In vivo overexpression of tissue-nonspecific alkaline phosphatase increases skeletal mineralization and affects the phosphorylation status of osteopontin

Authors

Sonoko Narisawa, Manisha C. Yadav, José Luis Millán

Abstract

Functional ablation of tissue-nonspecific alkaline phosphatase (TNAP) (Alpl-/- mice) leads to hypophosphatasia, characterized by rickets/osteomalacia attributable to elevated levels of extracellular inorganic pyrophosphate, a potent mineralization inhibitor. Osteopontin (OPN) is also elevated in the plasma and skeleton of Alpl-/- mice. Phosphorylated OPN is known to inhibit mineralization, however, the phosphorylation status of the increased OPN found in Alpl-/- mice is unknown. Here, we generated a transgenic mouse line expressing human TNAP under control of an osteoblast-specific Col1a1 promoter (Col1a1-Tnap). The transgene is expressed in osteoblasts, periosteum, and cortical bones, and plasma levels of TNAP in mice expressing Col1a1-Tnap are 10-20 times higher than those of wild-type mice. The Col1a1-Tnap animals are healthy and exhibit increased bone mineralization by microCT analysis. Crossbreeding of Col1a1-Tnap transgenic mice to Alpl-/- mice rescues the lethal hypophosphatasia phenotype characteristic of this disease model. Osteoblasts from [Col1a1-Tnap] mice mineralize better than non-transgenic controls and osteoblasts from [Col1a1-Tnap+/-; Alpl-/-] mice are able to mineralize to the level of Alpl+/- heterozygous osteoblasts, while Alpl-/- osteoblasts show no mineralization. We found that the increased levels of OPN in bone tissue of Alpl-/- mice are comprised of phosphorylated forms of OPN while WT and [Col1a1-Tnap+/-; Alpl-/-] mice had both phosphorylated and dephosphorylated forms of OPN. OPN from [Col1a1-Tnap] osteoblasts were more phosphorylated than non-transgenic control cells. Titanium dioxide-liquid chromatography and tandem mass spectrometry analysis revealed that OPN peptides derived from Alpl-/- bone and osteoblasts yielded a higher proportion of phosphorylated peptides than samples from WT mice, and at least two phosphopeptides, p(S174FQVS178DEQY182PDAT186DEDLT191)SHMK and FRIp(S299HELES304S305S306S307)EVN, with one non-localized site each, appear to be preferred sites of TNAP action on OPN. Our data suggest that the pro-mineralization role of TNAP may be related not only to its accepted pyrophosphatase activity but also to its ability to modify the phosphorylation status of OPN.

Link to Article

http://dx.doi.org/10.1002/jbmr.1901

Hexa-D-Arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype

Authors

Baozhi Yuan, Jian Q. Feng, Stephen Bowman, Ying Liu, Robert D. Blank, Iris Lindberg, Marc K. Drezner

Abstract

Inactivating mutations of PHEX/Phex underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum FGF-23 underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild type mice with the proprotein convertase (PC) inhibitor, Decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone, increases serum FGF-23 and produces the HYPphenotype. Since PC2 is uniquely co-localized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, while Signe1 (7B2) RNAi transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of proBMP1 to active BMP1, which resulted in limited cleavage of DMP1, and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.

Link to Article

http://dx.doi.org/10.1002/jbmr.1738

Inactivation of a Novel FGF23 Regulator, FAM20C, Leads to Hypophosphatemic Rickets in Mice

Authors

Xiaofang Wang, Suzhen Wang, Changcheng Li, Tian Gao, Ying Liu, Afsaneh Rangiani, Yao Sun, Jianjun Hao, Anne George, Yongbo Lu, Jay Groppe, Baozhi Yuan, Jian Q. Feng, Chunlin Qin

Abstract

Family with sequence similarity 20,-member C (FAM20C) is highly expressed in the mineralized tissues of mammals. Genetic studies showed that the loss-of-function mutations in FAM20C were associated with human lethal osteosclerotic bone dysplasia (Raine Syndrome), implying an inhibitory role of this molecule in bone formation. However, in vitro gain- and loss-of-function studies suggested that FAM20C promotes the differentiation and mineralization of mouse mesenchymal cells and odontoblasts. Recently, we generated Fam20c conditional knockout (cKO) mice in which Fam20c was globally inactivated (by crossbreeding with Sox2-Cre mice) or inactivated specifically in the mineralized tissues (by crossbreeding with 3.6 kb Col 1a1-Cre mice). Fam20c transgenic mice were also generated and crossbred with Fam20c cKO mice to introduce the transgene in the knockout background. In vitro gain- and loss-of-function were examined by adding recombinant FAM20C to MC3T3-E1 cells and by lentiviral shRNA–mediated knockdown of FAM20C in human and mouse osteogenic cell lines. Surprisingly, both the global and mineralized tissue-specific cKO mice developed hypophosphatemic rickets (but not osteosclerosis), along with a significant downregulation of osteoblast differentiation markers and a dramatic elevation of fibroblast growth factor 23 (FGF23) in the serum and bone. The mice expressing the Fam20c transgene in the wild-type background showed no abnormalities, while the expression of the Fam20c transgene fully rescued the skeletal defects in the cKO mice. Recombinant FAM20C promoted the differentiation and mineralization of MC3T3-E1 cells. Knockdown of FAM20C led to a remarkable downregulation of DMP1, along with a significant upregulation of FGF23 in both human and mouse osteogenic cell lines. These results indicate that FAM20C is a bone formation “promoter” but not an “inhibitor” in mouse osteogenesis. We conclude that FAM20C may regulate osteogenesis through its direct role in facilitating osteoblast differentiation and its systemic regulation of phosphate homeostasis via the mediation of FGF23.

Link to Article

http://dx.doi.org/10.1371/journal.pgen.1002708

Dose response of bone-targeted enzyme replacement for murine hypophosphatasia

Authors

Manisha C. Yadav, Isabelle Lemire, Pierre Leonard, Guy Boileau, Laurent Blond, Martin Beliveau, Esther Cory, Robert L. Sah, Michael P. Whyte, Philippe Crine and José Luis Millán

Abstract

Hypophosphatasia (HPP) features rickets or osteomalacia from tissue-nonspecific alkaline phosphatase (TNSALP) deficiency due to deactivating mutations within the ALPL gene. Enzyme replacement therapy with a bone-targeted, recombinant TNSALP (sALP-FcD10, renamed ENB-0040) prevents manifestations of HPP when initiated at birth in TNSALP knockout (Akp2−/−) mice. Here, we evaluated the dose–response relationship of ENB-0040 to various phenotypic traits of Akp2−/− mice receiving daily subcutaneous (SC) injections of ENB-0040 from birth at 0.5, 2.0, or 8.2 mg/kg for 43 days. Radiographs, μCT, and histomorphometric analyses documented better bone mineralization with increasing doses of ENB-0040. We found a clear, positive correlation between ENB-0040 dose and prevention of mineralization defects of the feet, rib cage, lower limbs, and jaw bones. According to a dose–response model, the ED80 (the dose that prevents bone defects in 80% of mice) was 3.2, 2.8 and 2.9 mg/kg/day for these sites, respectively. Long bones seemed to respond to lower daily doses of ENB-0040. There was also a positive relationship between ENB-0040 dose and survival. Median survival, body weight, and bone length all improved with increasing doses of ENB-0040. Urinary PPi concentrations remained elevated in all treatment groups, indicating that while this parameter is a good biochemical marker for diagnosing HPP, it may not be a good follow up marker for evaluating response to treatment when administering bone-targeted TNSALP. These dose–response relationships strongly support the pharmacological efficacy of ENB-0040 for HPP, and provide the experimental basis for the therapeutic range of ENB-0040 chosen for clinical trials. We find a positive correlation between ENB-0040 dose and prevention of mineralization defects in a murine model of hypophosphatasia. We found a positive relationship between ENB-0040 dose and survival. Urinary pyrophosphate is not a good follow up marker for evaluating response when using bone-targeted TNSALP treatment. These studies help set the therapeutic range of ENB-0040 to be chosen for clinical trials.

Link to Article

http://dx.doi.org/10.1016/j.bone.2011.03.770