The amyloid-ß degradation intermediate Aß34 is pericyte-associated and reduced in brain capillaries of patients with Alzheimer's disease.

ABSTRACT

An impairment of amyloid ß-peptide (Aß) 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 ß-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 cleavage of Aß40 and Aß42 results in the formation of a common Aß34 intermediate which was found elevated in cerebrospinal fluid levels of patients at the earliest disease stages. To further investigate the role of Aß34 as a marker for amyloid clearance in AD, we performed a systematic and comprehensive analysis of Aß34 immunoreactivity in hippocampal and cortical post-mortem brain tissue from AD patients and non-demented elderly individuals. In early Braak stages, Aß34 was predominantly detectable in a subset of brain capillaries associated with pericytes, while in later disease stages, in clinically diagnosed AD, this pericyte-associated Aß34 immunoreactivity was largely lost. Aß34 was also detected in isolated human cortical microvessels associated with brain pericytes and its levels correlated with Aß40, but not with Aß42 levels. Moreover, a significantly decreased Aß34/Aß40 ratio was observed in microvessels from AD patients in comparison to non-demented controls suggesting a reduced proteolytic degradation of Aß40 to Aß34 in AD. In line with the hypothesis that pericytes at the neurovascular unit are major producers of Aß34, biochemical studies in cultured human primary pericytes revealed a time and dose dependent increase of Aß34 levels upon treatment with recombinant Aß40 peptides while Aß34 production was impaired when Aß40 uptake was reduced or BACE1 activity was inhibited. Collectively, our findings indicate that Aß34 is generated by a novel BACE1-mediated Aß 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.