Investigating the Role of SLC45A4 in GABA Metabolism

ABSTRACT Solute carrier (SLC) proteins are membrane transporters that govern the cross-membrane exchanges of glucose, amino acids, inorganic ions, and other small molecule metabolites. Many SLC genes have been shown to be causes of Mendelian diseases in humans, and a number of SLC transporters are important drug targets. However, due to myriad technical difficulties, a large fraction of SLC family members are still orphan transporters without known substrates, which represents both a significant knowledge gap and a huge opportunity for new drug development. In order to systematically study the biochemical functions of the orphan SLC transporters, a computational workflow was developed in our lab, which combines public transcriptomic and metabolomic datasets to uncover the metabolic function of SLC transporters. Using this association analysis, an uncharacterized gene, SLC45A4, was identified to be the single greatest determinant of γ-Aminobutyric acid (GABA) levels in human cancer cells. GABA, which is mostly known as an inhibitory neurotransmitter in the mammalian central nervous system, functions in peripheral tissues to regulate cell proliferation, differentiation, and migration. Given the importance of these functions in both cancer and normal cells, it is essential to understand how SLC45A4 functions in the context of GABA metabolism in non-neuronal cells. Using stable isotope tracing, it was found that SLC45A4 increases cellular GABA levels not by promoting GABA uptake but by facilitating GABA de novo synthesis, suggesting an entirely new mechanism of the regulation of GABA synthesis. Therefore, it is hypothesized that SLC45A4 encodes a subcellular ornithine transporter that supports GABA production. This program will move forward in three directions: Aim 1, determining the biochemical mechanism by which SLC45A4 promotes cellular GABA production; Aim 2, determining the subcellular compartmentalization of SLC45A4 transporter and GABA synthesis; and Aim 3, determining substrate preference and enzymatic properties of SLC45A4. The overall goal is to determine how SLC45A4 regulates de novo GABA synthesis, which contributes to the fundamental knowledge of human biochemistry and may provide new therapeutics targeting GABA metabolism.