A death domain-containing NLR receptor promotes lytic cell death by interacting with DD-caspase in Vibrio splendidus-challenged Apostichopus japonicus

Abstract

The NLR family serves as pattern recognition receptors that activate the innate immune response by sensing pathogenic factors. These receptors mediate pyroptosis and promote the release of inflammatory factors, thereby effectively defending against pathogen invasion. Invertebrates possess NLRs exhibiting diverse domain architectures, where distinct structural combinations contribute to functional diversification. However, the mechanistic basis of these evolutionary innovations remains poorly characterized. In this study, we cloned and identified a novel NLR from the Apostichopus japonicus, designated as AjDD-NLR, which is characterized by the combination of a death domain (DD) at its N-terminus, a nucleotide-binding and oligomerization domain (NACHT) in the middle, and a small number of leucine-rich repeat (LRR) motifs at its C-terminus, and this receptor lacked the recruitment domain (CARD) or pyrin domain (PYD) that are typical of common cytoplasmic NLR-NT. The AjDD-NLR expression was siginificantly induced in Apostichopus japonicus following Vibrio splendidus infection and in LPS-stimulated coelomocytes. PAMP binding assays revealed that AjDD-NLR recognizes diverse PAMPs via its C-terminal LRR domain.Mechanistically, a novel caspase (named AjDD-caspase), which also contains a DD domain, was found to be recruited by AjDD-NLR. Moreover, it was found that the lytic cell death promoted by overexpression of AjDD-NLR could be blocked by interfering with AjDD-caspase, which clearly demonstrated that AjDD-NLR regulates lytic cell death through the AjDD-caspase pathway. In conclusion, these results supported that AjDD-NLR regulates lytic cell death by recruiting AjDD-caspase.