Tissue-nonspecific alkaline phosphatase (TNAP) hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)). Deletion of the TNAP gene (Akp2) in mice results in hypophosphatasia characterized by elevated levels of PP(i) and poorly mineralized bones, which are rescued by deletion of nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) that generates PP(i). Mice deficient in NPP1 (Enpp1(-/-)), or defective in the PP(i) channeling function of ANK (ank/ank), have decreased levels of extracellular PP(i) and are hypermineralized. Given the similarity in function between ANK and NPP1 we crossbred Akp2(-/-) mice to ank/ank mice and found a partial normalization of the mineralization phenotypes and PP(i) levels. Examination of Enpp1(-/-) and ank/ank mice revealed that Enpp1(-/-) mice have a more severe hypermineralized phenotype than ank/ank mice and that NPP1 but not ANK localizes to matrix vesicles, suggesting that failure of ANK deficiency to correct hypomineralization in Akp2(-/-) mice reflects the lack of ANK activity in the matrix vesicle compartment. We also found that the mineralization inhibitor osteopontin (OPN) was increased in Akp2(-/-), and decreased in ank/ank mice. PP(i) and OPN levels were normalized in [Akp2(-/-); Enpp1(-/-)] and [Akp2(-/-); ank/ank] mice, at both the mRNA level and in serum. Wild-type osteoblasts treated with PP(i) showed an increase in OPN, and a decrease in Enpp1 and Ank expression. Thus TNAP, NPP1, and ANK coordinately regulate PP(i) and OPN levels. The hypomineralization observed in Akp2(-/-) mice arises from the combined inhibitory effects of PP(i) and OPN. In contrast, NPP1 or ANK deficiencies cause a decrease in the PP(i) and OPN pools that leads to hypermineralization.

Extracellular matrix (ECM) mineralization is a physiological process in bone and a pathological one in soft tissues. The mechanisms determining the spatial restriction of ECM mineralization to bone physiologically are poorly understood. Here we show that a normal extracellular phosphate concentration is required for bone mineralization, while lowering this concentration prevents mineralization of any ECM. However, simply raising extracellular phosphate concentration is not sufficient to induce pathological mineralization, this is because of the presence in all ECMs of pyrophosphate, an inhibitor of mineralization. ECM mineralization occurs only in bone because of the exclusive coexpression in osteoblasts of Type I collagen and Tnap, an enzyme that cleaves pyrophosphate. This dual requirement explains why Tnap ectopic expression in cells producing fibrillar collagen is sufficient to induce pathological mineralization. This study reveals that coexpression in osteoblasts of otherwise broadly expressed genes is necessary and sufficient to induce bone mineralization and provides evidence that pathological mineralization can be prevented by modulating extracellular phosphate concentration.[Keywords: ECM; mineralization; TNAP; pyrophosphate; collagen] Supplemental material is available at http://www.genesdev.org.

We report three novel inhibitors of the physiological pyrophosphatase activity of alkaline phosphatase and show that these compounds are capable of reducing calcification in two models of vascular calcification (i.e., they suppress in vitro calcification by cultured Enpp1 −/− VSMCs and they inhibit the increased pyrophosphatase activity in a rat aortic model).Introduction: Genetic ablation of tissue-nonspecific alkaline phosphatase (TNALP) leads to accumulation of the calcification inhibitor inorganic pyrophosphate (PP i ). TNALP deficiency ameliorates the hypermineralization phenotype in Enpp1 −/− and ank/ank mice, two models of osteoarthritis and soft tissue calcification. We surmised that the pharmacological inhibition of TNALP pyrophosphatase activity could be used to prevent/ suppress vascular calcification. Materials and Methods: Comprehensive chemical libraries were screened to identify novel drug-like compounds that could inhibit TNALP pyrophosphatase function at physiological pH. We used these novel compounds to block calcification by cultured vascular smooth muscle cells (VSMCs) and to inhibit the upregulated pyrophosphatase activity in a rat aortic calcification model. Results: Using VSMC cultures, we determined that Enpp1−/− and ank/ank VSMCs express higher TNALP levels and enhanced in vitro calcification compared with wildtype cells. By high-throughput screening, three novel compounds, 5361418, 5923412, and 5804079, were identified that inhibit TNALP pyrophosphatase function through an uncompetitive mechanism, with high affinity and specificity when measured at both pH 9.8 and 7.5. These compounds were shown to reduce the calcification by Enpp1 −/− VSMCs. Furthermore, using an ex vivo rat whole aorta PP i hydrolysis assay, we showed that pyrophosphatase activity was inhibited by all three lead compounds, with compound 5804079 being the most potent at pH 7.5. Conclusions: We conclude that TNALP is a druggable target for the treatment and/or prevention of ectopic calcification. The lead compounds identified in this study will serve as scaffolds for medicinal chemistry efforts to develop drugs for the treatment of soft tissue calcification.

PHOSPHO1 is a phosphatase highly expressed in bone. We studied its functional involvement in mineralization through the use of novel small molecule inhibitors. PHOSPHO1 expression was present within matrix vesicles, and inhibition of enzyme action caused a decrease in the ability of matrix vesicles to calcify. Introduction:The novel phosphatase, PHOSPHO1, belongs to the haloacid dehalogenase superfamily of hydrolases and is capable of cleaving phosphoethanolamine (PEA) and phosphocholine to generate inorganic phosphate. Our aims in this study were to examine the expression of PHOSPHO1 in murine mineralizing cells and matrix vesicles (MV) and to screen a series of small-molecule PHOSPHO1-specific inhibitors for their ability to pharmacologically inhibit the first step of MV-mediated mineralization. Materials and Methods: q-PCR and immunohistochemistry were used to study the expression and localization profiles of PHOSPHO1. Inhibitors of PHOSPHO1's PEA hydrolase activity were discovered using highthroughput screening of commercially available chemical libraries. To asses the efficacy of these inhibitors to inhibit MV mineralization, MVs were isolated from TNAP-deficient (Akp2

Increased levels of ePP i in mice deficient in TNALP (i.e., Akp2 −/− ) lead to elevated OPN concentrations. We examined the skeletal phenotype of mice lacking both OPN and TNALP and concluded that the increased OPN levels contribute to the hypophosphatasia phenotype characteristic of Akp2 −/− mice. We also found that extracellular OPN regulates the PP i output by osteoblasts. Introduction: Akp2−/− display mineralization deficiencies characterized by rickets/osteomalacia. This defect has been attributed to the increased levels of extracellular inorganic pyrophosphate (ePP i ), a substrate of tissue-nonspecific alkaline phosphatase (TNALP) and a potent inhibitor of mineral deposition. Because elevated levels of ePP i induce Opn gene expression, the Akp2 −/− mice also display increased levels of osteopontin (OPN), another inhibitor of mineralization. Materials and Methods: Akp2−/− mice were bred into the Opn −/− line. The resulting double knockout mice were analyzed for skeletal abnormalities by histology and CT. Calvarial osteoblasts were assayed for their ability to mineralize in vitro and were probed for changes in gene expression. Results: Mice lacking both Akp2 and Opn showed partial normalization at the histological level with regard to mineral deposition and BMD. However, high ePP i levels remained in Akp2 −/− mice. We found that Opn

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.

The role of the Rho-Rho kinase signaling pathway on osteoblast differentiation was investigated using primary mouse calvarial cells. The bacterial toxin PMT inhibited, whereas Rho-ROK inhibitors stimulated, osteoblast differentiation and bone nodule formation. These effects correlated with altered BMP-2 and -4 expression. These data show the importance of Rho-ROK signaling in osteoblast differentiation and bone formation. Introduction:The signal transduction pathways controlling osteoblast differentiation are not well understood. In this study, we used Pasteurella multocida toxin (PMT), a unique bacterial toxin that activates the small GTPase Rho, and specific Rho inhibitors to investigate the role of Rho in osteoblast differentiation and bone formation in vitro. Materials and Methods: Primary mouse calvarial osteoblast cultures were used to investigate the effects of recombinant PMT and Rho-Rho kinase (ROK) inhibitors on osteoblast differentiation and bone nodule formation. Osteoblast gene expression was analyzed using Northern blot and RT-PCR, and actin rearrangements were visualized after phalloidin staining and confocal microscopy. Results: PMT stimulated the proliferation of primary mouse calvarial cells and markedly inhibited the differentiation of osteoblast precursors to bone nodules with a concomitant inhibition of osteoblastic marker gene expression. There was no apparent causal relationship between the stimulation of proliferation and inhibition of differentiation. PMT caused cytoskeletal rearrangements because of activation of Rho, and the inhibition of bone nodules was completely reversed by the Rho inhibitor C3 transferase and partly reversed by inhibitors of the Rho effector, ROK. Interestingly, Rho and ROK inhibitors alone potently stimulated osteoblast differentiation, gene expression, and bone nodule formation. Finally, PMT inhibited, whereas ROK inhibitors stimulated, bone morphogenetic protein (BMP)-2 and -4 mRNA expression, providing a possible mechanism for their effects on bone nodule formation. Conclusions: These results show that PMT inhibits osteoblast differentiation through a mechanism involving the Rho-ROK pathway and that this pathway is an important negative regulator of osteoblast differentiation. Conversely, ROK inhibitors stimulate osteoblast differentiation and may be potentially useful as anabolic agents for bone.

Introduction: The vast majority of tobacco smokers seeking to quit will relapse within the first month of abstinence. Currently available smoking cessation agents have limited utility in increasing rates of smoking cessation and in some cases there are notable safety concerns related to their use. Hence, there is a pressing need to develop safer and more efficacious smoking cessation medications.