Hidden receptors for nerve growth factor in PC12 cells

Abstract

The binding of nerve growth factor (NGF) to its receptors in [rat pheochromocytoma] PC12 cells was studied in 2 experimental conditions: cell fixation paraformaldehyde followed by permeabilization of the plasma membrane with methanol and metabolic poisoning of living cells with NaN3. Paraformaldehyde fixation of PC12 cells causes a 60-70% reduction of NGF binding capacity: the original binding capacity is restored following permeabilization with methanol. A kinetic analysis of NGF binding under these conditions reveals a single homogeneous population of receptors at variance with experiments performed in living cells where 2 kinetically distinct types of NGF receptors were demonstrated. The results suggest that a proportion of the NGF receptors in PC12 cells is hidden, i.e., not available for binding to the ligand, and in a dynamic equilibrium with exposed receptors. The existence of hidden receptors is confirmed by treatment of PC12 cells with NaN3, which causes a 50% reduction in NGF binding capacity and protection from trypsin digestion of the remaining pool of hidden receptors. The latter become exposed at the cell surface following removal of NaN3. The data provide an interpretation for the as yet unsatisfactorily explained data on NGF receptors.