Pentraxin 2 (PTX2), also known as Serum Amyoid P Component (SAP), is a secreted serum glycoprotein that is a universal non-fibrillar component of amyloid deposits. These extracellular deposits of insoluble protein fibrils are the result of protein misfolding and can lead to tissue damage and disease (1, 2). PTX2 belongs to the pentraxin (pentaxin) superfamily, whose members have the characteristic pentagonal discoid arrangement of five non-covalently bound subunits. Pentraxins bind to a variety of molecules in a calcium-dependent lectin-like manner through a pattern-recognition-binding site (1, 4, 5). There are two known subfamilies of pentraxins, the classical or short pentraxin subfamily that includes the serum C-reactive Protein (CRP) and PTX2, and the fusion or long pentraxin subfamily whose members contain pentraxin-related carboxyl-terminal halves (1).
PTX2 and CRP share approximately 50% amino acid sequence identity (2, 5). They are produced and secreted by liver hepatocytes and circulates in plasma. Mouse PTX2 is a major acute-phase protein whose plasma concentrations increase dramatically during an acute phase response (2). In human where CRP is the major acute-phase protein, the plasma concentration of human PTX2 remains relatively constant in response to tissue-damage (2, 5).
PTX2 associates ubiquitously with all amyloid deposits that are implicated in a diverse range of diseases including Alzheimer’s and prion diseases, type 2 diabetes and various systemic amyloidoses (3, 6, 7). As a non-fibrillar component, PTX2 regulates the solubility of amyloid fibrils and protects them from degradation by proteolytic enzymes and phagocytic cells. In addition to its role in the pathogenesis of amyloidoses, PTX2 also has an important physiological function in innate immunity (8).