ALL-TRANS-RETINOIC ACID STIMULATES AND MAINTAINS NEURITE OUTGROWTH INNERVE GROWTH FACTOR-SUPPORTED DEVELOPING CHICK EMBRYONIC SYMPATHETIC NEURONS

In explanted embryonic chick sympathetic neurons, all-trans retinoic a cid (RA) as well as nerve growth factor (NGF) were found to be require d for neuronal survival and neurite outgrowth at early stages of devel opment (day 6.5-7) in agreement with previous work (Rodriguez-Tebar an d Rohrer [1991] Development 112:813-820). The dependence of neurons on all-trans RA for survival diminished at later stages of development. However, all-trans RA was found to be needed at all stages of developm ent in order to maximize neurite outgrowth. Further, removal of all-tr ans RA from the cultures led to a rapid degeneration of the formed neu rites, demonstrating the essentiality of all-trans RA for both the dev elopment of neurites, and for the maintenance of existing neurites in cultured embryonic sympathetic neurons. The mechanism whereby all-tran s RA exerts its effects on embryonic sympathetic neurons may involve a ctivation of the nuclear retinoic acid and retinoid-X receptor (RAR an d RXR) families. The results of Northern blot analyses and/or reverse transcriptase-polymerase chain reaction (RT-PCR) studies show that emb ryonic sympathetic ganglia express RAR beta, RAR gamma and RXR gamma m RNAs. RXR gamma mRNA is expressed at highest levels in immature neuron s that are not yet responsive to NGF (day 6.5-7) and message levels de cline with increasing developmental age. In contrast, RAR beta transcr ipt levels are barely detectable at day 6.5-7, and increase similar to 4-fold in ganglia from embryos at day 8.5-9 and decline thereafter, R T-PCR studies show that RAR gamma mRNA is expressed both early (day 6. 5-7) and late (day 15) in ganglionic development. Transcripts for the NGF receptors, p75(NGFR) and p140(trk) were also examined. The appeara nce of a single 2.7 kb p140(trk) transcript coincides with the time wh en RAR beta mRNA is maximally expressed, raising the possibility that NGF receptors may be targets of retinoid action. Evidence is also pres ented that all-trans RA may enhance neurite outgrowth by mechanisms ot her than simply inducing NGF-responsiveness of neurons. (C) 1996 Wiley -Liss, Inc.