Limited availability of ZBP1 restricts axonal mRNA localization and nerve regeneration capacity

Dnajc3 (HSP40) Enhances Axon Regeneration in the Mouse Optic Nerve

A forward genetics approach was used to identify genomic elements enhancing axon regeneration in the BXD recombinant mouse strains. Axon regeneration was induced by knocking down Pten in retinal ganglion cells (RGCs) using adeno-associated virus (AAV) to deliver an shRNA followed by an intravitreal injection of Zymosan with CPT-cAMP that produced a mild inflammatory response. RGC axons were damaged by optic nerve crush (ONC). Following a 12-day survival period, regenerating axons were labeled by intravitreal injection of Cholera Toxin B (CTB) conjugated with Alexa Fluor 647. Two days later, labeled axons within the optic nerve were examined to determine the number of regenerating axons and the distance they traveled down the optic nerve. The analysis revealed a surprising difference in the amount of axonal regeneration across all 33 BXD strains. There was a 7.5-fold difference in the number of regenerating axons and a 4-fold difference in distance traveled by regenerating axons. These data were used to generate an integral map defining genomic loci modulating the enhanced axonal regeneration. A quantitative trait locus modulating axon regeneration was identified on Chromosome 14 (115 to 119 Mb). Within this locus were 16 annotated genes. Subsequent testing revealed that one candidate gene, Dnajc3 , modulates axonal regeneration. Dnajc3 encodes Heat Shock Protein 40 (HSP40), which is a molecular chaperone. Knocking down Dnajc3 in the high regenerative strain (BXD90) led to a decreased regeneration response, while overexpression of Dnajc3 in a low regenerative strain (BXD34) resulted in an increased regeneration response. These findings suggest that Dnajc3 not only increases the number of regenerating axons, it also increases the distance those axons travel. This may prove to be critical for functional recovery in large mammals, where the distance axons travel to their target is considerably longer than that of the mouse. Thus, Dnajc3 may play a critical role for functional recovery in humans by increasing the number of regenerating axons and the distance the regenerating axons travel.

Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) in hematological diseases

The oncofetal mRNA-binding protein IGF2BP1 belongs to a conserved family of RNA-binding proteins. It primarily promotes RNA stability, regulates translation and RNA localization, and mediates gene expression through its downstream effectors. Numerous studies have demonstrated that IGF2BP1 plays crucial roles in embryogenesis and carcinogenesis. IGF2BP1-modulated cell proliferation, invasion, and chemo-resistance in solid tumors have attracted researchers' attention. Additionally, several studies have highlighted the importance of IGF2BP1 in hematologic malignancies and hematological genetic diseases, positioning it as a promising therapeutic target for hematological disorders. However, there is a lack of systematic summaries regarding the IGF2BP1 gene within the hematological field. In this review, we provide a comprehensive overview of the discovery and molecular structure of IGF2BP1, along with recent studies on its role in regulating embryogenesis. We also focus on the mechanisms by which IGF2BP1 regulates hematological malignancies through its interactions with its targeted mRNAs. Furthermore, we systematically elucidate the function and mechanism of IGF2BP1 in promoting fetal hemoglobin expression in adult hematopoietic stem/progenitor cells. Finally, we discuss the limitations and challenges of IGF2BP1 as a therapeutic target, offering insights into its prospects.

Diversity of Molecular Functions of RNA-Binding Ubiquitin Ligases from the MKRN Protein Family

Makorin RING finger protein family includes four members (MKRN1, MKRN2, MKRN3, and MKRN4) that belong to E3 ubiquitin ligases and play a key role in various biological processes, such as cell survival, cell differentiation, and innate and adaptive immunity. MKRN1 contributes to the tumor growth suppression, energy metabolism, anti-pathogen defense, and apoptosis and has a broad variety of targets, including hTERT, APC, FADD, p21, and various viral proteins. MKRN2 regulates cell proliferation, inflammatory response; its targets are p65, PKM2, STAT1, and other proteins. MKRN3 is a master regulator of puberty timing; it controls the levels of gonadotropin-releasing hormone in the arcuate nucleus neurons. MKRN4 is the least studied member of the MKRN protein family, however, it is known to contribute to the T cell activation by ubiquitination of serine/threonine kinase MAP4K3. Proteins of the MKRN family are associated with the development of numerous diseases, for example, systemic lupus erythematosus, central precocious puberty, Prader-Willi syndrome, degenerative lumbar spinal stenosis, inflammation, and cancer. In this review, we discuss the functional roles of all members of the MKRN protein family and their involvement in the development of diseases.

RNA-binding proteins in pain

RNAs are meticulously controlled by proteins. Through direct and indirect associations, every facet in the brief life of an mRNA is subject to regulation. RNA-binding proteins (RBPs) permeate biology. Here, we focus on their roles in pain. Chronic pain is among the largest challenges facing medicine and requires new strategies. Mounting pharmacologic and genetic evidence obtained in pre-clinical models suggests fundamental roles for a broad array of RBPs. We describe their diverse roles that span RNA modification, splicing, stability, translation, and decay. Finally, we highlight opportunities to expand our understanding of regulatory interactions that contribute to pain signaling. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Regulation RNA in Disease and Development > RNA in Disease.

Dynamic subtype- and context-specific subcellular RNA regulation in growth cones of developing neurons of the cerebral cortex

Molecular mechanisms that cells employ to compartmentalize function via localization of function-specific RNA and translation are only partially elucidated. We investigate long-range projection neurons of the cerebral cortex as highly polarized exemplars to elucidate dynamic regulation of RNA localization, stability, and translation within growth cones (GCs), leading tips of growing axons. Comparison of GC-localized transcriptomes between two distinct subtypes of projection neurons- interhemispheric-callosal and corticothalamic- across developmental stages identifies both distinct and shared subcellular machinery, and intriguingly highlights enrichment of genes associated with neurodevelopmental and neuropsychiatric disorders. Developmental context-specific components of GC-localized transcriptomes identify known and novel potential regulators of distinct phases of circuit formation: long-distance growth, target area innervation, and synapse formation. Further, we investigate mechanisms by which transcripts are enriched and dynamically regulated in GCs, and identify GC-enriched motifs in 3' untranslated regions. As one example, we identify cytoplasmic adenylation element binding protein 4 (CPEB4), an RNA binding protein regulating localization and translation of mRNAs encoding molecular machinery important for axonal branching and complexity. We also identify RNA binding motif single stranded interacting protein 1 (RBMS1) as a dynamically expressed regulator of RNA stabilization that enables successful callosal circuit formation. Subtly aberrant associative and integrative cortical circuitry can profoundly affect cortical function, often causing neurodevelopmental and neuropsychiatric disorders. Elucidation of context-specific subcellular RNA regulation for GC- and soma-localized molecular controls over precise circuit development, maintenance, and function offers generalizable insights for other polarized cells, and might contribute substantially to understanding neurodevelopmental and behavioral-cognitive disorders and toward targeted therapeutics.

Control of Selective mRNA Translation in Neuronal Subcellular Compartments in Health and Disease

In multiple cell types, mRNAs are transported to subcellular compartments, where local translation enables rapid, spatially localized, and specific responses to external stimuli. Mounting evidence has uncovered important roles played by local translation in vivo in axon survival, axon regeneration, and neural wiring, as well as strong links between dysregulation of local translation and neurologic disorders. Omic studies have revealed that >1000 mRNAs are present and can be selectively locally translated in the presynaptic and postsynaptic compartments from development to adulthood in vivo A large proportion of the locally translated mRNAs is specifically upregulated or downregulated in response to distinct extracellular signals. Given that the local translatome is large, selectively translated, and cue-specifically remodeled, a fundamental question concerns how selective translation is achieved locally. Here, we review the emerging regulatory mechanisms of local selective translation in neuronal subcellular compartments, their mRNA targets, and their orchestration. We discuss mechanisms of local selective translation that remain unexplored. Finally, we describe clinical implications and potential therapeutic strategies in light of the latest advances in gene therapy.

Customization of the translational complex regulates mRNA-specific translation to control CNS regeneration

In the adult mammalian central nervous system (CNS), axons fail to regenerate spontaneously after injury because of a combination of extrinsic and intrinsic factors. Despite recent advances targeting the intrinsic regenerative properties of adult neurons, the molecular mechanisms underlying axon regeneration are not fully understood. Here, we uncover a regulatory mechanism that controls the expression of key proteins involved in regeneration at the translational level. Our results show that mRNA-specific translation is critical for promoting axon regeneration. Indeed, we demonstrate that specific ribosome-interacting proteins, such as the protein Huntingtin (HTT), selectively control the translation of a specific subset of mRNAs. Moreover, modulating the expression of these translationally regulated mRNAs is crucial for promoting axon regeneration. Altogether, our findings highlight that selective translation through the customization of the translational complex is a key mechanism of axon regeneration with major implications in the development of therapeutic strategies for CNS repair.

Axonal mRNA binding of hnRNP A/B is crucial for axon targeting and maturation of olfactory sensory neurons

Spatiotemporal control of gene expression is important for neural development and function. Here, we show that heterogeneous nuclear ribonucleoprotein (hnRNP) A/B is highly expressed in developing olfactory sensory neurons (OSNs), and its knockout results in reduction in mature OSNs and aberrant targeting of OSN axons to the olfactory bulb. RNA immunoprecipitation analysis reveals that hnRNP A/B binds to a group of mRNAs that are highly related to axon projections and synapse assembly. Approximately 11% of the identified hnRNP A/B targets, including Pcdha and Ncam2, encode cell adhesion molecules. In Hnrnpab knockout mice, PCDHA and NCAM2 levels are significantly reduced at the axon terminals of OSNs. Furthermore, deletion of the hnRNP A/B-recognition motif in the 3' UTR of Pcdha leads to impaired PCDHA expression at the OSN axon terminals. Therefore, we propose that hnRNP A/B facilitates OSN maturation and axon projection by regulating the local expression of its target genes at axon terminals.

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.

Spatial patterns of the cap-binding complex eIF4F in human melanoma cells

As a central node of protein synthesis, the cap-binding complex, eukaryotic translation initiation factor 4 F (eIF4F), is involved in cell homeostasis, development and tumorigenesis. A large body of literature exists on the regulation and function of eIF4F in cancer cells, however the intracellular localization patterns of this complex are largely unknown. Since different subsets of mRNAs are translated in distinct subcellular compartments, understanding the distribution of translation initiation factors in the cell is of major interest. Here, we developed an in situ detection method for eIF4F at the single cell level. By using an image-based spot feature analysis pipeline as well as supervised machine learning, we identify five distinct spatial patterns of the eIF4F translation initiation complex in human melanoma cells. The quantity of eIF4F complex per cell correlated with the global mRNA translation activity, and its variation is dynamically regulated by cell state or extracellular stimuli. In contrast, the spatial patterns of eIF4F complexes at the single cell level could distinguish melanoma cells harboring different oncogenic driver mutations. This suggests that different tumorigenic contexts differentially regulate the subcellular localization of mRNA translation, with specific localization of eIF4F potentially associated with melanoma cell chemoresistance.

N6-Methyladenosine Methylation of mRNA in Cell Senescence

Cell senescence is the growth arrest caused by the accumulation of irreparable cell damage, which is involved in physiological and pathological processes and regulated by the post-transcriptional level. This regulation is performed by transcriptional regulators and driven by aging-related small RNAs, long non-coding RNAs, and RNA-binding proteins. N6-methyladenosine (m⁶A) is the most common chemical modification in eukaryotic mRNA, which can enhance or reduce the binding of transcriptional regulators. Increasing studies have confirmed the crucial role of m⁶A in controlling mRNA in various physiological processes. Remarkably, recent reports have indicated that abnormal methylation of m⁶A-related RNA may affect cell senescence. In this review, we clarified the association between m⁶A modification and cell senescence and analyzed the limitations of the current research.

Local translation in primary afferents and its contribution to pain

ABSTRACT

Chronic pain remains a significant problem due to its prevalence, impact, and limited therapeutic options. Progress in addressing chronic pain is dependent on a better understanding of underlying mechanisms. Although the available evidence suggests that changes within the central nervous system contribute to the initiation and maintenance of chronic pain, it also suggests that the primary afferent plays a critical role in all phases of the manifestation of chronic pain in most of those who suffer. Most notable among the changes in primary afferents is an increase in excitability or sensitization. A number of mechanisms have been identified that contribute to primary afferent sensitization with evidence for both increases in pronociceptive signaling molecules, such as voltage-gated sodium channels, and decreases in antinociceptive signaling molecules, such as voltage-dependent or calcium-dependent potassium channels. Furthermore, these changes in signaling molecules seem to reflect changes in gene expression as well as posttranslational processing. A mechanism of sensitization that has received far less attention, however, is the local or axonal translation of these signaling molecules. A growing body of evidence indicates that this process not only is dynamically regulated but also contributes to the initiation and maintenance of chronic pain. Here, we review the biology of local translation in primary afferents and its relevance to pain pathobiology.

Tagged actin mRNA dysregulation in IGF2BP1[Formula: see text] mice

Gene expression is tightly regulated by RNA-binding proteins (RBPs) to facilitate cell survival, differentiation, and migration. Previous reports have shown the importance of the Insulin-like Growth Factor II mRNA-Binding Protein (IGF2BP1/IMP1/ZBP1) in regulating RNA fate, including localization, transport, and translation. Here, we generated and characterized a knockout mouse to study RBP regulation. We report that IGF2BP1 is essential for proper brain development and neonatal survival. Specifically, these mice display disorganization in the developing neocortex, and further investigation revealed a loss of cortical marginal cell density at E17.5. We also investigated migratory cell populations in the IGF2BP1[Formula: see text] mice, using BrdU labeling, and detected fewer mitotically active cells in the cortical plate. Since RNA localization is important for cellular migration and directionality, we investigated the regulation of β-actin messenger RNA (mRNA), a well-characterized target with established roles in cell motility and development. To aid in our understanding of RBP and target mRNA regulation, we generated mice with endogenously labeled β-actin mRNA (IGF2BP1[Formula: see text]; β-actin-MS2[Formula: see text]). Using endogenously labeled β-actin transcripts, we report IGF2BP1[Formula: see text] neurons have increased transcription rates and total β-actin protein content. In addition, we found decreased transport and anchoring in knockout neurons. Overall, we present an important model for understanding RBP regulation of target mRNA.

Local mRNA translation and cytoskeletal reorganization: Mechanisms that tune neuronal responses

Neurons are highly polarized cells with significantly long axonal and dendritic extensions that can reach distances up to hundreds of centimeters away from the cell bodies in higher vertebrates. Their successful formation, maintenance, and proper function highly depend on the coordination of intricate molecular networks that allow axons and dendrites to quickly process information, and respond to a continuous and diverse cascade of environmental stimuli, often without enough time for communication with the soma. Two seemingly unrelated processes, essential for these rapid responses, and thus neuronal homeostasis and plasticity, are local mRNA translation and cytoskeletal reorganization. The axonal cytoskeleton is characterized by high stability and great plasticity; two contradictory attributes that emerge from the powerful cytoskeletal rearrangement dynamics. Cytoskeletal reorganization is crucial during nervous system development and in adulthood, ensuring the establishment of proper neuronal shape and polarity, as well as regulating intracellular transport and synaptic functions. Local mRNA translation is another mechanism with a well-established role in the developing and adult nervous system. It is pivotal for axonal guidance and arborization, synaptic formation, and function and seems to be a key player in processes activated after neuronal damage. Perturbations in the regulatory pathways of local translation and cytoskeletal reorganization contribute to various pathologies with diverse clinical manifestations, ranging from intellectual disabilities (ID) to autism spectrum disorders (ASD) and schizophrenia (SCZ). Despite the fact that both processes are essential for the orchestration of pathways critical for proper axonal and dendritic function, the interplay between them remains elusive. Here we review our current knowledge on the molecular mechanisms and specific interaction networks that regulate and potentially coordinate these interconnected processes.

RNA and neuronal function: the importance of post-transcriptional regulation

The brain represents an organ with a particularly high diversity of genes that undergo post-transcriptional gene regulation through multiple mechanisms that affect RNA metabolism and, consequently, brain function. This vast regulatory process in the brain allows for a tight spatiotemporal control over protein expression, a necessary factor due to the unique morphologies of neurons. The numerous mechanisms of post-transcriptional regulation or translational control of gene expression in the brain include alternative splicing, RNA editing, mRNA stability and transport. A large number of trans-elements such as RNA-binding proteins and micro RNAs bind to specific cis-elements on transcripts to dictate the fate of mRNAs including its stability, localization, activation and degradation. Several trans-elements are exemplary regulators of translation, employing multiple cofactors and regulatory machinery so as to influence mRNA fate. Networks of regulatory trans-elements exert control over key neuronal processes such as neurogenesis, synaptic transmission and plasticity. Perturbations in these networks may directly or indirectly cause neuropsychiatric and neurodegenerative disorders. We will be reviewing multiple mechanisms of gene regulation by trans-elements occurring specifically in neurons.

Diversity of bacterial small RNAs drives competitive strategies for a mutual chaperone

Hundreds of bacterial small RNAs (sRNAs) require the Hfq chaperone to regulate mRNA expression. Hfq is limiting, thus competition among sRNAs for binding to Hfq shapes the proteomes of individual cells. To understand how sRNAs compete for a common partner, we present a single-molecule fluorescence platform to simultaneously visualize binding and release of multiple sRNAs with Hfq. We show that RNA residents rarely dissociate on their own. Instead, clashes between residents and challengers on the same face of Hfq cause rapid exchange, whereas RNAs that recognize different surfaces may cohabit Hfq for several minutes before one RNA departs. The prevalence of these pathways depends on the structure of each RNA and how it interacts with Hfq. We propose that sRNA diversity creates many pairwise interactions with Hfq that allow for distinct biological outcomes: active exchange favors fast regulation whereas co-residence of dissimilar RNAs favors target co-recognition or target exclusion.

GIT1 Promotes Axonal Growth in an Inflammatory Environment by Promoting the Phosphorylation of MAP1B

Spinal cord injury (SCI) is a severe traumatic condition. The loss of the bundle of axons involved in motor conduction in the spinal cord after SCI is the main cause of motor function injury. Presently, axon regeneration in the spinal cord has been studied extensively, but it remains unclear how axon growth is regulated in an inflammatory environment at the cellular level. In the present study, GIT1 knockout (KO) mouse neurons were cultured in a microfluidic device to simulate the growth of axons in an inflammatory environment. The molecular regulation of axon growth in an inflammatory environment by GIT1 was then investigated. We found that the axon growth of GIT1 KO mouse neurons was restricted in an inflammatory environment. Further investigations revealed that in both axons and cell bodies in the inflammatory environment, GIT1 phosphorylated ERK, promoted the entry of Nrf2 into the nucleus, and promoted the transcription of MAP1B, thereby increasing the levels of MAP1B and p-MAP1B and promoting axon growth. We also found that MAP1B could be translated locally in axons and transported in cell bodies and axons. In conclusion, we found that GIT1 regulated axon growth in an inflammatory environment. This provided a theoretical basis for axon regeneration in an inflammatory environment after SCI to develop new treatment options for axon regeneration.

Failure to Upregulate the RNA Binding Protein ZBP After Injury Leads to Impaired Regeneration in a Rodent Model of Diabetic Peripheral Neuropathy

Most diabetes patients eventually suffer from peripheral nerve degeneration. Unfortunately, there is no treatment for the condition and its mechanisms are not well understood. There is, however, an emerging consensus that the inability of peripheral nerves to regenerate normally after injury contributes to the pathophysiology. We have previously shown that regeneration of peripheral axons requires local axonal translation of a pool of axonal mRNAs and that the levels and members of this axonal mRNA pool are altered in response to injury. Here, we show that following sciatic nerve injury in a streptozotocin rodent model of type I diabetes, this mobilization of RNAs into the injured axons is attenuated and correlates with decreased axonal regeneration. This failure of axonal RNA localization results from decreased levels of the RNA binding protein ZBP1. Over-expression of ZBP1 rescues the in vitro growth defect in injured dorsal root ganglion neurons from diabetic rodents. These results provide evidence that decreased neuronal responsiveness to injury in diabetes is due to a decreased ability to alter the pool of axonal mRNAs available for local translation, and may open new therapeutic opportunities for diabetic peripheral neuropathy.

Axonal mRNA localization and translation: local events with broad roles

Messenger RNA (mRNA) can be transported and targeted to different subcellular compartments and locally translated. Local translation is an evolutionally conserved mechanism that in mammals, provides an important tool to exquisitely regulate the subcellular proteome in different cell types, including neurons. Local translation in axons is involved in processes such as neuronal development, function, plasticity, and diseases. Here, we summarize the current progress on axonal mRNA transport and translation. We focus on the regulatory mechanisms governing how mRNAs are transported to axons and how they are locally translated in axons. We discuss the roles of axonally synthesized proteins, which either function locally in axons, or are retrogradely trafficked back to soma to achieve neuron-wide gene regulation. We also examine local translation in neurological diseases. Finally, we give a critical perspective on the remaining questions that could be answered to uncover the fundamental rules governing local translation, and discuss how this could lead to new therapeutic targets for neurological diseases.

Neuron-Specific IMP2 Overexpression by Synapsin Promoter-Driven AAV9: A Tool to Study Its Role in Axon Regeneration

Insulin-like growth factor II mRNA-binding protein (IMP) 2 is one of the three homologues (IMP1-3) that belong to a conserved family of mRNA-binding proteins. Its alternative splice product is aberrantly expressed in human hepatocellular carcinoma, and it is therefore identified as HCC. Previous works have indicated that IMP1/ZBP1 (zipcode binding protein) is critical in axon guidance and regeneration by regulating localization and translation of specific mRNAs. However, the role of IMP2 in the nervous system is largely unknown. We used the synapsin promoter-driven adeno-associated viral (AAV) 9 constructs for transgene expression both in vitro and in vivo. These viral vectors have proven to be effective to transduce the neuron-specific overexpression of IMP2 and HCC. Applying this viral vector in the injury-conditioned dorsal root ganglion (DRG) culture demonstrates that overexpression of IMP2 significantly inhibits axons regenerating from the neurons, whereas overexpression of HCC barely interrupts the process. Quantitative analysis of binding affinities of IMPs to β-actin mRNA reveals that it is closely associated with their roles in axon regeneration. Although IMPs share significant structural homology, the distinctive functions imply their different ability to localize specific mRNAs and to regulate the axonal translation.

Local Translation Across Neural Development: A Focus on Radial Glial Cells, Axons, and Synaptogenesis

In the past two decades, significant progress has been made in our understanding of mRNA localization and translation at distal sites in axons and dendrites. The existing literature shows that local translation is regulated in a temporally and spatially restricted manner and is critical throughout embryonic and post-embryonic life. Here, recent key findings about mRNA localization and local translation across the various stages of neural development, including neurogenesis, axon development, and synaptogenesis, are reviewed. In the early stages of development, mRNAs are localized and locally translated in the endfeet of radial glial cells, but much is still unexplored about their functional significance. Recent in vitro and in vivo studies have provided new information about the specific mechanisms regulating local translation during axon development, including growth cone guidance and axon branching. Later in development, localization and translation of mRNAs help mediate the major structural and functional changes that occur in the axon during synaptogenesis. Clinically, changes in local translation across all stages of neural development have important implications for understanding the etiology of several neurological disorders. Herein, local translation and mechanisms regulating this process across developmental stages are compared and discussed in the context of function and dysfunction.

The biological function of IGF2BPs and their role in tumorigenesis

The insulin-like growth factor-2 mRNA-binding proteins (IGF2BPs) pertain to a highly conservative RNA-binding family that works as a post-transcriptional fine-tuner for target transcripts. Emerging evidence suggests that IGF2BPs regulate RNA processing and metabolism, including stability, translation, and localization, and are involved in various cellular functions and pathophysiologies. In this review, we summarize the roles and molecular mechanisms of IGF2BPs in cancer development and progression. We mainly discuss the functional relevance of IGF2BPs in embryo development, neurogenesis, metabolism, RNA processing, and tumorigenesis. Understanding IGF2BPs role in tumor progression will provide new insight into cancer pathophysiology.

Walking the line: mechanisms underlying directional mRNA transport and localisation in neurons and beyond

Messenger RNA (mRNA) localisation enables a high degree of spatiotemporal control on protein synthesis, which contributes to establishing the asymmetric protein distribution required to set up and maintain cellular polarity. As such, a tight control of mRNA localisation is essential for many biological processes during development and in adulthood, such as body axes determination in Drosophila melanogaster and synaptic plasticity in neurons. The mechanisms controlling how mRNAs are localised, including diffusion and entrapment, local degradation and directed active transport, are largely conserved across evolution and have been under investigation for decades in different biological models. In this review, we will discuss the standing of the field regarding directional mRNA transport in light of the recent discovery that RNA can hitchhike on cytoplasmic organelles, such as endolysosomes, and the impact of these transport modalities on our understanding of neuronal function during development, adulthood and in neurodegeneration.

Neurodegeneration and axonal mRNA transportation

The prevalence of neurodegenerative diseases is accelerating in rapidly aging global population. Novel and effective diagnostic and therapeutic methods are required to tackle the global issue of neurodegeneration in the future. A better understanding of the potential molecular mechanism causing neurodegeneration can shed light on dysfunctional processes in diseased neurons, which can pave the way to design and synthesize novel targets for early diagnosis during the asymptomatic phase of the disease. Abnormal protein aggregation is a hallmark of neurodegenerative diseases which can hamper transportation of cargoes into axons. Recent evidence suggests that disruption of local protein synthesis has been observed in neurodegenerative diseases. Because of their highly asymmetric structure, highly polarized neurons require trafficking of cargoes from the cell body to different subcellular regions to meet the extensive demands of cellular physiology. Localization of mRNAs and subsequent local translation to corresponding proteins in axons is a mechanism which allows neurons to rapidly respond to external stimuli as well as establishing neuronal networks by synthesizing proteins on demand. Axonal protein synthesis is required for axon guidance, synapse formation and plasticity, axon maintenance and regeneration in response to injury. Different types of excitatory and inhibitory neurons in the central and peripheral nervous systems have been shown to localize mRNA. Rising evidence suggests that the repertoire of localizing mRNA in axons can change during aging, indicating a connection between axonal mRNA trafficking and aging diseases such as neurodegeneration. Here, I briefly review the latest findings on the importance of mRNA localization and local translation in neurons and the consequences of their disruption in neurodegenerative diseases. In addition, I discuss recent evidence that dysregulation of mRNA localization and local protein translation can contribute to the formation of neurodegenerative diseases such as Alzheimer's disease, Amyotrophic Lateral Sclerosis, and Spinal Muscular Atrophy. In addition, I discuss recent findings on mRNAs localizing to mitochondria in neurodegeneration.

The functional organization of axonal mRNA transport and translation

Axons extend for tremendously long distances from the neuronal soma and make use of localized mRNA translation to rapidly respond to different extracellular stimuli and physiological states. The locally synthesized proteins support many different functions in both developing and mature axons, raising questions about the mechanisms by which local translation is organized to ensure the appropriate responses to specific stimuli. Publications over the past few years have uncovered new mechanisms for regulating the axonal transport and localized translation of mRNAs, with several of these pathways converging on the regulation of cohorts of functionally related mRNAs - known as RNA regulons - that drive axon growth, axon guidance, injury responses, axon survival and even axonal mitochondrial function. Recent advances point to these different regulatory pathways as organizing platforms that allow the axon's proteome to be modulated to meet its physiological needs.

Identification of 3' UTR motifs required for mRNA localization to myelin sheaths in vivo

Myelin is a specialized membrane produced by oligodendrocytes that insulates and supports axons. Oligodendrocytes extend numerous cellular processes, as projections of the plasma membrane, and simultaneously wrap multiple layers of myelin membrane around target axons. Notably, myelin sheaths originating from the same oligodendrocyte are variable in size, suggesting local mechanisms regulate myelin sheath growth. Purified myelin contains ribosomes and hundreds of mRNAs, supporting a model that mRNA localization and local protein synthesis regulate sheath growth and maturation. However, the mechanisms by which mRNAs are selectively enriched in myelin sheaths are unclear. To investigate how mRNAs are targeted to myelin sheaths, we tested the hypothesis that transcripts are selected for myelin enrichment through consensus sequences in the 3' untranslated region (3' UTR). Using methods to visualize mRNA in living zebrafish larvae, we identified candidate 3' UTRs that were sufficient to localize mRNA to sheaths and enriched near growth zones of nascent membrane. We bioinformatically identified motifs common in 3' UTRs from 3 myelin-enriched transcripts and determined that these motifs are required and sufficient in a context-dependent manner for mRNA transport to myelin sheaths. Finally, we show that 1 motif is highly enriched in the myelin transcriptome, suggesting that this sequence is a global regulator of mRNA localization during developmental myelination.

The role of RNA-binding and ribosomal proteins as specific RNA translation regulators in cellular differentiation and carcinogenesis

Tight control of mRNA expression is required for cell differentiation; imbalanced regulation may lead to developmental disorders and cancer. The activity of the translational machinery (including ribosomes and translation factors) regulates the rate (slow or fast) of translation of encoded proteins, and the quality of these proteins highly depends on which mRNAs are available for translation. Specific RNA-binding and ribosomal proteins seem to play a key role in controlling gene expression to determine the differentiation fate of the cell. This demonstrates the important role of RNA-binding proteins, specific ribosome-binding proteins and microRNAs as key molecules in controlling the specific proteins required for the differentiation or dedifferentiation of cells. This delicate balance between specific proteins (in terms of quality and availability) and post-translational modifications occurring in the cytoplasm is crucial for cell differentiation, dedifferentiation and oncogenic potential. In this review, we report how defects in the regulation of mRNA translation can be dependent on specific proteins and can induce an imbalance between differentiation and dedifferentiation in cell fate determination.

Axon microdissection and transcriptome profiling reveals the in vivo RNA content of fully differentiated myelinated motor axons

Axonal protein synthesis has been shown to play a role in developmental and regenerative growth, as well as in the maintenance of the axoplasm in a steady state. Recent studies have begun to identify the mRNAs localized in axons, which could be translated locally under different conditions. Despite that by now hundreds or thousands of mRNAs have been shown to be localized into the axonal compartment of cultured neurons in vitro, knowledge of which mRNAs are localized in mature myelinated axons is quite limited. With the purpose of characterizing the transcriptome of mature myelinated motor axons of peripheral nervous systems, we modified the axon microdissection method devised by Koenig, enabling the isolation of the axoplasm RNA to perform RNA-seq analysis. The transcriptome analysis indicates that the number of RNAs detected in mature axons is lower in comparison with in vitro data, depleted of glial markers, and enriched in neuronal markers. The mature myelinated axons are enriched for mRNAs related to cytoskeleton, translation, and oxidative phosphorylation. Moreover, it was possible to define core genes present in axons when comparing our data with transcriptomic data of axons grown in different conditions. This work provides evidence that axon microdissection is a valuable method to obtain genome-wide data from mature and myelinated axons of the peripheral nervous system, and could be especially useful for the study of axonal involvement in neurodegenerative pathologies of motor neurons such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophies (SMA).

Intra-axonal mechanisms driving axon regeneration

Traumatic injury to the peripheral and central nervous systems very often causes axotomy, where an axon loses connections with its target resulting in loss of function. The axon segments distal to the injury site lose connection with the cell body and degenerate. Axotomized neurons in the periphery can spontaneously mount a regenerative response and reconnect to their denervated target tissues, though this is rarely complete in humans. In contrast, spontaneous regeneration rarely occurs after axotomy in the spinal cord and brain. Here, we concentrate on the mechanisms underlying this spontaneous regeneration in the peripheral nervous system, focusing on events initiated from the axon that support regenerative growth. We contrast this with what is known for axonal injury responses in the central nervous system. Considering the neuropathy focus of this special issue, we further draw parallels and distinctions between the injury-response mechanisms that initiate regenerative gene expression programs and those that are known to trigger axon degeneration.

Local Translation in Growth Cones and Presynapses, Two Axonal Compartments for Local Neuronal Functions

During neural development, growth cones, very motile compartments of tips of axons, lead axonal extension to the correct targets. Subsequently, presynapses, another axonal compartment with vigorous trafficking of synaptic vesicles, emerge to form functional synapses with postsynapses. In response to extracellular stimuli, the immediate supply of proteins by local translation within these two axonal compartments far from cell bodies confers high motility of growth cones and active vesicle trafficking in presynapses. Although local translation in growth cones and presynapses occurs at a very low level compared with cell bodies and even dendrites, recent progress in omics and visualization techniques with subcellular fractionation of these compartments has revealed the actual situation of local translation within these two axonal compartments. Here, the increasing evidence for local protein synthesis in growth cones and presynapses for axonal and synaptic functions has been reviewed. Furthermore, the mechanisms regulating local translation in these two compartments and pathophysiological conditions caused by dysregulated local translation are highlighted.

Conservation of a core neurite transcriptome across neuronal types and species

The intracellular localization of mRNAs allows neurons to control gene expression in neurite extensions (axons and dendrites) and respond rapidly to local stimuli. This plays an important role in diverse processes including neuronal growth and synaptic plasticity, which in turn serves as a foundation for learning and memory. Recent high-throughput analyses have revealed that neurites contain hundreds to thousands of mRNAs, but an analysis comparing the transcriptomes derived from these studies has been lacking. Here we analyze 20 datasets pertaining to neuronal mRNA localization across species and neuronal types and identify a conserved set of mRNAs that had robustly localized to neurites in a high number of the studies. The set includes mRNAs encoding for ribosomal proteins and other components of the translation machinery, mitochondrial proteins, cytoskeletal components, and proteins associated with neurite formation. Our combinatorial analysis provides a unique resource for future hypothesis-driven research. This article is categorized under: RNA Export and Localization > RNA Localization RNA Evolution and Genomics > Computational Analyses of RNA RNA Methods > RNA Analyses in Cells.

Mechanism and role of the intra-axonal Calreticulin translation in response to axonal injury

Following injury, sensory axons locally translate mRNAs that encode proteins needed for the response to injury, locally and through retrograde signaling, and for regeneration. In this study, we addressed the mechanism and role of axotomy-induced intra-axonal translation of the ER chaperone Calreticulin. In vivo peripheral nerve injury increased Calreticulin levels in sensory axons. Using an in vitro model system of sensory neurons amenable to mechanistic dissection we provide evidence that axotomy induces local translation of Calreticulin through PERK (protein kinase RNA-like endoplasmic reticulum kinase) mediated phosphorylation of eIF2α by a mechanism that requires both 5' and 3'UTRs (untranslated regions) elements in Calreticulin mRNA. ShRNA mediated depletion of Calreticulin or inhibition of PERK signaling increased axon retraction following axotomy. In contrast, expression of axonally targeted, but not somatically restricted, Calreticulin mRNA decreased retraction and promoted axon regeneration following axotomy in vitro. Collectively, these data indicate that the intra-axonal translation of Calreticulin in response to axotomy serves to minimize the ensuing retraction, and overexpression of axonally targeted Calreticulin mRNA promotes axon regeneration.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Sequential Maturation of Olfactory Sensory Neurons in the Mature Olfactory Epithelium

The formation of the olfactory nerve and olfactory bulb (OB) glomeruli begins embryonically in mice. However, the development of the olfactory system continues throughout life with the addition of new olfactory sensory neurons (OSNs) in the olfactory epithelium (OE). Much attention has been given to the perinatal innervation of the OB by OSN axons, but in the young adult the process of OSN maturation and axon targeting to the OB remains controversial. To address this gap in understanding, we used BrdU to label late-born OSNs in young adult mice at postnatal day 25 (P25-born OSNs) and timed their molecular maturation following basal cell division. We show that OSNs in young adults undergo a sequential molecular development with the expression of GAP 43 (growth-associated protein 43) > AC3 (adenylyl cyclase 3) > OMP (olfactory marker protein), consecutively, in a time frame of ∼8 d. To assess OSN axon development, we implemented an in vivo fate-mapping strategy to label P25-born OSNs with ZsGreen. Using sampling intervals of 24 h, we demonstrate the progressive extension of OSN axons in the OE, through the foramen of the cribriform plate, and onto the surface of the OB. OSN axons reached the OB and began to target and robustly innervate specific glomeruli ∼10 d following basal cell division, a time point at which OMP expression becomes evident. Our data demonstrate a sequential process of correlated axon extension and molecular maturation that is similar to that seen in the neonate, but on a slightly longer timescale and with regional differences in the OE.

Pan-Cancer Analysis of lncRNA Regulation Supports Their Targeting of Cancer Genes in Each Tumor Context

Long noncoding RNAs (lncRNAs) are commonly dysregulated in tumors, but only a handful are known to play pathophysiological roles in cancer. We inferred lncRNAs that dysregulate cancer pathways, oncogenes, and tumor suppressors (cancer genes) by modeling their effects on the activity of transcription factors, RNA-binding proteins, and microRNAs in 5,185 TCGA tumors and 1,019 ENCODE assays. Our predictions included hundreds of candidate onco- and tumor-suppressor lncRNAs (cancer lncRNAs) whose somatic alterations account for the dysregulation of dozens of cancer genes and pathways in each of 14 tumor contexts. To demonstrate proof of concept, we showed that perturbations targeting OIP5-AS1 (an inferred tumor suppressor) and TUG1 and WT1-AS (inferred onco-lncRNAs) dysregulated cancer genes and altered proliferation of breast and gynecologic cancer cells. Our analysis indicates that, although most lncRNAs are dysregulated in a tumor-specific manner, some, including OIP5-AS1, TUG1, NEAT1, MEG3, and TSIX, synergistically dysregulate cancer pathways in multiple tumor contexts.

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Hundreds of lncRNAs target cancer genes and pathways in each tumor context

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lncRNA copy numbers are predictive of target cancer gene dysregulation

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Most lncRNAs are predicted to be transcriptional or post-transcriptional specialists

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lncRNAs are predicted to synergistically regulate proliferation pathways in cancer

Properties of LINE-1 proteins and repeat element expression in the context of amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving loss of motor neurons and having no known cure and uncertain etiology. Several studies have drawn connections between altered retrotransposon expression and ALS. Certain features of the LINE-1 (L1) retrotransposon-encoded ORF1 protein (ORF1p) are analogous to those of neurodegeneration-associated RNA-binding proteins, including formation of cytoplasmic aggregates. In this study we explore these features and consider possible links between L1 expression and ALS.

RESULTS

We first considered factors that modulate aggregation and subcellular distribution of LINE-1 ORF1p, including nuclear localization. Changes to some ORF1p amino acid residues alter both retrotransposition efficiency and protein aggregation dynamics, and we found that one such polymorphism is present in endogenous L1s abundant in the human genome. We failed, however, to identify CRM1-mediated nuclear export signals in ORF1p nor strict involvement of cell cycle in endogenous ORF1p nuclear localization in human 2102Ep germline teratocarcinoma cells. Some proteins linked with ALS bind and colocalize with L1 ORF1p ribonucleoprotein particles in cytoplasmic RNA granules. Increased expression of several ALS-associated proteins, including TAR DNA Binding Protein (TDP-43), strongly limits cell culture retrotransposition, while some disease-related mutations modify these effects. Using quantitative reverse transcription PCR (RT-qPCR) of ALS tissues and reanalysis of publicly available RNA-Seq datasets, we asked if changes in expression of retrotransposons are associated with ALS. We found minimal altered expression in sporadic ALS tissues but confirmed a previous report of differential expression of many repeat subfamilies in C9orf72 gene-mutated ALS patients.

CONCLUSIONS

Here we extended understanding of the subcellular localization dynamics of the aggregation-prone LINE-1 ORF1p RNA-binding protein. However, we failed to find compelling evidence for misregulation of LINE-1 retrotransposons in sporadic ALS nor a clear effect of ALS-associated TDP-43 protein on L1 expression. In sum, our study reveals that the interplay of active retrotransposons and the molecular features of ALS are more complex than anticipated. Thus, the potential consequences of altered retrotransposon activity for ALS and other neurodegenerative disorders are worthy of continued investigation.

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

hnRNPs Interacting with mRNA Localization Motifs Define Axonal RNA Regulons

mRNA translation in axons enables neurons to introduce new proteins at sites distant from their cell body. mRNA-protein interactions drive this post-transcriptional regulation, yet knowledge of RNA binding proteins (RBP) in axons is limited. Here we used proteomics to identify RBPs interacting with the axonal localizing motifs of Nrn1, Hmgb1, Actb, and Gap43 mRNAs, revealing many novel RBPs in axons. Interestingly, no RBP is shared between all four RNA motifs, suggesting graded and overlapping specificities of RBP-mRNA pairings. A systematic assessment of axonal mRNAs interacting with hnRNP H1, hnRNP F, and hnRNP K, proteins that bound with high specificity to Nrn1 and Hmgb1, revealed that axonal mRNAs segregate into axon growth-associated RNA regulons based on hnRNP interactions. Axotomy increases axonal transport of hnRNPs H1, F, and K, depletion of these hnRNPs decreases axon growth and reduces axonal mRNA levels and axonal protein synthesis. Thus, subcellular hnRNP-interacting RNA regulons support neuronal growth and regeneration.

Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) in cancer

The insulin-like growth factor-2 mRNA-binding protein 1 (IGF2BP1) plays essential roles in embryogenesis and carcinogenesis. IGF2BP1 serves as a post-transcriptional fine-tuner regulating the expression of some essential mRNA targets required for the control of tumor cell proliferation and growth, invasion, and chemo-resistance, associating with a poor overall survival and metastasis in various types of human cancers. Therefore, IGF2BP1 has been traditionally regarded as an oncogene and potential therapeutic target for cancers. Nevertheless, a few studies have also demonstrated its tumor-suppressive role. However, the details about the contradictory functions of IGF2BP1 are unclear. The growing numbers of microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) have been identified as its direct regulators, during tumor cell proliferation, growth, and invasion in multiple cancers. Thus, the mechanisms of post-transcriptional modulation of gene expression mediated by IGF2BP1, miRNAs, and lncRNAs in determining the fate of the development of tissues and organs, as well as tumorigenesis, need to be elucidated. In this review, we summarized the tissue distribution, expression, and roles of IGF2BP1 in embryogenesis and tumorigenesis, and focused on modulation of the interconnectivity between IGF2BP1 and its targeted mRNAs or non-coding RNAs (ncRNAs). The potential use of inhibitors of IGF2BP1 and its related pathways in cancer therapy was also discussed.

Understanding axon guidance: are we nearly there yet?

During nervous system development, neurons extend axons to reach their targets and form functional circuits. The faulty assembly or disintegration of such circuits results in disorders of the nervous system. Thus, understanding the molecular mechanisms that guide axons and lead to neural circuit formation is of interest not only to developmental neuroscientists but also for a better comprehension of neural disorders. Recent studies have demonstrated how crosstalk between different families of guidance receptors can regulate axonal navigation at choice points, and how changes in growth cone behaviour at intermediate targets require changes in the surface expression of receptors. These changes can be achieved by a variety of mechanisms, including transcription, translation, protein-protein interactions, and the specific trafficking of proteins and mRNAs. Here, I review these axon guidance mechanisms, highlighting the most recent advances in the field that challenge the textbook model of axon guidance.

Axonal mRNA transport and translation at a glance

Localization and translation of mRNAs within different subcellular domains provides an important mechanism to spatially and temporally introduce new proteins in polarized cells. Neurons make use of this localized protein synthesis during initial growth, regeneration and functional maintenance of their axons. Although the first evidence for protein synthesis in axons dates back to 1960s, improved methodologies, including the ability to isolate axons to purity, highly sensitive RNA detection methods and imaging approaches, have shed new light on the complexity of the transcriptome of the axon and how it is regulated. Moreover, these efforts are now uncovering new roles for locally synthesized proteins in neurological diseases and injury responses. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of how axonal mRNA transport and translation are regulated, and discuss their emerging links to neurological disorders and neural repair.

Locally translated mTOR controls axonal local translation in nerve injury

How is protein synthesis initiated locally in neurons? We found that mTOR (mechanistic target of rapamycin) was activated and then up-regulated in injured axons, owing to local translation of mTOR messenger RNA (mRNA). This mRNA was transported into axons by the cell size-regulating RNA-binding protein nucleolin. Furthermore, mTOR controlled local translation in injured axons. This included regulation of its own translation and that of retrograde injury signaling molecules such as importin β1 and STAT3 (signal transducer and activator of transcription 3). Deletion of the mTOR 3' untranslated region (3'UTR) in mice reduced mTOR in axons and decreased local translation after nerve injury. Both pharmacological inhibition of mTOR in axons and deletion of the mTOR 3'UTR decreased proprioceptive neuronal survival after nerve injury. Thus, mRNA localization enables spatiotemporal control of mTOR pathways regulating local translation and long-range intracellular signaling.

CBX2 Inhibits Neurite Development by Regulating Neuron-Specific Genes Expression

Polycomb group (PcG) proteins regulate the epigenetic status of transcription regulatory states during development. Progression from pluripotency to differentiation requires the sequential activation and repression of different PcG target genes, however, the relationship between early patterning signals, PcG expression, and the development of the central nervous system is still unclear. Using various models of neuronal differentiation, we provide evidence that CBX2 is a negative regulator of neuronal differentiation. Knock-down of CBX2 expression promotes neurite development, while overexpression of CBX2 inhibits neurite development. Further, we found that CBX2 is a direct target gene of miR-124. During neuronal differentiation, CBX2 was decreased while miR-124 was increased. Mechanistically, CBX2 directly interacts with the promoter region of several neuro-associated genes and regulates their expression. We found that the neuron-specific GAP-43 gene could contribute to the stimulating effect on neurite development associated with inhibition of CBX2.

Regulation of Adult CNS Axonal Regeneration by the Post-transcriptional Regulator Cpeb1

Adult mammalian central nervous system (CNS) neurons are unable to regenerate following axonal injury, leading to permanent functional impairments. Yet, the reasons underlying this regeneration failure are not fully understood. Here, we studied the transcriptome and translatome shortly after spinal cord injury. Profiling of the total and ribosome-bound RNA in injured and naïve spinal cords identified a substantial post-transcriptional regulation of gene expression. In particular, transcripts associated with nervous system development were down-regulated in the total RNA fraction while remaining stably loaded onto ribosomes. Interestingly, motif association analysis of post-transcriptionally regulated transcripts identified the cytoplasmic polyadenylation element (CPE) as enriched in a subset of these transcripts that was more resistant to injury-induced reduction at the transcriptome level. Modulation of these transcripts by overexpression of the CPE binding protein, Cpeb1, in mouse and Drosophila CNS neurons promoted axonal regeneration following injury. Our study uncovered a global evolutionarily conserved post-transcriptional mechanism enhancing regeneration of injured CNS axons.

RNP Assembly Defects in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a motor neuron disease caused by mutations/deletions within the survival of motor neuron 1 (SMN1) gene that lead to a pathological reduction of SMN protein levels. SMN is part of a multiprotein complex, functioning as a molecular chaperone that facilitates the assembly of spliceosomal small nuclear ribonucleoproteins (snRNP). In addition to its role in spliceosome formation, SMN has also been found to interact with mRNA-binding proteins (mRBPs), and facilitate their assembly into mRNP transport granules. The association of protein and RNA in RNP complexes plays an important role in an extensive and diverse set of cellular processes that regulate neuronal growth, differentiation, and the maturation and plasticity of synapses. This review discusses the role of SMN in RNP assembly and localization, focusing on molecular defects that affect mRNA processing and may contribute to SMA pathology.

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.