WHOLE‐BODY DELETION OF THE PYRIDOXAL PHOSPHATASE CHRONOPHIN IN MICE ELEVATES GABA LEVELS IN THE BRAIN AND TRIGGERS ANXIETY‐LIKE BEHAVIOR

Abstract

Pyridoxal phosphatase (PDXP, or chronophin) is a haloacid dehalogenase‐type phosphatase dedicated to the metabolism of pyridoxal 5′‐phosphate (PLP). PLP, the biologically active form of vitamin B6, functions as a cofactor in the catalysis of more than 140 different enzymatic reactions. In mammals, PLP‐dependent enzymes are predominantly involved in the biosynthesis of neurotransmitters, in amino acid metabolism, and in glycogen breakdown. Despite these important functions, the physiological roles of mammalian PDXP have not yet been explored on the organismal level.To elucidate the roles of PDXP in vivo, we have engineered conditionally PDXP‐deficient mice. EIIa‐Cre driven homologous recombination resulted in whole‐body deletion of PDXP, and offspring were viable and fertile. The analysis of B6 vitamers by mass spectrometry revealed that PLP levels in the brains of newborn and adult PDXP‐deficient mice were elevated ~threefold compared to control mice. In addition, PLP levels were significantly increased in skeletal muscle and in red blood cells. Given the prominent expression of PDXP in the brain, we analyzed neurotransmitter expression levels. While the concentrations of dopamine, serotonin, norepinephrine and their metabolites were unchanged upon PDXP loss, GABA levels were increased by ~20% in the brains of PDXP knockout pups, and in the cerebellum and basal ganglia of adult PDXP‐deficient mice. These results point to a PLP‐dependent stimulation of the vesicular GABA‐producing enzyme glutamic acid decarboxylase‐2 (GAD2, also known as GAD65), whose activity is tightly controlled by PLP concentrations. Indeed, Western blot analyses showed that PDXP loss triggered GAD2 oligomerization, suggesting increased GAD2 activity. In behavioral studies, PDXP‐deficient mice showed no impairment in spontaneous locomotor activity. Interestingly, the open field test and elevated plus maze test revealed anxiety‐like behavior. Loss of PDXP expression also caused reduced latency to fall in the inverted screen test and decreased performance in the weights test. However, direct measurements of muscle strength with a grip strength meter and spontaneous treadmill running exercises showed that PDXP deficiency did not primarily decrease paw strength or affect motor coordination. Thus, the impaired performance of PDXP‐deficient mice in inverted screen and weights tests is likely a manifestation of their anxiety‐like behavior.Taken together, our results demonstrate that PDXP is a bona fide PLP phosphatase in vivo. We propose that disturbances in the GABAergic system during early postnatal brain development of PDXP‐deficient mice cause anxiety‐like behavior in adulthood. Targeted ablation of PDXP in neurons may provide a novel model system to study the role of early‐life stress for anxiety disorders in the adult organism.Support or Funding InformationThis study was supported by the Deutsche Forschungsgemeinschaft, grants SFB688 and FZ82 to A.G.