Cytoskeletal elements regulate the distribution of nerve growth factor receptors in PC12 cells

Nerve growth factor receptor (NGFR)-like immunoreactivity (IR) was studied in PC12 cells treated for 96 hr with NGF (40 ng/ml), using immunogold labeling and electron microscopic morphometric analysis. The cells were exposed to the anti-NGFR antibody 192-IgG, followed by immunoglobulin (IgG) conjugated with colloidal gold. PC12 cells exhibited occasional gold label (positive NGFR-IR) on all surfaces. Cells treated with colcemid (0.05 micrograms/ml) or cytochalasin D (2 micrograms/ml), which limit microtubule (MT) and microfilament (MF) formation, respectively, displayed an increased NGFR-IR in terms of gold labeling. NGFR-IR was also seen on taxol (0.85 micrograms/ml)-exposed cells, an agent that promotes MT assembly. Cells treated simultaneously with cytochalasin D and taxol had a dramatically augmented NGFR-IR on their surfaces, which exceeded levels obtained with either agent alone. Prominent NGFR-IR was localized frequently in coated endocytotic vesicles, in smooth endoplasmic reticulum, and in secondary multivesicular lysosomes, in both treated and untreated cells. The results suggest that a large number of NGFRs (positive NGFR-IR) in PC12 cells are cryptic and not available for ligand binding. Changes in cytoskeletal organization that may affect mobility of integral membrane proteins can modulate the distribution of NGFR-IR on neuronal surfaces.