Background: SLC10A4 belongs to the solute carrier family SLC10 whose founding members are the Na+/taurocholate\nco-transporting polypeptide (NTCP, SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT,\nSLC10A2). These carriers maintain the enterohepatic circulation of bile acids between the liver and the gut. SLC10A4\nwas identified as a novel member of the SLC10 carrier family with the highest phylogenetic relationship to NTCP. The\nSLC10A4 protein was detected in synaptic vesicles of cholinergic and monoaminergic neurons of the peripheral and\ncentral nervous system, suggesting a transport function for any kind of neurotransmitter. Therefore, in the present\nstudy, we performed systematic transport screenings for SLC10A4 and also aimed to identify the vesicular sorting\ndomain of the SLC10A4 protein.\nResults: We detected a vesicle-like expression pattern of the SLC10A4 protein in the neuronal cell lines SH-SY5Y and\nCAD. Differentiation of these cells to the neuronal phenotype altered neither SLC10A4 gene expression nor its vesicular\nexpression pattern. Functional transport studies with different neurotransmitters, bile acids and steroid sulfates\nwere performed in SLC10A4-transfected HEK293 cells, SLC10A4-transfected CAD cells and in Xenopus laevis oocytes.\nFor these studies, transport by the dopamine transporter DAT, the serotonin transporter SERT, the choline transporter\nCHT1, the vesicular monoamine transporter VMAT2, the organic cation transporter Oct1, and NTCP were used as positive\ncontrol. SLC10A4 failed to show transport activity for dopamine, serotonin, norepinephrine, histamine, acetylcholine,\ncholine, acetate, aspartate, glutamate, gamma-aminobutyric acid, pregnenolone sulfate, dehydroepiandrosterone\nsulfate, estrone-3-sulfate, and adenosine triphosphate, at least in the transport assays used. When the C-terminus\nof SLC10A4 was replaced by the homologous sequence of NTCP, the SLC10A4-NTCP chimeric protein revealed clear\nplasma membrane expression in CAD and HEK293 cells. But this chimera also did not show any transport activity,\neven when the N-terminal domain of SLC10A4 was deleted by mutagenesis.\nConclusions: Although different kinds of assays were used to screen for transport function, SLC10A4 failed to show\ntransport activity for a series of neurotransmitters and neuromodulators, indicating that SLC10A4 does not seem to\nrepresent a typical neurotransmitter transporter such as DAT, SERT, CHT1 or VMAT2.
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