Spinal phospholipase A2 in inflammatory hyperalgesia: role of group IVA cPLA2

The mu-opioid receptor agonist morphine, but not agonists at delta- or kappa-opioid receptors, induces peripheral antinociception mediated by cannabinoid receptors

BACKGROUND AND PURPOSE

Although participation of opioids in antinociception induced by cannabinoids has been documented, there is little information regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids could be involved in peripheral antinociception induced by activation of mu-, delta- and kappa-opioid receptors.

EXPERIMENTAL APPROACH

Nociceptive thresholds to mechanical stimulation of rat paws treated with intraplantar prostaglandin E2 (PGE2, 2 microg) to induce hyperalgesia were measured 3 h after injection using an algesimetric apparatus. Opioid agonists morphine (200 microg), (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) (80 microg), bremazocine (50 microg); cannabinoid receptor antagonists N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) (20-80 microg), 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl) methanone (AM630) (12.5-100 microg); and an inhibitor of methyl arachidonyl fluorophosphonate (MAFP) (1-4 microg) were also injected in the paw.

KEY RESULTS

The CB1-selective cannabinoid receptor antagonist AM251 completely reversed the peripheral antinociception induced by morphine in a dose-dependent manner. In contrast, the CB2-selective cannabinoid receptor antagonist AM630 elicited partial antagonism of this effect. In addition, the administration of the fatty acid amide hydrolase inhibitor, MAFP, enhanced the antinociception induced by morphine. The cannabinoid receptor antagonists AM251 and AM630 did not modify the antinociceptive effect of SNC80 or bremazocine. The antagonists alone did not cause any hyperalgesic or antinociceptive effect.

CONCLUSIONS AND IMPLICATIONS

Our results provide evidence for the involvement of endocannabinoids, in the peripheral antinociception induced by the mu-opioid receptor agonist morphine. The release of cannabinoids appears not to be involved in the peripheral antinociceptive effect induced by kappa- and delta-opioid receptor agonists.

Decreased phospholipase A2 activity in cerebrospinal fluid of patients with dementia

Phospholipase A2 (PLA2) is involved in important aspects of dementia, for example neurotransmission and memory processing, membrane function, choline availability, and antioxidative defense. Reduced PLA2-activity has been reported so far in blood samples and postmortem neuronal tissue in Alzheimer disease. For the first time, we studied PLA2 in cerebrospinal fluid (CSF) in Alzheimer disease (AD), vascular (VD), and mixed Alzheimer/vascular dementia (MD). Intracellular PLA2 was assessed in CSF of 16 AD, 12 VD, 15 MD patients, and 19 healthy control subjects. A fluorometric assay was applied using the PLA2-specific substrate NBDC6-HPC. Significantly reduced PLA2 activity was not only found in AD, but also in VD and MD. This finding was independent of demographic co-variates and medication. PLA2 results in CSF corroborate previous findings of impaired PLA2 function in Alzheimer's disease and extend these to patients with VD. They are likely to reflect an involvement of PLA2 impairment in a variety of pathomechanisms crucial in different dementia subtypes, in which disruption of cholinergic neurotransmission and disturbance of intact membrane function appear to be the key mechanisms.

Inhibition of spinal cytosolic phospholipase A(2) expression by an antisense oligonucleotide attenuates tissue injury-induced hyperalgesia

Activation of the spinal phospholipase A(2) (PLA(2)) -cyclooxygenase (COX) -prostaglandin signaling pathway is widely implicated in nociceptive processing. Although the role of spinal COX isoforms in pain signal transmission has been extensively characterized, our knowledge of PLA(2) enzymes in this cascade is limited. Among all PLA(2) groups, cytosolic calcium-dependent PLA(2) group IVA (cPLA(2)IVA) appears to be the predominant PLA(2) enzyme in the spinal cord. In the present study we sought to (i) characterize anatomical and cellular distribution and localization of cPLA(2)IVA in dorsal horn of rat spinal cord, (ii) verify efficacy and selectivity of intrathecal (IT) delivery of an antisense oligonucleotide (AS) targeting rat cPLA(2)IVA mRNA on spinal expression of this enzyme, and (iii) examine the effect of down-regulation of spinal cPLA(2)IVA on peripheral tissue injury-induced pain behavior. Here we demonstrate that cPLA(2)IVA is constitutively expressed in rat spinal cord, predominantly in dorsal horn neurons and oligodendrocytes but not in astrocytes or microglia. Intrathecal injection of AS significantly down-regulated both protein and gene expression of cPLA(2)IVA in rat spinal cord, while control missense oligonucleotide (MS) had no effect. Immunocytochemistry confirmed that the reduction occurred in neurons and oligodendrocytes. cPLA(2)IVA AS did not alter expression of several other PLA(2) isoforms, such as secretory PLA(2) (groups IIA and V) and calcium-independent PLA(2) (group VI), indicating that the AS was specific for cPLA(2)IVA. This selective knockdown of spinal cPLA(2)IVA did not change acute nociception (i.e. paw withdrawal thresholds to acute thermal stimuli and intradermal formalin-induced first phase flinching), however, it significantly attenuated formalin-induced hyperalgesia (i.e. second phase flinching behavior), which reflects spinal sensitization. Thus the present findings suggest that cPLA(2)IVA may specifically participate in spinal nociceptive processing.

Calcium binding rigidifies the C2 domain and the intradomain interaction of GIVA phospholipase A2 as revealed by hydrogen/deuterium exchange mass spectrometry

The GIVA phospholipase A(2) (PLA(2)) contains two domains: a calcium-binding domain (C2) and a catalytic domain. These domains are linked via a flexible tether. GIVA PLA(2) activity is Ca(2+)-dependent in that calcium binding promotes protein docking to the phospholipid membrane. In addition, the catalytic domain has a lid that covers the active site, presumably regulating GIVA PLA(2) activity. We now present studies that explore the dynamics and conformational changes of this enzyme in solution utilizing peptide amide hydrogen/deuterium (H/D) exchange coupled with liquid chromatography-mass spectrometry (DXMS) to probe the solvent accessibility and backbone flexibility of the C2 domain, the catalytic domain, and the intact GIVA PLA(2). We also analyzed the changes in H/D exchange of the intact GIVA PLA(2) upon Ca(2+) binding. The DXMS results showed a fast H/D-exchanging lid and a slow exchanging central core. The C2 domain showed two distinct regions: a fast exchanging region facing away from the catalytic domain and a slow exchanging region present in the "cleft" region between the C2 and catalytic domains. The slow exchanging region of the C2 domain is in tight proximity to the catalytic domain. The effects of Ca(2+) binding on GIVA PLA(2) are localized in the C2 domain and suggest that binding of two distinct Ca(2+) ions causes tightening up of the regions that surround the anion hole at the tip of the C2 domain. This conformational change may be the initial step in GIVA PLA(2) activation.

Coherent anti-Stokes Raman scattering imaging of myelin degradation reveals a calcium-dependent pathway in lyso-PtdCho-induced demyelination

Coherent anti-Stokes Raman scattering (CARS) microscopy, which allows vibrational imaging of myelin sheath in its natural state, was applied to characterize lysophosphatidylcholine (lyso-PtdCho)-induced myelin degradation in tissues and in vivo. After the injection of lyso-PtdCho into ex vivo spinal tissues or in vivo mouse sciatic nerves, myelin swelling characterized by the decrease of CARS intensity and loss of excitation polarization dependence was extensively observed. The swelling corresponds to myelin vesiculation and splitting observed by electron microscopy. The demyelination dynamics were quantified by the increase of g ratio measured from the CARS images. Treating spinal tissues with Ca2+ ionophore A23187 resulted in the same kind of myelin degradation as lyso-PtdCho. Moreover, the demyelination lesion size was significantly reduced upon preincubation of the spinal tissue with Ca2+ free Krebs' solution or a cytosolic phospholipase A2 (cPLA(2)) inhibitor or a calpain inhibitor. In accordance with the imaging results, removal of Ca2+ or addition of cPLA(2) inhibitor or calpain inhibitor in the Krebs' solution remarkably increased the mean compound action potential amplitude in lyso-PtdCho treated spinal tissues. Our results suggest that lyso-PtdCho induces myelin degradation via Ca(2+) influx into myelin and subsequent activation of cPLA(2) and calpain, which break down the myelin lipids and proteins. The current work also shows that CARS microscopy is a potentially powerful tool for the study of demyelination.

Effect of endotoxin treatment on the expression and localization of spinal cyclooxygenase, prostaglandin synthases, and PGD2 receptors

Systemic inflammation leads to increased expression of spinal cyclooxygenase (COX)-2 and to a subsequent increase of prostaglandin (PG) biosynthesis, which contribute to the development of hyperalgesia and allodynia. In this study, endotoxin caused a sequential induction of membrane bound prostaglandin E synthase-1 and lipocalin-type PGD synthase (L-PGDS) in the mouse spinal cord. L-PGDS expression was detected in the leptomeninges, oligodendrocytes, and interestingly, in discrete perivascular cells. Endotoxin-caused increase was most prominent in oligodendrocytes. Endotoxin-induced COX-2 and membrane bound prostaglandin E synthase-1 were restricted to the leptomeninges and perivascular cells. COX-1 was not influenced by endotoxin. We found COX-1 expressed in microglia, some of them in close proximity to L-PGDS-positive oligodendrocytes and co-localization of COX-1 with L-PGDS in perivascular and leptomeningeal cells under control conditions. It can be assumed, that PGD2 biosynthesis under control conditions is mediated via COX-1 and that during inflammation, increased PGD2 is dependent on COX-2. We found the PGD2 receptors DP1 and chemoattractant receptor homologous molecule expressed on T helper type 2 cells (CRTH2) localized in neurons of the dorsal, and motoneurons in the ventral horn. The localization of the PGD2 receptors DP1 and CRTH in spinal cord neurons, particularly in neurons of lamina I and II involved in the processing of nociceptive stimuli, supports a role of PGD2 under inflammatory conditions.

Coherent anti-Stokes Raman scattering imaging of myelin degradation reveals a calcium-dependent pathway in lyso-PtdCho-induced demyelination

Coherent anti-Stokes Raman scattering (CARS) microscopy, which allows vibrational imaging of myelin sheath in its natural state, was applied to characterize lysophosphatidylcholine (lyso-PtdCho)-induced myelin degradation in tissues and in vivo. After the injection of lyso-PtdCho into ex vivo spinal tissues or in vivo mouse sciatic nerves, myelin swelling characterized by the decrease of CARS intensity and loss of excitation polarization dependence was extensively observed. The swelling corresponds to myelin vesiculation and splitting observed by electron microscopy. The demyelination dynamics were quantified by the increase of g ratio measured from the CARS images. Treating spinal tissues with Ca2+ ionophore A23187 resulted in the same kind of myelin degradation as lyso-PtdCho. Moreover, the demyelination lesion size was significantly reduced upon preincubation of the spinal tissue with Ca2+ free Krebs' solution or a cytosolic phospholipase A2 (cPLA(2)) inhibitor or a calpain inhibitor. In accordance with the imaging results, removal of Ca2+ or addition of cPLA(2) inhibitor or calpain inhibitor in the Krebs' solution remarkably increased the mean compound action potential amplitude in lyso-PtdCho treated spinal tissues. Our results suggest that lyso-PtdCho induces myelin degradation via Ca(2+) influx into myelin and subsequent activation of cPLA(2) and calpain, which break down the myelin lipids and proteins. The current work also shows that CARS microscopy is a potentially powerful tool for the study of demyelination.

Altered spinal arachidonic acid turnover after peripheral nerve injury regulates regional glutamate concentration and neuropathic pain behaviors in rats

Spinal glutamate transporters (GT) have been implicated in the mechanisms of neuropathic pain; however, how spinal GT uptake activity is regulated remains unclear. Here we show that alteration of spinal arachidonic acid (AA) turnover after peripheral nerve injury regulated regional GT uptake activity and glutamate homeostasis. Chronic constriction nerve injury (CCI) in rats significantly reduced spinal GT uptake activity ((3)H-glutamate uptake) with an associated increase in extracellular AA and glutamate concentration from spinal microdialysates on postoperative day 8. AACOCF3 (a cytosolic phospholipase A2 inhibitor, 30mug) given intrathecally twice a day for postoperative day 1-7 reversed this CCI-induced spinal AA production, prevented the reduced spinal GT uptake activity and increased extracellular glutamate concentration. Conversely, alteration of spinal AA metabolism by diclofenac (a cyclooxygenase 1/2 inhibitor, 200mug) further reduced spinal GT uptake activity and increased extracellular glutamate concentration in CCI rats. GT uptake activity was also attenuated when AA (10 or 100nM) was directly added into spinal samples of naïve rats in an in vitro(3)H-glutamate uptake assay, indicating a direct inhibitory effect of AA on GT uptake activity. Consistent with these findings, AACOCF3 reduced the development of both thermal hyperalgesia and mechanical allodynia, whereas diclofenac exacerbated thermal hyperalgesia, in CCI rats. Thus, spinal AA turnover may serve as a regulator in CCI-induced changes in regional GT uptake activity, glutamate homeostasis, and neuropathic pain behaviors. These data suggest that regulating spinal AA turnover may be a useful approach to improving the clinical management of neuropathic pain.

Impairment of p38 MAPK-mediated cytosolic phospholipase A2 activation in the kidneys is associated with pathogenicity of Candida albicans

In studying the mechanisms underlying the susceptibility of the kidney to candidal infection, we previously reported that the reduced production of cytokines [i.e. tumour necrosis factor-alpha (TNF-alpha)] via platelet-activating factor (PAF)-induced activation of nuclear factor-kappaB (NF-kappaB) renders the organ susceptible to the fungal burden. In this study, we investigated the possibility that pathogenic Candida albicans may evade clearance and perhaps even multiply by inhibiting elements in the signalling pathway that lead to the production of TNF-alpha. The fungal burden of pathogenic C. albicans in the kidneys was 10(4)-10(5)-fold higher than that of a non-pathogenic strain. PAF-induced early activation of NF-kappaB and TNF-alpha mRNA expression were both observed in the kidneys of mice infected with non-pathogenic strains of C. albicans, but not in mice infected with pathogenic strains. Impairment of PAF-mediated early NF-kappaB activation following infection with pathogenic C. albicans was associated with the prevention of activation of the enzyme cytosolic phospholipase A(2) (cPLA(2)) as well as the upstream pathway of cPLA(2), p38 mitogen-activated protein kinase. Collectively, these findings indicate that C. albicans exerts its pathogenicity through impairing the production of anticandidal cytokines by preventing cPLA(2) activity. This novel mechanism provides insight into understanding pathogenic C. albicans and perhaps identifies a target for its treatment.

Effects of intracerebroventricular injections of free fatty acids, lysophospholipids, or platelet activating factor in a mouse model of orofacial pain

The present study was carried out to determine the effects of central nervous free fatty acids, lysophospholipids, or platelet activating factor (PAF), in a mouse facial carrageenan injection model of orofacial pain. Mice that received intracerebroventricular (I.C.V.) injection of arachidonic acid or oleic acid showed significantly reduced allodynia and behavioral responses to von Frey hair stimulation of a carrageenan-injected area of the face, at 8 h post-injection, compared to controls that received I.C.V. injection of vehicle. In contrast to free fatty acids, increased responses were observed in mice at 72 h after I.C.V. lysophosphatidic acid or lysophosphatidylcholine injection, and at 8 and 24 h after PAF injection, compared vehicle injected controls. Information regarding pro-nociceptive effect of specific brain lipids may be a useful basis for further studies to explore mechanism.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Differential inhibition of group IVA and group VIA phospholipases A2 by 2-oxoamides

Inhibitors of the Group IVA phospholipase A(2) (GIVA cPLA(2)) and GVIA iPLA(2) are useful tools for defining the roles of these enzymes in cellular signaling and inflammation. We have developed inhibitors of GVIA iPLA(2) building upon the 2-oxoamide backbone that are uncharged, containing ester groups. Although the most potent inhibitors of GVIA iPLA(2) also inhibited GIVA cPLA(2), there were three 2-oxoamide compounds that selectively and weakly inhibited GVIA iPLA(2). We further show that several potent 2-oxoamide inhibitors of GIVA cPLA(2) containing free carboxylic groups (Kokotos et al. J. Med. Chem. 2002, 45, 2891-2893) do not inhibit GVIA iPLA(2) and are, therefore, selective GIVA cPLA(2) inhibitors.

Calcium-independent phospholipase A2modulates cytosolic oxidant activity and contractile function in murine skeletal muscle cells

Phospholipase A2(PLA2) activity supports production of reactive oxygen species (ROS) by mammalian cells. In skeletal muscle, endogenous ROS modulate the force of muscle contraction. We tested the hypothesis that skeletal muscle cells constitutively express the calcium-independent PLA2(iPLA2) isoform and that iPLA2modulates both cytosolic oxidant activity and contractile function. Experiments utilized differentiated C2C12myotubes and a panel of striated muscles isolated from adult mice. Muscle preparations were processed for measurement of mRNA by real-time PCR, protein by immunoblot, cytosolic oxidant activity by the dichlorofluorescein oxidation assay, and contractile function by in vitro testing. We found that iPLA2was constitutively expressed by all muscles tested (myotubes, diaphragm, soleus, extensor digitorum longus, gastrocnemius, heart) and that mRNA and protein levels were generally similar among muscles. Selective iPLA2blockade by use of bromoenol lactone (10 μM) decreased cytosolic oxidant activity in myotubes and intact soleus muscle fibers. iPLA2blockade also inhibited contractile function of unfatigued soleus muscles, shifting the force-frequency relationship rightward and depressing force production during acute fatigue. Each of these changes could be reproduced by selective depletion of superoxide anions using superoxide dismutase (1 kU/ml). These findings suggest that constitutively expressed iPLA2modulates oxidant activity in skeletal muscle fibers by supporting ROS production, thereby influencing contractile properties and fatigue characteristics.

Distinguishing phospholipase A2 types in biological samples by employing group-specific assays in the presence of inhibitors

This manuscript reviews and updates radiolabel-based enzyme assays designed to distinguish the activity of phospholipase A2 (PLA2) types in biological samples. This approach should be useful in lipidomics studies. The assays were originally designed to differentiate between Group IVA cytosolic PLA2 (GIVA cPLA2), Group VIA calcium-independent PLA2 (GVIA iPLA2), Group IIA secreted PLA2 (GIIA sPLA2) and Group V secreted PLA2 (GV sPLA2). The specificity of these assays has now been confirmed using purified, recombinant human PLA2s and the utility of these assays is demonstrated with rat spinal cord homogenate as an example of a biological tissue sample of interest to the neuroscience community. Modifications to the original assays by the addition of group-specific inhibitors are presented to ensure the specificity of the assays and to further differentiate between recently identified PLA2s. Specific tests are suggested to confirm the specificity of each assay. Additionally, it was discovered that one commonly used GIVA cPLA2/GVIA iPLA2 inhibitor, methyl arachidonyl fluorophosphonate (MAFP) from one commercial source, was found to inhibit GIIA sPLA2 and GV sPLA2, but not GIVA cPLA2, presumably due to oxidation of the compound during shipment, resulting in a different molecule with altered specificity.

Systemic and Intrathecal Effects of a Novel Series of Phospholipase A2 Inhibitors on Hyperalgesia and Spinal Prostaglandin E2 Release

Phospholipase A(2) (PLA(2)) forms are expressed in spinal cord, and inhibiting spinal PLA(2) induces a potent antihyperalgesia. Here, we examined the antihyperalgesic effects after systemic and i.t. delivery of four compounds constructed with a common motif consisting of a 2-oxoamide with a hydrocarbon tail and a four-carbon tether. These molecules were characterized for their ability to block group IVA calcium-dependent PLA(2) (cPLA(2)) and group VIA calcium-independent PLA(2) (iPLA(2)) in inhibition assays using human recombinant enzyme. The rank ordering of potency in blocking group IVA cPLA(2) was AX048 (ethyl 4-[(2-oxohexadecanoyl)amino]butanoate), AX006 (4-[(2-oxohexadecanoyl)amino]butanoic acid), and AX057 (tert-butyl 4-[(2-oxohexadecanoyl)amino]butanoate) > AX010 (methyl 4-[(2-oxohexadecanoyl)amino]butanoate) and for inhibiting group VIA iPLA(2) was AX048, AX057 > AX006, and AX010. No agent altered recombinant cyclooxygenase activity. In vivo, i.t. (30 mug) and systemic (0.2-3 mg/kg i.p.) AX048 blocked carrageenan hyperalgesia and after systemic delivery in a model of spinally mediated hyperalgesia induced by i.t. substance P (SP). The other agents were without activity. In rats prepared with lumbar i.t. loop dialysis catheters, SP evoked spinal prostaglandin E(2) (PGE(2)) release. AX048 alone inhibited PGE(2) release. Intrathecal SR141617, a cannabinoid CB1 inhibitor at doses that blocked the effects of i.t. anandamide had no effect upon i.t. AX048. These results suggest that AX048 is the first systemically bioavailable compound with a significant affinity for group IVA cPLA(2), which produces a potent antihyperalgesia. The other agents, although demonstrating enzymatic activity in cell-free assays, appear unable to gain access to the intracellular PLA(2) toward which their action is targeted.

Spinal phospholipase A2 in inflammatory hyperalgesia: role of the small, secretory phospholipase A2

Current work emphasizes that peripheral tissue injury and inflammation results in a heightened sensitivity to subsequent noxious input (hyperalgesia) that is mediated in large part by the spinal synthesis and release of eicosanoids, in particular prostaglandins. Secreted phospholipase A(2)s (sPLA(2)s) form a class of structurally related enzymes that release arachidonic acid from cell membranes that is further processed to produce eicosanoids. We hypothesized that spinal sPLA(2)s may contribute to inflammation-induced hyperalgesia. Spinal cord tissue and cerebrospinal fluid were collected from rats for assessment of sPLA(2) protein expression and sPLA(2) activity. A basal sPLA(2) protein expression and activity was detected in spinal cord homogenate (87+/-17 pmol/min/mg), though no activity could be detected in cisternal cerebrospinal fluid, of naive rats. The sPLA(2) activity did not change in spinal cord tissue or cerebrospinal fluid assessed over 8 h after injection of carrageenan into the hind paw. However, the sPLA(2) activity observed in spinal cord homogenates was suppressed by addition of LY311727, a selective sPLA(2) inhibitor. To determine the role of this spinal sPLA(2) in hyperalgesia, we assessed the effects of lumbar intrathecal (IT) administration of LY311727 in rats with chronic IT catheters in three experimental models of hyperalgesia. IT LY311727 (3-30 microg) dose-dependently prevented intraplantar carrageenan-induced thermal hyperalgesia and formalin-induced flinching, at doses that had no effect on motor function. IT LY311727 also suppressed thermal hyperalgesia induced by IT injection of substance P (30 nmol). Using in vivo spinal microdialysis, we found that IT injection of LY311727 attenuated prostaglandin E(2) release into spinal dialysate otherwise evoked by the IT injection of substance P. Taken together, this work points to a role for constitutive sPLA(2)s in spinal nociceptive processing.