Triheptanoin, the triglyceride of heptanoate, is anticonvulsant in various epilepsy models. It is thought to improve energy metabolism in the epileptic brain by re‐filling the tricarboxylic acid (TCA) cycle with C4‐intermediates (anaplerosis). Here, we injected mice with [1,2‐13C]glucose 3.5–4 weeks after pilocarpine‐induced status epilepticus (SE) fed either a control or triheptanoin diet. Amounts of metabolites and incorporations of 13C were determined in extracts of cerebral cortices and hippocampal formation and enzyme activity and mRNA expression were quantified. The percentage enrichment with two 13C atoms in malate, citrate, succinate, and GABA was reduced in hippocampal formation of control‐fed SE compared with control mice. Except for succinate, these reductions were not found in triheptanoin‐fed SE mice, indicating that triheptanoin prevented a decrease of TCA cycle capacity. Compared to those on control diet, triheptanoin‐fed SE mice showed few changes in most other metabolite levels and their 13C labeling. Reduced pyruvate carboxylase mRNA and enzyme activity in forebrains and decreased [2,3‐13C]aspartate amounts in cortex suggest a pyruvate carboxylation independent source of C‐4 TCA cycle intermediates. Most likely anaplerosis was kept unchanged by carboxylation of propionyl‐CoA derived from heptanoate. Further studies are proposed to fully understand triheptanoin's effects on neuroglial metabolism and interaction.
Most ingested ethanol is metabolized in the liver to acetaldehyde and then to acetate, which can be oxidized by the brain. This project assessed whether chronic exposure to alcohol can increase cerebral oxidation of acetate. Through metabolism, acetate may contribute to long‐term adaptation to drinking. Two groups of adult male Sprague–Dawley rats were studied, one treated with ethanol vapor and the other given room air. After 3 weeks the rats received an intravenous infusion of [2‐13C]ethanol via a lateral tail vein for 2 h. As the liver converts ethanol to [2‐13C]acetate, some of the acetate enters the brain. Through oxidation the 13C is incorporated into the metabolic intermediate α‐ketoglutarate, which is converted to glutamate (Glu), glutamine (Gln), and GABA. These were observed by magnetic resonance spectroscopy and found to be 13C‐labeled primarily through the consumption of ethanol‐derived acetate. Brain Gln, Glu, and, GABA 13C enrichments, normalized to 13C‐acetate enrichments in the plasma, were higher in the chronically treated rats than in the ethanol‐naïve rats, suggesting increased cerebral uptake and oxidation of circulating acetate. Chronic ethanol exposure increased incorporation of systemically derived acetate into brain Gln, Glu, and GABA, key neurochemicals linked to brain energy metabolism and neurotransmission.
The mechanistic link of ketosis to neuroprotection under certain pathological conditions continues to be explored. We investigated whether chronic ketosis induced by ketogenic diet results in the partitioning of ketone bodies toward oxidative metabolism in brain. We hypothesized that diet‐induced ketosis results in increased shunting of ketone bodies toward citric acid cycle and amino acids with decreased carbon shunting from glucose. Rats were fed standard (STD) or ketogenic (KG) diets for 3.5 weeks and then infused with [U‐13C]glucose or [U‐13C]acetoacetate tracers. Concentrations and 13C‐labeling pattern of citric acid cycle intermediates and amino acids were analyzed from brain homogenates using stable isotopomer mass spectrometry analysis. The contribution of [U‐13C]glucose to acetyl‐CoA and amino acids decreased by ~ 30% in the KG group versus STD, whereas [U‐13C]acetoacetate contributions were more than two‐fold higher. The concentration of GABA remained constant across groups; however, the 13C labeling of GABA was markedly increased in the KG group infused with [U‐13C]acetoacetate compared to STD. This study reveals that there is a significant contribution of ketone bodies to oxidative metabolism and GABA in diet‐induced ketosis. We propose that this represents a fundamental mechanism of neuroprotection under pathological conditions.
Excitatory amino acid transporters (EAATs) regulate glutamatergic signal transmission by clearing extracellular glutamate. Dysfunction of these transporters has been implicated in the pathogenesis of various neurological disorders. Previous studies have shown that venom from the spider Parawixia bistriata and a purified compound (Parawixin1) stimulate EAAT2 activity and protect retinal tissue from ischemic damage. In the present study, the EAAT2 subtype specificity of this compound was explored, employing chimeric proteins between EAAT2 and EAAT3 transporter subtypes and mutants to characterize the structural region targeted by the compound. This identified a critical residue (Histidine‐71 in EAAT2 and Serine‐45 in EAAT3) in transmembrane domain 2 (TM2) to be important for the selectivity between EAAT2 and EAAT3 and for the activity of the venom. Using the identified residue in TM2 as a structural anchor, several neighboring amino acids within TM5 and TM8 were identified to also be important for the activity of the venom. This structural domain of the transporter lies at the interface of the rigid trimerization domain and the central substrate‐binding transport domain. Our studies suggest that the mechanism of glutamate transport enhancement involves an interaction with the transporter that facilitates the movement of the transport domain.
Munc13‐1 is a pre‐synaptic active‐zone protein essential for neurotransmitter release and involved in pre‐synaptic plasticity in brain. Ethanol, butanol, and octanol quenched the intrinsic fluorescence of the C1 domain of Munc13‐1 with EC50s of 52 mM, 26 mM, and 0.7 mM, respectively. Photoactive azialcohols photolabeled Munc13‐1 C1 exclusively at Glu‐582, which was identified by mass spectrometry. Mutation of Glu‐582 to alanine, leucine, and histidine reduced the alcohol binding two‐ to five‐fold. Circular dichroism studies suggested that binding of alcohol increased the stability of the wild‐type Munc13‐1 compared with the mutants. If Munc13‐1 plays some role in the neural effects of alcohol in vivo, changes in the activity of this protein should produce differences in the behavioral responses to ethanol. We tested this prediction with a loss‐of‐function mutation in the conserved Dunc‐13 in Drosophila melanogaster. The Dunc‐13P84200/+ heterozygotes have 50% wild‐type levels of Dunc‐13 mRNA and display a very robust increase in ethanol self‐administration. This phenotype is reversed by the expression of the rat Munc13‐1 protein within the Drosophila nervous system. The present studies indicate that Munc13‐1 C1 has binding site(s) for alcohols and Munc13‐1 activity is sufficient to restore normal self‐administration to Drosophila mutants deficient in Dunc‐13 activity.
The 13C‐labeling patterns in glutamate and glutamine from brain tissue are quite different after infusion of a mixture of 13C‐enriched glucose and acetate. Two processes contribute to this observation, oxidation of acetate by astrocytes but not neurons, and preferential incorporation of α‐ketoglutarate into glutamate in neurons, and incorporation of α‐ketoglutarate into glutamine in astrocytes. The acetate:glucose ratio, introduced previously for analysis of a single 13C NMR spectrum, provides a useful index of acetate and glucose oxidation in the brain tissue. However, quantitation of relative substrate oxidation at the cell compartment level has not been reported. A simple mathematical method is presented to quantify the ratio of acetate‐to‐glucose oxidation in astrocytes, based on the standard assumption that neurons do not oxidize acetate. Mice were infused with [1,2‐13C]acetate and [1,6‐13C]glucose, and proton decoupled 13C NMR spectra of cortex extracts were acquired. A fit of those spectra to the model indicated that 13C‐labeled acetate and glucose contributed approximately equally to acetyl‐CoA (0.96) in astrocytes. As this method relies on a single 13C NMR spectrum, it can be readily applied to multiple physiologic and pathologic conditions.
Ethanol is a known neuromodulatory agent with reported actions at a range of neurotransmitter receptors. Here, we measured the effect of alcohol on metabolism of [3‐13C]pyruvate in the adult Guinea pig brain cortical tissue slice and compared the outcomes to those from a library of ligands active in the GABAergic system as well as studying the metabolic fate of [1,2‐13C]ethanol. Analyses of metabolic profile clusters suggest that the significant reductions in metabolism induced by ethanol (10, 30 and 60 mM) are via action at neurotransmitter receptors, particularly α4β3δ receptors, whereas very low concentrations of ethanol may produce metabolic responses owing to release of GABA via GABA transporter 1 (GAT1) and the subsequent interaction of this GABA with local α5‐ or α1‐containing GABA(A)R. There was no measureable metabolism of [1,2‐13C]ethanol with no significant incorporation of 13C from [1,2‐13C]ethanol into any measured metabolite above natural abundance, although there were measurable effects on total metabolite sizes similar to those seen with unlabelled ethanol.
Energy metabolism supports both inhibitory and excitatory neurotransmission processes. This study investigated the specific contribution of astrocytic metabolism to γ‐aminobutyric acid (GABA) synthesis and inhibitory GABAergic neurotransmission that remained to be ilucidated in vivo. Therefore, we measured 13C incorporation into brain metabolites by dynamic 13C nuclear magnetic resonance spectroscopy at 14.1 T in rats under α‐chloralose anaesthesia during infusion of [1,6‐13C]glucose. The enhanced sensitivity at 14.1 T allowed to quantify incorporation of 13C into the three aliphatic carbons of GABA non‐invasively. Metabolic fluxes were determined with a mathematical model of brain metabolism comprising glial, glutamatergic and GABAergic compartments. GABA synthesis rate was 0.11 ± 0.01 μmol/g/min. GABA‐glutamine cycle was 0.053 ± 0.003 μmol/g/min and accounted for 22 ± 1% of total neurotransmitter cycling between neurons and glia. Cerebral glucose oxidation was 0.47 ± 0.02 μmol/g/min, of which 35 ± 1% and 7 ± 1% was diverted to the glutamatergic and GABAergic tricarboxylic acid cycles, respectively. The remaining fraction of glucose oxidation was in glia, where 12 ± 1% of the TCA cycle flux was dedicated to oxidation of GABA. 16 ± 2% of glutamine synthesis was provided to GABAergic neurons. We conclude that substantial metabolic activity occurs in GABAergic neurons and that glial metabolism supports both glutamatergic and GABAergic neurons in the living rat brain.
Chronic stress represents a major environmental risk factor for mood disorders in vulnerable individuals. The neurobiological mechanisms underlying these disorders involve serotonergic and endocannabinoid systems. In this study, we have investigated the relationships between these two neurochemical systems in emotional control using genetic and imaging tools. CB1 cannabinoid receptor knockout mice (KO) and wild‐type littermates (WT) were exposed to chronic restraint stress. Depressive‐like symptoms (anhedonia and helplessness) were produced by chronic stress exposure in WT mice. CB1 KO mice already showed these depressive‐like manifestations in non‐stress conditions and the same phenotype was observed after chronic restraint stress. Chronic stress similarly impaired long‐term memory in both genotypes. In addition, brain levels of serotonin transporter (5‐HTT) were assessed using positron emission tomography. Decreased brain 5‐HTT levels were revealed in CB1 KO mice under basal conditions, as well as in WT mice after chronic stress. Our results show that chronic restraint stress induced depressive‐like behavioral alterations and brain changes in 5‐HTT levels similarly to those revealed in CB1 KO mice in non‐stressed conditions. These results underline the relevance of chronic environmental stress on serotonergic and endocannabinoid transmission for the development of depressive symptoms.
Naked mole‐rats (NMRs) are the oldest‐living rodent species. Living underground in a thermally stable ecological niche, NMRs have evolved certain exceptional traits, resulting in sustained health spans, negligible cognitive decline, and a pronounced resistance to age‐related disease. Uncovering insights into mechanisms underlying these extraordinary traits involved in successful aging may conceivably provide crucial clues to extend the human life span and health span. One of the most fundamental processes inside the cell is the production of ATP, which is an essential fuel in driving all other energy‐requiring cellular activities. Not surprisingly, a prominent hallmark in age‐related diseases, such as neurodegeneration and cancer, is the impairment and dysregulation of metabolic pathways. Using a two‐dimensional polyacrylamide gel electrophoresis proteomics approach, alterations in expression and phosphorylation levels of metabolic proteins in the brains of NMRs, aged 2–24 years, were evaluated in an age‐dependent manner. We identified 13 proteins with altered levels and/or phosphorylation states that play key roles in various metabolic pathways including glycolysis, β‐oxidation, the malate‐aspartate shuttle, the Tricarboxylic Acid Cycle (TCA) cycle, the electron transport chain, NADPH production, as well as the production of glutamate. New insights into potential pathways involved in metabolic aspects of successful aging have been obtained by the identification of key proteins through which the NMR brain responds and adapts to the aging process and how the NMR brain adapted to resist age‐related degeneration.
(R)‐3‐[2,6‐cis‐Di(4‐methoxyphenethyl)piperidin‐1‐yl]propane‐1,2‐diol (GZ‐793A) inhibits methamphetamine‐evoked dopamine release from striatal slices and methamphetamine self‐administration in rats. GZ‐793A potently and selectively inhibits dopamine uptake at the vesicular monoamine transporter‐2 (VMAT2). This study determined GZ‐793A's ability to evoke [3H]dopamine release and inhibit methamphetamine‐evoked [3H]dopamine release from isolated striatal synaptic vesicles. Results show GZ‐793A concentration‐dependent [3H]dopamine release; nonlinear regression revealed a two‐site model of interaction with VMAT2 (High‐ and Low‐EC50 = 15.5 nM and 29.3 μM, respectively). Tetrabenazine and reserpine completely inhibited GZ‐793A‐evoked [3H]dopamine release, however, only at the High‐affinity site. Low concentrations of GZ‐793A that interact with the extravesicular dopamine uptake site and the High‐affinity intravesicular DA release site also inhibited methamphetamine‐evoked [3H]dopamine release from synaptic vesicles. A rightward shift in the methamphetamine concentration‐response was evident with increasing concentrations of GZ‐793A, and the Schild regression slope was 0.49 ± 0.08, consistent with surmountable allosteric inhibition. These results support a hypothetical model of GZ‐793A interaction at more than one site on the VMAT2 protein, which explains its potent inhibition of dopamine uptake, dopamine release via a High‐affinity tetrabenazine‐ and reserpine‐sensitive site, dopamine release via a Low‐affinity tetrabenazine‐ and reserpine‐insensitive site, and a low‐affinity interaction with the dihydrotetrabenazine binding site on VMAT2. GZ‐793A inhibition of the effects of methamphetamine supports its potential as a therapeutic agent for the treatment of methamphetamine abuse.
In this study, we have evaluated cerebral atrophy, neurometabolite homeostasis, and neural energetics in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridin (MPTP) model of Parkinson's disease. In addition, the efficacy of acute l ‐DOPA treatment for the reversal of altered metabolic functions was also evaluated. Cerebral atrophy and neurochemical profile were monitored in vivo using MRI and 1H MR Spectroscopy. Cerebral energetics was studied by 1H‐[13C]‐NMR spectroscopy in conjunction with infusion of 13C labeled [1,6‐13C2]glucose or [2‐13C]acetate. MPTP treatment led to reduction in paw grip strength and increased level of GABA and myo‐inositol in striatum and olfactory bulb. 13C Labeling of glutamate‐C4 (1.93 ± 0.24 vs. 1.48 ± 0.06 μmol/g), GABA‐C2 (0.24 ± 0.04 vs. 0.18 ± 0.02 μmol/g) and glutamaine‐C4 (0.26 ± 0.04 vs. 0.20 ± 0.04 μmol/g) from [1,6‐13C2]glucose was found to be decreased with MPTP exposure in striatum as well as in other brain regions. However, glutamine‐C4 labeling from [2‐13C]acetate was found to be increased in the striatum of the MPTP‐treated mice. Acute l ‐DOPA treatment failed to normalize the increased ventricular size and level of metabolites but recovered the paw grip strength and 13C labeling of amino acids from [1,6‐13C2]glucose and [2‐13C]acetate in MPTP‐treated mice. These data indicate that brain energy metabolism is impaired in Parkinson's disease and acute l ‐DOPA therapy could temporarily recover the cerebral metabolism.
As reported previously, in the lithium–pilocarpine model of temporal lobe epilepsy (TLE), carisbamate (CRS) produces strong neuroprotection, leads to milder absence‐like seizures, and prevents behavioral impairments in a subpopulation of rats. To understand the metabolic basis of these effects, here we injected 90 mg/kg CRS or vehicle twice daily for 7 days starting 1 h after status epilepticus (SE) induction in rats. Two months later, we injected [1‐13C]glucose and [1,2‐13C]acetate followed by head microwave fixation after 15 min. 13C incorporation into metabolites was analyzed using 13C magnetic resonance spectroscopy. We found that SE reduced neuronal mitochondrial metabolism in the absence but not in the presence of CRS. Reduction in glutamate level was prevented by CRS and aspartate levels were similar to controls only in rats displaying absence‐like seizures after treatment [CRS‐absence‐like epilepsy (ALE)]. Glutamine levels in CRS‐ALE rats were higher compared to controls in hippocampal formation and limbic structures while unchanged in rats displaying motor spontaneous recurrent seizures after treatment (CRS‐TLE). Astrocytic mitochondrial metabolism was reduced in CRS‐TLE, and either enhanced or unaffected in CRS‐ALE rats, which did not affect the transfer of glutamine from astrocytes to neurons. In conclusion, CRS prevents reduction in neuronal mitochondrial metabolism but its effect on astrocytes is likely key in determining outcome of treatment in this model.
Glutamate transport is a critical process in the brain that maintains low extracellular levels of glutamate to allow for efficient neurotransmission and prevent excitotoxicity. Loss of glutamate transport function is implicated in epilepsy, traumatic brain injury, and amyotrophic lateral sclerosis. It remains unclear whether or not glutamate transport can be modulated in these disease conditions to improve outcome. Here, we show that sirtuin (SIRT)4, a mitochondrial sirtuin, is up‐regulated in response to treatment with the potent excitotoxin kainic acid. Loss of SIRT4 leads to a more severe reaction to kainic acid and decreased glutamate transporter expression and function in the brain. Together, these results indicate a critical and novel stress response role for SIRT4 in promoting proper glutamate transport capacity and protecting against excitotoxicity.
Soluble N‐ethylmaleimide sensitive factor attachment protein receptors (SNAREs) are crucial for exocytosis, trafficking, and neurite outgrowth, where vesicular SNAREs are directed toward their partner target SNAREs: synaptosomal‐associated protein of 25 kDa and syntaxin. SNARE proteins are normally membrane bound, but can be cleaved and released by botulinum neurotoxins. We found that botulinum proteases types C and D can easily be transduced into endocrine cells using DNA‐transfection reagents. Following administration of the C and D proteases into normally refractory Neuro2A neuroblastoma cells, the SNARE proteins were cleaved with high efficiency within hours. Remarkably, botulinum protease exposures led to cytotoxicity evidenced by spectrophotometric assays and propidium iodide penetration into the nuclei. Direct delivery of SNARE fragments into the neuroblastoma cells reduced viability similar to botulinum proteases' application. We observed synergistic cytotoxic effects of the botulinum proteases, which may be explained by the release and interaction of soluble SNARE fragments. We show for the first time that previously observed cytotoxicity of botulinum neurotoxins/C in neurons could be achieved in cells of neuroendocrine origin with implications for medical uses of botulinum preparations.
The amyloid precursor protein (APP) is a type I transmembrane glycoprotein better known for its participation in the physiopathology of Alzheimer disease as the source of the beta amyloid fragment. However, the physiological functions of the full length protein and its proteolytic fragments have remained elusive. APP was first described as a cell‐surface receptor; nevertheless, increasing evidence highlighted APP as a cell adhesion molecule. In this review, we will focus on the current knowledge of the physiological role of APP as a cell adhesion molecule and its involvement in key events of neuronal development, such as migration, neurite outgrowth, growth cone pathfinding, and synaptogenesis. Finally, since APP is over‐expressed in Down syndrome individuals because of the extra copy of chromosome 21, in the last section of the review, we discuss the potential contribution of APP to the neuronal and synaptic defects described in this genetic condition.
Niemann Pick type C (NPC1) is a rare fatal hereditary cholesterol storage disease associated with a massive Purkinje cells loss. The mechanisms leading to neurodegeneration are still poorly understood. Different laboratories pointed to hypersensitivity to cytotoxic effects of statins (HMG‐CoA reductase inhibitors) in NPC1 and suggested an underlying lack of geranylgeranyl pyrophosphate (GGPP). GGPP is a non‐sterol isoprenoid essential for cell survival and differentiation. We measured GGPP levels in cerebella of a NPC1 mouse model and of wild‐type littermates and found a physiological increase of GGPP levels between post‐natal days 21 and 49 in wild‐type mice but not in NPC mice. This further supports the hypothesis that Purkinje cell loss may be due to an extremely low level of GGPP. The progressive Purkinje cell loss in NPC starts between p21 and p49. To test the hypothesis, we used long‐term organotypic slice cultures of NPC1 mice that display the natural history of NPC1 disease in vitro and tested if chronic administration of GGPP might prevent Purkinje cell loss. We did not see a beneficial effect. This suggests, in contrast to the expectations, that the relative lack of GGPP may not significantly contribute to mechanisms of Purkinje cell loss in NPC1.
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