Nanometer size diesel exhaust particles are selectively toxic to dopaminergic neurons: the role of microglia, phagocytosis, and NADPH oxidase

Nanosize titanium dioxide stimulates reactive oxygen species in brain microglia and damages neurons in vitro

BACKGROUND

Titanium dioxide is a widely used nanomaterial whose photo-reactivity suggests that it could damage biological targets (e.g., brain) through oxidative stress (OS).

OBJECTIVES

Brain cultures of immortalized mouse microglia (BV2), rat dopaminergic (DA) neurons (N27), and primary cultures of embryonic rat striatum, were exposed to Degussa P25, a commercially available TiO(2) nanomaterial. Physical properties of P25 were measured under conditions that paralleled biological measures.

FINDINGS

P25 rapidly aggregated in physiological buffer (800-1,900 nm; 25 degrees C) and exposure media (approximately 330 nm; 37 degrees C), and maintained a negative zeta potential in both buffer (-12.2 +/- 1.6 mV) and media (-9.1 +/- 1.2 mV). BV2 microglia exposed to P25 (2.5-120 ppm) responded with an immediate and prolonged release of reactive oxygen species (ROS). Hoechst nuclear stain was reduced after 24-hr (>or=100 ppm) and 48-hr (>or=2.5 ppm) exposure. Microarray analysis on P25-exposed BV2 microglia indicated up-regulation of inflammatory, apoptotic, and cell cycling pathways and down-regulation of energy metabolism. P25 (2.5-120 ppm) stimulated increases of intracellular ATP and caspase 3/7 activity in isolated N27 neurons (24-48 hr) but did not produce cytotoxicity after 72-hr exposure. Primary cultures of rat striatum exposed to P25 (5 ppm) showed a reduction of immunohistochemically stained neurons and microscopic evidence of neuronal apoptosis after 6-hr exposure. These findings indicate that P25 stimulates ROS in BV2 microglia and is nontoxic to isolated N27 neurons. However, P25 rapidly damages neurons at low concentrations in complex brain cultures, plausibly though microglial generated ROS.

Chronic microglial activation and progressive dopaminergic neurotoxicity

PD (Parkinson's disease) is characterized by the selective and progressive loss of DA neurons (dopaminergic neurons) in the substantia nigra. Inflammation and activation of microglia, the resident innate immune cell in the brain, have been strongly linked to neurodegenerative diseases, such as PD. Microglia can respond to immunological stimuli and neuronal death to produce a host of toxic factors, including cytokines and ROS (reactive oxygen species). Microglia can also become persistently activated after a single stimulus and maintain the elevated production of both cytokines and ROS, long after the instigating stimulus is gone. Current reports suggest that this chronic microglial activation may be fuelled by either dying/damaged neurons or autocrine and paracrine signals from local glial cells, such as cytokines. Here, we review proposed mechanisms responsible for chronic neuroinflammation and explain the interconnected relationship between deleterious microglial activation, DA neuron damage and neurodegenerative disease.

Changes in neurotransmitter levels and proinflammatory cytokine mRNA expressions in the mice olfactory bulb following nanoparticle exposure

Recently, there have been increasing reports that nano-sized component of particulate matter can reach the brain and may be associated with neurodegenerative diseases. Previously, our laboratory has studied the effect of intranasal instillation of nano-sized carbon black (CB) (14 nm and 95 nm) on brain cytokine and chemokine mRNA expressions and found that 14-nm CB increased IL-1 beta, TNF-alpha, CCL2 and CCL3 mRNA expressions in the olfactory bulb, not in the hippocampus of mice. To investigate the effect of a single administration of nanoparticles on neurotransmitters and proinflammatory cytokines in a mouse olfactory bulb, we performed in vivo microdialysis and real-time PCR methods. Ten-week-old male BALB/c mice were implanted with guide cannula in the right olfactory bulb and, 1 week later, were instilled vehicle or CB (14 nm, 250 microg) intranasally. Six hours after the nanoparticle instillation, the mice were intraperitoneally injected with normal saline or 50 mug of bacteria cell wall component lipoteichoic acid (LTA), which may potentiate CB-induced neurologic effect. Extracellular glutamate and glycine levels were significantly increased in the olfactory bulb of CB-instilled mice when compared with vehicle-instilled control mice. Moreover, we found that LTA further increased glutamate and glycine levels. However, no alteration of taurine and GABA levels was observed in the olfactory bulb of the same mice. We also detected immunological changes in the olfactory bulb 11 h after vehicle or CB instillation and found that IL-1 beta mRNA expression was significantly increased in CB- and LTA-treated mice when compared with control group. However, TNF-alpha mRNA expression was increased significantly in CB- and saline-treated mice when compared with control group. These findings suggest that nanoparticle CB may modulate the extracellular amino acid neurotransmitter levels and proinflammatory cytokine IL-1 beta mRNA expressions synergistically with LTA in the mice olfactory bulb.

Microglial PHOX and Mac-1 are essential to the enhanced dopaminergic neurodegeneration elicited by A30P and A53T mutant alpha-synuclein

alpha-Synuclein, a gene whose mutations, duplication, and triplication has been linked to autosomal dominant familial Parkinson's disease (fPD), appears to play a central role in the pathogenesis of sporadic PD (sPD) as well. Enhancement of neurodegeneration induced by mutant alpha-synuclein has been attributed to date largely to faster formation of alpha-synuclein aggregates in neurons. Recently, we reported that microglial activation enhances wild type (WT) alpha-synuclein-elicited dopaminergic neurodegeneration. In the present study, using a primary mesencephalic culture system, we tested whether mutated alpha-synuclein could activate microglia more powerfully than WT alpha-synuclein, thereby contributing to the accelerated neurodegeneration observed in fPD. The results showed that alpha-synuclein with the A30P or A53T mutations caused greater microglial activation than WT alpha-synuclein. Furthermore, the extent of microglial activation paralleled the degree of dopaminergic neurotoxicity induced by WT and mutant alpha-synuclein. Mutant alpha-synuclein also induced greater production of reactive oxygen species than WT alpha-synuclein by NADPH oxidase (PHOX), and PHOX activation was linked to direct activation of macrophage antigen-1 (Mac-1) receptor, rather than alpha-synuclein internalization via scavenger receptors. These results have, for the first time, demonstrated that microglia are also critical in enhanced neurotoxicity induced by mutant alpha-synuclein.

Diesel exhaust particles induce matrix metalloprotease-1 in human lung epithelial cells via a NADP(H) oxidase/NOX4 redox-dependent mechanism

Chronic exposure to particulate air pollution is associated with lung function impairment. To determine the molecular mechanism(s) of this phenomenon, we investigated, in an alveolar human epithelial cell line (A549), whether diesel exhaust particles (DEPs), a main component of particulate air pollution, modulates the expression and activity of the matrix metalloprotease (MMP)-1, a collagenase involved in alveolar wall degradation. Interaction of DEPs with cigarette smoke, which also produces structural and functional lung alterations, was also investigated. A noncytotoxic concentration of DEPs induced an increase in MMP-1 mRNA and protein expression and activity in A549 cells without modifying the expression of the MMP inhibitors TIMP-1 and -2. This effect was not potentiated when cells were coexposed to noncytotoxic concentrations of cigarette smoke condensate. DEP-induced MMP-1 was associated with increased ERK 1/2 phosphorylation and upregulation of expression and activity of the NADPH oxidase analog NOX4. Cell transfection with a NOX4 small interfering RNA prevented these phenomena, showing the critical role of a NOX4 ERK 1/2 pathway in DEP-induced MMP-1 expression and activity. Similar results to those observed in A549 cells were obtained in another human lung epithelial cell line, NCI-H292. Furthermore, experiments in mice intratracheally instilled with DEPs confirmed the in vitro findings, showing the induction of NOX4 and MMP-1 protein expression in alveolar epithelial cells. We conclude that alveolar alterations secondary to MMP-1 induction could explain lung function impairment associated with exposure to particulate pollution.

Gene expression in nanotoxicology research: Analysis by differential display in BALB3T3 fibroblasts exposed to cobalt particles and ions

Broadly defined, nanoscale materials are substances in which at least one critical dimension is less than 100 nm. Nanoscale materials are employed in several industrial applications as well as in biology and medicine. Despite their wide use, very little research has been carried out on the potential toxicity of nanoparticles. For this reason, we report on a molecular approach in nanotoxicology research. Using the differential display technique, we focused our attention on mRNA expression in a BALB3T3 A31-1-1 cell line that was not exposed and exposed for 72 h to 1 microM of cobalt microparticles (Co-mu), nanoparticles (Co-nano), and ions. In the experiments, we obtained 10 differentially expressed sequences. These genes represent candidate biomarkers capable of indicating specific cellular effects after Co-nano exposure. In addition, our results show that treatment with Co-nano somehow activates cellular pathways of defense and repair mechanisms. It is also evident that molecular techniques are valuable tools in nanotoxicology research, where they will certainly find wide use.

Systemic LPS causes chronic neuroinflammation and progressive neurodegeneration

Inflammation is implicated in the progressive nature of neurodegenerative diseases, such as Parkinson's disease, but the mechanisms are poorly understood. A single systemic lipopolysaccharide (LPS, 5 mg/kg, i.p.) or tumor necrosis factor alpha (TNFalpha, 0.25 mg/kg, i.p.) injection was administered in adult wild-type mice and in mice lacking TNFalpha receptors (TNF R1/R2(-/-)) to discern the mechanisms of inflammation transfer from the periphery to the brain and the neurodegenerative consequences. Systemic LPS administration resulted in rapid brain TNFalpha increase that remained elevated for 10 months, while peripheral TNFalpha (serum and liver) had subsided by 9 h (serum) and 1 week (liver). Systemic TNFalpha and LPS administration activated microglia and increased expression of brain pro-inflammatory factors (i.e., TNFalpha, MCP-1, IL-1beta, and NF-kappaB p65) in wild-type mice, but not in TNF R1/R2(-/-) mice. Further, LPS reduced the number of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra (SN) by 23% at 7-months post-treatment, which progressed to 47% at 10 months. Together, these data demonstrate that through TNFalpha, peripheral inflammation in adult animals can: (1) activate brain microglia to produce chronically elevated pro-inflammatory factors; (2) induce delayed and progressive loss of DA neurons in the SN. These findings provide valuable insight into the potential pathogenesis and self-propelling nature of Parkinson's disease.

Basic science; metallothionein I and II attenuate the thalamic microglial response following traumatic axotomy in the immature brain

The clinical manifestations of inflicted traumatic brain injury in infancy most commonly result from intracranial hemorrhage, axonal stretch and disruption, and cerebral edema. Often hypoxia ischemia is superimposed, leading to early forebrain and later thalamic neurodegeneration. Such acute and delayed cellular injury activates microglia in the CNS. Although activated microglia provide important benefits in response to injury, microglial release of reactive oxygen species can be harmful to axotomized neurons. We have previously shown that the antioxidants metallothionein I and II (MT I & II) promote geniculocortical neuronal survival after visual cortex lesioning. The purpose of this investigation was to determine the influence of MT I & II on the density and rate of thalamic microglial activation and accumulation following in vivo axotomy. We ablated the visual cortex of 10-day-old and adult MT I & II knock out (MT(-/-)) and wild-type mice and then determined the density of microglia in the dorsal lateral geniculate nucleus (dLGN) over time. Compared to the wild-type strain, microglial activation occurred earlier in both young and adult MT(-/-) mice. Similarly, microglial density was significantly greater in young MT(-/-) mice 30, 36, and 48 hours after injury, and 3, 4, and 5 days after injury in MT(-/-) adults. In both younger and older mice, time and MT I & II deficiency each contributed significantly to greater microglial density. Only in younger mice did MT I & II expression significantly slow the rate (density x time) of microglial accumulation. These results suggest that augmentation of MT I & II expression may provide therapeutic benefits to infants with inflicted brain injury.

Ion channel blockade attenuates aggregated alpha synuclein induction of microglial reactive oxygen species: relevance for the pathogenesis of Parkinson's disease

Brain mononuclear phagocyte (perivascular macrophage and microglia, MG) inflammatory neurotoxins play a principal role in the pathogenesis of Parkinson's disease; chief among these are reactive oxygen species (ROS). We posit that aggregated, misfolded and oxidized alpha-synuclein (a major constituent of Lewy bodies), released or secreted from dying dopaminergic neurons, induces microglial ROS production that is regulated by ion channels and as such affects disease progression. To address this hypothesis, we performed patch clamp recordings of outward ionic currents in murine microglia and characterized their links to ROS production during alpha-synuclein stimulation. Aggregated nitrated alpha-synuclein induced ROS production in a dose-dependent manner that was inhibited by voltage-gated potassium current blockade, and to a more limited degree, by chloride current blockade. Interestingly, ROS produced in MG primed with tumor necrosis factor alpha and activated with phorbol myristate acetate was attenuated by voltage-gated potassium current blockade and more completely by chloride current blockade. In contrast, amyloid beta or cell membrane extract failed to induce microglial ROS production. Similar results were obtained using bone marrow-derived macrophages. The association of ROS production with specific plasma membrane ion currents provides a link between regulation of microglial ion transport and oxygen free radical production. Understanding these linkages may lead to novel therapeutics for Parkinson's disease where modulation of redox-related stress may slow disease progression.

Charge-associated effects of fullerene derivatives on microbial structural integrity and central metabolism

The effects of four types of fullerene compounds (C60, C60-OH, C60-COOH, C60-NH2) were examined on two model microorganisms (Escherichia coli W3110 and Shewanella oneidensis MR-1). Positively charged C60-NH2 at concentrations as low as 10 mg/L inhibited growth and reduced substrate uptake for both microorganisms. Scanning electron microscopy (SEM) revealed damage to cellular structures. Neutrally charged C60 and C60-OH had mild negative effects on S. oneidensis MR-1, whereas the negatively charged C60-COOH did not affect either microorganism's growth. The effect of fullerene compounds on global metabolism was further investigated using [3-13C]L-lactate isotopic labeling, which tracks perturbations to metabolic reaction rates in bacteria by examining the change in the isotopic labeling pattern in the resulting metabolites (often amino acids).1-3 The 13C isotopomer analysis from all fullerene-exposed cultures revealed no significant differences in isotopomer distributions from unstressed cells. This result indicates that microbial central metabolism is robust to environmental stress inflicted by fullerene nanoparticles. In addition, although C60-NH2 compounds caused mechanical stress on the cell wall or membrane, both S. oneidensis MR-1 and E. coli W3110 can efficiently alleviate such stress by cell aggregation and precipitation of the toxic nanoparticles. The results presented here favor the hypothesis that fullerenes cause more membrane stress 4-6 than perturbation to energy metabolism.7.

The scoop on the fly brain: glial engulfment functions in Drosophila

Glial cells provide support and protection for neurons in the embryonic and adult brain, mediated in part through the phagocytic activity of glia. Glial cells engulf apoptotic cells and pruned neurites from the developing nervous system, and also clear degenerating neuronal debris from the adult brain after neural trauma. Studies indicate that Drosophila melanogaster is an ideal model system to elucidate the mechanisms of engulfment by glia. The recent studies reviewed here show that many features of glial engulfment are conserved across species and argue that work in Drosophila will provide valuable cellular and molecular insight into glial engulfment activity in mammals.

Microglia-mediated neurotoxicity: uncovering the molecular mechanisms

Mounting evidence indicates that microglial activation contributes to neuronal damage in neurodegenerative diseases. Recent studies show that in response to certain environmental toxins and endogenous proteins, microglia can enter an overactivated state and release reactive oxygen species (ROS) that cause neurotoxicity. Pattern recognition receptors expressed on the microglial surface seem to be one of the primary, common pathways by which diverse toxin signals are transduced into ROS production. Overactivated microglia can be detected using imaging techniques and therefore this knowledge offers an opportunity not only for early diagnosis but, importantly, for the development of targeted anti-inflammatory therapies that might slow or halt the progression of neurodegenerative disease.

Neuroinflammation, Oxidative Stress and the Pathogenesis of Parkinson's Disease

Neuroinflammatory processes play a significant role in the pathogenesis of Parkinson's disease (PD). Epidemiologic, animal, human, and therapeutic studies all support the presence of an neuroinflammatory cascade in disease. This is highlighted by the neurotoxic potential of microglia . In steady state, microglia serve to protect the nervous system by acting as debris scavengers, killers of microbial pathogens, and regulators of innate and adaptive immune responses. In neurodegenerative diseases, activated microglia affect neuronal injury and death through production of glutamate, pro-inflammatory factors, reactive oxygen species, quinolinic acid amongst others and by mobilization of adaptive immune responses and cell chemotaxis leading to transendothelial migration of immunocytes across the blood-brain barrier and perpetuation of neural damage. As disease progresses, inflammatory secretions engage neighboring glial cells, including astrocytes and endothelial cells, resulting in a vicious cycle of autocrine and paracrine amplification of inflammation perpetuating tissue injury. Such pathogenic processes contribute to neurodegeneration in PD. Research from others and our own laboratories seek to harness such inflammatory processes with the singular goal of developing therapeutic interventions that positively affect the tempo and progression of human disease.

An in vitro model of human dopaminergic neurons derived from embryonic stem cells: MPP+ toxicity and GDNF neuroprotection

Human embryonic stem cells (hESCs) can proliferate indefinitely yet also differentiate in vitro, allowing normal human neurons to be generated in unlimited numbers. Here, we describe the development of an in vitro neurotoxicity assay using human dopaminergic neurons derived from hESCs. We showed that the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)), which produces features of Parkinson's disease in humans, was toxic for hESC-derived dopaminergic neurons. Treatment with glial cell line-derived neurotrophic factor protected tyrosine hydroxylase-positive neurons against MPP(+)-induced apoptotic cell death and loss of neuronal processes as well as against the formation of intracellular reactive oxygen species. The availability of human dopaminergic neurons, derived from hESCs, therefore allows for the possibility of directly examining the unique features of human dopaminergic neurons with respect to their responses to pharmacological agents as well as environmental and chemical toxins.

Mutagenicity of diesel exhaust particles mediated by cell-particle interaction in mammalian cells

Diesel exhaust particle (DEP) has been identified as a class 2A human carcinogen and closely related to the increased incidence of respiratory allergy, cardiopulmonary morbidity and mortality, and risk of lung cancer. However, the molecular mechanisms of DEP mutagenicity/carcinogenicity are still largely unknown. In the present study, we focused on the mutagenicity of DEPs in human-hamster hybrid (A(L)) cells and evaluated the role of cell-particle interaction in mediating mutagenic process. We found that DEPs formed micron-sized aggregates in the medium and located mainly in large cytoplasmic vacuoles of cells by 24h treatment. The cellular granularity was increased by DEP treatment in a dose-dependent manner. DEPs resulted in a dose-dependent increase of mutation yield at CD59 locus in A(L) cells, while inflicting minimal cytotoxicity. There was a more than two-fold increase of mutation yield at CD59 locus in A(L) cells exposed to DEPs at a dose of 50mug/ml. Such induction was significantly reduced by concurrent treatment with phagocytosis inhibitors, cytochalasin B and ammonium chloride (p<0.05). These results provided direct evidence that DEPs was mutagenic in mammalian cells and that cell-particle interaction played an essential role in the process.

Protective effects of an anti-inflammatory cytokine, interleukin-4, on motoneuron toxicity induced by activated microglia

Microglia-mediated cytotoxicity has been implicated in models of neurodegenerative diseases, such as amyotrophic lateral sclerosis, Parkinson's disease and Alzheimer's disease, but few studies have documented how neuroprotective signals might mitigate such cytotoxicity. To explore the neuroprotective mechanism of anti-inflammatory cytokines, we applied interleukin-4 (IL-4) to primary microglial cultures activated by lipopolysaccharide as well as to activated microglia cocultured with primary motoneurons. lipopolysaccharide increased nitric oxide and superoxide (O(2) (.-)) and decreased insulin-like growth factor-1 (IGF-1) release from microglial cultures, and induced motoneuron injury in microglia-motoneuron cocultures. However, lipopolysaccharide had minimal effects on isolated motoneuron cultures. IL-4 interaction with microglial IL-4 receptors suppressed and nitric oxide release, and lessened lipopolysaccharide-induced microglia-mediated motoneuron injury. The extent of nitric oxide suppression correlated directly with the extent of motoneuron survival. Although IL-4 enhanced release of free IGF-1 from microglia in the absence of lipopolysaccharide, it did not enhance free IGF-1 release in the presence of lipopolysaccharide. These data suggest that IL-4 may provide a significant immunomodulatory signal which can protect against microglia-mediated neurotoxicity by suppressing the production and release of free radicals.

Translocation and potential neurological effects of fine and ultrafine particles a critical update

Particulate air pollution has been associated with respiratory and cardiovascular disease. Evidence for cardiovascular and neurodegenerative effects of ambient particles was reviewed as part of a workshop. The purpose of this critical update is to summarize the evidence presented for the mechanisms involved in the translocation of particles from the lung to other organs and to highlight the potential of particles to cause neurodegenerative effects.

Fine and ultrafine particles, after deposition on the surfactant film at the air-liquid interface, are displaced by surface forces exerted on them by surfactant film and may then interact with primary target cells upon this displacement. Ultrafine and fine particles can then penetrate through the different tissue compartments of the lungs and eventually reach the capillaries and circulating cells or constituents, e.g. erythrocytes. These particles are then translocated by the circulation to other organs including the liver, the spleen, the kidneys, the heart and the brain, where they may be deposited. It remains to be shown by which mechanisms ultrafine particles penetrate through pulmonary tissue and enter capillaries. In addition to translocation of ultrafine particles through the tissue, fine and coarse particles may be phagocytized by macrophages and dendritic cells which may carry the particles to lymph nodes in the lung or to those closely associated with the lungs. There is the potential for neurodegenerative consequence of particle entry to the brain. Histological evidence of neurodegeneration has been reported in both canine and human brains exposed to high ambient PM levels, suggesting the potential for neurotoxic consequences of PM-CNS entry. PM mediated damage may be caused by the oxidative stress pathway. Thus, oxidative stress due to nutrition, age, genetics among others may increase the susceptibility for neurodegenerative diseases. The relationship between PM exposure and CNS degeneration can also be detected under controlled experimental conditions. Transgenic mice (Apo E -/-), known to have high base line levels of oxidative stress, were exposed by inhalation to well characterized, concentrated ambient air pollution. Morphometric analysis of the CNS indicated unequivocally that the brain is a critical target for PM exposure and implicated oxidative stress as a predisposing factor that links PM exposure and susceptibility to neurodegeneration.

Together, these data present evidence for potential translocation of ambient particles on organs distant from the lung and the neurodegenerative consequences of exposure to air pollutants.

Interactive role of the toll-like receptor 4 and reactive oxygen species in LPS-induced microglia activation

Microglia are activated by lipopolysaccharide (LPS) to produce neurotoxic pro-inflammatory factors and reactive oxygen species (ROS). While a multitude of LPS receptors and corresponding pathways have been identified, the detailed mechanisms mediating the microglial response to LPS are unclear. Using mice lacking a functional toll-like receptor 4 (TLR4), we demonstrate that TLR4 and ROS work in concert to mediate microglia activation, where the contribution from each pathway is dependent on the concentration of LPS. Immunocytochemical staining of microglia in neuron-glia cultures with antibodies against F4/80 revealed that while TLR4(+/+) microglia were activated the low concentration of 1 ng/ml of LPS, TLR4(-/-) microglia exhibit activated morphology in response to LPS only at higher concentrations (100-1,000 ng/ml). Additionally, tumor necrosis factor-alpha (TNF-alpha) was only produced from higher concentrations (100-1,000 ng/ml) of LPS in TLR4(-/-) enriched microglia cultures. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, reduced TNF-alpha production from TLR4(-/-) microglia. The influence of TLR4 on LPS-induced superoxide production was tested in rat enriched microglia cultures, where the presence or absence of serum failed to show any effect on the superoxide production. Further, both TLR4(-/-) and TLR4(+/+) microglia showed a similar increase in extracellular superoxide production when exposed to LPS (1-1,000 ng/ml). These data indicate that LPS-induced superoxide production in microglia is independent of TLR4 and that ROS derived from the production of extracellular superoxide in microglia mediates the LPS-induced TNF-alpha response of both the TLR4-dependent and independent pathway.

Titanium dioxide (P25) produces reactive oxygen species in immortalized brain microglia (BV2): implications for nanoparticle neurotoxicity

Concerns with the environmental and health risk of widely distributed, commonly used nanoparticles are increasing. Nanosize titanium dioxide (TiO2) is used in air and water remediation and in numerous products designed for direct human use and consumption. Its effectiveness in deactivating pollutants and killing microorganisms relates to photoactivation and the resulting free radical activity. This property, coupled with its multiple potential exposure routes, indicates that nanosize TiO2 could pose a risk to biological targets that are sensitive to oxidative stress damage (e.g., brain). In this study, brain microglia (BV2) were exposed to a physicochemically characterized (i.e., dispersion stability, particle size distribution, and zeta potential) nanomaterial, Degussa P25, and cellular expressions of reactive oxygen species were measured with fluorescent probes. P25's zeta potentials, measured in cell culture media and physiological buffer were -11.6 +/- 1.2 mV and -9.25 +/- 0.73 mV, respectively. P25 aggregation was rapid in both media and buffer with the hydrodynamic diameter of stable P25 aggregates ranging from 826 nm to 2368 nm depending on the concentration. The biological response of BV2 microglia to noncytotoxic (2.5-120 ppm) concentrations of P25 was a rapid (<5 min) and sustained (120 min) release of reactive oxygen species. The time course of this release suggested that P25 not only stimulated the immediate "oxidative burst" response in microglia but also interfered with mitochondrial energy production. Transmission electron microscopy indicated that small groups of nanosized particles and micron-sized aggregates were engulfed bythe microglia and sequestered as intracytoplasmic aggregates after 6 and 18 h exposure to P25 (2.5 ppm). Cell viability was maintained at all test concentrations (2.5-120 ppm) over the 18 h exposure period. These data indicate that mouse microglia respond to Degussa P25 with cellular and morphological expressions of free radical formation.

Activation of the stress axis and neurochemical alterations in specific brain areas by concentrated ambient particle exposure with concomitant allergic airway disease

OBJECTIVE

Exposure to ambient particulate matter (PM) has been linked to respiratory diseases in people living in urban communities. The mechanism by which PM produces these diseases is not clear. We hypothesized that PM could act on the brain directly to stimulate the stress axis and predispose individuals to these diseases. The purpose of this study was to test if exposure to PM can affect brain areas involved in the regulation of neuroendocrine functions, especially the stress axis, and to study whether the presence of preexisting allergic airway disease aggravates the stress response.

DESIGN

Adult male rats (n = 8/group) with or without ovalbumin (OVA)-induced allergic airway disease were exposed to concentrated air particles containing PM with an aerodynamic diameter pound 2.5 microm (PM(2.5)) for 8 hr, generated from ambient air in an urban Grand Rapids, Michigan, community using a mobile air research laboratory (AirCARE 1). Control animals were exposed to normal air and were treated with saline.

MEASUREMENTS

A day after PM(2.5) exposure, animals were sacrificed and the brains were removed, frozen, and sectioned. The paraventricular nucleus (PVN) and other brain nuclei were microdissected, and the concentrations of aminergic neurotransmitters and their metabolites were measured using high-performance liquid chromatography with electrochemical detection. Serum corticosterone levels were measured using radioimmunoassay.

RESULTS

A significant increase in the concentration (mean +/- SE, pg/microg protein) of norepinephrine in the PVN was produced by exposure to concentrated ambient particles (CAPs) or OVA alone (12.45 +/- 2.7 and 15.84 +/- 2.8, respectively) or after sensitization with OVA (19.06 +/- 3.8) compared with controls (7.98 +/- 1.3 ; p < 0.05). Serum corticosterone (mean +/- SE, ng/mL) was significantly elevated in the OVA + CAPs group (242.786 +/- 33.315) and in the OVA-presensitized group (242.786 +/- 33.315) compared with CAP exposure alone (114.55 +/- 20.9). Exposure to CAPs (alone or in combination with OVA pretreatment) can activate the stress axis, and this could probably play a role in aggravating allergic airway disease.

Midbrain neuronal cultures from parkin mutant mice are resistant to nitric oxide-induced toxicity

Nitric oxide (NO) is a modulator of differentiation and survival of dopamine (DA) neurons. NO may play a role in the pathogenesis of Parkinson's disease (PD) since its levels are increased in parkinsonian brains and it can nitrosylate and alter the function of key proteins involved in the pathogenesis of PD. NO producing neurons are spared in parkinsonian brains suggesting that toxicity by NO can be compensated. Furthermore, the neurotoxic or neurotrophic effects of NO on DA neurons depend on the balance between NO levels and the intracellular levels of glutathione (GSH). We have investigated the effects of NO-donating agents on midbrain neuronal cultures from parkin-deficient mice. Parkin mutations are the most common genetic deficit observed in hereditary parkinsonism. These mice have abnormal DA release and metabolism, increased production of free radicals and a compensatory elevation of GSH. Cultures from parkin knockout (PK-KO) mice were more resistant than those of wild type (WT) to the neurotoxicity by NO, and the difference of susceptibility applied equally to DA, GABA and total number of neurons, and to astrocytes. NO-induced cell death was mainly apoptotic and could be reduced by caspase inhibitors. Cultures from PK-KO had greater levels of GSH than WT and, after treatment with NO, greater levels of S-nitrosoglutathione. The differences in susceptibility disappear when the synthesis of GSH is inhibited or the GSH chelated with diethyl maleate. Our data show that, contrary to the expectations, and related to the enhanced production of GSH in parkin knockout mice, parkin-deficient dopamine neurons are less susceptible to toxicity by NO.

Cellular effect of high doses of silica-coated quantum dot profiled with high throughput gene expression analysis and high content cellomics measurements

Quantum dots (Qdots) are now used extensively for labeling in biomedical research, and this use is predicted to grow because of their many advantages over alternative labeling methods. Uncoated Qdots made of core/shell CdSe/ZnS are toxic to cells because of the release of Cd2+ ions into the cellular environment. This problem has been partially overcome by coating Qdots with polymers, poly(ethylene glycol) (PEG), or other inert molecules. The most promising coating to date, for reducing toxicity, appears to be PEG. When PEG-coated silanized Qdots (PEG-silane-Qdots) are used to treat cells, toxicity is not observed, even at dosages above 10-20 nM, a concentration inducing death when cells are treated with polymer or mercaptoacid coated Qdots. Because of the importance of Qdots in current and future biomedical and clinical applications, we believe it is essential to more completely understand and verify this negative global response from cells treated with PEG-silane-Qdots. Consequently, we examined the molecular and cellular response of cells treated with two different dosages of PEG-silane-Qdots. Human fibroblasts were exposed to 8 and 80 nM of these Qdots, and both phenotypic as well as whole genome expression measurements were made. PEG-silane-Qdots did not induce any statistically significant cell cycle changes and minimal apoptosis/necrosis in lung fibroblasts (IMR-90) as measured by high content image analysis, regardless of the treatment dosage. A slight increase in apoptosis/necrosis was observed in treated human skin fibroblasts (HSF-42) at both the low and the high dosages. We performed genome-wide expression array analysis of HSF-42 exposed to doses 8 and 80 nM to link the global cell response to a molecular and genetic phenotype. We used a gene array containing approximately 22,000 total probe sets, containing 18,400 probe sets from known genes. Only approximately 50 genes (approximately 0.2% of all the genes tested) exhibited a statistically significant change in expression level of greater than 2-fold. Genes activated in treated cells included those involved in carbohydrate binding, intracellular vesicle formation, and cellular response to stress. Conversely, PEG-silane-Qdots induce a down-regulation of genes involved in controlling the M-phase progression of mitosis, spindle formation, and cytokinesis. Promoter analysis of these results reveals that expression changes may be attributed to the down-regulation of FOXM and BHLB2 transcription factors. Remarkably, PEG-silane-Qdots, unlike carbon nanotubes, do not activate genes indicative of a strong immune and inflammatory response or heavy-metal-related toxicity. The experimental evidence shows that CdSe/ZnS Qdots, if appropriately protected, induce negligible toxicity to the model cell system studied here, even when exposed to high dosages. This study indicates that PEG-coated silanized Qdots pose minimal impact to cells and are a very promising alternative to uncoated Qdots.

Potent regulation of microglia-derived oxidative stress and dopaminergic neuron survival: substance P vs. dynorphin

Unregulated microglial activation has been implicated as a pivotal factor contributing to Parkinson's disease. Using mesencephalic neuron-glia cultures, we address the novel possibility that peptides endogenous to the substantia nigra (SN), substance P and dynorphin (10(-13)-10(-14) M), are opposing mediators of microglial activation and consequent DA neurotoxicity. Here, we identify that substance P (10(-13)-10(-14) M) is selectively toxic to DA neurons in a microglia-dependent manner. Mechanistically, substance P (10(-13)-10(-14) M) activated microglial NADPH oxidase to produce extracellular superoxide and intracellular reactive oxygen species (ROS). Neuron-glia cultures from mice lacking a functional NADPH oxidase complex (PHOX-/-) were insensitive to substance P (10(-13)-10(-14) M) -induced loss of DA neuron function. Mixed glia cultures from (PHOX-/-) mice failed to show a significant increase in intracellular ROS in response to substance P compared with control cultures (PHOX+/+). Further, dynorphin (10(-14) M) inhibited substance P (10(-13) M) -induced loss of [3H] DA uptake. Here we demonstrate a tightly regulated mechanism governing microglia-derived oxidative stress, where the neuropeptide balance of dynorphin and substance P is critical to DA neuron survival.

Thrombin-induced oxidative stress contributes to the death of hippocampal neurons in vivo: role of microglial NADPH oxidase

The present study investigated whether thrombin, a potent microglial activator, can induce reactive oxygen species (ROS) generation through activation of microglial NADPH oxidase and if this may contribute to oxidative damage and consequent neurodegeneration. Seven days after intrahippocampal injection of thrombin, Nissl staining and immunohistochemistry using the neuronal-specific nuclear protein NeuN revealed a significant loss in hippocampal CA1 neurons. In parallel, thrombin-activated microglia, assessed by OX-42 and OX-6 immunohistochemistry, and ROS production, assessed by hydroethidine histochemistry, were observed in the hippocampal CA1 area in which degeneration of hippocampal neurons occurred. Reverse transcription-PCR at various time points after thrombin administration demonstrated an early and transient expression of inducible nitric oxide synthase (iNOS) and several proinflammatory cytokines. Western blot analysis and double-label immunohistochemistry showed an increase in the expression of and the localization of iNOS within microglia. Additional studies demonstrated that thrombin induced the upregulation of membrane (gp91(phox)) and cytosolic (p47(phox) and p67(phox)) components, translocation of cytosolic proteins (p47(phox), p67(phox), and Rac1) to the membrane, and p67(phox) expression of the NADPH oxidase in microglia in the hippocampus in vivo, indicating the activation of NADPH oxidase. The thrombin-induced oxidation of proteins and loss of hippocampal CA1 neurons were partially inhibited by an NADPH oxidase inhibitor and by an antioxidant. To our knowledge, the present study is the first to demonstrate that thrombin-induced neurotoxicity in the hippocampus in vivo is caused by microglial NADPH oxidase-mediated oxidative stress. This suggests that thrombin inhibition or enhancing antioxidants may be beneficial for the treatment of neurodegenerative diseases, such as Alzheimer's disease, that are associated with microglial-derived oxidative damage.

Microglia and inflammation-mediated neurodegeneration: multiple triggers with a common mechanism

Inflammation, a common denominator among the diverse list of neurodegenerative diseases, has recently been implicated as a critical mechanism responsible for the progressive nature of neurodegeneration. Microglia are the resident innate immune cells in the central nervous system and produce a barrage of factors (IL-1, TNFalpha, NO, PGE2, superoxide) that are toxic to neurons. Evidence supports that the unregulated activation of microglia in response to environmental toxins, endogenous proteins, and neuronal death results in the production of toxic factors that propagate neuronal injury. In the following review, we discuss the common thread of microglial activation across numerous neurodegenerative diseases, define current perceptions of how microglia are damaging neurons, and explain how the microglial response to neuronal damage results in a self-propelling cycle of neuron death.

Molecular characterization of the cytotoxic mechanism of multiwall carbon nanotubes and nano-onions on human skin fibroblast

The increasing use of nanotechnology in consumer products and medical applications underlies the importance of understanding its potential toxic effects to people and the environment. Although both fullerene and carbon nanotubes have been demonstrated to accumulate to cytotoxic levels within organs of various animal models and cell types and carbon nanomaterials have been exploited for cancer therapies, the molecular and cellular mechanisms for cytotoxicity of this class of nanomaterial are not yet fully apparent. To address this question, we have performed whole genome expression array analysis and high content image analysis based phenotypic measurements on human skin fibroblast cell populations exposed to multiwall carbon nano-onions (MWCNOs) and multiwall carbon nanotubes (MWCNTs). Here we demonstrate that exposing cells to MWCNOs and MWCNTs at cytotoxic doses induces cell cycle arrest and increases apoptosis/necrosis. Expression array analysis indicates that multiple cellular pathways are perturbed after exposure to these nanomaterials at these doses, with material-specific toxigenomic profiles observed. Moreover, there are also distinct qualitative and quantitative differences in gene expression profiles, with each material at different dosage levels (6 and 0.6 microg/mL for MWCNO and 0.6 and 0.06 microg/mL for MWCNT). MWCNO and MWCNT exposure activates genes involved in cellular transport, metabolism, cell cycle regulation, and stress response. MWCNTs induce genes indicative of a strong immune and inflammatory response within skin fibroblasts, while MWCNO changes are concentrated in genes induced in response to external stimuli. Promoter analysis of the microarray results demonstrate that interferon and p38/ERK-MAPK cascades are critical pathway components in the induced signal transduction contributing to the more adverse effects observed upon exposure to MWCNTs as compared to MWCNOs.

Inhibition of thrombin-induced microglial activation and NADPH oxidase by minocycline protects dopaminergic neurons in the substantia nigra in vivo

The present study shows that activation of microglial NADPH oxidase and production of reactive oxygen species (ROS) is associated with thrombin-induced degeneration of nigral dopaminergic neurons in vivo. Seven days after thrombin injection in the rat substantia nigra (SN), tyrosine hydroxylase immunocytochemistry showed a significant loss of nigral dopaminergic neurons. This cell death was accompanied by localization of terminal deoxynucleotidyl transferase-mediated fluorecein UTP nick-end labelling (TUNEL) staining within dopaminergic neurons. This neurotoxicity was antagonized by the semisynthetic tetracycline derivative, minocycline, and the observed neuroprotective effects were associated with the ability of minocycline to suppress NADPH oxidase-derived ROS production and pro-inflammatory cytokine expression, including interleukin-1beta and inducible nitric oxide synthase, from activated microglia. These results suggest that microglial NADPH oxidase may be a viable target for neuroprotection against oxidative damage.

Microglial NADPH oxidase is a novel target for femtomolar neuroprotection against oxidative stress

Inflammation has been increasingly recognized to contribute to the pathogenesis of Parkinson's disease. Several compounds are neuroprotective at femtomolar concentrations through the inhibition of inflammation. However, the mechanisms mediating femtomolar-acting compounds are poorly understood. Here we show that both gly-gly-phe (GGF), a tri-peptide contained in the dynorphin opioid peptide, and naloxone are neuroprotective at femtomolar concentrations against LPS-induced dopaminergic neurotoxicity through the reduction of microglial activation. Mechanistic studies demonstrated the critical role of NADPH oxidase in the GGF and naloxone inhibition of microglial activation and associated DA neurotoxicity. Pharmacophore analysis of the neuroprotective dynorphin peptides and naloxone revealed common chemical properties (hydrogen bond acceptor, hydrogen bond donor, positive ionizable, hydrophobic) of these femtomolar-acting compounds. These results support a common high-affinity site of action for several femtomolar-acting compounds, where NADPH oxidase is the critical mechanism governing neuroprotection, suggesting a novel avenue of anti-inflammatory and neuroprotective therapy.