Nitric oxide from neuronal nitric oxide synthase sensitises neurons to hypoxia-induced death via competitive inhibition of cytochrome oxidase

Astragalin and Isoquercitrin Isolated from Aster scaber Suppress LPS-Induced Neuroinflammatory Responses in Microglia and Mice

The current study investigated the anti-neuroinflammatory effects and mechanisms of astragalin (Ast) and isoquercitrin (Que) isolated from chamchwi (Aster scaber Thunb.) in the lipopolysaccharide (LPS)-activated microglia and hippocampus of LPS induced mice. LPS induced increased cytotoxicity, nitric oxide (NO) production, antioxidant activity, reactive oxygen species (ROS), inducible nitric oxide synthase (iNOS) expression, the release of pro-inflammatory cytokines, protein kinase B phosphorylation, and mitogen-activated protein kinases (MAPK) phosphorylation in LPS-treated microglial cells. Intraperitoneal injection of LPS also induced neuroinflammatory effects in the murine hippocampus. Ast and Que significantly reduced LPS-induced production of NO, iNOS, and pro-inflammatory cytokines in the microglia and hippocampus of mice. Therefore, anti-inflammatory effects on MAPK signaling pathways mediate microglial cell and hippocampus inflammation. In LPS-activated microglia and hippocampus of LPS-induced mice, Ast or Que inhibited MAPK kinase phosphorylation by extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 signaling proteins. Ast and Que inhibited LPS-induced ROS generation in microglia and increased 1,1-diphenyl-2-picrylhydrazyl radical scavenging. In addition, LPS treatment increased the heme oxygenase-1 level, which was further elevated after Ast or Que treatments. Ast and Que exert anti-neuroinflammatory activity by down-regulation of MAPKs signaling pathways in LPS-activated microglia and hippocampus of mice.

Proteostasis Disturbances and Inflammation in Neurodegenerative Diseases

Protein homeostasis (proteostasis) disturbances and inflammation are evident in normal aging and some age-related neurodegenerative diseases. While the proteostasis network maintains the integrity of intracellular and extracellular functional proteins, inflammation is a biological response to harmful stimuli. Cellular stress conditions can cause protein damage, thus exacerbating protein misfolding and leading to an eventual overload of the degradation system. The regulation of proteostasis network is particularly important in postmitotic neurons due to their limited regenerative capacity. Therefore, maintaining balanced protein synthesis, handling unfolding, refolding, and degrading misfolded proteins are essential to preserve all cellular functions in the central nervous sysytem. Failing proteostasis may trigger inflammatory responses in glial cells, and the consequent release of inflammatory mediators may lead to disturbances in proteostasis. Here, we review the mechanisms of proteostasis and inflammatory response, emphasizing their role in the pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Furthermore, we discuss the interplay between proteostatic stress and excessive immune response that activates inflammation and leads to dysfunctional proteostasis.

Neuronal Cell Death

Neuronal cell death occurs extensively during development and pathology, where it is especially important because of the limited capacity of adult neurons to proliferate or be replaced. The concept of cell death used to be simple as there were just two or three types, so we just had to work out which type was involved in our particular pathology and then block it. However, we now know that there are at least a dozen ways for neurons to die, that blocking a particular mechanism of cell death may not prevent the cell from dying, and that non-neuronal cells also contribute to neuronal death. We review here the mechanisms of neuronal death by intrinsic and extrinsic apoptosis, oncosis, necroptosis, parthanatos, ferroptosis, sarmoptosis, autophagic cell death, autosis, autolysis, paraptosis, pyroptosis, phagoptosis, and mitochondrial permeability transition. We next explore the mechanisms of neuronal death during development, and those induced by axotomy, aberrant cell-cycle reentry, glutamate (excitoxicity and oxytosis), loss of connected neurons, aggregated proteins and the unfolded protein response, oxidants, inflammation, and microglia. We then reassess which forms of cell death occur in stroke and Alzheimer's disease, two of the most important pathologies involving neuronal cell death. We also discuss why it has been so difficult to pinpoint the type of neuronal death involved, if and why the mechanism of neuronal death matters, the molecular overlap and interplay between death subroutines, and the therapeutic implications of these multiple overlapping forms of neuronal death.

Modeling the detailed kinetics of mitochondrial cytochrome c oxidase: Catalytic mechanism and nitric oxide inhibition

Cytochrome c oxidase (CcO) catalyzes the exothermic reduction of O₂ to H₂O by using electrons from cytochrome c, and hence plays a crucial role in ATP production. Although details on the enzyme structure and redox centers involved in O₂ reduction have been known, there still remains a considerable ambiguity on its mechanism of action, e.g., the number of sequential electrons donated to O₂ in each catalytic step, the sites of protonation and proton pumping, and nitric oxide (NO) inhibition mechanism. In this work, we developed a thermodynamically constrained mechanistic mathematical model for the catalytic action of CcO based on available kinetic data. The model considers a minimal number of redox centers on CcO and couples electron transfer and proton pumping driven by proton motive force (PMF), and accounts for the inhibitory effects of NO on the reaction kinetics. The model is able to fit well all the available kinetic data under diverse experimental conditions with a physiologically realistic unique parameter set. The model predictions show that: 1) the apparent K_m of O₂ varies considerably and increases from fully reduced to fully oxidized cytochrome c depending on pH and the energy state of mitochondria, and 2) the intermediate enzyme states depend on pH and cytochrome c redox fraction and play a central role in coupling mitochondrial respiration to PMF. The developed CcO model can easily be integrated into existing mitochondrial bioenergetics models to understand the role of the enzyme in controlling oxidative phosphorylation in normal and disease conditions.

Up-Regulation of Neuronal Nitric Oxide Synthase Expression by Cobalt Chloride Through a HIF-1α Mechanism in Neuroblastoma Cells

Nitric oxide (NO) plays a dual role in response to neural hypoxia. NO is synthesized by three isoforms of nitric oxide synthase (NOS), among which the neuronal NOS (nNOS) is predominant in the nervous system. Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor that is induced under hypoxic conditions, but its correlation with nNOS remains unclear. In the present study, we aimed at clarifying the regulation pattern of the nNOS expression in response to cobalt chloride (CoCl2), a widely used chemical mimic of hypoxia, and the role of HIF-1α in this process in neuroblastoma cells. We found CoCl2 evidently increased the nNOS expression and NO production in human neuroblastoma SK-N-SH cells, but the effect of CoCl2 on NO was partially abrogated by 7-nitroindazole, a selective inhibitor for nNOS. Importantly, we identified a hypoxia response element (HRE) within the nNOS promoter, to which HIF-1α may bind, and CoCl2 greatly enhanced the HIF-1α expression and its binding to the HRE. Meanwhile, we demonstrated that this HRE was functionally important for the activation of the nNOS transcription, and CoCl2 increased the transcriptional activity of the nNOS promoter through this HRE. Taken together, our study shows that CoCl2 may induce the nNOS expression and NO production through a HIF-1α mechanism in neuroblastoma cells, which may provide a potential target for the treatment of neurological hypoxic disorders caused by NO dysregulation.

Implications of glial nitric oxide in neurodegenerative diseases

Nitric oxide (NO) is a pleiotropic janus-faced molecule synthesized by nitric oxide synthases (NOS) which plays a critical role in a number of physiological and pathological processes in humans. The physiological roles of NO depend on its local concentrations, as well as its availability and the nature of downstream target molecules. Its double-edged sword action has been linked to neurodegenerative disorders. Excessive NO production, as the evoked by inflammatory signals, has been identified as one of the major causative reasons for the pathogenesis of several neurodegenerative diseases. Moreover, excessive NO synthesis under neuroinflammation leads to the formation of reactive nitrogen species and neuronal cell death. There is an intimate relation between microglial activation, NO and neuroinflammation in the human brain. The role of NO in neuroinflammation has been defined in animal models where this neurotransmitter can modulate the inflammatory process acting on key regulatory pathways, such as those associated with excitotoxicity processes induced by glutamate accumulation and microglial activation. Activated glia express inducible NOS and produce NO that triggers calcium mobilization from the endoplasmic reticulum, activating the release of vesicular glutamate from astroglial cells resulting in neuronal death. This change in microglia potentially contributes to the increased age-associated susceptibility and neurodegeneration. In the current review, information is provided about the role of NO, glial activation and age-related processes in the central nervous system (CNS) that may be helpful in the isolation of new therapeutic targets for aging and neurodegenerative diseases.

Inflammation, Iron, Energy Failure, and Oxidative Stress in the Pathogenesis of Multiple Sclerosis

Multiple sclerosis is a chronic inflammatory demyelinating disease of the central nervous system. Different trigger pathologies have been suggested by the primary cytodegenerative “inside-out” and primary inflammation-driven “outside-in” hypotheses. Recent data indicate that mitochondrial injury and subsequent energy failure are key factors in the induction of demyelination and neurodegeneration. The brain weighs only a few percent of the body mass but accounts for approximately 20% of the total basal oxygen consumption of mitochondria. Oxidative stress induces mitochondrial injury in patients with multiple sclerosis and energy failure in the central nervous system of susceptible individuals. The interconnected mechanisms responsible for free radical production in patients with multiple sclerosis are as follows: (i) inflammation-induced production of free radicals by activated immune cells, (ii) liberation of iron from the myelin sheets during demyelination, and (iii) mitochondrial injury and thus energy failure-related free radical production. In the present review, the different sources of oxidative stress and their relationships to patients with multiple sclerosis considering tissue injury mechanisms and clinical aspects have been discussed.

The Influence of Vitamin D Treatment on the Inducible Nitric Oxide Synthase (INOS) Expression in Primary Hippocampal Neurons

INTRODUCTION

Neurodegeneration is a process that is characterized by the loss of neuronal structure and function and eventually ends with neuronal death. An elevated level of inducible nitric oxide synthase (iNOS) is suggested to accompany this process by inducing oxidative and nitrosative damage. Vitamin D is reported to protect glial cells against neurotoxicity via suppressing iNOS synthesis. Though there was no data about whether iNOS is regulated by vitamin D in hippocampal neurons. In this study our aim was to determine any alteration in iNOS expression of hippocampal neurons in response to vitamin D treatment.

METHOD

Twenty four and 48 hours of vitamin D treatments were performed on primary hippocampal neuron cultures that were prepared from Sprague dawley rat embryos (E18). The alterations in the iNOS mRNA expression were determined with quantative real time polymerase chain reaction (qRT-PCR). The cytotoxicity levels of each group were investigated by the measurement of lactate dehydrogenase (LDH) that is released to culture medium.

RESULTS

No difference was observed between groups in 24 hours of treatment regarding the iNOS expression. Though the iNOS mRNA level of vitamin D treated group was significantly lower than that of control group on the 48th hours of treatment (p<.001). Vitamin D treatment also attenuated the LDH release which is an indicator of cytotoxicity (p<.001).

CONCLUSION

Our results indicated that vitamin D has the potential to prevent oxidative damage by suppressing iNOS expression.

Inhibition of autophagy and glycolysis by nitric oxide during hypoxia-reoxygenation impairs cellular bioenergetics and promotes cell death in primary neurons

Excessive nitric oxide (NO) production is known to damage mitochondrial proteins and the autophagy repair pathway and so can potentially contribute to neurotoxicity. Accordingly, we hypothesized that protection against protein damage from reactive oxygen and nitrogen species under conditions of low oxygen by the autophagy pathway in neurons would be impaired by NO and enhance bioenergetic dysfunction. Rat primary cortical neurons had the same basal cellular respiration in hypoxia as in normoxia, whereas NO-exposed cells exhibited a gradual decrease in mitochondrial respiration in hypoxia. Upon reoxygenation, the respiration in NO-treated cells did not recover to prehypoxic levels. Hypoxia-reoxygenation in the presence of NO was associated with inhibition of autophagy, and the inability to recover during reoxygenation was exacerbated by an inhibitor of autophagy, 3-methyladenine. The effects of hypoxia could be recapitulated by inhibiting glycolytic flux under normoxic conditions. Under both normoxic and hypoxic conditions NO exposure induced immediate stimulation of glycolysis, but prolonged NO exposure, associated with irreversible inhibition of mitochondrial respiration in hypoxia, inhibited glycolysis. Importantly, we found that NO inhibited basal respiration under normoxic conditions only when glucose was absent from the medium or glycolysis was inhibited by 2-deoxy-d-glucose, revealing a novel NO-dependent mechanism for the inhibition of mitochondrial respiration that is modulated by glycolysis. Taken together these data suggest an oxygen-dependent interaction between mitochondrial respiration, glycolysis, and autophagy in protecting neuronal cells exposed to NO. Importantly, they indicate that mitochondrial dysfunction is intimately linked to a failure of glycolytic flux induced by exposure to NO. In addition, these studies provide new insights into the understanding of how autophagy and NO may play interactive roles in neuroinflammation-induced cellular damage, which is pertinent to our understanding of the pathology of neurodegenerative diseases in which excessive NO is generated.

Deletion of UCP2 in iNOS deficient mice reduces the severity of the disease during experimental autoimmune encephalomyelitis

Uncoupling protein 2 is a member of the mitochondrial anion carrier family that is widely expressed in neurons and the immune cells of humans. Deletion of Ucp2 gene in mice pre-activates the immune system leading to higher resistance toward infection and to an increased susceptibility to develop chronic inflammatory diseases as previously exemplified with the Experimental Autoimmune Encephalomyelitis (EAE), a mouse model for multiple sclerosis. Given that oxidative stress is enhanced in Ucp2−/− mice and that nitric oxide (NO) also plays a critical function in redox balance and in chronic inflammation, we generated mice deficient for both Ucp2 and iNos genes and submitted them to EAE. Mice lacking iNos gene exhibited the highest clinical score (3.4+/−0.5 p<0.05). Surprisingly, mice deficient for both genes developed milder disease with reduced immune cell infiltration, cytokines and ROS production as compared to iNos−/− mice.

Inflammatory neurodegeneration and mechanisms of microglial killing of neurons

Inflammatory neurodegeneration contributes to a wide variety of brain pathologies. A number of mechanisms by which inflammatory-activated microglia and astrocytes kill neurons have been identified in culture. These include: (1) acute activation of the phagocyte NADPH oxidase (PHOX) found in microglia, (2) expression of the inducible nitric oxide synthase (iNOS) in glia, and (3) microglial phagocytosis of neurons. Activation of PHOX (by cytokines, beta-amyloid, prion protein, lipopolysaccharide, ATP, or arachidonate) causes microglial proliferation and inflammatory activation; thus, PHOX is a key regulator of inflammation. However, activation of PHOX alone causes little or no death, but when combined with iNOS expression results in apparent apoptosis via peroxynitrite production. Nitric oxide (NO) from iNOS expression also strongly synergizes with hypoxia to induce neuronal death because NO inhibits cytochrome oxidase in competition with oxygen, resulting in glutamate release and excitotoxicity. Finally, microglial phagocytosis of these stressed neurons may contribute to their loss.

The NO donor DETA-NONOate reversibly activates an inward current in neurones and is not mediated by the released nitric oxide

BACKGROUND AND PURPOSE

It has been previously shown that high levels of nitric oxide (NO), from NO donors, kill neurones, but the mechanisms are unclear.

EXPERIMENTAL APPROACH

The effects of NO donors on the electrical properties of rat cultured cerebellar granule cells (CGC neurones) were investigated using the whole-cell patch-clamp technique.

KEY RESULTS

The NO donor (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate or NOC-18) caused a rapid, persistent, but fully reversible inward current that was associated with an increase in baseline noise and was concentration dependent (100 microM-10 mM). The response to 3 mM DETA-NONOate was completely inhibited by 1 mM gadolinium, but not by NO scavengers (1 mM haemoglobin or 1 mM PTIO) or glutamate receptor antagonists (10 microM MK-801 or 60 microM CNQX). Application of decomposed 3 mM DETA-NONOate or 3 mM nitrite had no effect. In contrast, the NO donor S-nitrosoglutathione (GSNO) caused a rapid, persistent, but fully reversible outward current that was also concentration dependent (1-10 mM). The 3 mM GSNO response was unaltered by NO scavengers, glutamate antagonists or gadolinium, but was mimicked by decomposed 3 mM GSNO and 3 mM oxidized glutathione.

CONCLUSIONS AND IMPLICATIONS

These results suggest that DETA-NONOate directly activates cation-selective channels, causing an inward current in CGCs. In contrast, GSNO causes an outward current in these cells. Some of the effects of these NO donors are independent of NO, and thus caution is required in interpreting results when using high concentrations of these compounds.

Mechanisms of inflammatory neurodegeneration: iNOS and NADPH oxidase

Inflammation contributes to a wide variety of brain pathologies, apparently via glia killing neurons. A number of mechanisms by which inflammatory-activated microglia and astrocytes kill neurons have been identified in culture. These include iNOS (inducible nitric oxide synthase), which is expressed in glia only during inflammation, and PHOX (phagocytic NADPH oxidase) found in microglia and acutely activated by inflammation. High levels of iNOS expression in glia cause (i) NO (nitric oxide) inhibition of neuronal respiration, resulting in neuronal depolarization and glutamate release, followed by excitotoxicity, and (ii) glutamate release from astrocytes via calcium-dependent vesicular release. Hypoxia strongly synergizes with iNOS expression to induce neuronal death via mechanism (i), because NO inhibits cytochrome oxidase in competition with oxygen. Activation of PHOX (by cytokines, beta-amyloid, prion protein, ATP or arachidonate) causes microglial proliferation and inflammatory activation; thus PHOX is a key regulator of inflammation. Activation of PHOX alone causes no death, but when combined with expressed iNOS results in extensive neuronal death via peroxynitrite production.