THE ROLE OF TMEM163 IN ZINC HOMEOSTASIS

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PROJECT SUMMARY Zinc homeostasis in many organisms, including humans, is achieved by tissue-specific and highly conserved low molecular weight proteins known as the zinc transporter (ZnT) and Zrt- and Irt-like protein (ZIP) families. Several redundant ZnT and ZIP transporters have been identified in tissues that confer zinc efflux and influx property, respectively. A strict regulation of intracellular zinc levels exists in many tissues, especially in the brain, because this tissue contains a sizeable amount of chelatable zinc pool that is co- released with glutamate during normal neuronal communication. Indeed, the ZnT3 zinc effluxer has been shown to be important in sequestering and shuttling zinc into glutamatergic vesicles and that knocking out this protein obliterates vesicular zinc compartmentalization in the brain. We identified Transmembrane 163 (TMEM163), a zinc-binding protein and transporter, as a protein interactor of the TRPML1 ion channel. Loss of TRPML1 function causes Mucolipidosis IV disease. Meanwhile, TMEM163 was recently reported to modulate pain perception via its interaction with the neuronal P2X3 receptor ion channel. TMEM163 also known as synaptic vesicle 31 (SV31) protein was first identified in rat brain synaptosomes and exists as a dimer. TMEM163 localizes within the plasma membrane and vesicular compartments such as synaptic vesicles and lysosomes. We have preliminary evidence that TMEM163 interacts with the ZnT3 zinc efflux transporters. Functional zinc flux assays show that the efflux activity of TMEM163-ZnT3 heterodimers parallels that of their respective homodimer isoforms. These results not only confirm that TMEM163 is a zinc effluxer, but that its heterodimerization with a related zinc transporter adds to a repertoire of homeostatic control for intracellular zinc levels. Thus, it appears that TMEM163 is important for the maintenance of brain zinc homeostasis that is independent of, or in conjunction with, another zinc efflux transporter. The overarching goal of this project is to investigate the biological significance of human TMEM163 using its mouse Tmem163 counterpart. To this end, we will use a Tmem163 knockout (KO) mouse to determine changes in chelatable zinc distribution patterns in the brain using histochemical, histo- fluorescence, and biochemical techniques. We will also examine the Tmem163 KO phenotype by analyzing the brain transcriptome using RNA sequencing to establish whether genetic compensation (a well-known biological phenomenon) by other zinc transporter genes results from the loss of Tmem163 function. Overall, this project could fill current gaps in knowledge on the biological function of TMEM163, and could provide insights on how cells or tissues devoid of TMEM163 impact human health or human disease processes where zinc dyshomeostasis has been implicated.