Renal amino acid transport: cellular and molecular events from clearance studies to frog eggs

The taurine transporter: mechanisms of regulation

Taurine transport undergoes an adaptive response to changes in taurine availability. Unlike most amino acids, taurine is not metabolized or incorporated into protein but remains free in the intracellular water. Most amino acids are reabsorbed at rates of 98-99%, but reabsorption of taurine may range from 40% to 99.5%. Factors that influence taurine accumulation include ionic environment, electrochemical charge, and post-translational and transcriptional factors. Among these are protein kinase C (PKC) activation and transactivation or repression by proto-oncogenes such as WT1, c-Jun, c-Myb and p53. Renal adaptive regulation of the taurine transporter (TauT) was studied in vivo and in vitro. Site-directed mutagenesis and the oocyte expression system were used to study post-translational regulation of the TauT by PKC. Reporter genes and Northern and Western blots were used to study transcriptional regulation of the taurine transporter gene (TauT). We demonstrated that (i) the body pool of taurine is controlled through renal adaptive regulation of TauT in response to taurine availability; (ii) ionic environment, electrochemical charge, pH, and developmental ontogeny influence renal taurine accumulation; (iii) the fourth segment of TauT is involved in the gating of taurine across the cell membrane, which is controlled by PKC phosphorylation of serine 322 at the post-translational level; (iv) expression of TauT is repressed by the p53 tumour suppressor gene and is transactivated by proto-oncogenes such as WT1, c-Jun, and c-Myb; and (v) over-expression of TauT protects renal cells from cisplatin-induced nephrotoxicity.

The development of pediatric nephrology

Pediatric nephrology, as a discipline, arose from descriptive studies of childhood glomerulonephritis in Europe and the field of pediatric metabolism in the United States. While pediatric scientists before 1950 were concerned with fluid and electrolyte metabolism, regulation of intracellular and extracellular fluid, acid-base homeostasis, and parenteral fluid therapy, the defined field of nephrology developed after the Second World War around six major advances: ACTH and glucocorticoid therapy for nephrotic syndrome; renal biopsy to diagnose glomerular disease; the role of immunologic factors in glomerular injury; the use of dialysis as renal replacement therapy; renal transplantation as the optimal form of therapy in children with end stage renal failure; and recognition of renal disease in the etiology of 80% of cases of childhood hypertension. These discoveries led to focused research, the definition of specific training in nephrology, establishment of an American, European, and an International Society of Pediatric Nephrology, as well as an American Sub-Board of Pediatric Nephrology, and the inception of a journal, Pediatric Nephrology, now in its 15th year. Major research themes have included developmental nephrology, transplantation immunology, and concerns about growth in children with renal disease. Many clinical entities have been described in detail, some of which are almost confined to children. The scientific basis of pediatric nephrology, ongoing patient care needs, and its technical aspects - renal biopsy, dialysis and transplantation - assure its continuing future as a major pediatric discipline on all continents.

Uptake of neutral alpha- and beta-amino acids by human proximal tubular cells

The transport characteristics of amino acids in primary cell cultures from the proximal tubule of human adults (AHKE cells) were examined, using alpha-aminoisobutyric acid (AIB) and beta-alanine as representatives of alpha- and beta-amino acids, respectively. The Na(+)-gradient dependent influx of AIB occurred by a single, saturable transport system, whereas the Na(+)-gradient dependent uptake data for beta-alanine could be described in terms of two-independent transport components as well as one-transport one-leak model with identical kinetic constants for the high-affinity system. Competition experiments revealed that all the neutral amino acids tested reduced the uptake of AIB, whereas there was no effect of taurine, L-aspartic acid, and L-arginine. By contrast, the influx of beta-alanine was only drastically reduced by beta-amino acids, whereas the inhibition by neutral alpha-amino acids was relatively low. Nor did L-arginine and L-aspartic acid affect the uptake of beta-alanine into AHKE cells. Comparison with the results obtained for normal (NHKE) and immortalized (IHKE) embryonic cells suggested an unaltered expression of the types of transport carriers for neutral alpha- and beta-amino acids in the embryonic and AHKE cells. However, the uptake capacity of the above-mentioned transport proteins was relatively smaller in the embryonic kidney compared with the adult human kidney, which may explain, at least partly, the phenomenon of physiologic amino aciduria in neonates.

Taurine in pediatric nutrition

The past 20 years have seen the status of taurine change from an end product of methionine and cysteine metabolism and substance conjugated to bile acids to that of an important, and sometimes essential, nutrient. It is now added to most synthetic human infant formulas and pediatric parenteral solutions throughout the world. This article describes the research that led to this end.