Cellular Dynamics Underlying Regeneration of Damaged Axons Differs from Initial Axon Development

While long-distance regeneration may be limited in mammalian species, it is becoming apparent that damaged mature neurons retain some capacity for attempted regeneration and that the adult CNS is not entirely inhibitory to axon growth. Our investigations show that there are critical intrinsic features of postinjury axonal regeneration that differ from initial axon development, and that these distinct differences may account for the limited and inappropriate regenerative response that currently characterizes the mature CNS. We compared the neurochemical and dynamic characteristics of developing axons to relatively mature regenerating axons, utilizing an in vitro model of axonal transection to long-term cultured rat cortical neurons. Immunolabelling studies revealed that regenerating and developing axons have a similar localization of cytoskeletal proteins, but the tips of regenerating axons, although morphologically similar, were smaller with reduced fillopodial extension, relative to developmental growth cones. Live imaging demonstrated that regenerating axons exhibited significantly less outgrowth than developmental neurites. Furthermore, growth cones of regenerating axons had a significant reduction in pausing, considered vital for interstitial branching and pathfinding, than did developmental growth cones. In addition, unlike developing axons, the regenerating axons were unresponsive to the growth factors BDNF and GDNF. Thus, although similar in their cytoskeletal composition, the growth cones of regenerative sprouts differed from their developmental counterparts in their size, their dynamic behaviour and their ability to respond to critical growth factors. These intrinsic differences may account for the inability of post-traumatic locally sprouting axons to make accurate pathway decisions and successfully respond to trauma.