Melittin increases AMPA receptor affinity in rat brain synaptoneurosomes

Familial Alzheimer's Disease Neurons Bearing Mutations in PSEN1 Display Increased Calcium Responses to AMPA as an Early Calcium Dysregulation Phenotype

Familial Alzheimer's disease (FAD) can be caused by mutations in PSEN1 that encode presenilin-1, a component of the gamma-secretase complex that cleaves amyloid precursor protein. Alterations in calcium (Ca2+) homeostasis and glutamate signaling are implicated in the pathogenesis of FAD; however, it has been difficult to assess in humans whether or not these phenotypes are the result of amyloid or tau pathology. This study aimed to assess the early calcium and glutamate phenotypes of FAD by measuring the Ca2+ response of induced pluripotent stem cell (iPSC)-derived neurons bearing PSEN1 mutations to glutamate and the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate compared to isogenic control and healthy lines. The data show that in early neurons, even in the absence of amyloid and tau phenotypes, FAD neurons exhibit increased Ca2+ responses to glutamate and AMPA, but not NMDA or kainate. Together, this suggests that PSEN1 mutations alter Ca2+ and glutamate signaling as an early phenotype of FAD.

Acetylcholine and norepinephrine mediate GABAergic but not glycinergic transmission enhancement by melittin in adult rat substantia gelatinosa neurons

GABAergic and glycinergic inhibitory synaptic transmissions in substantia gelatinosa (SG; lamina II of Rexed) neurons of the spinal dorsal horn play an important role in regulating nociceptive transmission from the periphery. It has not yet been well known whether each of the inhibitory transmissions plays a distinct role in the regulation. We report an involvement of neurotransmitters in GABAergic but not glycinergic transmission enhancement produced by the PLA2 activator melittin, where the whole-cell patch-clamp technique is applied to the SG neurons of adult rat spinal cord slices. Glycinergic but not GABAergic spontaneous inhibitory postsynaptic current (sIPSC) was increased in frequency and amplitude by melittin in the presence of nicotinic, muscarinic acetylcholine, and α1-adrenergic receptor antagonists (mecamylamine, atropine, and WB-4101, respectively). GABAergic transmission enhancement produced by melittin was unaffected by the 5-hydroxytryptamine 3 receptor and P2X receptor antagonists (ICS-205,930 and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid, respectively). Nicotinic and muscarinic acetylcholine receptor agonists [(−)-nicotine and carbamoylcholine, respectively] and norepinephrine, as well as melittin, increased GABAergic sIPSC frequency and amplitude. A repeated application of (−)-nicotine, carbamoylcholine, and norepinephrine, but not melittin, at an interval of 30 min produced a similar transmission enhancement. These results indicate that melittin produces the release of acetylcholine and norepinephrine, which activate (nicotinic and muscarinic) acetylcholine and α1-adrenergic receptors, respectively, resulting in GABAergic but not glycinergic transmission enhancement in SG neurons. The desensitization of a system leading to the acetylcholine and norepinephrine release is slow in recovery. This distinction in modulation between GABAergic and glycinergic transmissions may play a role in regulating nociceptive transmission.

Phospholipase A2 activation as a therapeutic approach for cognitive enhancement in early-stage Alzheimer disease

RATIONALE

Alzheimer disease (AD) is the leading cause of dementia in the elderly and has no known cure. Evidence suggests that reduced activity of specific subtypes of intracellular phospholipases A2 (cPLA2 and iPLA2) is an early event in AD and may contribute to memory impairment and neuropathology in the disease.

OBJECTIVE

The objective of this study was to review the literature focusing on the therapeutic role of PLA2 stimulation by cognitive training and positive modulators, or of supplementation with arachidonic acid (PLA2 product) in facilitating memory function and synaptic transmission and plasticity in either research animals or human subjects.

METHODS

MEDLINE database was searched (no date restrictions) for published articles using the keywords Alzheimer disease (mild, moderate, severe), mild cognitive impairment, healthy elderly, rats, mice, phospholipase A(2), phospholipid metabolism, phosphatidylcholine, arachidonic acid, cognitive training, learning, memory, long-term potentiation, protein kinases, dietary lipid compounds, cell proliferation, neurogenesis, and neuritogenesis. Reference lists of the identified articles were checked to select additional studies of interest.

RESULTS

Overall, the data suggest that PLA2 activation is induced in the healthy brain during learning and memory. Furthermore, learning seems to regulate endogenous neurogenesis, which has been observed in AD brains. Finally, PLA2 appears to be implicated in homeostatic processes related to neurite outgrowth and differentiation in both neurodevelopmental processes and response to neuronal injury.

CONCLUSION

The use of positive modulators of PLA2 (especially of cPLA2 and iPLA2) or supplementation with dietary lipid compounds (e.g., arachidonic acid) in combination with cognitive training could be a valuable therapeutic strategy for cognitive enhancement in early-stage AD.

Conditioning training and retrieval increase phospholipase A(2) activity in the cerebral cortex of rats

In rats, phospholipase A(2) (PLA(2)) activity was found to be increased in the hippocampus immediately after training and retrieval of a contextual fear conditioning paradigm (step-down inhibitory avoidance [IA] task). In the present study we investigated whether PLA(2) is also activated in the cerebral cortex of rats in association with contextual fear learning and retrieval. We observed that IA training induces a rapid (immediately after training) and long-lasting (3 h after training) activation of PLA(2) in both frontal and parietal cortices. However, immediately after retrieval (measured 24 h after training), PLA(2) activity was increased just in the parietal cortex. These findings suggest that PLA(2) activity is differentially required in the frontal and parietal cortices for the mechanisms of contextual learning and retrieval. Because reduced brain PLA(2) activity has been reported in Alzheimer disease, our results suggest that stimulation of PLA(2) activity may offer new treatment strategies for this disease.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Phospholipase A2 Activation Enhances Inhibitory Synaptic Transmission in Rat Substantia Gelatinosa Neurons

Phospholipase A2 (PLA2) activation enhances glutamatergic excitatory synaptic transmission in substantia gelatinosa (SG) neurons, which play a pivotal role in regulating nociceptive transmission in the spinal cord. By using melittin as a tool to activate PLA2, we examined the effect of PLA2 activation on spontaneous inhibitory postsynaptic currents (sIPSCs) recorded at 0 mV in SG neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique. Melittin enhanced the frequency and amplitude of GABAergic and glycinergic sIPSCs. The enhancement of GABAergic but not glycinergic transmission was largely depressed by Na+ channel blocker tetrodotoxin or glutamate-receptor antagonists (6-cyano-7-nitroquinoxaline-2,3-dione and/or dl-2-amino-5-phosphonovaleric acid) and also in a Ca2+-free Krebs solution. The effects of melittin on glycinergic sIPSC frequency and amplitude were dose-dependent with an effective concentration of ∼0.7 μM for half-maximal effect and were depressed by PLA2 inhibitor 4-bromophenacyl bromide or aristolochic acid. The melittin-induced enhancement of glycinergic transmission was depressed by lipoxygenase inhibitor nordihydroguaiaretic acid but not cyclooxygenase inhibitor indomethacin. These results indicate that the activation of PLA2 in the SG enhances GABAergic and glycinergic inhibitory transmission in SG neurons. The former action is mediated by glutamate-receptor activation and neuronal activity increase, possibly the facilitatory effect of PLA2 activation on excitatory transmission, whereas the latter action is due to PLA2 and subsequent lipoxygenase activation and is independent of extracellular Ca2+. It is suggested that PLA2 activation in the SG could enhance not only excitatory but also inhibitory transmission, resulting in the modulation of nociception.

The role of phospholipases A2 in schizophrenia

A range of neurotransmitter systems have been implicated in the pathogenesis of schizophrenia based on the antidopaminergic activities of antipsychotic medications, and chemicals that can induce psychotic-like symptoms, such as ketamine or PCP. Such neurotransmitter systems often mediate their cellular response via G-protein-coupled release of arachidonic acid (AA) via the activation of phospholipases A2 (PLA2s). The interaction of three PLA2s are important for the regulation of the release of AA--phospholipase A2 Group 2 A, phospholipase A2 Group 4A and phospholipase A2 Group 6A. Gene variations of these three key enzymes have been associated with schizophrenia with conflicting results. Preclinical data suggest that the activity of these three enzymes are associated with monoaminergic neurotransmission, and may contribute to the differential efficacy of antipsychotic medications, as well as other biological changes thought to underlie schizophrenia, such as altered neurodevelopment and synaptic remodelling. We review the evidence and discuss the potential roles of these three key enzymes for schizophrenia with particular emphasis on published association studies.

Phospholipase A2 activation by melittin enhances spontaneous glutamatergic excitatory transmission in rat substantia gelatinosa neurons

In order to know a role of phospholipase A2 in modulating nociceptive transmission, the effect of a secreted phospholipase A2 activator melittin on spontaneous glutamatergic excitatory transmission was investigated in substantia gelatinosa neurons of an adult rat spinal cord slice by using the whole-cell patch-clamp technique. Bath-applied melittin at concentrations higher than 0.5 microM increased both the amplitude and the frequency of spontaneous excitatory postsynaptic current in a manner independent of tetrodotoxin; the latter effect of which was examined in detail. In 80% of the neurons examined (n = 64), melittin superfused for 3 min gradually increased spontaneous excitatory postsynaptic current frequency (by 65+/-6% at 1 microM; n = 51) in a dose-dependent manner (effective concentration for half-maximal effect = 1.1 microM). This effect subsided within 3 min after washout. The spontaneous excitatory postsynaptic current frequency increase produced by melittin was reduced by the phospholipase A2 inhibitor 4-bromophenacryl bromide (10 microM) while being unaffected by the cyclooxygenase inhibitor indomethacin (100 microM) and the lipoxygenase inhibitor nordihydroguaiaretic acid (100 microM). A similar increase in spontaneous excitatory postsynaptic current frequency was produced by exogenous arachidonic acid (50 microM); this effect was also unaffected by the cyclooxygenase or lipoxygenase inhibitor. Melittin failed to increase spontaneous excitatory postsynaptic current frequency in a nominally Ca2+-free or La3+-containing Krebs solution. We conclude that melittin increases the spontaneous release of L-glutamate to substantia gelatinosa neurons by activating secreted phospholipase A2 and increasing Ca2+ influx through voltage-gated Ca2+ channels in nerve terminals, probably with an involvement of arachidonic acid but not its metabolites produced by cyclooxygenase and lipoxygenase. Considering that the substantia gelatinosa plays an important role in regulating nociceptive transmission, it is suggested that this transmission may be positively modulated by secreted phospholipase A2 activation in the substantia gelatinosa.

Electrophysiology and plasticity in isolated postsynaptic densities

The organization and regulation of excitatory synapses in the mammalian CNS entails complex molecular and cellular processes. In the postsynaptic membrane, scaffolding proteins bring together glutamate receptors with multiple regulatory proteins involved in signal transduction. This gives rise to an elaborate postsynaptic structure known as the postsynaptic density (PSD). This protein network plays a critical role in the regulation of glutamate receptor function and thus in synaptic plasticity. To study this regulation, we have developed a system in which ionotropic glutamate receptors (iGluRs) can be recorded, in the steady state, by the patch clamp technique in isolated PSDs incorporated into giant liposomes. In this preparation, ionotropic glutamate receptors maintain their characteristic physiological and pharmacological properties. The recordings reflect the presence of channel clusters, as multiple conductance and subconductance states are observed. Each of the receptor subtypes is activated by a specific set of kinases that are activated differentially by Ca(2+): the "kainate receptor kinases" are active even in the presence of EGTA, i.e. they are not calcium-dependent; the "N-methyl-D-aspartate receptor (NMDAR) channel kinases" are active in the presence of submicromolar calcium concentrations, whereas the "alpha-amino-3- hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor kinases" need microM calcium for activation. The NMDA receptor showed its characteristic voltage-dependent Mg(2+) blockade, and activation by phosphorylation was in part a consequence of a relief of Mg(2+) blockade. These results allow us to propose a model in which phosphorylation of NMDA receptors can contribute to a long-lasting and self-maintained change in synaptic function. The experimental approach we present will allow us to test the functional consequence of activation of the multiple signal transduction pathways thought to regulate excitatory neurotransmission in the adult CNS.

The Caveolin Scaffolding Domain Modifies 2-Amino-3-hydroxy-5-methyl-4-isoxazole Propionate Receptor Binding Properties by Inhibiting Phospholipase A2 Activity

Activation of the enzyme phospholipase (PLA 2) has been proposed to be part of the molecular mechanism involved in the alteration of 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptor responsiveness during long term changes in synaptic plasticity (long term potentiation). This study assesses the effect of the caveolin-1 scaffolding domain (CSD) on the activity of the regulatory enzyme PLA2. Caveolin-1 is a 22-kDa cholesterol-binding membrane protein known to inhibit the activity of most of its interacting partners. Our results show that the calcium-dependent cytosolic form of PLA2 (cPLA2) and caveolin-1 co-localized in mouse primary hippocampal neuron cultures and that they were co-immunoprecipitated from mouse hippocampal homogenates. A peptide corresponding to the scaffolding domain of caveolin-1 (Cav-(82-101)) dramatically inhibited cPLA2 activity in purified hippocampal synaptoneurosomes. Activation of endogenous PLA2 activity with KCl or melittin increased the binding of [3H]AMPA to its receptor. This effect was almost completely abolished by the addition of the CSD peptide to these preparations. Moreover, we demonstrated that the inhibitory action of the CSD peptide on AMPA receptor binding properties is specific (because a scrambled version of this peptide failed to have any effect) and that it is mediated by an inhibition of PLA2 enzymatic activity (because the CSD peptide failed to have an effect in membrane preparations lacking endogenous PLA2 activity). These results raised the possibility that caveolin-1, via the inhibition of cPLA2 enzymatic activity, may interfere with synaptic facilitation and long term potentiation formation in the hippocampus.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

Melittin enhances amino acid and free fatty acid release from the in vivo cerebral cortex

The effects of the phospholipase activator melittin on amino acid and free fatty acid release from the rat cerebral cortex were monitored and compared with those of a secretory PLA(2), using a cortical cup technique with topical application of these agents in artificial cerebrospinal fluid. Melittin (10 microg/ml; 3.5 microM) elicited a rapid increase in the levels of superfusate amino acids; aspartate, glutamate, GABA, glycine, taurine, glutamine, phosphoethanolamine, alanine, serine and the free fatty acids arachidonic, linoleic, palmitic and oleic acid. PLA(2) (25 microg/ml) also enhanced amino acid efflux but its effects were significantly slower to develop than those of melittin. The results confirm previous indications of an ability of phospholipases to augment extracellular levels of several amino acids, including the excitotoxins glutamate and aspartate, and further implicate phospholipase activation as a significant contributor to cerebral ischemic injury. Melittin has the potential to be a useful tool with which to evaluate the role of phospholipases in ischemia injury.

The neuromessenger platelet-activating factor in plasticity and neurodegeneration

Synaptic activation leads to the formation of arachidonic acid, platelet-activating factor (PAF, 1-O-alkyl-2-acyl-sn-3-phosphocholine) and other lipid messengers. PAF is a potent bioactive phospholipid in synaptic plasticity. PAF enhances presynaptic glutamate release, is a retrograde messenger in long-term potentiation and enhances memory formation. PAF also couples synaptic events with gene expression by stimulating a FOS/JUN/AP-1 transcriptional signaling system, as well as transcription of COX-2 (inducible prostaglandin synthase). Since the COX-2 gene is also involved in synaptic plasticity, the PAF-COX-2 pathway may have physiological significance. Seizures, ischemia and other forms of brain injury promote phospholipase A2 (PLA2) overactivation, resulting in the accumulation of bioactive lipids at the synapse. PAF, under these pathological conditions, behaves as a neuronal injury messenger by at least two mechanisms: (a) enhancing glutamate release; and, (b) by sustained augmentation of COX-2 transcription. These events link PAF with neurodegeneration. The upstream intracellular pathways of signal transduction involved in neuronal or photoreceptor cell apoptosis are not well understood and involve stress sensitive kinases. PAF is a transcriptional activator of the COX-2 gene. BN 50730, a potent intracellular PAF antagonist, blocks COX-2 induction. COX-2 transcription and protein expression are upregulated in the hippocampus in kainic acid induced epileptogenesis. There is a selectively elevated induction of COX-2 (72-fold) by kainic acid preceding neuronal cell death. BN 50730 administered by i.c.v. injection blocks seizure-induced COX-2 induction. Overall, PAF is a dual modulator of neural function and becomes an endogenous neurotoxin when over produced.

Bidirectional modulation of AMPA receptor properties by exogenous phospholipase A2 in the hippocampus

The synaptic modifications underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission in various brain structures may result from changes in the properties of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors. In the present study, we report that treatment of rat synaptoneurosomes with increasing concentrations of phospholipase A2 (PLA2) produces a biphasic effect on AMPA receptor binding, with low concentrations causing a decrease and high concentrations an increase in agonist binding. Analysis of the saturation kinetics of 3H-AMPA binding revealed that the biphasic effect of PLA2 was due to modifications in receptor affinity and not to changes in the maximum number of binding sites for AMPA receptors. The 12-lipoxygenase inhibitors preferentially reduced PLA2-induced decrease in AMPA binding and treatment of hippocampal synaptoneurosomes with arachidonic acid (AA) or 12-HPETE, the first metabolite generated from the hydrolysis of AA by 12-lipoxygenases, decreased 3H-AMPA binding. Moreover, electrophysiological experiments indicated that the 12-lipoxygenase inhibitor baicalein totally blocked LTD formation in area CA1 of hippocampal slices. The decrease in 3H-AMPA binding elicited by low concentrations of PLA2, as well as the level of LTD, were partially reduced by AA-861, a 5-lipoxygenase inhibitor, while the cyclooxygenase inhibitor indomethacin did not prevent LTD formation or the effects of PLA2 on 3H-AMPA binding. Our results provide evidence for a possible involvement of lipoxygenase metabolites in the regulation of AMPA receptor during synaptic depression. In addition, they strongly support the idea that the same biochemical pathway, i.e., NMDA receptor activation and endogenous PLA2 stimulation, may represent a common mechanism resulting in AMPA receptor alterations for both LTP and LTD formation.

Developmental changes in depolarization-mediated AMPA receptor modifications and potassium-induced long-term potentiation

In the present study, we examined the KCl-induced increase in [3H] amino-3-hydroxy-5-methylisoxazole-4-propionate ([3H]AMPA) binding in telencephalic synaptoneurosomes and potentiation of synaptic transmission (KLTP) in hippocampal slices during development in rats. As previously reported, KCI-induced depolarization of telencephalic synaptoneurosomes resulted in a 40 +/- 5% increase in [3H]AMPA binding to membrane fractions in adult rats (3 months old). KCI-induced increase in [3H]AMPA binding was reduced to 24 +/- 5% and 15 +/- 5% at postnatal days (PND) 25-30 and PND 15-20 respectively, and was only 6 +/- 5% at PND 5-10. KLTP in area CA1 of hippocampus was most pronounced in adult slices (40 +/- 5%), and was reduced to 30 +/- 5% in slices prepared from PND 25-30 animals; KCI-induced LTP was absent in CA1 hippocampal slices prepared from PND 5-10 animals (3 +/- 5%). The decrease in KCI-induced changes in AMPA receptor binding in young animals was also associated with an altered capacity of the bee venom peptide, mellitin (a phospholipase A2 (PLA2) activator), to increase [3H]AMPA binding in synaptoneurosomes. The smaller effect of mellitin on [3H]AMPA binding in young animals was not due to a decreased ability of this peptide to release [3H]arachidonate from synaptoneuro-somes. The parallel modifications in the extent of depolarization-induced change in AMPA receptor binding and excitatory synaptic transmission during development further support the hypothesis that alterations in AMPA receptor properties may play a critical role in synaptic plasticity.