Ablation of <i>Enpp6</i> Results in Transient Bone Hypomineralization

Abstract

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 <i>Enpp6</i> (<i>Enpp6</i> ^<i>-/-</i> ). Micro-computed tomography of the trabecular compartment revealed transient hypomineralization in <i>Enpp6</i> ^<i>-/-</i> tibias (<i>p</i> < 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 <i>Enpp6</i> ^<i>-/-</i> mice (<i>p</i> < 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 (<i>p</i> > 0.01) and osteoid surface (<i>p</i> < 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 <i>Enpp6</i> ^<i>-/-</i> 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. © 2020 The Authors. <i>JBMR Plus</i> published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.