Kdo₂‐lipid A: structural diversity and impact on immunopharmacology

Abstract

3-deoxy-d-manno-octulosonic acid-lipid A (Kdo2-lipid A) is the essential component of lipopolysaccharide in most Gram-negative bacteria and the minimal structural component to sustain bacterial viability. It serves as the active component of lipopolysaccharide to stimulate potent host immune responses through the complex of Toll-like-receptor 4 (TLR4) and myeloid differentiation protein 2. The entire biosynthetic pathway of Escherichia coli Kdo2-lipid A has been elucidated and the nine enzymes of the pathway are shared by most Gram-negative bacteria, indicating conserved Kdo2-lipid A structure across different species. Yet many bacteria can modify the structure of their Kdo2-lipid A which serves as a strategy to modulate bacterial virulence and adapt to different growth environments as well as to avoid recognition by the mammalian innate immune systems. Key enzymes and receptors involved in Kdo2-lipid A biosynthesis, structural modification and its interaction with the TLR4 pathway represent a clear opportunity for immunopharmacological exploitation. These include the development of novel antibiotics targeting key biosynthetic enzymes and utilization of structurally modified Kdo2-lipid A or correspondingly engineered live bacteria as vaccines and adjuvants. Kdo2-lipid A/TLR4 antagonists can also be applied in anti-inflammatory interventions. This review summarizes recent knowledge on both the fundamental processes of Kdo2-lipid A biosynthesis, structural modification and immune stimulation, and applied research on pharmacological exploitations of these processes for therapeutic development.

Tandem mass spectrometry determination of the putative structure of a heterogeneous mixture of Lipid As isolated from the lipopolysaccharide of the Gram‐negative bacteria Aeromonas liquefaciens SJ‐19a

“…At that point, the lipid A s mixture was successively precipitated from the LPS by hydrolysis with 1% acetic acid for 2 h at 100 °C and purified by chromatography using Sephadex G‐50 and Sephadex G‐15 (Sigma‐Aldrich, St. Louis, MO. USA) …”

“…It is vital to mention that during the acid hydrolysis step, possible chemical degradation occurs, enhancing the risk of losing acid labile functional groups and the production of artifacts, thus resulting in the heterogeneity of lipid A . In addition, this heterogeneity may result from under‐acylation due to incomplete biosynthesis of the LPS . Although lipid A fatty acids usually contain 10 to 16 carbon atom chains, it has been shown that other unusual carbon chains such as C‐20 can exist within some isolates .…”

“…Variation in the lipid A structure increases by changes in acylation, phosphorylation, and glycosylation. These changes can be affected by factors such as pH, Mg 2 + concentration, and the presence of cationic peptides . Nevertheless, the incomplete biosynthesis of lipid A is mainly responsible for directing and regulating the type of fatty acid substitution of the lipid A and it is often the main cause for producing such a heterogeneous mixture in a single preparation.…”

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

“…[12,13] Consequently,d evelopment of approaches for inhibitiono ft he LPS-inducedT LR4 activation hasb een af ocus of academic and pharmaceuticalr esearchi nt he past two decades. [14,15] It was unequivocally shown that down-regulation of TLR4-mediated signaling is beneficial for treatment of chronic and acute inflammation-related conditions including arthritis, [16] asthma, [17] neuroinflammation, [18] viral respiratory infection, [19] influenza [20] and sepsis [12,15,[21][22][23] Quite av arietyo fd ifferent tactics have been suggested for the inhibition of the deleteriousi nnate immune responses induced by LPS, which include deactivation of LPS by cationic antimicrobialp eptides; [24][25][26] intervention with the LPS transfer cascade involving LBP (LPS-binding protein) and CD14 (cluster of differentiation 14) [27][28][29][30] including development of TLR4 neutralizing antibodies. [31,32] Several innovative molecules with no similarity to Lipid Ah ave been proposed as candidates for blocking LPStriggered pro-inflammatory signaling.…”

Structural investigation by tandem mass spectrometry analysis of a heterogeneous mixture of Lipid An isolated from the lipopolysaccharide of Aeromonas hydrophila SJ‐55Ra

“…It provides the anchor that secures the molecule within the membrane, while the polysaccharide component interacts with the external environment, including the defence of the animal or plant host species. The lipid A component is the primary immune‐stimulatory centre of LPS and is considered the most toxic part within the LPS biomolecule …”