Effects of Ca/Pi ratio, Ca2+ x Pi ion product, and pH of incubation fluid on accumulation of 45Ca2+ by matrix vesicles in vitro

Shedding Light on the Role of Na,K-ATPase as a Phosphatase during Matrix-Vesicle-Mediated Mineralization

Matrix vesicles (MVs) contain the whole machinery necessary to initiate apatite formation in their lumen. We suspected that, in addition to tissue-nonspecific alkaline phosphatase (TNAP), Na,K,-ATPase (NKA) could be involved in supplying phopshate (P_i) in the early stages of MV-mediated mineralization. MVs were extracted from the growth plate cartilage of chicken embryos. Their average mean diameters were determined by Dynamic Light Scattering (DLS) (212 ± 19 nm) and by Atomic Force Microcopy (AFM) (180 ± 85 nm). The MVs had a specific activity for TNAP of 9.2 ± 4.6 U·mg^-1 confirming that the MVs were mineralization competent. The ability to hydrolyze ATP was assayed by a colorimetric method and by ³¹P NMR with and without Levamisole and SBI-425 (two TNAP inhibitors), ouabain (an NKA inhibitor), and ARL-67156 (an NTPDase1, NTPDase3 and Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) competitive inhibitor). The mineralization profile served to monitor the formation of precipitated calcium phosphate complexes, while IR spectroscopy allowed the identification of apatite. Proteoliposomes containing NKA with either dipalmitoylphosphatidylcholine (DPPC) or a mixture of 1:1 of DPPC and dipalmitoylphosphatidylethanolamine (DPPE) served to verify if the proteoliposomes were able to initiate mineral formation. Around 69-72% of the total ATP hydrolysis by MVs was inhibited by 5 mM Levamisole, which indicated that TNAP was the main enzyme hydrolyzing ATP. The addition of 0.1 mM of ARL-67156 inhibited 8-13.7% of the total ATP hydrolysis in MVs, suggesting that NTPDase1, NTPDase3, and/or NPP1 could also participate in ATP hydrolysis. Ouabain (3 mM) inhibited 3-8% of the total ATP hydrolysis by MVs, suggesting that NKA contributed only a small percentage of the total ATP hydrolysis. MVs induced mineralization via ATP hydrolysis that was significantly inhibited by Levamisole and also by cleaving TNAP from MVs, confirming that TNAP is the main enzyme hydrolyzing this substrate, while the addition of either ARL-6715 or ouabain had a lesser effect on mineralization. DPPC:DPPE (1:1)-NKA liposome in the presence of a nucleator (PS-CPLX) was more efficient in mineralizing compared with a DPPC-NKA liposome due to a better orientation of the NKA active site. Both types of proteoliposomes were able to induce apatite formation, as evidenced by the presence of the 1040 cm^-1 band. Taken together, the findings indicated that the hydrolysis of ATP was dominated by TNAP and other phosphatases present in MVs, while only 3-8% of the total hydrolysis of ATP could be attributed to NKA. It was hypothesized that the loss of Na/K asymmetry in MVs could be caused by a complete depletion of ATP inside MVs, impairing the maintenance of symmetry by NKA. Our study carried out on NKA-liposomes confirmed that NKA could contribute to mineral formation inside MVs, which might complement the known action of PHOSPHO1 in the MV lumen.

Lipid composition modulates ATP hydrolysis and calcium phosphate mineral propagation by TNAP-harboring proteoliposomes

Bone biomineralization is mediated by a special class of extracellular vesicles, named matrix vesicles (MVs), released by osteogenic cells. The MV membrane is enriched in sphingomyelin (SM), cholesterol (Chol) and tissue non-specific alkaline phosphatase (TNAP) compared with the parent cells' plasma membrane. TNAP is an ATP phosphohydrolase bound to cell and MV membranes via a glycosylphosphatidylinositol (GPI) anchor. Previous studies have shown that the lipid microenvironment influences the catalytic activity of enzymes incorporated into lipid bilayers. However, there is a lack of information about how the lipid microenvironment controls the ability of MV membrane-bound enzymes to induce mineral precipitation. Herein, we used TNAP-harboring proteoliposomes made of either pure dimyristoylphosphatidylcholine (DMPC) or DMPC mixed with either Chol, SM or both of them as MV biomimetic systems to evaluate how the composition modulates the lipid microenvironment and, in turn, TNAP incorporation into the lipid bilayer by means of calorimetry. These results were correlated with the proteoliposomes' catalytic activity and ability to induce the precipitation of amorphous calcium phosphate (ACP) in vitro. DMPC:SM proteoliposomes displayed the highest efficiency of mineral propagation, apparent affinity for ATP and substrate hydrolysis efficiency, which correlated with their highest degree of membrane organization (highest ΔH), among the tested proteoliposomes. Results obtained from turbidimetry and Fourier transformed infrared (FTIR) spectroscopy showed that the tested proteoliposomes induced ACP precipitation with the order DMPC:SM>DMPC:Chol:SM≈DMPC:Chol>DMPC which correlated with the lipid organization and the presence of SM in the proteoliposome membrane. Our study arises important insights regarding the physical properties and role of lipid organization in MV-mediated mineralization.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Effect of the presence of cholesterol in the interfacial microenvironment on the modulation of the alkaline phosphatase activity during in vitro mineralization

Mineralization of the skeleton starts within cell-derived matrix vesicles (MVs); then, minerals propagate to the extracellular collagenous matrix. Tissue-nonspecific alkaline phosphatase (TNAP) degrades inorganic pyrophosphate (PP_i), a potent inhibitor of mineralization, and contributes P_i (Phosphate) from ATP to initiate mineralization. Compared to the plasma membrane, MVs are rich in Cholesterol (Chol) (∼32%) and TNAP, but how Chol influences TNAP activity remains unclear. We have reconstituted TNAP in liposomes of dipalmitoylphosphatidylcholine (DPPC) or dioleoylphosphatidylcholine (DOPC) combined with Chol or its derivatives Cholestenone (Achol) and Ergosterol (Ergo). DPPC plus 36% sterols in liposome increased the catalytic activity of TNAP toward ATP. The presence of Chol also increased the propagation of minerals by 3.4-fold. The catalytic efficiency of TNAP toward ATP was fourfold lower in DOPC proteoliposomes as compared to DPPC proteoliposomes. DOPC proteoliposomes also increased biomineralization by 2.8-fold as compared to DPPC proteoliposomes. TNAP catalyzed the hydrolysis of ATP more efficiently in the case of the proteoliposome consisting of DOPC with 36% Chol. The same behavior emerged with Achol and Ergo. The organization of the lipid and the structure of the sterol influenced the surface tension (γ), the TNAP phosphohydrolytic activity in the monolayer, and the TNAP catalytic efficiency in the bilayers. Membranes in the Lα phase (Achol) provided better kinetic parameters as compared to membranes in the Lo phase (Chol and Ergo). In conclusion, the physical properties and the lateral organization of lipids in proteoliposomes are crucial to control mineral propagation mediated by TNAP activity during mineralization.

Mesenchymal stem cell proliferation and mineralization but not osteogenic differentiation are strongly affected by extracellular pH

Osteomyelitis is a serious complication in oral and maxillofacial surgery affecting bone healing. Bone remodeling is not only controlled by cellular components but also by ionic and molecular composition of the extracellular fluids in which calcium phosphate salts are precipitated in a pH dependent manner.

OBJECTIVE

To determine the effect of pH on self-renewal, osteogenic differentiation and matrix mineralization of mesenchymal stem cells (MSCs).

METHODS

We selected three different pH values; acidic (6.3, 6.7), physiological (7.0-8.0) and severe alkaline (8.5). MSCs were cultured at different pH ranges, cell viability measured by WST-1, apoptosis detected by JC-1, senescence was analyzed by β-galactosidase whereas mineralization was detected by Alizarin Red and osteogenic differentiation analyzed by Real-time PCR.

RESULTS

Self-renewal was affected by pH as well as matrix mineralization in which pH other than physiologic inhibited the deposition of extracellular matrix but did not affect MSCs differentiation as osteoblast markers were upregulated. The expression of osteocalcin and alkaline phosphatase activity was upregulated whereas osteopontin was downregulated under acidic pH.

CONCLUSION

pH affected MSCs self-renewal and mineralization without influencing osteogenic differentiation. Thus, future therapies, based on shifting acid-base balance toward the alkaline direction might be beneficial for prevention or treatment of osteomyelitis.

Effects of pH on the production of phosphate and pyrophosphate by matrix vesicles' biomimetics

During endochondral bone formation, chondrocytes and osteoblasts synthesize and mineralize the extracellular matrix through a process that initiates within matrix vesicles (MVs) and ends with bone mineral propagation onto the collagenous scaffold. pH gradients have been identified in the growth plate of long bones, but how pH changes affect the initiation of skeletal mineralization is not known. Tissue-nonspecific alkaline phosphatase (TNAP) degrades extracellular inorganic pyrophosphate (PPi), a mineralization inhibitor produced by ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1), while contributing Pi from ATP to initiate mineralization. TNAP and NPP1, alone or combined, were reconstituted in dipalmitoylphosphatidylcholine liposomes to mimic the microenvironment of MVs. The hydrolysis of ATP, ADP, AMP, and PPi was studied at pH 8 and 9 and compared to the data determined at pH 7.4. While catalytic efficiencies in general were higher at alkaline pH, PPi hydrolysis was maximal at pH 8 and indicated a preferential utilization of PPi over ATP at pH 8 versus 9. In addition, all proteoliposomes induced mineral formation when incubated in a synthetic cartilage lymph containing 1 mM ATP as substrate and amorphous calcium phosphate or calcium-phosphate-phosphatidylserine complexes as nucleators. Propagation of mineralization was significantly more efficient at pH 7.5 and 8 than at pH 9. Since a slight pH elevation from 7.4 to 8 promotes considerably more hydrolysis of ATP, ADP, and AMP primarily by TNAP, this small pH change facilitates mineralization, especially via upregulated PPi hydrolysis by both NPP1 and TNAP, further elevating the Pi/PPi ratio, thus enhancing bone mineralization.

Idiopathic Recurrent Calcium Urolithiasis (IRCU): pathophysiology evaluated in light of oxidative metabolism, without and with variation of several biomarkers in fasting urine and plasma--a comparison of stone-free and -bearing male patients, emphasizing mineral, acid-base, blood pressure and protein status

BACKGROUND

IRCU is traditionally considered as life?style disease (associations with, among others, overweight, obesity, hypertension, type-2 diabetes), arising from excess, in 24 h urine, of calcium (Ca) salts (calcium oxalate (CaOx), calcium phosphate (CaPi)), supersaturation of, and crystallization in, tubular fluid and urine, causing crystal-induced epithelial cell damage, proteinuria, crystal aggregation and uroliths.

METHODS

Another picture emerges from the present uncontrolled study of 154 male adult IRCU patients (75 stone-bearing (SB) and 79 age-matched stone-free (SF)), in whom stone-forming and other parameters in fasting urine and plasma were contrasted with five biomarkers (see footnote) of oxidative metabolism (OM), without and with variation of markers.

RESULTS

1) In SB vs. SF unstratified OM biomarkers were statistically unchanged, but the majority of patients was overweight; despite, in SB vs. SF urine pH, total and non-albumin protein concentration were elevated, fractional urinary uric acid excretion and blood bicarbonate decreased, whereas urine volume, sodium, supersaturation with CaOx and CaPi (as hydroxyapatite) were unchanged; 2) upon variation of OM markers (strata below and above median) numerous stone parameters differed significantly, among others urine volume, total protein, Ca/Pi ratio, pH, sodium, potassium, plasma Ca/Pi ratio and parathyroid hormone, blood pressure, renal excretion of non-albumin protein and other substances; 3) a significant shift from SF to SB patients occurred with increase of urine pH, decrease of blood bicarbonate, and increase of diastolic blood pressure, whereas increase of plasma uric acid impacted only marginally; 4) in both SF and SB patients a strong curvilinear relationship links a rise of urine Ca/Pi to urine Ca/Pi divided by plasma Ca/Pi, but in SB urine Ca/Pi failed to correlate significantly with urine hydroxyapatite supersaturation; 5) also in SB, plasma Ca/Pi and urinary nitrate were negatively correlated, whereas in SF plasma Ca/Pi ratio, PTH and body mass index correlated positively; 6) multivariate regression analysis revealed that PTH, body mass index and nitrate together could explain 22 (p = 0.002) and only 7 (p = 0.06) per cent of variation of plasma Ca/Pi in SF and SB, respectively.

CONCLUSIONS

In IRCU a) numerous constituents of fasting urine, plasma, blood and blood pressure change in response to variation of OM biomarkers, suggesting involvement of OM imbalance as factor in functional deterioration of tissue; b) in the majority of patients a positive exponential relationship links urine Ca/Pi to urine Ca/Pi divided by plasma Ca/Pi, presumably to accumulate Ca outside tubular lumen, thereby minimizing intratubular and urinary Ca salt crystallization; c) alteration of interactions of low urine nitrate, PTH and Ca/Pi in plasma may be of importance in formation of new Ca stone and co-regulation of dynamics of blood vasculature; d) overweight, combined with OM-modified renal interstitial environment appears to facilitate these processes, carrying the risk that CaPi mineral develops within or/and close to blood vessel tissue, and spreads towards urothelium. - For future research focussing on IRCU pathogenesis studies are recommended on the role of affluent lifestyle mediated renal ischemia, mild hypertensive nephropathy, rise of uric acid precursor oxypurines and uricemia, clarifying also why loss of significance of interrelationships of OM biomarkers with traditional Ca stone risk factors is characteristic for SB patients.

Biomimetic Mineralization of an Aligned, Self-Assembled Collagenous Matrix

An in-vitro method of mineralizing an aligned, self-assembled collagenous matrix is presented. Reconstituted collagen fibers were mineralized by exposure to saturated solutions of calcium and phosphate of varying pH in a double diffusion chamber for seven days at room temperature. Microscopic investigation of the mineral precipitate within the fibers indicate the formation of hydroxyapatite crystals with features comparable to mineral observed in bone and avian tendon. Mechanical test results indicate that tensile strength and tangent modulus increase after mineralization in comparison to unmineralized control fibers. These results suggest that mineralization of collagen fiber in-vitro may parallel some of the events seen in mineralization of bone and turkey tendon. In addition, mineralized collagen fibers may be useful in the design of composites for the replacement or augmentation of hard tissue

Inorganic pyrophosphate as a regulator of hydroxyapatite or calcium pyrophosphate dihydrate mineral deposition by matrix vesicles

OBJECTIVE

Pathological mineralization is induced by unbalance between pro- and anti-mineralization factors. In calcifying osteoarthritic joints, articular chondrocytes undergo terminal differentiation similar to that in growth plate cartilage and release matrix vesicles (MVs) responsible for hydroxyapatite (HA) or calcium pyrophosphate dihydrate (CPPD) deposition. Inorganic pyrophosphate (PP(i)) is a likely source of inorganic phosphate (P(i)) to sustain HA formation when hydrolyzed but also a potent inhibitor preventing apatite mineral deposition and growth. Moreover, an excess of PP(i) can lead to CPPD formation, a marker of pathological calcification in osteoarthritic joints. It was suggested that the P(i)/PP(i) ratio during biomineralization is a turning point between physiological and pathological mineralization. The aim of this work was to determine the conditions favoring either HA or CPPD formation initiated by MVs.

METHODS

MVs were isolated from 17-day-old chicken embryo growth plate cartilages and subjected to mineralization in the presence of various P(i)/PP(i) ratios. The mineralization kinetics and the chemical composition of minerals were determined, respectively, by light scattering and infrared spectroscopy.

RESULTS

The formation of HA is optimal when the P(i)/PP(i) molar ratio is above 140, but is completely inhibited when the ratio decreases below 70. The retardation of any mineral formation is maximal at P(i)/PP(i) ratio around 30. CPPD is exclusively produced by MVs when the ratio is below 6, but it is inhibited for the ratio exceeding 25.

CONCLUSIONS

Our findings are consistent with the P(i)/PP(i) ratio being a determinant factor leading to pathological mineralization or its inhibition.

Bone resembling apatite by amorphous-to-crystalline transition driven self-organisation

Calcium apatite is the main inorganic constituent of mammalian hard tissues such as bones and teeth. Its formation in vivo is likely to be preceded by a transient amorphous phase. If so, the amorphous-to-crystalline transition would have some crucial role in the biomineralisation process. To investigate this possibility, a two-step biomimetic experiment was designed. First, a stable amorphous calcium apatite precursor was synthesized in simulated body fluid (SBF) and was then transformed into a low crystalline apatite. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, vacuum FTIR, inductively coupled plasma-atomic emission spectrometry (ICP-AES), scanning electron microscopy (SEM) and N(2) adsorption measurements were used to characterise both the precursor and the apatite. The latter exhibits numerous bone-like features including lack of OH, nanometer size, low crystallinity, etc. An amorphous-to-crystalline transition driven self-organisation is observed. The amorphous precursor seems to be the essential step for the creation of bone resembling apatite.

Functional involvement of PHOSPHO1 in matrix vesicle-mediated skeletal mineralization

PHOSPHO1 is a phosphatase highly expressed in bone. We studied its functional involvement in mineralization through the use of novel small molecule inhibitors. PHOSPHO1 expression was present within matrix vesicles, and inhibition of enzyme action caused a decrease in the ability of matrix vesicles to calcify.

INTRODUCTION

The novel phosphatase, PHOSPHO1, belongs to the haloacid dehalogenase superfamily of hydrolases and is capable of cleaving phosphoethanolamine (PEA) and phosphocholine to generate inorganic phosphate. Our aims in this study were to examine the expression of PHOSPHO1 in murine mineralizing cells and matrix vesicles (MV) and to screen a series of small-molecule PHOSPHO1-specific inhibitors for their ability to pharmacologically inhibit the first step of MV-mediated mineralization.

MATERIALS AND METHODS

q-PCR and immunohistochemistry were used to study the expression and localization profiles of PHOSPHO1. Inhibitors of PHOSPHO1's PEA hydrolase activity were discovered using high-throughput screening of commercially available chemical libraries. To asses the efficacy of these inhibitors to inhibit MV mineralization, MVs were isolated from TNAP-deficient (Akp2(-/-)) osteoblasts and induced to calcify in their presence.

RESULTS

q-PCR revealed a 120-fold higher level of PHOSPHO1 expression in bone compared with a range of soft tissues. The enzyme was immunolocalized to the early hypertrophic chondrocytes of the growth plate and to osteoblasts of trabecular surfaces and infilling primary osteons of cortical bone. Isolated MVs also contained PHOSPHO1. PEA hydrolase activity was observed in sonicated MVs from Akp2(-/-) osteoblasts but not intact MVs. Inhibitors to PHOSPHO1 were identified and characterized. Lansoprazole and SCH202676 inhibited the mineralization of MVs from Akp2(-/-) osteoblasts by 56.8% and 70.7%, respectively.

CONCLUSIONS

The results show that PHOSPHO1 localization is restricted to mineralizing regions of bone and growth plate and that the enzyme present within MVs is in an active state, inhibition of which decreases the capacity of MVs to mineralize. These data further support our hypothesis that PHOSPHO1 plays a role in the initiation of matrix mineralization.

Effects of analogues of inorganic phosphate and sodium ion on mineralization of matrix vesicles isolated from growth plate cartilage of normal rapidly growing chickens

The mechanism of matrix vesicle (MV) mineralization was studied using MVs isolated from normal growth plate tissue, as well as several putative intermediates in the MV mineralization pathway--amorphous calcium phosphate (ACP), calcium phosphate phosphatidylserine complex (CPLX) and hydroxyapatite (HAP). Radionuclide uptake and increase in turbidity were used to monitor mineral formation during incubation in synthetic cartilage lymph (SCL). Inhibitors of phosphate (Pi) metabolism, as well as replacing Na(+) with various cations, were used to study MV Pi transport, which had been thought to be Na(+)-dependent. MVs induced rapid mineralization approximately 3 h after addition to SCL; CPLX and HAP caused almost immediate induction; ACP required approximately 1 h. Phosphonoformate (PFA), a Pi analog, potently delayed the onset and reduced the rate of mineral formation of MV and the intermediates with IC(50)'s of 3-6 microM and approximately 10 microM, respectively. PFA:Pi molar ratios required to reduce the rate of rapid mineralization by 50% were approximately 1:30 for ACP, approximately 1:20 for HAP, approximately 1:3.3 for CPLX, and approximately 1:2.0 for MVs. MV mineralization was not found to be strictly Na(+)-dependent: substitution of Li(+) or K(+) for Na(+) had minimal effect; while N-methyl D-glucamine (NMG(+)) was totally inhibitory, choline(+) was clearly stimulatory. Na(+) substitutions had minimal effect on HAP- and CPLX-seeded mineral formation. However with ACP, NMG(+) totally blocked and choline(+) stimulated, just as they did MV mineralization. Thus, kinetic analyses indicate that ACP is a key intermediate, nevertheless, formation of CPLX appears to be the rate-limiting factor in MV mineralization.

Modeling of matrix vesicle biomineralization using large unilamellar vesicles

Stable, large unilamellar vesicles (LUV) have been constructed that model matrix vesicles (MV) in inducing de novo mineral formation when incubated in synthetic cartilage lymph (SCL). Using a dialysis method for incorporation of predetermined pure lipid, electrolyte and protein constituents, the detergent n-octyl beta-D-glucopyranoside enabled formation of stable, impermeable LUV with a diameter ( approximately 300 nm), lipid composition (phosphatidylcholine-phosphatidylserine-cholesterol, 7:2:2, molar ratio) and enclosed inorganic phosphate level (25-100 mM) similar to that of native MV. Mineral formation by these LUVs was measured by 45Ca(2+) uptake and FTIR analysis following incubation in SCL. Addition of the ionophore A23187 to SCL enabled 45Ca(2+) uptake comparable to that of native MV. FTIR analysis revealed that crystalline mineral formed in the LUV during incubation in SCL, but not in the absence of ionophore. This mineral had an IR absorption spectrum like that of the acid-phosphate-rich, octacalcium phosphate-like mineral formed by native MV. Perturbing the LUV membrane with either detergents or phospholipase A(2) following prior incubation in SCL enabled egress of mineral crystallites from the vesicle lumen, stimulating further mineral formation. Annexin V, a major protein in native MV with known Ca(2+) channel activity, incorporated into the LUV lumen or added to the external medium, induced only limited 45Ca(2+) uptake. This indicates that additional factors are required for annexin V to form Ca(2+) channels. Nevertheless for the first time, stable LUVs have been constructed with MV-like lipid, electrolyte, and protein composition and size that induce formation of mineral like that formed by native MV.

Physicochemical characterization of the nucleational core of matrix vesicles

While previous studies revealed that matrix vesicles (MV) contain a nucleational core (NC) that converts to apatite when incubated with synthetic cartilage lymph, the initial mineral phase present in MV is not well characterized. This study explored the physicochemical nature of this Ca2+ and Pi-rich NC. MV, isolated from growth plate cartilage, were analyzed directly by solid-state 31P NMR, or incubated with hydrazine or NaOCl to remove organic constituents. Other samples of MV were subjected to sequential treatments with enzymes, salt solutions, and detergents to expose the NC. We examined the NC using transmission electron microscopy, energy-dispersive analysis with x-rays, and electron and x-ray diffraction, Fourier transform-infrared spectroscopy, high performance thin-layer chromatographic analysis, and SDS-polyacrylamide gel electrophoresis. We found that most of the MV proteins and lipids could be removed without destroying the NC; however, NaOCl treatment annihilated its activity. SDS-polyacrylamide gel electrophoresis showed that annexin V, a phosphatidylserine (PS)-dependent Ca2+-binding protein, was the major protein in the NC; high performance thin-layer chromatographic analysis revealed that the detergents removed the majority of the polar lipids, but left significant free cholesterol and fatty acids, and small but critical amounts of PS. Transmission electron microscopy showed that the NC was composed of clusters of approximately 1.0 nm subunits, which energy-dispersive analysis with x-rays revealed contained Ca and Pi with a Ca/P ratio of 1.06 +/- 0. 01. Electron diffraction, x-ray diffraction, and Fourier transform-infrared analysis all indicated that the NC was noncrystalline. 1H-Cross-polarization 31P NMR indicated that the solid phase of MV was an HPO42--rich mixture of amorphous calcium phosphate and a complex of PS, Ca2+, and Pi. Taken together, our findings indicate that the NC of MV is composed of an acid-phosphate-rich amorphous calcium phosphate intermixed with PS-Ca2+-Pi, annexin V, and other proteins and lipids.

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.

Characterization and reconstitution of the nucleational complex responsible for mineral formation by growth plate cartilage matrix vesicles

Previous studies revealed that matrix vesicles (MV) have an acid-labile nucleationally active core (ALNAC) essential for mineral formation; current studies were aimed at characterizing and reconstituting ALNAC. SDS-PAGE and FTIR analyses revealed the presence of lipids, proteins and amorphous calcium phosphate (ACP) in ALNAC. Extraction with chloroform-methanol reduced, but did not destroy MV calcification; treatment with chloroform-methanol-HCl destroyed all activity. This acidic solvent extracted the annexins, (phosphatidylserine (PS)-dependent Ca(2+)-binding proteins), and dissociated PS-Ca(2+)-Pi complexes present in the MV. Attempts to reconstitute ALNAC, centered on the Ca(2+)-PS-Pi complex. Various pure lipids, electrolytes and proteins were combined to form a synthetic nucleationally active complex (SNAC), analyzing the rate of Ca2+ uptake. Inclusion of phosphatidylethanolamine (PE) or sphingomyelin (SM) with PS, or Mg2+ or Zn2+ with Ca2+, strongly inhibited activity; incorporation of annexin V increased SNAC activity. Thus, approaching from either deconstruction or reconstruction, it appears that ALNAC is composed of ACP complexed with PS and the annexins. Other lipids, proteins and electrolytes modulate its activity. These findings also indicate how ALNAC must be formed in vivo.

Morphological and biochemical characterization of mineralizing primary cultures of avian growth plate chondrocytes: evidence for cellular processing of Ca2+ and Pi prior to matrix mineralization

Advances in the culture of mineralizing growth plate chondrocytes provided an opportunity to study endochondral calcification under controlled conditions. Here we report that these cultures synthesize large amounts of proteins characteristically associated with mineralization: type II and X collagens, sulfated proteoglycans, alkaline phosphatase, and the bone-related proteins, osteonectin and osteopontin. Certain chondrocytes appeared to accumulate large amounts of Ca2+ and Pi during the mineralization process: laser confocal imaging revealed high levels of intracellular Ca2+ in their periphery and X-ray microanalytical mapping revealed the presence of many Ca(2+)- and Pi-rich cell surface structures ranging from filamentous processes 0.14 +/- 0.02 microns by 0.5-2.0 microns, to spherical globules 0.70 +/- 0.27 microns in diameter. Removal of organic matter with alkaline sodium hypochlorite revealed numerous deposits of globular (0.77 +/- 0.19 micron) mineral (calcospherites) in the lacunae around these cells. The size and spatial distribution of these mineral deposits closely corresponded to the Ca(2+)-rich cell surface blebs. The globular mineral progressively transformed into clusters of crystallites. Taken with earlier studies, these findings indicate that cellular uptake of Ca2+ and Pi leads to formation of complexes of amorphous calcium phosphate, membrane lipids, and proteins that are released as cell surface blebs analogous to matrix vesicles. These structures initiate development of crystalline mineral. Thus, the current findings support the concept that the peripheral intracellular accumulation of Ca2+ and Pi is directly involved in endochondral calcification.

A facilitative role for carbonic anhydrase activity in matrix vesicle mineralization

Carbonic anhydrase (CA) which catalyzes the reversible hydrolysis of carbon dioxide is known to be important in osteoclastic bone resorption, however, suggested roles in calcium phosphate mineral formation have not been previously demonstrated. Biochemical evidence is provided for the presence of CA in growth plate matrix vesicles (MV) and the level of activity determined by enzyme assay. Inhibition of CA activity with the specific inhibitor acetazolamide resulted in reduced rates of MV mineralization. Other inhibitor studies showed that MV mineralization was also impaired by 4,4-diisothiocyanatostilbene-2, 2-disulfonic acid (DIDS), a blocker of membrane bicarbonate channels. No evidence was found for the presence of any proton pumps or channels. When acetazolamide and DIDS were combined, their inhibitory effects on MV mineralization were additive. These findings suggest that MV possess a pH regulation system composed of carbonic anhydrase and a putative bicarbonate channel. This system may function in the MV by providing intraluminal buffering capacity. The control of intravesicular pH is important for the stabilization of the acid-labile nucleational core complex and in preventing the build-up of protons during calcium phosphate phase transformations.

Carbonic anhydrase localization in normal and osteochondrotic joint cartilage of growing pigs

The histochemical localization of carbonic anhydrase in the normal and osteochondrotic epiphyseal growth cartilage from 15 growing pigs (6 to 18 weeks old) was studied. All animals were clinically normal. The entire thickness of the articular-epiphyseal cartilage complex from the femoral condyles was fixed in 2% glutaraldehyde and embedded in a water-soluble glycolmethacrylate. Sections (1-2 microns) were incubated on the surface of a medium containing cobalt, phosphate, and bicarbonate. A black precipitate formed at sites of enzymatic activity. This method shows the activity of all different isoenzymes of carbonic anhydrase. The specificity was checked by adding the carbonic anhydrase inhibitor acetazolamide to the incubation medium. Osteochondrosis in the epiphyseal growth cartilage was characterized by chondronecrotic areas in resting, proliferative, hypertrophic, and calcifying regions. When the hypertrophic and calcifying regions were involved, insufficient cartilage calcification and focally impaired ossification were seen. The chondronecrotic areas were surrounded by groups of morphologically viable cells, or so-called "clusters." Carbonic anhydrase was present in chondrocytes of hypertrophic and calcifying regions of the normal growth cartilage and in osteoclasts and erythrocytes. No evidence of carbonic anhydrase activity was found in the articular cartilage or in the resting region of normal growth cartilage in any of the pigs. No enzyme activity was found in the osteochondrotic cartilage, either in clusters or dead cells. The lack of carbonic anhydrase in the osteochondrotic cartilage demonstrated in this study may result in an inability to produce the alkaline matrix necessary for calcification and could be one reason for the insufficient calcification typical of this cartilage.

Initiation and progression of mineralization of bone nodules formed in vitro: the role of alkaline phosphatase and organic phosphate

Osteoid nodules form but do not mineralize in fetal rat calvaria cell cultures grown in alpha-minimal essential medium with 10% fetal bovine serum in the absence of Na beta-glycerophosphate (beta-GP). To study factors involved in the initiation and progression of mineralization, cultures were treated with beta-GP and radiolabelled with 0.1-0.2 microCi/ml 45Ca after nodules had formed (17-19 days in medium without beta-GP). Concentrations of beta-GP from 1 to 14 mM induced a dose-dependent increase in 45Ca uptake. 45Ca uptake was restricted to nodule-containing cultures and did not occur in cultures without nodules. Continuous labelling over 72 h compared with 2 h pulses over the same time period showed that little mineralization occurred over the first 8-12 h and that the rate of mineralization was maximal and constant after 24 h exposure to beta-GP. Calcium uptake from medium was slow during the first 12 h of beta-GP exposure but increased rapidly thereafter until the medium calcium concentration reached a steady state of between 0.5 and 0.6 mM. Measurement of calcium concentration in the medium after mineralization had been initiated (24 h after beta-GP exposure) showed a linear calcium uptake into nodules (r = 0.990) over a 7 h period at a rate of 9.2 micrograms calcium/h/culture. Initiation of mineralization was prevented by 100 microM levamisole, but not by 100 microM dexamisole. When 100 microM levamisole was added 24 h after mineralization had been initiated by the addition of beta-GP, the progression of mineralization was unaffected. Similarly, after mineralization had been initiated for 24 h by 10 mM beta-GP, mineralization continued independent of the presence of beta-GP. The data show that the initiation and progression of mineralization are separate phenomena and that organic phosphate and alkaline phosphatase play a crucial role in the initiation of mineralization but are not required for the continuation of mineralization of bone nodules.