Department of Pathology, Yale University School of Medicine, New Haven, CT, USA.
Department of Osteology and Biomechanics, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
J Bone Miner Res. 2022 Sep;37(9):1733-1749. doi: 10.1002/jbmr.4640. Epub 2022 Jul 29.
Biallelic ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) deficiency induces vascular/soft tissue calcifications in generalized arterial calcification of infancy (GACI), and low bone mass with phosphate-wasting rickets in GACI survivors (autosomal hypophosphatemic rickets type-2). ENPP1 haploinsufficiency induces early-onset osteoporosis and mild phosphate wasting in adults. Both conditions demonstrate the unusual combination of reduced accrual of skeletal mineral, yet excess and progressive heterotopic mineralization. ENPP1 is the only enzyme that generates extracellular pyrophosphate (PPi), a potent inhibitor of both bone and heterotopic mineralization. Life-threatening vascular calcification in ENPP1 deficiency is due to decreased plasma PPi; however, the mechanism by which osteopenia results is not apparent from an understanding of the enzyme's catalytic activity. To probe for catalysis-independent ENPP1 pathways regulating bone, we developed a murine model uncoupling ENPP1 protein signaling from ENPP1 catalysis, Enpp1 mice. In contrast to Enpp1 mice, which lack ENPP1, Enpp1 mice have normal trabecular bone microarchitecture and favorable biomechanical properties. However, both models demonstrate low plasma Pi and PPi, increased fibroblast growth factor 23 (FGF23), and by 23 weeks, osteomalacia demonstrating equivalent phosphate wasting in both models. Reflecting findings in whole bone, calvarial cell cultures from Enpp1 mice demonstrated markedly decreased calcification, elevated transcription of Sfrp1, and decreased nuclear β-catenin signaling compared to wild-type (WT) and Enpp1 cultures. Finally, the decreased calcification and nuclear β-catenin signaling observed in Enpp1 cultures was restored to WT levels by knockout of Sfrp1. Collectively, our findings demonstrate that catalysis-independent ENPP1 signaling pathways regulate bone mass via the expression of soluble Wnt inhibitors such as secreted frizzled-related protein 1 (SFRP1), whereas catalysis dependent pathways regulate phosphate homeostasis through the regulation of plasma FGF23. © 2022 American Society for Bone and Mineral Research (ASBMR).
双等位基因核苷酸焦磷酸酶/磷酸二酯酶 1 (ENPP1) 缺乏会导致婴儿全身性动脉钙化（GACI）中的血管/软组织钙化，以及 GACI 幸存者（常染色体低磷性佝偻病 2 型）中的低骨量伴磷酸盐性佝偻病。ENPP1 杂合不足会导致成年人早发性骨质疏松症和轻度磷酸盐丢失。这两种情况都表现出骨骼矿物质积累减少，而异位矿化过度和进行性的异常组合。ENPP1 是唯一能产生细胞外焦磷酸盐（PPi）的酶，PPi 是骨和异位矿化的有效抑制剂。ENPP1 缺乏导致危及生命的血管钙化是由于血浆 PPi 减少所致；然而，从对酶的催化活性的理解来看，导致骨质疏松症的机制并不明显。为了探索与催化无关的调节骨骼的 ENPP1 途径，我们开发了一种将 ENPP1 蛋白信号与 ENPP1 催化分离的小鼠模型，即 Enpp1 小鼠。与缺乏 ENPP1 的 Enpp1 小鼠不同，Enpp1 小鼠具有正常的小梁骨微观结构和有利的生物力学特性。然而，两种模型都表现出低血浆 Pi 和 PPi，成纤维细胞生长因子 23（FGF23）增加，到 23 周时，骨软化症表现出两种模型中同等的磷酸盐丢失。反映了整个骨骼的发现，与野生型（WT）和 Enpp1 培养物相比，Enpp1 小鼠的颅骨细胞培养物的钙化明显减少，Sfrp1 的转录升高，核 β-连环蛋白信号降低。最后，通过敲除 Sfrp1，使 Enpp1 培养物中观察到的降低的钙化和核 β-连环蛋白信号恢复到 WT 水平。总的来说，我们的研究结果表明，与催化无关的 ENPP1 信号通路通过表达可溶性 Wnt 抑制剂（如分泌卷曲相关蛋白 1（SFRP1））来调节骨量，而依赖于催化的途径通过调节血浆 FGF23 来调节磷酸盐稳态。
