NEUROTROPHINS INDUCE SPHINGOMYELIN HYDROLYSIS - MODULATION BY COEXPRESSION OF P75(NTR) WITH TRK RECEPTORS

We examined neurotrophin-induced sphingomyelin hydrolysis in cells expressing solely the law affinity neurotrophin receptor, p75(NTR), and in PC12 cells that coexpress p75(NTR) and Trk receptors, Nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3 (NT-3), and NT-5 stimulated sphingomyelin hydrolysis with similar kinetics in p75(NTR)-NIH-3T3 cells, Although brain-derived neurotrophic factor (10 ng/ml) was slightly more potent than NGF at inducing sphingomyelin hydrolysis, NT-3 and NT-5 induced significant hydrolysis (30-35%) at 0.1 to 1 ng/ml in p75(NTR)-NIH-3T3 cells, NT-3 did not induce sphingomyelin hydrolysis in Trk C-NIH-3T3 cells nor in cells expressing a mutated p75(NTR) containing a 57-amino acid cytoplasmic deletion, thus demonstrating the role of p75(NTR) in this signal transduction pathway, In p75(NTR)-NIH-3T3 cells, neurotrophin-induced sphingomyelin hydrolysis 1) localized to an internal pool of sphingomyelin, 2) was not a consequence of receptor internalization, and 3) showed no specificity with respect to the molecular species of sphingomyelin hydrolyzed, In contrast to cells expressing solely p75(NTR), NGF (100 ng/ml) did not induce sphingomyelin hydrolysis in PC12 cells, Interestingly, NT-3 (10 ng/ml) induced the same extent of sphingomyelin hydrolysis in PC12 cells as was apparent in p75(NTR)-NIH-3T3 cells. However, in the presence of NGF, NT-3 was unable to induce sphingomyelin hydrolysis, raising the possibility that Trk was modulating p75(NTR)-dependent sphingomyelin hydrolysis, Inhibition of Trk tyrosine kinase activity with 200 nM K252a enabled both NGF and NT-3 in the presence of NGF to induce sphingomyelin hydrolysis, These data support that p75(NTR) serves as a common signaling receptor for neurotrophins through induction of sphingomyelin hydrolysis and that crosstalk pathways exist between Trk and p75(NTR)-dependent signaling pathways.