The presence of skeletal hypomineralization was confirmed in mice lacking the gene for bone alkaline phosphatase, ie, the tissue-non-specific isozyme of alkaline phosphatase (TNAP). In this study, a detailed characterization of the ultrastructural localization, the relative amount and ultrastructural morphology of bone mineral was carried out in tibial growth plates and in subjacent metaphyseal bone of 10-day-old TNAP knockout mice. Alizarin red staining, microcomputerized tomography (micro CT), and FTIR imaging spectroscopy (FT-IRIS) confirmed a significant overall decrease of mineral density in the cartilage and bone matrix of TNAP-deficient mice. Transmission electron microscopy (TEM) showed diminished mineral in growth plate cartilage and in newly formed bone matrix. High resolution TEM indicated that mineral crystals were initiated, as is normal, within matrix vesicles (MVs) of the growth plate and bone of TNAP-deficient mice. However, mineral crystal proliferation and growth was inhibited in the matrix surrounding MVs, as is the case in the hereditary human disease hypophosphatasia. These data suggest that hypomineralization in TNAP-deficient mice results primarily from an inability of initial mineral crystals within MVs to self-nucleate and to proliferate beyond the protective confines of the MV membrane. This failure of the second stage of mineral formation may be caused by an excess of the mineral inhibitor pyrophosphate (PPi) in the extracellular fluid around MVs. In normal circumstances, PPi is hydrolyzed by the TNAP of MVs' outer membrane yielding monophosphate ions (Pi) for incorporation into bone mineral. Thus, with TNAP deficiency a buildup of mineral-inhibiting PPi would be expected at the perimeter of MVs.

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.

No abstract

No abstract

The distribution and staining intensity of bone morphogenetic proteins (BMPs) 2, 4, 6, and 7 were assessed by immunohistochemistry in ectopic bone induced in Nu/Nu mice by Saos-2 cell derived implants. Devitalized Saos-2 cells or their extracts can induce endochondral bone formation when implanted subcutaneously into Nu/Nu mice. BMP staining was mostly cytoplasmic. The most intense BMP staining was seen in hypertrophic and apoptotic chondrocytes, osteoprogenitor cells such as periosteal and perivascular cells, and osteoblasts. BMP staining in osteocytes and osteoclasts was variable, ranging from undetectable to intensely stained, and from minimal to moderately stained in megakaryocytes of the induced bone marrow. BMP-2, 4, 6, and 7 staining in Saos-2 implant-induced bone indicates the following: (1) Saos-2 cell products promote expression of BMPs by host osteoprogenitor cells, which in turn, leads to bone and marrow formation at ectopic sites; (2) strong BMP staining is seen in maturing chondrocytes, and thus may play a role in chondrocyte differentiation and/or apoptosis; (3) BMP expression in perivascular and periosteal cells indicates that osteoprogenitor cells also express BMP; (4) BMP release by osteoclasts may promote osteoblastic differentiation at sites of bone remodeling. These new data can be useful in understanding the role of BMPs in promoting clinical bone repair and in various pathologic conditions. ß

A simple and non-radioactive technique based on O-cresolpthalein complexone assay was developed to study in vitro non-radioactive calcium ( 40 Ca) deposition by isolated matrix vesicles. Using this technique, the effect of various phosphoester substrates including ATP, AMP and β-GP on in vitro MV-calcification was studied. O-cresolpthalein complexone assay with non-radioactive calcium demonstrated that AMP or β-GP were more effective in promoting calcium deposition by isolated MVs than ATP. The application of this nonradioactive technique, which is highly sensitive and simple, would offer a useful alternative approach to the routinely used radiometric biomineralization assay which employs radioactive 45 Ca.