Retinoic acid and neurotrophins collaborate to regulate neurogenesis in adult-derived neural stem cell cultures

The adult rat hippocampus contains fibroblast growth factor 2-responsive st em cells that are self-renewing and have the ability to generate both neuro ns and glia in vitro, but little is known about the molecular events that r egulate stem cell differentiation. Hippocampus-derived stem cell clones wer e used to examine the effects of retinoic acid (RA) on neuronal differentia tion. Exposure to RA caused an immediate up-regulation of NeuroD, increased p21 expression, and concurrent exit from cell cycle, These changes were ac companied by a threefold increase in the number of cells differentiating in to immature neurons. An accompanying effect of RA was to sustain or up-regu late trkA, trkB, trkC, and p75NGFR expression, Without RA treatment, cells were minimally responsive to neurotrophins (NTs), whereas the sequential ap plication of RA followed by brain derived neorotrophic factor or NT-3 led t o a significant increase in neurons displaying mature gamma-a-minobutyric a cid, acetylcholinesterase, tyrosine hydroxylase, or calbindin phenotypes. A lthough NTs promoted maturation, they had little effect on the total number of neurons generated, suggesting that RA and neurotrophins acted at distin ct stages in neurogenesis. RA first promoted the acquisition of a neuronal fate, and NTs subsequently enhanced maturation by way of RA-dependent expre ssion of the Trk receptors, In combination, these sequential effects were s ufficient to stimulate stem cell-derived progenitors to differentiate into neurons displaying a variety of transmitter phenotypes. (C) 1999 John Wiley & Sons, Inc.