Cellular localization of neuraminidases in cultured human fibroblasts

Comparative enzymology, biochemistry and pathophysiology of human exo-alpha-sialidases (neuraminidases)

This review summarizes the current research on human exo-alpha-sialidase (sialidase, neuraminidase). Where appropriate, the properties of viral, bacterial, and human sialidases have been compared. Sialic acids are implicated in diverse physiological processes. Sialidases, as enzymes acting upon sialic acids, assume importance as well. Sialidases hydrolyze the terminal, non-reducing, sialic acid linkage in glycoproteins, glycolipids, gangliosides, polysaccharides, and synthetic molecules. Therefore, a variety of assays are available to measure sialidase activity. Human sialidase is present in several organs and cells. Its cellular distribution could be cytosolic, lysosomal, or in the membrane. Human sialidase occurs in a high molecular-mass complex with several other proteins, including cathepsin A and beta-galactosidase. Multi-protein complexation is important for the in vivo integrity and catalytic activity of the sialidase. However, multi-protein complexation, the occurrence of isoenzymes, diverse subcellular localization, thermal instability, and membrane association have all contributed to difficulties in purifying and characterizing human sialidases. Human sialidase isoenzymes have recently been cloned and sequenced. Even though crystal structures for the human sialidases are not available, the highly conserved regions of the sialidase from various organisms have facilitated molecular modeling of the human enzyme and raise interesting evolutionary questions. While the molecular mechanisms vary, genetic defects leading to human sialidase deficiency are closely associated with at least two well-known human diseases, namely sialidosis and galactosialidosis. No therapy is currently available for either disease. A thorough investigation of human sialidases is therefore crucial to human health.

Mucolipidosis type IV: abnormal transport of lipids to lysosomes

Mucolipidosis type IV (MLIV) is a lysosomal storage disorder in which various phospholipids and gangliosides accumulate. The cause of this storage has not yet been identified. Loading experiments in cultured fibroblasts with radioactive phosphatidylcholine ([14C]PC) labelled either in the acyl groups or in the choline residue, indicated increased retention of this lipid in the lysosomes of these patients. Chase experiments in intact fibroblasts, on the other hand, indicated normal degradation and discharge of the radioactive PC in MLIV lysosomes. This was further supported by measurements of the degradation of [14C]PC by isolated lysosomes which indicated normal breakdown of PC in MLIV. Cultured fibroblasts from Hunter (MPSII) patients, which contain enlarged and numerous lysosomes, did not store [14C]PC when compared to normal controls, indicating that the storage of this lipid in MLIV is not a secondary phenomenon caused by the presence of enlarged and numerous lysosomes in these cells. Incubation of [14C]PC at 18 degrees C limits the endocytosis process only up to early endosomes. This temperature did not yield increased retention of [14C]PC in MLIV, indicating that accumulation occurs only after the PC reached late endosomes or the lysosomes. The data indicate that PC as well as other phospholipids and gangliosides accumulate in MLIV apparently owing to a defect in the endocytosis process of membranous components. This defect apparently leads to excessive transportation of these macromolecules into lysosomes rather than their recycling to the plasma membrane. The endocytosis of membrane components is different from receptor-mediated endocytosis which is not affected in MLIV. Once the membrane macromolecules reach the lysosomes in MLIV they are normally catabolized and normally discharged. This may explain the heterogeneity of the stored materials in MLIV. The normal catabolism of macromolecules in the lysosomes is reflected in the minor deterioration in the clinical manifestations of these patients.

Cell-contact mediated modulation of the sialylation of contactinhibin

Contactinhibin was found to be involved in contact-dependent inhibition of growth. The growth inhibitory activity of contactinhibin is mediated by N-linked oligosaccharides with desialylated beta-glycosidically linked, terminal galactose residues. Here we show that in sparse human fibroblasts contactinhibin was expressed in a biologically inactive, highly sialylated form both on the plasma membrane and intracellularly, while in confluent cells plasma membrane localized contactinhibin was present in a biologically active, low sialylated form. Plasma membranes were shown to contain a glycoprotein sialidase which is suggested to be engaged in the activation of contactinhibin in a cell contact-dependent manner.

Degradation of gangliosides by the lysosomal sialidase requires an activator protein

Lysosomal sialidase, which was formerly believed to degrade only water-soluble substrates but not glycolipids, cleaves ganglioside substrates II3NeuNAc-LacCer, IV3NeuNAc, II3NeuNAc-GgOse4Cer, IV3 NeuNAc, II3(NeuNAc)2-GgOse4Cer when these are dispersed either with an appropriate detergent (taurodeoxycholate) or with the sulfatide activator protein, a physiologic lipid solubilizer required for the lysosomal hydrolysis of other glycolipids by water-soluble hydrolases. In the presence of the activator protein, time and protein dependence were linear within wide limits, while the detergent rapidly inactivated the enzyme. The disialo group of the b-series gangliosides was only poorly attacked by the enzyme when the lipids were dispersed with the activator protein, whereas in the presence of the detergent, they were hydrolyzed as fast as terminal sialic acid residues. With the appropriate assay method, significant ganglioside sialidase activity could be demonstrated in the secondary lysosome fraction of normal skin fibroblasts but not of sialidosis fibroblasts. Our results support the notion that there is only one lysosomal sialidase, which degrades both the water-soluble and the membrane-bound sialyl glycoconjugates.

Biochemical characteristics and subcellular localizations of rat liver neuraminidase isozymes: a paradox resolved

A striking discrepancy in the abilities of two analytical approaches (fluorometric and electrophoretic) to detect the effect of a gene, Neu-2, on rat liver neuraminidase phenotypes led us to examine the biochemical and physical properties of the liver isozymes NEU-1 and NEU-2 that might be responsible for this difference. Cell fractionation via Percoll gradient centrifugation revealed NEU-1 activity almost exclusively in the lysosomal cell fraction, while NEU-2 was strictly cytosolic in distribution. The two isozymes were also found to differ in pH activity curves and optima (optima: 4.6-4.8 and 5.4-5.8 for NEU-1 and NEU-2, respectively) and in solubility characteristics (NEU-2 highly soluble; NEU-1 relatively insoluble but solubilized by freezing/thawing). Both isozymes were found to be freeze-thaw stable in crude, whole-cell extracts, but NEU-1 was destabilized in the enriched (partially purified) lysosomal subcellular fraction. Consideration of these properties relative to those described previously for unidentified cytosolic and membrane bound (lysosomal) rat liver neuraminidases (Tulsiani, D. R. P., and Carubelli, R., J. Biol. Chem. 245:1821, 1970) leads us to believe that NEU-2 also is destabilized by partial purification and that NEU-1 and NEU-2 have very different relative abundances within the cell. The biochemical and physical differences between NEU-1 and NEU-2 can account for the discrepant abilities of the fluorometric and electrophoretic approaches to detect the effects of Neu-2. Ways to increase the sensitivity of the fluorometric approach for quantitative assays of specific NEU-1 and NEU-2 activity are discussed.

The identification of lysosomal ganglioside sialidase in human cells

In this report we present evidence for the existence of a lysosomal ganglioside sialidase. The sialidase activity was solubilized by sonication and stimulated by cholate. The absence of ganglioside sialidase activity in sialidosis patients indicates that lysosomal sialidase is active towards gangliosides and glycoproteins. The plasma membranes were associated with two types of ganglioside sialidase activities, one was enhanced by cholate while the other was partially inhibited by this detergent.

Phosphatidylcholine storage in mucolipidosis IV

The accumulation of phosphatidylcholine (PC) in cultured fibroblasts of mucolipidosis IV (MLIV) patients was studied by subcellular fractionation on percoll gradients. Labelled PC accumulated in secondary lysosomes of the MLIV cells in significantly higher rates when compared to normal controls incubated with the precursors [32P]phosphate or [14C]choline. This accumulation was noted after 4 days of pulse and became more profound after 7 days of chase resulting in a 30-fold increase of this substance in the lysosomal fraction of MLIV compared to normal controls. On the other hand, no significant increase of radioactive PC was demonstrated in the buoyant fraction of the affected cells, and similarly the rate of disappearance of labeled PC from this fraction was identical in MLIV and controls. The retention of PC in lysosomes of MLIV could also be demonstrated following incubation of cultured fibroblasts with the radioactive phospholipid itself. In these experiments increased labelled PC in MLIV was also noted in endosomes, which are involved in the uptake process of PC enroute to the lysosomes. The metabolic defect leading to this storage in MLIV has not yet been identified, but these data indicate impairment in the lysosomal catabolism of phospholipids in MLIV.

Ganglioside GM3 sialidase activity in fibroblasts of normal individuals and of patients with sialidosis and mucolipidosis IV. Subcellular distribution and and some properties

Sensitive assays for the determination of the ganglioside sialidase activity of fibroblast homogenates were established using ganglioside GM3, 3H-labelled in the sphingosine moiety, as a substrate. Ganglioside GM3 sialidase activity was greatly stimulated by the presence of the non-ionic detergent Triton X-100 and was further enhanced by salts such as NaCl; the optimal pH was 4.5. The subcellular localization of this activity was determined by fractionation using free-flow electrophoresis and found to be exclusively associated with the marker for the plasma membrane, but not with that for lysosomes. This Triton-stimulated ganglioside sialidase activity was selectively inhibited by preincubating intact cells in the presence of millimolar concentrations of Cu2+, suggesting that the activity resides on the external surface of the plasma membrane. In normal fibroblasts homogenates, ganglioside GM3 sialidase was also greatly stimulated by sodium cholate. In contrast to the Triton X-100-activated reaction, however, it was not diminished by prior incubation of intact cells in the presence of Cu2+. Only after cell lysis was Cu2+ inhibitory. the cholate-stimulated ganglioside sialidase activity thus paralleled the behaviour of the lysosomal 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (4-MU-NeuAc) sialidase. In fibroblasts from sialidosis patients, the cholate-stimulated ganglioside GM3 sialidase activity, but not that of the Triton-activated enzyme, was profoundly diminished. In fibroblasts from patients with mucolipidosis IV (ML IV), both the Triton X-100- and the cholate-stimulated ganglioside GM3 sialidase activities were in the range of normal controls. The Triton-activated enzyme was associated with the plasma membrane in the same manner as in normal cells. Our findings suggest that, in human fibroblasts, there exist two sialidases that degrade ganglioside GM3: one on the external surface of the plasma membrane, and another that is localized in lysosomes and seems identical with the activity that acts on sialyloligosaccharides and 4-MU-NeuAc. As neither activity was found to be deficient in ML IV fibroblasts, our results argue against the hypothesis of a primary involvement of a ganglioside GM3 sialidase in the pathogenesis of ML IV.

The distinction between arylsulphatases in chorionic villi

The relatively high activity of arylsulphatase C (ASC) in the placenta is a potential risk for the misdiagnosis of arylsulphatase A (ASA) or arylsulphatase B (ASB) deficiency in chorionic villus sampling when assayed by synthetic substrates. A clear distinction between these enzymes can be achieved in either the direct villi or the cultured villi cells. Interestingly, the activity of ASC differed significantly in cultured villi cells when prepared by two different methods, namely, minced villi versus treatment with trypsin and collagenase, while ASA and ASB were not affected by these treatments. Whether ASC was directly affected by one of these treatments or whether a selection of cells with different ASC levels was achieved is not yet clear, but this phenomenon clearly indicates the importance of precise definition of CVS preparations to correlate with the enzyme activity data.

Phospholipids accumulation in mucolipidosis IV cultured fibroblasts

Cultured fibroblasts from mucolipidosis IV patients accumulated phospholipids when compared to normal controls or cells from other genotypes. The major stored compounds were identified as phosphatidylcholine, phosphatidylethanolamine and to a larger extent lysophosphatidylcholine and lysobisphosphatidic acid. Pulse chase experiments of 32P-labelled phospholipids showed increased retention of these compounds in the mucolipidosis IV lines throughout the pulse and chase periods. Phospholipase A1, A2, C, D and lysophospholipase showed normal activity in the mucolipidosis IV lines and thus the metabolic cause for this storage remains to be identified.

Cellular localization and substrate specificity of isoelectric forms of human liver neuraminidase activity

The four major isoelectric forms of human liver neuraminidase (with pI values between 3.4 and 4.8) have been isolated by preparative isoelectric focusing and characterized with regard to their substrate specificity using glycoprotein, glycopeptide, oligosaccharide and ganglioside natural substrates. All forms exhibited a rather broad linkage specificity and were capable of hydrolyzing sialic acid glycosidically linked alpha 2-3, alpha 2-6 and alpha 2-8, although differential rates of hydrolysis of the substrates were found for each form. The most acidic form 1 (pI 3.4) was most active on sialyl-lactose, whereas form 2 (pI 3.9) and 3 (pI 4.4) were most active on the more hydrophobic ganglioside substrates. Form 4 (pI 4.8) was most active on the low-Mr hydrophilic substrates (fetuin glycopeptide, sialyl-lactose). Each form was less active on the glycoprotein fetuin than on a glycopeptide derived from fetuin. Organelle-enriched fractions were prepared from fresh human liver tissue and neuraminidase activity on 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid was recovered in plasma membrane, microsomal, lysosomal and cytosolic preparations. Isoelectric focusing of the neuraminidase activity recovered in each of these preparations resulted in significantly different isoelectric profiles (number, relative amounts and pI values of forms) for each preparation. The differential substrate specificity of the isoelectric forms and the different isoelectric focusing profiles of neuraminidase activity recovered in subcellular-enriched fractions suggest that specific isoelectric forms with broad but defined substrate specificity are enriched at separate sites within the cell.

Regulation of liver cell ganglioside composition by extracellular fluid viscosity

The viscosity of plasma and extracellular fluid has been shown to be a regulator of lipoprotein production both in cultured hepatocytes and in vivo. The possibility that this extracellular effect on cell function involves modulation of cell surface membrane components was examined. In the present work, we studied the effect of medium viscosity on liver cell gangliosides known to be involved in various membrane functions and to be located predominantly at the cell surface membrane. Cultivation of isolated hepatocytes as primary cultures markedly reduced the ganglioside content, but this reduction process was attenuated by increasing the viscosity of the culture medium. Elevation of extracellular fluid viscosity inhibited the degradation of the cell gangliosides and secretion of lysosomal enzymes involved in ganglioside degradation. The cellular activity of these enzymes as well as the activity of enzymes involved in ganglioside synthesis, CMP-NANA:GM1 sialyltransferase, CMP-NANAP:GM3 sialyltransferase and UDP-galactose:GD2 galactosyltransferase, were not affected by modulation of the extracellular medium viscosity. It is proposed that the modulation of cell ganglioside content by extracellular fluid viscosity is due to an effect on enzymes involved in ganglioside catabolism.

Internalization of exogenous gangliosides in cultured skin fibroblasts for the diagnosis of mucolipidosis IV

The internalization of exogenous mixed brain gangliosides in ML IV cultured skin fibroblasts indicated an impairment of ganglioside catabolism in these cells. Incubation of ML IV, normal and various other lysosomal storage disorders cell lines for five days with exogenous tritium labelled GM3, GD1a or GT1 gangliosides allowed accurate quantitation of the retained gangliosides. This in vitro approach provides a reliable method for the diagnosis of ML IV.

A radiometric assay for ganglioside sialidase applied to the determination of the enzyme subcellular location in cultured human fibroblasts

A radiometric method for the assay of ganglioside sialidase in cultured human fibroblasts was set up. As substrate, highly radioactive (1.28 Ci/mmol) ganglioside GDla isotopically tritium-labeled at carbon C-3 of the long chain base was employed; the liberated, and TLC separated [3H]GM1 was determined by computer-assisted radiochromatoscanning. Under experimental conditions that provided a low and quite acceptable (4-5%) coefficient of variation, the detection limit of the method was 0.1 nmol of liberated GM1, using as low as 10 micrograms of fibroblast homogenate as protein. The detection limit could be lowered to 0.02-0.03 nmol, adopting conditions that, however, carried a higher analytical error (coefficient of variation over 10%). The content of ganglioside sialidase in human fibroblasts cultured in 75-cm2 plastic flasks was 5.8 +/- 2.5 (SD) nmol liberated GM1 h-1 mg protein-1. Subfractionation studies performed on fibroblast homogenate showed that the ganglioside sialidase was mainly associated with the light membrane subfraction that was rich in plasma and intracellular membranes. This subfraction displayed almost no sialidase activity on the artificial substrate 4-methylumbelliferyl-D-N-acetylneuraminic acid. A small but measurable ganglioside sialidase activity was also present in the lysosome-enriched subfraction, which contained a very high sialidase activity on the above artificial substrate. All this supports the hypothesis that human fibroblasts contain sialidases with different subcellular location and substrate specificity. Particularly, the sialidase acting on gangliosides seems to have two sites of subcellular location, a major one at the level of plasma membranes and/or intracellular organelles functionally related with the plasma membranes and a minor one in the lysosomes.

Ganglioside sialidase distribution in mucolipidosis type IV cultured fibroblasts

The subcellular distribution of ganglioside sialidase in Mucolipidosis IV (ML IV) cells was characterized by a series of Percoll gradients. Similar to normal cells, the enzyme cosedimented with plasma membrane markers, although this activity was reduced and exhibited decreased solubility in ML IV cells. Only trace amounts of ganglioside sialidase (less than 5%) was found in the lysosomes of normal cells. This activity was apparently reduced in ML IV cells but its minute activity in controls excluded further characterization of these differences. Plasma membranes on 6.7 and 5.6% Percoll gradients were biomodally distributed. Ganglioside sialidase in normal cells was found to be in both the heavier and the lighter membrane fractions, whereas the enzyme in ML IV cells was associated mainly with the denser membrane fraction. These data indicate that the enzyme in ML IV cells is characteristically different from normal in that it exhibits reduced activity and solubility and a different plasma membrane distribution.

Rat-liver lysosomal sialidase. Solubilization, substrate specificity and comparison with the cytosolic sialidase

Purified liver lysosomes, prepared from rats previously injected with Triton WR-1339, exhibited sialidase activity towards sialyllactose, fetuin, submaxillary mucin (bovine) and gangliosides, and could be disrupted hypotonically with little loss in these activities. After centrifugation, the activities with sialyllactose and fetuin were largely recovered in the supernatant, demonstrating that they were originally in the intralysosomal space. The activities towards submaxillary mucin and gangliosides, on the other hand, remained in the pellet. In the supernatant, activity with fetuin or orosomucoid was markedly reduced by protease inhibitors, suggesting that proteolysis of these glycoproteins may be prerequisite to sialidase activity. The intralysosomal sialidase was solubilized from the mitochondrial-lysosomal fraction of rat liver and partially purified by Sephadex G-200, or Sephadex G-200 followed by CM-cellulose. The enzyme was maximally active at pH 4.7 with sialyllactose as substrate and had a minimum relative molecular mass of 60 000 +/- 5000 by gel filtration; it hydrolyzed a variety of sialooligosaccharides , those containing (alpha 2----3)sialyl linkages being better substrates than those with (alpha 2----6)sialyl linkages. The enzyme failed to attack submaxillary mucin and gangliosides. It was also inactive towards fetuin, orosomucoid and transferrin but capable of hydrolyzing glycopeptides from pronase digest of fetuin. In contrast to the intralysosomal sialidase, the sialidase partially purified from rat liver cytosol by (NH4)2SO4 fractionation followed by chromatography on DEAE-cellulose and CM-cellulose hydrolyzed fetuin and orosomucoid to the extent about half that for sialyllactose. The enzyme was maximally active at pH 5.8 and had a relative molecular mass of approximately 60 000. It also hydrolyzed gangliosides but not submaxillary mucin.

Specificity studies on the oligosaccharide neuraminidase of human fibroblasts

Competition and thermal inactivation experiments with different potential natural substrates indicated that in homogenates of human fibroblasts one single enzyme is acting on both (alpha 2-3) and (alpha 2-6) sialosyl linkages of oligosaccharides and glycoproteins, but not of the ganglioside GM3. N-Acetylneuraminic and 2-deoxy-2,3-dehydro-N-acetylneuraminic acids are competitive inhibitors, whereas chondroitin 4-sulphate and the drug Suramin are potent inhibitors of undefined type.

Properties of endogenous, membrane-associated sialidase activity (N-acetylneuraminidase) of the goldfish visual system

The endogenous sialidase (N-acetylneuraminidase) activity of membranes prepared from goldfish retina and optic tectum displays characteristics similar to those reported for neural plasma membrane sialidases of other organisms. Endogenous membrane sialidase activity was found to be optimal at ph 4.0, and maximal release was obtained at 37-50 degrees C, above which temperature thermal instability of the preparations was observed. Optic nerve crush, which results in regeneration of retinal ganglion cell axons, did not result in significant changes in measured endogenous membrane sialidase activity in either the retina or the optic tectum. Enzymatic hydrolysis of membrane sialoglycolipid (ganglioside) accounted for about 70% of the total sialic acid released. Ganglioside GM1 accumulated as the major lipid product in both retina and tectum, indicating that the inner sialosylgalactosyl linkage in the ganglio oligosaccharide series was resistant to hydrolysis by the endogenous enzyme.

Characterization of human placental neuraminidases. Stability, substrate specificity and molecular weight

1. At least two components of neuraminidase can be distinguished on the basis of thermolability and sedimentability by using the artificial fluorogenic substrate 4-methylumbelliferyl N-acetyl-alpha-D-neuraminate. 2. In crude homogenates, thermodenaturation at 25 degrees C showed a biphasic curve corresponding to component A (half-life, 21 min) and B (half-life, 85 min). The two components were partially resolved by centrifugation. A being soluble and B sedimentable. Both had similar pH-activity curves (pH optimum, 4.4), Km values (A, 0.10 mM; B, 0.06 mM) and molecular weight as determined by radiation inactivation (A, 67000; B, 63000). 3. The soluble A form was still aggregated or bound to membranous debris since almost all neuraminidase activity was eluted near or at the void volume of a Sephacryl S-300 column. 4. Both soluble and sedimentable fractions of placenta hydrolysed the GD1A ganglioside and N-acetyl-neuraminyl-D-lactose linearly for 12 h but no fetuin hydrolysis was detected. 5. The neuraminidase activity with the artificial fluorogenic substrate was inhibited by N-acetylneuraminyl-D-lactose but not by the GD1A ganglioside. These preliminary results suggest that there exist two closely related enzymes hydrolysing both the artificial substrate and N-acetylneuraminyl-D-lactose and a third one hydrolysing the GD1A ganglioside exclusively.