Estibaliz Capetillo-Zarate

Intraneuronal β-amyloid accumulation and synapse pathology in Alzheimer’s disease

The aberrant accumulation of aggregated β-amyloid peptides (Aβ) as plaques is a hallmark of Alzheimer’s disease (AD) neuropathology and reduction of Aβ has become a leading direction of emerging experimental therapies for the disease. The mechanism(s) whereby Aβ is involved in the pathophysiology of the disease remain(s) poorly understood. Initially fibrils, and subsequently oligomers of extracellular Aβ have been viewed as the most important pathogenic form of Aβ in AD. More recently, the intraneuronal accumulation of Aβ 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 Aβ accumulation to the development of synaptic pathology and plaques in AD, and discuss the implications of intraneuronal β-amyloid for AD pathology, biology, diagnosis and therapy.

Dysregulation of the mTOR Pathway Mediates Impairment of Synaptic Plasticity in a Mouse Model of Alzheimer's Disease

Background 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 β-amyloid (Aβ)-induced synaptic dysfunction, which is considered to be critical in the pathogenesis of Alzheimers disease (AD). Methodology/Principal Findings We provide evidence that inhibition of mTOR signaling correlates with impairment in synaptic plasticity in hippocampal slices from an AD mouse model and in wild-type slices exposed to exogenous Aβ1-42. Importantly, by up-regulating mTOR signaling, glycogen synthase kinase 3 (GSK3) inhibitors rescued LTP in the AD mouse model, and genetic deletion of FK506-binding protein 12 (FKBP12) prevented Aβ-induced impairment in long-term potentiation (LTP). In addition, confocal microscopy demonstrated co-localization of intraneuronal Aβ42 with mTOR. Conclusions/Significance These data support the notion that the mTOR pathway modulates Aβ-related synaptic dysfunction in AD.

Synaptic Activity Reduces Intraneuronal Aβ, Promotes APP Transport to Synapses, and Protects against Aβ-Related Synaptic Alterations

A central question in Alzheimers disease research is what role synaptic activity plays in the disease process. Synaptic activity has been shown to induce β-amyloid peptide release into the extracellular space, and extracellular β-amyloid has been shown to be toxic to synapses. We now provide evidence that the well established synaptotoxicity of extracellular β-amyloid requires γ-secretase processing of amyloid precursor protein. Recent evidence supports an important role for intraneuronal β-amyloid in the pathogenesis of Alzheimers disease. We show that synaptic activity reduces intraneuronal β-amyloid and protects against β-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 β-amyloid with synaptic activity.

Co-occurrence of Alzheimer’s disease β-amyloid and tau pathologies at synapses

Although beta-amyloid (Abeta) plaques and tau neurofibrillary tangles are hallmarks of Alzheimers disease (AD) neuropathology, loss of synapses is considered the best correlate of cognitive decline in AD, rather than plaques or tangles. How pathological Abeta and tau aggregation relate to each other and to alterations in synapses remains unclear. Since aberrant tau phosphorylation occurs in amyloid precursor protein (APP) Swedish mutant transgenic mice, and since neurofibrillary tangles develop in triple transgenic mice harboring mutations in APP, tau and presenilin 1, we utilized these well-characterized mouse models to explore the relation between Abeta and tau pathologies. We now report that pathological accumulation of Abeta and hyperphosphorylation of tau develop concomitantly within synaptic terminals.

Effects of Synaptic Modulation on β-Amyloid, Synaptophysin, and Memory Performance in Alzheimer's Disease Transgenic Mice

Accumulation of β-amyloid (Aβ) and loss of synapses are hallmarks of Alzheimers disease (AD). How synaptic activity relates to Aβ accumulation and loss of synapses is a current topic of major interest. Synaptic activation promotes Aβ secretion, and chronic reduction of synaptic activity reduced Aβ 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 Aβ plaques but elevated intraneuronal Aβ immunoreactivity. These data support beneficial effects of synaptic activation on Aβ-related synaptic and behavioral impairment in AD.

Degradation of Alzheimer's amyloid fibrils by microglia requires delivery of ClC-7 to lysosomes

Microglial lysosomes lack ClC-7, which leads to incomplete lysosomal acidification. In quiescent microglia, ClC-7 is targeted for proteasomal degradation apparently by an endoplasmic reticulum-associated degradation (ERAD) pathway. Microglial activation recruits ClC-7 to lysosomes and causes lysosomal acidification, which leads to efficient amyloid degradation.

Accumulation of Intraneuronal β-Amyloid 42 Peptides Is Associated with Early Changes in Microtubule-Associated Protein 2 in Neurites and Synapses

Pathologic aggregation of β-amyloid (Aβ) peptide and the axonal microtubule-associated protein tau protein are hallmarks of Alzheimers disease (AD). Evidence supports that Aβ peptide accumulation precedes microtubule-related pathology, although the link between Aβ and tau remains unclear. We previously provided evidence for early co-localization of Aβ42 peptides and hyperphosphorylated tau within postsynaptic terminals of CA1 dendrites in the hippocampus of AD transgenic mice. Here, we explore the relation between Aβ 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 Aβ42 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 Aβ42 monomer and low molecular weight oligomer (M/LMW) peptide accumulation. Cumulative evidence suggests that accumulation of M/LMW Aβ42 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 Aβ peptide accumulation with pathology of MAP2 within neuronal processes and synaptic compartments early in the disease process reinforces the importance of intraneuronal Aβ accumulation in AD pathogenesis.

Transgenic Expression of Intraneuronal Aβ42 But Not Aβ40 Leads to Cellular Aβ Lesions, Degeneration, and Functional Impairment without Typical Alzheimer's Disease Pathology

An early role of amyloid-β peptide (Aβ) aggregation in Alzheimers disease pathogenesis is well established. However, the contribution of intracellular or extracellular forms of Aβ to the neurodegenerative process is a subject of considerable debate. We here describe transgenic mice expressing Aβ1–40 (APP47) and Aβ1–42 (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 Aβ concentration. The reduced solubility and increased aggregation of Aβ1–42 may impair its degradation. APP48 mice develop intracellular Aβ 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 Aβ1–40 lesions are found in APP47 mice. Neither APP47 nor APP48 nor the bigenic mice develop extracellular amyloid plaques. While intracellular membrane expression of Aβ1–42 in APP48 mice does not lead to the AD-typical lesions, Aβ aggregates develop within cells accompanied by considerable neurodegeneration.

Dispersible amyloid β-protein oligomers, protofibrils, and fibrils represent diffusible but not soluble aggregates: their role in neurodegeneration in amyloid precursor protein (APP) transgenic mice

Soluble amyloid β-protein (Aβ) aggregates have been identified in the Alzheimers disease (AD) brain. Dispersed Aβ 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 Aβ 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 Aβ-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 Aβ oligomers, protofibrils and fibrils precipitated with oligomer (A11) and protofibril/fibril (B10AP) specific antibodies than APP51/16 mice. These differences, rather than soluble Aβ and Aβ plaque pathology were associated with dendritic degeneration, neuron, and synapse loss in APP23 mice in comparison with APP51/16 mice. Immunoprecipitation of dispersible Aβ oligomers, protofibrils, and fibrils revealed that they were associated with APP C-terminal fragments (APP-CTFs). These results indicate that dispersible Aβ oligomers, protofibrils, and fibrils represent an important pool of Aβ aggregates in the brain that critically interact with membrane-associated APP C-terminal fragments. The concentration of dispersible Aβ aggregates, thereby, presumably determines its toxicity.

Impaired β-Amyloid Secretion in Alzheimer's Disease Pathogenesis

A central question in Alzheimers disease (AD) research is what role β-amyloid peptide (Aβ) plays in synaptic dysfunction. Synaptic activity increases Aβ secretion, potentially inhibiting synapses, but also decreases intraneuronal Aβ, protecting synapses. We now show that levels of secreted Aβ 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 Aβ and reduce intraneuronal Aβ becomes impaired in AD-transgenic but not wild-type neurons with time in culture. We demonstrate that synaptic activity promotes an increase in the Aβ-degrading protease neprilysin at the cell surface and a concomitant increase in colocalization with Aβ42. Remarkably, AD-transgenic but not wild-type neurons show reduced levels of neprilysin with time in culture. This impaired ability to secrete Aβ and reduce intraneuronal Aβ has important implications for the pathogenesis and treatment of AD.