Alterations of some membrane transport proteins in Alzheimer's disease: role of amyloid beta-peptide

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized clinically by progressive memory loss and decline in cognitive abilities and characterized pathologically by the presence of two types of abnormal deposits, i.e., senile plaques (SP) and neurofibrillary tangles (NFT), and by extensive synapse and neuronal loss. SP are composed of fibrillar amyloid beta-peptide (Abeta) surrounded by dystrophic neurites. Recent studies suggest two prospective mechanisms for Abeta-associated membrane dysfunction and subsequent neurotoxicity. One suggests that Abeta oligomers can form heterogeneous ion-channels in the cell membrane leading to cellular degeneration, while the second suggests insertion of Abeta oligomers in membrane lipid bilayers could induce the dysfunction of ion-channels or pumps by binding to or inducing oxidative modification of membrane proteins. In this review, we discuss the effects of Abeta on membrane proteins that are involved in cholinergic and glutamatergic pathways, and some ion-channels.     

Evidence of oxidative damage in Alzheimer's disease brain: central role for amyloid beta-peptide.

Amyloid beta-peptide (Abeta) is heavily deposited in the brains of Alzheimer's disease (AD) patients. Free-radical oxidative stress, particularly of neuronal lipids, proteins and DNA, is extensive in those AD brain areas in which Abeta is abundant. Recent research suggests that these observations might be linked, and it is postulated that Abeta-induced oxidative stress leads to neurodegeneration in AD brain. Consonant with this postulate, Abeta leads to neuronal lipid peroxidation, protein oxidation and DNA oxidation by means that are inhibited by free-radical antioxidants. Here, we summarize current research on phospholipid peroxidation, as well as protein and DNA oxidation, in AD brain, and discuss the potential role of Abeta in this oxidative stress.     

The role of membrane trafficking in the processing of amyloid precursor protein and production of amyloid peptides in Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and the release of inflammatory mediators by innate immune responses. β-Amyloid peptide (Aβ) is derived from sequential processing of the amyloid precursor protein (APP) by membrane-bound proteases, namely the β-secretase, BACE1, and γ-secretase. Membrane trafficking plays a key role in the regulation of APP processing as both APP and the processing secretases traffic along distinct pathways. Genome wide sequencing studies have identified several AD susceptibility genes which regulate membrane trafficking events. To understand the pathogenesis of AD it is critical that the cell biology of APP and Aβ production in neurons is well defined. This review discusses recent advances in unravelling the membrane trafficking events associated with the production of Aβ, and how AD susceptible alleles may perturb the sorting and transport of APP and BACE1. Mechanisms whereby inflammation may influence APP processing are also considered.     

Beyond amyloid: the next generation of Alzheimer's disease therapeutics

The precise pathological events that cause cognitive deficits in Alzheimer's disease remain to be determined. The most widely held view is that accumulation of amyloid beta peptide initiates the disease process; however, with more than eighteen amyloid-based therapeutic candidates currently in clinical trials, the targeting of amyloid alone may not be sufficient to improve functional deficits over the course of the disease. Alternative targets, such as the tau protein and apolipoprotein E, have thus been increasingly investigated, and in the future, therapeutic strategies will likely address events that are upstream of a more broadly construed pathological cascade that includes but is not limited to the generation and accumulation of amyloid beta. Consideration of such events provides the basis for an "indirect amyloid hypothesis," for which data are beginning to emerge. Although it is clinically defined by simple post-mortem criteria, Alzheimer's disease likely has a complex etiology, and effective treatments for this disease will become ever more urgent as the world's population ages.     

A protease activity associated with acetylcholinesterase releases the membrane-bound form of the amyloid protein precursor of Alzheimer's disease.

Amyloid deposits in the brains of patients with Alzheimer's disease (AD) contain a protein (beta A4) which is abnormally cleaved from a larger transmembrane precursor protein (APP). APP is believed to be normally released from membranes by the action of a protease referred to as APP secretase. Amyloid deposits have also been shown to contain the enzyme acetylcholinesterase (AChE). In this study, a protease activity associated with AChE was found to possess APP secretase activity, stimulating the release of a soluble 100K form of APP from HeLa cells transfected with an APP cDNA. The AChE-associated protease was strongly and specifically inhibited by soluble APP (10 nM) isolated from human brain. The AChE-associated protease cleaved a synthetic beta A4 peptide at the predicted cleavage site. As AChE is decreased in AD, a deficiency of its associated protease might explain why APP is abnormally processed in AD.     

Alzheimer's disease: immunohistochemical characterization of cerebral amyloid deposits

In Alzheimer cortex tissue sections, thioflavine stained three patterns of amyloid lesions: neurofibrillary tangles (NFT), senile plaques (SP) and vessel walls (amyloid angiopathy AA). An anti serum against Tau proteins detected NFT but neither SP nor AA. In contrast, an anti serum against beta protein amyloid (BP A4) revealed SP and AA but not NFT. A periodic acid pretreatment dramatically enhanced the anti-BP A4 immunolabelling corresponding to microplaques as well as a large amount of diffuse extracellular amyloid substance, but never stained NFT. Pretreatment of tissue sections with a mixture of endo and exoglycosidases gave identical results and corroborates the extraneuronal processing of BP A4 that appears in a glycosylated form in the extracellular compartment.     

Reassessing the amyloid cascade hypothesis of Alzheimer's disease

Since its inception, the amyloid cascade hypothesis has dominated the field of Alzheimer's disease (AD) research and has provided the intellectual framework for therapeutic intervention. Although the details of the hypothesis continue to evolve, its core principle has remained essentially unaltered. It posits that the amyloid-β peptides, derived from amyloid precursor protein (APP), are the root cause of AD. Substantial genetic and biochemical data support this view, and yet a number of findings also run contrary to its tenets. The presence of familial AD mutations in APP and presenilins, demonstration of Aβ toxicity, and studies in mouse models of AD all support the hypothesis, whereas the presence of Aβ plaques in normal individuals, the uncertain nature of the pathogenic Aβ species, and repeated disappointments with Aβ-centered therapeutic trials are inconsistent with the hypothesis. The current state of knowledge does not prove nor disprove the amyloid hypothesis, but rather points to the need for its reassessment. A view that Aβ is one of the factors, as opposed to the factor, that causes AD is more consistent with the present knowledge, and is more likely to promote comprehensive and effective therapeutic strategies.     

Neurotoxic traffic: uncovering the mechanics of amyloid production in Alzheimer's disease

Alzheimer's disease (AD) is thought by many to result from the accumulation of the neurotoxic amyloid-β (Aβ) peptide in brain parenchyma. The process by which Aβ is proteolytically derived from the larger amyloid precursor protein (APP) has been the focus of much attention in the AD research field over the past decade. Recently, several of the proteins directly involved in the generation of Aβ have been identified and characterized providing a number of viable therapeutic targets for the treatment of AD. However, the cellular mechanisms by which these proteins interact in the proteolytic processing of APP have not been well defined, nor are they readily apparent when one considers what is known about the intracellular localization and trafficking of the various participants. This article will review the underlying cell biology of Aβ production and discuss the mechanistic options for APP processing given the current knowledge of the proteases involved.     

The role of presenilin-1 in the gamma-secretase cleavage of the amyloid precursor protein of Alzheimer's disease

Presenilin-1 (PS1) is required for the release of the intracellular domain of Notch from the plasma membrane as well as for the cleavage of the amyloid precursor protein (APP) at the gamma-secretase cleavage site. It remains to be demonstrated whether PS1 acts by facilitating the activity of the protease concerned or is the protease itself. PS1 could have a gamma-secretase activity by itself or could traffic APP and Notch to the appropriate cellular compartment for processing. Human APP 695 and PS1 were coexpressed in Sf9 insect cells, in which endogenous gamma-secretase activity is not detected. In baculovirus-infected Sf9 cells, PS1 undergoes endoproteolysis and interacts with APP. However, PS1 does not cleave APP in Sf9 cells. In CHO cells, endocytosis of APP is required for Abeta secretion. Deletion of the cytoplasmic sequence of APP (APPDeltaC) inhibits both APP endocytosis and Abeta production. When APPDeltaC and PS1 are coexpressed in CHO cells, Abeta is secreted without endocytosis of APP. Taken together, these results conclusively show that, although PS1 does not cleave APP in Sf9 cells, PS1 allows the secretion of Abeta without endocytosis of APP by CHO cells.     

The role of cholesterol in pathogenesis of Alzheimer's disease: dual metabolic interaction between amyloid beta-protein and cholesterol

The implication that cholesterol plays an essential role in the pathogenesis of Alzheimer’s disease (AD) is based on the 1993 finding that the presence of apolipoprotein E (apoE) allele ε4 is a strong risk factor for developing AD. Since apoE is a regulator of lipid metabolism, it is reasonable to assume that lipids such as cholesterol are involved in the pathogenesis of AD. Recent epidemiological and biochemical studies have strengthened this assumption by demonstrating the association between cholesterol and AD, and by proving that the cellular cholesterol level regulates synthesis of amyloid β-protein (Aβ). Yet several studies have demonstrated that oligomeric Aβ affects the cellular cholesterol level, which in turn has a variety of effects on AD-related pathologies, including modulation of tau phosphorylation, synapse formation and maintenance of its function, and the neurodegenerative process. All these findings suggest that the involvement of cholesterol in the pathogenesis of AD is dualistic—it is involved in Aβ generation and in the amyloid cascade, leading to disruption of synaptic plasticity, promotion of tau phosphorylation, and eventual neurodegeneration. This review article describes recent findings that may lead to the development of a strategy for AD prevention by decreasing the cellular cholesterol level, and also focuses on the impact of Aβ on cholesterol metabolism in AD and mild cognitive impairment (MCI), which may result in promotion of the amyloid cascade at later stages of the AD process.     

Long-lasting acetylcholinesterase splice variations in anticholinesterase-treated Alzheimer's disease patients

Protein levels of different acetylcholinesterase (AChE) splice variants were explored by a combination of immunoblot techniques, using two different antibodies, directed against the C-terminus of the AChE-R splice variant or the core domain common to all variants. Both AChE-R and AChE-S splice variants as well as several heavier AChE complexes were detected in brain homogenates from the parietal cortex of patients with or without Alzheimer's disease (AD) as well as the cerebrospinal fluid (CSF) of AD patients, compatible with the assumption that CSF AChEs might originate from CNS neurons. Long-term changes in the composition of CSF AChE variants were further pursued in AD patients treated with rivastigmine (n = 11) or tacrine (n = 17) in comparison to untreated AD patients (n = 5). In untreated patients, AChE-R was markedly reduced as compared with the baseline level (37%), whereas the medium size AChE-S complex was increased by 32%. Intriguingly, tacrine produced a general and profound up-regulation of all detected AChE variants (up to 117%), whereas rivastigmine treatment caused a mild and selective up-regulation of AChE-R ( approximately 10%, p < 0.05). Moreover, the change in the ratio of AChE-R to AChE-S (R/S-ratio) strongly and positively correlated with sustained cognition at 12 months (p < 0.0001). Thus, evaluation of changes in the composition of CSF AChE variants may yield important information referring to the therapeutic efficacy and/or development of drug tolerance in AD patients treated with anti-cholinesterases.     

Interactions between the amyloid and cholinergic mechanisms in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disease characterized by memory and cognitive loss, the formation of senile plaques containing amyloid-beta (Abeta) peptide, degeneration of the cholinergic neurons and the development of neurofibrillary tangles. The build-up of Abeta is considered to be a central feature in the pathogenesis of AD. However, other critical molecular and neurochemical alterations too occur, such as a cholinergic dysfunction. As concerns the pathomechanism of the disease, both the amyloid cascade hypothesis and the cholinergic hypothesis of AD are widely accepted. This review surveys recent in vitro and in vivo experimental evidence relating to these two hypotheses. Bidirectional pathways linking them as regards the cholinergic neurotoxicity of Abeta and the regulatory mechanisms of cholinergic receptor activation or enzyme inhibition in the processing of the amyloid precursor protein are also discussed. Further work is warranted to elucidate the exact effects in the interactions between the cholinergic and amyloid hypotheses of the candidate drugs used in AD therapy.     

Oligodendrocytes damage in Alzheimer's disease: beta amyloid toxicity and inflammation

Research on Alzheimer's disease (AD) focuses mainly on neuronal death and synaptic impairment induced by beta-Amyloid peptide (Abeta), events at least partially mediated by astrocyte and microglia activation. However, substantial white matter damage and its consequences on brain function warrant the study of oligodendrocytes participation in the pathogenesis and progression of AD. Here, we analyze reports on oligodendrocytes' compromise in AD and discuss some experimental data indicative of Abeta toxicity in culture. We observed that 1 microM of fibrilogenic Abeta peptide damages oligodendrocytes in vitro: while pro-inflammatory molecules (1 microg/ ml LPS + 1 ng/ml IFNgamma) or the presence of astrocytes reduced the Abeta-induced damage. This agrees with our previous results showing an astrocyte-mediated protective effect over Abeta-induced damage on hippocampal cells and modulation of the activation of microglial cells in culture. Oligodendrocytes protection by astrocytes could be, either by reduction of Abeta fibrilogenesis/deposition or prevention of oxidative damage. Likewise, the decrease of Abeta-induced damage by proinflammatory molecules could reflect the production of trophic factors by activated oligodendrocytes and/or a metabolic activation as observed during myelination. Considering the association of inflammation with neurodegenerative diseases. oligodendrocytes impairment in AD patients could potentiate cell damage under pathological conditions.     

The many faces of amyloid beta in Alzheimer's disease

The 'amyloid cascade hypothesis' links amyloid beta peptide (Abeta) with the pathological process of Alzheimer's disease (AD) and it still awaits universal acceptance. Amyloid precursor protein (APP), through the actions of the gamma-secretase complex, eventually becomes a different Abetaspecies. The various Abeta species have proven to be difficult to investigate under physiological conditions, and the species of Abeta responsible for neurotoxicity has yet to be unequivocally identified. The two important Abeta peptides involved are Abeta(1-40) and Abeta(1-42), and each has been ascribed both toxic and beneficial attributes. The ratio between the two species can be important in AD etiology. Additionally, shorter variants of Abeta peptides such as Abeta(1-8), Abeta(9-16) and Abeta(16) have also been shown to be potential participants in AD pathology. Interestingly, a new 56-kDa Abeta peptide (Abeta*56) disrupts memory when injected into the brains of young rats. Transgenic mice models are complicated by the interplay between various human Abeta types and the mouse Abeta types in the mouse brains. However, the accumulation of Abeta(1-42) in the brains of transgenic C. elegans worms and Drosophila is indeed detrimental. A less investigated aspect of AD is epigenetics, but in time the investigation of the role of epigenetics in AD may add to our understanding of the development of AD.