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.

Cells of the craniofacial skeleton are derived from a common mesenchymal progenitor. The regulatory factors that control their differentiation into various cell lineages are unknown. To investigate the biological function of dentin matrix protein 1 (DMP1), an extracellular matrix gene involved in calcified tissue formation, stable transgenic cell lines and adenovirally infected cells overexpressing DMP1 were generated. The findings in this paper demonstrate that overexpression of DMP1 in pluripotent and mesenchyme-derived cells such as C3H10T1/2, MC3T3-E1, and RPC-C2A can induce these cells to differentiate and form functional odontoblast-like cells. Functional differentiation of odontoblasts requires unique sets of genes being turned on and off in a growth- and differentiation-specific manner. The genes studied include transcription factors like core binding factor 1 (Cbfa1), bone morphogenetic protein 2 (BMP2), and BMP4; early markers for extracellular matrix deposition like alkaline phosphatase (ALP), osteopontin, osteonectin, and osteocalcin; and late markers like DMP2 and dentin sialoprotein (DSP) that are expressed by terminally differentiated odontoblasts and are responsible for the formation of tissue-specific dentin matrix. However, this differentiation pathway was limited to mesenchyme-derived cells only. Other cell lines tested by the adenoviral expression system failed to express odontoblast-phenotypic specific genes. An in vitro mineralized nodule formation assay demonstrated that overexpressed cells could differentiate and form a mineralized matrix. Furthermore, we also demonstrate that phosphorylation of Cbfa1 (osteoblast-specific transcription factor) was not required for the expression of odontoblast-specific genes, indicating the involvement of other unidentified odontoblast-specific transcription factors or coactivators. Cell lines that differentiate into odontoblast-like cells are useful tools for studying the mechanism involved in the terminal differentiation process of these postmitotic cells.

Dentin matrix protein 1 (DMP1) is a bone- and teeth-specific protein initially identified from mineralized dentin. Here we report that DMP1 is primarily localized in the nuclear compartment of undifferentiated osteoblasts. In the nucleus, DMP1 acts as a transcriptional component for activation of osteoblast-specific genes like osteocalcin. During the early phase of osteoblast maturation, Ca²⁺ surges into the nucleus from the cytoplasm, triggering the phosphorylation of DMP1 by a nuclear isoform of casein kinase II. This phosphorylated DMP1 is then exported out into the extracellular matrix, where it regulates nucleation of hydroxyapatite. Thus, DMP1 is a unique molecule that initiates osteoblast differentiation by transcription in the nucleus and orchestrates mineralized matrix formation extracellularly, at later stages of osteoblast maturation. The data presented here represent a paradigm shift in the understanding of DMP1 function. This information is crucial in understanding normal bone formation, remodeling, fracture healing, and skeletal tissue repair.

Odontoblasts are postmitotic cells that differentiate from the dental papilla. These cells are responsible for producing the calcified dentin matrix. The pulp-odontoblast interphase contains undifferentiated mesenchymal stem cells, which have the ability to cytodifferentiate into odontoblast-like cells in response to specific signaling molecules. Dentin matrix protein 1 (DMP1) is one of the dentin noncollagenous extracellular matrix proteins that has been implicated in regulation of mineralization. In this study, we have examined the potential role of DMP1 in inducing cytodifferentiation of dental pulp stem cells into odontoblast-like cells and formation of reparative dentin in a rat model. Cavities were drilled and pulps exposed in maxillary first molars. Collagen matrix impregnated with recombinant DMP1 was implanted directly in Group 1, while calcium hydroxide, a commonly used pulp-capping agent was implanted in group 2, collagen matrix that was not impregnated with rDMP1 was implanted directly in group 3, which served as control. Each of these three groups was subdivided into two subgroups, A for 2 weeks time duration and B for 4 weeks duration. At the end of the time period the maxillae were excised, tissues were processed for histological and immunohistochemical evaluations. The results showed that DMP1 could act as a morphogen on undifferentiated mesenchymal cells present in the dentin–pulp complex. These differentiated cells had the potential to regenerate dentin-like tissue, which was confirmed by the presence of collagenous matrix, odontoblast specific markers and calcified deposits.