Reduced activities of thiamine-dependent enzymes in the brains and peripheral tissues of patients with Alzheimer's disease

Association of the dihydrolipoamide dehydrogenase gene with Alzheimer's disease in an Ashkenazi Jewish population

Abundant biochemical evidence links deficient activity of mitochondrial alpha-ketoglutarate dehydrogenase with neuropathologically confirmed Alzheimer's disease (AD). Reduced alpha-ketoglutarate dehydrogenase activity has also been associated with anti-mortem measures of clinical disability. One of the genes encoding this complex, namely, DLD, lies within a chromosome 7 region that is in linkage disequilibrium with AD. We therefore examined the hypothesis that variation in DLD is associated with AD risk. Denaturing HPLC was used to search for sequence variations in the coding and flanking regions of all exons of DLD, but no abundant variants that alter protein sequence were found. However, four common SNPs were identified and genotyped in a case-control series of 297 Caucasians from New York City, including 229 residents of a Jewish nursing home. Logistic regression analysis was performed for the four-locus DLD genotype, sex, and ApoE4 status to determine the association of these independent variables with AD. Significant associations with AD were observed for ApoE4 (P < 10(-6)) and sex combined with DLD genotype (P = 0.013). The association with the DLD genotypes appears only in the male population in both the Caucasian series (P = 0.0009, n = 83) and the Ashkenazi Jewish subseries (P = 0.017, n = 49). The DLD genotype appears to operate independently of APOE in conferring AD risk.

Substantial linkage disequilibrium across the dihydrolipoyl succinyltransferase gene region without Alzheimer's disease association

Association of the candidate gene DLST with late-onset Alzheimer's disease (LOAD) risk has been suggested on the basis of case-control studies. This gene, located on chromosome 14q24.3, encodes a subunit of a mitochondrial component known to be defective in AD, the alpha-ketoglutarate dehydrogenase complex. Positive reports have correlated different DLST alleles with LOAD, whereas other groups have failed to find any significant association. We therefore reexamined the association of DLST and LOAD in a more ethnically homogeneous series using three additional single nucleotide polymorphisms (SNP) located within or closely flanking either end of the DLST gene. Pairwise analysis of these SNPs indicated there was strong linkage disequilibrium across the DLST locus. Analysis of complex genotypes or haplotypes based upon all five SNP loci failed to identify a LOAD risk allele, suggesting that further studies of DLST in relation to AD are not warranted.

Mental retardation and hypotonia seen in the knock out mouse for Canavan disease is not due to succinate semialdehyde dehydrogenase deficiency

Canavan disease (CD) is an autosomal recessive disorder caused by aspartoacylase deficiency leading to accumulation of N-acetylaspartic acid and spongy degeneration of the brain. The mouse model for CD showed low levels of glutamate and gamma-aminobutyric acid (GABA) in the brain. Whether the low levels of glutamate and GABA observed in the CD mouse brain lead to abnormal production of glutamate-GABA associated enzymes and resulting succinate production is not obvious. While glutamate dehydrogenase and alpha-ketoglutarate dehydrogenase complex activities are lower in the cerebellum and brain stem of the CD mouse, alanine aminotransferase and succinate semialdehyde dehydrogenase (SSADH) activities and succinate level are similar to the levels observed in the wild type. Deficiency of SSADH has been suggested to be associated with mental retardation and hypotonia, similar to the clinical features of CD. The normal SSADH activity in the CD mouse brain suggests that mental retardation and hypotonia seen in the CD mouse is not due to SSADH activity and if documented also in patients with CD.

Mice deficient in dihydrolipoamide dehydrogenase show increased vulnerability to MPTP, malonate and 3-nitropropionic acid neurotoxicity

Altered energy metabolism, including reductions in activities of the key mitochondrial enzymes alpha-ketoglutarate dehydrogenase complex (KGDHC) and pyruvate dehydrogenase complex (PDHC), are characteristic of many neurodegenerative disorders including Alzheimer's Disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Dihydrolipoamide dehydrogenase is a critical subunit of KGDHC and PDHC. We tested whether mice that are deficient in dihydrolipoamide dehydrogenase (Dld+/-) show increased vulnerability to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), malonate and 3-nitropropionic acid (3-NP), which have been proposed for use in models of PD and HD. Administration of MPTP resulted in significantly greater depletion of tyrosine hydroxylase-positive neurons in the substantia nigra of Dld+/- mice than that seen in wild-type littermate controls. Striatal lesion volumes produced by malonate and 3-NP were significantly increased in Dld+/- mice. Studies of isolated brain mitochondria treated with 3-NP showed that both succinate-supported respiration and membrane potential were suppressed to a greater extent in Dld+/- mice. KGDHC activity was also found to be reduced in putamen from patients with HD. These findings provide further evidence that mitochondrial defects may contribute to the pathogenesis of neurodegenerative diseases.

Inhibition of alpha-ketoglutarate dehydrogenase complex promotes cytochrome c release from mitochondria, caspase-3 activation, and necrotic cell death

Mitochondrial dysfunction has been implicated in cell death in many neurodegenerative diseases. Diminished activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC), a key and arguably rate-limiting enzyme of the Krebs cycle, occurs in these disorders and may underlie decreased brain metabolism. The present studies used alpha-keto-beta-methyl-n-valeric acid (KMV), a structural analogue of alpha-ketoglutarate, to inhibit KGDHC activity to test effects of reduced KGDHC on mitochondrial function and cell death cascades in PC12 cells. KMV decreased in situ KGDHC activity by 52 +/- 7% (1 hr) or 65 +/- 4% (2 hr). Under the same conditions, KMV did not alter the mitochondrial membrane potential (MMP), as assessed with a method that detects changes as small as 5%. KMV also did not alter production of reactive oxygen species (ROS). However, KMV increased lactate dehydrogenase (LDH) release from cells by 100 +/- 4.7%, promoted translocation of mitochondrial cytochrome c to the cytosol, and activated caspase-3. Inhibition of the mitochondrial permeability transition pore (MPTP) by cyclosporin A (CsA) partially blocked this KMV-induced change in cytochrome c (-40%) and LDH (-15%) release, and prevented necrotic cell death. Thus, impairment of this key mitochondrial enzyme in PC12 cells may lead to cytochrome c release and caspase-3 activation by partial opening of the MPTP before the loss of mitochondrial membrane potentials.

Cerebrometabolic abnormalities in Alzheimer's disease

Extensive, replicated evidence in patients in vivo and in Alzheimer (AD) tissues in vitro indicates that impaired brain metabolism is one of the cardinal and essentially invariable events in AD. The degree of impairment in brain metabolism is proportional to the degree of clinical disability, both in vivo and in vitro. The 'cerebrometabolic lesion' cannot be attributed to 'slower thinking' or 'brain atrophy', because of quantitative considerations and because the metabolic lesion precedes the development of neuropsychological abnormalities or decreases in brain mass detectable by modern imaging techniques. The causes of the cerebrometabolic lesion in AD are not well defined. Free radicals seem likely to be involved, including free radicals generated from Alzheimer amyloid. Thus, the importance of the cerebrometabolic lesion is entirely compatible with most versions of the widely accepted 'amyloid cascade hypothesis' of AD. A variety of plausible, redundantly documented mechanisms are compatible with the proposal that the cerebrometabolic lesion is a proximate cause of the clinical disability in AD. In agreement with these findings, recent attempts to treat the cerebrometabolic lesion in AD have given encouraging preliminary results. The cerebrometabolic lesion in AD deserves further study.

Estrogen receptors and metabolic activity in the human tuberomamillary nucleus: changes in relation to sex, aging and Alzheimer’s disease

The human tuberomamillary nucleus (TMN), that is the sole source of histamine in the brain, is involved in arousal, learning and memory and is impaired in Alzheimer's disease (AD) as shown by the presence of cytoskeletal alterations, a reduction in the number of large neurons, a diminished neuronal metabolic activity and decreased histamine levels in the hypothalamus and cortex. Experimental data and the presence of sex hormone receptors suggest an important role of sex steroids in the regulation of the function of TMN neurons. Therefore, we investigated sex-, age- and Alzheimer-related changes in estrogen receptor alpha and beta (ERalpha and ERbeta) in the TMN. In addition, metabolic activity changes of TMN neurons were determined by measuring Golgi apparatus (GA) and cell size. In the present study, ERalpha immunocytochemical expression in AD patients did not differ from that in elderly controls. However, a larger amount of cytoplasmic ERbeta was found in the TMN cells of AD patients. Earlier studies, using the GA size as a parameter, have shown a clearly decreased metabolic activity in the TMN neurons in AD. In the present study, the size of the GA did not change during aging, indicating the absence of strong metabolic changes. Cell size of the TMN neurons appeared to increase during normal aging in men but not in women. Concluding, the enhanced cytoplasmic expression of ERbeta in the TMN may be involved in the diminished neuronal metabolism of these neurons in AD patients.

Changes in metabolic activity and estrogen receptors in the human medial mamillary nucleus: relation to sex, aging and Alzheimer's disease

The medial mamillary nucleus (MMN) is situated caudally in the human hypothalamus and is involved in memory processes. In search for putative sites of action in estrogen replacement therapy on memory both in aging and Alzheimer's disease (AD), we aimed at determining whether changes would occur in estrogen receptors (ER) or metabolic activity in the MMN neurons under these conditions in a sex-dependent way. The Golgi apparatus (GA) and cell size, that were previously shown to be good measures of changes in neuronal metabolic activity, were measured in the MMN of 10 young (20-50 years old), 11 elderly (56-76 years old) control men and women and 11 AD patients (54-78 years old). In addition, we investigated whether estrogen receptor alpha or beta (ERalpha or ERbeta) immunoreactivity was altered in the MMN in aging or AD. There were no sex- or AD-related differences in the GA or cell size in the MMN. Both the GA and cell size of the MMN neurons were found to be increased in postmenopausal compared to young control women accompanied by a decrease in the amount of nuclear ERbeta. The percentage of nuclear ERalpha-positive MMN neurons was markedly enhanced in AD patients compared to controls and most prominently in AD men. In AD patients the proportion of nuclear ERalpha-positive neurons was positively correlated to the Braak stages that indicate the progression of the disease. No differences in the proportion of ERbeta-positive neurons were observed between AD and control patients. We propose that estrogens play an inhibitory role with respect to the metabolic activity of human MMN, which is mediated via ERbeta. This inhibitory effect is diminished in postmenopausal women. The role of the enhanced nuclear ERalpha staining in AD, that was also found in other brain areas, remains to be elucidated.

Increased neuronal metabolic activity and estrogen receptors in the vertical limb of the diagonal band of Broca in Alzheimer's disease: relation to sex and aging

Changes in the interaction between sex hormones and the cholinergic system are presumed to play a role in cognitive decline in aging and Alzheimer's disease (AD). The hippocampus is one of the most strongly affected brain structures in AD and the vertical limb of the diagonal band of Broca (VDB) is its major source of innervation. In the present study we found, surprisingly, for the first time that the neuronal metabolic activity as measured by the size of the Golgi apparatus in the VDB gradually increases after the age of 50 years in controls and that this process starts earlier and is more pronounced in Alzheimer's disease patients. Neuronal metabolic activity in the VDB was significantly higher in AD than in control patients younger than 70 years of age and was higher in control patients over 70 years than in control patients younger than 70 years of age. The activation of VDB neurons during aging was accompanied by an increased nuclear estrogen receptor (ER) beta staining, which was stronger in patients over 70 years of age than in younger subjects (in both controls and AD patients). Interestingly, as in the nucleus basalis of Meynert, nuclear ERalpha expression was markedly enhanced in AD patients compared to controls independent of age. In addition, evidence was found for the influence of APOE genotype on ERalpha and ERbeta staining in the human VDB in aging and in AD. APOE genotype was positively correlated (epsilon 2 < epsilon 3 < epsilon 4) with the percentage of cytoplasm ERalpha-positive VDB neurons in elderly control male and female subjects and with both nuclear and cytoplasm ERbeta-positive neurons in control women. In conclusion, the VDB is compensatory activated and shows more nuclear ER expression in aging and AD in a sex- and APOE genotype-dependent way. So neither global degeneration or a strongly decreased neuronal metabolism nor a lack of sex hormone receptors in the VDB seems to contribute to the decline in cognition in aging or AD in which the hippocampus plays such a crucial role.

Deficits in a tricarboxylic acid cycle enzyme in brains from patients with Parkinson's disease

Parkinson's disease (PD) is associated with mitochondrial dysfunction, specifically a deficiency of complex I of the electron transport chain. Most, although not all, studies indicate that this deficiency is limited to brain regions with neurodegeneration. The current studies tested for deficiencies in other mitochondrial components in PD brain in a neuropathologically unaffected region where the abnormality cannot be attributed to secondary effects of neurodegeneration. The activity of a key (and arguably rate-limiting) tricarboxylic acid cycle enzyme, the alpha-ketoglutarate dehydrogenase complex (KGDHC), was measured in the cerebellum of patients with PD. Activity in 19 PD brains was 50.5% of that in 18 controls matched for age, sex, post-mortem interval, and method of preservation (P<0.0019). The protein subunits of KGDHC were present in normal amounts in PD brains, indicating a relatively discrete abnormality in the enzyme. The activities of another mitochondrial enzyme, glutamate dehydrogenase (GDH), were normal in PD brains. These results demonstrate that specific reductions in KGDHC occur even in pathologically unaffected areas in PD, where the decline is unlikely to be a non-specific result of neurodegeneration. Reductions in the activity of this enzyme, if widespread in the brain, may predispose vulnerable regions to further damage.

Inhibition of the alpha-ketoglutarate dehydrogenase complex alters mitochondrial function and cellular calcium regulation

Mitochondrial dysfunction occurs in many neurodegenerative diseases. The alpha-ketoglutarate dehydrogenase complex (KGDHC) catalyzes a key and arguably rate-limiting step of the tricarboxylic acid cycle (TCA). A reduction in the activity of the KGDHC occurs in brains and cells of patients with many of these disorders and may underlie the abnormal mitochondrial function. Abnormalities in calcium homeostasis also occur in fibroblasts from Alzheimer's disease (AD) patients and in cells bearing mutations that lead to AD. Thus, the present studies test whether the reduction of KGDHC activity can lead to the alterations in mitochondrial function and calcium homeostasis. alpha-Keto-beta-methyl-n-valeric acid (KMV) inhibits KGDHC activity in living N2a cells in a dose- and time-dependent manner. Surprisingly, concentration of KMV that inhibit in situ KGDHC by 80% does not alter the mitochondrial membrane potential (MMP). However, similar concentrations of KMV induce the release of cytochrome c from mitochondria into the cytosol, reduce basal [Ca(2+)](i) by 23% (P<0.005), and diminish the bradykinin (BK)-induced calcium release from the endoplasmic reticulum (ER) by 46% (P<0.005). This result suggests that diminished KGDHC activities do not lead to the Ca(2+) abnormalities in fibroblasts from AD patients or cells bearing PS-1 mutations. The increased release of cytochrome c with diminished KGDHC activities will be expected to activate other pathways including cell death cascades. Reductions in this key mitochondrial enzyme will likely make the cells more vulnerable to metabolic insults that promote cell death.

Acrolein inhibits NADH-linked mitochondrial enzyme activity: implications for Alzheimer's disease

In Alzheimer's disease (AD) brain increased lipid peroxidation and decreased energy utilization are found. Mitochondria membranes contain a significant amount of arachidonic and linoleic acids, precursors of lipid peroxidation products, 4-hydroxynonenal (HNE) and 2-propen-1-al (acrolein), that are extremely reactive. Both alkenals are increased in AD brain. In this study, we examined the effects of nanomolar levels of acrolein on the activities of pyruvate dehydrogenase (PDH) and Alpha-ketoglutarate dehydrogenase (KGDH), both reduced nicotinamide adenine dinucleotide (NADH)-linked mitochondrial enzymes. Acrolein decreased PDH and KGDH activities significantly in a dose-dependent manner. Using high performance liquid chromatography coupled to mass spectrometry (HPLC-MS), acrolein was found to bind lipoic acid, a component in both the PDH and KGDH complexes, most likely explaining the loss of enzyme activity. Acrolein also interacted with oxidized nicotinamide adenine dinucleotide (NAD(+)) in such a way as to decrease the production of NADH. Acrolein, which is increased in AD brain, may be partially responsible for the dysfunction of mitochondria and loss of energy found in AD brain by inhibition of PDH and KGDH activities, potentially contributing to the neurodegeneration in this disorder.

Inactivation of the 2-oxo acid dehydrogenase complexes upon generation of intrinsic radical species

Self-regulation of the 2-oxo acid dehydrogenase complexes during catalysis was studied. Radical species as side products of catalysis were detected by spin trapping, lucigenin fluorescence and ferricytochrome c reduction. Studies of the complexes after converting the bound lipoate or FAD cofactors to nonfunctional derivatives indicated that radicals are generated via FAD. In the presence of oxygen, the 2-oxo acid, CoA-dependent production of the superoxide anion radical was detected. In the absence of oxygen, a protein-bound radical concluded to be the thiyl radical of the complex-bound dihydrolipoate was trapped by alpha-phenyl-N-tert-butylnitrone. Another, carbon-centered, radical was trapped in anaerobic reaction of the complex with 2-oxoglutarate and CoA by 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO). Generation of radical species was accompanied by the enzyme inactivation. A superoxide scavenger, superoxide dismutase, did not protect the enzyme. However, a thiyl radical scavenger, thioredoxin, prevented the inactivation. It was concluded that the thiyl radical of the complex-bound dihydrolipoate induces the inactivation by 1e- oxidation of the 2-oxo acid dehydrogenase catalytic intermediate. A product of this oxidation, the DMPO-trapped radical fragment of the 2-oxo acid substrate, inactivates the first component of the complex. The inactivation prevents transformation of the 2-oxo acids in the absence of terminal substrate, NAD+. The self-regulation is modulated by thioredoxin which alleviates the adverse effect of the dihydrolipoate intermediate, thus stimulating production of reactive oxygen species by the complexes. The data point to a dual pro-oxidant action of the complex-bound dihydrolipoate, propagated through the first and third component enzymes and controlled by thioredoxin and the (NAD+ + NADH) pool.

The nitration product 5-nitro-gamma-tocopherol is increased in the Alzheimer brain

Oxidative stress and quasi-inflammatory processes recently have been recognized as contributing factors in the pathogenesis of Alzheimer's disease (AD). Reactive nitrating species have specifically been implicated in AD based on immunochemical and instrumental detection of nitrotyrosine in AD brain protein. The significance of lipid-phase nitration has not been investigated in AD. This study documents a significant two- to threefold increase in the lipid nitration product 5-nitro-gamma-tocopherol in affected regions of the AD brain as determined by high-performance liquid chromatography with electrochemical detection. In a bioassay to compare the relative potency of alpha-tocopherol and gamma-tocopherol against nitrative stress, rat brain mitochondria were exposed to the peroxynitrite-generating compound SIN-1. The oxidation-sensitive Kreb's cycle enzyme alpha-ketoglutarate dehydrogenase was inactivated by SIN-1, in a manner that could be significantly attenuated by gamma-tocopherol (at <10 microM) but not by alpha-tocopherol. These data indicate that nitric oxide-derived species are significant contributors to lipid oxidation in the AD brain. The findings are discussed in reference to the neuroinflammatory hypothesis of AD and the possible role of gamma-tocopherol as a major lipid-phase scavenger of reactive nitrogen species.

Mitochondrial impairment in the cerebellum of the patients with progressive supranuclear palsy

Abnormalities in energy metabolism and oxidative stress accompany many neurodegenerative diseases, including progressive supranuclear palsy (PSP). Previously, we showed decreased activities of a mitochondrial enzyme complex, alpha-ketoglutarate dehydrogenase complex (KGDHC), and marked increases in tissue malondialdehyde levels in post-mortem superior frontal cortex from the patients with PSP. The current study demonstrates that KGDHC is also significantly diminished (-58%) in the cerebellum from patients with PSP (n = 14), compared to age-matched control brains (n = 13). In contrast to cortex, markers of oxidative stress, such as malondialdehyde, tyrosine nitration or general protein carbonyl modification, did not increase in cerebellum. Furthermore, the protein levels of the individual components of KGDHC did not decline. The activities of two other mitochondrial enzymes were measured to determine whether the changes in KGDHC were selective. The activity of aconitase, a mitochondrial enzyme with an iron/sulfur cluster, is also significantly diminished (-50%), whereas glutamate dehydrogenase activity is unchanged. The present results suggest that the interaction of metabolic impairment and oxidative stress is region-specific in PSP brain. In cerebellum, reductions in KGDHC occur in the absence of increases in common measures of oxidative stress, and may underlie the metabolic deficits and contribute to pathological and clinical manifestation related to the cerebellum in patients with PSP.

No association between DLST gene and Alzheimer's disease or Wernicke-Korsakoff syndrome

Among many candidate genes for the genetically heterogeneous Alzheimer's disease (AD), only apolipoprotein E (ApoE) has been confirmed. Another candidate is the dihydrolipoyl succinyltransferase (DLST) gene, one of three components of thiamine-dependent mitochondrial alpha-ketoglutarate dehydrogenase complex (KGDHC), because KGDHC activity is reported reduced in AD patients. Also characterized by reduced KGDHC activity is another neuropsychiatric disease, Wernicke-Korsakoff syndrome (WKS), which results from thiamine deficiency. Examination of specific DLST gene polymorphism in 247 Japanese AD patients, 53 alcoholic WKS patients, and 368 nondemented Japanese control subjects revealed no significant differences in DLST genotypes and failed to replicate the findings of earlier studies indicating an association between DLST gene polymorphism and AD.

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

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

Oxidative biochemical markers; clues to understanding aging in long-lived species

Clues as to why long-lived species live so much longer than short-lived species may reside in the amount of reactive oxygen species (ROS) produced and their effect on damaging cell components (especially proteins) and alterations of crucial cellular processes. Rigorous evaluation of these concepts required critical comparisons of oxidative damage markers and/or parameters with assess difference in ROS flux and the critical age-modifying processes they influence. The limited experimental comparative results available implicate that ROS production per unit weight of total oxygen consumed is much less in the longer-lived species than in shorter-lived species. Mitochondria are the major site of ROS production. They are also the functional nexus for intracellular signaling thus modulating stress and growth factor mediated cellular survival, proliferation and apoptotic processes. Mitochondrial DNA mutations, perhaps caused by ROS, increase with age. Mutant mitochondria possess comparative replicative advantage, which leads to age-specific intracellular swarms. General inflammatory stress tends to increase with age. Disruption in coordinated cell-to-cell signaling triggered by alterations in intracellular signaling may be the basis of the age-related increases in tissue inflammation, which may explain some of the differences between long-lived species and short-lived species.

Mitochondrial dysfunction and oxidative stress in aging and neurodegenerative disease

A major risk factor for neurodegenerative diseases such as Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) is aging. Two processes that have been implicated in aging are free radical-induced oxidative damage and mitochondrial dysfunction. A progressive impairment of mitochondrial function and/or increased oxidative damage has been suggested to play critical roles in the pathogenesis of these neurodegenerative diseases. For example, decreased complex I activity, increased oxidative damage and altered activities of antioxidant defense enzymes have been demonstrated in PD. In AD, decrements in complex IV activity and increased oxidative damage have been reported. Reductions in complex II activity, increased cortical lactate levels and oxidative damage have been described in HD. Some familial ALS cases are associated with mutations in the gene for Cu,Zn superoxide dismutase (SOD1) while increased oxidative damage is observed in sporadic ALS. Studies in PSP have demonstrated regionally specific reductions in brain and muscle mitochondrial function, hypofrontality and increased oxidative damage. Altogether, the age-dependent onset and progressive course of these neurodegenerative diseases may ultimately highlight an association between aging, mitochondrial impairment and oxidative stress.

Dietary restriction attenuates the neuronal loss, induction of heme oxygenase-1 and blood-brain barrier breakdown induced by impaired oxidative metabolism

Experimental thiamine deficiency (TD) is a model of impaired oxidative metabolism associated with region-selective neuronal loss in the brain. Oxidative stress is a prominent feature of TD neuropathology, as evidenced by the accumulation of heme oxygenase-1 (HO-1), ferritin, reactive iron and superoxide dismutase in microglia, nitrotyrosine and 4-hydroxynonenal in neurons, as well as induction of endothelial nitric oxide synthase within the vulnerable areas. Dietary restriction (DR) reduces oxidative stress in several organ systems including the brain. DR increases lifespan and reduces neurodegeneration in a variety of models of neuronal injury. The possibility that DR can protect vulnerable neurons against TD-induced oxidative insults has not been tested. The current studies tested whether approximately 3 months of DR (60% of ad libitum intake) altered the response to TD. Six month-old ad libitum-fed or dietary restricted C57BL/6 mice received a thiamine-deficient diet either ad libitum, or under a DR regimen respectively for eleven days. The TD mice also received daily injections of the thiamine antagonist pyrithiamine. Control ad libitum-fed or DR mice received an unlimited amount, or 60% of ad libitum intake, respectively, of thiamine-supplemented diet. As in past studies, TD produced region-selective neuronal loss (-60%), HO-1 induction, and IgG extravasation in the thalamus of ad libitum-fed mice. DR attenuated the TD-induced neuronal loss (-30%), HO-1 induction and IgG extravasation in the thalamus. These studies suggest that oxidative damage is critical to the pathogenesis of TD, and that DR modulates the extent of free radical damage in the brain. Thus, TD is an important model for studying the relationship between aging, oxidative stress and nutrition.

Acceleration of phosphatidylcholine synthesis and breakdown by inhibitors of mitochondrial function in neuronal cells: a model of the membrane defect of Alzheimer's disease

Brain cells in Alzheimer's disease (AD) exhibit a membrane defect characterized by accelerated phospholipid turnover. The mechanism responsible for this defect remains unknown. Recent studies indicate that impairment of mitochondrial function is frequently observed in AD and may be responsible for certain aspects of its pathophysiology. We show that when PC12 cells are exposed to inhibitors of mitochondrial bioenergetics, the turnover of their major membrane phospholipid, phosphatidylcholine, is accelerated, producing a pattern of metabolic changes that mimics that observed in brains of AD patients. Abnormalities of mitochondrial function may therefore underlie the membrane defect in AD.

Zn2+ inhibits alpha-ketoglutarate-stimulated mitochondrial respiration and the isolated alpha-ketoglutarate dehydrogenase complex

Intracellular free Zn(2+) is elevated in a variety of pathological conditions, including ischemia-reperfusion injury and Alzheimer's disease. Impairment of mitochondrial respiration is also associated with these pathological conditions. To test whether elevated Zn(2+) and impaired respiration might be linked, respiration of isolated rat liver mitochondria was measured after addition of Zn(2+). Zn(2+) inhibition (K(i)(app) = approximately 1 micrometer) was observed for respiration stimulated by alpha-ketoglutarate at concentrations well within the range of intracellular Zn(2+) reported for cultured hepatocytes. The bc(1) complex is inhibited by Zn(2+) (Link, T. A., and von Jagow, G. (1995) J. Biol. Chem. 270, 25001-25006). However, respiration stimulated by succinate (K(i)(app) = approximately 6 micrometer) was less sensitive to Zn(2+), indicating the existence of a mitochondrial target for Zn(2+) upstream from bc(1) complex. Purified pig heart alpha-ketoglutarate dehydrogenase complex was strongly inhibited by Zn(2+) (K(i)(app) = 0.37 +/- 0.05 micrometer). Glutamate dehydrogenase was more resistant (K(i)(app) = 6 micrometer), malate dehydrogenase was unaffected, and succinate dehydrogenase was stimulated by Zn(2+). Zn(2+) inhibition of alpha-ketoglutarate dehydrogenase complex required enzyme cycling and was reversed by EDTA. Reversibility was inversely related to the duration of exposure and the concentration of Zn(2+). Physiological free Zn(2+) may modulate hepatic mitochondrial respiration by reversible inhibition of the alpha-ketoglutarate dehydrogenase complex. In contrast, extreme or chronic elevation of intracellular Zn(2+) could contribute to persistent reductions in mitochondrial respiration that have been observed in Zn(2+)-rich diseased tissues.

Oxidative stress and a key metabolic enzyme in Alzheimer brains, cultured cells, and an animal model of chronic oxidative deficits

Oxidative stress and diminished metabolism occur in several neurodegenerative disorders. Brains from Alzheimer's disease (AD) patients exhibit several indicators of oxidative stress and have reduced activities of the alpha-ketoglutarate dehydrogenase complex (KGDHC), a key mitochondrial enzyme. Whether these abnormalities are secondary to neurodegenerative processes or are inherent properties of the cells cannot be determined in autopsy brain. Studies in cultured fibroblasts suggest that AD-related differences in oxidative stress and KGDHC reflect inherent properties of AD cells. KGDHC is sensitive to oxidative stress whether the enzyme is studied in cells, in purified mitochondria, or as an isolated protein. Reductions of brain KGDHC in living rodents lead to oxidative stress and selective cell death. The results suggest that KGDHC participates in a deleterious cascade of events related to oxidative stress that are critical in selective neuronal loss in neurodegenerative diseases.

The alpha-ketoglutarate dehydrogenase complex in neurodegeneration

Altered energy metabolism is characteristic of many neurodegenerative disorders. Reductions in the key mitochondrial enzyme complex, the alpha-ketoglutarate dehydrogenase complex (KGDHC), occur in a number of neurodegenerative disorders including Alzheimer's Disease (AD). The reductions in KGDHC activity may be responsible for the decreases in brain metabolism, which occur in these disorders. KGDHC can be inactivated by several mechanisms, including the actions of free radicals (Reactive Oxygen Species, ROS). Other studies have associated specific forms of one of the genes encoding KGDHC (namely the DLST gene) with AD, Parkinson's disease, as well as other neurodegenerative diseases. Reductions in KGDHC activity can be plausibly linked to several aspects of brain dysfunction and neuropathology in a number of neurodegenerative diseases. Further studies are needed to assess mechanisms underlying the sensitivity of KGDHC to oxidative stress and the relation of KGDHC deficiency to selective vulnerability in neurodegenerative diseases.

Quantitative alpha-ketoglutarate dehydrogenase activity staining in brain sections and in cultured cells

The activity of a key mitochondrial enzyme, the alpha-ketoglutarate dehydrogenase complex (KGDHC), declines in the brains of patients with neurodegenerative diseases such as Alzheimer's disease, as well as in thiamine-deficient (TD) animals. The decreased activity often occurs without a reduction in enzyme protein, which negates the use of immunocytochemistry to study cellular or regional changes in enzyme activity within the brain. To overcome this limitation, an activity staining method using nitroblue tetrazolium was developed. The histochemical activity staining was standardized in cultured cells. The assay was linear with time and was highly specific for KGDHC. The dark-blue reaction product (formazan) formed a pattern that was consistent with mitochondrial localization. Treatment of the cultured cells with both reversible and irreversible inhibitors decreased formazan production, whereas conventional enzyme assays on cell lysates only revealed loss of KGDHC activity with irreversible inhibitors. The activity staining was also linear with time and highly specific for KGDHC activity in mouse brain sections. Staining occurred throughout the brain, and discrete neuronal populations exhibited particularly intense staining. The pattern of staining differed markedly from the distribution of KGDHC protein by immunocytochemistry. Generalized decreases in the intensity of activity staining that occurred in the TD brains compared to controls were comparable with the loss of KGDHC activity by conventional enzyme assay. Thus, the present study introduces a new histochemical method to measure KGDHC activity at the cellular and regional level, which will be useful to determine changes of in situ enzyme activity.

Metabolic impairment elicits brain cell type-selective changes in oxidative stress and cell death in culture

Abnormalities in oxidative metabolism and inflammation accompany many neurodegenerative diseases. Thiamine deficiency (TD) is an animal model in which chronic oxidative stress and inflammation lead to selective neuronal death, whereas other cell types show an inflammatory response. Therefore, the current studies determined the response of different brain cell types to TD and/or inflammation in vitro and tested whether their responses reflect inherent properties of the cells. The cells that have been implicated in TD-induced neurotoxicity, including neurons, microglia, astrocytes, and brain endothelial cells, as well as neuroblastoma and BV-2 microglial cell lines, were cultured in either thiamine-depleted media or in normal culture media with amprolium, a thiamine transport inhibitor. The activity levels of a key mitochondrial enzyme, alpha-ketoglutarate dehydrogenase complex (KGDHC), were uniquely distributed among different cell types: The highest activity was in the endothelial cells, and the lowest was in primary microglia and neurons. The unique distribution of the activity did not account for the selective response to TD. TD slightly inhibited general cellular dehydrogenases in all cell types, whereas it significantly reduced the activity of KGDHC exclusively in primary neurons and neuroblastoma cells. Among the cell types tested, only in neurons did TD induce apoptosis and cause the accumulation of 4-hydroxy-2-nonenal, a lipid peroxidation product. On the other hand, chronic lipopolysaccharide-induced inflammation significantly inhibited cellular dehydrogenase and KGDHC activities in microglia and astrocytes but not in neurons or endothelial cells. The results demonstrate that the selective cell changes during TD in vivo reflect inherent properties of the different brain cell types.

Inhibition of select mitochondrial enzymes in PC12 cells exposed to S-(1,1,2,2-tetrafluoroethyl)-L-cysteine

Many halogenated foreign compounds are detoxified by conversion to the corresponding cysteine S-conjugate, which is N-acetylated and excreted. However, several halogenated cysteine S-conjugates [e.g. S-(1,1,2,2-tetrafluoroethy)-L-cysteine (TFEC)] are converted to mitochondrial toxicants by cysteine S-conjugate beta-lyases. In the present work, we showed that TFEC appreciably inactivated highly purified alpha-ketoglutarate dehydrogenase complex (KGDHC) in the presence of a cysteine S-conjugate beta-lyase. Incubation of PC12 cells (which contain endogenous cysteine S-conjugate beta-lyase activity) with TFEC led to a concentration- and time-dependent loss of endogenous KGDHC activity. A 24-hr exposure to 1 mM TFEC decreased KGDHC activity in the cells by 90%. Although treatment with TFEC did not inhibit intrinsic pyruvate dehydrogenase complex (PDHC) activity, it inhibited dichloroacetate/Mg2+-mediated activation/dephosphorylation of PDHC in the PC12 cells by 90%. To determine the selectivity of enzymes targeted by TFEC, several cytosolic and mitochondrial enzymes involved in energy metabolism [malate dehydrogenase, glyceraldehyde 3-phosphate dehydrogenase, glutamate dehydrogenase, lactate dehydrogenase, cytosolic and mitochondrial aspartate aminotransferases (AspAT)] were also assayed in the PC12 cells exposed to 1 mM TFEC for 24 hr. Of these enzymes, only mitochondrial AspAT, a key enzyme of the malate-aspartate shuttle, was inhibited. The present results demonstrate a selective vulnerability of mitochondrial enzymes to toxic cysteine S-conjugates. The data indicate that TFEC may be a useful cellular/mitochondrial toxicant for elucidating the consequences of the diminished mitochondrial function that accompanies numerous neurodegenerative diseases.

Ionic and signal transduction alterations in Alzheimer's disease: relevance of studies on peripheral cells

Several lines of evidence indicate that Alzheimer's disease (AD) has systemic expression. Systemic changes are manifested as alterations in a number of molecular and cellular processes. Although, these alterations appear to have little or no consequence in peripheral systems, their parallel expression in the central nervous system (CNS) could account for the principal clinical manifestations of the disease. Recent research seems to indicate that alterations in ion channels, calcium homeostasis, and protein kinase C (PKC) can be linked and thereby constitute a model of pathophysiological relevance. Considering the difficulties of studying dynamic pathophysiological processes in the disease-ridden postmortem AD brain, peripheral tissues such as fibroblasts provide a suitable model to study molecular and cellular aspects of the disease.

Alcohol-thiamine interactions: an update on the pathogenesis of Wernicke encephalopathy

Wernicke encephalopathy is a neurological disorder commonly observed in chronic alcohol abuse, in patients with AIDS, and in other conditions of compromised nutritional status. The underlying cause of the disorder is thiamine deficiency. The present review highlights data focusing on alcohol-thiamine interactions and their relationship to the pathogenesis of Wernicke encephalopathy. Recent findings on the effects of alcohol on thiamine absorption and storage and on thiamine phosphorylation to the enzyme co-factor form (thiamine diphosphate) are discussed with regard to the postulated "biochemical lesion" of Wernicke encephalopathy. Also discussed are new findings on the molecular genetics of the thiamine-dependent enzyme transketolase in patients with Wernicke encephalopathy. A discussion of the hypotheses regarding the mechanisms underlying the phenomenon of selective neuronal cell death observed in this disorder including cerebral energy deficit, focal lactic acidosis, glutamate excitotoxicity, increased expression of immediate-early genes, free radicals and perturbations of the blood-brain barrier are presented. Finally, the possible role of thiamine deficiency in alcoholic peripheral neuropathy is reviewed.

Complementary and alternative medicines for Alzheimer's disease

The recent successes of large, multicenter clinical trials of acetylcholinesterase inhibitors for symptomatic treatment of Alzheimer's disease have spawned enthusiasm that this common and fatal neurologic disease is "treatable." A parallel explosion has occurred in the consumption of alternative medicines by the public seeking more effective, natural, or safer methods for treatment of dementia. Some of these medicines may, in fact, be biologically active in modulating the disease as well as producing side effects and interactions with accepted pharmaceuticals. This review brings to focus the scientific evidence presently available regarding such agents.

Metabolic impairment induces oxidative stress, compromises inflammatory responses, and inactivates a key mitochondrial enzyme in microglia

Microglial activation, oxidative stress, and dysfunctions in mitochondria, including the reduction of cytochrome oxidase activity, have been implicated in neurodegeneration. The current experiments tested the effects of reducing cytochrome oxidase activity on the ability of microglia to respond to inflammatory insults. Inhibition of cytochrome oxidase by azide reduced oxygen consumption and increased reactive oxygen species (ROS) production but did not affect cell viability. Azide also attenuated microglial activation, as measured by nitric oxide (NO.) production in response to lipopolysaccharide (LPS). It is surprising that the inhibition of cytochrome oxidase also diminished the activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC), a Krebs cycle enzyme. This reduction was exaggerated when the azide-treated microglia were also treated with LPS. The combination of the azide-stimulated ROS and LPS-induced NO. would likely cause peroxynitrite formation in microglia. Thus, the possibility that KGDHC was inactivated by peroxynitrite was tested. Peroxynitrite inhibited the activity of isolated KGDHC, nitrated tyrosine residues of all three KGDHC subunits, and reduced immunoreactivity to antibodies against two KGDHC components. Thus, our data suggest that inhibition of the mitochondrial respiratory chain diminishes aerobic energy metabolism, interferes with microglial inflammatory responses, and compromises mitochondrial function, including KGDHC activity, which is vulnerable to NO. and peroxynitrite that result from microglial activation. Thus, activation of metabolically compromised microglia can further diminish their oxidative capacity, creating a deleterious spiral that may contribute to neurodegeneration.

Properties and functions of the thiamin diphosphate dependent enzyme transketolase

This review highlights recent research on the properties and functions of the enzyme transketolase, which requires thiamin diphosphate and a divalent metal ion for its activity. The transketolase-catalysed reaction is part of the pentose phosphate pathway, where transketolase appears to control the non-oxidative branch of this pathway, although the overall flux of labelled substrates remains controversial. Yeast transketolase is one of several thiamin diphosphate dependent enzymes whose three-dimensional structures have been determined. Together with mutational analysis these structural data have led to detailed understanding of thiamin diphosphate catalysed reactions. In the homodimer transketolase the two catalytic sites, where dihydroxyethyl groups are transferred from ketose donors to aldose acceptors, are formed at the interface between the two subunits, where the thiazole and pyrimidine rings of thiamin diphosphate are bound. Transketolase is ubiquitous and more than 30 full-length sequences are known. The encoded protein sequences contain two motifs of high homology; one common to all thiamin diphosphate-dependent enzymes and the other a unique transketolase motif. All characterised transketolases have similar kinetic and physical properties, but the mammalian enzymes are more selective in substrate utilisation than the nonmammalian representatives. Since products of the transketolase-catalysed reaction serve as precursors for a number of synthetic compounds this enzyme has been exploited for industrial applications. Putative mutant forms of transketolase, once believed to predispose to disease, have not stood up to scrutiny. However, a modification of transketolase is a marker for Alzheimer's disease, and transketolase activity in erythrocytes is a measure of thiamin nutrition. The cornea contains a particularly high transketolase concentration, consistent with the proposal that pentose phosphate pathway activity has a role in the removal of light-generated radicals.

Affected enzyme activities in Alzheimer's disease are sensitive to antemortem hypoxia

Many enzyme activities in Alzheimer's disease (AD) are changed. Some of these enzyme activities are related to certain neurotransmitter systems. Enzymes in the brain can also be sensitive to antemortem hypoxia. In the present study it was determined if enzyme activities that are altered in AD are also subject to alteration by antemortem hypoxia. As an indicator of antemortem hypoxia brain lactate concentration was used. Enzyme activities measured were those of prolyl endopeptidase (PE), aminopeptidase (AP), phosphatidylinositol (PI) kinase, phosphatidylinositol phosphate kinase, alpha-ketoglutarate dehydrogenase (alpha-KGDH), choline acetyltransferase and beta-glucuronidase. All of these enzyme activities have been measured in AD patients before and several of them have been found to be decreased. In accordance with previous findings, PE, alpha-KGDH and ChAT activities were reduced in AD patients. PI kinase and beta-glucuronidase activities, however, were not reduced, contrary to previous findings. All enzyme activities, except that of beta-glucuronidase, correlated with brain lactate concentration, suggesting that antemortem hypoxia has a major influence on the activity of enzymes in the brain. PE, AP, alpha-KGDH and ChAT activities were still different between AD and control samples when these were matched for lactate concentration. The enzyme activities that were changed in AD were also significantly correlated with lactate concentration, an indicator of antemortem hypoxia, in brain specimens. This suggests that antemortem hypoxia and AD have some factor in common that may be responsible for changes in enzyme activities. Since both PE and alpha-KGDH are known to be sensitive to oxidative stress this factor could be oxidative stress.

Alpha-ketoglutarate dehydrogenase in Alzheimer brains bearing the APP670/671 mutation

Alzheimer's disease (AD) is associated with a striking reduction in the activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC). The deficiency occurs in brains from AD patients of undefined etiology, and in fibroblasts from both sporadic and familial AD cases. To further assess the nature of the abnormality of KGDHC in AD, KGDHC activities and immunoreactivities were analyzed in brains from AD patients bearing the Swedish APP670/671 mutation. This gene defect causes overproduction of the amyloid beta peptide. KGDHC activities were reduced by 55 to 57% compared with control values in the mutation-bearing AD cases in the medial temporal and superior frontal cortices. The immunochemical levels of KGDHC subunits Elk (-51%) and E2k (-76%) declined, whereas E3 concentrations were unchanged. The results suggest that mitochondrial dysfunction is a part of the pathophysiological process in AD even when the primary pathogenic cause is nonmitochondrial.

Thiamine deficiency-induced disruptions in the diurnal rhythm and regulation of body temperature in the rat

In the present study, diurnal rhythm and regulation of body temperature were monitored during and several weeks following pyrithiamine-induced thiamine deficiency (PTD group, n=8) or pairfeeding (control group, n=9). A significant decline of core body temperature and a disruption of its diurnal rhythm were observed at varying stages of PTD treatment. Following thiamine administration and return to thiamine-fortified chow, body temperature continued to fall and several days transpired before body temperature and its diurnal rhythm were returned to normal. When exposed to warm and cold environments, no significant group differences were observed in either the maximum temperature change or the time elapsed to reach maximal temperature change. Histological examination revealed necrotic lesions in thalamus and mammillary body in the PTD group characteristic of Wernicke's encephalopathy. No significant damage was observed in the medial preoptic and suprachiasmatic nuclei, brain regions involved in the regulation of body temperature and circadian rhythm. These findings suggest that hypothermia and disruption of the diurnal rhythm of body temperature can be reversed by restoration of adequate thiamine levels and are related to biochemical and physiological disturbances rather than gross structural changes.

Elevated adenosine monophosphate deaminase activity in Alzheimer's disease brain

Abnormal elevations in ammonia have been implicated in the pathogenesis of Alzheimer's disease. However, the biochemical mechanism(s) leading to increased ammonia in Alzheimer's disease have not yet been identified. A potential source of increased ammonia production is adenosine monophosphate (AMP) deaminase, an important enzyme in the regulation of the purine nucleotide cycle and adenylate energy charge. AMP deaminase activity is expressed in human brain and converts AMP to inosine monophosphate with the release of ammonia. We have investigated AMP deaminase activity in postmortem brain tissue from Alzheimer's disease subjects and age-matched controls. Compared to control brain, Alzheimer's disease brain AMP deaminase activity is 1.6- to 2.4-fold greater in the regions examined--the cerebellum, occipital cortex, and temporal cortex. Similar increases in AMP deaminase protein and mRNA levels are observed in Alzheimer's disease brain. These results suggest that increased AMP deaminase activity may augment ammonia levels in the brain in Alzheimer's disease.

Induction of nitric oxide synthase and microglial responses precede selective cell death induced by chronic impairment of oxidative metabolism

Abnormal oxidative processes including a reduction in thiamine-dependent enzymes accompany many neurodegenerative diseases. Thiamine deficiency (TD) models the cellular and molecular mechanisms by which chronic oxidative aberrations associated with thiamine-dependent enzyme deficits cause selective neurodegeneration. The mechanisms underlying selective cell death in TD are unknown. In rodent TD, the earliest region-specific pathological change is breakdown of the blood-brain barrier (BBB). The current studies tested whether nitric oxide and microglia are important in the initial events that couple BBB breakdown to selective neuronal loss. Enhanced expression of endothelial nitric oxide synthase and nicotinamide adenine dinucleotide phosphate diaphorase reactivity in microvessels, as well as the presence of numerous inducible nitric oxide synthase-immunoreactive microglia, accompanied the increases in BBB permeability. Nitric oxide synthase induction appears critical to TD pathology, because immunoreactivity for nitrotyrosine, a specific nitration product of peroxynitrite, also increased in axons of susceptible regions. In addition, TD elevated iron and the antioxidant protein ferritin in microvessels and in activated microglia, suggesting that these cells are responding to an oxidative challenge. All of these changes occurred in selectively vulnerable regions, preceding neuronal death. These findings are consistent with the hypothesis that the free radical-mediated BBB alterations permit entry of iron and extraneuronal proteins that set in motion a cascade of inflammatory responses culminating in selective neuronal loss. Thus, the TD model should help elucidate the relationship between oxidative deficits, BBB abnormalities, the inflammatory response, ferritin and iron elevation, and selective neurodegeneration.

Thiamine deficiency decreases steady-state transketolase and pyruvate dehydrogenase but not alpha-ketoglutarate dehydrogenase mRNA levels in three human cell types

Reductions in the levels and activities of enzymes that utilize thiamine diphosphate (ThDP) as a cofactor are thought to be responsible for the tissue damage suffered during thiamine deficiency. Although loss of cofactor can account in part for loss of enzyme activity, thiamine and its phosphorylated derivatives may also regulate the expression of the genes encoding these proteins. To examine this possibility, steady-state mRNA levels for three ThDP-dependent enzymes were measured in human fibroblasts, lymphoblasts and neuroblastoma cells cultured under conditions of thiamine sufficiency and deficiency. In all three cell types, the mRNA levels of transketolase and the E1beta subunit of pyruvate dehydrogenase complex were lower in thiamine-deficient cultures. In contrast, mRNA levels for a ThDP-binding subunit of alpha-ketoglutarate dehydrogenase, the E1 subunit did not differ. These results indicate that thiamine or a thiamine metabolite regulates the expression in humans of some, but not all, genes encoding ThDP-utilizing enzymes.

Peripheral markers in testing pathophysiological hypotheses and diagnosing Alzheimer's disease

Alterations in amyloid precursor protein (APP) metabolism, calcium regulation, oxidative metabolism, and transduction systems have been implicated in Alzheimer's disease (AD). Limitations to the use of postmortem brain for examining molecular mechanisms underscore the need to develop a human tissue model representative of the pathophysiological processes that characterize AD. The use of peripheral tissues, particularly of cultured skin fibroblasts derived from AD patients, could complement studies of autopsy samples and provide a useful tool with which to investigate such dynamic processes as signal transduction systems, ionic homeostasis, oxidative metabolism, and APP processing. Peripheral cells as well as body fluids (i.e., plasma and CSF) could also provide peripheral biological markers for the diagnosis of AD. The criteria required for a definite diagnosis of AD presently include clinical criteria in association with histopathologic evidence obtained from biopsy or autopsy. Thus, the use of peripheral markers as a diagnostic tool, either to predict or at least to confirm a diagnosis, may be of great importance.

Abnormalities in Alzheimer's disease fibroblasts bearing the APP670/671 mutation

Abnormalities in cultured fibroblasts from familial Alzheimer's Disease (FAD) cases uniquely enable the determination of how gene defects alter cell biology in living tissue from affected individuals. The current study focused on measures of calcium regulation and oxidative metabolism in fibroblast lines from controls and FAD individuals with the Swedish APP670/671 mutation. Bombesin-induced elevations in calcium in APP670/671 mutation-bearing lines were reduced by 40% (p < 0.05), a striking contrast to the 100% increase seen in sporadic AD and presenilin-1 (PS1) mutation-bearing cells in previously published studies. The APP670/671 mutation-bearing lines did not exhibit the exaggerated 4-bromo-A23187 releasable pool of calcium following 10 nM bradykinin, the enhanced sensitivity of calcium stores to low concentrations of bradykinin, nor the reduced activity of alpha-ketoglutarate dehydrogenase previously reported in cells from sporadic AD and mutant PS1 FAD. Thus, an altered regulation of internal calcium stores is common to all AD lines, but the calcium pool affected and the polarity of the alteration varies, apparently in association with particular gene mutations. Comparison of signal transduction in cell lines from multiple, genetically characterized AD families will allow testing of the hypothesis that these various pathogenic FAD abnormalities that lead to AD converge at the level of abnormal signal transduction.

Cloning and characterization of Helicobacter pylori succinyl CoA:acetoacetate CoA-transferase, a novel prokaryotic member of the CoA-transferase family

Sequencing of a fragment of Helicobacter pylori genome led to the identification of two open reading frames showing striking homology with Coenzyme A (CoA) transferases, enzymes catalyzing the reversible transfer of CoA from one carboxylic acid to another. The genes were present in all H. pylori strains tested by polymerase chain reaction or slot blotting but not in Campylobacter jejuni. Genes for the putative A and B subunits of H. pylori CoA-transferase were introduced into the bacterial expression vector pKK223-3 and expressed in Escherichia coli JM105 cells. Amino acid sequence comparisons, combined with measurements of enzyme activities using different CoA donors and acceptors, identified the H. pylori CoA-transferase as a succinyl CoA:acetoacetate CoA-transferase. This activity was consistently observed in different H. pylori strains. Antibodies raised against either recombinant A or B subunits recognized two distinct subunits of Mr approximately 26,000 and 24, 000 that are both necessary for H. pylori CoA-transferase function. The lack of alpha-ketoglutarate dehydrogenase and of succinyl CoA synthetase activities indicates that the generation of succinyl CoA is not mediated by the tricarboxylic acid cycle in H. pylori. We postulate the existence of an alternative pathway where the CoA-transferase is essential for energy metabolism.

Elevated levels of the Kearns-Sayre syndrome mitochondrial DNA deletion in temporal cortex of Alzheimer's patients

A mitochondrial hypothesis of Alzheimer's disease (AD) has been proposed based on a number of studies which establish altered oxidative phosphorylation (OXPHOS) and ATP synthesis in AD tissue. Four out of five complexes in the OXPHOS pathway are partly encoded by mitochondrial DNA (mtDNA); thus, this may be a crucial site of lesions that alter brain activity. We examined temporal cortex autopsy tissue for deleted mtDNA by PCR-based methods and Southern analysis. AD tissue was obtained from autopsy-confirmed cases that had a postmortem delay ranging from 5 to 27 h. Using a rat brain model system to examine postmortem effects by Southern analysis, no evidence of mtDNA degradation after 30 h of postmortem delay at room temperature was found. Nine tissue samples taken from AD autopsy brain (average age 68 years) and nine age-matched controls (average age 66 years) were assessed by serial dilution PCR for the 5 kb deletion (mtDNA delta 4977) previously associated with Kearns-Sayre syndrome. Using this method we determined that AD temporal cortex had a 6.5-fold greater frequency of mtDNA delta 4977 than controls (0.0593% vs. 0.0092%, p = 0.0269, one-tailed; p = 0.0530, two-tailed), indicating that damaged mtDNA preferentially accumulates in AD compared to aged brain.

Brain energy metabolizing enzymes in Alzheimer's disease: alpha-ketoglutarate dehydrogenase complex and cytochrome oxidase

PET observations of reduced cerebral glucose metabolism in AD could be explained by a defect in key energy metabolizing enzymes. In particular, levels of two enzymes, cytochrome oxidase (CO) and alpha-ketoglutarate dehydrogenase complex (alpha KGDHC) are generally assumed to be reliably reduced in postmortem brain of patients with AD. How strong is the evidence that brain CO and alpha KGDHC are reduced in AD? In our study CO activity and alpha KGDHC activity and protein subunit levels were measured in cerebral cortex of 19-29 AD patients and 29 control subjects. We found that mean CO activity in cerebral cortex was reduced by 16-26% in the AD group but with almost complete overlap between control and patient ranges. Since our publication in 1992, mean brain CO activity in AD was modestly reduced in 9 independent studies (p < 0.05 in 5). Activity of alpha KGDHC varied widely in control/AD subjects and is not useful as an enzyme marker. Cerebral cortical protein levels of E1-3 subunits, which showed much less variance, were reduced by 23-41% but with large overlap between control/patient groups. We concluded that decreased (i.e., below normal) brain CO and alpha KGDHC is a feature of some, but not all patients with AD. The possible causes and significance of the enzyme changes are discussed.

Importance of vascular changes in selective neurodegeneration with thiamine deficiency

These results demonstrate that early alterations in the BBB may underlie selective vulnerability in this model of chronic reduced oxidative metabolism. Changes in the BBB (IgG extravasation) precede alterations in APP processing and cell death. Since thiamine-dependent enzymes are also reduced in the brain in Alzheimer's disease, similar processes may be important in the pathophysiology of the disease.