BOUZAIENE, M.*; ANGERS, A.; ANCTIL, M.; Universit� de Montr�al, Canada: A retinoic acid-like receptor in interstitial cells of two colonial anthozoans (Cnidaria)
Retinoic acid is known to induce vertebrate stem cells to differentiate into a variety of cell types, including neurons. Although retinoic acid was reported to affect morphogenetic pattern specification in a cnidarian (the hydrozoan Hydractinia), the cellular targets of retinoids were not investigated. On the basis of (1) previously reported evidence of a retinoid RXR receptor in a jellyfish and (2) of a detected homology between sequences of an epitope of the human alpha RXR receptor antibody and of a coral nuclear receptor, we used Western immunoblotting and immunohistochemistry to investigate the presence and cellular distribution of a RXR-like receptor in the sea pansy Renilla koellikeri and the staghorn coral Acropora millepora (Cnidaria, Anthozoa). Western blots revealed a 64 KDa protein from a sea pansy extract in a band that co-migrated with the recombinant RXR protein and with a RXR protein from the rat brain. Using antibodies raised against two epitopes of human alpha RXR, we visualized clusters of immunoreactive interstitial (stem) cells in both R. koellikeri and A. millepora. These cells were absent in larvae and were present mainly in the polyp column of adult colonies where they occupy the basiectoderm near the ectoderm-mesoglea interface. RXR-like expression appears in interstitial cells with short processes but also in cells with more elongated, fine processes, thus indicating that the receptor is expressed in differentiation intermediates committed to the neuron pathway similar to those observed in hydra. These observations suggest that retinoic acid, acting through a RXR-like receptor, is involved in nerve cell specification in adult anthozoans and that this role was conserved throughout evolution. Further studies will examine the effects of retinoic acid on interstitial cells and neurons in culture. (Funded by Discovery Grant 6447-01 from NSERC of Canada)
