Discovery of Terminal Oxazole-Bearing Natural Products by a Targeted Metabologenomic Approach

A targeted metabologenomic method was developed to selectively discover terminal oxazole-bearing natural products from bacteria. For this, genes encoding oxazole cyclase, a key enzyme in terminal oxazole biosynthesis, were chosen as the genomic signature to screen bacterial strains that may produce oxazole-bearing compounds. Sixteen strains were identified from the screening of a bacterial DNA library (1,000 strains) using oxazole cyclase gene-targeting polymerase chain reaction (PCR) primers. The PCR amplicon sequences were subjected to phylogenetic analysis and classified into nine clades. 1H-13C coupled-HSQC NMR spectra obtained from the culture extracts of the hit strains enabled the unequivocal detection of the target compounds, including five new oxazole compounds, based on the unique 1JCH values and chemical shifts of oxazole: lenzioxazole (1) possessing an unprecedented cyclopentane, permafroxazole (2) bearing a tetraene conjugated with carboxylic acid, tenebriazine (3) incorporating two modified amino acids, and methyl-oxazolomycins A and B (4 and 5). Tenebriazine displayed inhibitory activity against pathogenic fungi, whereas methyl-oxazolomycins A and B (4 and 5) selectively showed anti-proliferative activity against estrogen receptor-positive breast cancer cells. This metabologenomic method enables the logical and efficient discovery of new microbial natural products with a target structural motif without the need for isotopic labeling.

Concomitant left atrial appendage closure with left ventricular assist device surgery can reduce ischemic cerebrovascular accidents

Background

It remains unknown if concomitant left atrial appendage closure (LAAC) at the time of left ventricular assist device (LVAD) surgery can reduce ischemic cerebrovascular accidents.

Purpose

The purpose of this study is to assess the impact of LAAC at LVAD surgery on the incidence of ischemic cerebrovascular accidents.

Methods

Between January 2012 and November 2021, 310 patients underwent LVAD surgery with HeartMate II or III. Out of 310 patients, 98 patients (31.6%) underwent concomitant LAAC. The cohort was divided into two groups: patients with LAAC (Group A, n=98) and without LAAC (Group B, n=212). To minimize device bias, LVAD surgery with HeartWare HVAD device was excluded. The ischemic cerebrovascular accident was defined as ischemic stroke, hemorrhagic stroke or transient ischemic attack. We reviewed early and long-term clinical outcomes. The incidence of ischemic cerebrovascular accidents was compared between two groups using the Kaplan-Meier method. We also investigated if LAAC was associated with ischemic cerebrovascular accidents by Cox proportional hazards analysis.

Results

There were no significant differences in baseline characteristics between two groups including age (Group A: 55.0±12.3 years old, Group B: 56.9±14.1 years old, p=0.26), preoperative CHADS2 score (Group A: 2.40±1.1, Group B: 2.58±1.1, p=0.19) and history of atrial fibrillation (Group A: 42.9%, Group B: 42.5%, p=0.95). In-hospital mortality was not significantly different between the two groups (Group A: 7.1%, Group B: 12.3%, p=0.16). In terms of postoperative complications, there were no significant differences between two groups in requiring extracorporeal membrane oxygenation, re-exploration for bleeding and newly required hemodialysis. Median follow up period was 474 days. Thirty-five patients (11.2%) developed ischemic cerebrovascular accidents (5 patients in Group A and 30 patients in Group B). The rate of freedom from ischemic cerebrovascular accidents in Group A (94.1% at 500 days and 94.1% at 1500 days) was significantly higher than that in Group B (88.2% at 500 days and 77.4% at 1500 days; log rank=0.024). In a Cox proportional hazards regression analysis including LAAC, age, history of atrial fibrillation, diabetes mellitus and Heartmate 3 device implantation, LAAC was associated with reducing the incidence of ischemic cerebrovascular accidents (hazard ratio 0.37, 95% CI 0.13–0.89, p=0.02).

Conclusion

Concomitant LAAC at the time of LVAD surgery can reduce ischemic cerebrovascular accidents without increasing perioperative mortality and complications.

Funding Acknowledgement

Type of funding sources: None.

Inhibitory Effects of Epipolythiodioxopiperazine Fungal Metabolites on Isocitrate Lyase in the Glyoxylate Cycle of Candida albicans

Four epipolythiodioxopiperazine fungal metabolites (1-4) isolated from the sponge-derived Aspergillus quadrilineatus FJJ093 were evaluated for their capacity to inhibit isocitrate lyase (ICL) in the glyoxylate cycle of Candida albicans. The structures of these compounds were elucidated using spectroscopic techniques and comparisons with previously reported data. We found secoemestrin C (1) (an epitetrathiodioxopiperazine derivative) to be a potent ICL inhibitor, with an inhibitory concentration of 4.77 ± 0.08 μM. Phenotypic analyses of ICL-deletion mutants via growth assays with acetate as the sole carbon source demonstrated that secoemestrin C (1) inhibited C. albicans ICL. Semi-quantitative reverse-transcription polymerase chain reaction analyses indicated that secoemestrin C (1) inhibits ICL mRNA expression in C. albicans under C₂-assimilating conditions.

Antibacterial Activity and Mode of Action of Lactoquinomycin A from Streptomyces bacillaris

This study aims to isolate and identify the structure of antibacterial compounds having potent activity on methicillin-resistant Staphylococcus aureus (MRSA) from marine actinomycetes, and also to identify their mode of action. Lactoquinomycin A (LQM-A) (compound 1) and its derivatives (2–4) were isolated from marine-derived Streptomyces bacillaris strain MBTC38, and their structures were determined using extensive spectroscopic methods. These compounds showed potent antibacterial activities against Gram-positive bacteria, with MIC values of 0.06–4 μg/mL. However, the tested compounds exhibited weak inhibitory activity against Gram-negative bacteria, although they were effective against Salmonella enterica (MIC = 0.03–1 μg/mL). LQM-A exhibited the most significant inhibitory activity against methicillin-resistant Staphylococcus aureus (MRSA) (MIC = 0.25–0.5 μg/mL), with a low incidence of resistance. An in vivo dual-reporter assay designed to distinguish between compounds that inhibit translation and those that induce DNA damage was employed to assess the mode of action of LQM-A. LQM-A-induced DNA damage and did not inhibit protein synthesis. The gel mobility shift assay showed that LQM-A switched plasmid DNA from the supercoiled to relaxed form in a time- and concentration-dependent manner. These data suggest that LQM-A intercalated into double-stranded DNA and damaged DNA repair.

Bioactive Bis(indole) Alkaloids from a Spongosorites sp. Sponge

Six new bis(indole) alkaloids (1-6) along with eight known ones of the topsentin class were isolated from a Spongosorites sp. sponge of Korea. Based on the results of combined spectroscopic analyses, the structures of spongosoritins A-D (1-4) were determined to possess a 2-methoxy-1-imidazole-5-one core connecting the indole moieties, and these were linked by a linear urea bridge for spongocarbamides A (5) and B (6). The absolute configurations of spongosoritins were assigned by electronic circular dichroism (ECD) computation. The new compounds exhibited moderate inhibition against transpeptidase sortase A and weak inhibition against human pathogenic bacteria and A549 and K562 cancer cell lines.

Sortase A-Inhibitory Coumarins from the Folk Medicinal Plant Poncirus trifoliata

Thirteen coumarins (1-13), including five new compounds (1-5), were isolated from the folk medicinal plant Poncirus trifoliata. Combined spectroscopic analyses revealed that coumarins 1-4 are bis-isoprenylated coumarins with diverse oxidation patterns, while 5 is an enantiomeric di-isoprenylated coumarin. The absolute configurations of the stereogenic centers in the isoprenyl chains were assigned through MTPA and MPA methods, and those of the known compounds triphasiol (6) and ponciol (7) were also assigned using similar methods. These coumarins inhibited significantly Staphylococcus aureus-derived sortase A (SrtA), a transpeptidase responsible for anchoring surface proteins to the peptidoglycan cell wall in Gram-positive bacteria. The present results obtained indicated that the bioactivity and underlying mechanism of action of these coumarins are associated with the inhibition of SrtA-mediated S. aureus adhesion to eukaryotic cell matrix proteins including fibrinogen and fibronectin, thus potentially serving as SrtA inhibitors.

Sortase A-Inhibitory Metabolites from a Marine-Derived Fungus Aspergillus sp

Seven alkaloidal compounds (2-8) and one polyketide (1) were isolated from a semisolid rice culture of the marine-derived fungus Aspergillus sp. F452. Structures of the isolated compounds were elucidated based on spectroscopic data and comparisons with previously reported data. The alkaloidal compounds (2-8) displayed weak to moderate inhibitory activities against Staphylococcus aureus-derived sortase A (SrtA) without affecting cell viability. Aspermytin A (1) strongly inhibited SrtA activity, with an IC₅₀ value of 146.0 μM, and significantly reduced bacterial adherence to fibronectin-coated surfaces. The present results indicate that the underlying mechanism of action of compound 1 is associated with the inhibition of SrtA-mediated S. aureus adhesion to fibronectin, thus potentially serving as an SrtA inhibitor.

Absolute Configuration and Antibiotic Activity of Piceamycin

The cultivation of a Streptomyces sp. SD53 strain isolated from the gut of the silkworm Bombyx mori produced two macrolactam natural products, piceamycin (1) and bombyxamycin C (2). The planar structures of 1 and 2 were identified by a combination of NMR, MS, and UV spectroscopic analyses. The absolute configurations were assigned based on chemical and chromatographic methods as well as ECD calculations. A new chromatography-based experimental method for determining the configurations of stereogenic centers β to nitrogen atoms in macrolactams was established and successfully applied in this report. These compounds exhibited significant bioactivities against the silkworm entomopathogen Bacillus thuringiensis and various human pathogens as well as human cancer cell lines. In particular, piceamycin potently inhibited Salmonella enterica and Proteus hauseri with MIC values of 0.083 μg/mL and 0.025 μg/mL, respectively. The biosynthetic pathway involved in the formation of the cyclopentenone moiety in piceamycin is discussed.

Donghaecyclinones A-C: New Cytotoxic Rearranged Angucyclinones from a Volcanic Island-Derived Marine Streptomyces sp

Chemical profiling of the Streptomyces sp. strain SUD119, which was isolated from a marine sediment sample collected from a volcanic island in Korea, led to the discovery of three new metabolites: donghaecyclinones A–C (1–3). The structures of 1–3 were found to be rearranged, multicyclic, angucyclinone-class compounds according to nuclear magnetic resonance (NMR) and mass spectrometry (MS) analyses. The configurations of their stereogenic centers were successfully assigned using a combination of quantum mechanics–based computational methods for calculating the NMR shielding tensor (DP4 and CP3) as well as electronic circular dichroism (ECD) along with a modified version of Mosher’s method. Donghaecyclinones A–C (1–3) displayed cytotoxicity against diverse human cancer cell lines (IC₅₀: 6.7–9.6 μM for 3).

Inhibitory Effects of Streptomyces sp. MBTH32 Metabolites on Sortase A and Sortase A-Mediated Cell Clumping of Staphylococcus aureus to Fibrinogen

Sortase A (SrtA), a type of transpeptidase responsible for anchoring surface proteins to the peptidoglycan cell wall, is important in the virulence of gram-positive bacteria. Three compounds were isolated from marine-derived Streptomyces sp. MBTH32 using various chromatography techniques. The structures of these compounds were determined based on spectroscopic data and comparisons with previously reported data. Among the metabolites tested, lumichrome showed strong inhibitory activity against Staphylococcus aureus SrtA without affecting cell viability. The results of cell clumping activity assessment suggest the potential for using this compound to treat S. aureus infection by inhibiting SrtA activity.