Activities of acid hydrolases in fibroblasts from normal and cystinotic children

Endo-lysosomal dysfunction in human proximal tubular epithelial cells deficient for lysosomal cystine transporter cystinosin

Nephropathic cystinosis is a lysosomal storage disorder caused by mutations in the CTNS gene encoding cystine transporter cystinosin that results in accumulation of amino acid cystine in the lysosomes throughout the body and especially affects kidneys. Early manifestations of the disease include renal Fanconi syndrome, a generalized proximal tubular dysfunction. Current therapy of cystinosis is based on cystine-lowering drug cysteamine that postpones the disease progression but offers no cure for the Fanconi syndrome. We studied the mechanisms of impaired reabsorption in human proximal tubular epithelial cells (PTEC) deficient for cystinosin and investigated the endo-lysosomal compartments of cystinosin-deficient PTEC by means of light and electron microscopy. We demonstrate that cystinosin-deficient cells had abnormal shape and distribution of the endo-lysosomal compartments and impaired endocytosis, with decreased surface expression of multiligand receptors and delayed lysosomal cargo processing. Treatment with cysteamine improved surface expression and lysosomal cargo processing but did not lead to a complete restoration and had no effect on the abnormal morphology of endo-lysosomal compartments. The obtained results improve our understanding of the mechanism of proximal tubular dysfunction in cystinosis and indicate that impaired protein reabsorption can, at least partially, be explained by abnormal trafficking of endosomal vesicles.

A study of the low beta-galactosidase activity in cystinotic fibroblasts: effects of cysteamine

beta-Galactosidase activity but not beta-glucuronidase, N-acetyl-beta-D-galactosaminidase or arylsulphatase A activity, is known to be significantly lower in cultured human skin fibroblasts from patients with cystinosis than in cells from control subjects. Incubation of cell homogenates with disulphide or thiol compounds did not affect beta-galactosidase activity, suggesting that decreased beta-galactosidase activity in affected cells was not caused by the presence of inhibiting substances or absence of activating substances. Incubating cells with 0.5 or 1.0 mmol/l cysteamine, a substance used in the clinical treatment of cystinosis because it depletes cells of excess cystine, greatly decreased beta-galactosidase activity in both cystinotic and normal cells. This effect is shown to result from enzyme instability in lysosomes with raised pH and increased thiol concentration. Thus, cysteamine, although effective in depleting cystinotic cells of excess cystine, may have the undesired side-effect of severely decreasing lysosomal beta-galactosidase.