Microalgae from the dinoflagellate group are known for their ability to survive in other animal cells. These tiny, one-celled organisms have engaged in mutually beneficial relationships with corals since primitive times. By transmitting essential nutrients to their hosts, dinoflagellates allow corals to thrive even in barren areas. A research team from the Center for Organismal Studies (COS) at the University of Heidelberg recently discovered that such symbioses within the cell essentially depend on the ability of algae to suppress the immune system of their host cell and thus avoid to be “spat out” again. At the same time, researchers have found evidence that this cellular immune response is an ancient, evolving immune mechanism that is more prevalent than previously assumed.
This mechanism is known as vomocytosis. Contrary to previous hypotheses, microalgae ingested by corals are not digested by the cell if they prove to be unsuitable as symbionts – that is, partners in a symbiotic relationship. Instead, they are “spit out” again in the process of vomocytosis. Special dinoflagellates are able to specifically suppress this immune response from their host cells in order to stay in the cell. A study by cell biologist Prof. Dr Annika Guse at COS demonstrated how they are able to do this. “The challenge for corals is to differentiate between beneficial and potentially harmful microorganisms. For their part, algae must bypass the host cell’s immune response, establish an intracellular niche where they can survive and coordinate their own cellular functions with them. of their host to exchange nutrients efficiently, ”explains the researcher.
So far, there has been no experimental evidence that could explain any of the conventional theories. Using the Exaiptasia diaphana (Aiptasia) model system of the sea anemone species, Prof. Guse’s team recently discovered how immune suppression by symbionts helps the host cell recognize and tolerate appropriate microalgae. long term. Aiptasia anemone larvae ingest environmental symbionts in the same way as coral larvae. Plus, their size and transparency make the larvae of this sea anemone perfect for high-resolution imaging and cellular experiments.
Aiptasia continually ingests various particles from the environment without distinguishing between appropriate and inappropriate particles or organisms. Incompatible particles are “spit out” again after some time. Symbionts avoid this process of vomocytosis, presumably by disrupting the toll receptor (TLR) signal pathways of the host cell. These receptors play a vital role in activating the cell’s own immune system and ensure that unwanted intruders are detected and eliminated. In most animals, toll-type receptors are controlled by the MyD88 gene. “We have been able to prove that the algae symbionts suppress MyD88 and thus initiate a symbiosis. This is how they evade vomocytosis, ”explains Professor Guse.
At the same time, the findings of the Heidelberg researchers indicate that vomocytosis involves a more prevalent mechanism than one might think. Until now, it was believed that the expulsion of harmful intruders was self-initiated to evade the partly highly specialized immune responses of the potential host cell. However, the study of the Aiptasia model suggests that this process can also be triggered by the host cell. The researchers therefore speculate that vomocytosis is an ancient, evolutionary immune mechanism that corals or cnidarians like Aiptasia use to select the appropriate symbionts. Teacher. Guse: “This suggests that vomocytosis is an important process that first led to the emergence of the intracellular way of life of coral symbionts.”
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