Many lines of evidence support that ?-amyloid (A?) peptides play an important role in Alzheimer's disease (AD), the most common cause of dementia. But despite much effort the molecular mechanisms of how A? contributes to AD remain unclear. While A? is generated from its precursor protein throughout life, the peptide is best known as the main component of amyloid plaques, the neuropathological hallmark of AD. Reduction in A? has been the major target of recent experimental therapies against AD. Unfortunately, human clinical trials targeting A? have not shown the hoped-for benefits. Thus, doubts have been growing about the role of A? as a therapeutic target. Here we review evidence supporting the involvement of A? in AD, highlight the importance of differentiating between various forms of A?, and suggest that a better understanding of A?'s precise pathophysiological role in the disease is important for correctly targeting it for potential future therapy.
Synaptic degeneration is one of the earliest hallmarks of Alzheimer disease. The molecular mechanism underlying this degeneration is not fully elucidated but one key player appears to be the synaptotoxic amyloid ?-peptide (A?). The exact localization of the production of A? and the mechanisms whereby A? is released remain elusive. We have earlier shown that A? can be produced in crude synaptic vesicle fractions and it has been reported that increased synaptic activity results in increased secreted but decreased intracellular A? levels. Therefore, we considered whether A? could be produced in synaptic vesicles and/or released through the same mechanisms as neurotransmitters in synaptic vesicle exocytosis. Small amounts of A? were found to be produced in pure synaptic vesicle preparations. We also studied the release of glutamate and A? from rat cortical nerve terminals (synaptosomes). We found that large amounts of A? were secreted from non-stimulated synaptosomes, from which glutamate was not released. On the contrary, we could not detect any differences in A? release between non-stimulated synaptosomes and synaptosomes stimulated with KCl or 4-aminopyridine, whereas glutamate release was readily inducible in this system. To conclude, our results indicate that the major release mechanism of A? from isolated nerve terminals differs from the synaptic release of glutamate and that the activity-dependent increase of secreted A?, reported by several groups using intact cells, is likely dependent on post-synaptic events, trafficking and/or protein synthesis mechanisms.
The progressive development of Alzheimer's disease (AD) pathology follows a spatiotemporal pattern in the human brain. In a transgenic (Tg) mouse model of AD expressing amyloid precursor protein (APP) with the arctic (E693G) mutation, pathology spreads along anatomically connected structures. Amyloid-? (A?) pathology first appears in the subiculum and is later detected in interconnected brain regions, including the retrosplenial cortex. We investigated whether the spatiotemporal pattern of A? pathology in the Tg APP arctic mice to interconnected brain structures can be interrupted by destroying neurons using a neurotoxin and thereby disconnecting the neural circuitry.
Decreases of the sex steroids, testosterone and estrogen, are associated with increased risk of Alzheimers disease. Testosterone and estrogen supplementation improves cognitive deficits in animal models of Alzheimers disease. Sex hormones play a role in the regulation of amyloid-? via induction of the amyloid-? degrading enzymes neprilysin and insulin-degrading enzyme. To mimic the effect of dihydrotestosterone (DHT), we administered a selective androgen receptor agonist, ACP-105, alone and in combination with the selective estrogen receptor ? (ER?) agonist AC-186 to male gonadectomized triple transgenic mice. We assessed long-term spatial memory in the Morris water maze, spontaneous locomotion, and anxiety-like behavior in the open field and in the elevated plus maze. We found that ACP-105 given alone decreases anxiety-like behavior. Furthermore, when ACP-105 is administered in combination with AC-186, they increase the amyloid-? degrading enzymes neprilysin and insulin-degrading enzyme and decrease amyloid-? levels in the brain as well as improve cognition. Interestingly, the androgen receptor level in the brain was increased by chronic treatment with the same combination treatment, ACP-105 and AC-186, not seen with DHT or ACP-105 alone. Based on these results, the beneficial effect of the selective ER? agonist as a potential therapeutic for Alzheimers disease warrants further investigation.
Age-related misfolding and aggregation of disease-linked proteins in selective brain regions is a characteristic of neurodegenerative diseases. Although neuropathological aggregates that characterize these various diseases are found at sites other than synapses, increasing evidence supports the idea that synapses are where the pathogenesis begins. Understanding these diseases is hampered by our lack of knowledge of what the normal functions of these proteins are and how they are affected by aging. Evidence has supported the idea that neurodegenerative disease-linked proteins have a common propensity for prion protein-like cell-to-cell propagation. However, it is not thought that the prion-like quality of these proteins/peptides that allows their cell-to-cell transmission implies a role for human-to-human spread in common age-related neurodegenerative diseases. It will be important to better understand the molecular and cellular mechanisms governing the role of these aggregating proteins in neural function, especially at synapses, how their propagation occurs and how pathogenesis is promoted by aging.
Pathologic aggregation of ?-amyloid (A?) peptide and the axonal microtubule-associated protein tau protein are hallmarks of Alzheimers disease (AD). Evidence supports that A? peptide accumulation precedes microtubule-related pathology, although the link between A? and tau remains unclear. We previously provided evidence for early co-localization of A?42 peptides and hyperphosphorylated tau within postsynaptic terminals of CA1 dendrites in the hippocampus of AD transgenic mice. Here, we explore the relation between A? peptide accumulation and the dendritic, microtubule-associated protein 2 (MAP2) in the well-characterized amyloid precursor protein Swedish mutant transgenic mouse (Tg2576). We provide evidence that localized intraneuronal accumulation of A?42 peptides is spatially associated with reductions of MAP2 in dendrites and postsynaptic compartments of Tg2576 mice at early ages. Our data support that reduction in MAP2 begins at sites of A?42 monomer and low molecular weight oligomer (M/LMW) peptide accumulation. Cumulative evidence suggests that accumulation of M/LMW A?42 peptides occurs early, before high molecular weight oligomerization and plaque formation. Since synaptic alteration is the best pathologic correlate of cognitive dysfunction in AD, the spatial association of M/LMW A? peptide accumulation with pathology of MAP2 within neuronal processes and synaptic compartments early in the disease process reinforces the importance of intraneuronal A? accumulation in AD pathogenesis.
A central question in Alzheimers disease (AD) research is what role ?-amyloid peptide (A?) plays in synaptic dysfunction. Synaptic activity increases A? secretion, potentially inhibiting synapses, but also decreases intraneuronal A?, protecting synapses. We now show that levels of secreted A? fall with time in culture in neurons of AD-transgenic mice, but not wild-type mice. Moreover, the ability of synaptic activity to elevate secreted A? and reduce intraneuronal A? becomes impaired in AD-transgenic but not wild-type neurons with time in culture. We demonstrate that synaptic activity promotes an increase in the A?-degrading protease neprilysin at the cell surface and a concomitant increase in colocalization with A?42. Remarkably, AD-transgenic but not wild-type neurons show reduced levels of neprilysin with time in culture. This impaired ability to secrete A? and reduce intraneuronal A? has important implications for the pathogenesis and treatment of AD.
Alzheimers disease, the most common neurodegenerative disease, is characterized by a progressive loss of synapses and accumulation of amyloid-beta (A?) peptides in the brain. Previous studies demonstrated that acute increase in synaptic activity in cultured hippocampal slices and mouse brains (Cirrito et al. Neuron 48: 913-922, 2005; Kamenetz et al. Neuron 37: 925-937, 2003) enhanced secretion of A?. Since synaptic activity promotes A? secretion, it could also affect the trafficking and processing of its precursor, the amyloid precursor protein (APP). Here, we describe a method to investigate the effect of acute synaptic activation on APP trafficking within dendrites.
?-Amyloid (A?) accumulation and aggregation are hallmarks of Alzheimers disease (AD). High-resolution three-dimensional (HR-3D) volumetric imaging allows for better analysis of fluorescence confocal microscopy and 3D visualization of A? pathology in brain. Early intraneuronal A? pathology was studied in AD transgenic mouse brains by HR-3D volumetric imaging. To better visualize and analyze the development of A? pathology, thioflavin S staining and immunofluorescence using antibodies against A?, fibrillar A?, and structural and synaptic neuronal proteins were performed in the brain tissue of Tg19959, wild-type, and Tg19959-YFP mice at different ages. Images obtained by confocal microscopy were reconstructed into three-dimensional volumetric datasets. Such volumetric imaging of CA1 hippocampus of AD transgenic mice showed intraneuronal onset of A?42 accumulation and fibrillization within cell bodies, neurites, and synapses before plaque formation. Notably, early fibrillar A? was evident within individual synaptic compartments, where it was associated with abnormal morphology. In dendrites, increasing intraneuronal thioflavin S correlated with decreases in neurofilament marker SMI32. Fibrillar A? aggregates could be seen piercing the cell membrane. These data support that A? fibrillization begins within AD vulnerable neurons, leading to disruption of cytoarchitecture and degeneration of spines and neurites. Thus, HR-3D volumetric image analysis allows for better visualization of intraneuronal A? pathology and provides new insights into plaque formation in AD.
Incomplete lysosomal acidification in microglia inhibits the degradation of fibrillar forms of Alzheimers amyloid ? peptide (fA?). Here we show that in primary microglia a chloride transporter, ClC-7, is not delivered efficiently to lysosomes, causing incomplete lysosomal acidification. ClC-7 protein is synthesized by microglia but it is mistargeted and appears to be degraded by an endoplasmic reticulum-associated degradation pathway. Activation of microglia with macrophage colony-stimulating factor induces trafficking of ClC-7 to lysosomes, leading to lysosomal acidification and increased fA? degradation. ClC-7 associates with another protein, Ostm1, which plays an important role in its correct lysosomal targeting. Expression of both ClC-7 and Ostm1 is increased in activated microglia, which can account for the increased delivery of ClC-7 to lysosomes. Our findings suggest a novel mechanism of lysosomal pH regulation in activated microglia that is required for fA? degradation.
Amyloid plaques, a well-known hallmark of Alzheimers disease (AD), are formed by aggregated ?-amyloid (A?). The cellular prion protein (PrPc) accumulates concomitantly with A? in amyloid plaques. One type of amyloid plaque, classified as a neuritic plaque, is composed of an amyloid core and surrounding dystrophic neurites. PrPc immunoreactivity reminiscent of dystrophic neurites is observed in neuritic plaques. Proteinase K treatment prior to immunohistochemistry removes PrPc immunoreactivity from amyloid plaques, whereas A? immunoreactivity is enhanced by this treatment. In the present study, we used a chemical pretreatment by a sarkosyl solution (0.1% sarkosyl, 75 mM NaOH, 2% NaCl), instead of proteinase K treatment, to evaluate PrPc accumulation within amyloid plaques. Since PrPc within amyloid plaques is removed by this chemical pretreatment, we can recognize that the PrP species deposits within amyloid plaques were PrPc. We could observe that PrPc accumulation in dystrophic neurites occurred differently compared with A? or hyperphosphorylated tau aggregation in the AD brain. These results could support the hypothesis that PrPc accumulation in dystrophic neurites reflects a response to impairments in cellular degradation, endocytosis, or transport mechanisms associated with AD rather than a non-specific cross-reactivity between PrPc and aggregated A? or tau.
Accumulation of ?-amyloid (A?) and loss of synapses are hallmarks of Alzheimers disease (AD). How synaptic activity relates to A? accumulation and loss of synapses is a current topic of major interest. Synaptic activation promotes A? secretion, and chronic reduction of synaptic activity reduced A? plaques in an AD transgenic mouse model. This suggested beneficial effects of reducing synaptic activity in AD. We now show that reduced synaptic activity causes detrimental effects on synapses and memory despite reducing plaques using two different models of chronic synaptic inhibition: deafferentation of the barrel cortex and administration of benzodiazepine. An interval of prolonged synaptic inhibition exacerbated loss of synaptophysin compared with synaptically more active brain in AD transgenic but not wild-type mice. Furthermore, an interval of benzodiazepine treatment, followed by a washout period, exacerbated memory impairment in AD transgenic mice. Exacerbation of synaptic and behavioral abnormalities occurred in the setting of reduced A? plaques but elevated intraneuronal A? immunoreactivity. These data support beneficial effects of synaptic activation on A?-related synaptic and behavioral impairment in AD.
The mammalian target of rapamycin (mTOR) is an evolutionarily conserved Ser/Thr protein kinase that plays a pivotal role in multiple fundamental biological processes, including synaptic plasticity. We explored the relationship between the mTOR pathway and ?-amyloid (A?)-induced synaptic dysfunction, which is considered to be critical in the pathogenesis of Alzheimers disease (AD).
The aberrant accumulation of aggregated beta-amyloid peptides (Abeta) as plaques is a hallmark of Alzheimers disease (AD) neuropathology and reduction of Abeta has become a leading direction of emerging experimental therapies for the disease. The mechanism(s) whereby Abeta is involved in the pathophysiology of the disease remain(s) poorly understood. Initially fibrils, and subsequently oligomers of extracellular Abeta have been viewed as the most important pathogenic form of Abeta in AD. More recently, the intraneuronal accumulation of Abeta has been described in the brain, although technical considerations and its relevance in AD have made this a controversial topic. Here, we review the emerging evidence linking intraneuronal Abeta accumulation to the development of synaptic pathology and plaques in AD, and discuss the implications of intraneuronal beta-amyloid for AD pathology, biology, diagnosis and therapy.
beta-Amyloid peptide accumulation plays a central role in the pathogenesis of Alzheimers disease. Aberrant beta-amyloid buildup in the brain has been shown to be present both in the extracellular space and within neurons. Synapses are important targets of beta-amyloid, and alterations in synapses better correlate with cognitive impairment than amyloid plaques or neurofibrillary tangles. The link between beta-amyloid and synapses became even tighter when it was discovered that beta-amyloid accumulates within synapses and that synaptic activity modulates beta-amyloid secretion. Currently, a central question in Alzheimers disease research is what role synaptic activity plays in the disease process, and how specifically beta-amyloid is involved in the synaptic dysfunction that characterizes the disease.
A central question in Alzheimers disease research is what role synaptic activity plays in the disease process. Synaptic activity has been shown to induce beta-amyloid peptide release into the extracellular space, and extracellular beta-amyloid has been shown to be toxic to synapses. We now provide evidence that the well established synaptotoxicity of extracellular beta-amyloid requires gamma-secretase processing of amyloid precursor protein. Recent evidence supports an important role for intraneuronal beta-amyloid in the pathogenesis of Alzheimers disease. We show that synaptic activity reduces intraneuronal beta-amyloid and protects against beta-amyloid-related synaptic alterations. We demonstrate that synaptic activity promotes the transport of the amyloid precursor protein to synapses using live cell imaging, and that the protease neprilysin is involved in reduction of intraneuronal beta-amyloid with synaptic activity.
Immunotherapy approaches for Alzheimer disease currently are among the leading therapeutic directions for the disease. Active and passive immunotherapy against the beta-amyloid peptides that aggregate and accumulate in the brain of those afflicted by the disease have been shown by numerous groups to reduce plaque pathology and improve behavior in transgenic mouse models of the disease. Several ongoing immunotherapy clinical trials for Alzheimer disease are in progress. The background and ongoing challenges for these immunological approaches for the treatment of Alzheimer disease are discussed.
Intrinsic optical emissions, such as autofluorescence and second harmonic generation (SHG), are potentially useful for functional fluorescence imaging and biomedical disease diagnosis for neurodegenerative diseases such as Alzheimers disease (AD). Here, using multiphoton and SHG microscopy, we identified sources of intrinsic emissions in ex vivo, acute brain slices from AD transgenic mouse models. We observed autofluorescence and SHG at senile plaques as well as characterized their emission spectra. The utility of intrinsic emissions was demonstrated by imaging senile plaque autofluorescence in conjunction with SHG from microtubule arrays to assess the polarity of microtubules near pathological lesions. Our results suggest that tissues from AD transgenic models contain distinct intrinsic emissions, which can provide valuable information about the disease mechanisms.
Oxidative stress is one of the earliest events in the pathogenesis of Alzheimers disease (AD) and can markedly exacerbate amyloid pathology. Modulation of antioxidant and anti-inflammatory pathways represents an important approach for AD therapy. Synthetic triterpenoids have been found to facilitate antioxidant response and reduce inflammation in several models. We investigated the effect of the triterpenoid, 2-Cyano-3,12-Dioxooleana-1,9-Dien-28-Oic acid-MethylAmide (CDDO-MA) in Tg19959 mice, which carry the human amyloid precursor protein with two mutations. These mice develop memory impairments and amyloid plaques as early as 2-3 months of age. CDDO-MA was provided with chow (800 mg/kg) from 1 to 4 months of age. CDDO-MA significantly improved spatial memory retention and reduced plaque burden, Abeta42 levels, microgliosis, and oxidative stress in Tg19959 mice.
Multiple lines of evidence have implicated ?-amyloid (A?) in the pathogenesis of Alzheimers disease (AD). However, the mechanism(s) whereby A? is involved in the disease process remains unclear. The dominant hypothesis in AD has been that A? initiates the disease via toxicity from secreted, extracellular A? aggregates. More recently, an alternative hypothesis has emerged focusing on a pool of A? that accumulates early on within AD vulnerable neurons of the brain. Although the topic of intraneuronal A? has been of major interest in the field, technical difficulties in detecting intraneuronal A? have also made this topic remarkably controversial. Here we review evidence pointing to the critical role of intraneuronal A? in AD and provide insights both into challenges faced in detecting intracellular A? and the prion-like properties of A?.
?-Amyloid (A?) plaques are a pathological hallmark of Alzheimers disease (AD) and multiple lines of evidence have linked A? with AD. However, synapse loss is known as the best pathological correlate of cognitive impairment in AD, and intraneuronal A? accumulation has been shown to precede plaque pathology. The progression of A? accumulation to synapse loss and plaque formation remains incomplete. The objective is to investigate the progression of intraneuronal A? accumulation in the brain.
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