A comparison of axonally transported proteins in the rat sciatic nerve by in vitro and in vivo techniques

Is the intrasomal phase of fast axonal transport driven by oscillations of intracellular calcium?

An hypothesis is presented suggesting that the delivery of vesicle-packaged protein from the neuronal soma to the axonal transport system is physiologically coupled to spontaneous fluctuations of intracellular calcium (Cai). Evidence is reviewed that oscillations of Cai, commonly detected as agonist- or voltage-triggered waves and spikes propagating through the cytosol, also occur as spontaneous events. Endogenously-generated oscillations are examined since intrasomal transport persists in the absence of extracellular signals or nerve impulse activity. Vesicle budding from the endoplasmic reticulum (ER) may be a key step at which anterograde transport is regulated by events related to the release and reuptake of ER stores of Ca2+.

Technique for injection into the sciatic nerve of the mouse for quantitative in vivo metabolic studies

In this paper we describe a technique for intraneural injections, applicable to mouse peripheral nerves, which, compared with previous techniques, reduces trauma to the nerves and increases the level and reduces the variability of label recovery. Our technique employs glass needles (tip diameter, 50 micron) linked to a peristaltic pump by polyethylene tubing to inject small volumes (in the microliter range) of radiolabeled substrate solutions into mouse sciatic nerves, and allows the recovery of 20.9 +/- 1.9% (mean +/- standard deviation) and 30.5 +/- 4.8% of the injected radioactivity for 2 microliter [3H]acetate and 0.5 microliter of [3H]stearate, respectively.

Protein synthesis and rapid axonal transport during regrowth of dorsal root axons

Damage to the sciatic nerve produces significant changes in the relative synthesis rates of some proteins in dorsal root ganglia and in the amounts of some fast axonally transported proteins in both the sciatic nerve and dorsal roots. We have now analyzed protein synthesis and axonal transport after cutting the other branch of dorsal root ganglia neurons, the dorsal roots. Two to three weeks after cutting the dorsal roots, [35S]methionine was used to label proteins in the dorsal root ganglia in vitro. Proteins synthesized in the dorsal root ganglia and transported along the sciatic nerve were analyzed on two-dimensional gels. All of the proteins previously observed to change after sciatic nerve damage were included in this study. No significant changes in proteins synthesized in dorsal root ganglia or rapidly transported along the sciatic nerve were detected. Axon regrowth from cut dorsal roots was observed by light and electron microscopy. Either the response to dorsal root damage is too small to be detected by our methods or changes in protein synthesis and fast axonal transport are not necessary for axon regrowth. When such changes do occur they may still aid in regrowth or be necessary for later stages in regeneration.

Changes in rapid transport of phospholipids in the rat sciatic nerve during axonal regeneration

Axonal transport of phospholipids in normal and regenerating sciatic nerve of the rat was studied. At various intervals after axotomy of the right sciatic nerve in the midthigh region and subsequent perineurial sutures of the transected fascicles, a mixture of 60 mu Ci [Me-14C]choline and 15 muCi [2-3H]glycerol in the region of the spinal motor neurons of the L5 and L6 segments was injected bilaterally. The amount of radioactive lipid (and in certain cases its distribution in various lipid classes) along the nerve was determined as a function of time. Three days after fascicular suture and 6 h after spinal cord injection of precursors, there was an accumulation of labeled phospholipids and sphingolipids in the transected sciatic nerve in the region immediately proximal to the site of suture. Nine days after, there was a marked increase in the accumulation of radioactivity in the distal segments of the injured nerve, which increased up to 14 days after cutting and disappeared as regeneration proceeded (21-45 days). In all segments of both normal and regenerating nerve fibers, as well as in L5 and L6 spinal cord segments, only phosphatidylcholine and sphingomyelin were labeled with [14C]choline. These results suggest that the regeneration process in a distal segment of a peripheral neuron, following cutting and fascicular repairing by surgical sutures, is sustained in the first 3 weeks by changes in the amount of phospholipids rapidly transported along the axon towards the site of nerve fiber outgrowth.

Axonal transport in transplanted frog sciatic nerve

Axonal transport was studied in transplanted frog sciatic nerves-dorsal ganglia for periods up to three months. The ganglia were either labeled prior to, or after varying periods of transplantation and the distribution of labeled proteins compared. The survival of the preparations was assessed by studying their ability to transmit compound action potentials, to maintain axonal transport and by their ultrastructural appearance. The sensory neurons retained their fast axonal transport and excitability during the transplantation periods. Morphologically about 30% of the axons appeared normal after one month of transplantation. The amount and distribution of protein incorporated radioactivity in the ganglia and the nerve changed with time but the decrease in the ganglia was not accompanied by a corresponding increase in the nerve. There was no evidence for a distinct slow phase of transport in spite of a functional fast transport system. The results indicate that either slow transport stops when the normal balance between release, degradation and recirculation is distributed by the presence of a ligature or that there is a continuous change in the composition of transported material with the time after labeling which is not reflected in altered transport characteristics.

Fast axonal transport in the presence of high Ca2+: evidence that microtubules are not required

Microtubules have long been associated with the mechanism of fast axoplasmic transport, although experimental evidence to support an involvement has been equivocal. Electron microscopic studies demonstrated that incubation of the axons of excised rat sciatic nerves in media containing 75 mM Ca2+ caused complete loss of microtubules within 6 hr. To evaluate the role of microtubules in fast anterograde transport, studies of transport in nerves exposed to these conditions were undertaken. Prior to measurement of axoplasmic transport, nerves ligated distal to the dorsal root ganglia were preincubated in vitro in 75 mM Ca2+ for 0-6 hr. Fast axonal transport was subsequently monitored by measuring the amount of trichloroacetic acid-insoluble radioactivity that accumulated at the ligature after incubation for 12-18 hr with L-[3H]proline. Nerves in which microtubules had been depolymerized by preincubation in high Ca2+ maintained control levels of transport. We conclude that intact microtubules are not required for fast anterograde axoplasmic transport.

Comparative analysis of rapidly transported axonal proteins in sensory neurons of the frog and rat

35S-labeled proteins carried by fast axonal transport in sciatic sensory axons of bullfrog and rat were separated electrophoretically on discontinuous polyacrylamide gradient slab gels. In contrast to the previously reported similarity in the electrophoretic profiles of rapidly transported proteins from functionally different neurons, we have found that there is very little correspondence in the profiles of these proteins in functionally similar neurons from two widely studied species. We also found very little correspondence between the two species in the profiles of locally synthesized sciatic nerve protein. The results demonstrate the difficulty inherent in comparing the electrophoretic profiles obtained using these two model systems for the study of rapidly transported axonal proteins. In particular, relationships between the major rapidly transported proteins in the two species could not be analyzed with this technique.