Molecular motors in the nervous system

Early in this century, neurobiologists noted that if nerve fibers were separated from their cell bodies, the fibers soon degenerated. This finding suggested that specific mechanisms must exist for transport of material into the axon (Scott, 1906; Ramon y Cajal, 1928). No direct evidence for a transport system was forthcoming until 1948, when Weiss and Hiscoe (1948) first described a slow “axoplasmic flow” of material. This slow flow subsequently was resolved into multiple, discrete rate components corresponding to slow axonal transport. The existence of additional, faster rate components for axonal transport, now called fast axonal transport, was recognized only later, when the rapid transport of a variety of membrane-bounded organelles was demonstrated by radioisotopic labeling (Lasek, 1967), histochemistry (Dahlstrom, 1967), and direct microscopic observation (Burdwood, 1965, J. Cell Biol., abstract). Subsequent characterization of axonal transport established its basic characteristics. The size, morphology, and specialized functions of the neuron permitted an unusually detailed analysis (seeTable 1 and earlier reviews, such as Grafstein and Forman, 1980; Brady, 1985a). As a result, the study of axonal transport has largely shaped our understanding of intracellular motility in both neuronal and nonneuronal cells. Characterization of axonal transport by radioisotopit labeling yielded information about composition, rates, and metabolic requirements, but gave little insight into underlying mechanisms. Little could be inferred about the motors responsible for translocation of material within the axon. Microscopic observation of living neurons provided a more direct view of movement in axons, but physical limitations to the resolution of cell structures were thought to be insurmountable. In 1981 the situation changed dramatically: video-enhanced contrast differential interference contrast (VEC-DIG) light microscopy was introduced. Individual microtubules could be detected (Allen et al., 1981), and synapticvesicles moving along microtubules became visible within living axons (Allen et al., 1982). Once the means to follow movement of individual organelles in real time became available, a preparation that permitted precise manipulation of experimental conditions was needed. An early use of VEC-DIC microscopy was the first description of the remarkable amount of bidirectional traffic in giant axons from the squid Loligo pealeii (Allen et al., 1982). Many years before, Bear et al. (1937) had demonstrated that axoplasm could be extruded from the giant axon as an intact cylinder. Properties of the isolated axoplasm had been characterized in some detail (Lasek, 1984), making VEC-DiC microscopic analysis of axoplasm a natural choice. Remarkably, even after extrusion, fast axonal transport continued unabated in isolated axoplasm for hours (Edrady et al., 1982). Isolated axoplasm from the giant axon had no plasma membrane or other permeability barriers, yet was readily maintained in an active state. Combining VEC-DIC microscopy with isolated axoplasm permitted rigorous dissection of the mechanisms for fast axonal transport using biochemical and pharmacological approaches (Brady et al., 1982, 1985).