Hydrogen peroxide-induced cell death in cultured Aplysia sensory neurons

Apoptotic effects of hydrogen peroxide and vitamin C on chicken embryonic fibroblasts: redox state and programmed cell death

The pro-apoptotic effects of hydrogen peroxide and the purported anti-apoptotic effects of Vitamin C on chicken embryonic fibroblasts were investigated. Hydrogen peroxide induced morphological changes in a dose dependent manner, and a myriad of autophagosomes were observed using transmission electron microscopy. Doxorubicin elicited alterations were not inhibited by co-incubation with Vitamin C except that mitochondrial structure was slightly improved. TUNEL assay, cytotoxicity analysis and flow cytometry revealed that the cytotoxicity, DNA fragmentation and apoptotic rates were dose dependent upon treatment with hydrogen peroxide. Calcium homeostasis was disrupted in a dose dependent manner, and cell cycle was blocked at G(2)/M checkpoint at low concentration and S/G(2) checkpoint at high concentration respectively upon treatment with hydrogen peroxide. The administration of Vitamin C only has a modest effect against doxorubicin induced apoptosis, calcium homeostasis disruption and cell cycle arrest. This research demonstrated that the elevation of reactive oxygen species is sufficient to induce the apoptosis of chicken embryonic fibroblasts, whereas the administration of Vitamin C does not necessarily have certain anti-apoptotic effects, especially when the stimulus is not directly linked with redox state.

Dopamine and paraquat enhance α-synuclein-induced alterations in membrane conductance

We have previously demonstrated that α-synuclein overexpression increases the membrane conductance of dopaminergic-like cells. Although α-synuclein is thought to play a central role in the pathogenesis of several neurodegenerative diseases including Parkinson's disease, multiple system atrophy, and diffuse Lewy body disease, the mechanism of action is not completely understood. In this study, we sought to determine whether multiple factors act together with α-synuclein to engender cell vulnerability through an augmentation of membrane conductance. In this article, we employed a cell model that mimics dopaminergic neurons coupled with α-synuclein overexpression and oxidative stressors. We demonstrate an enhancement of α-synuclein-induced toxicity in the presence of combined treatment with dopamine and paraquat, two molecules known to incite oxidative stress. In addition, we show that combined dopamine and paraquat treatment increases the expression of heme oxygenase-1, an antioxidant response protein. Finally, we demonstrate for the first time that combined treatment of dopaminergic cells with paraquat and dopamine enhances α-synuclein-induced leak channel properties resulting in increased membrane conductance. Importantly, these increases are most robust when both paraquat and dopamine are present suggesting the need for multiple oxidative insults to augment α-synuclein-induced disruption of membrane integrity.

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.

Cofilin expression induces cofilin-actin rod formation and disrupts synaptic structure and function in Aplysia synapses

Cofilin-actin rods are inclusion-like structures that are induced by certain chemical or physical stresses in cultured cells, and the rods formed in neurons are thought to be associated with neurodegeneration. Here, we cloned an Aplysia cofilin homolog and overexpressed it in cultured neurons. Overexpressed cofilin formed rod-like structures that included actin. The overall neuronal morphology was unaffected by cofilin overexpression; however, a decrease in number of synaptic varicosities was observed. Consistent with this structural change by cofilin overexpression, the synaptic strength was reduced, and furthermore, the long-term facilitation elicited by repeated pulses of 5-hydroxytryptamine was impaired in sensory-to-motor synapses. However, cofilin overexpression did not induce programmed cell death. These findings suggest that the formation of cofilin-actin rod-like structures can lead to neurodegeneration, and this might be a mechanism of rundown of neuronal and synaptic function without cell death in neurodegenerative diseases.

Neurotrophic activities of trk receptors conserved over 600 million years of evolution

The trk family of receptor tyrosine kinases is crucial for neuronal survival in the vertebrate nervous system, however both C. elegans and Drosophila lack genes encoding trks or their ligands. The only invertebrate representative of this gene family identified to date is Ltrk from the mollusk Lymnaea. Did trophic functions of trk receptors originate early in evolution, or were they an innovation of the vertebrates? Here we show that the Ltrk gene conserves a similar exon/intron order as mammalian trk genes in the region encoding defined extracellular motifs, including one exon encoding a putative variant immunoglobulin-like domain. Chimeric receptors containing the intracellular and transmembrane domains of Ltrk undergo ligand-induced autophosphorylation followed by MAP kinase activation in transfected cells. The chimeras are internalized similarly to TrkA in PC12 cells, and their stimulation leads to differentiation and neurite extension. Knock-down of endogenous Ltrk expression compromises outgrowth and survival of Lymnaea neurons cultured in CNS-conditioned medium. Thus, Ltrk is required for neuronal survival, suggesting that trophic activities of the trk receptor family originated before the divergence of molluscan and vertebrate lineages approximately 600 million years ago.

Thiol oxidation-mediated cell death in Aplysia cultured sensory neurons

Cellular thiol groups modulate various aspects of cellular function, including cell death. In this study, we found that a thiol oxidant, diamide, induced morphological changes such as cell swelling, membrane blebbing, and chromatin condensation in Aplysia cultured sensory neurons. Furthermore, diamide induced biphasic changes in the membrane potential, where hyperpolarization was followed by depolarization. Moreover, these diamide-induced cytotoxic effects were completely blocked by the equimolar addition of the disulfide reducing agent dithiothreitol (DTT). We also found that during H(2)O(2)-induced cell death, DTT attenuated cell swelling and membrane blebbing, but not DNA breakage, whereas the vitamin E analogue trolox attenuated DNA breakage, but not cell swelling and membrane blebbing. These results demonstrate that during H(2)O(2)-induced cell death, apoptotic features such as DNA breakage are mediated in part by free radical generation, whereas necrotic features such as cell swelling and membrane blebbing are primarily mediated by the oxidation of cellular thiol groups.

Effects of hydrogen peroxide on sodium current in acutely isolated rat hippocampal CA1 neurons

The effects of hydrogen peroxide (H2O2) on sodium currents (Na+ currents) in freshly dissociated rat hippocampal neurons were studied using the whole-cell patch-clamp techniques. H2O2 caused a reversible increase of the voltage-activated Na+ currents in a concentration- and voltage-dependent manner. The half-increasing concentration (EC50) of H2O2 on Na+ currents was 10.79 microM. In addition, 10 microM H2O2 shifted the steady-state inactivation curve of Na+ currents toward positive potential (control Vh = -64.58 +/- 1.22 mV, H2O2 Vh = -53.55 +/- 0.94 mV, n = 10, P < 0.01 without changing the slope factor). However, the steady-state activation curve was not affected. These results indicated that H2O2 could increase the amplitudes of Na+ currents and change the inactivation properties of Na+ channels even in very low concentration.

Altered excitability of intestinal neurons in primary culture caused by acute oxidative stress

Neurons were isolated from the intestine of guinea pigs and grown in primary culture for < or =15 days. Using conventional whole cell recording techniques, we demonstrated that the majority of neurons express a prolonged poststimulus afterhyperpolarization (slow AHP). These neurons also had large-amplitude (approximately 100 mV), broad-duration (approximately 2 ms) action potentials and generated a hyperpolarization activated inward current (Ih). Application of H2O2 (0.22-8.8 mM) hyperpolarized these neurons but not those lacking slow AHPs. The H2O2-induced hyperpolarization was followed by irreversible depolarization at higher concentrations (more than approximately 1 mM) of H2O2 while it was maintained after washout of submillimolar H2O2. The ionic mechanisms underlying the hyperpolarization included the suppression of Ih and the activation of an inwardly rectifying outward current, which was blocked by glybenclamide (25-50 microM) and TEA (30 mM). In addition, H2O2 suppressed the slow AHP and its underlying current. Internal perfusion of catalase and glutathione opposed the H2O2-mediated decrease in IsAHP. Our results indicate that acute oxidative stress has neuron- and conductance-specific actions in intestinal neurons that may underlie pathophysiological conditions.

Hydrogen peroxide modulates K+ ion currents in cultured Aplysia sensory neurons

Hydrogen peroxide (H(2)O(2)) causes oxidative stress and is considered a mediator of cell death in various organisms. Our previous studies showed that prolonged (>6 h) treatment of Aplysia sensory neurons with 1 mM H(2)O(2) produced hyperpolarization of the resting membrane potential, followed by apoptotic morphological changes. In this study, we examined the effect of H(2)O(2) on the membrane conductance of Aplysia sensory neurons. Hyperpolarization was induced by 10 mM H(2)O(2) within 1 h, and this was attributed to increased membrane conductance. In addition, treatment with 10 mM H(2)O(2) for 3 min produced immediate depolarization, which was due to decreased membrane conductance. The H(2)O(2)-induced hyperpolarization and depolarization were completely blocked by dithiothreitol, a disulfide-reducing agent. The later increase of membrane conductance induced by H(2)O(2) was completely blocked by 100 mM TEA, a K(+) channel blocker, suggesting that H(2)O(2)-induced hyperpolarization is due to the activation of K(+) conductance. However, the inhibition of K(+) efflux by TEA did not protect against H(2)O(2)-induced cell death in cultured Aplysia sensory neurons, which indicates that the signal pathway leading to H(2)O(2)-induced cell death is more complicated than expected.

Aggregate formation and the impairment of long-term synaptic facilitation by ectopic expression of mutant huntingtin in Aplysia neurons

Huntington's disease (HD) is caused by an expansion of a polyglutamine (polyQ) tract within huntingtin (htt) protein. To examine the cytotoxic effects of polyQ-expanded htt, we overexpressed an enhanced green fluorescent protein (EGFP)-tagged N-terminal fragment of htt with 150 glutamine residues (Nhtt150Q-EGFP) in Aplysia neurons. A combined confocal and electron microscopic study showed that Aplysia neurons expressing Nhtt150Q-EGFP displayed numerous abnormal aggregates (diameter 0.5-5 microm) of filamentous structures, which were formed rapidly (approximately 2 h) but which were sustained for at least 18 days in the cytoplasm. Furthermore, the overexpression of Nhtt150Q-EGFP in sensory cells impaired 5-hydroxytryptamine (5-HT)-induced long-term synaptic facilitation in sensori-motor synapses without affecting basal synaptic strength or short-term facilitation. This study demonstrates the stability of polyQ-based aggregates and their specific effects on long-term synaptic plasticity.

The ability of atypical antipsychotic drugs vs. haloperidol to protect PC12 cells against MPP+-induced apoptosis

The present study examined the effects of the atypical antipsychotic drugs clozapine, olanzapine, quetiapine and risperidone, on N-methyl-4-phenylpyridinium ion-induced apoptosis and DNA damage in PC12 cells, and explored the molecular mechanisms underlying these effects. Haloperidol, a typical antipsychotic drug, was used for comparison. Exposure of PC12 cells to 50 micro m N-methyl-4-phenylpyridinium ion for 24 h resulted in a 35-45% loss of cells in culture. Pretreatment with the aforementioned atypical antipsychotic drugs significantly reduced the N-methyl-4-phenylpyridinium ion-induced cell loss, whereas haloperidol (10-100 micro m) did not have this protective effect. Hoechst 33258 staining revealed the apoptotic nuclear features of the N-methyl-4-phenylpyridinium ion-induced cell death, and showed that the atypical antipsychotic drugs, but not haloperidol, effectively prevented PC12 cells from this N-methyl-4-phenylpyridinium ion-induced apoptosis. DNA fragmentation assays further confirmed the N-methyl-4-phenylpyridinium ion-induced nuclear fragmentation. Pretreatment with the atypical antipsychotic drugs completely prevented this nuclear fragmentation, whereas haloperidol only partially prevented it. In vitro oligonucleotide assays indicated an activation of a specific glycosylase that recognizes and cleaves bases (at the 8-hydroxyl-2-deoxyguanine site) that were damaged by N-methyl-4-phenylpyridinium ion. Pretreatment with the atypical antipsychotic drugs more effectively attenuated this N-methyl-4-phenylpyridinium ion-induced activation than did haloperidol. Northern blot analyses showed that the atypical antipsychotic drugs, but not haloperidol, blocked the N-methyl-4-phenylpyridinium ion-induced substantial increase of copper/zinc superoxide dismutase mRNA in PC12 cells. Atypical antipsychotic drugs slightly up-regulated the expression of copper/zinc superoxide dismutase mRNA, whereas haloperidol strongly increased the expression of copper/zinc superoxide dismutase mRNA. These data may account for the different therapeutic effects and side-effect profiles of typical and atypical antipsychotic drugs in schizophrenia.