This supplement is supported by Pamlab LLC.

Dr. McCaddon is senior honorary research fellow at Cardiff University, Cardiff School of Medicine, in North Wales, United Kingdom. Dr. Hudson is principal biochemist in the Department of Medical Biochemistry, Wrexham Maelor Hospital, in North Wales, United Kingdom.

Disclosures: Drs. McCaddon and Hudson are scientific advisors and shareholders of COBALZ Limited, a private limited company developing novel B-vitamin and antioxidant supplements.  COBALZ has granted certain U.S. rights concerning ‘Cerefolin NAC’ to Pamlab LLC.


 

Abstract

Alzheimer’s disease (AD) and dementia have enormous financial and social impacts on society. It is predicted that almost 36 million people will have dementia in 2010, a figure which is anticipated to double every 20 years as the world population ages.  Prevention of AD or slowing of the progression of AD would provide significant benefits. There are multiple ways in which vitamin B12, vitamin B6, folate, and homocysteine (Hcy) play a role in the pathogenesis of AD. Vitamin B12, vitamin B6, and folate deficiencies are associated with various cognitive disorders, including dementia. Neuroinflammatory oxidative stress occurs early in AD pathology. Total blood Hcy levels are utilized as a marker to assist in diagnosing such deficiencies. Hcy contributes to pathological cascades involving amyloid plaques and neurofibrillary tangles (NFTs). This review provides a thorough description of several factors involved in the development of the pathological changes associated with AD, such as neuroinflammatory oxidative stress and methylation, apoptosis, NFTs, amyloid plaques, and cerebrospinal fluid biomarkers. The review also considers the rationale for a combined B-vitamin and antioxidant supplement (Cerefolin NAC) in treating and slowing AD-related cognitive decline.


In this Expert Review Supplement, Andrew McCaddon, MD, and Peter R. Hudson, PhD provide a comprehensive review of factors involved in AD pathology as well as evidence supporting the use of a combined B-vitamin and antioxidant supplement (Cerefolin NAC) for AD-related cognitive decline. A commentary on this article is provided by leading AD expert Jeffrey L. Cummings, MD.

 

Introduction

Almost 36 million people will have dementia in 2010—an alarming figure set to double every 20 years with the “greying” of the world population.1 Alzheimer’s disease (AD) and dementia have enormous financial and social impacts on society. Prevention or illness delay of even a small percentage of cases would provide significant cost benefits for health-care systems.2 This review considers the rationale for a combined B-vitamin and antioxidant supplement (Cerefolin NAC) in treating and slowing AD-related cognitive decline.

 

B-Vitamins and Dementia

Vitamin B12 and folate deficiencies are associated with various cognitive disorders, including dementia.3 In the 1980s, plasma total homocysteine (tHcy) assays were introduced to assist in diagnosing these deficiencies. Hcy is derived from dietary methionine. Cells re-methylate Hcy to methionine using B12-dependent methionine synthase; 5-methyltetrahydrofolate (5-MTHF) acts as a methyl donor (Figure 1A).  Alternatively, Hcy is converted to cystathionine, and ultimately cysteine, by B6-dependent cystathionine b-synthase. Blood Hcy levels rise in B6, B12, and folate deficiencies.Higher levels are also associated with aging, smoking, male gender, renal impairment, and drugs including methotrexate, metformin, and levodopa.4

 

Using tHcy as a marker, B vitamin deficiencies were found to be highly prevalent in the elderly.5,6 This led to speculation that elevated blood Hcy, hyperhomocysteinemia, might occur commonly in dementias, including AD.7-9 Hyperhomocysteinemia implies impaired methylation reactions (hypomethylation),10 with predictable adverse effects for neurotransmitter synthesis and AD neuropathology. Hcy is also associated with vascular disease,11 itself a risk factor for dementia.12

Evidence for the “homocysteine hypothesis of dementia” came with reports of hyperhomocysteinemia in patients with clinically and pathologically confirmed AD.13,14 Raised blood levels were also observed in mild cognitive impairment (MCI) and vascular dementia.15,16 Although elevated Hcy could be a consequence of, or coincidental with, dementia it is now recognized to be associated with an increased risk for both cognitive decline and incident dementia.17-19

One curious feature of the relationship of Hcy with dementia is the absence of macrocytic anemia.20,21 The relationship is also independent of nutritional status,13,22 suggesting that rather than arising from dietary deficiency or malabsorption, it may reflect effects of oxidative stress on Hcy metabolism.23,24

 

Oxidative Damage,“Neuroinflammation,” and AD

Oxidative damage is a prominent feature of AD.25 Lipid peroxidation and levels of protein and nucleic acid oxidation are significantly increased in vulnerable brain regions.26 Such damage is not confined to AD, but also occurs in patients with amnestic MCI.27

There is also an association between AD and inflammation.28 Epidemiological studies link the use of anti-inflammatory drugs with a reduced risk for AD and expression of inflammatory mediators is increased in postmortem AD brains.29 Such “neuroinflammation” is likely a major driving force in the disease. Rather than being the primary lesion in AD, amyloid plaques and neurofibrillary tangles may be compensatory phenomena, ie, end-stage manifestations of cellular adaptation preceded by elevated markers of oxidative stress.30

 

Oxidative Stress and Methylation

Recycling of Hcy to methionine by methionine synthase (MS) requires vitamin B12 as co-factor and 5-MTHF  as methyl donor (Figure 1A). Methionine adenosyltransferase then converts methionine to S-adenosylmethionine (SAM)—a substrate for multitudinous cellular methylation reactions.

SAM synthesis is impaired by oxidative stress; cob(I)alamin, an intermediate in the MS reaction, is vulnerable to oxidative deactivation to cob(II)alamin.31 Reductive re-methylation of cob(II)alamin requires a methyl group donated by SAM itself. Oxidatively impaired MS activity also depletes folate stores via reduced polyglutamation—an essential prerequisite for cellular folate retention.32

Oxidative stress increases the requirement for, but decreases synthesis of, SAM.23 Naturally, an auto-corrective mechanism exists. Hepatic Hcy is metabolized via the transsulphuration pathway, culminating in synthesis of glutathione—an essential antioxidant for intracellular redox homeostasis.4 Neurones and other central nervous system (CNS) cells do not fully express this pathway, nor does brain tissue possess an alternative pathway for re-methylating Hcy.33 Hence, its capacity to metabolize Hcy is extremely vulnerable to oxidative stress and dependent on an adequate supply of folate and B12.

 

Oxidative Stress and Mitochondrial Dysfunction

Mitochondria are pivotal in cell life and death, producing energy in the form of adenosine triphosphate (ATP) and sequestering calcium, but also generating free radicals and serving as repositories for proteins regulating apoptosis. Perturbations in their function sensitize cells to neurotoxic insults and may initiate cell death.34 Alterations in energy metabolism occur early in AD. Energy consumption is drastically decreased in cortical and hippocampal regions, implying compromised mitochondrial function. This is accompanied by elevated reactive oxygen species (ROS), contributing to increased neuronal loss.35  One potential mechanism is the binding of amyloid-beta (Ab) with mitochondrial membrane proteins involved in adenosine diphosphate (ADP)/ATP transfer.36

Other processes can influence mitochondrial bioenergetics. Succinyl-CoA is an essential component of the mitochondrial citric acid cycle, which generates ATP via the mitochondrial electron-transport chain. One route of synthesis is from a-ketoglutarate via the enzyme complex, b-ketoglutarate dehydrogenase. Oxidative stress can impair this complex, compromising energy metabolism and further enhancing ROS formation in AD and other neurodegenerative diseases.37 

Succinyl-CoA is also synthesised from methylmalonyl-CoA via vitamin B12-dependent methylmalonyl-CoA mutase.38  While inhibition of this enzyme by ROS is still being investigated,39 B12 deficiency causes accumulation of methylmalonic acid (MMA). Evidence from the disorder methylmalonic aciduria suggests that neurodegeneration is associated with inhibition of the respiratory chain and tricarboxylic acid cycle not by MMA alone, but by synergistically acting alternative metabolites, in particular 2-methylcitric acid, malonic acid, and propionyl-CoA.40 Of relevance, a study of healthy elderly individuals showed a high prevalence of metabolically significant vitamin B12 deficiency, with increased MMA being associated with lower cognitive function scores.41

A potentially prudent strategy for maximal protection against these adverse metabolic insults upon mitochondria, and on energy production via the tricarboxylic acid cycle and electron transport chain, is  to optimize both glutathione synthesis and  vitamin B12 status.

 

Relationship Between Hcy, Hypomethylation, and AD Pathology

AD is characterized by intraneuronal neurofibrillary tangles and extracellular amyloid plaques. Hyperhomocysteinemia and hypomethylation influence the development of these lesions.

 

Neurofibrillary Tangle Formation

Neurofibrillary tangles (NFTs) are formed by the microtubule-associated protein tau. Tau is modulated by phosphorylation; the ability of tau to bind to and stabilize microtubules correlates inversely with its phosphorylation.42 Tau is highly phosphorylated in AD and other “tauopathies.” Disordered phosphorylation disrupts the normal co-localization of tau with microtubules, leading to hyperphosphorylation, tau-tau interactions, paired helical filaments, and ultimately aggregation into NFTs.43

Tau phosphorylation is regulated by competing effects of kinases and phosphatases; attention has focused on the kinases GSK3b and CDK5 and the phosphatase PP2A. PP2A actively de-phosphorylates abnormal tau.44

PP2A comprises regulatory and catalytic subunits; methylation of the latter is critical, suggesting that hypomethylation leads to tau hyperphosphorylation (Figure 1C).45 There is a negative correlation between phosphorylated tau and markers of methylation status in cerebrospinal fluid (CSF) of patients with various neurological disorders, including AD.46 Impaired folate and methylation status is closely linked to NFT formation,47,48 but preventable by supplementation in animal models.49,50 Interestingly, a recent study has also shown that GSK3β activity is increased in mice reared on a B-vitamin–deficient diet.51 The authors also confirmed previous reports of decreased substrate specificity for PP2A in folate deficient mice.

Pin1 is another important tau regulatory enzyme. It ensures that phosphorylated-tau is in the correct conformation for de-phosphorylation by PP2A.52 However, Pin1 is downregulated and oxidized in MCI and AD hippocampus, providing further evidence linking oxidative damage and NFT formation.53

 

Amyloid Plaque Formation

Amyloid precursor protein (APP) is cleaved by α­, β­,­ and γ­ secretases (Figure 1B). Normally, APP is cleaved by α­-secretase, releasing an N-­terminal fragment, sAPPα.  sAPPα is neuroprotective, participating in synapse formation and integrity of memory.54,55 Alternative cleavage of APP by β-secretase generates a secreted APP β peptide, sAPPβ. Cleavage by γ­-secretase of the remaining C-­terminal end of APP leads to formation of Aβ peptide, comprising 39­-43 amino acids, depending on the precise cleavage site. Aβ peptides subsequently aggregate into harmful amyloid plaques (AP).56

Similar to tau, Pin1 maintains APP in a configuration that reduces its metabolism by β-secretase, shifting cellular selectivity towards non-amyloidogenic APP processing.57  Thus, oxidative downregulation of Pin1 adversely influences Aβ formation and its subsequent aggregation into AP.52,58

Hypomethylation also contributes to Aβ production. The pathway for APP processing into Aβ involves β­-secretase and γ-­secretase activity. The γ­-secretase complex comprises four individual proteins: presenilin (PS1), nicastrin, APH-1, and PEN-2.59 PS1 is the catalytic subunit, and mutations in its gene are a risk factor for AD.60

The expression of β-secretase and PS1 are downregulated by DNA methylation. In vitro, deficiency of folate and vitamin B12 in cell culture medium reduces SAM levels with a consequent increase in PS1 and β­-secretase levels and increased Aβ production. Adding SAM to deficient medium restores normal gene expression and reduces Aβ levels.61 In vivo, folate deficient mice show increased APP phosphorylation in association with the expected changes in methylation in brain tissue.48 Similarly, hyperhomocysteinemic rats have elevated PS1 and a prominent spatial memory deficit which is reversible by folate and B12 supplementation.62 Elevated Hcy also augments the neurotoxicity of Aβ, at least in vitro, by potentiating oxidative stress.63

 

CSF Biomarkers in AD and MCI

There is evidence for the inter-relationships between Hcy, Aβ, tau, and oxidative stress in CSF. CSF levels of Aβ and tau are associated with progression from MCI to AD.64-67 The association between CSF phospho-tau and Hcy in AD suggests that hypomethylation links hyperhomocysteinemia and neurodegeneration.68,46 Oxidative stress markers, namely lipid peroxidation products (isoprostanes), accompany increases in Aβ, tau, and Hcy. CSF Aβ and isoprostane levels are probably the earliest markers for neuronal damage in AD.69 Brain tissue studies show that other lipid peroxidation products (4-hydroxynonenal and acrolein) are increased in selected regions of patients with MCI, suggesting that lipid peroxidation occurs early in AD pathogenesis.70

 

Neurochemistry

AD is characterized by deficits in the cholinergic neurotransmitter system, although there are also deficiencies in other neurotransmitter systems.71 Glutamate is an excitatory amino acid involved in cortico-cortical association pathways. The N-methyl-d-aspartate (NMDA) receptor is a marker for glutamate activity. NMDA receptors are present in high density in the cortex and hippocampus and play an important role in learning and memory.72 Elevated levels of oxidised Hcy derivatives and limited SAM availability due to vitamin B12 and folate deficiencies might adversely affect both glutamatergic and cholinergic systems.7

 

Glutamatergic

The NMDA receptor complex is a large protein assembly with different binding sites for different ligands, including an NMDA site, a strychnine insensitive glycine-binding site, and a binding site for non-competitive antagonists. Homocysteic acid and homocysteine sulphinic acid are oxidized derivatives of Hcy, and exert toxic effects on NMDA receptors (Figure 1D). These metabolites are 250-fold more efficient in disrupting neuronal networks than Hcy itself, and cause excess calcium influx, free-radical generation, collapse of the mitochondrial membrane potential and, eventually, neuronal death.73,74

 

Cholinergic

Neuronal choline is derived from intrasynaptic choline (via degradation of acetylcholine by acetylcholinesterase), extracellular choline (via a low affinity transport mechanism), and intraneuronal choline (via sequential methylation of membrane phosphatidylethanolamine [PE]).75 Intraneuronal choline will be depleted if SAM availability is limited7 (Figure 1E). Impaired MS activity  also induces the hepatic B12-independent betaine homocysteine methyltransferase pathway, betaine supplying a methyl group instead of methyl-folate.76 Since betaine is derived from choline oxidation, this will reduce extraneuronal choline supplies.7

Impaired PE methylation also influences transmembrane signal transmission. PE largely faces the cytoplasm, whereas phosphatidylcholine faces the extracellular space. The methylating enzymes (PEMT 1 and 2) are also asymmetrically distributed. Phospholipid methylation commences on the cytoplasmic side of the membrane and methylated phospholipids are translocated to the exterior. This increases membrane fluidity, and is coupled to calcium influx and release of intracellular secondary messengers.77

 

PARP Activation, DNA Repair, and Apoptosis

Gene expression is partly attenuated by methylated DNA stretches—CpG islands. Hypomethylation induces gene transcription and DNA strand breakage.78,79 In cultured neurones, Hcy itself induces breakages,80 probably via free-radical induced damage. In vivo, decreased thymidylate synthesis with subsequent uracil misincorporation into DNA probably also contributes.81 Uracil is excised from DNA, generating transient breaks requiring repair. Poly (ADP-ribose) polymerase (PARP) recognizes damaged DNA and prepares it for repair. However, with excessive damage, PARP triggers a cascade of events leading to cell death.82 PARP-controlled cell death is the major pathway for neuronal apoptosis. Hence, hypomethylation is closely linked with neuronal apoptosis (Figure 1F).

 

Cerebral Ischemia, Atrophy, and Blood Brain Barrier Abnormalities

Elevated Hcy is a risk factor for atherothrombotic disease, and folate supplementation is effective in secondary stroke prevention.83 AD commonly co-occurs with stroke, suggesting that hyperhomocysteinaemia and AD might also be partly linked via micro-vascular disease.84 Elevated Hcy is also associated with brain atrophy85,86 and blood-brain barrier (BBB) dysfunction,87 which is reversible by high-dose B-vitamin supplementation.88

 

Method of Action of Cerefolin NAC

Treatments for AD include cholinesterase inhibitors and the NMDA receptor antagonist memantine,89 although these are only indicated for patients with established disease.  Cerefolin NAC provides a unique option in early AD and MCI by addressing inter-related mechanisms associated with oxidative stress and B-vitamin deficiency (Figure 1). Open-label trials adopting a similar synergistic approach show considerable promise in early90 and late-stage AD.91

Unlike other folate supplements which contain synthetic folic acid, Cerefolin NAC contains the naturally occurring 5-MTHF (5.6 mg). This has an important advantage over folic acid. Folic acid can inhibit transport of 5-MTHF across the BBB.92 Hence, an accumulation of unmetabolized folic acid resulting from the use of alternative supplements might actually be detrimental in treating CNS disorders.

Cerefolin NAC also comprises N-acetylcysteine (NAC) (600 mg)—a membrane-permeable cysteine precursor rapidly hydrolyzed intracellularly to cysteine, a precursor of glutathione (GSH). Cysteine availability is the rate-limiting step in GSH synthesis. GSH is a major component of pathways protecting cells from oxidative stress and apoptosis. Other commonly used antioxidants, including vitamin C, vitamin K, and lipoic acid, neutralize free radicals but cannot replenish cysteine required for GSH synthesis.93 NAC itself is also an antioxidant and free-radical scavenger, and can additionally lower Hcy levels by increasing urinary excretion.94 In a double-blind trial of patients with probable AD, NAC improved nearly every outcome measure, although significant differences were obtained only for a subset of cognitive tasks.95

The third component of Cerefolin NAC is methylcobalamin (2 mg)—the co-factor for MS in the conversion of Hcy to methionine. High-dose oral vitamin B12 (1–2 mg/day) is as effective as intramuscular administration.96 GSH is required for  intracellular cobalamin processing.23,97 Hence, Cerefolin NAC might have advantages over other methylcobalamin formulations. Cobalamin itself might also act as a ROS scavenger,98 suppressing apoptosis and preventing cellular damage.99,100

 

Clinical Trials

Although Hcy-reducing clinical trials regarding dementia are disappointing, there are several important caveats.101  Most trials to date are of insufficient size and short duration.2 Also, lowering Hcy addresses only one of several pro-inflammatory mechanisms promoting oxidant stress and neurotoxicity.  Completed trials have only included patients with mildly elevated Hcy levels; the role of Hcy reduction in patients with more robustly elevated levels for both primary prevention and therapeutic treatment of dementia remains unknown.

Nevertheless, a recent expert review concluded that folate, B12, and Hcy levels should be determined in all dementia patients, and abnormal levels should be treated.83  Substitution of these vitamins may also improve cognitive function in the absence of overt deficiency.83 Given the close inter-dependent relationship between Hcy and oxidative stress, it is prudent to simultaneously administer antioxidants such as NAC when correcting such deficiencies. Several case studies, and two open-label studies, confirm the benefits of this synergistic approach.102,103,90,91

 

Summary

Neuroinflammatory oxidative stress occurs early in AD pathology. Elevated blood Hcy is a useful marker for such neuroinflammation. Hcy contributes to pathological cascades involving AP and NFTs. In AD, Hcy should be lowered by B-vitamin supplements and NAC.

 

References

1.   Alzheimer’s Disease International. World Alzheimer Report. 1-96. 21-9-2009. Available at: http://www.alz.co.uk/research/worldreport/. Accessed January 2010.
2.   Smith AD. The worldwide challenge of the dementias: a role for B vitamins and homocysteine? Food Nutr Bull. 2008;29:S143-S172.
3.   Moretti R, Torre P, Antonello RM, Cattaruzza T, Cazzato G, Bava A. Vitamin B12 and folate depletion in cognition: a review. Neurol India. 2004;52:310-318.
4.   Homocysteine in Health and Disease. Carmel R, Jacobsen DW, eds. Cambridge University Press. 2001.
5.   Pennypacker LC, Allen RH, Kelly JP, et al. High prevalence of cobalamin deficiency in elderly outpatients. J Am Geriatr Soc. 1992;40:1197-1204.
6.   Selhub J, Jacques PF, Wilson PW, Rush D, Rosenberg IH. Vitamin status and intake as primary determinants of homocysteinemia in an elderly population. JAMA. 1993;270:2693-2698.
7.   McCaddon A, Kelly CL. Alzheimer’s disease: a ‘cobalaminergic’ hypothesis. Med Hypotheses  1992;37:161-165.
8.   Regland B, Gottfries CG. Slowed synthesis of DNA and methionine is a pathogenetic mechanism common to dementia in Down’s syndrome, AIDS and Alzheimer’s disease? Med Hypotheses. 1992;38:11-19.
9.   Rosenberg IH, Miller J. Nutritional factors in physical and cognitive functions of elderly people. Am J Clin Nutr. 1992;55:1237s-1243s.
10. Miller AL. The methionine-homocysteine cycle and its effects on cognitive diseases. Altern Med Rev. 2003;8:7-19.
11. Zhou J, Austin RC. Contributions of hyperhomocysteinemia to atherosclerosis: causal relationship and potential mechanisms. Biofactors. 2009;35:120-129.
12. Qiu C, Kivipelto M, von Strauss E. Epidemiology of Alzheimer’s disease: occurrence, determinants, and strategies toward intervention. Dialogues Clin Neurosci. 2009;11:111-128.
13. McCaddon A, Davies G, Hudson P, Tandy S, Cattell H. Total serum homocysteine in senile dementia of Alzheimer type. Int J Geriatr Psychiatry. 1998;13:235-239.
14. Clarke R, Smith AD, Jobst KA, Refsum H, Sutton L, Ueland PM. Folate, vitamin B12, and serum total homocysteine levels in confirmed Alzheimer disease. Arch Neurol. 1998;55:1449-1455.
15. Lehmann M, Gottfries CG, Regland B. Identification of cognitive impairment in the elderly: homocysteine is an early marker. Dement Geriatr Cogn Disord. 1999;10:12-20.
16. Malaguarnera M, Ferri R, Bella R, Alagona G, Carnemolla A, Pennisi G. Homocysteine, vitamin B12 and folate in vascular dementia and in Alzheimer disease. Clin Chem Lab Med. 2004;42:1032-1035.
17. McCaddon A, Hudson P, Davies G, Hughes A, Williams JH, Wilkinson C. Homocysteine and cognitive decline in healthy elderly. Dement Geriatr Cogn Disord. 2001;12:309-313.
18. Seshadri S, Beiser A, Selhub J, et al. Plasma homocysteine as a risk factor for dementia and Alzheimer’s disease. N Engl J Med. 2002;346:476-483.
19. Ravaglia G, Forti P, Maioli F, et al. Homocysteine and folate as risk factors for dementia and Alzheimer disease. Am J Clin Nutr. 2005;82:636-643.
20. Karnaze DS, Carmel R. Low serum cobalamin levels in primary degenerative dementia. Do some patients harbor atypical cobalamin deficiency states? Arch Intern Med. 1987;147:429-431.
21. McCaddon A, Tandy S, Hudson P, et al. Absence of macrocytic anaemia in Alzheimer’s disease. Clin Lab Haematol. 2004;26:259-263.
22. McIlroy SP, Dynan KB, Lawson JT, Patterson CC, Passmore AP. Moderately elevated plasma homocysteine, methylenetetrahydrofolate reductase genotype, and risk for stroke, vascular dementia, and Alzheimer disease in Northern Ireland. Stroke. 2002;33:2351-2356.
23. McCaddon A, Regland B, Hudson P, Davies G. Functional vitamin B(12) deficiency and Alzheimer disease. Neurology. 2002;58:1395-1399.
24. Fuchs D, Jaeger M, Widner B, Wirleitner B, Artner-Dworzak E, Leblhuber F. Is hyperhomocysteinemia due to the oxidative depletion of folate rather than to insufficient dietary intake? Clin Chem Lab Med. 2001;39:691-694.
25. Sultana R, Butterfield DA. Role of oxidative stress in the progression of Alzheimer’s disease. J Alzheimers Dis. Epub 2009 Sep 11.
26. Lovell MA, Markesbery WR. Oxidative damage in mild cognitive impairment and early Alzheimer’s disease. J Neurosci Res. 2007;85:3036-3040.
27. Markesbery WR, Kryscio RJ, Lovell MA, Morrow JD. Lipid peroxidation is an early event in the brain in amnestic mild cognitive impairment. Ann Neurol. 2005;58:730-735.
28. McNaull BB, Todd S, McGuinness B, Passmore AP. Inflammation and anti-inflammatory strategies for Alzheimer’s disease – a mini-review. Gerontology. Epub 2009 Sep 10.
29. Wyss-Coray T. Inflammation in Alzheimer disease: driving force, bystander or beneficial response? Nat Med. 2006;12:1005-1015.
30. Moreira PI, Zhu X, Liu Q, et al. Compensatory responses induced by oxidative stress in Alzheimer disease. Biol Res. 2006;39:7-13.
31. Banerjee RV, Matthews RG. Cobalamin-dependent methionine synthase. FASEB J. 1990;4:1450-1459.
32. Scott JM, Weir DG. The methyl folate trap. A physiological response in man to prevent methyl group deficiency in kwashiorkor (methionine deficiency) and an explanation for folic-acid induced exacerbation of subacute combined degeneration in pernicious anaemia. Lancet.1981;2:337-340.
33. Molloy A, Weir G. Homocysteine and the nervous system. In: Carmel R, Jacobsen DW, eds. Homocysteine in Health and Disease.Cambridge University Press: Cambridge; 2001:183-197.
34. Onyango IG, Khan SM. Oxidative stress, mitochondrial dysfunction, and stress signaling in Alzheimer’s disease. Curr Alzheimer Res. 2006;3:339-349.
35. Pavlov PF, Petersen CH, Glaser E, Ankarcrona M. Mitochondrial accumulation of APP and Abeta: Significance for Alzheimer disease pathogenesis. J Cell Mol Med. Epub 2009 Sep 1.
36. Singh P, Suman S, Chandna S, Das TK. Possible role of amyloid-beta, adenine nucleotide translocase and cyclophilin-D interaction in mitochondrial dysfunction of Alzheimer’s disease. Bioinformation. 2009;3:440-445.
37. Gibson GE, Starkov A, Blass JP, Ratan RR, Beal MF. Cause and consequence: mitochondrial dysfunction initiates and propagates neuronal dysfunction, neuronal death and behavioral abnormalities in age-associated neurodegenerative diseases. Biochem biophys Acta. 2010;1802:122-134.
38. Green R, Miller JW. Vitamin B12. In: Zempleni J, Rucker RB, eds. Handbook of Vitamins.Taylor and Francis: Boca Raton, Florida; 2007:413-457.
39. Hubbard PA, Padovani D, Labunska T, Mahlstedt SA, Banerjee R, Drennan CL. Crystal structure and mutagenesis of the metallochaperone MeaB: insight into the causes of methylmalonic aciduria. J Biol Chem. 2007;282:31308-31316.
40. Kolker S, Schwab M, Horster F, et al. Methylmalonic acid, a biochemical hallmark of methylmalonic acidurias but no inhibitor of mitochondrial respiratory chain. J Biol Chem. 2003;278:47388-47393.
41. McCracken C, Hudson P, Ellis R, McCaddon A. Methylmalonic acid and cognitive function in the Medical Research Council Cognitive Function and Aging Study. Am J Clin Nutr. 2006;84:1406-1411.
42. Feijoo C, Campbell DG, Jakes R, Goedert M, Cuenda A. Evidence that phosphorylation of the microtubule-associated protein Tau by SAPK4/p38delta at Thr50 promotes microtubule assembly. J Cell Sci. 2005;118:397-408.
43. Stoothoff WH, Johnson GV. Tau phosphorylation: physiological and pathological consequences. Biochim Biophys Acta. 2005;1739:280-297.
44. Wang JZ, Grundke-Iqbal I, Iqbal K. Kinases and phosphatases and tau sites involved in Alzheimer neurofibrillary degeneration. Eur J Neurosci. 2007;25:59-68.
45. Vafai SB, Stock JB. Protein phosphatase 2A methylation: a link between elevated plasma homocysteine and Alzheimer’s Disease. FEBS Lett. 2002;518:1-4.
46. Obeid R, Kasoha M, Knapp JP, et al. Folate and methylation status in relation to phosphorylated tau protein(181P) and {beta}-amyloid(1-42) in cerebrospinal fluid. Clin Chem. 2007;53:1129-1136.
47. Sontag E, Nunbhakdi-Craig V, Sontag JM, et al. Protein phosphatase 2A methyltransferase links homocysteine metabolism with tau and amyloid precursor protein regulation. J Neurosci. 2007;27:2751-2759.
48. Sontag JM, Nunbhakdi-Craig V, Montgomery L, Arning E, Bottiglieri T, Sontag E. Folate deficiency induces in vitro and mouse brain region-specific downregulation of leucine carboxyl methyltransferase-1 and protein phosphatase 2A B(alpha) subunit expression that correlate with enhanced tau phosphorylation. J Neurosci. 2008;28:11477-11487.
49. Zhang CE, Tian Q, Wei W, et al. Homocysteine induces tau phosphorylation by inactivating protein phosphatase 2A in rat hippocampus. Neurobiol Aging. 2008;29:1654-1665.
50. Chan A, Rogers E, Shea TB. Dietary deficiency in folate and vitamin E under conditions of oxidative stress increases phospho-tau Levels: potentiation by ApoE4 and alleviation by s-adenosylmethionine. J Alzheimers Dis. 2009;110:831-836.
51. Nicolia V, Fuso A, Cavallaro RA, Di Luzio A, Scarpa S. B vitamin deficiency promotes tau phosphorylation through regulation of GSK3beta and PP2A. J Alzheimers Dis. Epub 2009 Nov 17.
52. Balastik M, Lim J, Pastorino L, Lu KP. Pin1 in Alzheimer’s disease: multiple substrates, one regulatory mechanism? Biochem Biophys Acta. 2007;1772:422-429.
53. Sultana R, Boyd-Kimball D, Poon HF, et al. Oxidative modification and down-regulation of Pin1 in Alzheimer’s disease hippocampus: a redox proteomics analysis. Neurobiol Aging. 2006;27:918-925.
54. Meziane H, Dodart JC, Mathis C, et al. Memory-enhancing effects of secreted forms of the beta-amyloid precursor protein in normal and amnestic mice. Proc Natl Acad Sci U.S.A. 1998;95:12683-12688.
55. Mattson MP, Cheng B, Culwell AR, Esch FS, Lieberburg I, Rydel RE. Evidence for excitoprotective and intraneuronal calcium-regulating roles for secreted forms of the beta-amyloid precursor protein. Neuron. 1993;10:243-254.
56. Finder VH, Glockshuber R. Amyloid-beta aggregation. Neurodegener Dis. 2007;4:13-27.
57. Butterfield DA, Poon HF, St Clair D, et al. Redox proteomics identification of oxidatively modified hippocampal proteins in mild cognitive impairment: insights into the development of Alzheimer’s disease. Neurobiol Dis. 2006;22:223-232.
58. Butterfield DA, Abdul HM, Opii W, et al. Pin1 in Alzheimer’s disease. J Neurochem. 2006;98:1697-1706.
59. Chen F, Hasegawa H, Schmitt-Ulms G, et al. TMP21 is a presenilin complex component that modulates gamma-secretase but not epsilon-secretase activity. Nature. 2006;440:1208-1212.
60. Willnow TE, Andersen OM. Pin-pointing APP processing. Mol Interv. 2006;6:137-139.
61. Fuso A, Seminara L, Cavallaro RA, D’Anselmi F, Scarpa S. S-adenosylmethionine/homocysteine cycle alterations modify DNA methylation status with consequent deregulation of PS1 and BACE and beta-amyloid production. Mol Cell Neurosci. 2005;28:195-204.
62. Zhang CE, Wei W, Liu YH, et al. Hyperhomocysteinemia increases beta-amyloid by enhancing expression of gamma-secretase and phosphorylation of amyloid precursor protein in rat brain. Am J Pathol. 2009;174:1481-1491.
63. Ho PI, Collins SC, Dhitavat S, et al. Homocysteine potentiates beta-amyloid neurotoxicity: role of oxidative stress. J Neurochem. 2001;78:249-253.
64. Hansson O, Zetterberg H, Buchhave P, Londos E, Blennow K, Minthon L. Association between CSF biomarkers and incipient Alzheimer’s disease in patients with mild cognitive impairment: a follow-up study. Lancet Neurol. 2006;5:228-234.
65. Blennow K, Zetterberg H. Cerebrospinal fluid biomarkers for Alzheimer’s disease. J Alzheimers Dis. 2009;18:413-417.
66. Lanari A, Parnetti L. Cerebrospinal fluid biomarkers and prediction of conversion in patients with mild cognitive impairment: 4-year follow-up in a routine clinical setting. ScientificWorldJournal. 2009;9:961-966.
67. Mattsson N, Zetterberg H, Hansson O, et al. CSF biomarkers and incipient Alzheimer disease in patients with mild cognitive impairment. JAMA. 2009;302:385-393.
68. Popp J, Lewczuk P, Linnebank M, et al. Homocysteine metabolism and cerebrospinal fluid markers for Alzheimer’s disease. J Alzheimers Dis. Epub 2009 Aug 3.
69. Glodzik-Sobanska L, Pirraglia E, Brys M, et al. The effects of normal aging and ApoE genotype on the levels of CSF biomarkers for Alzheimer’s disease. Neurobiol Aging. 2009;30:672-681.
70. Williams TI, Lynn BC, Markesbery WR, Lovell MA. Increased levels of 4-hydroxynonenal and acrolein, neurotoxic markers of lipid peroxidation, in the brain in mild cognitive impairment and early Alzheimer’s disease. Neurobiol Aging. 2006;27:1094-1099.
71. Doraiswamy PM. Non-cholinergic strategies for treating and preventing Alzheimer’s disease. CNS Drugs. 2002;16:811-824.
72. Magnusson KR. The aging of the NMDA receptor complex. Front Biosci. 1998;3:e70-e80.
73. Do KQ, Herrling PL, Streit P, Cuenod M. Release of neuroactive substances: homocysteic acid as an endogenous agonist of the NMDA receptor. J Neural Transm. 1988;72:185-190.
74. Lipton SA, Kim WK, Choi YB, et al. Neurotoxicity associated with dual actions of homocysteine at the N-methyl-D-aspartate receptor. Proc Natl Acad Sci U S A. 1997;94:5923-5928.
75. Blusztajn JK, Wurtman RJ. Choline and cholinergic neurons. Science. 1983;221:614-620.
76. Chanarin I, Deacon R, Lumb M, Muir M, Perry J. Cobalamin-folate interrelations: a critical review. Blood. 1985;66:479-489.
77. Hirata F, Axelrod J. Phospholipid methylation and biological signal transmission. Science. 1980;209:1082-1090.
78. Pogribny IP, Basnakian AG, Miller BJ, Lopatina NG, Poirier LA, James SJ. Breaks in genomic DNA and within the p53 gene are associated with hypomethylation in livers of folate/methyl-deficient rats. Cancer Res. 1995;55:1894-1901.
79. Fenech M. The role of folic acid and vitamin B12 in genomic stability of human cells. Mutat Res. 2001;475:57-67.
80. Kruman II, Culmsee C, Chan SL, et al. Homocysteine elicits a DNA damage response in neurons that promotes apoptosis and hypersensitivity to excitotoxicity. J Neurosci. 2000;20:6920-6926.
81. Blount BC, Mack MM, Wehr CM, et al. Folate deficiency causes uracil misincorporation into human DNA and chromosome breakage: implications for cancer and neuronal damage. Proc Natl Acad Sci USA. 1997;94:3290-3295.
82. Meli E, Pangallo M, Baronti R, et al. Poly(ADP-ribose) polymerase as a key player in excitotoxicity and post-ischemic brain damage. Toxicol Lett. 2003;139:153-162.
83. Stanger O, Fowler B, Piertzik K, et al. Homocysteine, folate and vitamin B12 in neuropsychiatric diseases: review and treatment recommendations. Expert Rev Neurother. 2009;9:1393-1412.
84. Morris MS. Homocysteine and Alzheimer’s disease. Lancet Neurol. 2003;2:425-428.
85. Sachdev PS. Homocysteine and brain atrophy. Prog Neuropsychopharmacol Biol Psychiatry. 2005;29:1152-1161.
86. Den Heijer T, Vermeer SE, Clarke R, et al. Homocysteine and brain atrophy on MRI of non-demented elderly. Brain. 2003;126:170-175.
87. Kamath AF, Chauhan AK, Kisucka J, et al. Elevated levels of homocysteine compromise blood-brain barrier integrity in mice. Blood. 2006;107:591-593.
88. Lehmann M, Regland B, Blennow K, Gottfries CG. Vitamin b(12)-b(6)-folate treatment improves blood-brain barrier function in patients with hyperhomocysteinaemia and mild cognitive impairment. Dement Geriatr Cogn Disord. 2003;16:145-150.
89. Smith DA. Treatment of Alzheimer’s disease in the long-term-care setting. Am J Health Syst Pharm. 2009;66:899-907.
90. Chan A, Paskavitz J, Remington R, Rasmussen S, Shea TB. Efficacy of a vitamin/nutriceutical formulation for early-stage Alzheimer’s disease: a 1-year, open-label pilot study with an 16-month caregiver extension. Am J Alzheimers Dis Other Demen. 2008;23:571-585.
91. Remington R, Chan A, Paskavitz J, Shea TB. Efficacy of a vitamin/nutriceutical formulation for moderate-stage to later-stage Alzheimer’s disease: a placebo-controlled pilot study. Am J Alzheimers Dis Other Demen. 2009;24:27-33.
92. Wollack JB, Makori B, Ahlawat S, et al. Characterization of folate uptake by choroid plexus epithelial cells in a rat primary culture model. J Neurochem. 2008;104:1494-1503.
93. Atkuri KR, Mantovani JJ, Herzenberg LA, Herzenberg LA. N-Acetylcysteine-a safe antidote for cysteine/glutathione deficiency. Curr Opin Pharmacol. 2007;7:355-359.
94. Ventura P, Panini R, Abbati G, Marchetti G, Salvioli G. Urinary and plasma homocysteine and cysteine levels during prolonged oral N-acetylcysteine therapy. Pharmacology. 2003;68:105-114.
95. Adair JC, Knoefel JE, Morgan N. Controlled trial of N-acetylcysteine for patients with probable Alzheimer’s disease. Neurology. 2001;57:1515-1517.
96. Butler CC, Vidal-Alaball J, Cannings-John R, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Fam Pract. 2006;23:279-285.
97. Kim J, Hannibal L, Gherasim C, Jacobsen DW, Banerjee R. A human B12 trafficking protein uses glutathione transferase activity for processing alkylcobalamins. J Biol Chem. 2009;284:33418-33424.
98. Suarez-Moreira E, Yun J, Birch CS, Williams JH, McCaddon A, Brasch NE. Vitamin B(12) and redox homeostasis: cob(II)alamin reacts with superoxide at rates approaching superoxide dismutase (SOD). J Am Chem Soc. 2009;131:15078-15079.
99. Birch CS, Brasch NE, McCaddon A, Williams JH. A novel role for vitamin B(12): Cobalamins are intracellular antioxidants in vitro. Free Radic Biol Med. 2009;47:184-188.
100. Richard E, Jorge-Finnigan A, Garcia-Villoria J, et al. Genetic and cellular studies of oxidative stress in methylmalonic aciduria (MMA) cobalamin deficiency type C (cblC) with homocystinuria (MMACHC). Hum Mutat. 2009;30:1558-1566.
101. Maron BA, Loscalzo J. The treatment of hyperhomocysteinemia. Annu Rev Med. 2009;60:39-54.
102. McCaddon A. Homocysteine and cognitive impairment; a case series in a general practice setting. Nutr J. 2006;5:6.
103. McCaddon A, Davies G. Co-administration of N-acetylcysteine, vitamin B12 and folate in cognitively impaired hyperhomocysteinaemic patients. Int J Geriatr Psychiatry. 2005;20:998-1000.

Return