SorCS1 variants and amyloid precursor protein (APP) are co‐transported in neurons but only SorCS1c modulates anterograde APP transport

Processing of amyloid precursor protein (APP) into amyloid-beta peptide (A beta) is crucial for the development of Alzheimer's disease (AD). Because this processing is highly dependent on its intracellular itinerary, altered subcellular targeting of APP is thought to directly affect the degree to which A beta is generated. The sorting receptor SorCS1 has been genetically linked to AD, but the underlying molecular mechanisms are poorly understood. We analyze two SorCS1 variants; one, SorCS1c, conveys internalization of surface-bound ligands whereas the other, SorCS1b, does not. In agreement with previous studies, we demonstrate co-immunoprecipitation and co-localization of both SorCS1 variants with APP. Our results suggest that SorCS1c and APP are internalized independently, although they mostly share a common post-endocytic pathway. We introduce functional Venus-tagged constructs to study SorCS1b and SorCS1c in living cells. Both variants are transported by fast anterograde axonal transport machinery and about 30% of anterograde APP-positive transport vesicles contain SorCS1. Co-expression of SorCS1b caused no change of APP transport kinetics, but SorCS1c reduced the anterograde transport rate of APP and increased the number of APP-positive stationary vesicles. These data suggest that SorCS1 and APP share trafficking pathways and that SorCS1c can retain APP from insertion into anterograde transport vesicles.