ABSTRACT
The signal transduction mechanisms in mollusks are still elusive since the genome information is incomplete and cell lines are not available. In previous study, we cloned a highly conserved Smad3 homolog (designated as Pf-Smad3) from the pearl oyster, Pinctada fucata. It seems that transforming growth factor beta (TGFbeta) signaling may play similar roles in the oyster as in vertebrate. Here we report a cDNA encoding an activin like receptor 1 homolog (designated as Pf-ALR1) of the oyster, another kind of TGFbeta superfamily member. Compared to the activin receptor-like kinases (ALK) in human, the amino acid sequence of Pf-ALR1 is more similar to that of ALK1, especially the L45 loop. Reverse transcription-polymerase chain reaction results indicate that Pf-ALR1 mRNA is expressed ubiquitously in the adult oyster. Thus, Pf-ALR1 may be important for many physiological processes in the oyster. To lay a basis for further investigation of the TGFbeta signal pathway functions in the oyster shell formation, in this report, the Pf-ALR1 mRNA expression in the oyster mantle was detected by in situ hybridization. The results show that Pf-ALR1 in the oyster mantle is mainly expressed at the inner epithelial cells of the outer fold and the outer epithelial cells of the middle fold, similarly as Pf-Smad3. The mRNA levels of Pf-ALR1 and Pf-Smad3 are all changed after shell notching. These results indicate that both Pf-ALR1 and Pf-Smad3 may take part in shell formation and repair. The results of drug treatment experiments with in-vitro cultured oyster mantle tissue cells demonstrate that the mRNA expression levels of Pf-Smad3, Pf-ALR1 and two oyster nuclear factor-kappaB (NF-kB) members can be adjusted and correlated. All our observations suggest that there should be similar TGFbeta signal pathways in the oyster and vertebrate. However, the potential functions of Pf-ALR1 and the relations of TGFbeta and NF-kB members in the oyster all need to be thoroughly investigated.
