Ablation of Enpp6 Results in Transient Bone Hypomineralization

Biomineralization is a fundamental process key to the development of the skeleton. The phosphatase orphan phosphatase 1 (PHOSPHO1), which likely functions within extracellular matrix vesicles, has emerged as a critical regulator of biomineralization. However, the biochemical pathways that generate intravesicular PHOSPHO1 substrates are currently unknown. We hypothesized that the enzyme ectonucleotide pyrophosphatase/phosphodiesterase 6 (ENPP6) is an upstream source of the PHOSPHO1 substrate. To test this, we characterized skeletal phenotypes of mice homozygous for a targeted deletion of Enpp6 (Enpp6 ^−/−). Micro-computed tomography of the trabecular compartment revealed transient hypomineralization in Enpp6 ^−/− tibias (p < 0.05) that normalized by 12 weeks of age. Whole-bone cortical analysis also revealed significantly hypomineralized proximal bone in 4- but not 12-week-old Enpp6 ^−/− mice (p < 0.05) compared with WT animals. Back-scattered SEM revealed a failure in 4-week-old trabecular bone of mineralization foci to propagate. Static histomorphometry revealed increased osteoid volume (p > 0.01) and osteoid surface (p < 0.05), which recovered by 12 weeks but was not accompanied by changes in osteoblast or osteoclast number. This study is the first to characterize the skeletal phenotype of Enpp6 ^−/− mice, revealing transient hypomineralization in young animals compared with WT controls. These data suggest that ENPP6 is important for bone mineralization and may function upstream of PHOSPHO1 as a novel means of generating its substrates inside matrix vesicles.