Distribution of transported proteins in the slow phase of axoplasmic flow. An electron microscopical autoradiographic study

Biogenesis of presynaptic terminal proteins

The delivery of proteins to the presynaptic terminals of guinea pig retinal ganglion cells by two of the major components of axonal transport, and the subsequent persistence and turnover of those proteins were examined in this study. Ganglion cell proteins were radiolabeled by intravitreal injection of radiolabeled amino acids and radioactive axonally transported proteins were analyzed in synaptosomes prepared from the superior colliculi. This procedure allowed examination of presynaptic components of ganglion cell synapses without having to compensate for postsynaptic or other unidentified contaminants. Each of the two major axonal transport components supplies a large number of proteins to the presynaptic terminal, in relative quantities similar although not identical to those seen in the axon. Proteins conveyed by the fast component of axonal transport reached the terminals by 3 h after intraocular injection, peaked by 24 h, and were largely undetectable by 15 days. Slow component b proteins reached the terminals by 12 days, peaked around 21 days, and persisted up to 63 days in the terminals. Proteins in both components demonstrated differential turnover relative to cotransported proteins once they reached the terminals. Differential turnover may account for change in relative concentration of a particular protein required to meet new functional demands on that protein once it enters the terminal.

Autoradiography of mossy fiber terminals in the fastigial nucleus of the cat

Terminal boutons of mossy fiber collaterals in the fastigial nucleus originating from the nucleus reticularis tegmenti pontis, the bulbar reticular formation, and the medial vestibular nucleus were studied with high-resolution autoradiography in order to examine their ultrastructural features and synaptic relations. Labeled mossy fiber boutons ranged in size from 0.5 to 5 micron in diameter, and they all contained clear and spherical vesicles in an electron-lucent matrix, mitochondria, and some fine tubular elements. These boutons form asymmetric synapses with dendritic profiles of different sizes. No evidence was found for mossy fiber termination on the soma of fastigial neurons. Two types of mossy fiber terminals were distinguished on the basis of the aggregation of synaptic vesicles: one type with clustered vesicles and one type with densely packed vesicles, occurring in equal number from all sources. Furthermore, the applicability of the congruity hypothesis is confirmed for the general identification of terminals.

Cerebellar nucleocortical projection with a survey of factors affecting the transport of radioactive tracers

The nucleocortical projection has been studied with the method of anterograde transport of tritiated amino acids and autoradiography. It was observed that this projection is made up of a small number of fibers. Counts of silver grain aggregates in projection sites of the granular layer were compared with counts from material on the cuneocerebellar projection. As it had been concluded in previous studies that the projection is quantitatively important, several modifications of the experimental conditions were tested. Radioactive leucine of high or low specific activity as well as a mixture of amino acids were used. It was observed that labeling is always more intense with a high specific activity tracer. Variations of the aspect and density of the projection were observed with survival periods lasting from a few hours to several days. With survival times between 9 and 15 hours, round aggregates of silver grains measuring from 10 to 20 microns were observed in the granular layer of labeled regions. At longer survival periods, the terminal structures in the granular layer appeared as coarse linear aggregates. It is suggested that the latter represent preterminal labeled fragments while the round accumulations correspond to terminal rosettes. Maximum density of the labeling in the granular layer occurred between 9 and 15 hours. At 2 days, the density was lowest, and at 5 or 6 days it was somewhat higher. Even under optimal conditions, the density of the nucleocortical terminals was low.

Contribution of axonal transport to the renewal of myelin phospholipids in peripheral nerves. II. Biochemical study

The classes of radioactive phospholipids appearing in the ciliary ganglion (CG) and especially in the myelin sheath of the intraorbital part of the oculomotor nerve (OMN) were determined after the intracerebral injection of [2-3H]glycerol and [methyl-14C]choline to chickens. Analysis of the radioactive compounds in water-soluble fractions and chloroform-methanol extracts was performed by thin-layer chromatography (TLC). The water-soluble content of the OMN and CG was much poorer in [2-3H]glycerol and metabolites than in [methyl-14C]choline and derivatives. All classes of glycerophospholipids were found to be axonally transported along the OMN and into the CG, but choline-phosphoglycerides (CPG) were largely predominant. In myelin fractions from the OMN, the specific radioactivity (SRA) of CPG labeled with [2-3H]glycerol reached a maximum earlier (40 h) than the SRA of CPG labeled with [methyl-14C]choline. A 25-fold enhancement of the [14C]SRA of sphingomyelin (SM) was observed between 12 h and 7 days. These results indicate that: (1) axonally transported phospholipids labeled with [2-3H]glycerol consist mainly of CPG; (2) small amounts of CPG are translocated from the axon to myelin; and (3) the progressive enrichment of myelin in [14C]CPG and, to a greater extent, SM draws attention to the importance of the base recycling for local synthesis of myelin phospholipids. Thus the axonal supply of Schwann cells with choline and the transfer of axonal phospholipids to myelin would probably contribute to the metabolic interdependence existing between neuron and glia.

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.

Protein degradation in the mouse visual system. I. Degradation of axonally transported and retinal proteins

The analysis of proteolysis in the nervous system is complicated by the heterogeneity of cell types, extensive reutilization of liberated amino acids, and artifacts that may arise when the integrity of the tissue is disrupted during experimentation. For these reasons, changes in proteolytic activity that are observed during brain development and in neuropathological states may often be difficult to interpret. To minimize these problems, we have developed a technique that permits protein degradation to be investigated specifically within axons of the mouse retinal ganglion cells (RGC). In the present study, the method has been used to examine the degradation of proteins conveyed in the slow phases of axoplasmic transport. When adult C57Bl/6J mice were injected intravitreally with L-[3H]proline, labeled proteins within the primary optic pathway (optic nerve and tract) after 5 days were almost exclusively the slow phase axonal proteins. The rate of degradation of these proteins was then determined within the excised, but otherwise intact, optic pathway by measuring the release of acid soluble radioactivity at 37 degrees C in vitro. At physiological pH, the amino acids released by proteolysis were extensively reutilized. Unless amino acid reutilization was prevented, protein degradative rates were artifactually lowered 3-fold. At least two proteolytic systems within RGC axons actively degraded the slowly transported axonal proteins. A 'neutral' system, stimulated by exogenous calcium ions, was optimally active within the physiological pH range (pH 7.0--7.8). The rate of protein degradation at pH 7.4 was uniform along the RGC axon. An 'acidic' system was optimally active with the incubation was carried out at pH 3.8. This proteolytic activity was calcium-independent and exhibited a proximodistal gradient within the RCG axon with higher activity proximally. Similar proteolytic activities were present in isolated intact retinas but in different proportions. The half-lives of axonal and retinal proteins were comparable to CNS protein half-lives estimated in vivo by methods that take amino acid reutilization into account. These and other recent findings demonstrate the utility of this neuron-specific approach in characterizing proteolytic processes within one cell type that may otherwise be obscured by proteolytic events in other cells when brain tissue is analyzed by conventional methods.

Potassium stimulated release of axonally transported radioactivity from slices of rabbit superior colliculus

The radioactivity in the trichloroacetic acid (TCA) soluble pool in the terminals of the retinal ganglion cells in the superior colliculus (SC) was studied one month after labelling of the nerve cell bodies in the retina with different radioactive amino acids. The TCA soluble fraction in the SC represented a few per cent of the total radioactivity of the isolated tissue and was mainly derived from protein degradation. The perfused slice of the SC responded to high K+ depolarization with an increased release of TCA-soluble radioactivity, while small changes occurred for TCA-precipitable fractions. The evoked release of TCA-soluble radioactivity was particularly prominent after labelling with [3H]glycine. The release was Ca2+-dependent and the response to repetitive depolarization indicated a continuous replenishment of the releasable pool.

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.

Electron microscope autoradiographic evidence for specific transneuronal transport in the mouse accessory olfactory bulb

The distribution of radioactive material was examined autoradiographically 8 h after application of [3H] proline to the vomeronasal organ in mice. Labelled material was transported along the axons of the vomeronasal nerves to their terminals in the glomerular layer of the accessory olfactory bulb (AOB). A lesser but consistent amount of radioactivity was found in the external plexiform layer (EPL) of the AOB. Electron microscopic autoradiography was used to determine which of the components of the EPL contained this labelled material. The method of proportional grain counts showed that the highest concentration of silver grains lay over the mitral cell dendrites, which are the elements immediately postsynaptic to the vomeronasal nerve axons. However, a fairly high proportion of grains also lay over the peripheral processes of granule cells. By application of a method of 'crossfire analysis' (which is explained in detail) it was possible to show that the observed grain distribution is best explained by the assumption that the radioactive material is confined to mitral cells, and the labelling over granule cell processes is due to crossfire from these sources. Im one animal at 5 days after [3H]proline administration label was found to have extended from mitral cells to granule cells, suggesting that the transsynaptically transported radioactive material, which was confined to the mitral cells at 8 h, may have become further redistributed at longer survivals. In a control experiment, [3H]proline was applied directly to the surface of the AOB. This gave rise to a completely different distribution of radioactivity in the EPL: radioactive material was present in all tissue components.

Fluids in the anterior part of the optic nerve in health and disease

New techniques have recently made it possible to study the flow of fluids (blood, axoplasm, and interstitial fluid) in the anterior part of the optic nerve. Blood flow has been reviewed previously; axoplasm and interstitial fluid are considered in this review. General concepts of axoplasmic transport (anterograde and retrograde) are outlined, and the role of axoplasmic transport in the pathogeneses of optic disc edema of various types, in glaucoma, and in ischemic and toxic optic neuropathies is discussed. The probable sources of interstitial fluid in the anterior part of the optic nerve are capillaries in the nerve itself, peripapillary choroid, vitreous, cerebrospinal fluid and possibly axoplasm in the local axons; the flow is defined by various barrier systems. The role of the interstitial fluid in the pathogeneses of optic edema (and associated phenomena) and in serous retinal detachment in the macular region associated with optic disc pit is discussed. Its involvement in the process of diffusion of retrobulbar medication into the optic nerve and vitreous is also considered.

A light microscopic, autoradiographic study of axoplasmic transport in the optic nerve head during ocular hypotony, increased intraocular pressure, and papilledema

Cyclocryotherapy of the monkey eye reliably produces transient increased intraocular pressure followed by prolonged hypotony, during which papilledema occurs. Axoplasmic transport was studied while the intraocular pressure was both increased and decreased after cyclocryotherapy by autoradiography following the intravitreal injection of tritiated leucine and proline. Surgical fistulization of the anterior chamber was also used to produce hypotony and papilledema. Significant alterations of both the rapid and the slow components of axoplasmic transport were demonstrated in the nerve head during increased intraocular pressure, and in ocular hypotony with papilledema.

A light microscopic, autoradiographic study of axoplasmic transport in the normal rhesus optic nerve head

A systematic study of axoplasmic transport in the normal optic nerve head is essential to the understanding of the role of this process in the pathogenesis of disease of the optic nerve. Twenty-one eyes from normal adult rhesus monkeys were subjected to intravitreal injections of tritiated leucine (3H) or proline (3H). Light microscopic autoradiographs were prepared on these eyes and optic nerves six hours to 60 days after injection. The isotope that was incorporated in cytoplasmic components associated with the slow phase of axoplasmic transport (3H leucine) became concentrated in the temporal region of the optic nerve head, unevenly distributed within axons in the axonal bundles, and apparently accumulated in axons or glial cells, or both, in the lamina choroidalis and scleralis as a broad pulse of isotope moved across the optic nerve head. Several anatomic factors, including the distribution of macular fibers, increasing amounts of glial cells within the same axonal bundles as the optic nerve passed from the lamina retinalis to the lamina scleralis, and the abrupt addition of myelin sheaths to axons behind the lamina scleralis, contributed to the complex distribution of isotope in the normal optic nerve head. Glial cell labeling was especially prominent with 3H proline.

Slow axoplasmic transport of mitochondria (MAO) and lactic dehydrogenase in mammalian nerve fibers

The axoplasmic transport of the enzyme monoamine oxidase (MAO) (monoamine:O2 oxidoreductase, (deaminating) EC 1.4.3.4), a marker for mitochondria, and lactic dehydrogenase (LDH) (L-lactate:NAD oxidoreductase, EC 1.1.1927), a soluble component of axoplasm, was studied in cat sciatic nerve. For both these enzymes a linear accumulation was found in the nerve proximal to ligations over a period of at least 20 h. In double-ligation experiments no evidence of a depletion of enzymes within the nerve segment was found over this period of time as would be the case if some portion of the enzymes was carried by fast axoplasmic transport. Both the soluble protein enzyme LDH and the mitochondria, shown by MAO, are thus considered to be moved down the nerve by slow axoplasmic transport. Some differences in the two materials were seen in the greater fall in the level of MAO compared to LDH within the double-ligated segment over the succeeding period from 20 to 48 h. These changes are considered with respect to the transport filament model as modified to take into account slow axoplasmic transport.

Demonstration of a pallido-nigral projection innervating dopaminergic neurons

Afferents to the substantia nigra from the neostriatum and globus pallidus were studied in the rat by means of the autoradiographic tracing technique. 3H-leucine was injected stereotaxically into either the globus pallidus or neostriatum. Twenty-four hours later the axoplasmic transport of labelled proteins to the substantia nigra was studied by light and electron microscopic autoradiography. In animals used for electron microscopy, degeneration of dopaminergic cells in the substantia nigra was induced by intraventricular injection of 6-hydroxydopamine 72 hours before sacrifice. After neostriatal injections, light microscopic analysis revealed heavy labelling of the globus pallidus and entopeduncular nucleus, but only background labelling of the subthalamic nucleus. There was preferential labelling of the zone reticulata of the substantia nigra, with significantly less labelling of the zone compacta. After pallidal injections, light microscopic analysis showed very light labelling of those parts of the caudate-putamen in the vicinity of the injection site. There was intense labelling of the subthalamic nucleus and heavy labelling of the entopeduncular nucleus. The zona compacta of the substantia nigra was also heavily labelled. There was considerably less labelling of the zona reticulata. The electron microscopic analyses showed that after neostriatal injections, autoradiographic grains in the substantia nigra were located preferentially over boutons which terminated on normal dendritic processes. After pallidal injections, however, grains were preferentially located over boutons synapsing with degenerating dendritic processes. The degeneration produced in these dopaminergic processes by 6-hydroxydopamine was invariably of the dark type. Except for the different association with degenerating vs. non-degenerating dendrites, the subcellular distribution of autoradiographic grains in the substantia nigra was the same after injection into either the globus pallidue or caudate-putamen. Approximately 80 percent of the grains were over axons or boutons which invariably made symmetrical synaptic contacts. These observations demonstrate the existence of a pallido-nigral projection which terminates preferentially on dopaminergic cells in the pars compacta of the substantia nigra. They also confirm previous studies indicating that the strionigral projection terminates mainly in the pars reticulata. These terminations appear to be principally to non-dopaminergic cells.

The auditory cortical projections onto the medial geniculate body in the cat. An experimental anatomical study with silver and autoradiographic methods

The auditory cortical projections to the medial geniculate body (MGB) were studied in the cat. Lesions carefully restricted to each one of the subdivisions of the auditory and peri-auditory cortex were made and degenerating fibres were mapped in the MGB. In other experiments L-[4-5-3-H]leucine was injected into the cortex of AI, AII and SF and its transport to the MGB was studied by autoradiography. The results show that fibres arise in the deep layers of the cortex of AI, pass through the deep subdivision of the dorsal nucleus of the MGB as well as through the magnocellular MGB. and end in the pars lateralis of the ventral nucleus of the MGB, Arising from AII, axons pass through the magnocellular MGB and end in the superficial and deep subdivisions of the dorsal nucleus extending to the most caudal part of the MGB. The ectosylvian posterior auditory cortex projects diffusely to all subdivisions of the MGB as well as to its magnocellular part. Of the peri-auditory areas, only the suprasylvian fringe projects to the parvicellular MGB, and it sends axons to the dorsal nucleus of the MGB. The SF and the ectosylvian anterior periauditory area send fibres to the magnocellular MGB. The insular cortex does not project to the MGB but sends heavy projections to lower nuclei of the auditory pathway. The complementary use of degeneration and autoradiographic methods shows that each method may be used to eliminate the drawbacks of the other. This strategy appears as being most adequate for the study of reciprocal projections such as those found between the MGB and the three auditory areas AI, AII, and the ectosylvian posterior auditory area.

Autoradiographic studies of the projections of the midbrain reticular formation: descending projections of nucleus cuneiformis

The descending projections of nucleus cuneiformis in the cat were traced by autoradiography in the transverse and sagittal planes following stereotaxically placed injections of 3H-leucine. Many descending axons are organized into distinct fiber systems, of which the largest and most well-defined crosses directly in the midbrain and descends through the ventromedial tegmentum of the brain stem. This fiber system first terminates profusely in n. reticularis tegmenti pontis and then proceeds through the rhombencephalic tegmentum emitting transversely oriented branches to n. reticularis pontis caudalis and gigantocellularis, the raphe magnus and the facial nucleus...