Arachidonic acid potentiates currents through Ca2+-permeable AMPA receptors by interacting with a CaMKII pathway

Regulation of GluA1 AMPA receptor through PKC phosphorylation induced by free fatty acid derivative HUHS2002

The present study investigated the effect of 4-[4-(Z)-hept-1-enyl-phenoxy] butyric acid (HUHS2002), a newly synthesized free fatty acid derivative, on α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor responses. HUHS2002 potentiated currents through GluA1 AMPA receptors expressed in Xenopus oocytes in a bell-shaped concentration (1 nM–1 μM)-dependent manner, the maximum reaching nearly 140 % of original amplitude at 100 nM. The potentiation was significantly inhibited by GF109203X, an inhibitor of protein kinase C (PKC), but not KN-93, an inhibitor of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). HUHS2002 had no potentiating effect on currents through mutant GluA1 AMPA receptors with replacement of Ser831, a PKC/CaMKII phosphorylation site, by Ala. In the in situ PKC assay using rat PC-12 cells, HUHS2002 significantly enhanced PKC activity, that is suppressed by GF109203X. Overall, the results of the present study show that HUHS2002 potentiates GluA1 AMPA receptor responses by activating PKC and phosphorylating the receptors at Ser831, regardless of CaMKII activation and phosphorylation.

Free fatty acid derivative HUHS2002 potentiates α7 ACh receptor responses through indirect activation of CaMKII

The present study examined the effect of 4-[4-(Z)-hept-1-enyl-phenoxy] butyric acid (HUHS2002), a free fatty acid derivative, on α7 acetylcholine (ACh) receptor responses. HUHS2002 potentiated whole-cell membrane currents through α7 ACh receptors expressed in Xenopus oocytes in a concentration (1-100 nM)-dependent manner, reaching about 140 % of the original amplitude at 100 nM 50 min after a 10-min treatment. The HUHS2002 effect was prevented by KN-93, an inhibitor of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), while it was not affected by GF109203X, an inhibitor of protein kinase C (PKC), or H-89, an inhibitor of protein kinase A (PKA). In the in situ CaMKII assay using cultured rat hippocampal neurons, HUHS2002 activated CaMKII and the activation was abolished by KN-93. In the cell-free assay of protein phosphatase 1 (PP1), HUHS2002 partially inhibited PP1 activity. Taken together, these results indicate that HUHS2002 potentiates α7 ACh receptor responses by indirectly activating CaMKII, possibly via inhibition of PP1.

1,2-dilinoleoyl-sn-glycero-3-phosphoethanolamine ameliorates age-related spatial memory deterioration by preventing neuronal cell death

BACKGROUND

Accumulating evidence has pointed that a variety of lipids could exert their beneficial actions against dementia including Alzheimer disease and age-related cognitive decline via diverse signaling pathways. Endoplasmic reticulum (ER) stress-induced neuronal apoptosis, on the other hand, is a critical factor for pathogenesis of neurodegenerative diseases such as Alzheimer disease and Parkinson disease, senile dementia, and ischemic neuronal damage. The present study examined the effects of 1,2-dilinoleoyl-sn-glycero-3-phosphoethanolamine (DLPhtEtn), a phospholipid, on ER stress-induced neuronal death and age-related cognitive disorders.

METHODS

PC-12 cell viability was assayed before and after treatment with amyloid-β(1-40) peptide or thapsigargin in the presence and absence of DLPhtEtn. A series of behavioral tests were performed for senescence-accelerated mouse-prone 8 (SAMP8) mice after 7-month oral administration with polyethylene glycol (PEG) or DLPhtEtn and then, the number of hippocampal neurons was counted.

RESULTS

Amyloid-β(1-40) peptide or thapsigargin is capable of causing ER stress-induced apoptosis. DLPhtEtn (30 μM) significantly inhibited PC-12 cell death induced by amyloid-β(1-40) peptide or thapsigargin. In the water maze test, oral administration with DLPhtEtn (1 mg/kg) for 7 months (three times a week) significantly shortened the prolonged retention latency for SAMP8 mice. In contrast, DLPhtEtn had no effect on the acquisition and retention latencies in both the open field test and the passive avoidance test for SAMP8 mice. Oral administration with DLPhtEtn (1 mg/kg) for 7 months prevented a decrease in the number of hippocampal neurons for SAMP8 mice.

CONCLUSION

The results of the present study show that DLPhtEtn ameliorates age-related spatial memory decline without affecting motor activities or fear memory, possibly by protecting hippocampal neuronal death. DLPhtEtn, thus, might exert its beneficial action against senile dementia and neurodegenerative diseases such as Alzheimer disease.

Phospholipase A2 and arachidonic acid in Alzheimer's disease

Essential fatty acids (EFA) play a critical role in the brain and regulate many of the processes altered in Alzheimer's disease (AD). Technical advances are allowing for the dissection of complex lipid pathways in normal and diseased states. Arachidonic acid (AA) and specific isoforms of phospholipase A(2) (PLA(2)) appear to be critical mediators in amyloid-beta (Abeta)-induced pathogenesis, leading to learning, memory, and behavioral impairments in mouse models of AD. These findings and ongoing research into lipid biology in AD and related disorders promise to reveal new pharmacological targets that may lead to better treatments for these devastating conditions.

DCP-LA stimulates AMPA receptor exocytosis through CaMKII activation due to PP-1 inhibition

The linoleic acid derivative 8-[2-(2-pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid (DCP-LA) activated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) by inhibiting protein phosphatase-1 (PP-1). DCP-LA induced a transient huge facilitation of synaptic transmission monitored from the CA1 region of rat hippocampal slices, which was largely inhibited by the CaMKII inhibitor KN-93. DCP-LA potentiated kainate-evoked whole-cell membrane currents for Xenopus oocytes expressing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors composed of the GluR1, GluR3, GluR1/GluR2, GluR1/GluR3, and GluR1/GluR2/GluR3 subunits, and the potentiation was significantly inhibited by KN-93. A similar potentiation was still found with mutant GluR1 (S831A) receptor lacking CaMKII phosphorylation site. The GluR1 and GluR2 subunits formed AMPA receptors in the rat hippocampus, and DCP-LA increased expression of both the subunits on the plasma membrane. The DCP-LA action was blocked by KN-93 and the exocytosis inhibitor botulinum toxin type A, but not by the endocytosis inhibitor phenylarsine oxide. DCP-LA, thus, appears to activate CaMKII through PP-1 inhibition, that stimulates AMPA receptor exocytosis to increase expression of the receptors on the plasma membrane, responsible for potentiate AMPA receptor responses and facilitation of hippocampal synaptic transmission.

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.

Tunicamycin inhibits NMDA and AMPA receptor responses independently of N-glycosylation

In a whole-cell patch-clamp configuration, currents through N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor channels were monitored in cultured rat hippocampal neurons, and those currents were depressed to 25 and 28% of basal levels, respectively, by 3-min treatment with tunicamycin (10 microM), an inhibitor of protein N-glycosylation. Tunicamycin (10 microM) reduced amplitude of population spikes elicited in the dentate gyrus of rat hippocampal slices, reaching 78% of basal levels 60 min after the beginning of treatment, and long-term potentiation (LTP) of the perforant path was never induced in the presence of tunicamycin. Tunicamycin, thus, appears to serve as a modulator for NMDA and AMPA receptors, regardless of N-glycosylation, thereby inhibiting neurotransmission and LTP in the dentate gyrus.

The aniracetam metabolite 2-pyrrolidinone induces a long-term enhancement in AMPA receptor responses via a CaMKII pathway

The present study was conducted to assess the effect of aniracetam and its metabolites, such as 2-pyrrolidinone, p-anisic acid, and anisamide butyrate, on the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, heteromerically formed of GluR1,2 (GluR1 and GluR2), GluR1,3 (GluR1 and GluR3), and GluR1,2,3 (GluR1, GluR2, and GluR3), expressed in Xenopus oocytes. 2-Pyrrolidinone potentiated kainate-evoked currents through GluR1,2,3 channels in a bell-shaped dose-dependent manner at concentrations ranged from 1 nM to 300 microM, with a maximal effect at 100 microM. The potentiation was long-lasting, reaching approximately 180% of basal levels 60 min after 5-min treatment with 2-pyrrolidinone at 100 microM. 2-Pyrrolidinone (100 microM) potentiated GluR1,3 channel currents as observed in GluR1,2,3, but instead it depressed GluR1,2 currents. Aniracetam and p-anisic acid potentiated GluR1,2,3 channel currents, but to a lesser extent, each about 130 and 103% of basal levels 60 min after treatment at 100 microM. In contrast, anisamide butyrate had no potentiating effect on the currents. Potentiation of GluR1,2,3 channel currents obtained with 2-pyrrolidinone was inhibited by KN-93, a selective inhibitor of calcium/calmodulin-dependent protein kinase (CaMKII), while it was not affected by GF109203X, a selective inhibitor of protein kinase C or H-89, a selective inhibitor of cAMP-dependent protein kinase. The results of the present study suggest that 2-pyrrolidinone persistently enhances activity of the Ca2+-permeable AMPA receptors, GluR1,3 and GluR1,2,3, by interacting with CaMKII.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Nefiracetam facilitates hippocampal neurotransmission by a mechanism independent of the piracetam and aniracetam action

Nefiracetam, a nootropic (cognition-enhancing) agent, facilitated neurotransmission in the dentate gyrus of rat hippocampal slices in a dose-dependent manner at concentrations ranged from 1 nM to 1 microM, being evident at 60-min washing-out of the drug. The facilitatory action was blocked by the nicotinic acetylcholine (ACh) receptor antagonists, alpha-bungarotoxin and mecamylamine. A similar facilitation was induced by the other nootropic agents, piracetam and aniracetam, but the facilitation was not inhibited by nicotinic ACh receptor antagonists and it did not occlude the potentiation induced by nefiracetam. In the Xenopus oocyte expression systems, nefiracetam potentiated currents through a variety of neuronal nicotinic ACh receptors (alpha 3beta 2, alpha 3beta 4, alpha 4 beta 2, alpha 4 beta 4, and alpha 7) to a different extent. In contrast, neither piracetam nor aniracetam had any potentiating action on alpha 7 receptor currents. While aniracetam delayed the decay time of currents through the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor, GluR1, -2, -3, expressed in oocytes, nefiracetam or piracetam had no effect on the currents. Nefiracetam, thus, appears to facilitate hippocampal neurotransmission by functionally targeting nicotinic ACh receptors, independently of the action of piracetam and aniracetam.