Increased PHOSPHO1 and alkaline phosphatase expression during the anabolic bone response to intermittent parathyroid hormone delivery.

The administration of intermittent parathyroid hormone (iPTH) is anabolic to the skeleton. Recent studies with cultured osteoblasts have revealed that the expression of PHOSPHO1, a bone-specific phosphatase essential for the initiation of mineralisation, is regulated by PTH. Therefore, this study sought to determine whether the bone anabolic response to iPTH involves modulation of expression of Phospho1 and of other enzymes critical for bone matrix mineralisation. To mimic iPTH treatment, primary murine osteoblasts were challenged with 50 nM PTH for 6 h in every 48 h period for 8 days (4 cycles), 14 days (7 cycles) and 20 days (10 cycles) in total. The expression of both Phospho1 and Smpd3 was almost completely inhibited after 4 cycles, whereas 10 cycles were required to stimulate a similar response in Alpl expression. To explore the in vivo role of PHOSPHO1 in PTH-mediated osteogenesis, the effects of 14- and 28-day iPTH (80 µg/kg/day) administration was assessed in male wild-type (WT) and Phospho1-/- mice. The expression of Phospho1, Alpl, Smpd3, Enpp1, Runx2 and Trps1 expression was enhanced in the femora of WT mice following iPTH administration but remained unchanged in the femora of Phospho1-/- mice. After 28 days of iPTH administration, the anabolic response in the femora of WT was greater than that noted in Phospho1-/- mice. Specifically, cortical and trabecular bone volume/total volume, as well as cortical thickness, were increased in femora of iPTH-treated WT but not in iPTH-treated Phospho1-/- mice. Trabecular bone osteoblast number was also increased in iPTH-treated WT mice but not in iPTH-treated Phospho1-/-  mice. The increased levels of Phospho1, Alpl, Enpp1 and Smpd3 in WT mice in response to iPTH administration is consistent with their contribution to the potent anabolic properties of iPTH in bone. Furthermore, as the anabolic response to iPTH was attenuated in mice deficient in PHOSPHO1, this suggests that the osteoanabolic effects of iPTH are at least partly mediated via bone mineralisation processes.

Culture of Murine Embryonic Metatarsals: A Physiological Model of Endochondral Ossification.

The fundamental process of endochondral ossification is under tight regulation in the healthy individual so as to prevent disturbed development and/or longitudinal bone growth. As such, it is imperative that we further our understanding of the underpinning molecular mechanisms involved in such disorders so as to provide advances towards human and animal patient benefit. The mouse metatarsal organ explant culture is a highly physiological ex vivo model for studying endochondral ossification and bone growth as the growth rate of the bones in culture mimic that observed in vivo. Uniquely, the metatarsal organ culture allows the examination of chondrocytes in different phases of chondrogenesis and maintains cell-cell and cell-matrix interactions, therefore providing conditions closer to the in vivo situation than cells in monolayer or 3D culture. This protocol describes in detail the intricate dissection of embryonic metatarsals from the hind limb of E15 murine embryos and the subsequent analyses that can be performed in order to examine endochondral ossification and longitudinal bone growth.

The Functional co-operativity of Tissue-Nonspecific Alkaline Phosphatase (TNAP) and PHOSPHO1 during initiation of Skeletal Mineralization.

Phosphatases are recognised to have important functions in the initiation of skeletal mineralization. Tissue-nonspecific alkaline phosphatase (TNAP) and PHOSPHO1 are indispensable for bone and cartilage mineralization but their functional relationship in the mineralization process remains unclear. In this study, we have used osteoblast and ex-vivo metatarsal cultures to obtain biochemical evidence for co-operativity and cross-talk between PHOSPHO1 and TNAP in the initiation of mineralization. Clones 14 and 24 of the MC3T3-E1 cell line were used in the initial studies. Clone 14 cells expressed high levels of PHOSPHO1 and low levels of TNAP and in the presence of ß-glycerol phosphate (BGP) or phosphocholine (P-Cho) as substrates and they mineralized their matrix strongly. In contrast clone 24 cells expressed high levels of TNAP and low levels of PHOSPHO1 and mineralized their matrix poorly. Lentiviral Phospho1 overexpression in clone 24 cells resulted in higher PHOSPHO1 and TNAP protein expression and increased levels of matrix mineralization. To uncouple the roles of PHOSPHO1 and TNAP in promoting matrix mineralization we used PHOSPHO1 (MLS-0263839) and TNAP (MLS-0038949) specific inhibitors, which individually reduced mineralization levels of Phospho1 overexpressing C24 cells, whereas the simultaneous addition of both inhibitors essentially abolished matrix mineralization (85 %; P<0.001). Using metatarsals from E15 mice as a physiological ex vivo model of mineralization, the response to both TNAP and PHOSPHO1 inhibitors appeared to be substrate dependent. Nevertheless, in the presence of BGP, mineralization was reduced by the TNAP inhibitor alone and almost completely eliminated by the co-incubation of both inhibitors. These data suggest critical non-redundant roles for PHOSPHO1 and TNAP during the initiation of osteoblast and chondrocyte mineralization.