Teleost fish models for the in vivo study of inflammasome signaling

Inflammation is an organism’s concerted response to damage and infection. For an inflammatory signal to spread it relies on specific signaling pathways to activate proinflammatory genes. One is set off by receptors that, after detecting an appropriate stimulus, nucleate the assembly of a large multimolecular signaling platform called the inflammasome. Currently, thanks to the advances in live microscopy, questions in immunology that can only be solved by live imaging are beginning to be addressed. In this work, we have established zebrafish and medaka as vertebrate model systems for the visualization of inflammasome signaling by three approaches. Since inflammasome assembly is driven by the aggregation of the adaptor molecule ASC, one approach was to study the dynamics of this molecule’s switch from a cellular cytoplasmic localization to a single aggregate, called a speck, using zebrafish. We saw that speck formation leads to pyroptosis, a proinflammatory type of cell death, in vivo. This is the first time this process is visualized in a live organism. Furthermore, we generated a zebrafish transgenic line with endogenously tagged ASC that can be used to study the role of inflammasome activation live in zebrafish infection models. Second, we used medaka to study the proinflammatory cytokine interleukin-1 (il1), whose activation is downstream of inflammasome assembly. We generated a transgenic line to track the transcriptional activation of the gene and the protein’s cleavage. Based on our results, we propose that il1 genes in teleost fish correspond genetically and functionally to both il1 paralogues in mammals, instead of only for il1β. Lastly, we generated a zebrafish reporter line to visualize and quantify NF-κB activity, a master regulator of proinflammatory genes. We show that the line has potential to be used in high-throughput screens. Overall, this work reveals unknown features of the functional role of the inflammasome signaling cascade in fish and its evolution