Alzheimer’s disease (AD) is a very common neurodegenerative disorder, chiefly caused by increased production of neurotoxic b-amyloid (Ab) peptide generated from proteolytic cleavage of b-amyloid protein precursor (APP). Except for familial AD arising from mutations in the APP and presenilin (PSEN) genes, the molecular mechanisms regulating the amyloidogenic processing of APP are largely unclear. Alcadein a/calsyntenin1 (ALCa/CLSTN1) is a neuronal type I transmembrane protein that forms a complex with APP, mediated by the neuronal adaptor protein X11-like (X11L or MINT2). Formation of the ALCa–X11L–APP tripartite complex suppresses Ab generation in vitro, and X11L-deficient mice exhibit enhanced amyloidogenic processing of endogenous APP. However, the role of ALCa in APP metabolism in vivo remains unclear. Here, by generating ALCa-deficient mice and using immunohistochemistry, immunoblotting, and co-immunoprecipitation analyses, we verified the role of ALCa in the suppression of amyloidogenic processing of endogenous APP in vivo. We observed that ALCa deficiency attenuates the association of X11L with APP, significantly enhances amyloidogenic b-site cleavage of APP, especially in endosomes, and increases the generation of endogenous Ab in the brain. Furthermore, we noted amyloid plaque formation in the brains of human APP-transgenic mice in an ALCa-deficient background. These results unveil a potential role of ALCa in protecting cerebral neurons from Ab-dependent pathogenicity in AD.
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