Role of spermine in amyloid beta-peptide-associated free radical-induced neurotoxicity

The polyamines, relatively low-molecular-weight aliphatic compounds, are the main inducers of eukaryotic cell growth and proliferation. Although polyamine requirements for cell growth are well defined, their role is still enigmatic. We have previously reported that amyloid beta-peptide (A beta), the main constituent of senile plaques in Alzheimer's disease (AD) brain, is toxic to neurons through a free radical-dependent oxidative stress mechanism and that A beta(1--42), the principal form of A beta in AD brain, causes an increase in polyamine metabolism manifested by up-regulated polyamine uptake and increased ornithine decarboxylase (ODC) activity. Both effects were prevented by the free radical scavenger vitamin E. Spermine has been reported to function directly as a free radical scavenger. In the current study, we aimed to address whether up-regulation of polyamine metabolism is a defense against, or a result of, A beta-induced oxidative stress by investigating the capability of spermine to quench A beta-associated free radicals in solution and to assert a protective function of spermine in neuronal culture against A beta. Pretreatment of cultured neurons with spermine prior to A beta exposure failed to prevent A beta-induced cell death. Indeed, A beta plus spermine added to cultured neurons was even more neurotoxic than either agent alone. Additionally, inhibition of the polyamine synthesis by difluoromethylornithine (DFMO) did not protect cells from A beta-induced free radical toxicity, and stimulation of the synthesis of putrescine and spermine by the aminopropyltransferase inhibitor S-adenosyl-1,8-diamino-thiooctane (AdoDATO), rather, further enhanced A beta-induced toxicity. Although spermine is capable of scavenging free radicals generated by A beta in solution as measured by electron paramagnetic resonance (EPR) spectroscopy, the up-regulated transport of exogenously added spermine together with A beta may lead to overaccumulation of a cellular spermine pool, with resulting enhanced neurotoxicity.

Cloning of dopamine, norepinephrine and serotonin transporters from monkey brain: relevance to cocaine sensitivity

We used RT-PCR to clone monoamine transporters from Macaca mulatta, Macaca fasicularis and Saimiri sciureus (dopamine transporter; DAT) and Macaca mulatta (norepinephrine transporter; NET and serotonin transporter; SERT). Monkey DAT, NET and SERT proteins were >98% homologous to human and, when expressed in HEK-293 cells, displayed drug affinities and uptake kinetics that were highly correlated with monkey brain or human monoamine transporters. In contrast to reports of other species, we discovered double (leucine for phenylalanine 143 and arginine for glutamine 509; Variant I) and single (proline for leucine 355; Variant II) amino acid variants of DAT. Variant I displayed dopamine transport kinetics and binding affinities for various DAT blockers (including cocaine) versus [3H] CFT (WIN 35, 428) that were identical to wild-type DAT (n=7 drugs; r(2)=0.991). However, we detected a six-fold difference in the affinity of cocaine versus [3H] cocaine between Variant I (IC(50): 488+/-102 nM, SEM, n=3) and wild-type DAT (IC(50): 79+/-8.2 nM, n=3, P<0.05). Variant II was localized intracellularly in HEK-293 cells, as detected by confocal microscopy, and had very low levels of binding and dopamine transport. Also discovered was a novel exon 5 splice variant of NET that displayed very low levels of transport and did not bind cocaine. With NetPhos analysis, we detected a number of highly conserved putative phosphorylation sites on extracellular as well as intracellular loops of the DAT, NET, and SERT, which may be functional for internalized transporters. The homology and functional similarity of human and monkey monoamine transporters further support the value of primates in investigating the role of monoamine transporters in substance abuse mechanisms, neuropsychiatric disorders and development of diagnostic and therapeutic agents.

Amyloid beta-peptide-associated free radical oxidative stress, neurotoxicity, and Alzheimer's disease

Given the increasing evidence of oxidative stress in AD brain and studies from different perspectives that appear to show a converging, central role for A beta in the pathogenesis and etiology of AD, insight into A beta-associated free radical oxidative stress will likely lead to a greater understanding of AD and, potentially, to better therapeutic strategies in this disorder. This article outlined methods to investigate markers of oxidative stress induced by A beta in brain membrane systems. Especially important are markers for protein oxidation, lipid peroxidation, and ROS generation by A beta. Oxidative stress and its sequelae are likely related to both necrotic and apoptotic mechanisms of neurotoxicity, and A beta-associated free radical oxidative stress may be of fundamental importance in Alzheimer's disease etiology and pathogenesis. The methods described here provide some means for investigating this possibility.

In vitro and in vivo oxidative stress associated with Alzheimer's amyloid beta-peptide (1-42)

The amyloid beta-peptide (A beta)-associated free radical oxidative stress model for neuronal death in Alzheimer's disease (AD) brain predicts that neuronal protein oxidation is a consequence of A beta-associated free radicals [8]. In this study we have used both in vitro and in vivo models of beta-amyloid (A beta) toxicity to detect free radical induced oxidative stress by the measure of protein carbonyl levels. These model systems employed cultured hippocampal neurons exposed to exogenous synthetic A beta(1-42) and transgenic Caenorhabditis elegans (C. elegans) animals expressing A beta(1-42). We also investigated the importance of the A beta(1-42) Met35 residue for free radical formation in peptide solution and for peptide-induced protein oxidation and neuronal toxicity in these model systems. A beta(1-42) in solution yielded an EPR spectrum, suggesting that free radicals are associated with this peptide; however, neither the reverse [A beta(42-1)] nor methionine-substituted peptide [A beta(1-42)Met35Nlc] gave significant EPR spectra, suggesting the importance of the methionine residue in free radical formation. A beta(1-42) addition to cultured hippocampal neurons led to both neurotoxicity (30.1% cell death, p < 0.001) and increased protein oxidation (158% of controls, p < 0.001). and both of those effects were not observed with reverse or Met35Nle substituted peptides. C. elegans transgenic animals expressing human A beta(1-42) also had significantly increased in vivo protein carbonyls (176% of control animals, p < 0.001), consistent with our model. In contrast, transgenic animals with a Met35cys substitution in A beta(1-42) showed no increased protein carbonyls in vivo, in support of the hypothesis that methionine is important in A beta-associated free radical oxidative stress. These results are discussed with reference to the A beta-associated free radical oxidative stress model of neurotoxicity in AD brain.

Alzheimer's amyloid beta-peptide associated free radicals increase rat embryonic neuronal polyamine uptake and ornithine decarboxylase activity: protective effect of vitamin E

Recent evidence indicates that alterations in brain polyamine metabolism may be critical for nerve cell survival after a free radical initiated neurodegenerative process. It has been shown previously that A beta(1-42) and A beta(25-35) are toxic to neurons through a free radical dependent oxidative mechanism. Treatment of rat embryonic hippocampal neuronal cultures with A beta-peptides increased ornithine decarboxylase (ODC) activity and spermidine uptake, suggesting that oxidative stress upregulates the polyamine mechanism for the repair of free radical damage. Pretreatment of the cells with vitamin E prior to A beta exposure decreased ODC activity and spermidine uptake to control level. This study is the first to demonstrate that A beta treated cells show an increased polyamine metabolism in response to free radical mediated oxidative stress and that the free radical scavenger vitamin E prevents these attenuations. These results are discussed with reference to Alzheimer's disease.

The antioxidant vitamin E modulates amyloid beta-peptide-induced creatine kinase activity inhibition and increased protein oxidation: implications for the free radical hypothesis of Alzheimer's disease

Amyloid beta-peptide (Abeta), the main constituent of senile plaques in Alzheimer's disease (AD) brain, is hypothesized to be a key factor in the neurodegeneration seen in AD. Recently it has been shown by us and others that the neurotoxicity of Abeta occurs in conjunction with free radical oxidative stress associated with the peptide. Abeta(1-40) and several other fragments of the Abeta sequence are associated with free radicals in solution that are detectable using electron paramagnetic resonance spectroscopy. These free radicals were shown to attack brain cell membranes, initiate lipid peroxidation, increase Ca2+ influx and damage membrane and cytosolic proteins. In AD brain obtained under rapid autopsy protocol, the activity of the oxidatively-sensitive enzyme creatine kinase was shown to be significantly reduced. We reasoned that Abeta-associated free radical-induced modification of creatine kinase activity and other markers of cellular damage might be modulated by free radical scavengers. Accordingly, this study demonstrates that vitamin E can modulate Abeta(25-35)-induced oxidative damage to creatine kinase and cellular proteins in cultured embryonic hippocampal neurons. These results, consistent with the hypothesis of free radical-mediated Abeta toxicity in AD, are discussed with deference to potential free radical scavengers as therapeutic agents for slowing the progression of AD.

Temporal relations among amyloid beta-peptide-induced free-radical oxidative stress, neuronal toxicity, and neuronal defensive responses

Amyloid beta-peptide (Abeta), the main constituent of senile plaques in Alzheimer's disease (AD) brain, is hypothesized to be a key factor in the neurodegeneration seen in AD. Recently it has been shown that the neurotoxicity of Abeta occurs in conjunction with free-radical oxidative stress associated with the peptide. In the present study, we investigated the temporal relations among the formation of Abeta-associated free radicals, the oxidative damage to, and the activation of antioxidant defense mechanisms in rat embryonic hippocampal neuronal culture subjected to toxic Abeta(25-35). Temporal electron paramagnetic resonance (EPR) spectroscopy results show that synthetic Abeta(25-35) forms free radicals rapidly after solubilization with a high signal intensity at initial time points. At those time points, neuronal toxicity and oxidative stress gradually increase as assessed by reduction of 3-[4,5-dimethylthiazol-2-yl)-2,5-diphenyl] tetrazolium bromide, trypan blue exclusion, formation of reactive oxygen species, and detection of protein carbonyl levels. The latter occurs before neurotoxicity. When the EPR signal intensity of Abeta solution decreases at later time points, neuronal toxicity levels off and remains the same until the end of the experiment. The oxidative-sensitive enzyme creatine kinase (CK) (brain isoform) (CK-BB) content increases at initial points of the Abeta treatment in correlation with the EPR signal to keep the CK activity constant, presumably to overcome the Abeta-induced oxidative insult. CK-BB content returns to normal levels by the end of the experiment. CK activity normalized to CK content implies the presence of inactivated CK molecules during the treatment. Both Mn SOD and Cu/Zn superoxide dismutase (SOD) mRNA levels show robust increases initially, which later return to control level with decreasing oxidative insult. These results are consistent with the notion that Abeta(25-35) promotes a rapid free-radical oxidative stress to neurons, which respond by modulating various oxidative stress-handling genes.