Proteolytic Processing of the Alzheimer's Disease Amyloid Precursor Protein within Its Cytoplasmic Domain by Caspase-like Proteases

Abstract

Alzheimer's disease is characterized by neurodegeneration and deposition of ␤A4, a peptide that is proteolytically released from the amyloid precursor protein (APP). Missense mutations in the genes coding for APP and for the polytopic membrane proteins presenilin (PS) 1 and PS2 have been linked to familial forms of early-onset Alzheimer's disease. Overexpression of presenilins, especially that of PS2, induces increased susceptibility for apoptosis that is even more pronounced in cells expressing presenilin mutants. Additionally, presenilins themselves are targets for activated caspases in apoptotic cells. When we analyzed APP in COS-7 cells overexpressing PS2, we observed proteolytic processing close to the APP carboxyl terminus. Proteolytic conversion was increased in the presence of PS2-I, which encodes one of the known PS2 pathogenic muta- Neurodegeneration in selected brain regions is one of the pathological features of Alzheimer's disease, together with amyloid deposition in extracellular plaques, congophilic angiopathy, and intracellular tangles. Amyloid deposited in senile plaques is mainly composed of ␤A4, a peptide consisting of 40 -42 residues, which derives by proteolytic processing from its cognate precursor protein termed APP 1 (1). Mutations in the APP gene cause a rare form of early-onset familial Alzheimer's disease (FAD) by raising the levels of total ␤A4 or by increasing the ratio of the longer form of ␤A4 that ends at residue 42 to the form ending at residue 40 (2-5). Additionally, the expression of APP carrying those mutations has been reported to induce apoptosis in cells, suggesting a probable link between apoptotic pathways and neurodegeneration in Alzheimer's disease (6 -8).Mutations causative for the majority of FAD cases have been identified recently in genes encoding presenilin (PS) 1 and PS2 (9 -11). Missense mutations in these genes were also found to alter APP processing in a pathological manner by increasing the relative concentration of ␤A4 ending at residue 42 (12-16). Both proteins are endoproteolytically converted by two alternative pathways: (i) in the first pathway, cleavage occurs within a highly hydrophobic region of the presenilins (16 -18), and (ii) in the second pathway, presenilins become converted by caspase-3 family proteases at about 30 residues distal to the normal cleavage site (19 -20). The corresponding caspase-derived fragments have been detected in all tissues and stages of animal development, suggesting a role of PS processing in the cellular response to apoptotic signals (19,21). Furthermore, overexpression of PS2 in transfected cells has been shown to increase susceptibility to apoptotic cell death. The latter is even more pronounced in cells expressing mutant PS2-I that encodes one of the known PS2 missense mutations changing Asn 141 into Ile (22, 23).When we analyzed carboxyl-terminal fragments of APP in cells coexpressing PS2 or PS2-I, we observed proteolytic processing of APP within the cytoplasmic domain that was augmented for mutated PS2-I (see below)....

Trafficking and Proteolytic Processing of APP

“…Later it was shown that APP is degraded in lysosomes to amyloidogenic and nonamyloidogenic fragments Haass et al 1992). In addition, APP was shown to be a caspase substrate (Weidemann et al 1999;Lu et al 2003), but the impact of this processing on Ab generation and/or AD pathology is probably minor (Harris et al 2010). …”

The Amyloidogenic Pathway of Amyloid Precursor Protein (APP) Is Independent of Its Cleavage by Caspases

“…A number of laboratories have recently demonstrated that APP can be cleaved in the cytoplasmic domain by caspases after the aspartate residue at position 664, Val-Glu-Val-Asp 664 2Ala, (APP 695 numbering, or Asp 720 using APP 751 numbering) (7)(8)(9)(10)(11)(12). This cleavage would generate a C-terminal-truncated APP molecule that is ϳ3.5 kDa shorter (APP⌬C31) (7,12).…”

Caspase Activation Increases β-Amyloid Generation Independently of Caspase Cleavage of the β-Amyloid Precursor Protein (APP)

“…Each of the four caspases, 3, 6, 7, and 8, have been shown to cleave APP in in vitro assays, and a major caspase site has been identified at Asp-720 (VEVD), resulting in the release of a fragment containing the last 31 amino-acids of APP (C31) and the production of APP⌬C31 (lacking Ala 721 to Asn 751 ) (5)(6)(7)(8). Two other major caspase sites have been recently identified at the N-terminal domain of APP Asp 197 and Asp 219 , leading to the production of a ϳ90 kDa fragment (APP⌬N).…”