Plasminogen activator in the developing rat cerebellum: biosynthesis and localization in granular neurons

The histochemical localization of plasminogen activator (PA) and the level of the translatable mRNA species coding for active PA were analyzed during ontogenesis of normal and of irradiation-agranulated rat cerebellum. Autoradiographic localization of PA activity was performed by plasminogen-dependent fixation of [125I]fibrin degradation products to frozen sections of developing rat cerebellum. Both the immature external and the adult internal granular layers were intensely labeled, in addition to labeling of meninges. In the irradiation-aggranulated cerebellum, PA labeling could be observed in residual granular neurons which went through their final division prior to the irradiation protocol. The concentration of the mRNA species directing the synthesis of catalytically active PA (PAmRNA) was monitored by an in ovo bioassay, using Xenopus oocytes as a translation system. A major species of 80,000 and a minor species of 50,000 apparent molecular weight of active PA were translated by mRNA from either control or X-irradiated cerebellum throughout ontogenesis. These could be detected by electrophoretic analysis of extracts and incubation media of microinjected oocytes. Both the content and the concentration of PAmRNA were found to be the highest at the stage of cerebellar development when granular neurons proliferate and migrate. These observations suggest that a major portion of the PA activity in the rat cerebellum is synthesized and localized in granular neurons through cerebellar ontogenesis, and that PA activity in the developing cerebellum is largely determined by the level of translatable mRNA coding for this enzyme.