Harmey Dympna, Hessle Lovisa, Narisawa Sonoko, Johnson Kristen A, Terkeltaub Robert, Millán José Luis
The Burnham Institute, La Jolla, CA 92037, USA.
Am J Pathol. 2004 Apr;164(4):1199-209. doi: 10.1016/S0002-9440(10)63208-7.
Tissue-nonspecific alkaline phosphatase (TNAP) hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)). Deletion of the TNAP gene (Akp2) in mice results in hypophosphatasia characterized by elevated levels of PP(i) and poorly mineralized bones, which are rescued by deletion of nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) that generates PP(i). Mice deficient in NPP1 (Enpp1(-/-)), or defective in the PP(i) channeling function of ANK (ank/ank), have decreased levels of extracellular PP(i) and are hypermineralized. Given the similarity in function between ANK and NPP1 we crossbred Akp2(-/-) mice to ank/ank mice and found a partial normalization of the mineralization phenotypes and PP(i) levels. Examination of Enpp1(-/-) and ank/ank mice revealed that Enpp1(-/-) mice have a more severe hypermineralized phenotype than ank/ank mice and that NPP1 but not ANK localizes to matrix vesicles, suggesting that failure of ANK deficiency to correct hypomineralization in Akp2(-/-) mice reflects the lack of ANK activity in the matrix vesicle compartment. We also found that the mineralization inhibitor osteopontin (OPN) was increased in Akp2(-/-), and decreased in ank/ank mice. PP(i) and OPN levels were normalized in [Akp2(-/-); Enpp1(-/-)] and [Akp2(-/-); ank/ank] mice, at both the mRNA level and in serum. Wild-type osteoblasts treated with PP(i) showed an increase in OPN, and a decrease in Enpp1 and Ank expression. Thus TNAP, NPP1, and ANK coordinately regulate PP(i) and OPN levels. The hypomineralization observed in Akp2(-/-) mice arises from the combined inhibitory effects of PP(i) and OPN. In contrast, NPP1 or ANK deficiencies cause a decrease in the PP(i) and OPN pools that leads to hypermineralization.
组织非特异性碱性磷酸酶（TNAP）可水解矿化抑制剂无机焦磷酸（PP(i)）。小鼠中TNAP基因（Akp2）的缺失会导致低磷酸酯酶症，其特征是PP(i)水平升高且骨骼矿化不良，而通过缺失产生PP(i)的核苷酸焦磷酸酶磷酸二酯酶1（NPP1）可挽救这种情况。缺乏NPP1（Enpp1(-/-)）或ANK（ank/ank）的PP(i)通道功能存在缺陷的小鼠，其细胞外PP(i)水平降低且矿化过度。鉴于ANK和NPP1在功能上的相似性，我们将Akp2(-/-)小鼠与ank/ank小鼠进行杂交，发现矿化表型和PP(i)水平部分恢复正常。对Enpp1(-/-)和ank/ank小鼠的检查显示，Enpp1(-/-)小鼠的矿化过度表型比ank/ank小鼠更严重，并且NPP1而非ANK定位于基质小泡，这表明ANK缺陷无法纠正Akp2(-/-)小鼠的矿化不足反映了基质小泡区室中ANK活性的缺乏。我们还发现矿化抑制剂骨桥蛋白（OPN）在Akp2(-/-)小鼠中增加，而在ank/ank小鼠中减少。在[Akp2(-/-); Enpp1(-/-)]和[Akp2(-/-); ank/ank]小鼠中，PP(i)和OPN水平在mRNA水平和血清中均恢复正常。用PP(i)处理的野生型成骨细胞显示OPN增加，而Enpp1和Ank表达减少。因此，TNAP、NPP1和ANK协同调节PP(i)和OPN水平。在Akp2(-/-)小鼠中观察到的矿化不足源于PP(i)和OPN的联合抑制作用。相反，NPP1或ANK缺陷导致PP(i)和OPN池减少，从而导致矿化过度。
