Prion protein is decreased in Alzheimer's brain and inversely correlates with BACE1 activity, amyloid-β levels and Braak stage.

The cellular prion protein (PrP(C)) has been implicated in the development of Alzheimer's disease (AD). PrP(C) decreases amyloid-β (Aβ) production, which is involved in AD pathogenesis, by inhibiting β-secretase (BACE1) activity. Contactin 5 (CNTN5) has also been implicated in the development of AD by a genome-wide association study. Here we measured PrP(C) and CNTN5 in frontal cortex samples from 24 sporadic AD and 24 age-matched control brains and correlated the expression of these proteins with markers of AD. PrP(C) was decreased in sporadic AD compared to controls (by 49%, p = 0.014) but there was no difference in CNTN5 between sporadic AD and controls (p = 0.217). PrP(C) significantly inversely correlated with BACE1 activity (rs = -0.358, p = 0.006), Aβ load (rs = -0.456, p = 0.001), soluble Aβ (rs = -0.283, p = 0.026) and insoluble Aβ (rs = -0.353, p = 0.007) and PrP(C) also significantly inversely correlated with the stage of disease, as indicated by Braak tangle stage (rs = -0.377, p = 0.007). CNTN5 did not correlate with Aβ load (rs = 0.040, p = 0.393), soluble Aβ (rs = 0.113, p = 0.223) or insoluble Aβ (rs = 0.169, p = 0.125). PrP(C) was also measured in frontal cortex samples from 9 Down's syndrome (DS) and 8 age-matched control brains. In contrast to sporadic AD, there was no difference in PrP(C) in the DS brains compared to controls (p = 0.625). These data are consistent with a role for PrP(C) in regulating Aβ production and indicate that brain PrP(C) level may be important in influencing the onset and progression of sporadic AD.