Interaction of the Cytosolic Domains of sorLA/LR11 with the Amyloid Precursor Protein (APP) and β-Secretase β-Site APP-Cleaving Enzyme

Abstract

sorLA is a recently identified neuronal receptor for amyloid precursor protein (APP) that is known to interact with APP and affect its intracellular transport and processing. Decreased levels of sorLA in the brain of Alzheimer's disease (AD) patients and elevated levels of amyloid-␤ peptide (A␤) in sorLA-deficient mice point to the importance of the receptor in this neurodegenerative disorder. We analyzed APP cleavage in an APP-shedding assay and found that both sorLA and, surprisingly, a sorLA tail construct inhibited APP cleavage in a ␤-site APP-cleaving enzyme (BACE)-dependent manner. In line with this finding, sorLA and the sorLA tail significantly reduced secreted A␤ levels when BACE was overexpressed, suggesting that sorLA influences ␤-cleavage. To understand the effect of sorLA on APP cleavage by BACE, we analyzed whether sorLA interacts with APP and/or BACE. Because both full-length sorLA and sorLA C-terminal tail constructs were functionally relevant for APP processing, we analyzed sorLA-APP for a potential cytoplasmatic interaction domain. sorLA and C99 coimmunoprecipitated, pointing toward the existence of a new cytoplasmatic interaction site between sorLA and APP. Moreover, sorLA and BACE also coimmunoprecipitate. Thus, sorLA interacts both with BACE and APP and might therefore directly affect BACE-APP complex formation. To test whether sorLA impacts BACE-APP interactions, we used a fluorescence resonance energy transfer assay to evaluate BACE-APP interactions in cells. We discovered that sorLA significantly reduced BACE-APP interactions in Golgi. We postulate that sorLA acts as a trafficking receptor that prevents BACE-APP interactions and hence BACE cleavage of APP.

SorLA Complement-type Repeat Domains Protect the Amyloid Precursor Protein against Processing

“…Spoelgen et al (36) previously identified a binding site in the cytoplasmic tail. We speculate that the use of different cell lines (SH-SY5Y cells in our study versus N2a cells by Spoelgen et al (36)) may explain why we see no interaction between the cytoplasmic tails.…”

“…Nevertheless, we found that SorLA-⌬CR did not decrease the processing of APP, providing strong evidence of a specific role for SorLA in determining the intracellular trafficking and fate of APP. However, SorLA has also been shown to bind BACE (36), and it is therefore possible that APP and BACE compete for binding to the same region of SorLA. Thus, our mutants may also affect the ability to bind and sort BACE, although we believe that APP is the preferred binding partner of SorLA.…”

“…Linkage between the SorLA and APP cytoplasmic domains by adaptor proteins has recently been suggested (36). Therefore, using SorLA-⌬CR, we tested whether association between the cytoplasmic tails is sufficient for complex formation.…”

GGA1 Acts as a Spatial Switch Altering Amyloid Precursor Protein Trafficking and Processing

“…It further acts as a switch for substrate specificity of ␥-secretase because notch cleavage is, in contrast to APP ␥-cleavage, not affected. It has been shown recently that other proteins involved in protein transport, including syntaxin1a (Khvotchev and Sudhof, 2004) and LR11/sorLA (Scherzer et al, 2004;Andersen et al, 2005;Spoelgen et al, 2006), can impact APP processing. Recent data from BACE transgenic mice also support the idea that the location of BACE, even more so than the amount of BACE in a cell, influences A␤ generation (Lee et al, 2005).…”

Regulation of β cleavage of amyloid precursor protein

“…The interaction of SorLA with the retromer mediates the recycling of APP from endosome to TGN, precluding the preferential  cleavage in endosome. Downregulation of SorLA enhances A formation in cultured cells and in mouse AD models [34,35] . The level of SorLA is reduced in sporadic AD patients, indicating that SorLA may play important roles in A production and AD pathogenesis.…”