Cu ions have been suggested to enhance the assembly and pathogenic potential of the Alzheimer's disease amyloid- (A) peptide. To explore this relationship in vivo, toxic-milk (tx J ) mice with a mutant ATPase7b transporter favoring elevated Cu levels were analyzed in combination with the transgenic (Tg) CRND8 amyloid precursor protein mice exhibiting robust A deposition. Unexpectedly, TgCRND8 mice homozygous for the recessive tx J mutation examined at 6 months of age exhibited a reduced number of amyloid plaques and diminished plasma A levels. In addition, homozygosity for tx J increased survival of young TgCRND8 mice and lowered endogenous CNS A at times before detectable increases in Cu in the CNS. These data suggest that the beneficial effect of the tx J mutation on CNS A burden may proceed by a previously undescribed mechanism, likely involving increased clearance of peripheral pools of A peptide. A lzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by extracellular deposition of amyloid- (A) as senile plaques and intracellular accumulation of hyperphosphorylated tau as neurofibrillary tangles (1). A is generated by secretase-mediated endoproteolysis of the amyloid precursor protein (APP), and familial AD mutations skew APP processing to favor production of pro-amyloidogenic forms of the peptide or net A production and thus drive disease pathogenesis. Although there is growing interest in defining pathogenic subvarieties of A [e.g., secreted oligomeric assemblies such as A-derived diffusible ligands (2) and intracellular forms (3)], modulators of APP and A biology in sporadic disease have remained more elusive. One area of particular interest concerns the role of transition metals.Cu and Zn ions are abundant in the normal brain (4-6), and direct measurements of metal levels have indicated altered homeostasis in AD (7-10). Interestingly, APP has a selective, high-affinity Cu-binding site in the extracellular (ecto-) domain that is capable of reducing Cu(II) to Cu(I) (11), and a recent structural analysis of this domain has revealed some similarity to previously identified Cu chaperone proteins (12). In addition to the ectodomain Cu-binding site of APP, A peptide also contains binding sites for . A-metal interaction may drive both fibril formation and free radical production (13,(16)(17)(18), findings potentially relevant to AD pathogenesis in vivo, given that metal chelators can resolubilize A aggregates from postmortem AD brain (19). On the other hand, studies of APP processing in cultured cells have revealed stimulation of the ␣-secretase pathway for APP processing by extracellular Cu ions (20). As this pathway cleaves the A domain of APP into two fragments, it has a potential to be anti-amyloidogenic. Prompted by these divergent observations, we devised a genetic experiment to investigate how Cu might modulate A-dependent pathologies in vivo by using the transgenic (Tg) CRND8 line of TgAPP mice (21,22) in conjunction with a mutant allele of the CuATPase7b c...