Periodic Reporting for period 4 - SYMCELLS (Resolving the molecular mechanisms of intracellular coral-algal symbiosis)
Période du rapport: 2021-12-01 au 2022-05-31
Research on coral symbiosis is also timely, since coral reefs are home to >25% of all marine species and provide food and income to millions of people. Thus coral reef ecosystems have major ecological and economic impacts for society. Yet, coral reefs are currently threatened by ‘coral bleaching’ due to climate change. Environmental stress such as the increase in sea water temperature leads to the breakdown of coral-algal symbiosis, which - if not reversed in a timely manner - leads to coral death. Understanding the molecular basis of the symbiotic interaction of corals and their symbionts provides the basis to understand the mechanisms of bleaching and thus the basis to develop effective means to mitigate coral reef loss.
Most corals acquire symbionts anew each generation during larval stages and the first objective of the project aims to uncover the fundamental mechanisms involved in symbiont acquisition and integration into host cells. Nutrient transfer from symbiont to host is vital to the survival of corals in nutrient poor environments and the second objective is to uncover key mechanisms of the metabolic transfer between the partners, as well as exploring the mechanisms of how symbionts can persist intracellularly inside host cells.
In addition, we investigated the mechanisms involved in symbiont persistence. The innate immune system of animals allows the detection and clearance of invading microorganisms to prevent infection. However, many intracellular parasites and symbionts have evolved mechanisms to escape immune detection and to establish an intracellular niche. To uncover the mechanisms used by the symbiont to avoid detection, we established a comparative approach with true symbionts and non-symbiotic algae. Using a combination of microscopy, life imaging, gene expression analysis and chemical perturbation experiments we find that uptake of microalgae is indiscriminate, but only true symbionts persist inside host cells. Symbiont uptake broadly suppresses host innate immunity, including the conserved TLR signalling pathway. Immune suppression allows symbionts to escape expulsion ('vomocytosis'), the canonical fate of non-symbiotic algae, and promote niche formation. This finding was unexpected because pathogens invading professional immune cells in mammals are mostly cleared by intracellular digestions and has implications for our understanding of the evolution of innate immunity and of an intracellular lifestyle.
Along the way we also developed a protocol and molecular tools for microinjection of Aiptasia eggs as a prerequisite to make the model system amenable for genetic manipulations.
Towards the end of the project, I expect to provide a first molecular framework of the individual steps involved in symbiosis establishment. The steps include symbiont recognition, uptake via phagocytosis, integration into host cell function, interaction with host defence mechanisms, metabolic transfer between the partners and spreading throughout the host organism. The mechanistic analysis of these steps will have broader implications within the fields of ecology and evolution, and specifically important ramifications on coral reef ecosystem health.