DEVELOPMENTAL-CHANGES IN GAP-43 EXPRESSION IN PRIMARY CULTURES OF RATCEREBELLAR GRANULE CELLS

GAP-43 is a growth-associated protein that has been implicated in the developmental outgrowth of axons. We have examined the profile of GAP- 43 levels in rat cerebellar granule cells during their development in vitro. During the first 1-2 days after plating, the majority of cells expressed neurites and after 8 days a complex neuronal network had dev eloped. In situ hybridization studies showed that GAP-43 mRNA levels r apidly increased to peak at 1-2 days and gradually returned to initial values after 7-8 days. Analysis of GAP-43 protein levels followed a s imilar transient profile. Initially, granule cell perikarya and struct ures associated with neuritogenesis all displayed GAP-43 immunoreactiv ity. In older cultures, perikaryal labelling was lost after 10 days wh ilst process staining decreased more gradually. During the first 48 ho urs detailed analysis of GAP-43 mRNA revealed two populations of granu le cells. It was suggested that cells with significant label originate d from the external germinal layer which displays much GAP-43 mRNA in cerebellar sections. Cells with little or no GAP-43, however, probably originated from the internal granular layer since this region display ed no specific labelling. Granule cells within clumps expressed more G AP-43 mRNA compared to isolated cells perhaps indicating cell-cell reg ulation of expression. These results describe the transient rise in GA P-43 protein and mRNA levels expressed by developing cerebellar granul e cell neurons in vitro and provide further evidence for the role GAP- 43 plays during neuritogenesis.