Ectonucleotide Pyrophosphatase/Phosphodiesterase (NPP)1-3 May Regulate Periodontal Homeostasis.

Objectives: The developing periodontium is sensitive to local levels of phosphate (P_i) and pyrophosphate (PP_i). Ectonucleotide pyrophosphatase/phosphodiesterases 1-3 (NPP1-3) are type-II transmembrane metalloenzymes with broad substrate specificity. While Enpp1 KO mice feature reduced PP_i and a cementum phenotype, nothing is known regarding the presence/role of NPP2 and NPP3 in the periodontium.  This research aimed to map NPP1,2, and 3 expression in the developing periodontium in wild-type (WT) mice, as well as in PP_i-deficient progressive ankylosis protein (Ank) and Enpp1 KO mice, to explore their roles in periodontal homeostasis.

Methods: Mandibles from WT, Enpp1 KO and ANK KO were harvested for immunohistochemistry at 23, 27, 33, 45 and 79 days post-coital (dpc).  Enpp1-3 gene expression was assayed by real-time PCR in human periodontal ligament (PDL) cells and murine cementoblasts (OCCM-30) under osteogenic conditions.

Results: In WT mice, NPP1 staining was observed in cementoblasts, osteoblasts, and odontoblasts, whereas NPP2 and NPP3 were localized to pulp and PDL tissues. Compared to WT, NPP1 expression was  increased markedly in Ank KO cementoblasts lining the cervical root from 27 to 79 dpc, whereas NPP2 and NPP3 were increased at days 27, 33 and 60 in pulp and PDL tissues. In Enpp1 KO mice, NPP2/3 expression was similar to WT.  In vitro, in cementoblasts, Enpp1 significantly increased with initiation of mineralization (about 3-fold), whereas Enpp2 and Enpp3 exhibited no pattern. In PDL cells, Enpp2 and Enpp3 were significantly increased at later stages of mineralization (about 4-fold), while Enpp1 expression was unaltered by osteogenic conditions.

Conclusions: Together, these findings suggest that each NPP has a distinct expression pattern in the developing periodontium.  While NPP1 is a key factor for cementum formation, NPP2 and NPP3 may be involved in regulation of PDL homeostasis.  NPP modulation should be considered as a potential target for reconstruction of periodontal tissues.