Flexible structures of SIBLING proteins, bone sialoprotein, and osteopontin

Osteopontin regulates dentin and alveolar bone development and mineralization

The periodontal complex is essential for tooth attachment and function and includes the mineralized tissues, cementum and alveolar bone, separated by the unmineralized periodontal ligament (PDL). To gain insights into factors regulating cementum-PDL and bone-PDL borders and protecting against ectopic calcification within the PDL, we employed a proteomic approach to analyze PDL tissue from progressive ankylosis knock-out (Ank^-/-) mice, featuring reduced PP_i, rapid cementogenesis, and excessive acellular cementum. Using this approach, we identified the matrix protein osteopontin (Spp1/OPN) as an elevated factor of interest in Ank^-/- mouse molar PDL. We studied the role of OPN in dental and periodontal development and function. During tooth development in wild-type (WT) mice, Spp1 mRNA was transiently expressed by cementoblasts and strongly by alveolar bone osteoblasts. Developmental analysis from 14 to 240days postnatal (dpn) indicated normal histological structures in Spp1^-/- comparable to WT control mice. Microcomputed tomography (micro-CT) analysis at 30 and 90dpn revealed significantly increased volumes and tissue mineral densities of Spp1^-/- mouse dentin and alveolar bone, while pulp and PDL volumes were decreased and tissue densities were increased. However, acellular cementum growth was unaltered in Spp1^-/- mice. Quantitative PCR of periodontal-derived mRNA failed to identify potential local compensators influencing cementum in Spp1^-/- vs. WT mice at 26dpn. We genetically deleted Spp1 on the Ank^-/- mouse background to determine whether increased Spp1/OPN was regulating periodontal tissues when the PDL space is challenged by hypercementosis in Ank^-/- mice. Ank^-/-; Spp1^-/- double deficient mice did not exhibit greater hypercementosis than that in Ank^-/- mice. Based on these data, we conclude that OPN has a non-redundant role regulating formation and mineralization of dentin and bone, influences tissue properties of PDL and pulp, but does not control acellular cementum apposition. These findings may inform therapies targeted at controlling soft tissue calcification.

Osteopontin as a novel substrate for the proprotein convertase 5/6 (PCSK5) in bone

Seven proprotein convertases cleave the basic amino acid consensus sequence K/R-X_n-K/R↓ (where n=0, 2, 4 or 6 variable amino acids) to activate precursor proteins. Despite similarities in substrate specificity, basic amino acid-specific proprotein convertases have a distinct tissue distribution allowing for enzymatic actions on tissue-resident substrates. Proprotein convertase 5/6 (PC5/6) has two splice variants - soluble PC5/6A and membrane-bound PC5/6B - and is expressed during mouse development in many tissues including bone and tooth, but little is known about the substrates for PC5/6 therein. Osteopontin (OPN) is an abundant bone extracellular matrix protein with roles in mineralization, cell adhesion and cell migration, and it has putative consensus sequence sites for cleavage by PC5/6, which may modify its function in bone. Since PC5/6-knockout mouse embryos show developmental abnormalities, and reduced overall mineralization, we designed this study to determine whether OPN is a substrate of PC5/6. In silico analysis of OPN protein sequences identified four potential PC5/6 consensus cleavage sites in human OPN, and three sites - including a noncanonical sequence - in mouse OPN. Ex vivo co-transfections with human OPN revealed complete OPN cleavage reducing full-length OPN (~70kDa) to an N-terminal fragment migrating at ~50kDa and two C-terminal fragments at ~18kDa and ~16kDa. Direct cleavage of OPN by PC5/6A - the predominant isoform expressed in human osteoblast cells - was confirmed by cell-free enzyme-substrate assays and by mass spectrometry. The latter was also used to investigate potential cleavage sites. Co-transfections of PC5/6 and mouse OPN showed partial cleavage of OPN into a C-terminal OPN fragment migrating at ~30kDa and an N-terminal fragment migrating at ~29kDa. Micro-computed tomography of PC5/6-knockout embryos at E18.5 confirmed a reduction in mineralized bone, and in situ hybridization performed on cryo-sections of normal mouse bone using Pcsk5 and Opn anti-sense and control-sense cRNA probes indicated the co-localization of the expression of these genes in bone cells. This mRNA expression profile was supported by semi-quantitative RT-PCR using osteoblast primary cultures, and cultured MC3T3-E1 osteoblast and MLO-Y4 osteocyte cell lines. Immunoblotting for OPN from mouse bone extracts showed altered OPN processing in PC5/6-knockout mice compared to wildtype mice. OPN fragments migrated at ~25kDa and ~16kDa in wildtype bone and were not present in PC5/6-deficient bone. In conclusion, this study demonstrates that Pcsk5 is expressed in bone-forming cells, and that OPN is a novel substrate for PC5/6. Cleavage of OPN by PC5/6 may modify the function of OPN in bone and/or modulate other enzymatic cleavages of OPN, leading to alterations in the bone phenotype.

The importance of the helical structure of a MamC-derived magnetite-interacting peptide for its function in magnetite formation

Biomineralization is the process of mineral formation by organisms and involves the uptake of ions from the environment in order to produce minerals, with the process generally being mediated by proteins. Most proteins that are involved in mineral interactions are predicted to contain disordered regions containing large numbers of negatively charged amino acids. Magnetotactic bacteria, which are used as a model system for iron biomineralization, are Gram-negative bacteria that can navigate through geomagnetic fields using a specific organelle, the magnetosome. Each organelle comprises a membrane-enveloped magnetic nanoparticle, magnetite, the formation of which is controlled by a specific set of proteins. One of the most abundant of these proteins is MamC, a small magnetosome-associated integral membrane protein that contains two transmembrane α-helices connected by an ∼21-amino-acid peptide. In vitro studies of this MamC peptide showed that it forms a helical structure that can interact with the magnetite surface and affect the size and shape of the growing crystal. Our results show that a disordered structure of the MamC magnetite-interacting component (MamC-MIC) abolishes its interaction with magnetite particles. Moreover, the size and shape of magnetite crystals grown in in vitro magnetite-precipitation experiments in the presence of this disordered peptide were different from the traits of crystals grown in the presence of other peptides or in the presence of the helical MIC. It is suggested that the helical structure of the MamC-MIC is important for its function during magnetite formation.

Is Osteopontin a Friend or Foe of Cell Apoptosis in Inflammatory Gastrointestinal and Liver Diseases?

Osteopontin (OPN) is involved in a variety of biological processes, including bone remodeling, innate immunity, acute and chronic inflammation, and cancer. The expression of OPN occurs in various tissues and cells, including intestinal epithelial cells and immune cells such as macrophages, dendritic cells, and T lymphocytes. OPN plays an important role in the efficient development of T helper 1 immune responses and cell survival by inhibiting apoptosis. The association of OPN with apoptosis has been investigated. In this review, we described the role of OPN in inflammatory gastrointestinal and liver diseases, focusing on the association of OPN with apoptosis. OPN changes its association with apoptosis depending on the type of disease and the phase of disease activity, acting as a promoter or a suppressor of inflammation and inflammatory carcinogenesis. It is essential that the roles of OPN in those diseases are elucidated, and treatments based on its mechanism are developed.

Overlapping functions of bone sialoprotein and pyrophosphate regulators in directing cementogenesis

Although acellular cementum is essential for tooth attachment, factors directing its development and regeneration remain poorly understood. Inorganic pyrophosphate (PP_i), a mineralization inhibitor, is a key regulator of cementum formation: tissue-nonspecific alkaline phosphatase (Alpl/TNAP) null mice (increased PP_i) feature deficient cementum, while progressive ankylosis protein (Ank/ANK) null mice (decreased PP_i) feature increased cementum. Bone sialoprotein (Bsp/BSP) and osteopontin (Spp1/OPN) are multifunctional extracellular matrix components of cementum proposed to have direct and indirect effects on cell activities and mineralization. Studies on dentoalveolar development of Bsp knockout (Bsp^-/-) mice revealed severely reduced acellular cementum, however underlying mechanisms remain unclear. The similarity in defective cementum phenotypes between Bsp^-/- mice and Alpl^-/- mice (the latter featuring elevated PP_i and OPN), prompted us to examine whether BSP is operating by modulating PP_i-associated genes. Genetic ablation of Bsp caused a 2-fold increase in circulating PP_i, altered mRNA expression of Alpl, Spp1, and Ank, and increased OPN protein in the periodontia. Generation of a Bsp knock-out (KO) cementoblast cell line revealed significantly decreased mineralization capacity, 50% increased PP_i in culture media, and increased Spp1 and Ank mRNA expression. While addition of 2μg/ml recombinant BSP altered Spp1, Ank, and Enpp1 expression in cementoblasts, changes resulting from this dose were not dependent on the integrin-binding RGD motif or MAPK/ERK signaling pathway. Decreasing PP_i by genetic ablation of Ank on the Bsp^-/- mouse background reestablished cementum formation, allowing >3-fold increased acellular cementum volume compared to wild-type (WT). However, deleting Ank did not fully compensate for the absence of BSP. Bsp^-/-; Ank^-/- double-deficient mice exhibited mean 20-27% reduced cementum thickness and volume compared to Ank^-/- mice. From these data, we conclude that the perturbations in PP_i metabolism are not solely driving the cementum pathology in Bsp^-/- mice, and that PP_i is more potent than BSP as a cementum regulator, as shown by the ability to override loss of BSP by lowering PP_i. We propose that BSP and PP_i work in concert to direct mineralization in cementum and likely other mineralized tissues.

Inhibition of FGFR Signaling Partially Rescues Hypophosphatemic Rickets in HMWFGF2 Tg Male Mice

Transgenic mice harboring high molecular weight fibroblast growth factor (FGF)2 isoforms (HMWTg) in osteoblast lineage cells phenocopy human X-linked hypophosphatemic rickets (XLH) and Hyp murine model of XLH demonstrating increased FGF23/FGF receptor signaling and hypophosphatemic rickets. Because HMWFGF2 was upregulated in bones of Hyp mice and abnormal FGF receptor (FGFR) signaling is important in XLH, HMWTg mice were used to examine the effect of the FGFR inhibitor NVP-BGJ398, now in clinical trials for cancer therapy, on hypophosphatemic rickets. Short-term treatment with NVP-BGJ398 rescued abnormal FGFR signaling and hypophosphatemia in HMWTg. Long-term treatment with NVP-BGJ398 normalized tail, tibia, and femur length. Four weeks NVP-BGJ398 treatment significantly increased total body bone mineral density (BMD) and bone mineral content (BMC) in HMWTg mice; however, at 8 weeks, total body BMD and BMC was indistinguishable among groups. Micro-computed tomography revealed decreased vertebral bone volume, trabecular number, and increased trabecular spacing, whereas femur trabecular tissue density was increased; however, NVP-BGJ398 rescued defective cortical bone mineralization, increased thickness, reduced porosity, and increased endosteal perimeter and cortical tissue density in HMWTg. NVP-BGJ398 improved femur cancellous bone, cortical bone structure, growth plate, and double labeling in cortical bone and also increased femur trabeculae double labeled surface, mineral apposition rate, bone formation rate, and osteoclast number and surface in HMWTg. The decreased NPT2a protein that is important for renal phosphate excretion was rescued by NVP-BGJ398 treatment. We conclude that NVP-BGJ398 partially rescued hypophosphatemic rickets in HMWTg. However, long-term treatment with NVP-BGJ398 further increased serum FGF23 that could exacerbate the mineralization defect.

Glycosylation status of bone sialoprotein and its role in mineralization

The highly glycosylated bone sialoprotein (BSP) is an abundant non-collagenous phosphoprotein in bone which enhances osteoblast differentiation and new bone deposition in vitro and in vivo. However, the structural details of its different glycosylation linkages have not been well studied and their functions in bone homeostasis are not clear. Previous studies suggested that the O-glycans, but not the N-glycans on BSP, are highly sialylated. Herein, we employed tandem mass spectrometry (MS/MS) to demonstrate that the N-glycanson the recombinant human integrin binding sialoprotein (rhiBSP) are also enriched in sialic acids (SAs) at their termini. We also identified multiple novel sites of N-glycan modification. Treatment of rhiBSP enhances osteoblast differentiation and mineralization of MC3T3-E1 cells and this effect could be partially reversed by efficient enzymatic removal of its N-glycans. Removal of all terminal SAs has a greater effect in reversing the effect of rhiBSP on osteogenesis, especially on mineralization, suggesting that sialylation at the termini of both N-glycans and O-glycans plays an important role in this regulation. Moreover, BSP-conjugated SAs may affect mineralization via ERK activation of VDR expression. Collectively, our results identified novel N-glycans enriched in SAs on the rhiBSP and demonstrated that SAs at both N- and O-glycans are important for BSP regulation of osteoblast differentiation and mineralization in vitro.

Structural properties of the intrinsically disordered, multiple calcium ion-binding otolith matrix macromolecule-64 (OMM-64)

Fish otoliths are calcium carbonate biominerals that are involved in hearing and balance sensing. An organic matrix plays a crucial role in their formation. Otolith matrix macromolecule-64 (OMM-64) is a highly acidic, calcium-binding protein (CBP) found in rainbow trout otoliths. It is a component of high-molecular-weight aggregates, which influence the size, shape and polymorph of calcium carbonate in vitro. In this study, a protocol for the efficient expression and purification of OMM-64 was developed. For the first time, the complete structural characteristics of OMM-64 were described. Various biophysical methods were combined to show that OMM-64 occurs as an intrinsically disordered monomer. Under denaturing conditions (pH, temperature) OMM-64 exhibits folding propensity. It was determined that OMM-64 binds approximately 61 calcium ions with millimolar affinity. The folding-unfolding experiments showed that calcium ions induced the collapse of OMM-64. The effect of other counter ions present in trout endolymph on OMM-64 conformational changes was studied. The significance of disordered properties of OMM-64 and the possible function of this protein is discussed.

Effect of Phosphorylation on a Human-like Osteopontin Peptide

The last decade established that the dynamic properties of the phosphoproteome are central to function and its modulation. The temporal dimension of phosphorylation effects remains nonetheless poorly understood, particularly for intrinsically disordered proteins. Osteopontin, selected for this study due to its key role in biomineralization, is expressed in many species and tissues to play a range of distinct roles. A notable property of highly phosphorylated isoforms of osteopontin is their ability to sequester nanoclusters of calcium phosphate to form a core-shell structure, in a fluid that is supersaturated but stable. In Biology, this process enables soft and hard tissues to coexist in the same organism with relative ease. Here, we extend our understanding of the effect of phosphorylation on a disordered protein, the recombinant human-like osteopontin rOPN. The solution structures of the phosphorylated and unphosphorylated rOPN were investigated by small-angle x-ray scattering and no significant changes were detected on the radius of gyration or maximum interatomic distance. The picosecond-to-nanosecond dynamics of the hydrated powders of the two rOPN forms were further compared by elastic and quasi-elastic incoherent neutron scattering. Phosphorylation was found to block some nanosecond side-chain motions while increasing the flexibility of other side chains on the faster timescale. Phosphorylation can thus selectively change the dynamic behavior of even a highly disordered protein such as osteopontin. Through such an effect on rOPN, phosphorylation can direct allosteric mechanisms, interactions with substrates, cofactors and, in this case, amorphous or crystalline biominerals.

Mineralization-inhibiting effects of transglutaminase-crosslinked polymeric osteopontin

Osteopontin (OPN) belongs to the SIBLING family (Small, Integrin-Binding LIgand N-linked Glycoproteins) of mineral-binding matrix proteins found in bones and teeth. OPN is a well-known inhibitor of matrix mineralization, and enzymatic modification of OPN can affect this inhibitory function. In bone, OPN exists both as a monomer and as a high-molecular-weight polymer - the latter is formed by transglutaminase-mediated crosslinking of glutamine and lysine residues in OPN to create homotypic protein assemblies. OPN can be covalently crosslinked by transglutaminase 2 (TG2) and Factor XIII-A. Polymeric OPN has increased binding to collagen and promotes osteoblast adhesion, but despite these initial observations, its role in mineralization is not clear. In this study, we investigated the effect of polymerized OPN on mineralization using a hydroxyapatite crystal growth assay and mineralizing MC3T3-E1 osteoblast cultures. In the cultures, endogenous polymeric OPN was detected after mineralization occurred. In cell-free conditions, TG2 was used to crosslink bovine OPN into its polymeric form, and atomic force microscopy and dynamic light scattering revealed variably-sized, large branched aggregates ranging across hundreds of nanometers. These OPN polymers inhibited the growth of hydroxyapatite crystals in solution at concentrations similar to monomeric OPN, although the crosslinking slightly reduced its inhibitory potency. When added to MC3T3-E1 osteoblast cultures, this exogenous polymeric OPN essentially did not inhibit mineralization when given during the later mineralization stages of culture; however, cultures treated early and then continuously with polymeric OPN throughout both the matrix assembly and mineral deposition stages showed reduced mineralization. Immunoblotting of protein extracts from these continuously treated cultures revealed exogenous OPN polymers incorporated into mature matrix that had not yet mineralized. These results suggest that in bone, the increased size and branched structure of crosslinked inhibitory polymeric OPN near the mineralization front could hinder it from accessing focal mineralization sites in the dense collagen-rich matrix, suggesting that OPN-crosslinking into polymers may represent a way to fine-tune the inhibitory potency of OPN on bone mineralization.

Inhibition of chondroitin sulfate glycosaminoglycans incorporation affected odontoblast differentiation in cultured embryonic mouse molars

Chondroitin sulfate proteoglycan (CSPG) is an important component of extracellular matrix (ECM), it is composed of a core protein and one or more chondroitin sulfate glycosaminoglycan side chains (CS-GAGs). To investigate the roles of its CS-GAGs in dentinogenesis, the mouse mandibular first molar tooth germs at early bell stage were cultivated with or without β-xyloside. As expected, the CS-GAGs were inhibited on their incorporation to CSPGs by β-xyloside, accompanied by the change of morphology of the cultured tooth germs. The histological results and the transmission electron microscopy (TEM) investigation indicated that β-xyloside exhibited obvious inhibiting effects on odontoblasts differentiation compared with the control group. Meanwhile the results of immunohistochemistry, in situ hybridization and quantitative RT-PCR for type I collagen, dentin matrix acidic phosphoprotein 1 and dentin sialophosphoprotein, the products of differentiated odontoblasts, further proved that odontoblasts differentiation was inhibited. Collagen fibers detected in TEM decreased and arranged in disorder as well. Thus we conclude that the inhibition of CS-GAGs incorporation to CSPGs can affect odontoblast differentiation in cultured embryonic mouse molars.

TNF-α has both stimulatory and inhibitory effects on mouse monocyte-derived osteoclastogenesis

Phenotypically different osteoclasts may be generated from different subsets of precursors. To what extent the formation of these osteoclasts is influenced or mediated by the inflammatory cytokine TNF-α, is unknown and was investigated in this study. The osteoclast precursors early blasts (CD31^hi Ly-6C^- ), myeloid blasts (CD31⁺ Ly-6C⁺ ), and monocytes (CD31^- Ly-6C^hi ) were sorted from mouse bone marrow using flow cytometry and cultured with M-CSF and RANKL, with or without TNF-α. Surprisingly, TNF-α prevented the differentiation of TRAcP⁺ osteoclasts generated from monocytes on plastic; an effect not seen with early blasts and myeloid blasts. This inhibitory effect could not be prevented by other cytokines such as IL-1β or IL-6. When monocytes were pre-cultured with M-CSF and RANKL followed by exposure to TNF-α, a stimulatory effect was found. TNF-α also stimulated monocytes' osteoclastogenesis when the cells were seeded on bone. Gene expression analysis showed that when TNF-α was added to monocytes cultured on plastic, RANK, NFATc1, and TRAcP were significantly down-regulated while TNF-αR1 and TNF-αR2 were up-regulated. FACS analysis showed a decreased uptake of fluorescently labeled RANKL in monocyte cultures in the presence of TNF-α, indicating an altered ratio of bound-RANK/unbound-RANK. Our findings suggest a diverse role of TNF-α on monocytes' osteoclastogenesis: it affects the RANK-signaling pathway therefore inhibits osteoclastogenesis when added at the onset of monocyte culturing. This can be prevented when monocytes were pre-cultured with M-CSF and RANKL, which ensures the binding of RANKL to RANK. This could be a mechanism to prevent unfavorable monocyte-derived osteoclast formation away from the bone.

Osteopontin -- a promising biomarker for cancer therapy

Osteopontin (OPN), a multifunctional protein, has emerged as a potentially valuable biomarker for diagnosing and treating cancers. Recent research focuses on its involvement in tumor biology including the cell proliferation, survival, angiogenesis, invasion, and metastasis. Understanding the molecular mechanisms and pharmacological effects of OPN in cancer development could lead to new targets for improving cancer diagnosis and treatment. This review explains how the structurally conserved domains of OPN are associated with OPN signaling mediators and CD44, and how the conserved OPN domains determine biological functions. The authors have reviewed representative works of OPN expression in breast cancer and colorectal cancer to elucidate the relationship between OPN and cancer/tumor biology. It has also been shown that the prognostic sensitivity in non-small cell lung cancer, hepatocellular carcinoma, gastric cancer, and ovarian cancer improved compared to the individual marker when OPN was analyzed in conjunction with other markers. The therapeutic approaches based on OPN inhibitors are discussed to illustrate recent research progress. Previous clinical data has indicated that OPN has played a unique role in cancer development, but further investigation is required to understand the underlying mechanism. More clinical trials are also required to examine the applicability and efficacy of OPN inhibitors in cancer therapy.

Bone Alkaline Phosphatase and Tartrate-Resistant Acid Phosphatase: Potential Co-regulators of Bone Mineralization

Phosphorylated osteopontin (OPN) inhibits hydroxyapatite crystal formation and growth, and bone alkaline phosphatase (BALP) promotes extracellular mineralization via the release of inorganic phosphate from the mineralization inhibitor inorganic pyrophosphate (PPi). Tartrate-resistant acid phosphatase (TRAP), produced by osteoclasts, osteoblasts, and osteocytes, exhibits potent phosphatase activity towards OPN; however, its potential capacity as a regulator of mineralization has not previously been addressed. We compared the efficiency of BALP and TRAP towards the endogenous substrates for BALP, i.e., PPi and pyridoxal 5'-phosphate (PLP), and their impact on mineralization in vitro via dephosphorylation of bovine milk OPN. TRAP showed higher phosphatase activity towards phosphorylated OPN and PPi compared to BALP, whereas the activity of TRAP and BALP towards PLP was comparable. Bovine milk OPN could be completely dephosphorylated by TRAP, liberating all its 28 phosphates, whereas BALP dephosphorylated at most 10 phosphates. OPN, dephosphorylated by either BALP or TRAP, showed a partially or completely attenuated phosphorylation-dependent inhibitory capacity, respectively, compared to native OPN on the formation of mineralized nodules. Thus, there are phosphorylations in OPN important for inhibition of mineralization that are removed by TRAP but not by BALP. In conclusion, our data indicate that both BALP and TRAP can alleviate the inhibitory effect of OPN on mineralization, suggesting a potential role for TRAP in skeletal mineralization. Further studies are warranted to explore the possible physiological relevance of TRAP in bone mineralization.

Osteopontin-metallothionein I/II interactions in experimental autoimmune encephalomyelitis

Osteopontin (OPN), an extracellular matrix (ECM) glyco-phosphoprotein, plays an important role in autoimmune-mediated demyelinating diseases, including multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). As an integrin and CD44 binding protein it participates in bidirectional communication between the ECM and target cells and affects transduction pathways that maintain neuronal and immune cell homeostasis. Its biological activity is also heavily influenced by microenvironment, which stimulates the cleavage of OPN and changes its functions. In this study we estimated the expression profile of OPN in neural tissues of DA rats during the first relapse of chronic relapsing EAE and investigated the relationship of OPN to metallothionein I+II (MTs), which play pivotal role in zinc-related cell homeostasis and in protection of CNS against cytokine-induced injury. The data showed that in EAE rats OPN mRNA and protein levels increased concurrently with the transcription of MTs and that within the spinal cord (SC) lysates EAE-afflicted rats had a higher content of OPN fragments of low molecular weight than untreated and CFA-treated rats. The expression of OPN and MTs was upregulated on ependymal, lymphoid and astroglial cells and on multiple αvβ3+ neurons in SC and in the brain (cortex, white matter, hippocampus, and cerebellum). Besides, multiple cells co-expressed OPN and MTs. Granular OPN signals were detected in secretory vesicles of Golgy (αvβ3 neurons) and in patches adjacent to the plasma membrane (subventricular zone). The findings imply that in demyelinating lesions are generated proteolytic OPN fragments and that OPN/MT interactions contribute to tissue remodeling during an autoimmune attack.

Periodontal disease and FAM20A mutations

Enamel-renal-gingival syndrome (ERGS; OMIM #204690), a rare autosomal recessive disorder caused by mutations in FAM20A, is characterized by nephrocalcinosis, nephrolithiasis, amelogenesis imperfecta, hypoplastic type, gingival fibromatosis and other dental abnormalities, including hypodontia and unerupted teeth with large dental follicles. We report three patients and their families with findings suggestive of ERGS. Mutation analysis of FAM20A was performed in all patients and their family members. Patients with homozygous frameshift and compound heterozygous mutations in FAM20A had typical clinical findings along with periodontitis. The other had a novel homozygous missense mutation in exon 10, mild gingival fibromatosis and renal calcifications. The periodontitis in our patients may be a syndrome component, and similar findings in previous reports suggest more than coincidence. Fam20a is an allosteric activator that increases Fam20c kinase activity. It is hypothesized that lack of FAM20A activation of FAM20C in our patients with FAM20A mutations might have caused amelogenesis imperfecta, abnormal bone remodeling and periodontitis. Nephrocalcinosis appears not to be a consistent finding of the syndrome and the missense mutation may correlate with mild gingival fibromatosis. Here we report three patients with homozygous or compound heterozygous mutations in FAM20A and findings that extend the phenotypic spectrum of this disorder, showing that protein truncation is associated with greater clinical severity.

Osteopontin splice variants and polymorphisms in cancer progression and prognosis

Osteopontin (OPN) is an extracellular matrix protein that is overexpressed in various cancers and promotes oncogenic features including cell proliferation, survival, migration, and angiogenesis, among others. OPN can participate in the regulation of the tumor microenvironment, affecting both cancer and neighboring cells. Here, we review the roles of OPN splice variants (a, b, c) in cancer development, progression, and prognosis, and also discuss the identities of isoforms 4 and 5. We also discussed how single-nucleotide polymorphisms (SNPs) of the OPN gene are an additional factor influencing the level of OPN in individuals, modulating the risks of cancer development and outcome.

Sildenafil, a Phosphodiesterase Type 5 Inhibitor, Downregulates Osteopontin in Human Peripheral Blood Mononuclear Cells

The aim of this study was to investigate the ability of sildenafil to regulate osteopontin (OPN) gene and protein in peripheral blood mononuclear cells (PBMCs) from healthy blood donors. OPN is expressed by a wide variety of cell types, including immune cells. OPN functions are linked to various physiological and pathological conditions. Sildenafil is a selective inhibitor of type 5 phosphodiesterase. Sildenafil has recently been found to have immunomodulatory effects in animal models and in studies performed in humans. PMA-stimulated and unstimulated PBMCs from 16 healthy blood donors (men) were cultured with sildenafil (at concentrations of 400 ng/ml and 4 µg/ml). OPN level in culture supernatants was measured by enzyme-linked immunosorbent assay. The analysis of OPN gene expression was performed by real-time PCR. Cell viability was assessed by trypan blue staining. PMA plus ionomycin stimulation of PBMCs resulted in a significant increase of OPN production and gene expression (p < 0.001). Sildenafil significantly decreased OPN secretion (p < 0.05) and gene expression (p < 0.05) in stimulated PBMCs; however, had no effect on OPN in unstimulated PBMCs. Sildenafil did not affect PBMCs viability. Sildenafil downregulates OPN in PBMCs from healthy men. Despite accumulating evidence for the immunomodulatory effects of sildenafil on human immune system cells, further studies are needed to determine if this drug affects the level of cGMP and NF-κB in PBMCs. In addition, it is needed to evaluate sildenafil’s activity in PBMCs from patients with elevated OPN levels.

Embracing the complexity of matricellular proteins: the functional and clinical significance of splice variation

Matricellular proteins influence wide-ranging fundamental cellular processes including cell adhesion, migration, growth and differentiation. They achieve this both through interactions with cell surface receptors and regulation of the matrix environment. Many matricellular proteins are also associated with diverse clinical disorders including cancer and diabetes. Alternative splicing is a precisely regulated process that can produce multiple isoforms with variable functions from a single gene. To date, the expression of alternate transcripts for the matricellular family has been reported for only a handful of genes. Here we analyse the evidence for alternative splicing across the matricellular family including the secreted protein acidic and rich in cysteine (SPARC), thrombospondin, tenascin and CCN families. We find that matricellular proteins have double the average number of splice variants per gene, and discuss the types of domain affected by splicing in matricellular proteins. We also review the clinical significance of alternative splicing for three specific matricellular proteins that have been relatively well characterised: osteopontin (OPN), tenascin-C (TNC) and periostin. Embracing the complexity of matricellular splice variants will be important for understanding the sometimes contradictory function of these powerful regulatory proteins, and for their effective clinical application as biomarkers and therapeutic targets.

Ameloblastin induces tumor suppressive phenotype and enhances chemosensitivity to doxorubicin via Src-Stat3 inactivation in osteosarcoma

Ameloblastin (AMBN), the most abundant non-amelogenin enamel matrix protein, plays a role in ameloblast differentiation. Previously, we found that AMBN promoted osteogenic differentiation via the interaction between CD63 and integrin β1, leading to the inactivation of Src; however, how AMBN affects the malignant behavior of osteosarcoma is still unclear. Osteosarcoma affects the bone and is associated with poor prognosis because of the high rate of pulmonary metastases and drug resistance. Here we demonstrated that stable overexpression of AMBN induced apoptosis and suppressed colony formation and cell migration via the inactivation of Src-Stat3 pathway in human osteosarcoma cells. Moreover, AMBN induced chemosensitivity to doxorubicin. Thus, AMBN induced a tumor suppressive phenotype and chemosensitivity to doxorubicin via the AMBN-Src-Stat3 axis in osteosarcoma. Indeed, immunohistochemical expression of AMBN was significantly correlated with better outcome of osteosarcoma patients. Our findings suggest that AMBN can be a new prognostic marker and therapeutic target for osteosarcoma combined with conventional doxorubicin treatment.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

The Dentin Matrix Protein 1 (Dmp1) is Specifically Expressed in Mineralized, but not Soft, Tissues during Development

Dentin Matrix Protein 1 ( Dmp1) was originally identified from dentin. However, its expression and function in vivo are not clear. To clarify these two issues, we have generated mice carrying a truncated Dmp1 gene by using gene targeting to replace exon 6 with a lacZ gene. Northern blot analysis shows the expected 5.8-kb Dmp1-lacZ fusion transcript and loss of the wild-type 2.8-kb Dmp1 transcript, confirmed by a lack of immunostaining for the protein. Using heterozygous animals, we demonstrate that Dmp1 is specific for mineralized tissues. Not previously shown, Dmp1 is also expressed in pulp cells. Dmp1-deficient embryos and newborns display no apparent gross abnormal phenotype, although there are a modest expansion of the hypertrophic chondrocyte zone and a modest increase in the long bone diameter. This suggests that DMP1 is not essential for early mouse skeletal or dental development.

Dentin Matrix Protein 1 (DMP1): New and Important Roles for Biomineralization and Phosphate Homeostasis

Previously, non-collagenous matrix proteins, such as DMP1, were viewed with little biological interest. The last decade of research has increased our understanding of DMP1, as it is now widely recognized that this protein is expressed in non-mineralized tissues, as well as in cancerous lesions. Protein chemistry studies have shown that the full length of DMP1, as a precursor, is cleaved into two distinct forms: the C-terminal and N-terminal fragments. Functional studies have demonstrated that DMP1 is essential in the maturation of odontoblasts and osteoblasts, as well as in mineralization via local and systemic mechanisms. The identification of DMP1 mutations in humans has led to the discovery of a novel disease: autosomal-recessive hypophosphatemic rickets. Furthermore, the regulation of phosphate homeostasis by DMP1 through FGF23, a newly identified hormone that is released from bone and targeted in the kidneys, sets a new direction for research that associates biomineralization with phosphate regulation.

Post-translational Modifications of SIBLING Proteins and Their Roles in Osteogenesis and Dentinogenesis

The extracellular matrix (ECM) of bone and dentin contains several non-collagenous proteins. One category of non-collagenous protein is termed the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family, that includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). These polyanionic SIBLING proteins are believed to play key biological roles in the mineralization of bone and dentin. Although the specific mechanisms involved in controlling bone and dentin formation are still unknown, it is clear that some functions of the SIBLING family members are dependent on the nature and extent of post-translational modifications (PTMs), such as phosphorylation, glycosylation, and proteolytic processing, since these PTMs would have significant effects on their structure. OPN and BSP are present in the ECM of bone and dentin as full-length forms, whereas amino acid sequencing indicates that DMP1 and DSPP exist as proteolytically processed fragments that result from scission of X-Asp bonds. We hypothesized that the processing of DMP1 and DSPP is catalyzed by the PHEX enzyme, since this protein, an endopeptidase that is predominantly expressed in bone and tooth, has a strong preference for cleavage at the NH2-terminus of aspartyl residue. We envision that the proteolytic processing of DMP1 and DSPP may be an activation process that plays a significant, crucial role in osteogenesis and dentinogenesis, and that a failure in this processing would cause defective mineralization in bone and dentin, as observed in X-linked hypophosphatemic rickets.

Is SPARC an Evolutionarily Conserved Collagen Chaperone?

The construction of collagen fiber scaffolds, which provide the structural integrity of the extracellular matrix of connective tissues and basement membranes, is initiated by a complex mechanism of protein-folding, whereby pro-collagen α-chains are assembled into triple-helical procollagen molecules. This unique assembly of the procollagen molecules is guided by several endoplasmic reticulum resident molecular chaperones, including HSP47, which dissociates from procollagen molecules prior to their transport from the endoplasmic reticulum into the cis-Golgi network. SPARC, an evolutionarily conserved collagen-binding glycoprotein, which is frequently co-expressed with collagen in rapidly remodeling tissues, binds to the triple-helical region of procollagen molecules. Analysis of data from genome projects indicates that specific amino acids and sequences in SPARC that are critical for collagen binding are evolutionarily conserved in organisms ranging from nematodes to mammals. Studies of invertebrates, which do not encode HSP47, indicate that SPARC expression is required for the deposition of collagen IV in basal lamina during embryonic development. In mammals, defects in collagen deposition have been observed in normal and wound-healing tissues in the absence of SPARC expression. Based on these and other observations, we propose that intracellular SPARC acts as a collagen molecular chaperone in the endoplasmic reticulum, and that in higher organisms, SPARC acts in concert with HSP47 to ensure that only correctly folded procollagen molecules exit the endoplasmic reticulum. In contrast to HSP47, SPARC is transported from the endoplasmic reticulum through the Golgi network and into secretory vesicles for exocytosis at the plasma membrane. Hence, SPARC may also play a role in regulating post-endoplasmic reticulum events that promote collagen fibrillogenesis.

Cancer Secretome May Influence BSP and DSP Expression in Human Salivary Gland Cells

One of the biggest challenges in managing head and neck cancers, especially salivary gland cancers, is the identification of secreted biomarkers of the disease that can be evaluated noninvasively. A relevant source of enriched tumor markers could potentially be found in the tumor secretome. Although numerous studies have evaluated secretomes from various cancers, the influence of the cancer secretome derived from salivary gland cancers on the behavior of normal cells has not yet been elucidated. Our data indicate that secretome derived from salivary gland cancer cells can influence the expression of two potential biomarkers of oral cancer-namely, bone sialoprotein (BSP) and dentin sialoprotein (DSP)-in normal salivary gland cells. Using routine immunohistochemistry, immunofluorescence, and immunoblotting techniques, we demonstrate an enrichment of BSP and DSP in human salivary gland (HSG) cancer tissue, unique localizations of BSP and DSP in HSG cancer cells, and enriched expression of BSP and DSP in normal salivary gland cells exposed to a cancer secretome. The secretome domain of the cancer microenvironment could alter signaling cascades responsible for normal cell proliferation, migration, and invasion, thus enhancing cancer cell survival and the potential for cancer progression. The cancer secretome may be critical in maintaining and stimulating "cancer-ness," thus potentially promoting specific hallmarks of metastasis.

Cellular and Molecular Pathways Leading to External Root Resorption

External apical root resorption during orthodontic treatment implicates specific molecular pathways that orchestrate nonphysiologic cellular activation. To date, a substantial number of in vitro and in vivo molecular, genomic, and proteomic studies have supplied data that provide new insights into root resorption. Recent mechanisms and developments reviewed here include the role of the cellular component-specifically, the balance of CD68⁺, iNOS⁺ M1- and CD68⁺, CD163⁺ M2-like macrophages associated with root resorption and root surface repair processes linked to the expression of the M1-associated proinflammatory cytokine tumor necrosis factor, inducible nitric oxide synthase, the M1 activator interferon γ, the M2 activator interleukin 4, and M2-associated anti-inflammatory interleukin 10 and arginase I. Insights into the role of mesenchymal dental pulp cells in attenuating dentin resorption in homeostasis are also reviewed. Data on recently deciphered molecular pathways are reviewed at the level of (1) clastic cell adhesion in the external apical root resorption process and the specific role of α/β integrins, osteopontin, and related extracellular matrix proteins; (2) clastic cell fusion and activation by the RANKL/RANK/OPG and ATP-P2RX7-IL1 pathways; and (3) regulatory mechanisms of root resorption repair by cementum at the proteomic and transcriptomic levels.

OPN-a induces muscle inflammation by increasing recruitment and activation of pro-inflammatory macrophages

New Findings

What is the central question of this study?

What is the functional relevance of OPN isoform expression in muscle pathology?

What is the main finding and its importance?

The full‐length human OPN‐a isoform is the most pro‐inflammatory isoform in the muscle microenvironment, acting on macrophages and myoblasts in an RGD‐integrin‐dependent manner. OPN‐a upregulates expression of tenascin‐C (TNC), a known Toll‐like receptor 4 (TLR4) agonist. Blocking TLR4 signalling inhibits the pro‐inflammatory effects of OPN‐a, suggesting that a potential mechanism of OPN action is by promoting TNC–TLR4 signalling.

Although osteopontin (OPN) is an important mediator of muscle remodelling in health and disease, functional differences in human spliced OPN variants in the muscle microenvironment have not been characterized. We thus sought to define the pro‐inflammatory activities of human OPN isoforms (OPN‐a, OPN‐b and OPN‐c) on cells present in regenerating muscle. OPN transcripts were quantified in normal and dystrophic human and dog muscle. Human macrophages and myoblasts were stimulated with recombinant human OPN protein isoforms, and cytokine mRNA and protein induction was assayed. OPN isoforms were greatly increased in dystrophic human (OPN‐a > OPN‐b > OPN‐c) and dog muscle (OPN‐a = OPN‐c). In healthy human muscle, mechanical loading also upregulated OPN‐a expression (eightfold; P < 0.01), but did not significantly upregulate OPN‐c expression (twofold; P > 0.05). In vitro, OPN‐a displayed the most pronounced pro‐inflammatory activity among isoforms, acting on both macrophages and myoblasts. In vitro and in vivo data revealed that OPN‐a upregulated tenascin‐C (TNC), a known Toll‐like receptor 4 (TLR4) agonist. Inhibition of TLR4 signalling attenuated OPN‐mediated macrophage cytokine production. In summary, OPN‐a is the most abundant and functionally active human spliced isoform in the skeletal muscle microenvironment. Here, OPN‐a promotes pro‐inflammatory signalling in both macrophages and myoblasts, possibly through induction of TNC–TLR4 signalling. Together, our findings suggest that specific targeting of OPN‐a and/or TNC signalling in the damaged muscle microenvironment may be of therapeutic relevance.

Dmp1 Null Mice Develop a Unique Osteoarthritis-like Phenotype

Patients with hypophosphatemia rickets (including DMP1 mutations) develop severe osteoarthritis (OA), although the mechanism is largely unknown. In this study, we first identified the expression of DMP1 in hypertrophic chondrocytes using immunohistochemistry (IHC) and X-gal analysis of Dmp1-knockout-lacZ-knockin heterozygous mice. Next, we characterized the OA-like phenotype in Dmp1 null mice from 7-week-old to one-year-old using multiple techniques, including X-ray, micro-CT, H&E staining, Goldner staining, scanning electronic microscopy, IHC assays, etc. We found a classical OA-like phenotype in Dmp1 null mice such as articular cartilage degradation, osteophyte formation, and subchondral osteosclerosis. These Dmp1 null mice also developed unique pathological changes, including a biphasic change in their articular cartilage from the initial expansion of hypertrophic chondrocytes at the age of 1-month to a quick diminished articular cartilage layer at the age of 3-months. Further, these null mice displayed severe enlarged knees and poorly formed bone with an expanded osteoid area. To address whether DMP1 plays a direct role in the articular cartilage, we deleted Dmp1 specifically in hypertrophic chondrocytes by crossing the Dmp1-loxP mice with Col X Cre mice. Interestingly, these conditional knockout mice didn't display notable defects in either the articular cartilage or the growth plate. Because of the hypophosphatemia remained in the entire life span of the Dmp1 null mice, we also investigated whether a high phosphate diet would improve the OA-like phenotype. A 8-week treatment of a high phosphate diet significantly rescued the OA-like defect in Dmp1 null mice, supporting the critical role of phosphate homeostasis in maintaining the healthy joint morphology and function. Taken together, this study demonstrates a unique OA-like phenotype in Dmp1 null mice, but a lack of the direct impact of DMP1 on chondrogenesis. Instead, the regulation of phosphate homeostasis by DMP1 via the axis of “FGF23-renal phosphorus reabsorption” is vital for maintaining a healthy joint.

Immunohistochemical analysis of dentin matrix protein 1 (Dmp1) phosphorylation by Fam20C in bone: implications for the induction of biomineralization

Dmp1 is an acidic phosphoprotein that is specifically expressed in osteocytes. During the secretory process, the full-length, precursor Dmp1 is cleaved into N- and C-terminal fragments. C-terminal Dmp1 is phosphorylated, becoming a highly negatively charged domain that may assist in bone mineralization by recruiting calcium ions and influencing subsequent mineral deposition. It has been recently reported that the Golgi-localized protein kinase Fam20C phosphorylates Dmp1 in vitro. To investigate this phosphorylation in situ, we determined the locations of phosphorylated Dmp1 and Fam20C in rat bones using immunohistochemistry. During osteocytogenesis, osteoblastic, osteoid, and young osteocytes (but not old osteocytes) express Dmp1 mRNA and contain Dmp1 protein in the Golgi apparatus. These Dmp1-producing cells were distributed across the surface layer of cortical bone. Using immunofluorescence, we found that N- and C-terminal Dmp1 fragments were predominantly distributed along the lacunar walls and canaliculi of mineralized bone, respectively, but were not present in the osteoid matrix. We also found that Fam20C and its substrate, C-terminal Dmp1, colocalized in the Golgi of osteoblastic, osteoid, and young osteocytes. Furthermore, phosphorylated C-terminal Dmp1 was present in the Golgi of young osteocytes. Double-labeling immunoelectron microscopy revealed that phosphorylated C-terminal Dmp1 localized to the canalicular wall in mineralized bone. These findings suggest that C-terminal Dmp1 is phosphorylated within osteocytes and then secreted into the pericanalicular matrix of mineralized bone. Phosphorylated, negatively charged C-terminal Dmp1 in the pericanalicular matrix may play an important role in bone mineralization by recruiting calcium ions.

Hierarchical Structures of Bone and Bioinspired Bone Tissue Engineering

Bone, as a mineralized composite of inorganic (mostly carbonated hydroxyapatite) and organic (mainly type I collagen) phases, possesses a unique combination of remarkable strength and toughness. Its excellent mechanical properties are related to its hierarchical structures and precise organization of the inorganic and organic phases at the nanoscale: Nanometer-sized hydroxyapatite crystals periodically deposit within the gap zones of collagen fibrils during bone biomineralization process. This hierarchical arrangement produces nanomechanical heterogeneities, which enable a mechanism for high energy dissipation and resistance to fracture. The excellent mechanical properties integrated with the hierarchical nanostructure of bone have inspired chemists and material scientists to develop biomimetic strategies for artificial bone grafts in tissue engineering (TE). This critical review provides a broad overview of the current mechanisms involved in bone biomineralization, and the relationship between bone hierarchical structures and the deformation mechanism. Our goal in this review is to inspire the application of these principles toward bone TE.

Predicting the Structure-Activity Relationship of Hydroxyapatite-Binding Peptides by Enhanced-Sampling Molecular Simulation

Understanding the molecular structural and energetic basis of the interactions between peptides and inorganic surfaces is critical to their applications in tissue engineering and biomimetic material synthesis. Despite recent experimental progresses in the identification and functionalization of hydroxyapatite (HAP)-binding peptides, the molecular mechanisms of their interactions with HAP surfaces are yet to be explored. In particular, the traditional method of molecular dynamics (MD) simulation suffers from insufficient sampling at the peptide-inorganic interface that renders the molecular-level observation dubious. Here we demonstrate that an integrated approach combining bioinformatics, MD, and metadynamics provides a powerful tool for investigating the structure-activity relationship of HAP-binding peptides. Four low charge density peptides, previously identified by phage display, have been considered. As revealed by bioinformatics and MD, the binding conformation of the peptides is controlled by both the sequence and the amino acid composition. It was found that formation of hydrogen bonds between lysine residue and phosphate ions on the surface dictates the binding of positively charged peptide to HAP. The binding affinities of the peptides to the surface are estimated by free energy calculation using parallel-tempering metadynamics, and the results compare favorably to measurements reported in previous experimental studies. The calculation suggests that the charge density of the peptide primarily controls the binding affinity to the surface, while the backbone secondary structure that may restrain side chain orientation toward the surface plays a minor role. We also report that the application of enhanced-sampling metadynamics effects a major advantage over the steered MD method by significantly improving the reliability of binding free energy calculation. In general, our novel integration of diverse sampling techniques should contribute to the rational design of surface-recognition peptides in biomedical applications.

A Tracer Study with Systemically and Locally Administered Dinitrophenylated Osteopontin

Osteopontin (OPN), a major non-collagenous matrix protein of bone, is also found in tissue fluids and in the circulation. It is still not clear whether circulating OPN contributes to bone formation. To elucidate this question, rat OPN was tagged with dinitrophenol groups and administered to rats either intravenously or by infusion with an osmotic minipump through a “surgical window” in the bone of the hemimandible. Dinitrophenylated rat albumin (ALB) was used as a control. The presence and distribution of tagged proteins were revealed by immunogold labeling on sections of tibia and alveolar bone. Tagged molecules of OPN were found in mineralization foci, surfaces and interfaces, and matrix accumulations among calcified collagen fibrils. Even though dinitrophenylated ALB was administered at several-fold higher concentrations, it did not accumulate in these sites. These results show that circulating OPN can be incorporated into specific compartments of forming bone and suggest that such molecules may play a more important role than previously suspected.

Products of dentin matrix protein-1 degradation by interleukin-1β-induced matrix metalloproteinase-3 promote proliferation of odontoblastic cells

We have previously reported that interleukin (IL)-1β induces matrix metalloproteinase (MMP)-3-regulated cell proliferation in mouse embryonic stem cell (ESC)-derived odontoblast-like cells, suggesting that MMP-3 plays a potentially unique physiological role in regeneration by odontoblast-like cells. MMPs are able to process virtually any component of the extracellular matrix, including collagen, laminin and bioactive molecules. Because odontoblasts produce dentin matrix protein-1 (DMP-1), we examined whether the degraded products of DMP-1 by MMP-3 contribute to enhanced proliferation in odontoblast-like cells. IL-1β increased mRNA and protein levels of odontoblastic marker proteins, including DMP-1, but not osteoblastic marker proteins, such as osteocalcin and osteopontin. The recombinant active form of MMP-3 could degrade DMP-1 protein but not osteocalcin and osteopontin in vitro. The exogenous degraded products of DMP-1 by MMP-3 resulted in increased proliferation of odontoblast-like cells in a dose-dependent manner. Treatment with a polyclonal antibody against DMP-1 suppressed IL-1β-induced cell proliferation to a basal level, but identical treatment had no effect on the IL-1β-induced increase in MMP-3 expression and activity. Treatment with siRNA against MMP-3 potently suppressed the IL-1β-induced increase in DMP-1 expression and suppressed cell proliferation (p < 0.05). Similarly, treatment with siRNAs against Wnt5a and Wnt5b suppressed the IL-1β-induced increase in DMP-1 expression and suppressed cell proliferation (p < 0.05). Rat KN-3 cells, representative of authentic odontoblasts, showed similar responses to the odontoblast-like cells. Taken together, our current study demonstrates the sequential involvement of Wnt5, MMP-3, DMP-1 expression, and DMP-1 degradation products by MMP-3, in effecting IL-1β-induced proliferation of ESC-derived odontoblast-like cells.

Matricellular proteins as regulators of cancer metastasis to bone

Metastasis is the major cause of death in cancer patients, and a frequent site of metastasis for many cancers is the bone marrow. Therefore, understanding the mechanisms underlying the metastatic process is necessary for future prevention and treatment. The tumor microenvironment is now known to play a role in the metastatic cascade, both at the primary tumor and in metastatic sites, and includes both cellular and non-cellular components. The extracellular matrix (ECM) provides structural support and signaling cues to cells. One particular group of molecules associated with the ECM, known as matricellular proteins, modulate multiple aspects of tumor biology, including growth, migration, invasion, angiogenesis and metastasis. These proteins are also important for normal function in the bone by regulating bone formation and bone resorption. Recent studies have described a link between some of these proteins and metastasis of various tumors to the bone. The aim of this review is to summarize what is currently known about matricellular protein influence on bone metastasis. Particular attention to the contribution of both tumor cells and non-malignant cells in the bone has been given.

Intrinsically disordered proteins and biomineralization

In vertebrates and invertebrates, biomineralization is controlled by the cell and the proteins they produce. A large number of these proteins are intrinsically disordered, gaining some secondary structure when they interact with their binding partners. These partners include the component ions of the mineral being deposited, the crystals themselves, the template on which the initial crystals form, and other intrinsically disordered proteins and peptides. This review speculates why intrinsically disordered proteins are so important for biomineralization, providing illustrations from the SIBLING (small integrin binding N-glycosylated) proteins and their peptides. It is concluded that the flexible structure, and the ability of the intrinsically disordered proteins to bind to a multitude of surfaces is crucial, but details on the precise-interactions, energetics and kinetics of binding remain to be determined.

The dentin phosphoprotein repeat region and inherited defects of dentin

Nonsyndromic dentin defects classified as type II dentin dysplasia and types II and III dentinogenesis imperfecta are caused by mutations in DSPP (dentin sialophosphoprotein). Most reported disease‐causing DSPP mutations occur within the repetitive DPP (dentin phosphoprotein) coding sequence. We characterized the DPP sequences of five probands with inherited dentin defects using single molecule real‐time (SMRT) DNA sequencing. Eight of the 10 sequences matched previously reported DPP length haplotypes and two were novel. Alignment with known DPP sequences showed 32 indels arranged in 36 different patterns. Sixteen of the 32 indels were not represented in more than one haplotype. The 25 haplotypes with confirmed indels were aligned to generate a tree that describes how the length variations might have evolved. Some indels were independently generated in multiple lines. A previously reported disease‐causing DSPP mutation in Family 1 was confirmed and its position clarified (c.3135delC; p.Ser1045Argfs*269). A novel frameshift mutation (c.3504_3508dup; p.Asp1170Alafs*146) caused the dentin defects in Family 2. A COL1A2 (c.2027G>A or p.Gly676Asp) missense mutation, discovered by whole‐exome sequencing, caused the dentin defects in Family 3. We conclude that SMRT sequencing characterizes the DPP repeat region without cloning and can improve our understanding of normal and pathological length variations in DSPP alleles.

Accelerated enamel mineralization in Dspp mutant mice

Dentin sialophosphoprotein (DSPP) is one of the major non-collagenous proteins present in dentin, cementum and alveolar bone; it is also transiently expressed by ameloblasts. In humans many mutations have been found in DSPP and are associated with two autosomal-dominant genetic diseases - dentinogenesis imperfecta II (DGI-II) and dentin dysplasia (DD). Both disorders result in the development of hypomineralized and mechanically compromised teeth. The erupted mature molars of Dspp(-/-) mice have a severe hypomineralized dentin phenotype. Since dentin and enamel formations are interdependent, we decided to investigate the process of enamel onset mineralization in young Dspp(-/-) animals. We focused our analysis on the constantly erupting mouse incisor, to capture all of the stages of odontogenesis in one tooth, and the unerupted first molars. Using high-resolution microCT, we revealed that the onset of enamel matrix deposition occurs closer to the cervical loop and both secretion and maturation of enamel are accelerated in Dspp(-/-) incisors compared to the Dspp(+/-) control. Importantly, these differences did not translate into major phenotypic differences in mature enamel in terms of the structural organization, mineral density or hardness. The only observable difference was the reduction in thickness of the outer enamel layer, while the total enamel thickness remained unchanged. We also observed a compromised dentin-enamel junction, leading to delamination between the dentin and enamel layers. The odontoblast processes were widened and lacked branching near the DEJ. Finally, for the first time we demonstrate expression of Dspp mRNA in secretory ameloblasts. In summary, our data show that DSPP is important for normal mineralization of both dentin and enamel.

Transglutaminase 2-Catalyzed Intramolecular Cross-Linking of Osteopontin

Osteopontin (OPN) is a multifunctional integrin-binding protein present in several tissues and body fluids. OPN is a substrate for the enzyme transglutaminase 2 (TG2), which catalyzes inter- and intramolecular cross-linking affecting the biological activity of the protein. Polymerization of OPN by intermolecular cross-linking has mostly been studied using relatively high TG2 concentrations, whereas the effect of lower concentrations of TG2 has remained unexplored. Here we show that TG2 at physiologically relevant concentrations predominantly catalyzes the formation of intramolecular cross-links in OPN. By site-directed mutagenesis and mass spectrometry, we demonstrate that Gln(42) and Gln(193) serve as the primary amine acceptor sites for isopeptide bond formation. We find that Gln(42) predominantly is linked to Lys(4) and that Gln(193) participates in a cross-link with Lys(154), Lys(157), or Lys(231). The formation of specific isopeptide bonds was not dependent on OPN phosphorylation, and similar patterns of cross-linking were observed in human and mouse OPN. Furthermore, we find that OPN purified from human urine contains the Lys(154)-Gln(193) isopeptide bond, indicating that intramolecular cross-linking of OPN occurs in vivo. Collectively, these data suggest that specific intramolecular cross-linking in the N- and C-terminal parts of OPN is most likely the dominant step in TG2-catalyzed modification of OPN.

The LPV Motif Is Essential for the Efficient Export of Secretory DMP1 From the Endoplasmic Reticulum

Dentin matrix protein 1 (DMP1) is found abundantly in the extracellular matrices of bone and dentin. Secretory DMP1 begins with a tripeptide of leucine-proline-valine (LPV) after the endoplasmic reticulum (ER)-entry signal peptide is cleaved. The goal of this study was to determine the role of the LPV motif in the secretion of DMP1. A series of DNA constructs was generated to express various forms of DMP1 with or without the LPV motif. These constructs were transfected into a preosteoblast cell line, the MC3T3-E1 cells, and the subcellular localization and secretion of various forms of DMP1 were examined by immunofluorescent staining and Western-blotting analyses. Immunofluorescent staining showed that the LPV-containing DMP1 variants were primarily localized in the Golgi complex, whereas the LPV-lacking DMP1 variants were found abundantly within the ER. Western-blotting analyses demonstrated that the LPV-containing DMP1 variants were rapidly secreted from the transfected cells, as they did not accumulate within the cells, and the amounts increased in the conditioned media over time. In contrast, the LPV-lacking DMP1 variants were predominantly retained within the cells, and only small amounts were secreted out of the cells over time. These results suggest that the LPV motif is essential for the efficient export of secretory DMP1 from the ER to the Golgi complex.

Strain-guided mineralization in the bone-PDL-cementum complex of a rat periodontium

Objective

The objective of this study was to investigate the effect of mechanical strain by mapping physicochemical properties at periodontal ligament (PDL)–bone and PDL–cementum attachment sites and within the tissues per se.

Design

Accentuated mechanical strain was induced by applying a unidirectional force of 0.06 N for 14 days on molars in a rat model. The associated changes in functional space between the tooth and bone, mineral forming and resorbing events at the PDL–bone and PDL–cementum attachment sites were identified by using micro-X-ray computed tomography (micro-XCT), atomic force microscopy (AFM), dynamic histomorphometry, Raman microspectroscopy, and AFM-based nanoindentation technique. Results from these analytical techniques were correlated with histochemical strains specific to low and high molecular weight GAGs, including biglycan, and osteoclast distribution through tartrate resistant acid phosphatase (TRAP) staining.

Results

Unique chemical and mechanical qualities including heterogeneous bony fingers with hygroscopic Sharpey's fibers contributing to a higher organic (amide III — 1240 cm^− 1) to inorganic (phosphate — 960 cm^− 1) ratio, with lower average elastic modulus of 8 GPa versus 12 GPa in unadapted regions were identified. Furthermore, an increased presence of elemental Zn in cement lines and mineralizing fronts of PDL–bone was observed. Adapted regions containing bony fingers exhibited woven bone-like architecture and these regions rich in biglycan (BGN) and bone sialoprotein (BSP) also contained high-molecular weight polysaccharides predominantly at the site of polarized bone growth.

Conclusions

From a fundamental science perspective the shift in local properties due to strain amplification at the soft–hard tissue attachment sites is governed by semiautonomous cellular events at the PDL–bone and PDL–cementum sites. Over time, these strain-mediated events can alter the physicochemical properties of tissues per se, and consequently the overall biomechanics of the bone–PDL–tooth complex. From a clinical perspective, the shifts in magnitude and duration of forces on the periodontal ligament can prompt a shift in physiologic mineral apposition in cementum and alveolar bone albeit of an adapted quality owing to the rapid mechanical translation of the tooth.

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Load-mediated shifts in mechanical strains will prompt self-governing zones at PDL-cementum and PDL-bone entheses.

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The intensity of strain amplification is predominantly felt at the entheses as it is a region where disparate materials attach.

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Physicochemical observations at the PDL-bone enthesial zone are not directly correlated to the events at PDL-cementum zone.

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Rapid shifts in PDL strain can prompt a shift in mineral apposition at respective entheses albeit of an adapted quality.

Dynamic footprint of sequestration in the molecular fluctuations of osteopontin

The sequestration of calcium phosphate by unfolded proteins is fundamental to the stabilization of biofluids supersaturated with respect to hydroxyapatite, such as milk, blood or urine. The unfolded state of osteopontin (OPN) is thought to be a prerequisite for this activity, which leads to the formation of core-shell calcium phosphate nanoclusters. We report on the structures and dynamics of a native OPN peptide from bovine milk, studied by neutron spectroscopy and small-angle X-ray and neutron scattering. The effects of sequestration are quantified on the nanosecond- ångström resolution by elastic incoherent neutron scattering. The molecular fluctuations of the free phosphopeptide are in agreement with a highly flexible protein. An increased resilience to diffusive motions of OPN is corroborated by molecular fluctuations similar to those observed for globular proteins, yet retaining conformational flexibilities. The results bring insight into the modulation of the activity of OPN and phosphopeptides with a role in the control of biomineralization. The quantification of such effects provides an important handle for the future design of new peptides based on the dynamics-activity relationship.

Osteopontin in Immune-mediated Diseases

Since its initial identification as one of the genes most highly upregulated upon T-cell activation, osteopontin (or Eta-1, as it was designated then) has been demonstrated to have many roles in the regulation of the immune response on multiple levels. It contributes to the development of immune-mediated and inflammatory diseases, and it regulates the host response to infection. In some cases, the mechanisms of these effects have been elucidated, while other mechanistic functions of the protein remain obscure. The protein itself makes these analyses complex, since it binds to a series of different integrins, and in addition to its classically secreted form, an intracellular form of osteopontin has been identified, which participates in several aspects of immune regulation. In this review, we focus on the role of osteopontin in a series of immune-related diseases, particularly those where significant advances have been made in recent years: multiple sclerosis, rheumatoid arthritis, lupus and related diseases, Sjögren's disease, colitis, and 1 area of inflammatory pathology, alcoholic and nonalcoholic liver diseases. A recurring theme in these diseases is a link between osteopontin and pathogenic T cells, particularly T helper 17 cells, where osteopontin produced by dendritic cells supports IL-17 expression, contributing to pathology. In addition, a role for osteopontin in B-cell differentiation is becoming clear. In general, osteopontin contributes to pathology in these diseases, but there are examples where it has a protective role; deciphering the mechanisms underlying these differences and the specific receptors for osteopontin will be a research challenge for the future. Aside from its newly discovered role in the development of Sjögren's disease, the role of osteopontin in inflammatory conditions in the oral cavity is still poorly understood. Elucidation of this role will be of interest.