The anxiety like phenotype of PDXP KO
The anxiety-like phenotype of PDXP-KO mice was unexpected, given the physiological role of GABA in counteracting anxiety via GABA-A receptors [58,65,66]. However, PLP and GABA levels were already elevated postnatally in our mouse model. GABA levels affect brain development  and synaptogenesis , and can thereby impact stress responses later in life. For example, postnatal administration of the GABA-A receptor agonist muscimol results in anxiety-like behavior in 3-months old mice . The EIIa-Cre driven, embryonic ablation of PDXP might affect brain development (through GABA and/or other mechanisms), and influence stress susceptibility and anxiety in that way. PDXP loss might also alter the balance between excitatory and inhibitory transmission in specific synapses in the adult organism. For example, GABA co-released with glutamate controls the activity of the lateral habenula, a region in the dorsal thalamus that regulates mood and anxiety . Finally, we found elevated d/l-serine levels (stereoisomers were not resolved in our analysis) in the cortex, and decreased glycine concentrations in the hindbrains of PDXP-KO mice. Serine racemase is a PLP-dependent enzyme, and d-serine is a co-agonist at N-methyl-d-aspartic Elafibranor (GFT505) (NMDA) glutamate receptors in the brain. Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem, and has excitatory effects in the cerebral cortex due to its agonistic activity at glutamatergic NMDA receptors. These changes may contribute to the observed behavioral phenotypes. Further work is necessary to investigate the electrophysiological consequences of PDXP loss in specific brain regions and synapses, and to characterize the involved receptors and signaling pathways in our mouse model . In addition to a GABA-dependent decrease in muscle tone , the apparent motor deficits of PDXP-KO mice might be influenced by increased anxiety. In addition, age-dependent processes related to continuously elevated cellular PLP levels may contribute to the observed phenotype. An increased anxiety-like behavior could add to the retardation of spatial learning that was observed when PLPP/CIN (PDXP)-deficient mice were tested in the Morris water maze in another study . The water maze imposes a much stronger aversive stimulus and more stressful conditions on mice than the dry-land Barnes maze used in our study, and increased stress and anxiety may confound the assay outcome . The discrepant results obtained in the two PDXP-KO mouse models might also be linked to differences in the analyzed mouse strains, as well as in animal age, -sex or -housing conditions. The mice investigated in our present study were conditionally PDXP-deficient males of the indicated ages; animals were on a C57BL/6J background and maintained under specific pathogen-free conditions. Kim et al. have described mice with a constitutive PDXP knockout on a mixed 129/SvEv-C57BL/6J albino background; age, sex and housing conditions of the animals were not reported . The inbred strain background can impact behavioral measures, and learning and memory is an age-dependent process [, , ]. Previous work by us and others has investigated the cofilin regulatory function of PDXP [, , , , , ], and a recent study has reported effects of PDXP on hippocampal cofilin phosphoregulation in vivo . Yet, a direct cofilin phosphatase activity of PDXP is difficult to reconcile with crystallographic work from different laboratories, including our own [21,38,80] (Protein Data Bank entries 2OYC, 2P69, 2P27, 2CFR, 2CFS, 2CFT, 5GYN, 4BX3, 4BXO, 4BX2, 4BKM, 5AES). PLP is deeply buried within the catalytic cleft of PDXP , and the active site is occluded by a large capping domain typical of small molecule-directed, haloacid dehalogenase (HAD)-type hydrolases [81,82]. Given these structural features, extensive conformational changes would be required to accommodate a protein substrate. Without structural information on the mechanism of cofilin dephosphorylation by PDXP, it currently appears more likely that the effects of PDXP on phosphocofilin that are found in some studies are caused by indirect mechanisms.