Expression of EGFP/SDCT1 fusion protein, subcellular localization signal analysis, tissue distribution and electrophysiological function study

NaDC3 Induces Premature Cellular Senescence by Promoting Transport of Krebs Cycle Intermediates, Increasing NADH, and Exacerbating Oxidative Damage

High-affinity sodium-dependent dicarboxylate cotransporter 3 (NaDC3) is a key metabolism-regulating membrane protein responsible for transport of Krebs cycle intermediates. NaDC3 is upregulated as organs age, but knowledge regarding the underlying mechanisms by which NaDC3 modulates mammalian aging is limited. In this study, we showed that NaDC3 overexpression accelerated cellular senescence in young human diploid cells (MRC-5 and WI-38) and primary renal tubular cells, leading to cell cycle arrest in G1 phase and increased expression of senescent biomarkers, senescence-associated β-galactosidase and p16. Intracellular levels of reactive oxygen species, 8-hydroxy-2'-deoxyguanosine, malondialdehyde, and carbonyl were significantly enhanced, and activities of respiratory complexes I and III and ATP level were significantly decreased in NaDC3-infected cells. Stressful premature senescent phenotypes induced by NaDC3 were markedly ameliorated via treatment with the antioxidants Tiron and Tempol. High expression of NaDC3 caused a prominent increase in intracellular levels of Krebs cycle intermediates and NADH. Exogenous NADH and NAD(+) may aggravate and attenuate the aging phenotypes induced by NaDC3, respectively. These results suggest that NaDC3 can induce premature cellular senescence by promoting the transport of Krebs cycle intermediates, increasing generation of NADH and reactive oxygen species and leading to oxidative damage. Our results clarify the aging signaling pathway regulated by NaDC3.

Activity assay of membrane transport proteins

Membrane transport proteins are integral membrane proteins and considered as potential drug targets. Activity assay of transport proteins is essential for developing drugs to target these proteins. Major issues related to activity assessment of transport proteins include availability of transporters, transport activity of transporters, and interactions between ligands and transporters. Researchers need to consider the physiological status of proteins (bound in lipid membranes or purified), availability and specificity of substrates, and the purpose of the activity assay (screening, identifying, or comparing substrates and inhibitors) before choosing appropriate assay strategies and techniques. Transport proteins bound in vesicular membranes can be assayed for transporting substrate across membranes by means of uptake assay or entrance counterflow assay. Alternatively, transport proteins can be assayed for interactions with ligands by using techniques such as isothermal titration calorimetry, nuclear magnetic resonance spectroscopy, or surface plasmon resonance. Other methods and techniques such as fluorometry, scintillation proximity assay, electrophysiological assay, or stopped-flow assay could also be used for activity assay of transport proteins. In this paper the major strategies and techniques for activity assessment of membrane transport proteins are reviewed.

Membrane topology structure of human high-affinity, sodium-dependent dicarboxylate transporter

High-affinity, sodium-dependent dicarboxylate transporter (NaDC3) is responsible for transport of Krebs cycle intermediates and may involve in regulation of aging and life span. Hydropathy analysis predicts that NaDC3 contains 11 or 12 hydrophobic transmembrane (TM) domains. However, the actual membrane topological structure of NaDC3 remains unknown. In this study, confocal immunofluorescence microscopy and membrane biotinylation of epitope-tagged N and C termini of NaDC3 provide evidence of an extracellular C terminus and an intracellular N terminus, indicating an odd number of transmembrane regions. The position of hydrophilic loops within NaDC3 was identified with antibodies against the loops domains combined with cysteine accessibility methods. A confocal image of membrane localization and transport activity assay of the cysteine insertion mutants show behavior similar to that of wild-type NaDC3 in transfected HEK293 cells, suggesting that these mutants retain a native protein configuration. We find that NaDC3 contains 11 transmembrane helices. The loops 1, 3, 5, 7, and 9 face the extracellular side, and loops 2, 4, 6, and 10 face the cytoplasmic side. A re-entrant loop-like structure between TM8 and TM9 may protrude into the membrane. Our results support the topography of 11 transmembrane domains with an extracellular C terminus and an intracellular N terminus of NaDC3, and for the first time provide experimental evidence for a novel topological model for NaDC3.

Identification of basolateral membrane targeting signal of human sodium-dependent dicarboxylate transporter 3

Sodium-dependent dicarboxylate transporters (NaDC) include low-affinity NaDC1 and high-affinity NaDC3. Despite high similarities structurally and functionally, both are localized to opposite surfaces of renal tubular cells. The molecular mechanisms and localization signals leading to this polarized distribution remain unknown. In this study, distribution of NaDC3 in human kidney tissue was firstly observed by immunohistochemistry and immunofluorescence. Then, EGFP-fused wild-type, NH2- and COOH-terminal deletion and point mutants of NaDC3, and chimera between NaDC3 and NaDC1, were generated and transfected into polarized renal cells lines, LLC-PK1 and MDCK. Their subcellular localizations were analyzed by laser confocal microscopy. Immunolocalization results revealed that NaDC3 was expressed at basolateral membrane of human renal proximal tubular epithelia. Confocal examinations showed that wild-type NaDC3 was targeted to the basolateral membrane of MDCK and LLC-PK1. Deletion mutations indicated that the basolateral targeting signal of NaDC3 located within a short sequence AKKVWSARR of its amino-terminal cytoplasmic domain. Addition of this sequence could redirect apical NaDC1 to the basolateral membrane of LLC-PK1. Point mutagenesis revealed that mutation of either of two hydrophobic amino acids V and W in this short sequence largely redirected NaDC3 to both apical and basolateral surfaces of LLC-PK, indicating that the two hydrophobic amino acids are critical for the basolateral targeting of NaDC3. Our studies provide direct evidence of the localization of NaDC3 at the basolateral membrane of human renal proximal tubule cells and identify a di-hydrophobic amino acid motif VW as basolateral localization signal in the N-terminal cytoplasmic domain of NaDC3.