What are the facts and artifacts of the pathogenesis and etiology of Alzheimer disease?

Over the past decade, an increased clinical awareness, together with advances in biochemical, cellular, and molecular analyses, have catapulted the study of Alzheimer disease to the forefront of biomedical research. During this time, a great number of theories, regarding disease pathogenesis, have come and gone but several have persisted. Here, we critically evaluate these theories in an attempt to delineate the facts from the artifacts.

Dimethylargininase, a nitric oxide regulatory protein, in Alzheimer disease

In this study, we show that dimethylargininase, a zinc protein involved in the regulation of nitric oxide synthase, is specifically elevated in neurons displaying cytoskeletal abnormalities and oxidative stress in Alzheimer disease (AD) while none of this enzyme was found in neurons in age-matched control cases. Seen in the context of earlier studies showing widespread nitric oxide related damage in AD and the role of dimethylargininase to activate nitric oxide synthetase, through catalytic removal of its endogenous inhibitors, these findings indicate major alterations in nitric oxide regulation in AD. Further, that low levels of zinc specifically inhibit dimethylargininase may provide a link between the numerous studies showing specific abnormalities in zinc and oxidative stress. Finally, our results provide additional evidence that oxidative stress- and nitric oxide-mediated events play important roles in the pathogenesis of AD.

A randomized trial of granulocyte colony-stimulating factor (Neupogen) starting day 1 vs day 7 post-autologous stem cell transplantation

The purpose of the study was to evaluate the effect of delayed granulocyte colony-stimulating factor (G-CSF) use on hematopoietic recovery post-autologous peripheral blood progenitor cell (PBPC) transplantation. Patients were randomized to begin G-CSF on day +1 or day +7 post transplantation. Thirty-seven patients with lymphoma or myeloma undergoing high-dose therapy and autologous PBPC rescue were randomized to daily subcutaneous G-CSF beginning on day +1 or day +7 post-transplant. Patients < or =70 kg received 300 microg/day and >70 kg 480 microg/day. All patients were reinfused with PBPCs with a CD34+ cell count >2.0 x 10(6)/kg. Baseline characteristics of age, sex and CD34+ cell count were similar between the two arms, the median CD34+ cell count being 5.87 x 10(6)/kg in the day +1 group and 7.70 x 10(6)/kg in the day +7 group (P=0.7). The median time to reach a neutrophil count of >0.5 x 10(9)/l was 9 days in the day +1 arm and 10 days in the day +7 arm, a difference which was not statistically significant (P=0.68). Similarly, there was no difference in median days to platelet recovery >20000 x 10(9)/l, which was 10 days in the day +1 arm and 11 days in the day +7 arm (P=0.83). There was also no significant difference in the median duration of febrile neutropenia (4 vs 6 days; P=0.7), intravenous antibiotic use (7 vs 8 days; P=0.54) or median number of red blood cell transfusions (4 vs 7 units; P=0.82) between the two arms. Median length of hospital stay was 11 days post-PBPC reinfusion in both groups. The median number of G-CSF injections used was 8 in the day +1 group and 3 in the day +7 group (P < 0.0001). There is no significant difference in time to neutrophil or platelet recovery when G-CSF is initiated on day +7 compared to day +1 post-autologous PBPC transplantation. There is also no difference in number of febrile neutropenic or antibiotic days, number of red blood cell transfusions or length of hospital stay. The number of doses of G-CSF used per transplant is significantly reduced with delayed initiation, resulting in a significant reduction in drug costs. For patients with an adequately mobilized PBPC graft, the initiation of G-CSF can be delayed until day +7 post-PBPC reinfusion.

CI-ATPase and Na+/K(+)-ATPase activities in Alzheimer's disease brains

The enzyme activities and the protein levels of Cl(-)-ATPase and Na+/K(+)-ATPase were examined in Alzheimer's disease (AD) brains. Cl(-)-ATPase and Na+/K(+)-ATPase activities in AD brains (n = 13) were significantly lower than those in age-matched control brains (n = 12). In contrast, there was no significant difference in anion-insensitive Mg2(+)-ATPase activity between the two groups. Western blot analysis revealed that the protein levels of Cl(-)-ATPase, Na+/K(+)-ATPase and neuron specific Na+/K(+)-ATPase alpha3 isoform were also significantly reduced in AD brains, while the amount of protein disulfide isomerase, one of the house keeping membrane proteins, was not different between the two groups. The data first demonstrated that Cl(-)-ATPase and Na+/K(+)-ATPase are selectively impaired in AD brains, which may reduce the gradients of Na(+), K(+) and Cl(-) across the cell membranes to cause excitotoxic cellular response and the resulting neuronal death.

Altered glycosylation pattern of proteins in Alzheimer disease

Post-translational modifications due to glycosylation of proteins in human brains from patients with Alzheimer disease (AD) were analyzed using lectin histochemistry. Results indicate a significant increase in the production of O-glycosylated (containing Galbeta1,3GalNAc alpha1,0 Ser/Thr or GalNAc alpha1,0 Ser/Thr) proteins in neuritic plaques and neurofibrillary tangles which are the major histopathological hallmarks of AD brains. These alterations were determined by positive labelling with lectins obtained from Amaranthus leucocarpus (ALL) and Macrobrachium rosenbergii (MRL) respectively. Immunohistochemistry indicated that the lectin-staining labelled specifically both neurofibrillary tangles and neuritic plaques. In contrast, lectins labelling was restricted to microvessels in normal control brains. These results provide evidence that modifications of the specific glycosylation patterns are closely related with the presence of the hallmark lesions of this disease, suggesting that an abnormal enzymatic processing of proteins may be an early event in the neuronal degeneration which characterises AD.

Alzheimer's disease--synergistic effects of glucose deficit, oxidative stress and advanced glycation endproducts

Many approaches have been undertaken to understand Alzheimer's disease (AD) but the heterogeneity of the etiologic factors makes it difficult to define the clinically most important factor determining the onset and progression of the disease. However, there is increasing evidence that the previously so-called "secondary factors" such as a disturbed glucose metabolism, oxidative stress and formation of "advanced glycation endproducts" (AGEs) and their interaction in a vicious cycle are also important for the onset and progression of AD. AGEs are protein modifications that contribute to the formation of the histopathological and biochemical hallmarks of AD: amyloid plaques, neurofibrillary tangles and activated microglia. Oxidative modifications are formed by a complex cascade of dehydration, oxidation and cyclisation reactions, subsequent to a non-enzymatic reaction of sugars with amino groups of proteins. Accumulation of AGE-crosslinked proteins throughout life is a general phenomenon of ageing. However, AGEs are more than just markers of ageing since they can also exert adverse biologic effects on tissues and cells, including the activation of intracellular signal transduction pathways, leading to the upregulation of cytokine and free radical production (oxidative stress). Oxidative stress is involved in various divergent events leading to cell damage, including an increase in membrane rigidity, DNA strand breaks and an impairment in glucose uptake. In addition, other age-related metabolic changes such as depletion of antioxidants or decreased energy production by a disturbed glucose metabolism diminish the ability of the cell to cope with the effects of radical-induced membrane, protein and DNA damage. With our improving understanding of the molecular basis for the clinical symptoms of dementia, it is hoped that the elucidation of the etiologic causes, particularly the positive feedback loops involving radical damage and a reduced glucose metabolism, will help to develop novel "neuroprotective" treatment strategies able to interrupt this vicious cycle of oxidative stress and energy shortage in AD.

Cytochemical demonstration of oxidative damage in Alzheimer disease by immunochemical enhancement of the carbonyl reaction with 2,4-dinitrophenylhydrazine

Formation of carbonyls derived from lipids, proteins, carbohydrates, and nucleic acids is common during oxidative stress. For example, metal-catalyzed, "site-specific" oxidation of several amino acid side-chains produces aldehydes or ketones, and peroxidation of lipids generates reactive aldehydes such as malondialdehyde and hydroxynonenal. Here, using in situ 2,4-dinitrophenylhydrazine labeling linked to an antibody system, we describe a highly sensitive and specific cytochemical technique to specifically localize biomacromolecule-bound carbonyl reactivity. When this technique was applied to tissues from cases of Alzheimer disease, in which oxidative events including lipoperoxidative, glycoxidative, and other oxidative protein modifications have been reported, we detected free carbonyls not only in the disease-related intraneuronal lesions but also in other neurons. In marked contrast, free carbonyls were not found in neurons or glia in age-matched control cases. Importantly, this assay was highly specific for detecting disease-related oxidative damage because the site of oxidative damage can be assessed in the midst of concurrent age-related increases in free carbonyls in vascular basement membrane that would contaminate biochemical samples subjected to bulk analysis. These findings demonstrate that oxidative imbalance and stress are key elements in the pathogenesis of Alzheimer disease.

Amyloid-beta deposition in Alzheimer transgenic mice is associated with oxidative stress

Increased awareness for a role of oxidative stress in the pathogenesis of Alzheimer's disease has highlighted the issue of whether oxidative damage is a fundamental step in the pathogenesis or instead results from disease-associated pathology. In vitro experiments support both possibilities: Oxidative stress increases amyloid-beta production, and, conversely, amyloid-beta increases oxidative damage. To address the relationship between amyloid-beta and oxidative stress in vivo, we examined, using an array of oxidative markers, transgenic mice that overexpress amyloid-beta precursor protein and, as in Alzheimer's disease, develop characteristic amyloid-beta deposits within the brain parenchyma. Transgenic animals show the same type of oxidative damage that is found in Alzheimer's disease, and it is important that this damage directly correlates with the presence of amyloid-beta deposits. The significance of these studies is twofold. First, they provide evidence that amyloid-beta and oxidative damage are inextricably linked in vivo. Second, they support the use of transgenic animals for the development of antioxidant therapeutic strategies.

Cerebrovascular muscle atrophy is a feature of Alzheimer's disease

We examined vascular amyloid-beta deposition and other abnormalities in the posterior cerebral artery of consecutive cases of Alzheimer's disease (AD) compared to controls. Smooth muscle atrophy was a consistent feature in the cases of AD examined (p<0.01) and was surprisingly independent of adjacent amyloid-beta deposition. These findings suggest that vascular abnormalities are a consistent feature in AD and may be an important contributor to the pathogenesis and complications of AD.

Evidence of oxidative stress and in vivo neurotoxicity of beta-amyloid in a transgenic mouse model of Alzheimer's disease: a chronic oxidative paradigm for testing antioxidant therapies in vivo

Increased expression of antioxidant enzymes and heat-shock proteins are key markers of oxidative stress. Such proteins are abnormally present within the neuropathological lesions of Alzheimer's disease (AD), suggesting that oxidative stress may play significant but yet undefined roles in this disorder. To gain further insight into the role of oxidative stress in AD, we studied the expression of CuZn superoxide dismutase (SOD) and hemoxygenase-1 (HO-1), two established markers of oxidative stress, in a transgenic mouse model of AD. Immunohistochemistry with anti-SOD and anti-HO-1 antibodies revealed a very pronounced increase of these proteins only in aged transgene-positive mice. Interestingly, the distribution of the oxidative burden was largely overlapping with dystrophic neuritic elements in the mice as highlighted with anti-ubiquitin antibodies. Because the most conspicuous alterations were identified around amyloid (Abeta) deposits, our results provide strong support for the hypothesis that Abeta is neurotoxic in vivo and that such toxicity is mediated by free radicals. To obtain additional experimental evidence for such an interpretation (ie, a cause-effect relationship between Abeta and oxidative neurotoxicity), PC12 cells were exposed to increasing concentrations of Abeta or to oxidative stress. In agreement with the in vivo findings, either treatment caused marked induction of SOD or HO-1 in a dose-dependent fashion. These results validate the transgenic approach for the study of oxidative stress in AD and for the evaluation of antioxidant therapies in vivo.

Abnormal localization of iron regulatory protein in Alzheimer's disease

A role for altered iron metabolism in the pathogenesis of Alzheimer's disease has been suggested by several reports associating the cardinal neuropathologic lesions with markers of free radical-induced damage and redox-active iron. We hypothesized that the abnormal distribution of iron in Alzheimer brain might result from alterations in iron regulatory proteins (IRP) such as IRP-1 and IRP-2, the main control elements of cellular iron homeostasis. Here, we report that while IRP-1 is present at similar levels in both Alzheimer and control brain tissue, IRP-2 shows striking differences and is associated with intraneuronal lesions, including neurofibrillary tangles, senile plaque neurites and neuropil threads. Since IRP-2 colocalizes with redox-active iron, our results suggest that alterations in IRP-2 might be directly linked to impaired iron homeostasis in Alzheimer's disease.

Advanced lipid peroxidation end-products in Alexander's disease

Rosenthal fibers (RF), intra-astrocytic hyaline inclusions, accumulate in various pathological conditions and are the histological hallmark of Alexander's disease. While the major protein components of RF have been identified, the factors accounting for their pathogenesis, accumulation, and insolubility are largely unknown. In this study, we immunohistochemically examined three cases of Alexander's disease using antibodies to a lysine-derived pyrrole modification arising from 4-hydroxy-2-nonenal, a highly cytotoxic reactive aldehyde produced by lipid peroxidation. In all the cases of Alexander's disease examined, strong immunolabeling of RF by the antibodies to 4-hydroxy-2-nonenal pyrrole adducts were noted. By contrast, age-matched control cases showed no immunoreactivity. These results indicate that modification of protein by lipid peroxidation adducts may play an important role in the formation of RF as well as in the pathogenesis of Alexander's disease. Furthermore, taken together with our previous data indicating advanced Maillard reaction end products in RF, it seems that post-translational modification of RF, initiated by oxidative stress, is critical for both the accumulation and the insolubility of RF, and therefore, by inference, in the pathogenesis of Alexander's disease.

Early glycoxidation damage in brains from Down's syndrome

In Down's syndrome, the presence of three copies of chromosome 21 is associated with premature aging and progressive mental retardation sharing the pathological features of Alzheimer disease. Early cortical dysgenesis and late neuronal degeneration are probably caused by an overproduction of amyloid beta-peptide, followed by an increased cellular oxidation. Interestingly, chromosome 21 codes for superoxide-dismutase and amyloid beta precursor resulting, in Down's syndrome, in an overflow of these gene products and metabolites. We studied Down's fetal brain cortex to evaluate the presence and amount of lipid and protein oxidation markers; moreover, we quantified two forms of glycation end products that are known to be involved in the process of cellular oxidation. All these parameters are significantly increased in Down's fetal brains in comparison to controls, providing the evidence that accelerated brain glycoxidation occurs very early in the life of Down's syndrome subjects.

Astrocytes regulate microglial phagocytosis of senile plaque cores of Alzheimer's disease

We have developed an in vitro model in which isolated senile plaque (SP) cores are presented to rat microglial cells in culture. Microglia rapidly phagocytosed, broke apart, and cleared SP cores. However, when cocultured with astrocytes, microglial phagocytosis was markedly suppressed, allowing the SPs to persist. Suppression of phagocytosis by astrocytes appears to be a general phenomena since microglia in the presence of astrocytes showed reduced capacity to phagocytose latex beads as well. The astrocyte effect on microglia is related in part to a diffusible factor(s) since astrocyte- but not fibroblast-conditioned media also reduced phagocytosis. These results suggest that while microglia have the capacity to phagocytose and remove SPs, astrocytes which lie in close association to microglia may help prevent the efficient clearance of SP material allowing them to persist in Alzheimer's disease.

Alteration of proteins regulating apoptosis, Bcl-2, Bcl-x, Bax, Bak, Bad, ICH-1 and CPP32, in Alzheimer's disease

Recently, apoptosis has been implicated in the selective neuronal loss of Alzheimer's disease (AD). Apoptosis is regulated by the B cell leukemia-2 gene product (Bcl-2) family (Bcl-2, Bcl-x, Bax, Bak and Bad) and the caspase family (ICH-1 and CPP32), with apoptosis being prevented by Bcl-2 and Bcl-x, and promoted by Bax, Bak, Bad, ICH-1 and CPP32. In the present study, we examined the levels of these proteins in the membranous and cytosolic fractions of temporal cortex in AD and control brain. In the membranous fraction, the levels of Bcl-2 alpha, Bcl-xL, Bcl-x beta, Bak and Bad were increased in AD. In the cytosolic fractions, the level of Bcl-x beta was increased, while Bcl-xL, Bax, Bak, and Bad and ICH-1L were unchanged. CPP32 was not detected in AD or control brain. These findings demonstrate a differential involvement of cell death-regulatory proteins in AD and suggest that Bak, Bad, Bcl-2 and Bcl-x are upregulated in AD brains.

Iron accumulation in Alzheimer disease is a source of redox-generated free radicals

Damage from free radicals has been demonstrated in susceptible neuronal populations in cases of Alzheimer disease. In this study, we investigated whether iron, a potent source of the highly reactive hydroxyl radical that is generated by the Fenton reaction with H2O2, might contribute to the source of radicals in Alzheimer disease. We found, using a modified histochemical technique that relies on the formation of mixed valence iron complexes, that redox-active iron is associated with the senile plaques and neurofibrillary tangles-the pathological hallmark lesions of this disease. This lesion-associated iron is able to participate in in situ oxidation and readily catalyzes an H2O2-dependent oxidation. Furthermore, removal of iron was completely effected using deferoxamine, after which iron could be rebound to the lesions. Characterization of the iron-binding site suggests that binding is dependent on available histidine residues and on protein conformation. Taken together, these findings indicate that iron accumulation could be an important contributor toward the oxidative damage of Alzheimer disease.

Advanced glycation modification of Rosenthal fibers in patients with Alexander disease

Rosenthal fibers, astrocytic inclusions that accumulate in various neoplastic and non-neoplastic conditions, are a characteristic of Alexander disease, a leukodystrophy of unknown etiology. Given that alphaB crystallin is the major protein component of Rosenthal fibers and that crystallins in the diabetic and aged lens are targets for advanced glycation end product modifications via the Maillard reaction we hypothesized that Rosenthal fibers might contain similar modifications. Using antibodies specific for two products of glycation, pyrraline and pentosidine, we showed labeling of Rosenthal fibers that may account for their insolubility and accumulation. These data suggest that advanced glycation end products may be critical to the pathogenesis of Alexander disease.