The Phosphate Transporter Proteins and Pyrophosphate in Matrix Vesicles

The Phosphate Transporter Proteins and Pyrophosphate in Matrix Vesicles 

The type El sodium-dependent phosphate (Na/Pi) transporters involved in mineralization are members of the inorganic phosphate transporter (PiT) family, which is conserved in all biology. El osteoclasts, a proton (H+) gradient instead of a sodium (Na+) gradient transports Pi. These transporters are antiports the Na+ or H+ is transported out as the Pi is transported in (Sects. 2.2.3 and 10.1.4). All these transporters are composed of repeating alpha helices that weave in and out of a membrane with intra- and extracellular turns (Fig. 9.7). A similar kind of structure is proposed for the annexins that mediate calcium ion transport. 

A physiologic phosphate concentration is required for bone mineralization. Lowering the concentration prevents mineralization, but raising it does not ensure precipitation because pyrophosphate is present to inhibit precipitation. The concentration of PPi in cartilage and bone is controlled by three enzymes, two on the outer surface of matrix vesicles (Fig. 9.5b). One is tissue-nonspecific alkaline phosphatase (TNAP), which decreases stromal pyrophosphate and the other is NTP-PPi hydrolase (also called plasma cell membrane glycoprotein-1), which increases it. The progressive ankylosis gene product (ANK protein) is expressed by osteoblasts to add to the pyrophosphate of the osteoid matrix from osteoblast cytosol. 

Mineralization therefore occurs in bone because of the exclusive co-expression in osteoblasts of type I collagen and tissue-nonspecific alkaline phosphatase (TNAP). The abnormal appearance of TNAP in any cell that also produces fibrillar collagen (ectopic TNAP expression) gives rise to pathological (nonbacterial) mineralization, which is outside the scope of this text. 

Osteoblasts secrete osteoid, a matrix rich in type I collagen fibers and vesicles. Precipitation of calcium phosphate is inhibited by a high concentration of pyrophosphate in stromal interstitial fluids, and a high concentration also of albumin and citrate in blood plasma. Pyrophosphate is derived from (1) transport out of the cytosol, and (2) synthesis from nucleoside triphosphates in the stromal interstitial fluid that permeates the osteoid matrix. Precipitation occurs only when calcium and phosphate ions are taken up into vesicles whose inner membrane is composed of phosphatidylserine. The high concentration of calcium and phosphate ions in the vesicle is mediated by annexin and type HI Pi Na-dependent transporters. This overwhelms the pyrophosphate and nucleation occurs. As the precipitate grows and ruptures the membrane, tissue-nonspecific alkaline phosphatase is activated to remove pyrophosphate from the osteoid matrix fluid so that calcium phosphate precipitates around phosphorylated serine residues within the collagen fibers. 

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