Adaptive diversity of innate immune receptor family short pentraxins in Murinae

Structural characterization of immune receptor family short pentraxins, C-reactive protein and serum amyloid P component, in primates

The short pentraxins C-reactive protein (CRP) and serum amyloid P component (SAP) are a family of pattern-recognition molecules that play versatile roles in innate immunity and inflammation. A comprehensive description is currently lacking as to the genetic characteristics of these molecules in primates. In the present study, we analyzed genetic changes of CRP and SAP genes in this phylogenic lineage. The results revealed that adaptive selection has brought about interspecific diversities of both genes. The adaptively selected amino acid changes have occurred in or adjacent to the structural domains involved in ligand- and effector-binding and homologous aggregation. Each gene, however, exhibits a striking lack of genetic variation in both commonly-used non-human primate models Macaca fascicularis and M. mulatta. These findings highlight basic facts on the genetic characteristics of primate short pentraxins and would contribute powerfully to the extrapolation of their functional insights and physiological outcomes from primate models to humans.

Evolution of C-Reactive Protein

C-reactive protein (CRP) is an evolutionarily conserved protein. From arthropods to humans, CRP has been found in every organism where the presence of CRP has been sought. Human CRP is a pentamer made up of five identical subunits which binds to phosphocholine (PCh) in a Ca²⁺-dependent manner. In various species, we define a protein as CRP if it has any two of the following three characteristics: First, it is a cyclic oligomer of almost identical subunits of molecular weight 20–30 kDa. Second, it binds to PCh in a Ca²⁺-dependent manner. Third, it exhibits immunological cross-reactivity with human CRP. In the arthropod horseshoe crab, CRP is a constitutively expressed protein, while in humans, CRP is an acute phase plasma protein and a component of the acute phase response. As the nature of CRP gene expression evolved from a constitutively expressed protein in arthropods to an acute phase protein in humans, the definition of CRP became distinctive. In humans, CRP can be distinguished from other homologous proteins such as serum amyloid P, but this is not the case for most other vertebrates and invertebrates. Literature indicates that the binding ability of CRP to PCh is less relevant than its binding to other ligands. Human CRP displays structure-based ligand-binding specificities, but it is not known if that is true for invertebrate CRP. During evolution, changes in the intrachain disulfide and interchain disulfide bonds and changes in the glycosylation status of CRP may be responsible for different structure-function relationships of CRP in various species. More studies of invertebrate CRP are needed to understand the reasons behind such evolution of CRP. Also, CRP evolved as a component of and along with the development of the immune system. It is important to understand the biology of ancient CRP molecules because the knowledge could be useful for immunodeficient individuals.

Recognition functions of pentameric C-reactive protein in cardiovascular disease

C-reactive protein (CRP) performs two recognition functions that are relevant to cardiovascular disease. First, in its native pentameric conformation, CRP recognizes molecules and cells with exposed phosphocholine (PCh) groups, such as microbial pathogens and damaged cells. PCh-containing ligand-bound CRP activates the complement system to destroy the ligand. Thus, the PCh-binding function of CRP is defensive if it occurs on foreign pathogens because it results in the killing of the pathogen via complement activation. On the other hand, the PCh-binding function of CRP is detrimental if it occurs on injured host cells because it causes more damage to the tissue via complement activation; this is how CRP worsens acute myocardial infarction and ischemia/reperfusion injury. Second, in its nonnative pentameric conformation, CRP also recognizes atherogenic low-density lipoprotein (LDL). Recent data suggest that the LDL-binding function of CRP is beneficial because it prevents formation of macrophage foam cells, attenuates inflammatory effects of LDL, inhibits LDL oxidation, and reduces proatherogenic effects of macrophages, raising the possibility that nonnative CRP may show atheroprotective effects in experimental animals. In conclusion, temporarily inhibiting the PCh-binding function of CRP along with facilitating localized presence of nonnative pentameric CRP could be a promising approach to treat atherosclerosis and myocardial infarction. There is no need to stop the biosynthesis of CRP.