Apo B100 similarities to viral proteins suggest basis for LDL-DNA binding and transfection capacity

A New Structural Model of Apolipoprotein B100 Based on Computational Modeling and Cross Linking

ApoB-100 is a member of a large lipid transfer protein superfamily and is one of the main apolipoproteins found on low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL) particles. Despite its clinical significance for the development of cardiovascular disease, there is limited information on apoB-100 structure. We have developed a novel method based on the “divide and conquer” algorithm, using PSIPRED software, by dividing apoB-100 into five subunits and 11 domains. Models of each domain were prepared using I-TASSER, DEMO, RoseTTAFold, Phyre2, and MODELLER. Subsequently, we used disuccinimidyl sulfoxide (DSSO), a new mass spectrometry cleavable cross-linker, and the known position of disulfide bonds to experimentally validate each model. We obtained 65 unique DSSO cross-links, of which 87.5% were within a 26 Å threshold in the final model. We also evaluated the positions of cysteine residues involved in the eight known disulfide bonds in apoB-100, and each pair was measured within the expected 5.6 Å constraint. Finally, multiple domains were combined by applying constraints based on detected long-range DSSO cross-links to generate five subunits, which were subsequently merged to achieve an uninterrupted architecture for apoB-100 around a lipoprotein particle. Moreover, the dynamics of apoB-100 during particle size transitions was examined by comparing VLDL and LDL computational models and using experimental cross-linking data. In addition, the proposed model of receptor ligand binding of apoB-100 provides new insights into some of its functions.

NET-associated citrullinated histones promote LDL aggregation and foam cell formation in vitro

Neutrophils have been recently identified in the atherosclerotic lesion and they can release neutrophil extracellular trap (NET) under the pro-inflammatory conditions prevailing in the lesion. Citrullinated histones (Cit-histones) are the major type of citrullinated proteins associated with NET release. Since elevated levels of citrullinated proteins have been detected in inflammatory diseases including atherosclerosis, this study analysed the role played by NET and Cit-histones in different atherogenic events in vitro. First, neutrophil recruitment and NET release in the presence of low-density lipoprotein (LDL) and oxidised LDL (Ox-LDL) were analysed by Boyden's chamber method and microscopy respectively. Then, LDL oxidation and LDL aggregation in the presence of NET and Cit-histones were analysed spectroscopically. Foam cell formation in the presence of NET or Cit-histone was studied by both microscopic and spectroscopic methods. While neutrophil recruitment was facilitated by Ox-LDL and not by LDL, the extent of NET release was significantly increased in the presence of both LDL and Ox-LDL. In the presence of NET, LDL oxidation, aggregation and foam cell formation were found to be increased. Cit-histones were found to accelerate LDL aggregation and foam cell formation at higher citrulline levels. Altogether, the results suggest that both NET and NET-associated Cit-histone released at the lesion can play major roles as pro-atherogenic mediators. Inhibiting the action of NET or Cit-histone would, therefore, be beneficial in slowing down atherosclerotic progression.

Identification of Receptor Ligands in Apo B100 Reveals Potential Functional Domains

LDL, VLDL and other members of the low-density lipoparticles (LLPs) enter cells through a large family of receptors. The actual receptor ligand(s) in apolipoprotein B100, one of the main proteins of LLP, remain(s) unknown. The objective of this study was to identify true receptor ligand(s) in apo B100, a molecule of 4563 residues. Apo B100 contains 33 analogues of Cardin-Weintraub arginine/lysine-based receptor ligand motifs and shares key lysine motifs and sequence similarity with the LDL receptor-associated protein, MESD, and heat shock proteins. Eleven FITC-labeled synthetic peptides of 21-42 residues, with at least one ligand, were tested for binding and internalization using HeLa cells. All peptides bind but display different binding capacities and patterns. Peptides B0013, B0582, B2366, and B2932 mediate endocytosis and appear in distinct sites in the cytoplasm. B0708 and B3181 bind and remain on the cell surface as aggregates/clusters. Peptides B3119 (Site A) and B3347 (Site B), the putative ligands, showed low binding and no cell entry capacity. Apo B100 regions in this study share similarities with related proteins of known function including chaperone proteins and Apo BEC stimulating protein, and not directly related proteins, e.g., the DNA-binding domain of interferon regulatory factors, MSX2-interacting protein, and snake venom Zinc metalloproteinase-disintegrin-like proteins.

Association of lipid profile alterations with severe forms of dengue in humans

Previous studies have shown a relationship between circulating lipids and dengue virus infection; however, the association of altered lipid profiles with severe dengue remains little studied. The aim of this study was to determine the association between circulating lipid content and severe dengue and/or platelet counts. Ninety-eight patients (2-66 years old) classified as having dengue without warning signs (DNWS), dengue with warning signs (DWWS), or severe dengue (SD) and 62 healthy individuals were studied. Blood samples were tested for NS1, anti-dengue IgM, platelet content, total cholesterol (TC), triglycerides (T), high-density lipoproteins (HDL), low-density lipoproteins (LDL) and very-low-density lipoproteins (VLDL). Lipid alterations were observed mainly in patients with SD. Increased T and VLDL was observed in SD, and increased HDL was observed in DWWS and SD. Decreased TC was found in all forms of dengue, and the lowest LDL values were found in SD. Platelet counts were significantly decreased in DWWS and SD when compare to DNWS. A positive correlation (p = 0.019) between LDL values and platelet counts and a negative correlation (p = 0.0162) between VLDL values and platelet counts were found. Lipid profile alterations were associated with severe dengue.

Analogs of LDL Receptor Ligand Motifs in Dengue Envelope and Capsid Proteins as Potential Codes for Cell Entry

It is established that cell entry of low density lipoprotein particles (LLPs) containing Apo B100 and Apo E is mediated by receptors and GAGs. Receptor ligand motifs, XBBBXXBX, XBBXBX, and ΨBΨXB, and mono- and bipartite NLS sequences are abundant in Apo E and Apo B100 as well as in envelope and capsid proteins of Dengue viruses 1-4 (DENV1-4). Synthetic, fluorescence-labeled peptides of sequences in DENV2 envelope protein, and DENV3 capsid that include these motifs were used to conduct a qualitative assessment of cell binding and entry capacity using HeLa cells. DENV2 envelope peptide, Dsp2EP, ⁰⁵⁶⁴Gly-Gly⁰⁵⁹⁵, was shown to bind and remain at the cell surface. In contrast, DENV3 capsid protein peptide, Dsp3CP, ⁰⁰⁰²Asn-Gln⁰⁰²⁸, readily enters HeLa cells and accumulates at discrete loci in the nucleus. FITC-labeled dengue synthetic peptides colocalize with Low Density Lipoprotein-CM-DiI and Apo E-CM-DiI to a degree that suggests that Dengue viruses may utilize cell entry pathways used by LLPs.

Lipophorins can adhere to dsRNA, bacteria and fungi present in the hemolymph of the desert locust: a role as general scavenger for pathogens in the open body cavity

Desert locusts are characterized by a highly sensitive and effective RNA interference (RNAi) response. Moreover, delivery of dsRNA into the open body cavity will elicit potent silencing effects throughout the body. On the other hand, many other insect species, such as Bombyx mori and Drosophila melanogaster, lack the ability to efficiently spread the RNAi-signal. In this study, we demonstrated that, in the serum of the desert locust, lipophorins adhere to dsRNA-fragments. Lipophorins can be subdivided into high density and low density lipophorins (HDLp and LDLp), according to their buoyant density, and we showed that both types of lipophorins can interact with dsRNA fragments. Furthermore, in the presence of (gram-positive) bacteria or fungi, LDLp induce the formation of pathogen aggregates, while no clear aggregation effects were detected in the presence of HDLp.

Biological mechanisms determining the success of RNA interference in insects

Insects constitute the largest group of animals on this planet, having a huge impact on our environment, as well as on our quality of life. RNA interference (RNAi) is a posttranscriptional gene silencing mechanism triggered by double-stranded (ds)RNA fragments. This process not only forms the basis of a widely used reverse genetics research method in many different eukaryotes but also holds great promise to contribute to the species-specific control of agricultural pests and to combat viral infections in beneficial and disease vectoring insects. However, in many economically important insect species, such as flies, mosquitoes, and caterpillars, systemic delivery of naked dsRNA does not trigger effective gene silencing. Although many components of the RNAi pathway have initially been deciphered in the fruit fly, Drosophila melanogaster, it will be of major importance to investigate this process in a wider variety of species, including dsRNA-sensitive insects such as locusts and beetles, to elucidate the factors responsible for the remarkable variability in RNAi efficiency, as observed in different insects. In this chapter, we review the current knowledge on the RNAi pathway, as well as the most recent insights into the mechanisms that might determine successful RNAi in insects.

The interaction between claudin-1 and dengue viral prM/M protein for its entry

Dengue disease is becoming a huge public health concern around the world as more than one-third of the world's population living in areas at risk of infection. In an effort to assess host factors interacting with dengue virus, we identified claudin-1, a major tight junction component, as an essential cell surface protein for dengue virus entry. When claudin-1 was knocked down in Huh 7.5 cells via shRNA, the amount of dengue virus entering host cells was reduced. Consequently, the progeny virus productions were decreased and dengue virus-induced CPE was prevented. Furthermore, restoring the expression of claudin-1 in the knockdown cells facilitated dengue virus entry. The interaction between claudin-1 and dengue viral prM protein was further demonstrated using the pull-down assay. Deletion of the extracellular loop 1 (ECL1) of claudin-1 abolished such interaction, so did point mutations C54A, C64A and I32M on ECL1. These results suggest that the interaction between viral protein prM and host protein claudin-1 was essential for dengue entry. Since host and viral factors involved in virus entry are promising therapeutic targets, determining the essential role of claudin-1 could lead to the discovery of entry inhibitors with attractive therapeutic potential against dengue disease.