Linking neurons to immunity: Lessons from Hydra

According to the Greek mythology (1), Heracles’ second labor was the destruction of the Lernaean Hydra, a fearsome fire-breathing monster with a dog-like body and nine snake heads. Many had tried to slay the Lernaean Hydra but in vain: Any head that was cut off regrew and multiplied. It is remarkable how consistent the narrative of this myth is with our recent understanding of tissue and organ regeneration. Heracles was successful not only because of his divine origin (he was the son of Zeus after all) and the help of Athena, but particularly because of the assistance he received from his nephew Iolaus, who was assigned the job of cauterizing the exposed stumps of the Hydra’s severed heads. Of course, we now understand why Iolaus’ contribution was so critical: His flame consumed the local stem cells and stopped them from generating a new head! In PNAS, Klimovich et al. (2) go one step further and provide a reasonable explanation as to why the Lernaean Hydra could remain healthy despite inhabiting the dirty waters of an unfathomable Peloponnesian swamp. Using Hydra as the experimental model organism, the authors define the molecular identity of a unique group of neurons that are responsible for the rhythmic contractions of the body and provide insight into the immune mechanisms they (and other neurons) employ to regulate the animal’s microbial environment. These findings reveal remarkable similarities between the pacemaker activities of the Hydra nervous system and the nervous system of the gut in vertebrates (including humans), advancing our understanding of the interaction between microbiota and host organ systems.Hydra , a freshwater metazoan of the phylum Cnidaria , was first introduced in the laboratory in 1740s (3), when the Swiss naturalist Abraham Trembley discovered that cutting the body of this animal in half resulted in the … [↵][1]1Email: vassilis.pachnis{at}crick.ac.uk. [1]: #xref-corresp-1-1