Impact of Single Acyclic Phosphonate Nucleotide (ZNA) Modifications on DNA Duplex Stability.

An acyclic phosphonate-linked nucleic acid backbone (ZNA) demonstrated the capability to support duplex formation and propagate genetic information in vivo, unveiling its potential for evolution into a synthetic genetic system (XNA). To determine the structural impact of such modification, modified Dickerson Drew DNA dodecamers (DDDs) were prepared by solid phase synthesis, each containing either an (R) or (S) isomeric form of a cytosine ZNA nucleotide. While the DDD is known to adopt a stable duplex, both duplex and hairpin forms were simultaneously observed for both modified oligonucleotides by NMR spectroscopy over a broad temperature range (5-65 °C). Diffusion-ordered spectroscopy (DOSY) experiments allowed to separate duplex and hairpin signals based on the different diffusion constants of both conformational states. For the oligomer containing (R)-ZNA, only the duplex form occurred at 5 °C, while it was not possible to determine by NMR a single hairpin conformation at higher temperatures. In the case of the (S)-ZNA nucleoside modified oligomer, both hairpin and duplex forms were observable at 0 °C, while a single hairpin conformation was detected at 37 °C, suggesting a higher destabilizing effect on dsDNA.

Structural insights into the morpholino nucleic acid/RNA duplex using the new XNA builder Ducque in a molecular modeling pipeline.

The field of synthetic nucleic acids with novel backbone structures [xenobiotic nucleic acids (XNAs)] has flourished due to the increased importance of XNA antisense oligonucleotides and aptamers in medicine, as well as the development of XNA processing enzymes and new XNA genetic materials. Molecular modeling on XNA structures can accelerate rational design in the field of XNAs as it contributes in understanding and predicting how changes in the sugar-phosphate backbone impact on the complementation properties of the nucleic acids. To support the development of novel XNA polymers, we present a first-in-class open-source program (Ducque) to build duplexes of nucleic acid analogs with customizable chemistry. A detailed procedure is described to extend the Ducque library with new user-defined XNA fragments using quantum mechanics (QM) and to generate QM-based force field parameters for molecular dynamics simulations within standard packages such as AMBER. The tool was used within a molecular modeling workflow to accurately reproduce a selection of experimental structures for nucleic acid duplexes with ribose-based as well as non-ribose-based nucleosides. Additionally, it was challenged to build duplexes of morpholino nucleic acids bound to complementary RNA sequences.

Characterization of an HNA aptamer suggests a non-canonical G-quadruplex motif.

Nucleic acids not only form the basis of heredity, but are increasingly a source of novel nano-structures, -devices and drugs. This has spurred the development of chemically modified alternatives (xeno nucleic acids (XNAs)) comprising chemical configurations not found in nature to extend their chemical and functional scope. XNAs can be evolved into ligands (XNA aptamers) that bind their targets with high affinity and specificity. However, detailed investigations into structural and functional aspects of XNA aptamers have been limited. Here we describe a detailed structure-function analysis of LYS-S8-19, a 1',5'-anhydrohexitol nucleic acid (HNA) aptamer to hen egg-white lysozyme (HEL). Mapping of the aptamer interaction interface with its cognate HEL target antigen revealed interaction epitopes, affinities, kinetics and hot-spots of binding energy similar to protein ligands such as anti-HEL-nanobodies. Truncation analysis and molecular dynamics (MD) simulations suggest that the HNA aptamer core motif folds into a novel and not previously observed HNA tertiary structure, comprising non-canonical hT-hA-hT/hT-hT-hT triplet and hG4-quadruplex structures, consistent with its recognition by two different G4-specific antibodies.

Polyketide Synthase-Mediated O-Methyloxime Formation in the Biosynthesis of the Oximidine Anticancer Agents.

Bacterial trans-acyltransferase polyketide synthases (trans-AT PKSs) are modular megaenzymes that employ unusual catalytic domains to assemble diverse bioactive natural products. One such PKS is responsible for the biosynthesis of the oximidine anticancer agents, oxime-substituted benzolactone enamides that inhibit vacuolar H+ -ATPases. Here, we describe the identification of the oximidine gene cluster in Pseudomonas baetica and the characterization of four novel oximidine variants, including a structurally simpler intermediate that retains potent anticancer activity. Using a combination of in vivo, in vitro and computational approaches, we experimentally elucidate the oximidine biosynthetic pathway and reveal an unprecedented mechanism for O-methyloxime formation. We show that this process involves a specialized monooxygenase and methyltransferase domain and provide insight into their activity, mechanism and specificity. Our findings expand the catalytic capabilities of trans-AT PKSs and identify potential strategies for the production of novel oximidine analogues.

Synthesis, antiviral activity, and computational study of ß-d-xylofuranosyl nucleoside phosphonates.

Molecular dynamics (MD) simulations provided insights into the favorable interactions between xylose nucleosides bearing a phosphonate moiety at their 3'-position and specific residues at the active site of the archetypal RNA-dependent RNA-polymerase (RdRp) of Enterovirus 71. Therefore, a series of xylosyl nucleoside phosphonates with adenine, uracil, cytosine, guanosine, and hypoxanthine as nucleobases were synthesized through multistep sequences starting from a single common precursor. Following antiviral activity evaluation, the adenine containing analogue was found to possess good antiviral activity against RNA viruses displaying an EC50 of 12 and 16 µM against measles virus (MeV) and enterovirus-68 (EV-68), respectively, whereas lacking cytotoxicity.

Common variable immunodeficiency in two kindreds with heterogeneous phenotypes caused by novel heterozygous NFKB1 mutations.

NFKB1 haploinsufficiengcy was first described in 2015 in three families with common variable immunodeficiency (CVID), presenting heterogeneously with symptoms of increased infectious susceptibility, skin lesions, malignant lymphoproliferation and autoimmunity. The described mutations all led to a rapid degradation of the mutant protein, resulting in a p50 haploinsufficient state. Since then, more than 50 other mutations have been reported, located throughout different domains of NFKB1 with the majority situated in the N-terminal Rel homology domain (RHD). The clinical spectrum has also expanded with possible disease manifestations in almost any organ system. In silico prediction tools are often used to estimate the pathogenicity of NFKB1 variants but to prove causality between disease and genetic findings, further downstream functional validation is required. In this report, we studied 2 families with CVID and two novel variants in NFKB1 (c.1638-2A>G and c.787G>C). Both mutations affected mRNA and/or protein expression of NFKB1 and resulted in excessive NLRP3 inflammasome activation in patient macrophages and upregulated interferon stimulated gene expression. Protein-protein interaction analysis demonstrated a loss of interaction with NFKB1 interaction partners for the p.V263L mutation. In conclusion, we proved pathogenicity of two novel variants in NFKB1 in two families with CVID characterized by variable and incomplete penetrance.

Synthesis, Structure-Activity Relationships, and Antiviral Profiling of 1-Heteroaryl-2-Alkoxyphenyl Analogs as Inhibitors of SARS-CoV-2 Replication.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, has led to a pandemic, that continues to be a huge public health burden. Despite the availability of vaccines, there is still a need for small-molecule antiviral drugs. In an effort to identify novel and drug-like hit matter that can be used for subsequent hit-to-lead optimization campaigns, we conducted a high-throughput screening of a 160 K compound library against SARS-CoV-2, yielding a 1-heteroaryl-2-alkoxyphenyl analog as a promising hit. Antiviral profiling revealed this compound was active against various beta-coronaviruses and preliminary mode-of-action experiments demonstrated that it interfered with viral entry. A systematic structure-activity relationship (SAR) study demonstrated that a 3- or 4-pyridyl moiety on the oxadiazole moiety is optimal, whereas the oxadiazole can be replaced by various other heteroaromatic cycles. In addition, the alkoxy group tolerates some structural diversity.

The ß2-adrenergic receptor in the apical membrane of intestinal enterocytes senses sugars to stimulate glucose uptake from the gut.

The presence of sugar in the gut causes induction of SGLT1, the sodium/glucose cotransporter in intestinal epithelial cells (enterocytes), and this is accompanied by stimulation of sugar absorption. Sugar sensing was suggested to involve a G-protein coupled receptor and cAMP - protein kinase A signalling, but the sugar receptor has remained unknown. We show strong expression and co-localization with SGLT1 of the ß2-adrenergic receptor (ß 2-AR) at the enterocyte apical membrane and reveal its role in stimulating glucose uptake from the gut by the sodium/glucose-linked transporter, SGLT1. Upon heterologous expression in different reporter systems, the ß 2-AR responds to multiple sugars in the mM range, consistent with estimated gut sugar levels after a meal. Most adrenergic receptor antagonists inhibit sugar signaling, while some differentially inhibit epinephrine and sugar responses. However, sugars did not inhibit binding of I125-cyanopindolol, a ß 2-AR antagonist, to the ligand-binding site in cell-free membrane preparations. This suggests different but interdependent binding sites. Glucose uptake into everted sacs from rat intestine was stimulated by epinephrine and sugars in a ß 2-AR-dependent manner. STD-NMR confirmed direct physical binding of glucose to the ß 2-AR. Oral administration of glucose with a non-bioavailable ß 2-AR antagonist lowered the subsequent increase in blood glucose levels, confirming a role for enterocyte apical ß 2-ARs in stimulating gut glucose uptake, and suggesting enterocyte ß 2-AR as novel drug target in diabetic and obese patients. Future work will have to reveal how glucose sensing by enterocytes and neuroendocrine cells is connected, and whether ß 2-ARs mediate glucose sensing also in other tissues.

Curing Cats with Feline Infectious Peritonitis with an Oral Multi-Component Drug Containing GS-441524.

Feline infectious peritonitis (FIP) caused by feline coronavirus (FCoV) is a common dis-ease in cats, fatal if untreated, and no effective treatment is currently legally available. The aim of this study was to evaluate efficacy and toxicity of the multi-component drug Xraphconn® in vitro and as oral treatment in cats with spontaneous FIP by examining survival rate, development of clinical and laboratory parameters, viral loads, anti-FCoV antibodies, and adverse effects. Mass spectrometry and nuclear magnetic resonance identified GS-441524 as an active component of Xraphconn®. Eighteen cats with FIP were prospectively followed up while being treated orally for 84 days. Values of key parameters on each examination day were compared to values before treatment initiation using linear mixed-effect models. Xraphconn® displayed high virucidal activity in cell culture. All cats recovered with dramatic improvement of clinical and laboratory parameters and massive reduction in viral loads within the first few days of treatment without serious adverse effects. Oral treatment with Xraphconn® containing GS-441524 was highly effective for FIP without causing serious adverse effects. This drug is an excellent option for the oral treatment of FIP and should be trialed as potential effective treatment option for other severe coronavirus-associated diseases across species.

Determination of tacrolimus, three mono-demethylated metabolites and a M1 tautomer in human whole blood by liquid chromatography - tandem mass spectrometry.

The immunosuppressant tacrolimus is the primary drug used in kidney transplantation to prevent organ rejection. A sensitive ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed to measure tacrolimus and its three known mono-demethylated metabolites 13-O-desmethyl tacrolimus (M1), 31-O-desmethyl tacrolimus (M2), 15-O-desmethyl tacrolimus (M3). By generating the metabolites to use as standards after incubation of tacrolimus with rat liver microsomes, we discovered multiple M1 peaks which we identified as two tautomers of M1. The M1 tautomer II was also successfully validated in this method. The separation and purification of the metabolites and tautomers were performed by semi-preparative liquid chromatography with UV-detection, while confirmation was done by UPLC-MS/MS and Nuclear Magnetic Resonance. For quantification an easy sample preparation was performed with zinc sulfate and acetonitrile as cell lyses and precipitation. Detection was performed in positive electrospray ionization. By better characterization of the metabolites and the tautomers, we could possibly explain insight into the clinical condition and thus adjust the immunosuppressant therapy individually per patient. Calibration curves were linear for all compounds. Precision was assessed according to the NCCLS EP5-T guideline, being below 15 % and mean recoveries were between 93 and 110 % for tacrolimus, its three metabolites and the M1 tautomer II. The validated method was successfully applied in a cohort of 20 patients after kidney transplantation.

Cyclic Peptides as T-Type Calcium Channel Blockers: Characterization and Molecular Mapping of the Binding Site.

T-type calcium (CaV3) channels play a crucial role in the generation and propagation of action potentials in excitable cells and are considered potential drug targets for the treatment of neurological and cardiovascular diseases. Given the limited pharmacological repertoire for these channels, there is a great need for novel potent and selective CaV3 channel inhibitors. In this study, we used Xenopus oocytes to heterologously express CaV3.1 channels and characterized the interaction with a small cyclic peptide, PnCS1. Using molecular modeling, PnCS1 was docked into the cryo-electron microscopy structure of the human CaV3.1 channel and molecular dynamics were performed on the resultant complex. The binding site of the peptide was mapped with the involvement of critical amino acids located in the pore region and fenestrations of the channel. More specifically, we found that PnCS1 reclines in the central cavity of the pore domain of the CaV3.1 channel and resides stably between the selectivity filter and the intracellular gate, blocking the conduction pathway of the channel. Using Multiple Attribute Positional Scanning approaches, we developed a series of PnCS1 analogues. These analogues had a reduced level of inhibition, confirming the importance of specific residues and corroborating our modeling. In summary, functional studies of PnCS1 on the CaV3.1 channel combined with molecular dynamics results provide the basis for understanding the molecular interactions of PnCS1 with CaV3.1 and are fundamental to structure-based drug discovery for treating CaV3 channelopathies.

Efficient and Accurate Potential Energy Surfaces of Puckering in Sugar-Modified Nucleosides.

Puckering of the sugar unit in nucleosides and nucleotides is an important structural aspect that directly influences the helical structure of nucleic acids. The preference for specific puckering modes in nucleic acids can be analyzed via in silico conformational analysis, but the large amount of conformations and the accuracy of the analysis leads to an extensive amount of computational time. In this paper, we show that the combination of geometry optimizations with the HF-3c method with single point energies at the RI-MP2 level results in accurate results for the puckering potential energy surface (PES) of DNA and RNA nucleosides while significantly reducing the necessary computational time. Applying this method to a series of known xeno nucleic acids (XNAs) allowed us to rapidly explore the puckering PES of each of the respective nucleosides and to explore the puckering PES of six-membered modified XNA (HNA and ß-homo-DNA) for the first time.

Functional and Molecular Characterization of the Halomicrobium sp. IBSBa Inulosucrase.

Fructans are fructose-based (poly)saccharides with inulin and levan being the best-known ones. Thanks to their health-related benefits, inulin-type fructans have been under the focus of scientific and industrial communities, though mostly represented by plant-based inulins, and rarely by microbial ones. Recently, it was discovered that some extremely halophilic Archaea are also able to synthesize fructans. Here, we describe the first in-depth functional and molecular characterization of an Archaeal inulosucrase from Halomicrobium sp. IBSBa (HmcIsc). The HmcIsc enzyme was recombinantly expressed and purified in Escherichia coli and shown to synthesize inulin as proven by nuclear magnetic resonance (NMR) analysis. In accordance with the halophilic lifestyle of its native host, the enzyme showed maximum activity at very high NaCl concentrations (3.5 M), with specific adaptations for that purpose. Phylogenetic analyses suggested that Archaeal inulosucrases have been acquired from halophilic bacilli through horizontal gene transfer, with a HX(H/F)T motif evolving further into a HXHT motif, together with a unique D residue creating the onset of a specific alternative acceptor binding groove. This work uncovers a novel area in fructan research, highlighting unexplored aspects of life in hypersaline habitats, and raising questions about the general physiological relevance of inulosucrases and their products in nature.

Stable Hairpin Structures Formed by Xylose-Based Nucleic Acids.

Xenobiology explores synthetic nucleic acid polymers as alternative carriers of genetic information to expand the central dogma. The xylo- and deoxyxylo-nucleic acids (XyNA and dXyNA), containing 3' epimers of riboses and deoxyriboses, are considered to be potential candidates for an orthogonal system. In this study, thermal and spectroscopic analyses show that XyNA and dXyNA form stable hairpins. The dXyNA hairpin structure determined by NMR spectroscopy contains a flexible loop that locks the stem into a stable ladder-like duplex with marginal right-handed helicity. The reduced flexibility of the dXyNA duplex observed in the stem of the hairpin demonstrates that folding of dXyNA yields more stable structures described so far.

Influence of circular permutations on the structure and stability of a six-fold circular symmetric designer protein.

The ß-propeller fold is adopted by a sequentially diverse family of repeat proteins with apparent rotational symmetry. While the structure is mostly stabilized by hydrophobic interactions, an additional stabilization is provided by hydrogen bonds between the N-and C-termini, which are almost invariably part of the same ß-sheet. This feature is often referred to as the "Velcro" closure. The positioning of the termini within a blade is variable and depends on the protein family. In order to investigate the influence of this location on protein structure, folding and stability, we created different circular permutants, and a circularized version, of the designer propeller protein named Pizza. This protein is perfectly symmetrical, possessing six identical repeats. While all mutants adopt the same structure, the proteins lacking the "Velcro" closure were found to be significantly less resistant to thermal and chemical denaturation. This could explain why such proteins are rarely observed in nature. Interestingly the most common "Velcro" configuration for this protein family was not the most stable among the Pizza variants tested. The circularized version shows dramatically improved stability, which could have implications for future applications.

Synthesis and Biological Evaluation of 1,3-Dideazapurine-Like 7-Amino-5-Hydroxymethyl-Benzimidazole Ribonucleoside Analogues as Aminoacyl-tRNA Synthetase Inhibitors.

Aminoacyl-tRNA synthetases (aaRSs) have become viable targets for the development of antimicrobial agents due to their crucial role in protein translation. A series of six amino acids were coupled to the purine-like 7-amino-5-hydroxymethylbenzimidazole nucleoside analogue following an optimized synthetic pathway. These compounds were designed as aaRS inhibitors and can be considered as 1,3-dideazaadenine analogues carrying a 2-hydroxymethyl substituent. Despite our intentions to obtain N1-glycosylated 4-aminobenzimidazole congeners, resembling the natural purine nucleosides glycosylated at the N9-position, we obtained the N3-glycosylated benzimidazole derivatives as the major products, resembling the respective purine N7-glycosylated nucleosides. A series of X-ray crystal structures of class I and II aaRSs in complex with newly synthesized compounds revealed interesting interactions of these "base-flipped" analogues with their targets. While the exocyclic amine of the flipped base mimics the reciprocal interaction of the N3-purine atom of aminoacyl-sulfamoyl adenosine (aaSA) congeners, the hydroxymethyl substituent of the flipped base apparently loses part of the standard interactions of the adenine N1 and the N6-amine as seen with aaSA analogues. Upon the evaluation of the inhibitory potency of the newly obtained analogues, nanomolar inhibitory activities were noted for the leucine and isoleucine analogues targeting class I aaRS enzymes, while rather weak inhibitory activity against the corresponding class II aaRSs was observed. This class bias could be further explained by detailed structural analysis.

Phenyltriazole-functionalized sulfamate inhibitors targeting tyrosyl- or isoleucyl-tRNA synthetase.

Antimicrobial resistance is considered as one of the major threats for the near future as the lack of effective treatments for various infections would cause more deaths than cancer by 2050. The development of new antibacterial drugs is considered as one of the cornerstones to tackle this problem. Aminoacyl-tRNA synthetases (aaRSs) are regarded as good targets to establish new therapies. Apart from being essential for cell viability, they are clinically validated. Indeed, mupirocin, an isoleucyl-tRNA synthetase (IleRS) inhibitor, is already commercially available as a topical treatment for MRSA infections. Unfortunately, resistance developed soon after its introduction on the market, hampering its clinical use. Therefore, there is an urgent need for new cellular targets or improved therapies. Follow-up research by Cubist Pharmaceuticals led to a series of selective and in vivo active aminoacyl-sulfamoyl aryltetrazole inhibitors targeting IleRS (e.g. CB 168). Here, we describe the synthesis of new IleRS and TyrRS inhibitors based on the Cubist Pharmaceuticals compounds, whereby the central ribose was substituted for a tetrahydropyran ring. Various linkers were evaluated connecting the six-membered ring with the base-mimicking part of the synthesized analogues. Out of eight novel molecules, a three-atom spacer to the phenyltriazole moiety, which was established using azide-alkyne click chemistry, appeared to be the optimized linker to inhibit IleRS. However, 11 (Ki,app = 88 ± 5.3 nM) and 36a (Ki,app = 114 ± 13.5 nM) did not reach the same level of inhibitory activity as for the known high-affinity natural adenylate-intermediate analogue isoleucyl-sulfamoyl adenosine (IleSA, CB 138; Ki,app = 1.9 ± 4.0 nM) and CB 168, which exhibit a comparable inhibitory activity as the native ligand. Therefore, 11 was docked into the active site of IleRS using a known crystal structure of T. thermophilus in complex with mupirocin. Here, we observed the loss of the crucial 3'- and 4'- hydroxyl group interactions with the target enzyme compared to CB 168 and mupirocin, which we suggest to be the reason for the limited decrease in enzyme affinity. Despite the lack of antibacterial activity, we believe that structurally optimizing these novel analogues via a structure-based approach could ultimately result in aaRS inhibitors which would help to tackle the antibiotic resistance problem.

The Kalimantacin Polyketide Antibiotics Inhibit Fatty Acid Biosynthesis in Staphylococcus aureus by Targeting the Enoyl-Acyl Carrier Protein Binding Site of FabI.

The enoyl-acyl carrier protein reductase enzyme FabI is essential for fatty acid biosynthesis in Staphylococcus aureus and represents a promising target for the development of novel, urgently needed anti-staphylococcal agents. Here, we elucidate the mode of action of the kalimantacin antibiotics, a novel class of FabI inhibitors with clinically-relevant activity against multidrug-resistant S. aureus. By combining X-ray crystallography with molecular dynamics simulations, in vitro kinetic studies and chemical derivatization experiments, we characterize the interaction between the antibiotics and their target, and we demonstrate that the kalimantacins bind in a unique conformation that differs significantly from the binding mode of other known FabI inhibitors. We also investigate mechanisms of acquired resistance in S. aureus and identify key residues in FabI that stabilize the binding of the antibiotics. Our findings provide intriguing insights into the mode of action of a novel class of FabI inhibitors that will inspire future anti-staphylococcal drug development.