Ablation of Enpp6 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. The biochemical pathways which generate intravesicular PHOSPHO1 substrates are however currently unknown. We hypothesized that the enzyme ectonucleotide pyrophosphatase/phosphodiesterase (ENPP6) is an upstream source of 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-/tibiae (p < 0.05) that normalized by 12 weeks of Accepted Article This article is protected by copyright. All rights reserved. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jbm4.10439 age. Whole-bone cortical analysis also revealed significantly hypomineralized proximal bone in 4-but not 12-week old Enpp6-/mice (p < 0.05) compared to wild-type animals. Backscattered scanning electron microscopy 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 to wild-type 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.

Spatial Lipidomic Profiling of Mouse Joint Tissue Demonstrates the Essential Role of PHOSPHO1 in Growth Plate Homeostasis

“…( 68 ) This observation aligns with the hypothesis that the specific distribution of these PCs and LPCs in the growth plate is linked to biomineralization as proposed. ( 51 , 52 ) Therefore, we aimed to determine if PCs and LPCs enriched in the growth plate are involved in the PHOSPHO1‐mediated accumulation of P i .…”

“…It has been proposed that MV membrane remodeling under the action of an unidentified phospholipase A2 (PLA2) enzyme converts phosphatidylcholine (PC) lipids to a lyso‐phosphatidylcholine (LPC) lipid intermediate, which is subsequently hydrolyzed by ectonucleotide pyrophosphatase/phosphodiesterase 6 (ENPP6) to generate the PCho substrate for PHOSPHO1. ( 51 ) The involvement of ENPP6 as an upstream mediator of PHOSPHO1 function was recently supported by the hypomineralized phenotype observed in ENPP6‐null mice. ( 52 ) Nevertheless, the precise role of PHOSPHO1 in lipid homeostasis and metabolism during bone mineralization in the growth plate is yet to be elucidated.…”

Matrix Vesicle-Mediated Mineralization and Osteocytic Regulation of Bone Mineralization

“…Consistently, Phospho1 −/− mice displayed hypo-mineralization of the bone at birth, biomechanical incompetency, scoliosis, and spontaneous greenstick fractures [ 61 , 62 ]. More recently, Dillon et al demonstrated ectonucleotide phosphodiesterase/pyrophosphatase member 6 ( Enpp6 ) −/− mice also revealed hypo-mineralization [ 63 ]. ENPP6 is a member of the nucleotide pyrophosphatase/phosphodiesterase family with lysophospholipase C activity, generating phosphocholine with a monoacylglycerol byproduct [ 64 , 65 , 66 ].…”