Cleavage Of Β-amyloid Precursor Protein Research Articles

Treatment with Simvastatin in Patients with Alzheimer’s Disease Lowers Both α- and β-Cleaved Amyloid Precursor Protein

We investigated the clinical and biological effects of cholesterol-lowering treatment with a statin in 19 patients with Alzheimer’s disease. They received simvastatin 20 mg/day for 12 weeks in an open trial. Primary efficacy parameters were the changes after 12 weeks in the cerebrospinal fluid (CSF) levels of β-amyloid₄₂ (Aβ₄₂), α-secretase-cleaved amyloid precursor protein (α-sAPP), β-secretase-cleaved APP (β-sAPP), tau, phospho-tau and the plasma levels of Aβ₄₂. A secondary efficacy parameter was the change in the Alzheimer’s Disease Assessment Scale-Cognition (ADAS-cog) score. After 12 weeks, CSF α-sAPP and CSF β-sAPP were significantly reduced (p < 0.001), but the CSF levels of tau, phospho-tau, Aβ₄₂ and the plasma levels of Aβ₄₂ were unchanged. The ADAS-cog score was slightly increased (p < 0.05). The results suggest that simvastatin acts directly on the processing of APP by inhibiting both the α- and the β-secretase pathways.

Memapsin 2 (β-secretase) cytosolic domain binds to the VHS domains of GGA1 and GGA2: implications on the endocytosis mechanism of memapsin 2

Memapsin 2, or β-secretase, is a membrane-anchored aspartic protease that initiates the cleavage of β-amyloid precursor protein (APP) leading to the production of β-amyloid peptide in the brain and the onset of Alzheimer’s disease. Memapsin 2 and APP are both endocytosed into endosomes for cleavage. Here we show that the cytosolic domain of memapsin 2, but not that of memapsin 1, binds the VHS domains of GGA1 and GGA2. Gel-immobilized VHS domains of GGA1 and GGA2 also bound to full-length memapsin 2 from cell mammalian lysates. Mutagenesis studies established that Asp 496, Leu 499 and Leu 500 were essential for the binding. The spacing of these three residues in memapsin 2 is identical to those in the cytosolic domains of mannose-6-phosphate receptors, sortilin and low density lipoprotein receptor-related protein 3. These observations suggest that the endocytosis and intracellular transport of memapsin 2, mediated by its cytosolic domain, may involve the binding of GGA1 and GGA2.

BACE1 interacts with nicastrin

β-Amyloid peptide (Aβ) is generated through the proteolytic cleavage of β-amyloid precursor protein (APP) by β- and γ-secretases. The β-secretase, BACE1, initiates Aβ formation followed by γ-cleavage within the APP transmembrane domain. Although BACE1 localizes in the transGolgi network (TGN), its physiological substrates and modulators are not known. In addition, the relationship to other secretase(s) also remains unidentified. Here, we demonstrate that BACE1 binds to nicastrin, a component of γ-secretase complexes, in vitro, and that nicastrin activates β-secretase activity in COS-7 cells.

Presenilin 1 negatively regulates beta-catenin/T cell factor/lymphoid enhancer factor-1 signaling independently of beta-amyloid precursor protein and notch processing.

In addition to its documented role in the proteolytic processing of Notch-1 and the beta-amyloid precursor protein, presenilin 1 (PS1) associates with beta-catenin. In this study, we show that this interaction plays a critical role in regulating beta-catenin/T Cell Factor/Lymphoid Enhancer Factor-1 (LEF) signaling. PS1 deficiency results in accumulation of cytosolic beta-catenin, leading to a beta-catenin/LEF-dependent increase in cyclin D1 transcription and accelerated entry into the S phase of the cell cycle. Conversely, PS1 specifically represses LEF-dependent transcription in a dose-dependent manner. The hyperproliferative response can be reversed by reintroducing PS1 expression or overexpressing axin, but not a PS1 mutant that does not bind beta-catenin (PS1Deltacat) or by two different familial Alzheimer's disease mutants. In contrast, PS1Deltacat restores Notch-1 proteolytic cleavage and Abeta generation in PS1-deficient cells, indicating that PS1 function in modulating beta-catenin levels can be separated from its roles in facilitating gamma-secretase cleavage of beta-amyloid precursor protein and in Notch-1 signaling. Finally, we show an altered response to Wnt signaling and impaired ubiquitination of beta-catenin in the absence of PS1, a phenotype that may account for the increased stability in PS1-deficient cells. Thus, PS1 adds to the molecules that are known to regulate the rapid turnover of beta-catenin.

β-Secretase revealed: starting gate for race to novel therapies for Alzheimer’s disease

Although remarkable progress has been made in the identification of BACE as β-secretase, there is still much to be done. For example, although BACE-mediated cleavage of APP might be important to the pathogenesis of AD, its role in normal cellular function remains unknown. This issue could be addressed by analysing transgenic mice that overexpress BACE, as well as mice that are deficient in both copies of the gene that encodes BACE. It will also be important to understand how BACE activity is regulated in the cell and how this regulation affects APP processing and Aβ production. For example, we already know that overexpression of BACE leads to decreased processing of APP by the alternative α-secretase pathway10xβ-secretase cleavage of Alzheimer’s amyloid precursor protein by the transmembrane aspartic protease BACE. Vassar, R. et al. Science. 1999; 286: 735–741Crossref | PubMed | Scopus (2413)See all References10. Similarly, protein kinase C (PKC)-mediated stimulation of α-secretase results in decreased β-secretase processing of APP, which suggests that these two enzymes might compete for cleavage of APP (Ref. 23xProtein kinase C-dependent α-secretase competes with β-secretase for cleavage of amyloid-β precursor protein in the trans-Golgi network. Skovronsky, D.M. et al. J. Biol. Chem. 2000; 275: 2568–2578Crossref | PubMed | Scopus (243)See all ReferencesRef. 23). Whether dysregulation of β-secretase or α-secretase contributes to the pathogenesis of AD has yet to be tested.It is tempting to speculate that polymorphisms in the BACE gene that affect BACE function might be linked to altered risks of AD. However, the BACE gene, which has been mapped to chromosome 11 (2xA pathogenic mutation for probable Alzheimer’s disease in the APP gene at the N-terminus of β-amyloid. Mullan, M. et al. Nat. Genet. 1992; 1: 345–347Crossref | PubMed | Scopus (813)See all References, 12xMembrane-anchored aspartyl protease with Alzheimer’s disease β-secretase activity. Yan, R. et al. Nature. 1999; 402: 533–537Crossref | PubMed | Scopus (1098)See all References), has not yet been associated genetically with AD. Interestingly, the BACE2 gene, like the gene encoding APP, maps to the obligate Down syndrome region of chromosome 21 (12xMembrane-anchored aspartyl protease with Alzheimer’s disease β-secretase activity. Yan, R. et al. Nature. 1999; 402: 533–537Crossref | PubMed | Scopus (1098)See all References, 22xBACE maps to chromosome 11 and a BACE homolog, BACE2, reside in the obligate down syndrome region of chromosome 21. Saunders, A.J. et al. Science. 1999; 286: 1255aCrossrefSee all References), and Down syndrome patients invariably develop amyloid plaques and AD-like pathology.Finally, the discovery of BACE as β-secretase greatly facilitates the prospects for the development of potential AD therapeutics. With a target protease now in hand, it will be possible to generate small-molecule inhibitors of BACE that could reduce Aβ production. On the basis of the success of protease inhibitors to retard progression of AIDS, the future propects for protease inhibition therapy for AD appear bright. Clinical trials of such BACE inhibitors will provide valuable clues to the pathogenesis of AD. By inhibiting Aβ production and senile plaque formation, these drugs will enable us to examine the link between senile plaques and dementia, thus providing the means for testing the amyloid cascade hypothesis in vivo.

Protein Kinase C-dependent α-Secretase Competes with β-Secretase for Cleavage of Amyloid-β Precursor Protein in the Trans-Golgi Network

The release of amyloidogenic amyloid-beta peptide (Abeta) from amyloid-beta precursor protein (APP) requires cleavage by beta- and gamma-secretases. In contrast, alpha-secretase cleaves APP within the Abeta sequence and precludes amyloidogenesis. Regulated and unregulated alpha-secretase activities have been reported, and the fraction of cellular alpha-secretase activity regulated by protein kinase C (PKC) has been attributed to the ADAM (a disintegrin and metalloprotease) family members TACE and ADAM-10. Although unregulated alpha-secretase cleavage of APP has been shown to occur at the cell surface, we sought to identify the intracellular site of PKC-regulated alpha-secretase APP cleavage. To accomplish this, we measured levels of secreted ectodomains and C-terminal fragments of APP generated by alpha-secretase (sAPPalpha) (C83) versus beta-secretase (sAPPbeta) (C99) and secreted Abeta in cultured cells treated with PKC and inhibitors of TACE/ADAM-10. We found that PKC stimulation increased sAPPalpha but decreased sAPPbeta levels by altering the competition between alpha- versus beta-secretase for APP within the same organelle rather than by perturbing APP trafficking. Moreover, data implicating the trans-Golgi network (TGN) as a major site for beta-secretase activity prompted us to hypothesize that PKC-regulated alpha-secretase(s) also reside in this organelle. To test this hypothesis, we performed studies demonstrating proteolytically mature TACE intracellularly, and we also showed that regulated alpha-secretase APP cleavage occurs in the TGN using an APP mutant construct targeted specifically to the TGN. By detecting regulated alpha-secretase APP cleavage in the TGN by TACE/ADAM-10, we reveal ADAM activity in a novel location. Finally, the competition between TACE/ADAM-10 and beta-secretase for intracellular APP cleavage may represent a novel target for the discovery of new therapeutic agents to treat Alzheimer's disease.

Involvement of Caspases in Proteolytic Cleavage of Alzheimer’s Amyloid-β Precursor Protein and Amyloidogenic Aβ Peptide Formation

The amyloid-β precursor protein (APP) is directly and efficiently cleaved by caspases during apoptosis, resulting in elevated amyloid-β (Aβ) peptide formation. The predominant site of caspase-mediated proteolysis is within the cytoplasmic tail of APP, and cleavage at this site occurs in hippocampal neurons in vivo following acute excitotoxic or ischemic brain injury. Caspase-3 is the predominant caspase involved in APP cleavage, consistent with its marked elevation in dying neurons of Alzheimer’s disease brains and colocalization of its APP cleavage product with Aβ in senile plaques. Caspases thus appear to play a dual role in proteolytic processing of APP and the resulting propensity for Aβ peptide formation, as well as in the ultimate apoptotic death of neurons in Alzheimer’s disease.

Two-way cleavage of β-amyloid precursor protein by multicatalytic proteinase: Shin-ichi Kojima and Hotoko Omori. Research Institute, Sumitomo Pharmaceuticals Co., Ltd., Osaka, 554 Japan

Two-way cleavage of β-amyloid precursor protein by multicatalytic proteinase: Shin-ichi Kojima and Hotoko Omori. Research Institute, Sumitomo Pharmaceuticals Co., Ltd., Osaka, 554 Japan