We have recently reported that the rickets and osteomalacia characteristic in tissue-nonspecific alkaline phosphatase (TNAP)-deficient mice (Akp2-/- mice) results from highly increased levels of inorganic pyrophosphate (PP), a calcification inhibitor PPi and a natural substrate of TNAP, and from the concomitant increase in the expression of skeletal osteopontin (OPN), another calcification inhibitor. These studies suggested the possibility of manipulating the PP/OPN axis as a means of affecting calcification. We recently tested this axis by surmising that transgenic mice over-expressing TNAP might be able to achieve tissular expression of TNAP sufficiently high to be able to lower circulating PPi and OPN concentrations to enhance bone mineral density (BMD) in these animals. Transgenic mice were generated by expressing human TNAP cDNA under control of the Apolipoprotein E promoter, which drives expression of TNAP primarily in the post- natal liver. We examined the expression levels of TNAP in tissues from mice carrying one copy or two copies of the ApoE-Tnap transgene and also from [Akp2-/-; ApoE-Tnap] mice, and examined the ability of their primary osteoblasts to calcify in culture. MicroCT analysis was used to measure BMD in long bones, vertebrae and calvaria. TNAP expression in ApoE-Tnap mice was major in the liver and kidney as expected, with lower but yet detectable levels in bone, brain and lung. Serum AP concentrations were 10 to 50-fold higher than age- matched sibling control wild-type (WT) mice. As predicted, serum levels of PPi and OPN were reduced in the transgenic animals. Furthermore, ?CT analysis of femur, vertebrae and calvaria revealed higher BMD in cancellous bone of ApoE-Tnap+ and ApoE-Tnap+/+ mice compared to WT mice. Thus, we have shown that increases in tissular and circulating levels of TNAP lead to higher BMD by reducing the effective levels of the calcification inhibitors PPi and OPN. These data provide a mechanistic interpretation for the correlation between AP and BMD that has been observed in humans and mice. Furthermore, these studies suggest the possibility that administration of recombinant TNAP itself, or of pharmacological activators of TNAP's pyrophosphatase activity, may serve as therapeutics drugs for the treatment of osteoporosis. Thus, this proposal aims at developing a sensitive assay for the discovery of TNAP activators that may serve as lead compounds for the development of drug-like molecules suitable for in vivo administration. We will use this assay to screen the small molecule repository (MLSMR) for activators of TNAP.
The specific aims are to: I) Identify small molecule compounds in the MLSMR that are highly specific activators of TNAP using a luminescence-based assay. II) Test confirmed positives in the secondary assay with natural substrates of TNAP and check for specificity against other recombinant phosphatases. III) Test confirmed positives for their ability to increase calcification in osteoblast cultures. The novel chemical probes to be identified in this way may ultimately lead to the novel therapy for the growing number of osteoporosis patients. Within the past five years this laboratory, in association with several collaborating groups, has focused on the factors that control mineralization and it is now clear that a main player in this process is phosphate in its two major forms, i.e., as inorganic phosphate (P)i and as inorganic pyrophosphate (PP). We have clearly shown i that the maintenance of a properly controlled extracellular P/PPi ratio is of paramount importance in promoting i healthy bone mineralization. Alterations in this ratio, either by genetic or pharmacologic means, can either correct or cause a pathologic state. Osteoporosis is characterized by an imbalance of osteoblast-mediated bone formation and osteoclast-mediated bone degradation, which results in overall increased bone resorption. Current treatments of osteoporosis aim at either reducing osteoclastic activity or augmenting osteoblastic function. In this proposal we will test a novel hypothesis, i.e., that we will be able to identify activators of TNAP's pyrophosphatase activity that will serve to promote degradation of PPi thus increasing the P/PPi ratio i to favor increased mineralization. We anticipate that this project will validate manipulating the P/PPi ratio as a i valuable therapeutic option to treating osteoporosis by affecting osteoblast-mediated mineral deposition. Such a strategy could be used as an alternative or as a complement to currently used drugs that decrease osteoclastic activity (bisphosphonate treatment) or increase osteoblasts numbers (PTH-like peptide treatment). ? ? ?
