The effect of (+) --cyanidanol on lysosomal enzymes of I-cell fibroblasts.

(+)--Cyanidanol, a water-soluble flavonoid, when added to cultured skin fibroblasts of a patient with I-cell disease raised the intracellular concentration of beta-galactosidase but did not affect the distribution of arylsulfatase. A, alpha-mannosidase or beta-glucuronidase. The elevated accumulation of 35SO4 by I-cell, Hunter and Maroteaux-Lamy fibroblasts was decreased by the addition of (+)--cyanidanol to the culture medium, but the degradation of previously labeled, intracellular glycosaminoglycans was not. It is concluded that (+)--cyanidanol does not produce a biochemical correction of the enzymic abnormalities existing in I-cell fibroblasts.     

Deficiency of UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase in organs of I-cell patients

A N-acetylglucosamine-1-phosphotransferase is involved in synthesis of a common phosphorylated recognition marker in lysosomal enzymes. Absence of this enzyme in liver, spleen, kidney and brain of two patients with I-cell disease is now reported. In these organs activities of lysosomal enzymes are close to normal. In contrast, in fibroblasts the absence of N-acetylglucosamine-1-phosphotransferase and of the common recognition marker are known to result in a severe intracellular deficiency of lysosomal enzymes. It is proposed that in certain organs the transport of lysosomal enzymes into lysosomes is mediated by alternative systems, which recognize structural features other than the phosphorylated recognition marker.     

Intracellular transport of acid alpha-glucosidase in human fibroblasts: evidence for involvement of phosphomannosyl receptor-independent system

Intracellular transport of two lysosomal enzymes, acid alpha-glucosidase and beta-hexosaminidase, was analyzed in human fibroblasts. The precursors of beta-hexosaminidase in normal fibroblasts were released from the membrane fraction by treatment with mannose 6-phosphate, but the precursor of alpha-glucosidase was not. Percoll density gradient centrifugation revealed a normal amount of acid alpha-glucosidase activity in heavy lysosomes in I-cell disease fibroblasts despite impaired maturation and defective phosphorylation, and beta-hexosaminidase activity was markedly reduced in lysosomes. It was concluded that the membrane-bound precursor of acid alpha-glucosidase is transported to lysosomes by a phosphomannosyl receptor-independent system although the enzyme lacks the recognition marker for the phosphomannosyl receptor and processing of an intermediate form to mature forms does not occur in this disease.     

I-cell disease: deficiency of extracellular hydrolase phosphorylation

The content of ³²P-phosphorylated residues of purified extracellular N-acetyl-β-hexosaminidase obtained from the fibroblasts of I-cell disease patients was compared to that of control cells hydrolase. The analyses indicated a 60-fold decrease of the radioactivity per unit enzyme activity in the hydrolase synthesized by the patient's fibroblasts compared to the normal enzyme. Electrofocusing demonstrated again a marked decrease in the ³²P-content of the I-cell hydrolase while the control enzyme showed the presence of radioactivity in both isozymes, namely hexosaminidase A and hexosaminidase B. Most of the radioactivity could be removed from the hydrolase following incubation with alkaline phosphatase, thus indicating its phosphoester linkage.Since phosphorylated sugar residues on lysosomal enzymes function as recognition marker for their transport to the lysosomal compartment and for their specific uptake by fibroblasts, the deficiency of phosphorylated residues on the I-cell hydrolase explains the low intracellular and high extracellular lysosomal enzyme levels observed in this disease.     

The subcellular localization of soluble and membrane-bound lysosomal enzymes in I-cell fibroblasts: a comparative immunocytochemical study

Using electron microscopic immunocytochemistry with gold probes, we have studied the localization of acid alpha-glucosidase, N-acetyl-beta-hexosaminidase and beta-glucocerebrosidase in cultured skin fibroblasts from control subjects and patients with mucolipidosis II (I-cell disease). In control fibroblasts, a random distribution of acid alpha-glucosidase and N-acetyl-beta-hexosaminidase within the lysosomes was observed, whereas beta-glucocerebrosidase was found to be localized on or near the lysosomal membrane. The observations confirm the soluble character of acid alpha-glucosidase and N-acetyl-beta-hexosaminidase and the membrane-bound character of beta-glucocerebrosidase. In I-cell fibroblasts an abnormal localization of the two soluble enzymes was found. Labeling in lysosomes was very weak, but instead, small 'presumptive' vesicles containing both enzymes were detected throughout the cytoplasm and close to the plasma membrane. These vesicles could be involved in the secretion of the two enzymes. In contrast, a normal membrane-bound lysosomal localization was observed for beta-glucocerebrosidase. It is concluded that the intracellular transport of beta-glucocerebrosidase to the lysosomes can occur even when the mannose-6-phosphate recognition system is defective. This explains the normal activity of beta-glucocerebrosidase in I-cells in contrast to the deficiency of most other lysosomal enzymes.     

Two clonal cell populations (mosaicism) in a 46,XY male with mucolipidosis II (I-cell disease)--an autosomal recessive disorder.

Cultured fibroblasts from a 46,XY male with an atypical form of mucolipidosis II (I-cell disease) had two distinct phenotypes. One population of these fibroblasts had the morphological and biochemical features characteristic of I-cell disease, while the remaining cells were indistinguishable from normal fibroblasts. Direct evidence that the patient was a mosaic, having two cell populations, was provided by the establishment of pure, stable clones of both wild type and I-cell fibroblasts from each of two biopsies obtained several months apart. Additionally, it was shown that the I-cell fibroblasts lacked UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosaminylphosphotransferase while the morphologically normal cells contained levels of this enzyme just below or at the lower end of the normal range.     

Light and heavy lysosomes: characterization of N-acetyl-{beta}-D-hexosaminidase isolated from normal and I-cell disease lymphoblasts

We previously reported that I-cell disease lymphoblasts maintainnormal or near-normal intracellular levels of lysosomal enzymes,even though N-acetylglucosamine-1-phosphotransferase activityis severely depressed or absent (Little et al., Biochem. J.,248, 151–159, 1987). The present study, employing subcellularfractionation on colloidal silica gradients, indicates thatboth light and heavy lysosomes isolated from I-cell diseaseand pseudo-Hurler polydystrophy lymphoblasts possess normalspecific activity levels of N-acetyl-ß-D-hexosaminidase,-D-mannosidase and ß-D-glucuronidase. These currentfindings are in contrast to those of cultured fibroblasts fromthe same patients, where decreased intralysosomal enzyme activitiesare found. Column chromatography on Ricinus communis revealedthat N-acetyl-ß-D-hexosaminidase in both heavy andlight I-cell disease lysosomal fractions from lymphoblasts possessesan increased number of accessible galactose residues (30–50%)as compared to the enzyme from the corresponding normal controls.Endo-ß-N-acetylglucos-aminidase H treatment of N-acetyl-ß-D-hexosaminidasefrom the I-cell lysosomal fractions suggests that the majorityof newly synthesized high-mannose-type oligosaccharide chainsare modified to complex-type carbohydrates prior to being transportedto lysosomes. This result from lymphoblasts differs from previousfindings with fibroblasts, where N-acetyl-ß-D-hexosaminidasefrom I-cell disease and pseudo-Hurler polydystrophy lysosomesexhibited properties associated with predominantly high-mannose-typeoligosaccharide chains. The current results imply that differentcell types may modify the carbohydrate side chains of lysosomalenzymes in a differential manner, and that selected cell typesmay also employ mechanisms other than the mannose-6-phosphatepathway for targeting lysosomal enzymes to lysosomes. I-cell disease lymphoblasts lysosomes mannose-6-phosphate oligosaccharide chains pseudo-Hurler polydystrophy     

Biosynthesis and processing of lysosomal acid phosphatase in cultured human cells

The biosynthesis of lysosomal acid phosphatase was studied in a normal human embryonic lung cell line, WI-38. Cells were labeled with radioactive leucine under a variety of conditions, the enzyme was immunoprecipitated using a monospecific antiserum raised against human liver lysosomal acid phosphatase, and the products were separated by electrophoresis and were visualized by fluorography. Lysosomal acid phosphatase constitutes 60% of the total tartrate-inhibitable acid phosphatase in WI-38. It is initially synthesized as a high-molecular-weight precursor polypeptide of 69 kDa. The precursor polypeptide is rapidly glycosylated and processed to a mature enzyme of 53-45 kDa via intermediates of 65 and 60 kDa in WI-38 cells. The 69-kDa precursor polypeptide is also converted to larger precursor polypeptides of 74 and 80 kDa. The multiplicity of precursor polypeptides is due at least in part to differences in the glycosylation and phosphorylation of the polypeptides. Sensitivity of phosphorylated oligosaccharide chains from precursor, mature and small polypeptides to endo-beta-hexosaminidase H-catalyzed cleavage suggests the presence of high-mannose phosphorylated oligosaccharide chains similar to those present on many other lysosomal enzymes. The effects of tunicamycin and ammonium chloride were also studied. In contrast to the effect of ammonium chloride on arylsulfatase A secretion, the lysosomal acid phosphatase in WI-38 cells was not secreted in the presence of NH4Cl. This is consistent with the existence of an alternate route for the transfer of lysosomal acid phosphatase into lysosomes. This alternate route may be the reason that I-cell fibroblasts contain a normal level of lysosomal acid phosphatase.     

Immunocytochemical localization of lysosomal acid phosphatase in normal and "I-cell" fibroblasts

This study represents the first example of immunological localization of lysosomal acid phosphatase. The intracellular localization of lysosomal acid phosphatase was investigated with immunocytochemical methods at the light and electron microscopical level in cultured fibroblasts obtained from normal subjects and from a patient with I-cell disease. Double-labeling studies using fluorescence microscopy showed that acid phosphatase is present in the same organelles as other hydrolases. At the electron microscopic level in control fibroblasts acid phosphatase was found in the rough endoplasmic reticulum, lysosomes, at the plasma membrane, in vesicles just below the plasma membrane and in multivesicular bodies. This localization was comparable with that of other lysosomal enzymes tested (acid alpha-glucosidase, N-acetyl-beta-hexosaminidase, beta-galactosidase). Acid phosphatase labeling was mainly found in association with the lysosomal membrane and with membranous material present within the lysosome. In I-cell fibroblasts the label was present in the same subcellular organelles but always associated with membranous structures. We suggest that the association of acid phosphatase with membranes might explain the normal enzyme activity found in I-cell fibroblasts.     

Synthesis and processing of alpha-galactosidase A in human fibroblasts. Evidence for different mutations in Fabry disease

The synthesis and processing of the human lysosomal enzyme alpha-galactosidase A was examined in normal and Fabry fibroblasts. In normal cells, alpha-galactosidase A was synthesized as an Mr = 50,500 precursor, which contained phosphate groups in oligosaccharide chains cleavable by endoglucosaminidase H. The precursor was processed via ill-defined intermediates to a mature Mr 46,000 form. Processing was complete within 3-7 days after synthesis. In the presence of NH4Cl and in I-cell fibroblasts, the majority of newly synthesized alpha-galactosidase A was secreted as an Mr = 52,000 form. For comparison, the processing and stability of alpha-galactosidase A were examined in fibroblasts from five unrelated patients with Fabry disease, which is caused by deficient alpha-galactosidase A activity. In one cell line, synthesis of immunologically cross-reacting polypeptides was not detectable. In another, the synthesis, processing, and stability of alpha-galactosidase A was indistinguishable from that in normal fibroblasts. In a third Fabry cell line, the mutation retarded the maturation of alpha-galactosidase A. Finally, in two cell lines, alpha-galactosidase A polypeptides were synthesized that were rapidly degraded following delivery to lysosomes. These results clearly indicate that Fabry disease comprises a heterogeneous group of mutations affecting synthesis, processing, and stability of alpha-galactosidase A.     

The effect of monensin on beta-hexosaminidase transport in normal and I-cell fibroblasts.

The carboxylic ionophore, monensin, blocks the migration of glycoprotein-containing vesicles from the Golgi region to the plasma membrane in fibroblasts resulting in an accumulation of secretory products in the Golgi cisternae. Treatment of cultured I-cell fibroblasts with monensin (0.5 muM) decreased the abnormal excretion of beta-hexosaminidase to 40% of untreated cultures within 15 min. A corresponding intracellular accumulation of the enzyme to greater than 200% of untreated cultured by 24 h was also observed. A small intracellular accumulation and slightly enhanced excretion of beta-hexosaminidase occurred in treated normal fibroblasts cultures. The intra- and extra-cellular distribution of newly synthesized beta-hexosaminidase in both monensin-treated normal and I-cell fibroblasts were electrophoretically indistinguishable from the four bands characteristic of I-cell intracellular beta-hexosaminidase. The excreted enzyme from both cultures was found to be a low- or no-uptake form. This form of beta-hexosaminidase may have been excreted from a secondary route preceding the site of the monensin effect. The similar findings in monensin-treated normal and I-cell cultures suggest that the subcellular site of the biochemical defect in I-cell disease is at a location after the site of the monensin effect i.e. late in the Golgi region or at a post-Golgi-region location.     

Mucopolysaccharidosis VI (Maroteaux-Lamy syndrome). An intermediate clinical phenotype caused by substitution of valine for glycine at position 137 of arylsulfatase B.

The Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI) is a lysosomal storage disease with autosomal recessive inheritance caused by deficiency of the enzyme arylsulfatase B. Severe, intermediate, and mild forms of the disease have been described. The molecular correlate of the clinical heterogeneity is not known at present. To identify the molecular defect in a patient with the intermediate form of the disease, arylsulfatase B mRNA from his fibroblasts was reverse-transcribed, amplified by the polymerase chain reaction, and subcloned. Three point mutations were detected by DNA sequence analysis, two of which, a silent A to G transition at nucleotide 1191 and a G to A transition at nucleotide 1126 resulting in a methionine for valine 376 substitution, were polymorphisms. A G to T transversion at nucleotide 410 causing a valine for glycine 137 substitution (G137V) was identified as the mutation underlying the Maroteaux-Lamy phenotype of the patient, who was homozygous for the allele. The kinetic parameters of the mutant arylsulfatase B enzyme toward a radiolabeled trisaccharide substrate were normal excluding an alteration of the active site. The G137V mutation did not affect the synthesis but severely reduced the stability of the arylsulfatase B precursor. While the wild type precursor is converted by limited proteolysis in late endosomes or lysosomes to a mature form, the majority of the mutant precursor was degraded presumably in a compartment proximal to the trans Golgi network and only a small amount escaped to the lysosomes accounting for the low residual enzyme activity in fibroblasts of a patient with the juvenile form of the disease.     

Influence of sialic acid on cell surface properties in I-cell disease fibroblasts

Summary Fibroblasts derived from patients with I-cell disease have been shown to accumulate many natural substrates including a three to fourfold increase in sialic acid content compared to that found in normal fibroblasts. This diverse accumulation of storage material is due to a massive deficiency of multiple lysosomal hydrolases as they are preferentially excreted into the culture fluid. There is evidence that the I-cell plasma membrane itself is abnormal with respect to certain transferase activities and in its sensitivity to freezing and Triton X-100. In this study, we have shown that a neuraminidase-sensitive substrate, and perhaps others in I-cell fibroblasts, contribute to an increased electronegativity of the I-cell fibroblast surface and to the cells' sensitivity to freezing. We also found that neuraminidase treatment of I-cell fibroblasts before preservative freezing in liquid nitrogen enables the cells to adapt more easily to subculture upon thawing. This project was supported in part by National Institutes of Health (NIH) BRSG Grant RR-05493, NIH Grant 1-R01-HD-11453-01-A1, National Science Foundation Grant PCM 77-05733, and Maternal and Child Health Service Project 417. Georgirene D. Vladutiu is the recipient of Research Career Development Award 1K04 HD 00312-01A1 from the National Institutes of Health.     

Comparative study of cystine clearance in cystinotic and I-cell fibroblasts upon exposure to cystine dimethyl ester

I-cell fibroblasts can accumulate cystine at levels comparable to those seen in homozygous cystinotic fibroblasts. Cystine accumulation in cystinosis is accounted for cystine clearance defect in situ. To unravel the question whether the same clearance defect or two different mechanisms cause cystine accumulation in I-cell disease, we used the cystine loading technique upon exposure of skin fibroblasts to radioactive cystine dimethyl ester. Normal, cystinotic and I-cell fibroblasts were exposed to radioactive cystine dimethyl ester, and the clearance of the generated radioactive cystine was measured. Cystinotic cells showed a marked defect in cystine clearance in situ, as compared to normal fibroblasts. In I-cell fibroblasts, we observed slow hydrolysis of cystine dimethyl ester to cystine, indicating low esterase activity, but no defect in clearance of the generated cystine. Cysteine production from the exogenous cystine dimethyl ester, presumably by cytoplasmic hydrolysis of the generated cystine, is normal in I-cell fibroblasts. Thus, our results indicate that, unlike cystinosis, there is no cystine clearance defect in situ for cystine in I-cell disease, and probably unrelated mechanisms cause cystine storage in cystinosis and I-cell disease.     

Biosynthesis and turnover of the phosphomannosyl receptor in human fibroblasts.

The phosphomannosyl receptor mediates intracellular targeting of newly synthesized acid hydrolases to lysosomes, and is also expressed as a pinocytosis receptor on the cell surface of fibroblasts. We have purified the phosphomannosyl receptor from bovine liver and produced rabbit antibodies to the bovine receptor. The antibodies partially blocked pinocytosis of human spleen beta-glucuronidase by fibroblasts, a process mediated by the phosphomannosyl receptor. Affinity-purified antibodies to the phosphomannosyl receptor were used to study the biosynthesis and turnover of the receptor in human fibroblasts. Phosphomannosyl receptor immunoprecipitated after a 15 min pulse-labelling of fibroblasts with [35S]methionine exhibited an identical mobility on sodium dodecyl sulphate/polyacrylamide gels as purified bovine liver phosphomannosyl receptor. Pulse-chase experiments for up to 3 days provided no evidence for changes in molecular weight attributable to post-translational processing of the phosphomannosyl receptor. Turnover studies determined that the half-life of the phosphomannosyl receptor in normal human fibroblasts was 24-29 h. The half-life of the receptor was slightly longer (32 h) in I-cell disease fibroblasts and normal fibroblasts exposed to leupeptin (32 h), slightly shorter in fibroblasts exposed to NH4Cl (23 h) and saturating amounts of ligand (21 h) and unaffected in cells exposed to mannose 6-phosphate (24 h). These studies show that the turnover of the phosphomannosyl receptor in fibroblasts is very slow, in contrast with its rate of internalization in endocytosis, and that its rate of degradation is not greatly altered by a variety of agents that affect lysosomal protein turnover and/or receptor-mediated endocytosis. These results suggest that the degradative activities of the lysosomes do not play an important role in phosphomannosyl receptor turnover in cultured fibroblasts.     

Compartmental distribution of beta-hexosaminidase isoenzymes in I-cell fibroblasts.

A characteristic of the human lysosomal disorder I-cell disease is an abnormal excretion of most lysosomal hydrolases, including beta-N-acetyl-D-glucosaminidase (EC 3.2.1.30; beta-hexosaminidase) by cultured skin fibroblasts. Treatment of I-cell cultures with cycloheximide or tunicamycin demonstrated that (1) I-cell fibroblasts rapidly excrete all newly synthesized beta-hexosaminidase, (2) two qualitatively distinct pools of beta-hexosaminidase isoenzymes exist inside I-cell fibroblasts, one of which is a rapid-turnover excretory pool, and (3) the induction of an abnormal glycosylation of beta-hexosaminidase by tunicamycin in normal or I-cell fibroblast cultures does not affect subsequent excretion of the enzyme.     

Effect of tunicamycin and cycloheximide on the secretion of acid hydrolases from I-cell cultured fibroblasts.

I-cell cultures fibroblasts secrete excessive amounts of N-acetyl-beta-D-hexosaminidase and alpha-L-fucosidase into the culture media as compared with normal fibroblasts. Addition of tunicamycin or cyd [14C]leucine (40--50%) into trichloroacetic acid-precipitable material decreased the secretion of these I-cell hydrolases to normal values within 24 h, but had no effect on the secretion of acid hydrolases from normal fibroblasts. These results indicate that I-cell cultured fibroblasts secrete at least two types of acid hydrolases: one is tunicamycin- and cycloheximide-sensitive and constitutes the greater proportion of the secreted hydrolases, and a smaller proportion is insensitive to tunicamycin and cycloheximide, similar t9 the acid hydrolases secreted by normal cultured fibroblasts.     

Evidence for the deficiency of β-glucosidase-activating factor in fibroblasts of patients with I-cell disease

Summary Reduced activity of -glucosidase was shown in the cultured skin fibroblasts of four patients with I-cell disease when the enzyme was tested without the use of detergents. In the presence of taurocholate and triton X100 -glucosidase activity was normal. This suggested a deficiency of a -glucosidase-activating factor in I-cell fibroblasts rather than of the enzyme itself. The deficiency of -glucosidase activity was corrected to some extent by mixing cell lysates, and more effectively by cocultivation and fusion of I-cell disease and Gaucher fibroblasts. These results present evidence for the presence of a -glucosidase-activating factor in normal and Gaucher fibroblasts. In fibroblasts of patients with I-cell disease this activator is probably deficient, as is the case for most lysosomal enzymes.     

Increased levels of sialic acid associated with a sialidase deficiency in I-cell disease (mucolipidosis II) fibroblasts.

Cultured fibroblasts from three unrelated patients with I-cell disease (mucolipidosis II) have a 3 to 4 fold increase in total sialic acid when compared to control fibroblasts. Sialic acid levels in a number of other lysosomal disorders, i.e., mucopolysaccharidosis I, II, III, VI, metachromatic leukodystrophy, G_M1 gangliosidosis, mannosidosis, Gaucher's and Sandhoff's disease are within the normal range suggesting that this is a finding specific for I-cells. Additionally, sonicates of cultured fibroblasts from controls were shown to have an acid sialidase capable of removing sialic acid from added fetuin at pH 4.2 in 0.05M acetate buffer. In contrast, I-cell fibroblasts, within the limits of the assay, lack this enzyme activity.     

The effects of sucrose loading on lysosomal hydrolases

Summary The addition of 88 mM sucrose to the culture medium of human skin fibroblasts from normal subjects caused remarkable increase in the intracellular lysosomal hydrolase activities. The mechanism of this induction by sucrose loading was carefully studied with several fibroblast strains of different inherited lysosomal storage disorders. In single lysosomal hydrolase defect such as GM₁-gangliosidosis, mannosidosis and Sandhoff disease, no induction of the deficient hydrolase was found with 88 mM sucrose loading. In contrast, sucrose loading caused normalization of intracellular lysosomal hydrolase activities in I-cell disease fibroblasts and cytoplasmic inclusion materials disappeared. Subsequent investigations reveal that I-cell disease cells are classified into three subgroups by the degree of hydrolase induction by sucrose loading; a high responding, an intermediate responding and a no-response group. The heterogeneity may be based upon different induction by sucrose loading of the enzyme, probably the residual phosphotransferase which is involved in the processing steps of lysosomal enzyme molecules. With the addition of mannose-6-phosphate and 10 mM NH₄Cl to cultured skin fibroblasts, it was shown that sucrose loading caused increased synthesis of lysosomal enzyme proteins. The result of the test with 2,4-dinitrophenol suggests that sucrose is indeed pinocytosed by cultured human skin fibroblasts and localized in lysosomes and that this event is the essential factor to trigger the induction of lysosomal hydrolases. Simultaneous loading of both invertase and sucrose in cultured cells caused no induction of -mannosidase activity. This result indicates that invertase is also pinocytosed, reaches the lysosomes and hydrolyzes sucrose in the lysosomes. Lysosomal overloading with sucrose resulted in induction of lysosomal hydrolases and invertase blocked the induction of -mannosidase activity. However, some induction still exists in -galactosidase and -fucosidase activity. Thus it is very likely that the induction of lysosomal hydrolases demands a complicated process.In this article, we investigated the effects of sucrose on the lysosomal hydrolases in cultured human skin fibroblasts of several inherited lysosomal storage disorders and normal subjects and discuss the possible mechanism. of the induction of lysosomal hydrolase activities by sucrose loading.