Asporin, also known as periodontal ligament-associated protein 1 (PLAP1) is a dimeric secreted extracellular matrix protein, which belongs to the small leucine-rich proteoglycan (SLRP) family. It consist out of 380 amino acids and has a highly conserved pro peptide sequence which contains a series of leucine rich repeats and are flanked by two cysteine residues in the C Terminal region. Further it has four cysteine residues that form disulphide bonds as well as aspartic acid repeats in the N-Terminal region.
High levels of Asporin can be found in aorta, uterus and osteoarthritic articular cartilage. Further, moderate levels of aspirin expression can be found in small intestine, heart liver, bladder, ovary, stomach, the adrenal-, thyroid-, and mammary gland. Lower levels of expression can be seen in the trachea, bone marrow and lung. Asporin is known to negatively regulate PDL differentiation and mineralisation as well as it inhibits BMP-dependent activation of SMAD proteins. Further, it directly binds to TGFb-1 subsequently, binds to collagen by way of its LRR domain. Through its interaction with TGFb-1, Asporin negatively regulates chondrogenesis in the articular cartilage by blocking the TGF-beta/receptor interaction on the cell surface and inhibiting canonical TGF-beta/Smad signaling. Moreover, it has the ability to bind calcium, giving it regulatory properties in osteoblast driven mineralisation and regulates FGF2 through direct and indirect interactions. Next to its regulatory properties in terms of cartilage and bone homeostasis, Asporin expression has also been linked to cancer invasion and progression. However, its value as biomarker remains to be established yet.
Asporin is a 380 amino acid long, secreted dimeric extracellular matrix protein, which functions as a direct binding partner for TGFβ-1 and, like Decorin and Biglycan, belongs to the der Small Leucine- Rich Proteins (SLRP) class I family. They are characterised by a high degree of homology at the genetic and molecular level and the presence of leucine- rich repeats (LRRs), which are flanked by disulfides formed by cysteine residues. Unlike other members of the SLRP class II family, Asporin does not contain a glycosaminoglycan binding site, but it has several unique aspartic acid repeats in the N-Terminal region. The main biological function of Asporin most likely consists in the regulation of TGF-β1 activity to which it binds directly.