Axonal transport of cytoskeletal proteins in oculomotor axons and their residence times in the axon terminals

Abstract

Cytoskeletal protein transport and metabolism are studied in the somatic motor and parasympathetic axons of the chicken oculomotor system. Kinetic analyses of pulse-labeled proteins indicate that the neurofilaments are transported 2-3 times more rapidly in the somatic motor axons than in the parasympathetic axons. By contrast, the transport rates of the slow component b (SCb) proteins are very similar in these axons. The parasympathetic axons terminate in the ciliary ganglion, and radiolabeling curves from the ciliary ganglion can be used to study the kinetics of cytoskeletal protein removal from the terminals. The rate of removal directly determines the residence time of the cytoskeletal proteins in the ciliary ganglion, and the residence time directly affects the shape and amplitude of the transport curves of the ganglion. A computer model was used to analyze these transport curves and to determine the half-residence time of the cytoskeletal proteins in the terminal regions. From the computer experiments, we estimate that the half-residence times of the neurofilament proteins actin and tubulin are 2, 3.5, and 7 d, respectively. The differences between the half-residence times of the cytoskeletal proteins indicate that the rate of protein removal from the terminals differs for these proteins. On the basis of these results, we propose that the removal mechanisms critically control the composition of the cytoskeleton in the terminal regions. Through their effects on its composition, the removal mechanisms have a crucial role in converting the cytoskeleton of the axon proper into the specialized cytoskeleton of the axon terminal.