Asporin competes with decorin for collagen binding, binds calcium and promotes osteoblast collagen mineralization

Abstract

The interactions of the ECM (extracellular matrix) protein asporin with ECM components have previously not been investigated. Here, we show that asporin binds collagen type I. This binding is inhibited by recombinant asporin fragment LRR (leucine-rich repeat) 10-12 and by full-length decorin, but not by biglycan. We demonstrate that the polyaspartate domain binds calcium and regulates hydroxyapatite formation in vitro. In the presence of asporin, the number of collagen nodules, and mRNA of osteoblastic markers Osterix and Runx2, were increased. Moreover, decorin or the collagen-binding asporin fragment LRR 10-12 inhibited the pro-osteoblastic activity of full-length asporin. Our results suggest that asporin and decorin compete for binding to collagen and that the polyaspartate in asporin directly regulates collagen mineralization. Therefore asporin has a role in osteoblast-driven collagen biomineralization activity. We also show that asporin can be expressed in Escherichia coli (Rosetta-gami) with correctly positioned cysteine bridges, and a similar system can possibly be used for the expression of other SLRPs (small LRR proteoglycans/proteins).

The small leucine-rich repeat secreted protein Asporin induces eyes in Xenopus embryos through the IGF signalling pathway

“…Although all three members share leucine-rich repeats (LRRs), ASPN differs from Decorin and Biglycan in the number and spacing of cysteine residues at its N-and C-terminals (Henry et al, 2001;Lorenzo et al, 2001). ASPN can bind type I Collagen and competes with Decorin (but not Biglycan) for the collagen-binding site (Kalamajski et al, 2009). Furthermore, ASPN has been shown to bind directly to TGFβ and BMP2 and prevents them from binding to their respective receptors (Nakajima et al, 2007;Tomoeda et al, 2008;Yamada et al, 2007), which is in good agreement with our findings (Fig.…”

“…2I,P; supplementary material Fig. S2A-C) (Kalamajski et al, 2009;Kizawa et al, 2005). In addition, SLRP members of other classes did not elicit an eye phenotype when they were overexpressed, the exceptions being the class II SLRP Lumican and class III SLRP Epiphycan, which occasionally induced a subtle eye phenotype ( Fig.…”

Differential proteomic analysis of a human breast tumor and its matched bone metastasis identifies cell membrane and extracellular proteins associated with bone metastasis

“…50 15 ASPN Asporin ↓ Asporin has a role in osteoblast-driven collagen biomineralization activity. 58 …”

“…In normal bone, lumican, mimecan, biglycan, decorin, and prolargin are all implicated in osteoblast-driven bone mineralization. [57][58][59][60][61][62] Despite their common role in normal bone formation, SLRPs appear to have distinct functions during cancer progression. In breast cancer tumors, lumican was reported as over-expressed and associated with high tumor grade, low estrogen receptor levels, and young age of patients.…”

“…61 22 DCN Decorin ↓ Asporin competes with decorin to promote osteoblast mineralization. 58 23 POSTN Periostin ↓ Periostin is expressed in vivo and in vitro during the stages of osteoblast differentiation and maturation. 84 24 PRELP Prolargin ↓ Prolargin reduces osteoclast number and activity in ovariectomized mice, underlying its physiological and/or pathological importance in skeletal remodeling.…”