Distinguishing self from nonself and the onset of defense effector mechanisms upon recognition of pathogens are essential for the survival of all life forms in the animal kingdom. The family of nucleotide -binding and oligomeriszation domain-like receptors (NLRs) was first identified in vertebrates and comprises a group of pivotal sensor protein of the innate immune system for microbial cell wall components or danger signals. Here, we provide first evidence that early diverging metazoans have large and complex NLR repertoires. The cnidarian NACHT/NB-ARC genes include novel combinations of domains, and the number of one specific type (NB-ARC and tetratricopeptide repeat containing) in Hydra is particularly large. We characterize the transcript structure and expression patterns of a selected HyNLR, HyNLR type 1 and describe putative interaction partners. In a heterologous expression system, we show induced proximity recruitment of an effector caspase (HyDD-Caspase) to the HyNLR type 1 protein upon oligomerization indicating a potential role of caspase activation downstream of NLR activation in Hydra. These results add substantially to our understanding of the ancestral innate immune repertoire as well as providing the first insights into putative cytoplasmic defense mechanisms at the base of animal evolution.

Signaling via the epidermal growth factor receptor (EGFR) pathway has emerged as one of the key mechanisms in the development of the central nervous system in Drosophila melanogaster. By contrast, little is known about the functions of EGFR signaling in the differentiated larval brain. Here, promoter-reporter lines of EGFR and its most prominent activating ligands, Spitz, Keren, and Vein, were used to identify the brain structures relevant for the EGFR pathway. Unexpectedly, promoter activity of all these pathway components was found in the mushroom bodies, which are known to be a higher brain center required for olfactory learning. We investigated the role of the EGFR pathway in this process by using different mutant larvae with reduced pan-neuronal EGFR signaling and those with reduced EGFR signaling in mushroom bodies only. Expression of a dominant-negative form of EGFR as well as silencing of the ligands via RNA interference was applied and resulted in significantly impaired olfactory learning performances. General defects in the ability to taste or smell as well as impaired EGFR signaling during embryonic development could be excluded as major reasons for this learning phenotype. In addition, targeted expression of a constitutively active form of the ligand Spitz also led to a significantly reduced learning ability. Thus, very low levels as well as very high levels of EGFR signaling are deleterious for olfactory learning and memory formation. We hypothesize that EGFR signaling in a certain range maintains a homeostatic situation in the mushroom bodies that is necessary for proper learning and memory.