| Abstract: | Nerve growth factor interacts with responsive cells by binding to cell surface membrane receptors. There are two different receptors on both embryonic sensory and sympathetic neurons, a high-affinity (type I) receptor and a lower-affinity (type II) receptor. Sequestration, which we have defined as bound nerve growth factor that becomes inaccessible to the external milieu with time, occurs through the type I receptor on both sensory and sympathetic neurons. We describe here a process subsequent to sequestration involving internalization and degradation of bound nerve growth factor and showing that bound nerve growth factor is not degraded under the following conditions: (1) low temperature, ie 4°C; (2) when a large excess of unlabeled nerve growth factor is added concomitantly with the labeled nerve growth factor and the temperature is raised from 4°C to 37°C; (3) when metabolic inhibitors sodium fluoride and dinitrophenol are added concomitantly with the labeled nerve growth factor and the temperature is raised from 4° to 37°C. On the other hand, conditions that allow bound nerve growth factor to be degraded are the following: (1) incubation of the sensory nerve cells at low temperature (ie, 4°C) only in the presence of labeled nerve growth factor, then raising the temperature to 37°C; (2) when sodium fluoride and dinitrophenol are added when the temperature is raised to 37°C; (3) when excess unlabeled nerve growth factor is added when the temperature is raised to 37°C. These studies are consistent with the idea that nerve growth factor has to bind to the cells in order to be degraded; however, binding is not sufficient for degradation to occur. Second, the bound nerve growth factor must be sequestered in order to be degraded. Third, the process of internalization of the bound nerve growth factor, unlike sequestration, is not an energy-dependent process. Thus, it seems reasonable to suggest the following steps for the interaction of nerve growth factor with responsive cells: binding to a cell surface membrane receptor, followed by sequestration of the bound nerve growth factor, and finally, internalization of the sequestered nerve growth factor. |
