Regulation of metamorphosis in ascidians involves NO/cGMP signaling and HSP90

Treatment of larvae of the ascidians Boltenia villosa (Family: Pyuridae) an d Cnemidocarpa finmarkiensis (Family: Styelidae) with drugs that inhibit th e function of the molecular chaperone HSP90 increased the frequency of tail resorption, the primary morphogenetic event of metamorphosis. If treatment was initiated at hatching, metamorphic events subsequent to tail resorptio n failed to occur, indicating an ongoing role for HSP90 during morphogenesi s. Removal of tails from heads of mature, but not newly hatched larvae, ind uced metamorphosis of the head. Decapitation experiments indicate that the capacity of tails to shorten in response to inhibition of HSP90 function re quires communication with heads. To identify candidate proteins with which HSP90 may interact to regulate metamorphosis, we noted that in mammalian ce lls, nitric oxide synthase (NOS) interacts with HSP90 and its activity is s ensitive to drugs that inhibit HSP90 function. In addition, nitric oxide (N O) signaling in the marine snail Ilyanassa obsoleta is an important regulat or of metamorphosis. Inhibition of NOS activity in these ascidian larvae wi th L-NAME increased the frequency of metamorphosis, consistent with a putat ive interaction of NOS and HSP90. NOS is present in tail muscle cells, impl icating them as targets for the drug treatments, consistent with the decapi tation experiments. Inhibition of soluble guanylyl cyclase, the most common effector of NO signaling, also increased the frequency of metamorphosis. I n contrast to treatment with anti-HSP90 drugs, metamorphosis induced with L -NAME or ODQ was complete. The results presented suggest that an HSP90-depe ndent, NO-based regulatory mechanism localized in tails represses ascidian metamorphosis. We discuss these results in relation to the induction of asc idian metamorphosis by several unrelated agents. J. Exp. Zool. 289:374-384, 2001. (C) 2001 Wiley-Liss, Inc.