成長円錐におけるニューロトロフィン–受容体間相互作用の１分子解析

The growth cone is a motile structure located at the distal tip of the nerve fiber. The 2.5S nerve growth factor (NGF) promotes the axonal growth and the survival of sensory neurons by reacting with their growth cones. We observed the behavior of single molecules of NGF on the growth cone of sensory neurons by using a fluorescent NGF, Cy3-NGF. Upon the application of 0.4 nM of Cy3-NGF, the growth cones responded within one minute of adding the stimulus by expanding their lamellipodia. Only 40 molecules of Cy3-NGF, which occupied less than 5% of the estimated total binding sites on a single growth cone, were required to initiate the motile responses. After binding to the receptor, Cy3-NGF displayed lateral diffusion on the membrane of the growth cones. The behavior of Cy3-NGF was shifted to a one-directional rearward movement toward the central region of the growth cone. The one-directional movement of Cy3-NGF displayed the same rate as the rearward flow of actin and the movements could be stopped by the application of the potent inhibitor of actin polymerization, latrunculin B. Molecules of Cy3-NGF were internalized in the vicinity of the central region of the growth cone during this rearward trafficking, as Cy3-NGF remained in the growth cone after the removal of Cy3-NGF from the receptors on the surface of growth cones. These results suggested that actin-driven trafficking of the NGF-receptor complex is an essential step to the accumulation and endocytosis of NGF at the growth cone. [J Physiol Sci. 2006;56 Suppl:S58]