Tissue Nonspecific Alkaline Phosphatase (TNAP) is a key player of bone mineralization and TNAP gene (ALPL) mutations in human are responsible for hypophosphatasia (HPP), a rare heritable disease affecting the mineralization of bones and teeth. Moreover, TNAP is also expressed by brain cells and the severe forms of HPP are associated with neurological disorders, including epilepsy and brain morphological anomalies. However TNAP’s role in the nervous system remains poorly understood. In order to investigate its neuronal functions, we aimed to identify without any a priori the metabolites regulated by TNAP in the nervous tissue. For this purpose we used 1H- and 31P NMR to analyze the brain metabolome of Alpl (Akp2) mice null for TNAP function, a well-described model of infantile HPP. Among 39 metabolites identified in brain extracts of one week-old animals, 8 displayed significantly different concentration in Akp2−/− compared to Akp2+/+ and Akp2+/− mice: cystathionine, adenosine, GABA, methionine, histidine, 3-methylhistidine, N-acetylaspartate (NAA) and N-acetyl-aspartyl-glutamate (NAAG), with cystathionine and adenosine levels displaying the strongest alteration. These metabolites identify several biochemical processes that directly or indirectly involve TNAP function, in particular through the regulation of ecto-nucleotide levels and of pyridoxal phosphate-dependent enzymes. Some of these metabolites are involved in neurotransmission (GABA, adenosine), in myelin synthesis (NAA, NAAG), and in the methionine cycle and transsulfuration pathway (cystathionine, methionine). Their disturbances may contribute to the neurodevelopmental and neurological phenotype of HPP.
Tissue-nonspecific alkaline phosphatase (TNAP) is necessary for skeletal mineralization by its ability to hydrolyze the mineralization inhibitor inorganic pyrophosphate (PP i ), 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-1β 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 μM of MLS-0038949 decreased the hydrolysis of AMP and ATP. However, TNAP inhibition did not significantly modulate ATP-and adenosineassociated 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 proinflammatory effects. In MSCs, TNAP did not participate in ATP hydrolysis nor in secretion of
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