Molecular cloning of the cDNA for an MDCK cell Na(+)- and Cl(-)-dependent taurine transporter that is regulated by hypertonicity.

Abstract

Cells in the hypertonic renal medulla maintain their intracellular ion concentration at isotonic levels, despite much higher concentrations of extracellular electrolytes, by accumulating high concentrations of nonperturbing small organic solutes termed osmolytes. Taurine has been identified as a nonperturbing osmolyte in the renal medulla and Madin-Darby canine kidney (MDCK) cells. In hypertonic medium, the increased accumulation of taurine in MDCK cells is the result of increased activity of a Na+-and Cl1-dependent taurine transporter. We have isolated a cDNA encoding a Na+-and Cl-dependent taurine transporter, whose sequence corresponds to a protein of 655 amino acids with igncant amino acid sequence similarity to previously cloned Na+-and Cldependent transporters, incuding the MDCK cell betaine/-aminobutyric acid transporter and several brain neurotransmitter transporters. Northern hybridization indicates that mRNA for the taurine transporter is present in renal cortex and medulla, ileal mucosa, brain, liver, and heart. The abndce of mRNA for the taurine transporter is increased in MDCK cells cultured in hypertonic medium, suggesting that regulation of transport activity by medium hypertonicity occurs at the level of mRNA accumulation. imum velocity (V.,.) of the taurine transporter in the basolateral plasma membrane without change in Km (4).Taurine transport in renal brush border membranes has also been well characterized (5). The activity of the cotransporter in the brush border of the proximal tubule contributes to whole-body homeostasis of taurine; activity increases in animals fed diets deficient in taurine and in sulfur-containing amino acids (6). The addition of 50 pAM taurine to taurine-free medium results in a decrease in the activity of the taurine transporter in MDCK cells (4,7). However, the rate of taurine transport was up-regulated by hypertonicity to the same degree as in cells cultured in taurine-free medium, suggesting that the regulation of taurine transport by hypertonicity and the regulation by medium taurine are independent of each other (4).In this report we describe the cloning of the cDNA for the MDCK cell taurine transporter. The sequence of the cDNAt indicates that the taurine transporter has considerable amino acid sequence similarity to the previously cloned Na+-and Cl--dependent transporters. Northern hybridizations indicate that the abundance of mRNA for the taurine transporter in MDCK cells is regulated by hypertonicity.Taurine (2-aminoethanesulfonic acid) is a major intracellular amino acid in mammals (1). It is involved in a number of important physiological processes, including bile acid conjugation in hepatocytes, modulation of calcium flux and neural excitability, osmoregulation, detoxification, and membrane stabilization (1).The cells of virtually all organisms respond to hypertonicity by the intracellular accumulation ofhigh concentrations of small organic solutes (osmolytes) that, in contrast to high concentrations of electrolytes, do not perturb the function of ...

The Family of Na+/Cl− Neurotransmitter Transporters

“…Although GAT2, GAT3, GAT4, and the taurine transporters exhibit >60% identity in their amino acid sequences, they are only =50% identical to GAT1 and =40% identical to members of the other subfamilies of Na~/C1 transporters. Despite the high sequence identity between the GABA and taurine transporters, the latter is not capable of GABA transport and GABA does not inhibit its taurine uptake activity (Liu et al, l992b;Uchida et al, 1992). In contrast, the taurine transporter is also an efficient /9-alanine transporter.…”

“…It includes the transporters GAT1 (Guastella et al, 1990;Liu et a!., 1992a), GAT2 (López-Corcuera et al, 1992), GAT3 , GAT4 , creatine , and taurine (Liu et al, 1992b;Smith et al, l992h;Uchida et al, 1992). Although GAT2, GAT3, GAT4, and the taurine transporters exhibit >60% identity in their amino acid sequences, they are only =50% identical to GAT1 and =40% identical to members of the other subfamilies of Na~/C1 transporters.…”

p38 and Activating Transcription Factor-2 Involvement in Osteoblast Osmotic Response to Elevated Extracellular Glucose

“…Immediate effects of a hyperosmotic response include cell shrinkage, followed by modulation of ion transporter activity (2,3), leading to a cell regulatory volume increase (4). Subsequently, in mammalian cells, genes encoding proteins involved in the accumulation of intracellular "compatible osmolytes" are induced within hours to days after exposure to hyperglycemia (5)(6)(7). Mechanisms accounting for these responses include activation of intracellular signaling pathways (8), nonenzymatic glycosylation of DNA and proteins (9), and altered cellular metabolism (10).…”

Phylogenetic Conservation of 4‐Aminobutyric Acid (GABA) Transporter Isoforms

“…Although the characterization of the GABA/noradrenaline transporter family originatcd froin the study of neurotransmission, it has turned out that their physiological roles are not restricted to this func- tion. Ainong its members are carricrs for taurine [9][10][11], bctaine [12, 131, /Lalanine r14-161 and crcatine r17, 1x1. Although taurine, betaine and /hlanine have also been discussed ;is potential neurotransmitters, they have diverse functions within and outside of the ncrvous systetn (e.g., as osmolytes), and transportcrs for these rnolecules urc also expressed in non-neural tissues.…”