Studies of nucleotides of growth-plate cartilage: evidence linking changes in cellular metabolism with cartilage calcification

Cited2 modulates hypoxia-inducible factor-dependent expression of vascular endothelial growth factor in nucleus pulposus cells of the rat intervertebral disc

OBJECTIVE

To determine whether nucleus pulposus cells of the intervertebral disc express hypoxia-inducible factor 2alpha (HIF-2alpha), and to assess the role of HIF-1 and HIF-2 in controlling cited2 and vascular endothelial growth factor (VEGF) expression.

METHODS

Rat cells were cultured under normoxic (21% O2) or hypoxic (2% O2) conditions, and expression and promoter activity of HIF-2 target genes were evaluated. Gain- or loss-of-function experiments were performed to investigate the contribution of HIF isoforms to cited2 activity as well as the role of cited2 in regulating VEGF expression.

RESULTS

We found that HIF-2alpha protein was expressed in vivo and that protein and messenger RNA expression were similar under both normoxic and hypoxic conditions. However, there was a significant increase in HIF-2alpha transactivation under hypoxic conditions. With respect to functional activity, unlike the case in most other tissues, HIF-2 failed to increase the transcriptional activities of superoxide dismutase 2 and frataxin, 2 common target genes involved in radical dismutation. However, under hypoxic conditions, HIF-2 preferentially regulated the expression and promoter activity of cited2, a p300 binding protein. When HIF-2alpha or HIF-1alpha was suppressed, cited2 promoter activity was inhibited. Finally, we showed that forced expression or suppression of cited2 resulted in corresponding changes in expression of VEGF, a common target gene for HIF-1 and HIF-2 in the nucleus pulposus cells.

CONCLUSION

Results of this study indicate that in nucleus pulposus cells, HIF-2 and HIF-1 modulate their own transcriptional activity through cited2. We suggest that the 2 arms of the regulatory circuit serve to maintain survival activities and inhibit angiogenesis in the healthy disc.

Normoxic stabilization of HIF-1alpha drives glycolytic metabolism and regulates aggrecan gene expression in nucleus pulposus cells of the rat intervertebral disk

The nucleus pulposus is an aggrecan-rich, avascular tissue that permits the intervertebral disk to resist compressive loads. In the disk, nucleus pulposus cells express hypoxia-inducible factor (HIF)-1alpha, a transcription factor that responds to oxygen tension and regulates glycolysis. The goal of the present study was to examine the importance of HIF-1alpha in rat nucleus pulposus cells and to probe the function of this transcription factor in terms of regulating aggrecan gene expression. We found that HIF-1alpha protein levels and mRNA stability were similar at 20 and 2% O(2); there was a small, but significant increase in HIF-1alpha transactivation domain activity in hypoxia. With respect to HIF-1alpha target genes GAPDH, GLUT-1, and GLUT-3, mRNA and protein levels were independent of the oxygen tension. Other than a modest increase in 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase reporter activity, the oxemic state did not change GAPDH, GLUT-1, and GLUT-3 promoter activities. Treatment of cells with 2-deoxyglucose (2-DG), a glycolytic inhibitor, resulted in a significant suppression in ATP synthesis in normoxia, whereas treatment with mitochondrial inhibitors did not affect ATP production and cell viability. However, measurement of the rate of fatty acid oxidation indicated that these cells contained functioning mitochondria. Finally, we showed that when HIF-1alpha was suppressed, irrespective of the oxemic state, there was a partial loss of aggrecan expression and promoter activity. Moreover, when cells were treated with 2-DG, there was inhibition in aggrecan promoter activity. Results of this study indicate that oxygen-independent stabilization of HIF-1alpha in nucleus pulposus cells is a metabolic adaptation that drives glycolysis and aggrecan expression.

Adenosine nucleotides in rat bone measured by ion-pair reversed-phase high-performance liquid chromatography: effect of hemorrhagic shock, with and without retransfusion of blood

The measurement of bone adenosine nucleotides (ATP. ADP. AMP) using a simple HPLC procedure is described for rat tibia; the response to hemorrhagic shock with and without blood retransfusion is also described. With respect to the measurement of nucleotides, a number of validation criteria are met. In the anesthetized intact rat (Normal) there was a declining gradient of the three nucleotides, expressed as nmol per g dry matter, from proximal over middle to distal diaphysis, with the mean ratio ATP/ADP (0.21, 0.20, 0.20) and the mean energy charge (0.34, 0.31, 0.30) being low. Irrespective of the anatomic site, hemorrhagic shock of 30-min duration evoked a further decrease versus Normal of ATP, ATP/ADP and energy charge. Blood retransfusion after shock kept nucleotides and other variables in the proximal and distal, but not the middle, diaphysis within normal limits. It was concluded that: (i) bone nucleotides are reliably measurable by HPLC, allowing the described method to be recommended for wider use in bone research and related areas; (ii) in contrast to more parenchymatous tissues, low ATP, ATP/ADP and energy charge may be characteristic for long bones, pointing towards different energy metabolism; and (iii) bone is a "shock organ", reflecting blood hypoperfusion, O2 deficiency and decreased ATP in this situation.

Chondrocyte death is linked to development of a mitochondrial membrane permeability transition in the growth plate

In the companion article, we reported that the local phosphate (Pi) concentration triggers apoptosis in epiphyseal chondrocytes. The goal of the current investigation was to evaluate the apoptotic process in relationship to the energy status of cells in the growth plate. For these studies, we used sections of the adolescent growth plate, as well as cells isolated from the tissue. We found that there was a maturation-dependent loss of mitochondrial function in growth plate chondrocytes and these cells generated energy by glycolysis. Since treatment with the uncoupler 2,4-dinitrophenol as well as the site-specific inhibitors antimycin A and rotenone failed to elicit a further increase in the activity of the glycolytic pathway, we concluded that oxidative metabolism was minimum in these cells. Flow cytometric studies of growth plate cells and confocal microscopy of growth plate sections using the mitochondrial probes Rh123 and DiOC6(3) provided unequivocal evidence that there was loss of mitochondrial membrane potential in hypertrophic cells. Furthermore, the intrinsic fluorescence of the flavoprotein lipoamide dehydrogenase complex of the electron transport chain revealed that the mitochondria were in an oxidized state. Finally, we assessed Bcl-2 expression in these cells. Although immunohistochemical and Western blot analysis showed that the chick cells contained a low level of the anti-apoptotic protein Bcl-2, reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that transcripts were present in chondrocytes. Based on these observations, we suggest that terminally differentiated chondrocytes undergo a maturation-dependent loss of mitochondrial function. In concert with the low expression of Bcl-2, they become sensitive to signals for programmed cell death. We hypothesize that Pi triggers apoptosis in these energy-compromised cells by promoting a mitochondrial membrane transition, thereby inducing the death process.

Three-dimensional culture of bovine chondrocytes in rotating-wall vessels

The Rotating-Wall Vessel (RWV) was used to culture chondrocytes for 36 d to observe the influence of low-shear and quiescent culture conditions allowing three-dimensional freedom on growth, differentiation, and extracellular matrix formation. Chondrocytes were freshly isolated from bovine cartilage and placed into the RWV with Cytodex-3 microcarriers. Nonadherent petri dishes were initiated with microcarriers as representative of standard culture conditions. In the RWV, large three-dimensional aggregates (5-7 mm) were formed in suspension. In addition, a large sheet of matrix adhered to the oxygenator core and vessel endcaps. Petri dish culture resulted in the formation of sheets of chondrocytes with no matrix production. Immunocytochemical analyses on histologic sections of tissue obtained from the RWV and the petri dish controls were performed with antibodies against fibronectin, collagen II, chondroitin-4-sulfate, chondroitin-6-sulfate, and vimentin. Results demonstrated increased signal in the RWV material while the petri dishes demonstrated a slight decrease in signal. In addition, differentiated chondrocytes were observed in sections of RWV material through 36 d, while few were observed in the sections of petri dish material. These results indicate that the unique conditions provided by the RWV afford access to cellular processes that signify the initiation of differentiation as well as production of normal matrix material.

Adaptation of chondrocytes to low oxygen tension: relationship between hypoxia and cellular metabolism

In endochondral bone, the growth cartilage is the site of rapid growth. Since the vascular supply to the cartilage is limited, it is widely assumed that cells of the cartilage are hypoxic and that limitations in the oxygen supply regulate the energetic state of the maturing cells. In this report, we evaluate the effects of oxygen tension on chondrocyte energy metabolism, thiol status, and expression of transcription elements, HIF and AP-1. Imposition of an hypoxic environment on cultured chondrocytes caused a proportional increase in glucose utilization and elevated levels of lactate synthesis. Although we observed a statistical increase in the activities of phosphofructokinase, pyruvate kinase, lactate dehydrogenase, and creatine kinase after exposure to lowered oxygen concentrations, the effect was small. The cultured cells exhibited a decreased utilization of glutamine, possibly due to down regulation of mitochondrial function and inhibition of oxidative deamination. With respect to total energy generation, we noted that these cells are quite capable of maintaining the energy charge of the cell at low oxygen tensions. Indeed, no changes in the absolute quantity of adenine nucleotides or the energy charge ratio was observed. Hypoxia caused a decrease in the glutathione content of cultured chondrocytes and a concomitant rise in cell and medium cysteine levels. It is likely that the fall in cell glutathione level is due to decreased synthesis of the tripeptide under reduced oxygen stress and the limited supply of glutamate. The observed rise in cellular and medium cysteine levels probably reflects an increase in the rate of degradation of glutathione and a decrease in synthesis of the peptide. To explore how cells transduce these metabolic effects, gel retardation assays were used to study chondrocyte HIF and AP-1 binding activities. Chondrocyte nuclear preparations bound an HIF-oligonucleotide; however, at low oxygen tensions, no increase in HIF binding was observed. In addition, we found that AP-1 binding activities in chondrocytes exposed to low oxygen tensions was elevated, although the response was lower than that exhibited by fibroblasts exposed to the same range of oxygen concentrations. We compared these results to HIF and AP-1 binding by cells in the growth plate. There was strong HIF and AP-1 binding throughout the plate, but no evidence of selective binding to any one zone. The results of the study lend strong support to the view that chondrocytes are very well adapted to low oxygen tensions; thus, under hypoxic conditions, there is a high level of expression of both HIF and AP-1, and energy conservation appears to be near-maximum.

Enzymes in mineralizing systems: state of the art

The hallmark of biological mineralization is the precise regulation of mineral deposition in space and time. The cells which produce mineralized tissues are themselves controlled by developmental programs and hormonal signals which result in regulation of gene expression and modulation of protein function. These signals are transduced into changes in enzyme levels and/or activity. Upon activation, cellular enzymes then act to synthesize the organic matrix and process it extracellularly, utilize metabolic energy to transport ions from the blood to the matrix, and to initiate the mineralization cascade. The first enzyme activity described in mineralizing tissues was alkaline phosphatase and it is still the best characterized enzyme in the mineralization process. Yet, important questions about the role of this protein remain unanswered, and it continues to occupy a central focus in mineralized tissue investigation. Other phosphatases, including protein tyrosine phosphatases are important in regulating tyrosine kinase mediated signals. Investigators have now begun to look closely at several groups of kinases which are also important for proper mineralization. As peptide hormones are important modulators of mineralized tissues, protein kinase A has always been presumed to play a key role in phosphorylating intracellular proteins. There is also considerable interest in protein kinase C, as well as tyrosine kinases in mineralized tissue signal transduction. Another group of kinases important in mineralized tissues are the enzymes which phosphorylate the matrix phosphoproteins. Of these, casein kinase II appears to be involved in intracellular and extracellular protein phosphorylation. Several enzymes present in the premineralized matrix are thought to be significant in triggering mineralization. Alkaline phosphatase may act at this level, but new data also suggests that metalloproteases and gelatinases, by modifying or digesting matrix components, may be important in the initiation of calcification.

Studies of chondrogenesis in rotating systems

A great deal of energy has been exerted over the years researching methods for regenerating and repairing bone and cartilage. Several techniques, especially bone implants and grafts, show great promise for providing a remedy for many skeletal disorders and chondrodystrophies. The bioreactor (rotating-wall vessel, RWV) is a cell culture system that creates a nurturing environment conducive to cell aggregation. Chondrocyte cultures have been studied as implants for repair and replacement of damaged and missing bone and cartilage since 1965 [Chesterman and Smith, J Bone Joint Surg 50B:184-197, 1965]. The ability to use large, tissue-like cartilage aggregates grown in the RWV would be of great clinical significance in treating skeletal disorders. In addition, the RWV may provide a superior method for studying chondrogenesis and chondrogenic mutations. Because the RWV is also reported to simulate many of the conditions of microgravity it is a very useful ground-based tool for studying how cell systems will react to microgravity.

Developmental regulation of creatine kinase activity in cells of the epiphyseal growth cartilage

During the process of endochondral bone formation, the maturing chondrocyte exhibits profound changes in energy metabolism. To explore the mechanism of energy conservation in cartilage we examined the expression of creatine kinase, an enzyme that catalyzes the formation of ATP in tissues under oxygen stress. Measurement of creatine kinase activity and cytochemical assessment of enzyme distribution clearly showed that the level of enzyme activity was related to chondrocyte maturation. Thus, as the cells hypertrophied, there was a progressive increase in creatine kinase activity. Similarly, an elevation in creatine kinase activity was noted in chondrocyte cultures as the cells assumed an hypertrophic state. When cartilage calcification was disturbed by rickets, there was a decrease in enzyme activity in the hypertrophic region. Studies were performed to examine the creatine kinase isozyme profile of cells of the epiphysis. In resting and proliferating cartilage, the isoform was MM. In hypertrophic cartilage, the predominant isoforms were MB and BB. In terms of the creatine phosphate content, the highest values were seen in the proliferative region; lower amounts were present in hypertrophic and resting cartilage; and no creatine phosphate was detected in calcified cartilage. These data suggest that turnover of creatine phosphate is greatest in the mineralized region of the epiphysis. The results of these investigations point to creatine kinase as being under developmental control. The activity of the enzyme in cartilage cells should serve as a marker of developmental events associated with chondrocyte proliferation, hypertrophy, and mineralization.

Creatine kinase activity is required for mineral deposition and matrix synthesis in endochondral growth cartilage

In earlier studies, we have drawn attention to the unique changes in energy metabolism that accompany the maturation of epiphyseal growth plate chondrocytes. The objective of this investigation was to examine the importance of the ATP generating enzyme creatine kinase (CK), in the development and mineralization of the growth plate. We inhibited CK function by administering beta-guanidinopropionic acid (beta-GPA) to rats in vivo and to cultured chick chondrocytes in vitro. We found that this agent inhibited normal development of cartilage. Disorganization of chondrocytes in the proliferative and hypertrophic zones, poor vascular invasion, and retention of calcified cartilage occurred in the long bones of beta-GPA-fed rats. beta-GPA caused a change in the electrophoretic mobility of type II and type X collagens. Inhibition of apatite formation in the bones of shell-less chick embryos was accompanied by a CK isoenzyme shift from a bone-specific phenotype to a CK isozyme profile similar to that of cartilage. The results of these studies indicate that CK activity is required for normal development of the growth plate and that interference with creatine phosphate metabolism results in profound changes in the synthesis of cartilage and the maturational activities of chondrocytes.

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

The energy state of resting and hypertrophic chondrocytes from growth plate was studied by 31P-NMR spectroscopy of superfused cartilage slices. The presence of phosphocreatine was demonstrated in both cell types, using a repetition time of 3 s. By comparing the decline in the nucleoside triphosphate level after adding blockers of the glycolysis or of the mitochondrial respiration, it was deduced that resting and hypertrophic chondrocytes use both metabolic pathways for energy production, but the glycolysis dominates. Hypertrophic cells rely more on the mitochondrial respiration than the resting cells.

Structural studies of the mineral phase of calcifying cartilage

The calcified cartilage of the epiphyseal growth plate of young calves has been studied by x-ray diffraction. Fourier transform infrared spectroscopy, magic angle 31P nuclear magnetic resonance spectroscopy, and chemical composition. The powdered tissue was separated by density centrifugation as a function of mineral content and thus qualitatively of the age of the calcium-phosphorus mineral phase. The individual density centrifugation fractions were examined separately. X-ray diffraction of the samples, especially of the lowest density fractions, revealed very poorly crystalline apatite. Fourier transform infrared spectroscopy and 31P nuclear magnetic resonance spectroscopy revealed the presence of significant amounts of nonapatitic phosphate ions. The concentration of such nonapatitic phosphates increases during the early stages of mineralization but then decreases as the mineral content steadily rises until full mineralization is achieved. The total concentration of carbonate ions was found to be much lower in calcified cartilage than in bone from the same organ (scapula). The carbonate ions are located in both A sites (OH-) and B sites (PO4(3-)), with a distribution similar to that found in bone mineral. However, discrepancies between infrared resolution factors of phosphate and carbonate bands are consistent with a heterogeneous distribution of carbonate ions in poorly organized domains of the solid phase of calcium phosphate. These initial studies permit one to characterize the calcium phosphate mineral phase as a very poorly crystalline, immature calcium phosphate apatite, rich in labile nonapatitic phosphate ions, with a low concentration of carbonate ions compared with bone mineral of the same animal, indeed from the bone of the same organ (scapula).

Differential deoxyadenosine toxicity to immature rabbit cartilage in vitro. A model for the chondro-osseous dysplasia of adenosine deaminase deficiency

Deoxyadenosine metabolism was investigated in rabbit growth plate and articular cartilage to elucidate the biochemical basis for the chondro-osseous dysplasia observed in adenosine deaminase (ADA) deficiency. Models of ADA deficiency, the combination of deoxy-adenosine and either of 2 ADA inhibitors, were selectively toxic to immature cartilage, supporting the hypothesis that the chondro-osseous dysplasia of ADA deficiency is the consequence of the enzyme deficiency. Depletion of ATP may play a role in the altered chondrocyte viability and function observed in this model.

Mechanism of de novo mineral formation by matrix vesicles

Matrix vesicles (MV) induce mineralization by compartmentalization of ion accumulation and crystal nucleation within membrane-enclosed extracellular microstructures. MV derive from cell surface microvilli by processes that cause selective enrichment of specific proteins, enzymes, lipids, and electrolytes. Incubated in synthetic cartilage lymph (SCL), MV accumulate Ca2+ and Pi, inducing mineral formation in a sequence of stages that can be altered by specific affectors. Rapid uptake of mineral ions by MV precedes formation of the first crystalline phase, octacalcium phosphate (OCP), which later converts to apatite (HAP). Early uptake of Ca2+ and Pi by MV is pH and protease sensitive, and is stimulated by o-phenanthroline (OP), a Zn2+ chelator. Recent studies reveal that a quantitatively major group of MV proteins bind to Ca2+ with high affinity in a lipid-dependent manner. These MV proteins appear to be involved in transport and accumulation of Ca2+ and Pi by MV, and may catalyze nucleation of the first mineral phase.

Adenine, guanine, and inosine nucleotides of chick growth cartilage: relationship between energy status and the mineralization process

The major aim of this investigation was to measure the nucleotide content of the developing chick epiphysis and to relate changes in nucleotide levels to chondrocyte maturation and the development of mineralization. Using a cryostat, sections of cartilage were isolated from the proximal head of the tibial growth cartilage, care being taken to preserve the metabolic integrity of the tissue. Sections were identified microscopically, pooled, and the nucleotide and nucleoside content of each sample determined by HPLC. Procedures used for the study were shown to minimize degradation of nucleotides. Their effectiveness was assessed through an evaluation of the rapid freezing technique and by examination of the effects of apatite on the recovery of endogenous and added nucleotides. Analysis of nucleotide levels in the growth cartilage indicated that chondrocytes undergo a profound change in energy metabolism during development and maturation. Thus, in the premineralized resting and proliferative zones, ATP and, to a lesser extent, GTP values were high, suggesting that the chondrocytes obtained metabolic energy through both glycolytic and mitochondrial oxidative processes. In the hypertrophic zone and in calcified cartilage, there was a profound decrease in the ATP concentration and a corresponding fall in the energy charge and the ATP/ADP ratios. The nucleotide levels in this zone indicated that there was increased reliance on nonoxidative metabolism. Measurement of nucleoside levels in premineralized cartilage suggested that there was little resynthesis of nucleotides through the salvage pathway. These observed changes in nucleotide values are consistent with earlier observations concerning chondrocyte redox and the low pO2 tension of the hypertrophic zone.2+off

Redox studies of the epiphyseal growth cartilage: pyridine nucleotide metabolism and the development of mineralization

The objective of this investigation was to examine the redox status of chondrocytes in normal and rachitic growth cartilages and to relate energy metabolism to cell maturation and the initiation of mineralization. The redox status was evaluated by chemical analysis and by microfluorimetric scanning of rapidly frozen, freeze-fractured tibial growth cartilages. In the normal epiphysis, the redox pattern of both avian and lagomorph cartilages were very similar. Thus, in the proliferative tissue zone the NAD/NADH ratio was high; in the hypertrophic zone, the cells appeared to be reduced. The sharp border between the two zones suggested that the redox shift may be associated with development of hypoxia. Induction of rickets resulted in a fivefold decrease in the total concentration of pyridine nucleotides in the proliferating and hypertrophic zones. Furthermore, the NAD/NADH ratio was profoundly disturbed. In the mineralizing zone, there was an accumulation of reduced pyridine nucleotide. Healing, initiated by administration of vitamin D to the rachitic birds, caused a rapid increase in NAD and NADH in all zones of the growth cartilage. It was concluded that vitamin D deficiency leads to changes in the energy metabolism of growth cartilage and that these changes were related to the defective mineralization of the rachitic tissue.

The deposition of calcium pyrophosphate by NTP pyrophosphohydrolase of matrix vesicles from fetal bovine epiphyseal cartilage

About 5 mumol CaPPi/mg protein was deposited within 3 h in the presence of the reaction mixtures containing 1 mM ATP, 2 mM Ca2+, 1 mM Pi, and 17 micrograms of purified NTP pyrophosphohydrolase. At 1 mM ATP, 50% of the deposition was inhibited by 0.5-1 mM of various substrate and product analogues including AMP, ADP, and ethylene hydroxyl diphosphonate. The magnitude of inhibition on NTP pyrophosphohydrolase activity was in the order of AMP = CMP = ADP greater than adenosine greater than adenine greater than NAD = NADP. AMP, CMP, ADP, and adenosine are competitive inhibitors. The modes of inhibition by adenine, NAD, and NADP differ from the competitive inhibition. Ribose, 3'-AMP, 2'-AMP, and cAMP did not inhibit the enzyme activity.

In vitro precipitation of calcium phosphate under intracellular conditions: formation of brushite from an amorphous precursor in the absence of ATP

Release of mitochondrial calcium has been shown to occur concomitant with mineral ion loading of matrix vesicles at the onset of mineralization in epiphyseal growth plate cartilage. Matrix vesicles contain amorphous calcium phosphate (ACP), a mineral form that usually results from rapid precipitation at high initial levels of Ca2+ and/or inorganic P (Pi). Since the cytosol of growth plate chondrocytes has been found to contain high levels of Pi, rapid release of mitochondrial Ca2+ into the cytosol may cause local precipitation of calcium phosphate and thus be coupled with matrix vesicle formation. Studies were carried out to determine the kinetics and nature of mineral formation that occur when small amounts of Ca2+ are added under various conditions to a Pi buffer composed of electrolytes matched in concentrations and pH to that of the cytosol of epiphyseal chondrocytes. Depending on the manner in which Ca2+ was added, ACP, dicalcium phosphate dihydrate (DCPD), or apatite (HA) first formed. In the presence of ATP, ACP was the only solid phase detected, being stable for at least 24 h. However, in its absence, ACP rapidly transformed into DCPD. Increasing the pH of the reaction buffer from 6.9 to 7.5 increased the amount of ACP initially formed, but DCPD was consistently found upon ACP transformation. Yet at pH 8.0, ACP persisted for at least 24 h. The amount of precipitate formed was proportional to the level of added Ca2+; precipitates formed when as little as 1.0 mmole was added per liter of buffer. Our findings support the possibility that rapid release of mitochondrial Ca2+ may cause localized intracellular precipitation of ACP. Since nascent ACP is known to stimulate membrane fusion and blebbing of vesicles, these findings may explain the presence of ACP in matrix vesicles. The rapid conversion of ACP to DCPD in the absence of ATP under these conditions may also explain the reported occurrence of DCPD in samples of early mineralizing tissue.

Morphochemical analysis of phosphorus pools in calcifying cartilage

The objective of this investigation was to measure phosphorus (P) levels in the epiphyseal growth cartilage and to relate pool sizes to chondrocyte maturation and tissue mineralization. To carry out these studies, we utilized a morphochemical technique that permitted measurements of insoluble mineral phosphate, soluble inorganic phosphate (Pi), low and high molecular weight phosphorylated macromolecules and lipid P in freeze-trapped histological sections. Analysis of the sections revealed that very low levels of P are present in pre-mineralized cartilage; at the mineralization front, a large increase in Pi is correlated with mineral formation. Moreover, with calcification of the cartilage, a decrease in the concentration of low molecular weight compounds was observed. It is suggested that these latter components may provide the initial source of Pi for the development of mineral. The results of the study support the view that metabolic regulation of P pool size may be a rate-limiting factor in the mineralization of cartilage.

The deposition of calcium pyrophosphate and phosphate by matrix vesicles isolated from fetal bovine epiphyseal cartilage

Since calcium (Ca) deposition by isolated fetal bovine matrix vesicles is selectively supported by nucleoside triphosphate, and since the Ca deposits appear to be amorphous by transmission electron microscopy, attempts were made to study further the nature of these Ca deposits. Calcification of isolated matrix vesicles was allowed to occur in a calcifying medium in which either inorganic phosphate (Pi) or [gamma-P]ATP was labeled with 32P. 32P in Ca P (pyrophosphate) deposits were analyzed by a Dowex 1 X 10 anion exchange chromatography. The results of the analysis indicate that the (32P) radioactivity was mainly associated with Pi when Pi in the calcifying media was labeled with 32P. In contrast, 32P was found to be associated with inorganic pyrophosphate (PPi) when [gamma-32P]ATP was used. Using a specific enzyme coupling assay for PPi, the presence of PPi in the Ca deposits was demonstrated. The amounts of Pi and PPi in the Ca deposits initiated by fetal calf matrix vesicles were found to be approximately equal. To exclude the possibility that the major part of PPi of Ca P deposit existed as adsorbed form, the deposition was performed under the conditions in which Pi was omitted from calcifying medium. The results of these experiments showed that substantial amount of PPi and Ca deposits remained the same and was not correlated to the amount of Pi in these deposits. In contrast, Pi of CaP was decreased if Pi was omitted from the calcifying medium. Thus, it appears that the major portion of PPi exists as mineral rather than adsorbed form.(ABSTRACT TRUNCATED AT 250 WORDS)

Microdissection--elemental analysis of the mineralizing growth cartilage of the normal and rachitic chick

The concentrations of elements in avian growth cartilage were studied by electron probe x-ray emission microanalysis (EDX). The cartilage was prepared for analysis by freezing, freeze-fracturing, freeze-drying, and carbon coating techniques. Cells and matrix fragments were removed from the tissue by microdissection with a tungsten needle in a scanning electron microscope (SEM) equipped with a real-time stereoscopic viewing facility. The samples were analyzed in the same SEM by EDX. Elemental analyses were performed on each fragment at a distance from the tissue sample, and hence background radiation due to the sample was eliminated. An important finding was that the intracellular potassium concentration of chondrocytes in calcified cartilage was similar to the levels in the premineralized zones. This observation supports the view that chondrocytes do not die in the process of, or as a consequence of, mineralization of the surrounding matrix. Calcium peaks were seen in the matrix at all levels and in chondrocytes immediately prior to mineralization. In contrast, phosphorus levels were always high in cells and low or absent from the premineralized matrix. At the mineralization front the appearance of a phosphorus peak in the matrix just preceded the deposition of mineral. We propose that the transfer of phosphorus from cell to matrix is a rate-limiting step in mineralization. Finally, when rachitic and normal cartilage were compared, little difference was seen in the profile of either intracellular or extracellular elements. However, in rickets the mineralized matrix remained soft in consistency. We suggest that this may reflect a phosphorus-related calcification defect that prevents growth and interlocking of the apatite crystallites.