Cell membranes and liposomes dissociate C‐reactive protein (CRP) to form a new, biologically active structural intermediate: mCRP_m

Abstract

Emerging evidence indicates that C-reactive protein (CRP) has at least two conformationally distinct isoforms, i.e., pentameric CRP (pCRP) and monomeric CRP (mCRP or CRP subunit). Both CRP isoforms are proposed to play roles in inflammation and may participate in the pathogenesis of cardiovascular disease. However, the origin of mCRP in situ and the interplay between the two CRP isoforms under physiological/pathological circumstances remain elusive. Herein, by probing conformational alteration, neoepitope expression, and direct visualization using electron-microscopy, we have shown that calcium-dependent binding of pCRP to membranes, including liposomes and cell membranes, led to a rapid but partial structural change, producing molecules that express CRP subunit antigenicity but with retained native pentameric conformation. This hybrid molecule is herein termed mCRP(m). The formation of mCRP(m) was associated with significantly enhanced complement fixation. mCRP(m) can further detach from membrane to form the well-recognized mCRP isoform converted in solution (mCRP(s)) and exert potent stimulatory effects on endothelial cells. The membrane-induced pCRP dissociation not only provides a physiologically relevant scenario for mCRP formation but may represent an important mechanism for regulating CRP function.

Regulated conformation changes in C-reactive protein orchestrate its role in atherogenesis

“…With this observation, we have provided compelling evidence that the lysoPC-dependent, multi-point binding of CRP to model or apoptotic cell membranes can overcome the stabilization effects of calcium and induce a rapid formation of a hybrid state [73]. This hybrid state, in the form of a separated subunit with near-native conformation, is termed mCRP m and eventually gives rise to mCRP with the aid of the relatively hydrophobic and fluidic characteristics of membranes [73].…”

“…In this regard, the rapid formation of mCRP m can override the size limitation such that even the dissociation of a single CRP molecule can provide multi-point binding to, for example, activate C1q. Therefore, the damaged membrane-induced stepwise dissociation of CRP can represent an efficient mechanism for prompt amplification of the bioactivities locally on membranes, as evidenced by the significantly enhanced CCP activation by mCRP m [73].…”

“…Due to the reduced binding affinity associated with the loss of multi-point attachment as the result of monomerization, mCRP m can detach from the membrane and undergo further conformational changes leading to the generation of extracellular mCRP [73], which may later reassociate with the membrane by direct insertion. Alternatively, analogous to the dissociation of CRP induced by immobilization onto plastic surfaces, the mild hydrophobicity of damaged membranes can also facilitate structural rearrangements in mCRP m for direct conversion to membrane-associated mCRP.…”

“…However, considering the relatively slow dynamics of the conversion process, the significant accumulation and effective actions of mCRP mostly likely occur in inflammatory loci where injured cells and damaged membranes are enriched. This has led us to propose that the dissociation of the CRP molecule is an activating mechanism that is required for the expression of enhanced bioactivities and is also a buffering mechanism that localizes the actions of mCRP into inflammatory loci to prevent the possible global effects directly induced by large-scale alterations in the serum levels of CRP [73]. Consistent with our proposal, immunohistochemical analyses using highly specific antibodies revealed that mCRP, not CRP, was the major isoform present in local lesions, including atherosclerotic plaques [55,76], diabetic kidneys [77] and stroke neovessels [78].…”

“…Moreover, Dr. Peter and colleagues [80] recently identified novel pathways for mCRP generation upon binding of CRP to amyloid aggregates, the typical pathological feature found in brains of persons with Alzheimer's disease, and lysoPC-enriched microparticles released by activated cells or isolated from blood of patients with myocardial infarction [81]. As disturbed membranes, these newly identified inducers also present multi-point binding sites and relative hydrophobicity, factors critical for driving the conversion of CRP to mCRP [73]. An additional factor, high membrane curvature, has recently been revealed by showing that CRP preferentially binds and dissociates on lipid-coated nanoparticles smaller than 28 nm [82].…”