In this review most of the various known, suspected, or postulated functions of osteopontin, a secreted highly acidic phosphoprotein, are discussed in terms of what we currently know about the protein. These include 1) binding of OPN both to cells via a GRGDS cell adhesion sequence that recognizes the alpha v beta 3 integrin and to extracellular matrix components via poorly characterized motifs, 2) regulation of the formation and remodeling of mineralized tissue, 3) recruiting and stimulating macrophages and lymphocytes as part of a nonspecific response to microbial infections, 4) multiple interactions with Ca2+ that likely influence OPN protein conformation and may be important in Ca(2+)-mediated or Ca(2+)-dependent processes, 5) inhibiting the growth of calcium oxalate crystals by disruption of the growing crystal lattice, 6) effects on gene expression, Ca2+ regulation, and nitric oxide production, and 7) involvement in cell migration. OPN production is frequently augmented when cell signaling pathways are activated by any of a variety of stimuli, for example in cancer cells.

Osteopontin: a protein with diverse functions.

The article gives an overview of phytic acid in food and of its significance for human nutrition. It summarises phytate sources in foods and discusses problems of phytic acid/phytate contents of food tables. Data on phytic acid intake are evaluated and daily phytic acid intake depending on food habits is assessed. Degradation of phytate during gastro-intestinal passage is summarised, the mechanism of phytate interacting with minerals and trace elements in the gastro-intestinal chyme described and the pathway of inositol phosphate hydrolysis in the gut presented. The present knowledge of phytate absorption is summarised and discussed. Effects of phytate on mineral and trace element bioavailability are reported and phytate degradation during processing and storage is described. Beneficial activities of dietary phytate such as its effects on calcification and kidney stone formation and on lowering blood glucose and lipids are reported. The antioxidative property of phytic acid and its potentional anticancerogenic activities are briefly surveyed. Development of the analysis of phytic acid and other inositol phosphates is described, problems of inositol phosphate determination and detection discussed and the need for standardisation of phytic acid analysis in foods argued.

https://wiebaktmee.nl/cms/pdf/Schlemmer%20_Mol_Nutr_Food_res_2009_Phytate_in_foods_and_significance_for_humans.pdf

Phytate in foods and significance for humans: food sources, intake, processing, bioavailability, protective role and analysis.

A review on calcium phosphate coatings produced using a sputtering process--an alternative to plasma spraying.

Many proteins found in mineralized tissues have been proposed to function as regulators of the mineralization process, either as nucleators or inhibitors of hydroxyapatite (HA) formation. We have studied the HA-nucleating and HA-inhibiting properties of proteins from bone [osteocalcin (OC), osteopontin (OPN), osteonectin (ON) and bone sialoprotein (BSP)], dentine [phosphophoryn (DPP)] and calcified cartilage [chondrocalcin (CC)] over a wide range of concentrations. Nucleation of HA was studied with a steady-state agarose gel system at sub-threshold [Ca] x [PO4] product. BSP and DPP exhibited nucleation activity at minimum concentrations of 0.3 microgram/ml (9 nM) and 10 micrograms/ml (67 nM) respectively. OC, OPN, ON and CC all lacked nucleation activity at concentrations up to 100 micrograms/ml. Inhibition of HA formation de novo was studied with calcium phosphate solutions buffered by autotitration. OPN was found to be a potent inhibitor of HA formation [IC50 = 0.32 microgram/ml (0.01 microM)] whereas OC was of lower potency [IC50 = 6.1 micrograms/ml (1.1 microM)]; BSP, ON and CC all lacked inhibitory activity at concentrations up to 10 micrograms/ml. The effect of OPN on HA formation de novo is mainly to inhibit crystal growth, whereas OC delays nucleation. These findings are consistent with the view that BSP and DPP may play roles in the initiation of mineralization in bone and dentine respectively. OPN seems to be the mineralized tissue protein most likely to function in the inhibition of HA formation, possibly by preventing phase separation in tissue fluids of high supersaturation.

Nucleation and inhibition of hydroxyapatite formation by mineralized tissue proteins.

Living organisms are capable of inducing the crystallization and deposition of a wide variety of minerals1 but the vertebrates mainly utilize the calcium phosphates in constructing their mineral phases in both normal circumstances in bone, dentin and tooth enamel and in pathological ectopic mineral deposits. The predominant form of the mineral in all situations is as carbonated apatite. However, the extent of mineralization in a particular tissue or organ is quite variable and crystallite size, crystal shape, and the packing and organization of the mineral crystals may also be variable. It is clear that the same physical chemical principles must apply to all, but it is equally clear that the organism must tightly regulate the local environment where the mineral is formed. This is an intrinsically complex problem because the mineral crystals of bone and dentin form in the extracellular matrix, external to the cells which are the ultimate regulators of the process.

Years of study of this complex set of problems has led to the general consensus that the cell-controlled processes of mineralization begin with the manufacture of an organic structure within which, or a compartment surface upon which, the mineral crystals may be initiated. The cells secrete and organize macromolecular structures which determine the ultimate character and orientations of the crystals subsequently initiated and grown, but in general, these structures do not themselves have the capacity to initiate mineralization. The incipient crystal nucleation event depends upon the interaction of the structural macromolecules with another set of secreted interactive macromolecules which locate specifically on the structural framework. These interactive molecules are doubly interactive, binding specifically to the framework on the one hand, and attracting the requisite mineral ions on the other to initiate the mineral ion clustering to a critical size that nucleates crystal growth. The biological fluids are supersaturated in mineral ion content with respect to the solubility of their biogenic mineral crystal phase, but unrestricted and uncontrolled crystal growth are not permissible in the vertebrate animal so there is a third line of macromolecular components interactive with the growing crystal surfaces that can limit growth rates, and do so selectively, controlling the crystal shape as well as size. A final component of the regulation is the availability of the crystal microions, pumped in controlled fashion from the cell into the extracellular matrix. The situation is shown schematically in Figure 1, wherein the solid, heavy arrows trace the construction of the structural matrix (a), the addition of the interactive macromolecules that activate the structural matrix (b) to bind the mineral ions (c), leading to growth of the oriented crystals (d), and finally, the addition of further macromolecules (e) to shape and delimit the crystal size to attain the mature mineralized matrix form. Note that in this paragraph, and in Figure 1 the nature of the macromolecules and mineral phase has not been specified. This is intentional to emphasize the point that in the majority of mineralizing tissues in animal systems the same overall scheme of events is likely to apply, although the structural scaffolds and mineral types can vary greatly.



Figure 1

A proposed generalized scheme for matrix-regulated mineralization reactions. Modified and reprinted with permission from Reference 280 [Veis, Reviews in Mineralogy & Geochemistry, V. 54]. Copyright 2003 Mineralogical Society of America.



The calcium and phosphate ion product (Ca × P) in the extracellular fluids (ECF) of the vertebrates are greater than the solubility products of most crystalline forms of calcium phosphate, with hydroxylapatite (HA) being the major least soluble form of calcium phosphate. Hence, HA would precipitate spontaneously if it were not for the inhibition of crystal formation by the reduction of the activity of these ions, and their sequestration, by various macromolecular components. Thus, the ECF proteins in Ca-rich milk and blood, for example, play important roles as inhibitors of apatite precipitation. The initial form of the Ca – P solid phase deposits in vitro have been shown to be disordered and exhibit a diffuse X-ray diffraction pattern, indicating that initially only a very short range ordering of calcium and phosphate ions relative to each other takes place, producing a structure called “amorphous calcium phosphate” (ACP) 2,3,4. Depending upon the conditions of temperature, pH, and ion concentrations, the solid phase may grow through a variety of forms from the ACP to (in order of decreasing solubility) brushite [CaHPO4·2H2O], whitlockite [Ca3(PO4)2 ·3H20], octacalcium phosphate [Ca8H2(PO4)6·5H2O], and hydroxyapatite [Ca10(PO4)6(OH)2]. In bone, dentin, cementum and enamel, in vivo, where the nucleation process and the final crystal composition are determined by the local environment created by the combination of the collagen or amelogenin structural matrix and particular NCP components, it is evident that the transition from ion aggregate to crystalline form is tightly regulated. A question of great importance for those interested in development of biomimetic models of bone is how the NCP might template the desired crystal forms.

The principal focus of this review, however, will be on the influence of the phosphoproteins of the matrix on the formation and growth of crystals in two systems: the small, plate-like crystals and crystal aggregates of carbonated apatite found in bone, dentin and cementum; and the large, high axial ratio rod-like crystal aggregates found in dental enamel. As pointed out many years ago5, the dentin and enamel are formed in apposition to each other and provide direct contrasts between the nature of the compartments created by the cells, the structural matrices produced, and the extracellular matrix macromolecules that control the nucleation, growth and structure of their carbonated apatite mineral phases. Here we consider these two systems separately.

/pdf/phosphorylated-proteins-and-control-over-apatite-nucleation-1tbiewjeur.pdf

Phosphorylated proteins and control over apatite nucleation, crystal growth, and inhibition.

Osteopontin-hydroxyapatite interactions in vitro: inhibition of hydroxyapatite formation and growth in a gelatin-gel.

Concentration-dependent effects of dentin phosphophoryn in the regulation of in vitro hydroxyapatite formation and growth.

The effects of several noncollagenous matrix proteins on hydroxyapatite formation and growth were studied in a dynamic collagen gel system. In this system growth plate proteoglycan aggregates at concentrations of 1-10 micrograms/ml were effective inhibitors, desulfated aggregates from brachymorphic mice were less effective. Phosphophoryn at 1-100 micrograms/ml had no effect on formation; 60-120 micrograms/ml retarded mineral growth. Type X collagen at concentrations of 50-300 micrograms/ml had no effect on formation or growth.

The effects of noncollagenous matrix proteins on hydroxyapatite formation and proliferation in a collagen gel system.

Proteoglycans from the brachymorphic (bm/bm) mouse have a reduced sulfate content due to the impaired activity of adenosine phosphosulfate phosphokinase in these animals. X-ray diffraction and infrared analyses of the mineral from the calcified cartilage of the bm/bm mice demonstrate the presence of significantly larger and more perfect hydroxyapatite crystals of lower carbonate to phosphate content than crystals found in the control animals. No differences were seen in the mineral content, crystallite size, CO3:PO4 ratio, or infrared splitting factors measured in the diaphyseal bone from these animals. Electron microscopic examination similarly shows larger, more disorganized crystals in the bm/bm animals' calcified cartilage as contrasted with controls.In vitro, proteoglycan aggregates from these dwarf mice are shown in a collagen gel-growth system to be less effective inhibitors of hydroxyapatite formation and growth than similarly size sulfated proteoglycans from agematched control animals. The proteoglycans from the control mice were comparable in inhibitory ability to proteoglycan aggregates extracted from fetal bovine epiphyses.

Michael Maresca

Papers

Osteopontin-hydroxyapatite interactions in vitro: inhibition of hydroxyapatite formation and growth in a gelatin-gel.

Concentration-dependent effects of dentin phosphophoryn in the regulation of in vitro hydroxyapatite formation and growth.

The effects of noncollagenous matrix proteins on hydroxyapatite formation and proliferation in a collagen gel system.

Hydroxyapatite formation in the presence of proteoglycans of reduced sulfate content: studies in the brachymorphic mouse.