Complement Protein C1q Recognizes a Conformationally Modified Form of the Prion Protein

Abstract

Several studies have suggested the implication of the classical complement pathway in the early stages of prion disease pathogenesis. To explore this hypothesis, surface plasmon resonance spectroscopy was used to test the ability of human C1q to recognize mouse PrP immobilized on a sensor chip. In this configuration, C1q bound avidly to PrP, with a K(D) of 5.4 nM (k(on) = 2.4 x 10(5) M(-1) s(-1); k(off) = 1.3 x 10(-3) s(-1)). The isolated C1q globular domain also bound to immobilized PrP, although with a higher K(D) (238 nM), due to a decreased k(on) (4.2 x 10(3) M(-1) s(-1)). Interaction was strongly enhanced by Cu(2+) ions, with a 10-fold increase in overall binding in the presence of 10 microM CuSO(4), without significant modification of the kinetic parameters. In contrast, using the same technique, no interaction was detected between immobilized C1q and soluble PrP. Likewise, gel filtration and chemical cross-linking analyses yielded no evidence for an interaction between these proteins in solution. Comparative analysis of the antigenic reactivity of soluble and immobilized PrP was performed by ELISA and surface plasmon resonance spectroscopy, respectively, using anti-PrP monoclonal antibodies. This analysis provides evidence that immobilized PrP undergoes a major conformational change in the sequence stretch 141GNDWEDRYYRENMYRYPNQ159 located in its C-terminal globular domain. It is concluded that immobilized PrP undergoes structural modifications that possibly mimic the conformational changes occurring during conversion to the pathological isoform and that C1q represents a natural sensor of these changes. Pathological implications of this recognition property are discussed in light of recent reports.

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.…”

“…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.…”

Interactions of the Extracellular Matrix Proteoglycans Decorin and Biglycan with C1q and Collectins

“…However, relative to the binding to intact C1q, the interaction of decorin and biglycan was far less than as compared with the binding of C1q and the gC1q domain to both proteoglycans. Furthermore, recent data indicate that interactions with isolated C1q CLR could be a result of the preparation, altering the physical-chemical properties of the molecule, and the binding characteristics of isolated CLR are different from those of intact C1q (26). Moreover, our experiments with inhibitory Abs against the gC1q domain clearly indicate that the primary interaction of proteoglycans with intact C1q involves the gC1q domain.…”

Molecular Mechanisms of Prion Pathogenesis

“…Indeed, mice that lack various complement factors including C1q (138), or that have been depleted of the C3 complement component (139), enjoy enhanced resistance to peripheral prion inoculation. C1q was shown to directly bind PrP in vitro (140). Human studies also point to a possible role for members of the classical complement cascade in prion pathogenesis (141); however, their precise role in prion disease is unknown.…”