TIME-RESOLVED SIGNALING PATHWAYS OF NERVE GROWTH-FACTOR DIVERGE DOWNSTREAM OF THE P140TRK RECEPTOR ACTIVATION BETWEEN CHICK SYMPATHETIC ANDDORSAL-ROOT GANGLION SENSORY NEURONS

We have recently shown that the small GTP binding protein p21ras is es sential for nerve growth factor (NGF)-mediated survival of peripheral embryonic chick dorsal root ganglia (DRG) sensory but not sympathetic neurons. To investigate at which level of the signaling cascade the pa thways diverge, we have studied the time-resolved pattern of NGF-stimu lated tyrosine phosphorylation of proteins within 4 h after addition o f the neurotrophin. In both chick sympathetic neurons [embryonic day ( E) 12] and DRG sensory neurons (E9) NGF induces within 1 min the autop hosphorylation of the receptor tyrosine kinase p140trk. However, the p attern of substrate protein tyrosine phosphorylation downstream of p14 0trk is distinctly different in both neuronal subtypes. In sympathetic neurons, we observe within 1 min the tyrosine phosphorylation of a ne w substrate protein, p105, reaching maximal levels at 3 min. Tyrosine phosphorylation of p105 remains elevated for up to 4 h. Subsequent to p105, NGF induces the tyrosine phosphorylation of p42, a protein belon ging to the family of mitogen-activated protein (MAP) kinases. This st imulation is transient, reaching maximal levels at 10 min and returnin g to very low levels already after 2 h. In DRG sensory neurons, tyrosi ne phosphorylation of p105 is weak and very short lived, disappearing already after treatment with NGF for 10 min. In contrast, activation o f MAP kinase p42 in DRG sensory neurons is more stable than in sympath etic neurons. All NGF-stimulated tyrosine phosphorylation events were inhibited by preincubation of neurons with the tropomyosin-related kin ase (trk) inhibitor K252a. We suggest the working hypothesis that pers istent tyrosine phosphorylation of p105 may play a role in the p21ras- independent NGF survival pathway of chick sympathetic neurons.