Inhibition of Na+-Dependent Transporters in Cystine-Loaded Human Renal Cells: Electrophysiological Studies on the Fanconi Syndrome of Cystinosis

Abstract

ABSTRACT. Cystinosis is the most common cause of the renal Fanconi syndrome in children, leading to severe electrolyte disturbances and growth failure. A defective lysosomal transporter, cystinosin, results in intralysosomal accumulation of cystine. Loading cells with cystine dimethyl ester (CDME) is the only available model for this disease. This model was used to present electrophysiologic studies on immortalized human kidney epithelial (IHKE-1) cells that had been derived from the proximal tubule with the slow whole-cell patch clamp technique. Basal membrane voltages (Vm) of IHKE-1 cells were −30.7 ± 0.4 mV (n= 151). CDME concentration-dependently altered Vmwith an initial depolarization (2.7 ± 0.2 mV;n= 76; 1 mM CDME) followed by a more pronounced hyperpolarization (−9.9 ± 1.0 mV;n= 49). Three Na+-dependent transporters were examined. Alanine (1 mM) depolarized IHKE-1 cells by 17.6 ± 0.7 mV (n= 59), and phosphate (1.8 mM) depolarized by 9.7 ± 1.1 mV (n= 18). Acidification of IHKE-1 cells with propionate (20 mM) resulted in a depolarization of Vmby 7.1 ± 0.3 mV (n= 21) followed by a repolarization by 2.9 ± 0.3 mV/min (n= 17), reflecting Na+/H+-exchanger activity. Acute addition of 1 mM CDME did not alter the alanine- and propionate-induced changes in Vm, but it reduced the phosphate-induced depolarization by 37 ± 9% (n= 10). Incubation with 1 mM CDME reduced the activity of all three transporters. Depolarizations by alanine and phosphate and the repolarization after propionate were inhibited by 57 ± 4% (n=30), 45 ± 9% (n= 9), and 78 ± 15% (n= 8), respectively. In conclusion, this study demonstrates that CDME acutely alters Vmof IHKE-1 cells and that at least three Na+-dependent transporters are inhibited, the Na+-phosphate cotransporter most sensitively. This might suggest that phosphate depletion and dissipation of the Na+-gradient are involved in the development of the Fanconi syndrome of cystinosis.