Keratinocytes Drive the Epithelial Hyperplasia Key to Sea Lice Resistance in Coho Salmon

Abstract

Background: Salmonid species have followed markedly divergent evolutionary trajectories in their interactions with sea lice. While sea lice parasitism poses significant economic, environmental, and animal welfare challenges for Atlantic salmon (Salmo salar) aquaculture, coho salmon (Oncorhynchus kisutch) exhibit near-complete resistance to sea lice, achieved through a potent epithelial hyperplasia response leading to rapid louse detachment. The molecular mechanisms underlying these divergent responses to sea lice are unknown. Results: We characterised the cellular and molecular responses of Atlantic salmon and coho salmon to sea lice using single-nuclei RNA sequencing. Juvenile fish were exposed to copepodid sea lice (Lepeophtheirus salmonis), and lice-attached pelvic fin and skin samples were collected 12h, 24h, 36h, 48h, and 60h after exposure, along with control samples. Comparative analysis of control and treatment samples revealed an immune and wound-healing response that was common to both species, but attenuated in Atlantic salmon, potentially reflecting greater sea louse immunomodulation. Our results revealed unique but complementary roles of three layers of keratinocytes in the epithelial hyperplasia response leading to rapid sea lice rejection in coho salmon. Our results suggest that basal keratinocytes direct the expansion and mobility of intermediate and, especially, superficial keratinocytes, which eventually encapsulate the parasite. Conclusion: Our results highlight the key role of keratinocytes to coho salmon's sea lice resistance, and the diverged biological response of the two salmonid host species when interacting with this parasite. This study has identified key pathways and candidate genes that could be manipulated using various biotechnological solutions to improve Atlantic salmon sea lice resistance.

Characterisation of mesenchymal stromal cells in the skin of Atlantic salmon

“…Unsupervised clustering of these 27,989 nuclei based on their transcriptomes revealed 18 distinct cell clusters (Figure 1A), potentially representing 18 different cell types. To assign putative cell types to the identified clusters, we referred to lists of marker genes from previously published works on the same tissues (16).…”

“…This finding is consistent with previous observations that this gene is highly expressed in MSCs and to a lesser extent in fibroblasts (28). PHATE cluster 2 was identified as fibroblasts by the expression of col12a1 , which was previously found to be a marker of fibroblasts in several salmonid species (16), as well as flrt2 (29), a fibroblast growth factor. The identity of cluster 4 as a bone precursor cell type is suggested by its expression of many genes associated with bone development, including pth1r (30), ptprd (31), runx3 (32,33), and cbfb (32).…”

“…In this study, mesenchymal stromal cells (MSCs) were identified and characterised in Atlantic salmon. These MSCs are embedded in cell populations previously classified as fibroblasts (16) and show high expression of the two paralogues of cd34, itga4 and itga5, which are known markers of MSCs in mammals (18)(19)(20). MSCs have been shown to be very similar to fibroblasts in morphology, proliferation dynamics, and immunomodulatory capacities, as well as in gene expression patterns (59), and some authors have even suggested that they are fibroblasts (60).…”

“…The use of animals for experiments was in accordance with the guidelines of the EU legislation (2010/63/U) as well as the Norwegian legislation on animal experimentation and was approved by the Norwegian Animal Research Authority (28346). (16,82)…”

“…The power of combining these technologies to understand skin biology has already been exploited in humans, for example, to unravel the role of different skin cell types during wound healing processes (15). Although both single-nucleus RNA-seq and spatial transcriptomics have been used separately to study skin in Atlantic salmon (16,17), cell expression patterns have not yet been investigated in their tissue context, which is crucial to gaining understanding of their function.…”