Local mRNA Translation Research Articles

Axonal endoplasmic reticulum tubules control local translation via P180/RRBP1-mediated ribosome interactions

Local mRNA translation in axons is critical for the spatiotemporal regulation of the axonal proteome. A wide variety of mRNAs are localized and translated in axons; however, how protein synthesis is regulated at specific subcellular sites in axons remains unclear. Here, we establish that the axonal endoplasmic reticulum (ER) supports axonal translation in developing rat hippocampal cultured neurons. Axonal ER tubule disruption impairs local translation and ribosome distribution. Using nanoscale resolution imaging, we find that ribosomes make frequent contacts with axonal ER tubules in a translation-dependent manner and are influenced by specific extrinsic cues. We identify P180/RRBP1 as an axonally distributed ribosome receptor that regulates local translation and binds to mRNAs enriched for axonal membrane proteins. Importantly, the impairment of axonal ER-ribosome interactions causes defects in axon morphology. Our results establish a role for the axonal ER in dynamically localizing mRNA translation, which is important for proper neuron development.

The lncRNA Malat1 is trafficked to the cytoplasm as a localized mRNA encoding a small peptide in neurons.

Synaptic function in neurons is modulated by local translation of mRNAs that are transported to distal portions of axons and dendrites. The metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) is broadly expressed across cell types, almost exclusively as a nuclear long noncoding RNA. We found that in differentiating neurons, a portion of Malat1 RNA redistributes to the cytoplasm. Depletion of Malat1 using antisense oligonucleotides (ASOs) stimulates the expression of particular pre- and postsynaptic proteins, implicating Malat1 in their regulation. Neuronal Malat1 is localized in puncta of both axons and dendrites that costain with Staufen1 protein, similar to neuronal RNA granules formed by locally translated mRNAs. Ribosome profiling of cultured mouse cortical neurons identified ribosome footprints within a 5' region of Malat1 containing short open reading frames. The upstream-most reading frame (M1) of the Malat1 locus was linked to the GFP-coding sequence in mouse embryonic stem cells. When these gene-edited cells were differentiated into glutamatergic neurons, the M1-GFP fusion protein was expressed. Antibody staining for the M1 peptide confirmed its presence in wild-type neurons and showed that M1 expression was enhanced by synaptic stimulation with KCl. Our results indicate that Malat1 serves as a cytoplasmic coding RNA in the brain that is both modulated by and modulates synaptic function.

Inhibition of Cpeb3 ribozyme elevates CPEB3 protein expression and polyadenylation of its target mRNAs and enhances object location memory.

A self-cleaving ribozyme that maps to an intron of the cytoplasmic polyadenylation element-binding protein 3 (Cpeb3) gene is thought to play a role in human episodic memory, but the underlying mechanisms mediating this effect are not known. We tested the activity of the murine sequence and found that the ribozyme's self-scission half-life matches the time it takes an RNA polymerase to reach the immediate downstream exon, suggesting that the ribozyme-dependent intron cleavage is tuned to co-transcriptional splicing of the Cpeb3 mRNA. Our studies also reveal that the murine ribozyme modulates maturation of its harboring mRNA in both cultured cortical neurons and the hippocampus: inhibition of the ribozyme using an antisense oligonucleotide leads to increased CPEB3 protein expression, which enhances polyadenylation and translation of localized plasticity-related target mRNAs, and subsequently strengthens hippocampal-dependent long-term memory. These findings reveal a previously unknown role for self-cleaving ribozyme activity in regulating experience-induced co-transcriptional and local translational processes required for learning and memory.

Dynamic local mRNA localization and translation occurs during the postnatal molecular maturation of perivascular astrocytic processes.

Astrocytes are highly ramified and send out perivascular processes (PvAPs) that entirely sheathe the brain's blood vessels. PvAPs are equipped with an enriched molecular repertoire that sustains astrocytic regulatory functions at the vascular interface. In the mouse, PvAP development starts after birth and is essentially complete by postnatal day (P) 15. Progressive molecular maturation also occurs over this period, with the acquisition of proteins enriched in PvAPs. The mechanisms controlling the development and molecular maturation of PvAPs have not been extensively characterized. We reported previously that mRNAs are distributed unequally in mature PvAPs and are locally translated. Since dynamic mRNA localization and local translation influence the cell's polarity, we hypothesized that they might sustain the postnatal maturation of PvAPs. Here, we used a combination of molecular biology and imaging approaches to demonstrate that the development of PvAPs is accompanied by the transport of mRNA and polysomal mRNA into PvAPs, the development of a rough endoplasmic reticulum (RER) network and Golgi cisternae, and local translation. By focusing on genes and proteins that are selectively or specifically expressed in astrocytes, we characterized the developmental profile of mRNAs, polysomal mRNAs and proteins in PvAPs from P5 to P60. We found that some polysomal mRNAs polarized progressively towards the PvAPs. Lastly, we found that expression and localization of mRNAs in developing PvAPs is perturbed in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts. Our results indicate that dynamic mRNA localization and local translation influence the postnatal maturation of PvAPs.

Synapse-specific burst coding sustained by local axonal translation

Neurotransmission in the brain is unreliable, suggesting that high-frequency spike bursts rather than individual spikes carry the neural code. For instance, cortical pyramidal neurons rely on bursts in memory formation. Protein synthesis is another key factor in long-term synaptic plasticity and learning but is widely considered unnecessary for synaptic transmission. Here, however, we show that burst neurotransmission at synapses between neocortical layer 5 pyramidal cells depends on axonal protein synthesis linked to presynaptic NMDA receptors and mTOR. We localized protein synthesis to axons with laser axotomy and puromycylation live imaging. We whole-cell recorded connected neurons to reveal how translation sustained readily releasable vesicle pool size and replenishment rate. We live imaged axons and found sparsely docked RNA granules, suggesting synapse-specific regulation. In agreement, translation boosted neurotransmission onto excitatory but not inhibitory basket or Martinotti cells. Local axonal mRNA translation is thus a hitherto unappreciated principle for sustaining burst coding at specific synapse types.

SNHG15-Mediated Localization of Nucleolin at the Cell Protrusions Regulates CDH2 mRNA Expression and Cell Invasion.

LncRNAs are emerging as important regulators of gene expression by controlling transcription in the nucleus and by modulating mRNA translation in the cytoplasm. In this study, we reveal a novel function of lncRNA SNHG15 in mediating breast cancer cell invasion through regulating the local translation of CDH2 mRNA. We show that SNHG15 preferentially localizes at the cellular protrusions or cell leading edge and that this localization is directed by IMP1, a multifunctional protein involved in many aspects of RNA regulation. We demonstrate that SNHG15 also forms a complex with nucleolin, allowing nucleolin to be co-transported with SNHG15 to the cell protrusions, where the accumulated nucleolin is able to bind to CDH2 mRNA. Interaction with nucleolin stabilizes local CDH2 mRNA and regulates its translation, thus promoting cell invasive potential. Our findings reveal an underlying mechanism by which lncRNA could serve as a carrier to transport a protein regulator into a specific cell compartment to enhance target mRNA expression.

FMRP Long-Range Transport and Degradation Are Mediated by Dynlrb1 in Sensory Neurons

The fragile X messenger ribonucleoprotein 1 (FMRP) is a multifunctional RNA-binding protein implicated in human neurodevelopmental and neurodegenerative disorders. FMRP mediates the localization and activity-dependent translation of its associated mRNAs through the formation of phase-separated condensates that are trafficked by microtubule-based motors in axons. Axonal transport and localized mRNA translation are critical processes for long-term neuronal survival and are closely linked to the pathogenesis of neurological diseases. FMRP dynein-mediated axonal trafficking is still largely unexplored but likely to constitute a key process underlying FMRP spatiotemporal translational regulation. Here, we show that dynein light chain roadblock 1 (Dynlrb1), a subunit of the dynein complex, is a critical regulator of FMRP function. In sensory axons, FMRP associates with endolysosomal organelles, likely through annexin A11, and is retrogradely trafficked by the dynein complex in a Dynlrb1-dependent manner. Moreover, Dynlrb1 silencing induced FMRP granule accumulation and repressed the translation of microtubule-associated protein 1b, one of its primary mRNA targets. Our findings suggest that Dynlrb1 regulates FMRP function through the control of its transport and targeted degradation.

Enhanced Local Delivery of Engineered IL-2 mRNA by Porous Silica Nanoparticles to Promote Effective Antitumor Immunity.

Localized expression of immunomodulatory molecules can stimulate immune responses against tumors in the tumor microenvironment while avoiding toxicities associated with systemic administration. In this study, we developed a polyethylenimine-modified porous silica nanoparticle (PPSN)-based delivery platform carrying cytokine mRNA for local immunotherapy in vivo. Our delivery platform was significantly more efficient than FDA-approved lipid nanoparticles for localized mRNA translation. We observed no off-target translation of mRNA in any organs and no evidence of systemic toxicity. Intratumoral injection of cytokine mRNA-loaded PPSNs led to high-level expression of protein within the tumor and stimulated immunogenic cancer cell death. Additionally, combining cytokine mRNA with an immune checkpoint inhibitor enhanced anticancer responses in several murine cancer models and enabled the inhibition of distant metastatic tumors. Our results demonstrate the potential of PPSNs-mediated mRNA delivery as a specific, effective, and safe platform for mRNA-based therapeutics in cancer immunotherapy.

Abstract P3040: Development Of A Protein Synthesis Network At The Sarcoplasmic Reticulum In Adult Cardiomyocytes

Introduction: The endoplasmic reticulum (ER) is the site of synthesis and folding of membrane and secretory proteins. In striated muscle cells, a specialized membrane system known as the sarcoplasmic reticulum (SR) sequesters Ca 2+ and carries key functions in excitation-contraction coupling. Textbooks often define the SR as the ER of muscle, but surprisingly, the biochemical and physiological relationship between the ER and SR and even whether they are distinct membrane networks or refer to the same structure remain unknown. Methods: To characterize the relationship between the ER and SR, we applied biochemical and imaging techniques, including immunofluorescence and election microscopy, fluorescence in situ hybridization, and STimulated Emission Depletion (STED) microscopy to ventricular myocytes, which allowed us to follow the differentiation of these membrane compartments in developing cardiomyocytes. Results: Using localization and expression levels of SERCA2, RyR2, KDEL and different markers of ribosomes, mRNA, and the secretory pathway, we found that the membrane compartment that exists in the early postnatal stage, resembles mostly the ER, and this membrane system decreases in postnatal development. The SR is the main membrane network that exists in the adult cardiomyocytes, replacing the ER, in all but a small perinuclear region. Both sites which appear to be interconnected, perform specialized tasks, with protein secretion through the classical ER-Golgi secretory pathway originating mainly from the perinuclear ER and Ca 2+ -dependent excitation-contraction coupling originating exclusively from the SR. Other functions, such as Ca 2+ intake or posttranslational protein processing occur in both compartments. Importantly, we show that in adult cardiomyocytes ribosomes are not only attached to the ER but also to the SR, where they actively perform mRNA translation and are major sites of protein synthesis. Conclusion: The ER and SR are two distinct coexisting membrane networks in cardiomyocytes that undergo molecular differentiation and structural organization during cardiac muscle development. Significant amounts of protein synthesis occur in adult cardiomyocytes at the SR and may serve localized mRNA translation near the sarcomere.

A local translation program regulates centriole amplification in the airway epithelium

Biogenesis of organelles requires targeting of a subset of proteins to specific subcellular domains by signal peptides or mechanisms controlling mRNA localization and local translation. How local distribution and translation of specific mRNAs for organelle biogenesis is achieved remains elusive and likely to be dependent on the cellular context. Here we identify Trinucleotide repeat containing-6a (Tnrc6a), a component of the miRNA pathway, distinctively localized to apical granules of differentiating airway multiciliated cells (MCCs) adjacent to centrioles. In spite of being enriched in TNRC6A and the miRNA-binding protein AGO2, they lack enzymes for mRNA degradation. Instead, we found these apical granules enriched in components of the mRNA translation machinery and newly synthesized proteins suggesting that they are specific hubs for target mRNA localization and local translation in MCCs. Consistent with this, Tnrc6a loss of function prevented formation of these granules and led to a broad reduction, rather than stabilization of miRNA targets. These included downregulation of key genes involved in ciliogenesis and was associated with defective multicilia formation both in vivo and in primary airway epithelial cultures. Similar analysis of Tnrc6a disruption in yolk sac showed stabilization of miRNA targets, highlighting the potential diversity of these mechanisms across organs.

EIF4A1-dependent mRNAs employ purine-rich 5'UTR sequences to activate localised eIF4A1-unwinding through eIF4A1-multimerisation to facilitate translation.

Altered eIF4A1 activity promotes translation of highly structured, eIF4A1-dependent oncogene mRNAs at root of oncogenic translational programmes. It remains unclear how these mRNAs recruit and activate eIF4A1 unwinding specifically to facilitate their preferential translation. Here, we show that single-stranded RNA sequence motifs specifically activate eIF4A1 unwinding allowing local RNA structural rearrangement and translation of eIF4A1-dependent mRNAs in cells. Our data demonstrate that eIF4A1-dependent mRNAs contain AG-rich motifs within their 5'UTR which specifically activate eIF4A1 unwinding of local RNA structure to facilitate translation. This mode of eIF4A1 regulation is used by mRNAs encoding components of mTORC-signalling and cell cycle progression, and renders these mRNAs particularly sensitive to eIF4A1-inhibition. Mechanistically, we show that binding of eIF4A1 to AG-rich sequences leads to multimerization of eIF4A1 with eIF4A1 subunits performing distinct enzymatic activities. Our structural data suggest that RNA-binding of multimeric eIF4A1 induces conformational changes in the RNA resulting in an optimal positioning of eIF4A1 proximal to the RNA duplex enabling efficient unwinding. Our data proposes a model in which AG-motifs in the 5'UTR of eIF4A1-dependent mRNAs specifically activate eIF4A1, enabling assembly of the helicase-competent multimeric eIF4A1 complex, and positioning these complexes proximal to stable localised RNA structure allowing ribosomal subunit scanning.

MRNA transport, translation, and decay in adult mammalian central nervous system axons

Localized mRNA translation regulates synapse function and axon maintenance, but how compartment-specific mRNA repertoires are regulated is largely unknown. We developed an axonal transcriptome capture method that allows deep sequencing of metabolically labeled mRNAs from retinal ganglion cell axon terminals in mouse. Comparing axonal-to-somal transcriptomes and axonal translatome-to-transcriptome enables genome-wide visualization of mRNA transport and translation and unveils potential regulators tuned to each process. FMRP and TDP-43 stand out as key regulators of transport, and experiments in Fmr1 knockout mice validate FMRP's role in the axonal transportation of synapse-related mRNAs. Pulse-and-chase experiments enable genome-wide assessment of mRNA stability in axons and reveal a strong coupling between mRNA translation and decay. Measuring the absolute mRNA abundance per axon terminal shows that the adult axonal transcriptome is stably maintained by persistent transport. Our datasets provide a rich resource for unique insights into RNA-based mechanisms in maintaining presynaptic structure and function invivo.

Different platforms for mitomiRs in mitochondria: Emerging facets in regulation of mitochondrial functions

Mitochondria are one of the central organelles involved in cellular energy metabolism and play a regulatory role in various human pathologies ranging from inborn errors of metabolism, cancer, inflammation, and infections. Mitochondrial DNA encodes limited number of genes that is not sufficient for its optimal functioning. Hence, mitochondria import ∼1500 of proteins and ncRNAs from the nucleus depending on energy requirement of cell, tissue size, complexity and diversity of functions. Mitochondrial outer membrane can serve as a platform for regulation of local translation of nuclear-encoded mRNAs of mitochondrial proteins (nmRNAmp); however, underlying molecular mechanism for translational regulation of nmRNAmp at mitochondria is unexplored. Emerging evidence now suggest that mitochondria are enriched with specific miRNAs known as mitomiRs, which may be nuclear or mitochondrial DNA encoded. MitomiRs may modulate mitochondrial function and metabolism by fine-tuning protein levels related to mitochondria. The discovery of mitomiRs raised the questions of elucidating molecular pathways for their biogenesis, translocation, action sites and mechanism of action. Here, we have reviewed the existing reports describing the role of mitomiRs in sub mitochondrial compartments and discussed possible molecular mechanisms of mitomiRs in the regulation of nmRNAmp and mitogenome encoded transcripts. Further understanding of mitomiRs will uncover their implication in various pathophysiological conditions associated with mitochondria.

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.

Intra-axonal translation of Khsrp mRNA slows axon regeneration by destabilizing localized mRNAs.

Axonally synthesized proteins support nerve regeneration through retrograde signaling and local growth mechanisms. RNA binding proteins (RBP) are needed for this and other aspects of post-transcriptional regulation of neuronal mRNAs, but only a limited number of axonal RBPs are known. We used targeted proteomics to profile RBPs in peripheral nerve axons. We detected 76 proteins with reported RNA binding activity in axoplasm, and levels of several change with axon injury and regeneration. RBPs with altered levels include KHSRP that decreases neurite outgrowth in developing CNS neurons. Axonal KHSRP levels rapidly increase after injury remaining elevated up to 28 days post axotomy. Khsrp mRNA localizes into axons and the rapid increase in axonal KHSRP is through local translation of Khsrp mRNA in axons. KHSRP can bind to mRNAs with 3’UTR AU-rich elements and targets those transcripts to the cytoplasmic exosome for degradation. KHSRP knockout mice show increased axonal levels of KHSRP target mRNAs, Gap43, Snap25, and Fubp1, following sciatic nerve injury and these mice show accelerated nerve regeneration in vivo. Together, our data indicate that axonal translation of the RNA binding protein Khsrp mRNA following nerve injury serves to promote decay of other axonal mRNAs and slow axon regeneration.

Steps toward the Elucidation of the Axonal Transcriptome of Human Peripheral Nerves

Recent studies conducted in rodents demonstrate the presence of translational machinery and mRNAs in peripheral axons. These studies also show that local translation of mRNAs in distal axons is important for axon growth, maintenance and regeneration and facilitates communication with the cell body. However, the question remains of which mRNAs are localized to axons of human peripheral neurons. In addition, an important component of axonal transport resides within the UnTranslated Region (UTR) of an mRNA. The 3’ UTR regulates mRNA subcellular localization, and these sequences can be highly divergent across species. The 3’ UTR contains cis-elements (or motifs) that are recognized by RNA-binding proteins (RBPs) for axonal transport. To address this in humans, we used human peripheral nerves collected from amputation surgeries combined with bulk-RNA and single-nuclei sequencing, in situ hybridization and Visium spatial transcriptomics. In our human sural nerve datasets, we detect sensory genes with known function in pain such as NTRK1 that are not expressed in non-neuronal cells present in sural nerves. This suggests that certain mRNAs may be transported to the peripheral sensory axon. We have conducted analysis of motifs-RBPs interactions for the putative axonal mRNAs. We identified RBPs, such as the fragile X mental retardation protein (FMRP), in the sural nerve. We are also using Visium spatial transcriptomics and publicly available datasets to characterize potential interactions between non-neuronal cells and nerve fibers. Our results demonstrate that immune cells present in the skin express several ligands that can interact with human sensory neurons. We detected several macrophage ligands that can interact with TRPV1, which is expressed in nociceptive fibers. Among those ligands are KNG1, FAAH and CALM1 which have known roles in peripheral pain. Maintaining the integrity of long axons is crucial and this work will provide new, fundamental insight into mechanisms that cause peripheral neuropathies. Recent studies conducted in rodents demonstrate the presence of translational machinery and mRNAs in peripheral axons. These studies also show that local translation of mRNAs in distal axons is important for axon growth, maintenance and regeneration and facilitates communication with the cell body. However, the question remains of which mRNAs are localized to axons of human peripheral neurons. In addition, an important component of axonal transport resides within the UnTranslated Region (UTR) of an mRNA. The 3’ UTR regulates mRNA subcellular localization, and these sequences can be highly divergent across species. The 3’ UTR contains cis-elements (or motifs) that are recognized by RNA-binding proteins (RBPs) for axonal transport. To address this in humans, we used human peripheral nerves collected from amputation surgeries combined with bulk-RNA and single-nuclei sequencing, in situ hybridization and Visium spatial transcriptomics. In our human sural nerve datasets, we detect sensory genes with known function in pain such as NTRK1 that are not expressed in non-neuronal cells present in sural nerves. This suggests that certain mRNAs may be transported to the peripheral sensory axon. We have conducted analysis of motifs-RBPs interactions for the putative axonal mRNAs. We identified RBPs, such as the fragile X mental retardation protein (FMRP), in the sural nerve. We are also using Visium spatial transcriptomics and publicly available datasets to characterize potential interactions between non-neuronal cells and nerve fibers. Our results demonstrate that immune cells present in the skin express several ligands that can interact with human sensory neurons. We detected several macrophage ligands that can interact with TRPV1, which is expressed in nociceptive fibers. Among those ligands are KNG1, FAAH and CALM1 which have known roles in peripheral pain. Maintaining the integrity of long axons is crucial and this work will provide new, fundamental insight into mechanisms that cause peripheral neuropathies.

Cotranslational Mechanisms of Protein Biogenesis and Complex Assembly in Eukaryotes.

The formation of protein complexes is crucial to most biological functions. The cellular mechanisms governing protein complex biogenesis are not yet well understood, but some principles of cotranslational and posttranslational assembly are beginning to emerge. In bacteria, this process is favored by operons encoding subunits of protein complexes. Eukaryotic cells do not have polycistronic mRNAs, raising the question of how they orchestrate the encounter of unassembled subunits. Here we review the constraints and mechanisms governing eukaryotic co- and posttranslational protein folding and assembly, including the influence of elongation rate on nascent chain targeting, folding, and chaperone interactions. Recent evidence shows that mRNAs encoding subunits of oligomeric assemblies can undergo localized translation and form cytoplasmic condensates that might facilitate the assembly of protein complexes. Understanding the interplay between localized mRNA translation and cotranslational proteostasis will be critical to defining protein complex assembly in vivo.

Dendritic distribution of CDK5 mRNA and p35 mRNA, and a glutamate-responsive increase of CDK5/p25 complex contribute to tau hyperphosphorylation

BackgroundIn Alzheimer's disease (AD), abnormally phosphorylated tau in the somatodendrite compartment of brain neurons causes synaptic loss, resulting in neuron death. Although the mechanism by which hyperphosphorylated tau appears in dendrites remains unclear, we have previously reported that local translation of tau mRNA and GSK3β mRNA in response to glutamatergic stimulation triggers an increase of tau protein and initiation of a cycle for amplification of reactivated preexisting GSK3β, respectively. In this study, we investigated the mechanism responsible for neural excitation-dependent activation of another major tau kinase, CDK5, within dendrites. MethodsPrimary hippocampal neurons were treated with glutamate and examined by in situ hybridization, immunocytochemistry and Western blotting. ResultsThe mRNAs for both CDK5 and its neural-specific activator, p35, were found to be constitutively distributed in dendrites. Glutamate treatment induced immediate local dendritic translation of these proteins as well as conversion of p35 to p25, which forms the hyper-activated CDK5/p25 complex. This neural excitation-dependent tau phosphorylation by CDK5 was suppressed in the presence of a calpain inhibitor or a NMDA receptor antagonist. ConclusionOur results indicate that in addition to an increase of dendritic tau and reactivation of preexisting GSK3β, increase and hyper-activation of CDK5 are evoked by translation of dendrite-distributed mRNAs upon NMDA receptor-mediated neural excitation. General significanceHyperphosphorylated tau with AD epitopes is locally produced in dendrites via translational activation of dendrite-distributed mRNAs in response to glutamatergic stimulation. Therefore, tau hyperphosphorylation may play a crucial role in synaptic transduction.

MtIF3 is locally translated in axons and regulates mitochondrial translation for axonal growth

BackgroundThe establishment and maintenance of functional neural connections relies on appropriate distribution and localization of mitochondria in neurites, as these organelles provide essential energy and metabolites. In particular, mitochondria are transported to axons and support local energy production to maintain energy-demanding neuronal processes including axon branching, growth, and regeneration. Additionally, local protein synthesis is required for structural and functional changes in axons, with nuclear-encoded mitochondrial mRNAs having been found localized in axons. However, it remains unclear whether these mRNAs are locally translated and whether the potential translated mitochondrial proteins are involved in the regulation of mitochondrial functions in axons. Here, we aim to further understand the purpose of such compartmentalization by focusing on the role of mitochondrial initiation factor 3 (mtIF3), whose nuclear-encoded transcripts have been shown to be present in axonal growth cones.ResultsWe demonstrate that brain-derived neurotrophic factor (BDNF) induces local translation of mtIF3 mRNA in axonal growth cones. Subsequently, mtIF3 protein is translocated into axonal mitochondria and promotes mitochondrial translation as assessed by our newly developed bimolecular fluorescence complementation sensor for the assembly of mitochondrial ribosomes. We further show that BDNF-induced axonal growth requires mtIF3-dependent mitochondrial translation in distal axons.ConclusionWe describe a previously unknown function of mitochondrial initiation factor 3 (mtIF3) in axonal protein synthesis and development. These findings provide insight into the way neurons adaptively control mitochondrial physiology and axonal development via local mtIF3 translation.

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

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