Sex And Brain Region-Specific Regulation Of Serotonin Transporter Activity In Synaptosomes In Guanine Nucleotide-Binding Protein G(Q) Alpha Knockout Mice

The regulation of the serotonin transporter (SERT) by guanine nucleotide-binding protein alpha (G alpha) q was investigated using G alpha q knockout mice. In the absence of G alpha q, SERT-mediated uptake of 5-hydroxytryptamine (5HT) was enhanced in midbrain and frontal cortex synaptosomes, but only in female mice. The mechanisms underlying this sexual dimorphism were investigated using quantitative western blot analysis revealing brain region-specific differences. In the frontal cortex, SERT protein expression was decreased in male knockout mice, seemingly explaining the sex-dependent variation in SERT activity. The differential expression of G alpha i1 in female mice contributes to the sex differences in the midbrain. In fact, G alpha i1 levels inversely correlate with 5HT uptake rates across both sexes and genotypes. Likely due to differential SERT regulation as well as sex differences in the expression of tryptophan hydroxylase 2, G alpha q knockout mice also displayed sex- and genotype-dependent alterations in total 5HT tissue levels as determined by high-performance liquid chromatography. G alpha q inhibitors, YM-254890 and BIM-46187, differentially affected SERT activity in both, synaptosomes and cultured cells. YM-254890 treatment mimicked the effect of G alpha q knockout in the frontal cortex. BIM-46187, which promotes the nucleotide-free form of G alpha proteins, substantially inhibited 5HT uptake, prompting us to hypothesise that G alpha q interacts with SERT similarly as with G-protein-coupled receptors and inhibits SERT activity by modulating transport-associated conformational changes. Taken together, our findings reveal a novel mechanism of SERT regulation and impact our understanding of sex differences in diseases associated with dysregulation of serotonin transmission, such as depression and anxiety.