Age-related changes in neuronal glucose uptake in response to glutamate and beta-amyloid

Altered hippocampal astroglial metabolism is associated with aging and preserved spatial learning and memory

An age-related decrease in hippocampal metabolism correlates with cognitive decline. Hippocampus-dependent learning and memory requires glutamatergic neurotransmission supported by glutamate-glutamine (GLU-GLN) cycling between neurons and astrocytes. We examined whether GLU-GLN cycling in hippocampal subregions (dentate gyrus and CA1) in Fischer 344 rats was altered with age and cognitive status. Hippocampal slices from young adult, aged cognitively-unimpaired (AU) and aged cognitively-impaired (AI) rats were incubated in artificial cerebrospinal fluid (aCSF) containing 1-13C-glucose to assess neural metabolism. Incorporation of 13C-glucose into glutamate and glutamine, measured by mass spectroscopy/liquid chromatography tandem mass spectroscopy, did not significantly differ between groups. However, when 13C-acetate, a preferential astrocytic metabolite, was used, a significant increase in 13C-labeled glutamate was observed in slices from AU rats. Taken together, the data suggest that resting state neural metabolism and GLU-GLN cycling may be preserved during aging when sufficient extracellular glucose is available, but that enhanced astroglial metabolism can occur under resting state conditions. This may be an aging-related compensatory change to maintain hippocampus-dependent cognitive function.

Age-Related Intraneuronal Aggregation of Amyloid-β in Endosomes, Mitochondria, Autophagosomes, and Lysosomes

 This work provides new insight into the age-related basis of Alzheimer’s disease (AD), the composition of intraneuronal amyloid (iAβ), and the mechanism of an age-related increase in iAβ in adult AD-model mouse neurons. A new end-specific antibody for Aβ₄₅ and another for aggregated forms of Aβ provide new insight into the composition of iAβ and the mechanism of accumulation in old adult neurons from the 3xTg-AD model mouse. iAβ levels containing aggregates of Aβ₄₅ increased 30-50-fold in neurons from young to old age and were further stimulated upon glutamate treatment. iAβ was 8 times more abundant in 3xTg-AD than non-transgenic neurons with imaged particle sizes following the same log-log distribution, suggesting a similar snow-ball mechanism of intracellular biogenesis. Pathologically misfolded and mislocalized Alz50 tau colocalized with iAβ and rapidly increased following a brief metabolic stress with glutamate. AβPP-CTF, Aβ₄₅, and aggregated Aβ colocalized most strongly with mitochondria and endosomes and less with lysosomes and autophagosomes. Differences in iAβ by sex were minor. These results suggest that incomplete carboxyl-terminal trimming of long Aβs by gamma-secretase produced large intracellular deposits which limited completion of autophagy in aged neurons. Understanding the mechanism of age-related changes in iAβ processing may lead to application of countermeasures to prolong dementia-free health span.

Global Metabolic Shifts in Age and Alzheimer's Disease Mouse Brains Pivot at NAD+/NADH Redox Sites

Age and Alzheimer’s disease (AD) share some common features such as cognitive impairments, memory loss, metabolic disturbances, bioenergetic deficits, and inflammation. Yet little is known on how systematic shifts in metabolic networks depend on age and AD. In this work, we investigated the global metabolomic alterations in non-transgenic (NTg) and triple-transgenic (3xTg-AD) mouse brain hippocampus as a function of age by using untargeted Ultrahigh Performance Liquid Chromatography-tandem Mass Spectroscopy (UPLC-MS/MS). We observed common metabolic patterns with aging in both NTg and 3xTg-AD brains involved in energy-generating pathways, fatty acids oxidation, glutamate, and sphingolipid metabolism. We found age-related downregulation of metabolites from reactions in glycolysis that consumed ATP and in the TCA cycle, especially at NAD⁺/NADH-dependent redox sites, where age- and AD-associated limitations in the free NADH may alter reactions. Conversely, metabolites increased in glycolytic reactions in which ATP is produced. With age, inputs to the TCA cycle were increased including fatty acid β-oxidation and glutamine. Overall age- and AD-related changes were > 2-fold when comparing the declines of upstream metabolites of NAD⁺/NADH-dependent reactions to the increases of downstream metabolites (p = 10^-5, n = 8 redox reactions). Inflammatory metabolites such as ceramides and sphingosine-1-phosphate also increased with age. Age-related decreases in glutamate, GABA, and sphingolipid were seen which worsened with AD genetic load in 3xTg-AD brains, possibly contributing to synaptic, learning- and memory-related deficits. The data support the novel hypothesis that age- and AD-associated metabolic shifts respond to NAD(P)⁺/NAD(P)H redox-dependent reactions, which may contribute to decreased energetic capacity.

The role of cell type-specific mitochondrial dysfunction in the pathogenesis of Alzheimer's disease

The decrease of metabolism in the brain has been observed as the important lesions of Alzheimer’s disease (AD) from the early stages of diagnosis. The cumulative evidence has reported that the failure of mitochondria, an organelle involved in diverse biological processes as well as energy production, maybe the cause or effect of the pathogenesis of AD. Both amyloid and tau pathologies have an impact upon mitochondria through physical interaction or indirect signaling pathways, resulting in the disruption of mitochondrial function and dynamics which can trigger AD. In addition, mitochondria are involved in different biological processes depending on the specific functions of each cell type in the brain. Thus, it is necessary to understand mitochondrial dysfunction as part of the pathological phenotypes of AD according to each cell type. In this review, we summarize that 1) the effects of AD pathology inducing mitochondrial dysfunction and 2) the contribution of mitochondrial dysfunction in each cell type to AD pathogenesis.

Computational singular perturbation analysis of brain lactate metabolism

Lactate in the brain is considered an important fuel and signalling molecule for neuronal activity, especially during neuronal activation. Whether lactate is shuttled from astrocytes to neurons or from neurons to astrocytes leads to the contradictory Astrocyte to Neuron Lactate Shuttle (ANLS) or Neuron to Astrocyte Lactate Shuttle (NALS) hypotheses, both of which are supported by extensive, but indirect, experimental evidence. This work explores the conditions favouring development of ANLS or NALS phenomenon on the basis of a model that can simulate both by employing the two parameter sets proposed by Simpson et al. (J Cereb. Blood Flow Metab., 27:1766, 2007) and Mangia et al. (J of Neurochemistry, 109:55, 2009). As most mathematical models governing brain metabolism processes, this model is multi-scale in character due to the wide range of time scales characterizing its dynamics. Therefore, we utilize the Computational Singular Perturbation (CSP) algorithm, which has been used extensively in multi-scale systems of reactive flows and biological systems, to identify components of the system that (i) generate the characteristic time scale and the fast/slow dynamics, (ii) participate to the expressions that approximate the surfaces of equilibria that develop in phase space and (iii) control the evolution of the process within the established surfaces of equilibria. It is shown that a decisive factor on whether the ANLS or NALS configuration will develop during neuronal activation is whether the lactate transport between astrocytes and interstitium contributes to the fast dynamics or not. When it does, lactate is mainly generated in astrocytes and the ANLS hypothesis is realised, while when it doesn't, lactate is mainly generated in neurons and the NALS hypothesis is realised. This scenario was tested in exercise conditions.

Age- and AD-related redox state of NADH in subcellular compartments by fluorescence lifetime imaging microscopy

Nicotinamide adenine dinucleotide (reduced form: NADH) serves as a vital redox-energy currency for reduction-oxidation homeostasis and fulfilling energetic demands. While NADH exists as free and bound forms, only free NADH is utilized for complex I to power oxidative phosphorylation, especially important in neurons. Here, we studied how much free NADH remains available for energy production in mitochondria of old living neurons. We hypothesize that free NADH in neurons from old mice is lower than the levels in young mice and even lower in neurons from the 3xTg-AD Alzheimer's disease (AD) mouse model. To assess free NADH, we used lifetime imaging of NADH autofluorescence with 2-photon excitation to be able to resolve the pool of NADH in mitochondria, cytoplasm, and nuclei. Primary neurons from old mice were characterized by a lower free/bound NADH ratio than young neurons from both non-transgenic (NTg) and more so in 3xTg-AD mice. Mitochondrial compartments maintained 26 to 41% more reducing NADH redox state than cytoplasm for each age, genotype, and sex. Aging diminished the mitochondrial free NADH concentration in NTg neurons by 43% and in 3xTg-AD by 50%. The lower free NADH with age suggests a decline in capacity to regenerate free NADH for energetic supply to power oxidative phosphorylation which further worsens in AD. Applying this non-invasive approach, we showed the most explicit measures yet of bioenergetic deficits in free NADH with aging at the subcellular level in live neurons from in-bred mice and an AD model.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Lack of appropriate stoichiometry: Strong evidence against an energetically important astrocyte-neuron lactate shuttle in brain

Glutamate-stimulated aerobic glycolysis in astrocytes coupled with lactate shuttling to neurons where it can be oxidized was proposed as a mechanism to couple excitatory neuronal activity with glucose utilization (CMR_glc ) during brain activation. From the outset, this model was not viable because it did not fulfill critical stoichiometric requirements: (i) Calculated glycolytic rates and measured lactate release rates were discordant in cultured astrocytes. (ii) Lactate oxidation requires oxygen consumption, but the oxygen-glucose index (OGI, calculated as CMR_O2 /CMR_glc ) fell during activation in human brain, and the small rise in CMR_O2 could not fully support oxidation of lactate produced by disproportionate increases in CMR_glc . (iii) Labeled products of glucose metabolism are not retained in activated rat brain, indicating rapid release of a highly labeled, diffusible metabolite identified as lactate, thereby explaining the CMR_glc -CMR_O2 mismatch. Additional independent lines of evidence against lactate shuttling include the following: astrocytic oxidation of glutamate after its uptake can help "pay" for its uptake without stimulating glycolysis; blockade of glutamate receptors during activation in vivo prevents upregulation of metabolism and lactate release without impairing glutamate uptake; blockade of β-adrenergic receptors prevents the fall in OGI in activated human and rat brain while allowing glutamate uptake; and neurons upregulate glucose utilization in vivo and in vitro under many stimulatory conditions. Studies in immature cultured cells are not appropriate models for lactate shuttling in adult brain because of their incomplete development of metabolic capability and astrocyte-neuron interactions. Astrocyte-neuron lactate shuttling does not make large, metabolically significant contributions to energetics of brain activation. © 2017 Wiley Periodicals, Inc.

Electroacupuncture improves cognitive deficits associated with AMPK activation in SAMP8 mice

Perturbations of brain energy metabolism are involved in Alzheimer's disease (AD). Adenosine monophosphate-activated kinase (AMPK) is a master energy sensor that monitors the levels of key energy metabolites. Electroacupuncture (EA) has demonstrated therapeutic potential for the treatment of AD. The effects of EA on cognitive functions and the changes of AMPK and its phosphorylated form (p-AMPK) expression were investigated in senescence-accelerated mouse prone 8 (SAMP8) mice. Cognitive function of SAMP8 mice was assessed using Morris water maze test after EA treatment. Then mice were sacrificed for immunohistochemistry and western blot analysis. EA stimulation significantly alleviated memory impairment of AD mice, and increased the levels of p-AMPK in the hippocampus. These results suggest that EA improved cognitive function associated with AMPK activation, AMPK may be a molecular target of EA in treating AD.

Efficient uptake and dissemination of scrapie prion protein by astrocytes and fibroblasts from adult hamster brain

Prion infections target neurons and lead to neuronal loss. However, the role of non-neuronal cells in the initiation and spread of infection throughout the brain remains unclear despite the fact these cells can also propagate prion infectivity. To evaluate how different brain cells process scrapie prion protein (PrPres) during acute infection, we exposed neuron-enriched and non-neuronal cell cultures from adult hamster brain to fluorescently-labeled purified PrPres and followed the cultures by live cell confocal imaging over time. Non-neuronal cells present in both types of cultures, specifically astrocytes and fibroblasts, internalized PrPres more efficiently than neurons. PrPres was trafficked to late endosomal/lysosomal compartments and rapidly transported throughout the cell bodies and processes of all cell types, including contacts between astrocytes and neurons. These observations suggest that astrocytes and meningeal fibroblasts play an as yet unappreciated role in prion infections via efficient uptake and dissemination of PrPres.

The metabolic response to excitotoxicity - lessons from single-cell imaging

Excitotoxicity is a pathological process implicated in neuronal death during ischaemia, traumatic brain injuries and neurodegenerative diseases. Excitotoxicity is caused by excess levels of glutamate and over-activation of NMDA or calcium-permeable AMPA receptors on neuronal membranes, leading to ionic influx, energetic stress and potential neuronal death. The metabolic response of neurons to excitotoxicity is complex and plays a key role in the ability of the neuron to adapt and recover from such an insult. Single-cell imaging is a powerful experimental technique that can be used to study the neuronal metabolic response to excitotoxicity in vitro and, increasingly, in vivo. Here, we review some of the knowledge of the neuronal metabolic response to excitotoxicity gained from in vitro single-cell imaging, including calcium and ATP dynamics and their effects on mitochondrial function, along with the contribution of glucose metabolism, oxidative stress and additional neuroprotective signalling mechanisms. Future work will combine knowledge gained from single-cell imaging with data from biochemical and computational techniques to garner holistic information about the metabolic response to excitotoxicity at the whole brain level and transfer this knowledge to a clinical setting.

Aging, the metabolic syndrome, and ischemic stroke: redefining the approach for studying the blood-brain barrier in a complex neurological disease

The blood-brain barrier (BBB) has many important functions in maintaining the brain's immune-privileged status. Endothelial cells, astrocytes, and pericytes have important roles in preserving vasculature integrity. As we age, cell senescence can contribute to BBB compromise. The compromised BBB allows an influx of inflammatory cytokines to enter the brain. These cytokines lead to neuronal and glial damage. Ultimately, the functional changes within the brain can cause age-related disease. One of the most prominent age-related diseases is ischemic stroke. Stroke is the largest cause of disability and is third largest cause of mortality in the United States. The biggest risk factors for stroke, besides age, are results of the metabolic syndrome. The metabolic syndrome, if unchecked, quickly advances to outcomes that include diabetes, hypertension, cardiovascular disease, and obesity. The contribution from these comorbidities to BBB compromise is great. Some of the common molecular pathways activated include: endoplasmic reticulum stress, reactive oxygen species formation, and glutamate excitotoxicity. In this chapter, we examine how age-related changes to cells within the central nervous system interact with comorbidities. We then look at how comorbidities lead to increased risk for stroke through BBB disruption. Finally, we discuss key molecular pathways of interest with a focus on therapeutic targets that warrant further investigation.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Dual-energy precursor and nuclear erythroid-related factor 2 activator treatment additively improve redox glutathione levels and neuron survival in aging and Alzheimer mouse neurons upstream of reactive oxygen species

To determine whether glutathione (GSH) loss or increased reactive oxygen species (ROS) are more important to neuron loss, aging, and Alzheimer's disease (AD), we stressed or boosted GSH levels in neurons isolated from aging 3xTg-AD neurons compared with those from age-matched nontransgenic (non-Tg) neurons. Here, using titrating with buthionine sulfoximine, an inhibitor of γ-glutamyl cysteine synthetase (GCL), we observed that GSH depletion increased neuronal death of 3xTg-AD cultured neurons at increasing rates across the age span, whereas non-Tg neurons were resistant to GSH depletion until old age. Remarkably, the rate of neuron loss with ROS did not increase in old age and was the same for both genotypes, which indicates that cognitive deficits in the AD model were not caused by ROS. Therefore, we targeted for neuroprotection activation of the redox sensitive transcription factor, nuclear erythroid-related factor 2 (Nrf2) by 18 alpha glycyrrhetinic acid to stimulate GSH synthesis through GCL. This balanced stimulation of a number of redox enzymes restored the lower levels of Nrf2 and GCL seen in 3xTg-AD neurons compared with those of non-Tg neurons and promoted translocation of Nrf2 to the nucleus. By combining the Nrf2 activator together with the NADH precursor, nicotinamide, we increased neuron survival against amyloid beta stress in an additive manner. These stress tests and neuroprotective treatments suggest that the redox environment is more important for neuron survival than ROS. The dual neuroprotective treatment with nicotinamide and an Nrf2 inducer indicates that these age-related and AD-related changes are reversible.

Reversible epigenetic histone modifications and Bdnf expression in neurons with aging and from a mouse model of Alzheimer's disease

With aging and Alzheimer's disease (AD), there is an increased sensitivity to stress along with declines in the memory-associated neurotrophin brain-derived neurotrophic factor in AD. We have replicated this aging phenotype in cultured neurons from aged mice despite being grown in the same environmental conditions as young neurons. This led us to hypothesize that age-related differences in epigenetic acetylation and methylation of histones are associated with age-related gene regulation. We cultured hippocampal/cortical neurons from the 3xTg-AD mouse model and from non-transgenic mice to quantify single cell acetylation and methylation levels across the life span. In non-transgenic neurons, H3 acetylation was unchanged with age, while H4 acetylation decreased with age of the donor. Compared to non-transgenic neurons, 3xTg-AD neurons had higher levels of H3 and H4 acetylation beginning at 4 months of age. In contrast to non-transgenic neurons, 3xTg-AD neurons increased acetylation with age; 3xTg-AD neurons also responded differently to inhibition of histone deacetylases at an early age. Importantly, treatment of non-transgenic neurons with the AD peptide Aβ also elevated levels of acetylation. We also examined the repressive function of histone H3 lysine 9 (H3K9) methylation. H3K9 methylation increased with age in non-transgenic neurons, which was amplified further in 3xTg-AD neurons. The dominant effect of higher H3K9 methylation was supported by lower Bdnf gene expression in non-transgenic and 3xTg-AD mice. These data show that the epigenetic states of non-transgenic and 3xTg-AD brain neurons are profoundly different and reversible, beginning at 4 months of age when the first memory deficits are reported.

Brain lactate metabolism: the discoveries and the controversies

Potential roles for lactate in the energetics of brain activation have changed radically during the past three decades, shifting from waste product to supplemental fuel and signaling molecule. Current models for lactate transport and metabolism involving cellular responses to excitatory neurotransmission are highly debated, owing, in part, to discordant results obtained in different experimental systems and conditions. Major conclusions drawn from tabular data summarizing results obtained in many laboratories are as follows: Glutamate-stimulated glycolysis is not an inherent property of all astrocyte cultures. Synaptosomes from the adult brain and many preparations of cultured neurons have high capacities to increase glucose transport, glycolysis, and glucose-supported respiration, and pathway rates are stimulated by glutamate and compounds that enhance metabolic demand. Lactate accumulation in activated tissue is a minor fraction of glucose metabolized and does not reflect pathway fluxes. Brain activation in subjects with low plasma lactate causes outward, brain-to-blood lactate gradients, and lactate is quickly released in substantial amounts. Lactate utilization by the adult brain increases during lactate infusions and strenuous exercise that markedly increase blood lactate levels. Lactate can be an 'opportunistic', glucose-sparing substrate when present in high amounts, but most evidence supports glucose as the major fuel for normal, activated brain.

A reversible early oxidized redox state that precedes macromolecular ROS damage in aging nontransgenic and 3xTg-AD mouse neurons

The brain depends on redox electrons from nicotinamide adenine dinucleotide (reduced form; NADH) to produce ATP and oxyradicals (reactive oxygen species [ROS]). Because ROS damage and mitochondrial dysregulation are prominent in aging and Alzheimer's disease (AD) and their relationship to the redox state is unclear, we wanted to know whether an oxidative redox shift precedes these markers and leads to macromolecular damage in a mouse model of AD. We used the 3xTg-AD mouse model, which displays cognitive deficits beginning at 4 months. Hippocampal/cortical neurons were isolated across the age span and cultured in common nutrients to control for possible hormonal and vascular differences. We found an increase of NAD(P)H levels and redox state in nontransgenic (non-Tg) neurons until middle age, followed by a decline in old age. The 3xTg-AD neurons maintained much lower resting NAD(P)H and redox states after 4 months, but the NADH regenerating capacity continuously declined with age beginning at 2 months. These redox characteristics were partially reversible with nicotinamide, a biosynthetic precursor of NAD+. Nicotinamide also protected against glutamate excitotoxicity. Compared with non-Tg neurons, 3xTg-AD neurons had more mitochondria/neuron and lower glutathione (GSH) levels that preceded age-related increases in ROS levels. These GSH deficits were again reversible with nicotinamide in 3xTg-AD neurons. Surprisingly, low macromolecular ROS damage was only elevated after 4 months in the 3xTg-AD neurons if antioxidants were removed. The present data suggest that a more oxidized redox state and a lower antioxidant GSH defense can be dissociated from neuronal ROS damage, changes that precede the onset of cognitive deficits in the 3xTg-AD model.

Reciprocal induction between α-synuclein and β-amyloid in adult rat neurons

In spite of definite roles for β-amyloid (Aβ) in familial Alzheimer's disease (AD), the cause of sporadic AD remains unknown. Amyloid senile plaques and Lewy body pathology frequently coexist in neocortical and hippocampal regions of AD and Parkinson's diseases. However, the relationship between Aβ and α-synuclein (α-Syn), the principle components in the pathological structures, in neuronal toxicity and the mechanisms of their interaction are not well studied. As Aβ and α-Syn accumulate in aging patients, the biological functions and toxicity of these polypeptides in the aging brain may be different from those in young brain. We examined the neurotoxicity influences of Aβ1-42 or α-Syn on mature neurons and the effects of Aβ1-42 or α-Syn on the production of endogenous α-Syn or Aβ1-40 reciprocally using a model of culture enriched with primary neurons from the hippocampus of adult rats. Treatment of neurons with high concentrations of Aβ1-42 or α-Syn caused significant apoptosis of neurons. Following Aβ1-42 treatment at sub apoptotic concentrations, both intra- and extra-cellular α-Syn levels were significantly increased. Reciprocally, the non-toxic levels of α-Syn treatment also increased intra- and extra-cellular Aβ1-40 levels. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, suppressed α-Syn-induced Aβ1-40 elevation, as well as Aβ1-42-induced α-Syn elevation. Thus, high concentrations of Aβ1-42 and α-Syn exert toxic effects on mature neurons; however, non-toxic concentration treatment of these polypeptides induced the production of each other reciprocally with possible involvement of PI3K pathway.

Proteomic study of amyloid beta (25-35) peptide exposure to neuronal cells: Impact on APE1/Ref-1's protein-protein interaction

The genotoxic, extracellular accumulation of amyloid β (Aβ) protein and subsequent neuronal cell death are associated with Alzheimer's disease (AD). APE1/Ref-1, the predominant apurinic/apyrimidinic (AP) endonuclease and essential in eukaryotic cells, plays a central role in the base excision repair (BER) pathway for repairing oxidized and alkylated bases and single-strand breaks (SSBs) in DNA. APE1/Ref-1 is also involved in the redox activation of several trans-acting factors (TFs) in various cell types, but little is known about its role in neuronal functions. There is emerging evidence for APE1/Ref-1's role in neuronal cells vulnerable in AD and other neurodegenerative disorders, as reflected in its nuclear accumulation in AD brains. An increase in APE1/Ref-1 has been shown to enhance neuronal survival after oxidative stress. To address whether APE1/Ref-1 level or its association with other proteins is responsible for this protective effect, we used 2-D proteomic analyses and identified cytoskeleton elements (i.e., tropomodulin 3, tropomyosin alpha-3 chain), enzymes involved in energy metabolism (i.e., pyruvate kinase M2, N-acetyl transferase, sulfotransferase 1c), proteins involved in stress response (i.e., leucine-rich and death domain, anti-NGF30), and heterogeneous nuclear ribonucleoprotien-H (hnRNP-H) as being associated with APE1/Ref-1 in Aβ(25-35)-treated rat pheochromocytoma PC12 and human neuroblastoma SH-SY5Y cell lines, two common neuronal precursor lines used in Aβ neurotoxicity studies. Because the levels of some of these proteins are affected in the brains of AD patients, our study suggests a neuroprotective role for APE1/Ref-1 via its association with those proteins and modulating their cellular functions during Aβ-mediated neurotoxicity.

Cerebral oxygen metabolism in patients with early Parkinson's disease

AIM

Decreased activity of the mitochondrial electron transport chain (ETC) has been implicated in the pathogenesis of Parkinson's disease (PD). This model would most likely predict a decrease in the rate of cerebral oxygen consumption (CMRO(2)). To test this hypothesis, we compared CMRO(2) and cerebral blood flow (CBF) PET scans from PD patients and healthy controls.

MATERIALS AND METHODS

Nine early-stage PD patients and 15 healthy age-matched controls underwent PET scans for quantitative mapping of CMRO(2) and CBF. Between-group differences were evaluated for absolute data and intensity-normalized values.

RESULTS

No group differences were detected in regional magnitudes of CMRO(2) or CBF. Upon normalization using the reference cluster method, significant relative CMRO(2) decreases were evident in widespread prefrontal, parieto-occipital, and lateral temporal regions. Sensory-motor and subcortical regions, brainstem, and the cerebellum were spared. A similar pattern was evident in normalized CBF data, as described previously.

CONCLUSION

While the data did not reveal substantially altered absolute CMRO(2) in brain of PD patients, employing data-driven intensity normalization revealed widespread relative CMRO(2) decreases in cerebral cortex. The detected pattern was very similar to that reported in earlier CBF and CMRglc studies of PD, and in the CBF images from the same subjects. Thus, the present results are consistent with the occurrence of parallel declines in CMRO(2), CBF, and CMRglc in spatially contiguous cortical regions in early PD, and support the hypothesis that ETC dysfunction could be a primary pathogenic mechanism in early PD.

AMP-activated protein kinase: a potential player in Alzheimer's disease

AMP-activated protein kinase (AMPK) stimulates energy production via glucose and lipid metabolism, whereas it inhibits energy consuming functions, such as protein and cholesterol synthesis. Increased cytoplasmic AMP and Ca(2+) levels are the major activators of neuronal AMPK signaling. Interestingly, Alzheimer's disease (AD) is associated with several abnormalities in neuronal energy metabolism, for example, decline in glucose uptake, mitochondrial dysfunctions and defects in cholesterol metabolism, and in addition, with problems in maintaining Ca(2+) homeostasis. Epidemiological studies have also revealed that many metabolic and cardiovascular diseases are risk factors for cognitive impairment and sporadic AD. Emerging studies indicate that AMPK signaling can regulate tau protein phosphorylation and amyloidogenesis, the major hallmarks of AD. AMPK is also a potent activator of autophagic degradation which seems to be suppressed in AD. All these observations imply that AMPK is involved in the pathogenesis of AD. However, the responses of AMPK activation are dependent on stimulation and the extent of activating stress. Evidently, AMPK signaling can repress and delay the appearance of AD pathology but later on, with increasing neuronal stress, it can trigger detrimental effects that augment AD pathogenesis. We will outline the potential role of AMPK function in respect to various aspects affecting AD pathogenesis.

Response to 'comment on recent modeling studies of astrocyte-neuron metabolic interactions': much ado about nothing

For many years, a tenet of cerebral metabolism held that glucose was the obligate energy substrate of the mammalian brain and that neuronal oxidative metabolism represented the majority of this glucose utilization. In 1994, Pellerin and Magistretti formulated the astrocyte-neuron lactate shuttle (ANLS) hypothesis, in which astrocytes, not neurons, metabolized glucose, with subsequent transport of the glycolytically derived lactate to fuel the energy needs of the neuron during neurotransmission. By considering the concentrations and kinetic characteristics of the nutrient transporter proteins, Simpson et al later supported the opposite view, in which lactate flows from neurons to astrocytes, thus leading to the neuron-astrocyte lactate shuttle (NALS). Most recently, a commentary was published in this journal attempting to discredit the NALS. This challenge has stimulated the present response in which we detail the inaccuracies of the commentary and further model several different possibilities. Although our simulations continue to support the predominance of neuronal glucose utilization during activation and neuronal to astrocytic lactate flow, the most important result is that, regardless of the direction of the flow, the overall contribution of lactate to cerebral glucose metabolism is found to be so small as to make this ongoing debate 'much ado about nothing'.

Comment on recent modeling studies of astrocyte-neuron metabolic interactions

Recent years have seen a surge in mathematical modeling of the various aspects of neuron-astrocyte interactions, and the field of brain energy metabolism is no exception in that regard. Despite the advent of biophysical models in the field, the long-lasting debate on the role of lactate in brain energy metabolism is still unresolved. Quite the contrary, it has been ported to the world of differential equations. Here, we summarize the present state of this discussion from the modeler's point of view and bring some crucial points to the attention of the non-mathematically proficient reader.

Amyloid-beta aggregates cause alterations of astrocytic metabolic phenotype: impact on neuronal viability

Amyloid-beta (Abeta) peptides play a key role in the pathogenesis of Alzheimer's disease and exert various toxic effects on neurons; however, relatively little is known about their influence on glial cells. Astrocytes play a pivotal role in brain homeostasis, contributing to the regulation of local energy metabolism and oxidative stress defense, two aspects of importance for neuronal viability and function. In the present study, we explored the effects of Abeta peptides on glucose metabolism in cultured astrocytes. Following Abeta(25-35) exposure, we observed an increase in glucose uptake and its various metabolic fates, i.e., glycolysis (coupled to lactate release), tricarboxylic acid cycle, pentose phosphate pathway, and incorporation into glycogen. Abeta increased hydrogen peroxide production as well as glutathione release into the extracellular space without affecting intracellular glutathione content. A causal link between the effects of Abeta on glucose metabolism and its aggregation and internalization into astrocytes through binding to members of the class A scavenger receptor family could be demonstrated. Using astrocyte-neuron cocultures, we observed that the overall modifications of astrocyte metabolism induced by Abeta impair neuronal viability. The effects of the Abeta(25-35) fragment were reproduced by Abeta(1-42) but not by Abeta(1-40). Finally, the phosphoinositide 3-kinase (PI3-kinase) pathway appears to be crucial in these events since both the changes in glucose utilization and the decrease in neuronal viability are prevented by LY294002, a PI3-kinase inhibitor. This set of observations indicates that Abeta aggregation and internalization into astrocytes profoundly alter their metabolic phenotype with deleterious consequences for neuronal viability.

PET studies of cerebral metabolism in Parkinson disease

A defect in cerebral energy production due to dysfunction of the mitochondrial electron transport system (ETS) has been postulated to be important in the pathogenesis of Parkinson Disease (PD). However, direct in vivo measurements of cerebral mitochondrial function are scant and inconsistent. We directly investigated cerebral mitochondrial function in vivo with positron emission tomography (PET) in 12 patients with early, never-medicated PD and 12 age-matched normal controls by combined measurements of the cerebral metabolic rate of oxygen (CMRO(2)) and the cerebral metabolic rate of glucose (CMRglc). Instead of the decrease in CMRO(2) and CMRO(2)/CMRglc molar ratio characteristic of defects in mitochondrial oxidative metabolism, there was a statistically significant 24% general increase in CMRO(2) and no change in CMRO(2)/CMRglc. Since PD symptoms were already manifest, reduced oxidative activity of the mitochondrial ETS cannot be a primary mechanism of neuronal death in early PD. This increase in metabolism could reflect the increased energy requirements of an injured brain or an uncoupling of ATP production from oxidation in the terminal stage of oxidative phosphorylation. Which is the case in early PD and whether these metabolic abnormalities are important in the pathogenesis of PD will require further study.

Compromised respiratory adaptation and thermoregulation in aging and age-related diseases

Mitochondrial dysfunction and reactive oxygen species (ROS) production are at the heart of the aging process and are thought to underpin age-related diseases. Mitochondria are not only the primary energy-generating system but also the dominant cellular source of metabolically derived ROS. Recent studies unravel the existence of mechanisms that serve to modulate the balance between energy metabolism and ROS production. Among these is the regulation of proton conductance across the inner mitochondrial membrane that affects the efficiency of respiration and heat production. The field of mitochondrial respiration research has provided important insight into the role of altered energy balance in obesity and diabetes. The notion that respiration and oxidative capacity are mechanistically linked is making significant headway into the field of aging and age-related diseases. Here we review the regulation of cellular energy and ROS balance in biological systems and survey some of the recent relevant studies that suggest that respiratory adaptation and thermodynamics are important in aging and age-related diseases.

Age-related toxicity of amyloid-beta associated with increased pERK and pCREB in primary hippocampal neurons: reversal by blueberry extract

Further clarification is needed to address the paradox that memory formation, aging and neurodegeneration all involve calcium influx, oxyradical production (ROS) and activation of certain signaling pathways. In aged rats and in APP/PS-1 mice, cognitive and hippocampal Ca(2+) dysregulation was reversed by food supplementation with a high antioxidant blueberry extract. Here, we studied whether neurons were an important target of blueberry extract and whether the mechanism involved altered ROS signaling through MAP kinase and cyclic-AMP response element binding protein (CREB), pathways known to be activated in response to amyloid-beta (Aβ). Primary hippocampal neurons were isolated and cultured from embryonic, middle-age or old-age (24 months) rats. Blueberry extract was found to be equally neuroprotective against Aβ neurotoxicity at all ages. Increases in Aβ toxicity with age were associated with age-related increases in immunoreactivity of neurons to pERK and an age-independent increase in pCREB. Treatment with blueberry extract strongly inhibited these increases in parallel with neuroprotection. Simultaneous labeling for ROS and for glutathione with dichlorofluorescein and monochlorobimane showed a mechanism of action of blueberry extract to involve transient ROS generation with an increase in the redox buffer glutathione. We conclude that the increased age-related susceptibility of old-age neurons to Aβ toxicity may be due to higher levels of activation of pERK and pCREB pathways that can be protected by blueberry extract through inhibition of both these pathways through an ROS stress response. These results suggest that the beneficial effects of blueberry extract may involve transient stress signaling and ROS protection that may translate into improved cognition in aging rats and APP/PS1 mice given blueberry extract.

Epigenetic oxidative redox shift (EORS) theory of aging unifies the free radical and insulin signaling theories

Harman's free radical theory of aging posits that oxidized macromolecules accumulate with age to decrease function and shorten life-span. However, nutritional and genetic interventions to boost anti-oxidants have generally failed to increase life-span. Furthermore, the free radical theory fails to explain why exercise causes higher levels of oxyradical damage, but generally promotes healthy aging. The separate anti-aging paradigms of genetic or caloric reductions in the insulin signaling pathway is thought to slow the rate of living to reduce metabolism, but recent evidence from Westbrook and Bartke suggests metabolism actually increases in long-lived mice. To unify these disparate theories and data, here, we propose the epigenetic oxidative redox shift (EORS) theory of aging. According to EORS, sedentary behavior associated with age triggers an oxidized redox shift and impaired mitochondrial function. In order to maintain resting energy levels, aerobic glycolysis is upregulated by redox-sensitive transcription factors. As emphasized by DeGrey, the need to supply NAD(+) for glucose oxidation and maintain redox balance with impaired mitochondrial NADH oxidoreductase requires the upregulation of other oxidoreductases. In contrast to the 2% inefficiency of mitochondrial reduction of oxygen to the oxyradical, these other oxidoreductases enable glycolytic energy production with a deleterious 100% efficiency in generating oxyradicals. To avoid this catastrophic cycle, lactate dehydrogenase is upregulated at the expense of lactic acid acidosis. This metabolic shift is epigenetically enforced, as is insulin resistance to reduce mitochondrial turnover. The low mitochondrial capacity for efficient production of energy reinforces a downward spiral of more sedentary behavior leading to accelerated aging, increased organ failure with stress, impaired immune and vascular functions and brain aging. Several steps in the pathway are amenable to reversal for exit from the vicious cycle of EORS. Examples from our work in the aging rodent brain as well as other aging models are provided.

Neuron survival in vitro is more influenced by the developmental age of the cells than by glucose condition

The objective of this study was to determine whether the sensitivity to varying glucose conditions differs for the peripheral and central nervous system neurons at different developmental stages. Ventral horn neurons (VHN) and dorsal root ganglion neurons (DRG) from rats of different postnatal ages were exposed to glucose-free or glucose-rich culture conditions. Following 24 h at those conditions, the number of protein gene product 9.5 positive (PGP(+)) DRG neurons and choline acetyltransferase positive (ChAT(+)) VHN were counted and their neurite lengths and soma diameters were measured. For both DRG and VHN, the highest number of cells with and without neurite outgrowth was seen when cells from postnatal day 4 donors were cultured, while the lowest cell numbers were when neurons were from donors early after birth and grown under glucose-free conditions. The length of the neurites and the soma diameter for VHN were not affected by either glucose level or age. DRG neurons, however, exhibited the shortest neurites and smallest soma diameter when neurons were obtained and cultured early after birth. Our results indicate that survival of neurons in vitro is more influenced by the developmental stage than by glucose concentrations.

Green tea (-)-epigallocatechin-3-gallate inhibits beta-amyloid-induced cognitive dysfunction through modification of secretase activity via inhibition of ERK and NF-kappaB pathways in mice

Alzheimer's disease (AD) is characterized by the extracellular deposition of beta-amyloid peptide (Abeta) in cerebral plaques. Abeta is derived from the beta-amyloid precursor protein (APP) by the enzymes alpha-, beta- and gamma-secretase. Compounds that enhance alpha-secretase, but inhibit beta- or gamma-secretase activity, have therapeutic potential in the treatment of AD. Green tea, or its major polyphenolic compound, has been shown to have neuroprotective effects. In this study, we investigated the possible effects of (-)-epigallocatechin-3-gallate (EGCG) on memory dysfunction caused by Abeta through the change of Abeta-induced secretase activities. Mice were pretreated with EGCG (1.5 or 3 mg/kg body weight in drinking water) for 3 wk before intracerebroventricular administration of 0.5 microg Abeta(1-42). EGCG dose-dependently reduced the Abeta(1-42)-induced memory dysfunction, which was evaluated using passive avoidance and water maze tests. Abeta(1-42) induced a decrease in brain alpha-secretase and increases in both brain beta- and gamma-secretase activities, which were reduced by EGCG. In the cortex and the hippocampus, expression of the metabolic products of the beta- and gamma-secretases from APP, C99, and Abeta also were dose-dependently suppressed by EGCG. Paralleled with the suppression of beta- and gamma-secretases by EGCG, we found that EGCG inhibited the activation of extracellular signal-regulated kinase and nuclear transcription factor-kappaB in the Abeta(1-42)-injected mouse brains. In addition, EGCG inhibited Abeta(1-42)-induced apoptotic neuronal cell death in the brain. To further test the ability of EGCG to affect memory, EGCG (3 mg/kg body weight) was administered in drinking water for 1 wk to genetically developed preseniline 2 (PS2) mutant AD mice. Compared with untreated mutant PS2 AD mice, treatment with EGCG enhanced memory function and brain alpha-secretase activity but reduced brain beta- and gamma-secretase activities as well as Abeta levels. Moreover, EGCG inhibited the fibrillization of Abeta in vitro with a half maximal inhibitory concentration of 7.5 mg/L. These studies suggest that EGCG may be a beneficial agent in the prevention of development or progression of AD.

Age-related deficiencies in complex I endogenous substrate availability and reserve capacity of complex IV in cortical neuron electron transport

Respiratory enzyme complex dysfunction is mechanistically involved in mitochondrial failure leading to neurodegenerative disease, but the pathway is unclear. Here, age-related differences in mitochondrial respiration were measured in both whole and permeabilized neurons from 9-month and 24-month adult rat cortex cultured in common conditions. After permeabilization, respiration increased in both ages of neurons with excess substrates. To dissect specific deficiencies in the respiratory chain, inhibitors for each respiratory chain complex were used to isolate their contributions. Relative to neurons from 9-month rats, in neurons isolated from 24-month rats, complexes I, III, and IV were more sensitive to selective inhibition. Flux control point analysis identified complex I in neurons isolated from 24-month rats as the most sensitive to endogenous substrate availability. The greatest age-related deficit in flux capacity occurred at complex IV with a 29% decrease in neurons isolated from 24-month rats relative to those from 9-month rats. The deficits in complexes I and III may contribute to a redox shift in the quinone pool within the electron transport chain, further extending these age-related deficits. Together these changes could lead to an age-related catastrophic decline in energy production and neuronal death.

Selective vulnerability of neurons in primary cultures and in neurodegenerative diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

Brain glucose hypometabolism and oxidative stress in preclinical Alzheimer's disease

One of the main features of Alzheimer's disease (AD) is the severe reduction of the cerebral metabolic rate for glucose (CMRglc). In vivo imaging using positron emission tomography with 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG-PET) demonstrates consistent and progressive CMRglc reductions in AD patients, the extent and topography of which correlate with symptom severity. Increasing evidence suggests that CMRglc reductions occur at the preclinical stages of AD. CMRglc reductions were observed on FDG-PET before the onset of disease in several groups of at-risk individuals, including patients with mild cognitive impairment (MCI), often a prodrome to AD; presymptomatic individuals carrying mutations responsible for early-onset familial AD; cognitively normal elderly individuals followed for several years until they declined to MCI and eventually to AD; normal, middle-aged individuals who expressed subjective memory complaints and were carriers of the apolipoprotein E epsilon-4 allele, a strong genetic risk factor for late-onset AD. However, the causes of the early metabolic dysfunction forerunning the onset of AD are not known. An increasing body of evidence indicates a deficient or altered energy metabolism that could change the overall oxidative microenvironment for neurons during the pathogenesis and progression of AD, leading to alterations in mitochondrial enzymes and in glucose metabolism in AD brain tissue. The present paper reviews findings that implicate hypometabolism and oxidative stress as crucial players in the initiation and progression of synaptic pathology in AD.

(-)-Epigallocatechin-3-gallate prevents lipopolysaccharide-induced elevation of beta-amyloid generation and memory deficiency

Neuroinflammation has been known to play a role in the pathogenesis of AD. Our previous study showed that lipopolysaccharide (LPS) induced memory impairment through the accumulation of Abeta via the increase of beta- and gamma-secretase. In this study, we investigated the possible preventive effect of (-)-epigallocatechin-3-gallate (EGCG) on memory deficiency caused by LPS through the inhibition of Abeta(1-42) generation. Oral treatment with EGCG (1.5 and 3 mg/kg, for 3 weeks) into drinking water ameliorated LPS (1 microg/mouse, i.c.v.)-induced memory deficiency in a dose dependent manner. In addition, EGCG also dose-dependently inhibited LPS-induced elevation of Abeta level through attenuation of LPS-induced beta- and gamma-secretase activities and expression of its metabolic products; C99 and Abeta. Moreover, EGCG prevented LPS-induced neuronal cell death as well as the expression of inflammatory proteins, inducible nitric oxide synthetase and cyclooxygenase-2. This study therefore suggests that EGCG prevents LPS-mediated apoptotic cell death through the inhibition of the elevation of Abeta via the inhibition of beta- and gamma-secretases, and thus EGCG can be a useful agent against neuroinflammation-associated development or progression of AD.

Cerebral mitochondrial metabolism in early Parkinson's disease

Abnormal cerebral energy metabolism owing to dysfunction of mitochondrial electron transport has been implicated in the pathogenesis of Parkinson's disease (PD). However, in vivo data of mitochondrial dysfunction have been inconsistent. We directly investigated mitochondrial oxidative metabolism in vivo in 12 patients with early, never-medicated PD and 12 age-matched normal controls by combined measurements of the cerebral metabolic rate of oxygen (CMRO(2)) and the cerebral metabolic rate of glucose (CMRglc) with positron emission tomography. The primary analysis showed a statistically significant 24% increase in bihemispheric CMRO(2) and no change in CMRO(2)/CMRglc. These findings are inconsistent with a defect in mitochondrial oxidative phosphorylation owing to reduced activity of the mitochondrial electron transport system (ETS). Because PD symptoms were already manifest, deficient energy production owing to a reduced activity of the mitochondrial ETS cannot be a primary mechanism of neuronal death in early PD. Alternatively, this general increase in CMRO(2) could be due not to an increased metabolic demand but to an uncoupling of ATP production from oxidation in the terminal stage of oxidative phosphorylation. Whether this is the case in early PD and whether it is important in the pathogenesis of PD will require further study.

Age-related changes to tumor necrosis factor receptors affect neuron survival in the presence of beta-amyloid

Inflammation including local accumulations of tumor necrosis factor alpha (TNF-alpha) is a part of Alzheimer's disease pathology and may exacerbate age-related neurodegeneration. Most studies on TNF-alpha and TNF neuronal receptors are conducted by using embryonic neurons. Few studies consider age-related deficits that may occur in neurons. Age-related changes in susceptibility to TNF-alpha through TNF receptor 1 (TNFR1) and receptor 2 (TNFR2) expression could increase susceptibility to beta-amyloid (1-42, Abeta42). Evidence is conflicting about which receptor mediates survival and/or apoptosis. We determined how aging affects receptor expression in cultured adult rat cortical neurons. Old neurons were more susceptible to Abeta42 toxicity than middle-aged neurons, and the addition of TNF-alpha was neuroprotective in middle-aged neurons, but exacerbated the toxicity from Abeta42 in old neurons. These pathologic and protective responses in old and middle-aged neurons, respectively, correlated with higher starting TNFR1 and TNFR2 mRNA levels in old vs. middle-aged neurons. Middle-aged neurons treated with TNF-alpha plus Abeta42 did not show an increase in either TNFR1 or TNFR2 mRNA, but old neurons showed an up-regulation in TNFR2 mRNA and not TNFR1 mRNA. Despite these mRNA changes, surface immunoreactivity of both TNFR1 and TNFR2 increased with the dose of TNF-alpha in middle-aged neurons. However, middle-aged neurons treated with TNF-alpha plus Abeta42 showed an up-regulation in both TNFR1 and TNFR2 surface expression, whereas old neurons failed to up-regulate surface expression of either receptor. These findings support the hypothesis that age-related changes in TNF-alpha surface receptor expression contribute to the neuronal loss associated with inflammation in Alzheimer's disease.

Critical age-related loss of cofactors of neuron cytochrome C oxidase reversed by estrogen

The mechanistic basis for the correlation between mitochondrial dysfunction and neurodegenerative disease is unclear, but evidence supports involvement of cytochrome C oxidase (CCO) deficits with age. Neurons isolated from the brains of 24 month and 9 month rats and cultured in common conditions provide a model of intrinsic neuronal aging. In situ CCO activity was decreased in 24 month neurons relative to 9 month neurons. Possible CCO-related deficits include holoenzyme activity, cofactor, and substrate. No difference was found between neurons from 24 month and 9 month rats in mitochondrial counts per neuron, CCO activity in submitochondrial particles, or basal respiration. Immunostaining for cytochrome C in individual mitochondria revealed an age-related deficit of this electron donor. 24 month neurons did not have adequate respiratory capacity to upregulate respiration after a glutamate stimulus, in spite of a two-fold upregulation of respiration seen in 9 month neurons. Respiration in 24 month neurons was inhibited by lower concentrations of potassium cyanide, suggesting a 50% deficit in functional enzyme in 24 month compared to 9 month neurons. In addition to cytochrome C, CCO requires cardiolipin to function. Staining with nonylacridine orange revealed an age-related deficit in cardiolipin. Treatment of 24 month neurons with 17-beta-estradiol restored cardiolipin levels (10 ng/mL) and upregulated respiration under glutamate stress (1 pg/mL). Attempts to induce mitochondrial turnover by neuronal multiplication also rejuvenated CCO activity in 24 month neurons. These data suggest cytochrome C and cardiolipin levels are deficient in 24 month neurons, preventing normal upregulation of respiration needed for oxidative phosphorylation in response to stress. Furthermore, the data suggest this deficit can be corrected with estrogen treatment.

Age-related decreases in NAD(P)H and glutathione cause redox declines before ATP loss during glutamate treatment of hippocampal neurons

Age-related glutamate excitotoxicity depends in an unknown manner on active mitochondria, which are key determinants of the cellular redox potential. Compared with embryonic and middle-aged neurons, old-aged rat hippocampal neurons have a lower resting reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and a lower redox ratio (NAD(P)H/flavin adenine nucleotide). Glutamate treatment resulted in an initial increase in NAD(P)H concentrations in all ages, followed by a profound calcium-dependent, age-related decline in NAD(P)H concentration and redox ratio. With complex I of the electron transport chain inhibited by rotenone, treatment with glutamate or ionomycin only resulted in the increase in NAD(P)H fluorescence. High-performance liquid chromatography analysis of adenine nucleotides in brain extracts showed 50% less nicotinamide adenine dinucleotide (NADH) and almost twice as much oxidized nicotinamide adenine dinucleotide, demonstrating a more oxidized ratio in old than middle-aged brain. Resting glutathione content also declined with age and further decreased with glutamate treatment without accompanying changes in adenosine triphosphate levels. We conclude that age does not affect production of NADH by dehydrogenases but that old-aged neurons consume more NADH and glutathione, leading to a catastrophic decline in redox ratio.

Changes in glutamate decarboxylase enzyme activity and tau-protein phosphorylation in the hippocampus of old rats exposed to chronic mild stress: reversal with the neuronal nitric oxide synthase inhibitor 7-nitroindazole

Effects of chronic stress are not completely understood. They may underlie depression and dementia. This study assessed the association between chronic stress, glutamate levels, tau-protein phosphorylation, and nitric-oxide in old rats exposed to chronic mild stress (CMS). Old (>15 months) male Wistar rats were exposed to CMS. Comparison groups included old and young control rats, young CMS-exposed, and old CMS-exposed rats treated with the neuronal nitric-oxide synthase (nNOS) enzyme inhibitor, 7-nitroindazole (20 mg/kg/day i.p.). Hippocampal glutamate levels and glutamate decarboxylase (GAD) activity were determined and tau protein phosphorylation was assessed. Age was a significant (p=0.025) source of variation in glutamate level [811.71+/-218.1, 665.9+/-124.9 micromol/g tissue protein (M+/-SD) in young and old control rats, respectively]. Old rats exposed to CMS were characterized by an increased risk to develop anhedonia. There was significant (p=0.035) decrease in GAD enzyme activity (-60.06%) and increased tau protein hyperphosphorylation in old rats exposed to CMS compared to control. Administration of 7-nitroindazole to CMS-exposed old rats significantly (p=0.002) increased GAD activity, decreased glutamate levels (7.19+/-3.19 vs. 763.9+/-91 micromol/g tissue protein; p=0.0005), and decreased phosphorylation of tau proteins compared to CMS exposed rats.

Age-related differences in NFkappaB translocation and Bcl-2/Bax ratio caused by TNFalpha and Abeta42 promote survival in middle-age neurons and death in old neurons

Alzheimer's disease is associated with an age-related accumulation of Abeta and inflammation. The inflammatory mediator, TNFalpha activates a signaling cascade involving NFkappaB translocation to the nucleus and a beneficial or detrimental transcriptional response, depending on the age of the neurons and the type of stress applied. Relative to treatment with Abeta42 alone, previously we found that TNFalpha plus Abeta42, applied to old rat neurons (24 month) is toxic, while the same treatment of middle-age neurons (10 month) is protective. In contrast to improved survival of middle-age rat cortical neurons, neurons from old rats are killed by TNFalpha plus Abeta42 despite greater p50 nuclear translocation. In middle-age neurons, blocking TNFR1 does not affect NFkappaB translocation, whereas blocking TNFR2 results in an increase in NFkappaB translocation. For old neurons, blocking either receptor, does not change NFkappaB translocation, but improves cell survival. To account for these effects on cell viability in response to TNF+Abeta, measures of the Bcl-2/Bax ratio positively correlate with survival. In the setting of old neurons, these results suggest that overactivated nuclear translocation of NFkappaB and lower Bcl-2 levels promote death that is reduced by inhibition of either TNFR1 or R2.

Isolation and culture of adult neurons and neurospheres

Here we present a protocol for extraction and culture of neurons from adult rat or mouse CNS. The method proscribes an optimized protease digestion of slices, control of osmolarity and pH outside the incubator with Hibernate and density gradient separation of neurons from debris. This protocol produces yields of millions of cortical, hippocampal neurons or neurosphere progenitors from each brain. The entire process of neuron isolation and culture takes less than 4 h. With suitable growth factors, adult neuron regeneration of axons and dendrites in culture proceeds over 1-3 weeks to allow controlled studies in pharmacology, electrophysiology, development, regeneration and neurotoxicology. Adult neurospheres can be collected in 1 week as a source of neuroprogenitors ethically preferred over embryonic or fetal sources. This protocol emphasizes two differences between neuron differentiation and neurosphere proliferation: adhesion dependence and the differentiating power of retinyl acetate.

Simultaneous age-related depolarization of mitochondrial membrane potential and increased mitochondrial reactive oxygen species production correlate with age-related glutamate excitotoxicity in rat hippocampal neurons

Mitochondria are implicated in glutamate excitotoxicity by causing bioenergetic collapse, loss of Ca(2+) homeostasis, and generation of reactive oxygen species (ROS), all of which become increasingly important clinically with age. Little is known about how aging affects the relative importance of mitochondrial membrane potential (DeltaPsi(m)) and ROS production. To determine aging affects on DeltaPsi(m) and ROS production in individual somal and axonal/dendritic mitochondria, we compared ROS production while simultaneously monitoring DeltaPsi(m) before and after glutamate treatment of live neurons from embryonic (day 18), middle-aged (9-12 months), and old (24 months) rats. At rest, old neuronal mitochondria 1) showed a higher rate of ROS production that was particularly strong in axonal/dendritic mitochondria relative to that in middle-age neurons, 2) were more depolarized in comparison with neurons of other ages, and 3) showed no differences in ROS or DeltaPsi(m) as a function of distance from the nucleus. All DeltaPsi(m) grouped into three classes of high (less than -120 mV), medium (-85 to -120 mV), and low (greater than -85 mV) polarization that shifted toward the lower classes with age at rest. Glutamate exposure dramatically depolarized the DeltaPsi(m) in parallel with greatly increased ROS production, with a surprising absence of an effect of age or distance from the nucleus on these mitochondrial parameters. These data suggest that old neurons are more susceptible to glutamate excitotoxicity because of an insidious depolarization of DeltaPsi(m) and rate of ROS generation at rest that lead to catastrophic failure of phosphorylative and reductive energy supplies under stress.

Mitoenergetic failure in Alzheimer disease

Brain cells are highly energy dependent for maintaining ion homeostasis during high metabolic activity. During active periods, full mitochondrial function is essential to generate ATP from electrons that originate with the oxidation of NADH. Decreasing brain metabolism is a significant cause of cognitive abnormalities of Alzheimer disease (AD), but it remains uncertain whether this is the cause of further pathology or whether synaptic loss results in a lower energy demand. Synapses are the first to show pathological symptoms in AD before the onset of clinical symptoms. Because synaptic function has high energy demands, interruption in mitochondrial energy supply could be the major factor in synaptic failure in AD. A newly discovered age-related decline in neuronal NADH and redox ratio may jeopardize this function. Mitochondrial dehydrogenases and several mutations affecting energy transfer are frequently altered in aging and AD. Thus, with the accumulation of genetic defects in mitochondria at the level of energy transfer, the issue of neuronal susceptibility to damage as a function of age and age-related disease becomes important. In an aging rat neuron model, mitochondria are both chronically depolarized and produce more reactive oxygen species with age. These concepts suggest that multiple treatment targets may be needed to reverse this multifactorial disease. This review summarizes new insights based on the interaction of mitoenergetic failure, glutamate excitotoxicity, and amyloid toxicity in the exacerbation of AD.

Ageing and neuronal vulnerability

Everyone ages, but only some will develop a neurodegenerative disorder in the process. Disease might occur when cells fail to respond adaptively to age-related increases in oxidative, metabolic and ionic stress, thereby resulting in the accumulation of damaged proteins, DNA and membranes. Determinants of neuronal vulnerability might include cell size and location, metabolism of disease-specific proteins and a repertoire of signal transduction pathways and stress resistance mechanisms. Emerging evidence on protein interaction networks that monitor and respond to the normal ageing process suggests that successful neural ageing is possible for most people, but also cautions that cures for neurodegenerative disorders are unlikely in the near future.

Effect of sesaminol glucosides on β-amyloid-induced PC12 cell death through antioxidant mechanisms

Several lines of evidence support that beta-amyloid (Abeta)-induced neurotoxicity is mediated through the generation of reactive oxygen species (ROS) and elevation of intracellular calcium. In this study, we have investigated protective effects of sesaminol glucosides on Abeta-induced oxidative cell death in cultured rat pheochromocytoma (PC12) cells. Sesaminol glucoside (50-250microg/ml) decreased Abeta(25-35)-induced ROS generation, formation of 8-oxodG, a form of oxidative DNA and elevation of intracellular calcium level concomitant with prevention of apoptotic cell death dose dependently. Sesaminol glucoside (50-250microg/ml) also effectively decreased Abeta1-42 and ADDL form of Abeta1-42 as well as the combination of H2O2 with FeSO4-induced cell damages. In mechanistic study, sesaminol glucosides attenuated Abeta25-35-induced activation of redox transcription factor nuclear factor-kappaB NF-kappaB through inhibition of p50 translocation and IkappaB phosphorylation, and blocked NF-kappaB-dependent luciferase activity in addition to the inhibitory effect on Abeta25-35-induced activation of ERK kinase signal pathway. Consistent with the inhibitory effect on Abeta25-35-induced stress-induced cell death, sesaminol glucosides decreased expression of pro-apoptotic gene p53, and Bax and caspase-3, but enhanced expression of anti-apoptotic Bcl-2. Moreover, the protective effects of sesaminol glucoside on Abeta25-35-induced ROS generation, NF-kappaB activation and cell death were further enhanced with glutathione. This study therefore suggests that sesaminol glucosides have protective effect on Abeta-induced neuronal cell death, and its effect may be through antioxidative property.