LOCATION OF PROFILIN AT PRESYNAPTIC SITES IN THE CEREBELLAR CORTEX - IMPLICATION FOR THE REGULATION OF THE ACTINPOLYMERIZATION STATE DURINGAXONAL ELONGATION AND SYNAPTOGENESIS

Profilin is a 15 kDa protein that binds actin monomers and inhibits th eir polymerization in vitro. The actin-profilin complex can be rapidly dissociated in vitro by phosphatidylinositol-4,5-bis-phosphate, provi ding a mechanism for regulating actin assembly-disassembly cycles duri ng cell motile events. We have used a polyclonal antibody to calf sple en profilin to analyse the developmental expression and cellular distr ibution of profilin in the rat cerebellum and cultured cortical neuron s. Immature neurons contain large amount of profilin both in vivo and in vitro. Immunofluorescence showed it to be present in developing neu rites and growth cones but not in the filopodia of cortical neurons in culture. Profilin immunoreactivity was intense in the parallel fibres , the granule cell axons of the cerebellar cortex, at the time when th ey are elongating. Purkinje cell dendrites were not labelled. Profilin immunostaining was present in presynaptic varicosities, but not in de ndritic spines within the molecular layer of juvenile and adult rats. The profilin concentration was higher in synaptosomes than in the tota l cerebellum during the second and third postnatal weeks, a period of intense synaptogenesis. Thus, profilin may help regulate actin polymer ization and depolymerization during axonal elongation and synaptogenes is. Its restriction to the presynaptic site in the adult suggests that it may also be involved in the regulation of the release of synaptic vesicles.