• The Role of the LDLR Family Member LRAD3 in the Brain

      Noyes, Nathaniel Christopher; Strickland, Dudley K. (2014)
      The LDLR family is a class of cell surface receptors important in a wide range of biological activities, including neuronal function and pathology. We discovered the highly conserved gene LDLRAD3 in a screen of the human EST database for novel LDLR-related genes expressed in the brain. The protein product for this gene, LRAD3, is highly conserved across species in amino acid sequence, specifically in functionally important protein domains and motifs. Therefore, we hypothesized that LRAD3 should have an important role in the brain. Indeed, we found LRAD3 to be highly expressed in the brain and specifically in neurons. We performed an unbiased screen searching for proteins that interact with LRAD3 in mouse brain and discovered several of the WW domain containing members of the Nedd4 family, including the E3 ligase Itch. This interaction results an increase in the ubiquitination activity of Itch that is dependent on the PPxY motifs in LRAD3. The increase in Itch activity leads to increased auto-ubiquitination and subsequent degradation of Itch. Finally, we demonstrate that the activation-induced degradation of Itch is also achieved by the protein Spartin and is therefore not unique to LRAD3. These results reveal that LRAD3 is a component of the ubiquitin proteasome system (UPS) similar to other Itch activating, PPxY containing proteins like NDFIP1 and Spartin. The UPS regulates proteins involved in Alzheimers Disease (AD) and the UPS is dysfunctional in AD. This led us to study the role of LRAD3 in the processing of the AD associated protein, APP, a protein regulated by a number of LDLR family members. We found that LRAD3 interacts with APP and this interaction is mediated through either their transmembrane domains or intracellular domains. The association of LRAD3 with APP increases the amyloidogenic processing of APP resulting in a decrease in sAPPα production and an increase in Aβ peptide production. Pulse-chase experiments revealed that LRAD3 expression significantly decreases the cellular half-life of mature APP. Our work indicates that LRAD3 may be an important component in AD, potentially through its regulation of the UPS.