Antibacterial Marinopyrroles and Pseudilins Act as Protonophores

Elucidating the mechanism of action (MoA) of antibacterial natural products is crucial to evaluating their potential as novel antibiotics. Marinopyrroles, pentachloropseudilin, and pentabromopseudilin are densely halogenated, hybrid pyrrole-phenol natural products with potent activity against Gram-positive bacterial pathogens like Staphylococcus aureus. However, the exact way they exert this antibacterial activity has not been established. In this study, we explore their structure-activity relationship, determine their spatial location in bacterial cells, and investigate their MoA. We show that the natural products share a common MoA based on membrane depolarization and dissipation of the proton motive force (PMF) that is essential for cell viability. The compounds show potent protonophore activity but do not appear to destroy the integrity of the cytoplasmic membrane via the formation of larger pores or interfere with the stability of the peptidoglycan sacculus. Thus, our current model for the antibacterial MoA of marinopyrrole, pentachloropseudilin, and pentabromopseudilin stipulates that the acidic compounds insert into the membrane and transport protons inside the cell. This MoA may explain many of the deleterious biological effects in mammalian cells, plants, phytoplankton, viruses, and protozoans that have been reported for these compounds.

Bioinformatics-guided discovery of biaryl-linked lasso peptides

Lasso peptides are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) that feature an isopeptide bond and a distinct lariat fold. A growing number of secondary modifications have been described that further decorate lasso peptide scaffolds. Using genome mining, we have discovered a pair of lasso peptide biosynthetic gene clusters (BGCs) that include cytochrome P450 genes. Using mass spectrometry, stable isotope incorporation, and extensive 2D-NMR spectrometry, we report the structural characterization of two unique examples of (C-N) biaryl-linked lasso peptides. Nocapeptin A, from Nocardia terpenica, is tailored with a Trp-Tyr crosslink, while longipepetin A, from Longimycelium tulufanense, features a Trp-Trp linkage. Besides the unusual bicyclic frame, a Met of longipepetin A undergoes S-methylation to yield a trivalent sulfonium, a heretofore unprecedented RiPP modification. A bioinformatic survey revealed additional lasso peptide BGCs containing P450 enzymes which await future characterization. Lastly, nocapeptin A bioactivity was assessed against a panel of human and bacterial cell lines with modest growth-suppression activity detected towards Micrococcus luteus.

Bioinformatics-Guided Discovery of Biaryl-Tailored Lasso Peptides

Lasso peptides are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) that feature an isopeptide bond and a distinct lariat fold. A growing number of secondary modifications have been described that further decorate lasso peptide scaffolds. Using genome mining, we have discovered a pair of lasso peptide biosynthetic gene clusters (BGCs) that include cytochrome P450 genes. Here, we report the structural characterization of two unique examples of (C-N) biaryl-containing lasso peptides. Nocapeptin A, from Nocardia terpenica, is tailored with Trp-Tyr crosslink while longipepetin A, from Longimycelium tulufanense, features Trp-Trp linkage. Besides the unusual bicyclic frame, longipepetin A receives an S-methylation by a new Met methyltransferase resulting in unprecedented sulfonium-bearing RiPP. Our bioinformatic survey revealed P450(s) and further maturating enzyme(s)-containing lasso BGCs awaiting future characterization.

The Cyanobacterial "Nutraceutical" Phycocyanobilin Inhibits Cysteine Protease Legumain

The blue biliprotein phycocyanin, produced by photo-autotrophic cyanobacteria including spirulina (Arthrospira) and marketed as a natural food supplement or "nutraceutical," is reported to have anti-inflammatory, antioxidant, immunomodulatory, and anticancer activity. These diverse biological activities have been specifically attributed to the phycocyanin chromophore, phycocyanobilin (PCB). However, the mechanism of action of PCB and the molecular targets responsible for the beneficial properties of PCB are not well understood. We have developed a procedure to rapidly cleave the PCB pigment from phycocyanin by ethanolysis and then characterized it as an electrophilic natural product that interacts covalently with thiol nucleophiles but lacks any appreciable cytotoxicity or antibacterial activity against common pathogens and gut microbes. We then designed alkyne-bearing PCB probes for use in chemical proteomics target deconvolution studies. Target identification and validation revealed the cysteine protease legumain (also known as asparaginyl endopeptidase, AEP) to be a target of PCB. Inhibition of this target may account for PCB's diverse reported biological activities.

Discovery of Ircinianin Lactones B and C-Two New Cyclic Sesterterpenes from the Marine Sponge Ircinia wistarii

Two new ircinianin-type sesterterpenoids, ircinianin lactone B and ircinianin lactone C (7 and 8), together with five known entities from the ircinianin compound family (1, 3–6) were isolated from the marine sponge Ircinia wistarii. Ircinianin lactones B and C (7 and 8) represent new ircinianin terpenoids with a modified oxidation pattern. Despite their labile nature, the structures could be established using a combination of spectroscopic data, including HRESIMS and 1D/2D NMR techniques, as well as computational chemistry and quantum-mechanical calculations. In a broad screening approach for biological activity, the class-defining compound ircinianin (1) showed moderate antiprotozoal activity against Plasmodium falciparum (IC₅₀ 25.4 μM) and Leishmania donovani (IC₅₀ 16.6 μM).

Synergetic Antimicrobial Activity and Mechanism of Clotrimazole-Linked CO-Releasing Molecules

Several metal-based carbon monoxide-releasing molecules (CORMs) are active CO donors with established antibacterial activity. Among them, CORM conjugates with azole antibiotics of type [Mn(CO)₃(2,2′-bipyridyl)(azole)]⁺ display important synergies against several microbes. We carried out a structure–activity relationship study based upon the lead structure of [Mn(CO)₃(Bpy)(Ctz)]⁺ by producing clotrimazole (Ctz) conjugates with varying metal and ligands. We concluded that the nature of the bidentate ligand strongly influences the bactericidal activity, with the substitution of bipyridyl by small bicyclic ligands leading to highly active clotrimazole conjugates. On the contrary, the metal did not influence the activity. We found that conjugate [Re(CO)₃(Bpy)(Ctz)]⁺ is more than the sum of its parts: while precursor [Re(CO)₃(Bpy)Br] has no antibacterial activity and clotrimazole shows only moderate minimal inhibitory concentrations, the potency of [Re(CO)₃(Bpy)(Ctz)]⁺ is one order of magnitude higher than that of clotrimazole, and the spectrum of bacterial target species includes Gram-positive and Gram-negative bacteria. The addition of [Re(CO)₃(Bpy)(Ctz)]⁺ to Staphylococcus aureus causes a general impact on the membrane topology, has inhibitory effects on peptidoglycan biosynthesis, and affects energy functions. The mechanism of action of this kind of CORM conjugates involves a sequence of events initiated by membrane insertion, followed by membrane disorganization, inhibition of peptidoglycan synthesis, CO release, and break down of the membrane potential. These results suggest that conjugation of CORMs to known antibiotics may produce useful structures with synergistic effects that increase the conjugate’s activity relative to that of the antibiotic alone.

High Plasticity of the Amicetin Biosynthetic Pathway in Streptomyces sp. SHP 22-7 Led to the Discovery of Streptcytosine P and Cytosaminomycins F and G and Facilitated the Production of 12F-Plicacetin

A chemical reinvestigation of the Indonesian strain Streptomyces sp. SHP 22-7 led to the isolation of three new pyrimidine nucleosides, along with six known analogues and zincphyrin. The structures of the new compounds (6, 7, 10) were elucidated by employing spectroscopic techniques (NMR, MS, CD, and IR) as well as enantioselective analyses of methyl branched side chain configurations. Application of the precursor-directed feeding approach led to the production and partial isolation of nine further pyrimidine analogues. The new compounds 6, 7, and 11 and three of the known compounds (2-4) were found to possess antimycobacterial and cytotoxic properties.

Nocathioamides, Uncovered by a Tunable Metabologenomic Approach, Define a Novel Class of Chimeric Lanthipeptides

The increasing number of available genomes, in combination with advanced genome mining techniques, unveiled a plethora of biosynthetic gene clusters (BGCs) coding for ribosomally synthesized and post-translationally modified peptides (RiPPs). The products of these BGCs often represent an enormous resource for new and bioactive compounds, but frequently, they cannot be readily isolated and remain cryptic. Here, we describe a tunable metabologenomic approach that recruits a synergism of bioinformatics in tandem with isotope- and NMR-guided platform to identify the product of an orphan RiPP gene cluster in the genomes of Nocardia terpenica IFM 0406 and 0706T . The application of this tactic resulted in the discovery of nocathioamides family as a founder of a new class of chimeric lanthipeptides I.

New Nocobactin Derivatives with Antimuscarinic Activity, Terpenibactins A-C, Revealed by Genome Mining of Nocardia terpenica IFM 0406

We report a genomics-guided exploration of the metabolic potential of the brasilicardin producer strain Nocardia terpenica IFM 0406. Bioinformatics analysis of the whole genome sequence revealed the presence of a biosynthetic gene cluster presumably responsible for the generation of formerly unknown nocobactin derivatives. Mass spectrometry-assisted isolation led to the identification of three new siderophores, terpenibactins A (1), B (2) and C (3), which belong to the class of nocobactins. Their structures were elucidated by employing spectroscopic techniques. Compounds 1-3 demonstrated inhibitory activity towards the muscarinic M3 receptor, while exhibiting only a low cytotoxicity.

Functional Characterisation of ClpP Mutations Conferring Resistance to Acyldepsipeptide Antibiotics in Firmicutes

Acyldepsipeptide (ADEP) is an exploratory antibiotic with a novel mechanism of action. ClpP, the proteolytic core of the caseinolytic protease, is deregulated towards unrestrained proteolysis. Here, we report on the mechanism of ADEP resistance in Firmicutes. This bacterial phylum contains important pathogens that are relevant for potential ADEP therapy. For Staphylococcus aureus, Bacillus subtilis, enterococci and streptococci, spontaneous ADEP-resistant mutants were selected in vitro at a rate of 10-6 . All isolates carried mutations in clpP. All mutated S. aureus ClpP proteins characterised in this study were functionally impaired; this increased our understanding of the mode of operation of ClpP. For molecular insights, crystal structures of S. aureus ClpP bound to ADEP4 were determined. Well-resolved N-terminal domains in the apo structure allow the pore-gating mechanism to be followed. The compilation of mutations presented here indicates residues relevant for ClpP function and suggests that ADEP resistance will occur at a lower rate during the infection process.

Nostotrebin 6 Related Cyclopentenediones and δ-Lactones with Broad Activity Spectrum Isolated from the Cultivation Medium of the Cyanobacterium Nostoc sp. CBT1153

Cyanobacteria are an interesting source of biologically active natural products, especially chemically diverse and potent protease inhibitors. On our search for inhibitors of the trypanosomal cysteine protease rhodesain, we identified the homodimeric cyclopentenedione (CPD) nostotrebin 6 (1) and new related monomeric, dimeric, and higher oligomeric compounds as the active substances in the medium extract of Nostoc sp. CBT1153. The oligomeric compounds are composed of two core monomeric structures, a trisubstituted CPD or a trisubstituted unsaturated δ-lactone. Nostotrebin 6 thus far has been the only known cyanobacterial CPD. It has been found to be active in a broad variety of assays, indicating that it might be a pan-assay interference compound (PAIN). Thus, we compared the antibacterial and cytotoxic activities as well as the rhodesain inhibition of selected compounds. Because a compound with a δ-lactone instead of a CPD core structure was equally active as nostotrebin 6, the bioactivities of these compounds seem to be based on the phenolic substructures rather than the CPD moiety. While the dimers were roughly equally potent, the monomer displayed slightly weaker activity, suggesting that the compounds show unspecific activity depending upon the number of free phenolic hydroxy groups per molecule.

Comparative Dose Accuracy of Durable and Patch Insulin Pumps Under Laboratory Conditions

Background: Recent studies demonstrate variable results of the accuracy with which patch pumps infuse insulin. Aim of this evaluation was to measure dose accuracies of the patch pump mylife™ OmniPod^® (OP) in comparison with the durable insulin pump MiniMed^® 640G (MM) simulating real-life clinical situations under laboratory conditions. Methods: Thirty-two OP and 15 MM were tested using insulin aspart at five different boluses (0.5, 1, 5, 10, and 15 international units [IU]) and three basal rates (0.2, 0.6, and 1.8 IU/h) at different time points during a 70 h investigation period. Owing to malfunctions only 22 OP and 11 MM could be analyzed. Dose accuracy was measured by an experimental setting based on IEC 60601-2-24:2012 with determination of weight differences of insulin collection tubes before and after experiments using a precision scale. A maximal tolerance of ±5% for boluses and basal rates was considered adequate according to IEC 60601-2-24:2012. Results: For the five boluses, the percentages of measurement results within the ±5% accuracy threshold were as follows: OP (18.6%, 26.5%, 89.0%, 96.0%, and 96.0%); MM (21.7%, 44.1%, 88.1%, 98.3%, and 100.0%). Both pumps were more accurate at higher bolus volumes (5, 10, and 15 IU), later bolus periods, and if the accuracy threshold was lowered to <10%, <15%, or >15%. For the three basal rates, the percentages within the ±5% accuracy threshold were as follows: OP (66.7%, 22.7%, and 16.7%); MM (14.3%, 0.0%, and 0.0%). Conclusion: This study demonstrates low accuracy for basal rates and single bolus deliveries at low insulin doses for both pump models. Clinicians should be aware of this variability when initiating insulin pump therapy especially in insulin-sensitive patients with low insulin dose requirements.

Mesenchymal stem cell (MSC)-mediated survival of insulin producing pancreatic β-cells during cellular stress involves signalling via Akt and ERK1/2

Mesenchymal stem cells (MSC) are of interest for cell therapy since their secreted factors mediate immunomodulation and support tissue regeneration. This study investigated the direct humoral interactions between MSC and pancreatic β-cells using human telomerase-immortalized MSC (hMSC-TERT) and rat insulinoma-derived INS-1E β-cells. hMSC-TERT supported survival of cocultured INS-1E β-cells during cellular stress by alloxan (ALX) and streptozotocin (STZ), but not in response to IL-1β. Accordingly, hMSC-TERT had no effect on inflammatory cytokine-related signalling via NF-kB and p-JNK but maintained p-Akt and upregulated p-ERK1/2. Inhibition of either p-Akt or p-ERK1/2 did not abolish protection by hMSC-TERT but activated the respective non-inhibited pathway. This suggests that one pathway compensates for the other. Main results were confirmed in mouse islets except hMSC-TERT-mediated upregulation of p-ERK1/2. Therefore, MSC promote β-cell survival by preservation of p-Akt signalling and further involve p-ERK1/2 activation in certain conditions such as loss of p-Akt or insulinoma background.