Yadav Manisha C, Simão Ana Maria Sper, Narisawa Sonoko, Huesa Carmen, McKee Marc D, Farquharson Colin, Millán José Luis
Sanford Children's Health Research Center, Sanford-Burnham Medical Research Institute, La Jolla, CA 92037, USA.
J Bone Miner Res. 2011 Feb;26(2):286-97. doi: 10.1002/jbmr.195. Epub 2010 Aug 3.
Endochondral ossification is a carefully orchestrated process mediated by promoters and inhibitors of mineralization. Phosphatases are implicated, but their identities and functions remain unclear. Alkaline phosphatase (TNAP) plays a crucial role promoting mineralization of the extracellular matrix by restricting the concentration of the calcification inhibitor inorganic pyrophosphate (PP(i)). Mutations in the TNAP gene cause hypophosphatasia, a heritable form of rickets and osteomalacia. Here we show that PHOSPHO1, a phosphatase with specificity for phosphoethanolamine and phosphocholine, plays a functional role in the initiation of calcification and that ablation of PHOSPHO1 and TNAP function prevents skeletal mineralization. Phospho1(-/-) mice display growth plate abnormalities, spontaneous fractures, bowed long bones, osteomalacia, and scoliosis in early life. Primary cultures of Phospho1(-/-) tibial growth plate chondrocytes and chondrocyte-derived matrix vesicles (MVs) show reduced mineralizing ability, and plasma samples from Phospho1(-/-) mice show reduced levels of TNAP and elevated plasma PP(i) concentrations. However, transgenic overexpression of TNAP does not correct the bone phenotype in Phospho1(-/-) mice despite normalization of their plasma PP(i) levels. In contrast, double ablation of PHOSPHO1 and TNAP function leads to the complete absence of skeletal mineralization and perinatal lethality. We conclude that PHOSPHO1 has a nonredundant functional role during endochondral ossification, and based on these data and a review of the current literature, we propose an inclusive model of skeletal calcification that involves intravesicular PHOSPHO1 function and P(i) influx into MVs in the initiation of mineralization and the functions of TNAP, nucleotide pyrophosphatase phosphodiesterase-1, and collagen in the extravesicular progression of mineralization.
软骨内成骨是一个由矿化促进因子和抑制因子介导的精心编排的过程。磷酸酶参与其中，但其身份和功能仍不清楚。碱性磷酸酶（TNAP）通过限制钙化抑制剂无机焦磷酸（PP(i)）的浓度，在促进细胞外基质矿化方面发挥关键作用。TNAP基因突变会导致低磷性佝偻病，这是一种遗传性佝偻病和骨软化症。在此，我们表明PHOSPHO1，一种对磷酸乙醇胺和磷酸胆碱具有特异性的磷酸酶，在钙化起始过程中发挥功能作用，并且PHOSPHO1和TNAP功能的缺失会阻止骨骼矿化。Phospho1(-/-)小鼠在生命早期表现出生长板异常、自发性骨折、长骨弯曲、骨软化症和脊柱侧弯。Phospho1(-/-)胫骨生长板软骨细胞和软骨细胞衍生的基质小泡（MVs）的原代培养显示矿化能力降低，Phospho1(-/-)小鼠的血浆样本显示TNAP水平降低且血浆PP(i)浓度升高。然而，尽管其血浆PP(i)水平恢复正常，但TNAP的转基因过表达并不能纠正Phospho1(-/-)小鼠的骨骼表型。相比之下，PHOSPHO1和TNAP功能的双重缺失导致骨骼矿化完全缺失和围产期致死。我们得出结论，PHOSPHO1在软骨内成骨过程中具有不可替代的功能作用，基于这些数据和对当前文献的综述，我们提出了一个包容性的骨骼钙化模型，该模型涉及矿化起始过程中囊泡内PHOSPHO1的功能以及磷酸根离子流入MVs，以及矿化囊泡外进展过程中TNAP、核苷酸焦磷酸酶磷酸二酯酶-1和胶原蛋白的功能。
