Energy state of chondrocytes assessed by 31P-NMR studies of preosseous cartilage

The 31P Spectral Modulus (PSM) as an Assay of Metabolic Status

The phosphorus-31 (31P) spectral modulus (PSM) is a measure of the metabolic status of cells, tissues, and organs. The PSM can be calculated from 31P nuclear magnetic resonance (31P NMR) spectra obtained from cell, tissue, or organ preparations. These 31P NMR spectra can be a measure of intact living cells, tissues, or organs, or appropriate biochemical extracts of such preparations. The 31P NMR spectrum is comprised of signals derived from organophosphate metabolites that resonate from 10 δ to -25 δ on the phosphorus chemical shift δ scale. The PSM is the ratio of the high-energy phosphate to that of the low-energy phosphate spectral integrals. These integrals may be conveniently grouped into high-energy and low-energy spectral regions, respectively, into 31P chemical shifts located between -0.13 δ to -25 δ and between 10 δ to -0.13 δ. High-energy phosphates are typically described as providing the energy necessary for the activity of cellular metabolism; chemically, they contain one or more phosphate anhydride bonds. This study demonstrates that, (1) in general, the higher the metabolic activity, the higher the PSM, and (2) the modulus calculation does not require a highly resolved 31P spectrum and can be calculated solely from the integral. The PSM was calculated among cells, tissues, and organs considered normal, diseased, and stressed. In diseased (mean 1.29 ± 0.73) and stressed (mean 1.23 ± 0.75) cells, tissues, and organs, PSM values are typically low or low relative to normal cells, tissues, or organs (mean 1.65 ± 0.90), following time-course measurements, in dynamic decline. The PSM is useful in determining the metabolic status of cells, tissues, or organs and can be employed as a calculable numeric assay for determining health status statically or over time. Calculation of the PSM can be carried out with spectra of low signal-to-noise; it relies on the minimal resolution required to detect an integral curve having a clear spectral integral inflection point at ca. -0.13 δ. Detection of an integral curve alone enables the calculation of a PSM even at levels of phosphorus concentration so low as to prevent detection of the individual or groups of metabolites, such as with in vivo or ex vivo cell, tissue, or organ determinations. This study (1) presents the foundations and fundamentals of the PSM, a living index of tissue metabolic health, and (2) demonstrates the use of spectral scan analysis in opening new vistas of biology and medicine for measuring the metabolic status of stressed and diseased tissues at a range of detectable levels for monitoring therapeutic interventions.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Osteoblasts induce glucose-derived ATP perturbations in chondrocytes through noncontact communication

Cartilage and subchondral bone communicate with each other through material and signal exchanges. However, direct evidence provided by experimental studies on their interactions is insufficient. In the present study, we establish a noncontact co-culture model with a transwell chamber to explore the energetic perturbations in chondrocytes influenced by osteoblasts. Our results indicate that osteoblasts induce more ATP generation in chondrocytes through an energetic shift characterized by enhanced glycolysis and impaired mitochondrial tricarboxylic acid cycle. Enhanced glycolysis is shown by an increase of secreted lactate and the upregulation of glycolytic enzymes, including glucose-6-phosphate isomerase (Gpi), liver type ATP-dependent 6-phosphofructokinase (Pfkl), fructose-bisphosphate aldolase C (Aldoc), glyceraldehyde-3-phosphate dehydrogenase (Gapdh), triosephosphate isomerase (Tpi1), and phosphoglycerate kinase 1 (Pgk1). Impaired mitochondrial tricarboxylic acid cycle is characterized by the downregulation of cytoplasmic aspartate aminotransferase (Got1) and mitochondrial citrate synthase (Cs). Osteoblasts induce the activation of Akt and P38 signaling to mediate ATP perturbations in chondrocytes. This study may deepen our understanding of the maintenance of metabolic homeostasis in the bone-cartilage unit.

Deletion of Glut1 in early postnatal cartilage reprograms chondrocytes toward enhanced glutamine oxidation

Glucose metabolism is fundamental for the functions of all tissues, including cartilage. Despite the emerging evidence related to glucose metabolism in the regulation of prenatal cartilage development, little is known about the role of glucose metabolism and its biochemical basis in postnatal cartilage growth and homeostasis. We show here that genetic deletion of the glucose transporter Glut1 in postnatal cartilage impairs cell proliferation and matrix production in growth plate (GPs) but paradoxically increases cartilage remnants in the metaphysis, resulting in shortening of long bones. On the other hand, articular cartilage (AC) with Glut1 deficiency presents diminished cellularity and loss of proteoglycans, which ultimately progress to cartilage fibrosis. Moreover, predisposition to Glut1 deficiency severely exacerbates injury-induced osteoarthritis. Regardless of the disparities in glucose metabolism between GP and AC chondrocytes under normal conditions, both types of chondrocytes demonstrate metabolic plasticity to enhance glutamine utilization and oxidation in the absence of glucose availability. However, uncontrolled glutamine flux causes collagen overmodification, thus affecting extracellular matrix remodeling in both cartilage compartments. These results uncover the pivotal and distinct roles of Glut1-mediated glucose metabolism in two of the postnatal cartilage compartments and link some cartilage abnormalities to altered glucose/glutamine metabolism.

Turning Nature's own processes into design strategies for living bone implant biomanufacturing: a decade of Developmental Engineering

A decade after the term developmental engineering (DE) was coined to indicate the use of developmental processes as blueprints for the design and development of engineered living implants, a myriad of proof-of-concept studies demonstrate the potential of this approach in small animal models. This review provides an overview of DE work, focusing on applications in bone regeneration. Enabling technologies allow to quantify the distance between in vitro processes and their developmental counterpart, as well as to design strategies to reduce that distance. By embedding Nature's robust mechanisms of action in engineered constructs, predictive large animal data and subsequent positive clinical outcomes can be gradually achieved. To this end, the development of next generation biofabrication technologies should provide the necessary scale and precision for robust living bone implant biomanufacturing.

Inhibition of 4-aminobutyrate aminotransferase protects against injury-induced osteoarthritis in mice

Recently we demonstrated that ablation of the DNA methyltransferase enzyme, Dnmt3b, resulted in catabolism and progression of osteoarthritis (OA) in murine articular cartilage through a mechanism involving increased mitochondrial respiration. In this study, we identify 4-aminobutyrate aminotransferase (Abat) as a downstream target of Dnmt3b. Abat is an enzyme that metabolizes γ-aminobutyric acid to succinate, a key intermediate in the tricarboxylic acid cycle. We show that Dnmt3b binds to the Abat promoter, increases methylation of a conserved CpG sequence just upstream of the transcriptional start site, and inhibits Abat expression. Dnmt3b deletion in articular chondrocytes results in reduced methylation of the CpG sequence in the Abat promoter, which subsequently increases expression of Abat. Increased Abat expression in chondrocytes leads to enhanced mitochondrial respiration and elevated expression of catabolic genes. Overexpression of Abat in murine knee joints via lentiviral injection results in accelerated cartilage degradation following surgical induction of OA. In contrast, lentiviral-based knockdown of Abat attenuates the expression of IL-1β-induced catabolic genes in primary murine articular chondrocytes in vitro and also protects against murine articular cartilage degradation in vivo. Strikingly, treatment with the FDA-approved small-molecule Abat inhibitor, vigabatrin, significantly prevents the development of injury-induced OA in mice. In summary, these studies establish Abat as an important new target for therapies to prevent OA.

GC-MS method for metabolic profiling of mouse femoral head articular cartilage reveals distinct effects of tissue culture and development

OBJECTIVE

The metabolic profile of cartilage is important to define as it relates to both normal and pathophysiological conditions. Our aim was to develop a precise, high-throughput method for gas/chromatography-mass/spectrometry (GC-MS) semi-targeted metabolic profiling of mouse cartilage.

METHOD

Femoral head (hip) cartilage was isolated from 5- and 15-week-old male C57BL/6J mice immediately after death for in vivo analyses. In vitro conditions were evaluated in 5-week-old samples cultured ±10% fetal bovine serum (FBS). We optimized cartilage processing for GC-MS analysis and evaluated group-specific differences by multivariate and parametric statistical analyses.

RESULTS

55 metabolites were identified in pooled cartilage (4 animals per sample), with 29 metabolites shared between in vivo and in vitro conditions. Multivariate analysis of these common metabolites demonstrated that culturing explants was the strongest factor altering cartilage metabolism, followed by age and serum starvation. In vitro culture altered the relative abundance of specific metabolites; whereas, cartilage development between five and 15-weeks of age reduced the levels of 36 out of 43 metabolites >2-fold, especially in TCA cycle and alanine, aspartate, and glutamate pathways. In vitro serum starvation depleted six out of 41 metabolites.

CONCLUSION

This study describes the first GC-MS method for mouse cartilage metabolite identification and quantification. We observed fundamental differences in femoral head cartilage metabolic profiles between in vivo and in vitro conditions, suggesting opportunities to optimize in vitro conditions for studying cartilage metabolism. In addition, the reductions in TCA cycle and amino acid metabolites during cartilage maturation illustrate the plasticity of chondrocyte metabolism during development.

The Metabolic Microenvironment Steers Bone Tissue Regeneration

Over the past years, basic findings in cancer research have revealed metabolic symbiosis between different cell types to cope with high energy demands under limited nutrient availability. Although this also applies to regenerating tissues with disrupted physiological nutrient and oxygen supply, the impact of this metabolic cooperation and metabolic reprogramming on cellular development, fate, and function during tissue regeneration has widely been neglected so far. With this review, we aim to provide a schematic overview on metabolic links that have a high potential to drive tissue regeneration. As bone is, aside from liver, the only tissue that can regenerate without excessive scar tissue formation, we will use bone healing as an exemplarily model system.

DNA methyltransferase 3b regulates articular cartilage homeostasis by altering metabolism

Osteoarthritis (OA) is the most common form of arthritis worldwide. It is a complex disease affecting the whole joint but is generally characterized by progressive degradation of articular cartilage. Recent genome-wide association screens have implicated distinct DNA methylation signatures in OA patients. We show that the de novo DNA methyltransferase (Dnmt) 3b, but not Dnmt3a, is present in healthy murine and human articular chondrocytes and its expression decreases in OA mouse models and in chondrocytes from human OA patients. Targeted deletion of Dnmt3b in murine articular chondrocytes results in an early-onset and progressive postnatal OA-like pathology. RNA-Seq and methylC-Seq analyses of Dnmt3b loss-of-function chondrocytes show that cellular metabolic processes are affected. Specifically, TCA metabolites and mitochondrial respiration are elevated. Importantly, a chondroprotective effect was found following Dnmt3b gain of function in murine articular chondrocytes in vitro and in vivo. This study shows that Dnmt3b plays a significant role in regulating postnatal articular cartilage homeostasis. Cellular pathways regulated by Dnmt3b in chondrocytes may provide novel targets for therapeutic approaches to treat OA.

Exercise training, creatine supplementation, and bone health in ovariectomized rats

Evidence suggests that creatine may have some beneficial effects on bone. The study aimed to investigate the effects of exercise alone or combined with creatine on bone health in ovariectomized rats. Findings show that exercise, but not creatine, has an important role in improving bone health.

INTRODUCTION

The aim of this study was to investigate the effects of exercise training alone or combined with creatine supplementation on bone health parameters in ovariectomized rats.

METHODS

Wistar rats were randomly allocated into one of five groups: (i) sham-operated, (ii) ovariectomized non-trained placebo-supplemented, (iii) ovariectomized non-trained creatine-supplemented, (iv) ovariectomized exercise-trained placebo-supplemented, and (v) ovariectomized exercise-trained creatine-supplemented. Downhill running training and/or creatine supplementation (300 mg/kg body weight) were administered for 12 weeks. Bone mineral content (BMC), bone mineral density (BMD), and biomechanical and histomorphometric parameters were assessed.

RESULTS

No interaction effects were observed for BMC and BMD at whole body, femur, and lumbar spine (p > 0.05). Importantly, a main effect of training was detected for whole body BMC and BMD (p = 0.003 and p < 0.001, respectively), femoral BMC and BMD (p = 0.005 and p < 0.001, respectively), and lumbar spine BMC and BMD (p < 0.001 and p < 0.001, respectively), suggesting that the trained animals had higher bone mass, irrespective of creatine supplementation. Main effects of training were also observed for maximal load (p < 0.001), stiffness (p < 0.001), and toughness (p = 0.046), indicating beneficial effects of exercise training on bone strength. Neither a main effect of supplementation nor an interaction effect was detected for biomechanical parameters (p > 0.05). No main or interaction effects were observed for any of the histomorphometric parameters evaluated (p > 0.05).

CONCLUSIONS

Exercise training, but not creatine supplementation, attenuated ovariectomy-induced bone loss in this rat model.

Bicarbonate-dependent pH(i) regulation by chondrocytes within the superficial zone of bovine articular cartilage

Control of chondrocyte pH (pH(i)) determines articular cartilage matrix metabolism. However, the transporters of chondrocytes in situ throughout cartilage zones are unclear, and we tested the hypothesis that chondocytes within the superficial zone (SZ) utilise a HCO(3) (-)-dependent system absent from other zones. Imaging of single BCECF-labelled cells was used to monitor the pH(i) of in situ chondrocytes within the cartilage zones, and also that of cells isolated from the SZ or full depth (FD) explants. Resting pH(i) and intrinsic buffering power (beta(i)) in HEPES-buffered saline was not different between SZ and DZ cells, however the pH(i) of SZ chondrocytes was lower in HCO(3) (-) saline. Ammonium pre-pulse was used to acid-load cells and pH(i) recovery by in situ or isolated SZ chondrocytes shown to be totally dependent on HCO(3) (-). pH(i) recovery rate was significantly (P < 0.05) greater for in situ cells, suggesting that isolation damaged the HCO(3) (-)-dependent system. Recovery of pH(i) by in situ cells was blocked by the anion transport inhibitor DIDS, and partially inhibited by EIPA probably non-specifically. Recovery of pH(i) by acidified MZ or DZ cells or those isolated from FD explants was not affected by HCO(3) (-) (P > 0.05). Na(+)-dependent HCO(3) (-)-(NBC) transporters were identified in SZ chondrocytes by fluorescence immunohistochemistry suggesting that this system might account for the HCO(3) (-)-dependent recovery of pH(i). Bovine articular cartilage chondrocytes possess a HCO(3) (-)-dependent transporter which plays a key role in pH(i) regulation in cells in the SZ, but not in chondrocytes within deeper cartilage zones.

Streptozotocin-induced diabetes influences the activity of ecto-nucleoside triphosphate diphosphohydrolase 1 of rat osseous plate membranes

We report the kinetic characterization of an ecto-nucleosidetriphosphate diphosphohydrolase 1 from rat osseous plate membranes in streptozotocin-induced diabetic rats, which arises during ectopic mineralization twenty days after a subcutaneous implantation of demineralized bone matrix, Insulin deficiency decreased the ecto-nucleoside triphosphate diphosphohydrolase activity from 1293.1 +/- 39.8 (control rats) to 556.0 +/- 8.2 nmol Pi/(min mg). Two families of ATP hydrolyzing sites showed cooperative effects with specific activities of 256.2 +/- 7.7 nmol Pi/(min mg) and 299.8 +/- 8.9 nmol Pi/(min mg), and studies on the stimulation of the enzyme by magnesium and calcium ions showed that the decrease in enzyme activity results from changes in the affinity of the enzyme for these ions. To our knowledge this is the first study associating the effects of type I diabetes with an ecto-nucleoside triphosphate diphosphohydrolase activity from rat osseous plate membranes.

Cells, rather than extracellular matrix, nucleate apatite in glutaraldehyde-treated vascular tissue

Glutaraldehyde (GA) causes a large increase in [Ca(2+)](i) and [P(i)](i) and calcification of porcine aortic valve fibroblasts. Calcification in GA-treated vascular tissue is likely to begin intracellularly, but the potential role of extracellular matrix has not been taken into account in earlier studies. To compare the role of cells and matrix in calcification, intestinal pouches made of a lipid-extracted rat small intestine were prepared. Lipid-extracted porcine aortic valves, or cells cultured from those same valves, were placed in intestinal pouches, sealed, fixed with GA, and grafted in rat subcutis. Cells in the pouches calcified in 3 weeks whereas the valvular matrix did not calcify for 9 weeks. Cellular calcification spread to the wall of the intestinal pouches and grew heavier after 9 weeks. Similarly, smooth muscle cells calcified exclusively in GA-treated rat aorta grafted in rat subcutis for 3 weeks. Calcification of extracellular matrix was seen after 9 weeks. Cells initiate calcification and extracellular matrix serves as a substrate for the subsequent growth of apatite in GA-treated vascular tissue.

Effect of oxygen tension and alginate encapsulation on restoration of the differentiated phenotype of passaged chondrocytes

The implantation of laboratory-grown tissue offers a valuable alternative approach to the treatment of cartilage defects. Procuring sufficient cell numbers for such tissue-engineered cartilage is a major problem since amplification of chondrocytes in culture typically leads to loss of normal cell phenotype yielding cartilage of inferior quality. In an effort to overcome this problem, we endeavored to regain the differentiated phenotype of chondrocytes after extensive proliferation in monolayer culture by modulating cell morphology and oxygen tension towards the in vivo state. Passaged cells were encapsulated in alginate hydrogel in an effort to regain the more rounded shape characteristic of differentiated chondrocytes. These cultures were exposed to reduced (5%-i.e., physiological), or control (20%) oxygen tensions. Both alginate encapsulation and reduced oxygen tension significantly upregulated collagen II and aggrecan core protein expression (differentiation markers). In fact, after 4 weeks in alginate at 5% oxygen, differentiated gene expression was comparable to primary chondrocytes. Collagen I expression (dedifferentiation marker) decreased dramatically after alginate entrapment, while reduced oxygen tension had no effect. It is concluded that alginate encapsulation and reduced oxygen tension help restore key differentiated phenotypic markers of passaged chondrocytes. These findings have important implications for cartilage tissue engineering, since they enable the increase in differentiated cell numbers needed for the in vitro development of functional cartilaginous tissue suitable for implantation.

Effect of oxygen tension on chondrocyte extracellular matrix accumulation

Since cartilage is mainly an avascular tissue, chondrocytes exist in a low-level oxygen environment in vivo. In the present study, we investigated the effect of oxygen tension (20%, 5% and 1% gas phase oxygen concentrations) over a 20-day period on the extracellular matrix accumulation of bovine articular chondrocytes in confluent surface cultures. Matrix accumulation was assessed by the amount of glycosaminoglycan and collagen deposited in the matrix. From initially confluent monolayers, the chondrocytes became distributed throughout a thick layer of extracellular matrix, thus forming a multicell-layer of tissue. Cells maintained their normal rounded shape, indicative of the differentiated phenotype, throughout the 20-day culture period. On a per culture and a per cell basis, the amount of collagen and glycosaminoglycan accumulation in the matrix was lower at the reduced oxygen tensions. Specifically, in 1% oxygen, matrix GAG content reached a steady-state level, with no net increase in GAG levels after two weeks, whereas in 20% oxygen, matrix GAG increased with time. It is concluded that oxygen has a significant effect on the amount of macromolecules accumulated in the extracellular matrix. The implications of these findings in growing cartilage constructs in vitro are discussed.

31P NMR spectroscopy of developing cartilage produced from chick chondrocytes in a hollow-fiber bioreactor

31P NMR was used to measure the concentrations and spin-lattice relaxation times of phosphorus-containing metabolites in neocartilage developing in an NMR-compatible hollow-fiber bioreactor over four weeks. Separate studies were performed for tissue developing from chondrocytes taken from the proximal and the distal sternum of the chick embryo. The metabolite ratio beta-ATP/Pi did not change significantly with development (proximal: beta-ATP/Pi = 0.38+/- 0.12 at one week, beta-ATP/Pi = 0.44+/-0.07 at four weeks, P< 0.63; distal: beta-ATP/Pi = 0.39+/-0.05 at one week, beta-ATP/Pi = 0.66+/- 0.26 at four weeks, P<0.28). ATP spin-lattice relaxation times were found to be comparable to those in muscle and brain tissue (proximal: T(1)(beta-ATP) = 0.5+/-0.06 sec at one week, T(1)(beta-ATP) = 0.4+/- 0.01 sec at four weeks; distal: T(1)(beta-ATP) = 0.3+/-0.12 sec at one week, T(1)(beta-ATP) = 0.4+/-0.04 sec at four weeks). A large increase in the spin-lattice relaxation time of inorganic phosphate, from 1.2+/-0.13 sec to 3.8+/-0.04 sec (P<0.0001) over four weeks of growth, was observed in tissue developing from chondrocytes harvested from the proximal sternum. No comparable increase in T(1)(Pi) was found in tissue developing from chondrocytes harvested from the distal portion of the sternum, which ossifies later in vivo.

Sensitivity of chondrocytes of growing cartilage to reactive oxygen species

Vascular invasion of calcified cartilage, during endochondral ossification, is initiated and sustained by invasive cells (endothelial cells and macrophages) which degrade the tissue by releasing lytic enzymes. Concurrently, reactive oxygen species (ROS) are also released by these cells and we hypothesize that ROS also contribute to the degradation of the tissue. As a preliminary approach to this problem, the antioxidant activities and the effect of ROS on hypertrophic cartilage and chondrocytes (HCs) were investigated. Compared to resting or articular chondrocytes, HCs exhibited higher catalase but lower SOD specific activities and lower PHGPx concentration, thus revealing a defence activity specific against H2O2. Moreover, dose-dependent depletion of ATP occurred after few minutes of exposure to ROS, and a long-term treatment (16 h incubation with ROS) promoted the release of LDH activity and a significant variation of the poly- to mono-unsaturated fatty acid ratio. Finally, the incubation of HCs with low ROS doses induced the release of sedimentable alkaline phosphatase activity (matrix vesicles). How the obtained results fit the in vivo occurring events is discussed.

Kinetic characterization of a membrane-specific ATPase from rat osseous plate and its possible significance on endochodral ossification

Treatment with phosphatidylinositol-specific phospholipase C of rat osseous plate membranes released up to 90-95% of alkaline phosphatase, but a specific ATPase activity (optimum pH = 7.5) remained bound to the membrane. The hydrolysis of ATP by this ATPase was negligible in the absence of magnesium or calcium ions. However, at millimolar concentrations of magnesium and calcium ions, the membrane-specific ATPase activity increased to about 560-600 U/mg, exhibiting two classes of ATP-hydrolysing sites, and site-site interactions. GTP, UTP, ITP, and CTP were also hydrolyzed by the membrane-specific ATPase. Oligomycin, ouabain, bafilomycin A1, thapsigargin, omeprazole, ethacrynic acid and EDTA slightly affected membrane-specific ATPase activity, while vanadate produced a 18% inhibition. The membrane-specific ATPase activity was insensitive to theophylline, but was inhibited 40% by levamisole. These data suggested that the membrane-specific ATPase activity present in osseous plate membranes, and alkaline phosphatase, were different proteins.

Evidence of the presence of a specific ATPase responsible for ATP-initiated calcification by matrix vesicles isolated from cartilage and bone

Accumulating evidence indicates that calcification by isolated mammalian matrix vesicles (MVs) can be initiated by ATP. Since ATP can be hydrolyzed by either a specific ATPase or by nonspecific alkaline phosphatase (ALP), it remains to be established whether ATPase or ALP mediates ATP-initiated Ca and Pi deposition. To support the hypothesis that specific ATPase is responsible for ATP-initiated calcification by MVs isolated from mammalian cartilage and bone, the effects of ATP analogs, ALP substrates, and specific inhibitors on ATP hydrolysis and ATP-initiated calcification were compared between intact MVs and monoclonal antibody affinity-purified MV ALP. ATP analogs such as ADP and AMP exerted marked inhibitory effects on both [gamma-32P]ATP hydrolysis and ATP-initiated calcification by intact MVs, whereas phosphomonoesters such as beta-glycerophosphate or phosphoethanolamine had no effect. In contrast to intact MVs, purified MV ALP failed to calcify, and its [gamma-32P]ATP hydrolytic activity was readily inhibited by phosphomonoesters. Additionally, [gamma-32P]ATP hydrolysis by purified ALP in contrast to that by intact vesicles was completely inhibited by l-tetramisole, a specific inhibitor of ALP, suggesting a loss of specific ATPase during purification. Vanadate inhibition of ATP hydrolysis by purified ALP can be decreased by increasing ATP concentrations. On the contrary, ATP concentrations did not affect vanadate inhibition of ATP hydrolysis by intact MVs if ALP activity was blocked by l-tetramisole. These observations, therefore, suggest that: 1) a portion of [gamma-32P]ATP hydrolysis by MVs is attributable to a specific ATPase, whereas the remaining activity is due to ALP; and 2) a specific ATPase, but not ALP, is responsible for ATP-dependent Ca- and Pi-depositing activity of MVs isolated from bone or cartilage.

Similarity in calcium channel activity of annexin V and matrix vesicles in planar lipid bilayers

Matrix vesicles (MVs), structures that accumulate Ca2+ during the initiation of mineral formation in growing bone, are rich in annexin V. When MVs are fused with planar phospholipid bilayers, a multiconductance Ca2+ channel is formed, with activity essentially identical to that observed when annexin V is delivered to the bilayer with phosphatidylserine liposomes. Ca2+ currents through this channel, from either MV or annexin V liposomes, are blocked by Zn2+, as is Ca2+ uptake by MV incubated in synthetic cartilage lymph. Blockage by Zn2+ was most effective when applied to the side containing the MV or liposomes. ATP and GTP differentially modulated the activity of this channel: ATP increased the amplitude of the current and the number of conductance states; GTP dramatically reduced the number of events and conductance states, leading to well-defined Ca2+ channel activity from either MV or the annexin V liposomes. In the distinctive effects of ATP, GTP, and Zn2+ on the Ca2+ channel activity observed in both the MV and the liposome systems, the common factor was the presence of annexin V. From this we conclude that Ca2+ entry into MV results from the presence of annexin V in these membrane-enclosed structures.

Matrix vesicles and focal proteoglycan aggregates are the nucleation sites revealed by the lanthanum incubation method: a correlated study on the hypertrophic zone of the rat epiphyseal cartilage

Correlated studies were performed with light and electron microscopy, and backscattered electron image in conjunction with X-ray microanalysis, of lanthanum-incubated epiphyseal cartilage of the young rat. The hall-mark of this procedure is the appearance of LaP electron-dense deposits (not present in control sections) in precise sites of the hypertrophic zone. The ultrastructural study revealed a dual nature of these sites: "dense matrix vesicles" and "focal filament aggregates". The dense matrix vesicles are a specific type of matrix vesicle with the intrinsic capacity of precipitating LaP mineral, as soon as they originate from the hypertrophic chondrocytes. Furthermore, the matrix vesicles were found to be heterogeneous because lanthanum-devoid, "light matrix vesicles" were also present. The focal filament aggregates, which were not recognized in unstained sections and in controls, are apparently focal concentrations of proteoglycans with high lanthanum binding capacity, although the presence in them of other components (e.g., type X collagen, C-propeptide of type II collagen) cannot be excluded. The were in close connection with the light matrix vesicles in the upper hypertrophic zone, and were loaded with a variable quantity of LaP irregular electron-dense deposits in the lower hypertrophic zone. These irregular deposits are similar to, but distinct from, calcification nodules. The lanthanum incubation method indirectly detects the matrix Ca-binding components (which bind La ions), and the calcification initiation sites (which precipitate a LaP-mineral phase). A sequence is proposed of successive steps of LaP nucleation within the focal filament aggregates, which possibly mimics calcium phosphate deposition. Such a sequence seems to require the participation not only of dense matrix vesicles, but also of the filamentous components of the focal aggregates, possibly together with the activity of alkaline phosphatase.

Opposing effects on mitochondrial membrane potential by malonate and levamisole, whose effect on cell-mediated mineralization is antagonistic

The act of chondrocyte preparation for primary, enchondral, mineralization is associated with a decline in mitochondrial respiration toward the end of the proliferative zone and the hypertrophic zone in the growth plate. Dexamethasone (Dex)-stimulated cultures of rat marrow stroma constitute a differentiation model simulating, in its energy metabolism, chondrocyte mineralization. In this model, early inhibition of succinate dehydrogenase (SDH) enriches the culture with mineralizing cells, whereas levamisole inhibits mineralization. Dex also increases mitochondrial membrane potential in stromal cells, especially on days 7-8 of stimulation. In the present study, suicide inhibition of SDH, by nitropropionic acid (NPA), in Dex-stimulated cells showed a dose-dependent increase in day 21 mineralization; the maximal effect was induced on days 2-4 of stimulation. Mineralization under 2-day-long exposure to NPA showed a similar trend to the previously studied effect of continuous exposure to malonate applied between days 3-11. Unlike malonate, the effect of NPA required its presence in the cultures for only 2 days and resulted in higher mineralization than that seen under 8 days of malonate. NPA delineated a period, days 2/4 to 7/9, in which inhibition of succinate oxidation is necessary to augment mineralization. During this period, NPA also exhibited OPC selection capacity. Early application of levamisole, under conditions previously shown to decrease day 21 mineralization, maintained mitochondrial membrane potential at the beginning of Dex stimulation but decreased or had little effect on it during days 5-10. By contrast, malonate previously found to increase day 21 mineralization decreased the membrane potential at the beginning of Dex stimulation but increased it later on day 7, or during days 5-10. These results indicate that during osteoprogenitor differentiation, before the mineralization stage, a surge in mitochondrial inner membrane potential during late matrix maturation may be a marker that heralds the extracellular matrix mineralization.

Control of matrix synthesis in isolated bovine chondrocytes by extracellular and intracellular pH

The effects of extracellular and intracellular pH on matrix synthesis by isolated bovine chondrocytes were studied using radioisotope incorporation (35SO4 and 3H proline) and fluorescence techniques. Matrix synthesis exhibited a bimodal relation with decreased extracellular pH; with slight reductions (7.4 > pH > 7.1), synthesis increased (by up to 50%), whereas in more acidic media (pH < 7.1), synthesis was inhibited by up to 75% of control levels. The pHi was largely unchanged with extracellular acidity over the range producing stimulation of matrix synthesis but fell when exposed to the more acidic media shown to have an inhibitory action on matrix synthesis. The inhibition of matrix synthesis by lactic acid addition was unaffected by the lactic acid transporter alpha-CHC, suggesting H+ transport by this pathway is small. Direct imposition of a sustained intracellular acidosis (pHi = 6.65) using ammonium prepulse with amiloride inhibited matrix synthesis by about 20%. These results show that matrix synthesis by chondrocytes was affected by extracellular pH, an action which could not be entirely explained by changes to pHi.

High hydrostatic pressures in traumatic joints require elevated synovial capillary pressure probably associated with arteriolar vasodilatation

Three out of the four Starling pressures were determined at arthroscopy of traumatic effusions of the knee. The range of the joint fluid hydrostatic pressure Pjoint was 5-83 cmH2O (0.5-8.1 kPa, 4-61 mmHg), that of the colloid osmotic pressure difference COPplasma-COPjoint 0-21.7 cmH2O. In 11 of 15 cases the sum Pjoint+COP difference exceeded 32.6 cmH2O (3.19 kPa, 24 mmHg), a high estimate of average capillary pressure at the level of the heart. The number of 'exceeding' cases was 8/15 if only 80% of the COP difference was considered effective. Pjoint and the COP difference oppose filtration of fluid from plasma into joints, indicating that mean capillary pressure, the only Starling pressure not determined, was elevated unless the effusions were being resorbed back into the blood. The findings can be explained by tamponade compensated by arteriolar vasodilatation, suspected to be metabolically mediated.