A PARADIGM FOR DISTINGUISHING THE ROLES OF MITOGENESIS AND TROPHISM IN NEURONAL PRECURSOR PROLIFERATION

During nervous system development, the generation of neuronal populati ons is subject to regulation by extracellular growth factors. Traditio nal views suggest that growth factors promote proliferation by increas ing the fraction of precursors that enter the mitotic cycle and subseq uently divide (that is, enhance mitosis). However, recent evidence ind icates that dividing precursors may undergo cell death. Consequently, a given molecule may also increase neuroblast proliferation by promoti ng survival of dividing precursors. In the present work, we developed a new approach to distinguish these two distinct effects of growth fac tors on dividing neuroblasts. By using a brief 6 h culture paradigm of embryonic day 15.5 sympathetic neuroblasts, we minimized cell death, thereby excluding the survival-promoting (trophic) activity of growth factors. In the absence of trophism, measured increases in [H-3]thymid ine incorporation reflected growth factor mitogenic activity only. Usi ng a well-characterized sympathetic model, we found that insulin, EGF and vasoactive intestinal peptide (VIP) increased [H-3]thymidine incor poration 30%, 20% and 46% respectively, consistent with their previous ly reported mitogenic activity. In contrast, neurotrophin-3 (NT3) and nerve growth factor (NGF), which serve as trophic signals for the neur oblasts, did not elicit any change in [H-3]thymidine incorporation, in dicating that the neurotrophins are not mitogenic for sympathetic prec ursors. This approach may be useful in distinguishing mitogenic and tr ophic regulation of proliferation in other brain precursor populations .