Rational Discovery of New Folds for Human Solute Carrier Proteins

Solute carrier superfamily (SLC) is the largest group responsible for transmembrane transport of substances in human cells. It includes more than 400 members that can be divided into 65 families according to their physiological function and sequence similarity. Different families of solute transporters can adopt the same or different folds that determines the working mechanism and reflects the evolutionary relationship between SLC members. Analysis of structural data shows that there are 13 different folds in the solute carrier superfamily covering 40 families and 342 members. To further study their working mechanism, we systematically explored the SLC superfamily to find more folds. Our results indicate that there are at least three new folds in the solute transporter superfamily among which the SLC44 family is experimentally verified to have a new fold. Our work has laid a foundation and provided important insights for the systematic and comprehensive study on the structure and function of solute carriers.

Kctd15 regulates nephron segment development by repressing Tfap2a activity

ABSTRACTA functional vertebrate kidney relies on structural units called nephrons, which are epithelial tubules with a sequence of segments each expressing a distinct repertoire of solute transporters. The transcriptiona`l codes driving regional specification, solute transporter program activation and terminal differentiation of segment populations remain poorly understood. Here, we demonstrate that the KCTD15 paralogs kctd15a and kctd15b function in concert to restrict distal early (DE)/thick ascending limb (TAL) segment lineage assignment in the developing zebrafish pronephros by repressing Tfap2a activity. During renal ontogeny, expression of these factors colocalized with tfap2a in distal tubule precursors. kctd15a/b loss primed nephron cells to adopt distal fates by driving slc12a1, kcnj1a.1 and stc1 expression. These phenotypes were the result of Tfap2a hyperactivity, where kctd15a/b-deficient embryos exhibited increased abundance of this transcription factor. Interestingly, tfap2a reciprocally promoted kctd15a and kctd15b transcription, unveiling a circuit of autoregulation operating in nephron progenitors. Concomitant kctd15b knockdown with tfap2a overexpression further expanded the DE population. Our study reveals that a transcription factor-repressor feedback module employs tight regulation of Tfap2a and Kctd15 kinetics to control nephron segment fate choice and differentiation during kidney development.

TDAG51 (T-Cell Death-Associated Gene 51) Is a Key Modulator of Vascular Calcification and Osteogenic Transdifferentiation of Arterial Smooth Muscle Cells

Objective: Cardiovascular disease is the primary cause of mortality in patients with chronic kidney disease. Vascular calcification (VC) in the medial layer of the vessel wall is a unique and prominent feature in patients with advanced chronic kidney disease and is now recognized as an important predictor and independent risk factor for cardiovascular and all-cause mortality in these patients. VC in chronic kidney disease is triggered by the transformation of vascular smooth muscle cells (VSMCs) into osteoblasts as a consequence of elevated circulating inorganic phosphate (P i ) levels, due to poor kidney function. The objective of our study was to investigate the role of TDAG51 (T-cell death-associated gene 51) in the development of medial VC. Methods and Results: Using primary mouse and human VSMCs, we found that TDAG51 is induced in VSMCs by P i and is expressed in the medial layer of calcified human vessels. Furthermore, the transcriptional activity of RUNX2 (Runt-related transcription factor 2), a well-established driver of P i -mediated VC, is reduced in TDAG51 −/− VSMCs. To explain these observations, we identified that TDAG51 −/− VSMCs express reduced levels of the type III sodium-dependent P i transporter, Pit-1, a solute transporter, a solute transporter, a solute transporter responsible for cellular P i uptake. Significantly, in response to hyperphosphatemia induced by vitamin D 3 , medial VC was attenuated in TDAG51 −/− mice. Conclusions: Our studies highlight TDAG51 as an important mediator of P i -induced VC in VSMCs through the downregulation of Pit-1. As such, TDAG51 may represent a therapeutic target for the prevention of VC and cardiovascular disease in patients with chronic kidney disease.

Role of Organic Solute Transporter Alpha/Beta in Hepatotoxic Bile Acid Transport and Drug Interactions

Organic solute transporter (OST) α/β is a key bile acid transporter expressed in various organs, including the liver under cholestatic conditions. However, little is known about the involvement of OSTα/β in bile acid-mediated drug-induced liver injury (DILI), a major safety concern in drug development. This study investigated whether OSTα/β preferentially transports more hepatotoxic, conjugated, primary bile acids and to what extent xenobiotics inhibit this transport. Kinetic studies with OSTα/β-overexpressing cells revealed that OSTα/β preferentially transported bile acids in the following order: taurochenodeoxycholate > glycochenodeoxycholate > taurocholate > glycocholate. The apparent half-maximal inhibitory concentrations for OSTα/β-mediated bile acid (5 µM) transport inhibition by fidaxomicin, troglitazone sulfate, and ethinyl estradiol were: 210, 334, and 1050 µM, respectively, for taurochenodeoxycholate; 97.6, 333, and 337 µM, respectively, for glycochenodeoxycholate; 140, 265, and 527 µM, respectively, for taurocholate; 59.8, 102, and 117 µM, respectively, for glycocholate. The potential role of OSTα/β in hepatocellular glycine-conjugated bile acid accumulation and cholestatic DILI was evaluated using sandwich-cultured human hepatocytes (SCHH). Treatment of SCHH with the farnesoid X receptor agonist chenodeoxycholate (100 µM) resulted in substantial OSTα/β induction, among other proteomic alterations, reducing glycochenodeoxycholate and glycocholate accumulation in cells+bile 4.0- and 4.5-fold, respectively. Treatment of SCHH with troglitazone and fidaxomicin together under cholestatic conditions resulted in increased hepatocellular toxicity compared with either compound alone, suggesting that OSTα/β inhibition may accentuate DILI. In conclusion, this study provides insights into the role of OSTα/β in preferential disposition of bile acids associated with hepatotoxicity, the impact of xenobiotics on OSTα/β-mediated bile acid transport, and the role of this transporter in SCHH and cholestatic DILI.