Conformationally constrained lipid A mimetics for exploration of structural basis of TLR4/MD-2 activation by lipopolysaccharide

Disease-Inspired Design of Biomimetic Tannic Acid-Based Hybrid Nanocarriers for Enhancing the Treatment of Bacterial-Induced Sepsis

This study explored the development of novel biomimetic tannic acid-based hybrid nanocarriers (HNs) for targeted delivery of ciprofloxacin (CIP-loaded TAH-NPs) against bacterial-induced sepsis. The prepared CIP-loaded TAH-NPs exhibited appropriate physicochemical characteristics and demonstrated biocompatibility and nonhemolytic properties. Computational simulations and microscale thermophoresis studies validated the strong binding affinity of tannic acid (TA) and its nanoformulation to human Toll-like receptor 4, surpassing that of the natural substrate lipopolysaccharide (LPS), suggesting a potential competitive inhibition against LPS-induced inflammatory responses. CIP released from TAH-NPs displayed a sustained release profile over 72 h. The in vitro antibacterial activity studies revealed that CIP-loaded TAH-NPs exhibited enhanced antibacterial efficacy and efflux pump inhibitory activity. Specifically, they showed a 3-fold increase in biofilm eradication activity against MRSA and a 2-fold increase against P. aeruginosa compared to bare CIP. Time-killing assays demonstrated complete bacterial clearance within 8 h of treatment with CIP-loaded TAH-NPs. In vitro DPPH scavenging and anti-inflammatory investigations confirmed the ability of the prepared hybrid nanosystem to neutralize reactive oxygen species (ROS) and modulate LPS-induced inflammatory responses. Collectively, these results suggest that CIP-loaded TAH-NPs may serve as an innovative nanocarrier for the effective and targeted delivery of antibiotics against bacterial-induced sepsis.

Nonreducing Sugar Scaffold Enables the Development of Immunomodulatory TLR4-specific LPS Mimetics with Picomolar Potency

Innate immune defense mechanisms against infection and cancer encompass the modulation of pattern recognition receptor (PRR)-mediated inflammation, including upregulation of various transcription factors and the activation of pro-inflammatory pathways important for immune surveillance. Dysfunction of PRRs-mediated signaling has been implicated in cancer and autoimmune diseases, while the overactivation of PRRs-driven responses during infection can lead to devastating consequences such as acute lung injury or sepsis. We used crystal structure-based design to develop immunomodulatory lipopolysaccharide (LPS) mimetics targeting one of the ubiquitous PRRs, Toll-like Receptor 4 (TLR4). Taking advantage of an exo-anomeric conformation and specific molecular shape of synthetic nonreducing β,β-diglucosamine, which was investigated by NMR, we developed two sets of lipid A mimicking glycolipids capable of either potently activating innate immune responses or inhibiting pro-inflammatory signaling. Stereoselective 1,1'-glycosylation towards fully orthogonally protected nonreducing GlcNβ(1↔1')βGlcN followed by stepwise assembly of differently functionalised phosphorylated glycolipids provided biologically active molecules that were evaluated for their ability to trigger or to inhibit cellular innate immune responses. Two LPS mimetics, identified as potent TLR4-specific inducers of the intracellular signaling pathways, serve as vaccine adjuvant- and immunotherapy candidates, while anionic glycolipids with TLR4-inhibitory potential hold therapeutic promise for the management of acute or chronic inflammation.

Pseudomonas aeruginosa Lipid A Structural Variants Induce Altered Immune Responses

Pseudomonas aeruginosa causes chronic lung infection in cystic fibrosis (CF), resulting in structural lung damage and progressive pulmonary decline. P. aeruginosa in the CF lung undergoes numerous changes, adapting to host-specific airway pressures while establishing chronic infection. P. aeruginosa undergoes lipid A structural modification during CF chronic infection that is not seen in any other disease state. Lipid A, the membrane anchor of LPS (i.e., endotoxin), comprises the majority of the outer membrane of Gram-negative bacteria and is a potent Toll-like receptor 4 (TLR4) agonist. The structure of P. aeruginosa lipid A is intimately linked with its recognition by TLR4 and subsequent immune response. Prior work has identified P. aeruginosa strains with altered lipid A structures that arise during chronic CF lung infection; however, the impact of the P. aeruginosa lipid A structure on airway disease has not been investigated. Here, we show that P. aeruginosa lipid A lacks PagL-mediated deacylation during human airway infection using a direct-from-sample mass spectrometry approach on human BAL fluid. This structure triggers increased proinflammatory cytokine production by primary human macrophages. Furthermore, alterations in lipid A 2-hydroxylation impact cytokine response in a site-specific manner, independent of CF transmembrane conductance regulator function. It is interesting that there is a CF-specific reduction in IL-8 secretion within the epithelial-cell compartment that only occurs in CF bronchial epithelial cells when infected with CF-adapted P. aeruginosa that lacks PagL-mediated lipid A deacylation. Taken together, we show that P. aeruginosa alters its lipid A structure during acute lung infection and that this lipid A structure induces stronger signaling through TLR4.

MANNosylation of Mesoporous Silica Nanoparticles Modifies TLR4 Localization and NF-κB Translocation in T24 Bladder Cancer Cells

D-mannose is widely used as non-antibiotic treatment for bacterial urinary tract infections. This application is based on a well-studied mechanism of binding to the type 1 bacterial pili and, therefore, blocking bacteria adhesion to the uroepithelial cells. To implement D-mannose into carrier systems, the mechanism of action of the sugar in the bladder environment is also relevant and requires investigation. Herein, two different MANNosylation strategies using mesoporous silica nanoparticles (MSNs) are described. The impact of different chemical linkers on bacterial adhesion and bladder cell response is studied via confocal microscopy imaging of the MSN interactions with the respective organisms. Cytotoxicity is assessed and the expression of Toll-like receptor 4 (TLR4) and caveolin-1 (CAV-1), in the presence or absence of simulated infection with bacterial lipopolysaccharide (LPS), is evaluated using the human urinary bladder cancer cell line T24. Further, localisation of the transcription factor NF-κB due to the MANNosylated materials is examined over time. The results show that MANNosylation modifies bacterial adhesion to the nanomaterials and significantly affects TLR4, caveolin-1, and NF-κB in bladder cells. These elements are essential components of the inflammatory cascade/pathogens response during urinary tract infections. These findings demonstrate that MANNosylation is a versatile tool to design hybrid nanocarriers for targeted biomedical applications.

Design, Synthesis, and Bioactivity Evaluation of a TF-Based Cancer Vaccine Candidate Using Lipid A Mimetics As a Built-In Adjuvant

This study describes the design and synthesis of five TF-based cancer vaccine candidates using a lipid A mimetic as the carrier and a built-in adjuvant. All synthesized conjugates elicited robust and consistent TF-specific immune responses in mice without external adjuvants. Immunological studies subsequently conducted in wild-type and TLR4 knockout C57BL/6 mice demonstrated that the activation of TLR4 was the main reason that the synthesized lipid A mimetics increased the TF-specific immune responses. All antisera induced by these conjugates can specifically recognize, bind to, and induce the lysis of TF-positive cancer cells. Moreover, representative conjugates 2 and 3 could effectively reduce the growth of tumors and prolong the survival time of mice in vivo, and the efficacies were better than glycoprotein TF-CRM197 with alum adjuvant. Lipid A mimetics could therefore be a promising platform for the development of new carbohydrate-based vaccine carriers with self-adjuvanting properties for the treatment of cancer.

BECC438b TLR4 agonist supports unique immune response profiles from nasal and muscular DTaP pertussis vaccines in murine challenge models

The protection afforded by acellular pertussis vaccines wanes over time, and there is a need to develop improved vaccine formulations. Options to improve the vaccines involve the utilization of different adjuvants and administration via different routes. While intramuscular (IM) vaccination provides a robust systemic immune response, intranasal (IN) vaccination theoretically induces a localized immune response within the nasal cavity. In the case of a Bordetella pertussis infection, IN vaccination results in an immune response that is similar to natural infection, which provides the longest duration of protection. Current acellular formulations utilize an alum adjuvant, and antibody levels wane over time. To overcome the current limitations with the acellular vaccine, we incorporated a novel TLR4 agonist, BECC438b, into both IM and IN acellular formulations to determine its ability to protect against infection in a murine airway challenge model. Following immunization and challenge, we observed that DTaP + BECC438b reduced bacterial burden within the lung and trachea for both administration routes when compared with mock-vaccinated and challenged (MVC) mice. Interestingly, IN administration of DTaP + BECC438b induced a Th1-polarized immune response, while IM vaccination polarized toward a Th2 immune response. RNA sequencing analysis of the lung demonstrated that DTaP + BECC438b activates biological pathways similar to natural infection. Additionally, IN administration of DTaP + BECC438b activated the expression of genes involved in a multitude of pathways associated with the immune system. Overall, these data suggest that BECC438b adjuvant and the IN vaccination route can impact efficacy and responses of pertussis vaccines in pre-clinical mouse models.

Combining lipid-mimicking-enabled transition metal and enzyme-mediated catalysis at the cell surface of E. coli

Being an essential multifunctional platform and interface to the extracellular environment, the cell membrane constitutes a valuable target for the modification and manipulation of cells and cellular behavior, as well as for the implementation of artificial, new-to-nature functionality. While bacterial cell surface functionalization via expression and presentation of recombinant proteins has extensively been applied, the corresponding application of functionalizable lipid mimetics has only rarely been reported. Herein, we describe an approach to equip E. coli cells with a lipid-mimicking, readily membrane-integrating imidazolium salt and a corresponding NHC-palladium complex that allows for flexible bacterial membrane surface functionalization and enables E. coli cells to perform cleavage of propargyl ethers present in the surrounding cell medium. We show that this approach can be combined with already established on-surface functionalization, such as bacterial surface display of enzymes, i.e. laccases, leading to a new type of cascade reaction. Overall, we envision the herein presented proof-of-concept studies to lay the foundation for a multifunctional toolbox that allows flexible and broadly applicable functionalization of bacterial membranes.

Exploring Species-Specificity in TLR4/MD-2 Inhibition with Amphiphilic Lipid A Mimicking Glycolipids

The Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) complex is a key receptor of the innate immune system and a major driver of inflammation that is responsible for the multifaceted defense response to Gram-negative infections. However, dysfunction in the tightly regulated mechanisms of TLR4-mediated signaling leads to the uncontrolled upregulation of local and systemic inflammation, often resulting in acute or chronic disease. Therefore, the TLR4/MD-2 receptor complex is an attractive target for the design and development of anti-inflammatory therapies which aim to control the unrestrained activation of TLR4-mediated signaling. Complex structure-activity relationships and species-specificity behind ligand recognition by the TLR4/MD-2 complex complicate the development of MD-2-specific TLR4 antagonists. The restriction of the conformational flexibility of the disaccharide polar head group is one of the key structural features of the newly developed lipid A-mimicking glycophospholipids, which are potential inhibitors of TLR4-mediated inflammation. Since phosphorylation has a crucial influence on MD-2-ligand interaction, glycolipids with variable numbers and positioning of phosphate groups were synthesized and evaluated for their ability to inhibit TLR4-mediated pro-inflammatory signaling in human and murine immune cells. A bis-phosphorylated glycolipid was found to have nanomolar antagonist activity on human TLR4 while acting as a partial agonist on murine TLR4. The glycolipid inhibited mTLR4/MD-2-mediated cytokine release, acting as an antagonist in the presence of lipopolysaccharide (LPS), but at the same time induced low-level cytokine production.

Therapeutic Targeting of TLR4 for Inflammation, Infection, and Cancer: A Perspective for Disaccharide Lipid A Mimetics

The Toll-like receptor 4 (TLR4) signaling pathway plays a central role in the prompt defense against infectious challenge and provides immediate response to Gram-negative bacterial infection. The TLR4/MD-2 complex can sense and respond to various pathogen-associated molecular patterns (PAMPs) with bacterial lipopolysaccharide (LPS) being the most potent and the most frequently occurring activator of the TLR4-mediated inflammation. TLR4 is believed to be both a friend and foe since improperly regulated TLR4 signaling can result in the overactivation of immune responses leading to sepsis, acute lung injury, or pathologic chronic inflammation involved in cancer and autoimmune disease. TLR4 is also considered a legitimate target for vaccine adjuvant development since its activation can boost the adaptive immune responses. The dual action of the TLR4 complex justifies the efforts in the development of both TLR4 antagonists as antisepsis drug candidates or remedies for chronic inflammatory diseases and TLR4 agonists as vaccine adjuvants or immunotherapeutics. In this review, we provide a brief overview of the biochemical evidences for possible pharmacologic applications of TLR4 ligands as therapeutics and report our systematic studies on the design, synthesis, and immunobiological evaluation of carbohydrate-based TLR4 antagonists with nanomolar affinity for MD-2 as well as disaccharide-based TLR4 agonists with picomolar affinity for the TLR4/MD-2 complex.

Semisynthetic Glycoconjugate Vaccine Candidates against Escherichia coli O25B Induce Functional IgG Antibodies in Mice

Extraintestinal pathogenic Escherichia coli (ExPEC) is a major health concern due to emerging antibiotic resistance. Along with O1A, O2, and O6A, E. coli O25B is a major serotype within the ExPEC group, which expresses a unique O-antigen. Clinical studies with a glycoconjugate vaccine of the above-mentioned O-types revealed O25B as the least immunogenic component, inducing relatively weak IgG titers. To evaluate the immunological properties of semisynthetic glycoconjugate vaccine candidates against E. coli O25B, we here report the chemical synthesis of an initial set of five O25B glycan antigens differing in length, from one to three repeat units, and frameshifts of the repeat unit. The oligosaccharide antigens were conjugated to the carrier protein CRM₁₉₇. The resulting semisynthetic glycoconjugates induced functional IgG antibodies in mice with opsonophagocytic activity against E. coli O25B. Three of the oligosaccharide-CRM₁₉₇ conjugates elicited functional IgGs in the same order of magnitude as a conventional CRM₁₉₇ glycoconjugate prepared with native O25B O-antigen and therefore represent promising vaccine candidates for further investigation. Binding studies with two monoclonal antibodies (mAbs) revealed nanomolar anti-O25B IgG responses with nanomolar K _D values and with varying binding epitopes. The immunogenicity and mAb binding data now allow for the rational design of additional synthetic antigens for future preclinical studies, with expected further improvements in the functional antibody responses. Moreover, acetylation of a rhamnose residue was shown to be likely dispensable for immunogenicity, as a deacylated antigen was able to elicit strong functional IgG responses. Our findings strongly support the feasibility of a semisynthetic glycoconjugate vaccine against E. coli O25B.

Multifaceted Computational Modeling in Glycoscience

Glycoscience assembles all the scientific disciplines involved in studying various molecules and macromolecules containing carbohydrates and complex glycans. Such an ensemble involves one of the most extensive sets of molecules in quantity and occurrence since they occur in all microorganisms and higher organisms. Once the compositions and sequences of these molecules are established, the determination of their three-dimensional structural and dynamical features is a step toward understanding the molecular basis underlying their properties and functions. The range of the relevant computational methods capable of addressing such issues is anchored by the specificity of stereoelectronic effects from quantum chemistry to mesoscale modeling throughout molecular dynamics and mechanics and coarse-grained and docking calculations. The Review leads the reader through the detailed presentations of the applications of computational modeling. The illustrations cover carbohydrate-carbohydrate interactions, glycolipids, and N- and O-linked glycans, emphasizing their role in SARS-CoV-2. The presentation continues with the structure of polysaccharides in solution and solid-state and lipopolysaccharides in membranes. The full range of protein-carbohydrate interactions is presented, as exemplified by carbohydrate-active enzymes, transporters, lectins, antibodies, and glycosaminoglycan binding proteins. A final section features a list of 150 tools and databases to help address the many issues of structural glycobioinformatics.

Lipid A Mimetics Based on Unnatural Disaccharide Scaffold as Potent TLR4 Agonists for Prospective Immunotherapeutics and Adjuvants

TLR4 is a key pattern recognition receptor that can sense pathogen- and danger- associated molecular patterns to activate the downstream signaling pathways which results in the upregulation of transcription factors and expression of interferons and cytokines to mediate protective pro-inflammatory responses involved in immune defense. Bacterial lipid A is the primary TLR4 ligand with very complex, species-specific, and barely predictable structure-activity relationships. Given that therapeutic targeting of TLR4 is an emerging tool for management of a variety of human diseases, the development of novel TLR4 activating biomolecules other than lipid A is of vast importance. We report on design, chemical synthesis and immunobiology of novel glycan-based lipid A-mimicking molecules that can activate human and murine TLR4-mediated signaling with picomolar affinity. Exploiting crystal structure - based design we have created novel disaccharide lipid A mimetics (DLAMs) where the inherently flexible β(1→6)-linked diglucosamine backbone of lipid A is exchanged with a conformationally restrained non-reducing βGlcN(1↔1')βGlcN scaffold. Excellent stereoselectivity in a challenging β,β-1,1' glycosylation was achieved by tuning the reactivities of donor and acceptor molecules using protective group manipulation strategy. Divergent streamlined synthesis of β,β-1,1'-linked diglucosamine-derived glycolipids entailing multiple long-chain (R)-3- acyloxyacyl residues and up two three phosphate groups was developed. Specific 3D-molecular shape and conformational rigidity of unnatural β,β-1,1'-linked diglucosamine combined with carefully optimized phosphorylation and acylation pattern ensured efficient induction of the TLR4-mediated signaling in a species-independent manner.

Lipopolysaccharide lipid A: A promising molecule for new immunity-based therapies and antibiotics

Lipopolysaccharides (LPS) are the main components of the external leaflet of the Gram-negative outer membrane and consist of three different moieties: lipid A, core oligosaccharide, and O-polysaccharide. The lipid A is a glucosamine disaccharide with different levels of acylation and phosphorylation, beside carrying, in certain cases, additional substituents on the sugar backbone. It is also the main immunostimulatory part of the LPS, as its recognition by the host immune system represents a fundamental event for detection of perilous microorganisms. Moreover, an uncontrolled immune response caused by a large amount of circulating LPS can lead to dramatic outcomes for human health, such as septic shock. The immunostimulant properties of an LPS incredibly vary depending on lipid A chemical structure, and for this reason, natural and synthetic variants of the lipid A are under study to develop new drugs that mimic or antagonise its natural effects. Here, we review past and recent findings on the lipid A as an antibiotic target and immune-therapeutic molecule, with a special attention on the crucial role of the chemical structure and its exploitation for conceiving novel strategies for treatment of several immune-related pathologies.

In silico investigation and surmounting of Lipopolysaccharide barrier in Gram-Negative Bacteria: How far has molecular dynamics Come?

Lipopolysaccharide (LPS), a main component of the outer membrane of Gram-negative bacteria, has crucial implications on both antibiotic resistance and the overstimulation of the host innate immune system. Fighting against these global concerns calls for the molecular understanding of the barrier function and immunostimulatory ability of LPS. Molecular dynamics (MD) simulations have become an invaluable tool for uncovering important findings in LPS research. While the reach of MD simulations for investigating the immunostimulatory ability of LPS has been already outlined, little attention has been paid to the role of MD simulations for exploring its barrier function and synthesis. Herein, we give an overview about the impact of MD simulations on gaining insight into the shield role and synthesis pathway of LPS, which have attracted considerable attention to discover molecules able to surmount antibiotic resistance, either circumventing LPS defenses or disrupting its synthesis. We specifically focus on the enhanced sampling and free energy calculation methods that have been combined with MD simulations to address such research. We also highlight the use of special-purpose MD supercomputers, the importance of appropriate LPS and ions parameterization to obtain reliable results, and the complementary views that MD and wet-lab experiments provide. Thereby, this work, which covers the last five years of research, apart from outlining the phenomena and strategies that are being explored, evidences the valuable insights that are gained by MD, which may be useful to advance antibiotic design, and what the prospects of this in silico method could be in LPS research.

Fighting Against Bacterial Lipopolysaccharide-Caused Infections through Molecular Dynamics Simulations: A Review

Lipopolysaccharide (LPS) is the primary component of the outer leaflet of Gram-negative bacterial outer membranes. LPS elicits an overwhelming immune response during infection, which can lead to life-threatening sepsis or septic shock for which no suitable treatment is available so far. As a result of the worldwide expanding multidrug-resistant bacteria, the occurrence and frequency of sepsis are expected to increase; thus, there is an urge to develop novel strategies for treating bacterial infections. In this regard, gaining an in-depth understanding about the ability of LPS to both stimulate the host immune system and interact with several molecules is crucial for fighting against LPS-caused infections and allowing for the rational design of novel antisepsis drugs, vaccines and LPS sequestration and detection methods. Molecular dynamics (MD) simulations, which are understood as being a computational microscope, have proven to be of significant value to understand LPS-related phenomena, driving and optimizing experimental research studies. In this work, a comprehensive review on the methods that can be combined with MD simulations, recently applied in LPS research, is provided. We focus especially on both enhanced sampling methods, which enable the exploration of more complex systems and access to larger time scales, and free energy calculation approaches. Thereby, apart from outlining several strategies for surmounting LPS-caused infections, this work reports the current state-of-the-art of the methods applied with MD simulations for moving a step forward in the development of such strategies.

Lipopolysaccharide Preparation Derived From Porphyromonas gingivalis Induces a Weaker Immuno-Inflammatory Response in BV-2 Microglial Cells Than Escherichia coli by Differentially Activating TLR2/4-Mediated NF-κB/STAT3 Signaling Pathways

Alzheimer’s disease (AD) is a degenerative disease of the central nervous system with unclear etiology and pathogenesis. In recent years, as the infectious theory and endotoxin hypothesis of AD has gained substantial attention, several studies have proposed that Porphyromonas gingivalis (P. gingivalis), one of the main pathogenic bacteria of chronic periodontitis, and the lipopolysaccharide (LPS) of P. gingivalis may lead to AD-like pathological changes and cognition impairment. However, research on the relationship between P. gingivalis-LPS and neuroinflammation is still lacking. Our study aimed to investigate the effects of P. gingivalis-LPS preparation on immuno-inflammation in microglial cells and further compared the differential inflammatory response induced by P. gingivalis-LPS and Escherichia coli (E. coli) LPS preparations. The results showed that P. gingivalis-LPS could upregulate the gene expression and release of pro-inflammatory factors in BV-2 microglial cells, including IL-1β, IL-6, TNF-α, IL-17, and IL-23. We also observed an increase in the level of Toll-like receptor 2/4 (TLR2/4) and NF-κB/STAT3 signaling. Moreover, the changes mentioned above were more significant in the E. coli-LPS group and the effects of both kinds of LPS could be differentially reversed by the administration of the TLR2 inhibitor C29 and TLR4 inhibitor TAK-242. The molecular simulation showed that the binding affinity of P. gingivalis-lipid A to TLR4-MD-2 was weaker than E. coli-lipid A, which was probably due to the presence of fewer acyl chains and phosphate groups of P. gingivalis-lipid A than E. coli-lipid A. We conclude that P. gingivalis-LPS could activate TLR2/4-mediated NF-κB/STAT3 signaling pathways, which ultimately resulted in an immune-inflammatory response in BV-2 microglia. In contrast to E. coli-LPS, P. gingivalis-LPS is a weaker TLR2/4 agonist and NF-κB/STAT3 signaling activator. Furthermore, the different fatty acid chains and phosphate groups between P. gingivalis-lipid A and E. coli-lipid A may be the reason for the weaker activating properties of P. gingivalis-LPS.

Tailored Modulation of Cellular Pro-inflammatory Responses With Disaccharide Lipid A Mimetics

Pro-inflammatory signaling mediated by Toll-like receptor 4 (TLR4)/myeloid differentiation-2 (MD-2) complex plays a crucial role in the instantaneous protection against infectious challenge and largely contributes to recovery from Gram-negative infection. Activation of TLR4 also boosts the adaptive immunity which is implemented in the development of vaccine adjuvants by application of minimally toxic TLR4 activating ligands. The modulation of pro-inflammatory responses via the TLR4 signaling pathway was found beneficial for management of acute and chronic inflammatory disorders including asthma, allergy, arthritis, Alzheimer disease pathology, sepsis, and cancer. The TLR4/MD-2 complex can recognize the terminal motif of Gram-negative bacterial lipopolysaccharide (LPS)—a glycophospholipid lipid A. Although immense progress in understanding the molecular basis of LPS-induced TLR4-mediated signaling has been achieved, gradual, and predictable TLR4 activation by structurally defined ligands has not yet been attained. We report on controllable modulation of cellular pro-inflammatory responses by application of novel synthetic glycolipids—disaccharide-based lipid A mimetics (DLAMs) having picomolar affinity for TLR4/MD-2. Using crystal structure inspired design we have developed endotoxin mimetics where the inherently flexible β(1 → 6)-linked diglucosamine backbone of lipid A is replaced by a conformationally restricted α,α-(1↔1)-linked disaccharide scaffold. The tertiary structure of the disaccharide skeleton of DLAMs mirrors the 3-dimensional shape of TLR4/MD-2 bound E. coli lipid A. Due to exceptional conformational rigidity of the sugar scaffold, the specific 3D organization of DLAM must be preserved upon interaction with proteins. These structural factors along with specific acylation and phosphorylation pattern can ensure picomolar affinity for TLR4 and permit efficient dimerization of TLR4/MD-2/DLAM complexes. Since the binding pose of lipid A in the binding pocket of MD-2 (±180°) is crucial for the expression of biological activity, the chemical structure of DLAMs was designed to permit a predefined binding orientation in the binding groove of MD-2, which ensured tailored and species-independent (human and mice) TLR4 activation. Manipulating phosphorylation and acylation pattern at the sugar moiety facing the secondary dimerization interface allowed for adjustable modulation of the TLR4-mediated signaling. Tailored modulation of cellular pro-inflammatory responses by distinct modifications of the molecular structure of DLAMs was attained in primary human and mouse immune cells, lung epithelial cells and TLR4 transfected HEK293 cells.

Lipopolysaccharide Recognition in the Crossroads of TLR4 and Caspase-4/11 Mediated Inflammatory Pathways

The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors-TLR4/MD-2 complex and caspase-4/11, respectively-are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a "basic" hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very "narrow" specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

Implications of the Human Gut-Brain and Gut-Cancer Axes for Future Nanomedicine

Recent clinical and pathological evidence have implicated the gut microbiota as a nexus for modulating the homeostasis of the human body, impacting conditions from cancer and dementia to obesity and social behavior. The connections between microbiota and human diseases offer numerous opportunities in medicine, most of which have limited or no therapeutic solutions available. In light of this paradigm-setting trend in science, this review aims to provide a comprehensive and timely summary of the mechanistic pathways governing the gut microbiota and their implications for nanomedicines targeting cancer and neurodegenerative diseases. Specifically, we discuss in parallel the beneficial and pathogenic relationship of the gut microbiota along the gut-brain and gut-cancer axes, elaborate on the impact of dysbiosis and the gastrointestinal corona on the efficacy of nanomedicines, and highlight a molecular mimicry that manipulates the universal cross-β backbone of bacterial amyloid to accelerate neurological disorders. This review further offers a forward-looking section on the rational design of cancer and dementia nanomedicines exploiting the gut-brain and gut-cancer axes.

Shortening the Lipid A Acyl Chains of Bordetella pertussis Enables Depletion of Lipopolysaccharide Endotoxic Activity

Whooping cough, or pertussis, is an acute respiratory infectious disease caused by the Gram-negative bacterium Bordetella pertussis. Whole-cell vaccines, which were introduced in the fifties of the previous century and proved to be effective, showed considerable reactogenicity and were replaced by subunit vaccines around the turn of the century. However, there is a considerable increase in the number of cases in industrialized countries. A possible strategy to improve vaccine-induced protection is the development of new, non-toxic, whole-cell pertussis vaccines. The reactogenicity of whole-cell pertussis vaccines is, to a large extent, derived from the lipid A moiety of the lipopolysaccharides (LPS) of the bacteria. Here, we engineered B. pertussis strains with altered lipid A structures by expressing genes for the acyltransferases LpxA, LpxD, and LpxL from other bacteria resulting in altered acyl-chain length at various positions. Whole cells and extracted LPS from the strains with shorter acyl chains showed reduced or no activation of the human Toll-like receptor 4 in HEK-Blue reporter cells, whilst a longer acyl chain increased activation. Pyrogenicity studies in rabbits confirmed the in vitro assays. These findings pave the way for the development of a new generation of whole-cell pertussis vaccines with acceptable side effects.

Chemical Strategies to Boost Cancer Vaccines

Personalized cancer vaccines (PCVs) are reinvigorating vaccine strategies in cancer immunotherapy. In contrast to adoptive T-cell therapy and checkpoint blockade, the PCV strategy modulates the innate and adaptive immune systems with broader activation to redeploy antitumor immunity with individualized tumor-specific antigens (neoantigens). Following a sequential scheme of tumor biopsy, mutation analysis, and epitope prediction, the administration of neoantigens with synthetic long peptide (SLP) or mRNA formulations dramatically improves the population and activity of antigen-specific CD4+ and CD8+ T cells. Despite the promising prospect of PCVs, there is still great potential for optimizing prevaccination procedures and vaccine potency. In particular, the arduous development of tumor-associated antigen (TAA)-based vaccines provides valuable experience and rational principles for augmenting vaccine potency which is expected to advance PCV through the design of adjuvants, delivery systems, and immunosuppressive tumor microenvironment (TME) reversion since current personalized vaccination simply admixes antigens with adjuvants. Considering the broader application of TAA-based vaccine design, these two strategies complement each other and can lead to both personalized and universal therapeutic methods. Chemical strategies provide vast opportunities for (1) exploring novel adjuvants, including synthetic molecules and materials with optimizable activity, (2) constructing efficient and precise delivery systems to avoid systemic diffusion, improve biosafety, target secondary lymphoid organs, and enhance antigen presentation, and (3) combining bioengineering methods to innovate improvements in conventional vaccination, "smartly" re-educate the TME, and modulate antitumor immunity. As chemical strategies have proven versatility, reliability, and universality in the design of T cell- and B cell-based antitumor vaccines, the union of such numerous chemical methods in vaccine construction is expected to provide new vigor and vitality in cancer treatment.

Exploring electrostatic patterns of human, murine, equine and canine TLR4/MD-2 receptors

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 IV_A.

Yersinia pseudotuberculosis-derived adhesins

Around fifteen surface components referred to adhesins have been identified in Yersinia pseudotuberculosis combining primarily microbiological, molecular and genetic, as well as immunochemical and biophysical methods. Y. pseudotuberculosis-derived adhesins vary in structure and chemical composition but they are mainly presented by protein molecules. Some of them were shown to participate not only in adhesive but in other pathogen-related physiological functions in the host-parasite interplay. Adhesins can mediate bacterial adhesion to eukaryotic cell either directly or via the extracellular matrix components. These adhesion molecules are encoded by chromosomal DNA excepting YadA protein which gene is located in the calcium-dependence plasmid pYV common for pathogenic yersisniae. An optimum temperature for adhesin biosynthesis is located close to the body temperature of warm-blooded animals; however, at low temperature only invasin InvA, full-length smooth lipopolysaccharide and porin OmpF are produced in Y. pseudotuberculosis. Several adhesins (Psa, InvA) can be expressed at low pH (corresponds to intracellular content), thereby defining pathogenic yersiniae as facultative intracellular parasites. Three human Yersinia genus pathogens differ by ability to produce adhesins. Y. pseudotuberculosis adherence to host cells or extracellular matrix components is determined by a cumulative adhesion-based activity, which expression depends on chemical composition and physicochemical environmental conditions. It’s proposed that at the initial stage of infectious process adherence of Y. pseudotuberculosis to intestinal epithelium is mediated by InvA protein and “smooth” LPS form. These adhesins are produced in bacterial cells at low (lower than 30°С) temperature occurring in environment from which a pathogen invades into the host. At later stages of pathogenesis, after penetrating through intestinal epithelium, bacterial cells produce other adhesins, which promote survival and dissemination primarily into the mesenteric lymph nodes and, possibly, liver and spleen. At later stages of pathogenesis, after penetrating through intestinal epithelium, bacterial cells produce other adhesins, which promote survival and dissemination primarily into the mesenteric lymph nodes and, perhaps, liver and spleen. Qualitative and quantitative spectrum of Y. pseudotuberculosis adhesins is determined by environmental parameters (intercellular space, intracellular content within the diverse eukaryotic cells).

Characterization of Lipid-Protein Interactions and Lipid-Mediated Modulation of Membrane Protein Function through Molecular Simulation

The cellular membrane constitutes one of the most fundamental compartments of a living cell, where key processes such as selective transport of material and exchange of information between the cell and its environment are mediated by proteins that are closely associated with the membrane. The heterogeneity of lipid composition of biological membranes and the effect of lipid molecules on the structure, dynamics, and function of membrane proteins are now widely recognized. Characterization of these functionally important lipid-protein interactions with experimental techniques is however still prohibitively challenging. Molecular dynamics (MD) simulations offer a powerful complementary approach with sufficient temporal and spatial resolutions to gain atomic-level structural information and energetics on lipid-protein interactions. In this review, we aim to provide a broad survey of MD simulations focusing on exploring lipid-protein interactions and characterizing lipid-modulated protein structure and dynamics that have been successful in providing novel insight into the mechanism of membrane protein function.

Disaccharide-Based Anionic Amphiphiles as Potent Inhibitors of Lipopolysaccharide-Induced Inflammation

Despite significant advances made in the last decade in the understanding of molecular mechanisms of sepsis and in the development of clinically relevant therapies, sepsis remains the leading cause of mortality in intensive care units with increasing incidence worldwide. Toll-like receptor 4 (TLR4)-a transmembrane pattern-recognition receptor responsible for propagating the immediate immune response to Gram-negative bacterial infection-plays a central role in the pathogenesis of sepsis and chronic inflammation-related disorders. TLR4 is complexed with the lipopolysaccharide (LPS)-sensing protein myeloid differentiation-2 (MD-2) which represents a preferred target for establishing new anti-inflammatory treatment strategies. Herein we report the development, facile synthesis, and biological evaluation of novel disaccharide-based TLR4⋅MD-2 antagonists with potent anti-endotoxic activity at micromolar concentrations. A series of synthetic anionic glycolipids entailing amide-linked β-ketoacyl lipid residues was prepared in a straightforward manner by using a single orthogonally protected nonreducing diglucosamine scaffold. Suppression of the LPS-induced release of interleukin-6 and tumor necrosis factor was monitored and confirmed in human immune cells (MNC and THP1) and mouse macrophages. Structure-activity relationship studies and molecular dynamics simulations revealed the structural basis for the high-affinity interaction between anionic glycolipids and MD-2, and highlighted two compounds as leads for the development of potential anti-inflammatory therapeutics.

Differential interactions of bacterial lipopolysaccharides with lipid membranes: implications for TRPA1-mediated chemosensation

Bacterial lipopolysaccharides (LPS) activate the TRPA1 cation channels in sensory neurons, leading to acute pain and inflammation in mice and to aversive behaviors in fruit flies. However, the precise mechanisms underlying this effect remain elusive. Here we assessed the hypothesis that TRPA1 is activated by mechanical perturbations induced upon LPS insertion in the plasma membrane. We asked whether the effects of different LPS on TRPA1 relate to their ability to induce mechanical alterations in artificial and cellular membranes. We found that LPS from E. coli, but not from S. minnesota, activates TRPA1. We then assessed the effects of these LPS on lipid membranes using dyes whose fluorescence properties change upon alteration of the local lipid environment. E. coli LPS was more effective than S. minnesota LPS in shifting Laurdan’s emission spectrum towards lower wavelengths, increasing the fluorescence anisotropy of diphenylhexatriene and reducing the fluorescence intensity of merocyanine 540. These data indicate that E. coli LPS induces stronger changes in the local lipid environment than S. minnesota LPS, paralleling its distinct ability to activate TRPA1. Our findings indicate that LPS activate TRPA1 by producing mechanical perturbations in the plasma membrane and suggest that TRPA1-mediated chemosensation may result from primary mechanosensory mechanisms.

Synthetic glycan-based TLR4 agonists targeting caspase-4/11 for the development of adjuvants and immunotherapeutics

The skewed molecular shape of the rigid α,α-(1↔1′)-linked disaccharide core of novel synthetic anionic glycan-based immunostimulants is accountable for potent and adjustable TLR4-mediated signaling which is dissociable from the induction of caspase-11 protease activity.

Gram-negative bacterial lipopolysaccharide (LPS)-induced Toll-like receptor 4 (TLR4) mediated pro-inflammatory signaling plays a key role in immunoprotection against infectious challenges and boosts adaptive immunity, whereas the activation of the cytosolic LPS receptor caspase-4/11 leads to cell death by pyroptosis and is deeply implicated in the development of sepsis. Despite tremendous advances in the understanding of the LPS–TLR4 interaction, predictably regulated TLR4 activation has not yet been achieved. The structural basis for the induction of caspase-4/11 protease activity by LPS is currently unknown. The modulation of innate and adaptive immune responses through the controlled induction of TLR4 signaling without triggering caspase-4/11 activity would open novel perspectives in the development of safe vaccine adjuvants and immunotherapeutics. We report the discovery of highly potent glycan-based immunostimulants with picomolar affinity for TLR4 which interact with caspase-4/11 and promote caspase-4/11 oligomerization while abolishing caspase-11 protease activity. The rigidity and twisted molecular shape of the α,α-(1↔1′)-linked disaccharide core of synthetic LPS mimicking anionic glycolipids accounted for both species-independent and adjustable TLR4-mediated NF-κB signaling and the modulation of caspase-4/11 activation. By the use of crystal structure based design and advanced synthetic chemistry we created a set of versatile probes for studying the structural basis of caspase-4/11 activation and established a chemical strategy for controllable TLR4 mediated cytokine release which is dissociable from the induction of caspase-11 protease activity.

Recent progress in the discovery of myeloid differentiation 2 (MD2) modulators for inflammatory diseases

Myeloid differentiation protein 2 (MD2), together with Toll-like receptor 4 (TLR4), binds lipopolysaccharide (LPS) with high affinity, inducing the formation of the activated homodimer LPS-MD2-TLR4. MD2 directly recognizes the Lipid A domain of LPS, leading to the activation of downstream signaling of cytokine and chemokine production, and initiation of inflammatory and immune responses. However, excessive activation and potent host responses generate severe inflammatory syndromes such as acute sepsis and septic shock. MD2 is increasingly being considered as an attractive pharmacological target for the development of potent anti-inflammatory agents. In this Keynote review, we provide a comprehensive overview of the recent advances in the structure and biology of MD2, and present MD2 modulators as promising agents for anti-inflammatory intervention.

Aminosugar-based immunomodulator lipid A: synthetic approaches

The immediate immune response to infection by Gram-negative bacteria depends on the structure of a lipopolysaccharide (LPS, also known as endotoxin), a complex glycolipid constituting the outer leaflet of the bacterial outer membrane. Recognition of picomolar quantities of pathogenic LPS by the germ-line encoded Toll-like Receptor 4 (TLR4) complex triggers the intracellular pro-inflammatory signaling cascade leading to the expression of cytokines, chemokines, prostaglandins and reactive oxygen species which manifest an acute inflammatory response to infection. The "endotoxic principle" of LPS resides in its amphiphilic membrane-bound fragment glycophospholipid lipid A which directly binds to the TLR4·MD-2 receptor complex. The lipid A content of LPS comprises a complex mixture of structural homologs varying in the acylation pattern, the length of the (R)-3-hydroxyacyl- and (R)-3-acyloxyacyl long-chain residues and in the phosphorylation status of the β(1→6)-linked diglucosamine backbone. The structural heterogeneity of the lipid A isolates obtained from bacterial cultures as well as possible contamination with other pro-inflammatory bacterial components makes it difficult to obtain unambiguous immunobiological data correlating specific structural features of lipid A with its endotoxic activity. Advanced understanding of the therapeutic significance of the TLR4-mediated modulation of the innate immune signaling and the central role of lipid A in the recognition of LPS by the innate immune system has led to a demand for well-defined materials for biological studies. Since effective synthetic chemistry is a prerequisite for the availability of homogeneous structurally distinct lipid A, the development of divergent and reproducible approaches for the synthesis of various types of lipid A has become a subject of considerable importance. This review focuses on recent advances in synthetic methodologies toward LPS substructures comprising lipid A and describes the synthesis and immunobiological properties of representative lipid A variants corresponding to different bacterial species. The main criteria for the choice of orthogonal protecting groups for hydroxyl and amino functions of synthetically assembled β(1→6)-linked diglucosamine backbone of lipid A which allows for a stepwise introduction of multiple functional groups into the molecule are discussed. Thorough consideration is also given to the synthesis of 1,1'-glycosyl phosphodiesters comprising partial structures of 4-amino-4-deoxy-β-L-arabinose modified Burkholderia lipid A and galactosamine-modified Francisella lipid A. Particular emphasis is put on the stereoselective construction of binary glycosyl phosphodiester fragments connecting the anomeric centers of two aminosugars as well as on the advanced P(III)-phosphorus chemistry behind the assembly of zwitterionic double glycosyl phosphodiesters.

Structure-Activity Relationship of Plesiomonas shigelloides Lipid A to the Production of TNF-α, IL-1β, and IL-6 by Human and Murine Macrophages

Plesiomonas shigelloides is a Gram-negative bacterium that is associated with diarrheal disease in humans. Lipopolysaccharide (LPS) is the main surface antigen and virulence factor of this bacterium. The lipid A (LA) moiety of LPS is the main region recognized by target cells of immune system. Here, we evaluated the biological activities of P. shigelloides LA for their abilities to induce the productions of proinflammatory cytokines (TNF-α, IL-1β, and IL-6) by human and murine macrophages [THP-1 macrophages and immortalized murine bone marrow-derived macrophages (iBMDM)]. Four native P. shigelloides LA preparations differing in their phosphoethanolamine (PEtn) substitution, length, number, and saturation of fatty acids were compared with Escherichia coli O55 LA. The bisphosphorylated, hexaacylated, and asymmetric forms of the P. shigelloides and E. coli LA molecules had similar activities in human and murine macrophages, indicating that shortening of the acyl chains in P. shigelloides LA had no effect on its in vitro activities. The PEtn decoration also had no impact on the interaction with the toll-like receptor 4/MD-2 receptor complex. The heptaacylated form of P. shigelloides LA decorated with 16:0 exhibited strong effect on proinflammatory activity, significantly decreasing the levels of all tested cytokines in both murine and human macrophages. Our results revealed that despite the presence of shorter acyl chains and an unsaturated acyl residue (16:1), the bisphosphorylated, hexaacylated, and asymmetric forms of P. shigelloides LA represent highly immunostimulatory structures.

The Role of Carbohydrates in the Lipopolysaccharide (LPS)/Toll-Like Receptor 4 (TLR4) Signalling

The interactions between sugar-containing molecules from the bacteria cell wall and pattern recognition receptors (PRR) on the plasma membrane or cytosol of specialized host cells are the first molecular events required for the activation of higher animal's immune response and inflammation. This review focuses on the role of carbohydrates of bacterial endotoxin (lipopolysaccharide, LPS, lipooligosaccharide, LOS, and lipid A), in the interaction with the host Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) complex. The lipid chains and the phosphorylated disaccharide core of lipid A moiety are responsible for the TLR4 agonist action of LPS, and the specific interaction between MD-2, TLR4, and lipid A are key to the formation of the activated complex (TLR4/MD-2/LPS)₂, which starts intracellular signalling leading to nuclear factors activation and to production of inflammatory cytokines. Subtle chemical variations in the lipid and sugar parts of lipid A cause dramatic changes in endotoxin activity and are also responsible for the switch from TLR4 agonism to antagonism. While the lipid A pharmacophore has been studied in detail and its structure-activity relationship is known, the contribution of core saccharides 3-deoxy-d-manno-octulosonic acid (Kdo) and heptosyl-2-keto-3-deoxy-octulosonate (Hep) to TLR4/MD-2 binding and activation by LPS and LOS has been investigated less extensively. This review focuses on the role of lipid A, but also of Kdo and Hep sugars in LPS/TLR4 signalling.

Species and mediator specific TLR4 antagonism in primary human and murine immune cells by βGlcN(1↔1)αGlc based lipid A mimetics

Immune stimulatory pathogen associated molecular patterns (PAMPs) are major drivers of infection pathology. Infections with Gram-negative bacteria or negatively polar and single stranded RNA influenza virus are prominent causes of morbidity and mortality. Toll-like receptor (TLR) 4 is a major host sensor for both of the two infections. In order to inhibit TLR4 driven immune activation we recently developed synthetic tetra-acylated lipid A mimetics based on a conformationally restricted βGlcN(1↔1)αGlcN disaccharide scaffold (DA-compounds) that antagonized ectopically overexpressed human and murine TLR4/MD-2 complexes. Here we comparatively analyzed human peripheral blood mononuclear cell (hPBMC) and murine bone marrow derived macrophage (mBM) activation upon 30 min of preincubation in vitro with six variably acylated DA-compounds. 16 h subsequent to consequent LPS challenge, we sampled culture supernatants for cytokine and NO concentration analysis. Four compounds significantly inhibited release of both TNF and IL-6 by hPBMCs upon LPS challenge. In contrast, three compounds effectively inhibited mBM production of MIP-2 and KC, and even five of them inhibited IL-6 and NO production. LPS driven like other TLR ligand driven mBM TNF release was largely unimpaired. The inhibitory effect was specific in that Clo75 driven cytokine release by both hPBMCs and mBMs was unimpaired by the compounds analyzed. Our results indicate biological species specificity of LPS antagonism by variably tetraacylated lipid A mimetics and validate three out of six DA-antagonists as promising candidates for development of therapeutically applicable anti-inflammatory compounds.

Modulating LPS signal transduction at the LPS receptor complex with synthetic Lipid A analogues

Sepsis, defined as a clinical syndrome brought about by an amplified and dysregulated inflammatory response to infections, is one of the leading causes of death worldwide. Despite persistent attempts to develop treatment strategies to manage sepsis in the clinical setting, the basic elements of treatment have not changed since the 1960s. As such, the development of effective therapies for reducing inflammatory reactions and end-organ dysfunction in critically ill patients with sepsis remains a global priority. Advances in understanding of the immune response to sepsis provide the opportunity to develop more effective pharmaceuticals. This article details current information on the modulation of the lipopolysaccharide (LPS) receptor complex with synthetic Lipid A mimetics. As the initial and most critical event in sepsis pathophysiology, the LPS receptor provides an attractive target for antisepsis agents. One of the well-studied approaches to sepsis therapy involves the use of derivatives of Lipid A, the membrane-anchor portion of an LPS, which is largely responsible for its endotoxic activity. This article describes the structural and conformational requirements influencing the ability of Lipid A analogues to compete with LPS for binding to the LPS receptor complex and to inhibit the induction of the signal transduction pathway by impairing LPS-initiated receptor dimerization.

Synthesis of anti-inflammatory α-and β-linked acetamidopyranosides as inhibitors of toll-like receptor 4 (TLR4)

The low-molecular weight isopropyl 2-acetamido-α-glucoside 16 (C34) inhibits toll-like receptor 4 (TLR4) in enterocytes and macrophages in vitro, and reduces systemic inflammation in mouse models of endotoxemia and necrotizing enterocolitis. We used a copper(II)-mediated solvolysis of anomeric oxazolines and an acid-mediated conversion of β-glucosamine and β-galactosamine pentaacetates to generate analogs of 16 at the anomeric carbon and at C-4 of the pyranose ring. These compounds were evaluated for their influence on TLR4-mediated inflammatory signaling in cultured enterocytes and monocytes. Their efficacy was confirmed using a NF-kB-luciferase reporter mouse, thus establishing the first structure-activity relationship (SAR) study in this series and identifying the more efficacious isopropyl 2-acetamido-α-galactoside 17.

Insights into the species-specific TLR4 signaling mechanism in response to Rhodobacter sphaeroides lipid A detection

TLR4 in complex with MD2 senses the presence of lipid A (LA) and initiates a signaling cascade that curb the infection. This complex is evolutionarily conserved and can initiate the immune system in response to a variety of LAs. In this study, molecular dynamics simulation (25 ns) was performed to elucidate the differential behavior of TLR4/MD2 complex in response to Rhodobacter sphaeroides lipid A (RsLA). Penta-acyl chain-containing RsLA is at the verge of agonist (6 acyl-chains) and antagonist (4 acyl-chains) structure, and activates the TLR4 pathway in horses and hamsters, while inhibiting in humans and murine. In the time-evolved coordinates, the promising factors that dictated the differential response included the local and global mobility pattern of complexes, solvent-accessible surface area of ligand, and surface charge distributions of TLR4 and MD2. We showed that the GlcN1-GlcN2 backbone acquires agonist (3FXI)-like configurations in horses and hamsters, while acquiring antagonist (2E59)-like configurations in humans and murine systems. Moreover, analysis of F126 behavior in the MD2 F126 loop (amino acids 123-129) and loop EF (81-89) suggested that certain sequence variations also contribute to species-specific response. This study underlines the TLR4 signaling mechanism and provides new therapeutic opportunities.

Anti-endotoxic activity and structural basis for human MD-2·TLR4 antagonism of tetraacylated lipid A mimetics based on βGlcN(1↔1)αGlcN scaffold

Interfering with LPS binding by the co-receptor protein myeloid differentiation factor 2 (MD-2) represents a useful approach for down-regulation of MD-2·TLR4-mediated innate immune signaling, which is implicated in the pathogenesis of a variety of human diseases, including sepsis syndrome. The antagonistic activity of a series of novel synthetic tetraacylated bis-phosphorylated glycolipids based on the βGlcN(1↔1)αGlcN scaffold was assessed in human monocytic macrophage-like cell line THP-1, dendritic cells and human epithelial cells. Two compounds were shown to inhibit efficiently the LPS-induced inflammatory signaling by down-regulation of the expression of TNF-α, IL-6, IL-8, IL-10 and IL-12 to background levels. The binding of the tetraacylated by (R)-3-hydroxy-fatty acids (2 × C_12, 2 × C₁₄), 4,4′-bisphosphorylated βGlcN(1↔1)αGlcN-based lipid A mimetic DA193 to human MD-2 was calculated to be 20-fold stronger than that of Escherichia coli lipid A. Potent antagonistic activity was related to a specific molecular shape induced by the β,α(1↔1)-diglucosamine backbone. ‘Co-planar’ relative arrangement of the GlcN rings was inflicted by the double exo-anomeric conformation around both glycosidic torsions in the rigid β,α(1↔1) linkage, which was ascertained using NOESY NMR experiments and confirmed by molecular dynamics simulation. In contrast to the native lipid A ligands, the binding affinity of βGlcN(1↔1)αGlcN-based lipid A mimetics to human MD-2 was independent on the orientation of the diglucosamine backbone of the synthetic antagonist within the binding pocket of hMD-2 (rotation by 180°) allowing for two equally efficient binding modes as shown by molecular dynamics simulation.

Chemistry of lipid A: at the heart of innate immunity

In many Gram-negative bacteria, lipopolysaccharide (LPS) and its lipid A moiety are pivotal for bacterial survival. Depending on its structure, lipid A carries the toxic properties of the LPS and acts as a potent elicitor of the host innate immune system via the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex. It often causes a wide variety of biological effects ranging from a remarkable enhancement of the resistance to the infection to an uncontrolled and massive immune response resulting in sepsis and septic shock. Since the bioactivity of lipid A is strongly influenced by its primary structure, a broad range of chemical syntheses of lipid A derivatives have made an enormous contribution to the characterization of lipid A bioactivity, providing novel pharmacological targets for the development of new biomedical therapies. Here, we describe and discuss the chemical aspects regarding lipid A and its role in innate immunity, from the (bio)synthesis, isolation and characterization to the molecular recognition at the atomic level.

Trehalose- and glucose-derived glycoamphiphiles: small-molecule and nanoparticle Toll-like receptor 4 (TLR4) modulators

An increasing number of pathologies have been linked to Toll-like receptor 4 (TLR4) activation and signaling, therefore new hit and lead compounds targeting this receptor activation process are urgently needed. We report on the synthesis and biological properties of glycolipids based on glucose and trehalose scaffolds which potently inhibit TLR4 activation and signaling in vitro and in vivo. Structure-activity relationship studies on these compounds indicate that the presence of fatty ester chains in the molecule is a primary prerequisite for biological activity and point to facial amphiphilicity as a preferred architecture for TLR4 antagonism. The cationic glycolipids here presented can be considered as new lead compounds for the development of drugs targeting TLR4 activation and signaling in infectious, inflammatory, and autoimmune diseases. Interestingly, the biological activity of the best drug candidate was retained after adsorption at the surface of colloidal gold nanoparticles, broadening the options for clinical development.

Vizantin inhibits endotoxin-mediated immune responses via the TLR 4/MD-2 complex

Vizantin has immunostimulating properties and anticancer activity. In this study, we investigated the molecular mechanism of immune activation by vizantin. THP-1 cells treated with small interfering RNA for TLR-4 abolished vizantin-induced macrophage activation processes such as chemokine release. In addition, compared with wild-type mice, the release of MIP-1β induced by vizantin in vivo was significantly decreased in TLR-4 knockout mice, but not in TLR-2 knockout mice. Vizantin induced the release of IL-8 when HEK293T cells were transiently cotransfected with TLR-4 and MD-2, but not when they were transfected with TLR-4 or MD-2 alone or with TLR-2 or TLR-2/MD-2. A dipyrromethene boron difluoride-conjugated vizantin colocalized with TLR-4/MD-2, but not with TLR-4 or MD-2 alone. A pull-down assay with vizantin-coated magnetic beads showed that vizantin bound to TLR-4/MD-2 in extracts from HEK293T cells expressing both TLR-4 and MD-2. Furthermore, vizantin blocked the LPS-induced release of TNF-α and IL-1β and inhibited death in mice. We also performed in silico docking simulation analysis of vizantin and MD-2 based on the structure of MD-2 complexed with the LPS antagonist E5564; the results suggested that vizantin could bind to the active pocket of MD-2. Our observations show that vizantin specifically binds to the TLR-4/MD-2 complex and that the vizantin receptor is identical to the LPS receptor. We conclude that vizantin could be an effective adjuvant and a therapeutic agent in the treatment of infectious diseases and the endotoxin shock caused by LPS.

Development of αGlcN(1↔1)αMan-based lipid A mimetics as a novel class of potent Toll-like receptor 4 agonists

The endotoxic portion of lipopolysaccharide (LPS), a glycophospholipid Lipid A, initiates the activation of the Toll-like Receptor 4 (TLR4)–myeloid differentiation factor 2 (MD-2) complex, which results in pro-inflammatory immune signaling. To unveil the structural requirements for TLR4·MD-2-specific ligands, we have developed conformationally restricted Lipid A mimetics wherein the flexible βGlcN(1→6)GlcN backbone of Lipid A is exchanged for a rigid trehalose-like αGlcN(1↔1)αMan scaffold resembling the molecular shape of TLR4·MD-2-bound E. coli Lipid A disclosed in the X-ray structure. A convergent synthetic route toward orthogonally protected αGlcN(1↔1)αMan disaccharide has been elaborated. The α,α-(1↔1) linkage was attained by the glycosylation of 2-N-carbamate-protected α-GlcN-lactol with N-phenyl-trifluoroacetimidate of 2-O-methylated mannose. Regioselective acylation with (R)-3-acyloxyacyl fatty acids and successive phosphorylation followed by global deprotection afforded bis- and monophosphorylated hexaacylated Lipid A mimetics. αGlcN(1↔1)αMan-based Lipid A mimetics (α,α-GM-LAM) induced potent activation of NF-κB signaling in hTLR4/hMD-2/CD14-transfected HEK293 cells and robust LPS-like cytokines expression in macrophages and dendritic cells. Thus, restricting the conformational flexibility of Lipid A by fixing the molecular shape of its carbohydrate backbone in the “agonistic” conformation attained by a rigid αGlcN(1↔1)αMan scaffold represents an efficient approach toward powerful and adjustable TLR4 activation.