Rapid transport of radioactivity in goldfish optic nerve following injections of labeled glucosamine

A Journey in Science: The Privilege of Exploring the Brain and the Immune System

Real innovations in medicine and science are historic and singular; the stories behind each occurrence are precious. At Molecular Medicine we have established the Anthony Cerami Award in Translational Medicine to document and preserve these histories. The monographs recount the seminal events as told in the voice of the original investigators who provided the crucial early insight. These essays capture the essence of discovery, chronicling the birth of ideas that created new fields of research; and launched trajectories that persisted and ultimately influenced how disease is prevented, diagnosed, and treated. In this volume, the Cerami Award Monograph is by Lawrence Steinman, MD, of Stanford University in California. A visionary in the field of neurology, this is the story of Dr. Steinman's scientific journey.

Extracellular proteins of goldfish optic tectum labeled by intraocular injection of 3H-proline

A prominent group of soluble glycoproteins with a molecular weight of 30K-40K and pI 5.0-5.6 was detected in various parts of the goldfish brain as well as in the optic nerves. Since these proteins are readily liberated from the tissue, we have designated them exoglycoproteins (X-GPs). The X-GPs in the optic tectum were found to be labeled after intraocular injection of radioactive tracers, in a manner consistent with the labeling of proteins transported in the optic axons. However, the labeling of X-GPs was blocked by intracranial injection of a protein synthesis inhibitor, whereas the labeling of axonally transported proteins was unaffected. This shows that the X-GPs can be synthesized locally within the brain. Nevertheless, when protein synthesis in the retina was blocked, the labeling of the X-GPs in the tectum was prevented, like the labeling of axonally transported proteins. Thus precursors for the synthesis of X-GPs can be derived from materials transported in the optic axons. This synthesis can occur in nonneuronal cells, as indicated by the incorporation of labeled amino acid into X-GPs in optic nerves directly exposed to the label. The synthesis of X-GPs was increased in regenerating nerves, suggesting that these proteins may play a role in regeneration. Partial amino acid sequencing of the proteins showed that they are identical to the proteins previously identified as "ependymins," which have been implicated in neuronal plasticity. There are minor differences in amino acid sequence among some individual spots.

Ganglioside synthesis and transport in regenerating sensory neurons of the rat sciatic nerve

The sciatic nerves of rats were crushed with fine forceps and allowed to survive for 3 or 7 days, at which time the 5th lumbar dorsal root ganglion was injected with [3H]glucosamine. Animals were killed 18 h later and the nerves proximal and distal to the crush site were cut into 3 mm segments. Gangliosides were purified from these segments, and radioactivity was separately measured in gangliosides, neutral glycolipids and glycoproteins. For all 3 fractions, radioactivity was distributed similarly between the crush site and point of maximum axonal elongation. A second smaller peak of ganglioside radioactivity was seen to span a few segments immediately distal to the point of maximum axonal elongation. We propose two possible explanations for this: (1) it represents ganglioside synthesis by Schwann cells (from blood-borne [3H]glucosamine) as part of the mitogenic response of these cells to the reappearance of axons; or (2) recently synthesized, transported gangliosides are released from the growth cone and taken up by adjacent mitogenic Schwann cells.

Phosphorylation of proteins in normal and regenerating goldfish optic nerve

Within 6 h after radiolabeled phosphate was injected into the eye of goldfish, labeled acid-soluble and acid-precipitable material began to appear in the optic nerve and subsequently also in the lobe of the optic tectum, to which the optic axons project. From the rate of appearance of the acid-precipitable material, a maximal velocity of axonal transport of 13-21 mm/day could be calculated, consistent with fast axonal transport group II. Examination of individual proteins by two-dimensional gel electrophoresis revealed that approximately 20 proteins were phosphorylated in normal and regenerating nerves. These ranged in molecular weight from approximately 18,000 to 180,000 and in pI from 4.4 to 6.9. Among them were several fast transported proteins, including protein 4, which is the equivalent of the growth-associated protein GAP-43. In addition, there was phosphorylation of some recognizable constituents of slow axonal transport, including alpha-tubulin, a neurofilament constituent (NF), and another intermediate filament protein characteristic of goldfish optic axons (ON2). At least some axonal proteins, therefore, may become phosphorylated as a result of the axonal transport of a phosphate carrier. Some of the proteins labeled by intraocular injection of 32P showed changes in phosphorylation during regeneration of the optic axons. By 3-4 weeks after an optic tract lesion, five proteins, including protein 4, showed a significant increase in labeling in the intact segment of nerve between the eye and the lesion, whereas at least four others (including ON2) showed a significant decrease. When local incorporation of radiolabeled phosphate into the nerve was examined by incubating nerve segments in 32P-containing medium, there was little or no labeling of the proteins that showed changes in phosphorylation during regeneration. Segments of either normal or regenerating nerves showed strong labeling of several other proteins, particularly a group ranging in molecular weight from 46,000 to 58,000 and in pI from 4.9 to 6.4. These proteins were presumably primarily of nonneuronal origin. Nevertheless, if degeneration of the axons had been caused by removal of the eye 1 week earlier, most of the labeling of these proteins was abolished. This suggests that phosphorylation of these proteins depends on the integrity of the optic axons.

Axonal transport characteristics of gangliosides in sensory axons of rat sciatic nerve

The distribution of axonally transported gangliosides and glycoproteins along the sciatic nerve was examined from 3 h to 4 weeks following injection of[3H]glucosamine into the fifth lumbar dorsal root ganglion of adult rats. Incorporation of labeled precursor into these glycoconjugates reached a maximal level in the ganglion within 6 h. Outflow patterns of radioactivity for glycoproteins showed a well-defined crest with a transport rate of approximately 330 mm/day. In contrast, the crest of transported gangliosides was continuously attenuated, implying a significant deposition along the axon, and an alternative method of calculating velocity was required. Analysis of accumulation of labeled material at double ligatures demonstrated both anterograde and retrograde transport of glycoproteins and gangliosides and allowed for the calculation of an anterograde transport rate of about 270 mm/day for each. Additional evidence of ganglioside transport is provided in that the TLC pattern of transported radioactive gangliosides accumulating at a ligature is significantly different from the pattern seen in the dorsal root ganglion or following intraneural administration of the labeled precursor. These data indicate that gangliosides are transported at the same rapid rate as glycoproteins but are subject to a more extensive exchange with stationary material than are glycoproteins.

The optic tectum regulates the transport of specific proteins in regenerating optic fibers of goldfish

The pattern of rapidly-transported proteins in regenerating optic fibers of the adult goldfish is regulated by interactions between these fibers and their main target, the optic tectum. When the optic fibers are allowed to interact with the tectum, the transport of proteins with molecular weights in the range of 110-145 kilodaltons (kDa) increases, whereas the transport of proteins in the 24-27 kDa range declines from the previously high level which has been induced by axotomy. If the optic fibers are prevented from interacting with the tectum, the transport of the 24-27 kDa proteins remains elevated for months. Amounts of other rapidly-transported retinal proteins (e.g. the acidic 43-49 kDa proteins that increase in regenerating optic fibers after axotomy) are relatively unaffected by tectal ablation.

Anterograde and retrograde axonal transport of native and derivatized wheat germ agglutinin in the visual system of the chicken

The anterograde and retrograde rates of axonal transport of the lectin wheat germ agglutinin (WGA) were investigated using native and derivatized lectins in embryonic (stage 39) and posthatch chickens. Anterograde transport rates in the retinotectal projection of posthatch animals ranged from 168 mm/day for native WGA to 345 mm/day for horseradish peroxidase conjugated WGA. Anterograde transport rates in embryos were at least 258 mm/day based on experiments employing tritium and horseradish peroxidase conjugates. Retrograde rates measured by appearance of label in the isthmo-optic nucleus in both embryonic and posthatch chickens were in the range of 150-180 mm/day. A fluorescein isothiocyanate conjugate of WGA was transported retrogradely but not anterogradely. When the extraocular eye muscles were labeled accidentally during injection, cells in the oculomotor or trochlear nuclei were more intensely labeled than neurons in the isthmo-optic nucleus. It was concluded that at least some conjugates of WGA, and possible the native lectin as well, travel in the fastest component of axonal transport. In view of the known intercellular movement of WGA from labeled presynaptic processes, it is recommended that survival times be kept short in experiments using WGA as a neuronal tracing agent (less than 24 h) to minimize the possibility of uptake and redistribution of the lectin by nearby cells.

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 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.

Intraocular injection of tetrodotoxin in goldfish decreases fast axonal transport of [3H]glucosamine-labeled materials in optic axons

When physiological activity in goldfish visual system was abolished by repeated intraocular injection of tetrodotoxin (TTX), the fast axonal transport of radioactive amino acid-labeled protein in the optic axons was unaltered. However, the TTX treatment reduced the amount of [3H]glucosamine-labeled glycolipids that were axonally transported to the optic tectum, and may have decreased their rate of turnover in the tectum. A similar though smaller effect was observed for glucosamine-containing glycoproteins. These alterations in axonal transport may be the basis for at least some of the deleterious effects of TTX on axonal regeneration in this system.

Ganglioside changes in the regenerating goldfish optic system: comparison with glycoproteins and phospholipids

Axonally transported radioactivity in sialoglycoconjugates, labeled by intraocular injection of [3H]N-acetylmannosamine, increased significantly during regeneration of goldfish optic axons at 30 degrees C. Ganglioside radioactivity showed the largest increase--approximately eightfold--in the optic nerve tract at 8 days after optic nerve crush while sialoglycoprotein radioactivity increased fourfold under the same conditions. As regeneration proceeded the magnitude of the increase in the nerve tract diminished for both glycoconjugates. In the optic tectum, however, transported radioactivities remained approximately twofold higher than controls between 15 and 25 days postcrush. The zwitterionic fraction of glycerophospholipids, labeled by intraocular injection of [14C]glycerol, also showed large increases during regeneration, but the acidic glycerophospholipids showed only modest increases. Thus while membrane components in general were elevated during the early stages of regeneration, the most pronounced increases occurred in gangliosides and certain glycerophospholipids. The significance of these changes in the regeneration process remain to be determined.

Biosynthesis and transport of gangliosides in peripheral nerve

Radiolabelled glucosamine was injected into L-7 dorsal root ganglion (DRG) of rabbits. At several different times after injection DRG, lumbosacral trunks (LST) and sciatic nerves (SN) removed and gangliosides extracted. Two and 3 weeks after injection the amounts of radioactivity in the ganglioside fractions of LST SN were significantly higher than at days 1 and 2. The TCA soluble radioactivity decreased dramatically over the same time period. Colchicine prevented the appearance of radiolabelled lipid in LST and SN. From these experiments we conclude that some ganglioside is synthesized in the neuronal cell bodies of DRG and transported in the axons of the sciatic nerve. In another experiment the sciatic nerve was transected and ends separated to prevent regeneration. Ganglioside synthesis and transport were studied in these animals the same way as the previous experiment. There was no difference the amount of radiolabelled ganglioside that was isolated from DRG or LST of transected compared with control nerves. The behavior of several potential acid soluble contaminants was studied in several steps used to isolate gangliosides. Of those studied only CMP-NeuAc could cause significant contamination of the final ganglioside preparation.

Antibodies to gangliosides inhibit goldfish optic nerve regeneration in vivo

Intraocular injection of antiserum to mixed ganglioside or to GM1 inhibited the regeneration of goldfish optic axons following an optic nerve crush. For example, injections of antiserum on 5 consecutive days beginning the day before the crush resulted in a decrease of about 40% in the axonal outgrowth distance measured at 10 days after the crush. The inhibition was observed even when the treatment was begun a few days after the lesion, and greater degrees of inhibition were observed when the treatment was given later in regeneration. This indicates that the antiganglioside serum interfered with axonal elongation more than with the initial sprout formation. The antiganglioside treatment did not impair the enlargement of the cell bodies and nucleoli that accompanies regeneration, nor did it affect fast axonal transport of protein, glycoprotein, or glycolipid in regenerating nerves. Thus inhibition of outgrowth by antiganglioside treatment was not mediated by a gross change in the metabolism of the regenerating neurons. Treatment of normal neurons with the antiserum produced a 20-30% increase in the amount of 3H-glucosamine-labeled glycoproteins and glycolipids conveyed by fast axonal transport. These results suggest that membrane gangliosides may normally influence the supply of axonally transported glycosylated macromolecules. However, the effect of antiganglioside on axonal transport of glycosylated molecules and on axonal outgrowth are not necessarily related to each other.

Uniform distribution and similar turnover rates of individual gangliosides along axons of retinal ganglion cells in the chicken

In 5-month-old chickens, an intracranial injection of N-[3H]acetylmannosamine led to a labeling of all optic lobe ganglioside species in a fashion paralleling the relative ganglioside distribution. In contrast, after an intraocular injection of the same precursor, the optic nerve and the optic lobe connected to the injected eye, possessed an exceptionally high labelling of GD1a (in comparison with GD1a-sialic acid), and only negligible incorporation of radioactivity into the myelin-specific GM4 and into a fraction migrating close to GM1. Subtracting both these very low labelling fractions from the total gave a percentage distribution of ganglioside sialic acid which now corresponded well to the distribution of radioactivity along the whole optic nerve, including the region of nerve terminals in the optic lobe. This pattern of ganglioside labelling, which indicates that GD1a carries about 60% of total ganglioside sialic acid of retinal ganglion cell axons, did not change remarkably during post-hatching development up to 5 months. Long-time incorporation studies revealed similar turnover rates of the main retinal ganglion cell gangliosides. The average half-lives were 34 (GD1a), 35 (GQ1b), 36.3 (GT1b) and 38.5 days (GD3). The findings suggest that the retinal ganglion cell axons and their presynaptic terminals possess a similar ganglioside pattern, characterized by a high content of GD1a.

Differential effects of cobalt on the initiation of fast axonal transport

Effects of Co2+ on the fast axonal transport of individual proteins were examined in vitro in bullfrog spinal/sciatic nerves. 35S-methionine-labeled proteins, fast-transported in control and Co2+-treated preparations were separated via two-dimensional gel electrophoresis. While the overall amount of protein transported was reduced, no qualitative differences could be seen when gel fluorographic patterns were compared. Quantitative analyses of the 48 most abundantly transported species revealed two significantly different populations (p less than 0.01) differentially sensitive to Co2+ and distinguishable to a large extent by molecular weight. Those proteins less sensitive to Co2+ ranged from approximately 20,000 to 35,000 daltons while those more sensitive to Co2+ were greater than approximately 35,000 daltons. The finding that all proteins are affected by Co2+ supports the proposal that fast-transported proteins are subject to a common Co2+-sensitive, Ca2+-requiring step. The observed differential effects are consistent with more than one Ca2+-dependent step occurring during the initiation phase of fast transport.

Selective effects of LSD and hyperthermia on the synthesis of synaptic proteins and glycoproteins

Protein synthesis in rabbit brain was inhibited following the intravenous injection of LSD. The incorporation of [35S]methionine into brain microsomal and synaptic fractions was decreased by 35-45% relative to control values. A selective increase was observed, however, in the relative labeling of a protein of molecular weight 75,000. Our previous studies have shown that LSD induces an increase in body temperature (i.e. hyperthermia) in rabbits. When LSD-induced hyperthermia was blocked the general reduction in labeling of microsomal and synaptic proteins was still apparent but the selective increase in relative labeling of the 75,000 dalton protein was not. Induction of hyperthermia by means other than LSD (i.e. elevation of ambient temperature) produced selective increases in the relative labeling of microsomal and synaptic proteins of molecular weight 75,000 and 95,000. These proteins are similar in molecular weight of two of the major 'heat shock' proteins whose synthesis is induced in several cultured cell lines following elevation of ambient temperature. Fractionation of [35S]methionine-labeled synaptic membranes by lectin affinity chromatography and analysis of [3H]fucose labeling patterns indicated that, in contrast to the general reduction in labeling of brain proteins, the synthesis of synaptic glycoproteins was not altered by LSD. The synthesis of glycosylated proteins present in other subcellular fractions was, however, reduced. These results suggest that LSD induced selective changes in the synthesis of brain proteins and that the synthesis of synaptic glycoproteins may be relatively resistant to drug administration.

Impaired retrograde axonal transport from a nerve crush in streptozotocin diabetic rats

The axonal transport of proteins in crushed nerves of streptozotocin (40 mg/kg) diabetic rats was investigated 4 weeks after induction of diabetes. 35S-methionine was used as a marker for protein and 3H-fucose as a marker for glycoprotein. The precursors were injected into the fifth lumbar spinal ganglion and the accumulation of TCA-insoluble activity proximal and distal to a sciatic nerve ligature was measured at different time intervals after application of a crush. The start of accumulation distal to the ligature was delayed by 1 hour for proteins as well as for glycoproteins. Furthermore, the total amount of accumulated protein after 19 h was decreased by 18% while the decrease was 21% for glycoprotein. By insulin treatment the differences could both be prevented and reversed after 3 days of normoglycaemia. These findings demonstrate an impaired response to a nerve crush and might be the explanation for the regenerative abnormalities of peripheral nerves in diabetes.

Slow components of axonal transport: two cytoskeletal networks

We have identified two slowly moving groups of axonally transported proteins in guinea pig retinal ganglion cell axons (4). The slowest group of proteins, designated slow component a (SCa), has a transport rate of 0.25 mm/d and consists of tubulin and neurofilament protein. The other slowly transported group of proteins, designated slow components b (SCb), has a transport rate of 2-3 mm/d and consists of many polypeptides, one of which is actin (4). Our analyses of the transport kinetics of the individual polypeptides of SCa and SCb indicate that (a) the polypeptides of SCa are transported coherently in the optic axons, (b) the polypeptides of SCb are also transported coherently but completely separately from the SCa polypeptides, and (c) the polypeptides of SCa differ completely from those comprising SCb. We relate these results to our general hypothesis that slow axonal transport represents the movements of structural complexes of proteins. Furthermore, it is proposed that SCa corresponds to the microtubule-neurofilament network, and that SCb represents the transport of the microfilament network together with the proteins complexed with microfilaments.

Axonal transport and metabolism of [3H]fucose- and [35S]-sulfate-labeled macromolecules in the rat visual system

The axonal transport of labeled macromolecules in retinal ganglion cells of rats was investigated from 1 to 20 days following intraocular injection of [3H]fucose and [35S]sulfate. Maximal incorporation of [3H]fucose into acid insoluble material in the retina was at 8 h, followed by a biphasic decline. Transported [3H]fucose (98% as glycoprotein) was in the optic nerve at 1 h, the optic tract and lateral geniculate body by 2 h, and the superior colliculus by 3 h after injection, indicating a rate of transport of approximately 200 mm/day. Radioactivity continued to accumulate in the superior colliculus for at least 8 h and began to decline rapidly by 24 h. Between 3 and 6 days levels rose again in both optic tract and superior colliculus before starting a gradual decline, indicating that a wave of rapidly transported material was delayed in leaving the retina. When proteins in the superior colliculus were fractionated by gel electrophoresis, the composition of the two fucosylated protein transport phases could be partially resolved. Radioactivity in individual gel peaks represented primarily in the first phase decayed with an average half-life of one day, althouth that in one prominent protein of molecular weight 280,000 turned over with a half-life of the order of 12 h. Radioactive peaks primarily in the second phase decayed with an average half-life of more than a week. Incorporation of [35S]sulfate into acid insoluble material in the retina was maximal at 1-2 h, after which there was a rapid loss of label. The appearance of [35S]sulfate in the optic tract, lateral geniculate body and superior colliculus preceded by a short time that of the [3H]fucose; indicating a shorter retinal processing time for this label. The total transported [35S]sulfate in the superior colliculus peaked by 4-8 h and had fallen by 65% at one day; no prominent second wave of transport was observed as was the case for [3H]fucose. Acid insoluble [35S]sulfate in the superior colliculus was equally divided between glycopeptides and glycosaminoglycans at all times examined, indicating that these macromolecules are transported at the same rate. [35S]Sulfate incorporated into various proteins fractionated by gel electrophoresis had heterogeneous turnover rates, the average being around 12 h. Radioactivity in one group of proteins, of molecular weight around 90,000, decayed with a half-life of only a few hours.

Subcellular and polypeptide distributions of slowly transported proteins in the garfish olfactory nerve

In the garfish olfactory nerve proteins labeled with [3H]leucine are transported by slow axonal flow as a well-defined crest of radioactivity. At 21 degrees C slow flow moves along the axon with a velocity of 0.92 +/- 0.02 mm/day. It has been possible to analyze 4 subcellular fractions (soluble, mitochondrial and 2 membranous) as well as their polypeptide composition, in areas of the nerve containing (1) the slow moving crest, (2) the material remaining in the nerve behind the crest, and (3) the labeling present in front of the slow crest. Analyses were done 70 and 110 days after isotope deposition. The crest of slow moving radioactivity is characterized by a close parallelism between labeling and protein concentration in the subcellular fractions as well as among the polypeptides constituting these fractions. The radioactivity is mainly associated with mol. wt. of 14,000, 30-45,000, 58,000 and 68,000. This last peak corresponds to a protein not labeled by fast transport, present only in the light membranous fraction. The composition of the moving crest remains essentially constant during the 40-day period investigated. Most of the slow-moving molecules remain in the axon behind the moving crest. This deposited material appears to be redistributed and/or to be turning over more rapidly than the molecules still moving in the crest. A large amount of radioactivity was recovered in front of the moving crest. This might be produced by molecules deposited by fast transport and by material released from the cell body at rates intermediate between the fast and slow phases of transport. The subcellular and polypeptide compositions of this area of the nerve remain constant and are intermediate between the compositions of fast and slow flow. The slowly transported labeled polypeptides in the mitochondrial fraction are of low molecular weight, and were found to be similar in the various areas of the nerve and at the two time points studied, and were even similar to the polypeptide distribution determined for fast transport.

Membrane glycolipids: regional synthesis and axonal transport in a single identified neuron of Aplysia californica

Glycolipids moving along an identified axon Aplysia californica were synthesized and incorporated into intracytoplasmic membranes solely in the perikaryon: direct injection of tritiated sugar into the axon revealed no local synthesis or exchange. There was no indication for transfer into axoplasm from glia. Insertion of glycolipids into nascent membranes occurs coordinately with insertion of protein components in the cell body.

A difference between the proteins conveyed in the fast component of axonal transport in guinea pig hypoglossal and vagus motor neurons

We compared the proteins transported in the fast component of guinea pig hypoglossal motor neurons with those of guinea pig vagus (preganglionic parasympathetic) neurons. The fast component proteins of hypoglossal and vagus neurons were radioactively labeled by injecting 3H-amino acids into the hypoglossal and vagus motor nuclei. The radioactive fast component proteins obtained from each system were then compared with each other by SDS-polyacrylamide slab gel electrophoresis and fluorography. These analyses revealed at least twenty polypeptides which appear common to the fast component of each neuronal system. In addition, we identified one difference between the proteins comprising the fast component of these neuronal systems. A polypeptide, molecular weight 50,000 daltons, present in the fast component of vagus neurons was not detected in the fast component of hypoglossal motor neurons. These observations are discussed with regard to the similarities and differences between these neuronal systems.

Axonal transport of lipid in goldfish optic axons

After injection of labeled glycerol, choline, or serine into the eye of goldfish, labeled lipids were axonally transported along the optic nerve to the optic tectum. Although the different precursors were presumably incorporated into somewhat different lipid populations, all three were approximately equally effective in labeling the lipids transported to the tectum, but the amount of transported material remaining in the nerve was different, being highest with choline and lowest with serine. The labeled lipids appeared in the tectum within 6 hr of the injection, indicating a fast rate of transport, but continued to accumulate over a period of 1--2 weeks, which presumably reflects the time course of their release from the cell body. Since there was a gradual increase in the proportion of labeled lipid in the tectum during this period, some other process in addition to fast axonal transport may have affected the distribution of the lipids along the optic axons. When [3H]choline was used as precursor, the transported material included a small amount of TCA-soluble material, which was probably mainly phosphorylcholine, with labeled acetylcholine appearing in only insignificant amounts. With serine, which gave rise to a large amount of axonally transported protein in addition to lipid, a late increase in the amount of labeled lipid in the tectum was seen, accompanied by a decrease in labeling of the protein fraction.

Two-dimensional gel electrophoresis of proteins in rapid axoplasmic transport

Two-dimensional electrophoresis of proteins has allowed high resolution analysis of the protein species rapidly transported in the frog sciatic nerve. The 7th, 8th and 9th dorsal root ganglia were selectively labeled with [3H]leucine or [35S]methionine in one compartment of a lucite chamber. Transport of TCA-precipitable material was monitored in the spinal roots and sciatic nerve kept in another compartment. Fastest transport rates were 75-90 mm/day at 18 decrees C. Ligation of the nerve 30 mm distal to the 8th ganglion at the beginning of the experiment resulted in accumulation of label during a 24 h period. This material was subjected to two-dimensional electrophoresis (pI 5-8; mol.wt. 10(4)-10(5) daltons) in 3 mm nerve segments. Autoradiographs or fluorographs from segments proximal to the ligature yielded a pattern of about 140 spots. Of these, at least 60 were considered to be independent protein species. Neither actin nor tubulin were present among these rapidly-transported, labeled proteins. No pattern was observed from segments distal to the ligature. Blocking protein synthesis with 18 micrometer anisomycin reduced the accumulation of label proximal to the ligature by 98%. Direct labeling of nerve segments produced patterns significantly different from the pattern of transported proteins.

Axonal transport and axonal processing of low molecular weight proteins from the abdominal ganglion of Aplysia

Axonal transport of proteins in nerves of the abdominal ganglion of Aplysia was observed after a 2 h incubation of the ganglion in tritiated amino acids. The transported proteins migrate as a series of discrete peaks, all apparently moving at a rate of 3 mm/h. This process is sensitive to both colchicine and vinblastine, the former agent reducing the amount of transported material without affecting the transport rate. The molecular weight distribution of the transported proteins, as revealed by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate (SDS), is basically unchanged for up to 20 h after labeling. Low molecular weight species (less than or equal to 18,000 daltons) make up 10-20% of the transported protein and appear to be enriched in leucine. These proteins undergo proteolytic cleavage during transport, eventually reaching a molecular weight of 3000 daltons or lower. It is suggested that these data reflect the axonal transport and processing of neurosecretory peptides synthesized by identifiable neurons of the ganglion.

Optic nerve axoplasm and papilledema

A detailed review of optic nerve axoplasm is presented. A number of hypotheses have been postulated for the pathogenesis of papilledema associated with increased intracranial pressure. These hypotheses, mechanical and nonmechanical, are critically evaluated in relation to five essential features of papilledema. Theories, as well as clinical and experimental studies, of axonal transport are reviewed, and a new hypothesis is proposed: Papilledema is primarily a mechanical, nonvascular phenomenon in which an excess amount of extracellular fluid is present in the prelaminar region of the optic disc and the accumulation of that fluid results from the leakage of axoplasm from optic nerve fibers which are compressed posterior to the lamina cribrosa of the optic disc. The authors believe that this is the only existing hypothesis consistent with all the known facts about papilledema. Discussions by Drs. J. Terry Ernest, Thomas R. Hedges, and S. S. Hayreh follow the review.

Incorporation of sialic acid into gangliosides and glycoproteins of the optic pathway following an intraocular injection of [N-3H]acetylmannosamine in the chicken

Following an intraocular (i.o.) injection of [N-3H]acetylmannosamine, low molecular soluble radioactive label is distributed rapidly along the optic pathway. At the same time glycoproteins and gangliosides were found radioactively labelled in the distal parts of the optic pathway. Negligible amounts of radioactivity were incorporated into the same structures via the blood. The present data confirm, that glycoproteins are transferred in the retinal ganglion cells by a rapid axonal transport mechanism towards the nerve endings. Unlike glycoproteins labelled gangliosides accumulated in the optic nerves up to 4 days after the precursor injection. Simultaneously radioactive material, soluble in TCA/PTA, disappeared from the optic nerves. The specific radioactivities (disint./min/mug NeuNAc) of the single ganglioside fractions of the optic lobes, which contain the nerve endings of the retinal ganglion cells, differed considerably. Especially one fraction, moving on TLC plates identically to 'GD1a', was up to 20 times higher labelled than the other gangliosides. After an intracerebral injection of the same precursor, radioactivity was incorporated into the ganglioside fractions of the optic lobes to the same extent.

Distribution along the axon and into various subcellular fractions of molecules labeled with (3H)leucine and rapidly transported in the garfish olfactory nerve

The distribution of molecules labeled with [3H]leucine by fast axoplasmic transport in vivo has been studied in the garfish olfactory nerve after incorporation of the amino acid by the olfactory mucosa. Owing to the size of the nerve, it has been possible to follow the fate of the labeled molecules in 10 different subcellular fractions of 6 consecutive nerve segments. Each segment represents a different part of the profile developed by the transported radioactive molecules. In order to determine the influence of the perikaryon (rate of protein synthesis and rate of protein release into the axon) transport was studied under 3 different conditions: (1) intact nerves (simply labeled with [3H]leucine); (2) nerves cut from the cell bodies 6 h after application of [3H]leucine; and (3) nerves pulse-chase labeled for 1 h. Several conclusions can be drawn. (1) The bulk of the rapidly transported molecules are membranous axonal proteins, as determined by enzyme markers. Most are found in subcellular fractions representing 17% of the total axonal protein. They are synthesized very rapidly in the cell bodies (less than 1 h after isotope deposition) and exhibit the highest specific activities measured. These high specific activities were found in the same axonal membrane fractions in both plateau and crest, suggesting that the membrane precursors are transported as particles rather than as subunits. (2) The majority of these proteins are released into the axon immediately after synthesis; however, at least 30% of the labeled axonal membranous proteins are not released with the fast wave itself but progressively over a long period of time. (3) The majority of the moving material, particularly in membranous fractions, is left behind the fast wave and is deposited in the axon. When the front base of the fast wve has covered 70% of the total nerve length, only 19% of the labeled material of the main axonal membranous fraction appears still to be moving. (4) Proteins with high specific activities are found near the cell bodies and may be the result of early axonal transport of amino acids, diffusing later into the surrounding cells and being incorporated into proteins. Some free amino acids are also transported along the axon.