The ART of Loss: Aβ Imaging in the Evaluation of Alzheimer’s Disease and other Dementias

Molecular neuroimaging based on annihilation radiation tomographic (ART) techniques such as positron emission tomography (PET), in conjunction with related biomarkers in plasma and cerebrospinal fluid (CSF), are proving valuable in the early and differential diagnosis of Alzheimer's disease (AD). With the advent of new therapeutic strategies aimed at reducing beta-amyloid (Abeta) burden in the brain to potentially prevent or delay functional and irreversible cognitive loss, there is increased interest in developing agents that allow assessment of Abeta burden in vivo. Abeta burden as assessed by molecular imaging matches histopathological reports of Abeta plaque distribution in aging and dementia and appears more accurate than FDG for the diagnosis of AD. Abeta imaging is also a very powerful tool in the differential diagnosis of AD from fronto-temporal dementia (FTD). Although Abeta burden as assessed by PET does not correlate with measures of cognitive decline in AD, it does correlate with memory impairment and rate of memory decline in mild cognitive impairment (MCI) and healthy older subjects. Approximately 30% of asymptomatic controls present cortical (11)C-PiB retention. These observations suggest that Abeta deposition is not part of normal ageing, supporting the hypothesis that Abeta deposition occurs well before the onset of symptoms and is likely to represent preclinical AD. Further longitudinal observations are required to confirm this hypothesis and to better elucidate the role of Abeta deposition in the course of Alzheimer's disease.     

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.     

Distinct destructive signal pathways of neuronal death in Alzheimer's disease

Abundant neuron loss is a major feature of Alzheimer's disease (AD). Hypotheses for this loss include abnormal amyloid precursor protein processing (i.e. excess Abeta production, protein aggregation or misfolding), oxidative stress, excitotoxicity and inflammation. Neuron loss is a major cause of dementia in AD; however, it seems that there is no definitive pathway that causes cell death in the AD brain. Here, we examine the hypotheses for neuron loss in AD and pose the argument that the means by which neurons degenerate is irrelevant for cognitive decline. The best treatment for cognitive decline is to prevent the toxicity that first sets the neuron on its path to destruction, which is the production of Abeta peptide.     

The interactive effect of the cholinergic system and acute ovarian suppression on the brain: an fMRI study

Recent evidence suggests that loss of ovarian function following ovariectomy is a risk factor for Alzheimer's disease (AD); however, the biological basis of this risk remains poorly understood. We carried out an fMRI study into the interaction between loss of ovarian function (after Gonadotropin Hormone Releasing Hormone agonist (GnRHa) treatment) and scopolamine (a cholinergic antagonist used to model the memory decline associated with aging and AD). Behaviorally, cholinergic depletion produced a deficit in verbal recognition performance in both GnRHa-treated women and wait list controls, but only GnRHa-treated women made more false positive errors with cholinergic depletion. Similarly, cholinergic depletion produced a decrease in activation in the left inferior frontal gyrus (LIFG; Brodmann area 45) - a brain region implicated in retrieving word meaning - in both groups, and activation in this area was further reduced following GnRHa treatment. These findings suggest biological mechanisms through which ovarian hormone suppression may interact with the cholinergic system and the LIFG. Furthermore, this interaction may provide a useful model to help explain reports of increased risk for cognitive decline and AD in women following ovariectomy.     

Alzheimer's disease: synapses gone cold

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by insidious cognitive decline and memory dysfunction. Synapse loss is the best pathological correlate of cognitive decline in AD and mounting evidence suggests that AD is primarily a disease of synaptic dysfunction. Soluble oligomeric forms of amyloid beta (Aβ), the peptide that aggregates to form senile plaques in the brain of AD patients, have been shown to be toxic to neuronal synapses both in vitro and in vivo. Aβ oligomers inhibit long-term potentiation (LTP) and facilitate long-term depression (LTD), electrophysiological correlates of memory formation. Furthermore, oligomeric Aβ has also been shown to induce synapse loss and cognitive impairment in animals. The molecular underpinnings of these observations are now being elucidated, and may provide clear therapeutic targets for effectively treating the disease. Here, we review recent findings concerning AD pathogenesis with a particular focus on how Aβ impacts synapses.     

Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer's disease

Extracellular amyloid plaques, intracellular neurofibrillary tangles, and loss of basal forebrain cholinergic neurons in the brains of Alzheimer's disease (AD) patients may be the end result of abnormalities in lipid metabolism and peroxidation that may be caused, or exacerbated, by beta-amyloid peptide (Abeta). Apolipoprotein E (apoE) is a major apolipoprotein in the brain, mediating the transport and clearance of lipids and Abeta. ApoE-dependent dendritic and synaptic regeneration may be less efficient with apoE4, and this may result in, or unmask, age-related neurodegenerative changes. The increased risk of AD associated with apoE4 may be modulated by diet, vascular risk factors, and genetic polymorphisms that affect the function of other transporter proteins and enzymes involved in brain lipid homeostasis. Diet and apoE lipoproteins influence membrane lipid raft composition and the properties of enzymes, transporter proteins, and receptors mediating Abeta production and degradation, tau phosphorylation, glutamate and glucose uptake, and neuronal signal transduction. The level and isoform of apoE may influence whether Abeta is likely to be metabolized or deposited. This review examines the current evidence for diet, lipid homeostasis, and apoE in the pathogenesis of AD. Effects on the cholinergic system and response to cholinesterase inhibitors by APOE allele carrier status are discussed briefly.     

Network dysfunction in Alzheimer's disease: does synaptic scaling drive disease progression?

Accumulation of beta-amyloid protein (Abeta) in the brain is a key feature of Alzheimer's disease (AD). The build-up of aggregated forms of Abeta leads to synaptic loss and to cognitive dysfunction. Although the pathways controlling production and aggregation of Abeta are well studied, the mechanisms that drive the spread of neurodegeneration in the brain are unclear. Here, the idea is presented that AD progresses as a consequence of synaptic scaling, a type of neuronal plasticity that helps maintain synaptic signal strength. Recent studies indicate that brain-derived neurotrophic factor, tumour necrosis factor-alpha and alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) regulate synaptic scaling in the AD brain. It is suggested that further studies on synaptic scaling in AD could reveal new targets for therapeutic drug development.     

Expression, purification, and characterization of recombinant human beta-amyloid42 peptide in Escherichia coli

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of cognitive function. Evidence indicates that abnormal processing and extracellular deposition of the beta-amyloid42 peptide, the longer form of proteolytic derivative of the transmembrane glycoprotein-amyloid precursor protein (APP), is a key step in the pathogenesis of AD. Since it is convenient and economical to obtain such a peptide biologically, in this study, we report for the first time a method to express in E. coli and purify beta-amyloid42 using glutathione-S-transferase (GST) fusion system. beta-Amyloid42 gene was inserted into a vector pGEX-4T-1 to construct a GST-fusion protein. The fusion protein GST-beta-amyloid42, expressed in BL21 (DE3) strain, was purified with GSH-affinity chromatography followed by thrombin cleavage. The digested product was further purified with an additional GSH-affinity and a Benzamidine chromatography step. After cleavage and purification, the beta-amyloid42 moiety showed the expected size of 4.5 kDa on Tricine-SDS-PAGE, and was further confirmed by Western blot. Moreover, the fibrillar recombinant beta-amyloid42 exhibited great aggregation activity and showed neurotoxicity on neuron cells in vitro. These results suggest that our method will be useful in obtaining a large quantity of recombinant beta-amyloid42 peptide for further physiological and biochemical studies.     

The neuroprotective peptide NAP inhibits the aggregation of the beta-amyloid peptide

Alzheimer's disease (AD) is characterized by brain plaques containing the beta-amyloid peptide (Abeta). One approach for treating AD is by blocking Abeta aggregation. Activity-dependent neuroprotective protein contains a peptide, NAP that protects neurons in culture against Abeta toxicity. Here, NAP was shown to inhibit Abeta aggregation using: (1) fluorimetry; (2) electron microscopy; (3) high-throughput screening of Abeta deposition onto a synthetic template (synthaloid); and (4) Congo Red staining of neurons. Further assays showed biotin-NAP binding to Abeta. These results suggest that part of the neuroprotective mechanism exerted by NAP is through modulation of toxic protein folding in the extracellular milieu.     

The role of nicotinic acetylcholine receptors in Alzheimer's disease.

The two hallmark lesions of Alzheimer's disease (AD) are extracellular amyloid plaques, mainly formed by a small peptide called amyloid-beta (Abeta), and neurofibrillary tangles, which are intracellular inclusions formed by aggregates of hyperphosphorylated tau protein. One of the major neurochemical features of AD is the marked reduction of nicotinic acetylcholine receptors in disease-relevant brain regions such as the cerebral cortex and hippocampus. This loss is further compounded by the loss of cholinergic cells, which contributes to the cognitive dysfunction. This observation has had a major impact on therapeutic treatments, as efforts to restore cholinergic function such as the administration of acetylcholinesterase inhibitors have been, until recently, the major treatment options available for AD. Understanding the relationship of these hallmark lesions with the plethora of other changes that occur in the AD brain has proven to be a difficult challenge to resolve. The utilization of transgenic mouse models, that recapitulate one or more neuropathological and neurochemical features of the AD brain is providing some inroads, as they offer a means to gain mechanistic insights into the disease process in an in vivo setting. In this review, we consider the role of nicotinic acetylcholine receptors in transgenic models and in AD.     

Antioxidants in the canine model of human aging

Oxidative damage can lead to neuronal dysfunction in the brain due to modifications to proteins, lipids and DNA/RNA. In both human and canine brain, oxidative damage progressively increases with age. In the Alzheimer's disease (AD) brain, oxidative damage is further exacerbated, possibly due to increased deposition of beta-amyloid (Aβ) peptide in senile plaques. These observations have led to the hypothesis that antioxidants may be beneficial for brain aging and AD. Aged dogs naturally develop AD-like neuropathology (Aβ) and cognitive dysfunction and are a useful animal model in which to test antioxidants. In a longitudinal study of aging beagles, a diet rich in antioxidants improved cognition, maintained cognition and reduced oxidative damage and Aβ pathology in treated animals. These data suggest that antioxidants may be beneficial for human brain aging and for AD, particularly as a preventative intervention. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.     

Maladie d’Alzheimer : inhibiteur de l’acétylcholinestérase,nouveau concept de médicament,apport de l’imagerie TEP

Alzheimer's disease (AD) is a neurodegenerative dementia which progressively destroys cognitive functions with effects on patient behaviour and social adaptation. More than 110,000 people per year in France suffer from this form of dementia where two-thirds are more than 80 years old. The pathological features are deposits in the human brain of beta-amyloid plaques and neurofibrillary tangles. Particularly, the cholinergic system is altered and treatment strategies in AD are centred around the development of cholinesterase inhibitor that improve cholinergic transmitter activity.     

Quantitative assessment of DNA fragmentation and beta-amyloid deposition in insular cortex and midfrontal gyrus from patients with Alzheimer's disease

It has been suggested that the neurodegeneration that occurs with Alzheimer's disease (AD) may result from apoptosis, a process of programmed cell death. Neuronal injury, induced by abnormal aggregates of beta-amyloid peptide, has been identified as an apoptotic trigger. In the present study, brain tissue samples were obtained from the insular cortex (INS) and midfrontal gyrus (MFG) of Alzheimer subjects and age-matched, nondemented controls. Tissue sections from all samples were alternately stained by an in situ TUNEL assay to identify 3' termini DNA strand breaks characteristic of apoptosis or immunohistochemically for beta-amyloid deposition in senile plaques. The incidence of DNA fragmentation detected in pyramidal neurons was relatively infrequent overall, but was significantly higher in AD compared to controls. AD subjects consistently exhibited a dense accumulation of plaques, with a twofold greater concentration in MFG as INS. There was no significant difference in pyramidal cell number regardless of subject or brain region. Taken together, our results indicate that the TUNEL assay may be revealing cell damage rather than cell loss. Our finding of a moderate correlation between the incidence of TUNEL-positive cells and plaque density implicates beta-amyloid as one of multiple factors provoking cell injury in AD. A notable contribution of this study is the identification of distinctive neuropathologies co-occurring in two brain regions interconnected with each other and with limbic and cortical areas typically damaged during AD.     

Caspase cleaved presenilin-1 is part of active gamma-secretase complexes

gamma-Secretase is a key enzyme involved in the processing of the beta-amyloid precursor protein into amyloid beta-peptides (Abeta). Abeta accumulates and forms plaques in Alzheimer's disease (AD) brains. A progressive neurodegeneration and cognitive decline occurs during the course of the disease, and Abeta is believed to be central for the molecular pathogenesis of AD. Apoptosis has been implicated as one of the mechanisms behind the neuronal cell loss seen in AD. We have studied preservation and activity of the gamma-secretase complex during apoptosis in neuroblastoma cells (SH-SY5Y) exposed to staurosporine (STS). We report that the known components (presenilin, Nicastrin, Aph-1 and Pen-2) interact and form active gamma-secretase complexes in apoptotic cells. In addition, the fragments corresponding to the PS1 N-terminal fragment and the caspase-cleaved PS1 C-terminal fragment (PS1-caspCTF) were found to form active gamma-secretase complexes when co-expressed in presenilin (PS) knockout cells. Interestingly, PS1-caspCTF replaced the normal PS1 C-terminal fragment and was co-immunoprecipitated with the gamma-secretase complex in SH-SY5Y cells exposed to STS. In addition, Abeta was detected in medium from apoptotic HEK APP(swe) cells. Together, the data show that gamma-secretase complexes containing PS1-caspCTF are active, and suggest that this proteolytic activity is also important in dying cells and may affect the progression of AD.     

The immunotherapy of Alzheimer's disease

Only a small percentage of patients with Alzheimer's disease benefit from current drug therapy and for only a relatively short time. This is not surprising as the goal of these drugs is to enhance existing cerebral function in Alzheimer patients and not to block the progression of cognitive decline. In contrast, immunotherapy is directed at clearing the neurotoxic amyloid beta peptide from the brain that directly or indirectly leads to cognitive decline in patients with Alzheimer's disease. The single trial of active immunization with the amyloid beta peptide provided suggestive evidence of a reduction in cerebral amyloid plaques and of stabilization in cognitive function of half the patients who developed good antibody responses to the amyloid beta peptide. However, 6% of actively immunized Alzheimer patients developed sterile meningoencephalitis that forced the cessation of the clinical trial. Passive immunotherapy in animal models of Alzheimer's disease has provided similar benefits comparable to those seen with active immunotherapy and has the potential of being effective in the half of Alzheimer's disease patients who do not make a significant anti-amyloid beta peptide antibody response and without inducing T-cell-mediated encephalitis. Published studies of 5 patients with sporadic Alzheimer disease treated with intravenous immunoglobulin containing anti-amyloid beta peptide antibodies showed that amyloid beta peptide was mobilized from the brain and cognitive decline was interrupted. Further studies of passive immunotherapy are urgently required to confirm these observations.     

Cdk5, a therapeutic target for Alzheimer's disease?

Alzheimer's disease (AD) represents the leading cause for senile dementia affecting more than 4 million people worldwide. AD patients display a triad of pathological features including brain atrophy caused by neuronal loss, beta-amyloid plaque and neurofibrillary tangles. We previously show that Cyclin-dependent kinase 5 (Cdk5) is deregulated in AD brains and may contribute to the pathogenesis of AD. In AD brains, a calpain cleavage product of its physiological regulator p35, p25 is elevated. p25 causes prolonged activation of Cdk5 and alteration of its substrate specificity. The implications of p25/Cdk5 in neurotoxicity, beta-amyloid plaque and neurofibrillary tangle pathology will be discussed.     

On the molecular basis linking Nerve Growth Factor (NGF) to Alzheimer's disease

1. Alzheimer's disease (AD) is pathologically defined by the deposition of amyloid peptide and neurofibrillary tangles and is characterized by a progressive loss of cognition and memory function, due to marked cortical cholinergic depletion. 2. Cholinergic cortical innervation is provided by basal forebrain cholinergic neurons. The neurotrophin Nerve Growth Factor (NGF) promotes survival and differentiation of basal forebrain cholinergic neurons. 3. This assertion has been at the basis of the hypothesis developed in the last 20 years, whereby NGF deprivation would be one of the factor involved in the etiology of sporadic forms of AD. 4. In this review, we shall summarize data that lead to the production and characterization of a mouse model for AD (AD11 anti-NGF mice), based on the expression of transgenic antibodies neutralizing NGF. The AD-like phenotype of AD11 mice will be discussed on the basis of recent studies that have posed NGF and its precursor pro-NGF back to the stage of AD-like neurodegeneration, showing the involvement of the precursor pro-NGF in one of the cascades leading to AD neurodegeneration.     

Androgen cell signaling pathways involved in neuroprotective actions

As a normal consequence of aging in men, testosterone levels significantly decline in both serum and brain. Age-related testosterone depletion results in increased risk of dysfunction and disease in androgen-responsive tissues, including brain. Recent evidence indicates that one deleterious effect of age-related testosterone loss in men is increased risk for Alzheimer's disease (AD). We discuss recent findings from our laboratory and others that identify androgen actions implicated in protecting the brain against neurodegenerative diseases and begin to define androgen cell signaling pathways that underlie these protective effects. Specifically, we focus on the roles of androgens as (1) endogenous negative regulators of beta-amyloid accumulation, a key event in AD pathogenesis, and (2) neuroprotective factors that utilize rapid non-genomic signaling to inhibit neuronal apoptosis. Continued elucidation of cell signaling pathways that contribute to protective actions of androgens should facilitate the development of targeted therapeutic strategies to combat AD and other age-related neurodegenerative diseases.     

Focal adhesions regulate Abeta signaling and cell death in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that results from a loss of synaptic transmission and ultimately cell death. The presenting pathology of AD includes neuritic plaques composed of beta-amyloid peptide (Abeta) and neurofibrillary tangles composed of hyperphosphorylated tau, with neuronal loss in specific brain regions. However, the mechanisms that induce neuronal cell loss remain elusive. Focal adhesion (FA) proteins assemble into intracellular complexes involved in integrin-mediated communication between the extracellular matrix and the actin cytoskeleton, regulating many cell physiological processes including the cell cycle. Interestingly, recent studies report that integrins bind to Abeta fibrils, mediating Abeta signal transmission from extracellular sites of Abeta deposits into the cell and ultimately to the nucleus. In this review, we will discuss the Abeta induced integrin/FA signaling pathways that mediate cell cycle activation and cell death.     

Caspase cleavage of members of the amyloid precursor family of proteins

The synapse loss and neuronal cell death characteristic of Alzheimer's disease (AD) are believed to result in large part from the neurotoxic effects of beta-amyloid peptide (Abeta), a 40-42 amino acid peptide(s) derived proteolytically from beta-amyloid precursor protein (APP). However, APP is also cleaved intracellularly to generate a second cytotoxic peptide, C31, and this cleavage event occurs in vivo as well as in vitro and preferentially in the brains of AD patients (Lu et al. 2000). Here we show that APPC31 is toxic to neurons in primary culture, and that like APP, the APP family members APLP1 and possibly APLP2 are cleaved by caspases at their C-termini. The carboxy-terminal peptide derived from caspase cleavage of APLP1 shows a degree of neurotoxicity comparable to APPC31. Our results suggest that even though APLP1 and APLP2 cannot generate Abeta, they may potentially contribute to the pathology of AD by generating peptide fragments whose toxicity is comparable to that of APPC31.