CONCENTRATION-DEPENDENT REGULATION OF NEURONAL GENE-EXPRESSION BY NERVE GROWTH-FACTOR

NGF is a neurotrophic protein that promotes the survival, growth, and differentiation of developing sympathetic neurons. To directly determi ne the effects of different concentrations of NGF on neuronal gene exp ression, we examined mRNAs encoding the p75 low-affinity NGF (LNGF) re ceptor, T-alpha-1 alpha-tubulin (T-alpha-1), and tyrosine hydroxylase (TH) in pure cultures of rat sympathetic neurons from postnatal day 1 superior cervical ganglia. Studies of the timecourse of gene expressio n during 2 wk in culture indicated that a 5-d incubation period would be optimal for the concentration-effect studies. Analysis of RNA isola ted from neurons cultured in 2-200 ng/ml 2.5S NGF for 5 d revealed tha t, as the NGF concentration increased, neurons expressed corresponding ly increased levels of all three mRNAs. Both LNGF receptor and TH mRNA s increased seven-fold, and T-alpha-1 mRNA increased four-fold in neur ons cultured in 200 versus 10 ng/ml NGF In contrast, T26 alpha-tubulin mRNA, which is constitutively expressed, did not alter as a function of NGF concentration. When neurons were initially cultured in 10 ng/ml NGF for 5 d, and then 200 ng/ml NGF was added, LNGF receptor, T-alpha -1, and TH mRNAs all increased within 48 h. The timecourse of inductio n differed: T-alpha-1 mRNA was maximal by 5 h, whereas LNGF receptor a nd TH mRNAs first began to increase at 12 h after the NGF increase. Th ese experiments show that NGF regulates expression of a subset of mRNA s important to neuronal growth and differentiation over a broad concen tration range, suggesting that the effects of NGF may be mediated by m ore than just a single receptor operating at one fixed affinity. These results also suggest a mechanism for coupling neuronal synthesis of a xonal proteins to increases in size of the innervated target territory during growth of the organism.