Haploinsufficient TNAP Mice Display Decreased Extracellular ATP Levels and Expression of Pannexin-1 Channels.

Álvaro Sebastián-Serrano,Miguel Díaz-Hernández,David C Henshall,Tobías Engel,Laura De Diego-García

doi:10.3389/fphar.2018.00170

Abstract

Hypophosphatasia (HPP) is a rare heritable metabolic bone disease caused by hypomorphic mutations in the ALPL (in human) or Akp2 (in mouse) gene, encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme. In addition to skeletal and dental malformations, severe forms of HPP are also characterized by the presence of spontaneous seizures. Initially, these seizures were attributed to an impairment of GABAergic neurotransmission caused by altered vitamin B6 metabolism. However, recent work by our group using knockout mice null for TNAP (TNAP-/-), a well-described model of infantile HPP, has revealed a deregulation of purinergic signaling contributing to the seizure phenotype. In the present study, we report that adult heterozygous (TNAP+/-) transgenic mice with decreased TNAP activity in the brain are more susceptible to adenosine 5′-triphosphate (ATP)-induced seizures. Interestingly, when we analyzed the extracellular levels of ATP in the cerebrospinal fluid, we found that TNAP+/- mice present lower levels than control mice. To elucidate the underlying mechanism, we evaluated the expression levels of other ectonucleotidases, as well as different proteins involved in ATP release, such as pannexin, connexins, and vesicular nucleotide transporter. Among these, Pannexin-1 (Panx1) was the only one showing diminished levels in the brains of TNAP+/- mice. Altogether, these findings suggest that a physiological regulation of extracellular ATP levels and Panx1 changes may compensate for the reduced TNAP activity in this model of HPP.

Highlights

Intracellular adenosine -triphosphate (ATP), best known as the main source of chemical energy along with its involvement in many crucial cellular processes, is widely recognized to act as either sole transmitter or co-transmitter in both the peripheral and central nervous system (CNS) (Burnstock, 2013; Rodrigues et al, 2015)
To elucidate the underlying mechanism, we evaluated the expression levels of other ectonucleotidases, as well as different proteins involved in ATP release, such as pannexin, connexins, and vesicular nucleotide transporter
We confirm that the brains from adult tissue-nonspecific alkaline phosphatase (TNAP)+/− mice show an overall decrease in alkaline phosphatase activity

Summary

Introduction

Intracellular ATP, best known as the main source of chemical energy along with its involvement in many crucial cellular processes, is widely recognized to act as either sole transmitter or co-transmitter in both the peripheral and CNS (Burnstock, 2013; Rodrigues et al, 2015). In the CNS, the extracellular concentration of ATP is tightly regulated under physiological conditions via different mechanism controlling its release and degradation (Burnstock, 2016). Additional mechanisms of nucleotide release include ATP-binding cassette transporters, connexin or pannexin hemichannels (Dubyak, 2006; Scemes et al, 2007). Rupture of the plasma membrane because of cell death or damage causes intracellular ATP to be directly released, thereby rapidly increasing extracellular ATP levels (Rodrigues et al, 2015)

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