Mannose receptor

Domain organisation The mannose receptor is a type I transmembrane protein, with an extracellular N-terminus and an intracellular C-terminus. It is first synthesised as an inactive precursor, but is proteolytically cleaved to its active form in the Golgi apparatus. N-terminal cysteine-rich domain The N-terminal cysteine-rich domain is homologous to the ricin B chain and binds to sulphated sugar moieties, with particularly high affinity for N-Acetylgalactosamine and galactose residues sulphated at positions 3 and 4 of their pyranose rings. Other ligands include chondroitin sulfates A and B, as well as sulphated Lewisx and Lewisa structures. C-type carbohydrate recognition domains (CRDs) The 8 tandem CRDs in the extracellular region of the mannose receptor share only 30% homology with each other. They each contain at least some of the amino acid residues necessary for Ca2+ and ligand binding, common to functional C-type CRDs. Only CRDs 4 and 5 contain all of the residues required for sugar binding, forming a protease-resistant ligand-binding core. The most common ligand is terminal mannose residues, but N-acetylglucosamine and fucose also bind. Individually, the CRDs bind mannose with only weak affinity. High affinity binding is thought to result from the clustering of multiple CRDs. This clustering allows for binding of multivalent, branched ligands such as high-mannose N-linked oligosaccharides. Conformation It has been suggested that the mannose receptor can exist in at least two different structural conformations. The C-type CRDs are each separated by linker regions of 10-20 amino acids containing a number of proline residues, whose cyclic side chain is fairly rigid and favours a conformation in which the N-terminal cysteine-rich domain is extended as far away from the plasma membrane as possible. Alternatively, interactions between neighbouring CRDs may hold them in close proximity to one another and cause the extracellular region of the receptor to bend, bringing the N-terminal cysteine-rich domain into close contact with the CRDs. This would position CRDs 4 and 5 furthest from the membrane to maximise their interaction with potential ligands. The resistance to proteolysis shown by CRDs 4 and 5 suggests physical interactions between the two domains does occur, thereby supporting the existence of this U-shaped conformation. The soluble protein consists of the entire extracellular region of the receptor and it may be involved in transport of mannosylated proteins away from sites of inflammation. Glycosylation The mannose receptor is heavily glycosylated and its N-linked glycosylation sites are highly conserved between mice and humans, indicating an important role for this post-translational modification. The presence of sialic acid residues on N-linked glycans of the mannose receptor is important for its role in binding both sulphated and mannosylated glycoproteins. Sialylation regulates multimerisation of the receptor, which is known to influence binding to sulphated glycoproteins. Terminal sialic acid residues are also known to be required for binding to mannosylated glycans. The absence of sialic acid reduces the receptors ability to bind and internalise mannosylated glycans, but does not affect its localisation to the plasma membrane or its endocytic activity. ==Function==

Phagocytosis of pathogens A number of pathogenic microorganisms, including C. albicans, Pneumocystis carinii and Leishmania donovani display glycans on their surfaces with terminal mannose residues that are recognised by the C-type CRDs of the mannose receptor, thereby acting as a marker of non-self. Upon recognition, the receptor internalises the bound pathogen and transports it to lysosomes for degradation via the phagocytic pathway. In this way, the mannose receptor acts as a pattern recognition receptor. The presence of a di-aromatic FENTLY (Phe-Glu-Asn-Thr-Leu-Tyr) sequence motif in the cytoplasmic tail of the receptor is vital for its clathrin-mediated internalization. Clathrin-mediated endocytosis The CRD regions of the mannose receptor on liver sinusoidal endothelial cells remove a number of waste material ranging from soluble macromolecules to large particulate matter. These include lysosomal enzymes, collagen α-chains, C-terminal propeptides of type I pro-collagens, and tissue plasminogen activator. Binding studies indicate that each liver sinusoidal endothelial cell expresses a surface pool of 20,000-25,000 mannose receptors. The mannose receptor on liver sinusoidal endothelial cell is a rapidly recycling receptor, with a Ke (endocytotic rate constant) of 4.12 min-1, which corresponds to a half-life of 10 s for the surface pool of receptor-ligand complexes. As opposed to macrophages that use the mannose receptors for phagocytosis of particulate matter >200 nm, the mannose receptor on liver sinusoidal endothelial cells mediates clathrin-mediated endocytosis of macromolecules and nanoparticles <200 nm. It is possible that the two receptors form a complex on the cell surface that facilitates signal transduction upon pathogenic challenge. Resolution of inflammation Another key role of the mannose receptor is to regulate the levels of molecules released into the circulation during the inflammatory response. In response to pathological events, glycoproteins including lysosomal hydrolases, tissue plasminogen activator and neutrophil myeloperoxidase are released to help fight off any invading microorganisms. Once the threat has subsided, these glycoproteins can be damaging to host tissues so their levels in the circulation must be strictly controlled. High-mannose oligosaccharides present on the surface of these glycoproteins act to mark their transient nature, since they are eventually recognised by the mannose receptor and removed from the circulation. Mannose receptor knockout mice are less able to clear these proteins, and show increased concentrations of a number of lysosomal hydrolases in the blood. Consistent with this function, the mannose receptor is expressed at low levels during inflammation and at high levels during the resolution of inflammation, to ensure inflammatory agents are removed from the circulation only at the appropriate time. Clearance of glycoprotein hormones The N-terminal cysteine-rich domain of the mannose receptor plays an important role in the recognition of sulphated glycoprotein hormones and their clearance from the circulation. Glycoprotein hormones such as lutropin, which triggers release of the egg during ovulation, must stimulate their receptors in pulses to avoid receptor desensitisation. Glycans on their surface are capped with sulphated N-Acetylgalactosamine (GalNAc), making them ligands for the cysteine-rich ricin homology domain of the mannose receptor. This tag ensures a cycle of release, stimulation, and removal from the circulation. Knockout mice lacking the enzyme required to add the sulphated GalNAc capping structure show longer half-lives for lutropin, which results in increased receptor activation and oestrogen production. Female knockout mice reach sexual maturity faster than their wild-type counterparts, have a longer oestrus cycle and produce more litters. Thus, the sulphated GalNAc tag is very important in regulating serum concentrations of certain glycoprotein hormones. ==Types==

Humans express two types of mannose receptors, each encoded by its own gene: ==Applications in health and disease==

The selective internalisation properties of the mannose receptor indicate a number of potential applications in health and disease. By manipulating the glycosylation of important bioactive proteins to a highly mannosylated state, their serum levels could be tightly regulated and they could be targeted specifically to cells expressing the mannose receptor. There is also potential for use of the mannose receptor as a target for improved macrophage activation and antigen presentation. MRC2/Endo180 interacts with Basigin/CD147 via its fourth C-type lectin domain to form a molecular epithelial-mesenchymal transition suppressor complex that if disrupted results in the induction of invasive prostate epithelial cell behavior associated with poor prostate cancer survival. Increased basement membrane stiffness due to its glycation can also trigger Endo180-dependent invasion of prostate epithelial cells and this bio-mechanical mechanism is associated with poor prostate cancer survival. It has been suggested that stabilization of the Endo180-CD147 epithelial-mesenchymal transition suppressor complex and targeting of the non-complexed form of Endo180 in invasive cells could have therapeutic benefit in the prevention of cancer progression and metastasis. ==References==