HSP90 expression in two migratory cell types during ascidian development: test cells deposit HSP90 on the larval tunic

Heat shock protein 90 (HSP90) is a ubiquitously expressed molecular chaperone that controls the folding, assembly and activity of proteins, many of which are involved in signal transduction. Recent work has shown that HSP90 is present extracellularly, indicating a heretofore under appreciated requirement for extracellular chaperoning, particularly among migratory cells. We applied immunological and surgical techniques to document the differential expression of HSP90 during ascidian development. Relative to other cell types during development, test cells and trunk lateral cells (TLCs), two migratory cell types in the ascidian Boltenia villosa, express elevated levels of HSP90. Late in embryogenesis, test cells deposit HSP90 onto the tunic, the second report of extracellular HSP90 during animal development. The pyurid ascidian Halocynthia igaboja and the styelids Cnemidocarpa finmarkiensis and Botrylloides violaceus all express HSP90 at elevated levels in larval mesenchyme, suggesting that this pattern of expression is widespread in the Ascidiaceae. We show that HSP90 expression in TLCs and test cells is coincident with the presence of HNK-1. Whereas in B. violaceous, cell populations expressing elevated levels of HSP90 are distinct from those expressing HNK-1, in B. villosa both these antigens are present in the TLCs. We evaluate existing hypotheses about test cell function and, in reference to the similarities between test cells and some of the descendants of TLCs, hypothesize that test cells are TLC descendants. Implications for the proposed evolutionary relationship between TLCs and neural crest are briefly discussed.