A cell-autonomous role for the vitamin B6 metabolism gene <i>PNPO</i> in <i>Drosophila</i> GABAergic neurons

In animals, the enzyme pyridox(am)ine 5'-phosphate oxidase (PNPO) is critical for synthesizing the active form of vitamin B6 (VB6), pyridoxal 5'-phosphate (PLP), from inactive vitamers. PLP is a required cofactor for many enzymatic reactions, including the synthesis of GABA and the monoamines. <i>PNPO</i> disruption in humans is associated with an array of epilepsy syndromes, while <i>Drosophila</i> harboring mutations in the sole <i>PNPO</i> ortholog, <i>sugarlethal</i> (<i>sgll</i>), display spontaneous seizures and shortened lifespans. These phenotypes are suppressed by PLP dietary supplementation and are exacerbated by restriction of dietary B6 vitamers. In the context of PNPO deficiency, it remains to be resolved what the specific contributions by cellular subpopulations in the nervous system are to the neurological phenotypes. We addressed this question in <i>sgll</i> mutants by expressing human <i>PNPO</i> (<i>hPNPO</i>) cDNA in cholinergic, glutamatergic, and GABAergic neurons as well as glia and measuring changes in survival and seizure phenotypes. We found <i>hPNPO</i> expression in GABAergic neurons largely restored lifespan and attenuated seizure activity, while glial expression also improved <i>sgll</i> phenotypes albeit to a lesser degree. In contrast, <i>hPNPO</i> expression in either cholinergic or glutamatergic neurons, accounting for most neurons in the fly brain, did not appreciably alter <i>sgll</i> phenotypes. We contrasted these observations with changes in <i>sgll</i> mutants induced by feeding GABA receptor modulators. The GABA_B agonist SKF-97541 reduced mortality, while GABA or GABA_A receptor modulators did not improve survival. Together, our data establish a cell-autonomous role for PNPO in GABAergic neurons to support brain function, especially under VB6-restricted conditions.