Solid-state 31P NMR spectroscopy of bone and bone substitutes

Nuclear magnetic resonance spectroscopy to quantify major extracellular matrix components in fibro-calcific aortic valve disease

Fibro-calcific aortic valve disease (FCAVD) is a pathological condition marked by overt fibrous and calcific extracellular matrix (ECM) accumulation that leads to valvular dysfunction and left ventricular outflow obstruction. Costly valve implantation is the only approved therapy. Multiple pharmacological interventions are under clinical investigation, however, none has proven clinically beneficial. This failure of translational approaches indicates incomplete understanding of the underlying pathomechanisms and may result from a limited toolbox of scientific methods to assess the cornerstones of FCAVD: lipid deposition, fibrous and calcific ECM accumulation. In this study, we evaluated magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to both, qualitatively and quantitatively assess these key elements of FCAVD pathogenesis. NMR spectra showed collagen, elastin, triacylglycerols, and phospholipids in human control and FCAVD tissue samples (n = 5). Calcification, measured by the hydroxyapatite content, was detectable in FCAVD tissues and in valve interstitial cells under procalcifying media conditions. Hydroxyapatite was significantly higher in FCAVD tissues than in controls (p < 0.05) as measured by 31P MAS NMR. The relative collagen content was lower in FCAVD tissues vs. controls (p < 0.05). Overall, we demonstrate the versatility of NMR spectroscopy as a diagnostic tool in preclinical FCAVD assessment.

Molecular conformations and dynamics in the extracellular matrix of mammalian structural tissues: Solid-state NMR spectroscopy approaches

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Solid-state NMR spectroscopy probes molecular conformation and dynamics in intact ECM.

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Collagen conformational dynamics has roles in mechanical properties of fibrils and cell adhesion.

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Solid-state NMR spectroscopy has shed new light on  the chemical structure of bone mineral.

Solid-state NMR spectroscopy has played an important role in multidisciplinary studies of the extracellular matrix. Here we review how solid-state NMR has been used to probe collagen molecular conformations, dynamics, post-translational modifications and non-enzymatic chemical changes, and in calcified tissues, the molecular structure of bone mineral and its interface with collagen. We conclude that NMR spectroscopy can deliver vital information that in combination with data from structural imaging techniques, can result in significant new insight into how the extracellular matrix plays its multiple roles.

Phase segregation in hydroxyfluorapatite solid solution at high temperatures studied by combined XRD/solid state NMR

Hydroxyfluorapatite solid solution segregates at high temperatures; the temperature of segregation is dependent on level of fluoride substitution.

The molecular environment of phosphorus in sewage sludge ash: implications for bioavailability

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

New methods to study the composition and structure of the extracellular matrix in natural and bioengineered tissues

The extracellular matrix (ECM) comprises a gel of numerous biopolymers that occurs in a multitude of biological tissues. The ECM provides the basic support and mechanical strength of skeletal tissue and is responsible for shape retention. At the same time, the ECM is responsible for the viscoelastic properties and the elasticity of soft tissues. As expected, there are several important diseases that affect and degenerate the ECM with severe consequences for its properties. Bioengineering is a promising approach to support the regenerative capacity of the body. Unfortunately, the biomechanical properties of bioengineered ECM often only poorly meet the standards of their native counterparts. Many bioengineered tissues are characterized by an increased glycosaminoglycan (GAG) but decreased collagen content. This leads to an enhanced water content that strongly alters the viscoelastic and thus the biomechanical properties. Therefore, compositional analysis is important to estimate the tissue quality. We will show that nuclear magnetic resonance (NMR) spectroscopy and soft-ionization mass spectrometry (MS) represent useful techniques for ECM research both in natural and bioengineered tissues. Both methods are strongly complimentary: while MS techniques such as matrix-assisted laser desorption and ionization (MALDI) are excellent and very sensitive analytical tools to determine the collagen and the GAG contents of tissues, NMR spectroscopy provides insight into the molecular architecture of the ECM, its dynamics and other important parameters such as the water content of the tissue as well as the diffusion of molecules within the ECM.

Reversible assembly of β-sheet nanocrystals within caddisfly silk

Nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) experiments reveal the structural importance of divalent cation-phosphate complexes in the formation of β-sheet nanocrystals from phosphorylated serine-rich regions within aquatic silk from caddisfly larvae of the species Hesperophyla consimilis. Wide angle XRD data on native caddisfly silk show that the silk contains a significant crystalline component with a repetitive orthorhombic unit cell aligned along the fiber axis with dimensions of 5.9 Å × 23.2 Å × 17.3 Å. These nanocrystalline domains depend on multivalent cations, which can be removed through chelation with ethylenediaminetetraacetic acid (EDTA). A comparison of wide angle X-ray diffraction data before and after EDTA treatment reveals that the integrated peak area of reflections corresponding to the nanocrystalline regions decreases by 15-25% while that of the amorphous background reflections increases by 20%, indicating a partial loss of crystallinity. (31)P solid-state NMR data on native caddisfly silk also show that the phosphorylated serine-rich motifs transform from a rigid environment to one that is highly mobile and water-solvated after treatment with EDTA. The removal of divalent cations through exchange and chelation has therefore caused a collapse of the β-sheet structure. However, NMR results show that the rigid phosphorus environment is mostly recovered after the silk is re-treated with calcium. The (31)P spin-lattice (T1) relaxation times were measured at 7.6 ± 3.1 and 1 ± 0.5 s for this calcium-recovered sample and the native silk sample, respectively. The shorter (31)P T1 relaxation times measured for the native silk sample are attributed to the presence of paramagnetic iron that is stripped away during EDTA chelation treatment and replaced with diamagnetic calcium.

Experimental aspect of solid-state nuclear magnetic resonance studies of biomaterials such as bones

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is increasingly becoming a popular technique to probe micro-structural details of biomaterial such as bone with pico-meter resolution. Due to high-resolution structural details probed by SSNMR methods, handling of bone samples and experimental protocol are very crucial aspects of study. We present here first report of the effect of various experimental protocols and handling methods of bone samples on measured SSNMR parameters. Various popular SSNMR experiments were performed on intact cortical bone sample collected from fresh animal, immediately after removal from animal systems, and results were compared with bone samples preserved in different conditions. We find that the best experimental conditions for SSNMR parameters of bones correspond to preservation at -20 °C and in 70% ethanol solution. Various other SSNMR parameters were compared corresponding to different experimental conditions. Our study has helped in finding best experimental protocol for SSNMR studies of bone. This study will be of further help in the application of SSNMR studies on large bone disease related animal model systems for statistically significant results.

The pore size of PLGA bone implants determines the de novo formation of bone tissue in tibial head defects in rats

PURPOSE

The influence of the pore size of biodegradable poly(lactic-co-glycolic acid) scaffolds on bone regeneration was investigated.

METHODS

Cylindrical poly(lactic-co-glycolic acid) scaffolds were implanted into a defect in the tibial head of rats. Pore sizes of 100-300, 300-500, and 500-710 μm were tested and compared to untreated defects as control. Two and four weeks after implantation, the specimens were explanted and defect regeneration and de novo extracellular matrix generation were investigated by MRI, quantitative solid-state NMR, and mass spectrometry.

RESULTS

The pore size of the scaffolds had a pronounced influence on the quantity of the extracellular matrix synthesized in the graft; most collagen was synthesized within the first 2 weeks of implantation, while the amount of hydroxyapatite increased in the second 2 weeks. After 4 weeks, the scaffolds contained large quantities of newly formed lamellar bone while the control defects were filled by inhomogenous woven bone. Best results were obtained for scaffolds of a pore size of 300-500 μm.

CONCLUSION

Our analysis showed that the structure and dynamics of the regenerated extracellular matrix was very similar to that of the native bone, suggesting that biomineralization was significantly enhanced by the choice of the most appropriate implant material.

31P and 13C solid-state NMR spectroscopy to study collagen synthesis and biomineralization in polymer-based bone implants

A combination of solid-state NMR spectroscopy and MRI was used to evaluate the formation of extracellular matrix in poly(D,L-lactide-co-glycolide) (PLGA) bone implants. Porous PLGA scaffolds were implanted into rat tibiae and analysed after 2, 4 or 8 weeks. MRI clearly delineated the implants within the cancellous bone. Differences in the trabecular structure of the implanted material and native bone were demonstrated. In addition, implants were analyzed by solid-state NMR spectroscopy under magic angle spinning. (13)C NMR spectra showed the unambiguous signature of collagen formed in the scaffolds, but also the characteristic signals of the PLGA matrix, indicating that resorption was not complete after 8 weeks. Furthermore, (31)P NMR spectroscopy detected the inorganic component of the matrix, which is composed of bioapatite. (31)P NMR spectra were quantified and this analysis revealed that the amount of inorganic extracellular matrix formed de novo was significantly lower than in native bone. This demonstrates that solid-state NMR spectroscopy, in particular in combination with MRI, can provide useful information on the composition and structure of the extracellular matrix, and serve as a tool to evaluate the quality of tissue engineering strategies.

Structural and spectroscopic characterization of a series of potassium- and/or sodium-substituted β-tricalcium phosphate

In this paper, we report X-ray diffraction investigations as well as Raman and solid-state (31)P and (23)Na magic angle spinning nuclear magnetic resonance (NMR) characterization of three series of calcium orthophosphates. The general formulae of the studied compounds are Ca(10.5-x/2)M(x)(PO(4))(7), where M=K or Na and x=0, 0.25, 0.50, 0.75, 1.0; and Ca(10)K(x)Na(1-x)(PO(4))(7), where x=0, 0.25, 0.5, 0.75, 1.0. These calcium orthophosphates are found to be isostructural with β-tricalcium phosphate (β-TCP, Ca(3)(PO(4))(2)) with the substitution of some calcium sites by potassium and/or sodium cations. The unit cell parameters vary continuously with the level of substitution, a characteristic of these solid solutions. The Raman spectra show the different vibrational bands of the phosphate groups PO(4), while the NMR chemical shifts are sensitive to the non-equivalent phosphorus and sodium ions present in these substituted samples. As both Raman and NMR spectroscopies are local probes, they offer tools to distinguish between these different phosphorus and phosphate groups, according to their structural site and local environment, especially the type of cation substituent. A convenient decomposition of the Raman and NMR spectra into Gaussian-Lorentzian components leads us to propose an assignment of the main observed bands of these substituted β-TCPs.

Quantitative monitoring of extracellular matrix production in bone implants by 13C and 31P solid-state nuclear magnetic resonance spectroscopy

We used (31)P and (13)C solid-state nuclear magnetic resonance (NMR) spectroscopy to detect and analyze the major organic and inorganic components (collagen type I and bioapatite) in natural rabbit bone and beta-tricalcium phosphate implants loaded with osteogenically differentiated mesenchymal stem cells. High-resolution solid-state NMR spectra were obtained using the magic-angle spinning (MAS) technique. The (31)P NMR spectra of bone specimens showed a single line characteristic of bone calcium phosphate. (13)C cross-polarization (CP) MAS NMR spectra of bone exhibited the characteristic signatures of collagen type I with good resolution for all major amino acids in collagen. Quantitative measurements of (13)C-(1)H dipolar couplings indicated that the collagen segments are very rigid, undergoing only small amplitude fluctuations with correlation times in the nanosecond range. In contrast, directly polarized (13)C MAS NMR spectra of rabbit bone were dominated by signals of highly mobile triglycerides. These quantitative investigations of natural bone may provide the basis for a quality control of various osteoinductive bone substitutes. We studied the formation of extracellular bone matrix in artificial mesenchymal stem cell-loaded beta-tricalcium phosphate matrices that were implanted into the femoral condyle of rabbits. The NMR spectra of these bone grafts were acquired 3 months after implantation. In the (31)P NMR spectra, beta-tricalcium phosphate and bone calcium phosphate could be distinguished quantitatively, allowing recording of the formation of the natural bone matrix. Further, (13)C CPMAS allowed detection of collagen type I that had been produced in the implants. Comparison with the spectroscopic data from natural bone allowed assessment of the quality of the bone substitute material.

Biophysic evaluation of bone quality-application of Fourier transform infrared spectroscopy and phosphorus-31 solid-state nuclear magnetic resonance spectroscopy-

In this review, we focus on findings obtained with biophysic techniques, Fourier transformed infrared (FTIR) spectroscopy and phosphorus-31 solid-state nuclear magnetic resonance (31P solid-state NMR) spectroscopy, which may allow us to evaluate bone quality and to predict bone strength. FTIR measures the absorption energy that produces an increase in the vibrational or rotational energy of atoms or groups of atoms within the molecule. FTIR spectroscopy allows us to examine the relative amount of minerals and matrix content and the arrangement of apatite and organic matrix. FTIR spectroscopy should become an important tool, because the relative amount of minerals and the arrangement of apatite and organic matrix could be a measure for evaluating bone quality. 31P solid-state NMR spectroscopy is useful for evaluating the quality of bone and predicting bone strength by calculating the spine-lattice relaxation time (T1) of bone. 31P solid-state NMR imaging can be used to measure quantitatively the mass of hydroxyapatite. The T1 relaxation time of both bone and deficient hydroxyapatite was much longer than that of pure hydroxyapatite. T1 relaxation time is one of the promising indices of bone quality.

Mineralization followup with the use of NMR spectroscopy and others

High-resolution solid state NMR spectroscopy appears to be a powerful method for a better understanding of bone structures and bone substitutes and implants. It is particularly efficient to estimate osteoformation via bioceramic bone colonization. Pathological calcification occurring in bioprosthetic heart valves and breast prostheses can be characterized as a complement to X-ray spectrometry.

Density of organic matrix of native mineralized bone measured by water- and fat-suppressed proton projection MRI

Water- and fat-suppressed projection MR imaging (WASPI) utilizes the large difference between the proton T(2) (*)s of the solid organic matrix and the fluid constituents of bone to suppress the fluid signals while preserving solid matrix signals. The solid constituents include collagen and some molecularly immobile water and exhibit very short T(2) (*). The fluid constituents include mobile water and fat, with long T(2) (*). In WASPI, chemical shift selective low-power pi/2 pulses excite mobile water and fat magnetization which is subsequently dephased by gradient pulses, while the magnetization of collagen and immobile water remains mostly in the z-direction. Additional selective pi pulses in alternate scans further cancel the residual water and fat magnetization. Following water and fat suppression, the matrix signal is excited by a short hard pulse and the free induction decay acquired in the presence of a gradient in a 3D projection method. WASPI was implemented on a 4.7 T MR imaging system and tested on phantoms and bone specimens, enabling excellent visualization of bone matrix. The bone matrix signal per unit volume of bovine trabecular specimens was measured by this MR technique and compared with that determined by chemical analysis. This method could be used in combination with bone mineral density measurement by solid state (31)P projection MRI to determine the degree of bone mineralization.

Nuclear magnetic resonance spectroscopy of bone substitutes

Calcium phosphate bone replacement biomaterials are widely used in different applications. Structure, composition, and organization are, before implantation, analyzed with different methods. Among them, X-ray diffraction is a recognized test. As bioresorption produces more amorphous material, the process is observed and quantified via scanning electron microscopy. Comparatively high-resolution 31P solid-state nuclear magnetic resonance spectroscopy is able to analyze raw ceramics composition and to estimate osteoformation.

Evaluation of human recombinant bone morphogenetic protein-2-loaded tricalcium phosphate implants in rabbits' bone defects

Porous beta-tricalcium phosphate (betaTCP) has osteoconductive properties. The adsorption of human recombinant bone morphogenetic protein-2 (rhBMP-2) onto TCP could realize an osteoinductive bone substitute. We evaluated it on an animal model using dual-energy X-ray absorptiometry (DEXA) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. BetaTCP cylinders loaded with rhBMP-2 were implanted into rabbits' femoral condyle bone defects, and betaTCP alone as control into the contralateral femur. We studied two different doses of rhBMP-2 (10 and 40 microg) on two groups of four animals. Evaluation consisted in radiography, histology, and histomorphometry, DEXA, and NMR spectroscopy using an original method of quantification. With both doses of rhBMP-2, we observed on radiographs an increase of trabecular bone around implants. Histology showed resorption of the ceramic, trabecular bone with osteoblasts and osteoid substance around the implants, and colonization inside the porous betaTCP by new bone formed. Histomorphometry showed that the osteoid surface (OS/BS) was greatest with the high dose of rhBMP-2. The difference was slight between the low dose of rhBMP-2 and control. DEXA showed a dose-dependent increase of bone mineral density of rhBMP-2-loaded betaTCP vs. control. NMR spectroscopy confirmed that the amount of new bone formed in betaTCP was greater when betaTCP carried rhBMP-2, and increased with the dose of rhBMP-2 used. We showed that betaTCP was a good matrix for rhBMP-2, which gave it osteoinductive properties in an orthotopic site, in a dose-dependent manner. Thus, such composite biomaterial seems to be of great interest in reconstructive bone surgery. Further studies are needed in clinical practice to determine optimal doses.

Quantitative solid-state NMR imaging of synthetic calcium phosphate implants

It is shown that solid-state phosphorus-31 nuclear magnetic resonance imaging can be used to measure quantitatively the mass of hydroxyapatite (HA), a synthetic calcium phosphate used as an orthopedic implant material, in the presence of bone. A three-dimensional projection reconstruction technique was used to produce solid-state images from 998 free induction decays sampled in the presence of a fixed amplitude field gradient whose direction was varied uniformly over the unit sphere. Chemical selection is achieved using T1 contrast, as the synthetic calcium phosphate has a shorter T1 (1.8 sec at 4.7 T) compared with the bone (approximately 15 sec at 4.7 T in vivo, 42 sec ex vivo). Experimental results demonstrating the linear relationship between image intensity and HA density in phantoms containing HA and silicon (IV) oxide, and HA and bone are shown. Chemically pure images of bone mineral and synthetic HA have been computed from images of New Zealand White rabbits acquired in vivo at two different recycle times. The technique can be used to follow noninvasively the resorption and remodeling of calcium phosphate implants in vivo.

Osseointegration in cortical sheep bone of calcium phosphate implants evaluated by PIXE method and histology

Osseointegration of porous calcium phosphate ceramics evolves in several stages once implanted. Histologic analysis has often been used to evaluate the mechanism of integration of this material. Histologic parameters can be completed by physical analysis to obtain a semiquantitative evaluation of the osseointegration process. The histologic observation of hydroxyapatite (HA)-ceramic-containing bone sections was associated with proton-induced X-ray emission (PIXE) analysis and the results obtained by both methods were compared. Porous HA-ceramic cylinders were implanted in cortical bone of sheep femurs for periods ranging from 2 to 36 weeks. Thick sections of the implant containing bone were made at the end of the implantation period. A scanning line with two proton impacts 0.5 mm apart was plotted from the edges of cortical bone across the implanted ceramic and the X-rays produced were determined. Calcium, phosphorus, zinc, strontium, and iron contents were measured. Following PIXE analysis, the sections were surface-stained and observed under a light microscope to define the osseointegration index. Two regions of the curves were identified for each element characterizing either the bone tissue or the ceramic. Zinc and strontium present in the bone tissue but absent from the ceramics appeared after the 8th and the 12th implantation weeks, respectively. Iron present in the implant decreased with time, and calcium and phosphorus contents tended to be the same at the end of the implantation period in both curve regions. Histologic observation showed that immature bone invaded the pores of the outer layer of the ceramic as early as 2 weeks after implantation. Ceramics were totally osseointegrated 20 weeks after implantation. Osseointegration was apparently still evolving as judged by the PIXE method when histologic integration was considered complete.

Solid state phosphorus-31 magnetic resonance imaging of bone mineral

Chemically selective solid state phosphorus-31 nuclear magnetic resonance (NMR) imaging of the mineral phase of bone and synthetic calcium phosphate models for bone mineral is demonstrated with microscopy-scale (about 5 mm field of view) apparatus at 6.0 T magnetic field strength. Pixel-by-pixel linear combination of image data from multiple radio frequency (RF) pulse sequences, chosen to develop contrast between chemical constituents of interest in the mineral, generates derived images showing the distribution of individual constituents. The technique combines the noninvasive character of magnetic resonance imaging (MRI) with the ability of solid state NMR spectroscopy to characterize subtle chemical variations in bone mineral, as well as to measure the amount of mineral. These methods are, in principle, extensible to larger dimensional scales suitable for live animal subjects or human limbs.