Calcium and collagen binding properties of osteopontin, bone sialoprotein, and bone acidic glycoprotein-75 from bone

Milk osteopontin has high iron-binding capacity and facilitates iron absorption in intestinal cells

Insufficient absorption of iron and the consequent development of iron deficiency have serious health consequences. Hence, identification and development of iron delivery systems that can increase the bioavailability and uptake of dietary iron are important. Osteopontin (OPN) is an acidic and highly phosphorylated integrin-binding protein found in milk where it exists as a full-length protein and as N-terminally derived fragments. Milk OPN can be taken up by enterocytes and transported across the intestinal barrier into the circulation. Milk OPN has previously been shown to bind calcium and magnesium. This study investigates milk OPN as a carrier of iron and its potential to increase iron absorption in intestinal cells. Full-length OPN and N-terminal fragments of OPN were shown to bind ∼30 and ∼10 mol of iron, respectively, and the phosphorylated residues were crucial for iron binding. Osteopontin retained iron bound after simulated gastrointestinal digestion. Immunodetection of digested OPN and OPN-Fe complexes showed that the OPN-Fe complexes were more resistant to pepsin digestion than OPN without bound iron. The cellular uptake of iron was investigated by measuring intracellular ferritin formation and mRNA expression of divalent metal transporter 1 in Caco-2 cells. Osteopontin increased the uptake of iron even in the presence of phytic acid, a dietary inhibitor of iron absorption. These data indicate that OPN can function as an iron carrier for use in alternative strategies for delivering iron in a bioavailable form for intestinal uptake.

Aspartic Acid Binding on Hydroxyapatite Nanoparticles with Varying Morphologies Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulation

Hydroxyapatite (HAP) exhibits a highly oriented hierarchical structure in biological hard tissues. The formation and selective crystalline orientation of HAP is a process that involves functional biomineralization proteins abundant in acidic residues. To obtain insights into the process of HAP mineralization and acidic residue binding, synthesized HAP with specific lattice planes including (001), (100), and (011) are structurally characterized following the adsorption of aspartic acid (Asp). The adsorption affinity of Asp on HAP surfaces is evaluated quantitatively and demonstrates a high dependency on the HAP morphological form. Among the synthesized HAP nanoparticles (NPs), Asp exhibits the strongest adsorption affinity to short HAP nanorods, which are composed of (100) and (011) lattice planes, followed by nanosheets with a preferential expression of the (001) facet, to which Asp displays a similar but slightly lower binding affinity. HAP nanowires, with the (100) lattice plane preferentially developed, show significantly lower affinity to Asp and evidence of multilayer adsorption compared to the previous two types of HAP NPs. A combination of solid-state NMR (SSNMR) techniques including 13C and 15N CP-MAS, relaxation measurements and 13C-31P Rotational Echo DOuble Resonance (REDOR) is utilized to characterize the molecular structure and dynamics of Asp-HAP bionano interfaces with 13C- and 15N-enriched Asp. REDOR is used to determine 13C-31P internuclear distances, providing insight into the Asp binding geometry where stronger 13C-31P dipolar couplings correlate with binding affinity determined from Langmuir isotherms. The carboxyl sites are identified as the primary binding groups, facilitated by their interaction with surface calcium sites. The Asp chelation conformations revealed by SSNMR are further refined with molecular dynamics (MD) simulation where specific models strongly agree between the SSNMR and MD models for the various surfaces.

BioAdhere: tailor-made bioadhesives for epiretinal visual prostheses

Introduction: Visual prostheses, i.e. epiretinal stimulating arrays, are a promising therapy in treating retinal dystrophies and degenerations. In the wake of a new generation of devices, an innovative method for epiretinal fixation of stimulator arrays is required. We present the development of tailor-made bioadhesive peptides (peptesives) for fixating epiretinal stimulating arrays omitting the use of traumatic retinal tacks. Materials and methods: Binding motifs on the stimulating array (poly[chloro-p-xylylene] (Parylene C)) and in the extracellular matrix of the retinal surface (collagens I and IV, laminin, fibronectin) were identified. The anchor peptides cecropin A (CecA), KH1, KH2 (author's initials) and osteopontin (OPN) were genetically fused to reporter proteins to assess their binding behavior to coated microtiter plates via fluorescence-based assays. Domain Z (DZ) of staphylococcal protein A was used as a separator to generate a bioadhesive peptide. Following ISO 10993 "biological evaluation of medical materials", direct and non-direct cytotoxicity testing (L-929 and R28 retinal progenitor cells) was performed. Lastly, the fixating capabilities of the peptesives were tested in proof-of-principle experiments. Results: The generation of the bioadhesive peptide required evaluation of the N- and C-anchoring of investigated APs. The YmPh-CecA construct showed the highest activity on Parylene C in comparison with the wildtype phytase without the anchor peptide. eGFP-OPN was binding to all four investigated ECM proteins (collagen I, laminin > collagen IV, fibronectin). The strongest binding to collagen I was observed for eGFP-KH1, while the strongest binding to fibronectin was observed for eGFP-KH2. The selectivity of binding was checked by incubating eGFP-CecA and eGFP-OPN on ECM proteins and on Parylene C, respectively. Direct and non-direct cytotoxicity testing of the peptide cecropin-A-DZ-OPN using L-929 and R28 cells showed good biocompatibility properties. Proof-of-concept experiments in post-mortem rabbit eyes suggested an increased adhesion of CecA-DZ-OPN-coated stimulating arrays. Conclusion: This is the first study to prove the applicability and biocompatibility of peptesives for the fixation of macroscopic objects.

Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Divergent Expression of SPARC, SPARC-L, and SCPP Genes During Jawed Vertebrate Cartilage Mineralization

While cartilage is an ancient tissue found both in protostomes and deuterostomes, its mineralization evolved more recently, within the vertebrate lineage. SPARC, SPARC-L, and the SCPP members (Secretory Calcium-binding PhosphoProtein genes which evolved from SPARC-L) are major players of dentine and bone mineralization, but their involvement in the emergence of the vertebrate mineralized cartilage remains unclear. We performed in situ hybridization on mineralizing cartilaginous skeletal elements of the frog Xenopus tropicalis (Xt) and the shark Scyliorhinus canicula (Sc) to examine the expression of SPARC (present in both species), SPARC-L (present in Sc only) and the SCPP members (present in Xt only). We show that while mineralizing cartilage expresses SPARC (but not SPARC-L) in Sc, it expresses the SCPP genes (but not SPARC) in Xt, and propose two possible evolutionary scenarios to explain these opposite expression patterns. In spite of these genetic divergences, our data draw the attention on an overlooked and evolutionarily conserved peripheral cartilage subdomain expressing SPARC or the SCPP genes and exhibiting a high propensity to mineralize.

Challenges and strategy in treatment with exosomes for cell-free-based tissue engineering in dentistry

In dentistry, problems of craniofacial, osteochondral, periodontal tissue, nerve, pulp or endodontics injuries, and osteoarthritis need regenerative therapy. The use of stem cells in dental tissue engineering pays a lot of increased attention, but there are challenges for its clinical applications. Therefore, cell-free-based tissue engineering using exosomes isolated from stem cells is regarded an alternative approach in regenerative dentistry. However, practical use of exosome is restricted by limited secretion capability of cells. For future regenerative treatment with exosomes, efficient strategies for large-scale clinical applications are being studied, including the use of ceramics-based scaffold to enhance exosome production and secretion which can resolve limited exosome secretory from the cells when compared with the existing methods available. Indeed, more research needs to be done on these strategies going forward.

Biomolecules Orchestrating Cardiovascular Calcification

Vascular calcification, once considered a degenerative, end-stage, and inevitable condition, is now recognized as a complex process regulated in a manner similar to skeletal bone at the molecular and cellular levels. Since the initial discovery of bone morphogenetic protein in calcified human atherosclerotic lesions, decades of research have now led to the recognition that the regulatory mechanisms and the biomolecules that control cardiovascular calcification overlap with those controlling skeletal mineralization. In this review, we focus on key biomolecules driving the ectopic calcification in the circulation and their regulation by metabolic, hormonal, and inflammatory stimuli. Although calcium deposits in the vessel wall introduce rupture stress at their edges facing applied tensile stress, they simultaneously reduce rupture stress at the orthogonal edges, leaving the net risk of plaque rupture and consequent cardiac events depending on local material strength. A clinically important consequence of the shared mechanisms between the vascular and bone tissues is that therapeutic agents designed to inhibit vascular calcification may adversely affect skeletal mineralization and vice versa. Thus, it is essential to consider both systems when developing therapeutic strategies.

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.

The Thermodynamics of Medial Vascular Calcification

Medial vascular calcification (MVC) is a degenerative process that involves the deposition of calcium in the arteries, with a high prevalence in chronic kidney disease (CKD), diabetes, and aging. Calcification is the process of precipitation largely of calcium phosphate, governed by the laws of thermodynamics that should be acknowledged in studies of this disease. Amorphous calcium phosphate (ACP) is the key constituent of early calcifications, mainly composed of Ca2+ and PO4 3- ions, which over time transform into hydroxyapatite (HAP) crystals. The supersaturation of ACP related to Ca2+ and PO4 3- activities establishes the risk of MVC, which can be modulated by the presence of promoter and inhibitor biomolecules. According to the thermodynamic parameters, the process of MVC implies: (i) an increase in Ca2+ and PO4 3- activities (rather than concentrations) exceeding the solubility product at the precipitating sites in the media; (ii) focally impaired equilibrium between promoter and inhibitor biomolecules; and (iii) the progression of HAP crystallization associated with nominal irreversibility of the process, even when the levels of Ca2+ and PO4 3- ions return to normal. Thus, physical-chemical processes in the media are fundamental to understanding MVC and represent the most critical factor for treatments' considerations. Any pathogenetical proposal must therefore comply with the laws of thermodynamics and their expression within the medial layer.

Milk proteins and their derived peptides on bone health: Biological functions, mechanisms, and prospects

Bone is a dynamic organ under constant metabolism (or remodeling), where a delicate balance between bone resorption and bone formation is maintained. Disruption of this coordinated bone remodeling results in bone diseases, such as osteoporosis, the most common bone disorder characterized by decreased bone mineral density and microarchitectural deterioration. Epidemiological and clinical evidence support that consumption of dairy products is beneficial for bone health; this benefit is often attributed to the presence of calcium, the physiological contributions of milk proteins on bone metabolism, however, are underestimated. Emerging evidence highlighted that not only milk proteins (including individual milk proteins) but also their derived peptides positively regulate bone remodeling and attenuate bone loss, via the regulation of cellular markers and signaling of osteoblasts and osteoclasts. This article aims to review current knowledge about the roles of milk proteins, with an emphasis on individual milk proteins, bioactive peptides derived from milk proteins, and effect of milk processing in particular fermentation, on bone metabolism, to highlight the potential uses of milk proteins in the prevention and treatment of osteoporosis, and, to discuss the knowledge gap and to recommend future research directions.

Biological Roles and Delivery Strategies for Ions to Promote Osteogenic Induction

Bone is the most studied tissue in the field of tissue regeneration. Even though it has intrinsic capability to regenerate upon injury, several pathologies and injuries could hamper the highly orchestrated bone formation and resorption process. Bone tissue engineering seeks to mimic the extracellular matrix of the tissue and the different biochemical pathways that lead to successful regeneration. For many years, the use of extrinsic factors (i.e., growth factors and drugs) to modulate these biological processes have been the preferred choice in the field. Even though it has been successful in some instances, this approach presents several drawbacks, such as safety-concerns, short release profile and half-time life of the compounds. On the other hand, the use of inorganic ions has attracted significant attention due to their therapeutic effects, stability and lower biological risks. Biomaterials play a key role in such strategies where they serve as a substrate for the incorporation and release of the ions. In this review, the methodologies used to incorporate ions in biomaterials is presented, highlighting the osteogenic properties of such ions and the roles of biomaterials in controlling their release.

A repeated triple lysine motif anchors complexes containing bone sialoprotein and the type XI collagen A1 chain involved in bone mineralization

While details remain unclear, initiation of woven bone mineralization is believed to be mediated by collagen and potentially nucleated by bone sialoprotein (BSP). Interestingly, our recent publication showed that BSP and type XI collagen form complexes in mineralizing osteoblastic cultures. To learn more, we examined the protein composition of extracellular sites of de novo hydroxyapatite deposition which were enriched in BSP and Col11a1 containing an alternatively spliced "6b" exonal sequence. An alternate splice variant "6a" sequence was not similarly co-localized. BSP and Col11a1 co-purify upon ion-exchange chromatography or immunoprecipitation. Binding of the Col11a1 "6b" exonal sequence to bone sialoprotein was demonstrated with overlapping peptides. Peptide 3, containing three unique lysine-triplet sequences, displayed the greatest binding to osteoblastic cultures; peptides containing fewer lysine triplet motifs or derived from the "6a" exon yielded dramatically lower binding. Similar results were obtained with 6-carboxyfluorescein (FAM)-conjugated peptides and western blots containing extracts from osteoblastic cultures. Mass spectroscopic mapping demonstrated that FAM-peptide 3 bound to 90 kDa BSP and its 18 to 60 kDa fragments, as well as to 110 kDa nucleolin. In osteoblastic cultures, FAM-peptide 3 localized to biomineralization foci (site of BSP) and to nucleoli (site of nucleolin). In bone sections, biotin-labeled peptide 3 bound to sites of new bone formation which were co-labeled with anti-BSP antibodies. These results establish the fluorescent peptide 3 conjugate as the first nonantibody-based method to identify BSP on western blots and in/on cells. Further examination of the "6b" splice variant interactions will likely reveal new insights into bone mineralization during development.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Inhibition of vascular calcification by inositol phosphates derivatized with ethylene glycol oligomers

Myo-inositol hexakisphosphate (IP6) is a natural product known to inhibit vascular calcification (VC), but with limited potency and low plasma exposure following bolus administration. Here we report the design of a series of inositol phosphate analogs as crystallization inhibitors, among which 4,6-di-O-(methoxy-diethyleneglycol)-myo-inositol-1,2,3,5-tetrakis(phosphate), (OEG2)2-IP4, displays increased in vitro activity, as well as more favorable pharmacokinetic and safety profiles than IP6 after subcutaneous injection. (OEG2)2-IP4 potently stabilizes calciprotein particle (CPP) growth, consistently demonstrates low micromolar activity in different in vitro models of VC (i.e., human serum, primary cell cultures, and tissue explants), and largely abolishes the development of VC in rodent models, while not causing toxicity related to serum calcium chelation. The data suggest a mechanism of action independent of the etiology of VC, whereby (OEG2)2-IP4 disrupts the nucleation and growth of pathological calcification.

Advances in Plant-Derived Scaffold Proteins

Scaffold proteins form critical biomatrices that support cell adhesion and proliferation for regenerative medicine and drug screening. The increasing demand for such applications urges solutions for cost effective and sustainable supplies of hypoallergenic and biocompatible scaffold proteins. Here, we summarize recent efforts in obtaining plant-derived biosynthetic spider silk analogue and the extracellular matrix protein, collagen. Both proteins are composed of a large number of tandem block repeats, which makes production in bacterial hosts challenging. Furthermore, post-translational modification of collagen is essential for its function which requires co-transformation of multiple copies of human prolyl 4-hydroxylase. We discuss our perspectives on how the GAANTRY system could potentially assist the production of native-sized spider dragline silk proteins and prolyl hydroxylated collagen. The potential of recombinant scaffold proteins in drug delivery and drug discovery is also addressed.

Thermal cycling effect on osteogenic differentiation of MC3T3-E1 cells loaded on 3D-porous Biphasic Calcium Phosphate (BCP) scaffolds for early osteogenesis

The application of heat stress on a defect site during the healing process is a promising technique for early bone regeneration. The primary goal of this study was to investigate the effect of periodic heat shock on bone formation. MC3T3-E1 cells were seeded onto biphasic calcium phosphate (BCP) scaffolds, followed by periodic heating to evaluate osteogenic differentiation. Heat was applied to cells seeded onto scaffolds at 41 °C for 1 h once, twice, and four times a day for seven days and their viability, morphology, and differentiation were analyzed. BCP scaffolds with interconnected porous structures mimic bone biology for cellular studies. MTT and confocal studies have shown that heat shock significantly increased cell proliferation without any toxic effects. Compared to non-heated samples, heat shock enhanced calcium deposition and mineralization, which could be visualized by SEM observation and Alizarin red S staining. Immunostaining images showed the localization of osteogenic proteins ALP and OPN on heat-shocked cells. qRT-PCR analysis revealed the presence of more osteospecific markers, osteopontin (OPN), osteocalcin, collagen type X, and Runx2, in the heat-shocked samples than in the non-heated sample. Periodic heat shock significantly upregulated both heat shock proteins (HSP70 and HSP27) in differentiated MC3T3-E1 cells. The results of this study demonstrated that periodically heat applied especially two times a day was better approach for osteogenic differentiation. Hence, this work provides a define temperature and time schedule for the development of a clinical heating device in future for early bone regeneration during the postsurgical period.

Biocompatibility Studies of Nanoengineered Polycaprolactone and Nanohydroxyapatite Scaffold for Craniomaxillofacial Bone Regeneration

Currently there is an increased demand for synthetic bone substitute materials (SBSMs) due to avoidance of donor-site surgery and morbidity. Attempts are done to create SBSM mimicking the bone microarchitecture for enhanced healing. In this study, the authors nanoengineered polycaprolactone (PCL) and nanohydroxyapatite (nHAp) composite scaffold by electrospinning. The nHAp is synthesized via hydrothermal process followed by microwave irradiation. In vitro biocompatibility evaluation with MG63 osteoblastic cell line showed enhanced cell proliferation in the PCL-nHAp scaffold than plain PCL by MTT assay and fluorescence microscopy. Increased osteogenesis in the PCL-nHAp scaffold was shown by the increased calcium load, alkaline phosphatase activity, and expression of osteogenic biomarkers namely osteocalcin, osteonectin, and osteopontin. In vivo studies conducted in rabbit femur bone defects showed increased bone regeneration in PCL-nHAp implanted defects. The results show that PCL-nHAp electrospun scaffold is biomimetic and highly osteogenic and thus a potential SBSM for critical size craniomaxillofacial bone defect applications.

Phosphorylation of Extracellular Bone Matrix Proteins and Its Contribution to Bone Fragility

Phosphorylation of bone matrix proteins is of fundamental importance to all vertebrates including humans. However, it is currently unknown whether increase or decline of total protein phosphorylation levels, particularly in hypophosphatemia-related osteoporosis, osteomalacia, and rickets, contribute to bone fracture. To address this gap, we combined biochemical measurements with mechanical evaluation of bone to discern fracture characteristics associated with age-related development of skeletal fragility in relation to total phosphorylation levels of bone matrix proteins and one of the key representatives of bone matrix phosphoproteins, osteopontin (OPN). Here for the first time, we report that as people age the total phosphorylation level declines by approximately 20% for bone matrix proteins and approximately 30% for OPN in the ninth decade of human life. Moreover, our results suggest that the decline of total protein phosphorylation of extracellular matrix (ECM) contributes to bone fragility, but less pronouncedly than glycation. We theorize that the separation of two sources of OPN negative charges, acidic backbone amino acids and phosphorylation, would be nature's means of assuring that OPN functions in both energy dissipation and biomineralization. We propose that total phosphorylation decline could be an important contributor to the development of osteoporosis, increased fracture risk and skeletal fragility. Targeting the enzymes kinase FamC20 and bone alkaline phosphatase involved in the regulation of matrix proteins' phosphorylation could be a means for the development of suitable therapeutic treatments. © 2018 American Society for Bone and Mineral Research.

Micro-Nanofibrillar Polycaprolactone Scaffolds as Translatable Osteoconductive Grafts for the Treatment of Musculoskeletal Defects without Infection

The treatment of musculoskeletal defects is currently limited by the tissue-regenerative materials available to orthopedic surgeons: autologous bone grafts only have a finite amount of harvestable material within a given patient, while allografts are prone to severe immunological complications and host rejection. With this motivation, the production of poly(ε-caprolactone) (PCL) scaffolds as synthetic, biomimetic biomaterials was investigated, with a specific focus on potential orthopedic translation. PCL scaffolds were produced through three different fabrication techniques: electrospinning (ES), rotary jet spinning (RJS), and airbrush (AB). ES and RJS were observed to produce microfibrillar scaffolds, while all AB products were nanofibrous. Osteoblast viability, within the PCL scaffolds, and the osteogenic phenotype were assessed in vitro through a combination of adherence, metabolic activity, proliferation, gene expression, alkaline phosphatase bioactivity, and calcium deposition assays. While the polymeric scaffolds induced slight reductions in initial osteoblast adhesion and metabolic activity, seeded cells were able to proliferate and demonstrate the bone formation phenotype. AB products demonstrated reduced bacterial surface colonization when inoculated with both Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacterial strains, in comparison to the microfibrous ES and RJS products, without any small-molecule antibiotics, antimicrobial peptides, or reactive nanomaterials included during scaffold synthesis.

Transformation of amorphous calcium phosphate to bone-like apatite

Mineralisation of calcium phosphates in bone has been proposed to proceed via an initial amorphous precursor phase which transforms into nanocrystalline, carbonated hydroxyapatite. While calcium phosphates have been under intense investigation, the exact steps during the crystallisation of spherical amorphous particles to platelet-like bone apatite are unclear. Herein, we demonstrate a detailed transformation mechanism of amorphous calcium phosphate spherical particles to apatite platelet-like crystals, within the confined nanodomains of a bone-inspired nanocomposite. The transformation is initiated under the presence of humidity, where nanocrystalline areas are formed and crystallisation advances via migration of nanometre sized clusters by forming steps at the growth front. We propose that such transformation is a possible crystallisation mechanism and is characteristic of calcium phosphates from a thermodynamic perspective and might be unrelated to the environment. Our observations provide insight into a crucial but unclear stage in bone mineralisation, the origins of the nanostructured, platelet-like bone apatite crystals.

Structural role of osteocalcin and osteopontin in energy dissipation in bone

Non-collagenous proteins are a vital component of bone matrix. Amongst them, osteocalcin (OC) and osteopontin (OPN) hold special significance due to their intimate interaction with the mineral and collagenous matrix in bone. Both proteins have been associated with microdamage and fracture, but their structural role in energy dissipation is unclear. This study used bone tissue from genetic deficient mice lacking OC and/or OPN and subjected them to a series of creep-fatigue-creep tests. To this end, whole tibiae were loaded in four-point bending to 70% stiffness loss which captured the three characteristic phases of fatigue associated with initiation, propagation, and coalescence of microdamage. Fatigue loading preceded and followed creep tests to determine creep and dampening parameters. Microdamage in the form of linear microcracks and diffuse damage were analyzed by histology. It was shown that OC and OPN were 'activated' following stiffness loss associated with fatigue damage where they facilitated creep and dampening parameters (i.e. increased energy dissipation). More specifically, post-fatigue creep rate and dampening were significantly greater in wild-types (WTs) than genetic deficient mice (p < 0.05). These results were supported by microdamage analysis which showed significant increase in creep-associated diffuse damage formation in WTs compared to genetic deficient groups (p < 0.05). Based on these findings, we propose that during local yield events, OC and OPN rely on ionic interactions of their charged side chains and on hydrogen bonding to dissipate energy in bone.

In Vitro Uptake of Hydroxyapatite Nanoparticles and Their Effect on Osteogenic Differentiation of Human Mesenchymal Stem Cells

There have been many applications in biomedical fields based on hydroxyapatite nanoparticles (HA NPs) over the past decades. However, the biocompatibility of HANPs is affected by exposure dose, particle size, and the way of contact with cells. The objective of this study is to investigate the effect of HA NPs with different sizes on osteogenesis using human mesenchymal stem cells (hMSCs). Three different-sized HA NPs (~50, ~100, and ~150 nm, resp.) were synthesized to study the cytotoxicity, cellular uptake, and effect on osteogenic differentiation of hMSCs. The results clearly showed that each size of HA NPs had dose-dependent cytotoxicity on hMSCs. It was found that HA NPs could be uptaken into hMSCs. The osteogenic differentiation of hMSCs was evaluated through alkaline phosphatase (ALP) activity measurement, ALP staining, immunofluorescent staining for osteopontin (OPN), and real-time polymerase chain reaction (RT-PCR) examination. As expected, HA NPs of all sizes could promote the differentiation of hMSCs towards osteoblast lineage. Among the three sizes, smaller-sized HA NPs (~50 and ~100 nm) appeared to be more effective in stimulating osteogenic differentiation of hMSCs.

Osteogenic stimulation of human dental pulp stem cells with a novel gelatin-hydroxyapatite-tricalcium phosphate scaffold

The aim of the present study was to construct and compare gelatin-HA-TCP scaffolds with a gelatin-only scaffold and to investigate the effect of the scaffold on osteogenic differentiation of human dental pulp stem cells. We developed a novel scaffold for bone tissue engineering via a solution casting/particle washing method, and the physical and mechanical properties of the scaffolds were examined using scanning electron microscopy and a universal testing machine, respectively. Scaffold cytotoxicity toward human dental pulp stem cells (hDPSCs) was evaluated with the CCK8 method, and hDPSC differentiation was evaluated with an alkaline phosphatase activity assay, alizarin red S staining, and reverse transcription-polymerase chain reaction (RT-PCR). Our results indicate that the gelatin-HA-TCP scaffolds exhibited good homogeneity, interconnected pores, and relatively high mechanical strength and water absorption rates. A significant increase in hDPSC proliferation and ALP activity that stimulated mineralization of the hDPSC-generated matrix was also seen on gelatin-HA-TCP scaffolds compared with the gelatin-only scaffolds. In addition, RT-PCR revealed that the gelatin-HA-TCP scaffold upregulated gene expression of the osteogenic markers Runx2, bone sialoprotein, and OSX. In conclusion, gelatin-HA-TCP scaffolds presented better mechanical properties, cytocompatibility and differentiation-inducing characteristics than gelatin scaffolds. These results indicate that the novel hydrogel gelatin-HA-TCP scaffolds may be a promising biomaterial for bone tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1851-1861, 2018.

t-Darpp is an elongated monomer that binds calcium and is phosphorylated by cyclin-dependent kinases 1 and 5

t‐Darpp (truncated isoform of dopamine‐ and cAMP‐regulated phosphoprotein) is a protein encoded by the PPP1R1B gene and is expressed in breast, colon, esophageal, gastric, and prostate cancers, as well as in normal adult brain striatal cells. Overexpression of t‐Darpp in cultured cells leads to increased protein kinase A activity and increased phosphorylation of AKT (protein kinase B). In HER2+ breast cancer cells, t‐Darpp confers resistance to the chemotherapeutic agent trastuzumab. To shed light on t‐Darpp function, we studied its secondary structure, oligomerization status, metal‐binding properties, and phosphorylation by cyclin‐dependent kinases 1 and 5. t‐Darpp exhibits 12% alpha helix, 29% beta strand, 24% beta turn, and 35% random coil structures. It binds calcium, but not other metals commonly found in biological systems. The T39 site, critical for t‐Darpp activation of the AKT signaling pathway, is a substrate for phosphorylation by cyclin‐dependent kinase 1 and cyclin‐dependent kinase 5. Gel filtration chromatography, sedimentation equilibrium analysis, blue native gel electrophoresis, and glutaraldehyde‐mediated cross‐linking experiments demonstrate that the majority of t‐Darpp exists as a monomer, but forms low levels (< 3%) of hetero‐oligomers with its longer isoform Darpp‐32. t‐Darpp has a large Stokes radius of 4.4 nm relative to its mass of 19 kDa, indicating that it has an elongated structure.

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.

Biophysical aspects of biomineralization

During the process of endochondral bone formation, chondrocytes and osteoblasts mineralize their extracellular matrix (ECM) by promoting the synthesis of hydroxyapatite (HA) seed crystals in the sheltered interior of membrane-limited matrix vesicles (MVs). Several lipid and proteins present in the membrane of the MVs mediate the interactions of MVs with the ECM and regulate the initial mineral deposition and posterior propagation. Among the proteins of MV membranes, ion transporters control the availability of phosphate and calcium needed for initial HA deposition. Phosphatases (orphan phosphatase 1, ectonucleotide pyrophosphatase/phosphodiesterase 1 and tissue-nonspecific alkaline phosphatase) play a crucial role in controlling the inorganic pyrophosphate/inorganic phosphate ratio that allows MV-mediated initiation of mineralization. The lipidic microenvironment can help in the nucleation process of first crystals and also plays a crucial physiological role in the function of MV-associated enzymes and transporters (type III sodium-dependent phosphate transporters, annexins and Na⁺/K⁺ ATPase). The whole process is mediated and regulated by the action of several molecules and steps, which make the process complex and highly regulated. Liposomes and proteoliposomes, as models of biological membranes, facilitate the understanding of lipid-protein interactions with emphasis on the properties of physicochemical and biochemical processes. In this review, we discuss the use of proteoliposomes as multiple protein carrier systems intended to mimic the various functions of MVs during the initiation and propagation of mineral growth in the course of biomineralization. We focus on studies applying biophysical tools to characterize the biomimetic models in order to gain an understanding of the importance of lipid-protein and lipid-lipid interfaces throughout the process.

Osteopontin: its potential role in cancer of children and young adults

Objective: Osteopontin (OPN) is aglyco-phosphoprotein, involved in tissue remodeling, inflammation and boneresorption. In various adult neoplasms OPN was shown to correlate with cancer progression, invasiveness and metastasis. Aim: to define the role of OPN in malignancies of children and young adults. Material and methods: a structured PubMed and Google Scholar literature analysis based on reports published in English between I'1995 and XII'2015. Results: 14 studies (four on hematological malignancies, four on bone tumors, three on CNS tumors, two on dendritic proliferative diseases and one on renal tumors) were identified. Higher levels of serum and cerebro-spinal fluid OPN protein, and high expressions of OPN mRNA and SPP1 gene were present in more aggressive and advanced childhood malignancies. In children with acute lymphoblastic leukemia with CNS involvement and with atypical teratoid/rhabdoid tumor (AT/RT) and medulloblastoma, the serum and CSF OPN levels reflected tumor bulk and response to therapy, while in children with AT/RT and multisystem Langerhans cell histiocytosis with high-risk organs involvement, high OPN serum levels correlated with poorer survival. To the contrary, in osteosarcoma, high OPN mRNA and SPP1 gene expressions correlated with better survival and good response to chemotherapy. Conclusions: The literature review suggests that OPN may play important roles in the development and progression of selected cancers of children and young adults, including acute lymphoblastic leukemia, malignant gliomas, AT/RT and Langerhans cell histiocytosis. However, limited number of published studies prevents from definite concluding on the clinical utility of OPN as a marker of diagnosis, prognosis and treatment monitoring in these pediatric cancers. Further studies performed in more numerous groups of patients with particular types of cancers of children and young adults are warranted.

Comprehensive Review on the Use of Graphene-Based Substrates for Regenerative Medicine and Biomedical Devices

Recent research suggests that graphene holds great potential in the biomedical field because of its extraordinary properties. Whereas initial attempts focused on the use of suspended graphene for drug delivery and bioimaging, more recent work has demonstrated its advantages for preparing substrates for tissue engineering and biomedical devices and products. Cells are known to interact with and respond to nanoparticles differently when presented in the form of a substrate than in the form of a suspension. In tissue engineering, a stable and supportive substrate or scaffold is needed to provide mechanical support, chemical stimuli, and biological signals to cells. This review compiles recent advances of the impact of both graphene and graphene-derived particles to prepare supporting substrates for tissue regeneration and devices as well as the associated cell response to multifunctional graphene substrates. We discuss the interaction of cells with pristine graphene, graphene oxide, functionalized graphene, and hybrid graphene particles in the form of coatings and composites. Such materials show excellent biological outcomes in vitro, in particular, for orthopedic and neural tissue engineering applications. Preliminary evaluation of these graphene-based materials in vivo reinforces their promise for tissue regeneration and implants. Although the reported findings of studies on graphene-based substrates are promising, several questions and concerns associated with their in vivo use persist. Possible strategies to examine these issues are presented.

Comparison of tissue transglutaminase 2 and bone biological markers osteocalcin, osteopontin and sclerostin expression in human osteoporosis and osteoarthritis

Osteoporosis (OP) and osteoarthritis (OA) are the most common joint diseases, with a high incidence in the elderly population. OP is characterized by trabecular bone remodeling and reabsorption, whereas articular cartilage and subchondral bone remodeling are major features of OA. Although classically considered as independent or even conflicting processes, clinical coexistence of OP and OA was recently described. Transglutaminase 2 (TG2) expression is considered a biomarker of OA, but its role in osteoporotic bone remodeling is still uncertain. We investigated TG2 and bone biological markers (Osteocalcin, Osteopontin, and Sclerostin) in osteoporotic and osteoarthritic osteocartilagineous tissue (n = 54) and human chondrocyte cultures in vitro by immunohistochemistry, immunofluorescence and RT-PCR. Histomorphometric evaluation of bone trabecular remodeling was also performed. In cartilage, TG2 expression was faint in control and OP and significantly less than in OA and OP + OA chondrocytes; the opposite was found for Osteocalcin, whereas Osteopontin and Sclerostin expression was similar. In the subchondral trabecular bone, osteocytes/osteoblasts TG2 expression was slight and similar comparing control, OP, OA, and OP + OA group, whereas Osteocalcin and Osteopontin expression was lower in OP compared to control, OA and OP + OA. Increased TG2 and reduced Osteocalcin expression were maintained in human osteoarthritic chondrocytes in vitro. Histomorphometric analysis confirmed reduced trabecular bone mass in OP and OP + OA compared with OA patients. TG2 represented a suitable biomarker of osteoarthritic chondrocyte activation, whereas osteocalcin and osteopontin characterized osteoporotic osteocyte/osteoblast changes; differences were lost in OP + OA patients, suggesting careful consideration when coexistence of the two diseases occurs.

Osteopontin protects against high phosphate-induced nephrocalcinosis and vascular calcification

Pathologic calcification is a significant cause of increased morbidity and mortality in patients with chronic kidney disease. The precise mechanisms of ectopic calcification are not fully elucidated, but it is known to be caused by an imbalance of procalcific and anticalcific factors. In the chronic kidney disease population, an elevated phosphate burden is both highly prevalent and a known risk factor for ectopic calcification. Here we tested whether osteopontin, an inhibitor of calcification, protects against high phosphate load-induced nephrocalcinosis and vascular calcification. Osteopontin knockout mice were placed on a high phosphate diet for 11 weeks. Osteopontin deficiency together with phosphate overload caused uremia, nephrocalcinosis characterized by substantial renal tubular and interstitial calcium deposition, and marked vascular calcification when compared with control mice. Although the osteopontin-deficient mice did not exhibit hypercalcemia or hyperphosphatemia, they did show abnormalities in the mineral metabolism hormone fibroblast growth factor-23. Thus, endogenous osteopontin plays a critical role in the prevention of phosphate-induced nephrocalcinosis and vascular calcification in response to high phosphate load. A better understanding of osteopontin's role in phosphate-induced calcification will hopefully lead to better biomarkers and therapies for this disease, especially in patients with chronic kidney disease and other at-risk populations.

A potential mechanism for amino acid-controlled crystal growth of hydroxyapatite

The mineral component of bone, dentin and calcified parts of avian tendon, hydroxyapatite (HAP), has non-stoichiometric composition (idealized as Ca₁₀(PO₄)₆(OH)₂), plate-like morphology and nanometer size. This unique crystal morphology contributes to the physico-chemical and biochemical properties of bone. Thus, understanding the mechanism for the controlled growth of plate-like HAP nanocrystals is significant in the study of bone biomineralization. Previous studies have shown that acidic non-collagenous proteins (ANCPs), which are enriched in the residues of acidic amino acids, may play an important role in HAP crystal growth modulation. In this study, glutamic acid (Glu) and phosphoserine (Ser-OPO₃) were used as model compounds to modify the synthesis of HAP nanocrystals. To identify the mechanisms of amino acids as regulators, X-ray diffraction (XRD), transmission electron microscopy (TEM) and solid state nuclear magnetic resonance (ssNMR) were used. The crystals obtained in the inorganic controls were needle-like, while crystals synthesized in the presence of the amino acids presented a plate-like morphology. The plate-like crystals had a preferred crystal orientation on (300) face, which was lacking in the inorganically grown crystals, indicating preferential adsorption and suppression of growth in specific crystal directions. Ser-OPO₃ was more efficient than Glu in modulating HAP nucleation and crystal growth. Furthermore, NMR revealed interactions between the charged side chain groups in amino acids and the crystal surfaces. These results were successfully explained through our MD simulations for the free energy calculation of amino acid binding on HAP crystal faces. The present study revealed that amino acids may act as effective regulators of HAP morphology without the need to invoke large NCPs in bone biomineralization and in designing bioinspired materials for orthopaedic and dental applications.

Calcium ion binding properties and the effect of phosphorylation on the intrinsically disordered Starmaker protein

Starmaker (Stm) is an intrinsically disordered protein (IDP) involved in otolith biomineralization in Danio rerio. Stm controls calcium carbonate crystal formation in vivo and in vitro. Phosphorylation of Stm affects its biomineralization properties. This study examined the effects of calcium ions and phosphorylation on the structure of Stm. We have shown that CK2 kinase phosphorylates 25 or 26 residues in Stm. Furthermore, we have demonstrated that Stm's affinity for calcium binding is dependent on its phosphorylation state. Phosphorylated Stm (StmP) has an estimated 30 ± 1 calcium binding sites per protein molecule with a dissociation constant (KD) of 61 ± 4 μM, while the unphosphorylated protein has 28 ± 3 sites and a KD of 210 ± 22 μM. Calcium ion binding induces a compaction of the Stm molecule, causing a significant decrease in its hydrodynamic radius and the formation of a secondary structure. The screening effect of Na(+) ions on calcium binding was also observed. Analysis of the hydrodynamic properties of Stm and StmP showed that Stm and StmP molecules adopt the structure of native coil-like proteins.

Expression and regulation of osteopontin in chronic rhinosinusitis with nasal polyps

BACKGROUND

Osteopontin (OPN) has been proposed to be associated with airway inflammation including asthma and chronic rhinosinusitis with nasal polyps (CRSwNP). This study sought to evaluate the expression and regulation of the OPN in CRSwNP patients.

METHODS

Nasal polyp (NP) tissues and normal tissues were collected from 30 CRSwNP patients and 16 control subjects. The expression and regulation of OPN, as well eosinophil (EOS) accumulation and activation, were examined in nasal tissues using DNA microarray, immunohistochemical (IHC), immunofluorescent (IF) staining and qPCR analysis. Moreover, the regulation of OPN in nasal epithelial cells and its effects on migration and activation of EOS were evaluated in vitro using flow cytometry, ELISA, qPCR and Western blot, etc.

RESULTS

DNA microarray analysis identified OPN as one of the 19 upregulated genes in polyp tissues. The mean number of OPN(+) cells in polyp tissues was found to be significantly increased compared with the normal controls (P < 0.01), and OPN(+) cells in polyp tissues significantly correlated with tissue eosinophilia [major basic protein (MBP)(+) cells; r = 0.51, P < 0.01]. Accordingly, the mRNA and protein levels of OPN in NP tissues were significantly higher than those in normal controls (P < 0.01). Poly I:C, flagellin and TLR-9 agonist CpG ODN, as well as TNF-α, IFN-γ, IL-6, IL-17A and TGF-β, significantly increased OPN mRNA expression in cultured PECs and NECs (P < 0.05). Recombinant human OPN significantly promoted the migration of EOS, as well as enhanced EOS cationic protein (ECP) production, in an in vitro dispersed NP cells (DNPCs) culture system (P < 0.05).

CONCLUSIONS

OPN promotes eosinophilic nasal inflammation in CRSwNP patients, which may represent a promising therapeutic target.

Potential of an amelogenin based peptide in promoting reminerlization of initial enamel caries

OBJECTIVE

In this study we give a preliminary study of a rationally designed small peptide, which is based on the enamel matrix protein amelogenin, to investigate its effect on remineralization of initial enamel caries lesions.

DESIGN

A novel peptide was designed and synthesized to investigate its effects on the remineralization of initial enamel carious lesions during pH cycling that simulates intra-oral conditions. Initial lesions were created in bovine enamel blocks, which were then pH-cycled for 12 days in the presence of 25μM peptide, 1g/L NaF (positive control), 50mM HEPES buffer(negative control). Before and after pH cycling, enamel blocks were analyzed by surface microhardness testing, polarized light microscopy and transverse microradiography.

RESULTS

Percentage of surface microhardness recovery (SMHR%) after pH cycling was significantly higher in peptide group than HEPES group. Lower lesion depth and less mineral mineral loss were found in peptide or NaF treatment groups after the cycling, and were significantly different to HEPES group. No significant differences were observed between the blocks treated with peptide and those treated with NaF.

CONCLUSSION

This study provides in vitro evidence that this amelogenin based peptide promotes enamel caries remineralization, offering a promising remineralizing biomaterial in initial enamel carious treatment.

Current Progress in Anticalcif ication for Bioprosthetic and Polymeric Heart Valves

The use of bioprosthetic valves fabricated from fixed heterograft tissue (porcine aortic valves or bovine pericardium) in heart valve replacement surgery is limited because of calcification-related failures. The mechanism of calcification of bioprosthetic valves is quite complex and has a variety of determinants, including host factors, tissue fixation conditions, and mechanical effects. Currently, there is no effective therapy to prevent calcification in clinical settings. This article reviews a variety of anticalcification strategies that are under investigation either in advanced animal models or in clinical trials. Bisphosphonates, such as ethan hydroxybisphosphonate (EHBP), inhibit calcium phosphate crystal formation. However, because of their systemic toxicity, they are used as either tissue treatments or polymeric site-specific delivery systems. Detergent treatment, such as sodium dodecyl sulfate (SDS), extracts almost all phospholipids from bioprosthetic heart valve cuspal tissue. Procedures, such as amino oleic acid pretreatment, inhibit calcium uptake. Polyurethane trileaflet valves, investigated as alternatives to bioprosthetic or mechanical valve prostheses, undergo intrinsic and thrombus-related calcification and degradation. Calcification- and thrombus-resistant polyurethanes synthesized in our laboratory by covalent linking of EHBP or heparin (either in bulk or on surface) by unique polyepoxidation chemistry are attractive candidates for further research. Tissue-engineered heart valves may have an important place in the future.

Effect of Local Sustainable Release of BMP2-VEGF from Nano-Cellulose Loaded in Sponge Biphasic Calcium Phosphate on Bone Regeneration

Bone regeneration is a coordinated process mainly regulated by multiple growth factors. Vascular endothelial growth factor (VEGF) stimulates angiogenesis and bone morphogenetic proteins (BMPs) induce osteogenesis during bone healing process. The aim of this study was to investigate how these growth factors released locally and sustainably from nano-cellulose (NC) simultaneously effect bone formation. A biphasic calcium phosphate (BCP)-NC-BMP2-VEGF (BNBV) scaffold was fabricated for this purpose. The sponge BCP scaffold was prepared by replica method and then loaded with 0.5% NC containing BMP2-VEGF. Growth factors were released from NC in a sustainable manner from 1 to 30 days. BNBV scaffolds showed higher cell attachment and proliferation behavior than the other scaffolds loaded with single growth factors. Bare BCP scaffolds and BNBV scaffolds seeded with rat bone marrow mesenchymal stem cells were implanted ectopically and orthotopically in nude mice for 4 weeks. No typical bone formation was exhibited in BNBV scaffolds in ectopic sites. BMP2 and VEGF showed positive effects on new bone formation in BNBV scaffolds, with and without seeded stem cells, in the orthotopic defects. This study demonstrated that the BNBV scaffold could be beneficial for improved bone regeneration. Stem cell incorporation into this scaffold could further enhance the bone healing process.

Differential influence of fluoride concentration on the synthesis of bone matrix glycoproteins within mineralizing bone cells in vitro

OBJECTIVE

This study investigated the influence of fluoride levels on the temporal synthesis of bone-associated glycoproteins, which have been assigned prominent roles in regulating crystal growth, size and shape during the mineralization process.

MATERIALS AND METHODS

Bone marrow stromal cells were isolated from male Wistar rats and cultured under mineralizing conditions, supplemented with 0 M, 10(-7) M or 10(-5) M sodium fluoride. The presence of bone-associated glycoproteins was examined 2-13 days post-reseeding by immunocytochemical localization. Results: All bone-associated glycoproteins increased in 10(-7) M fluoride, compared to untreated controls, particularly at days 6 and 13 in culture. Conversely, higher 10(-5) M fluoride concentrations decreased glycoprotein levels, compared to controls.

CONCLUSIONS

Results highlight a differential effect of fluoride concentration on glycoprotein synthesis by osteoblasts.

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.

Characterization of UO2(2+) binding to osteopontin, a highly phosphorylated protein: insights into potential mechanisms of uranyl accumulation in bones

Bones are one of the few organs in which uranyl (UO2(2+)) accumulates. This large dioxo-cation displays affinity for carboxylates, phenolates and phosphorylated functional groups in proteins. The noncollagenous protein osteopontin (OPN) plays an important role in bone homeostasis. It is mainly found in the extracellular matrix of mineralized tissues but also in body fluids such as milk, blood and urine. Furthermore, OPN is an intrinsically disordered protein, which, like other proteins of the SIBLING family, contains a polyaspartic acid sequence and numerous patterns of alternating acidic and phosphorylated residues. All these properties led to the hypothesis that this protein could be prone to UO2(2+) binding. In this work, a simple purification procedure enabling highly purified bovine (bOPN) and human OPN (hOPN) to be obtained was developed. Various biophysical approaches were set up to study the impact of phosphorylations on the affinity of OPN for UO2(2+) as well as the formation of stable complexes originating from structural changes induced by the binding of this metal cation. The results obtained suggest a new mechanism of the interaction of UO2(2+) with bone metabolism and a new role for OPN as a metal transporter.

Gelatin functionalized graphene oxide for mineralization of hydroxyapatite: biomimetic and in vitro evaluation

We report a facile modification of graphene oxide (GO) by gelatin to mimic charged proteins present in the extracellular matrix during bone formation. The bioinspired surface of GO-gelatin (GO-Gel) composite was used for biomimetic mineralization of hydroxyapatite (HA). A detailed structural and morphological characterization of the mineralized composite was performed. Additionally, MC3T3-E1 cells were cultured on the GO-Gel surfaces to observe various cellular activities and HA mineralization. Higher cellular activities such as cell adhesion, cell proliferation, and alkaline phosphatase activity (ALP) were observed on the GO-Gel surface compared with the GO or glass surface. The increase of ALP confirms that the proposed GO-Gel promotes the osteogenic differentiation of MC3T3-E1 cells. Moreover, the evidence of mineralization evaluated by scanning electron microscopy (SEM) and alizarin red staining (ARS) corroborate the idea that a native osteoid matrix is ultimately deposited. All these data suggest that the GO-Gel hybrids will have great potential as osteogenesis promoting scaffolds for successful application in bone surgery.

Biomimetic and cell-mediated mineralization of hydroxyapatite by carrageenan functionalized graphene oxide

In bone tissue engineering, it is imperative to design multifunctional biomaterials that can induce and assemble bonelike apatite that is close to natural bone. In this study, graphene oxide (GO) was functionalized by carrageenan. The resulting GO-carrageenan (GO-Car) composite was further used as a substrate for biomimetic and cell-mediated mineralization of hydroxyapatite (HA). It was confirmed that carrageenan on the GO surface facilitated the nucleation of HA. The observation of the effect of the GO-Car on the adhesion, morphology, and proliferation of MC3T3-E1 cells was investigated. In vitro studies clearly show the effectiveness of GO-Car in promoting HA mineralization and cell differentiation. The results of this study suggested that the GO-Car hybrid will be a promising material for bone regeneration and implantation.

Functionalization of biomaterials with small osteoinductive moieties

Human mesenchymal stem cells (MSCs) are currently recognized as a powerful cell source for regenerative medicine, notably for their capacity to differentiate into multiple cell types. The combination of MSCs with biomaterials functionalized with instructive cues can be used as a strategy to direct specific lineage commitment, and can thus improve the therapeutic efficacy of these cells. In terms of biomaterial design, one common approach is the functionalization of materials with ligands capable of directly binding to cell receptors and trigger specific differentiation signaling pathways. Other strategies focus on the use of moieties that have an indirect effect, acting, for example, as sequesters of bioactive ligands present in the extracellular milieu that, in turn, will interact with cells. Compared with complex biomolecules, the use of simple compounds, such as chemical moieties and peptides, and other small molecules can be advantageous by leading to less expensive and easily tunable biomaterial formulations. This review describes different strategies that have been used to promote substrate-mediated guidance of osteogenic differentiation of immature osteoblasts, osteoprogenitors and MSCs, through chemically conjugated small moieties, both in two- and three-dimensional set-ups. In each case, the selected moiety, the coupling strategy and the main findings of the study were highlighted. The latest advances and future perspectives in the field are also discussed.

Association of calcium and phosphate ions with collagen in the mineralization of vertebrate tissues

Among the vertebrate species, collagen is the most abundant protein and is associated with mineralization of their skeleton and dentition in all tissues except enamel. In such tissues, bones, calcifying tendon, dentin, and cementum are comprised principally of type I collagen, which has been proposed as a template for apatite mineral formation. Recent considerations of the interaction between type I collagen and calcium and phosphate ions as the major constituents of apatite have suggested that collagen polypeptide stereochemistry underlies binding of these ions at sites within collagen hole and overlap regions and leads to nucleation of crystals. The concept is fundamental to understanding both normal and abnormal mineralization, and it is reviewed in this article. Given this background, avenues for additional research studies in vertebrate mineralization will also be described. The latter include, for instance, how mineralization events subsequent to nucleation, that is, crystal growth and development, occur and whether they, too, are directed by collagen stereochemical parameters; whether mineralization can be expected in all spaces between collagen molecules; whether the side chains of charged amino acid residues actually point toward and into the hole and overlap collagen spaces to provide putative binding sites for calcium and phosphate ions; and what phenomena may be responsible for mineralization beyond hole and overlap zones and into extracellular tissue regions between collagen structural units. These questions will be discussed to provide a broader understanding of collagen contributions to potential mechanisms of vertebrate mineralization.

Osteopontin in bone mineral density of very old Brazilians

Recent evidence suggests that changes in plasma levels of osteopontin (OPN) may be a promising marker for early diagnosis of bone disorders. We hypothesized that a frequent OPN gene polymorphism may be useful for identifying very old individuals with alterations in plasma OPN levels and consequently at risk of abnormal bone density scores. Men and women (80 years or older) living in the Brazilian Federal District underwent assessments with dual energy X-ray absorptiometry for bone mineral density (BMD) of the femoral neck, femoral head and lumbosacral (L1 to S5) regions. Clinical inspection was performed to assess other physical traits and to exclude co-morbidities (cardiovascular, autoimmunity, infections or neoplastic diseases). Serum concentrations of OPN were determined with an enzyme-linked immunosorbent assay, whereas the A7385G polymorphism (rs1126772) was determined by direct sequencing of a polymerase chain reaction product. Among the two hundred and ten subjects enrolled, no differential scores for bone mineral density could be observed across genotypes, but a greater content of circulating OPN was found among carriers of the A allele (P ≤ 0.05) even after adjustments. Serum OPN levels were negatively correlated with femoral neck density (P = 0.050 for BMD; P = 0.032 for T scores) but not with scores of the other regions investigated. Analyses with the sample dichotomized to age and body mass revealed that this inverse relationship was noticeable only among those aged within the highest and weighing within the lowest intervals. Our findings indicate elevated circulating osteopontin levels in patients with decreased bone mineral density, consistent with a modest contribution of an OPN allelic variation to its expression. Assessing the clinical relevance of our findings demands forthcoming studies.

Chrysanthemum zawadskii extract protects osteoblastic cells from highly reducing sugar-induced oxidative damage

In this study, Chrysanthemum zawadskii extract (CZE) was investigated to determine its effects on 2-deoxy-D-ribose (dRib)-induced oxidative damage and cellular dysfunction in the MC3T3-E1 mouse osteoblastic cell line. Osteoblastic cells were treated with the highly reducing sugar, dRib, in the presence or absence of CZE. Cell viability, apoptosis and reactive oxygen species (ROS) production were subsequently examined. It was observed that dRib reduced cell survival, while it markedly increased the intracellular levels of ROS and apoptosis. However, pre-treatment of the cells with CZE attenuated all the dRib-induced effects. The antioxidant, N-acetyl-L-cysteine (NAC), also prevented dRib-induced oxidative cell damage. In addition, treatment with CZE resulted in a significant increase in alkaline phosphatase (ALP) activity and collagen content, as well as in the expression of genes associated with osteoblast differentiation [ALP, collagen, osteopontin (OPN), osteoprotegerin (OPG), bone sialoprotein (BSP), osteocalcin (OC) and bone morphogenetic protein (BMP)2, BMP4 and BMP7]. In mechanistic studies of the antioxidative potential of CZE, we found that CZE reversed the dRib-induced decrease in the expression of phosphatidylinositol 3-kinase (PI3K) and protein kinase B (AKT)1 and AKT2 genes, which are master regulators of survival-related signaling pathways. CZE also upregulated the gene expression of the antioxidant enzymes, superoxide dismutase (SOD)2, SOD3 and glutathione peroxidase 4 (GPx4), which was inhibited by dRib. Taken together, these results suggest that CZE attenuates dRib-induced cell damage in osteoblastic cells and may be useful for the treatment of diabetes-associated bone disease.