We have directly evaluated the effects of various intracellular second messengers including
cyclic nucleotides,
calcium ion, and
inositol polyphosphates on shape and motility of differentiating mouse
neuroblastoma cells. The messengers were microinjected into cells and the responses of the
soma, neurite, and growth cone were monitored using time-lapse video microscopy. Each messenger altered cell shape and motility in a characteristic manner.
Cyclic AMP promoted lamellipodial expansion, neurite outgrowth, and motility. The other injected messengers opposed motility.
Cyclic GMP caused motile structures to freeze and to retract permanently, while the inhibitory effects of
calcium injection were concentration-dependent. Small
calcium injections affected specifically actin-containing motile structures which froze and retracted temporarily. Intermediate
calcium injections caused a strong contraction at the site of injection in all cells. With large
injections, cells retracted long neurites, rounded up, and frequently began vigorous blebbing that continued to cell death.
Injections of the
inositol polyphosphates IP3(1,4,5) and IP4(1,4,5,6) mimicked the effects of small
calcium injections, as did electrical stimulation that elicited action potentials. The results suggest that in mouse
neuroblastoma cells, intracellular cAMP elevation increases cytoskeletal organization and promotes neurite extension perhaps through an enhancement of cell-substratum adhesion. On the other hand, a rise of intracellular cGMP or intracellular
calcium interferes directly with the function and organization of the actin-microfilament system. The integrated action of these second messenger systems may, therefore, operate in vivo to allow substances released from neighboring cells to regulate neuronal architecture.