RESUMO
Vitamin B6 deficiency has been linked to cognitive impairment in human brain disorders for decades. Still, the molecular mechanisms linking vitamin B6 to these pathologies remain poorly understood, and whether vitamin B6 supplementation improves cognition is unclear as well. Pyridoxal 5'-phosphate phosphatase (PDXP), an enzyme that controls levels of pyridoxal 5'-phosphate (PLP), the co-enzymatically active form of vitamin B6, may represent an alternative therapeutic entry point into vitamin B6-associated pathologies. However, pharmacological PDXP inhibitors to test this concept are lacking. We now identify a PDXP and age-dependent decline of PLP levels in the murine hippocampus that provides a rationale for the development of PDXP inhibitors. Using a combination of small-molecule screening, protein crystallography, and biolayer interferometry, we discover, visualize, and analyze 7,8-dihydroxyflavone (7,8-DHF) as a direct and potent PDXP inhibitor. 7,8-DHF binds and reversibly inhibits PDXP with low micromolar affinity and sub-micromolar potency. In mouse hippocampal neurons, 7,8-DHF increases PLP in a PDXP-dependent manner. These findings validate PDXP as a druggable target. Of note, 7,8-DHF is a well-studied molecule in brain disorder models, although its mechanism of action is actively debated. Our discovery of 7,8-DHF as a PDXP inhibitor offers novel mechanistic insights into the controversy surrounding 7,8-DHF-mediated effects in the brain.
Vitamin B6 is an important nutrient for optimal brain function, with deficiencies linked to impaired memory, learning and mood in various mental disorders. In older people, vitamin B6 deficiency is also associated with declining memory and dementia. Although this has been known for years, the precise role of vitamin B6 in these disorders and whether supplements can be used to treat or prevent them remained unclear. This is partly because vitamin B6 is actually an umbrella term for a small number of very similar and interchangeable molecules. Only one of these is 'bioactive', meaning it has a biological role in cells. However, therapeutic strategies aimed at increasing only the bioactive form of vitamin B6 are lacking. Previous work showed that disrupting the gene for an enzyme called pyridoxal phosphatase, which breaks down vitamin B6, improves memory and learning in mice. To investigate whether these effects could be mimicked by drug-like compounds, Brenner, Zink, Witzinger et al. used several biochemical and structural biology approaches to search for molecules that bind to and inhibit pyridoxal phosphatase. The experiments showed that a molecule called 7,8-dihydroxyflavone which was previously found to improve memory and learning in laboratory animals with brain disorders binds to pyridoxal phosphatase and inhibits its activity. This led to increased bioactive vitamin B6 levels in mouse brain cells involved in memory and learning. The findings of Brenner et al. suggest that inhibiting pyridoxal phosphatase to increase vitamin B6 levels in the brain could be used together with supplements. The identification of 7,8-dihydroxyflavone as a promising candidate drug is a first step in the discovery of more efficient pyridoxal phosphatase inhibitors. These will be useful experimental tools to directly study whether increasing the levels of bioactive vitamin B6 in the brain may help those with mental health conditions associated with impaired memory, learning and mood.