Biomarkers of Alzheimer disease in plasma

Plasma and serum biochemical markers proposed for Alzheimer disease (AD) are based on pathophysiologic processes such as amyloid plaque formation [amyloid β-protein (Aβ), Aβ autoantibodies, platelet amyloid precursor protein (APP) isoforms], inflammation (cytokines), oxidative stress (vitamin E, isoprostanes), lipid metabolism (apolipoprotein E, 24S-hydroxycholesterol), and vascular disease [homocysteine, lipoprotein (a)]. Most proteins or metabolites evaluated in plasma or serum thus far are, at best, biological correlates of AD: levels are statistically different in AD versus controls in some cohorts, but they lack sensitivity or specificity for diagnosis or for tracking response to therapy. Approaches combining panels of existing biomarkers or surveying the range of proteins in plasma (proteomics) show promise for discovering biomarker profiles that are characteristic of AD, yet distinct from nondemented patients or patients with other forms of dementia.     

Ryanodine receptor-mediated [Ca(2+)](i) release in glomus cells is independent of natural stimuli and does not participate in the chemosensory responses of the rat carotid body

Advanced glycation endproducts (AGEs), protein-bound oxidation products of sugars, have been shown to be involved in the pathophysiological processes of Alzheimer’s disease (AD). AGEs induce the expression of various pro-inflammatory cytokines and the inducible nitric oxide synthase (iNOS) leading to a state of oxidative stress. AGE modification and resulting crosslinking of protein deposits such as amyloid plaques may contribute to the oxidative stress occurring in AD. The aim of this study was to immunohistochemically compare the localization of AGEs and β-amyloid (Aβ) with iNOS in the temporal cortex (Area 22) of normal and AD brains. In aged normal individuals as well as early stage AD brains (i.e. no pathological findings in isocortical areas), a few astrocytes showed co-localization of AGE and iNOS in the upper neuronal layers, compared with no astrocytes detected in young controls. In late AD brains, there was a much denser accumulation of astrocytes co-localized with AGE and iNOS in the deeper and particularly upper neuronal layers. Also, numerous neurons with diffuse AGE but not iNOS reactivity and some AGE and iNOS-positive microglia were demonstrated, compared with only a few AGE-reactive neurons and no microglia in controls. Finally, astrocytes co-localized with AGE and iNOS as well as AGE and were found surrounding mature but not diffuse amyloid plaques in the AD brain. Our results show that AGE-positive astrocytes and microglia in the AD brain express iNOS and support the evidence of an AGE-induced oxidative stress occurring in the vicinity of the characteristic lesions of AD. Hence activation of microglia and astrocytes by AGEs with subsequent oxidative stress and cytokine release may be an important progression factor in AD.     

The plasma membrane redox system is impaired by amyloid β-peptide and in the hippocampus and cerebral cortex of 3xTgAD mice

Membrane-associated oxidative stress has been implicated in the synaptic dysfunction and neuronal degeneration that occurs in Alzheimer's disease (AD), but the underlying mechanisms are unknown. Enzymes of the plasma membrane redox system (PMRS) provide electrons for energy metabolism and recycling of antioxidants. Here, we show that activities of several PMRS enzymes are selectively decreased in plasma membranes from the hippocampus and cerebral cortex of 3xTgAD mice, an animal model of AD. Our results that indicate the decreased PMRS enzyme activities are associated with decreased levels of coenzyme Q₁₀ and increased levels of oxidative stress markers. Neurons overexpressing the PMRS enzymes (NQO1 or cytochrome b5 reductase) exhibit increased resistance to amyloid β-peptide (Aβ). If and to what extent Aβ is the cause of the impaired PMRS enzymes in the 3xTgAD mice is unknown. Because these mice also express mutant tau and presenilin-1, it is possible that one or more of the PMRS could be adversely affected by these mutations. Nevertheless, the results of our cell culture studies clearly show that exposure of neurons to Aβ1–42 is sufficient to impair PMRS enzymes. The impairment of the PMRS in an animal model of AD, and the ability of PMRS enzyme activities to protect neurons against Aβ-toxicity, suggest enhancement PMRS function as a novel approach for protecting neurons against oxidative damage in AD and related disorders.     

Cognitive impairment and Alzheimer’s disease: Links with oxidative stress and cholesterol metabolism

Oxidative stress has been implicated in the progression of a number of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease and amyotrophic lateral sclerosis. We carried out an in-depth study of cognitive impairment and its relationships with oxidative stress markers such as ferric-reducing ability of plasma (FRAP), plasma malondialdehyde and total antioxidative capacity (TAC), as well as cholesterol parameters, in two subsets of subjects, AD patients (n = 59) and a control group of neurologically normal subjects (n = 29), attending the University Hospital Salvador in Santiago, Chile. Cognitive impairment was assessed by a set of neuropsychological tests (Mini-Mental State Examination, Boston Naming Test, Ideomotor Praxia by imitation, Semantic Verbal Fluency of animals or words with initial A, Test of Memory Alteration, Frontal Assessment Battery), while the levels of those oxidative stress markers and cholesterol metabolism parameters were determined according with standard bioassays in fresh plasma samples of the two subgroups of patients. No significant differences were observed when the cholesterol parameters (low-, high-density lipoprotein, total cholesterol) of the AD group were compared with normal controls. Interestingly, a correlation was evidenced when the levels of cognitive impairment were analyzed with respect to the plasma antioxidant capacity (AOC) of patients. In this context, the subset of subjects exhibiting cognitive impairment were divided into two subgroups according with their Global Dementia Scale performance: a subgroup with mild AD and a subgroup with moderate to severe AD. Significant differences in AOC were found between subgroups. The different correlations between cognitive impairment of subgroups of subjects with the oxidative stress profile are discussed in the context of AD pathogenesis.     

Cerebrospinal fluid, serum and plasma protein oxidation in Alzheimer's disease

Objectives – Many studies have shown differences in carbonylation and nitration of individual proteins in brain and body fluids of Alzheimer’s disease (AD) patients. Therefore, we wanted to examine whether total levels of these oxidative stress markers of proteins were altered in AD. Patients and methods – Total levels of carbonyls and nitrotyrosine in cerebrospinal fluid, serum and plasma were measured in 22 AD patients and 18 age‐matched controls using commercially available enzyme immunoassay kits. Results – Protein carbonylation in cerebrospinal fluid did not differ between AD patients and controls but was decreased in APOE ?4 carriers as compared with non‐carriers. Serum but not plasma levels of carbonyls tended to be decreased in AD patients as compared with aged controls. Nitrotyrosine concentrations did not differ between the groups. Surrogate cerebrospinal fluid markers for AD, beta‐amyloid (1–42) and tau, correlated with blood carbonyl and nitrotyrosine levels. Conclusions – According to these preliminary data, changes in oxidative metabolism related to the pathogenesis of AD cannot be detected as increased cerebrospinal fluid, serum or plasma protein carbonylation or nitration.     

Zinc and its effects on oxidative stress in Alzheimer’s disease

Alzheimer’s disease (AD) is a progressive age-related neurodegenerative disorder. The patho-physiological characteristic of AD is abnormal deposition of fibrillar amyloid β protein, intracellular neurofibrillary tangles, oxidative damage and neuronal death in the brain. Zinc is an important trace element in human body regulating many physiological processes. Increasing evidence suggests that the etiology of AD may involve disruptions of zinc homeostasis, and oxidative stress facilitating reactive oxygen species production is an early and sustained event in AD disease progression. Both Zn deficiency and Zn overload may affect cellular Zn distribution and be linked to neurodegeneration in AD. Meanwhile, Zn may play paradoxical roles in initiating and inhibiting oxidative stress and neurotoxicity. This review will focus on aspects of the role of zinc in AD, which includes a large body of research regarding zinc dyshomeostasis and its relation with oxidative stress.     

Human serum transthyretin levels correlate inversely with Alzheimer's disease

Alzheimer's disease (AD) is histopathologically characterized by the presence of senile plaques, neurofibrillary tangles, and synapse loss. The main component of senile plaques is amyloid β-peptide (Aβ), which has been shown to induce oxidative stress in in vitro and in vivo studies. AD is associated with elevated levels of oxidative damage in brain and peripheral lymphocytes. Further Aβ has been found to be accumulated in mitochondria, which might contribute to the reported alterations in the mitochondrial morphology, and impaired mitochondrial energy metabolism in AD brain. Biomarkers are desperately needed for earlier diagnosis of AD and to monitor efficacy of new therapies. Hence, in the present study we show that markers of oxidative damage are elevated in mitochondria isolated from AD lymphocytes suggesting that these oxidative stress indices potentially could serve as a viable biomarker for AD.     

Genes and susceptible loci of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common and devastating neurodegenerative disease of the elderly. Many research findings on familial AD suggest that the mechanisms of the pathogenesis of the disorder is more complex although the overall neuropathology of all cases of AD is surprisingly very similar. Genetic studies on some families have shown that mutations in the genes encoding β-amyloid precursor protein and presenilins 1 and 2 are responsible for early-onset AD. In addition, apolipoprotein E gene allele E4 and the bleomycin hydrolase locus are shown to be genetic risk factors for late-onset AD in certain sporadic cases. Mitochondrial dysfunctions and age-related oxidative stress may also contribute to degenerative processes in AD. Although several studies support the amyloid cascade hypothesis as the mechanism of the disease, transgenic experiments and recent findings on a variant form of an AD family suggest that Aβ deposition may not be sufficient to cause AD. Identification in the future of other genetic, environmental, and age-related factors, may provide additional targets for therapies.     

Oxidatively modified proteins in Alzheimer’s disease (AD), mild cognitive impairment and animal models of AD: role of Abeta in pathogenesis

Oxidative stress has been implicated in the pathogenesis of a number of diseases including Alzheimer’s disease (AD). The oxidative stress hypothesis of AD pathogenesis, in part, is based on β-amyloid peptide (Aβ)-induced oxidative stress in both in vitro and in vivo studies. Oxidative modification of the protein may induce structural changes in a protein that might lead to its functional impairment. A number of oxidatively modified brain proteins were identified using redox proteomics in AD, mild cognitive impairment (MCI) and Aβ models of AD, which support a role of Aβ in the alteration of a number of biochemical and cellular processes such as energy metabolism, protein degradation, synaptic function, neuritic growth, neurotransmission, cellular defense system, long term potentiation involved in formation of memory, etc. All the redox proteomics-identified brain proteins fit well with the appearance of the three histopathological hallmarks of AD, i.e., synapse loss, amyloid plaque formation and neurofibrillary tangle formation and suggest a direct or indirect association of the identified proteins with the pathological and/or biochemical alterations in AD. Further, Aβ models of AD strongly support the notion that oxidative stress induced by Aβ may be a driving force in AD pathogenesis. Studies conducted on arguably the earliest stage of AD, MCI, may elucidate the mechanism(s) leading to AD pathogenesis by identifying early markers of the disease, and to develop therapeutic strategies to slow or prevent the progression of AD. In this review, we summarized our findings of redox proteomics identified oxidatively modified proteins in AD, MCI and AD models.     

Dual pathways mediate β‐amyloid stimulated glutathione release from astrocytes

Oxidative stress plays an important role in the progression of Alzheimer's disease (AD) and other neurodegenerative conditions. Glutathione (GSH), the major antioxidant in the central nervous system, is primarily synthesized and released by astrocytes. We determined if β‐amyloid (Aβ42), crucially involved in Alzheimer's disease, affected GSH release. Monomeric Aβ (mAβ) stimulated GSH release from cultured cortical astrocytes more effectively than oligomeric Aβ (oAβ) or fibrillary Aβ (fAβ). Monomeric Aβ increased the expression of the transporter ABCC1 (also referred to as MRP1) that is the main pathway for GSH release. GSH release from astrocytes, with or without mAβ stimulation, was reduced by pharmacological inhibition of ABCC1. Astrocytes robustly express connexin proteins, especially connexin43 (Cx43), and mAβ also stimulated Cx43 hemichannel‐mediated glutamate and GSH release. Aβ‐stimulation facilitated hemichannel opening in the presence of normal extracellular calcium by reducing astrocyte cholesterol level. Aβ treatment did not alter the intracellular concentration of reduced or oxidized glutathione. Using a mouse model of AD with early onset Aβ deposition (5xFAD), we found that cortical ABCC1 was significantly increased in temporal register with the surge of Aβ levels in these mice. ABCC1 levels remained elevated from 1.5 to 3.5 months of age in 5xFAD mice, before plunging to subcontrol levels when amyloid plaques appeared. Similarly, in cultured astrocytes, prolonged incubation with aggregated Aβ, but not mAβ, reduced induction of ABCC1 expression. These results support the hypothesis that in the early stage of AD pathogenesis, less aggregated Aβ increases GSH release from astrocytes (via ABCC1 transporters and Cx43 hemichannels) providing temporary protection from oxidative stress which promotes AD development. GLIA 2015;63:2208–2219     

蛋白质氧化与阿尔茨海默病

阿尔茨海默病（AD）是与老年相关的进展性神经变性疾病,主要分子病理学特征为脑内淀粉样蛋白（Aβ）沉积形成的老年斑和过度磷酸化tau蛋白所形成的神经纤维缠结.氧化应激是包括AD在内的神经变性病的一个重要发病机制.本文介绍了氧化应激后蛋白质发生改变的类型、AD进展期间脑内蛋白质的氧化、Aβ沉积和tau蛋白过度磷酸化的原因以及今后对AD进行预防与治疗的一些策略.     

Amyloid beta peptide, 4-hydroxynonenal and apoptosis

Considerable evidence suggests a role for oxidative stress in the pathogenesis of neuron degeneration in several neurodegenerative disorders including Alzheimer's disease (AD). Although debated, increasing evidence suggests that oxidative stress/damage (amyloid beta peptide, iron/hydrogen peroxide) or neurotoxic by-products of lipid peroxidation (4-hydroxy-2-nonenal, acrolein) lead to cell death through apoptosis or programmed cell death in AD. This review discusses current evidence supporting the role of oxidative stress/damage mediated apoptosis in in vitro models of neurodegeneration.     

Heat shock alters Alzheimer's β amyloid precursor protein expression in human endothelial cells

One of the pathological lesions in Alzheimer's disease (AD) is the amyloid or senile plaque. The plaque core is predominantly made up of amyloid beta peptide (Aβ), a 42–43 amino acid peptide derived from amyloid precursor protein (APP). APP is a membrane bound glycoprotein which is expressed ubiquitously is many cells. Although normal or pathological functions for APP are not well understood, several observations suggest that APP may play a role in cellular stress and inflammation at the endothelial cell/vascular barrier. APP is found in platelets and endothelial cells, it can inhibit a blood coagulation factor, and secreted APP can be neuroprotective. Changes in expression of APP during cellular stress or inflammation may contribute to pathological deposition of Aβ. In the present studies, expression of APP in human endothelial cells was examined following heat shock. In human umbilical vein endothelial cells (HUVECs) exposed to 42°C for 30 min, there was a five-to eight-fold increase in APP mRNA levels which peaked at 4 hr. The increase in APP mRNA was followed by an increase in APP protein immunoreactivity in the cytoplasm in a perinuclear Golgi-like region, and in discrete granular cytoplasmic structures. Immunoblot analysis of APP in the cell media found a transient increase in APP which peaked at 1 hr after heat shock. These results suggest that cellular stress induces the secretion of APP from endothelial cells followed by a subsequent increase in APP mRNA and protein synthesis. The upregulation of APP mRNA and protein supports a cellular stress role for APP. Wiley-Liss, Inc.     

Platelets and Alzheimer’s disease: Potential of APP as a biomarker

Platelets are the first peripheral source of amyloid precursor protein (APP). They possess the proteolytic machinery to produce Aβ and fragments similar to those produced in neurons, and thus offer an ex-vivo model to study APP processing and changes associated with Alzheimer’s disease (AD). Platelet process APP mostly through the α-secretase pathway to release soluble APP (sAPP). They produce small amounts of Aβ, predominantly Aβ₄₀ over Aβ₄₂. sAPP and Aβ are stored in α-granules and are released upon platelet activation by thrombin and collagen, and agents inducing platelet degranulation. A small proportion of full-length APP is present at the platelet surface and this increases by 3-fold upon platelet activation. Immunoblotting of platelet lysates detects APP as isoforms of 130 kDa and 106-110 kDa. The ratio of these of APP isoforms is significantly lower in patients with AD and mild cognitive impairment (MCI) than in healthy controls. This ratio follows a decrease that parallels cognitive decline and can predict conversion from MCI to AD. Alterations in the levels of α-secretase ADAM10 and in the enzymatic activities of α- and β-secretase observed in platelets of patients with AD are consistent with increased processing through the amyloidogenic pathway. β-APP cleaving enzyme activity is increased by 24% in platelet membranes of patients with MCI and by 17% in those with AD. Reports of changes in platelet APP expression with MCI and AD have been promising so far and merit further investigation as the search for blood biomarkers in AD, in particular at the prodromal stage, remains a priority and a challenge.     

Oxidative stress,perturbed calcium homeostasis,and immune dysfunction in Alzheimer's disease

Although Alzheimer's disease (AD) may not involve a transmissible agent, it does involve a pathogenic process similar to that of transmissible prion disorders (both involve a protein that adopts an abnormal pathogenic conformation in which it self-aggregates, forming amyloid deposits in and surrounding neurons) and viral dementias such as human immunodeficiency virus (HIV) encephalitis. The clinical presentation of patients with AD is dominated by cognitive deficits and emotional disturbances that result from dysfunction and degeneration of neurons in the limbic system and cerebral cortex. The pathogenic process in the brain involves deposition of insoluble aggregates of amyloid beta-peptide, oxidative stress and calcium dysregulation in neurons, and activation of inflammatory cytokine cascades involving microglia. However, AD patients also exhibit alterations in immune function. Studies of lymphocytes and lymphoblast cell lines from AD patients and age-matched normal control patients have documented alterations in cytokine and calcium signaling and increased levels of oxidative stress in immune cells from the AD patients. Studies of the pathogenic actions of mutations in presenilins and amyloid precursor protein that cause early-onset familial AD have established central roles for perturbed cellular calcium homeostasis and oxidative stress in the neurodegenerative process. Presenilin and amyloid precursor protein (APP) mutations also increase oxidative stress and perturb calcium signaling in lymphocytes in ways that alter their production of cytokines that are critical for proper immune responses. Immune dysfunction occurs prior to clinical symptoms in mouse models of AD, and brain cytokine responses to immune challenge are altered in presenilin mutant mice, suggesting a causal role for altered immune function in the disease process. Interestingly, immunization of AD mice with amyloid beta-peptide can stimulate the immune system to remove amyloid from the brain and can ameliorate memory deficits, suggesting that it may be possible to prevent AD by bolstering immune function.     

Widespread distribution of reticulon‐3 in various neurodegenerative diseases

Reticulons are a group of membrane‐bound proteins involved in diverse cellular functions, and are suggested to act as inhibitors of β‐secretase enzyme 1 (BACE1) activity that cleaves amyloid precursor protein. Reticulons are known to accumulate in the dystrophic neurites of Alzheimer's disease (AD), and studies have suggested that alterations in reticulons, such as increased aggregation, impair BACE1 binding, increasing amyloid‐β production, and facilitating reticulon deposition in dystrophic neurites. To further characterize the cellular distribution of reticulon, we examined reticulon‐3 expression in cases of AD, Parkinson's disease, and diffuse Lewy body disease. A more widespread cellular distribution of reticulon‐3 was noted than in previous reports, including deposits in dystrophic neurites, neuropil threads, granulovacuolar degeneration, glial cells, morphologically normal neurons in both hippocampal pyramidal cell layer and cerebral neocortex, and specifically neurofibrillary tangles and Lewy bodies. These results are compatible with reticulon alterations as nonspecific downstream stress responses, consistent with its expression during periods of endoplasmic reticulum stress. This emphasizes the increasing recognition that much of the AD pathological spectrum represents a response to the disease rather than cause, and emphasizes the importance of examining upstream processes, such as oxidative stress, that have functional effects prior to the onset of structural alterations.     

The heat shock/oxidative stress connection

Involvement of free-radical oxidations in the aging process has been a topic of interest since Harman's original contribution. Because of the close association between aging and Alzheimer disease (AD) and the qualitative similarity in the neuropathology of both conditions, it has been proposed by many investigators that oxidative stress may be important in AD. If such modality of injury was indeed involved, one should expect to find markers of oxidation and heat shock (since free radicals are key mediators of heat-shock induction) in brains of patients with AD. In fact, several studies documented abnormal expression of antioxidant enzymes and heat-shock proteins (HSP) along with other markers of oxidation in AD brains. We showed that abnormally expressed antioxidant enzymes are topographically associated with senile plaques and neurofibrillary tangles, and that the activity of these enzymes is (contrary to what one would expect) markedly reduced. These findings have recently been confirmed by other investigators. Despite a large amount of evidence that suggests an association between oxidative stress and the pathogenesis of AD, it is not yet known whether oxidative stress is a cause or consequence of the disorder. Future research efforts regarding the oxidative stress hypothesis of AD should include attempts, at generating AD pathology by oxidative means in laboratory animals, determining the role and integrity of the heat-shock response in AD, as well as that of various antioxidant systems, growth factors, and hormones with antioxidant and neuroprotective properties.     

Lipid peroxidation and oxidative imbalance: early functional events in Alzheimer's disease

Alzheimer's disease (AD) is a growing public health problem worldwide. Clinically, AD is a progressive neurodegenerative disorder characterized by a global cognitive decline. Accumulating evidence indicates that reactive oxygen species-mediated reactions, particularly of neuronal lipids, are extensive in those AD brain areas directly involved in the disease processes. Traditional views claim that oxidative-mediated tissue injury in the AD brain is the result of neurodegeneration. In recent years, numerous investigations have pointed to the functional importance of oxidative imbalance as a crucial event in mediating AD pathogenesis. The availability of specific and sensitive markers to monitor in vivo oxidative stress, in combination with studies performed in living patients with clinical diagnosis of AD are helping us to elucidate these issues. The evidence we have accumulated so far clearly indicates that oxidative imbalance and subsequent oxidative stress are early events during the evolution of the disease, and secondary to specific mechanism(s) present in AD but not in other neurodegenerative diseases. These new concepts implicate that this phenomenon may play a more important role in AD pathogenesis than previously anticipated, and that any therapeutic intervention targeting oxidative stress should be initiated at the earliest possible stage of the disease.     

Plasma biomarkers for mild cognitive impairment and Alzheimer's disease

Purpose of review: With the move toward development of disease modifying treatments, there is a need for more specific diagnosis of early Alzheimer's disease (AD) and mild cognitive impairment (MCI), plasma biomarkers are likely to play an important role in this. We review the current state of knowledge on plasma biomarkers for MCI and AD, including unbiased proteomics and very recent longitudinal studies.Recent findings: With the use of proteomics methodologies, some proteins have been identified as potential biomarkers in plasma and serum of AD patients, including alpha-1-antitrypsin, complement factor H, alpha-2-macroglobulin, apolipoprotein J, apolipoprotein A-I. The findings of cross-sectional studies of plasma amyloid beta (Aβ) levels are conflicting, but some recent longitudinal studies have shown that low plasma Aβ1–42 or Aβ1–40 levels, or Aβ1–42/Aβ1–40 ratio may be markers of cognitive decline. Other potential biomarkers for MCI and AD reflecting a variety of pathophysiological processes have been assessed, including isoprostanes and homocysteine (oxidative stress), total cholesterol and ApoE4 allele (lipoprotein metabolism), and cytokines and acute phase proteins (inflammation). A panel of 18 signal proteins was reported as markers of MCI and AD.Summary: A variety of potential plasma biomarkers for AD and MCI have been identified, however the findings need replication in longitudinal studies. This area of research promises to yield interesting results in the near future.