Holly L. Valentine

Multiexponential T2, magnetization transfer, and quantitative histology in white matter tracts of rat spinal cord.

Quantitative MRI measures of multiexponential T2 relaxation and magnetization transfer were acquired from six samples of excised and fixed rat spinal cord and compared with quantitative histology. MRI and histology data were analyzed from six white matter tracts, each of which possessed unique microanatomic characteristics (axon diameter and myelin thickness, in particular) but a relatively constant volume fraction of myelin. The results indicated that multiexponential T2 relaxation characteristics varied substantially with variation of microanatomy, while the magnetization transfer characteristics remained close to constant. The most‐often‐cited multiexponential T2 relaxation metric, myelin water fraction, varied by almost a factor of 2 between two regions with myelin volume fractions that differed by only ≈ 12%. Based on the quantitative histology, the proposed explanation for this variation was intercompartmental water exchange, which caused the underestimation of myelin water fraction and T2 values and is, presumably, a greater factor in white matter regions where axons are small and myelin is thin. In contrast to the multiexponential T2 relaxation observations, magnetization transfer metrics were relatively constant across white matter tracts and concluded to be relatively insensitive to intercompartmental water exchange. Magn Reson Med 63:902–909, 2010.

Brainstem Axonal Degeneration in Mice with Deletion of Selenoprotein P

Selenoprotein P is an abundant extracellular protein that is expressed in liver, brain, and other tissues. Studies in mice with the selenoprotein P gene deleted (Sepp −/− mice) have implicated the protein in maintaining brain selenium. Sepp −/− mice fed a normal or low selenium diet develop severe motor impairment and die, but Sepp −/− mice fed a high selenium diet remain clinically unimpaired. As an initial step to evaluate the effect of selenoprotein P deletion on central nervous system architecture, the brains and cervical spinal cords of Sepp −/− and Sepp + / + mice fed low or high selenium diets were examined by light and electron microscopy. Brains of Sepp −/− mice demonstrated no gross abnormalities. At the light microscopic level, however, Sepp −/− mice fed either the selenium deficient diet or the high selenium diet had enlarged dystrophic axons and degenerated axons in their brainstems and cervical spinal cords. No axonal lesions were observed in the Sepp + / + mice fed either diet. Electron microscopy demonstrated that the enlarged axons in the Sepp −/− mice were packed with organelles, suggesting a deficit in fast axonal transport. The similar severity of axonal lesions observed in Sepp −/− mice fed the 2 diets suggests that axonal dystrophy is a common phenotype for deletion of selenoprotein P regardless of selenium intake and that additional studies will be required to determine the pathogenesis of the neurological signs and mortality observed in Sepp −/−mice fed a low selenium diet.

Cytotoxic mechanisms of hydrosulfide anion and cyanide anion in primary rat hepatocyte cultures

Hydrogen sulfide and hydrogen cyanide are known to compromise mitochondrial respiration through inhibition of cytochrome c oxidase and this is generally considered to be their primary mechanism of toxicity. Experimental studies and the efficiency of current treatment protocols suggest that H(2)S may exert adverse physiological effects through additional mechanisms. To evaluate the role of alternative mechanisms in H(2)S toxicity, the relative contributions of electron transport inhibition, uncoupling of mitochondrial respiration, and opening of the mitochondrial permeability transition pore (MPTP) to hydrosulfide and cyanide anion cytotoxicity in primary hepatocyte cultures were examined. Supplementation of hepatocytes with the glycolytic substrate, fructose, rescued hepatocytes from cyanide anion induced toxicity, whereas fructose supplementation increased hydrosulfide anion toxicity suggesting that hydrosulfide anion may compromise glycolysis in hepatocytes. Although inhibitors of the MPTP opening were protective for hydrosulfide anion, they had no effect on cyanide anion toxicity, consistent with an involvement of the permeability transition pore in hydrosulfide anion toxicity but not cyanide anion toxicity. Exposure of isolated rat liver mitochondria to hydrosulfide did not result in large amplitude swelling suggesting that if H(2)S induces the permeability transition it does so indirectly through a mechanism requiring other cellular components. Hydrosulfide anion did not appear to be an uncoupler of mitochondrial respiration in hepatocytes based upon the inability of oligomycin and fructose to protect hepatocytes from hydrosulfide anion toxicity. These findings support mechanisms additional to inhibition of cytochrome c oxidase in hydrogen sulfide toxicity. Further investigations are required to assess the role of the permeability transition in H(2)S toxicity, determine whether similar affects occur in other cell types or in vivo and evaluate whether this may provide a basis for the design of more effective therapeutic measures for hydrogen sulfide intoxication.

Copper accumulation and lipid oxidation precede inflammation and myelin lesions in N,N-diethyldithiocarbamate peripheral myelinopathy.

Dithiocarbamates have a wide spectrum of applications in industry, agriculture and medicine with new applications being actively investigated. One adverse effect of dithiocarbamates is the neurotoxicity observed in humans and experimental animals. Results from previous studies have suggested that dithiocarbamates elevate copper and promote lipid oxidation within myelin membranes. In the current study, copper levels, lipid oxidation, protein oxidative damage and markers of inflammation were monitored as a function of N,N-diethyldithiocarbamate (DEDC) exposure duration in an established model for DEDC-mediated myelinopathy in the rat. Intra-abdominal administration of DEDC was performed using osmotic pumps for periods of 2, 4, and 8 weeks. Metals in brain, liver and tibial nerve were measured using ICP-MS and lipid oxidation assessed through HPLC measurement of malondialdehyde in tibial nerve, and GC/MS measurement of F(2) isoprostanes in sciatic nerve. Protein oxidative injury of sciatic nerve proteins was evaluated through quantification of 4-hydroxynonenal protein adducts using immunoassay, and inflammation monitored by quantifying levels of IgGs and activated macrophages using immunoassay and immunohistochemistry methods, respectively. Changes in these parameters were then correlated to the onset of structural lesions, determined by light and electron microscopy, to delineate the temporal relationship of copper accumulation and oxidative stress in peripheral nerve to the onset of myelin lesions. The data provide evidence that DEDC mediates lipid oxidation and elevation of total copper in peripheral nerve well before myelin lesions or activated macrophages are evident. This relationship is consistent with copper-mediated oxidative stress contributing to the myelinopathy.

Parenteral N, N-diethyldithiocarbamate produces segmental demyelination in the rat that is not dependent on cysteine carbamylation

Disulfiram, a dithiocarbamate drug used in alcohol aversion therapy, produces a peripheral neuropathy characterized in rats as segmental demyelination accompanied by generation of S-(diethylaminocarbonyl)cysteine (DETC-Cys) adducts. N,N-Diethyldithiocarbamate (DEDC) is a major metabolite of disulfiram that can undergo methylation and oxidation to S-methyl-N,N-diethylthiocarbamate (MeDETC) sulfoxide and sulfone, thought to be responsible for carbamylation of sulfhydryl functions by disulfiram. To assess the role of cysteine carbamylation in disulfiram toxicity, DEDC and MeDETC were administered parenterally to male Sprague-Dawley rats for 4 and 8 weeks. The roles of the disulfide linkage in disulfiram and of carbamylated glutathione metabolites were assessed by administering S-(diethylaminodithiocarbonyl)N-acetylcysteine (DS-NAC) and S-(diethylaminocarbonyl)-N-acetylcysteine (DETC-NAC), respectively, parenterally for 12 weeks. Following exposure, spinal cord-derived neurofilament preparations and hemoglobin were isolated and analyzed by RP-HPLC and LC/MS/MS for the presence of DETC-Cys adducts. Peripheral nerve sections were also obtained and examined by light and electron microscopy for morphological lesions. RP-HPLC analysis of globin preparations from DEDC-, MeDETC-, and DS-NAC-exposed animals demonstrated a late-eluting peak, identical to that reported for disulfiram-generated DETC-Cys adducts on the beta(3)-globin chain. DETC-NAC exposure did not result in detectable globin modification by RP-HPLC. The quantity of DETC-Cys adducts produced on globin and neurofilament preparations determined by LC/MS/MS was twofold greater for MeDETC than DEDC following equimolar doses of each compound. Primary myelin lesions consisting of demyelinated axons and myelin splitting were observed in peripheral nerves following exposure to DEDC for 8 weeks. No lesions were detected following exposure to MeDETC, DS-NAC, or DETC-NAC at any time point or dose level. These results are consistent with DEDC, but not the other metabolites, being a demyelinating agent and thus a potential proximate toxic species for disulfiram-mediated demyelination. The production of significantly greater levels of DETC-Cys adducts by MeDETC relative to DEDC in the absence of neurotoxicity for MeDETC is consistent with cysteine carbamylation not contributing to the demyelination produced by disulfiram and DEDC.

N,N-diethyldithiocarbamate promotes oxidative stress prior to myelin structural changes and increases myelin copper content

Dithiocarbamates are a commercially important class of compounds that can produce peripheral neuropathy in humans and experimental animals. Previous studies have supported a requirement for copper accumulation and enhanced lipid peroxidation in dithiocarbamate-mediated myelinopathy. The study presented here extends previous investigations in two areas. Firstly, although total copper levels have been shown to increase within the nerve it has not been determined whether copper is increased within the myelin compartment, the primary site of lesion development. Therefore, the distribution of copper in sciatic nerve was characterized using synchrotron X-ray fluorescence microscopy to determine whether the neurotoxic dithiocarbamate, N,N-diethyldithiocarbamate, increases copper levels in myelin. Secondly, because lipid peroxidation is an ongoing process in normal nerve and the levels of lipid peroxidation products produced by dithiocarbamate exposure demonstrated an unusual cumulative dose response in previous studies the biological impact of dithiocarbamate-mediated lipid peroxidation was evaluated. Experiments were performed to determine whether dithiocarbamate-mediated lipid peroxidation products elicit an antioxidant response through measuring the protein expression levels of three enzymes, superoxide dismutase 1, heme oxygenase 1, and glutathione transferase alpha, that are linked to the antioxidant response element promoter. To establish the potential of oxidative injury to contribute to myelin injury the temporal relationship of the antioxidant response to myelin injury was determined. Myelin structure in peripheral nerve was assessed using multi-exponential transverse relaxation measurements (MET(2)) as a function of exposure duration, and the temporal relationship of protein expression changes relative to the onset of changes in myelin integrity were determined. Initial assessments were also performed to explore the potential contribution of dithiocarbamate-mediated inhibition of proteasome function and inhibition of cuproenzyme activity to neurotoxicity, and also to assess the potential of dithiocarbamates to promote oxidative stress and injury within the central nervous system. These evaluations were performed using an established model for dithiocarbamate-mediated demyelination in the rat utilizing sciatic nerve, spinal cord and brain samples obtained from rats exposed to N,N-diethyldithiocarbamate (DEDC) by intra-abdominal pumps for periods of 2, 4, and 8 weeks and from non exposed controls. The data supported the ability of DEDC to increase copper within myelin and to enhance oxidative stress prior to structural changes detectable by MET(2). Evidence was also obtained that the excess copper produced by DEDC in the central nervous system is redox active and promotes oxidative injury.

Nitrogen Substituent Polarity Influences Dithiocarbamate-Mediated Lipid Oxidation, Nerve Copper Accumulation, and Myelin Injury

Dithiocarbamates have a wide spectrum of applications in industry, agriculture, and medicine, with new applications being investigated. Past studies have suggested that the neurotoxicity of some dithiocarbamates may result from copper accumulation, protein oxidative damage, and lipid oxidation. The polarity of a dithiocarbamates nitrogen substituents influences the lipophilicity of the copper complexes that it generates and thus potentially determines its ability to promote copper accumulation within nerve and induce myelin injury. In the current study, a series of dithiocarbamate-copper complexes differing in their lipophilicity were evaluated for their relative abilities to promote lipid peroxidation determined by malondialdehyde levels generated in an ethyl arachidonate oil-in-water emulsion. In a second component of this study, rats were exposed to either N,N-diethyldithiocarbamate or sarcosine dithiocarbamate; both generated dithiocarbamate-copper complexes that were lipid- and water-soluble, respectively. Following the exposures, brain, tibial nerve, spinal cord, and liver tissue copper levels were measured by inductively coupled mass spectroscopy to assess the relative abilities of these two dithiocarbamates to promote copper accumulation. Peripheral nerve injury was evaluated using grip strengths, nerve conduction velocities, and morphologic changes at the light microscope level. Additionally, the protein expression levels of glutathione transferase alpha and heme-oxygenase-1 in nerve were determined, and the quantity of protein carbonyls was measured to assess levels of oxidative stress and injury. The data provided evidence that dithiocarbamate-copper complexes are redox active and that the ability of dithiocarbamate complexes to promote lipid peroxidation is correlated to the lipophilicity of the complex. Consistent with neurotoxicity requiring the formation of a lipid-soluble copper complex, significant increases in copper accumulation, oxidative stress, and myelin injury were produced by N,N-diethyldithiocarbamate but not by sarcosine dithiocarbamate.

Peripheral nerve and brain differ in their capacity to resolve N,N-diethyldithiocarbamate-mediated elevations in copper and oxidative injury.

Previous studies have demonstrated that N,N-diethyldithiocarbamate (DEDC) elevates copper and promotes oxidative stress within the nervous system. However, whether these effects resolve following cessation of exposure or have the potential to persist and result in cumulative injury has not been determined. In this study, an established model for DEDC myelin injury in the rat was used to determine whether copper levels, oxidative stress, and neuromuscular deficits resolve following the cessation of DEDC exposure. Rats were exposed to DEDC for 8 weeks and then either euthanized or maintained for 2, 6 or 12 weeks after cessation of exposure. At each time point copper levels were measured by inductively coupled mass spectrometry to assess the ability of sciatic nerve, brain, spinal cord and liver to eliminate excess copper post-exposure. The protein expression levels of glutathione transferase alpha, heme oxygenase 1 and superoxide dismutase 1 in peripheral nerve and brain were also determined by western blot to assess levels of oxidative stress as a function of post-exposure duration. As an initial assessment of the bioavailability of the excess copper in brain the protein expression levels of copper chaperone for superoxide dismutase 1, and prion protein were determined by western blot as a function of exposure and post-exposure duration. Neuromuscular function in peripheral nerve was evaluated using grip strengths, nerve conduction velocities, and morphologic changes at the light microscope level. The data demonstrated that in peripheral nerve, copper levels and oxidative stress return to control levels within several weeks after cessation of exposure. Neuromuscular function also showed a trend towards pre-exposure values, although the resolution of myelin lesions was more delayed. In contrast, total copper and antioxidant enzyme levels remained significantly elevated in brain for longer post-exposure periods. The persistence of effects observed in brain suggests that the central nervous system is more susceptible to long-term cumulative adverse effects from dithiocarbamates. Additionally, significant changes in expression levels of chaperone for superoxide dismutase 1, and prion protein were observed consistent with at least a portion of the excess copper being bioactive.