Pro- and anti-mitogenic actions of pituitary adenylate cyclase-activating polypeptide in developing cerebral cortex: potential mediation by developmental switch of PAC1 receptor mRNA isoforms.

During corticogenesis, pituitary adenylate cyclase-activating polypeptide (PACAP; ADCYAP1) may contribute to proliferation control by activating PAC1 receptors of neural precursors in the embryonic ventricular zone. PAC1 receptors, specifically the hop and short isoforms, couple differentially to and activate distinct pathways that produce pro- or anti-mitogenic actions. Previously, we found that PACAP was an anti-mitogenic signal from embryonic day 13.5 (E13.5) onward both in culture and in vivo and activated cAMP signaling through the short isoform. However, we now find that mice deficient in PACAP exhibited a decrease in the BrdU labeling index (LI) in E9.5 cortex, suggesting that PACAP normally promotes proliferation at this stage. To further define mechanisms, we established a novel culture model in which the viability of very early cortical precursors (E9.5 mouse and E10.5 rat) could be maintained. At this stage, we found that PACAP evoked intracellular calcium fluxes and increased phospho-PKC levels, as well as stimulated G1 cyclin mRNAs and proteins, S-phase entry, and proliferation without affecting cell survival. Significantly, expression of hop receptor isoform was 24-fold greater than the short isoform at E10.5, a ratio that was reversed at E14.5 when short expression was 15-fold greater and PACAP inhibited mitogenesis. Enhanced hop isoform expression, elicited by in vitro treatment of E10.5 precursors with retinoic acid, correlated with sustained pro-mitogenic action of PACAP beyond the developmental switch. Conversely, depletion of hop receptor using short-hairpin RNA abolished PACAP mitogenic stimulation at E10.5. These observations suggest that PACAP elicits temporally specific effects on cortical proliferation via developmentally regulated expression of specific receptor isoforms.