Starvation differentially affects gene expression, immunity and pathogen susceptibility across symbiotic states in a model cnidarian

Mutualistic symbioses between cnidarians and photosynthetic algae are modulated by complex interactions between host immunity and environmental conditions. Here, we investigate how symbiosis interacts with food limitation to influence gene expression and stress response programming in the sea anemone Exaiptasia pallida (Aiptasia). Transcriptomic responses to starvation were similar between symbiotic and aposymbiotic Aiptasia; however, aposymbiotic anemone responses were stronger. Starved Aiptasia of both symbiotic states exhibited increased protein levels of immune-related transcription factor NF-κB, its associated gene pathways, and putative target genes. However, this starvation-induced increase in NF-κB correlated with increased immunity only in symbiotic anemones. Furthermore, starvation had opposite effects on Aiptasia susceptibility to pathogen and oxidative stress challenges, suggesting distinct energetic priorities under food scarce conditions. Finally, when we compared starvation responses in Aiptasia to those of a facultative coral and non-symbiotic anemone, 'defence' responses were similarly regulated in Aiptasia and the facultative coral, but not in the non-symbiotic anemone. This pattern suggests that capacity for symbiosis influences immune responses in cnidarians. In summary, expression of certain immune pathways-including NF-κB-does not necessarily predict susceptibility to pathogens, highlighting the complexities of cnidarian immunity and the influence of symbiosis under varying energetic demands.

Starvation decreases immunity and immune regulatory factor NF-κB in the starlet sea anemone Nematostella vectensis

Lack of proper nutrition has important consequences for the physiology of all organisms, and nutritional status can affect immunity, based on many studies in terrestrial animals. Here we show a positive correlation between nutrition and immunity in the sea anemone Nematostella vectensis. Gene expression profiling of adult anemones shows downregulation of genes involved in nutrient metabolism, cellular respiration, and immunity in starved animals. Starved adult anemones also have reduced protein levels and activity of immunity transcription factor NF-κB. Starved juvenile anemones have increased sensitivity to bacterial infection and also have lower NF-κB protein levels, as compared to fed controls. Weighted Gene Correlation Network Analysis (WGCNA) is used to identify significantly correlated gene networks that were downregulated with starvation. These experiments demonstrate a correlation between nutrition and immunity in an early diverged marine metazoan, and the results have implications for the survival of marine organisms as they encounter changing environments.

Molecular and Biochemical Approaches to Study the Evolution of NF-κB Signaling in Basal Metazoans

Extensive genomic and transcriptomic sequencing over the past decade has revealed NF-κB signaling pathway homologs in organisms basal to insects, for example, in members of the phyla Cnidaria (e.g., sea anemones, corals, hydra, jellyfish) and Porifera (sponges), and in several single-celled protists (e.g., Capsaspora owczarzaki, some choanoflagellates). Therefore, methods are required to study the function of NF-κB and its pathway members in early branching organisms, many of which do not have histories as model organisms. Here, we describe a combination of cellular, molecular, and biochemical techniques that have been used for studying NF-κB, and related pathway proteins, in some of these basal organisms. These methods are useful for studying the evolution of NF-κB signaling, and may be adaptable to the study of NF-κB in other non-model organisms.