Neurodevelopmental alterations and seizures developed by mouse model of infantile hypophosphatasia are associated with purinergic signalling deregulation

Abstract

Hypomorphic mutations in the gene encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme, ALPL in human or Akp2 in mice, cause hypophosphatasia (HPP), an inherited metabolic bone disease also characterized by spontaneous seizures. Initially, these seizures were attributed to the impairment of GABAergic neurotransmission caused by altered vitamin B6 (vit-B6) metabolism. However, clinical cases in human newborns and adults whose convulsions are refractory to pro-GABAergic drugs but controlled by the vit-B6 administration, suggest that other factors are involved. Here, to evaluate whether neurodevelopmental alterations are underlying the seizures associated to HPP, we performed morphological and functional characterization of postnatal homozygous TNAP null mice, a model of HPP. These analyses revealed that TNAP deficient mice present an increased proliferation of neural precursors, an altered neuronal morphology, and an augmented neuronal activity. We found that these alterations were associated with a partial downregulation of the purinergic P2X7 receptor (P2X7R). Even though deficient P2X7R mice present similar neurodevelopmental alterations, they do not develop neonatal seizures. Accordingly, we found that the additional blockage of P2X7R prevent convulsions and extend the lifespan of mice lacking TNAP. In agreement with these findings, we also found that exogenous administration of ATP or TNAP antagonists induced seizures in adult wild-type mice by activating P2X7R. Finally, our results also indicate that the anticonvulsive effects attributed to vit-B6 may be due to its capacity to block P2X7R. Altogether, these findings suggest that the purinergic signalling regulates the neurodevelopmental alteration and the neonatal seizures associated to HPP.

Disorders affecting vitamin B6 metabolism

“…In a mouse model of hypophosphatasia, low levels of the inhibitory neurotransmitter, GABA, were demonstrated in 1995 . However, seizures in patients with hypophosphatasia are refractory to pro‐GABAergic drugs and other experiments on the hypophosphatasia mouse show downregulation of P2X7 receptors and reduction of seizures by blockade of P2X7 receptors, suggesting the anticonvulsive effects of vitamin B 6 may be due to its capacity to block P2X7R receptors …”

“…46 However, seizures in patients with hypophosphatasia are refractory to pro-GABAergic drugs and other experiments on the hypophosphatasia mouse show downregulation of P2X7 receptors and reduction of seizures by blockade of P2X7 receptors, suggesting the anticonvulsive effects of vitamin B 6 may be due to its capacity to block P2X7R receptors. 47,48 Recent metabolomic profiling of the brains of mice with hypophosphatasia showed a complex range of abnormalities including low levels of GABA and another important neurotransmitter, adenosine, as well as abnormalities of metabolites involved in myelin synthesis (N-acetylaspartate [NAA], N-acetylglutamate [NAG]), and in the methionine cycle and transsulfuration pathway (cystathionine and methionine). 49 Studies on vitamin B 6 -deprived Neuro-2a cells showed reduced synthesis of glycine, serine, and 5-methyl-tetrahydrofolate.…”

Purinergic Signalling: Therapeutic Developments

“…A role for post-transcriptional control of the P2X7R expression has been proposed and therapeutic targeting of microRNA-22 was suggested to prevent development of epilepsy and inflammation ( Jimenez-Mateos et al, 2015 ). Purinergic signalling, via P2X7R, regulates neonatal seizures associated with hypophosphatasia, an inherited metabolic bone disease characterised by spontaneous seizures ( Sebastián-Serrano et al, 2016 ). P2X7R antagonists were effective against hypoxia-induced neonatal seizures in mice ( Rodriguez-Alvarez et al, 2017 ).…”

Pathophysiological Role of Purines and Pyrimidines in Neurodevelopment: Unveiling New Pharmacological Approaches to Congenital Brain Diseases

“…Although the exact consequences of these mutations on brain development have not been clarified yet, TNAP knock-out mice show abnormalities in myelination and synaptogenesis (Hanics et al, 2012 ), and subjects with hypophosphatasia suffer from seizures (Whyte, 2010 ), thus indicating defects in neurotransmission and/or in neurodevelopment (Fonta et al, 2015 ). Recent data have now started to link these defects and clinical manifestations to altered purinergic transmission, with the first demonstration of a direct involvement of an aberrant P2X7 activation in the alterations of hippocampal and cortex structure and in the development of seizures in TNAP −/− mice, due to high ATP concentrations as a consequence of reduced TNAP activity (Sebastián-Serrano et al, 2016 ). This observation would greatly help the development of possible new pharmacological approaches to the pathology.…”