Adnan Sali

Tissue-nonspecific alkaline phosphatase and plasma cell membrane glycoprotein-1 are central antagonistic regulators of bone mineralization

Osteoblasts mineralize bone matrix by promoting hydroxyapatite crystal formation and growth in the interior of membrane-limited matrix vesicles (MVs) and by propagating the crystals onto the collagenous extracellular matrix. Two osteoblast proteins, tissue-nonspecific alkaline phosphatase (TNAP) and plasma cell membrane glycoprotein-1 (PC-1) are involved in this process. Mutations in the TNAP gene result in the inborn error of metabolism known as hypophosphatasia, characterized by poorly mineralized bones, spontaneous fractures, and elevated extracellular concentrations of inorganic pyrophosphate (PPi). PPi suppresses the formation and growth of hydroxyapatite crystals. PPi is produced by the nucleoside triphosphate pyrophosphohydrolase activity of a family of isozymes, with PC-1 being the only member present in MVs. Mice with spontaneous mutations in the PC-1 gene have hypermineralization abnormalities that include osteoarthritis and ossification of the posterior longitudinal ligament of the spine. Here, we show the respective correction of bone mineralization abnormalities in knockout mice null for both the TNAP (Akp2) and PC-1 (Enpp1) genes. Each allele of Akp2 and Enpp1 has a measurable influence on mineralization status in vivo. Ex vivo experiments using cultured double-knockout osteoblasts and their MVs demonstrate normalization of PPi content and mineral deposition. Our data provide evidence that TNAP and PC-1 are key regulators of the extracellular PPi concentrations required for controlled bone mineralization. Our results suggest that inhibiting PC-1 function may be a viable therapeutic strategy for hypophosphatasia. Conversely, interfering with TNAP activity may correct pathological hyperossification because of PPi insufficiency.

Ecto‐phosphodiesterase/pyrophosphatase of lymphocytes and non‐lymphoid cells: structure and function of the PC‐1 family

Summary: Many developmentally regulated membrane proteins of lymphocytes are ecto‐enzymes, with their active sites on the external surface of the cell. These enzymes commonly have peptidase, phosphodiesterase or nucleotidase activity. Their biological roles are just beginning to be discovered. Although their expression is usually associated with particular stages of lymphoid differentiation, the same gene products are often expressed on the surface of certain non‐lymphoid cell types outside the immune system, indicating that their functions cannot be unique to lymphocytes, nor can they be ubiquitous. The plasma cell membrane protein PC‐1 (phosphodiesterase I; EC 3.1.4.1/nucleotide pyrophosphatase; EC 3.6.1.9), which was one of the first serological markers for lymphocyte subsets to be discovered, is a typical example. Within the immune system, PC‐1 is confined to plasma cells, which represent about 0.1% of lymphocytes. However, PC‐1 is also expressed on cells of the distal convoluted tubule of the kidney, chondrocytes, osteoblasts, epididymis and hepatocytes. Recent work has shown that PC‐1 is a member of a multigene family of ecto‐phosphodiesterases that currently has two other members, PD‐1α (autotaxin) and PD‐1β (B10). Within this family, the extracellular domains are highly conserved, especially around the active site. In contrast, the transmembrane and cytopiasmic domains are highly divergent. Individual members of the ecto‐phosphodiesterase family have distinct patterns of distribution in different cell types, and even within the same cell. For example, PC‐1 is present only on the basolateral surface of hepatocytes, while B10 (PD‐1β) is confined to the apical surface. Analysis of conservation and differences in the sequence of their cytoplasmic tails may illuminate intracellular tar‐getting signals. Ecto‐phosphodiesterases may play a part in diverse activities in different tissues, including recycling of nucleotides. They may also regulate the concentration of pharmacologically active extracellular compounds such as adenosine or its derivatives and ceil motility. Some members may modulate local concentrations of pyrophosphate, and hence influence calcification in bone and cartilage.