Reversible inhibition of acetylcholine synthesis and behavioural effects caused by 3-bromopyruvate

A Quantitative HILIC-MS/MS Assay of the Metabolic Response of Huh-7 Cells Exposed to 2,3,7,8-Tetrachlorodibenzo-p-Dioxin

A hydrophilic interaction liquid chromatography (HILIC)–ultra high-pressure liquid chromatography (UHPLC) coupled with tandem mass spectrometry (MS/MS) method was developed and applied to profile metabolite changes in human Huh-7 cells exposed to the potent aryl hydrocarbon receptor (AHR) ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Comparisons of sensitivity (limit of detection as low as 0.01 µM) and reproducibility (84% of compounds had an interday relative standard deviation (RSD) less than 10.0%; 83% of compounds had an intraday RSD less than 15.0%) were assessed for all the metabolites. The exposure of Huh-7 cells to the hepatotoxic carcinogen TCDD at low doses (1 nM and 10 nM for 4 h and 24 h, respectively) was reflected by the disturbance of amino acid metabolism, energy metabolism (glycolysis, TCA cycle), and nucleic acid metabolism. TCDD caused a significant decrease in amino acids such as serine, alanine, and proline while promoting an increase in arginine levels with 24 h treatment. Energy metabolism intermediates such as phosphoenolpyruvate and acetyl–CoA and nucleosides such as UMP, XMP, and CMP were also markedly decreased. These results support the application of HILIC–UHPLC–MS/MS for robust and reliable analysis of the cellular response to environmentally relevant toxicants at lower doses.

MCT1-mediated transport of a toxic molecule is an effective strategy for targeting glycolytic tumors

There is increasing evidence that oncogenic transformation modifies the metabolic program of cells. A common alteration is the upregulation of glycolysis, and efforts to target glycolytic enzymes for anti-cancer therapy are underway. Here, we performed a genome-wide haploid genetic screen to identify resistance mechanisms to 3-bromopyruvate (3-BrPA), a drug candidate that inhibits glycolysis in a poorly understood fashion. We identified the SLC16A1 gene product, MCT1, as the main determinant of 3-BrPA sensitivity. MCT1 is necessary and sufficient for 3-BrPA uptake by cancer cells. Additionally, MCT1 mRNA levels are the best predictor of 3-BrPA sensitivity and are most elevated in glycolytic cancer cells. Lastly, forced MCT1 expression in 3-BrPA resistant cancer cells sensitizes tumor xenografts to 3-BrPA treatment in vivo. Our results identify a potential biomarker for 3-BrPA sensitivity and provide proof of concept that the selectivity of cancer-expressed transporters can be exploited for delivering toxic molecules to tumors.

Hamster sperm capacitation: role of pyruvate dehydrogenase A and dihydrolipoamide dehydrogenase

Recently, we demonstrated that pyruvate dehydrogenase A2 (PDHA2) is tyrosine phosphorylated in capacitated hamster spermatozoa. In this report, using bromopyruvate (BP), an inhibitor of PDHA, we demonstrated that hamster sperm hyperactivation was blocked regardless of whether PDHA was inhibited prior to or after the onset of hyperactivation, but the acrosome reaction was blocked only if PDHA was inhibited prior to the onset of the acrosome reaction. Further, inhibition of PDHA activity did not inhibit capacitation-associated protein tyrosine phosphorylation observed in hamster spermatozoa. It is demonstrated that the essentiality of PDHA for sperm capacitation is probably dependent on its ability to generate effectors of capacitation such as reactive oxygen species (ROS) and cAMP, which are significantly decreased in the presence of BP. MICA (5-methoxyindole-2-carboxylic acid, a specific inhibitor of dihydrolipoamide dehydrogenase [DLD]), another component of the pyruvate dehydrogenase complex (PDHc), also significantly inhibited ROS generation and cAMP levels thus implying that these enzymes of the PDHc are required for ROS and cAMP generation. Furthermore, dibutryl cyclic adenosine monophosphate could significantly reverse the inhibition of hyperactivation observed in the presence of BP and inhibition of acrosome reaction observed in the presence of BP or MICA. The calcium ionophore, A23187, could also significantly reverse the inhibitory effect of BP and MICA on sperm acrosome reaction. These results establish that PDHA is required for hamster sperm hyperactivation and acrosome reaction, and DLD is required for hamster acrosome reaction. This study also provides evidence that ROS, cAMP, and calcium are involved downstream to PDHA.

Food for thought: fluctuations in brain extracellular glucose provide insight into the mechanisms of memory modulation

Extensive evidence indicates that peripheral or direct central glucose administration enhances cognitive processes in rodents and humans. These behavioral findings suggest that glucose acts directly on the brain to regulate neural processing, a function that seems incompatible with the traditional view that brain glucose levels are high and invariant except under extreme conditions. However, recent data suggest that the glucose levels of the brain's extracellular fluid are lower and more variable than previously supposed. In particular, the level of glucose in the extracellular fluid of a given brain area decreases substantially when a rat is performing a memory task for which the brain area is necessary. Together with results identifying downstream effects of such variance in glucose availability, the evidence leads to new thinking about glucose regulation of brain functions including memory.

Rapid eye movement sleep is reduced in prolactin-deficient mice

Prolactin (PRL) is implicated in the modulation of spontaneous rapid eye movement sleep (REMS). Previous models of hypoprolactinemic animals were characterized by changes in REMS, although associated deficits made it difficult to ascribe changes in REMS to reduced PRL. In the current studies, male PRL knock-out (KO) mice were used; these mice lack functional PRL but have no known additional deficits. Spontaneous REMS was reduced in the PRL KO mice compared with wild-type or heterozygous littermates. Infusion of PRL for 11-12 d into PRL KO mice restored their REMS to that occurring in wild-type or heterozygous controls. Six hours of sleep deprivation induced a non-REMS and a REMS rebound in both PRL KO mice and heterozygous littermates, although the REMS rebound in the KOs was substantially less. Vasoactive intestinal peptide (VIP) induced REMS responses in heterozygous mice but not in KO mice. Similarly, an ether stressor failed to enhance REMS in the PRL KOs but did in heterozygous littermates. Finally, hypothalamic mRNA levels for PRL, VIP, neural nitric oxide synthase (NOS), inducible NOS, and the interferon type I receptor were similar in KO and heterozygous mice. In contrast, tyrosine hydroxylase mRNA was lower in the PRL KO mice than in heterozygous controls and was restored to control values by infusion of PRL, suggesting a functioning short-loop negative feedback regulation in PRL KO mice. Data support the notion that PRL is involved in REMS regulation.

Short-term mental activation accelerates the age-related decline of high-energy phosphates in rat cerebral cortex

Aging in rats is associated with a significant decline in brain levels of energy-rich phosphates, including ATP and creatine phosphate. To test the effects of transient mental activity mediated by psychometric testing, and of metabolic inhibition of pyruvate dehydrogenase (PDH) (an enzyme complex that generates acetyl coenzyme A (CoA) to feed the mitochondrial tricarboxylic acid cycle), we compared adult (52 to 64-week-old) and aged (104-week-old) rats with and without intracerebral injections of the PDH inhibitor, bromopyruvate (BP), in the presence and in the absence of extensive psychometric testing by standard passive avoidance and hole board test paradigms. As compared with mental rest, short-term mental activation was associated with higher levels of energy-rich phosphates in the cerebral cortex of both adult and aged animals, but did not prevent the age-dependent decline in these phosphates. ATP turnover was markedly increased by mental activity, but was less pronounced in aged animals. In the hippocampus, less marked changes in the energy pool became obvious. The abnormalities in energy metabolism indicate an age-dependent and stress-accentuated reduction of the capacity to meet such energy-dependent demands as mixed function oxidation in the aged brain. BP did not change brain levels of energy-rich phosphates, indicating that the damage caused by decreased PDH activity can be compensated for both in adult and in aged animals.

The Contribution of Small Animal Positron Emission Tomography to the Neurosciences - A Critical Evaluation

This article presents an overview of those animal studies which so far have been performed with dedicated small animal positron emission tomographs in the field of the neurosciences. In vivo investigations focus on energy metabolism, perfusion and receptor/transporter binding in rat models of reinforcement, learning and memory, traumatic brain injury, epilepsy, depression, cardiovascular diseases--such as ischemia and focal stroke--and neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's disease. In the majority of studies, important novel aspects arise from the fact that the investigators made use of an option inherent to in vivo studies, namely to conduct longitudinal investigations on the same animals. Relevant findings pertain to the relationship of brain metabolism/perfusion and the cholinergic system, the regulation state of dopamine receptors upon cocaine administration and withdrawal, the regulation state of dopamine receptors and transporters in animal models of Parkinson's and Huntington's disease, and potential treatments of progressive dopaminergic depletion with adenoviral vectors, embryonic grafts, stem cells and nerve growth factors.

Deafferentation of the septo-hippocampal pathway in rats as a model of the metabolic events in Alzheimer's disease

Changes in the metabolic activity within the brain of patients suffering from Alzheimer's disease (AD) were investigated and compared with biochemical alterations in the hippocampus induced by fimbria/fornix transection in the rat. The deafferentation of the hippocampus results in a degeneration of cholinergic septo-hippocampal terminals accompanied by a persistent decrease of choline acetyltransferase (ChAT) and acetylcholine esterase (AChE) activities similar to the cholinergic malfunction in AD. In the animal model the [3H]-cytochalasin B binding to the glucose transporters was elevated up to the day 7 after surgery as was the activity of the phosphofructokinase (PFK) on day 3. A reactive astrogliosis could be evidenced by the upregulation of glial fibrillary acidic protein (GFAP). An increase of the PFK activity was also found in AD being accompanied by enhanced level of GFAP as well. A higher concentration of mRNA for all three isoenzymes of PFK was shown by reverse transcription (RT)-real time polymerase chain reaction (PCR) amplification. However, the pattern of PFK isoenzyme proteins and mRNAs did neither change in diseased human nor in the lesioned rat brain. The activities of the mitochondrial enzymes pyruvate dehydrogenase complex (PDHC) and cytochrome c oxidase (CO) were diminished in the lesioned rat hippocampus on day 7 as well as in AD brain. Subcellular fractionation showed that the activity of these enzymes was affected in the synaptosomal as well as in the extrasynaptosomal mitochondria indicating a loss of neuronal input and also a vulnerability of intrinsic hippocampal neurons and/or non-neuronal cells. The recovery of the mitochondrial enzyme activity in the animal model at later post lesion intervals may be the result of compensatory responses of surviving cells or of sprouting of other non-affected inputs. It is concluded that common metabolic mechanisms may underlie the concurrent degenerative and repair processes in the denervated hippocampus and the diseased Alzheimer brain.

Estrogen modulates spontaneous alternation and the cholinergic phenotype in the basal forebrain

We report that a small population of neurons expresses both choline acetyltransferase and classical estrogen receptor immunoreactivity and they are found primarily in the bed nucleus of the stria terminalis. In short-term ovariectomized ageing mice (24 months, n = 5) there were 41.0 +/- 4.1% fewer of these double-labeled cells than in young (five months, n = 5) short-term ovariectomized C57BL/6J mice. To study cholinergic neuron estrogen responsiveness, young mice (n = 8) were ovariectomized at puberty (five weeks). After three months half of the mice (n = 4) were given physiological levels of 17beta estradiol for 10 days. Bed nucleus double-labeled neurons increased by 32.9% (P < or = 0.003) in the young mice given estrogen. In a gel shift assay, double-stranded oligonucleotides with putative estrogen response elements from the choline acetyltransferase gene were used as competitors against estrogen receptor binding to consensus estrogen response elements. A sequence with 60% homology to the vitellogenin estrogen response element was found to compete at 500- and 1000-fold excess. Young mice (five months) with ovaries demonstrated significantly (P < or = 0.04) better performance in the spontaneous alternation T-maze test than did old (19 month) mice with ovaries (young = 66.3 +/- 3.3% correct choices; vs old = 55.0 +/- 4.0% in old mice with ovaries). Young mice (five months old), ovariectomized for one month and treated with estrogen, showed significantly more spontaneous alternation than ovariectomized controls (69.1 +/- 2.8% vs 58.3 +/- 3.9%; P < or = 0.04). Estrogen also increased spontaneous alternation in old, short-term ovariectomized mice (61.5 +/- 2.7% vs 48 +/- 3.3%; P < or = 0.005). In either young or old ovariectomized mice, estrogen increased spontaneous alternation to levels seen in young animals with ovaries. Estrogen increases the number of choline acetyltransferase-immunoreactive and choline acetyltransferase/estrogen receptor-immunoreactive cells in old or young mice lacking estrogen, and enhances working memory in old or young mice lacking estrogen. Our data suggest that estrogen may act at the level of the choline acetyltransferase gene, but in view of the limited distribution of cholinergic cells expressing the classical estrogen receptor, it is unlikely that these cells can account for a memory enhancing effect of estrogen replacement.

Cholinergic projection from the basal forebrain and cerebral glucose metabolism in rats: a dynamic PET study

To investigate the influence of cholinergic projections from the basal forebrain on cerebral cortex metabolism, we evaluated the cerebral metabolic rate of glucose (CMRGlu) after selective inhibition of cholinergic neurons in the rat basal forebrain using the pyruvate dehydrogenase complex inhibitor 3-bromopyruvic acid (BPA), and compared the results with those obtained after lesioning the basal forebrain with ibotenic acid, as well as with those from a sham-operated control group. CMRGlu was measured using positron emission tomography (PET) with [18F]-2-fluoro-2-deoxy-D-glucose (FDG). Three days after surgery, CMRGlu and k3 (phosphorylation of FDG) were reduced similarly in the frontal cortex on the BPA-injected side and in the ibotenic acid-treated group, whereas K1 (transport rate of FDG from the plasma to brain) showed no marked changes. At 3 weeks postoperatively, the CMRGlu and k3 of the frontal cortex in both groups recovered to levels similar to those of the sham-operated group. The main difference between the BPA and ibotenic acid groups was that CMRGlu showed mild reduction on the side contralateral to the operation in the former, while such reduction was confined to the ipsilateral hemisphere in the latter. The present results indicate that the cholinergic system in the basal forebrain regulates cerebral cortex glucose metabolism through direct excitation of cortical neurons.

Holeboard maze-learning deficits and brain monoaminergic neurotransmitter concentrations in rats after intracerebroventricular injection of 3-bromopyruvate

3-Bromopyruvate is a suicide inhibitor of pyruvate dehydrogenase complex in brain homogenates, and after intracerebral injection reduces acetylcholine tissue content and muscarinic cholinergic receptors in brain cortex and hippocampus for extended periods of time. A stereotaxic injection of 0.2 mumol 3-bromopyruvate was given twice into the cerebral ventricles of male Wistar rats. Ten weeks later, the animals were tested for learning deficits in a food-motivated complex holeboard test. 3-Bromopyruvate-treated rats showed an increased number of visits to nonfood-baited holes over a 5-day testing period (four trials per day) compared to sham-operated control rats, an increased number of visits to food-baited holes over the first 2 days of the testing period and an increased time for completing the task. There were no changes in brain monoaminergic neurotransmitter concentrations compared to controls. The results indicate that long-term learning deficits in a spatial discrimination paradigm are caused by 3-bromopyruvate, which might be related to a cholinergic deficit induced by a primary inhibition of brain glucose metabolism at the step of pyruvate dehydrogenase complex. This animal model may be useful for behavioral studies in relation to neurodegenerative diseases like dementia of Alzheimer type.

Acetylcholine and central respiratory control: perturbations of acetylcholine synthesis in the isolated brainstem of the neonatal rat

The brainstem neurochemical processes which support spontaneous ventilation are not known. Cholinergic transmission may play an important role. If this is true, perturbations in acetylcholine (ACh) turnover should alter ventilatory output in a predictable manner. Using the isolated superfused brainstem-spinal axis from the neonatal rat, the effects of modifiers of ACh synthesis on spontaneous C-4 (phrenic) output were determined. 3-Bromopyruvate and hydroxycitrate, inhibitors of acetyl-CoA (substrate for ACh synthesis) formation, caused depression of the C-4 output in a dose-dependent manner when added to the superfusate. Triethylcholine, a false-transmitter generating choline analog, caused a similar depression. Citrate, a cytosolic precursor to acetyl-CoA formation, caused stimulation of C-4 (phrenic) output. The stimulatory effects of citrate were blocked by the muscarinic cholinergic blocker, atropine. These findings are consistent with the view that the ACh synthetic pathway provides a continuous and important input to the normal brainstem elements that support ventilation.

Preliminary findings of high-dose thiamine in dementia of Alzheimer's type

Thiamine is important not only in the metabolism of acetylcholine but also in its release from the presynaptic neuron. Pathologic, clinical, and biochemical data suggest that thiamine deficiency is detrimental to the cholinergic system and that thiamine-dependent enzymes may be altered in Alzheimer's disease. Two previous studies reported contradictory results in patients with dementia of Alzheimer's type treated with 3 g/day of thiamine. In the present study, we examined the effects of 3 to 8 g/day thiamine administered orally. Our results suggest that thiamine at these pharmacologic dosages may have a mild beneficial effect in dementia of Alzheimer's type. The mechanism of the observed effect is unknown, but the findings warrant further investigation, not only for their therapeutic implications but for their possible etiologic clues. In addition, the results suggest long-term carry-over effects that should be considered in the design of future studies.