FIG.1.

(A) Selective recombination of LRP1 in brain and spinal cord from SynCre/LRP^lox/lox mice. Genomic DNA was prepared from the indicated tissues of SynCre/LRP^lox/lox mice. PCR analysis with primers specific for the recombined or nonrecombined allele, respectively, was performed. (Left) Recombination is detected in brain and spinal cord only. (Right) The nonrecombined allele is present in all tissues. (B) Reduced expression of LRP1 in SynCre/LRP^lox/lox neural tissue. (Top) Total brain RNA was prepared from two adult SynCre/LRP^lox/lox and two age-matched LRP^lox/lox mice. RNA samples were separated by agarose gel electrophoresis and analyzed by Northern blotting using an LRP1-specific probe. LRP1-mRNA in SynCre/LRP^lox/lox mice (lanes 3 and 4) is reduced significantly in comparison to LRP^lox/lox control animals (lanes 1 and 2). (Middle) Brain membrane preparations from adult SynCre/LRP^lox/lox and LRP^lox/lox mice were subjected to SDS-polyacrylamide gel electrophoresis and quantitative Western blotting with an LRP1 antibody. LRP1 protein levels are reduced by ∼50% in the brains of SynCre/LRP^lox/lox mice (lanes 3 and 4) compared to LRP^lox/lox mice (lanes 1 and 2). (Bottom) Ligand blotting with ¹²⁵I-labeled RAP confirms reduced expression of LRP1 in the brains of adult SynCre/LRP^lox/lox mice (lanes 3 and 4). RAP binds to the ligand binding domains of LRP1 and thus detects only the large 515-kDa subunit of the receptor. (C) LRP1 expression is lost in the majority of neurons from SynCre/LRP^lox/lox mice. Brain sections from adult SynCre/LRP^lox/lox (b, d, and f) and LRP^lox/lox control (a, c, and e) mice were examined for LRP1 expression by immunohistochemistry (a to d) and by in situ hybridization (e to f). Immunohistochemical staining with a polyclonal anti-LRP1 antibody reveals complete loss of LRP1 expression in most, but not all, neocortical neurons in SynCre/LRP^lox/lox (b) compared to LRP^lox/lox (a) mice. Recombination of the loxP-marked LRP1 allele and subsequent loss of LRP1 expression is particularly pronounced in large pyramidal neurons (arrows in panel d). In situ hybridization with an LRP1-specific probe confirms the absence of LRP1 mRNA in the majority of SynCre/LRP^lox/lox neurons (f).