Brady, J. and Herendeen, W. H. eds., Ben Jonson's 1616 Folio

239

231

Biogenic minerals found in teeth and bones are synthesized by precise cell-mediated mechanisms. They have superior mechanical properties due to their complex architecture. Control over biomineral properties can be accomplished by regulation of particle size, shape, crystal orientation, and polymorphic structure. In many organisms, biogenic minerals are assembled using a transient amorphous mineral phase. Here we report that organic constituents of bones and teeth, namely type I collagen and dentin matrix protein 1 (DMP1), are effective crystal modulators. They control nucleation of calcium phosphate polymorphs and the assembly of hierarchically ordered crystalline composite material. Both full-length recombinant DMP1 and post-translationally modified native DMP1 were able to nucleate hydroxyapatite (HAP) in the presence of type I collagen. However, the N-terminal domain of DMP1 (amino acid residues 1-334) inhibited HAP formation and stabilized the amorphous phase that was formed. During the nucleation and growth process, the initially formed metastable amorphous calcium phosphate phase transformed into thermodynamically stable crystalline hydroxyapatite in a precisely controlled manner. The organic matrix-mediated controlled transformation of amorphous calcium phosphate into crystalline HAP was confirmed by x-ray diffraction, selected area electron diffraction pattern, Raman spectroscopy, and elemental analysis. The mechanical properties of the protein-mediated HAP crystals were also determined as they reflect the material structure. Such understanding of biomolecule controls on biomineralization promises new insights into the controlled synthesis of crystalline structures.

Section: Characterization Of the Calcium Phosphate Deposits Obtained supporting

confidence: 81%

Section: Characterization Of the Calcium Phosphate Deposits Obtained mentioning

confidence: 54%

mentioning

confidence: 99%

See 1 more Smart Citation

Matrix Macromolecules in Hard Tissues Control the Nucleation and Hierarchical Assembly of Hydroxyapatite

Gajjeraman

Narayanan

Hao

et al. 2007

239

231

“…A common feature of these cells types is that all have been implicated in producing mineralizing extracellular matrix (43). It has been proposed that DMP1 plays a role in regulating the bioprocess of matrix mineralization (36,41,43,50,51). OF45 may play a similar role.…”

Section: Expression Of Of45 In Bone Marrowmentioning

confidence: 99%

Identification of Osteoblast/Osteocyte Factor 45 (OF45), a Bone-specific cDNA Encoding an RGD-containing Protein That Is Highly Expressed in Osteoblasts and Osteocytes

Petersen

Tkalcevic

Mansolf

et al. 2000

108

We describe the cloning and characterization of a novel bone-specific cDNA predicted to encode an extracellular matrix protein. This cDNA was identified by subtractive hybridization based upon its high expression in bone marrow-derived osteoblasts. By Northern blot analysis, we detected a single 2-kilobase mRNA transcript in bone, whereas no expression was detected in other tissues. Immunohistochemistry revealed that the protein was expressed highly in osteocytes within trabecular and cortical bone. RNA and protein expression analysis using in vivo marrow ablation as a model of bone remodeling demonstrated that this gene was expressed only in cells that were embedded within bone matrix in contrast to the earlier expression of known osteoblast markers. The cDNA was predicted to encode a serine/glycine-rich secreted peptide containing numerous potential phosphorylation sites and one RGD sequence motif. The interaction of RGD domain-containing peptides with integrins has been shown previously to regulate bone remodeling by promoting recruitment, attachment, and differentiation of osteoblasts and osteoclasts. Secretion of this RGD-containing protein from osteocytes has the potential to regulate cellular activities within the bone environment and thereby may impact bone homeostasis. We propose the name OF45 (osteoblast/osteocyte factor of 45 kDa) for this novel cDNA.Bone is a highly dynamic tissue that undergoes continual processes of remodeling and modeling (1). In the growing skeleton, the amount of mineralized bone formed exceeds the amount lost through resorption, whereas in the mature adult lost bone mineral is precisely balanced by an equivalent amount of formation, thereby preserving the integrity of the skeleton. Under certain conditions such as aging, postmenopausal estrogen deficiency, or prolonged steroid treatment, the amount of bone formed is not sufficient to compensate for the quantity lost by resorption. Over time, this imbalance results in reduced bone mass, compromises the structural integrity of the skeleton, and, ultimately, can lead to osteoporotic bone fractures. Osteoporosis and the resulting fractures are causes of significant morbidity and mortality within the aging population.Bone remodeling is a very complex process of tightly coordinated action by the bone resorbing osteoclasts and the bone forming osteoblasts. Osteoblasts are derived from a mesenchymal cell lineage and are responsible for the formation of new bone matrix in their differentiated state (2). In addition, osteoblasts produce factors that regulate the formation of osteoclasts and osteoclastic bone resorbing activity in response to endocrine signals such as parathyroid hormone and 1,25-dihydroxyvitamin D 3 . It has been postulated that bone loss associated with aging is caused by a defect in the osteoblast cell lineage (3-6). Either the mesenchymal precursor population is insufficient or has lost the capacity to proliferate and/or differentiate into sufficient numbers of functioning osteoblasts.

“…The NCPs are mostly acidic proteins, rich in glutamic acid, aspartic acid, and phosphorylated serine/threonine residues, with a high capacity for binding calcium ions and hydroxyapatite crystal surfaces. Therefore, they have been implicated in the regulation of mineral deposition during osteogenesis and dentinogenesis (1). Phosphorylation of threonine and serine residues takes place as a post-translational modification of the core protein via the action of casein kinases (2).…”

mentioning

confidence: 99%

Phosphorylation of Phosphophoryn Is Crucial for Its Function as a Mediator of Biomineralization

Ramachandran

Dahl

et al. 2005

161

157

Phosphoproteins of the organic matrix of bone and dentin have been implicated as regulators of the nucleation and growth of the inorganic Ca-P crystals of vertebrate bones and teeth. One such protein identified in the dentin matrix is phosphophoryn (PP). It is highly acidic in nature because of a high content of aspartic acid and phosphate groups on serines. The 244-residue carboxyl-terminal domain of rat PP, predominantly containing the aspartic acid-serine repeats, has been cloned, and the corresponding protein has been expressed recombinantly in Escherichia coli. This portion of PP, named DMP2 (dentin matrix protein 2), is not phosphorylated by the bacteria and thus provided a means to study the function of the phosphate groups, the major post-translational modification of native PP. The recombinant DMP2 (rDMP2) possessed much lower calcium binding capacity than native PP. Small angle x-ray scattering experiments demonstrated that PP folds to a compact globular structure upon calcium binding, whereas rDMP2 maintained an unfolded structure. In vitro nucleation experiments showed that PP could nucleate plate-like apatite crystals in pseudophysiological buffer, whereas rDMP2 failed to mediate the transformation of amorphous calcium phosphate to apatite crystals under the same experimental conditions. Collagen binding experiments demonstrated that PP favors the formation of collagen aggregates, whereas in the presence of rDMP2 thin fibrils are formed. Overall these results suggested that the phosphate moieties in phosphophoryn are important for its function as a mediator of dentin biomineralization.Mineralization is an essential requirement for the development of the mechanical properties of hard tissues such as bones and teeth. However, on a volume basis, the mineral constitutes only about 50% of a bone; the remaining extracellular organic matrix (ECM) 2 is a hydrated mixture of collagen and noncollagenous matrix proteins (NCPs). The NCPs constitute 5-10% of the total ECM and are associated with the mineral phase so strongly that the tissues need to be demineralized before those proteins can be extracted. The NCPs are mostly acidic proteins, rich in glutamic acid, aspartic acid, and phosphorylated serine/threonine residues, with a high capacity for binding calcium ions and hydroxyapatite crystal surfaces. Therefore, they have been implicated in the regulation of mineral deposition during osteogenesis and dentinogenesis (1). Phosphorylation of threonine and serine residues takes place as a post-translational modification of the core protein via the action of casein kinases (2). found that dentin mineralization was impaired in the presence of casein kinase inhibitors. Thus post-translational phosphorylation of NCPs is crucial for biomineralization.The major phosphoprotein of dentin is the Asp-and Ser-rich protein called phosphophoryn. The name phosphophoryn (PP) was coined to describe its exceedingly high content of phosphate groups. PP, as isolated from dentin, has a unique composition with aspartyl and seryl ...