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 c ontrol the transition from proliferation to differentiation, particula rly in the complex cerebral cortex. Although factors regulating prolif eration and differentiation have been identified, little is known abou t mechanisms governing the exit of precursors from the cell cycle. We now report that pituitary adenylate cyclase-activating polypeptide (PA CAP), a new member of the vasoactive intestinal peptide family express ed in embryonic brain, promotes this transition. In virtually pure cul tures of embryonic day 13.5 (E13.5) rat cortical precursors, PACAP inh ibited [H-3] thymidine incorporation by 43%, decreasing the proportion of mitotic cells. Moreover, the peptide promoted morphological and bi ochemical differentiation; PACAP elicited a 2-fold Increase in cells b earing neurites and a 30% increase in neurotrophin trkB receptor expre ssion, indicating that PACAP induced cell cycle withdrawal and promote d neuronal differentiation. The expression of PACAP ligand and recepto r 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 phosphorylat ed cAMP response element binding protein nuclear signaling. Furthermor e, treatment with the PACAP antagonist or neutralizing antibody increa sed DNA synthesis and proliferation, which is consistent with interrup tion of ongoing mitotic inhibition mediated by endogenous PACAP. Our o bservations suggest that cortical precursors produce PACAP as an autoc rine signal to elicit cell cycle withdrawal, inducing the transition f rom proliferation to neuronal differentiation.