Insights into Collagen Uptake by C-type Mannose Receptors from the Crystal Structure of Endo180 Domains 1-4

Lemon Balm (Melissa officinalis L.) Leaf Extract Promotes Endo180 Production in Dermal Fibroblasts and has Antiwrinkle Effect on Human Skin

BACKGROUND

The collagen receptor Endo180 participates in extracellular matrix remodeling by clearing the pericellular environment and recognizing and internalizing collagen degradation products. In photoaged skin, Endo180 expression in fibroblasts is decreased, and collagen fragments accumulate in the pericellular environment, leading to a decrease in type I collagen production and an increase in matrix metalloproteinase 1 production. This suggests that a decrease in Endo180 production may promote wrinkle formation by decreasing the dermal collagen fibril volume. Therefore, this study aimed to identify materials that promote Endo180 production in vitro and investigate whether promoting Endo180 production could prevent and improve wrinkles in vivo.

METHODS

Endo180 gene expression and protein production in fibroblasts were evaluated after screening 71 natural extracts. The conditioned medium of UVB-irradiated keratinocytes and Endo180 production-promoting extract were added to fibroblasts, and Endo180 and type I collagen production were evaluated. In a double-blind, randomized, placebo-controlled study, a cream formulated with an Endo180 production-promoting extract or placebo was topically administered to each side of the face of 20 healthy women twice daily for 8 weeks.

RESULTS

Screening results showed that 50 μg/mL of lemon balm (Melissa officinalis L.) leaf extract (MOLE) resulted in the highest levels of both Endo180 mRNA and protein at 178.1% and 127.4%, respectively. Its major component rosmarinic acid also promoted Endo180 production by 143.9% at a concentration of 20 μg/mL. MOLE at 200 μg/mL almost completely inhibited the decrease in Endo180 and type I collagen production in UVB-irradiated keratinocyte-conditioned medium. Furthermore, eye-corner wrinkles were reduced by treatment with the MOLE formulation compared to that in response to the placebo formulation.

CONCLUSIONS

MOLE may act as an antiwrinkle agent that inhibits the decline in collagen levels by promoting Endo180 production.

Novel human recombinant N-acetylgalactosamine-6-sulfate sulfatase produced in a glyco-engineered Escherichia coli strain

Mucopolysaccharidosis IVA (MPS IVA) is a lysosomal storage disease caused by mutations in the gene encoding the lysosomal enzyme N-acetylgalactosamine-6-sulfate sulfatase (GALNS), resulting in the accumulation of keratan sulfate (KS) and chondroitin-6-sulfate (C6S). Previously, it was reported the production of an active human recombinant GALNS (rGALNS) in E. coli BL21(DE3). However, this recombinant enzyme was not taken up by HEK293 cells or MPS IVA skin fibroblasts. Here, we leveraged a glyco-engineered E. coli strain to produce a recombinant human GALNS bearing the eukaryotic trimannosyl core N-glycan, Man3GlcNAc2 (rGALNSoptGly). The N-glycosylated GALNS was produced at 100 mL and 1.65 L scales, purified and characterized with respect to pH stability, enzyme kinetic parameters, cell uptake, and KS clearance. The results showed that the addition of trimannosyl core N-glycans enhanced both protein stability and substrate affinity. rGALNSoptGly was capture through a mannose receptor-mediated process. This enzyme was delivered to the lysosome, where it reduced KS storage in human MPS IVA fibroblasts. This study demonstrates the potential of a glyco-engineered E. coli for producing a fully functional GALNS enzyme. It may offer an economic approach for the biosynthesis of a therapeutic glycoprotein that could prove useful for MPS IVA treatment. This strategy could be extended to other lysosomal enzymes that rely on the presence of mannose N-glycans for cell uptake.

Development of Mannosylated Lipid Nanoparticles for mRNA Cancer Vaccine with High Antigen Presentation Efficiency and Immunomodulatory Capability

Insufficient accumulation of lipid nanoparticles (LNPs)-based mRNA vaccines in antigen presenting cells remains a key barrier to eliciting potent antitumor immune responses. Herein, we develop dendritic cells (DCs) targeting LNPs by taking advantage of mannose receptor-mediated endocytosis. Efficient delivery of mRNA to DCs is achieved in vitro and in vivo utilizing the sweet LNPs (STLNPs-Man). Intramuscular injection of mRNA vaccine (STLNPs-Man@mRNAOVA ) results in a four-fold higher uptake by DCs in comparison with commercially used LNPs. Benefiting from its DCs targeting ability, STLNPs-Man@mRNAOVA significantly promotes the antitumor performances, showing a comparable therapeutic efficacy by using one-fifth of the injection dosage as the vaccine prepared from normal LNPs, thus remarkably avoiding the side effects brought by conventional mRNA vaccines. More intriguingly, STLNPs-Man@mRNAOVA exhibits the ability to downregulate the expression of cytotoxic T-lymphocyte-associated protein 4 on T cells due to the blockade of CD206/CD45 axis, showing brilliant potentials in promoting antitumor efficacy combined with immune checkpoint blockade therapy.

Interaction of human dendritic cell receptor DEC205/CD205 with keratins

DEC205 (CD205) is one of the major endocytic receptors on dendritic cells and has been widely used as a receptor target in immune therapies. It has been shown that DEC205 can recognize dead cells through keratins in a pH-dependent manner. However, the mechanism underlying the interaction between DEC205 and keratins remains unclear. Here we determine the crystal structures of an N-terminal fragment of human DEC205 (CysR∼CTLD3). The structural data show that DEC205 shares similar overall features with the other mannose receptor family members such as the mannose receptor and Endo180, but the individual domains of DEC205 in the crystal structure exhibit distinct structural features that may lead to specific ligand binding properties of the molecule. Among them, CTLD3 of DEC205 adopts a unique fold of CTLD, which may correlate with the binding of keratins. Furthermore, we examine the interaction of DEC205 with keratins by mutagenesis and biochemical assays based on the structural information and identify an XGGGX motif on keratins that can be recognized by DEC205, thereby providing insights into the interaction between DEC205 and keratins. Overall, these findings not only improve the understanding of the diverse ligand specificities of the mannose receptor family members at the molecular level but may also give clues for the interactions of keratins with their binding partners in the corresponding pathways.

Structure of the transmembrane protein 2 (TMEM2) ectodomain and its apparent lack of hyaluronidase activity

Background: Hyaluronic acid (HA) is a major polysaccharide component of the extracellular matrix. HA has essential functions in tissue architecture and the regulation of cell behaviour. HA turnover needs to be finely balanced. Increased HA degradation is associated with cancer, inflammation, and other pathological situations. Transmembrane protein 2 (TMEM2) is a cell surface protein that has been reported to degrade HA into ~5 kDa fragments and play an essential role in systemic HA turnover. Methods: We produced the soluble TMEM2 ectodomain (residues 106-1383; sTMEM2) in human embryonic kidney cells (HEK293) and determined its structure using X-ray crystallography. We tested sTMEM2 hyaluronidase activity using fluorescently labelled HA and size fractionation of reaction products. We tested HA binding in solution and using a glycan microarray. Results: Our crystal structure of sTMEM2 confirms a remarkably accurate prediction by AlphaFold. sTMEM2 contains a parallel β-helix typical of other polysaccharide-degrading enzymes, but an active site cannot be assigned with confidence. A lectin-like domain is inserted into the β-helix and predicted to be functional in carbohydrate binding. A second lectin-like domain at the C-terminus is unlikely to bind carbohydrates. We did not observe HA binding in two assay formats, suggesting a modest affinity at best. Unexpectedly, we were unable to observe any HA degradation by sTMEM2. Our negative results set an upper limit for k _cat of approximately 10 ^-5 min ^-1. Conclusions: Although sTMEM2 contains domain types consistent with its suggested role in TMEM2 degradation, its hyaluronidase activity was undetectable. HA degradation by TMEM2 may require additional proteins and/or localisation at the cell surface.

A Novel CD206 Targeting Peptide Inhibits Bleomycin-Induced Pulmonary Fibrosis in Mice

Activated M2-polarized macrophages are drivers of pulmonary fibrosis in several clinical scenarios, including Idiopathic Pulmonary Fibrosis (IPF). In this study, we investigated the effects of targeting the CD206 receptor in M2-like macrophages with a novel synthetic analogue of a naturally occurring Host Defense Peptide (HDP), RP-832c, to decrease profibrotic cytokines. RP-832c selectively binds to CD206 on M2-polarized bone marrow-derived macrophages (BMDM) in vitro, resulting in a time-dependent decrease in CD206 expression and a transient increase in M1-macrophage marker TNF-α. To elucidate the antifibrotic effects of RP-832c, we used a murine model of bleomycin (BLM)-induced early-stage pulmonary fibrosis. RP-832c significantly reduced fibrosis in a dose-dependent manner, and decreased CD206, TGF-β1, and α-SMA expression in mouse lungs. Similarly, in an established model of lung fibrosis, RP-832c significantly decreased lung fibrosis and significantly decreased inflammatory cytokines TNF-α, IL-6, IL-10, IFN-γ, CXCL1/2, and fibrosis markers TGF-β1 and MMP-13. In comparison with the FDA-approved drugs Nintedanib and Pirfenidone, RP-832c exhibited a similar reduction in fibrosis compared to Pirfenidone, and to a greater extent than Nintedanib, with no apparent toxicities observed. In summary, our findings showed that inhibiting the profibrotic alternatively activated M2-like macrophages using a novel peptide, RP-832c, could reduce BLM-induced pulmonary fibrosis in mice, warranting the therapeutic potential of this peptide for patients with pulmonary fibrosis.

Structure of the transmembrane protein 2 (TMEM2) ectodomain and its lack of hyaluronidase activity

Background: Hyaluronic acid (HA) is a major polysaccharide component of the extracellular matrix. HA has essential functions in tissue architecture and the regulation of cell behaviour. HA turnover needs to be finely balanced. Increased HA degradation is associated with cancer, inflammation, and other pathological situations. Transmembrane protein 2 (TMEM2) is a cell surface protein that has been reported to degrade HA into ~5 kDa fragments and play an essential role in systemic HA turnover. Methods: We produced the soluble TMEM2 ectodomain (residues 106-1383; sTMEM2) in human embryonic kidney cells (HEK293) and determined its structure using X-ray crystallography. We tested sTMEM2 hyaluronidase activity using fluorescently labelled HA and size fractionation of reaction products. Results: Our crystal structure of sTMEM2 confirms a remarkably accurate prediction by AlphaFold. sTMEM2 contains a parallel β-helix typical of other polysaccharide-degrading enzymes, but an active site cannot be assigned with confidence. A lectin-like domain is inserted into the β-helix and predicted to be functional in carbohydrate binding. A second lectin-like domain at the C-terminus is unlikely to bind carbohydrates. Unexpectedly, we were unable to observe any HA degradation by sTMEM2. Our negative results set an upper limit for    kcat of approximately 10-5 min-1. Conclusions: Although sTMEM2 contains domain types consistent with its suggested role in TMEM2 degradation, its hyaluronidase activity was undetectable. HA degradation by TMEM2 may require additional proteins and/or localisation at the cell surface.

Structure of PLA2R reveals presentation of the dominant membranous nephropathy epitope and an immunogenic patch

Membranous nephropathy is an autoimmune kidney disease caused by autoantibodies targeting antigens present on glomerular podocytes, instigating a cascade leading to glomerular injury. The most prevalent circulating autoantibodies in membranous nephropathy are against phospholipase A2 receptor (PLA2R), a cell surface receptor. The dominant epitope in PLA2R is located within the cysteine-rich domain, yet high-resolution structure-based mapping is lacking. In this study, we define the key nonredundant amino acids in the dominant epitope of PLA2R involved in autoantibody binding. We further describe two essential regions within the dominant epitope and spacer requirements for a synthetic peptide of the epitope for drug discovery. In addition, using cryo-electron microscopy, we have determined the high-resolution structure of PLA2R to 3.4 Å resolution, which shows that the dominant epitope and key residues within the cysteine-rich domain are accessible at the cell surface. In addition, the structure of PLA2R not only suggests a different orientation of domains but also implicates a unique immunogenic signature in PLA2R responsible for inducing autoantibody formation and recognition.

uPARAP/Endo180: a multifaceted protein of mesenchymal cells

The urokinase plasminogen activator receptor-associated protein (uPARAP/Endo180) is already known to be a key collagen receptor involved in collagen internalization and degradation in mesenchymal cells and some macrophages. It is one of the four members of the mannose receptor family along with a macrophage mannose receptor (MMR), a phospholipase lipase receptor (PLA2R), and a dendritic receptor (DEC-205). As a clathrin-dependent endocytic receptor for collagen or large collagen fragments as well as through its association with urokinase (uPA) and its receptor (uPAR), uPARAP/Endo180 takes part in extracellular matrix (ECM) remodeling, cell chemotaxis and migration under physiological (tissue homeostasis and repair) and pathological (fibrosis, cancer) conditions. Recent advances that have shown an expanded contribution of this multifunctional protein across a broader range of biological processes, including vascular biology and innate immunity, are summarized in this paper. It has previously been demonstrated that uPARAP/Endo180 assists in lymphangiogenesis through its capacity to regulate the heterodimerization of vascular endothelial growth factor receptors (VEGFR-2 and VEGFR-3). Moreover, recent findings have demonstrated that it is also involved in the clearance of collectins and the regulation of the immune system, something which is currently being studied as a biomarker and a therapeutic target in a number of cancers.

Decreased levels of endocytic collagen receptor Endo180 in dermal fibroblasts lead to decreased production of type I collagen and increased expression of matrix metalloproteinase-1

BACKGROUND

Endo180 is involved in collagen remodeling by incorporating extracellular degraded collagen. Ultraviolet irradiation of dermal fibroblasts reduces Endo180 expression, which affects collagen fiber remodeling. However, it is unclear whether the decrease in Endo180 is directly related to the decrease in type I collagen fibers during photoaging. We aimed to clarify the relationship between Endo180 reduction and the decrease in type I collagen fibers observed in photoaged dermis.

METHODS

Endo180 was reduced in normal human dermal fibroblasts using RNAi. Endo180 knockdown cells were inoculated into collagen gels. The influence of Endo180 knockdown was evaluated by measuring mRNA expression of collagen fiber remodeling-related factors and collagen gel contraction. The collagen state and oxidative stress in the collagen gels were also measured.

RESULTS

Endo180 knockdown cells, which were confirmed by gelatin uptake inhibition, showed upregulation of matrix metalloproteinase-1 and downregulation of type I collagen mRNA expression when cultured in collagen gels. The contractility of the collagen gel was reduced by Endo180 knockdown. The collagen state in the extracellular matrix of the collagen gels containing Endo180 knockdown fibroblasts showed increased amounts of 3/4 fragmented collagen and denatured collagen and decreased type I collagen synthesis. In addition, an increase in intracellular oxidative stress was observed.

CONCLUSIONS

This study confirmed that the decrease in Endo180 caused a failure in collagen fiber formation and a decrease in collagen production, reproducing the photoaging dermal structural changes. This suggests that the decrease in Endo180 may be involved in wrinkle formation, which is a characteristic of photoaged skin.

Impairment of a distinct cancer-associated fibroblast population limits tumour growth and metastasis

Profiling studies have revealed considerable phenotypic heterogeneity in cancer-associated fibroblasts (CAFs) present within the tumour microenvironment, however, functional characterisation of different CAF subsets is hampered by the lack of specific markers defining these populations. Here we show that genetic deletion of the Endo180 (MRC2) receptor, predominantly expressed by a population of matrix-remodelling CAFs, profoundly limits tumour growth and metastasis; effects that can be recapitulated in 3D co-culture assays. This impairment results from a CAF-intrinsic contractility defect and reduced CAF viability, which coupled with the lack of phenotype in the normal mouse, demonstrates that upregulated Endo180 expression by a specific, potentially targetable CAF subset is required to generate a supportive tumour microenvironment. Further, characterisation of a tumour subline selected via serial in vivo passage for its ability to overcome these stromal defects provides important insight into, how tumour cells adapt to a non-activated stroma in the early stages of metastatic colonisation.

Endo180, a collagen binding receptor, is highly expressed in a subset of cancer-associated fibroblasts. The authors show, using knockout mice and 3D in vitro assays, that Endo180 depletion impairs tumour fibroblast contractility and viability resulting in reduced tumour growth and metastasis.

Effects of varying gelatin coating concentrations on RANKL induced osteoclastogenesis

Here, we assessed the effects of varying concentrations of gelatin coating on Receptor Activator of Nuclear Factor κ-B Ligand (RANKL)-induced RAW264.7 murine macrophage differentiation into osteoclast (OC) via osteoclastogenesis. The microstructures of coating surfaces with different concentrations of gelatin were examined by scanning electron microscopy and atomic force microscopy. Increased gelatin coating concentrations led to decreased gel rigidity but increased surface adhesion force attenuated OC differentiation and the decreased actin ring formation in RANKL-induced osteoclastogenesis. The decreased actin ring formation is associated with decreased lysosomal-associated membrane protein 1 (LAMP1) activity and bone resorption in the differentiated OCs with different gelatin coating concentrations as compared to the cells differentiated without gelatin coatings. In addition, increasing concentrations of gelatin coating attenuated the medium TGF-β1 protein levels and the expression levels of TGF-β and type-I (R1) and type-II (R2) TGF-β receptors in OCs, suggesting the gelatin-induced suppression of TGF-β signaling for the regulation of RNAKL-induced OC differentiation. Taken together, these findings showed that changes in gelatin coating concentrations, which were associated with altered gel thickness and substrate rigidity, might attenuate TGF-β signaling events to modulate OC differentiation and concomitant actin ring formation and bone matrix resorption in RANKL-induced osteoclastogenesis.

Positive-charge tuned gelatin hydrogel-siSPARC injectable for siRNA anti-scarring therapy in post glaucoma filtration surgery

Small interfering RNA (siRNA) therapy is a promising epigenetic silencing strategy. However, its widespread adoption has been severely impeded by its ineffective delivery into the cellular environment. Here, a biocompatible injectable gelatin-based hydrogel with positive-charge tuned surface charge is presented as an effective platform for siRNA protection and delivery. We demonstrate a two-step synthesis of a gelatin-tyramine (Gtn-Tyr) hydrogel with simultaneous charge tunability and crosslinking ability. We discuss how different physiochemical properties of the hydrogel interact with siSPARC (siRNA for secreted protein, acidic and rich in cysteine), and study the positive-charge tuned gelatin hydrogel as an effective delivery platform for siSPARC in anti-fibrotic treatment. Through in vitro studies using mouse tenon fibroblasts, the positive-charge tuned Gtn-Tyr hydrogel shows sustained siSPARC cellular internalization and effective SPARC silencing with excellent biocompatibility. Similarly, the same hydrogel platform delivering siSPARC in an in vivo assessment employing a rabbit model shows an effective reduction in subconjunctival scarring in post glaucoma filtration surgery, and is non-cytotoxic compared to a commonly used anti-scarring agent, mitomycin-C. Overall, the current siRNA delivery strategy involving the positive-charge tuned gelatin hydrogel shows effective delivery of gene silencing siSPARC for anti-fibrotic treatment. The current charge tunable hydrogel delivery system is simple to fabricate and highly scalable. We believe this delivery platform has strong translational potential for effective siRNA delivery and epigenetic silencing therapy.

Structural analysis of carbohydrate binding by the macrophage mannose receptor CD206

The human mannose receptor expressed on macrophages and hepatic endothelial cells scavenges released lysosomal enzymes, glycopeptide fragments of collagen, and pathogenic microorganisms and thus reduces damage following tissue injury. The receptor binds mannose, fucose, or N-acetylglucosamine (GlcNAc) residues on these targets. C-type carbohydrate-recognition domain 4 (CRD4) of the receptor contains the site for Ca2+-dependent interaction with sugars. To investigate the details of CRD4 binding, glycan array screening was used to identify oligosaccharide ligands. The strongest signals were for glycans that contain either Manα1-2Man constituents or fucose in various linkages. The mechanisms of binding to monosaccharides and oligosaccharide substructures present in many of these ligands were examined in multiple crystal structures of CRD4. Binding of mannose residues to CRD4 results primarily from interaction of the equatorial 3- and 4-OH groups with a conserved principal Ca2+ common to almost all sugar-binding C-type CRDs. In the Manα1-2Man complex, supplementary interactions with the reducing mannose residue explain the enhanced affinity for this disaccharide. Bound GlcNAc also interacts with the principal Ca2+ through equatorial 3- and 4-OH groups, whereas fucose residues can bind in several orientations, through either the 2- and 3-OH groups or the 3- and 4-OH groups. Secondary contacts with additional sugars in fucose-containing oligosaccharides, such as the Lewis-a trisaccharide, provide enhanced affinity for these glycans. These results explain many of the biologically important interactions of the mannose receptor with both mammalian glycoproteins and microbes such as yeast and suggest additional classes of ligands that have not been previously identified.

The cryo-EM structure of the endocytic receptor DEC-205

DEC-205 (CD205), a member of the macrophage mannose receptor protein family, is the prototypic endocytic receptor of dendritic cells, whose ligands include phosphorothioated cytosine-guanosine oligonucleotides, a motif often seen in bacterial or viral DNA. However, despite growing biological and clinical significance, little is known about the structural arrangement of this receptor or any of its family members. Here, we describe the 3.2 Å cryo-EM structure of human DEC-205, thereby illuminating the structure of the mannose receptor protein family. The DEC-205 monomer forms a compact structure comprising two intercalated rings of C-type lectin-like domains, where the N-terminal cysteine-rich and fibronectin domains reside at the central intersection. We establish a pH-dependent oligomerization pathway forming tetrameric DEC-205 using solution-based techniques and ultimately solved the 4.9 Å cryo-EM structure of the DEC-205 tetramer to identify the unfurling of the second lectin ring which enables tetramer formation. Furthermore, we suggest the relevance of this oligomerization pathway within a cellular setting, whereby cytosine-guanosine binding appeared to disrupt this cell-surface oligomer. Accordingly, we provide insight into the structure and oligomeric assembly of the DEC-205 receptor.

A Novel CD206 Targeting Peptide Inhibits Bleomycin Induced Pulmonary Fibrosis in Mice

Activated M2 polarized macrophages are drivers of pulmonary fibrosis in several clinical scenarios such as Acute Respiratory Disease Syndrome (ARDS) and Idiopathic Pulmonary Fibrosis (IPF), through the production of inflammatory and fibrosis-inducing cytokines. In this study, we investigated the effect of targeting the CD206 receptor with a novel fragment of a Host Defense Peptide (HDP), RP-832c to decrease cytokines that cause fibrosis. RP-832c selectively binds to CD206 on M2 polarized bone marrow derived macrophages (BMDM) in vitro , resulting in a time-dependent decrease in CD206 expression, and a transient increase in M1 marker TNFα, which resolves over a 24hr period. To elucidate the antifibrotic effect of RP-832c, we used a murine model of bleomycin (BLM) -induced early-stage pulmonary fibrosis. RP-832c significantly reduced bleomycin-induced fibrosis in a dosage dependent manner, as well as decreased CD206, TGF-β1 and α-SMA expression in mouse lungs. Interestingly we did not observe any changes in the resident alveolar macrophage marker CD170 expression. Similarly, in an established model of lung fibrosis, RP-832c significantly decreased fibrosis in the lung, as well as significantly decreased inflammatory cytokines TNFα, IL-6, IL-10, INF-γ, CXCL1/2, and fibrosis markers TGF-β1 and MMP-13. In comparison with FDA approved drugs, Nintedanib and Pirfenidone, RP-832c exhibited a similar reduction in fibrosis compared to Pirfenidone, and to a greater extent than Nintedanib, with no apparent toxicities observed on body weight or blood chemistry. In summary, RP-832c is a potential agent to mitigate the overactivity of M2 macrophages in pathogenesis several pulmonary fibrotic diseases, including SARS-CoV-2 induced lung fibrosis.

The collagen receptor uPARAP/Endo180 regulates collectins through unique structural elements in its FNII domain

C-type lectins that contain collagen-like domains are known as collectins. These proteins are present both in the circulation and in extravascular compartments and are central players of the innate immune system, contributing to first-line defenses against viral, bacterial, and fungal pathogens. The collectins mannose-binding lectin (MBL) and surfactant protein D (SP-D) are regulated by tissue fibroblasts at extravascular sites via an endocytic mechanism governed by urokinase plasminogen activator receptor-associated protein (uPARAP or Endo180), which is also a collagen receptor. Here, we investigated the molecular mechanisms that drive the uPARAP-mediated cellular uptake of MBL and SP-D. We found that the uptake depends on residues within a protruding loop in the fibronectin type-II (FNII) domain of uPARAP that are also critical for collagen uptake. Importantly, however, we also identified FNII domain residues having an exclusive role in collectin uptake. We noted that these residues are absent in the related collagen receptor, the mannose receptor (MR or CD206), which consistently does not interact with collectins. We also show that the second C-type lectin-like domain (CTLD2) is critical for the uptake of SP-D, but not MBL, indicating an additional level of complexity in the interactions between collectins and uPARAP. Finally, we demonstrate that the same molecular mechanisms enable uPARAP to engage MBL immobilized on the surface of pathogens, thereby expanding the potential biological implications of this interaction. Our study reveals molecular details of the receptor-mediated cellular regulation of collectins and offers critical clues for future investigations into collectin biology and pathology.

Structural basis of the pH-dependent conformational change of the N-terminal region of human mannose receptor/CD206

Mannose receptor (MR, CD206) is an immune receptor highly expressed on macrophages and plays important roles in glycoprotein clearance, immune response and matrix turnover. Previous studies have shown that MR recognizes multiple ligands and recycles between cell surface and endosomes, and the conformation and ligand binding of MR are regulated by environmental pH. However, due to the lack of high-resolution details, the mechanisms of the pH-dependent properties of MR have not been fully understood. Here we investigate the pH-dependent conformational change of MR by solving a series of crystal structures of MR N-terminal fragments (CysR~CTLD2/3) at pH ranging from 4.0 to 8.5. The results show that the CTLD3 domain plays a critical role in regulating the conformational change of the N-terminal region of MR by forming interactions with the CTLD2 domain specifically at acidic pH. Moreover, the structural data also show the conformational changes of the 4-SO₄-GalNAc binding pocket at the CysR domain, which might be relevant to the binding and release of the ligand. Overall, these results provide a model for the pH-dependent conformational change of the N-terminal region of MR that may help to understand its functional mechanism at molecular level.

Structural Insights into the pH-Dependent Conformational Change and Collagen Recognition of the Human Mannose Receptor

Mannose receptor (MR, CD206) is an endocytic receptor on microphages and dendritic cells. It recognizes multiple ligands and plays important roles in regulating immune responses and maintaining glycoprotein homeostasis. However, the structure and functional mechanism of MR remain unclear. Here we determine the crystal structures of the N-terminal fragments of MR and reveal the potential binding mode of collagen on the fibronectin II domain. The SAXS and other biophysical data suggest that MR adopts an extended conformation at physiological pH and undergoes conformational changes as pH decreases, resulting in a compact conformation in an acidic environment. Moreover, biochemical data show that MR binds to collagen in a Ca²⁺-enhanced manner at physiological pH, whereas Ca²⁺ has no effect on the binding at acidic pH. These results provide a model for the dynamic mechanism of MR regarding its ligand binding and release during the recycling between cell surface and endosomes.

Structural basis of laminin binding to the LARGE glycans on dystroglycan

Dystroglycan is a highly glycosylated extracellular matrix receptor with essential functions in skeletal muscle and the nervous system. Reduced matrix binding by α-dystroglycan (α-DG) due to perturbed glycosylation is a pathological feature of several forms of muscular dystrophy. Like-acetylglucosaminyltransferase (LARGE) synthesizes the matrix-binding heteropolysaccharide [-glucuronic acid-β1,3-xylose-α1,3-]n. Using a dual exoglycosidase digestion, we confirm that this polysaccharide is present on native α-DG from skeletal muscle. The atomic details of matrix binding were revealed by a high-resolution crystal structure of laminin-G-like (LG) domains 4 and 5 (LG4 and LG5) of laminin-α2 bound to a LARGE-synthesized oligosaccharide. A single glucuronic acid-β1,3-xylose disaccharide repeat straddles a Ca(2+) ion in the LG4 domain, with oxygen atoms from both sugars replacing Ca(2+)-bound water molecules. The chelating binding mode accounts for the high affinity of this protein-carbohydrate interaction. These results reveal a previously uncharacterized mechanism of carbohydrate recognition and provide a structural framework for elucidating the mechanisms underlying muscular dystrophy.

Crystal structures of the ligand-binding region of uPARAP: effect of calcium ion binding

The proteins of the mannose receptor (MR) family share a common domain organization and have a broad range of biological functions. Urokinase plasminogen activator receptor-associated protein (uPARAP) (or Endo180) is a member of this family and plays an important role in extracellular matrix remodelling through interaction with its ligands, including collagens and urokinase plasminogen activator receptor (uPAR). We report the crystal structures of the first four domains of uPARAP (also named the ligand-binding region, LBR) at pH 7.4 in Ca(2+)-bound and Ca(2+)-free forms. The first domain (cysteine-rich or CysR domain) folds into a new and unique conformation different from the β-trefoil fold of typical CysR domains. The so-called long loop regions (LLRs) of the C-type lectin-like domain (CTLD) 1 and 2 (the third and fourth domain) mediate the direct contacts between these domains. These LLRs undergo a Ca(2+)-dependent conformational change, and this is likely to be the key structural determinant affecting the overall conformation of uPARAP. Our results provide a molecular mechanism to support the structural flexibility of uPARAP, and shed light on the structural flexibility of other members of the MR family.