1,3-Dinitrobenzene inhibits mitochondrial complex II in rat and mouse brainstem and cortical astrocytes

Oral Toxicity of 2,4-Dinitroanisole in Rats

Subacute and subchronic studies were conducted to assess the toxicity of 2,4-dinitroanisole (DNAN) and to provide information important for protecting the health of military and civilian personnel. In the subchronic study, male and female Sprague-Dawley rats were dosed with DNAN via oral gavage at 0, 1.25, 5, 20, and 80 mg/kg/d. Likely owing to its conversion to 2,4-dinitrophenol, an inhibitor of energy homeostasis, DNAN caused an apparent increase in metabolism, leading to reduced feed efficiency ratios and body mass gains in males. Anemia, splenic enlargement, hemosiderosis, and extramedullary hematopoiesis indicated blood as a target organ, with females more sensitive than males. The DNAN was a testicular toxicant, causing decreased mass of testes and epididymides, as well as degeneration and atrophy of testicular seminiferous tubules and epididymal aspermia. Stereotypical behavior in males, gait irregularities, and cerebellar lesions indicated that DNAN is neurotoxic. Splenic enlargement, anemia, testicular toxicity, and neurotoxicity occurred only at or near lethal doses in the subchronic study.

1,3-Dinitrobenzene neurotoxicity - Passage effect in immortalized astrocytes

Age-related disturbances in astrocytic mitochondrial function are linked to loss of neuroprotection and decrements in neurological function. The immortalized rat neocortical astrocyte-derived cell line, DI-TNC1, provides a convenient model for the examination of cellular aging processes that are difficult to study in primary cell isolates from aged brain. Successive passages in culture may serve as a surrogate of aging in which time-dependent adaptation to culture conditions may result in altered responses to xenobiotic challenge. To investigate the hypothesis that astrocytic mitochondrial homeostatic function is decreased with time in culture, low passage DI-TNC1 astrocytes (LP; #2-8) and high passage DI-TNC1 astrocytes (HP; #17-28) were exposed to the mitochondrial neurotoxicant 1,3-dinitrobenzene (DNB). Cells were exposed in either monoculture or in co-culture with primary cortical neurons. Astrocyte mitochondrial membrane potential, morphology, ATP production and proliferation were monitored in monoculture, and the ability of DI-TNC1 cells to buffer K(+)-induced neuronal depolarization was examined in co-cultures. In HP DI-TNC1 cells, DNB exposure decreased proliferation, reduced mitochondrial membrane potential and significantly decreased mitochondrial form factor. Low passage DI-TNC1 cells effectively attenuated K(+)-induced neuronal depolarization in the presence of DNB whereas HP counterparts were unable to buffer K(+) in DNB challenge. Following DNB challenge, LP DI-TNC1 cells exhibited greater viability in co-culture than HP. The data provide compelling evidence that there is an abrupt phenotypic change in DI-TNC1 cells between passage #9-16 that significantly diminishes the ability of DI-TNC1 cells to compensate for neurotoxic challenge and provide neuroprotective spatial buffering. Whether or not these functional changes have an in vivo analog in aging brain remains to be determined.

1,3-dinitrobenzene induces age- and region-specific oxidation to mitochondria-related proteins in brain

Regions of the brain with high energy requirements are especially sensitive to perturbations in mitochondrial function. Hence, neurotoxicant exposures that target mitochondria in regions of high energy demand have the potential to accelerate mitochondrial damage inherently occurring during the aging process. 1,3-Dinitrobenzene (DNB) is a model neurotoxicant that selectively targets mitochondria in brainstem nuclei innervated by the eighth cranial nerve. This study investigates the role of age in the regional susceptibility of brain mitochondria-related proteins (MRPs) to oxidation following exposure to DNB. Male F344 rats (1 month old [young], 3 months old [adult], 18 months old [aged]) were exposed to 10 mg/kg DNB prior to mitochondrial isolation and histopathology experiments. Using a high-throughput proteomic approach, 3 important region- and age-related increases in DNB-induced MRP oxidation were determined: (1) brainstem mitochondria are ×3 more sensitive to DNB-induced oxidation than cortical mitochondria; (2) oxidation of brainstem MRPs is significantly higher than in cortical counterparts; and (3) MRPs from the brainstems of older rats are significantly more oxidized than those from young or adult rats. Furthermore, lower levels of DNB cause signs of intoxication (ataxia, chromodacryorrhea) and vacuolation of the susceptible neuropil in aged animals, while neither is observed in DNB-exposed young rats. Additionally, methemoglobin levels increase significantly in DNB-exposed adult and aged animals, but not young DNB-exposed animals. This suggests that oxidation of key MRPs observed in brainstem of aged animals is necessary for DNB-induced signs of intoxication and lesion formation. These results provide compelling evidence that environmental chemicals such as DNB may aid in the acceleration of injury to specific brain regions by inducing oxidation of sensitive mitochondrial proteins.

A comparative study of protein carbonylation and mitochondrial dysfunction using the neurotoxicants 1,3-dinitrobenzene, 3-nitropropionic acid, and 3-chloropropanediol

This comparative evaluation of neurotoxicants previously identified as models of chemical-induced mitochondrial dysfunction and energy deprivation demonstrated that subtoxic concentrations of 1,3-dinitrobenzene (1,3-DNB), 3-nitropropionic acid (3-NPA), and 3-chloropropanediol (3-CPD) each led to concentration-dependent loss of the mitochondrial membrane potential (ΔΨm) associated with similar patterns of protein carbonylation. Subtoxic concentrations of each neurotoxicant were determined by measuring DI TNC1 cell viability using the MTS cell proliferation assay. Although exposure 1 μM, 10 μM, and 100 μM concentrations of each toxicant did not result in loss of cell viability after 48 h, exposure to each toxicant at these concentrations led to concentration-dependent loss of tetramethyl rhodamine methyl ester (TMRM) fluorescence over the same exposure period. Preincubation with the antioxidant, deferoxamine, was effective in preventing loss of TMRM flurorescence. Through the combined use of two-dimensional polyacrylamide gel electrophoresis (2D PAGE) and Oxyblot analysis, this study demonstrated that exposure to each toxicant resulted in the formation of distinctly similar patterns of protein carbonylation comprised of specific proteins identified with tandem MS/MS. Our results provide insight as to how exposure to different neurotoxicants that enhance oxidative stress may, in fact, lead to mitochondrial injury and subsequent toxicity through selective, yet shared, pathways of protein modification by oxidative carbonylation.

Mixed inhibition of adenosine deaminase activity by 1,3-dinitrobenzene: a model for understanding cell-selective neurotoxicity in chemically-induced energy deprivation syndromes in brain

Astrocytes are acutely sensitive to 1,3-dinitrobenzene (1,3-DNB) while adjacent neurons are relatively unaffected, consistent with other chemically-induced energy deprivation syndromes. Previous studies have investigated the role of astrocytes in protecting neurons from hypoxia and chemical injury via adenosine release. Adenosine is considered neuroprotective, but it is rapidly removed by extracellular deaminases such as adenosine deaminase (ADA). The present study tested the hypothesis that ADA is inhibited by 1,3-DNB as a substrate mimic, thereby preventing adenosine catabolism. ADA was inhibited by 1,3-DNB with an IC(50) of 284 μM, Hill slope, n = 4.8 ± 0.4. Native gel electrophoresis showed that 1,3-DNB did not denature ADA. Furthermore, adding Triton X-100 (0.01-0.05%, wt/vol), Nonidet P-40 (0.0015-0.0036%, wt/vol), or bovine serum albumin (0.05 mg/ml or changing [ADA] (0.2 and 2 nM) did not substantially alter the 1,3-DNB IC(50) value. Likewise, dynamic light scattering showed no particle formation over a (1,3-DNB) range of 149-1043 μM. Kinetics revealed mixed inhibition with 1,3-DNB binding to ADA (K(I) = 520 ± 100 μM, n = 1 ± 0.6) and the ADA-adenosine complex (K(IS) = 262 ± 7 μM, n = 6 ± 0.6, indicating positive cooperativity). In accord with the kinetics, docking predicted binding of 1,3-DNB to the active site and three peripheral sites. In addition, exposure of DI TNC-1 astrocytes to 10-500 μM 1,3-DNB produced concentration-dependent increases in extracellular adenosine at 24 h. Overall, the results demonstrate that 1,3-DNB is a mixed inhibitor of ADA and may thus lead to increases in extracellular adenosine. The finding may provide insights to guide future work on chemically-induced energy deprivation.

1,3-Dinitrobenzene-induced metabolic impairment through selective inactivation of the pyruvate dehydrogenase complex

Prolonged exposure to the chemical intermediate, 1,3-dinitrobenzene (1,3-DNB), produces neuropathology in the central nervous system of rodents analogous to that observed in various conditions of acute energy deprivation including thiamine deficiency and Leigh's necrotizing encephalopathy. Increased production of reactive intermediates in addition to induction of oxidative stress has been implicated in the neurotoxic mechanism of 1,3-DNB, but a clear metabolic target has not been determined. Here we propose that similar to thiamine deficiency, the effects of 1,3-DNB on metabolic status may be due to inhibition of the thiamine-dependent α-ketoacid dehydrogenase complexes. The effects of 1,3-DNB on astroglial metabolic status and α-ketoacid dehydrogenase activity were evaluated using rat C6 glioma cells. Exposure to 1,3-DNB resulted in altered morphology and biochemical dysfunction consistent with disruption of oxidative energy metabolism. Cotreatment with acetyl-carnitine or acetoacetate attenuated morphological and metabolic effects of 1,3-DNB exposure as well as increased cell viability. 1,3-DNB exposure inhibited pyruvate dehydrogenase complex (PDHc) and the inhibition correlated with the loss of lipoic acid (LA) immunoreactivity, suggesting that modification of LA is a potential mechanism of inhibition. Treatment with antioxidants and thiol-containing compounds failed to protect against loss of LA. Alternatively, inhibition of dihydrolipoamide dehydrogenase, the E3 component of the complex attenuated loss of LA. Collectively, these data suggest that 1,3-DNB impairs oxidative energy metabolism through direct inhibition of the PDHc and that this impairment is due to perturbations in the function of protein-bound LA.

Animal models for neural diseases

"Animal Models of Neural Disease" was the focus of General Session 5 at a 2010 scientific symposium that was sponsored jointly by the Society of Toxicologic Pathology (STP) and the International Federation of Societies of Toxicologic Pathologists (IFSTP). The objective was to consider issues that dictate the choice of animal models for neuropathology-based studies used to investigate neurological diseases and novel therapeutic agents to treat them. In some cases, no animal model exists that recapitulates the attributes of the human disease (e.g., fibromyalgia syndrome). Alternatively, numerous animal models are available for other conditions, so an essential consideration is selecting the most appropriate experimental system (e.g., Alzheimer's disease). New technologies (e.g., genetically engineered rodent models) promise the opportunity to generate suitable animal models for syndromes that currently lack any in vivo animal model, while in vitro models offer the opportunity to evaluate xenobiotic effects in specific neural cell populations. The complex nature of neurological disease requires regular reassessment of available and potential options to ensure that animal-derived data sets support translational medicine efforts to improve public health.

Neurochemical and oedematous changes in 1,3-dinitrobenzene-induced astroglial injury in rat brain from a 1H-nuclear magnetic resonance perspective

1,3-Dinitrobenzene (1,3-DNB), an intermediate used in the chemical industry, has toxic effects in the brain and testes. It produces focal lesions with marked astroglial necrosis in the rat brain upon repeated administration. Astrocytic death occurs in parallel with elevated local blood flow and is followed by damage to the cerebral vasculature and neurones. (1)H-nuclear magnetic resonance spectroscopic analysis before the onset of astrocytic damage, showed a global elevation of lactate, whereas choline containing compounds increased in the non-vulnerable cerebral cortex, yet decreased in the vulnerable brainstem. Similarly, glutamate increased in the cerebral cortex, cerebellum and midbrain, but decreased in the susceptible brainstem. In vivo T2-weighted NMR imaging showed high signal intensities in brain nuclei shown to develop astroglial loss by conventional neuropathology at 24 hours after completion of dosing, but not at 6-10 hours. Hence the early neurochemical changes in susceptible areas contribute to the aetiology of degeneration, and those seen elsewhere may represent adaptive responses dependent on the particular phenotype of different cell groups and underlying metabolic relationships.

BCL-XL expression levels influence differential regional astrocytic susceptibility to 1,3-dinitrobenzene

The selective vulnerability of brainstem astrocytes to 1,3-dinitrobenzene is mediated by a 10-fold lower threshold for opening of the cyclosporin A-inhibitable mitochondrial permeability transition pore (mtPTP). BCL-XL, BAX and BCL-2 are members of the BCL-2 protein family known to regulate both apoptotic and necrotic cell death signaling at the mtPTP. The levels at which these proteins are expressed relative to one another, where in the cell they are located and whether they are post-translational modified contributes greatly to the balance in active agonistic to active antagonistic BCL-2 proteins, and this critical balance has been hypothesized to dictate regional astrocytic susceptibility to DNB. The effects of DNB on the balance in expression of the BCL-2 family proteins have been evaluated in F344 rat DNB-sensitive (brainstem) and non-sensitive (cortical) tissue homogenates and primary astrocytes. No significant treatment-related alterations in BCL-XL, BAX or BCL-2 protein expression are observed in rat tissue homogenates or primary astrocytes. However, moderate increases in BCL-XL are observed only in DNB-treated rat cortical astrocytes, and these increases may be sufficient to shift the constitutive balance in expression of antagonistic to agonistic BCL-2 proteins from a ratio which favors BAX to one in which BAX and BCL-XL are comparably expressed. Rat primary brainstem and cortical astrocytes are also transiently transfected with bcl-xl to evaluate whether or not moderate enhancement of BCL-XL protein expression levels are sufficient to alter regional sensitivity to DNB in vitro. BCL-XL overexpression minimizes DNB-induced inhibition of succinate dehydrogenase (complex II) activity and increases significantly the concentration of DNB required to induce MPT onset in primary brainstem and cortical astrocytes. Results from the current investigation suggest that modest region-specific alterations in the balance in expression of antagonistic to agonistic BCL-2 proteins may adequately explain differential regional sensitivity to DNB-induced mitochondrial dysfunction.

In situ imaging of mitochondrial outer-membrane pores using atomic force microscopy

Here we describe a technique for imaging of the outer contours of the mitochondrial membrane using atomic force microscopy, subsequent to or during a toxic or metabolic challenge. Pore formation in both glucose-challenged and 1,3-dinitrobenzene (DNB)-challenged mitochondria was observed using this technique. Our approach enables quantification of individual mitochondrial membrane pore formations. With this work, we have produced some of the highest resolution images of the outer contours of the in situ mitochondrial membrane published to date. These are potentially the first images of the component protein clusters at the time of formation of the mitochondrial membrane transition pore in situ. With the current work, we have extended the application of atomic force microscopy of mitochondrial membranes to fluid imaging. We have also begun to correlate 3-D surface features of mitochondria dotted with open membrane pores with features previously viewed with electron microscopy (EM) of fixed sections.

Regional variation in the activation threshold for 1,3-DNB-induced mitochondrial permeability transition in brainstem and cortical astrocytes

1,3-Dinitrobenzene (DNB) produces edematous, glio-vascular lesions in brainstem nuclei with high energy demands. Astrocytes in vulnerable brainstem nuclei appear to be an early and selective target of DNB and other nitroaromatic compounds, though the molecular basis of this susceptibility is poorly understood. It has been postulated that mitochondria are a principal target of DNB in sensitive cell types [Neuropathol. Appl. Neurobiol. 13 (5) (1987) 371], where redox-cycling of DNB increases levels of reactive oxygen species and disrupts cellular energy metabolism. The present study investigates the role of regional differences in activation of the mitochondrial permeability transition pore (mtPTP) by DNB in brainstem and cortical astrocytes and examines the expression of Bcl-2 proteins as potential regulators of mtPTP function. Neonatal rat astrocytes were cultured from both DNB-sensitive (brainstem) and insensitive (cortex) brain regions and evaluated for DNB-induced alterations in cell morphology and mitochondrial function. Exposure to DNB resulted in rapid changes in the morphology of brainstem astrocytes consistent with loss of ion homeostasis and initiation of necrotic cell death. These changes were not observed in cortical astrocytes at corresponding concentrations of DNB and were prevented in brainstem astrocytes by the mtPTP inhibitor, bongkrekic acid, suggesting that mitochondrial dysfunction is involved in DNB-induced morphological changes in brainstem astrocytes. Mitochondrial depolarization in brainstem astrocytes was observed at DNB concentrations as low as 10 microM, whereas no loss of mitochondrial membrane potential (DeltaPsi(mt)) occurred in cortical astrocytes at less than 100 microM DNB. DNB-induced loss of DeltaPsi(mt) followed apparent first-order kinetics, with EC(50)-values for half-maximal rates of mitochondrial depolarization of approximately 23 and approximately 290 microM in brainstem cortical astrocytes, respectively. DNB-induced mitochondrial depolarization was prevented by pretreatment with bongkrekic acid, indicating that loss of DeltaPsi(mt) was mediated by activation of the mtPTP. Inhibition of succinate dehydrogenase (SDH) activity occurred in astrocytes from both brain regions exposed to DNB and was blocked in brainstem, but not cortical, astrocytes by bongkrekic acid. Constitutive expression of Bcl-X(L) was high in cortical tissue and astrocytes, whereas Bax expression was low. However, Bax was highly expressed in brainstem tissue and astrocytes and Bcl-X(L) expression was markedly lower. The expression of Bcl-2 was similar in both brain regions. These data suggest that the selective vulnerability of brainstem astrocytes to DNB is due to a lower threshold for activation of the mtPTP that is be mediated, in part, by distinct expression patterns of Bcl-2 proteins rather than by intrinsic differences in susceptibility of the electron transport chain.