Transmembrane proteins BRI2 and amyloid precursor protein (APP) co-localize with amyloid ␤ (A␤) lesions in sporadic Alzheimer disease and mutations in both precursor proteins are linked to early-onset familial cases of cerebral amyloidosis associated with dementia and/or cerebral hemorrhage. A specific interaction between BRI2 and APP was unveiled by immunoprecipitation experiments using transfected and non-transfected cells. The use of deletion mutants further revealed that stretches 648 -719 of APP751 and 46 -106 of BRI2, both inclusive of the full transmembrane domains, are sufficient for the interaction. Removal of most of the APP and BRI2 extracellular domains without affecting the interaction implies that both proteins interact when are expressed on the same cell membrane (cis) rather than on adjacent cells (trans). The presence of BRI2 had a modulatory effect on APP processing, specifically increasing the levels of cellular APP as well as ␤-secretasegenerated COOH-terminal fragments while decreasing the levels of ␣-secretase-generated COOH-terminal fragments as well as the secretion of total APP and A␤ peptides. Determining the precise molecular pathways affected by the specific binding between APP and BRI2 could result in the identification of common therapeutic targets for these sporadic and familial neurodegenerative disorders. A␤,1 a 39 -42-amino acid peptide of unknown biological function normally present in biological fluids, is also the main constituent of parenchymal and vascular amyloid deposits characteristic of Alzheimer disease (AD) (reviewed in Ref. 1). It is an internal fragment of the larger type-I transmembrane amyloid precursor protein APP (also referred as A␤PP), which exists in several isoforms of different length, ranging from 695 to 770 residues (2, 3). From all these APP isoforms, A␤ is normally generated by proteolytic processing through the sequential action of ␤-and ␥-secretases. Within amyloid lesions, a number of unrelated components collectively known as amyloid-associated proteins (amyloid P-component, ␣1-antichymotrypsin, apoE, apoJ, complement components, vitronectin, extracellular matrix proteins, and APP, among others) co-localize with fibrillar and non-fibrillar A␤, as shown by immunohistochemical studies (4 -10). It is not clear whether these proteins are important for the mechanism of amyloidogenesis or just innocent bystanders. Recently, it was reported that a novel protein BRI2 was abundant in dystrophic neurites, in senile plaques, and around vessels in ischemic lesions in AD and was also detected in Lewy neurites in cases of dementia with Lewy bodies and Parkinson disease (11). Of interest, mutations at or near the stop codon of BRI2 are associated with dementia and cerebellar ataxia in kindreds of British (12) and Danish (13) origin.BRI2 is a type-II transmembrane protein encoded by the single gene BRI2 (also known as ITM2B and E25B) located on the long arm of chromosome 13 (12, 14 -16). BRI2 belongs to an evolutionary conserved multigene family comprising at least...

Presenilin-1 (PS1) has gained intensive attention in relation to Alzheimer’s disease, since it has been shown that PS1 mutations are linked to familial Alzheimer’s disease (FAD), and that PS1 is a member of the high molecular weight complex of γ-secretase, which generates the carboxyl end of β-amyloid peptide (γ-cleavage). A parallel line of evidence suggests that upon formation of cell-cell contacts, presenilin colocalizes with cadherins at the cell surface and stabilizes the cadherin-based adhesion complex. Under conditions stimulating cell-cell dissociation, cadherins are processed by a PS1/γ-secretase activity, promoting disassembly of adherens junctions, and resulting in the increase of cytosolic β-catenin, which is an important regulator of the Wnt/Wingless signaling pathway. PS1 also controls the cleavage of a number of transmembrane proteins at the interface of their transmembrane and cytosolic domains (Ε-cleavage), producing intracellular fragments with a putative transcriptional role. Remarkably, cleavage of N-cadherin by PS1 produces an intracellular fragment that downregulates CREB-mediated transcription, indicating a role of PS1 in gene expression. PS1 mutations associated with FAD abolish production of the N-cadherin intracellular fragment and thus fail to suppress CREB-dependent transcription. These findings suggest an alternative explanation for FAD that is separate from the widely accepted ‘amyloid hypothesis’: dysfunction in transcription regulatory mechanisms.

BRI2, a protein mutated in Familial British and Familial Danish Dementias, interacts with Amyloid Precursor Protein (APP) and reduces the levels of secreted APPβ (sAPPβ), which derives from APP cleavage by β-secretase (BACE1). Exploring the mechanisms of this effect, we obtained data that BRI2 decreases the cellular levels of BACE1 thus reducing the β-cleavage of APP. Deletion of N-terminal cytoplasmic or C-terminal extracellular sequences of BRI2 neither affected its interaction with BACE1 or APP (Fotinopoulou et al., 2005) nor the reduction in the levels of BACE1 and sAPPβ. These results suggest that BRI2 may prevent access of BACE1 to APP and the BRI2/BACE1 interaction may mediate the reduction in BACE1 levels. In support, BRI2 expression induced lysosomal but not proteasomal degradation of BACE1. In parallel, BRI2 expression was also found to reduce BACE1 mRNA levels by 50%. This study adds novel information regarding the mechanism by which BRI2 affects APP processing and BACE1 levels.

The amyloid-β protein precursor (AβPP) is a type-1 transmembrane protein involved in Alzheimer's disease (AD). It has become increasingly evident that AβPP, its protein-protein interactions, and its proteolytical fragments may affect calcium homeostasis and vice versa. In addition, there is evidence that calcium dysregulation contributes to AD. To study the role of AβPP in calcium homeostasis, we downregulated its expression in SH-SY5Y cells using shRNA (SH-SY5Y/AβPP-) or increased expression of AβPP695 by transfection (SH-SY5Y/AβPP+). The levels of cytosolic Ca2+ after treatment with thapsigargin, monensin, activation of capacitative calcium entry (CCE), and treatment with SKF, a store operated channel (SOCs) inhibitor, were measured by fura-2AM fluorimetry. SH-SY5Y/AβPP+ cells show reduced response to thapsigargin and reduced CCE, although this reduction is not statistically significant. On the other hand, we found that, relative to SH-SY5Y, SH-SY5Y/AβPP- cells show a significant increase in the response to thapsigargin but not in CCE and their SOCs were more susceptible to SKF inhibition. Additionally, downregulation of AβPP resulted in increased response to monensin that induces calcium release from acidic stores. The increase of calcium release from the endoplasmic reticulum and the acidic stores, when AβPP is downregulated, could be attributed to elevated Ca2+ content or to a dysregulation of Ca2+ transfer through their membranes. These data, along with already existing evidence regarding the role of AβPP in calcium homeostasis and the early occurring structural and functional abnormalities of endosomes, further substantiate the role of AβPP in calcium homeostasis and in AD.

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