The lysine cluster in the collagen-like domain of the scavenger receptor provides for its ligand binding and ligand specificity

Current Research Landscape of Marine-Derived Anti-Atherosclerotic Substances

Atherosclerosis is a chronic disease characterized by lipid accumulation and chronic inflammation of the arterial wall, which is the pathological basis for coronary heart disease, cerebrovascular disease and thromboembolic disease. Currently, there is a lack of low-cost therapeutic agents that effectively slow the progression of atherosclerosis. Therefore, the development of new drugs is urgently needed. The research and development of marine-derived drugs have gained increasing interest from researchers across the world. Many marine organisms provide a rich material basis for the development of atherosclerotic drugs. This review focuses on the latest technological advances in the structures and mechanisms of action of marine-derived anti-atherosclerotic substances and the challenges of the application of these substances including marine polysaccharides, proteins and peptides, polyunsaturated fatty acids and small molecule compounds. Here, we describe the theoretical basis of marine biological resources in the treatment of atherosclerosis.

Innate Immune Responses to Tuberculosis

Tuberculosis remains one of the greatest threats to human health. The causative bacterium, Mycobacterium tuberculosis, is acquired by the respiratory route. It is exquisitely adapted to humans and is a prototypic intracellular pathogen of macrophages, with alveolar macrophages being the primary conduit of infection and disease. However, M. tuberculosis bacilli interact with and are affected by several soluble and cellular components of the innate immune system which dictate the outcome of primary infection, most commonly a latently infected healthy human host, in whom the bacteria are held in check by the host immune response within the confines of tissue granuloma, the host histopathologic hallmark. Such individuals can develop active TB later in life with impairment in the immune system. In contrast, in a minority of infected individuals, the early host immune response fails to control bacterial growth, and progressive granulomatous disease develops, facilitating spread of the bacilli via infectious aerosols. The molecular details of the M. tuberculosis-host innate immune system interaction continue to be elucidated, particularly those occurring within the lung. However, it is clear that a number of complex processes are involved at the different stages of infection that may benefit either the bacterium or the host. In this article, we describe a contemporary view of the molecular events underlying the interaction between M. tuberculosis and a variety of cellular and soluble components and processes of the innate immune system.

Prospects for the therapeutic application of sulfated polysaccharides of brown algae in diseases of the cardiovascular system: review

CONTEXT

Fucoidans are water-soluble, highly sulfated, branched homo- and hetero-polysaccharides derived from the fibrillar cell walls and intercellular spaces of brown seaweeds of the class Phaeophyceae. Fucoidans possess mimetic properties of the natural ligands of protein receptors and regulate functions of biological systems via key signaling molecules.

OBJECTIVES

The aim of this review was to collect and combine all available scientific literature about the potential use of the fucoidans for diseases of cardiovascular system.

MATERIALS AND METHODS

The review has been compiled using references from major databases such as Web of Science, PubMed, Scopus, Elsevier, Springer and Google Scholar (up to September 2015). After obtaining all reports from database (a total number is about 580), the papers were carefully analyzed in order to find data related to the topic of this review (129 references).

RESULTS

An exhaustive survey of literature revealed that fucoidans possess a broad spectrum of biological activity, including anti-coagulant, hypolipidemic, anti-thrombotic, anti-inflammatory, immunomodulatory, anti-tumor, anti-adhesive and anti-hypertensive properties. Numerous investigations of fucoidans in diseases of the cardiovascular system mainly focus on pleiotropic anti-inflammatory effects. Fucoidans also possess pro-angiogenic and pro-vasculogenic properties.

CONCLUSION

A great number of investigations in the past years have demonstrated that fucoidans has great potential for in-depth investigation of their effects on cardiovascular system. Through this review, the authors hope to attract the attention of researchers to use fucoidan as mimetic of natural ligand receptor protein with the view of developing new formulations with an improved therapeutic value.

Macrophage immunoregulatory pathways in tuberculosis

Macrophages, the major host cells harboring Mycobacterium tuberculosis (M.tb), are a heterogeneous cell type depending on their tissue of origin and host they are derived from. Significant discord in macrophage responses to M.tb exists due to differences in M.tb strains and the various types of macrophages used to study tuberculosis (TB). This review will summarize current concepts regarding macrophage responses to M.tb infection, while pointing out relevant differences in experimental outcomes due to the use of divergent model systems. A brief description of the lung environment is included since there is increasing evidence that the alveolar macrophage (AM) has immunoregulatory properties that can delay optimal protective host immune responses. In this context, this review focuses on selected macrophage immunoregulatory pattern recognition receptors (PRRs), cytokines, negative regulators of inflammation, lipid mediators and microRNAs (miRNAs).

Direct recognition of superparamagnetic nanocrystals by macrophage scavenger receptor SR-AI

Scavenger receptors (SRs) are molecular pattern recognition receptors that have been shown to mediate opsonin-independent uptake of therapeutic and imaging nanoparticles, underlying the importance of SRs in nanomedicine. Unlike pathogens, engineered nanomaterials offer great flexibility in control of surface properties, allowing addressing specific questions regarding the molecular mechanisms of nanoparticle recognition. Recently, we showed that SR-type AI/II mediates opsonin-independent internalization of dextran superparamagnetic iron oxide (SPIO) nanoparticles via positively charged extracellular collagen-like domain. To understand the mechanism of opsonin-independent SPIO recognition, we tested the binding and uptake of nanoparticles with different surface coatings by SR-AI. SPIO coated with 10 kDa dextran was efficiently recognized and taken up by SR-AI transfected cells and J774 macrophages, while SPIO with 20 kDa dextran coating or cross-linked dextran hydrogel avoided the binding and uptake. Nanoparticle negative charge density and zeta-potential did not correlate with SR-AI binding/uptake efficiency. Additional experiments and computer modeling revealed that recognition of the iron oxide crystalline core by the positively charged collagen-like domain of SR-AI is sterically hindered by surface polymer coating. Importantly, the modeling revealed a strong complementarity between the surface Fe-OH groups of the magnetite crystal and the charged lysines of the collagen-like domain of SR-AI, suggesting a specific recognition of SPIO crystalline surface. These data provide an insight into the molecular recognition of nanocrystals by innate immunity receptors and the mechanisms whereby polymer coatings promote immune evasion.

Synthesis and biological applications of collagen-model triple-helical peptides

Triple-helical peptides (THPs) have been utilized as collagen models since the 1960s. The original focus for THP-based research was to unravel the structural determinants of collagen. In the last two decades, virtually all aspects of collagen structural biochemistry have been explored with THP models. More specifically, secondary amino acid analogs have been incorporated into THPs to more fully understand the forces that stabilize triple-helical structure. Heterotrimeric THPs have been utilized to better appreciate the contributions of chain sequence diversity on collagen function. The role of collagen as a cell signaling protein has been dissected using THPs that represent ligands for specific receptors. The mechanisms of collagenolysis have been investigated using THP substrates and inhibitors. Finally, THPs have been developed for biomaterial applications. These aspects of THP-based research are overviewed herein.

Macrophage scavenger receptor A mediates adhesion to apolipoproteins A-I and E

Macrophage scavenger receptor A (SR-A) is a multifunctional, multiligand pattern recognition receptor with roles in innate immunity, apoptotic cell clearance, and age-related degenerative pathologies, such as atherosclerosis and Alzheimer's disease. Known endogenous SR-A ligands are polyanionic and include modified lipoproteins, advanced glycation end products, and extracellular matrix proteins. No native plasma ligands have been identified, but it is known that SR-A recognition of unidentified serum components mediates integrin-independent macrophage adhesion, which may drive chronic local inflammation. In this study, we used a high-throughput fractionation and screening method to identify novel endogenous SR-A ligands that may mediate macrophage adhesion. SR-A was found to recognize the exchangeable apolipoproteins A-I and E (apo A-I and apo E, respectively) in both lipid-free and lipid-associated form, suggesting the shared amphipathic alpha-helix as a potential recognition motif. Adhesion of RAW 264.7 macrophages to surfaces coated with apo A-I and apo E4 proved to be integrin-independent and could be blocked by anti-SR-A antibodies. The presence of apo A-I and apo E in pathological deposits, such as atherosclerotic lesions and neurotoxic Alzheimer's plaques, suggests a possible contribution of SR-A-dependent adhesion of macrophages to an inflammatory microenvironment.

Structure-activity relations of nanolipoblockers with the atherogenic domain of human macrophage scavenger receptor A

Oxidized low density lipoprotein (oxLDL) uptake by macrophages is mediated by scavenger receptors and leads to unregulated cholesterol accumulation. Micellar nanolipoblockers (NLBs) consist of alkyl chains and polyethylene glycol on mucic acid. NLBs functionalized with anionic groups inhibit oxLDL uptake via the scavenger receptor A (SR-A). Molecular modeling and docking approaches were used to understand the structure-activity relationship (SAR) between NLBs and SR-A. Six NLB models were docked to the SR-A homology model to investigate charge placement and clustering. NLB models with the most favorable binding energy were also the most effective oxLDL inhibitors in THP-1 macrophages. Mutant SR-A models were generated by replacing charged residues with alanine. All charged residues in the region were necessary, with Lys60, Lys63, and Lys66 having the greatest effect on binding. We hypothesize that structural studies aided by theoretical modeling and docking can be used to design promising NLB candidates with optimal binding properties.

Modified low density lipoproteins binding requires a lysine cluster region in the murine macrophage scavenger receptor class A type II

Atherosclerosis is a consequence of lipid deposition and foam cell formation in the arterial wall. Macrophage scavenger receptor A II is involved in the uptake of modified low density lipoproteins. It contains an extracellular conserved lysine cluster which has been proposed to form a positively charged groove that interacts with acetylated low density lipoproteins (AcLDL). This study evaluated the role of the murine SRA-II and a lysine mutated SRA-II on AcLDL uptake. Fluorescence labeled AcLDL uptake was quantified using a Laser Scan Cytometer. A significant increase in fluorescence uptake was found in the cells transfected with SRA-II versus those with empty vector. Cells expressing the lysine mutated SRA-II also demonstrated a significant decrease in their uptake of AcLDL. This data supports the concept that the conserved lysine cluster in murine SRA-II is the binding region for AcLDL or contributes to the trimeric structure of SRA-II necessary for AcLDL binding.

Scavenger receptors: friend or foe in atherosclerosis?

PURPOSE OF REVIEW

Scavenger receptors were originally defined by their ability to bind and internalize modified lipoproteins. Nowadays the family of scavenger receptors is composed of structurally different surface receptors which recognize a broad pattern of common ligands which include, besides modified lipoproteins, apoptotic cells and pathogens. This review focuses on the role of scavenger receptors in the development of atherosclerotic lesions.

RECENT FINDINGS

Recent studies indicate that scavenger receptor A activity can be regulated by phosphorylation, glucosidases, 8-isoprostane, high glucose and nobiletin. Modulation of these regulatory components may beneficially influence scavenger receptor A's proatherogenic function. It appears that statins do lead to a reduction in CD36 transcription and could modulate in this way CD36-mediated atherosclerotic foam cell formation. Macrophage scavenger receptor BI appears to facilitate the development of small fatty streak lesions, whereas the formation of advanced atherosclerotic lesions is reduced, indicating a unique dual role for macrophage scavenger receptor BI in the pathogenesis of atherosclerosis.

SUMMARY

It is proposed that the presence of scavenger receptors in macrophages is beneficial because they remove potential deleterious material from the arterial wall. Inadequate handling of the internalized material by the macrophages will lead to foam cell formation. If adequate levels of ATP-binding cassette transporters and accepting HDL are present, however, the macrophage is perfectly able to metabolize and secrete the internalized atherogenic substances whereby HDL facilitates further transport from the arterial wall to the liver, leading to release in bile.

Control of scavenger receptor-mediated endocytosis by novel ligands of different length

The scavenger receptor recognized as a multiligand family of receptors falls in the group that is internalised through endocytosis. In this report we used several recombinant fragments of the tapeworm protein paramyosin, known to form filamentous dimers that bind collagenous structures as ligands of different length for the class A type I scavenger receptor (SR-AI). While native CHO cells are unresponsive to any of the recombinant fragments, it is shown that CHO cells transfected with this receptor efficiently internalise recombinant fragments that correspond to two thirds of the full-length paramyosin. In contrast, recombinant products corresponding to one-third of the full-length paramyiosin are not internalised. It is also shown that important molecules in the organization of the coated pit, are enriched when the two-thirds long paramyosin fragments were bound and internalised through the SR-AI. Moreover, internalisation of these fragments trigger a classical apoptotic pathway shown by the presence of TUNEL positive cells and the appearance of apoptotic bodies. We report paramyosin as a new ligand for the scavenger receptor and provide evidence supporting the notion that these receptors upon the formation of arrays with length-specific molecules, not only trigger endocytosis but also seem to regulate the synthesis of molecules involved in the organization of coated pits.

Inhibition of α-Glucosidases I and II Increases the Cell Surface Expression of Functional Class A Macrophage Scavenger Receptor (SR-A) by Extending Its Half-life

The class A scavenger receptor (SR-A) is a multifunctional trimeric membrane glycoprotein involved in atherogenesis. The mature receptor can mediate the binding and internalization of a number of specific ligands, including modified low-density lipoprotein. We have investigated the effects of inhibiting N-glycan processing on SR-A expression, distribution, and activity in the murine macrophage cell line RAW264.7. We have found that SR-A normally interacts with calnexin in the endoplasmic reticulum and in its mature form carries complex N-glycans. The imino sugar, N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of the N-glycan processing enzymes alpha-glucosidases I and II. Following NB-DNJ treatment SR-A became Endo H-sensitive, consistent with inhibition of N-glycan processing. A dose-dependent increase in cell surface expression of SR-A was observed in response to NB-DNJ treatment. The receptor on inhibitor-treated cells was still functional because the increased surface expression resulted in a proportional enhancement in the endocytosis of the ligand, acetylated low-density lipoprotein. The expression of SR-A on NB-DNJ cultured cells was further enhanced by co-treatment with interferon-gamma. Quantitative reverse transcriptase-PCR analysis did not show a significant difference in the amount of SR-A mRNA in NB-DNJ-treated RAW264.7 cells. However, the half-life of SR-A protein was significantly increased. These data indicate the retention of glucosylated N-glycans does not result in gross misfolding and degradation of this receptor or prevent its transport to the cell surface. SR-A interacts with calnexin and when the association is prevented changes in the recycling kinetics and rate of turnover of the receptor result, leading to enhanced cell surface expression.

Receptors, mediators, and mechanisms involved in bacterial sepsis and septic shock

Bacterial sepsis and septic shock result from the overproduction of inflammatory mediators as a consequence of the interaction of the immune system with bacteria and bacterial wall constituents in the body. Bacterial cell wall constituents such as lipopolysaccharide, peptidoglycans, and lipoteichoic acid are particularly responsible for the deleterious effects of bacteria. These constituents interact in the body with a large number of proteins and receptors, and this interaction determines the eventual inflammatory effect of the compounds. Within the circulation bacterial constituents interact with proteins such as plasma lipoproteins and lipopolysaccharide binding protein. The interaction of the bacterial constituents with receptors on the surface of mononuclear cells is mainly responsible for the induction of proinflammatory mediators by the bacterial constituents. The role of individual receptors such as the toll-like receptors and CD14 in the induction of proinflammatory cytokines and adhesion molecules is discussed in detail. In addition, the roles of a number of other receptors that bind bacterial compounds such as scavenger receptors and their modulating role in inflammation are described. Finally, the therapies for the treatment of bacterial sepsis and septic shock are discussed in relation to the action of the aforementioned receptors and proteins.

A role for class A scavenger receptor in dendritic cell nibbling from live cells

Monocyte-derived dendritic cells (DC) possess the unique capacity to capture Ag from live cells through intimate cell contact, a process referred to as nibbling. We sought to define the receptor(s) mediating DC nibbling. Uptake of fluorescently labeled plasma membrane from live cells by DC was inhibited by protease treatment and by a panel of polyanionic ligands, implicating scavenger receptors (SR) in this process. Differential expression of SR on DC and macrophages correlated with the capacity to acquire membrane from live cells. Internalized membrane colocalized with SR ligand and entered the endosomal pathway. DC very efficiently acquired and internalized gp100 tumor Ag expressed at the surface of viable adenocarcinoma cells via recombinant adenoviral infection. Cross-presentation of gp100 by DC to MHC class I-restricted T cells was inhibited by polyanionic SR ligand and an Ab to type A SR (SR-A), whereas Ab to the class B SR CD36, which mediates uptake of apoptotic cells, induced no inhibition. DC capture of fluorescently labeled membrane from live cells was partially blocked by SR-A-specific Ab, suggesting that other SR may also be contributing to nibbling. DC maturation resulted in a switch in expression from type II SR-A (SR-AII) to the SR-AI splice variant. Finally, SR-A was identified on interdigitating DC isolated from monkey lymph nodes. These findings define a novel role for SR-A, and suggest that Ag uptake from live cells by DC may be important in the generation of immunity and in the maintenance of peripheral tolerance in vivo.

Collagen model peptides: Sequence dependence of triple-helix stability

The triple helix is a specialized protein motif, found in all collagens as well as in noncollagenous proteins involved in host defense. Peptides will adopt a triple-helical conformation if the sequence contains its characteristic features of Gly as every third residue and a high content of Pro and Hyp residues. Such model peptides have proved amenable to structural studies by x-ray crystallography and NMR spectroscopy, suitable for thermodynamic and kinetic analysis, and a valuable tool in characterizing the binding activities of the collagen triple helix. A systematic approach to understanding the amino acid sequence dependence of the collagen triple helix has been initiated, based on a set of host-guest peptides of the form, (Gly-Pro-Hyp)(3)-Gly-X-Y-(Gly-Pro-Hyp)(4). Comparison of their thermal stabilities has led to a propensity scale for the X and Y positions, and the additivity of contributions of individual residues is now under investigation. The local and global stability of the collagen triple helix is normally modulated by the residues in the X and Y positions, with every third position occupied by Gly in fibril-forming collagens. However, in collagen diseases, such as osteogenesis imperfecta, a single Gly may be substituted by another residue. Host-guest studies where the Gly is replaced by various amino acids suggest that the identity of the residue in the Gly position affects the degree of destabilization and the clinical severity of the disease.

Oligonucleotide aggregates bind to the macrophage scavenger receptor

We have prepared a model receptor containing a Lys cluster (320-340) in the collagen-like domain of the bovine macrophage scavenger receptor, and have shown that it has a similar binding specificity to the native scavenger receptor. The native scavenger receptor is reported to bind the quadruplex structure of nucleotides. In this study, we analyzed the model receptor binding of nucleotides with various structures, random, parallel or antiparallel quadruplex and aggregate forms. This was carried out by direct binding assays using labeled oligonucleotides or surface plasmon resonance, and by an inhibition assay using Chinese hamster ovary (CHO) cells expressing the scavenger receptor. The results showed that the nucleotides forming the quadruplex structure did not exhibit any binding. Only the aggregate forms of the nucleotide could bind to the model receptor. They also inhibited the degradation of acetylated low density lipoprotein by CHO cells expressing the native scavenger receptor, whereas nucleotides that did not bind to the model receptor had no effect on cellular acetylated low density lipoprotein degradation. Our results suggest that the quadruplex structure is not essential but may be required for the formation of the nucleotide aggregates, which can interact with the scavenger receptor.

Receptors for oxidized low density lipoprotein

An increasing body of evidence indicates that oxidized low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis. One of the first biologic actions of oxidized LDL to be identified in vitro was its ability to interact with the 'acetyl LDL receptor' discovered by Goldstein and Brown. Over the past decade, considerable progress has been made in identifying and characterizing cell-surface receptors for oxidized LDL. Most of these receptors are thought to be multifunctional because they interact with several structurally different ligands, and accordingly have been termed 'scavenger receptors'. The objective of this article is to review the most important publications dealing with structure, ligand specificity, regulation, and function of scavenger receptors.

Dissection of the class A scavenger receptor by peptide engineering

The construction of portions of a protein provides information about its functional mechanisms. Peptide engineering is a recently developed technique to construct a small tertiary structure or a portion of a protein, and allows expansion to applications for bio- and physicochemistries. In this article, we focus on the class A scavenger receptor, which plays a key role in atherogenesis, and may be involved in other pathogenic processes. The receptor is mainly composed of simple tertiary structures, such as collagen and alpha-helical coiled coil structures, which have different functions. We constructed both the collagen-like and alpha-helical coiled coil domains by peptide engineering, and analysed the structure-function relationships of the receptor. Understanding the mechanisms of their functions at the amino acid level should help us to mimic the functional domains and to create de novo designed proteins with new functions.

Functional changes in scavenger receptor binding conformation are induced by charge mutants spanning the entire collagen domain

Macrophage scavenger receptors are trimeric integral membrane proteins that bind a diverse array of negatively charged ligands. They have been shown to play a role in the pathogenesis of atherosclerosis and in host responses to microbial infections. Earlier mutational studies demonstrated that the distal segment of the collagen domain of the receptor was critically important for high affinity ligand binding activity. In this study, mutations spanning the entire collagen domain were generated and binding was assayed in transfected cells, as well as in assays employing a secreted, receptor fusion protein. Many of the distal, positively charged C-terminal residues in the type II collagen domain of the receptor, previously reported to be essential for binding at 37 degreesC, were found not to be critical for binding at 4 degreesC. Conversely, more proximally charged residues of the collagen receptor that have not been previously mutated were shown to have substantial effects on binding that were also temperature-dependent. These data suggest that scavenger receptor ligand recognition depends on more complex conformational interactions, involving charged residues throughout the entire collagen domain, than was previously recognized.

Synthesis and stabilization of amino and carboxy terminal constrained collagenous peptides

Short collagenous peptides cross-linked at their amino and carboxy termini with Lys-Lys-dimer template(s) were synthesized, and the effect of the cross-linking on their stabilities was investigated by thermal denaturation experiments. Two chemoselective ligations were used for the construction of the amino and the carboxy cross-linked peptides. The thermal transition temperature (Tm) and the standard free energies (deltaG(o)) of the cross-linked collagenous peptides increased, and the thermal stabilization effect corresponded to an elongation by two units of the Gly-Pro-Hyp triad. The van't Hoff enthalpy (deltaH(o)) and the entropy (deltaS(o)) values of the cross-linked peptides increased with chain elongation, although the increments were smaller than those of the linear peptides. When the same chain lengths were compared, the deltaH(o) was increased and the deltaS(o) was nearly the same or increased by the cross-linking. These results suggest that the cross-linking of the collagenous peptides with the Lys-Lys-dimer template(s) for stabilization contributes to the enthalpic effect, rather than the entropic effect.

Involvement of specific mechanism in plasmid DNA uptake by mouse peritoneal macrophages

The binding and uptake of plasmid DNA encoding luciferase reporter gene (pCMV-Luc) were studied in vitro using cultured mouse peritoneal macrophages. A significant and time-dependent cellular association of [32P]pCMV-Luc with resident macrophages was observed at 37 degrees C and this decreased at 4 degrees C. The binding at 4 degrees C was saturable and a Scatchard plot gave a maximum binding capacity of 0.81 microgram/mg-protein and a dissociation constant of 0.30 microgram/ml. The binding of [32P]-pCMV-Luc was inhibited by polyinosinic acid, dextran sulfate and salmon sperm DNA, but not by polycytidylic acid, dextran and EDTA. A confocal microscopic study demonstrated that fluorescein-labeled pCMV-Luc was internalized at 37 degrees C while only cell surface binding occurred at 4 degrees C. No significant luciferase gene expression was obtained after incubation with a high concentration (100 micrograms/ml) of pCMV-Luc. These data suggest that plasmid DNA is taken up by macrophages via a mechanism mediated by a receptor like the macrophage scavenger receptor.