The mechanism of A beta production and its inhibition as a therapeutic strategy for the prevention of Alzheimer's disease

Deposition of amyloid beta peptide (A beta) as senile plaques or cerebrovascular amyloid characterizes the brains of patients with Alzheimer's disease (AD). A beta are composed of 40-42 amino acids that are proteolytically produced from its precursor beta APP. We have shown that the deposition of A beta ending at the 42nd residue (A beta 42) is one of the earliest pathological changes in AD brains. Genetic and cell biological evidence strongly suggest that mutations in beta APP or presenilin (PS) 1 and 2 genes cause AD through increase in production of A beta 42. Recently, PS1 and PS2 are shown to be the catalytic subunits of gamma-secretase that cleaves the intramembrane segments of beta APP and Notch. beta-amyloid hypothesis that emphasizes the primacy of A beta in the pathogenesis of AD is currently being verified by the new experimental therapeutic approaches, e.g., A beta vaccine therapy or administration of inhibitors of beta- or gamma-secretases. The mechanistic roles of newly identified co-factor proteins of PS (i.e., APH-1 and PEN-2) in gamma-secretase function, as well as recent advances in the development of gamma- or beta-secretase inhibitors will be discussed.     

Genetic basis of neurodegeneration in familial Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by two types of brain lesions, referred to as senile plaques and neurofibrillary tangles. Moreover, neuronal cell loss and synaptic degeneration appear in affected regions of the brain. A series of endoproteolytic cleavages of the amyloid precursor protein (APP) controlled by alpha, beta, and gamma-secretases leads to a formation of non-amyloidogenic (the alpha-secretase pathway) and amyloidogenic (the beta-secretase pathway) products which are essential for neurodegeneration. According to the "amyloid cascade hypothesis", the accumulation of amyloid beta (Abeta) peptides in the brain is a primary event in the pathogenesis of AD. One of the strong pieces of evidence supporting this hypothesis was the identification of pathogenic mutations within APP, presenilin 1 and presenilin 2 genes responsible for familial autosomal dominant AD. These mutations affect APP processing causing overproduction of Abeta42. Finding specific inhibitors of the Abeta42 generation is a major goal of AD drug development programs now and the key challenge for the treatment of the most devasting disease of human brain.     

Neuropathological,biochemical and genetic alterations in AD

The molecular and cellular processes that lead to the production of the amyloid beta (A beta) peptide and some of the processes associated with A beta fibrillogenesis and neurotoxicity have recently been elucidated. Experimental results have suggested that abnormalities in the processing of the beta-amyloid precursor protein (beta APP) are central to the pathogenesis of Alzheimer's disease (AD). beta APP processing includes two mutually exclusive proteolytic cleavage pathways, one involving the putative gamma-secretase enzyme, the identity of which remains unknown. Recent evidence has suggested the presenilin 1 and presenilin 2 genes are necessary for gamma-secretase activities. Another gene associated with susceptibility to AD is the apolipoprotein E (APOE) gene. Given the important role that abnormal processing of beta APP plays in the genesis of AD, most current efforts are directed at either modulating A beta peptide production or inhibiting its ability to aggregate into fibrils and cause neurotoxicity. To inhibit A beta production, one strategy might be to inhibit either beta-secretase or gamma-secretase. Several approaches to the inhibition of A beta aggregation are under investigation.     

Rationale for the development of cholinesterase inhibitors as anti-Alzheimer agents

Alzheimer's disease (AD) is characterized by progressive dementia caused by the loss of the presynaptic markers of the cholinergic system in the brain areas related to memory and learning and brain deposits of amyloid beta peptide (A beta) and neurofibrillary tangles (NFT). A small fraction of early onset familial AD (FAD) is caused by mutations in genes, such as the beta-amyloid precursor protein (APP) and presenilins that increase the load of A beta in the brain. These studies together with findings that A beta is neurotoxic in vitro, provide evidence that some aggregates of this peptide are the key to the pathogenesis of AD. The yield of A beta and the processing and turnover of APP are regulated by a number of pathways including apolipoprotein E, cholesterol and cholinergic agonists. Early studies showed that muscarinic agonists increased APP processing within the A beta sequence (sAPP alpha). More recently, we have presented evidence showing that some, but not all, anticholinesterases reduce secretion of sAPP alpha as well as A beta into the media suggesting that cholinergic agonists modulate A beta levels by multiple mechanisms. Herein we review the recent advances in understanding the function of cholinesterase (ChE) in the brain and the use of ChE-inhibitors in AD. We propose and support the position that the influence of cholinergic stimulation on amyloid formation is critical in light of the early targeting of the cholinergic basal forebrain in AD and the possibility that maintenance of this cholinergic tone might slow amyloid deposition. In this context, the dual action of certain cholinesterase inhibitors on their ability to increase acetylcholine levels and decrease amyloid burden assumes significance as it may identify a single drug to both arrest the progression of the disease as well as treat its symptoms. A new generation of acetyl- and butyryl cholinesterase inhibitors is being studied and tested in human clinical trials for AD. We critically discuss recent trends in AD research, from molecular and genetic to clinical areas, as it relates to the effects of cholinergic agents and their secondary effects on A beta. Finally, we examine different neurobiological mechanisms that provide the basis of new targets for AD drug development.     

The role of beta amyloid in Alzheimer's disease: still a cause of everything or the only one who got caught?

The beta amyloid (A beta) protein is a key molecule in the pathogenesis of Alzheimer's disease (AD). The tendency of the A beta peptide to aggregate, its reported neurotoxicity, and genetic linkage studies, have led to a hypothesis of AD pathogenesis that many AD researchers term the amyloid cascade hypothesis. In this hypothesis, an increased production of A beta results in neurodegeneration and ultimately dementia through a cascade of events. In the past 15 years, debate amongst AD researchers has arisen as to whether A beta is a cause or an effect of the pathogenic process. Recent in vitro and in vivo research has consolidated the theory that A beta is the primary cause, initiating secondary events, culminating in the neuropathological hallmarks associated with AD. This research has led to the development of therapeutic agents, currently in human clinical trials, which target A beta.     

Cholesterol and Alzheimer's disease: clinical and experimental models suggest interactions of different genetic, dietary and environmental risk factors

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cultured cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as 'secretases' participate in APP processing leading to the generation of either Abeta or non-amyloid proteins. However, the mechanisms of neurotoxicity of Abeta and the role of APP function in AD remain important unanswered questions. Although early studies recognized the loss of cholesterol and other lipids in the brain, these findings have been poorly connected with AD pathogenesis, despite the identification of the epsilon4 allele of APOE as a major risk factor in AD. The recent finding that cholesterol can modulate the yield of potentially toxic Abeta has boosted research on its role in AD. Consequently, several cholesterol-reducing drugs are currently being evaluated for the treatment of AD. The present review summarizes our current understanding of the relationship of AD pathogenesis with cholesterol, lipids and other genetic and environmental risk factors.     

The emerging utility of animal models of chronic neurodegenerative diseases

The two most common neurodegenerative diseases are Alzheimer's disease (AD) and Parkinson's disease (PD). The symptoms are caused by the initially selective degeneration of neuronal subpopulations involved in memory (AD) or movement control (PD). The cause of both diseases is unknown, but ageing is an inevitable risk factor. The identification of disease-associated genes was a breakthrough for the understanding of molecular mechanisms of neurodegeneration and has provided the basis for the establishment of cell culture and animal model systems, instrumental for target validation and drug screening. Familial AD is caused by mutations in the beta-amyloid precursor protein (betaAPP) and in the gene products responsible for its proteolytic processing, namely the presenilins. Transgenic mice expressing these mutant genes develop characteristic AD plaques in an age-dependent manner. A reduction of plaque burden and amelioration of cognitive decline in these animals was recently achieved by vaccination with amyloid beta-protein fibrils. The other hallmark lesion of AD, the neurofibrillary tangle, has been modelled recently in transgenic mice expressing mutant tau protein linked to frontotemporal dementia. PD is characterised by intraneuronal cytoplasmic deposits (Lewy bodies) of the PD-associated gene product alpha-synuclein. Transgenic expression of alpha-synuclein recreated hallmark features of PD in mice and fruit flies, establishing alpha-synuclein as PD-causing drug target. Moreover, environmental risk factors such as the pesticide rotenone have been used successfully to generate rodent models of PD. Lesion models of PD are being exploited for the development of experimental gene therapy and transplantation approaches.     

Gene identification in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia in the elderly population. Three genes have been identified that cause the less common early-onset, familial cases of the disease: the amyloid precursor (APP) protein gene on chromosome 21, the presenilin 1 (PSEN1) gene on chromosome 14 and the presenilin 2 (PSEN2) gene on chromosome 1. Mutations in these genes account for < 2% of the total number of AD cases. More than 50% of the cases are late-onset and related to the apolipoprotein E (APOE) gene on chromosome 19. The apolipoprotein E locus is a susceptibility gene, with polymorphisms affecting both risk and age-of-onset of the disease. Intense efforts are underway to identify additional susceptibility genes and promising regions on chromosomes 6, 9, 10 and 12 have been identified through whole genome scans. In addition, the genetic basis of several other non-AD inherited dementias has been unravelled. Discovery of the genetically relevant genes will aid in the elucidation of the pathogenesis of AD. The high-throughput tools of pharmacogenomics for gene-to-function-to-target studies can provide a quicker means of monitoring how mutations and polymorphisms affect model systems' adaptations to the altered genes, possibly identifying signal transduction or biochemical pathways. This relevant information can then be used for drug target selection and pharmacogenetics.     

Molecular approaches to the treatment, prophylaxis, and diagnosis of Alzheimer's disease: possible involvement of HRD1, a novel molecule related to endoplasmic reticulum stress, in Alzheimer's disease

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a protective mechanism against ER stress in which unfolded proteins accumulated in the ER are selectively transported to the cytosol for degradation by the ubiquitin-proteasome system. We cloned the novel ubiquitin ligase HRD1, which is involved in ERAD, and showed that HRD1 promoted amyloid precursor protein (APP) ubiquitination and degradation, resulting in decreased generation of amyloid β (Aβ). In addition, suppression of HRD1 expression caused APP accumulation and promoted Aβ generation associated with ER stress and apoptosis. Interestingly, HRD1 levels were significantly decreased in the cerebral cortex of patients with Alzheimer's disease (AD), and the brains of these patients experienced ER stress. Our recent study revealed that this decrease in HRD1 was due to its insolubilization; however, controversy persists about whether the decrease in HRD1 protein promotes Aβ generation or whether Aβ neurotoxicity causes the decrease in HRD1 protein levels. Here, we review current findings on the mechanism of HRD1 protein loss in the AD brain and the involvement of HRD1 in the pathogenesis of AD. Furthermore, we propose that HRD1 may be a target for novel AD therapeutics.     

Always around, never the same: pathways of amyloid beta induced neurodegeneration throughout the pathogenic cascade of Alzheimer's disease

There is an increasing amount of evidence showing the importance of intermediate aggregation species of amyloid beta (Abeta) in the pathogenic cascade of Alzheimer's disease (AD). Different Abeta assembly forms may mediate diverse toxic effects at different stages of the disease. Mouse models for AD suggest that intraneuronal accumulation of Abeta oligomers might be involved in AD pathogenesis at a very early stage of the disease. The detrimental effect of oligomeric Abeta on synaptic efficacy is suggested to be an early event in the pathogenic cascade. Also early neuronal responses as activation of the unfolded protein response are processes likely to be associated with the increased occurrence of oligomeric or low fibrillar Abeta in AD pathology. In later stages of AD pathology, the fibrillarity of Abeta increases, concomitantly with a neuroinflammatory response, followed by tau related neurofibrillary changes in end stage pathology. We will review recent findings in in vitro cell models, in vivo mouse models, and post mortem AD brain tissue in view of the effects of different Abeta peptide species on neurodegeneration during AD pathogenesis. Insight into the role of different Abeta species during AD pathogenesis is essential for the development of disease modifying drugs and therapeutical strategies.     

Identification of the role of presenilins beyond Alzheimer's disease.

Mutations in the genes encoding presenilin 1 (PS1) and presenilin 2 (PS2) account for the majority of the cases of familial early-onset Alzheimer's disease (FAD). Presenilins (PSs) facilitate the intramembraneous cleavage of amyloid precursor protein (APP), coined gamma-secretase cleavage, which generates beta-amyloid peptides (A beta). Considerable evidence suggests that FAD-linked PS variants exert their pathogenic influence by selectively elevating the levels of highly fibrillogenic A beta 42 peptides. In addition, numerous other functions have been ascribed to PSs based on subcellular localization, protein interactions, loss of function studies, and intramembraneous gamma-secretase cleavage of growing number of substrates. This review summarizes the diverse physiological functions that are regulated by PSs beyond APP metabolism.     

Amyloid deposition as the central event in the aetiology of Alzheimer's disease.

While there may be many causes of Alzheimer's disease (AD), the same pathological sequence of events, described here by John Hardy and David Allsop, is likely to occur in all cases. The recent discovery of a pathogenic mutation in the beta-amyloid precursor protein (APP) gene on chromosome 21 suggests that APP Mismetabolism and beta-amyloid deposition are the primary events in the disease process. The occurrence of AD in Down syndrome is consistent with this hypothesis. The pathological cascade for the disease process is most likely to be: beta-amyloid deposition----tau phosphorylation and tangle formation----neuronal death. The development of a biochemical understanding of this pathological cascade will facilitate rational design of drugs to intervene in this process.     

Molecular rationale for the pharmacological treatment of Alzheimer's disease

Cerebral deposition of amyloid plaques containing amyloid beta-peptide (Abeta) has traditionally been considered the central feature of Alzheimer's disease (AD). Abeta is derived from amyloid precursor protein (APP), which is cleaved by several different proteases: alpha-, beta- and gamma-secretase. In the past decade, however, the molecular pathogenesis of AD has been shown to involve alterations in several neurotransmitter, inflammatory, oxidative, and hormonal pathways that represent potential targets for AD prevention and treatment. Much research has shown a direct link between cholinergic impairment and altered APP processing as a major pathogenetic event in AD. Three highly probable mechanisms of APP regulation through inhibition of acetylcholinesterase are thus current topics of investigation. Indeed, acetylcholinesterase inhibitors appear to cause selective muscarinic activation of alpha-secretase and to induce the translation of APP mRNA; they may also restrict amyloid fibre assembly. Activation of N-methyl-D-aspartate receptors is considered a probable cause of chronic neurodegeneration in AD, and memantine has been widely used in some countries in AD patients to block cerebral N-methyl-D-aspartate receptors that normally respond to glutamate. Further studies are needed to determine whether antioxidants such as vitamins C and E are effective, through various mechanisms, in patients with mild-to-moderate AD. Additional data are also required for non-steroidal anti-inflammatory drugs, some of which appear to possess experimental effects that may ultimately prove favourable in AD patients. Statins also warrant further investigation, since they have activated alpha-secretase and they reduced Abeta generation and amyloid accumulation in a transgenic mouse model. beta-Secretase would seem to be an ideal target for anti-amyloid therapy in AD, but potential clinical and pharmacological issues, such as ensuring selectivity of inhibition, stability, and ease of blood-brain barrier penetration and cellular uptake, remain to be addressed for beta-secretase inhibitors. gamma-Secretase is not an easy candidate for pharmacological manipulation. Immunotherapeutic strategies have targeted Abeta directly; however, intensive investigation of indirect approaches to the management of AD with immunotherapy is now underway.     

Mitochondrial dysfunction, endoplasmic reticulum stress, and apoptosis in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder of late life characterized by insidious, chronic, and progressive memory impairment in association with the accumulation of senile plaques, neurofibrillary tangles, and massive loss of neurons. Apoptosis is believed to be an important contributor to progression and pathology of neurodegeneration in AD. There is considerable evidence that amyloid beta-peptide, a major component of senile plaques, has the capacity to activate intracellular apoptosis pathways leading to neuronal cell death. AD-related mutations in genes coding presenilins are also shown to cause neuronal apoptosis, by directly and indirectly regulating apoptotic signaling cascades. Recent evidence suggests that two intrinsic pathways, mitochondrial dysfunction and endoplasmic reticulum stress, are central in the execution of apoptosis in AD. This review summarizes recent progress of research in this field focused on the molecular mechanisms involved in neuronal apoptosis mediated by organelle dysfunction.     

Beta-amyloid therapies in Alzheimer's disease

Neurones in the brain produce beta-amyloid fragments from a larger precursor molecule termed the amyloid precursor protein (APP). When released from the cell, these protein fragments may accumulate in extracellular amyloid plaques and consequently hasten the onset and progression of Alzheimer's disease (AD). A beta fragments are generated through the action of specific proteases within the cell. Two of these enzymes, beta- and gamma-secretase, are particularly important in the formation of A beta as they cleave within the APP protein to give rise to the N-terminal and C-terminal ends of the A beta fragment, respectively. Consequently, many researchers are investigating therapeutic approaches that inhibit either beta- or gamma-secretase activity, with the ultimate goal of limiting A beta; production. An alternative AD therapeutic approach that is being investigated is to employ anti-A beta antibodies to dissolve plaques that have already formed. Both of these approaches focus on the possibility that accrual of A beta leads to neuronal degeneration and cognitive impairment characterised by AD and test the hypothesis that limiting A beta deposition in neuritic plaques may be an effective treatment for AD.     

Antioxidants as a potential therapy against age-related neurodegenerative diseases: amyloid Beta toxicity and Alzheimer's disease

Alzheimer's disease (AD) is a progressive age-related neurodegenerative disorder with distinct neuropathological features. Extracellular plaques, consisting of aggregated amyloid peptides of 39-43 amino acids are one of the most prominent pathological hallmarks of this disease. Although the exact neurochemical effector mechanism of Abeta aggregation is not yet elucidated, age-associated disturbances of metal ion metabolism have been proposed to promote the formation of aggregates from soluble Abeta. Oxidative stress is postulated to be a downstream effect of Abeta-metal ion interactions. Therefore, the modulation of brain metal metabolism and attenuation of oxidative stress by antioxidant molecules are proposed as a potential therapeutic intervention in AD. Here, we summarize the recent literature focused on APP/Abeta-metal ion interactions and the use of antioxidant metal chelators as potential therapy against AD.     

The induction of amyloid precursor protein and alpha-synuclein in rat hippocampal astrocytes by diethyldithiocarbamate and copper with or without glutathione

alpha-Synuclein is the major component of Lewy bodies. Its aggregation can be accelerated by copper, iron, or beta-amyloid (Abeta) and has been thought to provide a nucleation center during the formation of amyloid plaques. The main structural component of amyloid plaque is Abeta, which is derived from a larger protein, amyloid precursor protein (APP). Xenobiotics have been implicated in the etiology of the neurodegenerative disease. Mechanisms of diethyldithiocarbamate (DDC) neurotoxicity involve copper chelation and interactions with SH groups resulting in oxidative stress. In this study, rat hippocampal astrocytes were treated with DDC (75 microM), CuCl(2) (0.2 microM), or DDC (75 microM) plus CuCl(2) (0.2 microM) for 1h. Cells were allowed to recover with or without 10 mM GSH. Results showed an increase of APP and alpha-synuclein production occurring in a time-dependent manner. At 4 h post-treatment, cells contained small positively stained material deposited throughout the cytosol for APP and by 8 h post-treatment increases were seen in both APP and alpha-synuclein. Immunoblots supported immunocytochemical results. Glutathione (GSH) decreased the accumulation of these proteins at 8 h post-treatment.     

Human beta-secretase (BACE) and BACE inhibitors: progress report

A key step in the processing of the integral membrane protein APP, or Amyloid Precursor Protein is through the proteolytic cleavage by the enzyme beta-Secretase (BACE). The proteolysis of APP by BACE, followed by subsequent C-terminal cleavage(s) by gamma-secretase, results in the formation of the amyloid beta (Abeta) peptide. The principal component of the neuritic plaque found in the brains of Alzheimer's Disease (AD) patients is Abeta which is a neurotoxic and highly aggregatory peptide segment of APP. The amyloid hypothesis holds that the neuronal dysfunction and clinical manifestation of AD is a consequence of the long term deposition and accumulation of 40-42 amino-acid long Abeta peptides, and that this process leads to the onset and progression of AD. Due to the apparent causal relationship between Abeta and AD, the so-called "secretases" that produce Abeta have been targeted for development of inhibitors that might serve as therapeutic agents for treatment of this dreaded, and ever more prevalent disease. Herein will be discussed our current understanding of BACE, its role in the formation of neuritic plaques and the known inhibitors of the enzyme.     

Abeta, tau and alpha-synuclein and glial cells]

Many neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease are now characterized by abnormal accumulation of certain proteins in the brain. The key molecules include amyloid beta-protein (Abeta), tau and a-synuclein, all of which are involved in the pathogenesis and provide histopathological hallmarks of the diseases. Abeta is continuously produced in and removed from the brain. Microglia and astrocytes take up and degrade soluble Abeta. In the Alzheimer brain, once-deposited, insoluble Abeta is also removed by phagocytosis by activated microglia. The success of these removal processes, however, is at best partial. The phagocytic removal of insoluble Abeta is associated with neuroinflammation, a potentially neurotoxic reaction. Tau is accumulated in astrocytes under a diversity of pathological conditions in several forms: thorn-shaped astrocytes; tuft-shaped astrocytes; astrocytic plaques. Thorn-shaped astrocytes are associated with gliosis and are not disease-specific. Tuft-shaped astrocytes are characteristic of progressive supranuclear palsy (PSP) and astrocytic plaques of cortico-basal degeneration (CBD). Tau accumulation in oligodendrocytes is referred to as coiled bodies and occurs in PSP, CBD, Pick's disease and some other so-called taupathies. a-Synuclein is accumulated in oligodendrocytes, which is referred to as glial cytoplasmic inclusions (GCI). Occurrence of GCI is diagnostic to multiple system atrophy. Transgenic mouse models in which tau or alpha-synuclein is overexpressed in glial cells indicate that neuronal degeneration occurs following tau/alpha-synuclein accumulation in glial cells, supporting a notion that these abnormal glial cells play pathogenic roles.     

Stress proteins and regulation of microglial amyloid-beta phagocytosis

Recent studies have indicated that prolonged dysfunction and/or stress in the endoplasmic reticulum (ER) may contribute to pathogenesis and neurodegeneration. The disorder caused by misfolding and aggregation of proteins has been referred to as conformational disease, including Alzheimer's disease (AD). AD is characterized by the accumulation of extracellular amyloid-beta1-42 (A beta 42) fibrils with reactive microglia. Understanding the balance of production and clearance of A beta 42 is the key to elucidating amyloid plaque homeostasis. We have recently found that microglial phagocytosis of A beta 42 may be essentially driven by dynamic reorganization of the actin cytoskeleton through the pathway of WAVE and Rac1. In addition, an extracellular stress protein, such as Hsp90, enhances A beta 42 phagocytosis. HMGB1 inhibits microglial phagocytosis of A beta 42, and it binds A beta 42 and stabilizes the oligomerization. These results suggest that microglial clearance of A beta 42 may be another option for investigations in the search for a therapeutic strategy for AD, in addition to the study of production and degradation of A beta 42.