Lectins as drug targets : functional, structural, and pharmacological insights into E-selectin and FimH

Lectins are carbohydrate-binding proteins found throughout nature in plants, viruses, pro- and eukaryotes. Their carbohydrate specificity as well as their biological functions are highly diverse. Lectins in humans primarily serve as cell-surface receptors and are mainly involved in the immunogenic processes. Bacterial lectins are often involved in conferring pathogenesis by binding to carbohydrates of the host. In this thesis, structural, functional, and pharmacological insights on the mammalian lectin E-selectin and the bacterial lectin FimH are given.
Part E-selectin:
E-selectin is a C-type lectin involved in leukocyte recruitment during inflammation by binding to the tetrasaccharide ligand sialyl Lewisx (sLex). It is involved in numerous diseases, e.g. asthma, psoriasis, stroke, rheumatoid arthritis or cancer metastasis. Targeting E-selectin with glycomimetic antagonists is therefore in the focus of drug discovery. The following aspects of E-selectin were investigated:
-Publication 1: The thermodynamic driving forces of the interaction of E-selectin with sLex and glycomimetics thereof were investigated. We demonstrated that sLex binding is an entropy-driven process, which is an uncommon feature of carbohydrate-lectin interactions.
-Manuscript 1: The co-crystallization of E-selectin with sLex or a more potent glycomimetics thereof revealed a previously unseen induced fit of the binding site involving alterations in the first two domains. We showed that this induced fit occurs in solution and discuss the physiological relevance.
-Manuscript 2: A flexible and a pre-organized E-selectin antagonist were characterized for their kinetic and thermodynamic properties, which revealed an unexpected loss in entropy for the pre-organized antagonist. Co-crystallization with a series of antagonists revealed the reason for this behavior.
-Publication 2: The single nucleotide polymorphism which leads to the S128R mutation in E-selectin has been correlated with an increased risk of developing various diseases. We investigated the binding behavior of this mutant and demonstrated that glycomimetics are efficacious in inhibiting E-selectin-S128R mediated binding.
-Manuscript 3: Mice are able to express the N-glycolyl form of sLex, unlike humans. Therefore, the specificity of murine E-selectin might be altered. We investigated the binding specificity of murine E-selectin and evaluated the potency of antagonists designed for human E-selectin. We confirmed the efficacy of E-selectin antagonists towards murine E-selectin, thus demonstrating the validity of mouse models.
Part FimH:
The bacterial lectin FimH is presented by uropathogenic E. coli (UPEC) on the tip of type 1 pili and mediates the adhesion to mannosylated structure in the lower urinary tract. This interaction allows UPEC to colonize the bladder, the initial step in bladder infection. Mannoside-based FimH antagonists are under investigation as treatment for bladder infections.
-Manuscript 4: The goal of a drug discovery program aimed to develop a treatment for urinary tract infections is to identify high-affinity, orally available, and safe FimH antagonists. Starting from the carboxylate substituted biphenyl alpha-D-mannopyranoside, affinity as well as the relevant pharmacokinetic parameters (solubility, permeability, renal excretion) could be substantially improved by a bioisosteric approach.
-Manuscript 5: To comprehend and further develop potent FimH antagonists, structural data on ligand-protein interaction is essential. In this manuscript we present the X-ray co-crystal structures of FimH with three antagonist classes for which structural data were unavailable to date and provide an explanation for the observed entropy-enthalpy compensation by NMR.
-Manuscript 6: Crystallographic studies of FimH with alkyl- or aryl-substituted alpha-D-mannopyranosides have demonstrated alternative binding poses with differing involvement of the residues Tyr48 and Tyr137 at the binding site entrance. Thermodynamic and molecular modeling analysis provided insights into the importance of the tyrosine-gate.
-Manuscript 7: Several mutations of FimH are found in clinical isolates, which influence the binding phenotype of FimH by altering the interaction of the two FimH domains (lectin- and pilin domain). To date, FimH antagonists have never been tested on clinically relevant FimH variants. We demonstrated that antagonist affinity correlated with the binding behavior of different FimH variants.