Their

structures are similar; all display an Arg-Gly-Asp (RGD) motif which facilitates cell attachment, and all are commonly located on the human chromosome 4q21-23 [4], ERK inhibitor chemical structure [7] and [8]. In bone, MEPE is primarily expressed by osteocytes, but Mepe mRNA expression has also been observed in osteoblasts [9]. The expression of MEPE is increased during osteoblast matrix mineralization suggesting a function for MEPE in bone mineralization [10] and [11]. This has been further fuelled by analysis of the MEPE null mouse in which the ablation of MEPE leads to an increased bone mass due to increased numbers and activity of osteoblasts [12]. Furthermore, the overexpression of MEPE in mice, under the control of the Col1a1 promoter, leads to defective mineralization coupled with an increased level HDAC inhibitor of MEPE-ASARM peptides in bone [13]. The MEPE-overexpressing mice displayed wider epiphyseal growth plates, with associated expanded primary spongiosa and a significant decrease in mineral apposition rate [13]. Further studies in vitro have confirmed the inhibitory effect of MEPE on mineralization and have identified that MEPE is cleaved to a 2.2 kDa ASARM peptide which causes this effect [14] and [15]. The ASARM motif is located immediately downstream of a cathepsin B cleavage site, and it is responsible for the mineralization defect observed

in X-linked hypophosphatemic rickets, the most common form of inherited rickets [4], [14] and [15]. This defect can be reversed by administration of cathepsin inhibitors CAO74 or pepstatin [16]. PHEX PI-1840 plays a central role in the protection of MEPE from proteolytic cleavage by cathepsin B; it can bind to MEPE and prevent the release of the ASARM peptide [17]. The Hyp mouse, a spontaneous Phex knockout model, has an increased expression of cathepsin D, an upstream activator of cathepsin B [16]. Therefore PHEX may also assist in decreasing the activation of cathepsin B. Previous studies have shown that the post translational modification

of the MEPE-ASARM peptide is key to its functional role. MEPE has a number of potential casein kinase II phosphorylation motifs, and it is here that the ASARM peptide is phosphorylated at 3 serine residues [4]. This has been shown to inhibit mineralization in murine calvarial osteoblasts and in bone marrow stromal cells by the direct binding of the MEPE-ASARM peptide to HA crystals [14] and [18]. To elucidate the interactions of MEPE in the growth plate, this study was undertaken to examine the presence and function of MEPE and its ASARM peptide in growth plate matrix mineralization during the endochondral ossification process. The data indicated that MEPE is expressed by growth plate chondrocytes, in particular in the hypertrophic zone of chondrocytes consistent with a potential role in matrix mineralization.