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

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.

Tests for learning and memory in rodent regulatory studies

For decades, regulatory guidelines for safety assessment in rodents for drugs, chemicals, pesticides, and food additives with developmental neurotoxic potential have recommended a single test of learning and memory (L&M). In recent years some agencies have requested two such tests. Given the importance of higher cognitive function to health, and the fact that different types of L&M are mediated by different brain regions assessing higher functions represents a step forward in providing better evidence-based protection against adverse brain effects. Given the myriad of tests available for assessing L&M in rodents this leads to the question of which tests best fit regulatory guidelines. To address this question, we begin by describing the central role of two types of L&M essential to all mammalian species and the regions/networks that mediate them. We suggest that the tests recommended possess characteristics that make them well suited to the needs in regulatory safety studies. By brain region, these are (1) the hippocampus and entorhinal cortex for spatial navigation, which assesses explicit L&M for reference and episodic memory and (2) the striatum and related structures for egocentric navigation, which assesses implicit or procedural memory and path integration. Of the tests available, we suggest that in this context, the evidence supports the use of water mazes, specifically, the Morris water maze (MWM) for spatial L&M and the Cincinnati water maze (CWM) for egocentric/procedural L&M. We review the evidentiary basis for these tests, describe their use, and explain procedures that optimize their sensitivity.

Looking for answers far away from the soma-the (un)known axonal functions of TDP-43, and their contribution to early NMJ disruption in ALS

Axon degeneration and Neuromuscular Junction (NMJ) disruption are key pathologies in the fatal neurodegenerative disease Amyotrophic Lateral Sclerosis (ALS). Despite accumulating evidence that axons and NMJs are impacted at a very early stage of the disease, current knowledge about the mechanisms leading to their degeneration remains elusive. Cytoplasmic mislocalization and accumulation of the protein TDP-43 are considered key pathological hallmarks of ALS, as they occur in ~ 97% of ALS patients, both sporadic and familial. Recent studies have identified pathological accumulation of TDP-43 in intramuscular nerves of muscle biopsies collected from pre-diagnosed, early symptomatic ALS patients. These findings suggest a gain of function for TDP-43 in axons, which might facilitate early NMJ disruption. In this review, we dissect the process leading to axonal TDP-43 accumulation and phosphorylation, discuss the known and hypothesized roles TDP-43 plays in healthy axons, and review possible mechanisms that connect TDP-43 pathology to the axon and NMJ degeneration in ALS.

mRNA Localization and Local Translation of the Microtubule Severing Enzyme, Fidgetin-Like 2, in Polarization, Migration and Outgrowth

Cell motility requires strict spatiotemporal control of protein expression. During cell migration, mRNA localization and local translation in subcellular areas like the leading edge and protrusions are particularly advantageous for regulating the reorganization of the cytoskeleton. Fidgetin-Like 2 (FL2), a microtubule severing enzyme (MSE) that restricts migration and outgrowth, localizes to the leading edge of protrusions where it severs dynamic microtubules. FL2 is primarily expressed during development but in adulthood, is spatially upregulated at the leading edge minutes after injury. Here, we show mRNA localization and local translation in protrusions of polarized cells are responsible for FL2 leading edge expression after injury. The data suggests that the RNA binding protein IMP1 is involved in the translational regulation and stabilization of FL2 mRNA, in competition with the miRNA let-7. These data exemplify the role of local translation in microtubule network reorganization during migration and elucidate an unexplored MSE protein localization mechanism.

Lost in local translation: TDP-43 and FUS in axonal/neuromuscular junction maintenance and dysregulation in amyotrophic lateral sclerosis

Key early features of amyotrophic lateral sclerosis (ALS) are denervation of neuromuscular junctions and axonal degeneration. Motor neuron homeostasis relies on local translation through controlled regulation of axonal mRNA localization, transport, and stability. Yet the composition of the local transcriptome, translatome (mRNAs locally translated), and proteome during health and disease remains largely unexplored. This review covers recent discoveries on axonal translation as a critical mechanism for neuronal maintenance/survival. We focus on two RNA binding proteins, transactive response DNA binding protein-43 (TDP-43) and fused in sarcoma (FUS), whose mutations cause ALS and frontotemporal dementia (FTD). Emerging evidence points to their essential role in the maintenance of axons and synapses, including mRNA localization, transport, and local translation, and whose dysfunction may contribute to ALS. Finally, we describe recent advances in omics-based approaches mapping compartment-specific local RNA and protein compositions, which will be invaluable to elucidate fundamental local processes and identify key targets for therapy development.

Maintaining essential microtubule bundles in meter-long axons: a role for local tubulin biogenesis?

Axons are the narrow, up-to-meter long cellular processes of neurons that form the biological cables wiring our nervous system. Most axons must survive for an organism's lifetime, i.e. up to a century in humans. Axonal maintenance depends on loose bundles of microtubules that run without interruption all along axons. The continued turn-over and the extension of microtubule bundles during developmental, regenerative or plastic growth requires the availability of α/β-tubulin heterodimers up to a meter away from the cell body. The underlying regulation in axons is poorly understood and hardly features in past and contemporary research. Here we discuss potential mechanisms, particularly focussing on the possibility of local tubulin biogenesis in axons. Current knowledge might suggest that local translation of tubulin takes place in axons, but far less is known about the post-translational machinery of tubulin biogenesis involving three chaperone complexes: prefoldin, CCT and TBC. We discuss functional understanding of these chaperones from a range of model organisms including yeast, plants, flies and mice, and explain what is known from human diseases. Microtubules across species depend on these chaperones, and they are clearly required in the nervous system. However, most chaperones display a high degree of functional pleiotropy, partly through independent functions of individual subunits outside their complexes, thus posing a challenge to experimental studies. Notably, we found hardly any studies that investigate their presence and function particularly in axons, thus highlighting an important gap in our understanding of axon biology and pathology.

Receptor-Ribosome Coupling: A Link Between Extrinsic Signals and mRNA Translation in Neuronal Compartments

Axons receive extracellular signals that help to guide growth and synapse formation during development and to maintain neuronal function and survival during maturity. These signals relay information via cell surface receptors that can initiate local intracellular signaling at the site of binding, including local messenger RNA (mRNA) translation. Direct coupling of translational machinery to receptors provides an attractive way to activate this local mRNA translation and change the local proteome with high spatiotemporal resolution. Here, we first discuss the increasing evidence that different external stimuli trigger translation of specific subsets of mRNAs in axons via receptors and thus play a prominent role in various processes in both developing and mature neurons. We then discuss the receptor-mediated molecular mechanisms that regulate local mRNA translational with a focus on direct receptor-ribosome coupling. We advance the idea that receptor-ribosome coupling provides several advantages over other translational regulation mechanisms and is a common mechanism in cell communication.

Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Rat Sciatic Nerve Axoplasm Proteome Is Enriched with Ribosomal Proteins during Regeneration Processes

Axons are complex subcellular compartments that are extremely long in relation to cell bodies, especially in peripheral nerves. Many processes are required and regulated during axon injury, including anterograde and retrograde transport, glia-to-axon macromolecular transfer, and local axonal protein synthesis. Many in vitro omics approaches have been used to gain insight into these processes, but few have been applied in vivo. Here we adapted the osmotic ex vivo axoplasm isolation method and analyzed the adult rat sciatic-nerve-extruded axoplasm by label-free quantitative proteomics before and after injury. 2087 proteins groups were detected in the axoplasm, revealing translation machinery and microtubule-associated proteins as the most overrepresented biological processes. Ribosomal proteins (73) were detected in the uninjured axoplasm and increased their levels after injury but not within whole sciatic nerves. Meta-analysis showed that detected ribosomal proteins were present in in vitro axonal proteomes. Because local protein synthesis is important for protein localization, we were interested in detecting the most abundant newly synthesized axonal proteins in vivo. With an MS/MS-BONCAT approach, we detected 42 newly synthesized protein groups. Overall, our work indicates that proteomics profiling is useful for local axonal interrogation and suggests that ribosomal proteins may play an important role, especially during injury.

Local gene regulation in radial glia: Lessons from across the nervous system

Radial glial cells (RGCs) are progenitors of the cerebral cortex which produce both neurons and glia during development. Given their central role in development, RGC dysfunction can result in diverse neurodevelopmental disorders. RGCs have an elongated bipolar morphology that spans the entire radial width of the cortex and ends in basal endfeet connected to the pia. The basal process and endfeet are important for proper guidance of migrating neurons and are implicated in signaling. However, endfeet must function at a great distance from the cell body. This spatial separation suggests a role for local gene regulation in endfeet. Endfeet contain a local transcriptome enriched for cytoskeletal and signaling factors. These localized mRNAs are actively transported from the cell body and can be locally translated in endfeet. Yet, studies of local gene regulation in RGC endfeet are still in their infancy. Here, we draw comparisons of RGCs with foundational work in anatomically and phylogenetically related cell types, neurons and astrocytes. Our review highlights a striking overlap in the types of RNAs localized, as well as principles of local translation between these three cell types. Thus, studies in neurons, astrocytes and RGCs can mutually inform an understanding of RNA localization across the nervous system.

Growth Performance After Agouti-Signaling Protein 1 (Asip1) Overexpression in Transgenic Zebrafish

The melanocortin system is a key structure in the regulation of energy balance. Overexpression of inverse agonists, agouti-signaling protein (ASIP), and agouti-related protein (AGRP) results in increased food intake, linear growth, and body weight. ASIP regulates dorsal-ventral pigment polarity through melanocortin 1 receptor (MC1R) and overexpression induces obesity in mice by binding to central MC4R. Asip1 overexpression in transgenic zebrafish (asip1-Tg) enhances growth, yet experiments show fish overexpressing Asip1 do not develop obesity even under severe feeding regimes. Asip1-Tg fish do not need to eat more to grow larger and faster; thus, increased food efficiency can be observed. In addition, asip1-Tg fish reared at high density are able to grow far more than wild-type (WT) fish reared at low density, although asip1-Tg fish seem to be more sensitive to crowding stress than WT fish, thus making the melanocortin system a target for sustainable aquaculture, especially as the U.S. Food and Drug Association has recently approved transgenic fish trading.

Longitudinal biometal accumulation and Ca isotope composition of the Göttingen minipig brain

Biometals play a critical role in both the healthy and diseased brain's functioning. They accumulate in the normal aging brain, and are inherent to neurodegenerative disorders and their associated pathologies. A prominent example of this is the brain accumulation of metals such as Ca, Fe and Cu (and more ambiguously, Zn) associated with Alzheimer's disease (AD). The natural stable isotope compositions of such metals have also shown utility in constraining biological mechanisms, and in differentiating between healthy and diseased states, sometimes prior to conventional methods. Here we have detailed the distribution of the biologically relevant elements Mg, P, K, Ca, Fe, Cu and Zn in brain regions of Göttingen minipigs ranging in age from three months to nearly six years, including control animals and both a single- and double-transgenic model of AD (PS1, APP/PS1). Moreover, we have characterized the Ca isotope composition of the brain for the first time. Concentration data track rises in brain biometals with age, namely for Fe and Cu, as observed in the normal ageing brain and in AD, and biometal data point to increased soluble amyloid beta (Aβ) load prior to AD plaque identification via brain imaging. Calcium isotope results define the brain as the isotopically lightest permanent reservoir in the body, indicating that brain Ca dyshomeostasis may induce measurable isotopic disturbances in accessible downstream reservoirs such as biofluids.

Axonal mRNA translation in neurological disorders

It is increasingly recognized that local protein synthesis (LPS) contributes to fundamental aspects of axon biology, in both developing and mature neurons. Mutations in RNA-binding proteins (RBPs), as central players in LPS, and other proteins affecting RNA localization and translation are associated with a range of neurological disorders, suggesting disruption of LPS may be of pathological significance. In this review, we substantiate this hypothesis by examining the link between LPS and key axonal processes, and the implicated pathophysiological consequences of dysregulated LPS. First, we describe how the length and autonomy of axons result in an exceptional reliance on LPS. We next discuss the roles of LPS in maintaining axonal structural and functional polarity and axonal trafficking. We then consider how LPS facilitates the establishment of neuronal connectivity through regulation of axonal branching and pruning, how it mediates axonal survival into adulthood and its involvement in neuronal stress responses.

The prominent role of the cerebellum in the social learning of the phonological loop in working memory: How language was adaptively built from cerebellar inner speech required during stone-tool making

Based on advances in cerebellum research as to its cognitive, social, and language contributions to working memory, the purpose of this article is to describe new support for the prominent involvement of cerebellar internal models in the adaptive selection of language. Within this context it has been proposed that (1) cerebellar internal models of inner speech during stone-tool making accelerated the adaptive evolution of new cause-and-effect sequences of precision stone-tool knapping requirements, and (2) that these evolving cerebellar internal models coded (i.e., learned in corticonuclear microcomplexes) such cause-and-effect sequences as phonological counterparts and, these, when sent to the cerebral cortex, became new phonological working memory. This article describes newer supportive research findings on (1) the cerebellum's role in silent speech in working memory, and (2) recent findings on genetic aspects (FOXP2) of the role of silent speech in language evolution. It is concluded that within overall cerebro-cerebellar evolution, without the evolution of cerebellar coding of stone-tool making sequences of primitive working memory (beginning approximately 1.7 million years ago) language would not have evolved in the subsequent evolution of Homo sapiens.

mRNA localization and local translation in neurons

The spatial and temporal regulation of gene expression in neurons is an important step in creating functional and structural neuronal networks. The complexity of neurons require differential expression of various proteins in different compartments. Highly polarised cells, such as neurons, respond rapidly to different external stimuli by changing their local protein abundance and composition. Neurons can have extensions up to a meter away from their cell body in humans, so it is easy to envisage why they need to manage the synthesis of new proteins locally and on-demand. Recent research has demonstrated that neurons can control the expression of different proteins by localising translationally silent mRNAs, followed by subsequent translation. Neurons use mRNA localization and local translation to achieve different purposes during their life cycle. While developing neurons rely on mRNA localization for axon guidance and synaptogenesis, mature neurons can use mRNA localization for maintenance of essential physiological processes. mRNA localization also plays a role in response to neuron injury to regenerate and restore neuronal connections. Recent microscopic imaging techniques such as live imaging of fluorescently tagged molecules combined with genetic and biochemical studies in neurons have illustrated evolutionarily conserved mechanisms for targeting mRNAs into their correct compartments. This review provides an overview of mRNA localization and local translation in vertebrate and invertebrate neurons and discusses the mechanism by which mRNAs are trafficked into axons. Furthermore, the role of mRNA localization in synaptic activation, as well as axonal injury is explored.

α2A-adrenergic heteroreceptors are required for stress-induced reinstatement of cocaine conditioned place preference

The α2a-adrenergic receptor (α2a-AR) agonist guanfacine has been investigated as a potential treatment for substance use disorders. While decreasing stress-induced reinstatement of cocaine seeking in animal models and stress-induced craving in human studies, guanfacine has not been reported to decrease relapse rates. Although guanfacine engages α2a-AR autoreceptors, it also activates excitatory Gi-coupled heteroreceptors in the bed nucleus of the stria terminalis (BNST), a key brain region in driving stress-induced relapse. Thus, BNST α2a-AR heteroreceptor signaling might decrease the beneficial efficacy of guanfacine. We aimed to determine the role of α2a-AR heteroreceptors and BNST Gi-GPCR signaling in stress-induced reinstatement of cocaine conditioned place preference (CPP) and the effects of low dose guanfacine on BNST activity and stress-induced reinstatement. We used a genetic deletion strategy and the cocaine CPP procedure to first define the contributions of α2a-AR heteroreceptors to stress-induced reinstatement. Next, we mimicked BNST Gi-coupled α2a-AR heteroreceptor signaling using a Gi-coupled designer receptor exclusively activated by designer drug (Gi-DREADD) approach. Finally, we evaluated the effects of low-dose guanfacine on BNST cFOS immunoreactivity and stress-induced reinstatement. We show that α2a-AR heteroreceptor deletion disrupts stress-induced reinstatement and that BNST Gi-DREADD activation is sufficient to induce reinstatement. Importantly, we found that low-dose guanfacine does not increase BNST activity, but prevents stress-induced reinstatement. Our findings demonstrate a role for α2a-AR heteroreceptors and BNST Gi-GPCR signaling in stress-induced reinstatement of cocaine CPP and provide insight into the impact of dose on the efficacy of guanfacine as a treatment for stress-induced relapse of cocaine use.

Axonal precursor miRNAs hitchhike on endosomes and locally regulate the development of neural circuits

Various species of non‐coding RNAs (ncRNAs) are enriched in specific subcellular compartments, but the mechanisms orchestrating their localization and their local functions remain largely unknown. We investigated both aspects using the elongating retinal ganglion cell axon and its tip, the growth cone, as models. We reveal that specific endogenous precursor microRNAs (pre‐miRNAs) are actively trafficked to distal axons by hitchhiking primarily on late endosomes/lysosomes. Upon exposure to the axon guidance cue semaphorin 3A (Sema3A), pre‐miRNAs are processed specifically within axons into newly generated miRNAs, one of which, in turn, silences the basal translation of tubulin beta 3 class III (TUBB3), but not amyloid beta precursor protein (APP). At the organismal level, these mature miRNAs are required for growth cone steering and a fully functional visual system. Overall, our results uncover a novel mode of ncRNA transport from one cytosolic compartment to another within polarized cells. They also reveal that newly generated miRNAs are critical components of a ncRNA‐based signaling pathway that transduces environmental signals into the structural remodeling of subcellular compartments.

Local translation in neurons: visualization and function

Neurons are among the most compartmentalized and interactive of all cell types. Like all cells, neurons use proteins as the main sensors and effectors. The modification of the proteome in axons and dendrites is used to guide the formation of synaptic connections and to store information. In this Review, we discuss the data indicating that an important source of protein for dendrites, axons and their associated elements is provided by the local synthesis of proteins. We review the data indicating the presence of the machinery required for protein synthesis, the direct visualization and demonstration of protein synthesis, and the established functional roles for local translation for many different neuronal functions. Finally, we consider the open questions and future directions in this field.

Poor transcript-protein correlation in the brain: negatively correlating gene products reveal neuronal polarity as a potential cause

Transcription, translation, and turnover of transcripts and proteins are essential for cellular function. The contribution of those factors to protein levels is under debate, as transcript levels and cognate protein levels do not necessarily correlate due to regulation of translation and protein turnover. Here we propose neuronal polarity as a third factor that is particularly evident in the CNS, leading to considerable distances between somata and axon terminals. Consequently, transcript levels may negatively correlate with cognate protein levels in CNS regions, i.e., transcript and protein levels behave reciprocally. To test this hypothesis, we performed an integrative inter-omics study and analyzed three interconnected rat auditory brainstem regions (cochlear nuclear complex, CN; superior olivary complex, SOC; inferior colliculus, IC) and the rest of the brain as a reference. We obtained transcript and protein sets in these regions of interest (ROIs) by DNA microarrays and label-free mass spectrometry, and performed principal component and correlation analyses. We found 508 transcript|protein pairs and detected poor to moderate transcript|protein correlation in all ROIs, as evidenced by coefficients of determination from 0.34 to 0.54. We identified 57-80 negatively correlating gene products in the ROIs and intensively analyzed four of them for which the correlation was poorest. Three cognate proteins (Slc6a11, Syngr1, Tppp) were synaptic and hence candidates for a negative correlation because of protein transport into axon terminals. Thus, we systematically analyzed the negatively correlating gene products. Gene ontology analyses revealed overrepresented transport/synapse-related proteins, supporting our hypothesis. We present 30 synapse/transport-related proteins with poor transcript|protein correlation. In conclusion, our analyses support that protein transport in polar cells is a third factor that influences the protein level and, thereby, the transcript|protein correlation. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* and *Open Data* because it provided all relevant information to reproduce the study in the manuscript and because it made the data publicly available. The data can be accessed at https://osf.io/ha28n/. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Miles to go (mtgo) encodes FNDC3 proteins that interact with the chaperonin subunit CCT3 and are required for NMJ branching and growth in Drosophila

Analysis of mutants that affect formation and function of the Drosophila larval neuromuscular junction (NMJ) has provided valuable insight into genes required for neuronal branching and synaptic growth. We report that NMJ development in Drosophila requires both the Drosophila ortholog of FNDC3 genes; CG42389 (herein referred to as miles to go; mtgo), and CCT3, which encodes a chaperonin complex subunit. Loss of mtgo function causes late pupal lethality with most animals unable to escape the pupal case, while rare escapers exhibit an ataxic gait and reduced lifespan. NMJs in mtgo mutant larvae have dramatically reduced branching and growth and fewer synaptic boutons compared with control animals. Mutant larvae show normal locomotion but display an abnormal self-righting response and chemosensory deficits that suggest additional functions of mtgo within the nervous system. The pharate lethality in mtgo mutants can be rescued by both low-level pan- and neuronal-, but not muscle-specific expression of a mtgo transgene, supporting a neuronal-intrinsic requirement for mtgo in NMJ development. Mtgo encodes three similar proteins whose domain structure is most closely related to the vertebrate intracellular cytosolic membrane-anchored fibronectin type-III domain-containing protein 3 (FNDC3) protein family. Mtgo physically and genetically interacts with Drosophila CCT3, which encodes a subunit of the TRiC/CCT chaperonin complex required for maturation of actin, tubulin and other substrates. Drosophila larvae heterozygous for a mutation in CCT3 that reduces binding between CCT3 and MTGO also show abnormal NMJ development similar to that observed in mtgo null mutants. Hence, the intracellular FNDC3-ortholog MTGO and CCT3 can form a macromolecular complex, and are both required for NMJ development in Drosophila.

The Role of Axon Transport in Neuroprotection and Regeneration

Retinal ganglion cells and other central nervous system neurons fail to regenerate after injury. Understanding the obstacles to survival and regeneration, and overcoming them, is key to preserving and restoring function. While comparisons in the cellular changes seen in these non-regenerative cells with those that do have intrinsic regenerative ability has yielded many candidate genes for regenerative therapies, complete visual recovery has not yet been achieved. Insights gained from neurodegenerative diseases, like glaucoma, underscore the importance of axonal transport of organelles, mRNA, and effector proteins in injury and disease. Targeting molecular motor networks, and their cargoes, may be necessary for realizing complete axonal regeneration and vision restoration.

Rapid Cue-Specific Remodeling of the Nascent Axonal Proteome

Axonal protein synthesis and degradation are rapidly regulated by extrinsic signals during neural wiring, but the full landscape of proteomic changes remains unknown due to limitations in axon sampling and sensitivity. By combining pulsed stable isotope labeling of amino acids in cell culture with single-pot solid-phase-enhanced sample preparation, we characterized the nascent proteome of isolated retinal axons on an unparalleled rapid timescale (5 min). Our analysis detects 350 basally translated axonal proteins on average, including several linked to neurological disease. Axons stimulated by different cues (Netrin-1, BDNF, Sema3A) show distinct signatures with more than 100 different nascent protein species up- or downregulated within the first 5 min followed by further dynamic remodeling. Switching repulsion to attraction triggers opposite regulation of a subset of common nascent proteins. Our findings thus reveal the rapid remodeling of the axonal proteomic landscape by extrinsic cues and uncover a logic underlying attraction versus repulsion.

Beneficial Effects of Resveratrol-Mediated Inhibition of the mTOR Pathway in Spinal Cord Injury

Spinal cord injury (SCI) causes a high rate of morbidity and disability. The clinical features of SCI are divided into acute, subacute, and chronic phases according to its pathophysiological events. The mammalian target of rapamycin (mTOR) signaling pathway plays an important role in cell death and inflammation in the acute phase and neuroregeneration in the subacute/chronic phases at different times. Resveratrol has the potential of regulating cell growth, proliferation, metabolism, and angiogenesis through the mTOR signaling pathway. Herein, we explicate the role of resveratrol in the repair of SCI through the inhibition of the mTOR signaling pathway. The inhibition of the mTOR pathway by resveratrol has the potential of serving as a neuronal restorative mechanism following SCI.

Molecular control of local translation in axon development and maintenance

The tips of axons are often far away from the cell soma where most proteins are synthesized. Recent work has revealed that axonal mRNA transport and localised translation are key regulatory mechanisms that allow these distant outposts of the cell to respond rapidly to extrinsic factors and maintain axonal homeostasis. Here, we review recent evidence pointing to an increasingly broad role for local protein synthesis in controlling axon shape, synaptogenesis and axon survival by regulating diverse cellular processes such as vesicle trafficking, cytoskeletal remodelling and mitochondrial integrity. We further highlight current research on the regulatory mechanisms that coordinate the localization and translation of functionally linked mRNAs in axons.

To the end of the line: Axonal mRNA transport and local translation in health and neurodegenerative disease

Axons and growth cones, by their very nature far removed from the cell body, encounter unique environments and require distinct populations of proteins. It seems only natural, then, that they have developed mechanisms to locally synthesize a host of proteins required to perform their specialized functions. Acceptance of this ability has taken decades; however, there is now consensus that axons do indeed have the capacity for local translation, and that this capacity is even retained into adulthood. Accumulating evidence supports the role of locally synthesized proteins in the proper development, maintenance, and function of neurons, and newly emerging studies also suggest that disruption in this process has implications in a number of neurodevelopmental and neurodegenerative diseases. Here, we briefly review the long history of axonal mRNA localization and local translation, and the role that these locally synthesized proteins play in normal neuronal function. Additionally, we highlight the emerging evidence that dysregulation in these processes contributes to a wide range of pathophysiology, including neuropsychiatric disorders, Alzheimer's, and motor neuron diseases such as spinal muscular atrophy and Amyotrophic Lateral Sclerosis. © 2017 Wiley Periodicals, Inc. Develop. Neurobiol 78: 209-220, 2018.

RNA Docking and Local Translation Regulate Site-Specific Axon Remodeling In Vivo

Nascent proteins can be positioned rapidly at precise subcellular locations by local protein synthesis (LPS) to facilitate localized growth responses. Axon arbor architecture, a major determinant of synaptic connectivity, is shaped by localized growth responses, but it is unknown whether LPS influences these responses in vivo. Using high-resolution live imaging, we examined the spatiotemporal dynamics of RNA and LPS in retinal axons during arborization in vivo. Endogenous RNA tracking reveals that RNA granules dock at sites of branch emergence and invade stabilized branches. Live translation reporter analysis reveals that de novo β-actin hotspots colocalize with docked RNA granules at the bases and tips of new branches. Inhibition of axonal β-actin mRNA translation disrupts arbor dynamics primarily by reducing new branch emergence and leads to impoverished terminal arbors. The results demonstrate a requirement for LPS in building arbor complexity and suggest a key role for pre-synaptic LPS in assembling neural circuits.

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Tracking endogenous RNA shows that RNA docking predicts axon branch emergence in vivo

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Axon arbor complexity in vivo depends on local protein synthesis

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Axonal β-actin synthesis regulates branching by increased branch initiation

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Live imaging reveals de novo synthesis of β-actin hotspots during branch formation

Expanding Axonal Transcriptome Brings New Functions for Axonally Synthesized Proteins in Health and Disease

Intra-axonal protein synthesis has been shown to play critical roles in both development and repair of axons. Axons provide long-range connectivity in the nervous system, and disruption of their function and/or structure is seen in several neurological diseases and disorders. Axonally synthesized proteins or losses in axonally synthesized proteins contribute to neurodegenerative diseases, neuropathic pain, viral transport, and survival of axons. Increasing sensitivity of RNA detection and quantitation coupled with methods to isolate axons to purity has shown that a surprisingly complex transcriptome exists in axons. This extends across different species, neuronal populations, and physiological conditions. These studies have helped define the repertoire of neuronal mRNAs that can localize into axons and imply previously unrecognized functions for local translation in neurons. Here, we review the current state of transcriptomics studies of isolated axons, contrast axonal mRNA profiles between different neuronal types and growth states, and discuss how mRNA transport into and translation within axons contribute to neurological disorders.

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.

Electrospun nanofiber sheets incorporating methylcobalamin promote nerve regeneration and functional recovery in a rat sciatic nerve crush injury model

Peripheral nerve injury is one of common traumas. Although injured peripheral nerves have the capacity to regenerate, axon regeneration proceeds slowly and functional outcomes are often poor. Pharmacological enhancement of regeneration can play an important role in increasing functional recovery. In this study, we developed a novel electrospun nanofiber sheet incorporating methylcobalamin (MeCbl), one of the active forms of vitamin B12 homologues, to deliver it enough locally to the peripheral nerve injury site. We evaluated whether local administration of MeCbl at the nerve injury site was effective in promoting nerve regeneration. Electrospun nanofiber sheets gradually released MeCbl for at least 8weeks when tested in vitro. There was no adverse effect of nanofiber sheets on function in vivo of the peripheral nervous system. Local implantation of nanofiber sheets incorporating MeCbl contributed to the recovery of the motor and sensory function, the recovery of nerve conduction velocity, and the promotion of myelination after sciatic nerve injury, without affecting plasma concentration of MeCbl.

STATEMENT OF SIGNIFICANCE

Methylcobalamin (MeCbl) is a vitamin B12 analog and we previously reported its effectiveness in axonal outgrowth of neurons and differentiation of Schwann cells both in vitro and in vivo. Here we estimated the effect of local administered MeCbl with an electrospun nanofiber sheet on peripheral nerve injury. Local administration of MeCbl promoted functional recovery in a rat sciatic nerve crush injury model. These sheets are useful for nerve injury in continuity differently from artificial nerve conduits, which are useful only for nerve defects. We believe that the findings of this study are relevant to the scope of your journal and will be of interest to its readership.

Local translation in neuronal compartments: how local is local?

Efficient neuronal function depends on the continued modulation of the local neuronal proteome. Local protein synthesis plays a central role in tuning the neuronal proteome at specific neuronal regions. Various aspects of translation such as the localization of translational machinery, spatial spread of the newly translated proteins, and their site of action are carried out in specialized neuronal subcompartments to result in a localized functional outcome. In this review, we focus on the various aspects of these local translation compartments such as size, biochemical and organelle composition, structural boundaries, and temporal dynamics. We also discuss the apparent absence of definitive components of translation in these local compartments and the emerging state-of-the-art tools that could help dissecting these conundrums in greater detail in the future.

Messenger RNAs localized to distal projections of human stem cell derived neurons

The identification of mRNAs in distal projections of model organisms has led to the discovery of multiple proteins that are locally synthesized for functional roles such as axon guidance, injury signaling and regeneration. The extent to which local protein synthesis is conserved in human neurons is unknown. Here we used compartmentalized microfluidic chambers to characterize the transcriptome of distal projections of human embryonic stem cells differentiated using a protocol which enriched for glutamatergic neurons (hESC-neurons). Using gene expression analysis, we identified mRNAs proportionally enriched in these projections, representing a functionally unique local transcriptome as compared to the human neuronal transcriptome inclusive of somata. Further, we found that the most abundant mRNAs within these hESC-neuron projections were functionally similar to the axonal transcriptome of rat cortical neurons. We confirmed the presence of two well characterized axonal mRNAs in model organisms, β-actin and GAP43, within hESC-neuron projections using multiplexed single molecule RNA-FISH. Additionally, we report the novel finding that oxytocin mRNA localized to these human projections and confirmed its localization using RNA-FISH. This new evaluation of mRNA within human projections provides an important resource for studying local mRNA translation and has the potential to reveal both conserved and unique translation dependent mechanisms.

Global and local mechanisms sustain axonal proteostasis of transmembrane proteins

The control of neuronal protein homeostasis or proteostasis is tightly regulated both spatially and temporally, assuring accurate and integrated responses to external or intrinsic stimuli. Local or autonomous responses in dendritic and axonal compartments are crucial to sustain function during development, physiology and in response to damage or disease. Axons are responsible for generating and propagating electrical impulses in neurons, and the establishment and maintenance of their molecular composition are subject to extreme constraints exerted by length and size. Proteins that require the secretory pathway, such as receptors, transporters, ion channels or cell adhesion molecules, are fundamental for axonal function, but whether axons regulate their abundance autonomously and how they achieve this is not clear. Evidence supports the role of three complementary mechanisms to maintain proteostasis of these axonal proteins, namely vesicular transport, local translation and trafficking and transfer from supporting cells. Here, we review these mechanisms, their molecular machineries and contribution to neuronal function. We also examine the signaling pathways involved in local translation and their role during development and nerve injury. We discuss the relative contributions of a transport-controlled proteome directed by the soma (global regulation) versus a local-controlled proteome based on local translation or cell transfer (local regulation).

mTOR regulates peripheral nerve response to tensile strain

While excessive tensile strain can be detrimental to nerve function, strain can be a positive regulator of neuronal outgrowth. We used an in vivo rat model of sciatic nerve strain to investigate signaling mechanisms underlying peripheral nerve response to deformation. Nerves were deformed by 11% and did not demonstrate deficits in compound action potential latency or amplitude during or after 6 h of strain. As revealed by Western blotting, application of strain resulted in significant upregulation of mammalian target of rapamycin (mTOR) and S6 signaling in nerves, increased myelin basic protein (MBP) and β-actin levels, and increased phosphorylation of neurofilament subunit H (NF-H) compared with unstrained (sham) contralateral nerves (P < 0.05 for all comparisons, paired two-tailed t-test). Strain did not alter neuron-specific β3-tubulin or overall nerve tubulin levels compared with unstrained controls. Systemic rapamycin treatment, thought to selectively target mTOR complex 1 (mTORC1), suppressed mTOR/S6 signaling, reduced levels of MBP and overall tubulin, and decreased NF-H phosphorylation in nerves strained for 6 h, revealing a role for mTOR in increasing MBP expression and NF-H phosphorylation, and maintaining tubulin levels. Consistent with stretch-induced increases in MBP, immunolabeling revealed increased S6 signaling in Schwann cells of stretched nerves compared with unstretched nerves. In addition, application of strain to cultured adult dorsal root ganglion neurons showed an increase in axonal protein synthesis based on a puromycin incorporation assay, suggesting that neuronal translational pathways also respond to strain. This work has important implications for understanding mechanisms underlying nerve response to strain during development and regeneration.NEW & NOTEWORTHY Peripheral nerves experience tensile strain (stretch) during development and movement. Excessive strain impairs neuronal function, but moderate strains are accommodated by nerves and can promote neuronal growth; mechanisms underlying these phenomena are not well understood. We demonstrated that levels of several structural proteins increase following physiological levels of nerve strain and that expression of a subset of these proteins is regulated by mTOR. Our work has important implications for understanding nerve development and strain-based regenerative strategies.

Enhancement of Peripheral Nerve Regrowth by the Purine Nucleoside Analog and Cell Cycle Inhibitor, Roscovitine

Peripheral nerve regeneration is a slow process that can be associated with limited outcomes and thus a search for novel and effective therapy for peripheral nerve injury and disease is crucial. Here, we found that roscovitine, a synthetic purine nucleoside analog, enhances neurite outgrowth in neuronal-like PC12 cells. Furthermore, ex vivo analysis of pre-injured adult rat dorsal root ganglion (DRG) neurons showed that roscovitine enhances neurite regrowth in these cells. Likewise, in vivo transected sciatic nerves in rats locally perfused with roscovitine had augmented repopulation of new myelinated axons beyond the transection zone. By mass spectrometry, we found that roscovitine interacts with tubulin and actin. It interacts directly with tubulin and causes a dose-dependent induction of tubulin polymerization as well as enhances Guanosine-5′-triphosphate (GTP)-dependent tubulin polymerization. Conversely, roscovitine interacts indirectly with actin and counteracts the inhibitory effect of cyclin-dependent kinases 5 (Cdk5) on Actin-Related Proteins 2/3 (Arp2/3)-dependent actin polymerization, and thus, causes actin polymerization. Moreover, in the presence of neurotrophic factors such as nerve growth factor (NGF), roscovitine-enhanced neurite outgrowth is mediated by increased activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathways. Since microtubule and F-actin dynamics are critical for axonal regrowth, the ability of roscovitine to activate the ERK1/2 and p38 MAPK pathways and support polymerization of tubulin and actin indicate a major role for this purine nucleoside analog in the promotion of axonal regeneration. Together, our findings demonstrate a therapeutic potential for the purine nucleoside analog, roscovitine, in peripheral nerve injury.

A conceptual view at microtubule plus end dynamics in neuronal axons

Axons are the cable-like protrusions of neurons which wire up the nervous system. Polar bundles of microtubules (MTs) constitute their structural backbones and are highways for life-sustaining transport between proximal cell bodies and distal synapses. Any morphogenetic changes of axons during development, plastic rearrangement, regeneration or degeneration depend on dynamic changes of these MT bundles. A key mechanism for implementing such changes is the coordinated polymerisation and depolymerisation at the plus ends of MTs within these bundles. To gain an understanding of how such regulation can be achieved at the cellular level, we provide here an integrated overview of the extensive knowledge we have about the molecular mechanisms regulating MT de/polymerisation. We first summarise insights gained from work in vitro, then describe the machinery which supplies the essential tubulin building blocks, the protein complexes associating with MT plus ends, and MT shaft-based mechanisms that influence plus end dynamics. We briefly summarise the contribution of MT plus end dynamics to important cellular functions in axons, and conclude by discussing the challenges and potential strategies of integrating the existing molecular knowledge into conceptual understanding at the level of axons.

Quercetin promotes neurite growth through enhancing intracellular cAMP level and GAP-43 expression

The present study was designed to investigate the role of quercetin on neurite growth in N1E-115 cells and the underlying mechanisms. Quercetin was evaluated for its effects on cell numbers of neurites, neurite length, intracellular cAMP content, and Gap-43 expression in N1E-115 cells in vitro by use of microscopy, LANCE(tm) cAMP 384 kit, and Western blot analysis, respectively. Our results showed that quercetin could increase the neurite length in a concentration-dependent manner, but had no effect on the numbers of cells. Quercetin significantly increased the expression of cellular cAMP in a time- and concentration-dependent manner. The Gap-43 expression was up-regulated in a time-dependent manner. In conclusion, quercetin could promote neurite growth through increasing the intracellular cAMP level and Gap-43 expression.

Protein synthetic machinery and mRNA in regenerating tips of spinal cord axons in lamprey

Polyribosomes, mRNA, and other elements of translational machinery have been reported in peripheral nerves and in elongating injured axons of sensory neurons in vitro, primarily in growth cones. Evidence for involvement of local protein synthesis in regenerating central nervous system (CNS) axons is less extensive. We monitored regeneration of back-labeled lamprey spinal axons after spinal cord transection and detected mRNA in axon tips by in situ hybridization and microaspiration of their axoplasm. Poly(A)+mRNA was present in the axon tips, and was more abundant in actively regenerating tips than in static or retracting ones. Target-specific polymerase chain reaction (PCR) and in situ hybridization revealed plentiful mRNA for the low molecular neurofilament subunit and β-tubulin, but very little for β-actin, consistent with the morphology of their tips, which lack filopodia and lamellipodia. Electron microscopy showed ribosomes/polyribosomes in the distal parts of axon tips and in association with vesicle-like membranes, primarily in the tip. In one instance, there were structures with the appearance of rough endoplasmic reticulum. Immunohistochemistry showed patches of ribosomal protein S6 positivity in a similar distribution. The results suggest that local protein synthesis might be involved in the mechanism of axon regeneration in the lamprey spinal cord. J. Comp. Neurol. 524:3614-3640, 2016. © 2016 Wiley Periodicals, Inc.

Classical Conditioning and Modification of the Rabbit's (Oryctolagus Cuniculus) Unconditioned Nictitating Membrane Response

A fundamental tenet of behavior is that a reflex is automatic, unconscious, involuntary, and relatively invariant. However, we have discovered that a reflex can change dramatically as a function of classical conditioning, and this change can be demonstrated independently of the conditioned stimulus. We have termed this phenomenon conditioning-specific reflex modification (CRM). Although the behavioral laws and neural substrates of nonassociative reflex changes have been identified, the behavioral laws and neural substrates of CRM are only now being revealed. For example, CRM is similar to classical conditioning in that (a) it is a function of both the strength of conditioning and (b) the strength of the unconditioned stimulus, (c) it can be extinguished, and (d) it can be generalized from one unconditioned stimulus to another. Preliminary analysis suggests that CRM may have some features in common with post-traumatic stress disorder and may provide insights into treatment of the disorder.

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.

Glutamate Stimulates Local Protein Synthesis in the Axons of Rat Cortical Neurons by Activating α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors and Metabotropic Glutamate Receptors

Glutamate is the principal excitatory neurotransmitter in the mammalian CNS. By analyzing the metabolic incorporation of azidohomoalanine, a methionine analogue, in newly synthesized proteins, we find that glutamate treatments up-regulate protein translation not only in intact rat cortical neurons in culture but also in the axons emitting from cortical neurons before making synapses with target cells. The process by which glutamate stimulates local translation in axons begins with the binding of glutamate to the ionotropic AMPA receptors and metabotropic glutamate receptor 1 and members of group 2 metabotropic glutamate receptors on the plasma membrane. Subsequently, the activated mammalian target of rapamycin (mTOR) signaling pathway and the rise in Ca(2+), resulting from Ca(2+) influxes through calcium-permeable AMPA receptors, voltage-gated Ca(2+) channels, and transient receptor potential canonical channels, in axons stimulate the local translation machinery. For comparison, the enhancement effects of brain-derived neurotrophic factor (BDNF) on the local protein synthesis in cortical axons were also studied. The results indicate that Ca(2+) influxes via transient receptor potential canonical channels and activated the mTOR pathway in axons also mediate BDNF stimulation to local protein synthesis. However, glutamate- and BDNF-induced enhancements of translation in axons exhibit different kinetics. Moreover, Ca(2+) and mTOR signaling appear to play roles carrying different weights, respectively, in transducing glutamate- and BDNF-induced enhancements of axonal translation. Thus, our results indicate that exposure to transient increases of glutamate and more lasting increases of BDNF would stimulate local protein synthesis in migrating axons en route to their targets in the developing brain.

Axonally synthesized ATF4 transmits a neurodegenerative signal across brain regions

In Alzheimer's disease (AD) brain, exposure of axons to Aβ causes pathogenic changes that spread retrogradely by unknown mechanisms, affecting the entire neuron. We found that locally applied Aβ1-42 initiates axonal synthesis of a defined set of proteins including the transcription factor ATF4. Inhibition of local translation and retrograde transport or knockdown of axonal Atf4 mRNA abolished Aβ-induced ATF4 transcriptional activity and cell loss. Aβ1-42 injection into the dentate gyrus (DG) of mice caused loss of forebrain neurons whose axons project to the DG. Protein synthesis and Atf4 mRNA were upregulated in these axons, and coinjection of Atf4 siRNA into the DG reduced the effects of Aβ1-42 in the forebrain. ATF4 protein and transcripts were found with greater frequency in axons in the brain of AD patients. These results reveal an active role for intra-axonal translation in neurodegeneration and identify ATF4 as a mediator for the spread of AD pathology.

APC is an RNA-binding protein, and its interactome provides a link to neural development and microtubule assembly

Adenomatous polyposis coli (APC) is a microtubule plus-end scaffolding protein important in biology and disease. APC is implicated in RNA localization, although the mechanisms and functional significance remain unclear. We show APC is an RNA-binding protein and identify an RNA interactome by HITS-CLIP. Targets were highly enriched for APC-related functions, including microtubule organization, cell motility, cancer, and neurologic disease. Among the targets is β2B-tubulin, known to be required in human neuron and axon migration. We show β2B-tubulin is synthesized in axons and localizes preferentially to dynamic microtubules in the growth cone periphery. APC binds the β2B-tubulin 3' UTR; experiments interfering with this interaction reduced β2B-tubulin mRNA axonal localization and expression, depleted dynamic microtubules and the growth cone periphery, and impaired neuron migration. These results identify APC as a platform binding functionally related protein and RNA networks, and suggest a self-organizing model for the microtubule to localize synthesis of its own subunits.

In vitro growth conditions and development affect differential distributions of RNA in axonal growth cones and shafts of cultured rat hippocampal neurons

Local synthesis of proteins in the axons participates in axonogenesis and axon guidance to establish appropriate synaptic connections and confer plasticity. To study the transcripts present in the growth cones and axonal shafts of cultured rat hippocampal neurons, two chip devices, differing in their abilities to support axonal growth and branching, are designed and employed here to isolate large quantities of axonal materials. Cone-, shaft- and axon-residing transcripts with amounts higher than that of a somatodendritic transcript, Actg1 (γ-actin), are selected and classified. Since the chips are optically transparent, distribution of transcripts over axons can be studied by fluorescence in situ hybridization. Three transcripts, Cadm1 (cell adhesion molecule 1), Nefl (neurofilament light polypeptide), and Cfl1 (non-muscle cofilin) are confirmed to be preferentially localized to the growth cones, while Pfn2 (profilin2) is preferentially localized to the shafts of those axons growing on the chip that restricts axonal growth. The different growing conditions of axons on chips and on conventional coverslips do not affect the cone-preferred localization of Cadm1 and shaft-preferred localization of Pfn2, but affect the distributions of Nefl and Cfl1 over the axons at 14th day in vitro. Furthermore, the distributions of Cadm1 and Nefl over the axons growing on conventional coverslips undergo changes during in vitro development. Our results suggest a dynamic nature of the mechanisms regulating the distributions of transcripts in axonal substructures in a manner dependent upon both growth conditions and neuronal maturation.

Neurodegenerative changes are prevented by Erythropoietin in the pmn model of motoneuron degeneration

Motoneuron diseases are fatal neurodegenerative disorders characterized by a progressive loss of motoneurons, muscle weakness and premature death. The progressive motor neuronopathy (pmn) mutant mouse has been considered a good model for the autosomal recessive childhood form of spinal muscular atrophy (SMA). Here, we investigated the therapeutic potential of Erythropoietin (Epo) on this mutant mouse. Symptomatic or pre-symptomatic treatment with Epo significantly prolongs lifespan by 84.6% or 87.2% respectively. Epo preserves muscle strength and significantly attenuates behavioural motor deficits of mutant pmn mice. Histological and metabolic changes in the spinal cord evaluated by immunohistochemistry, western blot, and high-resolution (1)H-NMR spectroscopy were also greatly prevented by Epo-treatment. Our results illustrate the efficacy of Epo in improving quality of life of mutant pmn mice and open novel therapeutic pathways for motoneuron diseases.

Local translation of TC10 is required for membrane expansion during axon outgrowth

The surface of developing axons expands in a process mediated by the exocyst complex. The spatial-temporal regulation of the exocyst is only partially understood. Here we report that stimulated membrane enlargement in dorsal root ganglion (DRG) axons is triggered by intra-axonal synthesis of TC10, a small GTPase required for exocyst function. Induced membrane expansion and axon outgrowth are inhibited after axon-specific knockdown of TC10 mRNA. To determine the relationship of intra-axonal TC10 synthesis with the previously described stimulus-induced translation of the cytoskeletal regulator Par3, we investigate the signaling pathways controlling their local translation in response to NGF. Phosphoinositide 3-kinase (PI3K)-dependent activation of the Rheb-mTOR pathway triggers the simultaneous local synthesis of TC10 and Par3. These results reveal the importance of local translation in the control of membrane dynamics and demonstrate that localized, mTOR-dependent protein synthesis triggers the simultaneous activation of parallel pathways.

Increased axonal ribosome numbers is an early event in the pathogenesis of amyotrophic lateral sclerosis

Myelinating glia cells support axon survival and functions through mechanisms independent of myelination, and their dysfunction leads to axonal degeneration in several diseases. In amyotrophic lateral sclerosis (ALS), spinal motor neurons undergo retrograde degeneration, and slowing of axonal transport is an early event that in ALS mutant mice occurs well before motor neuron degeneration. Interestingly, in familial forms of ALS, Schwann cells have been proposed to slow disease progression. We demonstrated previously that Schwann cells transfer polyribosomes to diseased and regenerating axons, a possible rescue mechanism for disease-induced reductions in axonal proteins. Here, we investigated whether elevated levels of axonal ribosomes are also found in ALS, by analysis of a superoxide dismutase 1 (SOD1)^G93A mouse model for human familial ALS and a patient suffering from sporadic ALS. In both cases, we found that the disorder was associated with an increase in the population of axonal ribosomes in myelinated axons. Importantly, in SOD1^G93A mice, the appearance of axonal ribosomes preceded the manifestation of behavioral symptoms, indicating that upregulation of axonal ribosomes occurs early in the pathogenesis of ALS. In line with our previous studies, electron microscopy analysis showed that Schwann cells might serve as a source of axonal ribosomes in the disease-compromised axons. The early appearance of axonal ribosomes indicates an involvement of Schwann cells early in ALS neuropathology, and may serve as an early marker for disease-affected axons, not only in ALS, but also for other central and peripheral neurodegenerative disorders.

Regulation of mRNA transport, localization and translation in the nervous system of mammals (Review)

Post-transcriptional control of mRNA trafficking and metabolism plays a critical role in the actualization and fine tuning of the genetic program of cells, both in development and in differentiated tissues. Cis-acting signals, responsible for post-transcriptional regulation, reside in the RNA message itself, usually in untranslated regions, 5′ or 3′ to the coding sequence, and are recognized by trans-acting factors: RNA-binding proteins (RBPs) and/or non-coding RNAs (ncRNAs). ncRNAs bind short mRNA sequences usually present in the 3′-untranslated (3′-UTR) region of their target messages. RBPs recognize specific nucleotide sequences and/or secondary/tertiary structures. Most RBPs assemble on mRNA at the moment of transcription and shepherd it to its destination, somehow determining its final fate. Regulation of mRNA localization and metabolism has a particularly important role in the nervous system where local translation of pre-localized mRNAs has been implicated in developing axon and dendrite pathfinding, and in synapse formation. Moreover, activity-dependent mRNA trafficking and local translation may underlie long-lasting changes in synaptic efficacy, responsible for learning and memory. This review focuses on the role of RBPs in neuronal development and plasticity, as well as possible connections between ncRNAs and RBPs.