The ascidian prophenoloxidase activating system

Abstract

Phenoloxidases/tyrosinases initiate melanin synthesis in almost all organisms, and are involved in
different biological activities such as the colour change of human hair and the browning or blackening
of fruit skin etc. In many invertebrates, defence reactions are linked to phenoloxidase activity and/or
melanization. Contacts with foreign molecules are able to trigger the prophenoloxidase (proPO)
system that requires serine protease cleavage for activating the zymogen to phenoloxidase (PO). It is
generally accepted that the proPO system is fully expressed in arthropods, and, recently, progress in
the regulation of crustacean and insect proPO activation steps have been achieved. After cells were
stimulated by components of pathogen associated molecular pattern (PAMP), proPO activation takes
place via zimogenic serine proteinase in turn activated by PAMPs followed by cascade, spatial and
temporal control. The proPO activating system plays a defensive role in arthropods, molluscs, annelids, ascidians and the cephalochordate Branchiostoma belcheri. In the present paper, we report on ascidian proPO system and related molecules, with particular focus on the biochemical, cellular and molecular aspects of the Ciona intestinalis, proPO system of circulating hemocytes from naïve ascidians as well as of body wall following LPS inflammatory challenge.