Reorganization of brain spectrin (fodrin) during differentiation of PC12 cells.

Fodrin has been shown to redistribute dynamically between cytoplasmic and plasma membrane-associated compartments upon the differentiation of T lymphocytes. We studied the changes of distribution of fodrin in PC12 cells upon neuronal differentiation induced by nerve growth factor. To visualize preferentially the elements that were tightly associated with cytoskeletal structures, we performed immunofluorescence and immunoelectron microscopy on saponin-extracted cells. In undifferentiated PC12 cells, fodrin was distributed mostly underneath the plasma membrane. However, after the administration of nerve growth factor, perinuclear spot-like aggregates of fodrin appeared. Double-labeling immunofluorescence revealed that the cytoplasmic fodrin spot was co-localized with the intermediate filament proteins, peripherin and neurofilament. Immunogold electron microscopy showed that fodrin and neurofilament were localized in close association in the perinuclear regions enriched with intermediate filaments. With prolonged exposure to nerve growth factor, fodrin and intermediate filaments spread to the cytoplasm and neurites. These results suggest that there is a dynamic reorganization of fodrin during differentiation of PC12 cells, and that fodrin is first recruited in the perinuclear region closely associated with intermediate filaments. This dynamic reorganization of fodrin may represent important, previously unrecognized aspects of the morphological differentiation of neurons.