo, that it is nearly as effective as direct NMDA receptor antagonism, and that it has an extended therapeutic time window. As such, naproxen may be a particularly promising pharmaceutical for the treatment of neurological diseases associated with overactivation of NMDA receptors.

What synaptic lipid signaling tells us about seizure-induced damage and epileptogenesis

Glutamate, the most abundant excitatory neurotransmitter in the mammalian CNS, plays a central role in many neuronal functions, such as long-term potentiation, which is necessary for learning and memory formation. The fast excitatory glutamate neurotransmission is mediated by ionotropic receptors that include AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors, while the slow glutamate responses are mediated through its interaction with metabotropic receptors (mGluRs) coupled to G-proteins. During seizures, massive release of glutamate underlies excitotoxic neuronal damage as it triggers an overflow of calcium in postsynaptic neurons mediated by NMDA-gated channels. The early upstream postsynaptic events involve the activation of phospholipases, with the release of membrane-derived signaling molecules, such as free arachidonic acid (AA), eicosanoids, and platelet-activating factor (PAF). These bioactive lipids modulate the early neuronal responses to stimulation as they affect the activities of ion channels, receptors, and enzymes; and when released into the extracellular space, they can contribute to the modulation of presynaptic neurotransmitter release/re-uptake, and/or affect other neighboring neuronal/glial cells. The downstream postsynaptic events target the nucleus, leading to activation of gene-expression cascades. Syntheses of new proteins are the basis for seizure-induced sustained physiological and/or pathological changes that occur hours, days, or months later, such as synaptic reorganization and repair, and apoptotic/necrotic neuronal death. The intricate mesh of signaling pathways converging to the nucleus, and connecting upstream to downstream synaptic events, are at present the focus of many research efforts. We describe in this chapter how seizure-induced glutamate release activates the hydrolysis of membrane AA-phospholipids via phospholipase A2 (PLA2), PLC, and PLD, thus releasing bioactive lipids that, in turn, modulate neurotransmission. We discuss mechanisms through which lipid messengers, such as AA and PAF, may turn into injury mediators participating in seizure-induced brain damage.

Fatty acid-activated K+ channels in autonomic neurons: activation by an endogenous source of free fatty acids

Application of arachidonic acid evoked robust activation of large-conductance K+ channels in cell-attached and excised inside-out patches from acutely isolated chick ciliary ganglion neurons. A similar effect was produced by 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analogue of arachidonic acid. The unitary conductance of fatty acid-activated channels was 35-40 pS at +20 mV with physiological gradients of K+ and 165 pS at +20 mV with an extracellular K+ concentration of 37.5 mM and an intracellular K+ concentration of 150 mM. Gating of these channels in cell-attached patches was potentiated by membrane stretch. Channel gating evoked by both lipids was concentration-dependent, with detectable activation apparent at 4 microM in the majority of patches and maximal activation occurring between 32 and 64 microM. Gating was relatively voltage-independent. Large-conductance K+ channels were also activated in inside-out patches by the monounsaturated fatty acid 11-cis-eicosenoic acid but not by the fully saturated fatty acid arachidic acid. Application of 100 microM H2O2, an agent that activates cytosolic phospholipase A2, also caused activation of large-conductance K+ channels in intact neurons. The stimulatory effects of H2O2 were blocked by pretreatment with 20 microM 4-bromophenacyl bromide, an irreversible inhibitor of phospholipase A2. Therefore, mobilization of endogenous fatty acids can cause activation of large-conductance K+ channels in autonomic neurons.

Cortical impact injury in rats promotes a rapid and sustained increase in polyunsaturated free fatty acids and diacylglycerols

Neurotrauma activates the release of membrane phospholipid-derived second messengers, such as free arachidonic acid (20:4n-6, AA) and diacylglycerols (DAGs). In the present study, we analyze the effect of cortical impact injury of low-grade severity applied to the rat frontal right sensory-motor cortex (FRC) on the accumulation of free fatty acids (FFAs) and DAGs in eight brain areas 30 min and 24 hours after the insult. At these times, accumulation of FFAs and DAGs occurred mainly in the damaged FRC. The cerebellum was the only other brain area that displayed a significant accumulation of DAGs by day one post-injury. By 30 min, accumulation of free AA in the FRC displayed the greatest relative increase (300% over sham value), followed by free docosahexaenoic acid (22:6n-3, DHA, 150%), while both 20:4-DAGs and 22:6-DAGs were increased 100% over sham values. At day one, free 22:6 and 22:6-DAGs showed the greatest increase (590% and 230%, respectively). These results suggest that TBI elicits the hydrolysis of phospholipids enriched in excitable membranes, targeting early on 20:4-phospholipids (by 30 min post- trauma) and followed 24 hours later by preferential hydrolysis of DHA-phospholipids. These lipid metabolic changes may contribute to the initiation and maturation of neuronal and fiber track degeneration observed following cortical impact injury.

Secretory phospholipase A2 potentiates glutamate-induced rat striatal neuronal cell death in vivo

The secretory phospholipases A2 (sPLA2) OS2 (10, 20 and 50 pmol) or OS1, (50 pmol) purified from taipan snake Oxyuranus scutellatus scutellatus venom, and the excitatory amino acid glutamate (Glu) (2.5 and 5.0 micromol) were injected into the right striatum of male Wistar rats. Injection of 10 and 20 pmol OS2 caused no neurological abnormalities or tissue damage. OS2 (50 pmol) caused apathy and circling towards the injection side. Histology revealed an infarct at the injection site. Injection of 50 pmol OS1 showed very little or no signs of neurotoxicity. Injection of 2.5 micromol Glu caused no tissue damage or neurological abnormality. After injection of 5.0 micromol Glu, the animals initially circled towards the side of injection, and gradually developed generalized clonic convulsions. These animals showed a well demarcated striatal infarct. When non-toxic concentrations of 20 pmol OS2 and 2.5 micromol Glu were co-injected, a synergistic neurotoxicity was observed. Extensive histological damage occurred in the entire right hemisphere, and in several rats comprising part of the contralateral hemisphere. These animals were apathetic in the immediate hours following injection, with circling towards the side of injection in the following days. Thus, OS2 greatly potentiates glutamate excitoxicity in vivo.

Signal transduction in environmental neurotoxicity

Signal transduction is the process by which specific information is transferred from the cell surface to the cytosol and ultimately to the nucleus, leading to changes in gene expression. Since these chains of biochemical and molecular steps control the normal function of each cell, disruption of these processes would have a significant impact on cell physiology. Some of the major signal transduction pathways are briefly reviewed. The interactions of four chemicals (lead, ethanol, polychlorinated biphenyls, and trimethyltin) with different cell signaling systems, particularly the phospholipid hydrolysis/protein kinase C pathway, are discussed. The possible causal relationship of such cellular and molecular interactions with known signs and symptoms of neurotoxicity are highlighted.

Cloning, chromosomal mapping, and expression of a novel human secretory phospholipase A2

Secretory phospholipases A2 (sPLA2s) represent a rapidly expanding family of structurally related enzymes found in mammals as well as in insect and snake venoms. In this report, a cDNA coding for a novel sPLA2 has been isolated from human fetal lung, and its gene has been mapped to chromosome 16p13.1-p12. The mature sPLA2 protein has a molecular mass of 13.6 kDa, is acidic (pI 5.3), and made up of 123 amino acids. Key structural features of the sPLA2 include: (i) a long prepropeptide ending with an arginine doublet, (ii) 16 cysteines located at positions that are characteristic of both group I and group II sPLA2s, (iii) a C-terminal extension typical of group II sPLA2s, (iv) and the absence of elapid and pancreatic loops that are characteristic of group I sPLA2s. Based on these structural properties, this sPLA2 appears as a first member of a new group of sPLA2s, called group X. A 1.5-kilobase transcript coding for the human group X (hGX) sPLA2 was found in spleen, thymus, and peripheral blood leukocytes, while a less abundant 0.8-kilobase transcript was detected in the pancreas, lung, and colon. When the hGX sPLA2 cDNA was expressed in COS cells, sPLA2 activity preferentially accumulated in the culture medium, indicating that hGX sPLA2 is an actively secreted enzyme. It is maximally active at physiological pH and with 10 mM Ca2+. hGX sPLA2 prefers phosphatidylethanolamine and phosphatidylcholine liposomes to those of phosphatidylserine.