Christoph P. Sager

E-selectin ligand complexes adopt an extended high-affinity conformation

E-selectin is a cell-adhesion molecule of the vascular endothelium that promotes essential leukocyte rolling in the early inflammatory response by binding to glycoproteins containing the tetrasaccharide sialyl Lewisx (sLex). Efficient leukocyte recruitment under vascular flow conditions depends on an increased lifetime of E-selectin/ligand complexes under tensile force in a so-called catch-bond binding mode. Co-crystal structures of a representative fragment of the extracellular E-selectin region with sLex and a glycomimetic antagonist thereof reveal an extended E-selectin conformation, which is identified as a high-affinity binding state of E-selectin by molecular dynamics simulations. Small-angle X-ray scattering experiments demonstrate a direct link between ligand binding and E-selectin conformational transition under static conditions in solution. This permits tracing a series of concerted structural changes connecting ligand binding to conformational stretching as the structural basis of E-selectin catch-bond-mediated leukocyte recruitment. The detailed molecular view of the binding site paves the way for the design of a new generation of selectin antagonists. This is of special interest, since their therapeutic potential was recently demonstrated with the pan-selectin antagonists GMI-1070 (Rivipansel).

Biochemical analyses and molecular modeling explain the functional loss of 17β-hydroxysteroid dehydrogenase 3 mutant G133R in three Tunisian patients with 46, XY Disorders of Sex Development

Mutations in the HSD17B3 gene resulting in 17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) deficiency cause 46, XY Disorders of Sex Development (46, XY DSD). Approximately 40 different mutations in HSD17B3 have been reported; only few mutant enzymes have been mechanistically investigated. Here, we report novel compound heterozygous mutations in HSD17B3, composed of the nonsense mutation C206X and the missense mutation G133R, in three Tunisian patients from two non-consanguineous families. Mutants C206X and G133R were constructed by site-directed mutagenesis and expressed in HEK-293 cells. The truncated C206X enzyme, lacking part of the substrate binding pocket, was moderately expressed and completely lost its enzymatic activity. Wild-type 17β-HSD3 and mutant G133R showed comparable expression levels and intracellular localization. The conversion of Δ4-androstene-3,17-dione (androstenedione) to testosterone was almost completely abolished for mutant G133R compared with wild-type 17β-HSD3. To obtain further mechanistic insight, G133 was mutated to alanine, phenylalanine and glutamine. G133Q and G133F were almost completely inactive, whereas G133A displayed about 70% of wild-type activity. Sequence analysis revealed that G133 on 17β-HSD3 is located in a motif highly conserved in 17β-HSDs and other short-chain dehydrogenase/reductase (SDR) enzymes. A homology model of 17β-HSD3 predicted that arginine or any other bulky residue at position 133 causes steric hindrance of cofactor NADPH binding, whereas substrate binding seems to be unaffected. The results indicate an essential role of G133 in the arrangement of the cofactor binding pocket, thus explaining the loss-of-function of 17β-HSD3 mutant G133R in the patients investigated.

Urinary Tract Infection: Which Conformation of the Bacterial Lectin FimH Is Therapeutically Relevant?

Frequent antibiotic treatment of urinary tract infections has resulted in the emergence of antimicrobial resistance, necessitating alternative treatment options. One such approach centers around FimH antagonists that block the bacterial adhesin FimH, which would otherwise mediate binding of uropathogenic Escherichia coli to the host urothelium to trigger the infection. Although the FimH lectin can adopt three distinct conformations, the evaluation of FimH antagonists has mainly been performed with a truncated construct of FimH locked in one particular conformation. For a successful therapeutic application, however, FimH antagonists should be efficacious against all physiologically relevant conformations. Therefore, FimH constructs with the capacity to adopt various conformations were applied. By examining the binding properties of a series of FimH antagonists in terms of binding affinity and thermodynamics, we demonstrate that depending on the FimH construct, affinities may be overestimated by a constant factor of 2 orders of magnitude. In addition, we report several antagonists with excellent affinities for all FimH conformations.

LST-3TM12 is a member of the OATP1B family and a functional transporter

Graphical abstract Figure. No caption available. ABSTRACT Organic anion transporting polypeptides (OATPs) and particularly the two members of the OATP1B family are known for their role in pharmacokinetics. Both SLCO1B3 and SLCO1B1 are located on chromosome 12 encompassing the gene locus SLCO1B7. Hitherto, this particular gene has been assumed to be a pseudogene, even though there are published mRNA sequences linked to this chromosomal area. It was aim of this study to further investigate SLCO1B7 and the associated mRNA LST‐3TM12. In a first step, we aligned all mRNAs linked to the chromosomal region of SLCO1B‐transporters. This in silico analysis revealed that LST‐3TM12 is a product of splicing of SLCO1B3 and SLCO1B7, and encodes for a protein with twelve transmembrane domains. The existence of LST‐3TM12 mRNA was verified by polymerase chain reaction showing liver enriched expression. In addition, immunohistological staining showed that LST‐3TM12 protein was expressed in the endoplasmic reticulum (ER) of hepatocytes. Localization in the ER was further verified by immunoblot analysis showing high amounts of LST‐3TM12 in liver microsomes. Function of LST‐3TM12 was assessed by transport studies after heterologous expression in HeLa cells, where the transporter was shown to be expressed not only in the ER but also in the plasma membrane. Overexpression of LST‐3TM12 was associated with enhanced cellular accumulation of dehydroepiandrosterone sulfate (Vmax 300.2 pmol mg−1 min−1; Km 34.2 &mgr;m) and estradiol 17&bgr;‐glucuronide (Vmax 29.9 mol mg−1 min−1 and Km 32.8 &mgr;M). In conclusion, LST‐3TM12 is a functional splice variant of SLCO1B3 and SLCO1B7 expressed in the ER of human liver.

What contributes to an effective mannose recognition domain

In general, carbohydrate–lectin interactions are characterized by high specificity but also low affinity. The main reason for the low affinities are desolvation costs, due to the numerous hydroxy groups present on the ligand, together with the typically polar surface of the binding sites. Nonetheless, nature has evolved strategies to overcome this hurdle, most prominently in relation to carbohydrate–lectin interactions of the innate immune system but also in bacterial adhesion, a process key for the bacterium’s survival. In an effort to better understand the particular characteristics, which contribute to a successful carbohydrate recognition domain, the mannose-binding sites of six C-type lectins and of three bacterial adhesins were analyzed. One important finding is that the high enthalpic penalties caused by desolvation can only be compensated for by the number and quality of hydrogen bonds formed by each of the polar hydroxy groups engaged in the binding process. In addition, since mammalian mannose-binding sites are in general flat and solvent exposed, the half-lives of carbohydrate–lectin complexes are rather short since water molecules can easily access and displace the ligand from the binding site. In contrast, the bacterial lectin FimH benefits from a deep mannose-binding site, leading to a substantial improvement in the off-rate. Together with both a catch-bond mechanism (i.e., improvement of affinity under shear stress) and multivalency, two methods commonly utilized by pathogens, the affinity of the carbohydrate–FimH interaction can be further improved. Including those just described, the various approaches explored by nature to optimize selectivity and affinity of carbohydrate–lectin interactions offer interesting therapeutic perspectives for the development of carbohydrate-based drugs.

KinITC-One Method Supports both Thermodynamic and Kinetic SARs as Exemplified on FimH Antagonists.

Affinity data, such as dissociation constants (KD ) or inhibitory concentrations (IC50 ), are widely used in drug discovery. However, these parameters describe an equilibrium state, which is often not established in vivo due to pharmacokinetic effects and they are therefore not necessarily sufficient for evaluating drug efficacy. More accurate indicators for pharmacological activity are the kinetics of binding processes, as they shed light on the rate of formation of protein-ligand complexes and their half-life. Nonetheless, although highly desirable for medicinal chemistry programs, studies on structure-kinetic relationships (SKR) are still rare. With the recently introduced analytical tool kinITC this situation may change, since not only thermodynamic but also kinetic information of the binding process can be deduced from isothermal titration calorimetry (ITC) experiments. Using kinITC, ITC data of 29 mannosides binding to the bacterial adhesin FimH were re-analyzed to make their binding kinetics accessible. To validate these kinetic data, surface plasmon resonance (SPR) experiments were conducted. The kinetic analysis by kinITC revealed that the nanomolar affinities of the FimH antagonists arise from both (i) an optimized interaction between protein and ligand in the bound state (reduced off-rate constant koff ) and (ii) a stabilization of the transition state or a destabilization of the unbound state (increased on-rate constant kon ). Based on congeneric ligand modifications and structural input from co-crystal structures, a strong relationship between the formed hydrogen-bond network and koff could be concluded, whereas electrostatic interactions and conformational restrictions upon binding were found to have mainly an impact on kon .

FimH antagonists – solubility vs. permeability

Urinary tract infections (UTIs) caused by uropathogenic Escherichia coli (UPEC) are among the most prevalent infections worldwide. Since frequent antibiotic treatment favors the emergence of antibiotic resistance, efficient non-antibiotic strategies are urgently needed. The first step of the pathogenesis of UTI is the bacterial adherence to urothelial host cells, a process mediated by the mannose-binding adhesin FimH located at the tip of bacterial pili. In a preliminary study, biphenyl α-d-mannopyranosides with an electron-withdrawing carboxylate on the aglycone were identified as potent FimH antagonists. Although passive permeability could be established by masking the carboxylate as an ester, insufficient solubility and fast hydrolysis did not allow to maintain the therapeutic concentration in the bladder for the requested period of time. By modifying the substitution pattern, molecular planarity and symmetry of the biphenyl aglycone could be disrupted leading to improved solubility. In addition, when heteroatoms were introduced to the aglycone, antagonists with further improved solubility, metabolic stability as well as passive permeability were obtained. The best representative, the pyrrolylphenyl mannoside 42f exhibited therapeutic urine concentration for up to 6 h and is therefore a promising oral candidate for UTI prevention and/or treatment.

Drug Discovery Summit: 11th Swiss Course on Medicinal Chemistry

A summit amongst the summits: The 11(th) Swiss Course on Medicinal Chemistry was held in October 2014, again in the scenic setting of the Alps in Leysin, Switzerland. One hundred participants, mostly from industry, experienced a week of expert talks about numerous aspects of drug discovery and medicinal chemistry. In this conference report, we briefly summarize the essential topics of this event, while the most inspiring lectures are described in greater detail.