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.
?-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.
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.
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.
The role of cerebral amyloid angiopathy (CAA) in the pathogenesis of Alzheimers disease (AD) is not fully understood. Here, we studied whether CAA is associated with alterations in microvascularisation in transgenic mouse models and in the human brain. APP23 mice at 25-26 months of age exhibited severe CAA in thalamic vessels whereas APP51/16 mice did not. Wild-type littermates were free of CAA. We found CAA-related capillary occlusion within the thalamus of APP23 mice but not in APP51/16 and wild-type mice. Magnetic resonance angiography (MRA) showed blood flow alterations in the thalamic vessels of APP23 mice. CAA-related capillary occlusion in the branches of the thalamoperforating arteries of APP23 mice, thereby, corresponded to the occurrence of blood flow disturbances. Similarly, CAA-related capillary occlusion was observed in the human occipital cortex of AD cases but less frequently in controls. These results indicate that capillary CAA can result in capillary occlusion and is associated with cerebral blood flow disturbances providing an additional mechanism for toxic effects of the amyloid beta-protein in AD.
Soluble amyloid ?-protein (A?) aggregates have been identified in the Alzheimers disease (AD) brain. Dispersed A? aggregates in the brain parenchyma are different from soluble, membrane-associated and plaque-associated solid aggregates. They are in mixture with the extra- or intracellular fluid but can be separated from soluble proteins by ultracentrifugation. To clarify the role of dispersible A? aggregates for neurodegeneration we analyzed 2 different amyloid precursor protein (APP)-transgenic mouse models. APP23 mice overexpress human mutant APP with the Swedish mutation. APP51/16 mice express high levels of human wild type APP. Both mice develop A?-plaques. Dendritic degeneration, neuron loss, and loss of asymmetric synapses were seen in APP23 but not in APP51/16 mice. The soluble and dispersible fractions not separated from one another were received as supernatant after centrifugation of native forebrain homogenates at 14,000 × g. Subsequent ultracentrifugation separated the soluble, i.e., the supernatant, from the dispersible fraction, i.e., the resuspended pellet. The major biochemical difference between APP23 and APP51/16 mice was that APP23 mice exhibited higher levels of dispersible A? oligomers, protofibrils and fibrils precipitated with oligomer (A11) and protofibril/fibril (B10AP) specific antibodies than APP51/16 mice. These differences, rather than soluble A? and A? plaque pathology were associated with dendritic degeneration, neuron, and synapse loss in APP23 mice in comparison with APP51/16 mice. Immunoprecipitation of dispersible A? oligomers, protofibrils, and fibrils revealed that they were associated with APP C-terminal fragments (APP-CTFs). These results indicate that dispersible A? oligomers, protofibrils, and fibrils represent an important pool of A? aggregates in the brain that critically interact with membrane-associated APP C-terminal fragments. The concentration of dispersible A? aggregates, thereby, presumably determines its toxicity.
An early role of amyloid-? peptide (A?) aggregation in Alzheimers disease pathogenesis is well established. However, the contribution of intracellular or extracellular forms of A? to the neurodegenerative process is a subject of considerable debate. We here describe transgenic mice expressing A?1-40 (APP47) and A?1-42 (APP48) with a cleaved signal sequence to insert both peptides during synthesis into the endoplasmic reticulum. Although lower in transgene mRNA, APP48 mice reach a higher brain A? concentration. The reduced solubility and increased aggregation of A?1-42 may impair its degradation. APP48 mice develop intracellular A? lesions in dendrites and lysosomes. The hippocampal neuron number is reduced already at young age. The brain weight decreases during aging in conjunction with severe white matter atrophy. The mice show a motor impairment. Only very few A?1-40 lesions are found in APP47 mice. Neither APP47 nor APP48 nor the bigenic mice develop extracellular amyloid plaques. While intracellular membrane expression of A?1-42 in APP48 mice does not lead to the AD-typical lesions, A? aggregates develop within cells accompanied by considerable neurodegeneration.
?-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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