Pathogenic APP mutations near the gamma-secretase cleavage site differentially affect Abeta secretion and APP C-terminal fragment stability.
ABSTRACT 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.
- SourceAvailable from: Jorge Iván Castillo-Quan[Show abstract] [Hide abstract]
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.Frontiers in Aging Neuroscience 07/2014; 6:190. · 5.20 Impact Factor
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ABSTRACT: Early-onset Alzheimer's disease (EOAD) represents 1%–2% of the Alzheimer's disease (AD) cases, and it is generally characterized by a positive family history and a rapidly progressive symptomatology. Rare coding and fully penetrant variants in amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) are the only causative mutations reported for autosomal dominant AD. Thus, in this study we used exome sequencing data to rapidly screen rare coding variability in APP, PSEN1, and PSEN2, in a British cohort composed of 47 unrelated EOAD cases and 179 elderly controls, neuropathologically proven. We report 2 novel and likely pathogenic variants in PSEN1 (p.L166V and p.S230R). A comprehensive catalog of rare pathogenic variants in the AD Mendelian genes is pivotal for a premortem diagnosis of autosomal dominant EOAD and for the differential diagnosis with other early onset dementias such as frontotemporal dementia (FTD) and Creutzfeldt-Jakob disease (CJD).Neurobiology of Aging. 10/2014; 35(10):2422.e13–2422.e16.
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ABSTRACT: Alzheimer's disease (AD), a progressive neurodegenerative disorder that is the most common cause of dementia in the elderly, is characterized by the accumulation of amyloid-beta (Abeta) plaques and neurofibrillary tangles, as well as a progressive loss of synapses and neurons in the brain. The major pertinacious component of amyloid plaques is Abeta, a variably sized peptide derived from the integral membrane protein amyloid precursor protein (APP). The Abeta region of APP locates partly within its ecto- and trans-membrane domains. APP is cleaved by three proteases, designated as alpha-, beta-, and gamma-secretases. Processing by beta- and gamma-secretase cleaves the N- and C-terminal ends of the Abeta region, respectively, releasing Abeta, whereas alpha-secretase cleaves within the Abeta sequence, releasing soluble APPalpha (sAPPalpha). The gamma-secretase cleaves at several adjacent sites to yield Abeta species containing 39-43 amino acid residues. Both alpha- and beta-cleavage sites of human wild-type APP are located in APP672-699 region (ectodomain of beta-C-terminal fragment, ED-beta-CTF or ED-C99). Therefore, the amino acid residues within or near this region are definitely pivotal for human wild-type APP function and processing. Here, we report that one ED-C99-specific monoclonal antibody (mAbED-C99) blocks human wild-type APP endocytosis and shifts its processing from alpha- to beta-cleavage, as evidenced by elevated accumulation of cell surface full-length APP and beta-CTF together with reduced sAPPalpha and alpha-CTF levels. Moreover, mAbED-C99 enhances the interactions of APP with cholesterol. Consistently, intracerebroventricular injection of mAbED-C99 to human wild-type APP transgenic mice markedly increases membrane-associated beta-CTF. All these findings suggest that APP672-699 region is critical for human wild-type APP processing and may provide new clues for the pathogenesis of sporadic AD.Cell Death & Disease 08/2014; 5:e1374. · 6.04 Impact Factor