Axonal growth-associated proteins.

Abstract

Elongation ofaxons and active remodeling of their terminal arbors under­ lies the assembly of neural circuits during development, determines the success or fai lure of nerve regeneration, and may contribute to some forms of synaptic plast icity in adult brains. For most neurons, elongation of a principal axon is confined to a few days or weeks during development. Remodeling of axon terminal arbors also is most pronounced during transient critical late in development, although some forms of synaptic remodeling continue throughout lif e (Lichtman et aI19 87). Once past these epochs of axon elongation and dynamic sorting of synaptic terminals, it might be possible to stabilize principal axon branches or their terminal arbors by inactivating some of the molecular processes required for growth and synaptogenesis. Selective inactivation or retention of some growth-related processes in maturing neurons might then define some limits on the mechanisms available for synaptic remodeling in the adult nervous system. Studies of axon regeneration in vivo and in tissue culture indicate that some aspects of axon growth are indeed repressed in many adult neurons but can be re-induced under some conditio ns. Although such studies have considered primarily the elongation of primary axons, they also raise the possibility that neuronal processes underlying more subtle aspects of axon growth and synaptogenesis may be down-regulated chronically in mature neurons. At the molecular level, periods of axon outgrowth during devel­ opment and re-induction of axon growth for regeneration are correlated with large and specific changes in synthesis of a few proteins transported into the growing axons. This suggests the hypothesis that