Genomic structure and chromosomal location of the mouse macrophage C-type lectin gene

Differential regulation of C-type lectin expression on tolerogenic dendritic cell subsets

Antigen presenting cells (APC) express high levels of C-type lectins, which play a major role in cellular interactions as well as pathogen recognition and antigen presentation. The C-type lectin macrophage galactose-type lectin (MGL), expressed by dendritic cells (DC) and macrophages, mediates binding to glycoproteins and lipids that contain terminal GalNAc moieties. To investigate MGL expression patterns in more detail, we generated two new monoclonal antibodies and set up a quantitative real-time PCR analysis to determine MGL mRNA levels. MGL is not expressed by blood-resident plasmacytoid DC and thus represents an exclusive marker for myeloid-type APC. Dexamethasone treatment upregulated MGL expression on DC both at the protein and mRNA level in a time- and dose-dependent manner. In contrast, DC generated in the presence of IL-10 did not display enhanced MGL levels. Furthermore, dexamethasone and IL-10 also differentially regulated expression of other C-type lectins, such as DC-SIGN and Mannose Receptor. Our results demonstrate that depending on the local microenvironment, DC can adopt different C-type lectin profiles, which could have major influences on cell-cell interactions, antigen uptake and presentation.

Retardation of removal of radiation-induced apoptotic cells in developing neural tubes in macrophage galactose-type C-type lectin-1-deficient mouse embryos

MGL1/CD301a is a C-type lectin that recognizes galactose and N-acetylgalactosamine as monosaccharides and is expressed on limited populations of macrophages and dendritic cells at least in adult mice. In this study, pregnant mice with Mgl1+/- genotype were mated with Mgl1+/- or Mgl1-/- genotype males, and the embryos were used to assess a hypothesis that this molecule plays an important role in the clearance of apoptotic cells. After X-ray irradiation at 1 Gy of developing embryos at 10.5 days post coitus (d.p.c.), the number of Mgl1-/- pups was significantly reduced as compared with Mgl1+/+ pups. Distributions of MGL1-positive cells, MGL2-positive cells, and apoptotic cells were histologically examined in irradiated Mgl1+/+ embryos. MGL1-positive cells were detected in the neural tube in which many cells undergo apoptosis, whereas MGL2-positive cells were not observed. Biotinylated recombinant MGL1 bound a significant portion of the apoptotic cells. When Mgl1+/+ and Mgl1-/- embryos were examined for the presence of apoptotic cells, similar numbers of apoptotic cells gave rise, but the clearance of these cells was slower in Mgl1-/- embryos than in Mgl1+/+ embryos. These results strongly suggest that MGL1/CD301a is involved in the clearance of apoptotic cells. This process should be essential in the repair and normal development of X-ray-irradiated embryos.

Molecular cloning and characterization of a novel mouse macrophage C-type lectin, mMGL2, which has a distinct carbohydrate specificity from mMGL1

A novel mouse macrophage galactose-type C-type lectin 2 (mMGL2) was identified by BLAST analysis of expressed sequence tags. The sequence of mMGL2 is highly homologous to the mMGL, which should now be called mMGL1. The open reading frame of mMGL2 contains a sequence corresponding to a type II transmembrane protein with 332 amino acids having a single extracellular C-type lectin domain. The 3'-untranslated region included long terminal repeats of mouse early transposon. The Mgl2 gene was cloned from a 129/SvJ mouse genomic library and sequenced. The gene spans 7,136 base pairs and consists of 10 exons, which is similar to the genomic organization of mMGL1. The reverse transcriptase-PCR analysis indicates that mMGL2 is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner, also very similar to that of mMGL1. The mMGL2 mRNA was also detected in mMGL1-positive cells, which were sorted from thioglycollate-induced peritoneal cells with a mMGL1-specific monoclonal antibody, LOM-8.7. The soluble recombinant proteins of mMGL2 exhibited carbohydrate specificity for alpha- and beta-GalNAc-conjugated soluble polyacrylamides, whereas mMGL1 preferentially bound Lewis X-conjugated soluble polyacrylamides in solid phase assays. These two lectins may function cooperatively as recognition and endocytic molecules on macrophages and related cells.

Macrophage C-type lectin on bone marrow-derived immature dendritic cells is involved in the internalization of glycosylated antigens

Bone marrow-derived dendritic cells (DCs) were examined for the expression of the murine macrophage C-type lectin specific for galactose and N-acetylgalactosamine (mMGL). Flow cytometric analysis after double staining for MHC class II and mMGL with specific monoclonal antibodies indicated that mMGL was expressed on immature DCs with low to moderate levels of MHC class II and down-regulated during maturation. Immature DCs bound and internalized alpha-N-acetylgalactosaminides conjugated to soluble polyacrylamide (alpha-GalNAc polymers), whereas mature DCs and bone marrow cells did not. The two-color flow cytometric profiles indicated that the degree of alpha-GalNAc polymer bindings exactly coincided with the intensity of the binding of a mMGL-specific monoclonal antibody LOM-14. The internalized alpha-GalNAc polymers seemed to be transported to MHC class II compartments. Thus, mMGL is transiently expressed on bone marrow-derived DCs during their development and maturation and suggested to be involved in the uptake of glycosylated antigens for presentation.

Cloning, mapping and genomic organization of a fish C-type lectin gene from homozygous clones of rainbow trout (Oncorhynchus mykiss)

Utilizing a splenic cDNA library and rapid amplification of cDNA 5' ends (5'-RACE), a C-type lectin gene was cloned from a homozygous cloned rainbow trout. The 1176 bp cDNA contains a 714 bp open reading frame from which a 238-amino-acid (aa) (27 kDa) protein was deduced. It was confirmed that this protein belongs to the C-type animal lectins, and is a type II membrane receptor. The predicted protein from this sequence contains a 48 aa cytoplasmic domain, a 20 aa transmembrane domain (TM), a 46 aa stalk region and a 124 aa carbohydrate-recognition domain (CRD). The stalk region contains a leucine-zipper, and an N-glycosylation site was also found in the CRD. Sequence alignment and phylogenetic analysis of the CRD indicate that the protein has similarity with human dendritic cell immunoreceptor (DCIR), gp120 binding C-type lectin (gp120BCL) and mammalian hepatic lectins. The N-terminus (aa 4-183) has similarity with NKG2, a group of C-type lectin receptors important in human natural killer cell function. The genomic DNA (gDNA) containing this gene was amplified and sequenced. The 4569 bp gDNA contains five exons and four introns. The first three exons encode the cytoplasmic domain, the TM and stalk region, respectively. Unlike the other type II C-type lectin receptors in which the CRD was encoded by three exons, the CRD of this lectin was encoded by two exons. A transposon Tc1-like fragment was found in intron III. Intron IV is composed of a simple repeat. Tissue-specific expression of the gene was studied by RT-PCR, and it was mainly expressed in spleen and peripheral blood leukocyte (PBL). Using AluI to digest the fragment containing exon I, intron I and exon II, an RFLP was produced between the sequences of this gene in two cloned fish, OSU 142 and Arlee (AR). Seventy-one doubled haploids (DH) of OSU X AR were screened, and the gene was mapped to linkage group XIV on the published map (Young et al., Genetics 148 (1998) 839).

The amino-acid sequence of the abalone (Haliotis laevigata) nacre protein perlucin. Detection of a functional C-type lectin domain with galactose/mannose specificity

Perlucin isolated from abalone nacre consists of 155 amino acids including a glycosylated asparagine. The sequence of the first 130 amino acids shows a high similarity to the C-type carbohydrate-recognition domains of asialoglycoprotein receptors and other members of the group of C-type lectins but also a weaker similarity to related proteins without carbohydrate-binding activity. This C-type module is followed by a short C-terminal domain containing two almost identical sequence repeats with a length of 10 amino acids. Solid phase assays show a divalent metal ion-dependent binding of perlucin to (neo)glycoproteins containing D-galactose or D-mannose/D-glucose indicating that perlucin is a functional C-type lectin with broad carbohydrate-binding specificity. Our results also indicate that it may be difficult to predict carbohydrate-binding specificity and the occurrence of alternative binding configurations by amino-acid sequence comparisons and homology modeling.

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.