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

Abstract

Previous studies based on the use of serum as a source of C have shown that fibrils of β-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 Aβ1–42 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 Aβ1–42. Using C1 reconstituted from purified C1q, C1r, and C1s, it was shown that Aβ1–42 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 Aβ12–42 and the double mutant (D7N, E11Q) of Aβ1–42 did not yield C1 activation, providing further evidence that the C1 binding site of β-amyloid fibrils is located in the acidic N-terminal 1–11 region of the Aβ1–42 peptide. Binding studies performed using a solid phase assay provided strong evidence that C1q interacts with Aβ1–42 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 β-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.

Trimeric reassembly of the globular domain of human C1q

“…This is in good agreement with the funnel-shaped energy landscape theory which indicates that multiple folding routes exist and that depending on the pathways and the kinetic of the reactions, different thermodynamically stable functional isomers can be formed [37][38][39]. The concept of multiple energy minima conformational isomers seems to be particularly appropriate to describe protein with multiple ligand capacities, as are the gC1q and the C1q molecules [2,6,20,32,38]. The structural state of the gC1q protein after the treatment at pH 5.0, keeps some native-like structure, the secondary structure, in particular, suggesting that it has a structure closer to the native state than after the treatment at pH 1.7.…”

“…The globular domain of human C1q was generated essentially as described by Tacnet et al [6]. Briefly, C1q was treated with collagenase (C1q:collagenase ratio=15:1, w/w) for 16 h at 37 °C and purification was achieved by high-pressure gel permeation on a TSK-G2000 SW column (LKB).…”

“…As the recognition subunit of C1, C1q is the first player in the complement cascade and is able to recognize a plethora of different targets, including viruses, bacteria and even misfolded proteins such as amyloid and prions [2][3][4][5][6][7]. Though, the mechanism of such versatile recognition is unknown, it is clearly related to the structure of C1q which is a heterotrimer comprising three chains, A, B and C of similar lengths (~220 residues) and of homologous amino acid sequences.…”

Evidence That Complement Protein C1q Interacts with C-Reactive Protein through Its Globular Head Region

“…These are highly cationic sequences (20) with, for example, 5 basic residues in the 13 residues of C1qA 14 -26. The interactions ascribed to this segment of C1q could be mainly based on low affinity charge interactions, a hypothesis strengthened by the experimental detail that half-physiologic ionic strength buffers were used to detect the binding between C1qCLR and CRP (28).…”

“…C1q can bind various activators such as immunoglobulins IgG (16) and IgM (17), as well as non-Ig proteins like CRP (7), SAP (18), PTX3 (19), and ␤-amyloid fibrils (A␤) (20). A common binding site on C1q for all these activators may serve as target for potential inhibitors.…”