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Functional significance of calcium binding to tissue-nonspecific alkaline phosphatase.

ABSTRACT: The conserved active site of alkaline phosphatases (AP) contains catalytically important Zn2+ (M1 and M2) and Mg2+-sites (M3) and a fourth peripheral Ca2+ site (M4) of unknown significance. We have studied Ca2+ binding to M1-4 of tissue-nonspecific AP (TNAP), an enzyme crucial for skeletal mineralization, using recombinant TNAP and a series of M4 mutants. Ca2+ could substitute for Mg2+ at M3, with maximal activity for Ca2+/Zn2+-TNAP around 40% that of Mg2+/Zn2+-TNAP at pH 9.8 and 7.4. At pH 7.4, allosteric TNAP-activation at M3 by Ca2+ occurred faster than by Mg2+. Several TNAP M4 mutations eradicated TNAP activity, while others mildly influenced the affinity of Ca2+ and Mg2+ for M3 similarly, excluding a catalytic role for Ca2+ in the TNAP M4 site. At pH 9.8, Ca2+ competed with soluble Zn2+ for binding to M1 and M2 up to 1 mM and at higher concentrations, it even displaced M1- and M2-bound Zn2+, forming Ca2+/Ca2+-TNAP with a catalytic activity only 4-6% that of Mg2+/Zn2+-TNAP. At pH 7.4, competition with Zn2+ and its displacement from M1 and M2 required >10-fold higher Ca2+ concentrations, to generate weakly active Ca2+/Ca2+-TNAP. Thus, in a Ca2+-rich environment, such as during skeletal mineralization at pH 7.4, Ca2+ adequately activates Zn2+-TNAP at M3, but very high Ca2+ concentrations compete with available Zn2+ for binding to M1 and M2 and ultimately displace Zn2+ from the active site, virtually inactivating TNAP. Those ALPL mutations that substitute critical TNAP amino acids involved in coordinating Ca2+ to M4 cause hypophosphatasia because of their 3D-structural impact, but M4-bound Ca2+ is catalytically inactive. In conclusion, during skeletal mineralization, the building Ca2+ gradient first activates TNAP, but gradually inactivates it at high Ca2+ concentrations, toward completion of mineralization.

SUBMITTER: Hoylaerts MF

PROVIDER: S-EPMC4361680 | biostudies-literature | 2015

REPOSITORIES: biostudies-literature

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Functional significance of calcium binding to tissue-nonspecific alkaline phosphatase.

Hoylaerts Marc F MF Van Kerckhoven Soetkin S Kiffer-Moreira Tina T Sheen Campbell C Narisawa Sonoko S Millán José Luis JL

PloS one 20150316 3

The conserved active site of alkaline phosphatases (AP) contains catalytically important Zn2+ (M1 and M2) and Mg2+-sites (M3) and a fourth peripheral Ca2+ site (M4) of unknown significance. We have studied Ca2+ binding to M1-4 of tissue-nonspecific AP (TNAP), an enzyme crucial for skeletal mineralization, using recombinant TNAP and a series of M4 mutants. Ca2+ could substitute for Mg2+ at M3, with maximal activity for Ca2+/Zn2+-TNAP around 40% that of Mg2+/Zn2+-TNAP at pH 9.8 and 7.4. At pH 7.4, ...[more]

PMID: 25775211

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Project description:Tissue-nonspecific alkaline phosphatase (TNAP) is necessary for skeletal mineralization by its ability to hydrolyze the mineralization inhibitor inorganic pyrophosphate (PPi), which is mainly generated from extracellular ATP by ectonucleotide pyrophosphatase phosphodiesterase 1 (NPP1). Since children with TNAP deficiency develop bone metaphyseal auto-inflammations in addition to rickets, we hypothesized that TNAP also exerts anti-inflammatory effects relying on the hydrolysis of pro-inflammatory adenosine nucleotides into the anti-inflammatory adenosine. We explored this hypothesis in bone metaphyses of 7-day-old Alpl+/- mice (encoding TNAP), in mineralizing hypertrophic chondrocytes and osteoblasts, and non-mineralizing mesenchymal stem cells (MSCs) and neutrophils, which express TNAP and are present, or can be recruited in the metaphysis. Bone metaphyses of 7-day-old Alpl+/- mice had significantly increased levels of Il-1? and Il-6 and decreased levels of the anti-inflammatory Il-10 cytokine as compared with Alpl+/+ mice. In bone metaphyses, murine hypertrophic chondrocytes and osteoblasts, Alpl mRNA levels were much higher than those of the adenosine nucleotidases Npp1, Cd39 and Cd73. In hypertrophic chondrocytes, inhibition of TNAP with 25 ?M of MLS-0038949 decreased the hydrolysis of AMP and ATP. However, TNAP inhibition did not significantly modulate ATP- and adenosine-associated effects in these cells. We observed that part of TNAP proteins in hypertrophic chondrocytes was sent from the cell membrane to matrix vesicles, which may explain why TNAP participated in the hydrolysis of ATP but did not significantly modulate its autocrine pro-inflammatory effects. In MSCs, TNAP did not participate in ATP hydrolysis nor in secretion of inflammatory mediators. In contrast, in neutrophils, TNAP inhibition with MLS-0038949 significantly exacerbated ATP-associated activation and secretion of IL-1?, and extended cell survival. Collectively, these results demonstrate that TNAP is a nucleotidase in both hypertrophic chondrocytes and neutrophils, and that this nucleotidase function is associated with autocrine effects on inflammation only in neutrophils.

Project description:Hypophosphatasia is a rare heritable disorder caused by inactivating mutations in the gene (Alpl) that encodes tissue nonspecific alkaline phosphatase (TNAP). Hypophosphatasia with onset in infants and children can manifest as rickets. How TNAP deficiency leads to bone hypomineralization is well explained by TNAP's primary function of pyrophosphate hydrolysis when expressed in differentiated bone forming cells. How TNAP deficiency leads to abnormalities within endochondral growth plates is not yet known. Previous studies in hypophosphatemic mice showed that phosphate promotes chondrocyte maturation and apoptosis via MAPK signaling. Alpl-/- mice are not hypophosphatemic but TNAP activity does increase local levels of inorganic phosphate. Therefore, we hypothesize that TNAP influences endochondral bone development via MAPK. In support of this premise, here we demonstrate cranial base bone growth deficiency in Alpl-/- mice, utilize primary rib chondrocytes to show that TNAP influences chondrocyte maturation, apoptosis, and MAPK signaling in a cell autonomous manner; and demonstrate that similar chondrocyte signaling and apoptosis abnormalities are present in the cranial base synchondroses of Alpl-/- mice. Micro CT studies revealed diminished anterior cranial base bone and total cranial base lengths in Alpl-/- mice, that were prevented upon injection with mineral-targeted recombinant TNAP (strensiq). Histomorphometry of the inter-sphenoidal synchondrosis (cranial base growth plate) demonstrated significant expansion of the hypertrophic chondrocyte zone in Alpl-/- mice that was minimized upon treatment with recombinant TNAP. Alpl-/- primary rib chondrocytes exhibited diminished chondrocyte proliferation, aberrant mRNA expression, diminished hypertrophic chondrocyte apoptosis and diminished MAPK signaling. Diminished apoptosis and VEGF expression were also seen in 15 day-old cranial base synchondroses of Alpl-/- mice. MAPK signaling was significantly diminished in 5 day-old cranial base synchondroses of Alpl-/- mice. Together, our data suggests that TNAP is essential for the later stages of endochondral bone development including hypertrophic chondrocyte apoptosis and VEGF mediated recruitment of blood vessels for replacement of cartilage with bone. These changes may be mediated by diminished MAPK signaling in TNAP deficient chondrocytes due to diminished local inorganic phosphate production.

Dataset Information

Functional significance of calcium binding to tissue-nonspecific alkaline phosphatase.

Publications

Functional significance of calcium binding to tissue-nonspecific alkaline phosphatase.

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