Free radicals, proteolysis, and the degeneration of neurons in Alzheimer disease: how essential is the beta-amyloid link?

In 2024, the amyloid-cascade-hypothesis still remains a working hypothesis, no less but certainly no more

The amyloid-cascade-hypothesis of the pathogenesis of Alzheimer's disease (AD) was introduced 32 years ago, in 1992. From early on, this clear and straight forward hypothesis received a lot of attention, but also a lot of substantial criticism. Foremost, there have always been massive doubts that a complex age-associated disorder of the most intricate organ of the human body, the brain, can be explained by a linear, one-dimensional cause-and-effect model. The amyloid-cascade defines the generation, aggregation, and deposition of the amyloid beta peptide as the central pathogenic mechanism in AD, as the ultimate trigger of the disease, and, consequently, as the key pharmacological target. Certainly, the original 1992 version of this hypothesis has been refined by various means, and the 'formulating fathers' followed up with a few reappraisals and partly very open reflections in 2002, 2006, 2009, and 2016. However, up until today, for the supporters of this hypothesis, the central and initial steps of the cascade are believed to be driven by amyloid beta-even if now displayed somewhat more elaborate. In light of the recently published clinical results achieved with anti-amyloid antibodies, the controversy in the field about (1) the clinical meaningfulness of this approach, (2) the significance of clearance of the amyloid beta peptide, and last but not least (3) the relevance of the amyloid-cascade-hypothesis is gaining momentum. This review addresses the interesting manifestation of the amyloid-cascade-hypothesis as well as its ups and downs over the decades.

Characteristics of neuronal lipofuscin in the superior temporal gyrus in Alzheimer's disease do not differ from non-diseased controls: a comparison with disease-related changes in the superior frontal gyrus

Neuronal lipofuscin characteristics in the superior temporal gyrus from 21 patients with Alzheimer's disease (AD) and from 18 age-matched non-diseased subjects were compared with previously reported findings from the superior frontal gyrus. A discriminant function analysis of lipofuscin characteristics in the superior temporal gyrus did not provide a significant predictive level for cases whose diagnoses were correctly classified (56.4%, P=0.63). In contrast, AD-related decrease in the number of smaller lipofuscin regions in the neurons of the frontal gyrus was confirmed, and the same analysis of lipofuscin characteristics in this region gave a significant predictive level for membership of the AD group of 86.6% (P<0.001). The findings indicate that changes in neuronal lipofuscin related to AD, which may reflect an increased rate of lipofuscin formation, show differences between neocortical regions. This study provides additional information on the distribution of neuropathological characteristics in AD.

Risk factors for Alzheimer's disease: role of multiple antioxidants, non-steroidal anti-inflammatory and cholinergic agents alone or in combination in prevention and treatment

The etiology of Alzheimer's disease (AD) is not well understood. Etiologic factors, chronic inflammatory reactions, oxidative and nitrosylative stresses and high cholesterol levels are thought to be important for initiating and promoting neurodegenerative changes commonly found in AD brains. Even in familial AD, oxidative stress plays an important role in the early onset of the disease. Mitochondrial damage and proteasome inhibition represent early events in the pathogenesis of AD, whereas increased processing of amyloid precursor protein (APP) to beta-amyloid (Abeta) fragments (Abeta(40) and Abeta(42)) and formation of senile plaques and neurofibrillary tangles (NFTs) represent late events. We propose a hypothesis that in idiopathic AD, epigenetic components of neurons such as mitochondria, proteasomes and post-translation protein modifications (processing of amyloid precursor protein to beta-amyloid and hyperphosphorylation of tau), rather than nuclear genes, are the primary targets for the action of diverse groups of neurotoxins. Based on epidemiologic, laboratory and limited clinical studies, we propose that a combination of non steroidal anti-inflammatory drugs (NSAIDs) and appropriate levels and types of multiple micronutrients, including antioxidants, may be more effective than the individual agents in the prevention, and they, in combination with a cholinergic agent, may be more effective in the treatment of AD than the individual agents alone. In addition, agents, which can prevent formation of plaques or dissolve these plaques may further enhance the efficacy of our proposed treatment strategy.

Multiple antioxidants in the prevention and treatment of Alzheimer disease: analysis of biologic rationale

The etiology of Alzheimer disease (AD) is not well understood; therefore, neither prevention strategies nor long-term effective treatment modalities are available for this disease. Based on laboratory and clinical studies, it appears that reactive oxygen species (ROS) and reactive nitrogen species (RNS) that are generated extracellularly and intracellularly by various mechanisms are among the major intermediary risk factors that initiate and promote neurodegeneration in idiopathic AD. Therefore, multiple antioxidant supplements could be useful in the prevention of AD, and as an adjunct to standard therapy in the treatment of AD. The products of inflammatory reactions such as prostaglandins (PGs; PGE1 and PGA1), free radicals, cytokines, and complement proteins are neurotoxic. Nonsteroidal antiinflammatory drugs (NSAIDs), which inhibit the synthesis of PGs, reduce the rate of deterioration of cognitive functions in patients with advanced AD. Cholinergic drugs are routinely used in the treatment of AD to improve cognitive functions. Therefore, we propose that a combination of multiple antioxidants and NSAIDs may be more beneficial in the prevention of AD, and that this combination taken together with cholinergic drugs may be more effective in the treatment of AD than the individual agents alone. We also hypothesize that, in idiopathic AD, epigenetic components of neurons such as mitochondria, membranes, other membranous structures, and protein modifications--rather than the genes of neurons--are the primary targets for the action of neurotoxins including free radicals. In some familial AD, mutations in amyloid precursor protein and presenilins are associated with the risk of early onset of this disease; however, their mechanisms of action are not fully understood.

Mitochondrial genome lesions in the pathogenesis of sporadic Alzheimer's disease

BACKGROUND

The recent, magnificent results of molecular biology concerning beta-amyloid (betaA) metabolism in early onset Alzheimer's disease (AD) have generated a series of new findings and, in turn, a new etiological concept. Attention on the early events in the pathogenesis of AD has been shifted from the chromosomal abnormalities in the nucleus of nerve cells onto genetic changes in the mitochondrial genome. This offers a new pathogenetic approach which also opens new pharmacological challenges particularly for the episodic forms of AD.

OBJECTIVE

Alterations occurring at the mitochondrial genome result in major consequences of oxidative phosphorylation and, if a specific threshold is exceeded, they may constitute important causative events in the apoptosis of selected nerve cells. The fact that the main source of mitochondrial metabolism is its glucose turnover allows monitoring brain changes in glucose metabolism by 18F-2 deoxyglucose positron emission tomography. In the demented brain, a low glucose turnover causes a cholinergic deficit by decreasing the synthetic rate of acetyl coenzyme A (AcCoA). AcCoA represents the key substrate for the acetylation of choline to acetylcholine by choline acetyltransferase. The consistent energy need for AcCoA synthesis appears obvious when considering that 1 molecule of glucose generates just 2 molecules of AcCoA, but 38 molecules of ATP. In the brain, AcCoA is exclusively synthesized in the glycolitic pathway. Generation of betaA is increased if the synthetic rate of ATP drops below a critical threshold: under these conditions, the betaA precursor protein (betaAPP) is inserted only in part into synaptic membranes which have the highest betaAPP turnover. In conditions of short ATP supply, betaAPP is not split at the beta region by an ATP-activated protease and this results in a substantial increase in uncleaved betaA molecules.

CONCLUSION

Peroxidative alterations in mitochondrial DNA are of importance in degenerative diseases of postmitotic tissues, particularly in degenerative diseases. This offers a new pharmacological approach for the treatment of AD. Neurotrophic factors and estrogen seem to be the first pharmacological leads.

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.

Increased levels of alpha-1-antichymotrypsin in brains of patients with Alzheimer's disease correlate with activated astrocytes and are affected by APOE 4 genotype

Levels of alpha-1-antichymotrypsin (ACT) were higher in brain homogenates of patients with Alzheimer's disease (AD) than controls. Brain tissues from the same patients and controls were immunostained with antibodies specific for microglia or astrocytes, the leukocyte common antigen (CD45) and glial fibrillary acidic protein (GFAP), respectively. Both activated CD45 and GFAP cells were increased in AD. Astroglia were divided into scattered (CD45sc) and clustered microglia (CD45cl) or scattered (GFAPsc) and clustered astrocytes (GFAPcl). Clustered cells were defined according their tendency to form focal aggregates. CD45cl and GFAPcl cells were present only in AD brain, while CD45sc and GFAPsc positive cells were present either in AD or control brains, with AD brains showing increased numbers of both cell types. A positive correlation between brain ACT levels and the number of GFAPsc positive cells was present in AD. AD patients with APOE 4 allele showed increased levels of ACT and increased CD45sc positive cells. Elevated ACT levels in the brain of AD patients could be interpreted as a metabolic response of astrocytes which might modulate the potentially deleterious activation of microglia cells.

Pathogenesis of decreased glucose turnover and oxidative phosphorylation in ischemic and trauma-induced dementia of the Alzheimer type

The pathogenetic mechanisms causing a dementing brain disease after temporary ischemia, heat shock, or brain trauma are surveyed. These lesions increase beta amyloid precursor protein (beta APP) synthesis. This process is potentiated by an ischemic glutamate release that opens cellular Ca2+ channels, inhibiting glucose turnover and ATP production, which is, under these conditions, accompanied by the generation of beta amyloid (beta A), even in young persons. Beta amyloid starts a vicious circle by inactivating the glycolytic key enzyme, phosphofructokinase, which, with age, exhausts the functional reserve capacity of the brain. This demonstrates that beta A is an epiphenomenon of a dementing brain disease, triggered by the disturbance of glucose turnover and oxidative phosphorylation. Clinical studies have shown that a dementing brain disease can be clearly objectified and monitored by 18F-2-deoxyglucose PET studies. This paper looks briefly at pharmacologic approaches to this disease using models of temporary ischemia, the testing of 14C-deoxyglucose turnover, or examination with 31P magnetic resonance spectroscopy techniques. In conclusion, the key process of all dementing brain diseases of the Alzheimer type is a decreased glucose turnover and subsequently decreased oxidative phosphorylation, linked directly to a secondary amyloid formation and nerve cell atrophy.

Increased neuronal endocytosis and protease delivery to early endosomes in sporadic Alzheimer's disease: neuropathologic evidence for a mechanism of increased beta-amyloidogenesis

The early endosome is the first vacuolar compartment along the endocytic pathway. It is the site of internalization and initial processing of amyloid precursor protein (APP) and apolipoprotein E (ApoE), two proteins of etiological importance in Alzheimer's disease, and a putative site of beta-amyloid peptide (Abeta) formation. Here, we identify early endosomes in human pyramidal neurons, using specific compartmental markers and morphometry, and show that in Alzheimer's disease individual endosomes display up to 32-fold larger volumes than the normal average. Endosomal enlargement contributed to an average 2.5-fold larger total endosomal volume per neuron, implying a marked increase in endocytic activity. Endosomal alterations were evident in most pyramidal neurons in Alzheimer brain, detectable at early stages of the disease but absent in several other neurodegenerative disorders examined. In addition, mature and proenzyme forms of the proteases cathepsin B and cathepsin D, a candidate APP secretase, were identified in most early endosomes in Alzheimer brains but were detectable in only a minor proportion of endosomes in normal brain. Expression of the cation-dependent 46 kDa mannose 6-phosphate receptor was elevated in pyramidal neurons of Alzheimer brains, which could be a possible basis for the altered cathepsin trafficking pattern. Enhanced endocytic activity, coupled with increased trafficking to endosomes of proteases, which may have the ability under pathological conditions to generate Abeta, constitutes a potential mechanism by which beta-amyloidogenesis may become accelerated in sporadic AD and also be subject to influences by ApoE.

Rapid alteration of tau in oligodendrocytes after focal ischemic injury in the rat: involvement of free radicals

Glial inclusions containing the microtubule-associated protein tau are present in a variety of chronic neurodegenerative conditions. We now report a rapid and time-dependent increase of tau immunoreactivity within oligodendrocytes after focal cerebral ischemia in the rat. The number of tau positive oligodendrocytes in the ipsilateral subcortical white matter increased six- to eightfold by 40 minutes after permanent middle cerebral artery occlusion (MCAO). Tau was detected using antibodies that label both the N- and C-terminal of the protein, suggesting accumulation of full-length protein within these cells. Pretreatment with the spin trap agent alpha-phenyl-tert-butyl-nitrone (PBN)(100mg/kg) reduced the number of tau-positive oligodendrocytes by 55% in the subcortical white matter of the ischemic hemisphere compared with untreated animals at 40 minutes after MCAO. In contrast, pretreatment with glutamate receptor antagonists MK-801 (0.5 mg/kg) or 2,3-dihydroxy-6-nitro-7-sulpfamoyl-benzo(f)quinoxaline (NBQX) (2 x 30 mg/kg), failed to reduce the number of tau-positive oligodendrocytes after 40 minutes of ischemia. The results indicate that oligodendrocytes respond rapidly to an ischemic challenge and that free radical-mediated mechanisms are involved in the cascade leading to increased tau immunoreactivity.

Ubiquitin and Alzheimer's amyloid beta precursor protein colocalize to endosomes-lysosomes in cultured human cells

Chloroquine (CHQ)-sensitive cellular compartments, identified as endosomes-lysosomes (ELs), have been implicated in the proteolysis of amyloid beta precursor protein (A beta PP) in Alzheimer's disease. Here we show using immunocytochemistry and immunogold electron microscopy that not only A beta PP but also ubiquitin (Ub) co-localize to ELs in CHQ-treated human neuroblastoma (SK-N-SH) and glioblastoma (U-373). Immunoblotting analysis of cell lysates indicated a significant degree of CHQ-mediated interference in A beta PP metabolism in a time- and concentration-dependent manner. The implication is that abnormal intracellular accumulation of A beta PP and its C-terminal fragments beyond a certain threshold may trigger the Ub response. We hypothesize that Ub may play a role in A beta PP processing and/or trafficking to ELs, particularly in stress-related conditions.

Increased regional brain concentrations of ceruloplasmin in neurodegenerative disorders

Ceruloplasmin (CP), the major plasma anti-oxidant and copper transport protein, is synthesized in several tissues, including the brain. We compared regional brain concentrations of CP and copper between subjects with Alzheimer's disease (AD, n = 12), Parkinson's disease (PD, n = 14), Huntington's disease (HD, n = 11), progressive supranuclear palsy (PSP, n = 11), young adult normal controls (YC, n = 6) and elderly normal controls (EC, n = 7). Mean CP concentrations were significantly increased vs. EC (P < 0.05) in AD hippocampus, entorhinal cortex, frontal cortex, and putamen. PD hippocampus, frontal, temporal, and parietal cortices, and HD hippocampus, parietal cortex, and substantia nigra. Immunocytochemical staining for CP in AD hippocampus revealed marked staining within neurons, astrocytes, and neuritic plaques. Increased CP concentrations in brain in these disorders may indicate a localized acute phase-type response and/or a compensatory increase to oxidative stress.

The significance of glucose turnover in the brain in the pathogenetic mechanisms of Alzheimer's disease

This paper presents a comprehensive survey of the pathogenesis and pathophysiology of Alzheimer's disease (AD). Two mechanisms are of etiological importance in the development of a degenerative dementing brain disease: 1. Lesions in the mitochondrial genome that are caused by free radicals. Primary degenerative AD is characterized by a tendency to acquire random lesions within mitochondrial DNA that are produced by free radicals. The consequence of these lesions is a decrease in glucose turnover and a decline in oxidative phosphorylation. Point mutations on chromosome 21 are hypothesized to increase the susceptibility of mitochondrial DNA to lesions created by free radicals. 2. Ischemic brain lesions as well as traumatic brain damage cause an increase in the release of excitotoxic amino acids (glutamate, aspartate, etc.). These neurotransmitters increase CA(+2) influx into the nerve cell and significantly lower energy production. From a pathogenetic point of view, AD is characterized by a decrease in glucose turnover in the brain. The progression of AD can be monitored by F18- deoxyglucose PET studies. This technique also allows the recognition of patients who are prone to develop AD. The actual development of a cognitive deficit is a threshold phenomenon that occurs if glucose turnover in the hippocampus or temporoparietal cortex drops below a critical level of about 40% of the level of age-matched controls. The low glucose turnover in AD causes a cholinergic deficit by decreasing the synthesis of AcCoA, which is used by choline acetyltransferase in the acetylation of choline to acetylcholine. The decrease in glucose turnover also reduces oxidative phosphorylation. The resulting decrease in ATP triggers the hyperphosphorylation of tau protein by activating protein kinase 40erk. The hyperphosphorylation leads to the development of paired helical filaments. The generation of beta amyloid and the loss of neuronal synapses are also caused by a decrease in oxidative phosphorylation, since beta amyloid precursor proteins are not inserted into the membranes of nerve cells in the absence of a sufficient amount of ATP. This results in the generation of intact beta amyloid molecules and leads to amyloidosis in the brains of patients with Alzheimer's disease.