Analysis of axoplasmic RNA from invertebrate giant axons

Cryo-ET suggests tubulin chaperones form a subset of microtubule lumenal particles with a role in maintaining neuronal microtubules

The functional architecture of the long-lived neuronal microtubule (MT) cytoskeleton is maintained by various MT-associated proteins (MAPs), most of which are known to bind to the MT outer surface. However, electron microscopy (EM) has long ago revealed the presence of particles inside the lumens of neuronal MTs, of yet unknown identity and function. Here, we use cryogenic electron tomography (cryo-ET) to analyze the three-dimensional (3D) organization and structures of MT lumenal particles in primary hippocampal neurons, human induced pluripotent stem cell-derived neurons, and pluripotent and differentiated P19 cells. We obtain in situ density maps of several lumenal particles from the respective cells and detect common structural features underscoring their potential overarching functions. Mass spectrometry-based proteomics combined with structural modeling suggest that a subset of lumenal particles could be tubulin-binding cofactors (TBCs) bound to tubulin monomers. A different subset of smaller particles, which remains unidentified, exhibits densities that bridge across the MT protofilaments. We show that increased lumenal particle concentration within MTs is concomitant with neuronal differentiation and correlates with higher MT curvatures. Enrichment of lumenal particles around MT lattice defects and at freshly polymerized MT open-ends suggests a MT protective role. Together with the identified structural resemblance of a subset of particles to TBCs, these results hint at a role in local tubulin proteostasis for the maintenance of long-lived neuronal MTs.

The Trail of Axonal Protein Synthesis: Origins and Current Functional Landscapes

Local protein synthesis (LPS) in axons is now recognized as a physiological process, participating both in the maintenance of axonal function and diverse plastic phenomena. In the last decades of the 20th century, the existence and function of axonal LPS were topics of significant debate. Very early, axonal LPS was thought not to occur at all and was later accepted to play roles only during development or in response to specific conditions. However, compelling evidence supports its essential and pervasive role in axonal function in the mature nervous system. Remarkably, in the last five decades, Uruguayan neuroscientists have contributed significantly to demonstrating axonal LPS by studying motor and sensory axons of the peripheral nervous system of mammals, as well as giant axons of the squid and the Mauthner cell of fish. For LPS to occur, a highly regulated transport system must deliver the necessary macromolecules, such as mRNAs and ribosomes. This review discusses key findings related to the localization and abundance of axonal mRNAs and their translation levels, both in basal states and in response to physiological processes, such as learning and memory consolidation, as well as neurodevelopmental and neurodegenerative disorders, including Alzheimer's disease, autism spectrum disorder, and axonal injury. Moreover, we discuss the current understanding of axonal ribosomes, from their localization to the potential roles of locally translated ribosomal proteins, in the context of emerging research that highlights the regulatory roles of the ribosome in translation. Lastly, we address the main challenges and open questions for future studies.

Arginylation in a Partially Purified Fraction of 150 k xg Supernatants of Axoplasm and Injured Vertebrate Nerves

Transfer RNA-mediated posttranslational protein modification by arginine has been demonstrated in vitro in axoplasm extruded from the giant axons of squid and in injured and regenerating vertebrate nerves. In nerve and axoplasm, the highest activity is found in a fraction of a 150,000 g supernatant containing high molecular weight protein/RNA complexes but lacking molecules of <5 kDa. Arginylation (and protein modification by other amino acids) is not found in more purified, reconstituted fractions. The data are interpreted as indicating that it is critical to recover the reaction components in high molecular weight protein/RNA complexes in order to maintain maximum physiological activity. The level of arginylation is greatest in injured and growing vertebrate nerves compared with intact nerves, suggesting a role for these reactions in nerve injury/repair and during axonal growth.

Functional Genomics of Axons and Synapses to Understand Neurodegenerative Diseases

Functional genomics studies through transcriptomics, translatomics and proteomics have become increasingly important tools to understand the molecular basis of biological systems in the last decade. In most cases, when these approaches are applied to the nervous system, they are centered in cell bodies or somatodendritic compartments, as these are easier to isolate and, at least in vitro, contain most of the mRNA and proteins present in all neuronal compartments. However, key functional processes and many neuronal disorders are initiated by changes occurring far away from cell bodies, particularly in axons (axopathologies) and synapses (synaptopathies). Both neuronal compartments contain specific RNAs and proteins, which are known to vary depending on their anatomical distribution, developmental stage and function, and thus form the complex network of molecular pathways required for neuron connectivity. Modifications in these components due to metabolic, environmental, and/or genetic issues could trigger or exacerbate a neuronal disease. For this reason, detailed profiling and functional understanding of the precise changes in these compartments may thus yield new insights into the still intractable molecular basis of most neuronal disorders. In the case of synaptic dysfunctions or synaptopathies, they contribute to dozens of diseases in the human brain including neurodevelopmental (i.e., autism, Down syndrome, and epilepsy) as well as neurodegenerative disorders (i.e., Alzheimer's and Parkinson's diseases). Histological, biochemical, cellular, and general molecular biology techniques have been key in understanding these pathologies. Now, the growing number of omics approaches can add significant extra information at a high and wide resolution level and, used effectively, can lead to novel and insightful interpretations of the biological processes at play. This review describes current approaches that use transcriptomics, translatomics and proteomic related methods to analyze the axon and presynaptic elements, focusing on the relationship that axon and synapses have with neurodegenerative diseases.

Molecular mechanisms behind mRNA localization in axons

Messenger RNA (mRNA) localization allows spatiotemporal regulation of the proteome at the subcellular level. This is observed in the axons of neurons, where mRNA localization is involved in regulating neuronal development and function by orchestrating rapid adaptive responses to extracellular cues and the maintenance of axonal homeostasis through local translation. Here, we provide an overview of the key findings that have broadened our knowledge regarding how specific mRNAs are trafficked and localize to axons. In particular, we review transcriptomic studies investigating mRNA content in axons and the molecular principles underpinning how these mRNAs arrived there, including cis-acting mRNA sequences and trans-acting proteins playing a role. Further, we discuss evidence that links defective axonal mRNA localization and pathological outcomes.

Local protein synthesis is a ubiquitous feature of neuronal pre- and postsynaptic compartments

There is ample evidence for localization of messenger RNAs (mRNAs) and protein synthesis in neuronal dendrites; however, demonstrations of these processes in presynaptic terminals are limited. We used expansion microscopy to resolve pre- and postsynaptic compartments in rodent neurons. Most presynaptic terminals in the hippocampus and forebrain contained mRNA and ribosomes. We sorted fluorescently labeled mouse brain synaptosomes and then sequenced hundreds of mRNA species present within excitatory boutons. After brief metabolic labeling, >30% of all presynaptic terminals exhibited a signal, providing evidence for ongoing protein synthesis. We tested different classic plasticity paradigms and observed distinct patterns of rapid pre- and/or postsynaptic translation. Thus, presynaptic terminals are translationally competent, and local protein synthesis is differentially recruited to drive compartment-specific phenotypes that underlie different forms of plasticity.

Accumulating Evidence for Axonal Translation in Neuronal Homeostasis

The specialized structure of the neuron requires that homeostasis is sustained over the meter or more that may separate a cell body from its axonal terminus. Given this impressive distance and an axonal volume that is many times that of the cell body, how is such a compartment grown during development, re-grown after injury, and maintained throughout adulthood? While early answers to these questions focused on the local environment or the cell soma as supplying the needs of the axon, it is now well-established that the axon has some unique needs that can only be met from within. Decades of research have revealed local translation as an indispensable mechanism of axonal homeostasis during development and regeneration in both invertebrates and vertebrates. In contrast, the extent to which the adult, mammalian axonal proteome is maintained through local translation remains unclear and controversial. This mini-review aims to highlight important experiments that have helped to shape the field of axonal translation, to discuss conceptual arguments and recent evidence that supports local translation as important to the maintenance of adult axons, and to suggest experimental approaches that have the potential to further illuminate the role of axonal translation in neuronal homeostasis.

Tctp in Neuronal Circuitry Assembly

Although tctp expression in many areas of the human brain was reported more than 15 years ago, little was known about how it functions in neurons. The early notion that Tctp is primarily expressed in mitotic cells, together with reports suggesting a relative low abundance in the brain, has perhaps potentiated this almost complete disregard for the study of Tctp in the context of neuron biology. However, recent evidence has challenged this view, as a number of independent genome-wide profiling studies identified tctp mRNA among the most enriched in the axonal compartment across diverse neuronal populations, including embryonic retinal ganglion cells. Considering the emerging parallels between axon guidance and cancer cell invasion, the axonal expression of cancer-associated tctp was suggestive of it holding an unexplored role in the wiring of neuronal circuits. Our study revealed that Tctp is necessary for the accurate and timely development of axon projections during the formation of vertebrate retinal circuits via its association with the survival machinery of the axon. Globally, the findings indicate that compromised pro-survival signaling in Tctp-deficient axons results in mitochondrial dysfunction and a subsequent decrease in axonal mitochondrial density. These effects likely translate into a metabolic state inadequate to support the normal guidance and extension processes of a developing axon.

Degeneration, Trophic Interactions, and Repair of Severed Axons: A Reconsideration of Some Common Assumptions

We suggest that several interrelated properties of severed axons (degeneration, trophic dependencies, initial repair, and eventual repair) differ in important ways from commonly held assumptions about those properties. Specifically, (1) axotomy does not necessarily produce rapid degeneration of distal axonal segments because (2) the trophic maintenance of nerve axons does not necessarily depend entirely on proteins transported from the perikaryon—but instead axonal proteins can be trophically maintained by slowing their degradation and/or by acquiring new proteins via axonal synthesis or transfer from adjacent cells (e.g., glia). (3) The initial repair of severed distal or proximal segments occurs by barriers (seals) formed amid accumulations of vesicles and/or myelin delaminations induced by calcium influx at cut axonal ends—rather than by collapse and fusion of cut axolemmal leaflets. (4) The eventual repair of severed mammalian CNS axons does not necessarily have to occur by neuritic outgrowths, which slowly extend from cut proximal ends to possibly reestablish lost functions weeks to years after axotomy—but instead complete repair can be induced within minutes by polyethylene glycol to rejoin (fuse) the cut ends of surviving proximal and distal stumps. Strategies to repair CNS lesions based on fusion techniques combined with rehabilitative training and induced axonal outgrowth may soon provide therapies that can at least partially restore lost CNS functions.

Axonal maintenance, glia, exosomes, and heat shock proteins

Of all cellular specializations, the axon is especially distinctive because it is a narrow cylinder of specialized cytoplasm called axoplasm with a length that may be orders of magnitude greater than the diameter of the cell body from which it originates. Thus, the volume of axoplasm can be much greater than the cytoplasm in the cell body. This fact raises a logistical problem with regard to axonal maintenance. Many of the components of axoplasm, such as soluble proteins and cytoskeleton, are slowly transported, taking weeks to months to travel the length of axons longer than a few millimeters after being synthesized in the cell body. Furthermore, this slow rate of supply suggests that the axon itself might not have the capacity to respond fast enough to compensate for damage to transported macromolecules. Such damage is likely in view of the mechanical fragility of an axon, especially those innervating the limbs, as rapid limb motion with high impact, like running, subjects the axons in the limbs to considerable mechanical force. Some researchers have suggested that local, intra-axonal protein synthesis is the answer to this problem. However, the translational state of axonal RNAs remains controversial. We suggest that glial cells, which envelop all axons, whether myelinated or not, are the local sources of replacement and repair macromolecules for long axons. The plausibility of this hypothesis is reinforced by reviewing several decades of work on glia-axon macromolecular transfer, together with recent investigations of exosomes and other extracellular vesicles, as vehicles for the transmission of membrane and cytoplasmic components from one cell to another.

Arginylation in a Partially Purified Fraction of 150k × g Supernatants of Axoplasm and Injured Vertebrate Nerves

Transfer RNA-mediated posttranslational protein modification by arginine has been demonstrated in vitro in axoplasm extruded from the giant axons of squid and in injured and regenerating vertebrate nerves. In nerve and axoplasm, the highest activity is found in a fraction of a 150,000 × g supernatant containing high molecular weight protein/RNA complexes but lacking molecules of <5 kDa. Arginylation (and protein modification by other amino acids) is not found in more purified, reconstituted fractions. The data are interpreted as indicating that it is critical to recover the reaction components in high molecular weight protein/RNA complexes in order to maintain maximum physiological activity. The level of arginylation is greatest in injured and growing vertebrate nerves compared with intact nerves, suggesting a role for these reactions in nerve injury/repair and during axonal growth.

Zebrafish embryonic neurons transport messenger RNA to axons and growth cones in vivo

Although mRNA was once thought to be excluded from the axonal compartment, the existence of protein synthesis in growing or regenerating axons in culture is now generally accepted. However, its extent and functional importance remain a subject of intense investigation. Furthermore, unambiguous evidence of mRNA axonal transport and local translation in vivo, in the context of a whole developing organism is still lacking. Here, we provide direct evidence of the presence of mRNAs of the tubb5, nefma, and stmnb2 genes in several types of axons in the developing zebrafish (Danio rerio) embryo, with frequent accumulation at the growth cone. We further show that axonal localization of mRNA is a specific property of a subset of genes, as mRNAs of the huc and neurod genes, abundantly expressed in neurons, were not found in axons. We set up a reporter system in which the 3' untranslated region (UTR) of candidate mRNA, fused to a fluorescent protein coding sequence, was expressed in isolated neurons of the zebrafish embryo. Using this reporter, we identified in the 3'UTR of tubb5 mRNA a motif necessary and sufficient for axonal localization. Our work thus establishes the zebrafish as a model system to study axonal transport in a whole developing vertebrate organism, provides an experimental frame to assay this transport in vivo and to study its mechanisms, and identifies a new zipcode involved in axonal mRNA localization.

beta-Actin and beta-Tubulin are components of a heterogeneous mRNA population present in the squid giant axon

Previously, we have reported that the squid giant axon contains a heterogeneous population of polyadenylated mRNAs, as well as biologically active polyribosomes. To define the composition of this unique mRNA population, cDNA libraries were constructed to RNA obtained from the axoplasm of the squid giant axon and the parental cell bodies located in the giant fiber lobe. Here, we report that the giant axon contains mRNAs encoding beta-actin and beta-tubulin. The axonal location of these mRNA species was confirmed by in situ hybridization histochemistry, and their presence in the axoplasmic polyribosome fraction was demonstrated by polymerase chain reaction methodology. Taken together, these findings establish the identity of two relatively abundant members of the axonal mRNA population and suggest that key elements of the cytoskeleton are synthesized de novo in the squid giant axon.

Axonal mRNA localization and local protein synthesis in nervous system assembly, maintenance and repair

mRNAs can be targeted to specific neuronal subcellular domains, which enables rapid changes in the local proteome through local translation. This mRNA-based mechanism links extrinsic signals to spatially restricted cellular responses and can mediate stimulus-driven adaptive responses such as dendritic plasticity. Local mRNA translation also occurs in growing axons where it can mediate directional responses to guidance signals. Recent profiling studies have revealed that both growing and mature axons possess surprisingly complex and dynamic transcriptomes, thereby suggesting that axonal mRNA localization is highly regulated and has a role in a broad range of processes, a view that is increasingly being supported by new experimental evidence. Here, we review current knowledge on the roles and regulatory mechanisms of axonal mRNA translation and discuss emerging links to axon guidance, survival, regeneration and neurological disorders.

Local translation of mRNAs in neural development

Growing axons encounter numerous developmental signals to which they must promptly respond in order to properly form complex neural circuitry. In the axons, these signals are often transduced into a local increase or decrease in protein levels. Contrary to the traditional view that the cell bodies are the exclusive source of axonal proteins, it is becoming increasingly clear not only that de novo protein synthesis takes place in axons, but also that it is required for the axons to respond to certain signals. Here we review the current knowledge of local mRNA translation in developing neurons with a special focus on protein synthesis occurring in axons and growth cones.

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.

Axonal mRNA transport and localized translational regulation of kappa-opioid receptor in primary neurons of dorsal root ganglia

kappa-opioid receptor (KOR) is detected pre- and postsynaptically, but the subcellular localization, translation, and regulation of kor mRNA in presynaptic compartments of sensory neurons remain elusive. In situ hybridization detected axonal distribution of kor mRNA in primary neurons of dorsal root ganglia (DRG). The MS2-fused GFP tracked kor mRNA transport from DRG neuronal soma to axons, requiring its 5' and 3' UTRs. In Campenot chambers, axonal translation of kor mRNA was demonstrated for DRG neurons, which depended on its 5' UTR and was stimulated by KCl depolarization. KCl depolarization of DRG neurons rendered redistribution of kor mRNA from the postpolysomal fraction to the translationally active polysomal fraction. This study provided evidence for mRNA transport and regulation of presynaptic protein synthesis of nonstructural proteins like KOR in primary sensory neurons and demonstrated a mechanism of KCl depolarization-stimulated axonal mRNA redistribution for localized translational regulation.

RNA translation in axons

The cell body has classically been considered the exclusive source of axonal proteins. However, significant evidence has accumulated recently to support the view that protein synthesis can occur in axons themselves, remote from the cell body. Indeed, local translation in axons may be integral to aspects of synaptogenesis, long-term facilitation, and memory storage in invertebrate axons, and for growth cone navigation in response to environmental stimuli in developing vertebrate axons. Here we review the evidence supporting mRNA translation in axons and discuss the potential roles that local protein synthesis may play during development and subsequent neuronal function. We advance the view that local translation provides a rapid supply of nascent proteins in restricted axonal compartments that can potentially underlie long-term responses to transient stimuli.

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.

What is the signal for the posttranslational arginylation of proteins?

The N-terminal, posttranslational arginylation of proteins is ubiquitous in eukaryotic cells. Previous experiments, using purified components of the reaction incubated in the presence of exogenous substrates, have shown that only those proteins containing acidic residues at their N-terminals are arginylation substrates. However, data from experiments that used crude extracts of brain and nerve as the source of the arginylating molecules, suggest that the in vivo targets for arginylation are more complex than those demonstrated using purified components. One of the proposed functions for arginylation is as a signal for protein degradation and proteins that have undergone oxidative damage have been shown to be rapidly degraded. In the present experiments we have tested the hypothesis that the presence of an oxidatively damaged residue in a protein is a signal for its arginylation. These experiments have been performed by adding synthetic oxidized peptides to crude extracts of rat brain, incubating them with [3H]Arg and ATP and assaying for arginylated peptides using RP-HPLC. Results showed that while the oxidized A-chain of insulin was arginylated in this system, confirming previous experiments, other peptides containing oxidized residues were not. When a peptide containing Glu in the N-terminus was incubated under the same conditions it too was not a substrate for arginylation. These findings show that neither the presence of an N-terminal acidic residue nor an oxidized residue alone are sufficient to signal arginylation. Thus, another feature of the oxidized A-chain of insulin is required for arginylation. That feature remains to be identified.

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.

Synthesis of beta-tubulin, actin, and other proteins in axons of sympathetic neurons in compartmented cultures

The proteins needed for growth and maintenance of the axon are generally believed to be synthesized in the cell bodies and delivered to the axons by anterograde transport. However, recent reports suggest that some proteins can also be synthesized within axons. We used [35S]methionine metabolic labeling to investigate axonal protein synthesis in compartmented cultures of sympathetic neurons from newborn rats. Incubation of distal axons for 4 hr with [35S]methionine resulted in a highly specific pattern of labeled axonal proteins on SDS-PAGE, with 4 prominent bands in the 43-55 kDa range. The labeled proteins in axons were not synthesized in the cell bodies, because they were also produced by axons after the cell bodies had been removed. Two of the proteins were identified by immunoprecipitation as actin and beta-tubulin. Axons synthesized <1% of the actin and tubulin synthesized in the cell bodies and transported into the axons, and 75-85% inhibition of axonal protein synthesis by cycloheximide and puromycin failed to inhibit axonal elongation. Nonetheless, the specific production by axons of the major proteins of the axonal cytoskeleton suggests that axonal protein synthesis arises from specific mechanisms and likely has biological significance. One hypothetical scenario involves neurons with long axons in vivo in which losses from turnover during axonal transport may limit the availability of cell body synthesized proteins to the distal axons. In this case, a significant fraction of axonal proteins might be supplied by axonal synthesis, which could, therefore, play important roles in axonal maintenance, regeneration, and sprouting.

Axonal localization of neuropeptide-encoding mRNA in identified neurons of the snail Lymnaea stagnalis

mRNA transcripts encoding neuropeptides were detected, by means of in situ hybridization, in the axonal compartments of different types of identified neurons in the central nervous system of the pond snail Lymnaea stagnalis. All cell types studied contained axonal mRNA although the relative intensities of the hybridization signals (i.e., the intensity of the signal over the cell body versus that over the axonal compartment of a particular cell) varied greatly between the different cell types studied. Strong signals over the axonal compartment were obtained with an oligonucleotide probe specific for the molluscan insulin-related peptide gene III mRNA, whereas low signals were obtained, e.g., with a probe for the mRNA encoding the neuropeptide APG-Wamide. Furthermore, some neurons are known to express more than one neuropeptide gene, e.g., the molluscan insulin-related peptide-producing light green cells and the egg-laying hormone-producing caudo-dorsal cells; these cell types express 4 and 2 related neuropeptide genes, respectively. The results may indicate that the different neuropeptide transcripts within a neuron are transported selectively to the axonal compartment.

N-terminal arginylation and ubiquitin-mediated proteolysis in nerve regeneration

Damaged sciatic nerves of rats respond to injury within minutes by activating reactions that result in the transfer RNA-mediated posttranslational addition of several amino acids to a variety of cytoplasmic proteins. For the most part, the site of addition of individual amino acids and the identity of the target proteins is not known. However, arginine, one of the amino acids added in greatest amounts, has been shown to be covalently linked to the N-terminus of acceptor proteins. In other simpler eukaryotic cells, N-terminal arginylation results in degradation of the arginylated proteins via the ubiquitin proteolytic pathway. Recent experiments have shown that when proteins, obtained from sciatic nerves 2 h after injury, are arginylated in vitro, they form high molecular weight aggregates. Other experiments have shown that these arginylated proteins are immunoreactive to a monoclonal antibody to ubiquitin. These findings suggest that following injury to the sciatic nerve, proteins which are arginylated are candidates for ubiquitin mediated proteolysis. Injury to a nerve incapable of regeneration without experimental intervention, the rat optic nerve, does not result in activation of the arginylation reactions until 6 days following injury. Based on the temporal differences in response to injury of sciatic and optic nerves (2 h vs. 6 days), we propose that the lack of arginylation following injury to the CNS is related to its inability to mount a regenerative response. The association of Arg modification of damaged proteins with the ubiquitin-mediated degradation of those proteins, suggests that regenerative failure in the CNS may be related, in part, to a failure to degrade intracellular proteins at the site of injury.

Evidence that axonal tRNAs are resistant to RNase and ATPase and can be aminoacylated in the absence of exogenous ATP

A high molecular weight (HMW) fraction of the 150,000 g supernatant of rat brain homogenates contains protein-tRNA complexes which are able to incorporate [3H]Arg and [3H]Lys into tRNA. The aminoacylation of tRNA(Arg) was found to be dependent on ATP and inhibited by RNase. Conversely, the aminoacylation of tRNA(Lys) did not require exogenous ATP and was resistant to RNase and ATPase. In HMW fractions of regenerating rat sciatic nerves, the charging of both tRNA(Arg) and tRNA(Lys) was resistant to RNase and ATPase and did not require exogenous ATP. Because sciatic nerves are rich in axoplasm and tRNAs are known to be present in axons, we tested the hypothesis that degradative enzyme-resistant, ATP-tRNA complexes were of axonal origin. In HMW fractions from rat liver (containing no axons), both tRNA(Arg) and tRNA(Lys) were sensitive to RNase and required exogenous ATP for charging. But, in similar fractions of axoplasm obtained from the giant axon of squid, both tRNAs were insensitive to RNase and ATPase and did not require exogenous ATP for charging. These results suggest that tRNAs in axons are present in protected HMW complexes and contain endogenous stores of ATP. The presence of ATP in the HMW complexes was demonstrated by the luciferase-luciferin assay for ATP. The nature of the protection of tRNAs from RNases was examined by dissociating proteins from HMW complexes by boiling, treating with proteinase K, or overhomogenizing the tissue. These procedures failed to render brain tRNA(Lys) susceptible to RNase. But phenol-extracted, ethanol-precipitated brain tRNA(Lys) was sensitive to RNase, suggesting that the protection of tRNA(Lys) may be by a protease- and heat-resistant polypeptide or by a nonproteinaceous mechanism.

Expression of NF-L in both neuronal and nonneuronal cells of transgenic mice: increased neurofilament density in axons without affecting caliber

We have generated transgenic mice containing additional copies of the murine NF-L gene in order to examine the consequences of neurofilament-L overexpression on axonal morphology. Founder mice were constructed to carry a transgene in which the presumptive 5' promoter sequences of NF-L were replaced with the strong murine sarcoma virus long terminal repeat promoter. The transgenes were expressed prominently in several tissues, including skeletal muscle and kidney where NF-L accumulated to approximately 2% of cell protein. This was not accompanied by an overt phenotype, except that expression in lens led to cataract formation. In the brains of these animals, transgene RNA levels exceeded the endogenous NF-L RNAs by up to 20-fold, although no additional protein accumulated, indicating posttranscriptional regulation of NF-L expression. However, in peripheral neurons transgene RNA was approximately fourfold higher than endogenous NF-L mRNAs, and a corresponding increase in NF-L subunits was found in axons arising from these neurons. Myelinated nerve fibers of transgenic animals contained increased numbers of NFs, assembled predominantly of NF-L. This was reflected in an increase in the density of axonal NFs; axonal caliber was not affected.

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.

Neurofilament gene expression: a major determinant of axonal caliber

Within the wide spectrum of axonal diameters occurring in mammalian nerve fibers, each class of neurons has a relatively restricted range of axonal calibers. The control of caliber has functional significance because diameter is the principal determinant of conduction velocity in myelinated nerve fibers. Previous observations support the hypothesis that neurofilaments (NF) are major intrinsic determinants of axonal caliber in large myelinated nerve fibers. Following interruption of axons (axotomy) by crushing or cutting a peripheral nerve, caliber is reduced in the proximal axonal stumps, which extend from the cell bodies to the site of axotomy. (The distal axonal stumps, which are disconnected from the cell bodies, degenerate and are replaced by the outgrowth of regenerating axonal sprouts arising from the proximal stump). This reduction in axonal caliber in the proximal stumps is associated with a selective diminution in the amount of NF protein undergoing slow axonal transport in these axons, with a decrease in axonal NF content, and with reduced conduction velocity. The present report demonstrates that changes in axonal caliber after axotomy correlate with a selective alteration in NF gene expression. Hybridization with specific cDNAs was used to measure levels of mRNA encoding the 68-kDa neurofilament protein (NF68), beta-tubulin, and actin in lumbar sensory neurons of rat at various times after crushing the sciatic nerve. Between 4 and 42 days after axotomy by nerve crush, the levels of NF68 mRNA were reduced 2- to 3-fold. At the same times, the levels of tubulin and actin mRNAs were increased several-fold. These findings support the hypothesis that the expression of a single set of neuron-specific genes (encoding NF) directly determines axonal caliber, a feature of neuronal morphology with important consequences for physiology and behavior.

Alterations in mRNA translation products associated with regenerative responses in the retina

Translation products of mRNA from retinas of goldfish optic nerve (representing a regenerative CNS) and adult rabbit optic nerve (representing a nonregenerative CNS which can be induced to express regenerative characteristics) were examined by one- and two-dimensional gel electrophoresis. Translation products from retinas of the regenerating goldfish optic nerve included polypeptides barely detectable in the translation products of mRNA derived from retinas of uninjured controls. Some of these polypeptides, of apparent molecular weights 24-28, 43-49, 60, and 65 kilodaltons can be considered as growth-associated polypeptides described in other regenerative and developing systems. The induction of regeneration-associated characteristics in the injured adult rabbit optic nerve, "implanted" with diffusible substances from nonneuronal cells of regenerative or growing nerve, is reflected by changes in the mRNA translation products of the retina. Among such translation products are those of the following molecular weights: 16-18, 28, 32-35, 43-47, and 56-60 kilodaltons, and some higher-molecular-weight species.

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.

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.

Axonal transport of 4S RNA in the chick optic system

The axonal transport of tRNA has been investigated in the chick optic system. Chicks were injected with [3H]uridine intraocularly or intracranially and the RNA of the retina, nerve complex, and tecta separated by polyacrylamide gel electrophoresis and then counted. The ratio of tRNA to rRNA specific activities increased with time in both the nerve complex and contralateral tectum. The ratio increased more rapidly in the nerve complex than the tectum. However, no increase was observed in the case of intracranially injected animals. This is consistent with the axonal flow of tRNA. When [methyl-3H]methionine was used as precursor, the preferential labeling of 4S RNA to rRNA which resulted more clearly showed a transport of 4S RNA from the retinal cells to the tectum. In conclusion, it was found that about 40% of the radioactive RNA observed within the optic tectum 4 days after an intraocular injection of [3H]uridine was accounted for by 4S RNA which has flowed from the retina. However, the migration of a methylated RNA molecule of size 4S, but unrelated to rRNA, cannot be entirely eliminated.

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.

Cell-to-cell transfer of glial proteins to the squid giant axon. The glia-neuron protein trnasfer hypothesis

The hypothesis that glial cells synthesize proteins which are transferred to adjacent neurons was evaluated in the giant fiber of the squid (Loligo pealei). When giant fibers are separated from their neuron cell bodies and incubated in the presence of radioactive amino acids, labeled proteins appear in the glial cells and axoplasm. Labeled axonal proteins were detected by three methods: extrusion of the axoplasm from the giant fiber, autoradiography, and perfusion of the giant fiber. This protein synthesis is completely inhibited by puromycin but is not affected by chloramphenicol. The following evidence indicates that the labeled axonal proteins are not synthesized within the axon itself. (a) The axon does not contain a significant amount of ribosomes or ribosomal RNA. (b) Isolated axoplasm did not incorporate [(3)H]leucine into proteins. (c) Injection of Rnase into the giant axon did not reduce the appearance of newly synthesized proteins in the axoplasm of the giant fiber. These findings, coupled with other evidence, have led us to conclude that the adaxonal glial cells synthesize a class of proteins which are transferred to the giant axon. Analysis of the kinetics of this phenomenon indicates that some proteins are transferred to the axon within minutes of their synthesis in the glial cells. One or more of the steps in the transfer process appear to involve Ca++, since replacement of extracellular Ca++ by either Mg++ or Co++ significantly reduces the appearance of labeled proteins in the axon. A substantial fraction of newly synthesized glial proteins, possibly as much as 40 percent, are transferred to the giant axon. These proteins are heterogeneous and range in size from 12,000 to greater than 200,000 daltons. Comparisons of the amount of amino acid incorporation in glia cells and neuron cell bodies raise the possibility that the adaxonal glial cells may provide an important source of axonal proteins which is supplemental to that provided by axonal transport from the cell body. These findings are discussed with reference to a possible trophic effect of glia on neurons and metabolic cooperation between adaxonal glia and the axon.

The presence of transfer RNA in the axoplasm of the squid giant axon

Previous work has revealed that 4S RNA is the primary species of RNA in the axoplasm from the giant axons of the squid and Myxicola. This study shows that axoplasmic 4S RNA from the squid giant axon has the functional properties of tRNA. Axoplasmic RNA was charged with amino acids by aminoacyl-tRNA synthetases prepared from squid brain. Tthe aminoacylation was prevented by incubating the RNA with RNase prior to running the reaction. The amino acid-RNA complex was labile at pH 9, which is characteristic of the acyl linkage between an amino acid and its tRNA. Aminoacyl-tRNA synthetase activity was also present in the axoplasm, primarily in the soluble fraction.

Survival of functional synapses on crustacean neurons lacking cell bodies

Previous workers have demonstrated that some crustacean neurons remain capable of spike propagation and transmitter release and replenishment for months after removal of their perikarya. Here, it is shown that postsynaptic reactions to chemical synaptic input can also persist for months after removal of the soma of the postsynaptic neuron. Interneuron A of the crayfish abdominal cord receives chemically transmitting terminals of ipsilateral tactile afferents of the tail fan. The neuron's soma lies contralateral to its axon and dendrites at the caudal margin of the last abdominal ganglion. The region containing the soma was removed. Interneuron A unambiguously identified by receptive field, location, and size, survived and continued to respond sensitively to tactile input in better than 50% of the cases examined for more than 8 weeks. Cobalt filling of the active fiber in several 8-week-old preparations ruled out the possibility that the severed neurite had reconnected with a foreign soma.

The action of cycloheximide on the action potential and protein synthesis in medullated Xenopus axons

Cycloheximide depresses maximum rate of change in membrane potential observed during the rising phase of the action potential in single medullated axons of Xenopus. To,e course of depression is independent of cycloheximide concentration over a range that almost completely inhibits leucine incorporation into axonal proteins.

Transfer RNA may be axonally transported during regeneration of goldfish optic nerves

If [3H]uridine is injected into the eyes of goldfish during optic nerve regeneration, then the return of fibers to the optic tectum is accompanied by the appearance of [3H]RNA in the tectum. The amount of [3H]RNA arriving in the tectum is consistently greater than in non-regenerating controls and reaches maximum levels (more than 10 times controls) 24 days after optic nerve crush. When [14C]uridine is injected subarachnoidally 1 day prior to sacrificing, the amount of [14C]RNA in the tectum is approximately doubled throughout the regeneration period. In order to characterize the radioactive tectal RNA in these experiments, we have crushed the optic nerves of 15 fish, and 18 days later injected [3H]uridine into both eyes. Five days later [14C]uridine was injected subarachnoidally and all fish were sacrificed a day later. RNA was extracted and fractionated in 2.0% polyacrylamide gels. The amounts of 3H- and 14C-labeled ribosomal as well as small molecular weight RNAs were increased during regeneration. Analysis of the area under the 28S, 18S and 4-7S RNA peaks indicated a small increase in 14C radioactivity in each peak (1.2, 1.5, and 1.5 times control, respectively). On the other hand, 3H radioactivity showed the greatest increase in the 4-7S fraction (8.0 times control) whereas large molecular weight ribosomal fractions were approximately 3 times control. Electrophoresis of the RNA on 10% polyacrylamide gels demonstrated that all of the small molecular weight RNA was confined to the 4S (tRNA) peak. These results suggest that when optic nerves of goldfish regenerate, they may enter the tectum carrying 4S (transfer) RNA.

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.