Secreted venom allergen-like proteins of helminths: Conserved modulators of host responses in animals and plants.

Despite causing considerable damage to host tissue at the onset of parasitism, invasive helminths establish remarkably persistent infections in both animals and plants. Secretions released by these obligate parasites during host invasion are thought to be crucial for their persistence in infection. Helminth secretions are complex mixtures of molecules, most of which have unknown molecular targets and functions in host cells or tissues. Although the habitats of animal- and plant-parasitic helminths are very distinct, their secretions share the presence of a structurally conserved group of proteins called venom allergen-like proteins (VALs). Helminths abundantly secrete VALs during several stages of parasitism while inflicting extensive damage to host tissue. The tight association between the secretion of VALs and the onset of parasitism has triggered a particular interest in this group of proteins, as improved knowledge on their biological functions may assist in designing novel protection strategies against parasites in humans, livestock, and important food crops.

An Immuno-Suppressive Aphid Saliva Protein Is Delivered into the Cytosol of Plant Mesophyll Cells During Feeding.

Herbivore selection of plant hosts and plant responses to insect colonization have been subjects of intense investigations. A growing body of evidence suggests that, for successful colonization to occur, (effector/virulence) proteins in insect saliva must modulate plant defense responses to the benefit of the insect. A range of insect saliva proteins that modulate plant defense responses have been identified, but there is no direct evidence that these proteins are delivered into specific plant tissues and enter plant cells. Aphids and other sap-sucking insects of the order Hemiptera use their specialized mouthparts (stylets) to probe plant mesophyll cells until they reach the phloem cells for long-term feeding. Here, we show, by immunogold-labeling of ultrathin sections of aphid feeding sites, that an immuno-suppressive aphid effector localizes in the cytoplasm of mesophyll cells near aphid stylets but not in cells further away from aphid feeding sites. In contrast, another aphid effector protein localizes in the sheaths composed of gelling saliva that surround the aphid stylets. Thus, insects deliver effectors directly into plant tissue. Moreover, different aphid effectors locate extracellularly in the sheath saliva or are introduced into the cytoplasm of plant cells. [Formula: see text] Copyright © 2016 The Author(s). This is an open-access article distributed under the CC BY-NC-ND 4.0 International license .

The leucine-rich repeat receptor-like kinase BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 and the cytochrome P450 PHYTOALEXIN DEFICIENT3 contribute to innate immunity to aphids in Arabidopsis.

The importance of pathogen-associated molecular pattern-triggered immunity (PTI) against microbial pathogens has been recently demonstrated. However, it is currently unclear if this layer of immunity mediated by surface-localized pattern recognition receptors (PRRs) also plays a role in basal resistance to insects, such as aphids. Here, we show that PTI is an important component of plant innate immunity to insects. Extract of the green peach aphid (GPA; Myzus persicae) triggers responses characteristic of PTI in Arabidopsis (Arabidopsis thaliana). Two separate eliciting GPA-derived fractions trigger induced resistance to GPA that is dependent on the leucine-rich repeat receptor-like kinase BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1)/SOMATIC-EMBRYOGENESIS RECEPTOR-LIKE KINASE3, which is a key regulator of several leucine-rich repeat-containing PRRs. BAK1 is required for GPA elicitor-mediated induction of reactive oxygen species and callose deposition. Arabidopsis bak1 mutant plants are also compromised in immunity to the pea aphid (Acyrthosiphon pisum), for which Arabidopsis is normally a nonhost. Aphid-derived elicitors induce expression of PHYTOALEXIN DEFICIENT3 (PAD3), a key cytochrome P450 involved in the biosynthesis of camalexin, which is a major Arabidopsis phytoalexin that is toxic to GPA. PAD3 is also required for induced resistance to GPA, independently of BAK1 and reactive oxygen species production. Our results reveal that plant innate immunity to insects may involve early perception of elicitors by cell surface-localized PRRs, leading to subsequent downstream immune signaling.

Resistance of Arabidopsis thaliana to the green peach aphid, Myzus persicae, involves camalexin and is regulated by microRNAs.

· Small RNAs play important roles in resistance to plant viruses and the complex responses against pathogens and leaf-chewing insects. · We investigated whether small RNA pathways are involved in Arabidopsis resistance against a phloem-feeding insect, the green peach aphid (Myzus persicae). We used a 2-wk fecundity assay to assess aphid performance on Arabidopsis RNA silencing and defence pathway mutants. Quantitative real-time polymerase chain reaction was used to monitor the transcriptional activity of defence-related genes in plants of varying aphid susceptibility. High-performance liquid chromatography-mass spectrometry was employed to measure the accumulation of the antimicrobial compound camalexin. Artificial diet assays allowed the assessment of the effect of camalexin on aphid performance. · Myzus persicae produces significantly less progeny on Arabidopsis microRNA (miRNA) pathway mutants. Plants unable to process miRNAs respond to aphid infestation with increased induction of PHYTOALEXIN DEFICIENT3 (PAD3) and production of camalexin. Aphids ingest camalexin when feeding on Arabidopsis and are more successful on pad3 and cyp79b2/cyp79b3 mutants defective in camalexin production. Aphids produce less progeny on artificial diets containing camalexin. · Our data indicate that camalexin functions beyond antimicrobial defence to also include hemipteran insects. This work also highlights the extensive role of the miRNA-mediated regulation of secondary metabolic defence pathways with relevance to resistance against a hemipteran pest.

Generation, culture, and stimulation of small intestinal murine organoids in parasitology research.

Parasitic helminth worms frequently infect the gastrointestinal tract and interact with the intestinal epithelium and specialized cell types within it. Intestinal organoids derived from stem cells that line the intestine represent a transformational technology in the study of epithelial-parasite dialogue. Here, we present a protocol for establishing small intestine organoid cultures and administering parasite products of interest to these cultures. We then describe steps for evaluating their impact by microscopy, flow cytometry, immunohistology, and mRNA gene expression. For complete details on the use and execution of this protocol, please refer to Drurey et al. (2022).1.

BMP signaling in the intestinal epithelium drives a critical feedback loop to restrain IL-13-driven tuft cell hyperplasia.

The intestinal tract is a common site for various types of infections including viruses, bacteria, and helminths, each requiring specific modes of immune defense. The intestinal epithelium has a pivotal role in both immune initiation and effector stages, which are coordinated by lymphocyte cytokines such as IFNγ, IL-13, and IL-22. Here, we studied intestinal epithelial immune responses using organoid image analysis based on a convolutional neural network, transcriptomic analysis, and in vivo infection models. We found that IL-13 and IL-22 both induce genes associated with goblet cells, but the resulting goblet cell phenotypes are dichotomous. Moreover, only IL-13-driven goblet cells are associated with classical NOTCH signaling. We further showed that IL-13 induces the bone morphogenetic protein (BMP) pathway, which acts in a negative feedback loop on immune type 2-driven tuft cell hyperplasia. This is associated with inhibiting Sox4 expression to putatively limit the tuft cell progenitor population. Blocking ALK2, a BMP receptor, with the inhibitor dorsomorphin homolog 1 (DMH1) interrupted the feedback loop, resulting in greater tuft cell numbers both in vitro and in vivo after infection with Nippostrongylus brasiliensis. Together, this investigation of cytokine effector responses revealed an unexpected and critical role for the BMP pathway in regulating type 2 immunity, which can be exploited to tailor epithelial immune responses.

Intestinal epithelial tuft cell induction is negated by a murine helminth and its secreted products.

Helminth parasites are adept manipulators of the immune system, using multiple strategies to evade the host type 2 response. In the intestinal niche, the epithelium is crucial for initiating type 2 immunity via tuft cells, which together with goblet cells expand dramatically in response to the type 2 cytokines IL-4 and IL-13. However, it is not known whether helminths modulate these epithelial cell populations. In vitro, using small intestinal organoids, we found that excretory/secretory products (HpES) from Heligmosomoides polygyrus blocked the effects of IL-4/13, inhibiting tuft and goblet cell gene expression and expansion, and inducing spheroid growth characteristic of fetal epithelium and homeostatic repair. Similar outcomes were seen in organoids exposed to parasite larvae. In vivo, H. polygyrus infection inhibited tuft cell responses to heterologous Nippostrongylus brasiliensis infection or succinate, and HpES also reduced succinate-stimulated tuft cell expansion. Our results demonstrate that helminth parasites reshape their intestinal environment in a novel strategy for undermining the host protective response.

The IL-25-dependent tuft cell circuit driven by intestinal helminths requires macrophage migration inhibitory factor (MIF).

Macrophage migration inhibitory factor (MIF) is a key innate immune mediator with chemokine- and cytokine-like properties in the inflammatory pathway. While its actions on macrophages are well-studied, its effects on other cell types are less understood. Here we report that MIF is required for expansion of intestinal tuft cells during infection with the helminth Nippostrongylus brasiliensis. MIF-deficient mice show defective innate responses following infection, lacking intestinal epithelial tuft cell hyperplasia or upregulation of goblet cell RELMß, and fail to expand eosinophil, type 2 innate lymphoid cell (ILC2) and macrophage (M2) populations. Similar effects were observed in MIF-sufficient wild-type mice given the MIF inhibitor 4-IPP. MIF had no direct effect on epithelial cells in organoid cultures, and MIF-deficient intestinal stem cells could generate tuft cells in vitro in the presence of type 2 cytokines. In vivo the lack of MIF could be fully compensated by administration of IL-25, restoring tuft cell differentiation and goblet cell expression of RELM-ß, demonstrating its requirement upstream of the ILC2-tuft cell circuit. Both ILC2s and macrophages expressed the MIF receptor CXCR4, indicating that MIF may act as an essential co-factor on both cell types to activate responses to IL-25 in helminth infection.

Helminth extracellular vesicles: Interactions with the host immune system.

As long-lived parasites, helminths depend upon immunomodulation of their hosts for survival. The release of excretory-secretory (ES) products, including proteins, lipids and RNAs is how successful host manipulation is achieved. It has recently been discovered that the ES products of helminths contain extracellular vesicles (EVs), with every species investigated found to secrete these lipid-bound structures. EVs are perfect for packaging and delivering immune modulators to target cell types. This review outlines the research carried out on helminth EVs and their constituents thus far, as well as their interaction with components of the mammalian immune system. We discuss how targeting EVs will aid treatment of helminth infection and consider how EVs and their immunomodulatory cargo could be used as therapeutics as we progress through this exciting era.

Extracellular vesicles: new targets for vaccines against helminth parasites.

The hunt for effective vaccines against the major helminth diseases of humans has yet to bear fruit despite much effort over several decades. No individual parasite antigen has proved to elicit full protective immunity, suggesting that combinatorial strategies may be required. Recently it has been discovered that extracellular vesicles released by parasitic helminths contain multiple potential immune modulators, which could together be targeted by a future vaccine. Increasing knowledge of helminth extracellular vesicle components, both enclosed by and exposed on the membrane, will open up a new field of targets for an effective vaccine. This review discusses the interactions between helminth extracellular vesicles and the immune system discovered thus far, and the advantages of targeting these lipid-bound packages with a vaccine. In addition, we also comment upon specific antigens that may be the best targets for an anti-helminth vaccine. In the future, extensive knowledge of the parasites' full arsenal in controlling their host may finally provide us with the ideal target for a fully effective vaccine.

Heligmosomoides polygyrus Venom Allergen-like Protein-4 (HpVAL-4) is a sterol binding protein.

Heligmosomoides polygyrus bakeri is a model parasitic hookworm used to study animal and human helminth diseases. During infection, the parasite releases excretory/secretory products that modulate the immune system of the host. The most abundant protein family in excretory/secretory products comprises the venom allergen-like proteins (VALs), which are members of the SCP/TAPS (sperm-coating protein/Tpx/antigen 5/pathogenesis related-1/Sc7) superfamily. There are >30 secreted Heligmosomoides polygyrus VAL proteins (HpVALs) and these proteins are characterised by having either one or two 15 kDa CAP (cysteine-rich secretory protein (CRISP)/antigen 5/pathogenesis related-1) domains. The first known HpVAL structure, HpVAL-4, refined to 1.9 Šis reported. HpVAL-4 was produced as a homogeneously glycosylated protein in leaves of Nicotiana benthamiana infiltrated with recombinant plasmids, making this plant expression platform amenable for the production of biological products. The overall topology of HpVAL-4 is a three layered αßα sandwich between a short N-terminal loop and a C-terminal cysteine rich extension. The C-terminal cysteine rich extension has two strands stabilized by two disulfide bonds and superposes well with the previously reported extension from the human hookworm Necator americanus Ancylostoma secreted protein-2 (Na-ASP-2). The N-terminal loop is connected to alpha helix 2 via a disulfide bond previously observed in Na-ASP-2. HpVAL-4 has a central cavity that is more similar to the N-terminal CAP domain of the two CAP Na-ASP-1 from Necator americanus. Unlike Na-ASP-2, mammalian CRISP, and the C-terminal CAP domain of Na-ASP-1, the large central cavity of HpVAL-4 lacks the two histidines required to coordinate divalent cations. HpVAL-4 has both palmitate-binding and sterol-binding cavities and is able to complement the in vivo sterol export phenotype of yeast mutants lacking their endogenous CAP proteins. More studies are required to determine endogenous binding partners of HpVAL-4 and unravel the possible impact of sterol binding on immune-modulatory functions.

Crystal structure of Brugia malayi venom allergen-like protein-1 (BmVAL-1), a vaccine candidate for lymphatic filariasis.

Brugia malayi is a causative agent of lymphatic filariasis, a major tropical disease. The infective L3 parasite stage releases immunomodulatory proteins including the venom allergen-like proteins (VALs), which are members of the SCP/TAPS (Sperm-coating protein/Tpx/antigen 5/pathogenesis related-1/Sc7) superfamily. BmVAL-1 is a major target of host immunity with >90% of infected B. malayi microfilaraemic cases being seropositive for antibodies to BmVAL-1. This study is part of ongoing efforts to characterize the structures and functions of important B. malayi proteins. Recombinant BmVAL-1 was produced using a plant expression system, crystallized and the structure was solved by molecular replacement and refined to 2.1 Å, revealing the characteristic alpha/beta/alpha sandwich topology of eukaryotic SCP/TAPS proteins. The protein has more than 45% loop regions and these flexible loops connect the helices and strands, which are longer than predicted based on other parasite SCP/TAPS protein structures. The large central cavity of BmVAL-1 is a prototypical CRISP cavity with two histidines required to bind divalent cations. The caveolin-binding motif (CBM) that mediates sterol binding in SCP/TAPS proteins is large and open in BmVAL-1 and is N-glycosylated. N-glycosylation of the CBM does not affect the ability of BmVAL-1 to bind sterol in vitro. BmVAL-1 complements the in vivo sterol export phenotype of yeast mutants lacking their endogenous SCP/TAPS proteins. The in vitro sterol-binding affinity of BmVAL-1 is comparable with Pry1, a yeast sterol transporting SCP/TAPS protein. Sterol binding of BmVAL-1 is dependent on divalent cations. BmVAL-1 also has a large open palmitate-binding cavity, which binds palmitate comparably to tablysin-15, a lipid-binding SCP/TAPS protein. The central cavity, CBM and palmitate-binding cavity of BmVAL-1 are interconnected within the monomer with channels that can serve as pathways for water molecules, cations and small molecules.

The phosphorylation state of chloroplast transit peptides regulates preprotein import.

Import of nuclear encoded proteins into chloroplast is an essential and well-regulated mechanism. The cytosolic kinases STY8, STY17 and STY46 have been shown to phosphorylate chloroplast preprotein transit peptides advantaging the binding of a 14-3-3 dimer. Analyses of sty8 sty17 sty46 mutant plants revealed a role for the kinases in chloroplast differentiation, possibly due to lack of transit peptide phosphorylation. Moreover we could show that not only phosphorylation but also transit peptide dephosphorylation appears to be required for the fine regulation of the back-transport of nuclear encoded proteins to the chloroplast.