Regulation of ionotropic receptors by protein phosphorylation

Isolation of various forms of sterol beta-D-glucoside from the seed of Cycas circinalis: neurotoxicity and implications for ALS-parkinsonism dementia complex

The factors responsible for ALS-parkinsonism dementia complex (ALS-PDC), the unique neurological disorder of Guam, remain unresolved, but identification of causal factors could lead to clues for related neurodegenerative disorders elsewhere. Earlier studies focused on the consumption and toxicity of the seed of Cycas circinalis, a traditional staple of the indigenous diet, but found no convincing evidence for toxin-linked neurodegeneration. We have reassessed the issue in a series of in vitro bioassays designed to isolate non-water soluble compounds from washed cycad flour and have identified three sterol beta-d-glucosides as potential neurotoxins. These compounds give depolarizing field potentials in cortical slices, induce alterations in the activity of specific protein kinases, and cause release of glutamate. They are also highly toxic, leading to release of lactate dehydrogenase (LDH). Theaglycone form, however, is non-toxic. NMDA receptor antagonists block the actions of the sterol glucosides, but do not compete for binding to the NMDA receptor. The most probable mechanism leading to cell death may involve glutamate neuro/excitotoxicity. Mice fed cycad seed flour containing the isolated sterol glucosides show behavioral and neuropathological outcomes, including increased TdT-mediated biotin-dUTP nick-end labelling (TUNEL) positivity in various CNS regions. Astrocytes in culture showed increased caspase-3 labeling after exposure to sterol glucosides. The present results support the hypothesis that cycad consumption may be an important factor in the etiology of ALS-PDC and further suggest that some sterol glucosides may be involved in other neurodegenerative disorders.

Glutamatergic synaptic responses and long-term potentiation are impaired in the CA1 hippocampal area of calbindin D(28k)-deficient mice

The contribution of the cytosolic calcium binding protein calbindin D(28K) (CaBP) to glutamatergic neurotransmission and synaptic plasticity was investigated in hippocampal CA1 area of wild-type and antisense transgenic CaBP-deficient mice, with the use of extracellular recordings in the ex vivo slice preparation. The amplitude of non-N-methyl-D-aspartate receptor (non-NMDAr)-mediated extracellular field excitatory postsynaptic potentials (fEPSPs) recorded in control medium was significantly greater in CaBP-deficient mice, whereas the afferent fiber volley was not affected. In contrast, the amplitude of NMDAr-mediated fEPSPs isolated in a magnesium-free medium after blockade of non-NMDAr and GABAergic receptors was significantly depressed in these animals. No alteration in the magnitude of paired-pulse facilitation was found, indicating that the presynaptic calcium mechanisms controlling glutamate release were not altered in CaBP-deficient mice. The magnitude and time course of the short-term potentiation (STP) of fEPSPs induced by a 30 Hz conditioning stimulation, which was blocked by the NMDAr antagonist 2-amino-5-phosphonovalerate acid (2-APV), was not impaired in the transgenic mice, whereas long-term potentiation (LTP) induced by a 100 Hz tetanus was not maintained. The long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 15 min) in the presence of the GABA antagonist bicuculline was not altered. These results argue for a contribution of CaBP to the mechanisms responsible for the maintenance of long-term synaptic potentiation, at least in part by modulating the activation of NMDA receptors.

Learning-specific, time-dependent increases in hippocampal Ca2+/calmodulin-dependent protein kinase II activity and AMPA GluR1 subunit immunoreactivity

Ca2+/calmodulin-dependent protein kinase II (CAMK II) and one of its target, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), glutamate receptors have been shown to participate in both long-term potentiation (LTP) in the hippocampus, and in spatial, as well as in a variety, of learning paradigms. Recently, we were able to demonstrate that the intrahippocampal infusion of a specific inhibitor of CAMK II (KN62) provoked full retrograde amnesia of an inhibitory avoidance learning in rats when given immediately, but not 120 or 240 min, after training. Furthermore, this task is accompanied by a rapid, selective and reversible increase in hippocampal [3H] AMPA receptor binding. Here we report the effect of this aversively motivated learning task on CAMK II activity, and AMPA GluR1 subunit phosphorylation and immunoreactivity in the hippocampus. One trial inhibitory avoidance training is associated with a learning-specific, time-dependent increase (25-78%) in both total and Ca2+-independent activities of CAMK II in the hippocampus of rats killed immediately (0 min), but not 120 min, after training. In addition, immunoblotting experiments showed an increment in the amount of the alpha-subunit of CAMK II at 0, 30 and 120 min after training. An increase in the in vitro phosphorylation of alpha- and beta-subunits of CAMK II was also observed in hippocampal synaptosomal membranes (SPM) of trained rats killed immediately and 30 min post-training. In addition, inhibitory avoidance is accompanied by a 20% increase in GluR1 phosphorylation and a 33% increase in GluR1 immunoreactivity 120 min after training. No significant changes were observed in shocked animals. Phosphorylation of hippocampal SPM from naive control animals in conditions suitable for CAMK II activation resulted in a large increase in the density of [3H] AMPA binding (+ 100%). Taken together, these findings confirm and extend previous data suggesting that CAMK II and AMPA glutamate receptors in the hippocampus participate in the early phase of memory formation of an inhibitory avoidance learning.

Abnormal dephosphorylation effect on NMDA receptor regulation in ALS spinal cord

Previous studies have demonstrated a significant reduction of N-methyl-D-aspartate (NMDA) receptor binding in spinal cord sections from patients who died with amyotrophic lateral sclerosis (ALS) compared to that in control patients. The reduction in NMDA receptor binding in ALS could be increased toward control values by treatment with phorbol ester, suggesting a role for receptor protein phosphorylation in this disorder. In the present study we have evaluated the time course of recovery of [3H]MK-801 binding following phorbol ester treatment to assess protein phosphatase activity in spinal cord sections from ALS and control subjects. Phorbol ester-stimulated changes in [3H]MK-801 binding returned to untreated values significantly faster in ALS tissue compared to control and could not be blocked by the coapplication of the protein phosphatase inhibitors sodium vanadate or sodium beta-D-glycerol phosphate. Okadaic acid coapplication blocked recovery in both ALS and control tissue at a concentration range at which phosphatase 2B (calcineurin) would likely be inhibited. The results suggest that abnormal levels or activity of protein phosphatases, including calcineurin, may be involved in the abnormal levels of NMDA receptors in ALS and may play some role in the pathogenesis of the disease.

Neurodegenerative disorders in humans: the role of glutathione in oxidative stress-mediated neuronal death

Oxidative stress has been implicated in both normal aging and in various neurodegenerative disorders and may be a common mechanism underlying various forms of cell death including necrosis, apoptosis, and excitotoxicity. In this review, we develop the hypothesis that oxidative stress-mediated neuronal loss may be initiated by a decline in the antioxidant molecule glutathione (GSH). GSH plays multiple roles in the nervous system including free radical scavenger, redox modulator of ionotropic receptor activity, and possible neurotransmitter. GSH depletion can enhance oxidative stress and may also increase the levels of excitotoxic molecules; both types of action can initiate cell death in distinct neuronal populations. Evidence for a role of oxidative stress and diminished GSH status is presented for Lou Gehrig's disease (ALS), Parkinson's disease, and Alzheimer's disease. Potential links to the Guamanian variant of these diseases (ALS-PD complex) are discussed. In context to the above, we provide a GSH-depletion model of neurodegenerative disorders, suggest experimental verifications of this model, and propose potential therapeutic approaches for preventing or halting these diseases.

Memory formation: the sequence of biochemical events in the hippocampus and its connection to activity in other brain structures

Recent data have demonstrated a biochemical sequence of events in the rat hippocampus that is necessary for memory formation of inhibitory avoidance behavior. The sequence initially involves the activation of three different types of glutamate receptors followed by changes in second messengers and biochemical cascades led by enhanced activity of protein kinases A, C, and G and calcium-calmodulin protein kinase II, followed by changes in glutamate receptor subunits and binding properties and increased expression of constitutive and inducible transcription factors. The biochemical events are regulated early after training by hormonal and neurohumoral mechanisms related to alertness, anxiety, and stress, and 3-6 h after training by pathways related to mood and affect. The early modulation is mediated locally by GABAergic, cholinergic, and noradrenergic synapses and by putative retrograde synaptic messengers, and extrinsically by the amygdala and possibly the medial septum, which handle emotional components of memories and are direct or indirect sites of action for several hormones and neurotransmitters. The late modulation relies on dopamine D1, beta-noradrenergic, and 5HT1A receptors in the hippocampus and dopaminergic, noradrenergic, and serotoninergic pathways. Evidence indicates that hippocampal activity mediated by glutamate AMPA receptors must persist during at least 3 h after training in order for memories to be consolidated. Probably, this activity is transmitted to other areas, including the source of the dopaminergic, noradrenergic, and serotoninergic pathways, and the entorhinal and posterior parietal cortex. The entorhinal and posterior parietal cortex participate in memory consolidation minutes after the hippocampal chain of events starts, in both cases through glutamate NMDA receptor-mediated processes, and their intervention is necessary in order to complete memory consolidation. The hippocampus, amygdala, entorhinal cortex, and parietal cortex are involved in retrieval in the first few days after training; at 30 days from training only the entorhinal and parietal cortex are involved, and at 60 days only the parietal cortex is necessary for retrieval. Based on observations on other forms of hippocampal plasticity and on memory formation in the chick brain, it is suggested that the hippocampal chain of events that underlies memory formation is linked to long-term storage elsewhere through activity-dependent changes in cell connectivity.

Long-term regulation of opioid receptors in neuroblastoma and lymphoma cell lines

Long-term regulation of opioid binding was studied in the human neuroblastoma NMB and in the murine lymphoma R1.1 and R1.EGO cell lines. Binding was down-regulated following prolonged exposure to opioid agonists and up-regulated following exposure to antagonist. Down-regulation was inhibited by the metabolic blocker sodium-azide and by the protein kinase H-7. Up-regulation was blocked by the protein and mRNA synthesis blockers cycloheximide, alpha-amanitin and actinomycin D. A significant difference was found between the response of neuronal and immune cells to ethanol exposure: while opioid binding in neuroblastoma culture underwent a pronounced (75%) up-regulation, no effect of ethanol on opioid receptors in lymphoma cultures was detected. The described cell lines present an excellent experimental model to study long-term regulation of opioid receptors in the nervous and immune systems and to elucidate the biological effects of chronic use of opiates and alcohol.