Evidence for lysosomal enzyme recognition by human fibroblasts via a phosphorylated carbohydrate moiety

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.

Steps in the phosphorylation of the high mannose oligosaccharides of lysosomal enzymes

The phosphomannosyl recognition marker of acid hydrolases, which mediates their translocation to lysosomes, has been shown to be synthesized in two steps. First, N-acetylglucosamine 1-phosphate is transferred to an acceptor mannose by UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, resulting in a phosphate group in diester linkage between the outer N-acetylglucosamine and the inner mannose. Next, an a-N-acetylglucosaminyl phosphodiesterase removes the N-acetylglucosamine, leaving the phosphate in monoester linkage with the underlying mannose residue. This exposed phosphomannosyl residue serves as the essential component of a recognition marker which leads to binding to high-affinity receptors and subsequent translocation to lysosomes. We propose that the first enzyme in this scheme, N-acetylglucosaminylphosphotransferase, catalyses the initial, determining step by which newly synthesized acid hydrolases are distinguished from other newly synthesized glycoproteins and thus are eventually targeted to lysosomes. The absence of this enzyme activity, as in inclusion-cell (I-cell) disease and pseudo-Hurler polydystrophy, precludes the receptor-mediated targeting of newly synthesized acid hydrolases to lysosomes. As a consequence, the enzymes are secreted into the extracellular milieu.

Mannose 6-phosphate-independent endocytosis of beta-glucuronidase. II. Purification of a cation-dependent receptor from bovine liver

A new binding protein, which recognizes a specific peptide sequence from pronase digested bovine beta-glucuronidase, has been isolated from bovine liver membranes. Prior work has shown that this peptide (IIIb2) contains a Ser-X-Ser sequence, where X might be a posttranslational modified Trp. This receptor was detergent-extracted from total bovine liver membranes and purified by affinity chromatography on a bovine beta-glucuronidase-Sepharose and a IIIb2 peptide-Sepharose column. Binding of bovine beta-glucuronidase to the isolated receptor requires divalent cations, and their presence was necessary to maintain the receptor-ligand complex. Only the peptide sequence containing the fraction IIIb2 was able to impair the binding of the bovine enzyme to the receptor, no other peptide from bovine beta-glucuronidase had an effect on binding. When analyzed by SDS-PAGE under reducing conditions, two bands were observed, a major band of 78 kDa and a faint band of 72 kDa. Rabbit antibodies against this binding protein revealed the presence of the 78 kDa protein in membranes from bovine liver, human and bovine fibroblasts. These antibodies impaired human fibroblasts endocytosis of the bovine but not of the human beta-glucuronidase, which is taken up by a 300 kDa receptor that recognizes phosphomannosyl moieties in the enzyme.

Mannose 6-phosphate-independent endocytosis of beta-glucuronidase by human fibroblasts. I. Evidence for the existence of a membrane-binding activity

Prior work has shown that endocytosis of bovine beta-glucuronidase by human fibroblasts can be mediated by the existence of a Man6P-independent receptor for the recapture and targeting to lysosomes. In this study, we have isolated a peptide (IIIb2) from pronase digested bovine beta-glucuronidase that behaved as competitive inhibitor of the endocytosis of bovine beta-glucuronidase by human fibroblasts. This peptide contained a Ser-X-Ser sequence, where X is probably a posttranslational modified Trp. Antibodies raised against this peptide impaired the endocytosis of the bovine but not the human beta-glucuronidase, implying that the new recognition marker for the endocytosis of acid hydrolases might reside in a single discrete stretch of amino acid sequence. On the other hand, bovine beta-glucuronidase has been shown to bind specifically to receptors of human fibroblast membranes. The binding was saturable, divalent cation-dependent and was competitively inhibited by the IIIb2 peptide, but not by mannose 6-phosphate. Results presented suggested an interplay between manganese concentrations, temperature and pH on the dissociation of the beta-glucuronidase-receptor complexes. All together, these data reinforce the presence of two endocytic systems for the recapture and targeting of beta-glucuronidase in human fibroblasts.

Purification and characterization of recombinant human alpha-N-acetylglucosaminidase secreted by Chinese hamster ovary cells

alpha-N-Acetylglucosaminidase (EC 3.2.1.50) is a lysosomal enzyme that is deficient in the genetic disorder Sanfilippo syndrome type B. To study the human enzyme, we expressed its cDNA in Lec1 mutant Chinese hamster ovary (CHO) cells, which do not synthesize complex oligosaccharides. The enzyme was purified to apparent homogeneity from culture medium by chromatography on concanavalin A-Sepharose, Poros 20-heparin, and aminooctyl-agarose. The purified enzyme migrated as a single band of 83 kDa on SDS-PAGE and as two peaks corresponding to monomeric and dimeric forms on Sephacryl-300. It had an apparent K(m) of 0.22 mM toward 4-methylumbelliferyl-alpha-N-acetylglucosaminide and was competitively inhibited by two potential transition analogs, 2-acetamido-1,2-dideoxynojirimycin (K(i) = 0.45 microM) and 6-acetamido-6-deoxycastanospermine (K(i) = 0.087 microM). Activity was also inhibited by mercurials but not by N-ethylmaleimide or iodoacetamide, suggesting the presence of essential sulfhydryl residues that are buried. The purified enzyme preparation corrected the abnormal [(35)S]glycosaminoglycan catabolism of Sanfilippo B fibroblasts in a mannose 6-phosphate-inhibitable manner, but its effectiveness was surprisingly low. Metabolic labeling experiments showed that the recombinant alpha-N-acetylglucosaminidase secreted by CHO cells had only a trace of mannose 6-phosphate, probably derived from contaminating endogenous CHO enzyme. This contrasts with the presence of mannose 6-phosphate on naturally occurring alpha-N-acetylglucosaminidase secreted by diploid human fibroblasts and on recombinant human alpha-l-iduronidase secreted by the same CHO cells. Thus contrary to current belief, overexpressing CHO cells do not necessarily secrete recombinant lysosomal enzyme with the mannose 6-phosphate-targeting signal; this finding has implications for the preparation of such enzymes for therapeutic purposes.

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.

Long-term secretion of therapeutic proteins from genetically modified skeletal muscles

Protein delivery from genetically modified skeletal muscle has been reported previously. However, a stable and prolonged secretion was obtained in immunocompromised or newborn animals only. To evaluate the clinical relevance of this approach, we have transduced myoblasts from an adult beta-glucuronidase-deficient (MPS VII) mouse with retroviral vectors carrying either the human beta-glucuronidase cDNA or the murine erythropoietin (Epo) cDNA. The cells were then grafted into the tibialis anterior muscle of adult immunocompetent MPS VII recipients. Protein expression was controlled either by ubiquitous or muscle-specific transcriptional regulatory elements. Animals were analyzed over an 8-month period. The in situ detection of beta-glucuronidase activity revealed up to 60% of genetically modified myofibers in the recipient muscles. The human desmin promoter and enhancer showed the highest in vivo expression. Secretion of beta-glucuronidase induced a disappearance of lysosomal storage lesions in the liver and spleen of recipient animals. Delivery of Epo led to a permanent increase of hematocrit values over 3 months. These results showed that the transplantation of genetically modified myoblasts allowed a sustained secretion of recombinant proteins at therapeutic levels in immunocompetent adult mice. They suggest that the approach may be considered for human applications.

The two mannose 6-phosphate receptors transport distinct complements of lysosomal proteins

Mammalian cells express two different mannose 6-phosphate receptors (MPR 46 and MPR 300), which both mediate targeting of Man-6-P-containing lysosomal proteins to lysosomes. To assess the contribution of either and both MPRs to the transport of lysosomal proteins, fibroblasts were established from mouse embryos that were homozygous for disrupted alleles of either MPR 46 or MPR 300 or both MPRs. Fibroblasts missing both MPRs secreted most of the newly synthesized lysosomal proteins and were unable to maintain the catabolic function of lysosomes. The intracellular levels of lysosomal proteins decreased to < 20%, and undigested material accumulated in the lysosomal compartment. Fibroblasts lacking either MPR exhibited only a partial missorting and maintained, in general, half-normal to normal levels of lysosomal proteins. The same species of lysosomal proteins were found in secretions of double MPR-deficient fibroblasts as in secretions of single MPR-deficient fibroblasts, but at different ratios. This clearly indicates that neither MPR has an exclusive affinity for one or several lysosomal proteins. Furthermore, neither MPR can substitute in vivo for the loss of the other. It is proposed that the heterogeneity of the Man-6-P recognition marker within a lysosomal protein and among different lysosomal proteins has necessitated the evolution of two MPRs with complementary binding properties to ensure an efficient targeting of lysosomal proteins.

In vivo delivery of human alpha-L-iduronidase in mice implanted with neo-organs

Mucopolysaccharidose type I is a lysosomal storage disease caused by a deficiency in the enzyme alpha-L-iduronidase (IDUA). The existence of a secretory pathway for lysosomal enzymes and the capture of secreted molecules by distant cells through binding to mannose-6-phosphate receptors have provided a rationale for enzyme replacement therapy in lysosomal storage diseases. We have used genetically modified fibroblasts implanted into neo-organs as an in vivo delivery system for IDUA. The human IDUA cDNA was isolated and inserted into a retroviral vector where it was expressed from the phosphoglycerate kinase 1 gene promoter. MPS I fibroblasts transduced with this vector showed high levels of IDUA activity and secreted phosphorylated molecules that could be internalized by naive deficient cells. Neo-organs containing 2 x 10(7) IDUA-secreting cells were implanted into nude mice. Human and murine IDUA activities were measured in the liver and spleen of animals sacrificed 35-77 days after implantation. Human IDUA activity corresponded to 0.6-2.3% of the murine enzyme activity in the liver and to 0.1-0.3% in the spleen. These data indicated that human IDUA was secreted from neo-organs and internalized in distant tissues.

Long-term delivery of a lysosomal enzyme by genetically modified fibroblasts in dogs

We have evaluated the feasibility and efficacy of intraperitoneal implants (neo-organs) for protein delivery in large animals. Skin biopsies were taken from four healthy dogs. Primary fibroblast cultures were transduced with a retroviral vector coding for the human beta-glucuronidase. One to six lattices each containing 10(9) skin fibroblasts were implanted into the omentum of the donor animal. Laparotomies performed at regular intervals showed vascularized neo-organs without local inflammation. Human beta-glucuronidase levels equivalent to 0.8 to 3.1% of the endogenous canine activity were detected for up to 340 days on liver biopsy samples. These results indicate that neo-organs can be considered for the long-term delivery of therapeutic proteins or enzymes in humans.

Trypanosoma cruzi: inhibition of metacyclogenesis by mannose

Metacyclogenesis of Trypanosoma cruzi epimastigotes was evaluated in a medium supplemented with Triatoma infestans intestinal homogenate in the presence of sugars and derivates as are mannose, galactose, fucose, N-acetylglucosamine, mannose 6-P, and fructose 1,6-P at a concentration of 25 mM. Only mannose significantly inhibited metacyclogenesis. Sodium metaperiodate and trypsin treatment of the intestinal homogenate also inhibited differentiation. In our opinion there exists a proteinic factor in the intestine of the vector that promotes metacyclogenesis and is incorporated by the parasite. Treatment of the intestinal homogenate with alkaline phosphatase had no effect. Instead, high ionic strength in the medium (0.4 M NaCl) strongly inhibited metacyclogenesis indicating that, in these conditions, the possible binding of the differentiation factor to the parasite surface was inhibited.

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.

Redistribution of cathepsin B activity from the endosomal-lysosomal pathway in chick intestine within 3 min of calcium absorption

Earlier work has suggested that calcium-containing lysosomes are involved in 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-stimulated intestinal absorption of the divalent cation. In the present report immunofluorescent labelling studies on fixed frozen sections of chick intestine were undertaken to determine whether lysosomes could respond to calcium transport conditions in less than 5 min. Tissue prepared from vitamin D-deficient chicks dosed with vehicle or 1.3 nmol of 1,25(OH)2D3 15 h prior to use was immunofluorescently labelled for cathepsin B, a lysosomal protease. In the absence of calcium absorption, punctate staining was found in the region below the terminal web, and more diffusely in the cytoplasm. The intensity of staining was noticeably greater in sections from 1,25(OH)2D3-treated than control chicks. In sections prepared after 3 min of calcium absorption, cathepsin B staining was localized near the basal and lateral membranes of the epithelial cells. After 30 min of transport, the protease was found in the villus core regardless of vitamin D status; however, immunoreactivity within the epithelial cells of 1,25(OH)2D3-treated chick intestine had returned to pretransport intensity, whereas that of controls had not. To further investigate the specificity of the cathepsin B antibody, the intracellular compartmentalization of the protease was determined by biochemical methods. Using dosing procedures and calcium transport times equivalent to those for the immunofluorescent studies mucosae were collected by scraping, homogenized, and subcellular fractions prepared by a combination of differential and Percoll gradient centrifugation. In the absence of calcium transport, cathepsin B-specific activity was enhanced in whole homogenates, endocytic vesicles, and a lysosomal fraction prepared from intestinal epithelium of 1,25(OH)2D3-treated chicks, relative to vitamin D-deficient controls. After 3 min of calcium absorption, a profound (approximately 4-fold) decrease in endocytic vesicle cathepsin B activity was found regardless of vitamin D status, as well as a similar marked decrease in lysosomes prepared from vitamin D-deficient, but not -treated, chicks. After 30 min of calcium transport, endocytic vesicles prepared from either vitamin D-deficient or 1,25(OH)2D3-treated birds had recovered cathepsin B activity to pretransport levels. However, lysosomes prepared from rachitic chicks remained low in protease levels relative to equivalent fractions from dosed chicks. Thus, biochemical analysis of cathepsin B activity in putative endocytic vesicles and lysosomes supports the intracellular redistribution of protease visualized with immunofluorescence microscopy.

Role of the Golgi apparatus in cellular pathology

The Golgi apparatus response to pathological disorders is predominantly as an intermediary component of membrane biogenesis where it is involved in processing, sorting and secretion of materials via secretory granules, and in the formation of lysosomes. A common initial response of the Golgi apparatus to any stress is an alteration or cessation of secretory activity. In the transformed cell, the Golgi apparatus is altered both morphologically and biochemically, suggesting a shift from a secretory to a membrane-generating mode of functioning. However, since fewer or less well-developed Golgi apparatus are frequently found in transformed cells, analytical methods of membrane isolation developed for normal tissues may not always yield equivalent results when applied to tumors. Cell surface alterations characteristic of malignant cells may result from modifications occurring at the level of the Golgi apparatus. Some lysosomal dysfunctions may result from underglycosylation of acid hydrolases by the Golgi apparatus. The use of cell-free systems between endoplasmic reticulum and Golgi apparatus or within Golgi apparatus cisterane is providing a new approach to the elucidation of the role of the Golgi apparatus in normal as well as pathological states.

Uptake of extracellular enzyme by a novel pathway is a major determinant of cathepsin L levels in human macrophages

The phorbol myristate acetate (PMA)-differentiated myelomonocytic cell line, THP-1, and human alveolar macrophages contain the cysteine proteinase cathepsin L. This enzyme is synthesized as a 43-kD proenzyme and processed to the active 25-kD form. Differentiation of THP-1 cells in the presence of human serum resulted in an increase in the size of the vacuolar compartment and the accumulation of more 25-kD cathepsin L antigen, as compared with THP-1 cells differentiated in the presence of fetal calf serum. Cells cultured in both types of sera have equivalent levels of cathepsin L mRNA. Metabolic labeling experiments demonstrated equivalent rates of synthesis, processing to the active form, and persistence in both culture conditions. An extracellular source of enzyme was documented by immunoblotting human serum which demonstrated 25-kD cathepsin L antigen; furthermore, we demonstrated that both THP-1 cells, differentiated in human serum, and human alveolar macrophages take up the 43-kD proenzyme and process it to the 25-kD form. Thus, human serum contains a factor(s) that induces both a marked increase in the size of the vacuolar compartment in differentiated THP-1 cells and a novel pathway that is responsible for the uptake and processing of extracellular cathepsin L. The activity of this inducible pathway is a major determinant of levels of intracellular cathepsin L. Cathepsin L is a potent elastase and the regulation of its uptake and processing may play a role in the pathogenesis of disease processes characterized by the destruction of elastin, such as pulmonary emphysema.

Deficient glycosylation of arylsulfatase A in pseudo arylsulfatase-A deficiency

Deficient arylsulfatase-A activity is diagnostic of a neurodegenerative human lysosomal storage disease, metachromatic leukodystrophy. Paradoxically, similar enzyme deficiency also occurs in normal individuals, who are known as being pseudo arylsulfatase-A deficient. We showed previously that this phenotype is associated with a structural gene mutation that produces an exceptionally labile enzyme. We now report on the nature and consequence of this mutation. When the mutant arylsulfatase-A is deglycosylated by endoglycosidase H, only one smaller molecular species was generated, instead of the two from the normal enzyme. This is consistent with the loss of one of the two N-linked oligosaccharide side chains known to be present on the wild-type enzyme. Quantitative analysis of mannose and leucine incorporation showed that the mutant enzyme incorporated two- to tenfold less mannose than the normal enzyme on a molar basis. This deficient glycosylation was specific to arylsulfatase-A. Another lysosomal enzyme not affected in this mutation, beta-hexosaminidase, was glycosylated normally in the mutant cells. The remaining single oligosaccharide side chain released from the mutant arylsulfatase-A by pronase digestion was normally processed to complex and high-mannose forms. However, the high-mannose side chains contained 30% fewer phosphorylated residues than those of the normal enzyme. Nevertheless, this reduced level of phosphorylation did not prevent targeting of the mutant enzyme to the lysosomes, a process normally mediated through phosphorylated mannose residues. In conclusion, pseudo arylsulfatase-A deficiency is a unique human mutation associated with reduced glycosylation and phosphorylation of a lysosomal enzyme with the loss of one of the two carbohydrate side chains. The mutation results in greatly reduced enzyme stability, thus indicating a role for oligosaccharides in maintaining enzyme stability within the degradative environment of the lysosomes. However, the residual catalytic activity or subcellular targeting of the mutant enzyme was not affected. These properties probably account for the benign clinical presentation of pseudo arylsulfatase-A deficiency.

A calcium-binding, asparagine-linked oligosaccharide is involved in skeleton formation in the sea urchin embryo

We have previously identified a 130-kD cell surface protein that is involved in calcium uptake and skeleton formation by gastrula stage embryos of the sea urchin Strongylocentrotus purpuratus (Carson et al., 1985. Cell. 41:639-648). A monoclonal antibody designated mAb 1223 specifically recognizes the 130-kD protein and inhibits Ca+2 uptake and growth of the CaCO3 spicules produced by embryonic primary mesenchyme cells cultured in vitro. In this report, we demonstrate that the epitope recognized by mAb 1223 is located on an anionic, asparagine-linked oligosaccharide chain on the 130-kD protein. Combined enzymatic and chemical treatments indicate that the 1223 oligosaccharide contains fucose and sialic acid that is likely to be O-acetylated. Moreover, we show that the oligosaccharide chain containing the 1223 epitope specifically binds divalent cations, including Ca+2. We propose that one function of this negatively charged oligosaccharide moiety on the surfaces of primary mesenchyme cells is to facilitate binding and sequestration of Ca+2 ions from the blastocoelic fluid before internalization and subsequent deposition into the growing CaCO3 skeleton.

Adsorptive endocytosis of lysosomal enzymes by human fibroblasts: presence of two different functional systems that deliver an acid hydrolase to lysosomes

Endocytosis of human spleen beta-glucuronidase by human fibroblasts can be completely impaired by the competitive inhibitor mannose 6-phosphate or by pretreatment with acid phosphatase or endoglycosidases H or F. However, endocytosis of bovine spleen and liver beta-glucuronidase is partially impaired by the same treatments, suggesting that the bovine enzyme contains two endocytosis recognition markers located in separate enzyme domains. The mannose 6-phosphate recognition marker seems to be responsible for approximately 23% of the bovine enzyme endocytosis. The existence of two lysosomal endocytosis systems in human fibroblasts is supported by the following facts: (a) the rate of endocytosis of mannose 6-phosphate-containing human beta-glucuronidase was not affected by the presence of high levels of the bovine enzyme (which has only the other marker). (b) Anti-215K mannose 6-phosphate receptor antibodies selectively impair the endocytosis of the beta-glucuronidase containing mannose 6-phosphate. (c) Weak bases exert a differential effect on human and bovine endocytosis. beta-Glucuronidase internalized by either system is targeted to secondary lysosomes of human beta-glucuronidase-deficient fibroblasts, where it is able to degrade accumulated glycosaminoglycans. These results suggest that human fibroblasts have two different and independent endocytic systems for targeting of acid hydrolases to lysosomes.

Intracellular localization of beta-glucuronidase in fibroblasts after direct transfer from macrophages

The subcellular distribution of beta-glucuronidase acquired by deficient human fibroblasts during co-culture with peritoneal macrophages was compared with that taken up by receptor-mediated endocytosis. Labelled enzyme taken up via receptors was located initially in a low-density endosomal fraction and was transferred to lysosomes within a few minutes. The beta-glucuronidase acquired during 24 h of co-culture was present almost entirely within lysosomes and had a distribution profile identical with that of endogenous beta-hexosaminidase. Monensin prevented transfer of radiolabelled enzyme from endosomes to lysosomes and had a similar effect on the distribution of enzyme acquired by direct transfer, causing beta-glucuronidase to accumulate within endosomes. When the temperature was lowered from 37 degrees C to 19 degrees C, the rate of transfer of enzyme from endosomes to lysosomes was decreased during both direct transfer and indirect receptor-mediated endocytosis. These results show that a lysosomal enzyme acquired by direct transfer during cell-to-cell contact follows a similar intracellular route and has a similar distribution to that of enzymes taken up via cell-surface receptors.

Developmental aspects of the lysosomal apparatus. Membrane affinity of N-acetyl-beta-D-glucosaminidase in developing rat liver

The affinity of N-acetyl-beta-D-glucosaminidase by membranes of liver was studied in rats of different ages including fetuses at day 18 of gestation. It was found that membrane bound enzyme activity, extractable with 0.6 KCl, increases from fetal life to adulthood reaching a peak 9 days after birth. In binding assays it was found that the enzyme of fetal, 9 days old, or adult rat has high affinity for membranes of the corresponding age. These bindings were saturable and with a similar KD, but the number of receptor sites was lowest in the fetal stage, and reached a peak 9 days after birth. The fetal enzyme did not bind to adult membranes. These results suggest that the transport system of hepatic lysosomal enzymes undergoes post-natal changes which are synchronic with other parameters of lysosomal apparatus maturation studied by us and other authors as total enzyme activity and intracellular digestion of macromolecules.

Protein glycosylation in yeast

S. cerevisiae contains many mannose-rich glycoproteins that possess N- and O-linked carbohydrate chains, and both types may even occur on one and the same protein. The steps in the synthesis of asparagine-linked chains begin with assembly and transfer of the lipid-linked precursor to protein in a way common to all eucaryotes. Subsequent modifications lead to mannosyl extensions of various lengths, but complex type carbohydrate structures are not formed. Oligosaccharides O-linked to serine/threonine consist exclusively of mannose in S. cerevisiae. The mannose residue attached directly to the protein is transferred from Dol-P-Man in a unique way, which has been observed so far for fungal cells only. The cellular localization of the glycosylation reactions is summarized and the problem of transmembrane translocation of the sugar precursors at the ER and the Golgi is discussed. Some aspects of secretory (sec) and asparagine linked glycosylation (alg) mutants have been covered, and the various hypotheses related to the possible functions of this costly protein modification process are discussed. The article may also be helpful for those, who want to exploit the yeast's protein synthesizing machinery by genetically manipulating the cells.

beta-Galactosidase from rat epididymal fluid is bound by a recognition site attached to membranes of the epididymis different from the phosphomannosyl receptor

In order to know if the beta-galactosidase of the rat epididymal fluid, as other secreted acid hydrolases, carries a marker in its molecule, we studied the binding of this enzyme to cellular membranes of the epididymal tissue. The binding, like that mediated by the phosphomannosyl receptor, was saturable, did not require calcium, had a Kd in the nM range and was inhibited by phosphatase or metaperiodate treatment of the enzyme. However fructose 6-phosphate derivates were more effective competitive inhibitors than mannose 6-phosphate. The binding capacity of the membranes were extractable with Triton X-100 and incorporable into liposomes. Trypsin inhibited the binding capacity of Triton extracts but it did not affect the affinity of intact cellular membranes for beta-galactosidase. The results suggest that a phosphorylated carbohydrate of the enzyme is bound by a recognizing site of the cellular membranes different from the phosphomannosyl receptor.

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.

Improved purification and some properties of bovine lysosomal carboxypeptidase B

Lysosomal carboxypeptidase B (peptidyl-L-amino-acid hydrolase, EC 3.4.18.1) from bovine spleen purified to apparent homogeneity was found to have a molecular weight of 52 000 in the absence and of 25 000 in the presence of urea, determined by gel filtration, indicating the existence of two subunits of identical size. The amount of approx. 15% carbohydrate estimated after cleavage by endoglycosidase H was shown to be insignificant for enzymatic activity. The isoelectric focusing separated lysosomal carboxypeptidase B into several protein bands - each enzymatically active - with a range of isoelectric points between 4.6 and 5.2. The titration of the sulphydryl group in the active site of the enzyme with the proteinase inhibitor E-64 yielded one thiol group per molecule. A maximum of activation was achieved by the addition of selenocystamine together with dithioerythritol and EDTA in the incubation solution. Under these conditions the carboxypeptidase hydrolyzed benzoylglycylarginine (80 kat/mol enzyme), benzoylarginine amide (38 kat/mol enzyme) and carbobenzoxyglutaryltyrosine (110 kat/mol enzyme). Slight enzymatic activities towards benzoylarginine 2-naphthylamide and benzoylarginine p-nitroanilide could be measured. With the oxidized insulin B chain, lysosomal carboxypeptidase B exhibited only carboxypeptidase activity.

Comparison of biosynthesis and subcellular distribution of lysozyme and lysosomal enzymes in U937 monocytes

Using metabolic labelling and sucrose density fractionation we compared the synthesis of lysozyme and lysosomal enzymes in human monocytic U937 cells. In pulse-chase experiments in sucrose density gradients, the intracellular radioactively labelled lysozyme distributed similarly to cathepsin D and beta-hexosaminidase. With the aid of immunochemical detection in Western blots, the steady-state distribution of lysozyme was found to be slightly different from that of beta-hexosaminidase; relatively more lysozyme was present in fractions sedimenting between lysosomes and the Golgi apparatus. The observed distribution of the lysozyme antigen with a prominent peak in the lysosomal fraction was in striking contrast to the broad distribution of the lysozyme activity. The difference was explained by a bias in the determination of the activity of lysozyme by the 'lysoplate' diffusion assay.

Bovine generalised glycogenosis type II. Uptake of lysosomal alpha-glucosidase by cultured skeletal muscle and reversal of glycogen accumulation

acid alpha-glucosidase (EC 3.2.1.20) was purified from fetal bovine muscle by affinity chromatography on concanavalin A and Sephadex G-100 and added to the culture medium of mature muscle cultures from animals affected by glycogenosis type II. The enzyme activity in homogenates of treated cultures was significantly increased within 4 h of the addition of enzyme, was maximal by 18 h and the internalised activity was stable for at least 48 h after the removal of the enzyme from the culture medium. The acid alpha-glucosidase activity was internalised with an uptake constant of 300 nM and a Vmax of uptake of 133 nmol/h per mg protein. The glycogen concentration in affected cultures treated with exogenous acid alpha-glucosidase for 24 h had decreased by 20% and after a further 24 h of culture was comparable to the concentration observed in cultures from non-affected animals.

The effect of the lysosomotropic drug chloroquine on the binding of N-acetyl-beta-D-glucosaminidase to mannose-6-phosphate recognizing receptors of rat liver

The binding of N-acetyl-beta-D-glucosaminidase to rat liver receptors was studied in the presence of chloroquine. The association rate constant was not affected in the presence of the drug, while the dissociation rate constant and consequently the equilibrium dissociation binding constant significatively decreased. This results may explain effects of chloroquine on lysosomal enzyme transport found in cultured cells by other authors.

Iodinated fibroblast beta-glucuronidase as a ligand for receptor-mediated endocytosis

Antibodies raised to human placental beta-glucuronidase were shown to cross-react with the beta-glucuronidase secreted by mouse 3T3 fibroblasts, but did not react with other lysosomal enzymes. The beta-glucuronidase secreted by 3T3 cells was purified 15000-fold by chromatography on an affinity column made from this antibody and resolved into a single component, of Mr 68000, by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Iodinated samples of purified enzyme were taken up into mouse peritoneal macrophages by receptor-mediated endocytosis at a rate similar to that calculated previously for unlabelled enzyme, and uptake was competitively inhibited by yeast mannan. Binding of beta-glucuronidase to macrophages was saturable, with a Kd of 7 X 10(-9)l/mol, an affinity comparable with that calculated for the binding of mannosylated bovine serum albumin (Kd 1.3 X 10(-9)l/mol), a ligand specific for mannose receptors. Four times as many molecules of mannosylated albumin (12000) as of beta-glucuronidase (3000), however, bound to each cell. This purification and iodination procedure did not therefore have any adverse effect on the uptake properties of secreted beta-glucuronidase, and provides a ligand with which to investigate binding and specific endocytosis into a range of different types of cell.

Lysosomal enzyme binding to mouse P388D1 macrophage membranes lacking the 215-kDa mannose 6-phosphate receptor: evidence for the existence of a second mannose 6-phosphate receptor

Mouse P388D1 macrophages target newly synthesized acid hydrolases to lysosomes in spite of their lack of the 215-kDa mannose 6-phosphate (Man-6-P) receptor. We now report that these cells contain a membrane-associated Man-6-P receptor that is distinct from the previously described receptor. The new receptor binds lysosomal enzymes containing phosphomannosyl residues. This binding is inhibited by Man-6-P or by pretreatment of the lysosomal enzymes with alkaline phosphatase. Lysosomal enzyme binding occurs at neutral pH and dissociation of the bound ligand occurs at low pH values comparable to those found within endosomes or lysosomes. The new receptor differs from the 215-kDa Man-6-P receptor in two ways. It has an absolute requirement for divalent cations and is unable to bind Dictyostelium discoideum lysosomal enzymes, which contain methylphosphomannosyl residues rather than the usual phosphomannosyl monoesters. Based on the difference in cation requirement, we suggest that the 215-kDa receptor be referred to as Man-6-P receptor CI (cation independent) and the new receptor as Man-6-P receptor CD (cation dependent). We conclude that the Man-6-P receptor CD functions in the targeting of newly synthesized acid hydrolases to lysosomes in P388D1 macrophages.

Internalization of blocking antibodies against mannose-6-phosphate specific receptors

Antibodies against mannose-6-phosphate specific receptors inhibit the receptor-dependent endocytosis of exogenous lysosomal enzymes as well as the sorting of endogenous lysosomal enzymes. This inhibition was correlated with an apparent loss of the receptors. We report here that treatment of cells with the antibody results in the formation of receptor-antibody complexes that are not extracted by the procedure used for the solubilization of receptors prior to immunoprecipitation and detection of the receptor. The apparent loss of receptors is observed with both native antibody and the F(ab)2 fragments, but not with Fab fragments. In contrast the transport of lysosomal enzymes is inhibited by all three forms of the antibody. The inhibition is ascribed to masking by the antibody of the enzyme-binding site in the receptor. The inhibition of the sorting of endogenous lysosomal enzymes by antibodies added to the medium indicates that the mannose-6-phosphate specific receptors at the sorting site are in dynamic equilibrium with those at the cell surface. The receptor-antibody complexes formed at the cell surface appear to cycle between the cell surface and intracellular membranes. A fraction of the internalized antibodies dissociates from the receptors and is degraded after transfer into lysosomes. Complexing with Fab increases the concentration of the receptor in the lysosomes and decreases 2- to 3-fold the half-life of the receptor.

Binding and internalization of lysosomal enzymes by primary cultures of rat glia

Highly purified cultures of rat astrocytes and oligodendrocytes were examined for their ability to bind and internalize lysosomal enzymes. Astrocytes displayed a saturable uptake of beta-glucosidase and beta-galactosidase. The uptake was specifically inhibited by mannose-6-phosphate but not by several other sugars or sugar phosphates, indicating that the process was mediated by mannose-6-phosphate receptors. When cells were allowed to take up 125I-beta-glucosidase for 1 hr at 37 degrees C and subcellular organelles were isolated, the enzyme was shown to comigrate with a lysosomal organelle marker enzyme, suggesting that the enzyme was targeted to lysosomes. Astrocyte receptors were probed directly by binding of 125I labeled beta-glucosidase to astrocyte membranes at 4 degrees C. Binding was saturable and competitively inhibited by mannose-6-phosphate. In contrast to the astrocytes, cultured oligodendrocytes showed no specific binding or uptake of the lysosomal enzymes. Immunocytochemical staining of mixed glial cultures supported the biochemical data; only the astrocytes stained positive with anti-mannose-6-phosphate receptor antibodies.

Antibody to mannose 6-phosphate specific receptor induces receptor deficiency in human fibroblasts

Polyclonal antibodies to the mannose 6-phosphate specific receptor from human liver inhibited the endocytosis of lysosomal enzymes in fibroblasts by greater than 95% and enhanced 3-20-fold the secretion of precursors of lysosomal enzymes in these cells. Exposing fibroblasts for 4 h to antibody resulted in loss of greater than 90% of the membrane-bound receptors. If fibroblasts were treated with the antibody in the presence of CBZ-Phe-Ala-CHN2, an inhibitor of lysosomal cysteine proteinases, the receptor and smaller degradation products are recovered in dense lysosomes. In treated cells 18-58% of total receptor-related polypeptides were recovered in dense lysosomes. In control cells less than 4% of the receptor was found in the lysosomal fraction. We conclude from these results that normally the receptor is spared from lysosomal degradation. When tagged with antibody, however, the receptor is transported into lysosomes and degraded. The loss of intracellular receptors involved in segregation of newly synthesized lysosomal enzymes indicates an exchange between the former and the plasma membrane-bound receptors.

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.

Effect of monensin on intracellular transport and receptor-mediated endocytosis of lysosomal enzymes

In cultured human fibroblasts we observed that monensin, a Na+/H+-exchanging ionophore, (i) inhibits mannose 6-phosphate-sensitive endocytosis of a lysosomal enzyme, (ii) enhances secretion of the precursor of cathepsin D, while inhibiting secretion of the precursors of beta-hexosaminidase, (iii) induces secretion of mature beta-hexosaminidase and mature cathepsin D, and (iv) inhibits carbohydrate processing in and proteolytic maturation of the precursors remaining within the cells; this last effect appears to be secondary to an inhibition of the transport of the precursors. If the treated cells are transferred to a monensin-free medium, about half of the accumulated precursors are secreted, and the intracellular enzyme is converted into the mature form. Monensin blocks formation of complex oligosaccharides in lysosomal enzymes. In the presence of monensin, total phosphorylation of glycoproteins is partially inhibited, whereas the secreted glycoproteins are enriched in the phosphorylated species. The suggested inhibition by monensin of the transport within the Golgi apparatus [Tartakoff (1980) Int. Rev. Exp. Pathol. 22, 227-250] may be the cause of some of the effects observed in the present study (iv). Other effects (i, ii) are rather explained by interference by monensin with the acidification in the lysosomal and prelysosomal compartments, which appears to be necessary for the transport of endocytosed and of newly synthesized lysosomal enzymes.

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)

Direct enzyme transfer from lymphocytes is specific

Lymphocytes are known to interact directly with other cells in vivo and in vitro, and have recently been shown to transfer the lysosomal enzyme, beta-glucuronidase, to fibroblasts from patients with an inherited deficiency of the enzyme. This process requires cell-cell contact, is unaffected by inhibitors of 'classical' receptor-mediated endocytosis and is abolished by inhibitors of protein synthesis. Although it is not yet known to what extent the transfer of enzymes by direct cellular interaction is a general phenomenon, a similar mechanism could possibly be involved in the transfer of other lysosomal enzymes in vivo and in the exchange of protein in vitro. We show here that the direct transfer of enzymes from lymphocytes to fibroblasts is restricted to only certain lysosomal enzymes.

Post-translational processing reactions involved in the biosynthesis of lysosomal alpha-N-acetylgalactosaminidase in cultured human fibroblasts

The synthesis and processing of alpha-N-acetylgalactosaminidase and its oligosaccharides were studied by metabolic labeling of human skin fibroblasts with [2-3H]mannose or 32Pi, immunoprecipitation of the enzyme, gel electrophoresis of the immunoprecipitates, and examination of the radioactive oligosaccharides recovered from protein bands excised from the gels. The data suggest that the enzyme was first synthesized as a Mr = 65,000 precursor which was then processed to a mature Mr = 48,000 enzyme; only the Mr = 65,000 precursor was immunoprecipitated from the culture medium. The oligosaccharides were separated into two chromatographic species by Bio-Gel P-4 fractionation. The more retained species were determined to be high-mannose oligosaccharides containing 7 to 9 mannose residues. A portion of the more highly excluded oligosaccharides from the Mr = 65,000 band was hydrolyzed by alkaline phosphatase, and the resulting oligosaccharides migrated with the same mobility as Man8-9GlcNAc. This alkaline phosphatase-sensitive peak could also be labeled with 32Pi. These observations indicate that alpha-N-acetylgalactosaminidase was synthesized as a higher molecular weight precursor which contained phosphorylated high-mannose oligosaccharides.