Engineered zwitterionic phosphorylcholine monolayers for elucidating multivalent binding kinetics of C-reactive protein

The Multiple Faces of C-Reactive Protein-Physiological and Pathophysiological Implications in Cardiovascular Disease

C-reactive protein (CRP) is an intriguing protein which plays a variety of roles in either physiological or pathophysiological states. For years it has been regarded merely as a useful biomarker of infection, tissue injury and inflammation, and it was only in the early 80s that the modified isoforms (mCRP) of native CRP (nCRP) appeared. It soon became clear that the roles of native CRP should be clearly discriminated from those of the modified form and so the impacts of both isoforms were divided to a certain degree between physiological and pathophysiological states. For decades, CRP has been regarded only as a hallmark of inflammation; however, it has since been recognised as a significant predictor of future episodes of cardiovascular disease, independent of other risk factors. The existence of modified CRP isoforms and their possible relevance to various pathophysiological conditions, suggested over thirty years ago, has prompted the search for structural and functional dissimilarities between the pentameric nCRP and monomeric mCRP isoforms. New attempts to identify the possible relevance between the diversity of structures and their opposing functions have initiated a new era of research on C-reactive protein. This review discusses the biochemical aspects of CRP physiology, emphasizing the supposed relevance between the structural biology of CRP isoforms and their differentiated physiological and pathophysiological roles.

Critical Study of the Recognition between C-Reactive Protein and Surface-Immobilized Phosphorylcholine by Quartz Crystal Microbalance with Dissipation

C-reactive protein (CRP), a biomarker for cardiovascular disease, has been reported to have a strong affinity to zwitterionic phosphorylcholine (PC) groups in the presence of calcium ions. In addition, PC-immobilized surfaces have been used as a nonfouling coating to prevent nonspecific protein binding. By appropriately using the features of PC-immobilized surfaces, including specific recognition to CRP and nonfouling surface, it is reasonable to create an antibody-free biosensor for the specific capture of CRP. In this study, PC-functionalized 3,4-ethylenedioxythiophene (EDOT) monomers were used to prepare PC-immobilized surfaces. The density of PC groups on the surface can be fine-tuned by changing the composition of the monomer solutions for the electropolymerization. The density of PC group was confirmed by X-ray photoelectron spectroscopy (XPS). The specific interaction of CRP with PC groups was monitored by using a quartz crystal microbalance with dissipation (QCM-D). The amount of protein binding could be estimated by the reduction in frequency readout. Through the QCM-D measurement, we revealed the nonfouling property and the specific CRP capture from our PC-immobilized surfaces. Notably, the dissipation energy also dropped during the binding process between CRP and PC, indicating the release of water molecules from the PC groups during CRP adsorption. We anticipate that surface-bound water molecules are mainly released from areas near the immobilized PC groups. Based on Hofmeister series, we further examined the influence of ions by introducing four different anions including both kosmotrope (order maker) and chaotrope (disorder maker) into the buffer for the CRP binding test. The results showed that the concentration and the type of anions play an important role in CRP binding. The present fundamental study reveals deep insights into the recognition between CRP and surface-immobilized PC groups, which can facilitate the development of CRP sensing platforms.

Specific binding of human C-reactive protein towards supported monolayers of binary and engineered phospholipids

Circulating C-reactive protein (CRP) recognizes altered plasma membranes and activates complements systems in the acute phase of inflammation and infection in human. We have shown previously the calcium-independent adsorption of CRP toward 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and lysophosphatidylcholine (LPC) on supported phospholipid monolayers. Here, we extended our study to other phospholipids and additives to elucidate the pattern recognition of CRP using a surface plasmon resonance biosensor. Surface density and lateral fluidity depended on the type of phospholipids in the monolayers as characterized by SPR and fluorescence recovery after photobleaching measurements. CRP recognized 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) in the supported POPC monolayers without calcium at pH 7.4 and 5.5. As opposed to LPC, CRP did not recognize 3-sn-lysophosphatidylethanolamine in the POPC monolayers in calcium-free conditions. While, the addition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) or sphingomyelin to supported POPC monolayers blocked CRP adsorption. Calcium-dependent CRP binding was observed only at pH 5.5 on supported monolayers of engineered phospholipids with inverted headgroups relative to POPC. The complement 1q (C1q) protein recognized the active form of CRP on the supported phospholipid monolayers. The discovery of CRP recognition with these phospholipids aids our understanding of the activation dynamics of CRP with phospholipid-based biomaterials when used during the acute phase.

Conformational Changes in C-Reactive Protein Affect Binding to Curved Membranes in a Lipid Bilayer Model of the Apoptotic Cell Surface

C-reactive protein (CRP) is a serum protein that binds to damaged membranes through a phosphatidylcholine binding site. The membrane binding process can initiate the complement immune response and facilitates the clearance of apoptotic cells, likely aiding in the protection of autoimmunity. The initiation of an immune response relies on a conformation change from a native, pentameric form to a modified form, where the modified form binds complement proteins (i.e., C1q) and regulatory proteins substantially better than the native form. In vitro, this reactivity is observed when CRP is monomeric, and a modified form has also been observed at sites of inflammation. Despite evidence that the monomeric form has much higher affinities for almost all proteinaceous binding partners, the role of CRP conformation on lipid binding is yet unknown. In this work, we mimic the outer leaflet of apoptotic cell membranes using a nanopatterned substrate to create curved, supported lipid bilayers and then characterize how CRP conformation affects the interactions between CRP and target membranes. In this assay, the chemical composition and shape are separately tunable parameters. The lipids consisted primarily of palmitoyloleoylphosphatidylcholine, with and without lysophosphatidylcholine, and the curvature had a radius of 27-55 nm. Using this model system combined with quantitative fluorescence microscopy methods, CRP binding to lipid membranes was measured as a function of different conformations of CRP. The modified form of CRP bound curved membranes, but the pentameric form did not for the range of curvatures measured. Unlike most other curvature-sensing proteins, modified CRP accumulated more at a moderate curvature, rather than highly curved or flat regions, suggesting that the membrane bound form does not solely depend on a defect binding mechanism. The presence of lysophosphatidylcholine, a component of apoptotic membranes, increased CRP binding to all types of membranes. Overall, our results show that CRP interactions vary with protein form, lipid composition, and membrane shape. The mechanism by which CRP recognizes damaged membranes depends on the combination of all three.

Calcium-independent binding of human C-reactive protein to lysophosphatidylcholine in supported planar phospholipid monolayers

Details describing the molecular dynamics of inflammation biomarker human C-reactive protein (CRP) on plasma membranes containing bioactive lipid lysophosphatidylcholine (LPC) remain elusive. Here, we measured the binding kinetics of CRP to supported phospholipid monolayers deposited on an alkanethiol self-assembled monolayer on a planar gold substrate using surface plasmon resonance. Surprisingly, CRP binding to supported 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/LPC monolayers was calcium-independent although CRP binding to supported POPC monolayers was calcium-dependent. Binding inhibition assays indicate a specific interaction between CRP and the glycerophosphate group in LPC in the absence of calcium ions. Binding experiments on supported POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) monolayers further validated calcium-independent binding of CRP through the glycerophosphate moiety. Docking analysis predicted a new binding site for LPC in the absence of calcium ions, which is located on the opposite side of the known binding site for PC of cyclic pentameric CRP. These results using model plasma membranes should aid our understanding of the activation dynamics of CRP in altered local microenvironments of inflammation and infection.

STATEMENT OF SIGNIFICANCE

C-reactive protein (CRP), a major acute-phase pentraxin, binds to plasma membranes through the multivalent contacts with zwitterionic phosphorylcholine groups for activating classical complement systems. However, the interaction of CRP with phosphorylcholine-based biomaterials is unknown due to the lack of our understanding on the activation mechanism of CRP in altered local microenvironments. This paper reports the novel calcium-independent interaction of CRP to bioactive phospholipid lysophosphatidylcholine (LPC) in supported phospholipids monolayers as determined using SPR. Binding inhibition experiments indicate exposure of glycerophosphate moiety of LPC is responsible for the calcium-free interaction. Our study may explode the established concept that CRP requires calcium for binding to LPC on damaged cell membranes or biomaterials.