Concerted regulation of cementum by pyrophosphate regulatory agents

Objectives: Levels of the mineralization inhibitor pyrophosphate (PP_i) are controlled by antagonistic functions of factors that increase local PP_i (progressive ankylosis protein, ANK; ectonucleotide pyrophosphatase/phosphodiesterase-1, NPP1) and those that decrease PP_i (tissue-nonspecific alkaline phosphatase, Akp2/TNAP).  Transgenic mouse models have demonstrated tooth root acellular cementum is affected by PP_i dysregulation, but it remains unclear how regulatory factors work in concert to affect PP_i homeostasis and ultimately cementogenesis. 

Methods: Single and compound KO mouse models for Akp2, Ank, and Enpp1 were analyzed to determine the role of PP_i in controlling cementogenesis.  Periodontal development in KO vs. control mice was analyzed by microCT, histology, histomorphometry, and immunohistochemistry.     

Results: Excess PP_i during development (Akp2 KO) inhibited cementogenesis, while conversely, deficient PP_i (Ank or Enpp1 KO) encouraged increased cementum apposition to more than 10-fold WT.  Low PP_i and increased cementum conditions were associated with increased cementoblast expression of osteopontin (OPN), dentin matrix protein-1 (DMP1), and ANK or NPP1, all reflecting occurrence of excess mineralization.  Compound Akp2/Ank double KO (dKO) mice were prepared to determine effect on cementum when PP_i levels were relatively normalized by ablation of both ANK and TNAP.  Presence of cementum in dKO mice indicated that TNAP is not absolutely necessary for cementogenesis, but that PP_i removal is likely critical.  Regions of hypercementosis in the dKO (more than 5-fold WT) suggested reduced PP_i from loss of ANK was not sufficiently corrected by removing TNAP.  Increased OPN, DMP1, and NPP1 linked to increased dKO cementum showed that PP_i tunes not only cementum thickness, but also matrix properties.

Conclusions: PP_i is a central regulator of acellular cementum formation, and physiologic PP_i levels are carefully balanced by TNAP, ANK, and NPP1 in order to properly direct cementogenesis. The sensitivity of cementum to PP_i may provide an advantageous strategy for cementum regeneration via PP_i modulation.