A Toll-like receptor identified in Gigantidas platifrons and its potential role in the immune recognition of endosymbiotic methane oxidation bacteria

Abstract

Symbiosis with chemosynthetic bacteria is an important ecological strategy for the deep-sea megafaunas including mollusks, tubeworms and crustacean to obtain nutrients in hydrothermal vents and cold seeps. How the megafaunas recognize symbionts and establish the symbiosis has attracted much attention. Bathymodiolinae mussels are endemic species in both hydrothermal vents and cold seeps while the immune recognition mechanism underlying the symbiosis is not well understood due to the nonculturable symbionts. In previous study, a lipopolysaccharide (LPS) pull-down assay was conducted in Gigantidas platifrons to screen the pattern recognition receptors potentially involved in the recognition of symbiotic methane-oxidizing bacteria (MOB). Consequently, a total of 208 proteins including GpTLR13 were identified. Here the molecular structure, expression pattern and immune function of GpTLR13 were further analyzed. It was found that GpTLR13 could bind intensively with the lipid A structure of LPS through surface plasmon resonance analysis. The expression alternations of GpTLR13 transcripts during a 28-day of symbiont-depletion assay were investigated by real-time qPCR. As a result, a robust decrease of GpTLR13 transcripts was observed accompanying with the loss of symbionts, implying its participation in symbiosis. In addition, GpTLR13 transcripts were found expressed exclusively in the bacteriocytes of gills of G. platifrons by in situ hybridization. It was therefore speculated that GpTLR13 may be involved in the immune recognition of symbiotic methane-oxidizing bacteria by specifically recognizing the lipid A structure of LPS. However, the interaction between GpTLR13 and symbiotic MOB was failed to be addressed due to the nonculturable symbionts. Nevertheless, the present result has provided with a promising candidate as well as a new approach for the identification of symbiont-related genes in Bathymodiolinae mussels.

Interactions among deep-sea mussels and their epibiotic and endosymbiotic chemoautotrophic bacteria: Insights from multi-omics analysis

“…Previous studies have reported high levels of PRRs in the symbiotic gill tissue of bathymodiolines, such as G. platifrons , B. septemdierum , and B. azoricus ( Bettencourt et al, 2014 ; Chen et al, 2021 ; Ikuta et al, 2019 ; Li et al, 2020 ; Wang et al, 2019 ), suggesting their importance in symbiosis. Both TLRs and PGRPs have been suggested to participate in endosymbiont establishment and symbiosis regulation ( Li et al, 2021 ; Sun et al, 2017 ). TLRs and PGRPs also function in pathogenic infection, with the latter able to induce phagocytosis ( Bunet et al, 2019 ).…”

Symbioses from Cold Seeps

“…The internalized bacteria were then eliminated by antibacterial proteins such PRRs in the host play irreplaceable roles in the immune recognition of symbionts. To further decode the immune recognition of symbiosis, several PRR proteins that may interact with methanotrophic symbionts were studied to understand their molecular characteristics and expression patterns (Chen et al 2019;Li et al 2020Li et al , 2021. Six candidate PRRs, including leucine-rich-repeat-domain-containing protein, TLR13, TLR2, integrin, vacuolar sorting protein and matrix metalloproteinase 1, were selected by methane-oxidizing bacteria or lipopolysaccharide pull-down assays, suggesting their key role in symbiont recognition.…”