Distinction Between Differentiation, Cell Cycle, and Apoptosis Signals in PC12 Cells by the Nerve Growth Factor Mutant Delta 9/13, Which Is Selective for the p75 Neurotrophin Receptor

The common neurotrophin receptor p75 super(NTR) (low affinity nerve growth factor receptor) participates in the high-affinity binding with the TrkA nerve growth factor (NGF) receptor, may mediate apoptosis, and may signal independently in a cell-specific manner. The potential of p75 super(NTR) to signal independently of TrkA was investigated with an NGF mutant protein (NGF Delta 9/13) that binds poorly to TrkA (Woo et al. [1995] J. Biol. Chem. 270: 6278-6285). The NGF Delta 9/13 mutant does not activate TrkA autophosphorylation and fails to stimulate the normal NGF-induced growth arrest, demonstrating that TrkA activation is required to arrest PC12 cells at the NGF-activated G1/S cell cycle checkpoint. However, apoptosis is successfully blocked and cell survival is promoted by the NGF Delta 9/13 mutant in naive PC12 cells after serum withdrawal, suggesting that p75 super(NTR) can signal for survival autonomously of TrkA. Annexin V binding, an indication of apoptotic plasma membrane disruption, is inhibited by both NGF and the NGF Delta 9/13 mutant after serum deprivation. Both NGF and the NGF Delta 9/13 mutant inhibit the rapid apoptotic internucleosomal DNA cleavage of PC12 cells upon serum deprivation. Furthermore, the level of caspase3-like activity that is rapidly activated by serum withdrawal from PC12 cells is reduced by both the NGF Delta 9/13 protein and NGF. Finally, upon serum withdrawal, both NGF and the NGF Delta 9/13 mutant activate nuclear translocation of the transcriptional factor NF- Kappa B (nuclear factor Kappa B), a process involved in cell survival. These results are consistent with p75 super(NTR) inhibition of caspase-mediated apoptosis in PC12 cells. The different physiologic responses elicited by NGF Delta 9/13 indicate the potential for individual signaling by the two NGF receptors and also demonstrate the utility of NGF mutants for receptor-selective signal transduction.