A poly-proline II helix in YadA from Yersinia enterocolitica serotype O:9 facilitates heparin binding through electrostatic interactions

Poly-proline II helices are secondary structure motifs frequently found in ligand-binding sites. They exhibit increased flexibility and solvent exposure compared to the strongly hydrogen-bonded α-helices or β-strands and can therefore easily be misinterpreted as completely unstructured regions with an extremely high rotational freedom. Here, we show that the adhesin YadA of Yersinia enterocolitica serotype O:9 contains a poly-proline II helix interaction motif in the N-terminal region. The motif is involved in the interaction of YadAO:9 with heparin, a host glycosaminoglycan. We show that the basic residues within the N-terminal motif of YadA are required for electrostatic interactions with the sulfate groups of heparin. Biophysical methods including CD spectroscopy, solution-state NMR and SAXS all independently support the presence of a poly-proline helix allowing YadAO:9 binding to the rigid heparin. Lastly, we show that host cells deficient in sulfation of heparin and heparan sulfate are not targeted by YadAO:9 -mediated adhesion. We speculate that the YadAO:9 -heparin interaction plays an important and highly strain-specific role in the pathogenicity of Yersinia enterocolitica serotype O:9.

In Vitro Methods to Study the Golgi Apparatus Role in Proteoglycan and Glycosaminoglycan Synthesis

Subcellular fractionation is an introductory step in a variety of experimental approaches designed to study intracellular components, like membranes and organelle systems. Subcellular fractions enriched in membranes of the Golgi apparatus of mammalian cells have been isolated to address localization and activity of proteins, including enzymes, to study intracellular membrane transport mechanisms, and to reconstitute in vitro cellular processes associated with the Golgi apparatus. Here, I describe methods to purify Golgi membranes by subcellular fractionation, to assay nucleotide sulfate (PAPS) uptake into Golgi vesicles, and to measure sulfate incorporation into in vitro synthesized glycosaminoglycans.

The life cycle and enigmatic egress of coronaviruses

There has been considerable recent interest in the life cycle of Severe Acute Respiratory Syndrome Coronavirus‐2 (SARS‐CoV‐2), the causative agent of the Covid‐19 pandemic. Practically every step in CoV replication—from cell attachment and uptake via genome replication and expression to virion assembly has been considered as a specific event that potentially could be targeted by existing or novel drugs. Interference with cellular egress of progeny viruses could also be adopted as a possible therapeutic strategy; however, the situation is complicated by the fact that there is no broad consensus on how CoVs find their way out of their host cells. The viral nucleocapsid, consisting of the genomic RNA complexed with nucleocapsid proteins obtains a membrane envelope during virus budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)–Golgi interface. From here, several alternative routes for CoV extracellular release have been proposed. Strikingly, recent studies have shown that CoV infection leads to the disassembly of the Golgi ribbon and the mobilization of host cell compartments and protein machineries that are known to promote Golgi‐independent trafficking to the cell surface. Here, we discuss the life cycle of CoVs with a special focus on different possible pathways for virus egress.

Proteoglycan synthesis in conserved oligomeric Golgi subunit deficient HEK293T cells is affected differently, depending on the lacking subunit

The Conserved Oligomeric Golgi (COG) complex is an eight subunit protein complex associated with Golgi membranes. Genetic defects affecting individual COG subunits cause congenital disorders of glycosylation (CDGs), due to mislocalization of Golgi proteins involved in glycosylation mechanisms. While the resulting defects in N-and O-glycosylation have been extensively studied, no corresponding study of proteoglycan (PG) synthesis has been undertaken. We here show that glycosaminoglycan (GAG) modification of PGs is significantly reduced, regardless which COG subunit that is missing in HEK293T cells. Least reduction was observed for cells lacking COG1 and COG8 subunits, that bridge the A and B lobes of the complex. Lack of these subunits did not reduce GAG chain lengths of secreted PGs, which was reduced in cells lacking any other subunit (COG2-7). COG3 knock out (KO) cells had particularly reduced ability to polymerize GAG chains. For cell-associated GAGs, the mutant cell lines, except COG4 and COG7 KO, displayed longer GAG chains than wild-type cells, indicating that COG subunits play a role in cellular turnover of PGs. In light of the important roles PGs play in animal development, the effects KO of individual COG subunits have on GAG synthesis could explain the variable severity of COG associated CDGs.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.

Cell and context-dependent sorting of neuropathy-associated protein NDRG1 - insights from canine tissues and primary Schwann cell cultures

BACKGROUND

Mutations in the N-myc downstream-regulated gene 1 (NDRG1) can cause degenerative polyneuropathy in humans, dogs, and rodents. In humans, this motor and sensory neuropathy is known as Charcot-Marie-Tooth disease type 4D, and it is assumed that analogous canine diseases can be used as models for this disease. NDRG1 is also regarded as a metastasis-suppressor in several malignancies. The tissue distribution of NDRG1 has been described in humans and rodents, but this has not been studied in the dog.

RESULTS

By immunolabeling and Western blotting, we present a detailed mapping of NDRG1 in dog tissues and primary canine Schwann cell cultures, with particular emphasis on peripheral nerves. High levels of phosphorylated NDRG1 appear in distinct subcellular localizations of the Schwann cells, suggesting signaling-driven rerouting of the protein. In a nerve from an Alaskan malamute homozygous for the disease-causing Gly98Val mutation in NDRG1, this signal was absent. Furthermore, NDRG1 is present in canine epithelial cells, predominantly in the cytosolic compartment, often with basolateral localization. Constitutive expression also occurs in mesenchymal cells, including developing spermatids that are transiently positive for NDRG1. In some cells, NDRG1 localize to centrosomes.

CONCLUSIONS

Overall, canine NDRG1 shows a cell and context-dependent localization. Our data from peripheral nerves and primary Schwann cell cultures suggest that the subcellular localization of NDRG1 in Schwann cells is dynamically influenced by signaling events leading to reversible phosphorylation of the protein. We propose that disease-causing mutations in NDRG1 can disrupt signaling in myelinating Schwann cells, causing disturbance in myelin homeostasis and axonal-glial cross talk, thereby precipitating polyneuropathy.

Ephrin-B3 binds both cell-associated and secreted proteoglycans

The ephrin family of membrane proteins binds Eph tyrosine kinase receptors. We have previously shown that ephrin-B3 also binds to heparan sulfate proteoglycans (HSPGs). We now show that ephrin-B3 can bind both secretory and cell associated PGs, such as agrin, collagen XVIII, Perlecan, and CD44, and indicate that such interaction with cell associated PGs involves a complex including 20 and 45 kDa proteins. Ephrin-B3 binding to HEK-293T cells is blocked by a secretory variant of CD44 (v3-v10), while over-expression of membrane associated CD44 increased ephrin-B3 binding. In addition, ephrin-B3 precipitated CD44 expressed by the oral squamous carcinoma cell line H376. Moreover, ephrin-B3 binding affinities to heparin and CD44 in solution was strong. In conclusion, we have identified secretory and cell associated PGs with high ability to bind ephrin-B3 and suggest that ephrin-B3 can bind to a protein complex organized by a membrane associated PG.

The extracellular matrix of eggshell displays anti-inflammatory activities through NF-κB in LPS-triggered human immune cells

Avian eggshell membrane (ESM) is a natural biomaterial that has been used as an alternative natural bandage on burned and cut skin injuries for >400 years in Asian countries, and is available in large quantities from egg industries. Our aim was to characterize ESM that was separated and processed from egg waste, and to study whether this material possesses anti-inflammatory properties, making it suitable as an ingredient in industrial production of low cost wound healing products. Our results show that the processed ESM particles retain a fibrous structure similar to that observed for the native membrane, and contain collagen, and carbohydrate components such as hyaluronic acid and sulfated glycosaminoglycans, as well as N-glycans, mostly with uncharged structures. Furthermore, both processed ESM powder and the ESM-derived carbohydrate fraction had immunomodulation properties in monocytes and macrophage-like cells. Under inflammatory conditions induced by lipopolysaccharide, the ESM powder and the isolated carbohydrate fraction reduced the activity of the transcription factor nuclear factor-κB. The expression of the immune regulating receptors toll-like receptor 4 and ICAM-1, as well as the cell surface glycoprotein CD44, all important during inflammation response, were down-regulated by these fractions. Interestingly, our experiments show that the two fractions regulated cytokine secretion differently: ESM depressed inflammation by increased secretion of the anti-inflammatory cytokine IL-10 while the carbohydrate fraction reduced secretions of the pro inflammatory cytokines IL-1β and IL-6. Also, the phosphorylation of p65 and p50 subunits of nuclear factor-κB, as well as nuclear localization, differed between processed ESM powder and carbohydrate fraction, suggesting different down-stream regulation during inflammation. In conclusion, processed ESM powder and its soluble carbohydrate components possess anti-inflammatory properties, demonstrating the potential of ESM as a novel biological wound dressing for treatment of chronic inflammatory wounds.

Determination of 3'-phosphoadenosine-5'-phosphosulfate in cells and Golgi fractions using hydrophilic interaction liquid chromatography-mass spectrometry

3'-Phosphoadenosine-5'-phosphosulfate (PAPS) is a key player in the sulfation of biomolecules, but methods for selective measurements are lacking. A liquid chromatography-mass spectrometry (LC-MS) approach for measuring PAPS was developed. A central feature of the method was employing hydrophilic interaction liquid chromatography (HILIC), which is highly suited for separating very polar/charged compounds, and is compatible with electrospray MS. Using simple instrumentation, the analysis time per sample was below 10min and the method was characterized by easy sample preparation. The method was used to monitor decreasing levels of PAPS as function of sodium chlorate treatment (an inhibitor of PAPS synthesis) in whole-cell lysates as well as Golgi-fractions. The method allowed PAPS to be chromatographically separated from ADP and ATP, which can interfere with measurements if a less resolving LC-MS method is used.

Determinants of Glycosaminoglycan (GAG) Structure

Proteoglycans (PGs) are glycosylated proteins of biological importance at cell surfaces, in the extracellular matrix, and in the circulation. PGs are produced and modified by glycosaminoglycan (GAG) chains in the secretory pathway of animal cells. The most common GAG attachment site is a serine residue followed by a glycine (-ser-gly-), from which a linker tetrasaccharide extends and may continue as a heparan sulfate, a heparin, a chondroitin sulfate, or a dermatan sulfate GAG chain. Which type of GAG chain becomes attached to the linker tetrasaccharide is influenced by the structure of the protein core, modifications occurring to the linker tetrasaccharide itself, and the biochemical environment of the Golgi apparatus, where GAG polymerization and modification by sulfation and epimerization take place. The same cell type may produce different GAG chains that vary, depending on the extent of epimerization and sulfation. However, it is not known to what extent these differences are caused by compartmental segregation of protein cores en route through the secretory pathway or by differential recruitment of modifying enzymes during synthesis of different PGs. The topic of this review is how different aspects of protein structure, cellular biochemistry, and compartmentalization may influence GAG synthesis.

PAPST1 regulates sulfation of heparan sulfate proteoglycans in epithelial MDCK II cells

Proteoglycan (PG) sulfation depends on activated nucleotide sulfate, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Transporters in the Golgi membrane translocate PAPS from the cytoplasm into the organelle lumen where PG sulfation occurs. Silencing of PAPS transporter (PAPST) 1 in epithelial MDCK cells reduced PAPS uptake into Golgi vesicles. Surprisingly, at the same time sulfation of heparan sulfate (HS) was stimulated. The effect was pathway specific in polarized epithelial cells. Basolaterally secreted proteoglycans (PGs) displayed an altered HS sulfation pattern and increased growth factor binding capacity. In contrast, the sulfation pattern of apically secreted PGs was unchanged while the secretion was reduced. Regulation of PAPST1 allows epithelial cells to prioritize between PG sulfation in the apical and basolateral secretory routes at the level of the Golgi apparatus. This provides sulfation patterns that ensure PG functions at the extracellular level, such as growth factor binding.

Serglycin is implicated in the promotion of aggressive phenotype of breast cancer cells

Serglycin is a proteoglycan expressed by some malignant cells. It promotes metastasis and protects some tumor cells from complement system attack. In the present study, we show for the first time the in situ expression of serglycin by breast cancer cells by immunohistochemistry in patients' material. Moreover, we demonstrate high expression and constitutive secretion of serglycin in the aggressive MDA-MB-231 breast cancer cell line. Serglycin exhibited a strong cytoplasmic staining in these cells, observable at the cell periphery in a thread of filaments near the cell membrane, but also in filopodia-like structures. Serglycin was purified from conditioned medium of MDA-MB-231 cells, and represented the major proteoglycan secreted by these cells, having a molecular size of ~ 250 kDa and carrying chondroitin sulfate side chains, mainly composed of 4-sulfated (~ 87%), 6-sulfated (~ 10%) and non-sulfated (~ 3%) disaccharides. Purified serglycin inhibited early steps of both the classical and the lectin pathways of complement by binding to C1q and mannose-binding lectin. Stable expression of serglycin in less aggressive MCF-7 breast cancer cells induced their proliferation, anchorage-independent growth, migration and invasion. Interestingly, over-expression of serglycin lacking the glycosaminoglycan attachment sites failed to promote these cellular functions, suggesting that glycanation of serglycin is a pre-requisite for its oncogenic properties. Our findings suggest that serglycin promotes a more aggressive cancer cell phenotype and may protect breast cancer cells from complement attack supporting their survival and expansion.

Reduced sulfation of chondroitin sulfate but not heparan sulfate in kidneys of diabetic db/db mice

Heparan sulfate proteoglycans are hypothesized to contribute to the filtration barrier in kidney glomeruli and the glycocalyx of endothelial cells. To investigate potential changes in proteoglycans in diabetic kidney, we isolated glycosaminoglycans from kidney cortex from healthy db/+ and diabetic db/db mice. Disaccharide analysis of chondroitin sulfate revealed a significant decrease in the 4-O-sulfated disaccharides (D0a4) from 65% to 40%, whereas 6-O-sulfated disaccharides (D0a6) were reduced from 11% to 6%, with a corresponding increase in unsulfated disaccharides. In contrast, no structural differences were observed in heparan sulfate. Furthermore, no difference was found in the molar amount of glycosaminoglycans, or in the ratio of hyaluronan/heparan sulfate/chondroitin sulfate. Immunohistochemical staining for the heparan sulfate proteoglycan perlecan was similar in both types of material but reduced staining of 4-O-sulfated chondroitin and dermatan was observed in kidney sections from diabetic mice. In support of this, using qRT-PCR, a 53.5% decrease in the expression level of Chst-11 (chondroitin 4-O sulfotransferase) was demonstrated in diabetic kidney. These results suggest that changes in the sulfation of chondroitin need to be addressed in future studies on proteoglycans and kidney function in diabetes.

Proteoglycan synthesis and Golgi organization in polarized epithelial cells

A large number of complex glycosylation mechanisms take place in the Golgi apparatus. In epithelial cells, glycosylated protein molecules are transported to both the apical and the basolateral surface domains. Although the prevailing view is that the Golgi apparatus provides the same lumenal environment for glycosylation of apical and basolateral cargo proteins, there are indications that proteoglycans destined for the two opposite epithelial surfaces are exposed to different conditions in transit through the Golgi apparatus. We will here review data relating proteoglycan and glycoprotein synthesis to characteristics of the apical and basolateral secretory pathways in epithelial cells.

Glycosaminoglycan secretion in xyloside treated polarized human colon carcinoma Caco-2 cells

Polarized epithelial cells like Madin-Darby canine kidney (MDCK) and CaCo-2 cells synthesize and secrete proteoglycans (PGs), mostly of heparan sulphate (HS) type in direction of the basal extracellular matrix, but also some in the apical direction. MDCK cells possess the capacity to synthesize chondroitin sulphate (CS) PGs that are mainly secreted into the apical medium, a process that is enhanced in the presence of hexyl-beta-D: -xyloside. We have now tested the capacity of several xylosides to enhance glycosaminoglycan (GAG) chain secretion from the human colon carcinoma cell line CaCo-2 in the differentiated and non-differentiated state. In these cells, benzyl-beta-D: -xyloside was a potent initiator of CS chains, which for these cells were predominantly secreted into the basolateral medium. Xylosides with other aglycone groups mediated only minor changes in GAG secretion. Although benzyl-beta-D: -xyloside stimulated the basolateral CS-GAG secretion in both differentiated and undifferentiated CaCo-2 cells, basolateral secretion of trypsin-like activity was dramatically enhanced in undifferentiated cells, but not significantly altered in differentiated cells.

Epigenetics: methylation-associated repression of heparan sulfate 3-O-sulfotransferase gene expression contributes to the invasive phenotype of H-EMC-SS chondrosarcoma cells

Heparan sulfate proteoglycans (HSPGs), strategically located at the cell-tissue-organ interface, regulate major biological processes, including cell proliferation, migration, and adhesion. These vital functions are compromised in tumors, due, in part, to alterations in heparan sulfate (HS) expression and structure. How these modifications occur is largely unknown. Here, we investigated whether epigenetic abnormalities involving aberrant DNA methylation affect HS biosynthetic enzymes in cancer cells. Analysis of the methylation status of glycosyltransferase and sulfotransferase genes in H-HEMC-SS chondrosarcoma cells showed a typical hypermethylation profile of 3-OST sulfotransferase genes. Exposure of chondrosarcoma cells to 5-aza-2'-deoxycytidine (5-Aza-dc), a DNA-methyltransferase inhibitor, up-regulated expression of 3-OST1, 3-OST2, and 3-OST3A mRNAs, indicating that aberrant methylation affects transcription of these genes. Furthermore, HS expression was restored on 5-Aza-dc treatment or reintroduction of 3-OST expression, as shown by indirect immunofluorescence microscopy and/or analysis of HS chains by anion-exchange and gel-filtration chromatography. Notably, 5-Aza-dc treatment of HEMC cells or expression of 3-OST3A cDNA reduced their proliferative and invading properties and augmented adhesion of chondrosarcoma cells. These results provide the first evidence for specific epigenetic regulation of 3-OST genes resulting in altered HSPG sulfation and point to a defect of HS-3-O-sulfation as a factor in cancer progression.

A secretory Golgi bypass route to the apical surface domain of epithelial MDCK cells

Proteins leave the endoplasmic reticulum (ER) for the plasma membrane via the classical secretory pathway, but routes bypassing the Golgi apparatus have also been observed. Apical and basolateral protein secretion in epithelial Madin-Darby canine kidney (MDCK) cells display differential sensitivity to Brefeldin A (BFA), where low concentrations retard apical transport, while basolateral transport still proceeds through intact Golgi cisternae. We now describe that BFA-mediated retardation of glycoprotein and proteoglycan transport through the Golgi apparatus induces surface transport of molecules lacking Golgi modifications, possessing those acquired in the ER. Low concentrations of BFA induces apical Golgi bypass, while higher concentrations were required to induce basolateral Golgi bypass. Addition of the KDEL ER-retrieval sequence to model protein cores allowed observation of apical Golgi bypass in untreated MDCK cells. Basolateral Golgi bypass was only observed after the addition of BFA or upon cholesterol depletion. Thus, in MDCK cells, an apical Golgi bypass route can transport cargo from pre-Golgi organelles in untreated cells, while the basolateral bypass route is inducible.

Alternative translation initiation generates cytoplasmic sheep prion protein

Cytoplasmic localization of the prion protein (PrP) has been observed in different species and cell types. We have investigated this poorly understood phenomenon by expressing fusion proteins of sheep prion protein and green fluorescent protein ((GFP)PrP) in N2a cells, with variable sequence context surrounding the start codon Met(1). (GFP)PrP expressed with the wild-type sequence was transported normally through the secretory pathway to the cell surface with acquisition of N-glycan groups, but two N-terminal fragments of (GFP)PrP were detected intracellularly, starting in frame from Met(17). When (GFP)PrP was expressed with a compromised Kozak sequence ((GFP)PrP*), dispersed intracellular fluorescence was observed. A similar switch from pericellular to intracellular PrP localization was seen when analogous constructs of sheep PrP, without inserted GFP, were expressed, showing that this phenomenon is not caused by the GFP tag. Western blotting revealed a reduction in glycosylated forms of (GFP)PrP*, whereas the N-terminal fragments starting from Met(17) were still present. Formation of these N-terminal fragments was completely abolished when Met(17) was replaced by Thr, indicating that leaky ribosomal scanning occurs for normal sheep PrP and that translation from Met(17) is the cause of the aberrant cytoplasmic localization observed for a fraction of the protein. In contrast, the same phenomenon was not detected upon expression of similar constructs for mouse PrP. Analysis of samples from sheep brain allowed immunological detection of N-terminal PrP fragments, indicating that sheep PrP is subject to similar processing mechanisms in vivo.

Temporary fatigue and altered extracellular matrix in skeletal muscle during progression of heart failure in rats

Patients with congestive heart failure (CHF) experience increased skeletal muscle fatigue. The mechanism underlying this phenomenon is unknown, but a deranged extracellular matrix (ECM) might be a contributing factor. Hence, we examined ECM components and regulators in a rat postinfarction model of CHF. At various time points during a 3.5 mo-period after induction of CHF in rats by left coronary artery ligation, blood, interstitial fluid (IF), and muscles were sampled. Isoflurane anesthesia was employed during all surgical procedures. IF was extracted by wicks inserted intermuscularly in a hind limb. We measured cytokines in plasma and IF, whereas matrix metalloproteinase (MMP) activity and collagen content, as well as the level of glycosaminoglycans and hyaluronan were determined in hind limb muscle. In vivo fatigue protocols of the soleus muscle were performed at 42 and 112 days after induction of heart failure. We found that the MMP activity and collagen content in the skeletal muscles increased significantly at 42 days after induction of CHF, and these changes were time related to increased skeletal muscle fatigability. These parameters returned to sham levels at 112 days. VEGF in IF was significantly lower in CHF compared with sham-operated rats at 3 and 10 days, but no difference was observed at 112 days. We conclude that temporary alterations in the ECM, possibly triggered by VEGF, are related to a transient development of skeletal muscle fatigue in CHF.

Overexpression of the 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 increases sulfation of chondroitin sulfate in the apical pathway of MDCK II cells

The canine 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 fused to GFP was stably expressed with a typical Golgi localization in MDCK II cells (MDCK II-PAPST1). The capacity for PAPS uptake into Golgi vesicles was enhanced to almost three times that of Golgi vesicles isolated from untransfected cells. We have previously shown that chondroitin sulfate proteoglycans (CSPGs) are several times more intensely sulfated in the basolateral than the apical secretory pathway in MDCK II cells (Tveit H, Dick G, Skibeli V, Prydz K. 2005. A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells. J Biol Chem. 280:29596-29603). Here we demonstrate that increased availability of PAPS in the Golgi lumen enhances the sulfation of CSPG in the apical pathway several times, while sulfation of CSPGs in the basolateral pathway shows minor changes. Sulfation of heparan sulfate proteoglycans is essentially unchanged. Our data indicate that CSPG sulfation in the apical pathway of MDCK II cells occurs at suboptimal conditions, either because the sulfotransferases involved have high K(m) values, or there is a lower PAPS concentration in the lumen of the apical secretory route than in the basolateral counterpart.

Secretion of proteases in serglycin transfected Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells, which do not normally express the proteoglycan (PG) serglycin, were stably transfected with cDNA for human serglycin fused to a polyhistidine tag (His-tag). Clones with different levels of serglycin mRNA expression were generated. One clone with lower and one with higher serglycin mRNA expression were selected for this study. 35S-labelled serglycin in cell fractions and conditioned media was isolated using HisTrap affinity chromatography. Serglycin could also be detected in conditioned media using western blotting. To investigate the possible importance of serglycin linked to protease secretion, enzyme activities using chromogenic substrates and zymography were measured in cell fractions and serum-free conditioned media of the different clones. Cells were cultured in both the absence and presence of phorbol 12-myristate 13-acetate (PMA). In general, enzyme secretion was strongly enhanced by treatment with PMA. Our analyses revealed that the clone with the highest serglycin mRNA expression, level of HisTrap isolated 35S-labelled serglycin, and amount of serglycin core protein as detected by western blotting, also showed the highest secretion of proteases. Transfection of serglycin into MDCK cells clearly leads to changes in secretion levels of secreted endogenous proteases, and could provide further insight into the biosynthesis and secretion of serglycin and potential partner molecules.

Differences in the apical and basolateral pathways for glycosaminoglycan biosynthesis in Madin–Darby canine kidney cells

Serglycin with a green fluorescent protein tag (SG-GFP) expressed in epithelial Madin-Darby canine kidney cells is secreted mainly (85%) into the apical medium, but the glycosaminoglycan (GAG) chains on the SG-GFP protein core secreted basolaterally (15%) carry most of the sulfate added during biosynthesis (Tveit et al. (2005) J. Biol. Chem., 280, 29596-29603). Here we report further differences in apical and basolateral GAG synthesis. The less intensely sulfated chondroitin sulfate (CS) chains on apically secreted SG-GFP are longer than CS chains attached to basolateral SG-GFP, whereas the heparan sulfate (HS) chains are of similar lengths. When the supply of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) is limited by chlorate treatment, the synthesis machinery maintains sulfation of HS chains on basolateral SG-GFP until it is inhibited at 50 mM chlorate, whereas basolateral CS chains lose sulfate already at 12.5 mM chlorate and become longer. Apically, incorporation of 35S-sulfate into CS is reduced to a lesser extent at higher chlorate concentrations than basolateral CS, although apical CS is less intensely sulfated than basolateral CS in control cells. Similar to what was found for basolateral HS, sulfation of apical HS was not reduced at chlorate concentrations below 50 mM. Also, protein-free, xyloside-based GAG chains secreted basolaterally are more intensely sulfated than their apical counterpart, supporting the view that separate apical and basolateral pathways exist for GAG synthesis and sulfation. Introduction of benzyl beta-d-xyloside (BX) to the GAG synthesis machinery reduces the apical secretion of SG-GFP dramatically and also the modification of SG-GFP by HS.

Proteolytic processing of the ovine prion protein in cell cultures

The cellular compartment and purpose of the proteolytic processing of the prion protein (PrP) are still under debate. We have studied ovine PrP constructs expressed in four cell lines; murine neuroblastoma cells (N2a), human neuroblastoma cells (SH-SY5Y), dog kidney epithelial cells (MDCK), and human furin-deficient colon cancer cells (LoVo). Cleavage of PrP in LoVo cells indicates that the processing is furin independent. Neither is it reduced by some inhibitors of lysosomal proteinases, proteasomes or zinc-metalloproteinases, but incubation with bafilomycin A1, an inhibitor of vacuolar H+/ATPases, increases the amount of uncleaved PrP in the apical medium of MDCK cells. Mutations affecting the putative cleavage site near amino acid 113 reveal that the cleavage is independent of primary structure at this site. Absence of glycosylphosphatidylinositol anchor and glycan modifications does not influence the proteolytic processing of PrP. Our data indicate that PrP is cleaved during transit to the cell membrane.

A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells

We have grown polarized epithelial Madin-Darby canine kidney II (MDCK II) cells on filters in the presence of [(35)S]sulfate, [(3)H]glucosamine, or [(35)S]cysteine/[(35)S]methionine to study proteoglycan (PG) synthesis, sorting, and secretion to the apical and basolateral media. Whereas most of the [(35)S]sulfate label was recovered in basolateral PGs, the [(3)H]glucosamine label was predominantly incorporated into the glycosaminoglycan chains of apical PGs, indicating that basolateral PGs are more intensely sulfated than their apical counterparts. Expression of the PG serglycin with a green fluorescent protein tag (SG-GFP) in MDCK II cells produced a protein core secreted 85% apically, which was largely modified by chondroitin sulfate chains. Surprisingly, the 15% of secreted SG-GFP molecules recovered basolaterally were more heavily sulfated and displayed a different sulfation pattern than the apical counterpart. More detailed studies of the differential modification of apically and basolaterally secreted SG-GFP indicate that the protein cores have been designated to apical and basolateral transport platforms before pathway-specific, post-translational modifications have been completed.

Intracellular proteoglycans

Proteoglycans (PGs) are proteins with glycosaminoglycan chains, are ubiquitously expressed and have a wide range of functions. PGs in the extracellular matrix and on the cell surface have been the subject of extensive structural and functional studies. Less attention has so far been given to PGs located in intracellular compartments, although several reports suggest that these have biological functions in storage granules, the nucleus and other intracellular organelles. The purpose of this review is, therefore, to present some of these studies and to discuss possible functions linked to PGs located in different intracellular compartments. Reference will be made to publications relevant for the topics we present. It is beyond the scope of this review to cover all publications on PGs in intracellular locations.

Sulfation in the Golgi lumen of Madin-Darby canine kidney cells is inhibited by brefeldin A and depends on a factor present in the cytoplasm and on Golgi membranes

Madin-Darby canine kidney cells are more resistant than most other cell types to the classical effects of brefeldin A (BFA) treatment, the induction of retrograde transport of Golgi cisternae components to the endoplasmic reticulum. Here we show that sulfation of heparan sulfate proteoglycans (HSPGs), chondroitin sulfate proteoglycans (CSPGs), and proteins in the Golgi apparatus is dramatically reduced by low concentrations of BFA in which Golgi morphology is unaffected and secretion still takes place. BFA treatment seems to reduce sulfation by inhibition of the uptake of adenosine 3'-phosphate 5'-phosphosulfate (PAPS) into the Golgi lumen, and the inhibitory effect of BFA was similar for HSPGs, CSPGs, and proteins. This was different from the effect of chlorate, a well known inhibitor of PAPS synthesis in the cytoplasm. Low concentrations of chlorate (2-5 mm) inhibited sulfation of CSPGs and proteins only, whereas higher concentrations (15-30 mm) were required to inhibit sulfation of HSPGs. Golgi fractions pretreated with BFA had a reduced capacity for the synthesis of glycosaminoglycans (GAGs), but control level capacity could be restored by the addition of cytosol from various sources. This indicates that the PAPS pathway to the Golgi lumen depends on a BFA-sensitive factor that is present both on Golgi membranes and in the cytoplasm.

Effect of brefeldin A on heparan sulphate biosynthesis in Madin-Darby canine kidney cells

Brefeldin A (BFA) perturbs the organization of the Golgi apparatus, such that Golgi stack components are fused with the endoplasmic reticulum (ER) and separated from the trans-Golgi network. In many cell types, BFA blocks the secretion of macromolecules but still allows the action of Golgi enzymes in the ER. Treatment of cells with BFA has been reported to inhibit the secretion of heparan sulphate (HS) proteoglycans and alter the structure of their HS components, but the nature of such structural alterations has not been characterized in detail. We analysed the effect of BFA on HS biosynthesis in Madin-Darby canine kidney (MDCK) cells, in which the Golgi complex is more resistant towards BFA than in most other cell types. We found that MDCK cells were able to secrete HS proteoglycans in spite of BFA treatment. However, the secretion of HS was reduced and the secreted HS differed from that produced by untreated cells. In BFA-treated cells, two structurally distinct pools of HS were generated. One pool was similar to HS from control cells, with the exception that the 6-O-sulphation of glucosamine (GlcN) residues was reduced. In contrast, the other pool consisted of largely unmodified N-acetylheparosan polymers with a low (<20%) proportion of N-sulphated GlcN residues but a substantial proportion of N-unsubstituted GlcN units, indicating that it had been acted upon by N-deacetylases and partly by the N-sulphotransferases, but not by O-sulphotransferases. Together, these findings represent a previously unrecognized alteration in HS biosynthesis caused by BFA, and differ dramatically from our previous findings in MDCK cells pertaining to the undersulphation of HS caused by sodium chlorate treatment.

Internalization and stepwise degradation of heparan sulfate proteoglycans in rat hepatocytes

Intracellular transport and degradation of membrane anchored heparan sulfate proteoglycans (HSPGs) were studied in cultured rat hepatocytes labeled with [35S]sulfate and [3H]glucosamine. Pulse chase experiments showed that membrane anchored HSPGs were constitutively transported to the cell surface after completion of polymerization and modification of the glycosaminoglycan chains in the Golgi apparatus. The intact HSPGs had a relatively short residence time at the cell surface and in non-degrading compartments (T(1/2) approximately 2-3 h), while [35S]sulfate labeled degradation products were found in lysosomes, and to a lesser extent in late endosomes. These degradation products which were free heparan sulfate chains with little or no protein covalently attached, were approximately half the size of the original glycosaminoglycan chains and were the only degradation intermediate found in the course of HSPG catabolism in these cells. In cells incubated in the presence of the microtubule perturbant vinblastine, or in the presence of the vacuolar ATPase inhibitor bafilomycin A1, and in cells incubated at 19 degrees C, the endocytosed HSPGs were retained in endosomes and no degradation products were detected. Disruption of lysosomes with glycyl-phenylalanine 2-naphthylamide (GPN) revealed a GPN resistant degradative compartment with both intact and partially degraded HSPGs. This compartment probably corresponds to late endosomes. Treatment of hepatocytes with the thiol protease inhibitor leupeptin inhibited the final degradation of the protein moiety of the HSPGs. The protein portion seems to be degraded completely before the glycosaminoglycan chains are cleaved. The degradation of the glycosaminoglycan chains is rapid and complete with one observable intermediate.

Cholesterol depletion reduces apical transport capacity in epithelial Madin-Darby canine kidney cells

Reduction of the cholesterol level in membranes of epithelial Madin-Darby canine kidney (MDCK) cells reverses the apical-to-basolateral transport ratio of the apical membrane marker protein influenza virus haemagglutinin and the secreted glycoprotein gp80. At the same time, basolateral transport of the vesicular stomatitis virus G protein is unaffected [Keller and Simons (1998) J. Cell Biol. 140, 1357-1367]. To investigate whether cholesterol depletion influences apical sorting mechanisms specifically, or apical transport capacity more generally, we studied the effect of cholesterol depletion on the secretion of three different classes of molecules from the apical and basolateral surfaces of MDCK cell layers: glycoprotein gp80, sulphated proteoglycans and proteins, and non-glycosylated rat growth hormone. In each case, cholesterol depletion reduced the fraction secreted to the apical medium and increased the fraction secreted basolaterally. The fact that this was observed for all sulphated proteins and proteoglycans and for the non-glycosylated rat growth hormone, which is randomly secreted in untreated cells, indicates that cholesterol depletion reduces the apical transport capacity, rather than interfering with specific recognition and sorting processes.

The AGE product N epsilon-(carboxymethyl)lysine serum albumin is a modulator of proteoglycan expression in polarized cultured kidney epithelial cells

AIMS/HYPOTHESIS

Changes in kidney function in diabetes could be due to changes in the kidney basement membranes. Proteoglycans are important constituents of this kidney extracellular matrix. This study explored the possibility that advanced glycation end products affect proteoglycan synthesis in cultured kidney epithelial cells.

METHODS

Madin Darby Canine Kidney (MDCK) epithelial cells were cultured with either low glucose (5 mmol/l), low glucose with 10 micrograms/ml of N epsilon-(carboxymethyl)lysine bovine serum albumin (CML-BSA) or high glucose (25 mmol/l). From day 7-8 cells were labelled with either [35S]sulphate or [3H]glucosamine for 24 h. Labelled macromolecules were purified by gel and ion exchange chromatography, and isolated proteoglycans analysed by gel chromatography and electrophoresis.

RESULTS

The CML-BSA treatment reduced the proteoglycan synthesis in MDCK cells. Neither the type of glycosaminoglycan chains made nor the molecular size of the chains was affected.

CONCLUSION/INTERPRETATION

At concentrations found in the plasma of diabetes patients CML-BSA, decreases proteoglycan expression in kidney epithelial cells. Advanced glycation end products could, accordingly, promote pathological changes in kidneys of diabetics.

Proteoglycan synthesis is increased in cells with impaired clathrin-dependent endocytosis

Overexpression of a GTPase deficient dynamin mutant in HeLa dynK44A cells causes a block in clathrin-dependent endocytosis. When endocytosis is inhibited, these cells incorporate higher levels of [(35)S]sulfate into both cellular and secreted macromolecules and larger amounts of proteoglycans such as syndecan and perlecan are immunoprecipitated from [(35)S]sulfate-labelled lysates. Gel filtration and ion-exchange chromatography revealed that the increased [(35)S]sulfate incorporation into proteoglycans was not due to significant differences in size or density of negative charge of glycosaminoglycan chains attached to proteoglycan core proteins. On the other hand, measurements of the syndecan-1 mRNA level and of [(3)H]leucine-labelled perlecan after immunoprecipitation supported the idea that the increased [(35)S]sulfate incorporation into proteoglycans was due to a selective increase in the synthesis of proteoglycan core proteins. Interestingly, the activity of protein kinase C was increased in cells expressing mutant dynamin and inhibition of protein kinase C with BIM reduced the differences in [(35)S]sulfate incorporation between cells with normal and impaired clathrin-dependent endocytosis. Thus, the activation of protein kinase C observed upon inhibition of clathrin-dependent endocytosis may be responsible for the increased synthesis of proteoglycans.

Lithocholic acid and sulphated lithocholic acid differ in the ability to promote matrix metalloproteinase secretion in the human colon cancer cell line CaCo-2

The human colon carcinoma cell line CaCo-2 has the ability to sulphate the secondary bile acid lithocholic acid (LA), whereas other primary or secondary bile acids were not sulphated [Halvorsen, Kase, Prydz, Gharagozlian, Andresen and Kolset (1999) Biochem. J. 343, 533--539]. To study the biological implications of this modification, CaCo-2 cells were incubated with either LA or sulphated lithocholic acid (3-sulpholithocholic acid, SLA), and in some experiments with taurine-conjugated lithocholic acid. Increased secretion of matrix metalloproteinases (MMPs) correlates with transformation of colon epithelial cells. When CaCo-2 cells were incubated with LA, the secretion of MMP-2 was found to increase approx. 60% when analysed by gelatin zymography, and 80% when analysed by Western blotting. SLA, in contrast, did not affect the level of MMP-2 secretion, and after zymography the level of enzyme activity was 78% of control values after 18 h incubation. The secretion of MMPs is linked to increased cellular invasion and, in tumours, to increased capacity for metastasis. The ability of CaCo-2 cells to invade in a chamber assay was stimulated after exposure to LA, whereas SLA-treated cells did not differ from control cells. LA therefore seems to induce a more invasive CaCo-2 cell phenotype, as judged by the two parameters tested, whereas the sulphated counterpart, SLA, did not have these effects. Sulphation of LA in the colon may be an important mechanism to decrease the potential LA has to promote a malignant epithelial phenotype.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

Selective effects of sodium chlorate treatment on the sulfation of heparan sulfate

We have analyzed the effect of sodium chlorate treatment of Madin-Darby canine kidney cells on the structure of heparan sulfate (HS), to assess how the various sulfation reactions during HS biosynthesis are affected by decreased availability of the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate. Metabolically [(3)H]glucosamine-labeled HS was isolated from chlorate-treated and untreated Madin-Darby canine kidney cells and subjected to low pH nitrous acid cleavage. Saccharides representing (i) the N-sulfated domains, (ii) the domains of alternating N-acetylated and N-sulfated disaccharide units, and (iii) the N-acetylated domains were recovered and subjected to compositional disaccharide analysis. Upon treatment with 50 mM chlorate, overall O-sulfation of HS was inhibited by approximately 70%, whereas N-sulfation remained essentially unchanged. Low chlorate concentrations (5 or 20 mM) selectively reduced the 6-O-sulfation of HS, whereas treatment with 50 mM chlorate reduced both 2-O- and 6-O-sulfation. Analysis of saccharides representing the different domain types indicated that 6-O-sulfation was preferentially inhibited in the alternating domains. These data suggest that reduced 3'-phosphoadenosine 5'-phosphosulfate availability has distinct effects on the N- and O-sulfation of HS and that O-sulfation is affected in a domain-specific fashion.

Sulphation of lithocholic acid in the colon-carcinoma cell line CaCo-2

High levels of bile acids in the colon may correlate with an increased risk of colon cancer, but the underlying mechanisms are not known. Proteoglycan structures have been shown to change when human colon cells differentiate in vitro. The expression of [(35)S]sulphated molecules was used as a phenotypic marker to study the effects of bile acids on the human-colon-carcinoma cell line CaCo-2. [(35)S]sulphated compounds were isolated from the medium of cell fractions of cells metabolically labelled with [(35)S]sulphate in the absence and presence of cholic acid, deoxycholic acid, chenodeoxycholic acid and lithocholic acid (LA). Labelled molecules were analysed by gel chromatography, HPLC and SDS/PAGE in combination with chemical and enzymic methods. The expression of (35)S-labelled proteoglycans was not affected by any of the bile acids tested. However, the level of sulphated metabolites increased 7-18-fold in different experiments during a 22 h labelling period in the presence of an LA concentration of 10 microg/ml (26.6 nmol/ml) compared with controls. Further analyses showed that this was due, at least in part, to the sulphation of LA itself. This sulphation of LA was a rapid process followed by secretion back to the medium. Brefeldin A did not reduce the sulphation of LA, indicating that this conversion takes place in the cytosol, rather than in the Golgi apparatus of the CaCo-2 cells. LA in colon may be sulphated efficiently by the colonocytes to reduce the toxic effects of this particular bile acid. Sulphation may possibly be an important protective mechanism in the colon.

Fluid phase endocytosis and galactosyl receptor-mediated endocytosis employ different early endosomes

Endocytosis may originate both in coated pits and in uncoated regions of the plasma membrane. In hepatocytes it has been shown that fluid phase endocytosis (here defined as 'pinocytosis') is unaffected by treatments that arrest coated pit-mediated endocytosis, indicating that pinocytosis is primarily a clathrin-independent process. In this study we have tried to determine possible connections between pinocytosis and clathrin-dependent endocytosis in rat hepatocytes by means of subcellular fractionation, electron microscopy, and by assessing the influence of inhibitors of clathrin-dependent endocytosis on pinocytosis. As marker for clathrin-dependent endocytosis was used asialoorosomucoid (AOM) labelled with [(125)I]tyramine cellobiose ([(125)I]TC). [(125)I]TC-labelled bovine serum albumin ([(125)I]TC-BSA) was found to be a useful marker for pinocytosis. Its uptake in the cells is not saturable, and any remnants of [(125)I]TC-BSA associated with the cell surface could be removed by incubating the cells with 0.3% pronase at 0 degrees C for 60 min. The data obtained by electron microscopy and by subcellular fractionation suggested that early after initiation of uptake (<15 min) [(125)I]TC-BSA and [(125)I]TC-AOM were present in different endocytic vesicles. The two probes probably join prior to their entrance in the lysosomal compartment. The relation between endocytosis via coated pits and pinocytosis was also studied with techniques that induced a selective density shift either in the clathrin-dependent pathway (by AOM-HRP) or in the pinocytic pathway (by allowing uptake of AuBSA). Both treatments indicated that the two probes ([(125)I]TC-AOM and [(125)I]TC-BSA) were early after uptake, at least partly, in separate endocytic compartments. The different distribution of the fluid phase marker and the ligand (internalised via coated pits) was not due to a difference in the rate at which they enter a later compartment, since a lowering of the incubation temperature to 18 degrees C, which should keep the probes in the early endosomes, did not affect their early density distribution. Incubation of cells in a hypertonic medium reduced uptake both of [(125)I]TC-AOM and [(125)I]TC-BSA; the uptake of [(125)I]TC-AOM was, however, reduced much more than that of the fluid phase marker. This finding supports the notion that the two probes enter the cells via different routes.

Characterization of fibroblast growth factor 1 binding heparan sulfate domain

Fibroblast growth factors FGF-1 and FGF-2 mediate their biological effects via heparan sulfate-dependent interactions with cell surface FGF receptors. While the specific heparan sulfate domain binding to FGF-2 has been elucidated in some detail, limited information has been available concerning heparan sulfate structures involved in the recognition of FGF-1. In the current study we present evidence that the minimal FGF-1 binding heparan sulfate sequence comprises 5-7 monosaccharide units and contains a critical trisulfated IdoA(2-OSO3)-GlcNSO3(6-OSO3) disaccharide unit. N-Sulfated heparan sulfate decasaccharides depleted of FGF-1 binding domains showed dose-dependent and saturable binding to FGF-2. These data indicate that the FGF-1 binding domain is distinct from the minimal FGF-2 binding site, previously shown to contain an IdoA(2-OSO3) residue but no 6-O-sulfate groups. We further show that the FGF-1 binding heparan sulfate domain is expressed in human aorta heparan sulfate in an age-related manner in contrast to the constitutively expressed FGF-2 binding domain. Reduction of heparan sulfate O-sulfation by chlorate treatment of cells selectively impedes binding to FGF-1. The present data implicate the 6-O-sulfation of IdoA(2-OSO3)-GlcNSO3 units in cellular heparan sulfate in the control of the biological activity of FGF-1.

Apical secretion of chondroitin sulphate in polarized Madin-Darby canine kidney (MDCK) cells

Sugar moieties have been shown to contain sufficient and necessary information to target examples of secreted and transmembrane glycoproteins to the apical surface of epithelial MDCK cells. We have investigated if the sugar chains of proteoglycans, the glycosaminoglycans, also contain structural determinants for apical transport. Here we show that although 75% of the proteoglycan secretion from MDCK cell monolayers is into the basolateral medium, 75% of the proteoglycans of the chondroitin sulphate type are secreted apically. The sorting information in the chondroitin sulphate proteoglycans is localized to the sugar chains, since protein-free chondroitin sulphate chains, initiated on hexyl beta-D-thioxyloside, were also predominantly secreted to the apical medium.

Differentiation-associated modulation of heparan sulfate structure and function in CaCo-2 colon carcinoma cells

Heparan sulfate species expressed by different cell and tissue types differ in their structural and functional properties. Limited information is available on differences in regulation of heparan sulfate biosynthesis within a single tissue or cell population under different conditions. We have approached this question by studying the effect of cell differentiation on the biosynthesis and function of heparan sulfate in human colon carcinoma cells (CaCo-2). These cells undergo spontaneous differentiation in culture when grown on semipermeable supports; the differentiated cells show phenotypic similarity to small intestine enterocytes. Metabolically labeled heparan sulfate was isolated from the apical and basolateral media from cultures of differentiated and undifferentiated cells. Compositional analysis of disaccharides, derived from the contiguous N-sulfated regions of heparan sulfate, indicated a greater proportion of 2-O-sulfated iduronic acid units and a smaller amount of 6-O-sulfated glucosamine units in differentiated than in undifferentiated cells. By contrast, the overall degree of sulfation, the chain length and the size distribution of the N-acetylated regions were similar regardless the differentiation status of the cells. The structural changes were found to affect the binding of heparan sulfate to the long isoform of platelet-derived growth factor A chain but not to fibroblast growth factor 2. These findings show that heparan sulfate structures change during cell differentiation and that heparan sulfate-growth factor interactions may be affected by such changes.

Endocytosed ricin and asialoorosomucoid follow different intracellular pathways in hepatocytes

Earlier studies have suggested that fluid phase endocytosis in rat hepatocytes takes place via a clathrin-independent mechanism [1,2]. This observation suggests that a relatively large amount of plasma membrane outside coated pits may be involved in hepatic endocytosis. Ricin, which binds to galactose residues on glycoproteins and glycolipids, has, in this report, been used as a general marker for the plasma membrane of hepatocytes. The endocytosis of ricin was compared with that of asialoorosomucoid (AOM) which is taken up exclusively via clathrin-coated pits. Hypertonic medium has been shown to inhibit uptake via coated pits more effectively than clathrin-independent uptake [3-5]. It was found, in this study, that the addition of 100 mM sucrose to the incubation medium inhibited the uptake of 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) more extensively than that of 125I-tyramine-cellobiose-ricin (125I-TC-ricin), compatible with the notion that the two probes are internalised via different mechanisms. Subcellular fractionation experiments indicated that 125I-TC-ricin entered a denser endocytic organelle than that receiving 125I-TC-AOM. To determine whether the separation of the two probes was due to a different transport kinetics (i.e. that 125I-TC-ricin is transported more rapidly to a later, denser compartment than 125I-TC-AOM) the cells were incubated at 18 degreesC to allow a slower internalisation/transport of the labelled probes. The results obtained showed, again, that the early endosomes containing 125I-TC-ricin were significantly denser than those containing 125I-TC-AOM. We also employed the horseradish peroxidase (HRP)-diaminobenzidine (DAB) density shift technique of Courtoy et al. [6] to determine whether 125I-TC-ricin and 125I-TC-AOM were in separate endosomes early after their uptake. The results showed that early endosomes containing 125I-TC-AOM were density shifted whereas those containing 125I-TC-ricin were unaffected by the density shift procedure. The use of probes labelled with 125I-TC allowed us to identify compartments involved in the degradation of 125I-TC-AOM and 125I-TC-ricin, by measuring acid soluble radioactivities in the gradient fractions. It was found that 125I-TC-ricin was degraded mainly in endosomes, whereas 125I-TC-AOM, as expected, was degraded mainly in lysosomes.

[Proteoglycans and pathology--new aspects]

It is well documented that proteoglycans are involved in a wide range of pathological conditions. Recently published results in international journals provide new information on the role of proteoglycans in such conditions. A mutation in the gene encoding for a cell surface proteoglycan has been demonstrated in overgrowth syndromes. A proteoglycan has been isolated from urine and shown to induce cachexia in cancer patients. Furthermore, in both achondrogenesis and colon cancer, the reduced ability to sulphate proteoglycans is due to genetic defects in cellular sulfate transporters. Finally, fibrosis has been inhibited in glomerulonephritic mice by transferring the gene for decorin, a transforming growth factor beta-1 binding proteoglycan, into muscle tissue.