Osteonectin

Bone SPARC protein (osteonectin) Not bone-specific. 

Sauk JJ, Smith T, Silbergeld EK, et al. 1992. Lead inhibits secretion of osteonectin/sparc without significantly altering collagen or hsp47 production in osteoblast-like ros 17/2.8 cells. Toxicol Appl Pharmacol 116(2) 240-247. 

Clezardin, R, Malaval, L., Morel, M. C., et al, Osteonectin is an alpha-granule component involved with thrombospondin in platelet aggregation. J. Bone Miner Res. 6, 1059-1070 (1991). 

Figure 11.3 Studies in mutant mice, (a) Image of mineral to matrix in the growth plate of a young mouse lacking matrix gla protein (MGP—/—) and its wildtype control (MGP+/+). Note the gradient of mineral matrix ratios in the wildtype is not apparent in the knockout, (b) Mineral matrix increases at three sites in the cortices of the osteocalcin knockout (KO) mouse (6 month data), while crystallinity is decreased relative to wildtype (WT) controls, (c) In the osteonectin knockout mouse (4 months old) the collagen maturity assessed in terms of the 1660 1690 peak area ratio is increased on the periosteal and endosteal surface.

Delany, A. M., Arnling, M., Priemel, M., Howe, C., Baron, R. and Canalis, E. (2000) Osteopenia and decreased bone formation in osteonectin-deficient mice. J. Clin. Invest. 105, 915-23. 

Takano-Yamamoto T, Takemura T, Kitamura Y and Nomura S (1994) Site-specific expression of mRNAs for osteonectin, osteocalcin, and osteopontin revealed by in situ hybridization in rat periodontal ligament during physiological tooth movement. J. Histochem. Cytochem. 42 885-896. 

BM-40/osteonectin/SPARC Single chain (M, 35K) Calcium binding... 

Complete sequence analysis based on cDNA was achieved for osteonectin and SPARC and indicated a disulfide-bonded, four-domain structure for the protein (Fig. 12). Remarkable features of the structure are a cluster of 16 glutamic acid residues at the N terminus and an EF-hand domain located between the last two cysteine residues, both regions having the potential for calcium-binding (Bolander et al., 1988 Engel et al., 1987). Studies with BM-40 demonstrated a reversible change in a helix from 30 to 20% upon removal of calcium. The data indicate the... 

In addition, l,25(OH)2D3 is involved in a number of genomic events related to the synthesis of a number of proteins found in bone cells (procollagen [41], osteocalcin [42], osteonectin [43], matrix Gla protein [44]). 

Bone resorption in response to continuous mechanical deformation appears to be regulated by cells of osteoblast lineage such as preosteoblasts, osteoblasts, bone lining cells and osteocytes, and stretch-enhanced, osteoclastlike cell formation involves prostaglandins, but not PGE2 (Soma et al., 1996). Stretch-induced increases in osteopontin and osteonectin were inhibited by addition of the calcium channel antagonist nifedipine suggesting an important role for L-type calcium channels in early mechanical strain transduction pathways in osteoblasts. 

Fig. 2. Gradient (4-20%) acrylamide gel of bone extracts. Two hundred micrograms of modem bone extract (M) and 400 /ig of three fossil bone extracts from the Archaic (7000 years b.p.) Windover site were stained with Coomassie Brilliant Blue. Products in the molecular weight range of albumin, IgG (heavy chain), and osteonectin are annotated, and the three proteins were identified in Western blots of this gel in both the modem and fossil bone extracts.

Fig. 3. Gradient (4-20%) acrylamide gel of bone extracts from a fossil whale bone (10,000 years b.p.). Soluble extract (A) shows a range of molecular weights that are stained with Coomassie Brilliant Blue. The insoluble (in guanidine/EDTA) extract was heated in gel sample buffer at 100° for 30 min, and the buffer was removed. This insoluble extract also has a range of molecular weights that tend to be higher on average than the EDTA-soluble component. The soluble extract was digested with bacterial collagenase (B), and two products with molecular weights similar to albumin and osteonectin were revealed.

Osteonectin, which has a molecular weight about 32,000, is a phosphorylated glycoprotein It has a high affinity for collagen and bone mineral. It is thought that osteonectin coats the collagen fibers and facilitates the deposit of mineral on the collagen. 

Pacifici, M., Oshima, O., Pisher, L. VV., Young, M. R, Shapiro, I. M., and Leboy, P. S. (1990). Changes in osteonectin distribution and levels are associated with mineralization of the chicken tibia I grow th cartilage. Calcif. TfssHC Ini. 47,. 51-61. 

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