Immunohistological and western blot characterization of amyloidogenic proteins and ∃-site cleavage in resected human choroid plexus (CP) and cortical (Ctx) samples (n=5). Panels (A-C) show that BACE1 immunoreactivity (IR) is absent in the CP in BACE1 knockout (BACE1−/−) mouse brain (A), but exists in wild-type (BACE+/+) counterpart (B, C, pointed by arrows). Note the heavy BACE1 IR in the mossy fiber (mf) terminals in (B). Double immunofluorescence with bisbenzimide (Bis) counterstain shows that the CP cells exhibit colocalized IR for ∃-amyloid precusor protein (APP) (22C11 antibody) and BACE1 (D-F), and for 6E10 and presenilin (PS1) (G, H). APP and BACE1 protein levels are higher in human CP than cortical homogenates (I, K). ∃-Site APP cleavage products (C99 and C89) are present in CP extracts (J). ∃-Site APP cleavage activity is higher in the CP relative to cortical extracts (L). Scale bar = 1 mm in (A) applying to (B, D); equal to 200 :m for (C) and 50 :m for (E-H).