Pituitary adenylate cyclase-activating polypeptide is an autocrine inhibitor of mitosis in cultured cortical precursor cells

During brain development, an intricate array of signals is likely to control the transition from proliferation to differentiation, particularly in the complex cerebral cortex. Although factors regulating proliferation and differentiation have been identified, little is known about mechanisms governing the exit of precursors from the cell cycle. We now report that pituitary adenylate cyclase-activating polypeptide (PACAP), a new member of the vasoactive intestinal peptide family expressed in embryonic brain, promotes this transition. In virtually pure cultures of embryonic day 13.5 (E13.5) rat cortical precursors, PACAP inhibited [H-3] thymidine incorporation by 43%, decreasing the proportion of mitotic cells. Moreover, the peptide promoted morphological and biochemical differentiation; PACAP elicited a 2-fold Increase in cells bearing neurites and a 30% increase in neurotrophin trkB receptor expression, indicating that PACAP induced cell cycle withdrawal and promoted neuronal differentiation. The expression of PACAP ligand and receptor in precursors raised the possibility of autocrine function. Indeed, 85% of cells exhibited PACAP immunoreactivity while 64% expressed type I receptor, which, in turn, mediated cAMP activation and phosphorylated cAMP response element binding protein nuclear signaling. Furthermore, treatment with the PACAP antagonist or neutralizing antibody increased DNA synthesis and proliferation, which is consistent with interruption of ongoing mitotic inhibition mediated by endogenous PACAP. Our observations suggest that cortical precursors produce PACAP as an autocrine signal to elicit cell cycle withdrawal, inducing the transition from proliferation to neuronal differentiation.