Tissue distribution and subcellular localization of hyaluronan synthase isoenzymes

Hyaluronan synthases (HAS) are unique plasma membrane glycosyltransferases secreting this glycosaminoglycan directly to the extracellular space. The three HAS isoenzymes (HAS1, HAS2, and HAS3) expressed in mammalian cells differ in their enzymatic properties and regulation by external stimuli, but clearly distinct functions have not been established. To overview the expression of different HAS isoenzymes during embryonic development and their subcellular localization, we immunostained mouse embryonic samples and cultured cells with HAS antibodies, correlating their distribution to hyaluronan staining. Their subcellular localization was further studied by GFP–HAS fusion proteins. Intense hyaluronan staining was observed throughout the development in the tissues of mesodermal origin, like heart and cartilages, but also for example during the maturation of kidneys and stratified epithelia. In general, staining for one or several HASs correlated with hyaluronan staining. The staining of HAS2 was most widespread, both spatially and temporally, correlating with hyaluronan staining especially in early mesenchymal tissues and heart. While epithelial cells were mostly negative for HASs, stratified epithelia became HAS positive during differentiation. All HAS isoenzymes showed cytoplasmic immunoreactivity, both in tissue sections and cultured cells, while plasma membrane staining was also detected, often in cellular extensions. HAS1 had brightest signal in Golgi, HAS3 in Golgi and microvillous protrusions, whereas most of the endogenous HAS2 immunoreactivity was localized in the ER. This differential pattern was also observed with transfected GFP–HASs. The large proportion of intracellular HASs suggests that HAS forms a reserve that is transported to the plasma membrane for rapid activation of hyaluronan synthesis.     

Hyaluronan synthesis induces microvillus-like cell surface protrusions

Hyaluronan synthases (HASs) are plasma membrane enzymes that simultaneously elongate, bind, and extrude the growing hyaluronan chain directly into extracellular space. In cells transfected with green fluorescent protein (GFP)-tagged Has3, the dorsal surface was decorated by up to 150 slender, 3-20-microm-long microvillus-type plasma membrane protrusions, which also contained filamentous actin, the hyaluronan receptor CD44, and lipid raft microdomains. Enzymatic activity of HAS was required for the growth of the microvilli, which were not present in cells transfected with other GFP proteins or inactive GFP-Has3 mutants or in cells incubated with exogenous soluble hyaluronan. The microvilli induced by HAS3 were gradually withered by introduction of an inhibitor of hyaluronan synthesis and rapidly retracted by hyaluronidase digestion, whereas they were not affected by competition with hyaluronan oligosaccharides and disruption of the CD44 gene, suggesting independence of hyaluronan receptors. The data bring out the novel concept that the glycocalyx created by dense arrays of hyaluronan chains, tethered to HAS during biosynthesis, can induce and maintain prominent microvilli.     

'Piggy-back' transport of Xenopus hyaluronan synthase (XHAS1) via the secretory pathway to the plasma membrane

Hyaluronan is the sole glycosaminoglycan whose biosynthesis takes place directly at the plasma membrane. The mechanism by which hyaluronan synthase (HAS) becomes inserted there, as well as the question of how the enzyme discriminates between particular membrane species in polarized cells, are largely unknown. In vitro translation of HAS suggested that the nascent protein becomes stabilized in the presence of microsomal membranes, but would not insert spontaneously into membranes after being translated in the absence of those. We therefore monitored the membrane attachment of enzymatically active fusion proteins consisting of Xenopus HAS1 and green fluorescent protein shortly after de novo synthesis in Vero cells. Our data strongly suggest that HAS proteins are directly translated on the ER membrane without exhibiting an N-terminal signal sequence. From there the inactive protein is transferred to the plasma membrane via the secretory pathway. For unknown reasons, HAS inserted into membranes other than the plasma membrane remains inactive.     

4-Methylumbelliferone inhibits hyaluronan synthesis by depletion of cellular UDP-glucuronic acid and downregulation of hyaluronan synthase 2 and 3

Hyaluronan accumulation on cancer cells and their surrounding stroma predicts an unfavourable disease outcome, suggesting that hyaluronan enhances tumor growth and spreading. 4-Methylumbelliferone (4-MU) inhibits hyaluronan synthesis and retards cancer spreading in experimental animals through mechanisms not fully understood. These mechanisms were studied in A2058 melanoma cells, MCF-7 and MDA-MB-361 breast, SKOV-3 ovarian and UT-SCC118 squamous carcinoma cells by analysing hyaluronan synthesis, UDP-glucuronic acid (UDP-GlcUA) content, and hyaluronan synthase (HAS) mRNA levels. The maximal inhibition in hyaluronan synthesis ranged 22-80% in the cell lines tested. Active glucuronidation of 4-MU produced large quantities of 4-MU-glucuronide, depleting the cellular UDP-GlcUA pool. The maximal reduction varied between 38 and 95%. 4-MU also downregulated HAS mRNA levels: HAS3 was 84-60% lower in MDA-MB-361, A2058 and SKOV-3 cells. HAS2 was the major isoenzyme in MCF-7 cells and lowered by 81%, similar to 88% in A2058 cells. These data indicate that both HAS substrate and HAS2 and/or HAS3 mRNA are targeted by 4-MU. Despite different target point sensitivities, the reduction of hyaluronan caused by 4-MU was associated with a significant inhibition of cell migration, proliferation and invasion, supporting the importance of hyaluronan synthesis in cancer, and the therapeutic potential of hyaluronan synthesis inhibition.     

Fluorescence Resonance Energy Transfer (FRET) and Proximity Ligation Assays Reveal Functionally Relevant Homo- and Heteromeric Complexes among Hyaluronan Synthases HAS1, HAS2, and HAS3

In vertebrates, hyaluronan is produced in the plasma membrane from cytosolic UDP-sugar substrates by hyaluronan synthase 1–3 (HAS1–3) isoenzymes that transfer N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcUA) in alternative positions in the growing polysaccharide chain during its simultaneous extrusion into the extracellular space. It has been shown that HAS2 immunoprecipitates contain functional HAS2 homomers and also heteromers with HAS3 (Karousou, E., Kamiryo, M., Skandalis, S. S., Ruusala, A., Asteriou, T., Passi, A., Yamashita, H., Hellman, U., Heldin, C. H., and Heldin, P. (2010) The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination. J. Biol. Chem. 285, 23647–23654). Here we have systematically screened in live cells, potential interactions among the HAS isoenzymes using fluorescence resonance energy transfer (FRET) and flow cytometric quantification. We show that all HAS isoenzymes form homomeric and also heteromeric complexes with each other. The same complexes were detected both in Golgi apparatus and plasma membrane by using FRET microscopy and the acceptor photobleaching method. Proximity ligation assays with HAS antibodies confirmed the presence of HAS1-HAS2, HAS2-HAS2, and HAS2-HAS3 complexes between endogenously expressed HASs. C-terminal deletions revealed that the enzymes interact mainly via uncharacterized N-terminal 86-amino acid domain(s), but additional binding site(s) probably exist in their C-terminal parts. Of all the homomeric complexes HAS1 had the lowest and HAS3 the highest synthetic activity. Interestingly, HAS1 transfection reduced the synthesis of hyaluronan obtained by HAS2 and HAS3, suggesting functional cooperation between the isoenzymes. These data indicate a general tendency of HAS isoenzymes to form both homomeric and heteromeric complexes with potentially important functional consequences on hyaluronan synthesis.     

Methyl-β-cyclodextrin Suppresses Hyaluronan Synthesis by Down-regulation of Hyaluronan Synthase 2 through Inhibition of Akt

Hyaluronan synthases (HAS1–3) are integral plasma membrane proteins that synthesize hyaluronan, a cell surface and extracellular matrix polysaccharide necessary for many biological processes. It has been shown that HAS is partly localized in cholesterol-rich lipid rafts of MCF-7 cells, and cholesterol depletion with methyl-β-cyclodextrin (MβCD) suppresses hyaluronan secretion in smooth muscle cells. However, the mechanism by which cholesterol depletion inhibits hyaluronan production has remained unknown. We found that cholesterol depletion from MCF-7 cells by MβCD inhibits synthesis but does not decrease the molecular mass of hyaluronan, suggesting no major influence on HAS stability in the membrane. The inhibition of hyaluronan synthesis was not due to the availability of HAS substrates UDP-GlcUA and UDP-GlcNAc. Instead, MβCD specifically down-regulated the expression of HAS2 but not HAS1 or HAS3. Screening of signaling proteins after MβCD treatment revealed that phosphorylation of Akt and its downstream target p70S6 kinase, both members of phosphoinositide 3-kinase-Akt pathway, were inhibited. Inhibitors of this pathway suppressed hyaluronan synthesis and HAS2 expression in MCF-7 cells, suggesting that the reduced hyaluronan synthesis by MβCD is due to down-regulation of HAS2, mediated by the phosphoinositide 3-kinase-Akt-mTOR-p70S6K pathway.     

ADP-ribosylation factor 1 of Arabidopsis plays a critical role in intracellular trafficking and maintenance of endoplasmic reticulum morphology in Arabidopsis

ADP-ribosylation factors (Arf), a family of small GTP-binding proteins, play important roles in intracellular trafficking in animal and yeast cells. Here, we investigated the roles of two Arf homologs, Arf1 and Arf3 of Arabidopsis, in intracellular trafficking in plant cells. We generated dominant negative mutant forms of Arf 1 and Arf3 and examined their effect on trafficking of reporter proteins in protoplasts. Arf1[T31N] inhibited trafficking of H(+)-ATPase:green fluorescent protein (GFP) and sialyltransferase (ST):GFP to the plasma membrane and the Golgi apparatus. In addition, Arf1[T31N] caused relocalization of the Golgi reporter protein ST:GFP to the endoplasmic reticulum (ER). In protoplasts expressing Arf1[T31N], ST:red fluorescent protein remained in the ER, whereas H(+)-ATPase:GFP was mistargeted to another organelle. Also, expression of Arf1[T31N] in protoplasts resulted in profound changes in the morphology of the ER. The treatment of protoplasts with brefeldin A had exactly the same effect as Arf1[T31N] on various intracellular trafficking pathways. In contrast, Arf3[T31N] did not affect trafficking of any of these reporter proteins. Inhibition experiments using mutants with various domains swapped between Arf1 and Arf3 revealed that the N-terminal domain is interchangeable for trafficking inhibition. However, in addition to the T31N mutation, motifs in domains II, III, and IV of Arf1 were necessary for inhibition of trafficking of H(+)-ATPase:GFP. Together, these results strongly suggest that Arf1 plays a role in the intracellular trafficking of cargo proteins in Arabidopsis, and that Arf1 functions through a brefeldin A-sensitive factor.     

Involvement of the endoplasmic reticulum in the assembly and envelopment of African swine fever virus.

African swine fever (ASF) virus is a large enveloped DNA virus assembled in the cytoplasm of cells. In this study, the membrane compartments involved in the envelopment of ASF virus were investigated. A monoclonal antibody recognizing p73, the major structural protein of ASF virus, was generated to analyze the binding of p73 to membranes during the assembly of the virus. Approximately 50% of the intracellular pool of p73 associated with membranes as a peripheral membrane protein. Binding was rapid and complete within 15 min of synthesis. Subcellular membrane fractionation showed that newly synthesized p73 molecules cosedimented with endoplasmic reticulum (ER) membranes and remained associated with the ER during a 2-h chase. A similar distribution on gradients was recorded for p17, a structural membrane protein of ASF virus. The results suggested that the ER was involved in the assembly of ASF virus. A protease protection assay demonstrated a time-dependent envelopment of the membrane bound, but not cytosolic, pool of p73. Envelopment of p73 took place 1 h after binding to membranes and was completed 1 h before the first detection of p73 in virions secreted from cells. Envelopment was unaffected by brefeldin A and monensin, drugs that block membrane transport between the ER and Golgi. Taken together the results provide evidence for the binding of ASF virus structural proteins to a specific membrane compartment and implicate a role for the ER in the assembly and envelopment of ASF virus.     

Hyaluronan synthase 1 (HAS1) produces a cytokine-and glucose-inducible,CD44-dependent cell surface coat

Hyaluronan is a ubiquitous glycosaminoglycan involved in embryonic development, inflammation and cancer. In mammals, three hyaluronan synthase isoenzymes (HAS1-3) inserted in the plasma membrane produce hyaluronan directly on cell surface. The mRNA level and enzymatic activity of HAS1 are lower than those of HAS2 and HAS3 in many cells, obscuring the importance of HAS1. Here we demonstrate using immunocytochemistry and transfection of fluorescently tagged HAS1 that its enzymatic activity depends on the ER–Golgi–plasma membrane traffic, like reported for HAS2 and HAS3. When cultured in 5 mM glucose, HAS1-transfected MCF-7 cells show very little cell surface hyaluronan, detected with a fluorescent hyaluronan binding probe. However, a large hyaluronan coat was seen in cells grown in 20 mM glucose and 1 mM glucosamine, or treated with IL-1β, TNF-α, or TGF-β. The coats were mostly removed by the presence of hyaluronan hexasaccharides, or Hermes1 antibody, indicating that they depended on the CD44 receptor, which is in a contrast to the coat produced by HAS3, remaining attached to HAS3 itself. The findings suggest that HAS1-dependent coat is induced by inflammatory agents and glycemic stress, mediated by altered presentation of either CD44 or hyaluronan, and can offer a rapid cellular response to injury and inflammation.     

The syntaxins SYP31 and SYP81 control ER-Golgi trafficking in the plant secretory pathway

Overexpression of the Golgi and endoplasmic reticulum (ER) syntaxins SYP31 and SYP81 strongly inhibits constitutive secretion. By comparing the secreted reporter alpha-amylase with the ER-retained reporter alpha-amylase-HDEL, it was concluded that SYP81 overexpression inhibits both retrograde and anterograde transport, while SYP31 overexpression mainly affected anterograde transport. Of the other interacting SNAREs investigated, only the overexpression of MEMB11 led to an inhibition of protein secretion. Although the position of a fluorescent tag does not influence the correct localization of the fusion protein, only N-terminal-tagged SYP31 retained the ability of the untagged SNARE to inhibit transport. C-terminal-tagged SYP31 failed to exhibit this effect. Overexpression of both wild-type and N-terminal-tagged syntaxins caused standard Golgi marker proteins to redistribute into the ER. Nevertheless, green fluorescent protein (GFP)-SYP31 was still visible as fluorescent punctae, which, unlike SYP31-GFP, were resistant to brefeldin A treatment. Immunogold electron microscopy showed that endogenous SYP81 is not only present at the ER but also in the cis Golgi, indicating that this syntaxin cycles between these two organelles. However, when expressed at non-inhibitory levels, YFP-SYP81 was seen to locate principally to subdomains of the ER. These punctate structures were physically separated from the Golgi, suggesting that they might possibly reflect the position of ER import sites.     

Internalized Plasma Membrane Cholesterol Passes through an Endosome Compartment That Is Distinct from the Acid Vesicle–Lysosome Compartment

Cholesterol from the plasma membrane of MA-10 Leydig tumor cells is internalized into the cell and either esterified or used as substrate for steroid hormone synthesis. In the present studies we show that chloroquine and sphinganine cause LDL cholesterol and cholesteryl esters to accumulate in the cells. A lysosome fraction contained the excess cholesterol and cholesteryl esters. Both inhibitors blocked the conversion of plasma membrane cholesterol into intracellular cholesteryl esters and caused dose-dependent inhibition of dibutyryl-cAMP-stimulated progesterone synthesis. Radiolabeled cholesterol applied to the plasma membrane of MA-10 cells accumulated in the lysosome fraction of chloroquine and sphinganine-treated cells. Evidence that these inhibitors did not require the Golgi was provided by experiments using brefeldin A. Experiments utilizing a fluorescent cholesterol analogue and a lysosomal marker indicated that cholesterol entered the cells in structures that were different than the acidic vesicle–lysosome compartment. Consistent with this observation was the observation that the peak fluorescence fractions of cells subjected to density gradient centrifugation was of lower density than the lysosome fraction.     

MDA-MB-231 breast cancer cell viability,motility and matrix adhesion are regulated by a complex interplay of heparan sulfate,chondroitin−/dermatan sulfate and hyaluronan biosynthesis

Proteoglycans and glycosaminoglycans modulate numerous cellular processes relevant to tumour progression, including cell proliferation, cell-matrix interactions, cell motility and invasive growth. Among the glycosaminoglycans with a well-documented role in tumour progression are heparan sulphate, chondroitin/dermatan sulphate and hyaluronic acid/hyaluronan. While the mode of biosynthesis differs for sulphated glycosaminoglycans, which are synthesised in the ER and Golgi compartments, and hyaluronan, which is synthesized at the plasma membrane, these polysaccharides partially compete for common substrates. In this study, we employed a siRNA knockdown approach for heparan sulphate (EXT1) and heparan/chondroitin/dermatan sulphate-biosynthetic enzymes (β4GalT7) in the aggressive human breast cancer cell line MDA-MB-231 to study the impact on cell behaviour and hyaluronan biosynthesis. Knockdown of β4GalT7 expression resulted in a decrease in cell viability, motility and adhesion to fibronectin, while these parameters were unchanged in EXT1-silenced cells. Importantly, these changes were associated with a decreased expression of syndecan-1, decreased signalling response to HGF and an increase in the synthesis of hyaluronan, due to an upregulation of the hyaluronan synthases HAS2 and HAS3. Interestingly, EXT1-depleted cells showed a downregulation of the UDP-sugar transporter SLC35D1, whereas SLC35D2 was downregulated in β4GalT7-depleted cells, indicating an intricate regulatory network that connects all glycosaminoglycans synthesis. The results of our in vitro study suggest that a modulation of breast cancer cell behaviour via interference with heparan sulphate biosynthesis may result in a compensatory upregulation of hyaluronan biosynthesis. These findings have important implications for the development of glycosaminoglycan-targeted therapeutic approaches for malignant diseases.     

Brefeldin A induces a microtubule-dependent fusion of galactosyltransferase-containing vesicles with the rough endoplasmic reticulum

The fungal drug brefeldin A (BFA) has recently been found to induce a redistribution of medial- and cis-Golgi components to the endoplasmic reticulum (ER), raising the possibility of the existence of a retrograde pathway from the Golgi complex to the ER. Here, we demonstrate a BFA-induced reversible rearrangement of the trans-Golgi membrane protein galactosyltransferase (Gal-T) to the ER in HeLa cells. With immunofluorescence microscopy we have shown that BFA first caused a rapid change of Gal-T immunolabelling from a normal Golgi complex pattern to long and slender structures emanating from the cell centre and co-localizing with tubulin. Then immunofluorescence became ER-like. This effect was not dependent on ongoing protein synthesis and was reversed to normal within 120 min after removal of the drug. Restoration of the Golgi complex after removal of brefeldin A was energy-dependent but not mediated by microtubules nor dependent on protein synthesis. BFA-induced backflow of Gal-T was inhibited by nocodazole, a microtubule-disrupting agent. Immunoelectron microscopy showed that BFA treatment resulted in the fusion of Gal-T-containing vesicles with the ER. Furthermore, sucrose gradient centrifugation showed a significant shift in density of mature Gal-T polypeptides upon BFA treatment: about 40% of the enzyme migrated from its original density (1.13 g/ml) to the density of rough ER (1.19 g/ml). Thus, BFA caused microtubule-dependent vesicular backflow from a trans-Golgi component to the ER followed by fusion of the Golgi-derived vesicles with the ER.     

Stacks on tracks: the plant Golgi apparatus traffics on an actin/ER network †

We have visualized the relationship between the endoplasmic reticulum (ER) and Golgi in leaf cells of Nicotiana clevelandii by expression of two Golgi proteins fused to green fluorescent protein (GFP). A fusion of the trans -membrane domain (signal anchor sequence) of a rat sialyl transferase to GFP was targeted to the Golgi stacks. A second construct that expressed the Arabidopsis H/KDEL receptor homologue aERD2, fused to GFP, was targeted to both the Golgi apparatus and ER, allowing the relationship between these two organelles to be studied in living cells for the first time. The Golgi stacks were shown to move rapidly and extensively along the polygonal cortical ER network of leaf epidermal cells, without departing from the ER tubules. Co-localization of F-actin in the GFP-expressing cells revealed an underlying actin cytoskeleton that matched precisely the architecture of the ER network, while treatment of cells with the inhibitors cytochalasin D and N-ethylmaleimide revealed the dependency of Golgi movement on actin cables. These observations suggest that the leaf Golgi complex functions as a motile system of actin-directed stacks whose function is to pick up products from a relatively stationary ER system. Also, we demonstrate for the first time in vivo brefeldin A-induced retrograde transport of Golgi membrane protein to the ER.     

Rab10-mediated Endocytosis of the Hyaluronan Synthase HAS3 Regulates Hyaluronan Synthesis and Cell Adhesion to Collagen

Hyaluronan synthases (HAS1–3) are unique in that they are active only when located in the plasma membrane, where they extrude the growing hyaluronan (HA) directly into cell surface and extracellular space. Therefore, traffic of HAS to/from the plasma membrane is crucial for the synthesis of HA. In this study, we have identified Rab10 GTPase as the first protein known to be involved in the control of this traffic. Rab10 colocalized with HAS3 in intracellular vesicular structures and was co-immunoprecipitated with HAS3 from isolated endosomal vesicles. Rab10 silencing increased the plasma membrane residence of HAS3, resulting in a significant increase of HA secretion and an enlarged cell surface HA coat, whereas Rab10 overexpression suppressed HA synthesis. Rab10 silencing blocked the retrograde traffic of HAS3 from the plasma membrane to early endosomes. The cell surface HA coat impaired cell adhesion to type I collagen, as indicated by recovery of adhesion following hyaluronidase treatment. The data indicate a novel function for Rab10 in reducing cell surface HAS3, suppressing HA synthesis, and facilitating cell adhesion to type I collagen. These are processes important in tissue injury, inflammation, and malignant growth.     

Membrane protein transport between the endoplasmic reticulum and the Golgi in tobacco leaves is energy dependent but cytoskeleton independent: evidence from selective photobleaching

The mechanisms that control protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus are poorly characterized in plants. Here, we examine in tobacco leaves the structural relationship between Golgi and ER membranes using electron microscopy and demonstrate that Golgi membranes contain elements that are in close association and/or in direct contact with the ER. We further visualized protein trafficking between the ER and the Golgi using Golgi marker proteins tagged with green fluorescent protein. Using photobleaching techniques, we showed that Golgi membrane markers constitutively cycle to and from the Golgi in an energy-dependent and N-ethylmaleimide-sensitive manner. We found that membrane protein transport toward the Golgi occurs independently of the cytoskeleton and does not require the Golgi to be motile along the surface of the ER. Brefeldin A treatment blocked forward trafficking of Golgi proteins before their redistribution into the ER. Our results indicate that in plant cells, the Golgi apparatus is a dynamic membrane system whose components continuously traffic via membrane trafficking pathways regulated by brefeldin A- and N-ethylmaleimide-sensitive machinery.     

Intracellular Localization of the Peanut Clump Virus Replication Complex in Tobacco BY-2 Protoplasts Containing Green Fluorescent Protein-Labeled Endoplasmic Reticulum or Golgi Apparatus

RNA-1 of Peanut clump virus (PCV) encodes the proteins P131 and P191, containing the signature motifs of replication proteins, and P15, which regulates viral RNA accumulation. In PCV-infected protoplasts both P131 and P191 were immunodetected in the perinuclear region. Laser scanning confocal microscopy (LSCM) showed that P131 and P191 colocalized with neosynthesized 5-bromouridine 5'-triphosphate-labeled RNA and double-stranded RNA, demonstrating that they belong to the replication complex. On the contrary, the P15 fused to the enhanced green fluorescent protein (EGFP) never colocalized with the two proteins. In endoplasmic reticulum (ER)-GFP transgenic BY-2 protoplasts, the distribution of the green fluorescent-labeled ER was strongly modified by PCV infection. LSCM showed that both P131 and P191 colocalized with ER green fluorescent bodies accumulating around the nucleus during infection. The replication process was not inhibited by cerulenin and brefeldin A, suggesting that PCV replication does not depend on de novo-synthesized membrane and does not require transport through the Golgi apparatus. Electron microscopy of ultrathin sections of infected protoplasts showed aggregates of broken ER but also visualized vesicles, some of which resembled modified peroxisomes. The results suggest that accumulation of PCV during infection is accompanied by specific association of PCV RNA-1-encoded proteins with membranes of the ER and other organelles. The concomitant extensive rearrangement of these membranous structures leads to the formation of intracellular compartments in which synthesis and accumulation of the viral RNA occur in defined areas.