The impact of hydroxyapatite crystal structures and protein interactions on bone's mechanical properties

Hydroxyapatite (HAP) constitutes the primary mineral component of bones, and its crystal structure, along with the surface interaction with proteins, significantly influences the outstanding mechanical properties of bone. This study focuses on natural hydroxyapatite, constructing a surface model with calcium vacancy defects. Employing a representative model of aspartic acid residues, we delve into the adsorption mechanism on the crystal surface and scrutinize the adsorption forms of amino acid residues on HAP and calcium-deficient hydroxyapatite (CDHA) surfaces. The research also explores the impact of different environments on adsorption energy. Furthermore, a simplified sandwich structure of crystal-polypeptide-crystal is presented, analyzing the distribution of amino acid residue adsorption sites on the crystal surface of the polypeptide fragment. This investigation aims to elucidate how the stick-slip mechanism of polypeptide molecules on the crystal surface influences the mechanical properties of the system. By uncovering the interface mechanical behavior between HAP and osteopontin peptides, this article offers valuable theoretical insights for the construction and biomimetic design of biocomposites.

Thrombin cleavage of osteopontin initiates osteopontin's tumor-promoting activity

BACKGROUND

Osteopontin (OPN) is a multifunctional proinflammatory matricellular protein overexpressed in multiple human cancers and associated with tumor progression and metastases. Thrombin cleavage of OPN reveals a cryptic binding site for α4 β1 and α9 β1 integrins.

METHODS

Thrombin cleavage-resistant OPNR153A knock-in (OPN-KI) mice were generated and compared to OPN deficient mice (OPN-KO) and wild type (WT) mice in their ability to support growth of melanoma cells. Flow cytometry was used to analyze tumor infiltrating leukocytes.

RESULTS

OPN-KI mice engineered with a thrombin cleavage-resistant OPN had reduced B16 melanoma growth and fewer pulmonary metastases than WT mice. The tumor suppression phenotype of the OPN-KI mouse was identical to that observed in OPN-KO mice and was replicated in WT mice by pharmacologic inhibition of thrombin with dabigatran. Tumors isolated from OPN-KI mice had increased tumor-associated macrophages with an altered activation phenotype. Immunodeficient OPN-KI mice (NOG-OPN-KI) or macrophage-depleted OPN-KI mice did not exhibit the tumor suppression phenotype. As B16 cells do not express OPN, thrombin-cleaved fragments of host OPN suppress host antitumor immune response by functionally modulating the tumor-associated macrophages. YUMM3.1 cells, which express OPN, showed less tumor suppression in the OPN-KI and OPN-KO mice than B16 cells, but its growth was suppressed by dabigatran similar to B16 cells.

CONCLUSIONS

Thrombin cleavage of OPN, derived from the host and the tumor, initiates OPN's tumor-promoting activity in vivo.

Osteopontin on the Dental Implant Surface Promotes Direct Osteogenesis in Osseointegration

After dental implantation, osteopontin (OPN) is deposited on the hydroxyapatite (HA) blasted implant surface followed by direct osteogenesis, which is significantly disturbed in Opn-knockout (KO) mice. However, whether applying OPN on the implant surface promotes direct osteogenesis remains unclarified. This study analyzed the effects of various OPN modified protein/peptides coatings on the healing patterns of the bone-implant interface after immediately placed implantation in the maxilla of four-week-old Opn-KO and wild-type (WT) mice (n = 96). The decalcified samples were processed for immunohistochemistry for OPN and Ki67 and tartrate-resistant acid phosphatase histochemistry. In the WT mice, the proliferative activity in the HA binding peptide-OPN mimic peptide fusion coated group was significantly higher than that in the control group from day 3 to week 1, and the rates of OPN deposition and direct osteogenesis around the implant surface significantly increased in the recombinant-mouse-OPN (rOPN) group compared to the Gly-Arg-Gly-Asp-Ser peptide group in week 2. The rOPN group achieved the same rates of direct osteogenesis and osseointegration as those in the control group in a half period (week 2). None of the implant surfaces could rescue the direct osteogenesis in the healing process in the Opn-KO mice. These results suggest that the rOPN coated implant enhances direct osteogenesis during osseointegration following implantation.

Binding Mode Characterization of Osteopontin on Hydroxyapatite by Solution NMR Spectroscopy

Extracellular matrix glycoproteins play a major role in bone mineralization and modulation of osteogenesis. Among these, the intrinsically disordered protein osteopontin (OPN) is associated with the inhibition of formation, growth and proliferation of the bone mineral hydroxyapatite (HAP). Furthermore, post-translational modifications like phosphorylation can alter conformations and interaction properties of intrinsically disordered proteins (IDPs). Therefore, the actual interaction of OPN with a HAP surface on an atomic level and how this interaction is affected by phosphorylation is of great interest. Here, we study the interaction of full-length OPN on the surface of suspended HAP nanoparticles by solution NMR spectroscopy. We report the binding modes of this IDP and provide evidence for the influence of hyperphosphorylation on the binding character and an explanation for the differing roles in biomineralization. Our study moreover presents an easy and suitable option to measure interaction of nanoparticles in a stable suspension with full-length proteins.

Osteopontin regulates biomimetic calcium phosphate crystallization from disordered mineral layers covering apatite crystallites

Details of apatite formation and development in bone below the nanometer scale remain enigmatic. Regulation of mineralization was shown to be governed by the activity of non-collagenous proteins with many bone diseases stemming from improper activity of these proteins. Apatite crystal growth inhibition or enhancement is thought to involve direct interaction of these proteins with exposed faces of apatite crystals. However, experimental evidence of the molecular binding events that occur and that allow these proteins to exert their functions are lacking. Moreover, recent high-resolution measurements of apatite crystallites in bone have shown that individual crystallites are covered by a persistent layer of amorphous calcium phosphate. It is therefore unclear whether non-collagenous proteins can interact with the faces of the mineral crystallites directly and what are the consequences of the presence of a disordered mineral layer to their functionality. In this work, the regulatory effect of recombinant osteopontin on biomimetic apatite is shown to produce platelet-shaped apatite crystallites with disordered layers coating them. The protein is also shown to regulate the content and properties of the disordered mineral phase (and sublayers within it). Through solid-state NMR atomic carbon-phosphorous distance measurements, the protein is shown to be located in the disordered phases, reaching out to interact with the surfaces of the crystals only through very few sidechains. These observations suggest that non-phosphorylated osteopontin acts as regulator of the coating mineral layers and exerts its effect on apatite crystal growth processes mostly from afar with a limited number of contact points with the crystal.

A Non-Invasive Nanoprobe for In Vivo Photoacoustic Imaging of Vulnerable Atherosclerotic Plaque

Vulnerable atherosclerotic (AS) plaque is the major cause of cardiovascular death. However, clinical methods cannot directly identify the vulnerable AS plaque at molecule level. Herein, osteopontin antibody (OPN Ab) and NIR fluorescence molecules of ICG co-assembled Ti₃ C₂ nanosheets are reported as an advanced nanoprobe (OPN Ab/Ti₃ C₂ /ICG) with enhanced photoacoustic (PA) performance for direct and non-invasive in vivo visual imaging of vulnerable AS plaque. The designed OPN Ab/Ti₃ C₂ /ICG nanoprobes successfully realize obvious NIR fluorescence imaging toward foam cells as well as the vulnerable AS plaque slices. After intravenous injection of OPN Ab/Ti₃ C₂ /ICG nanoprobes into AS model mice, in vivo imaging results show a significantly enhanced PA signal in the aortic arch accumulated with vulnerable plaque, well indicating the remarkable feasibility of OPN Ab/Ti₃ C₂ /ICG nanoprobes to distinguish the vulnerable AS plaque. The proposed OPN Ab/Ti₃ C₂ /ICG nanoprobes not only overcome the clinical difficulty to differentiate vulnerable plaque, but also achieve the non-invasively specific in vivo imaging of vulnerable AS plaque at molecule level, greatly promoting the innovation of cardiovascular diagnosis technology.

Evaluation of Bioactivities of the Bovine Milk Lactoferrin-Osteopontin Complex in Infant Formulas

Human milk contains several bioactive proteins, including lactoferrin (LF) and osteopontin (OPN). These two proteins have been shown to form a complex, which shows increased bioactivities. Bovine LF and OPN can also form such a complex. We assessed bioactivities of the bovine LF-OPN complex (at molar ratios of LF:OPN = 3:1, 5:1, or 8:1) in a formula protein matrix, including LF, OPN, bovine whey protein hydrolysate, and α-lactalbumin. Our results show that the bovine LF-OPN complex together with formula proteins is resistant to in vitro digestion, stimulates intestinal cell proliferation (by 15-50%) and differentiation (by 30-50%), increases antibacterial activity (by 25-50%), and enhances intestinal immunity. The 3:1 ratio of LF to OPN exhibits the most potent effects, as compared with the other two ratios. In conclusion, adding bovine LF and OPN to infant formulas may result in increased stability of the two components and enhanced bioactivities, possibly improving outcomes in formula-fed infants.

Computational Nanomechanics of Noncollagenous Interfibrillar Interface in Bone

The noncollagenous interfibrillar interface in bone provides the critical function of transferring loads among collagen fibrils and their bundles, with adhesive mechanisms at this site thus significantly contributing to the mechanical properties of bone. Motivated by the experimental observations and hypotheses, a computational study is presented to elucidate the critical roles of two major proteins at the nanoscale interfibrillar interface, that is, osteopontin (OPN) and osteocalcin (OC) in bone. This study reveals the extremely high interfacial toughness of the OPN/OC composite. The previously proposed hypothesis of sacrificial bonds in the extracellular organic matrix is tested, and the remarkable mechanical properties of the nanoscale bone interface are attributed to the collaborative interactions between the OPN and OC proteins.

Nanostructure of mouse otoconia

Mammalian otoconia of the inner ear vestibular apparatus are calcium carbonate-containing mineralized structures critical for maintaining balance and detecting linear acceleration. The mineral phase of otoconia is calcite, which coherently diffracts X-rays much like a single-crystal. Otoconia contain osteopontin (OPN), a mineral-binding protein influencing mineralization processes in bones, teeth and avian eggshells, for example, and in pathologic mineral deposits. Here we describe mineral nanostructure and the distribution of OPN in mouse otoconia. Scanning electron microscopy and atomic force microscopy of intact and cleaved mouse otoconia revealed an internal nanostructure (~50 nm). Transmission electron microscopy and electron tomography of focused ion beam-prepared sections of otoconia confirmed this mineral nanostructure, and identified even smaller (~10 nm) nanograin dimensions. X-ray diffraction of mature otoconia (8-day-old mice) showed crystallite size in a similar range (73 nm and smaller). Raman and X-ray absorption spectroscopy - both methods being sensitive to the detection of crystalline and amorphous forms in the sample - showed no evidence of amorphous calcium carbonate in these mature otoconia. Scanning and transmission electron microscopy combined with colloidal-gold immunolabeling for OPN revealed that this protein was located at the surface of the otoconia, correlating with a site where surface nanostructure was observed. OPN addition to calcite growing in vitro produced similar surface nanostructure. These findings provide details on the composition and nanostructure of mammalian otoconia, and suggest that while OPN may influence surface rounding and surface nanostructure in otoconia, other incorporated proteins (also possibly including OPN) likely participate in creating internal nanostructure.

Sensitivity-enhanced three-dimensional and carbon-detected two-dimensional NMR of proteins using hyperpolarized water

Signal enhancements of up to two orders of magnitude in protein NMR can be achieved by employing HDO as a vector to introduce hyperpolarization into folded or intrinsically disordered proteins. In this approach, hyperpolarized HDO produced by dissolution-dynamic nuclear polarization (D-DNP) is mixed with a protein solution waiting in a high-field NMR spectrometer, whereupon amide proton exchange and nuclear Overhauser effects (NOE) transfer hyperpolarization to the protein and enable acquisition of a signal-enhanced high-resolution spectrum. To date, the use of this strategy has been limited to 1D and ¹H-¹⁵N 2D correlation experiments. Here we introduce 2D ¹³C-detected D-DNP, to reduce exchange-induced broadening and other relaxation penalties that can adversely affect proton-detected D-DNP experiments. We also introduce hyperpolarized 3D spectroscopy, opening the possibility of D-DNP studies of larger proteins and IDPs, where assignment and residue-specific investigation may be impeded by spectral crowding. The signal enhancements obtained depend in particular on the rates of chemical and magnetic exchange of the observed residues, thus resulting in non-uniform 'hyperpolarization-selective' signal enhancements. The resulting spectral sparsity, however, makes it possible to resolve and monitor individual amino acids in IDPs of over 200 residues at acquisition times of just over a minute. We apply the proposed experiments to two model systems: the compactly folded protein ubiquitin, and the intrinsically disordered protein (IDP) osteopontin (OPN).

The Impact of Glycosylation of Osteopontin on Urinary Stone Formation

Osteopontin (OPN) is a matrix glycoprotein of urinary calculi. This study aims to identify the role of aberrant glycosylation of OPN in urolithiasis. We retrospectively measured urinary glycosylated OPN normalized by urinary full-length-OPN levels in 110 urolithiasis patients and 157 healthy volunteers and 21 patients were prospectively longitudinal follow-up during stone treatment. The urinary full-length-OPN levels were measured using enzyme-linked immunosorbent assay and glycosylated OPN was measured using a lectin array and lectin blotting. The assays were evaluated using the area under the receiver operating characteristics curve to discriminate stone forming urolithiasis patients. In the retrospective cohort, urinary Gal3C-S lectin reactive- (Gal3C-S-) OPN/full-length-OPN, was significantly higher in the stone forming urolithiasis patients than in the healthy volunteers (p < 0.0001), with good discrimination (AUC, 0.953), 90% sensitivity, and 92% specificity. The Lycopersicon esculentum lectin analysis of urinary full-length-OPN showed that urinary full-length-OPN in stone forming urolithiasis patients had a polyLacNAc structure that was not observed in healthy volunteers. In the prospective longitudinal follow-up study, 92.8% of the stone-free urolithiasis group had Gal3C-S-OPN/full-length-OPN levels below the cutoff value after ureteroscopic lithotripsy (URS), whereas 71.4% of the residual-stone urolithiasis group did not show decreased levels after URS. Therefore, Gal3C-S-OPN/full-length-OPN levels could be used as a urolithiasis biomarker.

The Ambivalent Role of Proline Residues in an Intrinsically Disordered Protein: From Disorder Promoters to Compaction Facilitators

Intrinsically disordered proteins (IDPs) carry out many biological functions. They lack a stable three-dimensional structure, but rather adopt many different conformations in dynamic equilibrium. The interplay between local dynamics and global rearrangements is key for their function. In IDPs, proline residues are significantly enriched. Given their unique physicochemical and structural properties, a more detailed understanding of their potential role in stabilizing partially folded states in IDPs is highly desirable. Nuclear magnetic resonance (NMR) spectroscopy, and in particular ¹³C-detected NMR, is especially suitable to address these questions. We applied a ¹³C-detected strategy to study Osteopontin, a largely disordered IDP with a central compact region. By using the exquisite sensitivity and spectral resolution of these novel techniques, we gained unprecedented insight into cis-Pro populations, their local structural dynamics, and their role in mediating long-range contacts. Our findings clearly call for a reassessment of the structural and functional role of proline residues in IDPs. The emerging picture shows that proline residues have ambivalent structural roles. They are not simply disorder promoters but rather can, depending on the primary sequence context, act as nucleation sites for structural compaction in IDPs. These unexpected features provide a versatile mechanistic toolbox to enrich the conformational ensembles of IDPs with specific features for adapting to changing molecular and cellular environments.

The influence of osteopontin-guided collagen intrafibrillar mineralization on pericyte differentiation and vascularization of engineered bone scaffolds

Biomimetically mineralized collagen scaffolds are promising for bone regeneration, but vascularization of these materials remains to be addressed. Here, we engineered mineralized scaffolds using an osteopontin-guided polymer-induced liquid-precursor mineralization method to recapitulate bone's mineralized nanostructure. SEM images of mineralized samples confirmed the presence of collagen with intrafibrillar mineral, also EDS spectra and FTIR showed high peaks of calcium and phosphate, with a similar mineral/matrix ratio to native bone. Mineralization increased collagen compressive modulus up to 15-fold. To evaluate vasculature formation and pericyte-like differentiation, HUVECs and hMSCs were seeded in a 4:1 ratio in the scaffolds for 7 days. Moreover, we used RT-PCR to investigate the gene expression of pericyte markers ACTA2, desmin, CD13, NG2, and PDGFRβ. Confocal images showed that both nonmineralized and mineralized scaffolds enabled endothelial capillary network formation. However, vessels in the nonmineralized samples had longer vessel length, a larger number of junctions, and a higher presence of αSMA+ mural cells. RT-PCR analysis confirmed the downregulation of pericytic markers in mineralized samples. In conclusion, although both scaffolds enabled endothelial capillary network formation, mineralized scaffolds presented less pericyte-supported vessels. These observations suggest that specific scaffold characteristics may be required for efficient scaffold vascularization in future bone tissue engineering strategies. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1522-1532, 2019.

Osteopontin-derived synthetic peptide SVVYGLR has potent utility in the functional regeneration of oral and maxillofacial skeletal muscles

Oral and maxillofacial skeletal muscles are critical for oral motor functions, and severe damage to these muscles by trauma or surgery may lead to persistent functional impairment. This study investigated the effects of SVVYGLR (SV) peptide, a thrombin-cleaved osteopontin-derived motif, on histopathological wound healing and functional repair after severe injury of skeletal muscles. A rat model of volumetric muscle loss bilateral masseter muscle was developed. A single dose of SV-peptide or phosphate-buffered saline (PBS) was separately injected into the injured muscle belly. Histopathological and functional analyses were performed 1-8 weeks after the treatment. Behavioral analysis during free-feeding revealed that the feeding rate markedly increased in the SV-peptide group, in contrast, the PBS group showed fewer changes after the injury. Electromyogram recordings from injured muscles demonstrated amplification of rectified burst activity over time accompanied by increased maximal amplitude and duration in the SV-peptide group, in contrast, the PBS group showed moderate changes. A lissajous figure for bilateral masseter muscle activities also revealed superior functional recovery by the SV-peptide treatment. The SV-peptide also facilitated regeneration of muscles composed of matured myofibers with a greater diameter compared to the PBS group. In addition, granulation in the earlier period and fibrosis in the later period of wound healing were significantly inhibited by the SV-peptide treatment but not by the PBS treatment. Therefore, local application of the SV-peptide could help facilitate regeneration of muscles, inhibition of fibrosis, and improvement of functional impairment of oral and maxillofacial skeletal muscles damaged by severe trauma or surgery.

An antibody-free sample pretreatment method for osteopontin combined with MALDI-TOF MS/MS analysis

Osteopontin is an osteoblast-secreted protein with an aspartic acid-rich, highly phosphorylated, and glycosylated structure. Osteopontin can easily bind to integrins, tumor cells, extracellular matrix and calcium, and is related to bone diseases, various cancers, inflammation etc. Here, DEAE-Cibacron blue 3GA was used to extract recombinant osteopontin from human plasma, and to deplete abundant plasma proteins with an antibody-free method. Using selected buffer systems, osteopontin and human serum albumin could be bound to DEAE-Cibacron blue 3GA, while immunoglobulin G was excluded. The bound osteopontin could then be separated from albumin by using different sequential elution buffers. By this method, 1 μg/mL recombinant osteopontin could be separated from the major part of the most abundant proteins in human plasma. After trypsin digestion, the extracted osteopontin could be successfully detected and identified by MALDI-TOF MS/MS using the m/z 1854.898 peptide and its fragments.

Nanostructure, osteopontin, and mechanical properties of calcitic avian eggshell

Avian (and formerly dinosaur) eggshells form a hard, protective biomineralized chamber for embryonic growth-an evolutionary strategy that has existed for hundreds of millions of years. We show in the calcitic chicken eggshell how the mineral and organic phases organize hierarchically across different length scales and how variation in nanostructure across the shell thickness modifies its hardness, elastic modulus, and dissolution properties. We also show that the nanostructure changes during egg incubation, weakening the shell for chick hatching. Nanostructure and increased hardness were reproduced in synthetic calcite crystals grown in the presence of the prominent eggshell protein osteopontin. These results demonstrate the contribution of nanostructure to avian eggshell formation, mechanical properties, and dissolution.

Separation Options for Phosphorylated Osteopontin from Transgenic Microalgae Chlamydomonas reinhardtii

Correct folding and post-translational modifications are vital for therapeutic proteins to elicit their biological functions. Osteopontin (OPN), a bone regenerative protein present in a range of mammalian cells, is an acidic phosphoprotein with multiple potential phosphorylation sites. In this study, the ability of unicellular microalgae, Chlamydomonas reinhardtii, to produce phosphorylated recombinant OPN in its chloroplast is investigated. This study further explores the impact of phosphorylation and expression from a “plant-like” algae on separation of OPN. Chromatography resins ceramic hydroxyapatite (CHT) and Gallium-immobilized metal affinity chromatography (Ga-IMAC) were assessed for their binding specificity to phosphoproteins. Non-phosphorylated recombinant OPN expressed in E. coli was used to compare the specificity of interaction of the resins to phosphorylated OPN. We observed that CHT binds OPN by multimodal interactions and was better able to distinguish phosphorylated proteins in the presence of 250 mM NaCl. Ga-IMAC interaction with OPN was not selective to phosphorylation, irrespective of salt, as the resin bound OPN from both algal and bacterial sources. Anion exchange chromatography proved an efficient capture method to partially separate major phosphorylated host cell protein impurities such as Rubisco from OPN.

In Vitro Dialysis of Cytokine-Rich Plasma With High and Medium Cut-Off Membranes Reduces Its Procalcific Activity

Recently developed high-flux (HF) dialysis membranes with extended permeability provide better clearance of middle-sized molecules such as interleukins (ILs). Whether this modulation of inflammation influences the procalcific effects of septic plasma on vascular smooth muscle cells (VSMCs) is not known. To assess the effects of high cut-off (HCO) and medium cut-off (MCO) membranes on microinflammation and in vitro vascular calcification we developed a miniature dialysis model. Plasma samples from lipopolysaccharide-spiked blood were dialyzed with HF, HCO, and MCO membranes in an in vitro miniature dialysis model. Afterwards, IL-6 concentrations were determined in dialysate and plasma. Calcifying VSMCs were incubated with dialyzed plasma samples and vascular calcification was assessed. Osteopontin (OPN) and matrix Gla protein (MGP) were measured in VSMC supernatants. IL-6 plasma concentrations were markedly lower with HCO and MCO dialysis. VSMC calcification was significantly lower after incubation with MCO- and HCO-serum compared to HF plasma. MGP and OPN levels in supernatants were significantly lower in the MCO but not in the HCO group compared to HF. In vitro dialysis of cytokine-enriched plasma samples with MCO and HCO membranes reduces IL-6 levels. The induction of vascular calcification by cytokine-enriched plasma is reduced after HCO and MCO dialysis.

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.

Molecular Imaging of Vulnerable Atherosclerotic Plaques in Vivo with Osteopontin-Specific Upconversion Nanoprobes

Owing to the high mortality rate of cardiovascular diseases, developing novel noninvasive diagnostic methods becomes urgent and mandatory. It is well-known that the rupture of vulnerable plaques directly leads to deadly consequences. However, differentiating vulnerable plaques from stable plaques remains challenging in the clinic. In the current study, osteopontin (OPN), a secreted biomarker associated with macrophages and foamy macrophages, was selected as a target for identifying the vulnerable plaques. A dual modality imaging probe was constructed by covalently attaching an OPN antibody to NaGdF₄:Yb,Er@NaGdF₄ upconversion nanoparticles. Upon intravenous injection of the resulting probes, upconversion optical imaging was performed to visualize the plaques induced by altering the shear stress in carotid arteries of a mouse model. The imaging studies revealed that the signals of vulnerable and stable plagues induced by lowered shear stress and oscillatory shear stress, respectively, presented significantly different signal intensities, implying that the current probe and imaging strategy are potentially useful for a precise diagnosis of atherosclerosis plaques.

Osteopontin and the dento-osseous pathobiology of X-linked hypophosphatemia

Seven young patients with X-linked hypophosphatemia (XLH, having inactivating PHEX mutations) were discovered to accumulate osteopontin (OPN) at the sites of defective bone mineralization near osteocytes - the so-called hallmark periosteocytic (lacunar) "halos" of XLH. OPN was also localized in the pericanalicular matrix extending beyond the osteocyte lacunae, as well as in the hypomineralized matrix of tooth dentin. OPN, a potent inhibitor of mineralization normally degraded by PHEX, is a member of a family of acidic, phosphorylated, calcium-binding, extracellular matrix proteins known to regulate dental, skeletal, and pathologic mineralization. Associated with the increased amount of OPN (along with inhibitory OPN peptide fragments) in XLH bone matrix, we found an enlarged, hypomineralized, lacuno-canalicular network - a defective pattern of skeletal mineralization that decreases stiffness locally at: i) the cell-matrix interface in the pericellular environment of the mechanosensing osteocyte, and ii) the osteocyte's dendritic network of cell processes extending throughout the bone. Our findings of an excess of inhibitory OPN near osteocytes and their cell processes, and in dentin, spatially correlates with the defective mineralization observed at these sites in the skeleton and teeth of XLH patients. These changes likely contribute to the dento-osseous pathobiology of XLH, and participate in the aberrant bone adaptation and remodeling seen in XLH.

Peptide-based vaccination against OPN integrin binding sites does not improve cardio-metabolic disease in mice

Obesity causes insulin resistance via a chronic low-grade inflammation. This inflammation is characterized by elevated pro-inflammatory markers and macrophage accumulation in the adipose tissue (AT). AT inflammation is a key factor causing insulin resistance and thus type 2 diabetes, both linked to atherosclerotic cardiovascular disease. Osteopontin (OPN), a well-known inflammatory cytokine, is involved in obesity-linked complications including AT inflammation, insulin resistance, atherosclerosis and CVD. During inflammation, OPN is proteolytically cleaved by matrix metalloproteinases or thrombin leading to increased OPN activity. Therefore, OPN provides a new interesting target for immunological prevention and treatment of obesity-associated diseases. The aim of our study was to evaluate peptide-based vaccines against integrin binding sites of OPN and to examine whether these active immunotherapies are functional in reducing metabolic tissue inflammation, insulin resistance, and atherosclerosis in a cardio-metabolic (Ldlr^-/- mice) and a diet-induced obesity model (WT mice). However, atherosclerosis, insulin resistance and AT inflammation were not diminished after treatment with OPN-derived peptides in murine models. Lack of efficacy was based on a failure to induce antibodies capable to bind epitopes in the context of functional OPN protein. In conclusion, our data point to unexpected challenges in the immunotherapeutic targeting of adhesive motives, such as RGD containing sequences, on endogenous proteins.

Biological roles of milk osteopontin

PURPOSE OF REVIEW

Osteopontin (OPN) is an extensively phosphorylated acidic glycoprotein that is present at high concentration in human milk. Bovine milk OPN has recently become commercially available, and can thus be added to infant formula as well as other food products. Studies in experimental animals as well as a clinical trial in human infants reveal important biological roles for milk OPN.

RECENT FINDINGS

Newborn wild-type mice nursing knock-out dams lacking milk OPN showed impaired cognitive development, reduced brain OPN and myelin-related proteins as compared with pups nursing wild-type dams. Transcriptomic analysis of intestinal biopsies from infant rhesus monkeys fed formula with added bovine milk OPN showed that OPN positively regulated intestinal proliferation, cell migration, and cellular chemotaxis via binding to integrin receptors. A clinical trial on human infants fed formula with added bovine milk OPN showed significantly reduced levels of serum tumor necrosis factor a and fewer days of fever as compared with infants fed regular formula. Further, their immune cell profile was more similar to that of breast-fed infants.

SUMMARY

Milk OPN affects immune functions, and intestinal development in the newborn as well as brain development in a mouse model. Addition of bovine milk OPN to infant formula may improve the performance of formula-fed infants and make them more similar to breast-fed infants. Due to its multiple functions, milk OPN is also used for various adult applications.

Structure-function relationships in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is an important regulator in the balance of coagulation and fibrinolysis. TAFI is a metallocarboxypeptidase that circulates in plasma as zymogen. Activated TAFI (TAFIa) cleaves C-terminal lysine or arginine residues from peptide substrates. The removal of C-terminal lysine residues from partially degraded fibrin leads to reduced plasmin formation and thus attenuation of fibrinolysis. TAFI also plays a role in inflammatory processes via the removal of C-terminal arginine or lysine residues from bradykinin, thrombin-cleaved osteopontin, C3a, C5a and chemerin. TAFI has been studied extensively over the past three decades and recent publications provide a wealth of information, including crystal structures, mutants and structural data obtained with antibodies and peptides. In this review, we combined and compared available data on structure/function relationships of TAFI.

(1)H, (15)N, (13)C resonance assignment of human osteopontin

Osteopontin (OPN) is a 33.7 kDa intrinsically disordered protein and a member of the SIBLING family of proteins. OPN is bearing a signal peptide for secretion into the extracellular space, where it exerts its main physiological function, the control of calcium biomineralization. It is often involved in tumorigenic processes influencing proliferation, migration and survival, as well as the adhesive properties of cancer cells via CD44 and integrin signaling pathways. Here we report the nearly complete NMR chemical shift assignment of recombinant human osteopontin.

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.

[The highly expressed secreted phosphoprotein 1 gene in prostate cancer metastasis: a microarray-based bioinformatic analysis]

OBJECTIVE

To investigate the composition, function, and regulatory mechanisms of the secreted phosphoprotein 1 (SPP1) gene in metastatic prostate cancer.

METHODS

We obtained the data about the whole genomic expression profiles on prostate cancer metastasis from the GEO database, and performed data-mining and bioinformatic analysis using BRB-Array Tools and such softwares as Protparam, MotifScan, SignalP 4.0, TMHMM, NetPhos2.0, PredictProtein, GO, KEGG, and STRING.

RESULTS

Totally, 73 co-expressed differential genes in prostate cancer metastasis were identified, 21 up-regulated and 52 down-regulated (P <0.01). Bioinformatic analysis indicated that the highly expressed SPP1 gene encoded 314 amino acids and contained 2 N-glycosylation sites, 8 casein kinase II phosphorylation sites and 3 protein kinase C phosphorylation sites, playing essential roles in extracellular matrix (ECM) binding, ossification, osteoblast differentiation, cell adhesion, PI3K-Akt signaling pathway, focal adhesion, Toll-like receptor signaling pathway, and ECM-receptor interaction.

CONCLUSION

The bioinformatic method showed a high efficiency in analyzing microarray data and revealing internal biological information. SPP1 may play an important role in prostate cancer metastasis and become a novel biomarker for the diagnosis of prostate cancer metastasis and a new target for its treatment.

Osteopontin binds multiple calcium ions with high affinity and independently of phosphorylation status

Osteopontin (OPN) is an acidic, intrinsically disordered extracellular matrix protein with a capacity to modulate biomineralization in vitro and in vivo. The role of posttranslational modification of osteopontin has been intensively studied. Phosphorylation of OPN has been demonstrated to play a role in inhibition of biomineral formation and growth in vitro. Here, we used isothermal titration calorimetry (ITC) to investigate the ability of OPN to bind the divalent cations Ca(2+) and Mg(2+), both essential components of inorganic minerals in vivo. We found, that bovine OPN binds ~10 Ca(2+) ions with an apparent affinity ~50-fold tighter than Mg(2+), both regardless of OPN phosphorylation, and with affinities significantly stronger than previously reported. These results were confirmed using human derived OPN. This implies that a majority of the acidic residues within OPN must be engaged in calcium interaction under physiological conditions.

Role of osteopontin in the pathophysiology of cancer

Osteopontin (OPN) is a multifunctional cytokine that impacts cell proliferation, survival, drug resistance, invasion, and stem like behavior. Due to its critical involvement in regulating cellular functions, its aberrant expression and/or splicing is functionally responsible for undesirable alterations in disease pathologies, specifically cancer. It is implicated in promoting invasive and metastatic progression of many carcinomas. Due to its autocrine and paracrine activities OPN has been shown to be a crucial mediator of cellular cross talk and an influential factor in the tumor microenvironment. OPN has been implicated as a prognostic and diagnostic marker for several cancer types. It has also been explored as a possible target for treatment. In this article we hope to provide a broad perspective on the importance of OPN in the pathophysiology of cancer.

Cancer antigen 125, human epididymis 4, kallikrein 6, osteopontin and soluble mesothelin-related peptide immunocomplexed with immunoglobulin M in epithelial ovarian cancer diagnosis

BACKGROUND

Human epididymis protein 4 (HE4), kallikrein 6 (KLK6), osteopontin (OPN) and soluble mesothelin-related peptide (SMRP) are new promising biomarkers that could integrate CA125 in epithelial ovarian cancer (EOC) diagnosis. The autoantibody response to tumor antigens is a potential tool for improving the diagnostic performances of biomarkers. The aim of this study was to assess the diagnostic potential of these biomarkers in the form of free markers and immunocomplexed with immunoglobulin M (IgM). Moreover, we analyzed the association between these markers and clinico-pathological characteristics of EOC patients.

METHODS

Serum and plasma samples of 60 healthy controls, 60 ovarian benign cysts, 60 endometriosis and 60 EOCs, collected before any treatment, were tested for CICs and free antigens by immunoassays.

RESULTS

Immunocomplexes were characterized by poor sensitivity and specificity, since they allowed the detection only of a small number of EOC patients and were increased in patients with benign gynecological pathologies. However, the markers in the form of free antigens showed good diagnostic performances. Of note, CA125 and HE4 showed high sensitivity in the detection of the malignancy and HE4 emerged as a useful biomarker in differential diagnosis between EOC and endometriosis. Finally, elevated KLK6 and OPN, were associated with advanced FIGO stage, high grade disease, suboptimally debulked tumor and ascites.

CONCLUSIONS

This study confirms the diagnostic role of CA125, HE4, KLK6, OPN and SMRP, and for the first time showed that CA125, HE4, KLK6, OPN and SMRP immunocomplexed with IgM are not a potential tool for EOC diagnosis.

Reversible inhibition of calcium oxalate monohydrate growth by an osteopontin phosphopeptide

Calcium oxalate, primarily as calcium oxalate monohydrate (COM), is the primary constituent of most kidney stones. Certain proteins, such as osteopontin (OPN), inhibit stone formation. The complexity of stone formation and the effects of urinary proteins at various stages of the process make it hard to predict the exact physiological roles of these proteins in growth inhibition. The inhibition of crystallization due to adsorbed impurities is usually explained in terms of a model proposed in 1958 by Cabrera and Vermilyea. In this model, impurities adsorb to growth faces and pin growth steps, forcing them to curve, thus impeding their progress via the Gibbs-Thomson effect. To determine the role of OPN in the biomineralization of kidney stones, crystal growth on the {010} face of COM was examined in real time with atomic force microscopy in the presence of a synthetic peptide corresponding to amino acids 65-80 (hereafter referred to as pOPAR) of rat bone OPN. We observed clear changes in the morphology of the growth-step structure and a decrease in step velocity upon addition of pOPAR, which suggest adsorption of inhibitors on the {010} growth hillocks. Experiments in which pOPAR was replaced in the growth cell by a supersaturated solution showed that COM hillocks are able to fully recover to their preinhibited state. Our results suggest that recovery occurs through incorporation of the peptide into the growing crystal, rather than by, e.g., desorption from the growth face. This work provides new insights into the mechanism by which crystal growth is inhibited by adsorbants, with important implications for the design of therapeutic agents for kidney stone disease and other forms of pathological calcification.

The effect of adsorbed fibronectin and osteopontin on macrophage adhesion and morphology on hydrophilic and hydrophobic model surfaces

Macrophages play a crucial role in the host response to biomaterials. Here we investigated the effect of adsorbed fibronectin (FN) and osteopontin (OPN), two important proteins for tissue repair, on macrophage adhesion and morphology. Since cell-biomaterial interactions are modulated via proteins adsorbed onto biomaterial surfaces, FN and OPN were adsorbed on model self-assembled monolayers (SAMs) of alkanethiols on gold with different functional terminal groups (CH(3), OH and tetra(ethylene-glycol)). The initial interaction of inflammatory cells with a biomaterial is crucial for the ensuing phases of an inflammatory reaction. For this reason short-term cultures of primary human macrophages were performed. To account for the competitive adsorption of other proteins serum was added to the culture medium and the effect compared with serum-free medium cultures. In the presence of serum hydrophilic surfaces increased macrophage adhesion. In particular, FN induced a higher cell density, while OPN tended to decrease it. In serum-free medium cell adhesion was greater on hydrophobic surfaces, except for OPN-coated SAMs. Importantly, FN no longer enhanced macrophage adhesion, while OPN maintained its inhibitory effect. Cell polarization studies indicated that macrophage morphology variations induced by surface chemistry are overcome by pre-adsorbed OPN. Taken together our results show that in the presence of serum macrophage adhesion is promoted by FN hydrophilic surfaces, but impaired on OPN-coated surfaces. The effects of inhibited macrophage adhesion on macrophage fusion, and its relevance to the initial stages of the inflammatory response to biomaterials are discussed.

Secreted osteopontin is highly polymerized in human airways and fragmented in asthmatic airway secretions

BACKGROUND

Osteopontin (OPN) is a member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family and a cytokine with diverse biologic roles. OPN undergoes extensive post-translational modifications, including polymerization and proteolytic fragmentation, which alters its biologic activity. Recent studies suggest that OPN may contribute to the pathogenesis of asthma.

METHODOLOGY

To determine whether secreted OPN (sOPN) is polymerized in human airways and whether it is qualitatively different in asthma, we used immunoblotting to examine sOPN in bronchoalveolar lavage (BAL) fluid samples from 12 healthy and 21 asthmatic subjects (and in sputum samples from 27 healthy and 21 asthmatic subjects). All asthmatic subjects had mild to moderate asthma and abstained from corticosteroids during the study. Furthermore, we examined the relationship between airway sOPN and cellular inflammation.

PRINCIPAL FINDINGS

We found that sOPN in BAL fluid and sputum exists in polymeric, monomeric, and cleaved forms, with most of it in polymeric form. Compared to healthy subjects, asthmatic subjects had proportionately less polymeric sOPN and more monomeric and cleaved sOPN. Polymeric sOPN in BAL fluid was associated with increased alveolar macrophage counts in airways in all subjects.

CONCLUSIONS

These results suggest that sOPN in human airways (1) undergoes extensive post-translational modification by polymerization and proteolytic fragmentation, (2) is more fragmented and less polymerized in subjects with mild to moderate asthma, and (3) may contribute to recruitment or survival of alveolar macrophages.

Biochemical characterization of major bone-matrix proteins using nanoscale-size bone samples and proteomics methodology

There is growing evidence supporting the need for a broad scale investigation of the proteins and protein modifications in the organic matrix of bone and the use of these measures to predict fragility fractures. However, limitations in sample availability and high heterogeneity of bone tissue cause unique experimental and/or diagnostic problems. We addressed these by an innovative combination of laser capture microscopy with our newly developed liquid chromatography separation methods, followed by gel electrophoresis and mass spectrometry analysis. Our strategy allows in-depth analysis of very limited amounts of bone material, and thus, can be important to medical sciences, biology, forensic, anthropology, and archaeology. The developed strategy permitted unprecedented biochemical analyses of bone-matrix proteins, including collagen modifications, using nearly nanoscale amounts of exceptionally homogenous bone tissue. Dissection of fully mineralized bone-tissue at such degree of homogeneity has not been achieved before. Application of our strategy established that: (1) collagen in older interstitial bone contains higher levels of an advanced glycation end product pentosidine then younger osteonal tissue, an observation contrary to the published data; (2) the levels of two enzymatic crosslinks (pyridinoline and deoxypiridinoline) were higher in osteonal than interstitial tissue and agreed with data reported by others; (3) younger osteonal bone has higher amount of osteopontin and osteocalcin then older interstitial bone and this has not been shown before. Taken together, these data show that the level of fluorescent crosslinks in collagen and the amount of two major noncollagenous bone matrix proteins differ at the level of osteonal and interstitial tissue. We propose that this may have important implications for bone remodeling processes and bone microdamage formation.

The flexible polyelectrolyte hypothesis of protein-biomineral interaction

Biomineralization is characterized by a high degree of control over the location, nature, size, shape, and orientation of the crystals formed. For many years, it has been widely believed that the exquisitely precise nature of crystal formation in biological tissues is the result of stereochemically specific interactions between growing crystals and extracellular matrix proteins. That is, the ability of many mineralized tissue proteins to adsorb to particular faces of biominerals has been attributed to a steric and electrical complementarity between periodic regions of the polypeptide chain and arrays of ions on the crystal face. In recent years, however, evidence has accumulated that many mineral-associated proteins lack periodic structure even when adsorbed to crystals. It also appears that protein-crystal interactions involve a general electrostatic attraction rather than arrays of complementary charges. In the present work, we review these studies and present some relevant new findings involving the mineral-modulating phosphoprotein osteopontin. Using molecular dynamics simulations, we show that the adsorption of osteopontin peptides to hydroxyapatite crystals does not involve a unique conformation of the peptide molecule, and that the adsorbed peptides are not aligned with rows of Ca(2+) ions on the crystal face. Further, we show that the interface between osteopontin peptides and calcium oxalate monohydrate crystals consists of peptide regions of high electronegativity and crystal faces of high electropositivity. Collectively, the above-mentioned studies suggest that interactions between mineral-modulating proteins and biologically relevant crystals are primarily electrostatic in nature, and that molecular disorder assists these proteins in forming multiple bonds with cations of the crystal face.

[The role of osteopontin in cardiovascular diseases]

Osteopontin (OPN) is an acid phosphorylated glycoprotein secreted by cells of the immune system, epithelial cells, smooth muscle cells, osteoblasts, osteoclasts, tumor cells and many more. Osteopontin is a multifunctional protein. Due to characteristic molecular structure containing integrin bining domains, OPN can interact with several integrin receptors, thus it play a role in activation, adhesion and migration of many cell types in such processes as inflammation, tissue mineralization and tumor genesis. This broad biological action of osteopontin underlie its presumed role in the pathogenesis of cardiovascular diseases, including atherosclerosis and abdominal aortic aneurysm formation.

Hydroxyapatite growth inhibition by osteopontin hexapeptide sequences

The effects of three acidic hexapeptides on in vitro hydroxyapatite growth were characterized by pH-stat kinetic studies, adsorption isotherms, and molecular modeling. The three peptides, pSDEpSDE, SDESDE, and DDDDDD, are equal-length model compounds for the acidic sequences in osteopontin, a protein that inhibits mineral formation in both calcified and noncalcified tissues. Growth rates from 1.67 mM calcium and 1.00 mM phosphate solution were measured at pH 7.4 and 37 degrees C in 150 mM NaCl. pSDEpSDE was a strong growth inhibitor when preadsorbed onto hydroxyapatite (HA) seeds from > or = 0.67 mM solutions, concentrations where adsorption isotherms showed relatively complete surface coverage. The nonphosphorylated SDESDE control showed no growth inhibition. Although it adsorbed to almost the same extent as pSDEpSDE, it rapidly desorbed under the pH-stat growth conditions while pSDEpSDE did not. DDDDDD exhibited weak inhibition as its concentration was increased and similar adsorption/desorption behavior to pSDEpSDE. Molecular modeling yielded binding energy trends based on simple adsorption of peptides on the [100] surface that were consistent with observed inhibition, but not for the [001] surface. The relatively unfavorable binding energies for peptides on the [001] surface suggest that their absorption will be primarily on the [100] face. The kinetic and adsorption data are consistent with phosphorylation of osteopontin acting to control mineral formation.

The RGD domain of human osteopontin promotes tumor growth and metastasis through activation of survival pathways

BACKGROUND

Human osteopontin (OPN), a known tumor associated protein, exists in different isoforms, whose function is unclear. It also possesses a RGD domain, which has been implicated in diverse function. Here, we use genetic approaches to systematically investigate the function of the RGD domain in different OPN isoforms on tumor progression and metastasis for 2 different solid tumor models.

METHODOLOGY/PRINCIPAL FINDINGS

Using isoform-specific qRT-PCR, we found that OPN-A and B were the main isoforms overexpressed in evaluated human tumors, which included 4 soft tissue sarcomas, 24 lung and 30 head and neck carcinomas. Overexpression of either OPN-A or B in two different cell types promoted local tumor growth and lung metastasis in SCID mouse xenografts. However, expression of either isoform with the RGD domain either mutated or deleted decreased tumor growth and metastasis, and resulted in increased apoptosis by TUNEL staining. In vitro, whereas mutation of the RGD domain did not affect cell-cell adhesion, soft agar growth or cell migration, it increased apoptosis under hypoxia and serum starvation. This effect could be mitigated when the RGD mutant cells were treated with condition media containing WT OPN. Mechanistically, the RGD region of OPN inhibited apoptosis by inducing NF-kappaB activation and FAK phosphorylation. Inhibition of NF-kappaB (by siRNA to the p65 subunit) or FAK activation (by a inhibitor) significantly increased apoptosis under hypoxia in WT OPN cells, but not in RGD mutant cells.

CONCLUSION/SIGNIFICANCE

Unlike prior reports, our data suggest that the RGD domain of both OPN-A and B promote tumor growth and metastasis mainly by protecting cells against apoptosis under stressed conditions and not via migration or invasion. Future inhibitors directed against OPN should target multiple isoforms and should inhibit cell survival mechanisms that involve the RGD domain, FAK phosphorylation and NF-kappaB activation.

Roles of electrostatics and conformation in protein-crystal interactions

In vitro studies have shown that the phosphoprotein osteopontin (OPN) inhibits the nucleation and growth of hydroxyapatite (HA) and other biominerals. In vivo, OPN is believed to prevent the calcification of soft tissues. However, the nature of the interaction between OPN and HA is not understood. In the computational part of the present study, we used molecular dynamics simulations to predict the adsorption of 19 peptides, each 16 amino acids long and collectively covering the entire sequence of OPN, to the {100} face of HA. This analysis showed that there is an inverse relationship between predicted strength of adsorption and peptide isoelectric point (P<0.0001). Analysis of the OPN sequence by PONDR (Predictor of Naturally Disordered Regions) indicated that OPN sequences predicted to adsorb well to HA are highly disordered. In the experimental part of the study, we synthesized phosphorylated and non-phosphorylated peptides corresponding to OPN sequences 65–80 (pSHDHMDDDDDDDDDGD) and 220–235 (pSHEpSTEQSDAIDpSAEK). In agreement with the PONDR analysis, these were shown by circular dichroism spectroscopy to be largely disordered. A constant-composition/seeded growth assay was used to assess the HA-inhibiting potencies of the synthetic peptides. The phosphorylated versions of OPN65-80 (IC₅₀ = 1.93 µg/ml) and OPN220-235 (IC₅₀ = 1.48 µg/ml) are potent inhibitors of HA growth, as is the nonphosphorylated version of OPN65-80 (IC₅₀ = 2.97 µg/ml); the nonphosphorylated version of OPN220-235 has no measurable inhibitory activity. These findings suggest that the adsorption of acidic proteins to Ca²⁺-rich crystal faces of biominerals is governed by electrostatics and is facilitated by conformational flexibility of the polypeptide chain.

Surface aggregation of urinary proteins and aspartic Acid-rich peptides on the faces of calcium oxalate monohydrate investigated by in situ force microscopy

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)(6)DDD and (DDDG)(6)DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the (101) face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface.

Tissue engineering scaffolds for the regeneration of craniofacial bone

Current strategies for skeletal regeneration involve the use of autogenous and allogenic bone grafts that may not always be available or safe to use. One alternative is to develop materials for use as scaffolds for the tissue engineering of bone. We created architecturally nanofibrous scaffolds using the electrospinning technique. These calcium phosphate- based materials are porous, have a large surface-area-to-volume ratio and can be used to deliver drugs, biologics or cells for tissue engineering applications. Bone-matrix proteins were also conjugated to the surface of a polymer network of polycaprolactone and poly(2-hydroxyethyl methacrylate) to create a material with enhanced cellular responses. This biomimetic strategy resulted in favourable cell-surface interactions that will likely enhance bone-matrix synthesis and regeneration. These collective advancements enable the development of innovative scaffolds for applications in tissue engineering and bone regeneration.

Kinetics of calcium oxalate crystal growth in the presence of osteopontin isoforms: an analysis by scanning confocal interference microcopy

Proteins that inhibit the growth and aggregation of calcium oxalate crystals play important roles in the prevention of kidney stone disease. One such protein is osteopontin (OPN), which inhibits the formation of calcium oxalate monohydrate (COM) in a phosphorylation-dependent manner. To determine the role of phosphate groups in the inhibition of COM growth by OPN, we used scanning confocal interference microscopy to compare the effects of highly phosphorylated OPN from cow milk, less phosphorylated OPN from rat bone, and nonphosphorylated recombinant OPN. COM growth was measured in the principal crystallographic directions <001>, <010>, and <100>, representing lattice-ion addition to {121}, {010}, and {100} faces, respectively. While the shapes of growth curves were very consistent from crystal to crystal, absolute growth rates varied widely. To control for this, results were expressed as changes in the aspect ratios <010>/<001> and <100>/<001>. Compared to control, bone OPN increased <010>/<001> and had no effect on <100>/<001>; milk OPN had no effect on <010>/<001>and decreased <100>/<001>; recombinant OPN had no significant effect on either aspect ratio. These findings indicate that milk OPN interacts with COM crystal faces in order of preference {100} > {121} approximately {010}, whereas bone OPN interacts in order of preference {100} approximately {121} > {010}. As {100} is the most Ca(2+)-rich face of COM, while {010} is the least Ca(2+)-rich, it appears that the OPN-mediated inhibition of COM growth occurs through a nonspecific electrostatic interaction between Ca(2+) ions of the crystal and phosphate groups of the protein.

Osteopontin functions as an opsonin and facilitates phagocytosis by macrophages of hydroxyapatite-coated microspheres: implications for bone wound healing

Osteopontin (OPN) is a secreted protein abundant in mineralized tissue extracellular matrices and bodily fluids. Previously we have shown that mineralized debris at surgical wound sites in bone and teeth are coated by macrophage-derived OPN and phagocytosed. Here, we have performed opsonophagocytosis assays to determine whether OPN acts as an opsonin and facilitates phagocytosis by macrophages of protein- and hydroxyapatite mineral-coated microspheres. Moreover, we have examined the opsonization effects of monomer OPN versus OPN polymerized (crosslinked) by tissue transglutaminase 2. Murine macrophages J774A.1 were exposed to polystyrene-latex microspheres having different surface chemistries (non-ionic, aldehyde amidine, carboxyl and aliphatic amine) which were coated with either serum albumin, immunoglobulin, monomer OPN or polymer OPN. Similar experiments with the same protein coatings were performed using hydroxyapatite-covered microspheres. Internalization of microspheres by phagocytosis into macrophages was confirmed by co-localization with the (phago)lysosomal markers lysosome-associated membrane protein-1 (Lamp-1) and LysoTracker, and by light microscopy and transmission electron microscopy after serial sectioning of plastic/resin-embedded cells containing microspheres. OPN significantly increased phagocytosis of both microspheres and hydroxyapatite-covered microspheres compared to negative controls (albumin-coated and uncoated microspheres), with phagocytic indices similar to, or greater than, those of the positive control (IgG-coated). The effect of OPN and hydroxyapatite on microsphere phagocytosis was synergistic. Polymer OPN further enhanced the phagocytosis of aliphatic amine and aldehyde amidine microspheres. Taken together, these results indicate that OPN is an effective opsonin able to facilitate particle uptake (including mineralized particles) by macrophages.

Calcium oxalate crystals in fetal bovine serum: implications for cell culture, phagocytosis and biomineralization studies in vitro

Cell culture methods and models are key investigative tools for cell and molecular biology studies. Fetal bovine serum (FBS) is commonly used as an additive during cell culture since its constituents promote cell survival, proliferation and differentiation. Here we report that commercially available FBS from different major suppliers consistently contain precipitated, calcium oxalate crystals-either in the monohydrate (COM) or dihydrate (COD) form. Mineral structure and phase identification of the crystals were determined by X-ray diffraction, chemical composition by energy-dispersive X-ray microanalysis, and imaging and measurement of crystal growth steps by atomic force microscopy-all identified and confirmed crystallographic parameters for COM and COD. Proteins binding to the crystals were identified by immunoblotting, revealing the presence of osteopontin and fetuin-A (alpha(2)HS-glycoprotein)--known regulators of crystal growth found in serum. Macrophage cell cultures exposed to calcium oxalate crystals showed internalization of the crystals by phagocytosis in a process that induced disruption of cell-cell adhesion, release of reactive oxygen species and membrane damage, events that may be linked to the release of inflammatory cytokines by these cells into the culture media. In conclusion, calcium oxalate crystals found in commercially available FBS are toxic to cells, and their presence may confound results from in vitro studies where, amongst others, phagocytosis, biomineralization, renal cell and molecular biology, and drug and biomaterial testing are being examined.

Thrombin hydrolysis of human osteopontin is dependent on thrombin anion-binding exosites

The cytokine osteopontin (OPN) can be hydrolyzed by thrombin exposing a cryptic alpha(4)beta(1)/alpha(9)beta(1) integrin-binding motif (SVVYGLR), thereby acting as a potent cytokine for cells bearing these activated integrins. We show that purified milk OPN is a substrate for thrombin with a k(cat)/K(m) value of 1.14 x 10(5) m(-1) s(-1). Thrombin cleavage of OPN was inhibited by unsulfated hirugen (IC(50) = 1.2 +/- 0.2 microm), unfractionated heparin (IC(50) = 56.6 +/- 8.4 microg/ml) and low molecular weight (5 kDa) heparin (IC(50) = 31.0 +/- 7.9 microg/ml), indicating the involvement of both anion-binding exosite I (ABE-I) and anion-binding exosite II (ABE-II). Using a thrombin mutant library, we mapped residues important for recognition and cleavage of OPN within ABE-I and ABE-II. A peptide (OPN-(162-197)) was designed spanning the OPN thrombin cleavage site and a hirudin-like C-terminal tail domain. Thrombin cleaved OPN-(162-197) with a specificity constant of k(cat)/K(m) = 1.64 x 10(4) m(-1) s(-1). Representative ABE-I mutants (K65A, H66A, R68A, Y71A, and R73A) showed greatly impaired cleavage, whereas the ABE-II mutants were unaffected, suggesting that ABE-I interacts principally with the hirudin-like OPN domain C-terminal and contiguous to the thrombin cleavage site. Debye-Hückel slopes for milk OPN (-4.1 +/- 1.0) and OPN-(162-197) (-2.4 +/- 0.2) suggest that electrostatic interactions play an important role in thrombin recognition and cleavage of OPN. Thus, OPN is a bona fide substrate for thrombin, and generation of thrombin-cleaved OPN with enhanced pro-inflammatory properties provides another molecular link between coagulation and inflammation.

Osteopontin promotes the development of natural killer cells from hematopoietic stem cells

The detailed mechanisms driving the development of natural killer (NK) cells from hematopoietic stem cells remain to be clearly elucidated. Here, we show that osteopontin (OPN) is a key factor for NK development. OPN-deficient mice evidenced severe impairments of NK development in bone marrow (BM) and spleen in which the NK populations that express CD122 and NK cell receptors were reduced. However, the absence of intrinsic OPN expression did not affect NK development, whereas the absence of OPN in the microenvironment caused a significant reduction in NK population. The expression of OPN was induced by interleukin (IL)-15 in BM stromal cells, and the defect in NK differentiation in IL-15(-/-) hematopoietic precursor cells (HPC) was recovered by addition of recombinant OPN, suggesting that the microenvironmental OPN may be a key factor in IL-15-mediated NK differentiation. In addition, OPN-driven NK maturation was reduced in T-bet-deficient HPC, suggesting that T-bet is required for OPN-mediated NK development. Collectively, these results show that paracrine OPN signaling drives NK-lineage commitment, thus ultimately promoting NK cell development. Disclosure of potential conflicts of interest is found at the end of this article.

Backbone assignment of osteopontin, a cytokine and cell attachment protein implicated in tumorigenesis

OPN is an RGD-containing protein overexpressed in cells transformed by v-myc and v-mil(raf) oncogenes. Here we report the resonance assignment of recombinant quail OPN and provide NMR evidence that quail OPN is an intrinsically unstructured protein in solution.

High calcium concentration and calcium oxalate crystals cause significant inaccuracies in the measurement of urinary osteopontin by enzyme linked immunosorbent assay

Strong evidence that osteopontin (OPN) is a determinant of urolithiasis has prompted studies comparing the protein's urinary excretion in healthy subjects and stone formers. However, reported mean urinary values have varied widely, from <1 microg/mL to more than 20 times that value. Since OPN binds to CaOx crystals, the presence of crystals in urine may cause underestimation of its urinary levels. Using a commercial ELISA, we measured urinary OPN levels in the presence of endogenous or exogenous CaOx monohydrate (COM) and dihydrate (COD) crystals. OPN concentrations decreased in the presence of endogenous and exogenous CaOx crystals, but never below 2 microg/mL. Increasing the urinary calcium concentration decreased detectable OPN levels, possibly as a result of changes in the three-dimensional conformation of the protein. Because calcium concentration and the formation of CaOx crystals cannot be controlled in urine, the use of urinary OPN levels as a biomarker for any human pathology must be seriously questioned, but particularly for the investigation of stone formers in whom hypercalciuria and crystalluria are more common than in healthy subjects.

Control of osteopontin signaling and function by post-translational phosphorylation and protein folding

Osteopontin (OPN) plays roles in a variety of cellular processes from bone resorption and extracellular matrix (ECM) remodeling to immune cell activation and inhibition of apoptosis. Because it binds receptors (integrins, CD44 variants) typically engaged by ECM molecules, OPN acts as a "soluble" ECM molecule. A persistent theme throughout the characterization of how OPN functions has been the importance of phosphorylation. The source of the OPN used in specific experiments and the location of modified sites is an increasingly important consideration for OPN research. We review briefly some of the ways OPN impacts on the biology of mammalian systems with an emphasis on the importance of serine phosphorylation in modulating its signaling ability. We describe experiments that support the hypothesis that differences in the post-translational phosphorylation of OPN expressed by different cell types regulate how it impacts on target cells. Analyses of OPN's potential secondary structure reveal a possible beta-sheet conformation that offers an interpretation of certain experimental observations, specifically the effect of thrombin cleavage; it is consistent with an interaction between the C-terminal region of the protein and the central integrin-binding RGD sequence.