Identification and characterization of a murine receptor for galactose-terminated glycoproteins

Soluble Asialoglycoprotein Receptors Reflect the Apoptosis of Hepatocytes

Cell death is thought to take place through at least two distinct processes: apoptosis and necrosis. There is increasing evidence that dysregulation of the apoptotic program is involved in liver diseases. However, there is no method to simply evaluate apoptosis in the liver tissue at present. It has been reported that the expression of asialoglycoprotein receptors (AGPRs) increases with apoptosis, but there is no report until now that investigates the influence of soluble AGPRs on apoptosis of hepatocytes. Soluble AGPRs have been reported to be present in human serum under physiological conditions. In the present study, in order to investigate the correlation between apoptosis of hepatocytes and soluble AGPR, mouse soluble AGPRs were detected using SDS-PAGE and Western blot analysis was conducted using anti-extracellular mouse hepatic lectin-1 (Ex-MHL-1) antiserum (polyclonal rabbit serum). The mouse soluble AGPRs were present in culture medium and mouse serum when hepatocytes were damaged. The soluble AGPRs increased proportionately, as the number of dead hepatocytes increased. In addition, soluble AGPRs existed more when apoptotic cell death was observed in in vitro and in vivo than when necrotic cell death was observed. The extracellular moiety of MHL-1 exists in the culture medium and mouse serum as a soluble AGPR, but the detailed mechanism of releasing soluble AGPR from hepatocytes has not been revealed yet. We described the first evidence for the relation between quantity of soluble AGPRs with two kinds of cell death: necrosis and apoptosis. Based on the results of our study, soluble AGPRs might become a new marker of apoptosis in the liver tissue and be useful for clinical diagnosis and treatment for liver diseases.

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.

The Ashwell-Morell receptor

The Ashwell-Morell receptor (AMR) of hepatocytes, originally termed the hepatic asialoglycoprotein receptor, was the first cellular receptor to be identified and isolated and the first lectin to be detected in mammals. It is one of the multiple lectins of the C-type lectin family involved in recognition, binding, and clearance of asialoglycoproteins. We recently identified endogenous ligands of the AMR as desialylated prothrombotic components, including platelets and von Willebrand Factor [Ellies L. G., Ditto D., Levy G. G., Wahrenbrock M., Ginsburg D., Varki A., Le D. T., and Marth J. D. (2002). Sialyltransferase ST3Gal-IV operates as a dominant modifier of hemostasis by concealing asialoglycoprotein receptor ligands. Proc. Natl. Acad. Sci. USA 99: pp. 10042-10047; Grewal, P. K. Uchiyama, S., Ditto, D., Varki, N., Le, D. T., Nizet, V., Marth, J. D. (2008). The Ashwell receptor mitigates the lethal coagulopathy of sepsis. Nat. Medicine 14, pp. 648-655]. Among these components, clearance by the liver's AMR is enhanced by exposure of terminal galactose on the glycan chains. A physiological role for engaging the AMR in rapid clearance was identified as mitigating disseminating intravascular coagulopathy in sepsis to promote survival. This chapter overviews the endogenous ligands of the AMR as components of the coagulatory system, describes clearance mechanisms of the liver, and details hematology and coagulation assays used in mouse coagulation studies.

Impact of asialoglycoprotein receptor deficiency on the development of liver injury

The asialoglycoprotein (ASGP) receptor is a well-characterized hepatic receptor that is recycled via the common cellular process of receptor-mediated endocytosis (RME). The RME process plays an integral part in the proper trafficking and routing of receptors and ligands in the healthy cell. Thus, the mis-sorting or altered transport of proteins during RME is thought to play a role in several diseases associated with hepatocyte and liver dysfunction. Previously, we examined in detail alterations that occur in hepatocellular RME and associated receptor functions as a result of one particular liver injury, alcoholic liver disease (ALD). The studies revealed profound ethanol-mediated impairments to the ASGP receptor and the RME process, indicating the importance of this receptor and the maintenance of proper endocytic events in normal tissue. To further clarify these observations, studies were performed utilizing knockout mice (lacking a functional ASGP receptor) to which were administered several liver toxicants. In addition to alcohol, we examined the effects following administration of anti-Fas (CD95) antibody, carbon tetrachloride (CCl(4)) and lipopolysaccharide (LPS)/galactosamine. The results of these studies demonstrated that the knockout mice sustained enhanced liver injury in response to all of the treatments, as shown by increased indices of liver damage, such as enhancement of serum enzyme levels, histopathological scores, as well as hepatocellular death. Overall, the work completed to date suggests a possible link between hepatic receptors and liver injury. In particular, adequate function and content of the ASGP receptor may provide protection against various toxin-mediated liver diseases.

The Ashwell receptor mitigates the lethal coagulopathy of sepsis

The Ashwell receptor, the major lectin of hepatocytes, rapidly clears from blood circulation glycoproteins bearing glycan ligands that include galactose and N-acetylgalactosamine. This asialoglycoprotein receptor activity remains a key factor in the development and administration of glycoprotein pharmaceuticals, yet a biological purpose of the Ashwell receptor has remained elusive. We have identified endogenous ligands of the Ashwell receptor as glycoproteins and regulatory components in blood coagulation and thrombosis that include von Willebrand factor (vWF) and platelets. The Ashwell receptor normally modulates vWF homeostasis and is responsible for thrombocytopenia during systemic Streptococcus pneumoniae infection by eliminating platelets desialylated by the bacterium's neuraminidase. Hemostatic adaptation by the Ashwell receptor moderates the onset and severity of disseminated intravascular coagulation during sepsis and improves the probability of host survival.

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.

Hepatocyte behavior on synthetic glycopolymer matrix: inhibitory effect of receptor–ligand binding on hepatocyte spreading

The interaction of carbohydrate-based polymers with asialoglycoprotein receptors (ASGPRs) on the surface of hepatocytes has been used to design hepatocyte adhesion matrices. Therefore, we have characterized the interaction of ASGPR on the surface of hepatocytes with glycopolymer-coated surfaces. Since ASGPRs bound to glycopolymer surfaces escape from internalization and degradation, they were quantified by western blot analysis. The amount of hepatocyte ASGPRs that initially adhered to the glycopolymer surface was proportional to the concentration of the coated glycopolymer. We found that the initial adhesion of hepatocytes to the glycopolymer surface was enhanced by interactions with ASGPR, whereas interactions with ASGPR inhibited the post-adhesion process, a cell adhesion phenomenon that occurs following the initial adhesion. Furthermore, hepatocytes are much more spread on glycopolymer surfaces with lower coating density. Taken together, we suggest that the post-adhesion process triggered hepatocyte spreading on glycopolymer surfaces, and ASGPR-carbohydrate interactions act negatively on the post-adhesion mechanism as well as on hepatocyte spreading on glycopolymer surfaces depending on the density of coated glycopolymers.

Specific binding of glucose-derivatized polymers to the asialoglycoprotein receptor of mouse primary hepatocytes

In this study, we designed a novel amphiphilic poly-(p-N-vinylbenzyl-D-glucuronamide) (PV6Gna) modified at the 6-OH position of glucose for hepatocyte recognition to address the mechanism of the interaction between mouse primary hepatocytes and the PV6Gna. PV6Gna bound to lectins specific for glucose but not galactose as did other glucose-derivatized polymers. However, hepatocyte adhesion onto the PV6Gna surface was inhibited in the presence of galactose and its analogues but not in the presence of glucose and its analogues. We also showed that hepatocyte adhesion to the PV6Gna surface was inhibited dose dependently by asialofetuin (ASF). Interactions between soluble PV6Gna and hepatocytes were inhibited by GalNAc, ASF, and EGTA in flow cytometry analysis using fluorescein isothiocyanate-conjugated PV6Gna. Hepatocyte adhesion to the PV6Gna surface was inhibited more effectively by GalNAc than by methyl beta-D-galactose. In flow cytometry analysis and cell adhesion assay, ASF competed for the inhibition of interaction between PV6Gna and hepatocytes 0.5-4 x 10(5)-fold more effectively than did GalNAc. These results demonstrate involvement of asialoglycoprotein receptors (ASGPRs) in the interaction between PV6Gna and hepatocytes. Furthermore, to clarify the mechanism of the interaction between glycopolymers modified at the 6-OH position of glucose and the hepatocyte, we prepared a gel particle containing 6-O-methacryloyl-d-glucose (PMglc) synthesized by an enzymatic method. ASGPRs could be detected using Western blot analysis following precipitation with PMglc in hepatocyte cell lysate. The precipitation of ASGPRs was inhibited in the presence of galactose, ASF, PV6Gna, and EGTA. The precipitation was inhibited more effectively by GalNAc than by methyl beta-d-galactose. ASGPRs were rarely precipitated by PMglc in the cell lysate that had been treated with ASF-conjugated Sepharose. Taken together, we suggest that mouse primary hepatocytes adhere to the PV6Gna surface mediated by ASGPRs, which specifically interacted with the glycopolymers modified at the C-6 position of glucose.

Normal levels of serum glycoproteins maintained in beta-1, 4-galactosyltransferase I-knockout mice

The galactose-mediated clearance of serum glycoproteins from the circulation was evaluated using beta-1,4-galactosyltransferase (beta-1,4-GalT) I-knockout mice. Partial structural study of the oligosaccharides released from mouse serum glycoproteins revealed that 77.4% of the oligosaccharides from beta-1,4-GalT I(+/+) mouse contain galactose, while 7.7% of those from beta-1,4-GalT I(-/-) mouse were galactosylated. Under the conditions, no significant change in serum protein concentrations was observed between the normal and mutant mice. The results indicate that the hepatic asialoglycoprotein receptor-mediated system is not functioning in the clearance of endogenous serum glycoproteins.

Composition, antigenic properties and hepatocyte surface expression of the woodchuck asialoglycoprotein receptor

We have purified woodchuck hepatic asialoglycoprotein receptor (ASGPR) by ligand affinity chromatography and have identified it as a heterooligomeric complex comprised of two subunits with molecular masses of 40 and 47 kD, designated as woodchuck hepatic lectin 1 and 2 (WHL1 and WHL2), respectively. With the help of antisera generated against the soluble, bioactive woodchuck and rabbit ASGPRs and anti-subunit monospecific antibodies, distinct antigenic specificity of each of the ASGPR polypeptide subunits and interspecies immunologic cross-reactivity of the receptor polypeptides displaying comparable molecular masses were documented. In contrast to the purified woodchuck receptor, WHL2 antigenic reactivity was not identifiable in woodchuck hepatocyte plasma membranes unless the intact membranes were exposed to an asialylated ligand or a soluble membrane fraction was incubated with anti-receptor antibody. These findings imply that both WHL1 and WHL2 are expressed on the hepatocyte surface and contribute to ligand binding, since antibody specific to either subunit blocks ligand attachment. Our results also indicate that ligand binding modifies antigenic properties of the membrane expressed ASGPR.

The major form of the murine asialoglycoprotein receptor: cDNA sequence and expression in liver, testis and epididymis

Northern blot analysis of poly(A)+RNAs isolated from mouse liver or mouse testis (Te)/epididymis (Ep) reveals that both tissues express 1.5- and 7.5-kb transcripts which have extensive homology to the major form of the rat asialoglyco-protein receptor (ASGP-R). In situ hybridization studies have localized the expression of this ASGP-R-like transcript to late-stage sperm from Te and Ep of several different strains of mice. Swiss Webster mice express this ASGP-R-like transcript in late-stage spermatids at the time of release into the seminiferous tubule and in Ep sperm, while Balb/C, NIH Swiss and C57Bl/6 mice express this ASGP-R-like transcript predominantly in Ep sperm. cDNAs containing the entire coding region for this ASGP-R-like transcript have been cloned from mouse liver and mouse Te/Ep. These cDNAs are 100% identical in the coding region and 3'-untranslated region (UTR), but differ in the 5'-UTR. The gene encoding these cDNAs is called MHL-1, designating the major form of the mouse ASGP-R. The deduced amino acid (aa) sequence of MHL-1 shares 88% homology to the rat hepatic (He) lectin form 1 (RHL-1) and 78% homology to the human asialoglycoprotein receptor form 1 (H1). The three sites for N-linked glycosylation in the RHL-1 sequence are all conserved in the deduced MHL-1 sequence. Taken collectively, these data describe the cloning and sequencing of the MHL-1 cDNA and illustrate its deduced aa homology to RHL-1 and H1.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the "hepatic" asialoglycoprotein receptor in rat late-stage spermatids and epididymal sperm

Northern blot analysis of rat testicular (Te) poly(A)+RNA reveals that a transcript homologous to the major form of the asialoglycoprotein receptor (ASGP-R), designated RHL-1, is expressed as early as one week postnatally and that steady-state levels are approx. 8-times higher in the Te of an 8-week-old rat (sexually mature) as compared to an 84-week-old rat (aged). Partial cDNAs encoding RHL-1 and the minor form of the ASGP-R, designated RHL-2/3, have been cloned from two rat Te/epididymal (Ep) cDNA libraries and rat Te poly(A)+RNA. Sequence analysis of the Te/Ep RHL-1 cDNA and the Te/Ep RHL-2/3 cDNA indicates that these cDNAs are identical to the forms expressed in rat liver. Western blot analysis demonstrates the presence of a 49-kDa Te/Ep RHL-1-related protein band and a 54-kDa Te/Ep RHL-2/3-related protein band in both rat Te membrane fractions (MF) and rat Ep sperm MF. The RHL-1-related protein has been localized to late-stage Te spermatids at the time of release from the seminiferous tubules and to Ep sperm in the region of the sperm tail, referred to as the middle piece. Taken collectively, these data indicate that the authentic RHL-1 and RHL-2/3 genes of the ASGP-R are expressed in late-stage spermatids; however, the Te/Ep RHL-1-related protein differs in size from the hepatic RHL-1 polypeptide, possibly indicating a specific function of the RHL-1-related protein in spermatogenesis.

Determination of mouse major asialoglycoprotein receptor cDNA sequence

Hepatic lectins (asialoglycoprotein receptors) specifically recognize galactose-terminated glycoproteins and mediate endocytosis of these molecules. We now report the cloning and sequence of a cDNA encoding murine asialoglycoprotein receptor. It shows high homology with rat and human major receptor forms designated RHL-1 and HHL-1, respectively. They have many conserved regions, such as a transmembrane region, carbohydrate additional region and carbohydrate related region. According to the homology analysis, we concluded that the clone encodes the mouse major asialoglycoprotein receptor (MHL-1).

Differences in the abundance of variably spliced transcripts for the second asialoglycoprotein receptor polypeptide, H2, in normal and transformed human liver

The human hepatic asialoglycoprotein receptor comprises two homologous polypeptides designated H1 and H2. Two distinct complementary DNA clones encoding these receptor subunits have been previously isolated from the human hepatoblastoma cell line HepG2. We discovered that multiple variants of H2 transcripts exist both in HepG2 cells and in the normal human liver that, at least in part, appear to be the result of alternative splicing events. We have found that (a) the complementary DNA clone for H2 previously isolated from HepG2 cells, characterized by a 57-nucleotide insertion within the 5' end of the complementary DNA that is absent from H1, represented only one third of H2-related sequences in an unamplified normal human liver complementary DNA library and less than 10% of H2 clones in HepG2 cells; (b) the predominant message for H2 expressed in the liver and HepG2 cells, designated L-H2, appeared to represent the fully processed product of the gene encoding both L-H2 and H2; and (c) a variant H2 transcript existed in HepG2 cells, designated H2', that contained a novel, 5' 88-bp nucleotide insertion. Poly(A+) RNA analysis of the normal liver and HepG2 cells by complementary RNA hybridization and ribonuclease protection corroborated the observations made during the screening of complementary DNA libraries regarding the abundance of the various messages. A striking incongruity was found between the levels of messenger RNA containing the H2-specific 57-nucleotide sequence and the levels of polypeptide expressed in the liver and HepG2 cells as recognized by antiserum specifically raised against this sequence.

Mouse asialoglycoprotein receptor cDNA sequence: conservation of receptor genes during mammalian evolution

The asialoglycoprotein receptor internalizes galactose-terminated glycoproteins into mammalian hepatocytes for degradation in lysosomes. We report the cloning and sequencing of one murine asialoglycoprotein receptor cDNA which exhibits homology with rat and human receptor forms. Conserved regions may correlate with functional domains. The membrane-bound M (mouse) HL polypeptide does not contain a cleavable N-terminal signal sequence and is probably anchored to the membrane via an internal insertion sequence.

Assembly of a heterooligomeric asialoglycoprotein receptor complex during cell-free translation

We have translated RNAs for the two rat asialoglycoprotein receptor polypeptides together in a cell-free system containing dog pancreatic microsomes and immunoprecipitated the products with antibodies that distinguish the two proteins. In this system the proteins oligomerize, as judged by their coprecipitation with either of the subunit-specific antisera. Oligomerization does not occur between subunits synthesized without microsomes or between subunits synthesized on separate microsomes mixed during detergent solubilization. Thus, oligomerization occurs within the microsomal membrane. We calculate that oligomerization proceeds with an efficiency of approximately 85%. The receptor complex appears to represent a specific oligomer because it excludes a third membrane glycoprotein synthesized in the same reaction. Oligomerization of the asialoglycoprotein receptor in vitro should provide a useful system to study the assembly of a membrane-protein complex.

Expression of enzymatically active rat dipeptidyl peptidase IV in Chinese hamster ovary cells after transfection

Dipeptidyl peptidase IV (DPPIV) is a cell surface membrane glycoprotein expressed in many tissues. We have subcloned the coding region of a full-length cDNA for DPPIV into the inducible eukaryotic expression vector pMSG. The resulting construct was used to transfect Chinese hamster ovary (CHO) cells. Stable transformants were found to express DPPIV, and the expression is enhanced by dexamethasone. Metabolic labeling of the transfected cells with [35S]Met followed by immunoprecipitation revealed the presence of two specific products of apparent Mr 100,000 (100-kDa form) and 110,000 (110-kDa form), respectively. Pulse-chase experiments demonstrated that the 100-kDa form can be chased into the 110-kDa form, suggesting the 100-kDa form is the precursor of the 110-kDa form. Further studies with endo H treatment demonstrated that the carbohydrate structures are of the high-mannose type, and of the complex type for the 100- and 110-kDa forms, respectively. The 110-kDa form is present at the cell surface as shown by its accessibility to cell surface iodination. The DPPIV expressed on the cell surface is resistant to digestion by relatively high concentrations of trypsin. Studies also demonstrated that the surface DPPIV is fairly stable with a half-life for turnover of about 40 h. Furthermore, the DPPIV produced in the transfected cells displays specific dipeptidyl peptidase activity. The stably transfected cells that express enzymatically active DPPIV in an inducible manner will provide an excellent system for further biochemical, functional, and cell biological characterizations of DPPIV.

Expression of dipeptidyl peptidase IV in rat tissues is mainly regulated at the mRNA levels

Dipeptidyl peptidase IV (DPPIV) is a serine peptidase that cleaves N-terminal dipeptides from polypeptides when the second residue is a proline or an alanine. We have recently cloned cDNAs for rat gp110, a membrane glycoprotein with Mr of 110,000 isolated initially from rat liver. Studies reported here establish that the gp110 for which we have cloned cDNAs is DPPIV. Using the antibodies against and cDNA for DPPIV, we have assessed the tissue distribution of DPPIV by molecular approaches. Immunoblot analysis demonstrated that DPPIV is present in the kidney, lung, and small intestine at high levels, in the liver and spleen at moderate levels, and in the heart at low levels. The highest levels of mRNA for DPPIV were detected in the kidney and small intestine as compared to moderate levels found in the lung, liver, and spleen. The lowest levels of DPPIV mRNA were found in the stomach, testis, and heart. No detectable DPPIV protein and mRNA were found in brain or muscle. LDPPIV protein and mRNA are present at much lower levels in fetal livers as compared to the adult liver. Indirect immunofluorescence microscopy demonstrated that DPPIV is localized in the bile canaliculus of hematocytes and in the apical membrane domains of kidney tubule and small intestine. Further studies by Southern blot analysis indicate that DPPIV is encoded by a single gene.

Asialoglycoprotein receptor genes are linked on chromosome 11 in the mouse

The asialoglycoprotein receptor on the hepatocyte plasma membrane recognizes galactose-terminated glycoproteins and internalizes them for subsequent degradation in lysosomes. The rat receptor, also known as rat hepatic lectin (RHL), is comprised of three protein subunits called RHL-1, RHL-2, and RHL-3; two genes code for RHL-1 and RHL-2/3, respectively. We have cloned and sequenced the gene for RHL-2/3, and demonstrated that homologous asialoglycoprotein receptor genes exist in the mouse genome. Biochemical studies have demonstrated that receptor subunits exhibit the same temporal expression during development and function in a coordinate manner. This study asks if mouse receptor genes are linked and thus could possibly respond to shared cis-acting regulatory elements. Using restriction fragment length polymorphisms (RFLPs) and recombinant inbred lines, we mapped two closely linked mouse hepatic lectin (MHL) genes to chromosome 11. Asgr is designated to name asialoglycoprotein gene loci. Coordinate regulation of this linked gene family is discussed.