Tissue-specific Proteogenomic Analysis of Plutella xylostella Larval Midgut Using a Multialgorithm Pipeline

Reverse engineering high-level resistance to Bt Cry1Ac toxin in Plutella xylostella reveals a hormonal regulatory feedback pathway

Decoding the molecular mechanisms of insect resistance to Bacillus thuringiensis (Bt) toxins is crucial for the sustainable utilization of Bt-based bioinsecticides and transgenic crops. Our previous studies showed that a hormone-responsive transcription factor FOXO binds to an inserted short interspersed nuclear element (SINE, named SE2), causing MAP4K4 overexpression and resistance to Bt Cry1Ac toxin in Plutella xylostella. Furthermore, titers of two upstream signaling hormones (20-hydroxyecdysone and juvenile hormone) were also found to be elevated in the resistant strain, but it was unclear whether this was due to natural variation or a feedback pathway. Here, we established a homozygous knock-in strain (SE2-KI) using a reverse genetic approach to insert the SE2 retrotransposon into the MAP4K4 promoter of a Cry1Ac-susceptible strain. The SE2 insertion induced MAP4K4 overexpression, which in turn caused a downregulation of midgut receptors and an identical resistance phenotype to that seen in the evolved resistant strain. Moreover, SE2 insertion significantly increased the levels of two insect hormones providing definitive evidence for a positive feedback regulatory pathway. This study unveils an as yet uncharacterized hormonal regulatory feedback pathway orchestrating Cry1Ac resistance in P. xylostella, providing new insights into the molecular basis of Bt resistance and informing suitable field resistance management strategies.

Critical Analysis of Multi-Omic Data from a Strain of Plutella xylostella Resistant to Bacillus thuringiensis Cry1Ac Toxin

Rapid evolution of resistance in crop pests to Bacillus thuringiensis (Bt) products threatens their widespread use, especially as pests appear to develop resistance through a range of different physiological adaptations. With such a diverse range of mechanisms reported, researchers have resorted to multi-omic approaches to understand the molecular basis of resistance. Such approaches generate a lot of data making it difficult to establish where causal links between physiological changes and resistance exist. In this study, a combination of RNA-Seq and iTRAQ was used with a strain of diamondback moth, Plutella xylostella (L.), whose resistance mechanism is well understood. While some of the causal molecular changes in the resistant strain were detected, other previously verified changes were not detected. We suggest that while multi-omic studies have use in validating a proposed resistance mechanism, they are of limited value in identifying such a mechanism in the first place.

The regulation landscape of MAPK signaling cascade for thwarting Bacillus thuringiensis infection in an insect host

Host-pathogen interactions are central components of ecological networks where the MAPK signaling pathways act as central hubs of these complex interactions. We have previously shown that an insect hormone modulated MAPK signaling cascade participates as a general switch to trans-regulate differential expression of diverse midgut genes in the diamondback moth, Plutella xylostella (L.) to cope with the insecticidal action of Cry1Ac toxin, produced by the entomopathogenic bacterium Bacillus thuringiensis (Bt). The relationship between topology and functions of this four-tiered phosphorylation signaling cascade, however, is an uncharted territory. Here, we carried out a genome-wide characterization of all the MAPK orthologs in P. xylostella to define their phylogenetic relationships and to confirm their evolutionary conserved modules. Results from quantitative phosphoproteomic analyses, combined with functional validations studies using specific inhibitors and dsRNAs lead us to establish a MAPK "road map", where p38 and ERK MAPK signaling pathways, in large part, mount a resistance response against Bt toxins through regulating the differential expression of multiple Cry toxin receptors and their non-receptor paralogs in P. xylostella midgut. These data not only advance our understanding of host-pathogen interactions in agricultural pests, but also inform the future development of biopesticides that could suppress Cry resistance phenotypes.

Proteogenomics-Guided Evaluation of RNA-Seq Assembly and Protein Database Construction for Emergent Model Organisms

Proteogenomics is gaining momentum as, today, genomics, transcriptomics, and proteomics can be readily performed on any new species. This approach allows key alterations to molecular pathways to be identified when comparing conditions. For animals and plants, RNA-seq-informed proteomics is the most popular means of interpreting tandem mass spectrometry spectra acquired for species for which the genome has not yet been sequenced. It relies on high-performance de novo RNA-seq assembly and optimized translation strategies. Here, several pre-treatments for Illumina RNA-seq reads before assembly are explored to translate the resulting contigs into useful polypeptide sequences. Experimental transcriptomics and proteomics datasets acquired for individual Gammarus fossarum freshwater crustaceans are used, the most relevant procedure is defined by the ratio of MS/MS spectra assigned to peptide sequences. Removing reads with a mean quality score of less than 17-which represents a single probable nucleotide error on 150-bp reads-prior to assembly, increases the proteomics outcome. The best translation using Transdecoder is achieved with a minimal open reading frame length of 50 amino acids and systematic selection of ORFs longer than 900 nucleotides. Using these parameters, transcriptome assembly and translation informed by proteomics pave the way to further improvements in proteogenomics.

Ubc9 Interacts with and SUMOylates the TCR Adaptor SLP-76 for NFAT Transcription in T Cells

Although the immune adaptor SH2 domain containing leukocyte phosphoprotein of 76 kDa (SLP-76) integrates and propagates the TCR signaling, the regulation of SLP-76 during the TCR signaling is incompletely studied. In this article, we report that SLP-76 interacts with the small ubiquitin-like modifier (SUMO) E2 conjugase Ubc9 and is a substrate for Ubc9-mediated SUMOylation in human and mouse T cells. TCR stimulation promotes SLP-76-Ubc9 binding, accompanied by an increase in SLP-76 SUMOylation. Ubc9 binds to the extreme C terminus of SLP-76 spanning residues 516-533 and SUMOylates SLP-76 at two conserved residues K266 and K284. In addition, SLP-76 and Ubc9 synergizes to augment the TCR-mediated IL-2 transcription by NFAT in a manner dependent of SUMOylation of SLP-76. Moreover, although not affecting the TCR proximal signaling events, the Ubc9-mediated SUMOylation of SLP-76 is required for TCR-induced assembly of Ubc9-NFAT complex for IL-2 transcription. Together, these results suggest that Ubc9 modulates the function of SLP-76 in T cell activation both by direct interaction and by SUMOylation of SLP-76 and that the Ubc9-SLP-76 module acts as a novel regulatory complex in the control of T cell activation.

CRISPR/Cas9-mediated knockout of both the PxABCC2 and PxABCC3 genes confers high-level resistance to Bacillus thuringiensis Cry1Ac toxin in the diamondback moth, Plutella xylostella (L.)

Rapid evolution of resistance by insect pests severely jeopardizes the sustainable utilization of biopesticides and transgenic crops that produce insecticidal crystal proteins derived from the entomopathogenic bacterium Bacillus thuringiensis (Bt). Recently, high levels of resistance to Bt Cry1 toxins have been reported to be genetically linked to the mutation or down-regulation of ABC transporter subfamily C genes ABCC2 and ABCC3 in seven lepidopteran insects, including Plutella xylostella (L.). To further determine the causal relationship between alterations in the PxABCC2 and PxABCC3 genes and Cry1Ac resistance in P. xylostella, the novel CRISPR/Cas9 genome engineering system was utilized to successfully construct two knockout strains: the ABCC2KO strain is homozygous for a 4-bp deletion in exon 3 of the PxABCC2 gene, and the ABCC3KO strain is homozygous for a 5-bp deletion in exon 3 of the PxABCC3 gene, both of which can produce only truncated ABCC proteins. Bioassay results indicated that high levels of resistance to the Cry1Ac protoxin were observed in both the ABCC2KO (724-fold) and ABCC3KO (413-fold) strains compared to the original susceptible DBM1Ac-S strain. Subsequently, dominance degree and genetic complementation tests demonstrated that Cry1Ac resistance in both the knockout strains was incompletely recessive, and Cry1Ac resistance alleles were located in the classic BtR-1 resistance locus that harbored the PxABCC2 and PxABCC3 genes, similar to the near-isogenic resistant NIL-R strain. Moreover, qualitative toxin binding assays revealed that the binding of the Cry1Ac toxin to midgut brush border membrane vesicles (BBMVs) in both knockout strains was dramatically reduced compared to that in the susceptible DBM1Ac-S strain. In summary, our CRISPR/Cas9-mediated genome editing study presents, for the first time, in vivo reverse genetics evidence for both the ABCC2 and ABCC3 proteins as midgut functional receptors for Bt Cry1 toxins in insects, which provides new insight into the pivotal roles of both the ABCC2 and ABCC3 proteins in the complex molecular mechanism of insect resistance to Bt Cry1 toxins.

De novo comparative transcriptome analysis of a rare cicada, with identification of candidate genes related to adaptation to a novel host plant and drier habitats

Background

Although the importance of host plant chemistry in plant–insect interactions is widely recognized, our understanding about the genetic basis underlying the relationship between changes in midgut proteins and adaptation of plant-feeding insects to novel host plants and habitats is very limited. To address this knowledge gap, the transcriptional profiles of midguts among three populations of the cicada Subpsaltria yangi Chen were compared. Among which, the Hancheng (HC) and Fengxiang (FX) populations occurring in the Loess Plateau feed on Ziziphus jujuba Mill. var. spinosa (Bunge) Hu ex H. F. Chow, while the population occurring in a much drier habitat in the Helan (HL) Mountains is locally specialized on a chemically divergent plant, Ephedra lepidosperma C. Y. Cheng.

Results

Based on comparative analysis, 1826 (HL vs HC) differentially expressed genes (DEGs) and 723 DEGs (HL vs FX) were identified between the populations utilizing different host plants, including 20, 36, 2, 5 and 2 genes related to digestion, detoxification, oxidation-reduction, stress response and water-deprivation response, respectively, and 35 genes presumably associated with osmoregulation. However, only 183 DEGs were identified between the HC and FX populations, including two genes related to detoxification, two genes related to stress response, and one gene presumably associated with osmoregulation. These results suggest that the weakest expression differences were between the populations utilizing the same host plant and occurring in the closest habitats, which may help explain the metabolic mechanism of adaptation in S. yangi populations to novel host plants and new niches.

Conclusions

The observed differences in gene expression among S. yangi populations are consistent with the hypothesis that the host plant shift and habitat adaptation in the HL population was facilitated by differential regulation of genes related to digestion, detoxification, oxidation-reduction, stress response, water-deprivation response and osmoregulation. The results may inform future studies on the molecular mechanisms underlying the relationship between changes in midgut proteins and adaptation of herbivorous insects to novel host plants and new niches.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5547-y) contains supplementary material, which is available to authorized users.

Quantitative Proteomics Analysis of Membrane Proteins in Enterococcus faecalis With Low-Level Linezolid-Resistance

Despite increasing reports of low-level linezolid-resistant enterococci worldwide, the mechanism of this resistance remains poorly understood. Previous transcriptome studies of low-level linezolid-resistant Enterococcus faecalis isolates have demonstrated a number of significantly up-regulated genes potentially involved in mediation of drug resistance. However, whether the transcriptome faithfully reflects the proteome remains unknown. In this study, we performed quantitative proteomics analysis of membrane proteins in an E. faecalis isolate (P10748) with low-level linezolid-resistance in comparison with two linezolid-susceptible strains 3138 and ATCC 29212, all of which have been previously investigated by whole transcriptome analysis. A total of 8,197 peptides associated with 1,170 proteins were identified in all three isolates with false discovery rate (FDR) at 1% and P < 0.05. There were 14 significantly up-regulated and 6 significantly down-regulated proteins in strain P10748 compared to strains 3138 and ATCC 29212, which were in general positively correlated with transcription levels revealed in previous transcriptome studies. Our analysis suggests that the low-level linezolid-resistance in E. faecalis is conferred primarily by the ATP-binding cassette protein OptrA through ribosomal protection and, possibly, also by the enterococcal surface protein (Esp) and other proteins through biofilm formation. The genetic transfer of optrA is potentially regulated by the surface exclusion protein Sea1, conjugal transfer protein TraB, replication protein RepA and XRE family transcription regulator protein. This report represents the first investigation of the mechanisms of linezolid-resistance in E. faecalis by a quantitative proteomics approach.

Proteogenomic insights into uranium tolerance of a Chernobyl's Microbacterium bacterial isolate

Microbacterium oleivorans A9 is a uranium-tolerant actinobacteria isolated from the trench T22 located near the Chernobyl nuclear power plant. This site is contaminated with different radionuclides including uranium. To observe the molecular changes at the proteome level occurring in this strain upon uranyl exposure and understand molecular mechanisms explaining its uranium tolerance, we established its draft genome and used this raw information to perform an in-depth proteogenomics study. High-throughput proteomics were performed on cells exposed or not to 10μM uranyl nitrate sampled at three previously identified phases of uranyl tolerance. We experimentally detected and annotated 1532 proteins and highlighted a total of 591 proteins for which abundances were significantly differing between conditions. Notably, proteins involved in phosphate and iron metabolisms show high dynamics. A large ratio of proteins more abundant upon uranyl stress, are distant from functionally-annotated known proteins, highlighting the lack of fundamental knowledge regarding numerous key molecular players from soil bacteria.

BIOLOGICAL SIGNIFICANCE

Microbacterium oleivorans A9 is an interesting environmental model to understand biological processes engaged in tolerance to radionuclides. Using an innovative proteogenomics approach, we explored its molecular mechanisms involved in uranium tolerance. We sequenced its genome, interpreted high-throughput proteomic data against a six-reading frame ORF database deduced from the draft genome, annotated the identified proteins and compared protein abundances from cells exposed or not to uranyl stress after a cascade search. These data show that a complex cellular response to uranium occurs in Microbacterium oleivorans A9, where one third of the experimental proteome is modified. In particular, the uranyl stress perturbed the phosphate and iron metabolic pathways. Furthermore, several transporters have been identified to be specifically associated to uranyl stress, paving the way to the development of biotechnological tools for uranium decontamination.

Integrating transcriptome and proteome profiling: Strategies and applications

Discovering the gene expression signature associated with a cellular state is one of the basic quests in majority of biological studies. For most of the clinical and cellular manifestations, these molecular differences may be exhibited across multiple layers of gene regulation like genomic variations, gene expression, protein translation and post-translational modifications. These system wide variations are dynamic in nature and their crosstalk is overwhelmingly complex, thus analyzing them separately may not be very informative. This necessitates the integrative analysis of such multiple layers of information to understand the interplay of the individual components of the biological system. Recent developments in high throughput RNA sequencing and mass spectrometric (MS) technologies to probe transcripts and proteins made these as preferred methods for understanding global gene regulation. Subsequently, improvements in "big-data" analysis techniques enable novel conclusions to be drawn from integrative transcriptomic-proteomic analysis. The unified analyses of both these data types have been rewarding for several biological objectives like improving genome annotation, predicting RNA-protein quantities, deciphering gene regulations, discovering disease markers and drug targets. There are different ways in which transcriptomics and proteomics data can be integrated; each aiming for different research objectives. Here, we review various studies, approaches and computational tools targeted for integrative analysis of these two high-throughput omics methods.