A radiolabelled pulse for the simultaneous study of anterograde and retrograde axonal transport

Cytoplasmic dynein conversion at a crush injury in rat peripheral axons

Cytoplasmic dynein is a motor for retrograde axonal transport for movement of membranous organelles toward the neuronal cell body. However, cytoplasmic dynein is synthesized in the cell body and conveyed along the axon to nerve terminals. To characterize the axonal transport of cytoplasmic dynein in relation to synaptic vesicles and other membrane compartments, immunocytochemical and cytofluorimetric scanning analyses of crush-operated rat sciatic nerves were performed. Distal to the crush, the kinetics of dynein accumulation were consistent with its role in the retrograde transport of membranous organelles. During the initial 3 hr after crush, only small amounts of dynein-immunoreactive material accumulated proximal to the crush. This is consistent with metabolic labeling studies showing that most of the dynein moving in the anterograde direction is in the slow component of axonal transport. Thereafter, the rate of proximal accumulation of dynein increased, and by 8 hr postcrush a large amount of dynein immunoreactivity was observed. This accelerated accumulation may be due to recruitment of dynein from slow component b onto organelles proximal to the crush. Double labeling demonstrated that dynein immunoreactivity colocalized with synaptophysin, a transmembrane protein found in small, clear synaptic vesicles. In contrast, dynein immunoreactivity did not colocalize well with calcitonin gene-related peptide (CGRP), a peptide matrix marker for some large dense-cored vesicles. Finally, dynein immunoreactivity colocalized with the anterograde transport motor kinesin both proximal and distal to a crush, suggesting that kinesin may carry some dynein-containing membrane compartments during fast anterograde axonal transport.

Anterograde to retrograde reversal of fast axonal transport within cold blocked and rewarmed intact axons

The possibility that anterograde to retrograde reversal of axonal transport might take place in mid axon at a site distant from any nerve termination was investigated in sciatic nerve preparations from Xenopus laevis. The nerve, containing a pulse of anterogradely transported protein labeled with [35S]methionine, was kept in a two-compartment temperature controlled chamber. One compartment containing the proximal nerve was maintained at room temperature throughout the duration of an experiment while the second compartment containing the distal nerve, and separated from the first by a thermal barrier, was initially cooled to 3-4 degrees C and later warmed to room temperature. Transport of labeled proteins in the nerve was detected with a position-sensitive detector of ionizing radiation. With the distal portion of the nerve cold, the pulse of labeled protein transported up to the thermal barrier and stopped. When the distal part of the nerve was warmed to room temperature, retrograde and anterograde pulses of label propagated away from the thermal barrier with no time delay. The retrograde pulse could be collected on the distal side of a proximally placed tie and could be eliminated by treatment of the proximal nerve with vinblastine or dinitrophenol. Functional and structural evidence indicated that the cold block and thermal barrier were not destructive to the axons. Electron microscopy showed that the numerical density of axonal microtubules distal to the cold block was decreased about seven fold during the cold treatment and that this decrease could be prevented by 10 mumol/l taxol.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of brefeldin A on amphibian neurons: passage of newly synthesized proteins through the Golgi complex is not required for continued fast organelle transport in axons

The relation between the availability of newly synthesized protein and lipid and the axonal transport of optically detectable organelles was examined in peripheral nerve preparations of amphibia (Rana catesbeiana and Xenopus laevis) in which intracellular traffic from the endoplasmic reticulum to the Golgi complex was inhibited with brefeldin A (BFA). Accumulation of fast-transported radio-labeled protein or phospholipid proximal to a sciatic nerve ligature was monitored in vitro in preparations of dorsal root ganglia and sciatic nerve. Organelle transport was examined by computer-enhanced video microscopy of single myelinated axons. BFA reduced the amount of radiolabeled protein and lipid entering the fast-transport system of the axon without affecting either the synthesis or the transport rate of these molecules. The time course of the effect of BFA on axonal transport is consistent with an action at an early step in the intrasomal pathway, and with its action being related to the observed rapid (< 1 h) disassembly of the Golgi complex. At a concentration of BFA that reduced fast-transported protein by > 95%, no effect was observed on the flux or velocity of anterograde or retrograde organelle transport in axons for at least 20 h. Bidirectional axonal transport of organelles was similarly unaffected following suppression of protein synthesis by > 99%. The findings suggest that the anterograde flux of transport organelles is not critically dependent on a supply of newly synthesized membrane precursors. The possibilities are considered that anterograde organelles normally arise from membrane components supplied from a post-Golgi storage pool, as well as from recycled retrograde organelles.

Reversal of rapidly transported protein and organelles at an axonal lesion

The time required for both rapid axonally transported organelles (vesicles and tubulo-vesicular structures) and proteins to undergo anterograde to retrograde reversal at a crush site was examined using sciatic nerve preparations obtained from Xenopus laevis. The transport and reversal of a pulse of newly synthesized 35S-labeled proteins was studied with a position-sensitive detector of ionizing radiation. Organelle transport and reversal were studied using video microscopy. Both protein and organelle reversal were assessed in two bathing media: a physiological saline and a medium that was compatible with the intracellular environment (internal medium). The time required for protein transport to reverse at a ligature was determined as a function of the time interval between the application of the ligature and the arrival of the pulse at the ligature (lesion time). In physiological saline, reversal times were greatest, about 3.5 h, when the lesion time was 1 h or less and decreased to approximately 1.5 h for lesion times of 4-12 h. When corrected for the approximately 2 mm length of degeneration caused by the saline, the results were similar to those obtained in internal medium and indicated a minimal reversal time for proteins of about 2 h. Organelle transport was examined close to narrow lesions in single myelinated axons. That the organelles moving away from the lesion represented organelles that had undergone reversed transport was suggested by observation of the reversal of individual organelles, and by a correlation between the flux of organelles towards and away from the lesion. Analysis of organelle flux within and adjacent to a segment of axon isolated by two lesions indicated that 70-80% of organelles moving away from a lesion represented reversed transport. Observations in internal medium were consistent with a reversal time of < 15 min, and in physiological saline < 30 min. The substantially smaller reversal time for organelle transport as compared to protein transport is consistent either with the existence of two types of organelles with different reversal times and hence different reversal mechanisms, or with the possibility that during reversal proteins are off-loaded from carrier organelles and subsequently up-loaded to different organelles.

Reversal of rapid axonal transport at a lesion: leupeptin inhibits reversed protein transport, but does not inhibit reversed organelle transport

The hypothesis that the reversal of rapid axonal transport requires a proteolytic conversion of anterograde transport vesicles was examined using sciatic nerve preparations from Xenopus laevis and leupeptin as an inhibitor of proteolysis. The transport of newly synthesized 35S-labeled proteins was studied with a position-sensitive detector of radiation. Organelle transport in isolated myelinated axons was studied by video microscopy. Leupeptin (0.1-0.4 mM) reduced the anterograde-to-retrograde reversal of protein transport adjacent to an axonal lesion. In experiments in which organelle transport was observed close to lesions in axons maintained in a medium compatible with intracellular function, 1.0 mM leupeptin inhibited neither the anterograde-to-retrograde nor the retrograde-to-anterograde reversal of organelle transport. In addition, in experiments in which conditions approximated those used to study protein transport, organelle transport away from the lesion was not inhibited by 1.0 mM leupeptin. A comparison of the morphology of rapidly transported organelles that underwent anterograde transport to the morphology of those that returned from a lesion (with or without the presence of leupeptin) provided no evidence that a morphological conversion was a necessary step in transport reversal.

Junction between parent and daughter axons in regenerating myelinated nerve: properties of structure and rapid axonal transport

The primary aim of this work was to investigate the properties of rapid axonal transport in regenerating myelinated axons in the sciatic nerve of Xenopus laevis, with particular attention to events at the junction between the proximal, intact axon (the "parent") and the distal, newly formed axon (the "daughter"). Morphological studies indicated that all myelinated axons initiated regeneration and that at least 80% of these axons regenerated at a rate of 1 mm/day or greater (20 degrees C). The ultrastructure of the junctional region was examined at regeneration times between 3 days and 20 weeks. The main qualitative change in the junctional axoplasm over this period was in its content of particulate organelles. At times up to 2 weeks regeneration, the junction contained abnormal numbers of 50 nm diameter vesicles and 10 nm granules. Between 2 and 5 weeks the junction showed in addition a peripheral rim of large membrane-bounded organelles around a central core of microtubules and neurofilaments. At longer times the numbers of large membrane-bounded organelles diminished and all junctions contained prominent accumulations of 10 nm granules. The rate of rapid axonal transport of protein was similar in parent and daughter axons. Compared to the parent axons, a 2-5 times greater amount of protein was deposited to a stationary phase in daughter axons. Specimens of nerve that were subjected to mechanical stress during the removal of the perineurium showed a large accumulation of rapidly transported protein in the region of the crush at regeneration times up to 40 days; some of the accumulated protein was subsequently transported retrogradely. Video microscopy of isolated axons supplied evidence that the transport deficit in mechanically stressed nerve was a partial block of anterograde vesicle transport, plus a reversal of anterograde transport, at the junction of parent with daughter axons. No structural changes were detected in mechanically stressed nerve. The results show that the junction between parent and daughter myelinated axons is a region with distinct morphology at which the dynamics of anterograde axonal transport may change dramatically.

Loss of material from the retrograde axonal transport system in frog sciatic nerve

Rapid axonal transport was studied in sciatic nerve preparations of the amphibian Xenopus laevis maintained in vitro at 23.0 +/- 0.2 degrees C. A pulse of [35S]methionine-labeled material was allowed to move in the anterograde direction until encountering a lesion, at which a portion of the pulse reversed directions and moved in the retrograde direction. By constricting the nerve during the course of the experiment, it was possible to prevent continuous return of label from the lesion, thus creating a retrogradely moving pulse that contained a defined quantity of radiolabel. Movement of both the anterograde and the retrograde pulse were monitored continuously for up to 24 h using a position-sensitive detector of ionizing radiation. The front and the back edge of the anterograde pulse were found to move at the rates of (mm/day) 179.9 +/- 3.9 (+/- SEM) and 149.9 +/- 5.9, respectively, and the front and the back edge of the retrograde pulse moved at the rates of 155.8 +/- 11.3 and 84.6 +/- 2.9, respectively. By comparison of the quantity of label lost to the stationary phase to the quantity of label calculated to have been present in the anterograde pulse, it was determined that 0.068 +/- 0.009 of the anterograde pulse is lost to each 3.18-mm region of nerve. Comparison of the quantity of label calculated to have been present in the retrograde pulse to that in the anterograde pulse revealed that 0.057 +/- 0.014 of the retrograde pulse is lost to each 3.18-mm region of nerve. It is concluded that protein originating in the cell body and which reverses its direction of transport at a lesion can be lost from the retrograde axonal transport system.

The release of axonally transported material from an in vitro amphibian sciatic nerve preparation

The rapid axonal transport of a pulse of [35S]methionine-labelled material was used to study the release of transported material from amphibian nerve maintained in vitro. Following creation of a moving pulse of activity in a dorsal root ganglion-sciatic nerve preparation, the ganglion was removed and the nerve placed in a three-compartment tray, the section of nerve in the middle compartment containing no truncated branches (unbranched section). All three compartments were filled with a saline solution that in some studies contained nonradioactive methionine (1.0 mmol/L). Analysis of studies in which nonradioactive methionine was absent revealed that labelled material appeared in the bathing solution of the end compartments that contained truncated branches, but not in the solution of the middle (unbranched) compartment. The quantity of label released in the branched compartments was approximately 6% of that remaining in the corresponding section of nerve following an 18-20 h incubation period. However, when nonradioactive methionine was present, all compartments showed an additional activity in the bathing solution of approximately 10% of that remaining in the nerve. In another study in which a position-sensitive detector of ionizing radiation was used to monitor progress of the pulse, it was found that activity did not enter the bathing solution of a compartment prior to the pulse of activity. It is concluded that in the absence of methionine from the bathing solution, axonally transported material is released only from regions of nerve that contain severed axons; however, the presence of methionine allows transported material to be released from nerve containing intact axons. Ultrafiltration studies and thin-layer chromatography revealed the majority of material released to be of low-molecular weight (less than 30,000 daltons) and not free [35S]methionine.