Putrescine acetyltransferase (PAT/SAT1) dependent GABA synthesis in astrocytes

GABA synthesis in astrocytes mediates tonic inhibition to regulate patho-physiological processes in various brain regions. Monoamine oxidase B (MAO-B) has been known to be the most important metabolic enzyme for synthesizing GABA from the putrescine degradation pathway. MAO-B converts N1-acetylputrescine to N1-acetyl-{gamma}-aminobutyraldehyde and hydrogen peroxide (H2O2). Putrescine acetyltransferase (PAT), also known as spermidine and spermine N1-acetyltransferase 1 (SAT1), has been thought to be a feasible candidate enzyme for converting putrescine to N1-acetylputrescine. However, it has not been rigorously investigated or determined whether PAT/SAT1 contributes to GABA synthesis in astrocytes. To investigate the contribution of PAT/SAT1 to GABA synthesis in astrocytes, we conducted sniffer patch and whole-cell patch experiments with gene silencing of PAT/SAT1 by Sat1 shRNA expression. Our results showed that the gene silencing of PAT/SAT1 significantly decreased the MAO-B-dependent GABA synthesis, which was induced by putrescine incubation, leading to decreased Ca2+-dependent release of GABA in vitro. Additionally, we found that, from the brain slice ex vivo, putrescine incubation induces tonic GABA inhibition in dentate gyrus granule cells, which can be inhibited by MAO-B inhibitor, selegiline. Consistent with our in vitro results, astrocytic gene silencing of PAT/SAT1 significantly reduced putrescine incubation-induced tonic GABA current, possibly by converting putrescine to N1-acetylputrescine, a substrate of MAO-B. Our findings emphasize a crucial role of PAT/SAT1 in MAO-B-dependent GABA synthesis in astrocytes.