Structural but not functional conservation of an immune molecule: a tachylectin-like gene in Hydractinia

Tachylectin-related proteins are a recently characterized group of pattern recognition molecules, functioning in the innate immunity of various animals, from the ancient sponges to vertebrates. Tachylectins are characterized by six internal tandem repeats forming β-propeller domains. We have identified and characterized a tachylectin-related gene in the colonial marine hydroid, Hydractinia echinata. The predicted gene product, termed CTRN, contained an N-terminal signal peptide and had a well-conserved tachylectin-like structure. RT-PCR analyses revealed only post-metamorphic expression while no mRNA was detected during embryonic development or in planula larvae. Exposure of colonies to LPS under conditions known to activate an immune response in Hydractinia did not result in upregulation of the gene. In situ hybridization analysis of metamorphosed animals detected CTRN transcripts only in a small subpopulation of neurons and their precursor cells, localized in a ring-like structure around the mouth of polyps. The same ring-like structure of CTRN expressing neurons was also observed in young polyp buds, predicting the position of the future mouth. This type of expression pattern can hardly be attributed to an immune-relevant gene. Thus, despite high structural similarity to tachylectins, this cnidarian member of this group seems to be an exception to all other tachylectins identified so far as it seems to have no function in cnidarian innate immunity.