Critical Role of Arg/Lys343 in the Species-Dependent Recognition of Phosphatidylinositol by Pulmonary Surfactant Protein D,

Aberrant Cellular Glycosylation May Increase the Ability of Influenza Viruses to Escape Host Immune Responses through Modification of the Viral Glycome

Individuals with metabolic dysregulation of cellular glycosylation often experience severe influenza disease, with a poor immune response to the virus and low vaccine efficacy. Here, we investigate the consequences of aberrant cellular glycosylation for the glycome and the biology of influenza virus. We transiently induced aberrant N-linked glycosylation in cultured cells with an oligosaccharyltransferase inhibitor, NGI-1. Cells treated with NGI-1 produced morphologically unaltered viable influenza virus with sequence-neutral glycosylation changes (primarily reduced site occupancy) in the hemagglutinin and neuraminidase proteins. Hemagglutinin with reduced glycan occupancy required a higher concentration of surfactant protein D (an important innate immunity respiratory tract collectin) for inhibition compared to that with normal glycan occupancy. Immunization of mice with NGI-1-treated virus significantly reduced antihemagglutinin and antineuraminidase titers of total serum antibody and reduced hemagglutinin protective antibody responses. Our data suggest that aberrant cellular glycosylation may increase the risk of severe influenza as a result of the increased ability of glycome-modified influenza viruses to evade the immune response.

Role of inositol to improve surfactant functions and reduce IL-6 levels: A potential adjuvant strategy for SARS-CoV-2 pneumonia?

To date, the spread of SARS-CoV-2 infection is increasing worldwide and represents a primary healthcare emergency. Although the infection can be asymptomatic, several cases develop severe pneumonia and acute respiratory distress syndrome (ARDS) characterized by high levels of pro-inflammatory cytokines, primarily interleukin (IL)-6. Based on available data, the severity of ARDS and serum levels of IL-6 are key determinants for the prognosis.

In this scenario, available in vitro and in vivo data suggested that myo-inositol is able to increase the synthesis and function of the surfactant phosphatidylinositol, acting on the phosphoinositide 3-kinase (PI3K)-regulated signaling, with amelioration of both immune system and oxygenation at the bronchoalveolar level. In addition, myo-inositol has been found able to decrease the levels of IL-6 in several experimental settings, due to an effect on the inositol-requiring enzyme 1 (IRE1)-X-box-binding protein 1 (XBP1) and on the signal transducer and activator of transcription 3 (STAT3) pathways. In this scenario, treatment with myo-inositol may be able to reduce IL-6 dependent inflammatory response and improve oxygenation in patients with severe ARDS by SARS-CoV-2. In addition, the action of myo-inositol on IRE1 endonuclease activity may also inhibit the replication of SARS-CoV-2, as was reported for the respiratory syncytial virus. Since the available data are extremely limited, if this potential therapeutic approach will be considered valid in the clinical practice, the necessary future investigations should aim to identify the best dose, administration route (oral, intravenous and/or aerosol nebulization), and cluster(s) of patients which may get beneficial effects from this treatment.

Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with N-acetylglucosamine

Bovine conglutinin is an immune protein that is involved in host resistance to microbes and parasites and interacts with complement component iC3b, agglutinates erythrocytes, and neutralizes influenza A virus. Here, we determined the high-resolution (0.97–1.46 Å) crystal structures with and without bound ligand of a recombinant fragment of conglutinin's C-terminal carbohydrate-recognition domain (CRD). The structures disclosed that the high-affinity ligand N-acetyl-d-glucosamine (GlcNAc) binds in the collectin CRD calcium site by interacting with the O3′ and O4′ hydroxyls alongside additional specific interactions of the N-acetyl group oxygen and nitrogen with Lys-343 and Asp-320, respectively. These residues, unique to conglutinin and differing both in sequence and in location from those in other collectins, result in specific, high-affinity binding for GlcNAc. The binding pocket flanking residue Val-339, unlike the equivalent Arg-343 in the homologous human surfactant protein D, is sufficiently small to allow conglutinin Lys-343 access to the bound ligand, whereas Asp-320 lies in an extended loop proximal to the ligand-binding site and bounded at both ends by conserved residues that coordinate to both calcium and ligand. This loop becomes ordered on ligand binding. The electron density revealed both α and β anomers of GlcNAc, consistent with the added α/βGlcNAc mixture. Crystals soaked with α1–2 mannobiose, a putative component of iC3b, reported to bind to conglutinin, failed to reveal bound ligand, suggesting a requirement for presentation of mannobiose as part of an extended physiological ligand. These results reveal a highly specific GlcNAc-binding pocket in conglutinin and a novel collectin mode of carbohydrate recognition.

Structural definition of hSP-D recognition of Salmonella enterica LPS inner core oligosaccharides reveals alternative binding modes for the same LPS

The crystal structures of a biologically and therapeutically active recombinant homotrimeric fragment of native human SP-D (hSP-D) complexed with the inner core oligosaccharide of the Salmonella enterica sv Minnesota rough strains R5 and R7 (rough mutant chemotypes Rc and Rd1) have been determined. The structures reveal that hSP-D specifically and preferentially targets the LPS inner core via the innermost conserved Hep-Kdo pair with the flexibility for alternative recognition when this preferred epitope is not available for binding. Hep-Kdo binding is achieved through calcium dependent recognition of the heptose dihydroxyethyl side chain coupled with specific interactions between the Kdo and the binding site flanking residues Arg343 and Asp325 with evidence for an extended binding site for LPS inner cores containing multiple Kdo residues. In one subunit of the R5-bound structure this preferred mode of binding is precluded by the crystal lattice and oligosaccharide is bound through the terminal inner core glucose. The structures presented here thus provide unique multiple insights into the recognition and binding of bacterial LPS by hSP-D. Not only is it demonstrated that hSP-D targets the highly conserved LPS proximal inner core Hep-Kdo motif, but also that hSP-D can recognise either terminal or non-terminal sugars and has the flexibility and versatility to adopt alternative strategies for bacterial recognition, utilising alternative LPS epitopes when the preferred inner core Hep-Kdo disaccharide is not available for binding.

Differential Ligand Binding Specificities of the Pulmonary Collectins Are Determined by the Conformational Freedom of a Surface Loop

Lung surfactant proteins (SPs) play critical roles in surfactant function and innate immunity. SP-A and SP-D, members of the collectin family of C-type lectins, exhibit distinct ligand specificities, effects on surfactant structure, and host defense functions despite extensive structural homology. SP-A binds to dipalmitoylphosphatidylcholine (DPPC), the major surfactant lipid component, but not phosphatidylinositol (PI), whereas SP-D shows the opposite preference. Additionally, SP-A and SP-D recognize widely divergent pathogen-associated molecular patterns. Previous studies suggested that a ligand-induced surface loop conformational change unique to SP-A contributes to lipid binding affinity. To test this hypothesis and define the structural features of SP-A and SP-D that determine their ligand binding specificities, a structure-guided approach was used to introduce key features of SP-D into SP-A. A quadruple mutant (E171D/P175E/R197N/K203D) that introduced an SP-D-like loop-stabilizing calcium binding site into the carbohydrate recognition domain was found to interconvert SP-A ligand binding preferences to an SP-D phenotype, exchanging DPPC for PI specificity, and resulting in the loss of lipid A binding and the acquisition of more avid mannan binding properties. Mutants with constituent single or triple mutations showed alterations in their lipid and sugar binding properties that were intermediate between those of SP-A and SP-D. Structures of mutant complexes with inositol or methyl-mannose revealed an attenuation of the ligand-induced conformational change relative to wild-type SP-A. These studies suggest that flexibility in a key surface loop supports the distinctive lipid binding functions of SP-A, thus contributing to its multiple functions in surfactant structure and regulation, and host defense.

Integrated Omics and Computational Glycobiology Reveal Structural Basis for Influenza A Virus Glycan Microheterogeneity and Host Interactions

Despite sustained biomedical research effort, influenza A virus remains an imminent threat to the world population and a major healthcare burden. The challenge in developing vaccines against influenza is the ability of the virus to mutate rapidly in response to selective immune pressure. Hemagglutinin is the predominant surface glycoprotein and the primary determinant of antigenicity, virulence and zoonotic potential. Mutations leading to changes in the number of HA glycosylation sites are often reported. Such genetic sequencing studies predict at best the disruption or creation of sequons for N-linked glycosylation; they do not reflect actual phenotypic changes in HA structure. Therefore, combined analysis of glycan micro and macro-heterogeneity and bioassays will better define the relationships among glycosylation, viral bioactivity and evolution. We present a study that integrates proteomics, glycomics and glycoproteomics of HA before and after adaptation to innate immune system pressure. We combined this information with glycan array and immune lectin binding data to correlate the phenotypic changes with biological activity. Underprocessed glycoforms predominated at the glycosylation sites found to be involved in viral evolution in response to selection pressures and interactions with innate immune-lectins. To understand the structural basis for site-specific glycan microheterogeneity at these sites, we performed structural modeling and molecular dynamics simulations. We observed that the presence of immature, high-mannose type glycans at a particular site correlated with reduced accessibility to glycan remodeling enzymes. Further, the high mannose glycans at sites implicated in immune lectin recognition were predicted to be capable of forming trimeric interactions with the immune-lectin surfactant protein-D.

Crystal Structure of a Complex of Surfactant Protein D (SP-D) and Haemophilus influenzae Lipopolysaccharide Reveals Shielding of Core Structures in SP-D-Resistant Strains

The carbohydrate recognition domains (CRDs) of lung collectin surfactant protein D (SP-D) recognize sugar patterns on the surface of lung pathogens and promote phagocytosis. Using Haemophilus influenzae Eagan strains expressing well-characterized lipopolysaccharide (LPS) surface structures of various levels of complexity, we show that bacterial recognition and binding by SP-D is inversely related to LPS chain extent and complexity. The crystal structure of a biologically active recombinant trimeric SP-D CRD complexed with a delipidated Eagan 4A LPS suggests that efficient LPS recognition by SP-D requires multiple binding interactions utilizing the three major ligand-binding determinants in the SP-D binding pocket, with Ca-dependent binding of inner-core heptose accompanied by interaction of anhydro-Kdo (4,7-anhydro-3-deoxy-d-manno-oct-2-ulosonic acid) with Arg343 and Asp325. Combined with enzyme-linked immunosorbent assays (ELISAs) and fluorescence-activated cell sorter (FACS) binding analyses, our results show that extended LPS structures previously thought to be targets for collectins are important in shielding the more vulnerable sites in the LPS core, revealing a mechanism by which pathogens with complex LPS extensions efficiently evade a first-line mucosal innate immune defense. The structure also reveals for the first time the dominant form of anhydro-Kdo.

Protective effects of surfactant protein D treatment in 1,3-β-glucan-modulated allergic inflammation

Surfactant protein D (SP-D) is a pulmonary collectin important in lung immunity. SP-D-deficient mice (Sftpd(-/-)) are reported to be susceptible to ovalbumin (OVA)- and fungal allergen-induced pulmonary inflammation, while treatment with exogenous SP-D has therapeutic effects in such disease models. β-Glucans are a diverse group of polysaccharides previously suggested to serve as fungal ligands for SP-D. We set out to investigate if SP-D could interact with 1,3-β-glucan and attenuate allergic pulmonary inflammation in the presence of 1,3-β-glucan. Allergic airway disease was induced in Sftpd(-/-) and Sftpd(+/+) mice by OVA sensitization and subsequent challenge with OVA, 1,3-β-glucan, or OVA/1,3-β-glucan together. Mice in the combined treatment group were further treated with a high dose of recombinant fragment of human SP-D (rfhSP-D). We demonstrated direct interaction between SP-D and 1,3-β-glucan. OVA-induced mucous cell metaplasia was increased in Sftpd(-/-) mice, supporting previously reported protective effects of endogenous SP-D in allergy. OVA-induced parenchymal CCL11 levels and eosinophilic infiltration in bronchoalveolar lavage were unaffected by 1,3-β-glucan, but were reversed with rfhSP-D treatment. 1,3-β-Glucan treatment did, however, induce pulmonary neutrophilic infiltration and increased TNF-α levels in bronchoalveolar lavage, independently of OVA-induced allergy. This infiltration was also reversed by treatment with rfhSP-D. 1,3-β-Glucan reduced OVA-induced mucous cell metaplasia, T helper 2 cytokines, and IFN-γ production. rfhSP-D treatment further reduced mucous metaplasia and T helper 2 cytokine secretion to background levels. In summary, rfhSP-D treatment resulted in attenuation of both allergic inflammation and 1,3-β-glucan-mediated neutrophilic inflammation. Our data suggest that treatment with high-dose SP-D protects from mold-induced exacerbations of allergic asthma.

Structure-function relationships in pulmonary surfactant membranes: from biophysics to therapy

Pulmonary surfactant is an essential lipid-protein complex to maintain an operative respiratory surface at the mammalian lungs. It reduces surface tension at the alveolar air-liquid interface to stabilise the lungs against physical forces operating along the compression-expansion breathing cycles. At the same time, surfactant integrates elements establishing a primary barrier against the entry of pathogens. Lack or deficiencies of the surfactant system are associated with respiratory pathologies, which treatment often includes supplementation with exogenous materials. The present review summarises current models on the molecular mechanisms of surfactant function, with particular emphasis in its biophysical properties to stabilise the lungs and the molecular alterations connecting impaired surfactant with diseased organs. It also provides a perspective on the current surfactant-based strategies to treat respiratory pathologies. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Unifying thoracic biomarkers: surfactant protein-D and beyond

Chronic obstructive pulmonary disease (COPD) is a progressive disorder that affects 300 million people worldwide and is responsible for 3 million deaths annually. Currently, there are no accepted biomarkers of COPD, which has impaired drug development and management of patients with COPD. Pneumoproteins, which are proteins synthesized predominantly in the lungs, are promising blood biomarkers because they have high specificity for lung disease. The most promising is surfactant protein-D, which is synthesized largely in Type 2 pneumocytes, and its blood concentrations have been associated with COPD and with certain clinical end points such as mortality. In this paper, we discuss surfactant protein-D and other pneumoproteins as promising biomarkers of COPD.

Mutagenesis of surfactant protein D informed by evolution and x-ray crystallography enhances defenses against influenza A virus in vivo

The recognition of influenza A virus (IAV) by surfactant protein D (SP-D) is mediated by interactions between the SP-D carbohydrate recognition domains (CRD) and glycans displayed on envelope glycoproteins. Although native human SP-D shows potent antiviral and aggregating activity, trimeric recombinant neck+CRDs (NCRDs) show little or no capacity to influence IAV infection. A mutant trimeric NCRD, D325A/R343V, showed marked hemagglutination inhibition and viral neutralization, with viral aggregation and aggregation-dependent viral uptake by neutrophils. D325A/R343V exhibited glucose-sensitive binding to Phil82 hemagglutinin trimer (HA) by surface plasmon resonance. By contrast, there was very low binding to the HA trimer from another virus (PR8) that lacks glycans on the HA head. Mass spectrometry demonstrated the presence of high mannose glycans on the Phil82 HA at positions known to contribute to IAV binding. Molecular modeling predicted an enhanced capacity for bridging interactions between HA glycans and D325A/R343V. Finally, the trimeric D325A/R343V NCRD decreased morbidity and increased viral clearance in a murine model of IAV infection using a reassortant A/WSN/33 virus with a more heavily glycosylated HA. The combined data support a model in which altered binding by a truncated mutant SP-D to IAV HA glycans facilitates viral aggregation, leading to significant viral neutralization in vitro and in vivo. These studies demonstrate the potential utility of homology modeling and protein structure analysis for engineering effective collectin antivirals as in vivo therapeutics.

Crystallographic complexes of surfactant protein A and carbohydrates reveal ligand-induced conformational change

Surfactant protein A (SP-A), a C-type lectin, plays an important role in innate lung host defense against inhaled pathogens. Crystallographic SP-A·ligand complexes have not been reported to date, limiting available molecular information about SP-A interactions with microbial surface components. This study describes crystal structures of calcium-dependent complexes of the C-terminal neck and carbohydrate recognition domain of SP-A with d-mannose, D-α-methylmannose, and glycerol, which represent subdomains of glycans on pathogen surfaces. Comparison of these complexes with the unliganded SP-A neck and carbohydrate recognition domain revealed an unexpected ligand-associated conformational change in the loop region surrounding the lectin site, one not previously reported for the lectin homologs SP-D and mannan-binding lectin. The net result of the conformational change is that the SP-A lectin site and the surrounding loop region become more compact. The Glu-202 side chain of unliganded SP-A extends out into the solvent and away from the calcium ion; however, in the complexes, the Glu-202 side chain translocates 12.8 Å to bind the calcium. The availability of Glu-202, together with positional changes involving water molecules, creates a more favorable hydrogen bonding environment for carbohydrate ligands. The Lys-203 side chain reorients as well, extending outward into the solvent in the complexes, thereby opening up a small cation-friendly cavity occupied by a sodium ion. Binding of this cation brings the large loop, which forms one wall of the lectin site, and the adjacent small loop closer together. The ability to undergo conformational changes may help SP-A adapt to different ligand classes, including microbial glycolipids and surfactant lipids.

Insight into the dynamic interaction of different carbohydrates with human surfactant protein D: molecular dynamics simulations

The unbinding process of three monosaccharides--galactose, glucose, and mannose--from human surfactant protein D (hSP-D) was investigated by the molecular docking and molecular dynamics methods to explore the cause of different dynamic interaction between these monosaccharides and the protein. The results show that the low affinity of galactose for hSP-D is attributed to the different binding conformation from the other two monosaccharides. The sugar coordinates to the calcium ion by the hydroxyl groups in the C2 and C3 atoms, so it cannot form the effective interaction with hSP-D. Glucose and mannose have similar binding conformations with hSP-D. Their difference in the affinity is induced by the interaction between the hydroxyl group in the C2 atom and the residue Asp325. The direction of the hydroxyl group in mannose results in the formation of the hydrogen bond with Asp325 and further makes mannose hydrogen-bond to the residues Glu329 and Arg343 by the hydroxyl groups in the C3, C4, and C6 atoms. As glucose only forms three hydrogen bonds with the residues Glu321, Asn323, and Glu329 by the hydroxyl groups in the C3 and C4 atoms, its interaction with hSP-D is weaker than that of mannose. Thus glucose has a lower energy barrier of dissociation. This work could provide the more penetrating understanding of hSP-D physiological functions.

Increasing antiviral activity of surfactant protein d trimers by introducing residues from bovine serum collectins: dissociation of mannan-binding and antiviral activity

Collectins contribute to host defence through interactions with glycoconjugates on pathogen surfaces. We have prepared recombinant trimeric neck and carbohydrate recognition domains (NCRD) of collectins, and we now show that the NCRD of bovine conglutinin and CL-46 (like that of CL-43) have greater intrinsic antiviral activity for influenza A virus (IAV) than the human SP-D NCRD (hSP-D-NCRD). The three serum collectins differ from SP-D by having insertions adjacent to amino acid 325 and substitution of hydrophobic residues for arginine 343. We previously showed that a three amino acid (RAK) insertion, as found in CL-43, increases antiviral activity and mannan-binding activity of the hSP-D-NCRD, while the substitution of valine at 343, as in conglutinin, more strongly increased these activities. Mannan-binding activity of collectins has been considered to predict for ability to bind to high mannose glycans on viruses or other pathogens. We now show, however, that combined mutants containing the RAK insertion and R343V or R343I substitutions have greatly increased mannan-binding ability, but lower IAV binding or inhibiting activity than mutants containing R343V or R343I substitutions only. These findings indicate differences in the recognition of glycan structures of mannan and IAV by the NCRD and emphasize the importance of the flanking sequences in determining the differing interactions of human SP-D and bovine serum collectins with mannose-rich glycoconjugates on IAV and other pathogens. Of interest, we show conservation of some monoclonal antibody-binding epitopes between bovine collectin NCRD and hSP-D, suggesting shared structural motifs.

Monoclonal antibody-assisted structure-function analysis of the carbohydrate recognition domain of surfactant protein D

Surfactant protein D (SP-D) plays important roles in host defense against a variety of pathogens including influenza A virus (IAV). Ligand binding by SP-D is mediated by the trimeric neck and carbohydrate recognition domain (NCRD). We used monoclonal antibodies (mAbs) against human SP-D and a panel of mutant collectin NCRD constructs to identify functionally and structurally important epitopes. The ability of SP-D to bind to IAV and mannan involved partially overlapping binding sites that are distinct from those involved in binding to the glycoprotein-340 (gp-340) scavenger receptor protein. A species-specific motif (D324,D325,R343), which has been implicated in the specific binding of several ligands, contributes to recognition by mAbs that block antiviral or mannan binding activity. D325, in particular, is involved in the epitopes of these blocking mAbs. Conversely, the interspecies substitution of arginine for Lys343 in the rat NCRD (rK343R) conferred binding to two of the mAbs. The single site substitution of alanine for R349 or E347 resulted in highly selective alterations in mAb binding and caused decreased antiviral activity. Mutations at Glu333 (E333A), Trp340 (W340F), and Phe335 (F335A), which abrogated antiviral activity, were associated with decreased binding to multiple blocking mAbs, consistent with critical structural roles. More conservative substitutions at 335, which showed a significant increase in neutralization activity, caused selective loss of binding to one mAb. The analysis reveals, for the first time, an extended binding site for IAV; calcium-dependent antiviral activity involves residues flanking the primary carbohydrate binding site as well as more remote residues displayed on the carbohydrate recognition domain surface.

Review: Structural determinants of pattern recognition by lung collectins

Host defense roles for the lung collectins, surfactant protein A (SP-A) and surfactant protein D (SP-D), were first suspected in the 1980s when molecular characterization revealed their sequence homology to the acute phase reactant of serum, mannose-binding lectin. Surfactant protein A and SP-D have since been shown to play diverse and important roles in innate immunity and pulmonary homeostasis. Their location in surfactant ideally positions them to interact with air-space pathogens. Despite extensive structural similarity, the two proteins show many functional differences and considerable divergence in their interactions with microbial surface components, surfactant lipids, and other ligands. Recent crystallographic studies have provided many new insights relating to these observed differences. Although both proteins can participate in calcium-dependent interactions with sugars and other polyols, they display significant differences in the spatial orientation, charge, and hydrophobicity of their binding surfaces. Surfactant protein D appears particularly adapted to interactions with complex carbohydrates and anionic phospholipids, such as phosphatidylinositol. By contrast, SP-A shows features consistent with its preference for lipid ligands, including lipid A and the major surfactant lipid, dipalmitoylphosphatidylcholine. Current research suggests that structural biology approaches will help to elucidate the molecular basis of pulmonary collectin-ligand recognition and facilitate development of new therapeutics based upon SP-A and SP-D.

Surfactant protein D interacts with alpha2-macroglobulin and increases its innate immune potential

Surfactant protein D (SP-D) is an innate immune collectin that recognizes microbes via its carbohydrate recognition domains, agglutinates bacteria, and forms immune complexes. During microbial infections, proteases, such as elastases, cleave the carbohydrate recognition domains and can inactivate the innate immune functions of SP-D. Host responses to counterbalance the reduction of SP-D-mediated innate immune response under these conditions are not clearly understood. We have unexpectedly identified that SP-D could interact with protein fractions containing ovomucin and ovomacroglobulin. Here, we show that SP-D interacts with human alpha(2)-macroglobulin (A2M), a protease inhibitor present in the lungs and serum. Using enzyme-linked immunosorbent assays, surface plasmon resonance, and carbohydrate competition assays, we show that SP-D interacts with A2M both in solid phase (K(D) of 7.33 nM) and in solution via lectin-carbohydrate interactions under physiological calcium conditions. Bacterial agglutination assays further show that SP-D x A2M complexes increase the ability of SP-D to agglutinate bacteria. Western blot analyses show that SP-D, but not A2M, avidly binds bacteria. Interestingly, intact and activated A2M also protect SP-D against elastase-mediated degradation, and the cleaved A2M still interacts with SP-D and is able to enhance its agglutination abilities. We also found that SP-D and A2M can interact with each other in the airway-lining fluid. Therefore, we propose that SP-D utilizes a novel mechanism in which the collectin interacts with protease inhibitor A2M to decrease its degradation and to concurrently increase its innate immune function. These interactions particularly enhance bacterial agglutination and immune complex formation.

Structural characterisation of ligand-binding determinants in human lung surfactant protein D: influence of Asp325

The crystal structures of a biologically and therapeutically active recombinant homotrimeric fragment of human lung surfactant protein D with a series of bound ligands have been determined. While the structures reveal various different binding modes, all utilise a similarly positioned pair of mannose-type O3' and O4' hydroxyls with no direct interaction between any non-terminal sugar and protein. The orientation, position, and interactions of the bound terminal sugar depend on the sugar itself, the presence and form of glycosidic linkage, and the environment in the crystal, which, via Asp325, places stereochemical and electronic constraints, different for the three different subunits in the homotrimer, on the ligand-binding site. As a direct consequence of this influence, the other binding-pocket flanking residue, Arg343, exhibits variable conformation and variable interactions with bound ligand and leaves open to question which orientation of terminal mannobiose, and of other terminal disaccharides, may be present in extended physiological ligands. The combined structural evidence shows that there is significant flexibility in recognition; that Asp325, in addition to Arg343, is an important determinant of ligand selectivity, recognition, and binding; and that differences in crystal contact interfaces exert, through Asp325, significant influence on preferred binding modes.

Critical role of amino acid position 343 of surfactant protein-D in the selective binding of glycolipids from Mycobacterium tuberculosis

Surfactant protein D (SP-D), a lectin that recognizes carbohydrates via its C-type carbohydrate recognition domains (CRDs), regulates Mycobacterium tuberculosis (M.tb)-macrophage interactions via recognition of M.tb mannosylated cell wall components. SP-D binds to, agglutinates, and reduces phagocytosis and intracellular growth of M.tb. Species-specific variations in the CRD amino acid sequence contribute to carbohydrate recognition preferences and have been exploited to enhance the antimicrobial properties of SP-D in vitro. Here, we characterized the binding interaction between several wild-type and mutant SP-D neck + CRD trimeric subunits (NCRDs) and pathogenic and nonpathogenic mycobacterial species. Specific amino acid substitutions (i.e., the 343-amino-acid position) that flank the carbohydrate binding groove led to significant increases in binding of only virulent and attenuated M.tb strains and to a lesser extent M. marinum, whereas there was negligible binding to M. avium complex and M. smegmatis. Moreover, a nonconserved mutation at the critical 321-amino-acid position (involved in Ca(2+) coordination) abrogated binding to M.tb and M. marinum. We further characterized the binding of NCRDs to the predominant surface-exposed mannosylated lipoglycans of the M.tb cell envelope. Results showed a binding pattern that is dependent on the nature of the side chain of the 343-amino-acid position flanking the SP-D CRD binding groove and the nature of the terminal mannosyl sugar linkages of the mycobacterial lipoglycans. We conclude that the 343 position is critical in defining the binding pattern of SP-D proteins to M.tb and its mannosylated cell envelope components.

Recognition of mannosylated ligands and influenza A virus by human surfactant protein D: contributions of an extended site and residue 343

Surfactant protein D (SP-D) plays important roles in antiviral host defense. Although SP-D shows a preference for glucose/maltose, the protein also recognizes d-mannose and a variety of mannose-rich microbial ligands. This latter preference prompted an examination of the mechanisms of mannose recognition, particularly as they relate to high-mannose viral glycans. Trimeric neck plus carbohydrate recognition domains from human SP-D (hNCRD) preferred alpha1-2-linked dimannose (DM) over the branched trimannose (TM) core, alpha1-3 or alpha1-6 DM, or D-mannose. Previous studies have shown residues flanking the carbohydrate binding site can fine-tune ligand recognition. A mutant with valine at 343 (R343V) showed enhanced binding to mannan relative to wild type and R343A. No alteration in affinity was observed for D-mannose or for alpha1-3- or alpha1-6-linked DM; however, substantially increased affinity was observed for alpha1-2 DM. Both proteins showed efficient recognition of linear and branched subdomains of high-mannose glycans on carbohydrate microarrays, and R343V showed increased binding to a subset of the oligosaccharides. Crystallographic analysis of an R343V complex with 1,2-DM showed a novel mode of binding. The disaccharide is bound to calcium by the reducing sugar ring, and a stabilizing H-bond is formed between the 2-OH of the nonreducing sugar ring and Arg349. Although hNCRDs show negligible binding to influenza A virus (IAV), R343V showed markedly enhanced viral neutralizing activity. Hydrophobic substitutions for Arg343 selectively blocked binding of a monoclonal antibody (Hyb 246-05) that inhibits IAV binding activity. Our findings demonstrate an extended ligand binding site for mannosylated ligands and the significant contribution of the 343 side chain to specific recognition of multivalent microbial ligands, including high-mannose viral glycans.

Surfactant protein-D regulates the postnatal maturation of pulmonary surfactant lipid pool sizes

Surfactant protein (SP)-D plays an important role in host defense and pulmonary surfactant homeostasis. In SP-D-deficient (Sftpd(-/-)) mice, the abnormal large surfactant forms seen at the ultrastructural level are taken up inefficiently by type II cells, resulting in an over threefold increase in the surfactant pool size. The mechanisms by which SP-D influences surfactant ultrastructure are unknown. We hypothesized that SP-D binds to surfactant immediately after being secreted and influences surfactant ultrastructure conversion. In newborn and adult sheep lungs, immunogold-labeled SP-D was associated with both lamellated membranous lipid structures of newly secreted surfactant and with small aggregate surfactant but not with tubular myelin. Since SP-D preferentially binds to phosphatidylinositol (PI) in vitro, the postnatal changes in PI were assessed. PI content in the bronchoalveolar lavage fluid increased after birth and peaked at 2-5 days of age, a time of rapid conversion of surfactant forms that is associated with the peak of surfactant lipid pool size. SP-D selectively interacted with PI-rich liposomes in vitro, causing their lysis. Similarly, the abnormal surfactant ultrastructure in Sftpd(-/-) mice was corrected by the addition of SP-D or melittin, and both peptides caused lysis of lipid vesicles. The normal conversion of surfactant ultrastructure requires SP-D that preferentially interacts with PI-rich, newly secreted surfactant, causing lysis of surfactant lipid membranes, converting the lipid forms into smaller surfactant lamellated structures that are critical for surfactant uptake by type II cells and normal surfactant homeostasis. SP-D regulates the dramatic decreases in the surfactant pool size that occurs in the newborn period.

Modification of surfactant protein D by reactive oxygen-nitrogen intermediates is accompanied by loss of aggregating activity, in vitro and in vivo

Surfactant protein D (SP-D) is an important effector of innate immunity. We have previously shown that SP-D accumulates at sites of acute bacterial infection and neutrophil infiltration, a setting associated with the release of reactive species such as peroxynitrite. Incubation of native SP-D or trimeric SP-D lectin domains (NCRDs) with peroxynitrite resulted in nitration and nondisulfide cross-linking. Modifications were blocked by peroxynitrite scavengers or pH inactivation of peroxynitrite, and mass spectroscopy confirmed nitration of conserved tyrosine residues within the C-terminal neck and lectin domains. Mutant NCRDs lacking one or more of the tyrosines allowed us to demonstrate preferential nitration of Tyr314 and the formation of Tyr228-dependent cross-links. Although there was no effect of peroxynitrite or tyrosine mutations on lectin activity, incubation of SP-D dodecamers or murine lavage with peroxynitrite decreased the SP-D-dependent aggregation of lipopolysaccharide-coated beads, supporting our hypothesis that defective aggregation results from abnormal cross-linking. We also observed nitration, cross-linking of SP-D, and a significant decrease in SP-D-dependent aggregating activity in the lavage of mice acutely exposed to nitrogen dioxide. Thus, modification of SP-D by reactive oxygen-nitrogen species could contribute to alterations in the structure and function of SP-D at sites of inflammation in vivo.

Interaction of recombinant surfactant protein D with lipopolysaccharide: conformation and orientation of bound protein by IRRAS and simulations

Effective innate host defense requires early recognition of pathogens. Surfactant protein D (SP-D), shown to play a role in host defense, binds to the lipopolysaccharide (LPS) component of Gram-negative bacterial membranes. Binding takes place via the carbohydrate recognition domain (CRD) of SP-D. Recombinant trimeric neck+CRDs (NCRD) have proven valuable in biophysical studies of specific interactions. Although X-ray crystallography has provided atomic level information on NCRD binding to carbohydrates and other ligands, molecular level information about interactions between SP-D and biological ligands under physiologically relevant conditions is lacking. Infrared reflection-absorption spectroscopy (IRRAS) provides molecular structure information from films at the air/water interface where protein adsorption to LPS monolayers serves as a model for protein-lipid interaction. In the current studies, we examine the adsorption of NCRDs to Rd 1 LPS monolayers using surface pressure measurements and IRRAS. Measurements of surface pressure, Amide I band intensities, and LPS acyl chain conformational ordering, along with the introduction of EDTA, permit discrimination of Ca (2+)-mediated binding from nonspecific protein adsorption. The findings support the concept of specific binding between the CRD and heptoses in the core region of LPS. In addition, a novel simulation method that accurately predicts the IR Amide I contour from X-ray coordinates of NCRD SP-D is applied and coupled to quantitative IRRAS equations providing information on protein orientation. Marked differences in orientation are found when the NCRD binds to LPS compared to nonspecific adsorption. The geometry suggests that all three CRDs are simultaneously bound to LPS under conditions that support the Ca (2+)-mediated interaction.

Critical role for cross-linking of trimeric lectin domains of surfactant protein D in antiviral activity against influenza A virus

Collectins are multimeric host defence lectins with trimeric CRDs (carbohydrate-recognition domains) and collagen and N-terminal domains that form higher-order structures composed of four or more trimers. Recombinant trimers composed of only the CRD and adjacent neck domain (termed NCRD) retain binding activity for some ligands and mediate some functional activities. The lung collectin SP-D (surfactant protein D) has strong neutralizing activity for IAVs (influenza A viruses) in vitro and in vivo, however, the NCRD derived from SP-D has weak viral-binding ability and lacks neutralizing activity. Using a panel of mAbs (monoclonal antibodies) directed against the NCRD in the present study we show that mAbs binding near the lectin site inhibit antiviral activity of full-length SP-D, but mAbs which bind other sites on the CRD do not. Two of the non-blocking mAbs significantly increased binding and antiviral activity of NCRDs as assessed by haemagglutination and neuraminidase inhibition and by viral neutralization. mAb-mediated cross-linking also enabled NCRDs to induce viral aggregation and to increase viral uptake by neutrophils and virus-induced respiratory burst responses by these cells. These results show that antiviral activities of SP-D can be reproduced without the N-terminal and collagen domains and that cross-linking of NCRDs is essential for antiviral activity of SP-D with respect to IAV.