Sedimentation analysis of ribonucleic acid extracted from isolated Mauthner nerve fibre components

Myosin Va but Not nNOSα is Significantly Reduced in Jejunal Musculomotor Nerve Terminals in Diabetes Mellitus

Nitric oxide (NO) mediated slow inhibitory junction potential and mechanical relaxation after electrical field stimulation (EFS) is impaired in diabetes mellitus. Externally added NO donor restore nitrergic function, indicating that this reduction result from diminution of NO synthesis within the pre-junctional nerve terminals. The present study aimed to investigate two specific aims that may potentially provide pathophysiological insights into diabetic nitrergic neuropathy. Specifically, alteration in nNOSα contents within jejunal nerve terminals and a local subcortical transporter myosin Va was tested 16 weeks after induction of diabetes by low dose streptozotocin (STZ) in male Wistar rats. The results show that diabetic rats, in contrast to vehicle treated animals, have: (a) nearly absent myosin Va expression in nerve terminals of axons innervating smooth muscles and (b) significant decrease of myosin Va in neuronal soma of myenteric plexus. In contrast, nNOSα staining in diabetic jejunum neuromuscular strips showed near intact expression in neuronal cell bodies. The space occupancy of nitrergic nerve fibers was comparable between groups. Normal concentration of nNOSα was visualized within a majority of nitrergic terminals in diabetes, suggesting intact axonal transport of nNOSα to distant nerve terminals. These results reveal the dissociation between presences of nNOSα in the nerve terminals but deficiency of its transporter myosin Va in the jejunum of diabetic rats. This significant observation of reduced motor protein myosin Va within jejunal nerve terminals may potentially explain impairment of pre-junctional NO synthesis during EFS of diabetic gut neuromuscular strips despite presence of the nitrergic synthetic enzyme nNOSα.

Protein synthesis in nerve terminals and the glia-neuron unit

The progressive philogenetic lengthening of axonal processes and the increase in complexity of terminal axonal arborizations markedly augmented the demands of the neuronal cytoplasmic mass on somatic gene expression. It is proposed that in an adaptive response to this challenge, novel gene expression functions developed in the axon compartment, consisting of axonal and presynaptic translation systems that rely on the delivery of transcripts synthesized in adjacent glial cells. Such intercellular mode of gene expression would allow more rapid plastic changes to occur in spatially restricted neuronal domains, down to the size of individual synapses. The cell body contribution to local gene expression in well-differentiated neurons remains to be defined. The history of this concept and the experimental evidence supporting its validity are critically discussed in this article. The merit of this perspective lies with the recognition that plasticity events represent a major occurrence in the brain, and that they largely occur at synaptic sites, including presynaptic endings.

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.

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.

Local synthesis of axonal and presynaptic RNA in squid model systems

The presence of active systems of protein synthesis in axons and nerve endings raises the question of the cellular origin of the corresponding RNAs. Our present experiments demonstrate that, besides a possible derivation from neuronal cell bodies, axoplasmic RNAs originate in periaxonal glial cells and presynaptic RNAs derive from nearby cells, presumably glial cells. Indeed, in perfused squid giant axons, delivery of newly synthesized RNA to the axon perfusate is strongly stimulated by axonal depolarization or agonists of glial glutamate and acetylcholine receptors. Likewise, incubation of squid optic lobe slices with [3H]uridine leads to a marked accumulation of [3H]RNA in the large synaptosomes derived from the nerve terminals of retinal photoreceptor neurons. As the cell bodies of these neurons lie outside the optic lobe, the data demonstrate that presynaptic RNA is locally synthesized, presumably by perisynaptic glial cells. Overall, our results support the view that axons and presynaptic regions are endowed with local systems of gene expression which may prove essential for the maintenance and plasticity of these extrasomatic neuronal domains.

Beyond the initial axon segment of the spinal motor axon: fasciculated microtubules and polyribosomal clusters

Dense undercoating, microtubular fascicles and scattered polyribosomal clusters have until now been considered to be the three structural features of the initial segment, and were thought not to extend beyond the initial segment into the myelinated parts of the axon. The aim of the present study was to make clear whether there is a sudden change in morphology between the unmyelinated and myelinated part. We followed spinal motor axons from the initial segment to the first internode by conventional electron microscopy and serial sectioning, and found that the microtubular fascicles and polyribosomal clusters do exist in the internodal axoplasm. The fasciculated microtubules were observed mainly in the first paranode. The polyribosomal clusters were found along the course of the first internode at a random distance, however, they occurred mainly in the proximal part of the first internode. The proportion of sections in which ribosomes were found, i.e. the incidence of ribosomes, in the first 30-microm-long portion was 71 +/- 24% (mean +/- SD, n = 4), and significantly different from that in the second 30-microm-long portion (3.2 +/- 1.3%) (mean +/- SD, n = 4) (P < 0.005). The more distal part of the first internode was not investigated.

Axonal and presynaptic protein synthesis: new insights into the biology of the neuron

The presence of a local mRNA translation system in axons and terminals was proposed almost 40 years ago. Over the ensuing period, an impressive body of evidence has grown to support this proposal -- yet the nerve cell body is still considered to be the only source of axonal and presynaptic proteins. To dispel this lingering neglect, we now present the wealth of recent observations bearing on this central idea, and consider their impact on our understanding of the biology of the neuron. We demonstrate that extrasomatic translation sites, which are now well recognized in dendrites, are also present in axonal and presynaptic compartments.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Protein-synthesizing machinery in the axon compartment

Contrary to the prevailing view that the axon lacks the capacity to synthesize proteins, a substantial body of evidence points to the existence of a metabolically active endogenous translational machinery. The machinery appears to be largely localized in the cortical zone of the axon, where, in vertebrate axons, it is distributed longitudinally as intermittent, discrete domains, called periaxoplasmic plaques. Studies, based on translation assays and probes of RNA transcripts in axon models such as the squid giant axon and selected vertebrate axons, provide evidence of locally synthesized proteins, most of which appear to be constituents of the slow axoplasmic transport rate groups. Metabolic and molecular biological findings are consistent with the view that the synthesis of proteins undergoing local turnover in the axonal compartment of macroneurons depends on the activity of an endogenous translational machinery. The documented presence of a metabolically active machinery in presynaptic terminals of squid photoreceptor neurons is also described. Finally, potential sources of axoplasmic RNAs comprising the machinery, which may include the ensheathing cell of the axon, as well as the cognate cell body, are also discussed.

Visualization of polyribosomes in the postsynaptic area of the squid giant synapse by electron spectroscopic imaging

Giant axons from squids (Loligo vulgaris and Loligo pealii) were studied by conventional electron microscopy and by electron spectroscopic imaging: electrons with a characteristic energy loss due to interaction with phosphorus atoms were used to image phosphorus-rich elements such as ribosomes. Patches of granular material were seen in the proximal region of giant axons, especially in the postsynaptic area of the giant synapse, but not on the presynaptic side. They were rare or absent in the more distal part of the axon. With electron spectroscopic imaging the patches emitted strong phosphorus signals, indistinguishable from signals of ribosomes in neuronal perikarya, confirming their identification as RNA-rich elements. In the part of the axon near the perikarya in the giant fibre lobe, the patches consisted of sacs of rough endoplasmic reticulum. It is suggested that patches of rough endoplasmic reticulum are transported to the synaptic region where ribosomes detach from the cisterns and aggregate into polyribosomes.

Axoplasmic RNA species synthesized in the isolated squid giant axon

Isolated squid stellate nerves and giant fiber lobes were incubated for 8 hr in Millipore filtered sea water containing [3H]uridine. The electrophoretic patterns of radioactive RNA purified from the axoplasm of the giant axon and from the giant fiber lobe (cell bodies of the giant axon) demonstrated the presence of RNA species with mobilities corresponding to tRNA and rRNA. The presence of labeled rRNAs was confirmed by the behavior of the large rRNA component (31S) which, in the squid, readily dissociates into its two constituent moyeties (17S and 20S). Comparable results were obtained with the axonal sheath and the stellate nerve. In all the electrophoretic patterns, additional species of radioactive RNA migrated between the 4S and the 20S markers, i.e. with mobilities corresponding to presumptive mRNAs. Chromatographic analysis of the purified RNAs on oligo(dT)cellulose indicated the presence of labeled poly(A)+ RNA in all tissue samples. Radioactive poly(A)+ RNA represented approximately 1% of the total labeled RNA in the axoplasm, axonal sheath and stellate nerve, but more than 2% in the giant fiber lobe. The labeled poly(A)+ RNAs of the giant fibre lobe showed a prevalence of larger species in comparison to the axonal sheath and stellate nerve. In conclusion, the axoplasmic RNAs synthesized by the isolated squid giant axon appear to include all the major classes of axoplasmic RNAs, that is rRNA, tRNA and mRNA.

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.

Axoplasmic incorporation of amino acids in a myelinated fiber exceeds that of its soma: a radioautographic study

The axoplasmic incorporation of amino acids was studied in the giant Mauthner axon to determine its magnitude, to estimate the half-life of the resulting material, and to establish whether or not it undergoes transport. The incorporation of labeled leucine, lysine, and proline was followed by radioautography. The tracers were injected away from the cell body, either in the floor of the IV ventricle or into the spinal cord. Radioautographic reaction on the Mauthner somata was barely discernible. However, at the site of injection, for survivals to 5.6 days, a peak of reaction was observed on the Mauthner axoplasm, with a sharp and symmetric decrement in both central and distal directions; the intensity of the peak decreased with survival (14% at 5.6 days). In intact fibers, the intensity of the reaction was 1.4% compared with that over the Nissl substance of neighboring somata; by contrast, in severed fibers whose axoplasm had been directly exposed to the tracers the reaction rose to 4.1%. At early times, the intensity of the response of the axoplasm as a function of survival of the fish did not have a lag time. In high-resolution radioautograms, most of the grains overlay the ground axoplasm. Cycloheximide depressed the reaction over the fiber open to the extracellular space by 55% whereas only by 20% over the intact fiber. Adsorption of free amino acids or their binding to tRNA were ruled out as a cause of the radioautographic response. Our results indicated that incorporation of amino acids into macromolecules of the Mauthner fiber occurred locally, and its magnitude over the whole axoplasm was estimated to be one order of magnitude larger than that occurring in the soma. The bulk of the material resulting from this incorporation did not undergo transport and survived some days. We suggest that the amino acids were incorporated by a ribosomal mechanism.

Local protein synthesizing activity in axonal fields regenerating in vitro

The goldfish retinal explant system of Landreth and Agranoff was used to study endogenous protein synthesizing activity of retinal ganglion cell axons regenerating in culture. Light and electron microscopic examination of axonal fields showed that axons were free of nonneural cell investment. Decentralized axons were incubated with a mixture of tritiated amino acids, and direct quantitative microanalysis of protein and tritium radioactivity was carried out on individual axonal fields. Our findings showed that radioactive amino acids were incorporated into axonal protein in a manner that inhibited significantly by cycloheximide, but not by chloramphenicol. Decentralized axons appeared to maintain their viability for at least 3-4 h. Axonal fields maintaining their central connections to the explant incorporated 3H-amino acids at an apparent rate that was similar to decentralized axonal fields. Labeled material transported into axonal fields from ganglion cell bodies appeared in significant amounts after a delay of 2-3 h. Fluorographic patterns of axonal proteins after labeling with either 3H-amino acids or [35S]methionine and separated by microelectrophoresis indicated that primarily tubulin and, to a lesser extent, actin were labeled. Our findings indicate that goldfish retinal ganglion cell axons regenerating in vitro exhibit measureable endogenous protein-synthesizing activity.

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.

Ribosomal RNA in Mauthner axon: implications for a protein synthesizing machinery in the myelinated axon

RNA was extracted from myelin-free Mauthner axons of the goldfish on a microscale and fractionated by microelectrophoresis. Microextracts showed the presence of nominal 26 SE, 18 SE, 5 SE and 4 SE components, which co-migrated with rRNA from fish brain. In addition, a non-ribosomal 15 SE component was present in axon microextracts, but not in RNA extracts of fish brain or of myelin sheath from Mauthner axon, indicating an unusual enrichment of a putative mRNA class. Evidence was presented to support the contention that axonal rRNA was not due to contamination from the myelin sheath. Possible reasons for the lack of ultrastructural evidence for axoplasmic ribosomes are discussed.

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.

Biochemical studies of trophic dependences in crayfish giant axons

Data from previous histological studies indicate that long-term survival of crayfish medial giant axons might be due in part to trophic support from cells of the surrounding glial sheath which often hypertrophy in response to transection of the medial giants. The biochemical studies reported herein show that segments from transected ventral nerve cords (VNC) always incorporate more [3H]leucine into protein than do corresponding segments from intact VNCs. Furthermore, the relative amount of [3H]leucine incorporation in severed segments seems to be influenced by distance and direction from the lesion site as well as time after lesioning. Similar spatiotemporal parameters were previously shown to be correlated with extent of glial hypertrophy around severed medial giant axons. Quantitative autoradiography of medial giant axons after incubation in [3H]leucine revealed that the grain density of label in glial sheaths surrounding severed medial giants was over two-fold greater than in sheaths around corresponding control axons. Moreover, the grain density in the axoplasm of severed medial giants was nearly four-fold greater than the grain density in the axoplasm of control axons. Data from experiments using short or long labeling intervals suggests that labeling in the medial giant axoplasm may be due more to transfer from glial sheath cells than from inherent axonal synthetic mechanisms. In light of this and other data, we concluded that long-term survival of severed medial giant axons is probably due to the direct transfer of trophic substances from cells of the glial sheath into the axon.

Protein metabolism in transected peripheral nerves of the crayfish

The significance of the protein metabolism in crayfish peripheral nerve was studied in relation the ability of crayfish motor axons to survive for over 200 days following axotomy. In contrast to frog peripheral nerves, the crayfish nerves appear to more closely resemble ganglia in their profiles of synthesis expressed on sodium dodecyl sulfate (SDS) gels, and have higher incorporation rates of [3H]leucine into protein than ganglia. Since anisomycin inhibits over 95% of protein synthesis in crayfish peripheral nerve, it was concluded that this local protein synthesis was dependent upon a eukaryotic ribosomal mechanism. Radioautography of isolated nerves reveals newly synthesized proteins in glial sheaths, and also within the axoplasm of large motor fibers. Based upon the data available at present, a hypothesis that the glia surrounding the axons are responsible for the local protein synthesis, and that some of these newly synthesized proteins are transported into the axon, is presented. Transection of crayfish peripheral nerves proximal to the neuron cell bodies produced a more than two-fold increase in [3H]leucine incorporation, but no significant changes in labeling profiles of the proteins on SDS gels. The data suggest that while an active local protein synthesis may be necessary for the maintenance of several crayfish motor axons, it is not a sufficient condition.

Quantitative autoradiographic study of labeled RNA in rabbit optic nerve after intraocular injection of (3H)uridine

The distribution of labeled RNA in the optic nerve of the rabbit was studied by quantitative ultrastructural autoradiography after the intraocular injection of [(3)H]uridine. The highest density of silver grains related to [(3)H]RNA (27-40 grains/100 microm(2)) was found in glial cell perikarya; a slightly lower density was present in the glial nuclei (19-20 grains/100 microm(2)). Axons (4-5 grains/100 microm(2)) and myelin (2-3 grains/100 microm(2)) had the lowest grain densities. 74-83% of all counted grains were located outside the axons. By comparing the grain density distribution over the axon with that expected in the case of an exclusive labeling of the surrounding myelin and glial cell processes, it was concluded that the axons contained a number of grains representing [(3)H]RNA significantly higher than that expected to scatter from myelin and glial processes. Most of these grains were concentrated at the periphery of the axon and were not related to axonal mitochondria.