Distinction between differentiation, cell cycle, and apoptosis signals in PC12 cells by the nerve growth factor mutant delta9/13, which is selective for the p75 neurotrophin receptor.
The common neurotrophin receptor p75(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(NTR) to signal independently of TrkA was investigated with an NGF mutant protein (NGFdelta9/13) that binds poorly to TrkA (Woo et al.  J. Biol. Chem. 270:6278-6285). The NGFdelta9/13 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 NGFdelta9/13 mutant in naive PC12 cells after serum withdrawal, suggesting that p75(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 NGFdelta9/13 mutant after serum deprivation. Both NGF and the NGFdelta9/13 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 NGFdelta9/13 protein and NGF. Finally, upon serum withdrawal, both NGF and the NGFdelta9/13 mutant activate nuclear translocation of the transcriptional factor NF-kappaB (nuclear factor kappaB), a process involved in cell survival. These results are consistent with p75(NTR) inhibition of caspase-mediated apoptosis in PC12 cells. The different physiologic responses elicited by NGFdelta9/13 indicate the potential for individual signaling by the two NGF receptors and also demonstrate the utility of NGF mutants for receptor-selective signal transduction.
Hughes, AL; Messineo-Jones, D; Lad, SP; Neet, KE
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