Exploring the Active Site of the Antibacterial Target MraY by Modified Tunicamycins

Mechanism of action of pseudopteroxazole and pseudopterosin G: Diterpenes from marine origin

Pseudopteroxazole (Ptx) and the pseudopterosins are marine natural products with promising antibacterial potential. While Ptx has attracted interest for its antimycobacterial activity, pseudopterosins are active against several clinically relevant pathogens. Both compound classes exhibit low cytotoxicity and accessibility to targeted synthesis, yet their antibacterial mechanisms remain elusive. In this study, we investigated the modes of action of Ptx and pseudopterosin G (PsG) in Bacillus subtilis employing an unbiased approach that combines gel-based proteomics with a mathematical similarity analysis of response profiles. Proteomic responses to sublethal concentrations of Ptx and PsG were compared to a library of antibiotic stress response profiles revealing that both induce a stress response characteristic for agents targeting the bacterial cell envelope by interfering with membrane-bound steps of cell wall biosynthesis. Microscopy-based assays confirmed that both compounds compromise the integrity of the bacterial cell wall without disrupting the membrane potential. Furthermore, LC-MSE analysis showed that the greater potency of PsG against B. subtilis, reflected in a lower MIC and a more pronounced proteomic response, may be rooted in a more effective association with and penetration of B. subtilis cells. We conclude that Ptx and PsG target the integrity of the gram-positive cell wall.

The tunicamycin derivative TunR2 exhibits potent antibiotic properties with low toxicity in an in vivo Mycobacterium marinum-zebrafish TB infection model

Tunicamycins (TUN) are well-defined, Streptomyces-derived natural products that inhibit protein N-glycosylation in eukaryotes, and by a conserved mechanism also block bacterial cell wall biosynthesis. TUN inhibits the polyprenylphosphate-N-acetyl-hexosamine-1-phospho-transferases (PNPT), an essential family of enzymes found in both bacteria and eukaryotes. We have previously published the development of chemically modified TUN, called TunR1 and TunR2, that have considerably reduced activity on eukaryotes but that retain the potent antibacterial properties. A mechanism for this reduced toxicity has also been reported. TunR1 and TunR2 have been tested against mammalian cell lines in culture and against live insect cells but, until now, no in vivo evaluation has been undertaken for vertebrates. In the current work, TUN, TunR1, and TunR2 are investigated for their relative toxicity and antimycobacterial activity in zebrafish using a well-established Mycobacterium marinum (M. marinum) infection system, a model for studying human Mycobacterium tuberculosis infections. We also report the relative ability to activate the unfolded protein response (UPR), the known mechanism for the eukaryotic toxicity observed with TUN treatment. Importantly, TunR1 and TunR2 retained their antimicrobial properties, as evidenced by a reduction in M. marinum bacterial burden, compared to DMSO-treated zebrafish. In summary, findings from this study highlight the characteristics of recently developed TUN derivatives, mainly TunR2, and its potential for use as a novel anti-bacterial agent for veterinary and potential medical purposes.

Precursor-Directed Biosynthesis and Biological Testing of omega-Alicyclic- and neo-Branched Tunicamycin N-Acyl Variants

Tunicamycins (TUNs) are Streptomyces-derived natural products, widely used to block protein N-glycosylation in eukaryotes or cell wall biosynthesis in bacteria. Modified or synthetic TUN analogues that uncouple these activities have considerable potential as novel mode-of-action antibacterial agents. Chemically modified TUNs reported previously with attenuated activity on yeast have pinpointed eukaryotic-specific chemophores in the uridyl group and the N-acyl chain length and terminal branching pattern. A small molecule screen of fatty acid biosynthetic primers identified several novel alicyclic- and neo-branched TUN N-acyl variants, with primer incorporation at the terminal omega-acyl position. TUNs with unique 5- and 6-carbon ω-cycloalkane and ω-cycloalkene acyl chains are produced under fermentation and in yields comparable with the native TUN. The purification, structural assignments, and the comparable antimicrobial properties of 15 of these compounds are reported, greatly extending the structural diversity of this class of compounds for potential medicinal and agricultural applications.

Berberine improves liver injury induced glucose and lipid metabolic disorders via alleviating ER stress of hepatocytes and modulating gut microbiota in mice

Liver injury mediated by endoplasmic reticulum (ER) stress can cause many kinds of liver diseases including hepatic glucose and lipid metabolic disorders, and long term liver injury would lead to cirrhosis and hepatic cancer. Therefore, effective drugs for treating liver injury are urgent in need. Berberine is a multifunctional drug of traditional Chinese medicine, and it can improve various liver diseases. To study the effects of berberine on ER stress-induced liver injury, tunicamycin was administrated to C57BL/6 mice with or without berberine pre-treatment. H&E staining was used to check the morphology and histology of liver tissues. The serum and liver tissues were harvested to test biochemical indexes and the expression levels of genes related with glucose and lipid metabolism, ER stress and unfold protein response (UPR). 16S rDNA sequence technology was conducted to check the fecal microbiota. Pre-administration with berberine could alleviate the excess accumulation of triglyceride (TG) in the liver of mice treated with tunicamycin. Tunicamycin administration caused significant increase of the expression level of genes related to ER stress and UPR, such as CHOP, Grp78 and ATF6, but the berberine pre-treatment could significantly downregulate the expression level of these genes. Tunicamycin administration resulted in increased ratio of Prevotellaceae to Erysipelotrichaceae at the family level of the fecal microbiota in mice, and this trend was reversed by the pre-treatment of berberine. These results demonstrated that berberine could improve liver injury induced hepatic metabolic disorders through relieving ER stress in hepatocytes and regulating gut microbiota in mice.

Branched Chain Lipid Metabolism As a Determinant of the N-Acyl Variation of Streptomyces Natural Products

Branched-chain fatty acids (BCFA) are encountered in Gram-positive bacteria, but less so in other organisms. The bacterial BCFA in membranes are typically saturated, with both odd- and even-numbered carbon chain lengths, and with methyl branches at either the ω-1 (iso) or ω-2 (anteiso) positions. The acylation with BCFA also contributes to the structural diversity of microbial natural products and potentially modulates biological activity. For the tunicamycin (TUN) family of natural products, the toxicity toward eukaryotes is highly dependent upon N-acylation with trans-2,3-unsaturated BCFA. The loss of the 2,3-unsaturation gives modified TUN with reduced eukaryotic toxicity but crucially with retention of the synergistic enhancement of the β-lactam group of antibiotics. Here, we infer from genomics, mass spectrometry, and deuterium labeling that the trans-2,3-unsaturated TUN variants and the saturated cellular lipids found in TUN-producing Streptomyces are derived from the same pool of BCFA metabolites. Moreover, non-natural primers of BCFA metabolism are selectively incorporated into the cellular lipids of TUN-producing Streptomyces and concomitantly produce structurally novel neo-branched TUN N-acyl variants.