Evidence for intercellular coupling and connexin-like protein in the luminescent endoderm of Renilla koellikeri (Cnidaria, Anthozoa)

Gap junction plaques are abundant in Hydrozoa, where they play an important role in signal propagation through epithelia and nerve nets, but they have not been found in the two other classes of Cnidaria, the Scyphozoa and the Anthozoa. Here several lines of evidence are presented that point to the existence of intercellular coupling in tissues of the anthozoan Renilla koellikeri, especially in the luminescent endoderm. Dye-exchange experiments show that calcein vital stains spread between cultured cells after their reassociation. Polyp luminescence evoked by KCl depolarization, electrical stimulation, or beta-adrenergic agonists was largely and reversibly suppressed in the presence of the gap junction uncouplers octanol, heptanol, and low pH sodium acetate. A connexin43-like protein was isolated on Western blots of R. koellikeri membrane extracts by using a monoclonal connexin-43 antibody. Loading this antibody in R. koellikeri tissues resulted in the suppression of luminescence evoked by electrical stimulation. Immunohistochemical investigations using this antibody revealed mostly punctate immunostaining associated with endodermal cells of the luminescent tissue and with the mesogleal nerve net. Electron microscopic observations confirmed the absence of conventional gap junction plaques in these tissues, but revealed the presence of tiny zones of close membrane apposition between light-emitting and other endodermal cells, with gaps of 2-4 nm. Taken together, these results are consistent with the notion of the existence in R. koellikeri of intercellular coupling (1) involved in local transmission of luminescence signals, and (2) mediated by connexin43-based connexons that are not assembled into typical gap junction plaques.