Local translation and mRNA trafficking in axon pathfinding

Force: A messenger of axon outgrowth

The axon is a sophisticated macromolecular machine composed of interrelated parts that transmit signals like spur gears transfer motion between parallel shafts. The growth cone is a fine sensor that integrates mechanical and chemical cues and transduces these signals through the generation of a traction force that pushes the tip and pulls the axon shaft forward. The axon shaft, in turn, senses this pulling force and transduces this signal in an orchestrated response, coordinating cytoskeleton remodeling and intercalated mass addition to sustain and support the advancing of the tip. Extensive research suggests that the direct application of active force is per se a powerful inducer of axon growth, potentially bypassing the contribution of the growth cone. This review provides a critical perspective on current knowledge of how the force is a messenger of axon growth and its mode of action for controlling navigation, including aspects that remain unclear. It also focuses on novel approaches and tools designed to mechanically manipulate axons, and discusses their implications in terms of potential novel therapies for re-wiring the nervous system.

Control of circadian rhythm on cortical excitability and synaptic plasticity

Living organisms navigate through a cyclic world: activity, feeding, social interactions are all organized along the periodic succession of night and day. At the cellular level, periodic activity is controlled by the molecular machinery driving the circadian regulation of cellular homeostasis. This mechanism adapts cell function to the external environment and its crucial importance is underlined by its robustness and redundancy. The cell autonomous clock regulates cell function by the circadian modulation of mTOR, a master controller of protein synthesis. Importantly, mTOR integrates the circadian modulation with synaptic activity and extracellular signals through a complex signaling network that includes the RAS-ERK pathway. The relationship between mTOR and the circadian clock is bidirectional, since mTOR can feedback on the cellular clock to shift the cycle to maintain the alignment with the environmental conditions. The mTOR and ERK pathways are crucial determinants of synaptic plasticity and function and thus it is not surprising that alterations of the circadian clock cause defective responses to environmental challenges, as witnessed by the bi-directional relationship between brain disorders and impaired circadian regulation. In physiological conditions, the feedback between the intrinsic clock and the mTOR pathway suggests that also synaptic plasticity should undergo circadian regulation.

Dynamic remodeling of ribosomes and endoplasmic reticulum in axon terminals of motoneurons

In neurons, the endoplasmic reticulum (ER) forms a highly dynamic network that enters axons and presynaptic terminals and plays a central role in Ca2+ homeostasis and synapse maintenance; however, the underlying mechanisms involved in regulation of its dynamic remodeling as well as its function in axon development and presynaptic differentiation remain elusive. Here, we used high-resolution microscopy and live-cell imaging to investigate rapid movements of the ER and ribosomes in axons of cultured motoneurons after stimulation with brain-derived neurotrophic factor. Our results indicate that the ER extends into axonal growth cone filopodia, where its integrity and dynamic remodeling are regulated mainly by actin and the actin-based motor protein myosin VI (encoded by Myo6). Additionally, we found that in axonal growth cones, ribosomes assemble into 80S subunits within seconds and associate with the ER in response to extracellular stimuli, which describes a novel function of axonal ER in dynamic regulation of local translation. This article has an associated First Person interview with Chunchu Deng, joint first author of the paper.

Altered ribosomal function and protein synthesis caused by tau

The synthesis of new proteins is a fundamental aspect of cellular life and is required for many neurological processes, including the formation, updating and extinction of long-term memories. Protein synthesis is impaired in neurodegenerative diseases including tauopathies, in which pathology is caused by aberrant changes to the microtubule-associated protein tau. We recently showed that both global de novo protein synthesis and the synthesis of select ribosomal proteins (RPs) are decreased in mouse models of frontotemporal dementia (FTD) which express mutant forms of tau. However, a comprehensive analysis of the effect of FTD-mutant tau on ribosomes is lacking. Here we used polysome profiling, de novo protein labelling and mass spectrometry-based proteomics to examine how ribosomes are altered in models of FTD. We identified 10 RPs which were decreased in abundance in primary neurons taken from the K3 mouse model of FTD. We further demonstrate that expression of human tau (hTau) decreases both protein synthesis and biogenesis of the 60S ribosomal subunit, with these effects being exacerbated in the presence of FTD-associated tau mutations. Lastly, we demonstrate that expression of the amino-terminal projection domain of hTau is sufficient to reduce protein synthesis and ribosomal biogenesis. Together, these data reinforce a role for tau in impairing ribosomal function.

The TRIM9/TRIM67 neuronal interactome reveals novel activators of morphogenesis

TRIM9 and TRIM67 are neuronally enriched E3 ubiquitin ligases essential for appropriate morphogenesis of cortical and hippocampal neurons and fidelitous responses to the axon guidance cue netrin-1. Deletion of murine Trim9 or Trim67 results in neuroanatomical defects and striking behavioral deficits, particularly in spatial learning and memory. TRIM9 and TRIM67 interact with cytoskeletal and exocytic proteins, but the full interactome is not known. Here we performed the unbiased proximity-dependent biotin identification (BioID) approach to define TRIM9 and TRIM67 protein-protein proximity network in developing cortical neurons and identified putative neuronal TRIM interaction partners. Candidates included cytoskeletal regulators, cytosolic protein transporters, exocytosis and endocytosis regulators, and proteins necessary for synaptic regulation. A subset of high-priority candidates was validated, including Myo16, Coro1A, MAP1B, ExoC1, GRIP1, PRG-1, and KIF1A. For a subset of validated candidates, we utilized total internal reflection fluorescence microscopy to demonstrate dynamic colocalization with TRIM proteins at the axonal periphery, including at the tips of filopodia. Further analysis demonstrated that the RNA interference-based knockdown of the unconventional myosin Myo16 in cortical neurons altered growth cone filopodia density and axonal branching patterns in a TRIM9- and netrin-1-dependent manner. Future analysis of other validated candidates will likely identify novel proteins and mechanisms by which TRIM9 and TRIM67 regulate neuronal form and function. [Media: see text].

PDCD4 regulates axonal growth by translational repression of neurite growth-related genes and is modulated during nerve injury responses

Programmed cell death 4 (PDCD4) protein is a tumor suppressor that inhibits translation through the mTOR-dependent initiation factor EIF4A, but its functional role and mRNA targets in neurons remain largely unknown. Our work identified that PDCD4 is highly expressed in axons and dendrites of CNS and PNS neurons. Using loss- and gain-of-function experiments in cortical and dorsal root ganglia primary neurons, we demonstrated the capacity of PDCD4 to negatively control axonal growth. To explore PDCD4 transcriptome and translatome targets, we used Ribo-seq and uncovered a list of potential targets with known functions as axon/neurite outgrowth regulators. In addition, we observed that PDCD4 can be locally synthesized in adult axons in vivo, and its levels decrease at the site of peripheral nerve injury and before nerve regeneration. Overall, our findings demonstrate that PDCD4 can act as a new regulator of axonal growth via the selective control of translation, providing a target mechanism for axon regeneration and neuronal plasticity processes in neurons.

Synaptogenesis Stimulates a Proteasome-Mediated Ribosome Reduction in Axons

Ribosomes and a subset of cellular mRNAs are trafficked into axons of developing neurons. The axonal localization of translational machinery allows new proteins to be rapidly and locally synthesized during axonal growth and pathfinding. However, in mature neurons, axonal ribosomes are significantly reduced or even absent. The mechanism that elicits this removal is currently unknown. Here, we demonstrate that synapse formation is the trigger for ribosome reduction in mature axons. In vivo analysis shows that axonal ribosome levels decrease in rat brain at a developmental stage coincident with synapse formation. Next, we observe in vitro that different synaptogenic inducers trigger an overall decrease of ribosomal proteins and rRNA in the axons of spinal motor neurons. We further observe that this process is dependent on the ubiquitin-proteasome system but not on autophagy. Together, these data identify synaptogenesis as the long missing biological trigger that leads to ribosome disappearance during axonal maturation.

The mechanism behind the striking loss of ribosomes from axons during neuronal maturation is unknown. Using in vivo and in vitro models, including neuron-muscle co-cultures and combining biochemistry and imaging techniques, Costa et al. demonstrate that synapse formation triggers ribosome reduction by a mechanism involving the ubiquitin-proteasome system.

Distinct regulation of bioenergetics and translation by group I mGluR and NMDAR

Neuronal activity is responsible for the high energy consumption in the brain. However, the cellular mechanisms draining ATP upon the arrival of a stimulus are yet to be explored systematically at the post-synapse. Here, we provide evidence that a significant fraction of ATP is consumed upon glutamate stimulation to energize mGluR-induced protein synthesis. We find that both mGluR and NMDAR alter protein synthesis and ATP consumption with distinct kinetics at the synaptic-dendritic compartments. While mGluR activation leads to a rapid and sustained reduction in neuronal ATP levels, NMDAR activation has no immediate impact on the same. ATP consumption correlates inversely with the kinetics of protein synthesis for both receptors. We observe a persistent elevation in protein synthesis within 5 minutes of mGluR activation and a robust inhibition of the same within 2 minutes of NMDAR activation, assessed by the phosphorylation status of eEF2 and metabolic labeling. However, a delayed protein synthesis-dependent ATP expenditure ensues after 15 minutes of NMDAR stimulation. We identify a central role for AMPK in the correlation between protein synthesis and ATP consumption. AMPK is dephosphorylated and inhibited upon mGluR activation, while it is phosphorylated upon NMDAR activation. Perturbing AMPK activity disrupts receptor-specific modulations of eEF2 phosphorylation and protein synthesis. Our observations, therefore, demonstrate that the regulation of the AMPK-eEF2 signaling axis by glutamate receptors alters neuronal protein synthesis and bioenergetics.

Upregulation of Neuronal Rheb(S16H) for Hippocampal Protection in the Adult Brain

Ras homolog protein enriched in brain (Rheb) is a key activator of mammalian target of rapamycin complex 1 (mTORC1). The activation of mTORC1 by Rheb is associated with various processes such as protein synthesis, neuronal growth, differentiation, axonal regeneration, energy homeostasis, autophagy, and amino acid uptake. In addition, Rheb-mTORC1 signaling plays a crucial role in preventing the neurodegeneration of hippocampal neurons in the adult brain. Increasing evidence suggests that the constitutive activation of Rheb has beneficial effects against neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Our recent studies revealed that adeno-associated virus serotype 1 (AAV1) transduction with Rheb(S16H), a constitutively active form of Rheb, exhibits neuroprotective properties through the induction of various neurotrophic factors, promoting neurotrophic interactions between neurons and astrocytes in the hippocampus of the adult brain. This review provides compelling evidence for the therapeutic potential of AAV1-Rheb(S16H) transduction in the hippocampus of the adult brain by exploring its neuroprotective effects and mechanisms.

Could an Impairment in Local Translation of mRNAs in Glia be Contributing to Pathogenesis in ALS?

One of the key pathways implicated in amyotrophic lateral sclerosis (ALS) pathogenesis is abnormal RNA processing. Studies to date have focussed on defects in RNA stability, splicing, and translation, but this review article will focus on the largely overlooked RNA processing mechanism of RNA trafficking, with particular emphasis on the importance of glia. In the central nervous system (CNS), oligodendrocytes can extend processes to myelinate and metabolically support up to 50 axons and astrocytes can extend processes to cover up to 100,000 synapses, all with differing local functional requirements. Furthermore, many of the proteins required in these processes are large, aggregation-prone proteins which would be difficult to transport in their fully translated, terminally-folded state. This, therefore, highlights a critical requirement in these cells for local control of protein translation, which is achieved through specific trafficking of mRNAs to each process and local translation therein. Given that a large number of RNA-binding proteins have been implicated in ALS, and RNA-binding proteins are essential for trafficking mRNAs from the nucleus to glial processes for local translation, RNA misprocessing in glial cells is a likely source of cellular dysfunction in ALS. To date, neurons have been the focus of ALS research, but an intrinsic deficit in glia, namely astrocytes and oligodendrocytes, could have an additive effect on declining neuronal function in ALS. This review article aims to highlight the key evidence that supports the contention that RNA trafficking deficits in astrocytes and oligodendrocytes may contribute to in ALS.

Mechanistic Target of Rapamycin Pathway in Epileptic Disorders

The mechanistic target of rapamycin (mTOR) pathway coordinates the metabolic activity of eukaryotic cells through environmental signals, including nutrients, energy, growth factors, and oxygen. In the nervous system, the mTOR pathway regulates fundamental biological processes associated with neural development and neurodegeneration. Intriguingly, genes that constitute the mTOR pathway have been found to be germline and somatic mutation from patients with various epileptic disorders. Hyperactivation of the mTOR pathway due to said mutations has garnered increasing attention as culprits of these conditions : somatic mutations, in particular, in epileptic foci have recently been identified as a major genetic cause of intractable focal epilepsy, such as focal cortical dysplasia. Meanwhile, epilepsy models with aberrant activation of the mTOR pathway have helped elucidate the role of the mTOR pathway in epileptogenesis, and evidence from epilepsy models of human mutations recapitulating the features of epileptic patients has indicated that mTOR inhibitors may be of use in treating epilepsy associated with mutations in mTOR pathway genes. Here, we review recent advances in the molecular and genetic understanding of mTOR signaling in epileptic disorders. In particular, we focus on the development of and limitations to therapies targeting the mTOR pathway to treat epileptic seizures. We also discuss future perspectives on mTOR inhibition therapies and special diagnostic methods for intractable epilepsies caused by brain somatic mutations.

Apolipoprotein E Affects In Vitro Axonal Growth and Regeneration via the MAPK Signaling Pathway

Following central nervous system injury in mammals, failed axonal regeneration is closely related to dysneuria. Previous studies have shown that the obvious effects of apolipoprotein E (ApoE) on traumatic brain injury (TBI) were associated with an axonal mechanism. However, little information on the actions of ApoE and its isoforms on axonal regeneration following TBI was provided. In our study, the cerebral cortices of ApoE-deficient (ApoE^-/-) and wild-type (ApoE^+/+) mice were cultured in vitro, and an axonal transection model was established. Interventions included the conditioned medium of astrocytes, human recombinant ApoE2/3/4 isoforms and inhibitors of the JNK/ERK/p38 pathway. Axonal growth and regeneration were evaluated by measuring the maximum distance and area of the axons. The expression levels of β-tubulin III, MAP2, ApoE, p-JNK, p-ERK and p-p38 were detected by immunofluorescence and western blotting. The results showed that ApoE mRNA and protein were expressed in intact axons and regenerated axons. Axonal growth and regeneration were attenuated in ApoE^-/- mice but recovered by exogenous ApoE. Human recombinant ApoE3 positively influenced axonal growth and regeneration; these effects were mediated by the JNK/ERK/p38 pathway. These results suggest ApoE and its isoforms may have influenced axonal growth and regeneration via the MAPK signaling pathway in vitro.

RACK1 is necessary for the formation of point contacts and regulates axon growth

Receptor for activated C kinase 1 (RACK1) is a multifunctional ribosomal scaffolding protein that can interact with multiple signaling molecules concurrently through its seven WD40 repeats. We recently found that RACK1 is localized to mammalian growth cones, prompting an investigation into its role during neural development. Here, we show for the first time that RACK1 localizes to point contacts within mouse cortical growth cones. Point contacts are adhesion sites that link the actin network within growth cones to the extracellular matrix, and are necessary for appropriate axon guidance. Our experiments show that RACK1 is necessary for point contact formation. Brain-derived neurotrophic factor (BDNF) stimulates an increase in point contact density, which was eliminated by RACK1 shRNA or overexpression of a nonphosphorylatable mutant form of RACK1. We also found that axonal growth requires both RACK1 expression and phosphorylation. We have previously shown that the local translation of β-actin mRNA within growth cones is necessary for appropriate axon guidance and is dependent on RACK1. Thus, we examined the location of members of the local translation complex relative to point contacts. Indeed, both β-actin mRNA and RACK1 colocalize with point contacts, and this colocalization increases following BDNF stimulation. This implies the novel finding that local translation is regulated at point contacts. Taken together, these data suggest that point contacts are a targeted site of local translation within growth cones, and RACK1 is a critical member of the point contact complex and necessary for appropriate neural development. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1038-1056, 2017.

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.

Axons provide the secretory machinery for trafficking of voltage-gated sodium channels in peripheral nerve

The regulation of the axonal proteome is key to generate and maintain neural function. Fast and slow axoplasmic waves have been known for decades, but alternative mechanisms to control the abundance of axonal proteins based on local synthesis have also been identified. The presence of the endoplasmic reticulum has been documented in peripheral axons, but it is still unknown whether this localized organelle participates in the delivery of axonal membrane proteins. Voltage-gated sodium channels are responsible for action potentials and are mostly concentrated in the axon initial segment and nodes of Ranvier. Despite their fundamental role, little is known about the intracellular trafficking mechanisms that govern their availability in mature axons. Here we describe the secretory machinery in axons and its contribution to plasma membrane delivery of sodium channels. The distribution of axonal secretory components was evaluated in axons of the sciatic nerve and in spinal nerve axons after in vivo electroporation. Intracellular protein trafficking was pharmacologically blocked in vivo and in vitro. Axonal voltage-gated sodium channel mRNA and local trafficking were examined by RT-PCR and a retention-release methodology. We demonstrate that mature axons contain components of the endoplasmic reticulum and other biosynthetic organelles. Axonal organelles and sodium channel localization are sensitive to local blockade of the endoplasmic reticulum to Golgi transport. More importantly, secretory organelles are capable of delivering sodium channels to the plasma membrane in isolated axons, demonstrating an intrinsic capacity of the axonal biosynthetic route in regulating the axonal proteome in mammalian axons.

Netrin-1 induces local translation of down syndrome cell adhesion molecule in axonal growth cones

Down syndrome cell adhesion molecule (DSCAM) plays an important role in many neurodevelopmental processes such as axon guidance, dendrite arborization, and synapse formation. DSCAM is located in the Down syndrome trisomic region of human chromosome 21 and may contribute to the Down syndrome brain phenotype, which includes a reduction in the formation of long-distance connectivity. The local translation of a select group of mRNA transcripts within growth cones is necessary for the formation of appropriate neuronal connectivity. Interestingly, we have found that Dscam mRNA is localized to growth cones of mouse hippocampal neurons, and is dynamically regulated in response to the axon guidance molecule, netrin-1. Furthermore, netrin-1 stimulation results in an increase in locally translated DSCAM protein in growth cones. Deleted in colorectal cancer (DCC), a netrin-1 receptor, is required for the netrin-1-induced increase in Dscam mRNA local translation. We also find that two RNA-binding proteins-fragile X mental retardation protein (FMRP) and cytoplasmic polyadenylation element binding protein (CPEB)-colocalize with Dscam mRNA in growth cones, suggesting their regulation of Dscam mRNA localization and translation. Finally, overexpression of DSCAM in mouse cortical neurons results in a severe stunting of axon outgrowth and branching, suggesting that an increase in DSCAM protein results in a structural change having functional consequences. Taken together, these results suggest that netrin-1-induced local translation of Dscam mRNA during embryonic development may be an important mechanism to regulate axon growth and guidance in the developing nervous system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 799-816, 2016.

Coupled local translation and degradation regulate growth cone collapse

Local translation mediates axonal responses to Semaphorin3A (Sema3A) and other guidance cues. However, only a subset of the axonal proteome is locally synthesized, while most proteins are trafficked from the soma. The reason why only specific proteins are locally synthesized is unknown. Here we show that local protein synthesis and degradation are linked events in growth cones. We find that growth cones exhibit high levels of ubiquitination and that local signaling pathways trigger the ubiquitination and degradation of RhoA, a mediator of Sema3A-induced growth cone collapse. Inhibition of RhoA degradation is sufficient to remove the protein-synthesis requirement for Sema3A-induced growth cone collapse. In addition to RhoA, we find that locally translated proteins are the main targets of the ubiquitin-proteasome system in growth cones. Thus, local protein degradation is a major feature of growth cones and creates a requirement for local translation to replenish proteins needed to maintain growth cone responses.

The neurology of mTOR

The mechanistic target of rapamycin (mTOR) signaling pathway is a crucial cellular signaling hub that, like the nervous system itself, integrates internal and external cues to elicit critical outputs including growth control, protein synthesis, gene expression, and metabolic balance. The importance of mTOR signaling to brain function is underscored by the myriad disorders in which mTOR pathway dysfunction is implicated, such as autism, epilepsy, and neurodegenerative disorders. Pharmacological manipulation of mTOR signaling holds therapeutic promise and has entered clinical trials for several disorders. Here, we review the functions of mTOR signaling in the normal and pathological brain, highlighting ongoing efforts to translate our understanding of cellular physiology into direct medical benefit for neurological disorders.

SMN regulates axonal local translation via miR-183/mTOR pathway

Reduced expression of SMN protein causes spinal muscular atrophy (SMA), a neurodegenerative disorder leading to motor neuron dysfunction and loss. However, the molecular mechanisms by which SMN regulates neuronal dysfunction are not fully understood. Here, we report that reduced SMN protein level alters miRNA expression and distribution in neurons. In particular, miR-183 levels are increased in neurites of SMN-deficient neurons. We demonstrate that miR-183 regulates translation of mTor via direct binding to its 3' UTR. Interestingly, local axonal translation of mTor is reduced in SMN-deficient neurons, and this can be recovered by miR-183 inhibition. Finally, inhibition of miR-183 expression in the spinal cord of an SMA mouse model prolongs survival and improves motor function of Smn-mutant mice. Together, these observations suggest that axonal miRNAs and the mTOR pathway are previously unidentified molecular mechanisms contributing to SMA pathology.

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.

A comparative quantitative assessment of axonal and dendritic mRNA transport in maturing hippocampal neurons

Translation of mRNA in axons and dendrites enables a rapid supply of proteins to specific sites of localization within the neuron. Distinct mRNA-containing cargoes, including granules and mitochondrial mRNA, are transported within neuronal projections. The distributions of these cargoes appear to change during neuronal development, but details on the dynamics of mRNA transport during these transitions remain to be elucidated. For this study, we have developed imaging and image processing methods to quantify several transport parameters that can define the dynamics of RNA transport and localization. Using these methods, we characterized the transport of mitochondrial and non-mitochondrial mRNA in differentiated axons and dendrites of cultured hippocampal neurons varying in developmental maturity. Our results suggest differences in the transport profiles of mitochondrial and non-mitochondrial mRNA, and differences in transport parameters at different time points, and between axons and dendrites. Furthermore, within the non-mitochondrial mRNA pool, we observed two distinct populations that differed in their fluorescence intensity and velocity. The net axonal velocity of the brighter pool was highest at day 7 (0.002±0.001 µm/s, mean ± SEM), raising the possibility of a presynaptic requirement for mRNA during early stages of synapse formation. In contrast, the net dendritic velocity of the brighter pool increased steadily as neurons matured, with a significant difference between day 12 (0.0013±0.0006 µm/s ) and day 4 (−0.003±0.001 µm/s) suggesting a postsynaptic role for mRNAs in more mature neurons. The dim population showed similar trends, though velocities were two orders of magnitude higher than of the bright particles. This study provides a baseline for further studies on mRNA transport, and has important implications for the regulation of neuronal plasticity during neuronal development and in response to neuronal injury.

Sonic hedgehog and Wnt: antagonists in morphogenesis but collaborators in axon guidance

As indicated by their name, morphogens were first identified for their role in the formation of tissues early in development. Secreted from a source, they spread through the tissue to form gradients by which they affect the differentiation of precursor cells in a concentration-dependent manner. In this context, the antagonistic roles of the morphogens of the Wnt family and Sonic hedgehog (Shh) in the specification of cell types along the dorso-ventral axis of the neural tube have been studied in detail. However, more recently, morphogens have been demonstrated to act well beyond the early stages of nervous system development, as additional roles of morphogen gradients in vertebrate neural circuit formation have been identified. Both Wnt and Shh affect neural circuit formation at several stages by their influence on neurite extension, axon pathfinding and synapse formation. In this review, we will summarize the mechanisms of morphogen function during axon guidance in the vertebrate nervous system.

Target-derived neurotrophins coordinate transcription and transport of bclw to prevent axonal degeneration

Establishment of neuronal circuitry depends on both formation and refinement of neural connections. During this process, target-derived neurotrophins regulate both transcription and translation to enable selective axon survival or elimination. However, it is not known whether retrograde signaling pathways that control transcription are coordinated with neurotrophin-regulated actions that transpire in the axon. Here we report that target-derived neurotrophins coordinate transcription of the antiapoptotic gene bclw with transport of bclw mRNA to the axon, and thereby prevent axonal degeneration in rat and mouse sensory neurons. We show that neurotrophin stimulation of nerve terminals elicits new bclw transcripts that are immediately transported to the axons and translated into protein. Bclw interacts with Bax and suppresses the caspase6 apoptotic cascade that fosters axonal degeneration. The scope of bclw regulation at the levels of transcription, transport, and translation provides a mechanism whereby sustained neurotrophin stimulation can be integrated over time, so that axonal survival is restricted to neurons connected within a stable circuit.

14-3-3 proteins regulate retinal axon growth by modulating ADF/cofilin activity

Precise navigation of axons to their targets is critical for establishing proper neuronal networks during development. Axon elongation, whereby axons extend far beyond the site of initiation to reach their target cells, is an essential step in this process, but the precise molecular pathways that regulate axon growth remain uncharacterized. Here we show that 14-3-3/14-3-3ς proteins-adaptor proteins that modulate diverse cellular processes including cytoskeletal dynamics-play a critical role in Xenopus retinal ganglion cell (RGC) axon elongation in vivo and in vitro. We have identified the expression of 14-3-3/14-3-3ς transcripts and proteins in retinal growth cones, with higher levels of expression occurring during the phase of rapid pathway extension. Competitive inhibition of 14-3-3/14-3-3ς by expression of a genetically encoded peptide, R18, in RGCs resulted in a marked decrease in the length of the initial retinotectal projection in vivo and a corresponding decrease in axon elongation rate in vitro (30-40%). Furthermore, 14-3-3/14-3-3ς (R1) co-localized with Xenopus actin depolymerizing factor (ADF)/cofilin (XAC) in RGC growth cones. Inhibition of 14-3-3/14-3-3ς function with either R18 or morpholinos reduced the level of inactive pXAC and increased the sensitivity to collapse by the repulsive cue, Slit2. Collectively, these results demonstrate that14-3-3/14-3-3ς participates in the regulation of retinal axon elongation, in part by modulating XAC activity.

Scp160-dependent mRNA trafficking mediates pheromone gradient sensing and chemotropism in yeast

mRNAs encoding polarity and secretion factors (POLs) target the incipient bud site in yeast for localized translation during division. In pheromone-treated cells we now find that these mRNAs are also localized to the yeast-mating projection (shmoo) tip. However, in contrast to the budding program, neither the She2 nor She3 proteins are involved. Instead, the Scp160 RNA-binding protein binds POL and mating pathway mRNAs and regulates their spatial distribution in a Myo4- and cortical ER-dependent fashion. RNA binding by Scp160 is stimulated by activation of Gpa1, the G protein α subunit regulated by the pheromone receptor, and is required for pheromone gradient sensing, as well as subsequent chemotropic growth and cell-cell mating. These effects are incurred independently of obvious changes in translation; thus, mRNA trafficking is required for chemotropism and completion of the mating program. This is, to our knowledge, the first demonstration of ligand-activated RNA targeting in the development of a simple eukaryote.

Lysophosphatidic acid differentially regulates axonal mRNA translation through 5'UTR elements

Sensory neurons transport a complex population of mRNAs into their axons, including many encoding ER chaperone proteins. Transport of the mRNA encoding the ER chaperone protein calreticulin is regulated through 3'UTR elements. In other cellular systems, translation of chaperone protein mRNAs can be regulated by ER stress. Here, we have asked if the translation of axonal calreticulin mRNA is regulated in a different manner than its transport into axons. Treatment with lysophosphatidic acid, which is known to trigger axon retraction and stimulate ER Ca(2+) release, caused a translation-dependent increase in axonal calreticulin protein levels. RNA sequences in the 5'UTR of calreticulin confer this translational control through a mechanism that requires an inactivating phosphorylation of eIF2α. In contrast to calreticulin, these signaling events do not activate axonal translation through β-actin's 5'UTR. Together, these data indicate that stimulation of ER stress can regulate specificity of localized mRNA translation through 5'UTR elements.

Control of cytoplasmic mRNA localization

mRNA localization is a mechanism used by various organisms to control the spatial and temporal production of proteins. This process is a highly regulated event that requires multiple cis- and trans-acting elements that mediate the accurate localization of target mRNAs. The intrinsic nature of localization elements, together with their interaction with different RNA-binding proteins, establishes control mechanisms that can oversee the transcript from its birth in the nucleus to its specific final destination. In this review, we aim to summarize the different mechanisms of mRNA localization, with a particular focus on the various control mechanisms that affect the localization of mRNAs in the cytoplasm.

A novel mRNA affinity purification technique for the identification of interacting proteins and transcripts in ribonucleoprotein complexes

Intracellular mRNA targeting and localized translation are potential determinants for protein localization. To facilitate targeting, mRNAs possess specific cis-acting sequence motifs that are recognized by trans-acting RNA-binding proteins (RBPs). While many mRNAs are trafficked, our knowledge of the RBPs involved and presence of additional transcripts within these ribonucleoprotein (RNP) complexes is limited. To facilitate the identification of RBPs and transcripts that bind to specific mRNAs, we developed RNA-binding protein purification and identification (RaPID), a novel technique that allows for the affinity purification of MS2 aptamer-tagged mRNAs and subsequent detection of bound RBPs and transcripts using mass-spectometry and RT-PCR, respectively. RaPID effectively isolated specific mRNAs from both yeast and mammalian cells, and identified known mRNA-RBP interactions (e.g., ASH1-She2; β-Actin-IMP1). By isolating tagged OXA1 mRNA using RaPID, we could identify a yeast COPI subunit (i.e., Sec27) as a candidate interacting protein. This finding was strengthened by the observation that a portion of OXA1 mRNA was delocalized in a sec27-1 temperature-sensitive mutant at restrictive temperatures. Finally, RaPID could also be used to show biochemically the coexistence of different RNA species within the same RNP complex (e.g., coprecipitation of the yeast SRO7, WSC2, SEC3, and IST2 mRNAs with ASH1 mRNA) for the first time.

Subcellular communication through RNA transport and localized protein synthesis

Interest in the mechanisms of subcellular localization of mRNAs and the effects of localized translation has increased over the last decade. Polarized eukaryotic cells transport mRNA-protein complexes to subcellular sites, where translation of the mRNAs can be regulated by physiological stimuli. The long distances separating distal neuronal processes from their cell body have made neurons a useful model system for dissecting mechanisms of mRNA trafficking. Both the dendritic and axonal processes of neurons have been shown to have protein synthetic capacity and the diversity of mRNAs discovered in these processes continues to increase. Localized translation of mRNAs requires a co-ordinated effort by the cell body to target both mRNAs and necessary translational machinery into distal sites, as well as temporal control of individual mRNA translation. In addition to altering protein composition locally at the site of translation, some of the proteins generated in injured nerves retrogradely signal to the cell body, providing both temporal and spatial information on events occurring at distant subcellular sites.

In vivo activation of channelrhodopsin-2 reveals that normal patterns of spontaneous activity are required for motoneuron guidance and maintenance of guidance molecules

Spontaneous, highly rhythmic episodes of propagating bursting activity are present early during the development of chick and mouse spinal cords. Acetylcholine, and GABA and glycine, which are both excitatory at this stage, provide the excitatory drive. It was previously shown that a moderate decrease in the frequency of bursting activity, caused by in ovo application of the GABA(A) receptor blocker, picrotoxin, resulted in motoneurons making dorsal-ventral (D-V) pathfinding errors in the limb and in the altered expression of guidance molecules associated with this decision. To distinguish whether the pathfinding errors were caused by perturbation of the normal frequency of bursting activity or interference with GABA(A) receptor signaling, chick embryos were chronically treated in ovo with picrotoxin to block GABA(A) receptors, while light activation by channelrhodopsin-2 was used to restore bursting activity to the control frequency. The restoration of normal patterns of neural activity in the presence of picrotoxin prevented the D-V pathfinding errors in the limb and maintained the normal expression levels of EphA4, EphB1, and polysialic acid on neural cell adhesion molecule, three molecules previously shown to be necessary for this pathfinding choice. These observations demonstrate that developing spinal motor circuits are highly sensitive to the precise frequency and pattern of spontaneous activity, and that any drugs that alter this activity could result in developmental defects.

Anchoring local translation in neurons

In probing how protein synthesis in neurons is coupled to extracellular stimuli, Tcherkezian et al. (2010) discover that the receptor protein DCC anchors components of the translation machinery at the plasma membrane. Binding of DCC to its ligand, netrin, triggers the release of these components to initiate spatially restricted protein synthesis.

Fragile X Mental Retardation Protein is Involved in Protein Synthesis-Dependent Collapse of Growth Cones Induced by Semaphorin-3A

Fragile X syndrome, the most frequent form of familial mental retardation, is caused by mutation of the Fmr1 gene. Fmr1 encodes the fragile X mental retardation protein (FMRP), an mRNA binding protein regulating local, postsynaptic mRNA translation along dendrites necessary for long-term synaptic plasticity. However, recent studies on FMRP localization in axons and growth cones suggest a possible function in the regulation of local protein synthesis needed for axon guidance. Here, we have demonstrated that FMRP is involved in axonal and growth cone responses induced by the axon guidance factor, Semaphorin-3A (Sema3A). In cultured hippocampal neurons from wild type mice, Sema3A-induced growth cone collapse was protein synthesis-dependent. In contrast, Sema3A-induced growth cone collapse was attenuated in Fmr1 knock-out (KO) neurons and insensitive to protein synthesis inhibitors, suggesting that FMRP is involved in protein synthesis-dependent growth cone collapse. Sema3A increased phosphorylation of eukaryotic initiation factor 4E (eIF4E), an indicator of local translation, in distal axons and growth cones of wild type, but not Fmr1 KO neurons. Furthermore, Sema3A rapidly induced a protein synthesis-dependent increase in levels of microtubule associated protein 1B (MAP1B) in distal axons of wild type neurons, but this response was attenuated in Fmr1 KO neurons. These results suggest a possible role of FMRP to regulate local translation and axonal protein localization in response to Sema3A. This study reveals a new link between FMRP and semaphorin signaling in vitro, and raises the possibility that FMRP may have a critical role in semaphorin signaling in axon guidance during brain development.

Actin in axons: stable scaffolds and dynamic filaments

Actin filaments are thin polymers of the 42 kD protein actin. In mature axons a network of subaxolemmal actin filaments provide stability for membrane integrity and a substrate for short distance transport of cargos. In developing neurons dynamic regulation of actin polymerization and organization mediates axonal morphogenesis and axonal pathfinding to synaptic targets. Other changes in axonal shape, collateral branching, branch retraction, and axonal regeneration, also depend on actin filament dynamics. Actin filament organization is regulated by a diversity of actin-binding proteins (ABP). ABP are the focus of complex extrinsic and intrinsic signaling pathways, and many neurological pathologies and dysfunctions arise from defective regulation of ABP function.

Regulation of mRNA transport and translation in axons

Movement of mRNAs into axons occurs by active transport by microtubules through the activity of molecular motor proteins. mRNAs are sequestered into granular-like particles, referred to as transport ribonucleoprotein particles (RNPs) that mediate transport into the axonal compartment. The interaction of mRNA binding proteins with targeted mRNA is a key event in regulating axonal mRNA localization and subsequent localized translation of mRNAs. Several growth-modulating stimuli have been shown to regulate axonal mRNA localization. These do so by activating specific intracellular signaling pathways that converge upon RNA binding proteins and other components of the transport RNP to regulate their activity specifically. Transport can be both positively and negatively regulated by individual stimuli with regard to individual mRNAs. Consequently, there is exquisite specificity for regulating the axon's composition of mRNAs and proteins that control expression in the axon. Finally, recent studies indicate that axotomy can also trigger changes in axonal mRNA composition by specifically shifting the populations of mRNAs that are transported into distal axons.