Nr. Lu et al., A PARADIGM FOR DISTINGUISHING THE ROLES OF MITOGENESIS AND TROPHISM IN NEURONAL PRECURSOR PROLIFERATION, Developmental brain research, 94(1), 1996, pp. 31-36
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
.