Spatial-Temporal Patterns and Inflammatory Factors of Bone Matrix Remodeling

Figure 2

The two patterns of ACP/HA formation by MVs. For one pattern, iNTP transforms into iPi through ATPase, and iPPi transforms into iPi by iPPase. Then, iPPi is secreted out of MVs by ANK, and iNTP transports into ECM as ePPi. And iNTP could transform into iPPi by NPP1. Extracellular PPi and iNTP hydrolyze to Pi via TNAP [21]. In the meantime, intracellular Ca²⁺ and iPi translate into the matrix. ePi and eCa²⁺ synthesize HA or ACP. On the other hand, PHOSPHO1 activates the formation of apatite crystals by hydrolyzing phospholipid and collaborating with phosphoethanolamine and phosphocholine [23]. The annexin channels allow a mass of Ca²⁺ to enter into MVs [24]. It is possible that lipids in the membranes of MVs are nucleation sites, and Ca²⁺, Pi, and some other proteins in MVs constitute specific structures, where HA or ACP could crystallize [25]. MVs: matrix vesicles; iNTP: intracellular nucleoside triphosphate; ATPase: adenosine triphosphatase; iPPi: intracellular pyrophosphate pool; iPPase: intracellular pyrophosphatase; ANK: ankylosis protein; eNTP: extracellular nucleoside triphosphate; NPP1: nucleotide pyrophosphatase phosphodiesterase 1; ePPi: extracellular pyrophosphate pool; TNAP: tissue nonspecific alkaline phosphatase; ACP: amorphous calcium phosphate; HA: hydroxyapatite; ePi: extracellular Pi; iPi: intracellular Pi; eCa²⁺: extracellular Ca²⁺; iCa²⁺: intracellular Ca²⁺.