Probing the structure-function relationship of nerve growth factor.
We compared the receptor binding, antigenicity, biological activation, and cell-mediated proteolytic degradation properties of mouse nerve growth factor (mNGF) and human NGF (hNGF). The affinity of hNGF toward human NGF-receptor is greater than that of mNGF, but the affinity of mNGF toward rat NGF-receptor is greater than that of hNGF. Thus, the specificity of the interaction between NGF and its receptor resides both on the NGF and on its receptor. Using a group of anti-NGF monoclonal antibodies that competitively inhibit the binding of NGF to receptor, sites differing between mNGF and hNGF were detected. Together, these results indicate that the sites on hNGF and mNGF, responsible for binding to NGF-receptor, are similar but not identical. In comparing the relative abilities of mNGF and hNGF to stimulate a biological response in PC12 cells, we observed that mNGF was better at stimulating neurite outgrowth than was hNGF, consistent with the differences observed for receptor binding affinity. However, the ED50 for biological activation is approximately 100-fold lower than the Kd for receptor occupancy, and, thus, the dose-response curve is not consistent with a simple activation proportional to receptor occupancy. The data are consistent with a model requiring a low-level threshold occupancy of NGF-receptor (Kd = 10(-9) M) in order to stimulate full biological activity. Finally, we observed the degradation of NGF by PC12 cells. We found that the NGF molecule is significantly degraded via a receptor-mediated uptake mechanism. Together, the data provide insight into regions of the NGF molecule involved in contacts with the receptor leading to formation of the NGF:NGF-receptor complex. Additionally, they establish the link between occupancy of receptor and biological activation and the requirement for receptor-mediated uptake in order to degrade NGF proteolytically in cultured PC12 cells.