EXPRESSION OF PAC-1, AN EPITOPE ASSOCIATED WITH 2 SYNAPSE-ENRICHED GLYCOPROTEINS AND A NEURONAL CYTOSKELETON-ASSOCIATED POLYPEPTIDE IN DEVELOPING FOREBRAIN NEURONS

The monoclonal antibody PAC 1 (postsynaptic density and cytoskeleton e nriched) recognizes an epitope present on two postsynaptic density-enr iched glycoproteins of 130,000 (postsynaptic density-enriched glycopro tein 130) and 117,000 mol. wt (postsynaptic density-enriched glycoprot ein 117), and a cytoskeleton-enriched polypeptide of 155,000 mol. wt ( cp155). The PAC 1 antibody has been used to study the development of t he PAC 1 antigens in the developing rat forebrain in vivo and in tissu e culture. cp155 is detected by embryonic day 14 and its level continu es to rise until the sixth postnatal week. Postsynaptic density-enrich ed glycoproteins 130 and 117 are also expressed in embryonic brain alt hough the level of postsynaptic density-enriched glycoprotein 130 init ially increases more rapidly than that of postsynaptic density-enriche d glycoprotein 117. Peak values are observed at postnatal days 4 (post synaptic density-enriched glycoprotein 117) and 9 (postsynaptic densit y-enriched glycoprotein 130). The level of post synaptic density-enric hed glycoprotein 117 subsequently decreases to some 50% of the peak va lue by postnatal day 42. Immunocytochemical studies show that PAC 1 im munoreactivity in developing cerebral cortex, detectable by postnatal day 0, is primarily associated with the perikarya and dendrites of pyr amidal cells. The immunoreactivity develops as patches of PAC 1-positi ve neurons, uniform staining of the cortex only being fully establishe d after postnatal day 9. Double-immunofluorescence labelling studies o f forebrain cultures prepared from embryonic day 18 animals shows that many, but not all, growth-associated protein 43-positive neurons exhi bit PAC 1 immunoreactivity. Some non-neuronal cells also stain with th e PAC 1 monoclonal antibody. The growth cones of cultured neurons exhi bit PAC 1 immunoreactivity and the PAC 1 antigens are detected on immu nodeveloped western blots of isolated growth cones. The PAC 1 epitope is intracellular, but immunoreactivity does not co-localize with F-act in as detected by rhod-amine-phalloidin or with tubulin immunoreactivi ty. Postsynaptic density-enriched glycoprotein 130 is readily detected on PAC 1 immunodeveloped western blots of forebrain cultures maintain ed for up to 14 days in vitro. Postsynaptic density-enriched glycoprot ein 117 is only poorly expressed by these cultures. The PAC 1 glycopro teins are present in forebrain synaptic membranes and postsynaptic den sities at an early stage of development. The synaptic membrane level o f postsynaptic density-enriched glycoprotein 130 and postsynaptic dens ity-enriched glycoprotein 117 increases markedly between postnatal day s 3 and 8. The level of both glycoproteins detected in postsynaptic de nsities remain virtually constant from postnatal days 9-90. These resu lts are consistent with functional roles for these molecules in neuron al and synapse development.