Roles of alkaline phosphatase and labile internal mineral in matrix vesicle-mediated calcification. Effect of selective release of membrane-bound alkaline phosphatase and treatment with isosmotic pH 6 buffer

Biocompatibility and bioactive potential of NeoPUTTY calcium silicate-based cement: An in vivo study in rats

AIM

To evaluate the inflammatory reaction and the ability to induce mineralization activity of a new repair material, NeoPUTTY (NPutty; NuSmile, USA), in comparison with Bio-C Repair (BC; Angelus, Brazil) and MTA Repair HP (MTA HP; Angelus, Brazil).

METHODOLOGY

Polyethylene tubes were filled with materials or kept empty (control group, CG) and implanted in subcutaneous tissue of rats for 7, 15, 30, and 60 days (n = 6/group). Capsule thickness, number of inflammatory cells (ICs), fibroblasts, collagen content, and von Kossa analysis were performed. Unstained sections were evaluated under polarized light and by immunohistochemistry for osteocalcin (OCN). Data were submitted to two-way anova followed by Tukey's test (p ≤ .05), except for OCN. OCN data were submitted to Kruskal-Wallis and Dunn and Friedman post hoc tests followed by the Nemenyi test at a significance level of 5%.

RESULTS

At 7, 15, and 30 days, thick capsules containing numerous ICs were seen around the materials. At 60 days, a moderate inflammatory reaction was observed for NPutty, BC while MTA HP presented thin capsules with moderate inflammatory cells. In all periods, NPutty specimens contained the highest values of ICs (p < .05). From 7 to 60 days, the number of ICs reduced significantly while an increase in the number of fibroblasts and birefringent collagen content was observed. At 7 and 15 days, no significant difference was observed in the immunoexpression of OCN (p > .05). At 30 and 60 days, NPutty showed the lowest values of OCN (p < .05). At 60 days, a similar immunoexpression was observed for BC and MTA HP (p > .05). In all time intervals, capsules around NPutty, BC, and MTA HP showed von Kossa-positive and birefringent structures.

CONCLUSIONS

Despite the greater inflammatory reaction promoted by NeoPutty than BC and MTA HP, the reduction in the thickness of capsules, the increase in the number of fibroblasts, and the reduction in the number of ICs indicate that this bioceramic material is biocompatible Furthermore, NeoPutty presents the ability to induce mineralization activity.

Comparison of Bio-C Pulpo and MTA Repair HP with White MTA: effect on liver parameters and evaluation of biocompatibility and bioactivity in rats

AIM

To evaluate the tissue response promoted by Bio-C Pulpo (Bio), MTA Repair HP (MTA-HP) and White MTA (WMTA) and whether these materials cause liver changes in a rat experimental model.

METHODOLOGY

Polyethylene tubes filled with Bio, MTA-HP and WMTA, and empty tubes (control group, CG) were implanted into the subcutaneous tissues of rats for 7, 15, 30 and 60 days. Inflammatory reaction score (IRS), capsule thickness, number of inflammatory cells (IC), von Kossa reaction, interleukin-6 (IL-6) and alkaline phosphatase (ALP) immunohistochemistry reactions were performed. Combined methods, von Kossa followed by immunohistochemistry for detection of ALP, were performed. At 60 days, the serum glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) levels were measured and liver fragments were collected for histological analysis; the data were assessed by one-way ANOVA analysis followed by Sidak's post-test. The biocompatibility and bioactivity data were subjected to the two-way ANOVA analysis followed by Tukey post hoc test, except the IRS. The IRS data were subjected to the Kruskal-Wallis ANOVA non-parametric test followed by Dunn's test (p ≤ .05).

RESULTS

No significant difference was detected in serum GOT and GPT concentrations and in the number of hepatocytes among the experimental and CG samples. Although Bio-C Pulpo had the highest IC and IL-6-immunolabelled cells (p < 0.0001) at all periods, no significant difference was observed in the IRS among the materials, except at 60 days. In this period, the WMTA had lower IRS. All groups had a significant reduction in the capsule thickness and in the number of IC and IL-6-immunolabelled cells over time. Bio-C Pulpo, MTA-HP and WMTA specimens had greater immunoexpression of ALP than CG (p < .0001). At all periods, von Kossa-positive and birefringent structures were observed in the capsules around the materials. ALP-immunolabelled cells were also seen near von Kossa-positive structures.

CONCLUSIONS

Bio-C Pulpo, MTA-HP and WMTA materials did not cause morphological changes in the liver and no significant alteration in the serum GOT and GPT levels. Moreover, these bioceramic materials were biocompatible and exhibited bioactive potential. However, Bio-C Pulpo induced greater inflammatory infiltrate than MTA-HP and WMTA at all periods.

Role of matrix vesicles and crystal ghosts in bio-mineralization

Matrix vesicles (MVs) are extracellular membrane-bound vesicles of about ~ 50-200 nm in diameter that play a role in the bio-mineralization process of hard tissue formation. The present review is based on the empirical phenomenon of primary mineralization process via matrix vesicle-mediated mechanism with special reference to crystal ghosts as well as the mechanism on the organic-inorganic relationship between matrix vesicles and crystal ghosts, and the transformation that these structures undergo during bio-mineralization.

Phosphatidylserine controls calcium phosphate nucleation and growth on lipid monolayers: A physicochemical understanding of matrix vesicle-driven biomineralization

Bone biomineralization is an exquisite process by which a hierarchically organized mineral matrix is formed. Growing evidence has uncovered the involvement of one class of extracellular vesicles, named matrix vesicles (MVs), in the formation and delivery of the first mineral nuclei to direct collagen mineralization. MVs are released by mineralization-competent cells equipped with a specific biochemical machinery to initiate mineral formation. However, little is known about the mechanisms by which MVs can trigger this process. Here, we present a combination of in situ investigations and ex vivo analysis of MVs extracted from growing-femurs of chicken embryos to investigate the role played by phosphatidylserine (PS) in the formation of mineral nuclei. By using self-assembled Langmuir monolayers, we reconstructed the nucleation core - a PS-enriched motif thought to trigger mineral formation in the lumen of MVs. In situ infrared spectroscopy of Langmuir monolayers and ex situ analysis by transmission electron microscopy evidenced that mineralization was achieved in supersaturated solutions only when PS was present. PS nucleated amorphous calcium phosphate that converted into biomimetic apatite. By using monolayers containing lipids extracted from native MVs, mineral formation was also evidenced in a manner that resembles the artificial PS-enriched monolayers. PS-enrichment in lipid monolayers creates nanodomains for local increase of supersaturation, leading to the nucleation of ACP at the interface through a multistep process. We posited that PS-mediated nucleation could be a predominant mechanism to produce the very first mineral nuclei during MV-driven bone/cartilage biomineralization.

Quantitative atomic force microscopy provides new insight into matrix vesicle mineralization

Matrix vesicles (MVs) are a class of extracellular vesicles that initiate mineralization in cartilage, bone, and other vertebrate tissues by accumulating calcium ions (Ca²⁺) and inorganic phosphate (P_i) within their lumen and forming a nucleation core (NC). After further sequestration of Ca²⁺ and P_i, the NC transforms into crystalline complexes. Direct evidence of the existence of the NC and its maturation have been provided solely by analyses of dried samples. We isolated MVs from chicken embryo cartilage and used atomic force microscopy peak force quantitative nanomechanical property mapping (AFM-PFQNM) to measure the nanomechanical and morphological properties of individual MVs under both mineralizing (+Ca²⁺) and non-mineralizing (-Ca²⁺) fluid conditions. The elastic modulus of MVs significantly increased by 4-fold after incubation in mineralization buffer. From AFM mapping data, we inferred the morphological changes of MVs as mineralization progresses: prior to mineralization, a punctate feature, the NC, is present within MVs and this feature grows and stiffens during mineralization until it occupies most of the MV lumen. Dynamic light scattering showed a significant increase in hydrodynamic diameter and no change in the zeta potential of hydrated MVs after incubation with Ca²⁺. This validates that crystalline complexes, which are strongly negative relative to MVs, were forming within the lumen of MVs. These data were substantiated by transmission electron microscopy energy dispersive X-ray and Fourier transform infrared spectroscopic analyses of dried MVs, which provide evidence that the complexes increased in size, crystallinity, and Ca/P ratio within MVs during the mineralization process.

Matrix vesicles from chondrocytes and osteoblasts: Their biogenesis, properties, functions and biomimetic models

BACKGROUND

Matrix vesicles (MVs) are released from hypertrophic chondrocytes and from mature osteoblasts, the cells responsible for endochondral and membranous ossification. Under pathological conditions, they can also be released from cells of non-skeletal tissues such as vascular smooth muscle cells. MVs are extracellular vesicles of approximately 100-300nm diameter harboring the biochemical machinery needed to induce mineralization.

SCOPE OF THE REVIEW

The review comprehensively delineates our current knowledge of MV biology and highlights open questions aiming to stimulate further research. The review is constructed as a series of questions addressing issues of MVs ranging from their biogenesis and functions, to biomimetic models. It critically evaluates experimental data including their isolation and characterization methods, like lipidomics, proteomics, transmission electron microscopy, atomic force microscopy and proteoliposome models mimicking MVs.

MAJOR CONCLUSIONS

MVs have a relatively well-defined function as initiators of mineralization. They bind to collagen and their composition reflects the composition of lipid rafts. We call attention to the as yet unclear mechanisms leading to the biogenesis of MVs, and how minerals form and when they are formed. We discuss the prospects of employing upcoming experimental models to deepen our understanding of MV-mediated mineralization and mineralization disorders such as the use of reconstituted lipid vesicles, proteoliposomes and, native sample preparations and high-resolution technologies.

GENERAL SIGNIFICANCE

MVs have been extensively investigated owing to their roles in skeletal and ectopic mineralization. MVs serve as a model system for lipid raft structures, and for the mechanisms of genesis and release of extracellular vesicles.

The Expression of PHOSPHO1, nSMase2 and TNAP is Coordinately Regulated by Continuous PTH Exposure in Mineralising Osteoblast Cultures

Sustained exposure to high levels of parathyroid hormone (PTH), as observed in hyperparathyroidism, is catabolic to bone. The increase in the RANKL/OPG ratio in response to continuous PTH, resulting in increased osteoclastogenesis, is well established. However, the effects of prolonged PTH exposure on key regulators of skeletal mineralisation have yet to be investigated. This study sought to examine the temporal expression of PHOSPHO1, TNAP and nSMase2 in mineralising osteoblast-like cell cultures and to investigate the effects of continuous PTH exposure on the expression of these enzymes in vitro. PHOSPHO1, nSMase2 and TNAP expression in cultured MC3T3-C14 cells significantly increased from day 0 to day 10. PTH induced a rapid downregulation of Phospho1 and Smpd3 gene expression in MC3T3-C14 cells and cultured hemi-calvariae. Alpl was differentially regulated by PTH, displaying upregulation in cultured MC3T3-C14 cells and downregulation in hemi-calvariae. PTH was also able to abolish the stimulatory effects of bone morphogenic protein 2 (BMP-2) on Smpd3 and Phospho1 expression. The effects of PTH on Phospho1 expression were mimicked with the cAMP agonist forskolin and blocked by the PKA inhibitor PKI (5-24), highlighting a role for the cAMP/PKA pathway in this regulation. The potent down-regulation of Phospho1 and Smpd3 in osteoblasts in response to continuous PTH may provide a novel explanation for the catabolic effects on the skeleton of such an exposure. Furthermore, our findings support the hypothesis that PHOSPHO1, nSMase2 and TNAP function cooperatively in the initiation of skeletal mineralisation.

A Molecular View of Pathological Microcalcification in Breast Cancer

Breast microcalcification is a potential diagnostic indicator for non-palpable breast cancers. Microcalcification type I (calcium oxalate) is restricted to benign tissue, whereas type II (calcium hydroxyapatite) occurs both in benign as well as in malignant lesions. Microcalcification is a pathological complication of the mammary gland. Over the past few decades, much attention has been paid to exploit this property, which forms the basis for advances in diagnostic procedures and imaging techniques. The mechanism of its formation is still poorly understood. Hence, in this paper, we have attempted to address the molecular mechanism of microcalcification in breast cancer. The central theme of this communication is "how a subpopulation of heterogeneous breast tumor cells attains an osteoblast-like phenotype, and what activities drive the process of pathophysiological microcalcification, especially at the invasive or infiltrating front of breast tumors". The role of bone morphogenetic proteins (BMPs) and tumor associated macrophages (TAMs) along with epithelial to mesenchymal transition (EMT) in manipulating this pathological process has been highlighted. Therefore, this review offers a novel insight into the mechanism underlying the development of microcalcification in breast carcinomas.

Nuclear factor I-C expression pattern in developing teeth and its important role in odontogenic differentiation of human molar stem cells from the apical papilla

Nuclear factor I-C (NFIC) has an important role in the development of murine dental roots, but its role in human root formation is unreported. We thus elucidated the regulatory role of NFIC in the differentiation of human stem cells from the apical papilla (hSCAPs). The first step for this was to determine the expression of NFIC in human teeth, and it was found that NFIC expression was restricted to the odontoblasts and preodontoblasts of the developing molars of humans and mice. NFIC was found to be expressed in odontoblast-like cells after the subcutaneous transplantation of hSCAPs. NFIC expression was concomitant with dentin sialophosphoprotein (DSPP) in the mineralization of hSCAPs. NFIC knockdown in hSCAPs significantly inhibited expression of DSPP and promoted that of dentin matrix protein 1 (DMP1), meanwhile upregulated the expression of TGF-β1 and downregulated SMAD3 and SMAD4. NFIC expression was significantly upregulated after TGF-β1 treatment in hSCAPs. NFIC knockdown prolonged G1 phase of the cell cycle, but had no effect on cell proliferation and migration. These results suggest that NFIC is involved in the development of human root dentin and the regulation of odontoblastic differentiation of hSCAPs. NFIC may participate in the DMP1-DSPP signaling pathway and comprises a complex signaling cycle with TGF-β1.

The role of phosphatases in the initiation of skeletal mineralization

Endochondral ossification is a carefully orchestrated process mediated by promoters and inhibitors of mineralization. Phosphatases are implicated, but their identities and functions remain unclear. Mutations in the tissue-nonspecific alkaline phosphatase (TNAP) gene cause hypophosphatasia, a heritable form of rickets and osteomalacia, caused by an arrest in the propagation of hydroxyapatite (HA) crystals onto the collagenous extracellular matrix due to accumulation of extracellular inorganic pyrophosphate (PPi), a physiological TNAP substrate and a potent calcification inhibitor. However, TNAP knockout (Alpl(-/-)) mice are born with a mineralized skeleton and have HA crystals in their chondrocyte- and osteoblast-derived matrix vesicles (MVs). We have shown that PHOSPHO1, a soluble phosphatase with specificity for two molecules present in MVs, phosphoethanolamine and phosphocholine, is responsible for initiating HA crystal formation inside MVs and that PHOSPHO1 and TNAP have nonredundant functional roles during endochondral ossification. Double ablation of PHOSPHO1 and TNAP function leads to the complete absence of skeletal mineralization and perinatal lethality, despite normal systemic phosphate and calcium levels. This strongly suggests that the Pi needed for initiation of MV-mediated mineralization is produced locally in the perivesicular space. As both TNAP and nucleoside pyrophosphohydrolase-1 (NPP1) behave as potent ATPases and pyrophosphatases in the MV compartment, our current model of the mechanisms of skeletal mineralization implicate intravesicular PHOSPHO1 function and Pi influx into MVs in the initiation of mineralization and the functions of TNAP and NPP1 in the extravesicular progression of mineralization.

Compounded PHOSPHO1/ALPL deficiencies reduce dentin mineralization

Phosphatases are involved in bone and tooth mineralization, but their mechanisms of action are not completely understood. Tissue-nonspecific alkaline phosphatase (TNAP, ALPL) regulates inhibitory extracellular pyrophosphate through its pyrophosphatase activity to control mineral propagation in the matrix; mice without TNAP lack acellular cementum, and have mineralization defects in dentin, enamel, and bone. PHOSPHO1 is a phosphatase found within membrane-bounded matrix vesicles in mineralized tissues, and double ablation of Alpl and Phospho1 in mice leads to a complete absence of skeletal mineralization. Here, we describe mineralization abnormalities in the teeth of Phospho1(-/-) mice, and in compound knockout mice lacking Phospho1 and one allele of Alpl (Phospho1(-/-);Alpl(+/-) ). In wild-type mice, PHOSPHO1 and TNAP co-localized to odontoblasts at early stages of dentinogenesis, coincident with the early mineralization of mantle dentin. In Phospho1 knockout mice, radiography, micro-computed tomography, histology, and transmission electron microscopy all demonstrated mineralization abnormalities of incisor dentin, with the most remarkable findings being reduced overall mineralization coincident with decreased matrix vesicle mineralization in the Phospho1(-/-) mice, and the almost complete absence of matrix vesicles in the Phospho1(-/-);Alpl(+/-) mice, whose incisors showed a further reduction in mineralization. Results from this study support prominent non-redundant roles for both PHOSPHO1 and TNAP in dentin mineralization.

Isolation and characteristics of matrix vesicles

Mineralizing matrix vesicles (MVs) are extracellular organelles produced by chondrocytes, osteoblasts, and odontoblasts under physiological conditions and by vascular smooth muscle cells under pathological conditions. MVs are involved in the early stage of mineralization allowing calcium and phosphate to accumulate, and therefore providing an optimal environment facilitating hydroxyapatite formation. Here, we describe the isolation of MVs from osteoblasts and chondrocytes and present their main characteristics.

Effect of race and genetics on vitamin D metabolism, bone and vascular health

The pathophysiology of chronic kidney disease-mineral and bone disorder accounts for an inverse relationship between bone mineralization and vascular calcification in progressive nephropathy. Inverse associations between bone mineral density (BMD) and calcified atherosclerotic plaque are also observed in individuals of European and African ancestry without nephropathy, suggesting a mechanistic link between these processes that is independent of kidney disease. Despite lower dietary calcium intake and serum 25-hydroxyvitamin D (25(OH)D) concentrations, African Americans have higher BMD and develop osteoporosis less frequently than do European Americans. Moreover, despite having more risk factors for cardiovascular disease, African Americans have a lower incidence and severity of calcified atherosclerotic plaque formation than do European Americans. Strikingly, evidence is now revealing that serum 25(OH)D and/or 1,25 dihydroxyvitamin D levels associate positively with atherosclerosis but negatively with BMD in African Americans; by contrast, vitamin D levels associate negatively with atherosclerosis and positively with BMD in individuals of European ancestry. Biologic phenomena, therefore, seem to contribute to population-specific differences in vitamin D metabolism, bone and vascular health. Genetic and mechanistic approaches used to explore these differences should further our understanding of bone-blood vessel relationships and explain how African ancestry protects from osteoporosis and calcified atherosclerotic plaque, provided that access of African Americans to health care is equivalent to individuals of European ethnic origin. Ultimately, in our opinion, a new mechanistic understanding of the relationships between bone mineralization and vascular calcification will produce novel approaches for disease prevention in aging populations.

Relationships between calcified atherosclerotic plaque and bone mineral density in African Americans with type 2 diabetes

Inverse relationships have been reported between bone mineral density (BMD) and calcified atherosclerotic plaque (CP). This suggests these processes may be related. We examined relationships between BMD and CP in 753 African Americans with type 2 diabetes from 664 families, accounting for the effects of modifiable cardiovascular disease (CVD) risk factors. Association analyses were performed using generalized estimating equations (GEEs) to assess cross-sectional relationships between computed tomography-determined measures of thoracic and lumbar vertebral volumetric BMD (vBMD) and CP in the coronary and carotid arteries and infrarenal aorta. Significant inverse associations were seen between thoracic and lumbar vBMD and CP in all three vascular beds in unadjusted analyses. A fully adjusted model accounting for age, sex, body mass index, systolic blood pressure, low-density lipoprotein cholesterol, C-reactive protein, hemoglobin A(1c), smoking, and hormone-replacement therapy revealed significant inverse associations between thoracic vBMD and CP in coronary and carotid arteries and aorta, whereas lumbar vBMD was associated with CP in coronary artery and aorta. Inverse associations exist between vertebral BMD and calcified atherosclerotic plaque in African-American men and women with type 2 diabetes. This relationship was independent of conventional CVD risk factors and supports the hypothesis that bone metabolism and atherosclerotic plaque mineralization are related processes.

Proteomic characterization of biogenesis and functions of matrix vesicles released from mineralizing human osteoblast-like cells

Matrix vesicles (MVs), released by budding from apical microvilli of osteoblasts during bone formation and development, are involved in the initiation of mineralization by promoting the formation of hydroxyapatite in their lumen. To gain additional insights into MV biogenesis and functions, MVs and apical microvilli were co-isolated from mineralizing osteoblast-like Saos-2 cells and their proteomes were characterized using LC-ESI-MS/MS and compared. In total, 282 MV and 451 microvillar proteins were identified. Of those, 262 were common in both preparations, confirming that MVs originate from apical microvilli. The occurrence of vesicular trafficking molecules (e.g. Rab proteins) and of the on-site protein synthetic machinery suggests that cell polarization and apical targeting are required for the incorporation of specific lipids and proteins at the site of MV formation. MV release from microvilli may be driven by actions of actin-severing proteins (gelsolin, cofilin 1) and contractile motor proteins (myosins). In addition to the already known proteins involved in MV-mediated mineralization, new MV residents were detected, such as inorganic pyrophosphatase 1, SLC4A7 sodium bicarbonate cotransporter or sphingomyelin phosphodiesterase 3, providing additional insights into MV functions.

Loss of skeletal mineralization by the simultaneous ablation of PHOSPHO1 and alkaline phosphatase function: a unified model of the mechanisms of initiation of skeletal calcification

Endochondral ossification is a carefully orchestrated process mediated by promoters and inhibitors of mineralization. Phosphatases are implicated, but their identities and functions remain unclear. Alkaline phosphatase (TNAP) plays a crucial role promoting mineralization of the extracellular matrix by restricting the concentration of the calcification inhibitor inorganic pyrophosphate (PP(i)). Mutations in the TNAP gene cause hypophosphatasia, a heritable form of rickets and osteomalacia. Here we show that PHOSPHO1, a phosphatase with specificity for phosphoethanolamine and phosphocholine, plays a functional role in the initiation of calcification and that ablation of PHOSPHO1 and TNAP function prevents skeletal mineralization. Phospho1(-/-) mice display growth plate abnormalities, spontaneous fractures, bowed long bones, osteomalacia, and scoliosis in early life. Primary cultures of Phospho1(-/-) tibial growth plate chondrocytes and chondrocyte-derived matrix vesicles (MVs) show reduced mineralizing ability, and plasma samples from Phospho1(-/-) mice show reduced levels of TNAP and elevated plasma PP(i) concentrations. However, transgenic overexpression of TNAP does not correct the bone phenotype in Phospho1(-/-) mice despite normalization of their plasma PP(i) levels. In contrast, double ablation of PHOSPHO1 and TNAP function leads to the complete absence of skeletal mineralization and perinatal lethality. We conclude that PHOSPHO1 has a nonredundant functional role during endochondral ossification, and based on these data and a review of the current literature, we propose an inclusive model of skeletal calcification that involves intravesicular PHOSPHO1 function and P(i) influx into MVs in the initiation of mineralization and the functions of TNAP, nucleotide pyrophosphatase phosphodiesterase-1, and collagen in the extravesicular progression of mineralization.

Sinomenine, theophylline, cysteine, and levamisole: Comparisons of their kinetic effects on mineral formation induced by matrix vesicles

The effects of sinomenine (SIN, an alkaloid extracted from the Chinese medicinal plant Sinomenium acutum used for centuries to treat rheumatic disease, including rheumatoid arthritis) on apatitic nucleation and matrix vesicle (MV)-induced mineral formation were compared with those of cysteine, levamisole, and theophylline. We found that SIN was not an inhibitor of tissue non-specific alkaline phosphatase (TNAP), a marker of biological mineralization, but confirmed that cysteine, levamisole, and theophylline were. Further, none of these four molecules directly affected the nucleation of hydroxyapatite (HA) formation, in contrast to pyrophosphate (PP(i)) which did. Incubation of 0.25-1.0mM cysteine, theophylline, or levamisole with MVs in synthetic cartilage lymph (SCL) containing AMP and Ca(2+), but not inorganic phosphate (P(i)), prolonged the induction time of mineral formation, apparently by inhibiting TNAP activity. SIN at the same levels neither inhibited TNAP activity nor affected the induction time of MV mineral formation. However, SIN did markedly delay MV-induced mineral formation in SCL containing P(i) (instead of AMP) in a manner similar to theophylline, but to a lesser extent than levamisole. Cysteine did not delay, in fact it slightly accelerated MV-induced mineral formation in Pi-containing SCL. These findings suggest that levamisole, SIN and theophylline may directly affect Ca(2+) and/or P(i) accretion during mineral formation; however, TNAP was not directly involved. The possible roles of annexins and other ion transporters, such as proteins of the solute carrier family implicated in Ca(2+) and P(i) influx are discussed.

Role of matrix vesicles in biomineralization

BACKGROUND

Matrix vesicles have been implicated in the mineralization of calcified cartilage, bone and dentin for more than 40 years. During this period, their exact role, if any in the nucleation of hydroxyapatite mineral, and its subsequent association with the collagen fibrils in the organic matrix has been debated and remains controversial.

SCOPE OF REVIEW

This review summarizes studies spanning the whole history of matrix vesicles, but emphasizes recent findings and several hypotheses which have been recently introduced to explain in greater detail how matrix vesicles function in biomineralization.

MAJOR CONCLUSIONS

It is now generally accepted that matrix vesicles have some role(s) in mineralization; that they are the initial site of mineral formation; that MV bud from the plasma membrane of mineral forming cells, but that they take with them only a subset of the materials found in the parent membrane; that the three proteins, alkaline phosphatase, nucleotide pyrophosphatase phosphodiesterase and annexin V have important roles in the process and that matrix vesicles participate in regulating the concentration of PPi in the matrix. In contrast, many open questions remain to be answered.

GENERAL SIGNIFICANCE

Understanding the role of matrix vesicles in biomineralization will increase our knowledge of this important process.

The presence of PHOSPHO1 in matrix vesicles and its developmental expression prior to skeletal mineralization

PHOSPHO1 is a phosphoethanolamine/phosphocholine phosphatase that has previously been implicated in generating inorganic phosphate (P(i)) for matrix mineralization. In this study, we have investigated PHOSPHO1 mRNA expression during embryonic development in the chick. Whole-mount in situ hybridization indicated that PHOSPHO1 expression occurred prior to E6.5 and was initially restricted to the bone collar within the mid-shaft of the diaphysis of long bones but by E11.5 expression was observed over the entire length of the diaphysis. Alcian blue/alizarin red staining revealed that PHOSPHO1 expression seen in the primary regions of ossification preceded the deposition of mineral, suggesting that it is involved in the initial events of mineral formation. We isolated MVs from growth plate chondrocytes and confirmed the presence of high levels of PHOSPHO1 by immunoblotting. Expression of PHOSPHO1, like TNAP activity, was found to be up-regulated in MVs isolated from chondrocytes induced to differentiate by the addition of ascorbic acid. This suggests that both enzymes may be regulated by similar mechanisms. These studies provide for the first time direct evidence that PHOSPHO1 is present in MVs, and its developmental expression pattern is consistent with a role in the early stages of matrix mineralization.

Effects of Caponization and Different Exogenous Androgen on the Bone Characteristics of Male Chickens

The effects of caponization and androgen implantation on the bone characteristics of male chickens were evaluated. Healthy Single Comb White Leghorn cockerels were caponized or sham operated (sham) at 12 wk old. Sixteen birds from each group were selected for a 14-wk experiment in trial 1. Sixteen birds from the sham group and 64 from the caponized group (randomly allocated into 4 treatments) were implanted with 10.4 +/- 0.4 mg (1.62-mm i.d., 3.6-mm o.d.) of cholesterol, testosterone (TES), 5alpha-dihydrotestosterone (5alpha-DHT), or 19-nortestosterone (19-NorT) and were assigned to trial 2 for a 14-wk experiment. The results from trial 1 showed that caponization increased BW (P < 0.05) and decreased tibia stress, ash content, and P content with higher blood P concentration (P < 0.05) as compared with the sham group. In trial 2, the cholesterol implantation group showed the lowest tibia breaking strength, bending moment, stress, and ash content (P < 0.05). The 19-NorT implantation group showed decreased (P < 0.05) blood Ca and P concentration but increased tibia ash and P content, reaching the same level as the sham group (P > 0.05). The adverse effects of caponization on bone characteristics could be improved using androgen implantation. Among the implantation groups, the 19-NorT implantation group showed the best improvement in tibia breaking strength and bending moment, followed by the TES and 5alpha-DHT groups. The TES group showed the best improvement in tibia stress, followed by the 19-NorT and 5alpha-DHT groups.

A comparative analysis of strategies for isolation of matrix vesicles

Matrix vesicles (MVs) are extracellular organelles involved in the initial steps of mineralization. MVs are isolated by two methods. The first isolation method of MVs starts with collagenase digestion of osseous tissues, followed by two differential centrifugations. The second isolation method does not use proteases but rather starts with differential centrifugation, followed by a fractionation on a sucrose gradient. The first method results in a homogeneous population of MVs with higher cholesterol/lipid content, alkaline phosphatase activity, and mineral formation rate as compared with MVs isolated by the second method. The second method leads to higher protein diversity as compared with MVs isolated according to the first method. Due to their distinct protein composition, lipid-to-protein and cholesterol-to-phospholipid ratios, and differences in rates of mineral formation, both types of isolated MVs are crucial for proteomic analysis and for understanding the regulation of mineralization process at the molecular level.

Clinical manifestations and pathogenesis of hydroxyapatite crystal deposition in juvenile dermatomyositis

Pathologic deposition of mineral in the form of bone-like hydroxyapatite is a frequent occurrence in juvenile dermatomyositis (JDM) and other connective tissue diseases. Although the sizes of the mineral crystals in JDM are similar to those in bone, there is much more mineral in the deposits than there is in bone. Bone matrix proteins also accumulate associated with the deposits. The reasons for the formation of these deposits are not known. It is our hypothesis that persistent inflammation is a component of JDM and other hydroxyapatite deposition diseases. Other contributing factors are genetic, environmental, and physical chemical. This paper discusses the influence of inflammation on the deposition of hydroxyapatite, with emphasis on the clinical and environmental factors that may facilitate the formation of calcific deposits in JDM.

Phosphodiesterase activity of alkaline phosphatase in ATP-initiated Ca(2+) and phosphate deposition in isolated chicken matrix vesicles

Inorganic pyrophosphate is a potent inhibitor of bone mineralization by preventing the seeding of calcium-phosphate complexes. Plasma cell membrane glycoprotein-1 and tissue nonspecific alkaline phosphatase were reported to be antagonistic regulators of mineralization toward inorganic pyrophosphate formation (by plasma cell membrane glycoprotein-1) and degradation (by tissue nonspecific alkaline phosphatase) under physiological conditions. In addition, they possess broad overlapping enzymatic functions. Therefore, we examined the roles of tissue nonspecific alkaline phosphatase within matrix vesicles isolated from femurs of 17-day-old chick embryos, under conditions where these both antagonistic and overlapping functions could be evidenced. Addition of 25 microM ATP significantly increased duration of mineralization process mediated by matrix vesicles, while supplementation of mineralization medium with levamisole, an alkaline phosphatase inhibitor, reduces the ATP-induced retardation of mineral formation. Phosphodiesterase activity of tissue nonspecific alkaline phosphatase for bis-p-nitrophenyl phosphate was confirmed, the rate of this phosphodiesterase activity is in the same range as that of phosphomonoesterase activity for p-nitrophenyl phosphate under physiological pH. In addition, tissue nonspecific alkaline phosphatase at pH 7.4 can hydrolyze ADPR. On the basis of these observations, it can be concluded that tissue nonspecific alkaline phosphatase, acting as a phosphomonoesterase, could hydrolyze free phosphate esters such as pyrophosphate and ATP, while as phosphodiesterase could contribute, together with plasma cell membrane glycoprotein-1, in the production of pyrophosphate from ATP.

Human PHOSPHO1 exhibits high specific phosphoethanolamine and phosphocholine phosphatase activities

Human PHOSPHO1 is a phosphatase enzyme for which expression is upregulated in mineralizing cells. This enzyme has been implicated in the generation of P(i) for matrix mineralization, a process central to skeletal development. PHOSPHO1 is a member of the haloacid dehalogenase (HAD) superfamily of Mg2+-dependent hydrolases. However, substrates for PHOSPHO1 are, as yet, unidentified and little is known about its activity. We show here that PHOSPHO1 exhibits high specific activities toward phosphoethanolamine (PEA) and phosphocholine (PCho). Optimal enzymic activity was observed at approx. pH 6.7. The enzyme shows a high specific Mg2+-dependence, with apparent K(m) values of 3.0 microM for PEA and 11.4 microM for PCho. These results provide a novel mechanism for the generation of P(i) in mineralizing cells from PEA and PCho.

Primary culture of rat growth plate chondrocytes: an in vitro model of growth plate histotype, matrix vesicle biogenesis and mineralization

During endochondral ossification (EO), cartilage is replaced by bone. Chondrocytes of growth plate undergo proliferation, maturation, hypertrophy, matrix vesicle (MV) biogenesis and programmed cell death (PCD, apoptosis). The in vitro system presented here provides a potential experimental model for studying in vitro differentiation and MV biogenesis in chondrocyte cultures. Chondrocytes were obtained from collagenase-digested tibial and femoral growth plate cartilage of 7-week-old rachitic rats. The isolated chondrocytes were plated as monolayers at a density of 0.5 x 10(6) cells per 35-mm plate and grown for 17 days in BGJ(b) medium supplemented with 10% fetal bovine serum, 50 microg/ml ascorbic acid. Light microscopy revealed Sirius red-positive, apparent bone matrix in layers at the surfaces of cartilaginous nodules that developed in the cultures. The central matrix was largely alcian blue staining thus resembling cartilage matrix. Electron microscopy revealed superficial areas of bone like matrix with large banded collagen fibrils, consistent with type I collagen. Most of the central matrix was cartilaginous, with small fibrils, randomly arranged consistent with type II collagen. The presence of peripheral type I and central type II and type X collagen was confirmed by immunohistochemical staining. Immunohistochemistry with anti-Bone morphogenetic proteins 2, 4 and 6 showed that BMP expression is associated with maturing hypertrophic central chondrocytes, many of which were TUNEL positive and undergoing cell death with plasma membrane breaks, hydropic swelling and cell fragmentation. During early mineralization, small radial clusters of hydroxyapatite-like mineral were associated with matrix vesicles. Collagenase digestion-released MVs from the cultures showed a high specific activity for alkaline phosphatase and demonstrated a pattern of AMP-stimulated nonradioactive (40)Calcium deposition comparable to that observed with native MVs. These studies confirm that primary cultures of rat growth plate chondrocytes are a reasonable in vitro model of growth plate histotype, MV biogenesis and programmed cell death.

Further characterization of ATP-initiated calcification by matrix vesicles isolated from rachitic rat cartilage. Membrane perturbation by detergents and deposition of calcium pyrophosphate by rachitic matrix vesicles

Although membrane associated enzymes such as ATPase, alkaline phosphatase, and NTP pyrophosphohydrolase in matrix vesicles (MVs) may underlie the mechanisms of ATP-promoted calcification, prior to the current investigation, the role of the MV membrane in calcification had not been addressed. In this study, various perturbations were introduced to the MV membrane in in vitro calcification systems to determine ideal conditions for ATP-initiated calcification by MVs isolated from rachitic rat epiphyseal cartilage. Membrane integrity appears to be required, since the rupture of the vesicular membrane by vigorously mixing with 10% butanol abolished calcification. In contrast, a mild treatment of MVs with low concentrations (e.g., 0.01%, which is much below the critical concentration for micelle formation) of either neutral Triton X-100 or anionic deoxycholate stimulated calcification by >2-fold, without inducing obvious changes in vesicular appearance. Fourier transform infrared spectroscopic studies were done to identify the mineral phase formed in these experiments. For the first time, rachitic MVs were shown to induce the formation of a calcium pyrophosphate dihydrate-like phase after their exposure to calcifying medium with 1 mM ATP. The integration of spectral areas indicated that calcification was enhanced by Triton X-100. The detergent effect was reversible and appeared to be not mediated through activation of ATPase, alkaline phosphatase, or ATP pyrophosphohydrolase. In contrast to neutral Triton X-100 and anionic deoxycholate, cationic cetyltrimethylammonium bromide inhibited both ATPase activity (I50=10 microM) and ATP-initiated calcification. These observations suggest that membrane perturbations can affect calcification and that the presence of NTP-pyrophosphohydrolase in MVs may play a role in the deposition of CaPPi in rachitic cartilage.

Identification and cloning of a novel phosphatase expressed at high levels in differentiating growth plate chondrocytes

Growth plate chondrocytes progress through a proliferative phase before acquiring a terminally-differentiated phenotype. In this study we used Percoll density gradients to separate chick growth plate chondrocytes into populations of different maturational phenotype. By applying agarose gel differential display to these populations we cloned a cDNA encoding a novel 268 amino acid protein (3X11A). 3X11A contains two peptide motifs that are conserved in a recently identified superfamily of phosphotransferases. It is likely that 3X11A is a phosphatase, but its substrate specificity remains uncertain. 3X11A expression is upregulated 5-fold during chondrocyte terminal differentiation and its expression is approximately 100-fold higher in hypertrophic chondrocytes than in non-chondrogenic tissues. This suggests that 3X11A participates in a biochemical pathway that is particularly active in differentiating chondrocytes.

Effects of polyelectrolyte complex (PEC) on human periodontal ligament fibroblast (HPLF) function. II. Enhancement of HPLF differentiation and aggregation on PEC by L-ascorbic acid and dexamethasone

In addition to many types of extra cellular matrix (ECM) in vivo, cells are stimulated by many types of vitamins, hormones, growth factors, etc. In this paper the effects of L-ascorbic acid 2-phosphate (Asc-2P) and dexamethasone (Dex) on proliferation and differentiation of human periodontal ligament fibroblast (HPLF) using polyelectrolyte complex (PEC) as a matrix in vitro will be discussed. The PEC was composed of chitosan as a polycation, with carboxymethyl (CPEC) or sulfated chitin (SPEC). Asc-2P (0.2 mM) inhibited the growth of HPLF on CPEC, but promoted the growth on SPEC. Moreover, the aggregation of HPLF on CPEC was inhibited by Asc-2P, but that on SPEC was induced in the presence of Asc-2P and Dex. Although Asc-2P reduced an increase in alkaline phosphatase (ALPase) activity of HPLF on CPEC as well, it induced a twofold increase in ALPase activities on SPEC and TCD. Furthermore, in the medium containing Asc-2P and 100 mM of Dex, cell growth was inhibited, but ALPase activity was promoted on both SPEC and TCD to form many aggregates on SPEC. ALPase activity increased by twofold over that of HPLF cultured in the medium containing only Asc-2P. Therefore, it is suggested that the cell functions of HPLF are controlled by the combination of PEC and additives.

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.

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.

Characteristics and regulation of Pi transport in osteogenic cells for bone metabolism

Inorganic phosphate (Pi) is an essential element in the development of osteogenic cells. The translocation of Pi from the systemic to the skeletal extracellular compartment appears to be an important function of osteoblastic cells. The plasma membrane of osteogenic cells is endowed with a sodium-dependent Pi transport system that is regulated by osteotropic factors such as parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrP), insulin-like growth factor-1 (IGF-1), platelet-derived growth factor (PDGF) and fluoride. A similar Pi transport system has been recently identified in matrix vesicles derived from the plasma membrane of osteogenic cells, such as epiphyseal chondrocytes or osteoblastic cells. Matrix vesicles are extracellular structures which are considered to play an important role in endochondral and membranous calcification. Pi transport appears to be the driving force responsible for the accumulation of mineral inside the matrix vesicles and thereby can be considered as a pivotal determinant in the induction of the calcification process. Furthermore, modulation of the activity of the Pi transport at the level of the plasma membrane of osteogenic cells by osteotropic factors is transferred to the matrix vesicles derived from these cells. This notion implies that hormonal and other environmental factors, such as Pi itself and calcium, which have a direct impact on the Pi transport activity of osteogenic cells can also influence the capacity of the matrix vesicles to initiate the mineralization of the bone matrix. The cellular mechanisms involved in the regulation of Pi transport by osteotropic factors have been recently investigated. For the PTH/PTHrP regulatory effect, cAMP appears to be the main mediator and the response does not require the de novo synthesis of proteins. For the effects of IGF-1, PDGF and fluoride, tyrosine phosphorylation processes are involved and responses are dependent upon the de novo synthesis of proteins. The molecules responsible for activation of these signaling pathways are currently under investigation. Such an investigation may improve our understanding of the mechanisms underlying the differentiation processes of osteogenesis such as the calcification of the extracellular matrix.

Identification and characterization of various differentiative growth plate chondrocytes from porcine by countercurrent centrifugal elutriation

Countercurrent centrifugal elutriation was used to separate growth plate chondrocytes from porcine basing on their differences in sizes and densities. Eighteen fractions of cells with different sizes and densities were obtained. The mean cellular volumes increased progressively in each of successive fractions, and that increase was associated with specific phenotypic changes, such as biochemical differences in DNA synthesis, proteoglycan synthesis, and activities of alkaline phosphatase. Three distinct chondrocyte subpopulations with their unique characteristics were identified among the elutriated fractions. The resting chondrocytes were found to be small in size and quiescent. The hypertrophic chondrocytes were found to be large in size and metabolically active both in alkaline phosphatase and in proteoglycan productions. The proliferative chondrocytes exhibited a high DNA synthesis rate, and their sizes were found to be between those of the resting and hypertrophic chondrocytes.

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.

Bovine enamel organ cells express tissue non-specific alkaline phosphatase mRNA

Alkaline phosphatase (AP) is expressed at high levels in all mineralizing tissues, and the isoform identified in developing enamel has biochemical properties similar to that found in bone. While the bone AP is referred to as the liver/bone/kidney or tissue non-specific (TNS) form, other APs are highly specific for tissue of expression. To determine unequivocally the AP isoform made by enamel organ cells, we constructed a fetal bovine enamel organ cDNA library, which yielded eight AP cDNA clones. In each case, the DNA sequence was homologous to the partial cDNA reported for bovine kidney AP (Garattini et al., 1987). It is concluded that enamel organ cells express the TNS-AP isoform. The extended 3' untranslated region of the cDNA has considerable homology to human TNS-AP, and the conservation of sequence suggests that the 3' end may have a role in post-transcriptional regulation of expression.

Characterization of the phosphatidylinositol-specific phospholipase C-released form of rat osseous plate alkaline phosphatase and its possible significance on endochondral ossification

Alkaline phosphatase activity was released up to 100% from the membrane by incubating the rat osseous plate membrane-bound enzyme with phosphatidylinositol-specific phospholipase C. The molecular weight of the released enzyme was 145,000 on Sephacryl S-300 gel filtration and 66,000 on PAGE-SDS, suggesting a dimeric structure. Solubilization of the membrane-bound enzyme with phospholipase C did not destroy its ability to hydrolyse PNPP, ATP and pyrophosphate. The hydrolysis of ATP and PNPP by phosphatidylinositol-specific phospholipase C-released enzyme exhibited 'Michaelian' kinetics with K0.5 = 70 and 979 microM, respectively. For pyrophosphate, K0.5 was 128 microM and site-site interactions were observed (n = 1.4). Magnesium ions were stimulatory (K0.5 = 1.5 mM) and zinc ions were a powerful noncompetitive inhibitor (Ki = 6.2 microM) of phosphatidylinositol-specific phospholipase C-released enzyme. Phosphatidylinositol-specific phospholipase C-released alkaline phosphatase was relatively stable at 40 degrees C. However, with increasing temperature from 40-60 degrees C, the enzyme was inactivated rapidly following first order kinetics and thermal inactivation constants varied from 5.08 x 10(-4) min-1 to 0.684 min-1. Treatment of phosphatydilinositol-specific phospholipase C-released alkaline phosphatase with Chellex 100 depleted to 5% its original PNPPase activity. Magnesium (K0.5 = 29.5 microM), manganese (K0.5 = 5 microM) and cobalt ions (K0.5 = 10.1 microM) restored the activity of Chelex-treated enzyme, demonstrating its metalloenzyme nature. The stimulation of Chelex-treated enzyme by calcium ions (K0.5 = 653 microM) was less effective (only 26%) and occurred with site-site interactions (n = 0.7). Zinc ions had no stimulatory effects. The possibility that the soluble form of the enzyme, detected during endochondral ossification, would arise by the hydrolysis of the Pl-anchored form of osseous plate alkaline phosphatase is discussed.

Defect in formation of functional matrix vesicles by growth plate chondrocytes in avian tibial dyschondroplasia: evidence of defective tissue vascularization

Avian tibial dyschondroplasia (ATD), a disease characterized by an almost total lack of mineralization in affected areas of growth plate cartilage, may involve defective matrix vesicle (MV) mineralization. To explore the biochemical defect in ATD, both normal and diseased tissue were analyzed for the amount of isolatable MVs, their chemical composition, and their ability to induce mineral formation. We found significantly fewer MVs in ATD tissue, and in contrast to normal MVs, which rapidly mineralized when incubated in synthetic cartilage lymph, those isolated from ATD lesions induced only limited mineralization even after prolonged incubation. Analysis by detergent extraction revealed a nearly dysfunctional nucleational core in ATD MVs. Thus, in ATD tissue, there is a defect in the formation of MVs, and those that form are nearly inactive. There were also alterations in the lipid-dependent Ca2+(-)binding proteins (annexins) in ATD MVs. There were lower levels of annexins II and VI in endogenously produced collagenase-released matrix vesicles (CRMVs), but not in matrix vesicle-enriched microsomes (MVEMs) produced by tissue homogenization. These findings indicate that there is insufficient Ca2+ in ATD cells to enable incorporation of the annexins into MVs. Finally, there was evidence of phospholipid breakdown in ATD MVs, as well as in ATD tissue generally. This indicated that the ATD lesions were becoming necrotic. Taken together, these findings indicate that there is a defect in tissue vascularization such that the supply of mineral ions and nutrients to ATD cartilage is inadequate to support normal MV formation and subsequent mineralization.

Mineralization of alkaline phosphatase-complexed collagen implants in the rat in relation to serum inorganic phosphate

The present study was designed to determine the relationship between mineralization of collagenous matrices and serum levels of calcium and inorganic phosphate. Collagen slices were prepared from bovine dentin or cortical bone and complexed with varying amounts of intestinal alkaline phosphatase (ALP). The enzyme was added to induce de novo mineralization. The ALP-complexed slices were implanted subcutaneously over the skull and in the dorsolateral aspect of the abdominal wall in female Wistar rats of various ages (5-, 10-, 20-, or 35-week-old) and in young male rats fed on a low-P diet. After 1-4 weeks, the implants were removed and analyzed for calcium and phosphate content. In addition, serum levels of calcium and phosphate (total and inorganic) were determined. It was shown that the highest mineral influx occurred in the younger rats (which were also highest in serum P(i)), whereas almost no mineral uptake occurred in the older ones. Also in rats fed on a low-P diet (which were low in serum P(i), a strongly decreased mineral influx was noted. In all animal groups a positive correlation was found between the degree of mineralization and serum P(i). No distinct relationship was found between serum Ca/organic phosphate levels and mineral influx in the implants. In vitro incubation of ALP-collagen conjugates in serum from younger and older rats confirmed our view that serum P(i), besides local levels of ALP, is important in de novo mineral deposition. For accretion of mineral in partially remineralized collagenous carriers, ALP activity was not required.

Interpretation and clinical significance of alkaline phosphatase isoenzyme patterns

Alkaline phosphatase (ALP, EC 3.1.3.1) is a membrane-bound metalloenzyme that consists of a group of true isoenzymes, all glycoproteins, encoded for by at least four different gene loci: tissue-nonspecific, intestinal, placental, and germ-cell ALP. Through posttranslational modifications of the tissue-nonspecific gene, for example, through differences in carbohydrate composition, bone and liver ALP are formed. Nowadays, most commercially available methods for separating or measuring ALP isoenzymes are easy to perform and sensitive and allow for reproducible and quantitative results. As more isoenzymes and isoforms have been characterized, confusion has arisen due to the many different names they were given. For the sake of simplicity and because of structural analogies, we propose an alternative nomenclature for the ALP isoenzymes and isoforms based on their structural characteristics: soluble, dimeric (Sol), anchor-bearing (Anch), and membrane-bound (Mem) liver, bone, intestinal, and placental ALP. Together with lipoprotein-bound liver ALP and immunoglobulin-bound ALP, these names largely fit the many forms of ALP one can encounter in human serum and tissues. The clinically relevant isoenzymes are sol-liver, Mem-liver, lipoprotein-bound liver, and Sol-intestinal ALP in liver diseases, and Sol-bone and Anch-bone ALP in bone diseases. Many different isoenzyme patterns can be found in malignancies and renal diseases. This test provides the clinician with valuable information for diagnostic purposes as well as for follow-up of patients and monitoring of treatment. However, ALP isoenzyme determination will only provide clinically useful information if the patterns are correctly interpreted. In this respect, care should be taken to use the proper reference ranges, taking into account the age and sex of the patient. A normal total ALP activity does not rule out the presence of an abnormal isoenzyme pattern, particularly in children. Separating ALP into its isoenzymes adds considerable value to the mere assay of total ALP activity.

In vitro Ca deposition by rat matrix vesicles: is the membrane association of alkaline phosphatase essential for matrix vesicle-mediated calcium deposition?

1. Phosphatidylinositol phospholipase C (PI-PLC) treatment of rachitic rat matrix vesicles (MVs) released about 80% of membrane-bound alkaline phosphatase (ALP), AMPase, PPiase into the media. 2. About 20% hydrolytic activity was not released from MV membranes by PI-PLC treatment. 3. SDS-polyacrylamide gel electrophoresis and Western blot analysis showed only one immunoreactive protein corresponding to the molecular weight of ALP present in the soluble fraction after PI-PLC treatment. 4. The specific activity of the released ALP was at least 5-fold higher than the residual activity. 5. After PI-PLC treatment, MVs also demonstrated an 80% reduction of AMP- or beta GP-dependent calcium deposition. 6. The soluble fraction containing 80% of ALP activity was unable to support calcium deposition. The mixing of the soluble and insoluble fractions after PI-PLC treatment failed to fully restore calcium-depositing activity.

Allosteric modulation by ATP, calcium and magnesium ions of rat osseous plate alkaline phosphatase

Alkaline phosphatase from rat osseous plate is allosterically modulated by ATP, calcium and magnesium at pH 7.5. At pH 9.4, the hydrolysis of ATP and PNPP follows Michaelis-Menten kinetics with K0.5 values of 154 microM and 42 microM, respectively. However, at pH 7.5 both substrates exhibit more complex saturation curves, while only ATP exhibited site-site interactions. Ca(2+)-ATP and Mg(2+)-ATP were effective substrates for the enzyme, while the specific activity of the enzyme for the hydrolysis of ATP at pH 7.5 was 800-900 U/mg and was independent of the ion species. ATP, but not PNPP, was hydrolyzed slowly in the absence of metal ions with a specific activity of 140 U/mg. These data demonstrate that in vitro and at pH 7.5 rat osseous plate alkaline phosphatase is an active calcium or magnesium-activated ATPase.

Effects of hydroxyapatite implants on primary mineralization during rat tibial healing: biochemical and morphometric analyses

The effect of 40- to 60-mesh hydroxyapatite (HA) granules (Calcitek, Inc., Carlsbad, CA) on the process of primary mineralization during bone healing was examined following insertion of the HA granules into rat tibial bone after marrow ablation. Response to HA was assessed by monitoring morphometric and biochemical changes in matrix vesicles, which are extracellular organelles associated with initial calcification. Following insertion of HA, matrix vesicle-enriched membranes (MVEMs) were isolated from the tissue adjacent to the implant and from the endosteum of the contralateral limb at 3, 6, 14, and 21 days and from a nonimplanted control group (t = 0). MVEM alkaline phosphatase- and phospholipase A2-specific activities were increased on days 6 (peak) and 14; phosphatidylserine content was also elevated on days 6 and 14 (peak). Comparable changes were seen in the contralateral limb but at lesser magnitudes. Morphological changes were observed as well. The number of matrix vesicles/micron2 matrix increased on days 6 (peak) and 14. The mean diameter of the matrix vesicles was elevated on days 6 (peak), 14, and 21. Mean distance from the calcifying front increased on day 6 but was decreased on days 14 and 21. These results indicated that HA behaves like bone-bonding implants in that there is a stimulation of matrix vesicle enzymes, increased phosphatidylserine content, and increase numbers of matrix vesicles. However, the increases occur only after 6 days postimplantation, indicating a delay in response when compared to normal healing. This delay is confirmed by the morphometric measurements. HA causes a reduction in the response associated with marrow ablation. In addition, the effects of HA are comparable locally and systemically but with different intensity. These observations suggest that osteogenic cells are able to compensate for the inhibitory effects of HA and primary calcification involves normal matrix vesicle production and maturation, if somewhat delayed and reduced in magnitude. The ability to support primary mineral formation may contribute to the successful bonding of HA with surrounding osseous tissue.

Modulation of matrix vesicle enzyme activity and phosphatidylserine content by ceramic implant materials during endosteal bone healing

This study examined effects of bone bonding and nonbonding implants on parameters associated with matrix vesicle-mediated primary bone formation, matrix vesicle alkaline phosphatase and phospholipase A2 specific activities, and phosphatidylserine content. Tibia marrow ablation followed by implantation of KG-Cera, Mina 13 (bonding), KGy-213, or M 8/1 (nonbonding) was used as the experimental model. Postsurgery, matrix vesicle-enriched microsomes (MVEM) were isolated from implanted and contralateral limbs. MVEM alkaline phosphatase and phospholipase A2 were stimulated adjacent to bonding implants with similar, though reduced, effects contralaterally. Alkaline phosphatase exhibited slight stimulation in nonbonding tissue; phospholipase A2 was inhibited or unchanged in treated and contralateral limbs. Phosphatidylserine content of MVEM was differentially affected by the implant materials. Thus, MVEM are modulated by implant materials locally and systemically. The data demonstrate that the model is a biologically relevant diagnostic for assessing the tissue/implant interface, primary calcification is affected by implant materials, and implant-specific effects are detected in the contralateral unimplanted limb.

In vitro calcium deposition by rachitic rat matrix vesicles: nucleoside triphosphate supported calcium deposition

The present study was designed to test whether ATP at serum levels can support matrix vesicle-mediated Ca deposition while the final Ca x P ion product is maintained at or below serum or cartilage fluid levels. Rachitic rat epiphyseal cartilage matrix vesicles (40 micrograms protein/ml) in a simple calcifying solution (without exogenously added Pi) containing 50 mM Tris, pH 7.6 at 37 degrees C, 0.1 M NaCl, 1.35 mM CaCl2, 1 mM ATP, deposited about 500 nmol Ca/mg protein after 5 h. The amount of Ca deposited increased with increases in incubation time, concentrations of ATP, Ca2+, hydroxide, and matrix vesicle protein. UTP, GTP, and CTP were equally effective in supporting Ca deposition by matrix vesicles. ATP-alpha,beta-methylene and ATP-beta,gamma-methylene were inhibitory for ATP-dependent Ca deposition. Experiments with limiting amounts of ATP and Ca2+ available in the calcifying solution indicated that ATP concentration at serum levels, in the presence of Ca x P ion products at serum or cartilage fluid levels, can support matrix vesicle-mediated Ca deposition.