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

Cytomatrix protein residence times differ significantly between the tract and the terminal segments of optic axons

The window method of radiolabeled protein analysis was used to study the transport kinetics of axonally transported cytomatrix proteins as they move through segments of mouse optic axons. Three slow component b (SCb) proteins--actin, a 30 kDa protein, and clathrin--were radiolabeled in the eye and were followed for up to 119 days by quantitative one-dimensional gel electrophoresis. These proteins appeared first in the optic nerve, next in the tract, and last in the superior colliculus. All of the radiolabeled proteins had passed through the optic axons and had been effectively removed from the terminals by 119 days. Two different axonal segments ('windows') were examined in detail: a segment of the axon shaft region in the optic tract, and a segment of axon terminal region in the midbrain superior colliculus. The median transit times of the 3 proteins were 53-100% longer in the colliculus than in the tract, and the pulse transients (the total area under the transport curve in each window) were 180-350% larger in the colliculus than in the tract. These results indicate that at least certain cytomatrix and cytoskeletal proteins have longer residence times in the terminal regions than in the axon proper.

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.

Complex compartmentation of tyrosine sulfate-containing proteins undergoing fast axonal transport

The compartmentation of fast-transported proteins that possess sulfated tyrosine residues--sulfoproteins--has been examined for further resolution of the possible significance of sulfated tyrosine in routing and delivery of fast-transported proteins. In vitro fast axonal transport of [35S]methionine- or 35SO4-labeled proteins was measured in dorsal root ganglion neurons for analysis of protein compartmentation en route and in synaptic regions. When membrane fractions were exposed to Na2CO3 for separation of "lumenal" and peripheral membrane proteins from integral components of the membrane, approximately 20% of the [35S]methionine incorporated into fast-transported proteins was present in a carbonate-releasable form in the axon, whereas 53% of the incorporated 35SO4 was released by carbonate. Eighty percent of the 35SO4 in this releasable fraction was acid labile, typical of sulfate ester-linked to tyrosine. Sulfoproteins were also detected in synaptosomes and were released into the extracellular medium in a calcium-dependent fashion, an observation suggesting that fast-transported sulfoproteins are secreted. Of the remaining 47% of the fast-transported 35SO4-labeled proteins resistant to carbonate treatment (the integral membrane protein fraction), nearly 60% of the 35SO4 was acid labile. Other membrane stripping agents, such as 0.1 M NaOH, 0.5 M NaCl, or mild trypsin treatment, failed to remove acid-labile 35SO4-labeled species from carbonate-treated membrane. Quantitative comparisons of several of the most abundant sulfoproteins resolved via two-dimensional gel electrophoresis confirmed that approximately 7% of each of the species remained associated with carbonate-treated membranes, presumably as integral membrane components.(ABSTRACT TRUNCATED AT 250 WORDS)

In situ degradation of rapidly transported proteins in nerve terminals of retinal ganglion cells

The in situ degradation of rapidly transported proteins in nerve terminals of retinal ganglion cells was studied in pieces of the superior colliculus of the rabbit. Proteolytic degradation was found to be dependent upon extracellular calcium and intact calcium channels. Degradation was inhibited by leupeptin and SH-group blocking agents.

Transneuronal transport in the vestibular and auditory systems of the squirrel monkey and the arctic ground squirrel. I. Vestibular system

Transneuronal transport of [3H]proline, [3H]fucose, and [3H]leucine in various combinations from pledgets implanted in the ampulla of a single semicircular duct was studied in the squirrel monkey and arctic ground squirrel after long survival periods. Tritiated amino acids implanted in any single ampulla resulted in labeling of nearly all vestibular and auditory receptors, nearly all cells of the vestibular and spiral ganglia and central transport via nearly all root fibers of both nerves. Primary vestibular fibers were distributed to the vestibular nuclei (VN) and specific parts of the cerebellum in the pattern previously described. Transneuronal transport of [3H]proline by vestibular neurons was present in all known secondary pathways, except those projecting to thalamic nuclei. Observations were similar in both species, except for small differences in commissural vestibular projections. Major commissural transport was to all parts of the opposite medial vestibular nucleus (MVN) and to peripheral parts of the superior vestibular nucleus (SVN), but some transport was present in all contralateral VN, including ventral cell group y. Descending transneuronal transport was evident in vestibulospinal tract (VST) ipsilaterally and in the medial longitudinal fasciculus (MLF) bilaterally. Both [3H]proline and [3]fucose were transported transneuronally to the ipsilateral abducens nucleus (AN); with long survivals [3H]proline was transported peripherally via the ipsilateral abducens nerve root. Ascending transport in the MLF was bilateral, asymmetric and greatest contralaterally. Fibers entered the contralateral MLF near the AN and the lateral wing of the ipsilateral MLF rostral to most of the VN. Terminals in the trochlear nuclei (TN) were bilateral and greatest contralaterally. In the monkey terminals in ipsilateral oculomotor complex (OMC) were distributed uniformly in all subdivisions, except for the medial rectus subdivision (MRS), where terminals were more numerous. The greatest density of terminals was present contralaterally in the superior rectus subdivision (SRS) of the OMC; only sparse terminals were present in the MRS on that side. Transport in the ipsilateral abducens nerve roots in the monkey and the virtual absence of transport to the MRS of the contralateral OMC suggested transneuronal transport to abducens motor neurons, but not to internuclear neurons (AIN). The AIN project only to the MRS of the contralateral OMC and do not appear to receive vestibular input. Comparable observations were made in the AN, TN and OMC of the ground squirrel, although the representation of the extraocular muscles in the OMC is unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Transneuronal transport in the vestibular and auditory systems of the squirrel monkey and the arctic ground squirrel. II. Auditory system

Transneuronal transport in the auditory system of the squirrel monkey and the arctic ground squirrel was studied after implantation of tritiated protein or glycoprotein precursors into the ampulla of a single semicircular duct. In both species, essentially the same pattern of transneuronal transport extended beyond the cochlear nuclei to the central nucleus of the inferior colliculus (CNIC), after survival periods ranging from 9 to 33 days. Animals displayed dense labeling over nearly all auditory receptors, nearly all portions of the spiral ganglion and throughout the cochlear nuclei (CN). Labeled fibers, mainly in the ventral acoustic stria, terminated over the ipsilateral lateral superior olive (LSO) and the lateral aspect of medial superior olive (MSO). Fibers continuing medially, decussated in an orderly manner, and terminated over the opposite medial nucleus of the trapezoid body (MNTB) and medial aspect of MSO. Labeled fibers projecting into the opposite lateral lemniscus (LL) terminated in the ventral nucleus of the lateral lemniscus (VNLL) and the CNIC. Fibers, but few terminals, were noted over the dorsal nucleus of the LL. The ipsilateral LL contained comparatively few labeled fibers, but sparse terminations occurred over portions of VNLL and CNIC. No transport of [3H]precursors was noted in the peripheral nuclei of the inferior colliculus or in the medial geneculate body on either side. Massive transport via the contralateral LL and the profuse terminals in the opposite CNIC suggested transneuronal transport via secondary and higher order auditory fibers. Although the largest number of fibers in the contralateral LL probably arose from the cochlear nuclei, higher order fibers also may have arisen from the ipsilateral LSO and the contralateral MSO and VNLL. Small numbers of fibers in both species descended from the region of the superior olivary complex (SOC) ventral to the facial motor nucleus. In the ground squirrel, scant auditory projections were traced into the opposite cochlear nuclei. Tritiated precursors in the endolymph passed most readily from labyrinth to cochlea, and transneuronal transport was more extensive in the auditory pathways than in the vestibular system at comparable times. Centrally transported [3H]fucose was cleared more promptly than [3H]proline in monkeys.

The effect of intraocular injection of tetrodotoxin on fast axonal transport of [3H]proline- and [3H]fucose-labeled materials in the developing rat optic nerve

The fast axonal transport of [3H]proline-labeled proteins and [3H]fucose-labeled glycoproteins delivered to the dorsal lateral geniculate nucleus in the developing rat optic nerve was investigated during tetrodotoxin-induced monocular impulse blockade. Repeated intraocular injections of various dosages of tetrodotoxin or citrate buffer vehicle were made every two days in rats aged 5-21 days postnatal, and the accumulation of rapidly transported radioactivity in the lateral geniculate nucleus measured between three and twelve hours post-injection at each age. The effectiveness of prolonged tetrodotoxin treatment was monitored by loss of the pupillary light reflex and the level of cytochrome oxidase activity in the contralateral superior colliculus and dorsal lateral geniculate nucleus. Numbers of optic axons proximal to the chiasm and the frequency of retinal ganglion cells per unit distance from the optic disc were examined for signs of tetrodotoxin-induced degeneration of the retinofugal pathway. Tetrodotoxin-treatment reduced the amount of fucosyl glycoproteins, but not proline-labeled proteins, axonally transported to the lateral geniculate nucleus during the first three weeks of postnatal development. Other studies indicated that tetrodotoxin significantly reduced the incorporation of [3H]fucose into retinal proteins indicating that the reduction in transport was probably due to a decrease in precursor incorporation into retinal ganglion cells. Electron microscopy of ganglion cells at 21 days revealed dilated and vacuolated Golgi cisternae associated with tetrodotoxin treatment, suggesting that tetrodotoxin may alter fucose metabolism by secondarily disrupting Golgi organization. Other protein synthetic machinery in these cells, including ribosomes and rough endoplasmic reticulum, appeared normal throughout tetrodotoxin treatment. These data indicate that Na+-dependent optic impulse activity may be indirectly related to the axonal transport of glycoproteins during early postnatal development by mediating the incorporation of precursor into glycoproteins at the Golgi apparatus and their subsequent entrance into the fast transport system.

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.

Fast axonal transport of tyrosine sulfate-containing proteins: preferential routing of sulfoproteins toward nerve terminals

The presence of a subset of fast-transported proteins containing sulfate while lacking carbohydrate residues [Stone et al. (1983). J. Neurochem. 41:1085-1089] was confirmed by two-dimensional gel electrophoretic analysis of individual fast-transported proteins double-labeled with 35SO4 and [3H]mannose. Analysis by high-pressure liquid chromatography revealed that the sulfate moieties of these "sulfoproteins" are linked to tyrosine residues. Separation of fast-transported 35SO4-labeled proteins delivered to local regions of axon from proteins en route toward terminal regions demonstrated, on the basis of acid lability of tyrosine-bound sulfate, that the sulfoproteins were localized preferentially in the wavefront of fast-transported proteins. Analysis of individual sulfoproteins confirmed differential transport in that sulfoproteins were present at threefold greater amount in the wavefront than in material off-loaded to local regions of the axon. By contrast, nonsulfated species of molecular weights similar to those of the sulfoproteins were detected in nearly equal amounts in both regions of the transport profile. Treatment of nerve segments containing total 35SO4-labeled fast-transported proteins with sodium carbonate led to solubilization of half the protein-bound sulfate. Exposure of the solubilized proteins to mild acid resulted in the release of approximately 80% of the 35SO4 associated with this fraction. Two-dimensional gel patterns displaying carbonate releasable or nonreleasable fractions are consistent with the most abundantly labeled sulfoproteins being transported within membrane-bound organelles. In terms of apparent destination and subcellular compartmentalization, the sulfoproteins meet critical requirements for consideration as secretable fast-transported proteins.

Postnatal changes in [3H]fucosyl glycoconjugates axonally transported into hamster optic nerve endings

The subsynaptosomal distribution of [3H]fucosyl glycoproteins axonally transported into the optic nerve endings of neonatal and adult hamsters changed dramatically at eye-opening. In 12 day-old previsual hamsters, the highest concentration of incorporated fucose was in the axoplasmic reticulum/synaptic vesicle fraction (51%), with only 6% in the dense synaptic membrane fraction. By the end of the eye-opening period four days later proportional labeling of the dense synaptic membrane fraction had increased four-fold to 23% of total sub-synaptosomal radioactivity. Labeling of the synaptic membrane doubled again in adults (41%). Total synaptosomal radioactivity was greatest in 16 day-olds. These results imply that utilization o [3H]fucose by the retinal ganglion cells, as well as composition of the synaptic membrane, change in association with the onset of functional visual activity.

Fast-transported glycoproteins and nonglycosylated proteins contain sulfate

35SO4-labeled fast-transported proteins of bullfrog dorsal root ganglion neurons were separated by two-dimensional gel electrophoresis, and their mobilities were compared to similar species labeled with [3H]mannose or [3H]fucose. Fluorography revealed regions of poorly resolved, high molecular weight material, likely to represent sulfated proteoglycans, as well as many well resolved spots that corresponded in mobility to individual [35S]methionine-labeled fast-transported proteins. The majority of these well resolved spots appeared as "families," previously identified as glycoproteins based on their labeling with sugars. Thus, sulfate can be a contributor to the carbohydrate side-chain charge that underlies microheterogeneity. The most heavily 35SO4-labeled species, however, corresponded to fast-transported proteins that were not labeled by either sugar. The relative acid labilities of 35SO4 associated with individual species cut from the gel confirmed the assignments of these spots as glycoproteins or nonglycoproteins. A group of spots intermediate in their acid lability was also detected, suggesting that some proteins may contain sulfate linked to carbohydrate as well as to amino acid residues.

Glycosylation as a criterion for defining subpopulations of fast-transported proteins

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.

Composition and subcellular distribution of glycoproteins and glycosaminoglycans undergoing axonal transport in garfish olfactory nerves

The study examined the subcellular distribution of [3H]glucosamine-labeled glycoconjugates undergoing axonal transport in 100,000 x g soluble and two membranous subfractions of the garfish olfactory nerve. Analysis was made of intact glycoconjugates and of glycopeptides and glycosaminoglycans derived from these molecules by limit protease digestion. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed labeling of a variety of high-molecular-weight molecules with a lower molecular weight distribution in the soluble fraction than in the membranous fractions. Following protease digestion, nearly two-thirds of transported radioactivity in glycopeptides was recovered in the plasma membrane-enriched subfraction, with the remainder equally divided between soluble and higher density membrane fraction. Comparison of the distribution of glycopeptide radioactivity and chemically assayed hexosamine revealed transport labeling of a large variety of different-sized neutral and acidic glycopeptides in all subfractions. Transport labeling of most glycoprotein carbohydrate chains was in proportion of their hexosamine content. Transported glycosaminoglycan label was most heavily concentrated in the plasma membrane fraction, whereas hexosamine was most concentrated in the higher density membrane fraction. The labeling pattern suggested both transported and nontransported pools of these molecules. The specific glycosaminoglycans chondroitin sulfate and heparan sulfate were recovered in all subfractions, whereas hyaluronic acid was confined to the soluble fraction.

Different polypeptides are rapidly transported in auditory and optic neurons

Rapidly transported proteins and glycoproteins in the auditory and optic nerves of the guinea pig were analyzed by electrophoresis and two-dimensional electrofocusing/electrophoresis. Proteins transported in the auditory nerve were analyzed in the cochlear nucleus 3 h after cochlear injection of radioactive precursor, and proteins transported in the optic nerve were analyzed in the superior colliculus 6 h after intraocular injection of radioactive precursor. Two-dimensional analysis showed that several rapidly transported polypeptides were present in one system, but not in the other. By use of [3H]fucose as a precursor or by separating [35S]methionine-labeled polypeptides on immobilized concanavalin A or wheat germ agglutinin, it was shown that most of the proteins transported in only one system are glycoproteins. As previously reported a polypeptide of molecular weight 140,000 was a major labeled species in the auditory nerve. This polypeptide was also found in the optic nerve, but only as a minor species. Two other polypeptides with molecular weights and isoelectric points similar to those of the 140,000 molecular weight polypeptide were present in both systems, but were much more abundant in the optic nerve. The major labeled polypeptide in both systems had a molecular weight of 25,000.

Axonal transport of the mitochondria-specific lipid, diphosphatidylglycerol, in the rat visual system

Rats 24 d old were injected intraocularly with [2-3H]glycerol and [35S]methionine and killed 1 h-60 d later. 35S label in protein and 3H label in total phospholipid and a mitochondria-specific lipid, diphosphatidylglycerol(DPG), were determined in optic pathway structures (retinas, optic nerves, optic tracts, lateral geniculate bodies, and superior colliculi). Incorporation of label into retinal protein and phospholipid was nearly maximal 1 h postinjection, after which the label appeared in successive optic pathway structures. Based on the time difference between the arrival of label in the optic tract and superior colliculus, it was calculated that protein and phospholipid were transported at a rate of about 400 mm/d, and DPG at about half this rate. Transported labeled phospholipid and DPG, which initially comprised 3-5% of the lipid label, continued to accumulate in the visual structures for 6-8 d postinjection. The distribution of transported material among the optic pathway structures as a function of time differed markedly for different labeled macromolecules. Rapidly transported proteins distributed preferentially to the nerve endings (superior colliculus and lateral geniculate). Total phospholipid quickly established a pattern of comparable labeling of axon (optic nerve and tract) and nerve endings. In contrast, the distribution of transported labeled DPG gradually shifted toward the nerve ending and stabilized by 2-4 d. A model is proposed in which apparent "transport" of mitochondria is actually the result of random bidirectional saltatory movements of individual mitochondria which equilibrate them among cell body, axon, and nerve ending pools.

Axonal transport of proteins and glycoproteins in the rat nigro-striatal pathway and the effects of 6-hydroxydopamine

Following stereotaxic injection of [35S]methionine into the substantia nigra of adult rats, there was rapid local incorporation of radioactivity into acid-insoluble material. Incorporation peaked by 4 h and then decreased. In contrast, acid-precipitable radioactivity in the corpus striatum (the major projection site of the substantia nigra) rose markedly between 1 and 8 h followed by a plateau period and another even more marked increase between 24 h and 6 days. Experiments involving injection of [3H]fucose gave similar results except that most of the acid-precipitable radioactivity in the striatum appeared in an early wave. In each case radioactivity in the contralateral striatum was less than 11% of that on the ipsilateral side. Stereotaxic injection of colchicine (20 microgram) into the nigrostriatal pathway (within the median forebrain bundle) blocked transport of [35S]protein and [3H]glycoprotein by 90% and 50%, respectively. In animals treated with 6-hydroxydopamine (6-OHDA; treated neonatally or as adults) the accumulation of striatal [35S]protein was reduced to 7 to 26% of control levels; striatal [3H]glycoprotein was also reduced, but not as much (29% to 73% of control). In control experiments, [3H]DOPA wa injected into the substantia nigra, and [3H]dopamine was measured in corpus striatum; 6-OHDA treatment reduced the amounts of striatal [3H]dopamine recovered to 3% of control values. The failure of colchicine or 6-OHDA to block transport of incorporated fucose as effectively as the transport of incorporated methionine is possible due to greater diffusion of fucose away from the injection site to non-dopaminergic nuclei projecting to the striatum. The molecular weight distribution of radioactive proteins at the substantia nigra and corpus striatum was analyzed by polyacrylamide gel electrophoresis. For both [35S]methionine and [3H]fucose, the gel electrophoretic pattern of radioactive proteins in the injection site (substantia nigra) was complex and did not change greatly between 2 h and 6 days. At the projection site (striatum) the electrophoretic distribution pattern was initially different from that of the substantia nigra, and changed markedly over the course of several days. In 6-OHDA-treated animals (treated neonatally or as adults), the bulk of proteins transported in nigro-striatal non-dopaminergic neurons appears to be very similar to that transported in the intact pathway in control rats. However, in striata of 6-OHDA-treated animals, a consistent reduction in striatal 35S- and 3H-radioactivitiy was observed in proteins with molecular weight from about 67,000 to 77,000. Assuming that the 6-OHDA treatment did not substantially affect the non-dopaminergic neurons, we interpret this to mean that some of the proteins in this molecular weight range are transported primarily by dopaminergic neurons.