Identification of a Stable Fragment of the Alzheimer Amyloid Precursor Containing the β-Protein in Brain Microvessels

Altered proteolysis of the β-amyloid precursor protein (βAPP) resulting in release of the ≈40-residue amyloid β-protein (AβP) may be a seminal pathogenetic event in Alzheimer disease. Using region-specific βAPP antibodies, we searched for stable proteolytic intermediates containing the intact AβP region in brain tissue. A 22-kDa βAPP fragment was selectively detected in microvessels purified from cerebral cortex and other brain regions. On immunoblots, the 22-kDa band is labeled by five distinct antisera to βAPP carboxyl-terminal peptides and by affinity-purified antibodies to the recombinant proteins βAPP444-592and βAPP592-695, which flank the AβP region. The protein is virtually undetectable in whole-brain homogenates or microvessel-free fractions of brain. The protein is extractable from microvessels in Triton X-100 and other detergents, indicating its membrane association. In comparison with cortical microvessels, microvessels purified from white matter, cerebellum, and nonneural tissues contain lower amounts of the 22-kDa protein. The protein is found in microvessels of both normal and Alzheimer disease brains and occurs in low amounts in microvessels from fresh bovine brain. The size and specific immunoreactivity of the 22-kDa protein indicate that it is a stable fragment of βAPP containing the intact AβP. The occurrence of this potentially amyloidogenic intermediate in microvessels is consistent with a vascular or hematogenous origin for some AβP deposits in Alzheimer disease.