Amyloid precursor protein in unique cholesterol-rich microdomains different from caveolae-like domains.

To determine the localization of the amyloid precursor protein (APP) on the cellular membrane, we performed membrane fractionation of cultured cells including that of Madin-Darby canine kidney (MDCK) and P19 cells transfected with human APP cDNA, non-transfected SH-SY5Y cells, and rat cerebral cortices. In MDCK cells, APP was exclusively present in abundance in the supernatant following solubilization of the plasma membranes using Triton X-100, and in high-density fractions of sucrose density gradient fractionation (SDGF) following Triton X-100 solubilization of whole cellular membranes. Caveolin-1 was not cofractionated with APP. In experiments using P19 cells and rat cerebral cortices, we detected two isoforms of APP. The APP with the apparently lower molecular weight (immature type) coexisted in abundance with integrin in the high-density fractions, whereas the APP with the apparently higher molecular weight (mature type) was recovered predominantly in the low-density fractions with cholesterol and GM1 gangliosides, the concentrations of which were higher than those in the bulk plasma membranes, but lower than those in caveolae-like domains (CLDs), following SDGF of Triton X-100-solubilized cellular membranes. The results of this study suggest the following; first, APP is not present in abundance in caveolae or CLDs, but is in unique cholesterol-rich microdomains; second, the targeting of APP to these unique microdomains may be linked to the maturation of APP in some cells.