Retrograde Axonal Transport of Gangliosides and Glycoproteins in the Motoneurons of Rat Sciatic Nerve

Axonal transport of glycoconjugates was studied in the motoneurons of rat sciatic nerve following injection of [3H]glucosamine into the lumbosacral spinal cord. After varying time intervals, the sciatic nerve was exposed, and two ligatures were tied for collection of materials undergoing anterograde and retrograde transport. Gangliosides and glycoproteins were found to undergo fast anterograde transport, estimated at 284-446 mm/day. Both classes underwent retrograde transport as well, with labeled glycoproteins returning slightly ahead of labeled gangliosides. Only minor quantities of labeled proteoglycans were detected. Purified gangliosides extracted from nerve segments were fractionated according to sialic acid number on diethylaminoethyl-Sephadex; the distributional pattern tended to resemble that of brain gangliosides. The similarity between anterograde and retrograde patterns suggested absence of metabolic changes in gangliosides entering and leaving the axon-nerve terminal structures.     

Axonal Transport of Polyamines in Intact and Regenerating Axons of the Rat Sciatic Nerve

The axonal transport of putrescine or its polyamine derivatives spermidine or spermine is a subject of some debate. We investigated this question by injecting [3H]putrescine into the lumbar spinal cord of the rat and measuring the accumulation of radioactivity central to ligatures placed on intact and regenerating sciatic nerves. In normal nerves, approximately twice as much radioactivity built up proximal to these ligatures 2 or 3 days after injection than at more distal ligatures used to control for accumulation of radioactivity which might be due to tissue damage alone. In regenerating nerves the amount of radioactivity accumulating at the ligature was approximately five times that at the distal ligature and two to three times greater than in intact nerves. The identity of the radioactivity in regenerating nerves, determined on an amino acid analyzer, was found to be primarily spermidine and an unknown compound that migrated as a frontal elution peak. Autoradiographic analysis showed that the radioactivity was largely confined to axons, but a significant amount of the silver grains was associated with Schwann cells and myelin sheaths surrounding labeled axons in both intact and regenerating nerves. The data indicate that polyamine derivatives of putrescine are transported axonally in rat sciatic nerves, and some of this transported material accumulates in Schwann cells surrounding the labeled axons. These processes are apparently augmented during regeneration of the injured axons.     

Changes in Rapid Transport of Phospholipids in the Rat Sciatic Nerve During Axonal Regeneration

Abstract: 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 μCi [Me-^HC]choline and 15 μCi [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.     

Differences in the Kinetics of Axonal Transport for Individual Lipid Classes in Rat Sciatic Nerve

Lipid precursors ([2-3H]glycerol for phospholipids and [3H]acetate for cholesterol) were injected into the L-5 dorsal root ganglion of adult rats. At various times, animals were killed, the ganglion and consecutive 5-mm segments of sciatic nerve were dissected, and lipids were extracted and analyzed by TLC. Individual lipid classes exhibited markedly different transport patterns. The crest of radioactive phosphatidylcholine moved as a sharply defined front at about 300 mm/day, with a relatively flat plateau behind the moving crest. Although some radioactive phosphatidylethanolamine also moved at the same rate, the crest was continually attenuated as it moved so that a gradient of radioactive phosphatidylethanolamine along the axon was maintained for several days. Transported diphosphatidylglycerol exhibited a defined crest, as did phosphatidylcholine, but moved at about half the rate. Labeled cholesterol was transported at a rapid rate similar to that for phosphatidylcholine and phosphatidylethanolamine, but like phosphatidylethanolamine, the initial moving crest of radioactivity was continually attenuated. Relative to the phospholipids, cholesterol showed a more prolonged period of accumulation in the axons and was more metabolically stable. We propose that most labeled phosphatidylcholine, phosphatidylethanolamine, and cholesterol is transported in similar (or the same) rapidly moving membranous particles. Once incorporated into these particles, molecules of phosphatidylcholine tend to maintain associated with them during transport. In contrast, molecules of phosphatidylethanolamine and cholesterol in these transported particles exchange extensively with unlabeled molecules in stationary axonal structures. Diphosphatidylglycerol, localized in a specialized organelle, the mitochondrion, is transported at a slower rate than other phospholipids, and does not exchange with other structures.     

Reduced Axonal Transport of the G4 Molecular Form of Acetylcholinesterase in the Rat Sciatic Nerve During Aging

Aging in the sciatic nerve of the rat is characterized by various alterations, mainly cytoskeletal impairment, the presence of residual bodies and glycogen deposits, and axonal dystrophies. These alterations could form a mechanical blockade in the axoplasm and disturb the axoplasmic transports. However, morphometric studies on the fiber distribution indicate that the increase of the axoplasmic compartment during aging could obviate this mechanical blockade. Analysis of the axoplasmic transport, using acetylcholinesterase (AChE) molecular forms as markers, demonstrates a reduction in the total AChE flow rate, which is entirely accounted for by a significant bidirectional 40-60% decrease in the rapid axonal transport of the G4 molecular form. However, the slow axoplasmic flow of G1 + G2 forms, as well as the rapid transport of the A12 form of AChE, remain unchanged. Our results support the hypothesis that the alterations observed in aged nerves might be related either to the impairment in the rapid transport of specific factor(s) or to modified exchanges between rapidly transported and stationary material along the nerves, rather than to a general defect in the axonal transport mechanisms themselves.     

Acrylamide-Induced Increases in Deposition of Axonally Transported Glycoproteins in Rat Sciatic Nerve

The axonal transport of proteins, glycoproteins, and gangliosides in sensory neurons of the sciatic nerve was examined in adult rats exposed to acrylamide via intraperitoneal injection (40 mg/kg of body weight/day for nine consecutive days). The L5 dorsal root ganglion was injected with either [35S]methionine to label proteins or [3H]glucosamine to label, more specifically, glycoproteins and gangliosides. At times ranging from 2 to 6 h later, the sciatic nerve and injected ganglion were excised and radioactivity in consecutive 5-mm segments determined. In both control and acrylamide-treated animals, outflow profiles of [35S]methionine-labeled proteins showed a well defined crest which moved down the nerve at a rate of approximately 340 mm/day. Similar outflow profiles and transport rates were seen for [3H]glucosamine-labeled glycoproteins in control animals. However, in animals treated with acrylamide, the crest of transported labeled glycoprotein was severely attenuated as it moved down the nerve. This finding suggests that in acrylamide-treated animals, axonally transported glycoproteins were preferentially transferred (unloaded or exchanged against unlabeled molecules) from the transport vector to stationary axonal structures. We also examined the clearance of axonally transported glycoproteins distal to a ligature on the nerve. The observed impairment of clearance in acrylamide-treated animals relative to controls is supportive of the above hypothesis. Acrylamide may directly affect the mechanism by which axonally transported material is unloaded from the transport vector. Alternatively, the increased rate of unloading might reflect an acrylamide-induced increase in the demand for axonally transported material.     

Axonal Transport of Glycoconjugates in the Rat Visual System

Long-Evans rats at 45 days of age were injected intraocularly with 25 mu Ci of [3H]glucosamine. Incorporation of radioactivity into retinal gangliosides, glycoproteins, and glycosaminoglycans (GAGs) was determined at various times after injection. Portions of all three classes of radioactive macromolecules were committed to rapid axonal transport in the retinal ganglion cells. With respect to gangliosides about 60% of those synthesized in the retina were retained in that structure, 30% were committed to transport to regions containing the nerve terminal structures (lateral geniculate body and superior colliculus), and about 10% were deposited in stationary structures of the axons (optic nerve and tract). With the exception of ganglioside GD3 the molecular species distribution of gangliosides synthesized in the retina matched that committed to transport. In contrast to gangliosides a smaller fraction of newly synthesized retinal glycoprotein (less than 12% of that synthesized in the retina) was committed to rapid transport to nerve ending regions and only about 0.5% was retained in the nerve and tract. The molecular-weight distribution of glycoproteins committed to transport differed quantitatively from that of the retina. With respect to GAGs an even smaller portion (1-2%) of that synthesized in the retina was committed to rapid transport; of this portion almost all was recovered in nerve terminal-containing structures. A constant proportion of each retinal GAG species was transported to the superior colliculus. We suggest that most of the retinal gangliosides are synthesized in neurons and preferentially in ganglion cells (possibly a function of the large surface membrane area supported by these cells). Subcellular fractionation experiments indicated that transported gangliosides, glycoproteins, and GAGs may be preferentially distributed into different subcellular compartments.     

Axonal Transport of Choline Lipids in Normal and Regenerating Rat Sciatic Nerve

Abstract: Biochemical methods were used to study the time course of transport of choline phospholipids (labeled by the injection of [³H]choline into the ventral horn of the lumbar spinal cord) in rat sciatic nerve. Autoradiographic methods were used to localize the transported lipid within motor axons. Transported phospholipid, primarily phosphatidylcholine, present in the nerve at 6 h, continued to accumulate over the following 12 days. No discrete waves of transported lipid were observed (a small wave of radioactive phospholipid moving at the high rate would have been missed); the amounts of radioactive lipid increased uniformly along the entire sciatic nerve. In light-microscope autoradiographs, a class of large-caliber axons, presumably motor axons, retained the labeled lipid. Some lipid, even at 6 h, was seen within the myelin sheaths. Later, the labeling of the myelin relative to axon increased. The continued accumulation of choline phospholipids in the axons probably signifies their prolonged release from cell bodies and their retention in various axonal membranes, including the axolemma. The build-up of these phospholipids in myelin probably represents their transfer from the axons to the myelin sheaths surrounding them. When nerves are crushed and allowed to regenerate for 6 or 12 days, choline phospholipids transported during these times enter the regenerating nerve. In light and electron microscope autoradiographs, transported lipid was seen to be localized primarily in the regenerating axons. However, grains overlay the adjacent Schwann cell cytoplasm, indicating transported lipids were transferred from the regenerating axons to the associated Schwann cells. In addition, some cells not associated with growing axons were labeled, suggesting that phosphatidylcholine and possibly acetylcholine, carried to the regenerating axons by axonal transport, were actively metabolized in the terminal, with released choline label being used by other cells. These results demonstrate that axonal transport supplies mature and growing axons and their glial cells with choline phospholipids.     

Anterograde Axonal Transport of Peptidylglycine α-Amidating Monooxygenase in Rat Sciatic Nerves

Axonal transport of peptidylglycine alpha-amidating monooxygenase (PAM) activity was studied in rat sciatic nerves from 12 to 120 h after double ligations. The anterograde axonal transport increased and reached a plateau between 48 and 72 h and then decreased. The flow rate was 100 mm/day, and the molecular mass of the active entity was 70 kDa, which was determined by gel filtration. In contrast, there was no evidence for significant retrograde axonal transport. Anterograde axonal transport of immunoreactive cholecystokinin, a carboxy-terminal-amidated putative neuropeptide, was also found. These results suggest that PAM is transported by a rapid axonal flow and may play a role as a processing enzyme during transport or in the terminals of rat sciatic nerves.     

Retrograde Axonal Transport of Locally Synthesized Phosphoinositides in the Rat Sciatic Nerve

Although autoradiography has demonstrated local incorporation of [3H]inositol into axonal phospholipids after intraneural injection, retrograde axonal transport of phosphatidylinositol has only been demonstrated after injection of lipid precursor into the cell body regions (L4 and L5 dorsal root ganglia) of the sciatic nerve. We now report the retrograde axonal transport of inositol phospholipids synthesized locally in the axons. Following microinjection of myo-[3H]inositol into the rat sciatic nerve (50-55 mm distal to L4 and L5 dorsal root ganglia), a time-dependent accumulation of 3H label occurred in the dorsal root ganglia ipsilateral to the injection site. The ratio of dpm present in the ipsilateral dorsal root ganglia to that in the contralateral dorsal root ganglia was not significantly different from unity between 2 and 8 h following isotope injection but increased to 10-12-fold between 24 and 72 h following precursor injection. By 24 h following precursor injection, the ipsilateral/contralateral ratio of the water-soluble label in the dorsal root ganglia still remained approximately 1.0, whereas the corresponding ratio in the chloroform/methanol-soluble fraction was approximately 20. The time course of appearance of labeled lipids in the ipsilateral dorsal root ganglia after injection of precursor into the nerve at various distances from the dorsal root ganglia indicated a transport rate of at least 5 mm/h. Accumulation of label in the dorsal root ganglia could be prevented by intraneural injection of colchicine or ligation of the sciatic nerve between the dorsal root ganglia and the isotope injection site. These results demonstrate that inositol phospholipids synthesized locally in the sciatic nerve are retrogradely transported back to the nerve cell bodies located in the dorsal root ganglia.     

Retrograde Axonal Transport of Phospholipid in Rat Sciatic Nerve

Abstract: Retrograde axonal transport of phospholipid was studied in rat sciatic motoneuron axons by placing collection crushes on the nerve at intervals after injection of [methyl-³H]choline into the lumbosacral spinal cord, and allowing labelled material undergoing anterograde or retrograde movement to accumulate adjacent to the collection crushes. Control experiments showed that the accumulations of label were not a result of local uptake of circulating precursor. The majority of the ³H label was associated with phosphatidylcholine. Accumulation of label at the distal collection crush, representing retrograde transport, was observed subsequent to the anterograde transport of phospholipid. In comparison with previous study on retrograde transport of protein, the following points were noted: (1) onset of retrograde transport occurred at approximately the same time after precursor injection (10–20 h) for both protein and phospholipid; (2) retrograde transport of lipids was more prolonged: maximum retrograde transport occurred later for phospholipid (30 h) than for protein (15–20 h), and declined to half-maximum between 49 and 99 h, compared to a corresponding value of 24–28 h for protein; (3) the proportion of total anterograde-transported activity subsequently undergoing retrograde transport was less in the case of phospholipid, at least over the time interval studied (up to 99 h after precursor injection). The similar times of onset of retrograde transport of phospholipid and protein support the concept of retrograde transport as a recycling mechanism returning to the cell body membrane fragments that were earlier transported into the axon. Coordinated retrograde transport of labelled protein and phospholipid components of the recycled membranes would be predicted. Differences between protein and phospholipid in the subsequent time course and amount of retrograde transport may reflect differences in axonal handling of protein and lipid. Both the more prolonged outflow of labelled lipids from cell body into axon and exchange with a distal pool of unlabelled phospholipid may account for the prolonged time course of retrograde transport of labelled lipid.     

Destinations of Some Fast-Transported Proteins in Sensory Neurons of Bullfrog Sciatic Nerve

Many characteristics of proteins that are fast axonally transported have been described, but the destinations of most within the neuron remain unknown. We have studied the destinations of some fast-transported proteins in sensory neurons of the bullfrog sciatic nerve, specifically to determine which may be deposited in axons and which may be destined for more distal, possibly terminal, areas. Dorsal root ganglia were pulse-labeled with [35S]methionine in vitro, following which they were separated from the sciatic nerve. After additional periods of transport, radioactive proteins from two areas of the nerve were separated by two-dimensional polyacrylamide gel electrophoresis and used to develop x-ray film. The first area contained the wavefront of transported radioactivity (wavefront region), whereas the second area was taken from nerve through which the wavefront had already passed (plateau region). The amount of radioactivity in certain fast-transported protein species from each area was determined by computer analysis of digitized video images of fluorographs. Certain proteins were preferentially left behind the wavefront and, therefore, may supply axon and possibly other nerve components, whereas other proteins were found almost exclusively in the wavefront and, hence, may supply more distal, possibly terminal, areas.     

A Rapid Anterograde Axonal Transport of Carboxypeptidase H in Rat Sciatic Nerves

Abstract: Using the highly sensitive HPLC-fluorophotometry technique, anterograde and retrograde axonal transport of carboxypeptidase H (CPH), a putative pro-hormone processing enzyme that removes a basic amino acid from the C-terminus of a precursor peptide, was measured 12–72 h after double ligations of rat sciatic nerves. CPH-like activity in rat sciatic nerves was 60-fold lower than that in the pituitary gland. CPH-like enzyme activity was rapidly accumulated in the proximal segment and peaked 48 h after ligation. The axonal flow was 100 mm/day, indicating that CPH in rat sciatic nerves is rapidly transported to the nerve terminals as an active form. The properties of the enzyme were similar to those of CPH in the brain: The pH optimum is at 5.5, and the molecular mass is ∼50 kDa. These results suggest that active CPH in the PNS is transported by a rapid anterograde axonal flow and may play a role in converting proneuropeptides to active neuropeptides under the axonal transport.     

Differential Axonal Transport of Soluble and Insoluble τ in the Rat Sciatic Nerve

Abstract: Axonal transport of microtubule-associated protein τ was studied in the motor fibers of the rat sciatic nerve 1–4 weeks after labeling of the spinal cord with [³⁵S]methionine. As 60–70% of low molecular weight τ in this system was found to be insoluble in 1% Triton-containing buffer, labeled proteins in 6-mm consecutive nerve segments were first separated into Triton-soluble and insoluble fractions. Two-dimensional gel electrophoresis and immunoblotting with anti-tau antibody confirmed the presence of τ among labeled, transported proteins in both fractions. Isoform composition of labeled τ was similar to that of bulk axonal τ, the most acidic species with apparent molecular mass of 66 kDa being the major component. Transport profiles obtained by measuring radioactivities associated with this major isoform showed that soluble and insoluble τ were transported at different rates. Insoluble τ, which contained the majority of τ-associated radioactivity, was transported at 1.7 mm/day in slow component a (SCa), whereas soluble τ was transported faster, at 3 mm/day, corresponding to the rate of slow component b (SCb). Cotransport of insoluble τ with insoluble tubulin in SCa suggests its association with stable microtubules.     

Slow Axonal Transport of Structural Polypeptides in Rat, Early Changes in Streptozocin Diabetes, and Effect of Insulin Treatment

The synthesis and transport of slowly transported polypeptides in sciatic nerves of rats was investigated by [35S]methionine pulse labeling and gel electrophoresis in control, diabetic, and insulin-treated diabetic rats. To detect very early changes diabetes was induced by streptozocin only 5 days prior to the labeling of the dorsal root ganglion cells. Fourteen days were allowed for axonal transport. In this experimental system, the neurofilament triplet is transported at an apparent velocity of 1.1 +/- 0.1 mm/day (mean +/- SD). The actin-related complex, including actin and two polypeptides of 87 kilodaltons and 37 kilodaltons, was transported at a velocity of 2.6 +/- 0.2 mm/day. For alpha- and beta-tubulin we found an apparent transport velocity of 2.2 +/- 0.1 mm/day, placing it between actin and the neurofilament triplet. The diabetic rats had a selective 32% decrease in the amount of the heaviest neurofilament subunit: 0.47 +/- 0.19% of trichloroacetic acid-insoluble radioactivity versus 0.69 +/- 0.17% in controls; 2p less than 0.05. This decrease was associated with a proximal accumulation of the two lighter neurofilament subunits. Insulin treatment of a diabetic group failed to normalize the changes of axonal transport and additional changes suggesting a hypoglycemic injury was observed.     

Locally Synthesized Phosphatidylcholine, but Not Protein, Undergoes Rapid Retrograde Axonal Transport in the Rat Sciatic Nerve

Abstract: Retrograde axonal transport of phosphatidylcholine in the sciatic nerve has been demonstrated only after injection of lipid precursors into the cell body region. We now report, however, that after microinjection (1 μl) of [methyl-³H]choline chloride into the rat sciatic nerve (35-40 mm distal to the L4 and L5 dorsal root ganglia), time-dependent accumulation of ³H-labeled material occurred in dorsal root ganglia ipsilateral, but not contralateral, to the injection site. The level of radioactivity in the ipsilateral dorsal root ganglia was minimal at 2 h after isotope injection but was significantly increased at 7, 24, 48, and 72 h after intraneural isotope injection (n = 3–8 per time point); at these time points, all of the radiolabel in the chloroform/methanol extract of the ipsilateral dorsal root ganglia was present in phosphatidylcholine. The radioactivity in the water-soluble fraction did not show a time-dependent accumulation in the ipsilateral dorsal root ganglia as compared with the contralateral DRGs, ruling out transport or diffusion of precursor molecules. In addition, colchicine injection into the sciatic nerve proximal to the isotope injection site prevented the accumulation of radiolabel in the ipsilateral dorsal root ganglia. Therefore, this time-dependent accumulation of radiolabeled phosphatidylcholine in the ipsilateral dorsal root ganglia is most likely due to retrograde axonal transport of locally synthesized phospholipid material. Moreover, 24 h after injection of both [³H]choline and [³⁵S]-methionine into the sciatic nerve, the ipsilateral/contralateral ratio of radiolabel was 11.7 for ³H but only 1.1 for ³⁵S. indicating that only locally synthesized choline phospholipids, but not protein, were retrogradely transported.     

A Ganglioside Species (GD1α) Migrates at a Slow Rate and CMP-Sialic Acid Severalfold Faster in Xenopus Sciatic Nerve: Fluorographic Demonstration

The ninth dorsal root ganglion of adult Xenopus laevis was labeled with N-acetyl-D-[6-3H]mannosamine, and intraaxonal migration of gangliosides was examined by analysis of the chloroform/methanol extract of each of 5-mm consecutive nerve segments by TLC coupled with fluorography. A unique disialoganglioside (GD1 alpha), which amounted to up to 83% of the total ganglioside in this nerve, migrated at 1-2 mm/day at 15 degrees C. This contrasts with the rapid transport of other ganglioside species previously reported in the optic systems of goldfish, rabbits, chickens, and rats. Fluorographic analysis also revealed a trichloroacetic acid-soluble substance migrating at a velocity of approximately 8 mm/day at 15 degrees C. The substance was considered to be CMP-sialic acid on the basis of observations that it comigrates with authentic CMP-N-acetylneuraminic acid in TLC developed with two different solvent systems, it is very labile to weak acid but resistant to neuraminidase from Vibrio cholerae, it is converted to N-acetylmannosamine when treated first with weak acid and subsequently with N-acetylneuraminic acid aldolase, and it has a beta-sialosyl group in its structure. Because CMP-sialic acid is believed to be the sole sialosyl donor in the cells, its migration in axons toward terminals, together with the previous demonstration of sialyltransferase activity in the synaptosomal plasma membrane, strongly supports the possibility that sialosylation of gangliosides and probably of other sialoglycoproteins is not confined to the Golgi apparatus, but can also occur after the compounds are committed to axonal transport.     

Axonal Transport, Deposition, and Metabolic Turnover of Glycoproteins in the Rat Optic Pathway

Abstract: Following intraocular injection of [³H]fucose in the rat, radioactive glycoproteins are rapidly transported to the nerve terminals in at least two waves, one with a peak at 8 h and a second with a peak at about a week. The molecular weight distribution of radioactive peptides in ach transport wave as determined by gel electrophoresis in buffers containing sodium dodecyl sulfate is very similar. Most of the many glycopeptides in the first wave of rapid transport pass through the optic tract in unison (apparent half-life of about 15 h) and are preferentially destined for the nerve endings. However, two proteins of apparent M. W. 28,000 and 49,000 are preferentially retained in the axons. The remaining proteins, after reaching the nerve endings (superior colliculus), decay with apparent half-lives ranging from 17 to 34 h. During the second wave a large amount of the 28,000 and 49,000 M. W. peptides are again preferentially retained in the axons. The remaining proteins, on reaching the nerve endings, decay with apparent half-lives ranging from 5 to 9 days. Subcellular fractionation of the superior colliculus supports the hypothesis that the 49,000 and 28,000 M. W. peptides are the predominantly labeled glycoproteins present in myelinated axons (representing over 50% of the radioactive glycoproteins 7 days following injection), although they are probably also present in membranes of the nerve endings. A comparison with glycoprotein transport in other tracts (geniculocortical and nigrostriatal tracts) suggests that glycoprotein transport in these pathways has many similarities to glycoprotein transport in the retinal ganglion cells, and that the optic system is a good general model for axonal transport in the CNS.