Synaptic mechanisms of cadmium neurotoxicity

正Cadmium(Cd) is a toxic heavy metal ubiquitously distributed in the environment(water, air, food, smoke) with extreme ability to accumulate in the human body due to its delayed clearance(half-life time 15–30 years). Consequently, prolonged exposure to low doses of Cd causes multi-organ toxicity. Remarkably,     

Subcellular mechanisms of lead neurotoxicity

The neurotoxic effects of inorganic lead (Pb) involve inhibition of calcium-dependent acetylcholine release and increases in calcium-dependent dopamine release. These apparently differential effects of Pb are associated with differing Pb-calcium (Ca) interactions: Pb blocks 45Ca binding to peripheral cholinergic ganglia and increases 45Ca binding to synaptosomes prepared from caudate nucleus (CN). Pb-induced increases in CN 45Ca binding did not result from nonspecific disruption of selective ion permeability of the membrane. Also, the Na-K ATPase-linked Ca extrusion system of synpatosomes was not affected by Pb. A Pb-sodium (Na) interaction was found such that elevation of intrasynaptosomal Na reversed effects of Pb on 45Ca binding. The intracellular localization of this effect appeared to be primarily at the mitochondrial level. Pb inhibited Na-induced release of 45Ca from preloaded mitochondria. This action may be translated into increased transmembrane flux of exogenous Ca, and thence into increased exocytotic events at the synapse. The apparently neurotransmitter-specific effects of Pb, cholinergic inhibition and dopaminergic augmentation, are hypothesized to result from different Pb-Ca interactions which are determined by the specific localization of Pb within nerve endings.     

Oxidative mechanisms underlying methyl mercury neurotoxicity

Cerebellar granule cells from 5-12-day-old rats can be incubated in suspension at 37 degrees C for up to 3 hr with minimal decline in viability. Methyl mercury was found to produce time- and concentration-dependent cell killing with greater than 85% cell death after 3 hr exposure to a concentration of 20 microM. Previously characterized inhibition of protein and RNA synthesis as well as known methyl mercury-induced defects in cellular ATP production have been shown to be incapable of causing this degree of cell death. Here we report that methyl mercury induced a concentration-dependent increase in membrane lipoperoxidation and a rapid decline in reduced glutathione in this cerebellar neuronal preparation. Hydrogen peroxide at 5 mM was found to closely reproduce each of the cytotoxic effects manifested by methyl mercury suggesting that oxidizing conditions produced by methyl mercury may account for the observed cell death. Methyl mercury-induced lipoperoxidation was not the cause of cell death since malonaldehyde production could be blocked by alpha-tocopherol or EDTA without appreciable protecting against cell death. Significant protection from methyl mercury-induced cell death was observed, with EGTA, deferoxamine and KCN. We propose that oxidative events contribute to the toxic mechanism of action of methyl mercury in isolated cerebellar granule neurons.     

Estradiol enhances the neurotoxicity of glutamate in GT1-7 cells through an estrogen receptor-dependent mechanism

Glutamate plays an important role in neuroendocrine regulation of reproduction through acting on the N-methyl-D-asparate receptor (NMDAR) in the preoptic area (POA). However, a larger dose of glutamate is neurotoxic. Estradiol (E2) increases the responsiveness of neurons to glutamate through activation and/or expression of NMDAR. In order to investigate whether estradiol modulates the neurotoxic effect of glutamate on the neurons through estrogen receptor (ER), immortalized GT1-7 cells, which simultaneously express ER and NMDAR were used. Tamoxifen and ICI 182,780, ER antagonist, were used to investigate whether the ER is involved in the effect of estradiol on glutamate-induced neurotoxicity. MK-801, a NMDAR antagonist, was used to confirm the enhancement of NMDAR-mediated neurotoxicity by estradiol. Neurotoxicity was evaluated by cell viability and LDH efflux. Cell death was observed by flow cytometry and DNA fragmentation. The results showed that: (1) estradiol (10 nM, incubated for 3 days) significantly enhanced the glutamate-induced neuronal death; (2) the percentages of necrosis and apoptosis were elevated after glutamate treatment, and estradiol significantly enhanced the glutamate-induced cell death; (3) glutamate-induced DNA fragmentation was enhanced by E2-pretreatment; (4) the induction of cell death and increase of LDH efflux after glutamate treatment were also enhanced by E2-pretreatment; (5) both the tamoxifen and ICI 182,780 abolished the estradiol-enhanced NMDAR expression and neurotoxicity of glutamate; (6) higher dose of MK-801 (2 microM) was needed in E2-pretreated cells than in non-E2-pretreated group to block the glutamate-induced neurotoxicity. These results suggested that pretreatment of estradiol might enhance the expression of NMDAR and subsequent glutamate-induced neurotoxicity on the GT1-7 cells through an ER-dependent manner.     

Biochemical mechanisms of developmental neurotoxicity of methylmercury

Methylmercury has been designated a "behavioral teratogen" because of its ability to evoke abnormalities in the absence of gross morphological damage to the developing brain. Recent work indicates that exposure to doses of methylmercury associated with neurobehavioral actions causes early alterations in brain ornithine decarboxylase, an enzyme whose activity is related to the coordination of cellular maturation. These effects are followed by regionally-targeted perturbation of cell replication and differentiation, indicated by measurements of nucleic acid and protein synthesis and levels. Neurobehavioral disturbances are associated with postnatal alterations in synaptogenesis and synaptic activity, as exemplified by studies in catecholaminergic pathways. Thus, methylmercury alters neurotransmitter uptake and turnover in presynaptic terminals, as well as development of postsynaptic adrenergic receptor binding sites. These changes result in aberrant signal transmission across the synapse, with consequent effects on synaptic function and ultimately on the communication of trophic developmental signals which ordinarily pass from neuron to target tissue. Although the specific linkages among the various biochemical effects of developmental exposure to methylmercury remain to be elucidated, studies of this type can serve as a model with which to understand the subcellular mechanisms underlying behavioral teratogenesis.     

Neurotoxins and neurotoxicity mechanisms. An overview

Neurotoxlns represent unique chemical tools, providing a means to 1) gain insight into cellular mechanisms of apopotosis and necrosis, 2) achieve a morphological template for studies otherwise unattainable, 3) specifically produce a singular phenotype of denervation, and 4) provide the starting point to delve into processes and mechanisms of nerve regeneration and sprouting. There are many other notable uses of neurotoxins in neuroscience research, and ever more being discovered each year. The objective of this review paper is to highlight the broad areas of neuroscience in which neurotoxins and neurotoxicity mechanism come into play. This shifts the focus away from neurotoxins per se, and onto the major problems under study today. Neurotoxins broadly defined are used to explore neurodegenerative disorders, psychiatric disorders and substance use disorders. Neurotoxic mechanisms relating to protein aggregates are indigenous to Alzheimer disease, Parkinson’s disease. NeuroAIDS is a disorder in which microglia and macrophages have enormous import. The gap between the immune system and nervous system has been bridged, as neuroinflammation is now considered to be part of the neurodegenerative process. Related mechanisms now arise in the process of neurogenesis. Accordingly, the entire spectrum of neuroscience is within the purview of neurotoxins and neurotoxicity mechanisms. Highlights on discoveries in the areas noted, and on selective neurotoxins, are included, mainly from the past 2 to 3 years.     

Phenidone prevents kainate-induced neurotoxicity via antioxidant mechanisms

Acculmulating evidence indicates that a marked generation of oxygen free radicals derived from the metabolism of arachidonic acid causes neurodegeneration. Recently, we have demonstrated that the novel antioxidant actions mediated by phenidone, a dual inhibitor of cyclooxygenase/lipoxygenase pathways, may play a crucial role in preventing neuroexcitotoxicity in vitro [Neurosci. Lett. 272 (1999) 91], and that phenidone significantly attenuates kainic acid (KA)-induced seizures via inhibiting the synthesis of Fos-related antigen protein [Brain Res. 782 (1998) 337]. In order to extend our understanding of the pharmacological intervention of phenidone, we evaluated the antioxidant activity of this compound in vivo in the present study. In order to better understand the significance of a blockade of both the cyclooxygenase and lipoxygenase pathways, we studied the effects of aspirin (ASP; a non-selective inhibitor of cyclooxygenase), NS-398 (a selective inhibitor of cyclooxygenase-2), esculetin (an inhibitor of lipoxygenase) and phenidone on lipid peroxidation, protein oxidation, and glutathione (GSH) status in the rat hippocampus after KA administration. ASP (7.5 or 15 mg/kg), NS-398 (10 or 20 mg/kg), esculetin (5 or 10 mg/kg) or phenidone (25, 50 or 100 mg/kg) was administered orally five times every 12 h before the injection of KA (10 mg/kg, i.p.). The KA-induced toxic behavioral signs, oxidative stress (lipid peroxidation and protein oxidation), impairment of GSH status, and the loss of hippocampal neurons were dose-dependently attenuated by the phenidone, NS-398+esculetin, and ASP+esculetin. However, ASP, NS-398 and esculetin alone failed to protect against the neurotoxicities induced by KA. Therefore, the results suggest that protection by blockade of both cyclooxygenase and lipoxygenase pathways against KA-induced neuroexcitotoxicity is via antioxidant actions. However, a novel anticonvulsant/neuroprotective effect mediated by phenidone remains to be further characterized.     

Fenobucarb-induced developmental neurotoxicity and mechanisms in zebrafish

Fenobucarb (2-sec-butylphenyl methylcarbamate, BPMC) is an extensively used carbamate insecticide. Its developmental neurotoxicity and the underlying mechanisms have not been well investigated. In this study, zebrafish embryos were exposed to various concentrations of BPMC from 6 hpf (hours post fertilization, hpf) to 120 hpf. BPMC induced developmental toxicity with reduced motility in larval zebrafish. The spinal cord neutrophil infiltration, increased ROS production, caspase 3 and 9 activation, central nerve and peripheral motor neuron damage, axon and myelin degeneration were observed in zebrafish treated with BPMC generally in a dose-dependent manner. The expression of eight marker genes for nervous system function or development, namely, a1-tubulin, shha, elavl3, gap43, syn2a, gfap, mbp and manf, was significantly downregulated following BPMC exposure. AChE activity reduction and ache gene expression suppression was also found significantly in BPMC-treated zebrafish. These results indicate that BPMC is highly toxic to zebrafish and that BPMC induces zebrafish developmental neurotoxicity through pathways involved in inflammation, oxidative stress, degeneration and apoptosis.     

Microparticles from patients with multiple organ dysfunction syndrome and sepsis support coagulation through multiple mechanisms

AIM: We investigated the occurrence and thrombin generating mechanisms of circulating microparticles (MP) in patients with multiple organ dysfunction syndrome (MODS) and sepsis. METHODS: MP, isolated from blood of patients (n = 9) and healthy controls (n = 14), were stained with cell-specific monoclonal antibodies (MoAbs) or anti-tissue factor (anti-TF) MoAb and annexin V, and analyzed by flow cytometry. To assess their thrombin-generating capacity, MP were reconstituted in normal plasma. The coagulation activation status in vivo was quantified by plasma prothrombin fragment F1+2- and thrombin-antithrombin (TAT) measurements. RESULTS: Annexin V-positive MP in the patients originated predominantly from platelets (PMP), and to a lesser extent from erythrocytes, endothelial cells (EMP) and granulocytes (GMP). Compared to healthy controls, the numbers of annexin V-positive PMP and TF-exposing MP were decreased (p = <0.001 for both), EMP were decreased (E-selectin, p = 0.003) or found equal (CD144, p = 0.063), erythrocyte-derived MP were equal (p = 0.726), and GMP were increased (p = 0.008). GMP numbers correlated with plasma concentrations of elastase (r = 0.70, p = 0.036), but not with C-reactive-protein or interleukin-6 concentrations. Patient samples also contained reduced numbers of annexin V-negative PMP, and increased numbers of erythrocyte-derived MP and GMP (p = 0.005, p = 0.021 and p <0.001, respectively). Patient MP triggered thrombin formation, which was reduced compared to the healthy controls (p = 0.008) and strongly inhibited by an anti-factor XII MoAb (two patients), by anti-factor XI MoAb (eight patients) or by anti-TF MoAb (four patients). Concentrations of F1+2 and TAT were elevated (p = 0.005 and p = 0.001, respectively) and correlated inversely with the number of circulating MP (and r = -0.51, p = 0.013, and r = -0.65, p = 0.001, respectively) and their thrombin generation capacity (F1+2: r= -0.62, p = 0.013). CONCLUSIONS: In patients with MODS and sepsis relatively low numbers of MP are present that differ from controls in their cellular origin, numbers and coagulation activation mechanisms.     

Mercury neurotoxicity: mechanisms of blood-brain barrier transport

Mercury exists in a wide variety of physical and chemical states, each of which has unique characteristics of target organ toxicity. The classic symptoms associated with exposure to elemental mercury vapor (Hg0) and methylmercury (CH3Hg+; MeHg) involve the central nervous system (CNS), while the kidney is the target organ for the mono- and divalent salts of mercury (Hg+ and Hg++, respectively). Physical properties and redox potentials determine the qualitative and quantitative differences in toxicity among inorganic mercury compounds, while the ability of MeHg to cross the blood-brain barrier accounts for its accumulation in the CNS and a clinical picture that is dominated by neurological disturbances. This review gives an up-to-date account of mercury's physical and chemical properties and its interaction with biologically active sites pertinent to transport across the blood-brain barrier, a major regulator of the CNS millieu.     

Bone marrow stromal cells protect and repair damaged neurons through multiple mechanisms

A surprising shortage of information surrounds the mechanism by which bone marrow stromal cells (BMSC) restore lost neurologic functions when transplanted into the damaged central nervous system. To clarify the issue, the BMSC were cocultured with the neurons using two paradigms: the cell-mixing coculture technique and three-dimensional coculture technique. The green fluorescent protein (GFP)-expressing BMSC were cocultured with the PKH-26-labelled neurons, using cell mixing coculture technique. GFP-positive, PKH-26-negative cells morphologically simulated the neurons and significantly increased the expression of MAP-2, Tuj-1, nestin, and GFAP. GFP/nestin-positive, PKH-26-negative cells increased from 13.6% +/- 6.7% to 32.1% +/- 15.5% over 7 days of coculture. They further enhanced Tuj-1 expression when cocultured with neurons exposed to 100 microM of glutamate for 10 min. About 20-30% of GFP-positive cells became positive for PKH-26 through coculture with the neurons, but the doubly positive cells did not increase when cocultured with glutamate-exposed neurons. Alternatively, the BMSC significantly ameliorated glutamate-induced neuronal damage when cocultured with the three-dimensional coculture technique. The protective effect was more prominent when coculture was started prior to glutamate exposure than when coculture was started just after glutamate exposure. ELISA analysis revealed that the BMSC physiologically produce NGF, BDNF, SDF-1alpha, HGF, TGFbeta-1, and IGF-1 and significantly enhanced the production of NGF and BDNF when cocultured with glutamate-exposed neurons. These findings strongly suggest that the BMSC may protect and repair the damaged neurons through multiple mechanisms, including transdifferentiation, cell fusion, and production of growth factors.     

Aluminum enhances glutamate-mediated neurotoxicity in organotypic cultures of rat hippocampus

Both, glutamate (GLU) and aluminum (Al) have been implicated in neuronal damage and/or death in certain human neurodegenerative disorders. Recent evidence suggests that aluminum (Al) may potentiate the increase in glutamate-induced intracellular calcium overload. The present ultrastructural study was undertaken to determine the effect of Al on the development of GLU-mediated neurotoxicity in tissue culture conditions. The experiments were performed on organotypic cultures of rat hippocampus treated with low, subtoxic concentration of GLU (50 microM) and AlCl3 (400 microM) added to the growth medium separately or in combination. The exposure of cultures to GLU in the presence of Al3+ ions for up to 24 hours resulted in the development of typical excitotoxic neuronal changes, whereas separate GLU treatment at subtoxic doses or single Al application did not produce any apparent tissue damage. The neuronal lesions resulting from the combined application of GLU plus Al consisted predominately of more or less pronounced mitochondrial abnormalities, which are characteristic for early excitotoxic events. Severe swelling of the mitochondria led to the disruption of their internal structure and finally resulted in an apparent microvacuolization of the perikaryal cytoplasm of some pyramidal neurons. The present morphological data evidenced that Al is capable to potentiate the GLU-induced degenerative changes in hippocampal neurons in vitro. This supports the view of a possible role of Al in the process of neurodegeneration and suggests that Al may participate in the development of glutamate-mediated excitotoxic neuronal injury under certain pathological conditions.     

Co-administration of low molecular weight heparin enhances the profibrinolytic effect of warfarin through different mechanisms

Background and Objective

Treatment with vitamin K antagonists (VKA) reduces fibrinolytic resistance through the inhibition of thrombin-mediated activation of thrombin activatable fibrinolysis inhibitor (TAFI). Because low-molecular weight heparin (LMWH) is co-administered with VKA during initiation of anticoagulant treatment, we evaluated the effect of dual anticoagulation on fibrinolytic resistance.

Patients and Methods

Two groups of patients were studied: 1) patients on stable warfarin; 2) patients starting oral anticoagulant therapy, who were evaluated during dual anticoagulation and after enoxaparin withdrawal. Only samples with an INR between 2 and 3 were compared. The resistance of clots to t-PA-induced fibrinolysis was evaluated in blood and plasma by thromboelastography (TEG) and turbidimetry, respectively.

Results

In patients on dual anticoagulation, blood fibrinolysis time (TEG) was significantly shorter than in patients on warfarin alone and significantly correlated with LMWH level. The profibrinolytic effect was partly ascribable to a reduction of thrombin-dependent TAFI activation: 1) thrombin and TAFIa generation were significantly reduced by dual anticoagulation; 2) the addition of enoxaparin to warfarin-blood reduced TAFI-mediated fibrinolysis inhibition. Patients on dual anticoagulation also displayed a reduction in clot strength, a phenomenon known to reduce fibrinolytic resistance. The profibrinolytic effect of LMWH co-administration was not seen in plasma, likely because TAFIa generation was below the threshold required to inhibit fibrinolysis.

Conclusions

Co-administration of LMWH in patients under VKA reduces the fibrinolytic resistance of blood clots via TAFI-dependent and TAFI-independent mechanisms. Further studies are warranted to assess the clinical implications of these findings.     

MPTP: a review of its mechanisms of neurotoxicity

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes damage to substantia nigra pars compacta (SNpc) dopaminergic (DA) neurons as seen in Parkinson's disease (PD). After systemic administration of MPTP, its active metabolite, MPP⁺, accumulates within SNpc DA neurons, where it inhibits ATP production and stimulates superoxide radical formation. The produced superoxide radicals react with nitric oxide (NO) to produce peroxynitrite, a highly reactive tissue-damaging species that damages proteins by oxidation and nitration. Only selected proteins appear nitrated, and among these is found tyrosine hydroxylase (TH), the rate limiting enzyme in DA synthesis, and the pre-synaptic protein α-synuclein. Peroxynitrite also nicks DNA, which, in turn, activates poly(ADP-ribose) polymerase (PARP). PARP activation consumes ATP, and thus acutely depletes the cell energy stores. This latter event aggravates the preexisting energy failure due to MPP⁺-induced mitochondrial respiration blockade and precipitates cell death. On the other hand, MPP⁺ also activates highly regulated cell death-associated molecular pathways that participate in the relentless demise of neurons in PD. Altogether, these findings support the view that MPTP's deleterious cascade of events include mitochondrial respiration deficit, oxidative stress, energy failure and activation of apoptotic genetic programs. Because of the similarity between the MPTP mouse model and PD, it is tempting to propose that a similar scenario applies to the pathogenesis of PD.     

Receptor mechanisms and circuitry underlying NMDA antagonist neurotoxicity.

NMDA glutamate receptor antagonists are used in clinical anesthesia, and are being developed as therapeutic agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the ability of these agents to produce neurotoxicity in adult rats and psychosis in adult humans compromises their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic disorders, like Alzheimer's disease and schizophrenia. Thus, understanding the mechanism underlying NRHypo-induced neurotoxicity and psychosis could have significant clinically relevant benefits. NRHypo neurotoxicity can be prevented by several classes of agents (e.g. antimuscarinics, non-NMDA glutamate antagonists, and alpha(2) adrenergic agonists) suggesting that the mechanism of neurotoxicity is complex. In the present study a series of experiments was undertaken to more definitively define the receptors and complex neural circuitry underlying NRHypo neurotoxicity. Injection of either the muscarinic antagonist scopolamine or the non-NMDA antagonist NBQX directly into the cortex prevented NRHypo neurotoxicity. Clonidine, an alpha(2) adrenergic agonist, protected against the neurotoxicity when injected into the basal forebrain. The combined injection of muscarinic and non-NMDA Glu agonists reproduced the neurotoxic reaction. Based on these and other results, we conclude that the mechanism is indirect, and involves a complex network disturbance, whereby blockade of NMDA receptors on inhibitory neurons in multiple subcortical brain regions, disinhibits glutamatergic and cholinergic projections to the cerebral cortex. Simultaneous excitotoxic stimulation of muscarinic (m(3)) and glutamate (AMPA/kainate) receptors on cerebrocortical neurons appears to be the proximal mechanism by which the neurotoxic and psychotomimetic effects of NRHypo are mediated.     

Beta-N-methylamino-L-alanine. Chronic oral administration is not neurotoxic to mice

Repeated dietary consumption of the neurotoxic amino acid beta-N-methylamino-L-alanine (BMAA), found in the seeds of Cycas circinalis, has been postulated as causing both amyotrophic lateral sclerosis (ALS) and the parkinsonism-dementia syndrome (PD) that were formerly very prevalent among the indigenous people of the Marianas Islands. Cynomolgus monkeys fed BMAA have been reported to develop behavioral and neuropathological changes like those found in human ALS. We gave large amounts of BMAA, totaling 15.5 g/kg of the L-isomer, by gavage to mice over 11 weeks without observing any behavioral abnormalities. When killed, these animals showed none of the neurochemical or neuropathological changes that would be expected in ALS or Parkinson's disease. Their striatal dopamine contents were normal, and there were no reductions in the contents of glutamate and aspartate in cerebral cortex like those encountered in sporadic human ALS. The results of this experiment do not support chronic ingestion of BMAA as the causative factor for Guamanian ALS or PD.