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-Amyloid Fibrils Activate the C1 Complex of Complement Under Physiological Conditions: Evidence for a Binding Site for A on the C1q Globular Regions¹

Laboratoire d’Enzymologie Moléculaire, Institut de Biologie Structurale, Grenoble, France

Previous studies based on the use of serum as a source of C have shown that fibrils of -amyloid peptides that accumulate in the brain of patients with Alzheimer’s disease have the ability to bind C1q and activate the classical C pathway. The objective of the present work was to test the ability of fibrils of peptide A1–42 to trigger direct activation of the C1 complex and to carry out further investigations on the site(s) of C1q involved in the interaction with A1–42. Using C1 reconstituted from purified C1q, C1r, and C1s, it was shown that A1–42 fibrils trigger direct C1 activation both in the absence of C1 inhibitor and at C1 inhibitor:C1 ratios up to 8:0, i.e., under conditions consistent with the physiological context in serum. The truncated peptide A12–42 and the double mutant (D7N, E11Q) of A1–42 did not yield C1 activation, providing further evidence that the C1 binding site of -amyloid fibrils is located in the acidic N-terminal 1–11 region of the A1–42 peptide. Binding studies performed using a solid phase assay provided strong evidence that C1q interacts with A1–42 fibrils through its C-terminal globular regions. In contrast to previous studies based on a different experimental design, no significant involvement of the C1q collagen-like domain was detected. These findings were confirmed by additional experiments based on C1 activation and C4 consumption assays. These observations provide direct evidence of the ability of -amyloid fibrils to trigger activation of the classical C pathway and further support the hypothesis that C activation may be a component of the pathogenesis of Alzheimer’s disease.

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