Cystine transport is defective in isolated leukocyte lysosomes from patients with cystinosis

Cystine storage in cultured myotubes from patients with nephropathic cystinosis

Sorted muscle cells, cultured from a patient with nephropathic cystinosis, stored 100 times normal amounts of cystine. Subcellular fractionation and density-gradient centrifugation confirmed that the cystine was located in a lysosomal compartment. 2. Myoblasts from cystinotic patients in culture underwent fusion to myotubes in a normal fashion. 3. The free thiol cysteamine effectively depleted cystinotic-muscle cells of cystine. 4. Cultured myoblast and myotubes offered a unique system for investigating the effects of lysosomal storage on differentiated cell functions.

Cysteamine therapy for children with nephropathic cystinosis

We treated 93 children with nephropathic cystinosis with oral cysteamine (mean dose, 51.3 mg per kilogram of body weight per day) for up to 73 months. This agent is known to be effective in depleting cells of cystine. In our study, the mean cystine depletion from leukocytes was 82 percent. A historical control group of 55 children received either ascorbic acid (27 children) or placebo (28). At age six, 2 of 17 controls had a serum creatinine level less than 1.0 mg per deciliter, as compared with 17 of 27 patients treated with cysteamine for at least one year (odds ratio, 12.8; 95 percent confidence interval, 2.1 to 33.9). At the end of the study, creatinine clearance was higher in the cysteamine group than in the control group (38.5 vs. 29.7 ml per minute per 1.73 m2; 95 percent confidence limits on the difference, 1.8 and 15.8), even though the cysteamine group was on average 1.4 years older than the control group. Cysteamine also improved growth; those in the cysteamine group between two and three years of age grew at 93 percent of the normal velocity, as compared with 54 percent in the control group. Fourteen percent of the patients could not tolerate the taste and smell of cysteamine. Concurrent controls treated in a blinded fashion with a placebo were not included in this study. With this limitation in mind, we conclude that oral cysteamine, by depleting cells of cystine, helps maintain renal glomerular function, improves growth, and constitutes the current treatment of choice for nephropathic cystinosis.

Removal of corneal crystals by topical cysteamine in nephropathic cystinosis

In patients with nephropathic cystinosis, corneal crystals develop by one year of age; they progressively accumulate and eventually cause recurrent corneal erosions and photophobia. After an in vitro study of cystinotic corneal stromal cells showed cystine depletion by cysteamine and after topical cysteamine was determined to be nontoxic in rabbits, we performed a controlled double-blind clinical trial of 10 mM cysteamine eyedrops in young patients with cystinosis, using one eye for treatment and the other as the control. Two children begun on the protocol before two years of age had a striking decrease in the number of corneal crystals in the cysteamine-treated eye within four to five months of entering the study. Cysteamine eyedrops appear to be safe and efficacious in the short-term treatment of patients with cystinosis who are under two years of age. The long-term value of such treatment and its effectiveness in older patients remain to be determined.

Prenatal diagnosis of cystinosis utilizing chorionic villus sampling

The prenatal diagnosis of cystinosis is currently based on the increased amount of free-cystine present in amniotic fluid cells. Amniocyte cultures must be grown for at least 2 weeks to obtain sufficient cells for such measurements. Thus, the diagnosis cannot be made until close to 20 weeks gestational age by this method. We report a case in which chorionic villi were used for direct cystine measurement resulting in the in utero diagnosis of cystinosis at 9 weeks gestational age. The diagnosis was confirmed by the study of cultured chorionic villus cells, and of the 10-week abortus.

Elevated temperature produces cystine depletion in cystinotic fibroblasts

Increasing the incubation temperature of cystinotic fibroblasts to 40 or 43 degrees C produces a 70-80% decrease in lysosomal cystine content within 24-48 h. This effect is probably mediated by an altered substrate affinity for another lysosomal transport protein.

Course of nephropathic cystinosis after age 10 years

We identified 80 patients with nephropathic cystinosis older than age 10 years in the United States and Canada. The oldest reported individual was 26 years of age. Ninety percent of patients had received at least one renal allograft. Age at the time of first transplant varied between 7 and 17 years (mean 10.0 years). Almost three fourths of the patients required thyroid replacement, 27% had splenomegaly, and 42% had hepatomegaly. Photophobia was noted in 86% of patients, decreased visual acuity in 32%, and corneal ulcerations in 15%. Neurologic involvement, renal osteodystrophy, and diabetes mellitus were unusual. All these late complications of nephropathic cystinosis contribute to a description of the natural history of the disease and provide a rationale for the therapeutic use of cystine-depleting agents after renal transplantation.

Free N-acetylneuraminic acid (NANA) storage disorders: evidence for defective NANA transport across the lysosomal membrane

To study the biochemical defect underlying N-acetylneuraminic acid (NANA) storage disorders (NSD), a tritium-labeled NANA-methylester was prepared and its metabolism was studied in normal and mutant human fibroblasts. The uptake of methylester, its conversion into free NANA, and the release of free NANA was studied in lysosome-enriched fractions. In three clinically different types of NSD accumulation of free NANA was observed and the half-life of this compound was significantly increased. Our observations indicate the existence of a transport system for NANA across the lysosomal membrane, which is deficient in all variants of NSD.

Impaired clearance of free cystine from lysosome-enriched granular fractions of I-cell-disease fibroblasts

Cultured fibroblasts from patients with I-cell disease (mucolipidosis II) accumulate excessive amounts of free cystine, similarly to cells from patients with nephropathic cystinosis, a disorder of lysosomal cystine transport. To clarify whether the intralysosomal accumulation of cystine in I-cell-disease fibroblasts was due to a defective disposal mechanism, we measured the rates of clearance of free [35S]cystine from intact normal, cystinotic and I-cell-disease fibroblasts. Loss of radioactivity from the two mutant cell types occurred slowly (t 1/2 = 500 min) compared with the rapid loss from normal cells (t 1/2 = 40 min). Lysosome-rich granular fractions isolated from three different cystine-loaded normal, cystinotic and I-cell-disease fibroblast strains were similarly examined for non-radioactive cystine egress. Normal granular fractions lost cystine rapidly (mean t 1/2 = 43 min), whereas cystinotic granular fractions did not lose any cystine (mean t 1/2 = infinity). I-cell-disease granular fractions displayed prolonged half-times for cystine disposal (mean = 108 min), suggesting that I-cell-disease fibroblasts, like cystinotic cells, possess a defective carrier mechanism for cystine transport.

Cystine transport in purified rat liver lysosomes

Amino acid efflux from highly purified rat liver lysosomes exposed to the methyl ester derivatives of leucine, methionine, tyrosine and cystine was examined. The lysosomal efflux of leucine, methionine and tyrosine was unaffected by the presence of MgATP, whereas cystine efflux was stimulated by MgATP. Exposure of lysosomes to 2 mM-MgATP resulted in lysosomal acidification and a 0.5 pH unit increase in the lysosomal pH gradient through the action of a proton-pumping ATPase. Cystine efflux was also stimulated when the lysosomal proton gradient was increased through changes in buffer pH. Decreasing the lysosomal proton gradient with ionophores resulted in diminished cystine efflux. Bivalent cations had no effect on the lysosomal efflux of leucine, methionine and tyrosine. However, cystine efflux was stimulated by the presence of bivalent cations even when the lysosomal proton gradient was minimized. Cation-stimulated cystine efflux was inhibited by the presence of the calcium ionophore A23187, which altered the lysosomal membrane potential. Cystine efflux from lysosomes appears to be uniquely dependent on pH gradients and cation concentrations.

Studies of lysosomal sialic acid metabolism: retention of sialic acid by Salla disease lysosomes

Purified rat liver lysosomes were incubated in 0.2 M sialic acid resulting in an increase in lysosomal free sialic acid of 3.8 +/- 1.5 nmol/unit beta hexosaminidase. Sialic acid loss by these lysosomes was stimulated 2-3 fold by 25 mM sodium phosphate. Loss of sialic acid by lysosomes from cultured human diploid fibroblasts was similar to that observed in rat liver lysosomes while loss of sialic acid by lysosomes from cultured fibroblasts from a patient with infantile Salla disease occurred much more slowly. Salla disease appears to be the consequence of defective lysosomal transport of sialic acid and is analogous to cystinosis, a disorder of lysosomal amino acid transport.

Defective sialic acid egress from isolated fibroblast lysosomes of patients with Salla disease

Normal fibroblasts exposed to N-acetylmannosamine yielded lysosome-rich granular fractions loaded with free (unbound) sialic acid, whose velocity of egress increased with increasing initial loading. Fibroblast granular fractions of patients with Salla disease exhibited negligible egress of sialic acid, whether endogenous or derived from N-acetylmannosamine exposure. Salla disease represents the first disorder demonstrated to be caused by defective transport of a monosaccharide out of cellular lysosomes.

Lysosomal degradation of glycoproteins and glycosaminoglycans. Efflux and recycling of sulphate and N-acetylhexosamines

Lysosomal degradation of the carbohydrate portion of glycoproteins and glycosaminoglycans produces monosaccharides and sulphate, which must efflux from the lysosomes before re-entering biosynthetic pathways. We examined the degradation of glycoproteins and glycosaminoglycans by lysosomes isolated from cultured human diploid fibroblasts. Cells were grown for 24 h in medium containing [3H]glucosamine and [35S]sulphate. When lysosomes are isolated from these cells, they contain label primarily in macromolecules (glycoproteins and glycosaminoglycans). Glycoprotein degradation by isolated lysosomes was followed by measuring the release of tritiated sugars from macromolecules and efflux of these sugars from the organelles. Glycosaminoglycan degradation was monitored by the release of both tritiated sugars and [35S]sulphate. During macromolecule degradation, the total amounts of free [35S]sulphate, N-acetyl[3H]glucosamine and N-acetyl[3H]galactosamine found outside the lysosome parallels the amounts of these products released by degradation. The total degradation of glycoproteins and glycosaminoglycans by intact cultured cells was also examined. The lysosomal contribution to degradation was assessed by measuring inhibition by the lysosomotropic amine NH4Cl. After 48 h incubation, inhibition by NH4Cl exceeded 55% of glycoprotein and 72% of glycosaminoglycan degradation. Recycling of [3H]hexosamines and [35S]sulphate by intact cells was estimated by measuring the appearance of 'newly synthesized' radioactively labelled macromolecules in the medium. Sulphate does not appear to be appreciably recycled. N-Acetylglucosamine and N-acetylgalactosamine, on the other hand, are reutilized to a significant extent.

Further studies on the effect of chloroquine on the uptake, metabolism and intracellular translocation of [35S]cystine in cystinotic fibroblasts

The present study uses the lysosomotropic drug chloroquine to investigate the mechanisms by which exogenous [35S]cystine is able to label the intracellular (intralysosomal) cystine pool(s) in cystinotic fibroblasts. When cystinotic fibroblasts were labelled for short periods of time (8 h or less), chloroquine (20 microM) inhibited the labelling of the intracellular cystine pool(s). However, when the cells were labelled for longer periods of time (24 h or more) chloroquine stimulated the labelling of the intracellular cystine pool(s). The short-term effect was selectively abolished when the cells were washed free of chloroquine, while the long-term effect was selectively abolished when the medium was depleted of cystine. Two routes of translocation of exogenous cystine to the lysosomes could be defined. One route was fast, had a low capacity, was inhibited by chloroquine and increased with increasing medium pH, while the other route was slow, had a high capacity, was stimulated by chloroquine and was more active at low pH. The former pathway probably consisted of plasma membrane transport of cystine into the cytosol followed by direct or indirect transport into the lysosomes. The latter route possibly consisted of pinocytosis with fusion of the cystine-containing pinosomes with lysosomes.

Cysteamine depletes cystinotic leucocyte granular fractions of cystine by the mechanism of disulphide interchange

Cystinotic lysosome-rich leucocyte granular fractions, loaded with [35S]cystine, were exposed to different cystine-depleting agents. During a 30 min incubation at 37 degrees C, untreated cystinotic granular fractions lost negligible [35S]cystine when corrected for lysosome rupture. Granular fractions exposed to 0.1 mM-cysteamine lost 64% of their initial cystine, and hexosaminidase activity was decreased by 10%. This was accompanied by the formation of high concentrations of [35S]cysteine-cysteamine mixed disulphide within the granular-fraction pellet, and, in the presence of N-ethylmaleimide, increasing amounts of [35S]cysteine-N-ethylmaleimide adduct outside the granular fraction. In separate experiments, [35S]cystine exited cystinotic leucocyte lysosomes at a negligible rate (half-times 199 and 293 min), but [35S]cysteine-cysteamine mixed disulphide exhibited substantial egress (half-times 66 and 88 min) and was recovered intact outside the granular-fraction pellet. We conclude that cysteamine depletes lysosomes of cystine by participating in a thiol-disulphide interchange reaction to produce cysteine and cysteine-cysteamine mixed disulphide, both of which traverse the cystinotic leucocyte lysosomal membrane.

Defect in vitamin B12 release from lysosomes: newly described inborn error of vitamin B12 metabolism

Cultured diploid fibroblasts from a patient with a previously undescribed inborn error of cobalamin metabolism accumulate unmetabolized, nonprotein-bound vitamin B12 in lysosomes. These cells are able to endocytose the transcobalamin II-B12 complex and to release B12 from transcobalamin II. The freed vitamin B12 is not released from lysosomes into the cytoplasm of the cell. This suggests that there is a specific lysosomal transport mechanism for vitamin B12 in the human.

pH effects on cystine transport in lysosome-rich leucocyte granular fractions

Inhibitors of lysosomal acidification (4,4'-di-isothiocyanostilbene-2,2'-disulphonate, NN'-dicyclohexylcarbodi-imide, carbonyl cyanide m-chlorophenylhydrazone, NH4Cl and methylamine hydrochloride) did not alter cystine egress or countertransport in polymorphonuclear-leucocyte lysosome-rich granular fractions at pH 7.0. Together, 2 mM-MgCl2/MgATP and 90 mM-KCl stimulated cystine egress 2-fold, but this effect also was not influenced by inhibitors of ATP-dependent lysosomal acidification. MgCl2/MgATP stimulated cystine transport at pH 5.5, but the effect also occurred with MgCl2, MgSO4 or MnCl2 alone, was prevented by chelation, and was not seen with NaATP; therefore, it was considered a bivalent-cation, not an ATP, effect. Proton-pump-mediated acidification of lysosomes does not appear to be required for cystine transport in normal polymorphonuclear-leucocyte granular fractions, as reported for lymphoblast lysosomes.

Plasma and muscle free carnitine deficiency due to renal Fanconi syndrome

Plasma and urine free and acyl carnitine were measured in 19 children with nephropathic cystinosis and renal Fanconi syndrome. Each patient exhibited a deficiency of plasma free carnitine (mean 11.7 +/- 4.0 [SD] nmol/ml) compared with normal control values (42.0 +/- 9.0 nmol/ml) (P less than 0.001). Mean plasma acyl carnitine in the cystinotic subjects was normal. Four subjects with Fanconi syndrome but not cystinosis displayed the same abnormal pattern of plasma carnitine levels; controls with acidosis or a lysosomal storage disorder (Fabry disease), but not Fanconi syndrome, had entirely normal plasma carnitine levels. Two postrenal transplant subjects with cystinosis but without Fanconi syndrome also had normal plasma carnitine levels. Absolute amounts of urinary free carnitine were elevated in cystinotic individuals with Fanconi syndrome. In all 21 subjects with several different etiologies for the Fanconi syndrome, the mean fractional excretion of free carnitine (33%) as well as acyl carnitine (26%) greatly exceeded normal values (3 and 5%, respectively). Total free carnitine excretion in Fanconi syndrome patients correlated with total amino acid excretion (r = 0.76). Two cystinotic patients fasted for 24 h and one idiopathic Fanconi syndrome patient fasted for 5 h showed normal increases in plasma beta-hydroxybutyrate and acetoacetate, which suggested that hepatic fatty acid oxidation was intact despite very low plasma free carnitine levels. Muscle biopsies from two cystinotic subjects with Fanconi syndrome and plasma carnitine deficiency had 8.5 and 13.1 nmol free carnitine per milligram of noncollagen protein, respectively (normal controls, 22.3 and 17.1); total carnitines were 11.8 and 13.3 nmol/mg noncollagen protein (controls 33.5, 20.0). One biopsy revealed a mild increase in lipid droplets. The other showed mild myopathic features with variation in muscle fiber size, small vacuoles, and an increase in lipid droplets. In renal Fanconi syndrome, failure to reabsorb free and acyl carnitine results in a secondary plasma and muscle free carnitine deficiency.

In vivo alteration of a mutant human protein using the free thiol cysteamine

Inborn errors of metabolism in which there is a mutant protein due to a cysteine for arginine substitution may be amenable to treatment with the free thiol cysteamine. Evidence for this derives from patients with type III hyperlipoproteinemia, who are homozygous for apolipoprotein E2, which differs in charge and in vitro function based on a single such amino acid substitution. The plasma of a type III hyperlipoproteinemic patient, when made at least 50 microM with respect to cysteamine in vitro, demonstrated a charge shift of the apolipoprotein E isoelectric focusing pattern from the E2 to the normal E3 and E4 positions. Two children treated for cystinosis with cysteamine each exhibited some charge alteration of their apoE3 to a form migrating in the apoE4 position. The use of thiol reagents such as cysteamine to specifically alter selected mutant human proteins, such as antithrombin III Toyama, may be added to our therapeutic armamentarium in the treatment of life-threatening metabolic disorders.

Biochemical phenotyping of a single sibship with both cystinosis and Fabry disease

Two lysosomal storage diseases, cystinosis and Fabry disease, were diagnosed clinically in different members of a single sibship. The possibility that the affected sister and brother might have related disorders with disparate manifestations was pursued. The four principal family members were tested for heterozygote status with respect to serum and leukocyte alpha-galactosidase A activity, urinary trihexosylceramide excretion, and the capacity to engage in cystine counter-transport across leukocyte lysosome membranes. Results were consistent with classical autosomal recessive inheritance in the case of cystinosis and X-linked inheritance for Fabry disease, confirming that this family represents an example of two rare disorders occurring in the same sibship.

The effect of ions and ionophores on cystine egress from human leucocyte lysosome-rich granular fraction

This paper describes the stimulation of exodus of cystine from lysosome-rich granular fractions by potassium. Potassium permeability into lysosomes is low, but in the presence of an ionophore or permeable anion, the movement of K+ into lysosomes caused a large stimulation of cystine exodus. Lysosomal preparations from leucocytes of cystinotic patients, which lack carrier-mediated cystine transport, also manifested stimulation of cystine egress by valinomycin and K+. This suggests that potassium-dependent cystine egress involves a carrier different from that defective in cystinosis, or occurs through a non-carrier-mediated mechanism.

Cystine accumulation and clearance in normal and cystinotic fibroblasts exposed to cystine dimethyl ester

Exposure of cultured skin fibroblasts of normals and cystinotic patients to 0.5 mmol/l[35S]cystine dimethyl ester for 30 min resulted in an accumulation of cystine in excess to that naturally occurring in cystinotic skin fibroblasts. These equal levels of cystine accumulation achieved in both cystinotic and normal cells, permitted comparative experiments to look for differences in cystine disposal between normal and cystinotic cells. Cystinotic fibroblasts demonstrated very low cystine clearance with a lower ratio of cysteine-N-ethylmaleimide to cystine than normal. The results on cystinotic fibroblasts are consistent with those observed in leucocytes, suggesting that fibroblasts can be useful in further studies to elucidate the clearance defect of cystine in cystinosis as well as its potential in antenatal diagnosis.

Pantethine and cystamine deplete cystine from cystinotic fibroblasts via efflux of cysteamine-cysteine mixed disulfide

Children suffering from cystinosis, a genetic disease characterized by high levels of lysosomal cystine, are currently being treated with cysteamine to lower the cystine levels in their cells. In fibroblasts from these patients, cysteamine and its disulfide, cystamine, are equally effective in lowering cystine levels. We recently reported that pantethine, a dietary precursor of coenzyme A, depletes cystine from cultured, cystinotic fibroblasts as effectively as cystamine. To determine the mechanism of action of pantethine, and of cystamine, we have compared the fate of [35S]cystine-derived metabolites in the presence and absence of these agents. The results indicate that the ability of pantethine to deplete cystine resides in its being a metabolic precursor of cysteamine. Furthermore, both pantethine and cystamine act by generating the mixed disulfide of cysteamine and cysteine in the lysosomes, which is then rapidly excreted from the cells. The fall in intracellular [35S]cystine caused by these agents was not accompanied by a comparable increase in any intracellular metabolite; rather, it could be accounted for by the appearance of mixed disulfide in the medium. There was no accumulation of mixed disulfide in the cells. Radioactivity in cytoplasmic glutathione was, however, increased by cystamine or pantethine. Thus, cysteamine (formed intracellularly in these experiments) undergoes thiol-disulfide exchange with cystine in the lysosomes, producing cysteamine-cysteine mixed disulfide and free cysteine, which enter the cytoplasm. The free cysteine is available to several pathways, including oxidation to the disulfide or the mixed disulfide, and synthesis of glutathione. The mixed disulfide is excreted from the cell, which ultimately depletes the cell of its excess cystine.

Characteristics of cystine counter-transport in normal and cystinotic lysosome-rich leucocyte granular fractions

Normal leucocyte lysosome-rich granular fractions exhibited counter-transport of cystine, confirming that cystine transport across the lysosomal membrane is carrier-mediated. The trans-activation of cystine transport was temperature-dependent but relatively independent of the external Na+ or K+ concentration in phosphate buffer. Counter-transport, measured as uptake of exogenous [3H]cystine, increased with increasing intralysosomal cystine content up to approx. 3 nmol of half-cystine/unit of hexosaminidase activity. The amount of [3H]cystine entering lysosomes loaded with unlabelled cystine decreased when unlabelled cystine was added to the extralysosomal medium. Lysosomal cystine counter-transport was stereospecific for the L-isomer. Cystathionine, cystamine and cysteamine-cysteine mixed disulphide gave evidence of sharing the lysosomal cystine-transport system, although at lower activity than cystine. Other tested amino acids, including arginine, glutamate and homocystine, were inactive in this system. Nine leucocyte lysosome-rich preparations from eight different cystinotic patients displayed virtually no counter-transport of cystine, conclusively establishing that a carrier-mediated system for cystine transport is dysfunctional in cystinotic lysosomes.

Effects of cysteamine and other thiols on cystine egress from isolated lysosome-rich fractions of cystinotic and normal leucocytes

After loading isolated normal and cystinotic white blood cell lysosomes with radioactive amino acid methyl esters, egress of the corresponding amino acids was measured in the presence or absence of reducing agents. Cysteamine greatly enhanced disposal of cystine- and, to a lesser extent, cysteine-loaded lysosomes, resulting in final similar rates of radioactive clearance. Cysteamine did not alter leucine egress. Cysteamine comparably accelerated cystine egress from normal and cystinotic lysosomes. Reduced glutathione, ascorbic acid, coenzyme A and acetylcoenzyme A, which are much less effective cystine-depleting agents on intact cystinotic cells than cysteamine, failed to increase egress rates from isolated lysosomes loaded with cystine. We believe this in vitro system can help in the future in evaluating drug efficacy in reducing cystine content in cystinotics in addition to the already existing tissue culture technique.

Sugar transport in rat liver lysosomes. Direct demonstration by using labelled sugars

Purified rat liver lysosomes ('tritosomes') were prepared from rats injected with Triton WR-1339. 2. The water space of tritosomes, measured by using [3H]water and [14C]sucrose, was 2.15 +/- 0.72 microliter/mg of protein (mean +/- S.E.M., n = 12). 3. Tritosomes, when compared with a crude preparation of normal lysosomes by an indirect method of study, showed sugar specificity but decreased stereospecificity of sugar uptake. 4. At 125 mM the relative rates of net uptake of D-[14C]ribose, D-[14C]- or D-[3H]glucose and 2-deoxy-D-[3H]glucose were the same as that inferred from the indirect study. 5. The entry of D-[3H]glucose into tritosomes showed concentration-dependence suggestive of saturation, with a Km of 48 +/- 18 mM (4). 6. D- and L-glucose, D-ribose, 2-deoxy-D-glucose and D-mannose competed with D-[14C]glucose or D-[14C]ribose for uptake. 7. Cytochalasin B inhibited D-[3H]glucose uptake. 8. Uptake of 1 mM-L-[14C]glucose was slower than for 1 mM-D-[14C]glucose. 9. It is concluded that a facilitated-diffusion transport system is present in purified rat liver lysosomes.