Identification of altered brain metabolites associated with TNAP activity in a mouse model of hypophosphatasia using untargeted NMR ‐based metabolomics analysis

Tissue non‐specific alkaline phosphatase (TNAP) is a key player of bone mineralization andTNAPgene (ALPL) mutations in human are responsible for hypophosphatasia (HPP), a rare heritable disease affecting the mineralization of bones and teeth. Moreover,TNAPis also expressed by brain cells and the severe forms ofHPPare associated with neurological disorders, including epilepsy and brain morphological anomalies. However,TNAP's role in the nervous system remains poorly understood. To investigate its neuronal functions, we aimed to identify without anya priorithe metabolites regulated byTNAPin the nervous tissue. For this purpose we used1H‐ and31P NMRto analyze the brain metabolome ofAlpl(Akp2) mice null forTNAPfunction, a well‐described model of infantileHPP. Among 39 metabolites identified in brain extracts of 1‐week‐old animals, eight displayed significantly different concentration inAkp2−/−compared toAkp2+/+andAkp2+/−mice: cystathionine, adenosine,GABA, methionine, histidine, 3‐methylhistidine, N‐acetylaspartate (NAA), and N‐acetyl‐aspartyl‐glutamate, with cystathionine and adenosine levels displaying the strongest alteration. These metabolites identify several biochemical processes that directly or indirectly involveTNAPfunction, 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 ofHPP.