Subcellular targeting and function of osteoblast nucleotide pyrophosphatase phosphodiesterase 1

Homozygous splice-site variant in ENPP1 underlies generalized arterial calcification of infancy

ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) plays a critical role by converting extracellular ATP to AMP, generating extracellular PPi, a potential inhibitor of calcification. Pathogenic variants in the ENPP1 cause generalized arterial calcification of infancy (GACI [OMIM 208000]). GACI, is an ultra-rare disease characterized by early-onset calcification of large and medium-sized arteries, leading to severe cardiovascular complications such as heart failure, pulmonary stenosis (PS), hypertension, and more. In this study, we report a novel homozygous splice-site pathogenic variant in ENPP1 (NM_006208, c.2230 + 5G > A; p.Asp701Asnfs*2) residing in C-terminal nuclease-like domain (NLD) of ENPP1 protein in a Pakistani family diagnosed with severe valvular PS and mild right ventricular hypertrophy (RVH). cDNA assays confirmed the skipping of exon 21, and the splice product underwent nonsense-mediated decay. Functional studies on fibroblasts from the patient demonstrated increased calcification and decreased enzymatic activity of ENPP1, recapitulating the hallmarks of GACI. By combining genetic analysis with the in vitro study, we substantiate that ENPP1:c.2230 + 5G > A variant is pathogenic, underscoring its role in the development of GACI.

ENPP1 deficiency: A clinical update on the relevance of individual variants using a locus-specific patient database

Loss-of-function variants in the ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) cause ENPP1 Deficiency, a rare disorder characterized by pathological calcification, neointimal proliferation, and impaired bone mineralization. The consequence of ENPP1 Deficiency is a broad range of age dependent symptoms and morbidities including cardiovascular complications and 50% mortality in infants, autosomal recessive hypophosphatemic rickets type 2 (ARHR2) in children, and joint pain, osteomalacia and enthesopathies in adults. Recent research continues to add to the growing clinical presentation profile as well as expanding the role of ENPP1 itself. Here we review the current knowledge on the spectrum of clinical and genetic findings of ENPP1 Deficiency reported in patients diagnosed with GACI or ARHR2 phenotypes using a comprehensive database of known ENPP1 variants with associated clinical data. A total of 108 genotypes were identified from 154 patients. Of the 109 ENPP1 variants reviewed, 72.5% were demonstrably disease-causing, a threefold increase in pathogenic/likely pathogenic variants over other databases. There is substantial heterogeneity in disease severity, even among patients with the same variant. The approach to creating a continuously curated database of ENPP1 variants accessible to clinicians is necessary to increase the diagnostic yield of clinical genetic testing and accelerate diagnosis of ENPP1 Deficiency.

Apigenin Modulates AnxA6- and TNAP-Mediated Osteoblast Mineralization

Mineralization-competent cells like osteoblasts and chondrocytes release matrix vesicles (MVs) which accumulate Ca2+ and Pi, creating an optimal environment for apatite formation. The mineralization process requires the involvement of proteins, such as annexins (Anx) and tissue-nonspecific alkaline phosphatase (TNAP), as well as low molecular-weight compounds. Apigenin, a flavonoid compound, has been reported to affect bone metabolism, but there are doubts about its mechanism of action under physiological and pathological conditions. In this report, apigenin potency to modulate annexin A6 (AnxA6)- and TNAP-mediated osteoblast mineralization was explored using three cell lines: human fetal osteoblastic hFOB 1.19, human osteosarcoma Saos-2, and human coronary artery smooth muscle cells HCASMC. We compared the mineralization competence, the morphology and composition of minerals, and the protein distribution in control and apigenin-treated cells and vesicles. The mineralization ability was monitored by AR-S/CPC analysis, and TNAP activity was determined by ELISA assay. Apigenin affected the mineral structure and modulated TNAP activity depending on the concentration. We also observed increased mineralization in Saos-2 cells. Based on TEM-EDX, we found that apigenin influenced the mineral composition. This flavonoid also disturbed the intracellular distribution of AnxA6 and TNAP, especially blocking AnxA6 aggregation and TNAP attachment to the membrane, as examined by FM analysis of cells and TEM-gold analysis of vesicles. In summary, apigenin modulates the mineralization process by regulating AnxA6 and TNAP, as well as through various effects on normal and cancer bone tissues or atherosclerotic soft tissue.

Extracellular ATP and its derivatives provide spatiotemporal guidance for bone adaptation to wide spectrum of physical forces

ATP is a ubiquitous intracellular molecule critical for cellular bioenergetics. ATP is released in response to mechanical stimulation through vesicular release, small tears in cellular plasma membranes, or when cells are destroyed by traumatic forces. Extracellular ATP is degraded by ecto-ATPases to form ADP and eventually adenosine. ATP, ADP, and adenosine signal through purinergic receptors, including seven P2X ATP-gated cation channels, seven G-protein coupled P2Y receptors responsive to ATP and ADP, and four P1 receptors stimulated by adenosine. The goal of this review is to build a conceptual model of the role of different components of this complex system in coordinating cellular responses that are appropriate to the degree of mechanical stimulation, cell proximity to the location of mechanical injury, and time from the event. We propose that route and amount of ATP release depend on the scale of mechanical forces, ranging from vesicular release of small ATP boluses upon membrane deformation, to leakage of ATP through resealable plasma membrane tears, to spillage of cellular content due to destructive forces. Correspondingly, different P2 receptors responsive to ATP will be activated according to their affinity at the site of mechanical stimulation. ATP is a small molecule that readily diffuses through the environment, bringing the signal to the surrounding cells. ATP is also degraded to ADP which can stimulate a distinct set of P2 receptors. We propose that depending on the magnitude of mechanical forces and distance from the site of their application, ATP/ADP profiles will be different, allowing the relay of information about tissue level injury and proximity. Lastly, ADP is degraded to adenosine acting via its P1 receptors. The presence of large amounts of adenosine without ATP, indicates that an active source of ATP release is no longer present, initiating the transition to the recovery phase. This model consolidates the knowledge regarding the individual components of the purinergic system into a conceptual framework of choreographed responses to physical forces.

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Cellular bioenergetic molecule ATP is released when cell is mechanically stimulated.

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ATP release is proportional to the amount of cellular damage.

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ATP diffusion and transformation to ADP indicates the proximity to the damage.

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Purinergic receptors form a network choreographing cell response to physical forces.

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Complete transformation of ATP to adenosine initiates the recovery phase.

In Search of a Role for Extracellular Purine Enzymes in Bone Function

Bone is one of the major tissues that undergoes continuous remodeling throughout life, thus ensuring both organic body growth during development and protection of internal organs as well as repair of trauma during adulthood. Many endogenous substances contribute to bone homeostasis, including purines. Their role has increasingly emerged in recent decades as compounds which, by interacting with specific receptors, can help determine adequate responses of bone cells to physiological or pathological stimuli. Equally, it is recognized that the activity of purines is closely dependent on their interconversion or metabolic degradation ensured by a series of enzymes present at extracellular level as predominantly bound to the cell membrane or, also, as soluble isoforms. While the effects of purines mediated by their receptor interactions have sufficiently, even though not entirely, been characterized in many tissues including bone, those promoted by the extracellular enzymes providing for purine metabolism have not been. In this review, we will try to circumstantiate the presence and the role of these enzymes in bone to define their close relationship with purine activities in maintaining bone homeostasis in normal or pathological conditions.

Annexins A2, A6 and Fetuin-A Affect the Process of Mineralization in Vesicles Derived from Human Osteoblastic hFOB 1.19 and Osteosarcoma Saos-2 Cells

The mineralization process is initiated by osteoblasts and chondrocytes during intramembranous and endochondral ossifications, respectively. Both types of cells release matrix vesicles (MVs), which accumulate P_i and Ca²⁺ and form apatites in their lumen. Tissue non-specific alkaline phosphatase (TNAP), a mineralization marker, is highly enriched in MVs, in which it removes inorganic pyrophosphate (PP_i), an inhibitor of apatite formation. MVs then bud from the microvilli of mature osteoblasts or hypertrophic chondrocytes and, thanks to the action of the acto-myosin cortex, become released to the extracellular matrix (ECM), where they bind to collagen fibers and propagate mineral growth. In this report, we compared the mineralization ability of human fetal osteoblastic cell line (hFOB 1.19 cells) with that of osteosarcoma cell line (Saos-2 cells). Both types of cells were able to mineralize in an osteogenic medium containing ascorbic acid and beta glycerophosphate. The composition of calcium and phosphate compounds in cytoplasmic vesicles was distinct from that in extracellular vesicles (mostly MVs) released after collagenase-digestion. Apatites were identified only in MVs derived from Saos-2 cells, while MVs from hFOB 1.19 cells contained amorphous calcium phosphate complexes. In addition, AnxA6 and AnxA2 (nucleators of mineralization) increased mineralization in the sub-membrane region in strongly mineralizing Saos-2 osteosarcoma, where they co-localized with TNAP, whereas in less mineralizing hFOB 1.19 osteoblasts, AnxA6, and AnxA2 co-localizations with TNAP were less visible in the membrane. We also observed a reduction in the level of fetuin-A (FetuA), an inhibitor of mineralization in ECM, following treatment with TNAP and Ca channels inhibitors, especially in osteosarcoma cells. Moreover, a fraction of FetuA was translocated from the cytoplasm towards the plasma membrane during the stimulation of Saos-2 cells, while this displacement was less pronounced in stimulated hFOB 19 cells. In summary, osteosarcoma Saos-2 cells had a better ability to mineralize than osteoblastic hFOB 1.19 cells. The formation of apatites was observed in Saos-2 cells, while only complexes of calcium and phosphate were identified in hFOB 1.19 cells. This was also evidenced by a more pronounced accumulation of AnxA2, AnxA6, FetuA in the plasma membrane, where they were partly co-localized with TNAP in Saos-2 cells, in comparison to hFOB 1.19 cells. This suggests that both activators (AnxA2, AnxA6) and inhibitors (FetuA) of mineralization were recruited to the membrane and co-localized with TNAP to take part in the process of mineralization.

Src and ROCK Kinases Differentially Regulate Mineralization of Human Osteosarcoma Saos-2 Cells

Osteoblasts initiate bone mineralization by releasing matrix vesicles (MVs) into the extracellular matrix (ECM). MVs promote the nucleation process of apatite formation from Ca²⁺ and P_i in their lumen and bud from the microvilli of osteoblasts during bone development. Tissue non-specific alkaline phosphatase (TNAP) as well as annexins (among them, AnxA6) are abundant proteins in MVs that are engaged in mineralization. In addition, sarcoma proto-oncogene tyrosine-protein (Src) kinase and Rho-associated coiled-coil (ROCK) kinases, which are involved in vesicular transport, may also regulate the mineralization process. Upon stimulation in osteogenic medium containing 50 μg/mL of ascorbic acid (AA) and 7.5 mM of β-glycerophosphate (β-GP), human osteosarcoma Saos-2 cells initiated mineralization, as evidenced by Alizarin Red-S (AR-S) staining, TNAP activity, and the partial translocation of AnxA6 from cytoplasm to the plasma membrane. The addition of 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d] pyrimidine (PP2), which is an inhibitor of Src kinase, significantly inhibited the mineralization process when evaluated by the above criteria. In contrast, the addition of (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexane carboxamide hydrochloride (Y-27632), which is an inhibitor of ROCK kinase, did not affect significantly the mineralization induced in stimulated Saos-2 cells as denoted by AR-S and TNAP activity. In conclusion, mineralization by human osteosarcoma Saos-2 cells seems to be differently regulated by Src and ROCK kinases.

ENPP1-Fc prevents neointima formation in generalized arterial calcification of infancy through the generation of AMP

Generalized arterial calcification of infancy (GACI) is associated with widespread arterial calcification and stenoses and is caused by mutations in ENPP1. ENPP1 encodes for ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), which cleaves ATP to generate inorganic pyrophosphate (PP_i) and adenosine monophosphate (AMP) extracellularly. The current study was designed to define the prevalence of arterial stenoses in GACI individuals and to identify the mechanism through which ENPP1 deficiency causes intimal proliferation. Furthermore, we aimed to effectively prevent and treat neointima formation in an animal model of GACI through the systemic administration of recombinant human (rh)ENPP1-Fc protein. Based on a literature review, we report that arterial stenoses are present in at least 72.4% of GACI cases. We evaluated the effect of rhENPP1-Fc on ENPP1-silenced human vascular smooth muscle cells (VSMCs) and on induced intimal proliferation in Enpp1-deficient ttw/ttw mice treated with carotid ligation. We demonstrate that silencing ENPP1 in VSMCs resulted in a tenfold increase in proliferation relative to that of cells transfected with negative control siRNA. The addition of rhENPP1-Fc, AMP or adenosine restored the silenced ENPP1-associated proliferation. In contrast, neither PP_i nor etidronate, a current off-label treatment for GACI, had an effect on VSMC proliferation. Furthermore, subcutaneous rhENPP1-Fc protein replacement was effective in preventing and treating intimal hyperplasia induced by carotid ligation in an animal model of GACI. We conclude that ENPP1 inhibits neointima formation by generating  AMP. RhENPP1-Fc may serve as an approach for the effective prevention and treatment of arterial stenoses in GACI.

A protein replacement therapy may prove useful in tackling calcification and narrowing of the arteries in babies with a severe genetic disorder. Generalized Arterial Calcification of Infancy (GACI) is a rare condition in which infants’ arteries become calcified and their blood vessels internally scarred. It often leads to congestive heart failure. The ENPP1 gene encodes a protein that is crucial to preventing excess calcium build-up in the body. Mutations in the ENPP1 gene lead to GACI, but no therapies for the condition exist. Now, Frank Rutsch at Muenster University Children’s Hospital in Germany and co-workers have shown that administering a protein replacement can inhibit blood vessel scarring and arterial clogging in GACI mice models and in human stem cell cultures. The protein replacement boosts production of a key metabolic molecule called adenosine monophosphate.

Characteristics of minerals in vesicles produced by human osteoblasts hFOB 1.19 and osteosarcoma Saos-2 cells stimulated for mineralization

Bone cells control initial steps of mineralization by producing extracellular matrix (ECM) proteins and releasing vesicles that trigger apatite nucleation. Using transmission electron microscopy with energy dispersive X-ray microanalysis (TEM-EDX) we compared the quality of minerals in vesicles produced by two distinct human cell lines: fetal osteoblastic hFOB 1.19 and osteosarcoma Saos-2. Both cell lines, subjected to osteogenic medium with ascorbic acid (AA) and β-glycerophosphate (β-GP), undergo the entire osteoblastic differentiation program from proliferation to mineralization, produce the ECM and spontaneously release vesicles. We observed that Saos-2 cells mineralized better than hFOB 1.19, as probed by Alizarin Red-S (AR-S) staining, tissue nonspecific alkaline phosphatase (TNAP) activity and by analyzing the composition of minerals in vesicles. Vesicles released from Saos-2 cells contained and were surrounded by more minerals than vesicles released from hFOB 1.19. In addition, there were more F and Cl substituted apatites in vesicles from hFOB 1.19 than in those from Saos-2 cells as determined by ion ratios. Saos-2 and h-FOB 1.19 cells revealed distinct mineralization profiles, indicating that the process of mineralization may proceed differently in various types of cells. Our findings suggest that TNAP activity is correlated with the relative proportions of mineral-filled vesicles and mineral-surrounded vesicles. The origin of vesicles and their properties predetermine the onset of mineralization at the cellular level.

Effects of Different Variants in the ENPP1 Gene on the Functional Properties of Ectonucleotide Pyrophosphatase/Phosphodiesterase Family Member 1

Ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (E-NPP1), encoded by ENPP1, is a plasma membrane protein that generates inorganic pyrophosphate (PP_i ), a physiologic inhibitor of hydroxyapatite formation. In humans, variants in ENPP1 are associated with generalized arterial calcification of infancy, an autosomal-recessive condition causing premature onset of arterial calcification and intimal proliferation resulting in stenoses. ENPP1 variants also cause pseudoxanthoma elasticum characterized by ectopic calcification of soft connective tissues. To determine the functional impact of ENPP1 missense variants, we analyzed 13 putative pathogenic variants in vitro regarding their functional properties, that is, activity, localization, and PP_i generation. Transfection of eight of the 13 variants led to complete loss of NPP activity, whereas four mutants (c.1412A > G, p.Tyr471Cys; c.1510A > C, p.Ser504Arg; c.1976A > G, p.Tyr659Cys; c.2330A > G, p.His777Arg) showed residual activity compared with wild-type E-NPP1. One putative pathologic variant (c.2462 G > A, p.Arg821His) showed normal activity. The five mutants with normal or residual E-NPP1 enzyme activity were still able to generate PP_i and localized in the plasma membrane. In this study, we identified a functional ENPP1 polymorphism, which was expected to be pathogenic till now. Furthermore, we identified four mutants (p.Tyr471Cys, p.Ser504Arg, p.Tyr659Cys, p.His777Arg) with residual E-NPP1 function, which would be potential therapeutical targets for conformational-stabilizing agents.

Improved preparation of 4(5)-aryl-2-(β-d-glucopyranosyl)-imidazoles, the most efficient glucose analogue inhibitors of glycogen phosphorylase

Large scale (up to 20 g) preparation ofIandIIallowed the best inhibitors of glycogen phosphorylaseIIIto be synthesized in close to 60% overall yields fromI.

Acidosis is a key regulator of osteoblast ecto-nucleotidase pyrophosphatase/phosphodiesterase 1 (NPP1) expression and activity

Previous work has shown that acidosis prevents bone nodule formation by osteoblasts in vitro by inhibiting mineralisation of the collagenous matrix. The ratio of phosphate (P_i) to pyrophosphate (PP_i) in the bone microenvironment is a fundamental regulator of bone mineralisation. Both P_i and PP_i, a potent inhibitor of mineralisation, are generated from extracellular nucleotides by the actions of ecto‐nucleotidases. This study investigated the expression and activity of ecto‐nucleotidases by osteoblasts under normal and acid conditions. We found that osteoblasts express mRNA for a number of ecto‐nucleotidases including NTPdase 1–6 (ecto‐nucleoside triphosphate diphosphohydrolase) and NPP1‐3 (ecto‐nucleotide pyrophosphatase/phosphodiesterase). The rank order of mRNA expression in differentiating rat osteoblasts (day 7) was Enpp1 > NTPdase 4 > NTPdase 6 > NTPdase 5 > alkaline phosphatase > ecto‐5‐nucleotidase > Enpp3 > NTPdase 1 > NTPdase 3 > Enpp2 > NTPdase 2. Acidosis (pH 6.9) upregulated NPP1 mRNA (2.8‐fold) and protein expression at all stages of osteoblast differentiation compared to physiological pH (pH 7.4); expression of other ecto‐nucleotidases was unaffected. Furthermore, total NPP activity was increased up to 53% in osteoblasts cultured in acid conditions (P < 0.001). Release of ATP, one of the key substrates for NPP1, from osteoblasts, was unaffected by acidosis. Further studies showed that mineralised bone formation by osteoblasts cultured from NPP1 knockout mice was increased compared with wildtypes (2.5‐fold, P < 0.001) and was partially resistant to the inhibitory effect of acidosis. These results indicate that increased NPP1 expression and activity might contribute to the decreased mineralisation observed when osteoblasts are exposed to acid conditions. J. Cell. Physiol. 230: 3049–3056, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc.

Azanitrile Cathepsin K Inhibitors: Effects on Cell Toxicity, Osteoblast-Induced Mineralization and Osteoclast-Mediated Bone Resorption

Aim

The cysteine protease cathepsin K (CatK), abundantly expressed in osteoclasts, is responsible for the degradation of bone matrix proteins, including collagen type 1. Thus, CatK is an attractive target for new anti-resorptive osteoporosis therapies, but the wider effects of CatK inhibitors on bone cells also need to be evaluated to assess their effects on bone. Therefore, we selected, among a series of synthetized isothiosemicarbazides, two molecules which are highly selective CatK inhibitors (CKIs) to test their effects on osteoblasts and osteoclasts.

Research Design and Methods

Cell viability upon treatment of CKIs were was assayed on human osteoblast-like Saos-2, mouse monocyte cell line RAW 264.7 and mature mouse osteoclasts differentiated from bone marrow. Osteoblast-induced mineralization in Saos-2 cells and in mouse primary osteoblasts from calvaria, with or without CKIs,; were was monitored by Alizarin Red staining and alkaline phosphatase activity, while osteoclast-induced bone resorption was performed on bovine slices.

Results

Treatments with two CKIs, CKI-8 and CKI-13 in human osteoblast-like Saos-2, murine RAW 264.7 macrophages stimulated with RANKL and mouse osteoclasts differentiated from bone marrow stimulated with RANKL and MCSF were found not to be toxic at doses of up to 100 nM. As probed by Alizarin Red staining, CKI-8 did not inhibit osteoblast-induced mineralization in mouse primary osteoblasts as well as in osteoblast-like Saos-2 cells. However, CKI-13 led to a reduction in mineralization of around 40% at 10–100 nM concentrations in osteoblast-like Saos-2 cells while it did not in primary cells. After a 48-hour incubation, both CKI-8 and CKI-13 decreased bone resorption on bovine bone slices. CKI-13 was more efficient than the commercial inhibitor E-64 in inhibiting bone resorption induced by osteoclasts on bovine bone slices. Both CKI-8 and CKI-13 created smaller bone resorption pits on bovine bone slices, suggesting that the mobility of osteoclasts was slowed down by the addition of CKI-8 and CKI-13.

Conclusion

CKI-8 and CKI-13 screened here show promise as antiresorptive osteoporosis therapeutics but some off target effects on osteoblasts were found with CKI-13.

Endocrine role of bone: recent and emerging perspectives beyond osteocalcin

Recent developments in endocrinology, made possible by the combination of mouse genetics, integrative physiology and clinical observations have resulted in rapid and unanticipated advances in the field of skeletal biology. Indeed, the skeleton, classically viewed as a structural scaffold necessary for mobility, and regulator of calcium-phosphorus homoeostasis and maintenance of the haematopoietic niche has now been identified as an important regulator of male fertility and whole-body glucose metabolism, in addition to the classical insulin target tissues. These seminal findings confirm bone to be a true endocrine organ. This review is intended to detail the key events commencing from the elucidation of osteocalcin (OC) in bone metabolism to identification of new and emerging candidates that may regulate energy metabolism independently of OC.

Counter-regulatory phosphatases TNAP and NPP1 temporally regulate tooth root cementogenesis

Cementum is critical for anchoring the insertion of periodontal ligament fibers to the tooth root. Several aspects of cementogenesis remain unclear, including differences between acellular cementum and cellular cementum, and between cementum and bone. Biomineralization is regulated by the ratio of inorganic phosphate (P_i) to mineral inhibitor pyrophosphate (PP_i), where local P_i and PP_i concentrations are controlled by phosphatases including tissue-nonspecific alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). The focus of this study was to define the roles of these phosphatases in cementogenesis. TNAP was associated with earliest cementoblasts near forming acellular and cellular cementum. With loss of TNAP in the Alpl null mouse, acellular cementum was inhibited, while cellular cementum production increased, albeit as hypomineralized cementoid. In contrast, NPP1 was detected in cementoblasts after acellular cementum formation, and at low levels around cellular cementum. Loss of NPP1 in the Enpp1 null mouse increased acellular cementum, with little effect on cellular cementum. Developmental patterns were recapitulated in a mouse model for acellular cementum regeneration, with early TNAP expression and later NPP1 expression. In vitro, cementoblasts expressed Alpl gene/protein early, whereas Enpp1 gene/protein expression was significantly induced only under mineralization conditions. These patterns were confirmed in human teeth, including widespread TNAP, and NPP1 restricted to cementoblasts lining acellular cementum. These studies suggest that early TNAP expression creates a low PP_i environment promoting acellular cementum initiation, while later NPP1 expression increases PP_i, restricting acellular cementum apposition. Alterations in PP_i have little effect on cellular cementum formation, though matrix mineralization is affected.

Mineralisation of collagen rich soft tissues and osteocyte lacunae in Enpp1(-/-) mice

Ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyse nucleotide triphosphates to the corresponding nucleotide monophosphates and the mineralisation inhibitor, pyrophosphate (PPi). This study examined the role of NPP1 in osteocytes, osteoclasts and cortical bone, using a mouse model lacking NPP1 (Enpp1(-/-)). We used microcomputed tomography (μCT) to investigate how NPP1 deletion affects cortical bone structure; excised humerus bones from 8, 15 and 22-week old mice were scanned at 0.9 μm. Although no changes were evident in the cortical bone of 8-week old Enpp1(-/-) mice, significant differences were observed in older animals. Cortical bone volume was decreased 28% in 22-week Enpp1(-/-) mice, whilst cortical porosity was reduced 30% and 60% at 15 and 22-weeks, respectively. This was accompanied by up to a 15% decrease in closed pore diameter and a 55% reduction in the number of pores. Cortical thickness was reduced up to 35% in 15 and 22-week Enpp1(-/-) animals and the endosteal diameter was increased up to 23%. Thus, the cortical bone from Enpp1(-/-) mice was thinner and less porous, with a larger marrow space. Scanning electron microscopy (SEM) revealed a decrease in the size and number of blood vessel channels in the cortical bone as well as a 40% reduction in the mean plan area of osteocyte lacunae. We noted that the number of viable osteocytes isolated from the long bones of Enpp1(-/-) mice was decreased ≤50%. In contrast, osteoclast formation and resorptive activity were unaffected by NPP1 deletion. μCT and histological analysis of Enpp1(-/-) mice also revealed calcification of the joints and vertebrae as well as soft tissues including the whisker follicles, ear pinna and trachea. This calcification worsened as the animals aged. Together, these data highlight the key role of NPP1 in regulating calcification of both soft and skeletal tissues.

Imidazopyridine- and purine-thioacetamide derivatives: potent inhibitors of nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1)

Nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) belongs to the family of ecto-nucleotidases, which control extracellular nucleotide, nucleoside, and (di)phosphate levels. To study the (patho)physiological roles of NPP1 potent and selective inhibitors with drug-like properties are required. Therefore, a compound library was screened for NPP1 inhibitors using a colorimetric assay with p-nitrophenyl 5'-thymidine monophosphate (p-Nph-5'-TMP) as an artificial substrate. This led to the discovery of 2-(3H-imidazo[4,5-b]pyridin-2-ylthio)-N-(3,4-dimethoxyphenyl)acetamide (5a) as a hit compound with a Ki value of 217 nM. Subsequent structure-activity relationship studies led to the development of purine and imidazo[4,5-b]pyridine analogues with high inhibitory potency (Ki values of 5.00 nM and 29.6 nM, respectively) when assayed with p-Nph-5'-TMP as a substrate. Surprisingly, the compounds were significantly less potent when tested versus ATP as a substrate, with Ki values in the low micromolar range. A prototypic inhibitor was investigated for its mechanism of inhibition and found to be competitive versus both substrates.

Deficiency of the bone mineralization inhibitor NPP1 protects mice against obesity and diabetes

The emergence of bone as an endocrine regulator has prompted a re-evaluation of the role of bone mineralization factors in the development of metabolic disease. Ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) controls bone mineralization through the generation of pyrophosphate, and levels of NPP1 are elevated both in dermal fibroblast cultures and muscle of individuals with insulin resistance. We investigated the metabolic phenotype associated with impaired bone metabolism in mice lacking the gene that encodes NPP1 (Enpp1^−/− mice). Enpp1^−/− mice exhibited mildly improved glucose homeostasis on a normal diet but showed a pronounced resistance to obesity and insulin resistance in response to chronic high-fat feeding. Enpp1^−/− mice had increased levels of the insulin-sensitizing bone-derived hormone osteocalcin but unchanged insulin signalling within osteoblasts. A fuller understanding of the pathways of NPP1 could inform the development of novel therapeutic strategies for treating insulin resistance.

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

Osteotropic cancer diagnosis by an osteocalcin inspired molecular imaging mimetic

BACKGROUND

Although microcalcifications of hydroxyapatite can be found in both benign and malignant osteotropic tumors, they are mostly seen in proliferative lesions, including carcinoma. The aim of this present study is to develop a molecular imaging contrast agent for selective identification of hydroxyapatite calcification in human osteotropic tumor tissues ex vivo and in human osteosarcoma cells in vitro.

METHODS

A bioinspired biomarker, hydroxyapatite binding peptide (HABP), was designed to mimic natural protein osteocalcin property in vivo. A fluorescein isothiocyanate dye conjugated HABP (HABP-19) was utilized to characterize hydroxyapatite on human osteotropic tumor tissue sections ex vivo and to selectively image hydroxyapatite calcifications in human osteosarcoma cells in vitro.

RESULTS

Using a HABP-19 molecular imaging probe, we have shown that it is possible to selectively image hydroxyapatite calcifications in osteotropic cancers ex vivo and in human SaOS-2 osteosarcoma cells in vitro.

CONCLUSION

Hydroxyapatite calcifications were selectively detected in osteotropic tissues ex vivo and in the early stage of the calcification process of SaOS-2 human osteosarcoma in vitro using our HABP-19 molecular imaging probe. This new target-selective molecular imaging probe makes it possible to study the earliest events associated with hydroxyapatite deposition in various osteotropic cancers at the cellular and molecular levels.

GENERAL SIGNIFICANCE

It potentially could be used to diagnose and treat osteotropic cancer or to anchor therapeutic agents directing the local distribution of desired therapy at calcified sites.

Structure of NPP1, an ectonucleotide pyrophosphatase/phosphodiesterase involved in tissue calcification

Ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) converts extracellular nucleotides into inorganic pyrophosphate, whereas its close relative NPP2/autotaxin hydrolyzes lysophospholipids. NPP1 regulates calcification in mineralization-competent tissues, and a lack of NPP1 function underlies calcification disorders. Here, we show that NPP1 forms homodimers via intramembrane disulfide bonding, but is also processed intracellularly to a secreted monomer. The structure of secreted NPP1 reveals a characteristic bimetallic active site and a nucleotide-binding groove, but it lacks the lipid-binding pocket and open tunnel present in NPP2. A loop adjacent to the nucleotide-binding site, which is disordered in NPP2, is well ordered in NPP1 and might promote nucleotide binding. Remarkably, the N-terminal somatomedin B-like domains of NPP1, unlike those in NPP2, are flexible and do not contact the catalytic domain. Our results provide a structural basis for the nucleotide pyrophosphatase activity of NPP1 and help to understand how disease-causing mutations may affect NPP1 structure and function.

Detection of hydroxyapatite in calcified cardiovascular tissues

OBJECTIVE

The objective of this study is to develop a method for selective detection of the calcific (hydroxyapatite) component in human aortic smooth muscle cells in vitro and in calcified cardiovascular tissues ex vivo. This method uses a novel optical molecular imaging contrast dye, Cy-HABP-19, to target calcified cells and tissues.

METHODS

A peptide that mimics the binding affinity of osteocalcin was used to label hydroxyapatite in vitro and ex vivo. Morphological changes in vascular smooth muscle cells were evaluated at an early stage of the mineralization process induced by extrinsic stimuli, osteogenic factors and a magnetic suspension cell culture. Hydroxyapatite components were detected in monolayers of these cells in the presence of osteogenic factors and a magnetic suspension environment.

RESULTS

Atherosclerotic plaque contains multiple components including lipidic, fibrotic, thrombotic, and calcific materials. Using optical imaging and the Cy-HABP-19 molecular imaging probe, we demonstrated that hydroxyapatite components could be selectively distinguished from various calcium salts in human aortic smooth muscle cells in vitro and in calcified cardiovascular tissues, carotid endarterectomy samples and aortic valves, ex vivo.

CONCLUSION

Hydroxyapatite deposits in cardiovascular tissues were selectively detected in the early stage of the calcification process using our Cy-HABP-19 probe. This new probe makes it possible to study the earliest events associated with vascular hydroxyapatite deposition at the cellular and molecular levels. This target-selective molecular imaging probe approach holds high potential for revealing early pathophysiological changes, leading to progression, regression, or stabilization of cardiovascular diseases.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

Carbonic anhydrase I (CA1) is involved in the process of bone formation and is susceptible to ankylosing spondylitis

Introduction

Ankylosing spondylitis (AS) is characterized by abnormal bone formation in the spine and the sacroiliac joints. In vitro assays demonstrate that carbonic anhydrase I (CA1) promotes calcium precipitation. This study investigated the function of CA1 for bio-mineralization and determined if common polymorphisms in the CA1 gene might contribute to AS risk.

Methods

Calcification was induced in Saos-2 cells, a human osteosarcoma cell line, with ascorbic acid and β-glycerophosphate. Calcification was determined by Alizarin Red-S (AR-S) staining. Expressions of CA1, alkaline phosphatase (ALP), bone sialoprotein (BSP), osteocalcin (OCN), osterix (OSX) and runt-related transcription factor-2 (Runx2) were determined by real-time PCR and western blotting. The cells were also treated with acetazolamide, an anti-carbonic anhydrase drug. Genotyping was performed using Illumina VeraCode microarray in a case-control study including 51 AS patients, 267 rheumatoid arthritis (RA) patients and 160 healthy controls. The result was confirmed by Taqman assay, including 258 AS patients, 288 RA patients and 288 healthy controls.

Results

Following the induction of calcification, Saos-2 cells produced large amounts of calcium-rich deposits. Increased transcriptions of CA1, ALP, BSP, OCN, OSX and Runx2, essential genes for ossification, were detected in the cultured cells. Following treatmen with acetazolamide, the expression of CA1 obviously declined and mineralized nodule formation was also decreased. Illumina microarray indicates that SNP at rs7841425 also showed significant differences in allelic frequency (P = 0.01396) and genotypic frequency (P = 0.005902) between AS cases and controls. In addition, SNP at rs7827474 showed significant differences in allelic frequency (P = 5.83E-04) and genotypic frequency (P = 0.000186) between RA cases and controls (P values were adjusted to multiple comparisons). The Taqman assay revealed that rs725605 demonstrated statistically significant evidence of allele frequency (P = 0.022307) and gene frequency (P = 0.007731) for association with AS. This SNP did not show significant differences in allelic frequencies and gene frequencies between RA patients and controls.

Conclusions

CA1 may play an essential role in bio-mineralization and new bone formation. The gene encoding CA1 is susceptible to AS.

Diadenosine 5',5''-(boranated)polyphosphonate analogues as selective nucleotide pyrophosphatase/phosphodiesterase inhibitors

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyze extracellular nucleotides and dinucleotides and thus control purinergic signaling. Enhanced NPP activity is implicated in health disorders such as osteoarthritis and cancer. We designed novel diadenosine polyphosphonate derivatives as potential NPP inhibitors. Analogues 1-4 bear a phosphonate and/or boranophosphate group and/or a 2'-H atom instead of a 2'-OH group. In comparison to ATP, analogues 1-4 were barely hydrolyzed by human NTPDase1, -2, -3, and -8 (<5% hydrolysis) and NPP1 and -3 (≤ 13%) and were not hydrolyzed by ecto-5'-nucleotidase, unlike AMP. These derivatives did not affect NTPDase activity, and analogues 1 and 2 did not inhibit ecto-5'-nucleotidase. All analogues blocked ∼80% of the NPP2-dependent hydrolysis of pnp-TMP, a specific NPP substrate, and inhibited the catabolism of pnp-TMP (K(i) and IC₅₀ both found to be between 10 and 60 μM), Ap₅A, and ATP by NPP1. The activity of NPP3 was inhibited to a lesser extent by the new analogues, with compounds 1 and 4 being the most effective in that respect. The analogues dramatically reduced the level of hydrolysis of pnp-TMP at the cell surface of both osteocarcinoma and colon cancer cells. Importantly, analogues 1-4 exhibited significantly reduced agonistic activity toward human P2Y₁,₁₁) receptors (except for analogue 1) and no activity with human P2Y₂ receptor. Our data provide strong evidence that analogue 2 is the first specific NPP inhibitor to be described.

Inorganic pyrophosphatase induces type I collagen in osteoblasts

INTRODUCTION

The physiologic selectivity of calcification in bone tissue reflects selective co-expression by osteoblasts of fibrillar collagen I and of tissue nonspecific alkaline phosphatase (TNAP), which hydrolyzes the calcification inhibitor pyrophosphate (PP(i)) and generates phosphate (P(i)). Humans and mice deficient in the PP(i)-generating ecto-enzyme NPP1 demonstrate soft tissue calcification, occurring at sites of extracellular matrix expansion. Significantly, the function in osteoblasts of cytosolic inorganic pyrophosphatase (abbreviated iPP(i)ase), which generates P(i) via PP(i) hydrolysis with neutral pH optimum, remains unknown. We assessed iPP(i)ase in Enpp1(-/-) and wild type (WT) mouse osteoblasts and we tested the hypothesis that iPP(i)ase regulates collagen I expression.

METHODS

We treated mouse calvarial osteoblasts with ascorbate and beta-glycerol phosphate to promote calcification, and we assessed cytosolic P(i) and PP(i) levels, sodium-dependent P(i) uptake, Pit-1 P(i) co-transporter expression, and iPP(i)ase and TNAP activity and expression. We also assessed the function of transfected Ppa1 in osteoblasts.

RESULTS

Inorganic pyrophosphatase but not TNAP was elevated in Enpp1(-/-) calvariae in situ. Cultured primary Enpp1(-/-) calvarial osteoblasts demonstrated increased calcification despite flat TNAP activity rather than physiologic TNAP up-regulation seen in WT osteoblasts. Despite decreased cytosolic PP(i) in early culture, Enpp1(-/-) osteoblasts maintained cytosolic P(i) levels comparable to WT osteoblasts, in association with increased iPP(i)ase, enhanced sodium-dependent P(i) uptake and expression of Pit-1, and markedly increased collagen I synthesis. Suppression of collagen synthesis in Enpp1(-/-) osteoblasts using 3,4-dehydroproline markedly suppressed calcification. Last, transfection of Ppa1 in WT osteoblasts increased cytosolic P(i) and decreased cytosolic but not extracellular PP(i), and induced both collagen I synthesis and calcification.

CONCLUSIONS

Increased iPP(i)ase is associated with "P(i) hunger" and increased calcification by NPP1-deficient osteoblasts. Furthermore, iPP(i)ase induces collagen I at the levels of mRNA expression and synthesis and, unlike TNAP, stimulates calcification by osteoblasts without reducing the extracellular concentration of the hydroxyapatite crystal inhibitor PP(i).

Matrix vesicles originate from apical membrane microvilli of mineralizing osteoblast-like Saos-2 cells

In bone, mineralization is tightly regulated by osteoblasts and hypertrophic chondrocytes which release matrix vesicles (MVs) and control extracellular ionic conditions and matrix composition. MVs are the initial sites of hydroxyapatite (HA) mineral formation. Despite growing knowledge about their morphology and function, their biogenesis is not well understood. The purpose of this work was to determine the source of MVs in osteoblast lineage, Saos-2 cells, and to check whether MVs originated from microvilli. Microvilli were isolated from the apical plasma membrane of Saos-2 cells. Their morphology, structure, and function were compared with those of MVs. The role of actin network in MV release was investigated by using microfilament perturbing drugs. When examined by electron microscopy MVs and microvillar vesicles were found to exhibit similar morphology with trilaminar membranes and diameters in the same range. Both types of vesicles were able to induce HA formation. Their electrophoretic profiles displayed analogous enrichment in alkaline phosphatase, Na(+)/K(+) ATPase, and annexins A2 and A6. MVs and microvillar vesicles exhibited almost the same lipid composition with a higher content of cholesterol, sphingomyelin, and phosphatidylserine as compared to plasma membrane. Finally, cytochalasin D, which inhibits actin polymerization, was found to stimulate release of MVs. Our findings were consistent with the hypothesis that MVs originated from cell microvilli and that actin filament disassembly was involved in their biogenesis.

Phosphate: known and potential roles during development and regeneration of teeth and supporting structures

Inorganic phosphate (P(i)) is abundant in cells and tissues as an important component of nucleic acids and phospholipids, a source of high-energy bonds in nucleoside triphosphates, a substrate for kinases and phosphatases, and a regulator of intracellular signaling. The majority of the body's P(i) exists in the mineralized matrix of bones and teeth. Systemic P(i) metabolism is regulated by a cast of hormones, phosphatonins, and other factors via the bone-kidney-intestine axis. Mineralization in bones and teeth is in turn affected by homeostasis of P(i) and inorganic pyrophosphate (PPi), with further regulation of the P(i)/PP(i) ratio by cellular enzymes and transporters. Much has been learned by analyzing the molecular basis for changes in mineralized tissue development in mutant and knock-out mice with altered P(i) metabolism. This review focuses on factors regulating systemic and local P(i) homeostasis and their known and putative effects on the hard tissues of the oral cavity. By understanding the role of P(i) metabolism in the development and maintenance of the oral mineralized tissues, it will be possible to develop improved regenerative approaches.

Phosphate and pyrophosphate mediate PKA-induced vascular cell calcification

Vascular calcification is associated with increased cardiovascular risk and occurs by osteochondrogenic differentiation of vascular cells. Many of the same regulatory factors that control skeletal mineralization, including the complex metabolic pathway controlling levels of the activator, inorganic phosphate, and the potent inhibitor, pyrophosphate, also govern vascular calcification. We previously found that the cAMP/PKA signaling pathway mediates in vitro vascular cell calcification induced by inflammatory factors including tumor necrosis factor-alpha 1 and oxidized phospholipids. In this report, we tested whether this signaling pathway modulates phosphate and pyrophosphate metabolism. Treatment of primary murine aortic cells with the PKA activator, forskolin, significantly induced osteoblastic differentiation markers, including alkaline phosphatase (ALP), osteopontin, and osteocalcin as well as the pyrophosphate generator, ectonucleotide-pyrophosphatase/phosphodiesterase-1 (Enpp1) and the pyrophosphate transporter, ankylosis protein, but not the sodium/phosphate cotransporter, Pit-1. In the presence of a substrate for ALP, beta-glycerophosphate, which generates inorganic phosphate, forskolin also enhanced matrix mineralization. Inhibitors of ALP or Pit-1 abrogated forskolin-induced osteopontin expression and mineralization but not forskolin-induced osteocalcin or ALP. These results suggest that phosphate is necessary for PKA-induced calcification of vascular cells and that the extent of PKA-induced calcification is controlled by feedback induction of the inhibitor, pyrophosphate.

Extracellular nucleotides block bone mineralization in vitro: evidence for dual inhibitory mechanisms involving both P2Y2 receptors and pyrophosphate

Extracellular nucleotides, signaling through P2 receptors, may act as local regulators of bone cell function. We investigated the effects of nucleotide agonists [ATP, ADP, uridine triphosphate (UTP), and uridine diphosphate] and pyrophosphate (PPi, a key physiological inhibitor of mineralization) on the deposition and mineralization of collagenous matrix by primary osteoblasts derived from rat calvariae. Our results show that extracellular ATP, UTP, and PPi strongly and selectively blocked the mineralization of matrix nodules; ADP and uridine diphosphate were without effect. Significant inhibition of mineralization occurred in the presence of relatively low concentrations of ATP, UTP, or PPi (1-10 microm), without affecting production of fibrillar or soluble collagen. In cultures treated with 10 microm ATP or UTP, the expression and activity of alkaline phosphatase, which promotes mineralization by hydrolyzing PPi, was inhibited. The potent inhibitory actions of ATP and UTP on bone mineralization are consistent pharmacologically with mediation by the P2Y(2) receptor, which is strongly expressed by mature osteoblasts. In support of this notion, we found 9-17% increases in bone mineral content of hindlimbs of P2Y(2)-deficient mice. We also found that osteoblasts express ectonucleotide phosphodiesterase/pyrophosphatase-1, an ectonucleotidase that hydrolyzes nucleotide triphosphates to yield PPi, and that addition of 10 microm ATP or UTP to osteoblast cultures generated 2 microm PPi within 10 min. Thus, a component of the profound inhibitory action of ATP and UTP on bone mineralization could be mediated directly by PPi, independently of P2 receptors.

Porcine ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1/CD203a): cloning, transcription, expression, mapping, and identification of an NPP1/CD203a epitope for swine workshop cluster 9 (SWC9) monoclonal antibodies

Swine workshop cluster 9 (SWC9) antibody identifying a porcine epitope on macrophages and thymocytes was used to precipitate and characterize the molecule from biotinylated macrophages and to obtain peptide sequence by mass spectrometry. The protein was identified as ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1/CD203a). The porcine NPP1/CD203a encoding gene was mapped to chromosome 1 using a radiation hybrid panel, and transcription was investigated by RT-PCR analysis of several tissues. The cDNA was cloned and introduced into COS7 cells resulting in expression of functionally active enzyme and verification of the specificity of an SWC9 reacting monoclonal antibody. The antibody was used for immunohistochemical examination of various porcine tissues. Most prominent expression of NPP1/CD203a was found in lung macrophages and liver sinusoids.

Physiologic and pathologic functions of the NPP nucleotide pyrophosphatase/phosphodiesterase family focusing on NPP1 in calcification

The catabolism of ATP and other nucleotides participates partly in the important function of nucleotide salvage by activated cells and also in removal or de novo generation of compounds including ATP, ADP, and adenosine that stimulate purinergic signaling. Seven nucleotide pyrophosphatase/phosphodiesterase NPP family members have been identified to date. These isoenzymes, related by up conservation of catalytic domains and certain other modular domains, exert generally non-redundant functions via distinctions in substrates and/or cellular localization. But they share the capacity to hydrolyze phosphodiester or pyrophosphate bonds, though generally acting on distinct substrates that include nucleoside triphosphates, lysophospholipids and choline phosphate esters. PP_i generation from nucleoside triphosphates, catalyzed by NPP1 in tissues including cartilage, bone, and artery media smooth muscle cells, supports normal tissue extracellular PP_i levels. Balance in PP_i generation relative to PP_i degradation by pyrophosphatases holds extracellular PP_i levels in check. Moreover, physiologic levels of extracellular PP_i suppress hydroxyapatite crystal growth, but concurrently providing a reservoir for generation of pro-mineralizing P_i. Extracellular PP_i levels must be supported by cells in mineralization-competent tissues to prevent pathologic calcification. This support mechanism becomes dysregulated in aging cartilage, where extracellular PP_i excess, mediated in part by upregulated NPP1 expression stimulates calcification. PP_i generated by NPP1modulates not only hydroxyapatite crystal growth but also chondrogenesis and expression of the mineralization regulator osteopontin. This review pays particular attention to the role of NPP1-catalyzed PP_i generation in the pathogenesis of certain disorders associated with pathologic calcification.

Modulation of purinergic signaling by NPP-type ectophosphodiesterases

Extracellular nucleotides can elicit a wide array of cellular responses by binding to specific purinergic receptors. The level of ectonucleotides is dynamically controlled by their release from cells, synthesis by ectonucleoside diphosphokinases and ectoadenylate kinases, and hydrolysis by ectonucleotidases. One of the four structurally unrelated families of ectonucleotidases is represented by the NPP-type ectophosphodiesterases. Three of the seven members of the NPP family, namely NPP1–3, are known to hydrolyze nucleotides. The enzymatic action of NPP1–3 (in)directly results in the termination of nucleotide signaling, the salvage of nucleotides and/or the generation of new messengers like ADP, adenosine or pyrophosphate. NPP2 is unique in that it hydrolyzes both nucleotides and lysophospholipids and, thereby, generates products that could synergistically promote cell motility. We review here the enzymatic properties of NPPs and analyze current evidence that links their nucleotide-hydrolyzing capability to epithelial and neural functions, the immune response and cell motility.

Establishment of Correlation between in Vitro Enzyme Binding Potency and in Vivo Pharmacological Activity: Application to Liver Glycogen Phosphorylase a Inhibitors

In drug discovery, establishing a correlation between in vitro potency and in vivo activity is critical for the validation of the selected target and for developing confidence in the in vitro screening strategy. The present study developed a competition equilibrium dialysis assay using a 96-well dialysis technique to determine the intrinsic Kd for 13 inhibitors of human liver glycogen phosphorylase a (GPa) in the presence of liver homogenate to mimic the physiological environment. The results provided evidence that binding of an inhibitor to GPa was affected by extra cofactors present in the liver homogenate. A good correlation was demonstrated between the in vitro Kd determined under liver homogenate environment and free liver concentration of an inhibitor at the minimum efficacious dose in diabetic ob/ob mice. This study revealed important elements (such as endogenous cofactors missing from the in vitro assay and free concentration at the target tissue) that contributed to a better understanding of the linkage between in vitro and in vivo activity. The approach developed here may be applied to many drugs in pharmacology studies in which the correlation between in vitro and in vivo activities for the target tissue (such as solid tumors, brain, and liver) is critical.

NPP-type ectophosphodiesterases: unity in diversity

Nucleotide pyrophosphatase/phosphodiesterase (NPP)-type ectophosphodiesterases are found at the cell surface as type-I or type-II transmembrane proteins, but are also found extracellularly as secreted or shedded enzymes. They hydrolyze pyrophosphate or phosphodiester bonds in a variety of extracellular compounds including nucleotides, (lyso)phospholipids and choline phosphate esters. Despite their structurally related catalytic domain, each enzyme has well-defined substrate specificity. Catalysis by NPPs affects processes as diverse as cell proliferation and motility, angiogenesis, bone mineralization and digestion. In addition, there is emerging evidence for non-catalytic functions of NPPs in cell signaling. NPP-type ectophosphodiesterases are also implicated in the pathophysiology of cancer, insulin resistance and calcification diseases, and they hold great promise as easily accessible therapeutic targets.

Sustained osteomalacia of long bones despite major improvement in other hypophosphatasia-related mineral deficits in tissue nonspecific alkaline phosphatase/nucleotide pyrophosphatase phosphodiesterase 1 double-deficient mice

We have shown previously that the hypomineralization defects of the calvarium and vertebrae of tissue nonspecific alkaline phosphatase (TNAP)-deficient (Akp2-/-) hypophosphatasia mice are rescued by simultaneous deletion of the Enpp1 gene, which encodes nucleotide pyrophosphatase phosphodiesterase 1 (NPP1). Conversely, the hyperossification in the vertebral apophyses typical of Enpp1-/- mice is corrected in [Akp2-/-; Enpp1-/-] double-knockout mice. Here we have examined the appendicular skeletons of Akp2-/-, Enpp1-/-, and [Akp2-/-; Enpp1-/-] mice to ascertain the degree of rescue afforded at these skeletal sites. Alizarin red and Alcian blue whole mount analysis of the skeletons from wild-type, Akp2-/-, and [Akp2-/-; Enpp1-/-] mice revealed that although calvarium and vertebrae of double-knockout mice were normalized with respect to mineral deposition, the femur and tibia were not. Using several different methodologies, we found reduced mineralization not only in Akp2-/- but also in Enpp1-/- and [Akp2-/-; Enpp1-/-] femurs and tibias. Analysis of calvarial- and bone marrow-derived osteoblasts for mineralized nodule formation in vitro showed increased mineral deposition by Enpp1-/- calvarial osteoblasts but decreased mineral deposition by Enpp1-/- long bone marrow-derived osteoblasts in comparison to wild-type cells. Thus, the osteomalacia of Akp2-/- mice and the hypomineralized phenotype of the long bones of Enpp1-/- mice are not rescued by simultaneous deletion of TNAP and NPP1 functions.