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.     

Molecular mechanisms for neuronal cell death by Alzheimer's amyloid precursor protein-relevant insults

To develop a therapeutic intervention for Alzheimer's disease (AD), it is necessary to clarify the mechanisms underlying the pathogenesis of AD, in which senile plaques, neurofibrillary tangles and neuronal loss in the cerebrum are the central abnormalities. A number of studies have focused on the major component of the senile plaques, which is amyloid-beta (Abeta) and its precursor protein APP, and have investigated the roles of these molecules in the onset, progression and inhibition of AD. For multiple reasons, however, their roles in AD, especially in neuronal death, remain elusive and a unified concept for their roles has not yet been established. Recently, it has been found that APP functions normally as a neuronal surface transmembrane protein. In this article, we review the molecular mechanisms of neuronal cell death by these APP-relevant insults and discuss the functions of APP in regard to intracellular signal transducers, including c-Jun N-terminal kinase. We also revise the roles of Abeta in neuronal death and survival.     

Molecular mechanisms mediating pathological plasticity in Huntington's disease and Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease and Alzheimer's disease, although very different in etiology, share common degenerative processes. These include neuronal dysfunction, decreased neural connectivity, and disruption of cellular plasticity. Understanding the molecular mechanisms underlying the neural plasticity deficits in these devastating conditions may lead the way toward new therapeutic targets, both disease-specific and more generalized, which can ameliorate degenerative cognitive deficits. Furthermore, investigations of 'pathological plasticity' in these diseases lend insight into normal brain function. This review will present evidence for altered plasticity in Huntington's and Alzheimer's diseases, relate these findings to symptomatology, and review possible causes and commonalities.     

Plaque busters: strategies to inhibit amyloid formation in Alzheimer's disease.

Alzheimer's disease is a devastating degenerative disorder of the brain for which there is no cure or effective treatment. Although the etiology of Alzheimer's disease is not fully understood, recent research suggests that deposition of cerebral amyloid plaques is central to the disease process. Therefore, an attractive therapeutic strategy for Alzheimer's disease is to prevent, reduce or reverse amyloid deposition in the brain. Several small chemical compounds, synthetic peptides and natural proteins have been described that inhibit amyloid formation or amyloid neurotoxicity in vitro. The effect of these and other compounds now needs to be tested in vivo and the ability of amyloid inhibitors to halt the progression of Alzheimer's disease in humans needs to be evaluated.     

Molecular biology of the amyloid of Alzheimer's disease. An overview.

Alzheimer's disease is a progressive neurodegenerative disorder that affects a significant percentage of elderly individuals. Degenerative nerve cells express atypical proteins, and amyloid is deposited. The hallmark event of Alzheimer's disease is the deposition of amyloid as insoluble fibrous masses in extracellular neuritic plaques and around the walls of cerebral blood vessels. This review will focus on the advances on the knowledge of Alzheimer's amyloid, because it is becoming increasingly clear that the deposition of amyloid on neuritic plaques in the brain represents the earliest and most characteristic pathological feature of Alzheimer's disease. The main component of amyloid is a 4.2-4.5 KDa hydrophobic peptide, named amyloid beta-peptide, that is codified in chromosome 21 as part of a much larger precursor protein. The study of the mechanism by which the amyloid beta-peptide arises from the amyloid precursor protein is very important in order to understand the biological basis of amyloid deposition and its role in Alzheimer's disease.     

Late endocytic dysfunction as a putative cause of amyloid fibril formation in Alzheimer's disease

The assembly of amyloid β-protein to amyloid fibrils is a critical event in Alzheimer's disease. Evidence exists that endocytic pathway abnormalities, including the enlargement of early endosomes, precede the extraneuronal amyloid fibril deposition in the brain. We determined whether endocytic dysfunction potently promotes the assembly of amyloid β-protein on the surface of cultured cells. Blocking the early endocytic pathway by clathrin suppression, inactivation of small GTPases, removal of membrane cholesterol, and Rab5 knockdown did not result in amyloid fibril formation on the cell surface from exogenously added soluble amyloid β-protein. In contrast, blocking the late endocytic pathway by Rab7 suppression markedly induced the amyloid fibril formation in addition to the enlargement of early endosomes. Notably, a monoclonal antibody specific to GM1-ganglioside-bound amyloid β-protein, an endogenous seed for Alzheimer amyloid, completely blocks the amyloid fibril formation. Our results suggest that late but not early endocytic dysfunction contributes to the amyloid fibril formation by facilitating the generation of amyloid seed in the Alzheimer's brain.     

Molecular and cellular mechanisms for Alzheimer's disease: understanding APP metabolism

Alzheimer's disease (AD) is the most common neurodegenerative disease associated with aging. One important pathologic feature of AD is the formation of extracellular senile plaques in the brain, whose major components are small peptides called beta-amyloid (Abeta) that are derived from beta-amyloid precursor protein (APP) through sequential cleavages by beta-secretase and gamma-secretase. Because of the critical role of Abeta in the pathogenesis of AD, unraveling the cellular and molecular events underlying APP/Abeta metabolism has been and remains, of paramount importance to AD research. In this article we will focus on the regulation of APP metabolism leading to Abeta generation. We will review current knowledge of the secretases (alpha-, beta-, and gamma-secretases) involved in APP processing and various molecular and cellular mechanisms underlying intracellular trafficking of APP, which is a highly regulated process and whose disturbance has direct impacts on the production of Abeta.     

Molecular mechanisms for Alzheimer's disease: implications for neuroimaging and therapeutics

Alzheimer's disease is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are beta-amyloid (Abeta) plaques, neurofibrillary tangles, synaptic loss and reactive gliosis. The current therapeutic effort is directed towards developing drugs that reduce Abeta burden or toxicity by inhibiting secretase cleavage, Abeta aggregation, Abeta toxicity, Abeta metal interactions or by promoting Abeta clearance. A number of clinical trials are currently in progress based on these different therapeutic strategies and they should indicate which, if any, of these approaches will be efficacious. Current diagnosis of Alzheimer's disease is made by clinical, neuropsychologic and neuroimaging assessments. Routine structural neuroimaging evaluation with computed tomography and magnetic resonance imaging is based on non-specific features such as atrophy, a late feature in the progression of the disease, hence the crucial importance of developing new approaches for early and specific recognition at the prodromal stages of Alzheimer's disease. Functional neuroimaging techniques such as functional magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography and single photon emission computed tomography, possibly in conjunction with other related Abeta biomarkers in plasma and CSF, could prove to be valuable in the differential diagnosis of Alzheimer's disease, as well as in assessing prognosis. With the advent of new therapeutic strategies there is increasing interest in the development of magnetic resonance imaging contrast agents and positron emission tomography and single photon emission computed tomography radioligands that will permit the assessment of Abeta burden in vivo.     

Molecular determinants of amyloid deposition in Alzheimer's disease: conformational studies of synthetic beta-protein fragments

The amyloid beta-protein (1-42) is a major constituent of the abnormal extracellular amyloid plaque that characterizes the brains of victims of Alzheimer's disease. Two peptides, with sequences derived from the previously unexplored C-terminal region of the beta-protein, beta 26-33 (H2N-SNKGAIIG-CO2H) and beta 34-42 (H2N-LMVGGVVIA-CO2H), were synthesized and purified, and their solubility and conformational properties were analyzed. Peptide beta 26-33 was found to be freely soluble in water; however, peptide beta 34-42 was virtually insoluble in aqueous media, including 6 M guanidinium thiocyanate. The peptides formed assemblies having distinct fibrillar morphologies and different dimensions as observed by electron microscopy of negatively stained samples. X-ray diffraction revealed that the peptide conformation in the fibrils was cross-beta. A correlation between solubility and beta-structure formation was inferred from FTIR studies: beta 26-33, when dissolved in water, existed as a random coil, whereas the water-insoluble peptide beta 34-42 possessed antiparallel beta-sheet structure in the solid state. Solubilization of beta 34-42 in organic media resulted in the disappearance of beta-structure. These data suggest that the sequence 34-42, by virtue of its ability to form unusually stable beta-structure, is a major contributor to the insolubility of the beta-protein and may nucleate the formation of the fibrils that constitute amyloid plaque.     

Molecular studies in Alzheimer's disease

In the past decade, there has been an explosion of information relating to the molecular neurobiology of Alzheimer's disease (AD). Molecular dissection of the neuropathology of AD has provided insight into the pathogenesis of this disease and has defined areas where investigation may prove useful in elucidating the cause of this disorder and suggest new treatments.     

Multiple mechanisms underlie neurotoxicity by different types of Alzheimer's disease mutations of amyloid precursor protein

We examined a neuronal cell system in which single-cell expression of either familial Alzheimer's disease (FAD) gene V642I-APP or K595N/M596L-APP (NL-APP) in an inducible plasmid was controlled without affecting transfection efficiency. This system revealed that (i) low expression of both mutants exerted toxicity sensitive to both Ac-DEVD-CHO (DEVD) and glutathione ethyl ester (GEE), whereas wild-type APP (wtAPP) only at higher expression levels caused GEE/DEVD-resistant death to lesser degrees; (ii) toxicity by the V642I mutation was entirely GEE/DEVD sensitive; and (iii) toxicity by higher expression of NL-APP was GEE/DEVD resistant. The GEE/DEVD-sensitive death was sensitive to pertussis toxin and was due to G(o)-interacting His(657)-Lys(676) domain. The GEE/DEVD-resistant death was due to C-terminal Met(677)-Asn(695). APP mutants lacking either domain unraveled elaborate intracellular cross-talk between these domains. E618Q-APP, responsible for non-AD type of a human disease, only exerted GEE/DEVD-resistant death at higher expression. Therefore, (i) different FAD mutations in APP cause neuronal cell death through different cytoplasmic domains via different sets of mechanisms; (ii) expression levels of FAD genes are critical in activating specific death mechanisms; and (iii) toxicity by low expression of both mutants most likely reflects the pathogenetic mechanism of FAD.     

Detecting bioactive amyloid beta peptide species in Alzheimer's disease

Amyloid beta peptide (A beta) is believed to play a central role in the pathogenesis of Alzheimer's disease (AD). However, the form of A beta that induces neurodegeneration in AD, defined here as bioactive A beta, is not clear. Preventing the formation of bioactive A beta or inactivating previously formed bioactive A beta should be a promising approach to treat AD. We have previously developed a cell-based assay for the detection of bioactive A beta species. The assay is based upon the correlation between the ability of an A beta sample to induce a unique form of cellular MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] formazan exocytosis, and its ability to activate glia and induce neurotoxicity. Here, we show that this cell-based assay is not only useful for a cellular model of A beta amyloidogenesis but is also able to detect bioactive A beta species in a transgenic mouse model of AD, as well as in post-mortem cortex samples from AD patients. There is a good correlation between the extent of glia activation and the level of bioactive A beta species in the mouse brain. A promising deuteroporphyrin that can inactivate bioactive A beta species was also identified using this assay. These novel insights and findings should have important implications for the treatment of AD.     

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.