Axonal Transport: Components, Mechanisms, and Specificity

Abstract

The neuron is a regionally differentiated cell. Axonal transport is the pro­ cess by which materials, chiefly membranous and metabolically active, are moved rapidly from one region to another. The study of transport has recently attracted neurobiologists with widely different interests. Some are drawn to it because of their primary interest in neuronal physiology and biochemistry; these are eager to understand transport in order to compre­ hend the cellular dynamics of neuronal functioning. Others, interested in the organization of the brain, have used transport as a neuroanatomical tool. Axonal transport occurs in two directions, from the cell body to terminals (anterograde) and from terminals to the cell body (retrograde). Once it is recognized that almost all macromolecular synthesis occurs in the peri­ caryon, the functional significance of anterograde transport becomes obvi­ ous. Essentially all subcellular organelles and membrane components in axons, dendrites, and nerve terminals must originate in the cell body, if not fully assembled, then at least in some precursor form. Transport also occurs in the retrograde direction, returning worn out materials from nerve termi­ nals to the cell body for either degradation or restoration. Although its role in the pathogenesis of neurotropic viral diseases was suggested in the 1920s (see Baringer 1975), retrograde transport only recently has been shown to play an important part in the life cycle of the synaptic vesicle (see Heuser & Reese 1974, Holtzman 1977). Both anterograde and retrograde axonal transport have also been widely used during the past decade as the basis for