Total Synthesis of Acanthodoral Using a Rearrangement Strategy.

We present the second total synthesis of (±)-acanthodoral, a sesquiterpenoid derived from the marine nudibranch Acanthodoris nanaimoensis. Our approach involves a concise three-step transformation from a previously reported compound, resulting in the formation of a less strained precursor of the bicyclo[3.1.1]heptane core and both all-carbon quaternary stereocenters characteristic of the natural product. Notably, this synthetic route incorporates two pivotal steps: a Sm(II)-induced 1,2-rearrangement and a semipinacol rearrangement.

Siderophore conjugation with cleavable linkers boosts the potency of RNA polymerase inhibitors against multidrug-resistant E. coli.

The growing antibiotic resistance, foremost in Gram-negative bacteria, requires novel therapeutic approaches. We aimed to enhance the potency of well-established antibiotics targeting the RNA polymerase (RNAP) by utilizing the microbial iron transport machinery to improve drug translocation across their cell membrane. As covalent modifications resulted in moderate-low antibiotic activity, cleavable linkers were designed that permit a release of the antibiotic payload inside the bacteria and unperturbed target binding. A panel of ten cleavable siderophore-ciprofloxacin conjugates with systematic variation at the chelator and the linker moiety was used to identify the quinone trimethyl lock in conjugates 8 and 12 as the superior linker system, displaying minimal inhibitory concentrations (MICs) of ≤1 µM. Then, rifamycins, sorangicin A and corallopyronin A, representatives of three structurally and mechanistically different natural product RNAP inhibitor classes, were conjugated via the quinone linker to hexadentate hydroxamate and catecholate siderophores in 15-19 synthetic steps. MIC assays revealed an up to 32-fold increase in antibiotic activity against multidrug-resistant E. coli for conjugates such as 24 or 29 compared to free rifamycin. Experiments with knockout mutants in the transport system showed that translocation and antibiotic effects were conferred by several outer membrane receptors, whose coupling to the TonB protein was essential for activity. A functional release mechanism was demonstrated analytically by enzyme assays in vitro, and a combination of subcellular fractionation and quantitative mass spectrometry proved cellular uptake of the conjugate, release of the antibiotic, and its increased accumulation in the cytosol of bacteria. The study demonstrates how the potency of existing antibiotics against resistant Gram-negative pathogens can be boosted by adding functions for active transport and intracellular release.

Tetrazine-induced activation of a trimethyl lock as a click-to-release system for protected doxorubicin.

We herein report a novel chemically triggered click-to-release system, that combines the trimethyl lock (TML) lactonization with the bioorthogonal inverse electron demand Diels-Alder (IEDDA) reaction of a vinyl ether and a tetrazine. Kinetic studies were carried out on a vinyl phenol model system with six tetrazines using NMR and UV/Vis spectroscopy, revealing that within the three step sequence the IEDDA reaction was rate-limiting. The reaction rates were enhanced by increasing the electrophilicity of the tetrazine, while balancing reactivity and stability of the tetrazines. The anticancer drug doxorubicin was conjugated to a vinyl-modified TML. Its subsequent liberation from vinyl-TML was triggered by dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate and followed quantitatively by NMR, thereby providing a proof-of-concept for the tetrazine/TML click-to-release system. In addition the applicability of the reaction under physiolgoical conditions could be shown.

Total synthesis and mechanism of action of the antibiotic armeniaspirol A.

Emerging antimicrobial resistance urges the discovery of antibiotics with unexplored, resistance-breaking mechanisms. Armeniaspirols represent a novel class of antibiotics with a unique spiro[4.4]non-8-ene scaffold and potent activities against Gram-positive pathogens. We report a concise total synthesis of (±) armeniaspirol A in six steps with a yield of 20.3% that includes the formation of the spirocycle through a copper-catalyzed radical cross-coupling reaction. In mechanistic biological experiments, armeniaspirol A exerted potent membrane depolarization, accounting for the pH-dependent antibiotic activity. Armeniaspirol A also disrupted the membrane potential and decreased oxygen consumption in mitochondria. In planar lipid bilayers and in unilamellar vesicles, armeniaspirol A transported protons across membranes in a protein-independent manner, demonstrating that armeniaspirol A acted as a protonophore. We provide evidence that this mechanism might account for the antibiotic activity of multiple chloropyrrole-containing natural products isolated from various origins that share a 4-acylphenol moiety coupled to chloropyrrole as a joint pharmacophore. We additionally describe an efflux-mediated mechanism of resistance against armeniaspirols.

Polysialylation at Early Stages of Oligodendrocyte Differentiation Promotes Myelin Repair.

Polysialic acid is a glycan modification of the neural cell adhesion molecule (NCAM) produced by the polysialyltransferases ST8SIA2 and ST8SIA4. Polysialic acid has been detected in multiple sclerosis plaques, but its beneficial or adverse role in remyelination is elusive. Here, we show that, despite a developmental delay, myelination at the onset and during cuprizone-induced demyelination was unaffected in male Ncam1-/- or St8sia2-/- mice. However, remyelination, restoration of oligodendrocyte densities, and motor recovery after the cessation of cuprizone treatment were compromised. Impaired differentiation of NCAM- or ST8SIA2-negative oligodendrocyte precursors suggested an underlying cell-autonomous mechanism. In contrast, premature differentiation in ST8SIA4-negative cultures explained the accelerated remyelination previously observed in St8sia4-/- mice. mRNA profiling during differentiation of human stem cell-derived and primary murine oligodendrocytes indicated that the opposing roles of ST8SIA2 and ST8SIA4 arise from sequential expression. We also provide evidence that potentiation of ST8SIA2 by 9-cis-retinoic acid and artificial polysialylation of oligodendrocyte precursors by a bacterial polysialyltransferase are mechanisms to promote oligodendrocytic differentiation. Thus, differential targeting of polysialyltransferases and polysialic acid engineering are promising strategies to advance the treatment of demyelinating diseases.SIGNIFICANCE STATEMENT The beneficial or adverse role of polysialic acid (polySia) in myelin repair is a long-standing question. As a modification of the neural cell adhesion molecule (NCAM), polySia is produced by the polysialyltransferases ST8SIA2 and ST8SIA4. Here we demonstrate that NCAM and ST8SIA2 promote oligodendrocyte differentiation and myelin repair as well as motor recovery after cuprizone-induced demyelination. In contrast, ST8SIA4 delays oligodendrocyte differentiation, explaining its adverse role in remyelination. These opposing roles of the polysialyltransferases are based on different expression profiles. 9-cis-retinoic acid enhances ST8SIA2 expression, providing a mechanism for understanding how it supports oligodendrocyte differentiation and remyelination. Furthermore, artificial polysialylation of the cell surface promotes oligodendrocyte differentiation. Thus, boosting ST8SIA2 and engineering of polySia are promising strategies for improving myelin repair.

Engineering the product profile of a polysialyltransferase.

Oligo- and polysaccharides have myriad applications as therapeutic reagents from glycoconjugate vaccines to matrices for tissue engineering. Polysaccharide length may vary over several orders of magnitude and is a critical determinant of both their physical properties and biological activities. Therefore, the tailored synthesis of oligo- and polysaccharides of defined size is a major goal for glycoengineering. By mutagenesis and screening of a bacterial polysialyltransferase (polyST), we identified a single-residue switch that controls the size distribution of polymeric products. Specific substitutions at this site yielded distributive enzymes that synthesize polysaccharides with narrow size distribution ideal for glycoengineering applications. Mechanistic investigation revealed that the wild-type enzyme has an extended binding site that accommodates at least 20 residues of the growing polymer; changes in affinity along this binding site allow fine-tuning of the enzyme's product distribution.

A single amino acid toggles Escherichia coli polysialyltransferases between mono- and bifunctionality.

The enteropathogenic Escherichia coli K92 synthesizes a unique capsule consisting of polysialic acid (polySia) with alternating α2,8- and α2,9-linkages. The fact that a single enzyme is responsible for the synthesis of these alternating regioisomeric linkages raises questions as to how this controlled bifunctionality is achieved mechanistically. Aiming to identify the sequence elements responsible for dual regiospecificity, we have utilized a high-throughput polysialyltransferase (polyST) activity screen to explore the relevant sequence space between this enzyme and its close monofunctional homolog from E. coli K1. The linkage specificity of selected mutants was subsequently confirmed using a polySia permethylation linkage analysis technique. We have identified a single amino acid exchange at residue 52 that toggles these enzymes between mono and dual regiospecificity. The results have implications for the mechanism by which the E. coli K92 polyST achieves bifunctional elongation.