Molecular basis of cnidarian endosymbiosis and its response to environmental change
Final Report Abstract
Mutualistic symbioses are key to evolutionary and ecological novelties. Eukaryotic cells evolved through the acquisition of bacteria, and coral reef ecosystems depend on the endosymbiosis between corals and photosynthetic dinoflagellates. Most corals produce non-symbiotic larvae that acquire symbionts anew each generation via phagocytosis into host endodermal cells. After acquisition, symbionts reside inside specialized phagosomes (“symbiosomes”) in the endodermal cells. We have only a limited understanding of symbiosis, primarily because corals are unsuitable as laboratory systems and only spawn once annually. My lab has established the larvae of Aiptasia, a marine sea anemone and emerging model for coral symbiosis, as a platform for studying the molecular mechanisms of host-symbiont interaction. During this project, we developed a robust protocol to induce Aiptasia spawning in the laboratory, analyzed embryonic and larval development, established a novel symbiosis-establishment assay and assessed the efficiency of symbiont phagocytosis in relation to larval development, and completed a first comparison of symbiosis specificity between Aiptasia and coral larvae collected in the field. We participated in sequencing the Aiptasia genome and identified a first set of candidate genes involved in symbiosis establishment by RNA-Seq. We have developed molecular tools such as in situ hybridization, confocal microscopy, protein extraction and metabolomics assays. We have developed cell state specific transcriptomics, and are currently optimizing functional approaches including genome engineering and to further extend our toolbox. Building up on these achievements we have focused on fundamental questions of endosymbiosis: What are the molecular mechanisms of symbiont acquisition? How does the symbiont avoid detection? How are symbiont-derived nutrients integrated and how does metabolic exchange work? Acquisition & avoiding detection . The mechanism of symbiont uptake is unclear. Using a comparative approach with non-symbiotic algae, we find that an unspecific trial-and-error mechanism is used to phagocytose particles. We provide evidence that symbionts, similar to some pathogens, enter the Aiptasia and coral host cells via integrin-mediated phagocytosis by binding to a conserved RGD-motif. Surprisingly, we found that non-symbiotic algae are removed by expulsion (and not by digestion) which is reminiscent to ‘vomocytosis’, a process observed in amoeba and macrophages to remove certain fungal pathogens. We demonstrate that a broad-scale transcriptional suppression of the host immunity, likely by targeting the adapter MyD88 to suppress TLR signaling, is key for symbionts to avoid ‘vomocytosis’ initially, but it is not affecting symbiosis stability. Metabolic integration. All animals have to eat to survive. Endosymbiosis is a unique way to acquire nutrients from within the cells. We find that symbiosis establishment results in a metabolic switch, triggered by the highly conserved mTORC1 (mechanistic target of rapamycin complex 1) signaling. Specifically, mTORC1 signaling is elevated in symbiotic anemones, resembling a feeding response. Moreover, symbiosis establishment enhances lipid content and cell proliferation in Aiptasia larvae. We propose that symbiont-derived nutrients are sensed at the symbiosome and activate the evolutionarily conserved mTORC1 pathway to promote growth in endosymbiotic cnidarians. Metabolic exchange. The nutrient transfer from symbionts to host is key for ecosystem function. One example are sterols, essential building blocks for membranes in all animal cells. We found that sterolauxotroph anemones and corals receive their bulk sterols from their symbionts. Host sterol composition is flexible and varies with the symbiont type housed. During evolution, symbiotic hosts have expanded their suite of sterol-binding NPC2 proteins which may be specifically adapted to the low pH within the ‘symbiosome’. Taken together, we have developed a comprehensive experimental toolkit for Aiptasia to dissect endosymbiosis at a mechanistic level and generated a first molecular understanding of key aspect of symbiosis establishment. Our work provides the basis to better understand cnidarian-dinoflagellate endosymbiosis in the context of ecology and evolution, a prerequisite to combat loss of coral reefs due to climate change.
Publications
- (2015). Induction of gametogenesis in the cnidarian endosymbiosis model Aiptasia sp. Scientific Reports 5, 15677
Grawunder, D., Hambleton, E.A., Bucher, M., Wolfowicz, I., Bechtoldt, N. & Guse, A.
(See online at https://doi.org/10.1038/srep15677) - (2015). The genome of Aiptasia, a sea anemone model for coral symbiosis. Proceedings of the National Academy of Sciences USA 112, 11893-8
Baumgarten, S., Simakov, O., Esherick, L.Y., Liew, Y.J., Lehnert, E.M., Michell, C.T., Li, Y., Hambleton, E.A., Guse, A., Oates, M.E., Gough, J., Weis, V.M., Aranda, M., Pringle, J.R. & Voolstra, C.R.
(See online at https://doi.org/10.1073/pnas.1513318112) - (2016). Development and symbiosis establishment in the cnidarian endosymbiosis model Aiptasia sp. Scientific Reports 6, 19867
Bucher, M., Wolfowicz, I., Voss, P.A., Hambleton, E.A. & Guse, A.
(See online at https://doi.org/10.1038/srep19867) - Aiptasia sp. larvae as a model to reveal mechanisms of symbiont selection in cnidarians. Scientific Reports 6, 32366
Wolfowicz, I., Baumgarten, S., Voss, P.A., Hambleton, E.A., Voolstra, C.R., Hatta, M., Guse, A.
(See online at https://doi.org/10.1038/srep32366) - (2018). Microinjection to deliver protein, mRNA, and DNA into zygotes of the cnidarian endosymbiosis model Aiptasia sp. Scientific Reports 8, 16437
Jones, V.A.S., Bucher, M., Hambleton, E.A., Guse, A.
(See online at https://doi.org/10.1038/s41598-018-34773-1) - (2018). Sterol transfer by atypical cholesterol-binding NPC2 proteins in coral-algal symbiosis. eLife, 439231
Hambleton, E.A., Jones, V.A.S., Maegele, I., Kvaskoff, D., Sachsenheimer, T., Guse, A.
(See online at https://doi.org/10.7554/eLife.43923) - Dinoflagellate symbionts escape vomocytosis by host cell immune suppression
Jacobovitz, M.R., Rupp, S., Voss, P.A., Gornik S., Guse, A.
(See online at https://doi.org/10.1101/864579) - Nutrient-dependent mTORC1 signaling in coral-algal symbiosis
Voss, P.A., Gornik S., Jacobovitz, M.R., Rupp, S., Dörr, M., Guse, A.
(See online at https://doi.org/10.1101/723312) - High temperature inhibits cnidariandinoflagellate symbiosis establishment through nitric oxide signaling
Hill, L., Salgado, L.T., Salomon, P.S, Paulo S, Guse, A.
(See online at https://doi.org/10.1101/2020.05.13.086868) - Photoreceptor complexity accompanies adaptation to challenging marine environments in Anthozoa
Gornik S., Bergheim, B.G., Foulkes, N.S., Guse, A
(See online at https://doi.org/10.1101/2020.05.28.118018)
