Periodic Reporting for period 3 - AntiViralEvo (Unravelling the evolution of antiviral sensors and response systems in animals using the phylum Cnidaria)
Reporting period: 2023-05-01 to 2024-10-31
Viruses and their hosts are locked in an ongoing evolutionary arms race since the dawn of life on Earth. This antagonistic relationship is considered a major force in evolution and a major reason for the exceptional diversity of antiviral systems in animals. Antiviral immunity in vertebrates such as fish, rodents and humans, relies on the interferon pathway that enables infected cells to alert neighbouring cells against incoming infection and to recruit immune system cells to battle the virus. In invertebrates such as insects and nematode worms which lack interferons, antiviral immunity is believed to be based mostly on RNA interference, a pathway that directly cleaves and inactivates viral RNA. Project AntiViralEvo studies the original mode of action of these systems in the latest common ancestor of all these groups to determine how antiviral immunity was triggered in early animals. In this project we utilize the sea anemone Nematostella vectensis, a representative model species of Cnidaria, a phylum that diverged approximately 600 million years ago from other animals and includes in addition to sea anemones also corals, jellyfish and hydroids. Nematostella is a well-studied lab model and application of advanced molecular and gene manipulation tools will help to decipher the system this cnidarian employs for battling RNA viruses and to answer important questions regarding the evolution of antiviral immunity and its ancestral state in animals. Further, this project will also test in its late phase whether the findings in Nematostella in its initial phase also extend to other cnidarians, such as the far-related sea anemone, Exaiptasia pallida and the reef-building coral Stylophora pistillata. This will provide a wider view regarding the composition and evolution of the antiviral immune system in Hexacorallia (sea anemones and reef-building corals). Moreover, this branch of the project could also have important implications for ecology, as coral reefs are pillars for marine biodiversity and corals are dying at alarming rates for many different reasons in the last few decades. Whereas bacteria were shown to play a role in this decline, the role of viruses and the coral immune systems that fight them remain mostly uncharted. Thus, this project may also shed light on this important ecological topic.
The mechanisms for sensing and defeating RNA viruses vastly differ between vertebrate and invertebrate bilaterian animals which hinders our understanding of their evolutionary origin. In the first 18 months of the project we characterized the gene expression response (i.e. which genes are activated and inactivated) to a common viral mimic, double-stranded RNA, in a non-bilaterian animal, the cnidarian Nematostella vectensis. Strikingly, we revealed that the innate immune response of Nematostella displays similarities to both invertebrate and vertebrate systems as we find activation of distinct components similar to those found in the interferon system of vertebrates and also to those found in the RNA interference system of flies and worms. This result suggests an ancient origin of these antiviral immune mechanisms and that the last common ancestor of cnidarians and bilaterian animals had a highly sophisticated antiviral system inside its cells. Unexpectedly, after cnidarians separated from the rest of animals, ancient vertebrates and invertebrates lost parts of their antiviral immune systems, whereas cnidarians retained the ancestral complexity. Further, the results of our biochemical and genetic assays indicate that the retinoic acid-inducible gene I-like receptors (RLRs), known to detect viral double-stranded RNA in vertebrates and worms, are essential to initiate the antiviral immune response in Nematostella. Altogether, we provided functional evidence that the RLR-mediated sensing of viral RNA originated before the split of cnidarians from other animals more than half a billion years ago.
As stated above, the results obtained in the first reporting period (18 months) of this project already revealed unexpected complexity of the ancient antiviral immune system in animals. This is an important progress in our understanding of this topic. On a more technical aspect, we developed and generated transgenic lines of Nematostella that express reporter genes under a highly ubiquitous promoter, which is an important research tool in this system. Further, we generated transgenic lines of the same species that utilize this promoter for over-expressing a Nemaotstella receptor of viral double-stranded RNA (RLR) with a peptide tag that enables its efficient biochemical isolation and detection. We used this important tool for testing and characterizing the biochemical traits of this receptor. Our expectation is that by the end of this project we will learn much more about the evolution of antiviral immunity in animals and that we will develop more cutting-edge transgenic tools in Nematostella that will benefit the scientific community using this species as a lab model organism.