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

Biological Roles of Local Protein Synthesis in Axons: A Journey of Discovery

The remit of this review is to give an autobiographical account of our discovery of the role of local protein synthesis in axon guidance. The paper reporting our initial findings was published in 2001. Here, I describe some of the work that led to this publication, the skepticism our findings initially received, and the subsequent exciting years of follow-up work that helped gradually to convince the neuroscience community of the existence and functional importance of local protein synthesis in multiple aspects of axon biology-guidance, branching, synaptogenesis, and maintenance. The journey has been an exhilarating one, taking me into a new field of RNA biology, with many unexpected twists and turns. In retelling it here, I have tried to recall the major influences on my thinking at the time rather than give a comprehensive review, and I apologize for any omissions due to my own ignorance during that era.

European Society of Toxicologic Pathology (Pathology 2.0 Molecular Pathology Special Interest Group): Review of In Situ Hybridization Techniques for Drug Research and Development

In situ hybridization (ISH) is used for the localization of specific nucleic acid sequences in cells or tissues by complementary binding of a nucleotide probe to a specific target nucleic acid sequence. In the last years, the specificity and sensitivity of ISH assays were improved by innovative techniques like synthetic nucleic acids and tandem oligonucleotide probes combined with signal amplification methods like branched DNA, hybridization chain reaction and tyramide signal amplification. These improvements increased the application spectrum for ISH on formalin-fixed paraffin-embedded tissues. ISH is a powerful tool to investigate DNA, mRNA transcripts, regulatory noncoding RNA, and therapeutic oligonucleotides. ISH can be used to obtain spatial information of a cell type, subcellular localization, or expression levels of targets. Since immunohistochemistry and ISH share similar workflows, their combination can address simultaneous transcriptomics and proteomics questions. The goal of this review paper is to revisit the current state of the scientific approaches in ISH and its application in drug research and development.

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.

Presynaptic protein synthesis and brain plasticity: From physiology to neuropathology

To form and maintain extremely intricate and functional neural circuitry, mammalian neurons are typically endowed with highly arborized dendrites and a long axon. The synapses that link neurons to neurons or to other cells are numerous and often too remote for the cell body to make and deliver new proteins to the right place in time. Moreover, synapses undergo continuous activity-dependent changes in their number and strength, establishing the basis of neural plasticity. The innate dilemma is then how a highly complex neuron provides new proteins for its cytoplasmic periphery and individual synapses to support synaptic plasticity. Here, we review a growing body of evidence that local protein synthesis in discrete sites of the axon and presynaptic terminals plays crucial roles in synaptic plasticity, and that deregulation of this local translation system is implicated in various pathologies of the nervous system.

A technical review and guide to RNA fluorescence in situ hybridization

RNA-fluorescence in situ hybridization (FISH) is a powerful tool to visualize target messenger RNA transcripts in cultured cells, tissue sections or whole-mount preparations. As the technique has been developed over time, an ever-increasing number of divergent protocols have been published. There is now a broad selection of options available to facilitate proper tissue preparation, hybridization, and post-hybridization background removal to achieve optimal results. Here we review the technical aspects of RNA-FISH, examining the most common methods associated with different sample types including cytological preparations and whole-mounts. We discuss the application of commonly used reagents for tissue preparation, hybridization, and post-hybridization washing and provide explanations of the functional roles for each reagent. We also discuss the available probe types and necessary controls to accurately visualize gene expression. Finally, we review the most recent advances in FISH technology that facilitate both highly multiplexed experiments and signal amplification for individual targets. Taken together, this information will guide the methods development process for investigators that seek to perform FISH in organisms that lack documented or optimized protocols.

Adenylyl cyclase A mRNA localized at the back of cells is actively translated in live chemotaxing Dictyostelium

Dictyostelium discoideum cells transport adenylyl cyclase A (ACA)-containing vesicles to the back of polarized cells to relay exogenous cAMP signals during chemotaxis. Fluorescence in situ hybridization (FISH) experiments showed that ACA mRNA is also asymmetrically distributed at the back of polarized cells. By using the MS2 bacteriophage system, we now visualize the distribution of ACA mRNA in live chemotaxing cells. We found that the ACA mRNA localization is not dependent on the translation of the protein product and requires multiple cis-acting elements within the ACA-coding sequence. We show that ACA mRNA is associated with actively translating ribosomes and is transported along microtubules towards the back of cells. By monitoring the recovery of ACA-YFP after photobleaching, we observed that local translation of ACA-YFP occurs at the back of cells. These data represent a novel functional role for localized translation in the relay of chemotactic signals during chemotaxis.

Omics approaches for subcellular translation studies

Koppel & Fainzilber review translatomics and proteomics methods for studying protein synthesis at subcellular resolution.

Squid Giant Axon Contains Neurofilament Protein mRNA but does not Synthesize Neurofilament Proteins

When isolated squid giant axons are incubated in radioactive amino acids, abundant newly synthesized proteins are found in the axoplasm. These proteins are translated in the adaxonal Schwann cells and subsequently transferred into the giant axon. The question as to whether any de novo protein synthesis occurs in the giant axon itself is difficult to resolve because the small contribution of the proteins possibly synthesized intra-axonally is not easily distinguished from the large amounts of the proteins being supplied from the Schwann cells. In this paper, we reexamine this issue by studying the synthesis of endogenous neurofilament (NF) proteins in the axon. Our laboratory previously showed that NF mRNA and protein are present in the squid giant axon, but not in the surrounding adaxonal glia. Therefore, if the isolated squid axon could be shown to contain newly synthesized NF protein de novo, it could not arise from the adaxonal glia. The results of experiments in this paper show that abundant 3H-labeled NF protein is synthesized in the squid giant fiber lobe containing the giant axon's neuronal cell bodies, but despite the presence of NF mRNA in the giant axon no labeled NF protein is detected in the giant axon. This lends support to the glia-axon protein transfer hypothesis which posits that the squid giant axon obtains newly synthesized protein by Schwann cell transfer and not through intra-axonal protein synthesis, and further suggests that the NF mRNA in the axon is in a translationally repressed state.

Monitoring mRNA Translation in Neuronal Processes Using Fluorescent Non-Canonical Amino Acid Tagging

A steady accumulation of experimental data argues that protein synthesis in neurons is not merely restricted to the somatic compartment, but also occurs in several discrete cellular micro-domains. Local protein synthesis is critical for the establishment of synaptic plasticity in mature dendrites and in directing the growth cones of immature axons, and has been associated with cognitive impairment in mice and humans. Although in recent years a number of important mechanisms governing this process have been described, it remains technically challenging to precisely monitor local protein synthesis in individual neuronal cell parts independent from the soma. This report presents the utility of employing microfluidic chambers for the isolation and treatment of single neuronal cellular compartments. Furthermore, it is demonstrated that a protein synthesis assay, based on fluorescent non-canonical amino acid tagging (FUNCAT), can be combined with this cell culture system to label nascent proteins within a discrete structural and functional domain of the neuron. Together, these techniques could be employed for the detection of protein synthesis within developing and mature neurites, offering an effective approach to elucidate novel mechanisms controlling synaptic maintenance and plasticity.

Intra-axonal protein synthesis in development and beyond

Proteins can be locally produced in the periphery of a cell, allowing a rapid and spatially precise response to the changes in its environment. This process is especially relevant in highly polarized and morphologically complex cells such as neurons. The study of local translation in axons has evolved from being primarily focused on developing axons, to the notion that also mature axons can produce proteins. Axonal translation has been implied in several physiological and pathological conditions, and in all cases it shares common molecular actors and pathways as well as regulatory mechanisms. Here, we review the main findings in these fields, and attempt to highlight shared principles.

The central dogma decentralized: new perspectives on RNA function and local translation in neurons

The elaborate morphology of neurons together with the information processing that occurs in remote dendritic and axonal compartments makes the use of decentralized cell biological machines necessary. Recent years have witnessed a revolution in our understanding of signaling in neuronal compartments and the manifold functions of a variety of RNA molecules that regulate protein translation and other cellular functions. Here we discuss the view that mRNA localization and RNA-regulated and localized translation underlie many fundamental neuronal processes and highlight key issues for future experiments.

Intra-axonal synthesis of eukaryotic translation initiation factors regulates local protein synthesis and axon growth in rat sympathetic neurons

Axonal protein synthesis is a complex process involving selective mRNA localization and translational regulation. In this study, using in situ hybridization and metabolic labeling, we show that the mRNAs encoding eukaryotic translation initiation factors eIF2B2 and eIF4G2 are present in the axons of rat sympathetic neurons and are locally translated. We also report that a noncoding microRNA, miR16, modulates the axonal expression of eIF2B2 and eIF4G2. Transfection of axons with precursor miR16 and anti-miR16 showed that local miR16 levels modulated axonal eIF2B2 and eIF4G2 mRNA and protein levels, as well as axon outgrowth. siRNA-mediated knock-down of axonal eIF2B2 and eIF4G2 mRNA also resulted in a significant decrease in axonal eIF2B2 and eIF4G2 protein. Moreover, results of metabolic labeling studies showed that downregulation of axonal eIF2B2 and eIF4G2 expression also inhibited local protein synthesis and axon growth. Together, these data provide evidence that miR16 mediates axonal growth, at least in part, by regulating the local protein synthesis of eukaryotic translation initiation factors eIF2B2 and eIF4G2 in the axon.

Differing semaphorin 3A concentrations trigger distinct signaling mechanisms in growth cone collapse

Semaphorin-3A (Sema3A) is a major guidance cue in the developing nervous system. Previous studies have revealed a dependence of responses to Sema3A on local protein synthesis (PS) in axonal growth cones, but a recent study has called this dependence into question. To understand the basis of this discrepancy we used the growth cone collapse assay on chick dorsal root ganglion neurons. We show that the dependence of growth cone collapse on protein synthesis varies according to Sema3A concentration, from near-total at low concentration (<100 ng/ml) to minimal at high concentration (>625 ng/ml). Further, we show that neuropilin-1 (NP-1) mediates both PS-dependent and PS-independent collapse. Our findings are consistent with the operation of at least two distinct Sema3A signaling pathways: one that is PS-dependent, involving mammalian target of rapamycin, and one that is PS-independent, involving GSK-3β activation and operative at all concentrations of Sema3A examined. The results provide a plausible explanation for the discrepancy in PS-dependence reported in the literature, and indicate that different signaling pathways activated within growth cones can be modulated by changing the concentration of the same guidance cue.

Caltubin, a novel molluscan tubulin-interacting protein, promotes axonal growth and attenuates axonal degeneration of rodent neurons

Axotomized central neurons of most invertebrate species demonstrate a strong regenerative capacity, and as such may provide valuable molecular insights and new tools to promote axonal regeneration in injured mammalian neurons. In this study, we identified a novel molluscan protein, caltubin, ubiquitously expressed in central neurons of Lymnaea stagnalis and locally synthesized in regenerating neurites. Reduction of caltubin levels by gene silencing inhibits the outgrowth and regenerative ability of adult Lymnaea neurons and decreases local α- and β-tubulin levels in neurites. Caltubin binds to α- and/or β-tubulin in both Lymnaea and rodent neurons. Expression of caltubin in PC12 cells and mouse cortical neurons promotes NGF-induced axonal outgrowth and attenuates axonal retraction after injury. This is the first study illustrating that a xenoprotein can enhance outgrowth and prevent degeneration of injured mammalian neurons. These results may open up new avenues in molecular repair strategies through the insertion of molecular components of invertebrate regenerative pathways into mammalian neurons.

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.

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.

Axon viability and mitochondrial function are dependent on local protein synthesis in sympathetic neurons

(1) Axons contain numerous mRNAs and a local protein synthetic system that can be regulated independently of the cell body. (2) In this study, cultured primary sympathetic neurons were employed, to assess the effect of local protein synthesis blockade on axon viability and mitochondrial function. (3) Inhibition of local protein synthesis reduced newly synthesized axonal proteins by 65% and resulted in axon retraction after 6 h. Acute inhibition of local protein synthesis also resulted in a significant decrease in the membrane potential of axonal mitochondria. Likewise, blockade of local protein transport into the mitochondria by transfection of the axons with Hsp90 C-terminal domain decreased the mitochondrial membrane potential by 65%. Moreover, inhibition of the local protein synthetic system also reduced the ability of mitochondria to restore axonal levels of ATP after KCl-induced depolarization. (4) Taken together, these results indicate that the local protein synthetic system plays an important role in mitochondrial function and the maintenance of the axon.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

New insights into neuronal regeneration: the role of axonal protein synthesis in pathfinding and axonal extension

Protein synthesis in dendrites has become an accepted cellular mechanism that contributes to activity-dependent responses in the post-synaptic neuron. Although it was argued that protein synthesis does not occur in axons, early studies from a number of groups provided evidence for the presence of RNAs and active protein synthesis machinery in both invertebrate and vertebrate axons. Work over the past decade has confirmed these early findings and has proven the capability of axons to locally synthesize some of their own proteins. The functional significance of this localized protein synthesis remained largely unknown until recent years. Recent studies have shown that mRNA translation in developing and mature axons plays a role in axonal growth. In developing axons, protein synthesis allows the distal axon to autonomously respond to guidance cues by rapidly changing its direction of outgrowth. In addition, local proteolysis of axonal proteins contributes axonal guidance and growth cone initiation. This local synthesis and degradation of proteins are likely to provide novel insights into how growing axons navigate through their complex environment. In mature axons, injury triggers formation of a growth cone through localized protein synthesis, and moreover, in these injured axons locally synthesized proteins provide a retrogradely transported signal that can enhance regenerative responses. The intrinsic capability for axons to autonomously regulate local protein levels can be modulated by exogenous stimuli providing opportunities for enhancing regeneration. In this review, the concept of axonal protein synthesis is discussed from a historical perspective. Further, the implications of axonal protein synthesis and proteolysis for neural repair are considered.

Nerve terminals of squid photoreceptor neurons contain a heterogeneous population of mRNAs and translate a transfected reporter mRNA

It is now well established that the distal structural/functional domains of the neuron contain 2a diverse population of mRNAs that program the local synthesis of protein. However, there is still a paucity of information on the composition and function of these mRNA populations in the adult nervous system. To generate empirically, hypotheses regarding the function of the local protein synthetic system, we have compared the mRNAs present in the squid giant axon and its parental cell bodies using differential mRNA display as an unbiased screen. The results of this screen facilitated the identification of 31 mRNAs that encoded cytoskeletal proteins, translation factors, ribosomal proteins, molecular motors, metabolic enzymes, nuclear-encoded mitochondrial mRNAs, and a molecular chaperone. Results of cell fractionation and RT-PCR analyses established that several of these mRNAs were present in polysomes present in the presynaptic nerve terminal of photoreceptor neurons, indicating that these mRNAs were being actively translated. Findings derived from in vitro transfection studies established that these isolated nerve terminals had the ability to translate a heterologous reporter mRNA. Based upon these data, it is hypothesized that the local protein synthetic system plays an important role in the maintenance/remodelling of the cytoarchitecture of the axon and nerve terminal, maintenance of the axon transport and mRNA translation systems, as well as contributing to the viability and function of the local mitochondria.

Subcellular compartmentation of neuronal protein synthesis: new insights into the biology of the neuron

During the past few years, it has become well established that the distal structural/functional domains of the neuron contain numerous mRNAs. However, there is a paucity of information on the composition and function of these unique mRNA populations. In this article, we review recent evidence to support the hypothesis that protein synthesis occurs in multiple subcellular compartments in the neuron, to include the axon and presynaptic nerve terminal. The studies we describe use the squid giant axon and photoreceptor neuron as model invertebrate motor and sensory systems, respectively. Initial cell-free translation studies and molecular hybridization analysis established that the giant axon contained a heterogeneous population of polyadenylated mRNAs. The application of differential mRNA display methodology greatly facilitated the isolation and identification of 29 of these mRNAs, which encode cytoskeletal proteins, molecular motors, translation factors, various nuclear-encoded mitochondrial mRNAs, and several novel mRNA species. RT-PCR analysis of RNA from squid brain synaptosomes confirmed the presence of these mRNAs in the presynaptic nerve terminal. The presence of these mRNAs in polysomes purified from the synaptosomal fraction establish that these messengers are actively translated in the terminal. Results of in vitro labeling studies demonstrate that a significant fraction of the nuclear-encoded mitochondrial protein derives from the local synthesis in the terminal. This finding calls attention to the intimacy of the relationship that has evolved between the nerve terminal and its energy-generating system. The role that local protein synthesis might play in the mammalian nervous system and in the neuronal response to stress is discussed.

Stimulus-dependent translocation of egg-laying hormone encoding mRNA into the axonal compartment of the neuroendocrine caudodorsal cells

To get insight into the stimulus-dependent translocation of mRNA encoding neuropeptides to the axonal compartment of neurons, we investigated this process in the egg-laying hormone producing caudodorsal cells of the mollusk Lymnaea stagnalis. The axonal compartment including the nerve terminals of these neurons harbors high amounts of mRNA encoding the egg-laying hormone precursor. We determined how a sensory stimulus, that results in egg-laying, affected the amount of egg-laying hormone encoding transcripts in the axon endings. Four hours after stimulation high amounts of transcripts were detected in the axonal compartment and maximum values were reached after 8 h. Transcript levels in the somata were affected in a similar fashion, although the increase was not as pronounced as in the axons. Next, we investigated the ultrastructural localization of egg-laying hormone encoding transcripts in axons and axon terminals by means of electron microscopic in situ hybridization and showed that transcripts were localized in the axoplasm. By means of conventional electron microscopy we showed that axon terminals of egg-laying hormone producing neurons contained large amounts of polyribosomes. Together, these data support the notion that egg-laying hormone encoding transcripts are translated in the axonal compartment.

Cloning and characterization of an ionotropic glutamate receptor subunit expressed in the squid nervous system

In this paper we describe the cloning of a putative ionotropic glutamate receptor subunit, SqGluR, and its distribution in the nervous system of the squid. A full-length cDNA was assembled from a cDNA library of the stellate ganglion/giant fibre lobe complex of Loligo opalescens. The deduced amino acid sequence of the mature SqGluR displayed 44-46% amino acid identity with mammalian GluR1-GluR4 and 53% with Lym-eGluR1 from Lymnaea stagnalis. In situ hybridizations in adult squid confirmed that the SqGluR mRNA is abundant in giant fibre lobe neurons, in large, presumptive motor neurons of the stellate ganglion proper and in the supraoesophageal and optic lobes of the central nervous system. In newborn squid, SqGluR mRNA expression was detected throughout the nervous system but not elsewhere. A synthetic peptide corresponding to the last 15 amino acids of the SqGluR C-terminus was used to generate polyclonal antibodies, which were used for immunoblot analysis to demonstrate widespread expression in the squid central and peripheral nervous systems. Injection of the synthetic peptide into the postsynaptic side of the giant synapse inhibited synaptic transmission.

Identification of a novel membrane-associated protein expressed in neurons of the squid and rodent nervous systems

In a previous communication, we reported the isolation of a novel cDNA clone (pA6) from a library constructed from squid axonal mRNAs. The partial cDNA clone contained a unique open reading frame that encoded 84 amino acids and was complementary to a moderately abundant mRNA approximately 550-600 nucleotides in length [Chun et al., J. Neurosci. Res. 49 (1997) 144-153]. In this report, we identify the pA6 gene product, and characterize its expression in the squid and rodent brain. Results of immunoblot analyses conducted in squid, using a polyclonal antibody raised against a synthetic peptide corresponding to the C-terminus of the putative protein, established the presence of two pA6 immunoreactive proteins of approximately 14 kDa and 26 kDa in size. In contrast, mouse brain contained only a single 26-kDa immunoreactive species. In both the squid and mouse brain, the expression of pA6 appears highly selective, being detected in certain neurons but not in non-neuronal cells, as judged by both in situ hybridization and immunocytochemistry. Findings derived from light microscopic, double-label immunohistofluorescence studies indicate that pA6 protein co-localizes with prohibitin, a mitochondrial marker protein. Consistent with these results, electron microscopy localized pA6 immunoreactivity to several membrane compartments to include the outer membrane of mitochondria, as well as to the smooth endoplasmic reticulum and tubulovesicles in dendrites, axons, and axon terminals of neurons in the rat brain. Taken together, these findings indicate that pA6 is a novel, membrane-associated protein, which is expressed in the distal structural/functional domains of neurons in both the invertebrate and vertebrate nervous systems.

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 synaptosomes: a proteomics analysis

A proteomics approach was used to identify the translation products of a unique synaptic model system, squid optic lobe synaptosomes. Unlike its vertebrate counterparts, this preparation is largely free of perikaryal cell fragments and consists predominantly of pre-synaptic terminals derived from retinal photoreceptor neurones. We metabolically labelled synaptosomes with [(35)S] methionine and applied two-dimensional gel electrophoresis to resolve newly synthesized proteins at high resolution. Autoradiographs of blotted two-dimensional gels revealed de novo synthesis of about 80 different proteins, 18 of which could be matched to silver-stained gels that were run in parallel. In-gel digestion of the matched spots and mass spectrometric analyses revealed the identities of various cytosolic enzymes, cytoskeletal proteins, molecular chaperones and nuclear-encoded mitochondrial proteins. A number of novel proteins (i.e. not matching with database sequences) were also detected. In situ hybridization was employed to confirm the presence of mRNA and rRNA in synaptosomes. Together, our data show that pre-synaptic endings of squid photoreceptor neurones actively synthesize a wide variety of proteins involved in synaptic functioning, such as transmitter recycling, energy supply and synaptic architecture.

A functional role for intra-axonal protein synthesis during axonal regeneration from adult sensory neurons

Although intradendritic protein synthesis has been documented in adult neurons, the question of whether axons actively synthesize proteins remains controversial. Adult sensory neurons that are conditioned by axonal crush can rapidly extend processes in vitro by regulating the translation of existing mRNAs (Twiss et al., 2000). These regenerating processes contain axonal but not dendritic proteins. Here we show that these axonal processes of adult sensory neurons cultured after conditioning injury contain ribosomal proteins, translational initiation factors, and rRNA. Pure preparations of regenerating axons separated from the DRG cell bodies can actively synthesize proteins in vitro and contain ribosome-bound beta-actin and neurofilament mRNAs. Blocking protein synthesis in these regenerating sensory axons causes a rapid retraction of their growth cones when communication with the cell body is blocked by axotomy or colchicine treatment. These findings indicate that axons of adult mammalian neurons can synthesize proteins and suggest that, under some circumstances, intra-axonal translation contributes to structural integrity of the growth cone in regenerating axons. By immunofluorescence, translation factors, ribosomal proteins, and rRNA were also detected in motor axons of ventral spinal roots analyzed after 7 d in vivo after a peripheral axonal crush injury. Thus, adult motor neurons are also likely capable of intra-axonal protein synthesis in vivo after axonal injury.

Local synthesis of nuclear-encoded mitochondrial proteins in the presynaptic nerve terminal

One of the central tenets in neuroscience has been that the protein constituents of distal compartments of the neuron (e.g., the axon and nerve terminal) are synthesized in the nerve cell body and are subsequently transported to their ultimate sites of function. In contrast to this postulate, we have established previously that a heterogeneous population of mRNAs and biologically active polyribosomes exist in the giant axon and presynaptic nerve terminals of the photoreceptor neurons in squid. We report that these mRNA populations contain mRNAs for nuclear-encoded mitochondrial proteins to include: cytochrome oxidase subunit 17, propionyl-CoA carboxylase (EC 6.4.1.3), dihydrolipoamide dehydrogenase (EC 1.8.1.4), and coenzyme Q subunit 7. The mRNA for heat shock protein 70, a chaperone protein known to be involved in the import of proteins into mitochondria, has also been identified. Electrophoretic gel analysis of newly synthesized proteins in the synaptosomal fraction isolated from the squid optic lobe revealed that the large presynaptic terminals of the photoreceptor neuron contain a cytoplasmic protein synthetic system. Importantly, a significant amount of the cycloheximide resistant proteins locally synthesized in the terminal becomes associated with mitochondria. PCR analysis of RNA from synaptosomal polysomes establishes that COX17 and CoQ7 mRNAs are being actively translated. Taken together, these findings indicate that proteins required for the maintenance of mitochondrial function are synthesized locally in the presynaptic nerve terminal, and call attention to the intimacy of the relationship between the terminal and its energy generating system. J. Neurosci. Res. 64:447-453, 2001. Published 2001 Wiley-Liss, Inc.

Cryptic peripheral ribosomal domains distributed intermittently along mammalian myelinated axons

A growing body of metabolic and molecular evidence of an endogenous protein-synthesizing machinery in the mature axon is a challenge to the prevailing dogma that the latter is dependent exclusively on slow axoplasmic transport to maintain protein mass in a steady state. However, evidence for a systematic occurrence of ribosomes in mature vertebrate axons has been lacking until recently, when restricted ribosomal domains, called "periaxoplasmic plaques," were described in goldfish CNS myelinated axons. Comparable restricted RNA/ribosomal "plaque" domains now have been identified in myelinated axons of lumbar spinal nerve roots in rabbit and rat on the basis of RNase sensitivity of YOYO-1-binding fluorescence, immunofluorescence of ribosome-specific antibodies, and ribosome phosphorus mapping by electron spectroscopic imaging (ESI). The findings were derived from examination of the axoplasm isolated from myelinated fibers as axoplasmic whole mounts and delipidated spinal nerve roots. Ribosomal periaxoplasmic plaque domains in rabbit axons were typically narrow ( approximately 2 microm), elongated ( approximately 10 microm) sites that frequently were marked by a protruding structure. The domain complexity included an apparent ribosome-binding matrix. The small size, random distribution, and variable intermittent axial spacing of plaques around the periphery of axoplasm near the axon-myelin border are likely reasons why their systematic occurrence has remained undetected in ensheathed axons. The periodic but regular incidence of ribosomal domains provides a structural basis for previous metabolic evidence of protein synthesis in myelinated axons.

Neurofilament mRNAs are present and translated in the normal and severed sciatic nerve

Local protein synthesis within axons has been studied on a limited scale. In the present study, several techniques were used to investigate this synthesis in sciatic nerve, and to show that it increases after damage to the axon. Neurofilament (NF) mRNAs were probed by RT-PCR, Northern blot and in situ hybridization in axons of intact rat sciatic nerve, and in proximal or distal stumps after sciatic nerve transection. RT-PCR demonstrated the presence of NF-L, NF-M and NF-H mRNAs in intact sciatic nerve, as well as in proximal and distal stumps of severed nerves. Northern blot analysis of severed nerve detected NF-L and NF-M, but not NF-H. This technique did not detect the three NFs mRNAs in intact nerve. Detection of NF-L and NF-M mRNA in injured nerve, however, indicated that there was an up-regulation in response to nerve injury. In situ hybridization showed that NF-L mRNA was localized in the Schwann cell perinuclear area, in the myelin sheath, and at the boundary between myelin sheath and cortical axoplasm. RNA and protein synthesizing activities were always greater in proximal as compared to distal stumps. NF triplet proteins were also shown to be synthesized de novo in the proximal stump. The detection of neurofilament mRNAs in nerves, their possible upregulation during injury and the synthesis of neurofilament protein triplet in the proximal stumps, suggest that these mRNAs may be involved in nerve regeneration, providing a novel point of view of this phenomenon.

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.

Ribosomes and polyribosomes are present in the squid giant axon: an immunocytochemical study

Ribosomes and polyribosomes were detected by immuno-electron microscopy in the giant axon and small axons of the squid using a polyclonal antibody against rat brain ribosomes. The ribosomal fraction used as antigen was purified by ultracentrifugation on a sucrose density gradient and shown to contain ribosomal RNAs and native ribosomes. The polyclonal antibody raised in rabbits reacted with at least ten proteins on immunoblots of purified rat brain ribosomes as well as with a set of multiple ribosomal proteins prepared from the squid giant fiber lobe. Immunoreactions were performed on cryostat sections of the stellate nerve cut at a distance of more than 3 cm from the stellate ganglion, using pre-embedding techniques. Ribosomes and polyribosomes were identified within the giant axon and small axons using electron microscopic methods, following binding of peroxidase-conjugated anti-rabbit IgG secondary antibody. Polysomes were more frequently localized in peripheral axoplasm, including the cortical layer of the giant axon, and were generally associated with unidentified cytoskeletal filaments or with dense matrix material. The immunochemical demonstration of ribosomes and polyribosomes in the giant axon and small axons of the squid confirms similar observations in the squid and the goldfish obtained with the method of electron spectroscopic imaging, and strongly supports the view that a local system of protein synthesis is present in axons. The immunochemical method here described offers an alternative tool for the selective identification of ribosomes, and is likely to prove of value in the analyses of other axonal systems.

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.

Sorting of beta-actin mRNA and protein to neurites and growth cones in culture

The transport of mRNAs into developing dendrites and axons may be a basic mechanism to localize cytoskeletal proteins to growth cones and influence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localization patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones and filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to be peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to neuronal processes and growth cones, unlike gamma-actin mRNAs, which were restricted to the cell body. The rapid localization of beta-actin mRNA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hybridization and image-processing methods, we showed that the distribution of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs were detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with translational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequence-specific mechanism to synthesize cytoskeletal proteins directly within processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.

Active polysomes are present in the large presynaptic endings of the synaptosomal fraction from squid brain

Previous data have suggested that the large nerve terminals present in the synaptosomal fraction from squid optic lobe are capable of protein synthesis (Crispino et al., 1993a,b). We have further examined this issue by comparing the translation products of synaptosomal and microsomal polysomes. Both preparations programmed an active process of translation, which was completely abolished by their previous treatment with EDTA. After immunoabsorption of the newly synthesized neurofilament (NF) proteins, the labeling ratio of the 60 and 70 kDa NF proteins was found to differ, in agreement with comparable differences obtained with intact synaptosomes. These observations indicate that the set of mRNAs translated by synaptosomes differs from that translated by nerve cell bodies. Hence, because NF proteins are neuron-specific, they support the view that the active synaptosomal polysomes are mostly localized in the large nerve terminals that represent the most abundant neuronal component of the fraction. This hypothesis was confirmed (1) by electron spectroscopic data demonstrating the presence of ribosomes and polysomes within the large nerve endings of the synaptosomal fraction, as well as in the carrot-like nerve endings of the retinal photoreceptors that constitute the only large terminals in the optic lobe, and (2) by light and high resolution autoradiography of synaptosomal samples incubated with [3H]leucine, showing that most labeled proteins are associated with the large nerve endings. This response was abolished by cycloheximide. Taken together, the data provide the first unequivocal demonstration that presynaptic nerve terminals are capable of protein synthesis.

Characterization of squid enolase mRNA: sequence analysis, tissue distribution, and axonal localization

Enolase is a glycolytic enzyme whose amino acid sequence is highly conserved across a wide range of animal species. In mammals, enolase is known to be a dimeric protein composed of distinct but closely related subunits: alpha (non-neuronal), beta (muscle-specific), and gamma (neuron-specific). However, little information is available on the primary sequence of enolase in invertebrates. Here we report the isolation of two overlapping cDNA clones and the putative primary structure of the enzyme from the squid (Loligo pealii) nervous system. The composite sequence of those cDNA clones is 1575 bp and contains the entire coding region (1302 bp), as well as 66 and 207 bp of 5' and 3' untranslated sequence, respectively. Cross-species comparison of enolase primary structure reveals that squid enolase shares over 70% sequence identity to vertebrate forms of the enzyme. The greatest degree of sequence similarity was manifest to the alpha isoform of the human homologue. Results of Northern analysis revealed a single 1.6 kb mRNA species, the relative abundance of which differs approximately 10-fold between various tissues. Interestingly, evidence derived from in situ hybridization and polymerase chain reaction experiments indicate that the mRNA encoding enolase is present in the squid giant axon.

RNA in the axonal domain: a new dimension in neuronal functioning?

In the last three decades it has become evident that in a wide variety of animals, the axonal compartment of neurons harbours a variety of RNA molecules, such as transfer RNA, ribosomal RNA and messenger RNA. In this paper data will be reviewed on the origin, the selective transport, subcellular localization and functional aspects of the occurrence of RNA molecules in the axonal domain.

N-terminal arginylation of proteins in explants of injured sciatic nerves and embryonic brains of rats

Posttranslational modification of proteins by arginine and lysine has been demonstrated in crude extracts of vertebrate nerves and brain but not in intact cells. In the present experiments we have exploited the fact that Arg is added posttranslationally only at the N-terminus of target proteins, to demonstrate these reactions in intact cells of sciatic nerves and embryonic brains of rats. Sciatic nerves were crushed in anaesthesized rats and 2 hrs later segments of nerve, including the site of the crush, were removed and incubated in media containing [3H]Arg. Incorporation of [3H]Arg into total proteins was analyzed by acid precipitation and the presence of label at the N-terminus was determined by a modification of the Edman degradation procedure. Approximately 25% of protein bound [3H]Arg was released from the N-terminus by the Edman reaction indicating that it was added posttranslationally rather than through protein synthesis. N-terminal labeling was not detectable in nerves not crushed prior to explant and incubation. Slices of embryonic day 20 visual cortex, when incubated under similar conditions as injured sciatic nerves, also showed approximately 25% of the protein incorporated [3H]Arg at the N-terminus, while arginylation was not detectable in adult rat brain slices. Since Lys is not added posttranslationally to the N-terminus, we have attempted to observe lysylation of proteins in intact cells by using cycloheximide (Cx) to block protein synthesis without interfering with protein modification. The posttranslational incorporation of Arg/Lys into proteins was found to be insensitive to up to 2.0 mM Cx in tissue extracts (in vitro).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurofilament proteins are synthesized in nerve endings from squid brain

It is generally believed that the proteins of the nerve endings are synthesized on perikaryal polysomes and are eventually delivered to the presynaptic domain by axoplasmic flow. At variance with this view, we have reported previously that a synaptosomal fraction from squid brain actively synthesizes proteins whose electrophoretic profile differs substantially from that of the proteins made in nerve cell bodies, axons, or glial cells, i.e., by the possible contaminants of the synaptosomal fraction. Using western analyses and immunoabsorption methods, we report now that (a) the translation products of the squid synaptosomal fraction include neurofilament (NF) proteins and (b) the electrophoretic pattern of the synaptosomal newly synthesized NF proteins is drastically different from that of the NF proteins synthesized by nerve cell bodies. The latter results exclude the possibility that NF proteins synthesized by the synaptosomal fraction originate in fragments of nerve cell bodies possibly contaminating the synaptosomal fraction. They rather indicate that in squid brain, nerve terminals synthesize NF proteins.

Discordant estimates of heterologous promoter activity as determined by reporter gene mRNA levels and enzyme activity

In this study, the human cytomegalovirus (CMV) promoter fused to the lacZ (beta-gal) reporter gene was transfected into neuroblastoma SK-N-BE(2)-C cells, and phorbol ester-stimulated promoter activity assessed by both PCR quantitation of reporter gene mRNA levels and enzyme activity. Surprisingly, significant differences were observed in the induction profile of CMV promoter activity as judged by these two independent methods of analysis. For example, at 24 hrs post-transfection beta-gal activity was elevated 7.3-fold in phorbol ester-treated cells, whereas 2.4-fold increases were observed in the cognate mRNA levels. These findings demonstrate the efficacy of quantitative PCR methodology to evaluate promoter activity in DNA-mediated cell transfection analyses, and raise a cautionary note on the reliance of reporter gene enzyme activity to estimate the transcriptional activity of heterologous promoters.