Development of a monoclonal antibody with equal reactivity to ustiloxins A and B for quantification of main cyclopeptide mycotoxins in rice samples

Ustiloxin A impairs oocyte quality by disrupting organelles function

Oocyte quality is pivotal for fertilization and early embryonic development. Ustiloxin A (UA), is an emerging mycotoxin that has been frequently detected in rice and paddy. Because UA has been reported to be phytotoxic and cytotoxic, it poses a potential hazard to human and animal health. However, the effects of UA on oocyte maturation remain unknown. Here, we investigated the effects of acute UA exposure on mouse oocyte maturation. First, UA exposure inhibited oocyte maturation in a concentration-dependent manner and induced meiotic arrest by disrupting spindle assembly and reducing actin density. Moreover, mitochondrial function was substantially disrupted in oocytes upon UA exposure. Aberrant mitochondrial distribution, substantial downregulation of mitochondrial dynamics-associated genes Mfn1, Mfn2 and Fis1, decreased membrane potential and TOM20 expression were observed in UA-exposed oocytes; these effects further led to oxidative stress and DNA damage. Furthermore, UA induced ER and Golgi dysfunction and triggered ER stress by increasing GRP78 expression, which ultimately resulted in autophagy and early apoptosis in oocytes. Therefore, these results demonstrate that UA impairs oocyte quality by disrupting organelles function, providing new insight into the influence of UA on female reproduction in mammals.

UvHOS3-mediated histone deacetylation is essential for virulence and negatively regulates ustilaginoidin biosynthesis in Ustilaginoidea virens

Ustilaginoidea virens is the causal agent of rice false smut, which has recently become one of the most important rice diseases worldwide. Ustilaginoidins, a major type of mycotoxins produced in false smut balls, greatly deteriorates grain quality. Histone acetylation and deacetylation are involved in regulating secondary metabolism in fungi. However, little is yet known on the functions of histone deacetylases (HDACs) in virulence and mycotoxin biosynthesis in U. virens. Here, we characterized the functions of the HDAC UvHOS3 in U. virens. The ΔUvhos3 deletion mutant exhibited the phenotypes of retarded growth, increased mycelial branches and reduced conidiation and virulence. The ΔUvhos3 mutants were more sensitive to sorbitol, sodium dodecyl sulphate and oxidative stress/H2 O2 . ΔUvhos3 generated significantly more ustilaginoidins. RNA-Seq and metabolomics analyses also revealed that UvHOS3 is a key negative player in regulating secondary metabolism, especially mycotoxin biosynthesis. Notably, UvHOS3 mediates deacetylation of H3 and H4 at H3K9, H3K18, H3K27 and H4K8 residues. Chromatin immunoprecipitation assays indicated that UvHOS3 regulates mycotoxin biosynthesis, particularly for ustilaginoidin and sorbicillinoid production, by modulating the acetylation level of H3K18. Collectively, this study deepens the understanding of molecular mechanisms of the HDAC UvHOS3 in regulating virulence and mycotoxin biosynthesis in phytopathogenic fungi.

Full-length transcriptomics study of Ustiloxins-induced hepatocyte injury

Ustiloxins is a mycotoxin produced by the metabolism of Rice false smut. Studies have shown that Ustiloxins may be toxic to animals, but there is still a lack of toxicological evidence. The liver, as the main organ for the biotransformation of foreign chemicals, may be the direct target organ of Ustiloxins toxicity. In this study, we found that cell viability decreased in a dose- and time-dependent manner when BNL CL.2 cells were treated with different concentrations of Ustiloxins (0, 5, 10, 20, 30, 40, 60, 80, 100, 150 and 200 μg/mL) for 24 and 48 h. In addition, scanning electron microscope observation showed that the cell membrane of the experimental group was damaged, with the appearance of apoptotic bodies. Moreover, the ROS and GSH levels were significantly increased in cells exposed to Ustiloxins. We analyzed the key action targets of Ustiloxins on hepatocyte injury using full-length transcriptomics. A total of 1099 differentially expressed genes were screened, of which 473 genes were up-regulated, and 626 genes were down-regulated. Besides, we also found that the expression of MCM7 and CDC45 in BNL CL.2 cells treated with Ustiloxins decreased, and the expression of CCl-2, CYP1b1, CYP4f13, and GSTM1 increased according to qRT-PCR. Ustiloxins might change CYP450 and GST-related genes, affect DNA replication and cell cycle, and lead to oxidative stress and liver cell injury.

Inhibition of rice germination by ustiloxin A involves alteration in carbon metabolism and amino acid utilization

Ustiloxins are the main mycotoxin in rice false smut, a devastating disease caused by Ustilaginoidea virens. A typical phytotoxicity of ustiloxins is strong inhibition of seed germination, but the physiological mechanism is not clear. Here, we show that the inhibition of rice germination by ustiloxin A (UA) is dose-dependent. The sugar availability in UA-treated embryo was lower while the starch residue in endosperm was higher. The transcripts and metabolites responsive to typical UA treatment were investigated. The expression of several SWEET genes responsible for sugar transport in embryo was down-regulated by UA. Glycolysis and pentose phosphate processes in embryo were transcriptionally repressed. Most of the amino acids detected in endosperm and embryo were variously decreased. Ribosomal RNAs for growth were inhibited while the secondary metabolite salicylic acid was also decreased under UA. Hence, we propose that the inhibition of seed germination by UA involves the block of sugar transport from endosperm to embryo, leading to altered carbon metabolism and amino acid utilization in rice plants. Our analysis provides a framework for understanding of the molecular mechanisms of ustiloxins on rice growth and in pathogen infection.

Ustiloxin A inhibits proliferation of renal tubular epithelial cells in vitro and induces renal injury in mice by disrupting structure and respiratory function of mitochondria

Recently, we found that Ustiloxin A (UA, a mycotoxin) was widely detected in paddy environment and rice samples from several countries, and was also detected in human urine samples from China. However, the current knowledge about the health risks of UA are limited. In this research, the cytotoxicity of UA in mice renal tubular epithelial cells (mRTECs) was evaluated, and the results indicated that UA arrested cell cycle in G2/M phase via altering cellular morphology and microtubule, and inhibited the proliferation and division of mRTECs. Furthermore, UA could inhibit mitochondrial respiration via binding to the CoQ-binding site in dihydro-orotate dehydrogenase (DHODH) protein, and resulted in mitochondrial damage. These adverse effects of UA on mitochondria might be responsible for the cytotoxicity observed in vitro. In vivo, UA at concentrations that were comparable to the realistic concentrations of human exposure induced renal insufficiency in mice, and this might be associated with the renal mitochondrial damage in mice. However, exposure to UA at those realistic concentrations did not promote the progression from renal insufficiency to renal fibrosis and chronic kidney disease was not observed in mice.

Occurrence and translocation of ustiloxins in rice false smut-occurred paddy fields, Hubei, China

Ustiloxin A (UA) and ustiloxin B (UB), two major mycotoxins produced by the pathogen of rice false smut (RFS) during rice cultivation, have attracted increasing attentions due to their potential health risks. However, limited data are available about their occurrence and fate in paddy fields and contamination profiles in rice. In this study, a field study was performed to investigate the occurrence and translocation of UA and UB in RFS-occurred paddies. For the first time to our knowledge, we reported a ubiquitous occurrence of the two ustiloxins in the paddy water (range: 0.01-3.46 μg/L for UA and <0.02-1.15 μg/L for UB) and brown rice (range: 0.09-154.08 μg/kg for UA and <0.09-23.57 μg/kg for UB). A significant positive correlation was observed between ustiloxin levels in paddy water and brown rice (r_s = 0.48-0.79, p < 0.01). The occurrence of ustiloxin uptake in water-rice system was also evidenced by the rice exposure experiment, suggesting paddy water might be an important source for ustiloxin accumulation in rice. These results suggested that the contamination of ustiloxins in rice might occur widely, which was supported by the significantly high detection frequencies of UA (96.6%) and UB (62.4%) in polished rice (149 samples) from Hubei Province, China. The total concentrations of ustiloxins in the polished rice samples collected from Hubei Province ranged from <20.7 ng/kg (LOD) to 55.1 μg/kg (dry weight). Further studies are needed to evaluate the potential risks of ustiloxin exposure in the environment and humans.

Global distribution of ustiloxins in rice and their male-biased hepatotoxicity

Ustiloxins, a group of bioactive metabolites produced by the pathogen of rice false smut (RFS), have emerged as ubiquitous contaminants in RFS-occurred paddy fields and could accumulate in rice. Nevertheless, the prevalence of ustiloxins in rice and exposure risks of humans are limited. In this study, concentrations of ustiloxin A (UA) and ustiloxins B (UB), which are two predominant ustiloxins, were measured in 240 rice samples from China and 72 rice samples from 12 other counties. The detection rates (DRs) of UA and UB were 82.1% and 49.3%, respectively, and their concentrations in rice ranged from below detection limit (LOD: 0.22 μg/kg) to 85.96 μg/kg dw. Furthermore, for the first time, we reported the occurrence of UA (DR = 22.8%) in urine collected from residues of Enshi city, China. Urinary UA were significantly correlated with the activities of alanine aminotransferase in male, and this male-biased hepatotoxicity was further confirmed in mice exposure experiment. This study for the first time reported the widespread geographical distribution of ustiloxins in rice, as well as emphasized the occurrence of internal exposure and potential health risk in humans.

Development of Generic Immuno-Magnetic Bead-Based Enzyme-Linked Immunoassay for Ustiloxins in Rice Coupled with Enrichment

Ustiloxins are a group of mycotoxins produced by rice false smut pathogen. Previous studies have shown that the false smut balls contain six types of ustiloxins, and these toxins are toxic to living organisms. Thus, immunoassay for on-site monitoring of ustiloxins in rice is urgently required. The current immunoassays are only for detecting single ustiloxin, and they cannot meet the demand for synchronous and rapid detection of the group toxins. Therefore, this study designed and synthesized a generic antigen with ustiloxin G as material based on the common structure of the mycotoxins. Ustiloxin G was conjugated to two carrier proteins including bovine serum albumin (BSA) and ovalbvmin (OVA) by carbon diimide method. The mice were immunized with ustiloxin-G-BSA to generate the antibody serum, which was further purified to obtain the generic antibody against ustiloxins. The conjugated ustiloxin G-OVA and generic antibodies were used for establishing the enzyme-linked immunosorbent assay (ELISA) for ustiloxin detection and optimizing experiment conditions. The characterization of the antibody showed that the semi-inhibitory concentrations (IC50) of ustiloxin A, B, and G were 0.53, 0.34, and 0.06 µg/mL, respectively, and that their corresponding cross-reactivities were 11.9%, 18.4%, and 100%, respectively. To increase ELISA detection efficiency, generic antibody was combined with magnetic beads to obtain sensitive and class-specific immune-magnetic beads. Based on these immuno-magnetic beads, a high-efficiency enzyme-linked immunoassay method was developed for ustiloxin detection, whose sensitivity to ustiloxin A, B, and G was improved to 0.15 µg/mL, 0.14 µg/mL, and 0.04 µg/mL, respectively. The method accuracy was evaluated by spiking ustiloxin G as standard, and the spiked samples were tested by the immune-magnetic bead-based ELISA. The result showed the ustiloxin G recoveries ranged from 101.9% to 116.4% and were accepted by a standard HPLC method, indicating that our developed method would be promising for on-site monitoring of ustiloxins in rice.

Detection of Ustiloxin A in urine by ultra-high-performance liquid chromatography-tandem mass spectrometry coupled with two-step solid-phase extraction

Due to global outbreak of rice false smut disease, ustiloxin A (UA) was detected in rice. However, accurate methods for monitoring UA in human body fluids were lacking. In this context, a UPLC-MS/MS method based on two-step SPE was constructed for measuring UA in urine. The limits of UA quantification in human and mice urine were 58.3 and 108.7 ng/L, respectively. The proposed method was applied to detect UA in urine samples collected from human and mice. After dietary exposure, the contents of UA in mice urine were from 6.03 to 16.76 μg/g of creatine, accounting for approximate 14% of daily intake dose. Furthermore, due to the trace residues in rice (78-109 ng/kg), no detectable UA was observed in the urine of 20 volunteers. To the best of our knowledge, it is the first time to report the occurrence of UA in mammal urine.