The amyloid-β degradation intermediate Aβ34 is pericyte-associated and reduced in brain capillaries of patients with Alzheimer's disease

An impairment of amyloid beta-peptide (A beta) clearance is suggested to play a key role in the pathogenesis of sporadic Alzheimer's disease (AD). Amyloid degradation is mediated by various mechanisms including fragmentation by enzymes like neprilysin, matrix metalloproteinases (MMPs) and a recently identified amyloidolytic activity of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 cleavage of A beta 40 and A beta 42 results in the formation of a common A beta 34 intermediate which was found elevated in cerebrospinal fluid levels of patients at the earliest disease stages. To further investigate the role of A beta 34 as a marker for amyloid clearance in AD, we performed a systematic and comprehensive analysis of A beta 34 immunoreactivity in hippocampal and cortical post-mortem brain tissue from AD patients and non-demented elderly individuals. In early Braak stages, A beta 34 was predominantly detectable in a subset of brain capillaries associated with pericytes, while in later disease stages, in clinically diagnosed AD, this pericyte-associated A beta 34 immunoreactivity was largely lost. A beta 34 was also detected in isolated human cortical microvessels associated with brain pericytes and its levels correlated with A beta 40, but not with A beta 42 levels. Moreover, a significantly decreased A beta 34/A beta 40 ratio was observed in microvessels from AD patients in comparison to non-demented controls suggesting a reduced proteolytic degradation of A beta 40 to A beta 34 in AD. In line with the hypothesis that pericytes at the neurovascular unit are major producers of A beta 34, biochemical studies in cultured human primary pericytes revealed a time and dose dependent increase of A beta 34 levels upon treatment with recombinant A beta 40 peptides while A beta 34 production was impaired when A beta 40 uptake was reduced or BACE1 activity was inhibited. Collectively, our findings indicate that A beta 34 is generated by a novel BACE1-mediated A beta clearance pathway in pericytes of brain capillaries. As amyloid clearance is significantly reduced in AD, impairment of this pathway might be a major driver of the pathogenesis in sporadic AD.