SORLA‐retromer complex formation rely on receptor dimerization within the endosome

Abstract

AbstractBackgroundSORLA acts as an endosome recycling receptor together with the retromer complex to escort cargo through the endo‐lysosomal tubular network. SORLA is encoded by the SORL1 gene, which is the gene most frequently mutated in patients with Alzheimer’s disease, although it is still not clear which of the many variants are (the most) pathogenic. Here, we studied the biochemistry and cell biology of a SORL1 variant that is found in several European families.MethodsWe used transfected cells to study SORLA maturation and shedding of the soluble sSORLA by Western blotting. Expression of non‐mutated and mutated receptor at the cell surface was determined by flow cytometry, and the intracellular localization and overlap with markers of Endoplasmic reticulum and endosome compartments investigated by immunocytochemistry and confocal microscopy. The cellular distribution of receptor dimerization was visualized using bimolecular fluorescence complementation and proximity ligation assays, and quantified using the GFP‐trap method.ResultsWe showed that non‐mutated SORLA forms a dimer in retromer‐coated endosome structures, while the pathogenic SORLA protein is unable to form dimers, although it traffics to the endosome. This lack of dimerization in the endosome correlates with strongly decreased receptor maturation, expression at the cell surface, and shedding of its luminal sSORLA domain.ConclusionSORLA dimer formation is required for its association with retromer, which is important for efficient receptor maturation and delivery to the cell surface. Decreased cell surface expression of mutated protein with impaired dimerization leads to less shedding, showing how maturation and sSORLA production can reliably inform on the pathogenicity of SORL1 genetic variants in Alzheimer’s disease.