Pathogenic APP mutations near the -secretase cleavage site differentially affect Aβ secretion and APP C-terminal fragment stability
Laboratory of Neurogenetics, Department of Molecular Genetics, Flanders Interuniversity Institute for Biotechnology, University of Antwerp, Belgium. Human Molecular Genetics
(Impact Factor: 6.39).
Release of amyloid beta (Abeta) from the amyloid precursor protein (APP) requires cleavages by beta- and gamma-secretases and plays a crucial role in Alzheimer's disease (AD) pathogenesis. Missense mutations in the APP gene causing familial AD are clustered around the beta-, alpha- and particular gamma-secretase cleavage sites. We systematically compare in primary neurons the effect on APP processing of a series of clinical APP mutations (two of which not characterized before) located in close proximity to the gamma-secretase cleavage site. We confirm and extend previous observations showing that all these mutations (T714I, V715M, V715A, I716V, V717I and V717L) affect gamma-secretase cleavage causing an increased relative ratio of Abeta42 to Abeta40. Taking advantage of these extended series of APP mutations we were able to demonstrate an inverse correlation between these ratios and the age at onset of the disease in the different families. In addition, a subset of mutations caused the accumulation of APP C-terminal fragments indicating that these mutations also influence the stability of APP C-terminal fragments. However, it is unlikely that these fragments contribute significantly to the disease process.
Available from: Manish K. Tiwari
- "Among the isoforms of A␤ , the longer tend to be more hydrophobic and thus more prone to aggregation  , and of the two major isoforms, A␤ 42 (typically 10% of the total amyloid load, but the major form in deposited plaques) is more cytotoxic than the A␤ 40 isoform . Consistent with this, several A␤PP mutations likely disturb ␥-secretase activity to increase the A␤ 42 /A␤ 40 ratio . Despite these advances, the activeconformationsandchemicalpropertiesofthenormal A␤ monomers that serve as precursors in oligomer formation are currently unknown and hard to study experimentally due to their intrinsic disorder . "
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ABSTRACT: Protein aggregation is a hallmark of many neurodegenerative disorders. Alzheimer's disease (AD) is directly linked to deposits of amyloid-β (Aβ) derived from the amyloid-β protein precursor (AβPP), and multiple experimental studies have investigated the aggregation behavior of these amyloids. The present paper reports modeling of the aggregation propensities and cell toxicities of genetic variants of Aβ known to increase disease risk. From correlation to experimental data, and using four distinct experimental structures to test structural sensitivity, we find that the Spatial Aggregation Propensity (SAP) formalism can describe the relative experimental aggregation propensities of Aβ 42 variants (R2 = 0.49 and 0.70, p∼0.02 and 0.002, for 1IYT and 1Z0Q conformations using a probe radius of 10 Å). Our analysis finds correlation between the reduction in hydrophilic surface and experimental aggregation propensities. Finally, we show that experimental cell toxicities of Aβ variants are well described by computed SAP values, suggesting direct interplay between aggregation propensity and cell toxicity and providing a step toward a first computational estimator of Aβ toxicity. The present study contributes to our understanding of amyloid aggregation and suggests a method to predict aggregation propensity and toxicity of Aβ variants, and potentially to reduce aggregation propensities of amyloids by molecular intervention directed toward specific conformations of the peptides.
Available from: Jorge Iván Castillo-Quan
- "and may therefore not include other potential/additional benefits of lithium on Aβ toxicity. As well as the increased ratio of Aβ42 to Aβ40 peptide observed in familial AD cases with APP mutations (De Jonghe et al., 2001), increased levels of APP could also contribute to AD pathogenesis. Indeed, patients with Down syndrome have a high risk of developing AD possibly due to trisomy of the APP gene which leads to increased APP expression (Wiseman et al., 2009). "
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ABSTRACT: The greatest risk factor for Alzheimer's disease (AD) is age, and changes in the ageing nervous system are likely contributors to AD pathology. Amyloid beta (Aβ) accumulation, which occurs as a result of the amyloidogenic processing of amyloid precursor protein (APP), is thought to initiate the pathogenesis of AD, eventually leading to neuronal cell death. Previously, we developed an adult-onset Drosophila model of AD. Mutant Aβ42 accumulation led to increased mortality and neuronal dysfunction in the adult flies. Furthermore, we showed that lithium reduced Aβ42 protein, but not mRNA, and was able to rescue Aβ42-induced toxicity. In the current study, we investigated the mechanism/s by which lithium modulates Aβ42 protein levels and Aβ42 induced toxicity in the fly model. We found that lithium caused a reduction in protein synthesis in Drosophila and hence the level of Aβ42. At both the low and high doses tested, lithium rescued the locomotory defects induced by Aβ42, but it rescued lifespan only at lower doses, suggesting that long-term, high-dose lithium treatment may have induced toxicity. Lithium also down-regulated translation in the fission yeast Schizosaccharomyces pombe associated with increased chronological lifespan. Our data highlight a role for lithium and reduced protein synthesis as potential therapeutic targets for AD pathogenesis.
Available from: SangYun Kim
- "Mutations near the cleavage site of alpha secretase (Glu693Lys, Glu693Gly, Glu693del, Asp694Asn) might change the processing of APP, in enhancing the proteolytic resistance of Abeta peptide.16,17 De Jonghe et al studied the APP mutations near the gamma secretase cleavage site.13 Missense mutations at codon 714-715 of APP decreased the secretion of Abeta 40, and the mutations at codon 716-717 increased the production and secretion of Abeta 42. "
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ABSTRACT: Alzheimer's disease (AD) is a complex and heterogeneous neurodegenerative disorder, classified as either early onset (under 65 years of age), or late onset (over 65 years of age). Three main genes are involved in early onset AD: amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2). The apolipoprotein E (APOE) E4 allele has been found to be a main risk factor for late-onset Alzheimer's disease. Additionally, genome-wide association studies (GWASs) have identified several genes that might be potential risk factors for AD, including clusterin (CLU), complement receptor 1 (CR1), phosphatidylinositol binding clathrin assembly protein (PICALM), and sortilin-related receptor (SORL1). Recent studies have discovered additional novel genes that might be involved in late-onset AD, such as triggering receptor expressed on myeloid cells 2 (TREM2) and cluster of differentiation 33 (CD33). Identification of new AD-related genes is important for better understanding of the pathomechanisms leading to neurodegeneration. Since the differential diagnoses of neurodegenerative disorders are difficult, especially in the early stages, genetic testing is essential for diagnostic processes. Next-generation sequencing studies have been successfully used for detecting mutations, monitoring the epigenetic changes, and analyzing transcriptomes. These studies may be a promising approach toward understanding the complete genetic mechanisms of diverse genetic disorders such as AD.
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