Synthesis and immunobiological activity of an original series of acyclic lipid a mimics based on a pseudodipeptide backbone

pH-Dependent physicochemical properties of ornithine lipid in mono- and bilayers

In certain bacteria, phosphatidylethanolamine lipids (PEL) get largely replaced by phosphate-free ornithine lipids (OL) under conditions of phosphate starvation. It has so far been unknown how much these two lipid types deviate in their physicochemical properties, and how strongly bacteria thus have to adapt in order to compensate for the difference. Here, we use differential scanning calorimetry, X-ray scattering, and X-ray fluorescence to investigate the properties of OL with saturated C14 alkyl chains in mono- and bilayers. OL is found to have a greater tendency than chain-analogous PEL to form ordered structures and, in contrast to PEL, even a molecular superlattice based on a hydrogen bonding network between the headgroups. This superlattice is virtually electrically uncharged and persists over a wide pH range. Our results indicate that OL and PEL behave very differently in ordered single-component membranes but may behave more similarly in fluid multicomponent membranes.

Progress in the synthesis and biological evaluation of lipid A and its derivatives

Lipid A is one of the core structures of bacterial lipopolysaccharides (LPSs), and it is mainly responsible for the strong immunostimulatory activities of LPS through interactions with the Toll-like receptors and other molecules in the human immune system. To obtain structurally homogeneous and well-defined lipid As and its derivatives in quantities meaningful for various biological studies and applications, their chemical synthesis has become a focal point. This review has provided a survey of significant progresses made in the synthesis of lipid A, and its derivatives that carry diverse saturated and unsaturated lipids, have the phosphate group at its reducing end replaced with a more stable phosphate or carboxyl group, or lack the reducing end phosphate or both phosphate groups, as well as progresses in the synthesis of LPS analogs and other lipid A conjugates. These synthetic molecules have facilitated the elucidation of the structure-activity relationships of lipid A useful for the design and development of lipid A based therapeutics, such as those utilized to treat sepsis, and other medical applications, for example the use of monophosphoryl lipid A as a carrier molecule for the study of fully synthetic self-adjuvanting conjugate vaccines. These topics are also briefly covered in the current review.

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.

Design and synthesis of nucleolipids as possible activated precursors for oligomer formation via intramolecular catalysis: stability study and supramolecular organization

BACKGROUND

Fatty acid vesicles are an important part of protocell models currently studied. As protocells can be considered as pre-biological precursors of cells, the models try to contribute to a better understanding of the (cellular) origin of life and emphasize on 2 major aspects: compartmentalization and replication. It has been demonstrated that lipid-based membranes are amenable to growth and division (shell replication). Furthermore compartmentalization creates a unique micro-environment in which biomolecules can accumulate and reactions can occur. Pioneering research by Sugawara, Deamer, Luisi, Szostak and Rasmussen gave more insight in obtaining autocatalytic, self-replicating vesicles capable of containing and reproducing nucleic acid sequences (core replication). Linking both core and shell replication is a challenging feat requiring thorough understanding of membrane dynamics and (auto)catalytic systems. A possible solution may lie in a class of compounds called nucleolipids, who combine a nucleoside, nucleotide or nucleobase with a lipophilic moiety. Early contributions by the group of Yanagawa mentions the prebiotic significance (as a primitive helical template) arising from the supramolecular organization of these compounds. Further contributions, exploring the supramolecular scope regarding phospoliponucleosides (e.g. 5'-dioleylphosphatidyl derivatives of adenosine, uridine and cytidine) can be accounted to Baglioni, Luisi and Berti. This emerging field of amphiphiles is being investigated for surface behavior, supramolecular assembly and even drug ability.

RESULTS

A series of α/β-hydroxy fatty acids and α-amino fatty acids, covalently bound to nucleoside-5'-monophosphates via a hydroxyl or amino group on the fatty acid was examined for spontaneous self-assembly in spherical aggregates and their stability towards intramolecular cleavage. Staining the resulting hydrophobic aggregates with BODIPY-dyes followed by fluorescent microscopy gave several distinct images of vesicles varying from small, isolated spheres to higher order aggregates and large, multimicrometer sized particles. Other observations include rod-like vesicle precursors. NMR was used to assess the stability of a representative sample of nucleolipids. 1D ³¹P NMR revealed that β-hydroxy fatty acids containing nucleotides were pH-stable while the α-analogs are acid labile. Degradation products identified by [¹H-³¹P] heteroTOCSY revealed that phosphoesters are cleaved between sugar and phosphate, while phosphoramidates are also cleaved at the lipid-phosphate bond. For the latter compounds, the ratio between both degradation pathways is influenced by the nucleobase moiety. However no oligomerization of nucleotides was observed; nor the formation of 3'-5'-cyclic nucleotides, possible intermediates for oligonucleotide synthesis.

CONCLUSIONS

The nucleolipids with a deoxyribose sugar moiety form small or large vesicles, rod-like structures, vesicle aggregates or large vesicles. Some of these aggregates can be considered as intermediate forms in vesicle formation or division. However, we could not observe nucleotide polymerization or cyclic nucleotide function of these nucleolipids, regardless of the sugar moiety that is investigated (deoxyribose, ribose, xylose). To unravel this observation, the chemical stability of the constructs was studied. While the nucleolipids containing β-hydroxy fatty acids are stable as well in base as in acid circumstances, others degraded in acidic conditions. Phosphoramidate nucleolipids hydrolyzed by P-N as well as P-O bond cleavage where the ratio between both pathways depends on the nucleobase. Diester constructs with an α-hydroxy stearic acid degraded exclusively by hydrolysis of the 5'-O-nucleoside ester bond. As the compounds are too stable and harsh conditions would destruct the material itself, more reactive species such as lipid imidazolates of nucleotides need to be synthesized to further analyze the potential polymerization process. Graphical AbstractVesicle information of a nucleolipid consisting of a nucleoside 5'-monophosphate and a α-hydroxy fatty acid.

Adjuvants containing natural and synthetic Toll-like receptor 4 ligands

The last decade has seen an increased focus on the development of adjuvants for vaccines, and several novel adjuvants are now in licensed products or in late-stage clinical development. These advancements have been aided by the discovery of receptors and signaling pathways of the innate immune system and an increased understanding of how these innate responses influence the adaptive immune response. Successful vaccine development relies on knowledge of which adjuvants to use and the proper formulation of adjuvants and antigens to achieve safe, stable and immunogenic vaccines. In this review, the authors focus on the current use of natural and synthetic lipopolysaccharide analogues that retain their adjuvant properties with reduced toxicity compared with the parent compound for use in emerging vaccines. The authors review how these compounds initiate signal transduction through Toll-like receptor 4, insights from structure-function studies and how formulation parameters can influence their effectiveness as vaccine adjuvants.

Development of vizantin, a safe immunostimulant, based on the structure-activity relationship of trehalose-6,6'-dicorynomycolate

Vizantin, 6,6'-bis-O-(3-nonyldodecanoyl)-α,α'-trehalose, was developed as a safe immunostimulator on the basis of a structure-activity relationship (SAR) study with trehalose 6,6'-dicorynomycolate (TDCM). It was possible to synthesize vizantin on a large scale more easily than in the case of TDCM, and the compound exhibited more potent prophylactic effect on experimental lung metastasis of B16-F0 melanoma cells. Because vizantin stimulated human macrophages, it is a promising candidate for clinical application.

Toll-like receptor activation of human cells by synthetic triacylated lipid A-like molecules

Recognition of microbial molecules by mammalian host receptors is essential to mount an immune response. Hexaacylated LPS is the prototypic example of a bacterial molecule recognized by the receptor complex TLR4/MD-2 with its lipid A moiety, whereas bacterial lipopeptides are recognized by TLR2. Here we show that a series of synthetic triacylated lipid A-like molecules are weak Toll-like receptor (TLR) agonists (mainly TLR2 agonists) but very potent TLR4/MD-2 antagonists (submicromolar range). Not only do they block human cell responses to LPS but also to whole gram-negative bacteria, and they inhibit the phagocytosis of gram-negative bacteria. These compounds may represent promising immunomodulatory agents.

Enatiomerically pure hydroxycarboxylic acids: current approaches and future perspectives

The growing awareness of the importance of chirality in conjunction with biological activity has led to an increasing demand for efficient methods for the industrial synthesis of enantiomerically pure compounds. Polyhydroxyalkanotes (PHAs) are a family of polyesters consisting of over 140 chiral R-hydroxycarboxylic acids (R-HAs), representing a promising source for obtaining chiral chemicals from renewable carbon sources. Although some R-HAs have been produced for some time and certain knowledge of the production processes has been gained, large-scale production has not yet been possible. In this article, through analysis of the current advances in production of these acids, we present guidelines for future developments in biotechnological processes for R-HA production.

99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: immune therapies of type 1 diabetes: new opportunities based on the hygiene hypothesis

Insulin-dependent (type 1) diabetes is a prototypic organ-specific autoimmune disease resulting from the selective destruction of insulin-secreting beta cells within pancreatic islets of Langerhans by an immune-mediated inflammation involving autoreactive CD4(+) and CD8(+) T lymphocytes which infiltrate pancreatic islets. Current treatment is substitutive, i.e. chronic use of exogenous insulin which, in spite of significant advances, is still associated with major constraints (multiple daily injections, risks of hypoglycaemia) and lack of effectiveness over the long term in preventing severe degenerative complications. Finding a cure for autoimmune diabetes by establishing effective immune-based therapies is a real medical health challenge, as the disease incidence increases steadily in industrialized countries. As the disease affects mainly children and young adults, any candidate immune therapy must therefore be safe and avoid a sustained depression of immune responses with the attendant problems of recurrent infection and drug toxicity. Thus, inducing or restoring immune tolerance to target autoantigens, controlling the pathogenic response while preserving the host reactivity to exogenous/unrelated antigens, appears to be the ideal approach. Our objective is to review the major progress accomplished over the last 20 years towards that aim. In addition, we would like to present another interesting possibility to access new preventive strategies based on the 'hygiene hypothesis', which proposes a causal link between the increasing incidence of autoimmune diseases, including diabetes, and the decrease of the infectious burden. The underlying rationale is to identify microbial-derived compounds mediating the protective activity of infections which could be developed therapeutically.

Synthetic and natural TLR4 agonists as safe and effective vaccine adjuvants

Natural derivatives and synthetic analogues of lipopolysaccharide are potent stimulators of the mammalian immune system. Retained adjuvant activity with reduced toxicity was obtained by the development of monophosphoryl lipid A (MPL((R))), which is approved for use in several vaccine products. Ongoing research and development of synthetic TLR4 agonists may offer increased purity and biological activity with reduced cost. Extensive research has elucidated the mechanism of action of TLR4 agonists and structure-function relationships. Moreover, the formulation of TLR4 agonists has been shown to significantly affect the type and magnitude of elicited immune response. TLR4 agonists comprise a promising class of adjuvants for safe and effective vaccines.

Glycolipids and benzylammonium lipids as novel antisepsis agents: synthesis and biological characterization

New glycolipids and a benzylammonium lipid were rationally designed by varying the chemical structure of a D-glucose-derived hit compound active as lipid A antagonist. We report the synthesis of these compounds, their in vitro activity as lipid A antagonists on HEK cells, and the capacity to inhibit LPS-induced septic shock in vivo. The lack of toxicity and the good in vivo activity suggest the use of some compounds of the panel as hits for antisepsis drug development.

Lipid Core Peptide System for Gene, Drug, and Vaccine Delivery

A vast number of biologically active compounds await efficient delivery to become therapeutic agents. Lipidation has been demonstrated to be a convenient and useful approach to improve the stability and transport across biological membranes of potential drug molecules. The lipid core peptide (LCP) system has emerged as a promising lipidation tool because of its versatile features. This review discusses the progress in the development of the LCP system to improve cell permeability of nucleotides, physicochemical properties of potential drugs, and vaccine immunogenicity. Emphasis was put on the application of the LCP system to deliver antigens for the prevention of group A streptococcus infection, novel techniques of conjugation of target molecules to the LCP, and new alterations of the LCP system itself.

Agonistic and antagonistic properties of a Rhizobium sin-1 lipid A modified by an ether-linked lipid

LPS from Rhizobium sin-1 (R. sin-1) can antagonize the production of tumor necrosis factor alpha (TNF-alpha) by E. coli LPS in human monocytic cells. Therefore these compounds provide interesting leads for the development of therapeutics for the prevention or treatment of septic shock. Detailed structure activity relationship studies have, however, been hampered by the propensity of these compounds to undergo beta-elimination to give biological inactive enone derivatives. To address this problem, we have chemically synthesized in a convergent manner a R. sin-1 lipid A derivative in which the beta-hydroxy ester at C-3 of the proximal sugar unit has been replaced by an ether linked moiety. As expected, this derivative exhibited a much-improved chemical stability. Furthermore, its ability to antagonize TNF-alpha production induced by enteric LPS was only slightly smaller than that of the parent ester modified derivative demonstrating that the ether-linked lipids affect biological activities only marginally. Furthermore, it has been shown for the first time that R. sin-1 LPS and the ether modified lipid A are also able to antagonize the production of the cytokine interferon-inducible protein 10, which arises from the TRIF-dependent pathway. The latter pathway was somewhat more potently inhibited than the MyD88-dependent pathway. Furthermore, it was observed that the natural LPS possesses much greater activity than the synthetic and isolated lipid As, which indicates that di-KDO moiety is important for optimal biological activity. It has also been found that isolated R. sin-1 LPS and lipid A agonize a mouse macrophage cell line to induce the production of TNF-alpha and interferon beta in a Toll-like receptor 4-dependent manner demonstrating species specific properties.