Identification of rat hippocampal mRNAs altered by the mitochondrial toxicant, 3-NPA.

3-Nitropropionic acid (3-NPA) is a model mitochondrial inhibitor that causes selective neurodegeneration in brain. 3-NPA-induced neurodegeneration occurs via a secondary neurotoxicity, caused initially by ATP depletion and redox changes in the cell. It is known that the hippocampal degeneration caused by mitochondrial dysfunction affects learning and memory, cognitive functions commonly disturbed in neurodegenerative diseases. The 3-NPA- treated animal model can be used to study molecular mechanisms underlying selective degeneration in the brain. In this study, a microarray approach was utilized to define changes in the expression of 530 genes in the rat hippocampus after acute exposure to 3-NPA at 30 mg/kg, sc. The microarray data were collected at 30 min, 2 h, and 4 h post-3-NPA. Statistical modeling using an ANOVA mixed model applied to Van der Waerden scores of rank-transformed intensity data was used to assign statistical significance to 44 transcripts. These transcripts represent genes associated with energy metabolism, calcium homeostasis, the cytoskeleton, neurotransmitter metabolism, and other cellular functions. Changes in the transcripts of genes encoding 2 transporters [blood-brain specific anion transporter (Slco1c1) and sodium-dependent inorganic phosphate cotransporter (Slc17a7)] were confirmed by real-time RT-PCR. In conclusion, this study identified 2 new potential targets for enhancement of neuroprotection or inhibition of neurodegeneration associated with ATP depletion in the hippocampus.

L-carnitine and neuroprotection in the animal model of mitochondrial dysfunction.

We have shown previously that pretreatment with l-carnitine (LC) prior to 3-nitropropionic acid (3-NPA) exposure, while not significantly attenuating succinate dehydrogenase (SDH) inhibition, prevented hypothermia and oxidative stress. The plant and fungal toxin, 3-NPA, acts as an inhibitor of mitochondrial function via irreversible inactivation of the mitochondrial inner membrane enzyme, SDH. Inhibition of SDH disturbs electron transport, leading to cellular energy deficits and oxidative stress-related neuronal injury. In the study presented here, a neurohistological method was applied to examine the mitochondriotropic effect of LC pretreatment against 3-NPA-induced neurotoxicity. Twenty adult male Sprague-Dawley rats randomly divided into two groups (n = 10/group) were injected twice with 3-NPA at 30 mg/kg sc, at 2 days apart, or received LC pretreatment at 100 mg/kg, at 30-40 min before 3-NPA administration. Rats in both groups were perfused 7 days later and their brains harvested. Degenerating neurons were identified and localized via the fluorescent marker Fluoro-Jade B. Data analysis showed that LC was protective against 3-NPA-induced toxicity, as reflected by both reduced mortality and significantly reduced neuronal degeneration.

The differential JunB responses to inhibition of succinate dehydrogenase in rat hippocampus and liver.

The inhibitor of mitochondrial enzyme succinate dehydrogenase, 3-nitropropionic acid (3-NPA), induces cellular energy deficit followed by oxidative stress, secondary excitotoxicity and neuronal degeneration. The fast activation of Jun and Fos proteins and other proteins encoding inducible transcription factors (ITFs) occurs in most tissues upon exposure to a variety of stressors including exposure to mitochondrial inhibitors. However, the consequences of this activation can differ dramatically in different organs. For example, while activation of the same ITFs may lead to apoptosis and necrosis in neurons it may stimulate liver regeneration. Here, we report the alterations in mRNAs levels of c-Fos, JunB, and Krox20 proteins induced in the rat brain and liver by the acute exposure to 3-NPA at 30 mg/kg, s.c. While the increase of c-fos transcripts was observed in both the hippocampus and liver, the junb transcript increased in the hippocampus but decreased in the liver. No changes were observed in krox-20 mRNA in the hippocampus. Interestingly, there was a large variation in krox-20 mRNA levels in the liver among animals within the same experimental group. In conclusion, out of the three ITFs transcripts examined here junb may activate different pathways depending on the tissue as indicated by differential responses to mitochondrial inhibition in the hippocampus and liver.

Neuroprotective effect of L-carnitine in the 3-nitropropionic acid (3-NPA)-evoked neurotoxicity in rats.

A plant and fungal toxin, 3-NPA, acts as an inhibitor of mitochondrial function via irreversible inactivation of the mitochondrial inner membrane enzyme, succinate dehydrogenase (SDH). Inhibition of SDH disturbs electron transport and leads to cellular energy deficits and neuronal injury. We have shown that pretreatment with l-carnitine, while not significantly attenuating SDH inhibition, prevented hypothermia and oxidative stress-associated increased activity of free radical-scavenging enzymes. Here, a neurohistological method was applied to examine the effect of carnitine pretreatment against 3-NPA-induced neurotoxicity. Twenty adult male Sprague-Dawley rats were randomly divided into two groups (n = 10/group). Rats in the first group were injected twice with 3-NPA at 30 mg/kg s.c., 2 days apart, and the second group of animals received l-carnitine pretreatment at 100 mg/kg 30-40 min before 3-NPA administration. Rats in both groups were perfused 7 days later and their brains harvested. Degenerating neurons were identified and localized via the fluorescent marker Fluoro-Jade B. In the three animals that survived 3-NPA dosing, one exhibited no pathology, one exhibited moderate unilateral damage to the striatum, and the third exhibited extensive bilateral neuronal degeneration in multiple forebrain regions. In the seven surviving animals that received l-carnitine prior to 3-NPA insult, six exhibited no lesions, while one exhibited a modest unilateral lesion in the striatum. It appears that l-carnitine is protective against 3-NPA-induced toxicity, as reflected by both reduced mortality and significantly reduced neuronal degeneration.

Polymorphisms in human soluble epoxide hydrolase.

Human soluble epoxide hydrolase (hsEH) metabolizes a variety of epoxides to the corresponding vicinal diols. Arachidonic and linoleic acid epoxides are thought to be endogenous substrates for hsEH. Enzyme activity in humans shows high interindividual variation (e.g., 500-fold in liver) suggesting the existence of regulatory and/or structural gene polymorphisms. We resequenced each of the 19 exons of the hsEH gene (EPHX2) from 72 persons representing black, Asian, and white populations. A variety of polymorphisms was found, six of which result in amino acid substitutions. Amino acid variants were localized on the crystal structure of the mouse sEH, resulting in the prediction that at least two of these (Arg287Gln and Arg103Cys) might significantly affect enzyme function. The six variants of the hsEH cDNA corresponding to each single polymorphism and one corresponding to a double polymorphism were then constructed by site-directed mutagenesis and expressed in insect cells. As predicted, Arg287Gln and the double mutant Arg287Gln/Arg103Cys showed decreased enzyme activity using trans-stilbene oxide, trans-diphenylpropene oxide, and 14,15-epoxyeicosatrienoic acid as substrates. Lys55Arg and Cys154Tyr mutants had elevated activity for all three substrates. Detailed kinetic studies revealed that the double mutant Arg287Gln/Arg103Cys showed significant differences in Km and Vmax. In addition, stability studies showed that the double mutant was less stable than wild-type protein when incubated at 37 degrees C. These results suggest that at least six hsEH variants exist in the human population and that at least four of these may influence hsEH-mediated metabolism of exogenous and endogenous epoxide substrates in vivo.

Global transcription analysis of Krebs tricarboxylic acid cycle mutants reveals an alternating pattern of gene expression and effects on hypoxic and oxidative genes.

To understand the many roles of the Krebs tricarboxylic acid (TCA) cycle in cell function, we used DNA microarrays to examine gene expression in response to TCA cycle dysfunction. mRNA was analyzed from yeast strains harboring defects in each of 15 genes that encode subunits of the eight TCA cycle enzymes. The expression of >400 genes changed at least threefold in response to TCA cycle dysfunction. Many genes displayed a common response to TCA cycle dysfunction indicative of a shift away from oxidative metabolism. Another set of genes displayed a pairwise, alternating pattern of expression in response to contiguous TCA cycle enzyme defects: expression was elevated in aconitase and isocitrate dehydrogenase mutants, diminished in alpha-ketoglutarate dehydrogenase and succinyl-CoA ligase mutants, elevated again in succinate dehydrogenase and fumarase mutants, and diminished again in malate dehydrogenase and citrate synthase mutants. This pattern correlated with previously defined TCA cycle growth-enhancing mutations and suggested a novel metabolic signaling pathway monitoring TCA cycle function. Expression of hypoxic/anaerobic genes was elevated in alpha-ketoglutarate dehydrogenase mutants, whereas expression of oxidative genes was diminished, consistent with a heme signaling defect caused by inadequate levels of the heme precursor, succinyl-CoA. These studies have revealed extensive responses to changes in TCA cycle function and have uncovered new and unexpected metabolic networks that are wired into the TCA cycle.