The three-dimensional structure of bovine calcium ion-bound osteocalcin using 1H NMR spectroscopy

Osteocalcin improves glucose tolerance, insulin sensitivity and secretion in older male mice

Osteocalcin deficient mice (OC-/-), on a mixed 129/BL6J background, were reported to show glucose intolerance, insulin insensitivity and reduced insulin secretion at 1-6 mos of age. This is controversial as two studies in OC-/- mice on different backgrounds (C3H/BL6 (5-6 mos.) and C57BL/6N (5 and 9 mos.)) found no effect on glucose metabolism. To determine the role of OC in glucose metabolism we conducted glucose tolerance tests (GTT), insulin tolerances tests (ITT) and glucose stimulated insulin secretion (GSIS) on 6 and 9.5 month-old male OC-/- and OC+/+ mice on a pure C57BL/6J background and fed a normal chow diet. All results were analyzed with a two-way repeated measures ANOVA. The GTT results showed no effect on males at 6 months of age but glucose intolerance was significantly increased (p < 0.05) in male OC-/- mice at 9.5 months of age. The ITT results indicated significantly increased insulin resistance in male OC-/- mice. Glucose stimulated insulin secretion (GSIS) showed insulin significantly (p < 0.05) reduced in OC-/- at several time points. Mouse Osteocalcin injected into OC-/- mice decreased the glucose level. Our results confirm the role of OC in glucose metabolism and insulin sensitivity and demonstrate a role in insulin secretion in older male mice on a C57BL/6J background. Differences in background, age, or experimental procedures could explain controversial results. A delayed onset of the effect of OC on glucose metabolism at 9.5 months in male C57BL/6J mice highlights the importance of background on phenotype. Consideration of genetic background and age may be beneficial for human studies on osteocalcin and glucose homeostasis and may be relevant to the elderly where osteocalcin is reduced.

Osteocalcin: Promoter or Inhibitor of Hydroxyapatite Growth?

Osteocalcin is the most abundant noncollagenous bone protein and the functions in bone remineralization as well as in inhibition of bone growth have remained unclear. In this contribution, we explain the dual role of osteocalcin in the nucleation of new calcium phosphate during bone remodeling and in the inhibition of hydroxyapatite crystal growth at the molecular scale. The mechanism was derived using pH-resolved all-atom models for the protein, phosphate species, and hydroxyapatite, along with molecular dynamics simulations and experimental and clinical observations. Osteocalcin binds to (hkl) hydroxyapatite surfaces through multiple residues, identified in this work, and the fingerprint of binding residues varies as a function of the (hkl) crystal facet and pH value. On balance, the affinity of osteocalcin to hydroxyapatite slows down crystal growth. The unique tricalcium γ-carboxylglutamic acid (Gla) domain hereby rarely adsorbs to hydroxyapatite surfaces and faces instead toward the solution. The Gla domain enables prenucleation of calcium phosphate for new bone formation at a slightly acidic pH of 5. The growth of prenucleation clusters of calcium phosphate continues upon increase in pH value from 5 to 7 and is much less favorable, or not observed, on the native osteocalcin structure at and above neutral pH values of 7. The results provide mechanistic insight into the early stages of bone remodeling from the molecular scale, help inform mutations of osteocalcin to modify binding to apatites, support drug design, and guide toward potential cures for osteoporosis and hyperosteogeny.

Solution NMR structure of cementum protein 1 derived peptide (CEMP1-p1) and its role in the mineralization process

We report the characterization of the three-dimensional structure of the CEMP1-p1 peptide [MGTSSTDSQQAQHRRCSTSN: corresponding to residues 1-20 of the N-terminus of cementum protein 1 (CEMP1)]. This peptide imitates the capacity of CEMP1 to stimulate hydroxyapatite (HA) crystal nucleation and growth, and promotes the differentiation of periodontal ligament cells into a cementoblastic phenotype. Additionally, in experimental models of critical-sized calvarial defects in Wistar rats, CEMP1-p1 has shown osteogenic properties that enhanced the physiological deposition and maturation of newly formed bone. In this work, studies of CEMP1-p1 by circular dichroism (CD) and nuclear magnetic resonance (NMR) were performed in trifluoroethanol D2 (TFED2) and aqueous solution to determine the 3D structure of the peptide. Using the 3D model, experimental data from HA crystals formation and calcium fluorescence emission, we explain the biological mechanisms involved in CEMP1-p1 activity to promote calcium recruitment and its affinity to HA crystals. This information is valuable because it proposes, for the first time, a plausible molecular mechanism during the mineralization process, from a specific cementum protein-derived peptide.

Lead induced differences in bone properties in osteocalcin +/+ and −/− female mice

Lead (Pb) toxicity is a major health problem and bone is the major reservoir. Lead is detrimental to bone, affects bone remodeling and is associated with elderly fractures. Osteocalcin (OC) affects bone remodeling, improves fracture resistance and decreases with age and in some diseases. The effect of lead in osteocalcin depleted bone is unknown and of interest. We compared bone mineral properties of control and Pb exposed (from 2 to 6 months) femora from female adult C57BL6 OC+/+ and OC-/- mice using Fourier Transform Infrared Imaging (FTIRI), Micro-computed tomography (uCT), bone biomechanical measurements and serum turnover markers (P1NP, CTX). Lead significantly increased turnover in OC+/+ and in OC-/- bones producing increased total volume, area and marrow area/total area with decreased BV/TV compared to controls. The increased turnover decreased mineral/matrix vs. Oc+/+ and increased mineral/matrix and crystallinity vs. OC-/-. PbOC-/- had increased bone formation, cross-sectional area (Imin) and decreased collagen maturity compared OC-/- and PbOC+/+. Imbalanced turnover in PbOC-/- confirmed the role of osteocalcin as a coupler of formation and resorption. Bone strength and stiffness were reduced in OC-/- and PbOC-/- due to reduced material properties vs. OC+/+ and PbOC+/+ respectively. The PbOC-/- bones had increased area to compensate for weaker material properties but were not proportionally stronger for increased size. However, at low lead levels osteocalcin plays the major role in bone strength suggesting increased fracture risk in low Pb²⁺ exposed elderly could be due to reduced osteocalcin as well. Years of low lead exposure or higher blood lead levels may have an additional effect on bone strength.

Structural role of osteocalcin and its modification in bone fracture

Osteocalcin (OC), an abundant non-collagenous protein in bone extracellular matrix, plays a vital role in both its biological and mechanical function. OC undergoes post-translational modification, such as glycation; however, it remains unknown whether glycation of OC affects bone's resistance to fracture. Here, for the first time, we demonstrate the formation of pentosidine, an advanced glycation end-product (AGE) cross-link on mouse OC analyzed by ultra-performance liquid chromatography. Next, we establish that the presence of OC in mouse bone matrix is associated with lower interlamellar separation (distance) and thicker bridges spanning the lamellae, both of which are critical for maintaining bone's structural integrity. Furthermore, to determine the impact of modification of OC by glycation on bone toughness, we glycated bone samples in vitro from wild-type (WT) and osteocalcin deficient (Oc-/-) mice, and compared the differences in total fluorescent AGEs and fracture toughness between the Oc -/- glycated and control mouse bones and the WT glycated and control mouse bones. We determined that glycation resulted in significantly higher AGEs in WT compared to Oc-/- mouse bones (delta-WT > delta-OC, p = 0.025). This observed change corresponded to a significant decrease in fracture toughness between WT and Oc-/- mice (delta-WT vs delta-OC, p = 0.018). Thus, we propose a molecular deformation and fracture mechanics model that corroborates our experimental findings and provides evidence to support a 37%-90% loss in energy dissipation of OC due to formation of pentosidine cross-link by glycation. We anticipate that our study will aid in elucidating the effects of a major non-collagenous bone matrix protein, osteocalcin, and its modifications on bone fragility and help identify potential therapeutic targets for maintaining skeletal health.

Sequential Release of Panax Notoginseng Saponins and Osteopractic Total Flavone from Poly (L-Lactic Acid) Scaffold for Treating Glucocorticoid-Associated Osteonecrosis of Femoral Head

Glucocorticoids inhibit angiogenesis in the femoral head, which fails to nourish the bone tissue and leads to osteonecrosis. Restoring angiogenesis is not only essential for vessel formation, but also crucial for osteogenesis. Poly (L-lactic acid) (PLLA) is commonly used in the bone tissue engineering field. Panax notoginseng saponins (PNS) and osteopractic total flavone (OTF) promote angiogenesis and osteogenesis, respectively. We designed a sequentially releasing PLLA scaffold including PLLA loaded with OTF (inner layer) and PLLA loaded with PNS (outer layer). We assessed the osteogenic effect of angiogenesis in this scaffold by comparing it with the one-layered scaffold (PLLA embedded with OTF and PNS) in vivo. Results from the micro-CT showed that the data of bone mineral density (BMD), bone volume (BV), and percent bone volume (BV/TV) in the PO-PP group were significantly higher than those in the POP group (p < 0.01). Histological analyses show that the PO-PP scaffold exhibits better angiogenic and osteogenic effects compared with the one-layered scaffold. These might result from the different structures between them, where the sequential release of a bi-layer scaffold achieves the osteogenic effect of vascularization by initially releasing PNS in the outer layer. We further explored the possible mechanism by an immunohistochemistry analysis and an immunofluorescence assay. The results showed that the protein expressions of vascular endothelial growth factor (VEGF) and platelet endothelial cell adhesion molecule-1(CD31) in the PO-PP scaffold were significantly higher than those in the POP scaffold (p < 0.01); the protein expressions of osteocalcin (OCN), osteopontin (OPN), and alkaline phosphatase (ALP) in the PO-PP scaffold were significantly higher than those in the POP scaffold (p < 0.05). Upregulating the expressions of angiogenic and osteogenic proteins might be the possible mechanism.

Atomic Details of Biomineralization Proteins Inspiring Protein Design and Reengineering for Functional Biominerals

Biominerals are extraordinary materials that provide organisms with a variety of functions to support life. The synthesis of biominerals and organization at the macroscopic level is a consequence of the interactions of these materials with proteins. The association of biominerals and proteins is very ancient and has sparked a wealth of research across biological, medical and material sciences. Calcium carbonate, hydroxyapatite, and silica represent widespread natural biominerals. The atomic details of the interface between macromolecules and these biominerals is very intriguing from a chemical perspective, considering the association of chemical entities that are structurally different. With this review I provide an overview of the available structural studies of biomineralization proteins, explored from the Protein Data Bank (wwPDB) archive and scientific literature, and of how these studies are inspiring the design and engineering of proteins able to synthesize novel biominerals. The progression of this review from classical template proteins to silica polymerization seeks to benefit researchers involved in various interdisciplinary aspects of a biomineralization project, who need background information and a quick update on advances in the field. Lessons learned from structural studies are exemplary and will guide new projects for the imaging of new hybrid biomineral/protein superstructures at the atomic level.

Undercarboxylated, but not Carboxylated, Osteocalcin suppresses TNF-α induced inflammatory signaling pathway in Myoblast

Undercarboxylated osteocalcin (ucOCN) has been considered to be an important endocrine factor, especially to regulate bone and energy metabolism. Even with the mounting evidence showing the consistent inverse correlation of ucOCN levels in chronic inflammatory diseases, however, the mechanism underlying the involvement of ucOCN in the muscular inflammation has not been fully understood. In the present study, we explored 1) the endocrine role of ucOCN in the regulation of inflammation in C2C12 myoblasts and primary myoblasts and the underlying intracellular signaling mechanisms, and 2) whether G protein–coupled receptor family C group 6 member A (GPRC6A) is the ucOCN-sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts. ucOCN suppressed the tumor necrosis factor-α (TNF-α)–induced expressions of major inflammatory cytokines, including interleukin-1β (IL-1β) and inhibited the TNF-α–stimulated activities of transcription factors, including NF-κB, in C2C12 and primary myoblasts. Both knockdown and knockout of GPRC6A, by using siRNA or a CRISPR/CAS9 system, respectively, did not reverse the effect of ucOCN on IL-1β expression in myoblasts. Interestingly, TNF-α–induced IL-1β expression was inhibited by knockdown or deletion of GPRC6A itself, regardless of the ucOCN treatment. ucOCN was rapidly internalized into the cytoplasmic region via caveolae-mediated endocytosis, suggesting the presence of new target proteins in the cell membrane and/or in the cytoplasm for interaction with ucOCN in myoblasts. Taken together, these findings indicate that ucOCN suppresses the TNF-α–induced inflammatory signaling pathway in myoblasts. GPRC6A is not a sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts.

Vitamin K-Dependent Protein Activation: Normal Gamma-Glutamyl Carboxylation and Disruption in Disease

Vitamin K-dependent (VKD) proteins undergo an unusual post-translational modification, which is the conversion of specific Glu residues to carboxylated Glu (Gla). Gla generation is required for the activation of VKD proteins, and occurs in the endoplasmic reticulum during their secretion to either the cell surface or from the cell. The gamma-glutamyl carboxylase produces Gla using reduced vitamin K, which becomes oxygenated to vitamin K epoxide. Reduced vitamin K is then regenerated by a vitamin K oxidoreductase (VKORC1), and this interconversion of oxygenated and reduced vitamin K is referred to as the vitamin K cycle. Many of the VKD proteins support hemostasis, which is suppressed during therapy with warfarin that inhibits VKORC1 activity. VKD proteins also impact a broad range of physiologies beyond hemostasis, which includes regulation of calcification, apoptosis, complement, growth control, signal transduction and angiogenesis. The review covers the roles of VKD proteins, how they become activated, and how disruption of carboxylation can lead to disease. VKD proteins contain clusters of Gla residues that form a calcium-binding module important for activity, and carboxylase processivity allows the generation of multiple Glas. The review discusses how impaired carboxylase processivity results in the pseudoxanthoma elasticum-like disease.

The carboxylation status of osteocalcin has important consequences for its structure and dynamics

BACKGROUND

The carboxylation status of Osteocalcin (Ocn) not only influences formation and structure in bones but also has important endocrine functions affecting energy metabolism and expenditure. In this study, the role of γ-carboxylation of the glutamate residues in the structure-dynamics-function relationship in Ocn is investigated.

METHODS

Three forms of Ocn, differentially carboxylated at the Glu-17, 21 and 24 residues, along with a mutated form of Ocn carrying Glu/Ala mutations, are modeled and simulated using molecular dynamics (MD) simulation in the presence of calcium ions.

RESULTS

Characterization of the global conformational dynamics of Ocn, described in terms of the orientational variations within its 3-helical domain, highlights large structural variations in the non-carboxylated osteocalcin (nOcn). The bi-carboxylated Ocn (bOcn) and tri-carboxylated (tOcn) species, in contrast, display relatively rigid tertiary structures, with the dynamics of most regions strongly correlated. Radial distribution functions calculated for both bOcn and tOcn show long-range ordering of the calcium ion distribution around the carboxylated glutamate (γGlu) residues, likely playing an important role in promoting stability of these Ocns. Additionally, the same calcium ions are observed to coordinate with neighboring γGlu, better shielding their negative charges and in turn stabilizing these systems more than do the singly coordinating calcium ions observed in the case of nOcn. bOcn is also found to exhibit a more helical C-terminal structure, that has been shown to activate its cellular receptor GPRC6A, highlighting the allosteric role of Ocn carboxylation in modulating the stability and binding potential of the active C-terminal.

CONCLUSIONS

The carboxylation status of Ocn as well and its calcium coordination appear to have a direct influence on Ocn structure and dynamics, possibly leading to the known differences in Ocn biological function.

GENERAL SIGNIFICANCE

Modification of Ocn sequence or its carboxylation state may provide the blueprint for developing high-affinity peptides targeting its cellular receptor GPRC6A, with therapeutic potential for treatment of metabolic disorders.

Injectable thermosensitive hybrid hydrogel containing graphene oxide and chitosan as dental pulp stem cells scaffold for bone tissue engineering

Here, we fabricated thermosensitive injectable hydrogel containing poly (N-isopropylacrylamide) (PNIPAAm)-based copolymer/graphene oxide (GO) composite with different feed ratio to chitosan (CS) as a natural polymer through physical and chemical crosslinking for the proliferation and differentiation of the human dental pulp stem cells (hDPSCs) to the osteoblasts. The PNIPAAm copolymer/GO composite was synthesized by free-radical copolymerization of (N-isopropylacrylamide) (NIPAAm), itaconic acid (IA) and maleic anhydride-modified poly(ethylene glycol) (PEG) in the presence of GO and used for the preparation of the hydrogels. The formulated hydrogels were evaluated for the porous architecture, rheological behavior, compressive strength, swelling property, in vitro degradation, hemocompatibility, biocompatibility, and differentiation. The hydrogel could enhance the deposition of minerals and the activity of alkaline phosphatase (ALP), in large part attributable to the oxygen and amine-containing functional groups of GO and CS. The engineered hydrogel could also upregulate the expression of the Runt-related transcription factor 2 and osteocalcin in the hDPSCs cultivated in both the normal and osteogenic media. It seems to promote the absorption of osteogenic inducer too. Based on our findings, the engineered hydrogel demonstrated the osteogenic potential, upon which it is proposed as a constructing scaffold in bone tissue engineering for the transplantation of hDPSCs.

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.

Growth Factor Engineering Strategies for Regenerative Medicine Applications

Growth factors are critical molecules for tissue repair and regeneration. Therefore, recombinant growth factors have raised a lot of hope for regenerative medicine applications. While using growth factors to promote tissue healing has widely shown promising results in pre-clinical settings, their success in the clinic is not a forgone conclusion. Indeed, translation of growth factors is often limited by their short half-life, rapid diffusion from the delivery site, and low cost-effectiveness. Trying to circumvent those limitations by the use of supraphysiological doses has led to serious side-effects in many cases and therefore innovative technologies are required to improve growth factor-based regenerative strategies. In this review, we present protein engineering approaches seeking to improve growth factor delivery and efficacy while reducing doses and side effects. We focus on engineering strategies seeking to improve affinity of growth factors for biomaterials or the endogenous extracellular matrix. Then, we discuss some examples of increasing growth factor stability and bioactivity, and propose new lines of research that the field of growth factor engineering for regenerative medicine may adopt in the future.

Osteocalcin affects bone mineral and mechanical properties in female mice

Osteocalcin is one of the most abundant noncollagenous proteins in bone. Phenotypes of osteocalcin knock-out mice (OC-/-) may vary on different backgrounds and with sex. Previous studies using adult female (OC-/-) mice on a mixed genetic background (129/B6) showed osteocalcin inhibited bone formation leading to weaker bone in wild-type (OC+/+). Yet on a pure (B6) genetic background male mice revealed osteocalcin improved fracture resistance and OC-/- bones were more prone to fracture. Osteocalcin is decreased with age and in some diseases (diabetes) where bone weakness is observed. The effect of osteocalcin in adult female bone from mice on a pure B6 background is unknown. We investigated differences in bone mineral properties and bone strength in female adult (6 months) (OC+/+) and (OC-/-) mice on a pure C57BL/6J background using Fourier Transform Infrared Imaging (FTIRI), micro-computed tomography (uCT), biomechanical measurements, histomorphometry and serum turnover markers (P1NP, CTX). Similar to female age matched mice on the (129/C57) background we found B6 OC-/- mice had a higher bone formation rate, no change in bone resorption, more immature mineral, decreased crystallinity and increased trabecular bone as compared to OC+/+. In contrast, the OC-/- mice on a pure B6 background had a lower bone mineral density, lower mineral to matrix ratio resulting in reduced stiffness and weaker bone strength. Our results demonstrate some properties of the OC-/- phenotype are dependent on genetic background. This may suggest that reduced osteocalcin may contribute to fracture and weaker bone in some groups of elderly and adults with diseases where osteocalcin is low.

Undercarboxylated osteocalcin downregulates pancreatic lipase expression in an ATF4-dependent manner in pancreatic acinar cells

Osteocalcin is an osteoblast-specific secreted protein that has been associated with endocrine roles in multiple aspects of energy metabolism. We examined whether undercarboxylated osteocalcin (ucOC) downregulates pancreatic lipase (PNLIP) expression in pancreatic acinar cells and then identified the downstream signaling pathway involved. We previously demonstrated that β adrenergic blockade attenuates body weight/fat mass gain in high-fat diet-fed mice and that this effect is associated with decreased PNLIP expression in pancreatic acinar cells. In the present study, we first confirmed that the serum ucOC level is inversely correlated with PNLIP expression, i.e., mice exhibiting high serum levels of ucOC showed low PNLIP levels in the pancreas. In in vitro experiments using primary pancreatic acinar and 266-6 cells, ucOC downregulated PNLIP expression. cAMP/PKA signaling inhibitors significantly reversed ucOC-induced downregulation of PNLIP expression. ucOC promoted the phosphorylation of cAMP response element-binding protein 2 (ATF4). Overexpression of ATF4 significantly suppressed PNLIP expression. Knockdown of ATF4 by siRNA reversed the ucOC-induced downregulation of PNLIP expression. A luciferase reporter assay showed that ucOC suppressed PNLIP promoter transactivation. Chromatin immunoprecipitation and a luciferase reporter assay demonstrated that ATF4 directly bound to the CRE on the mouse PNLIP promoter and suppressed PNLIP transactivation. Knockdown of G-protein coupled receptor 6A (Gprc6a), a candidate receptor for mediating the response to ucOC in the bone-pancreas endocrine loop, by siRNA reversed the downregulating effect of ucOC on PNLIP expression. Taken together, ucOC downregulates pancreatic lipase expression in a cAMP/protein kinase A/ATF4-dependent manner. Gprc6a is a potential osteocalcin-sensing receptor that regulates PNLIP expression in pancreatic acinar cells.

High Fat Mixed Meal Tolerance Test Leads to Suppression of Osteocalcin Decrease in Obese Insulin Resistant Subjects Compared to Healthy Adults

Nutrients influence bone turnover. Carboxylated osteocalcin (Gla-OC) participates in bone formation whereas its undercarboxylated form (Glu-OC) acts as a hormone in glucose metabolism. The aim of the study was to determine the responses of Gla-OC, Glu-OC, and total-OC (calculated as the sum of Gla-OC and Glu-OC) to a high fat mixed meal tolerance test (HFMTT) in non-obese (body mass index (BMI) < 30 kg/m², n = 24) and obese subjects (30 < BMI < 40 kg/m², n = 70) (both sexes, aged 25–65 years). Serum Gla-OC and Glu-OC were measured at baseline as well as at 2 and 6 h during a HFMTT by enzyme-linked immunosorbent assay (ELISA). Baseline Gla-OC, Glu-OC, and total-OC levels were lower in obese individuals compared to non-obese participants (p = 0.037, p = 0.016 and p = 0.005, respectively). The decrease in Gla-OC and total-OC, but not in Glu-OC, concentrations during the HFMTT was suppressed in obese, but not in non-obese controls (p < 0.05, p < 0.01, p = 0.08, respectively). Subjects with the highest homeostatic model assessment for insulin resistance (HOMA-IR) index values had a less pronounced decrease in total-OC compared to patients with values of HOMA-IR index in the 1st quartile (p < 0.05). Net incremental area under Gla-OC inversely correlated with adiponectin (rho = −0.35, p = 0.001). Increase in insulin sensitivity and adiponectin level in obese subjects could beneficially influence postprandial bone turnover expressed by osteocalcin concentration.

Synergistic effect of extracellularly supplemented osteopontin and osteocalcin on stem cell proliferation, osteogenic differentiation, and angiogenic properties

A high demand for functional bone grafts is being observed worldwide, especially due to the increased life expectancy. Osteoinductive components should be incorporated into functional bone grafts, accelerating cell recruitment, cell proliferation, angiogenesis, and new bone formation at a defect site. Noncollagenous bone matrix proteins, especially osteopontin (OPN) and osteocalcin (OC), have been reported to regulate some physiological process, such as cell migration and bone mineralization. However, the effects of OPN and OC on cell proliferation, osteogenic differentiation, mineralization, and angiogenesis are still undefined. Therefore, we assessed the exogenous effect of OPN and OC supplementation on human bone marrow mesenchymal stem/stromal cells (hBM MSC) proliferation and osteogenic differentiation. OPN dose-dependently increased the proliferation of hBM MSC, as well as improved the angiogenic properties of human umbilical vein endothelial cells (HUVEC) by increasing the capillary-like tube formation in vitro. On the other hand, OC enhanced the differentiation of hBM MSC into osteoblasts and demonstrated an increase in extracellular calcium levels and alkaline phosphatase activity, as well as higher messenger RNA levels of mature osteogenic markers osteopontin and osteocalcin. In vivo assessment of OC/OPN-enhanced scaffolds in a critical-sized defect rabbit long-bone model revealed no infection, while new bone was being formed. Taken together, these results suggest that OC and OPN stimulate bone regeneration by inducing stem cell proliferation, osteogenesis and by enhancing angiogenic properties. The synergistic effect of OC and OPN observed in this study can be applied as an attractive strategy for bone regeneration therapeutics by targeting different vital cellular processes.

First evidence of octacalcium phosphate@osteocalcin nanocomplex as skeletal bone component directing collagen triple-helix nanofibril mineralization

Tibia trabeculae and vertebrae of rats as well as human femur were investigated by high-resolution TEM at the atomic scale in order to reveal snapshots of the morphogenetic processes of local bone ultrastructure formation. By taking into account reflections of hydroxyapatite for Fourier filtering the appearance of individual alpha-chains within the triple-helix clearly shows that bone bears the feature of an intergrowth composite structure extending from the atomic to the nanoscale, thus representing a molecular composite of collagen and apatite. Careful Fourier analysis reveals that the non-collagenous protein osteocalcin is present directly combined with octacalcium phosphate. Besides single spherical specimen of about 2 nm in diameter, osteocalcin is spread between and over collagen fibrils and is often observed as pearl necklace strings. In high-resolution TEM, the three binding sites of the γ-carboxylated glutamic acid groups of the mineralized osteocalcin were successfully imaged, which provide the chemical binding to octacalcium phosphate. Osteocalcin is attached to the collagen structure and interacts with the Ca-sites on the (100) dominated hydroxyapatite platelets with Ca-Ca distances of about 9.5 Å. Thus, osteocalcin takes on the functions of Ca-ion transport and suppression of hydroxyapatite expansion.

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.

Vitamin K‑dependent proteins involved in bone and cardiovascular health (Review)

In postmenopausal women and elderly men, bone density decreases with age and vascular calcification is aggravated. This condition is closely associated with vitamin K2 deficiency. A total of 17 different vitamin K-dependent proteins have been identified to date. Vitamin K-dependent proteins are located within the bone, heart and blood vessels. For instance, carboxylated osteocalcin is beneficial for bone and aids the deposition of calcium into the bone matrix. Carboxylated matrix Gla protein effectively protects blood vessels and may prevent calcification within the vascular wall. Furthermore, carboxylated Gla-rich protein has been reported to act as an inhibitor in the calcification of the cardiovascular system, while growth arrest-specific protein-6 protects endothelial cells and vascular smooth muscle cells, resists apoptosis and inhibits the calcification of blood vessels by inhibiting the apoptosis of vascular smooth muscle cells. In addition, periostin may promote the differentiation, aggregation, adhesion and proliferation of osteoblasts. Periostin also occurs in the heart and may be associated with the reconstruction of heart function. These vitamin K-dependent proteins may exert their functions following γ-carboxylation with vitamin K, and different vitamin K-dependent proteins may exhibit synergistic effects or antagonistic effects on each other. In the cardiovascular system with vitamin K antagonist supplement or vitamin K deficiency, calcification occurs in the endothelium of blood vessels and vascular smooth muscle cells are transformed into osteoblast-like cells, a phenomenon that resembles bone growth. Both the bone and cardiovascular system are closely associated during embryonic development. Thus, the present study hypothesized that embryonic developmental position and tissue calcification may have a certain association for the bone and the cardiovascular system. This review describes and briefly discusses several important vitamin K-dependent proteins that serve an important role in bone and the cardiovascular system. The results of the review suggest that the vascular calcification and osteogenic differentiation of vascular smooth muscle cells may be associated with the location of the bone and cardiovascular system during embryonic development.

Super High-Flux Continuous Venovenous Hemodialysis Using Regional Citrate Anticoagulation: Long-Term Stability of Middle Molecule Clearance

Continuous renal replacement therapy is a standard treatment in critically ill patients with acute kidney injury. All CRRT techniques provide a high low-molecular weight clearance but even with hemofiltration, clearance of middle molecules is low. We investigated whether a new super high-flux hemofilter provides effective and sustained middle molecule clearance during citrate-anticoagulated continuous venovenous hemodialysis for up to 72 h. We included 14 critically ill patients with AKI-KDIGO-III in a prospective observational trial. We measured/calculated blood and urine concentrations, clearances and sieving coefficients of eight molecules with molecular weights from 60 to 66 kDa, hemodynamic parameters and SAPS-II scores. All filters were patent at 72 h. Clearance and sieving coefficients of small solutes were high and sustained over time, those for larger solutes decreased over 72 h but remained high enough to decrease blood concentrations of solutes up to 25 kDa. Albumin serum levels remained unaffected. Catecholamine doses and SAPS-II scores decreased significantly. This new hemofilter may improve blood purification in critically ill patients with AKI.

Effect of vitamin K2 on type 2 diabetes mellitus: A review

Type 2 diabetes mellitus (T2DM) continue to be a major public health problem around the world that frequently presents with microvascular and macrovascular complications. Individuals with T2DM are not only suffering from significant emotional and physical misery, but also at increased risk of dying from severe complications. In recent years, evidence from prospective observational studies and clinical trials has shown T2DM risk reduction with vitamin K2 supplementation. We thus did an overview of currently available studies to assess the effect of vitamin K2 supplementation on insulin sensitivity, glycaemic control and reviewed the underlying mechanisms. We proposed that vitamin K2 improved insulin sensitivity through involvement of vitamin K-dependent-protein osteocalcin, anti-inflammatory properties, and lipid-lowering effects. Vitamin K2 had a better effect than vitamin K1 on T2DM. The interpretation of this review will increase comprehension of the development of a therapeutic strategy to prevent and treat T2DM.

Biomolecular regulation, composition and nanoarchitecture of bone mineral

Tough natural nanocomposites like bone, nacre and sea sponges contain within their hierarchy, a mineral (phosphate, silicate or carbonate) phase that interacts with an organic phase. In bone, the role of mineral ultrastructure (organization, morphology, composition) is crucial to the mechanical and biological properties of the tissue. Better understanding of mineral interaction with the organic matrix, in particular non-collagenous proteins, osteocalcin (OC) and osteopontin (OPN), can lead to better design of biomimetic materials. Using small angle x-ray scattering (SAXS) and wavelength dispersive spectroscopy (WDS) on single (OC-/- and OPN-/-) and double (OC-OPN-/-;-/-) genetic knockout mice bones, we demonstrate that both osteocalcin and osteopontin have specific roles in the biomolecular regulation of mineral in bone and together they are major determinants of the quality of bone mineral. Specifically, for the first time, we show that proteins osteocalcin and osteopontin regulate bone mineral crystal size and organization in a codependent manner, while they independently determine crystal shape. We found that OC is more dominant in the regulation of the physical properties of bone mineral, while OPN is more dominant in the regulation of the mineral composition.

Orientation and conformation of osteocalcin adsorbed onto calcium phosphate and silica surfaces

Adsorption isotherms, circular dichroism (CD) spectroscopy, x-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to investigate the adsorption of human osteocalcin (hOC) and decarboxylated (i.e., Gla converted back to Glu) hOC (dhOC) onto various calcium phosphate surfaces as well as silica surfaces. The adsorption isotherms and XPS nitrogen signals were used to track the amount of adsorbed hOC and dhOC. The intensities of key ToF-SIMS amino acid fragments were used to assess changes in the structure of adsorbed hOC and dhOC. CD spectra were used to investigate the secondary structure of OC. The largest differences were observed when the proteins were adsorbed onto silica versus calcium phosphate surfaces. Similar amounts (3-4 at. % N) of hOC and dhOC were adsorbed onto the silica surface. Higher amounts of hOC and dhOC were adsorbed on all the calcium phosphate surfaces. The ToF-SIMS data showed that the intensity of the Cys amino acid fragment, normalized to intensity of all amino acid fragments, was significantly higher (∼×10) when the proteins were adsorbed onto silica. Since in the native OC structure the cysteines are located in the center of three α-helices, this indicates both hOC and dhOC are more denatured on the silica surface. As hOC and dhOC denature upon adsorption to the silica surface, the cysteines become more exposed and are more readily detected by ToF-SIMS. No significant differences were detected between hOC and dhOC adsorbed onto the silica surface, but small differences were observed between hOC and dhOC adsorbed onto the calcium phosphate surfaces. In the OC structure, the α-3 helix is located above the α-1 and α-2 helices. Small differences in the ToF-SIMS intensities from amino acid fragments characteristic of each helical unit (Asn for α-1; His for α-2; and Phe for α-3) suggests either slight changes in the orientation or a slight uncovering of the α-1 and α-2 for adsorbed dhOC. XPS showed that similar amounts of hOC and dhOC were absorbed onto hydroxyapaptite and octacalcium phosphate surfaces, but ToF-SIMS detected some small differences in the amino acid fragment intensities on these surfaces for adsorbed hOC and dhOC.

Carboxylated and undercarboxylated osteocalcin in metabolic complications of human obesity and prediabetes

BACKGROUND

Carboxylated osteocalcin (Gla-OC) participates in bone remodeling, whereas the undercarboxylated form (Glu-OC) takes part in energy metabolism. This study was undertaken to compare the blood levels of Glu-OC and Gla-OC in nonobese, healthy obese, and prediabetic volunteers and correlate it with the metabolic markers of insulin resistance and early markers of inflammation.

METHODS

Nonobese (body mass index [BMI] <30 kg/m² ; n = 34) and obese subjects (30 <BMI <40 kg/m² ; n = 98), both sexes, aged 25 to 65 years, were divided into healthy control, normal weight subjects, healthy obese, and obese with biochemical markers of prediabetes. The subgroups with obesity and low or high Gla-OC or Glu-OC were also considered for statistical analysis. After 2 weeks of diet standardization, venous blood was sampled for the determination of Gla-OC, Glu-OC, lipid profile, parameters of inflammation (hsCRP, interleukin 6, sE-selectin, sPECAM-1, and monocyte chemoattractant protein 1), and adipokines (leptin, adiponectin, visfatin, and resistin).

RESULTS

Gla-OC in obese patients was significantly lower compared to nonobese ones (11.36 ± 0.39 vs 12.69 ± 0.90 ng/mL, P = .048) and weakly correlated with hsCRP (r = -0.18, P = .042), visfatin concentration (r = -0.19, P = .033), and BMI (r = -0.17, P = .047). Glu-OC was negatively associated with fasting insulin levels (r = -0.18, P = .049) and reduced in prediabetic individuals compared with healthy obese volunteers (3.04 ± 0.28 vs 4.48 ± 0.57, P = .025).

CONCLUSIONS

Decreased blood concentration of Glu-OC may be a selective early symptom of insulin resistance in obesity, whereas the decreased level of Gla-OC seems to be associated with the appearance of early markers of low grade inflammation accompanying obesity.

Bone Health and Osteoporosis: The Role of Vitamin K and Potential Antagonism by Anticoagulants

BACKGROUND

Vitamin K's effects extend beyond blood clotting to include a role in bone metabolism and potential protection against osteoporosis. Vitamin K is required for the gamma-carboxylation of osteocalcin. Likewise, this gamma-carboxylation also occurs in the liver for several coagulation proteins. This mechanism is interrupted by coumarin-based anticoagulants in both the liver and bone.

METHODS

A thorough review of the literature on vitamin K, osteocalcin and their role in bone metabolism and osteoporosis, as well as the potential bone effects of anticoagulant therapy was conducted.

CONCLUSIONS

Epidemiological studies and clinical trials consistently indicate that vitamin K has a positive effect on bone mineral density and decreases fracture risk. Typical dietary intakes of vitamin K are below the levels associated with better BMD and reduced fracture risk; thus issues of increasing dietary intakes, supplementation, and/or fortification arise. To effectively address these issues, large-scale, intervention trials of vitamin K are needed. The effects of coumarin-based anticoagulants on bone health are more ambiguous, with retrospective studies suggesting that long-term therapy adversely affects vertebral BMD and fracture risk. Anticoagulants that do not affect vitamin K metabolism are now available and make clinical trials feasible to answer the question of whether coumarins adversely affect bone. The research suggests that at a minimum, clinicians should carefully assess anticoagulated patients for osteoporosis risk, monitor BMD, and refer them to dietitians for dietary and supplement advice on bone health. Further research is needed to make more efficacious decisions about vitamin K intake, anticoagulant therapy, and bone health.

Adsorption of benzoxaboroles on hydroxyapatite phases

Benzoxaboroles are a family of molecules that are finding an increasing number of applications in the biomedical field, particularly as a "privileged scaffold" for the design of new drugs. Here, for the first time, we determine the interaction of these molecules with hydroxyapatites, in view of establishing (i) how benzoxaborole drugs may adsorb onto biological apatites, as this could impact on their bioavailability, and (ii) how apatite-based materials can be used for their formulation. Studies on the adsorption of the benzoxaborole motif (C7H7BO2, referred to as BBzx) on two different apatite phases were thus performed, using a ceramic hydroxyapatite (HAceram) and a nanocrystalline hydroxyapatite (HAnano), the latter having a structure and composition more similar to the one found in bone mineral. In both cases, the grafting kinetics and mechanism were studied, and demonstration of the surface attachment of the benzoxaborole under the form of a tetrahedral benzoxaborolate anion was established using (11)B solid state NMR (including (11)B-(31)P correlation experiments). Irrespective of the apatite used, the grafting density of the benzoxaborolates was found to be low, and more generally, these anions demonstrated a poor affinity for apatite surfaces, notably in comparison with other anions commonly found in biological media, such as carboxylates and (organo)phosphates. The study was then extended to the adsorption of a molecule with antimicrobial and antifungal properties (3-piperazine-bis(benzoxaborole)), showing, on a more general perspective, how hydroxyapatites can be used for the development of novel formulations of benzoxaborole drugs.

STATEMENT OF SIGNIFICANCE

Benzoxaboroles are an emerging family of molecules which have attracted much attention in the biomedical field, notably for the design of new drugs. However, the way in which these molecules, once introduced in the body, may interact with bone mineral is still unknown, and the possibility of associating benzoxaboroles to calcium phosphates for drug-formulation purposes has not been looked into. Here, we describe the first study of the adsorption of benzoxaboroles on hydroxyapatite, which is the main mineral phase present in bone. We describe the mode of grafting of benzoxaboroles on this material, and show that they only weakly bind to its surface, especially in comparison to other ionic species commonly found in physiological media, such as phosphates and carboxylates. This demonstrates that administered benzoxaborole drugs are unlikely to remain adsorbed on hydroxyapatite surfaces for long periods of time, which means that their biodistribution will not be affected by such phenomena. Moreover, this work shows that the formulation of benzoxaborole drugs by association to calcium phosphates like hydroxyapatite will lead to a rapid release of the molecules.

Evidence for Osteocalcin Binding and Activation of GPRC6A in β-Cells

The possibility that G protein-coupled receptor family C member A (GPRC6A) is the osteocalcin (Ocn)-sensing G protein-coupled receptor that directly regulates pancreatic β-cell functions is controversial. In the current study, we found that Ocn and an Ocn-derived C-terminal hexapeptide directly activate GPRC6A-dependent ERK signaling in vitro. Computational models probe the structural basis of Ocn binding to GPRC6A and predict that the C-terminal hexapeptide docks to the extracellular side of the transmembrane domain of GPRC6A. Consistent with the modeling, mutations in the computationally identified binding pocket of GPRC6A reduced Ocn and C-terminal hexapeptide activation of this receptor. In addition, selective deletion of Gprc6a in β-cells (Gprc6a(β)(-cell-cko)) by crossing Gprc6a(flox/flox) mice with Ins2-Cre mice resulted in reduced pancreatic weight, islet number, insulin protein content, and insulin message expression. Both islet size and β-cell proliferation were reduced in Gprc6a(β)(-cell-cko) compared with control mice. Gprc6a(β)(-cell-cko) exhibited abnormal glucose tolerance, but normal insulin sensitivity. Islets isolated from Gprc6a(β)(-cell-cko) mice showed reduced insulin simulation index in response to Ocn. These data establish the structural basis for Ocn direct activation of GPRC6A and confirm a role for GPRC6A in regulating β-cell proliferation and insulin secretion.

How does the pathophysiological context influence delivery of bone growth factors?

"Orthobiologics" represents an important category of therapeutics for the regeneration of bone defects caused by injuries or diseases, and bone growth factors are a particularly rapidly growing sub-category. Clinical application of bone growth factors has accelerated in the last two decades with the introduction of BMPs into clinical bone repair. Optimal use of growth factor-mediated treatments heavily relies on controlled delivery, which can substantially influence the local growth factor dose, release kinetics, and biological activity. The characteristics of the surrounding environment, or "context", during delivery can dictate growth factor loading efficiency, release and biological activity. This review discusses the influence of the surrounding environment on therapeutic delivery of bone growth factors. We specifically focus on pathophysiological components, including soluble components and cells, and how they can actively influence the therapeutic delivery and perhaps efficacy of bone growth factors.

Matrix Gla protein and osteocalcin: from gene duplication to neofunctionalization

Osteocalcin (OC or bone Gla protein, BGP) and matrix Gla protein (MGP) are two members of the growing family of vitamin K-dependent (VKD) proteins. They were the first VKD proteins found not to be involved in coagulation and synthesized outside the liver. Both proteins were isolated from bone although it is now known that only OC is synthesized by bone cells under normal physiological conditions, but since both proteins can bind calcium and hydroxyapatite, they can also accumulate in bone. Both OC and MGP share similar structural features, both in terms of protein domains and gene organization. OC gene is likely to have appeared from MGP through a tandem gene duplication that occurred concomitantly with the appearance of the bony vertebrates. Despite their relatively close relationship and the fact that both can bind calcium and affect mineralization, their functions are not redundant and they also have other unrelated functions. Interestingly, these two proteins appear to have followed quite different evolutionary strategies in order to acquire novel functionalities, with OC following a gene duplication strategy while MGP variability was obtained mostly by the use of multiple promoters and alternative splicing, leading to proteins with additional functional characteristics and alternative gene regulatory pathways.

Osteocalcin levels are inversely associated with Hba1c and BMI in adult subjects with long-standing type 1 diabetes

PURPOSE

Diabetic osteopathy is an upcoming complication of diabetes characterized by osteoporosis, increased risk for bone fractures and alterations in bone metabolism. Osteocalcin (OC) is a bone-specific protein produced by osteoblasts involved in the regulation of glucose and energy metabolism. The aim of this study is to determine whether OC serum levels are correlated with metabolic control in adult subjects with type one diabetes mellitus (T1DM).

METHODS

A cross-sectional study was conducted on 93 subjects (51 men) with mean age, disease duration and body mass index (BMI) of 39.9 ± 12.3, 17.2 ± 12.6 years and 24.5 ± 3.4 kg/m(2), respectively. Blood samples were drawn to measure levels of hemoglobin A1c (HbA1c), OC, 25-OH vitamin D and PTH.

RESULTS

Significant inverse correlations were found between OC and HbA1c (r = -0.295, P = 0.004) and between OC and BMI (r = -0.218, P = 0.037). These correlations were confirmed also among men in the analyses by gender [HbA1c vs OC: r = -0.363, P = 0.009; BMI vs OC: r = -0.291, P = 0.043], and similar but nonsignificant trends were confirmed among women. A significant difference in mean OC was also found between the lowest and the highest HbA1c tertile (22.3 ± 10.0 vs 16.9 ± 8.0 ng/mL, P = 0.025).

CONCLUSIONS

These data show that in T1DM of long duration, OC serum levels are inversely associated with HbA1c and BMI, supporting the hypothesis that a poor glycemic control can affect osteoblast function.

NMR investigation of the role of osteocalcin and osteopontin at the organic-inorganic interface in bone

Mechanical resilience of bone tissue decreases with age. The ability to comprehensively probe and understand bone properties could help alleviate this problem. One important aspect of bone quality that has recently been made evident is the presence of dilatational bands formed by osteocalcin (OC) and osteopontin (OPN), which contribute to fracture toughness. However, experimental evidence of the structural role of these two proteins at the organic-mineral interface in bone is still needed. Solid state nuclear magnetic resonance (SSNMR) is emerging as a useful technique in probing molecular level aspects of bone. Here, we present the first SSNMR study of bone tissue from genetically modified mice lacking OC and/or OPN. Probing the mineral phase, the organic matrix and their interface revealed that, despite the absence of OC and OPN, the organic matrix and mineral were well preserved, and the overall exposure of collagen to hydroxyapatite (HA) nanoparticles was hardly affected. However, the proximity to the HA surface was slightly increased for a number of bone components including less abundant amino acids like lysine, suggesting that this is how the tissue compensates for the lack of OC and OPN. Taken together, the NMR data supports the recently proposed model, in which the contribution of OC-OPN to fracture toughness is related to their presence at the extrafibrillar organic-mineral interfaces, where they reinforce the network of mineralized fibrils and form dilatational bands. In an effort toward further understanding the structural role of individual amino acids of low abundance in bone, we then explored the possibility of specific (13)C enrichment of mouse bone, and report the first SSNMR spectra of 97% (13)C lysine-enriched tissue. Results show that such isotopic enrichment allows valuable molecular-level structural information to be extracted, and sheds light on post-translational modifications undergone by specific amino acids in vivo.

Peptides of Matrix Gla protein inhibit nucleation and growth of hydroxyapatite and calcium oxalate monohydrate crystals

Matrix Gla protein (MGP) is a phosphorylated and γ-carboxylated protein that has been shown to prevent the deposition of hydroxyapatite crystals in the walls of blood vessels. MGP is also expressed in kidney and may inhibit the formation of kidney stones, which mainly consist of another crystalline phase, calcium oxalate monohydrate. To determine the mechanism by which MGP prevents soft-tissue calcification, we have synthesized peptides corresponding to the phosphorylated and γ-carboxylated sequences of human MGP in both post-translationally modified and non-modified forms. The effects of these peptides on hydroxyapatite formation and calcium oxalate crystallization were quantified using dynamic light scattering and scanning electron microscopy, respectively. Peptides YGlapS (MGP1-14: YγEpSHEpSMEpSYELNP), YEpS (YEpSHEpSMEpSYELNP), YGlaS (YγESHESMESYELNP) and SK-Gla (MGP43-56: SKPVHγELNRγEACDD) inhibited formation of hydroxyapatite in order of potency YGlapS > YEpS > YGlaS > SK-Gla. The effects of YGlapS, YEpS and YGlaS on hydroxyapatite formation were on both crystal nucleation and growth; the effect of SK-Gla was on nucleation. YGlapS and YEpS significantly inhibited the growth of calcium oxalate monohydrate crystals, while simultaneously promoting the formation of calcium oxalate dihydrate. The effects of these phosphopeptides on calcium oxalate monohydrate formation were on growth of crystals rather than nucleation. We have shown that the use of dynamic light scattering allows inhibitors of hydroxyapatite nucleation and growth to be distinguished. We have also demonstrated for the first time that MGP peptides inhibit the formation of calcium oxalate monohydrate. Based on the latter finding, we propose that MGP function not only to prevent blood-vessel calcification but also to inhibit stone formation in kidney.

X-ray crystal structure of bovine 3 Glu-osteocalcin

The 3 Glu form of osteocalcin (3 Glu-OCN) is increased in serum during low vitamin K intake or oral anticoagulant use (warfarin). Previous reports using circular dichroism show it is less structured than 3 Gla Ca²⁺-osteocalcin and does not bind strongly to bone mineral. Recent studies have suggested a role for 3 Glu-OCN as a potential regulator of glucose metabolism. A G-protein-coupled receptor, GPRC6a, found in the pancreas and testes was identified as the putative osteocalcin receptor. The purpose of this study is to determine the high-resolution structure of bovine 3 Glu-OCN, using X-ray crystallography, to understand molecular interactions with mineral and the GPRC6a receptor. Diffraction quality crystals of thermally decarboxylated bovine osteocalcin were grown, and the crystal structure was determined to 1.88 Å resolution. The final refined structure contained residues 17-47 and, like 3 Gla Ca²⁺-OCN, consisted of three α-helices surrounding a hydrophobic core, a C23-C29 disulfide bond between two of the helices, and no bound Ca²⁺. Thus, the helical structure of 3 Glu-OCN is Ca²⁺-independent but similar to that of 3 Gla Ca²⁺-OCN. A reduced level of mineral binding could result from a lower number of Ca²⁺ coordinating ligands on 3 Glu-OCN. The structure suggests the GPRC6a receptor may respond to helical osteocalcin and will aid in providing molecular mechanistic insight into the role of 3 Glu-OCN in glucose homeostasis.

Ion Mobility Spectrometry-Mass Spectrometry of Intrinsically Unfolded Proteins: Trying to Put Order into Disorder

Intrinsically disordered proteins do not adopt well-defined native structures and therefore present an intriguing challenge in terms of structural elucidation as they are relatively inaccessible to traditional approaches such as NMR and X-ray crystallography. Many members of this important group of proteins have a distinct biological function and frequently undergo a conformational change on binding to their physiological targets which can in turn modulate their function. Furthermore, many intrinsically unstructured proteins are associated with a wide range of major diseases including cancer and amyloid-related disorders. Here, electrospray ionisation-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) has been used to probe the conformational characteristics of two intrinsically disordered proteins: apo-cytochrome c and apo-osteocalcin. Both proteins are structured in their holo-states when bound to their respective substrates, but disordered in their apo-states. Here, the conformational properties of the holo- and the apo-protein forms for both species have been analysed and their mass spectral data and ion mobility spectrometry-derived collision cross-sectional areas, indicative of their physical size, compared to study the relationship between substrate binding and tertiary structure. In both cases, the intrinsically unstructured apo-states populated multiple conformations with larger cross-sectional areas than their holo-analogues, suggesting that intrinsic disorder in proteins does not preclude the formation of preferred conformations. Additionally, analysis of truncated analogues of osteocalcin has located the region of the protein responsible for the conformational changes detected upon metal cation binding. Together, the data illustrate the scope and utility of ESI-IMS-MS for studying the characteristics and properties of intrinsically disordered proteins whose analysis by other techniques is limited.

The role of osteocalcin in human glucose metabolism: marker or mediator?

Increasing evidence supports an association between the skeleton and energy metabolism. These interactions are mediated by a variety of hormones, cytokines and nutrients. Here, the evidence for a role of osteocalcin in the regulation of glucose metabolism in humans is reviewed. Osteocalcin is a bone matrix protein that regulates hydroxyapatite size and shape through its vitamin-K-dependent, γ-carboxylated form. The concentration of osteocalcin in the circulation is a measure of bone formation. The undercarboxylated form of osteocalcin is active in glucose metabolism in mice. Total serum osteocalcin concentrations in humans are inversely associated with measures of glucose metabolism; however, human data are inconclusive with regard to the role of uncarboxylated osteocalcin in glucose metabolism because most studies do not account for the influence of vitamin K on the proportion of undercarboxylated osteocalcin or differentiate between the total and uncarboxylated forms of osteocalcin. Furthermore, most human studies do not concomitantly measure other bone turnover markers to isolate the role of osteocalcin as a measure of bone formation from its effect on glucose metabolism. Carefully designed studies are required to define the role of osteocalcin and its carboxylated or undercarboxylated forms in the regulation of glucose metabolism in humans.

Solid state NMR investigation of intact human bone quality: balancing issues and insight into the structure at the organic-mineral interface

Age-related bone fragility fractures present a significant problem for public health. Measures of bone quality are increasingly recognized to complement the conventional bone mineral density (BMD) based assessment of fracture risk. The ability to probe and understand bone quality at the molecular level is desirable in order to unravel how the structure of organic matrix and its association with mineral contribute to the overall mechanical properties. The (13)C{(31)P} REDOR MAS NMR (Rotational Echo Double Resonance Magic Angle Spinning Nuclear Magnetic Resonance) technique is uniquely suited for the study of the structure of the organic-mineral interface in bone. For the first time, we have applied it successfully to analyze the structure of intact (non-powdered) human cortical bone samples, from young healthy and old osteoporotic donors. Loading problems associated with the rapid rotation of intact bone were solved using a Finite Element Analysis (FEA) approach, and a method allowing osteoporotic samples to be balanced and spun reproducibly is described. REDOR NMR parameters were set to allow insight into the arrangement of the amino acids at the mineral interface to be accessed, and SVD (Singular Value Decomposition) was applied to enhance the signal to noise ratio and enable a better analysis of the data. From the REDOR data, it was found that carbon atoms belonging to citrate/glucosaminoglycans (GAGs) are closest to the mineral surface regardless of age or site. In contrast, the arrangement of the collagen backbone at the interface varied with site and age. The relative proximity of two of the main amino acids in bone matrix proteins, hydroxyproline and alanine, with respect to the mineral phase was analyzed in more detail, and discussed in view of glycation measurements which were carried out on the tissues. Overall, this work shows that the (13)C{(31)P} REDOR NMR approach could be used as a complementary technique to assess a novel aspect of bone quality, the organic-mineral interface structure.

Vitamin K-dependent carboxylation of osteocalcin: friend or foe?

Osteocalcin originates from osteoblastic synthesis and is deposited into bone or released into circulation, where it correlates with histological measures of bone formation. The presence of 3 vitamin K-dependent γ carboxyglutamic acid residues is critical for osteocalcin's structure, which appears to regulate the maturation of bone mineral. In humans, the percentage of the circulating osteocalcin that is not γ-carboxylated (percent ucOC) is used as a biomarker of vitamin K status. In contrast, when ucOC is not corrected for total osteocalcin, the interpretation of this measure is confounded by osteoblastic activity, independent of vitamin K. Observational studies using percent ucOC have led to the conclusion that vitamin K insufficiency leads to age-related bone loss. However, clinical trials do not provide overall support for the suggestion that vitamin K supplementation of the general population will reduce bone loss or fracture risk. More recently, results from in vitro and in vivo studies using animal models indicate that ucOC is an active hormone with a positive role in glucose metabolism. By inference, vitamin K, which decreases ucOC, would have a detrimental effect. However, in humans this hypothesis is not supported by the limited data available, nor is it supported by what has been established regarding osteocalcin chemistry. In summary, the specific function of osteocalcin in bone and glucose metabolism has yet to be elucidated.

The ratio 1660/1690 cm(-1) measured by infrared microspectroscopy is not specific of enzymatic collagen cross-links in bone tissue

In postmenopausal osteoporosis, an impairment in enzymatic cross-links (ECL) occurs, leading in part to a decline in bone biomechanical properties. Biochemical methods by high performance liquid chromatography (HPLC) are currently used to measure ECL. Another method has been proposed, by Fourier Transform InfraRed Imaging (FTIRI), to measure a mature PYD/immature DHLNL cross-links ratio, using the 1660/1690 cm⁻¹ area ratio in the amide I band. However, in bone, the amide I band composition is complex (collagens, non-collagenous proteins, water vibrations) and the 1660/1690 cm⁻¹ by FTIRI has never been directly correlated with the PYD/DHLNL by HPLC. A study design using lathyritic rats, characterized by a decrease in the formation of ECL due to the inhibition of lysyl oxidase, was used in order to determine the evolution of 1660/1690 cm⁻¹ by FTIR Microspectroscopy in bone tissue and compare to the ECL quantified by HPLC. The actual amount of ECL was quantified by HPLC on cortical bone from control and lathyritic rats. The lathyritic group exhibited a decrease of 78% of pyridinoline content compared to the control group. The 1660/1690 cm⁻¹ area ratio was increased within center bone compared to inner bone, and this was also correlated with an increase in both mineral maturity and mineralization index. However, no difference in the 1660/1690 cm⁻¹ ratio was found between control and lathyritic rats. Those results were confirmed by principal component analysis performed on multispectral infrared images. In bovine bone, in which PYD was physically destructed by UV-photolysis, the PYD/DHLNL (measured by HPLC) was strongly decreased, whereas the 1660/1690 cm⁻¹ was unmodified. In conclusion, the 1660/1690 cm⁻¹ is not related to the PYD/DHLNL ratio, but increased with age of bone mineral, suggesting that a modification of this ratio could be mainly due to a modification of the collagen secondary structure related to the mineralization process.

Biomaterials that regulate growth factor activity via bioinspired interactions

Growth factor activity is localized within the natural extracellular matrix (ECM) by specific non-covalent interactions with core ECM biomolecules, such as proteins and proteoglycans. Recently, these interactions have inspired us and others to develop synthetic biomaterials that can non-covalently regulate growth factor activity for tissue engineering applications. For example, biomaterials covalently or non-covalently modified with heparin glycosaminoglycans can augment growth factor release strategies. In addition, recent studies demonstrate that biomaterials modified with heparin-binding peptides can sequester cell-secreted heparin proteoglycans and, in turn, sequester growth factors and regulate stem cell behavior. Another set of studies show that modular versions of growth factor molecules can be designed to interact with specific components of natural and synthetic ECMs, including collagen and hydroxyapatite. In addition, layer-by-layer assemblies of GAGs and other natural polyelectrolytes retain growth factors at a cell-material interface via specific non-covalent interactions. This review will detail the various bioinspired strategies being used to non-covalently localize growth factor activity within biomaterials, and will highlight in vivo examples of the efficacy of these materials to promote tissue regeneration.

Influence of different modifications of a calcium phosphate bone cement on adhesion, proliferation, and osteogenic differentiation of human bone marrow stromal cells

Collagen and noncollagenous proteins of the extracellular bone matrix are able to stimulate bone cell activities and bone healing. The modification of calcium phosphate bone cements used as temporary bone replacement materials with these proteins seems to be a promising approach to accelerate new bone formation. In this study, we investigated adhesion, proliferation, and osteogenic differentiation of human bone marrow stromal cells (hBMSC) on Biocement D/collagen composites which have been modified with osteocalcin and O-phospho-L-serine. Modification with osteocalcin was carried out by its addition to the cement precursor before setting as well as by functionalization of the cement samples after setting and sterilization. hBMSC were cultured on these samples for 28 days with and without osteogenic supplements. We found a positive impact especially of the phosphoserine-modifications but also of both osteocalcin-modifications on differentiation of hBMSC indicated by higher expression of the osteoblastic markers matrix metalloproteinase-13 and bone sialo protein II. For hBMSC cultured on phosphoserine-containing composites, an increased proliferation has been observed. However, in case of the osteocalcin-modified samples, only osteocalcin adsorbed after setting and sterilization of the cement samples was able to promote initial adhesion and proliferation of hBMSC. The addition of osteocalcin before setting results in a finer microstructure but the biological activity of osteocalcin might be impaired due to the sterilization process. Thus, our data indicate that the initial adhesion and proliferation of hBMSC is enhanced rather by the biological activity of osteocalcin than by the finer microstructure.

Passage through vertebrate gap junctions of 17/18kDa molecules is primarily dependent upon molecular configuration

In fish, amphibians and mammals, gap junctions of some cells allow passage of elongate molecules as large as 18kDa, while excluding smaller, less elongate molecules. Fluorescently labeled Calmodulin (17kDa) and fluorescently labeled Troponin-C (18kDa), when microinjected into oocytes of Danio rerio, Xenopus laevis or Mus domestica, were able to transit the gap junctions between these oocytes and the granulosa cells which surrounded them. Co-microinjected with these Ca(2+)-binding proteins, Texas-red-labeled dextran (10kDa) remained in the microinjected cell. Osteocalcin (6kDa), also a Ca(2+)-binding protein, but with a wide "V" shape proved unable to transit these gap junctions. Calmodulin, but not Troponin-C, was able to transit gap junctions of gonadotropin treated WB cells in culture. We show evidence that molecules as large as 18kDa can pass through some vertebrate gap junctions, both homologous and heterologous, and that it is primarily molecular configuration which governs gap junctional permeability.

Solution- and adsorbed-state structural ensembles predicted for the statherin-hydroxyapatite system

We have developed a multiscale structure prediction technique to study solution- and adsorbed-state ensembles of biomineralization proteins. The algorithm employs a Metropolis Monte Carlo-plus-minimization strategy that varies all torsional and rigid-body protein degrees of freedom. We applied the technique to fold statherin, starting from a fully extended peptide chain in solution, in the presence of hydroxyapatite (HAp) (001), (010), and (100) monoclinic crystals. Blind (unbiased) predictions capture experimentally observed macroscopic and high-resolution structural features and show minimal statherin structural change upon adsorption. The dominant structural difference between solution and adsorbed states is an experimentally observed folding event in statherin's helical binding domain. Whereas predicted statherin conformers vary slightly at three different HAp crystal faces, geometric and chemical similarities of the surfaces allow structurally promiscuous binding. Finally, we compare blind predictions with those obtained from simulation biased to satisfy all previously published solid-state NMR (ssNMR) distance and angle measurements (acquired from HAp-adsorbed statherin). Atomic clashes in these structures suggest a plausible, alternative interpretation of some ssNMR measurements as intermolecular rather than intramolecular. This work demonstrates that a combination of ssNMR and structure prediction could effectively determine high-resolution protein structures at biomineral interfaces.

The (1)H NMR structure of bovine Pb(2+)-osteocalcin and implications for lead toxicity

Structural information on the effect of Pb(2+) on proteins under physiologically relevant conditions is largely unknown. We have previously shown that low levels of lead increased the amount of osteocalcin bound to hydroxyapatite (BBA 1535:153). This suggested that lead induced a more compact structure in the protein. We have determined the 3D structure of Pb(2+)-osteocalcin (49 amino acids), a bone protein from a target tissue, using (1)H 2D NMR techniques. Lead, at a stoichiometry of only 1:1, induced a similar fold in the protein as that induced by Ca(2+) at a stoichiometry of 3:1. The structure consisted of an unstructured N-terminus and an ordered C-terminal consisting of a hydrophobic core (residues 16-49). The genetic algorithm-molecular dynamics simulation predicted the lead ion was coordinated by the Gla 24 and Gla 21 residues. It is proposed that mineral binding occurs via uncoordinated Gla oxygen ions binding to calcium in hydroxyapatite. A comparison of Pb(2+)- and Ca(2+)-osteocalcin suggests Pb(2+), at a lower stoichiometry, may induce similar conformational changes in proteins and subsequent molecular processes normally controlled by calcium alone. This may contribute to a molecular mechanism of lead toxicity for calcium binding proteins. Lead exposure may alter the amount of mineral bound osteocalcin and contribute to abnormal bone remodeling.

Importance of molecular configuration in gap junctional permeability

Gap junctions between insect oocytes and follicular epithelial cells allow transit of elongate Ca(2+)-binding proteins Calmodulin (CaM, 17 kDa) and Troponin-C (Trop-C, 18 kDa), but not multi-branched dextran (10 kDa) nor the Ca(2+)-binding protein Osteocalcin (Osteo, 6 kDa). By microinjection of fluorescently labeled versions of each of these molecules we were able to obtain visual evidence that, despite their lesser molecular weight, molecules with greater cross-sections were unable to transit these gap junctions, while heavier but elongate molecules could. While CaM had previously been shown to pass through gap junctions from oocytes to their surrounding epithelial cells, the ability of CaM and Trop-C to transit the gap junctions between adjacent epithelial cells had not been demonstrated. Evidence shown here demonstrates that the homologous gap junctions among epithelial cells, like the heterologous gap junctions between epithelial cells and the oocyte they surround, allow transit of elongate molecules up to at least 18 kDa. Furthermore, the evidence for four different molecules of differing molecular weights and configurations supports the hypothesis that it is molecular configuration, not chemical activity, that primarily determines the observed permeability of gap junctions to molecules 5-6 times larger than the molecular weight limit previously acknowledged.