A literature review concerning the unexpected species differences of the vertebrate innate immune response to lipid IVA was published in CSBJ prior to the present computational study to address the unpaired activity-sequence correlation of prototypic E. coli -type lipid A and its precursor lipid IVA regarding human, murine, equine and canine species. To this end, their sequences and structures of hitherto known Toll-like receptor 4 (TLR4) and myeloid differentiation factor 2 (MD-2) complexes were aligned and their differential side chain patterns studied. If required due to the lack of the corresponding X-ray crystallographic data, three-dimensional models of TLR4/MD-2/ligand complexes were generated using mono and dimeric crystal structures as templates and in silico docking of the prototypic ligands lipid A, lipid IVA and Eritoran. All differential amino acids were mapped to pinpoint species dependency on an atomic scale, i.e. the possible concert of mechanistically relevant side chains. In its most abstract and general form the three-dimensional (3D-) models devise a triangular interface or “wedge” where molecular interactions between TLR4, MD-2 and ligand itself take place. This study identifies two areas in the wedge related to either agonism or antagonism reflecting why ligands like lipid IVA can possess a species dependent dual activity. Lipid IVA represents an imperfect (underacylated and backbone-flipped), low affinity ligand of mammalian TLR4/MD-2 complexes. Its specific but weak antagonistic activity in the human system is in particular due to the loss of phosphate attraction in the wedge-shaped region conferred by nonhomologous residue changes when compared to crystal and modeled structures of the corresponding murine and equine TLR4/MD-2 complexes. The counter-TLR4/MD-2 unit was also taken into account since agonist-mediated dimerization in a defined m-shaped complex composed of two TLR4/MD-2/agonist subunits triggers intracellular signaling during the innate immune response to bacterial endotoxin exposure.

In view of the serious health problems concerning infectious diseases in heavily populated areas, we followed the strategy of lead compound diversification to evaluate the near-by chemical space for new organic compounds. To this end, twenty derivatives of nitazoxanide (NTZ) were synthesized and tested for activity against Entamoeba histolytica parasites. To ensure drug-likeliness and activity relatedness of the new compounds, the synthetic work was assisted by a quantitative structure-activity relationships study (QSAR). Many of the inherent downsides – well-known to QSAR practitioners – we circumvented thanks to workarounds which we proposed in prior QSAR publication. To gain further mechanistic insight on a molecular level, ligand-enzyme docking simulations were carried out since NTZ is known to inhibit the protozoal pyruvate ferredoxin oxidoreductase (PFOR) enzyme as its biomolecular target.

Electrostatic interactions between phosphate anions and Toll-like receptor 4 / Myeloid differentiation factor-2 (TLR4/MD-2) protein complexes of human, murine, equine and canine species were computed. Such knowledge can provide mechanistic information about recognising LPS-like ligands, since anionic phosphate groups belong to the structural features of LPS with their diphosphorylated diglucosamine backbone. Sequence composition analyses, electrostatic interaction potentials and docked energies as well as molecular dynamics studies evaluated the phosphate interactions within the triangular LPS binding site (wedge). According to electrostatic analyses, human, horse and dog wedges possess phosphate-binding sites with indistinct positive and negative charge distributions, but the murine wedge shows a unique strong negative net charge at the site where antagonists bind in other species (Pan). Docking of a phosphate mono-anion (probe) confirmed its repulsion at this Pan site, but the Pag site of the murine wedge attracted the probe. It is occupied by phosphate groups of agonists in other species (Pag). Molecular dynamics trajectories show a variable degree of random walk across the wedges, that is, not following electrostatic preferences (neither Pag nor Pan). In summary, two opposing electrostatic patterns exist –murine versus human, equine and canine species – all of which reflect the potential dual activity mode of under-acylated ligands such as lipid IVA.

As a part of innate immunity mechanisms, the Toll-like receptor (TLR) signaling pathway serves as one of the mainstay lines of defense against pathogenic microorganisms and cell dysfunction. Nevertheless, TLR overactivation induces a systemic proinflammatory environment compromising organ function or causing the patient’s death. TLRs modulators, specially those focused for TLR4, remain a promising approach for inflammatory diseases treatment, being peptide-based therapy a trendy approach. Heat shock protein 60 (HSP60) not only plays a pivotal role in the development of several maladies with strong inflammatory components but also HSP60 peptides possess anti-inflammatory properties in TLR4-mediated diseases, such as diabetes, arthritis, and atherosclerosis. The experimental treatment using HSP60 peptides has proven to be protective in preclinical models of the heart by hampering inflammation and modulating the activity of immune cells. Nonetheless, the effect that these peptides may exert directly on cells that express TLR and its role to inhibit overactivation remain elusive. The aim of this study is to evaluate by molecular docking, a 15 amino acid long-HSP60 peptide (Peptide-2) in the lipopolysaccharide (LPS) binding site of TLR4/MD2, finding most Peptide-2 resulting conformations posed into the hydrophobic pocket of MD2. This observation is supported by binding energy obtained for the control antagonist Eritoran, close to those of Peptide-2. This last does not undergo drastic structural changes, moving into a delimited space, and maintaining the same orientation during molecular dynamics simulation. Based on the two computational techniques applied, interaction patterns were defined for Peptide-2. With these results, it is plausible to propose a peptidic approach for TLR4 modulation as a new innovative therapy to the treatment of TLR4-related cardiovascular diseases.

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