Differential Transport of Protein in Axons: Comparison Between the Sciatic Nerve and Dorsal Columns of Cats

Abstract

Fast axoplasmic transport was studied in dorsal root ganglion cells of the cat. Proteins carried in the fast axoplasmic flow were labeled after an intraganglionic injection of L-[4,5-3Hlleucine. The rate of transport was 380 + 26 mm/day in both the central and peripheral branches of the bifurcating axons that arise from cells of the dorsal root ganglion. The amount of radioactivity transported centrally, through the dorsal roots into the spinal cord, was about 50% of that moving peripherally, through the sensory fibers of the sciatic nerve. Labeled material appears to be transported principally in a bound form, as 70-80% of the radioactivity was insoluble in 0.01 M potassium phosphate buffer at pH 7.4. With multiple extractions, a fractionation procedure was developed by which 94-96% of the total transported radioactivity could be solubilized. The proteins carried by fast axoplasmic transport through the dorsal columns and through the sciatic nerve were compared by electrophoresis of extracted fractions on Na dodecyl sulfate-polyacrylamide gels.Different patterns of radioactivity are seen in the electrophoretograms, suggesting that the cells of the dorsal root ganglion may possess the ability to commit different proteins to transport through two branches of a single bifurcating axon.Neurons elaborate various protein and particulate matter that is carried through the axon to the nerve terminals by fast axoplasmic transport (1, 2). This phenomenon has been demonstrated in sensory fibers of the sciatic nerve after injection of [3HIleucine into the seventh lumbar dorsal root ganglia (Lq DRG) of cats (3, 4). Labeled material also moves from the DRG through the dorsal root fibers into the central nervous system (3). Recent work has confirmed the importance of fast transport in the control and maintenance of synaptic function (5, 6).In this paper, we consider the question: Can different proteins be transported through different branches of a bifurcating axon? The cells of the Lq DRG in cats provide a useful experimental system. Each of these cells sends out a single bifurcating fiber (7). From the bifurcation, one branch extends toward the periphery as a sensory fiber in the sciatic nerve. The other branch travels through the dorsal roots and into the spinal cord, where the fiber either terminates within a short distance or extends up the dorsal columns to the dorsal column nuclei in the medulla. In this study, labeling with radioactive precursors was used to compare material transported through the peripheral fibers, in the sciatic nerve, with that transported through the central fibers, in the dorsal columns. Although the rate of movement and the general Abbreviations: DRG, dorsal root ganglia; L7, lumbar seventh. 1521 solubility characteristics of the labeled material were the same for material moving in both directions, different transported proteins were found in the two branches. MATERIALS AND METHODSCats (between 2.5 and 4.0 kg) were anesthetized by intraperitoneal injection of sodium pentobarbital (aver...

Routing of transported materials in the dorsal root and nerve fiber branches of the dorsal root ganglion

“…This was shown by the greater crest height of labeled proteins in the sciatic nerve as compared to the dorsal root following injection of the L7 dorsal root ganglion with "H-leucine (Ochs, 1972a). A similar asymmetry of outflow was reported for the cat by Anderson and McClure (1973), the rat by Komiya and Kurokama (1978), and for branches of the nodose ganglion of the vagus (Watson, et al, 1975). Komiya and Kurokawa (1978) did not see the same asymmetry for slow transport, but differences in the slopes of outflow in the dorsal roots and sensory nerves of the cat as longer times had been shown by Lasek (1968) which suggested either a difference in rate or a similar asymmetry of the amount of outflow carried by slow transport.…”

“…There is a need to supply the terminals of the dorsal roots with components required for synaptic transmission and yet other components to the terminals of the sensory nerve fibers that are specially required for the process of transduction (Ochs, 1972). A difference in protein composition in the two branches was claimed by Anderson and McClure (1973) but denied by White and White (1977) and cf. Karlsson and Sjostrand (1972).…”

Characterization of proteins transported at different rates by axoplasmic flow in the dorsal root afferents of rats

“…Also, we found significant radioactivity in proteins of high molecular weight (>250,000 daltons) while Anderson and McClure (1973) found no radioactivity in these high molecular weight proteins.…”

“…Anderson and McClure (1973) found markedly different electrophoretic patterns in the dorsal column as opposed to the sciatic nerve of cats at an unspecified time between 4 and 16 hours following injection of L7 DRG with [3H] leucine. This finding led them to suggest that a given cell can transport different proteins through different branches of a bifurcating axon.…”

Protein Synthesis and Rapid Axonal Transport During Regrowth of Dorsal Root Axons

“…Although the velocity of rapid transport is the same (Ochs, 19721, less material is rapidly transported along the roots than the nerve (Lasek, 1968). Three to five times less material has been reported to be rapidly transported in mammalian dorsal roots (Ochs, 1972;Anderson and McClure, 1973;Ochs et al, 1978). In two studies of the amounts of rapidly transported label at ligatures on peripheral and central branches of frog DRG neurons, we have found peripheral/ central ratios of 7 (Stone and Wilson, 1979) and 3-6 (Perry and Wilson, 1981).…”