Fig. 5. Processing of APP751 in LRP–/– cells can be restored by expression of an LRP-CT construct. (A) LRP–/– fibroblasts transfected with APP751 were infected with a retrovirus expressing the LRP-CT construct and immunoblotted with the LRP C-terminal antibody 1704. No signal is seen in the LRP–/– cells (middle lanes, top panel). The LRP-CT protein is a truncated β-subunit (right lanes, LRP–/– LRP-CT) and migrates faster than the authentic β-subunit (left lanes, LRP+/–) and, as expected, the full-length LRP species (α + β) is absent in the LRP–/– LRP-CT cells (right lanes, top panel). In the bottom panel, immunoblotting of APP-αCTF was carried out with the APP C-terminal antibody CT15. Note that following expression of the LRP-CT construct in LRP–/– cells, the normally low levels of APP-CTF in LRP–/– cells (middle lanes, bottom panel) are now restored to the level seen in LRP+/– cells (compare right with left lanes, bottom panel). (B) Levels of APPs in medium of LRP+/–, LRP–/– and LRP–/– LRP-CT cells were determined by immunoprecipitation/western blotting as before. As with APP-CTF levels, expression of LRP-CT in LRP–/– cells restores the abnormal levels of APPs. Similarly to APP695, APPs release derived from APP751 is decreased after introduction of LRP-CT in LRP–/– cells.