Developmental pattern of plasminogen activator activity in chick optic lobe

Plasminogen activators are serine proteases which play a key role in morphogenesis and tissue remodelling. Two different molecular types, tissue‐type and urokinase‐type, were identified and they were postulated to play a role in neural development. The developing chick optic lobe plays a central role in processing visual information. In previous studies we demonstrated the occurrence of high levels of plasminogen activator activity in this model. The aim of the present paper is to study the temporal pattern of expression of this activity and characterize the type of plasminogen activator expressed in the developing optic lobe. Using soluble fractions derived by ultracentrifugation from Triton X‐100‐treated membrane fractions we measured the protease activity with a radial fibrinolytic assay. Employing different inhibitors of fibrinolytic activity and a zymographic assay, we showed that the developing optic lobe expresses only one type of plasminogen activator which corresponds to an urokinase‐type of 70 kDa. Our results indicate that peaks of protease activity temporally correlate with massive neuronal migration, neurite outgrowth and synapse formation and maturation. This suggests that a plasminogen activator could play a role in these developmental events. This consistent pattern of variability strongly suggests that it is developmentally regulated and, if so, it could be a reliable parameter to study neural plastic changes induced by modifications in the environmental stimulation.