HSP90 and MAPK activation are required for ampulla development in the direct‐developing ascidian Molgula pacifica

Abstract

Abstract. Heat shock protein 90 (HSP90) functions as a major chaperone protein that interacts with a limited number of specific client proteins including some transcription factors, many kinases including mitogen‐activated protein kinase (MAPK), and cytoskeletal proteins. The antibiotic geldanamycin (GA) is frequently used as an in vivo inhibitor of HSP90 function and the organic compound U0126 inhibits the activation of MAPK in live cells. In the present study, embryos of Molgula pacifica, at the gastrula stage, were continuously treated with 5 μmol L−1 GA or 10 μmol L−1 U0126 for 4 d, and the resulting juveniles were examined for possible morphological defects. The key finding of the present study is that HSP90‐dependent MAPK signaling appears to be required for the outgrowth of ampullae and for the development of coelomic progenitor cells. The effect of GA treatment seemed to be relatively specific because it had a minimal effect on other morphogenetic events, including the development of the anterior–posterior axis and tunic development. The possibility that GA treatment inhibits ampulla outgrowth by disrupting normal HSP90‐actin cytoskeleton interactions within the cytoplasm of ampulla epidermal cells was examined using actin immunofluorescence. The results, however, indicate that GA treatment does not grossly disrupt actin‐based cytoskeletons in epidermal cells that form ampullae. Furthermore, terminal deoxynucleotidyl transferase‐mediated dUTP nick end labeling experiments suggest that treatment with 5 μmol L−1 GA does not significantly promote cell death during development in M. pacifica. Mapping the region in juveniles, where GA‐sensitive coelomic progenitor cells originate, was studied using cell deletion methods, and the results suggest that the potential to form coelomic progenitor cells is restricted to the posterior region at day 1 of development.