Thioarylation of 6-Amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic Acid (IminoKdo): Access to 3,6-Disubstituted Picolinates and Mechanistic Insights

In this work, we present a metal-free coupling protocol for the regio- and stereoselective C3-thioarylation of 6-amino-2,3,6-trideoxy-d-manno-oct-2-ulosonic acid (iminoKdo). The developed procedure enables the coupling of electron-rich, electron-deficient, and hindered arylthiols, providing a series of C3-modified iminoKdo derivatives in moderate to good yields. Elucidation of active species through controlled experimental studies and time-lapse 31 P NMR analysis provides insights into the reaction mechanism. We demonstrate that, following a tandem Staudinger/aza-Wittig reaction of an azido-containing keto ester, an inseparable equimolar mixture of imine/enamine is formed. The enamine then undergoes a Stork-like nucleophilic attack with the in situ-formed disulfide reagent, resulting in the formation of the coupling products. Additionally, we describe a rarely reported acid-promoted aromatization of the C3-thioarylated iminoKdo skeleton into 3,6-disubstituted picolinates, which are reminiscent of dichotomines.

Evaluation of Azido 3-Deoxy-d-manno-oct-2-ulosonic Acid (Kdo) Analogues for Click Chemistry-Mediated Metabolic Labeling of Myxococcus xanthus DZ2 Lipopolysaccharide

Metabolic labeling paired with click chemistry is a powerful approach for selectively imaging the surfaces of diverse bacteria. Herein, we explored the feasibility of labeling the lipopolysaccharide (LPS) of Myxococcus xanthus-a Gram-negative predatory social bacterium known to display complex outer membrane (OM) dynamics-via growth in the presence of distinct azido (-N3) analogues of 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo). Determination of the LPS carbohydrate structure from strain DZ2 revealed the presence of one Kdo sugar in the core oligosaccharide, modified with phosphoethanolamine. The production of 8-azido-8-deoxy-Kdo (8-N3-Kdo) was then greatly improved over previous reports via optimization of the synthesis of its 5-azido-5-deoxy-d-arabinose precursor to yield gram amounts. The novel analogue 7-azido-7-deoxy-Kdo (7-N3-Kdo) was also synthesized, with both analogues capable of undergoing in vitro strain-promoted azide-alkyne cycloaddition (SPAAC) "click" chemistry reactions. Slower and faster growth of M. xanthus was displayed in the presence of 8-N3-Kdo and 7-N3-Kdo (respectively) compared to untreated cells, with differences also seen for single-cell gliding motility and type IV pilus-dependent swarm community expansion. While the surfaces of 8-N3-Kdo-grown cells were fluorescently labeled following treatment with dibenzocyclooctyne-linked fluorophores, the surfaces of 7-N3-Kdo-grown cells could not undergo fluorescent tagging. Activity analysis of the KdsB enzyme required to activate Kdo prior to its integration into nascent LPS molecules revealed that while 8-N3-Kdo is indeed a substrate of the enzyme, 7-N3-Kdo is not. Though a lack of M. xanthus cell aggregation was shown to expedite growth in liquid culture, 7-N3-Kdo-grown cells did not manifest differences in intrinsic clumping relative to untreated cells, suggesting that 7-N3-Kdo may instead be catabolized by the cells. Ultimately, these data provide important insights into the synthesis and cellular processing of valuable metabolic labels and establish a basis for the elucidation of fundamental principles of OM dynamism in live bacterial cells.

Revisiting Leishmania GP63 host cell targets reveals a limited spectrum of substrates

Colonization of host phagocytic cells by Leishmania metacyclic promastigotes involves several parasite effectors, including the zinc-dependent metalloprotease GP63. The major mode of action of this virulence factor entails the cleavage/degradation of host cell proteins. Given the potent proteolytic activity of GP63, identification of its substrates requires the adequate preparation of cell lysates to prevent artefactual degradation during cell processing. In the present study, we re-examined the cleavage/degradation of reported GP63 substrates when GP63 activity was efficiently neutralized during the preparation of cell lysates. To this end, we infected bone marrow-derived macrophages with either wild type, Δgp63, and Δgp63+GP63 L. major metacyclic promastigotes for various time points. We prepared cell lysates in the absence or presence of the zinc-metalloprotease inhibitor 1,10-phenanthroline and examined the levels and integrity of ten previously reported host cell GP63 substrates. Inhibition of GP63 activity with 1,10-phenanthroline during the processing of macrophages prevented the cleavage/degradation of several previously described GP63 targets, including PTP-PEST, mTOR, p65RelA, c-Jun, VAMP3, and NLRP3. Conversely, we confirmed that SHP-1, Synaptotagmin XI, VAMP8, and Syntaxin-5 are bona fide GP63 substrates. These results point to the importance of efficiently inhibiting GP63 activity during the preparation of Leishmania-infected host cell lysates. In addition, our results indicate that the role of GP63 in Leishmania pathogenesis must be re-evaluated.

Liquid biopsies for omics-based analysis in sentinel mussels

Liquid biopsy of plasma is a simple and non-invasive technology that holds great promise in biomedical research. It is based on the analysis of nucleic acid-based biomarkers with predictive potential. In the present work, we have combined this concept with the FTA technology for sentinel mussels. We found that hemocytes collected from liquid biopsies can be readily fixed on FTA cards and used for long-term transcriptome analysis. We also showed that liquid biopsy is easily adaptable for metagenomic analysis of bacterial profiles of mussels. We finally provide evidence that liquid biopsies contained circulating cell-free DNA (ccfDNA) which can be used as an easily accessible genomic reservoir. Sampling of FTA-fixed circulating nucleic acids is stable at room temperature and does not necessitate a cold-chain protection. It showed comparable performance to frozen samples and is ideally adapted for sampling in remote areas, most notably in polar regions threatened by anthropogenic activities. From an ethical point of view, this minimally-invasive and non-lethal approach further reduces incidental mortality associated with conventional tissue sampling. This liquid biopsy-based approach should thus facilitate biobanking activities and development of omics-based biomarkers in mussels to assess the quality of aquatic ecosystems.