Antagonistic fungal enterotoxins intersect at multiple levels with host innate immune defences

Abstract

Animals and plants need to defend themselves from pathogen attack. Their defences drive innovation in virulence mechanisms, leading to never-ending cycles of co-evolution in both hosts and pathogens. A full understanding of host immunity therefore requires examination of pathogen virulence strategies. Here, we take advantage of the well-studied innate immune system of Caenorhabditis elegans to dissect the action of two virulence factors from its natural fungal pathogen Drechmeria coniospora. We show that these two enterotoxins have strikingly different effects when expressed individually in the nematode epidermis. One is able to interfere with diverse aspects of host cell biology, altering vesicle trafficking and preventing the key STAT-like transcription factor STA-2 from activating defensive antimicrobial peptide gene expression. The second, potentially as a consequence of a host surveillance mechanism, increases STA-2 levels in the nucleus, modifies the nucleolus, and causes increased defence gene expression. Our results highlight the remarkably complex and potentially antagonistic mechanisms that come into play in the interaction between co-evolved hosts and pathogens.

C. elegans: out on an evolutionary limb

“…The extra genes appear to have arisen by duplication and diversification of existing C. elegans genes, and include several encoding G-protein coupled receptors (GPCR), a class of proteins known to be important for host-pathogen interactions (see below), as well as nematode-specific proteins. One, F40H7.12, also called ifas-1, for "Inducible FAScin Domain containing," has been the focus of our attention recently as its expression is induced by natural fungal infection (Omi et al 2021), and appears to be part of a poorly characterised innate immune defence mechanism (Zhang et al 2021b). Another hyper-divergent region encompasses a cluster of paralogous genes that encode antimicrobial peptides (AMPs) of the NLP class, that show inter-species variability indicative of positive selection (Pujol et al 2012(Pujol et al , 2008b and, as explained below, play multiple important roles in innate immunity against natural fungal infection (Harding and Ewbank 2021).…”

The predicted bZIP transcription factor ZIP-1 promotes resistance to intracellular infection in Caenorhabditis elegans

“…Here we found no effect of zip-10 RNAi on pals-5 p::GFP induction upon heat stress. We also analyzed the STAT-like transcription factor sta-2 , which is important for response to the fungal pathogen Drechmeria coniospora , which penetrates and grows inside epidermal cells (20, 21). Here as well, we did not find an effect of sta-2 RNAi on induction of pals-5 p::GFP expression (Fig.…”

Increased pathogenicity of the nematophagous fungus Drechmeria coniospora following long-term laboratory culture

“…Our established D. coniospora transformation method (He and Ewbank, 2017) no longer functions and we are not currently able to alter specifically the fungal genome. As an alternative, one can heterologously express candidate virulence factors in C. elegans (Zhang et al, 2021), but this is only appropriate for D. coniospora proteins that are predicted to be secreted into the host, which is not the case for any of the candidates identified in the current study. The most striking molecular change concerned DcCPKA.…”

“…The remaining Dan2 protein sequences were aligned by TBLASTN against the Swe2 genome. Those matching an existing predicted Swe2 protein-coding gene were considered likely to reflect events of gene expansion in Dan2 and not pursued further, with the exception of genes encoding proteins with an enterotoxin alpha domain (PFAM: PF01375), which was the subject of manual annotation (Zhang et al, 2021). Then Dan2 protein sequences that aligned with predicted coding sequence from Swe2 with an identity greater than 90% and for which the main exon was more than 80% of the predicted coding sequence for the gene were manually curated and added to the set of predicted protein-coding genes in Swe2.…”

“…Only two out of these 23 enterotoxins have been characterised in any detail. They play specific, partly antagonistic roles, interfering with host defence signalling pathways (Zhang et al, 2021). These examples represent a tiny fraction of the hundreds of potential virulence factors encoded within the D. coniospora genome.…”