Activation of the NLRP3 Inflammasome by Hyaboron, a New Asymmetric Boron-Containing Macrodiolide from the Myxobacterium Hyalangium minutum

Recent advances in discovery and biosynthesis of natural products from myxobacteria: an overview from 2017 to 2023

Covering: 2017.01 to 2023.11Natural products biosynthesized by myxobacteria are appealing due to their sophisticated chemical skeletons, remarkable biological activities, and intriguing biosynthetic enzymology. This review aims to systematically summarize the advances in the discovery methods, new structures, and bioactivities of myxobacterial NPs reported in the period of 2017-2023. In addition, the peculiar biosynthetic pathways of several structural families are also highlighted.

Hyalangiumruber sp. nov, characterization of a novel myxobacterium strain s54d21 and their secondary metabolites

Myxobacteria are special bacteria with wide adaptability, which are rich sources of structurally diverse natural products with intriguing biological properties. Here, a gram-negative myxobacterium strain s54d21T was isolated from the sediment of a wetland park in China using the Escherichia coli baiting method. Based on 16S rRNA gene sequence and genomic data, the strain was demonstrated to be a novel species of a rare genus Hyalangium, designated Hyalangium ruber sp. nov (type strain s54d21T = GDMCC 1.1945T = JCM 39263T). The subsequent chemical investigation of the strain s54d21T led to the isolation of three rare 3,5,6-trisubstituted 2(1H)-pyrazinones, namely, hyalanones A-C (1-3), together with a known macrolactin A (4). Those new structures and their absolute configurations were unambiguously assigned by extensive analyses of spectroscopic data and density functional theory (DFT) calculations. In biological assays, compound 4 exhibited moderate cytotoxic activities against human cell lines RKO, A549, and NCM460 with IC50 values ranging from 27.21 to 32.14 μM.

Characterization of phytopathogen-preying Hyalangium versicolor sp. nov., and proposal for the reclassification of Cystobacter gracilis as Hyalangium gracile comb. Nov

A Gram-staining-negative, aerobic, and rod-shaped with tapered end myxobacterium, designed as strain H56D21^T, was isolated from forest soil sampled from the Diaoluo Mountain National Nature Reserve located in Hainan Province, PR China. It showed prey ability on two kinds of phytopathogens including both fungi (Fusarium solani, Fusarium graminearum, and Fusarium oxysporum) and bacteria (Ralstonia solanacearum and Xanthomonas campestris). Phylogenetic analyses based on the 16S rRNA gene and core genes  sequences revealed that strain H56D21^T belonged to the genus Hyalangium and was most closely related to Cystobacter gracilis DSM 14753 ^T and Hyalangium minutum DSM 14724 ^T. Genome comparison showed 85.6% and 82.3% of average nucleotide identity between strain H56D21^T and the above two type strains and 29.8% and 25.1% of digital DNA-DNA hybridization , respectively. The novel strain had a large genome size of 13.56 Mbp and a high  DNA G + C content of 67.1%. Genome annotation identified 46 secondary metabolite biosynthesis gene clusters and 187 CAZymes-encoding genes. The major fatty acids contained iso-C_15:0, iso-C_15:0 DMA, C_16:1 ω5c, and iso-C_17:0. The dominant respiratory quinone was menaquinone 8. Based on the phenotypic, chemotaxonomic and phylogenetic analyses, we suggested that strain H56D21^T should represent a novel species of the genus Hyalangium with a proposed name of Hyalangium versicolor sp. nov. (type strain H56D21^T = GDMCC 1.1944 ^T = KCTC 82613 ^T) and Cystobacter gracilis should be reclassified as Hyalangium gracile comb. nov.

A Drimane Meroterpenoid Borate as a Synchronous Ca+ Oscillation Inhibitor from the Coral-Associated Fungus Alternaria sp. ZH-15

Territrem F (1), a drimane meroterpenoid bearing a unique borate ring system, was isolated together with its diol precursor territrem B (2) from the fungus Alternaria sp. ZH-15 associated with the soft coral Lobophytum crassum collected in the South China Sea. The structure of the new compound was elucidated by spectroscopic analysis and an X-ray single-crystal diffraction study, representing a new type of boron-containing natural product. Both compounds significantly inhibited spontaneous synchronous Ca²⁺ oscillations (SCOs) and epileptic discharges induced by 4-aminopyridine, showing the potential for antiepileptic drug research. The 5,9-boronic ester derivative of 2 did not change its SCO inhibitory activity.

Composites of Nucleic Acids and Boron Clusters (C2B10H12) as Functional Nanoparticles for Downregulation of EGFR Oncogene in Cancer Cells

Epidermal growth factor receptor (EGFR) is one of the most promising molecular targets for anticancer therapy. We used boron clusters as a platform for generation of new materials. For this, functional DNA constructs conjugated with boron clusters (B-ASOs) were developed. These B-ASOs, built from 1,2-dicarba-closo-dodecaborane linked with two anti-EGFR antisense oligonucleotides (ASOs), form with their complementary congeners torus-like nanostructures, as previously shown by atomic force microscope (AFM) and transmission electron cryo-microscopy (cryo-TEM) imaging. In the present work, deepened studies were carried out on B-ASO's properties. In solution, B-ASOs formed four dominant complexes as confirmed by non-denaturing polyacrylamide gel electrophoresis (PAGE). These complexes exhibited increased stability in cell lysate comparing to the non-modified ASO. Fluorescently labeled B-ASOs localized mostly in the cytoplasm and decreased EGFR expression by activating RNase H. Moreover, the B-ASO complexes altered the cancer cell phenotype, decreased cell migration rate, and arrested the cells in the S phase of cell cycle. The 1,2-dicarba-closo-dodecaborane-containing nanostructures did not activate NLRP3 inflammasome in human macrophages. In addition, as shown by inductively coupled plasma mass spectrometry (ICP MS), these nanostructures effectively penetrated the human squamous carcinoma cells (A431), showing their potential applicability as anticancer agents.

A conserved coccidian gene is involved in Toxoplasma sensitivity to the anti-apicomplexan compound, tartrolon E

New treatments for the diseases caused by apicomplexans are needed. Recently, we determined that tartrolon E (trtE), a secondary metabolite derived from a shipworm symbiotic bacterium, has broad-spectrum anti-apicomplexan parasite activity. TrtE inhibits apicomplexans at nM concentrations in vitro, including Cryptosporidium parvum, Toxoplasma gondii, Sarcocystis neurona, Plasmodium falciparum, Babesia spp. and Theileria equi. To investigate the mechanism of action of trtE against apicomplexan parasites, we examined changes in the transcriptome of trtE-treated T. gondii parasites. RNA-Seq data revealed that the gene, TGGT1_272370, which is broadly conserved in the coccidia, is significantly upregulated within 4 h of treatment. Using bioinformatics and proteome data available on ToxoDB, we determined that the protein product of this tartrolon E responsive gene (trg) has multiple transmembrane domains, a phosphorylation site, and localizes to the plasma membrane. Deletion of trg in a luciferase-expressing T. gondii strain by CRISPR/Cas9 resulted in a 68% increase in parasite resistance to trtE treatment, supporting a role for the trg protein product in the response of T. gondii to trtE treatment. Trg is conserved in the coccidia, but not in more distantly related apicomplexans, indicating that this response to trtE may be unique to the coccidians, and other mechanisms may be operating in other trtE-sensitive apicomplexans. Uncovering the mechanisms by which trtE inhibits apicomplexans may identify shared pathways critical to apicomplexan parasite survival and advance the search for new treatments.

A symbiotic bacterium of shipworms produces a compound with broad spectrum anti-apicomplexan activity

Apicomplexan parasites cause severe disease in both humans and their domesticated animals. Since these parasites readily develop drug resistance, development of new, effective drugs to treat infection caused by these parasites is an ongoing challenge for the medical and veterinary communities. We hypothesized that invertebrate-bacterial symbioses might be a rich source of anti-apicomplexan compounds because invertebrates are susceptible to infections with gregarines, parasites that are ancestral to all apicomplexans. We chose to explore the therapeutic potential of shipworm symbiotic bacteria as they are bona fide symbionts, are easily grown in axenic culture and have genomes rich in secondary metabolite loci [1,2]. Two strains of the shipworm symbiotic bacterium, Teredinibacter turnerae, were screened for activity against Toxoplasma gondii and one strain, T7901, exhibited activity against intracellular stages of the parasite. Bioassay-guided fractionation identified tartrolon E (trtE) as the source of the activity. TrtE has an EC50 of 3 nM against T. gondii, acts directly on the parasite itself and kills the parasites after two hours of treatment. TrtE exhibits nanomolar to picomolar level activity against Cryptosporidium, Plasmodium, Babesia, Theileria, and Sarcocystis; parasites representing all branches of the apicomplexan phylogenetic tree. The compound also proved effective against Cryptosporidium parvum infection in neonatal mice, indicating that trtE may be a potential lead compound for preclinical development. Identification of a promising new compound after such limited screening strongly encourages further mining of invertebrate symbionts for new anti-parasitic therapeutics.

Recent progress of antibacterial natural products: Future antibiotics candidates

The discovery of novel antibacterial molecules plays a key role in solving the current antibiotic crisis issue. Natural products have long been an important source of drug discovery. Herein, we reviewed 256 natural products from 11 structural classes in the period of 2016-01/2020, which were selected by SciFinder with new compounds or new structures and MICs lower than 10 μg/mL or 10 μM as criterions. This review will provide some effective antibacterial lead compounds for medicinal chemists, which will promote the antibiotics research based on natural products to the next level.

New Peptaibiotics and a Cyclodepsipeptide from Ijuhya vitellina: Isolation, Identification, Cytotoxic and Nematicidal Activities

Fungal associations with nematodes have attracted scientific attention because of the need to develop new biocontrol agents. In this context, Ijuhya vitellina, an antagonistic fungus previously isolated from the plant parasitic cyst nematode Heterodera filipjevi, was selected to carry out an in-depth metabolomic study for its active metabolites. Herein, three new nonapeptide peptaibols with leucinostatin based sequences were isolated and identified by 1, 2D NMR, and HR-ESI-MS-MS. The absolute configuration was assigned based on Marfay’s analysis and Mosher ester formation. The new leucinostatins manifested moderate nematicidal effect against the plant pathogenic nematode Pratylenchus penetrans with LD₉₀ values ranging from 5 to 7 µg/mL. Furthermore, a cyclodepsipeptide, named arthrichitin D, with five amino acid residues attached to a 3-hydroxy-2,4-dimethylhexadeca-4,6-dienoic fatty acid chain was discovered and showed weak nematicidal effect against Caenorhabditis elegans. Chaetoglobosin B and its 19-O-acetyl derivative were also obtained as minor metabolites, and the activity of chaetoglobosin B on the actin cytoskeleton of mammalian cells was assessed.

BAX/BAK-Induced Apoptosis Results in Caspase-8-Dependent IL-1β Maturation in Macrophages

IL-1β is a cytokine of pivotal importance to the orchestration of inflammatory responses. Synthesized as an inactive pro-cytokine, IL-1β requires proteolytic maturation to gain biological activity. Here, we identify intrinsic apoptosis as a non-canonical trigger of IL-1β maturation. Guided by the discovery of the immunomodulatory activity of vioprolides, cyclic peptides isolated from myxobacteria, we observe IL-1β maturation independent of canonical inflammasome pathways, yet dependent on intrinsic apoptosis. Mechanistically, vioprolides inhibit MCL-1 and BCL2, which in turn triggers BAX/BAK-dependent mitochondrial outer membrane permeabilization (MOMP). Induction of MOMP results in the release of pro-apoptotic factors initiating intrinsic apoptosis, as well as the depletion of IAPs (inhibitors of apoptosis proteins). IAP depletion, in turn, operates upstream of ripoptosome complex formation, subsequently resulting in caspase-8-dependent IL-1β maturation. These results establish the ripoptosome/caspase-8 complex as a pro-inflammatory checkpoint that senses the perturbation of mitochondrial integrity.

Caspases in Cell Death, Inflammation, and Disease

Caspases are an evolutionary conserved family of cysteine proteases that are centrally involved in cell death and inflammation responses. A wealth of foundational insight into the molecular mechanisms that control caspase activation has emerged in recent years. Important advancements include the identification of additional inflammasome platforms and pathways that regulate activation of inflammatory caspases; the discovery of gasdermin D as the effector of pyroptosis and interleukin (IL)-1 and IL-18 secretion; and the existence of substantial crosstalk between inflammatory and apoptotic initiator caspases. A better understanding of the mechanisms regulating caspase activation has supported initial efforts to modulate dysfunctional cell death and inflammation pathways in a suite of communicable, inflammatory, malignant, metabolic, and neurodegenerative diseases. Here, we review current understanding of caspase biology with a prime focus on the inflammatory caspases and outline important topics for future experimentation.

Effects of boron-containing compounds on immune responses: review and patenting trends

INTRODUCTION

Boron-containing compounds induce effects on immune responses. Such effects are interesting to the biomedical field for the development of therapeutic tools to modulate the immune system.

AREAS COVERED

The scope of BCC use to modify immune responses is expanding, mainly with regard to inflammatory diseases. The information was organized to demonstrate the breadth of reported effects. BCCs act as modulators of innate and adaptive immunity, with the former including regulation of cluster differentiation and cytokine production. In addition, BCCs exert effects on inflammation induced by infectious and noninfectious agents, and there are also reports regarding their effects on mechanisms involving hypersensitivity and transplants. Finally, the authors discuss the beneficial effects of BCCs on pathologies involving various targets and mechanisms.

EXPERT OPINION

Some BCCs are currently used as drugs in humans. The mechanisms by which these BCCs modulate immune responses, as well as the required structure-activity relationship for each observed mechanism of action, should be clarified. The former will allow for the development of improved immunomodulatory drugs with extensive applications in medicine. Patenting trends involve claims concerning the synthesis and actions of identified molecules with a defined profile regarding cytokines, cell differentiation, proliferation, and antibody production.