Molecular characterization of the rat Kupffer cell glycoprotein receptor

Host genetic backgrounds: the key to determining parasite-host adaptation

Parasitic diseases pose a significant threat to global public health, particularly in developing countries. Host genetic factors play a crucial role in determining susceptibility and resistance to infection. Recent advances in molecular and biological technologies have enabled significant breakthroughs in understanding the impact of host genes on parasite adaptation. In this comprehensive review, we analyze the host genetic factors that influence parasite adaptation, including hormones, nitric oxide, immune cells, cytokine gene polymorphisms, parasite-specific receptors, and metabolites. We also establish an interactive network to better illustrate the complex relationship between host genetic factors and parasite-host adaptation. Additionally, we discuss future directions and collaborative research priorities in the parasite-host adaptation field, including investigating the impact of host genes on the microbiome, developing more sophisticated models, identifying and characterizing parasite-specific receptors, utilizing patient-derived sera as diagnostic and therapeutic tools, and developing novel treatments and management strategies targeting specific host genetic factors. This review highlights the need for a comprehensive and systematic approach to investigating the underlying mechanisms of parasite-host adaptation, which requires interdisciplinary collaborations among biologists, geneticists, immunologists, and clinicians. By deepening our understanding of the complex interactions between host genetics and parasite adaptation, we can develop more effective and targeted interventions to prevent and treat parasitic diseases. Overall, this review provides a valuable resource for researchers and clinicians working in the parasitology field and offers insights into the future directions of this critical research area.

Mucins Shed from the Laminated Layer in Cystic Echinococcosis Are Captured by Kupffer Cells via the Lectin Receptor Clec4F

Cystic echinococcosis is caused by the larval stages (hydatids) of cestode parasites belonging to the species cluster Echinococcus granulosus sensu lato, with E. granulosus sensu stricto being the main infecting species. Hydatids are bladderlike structures that attain large sizes within various internal organs of livestock ungulates and humans. Hydatids are protected by the massive acellular laminated layer (LL), composed mainly of mucins. Parasite growth requires LL turnover, and abundant LL-derived particles are found at infection sites in infected humans, raising the question of how LL materials are dealt with by the hosts. In this article, we show that E. granulosus sensu stricto LL mucins injected into mice are taken up by Kupffer cells, the liver macrophages exposed to the vascular space. This uptake is largely dependent on the intact mucin glycans and on Clec4F, a C-type lectin receptor which, in rodents, is selectively expressed in Kupffer cells. This uptake mechanism operates on mucins injected both in soluble form intravenously (i.v.) and in particulate form intraperitoneally (i.p.). In mice harboring intraperitoneal infections by the same species, LL mucins were found essentially only at the infection site and in the liver, where they were taken up by Kupffer cells via Clec4F. Therefore, shed LL materials circulate in the host, and Kupffer cells can act as a sink for these materials, even when the parasite grows in sites other than the liver.

Early detection of the initial stages of LED light-triggered non-alcoholic fatty liver disease by wax physisorption kinetics-Fourier transform infrared imaging

Light-emitting diodes (LEDs), particularly in the blue waveform range, are regarded as a major source of circadian rhythm dysregulation. A circadian rhythm dysregulation induced by blue LEDs is associated with non-alcoholic fatty liver disease (NAFLD). Hepatocellular accumulation of lipids is a key event in the early stages of NAFLD. Kupffer cells (KCs) have been reported to be lost in the early onset of NAFLD followed by an inflammatory reaction that alters the liver response to lipid overload. This study focused on the detection of the initial stages (subpathological stages) of LED light-triggered NAFLD. Mice were exposed to either blue or white LED irradiation for 44 weeks. Synchrotron radiation-based Fourier-transform infrared microspectroscopy (SR-FTIRM) and wax physisorption kinetic-Fourier transform infrared (WPK-FTIR) imaging were used to evaluate the ratio of lipid to protein and the glycosylation of glycoprotein, respectively. Immunohistopathological studies on KCs and circadian-related proteins were performed. Although liver biopsy showed normal pathology, an SR-FTIRM study revealed a high hepatic lipid-to-protein ratio after receiving LED illumination. The results of WPK-FTIR demonstrated that a high inflammation index was found in the high irradiance of the blue LED illumnation group. These groups showed a decrease in KC number and an increase in Bmal1 and Reverbα circadian protein expression. These findings provide explanations for the reduction of KCs without subsequent inflammation. A significant reduction of Per2 and Cry1 expression is correlated with the findings of WPK-FTIR imaging. WPK-FTIR is a sensitive method for detecting initiative stages of NAFLD induced by long-term blue LED illumination.

A Comprehensive Review on Liver Targeting: Emphasis on Nanotechnology- based Molecular Targets and Receptors Mediated Approaches

BACKGROUND

The pathogenesis of hepatic diseases involves several cells, which complicates the delivery of pharmaceutical agents. Many severe liver diseases affecting the worldwide population cannot be effectively treated. Major hindrances or challenges are natural physiological barriers and non-specific targeting of drugs administered, leading to inefficient treatment. Hence, there is an earnest need to look for novel therapeutic strategies to overcome these hindrances. A kind of literature has reported that drug safety and efficacy are incredibly raised when a drug is incorporated inside or attached to a polymeric material of either hydrophilic or lipophilic nature. This has driven the dynamic investigation for developing novel biodegradable materials, drug delivery carriers, target-specific drug delivery systems, and many other novel approaches.

OBJECTIVE

Present review is devoted to summarizing receptor-based liver cell targeting using different modified novel synthetic drug delivery carriers. It also highlights recent progress in drug targeting to diseased liver mediated by various receptors, including asialoglycoprotein, mannose and galactose receptor, Fc receptor, low-density lipoprotein, glycyrrhetinic, and bile acid receptor. The essential consideration is given to treating liver cancer targeting using nanoparticulate systems, proteins, viral and non-viral vectors, homing peptides and gene delivery.

CONCLUSION

Receptors based targeting approach is one such approach that was explored by researchers to develop novel formulations which can ensure site-specific drug delivery. Several receptors are on the surfaces of liver cells, which are highly overexpressed in various disease conditions. They all are helpful for the treatment of liver cancer.

Schistosoma mansoni α-N-acetylgalactosaminidase (SmNAGAL) regulates coordinated parasite movement and egg production

α-galactosidase (α-GAL) and α-N-acetylgalactosaminidase (α-NAGAL) are two glycosyl hydrolases responsible for maintaining cellular homeostasis by regulating glycan substrates on proteins and lipids. Mutations in the human genes encoding either enzyme lead to neurological and neuromuscular impairments seen in both Fabry- and Schindler/Kanzaki- diseases. Here, we investigate whether the parasitic blood fluke Schistosoma mansoni, responsible for the neglected tropical disease schistosomiasis, also contains functionally important α-GAL and α-NAGAL proteins. As infection, parasite maturation and host interactions are all governed by carefully-regulated glycosylation processes, inhibiting S. mansoni's α-GAL and α-NAGAL activities could lead to the development of novel chemotherapeutics. Sequence and phylogenetic analyses of putative α-GAL/α-NAGAL protein types showed Smp_089290 to be the only S. mansoni protein to contain the functional amino acid residues necessary for α-GAL/α-NAGAL substrate cleavage. Both α-GAL and α-NAGAL enzymatic activities were higher in females compared to males (p<0.05; α-NAGAL > α-GAL), which was consistent with smp_089290's female biased expression. Spatial localisation of smp_089290 revealed accumulation in parenchymal cells, neuronal cells, and the vitellaria and mature vitellocytes of the adult schistosome. siRNA-mediated knockdown (>90%) of smp_089290 in adult worms significantly inhibited α-NAGAL activity when compared to control worms (siLuc treated males, p<0.01; siLuc treated females, p<0.05). No significant reductions in α-GAL activities were observed in the same extracts. Despite this, decreases in α-NAGAL activities correlated with a significant inhibition in adult worm motility as well as in egg production. Programmed CRISPR/Cas9 editing of smp_089290 in adult worms confirmed the egg reduction phenotype. Based on these results, Smp_089290 was determined to act predominantly as an α-NAGAL (hereafter termed SmNAGAL) in schistosome parasites where it participates in coordinating movement and oviposition processes. Further characterisation of SmNAGAL and other functionally important glycosyl hydrolases may lead to the development of a novel anthelmintic class of compounds.

Kupffer cell receptor CLEC4F is important for the destruction of desialylated platelets in mice

The liver has recently been identified as a major organ for destruction of desialylated platelets. However, the underlying mechanism remains unclear. Kupffer cells, which are professional phagocytic cells in the liver, comprise the largest population of resident tissue macrophages in the body. Kupffer cells express a C-type lectin receptor, CLEC4F, that recognizes desialylated glycans with an unclear in vivo role in mediating platelet destruction. In this study, we generated a CLEC4F-deficient mouse model (Clec4f^-/-) and found that CLEC4F was specifically expressed by Kupffer cells. Using the Clec4f^-/- mice and a newly generated platelet-specific reporter mouse line, we revealed a critical role for CLEC4F on Kupffer cells in mediating destruction of desialylated platelets in the liver in vivo. Platelet clearance experiments and ultrastructural analysis revealed that desialylated platelets were phagocytized predominantly by Kupffer cells in a CLEC4F-dependent manner in mice. Collectively, these findings identify CLEC4F as a Kupffer cell receptor important for the destruction of desialylated platelets induced by bacteria-derived neuraminidases, which provide new insights into the pathogenesis of thrombocytopenia in disease conditions such as sepsis.

N-Acetyl Galactosamine Targeting: Paving the Way for Clinical Application of Nucleotide Medicines in Cardiovascular Diseases

While the promise of oligonucleotide therapeutics, such as (chemically modified) ASO (antisense oligonucleotides) and short interfering RNAs, is undisputed from their introduction onwards, their unfavorable pharmacokinetics and intrinsic capacity to mobilize innate immune responses, were limiting widespread clinical use. However, these major setbacks have been tackled by breakthroughs in chemistry, stability and delivery. When aiming an intervention hepatic targets, such as lipid and sugar metabolism, coagulation, not to mention cancer and virus infection, introduction of N-acetylgalactosamine aided targeting technology has advanced the field profoundly and by now a dozen of N-acetylgalactosamine therapeutics for these indications have been approved for clinical use or have progressed to clinical trial stage 2 to 3 testing. This technology, in combination with major advances in oligonucleotide stability allows safe and durable intervention in targets that were previously deemed undruggable, such as Lp(a) and PCSK9 (proprotein convertase subtilisin/kexin type 9), at high efficacy and specificity, often with as little as 2 doses per year. Their successful use even the most visionary would not have predicted 2 decades ago. Here, we will review the evolution of N-acetylgalactosamine technology. We shall outline their fundamental design principles and merits, and their application for the delivery of oligonucleotide therapeutics to the liver. Finally, we will discuss the perspectives of N-acetylgalactosamine technology and propose directions for future research in receptor targeted delivery of these gene medicines.

Mammalian lectin arrays for screening host-microbe interactions

Many members of the C-type lectin family of glycan-binding receptors have been ascribed roles in the recognition of microorganisms and serve as key receptors in the innate immune response to pathogens. Other mammalian receptors have become targets through which pathogens enter target cells. These receptor roles have often been documented with binding studies involving individual pairs of receptors and microorganisms. To provide a systematic overview of interactions between microbes and the large complement of C-type lectins, here we developed a lectin array and suitable protocols for labeling of microbes that could be used to probe this array. The array contains C-type lectins from cow, chosen as a model organism of agricultural interest for which the relevant pathogen–receptor interactions have not been previously investigated in detail. Screening with yeast cells and various strains of both Gram-positive and -negative bacteria revealed distinct binding patterns, which in some cases could be explained by binding to lipopolysaccharides or capsular polysaccharides, but in other cases they suggested the presence of novel glycan targets on many of the microorganisms. These results are consistent with interactions previously ascribed to the receptors, but they also highlight binding to additional sugar targets that have not previously been recognized. Our findings indicate that mammalian lectin arrays represent unique discovery tools for identifying both novel ligands and new receptor functions.

DNA Framework-Programmed Cell Capture via Topology-Engineered Receptor-Ligand Interactions

Receptor-ligand interactions (RLIs) that play pivotal roles in living organisms are often depicted with the classic keys-and-locks model. Nevertheless, RLIs on the cell surface are generally highly complex and nonlinear, partially due to the noncontinuous and dynamic distribution of receptors on extracellular membranes. Here, we develop a tetrahedral DNA framework (TDF)-programmed approach to topologically engineer RLIs on the cell membrane, which enables active recruitment-binding of clustered receptors for high-affinity capture of circulating tumor cells (CTCs). The four vertices of a TDF afford orthogonal anchoring of ligands with spatial organization, based on which we synthesized n-simplexes harboring 1-3 aptamers targeting epithelial cell adhesion molecule (EpCAM) that are overexpressed on the membrane of tumor cells. The 2-simplex with three aptamers not only shows increased binding affinity (∼19-fold) but prevents endocytosis by cells. By using 2-simplex as the capture probe, we demonstrate the high-efficiency CTC capture, which is challenged in real clinical breast cancer patient samples. This TDF-programmed platform thus provides a powerful means for studying RLIs in physiological settings and for cancer diagnosis.

Trimeric structure of the mouse Kupffer cell C-type lectin receptor Clec4f

The C-type lectin receptor Clec4f has been identified as a specific surface marker for Kupffer cells, although its ortholog is absent in humans and its biological function remains elusive. Here, we report the crystal structure of a truncated mouse trimeric Clec4f. The orientation between the carbohydrate-recognition domain of Clec4f and its neck region differs from other C-type lectins, resulting in an observed distance of 45 Å between the glycan-binding sites within the Clec4f trimer. Interestingly, the trimeric coiled-coil interface within its heptad neck region contains multiple polyglutamine interactions instead of the predominantly hydrophobic leucine zipper found in other C-type lectin receptors. The Clec4f trimeric structure displays unique features regarding its assembly and ligand recognition, shedding light on the evolution and diversity of the C-type lectin family.

Absence of a human ortholog of rodent Kupffer cell galactose-binding receptor encoded by the CLEC4f gene

The murine CLEC4f gene encodes the Kupffer cell receptor, a galactose-binding receptor containing a C-type carbohydrate-recognition domain. Orthologs have been identified in nearly 100 species. The receptors from rat and mouse have previously been characterized and data presented here show that functional CLEC4f protein is expressed in domestic cattle (Bos taurus). However, the human CLEC4f gene does not encode a functional receptor because a mutation in the splice acceptor site of the final exon prevents appropriate splicing and a missense mutation disrupts the sugar-binding site. Transcriptomic and PCR analysis of transcripts confirms the absence of a spliced transcript containing the final exon and only background levels of transcripts are detected in human tissues. These mutations are also present in the CLEC4f gene in Neanderthals. In contrast to humans, closely related species, including chimpanzees, do have CLEC4f genes that encode full-length receptors. Affinity chromatography and glycan array results demonstrate that the chimpanzee, bovine and murine proteins all bind to galactose, but they show preferences for different subsets of galactose-containing glycans. In non-human primates, the receptor is expressed in spleen rather than in liver. The results indicate that the CLEC4f protein probably has distinct functions in different species. Absence of the receptor precludes using it for targeting of glycoconjugates to cells in human liver. The fact that CLEC4f protein is expressed in spleen in non-human primates and the close evolutionary relationship of the CLEC4f protein to langerin (CD207) suggest that it may function in the immune system, possibly as a pathogen receptor.

Liver Resident Macrophages (Kupffer Cells) Share Several Functional Antigens in Common with Endothelial Cells

The identification and specific functions of Kupffer cells (KCs), a liver resident macrophage subpopulation, are still unclear. We compared KCs with peritoneal macrophages using cDNA microarray analysis and found that these cells share some antigens with endothelial cells. KCs highly express VCAM-1 and VEGF receptors (VEGF-Rs) at transcriptional and protein levels. VCAM-1 mediates the functional binding of KCs with lymphocytes and induces KC activation. Among the VEGF receptors, VEGF-R2 and VEGF-R3 were expressed on the KCs, while VEGF-R1 was expressed on other tissue macrophage subsets. VEGF120, a ligand of both VEGF-R1 and VEGF-R2, transduced strong survival and chemotactic signals through the KCs, when compared to PIGF, a VEGF-R1 ligand, indicating that VEGF-R2 plays significant roles in regulating KC activities. Expression of the VEGF-Rs was regulated by TLR4 signalling. These results suggest that the function of KCs is partly regulated by the common antigens shared with endothelial cells.

The laminated layer: Recent advances and insights into Echinococcus biology and evolution

The laminated layer is the unique mucin-based extracellular matrix that protects Echinococcus larvae, and thus to an important extent, shapes host-parasite relationships in the larval echinococcoses. In 2011, we published twin reviews summarizing what was known about this structure. Since then, important advances have been made. Complete genomes and some RNAseq data are now available for E. multilocularis and E. granulosus, leading to the inference that the E. multilocularis LL is probably formed by a single type of mucin backbone, while a second apomucin subfamily additionally contributes to the E. granulosus LL. Previously suspected differences between E. granulosus and E. multilocularis in mucin glycan size have been confirmed and pinned down to the virtual absence of Galβ1-3 chains in E. multilocularis. The LL carbohydrates from both species have been found to interact selectively with the Kupffer cell receptor expressed in rodent liver macrophages, highlighting the ancestral adaptations to rodents as intermediate hosts and to the liver as infection site. Finally, LL particles have been shown to possess carbohydrate-independent mechanisms profoundly conditioning non-liver-specific dendritic cells and macrophages. These advances are discussed in an integrated way, and in the context of the newly determined phylogeny of Echinococcus and its taenid relatives.

Delivery of an enzyme-IGFII fusion protein to the mouse brain is therapeutic for mucopolysaccharidosis type IIIB

Mucopolysaccharidosis type IIIB (MPS IIIB, Sanfilippo syndrome type B) is a lysosomal storage disease characterized by profound intellectual disability, dementia, and a lifespan of about two decades. The cause is mutation in the gene encoding α-N-acetylglucosaminidase (NAGLU), deficiency of NAGLU, and accumulation of heparan sulfate. Impediments to enzyme replacement therapy are the absence of mannose 6-phosphate on recombinant human NAGLU and the blood-brain barrier. To overcome the first impediment, a fusion protein of recombinant NAGLU and a fragment of insulin-like growth factor II (IGFII) was prepared for endocytosis by the mannose 6-phosphate/IGFII receptor. To bypass the blood-brain barrier, the fusion protein ("enzyme") in artificial cerebrospinal fluid ("vehicle") was administered intracerebroventricularly to the brain of adult MPS IIIB mice, four times over 2 wk. The brains were analyzed 1-28 d later and compared with brains of MPS IIIB mice that received vehicle alone or control (heterozygous) mice that received vehicle. There was marked uptake of the administered enzyme in many parts of the brain, where it persisted with a half-life of approximately 10 d. Heparan sulfate, and especially disease-specific heparan sulfate, was reduced to control level. A number of secondary accumulations in neurons [β-hexosaminidase, LAMP1(lysosome-associated membrane protein 1), SCMAS (subunit c of mitochondrial ATP synthase), glypican 5, β-amyloid, P-tau] were reduced almost to control level. CD68, a microglial protein, was reduced halfway. A large amount of enzyme also appeared in liver cells, where it reduced heparan sulfate and β-hexosaminidase accumulation to control levels. These results suggest the feasibility of enzyme replacement therapy for MPS IIIB.

The surface carbohydrates of the Echinococcus granulosus larva interact selectively with the rodent Kupffer cell receptor

The larvae of the cestodes belonging to the genus Echinococcus dwell primarily in mammalian liver. They are protected by the laminated layer (LL), an acellular mucin-based structure. The glycans decorating these mucins constitute the overwhelming majority of molecules exposed by these larvae to their hosts. However, their decoding by host innate immunity has not been studied. Out of 36 mammalian innate receptors with carbohydrate-binding domains, expressed as Fc fusions, only the mouse Kupffer cell receptor (KCR; CLEC4F) bound significantly to the Echinococcus granulosus LL mucins. The receptor also bound the Echinococcus multilocularis LL. Out of several synthetic glycans representing Echinococcus LL structures, the KCR bound strongly in particular to those ending in Galα1-4Galβ1-3 or Galα1-4Galβ1-4GlcNAc, both characteristic LL carbohydrate motifs. LL carbohydrates may be optimized to interact with the KCR, expressed only in liver macrophages, cells known to contribute to the tolerogenic antigen presentation that is characteristic of this organ.

CLEC4F is an inducible C-type lectin in F4/80-positive cells and is involved in alpha-galactosylceramide presentation in liver

CLEC4F, a member of C-type lectin, was first purified from rat liver extract with high binding affinity to fucose, galactose (Gal), N-acetylgalactosamine (GalNAc), and un-sialylated glucosphingolipids with GalNAc or Gal terminus. However, the biological functions of CLEC4F have not been elucidated. To address this question, we examined the expression and distribution of murine CLEC4F, determined its binding specificity by glycan array, and investigated its function using CLEC4F knockout (Clec4f-/-) mice. We found that CLEC4F is a heavily glycosylated membrane protein co-expressed with F4/80 on Kupffer cells. In contrast to F4/80, CLEC4F is detectable in fetal livers at embryonic day 11.5 (E11.5) but not in yolk sac, suggesting the expression of CLEC4F is induced as cells migrate from yolk cells to the liver. Even though CLEC4F is not detectable in tissues outside liver, both residential Kupffer cells and infiltrating mononuclear cells surrounding liver abscesses are CLEC4F-positive upon Listeria monocytogenes (L. monocytogenes) infection. While CLEC4F has strong binding to Gal and GalNAc, terminal fucosylation inhibits CLEC4F recognition to several glycans such as Fucosyl GM1, Globo H, Bb3∼4 and other fucosyl-glycans. Moreover, CLEC4F interacts with alpha-galactosylceramide (α-GalCer) in a calcium-dependent manner and participates in the presentation of α-GalCer to natural killer T (NKT) cells. This suggests that CLEC4F is a C-type lectin with diverse binding specificity expressed on residential Kupffer cells and infiltrating monocytes in the liver, and may play an important role to modulate glycolipids presentation on Kupffer cells.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

Galactose-modified cationic liposomes as a liver-targeting delivery system for small interfering RNA

We have developed a galactose-modified cationic liposome for delivery of small interfering RNA (siRNA) to the liver. The liposomes were designed to be transported into hepatocytes via the asialoglycoprotein receptor, which recognizes galactose residues. The liposomes contained a novel galactose-modified lipid, 1,2-dioleoyl-sn-glycerol-3-phosphatidyl-N-(1-deoxylactito-1-yl)ethanolamine (GDOPE). Delivery of siRNA to hepatocytes by the liposomes was evaluated by measuring the gene-silencing activity of liposome : siRNA complexes in two human hepatoma cell lines. A formulation with a cationic lipid : GDOPE ratio of 3 : 5 by weight, LIC-G5, showed the strongest activity. In mice, intravenous injection of LIC-G5 complexed with (3)H-labeled siRNA led to accumulation of radioactivity in the liver. When the hepatic cellular uptake was determined after intravenous injection into mice followed by collagenase liver perfusion, the distribution of siRNA to parenchymal cells was 1.9 times higher when LIC-G5 rather than nongalactosylated LIC was used as the carrier. The concentration of siRNA accumulated was 45 µg/ml, 30 times the concentration that produced strong gene silencing in vitro and therefore presumably sufficient for a therapeutic effect. Because increasing the cationic-lipid content of a liposome carrier generally enhances the uptake of siRNA by the liver at the expense of increased cell toxicity, we used only a moderate amount of cationic lipid in our galactose-modified carrier. LIC-G5 enhanced the uptake of siRNA by the liver without cytotoxic effects and is a promising candidate delivery system for liver-targeted siRNA therapy.

Mouse LSECtin as a model for a human Ebola virus receptor

The biochemical properties of mouse LSECtin, a glycan-binding receptor that is a member of the C-type lectin family found on sinusoidal endothelial cells, have been investigated. The C-type carbohydrate-recognition domain of mouse LSECtin, expressed in bacteria, has been used in solid-phase binding assays, and a tetramerized form has been used to probe a glycan array. In spite of sequence differences near the glycan-binding sites, the mouse receptor closely mimics the properties of the human receptor, showing high affinity binding to glycans bearing terminal GlcNAcβ1-2Man motifs. Site-directed mutagenesis has been used to confirm that residues near the binding site that differ between the human and the mouse proteins do not affect this binding specificity. Mouse and human LSECtin have been shown to bind Ebola virus glycoprotein with equivalent affinities, and the GlcNAcβ1-2Man disaccharide has been demonstrated to be an effective inhibitor of this interaction. These studies provide a basis for using mouse LSECtin, and knockout mice lacking this receptor, to model the biological properties of the human receptor.

Profiling carbohydrate-receptor interaction with recombinant innate immunity receptor-Fc fusion proteins

The recognition of bacteria, viruses, fungi, and other microbes is controlled by host immune cells, which are equipped with many innate immunity receptors, such as Toll-like receptors, C-type lectin receptors, and immunoglobulin-like receptors. Our studies indicate that the immune modulating properties of many herbal drugs, for instance, the medicinal fungus Reishi (Ganoderma lucidum) and Cordyceps sinensis, could be attributed to their polysaccharide components. These polysaccharides specifically interact with and activate surface receptors involved in innate immunity. However, due to the complexity of polysaccharides and their various sources from medicinal fungi, quantitative analysis of medicinal polysaccharide extracts with regard to their functions represents a major challenge. To profile carbohydrate-immune receptor interactions, the extracellular domains of 17 receptors were cloned as Fc-fusion proteins, such that their interactions with immobilized polysaccharides could be probed in an enzyme-linked immunosorbent assay. The results show that several innate immune receptors, including Dectin-1, DC-SIGN, Langerin, Kupffer cell receptor, macrophage mannose receptor, TLR2, and TLR4, interact with the polysaccharide extracts from G. lucidum (GLPS). This analysis revealed distinct polysaccharide profiles from different sources of medicinal fungi, and the innate immune receptor-based enzyme-linked immunosorbent assay described here can serve as a high-throughput profiling method for the characterization and quality control of medicinal polysaccharides. It also provides a means to dissect the molecular mechanism of medicinal polysaccharide-induced immunomodulation events.

Autonomous tetramerization domains in the glycan-binding receptors DC-SIGN and DC-SIGNR

Multivalent binding of glycans on pathogens and on mammalian cells by the receptors DC-SIGN (CD209) and DC-SIGNR (L-SIGN, CD299) is dependent on correct disposition of the C-type carbohydrate-recognition domains projected at the C-terminal ends of necks at the cell surface. In the work reported here, neck domains of DC-SIGN and DC-SIGNR expressed in isolation are shown to form tetramers in the absence of the CRDs. Stability analysis indicates that interactions between the neck domains account fully for the stability of the tetrameric extracellular portions of the receptors. The neck domains are approximately 40% alpha-helical based on circular dichroism analysis. However, in contrast to other glycan-binding receptors in which fully helical neck regions are intimately associated with C-terminal C-type CRDs, the neck domains in DC-SIGN and DC-SIGNR act as autonomous tetramerization domains and the neck domains and CRDs are organized independently. Neck domains from polymorphic forms of DC-SIGNR that lack some of the repeat sequences show modestly reduced stability, but differences near the C-terminal end of the neck domains lead to significantly enhanced stability of DC-SIGNR tetramers compared to DC-SIGN.

Virulence attenuation of a UDP-galactose/N-acetylglucosamine beta1,4 galactosyltransferase expressing Leishmania donovani promastigote

Protozoan parasites of the genus Leishmania are the causative agent of leishmaniasis, a disease whose manifestations in humans range from mild cutaneous lesions to fatal visceral infections. Human visceral leishmaniasis is caused by Leishmania donovani. Long-term culture in vitro leads to the attenuation of the parasite. This loss of parasite virulence is associated with the expression of a developmentally regulated UDP-Galactose/N-acetylglucosamine beta 1-4 galactosyltransferase and galactose terminal glycoconjugates as determined by their agglutination with the pea nut agglutinin (PNA). Thus, all promastigotes passaged for more than 11 times were 100% agglutinated with PNA, and represent a homogeneous population of avirulent parasites. Identical concentrations of PNA failed to agglutinate promastigotes passaged for < or =5 times. These PNA(-) promastigotes were virulent. Promastigotes passaged from 5 to 10 times showed a mixed population. The identity of populations defined by virulence and PNA agglutination was confirmed by isolating PNA(+) avirulent and PNA(-) virulent clones from the 7th passage promastigotes. Only the PNA(+) clones triggered macrophage microbicidal activity. The PNA(+) clones lacked lipophosphoglycan. Intravenous administration of [(14)C] galactose-labeled parasite in BALB/c mice resulted in rapid clearance of the parasite from blood with a concomitant accumulation in the liver. By enzymatic assay and RT-PCR we have shown the association of a UDP-Galactose/N-acetylglucosamine beta1,4 galactosyltransferase with only the attenuated clones. By immunofluorescence we demonstrated that the enzyme is located in the Golgi apparatus. By western blot analysis and SDS-PAGE of the affinity-purified protein, we have been able to identify a 29 KDa galactose terminal protein from the avirulent clones.

The glycosylation and pharmacokinetics of CTLA4Ig produced in rice cells

Cytotoxic T-lymphocyte antigen 4-immunoglobulin (CTLA4Ig) has immunosuppressive activity and the ability to induce immune tolerance. There has been no report of its glycosylation ratio or of the role of its glycans. We investigated the terminal sialylation of rice cell-derived recombinant human CTLA4Ig (rrhCTLA4Ig) using lectins. The glycosylation ratios of rrhCTLA4Ig and Chinese hamster ovary (CHO) cell-derived recombinant human CTLA4Ig (crhCTLA4Ig) were evaluated by chemical deglycosylation. After intravenous (i.v.) or subcutaneous (s.c.) administration to rats, the pharmacokinetics of rrhCTLA4Ig and crhCTLA4Ig as well as of their deglycosylated forms were evaluated. rrhCTLA4Ig does not have terminal sialic acids and its glycosylation ratio was slightly lower than that of crhCTLA4Ig. Its terminal elimination half-life (T(1/2)) was shorter than that of crhCTLA4Ig following i.v. administration. However, the half-life was significantly prolonged and was similar with that of crhCTLA4Ig following s.c. administration. Moreover, the deglycosylated forms of both preparations were cleared from the circulation faster than the native forms. These results suggest that the presence of glycans on rrhCTLA4Ig and crhCTA4Ig are important for their in vivo stability. In addition, the glycan structure of rrhCTLA4Ig is more effective in maintaining in vivo stability after s.c. administration than after i.v. administration although the glycans on rrhCTLA4Ig lack terminal sialic acids, suggesting that its glycans have the potential for in vivo stability.

A novel mechanism for LSECtin binding to Ebola virus surface glycoprotein through truncated glycans

LSECtin is a member of the C-type lectin family of glycan-binding receptors that is expressed on sinusoidal endothelial cells of the liver and lymph nodes. To compare the sugar and pathogen binding properties of LSECtin with those of related but more extensively characterized receptors, such as DC-SIGN, a soluble fragment of LSECtin consisting of the C-terminal carbohydrate-recognition domain has been expressed in bacteria. A biotin-tagged version of the protein was also generated and complexed with streptavidin to create tetramers. These forms of the carbohydrate-recognition domain were used to probe a glycan array and to characterize binding to oligosaccharide and glycoprotein ligands. LSECtin binds with high selectivity to glycoproteins terminating in GlcNAcbeta1-2Man. The inhibition constant for this disaccharide is 3.5 microm, making it one of the best low molecular weight ligands known for any C-type lectin. As a result of the selective binding of this disaccharide unit, the receptor recognizes glycoproteins with a truncated complex and hybrid N-linked glycans on glycoproteins. Glycan analysis of the surface glycoprotein of Ebola virus reveals the presence of such truncated glycans, explaining the ability of LSECtin to facilitate infection by Ebola virus. High mannose glycans are also present on the viral glycoprotein, which explains why DC-SIGN also binds to this virus. Thus, multiple receptors interact with surface glycoproteins of enveloped viruses that bear different types of relatively poorly processed glycans.

Widely Divergent Biochemical Properties of the Complete Set of Mouse DC-SIGN-related Proteins

The mouse genome sequence has been examined to identify the complete set of proteins related to the human glycanbinding receptor, DC-SIGN. In addition to five SIGNR proteins previously described, a pseudogene, encoding a hypothetical SIGNR6, and a further two expressed proteins, SIGNR7 and SIGNR8, have been identified. The ligand-binding properties of these novel proteins and of the previously described mouse SIGNs have been systematically investigated in order to define the mouse proteins that most resemble human DC-SIGN and DC-SIGNR. Results from screening of a glycan array demonstrate that only mouse SIGNR3 shares with human DC-SIGN the ability to bind both high mannose and fucose-terminated glycans in this format and to mediate endocytosis. The finding that neither SIGNR1 nor SIGNR5 binds with high affinity to specific ligands in a large panel of mammalian glycans is consistent with the suggestion that these receptors bind surface polysaccharides on bacterial and fungal pathogens in a manner analogous to serum mannose-binding protein. The data also reveal that two of the mouse SIGNs have unusual binding specificities that have not been previously described for members of the C-type lectin family; the newly identified SIGNR7 binds preferentially to the 6-sulfo-sialyl Lewis(x) oligosaccharide, whereas SIGNR2 binds almost exclusively to glycans that bear terminal GlcNAc residues. The results presented demonstrate that the mouse homologs of DC-SIGN have a diverse set of ligand-binding and intracellular trafficking properties, some of which are distinct from the properties of any of the human receptors.

Thomsen-Friedenreich oncofetal antigen in Schistosoma mansoni : localization and immunogenicity in experimental mouse infection

Our preliminary observation, that sera from schistosomiasis patients react with carcinomas, raised the possibility of antigenic cross-reactivity. We here extend this observation to show that mice experimentally infected with Schistosoma mansoni react with human urothelial and transitional bladder carcinomas and also with a gastric carcinoma cell line, AGS. To identify cross-reacting epitopes, we looked for the expression of carcinoma markers in schistosome worms and eggs using monoclonal antibodies against tumour antigens MUC1, Tn and TF (also known as the oncofetal Thomsen-Friedenreich antigen or T antigen). Immunohistochemical staining showed that the TF-epitope is present in adult intravascular S. mansoni worms and eggs deposited in tissues of infected animals. The localization of TF-immuno-reactive material in schistosomes was seen at the parasite surface between male and female worms and around trapped eggs in the liver. This localization is consistent with the antigen being secreted. Mice experimentally infected with S. mansoni, developed circulating antibodies against the TF-epitope (identified as Gal(beta1-3) GalNAc-O-R) as seen in ELISA using TF-expressing asialoglycophorin (AGP) as antigen. The observed anti-TF response in S. mansoni-infected mice reflects the complexity of host-parasite interactions in this infection.

Two categories of mammalian galactose-binding receptors distinguished by glycan array profiling

Profiling of the four known galactose-binding receptors in the C-type lectin family has been undertaken in parallel on a glycan array. The results are generally consistent with those of previous assays using various different formats, but they provide a direct comparison of the properties of the four receptors, revealing that they fall into two distinct groups. The major subunit of the rat asialoglycoprotein receptor and the rat Kupffer cell receptor show similar broad preferences for GalNAc-terminated glycans, while the rat macrophage galactose lectin and the human scavenger receptor C-type lectin (SRCL) bind more restricted sets of glycans. Both of these receptors bind to Lewis x-type structures, but the macrophage galactose lectin also interacts strongly with biantennary galactose- and GalNAc-terminated glycans. Although the similar glycan-binding profiles for the asialoglycoprotein receptor and the Kupffer cell receptor might suggest that these receptors are functionally redundant, analysis of fibroblasts transfected with full-length Kupffer cell receptor reveals that they fail to endocytose glycosylated ligand.

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

Unique sequence and expression profiles of rat galectins-5 and -9 as a result of species-specific gene divergence

Presence of species-specific gene divergence in a protein family prompts to thoroughly study structural aspects and expression profiles of the products. We herein focus on two members of an adhesion/growth-regulatory group of endogenous lectins, i.e. galectins-5 and -9. After first ascertaining species specificity of occurrence of galectin-5, constituted by a short section of rat galectin-9's N-terminal part and its C-terminal carbohydrate recognition domain, by database mining, we next detected and defined sequence differences in the proximal promoter region between the two genes. The ensuing hypothesis for distinct expression profiles was tested first by RT-PCR and then by immunohistochemistry. For the latter purpose, we employed antibodies rigorously controlled for absence of cross-reactivity including assays with various other galectins and, if necessary, refined by chromatographic removal of bi- or oligospecific activities. Indeed, the galectins have non-identical expression profiles, qualitative differences, e.g. seen for galectin-5-positive bone marrow and erythrocytes or for hitherto unknown expression in cells of the theca folliculi and galectin-9-positive skin epidermis and esophageal epithelium. Lack of hepatocyte or renal cortex staining separates these two expression profiles in rat from localization of galectin-9 in mouse. Interspecies extrapolation in a case of a galectin involved in unique gene divergence may thus not be valid. The presented results on galectin-5 relative to galectin-9 intimate distinct functions especially in erythropoiesis and imply currently unknown mechanisms to compensate its absence from the galectin network in other mammals.

Ligands of the asialoglycoprotein receptor for targeted gene delivery, part 1: Synthesis of and binding studies with biotinylated cluster glycosides containing N-acetylgalactosamine

In order to develop the non-viral Bioplex vector system for targeted delivery of genes to hepatocytes, we have evaluated the structure-function relationship for a number of synthetic ligands designed for specific interaction with the hepatic lectin ASGPr. Biotinylated ligand derivatives containing two, three or six beta-linked N-acetylgalactosamine (GalNAc) residues were synthesized, bound to fluorescent-labeled streptavidin and tested for binding and uptake to HepG2 cells using flow cytometry analysis (FACS). Uptake efficiency increased with number of displayed GalNAc units per ligand, in a receptor dependent manner. Thus, a derivative displaying six GalNAc units showed the highest uptake efficacy both in terms of number of internalizing cells and increased amount of material taken up per each cell. However, this higher efficiency was shown to be due not so much to higher number of sugar units, but to higher accessibility of the sugar units for interaction with the receptor (longer spacer). Improving the flexibility and accessibility of a trimeric GalNAc ligand through use of a longer spacer markedly influenced the uptake efficiency, while increasing the number of GalNAc units per ligand above three only provided a minor contribution to the overall affinity. We hereby report the details of the chemical synthesis of the ligands and the structure-function studies in vitro.

Cell surface biology mediated by low affinity multivalent protein-glycan interactions

Glycan-binding proteins mediate diverse aspects of cell biology including pathogen recognition of host cells, cell trafficking, endocytosis and modulation of cell signaling. This is accomplished despite the intrinsic low affinity for their ligands through multivalent interactions that increase effective affinity and adhesive force. Recent successes in the rational design of high-affinity ligands for glycan-binding proteins offer the promise to create well-defined tools for exploring the structure and functions of this class of receptors.

Sequence and expression of C-type lectin receptors in Atlantic salmon (Salmo salar)

The diverse receptors of the C-type lectin superfamily play key roles in innate immunity. In mammals, cell surface receptors with C-type lectin domains are involved in pathogen recognition and in immune response, and in some cases are exploited by pathogens to gain entry into cells. This study reports on sequence and expression analysis of three paralogous group II C-type lectins from the teleost fish Atlantic salmon (Salmo salar). Each of the receptors showed similarity to immune-relevant mammalian receptors in terms of amino acid sequence and overall organization within the C-type lectin-like domain (CTLD). Two of the three have cytoplasmic motifs consistent with the immunoreceptor tyrosine-based activation motifs (ITAM), which are known to modulate downstream functions in leukocytes. All three C-type lectin receptors were expressed in multiple tissues of healthy fish, including peripheral blood leukocytes and salmon head kidney cells (SHK-1). Each receptor was up-regulated in salmon liver in response to infection by Aeromonas salmonicida and one receptor was substantially up-regulated in cultured SHK-1 cells in response to lipopolysaccharide (LPS). Putative binding sites for the CAAT-enhancer-binding protein (C/EBP) family of transcription factors in the regulatory regions of these C-type lectin genes may mediate their response to bacteria and LPS in salmon leukocytes. The identification of these types of receptors in distinct populations of cells within the immune system will provide important markers for identifying and categorizing the state of differentiation or activation of these cells and lead to further understanding of the interaction between the salmon host and multiple pathogens.

Victor Ginsburg's influence on my research of the role of sialic acids in biological recognition

Sialic acids are monosaccharides with relatively strong acidity which belong to the most important molecules of higher animals and also occur in some microorganisms. They are bound to complex carbohydrates and occupy prominent positions, especially in cell membranes. Their structural diversity is high and, correspondingly, the mechanisms for their biosynthesis complex. Sialic acids are involved in a great number of cell functions. Due to their cell surface location these acidic molecules shield macromolecules and cells from enzymatic and immunological attacks and thus contribute to innate immunity. In contrast to this masking role, enabling, for example, blood cells and serum glycoproteins a longer life-time, sialic acids also represent recognition sites for various physiological receptors, such as the selectins and siglecs, as well as for toxins and microorganisms and thus allow their colonization. The recognition function of sialic acids can again be masked by O-acetylation, which modifies the interaction with receptors. Many viruses use sialic acids for the infection of cells. As sialic acids play also a decisive role in tumor biology, they prove to be rather versatile molecules that modulate biological and pathological cellular events in a sensitive way. Thus, they are most prominent representatives of mediators of molecular and cellular recognition.

Galactose-specific asialoglycoprotein receptor is involved in lipoprotein (a) catabolism

Lp(a) [lipoprotein (a)] is a highly atherogenic plasma lipoprotein assembled from low-density lipoprotein and the glycoprotein apolipoprotein (a). The rate of Lp(a) biosynthesis correlates significantly with plasma Lp(a) concentrations, whereas the fractional catabolic rate does not have much influence. So far, little is known about Lp(a) catabolism. To study the site and mode of Lp(a) catabolism, native or sialidase-treated Lp(a) was injected into hedgehogs or ASGPR (asialoglycoprotein receptor)-knockout (ASGPR-) mice or wild-type (ASGPR+) mice, and the decay of the plasma Lp(a) concentration was followed. COS-7 cells were transfected with high- (HL-1) and low-molecular-mass ASGPR subunits (HL-2), and binding and degradation of intact or desialylated Lp(a) were measured. In hedgehogs, one of the few species that synthesize Lp(a), most of the Lp(a) was taken up by the liver, followed by kidney and spleen. Lp(a) and asialo-Lp(a) were catabolized with apparent half-lives of 13.8 and 0.55 h respectively. Asialo-orosomucoide increased both half-lives significantly. In mice, the apparent half-life of Lp(a) was 4-6 h. Catabolism of native Lp(a) by wild-type mice was significantly faster compared with ASGPR- mice and there was a significantly greater accumulation of Lp(a) in the liver of ASGPR+ mice compared with ASGPR- mice. The catabolism of asialo-Lp(a) in ASGPR- mice was 8-fold faster when compared with native Lp(a) in wild-type mice. Transfected COS-7 cells expressing functional ASGPR showed approx. 5-fold greater binding and 2-fold faster degradation of native Lp(a) compared with control cells. Our results for the first time demonstrate a physiological function of ASGPR in the catabolism of Lp(a).