Mice lacking tissue non–specific alkaline phosphatase die from seizures due to defective metabolism of vitamin B–6

Abstract

In humans, deficiency of the tissue non-specific alkaline phosphatase (TNAP) gene is associated with defective skeletal mineralization. In contrast, mice lacking TNAP generated by homologous recombination using embryonic stem (ES) cells have normal skeletal development. However, at approximately two weeks after birth, homozygous mutant mice develop seizures which are subsequently fatal. Defective metabolism of pyridoxal 5'-phosphate (PLP), characterized by elevated serum PLP levels, results in reduced levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the brain. The mutant seizure phenotype can be rescued by the administration of pyridoxal and a semi-solid diet. Rescued animals subsequently develop defective dentition. This study reveals essential physiological functions of TNAP in the mouse.

Vitamin B-6 Metabolism and Interactions with TNAP

“…This was confirmed in knockout mice lacking TNAP which died from seizures (Waymire et al 1995). Pyridoxal phosphate in cerebrospinal fluid was above the normal range in three infants with hypophosphatasia (Baumgartner-Sigl et al 2007;Hofmann et al 2013;Belachew et al 2013) supporting the hypothesis that low alkaline phosphatase activity restricted uptake of vitamin B-6 by the brain.…”

“…However, pyridoxal phosphate declined about 46 % while pyridoxamine phosphate declined only 13 %. In the brains of TNAP deficient knockout mice pyridoxal phosphate was decreased to 35 % of control values while pyridoxamine phosphate was 65 % of controls (Waymire et al 1995). While pyridoxamine phosphate may be more resistant to depletion, when rats were exposed to a low vitamin B-6 diet throughout gestation and lactation, all B-6 vitamers were significantly reduced in the neocortex at 30 days postnatal (Groziak and Kirksey 1987).…”

“…At 10-14 days postnatal brain GABA concentrations were reduced 50 % in TNAP knockout mice compared to normal controls (Waymire et al 1995). Similarly, in an infant with hypophosphatasia and West syndrome GABA was less than 0.005 nmol/mL in the cerebrospinal fluid (Yamamoto et al 2004).…”

“…Pyridoxal phosphate in cerebrospinal fluid was above the normal range in three infants with hypophosphatasia (Baumgartner-Sigl et al 2007;Hofmann et al 2013;Belachew et al 2013) supporting the hypothesis that low alkaline phosphatase activity restricted uptake of vitamin B-6 by the brain. Waymire et al (1995) hypothesized that the seizures resulted from decreased concentration of the inhibitory neurotransmitter, gamma-aminobutyrate (GABA). GABA is produced from glutamic acid by glutamate decarboxylase (EC 4.1.1.15), which requires pyridoxal phosphate.…”

“…Since then there have been multiple reports in which vitamin B-6 supplementation was beneficial (Sia et al 1975;Litmanovitz et al 2002;Nunes et al 2002;Yamamoto et al 2004;Arun et al 2005;Baumgartner-Sigl et al 2007;Balasubramaniam et al 2010;Demirbilek et al 2012;Belachew et al 2013). Vitamin B-6 supplementation also corrected seizures in mice lacking alkaline phosphatase (Waymire et al 1995;Narisawa et al 2001). Finally, pyridoxal phosphate concentrations in plasma are markedly elevated in hypophosphatasia (Whyte et al 1985) demonstrating an important physiological role for alkaline phosphatase in vitamin B-6 metabolism.…”