Amyloid Precursor Protein (APP) May Act as a Substrate and a Recognition Unit for CRL4CRBN and Stub1 E3 Ligases Facilitating Ubiquitination of Proteins Involved in Presynaptic Functions and Neurodegeneration

Abstract The Amyloid precursor protein (APP), whose mutations cause Alzheimer's disease, plays an important in vivo role and facilitates transmitter release. Since the APP cytosolic region (ACR) is essential for these functions, we have characterized its brain interactome. We found that the ACR interacts with proteins that regulate the Ubiquitin-Proteasome-system, predominantly with the E3 ubiquitin-protein ligases Stub1, which binds the NH2-terminus of the ACR, and CRL4CRBN., which is formed by Cul4a/b, Ddb1 and Crbn, and interacts with the COOH-terminus of the ACR via Crbn. APP shares essential functions with APP-like protein-2 (APLP2) but not APP-like protein-1 (APLP1). Noteworthy, APLP2, but not APLP1, interacts with Stub1 and CRL4CRBN, pointing to a functional pathway shared only by APP and APLP2. In vitro ubiquitination/ubiquitome analysis indicates that these E3 ligases are enzymatically active and ubiquitinate the ACR residues K649/650/651/676/688. Deletion of Crbn reduces ubiquitination of K676 suggesting that K676 is physiologically ubiquitinated by CRL4CRBN. The ACR facilitated in vitro ubiquitination of presynaptic proteins that regulate exocytosis, suggesting a mechanism by which APP tunes transmitter release. Other dementia-related proteins, namely tau and ApoE, interact with and are ubiquitinated via the ACR in vitro. This, and the evidence that CRBN and CUL4B are linked to Intellectual disability, prompts to hypothesize a pathogenic mechanism, in which APP acts as a modulator of E3 ubiquitin-protein ligase(s), shared by distinct neuronal disorders. The well-described accumulation of ubiquitinated protein inclusions in neurodegenerative diseases and the link between the Ubiquitin-Proteasome-system (UPS) and neurodegeneration make this concept plausible.