Bone sialoprotein explained

Bone sialoprotein (BSP) is a component of mineralized tissues such as bone, dentin, cementum and calcified cartilage. BSP is a significant component of the bone extracellular matrix and has been suggested to constitute approximately 8% of all non-collagenous proteins found in bone and cementum.^[1] BSP, a SIBLING protein, was originally isolated from bovine cortical bone as a 23-kDa glycopeptide with high sialic acid content.^{[2] [3]}

The human variant of BSP is called bone sialoprotein 2 also known as cell-binding sialoprotein or integrin-binding sialoprotein and is encoded by the IBSP gene.^[4]

Structure

Native BSP has an apparent molecular weight of 60-80 kDa based on SDS-PAGE, which is a considerable deviation from the predicted weight (based on cDNA sequence) of approximately 33 kDa.^[5] The mammalian BSP cDNAs encode for proteins averaging 317 amino acids, which includes the 16-residue preprotein secretory signal peptide. Among the mammalian cDNAs currently characterized, there is an approximate 45% conservation of sequence identity and a further 10-23% conservative substitution. The protein is highly acidic (pKa of ~ 3.9)^[6] and contains a large amount of Glu residues, constituting ~22% of the total amino acid.

Secondary structure prediction and hydrophobicity analyses suggest that the primary sequence of BSP has an open, flexible structure with the potential to form regions of α-helix and some β-sheet.^[7] However, the majority of studies have demonstrated that BSP has no α-helical or β-sheet structure by 1D NMR^{[6] [8]} and circular dichroism.^[9] Analysis of native protein by electron microscopy confirm that the protein has an extended structure approximately 40 nm in length.^[10] This flexible conformation suggests that the protein has few structural domains, however it has been suggested that there may be several spatially segmented functional domains including a hydrophobic collagen-binding domain (rattus norvegicus residues 36–57),^[11] a hydroxyapatite-nucleating region of contiguous glutamic acid residues (rattus norvegicus residues 78–85, 155–164)^[9] and a classical integrin-binding motif (RGD) near the C-terminal (rattus norvegicus residues 288–291).

BSP has been demonstrated to be extensively post-translationally modified, with carbohydrates and other modifications comprising approximately 50% of the molecular weight of the native protein.^{[12] [13]} These modifications, which include N- and O-linked glycosylation, tyrosine sulfation and serine and threonine phosphorylation, make the protein highly heterogeneous.

A 3D model of human bone sialoprotein has been developed using molecular modelling techniques, as shown in the picture above. The model suggests that the protein provides a flexible template for the rapid self-assembly of calcium and phosphate ions, so nucleating the growth of hydroxyapatite crystals.^[14]

Function

The amount of BSP in bone and dentin is roughly equal,^[15] however the function of BSP in these mineralized tissues is not known. One possibility is that BSP acts as a nucleus for the formation of the first apatite crystals.^[16] As the apatite forms along the collagen fibres within the extracellular matrix, BSP could then help direct, redirect or inhibit the crystal growth.

Additional roles of BSP are angiogenesis and protection from complement-mediated cell lysis. Regulation of the BSP gene is important to bone matrix mineralization and tumor growth in bone.^[17]

Further reading

• Karadag A, Fisher LW . Bone sialoprotein enhances migration of bone marrow stromal cells through matrices by bridging MMP-2 to alpha(v)beta3-integrin . J. Bone Miner. Res. . 21 . 10 . 1627–36 . 2006 . 16995818 . 10.1359/jbmr.060710 . 84886034 . free .
• Barnes GL, Javed A, Waller SM . Osteoblast-related transcription factors Runx2 (Cbfa1/AML3) and MSX2 mediate the expression of bone sialoprotein in human metastatic breast cancer cells . Cancer Res. . 63 . 10 . 2631–7 . 2003 . 12750290 . etal.
• Carlinfante G, Vassiliou D, Svensson O . Differential expression of osteopontin and bone sialoprotein in bone metastasis of breast and prostate carcinoma . Clin. Exp. Metastasis . 20 . 5 . 437–44 . 2003 . 14524533 . 10.1023/A:1025419708343 . 341938 . etal.
• Hwang Q, Cheifetz S, Overall CM . Bone sialoprotein does not interact with pro-gelatinase A (MMP-2) or mediate MMP-2 activation . BMC Cancer . 9. 121 . 2009 . 19386107 . 2679042 . 10.1186/1471-2407-9-121 . etal . free .
• Styrkarsdottir U, Halldorsson BV, Gretarsdottir S . New sequence variants associated with bone mineral density . Nat. Genet. . 41 . 1 . 15–7 . 2009 . 19079262 . 10.1038/ng.284 . 9876454 . etal.
• Zhang L, Hou X, Lu S . Predictive significance of bone sialoprotein and osteopontin for bone metastases in resected Chinese non-small-cell lung cancer patients: a large cohort retrospective study . Lung Cancer . 67 . 1 . 114–9 . 2010 . 19376608 . 10.1016/j.lungcan.2009.03.017 . etal.
• Roca H, Phimphilai M, Gopalakrishnan R . Cooperative interactions between RUNX2 and homeodomain protein-binding sites are critical for the osteoblast-specific expression of the bone sialoprotein gene . J. Biol. Chem. . 280 . 35 . 30845–55 . 2005 . 16000302 . 10.1074/jbc.M503942200 . etal. free .
• Lamour V, Detry C, Sanchez C . Runx2- and histone deacetylase 3-mediated repression is relieved in differentiating human osteoblast cells to allow high bone sialoprotein expression . J. Biol. Chem. . 282 . 50 . 36240–9 . 2007 . 17956871 . 10.1074/jbc.M705833200 . etal. free .
• Ogata Y . Bone sialoprotein and its transcriptional regulatory mechanism . Journal of Periodontal Research . 43 . 2 . 127–35 . 2008 . 18302613 . 10.1111/j.1600-0765.2007.01014.x .
• Papotti M, Kalebic T, Volante M . Bone sialoprotein is predictive of bone metastases in resectable non-small-cell lung cancer: a retrospective case-control study . J. Clin. Oncol. . 24 . 30 . 4818–24 . 2006 . 17050866 . 10.1200/JCO.2006.06.1952 . etal.
• Frank O, Heim M, Jakob M . Real-time quantitative RT-PCR analysis of human bone marrow stromal cells during osteogenic differentiation in vitro . J. Cell. Biochem. . 85 . 4 . 737–46 . 2002 . 11968014 . 10.1002/jcb.10174 . 23595289 . etal. free .
• Yerges LM, Klei L, Cauley JA . High-Density Association Study of 383 Candidate Genes for Volumetric BMD at the Femoral Neck and Lumbar Spine Among Older Men . J. Bone Miner. Res. . 24 . 12 . 2039–49 . 2009 . 19453261 . 2791518 . 10.1359/jbmr.090524 . etal.
• Gordon JA, Sodek J, Hunter GK, Goldberg HA . Bone sialoprotein stimulates focal adhesion-related signaling pathways: role in migration and survival of breast and prostate cancer cells . J. Cell. Biochem. . 107 . 6 . 1118–28 . 2009 . 19492334 . 10.1002/jcb.22211 . 36937586 .
• Araki S, Mezawa M, Sasaki Y . Parathyroid hormone regulation of the human bone sialoprotein gene transcription is mediated through two cAMP response elements . J. Cell. Biochem. . 106 . 4 . 618–25 . 2009 . 19127545 . 10.1002/jcb.22039 . 5586385 . etal.
• Wuttke M, Müller S, Nitsche DP, Paulsson M, Hanisch FG, Maurer P . Structural characterization of human recombinant and bone-derived bone sialoprotein. Functional implications for cell attachment and hydroxyapatite binding . J. Biol. Chem. . 276 . 39 . 36839–48 . September 2001 . 11459848 . 10.1074/jbc.M105689200 . free .
• Hilbig H, Wiener T, Armbruster FP . Effects of dental implant surfaces on the expression of bone sialoprotein in cells derived from human mandibular bone . Med. Sci. Monit. . 11 . 4 . BR111–5 . 2005 . 15795688 . etal.
• Koller DL, Ichikawa S, Lai D . Genome-Wide Association Study of Bone Mineral Density in Premenopausal European-American Women and Replication in African-American Women . J. Clin. Endocrinol. Metab. . 95 . 4 . 1802–9 . 2010 . 20164292 . 2853986 . 10.1210/jc.2009-1903 . etal.
• Strausberg RL, Feingold EA, Grouse LH . Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences . Proc. Natl. Acad. Sci. U.S.A. . 99 . 26 . 16899–903 . 2002 . 12477932 . 139241 . 10.1073/pnas.242603899 . 2002PNAS...9916899M . etal. free .
• Fujisawa R . [Recent advances in research on bone matrix proteins] . Nippon Rinsho . 60 . 72–8 . 2002 . Suppl 3 . 11979972 .
• Loibl S, Königs A, Kaufmann M, Costa SD, Bischoff J . [PTHrP and bone sialoprotein as prognostic markers for developing bone metastases in breast cancer patients] . de . Zentralbl Gynakol . 128 . 6 . 330–5 . December 2006 . 17213971 . 10.1055/s-2006-942314 . 71661648 .
• Uccello M, Malaguarnera G, Vacante M . Serum bone sialoprotein levels and bone metastases . J. Cancer Res. Ther. . 7 . 2 . 115–9 . 2011 . 21768695 . 10.4103/0973-1482.82912 . etal. free .

Notes and References

1. Fisher LW, McBride OW, Termine JD, Young MF . Human bone sialoprotein. Deduced protein sequence and chromosomal localization . J. Biol. Chem. . 265 . 4 . 2347–51 . February 1990 . 10.1016/S0021-9258(19)39982-X . 2404984 . free .
2. Williams PA, Peacocke AR . The physical properties of a glycoprotein from bovine cortical bone (bone sialoprotein) . Biochim. Biophys. Acta . 101 . 3 . 327–35 . November 1965 . 5862222 . 10.1016/0926-6534(65)90011-4.
3. Herring GM . Comparison of bovine bone sialoprotein and serum orosomucoid . Nature . 201 . 4920. 709 . February 1964 . 14139700 . 10.1038/201709a0. 1964Natur.201..709H . 4210187 . free .
4. Kerr JM, Fisher LW, Termine JD, Wang MG, McBride OW, Young MF . The human bone sialoprotein gene (IBSP): genomic localization and characterization . Genomics . 17 . 2 . 408–15 . August 1993 . 8406493 . 10.1006/geno.1993.1340 .
5. Fisher LW, Whitson SW, Avioli LV, Termine JD . Matrix sialoprotein of developing bone . J. Biol. Chem. . 258 . 20 . 12723–7 . October 1983 . 10.1016/S0021-9258(17)44236-0 . 6355090 . free .
6. Stubbs JT, Mintz KP, Eanes ED, Torchia DA, Fisher LW . Characterization of native and recombinant bone sialoprotein: delineation of the mineral-binding and cell adhesion domains and structural analysis of the RGD domain . J. Bone Miner. Res. . 12 . 8 . 1210–22 . August 1997 . 9258751 . 10.1359/jbmr.1997.12.8.1210. 26407786 . free .
7. Shapiro HS, Chen J, Wrana JL, Zhang Q, Blum M, Sodek J . Characterization of porcine bone sialoprotein: primary structure and cellular expression . Matrix . 13 . 6 . 431–40 . November 1993 . 8309422 . 10.1016/s0934-8832(11)80109-5.
8. Fisher LW, Torchia DA, Fohr B, Young MF, Fedarko NS . Flexible structures of SIBLING proteins, bone sialoprotein, and osteopontin . Biochem. Biophys. Res. Commun. . 280 . 2 . 460–5 . January 2001 . 11162539 . 10.1006/bbrc.2000.4146 .
9. Tye CE, Rattray KR, Warner KJ, Gordon JA, Sodek J, Hunter GK, Goldberg HA . Delineation of the hydroxyapatite-nucleating domains of bone sialoprotein . J. Biol. Chem. . 278 . 10 . 7949–55 . March 2003 . 12493752 . 10.1074/jbc.M211915200 . free .
10. Oldberg A, Franzén A, Heinegård D . The primary structure of a cell-binding bone sialoprotein . J. Biol. Chem. . 263 . 36 . 19430–2 . December 1988 . 10.1016/S0021-9258(19)77651-0 . 3198635 . free .
11. Tye CE, Hunter GK, Goldberg HA . Identification of the type I collagen-binding domain of bone sialoprotein and characterization of the mechanism of interaction . J. Biol. Chem. . 280 . 14 . 13487–92 . April 2005 . 15703183 . 10.1074/jbc.M408923200 . free .
12. Kinne RW, Fisher LW . Keratan sulfate proteoglycan in rabbit compact bone is bone sialoprotein II . J. Biol. Chem. . 262 . 21 . 10206–11 . July 1987 . 10.1016/S0021-9258(18)61099-3 . 2956253 . free .
13. Ganss B, Kim RH, Sodek J . Bone sialoprotein . Crit. Rev. Oral Biol. Med. . 10 . 1 . 79–98 . 1999 . 10759428 . 10.1177/10454411990100010401. free .
14. Vincent K, Durrant MC . A structural and functional model for human bone sialoprotein . J. Mol. Graph. Model. . 39 . 108–117 . 2013. 23261880 . 10.1016/j.jmgm.2012.10.007 .
15. Qin C, Brunn JC, Jones J, George A, Ramachandran A, Gorski JP, Butler WT . A comparative study of sialic acid-rich proteins in rat bone and dentin . Eur. J. Oral Sci. . 109 . 2 . 133–41 . April 2001 . 11347657 . 10.1034/j.1600-0722.2001.00001.x.
16. Hunter GK, Goldberg HA . Modulation of crystal formation by bone phosphoproteins: role of glutamic acid-rich sequences in the nucleation of hydroxyapatite by bone sialoprotein . Biochem. J. . 302 (Pt 1) . Pt 1. 175–9 . August 1994 . 7915111 . 1137206 . 10.1042/bj3020175.
17. Ogata Y . Bone sialoprotein and its transcriptional regulatory mechanism . Journal of Periodontal Research . 43 . 2 . 127–35 . April 2008 . 18302613 . 10.1111/j.1600-0765.2007.01014.x .