Molecular cloning and sequence analysis of cDNA encoding the macrophage lectin specific for galactose and N-acetylgalactosamine

Ligand Recognition by the Macrophage Galactose-Type C-Type Lectin: Self or Non-Self?-A Way to Trick the Host's Immune System

The cells and numerous macromolecules of living organisms carry an array of simple and complex carbohydrates on their surface, which may be recognized by many types of proteins, including lectins. Human macrophage galactose-type lectin (MGL, also known as hMGL/CLEC10A/CD301) is a C-type lectin receptor expressed on professional antigen-presenting cells (APCs) specific to glycans containing terminal GalNAc residue, such as Tn antigen or LacdiNAc but also sialylated Tn antigens. Macrophage galactose-type lectin (MGL) exhibits immunosuppressive properties, thus facilitating the maintenance of immune homeostasis. Hence, MGL is exploited by tumors and some pathogens to trick the host immune system and induce an immunosuppressive environment to escape immune control. The aims of this article are to discuss the immunological outcomes of human MGL ligand recognition, provide insights into the molecular aspects of these interactions, and review the MGL ligands discovered so far. Lastly, based on the human fetoembryonic defense system (Hu-FEDS) hypothesis, this paper raises the question as to whether MGL-mediated interactions may be relevant in the development of maternal tolerance toward male gametes and the fetus.

Active transport nanochelators for the reduction of liver iron burden in iron overload

Liver dysfunction and failure account for a major portion of premature deaths in patients suffering from various iron associated pathogeneses, particularly primary and secondary iron overload disorders, despite intensive treatment. The liver is a central player in iron homeostasis and a major iron storage organ, and currently, there are no active approaches for the excretion of excess liver iron. Herein, we report a new method for the rapid reduction of iron burden in iron overload diseases by developing a new class of liver targeted nanochelators with favorable pharmacokinetics and biodistribution. The new nanochelators bypass the reticuloendothelial system and specifically target hepatocytes without non-specific accumulation in other organs. The targeted nanochelators bound and neutralized excess iron in the liver and from the vasculature and, eventually leading to rapid hepatobiliary excretion of labile iron. Further, these rapidly excreted nanochelators did not induce toxicity in the liver, were highly cytocompatible in both iron overload and non-loaded conditions, and were promising in mitigating iron triggered free radical oxidative damage. These studies provide key insights into the development of organ targeted nanochelating systems and the rapid reduction of iron burden in vivo. This methodology allows for further development of nanotherapeutics for specific iron overload diseases.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

ASGR1 and ASGR2, the Genes that Encode the Asialoglycoprotein Receptor (Ashwell Receptor), Are Expressed in Peripheral Blood Monocytes and Show Interindividual Differences in Transcript Profile

Background. The asialoglycoprotein receptor (ASGPR) is a hepatic receptor that mediates removal of potentially hazardous glycoconjugates from blood in health and disease. The receptor comprises two proteins, asialoglycoprotein receptor 1 and 2 (ASGR1 and ASGR2), encoded by the genes ASGR1 and ASGR2. Design and Methods. Using reverse transcription amplification (RT-PCR), expression of ASGR1 and ASGR2 was investigated in human peripheral blood monocytes. Results. Monocytes were found to express ASGR1 and ASGR2 transcripts. Correctly spliced transcript variants encoding different isoforms of ASGR1 and ASGR2 were present in monocytes. The profile of transcript variants from both ASGR1 and ASGR2 differed among individuals. Transcript expression levels were compared with the hepatocyte cell line HepG2 which produces high levels of ASGPR. Monocyte transcripts were 4 to 6 orders of magnitude less than in HepG2 but nonetheless readily detectable using standard RT-PCR. The monocyte cell line THP1 gave similar results to monocytes harvested from peripheral blood, indicating it may provide a suitable model system for studying ASGPR function in this cell type. Conclusions. Monocytes transcribe and correctly process transcripts encoding the constituent proteins of the ASGPR. Monocytes may therefore represent a mobile pool of the receptor, capable of reaching sites remote from the liver.

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.

Identification of novel contributions to high-affinity glycoprotein-receptor interactions using engineered ligands

Engineered receptor fragments and glycoprotein ligands employed in different assay formats have been used to dissect the basis for the dramatic enhancement of binding of two model membrane receptors, dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and the macrophage galactose lectin, to glycoprotein ligands compared to simple sugars. These approaches make it possible to quantify the importance of two major factors that combine to enhance the affinity of single carbohydrate-recognition domains (CRDs) for glycoprotein ligands by 100-to 300-fold. First, the presence of extended binding sites within a single CRD can enhance interaction with branched glycans, resulting in increases of fivefold to 20-fold in affinity. Second, presentation of glycans on a glycoprotein surface increases affinity by 15-to 20-fold, possibly due to low-specificity interactions with the surface of the protein or restriction in the conformation of the glycans. In contrast, when solution-phase networking is avoided, enhancement due to binding of multiple branches of a glycan to multiple CRDs in the oligomeric forms of these receptors is minimal and binding of a receptor oligomer to multiple glycans on a single glycoprotein makes only a twofold contribution to overall affinity. Thus, in these cases, multivalent interactions of individual glycoproteins with individual receptor oligomers have a limited role in achieving high affinity. These findings, combined with considerations of membrane receptor geometry, are consistent with the idea that further enhancement of the binding to multivalent glycoprotein ligands requires interaction of multiple receptor oligomers with the ligands.

Evidence for an asialoglycoprotein receptor on nonparenchymal cells for O-linked glycoproteins

B cell-activating factor receptor 3 (BR3)-Fc is an IgG1-receptor dimeric fusion protein that has multiple O-linked glycosylation sites and sialylation levels that can vary in the manufacturing process. Increased sialic acid levels resulted from increased site occupancy with the O-linked N-acetylgalactosamine (GalNAc-Gal), but because the ratio of sialic acid per mole of oligosaccharide remained approximately 1, this led to increased asialo terminal GalNAc. Previous studies have demonstrated an effect of terminal asialo Gal or GalNAc on the clearance of glycoproteins due to uptake and degradation by lectin receptors in the liver. However, the previous studies examined N-linked oligosaccharides, and there are less data regarding O-linked oligosaccharides. The objective of these studies was to determine the effects on the pharmacokinetics and distribution of the asialo terminal GalNAc and varying amounts of sialic acid residues on BR3-Fc. The results of the data presented here suggest that exposed Gal on the desialylated BR3-Fc led to rapid clearance due to uptake and degradation in the liver that was associated with nonparenchymal cells. It is interesting to note that the data indicated a decreased clearance and increased exposure of BR3-Fc as the sialic acid levels increased, even though increased sialic acid was associated with increased asialo GalNAc. Therefore, the exposed GalNAc did not seem to play a role in the clearance of BR3-Fc; although the Gal linked to the hydroxyl group at position 3 may have prevented an interaction. Because we did not see uptake of desialylated BR3-Fc in hepatocytes where the asialoglycoprotein receptor is localized, this nonparenchymal cell lectin may have preference for O-linked glycoproteins.

Galactosylated low molecular weight chitosan as a carrier delivering oligonucleotides to Kupffer cells instead of hepatocytes in vivo

The in vivo cellular localization of oligodeoxynucleotides (ODNs) delivered by galactosylated low molecular weight chitosan (gal-LMWC) was investigated. The gal-LMWCs preference for Kupffer cells was confirmed by in vivo and in vitro experiments. Furthermore, asialoglycoprotein receptor (ASGPr) was studied as a possible surface lectin which may involved in the endocytosis of the gal-LMWC/ODN complexes. Results showed that the gal-LMWC/ODN complex accumulated in liver when injected intravenously (i.v.). Further studies revealed that 50.6% of the complex was taken up by Kupffer cells in liver, 33.2% was taken up by endothelial cells, and only 16.2% of the complex was taken up by parenchymal cells. In vitro results also confirmed the affinity of gal-LMWC to murine Kupffer cells. Inhibition of the transfection by lactose and N-acetyl galactosamine (GalNAc) suggested that the particles might enter macrophages via ASGPr and the inhibition by LMWC implied that there might be other lectins involved in the endocytosis. In summary, our studies revealed that gal-LMWC/ODN complex is inclined to enter into Kupffer cells rather than into liver parenchymal cells in vivo. Galactosylation may not be a proper means for targeting chitosan/DNA nanoparticles to hepatocytes but it does have the potential to be a Kupffer cells targeting strategy especially for delivering drugs for antiinflammation.

Expression and tissue localization of collectin placenta 1 (CL-P1, SRCL) in human tissues

Collectin placenta-1 (CL-P1), also known as scavenger receptor with C-type lectin (SRCL), is a type II membrane glycoprotein that shares structural features with both collectins and type A scavenger receptors. CL-P1 was originally cloned from the placenta and found to be associated with endothelial cells. It binds via its lectin domain to desialyated Lewis X containing glycoproteins and it is able to facilitate internalization of bound ligands. Via positively charged residues in the collagen-like region it binds to negatively charged components of microbial membranes. It has previously been proposed that CL-P1 plays a role in the host defense system and in the clearance of glycoproteins from the blood. With the aims of determining the detailed tissue expression of human CL-P1 we expressed CL-P1 recombinantly in both E. coli and CHO cells, and raised monoclonal antibodies against human CL-P1. Three monoclonal antibodies were characterized and used in immunohistochemical analyses of a panel of cryo- and formalin-fixed sections. We find that CL-P1 mainly associates with cytotrophoblasts and syncytiotrophoblasts of the placenta, alveolar macrophages and to a less degree with macrophage-like and stromal cells of the tonsils. By real-time RT-PCR we verified that the placenta is also the main organ of CL-P1 synthesis. The only source of endothelial cells whereto CL-P1 associates are umbilical cord vein endothelial cells (human umbilical vein endothelial cells, HUVEC). In vitro cultured HUVECs express both the CL-P1 mRNA and show anti-CL-P1 immunoreactivity but CL-P1 locates mainly to the cytosol and not to the membrane of these cells. We conclude that CL-P1 is not a common membrane protein on endothelial cells found in normal tissues under steady state conditions.

Trivalent, Gal/GalNAc-containing ligands designed for the asialoglycoprotein receptor

A series of novel, fluorescent ligands designed to bind with high affinity and specificity to the asialoglycoprotein receptor (ASGP-R) has been synthesized and tested on human liver cells. The compounds bear three non-reducing, beta-linked Gal or GalNAc moieties linked to flexible spacers for an optimal spatial interaction with the binding site of the ASGP-R. The final constructs were selectively endocytosed by HepG2 cells derived from parenchymal liver cells-the major human liver cell type-in a process that was visualized with the aid of fluorescence microscopy. Furthermore, the internalization was analyzed with flow cytometry, which showed the process to be receptor-mediated and selective. The compounds described in this work could serve as valuable tools for studying hepatic endocytosis, and are suited as carriers for site-specific drug delivery to the liver.

Sweet preferences of MGL: carbohydrate specificity and function

C-type lectins play important roles in both innate and adaptive immune responses. In contrast to the mannose- or fucose-specific C-type lectins DC-SIGN and mannose receptor, the galactose-type lectins, of which only macrophage galactose-type lectin (MGL) is found within the immune system, are less well known. MGL is selectively expressed by immature dendritic cells and macrophages with elevated levels on tolerogenic or alternatively activated subsets. Human MGL has an exclusive specificity for rare terminal GalNAc structures, which are revealed on the tumor-associated mucin MUC1 and CD45 on effector T cells. These findings implicate MGL in the homeostatic control of adaptive immunity. We discuss here the functional similarities and differences between MGL orthologs and compare MGL to its closest homolog, the liver-specific asialoglycoprotein receptor (ASGP-R).

LPS suppresses expression of asialoglycoprotein-binding protein through TLR4 in thioglycolate-elicited peritoneal macrophages

Macrophages are known to express various types of endocytosis receptors that mediate the removal of foreign pathogens. Macrophage asialoglycoprotein-binding protein (M-ASGP-BP) is a Gal/GalNAc-specific lectin, which functions as an endocytosis receptor. We found here that LPS is able to down-regulate the mRNA expression of M-ASGP-BP in a time-dependent manner using thioglycolate-elicited rat and mouse peritoneal macrophages. However, LPS does not modulate the mRNA expression of M-ASGP-BP from macrophages of C3H/HeN mice, which have a point mutation of TLR4, the primary LPS receptor. Furthermore, an inhibitor of NF-kappaB was observed to efficiently block the suppressive effect of LPS on M-ASGP-BP as well as to inhibit the phosphorylated IkappaB. These results demonstrate that the mRNA expression of M-ASGP-BP is down-regulated by the LPS-mediated TLR4 pathway involving NF-kappaB activation, suggesting that engagement of M-ASGP-BP by LPS may yield a negative signal that interferes with the LPS-induced positive signals mediated by proinflammatory cytokines.

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.

Microdamage and apoptosis

Microdamage of healthy bone leads to targeted removal and repair of the damage. This process must involve the production of specific targeting signals. The identity of these signals is unknown but constitutes a legitimate research goal since it is this targeting process which appears to become impaired in ageing and disease. Here we discuss the potential role of the matrix bound osteocyte in the sensing and targeting of microdamage. In particular we will review current understanding concerning the apoptotic death of osteocytes at sites of microdamage and discuss the potential physiological significance of these findings in the light of knowledge of the significance of apoptosis in other cell systems.

Generation of mice deficient for macrophage galactose- and N-acetylgalactosamine-specific lectin: limited role in lymphoid and erythroid homeostasis and evidence for multiple lectins

Macrophage receptors function in pattern recognition for the induction of innate immunity, in cellular communication to mediate the regulation of adaptive immune responses, and in the clearance of some glycosylated cells or glycoproteins from the circulation. They also function in homeostasis by initiating the engulfment of apoptotic cells. Evidence has suggested that macrophage receptors function to recognize cells that are destined for programmed cell death but not yet overtly apoptotic. We have examined the function of a macrophage receptor specific for unsialylated glycoproteins, known as the mouse macrophage galactose- and N-acetylgalactosamine-specific lectin (mMGL) (Ii et al., J. Biol. Chem. 265:11295-11298, 1990; Sato et al., J. Biochem. [Tokyo] 111:331-336, 1992; Yamamoto et al., Biochemistry 33:8159-8166, 1994). With targeted disruption, we tested whether mMGL is necessary for macrophage function, controlled thymic development, the loss of activated CD8 T cells, and the turnover of red blood cells. Evidence indicates that mMGL may play a nonessential role in several of these macrophage functions. Experiments are presented that indicate the existence of another galactose- and N-acetylgalactosamine-recognizing lectin distinct from mMGL. This may explain the absence of a strong phenotype in mMGL-deficient mice.

The macrophage C-type lectin specific for galactose/N-acetylgalactosamine is an endocytic receptor expressed on monocyte-derived immature dendritic cells

Lectins on antigen presenting cells are potentially involved in the antigen uptake and the cellular recognition and trafficking. Serial analysis of gene expression in monocyte-derived dendritic cells (DCs), monocytes, and macrophages revealed that 7 of the 19 C-type lectin mRNA were present in immature DCs. Two of these, the macrophage mannose receptor and the macrophage lectin specific for galactose/N-acetylgalactosamine (MGL), were found only in immature DCs, as confirmed by reverse transcriptase-PCR and flow cytometric analysis. By subcloning and sequencing the amplified mRNA, we obtained nucleotide sequences encoding seven different human MGL (hMGL) subtypes, which were apparently derived from alternatively spliced mRNA. In addition, the hMGL gene locus on human chromosome 17p13 contains one gene. A single nucleotide polymorphism was identified at a position in exon 3 that corresponds to the cytoplasmic region proximal to the transmembrane domain. Of all the splicing variants, the hMGL variant 6C was expressed at the highest levels on immature DCs from all donors tested. Immature DCs could incorporate alpha-GalNAc-modified soluble acrylamide polymers, and this was significantly inhibited by pretreatment of the cells with an anti-hMGL monoclonal antibody that blocks the lectin-carbohydrate interaction. We propose that hMGL is a marker of imDCs and that it functions as an endocytic receptor for glycosylated antigens.

Human macrophage lectin specific for galactose/N-acetylgalactosamine is a marker for cells at an intermediate stage in their differentiation from monocytes into macrophages

We studied the expression of a human macrophage lectin specific for galactose/N-acetylgalactosamine (hMGL) during macrophage differentiation. The expression of hMGL during the in vitro differentiation induced by human serum was examined by immunostaining and Western blotting with a specific mAb, MLD-1, as well as with RT-PCR analysis. hMGL was detected on cells at an intermediate stage of differentiation. These cells were round, slightly larger in size (12.7 +/- 0.2 microm) than monocytes (9.8 +/- 0.1 microm) and expressed the macrophage marker CD14, but lacked the dendritic cell marker CD1a. The highest levels of expression occurred after 2-4 days of culture. At this time point, MLD-1 prominently stained 20-40% of the cells. Monocytes cultured for 16 h or fully differentiated monocyte-derived macrophages were negative or weak for hMGL expression. Similar transient expression was also observed during granulocyte macrophage colony stimulating factor- or macrophage colony stimulating factor-dependent macrophage differentiation. The lectin was characterized as a functional endocytic receptor for glycosylated macromolecules, since the uptake of carbohydrate polymers was partially inhibited by the addition of MLD-1. The distribution of hMGL(+) cells in normal human skin was found by immunostaining to be mainly in the upper dermis distant from vascular structures. More than 90% of the hMGL(+) cells were double stained with anti-CD68 mAb and constituted approximately 20% of the CD68(+) cells. We suggest that the dermal hMGL(+) cells are a subset of differentiated cells derived from monocytes and that hMGL is a unique marker for cells at an intermediate stage of macrophage differentiation.

Kupffer cell-mediated recruitment of rat dendritic cells to the liver: roles of N-acetylgalactosamine-specific sugar receptors

BACKGROUND & AIMS

Tissue recruitment of dendritic cells (DCs) is essential for antigen presentation. This study aimed to examine cellular and molecular mechanisms for DC recruitment to the liver.

METHODS

Purified rat DCs were injected into circulation and their traffics were analyzed in normal and Kupffer cell-depleted rats by intravital confocal microscopy and immunohistology. Affinities of DCs to sinusoidal cells were examined by a cell-binding assay. DC precursor recruitment was induced by particulate injection.

RESULTS

Both DC precursors and DCs at the antigen-transporting stage could be recruited to the liver, and their majority initially showed a selective binding to Kupffer cells. In the Kupffer cell-depleted rats, DCs could neither be recruited to the liver nor adhere to sinusoidal walls. Pretreatment with varied monosaccharides showed that sugar residues consisting of N-acetylgalactosamine were necessary for this binding. The binding was calcium-dependent, implying the C-type lectin involvement. Furthermore, DCs could endocytose N-acetylgalactosamine polymers in a receptor-specific manner.

CONCLUSIONS

The DC-Kupffer cell binding through N-acetylgalactosamine-specific C-type lectin-like receptors is crucial for DC recruitment to the liver. Rat DCs at least partly possess receptors for endocytosis of galactosylated antigens. These DC receptors as well as Kupffer cell lectins are presumably responsible for this binding.

Immature human dendritic cells express asialoglycoprotein receptor isoforms for efficient receptor-mediated endocytosis

In a search for genes expressed by dendritic cells (DC), we have cloned cDNAs encoding different forms of an asialoglycoprotein receptor (ASGPR). The DC-ASGPR represents long and short isoforms of human macrophage lectin, a Ca(2+)-dependent type II transmembrane lectin displaying considerable homology with the H1 and H2 subunits of the hepatic ASGPR. Immunoprecipitation from DC using an anti-DC-ASGPR mAb yielded a major 40-kDa protein with an isoelectric point of 8.2. DC-ASGPR mRNA was observed predominantly in immune tissues. Both isoforms were detected in DC and granulocytes, but not in T, B, or NK cells, or monocytes. DC-ASGPR species were restricted to the CD14-derived DC obtained from CD34(+) progenitors, while absent from the CD1a-derived subset. Accordingly, both monocyte-derived DC and tonsillar interstitial-type DC expressed DC-ASGPR protein, while Langerhans-type cells did not. Furthermore, DC-ASGPR is a feature of immaturity, as expression was lost upon CD40 activation. In agreement with the presence of tyrosine-based and dileucine motifs in the intracytoplasmic domain, mAb against DC-ASGPR was rapidly internalized by DC at 37 degrees C. Finally, intracellular DC-ASGPR was localized to early endosomes, suggesting that the receptor recycles to the cell surface following internalization of ligand. Our findings identify DC-ASGPR/human macrophage lectin as a feature of immature DC, and as another lectin important for the specialized Ag-capture function of DC.

Determination of the upper size limit for uptake and processing of ligands by the asialoglycoprotein receptor on hepatocytes in vitro and in vivo

The asialoglycoprotein receptor (ASGPr) on hepatocytes plays a role in the clearance of desialylated proteins from the serum. Although its sugar preference (N-acetylgalactosamine (GalNAc) >> galactose) and the effects of ligand valency (tetraantennary > triantennary >> diantennary >> monoantennary) and sugar spacing (20 A 10 A 4 A) are well documented, the effect of particle size on recognition and uptake of ligands by the receptor is poorly defined. In the present study, we assessed the maximum ligand size that still allows effective processing by the ASGPr of mouse hepatocytes in vivo and in vitro. Here too, we synthesized a novel glycolipid, which possesses a highly hydrophobic steroid moiety for stable incorporation into liposomes, and a triantennary GalNAc(3)-terminated cluster glycoside with a high nanomolar affinity (2 nm) for the ASGPr. Incorporation of the glycolipid into small (30 nm) [(3)H]cholesteryl oleate-labeled long circulating liposomes (1-50%, w/w) caused a concentration-dependent increase in particle clearance that was liver-specific (reaching 85 +/- 7% of the injected dose at 30 min after injection) and mediated by the ASGPr on hepatocytes, as shown by competition studies with asialoorosomucoid in vivo. By using glycolipid-laden liposomes of various sizes between 30 and 90 nm, it was demonstrated that particles with a diameter of >70 nm could no longer be recognized and processed by the ASGPr in vivo. This threshold size for effective uptake was not related to the physical barrier raised by the fenestrated sinusoidal endothelium, which shields hepatocytes from the circulation, because similar results were obtained by studying the uptake of liposomes on isolated mouse hepatocytes in vitro. From these data we conclude that in addition to the species, valency, and orientation of sugar residues, size is also an important determinant for effective recognition and processing of substrates by the ASGPr. Therefore, these data have important implications for the design of ASGPr-specific carriers that are aimed at hepatocyte-directed delivery of drugs and genes.

A macrophage protein, Ym1, transiently expressed during inflammation is a novel mammalian lectin

Oral infections of mice with Trichinella spiralis induce activation of peritoneal exudate cells to transiently express and secrete a crystallizable protein Ym1. Purification of Ym1 to homogeneity was achieved. It is a single chain polypeptide (45 kDa) with a strong tendency to crystallize at its isoelectric point (pI 5.7). Co-expression of Ym1 with Mac-1 and scavenger receptor pinpoints macrophages as its main producer. Protein microsequencing data provide information required for full-length cDNA cloning from libraries constructed from activated peritoneal exudate cells. A single open reading frame of 398 amino acids with a leader peptide (21 residues) typical of secretory protein was deduced and later deposited in GenBank (accession number M94584) in 1992. By means of surface plasmon resonance analyses, Ym1 has been shown to exhibit binding specificity to saccharides with a free amine group, such as GlcN, GalN, or GlcN polymers, but it failed to bind to other saccharides. The interaction is pH-dependent but Ca2+ and Mg2+ ion-independent. The binding avidity of Ym1 to GlcN oligosaccharides was enhanced by more than 1000-fold due to the clustering effect. Specific binding of Ym1 to heparin suggests that heparin/heparan sulfate may be its physiological ligand in vivo during inflammation and/or tissue remodeling. Although it shares approximately 30% homology with microbial chitinases, no chitinase activity was found associated with Ym1. Genomic Southern blot analyses suggest that Ym1 may represent a member of a novel lectin gene family.

Asialoglycoprotein receptors on rat dendritic cells: possible roles for binding with Kupffer cells and ingesting virus particles

Rat dendritic cells selectively bind to Kupffer cells in vitro. The present study aimed to reveal adhesion molecules on dendritic cells and their roles in the host defense system. The in situ binding assay to examine the effects of pretreatment of dendritic cells with various kinds of monosaccharides suggested that N-acetylgalactosamine was necessary for the binding of dendritic cells to Kupffer cells. This binding was also attenuated when dendritic cells were injected into an ex vivo liver perfusion circuit together with N-acetyl-galactosamine. It was further shown that the majority of rat lymph dendritic cells and some interdigitating dendritic cells in the lymph nodes possessed asialoglycoprotein receptors specific for N-acetylgalactosamine/galactose as detected by immunostaining. Lymph dendritic cells could ingest virus particles in vitro, even though these cells showed no phagocytic activity for latex particles. The results indicate that rat dendritic cells possess asialoglycoprotein receptors which are probably utilized to recognize Kupffer cells for their recruitment to the liver and possibly to recognize virus particles prior to phagocytosis.

Asialoglycoprotein receptor deficiency in mice lacking the major receptor subunit. Its obligate requirement for the stable expression of oligomeric receptor

The asialoglycoprotein receptor is an abundant hetero-oligomeric endocytic receptor that is predominantly expressed on the sinusoidal surface of the hepatocytes. A number of physiological and pathophysiological functions have been ascribed to this hepatic lectin (HL), the removal of desialylated serum glycoproteins and apoptotic cells, clearance of lipoproteins, and the sites of entry for hepatotropic viruses. The assembly of two homologous subunits, HL-1 and HL-2, is required to form functional, high affinity receptors on the cell surface. However, the importance of the individual subunits for receptor transport to the cell surface is controversial. We have previously generated HL-2-deficient mice and showed that the expression of HL-1 was significantly reduced, and the functional activity as the asialoglycoprotein receptor was virtually eliminated. However, we failed to detect phenotypic abnormalities. To explore the significance of the major HL-1 subunit for receptor expression and function in vivo, we have disrupted the HL-1 gene in mice. Homozygous HL-1-deficient animals are superficially normal. HL-2 expression in the liver is virtually abrogated, indicating that HL-1 is strictly required for the stable expression of HL-2. Although these mice are almost unable to clear asialo-orosomucoid, a high affinity ligand for asialoglycoprotein receptor, they do not accumulate desialylated glycoproteins or lipoproteins in the plasma.

Expression of macrophage asialoglycoprotein-binding protein is induced through MAPK classical pathway

Macrophage asialoglycoprotein-binding protein (M-ASGP-BP) is a Gal/GalNAc-specific lectin, which functions as an endocytosis receptor. We found that the expression of M-ASGP-BP mRNA in bone marrow cells was induced during the differentiation into macrophages. To investigate the mechanism by which M-ASGP-BP mRNA expression is induced, we used U937 cells as a model. Treatment of U937 cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in M-ASGP-BP mRNA expression within 6 h. This induction was completely inhibited by PKC inhibitors, calphostin C, and staurosporine. Furthermore, MAP kinase inhibitors PD98059, but not SB202190, blocked M-ASGP-BP mRNA expression. These data indicate that M-ASGP-BP mRNA expression occurs through the activation of PKC and the MAPK classical pathway in the course of cell differentiation into macrophages.

Enhanced adhesion of oxidized mouse polymorphonuclear leukocytes to macrophages by a cell-surface sugar-dependent mechanism

Mouse thioglycollate-induced peritoneal macrophages effectively, in the absence of serum, recognized mouse polymorphonuclear leukocytes (PMNs) mildly oxidized with diamide, superoxide (hypoxanthine/xanthine oxidase) or t-butyhydroperoxide, or modified with N-ethylmaleimide (NEM). The recognition reached a maximum when PMNs were treated wtih each of the reagents at relatively low concentrations, and the recognition was decreased on treatment with reagents at higher concentrations. Glutathione depletion in the diamide-oxidized PMNs may cause enhanced adhesion to macrophages. Sialylated sugar chains attached to a peptide chain in glycophorin A and sialylated poly-N-acetyllactosaminyl sugar chains in lactoferrin and band 3 glycoprotein effectively inhibited the increased adhesion of the diamide-oxidized PMNs. Enzymatic removal of sialyl residues and the degradation of poly-N-acetyllactosaminyl sugar chains by pretreatment of PMNs with neuraminidase or endo-beta-galactosidase, respectively, lost their increasing ability for macrophage adhesion after oxidation with diamide, superoxide or t-butylhydroperoxide. Clustered sialylated poly-N-acetyllactosaminyl sugar chains on the cell surface may be involved in the increased adhesion of the oxidized PMNs to macrophages.

Representational difference analysis using myeloid cells from C/EBPε deletional mice

C/EBPε is a recently cloned member of the C/EBP family of transcriptional factors. Previous studies demonstrated that the expression of this gene is tightly regulated in a tissue specific manner; it is expressed exclusively in myeloid cells. C/EBPε-deficient mice developed normally but failed to generate functional neutrophils and eosinophils, and these mice died of opportunistic infections suggesting that C/EBPε may play a central role in myeloid differentiation. To identify myelomonocytic genes regulated by the C/EBPε gene, we performed representational difference analysis (RDA), a polymerase chain reaction (PCR)-based subtractive hybridization using neutrophils and macrophages from wild-type and C/EBPε knockout mice. We identified a set of differentially expressed genes, including chemokines specific to myelomonocytic cells. Several novel genes were identified that were differentially expressed in normal myelomonocytic cells. Taken together, we have found several genes whose expression might be enhanced by C/EBPε.

Representational difference analysis using myeloid cells from C/EBPε deletional mice

C/EBPε is a recently cloned member of the C/EBP family of transcriptional factors. Previous studies demonstrated that the expression of this gene is tightly regulated in a tissue specific manner; it is expressed exclusively in myeloid cells. C/EBPε-deficient mice developed normally but failed to generate functional neutrophils and eosinophils, and these mice died of opportunistic infections suggesting that C/EBPε may play a central role in myeloid differentiation. To identify myelomonocytic genes regulated by the C/EBPε gene, we performed representational difference analysis (RDA), a polymerase chain reaction (PCR)-based subtractive hybridization using neutrophils and macrophages from wild-type and C/EBPε knockout mice. We identified a set of differentially expressed genes, including chemokines specific to myelomonocytic cells. Several novel genes were identified that were differentially expressed in normal myelomonocytic cells. Taken together, we have found several genes whose expression might be enhanced by C/EBPε.

Role of the macrophage galactose lectin in the uptake of desialylated LDL

Desialylated low density lipoprotein (LDL) is rapidly taken up and accumulated by both peripheral blood monocytes and cells isolated from human arterial intima consisting predominantly of smooth muscle cells. It is shown that thioglycollate (TG)-elicited mouse macrophages and mouse peritoneal macrophages stimulated with lipopolysaccharide (LPS) show increased expression of a membrane-bound, galactose-specific lectin that could be responsible for this uptake. In LPS-stimulated macrophages accumulation of desialylated LDL is increased ca. 2.6-fold. Accumulation of acetylated LDL in the same cells is reduced, suggesting that the galactose-specific lectin might be responsible for the uptake of desialylated LDL. Transfection of cells with the mouse macrophage Gal/GalNAc-specific lectin (MMGL) increased their capacity to take up asialofetuin (ASF) and, to a smaller extent, desialylated LDL. The uptake of desialylated LDL was small, most likely due to the high k(d) of MMGL for biantennary oligosaccharides as found on LDL, and low concentration of LDL achieved in tissue culture experiments. The data suggest that the expression of galactose-specific lectins can be elevated under inflammatory conditions, and that these receptors could contribute to foam cell formation under conditions of high desialylated LDL concentration, as might be found in arterial intima.

Galactosyl receptor, a cell surface C-type lectin of normal and tumoral prostate epithelial cells with binding affinity to endothelial cells

BACKGROUND

The mechanism of bone metastasis of prostate cancer involves the interaction of cell surface receptor(s) on cancer cells with ligand(s) on bone marrow endothelial cell surfaces. The rat galactosyl receptor gene generates two mRNA species by differential splicing: one species encodes a protein identical to the minor form of hepatocyte asialoglycoprotein receptor and displays a galactose/N-acetyl-galactosamine-recognition domain; the other encodes a protein with identical intracellular and transmembrane domains but with a different extracellular domain lacking the carbohydrate-recognition domain (CRD). Both proteins appear to coexist as a heterooligomer on the surface of normal mouse, rat, and human prostate epithelial cells and human prostate cancer cells, including the PC-3 cell line. The CRD of galactosyl receptor mediates adhesion of normal and tumoral prostate cells to the surfaces of a human bone marrow endothelial cell line. The use of inhibitors targeting the CRD would be very valuable in hindering the binding of prostate cancer cells to endothelial cells, thus decreasing the incidence of hematogenous metastasis to bone.

METHODS

Molecular biology, immunohistochemistry, flow cytometry, and a cell aggregation assay were used to determine the expression and role of the galactosyl receptor in cell adhesion.

RESULTS

Immunoblotting experiments demonstrated that each component of the heterooligomer has a mass of 54 kDa, ascribed in part to associated carbohydrates. An oligonucleotide probe showed the presence of both galactosyl receptor forms in rat prostate and testis, but not in liver, kidney, and spleen. Antibodies to the CRD and a segment of the nonhomologous extracellular domain of the galactosyl receptor blocked cell adhesion to endothelial cell monolayers.

CONCLUSIONS

The galactosyl receptor provides a valuable target for the development and use of synthetic ligands capable of disrupting endothelial cell-prostate cancer cell interaction, the first step in prostate cancer bone metastasis.

Epitope mapping of monoclonal antibodies specific for a macrophage lectin: a calcium-dependent epitope is in the carbohydrate recognition domain

Mouse macrophage galactose/N-acetylgalactosamine-specific calcium-type lectin (mMGL) has a calcium-dependent conformational epitope which is a ligand-induced binding site. A monoclonal antibody (mAb) specific for this epitope (LOM-11) stabilize lectin activity. We performed mapping for this conformational epitope using trypsin fragments that contain a carbohydrate recognition domain (CRD) and chimeric recombinant proteins between mMGL and a human counterpart of this molecule. Binding site for the mAb LOM-11 was mapped within the C-terminal 59 amino acids of CRD. Binding sites for all four mAbs that block carbohydrate ligand binding were also mapped in the C-terminal half of CRD. These results indicated that the calcium-dependent site potentially involved in protein-protein interaction, regulatory or for coordinated binding, is mapped within CRD in addition to the independent carbohydrate binding site, and that both of the distinct sites may have spatial proximity.

Identification of amino acid residues that determine pH dependence of ligand binding to the asialoglycoprotein receptor during endocytosis

The rat hepatic asialoglycoprotein receptor mediates clearance of galactose- and N-acetylgalactosamine-terminated glycoproteins by endocytosis, binding ligands through a C-type, Ca(2+)-dependent carbohydrate-recognition domain (CRD) at extracellular pH and releasing them at lower pH in endosomes. At physiological Ca(2+) concentrations, the midpoint for ligand release from the CRD of the major subunit of the receptor is pH 7.1. In contrast, the midpoint is pH 5.0 for a galactose-binding derivative of the homologous C-type CRD of serum mannose-binding protein, which would thus not efficiently release ligand at an endosomal pH of 5.4. Site-directed mutagenesis of the CRD from the major subunit of the asialoglycoprotein receptor has been used to identify residues that are essential for efficient release of ligand at endosomal pH. The effects of changes to residues His(256), Asp(266), and Arg(270) singly and in combination indicate that these residues reduce the affinity of the CRD for Ca(2+), so that ligands are released at physiological Ca(2+) concentrations. The proximity of these three residues to the ligand-binding site at Ca(2+) site 2 of the domain suggests that they form a pH-sensitive switch for Ca(2+) and ligand binding. Introduction of histidine and aspartic acid residues into the mannose-binding protein CRD at positions equivalent to His(256) and Asp(266) raises the pH for half-maximal binding of ligand to 6.1. The results, as well as sequence comparisons with other C-type CRDs, confirm the importance of these residues in conferring appropriate pH dependence in this family of domains.

Sialoforms of dipeptidylpeptidase IV from rat kidney and liver

Dipeptidylpeptidase IV (DPP IV, CD26), a serine-type exo- and endopeptidase found in the cell surface membrane of many tissues, was employed as a model membrane glycoprotein to study the expression of sialoforms on cell surface glycoproteins. Native, enzymatically active DPP IV was purified from plasma membranes of kidney and liver by lectin affinity chromatography in conjunction with crown ether anion exchange chromatography. The enzyme was gradient-eluted in continuous fractions, all showing a single polypeptide band of about 100 kDa when separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing, denaturing conditions. Analysis of the purified DPP IV by isoelectric focusing (IEF) showed that it consists of several polypeptides of different isoelectric points (IP) ranging from 5.5 to 7.0. In vitro- desialylation of the enzyme and subsequent isoelectric focusing revealed that the differences in isoelectric points were due to differences in the degree of sialylation. Differences in the degree of sialylation between the fractions were also demonstrated by SDS-PAGE under nonreducing and nondenaturing conditions. Increased sialylation of the enzyme as demonstrated by isoelectric focusing resulted in increased migration velocity in nonreducing and nondenaturing SDS-polyacrylamide gels. In vitro -desialylation of the enzyme and its resialylation confirmed that sialylation was responsible for this extraordinary migration behavior. The native enzyme was predominantly sialylated via alpha 2, 6-linkage, as shown by lectin affinity blotting employing Sambucus nigra agglutinin (SNA) and Maackia amurensis agglutinin (MAA). These findings demonstrate that a distinct membrane glycoprotein may exist in various sialoforms, distinguished from each other by a different number of sialic acid residues. Moreover, these sialoforms can be individually purified by crown ether anion exchange chromatography.

A Novel LPS-Inducible C-Type Lectin Is a Transcriptional Target of NF-IL6 in Macrophages

C-type lectins serve multiple functions through recognizing carbohydrate chains. Here we report a novel C-type lectin, macrophage-inducible C-type lectin (Mincle), as a downstream target of NF-IL6 in macrophages. NF-IL6 belongs to the CCAAT/enhancer binding protein (C/EBP) of transcription factors and plays a crucial role in activated macrophages. However, what particular genes are regulated by NF-IL6 has been poorly defined in macrophages. Identification of downstream targets is required to elucidate the function of NF-IL6 in more detail. To identify downstream genes of NF-IL6, we screened a subtraction library constructed from wild-type and NF-IL6-deficient peritoneal macrophages and isolated Mincle that exhibits the highest homology to the members of group II C-type lectins. Mincle mRNA expression was strongly induced in response to several inflammatory stimuli, such as LPS, TNF-α, IL-6, and IFN-γ in wild-type macrophages. In contrast, NF-IL6-deficient macrophages displayed a much lower level of Mincle mRNA induction following treatment with these inflammatory reagents. The mouse Mincle proximal promoter region contains an indispensable NF-IL6 binding element, demonstrating that Mincle is a direct target of NF-IL6. The Mincle gene locus was mapped at 0.6 centiMorgans proximal to CD4 on mouse chromosome 6.

Coordinated binding of sugar, calcium, and antibody to macrophage C-type lectin

Mouse macrophage galactose/N-acetylgalactosamine-specific C-type lectin (MMGL) is a type II transmembrane glycoprotein belonging to the C-type lectin family. Our development of monoclonal antibodies led us to discover that a calcium-dependent conformational change is detected by an antibody (termed mAb LOM-11) and that the antibody's binding to the respective site locks the lectin in an active conformation. These findings correspond to the divalent cation-mediated regulatory mechanisms in a family of cell adhesion molecule integrins that have gained much attention. We now provide direct evidence that mAb LOM-11 increases the affinity of the lectin for calcium ions as a mechanism for the conformational lock using a soluble recombinant form of MMGL (rML) produced in bacteria. Furthermore, we discovered by using an enzyme-linked immunosorbent assay that specific monosaccharides induced a binding site for mAb LOM-11 on the immobilized rML under low calcium environments. We also demonstrated that cell surface MMGL on a transfectant cell line underwent a conformational change upon addition of calcium or ligands, as detected by the binding of mAb LOM-11. These properties are reminiscent of ligand-induced binding sites defined for integrins. The present results suggest a possibility that the mAb LOM-11 binding site on the lectin may be a site at which protein-protein interaction helps to fine tune the specificity of the C-type lectins by means of coordinated recognition mechanisms.

All low density lipoprotein particles are partially desialylated in plasma

Desialylation has been proposed as a natural modification of low density lipoprotein (LDL) increasing atherogenicity. The galactose (Gal)-specific lectin, Ricinus communis agglutinin I (RCA120), has been used to analyse LDL prepared by different methods and it was found that more than 96% of LDL binds to the lectin. The bound LDL could be eluted with Gal or Lactose (Lac), but not with sialic acid, mannose (Man), glucose (Glu) or sodium chloride, indicating that binding occurs via exposed Gal residues on the LDL particle. When freshly isolated whole plasma was loaded on an RCA120 column, apo B-containing lipoproteins (including LDL) were quantitatively bound, whereas other glycosylated serum proteins, like transferrin, were not. Thus desialylation of LDL is not a consequence of its isolation from plasma, or a general property of all serum proteins. Analysis of apolipoprotein B from LDL indicates that only monodesialylated oligosaccharide chains are present, consistent with the rapid clearance of particles having biantennary Gal residues exposed.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

A molecular model of the carbohydrate recognition domain of a rat macrophage lectin and analysis of its binding site

A three-dimensional model of the carbohydrate recognition domain of a rat macrophage C-type lectin has been constructed by comparative modeling and assessed by inverse folding analysis. Comparative modeling in the presence of low sequence similarity was based on information provided by comparison of X-ray structures and sequence-structure alignments. The sequence-structure compatibility of the model was sound. Its binding site was analyzed in comparison to the X-ray structure of a galactose-specific mutant of the mannose-binding protein. The specificity of the macrophage lectin was discussed in light of mutagenesis data on asialoglycoprotein receptors.

Selective sugar binding to the carbohydrate recognition domains of the rat hepatic and macrophage asialoglycoprotein receptors

Asialoglycoprotein receptors on the surfaces of both hepatocytes and peritoneal macrophages bind terminal galactose residues of desialylated glycoproteins and mediate endocytosis and eventual degradation of these ligands. The hepatic receptor binds oligosaccharides with terminal N-acetylgalactosamine residues more tightly than ligands with terminal galactose residues, but the macrophage receptor shows no such differential binding affinity. Carbohydrate recognition domains from the macrophage receptor and the major subunit of the hepatic receptor have been expressed in a bacterial system and have been shown to retain the distinct binding selectivities of the receptors from which they derive. Binding of a series of N-acyl derivatives of galactosamine suggests that the 2-substituent of these sugars interacts with the surface of the hepatic receptor with highest affinity binding observed for the N-propionyl derivative. Chimeric sugar-binding domains have been used to identify three regions of the hepatic receptor that are essential for establishing selectivity for N-acetylgalactosamine over galactose. Based on these results and the orientation of N-acetylgalactosamine when bound to an homologous galactose-binding mutant of rat serum mannose-binding protein, a fourth region likely to interact with N-acetylgalactosamine has been identified and probed by site-directed mutagenesis. The results of these studies define a binding pocket for the 2-substituent of N-acetylgalactosamine in the hepatic asialoglycoprotein receptor.