NgR1 is cleaved at the cell surface by MT1-, MT2-, and MT3-MMPs. A, Western blot analysis of cell lysates and conditioned medium from COS-7 cells cotransfected with mouse NgR1 and FLAG-tagged MT1-, MT2-, MT3-, MT4-, MT5-, and MT6-MMPs (MT1–MT6, respectively) or TACE. Cotransfection with MT1-, MT2-, MT3-, and MT5-MMPs promoted NgR1 shedding. Anti-mouse NgR1 antibody was used to detect full-length and shed NgR1. MT3-MMP is apparent only in cell lysates with longer Western blot exposures (see lower panel). B, in vitro cleavage of recombinant NgR1 with recombinant (rec) MT1-, MT2-, MT3-, and MT5-MMPs or TACE. C, Western blot analysis of conditioned medium and cell-surface biotinylated proteins (CSB) from cortical neurons treated with 1 μm recombinant MT1-, MT2-, and MT3-MMPs or TACE.