Description

The natural fungal pathogen Drechmeria coniospora pierces the worm’s cuticle and its hyphae grow throughout the organism. In the epidermis, this triggers a rapid increase in the expression of genes from the nlp (for neuro-peptide-like protein) and cnc (caenacin) families. These genes encode structurally-related antimicrobial peptides (AMPs). We have defined major signalling pathways required for the regulation of nlp‑29 gene expression. Two of them, one specific for infection and the second also activated by wounding, act upstream of a highly conserved p38 MAPK signalling cascade. The induction of cnc‑2 upon infection, on the other hand, is independent of PMK-1/p38 MAPK, but requires DBL-1/TGFß produced by certain neuronal cells, acting via a non-canonical TGFß pathway in epidermal cells. The STAT transcription factor-like protein, STA‑2, is essential for both the PMK-1/p38 MAPK, and DBL-1/TGFß immune signalling pathways, to govern the transcriptional response to fungal infection in the epidermis (reviewed in Kim and Ewbank, 2018; Martineau et al., 2021).

In the absence of infection, expression of a constitutively active Gα protein (GPA-12gf) in the adult epidermis leads to higher expression of AMP genes of both the nlp and cnc families (Labed et al., 2012). More generally, there is a considerable overlap between the genes induced by infection (Engelmann et al., 2011) and those up-regulated upon expression of GPA-12gf (Lee et al., 2018). To broaden our understanding of the host response to fungal infection, we selected 10 strongly-induced genes for validation through qRT-PCR. We confirmed that for 9 of them, expression was increased in both the infected and the GPA-12gf samples. They included members of the nlp and cnc families, but also several other genes predicted to encode small secreted peptides, like F48C1.9 (Omi and Pujol, 2019), fip-6 (Pujol et al., 2012), F57H12.6 and ora-1. For only one, F40H7.12, expression was induced by fungal infection but not in the GPA-12gf background (Figure 1A). The gene encodes a protein with a fascin domain, associated with actin binding. Two other C. elegans genes (F09C6.1 and Y105C5B.14) are predicted to encode proteins with a fascin domain (Figure 1B). Data in Wormbase indicates that their expression increases upon exposure to a variety of stresses. We therefore called this family ifas for “inducible fascin domain”. We determined that the induction of F40H7.12/ifas-1 upon fungal infection does not require sta-2, unlike nlp-29, cnc-2 or fip-6 (Figure 1C).

We made two different reporter transgenes to study the gene’s expression pattern, both containing the short (800 bp) intergenic region separating ifas-1 from its upstream gene, one with the 3’ UTR of unc-54, the other with its own 3’ UTR. Transgenic strains produced with the two constructs behaved similarly. A constitutive expression was observed in a subset of neurons including neurons in the head, lateral neurons, including the CAN neurons, the HSN neurons and a subset of retrovesicular ganglion neurons (Figure 1D). Upon fungal infection and wounding, an induction was observed in the epidermis. While the induction was robust and reproducible, it was only observed in less than 20% of the worms (Figure 1E-F). This may be because the reporter constructs do not contain all the regulatory elements required to reflect endogenous gene expression. Future characterization of ifas-1 is expected to reveal previously unexplored aspects of the innate immune response to epidermal fungal infection.