Stefana M. Petrescu

Conformation-Independent Binding of Monoglucosylated Ribonuclease B to Calnexin

Calnexin is a membrane protein of the endoplasmic reticulum that associates transiently with newly synthesized N-linked glycoproteins in vivo. Using defined components, the binding of ribonuclease B (RNase B) Man7-Man9 glycoforms to the luminal domain of calnexin was observed in vitro only if RNase B was monoglucosylated. Binding was independent of the conformation of the glycoprotein. Calnexin protected monoglucosylated RNase B from the action of glucosidase II and PNGase F but not from that of Endo H, which completely released the protein from calnexin. These observations directly demonstrate that calnexin can act exclusively as a lectin.

Targeted drug delivery

Cancer chemotherapy drugs are neither specific, i.e., they do not act exclusively on the metabolic pathways of cancer cells, nor are they targeted solely toward cancer cells. However, recent research has begun to address, in part, the latter issue. Improved delivery of chemotherapeutic agents to tumor tissue in man appears to be an achievable goal in the next decade. Improved drug delivery includes developing predictive models that allow for laboratory assessment of the best treatment for a patients cancer without exposing the patient to an empirical trial or to the possible morbidity from exposure to a less useful drug, or to the loss of time in the fight against cancer because of ineffectual therapy. Monoclonal antibodies directed against tumor‐associated antigens have the potential to achieve major advances in targeted drug delivery. Monoclonal antibodies may have direct antitumor effects, or they can be used as “homing devices” when attached to a payload and can guide diagnostic or therapeutic agents to the targeted tissues. Carrier systems of all types have become available; these include liposomes and polymeric compounds which can carry drugs, radionuclides, toxins, or other materials in a protected environment. These carriers can also be bound to monoclonal antibodies for possible targeted delivery. Pharmacological sanctuaries have been recognized as a problem in cancer treatment. The best known of these is the central nervous system (CNS). Techniques to temporarily disrupt the blood‐brain barrier are now appearing. Mechanisms to administer therapy directly into the CNS are also being reassessed. Implantable pumps and reservoirs have been used to treat selected organs or for regional perfusions. Other treatments that are regional in scope include administration directly into a cavity or into a tumor. Computerized implantable devices should play a major role in cancer therapy in the future, in pain control as well as antibiotic and hormone administration. In recent years, mathematical models have been developed that can more accurately predict drug distribution and metabolism in various tissues of the body. Such models point the way to more logical designs of chemotherapeutic administration. The expanded use of autologous bone marrow transplantation, along with improving techniques of “purging” the marrow of tumor cells before reinfusion can be anticipated. Pro‐drugs are substances that must be biotransformed in vivo to exert their pharmacologic effect. Certain pro‐drugs that may be more effective or resistance‐avoiding analogues of estab lished chemotherapeutic drugs are currently under development and offer considerable promise. A new era of improved drug delivery is achievable and can lead to greater efficacy of treatment regimens and a higher cure rate while at the same time reducing toxicity. This discussion deals primarily with the currently emerging therapies of monoclonal antibodies, liposomes and intra‐arterial infusions. Cancer 58:573‐583, 1986.The use of pH-sensitive liposomes enhances the delivery of drugs that target the endoplasmic reticulum, reducing the required dosage compared to direct administration of the drug without such liposomes. In particular, antiviral compounds such as N-butyldeoxynojirnmycin can be used in lower amounts when administered in a pH-sensitive liposome.

The solution NMR structure of glucosylated N-glycans involved in the early stages of glycoprotein biosynthesis and folding

Glucosylated oligomannose N‐linked oligosaccharides (GlcxMan9GlcNAc2 where x = 1–3) are not normally found on mature glycoproteins but are involved in the early stages of glycoprotein biosynthesis and folding as (i) recognition elements during protein N‐glycosylation and chaperone recognition and (ii) substrates in the initial steps of N‐glycan processing. By inhibiting the first steps of glycan processing in CHO cells using the α‐glucosidase inhibitor N‐butyl‐deoxynojirimycin, we have produced sufficient Glc3Man7GlcNAc2 for structural analysis by nuclear magnetic resonance (NMR) spectroscopy. Our results show the glucosyl cap to have a single, well‐defined conformation independent of the rest of the saccharide. Comparison with the conformation of Man9GlcNAc2, previously determined by NMR and molecular dynamics, shows the mannose residues to be largely unaffected by the presence of the glucosyl cap. Sequential enzymatic cleavage of the glucose residues does not affect the conformation of the remaining saccharide. Modelling of the Glc3 Man9GlcNAc2, Glc2Man9GlcNAc2 and Glc1Man9 GlcNAc2 conformations shows the glucose residues to be fully accessible for recognition. A more detailed analysis of the conformations allows potential recognition epitopes on the glycans to be identified and can form the basis for understanding the specificity of the glucosidases and chaperones (such as calnexin) that recognize these glycans, with implications for their mechanisms of action.

Inhibition of N-Glycan Processing in B16 Melanoma Cells Results in Inactivation of Tyrosinase but Does Not Prevent Its Transport to the Melanosome

Tyrosinase is the key enzyme in melanin biosynthesis, catalyzing multiple steps in this pathway. The mature glycoprotein is transported from the Golgi to the melanosome where melanin biosynthesis occurs. In this study, we have investigated the effects of inhibitors of N-glycan processing on the synthesis, transport, and catalytic activity of tyrosinase. When B16 mouse melanoma cells were cultured in the presence ofN-butyldeoxynojirimycin, an inhibitor of the endoplasmic reticulum-processing enzymes α-glucosidases I and II, the enzyme was synthesized and transported to the melanosome but almost completely lacked catalytic activity. The cells contained only 2% of the melanin found in untreated cells. Structural analysis of theN-glycans from N-butyldeoxynojirimycin-treated B16 cells demonstrated that three oligosaccharide structures (Glc3Man7–9) predominated. Removal of the glucose residues with α-glucosidases I and II failed to restore enzymatic activity, suggesting that the glucosylatedN-glycans do not sterically interfere with the enzyme’s active sites. The mannosidase inhibitor deoxymannojirimycin had no effect on catalytic activity suggesting that the retention of glucosylated N-glycans results in the inactivation of this enzyme. The retention of glucosylated N-glycans does not therefore result in misfolding and degradation of the glycoprotein, as the enzyme is transported to the melanosome, but may cause conformational changes in its catalytic domains.

Hepatitis B Virus Requires Intact Caveolin-1 Function for Productive Infection in HepaRG Cells

ABSTRACT Investigation of the entry pathways of hepatitis B virus (HBV), a member of the family Hepadnaviridae, has been hampered by the lack of versatile in vitro infectivity models. Most concepts of hepadnaviral infection come from the more robust duck HBV system; however, whether the two viruses use the same mechanisms to invade target cells is still a matter of controversy. In this study, we investigate the role of an important plasma membrane component, caveolin-1 (Cav-1), in HBV infection. Caveolins are the main structural components of caveolae, plasma membrane microdomains enriched in cholesterol and sphingolipids, which are involved in the endocytosis of numerous ligands and complex signaling pathways within the cell. We used the HepaRG cell line permissive for HBV infection to stably express dominant-negative Cav-1 and dynamin-2, a GTPase involved in vesicle formation at the plasma membrane and other organelles. The endocytic properties of the newly established cell lines, designated HepaRGCav-1, HepaRGCav-1Δ1-81, HepaRGDyn-2, and HepaRGDyn-2K44A, were validated using specific markers for different entry routes. The cells maintained their properties during cell culture, supported differentiation, and were permissive for HBV infection. The levels of both HBV transcripts and antigens were significantly decreased in cells expressing the mutant proteins, while viral replication was not directly affected. Chemical inhibitors that specifically inhibit clathrin-mediated endocytosis had no effect on HBV infection. We concluded that HBV requires a Cav-1-mediated entry pathway to initiate productive infection in HepaRG cells.

Folding and Maturation of Tyrosinase-related Protein-1 Are Regulated by the Post-translational Formation of Disulfide Bonds and by N-Glycan Processing

In this study we have explored the endoplasmic reticulum associated events accompanying the maturation of the tyrosinase-related protein-1 (TRP-1) nascent chain synthesized in mouse melanoma cells. We show that TRP-1 folding process occurs much more rapidly than for tyrosinase, a highly homologous protein, being completed post-translationally by the formation of critical disulfide bonds. In cells pretreated with dithiothreitol (DTT), unfolded TRP-1 is retained in the endoplasmic reticulum by a prolonged interaction with calnexin and BiP before being targeted for degradation. The TRP-1 chain was able to fold into DTT-resistant conformations both in the presence or absence of α-glucosidase inhibitors, but folding occurred through different pathways. During the normal folding pathway, TRP-1 interacts with calnexin. In the presence of α-glucosidase inhibitors, the interaction with calnexin is prevented, with TRP-1 folding being assisted by BiP. In this case, the process has similar kinetics to that of untreated TRP-1 and yields a compact form insensitive to DTT as well. However, this form has different thermal denaturation properties than the native conformation. We conclude that disulfide bridge burring is crucial for the TRP-1 export. This suggests that although various folding pathways may complete this process, the native form may be acquired only through the normal unperturbed pathway.

Differentiation of mesenchymal stem cells onto highly adherent radio frequency-sputtered carbonated hydroxylapatite thin films.

In this work, an improved version of the radio frequency magnetron sputtering (RF-MS) technique was used to prepare highly adherent B-type carbonated hydroxylapatite (B-CHA) thin films. Fourier transform infrared spectroscopy (FTIR) and grazing incidence X-ray diffraction studies proved that the coatings maintained the composition and revealed the polycrystalline structure of HA. Scanning electron microscopy analysis showed that the CHA films are rough and exhibit a homogeneous microstructure. Energy-dispersive X-ray spectroscopy (EDX) mapping demonstrated a uniform distribution of the Ca and P cations while a Ca/P ratio of 1.8 was found. In addition, the FTIR experiments showed a remarkable reproducibility of the nanostructures. Human mesenchymal stem cells (hMSCs), in vitro differentiated osteoblasts, and explanted bone cells were grown over the surface of CHA coatings for periods between a few hours and 21 days. Osteoprogenitor cells maintained viability and characteristic morphology after adhesion on CHA coatings. Ki67-positive osteoblasts were the evidence of cell proliferation events. Cells showed positive staining for markers of osteoblast phenotype such as collagen type I, bone sialoprotein and osteonectin. Our data showed the formation of mineralized foci by differentiation of hMSCs to human primary osteoblasts after cultivation in osteogenic media on RF-sputtered films. The results demonstrate the capacity of B-type CHA coating to support MSCs adhesion and osteogenic differentiation ability.

Levan Nanostructured Thin Films by MAPLE Assembling

Synthesis of nanostructured thin films of pure and oxidized levan exopolysaccharide by matrix-assisted pulsed laser evaporation is reported. Solutions of pure exopolysaccharides in dimethyl sulfoxide were frozen in liquid nitrogen to obtain solid cryogenic pellets that have been used as targets in pulsed laser evaporation experiments with a KrF* excimer source. The expulsed material was collected and assembled onto glass slides and Si wafers. The contact angle studies evidenced a higher hydrophilic behavior in the case of oxidized levan structures because of the presence of acidic aldehyde-hydrogen bonds of the coating formed after oxidation. The obtained films preserved the base material composition as confirmed by Fourier transform infrared spectroscopy. They were compact with high specific surface areas, as demonstrated by scanning electron and atomic force microscopy investigations. In vitro colorimetric assays revealed a high potential for cell proliferation for all coatings with certain predominance for oxidized levan.

Activation of ERAD Pathway by Human Hepatitis B Virus Modulates Viral and Subviral Particle Production

Hepatitis B virus (HBV) belongs to the Hepadnaviridae family of enveloped DNA viruses. It was previously shown that HBV can induce endoplasmic reticulum (ER) stress and activate the IRE1-XBP1 pathway of the unfolded protein response (UPR), through the expression of the viral regulatory protein X (HBx). However, it remained obscure whether or not this activation had any functional consequences on the target genes of the UPR pathway. Of these targets, the ER degradation-enhancing, mannosidase-like proteins (EDEMs) are thought to play an important role in relieving the ER stress during UPR, by recognizing terminally misfolded glycoproteins and delivering them to the ER-associated degradation (ERAD). In this study, we investigated the role of EDEMs in the HBV life-cycle. We found that synthesis of EDEMs (EDEM1 and its homologues, EDEM2 and EDEM3) is significantly up-regulated in cells with persistent or transient HBV replication. Co-expression of the wild-type HBV envelope proteins with EDEM1 resulted in their massive degradation, a process reversed by EDEM1 silencing. Surprisingly, the autophagy/lysosomes, rather than the proteasome were involved in disposal of the HBV envelope proteins. Importantly, inhibition of the endogenous EDEM1 expression in HBV replicating cells significantly increased secretion of both, enveloped virus and subviral particles. This is the first report showing that HBV activates the ERAD pathway, which, in turn, reduces the amount of envelope proteins, possibly as a mechanism to control the level of virus particles in infected cells and facilitate the establishment of chronic infections.

pH-sensitive liposomes are efficient carriers for endoplasmic reticulum-targeted drugs in mouse melanoma cells

Tyrosinase, the key enzyme of melanin biosynthesis, is inactivated in melanoma cells following the incubation with the imino-sugar N-butyldeoxynojirimycin, an inhibitor of the endoplasmic reticulum N-glycosylation processing. We have previously shown that tyrosinase inhibition requires high NB-DNJ concentrations, suggesting an inefficient cellular uptake of the drug. Here we show that the use of pH-sensitive liposomes composed of dioleoylphosphatidylethanolamine and cholesteryl hemisuccinate for the delivery of NB-DNJ reduced the required dose for tyrosinase inhibition by a factor of 1000. The results indicate that these pH-sensitive liposomes are efficient carriers for imino-sugars delivery in the endoplasmic reticulum of mammalian cells.