Phosphohexosyl recognition is a general characteristic of pinocytosis of lysosomal glycosidases by human fibroblasts

Enzymes approved for human therapy: indications, mechanisms and adverse effects

Research and drug developments fostered under orphan drug product development programs have greatly assisted the introduction of efficient and safe enzyme-based therapies for a range of rare disorders. The introduction and regulatory approval of 20 different recombinant enzymes has enabled, often for the first time, effective enzyme-replacement therapy for some lysosomal storage disorders, including Gaucher (imiglucerase, taliglucerase, and velaglucerase), Fabry (agalsidase alfa and beta), and Pompe (alglucosidase alfa) diseases and mucopolysaccharidoses I (laronidase), II (idursulfase), IVA (elosulfase), and VI (galsulfase). Approved recombinant enzymes are also now used as therapy for myocardial infarction (alteplase, reteplase, and tenecteplase), cystic fibrosis (dornase alfa), chronic gout (pegloticase), tumor lysis syndrome (rasburicase), leukemia (L-asparaginase), some collagen-based disorders such as Dupuytren's contracture (collagenase), severe combined immunodeficiency disease (pegademase bovine), detoxification of methotrexate (glucarpidase), and vitreomacular adhesion (ocriplasmin). The development of these efficacious and safe enzyme-based therapies has occurred hand in hand with some remarkable advances in the preparation of the often specifically designed recombinant enzymes; the manufacturing expertise necessary for commercial production; our understanding of underlying mechanisms operative in the different diseases; and the mechanisms of action of the relevant recombinant enzymes. Together with information on these mechanisms, safety findings recorded so far on the various adverse events and problems of immunogenicity of the recombinant enzymes used for therapy are presented.

Different Pathways to the Lysosome: Sorting out Alternatives

Considerable research supports a model in which hydrolytic enzymes of mammalian lysosomes are sorted to their destinations in a receptor-dependent mechanism. The ligand for the mammalian sorting receptors is mannose 6-phosphate (M6P). Two M6P receptors have been defined in mammals. Here, we review the foundational evidence supporting this mechanism and highlight the remaining gaps in our understanding of the mammalian mechanism, including evidence for M6P-independent sorting, and its relevance to lysosomal enzyme sorting in metazoa.

A novel mouse model of a patient mucolipidosis II mutation recapitulates disease pathology

Mucolipidosis II (MLII) is a lysosomal storage disorder caused by loss of N-acetylglucosamine-1-phosphotransferase, which tags lysosomal enzymes with a mannose 6-phosphate marker for transport to the lysosome. In MLII, the loss of this marker leads to deficiency of multiple enzymes and non-enzymatic proteins in the lysosome, leading to the storage of multiple substrates. Here we present a novel mouse model of MLII homozygous for a patient mutation in the GNPTAB gene. Whereas the current gene knock-out mouse model of MLII lacks some of the characteristic features of the human disease, our novel mouse model more fully recapitulates the human pathology, showing growth retardation, skeletal and facial abnormalities, increased circulating lysosomal enzymatic activities, intracellular lysosomal storage, and reduced life span. Importantly, MLII behavioral deficits are characterized for the first time, including impaired motor function and psychomotor retardation. Histological analysis of the brain revealed progressive neurodegeneration in the cerebellum with severe Purkinje cell loss as the underlying cause of the ataxic gait. In addition, based on the loss of Npc2 (Niemann-Pick type C 2) protein expression in the brain, the mice were treated with 2-hydroxypropyl-β-cyclodextrin, a drug previously reported to rescue Purkinje cell death in a mouse model of Niemann-Pick type C disease. No improvement in brain pathology was observed. This indicates that cerebellar degeneration is not primarily triggered by loss of Npc2 function. This study emphasizes the value of modeling MLII patient mutations to generate clinically relevant mouse mutants to elucidate the pathogenic molecular pathways of MLII and address their amenability to therapy.

An adult patient with mucolipidosis III alpha/beta presenting with parkinsonism

A 36-year-old man with mucolipidosis type III alpha/beta presented with hypoactivity, mutism, muscle rigidity, and involuntary movement. The involuntary movement was interpreted to be tremor at rest on physical examination and surface electromyography, which revealed mostly asynchronous contractions at 3-4 Hz of the biceps and triceps brachii muscles. All these symptoms were consistent with abnormalities of parkinsonism, which is caused by an insult to the basal ganglia that permeates the entire basal ganglia-thalamocortical circuitry. This report is the first to present a case of mucolipidosis type III alpha/beta in association with parkinsonism.

Lysosomal dysfunction causes neurodegeneration in mucolipidosis II 'knock-in' mice

Mucolipidosis II is a neurometabolic lysosomal trafficking disorder of infancy caused by loss of mannose 6-phosphate targeting signals on lysosomal proteins, leading to lysosomal dysfunction and accumulation of non-degraded material. However, the identity of storage material and mechanisms of neurodegeneration in mucolipidosis II are unknown. We have generated 'knock-in' mice with a common mucolipidosis II patient mutation that show growth retardation, progressive brain atrophy, skeletal abnormalities, elevated lysosomal enzyme activities in serum, lysosomal storage in fibroblasts and brain and premature death, closely mimicking the mucolipidosis II disease in humans. The examination of affected mouse brains at different ages by immunohistochemistry, ultrastructural analysis, immunoblotting and mass spectrometric analyses of glycans and anionic lipids revealed that the expression and proteolytic processing of distinct lysosomal proteins such as α-l-fucosidase, β-hexosaminidase, α-mannosidase or Niemann-Pick C2 protein are more significantly impacted by the loss of mannose 6-phosphate residues than enzymes reaching lysosomes independently of this targeting mechanism. As a consequence, fucosylated N-glycans, GM2 and GM3 gangliosides, cholesterol and bis(monoacylglycero)phosphate accumulate progressively in the brain of mucolipidosis II mice. Prominent astrogliosis and the accumulation of organelles and storage material in focally swollen axons were observed in the cerebellum and were accompanied by a loss of Purkinje cells. Moreover, an increased neuronal level of the microtubule-associated protein 1 light chain 3 and the formation of p62-positive neuronal aggregates indicate an impairment of constitutive autophagy in the mucolipidosis II brain. Our findings demonstrate the essential role of mannose 6-phosphate for selected lysosomal proteins to maintain the capability for degradation of sequestered components in lysosomes and autophagolysosomes and prevent neurodegeneration. These lysosomal proteins might be a potential target for a valid therapeutic approach for mucolipidosis II disease.

Mannose 6-phosphate receptor homology (MRH) domain-containing lectins in the secretory pathway

BACKGROUND

The mannose 6-phosphate receptor homology (MRH) domain-containing family of proteins, which include recycling receptors (mannose 6-phosphate receptors, MPRs), resident endoplasmic reticulum (ER) proteins (glucosidase II β-subunit, XTP3-B, OS-9), and a Golgi glycosyltransferase (GlcNAc-phosphotransferase γ-subunit), are characterized by the presence of one or more MRH domains. Many MRH domains act as lectins and bind specific phosphorylated (MPRs) or non-phosphorylated (glucosidase II β-subunit, XTP3-B and OS-9) high mannose-type N-glycans. The MPRs are the only proteins known to bind mannose 6-phosphate (Man-6-P) residues via their MRH domains.

SCOPE OF REVIEW

Recent biochemical and structural studies that have provided valuable insight into the glycan specificity and mechanisms of carbohydrate recognition by this diverse group of MRH domain-containing proteins are highlighted.

MAJOR CONCLUSIONS

Currently, three-dimensional structures are known for ten MRH domains, revealing the conservation of a similar fold. OS-9 and the MPRs use the same four residues (Gln, Arg, Glu, and Tyr) to bind mannose.

GENERAL SIGNIFICANCE

The MRH domain-containing proteins play key roles in the secretory pathway: glucosidase II, XTP3-B, and OS-9 are involved in the recognition of nascent glycoproteins, whereas the MPRs play an essential role in lysosome biogenesis by targeting Man-6-P-containing lysosomal enzymes to the lysosome.

N-glycosylation of the human melanocortin 1 receptor: occupancy of glycosylation sequons and functional role

The melanocortin 1 receptor (MC1R), a major determinant of skin pigmentation and phototype, mediates the actions of α-melanocyte-stimulating hormone on melanocytes and is critical for melanocyte proliferation and differentiation. MC1R has two putative N-glycosylation targets, Asn15 and Asn29. It has been shown that MC1R is a glycoprotein with an unusual sensitivity to endoglycosidase H digestion. However, the occupancy and functional importance of each specific glycosylation sequon remains unknown. We demonstrate that MC1R is N-glycosylated at Asn15 and Asn29, with structurally and functionally different glycan chains. N-glycosylation is not necessary for high affinity agonist binding or functional coupling but has a strong effect on the availability of MC1R molecules on the plasma membrane, most likely by a combination of improved forward trafficking and decreased internalization. Finally, we found that MC1R variants exhibit different degrees of glycosylation which do not show a simple correlation with their functional status or intracellular trafficking.

Carbohydrate recognition by the mannose-6-phosphate receptors

The two P-type lectins, the 46kDa cation-dependent mannose-6-phosphate (Man-6-P) receptor (CD-MPR), and the 300kDa cation-independent Man-6-P receptor (CI-MPR), are the founding members of the growing family of mannose-6-phosphate receptor homology (MRH) proteins. A major cellular function of the MPRs is to transport Man-6-P-containing acid hydrolases from the Golgi to endosomal/lysosomal compartments. Recent advances in the structural analyses of both CD-MPR and CI-MPR have revealed the structural basis for phosphomannosyl recognition by these receptors and provided insights into how the receptors load and unload their cargo. A surprising finding is that the CD-MPR is dynamic, with at least two stable quaternary states, the open (ligand-bound) and closed (ligand-free) conformations, similar to those of hemoglobin. Ligand binding stabilizes the open conformation; changes in the pH of the environment at the cell surface and in endosomal compartments weaken the ligand-receptor interaction and/or weaken the electrostatic interactions at the subunit interface, resulting in the closed conformation.

Intracellular localization and trafficking of serine proteinase AhSub and cysteine proteinase AhCP of Acanthamoeba healyi

Proteinases have been proposed to play important roles in pathogenesis and various biologic actions in Acanthamoeba. Although genetic characteristics of several proteases of Acanthamoeba have been reported, the intracellular localization and trafficking of these enzymes has yet to be studied. In the present study, we analyzed the intracellular localization and trafficking of two proteinases, AhSub and AhCP, of Acanthamoeba healyi by transient transfection. Full-length AhSub-enhanced green fluorescent protein (EGFP) fusion protein was found in intracellular vesicle-like structures of transfected amoebae. Time-lapse photographs confirmed the secretion of the fluorescent material of the vesicle toward the extracellular space. The mutated AhSub, of which the pre or prepro region was deleted, was found to localize diffusely throughout the cytoplasm of the amoeba rather than concentrated in the secretory vesicle. Transfection of the construct containing the pre region only showed the same localization and trafficking of the full-length AhSub. A cysteine proteinase AhCP-EGFP fusion protein showed similar localization in the vesicle-like structure in the amoeba. However, using Lyso Tracker analysis, these vesicular structures of AhCP were confirmed to be lysosomes rather than secretory vesicles. The AhCP construct with a deletion of the prepro region showed a dispersed distribution of fluorescence in the cytoplasm of the cells. These results indicated that AhSub and AhCP would play different roles in Acanthameoba biology and that the pre region of AhSub and pro region of AhCP are important for proper intracellular localization and trafficking of each proteinase.

Cationic sites on granzyme B contribute to cytotoxicity by promoting its uptake into target cells

Granzyme B (GrB) is a key effector of cytotoxic lymphocyte-mediated cell death. It is delivered to target cells bound to the proteoglycan serglycin, but how it crosses the plasma membrane and accesses substrates in the cytoplasm is poorly understood. Here we identify two cationic sequences on GrB that facilitate its binding and uptake. Mutation of cationic sequence 1 (cs1) prevents accumulation of GrB in a distinctive intracellular compartment and reduces cytotoxicity 20-fold. Mutation of cs2 reduces accumulation in this intracellular compartment and cytotoxicity two- to threefold. We also show that GrB-mediated cytotoxicity is abrogated by heparin and that target cells deficient in cell surface sulfate or glycosaminoglycans resist GrB. However, heparin does not completely prevent GrB internalization and chondroitin 4-sulfate does not inhibit cytotoxicity, suggesting that glycosaminoglycans are not essential GrB receptors. We propose that GrB enters cells by nonselective adsorptive pinocytosis, exchanging from chondroitin sulfate on serglycin to anionic components of the cell surface. In this electrostatic "exchange-adsorption" model, cs1 and cs2 participate in binding of GrB to the cell surface, thereby promoting its uptake and eventual release into the cytoplasm.

Mannose 6-Phosphorylated Proteins Are Required for Tumor Necrosis Factor-induced Apoptosis

Whereas caspases are essential components in apoptosis, other proteases seem to be involved in programmed cell death. This study investigated the role of lysosomal mannose 6-phosphorylated proteins in tumor necrosis factor (TNF)-induced apoptosis. We report that fibroblasts isolated from patients affected with inclusion-cell disease (ICD), having a deficient activity of almost all lysosomal hydrolases, are resistant to the toxic effect of TNF. These mutant cells exhibited a defect in TNF-induced caspase activation, Bid cleavage, and release of cytochrome c. In contrast, TNF-induced p42/p44 MAPK activation and CD54 expression remained unaltered. Human ICD lymphoblasts and fibroblasts derived from mice nullizygous for Igf2 and the two mannose 6-phosphate (M6P) receptors, Mpr300 and Mpr46, which develop an ICD-like phenotype, were also resistant to CD95 ligand and TNF, respectively. Moreover, correction of the lysosomal enzyme defect of ICD fibroblasts, using a medium enriched in M6P-containing proteins, enabled restoration of sensitivity to TNF. This effect was blocked by exogenous M6P but not by cathepsin B or L inhibitors. Altogether, these findings suggest that some M6P-bearing glycoproteins modulate the susceptibility to TNF-induced apoptosis. As a matter of fact, exogenous tripeptidyl peptidase 1, a lysosomal carboxypeptidase, could sensitize ICD fibroblasts to TNF. These observations highlight the hitherto unrecognized role of some mannose 6-phosphorylated proteins such as tripeptidyl peptidase 1 in the apoptotic cascade triggered by TNF.

Coiling phagocytosis of Borrelia burgdorferi by primary human macrophages is controlled by CDC42Hs and Rac1 and involves recruitment of Wiskott-Aldrich syndrome protein and Arp2/3 complex

Lyme borreliosis is a multisystemic disorder primarily affecting the skin, nervous system, and joints. It is caused by the spirochete Borrelia burgdorferi sensu lato and is transmitted via ticks of the Ixodidae family. Persistence of borreliae within macrophages has been implicated in the often chronic history of borreliosis. The uptake of B. burgdorferi by professional phagocytes occurs predominantly by coiling phagocytosis, a host cell-driven process in which single pseudopods wrap around and engulf the spirochetes. In the present study, we investigated the molecular machinery and the signal transduction pathways controlling the formation of these unique uptake structures. We found that the phagocytosis of borreliae by primary human macrophages is accompanied by the formation of f-actin-rich structures, which in their morphological organization correspond well to the earlier described coiling pseudopods. Further experiments revealed that Wiskott-Aldrich Syndrome protein and Arp2/3 complex, major regulators of actin polymerization, are also recruited to these sites of actin accumulation. In addition, inhibition of an upstream regulator of Wiskott-Aldrich Syndrome protein, the Rho-family GTPase CDC42Hs, greatly inhibited the occurrence of borrelia-induced phagocytic uptake structures. Inhibition of Rac1, another Rho family GTPase, had a less-pronounced inhibitory effect, while blocking of Rho activity showed no discernible influence. These results suggest that basic mechanisms of actin polymerization that control other types of phagocytosis are also functional in the formation of the morphologically unique uptake structures in coiling phagocytosis. Our findings should enhance the understanding of the infection process of B. burgdorferi and contribute to devising new strategies for countering Lyme disease.

Mannose 6-phosphate/insulin-like growth factor II receptor is a death receptor for granzyme B during cytotoxic T cell-induced apoptosis

The serine proteinase granzyme B is crucial for the rapid induction of target cell apoptosis by cytotoxic T cells. Granzyme B was recently demonstrated to enter cells in a perforin-independent manner, thus predicting the existence of a cell surface receptor(s). We now present evidence that this receptor is the cation-independent mannose 6-phosphate/insulin-like growth factor receptor (CI-MPR). Inhibition of the granzyme B-CI-MPR interaction prevented granzyme B cell surface binding, uptake, and the induction of apoptosis. Significantly, expression of the CI-MPR was essential for cytotoxic T cell-mediated apoptosis of target cells in vitro and for the rejection of allogeneic cells in vivo. These results suggest a novel target for immunotherapy and a potential mechanism used by tumors for immune evasion.

Mutational analysis of the binding site residues of the bovine cation-dependent mannose 6-phosphate receptor

Mannose 6-phosphate receptors (MPRs) deliver soluble acid hydrolases to the lysosome in higher eukaryotic cells. The two MPRs, the cation-dependent MPR (CD-MPR) and the insulin-like growth factor II/cation-independent MPR, carry out this process by binding with high affinity to mannose 6-phosphate residues found on the N-linked oligosaccharides of their ligands. To elucidate the key amino acids involved in conveying this carbohydrate specificity, site-directed mutagenesis studies were conducted on the extracytoplasmic domain of the bovine CD-MPR. Single amino acid substitutions of the residues that form the binding pocket were generated, and the mutant constructs were expressed in transiently transfected COS-1 cells. Following metabolic labeling, mutant CD-MPRs were tested for their ability to bind pentamannosyl phosphate-containing affinity columns. Of the eight amino acids mutated, four (Gln-66, Arg-111, Glu-133, and Tyr-143) were found to be essential for ligand binding. In addition, mutation of the single histidine residue, His-105, within the binding site diminished the binding of the receptor to ligand, but did not eliminate the ability of the CD-MPR to release ligand under acidic conditions.

Calcitonin stimulates lysosomal enzyme release and uptake in LLC-PK1 cells

Renal tubular targeted hormones increase urinary excretion of a lysosomal enzyme, N-acetyl-beta-D-glucosaminidase (NAG). To elucidate the mechanism of this event, the calcitonin effect on NAG handling by LLC-PK1 cells was examined. Calcitonin (1 nM to 1 microM), phorbol myristate (10 nM to 1 microM), and ionomycin (1 to 10 microM) promoted NAG release without any increase in lactate dehydrogenase release or any reduction of mitochondrial dehydrogenase activity. Treatment with 100 nM calphostin C or 50 microM KN-93 partially reversed the calcitonin effect on NAG release. Calcitonin promoted secretion of fluorescence ceramide, a reporter of protein transport from Golgi apparatus to cell surface. Calcitonin-stimulated NAG release was partially inhibited by 10 microg/ml brefeldin A, a blocker of protein transport through the Golgi apparatus. Calcitonin accelerated cellular uptake of exogenous NAG, which was inhibited by low temperature, 0.1 mM monodansyl cadaverine (receptor-mediated endocytosis inhibitor), and 10 mM mannose-6-phosphate. Furthermore, calcitonin promoted progression of intracellular membranes stained by a fluorescence membrane marker, styryl pyridinium dye, from cell periphery to perinuclear regions (commonly referred to as recycling vesicles) and increased dye release from preloaded cells. Fluorescence release from the cells pre-loaded with FITC-labeled NAG or albumin was also stimulated by calcitonin. These calcitonin effects on endocytotic and re-exocytotic pathways were inhibited by 100 nM cytochalasin D, 100 nM nocodazole, 0.1 to 1 microM bafilomycin A1, or 0.1 mM monodansyl cadaverine. Increased urinary NAG excretion has been considered to reflect renal tubular damage. However, it was demonstrated here that stimulation of secretory and recycling pathways may be an alternative mechanism for calcitonin-induced enzymuria, which will become a new indicator of renal tubular response to this hormone.

alpha-Glucosidase and N-acetylglucosamine-6-sulphatase are the major mannose-6-phosphate glycoproteins in human urine

Most newly synthesized lysosomal enzymes contain a transient carbohydrate modification, mannose 6-phosphate (Man-6-P), which signals their vesicular transport from the Golgi to the lysosome via Man-6-P receptors (MPRs). We have examined Man-6-P glycoproteins in human urine by using a purified soluble fragment of the soluble cation-independent MPR (sCI-MPR) as a preparative and analytical affinity reagent. In a survey of urine samples from seven healthy subjects, the pattern of Man-6-P glycoproteins detected with iodinated sCI-MPR as a probe in a blotting assay was essentially identical in each, regardless of sex or age. Two bands of approx. 100 and 110 kDa were particularly prominent. Man-6-P glycoproteins in human urine were purified by affinity chromatography on immobilized sCI-MPR. Seven distinct bands revealed by SDS/PAGE and Coomassie Blue staining were subjected to N-terminal sequence analysis. The prominent 100 and 110 kDa Man-6-P glycoproteins were identified as N-acetylglucosamine-6-sulphatase and alpha-glucosidase respectively. This identification was confirmed by molecular mass determinations on the two major bands after deglycosylation. Sequence analysis revealed arylsulphatase A and several previously unidentified proteins as minor species. Man-6-P glycoproteins were also purified on an analytical scale to determine the proportion of a number of lysosomal enzyme activities represented by the mannose-6-phosphorylated forms. The lysosomal enzymes in urine containing the highest proportion of mannose-6-phosphorylated form were beta-mannosidase (82%), hexosaminidase (27%) and alpha-glucosidase (24%). The profiles of Man-6-P glycoproteins detected by blotting in urine and plasma were not similar, suggesting that the urinary species are not derived from the bloodstream.

Release behaviour of 5-fluorouracil from chitosan-gel microspheres immobilizing 5-fluorouracil derivative coated with polysaccharides and their cell specific recognition

In order to provide a device releasing drugs in a controlled manner and having targetability to specific organs or cells, chitosan-gel microspheres, CMS, crosslinked with glutaraldehyde, immobilizing 1-[N-(5-aminopentyl) carbamoyl]-5-fluorouracil, 1, coated with anionic polysaccharides, such as 6-O-carboxymethyl-N-acetyl-alpha-1,4-polygalactosamine (CM-NAPGA), 6-O-carboxymethyl-chitin, alginic acid and heparin, by polyelectrolyte complex membrane formation were prepared. When chitosan was crosslinked with glutaraldehyde, 1 was simultaneously immobilized into CMS by Schiff's base formation. Average diameter of CMS obtained was estimated to be about 0.5-1.0 micron by SEM observation. In physiological saline media, only free 5-FU was released from the CMS but 1 and any 5-FU derivative was not. Release rate of 5-FU from the CMS was reduced by coating with polyelectrolyte complex membrane of cationic chitosan and anionic polysaccharides. CMS coated with CM-NAPGA showed a lectin-mediated specific aggregation phenomenon by addition of Abrus precatorius agglutinin. Moreover, the CMS immobilizing 1 coated with CM-NAPGA showed higher growth-inhibitory effect against SK-Hep-1 (human hepatoma) cells in vitro than the CMS coated with other polysaccharides.

Binding receptors for alpha-L-fucosidase in human B-lymphoid cell lines

An established mechanism for directing newly made acid hydrolases to lysosomes involves acquisition of mannose 6-phosphate residues by the carbohydrate portion of acid hydrolases followed by binding to specific membrane-bound transport receptors and delivery to lysosomes. Two distinct phosphomannosyl receptors (CI-MPR and CD-MPR) have been identified. Alternative mechanisms for trafficking acid hydrolases exist. This report examines means for the possible receptor-mediated intracellular transport of alpha-L- fucosidase in lymphoid cells. The binding of alpha-L-fucosidase to intact cells and to total cell membrane preparations, in conjunction with immunoassays of solubilized membrane preparations, revealed the presence of CI-MPR and CD-MPR on human lymphoid and fibroblast cell lines. The mean level of CD-MPR in nine lymphoid cell lines was 7.2-fold greater than CI-MPR. The mean level of CI-MPR in two fibroblast lines was 3.8-fold greater than CD-MPR. The mean content of CI-MPR was 19.5-fold greater in the fibroblasts than in the lymphoid cells. The CD-MPR content of fibroblasts and lymphoid cells was nearly equivalent. Among these cell lines were a fibroblast and a lymphoid line from the same individual. These results indicate that human B-lymphoid cells are deficient in CI-MPR and suggest that modulation of expression of CI-MPR and CD-MPR in lymphoid cells differs from that in fibroblasts, including cell lines with identical genomes. No specific receptor capable of binding alpha-L-fucosidase independent of mannose 6-phosphate was demonstrable, despite published results that support the existence of a mannose 6-phosphate independent trafficking mechanism in lymphoid cells for this enzyme.

Cell- and ligand-specific dephosphorylation of acid hydrolases: evidence that the mannose 6-phosphatase is controlled by compartmentalization

Mouse L cells that possess the cation-independent mannose 6-phosphate (Man 6-P)/insulin-like growth factor (IGF) II receptor change the extent to which they dephosphorylate endocytosed acid hydrolases in response to serum (Einstein, R., and C. A. Gabel. 1989. J. Cell Biol. 109:1037-1046). To investigate the mechanism by which dephosphorylation competence is regulated, the dephosphorylation of individual acid hydrolases was studied in Man 6-P/IGF II receptor-positive and -deficient cell lines. 125I-labeled Man 6-P-containing acid hydrolases were proteolytically processed but remained phosphorylated when endocytosed by receptor-positive L cells maintained in the absence of serum; after the addition of serum, however, the cell-associated hydrolases were dephosphorylated. Individual hydrolases were dephosphorylated at distinct rates and to different extents. In contrast, the same hydrolases were dephosphorylated equally and completely after entry into Man 6-P/IGF II receptor-positive Chinese hamster ovary (CHO) cells. The dephosphorylation competence of Man 6-P/IGF II receptor-deficient mouse J774 cells was more limited. beta-Glucuronidase produced by these cells underwent a limited dephosphorylation in transit to lysosomes such that diphosphorylated oligosaccharides were converted to monophosphorylated species. The overall quantity of phosphorylated oligosaccharides associated with the enzyme, however, did not decrease within the lysosomal compartment. Likewise, beta-glucuronidase was not dephosphorylated when introduced into J774 cells via Fc receptor-mediated endocytosis. The CHO and J774 cell lysosomes, therefore, display opposite extremes with respect to their capacity to dephosphorylate acid hydrolases; within CHO cell lysosomes acid hydrolases are rapidly and efficiently dephosphorylated, but within J774 cell lysosomes the same acid hydrolases remain phosphorylated. This difference in processing indicates that lysosomes themselves exist in a dephosphorylation-competent and -incompetent state. Man 6-P-bearing acid hydrolases endocytosed by the L+ cells in the absence of serum were not distributed uniformly throughout the lysosomal compartment. The change in the dephosphorylation competence of L cells in response to serum suggests, therefore, that these cells contain multiple populations of lysosomes that differ with respect to their content of a mannose 6-phosphatase, and that serum factors affect the distribution of hydrolases between the different compartments.

Cell contact induces the synthesis of a lysosomal enzyme precursor in lymphocytes and its direct transfer to fibroblasts

The activity of a lysosomal enzyme, alpha-D-mannosidase (EC 3.2.1.24), increased markedly in normal lymphocytes when they were cultured together with fibroblasts from a patient with an inherited deficiency of this enzyme. Cell-to-cell contact was obligatory for this increase in activity, which also required new protein synthesis. The enzyme induced in the co-cultured lymphocytes was a high molecular weight form of alpha-D-mannosidase that was not detected in lymphocytes cultured alone, which had only the low molecular weight mature enzyme. It was this precursor form alone that was directly transferred to the mannosidosis fibroblasts, where it was present initially in organelles of low density. When the culture period was extended the lymphocyte precursor enzyme was transported to the heavy lysosomes in the recipient cells, and correctly processed to the functionally effective mature enzyme.

Lymphocytes transfer only the lysosomal form of alpha-D-mannosidase during cell-to-cell contact

We have examined the changes in the activities of the different types of alpha-D-mannosidase when fibroblasts from patients deficient in the lysosomal form of the enzyme are cultured together with normal lymphocytes. Our results show that whereas the mannosidosis cells acquired high levels of this enzyme, the activities of both the Golgi and the endoplasmic reticulum forms of alpha-D-mannosidase remained the same as in the fibroblasts cultured alone in the absence of lymphocytes. The increase in the activity of the lysosomal enzyme in the cocultured fibroblasts was not affected by the presence of mannose 6-phosphate or alpha-methyl mannoside, inhibitors of receptor- and lectin-mediated uptake of lysosomal enzymes, respectively, but it did require cell-to-cell contact. Ion-exchange HPLC and electrophoresis in polyacrylamide gradient gels showed that the acquired enzyme had the same elution profile and molecular size as the lysosomal form of the enzyme present in the lymphocytes. Immunoprecipitation studies using antibody specific for the lymphocyte type of lysosomal alpha-D-mannosidase confirmed that the increased activity in the cocultured mannosidosis cells resulted from the acquisition of the lymphocyte enzyme. Cytochemical examination revealed, however, that the transferred lymphocyte enzyme was localized in cytoplasmic organelles in the peripheral regions of the recipient fibroblasts. These results show that lymphocytes transfer only the lysosomal form of alpha-D-mannosidase during cell-to-cell contact with mannosidosis cells.

Characteristics of lysosomal phosphomannosyl-enzyme receptors in the rat heart

The receptor system recognizing mannose 6-phosphate groups of lysosomal enzymes has been characterized, e.g. in fibroblasts and liver cells. The purpose of this study was to demonstrate the presence of a phosphomannosyl receptor system in rat heart muscle. The characterization of receptors was accomplished with beta-N-acetylglucosaminidase (beta-GA) secreted by rat embryo fibroblasts after ammonium chloride stimulation. The receptor binding of ligand enzymes was saturated by adding increasing concentrations of beta-GA and the binding increased linearly when the content of membrane protein was increased. The binding of beta-GA was inhibited by mannose and glucose phosphates, especially mannose 6-phosphate. Mannose itself did not inhibit binding of the enzyme, showing that the binding was not mediated by mannose receptors. Alkaline phosphatase treatment of beta-GA decreased the binding of ligand enzymes to receptors. Alkaline conditions increased the dissociation of receptor-ligand complexes, whereas the dissociation was minimal between pH 5.5 and 6.5. The proportion of endogenous beta-GA activity in membranes, probably representing receptor-bound location, varied between 40 and 55% of the total activity in various parts of rat cardiac muscle. The differences in the content of phosphomannosyl receptors, however, were insignificant between various cardiac muscle samples. At the organelle level the highest specific binding capacity, as well as the highest endogenous beta-GA activity, was in the sarcolemmal fraction. These results suggest that phosphomannosyl receptors also function in the endocytosis and transport of lysosomal enzymes in cardiomyocytes, as well as in several other cell types studied.

Glucose phosphotransferase and intracellular trafficking

Glycoproteins containing phosphodiester-linked glucose residues have recently been described. The synthesis of this structure occurs due to the intact transfer of alpha glucose-1-phosphate from UDP-glucose and is catalyzed by the enzyme glucose phosphotransferase (GlcPTase). The endogenous acceptors for GlcPTase have been characterized as to molecular weight following incubation of selected homogenates with (beta 32P)UDP-glucose. These glycoproteins are distinct from the lysosomal hydrolases recognized by the GlcNAc phosphotransferase. The transfer of 32P from (beta 32P)UDP-Glc can also be detected when the nucleotide sugar is microinjected into the cytoplasm of individual neurons in Aplysia. The phosphorylated acceptors in this system seem to be predominantly two glycoproteins that are subjected to rapid axoplasmic transport. The possible role of this post-translational modification in the intracellular trafficking of a subset of newly synthesized glycoproteins is discussed.

Heterozygosity for phosphodiester glycosidase deficiency: a novel human mutation of lysosomal enzyme processing

We have carried out studies on the fibroblasts of III-3, a clinically normal Lebanese individual previously reported to have abnormally high plasma lysosomal enzyme levels. Mannose-6-phosphate (man-6-P) receptors in III-3 fibroblasts were found to be functioning normally, but the cells had only half normal levels of phosphodiester glycosidase activity. Pinocytosis of III-3 fibroblast secreted beta-hexosaminidase B (hex B) into Sandhoff disease fibroblasts was 18% of control, and the apparent KD for binding of III-3 hex B to man-6-P receptors was 3.7 X 10(-9) M compared to 1.25 X 10(-9) M for control enzyme. Hex B secreted by III-3 fibroblasts included an enzyme pool less electro-negative than control enzyme which had a very low affinity for man-6-P receptors and which did not bind to DEAE-Sephadex. Treatment of this abnormal hex B with exogenous placental phosphodiester glycosidase increased its binding to man-6-P receptors three-fold. Secretion rates of seven lysosomal enzymes from III-3 fibroblasts were, on average, twice as great as rates measured for two I-cell disease heterozygote fibroblast lines. The results suggest that III-3 fibroblasts are heterozygous for phosphodiester glycosidase deficiency. The possibility that an individual homozygous for this enzyme deficiency would develop I-cell disease is discussed.

Endocytosis of mannose-6-phosphate binding sites by mouse T-lymphoma cells

The endocytosis and intracellular transport of mannose-6-phosphate conjugated to bovine serum albumin (Man-6-P:BSA) by mouse T-lymphoma cells were investigated in detail using several methods of analysis, both morphological and biochemical. Man-6-P:BSA was labeled with fluorescein or 125I and used to locate both surface and intracellular Man-6-P binding sites by light or electron microscopy, respectively. Incubation of cells with either fluorescent- or 125I-labeled Man-6-P:BSA at 0 degree C revealed a uniform distribution of the Man-6-P binding sites over the cell surface. Competition experiments indicate that the Man-6-P:BSA binding sites on the cell surface are the same receptors that can recognize lysosomal hydrolases. After as little as 1 min incubation at 37 degrees C, endocytosis of Man-6-P binding sites was clearly observed to occur through regions of the plasma membrane and via vesicles that also bound anticlathrin antibody. After a 5-15-min incubation of cells at 37 degrees C, the internalized ligand was detected first in the cis region of the Golgi apparatus and then in the Golgi stacks using both autoradiography and immunocytochemistry to visualize the ligand. The appearance of Man-6-P:BSA in the Golgi region after 15-30 min was confirmed by subcellular fractionation, which demonstrated an accumulation of Man-6-P:BSA in light membrane fractions that corresponded with the Golgi fractions. After a 30-min incubation at 37 degrees C, the internalized Man-6-P binding sites were localized primarily in lysosomal structures whose membrane but not lumen co-stained for acid phosphatase. These results demonstrate a temporal participation of clathrin-containing coated vesicles during the initial endocytosis of Man-6-P binding sites and that one step in the Man-6-P:BSA transport pathway between plasma membrane and the lysosomal structure can involve a transit through the Golgi stacks.

Membrane lectins on human monocytes. Maturation-dependent modulation of 6-phosphomannose and mannose receptors

Freshly isolated human monocytes, which do not contain cell-surface mannose-specific receptors, bind mannose 6-phosphate and actively endocytose mannose 6-phosphate-bearing neoglycoproteins (6-P-Man-F-BSA). Three days after isolation, human monocytes endocytose very actively 6-P-Man-F-BSA as well as Man-F-BSA, and the endocytosed neoglycoproteins are rapidly degraded. These results were obtained in quantitative flow cytofluorometry by using a panel of fluoresceinylated sugar-substituted serum albumins (neoglycoproteins). Thus, in contrast to mannose receptors which appear only after maturation, mannose 6-phosphate receptors are already present on freshly isolated human monocytes.

Detection of sugar-binding proteins in membrane-depleted nuclei

Nuclear sugar-binding proteins were detected in membrane-depleted nuclei isolated from hamster BHK cells and mouse L 1210 leukemia cells by means of fluorescein-labelled neoglycoproteins. In fluorescence microscopy, the fluorescence was seen throughout the nucleus but was generally brighter over the nucleoli than over the rest of the nucleus. Flow cytofluorometry analysis demonstrated the presence of nuclear sugar-binding proteins for synthetic glycoproteins associated with different sugar residues. Among the nine neoglycoproteins used, four neoglycoproteins (namely alpha-rhamnosylated, alpha-glucosylated, N-acetyl-beta-glucosaminylated and alpha-mannosylated-6P-serum albumin) strongly labelled nuclei. Various controls strongly argue for the specificity of the nuclear labelling. The possibility that some of the sugar-binding proteins might correspond to endogenous nuclear lectins is considered.

Modification of the carbohydrate in ricin with metaperiodate-cyanoborohydride mixtures. Effects on toxicity and in vivo distribution

Attempts to target antibody-ricin conjugates (immunotoxins) to designated cell types in vivo may be thwarted by their rapid clearance by hepatic reticuloendothelial cells which have receptors that recognise oligosaccharide side chains on the toxin. The B-chain of ricin contains high mannose type oligosaccharides and the A-chain contains a complex unit (GlcNAc)2-Fuc-Xyl-(Man)4-6, all of which potentially could be recognised by the reticuloendothelial system. Treatment of ricin with a mixture of sodium metaperiodate and sodium cyanoborohydride at pH 3.5 resulted in oxidative cleavage of the carbohydrates and reduction of the aldehyde groups thus formed to primary alcohols. By conducting the modification procedure at acidic pH, both the possibility of Schiff's base formation between the aldehyde groups and amino groups in the protein and the possibility of non-specific oxidation of amino acids were minimised. The extent of the carbohydrate modification depended on the duration of treatment, resulting maximally in the destruction of 13 of the 18 mannose residues and of all xylose and fucose. The toxicity of the modified toxin to cells in culture declined by up to 90% as the carbohydrate was destroyed. This was not due to a reduced ability of the B-chain to bind to cells or of the A-chain to inactivate ribosomes. In contrast to the in vitro results, the toxicity of the modified toxin to mice and rats was elevated by up to fourfold. The modification greatly reduced the clearance of the toxin by non-parenchymal cells in the liver and prevented the damage to hepatic Kupffer and sinusoidal cells and to the red pulp of the spleen that is inflicted by the native toxin. The elevated toxicity to animals appears to be because the modified toxin evades the reticuloendothelial system and persists in the bloodstream for longer periods, thus resulting in lethal damage to vital tissues in the animal at lower dosage. The results suggest that immunotoxins prepared from modified ricin would not be readily cleared by the reticuloendothelial system and so be more effective at killing their target cells.

Beta-glucuronidase-positive erythrocytic inclusion bodies--a hitherto unknown phenomenon

A combined cytochemical and electron microscopical study has delineated a new type of an erythrocytic inclusion body. Enzyme cytochemically these inclusions are characterized by beta-glucuronidase as a marker enzyme. In part, the inclusions may contain acid phosphatase and ferritin. The inclusions develop in mature erythrocytes since beta-glucuronidase normally does not occur in erythroblasts and, in general, this type of inclusion body is not found in erythroblasts. Based upon our preliminary findings, the hypothesis is extended that beta-glucuronidase is taken up via receptor-mediated endocytosis into erythrocytes and is finally put into clustered cytolysosomal vaculoes, that account for the inclusion bodies as seen at light microscopy. Exogenous beta-glucuronidase might be contributed for by breakdown of cells (e.g. hepatocytes) producing this enzyme in considerable amounts numbers. This view is corroborated by the observation that most patients with beta-glucuronidase-positive inclusions suffered from various chronic disorders of the liver.

Receptor-mediated transport of acid hydrolases to lysosomes

Lysosomal enzymes are the products of 40-50 unlinked genes in the nucleus. Like membrane and secretory proteins, they are synthesized on membrane-bound ribosomes in the rough endoplasmic reticulum. They receive high-mannose oligosaccharide chains from lipid-linked intermediates on asparagine residues. They must be sorted from other proteins present in the lumen of the endoplasmic reticulum and delivered to the lysosomes. The best understood mechanism for this sorting and delivery involves the Man 6-P recognition system. The newly synthesized acid hydrolases acquire Man 6-P residues by a two-step reaction. First, GlcNAc 1-P is transferred to the C-6 position of the mannose residues which are present on the asparagine-linked high-mannose oligosaccharides. Then, N-acetylglucosamine residues are removed by the N-acetylglucosaminyl phosphoglycosidase to generate the Man 6-P monoester, which is capable of binding the Man 6-P receptor. Phosphorylated enzymes can then bind to Man 6-P receptors which collect into vesicles and bud off for delivery of enzymes to lysosomes. The region of the Golgi apparatus where the receptors containing newly synthesized enzymes bud off is not yet clear. Enzymes which fail to bind receptors are secreted. Some cell types express on their cell surfaces receptors which are capable of recapturing phosphorylated enzyme by receptor-mediated endocytosis. This secretion - recapture pathway provides an alternate route to lysosomes. Following delivery of enzyme to lysosomes, the enzymes undergo post-lysosomal processing by acid phosphotases, which remove the phosphomonoester groups, and acid proteases which reduce their size and trim off excess polypeptides. Although the evidence is very persuasive that enzymes can reach lysosomes by pathways that do not depend on the Man 6-P receptor, the mechanisms of Man 6-P receptor-independent segregation of acid hydrolases to lysosomes are totally unclear. In addition to this question, there are two other significant questions that remain to be answered. One of these is the precise intracellular route of newly synthesized enzyme. Where does the enzyme first bind receptor, and where does receptor actually bud off the Golgi apparatus to effect sorting? The second major question is really the central question of the mechanism of sorting of acid hydrolases. Although we know now that the sorting is effected through an enzyme which phosphorylates acid hydrolases, the question remains: How does the processing phosphotransferase distinguish acid hydrolases from other glycoproteins in the endoplasmic reticulum?(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced vesicular ingestion of nonenzymatically glucosylated proteins by capillary endothelium

Isolated capillary endothelial cells exhibit an enhanced vesicular ingestion of nonenzymatically glucosylated myoglobin and ovalbumin. Proteins were incubated in the presence of glucose and the extent of nonenzymatic glucosylation was assessed using NaB3H4 reduction. Unmodified and glucosylated proteins were fluorescently labeled, and vesicular ingestion was quantified using a system of isolated capillary endothelium. Glucosylated myoglobin exhibited a 3.5-fold greater rate of vesicular injection as compared to unmodified myoglobin during the initial 30 min of ingestion. Glucosylated ovalbumin was vesicularly ingested at a rate 15-fold greater than the rate observed for unmodified ovalbumin during the initial 30 min of ingestion. These results indicate that glucosylation of small proteins may alter their subsequent recognition by the endothelial cell plasma membrane and result in increased sequestration within micropinocytic vesicles.

Raman and infrared studies of homogeneous forms of acid phosphatase from rat liver

Raman spectra of acid phosphatase (orthophosphoric-monoester phosphohydrolase (acid optimum), EC 3.1.3.2) forms from rat liver in water solution, and infrared spectra of the same forms as thin films, have been investigated. The spectra show strong bands belonging to phosphodiester or phosphomonoester residues. These groups are modified during the postsynthetic modification of acid phosphatase and are probably connected with the process of bonding and splitting of mannose 6-phosphate and N-acetylglucosamine, in agreement with previous biochemical models for the intracellular transport of newly synthesized lysosomal hydrolases to lysosomes. Some other bands in the infrared spectra are assigned to vibrations of N-H groups which may belong to N-acetylglucosamine.

Five related Lebanese individuals with high plasma lysosomal hydrolases: a new defect in mannose-6-phosphate receptor recognition?

Five healthy related individuals in 3 generations of a Lebanese family have been found to have highly elevated plasma lysosomal enzyme levels inherited as a dominant Mendelian trait. The same enzymes in other extracellular fluids were within normal limits. While the pattern and extent of plasma enzyme elevation was similar to that found in mucolipidoses II and III, the physicochemical properties of the elevated enzymes were different from those of both control and I-cell disease plasma. Secretion of lysosomal hydrolases into cell media by fibroblasts from one of the individuals was increased two to seven times more than that from controls. The results suggest faulty recognition between lysosomal hydrolases and mannose-6-phosphate receptors. This could be caused by a defect either in the phosphodiesterase that normally uncovers mannose-6-phosphate hydrolase markers or in the mannose-6-phosphate receptor itself.

Transport and processing of beta-hexosaminidase in normal and mucolipidosis-II cultured fibroblasts. Effect of monensin and nigericin

The univalent-cation ionophores monensin (4.0 microM) and nigericin (0.5 microM) inhibited the abnormal excretion of beta-hexosaminidase from mucolipidosis-II cultured fibroblasts by 62 and 76% respectively, with a corresponding intracellular accumulation of the enzyme. As shown by lectin binding, the enzyme which accumulated in monensin-treated cells did not contain galactose residues, whereas the corresponding enzyme from nigericin-treated cells was galactosylated. The results suggest that monensin acts at an early point in the process of hydrolase glycosylation, and nigericin acts later, both presumably within the Golgi region, allowing the accumulation of different glycosylated forms of the enzyme. The intra- and extra-cellular distribution of beta-hexosaminidase in ionophore-treated normal cells was essentially unchanged, whereas concanavalin A precipitability of excreted enzyme was increased and its ability to be taken up by deficient fibroblasts was decreased. The bivalent-cation ionophore A23187 (1 microM) reduced beta-hexosaminidase excretion from mucolipidosis-II cells by 82% and by 96% when used with EGTA (1 mM). However, there was no intracellular accumulation of enzyme, suggesting that the effect of this ionophore was restricted to the inhibition of synthesis. It therefore appears that the actual transport of beta-hexosaminidase in mucolipidosis-II cells is affected by univalent-cation ionophores in a selective manner. These findings suggest that individual ionophores could be used to identify the sites of hydrolase oligosaccharide processing in the Golgi region by causing intermediate glycosylated forms of the transported hydrolase to accumulate in a specific Golgi compartment preceding the blocking site of the ionophore.

Effects of exogenous amines on mammalian cells, with particular reference to membrane flow

We have reviewed the evidence that amines accumulate in intracellular vesicles of low pH, such as lysosomes and endosomes. There is consequent elevation of intravesicular pH, and inhibition of receptor-ligand dissociation often results from this pH change. We have argued that the capacity for fusion of such vesicles is also reduced by the high pH. We suggest that the variety of effects of amines on membrane flow and macromolecular transport we describe are at least partly due to such reduced fusion (Figs. 1 and 2). We propose that an internal low pH may facilitate heterologous vesicle-vesicle and vesicle-plasma membrane fusion. There is some evidence that clathrin can accelerate phospholipid vesicle fusion in vitro at low pH (Blumenthal et al., 1983) but no direct evidence on the role of intravesicular pH. This idea is consistent not only with the preceding discussion, but also with the fact that the intracellular membrane-bound compartments least involved in fusion events (e.g. mitochondria) are of neutral or alkaline internal pH. Membrane fusion is certainly required for the formation of vesicles at the periphery of the Golgi apparatus, and possibly earlier in the transport and processing of biosynthetic products in the Golgi (Bergeron et al., 1982). Thus the accumulation of amines in the Golgi may be responsible for several effects on the flow of macromolecules along their translocation pathways. The status of the plasma membrane in this view is complex. It might be argued that the pH dictating the fusion step in endocytosis is that of the extracellular fluid, in which case the inhibitory effects of amines on this process are not explained. However, the rapidity of acidification of the newly formed endocytic vesicles allows the possibility that plasma membrane invaginations might temporarily sequester areas which are of lower pH than that of the bulk extracellular fluid even before fusion, since the proton pumping enzyme(s) are probably present on the plasma membrane. Were this the case, then an acid pH could again be a factor determining membrane fusion at the plasma membrane. The inhibition of endocytosis by weak bases thus may again reflect elevation of pH in a sequestered compartment. From the data on the dependence of response on the concentration of amines, we anticipate that most responses involving membrane flow will be biphasic, with inhibitory effects at low amine concentration, giving way to stimulatory ones at higher concentrations. We suggest that the reported dichotomy between different amines in intracellular membrane fusion systems (D'Arcy Hart, 1982) may result from this concentration dependence.(ABSTRACT TRUNCATED AT 400 WORDS)

I-cell disease (mucolipidosis II). Differential expression in satellite cells and mature muscle fibers

A well documented case of I-cell disease is presented. Light- and electron-microscopic studies of muscle revealed marked accumulation of characteristic I-cell inclusions in satellite cells and only scattered autophagic vacuoles in muscle fibers. Correlation with previous tissue culture studies indicated an amelioration of structural abnormalities with differentiation from satellite cell to mature muscle fiber. Histochemically, the muscle demonstrated paucity of type I fibers without evidence of denervation thus suggesting a developmental disturbance in motor unit organization. Selective type I fiber dysfunction and reduced satellite cell regenerative capacity may be related factors in the neuromuscular disability of patients with I-cell disease.

Specificity of thyroglobulin interactions with thyroid cells and membranes

Homologous species specificity is demonstrated with bovine and human thyroglobulin in which the two terminal sugars of the B carbohydrate chain, sialic acid and galactose have been removed by enzymatic hydrolysis. The species specificity is demonstrated by measuring the ability of the deglycosylated thyroglobulin derivatives to inhibit thyrotropin-induced increases in cAMP in human, rat and bovine thyroid cells in culture. Thus human-human or bovine-bovine interactions have higher activity coefficients by at least an order of magnitude than their heterologous counterparts. The homologous interactions are confirmed in binding studies and shown to be associated with negligible degradation of the bound ligand over a 24 hour period.

Recognition and receptor-mediated uptake of phosphorylated high mannose-type oligosaccharides by cultured human fibroblasts

The intracellular transport of newly synthesized lysosomal hydrolases to lysosomes requires the presence of one or more phosphorylated high mannose-type oligosaccharides per enzyme. A receptor that mediates mannose-6-PO4-specific uptake of lysosomal enzymes is expressed on the surface of fibroblasts and presumably accounts for the intracellular transport of newly synthesized enzymes to the lysosome. In this study, we examined the internalization of lysosomal enzyme-derived phosphorylated oligosaccharides by cultured human fibroblasts. Oligosaccharides of known specific activity bearing a single phosphate in monoester linkage were internalized with Kuptake of 3.2 X 10(-7) M, whereas oligosaccharides bearing two phosphates in monoester linkage were internalized with a Kuptake of 3.9 X 10(-8) M. Thus, phosphorylated high mannose-type oligosaccharides appear to be the minimal structure required for recognition and uptake by the fibroblast receptor. The finding that the Kuptake for monophosphorylated oligosaccharides is 100-fold less than the reported Ki for mannose-6-phosphate indicates that the fibroblast phosphomannosyl receptor contains a binding site that recognizes features of the oligosaccharide in addition to mannose-6-phosphate.

Comparative kinetics of phosphomannosyl receptor-mediated pinocytosis of fibroblast secretion acid hydrolases and glycopeptides prepared from them

In a previous report we demonstrated that phosphorylated oligosaccharides isolated from acid hydrolases were subject to pinocytosis by phosphomannosyl receptors present on the cell surface of human fibroblasts [9]. However, limiting quantities of oligosaccharides precluded detailed comparison of the kinetics of pinocytosis of these phosphorylated oligosaccharides to those of the acid hydrolases from which they were derived. In this report we present studies comparing the kinetics of pinocytosis of acid hydrolases from NH4Cl-induced fibroblast secretions with those of concanavalin A-binding glycopeptides prepared from them by pronase digestion. The uptake of both secretion acid hydrolases and 125I-labeled glycopeptides was linear for at least 3 hr, saturable, inhibited competitively by mannose 6-phosphate, and destroyed by prior treatment of the ligand with alkaline phosphatase. The inhibition constants of excess unlabeled glycopeptide for the uptake of 125I-labeled glycopeptides (Ki of 1.5 X 10(-6) M) and for the uptake of secretion acid hydrolases (Ki of 2.2 X 10(-6) M) were remarkably similar. Furthermore, the Ki for mannose 6-phosphate inhibition of pinocytosis of glycopeptide uptake (3 X 10(-5) M) compares closely to that previously determined for the pinocytosis of intact "high-uptake" acid hydrolases (3-6 X 10(-5) M). "High-uptake" fractions of both ligands were prepared and quantified by affinity chromatography on immobilized phosphomannosyl receptors purified from bovine liver. Only 10% of the concanavalin A-binding glycopeptides bound to the immobilized phosphomannosyl receptors, while 80% of the acid hydrolases from which they were prepared bound and were eluted with 10 mM mannose 6-phosphate. However, the fraction of each type of ligand that binds to the immobilized phosphomannosyl receptors accounts for all the uptake activity of that ligand.

Sugar-specific endocytosis of glycoproteins by Lewis lung carcinoma cells

Lewis lung carcinoma cells from tumors, metastasis nodules, or from culture bind fluorescent derivatives of neoglycoproteins containing alpha-D-glucose residues: This binding is competitively inhibited by neoglycoproteins containing alpha-D-glucose, by mannan, and by several other neoglycoproteins. Cell binding and uptake of the fluorescent derivatives of the neoglycoproteins was quantified by lysing the cells with an alkylpolyol (MAC 19 or MAC 18) and measuring the fluorescence intensity of the supernatant. The amount of cell-associated neoglycoprotein was higher at 37 degrees C than at 4 degrees C with LLC from tumor. The binding and uptake were inhibited by glycoconjugates containing alpha-D-glucose. These results suggest the presence of sugar specific receptors in Lewis lung carcinoma cells which are involved in a sugar-specific binding and endocytosis phenomenon. The implication of the existence of a carbohydrate-binding protein on the surface of Lewis lung carcinoma cells are discussed with regard to the in vivo behaviour of these cells, especially in relation to their metastatic properties and to the possibility of using neoglycoproteins as specific carriers of cytotoxic drugs. Hybrid molecules of gelonin and neoglycoprotein containing alpha-D-glucose were used as targetted toxin: The targetted toxin was found to bind to and to enter the intact cells and was 100 times more toxic than free drug.