Distribution of [3H]RNA in goldfish optic tectum following intraocular or intracranial injection of [3H]uridine. Evidence of axonal migration of RNA in regenerating optic fibers

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

N-terminal arginylation of sciatic nerve and brain proteins following injury

N-terminal protein arginylation has been demonstrated in vitro and in situ and has been reported to increase following injury to sciatic nerves of rats. The present study attempts to demonstrate these reactions in vivo by applying [3H]Arg to the cut end of sciatic nerves in anesthetized rats and assaying for N-terminal arginylation using Edman chemistry and acid precipitation of labeled proteins in the proximal nerve segment. No evidence was found for arginylation in an aqueous soluble fraction. However, N-terminal arginylation was detected in a urea soluble fraction at 2 hours after nerve crush. The data show that arginylation of rat sciatic nerve proteins occurs in vivo and suggest that the arginylated proteins formed an aqueous insoluble/urea soluble aggregate after arginylation. In other experiments, rat brains were injured and assayed for arginylation in vitro to test the hypothesis that injury causes an up-regulation of these reactions. Results showed an activation of the reaction at 2 hours post crush and indicate that increases in N-terminal arginylation are likely to be a general response to injury in nervous tissue.

Comparison of posttranslational protein modification by amino acid addition after crush injury to sciatic and optic nerves of rats

Posttranslational protein modifications by the addition of amino acids are reactions which occur in intact sciatic and optic nerves of rats. The nerves differ, however, in that 2 h after crush injury these reactions are activated in sciatic but not in optic nerves. As sciatic nerves will eventually regenerate, whereas optic nerves will not, we have proposed that the activation of these reactions is correlated with the ability of a nerve to regenerate. The current experiments examined the posttranslational addition of amino acids to proteins at times greater than 2 h after nerve crush, during sciatic nerve regeneration and optic nerve degeneration. We also examined the optic nerve for morphologic correlates to changes in protein modification and partially characterized the proteins modified by [3H]Lys in the regenerating sciatic nerve using two-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). In a segment of sciatic nerve taken from a region just proximal to the site of crush, protein modification by covalent addition of [3H]Arg, [3H]Lys and [3H]Leu increased during both posttraumatic (2 h postcrush) and regenerative (6 days and 14 days postcrush) stages. Two-dimensional PAGE of [3H]Lys modified sciatic nerve proteins 6 days after crush injury showed labeling of proteins having molecular masses in the 18,000- to 20,000-, 30,000- to 40,000-, and 80,000- to 100,000-Da ranges, with neutral or basic isoelectric points (pI 7.1 to 8.0). In the retinal portion of the crushed optic nerve, incorporation of the same amino acids was unchanged or depressed to 21 days postcrush, except at 6 days postcrush when the incorporation of all three amino acids into proteins was increased threefold. These increases correlated with the appearance of terminal end bulbs in the portion of nerve analyzed. Histological examination of each nerve 2 h postcrush showed marked edema in the optic but not the sciatic nerve, a condition which may be related to the ability of sciatic and inability of optic nerves to activate protein modification reactions.

Posttranslational protein modification by amino acid addition in regenerating optic nerves of goldfish

Previous experiments have demonstrated that 4S RNA, (tRNA), is transported axonally during the reconnection and maturation of regenerating optic nerves of goldfish. The present experiments were performed to determine if tRNA is transported axonally during elongation of these regenerating nerves and whether, as has been demonstrated in other systems, it participates in posttranslational protein modification (PTPM). [3H]Uridine was injected into both eyes of fish with intact optic nerves and 0, 2, 4, or 8 days after bilateral optic nerve cut. Fish were killed 2 days after injection, and [3H]RNA was isolated from retinae and nerves by phenol extraction and ethanol precipitation. [3H]RNA was fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Although the percentage of [3H]4S RNA remained constant in all retinal and control nerve samples, regenerating nerves showed a twofold increase by 6 days after injury, suggesting that [3H]4S RNA is transported axonally in regenerating nerves as early as 6 days after injury. In other experiments, the 150,000-g supernatant of optic nerves was analyzed for incorporation of 3H-amino acids into proteins. No incorporation of 3H-amino acid was found in the soluble supernatant, but when the supernatant was passed through a Sephacryl S-200 column (removing molecules less than 20,000 daltons), [3H]Arg, [3H]Lys, and [3H]Leu were incorporated into proteins. This posttranslational addition of amino acids was greater (1.4-5 times for Lys and 2-13 times for Leu) in regenerating optic nerves than nonregenerating nerves, and the growing tips of regenerating nerves incorporated 5-15 times more [3H]Lys and [3H]Leu into proteins than did the shafts.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraocular tetrodotoxin reduces axonal transport and transcellular transfer of adenosine and other nucleosides in the visual system of goldfish

Intraocular injection of tetrodotoxin (TTX) in goldfish, which abolishes physiological activity in the optic axons, decreased by up to about 30% the amount of radioactively labeled adenosine, uridine and guanosine (and their nucleotide derivatives) that was axonally transported in the optic nerve. The amount of labeled nucleoside that reached the optic tectum and became incorporated into RNA in the postsynaptic tectal neurons and glial cells was reduced by up to about 50%. There was no change, however, in the amount of transported nucleoside that became incorporated into RNA in the optic nerve glia. The TTX-induced changes were eliminated when axonal transport was blocked with vincristine, indicating that this change did not involve material moving along the nerve by diffusion. If the TTX injection was delayed until several hours after labeling of the transported materials, the transported labeled nucleoside in the nerve was reduced very little, but the RNA labeling in the tectum was reduced just as much as when TTX was given prior to labeling. This indicates that the labeling of the tectal cells was affected more by the level of activity in the pathway than by the amount of transported nucleoside reaching the optic nerve terminals. It appears likely, therefore, that the process most affected by the decrease in physiological activity is the release of nucleoside from the terminals of the presynaptic neurons or its uptake into postsynaptic tectal neurons and glia. The fact that physiological activity may modify the amount of axonally transported nucleosides made available for metabolism (including RNA synthesis) in postsynaptic neurons may provide an explanation for activity-linked neurotrophic effects.

Posttranslational protein modification by amino acid addition in intact and regenerating axons of the rat sciatic nerve

Experiments were performed to determine whether posttranslational addition of amino acids to axonal proteins occurs in axons of the rat sciatic nerve. Two ligatures were placed 1 cm apart on sciatic nerves. Six days later, segments proximal to each ligature were removed, homogenized, centrifuged at 150,000 X g, and analyzed for the ability to incorporate 3H-amino acids into proteins. No incorporation of amino acids into proteins was found in the high-speed supernatant, but when the supernatant was passed through a Sephacryl S-200 chromatography column (removing molecules less than 20 kD), [3H]arginine, lysine, leucine and aspartic acid were incorporated into proteins in both proximal and distal nerve segments. Small but consistently greater amounts of radioactivity were incorporated into proteins in proximal segments compared with distal segments, indicating that the components necessary for the reaction are transported axonally. This reaction represents the posttranslational incorporation of a variety of amino acids into proteins of rat sciatic nerve axons. Other experiments showed that the incorporation of amino acids into proteins is by covalent bonding, that the amino acid donor is likely to be tRNA, and that the reaction is inhibited in vivo by a substance whose molecular mass is less than 20 kD. This inhibition is not affected by incubation with physiological concentrations of unlabeled amino acids, by boiling, or by treatment with Proteinase K. When the axonally transported component of the reaction was determined in regenerating nerves, the amount of incorporation of amino acids into protein was 15-150 times that in intact nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that multiple species of aminoacylated transfer RNA are present in regenerating optic axons of goldfish

This study reports that 4S RNA present in regenerating optic axons of goldfish is likely to be transfer RNA. Evidence is also presented which indicates that this transfer RNA is similar to transfer RNA found in tectal cells and that its aminocylation is likely to occur both in retinal ganglion cells prior to axonal transport as well as in the axon itself. Fish with regenerating optic nerves received intraocular injections of [3H]uridine followed 4 days later by intracranial injections of [14C]uridine. Radioactive tectal 4S RNA was isolated 6 days after [3H]uridine injections and chromatographed by BD cellulose chromatography. Optical density as well as radioactivity profiles for both [14C]4S RNA (from tectal cells) and [3H]4S RNA (90% of which originated from regenerating optic axons) were found to be similar to E. coli transfer RNA optical density profiles, indicating that the intra-axonal 4S RNA is likely to be transfer RNA. Moreover, comparisons of 3H/14C suggest that intra-axonal and cellular 4S RNAs are composed of similar species of transfer RNA. Results of other experiments indicate that aminoacylation of axonally transported tRNA occurs both in the retina and in optic axons subsequent to axonal transport.

Origin of axoplasmic RNA in the squid giant fiber

The origin of axoplasmic RNA in the squid giant fiber was investigated after exposure of the giant axon or of the giant fiber lobe to [3H]uridine. The occurrence of a local process of synthesis was indicated by the accumulation of labeled axoplasmic RNA in isolated axons incubated with the radioactive precursor. Similar results were obtained in vivo after injection of [3H]uridine near the stellate nerve at a sizable distance from the ganglion. Exposure of the giant fiber lobe to [3H]uridine under in vivo and in vitro conditions was followed by the appearance of labeled RNA in the axoplasm and in the axonal sheath. While the latter process is attributed to incorporation of precursor by sheath cells, a sizable fraction of the radioactive RNA accumulating in the axoplasmic is likely to originate from neuronal perikarya by a process of axonal transport.

4S RNA is transported axonally in normal and regenerating axons of the sciatic nerves of rats

Experiments were designed to determine if following injection of [3H]uridine into the lumbar spinal cord of the rat, [3H]RNA could be demonstrated within axons of the sciatic nerve, and if 4S RNA is the predominant RNA species present in these axons. In one experiment the left sciatic nerve of a rat was crushed. Two days later 170 microCi of [3H]uridine was injected into the vicinity of the lumbar ventral horn cells. Ten days after injection, rats were sacrificed and sciatic nerves were prepared for autoradiography. Photomicrographs were taken of labeled areas of intact and regenerating nerves and grains were counted over Schwann cells, myelin, axons and other unspecified areas. In both intact and regenerating sciatic nerves more than 20% of the silver grains were associated with motor axons and approximately 40% were found over cytoplasm of Schwann cells surrounding these axons. These data indicate an intra-axonal localization of RNA in sciatic nerve axons, as well as an active transfer of RNA precursors from axons to their surrounding Schwann cels. In separate studies, the left sciatic nerve was crushed and 10 days later [3H]uridine was bilaterally injected intraspinally into 6 rats. Four control rats were sacrificed at 14 or 20 days after injection. In the remaining 2 rats the sciatic nerve was cut 14 days after injection and the distal part of the nerve was allowed to degenerate for 6 days before sacrificing the rat. Thus, the distal portion of the nerve contained Schwann cells labeled by axonal transport but lacked intact axons. RNA was isolated from experimental and control nerve segments by hot phenol extraction and ethanol precipitation. RNA species (28S, 18S and 4S) were separated by polyacrylamide gel electrophoresis and radioactivity was measured in a liquid scintillation counter. Control groups had RNA profiles similar to those already described, with greater than 30% of the radioactivity present as 4S RNA. The proximal portions of nerve taken from the group in which nerves were cut, had a similar amount of radioactivity present as 4S RNA. However, in the distal segments of these nerves (in which the axons had degenerated thus creating an 'axon-less' nerve) the amount of radioactivity in the 4S peak decreased to approximately 15% of the total RNA, suggesting that 4S RNA is the predominant if not the only RNA present in these axons. These results strongly indicate that both intact and regenerating sciatic nerves of rats selectively transport 4S RNA along their motor axons.

Lack of glia-axon transfer of proteins in the normal optic system of goldfish

An ultrastructural autoradiographic study of the goldfish optic tectum was carried out to determine whether proteins synthesized in glial cells are transferred into adjacent optic axons. Goldfish were injected intracranially over the optic tecta with [3H]leucine and fixed by perfusion 30 min, 4 and 24 h later. All unincorporated precursors were removed by repeated washings with fixatives, and tissue slices from the optic tecta were embedded and processed for electrom microscopy autoradiography (EMA). The densities of the silver grains were determined over optic axons and their myelin sheaths. The densities over the axons were lower than those over the myelin sheaths at all time intervals. The density of the intraaxonal grains, in absolute terms as well as relative to that of the myelin, was highest in the 4 h experiment. Analysis of the distribution of the grain densities over the myelin sheath and over concentric axonal compartments was carried out at this time interval to determine whether the grain density over the axon represented intraaxonal [3H]proteins or was only the result of grains 'scattered' from [3H]proteins located in the surrounding myelin sheaths. When the experimental distribution of the grain densities over the axon was compared with the theoretical distribution expected over the axon if only the myelin sheaths were labeled, no significant difference was found. This indicates that the silver grains present in the axons were scattered from the adjacent myelin sheath and did not represent intraaxonal radioactivity. It is therefore concluded that in our system there is not a quantitatively significant transfer of proteins between glial cells and adjacent axons.

Axonal growth during regeneration: a quantitative autoradiographic study

The intraaxonal distribution of labeled glycoproteins in the regenerating hypoglossal nerve of the rabbit was studied by use of quantitative electron microscope autoradiography. 9 d after nerve crush, glycoproteins were labeled by the administration of [3H]fucose to the medulla. The distribution of transported 3H-labeled glycoproteins was determined 18 h later in segments of the regenerating nerve and in the contralateral, intact nerve. At the regenerating tip, the distribution was determined both in growth cones and in non-growth cone axons, 6 and 18 h after labeling. The distribution within the non-growth cone axons of the tips was quite different at 6 and 18 h. At 6 h, the axolemma region contained < 10% of the radioactivity; at 18 h, it contained virtually all the radioactivity. In contrast, the distribution within the growth cones was similar at both time intervals, with 30% of the radioactivity over the axolemmal region. Additional segments of the regenerating nerve also showed a preferential labeling of the axolemmal region. In the intact nerve, 3H-labeled glycoproteins were uniformly distributed. These results suggest that: (a) in this system the labeled glycoproteins reaching the tip of the regenerating axons are inserted into the axolemma between 6 and 18 h after leaving the neuronal perikaryon; (b) at the times studied, there is a fairly constant ratio between glycoproteins reaching the growth cone through axoplasmic transport and glycoproteins inserted into the growth cone axolemma; (c) the axolemma elongates by continuous insertion of membrane precursors at the growth cone; the growth cone then advances, leaving behind an immature axon with a newly formed axolemma; and (d) glycoproteins are preferentially inserted into the axolemma along the entire regenerating axon.

4S RNA is present in regenerating optic axons of goldfish

Regenerating optic axons of goldfish were loaded with [3H]RNA by injecting [3H]uridine into the eye and allowing time for the radioactivity to be delivered to the optic tectum. The axons were subsequently removed from the tecta by cutting the optic nerve and allowing the optic axons in the tectum to degenerate. Analysis of tectal [3H]RNA by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a selective loss of tritiated 4S RNA and not ribosomal RNA from the denervated tecta. These results support the hypothesis that regenerating optic axons of goldfish grow back into the tectum carrying 4C but not ribosomal RNA.

Axonal transport of nucleosides, nucleotides and 4S RNA in the neonatal rat visual system

The axonal migration of RNA, the nucleoside uridine and its nucleotide derivates (NS/NT) were compared in neonatal and young adult rat optic axons. Tritiated uridine was injected into right eyes of developing (1- or 4-day-old) and young adult (40-day-old) rats which were sacrificed at times after injection ranging from 6 h to 20 days. Right and left lateral geniculates were removed and assayed for trichloroacetic acid soluble (NS/NT) and RNA radioactivity. Left minus right geniculate (L-RLG) radioactivity was used as an index of axonally migrating radioactivity. Results showed that uridine and its phosphorylated derivatives were transported along both neonatal and young adult rat optic axons. Greater than 90% of right geniculate (blood-borne) TCA soluble radioactivity was metabolized to volatile substances (probably 3H2O) by three days after injection, leaving approximately 3% of the neonatal and approximately 10% of the adult activity as [3H]NS/NT. In left geniculate fractions (containing transported material) approximately 15% and 40% of total TCA soluble radioactivity was present as [3H]NS/NT in neonates and adults, respectively. Thus, axonal NS/NT appears to be relatively protected from degradation when compared with blood-borne NS/NT. The amount of L-RLG [3H]RNA in the neonates was 10 times higher than in young adults. Peaks of neonatal [3H]RNA occurred at 5 and 10 days after birth, whether injections were made at 1 or 4 days of age indicating that this [3H]RNA may be linked to developmental events. Gel electrophoretic analysis of neonatal geniculate RNA indicated that a small portion of the [3H]RNA in the first peak represented axonally transported 4S RNA. The remainder of the L-RLG [3H]RNA in the neonates was probably due to a rapid and efficient incorporation of axonally transported [3H]NS/NT into extraaxonal geniculate RNA. In contrast, little or no axonal RNA transport could be demonstrated in the young adults.

Evidence that 4S RNA is axonally transported in normal and regenerating rat sciatic nerves

Studies in regenerating goldfish optic nerves indicate that RNA may be axonally transported during optic nerve regeneration14,18,19. The present study was performed to determine if the axonal migration of RNA could be demonstrated during regeneration of the rat sciatic nerve. Rats, which had only the left sciatic nerve crushed 10 days earlier, were injected bilaterally with [3H]uridine into the spinal cord at segmental levels L5 and L6, thus labeling ventral horn cells giving rise to the sciatic nerve. Six, 14 and 20 days later rats were sacrificed by cardiac perfusion of saline followed by 10% formaldehyde. Formaldehyde-precipitable radioactivity, identified as [3H]RNA, was 4--5 times greater in the regenerating sciatic nerve compared to the normal nerve and moved without impediment beyond the point of the crush into the regenerating portion of the nerve. The axonal migration of free unincorporated labeled RNA precursors was also demonstrated, raising the possibility that the distribution of [3H]RNA along the sciatic nerve might be entirely extra-axonal; i.e., free [3H]uridine is taken up by Schwann cells from the axon where it is incorporated into [3H]RNA. This interpretation of the data would also result in the appearance of a proximodistal distribution of RNA associated radioactivity. To determine whether any sciatic nerve [3H]RNA was due to axonal transport, rats which had only the left sciatic nerve crushed 10 days earlier were injected bilaterally with [3H]uridine into the spinal cord. Fourteen days after injection, rats were sacrificed and radioactivity present in the nerve was confirmed as RNA by SDS polyacrylamide gel electrophoresis. Radioactivity in the various RNA species 14 days after intraspinal injection showed the following distribution: 28 + 18S RNA--normal 39.3% +/- 2.1; regenerating 45.4% +/- 1.6; 4S RNA--normal 43.0% +/- 1.3; regenerating 46.8% +/- 2.7. Similar characterization of sciatic nerve RNA 1 or 3 days following the intravenous administration of [3H]uridine gave the following distribution: 28 + 18S RNA--normal 72.4% +/- 3.0; regenerating 75.0% +/- 3.6; 4S RNA--normal 7.7% +/- 1.3; regenerating 10.7% +/- 0.8. The intraspinal injection of [3H]uridine would label Schwann cell RNA and, in addition, any species of intra-axonal RNA, while intravenous injections would label Schwann cell RNA and not axonal RNA. If 4S RNA is in the axon, one would predict relatively more labeled 4S RNA following intraspinal injections than following intravenous injections. The data demonstrate an enrichment of 4S RNA in both normal and regenerating rat sciatic nerve following the intraspinal but not following the intravenous injection of labeled precursor. Therefore, we suggest that 4S RNA migrates axonally in both normal and regenerating sciatic nerves of rats.