•Axonal protein synthesis occurs in healthy mature axons in vivo.•Protein synthetic and processing machinery may take different forms in axon and soma.•Functionally related proteins are locally ...synthesized by coordinate mRNA translation.•Presynaptic, mitochondrial and ribosomal proteins are synthesized in mature axons.•Dysfunctional axonal translation may trigger axon destruction and neurodegeneration.
Distal axons, remote from their cell bodies and nuclei, must survive the lifetime of an organism. Recent studies have provided compelling evidence that proteins are locally synthesized in healthy, mature central nervous system axons and presynaptic terminals in vivo. Presynaptic, mitochondrial and ribosomal proteins are locally synthesized in most adult axons of diverse cell types, linking local translation to axon function and survival. Accordingly, inhibiting the intra-axonal translation of key mRNAs or the function of their translational regulators causes dying-back axon degeneration, and human mutations in RNA metabolic pathways are increasingly being associated with neurodegenerative diseases that accompany axon degeneration. Here, we summarize recent relevant findings in a highly simplified ‘RNA operon’-based model and discuss open questions and future directions.
mRNAs can be targeted to specific neuronal subcellular domains, which enables rapid changes in the local proteome through local translation. This mRNA-based mechanism links extrinsic signals to ...spatially restricted cellular responses and can mediate stimulus-driven adaptive responses such as dendritic plasticity. Local mRNA translation also occurs in growing axons where it can mediate directional responses to guidance signals. Recent profiling studies have revealed that both growing and mature axons possess surprisingly complex and dynamic transcriptomes, thereby suggesting that axonal mRNA localization is highly regulated and has a role in a broad range of processes, a view that is increasingly being supported by new experimental evidence. Here, we review current knowledge on the roles and regulatory mechanisms of axonal mRNA translation and discuss emerging links to axon guidance, survival, regeneration and neurological disorders.
The subcellular position of a protein is a key determinant of its function. Mounting evidence indicates that RNA localization, where specific mRNAs are transported subcellularly and subsequently ...translated in response to localized signals, is an evolutionarily conserved mechanism to control protein localization. On-site synthesis confers novel signaling properties to a protein and helps to maintain local proteome homeostasis. Local translation plays particularly important roles in distal neuronal compartments, and dysregulated RNA localization and translation cause defects in neuronal wiring and survival. Here, we discuss key findings in this area and possible implications of this adaptable and swift mechanism for spatial control of gene function.
During the maternal-to-zygotic transition (MZT), maternal RNAs are actively degraded and replaced by newly synthesized zygotic transcripts in a highly coordinated manner. However, it remains largely ...unknown how maternal mRNA decay is triggered in early vertebrate embryos. Here, through genome-wide profiling of RNA abundance and 3′ modification, we show that uridylation is induced at the onset of maternal mRNA clearance. The temporal control of uridylation is conserved in vertebrates. When the homologs of terminal uridylyltransferases TUT4 and TUT7 (TUT4/7) are depleted in zebrafish and Xenopus, maternal mRNA clearance is significantly delayed, leading to developmental defects during gastrulation. Short-tailed mRNAs are selectively uridylated by TUT4/7, with the highly uridylated transcripts degraded faster during the MZT than those with unmodified poly(A) tails. Our study demonstrates that uridylation plays a crucial role in timely mRNA degradation, thereby allowing the progression of early development.
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•TUT4 and TUT7 are conserved writers of U tail in vertebrates•mRNA uridylation is induced during early embryogenesis•Uridylation facilitates the programmed elimination of maternal RNAs•TUT4 and TUT7 are required for gastrulation in zebrafish and Xenopus
An embryo begins its life with maternally deposited RNAs that undergo timely translation and degradation. Chang et al. show that terminal uridylyltransferases add uridine tails to trigger decay of short poly(A)-tailed RNAs and that uridylation is critical for temporal organization of transcriptome in early vertebrate embryos.
Neurons make long-distance connections via their axons, and the accuracy and stability of these connections are crucial for brain function. Research using various animal models showed that the ...molecular and cellular mechanisms underlying the assembly and maintenance of neuronal circuitry are highly conserved in vertebrates. Therefore, to gain a deeper understanding of brain development and maintenance, an efficient vertebrate model is required, where the axons of a defined neuronal cell type can be genetically manipulated and selectively visualized
. Placental mammals pose an experimental challenge, as time-consuming breeding of genetically modified animals is required due to their
development.
, the most commonly used amphibian model, offers comparative advantages, since their embryos
during which embryological manipulations can be performed. However, the tetraploidy of the
genome makes them not ideal for genetic studies. Here, we use
, a diploid amphibian species, to visualize axonal pathfinding and degeneration of a single central nervous system neuronal cell type, the retinal ganglion cell (RGC). First, we show that RGC axons follow the developmental trajectory previously described in
with a slightly different timeline. Second, we demonstrate that co-electroporation of DNA and/or oligonucleotides enables the visualization of gene function-altered RGC axons in an intact brain. Finally, using this method, we show that the axon-autonomous, Sarm1-dependent axon destruction program operates in
. Taken together, the present study demonstrates that the visual system of
is a highly efficient model to identify new molecular mechanisms underlying axon guidance and survival.
Ribosome assembly occurs mainly in the nucleolus, yet recent studies have revealed robust enrichment and translation of mRNAs encoding many ribosomal proteins (RPs) in axons, far away from neuronal ...cell bodies. Here, we report a physical and functional interaction between locally synthesized RPs and ribosomes in the axon. We show that axonal RP translation is regulated through a sequence motif, CUIC, that forms an RNA-loop structure in the region immediately upstream of the initiation codon. Using imaging and subcellular proteomics techniques, we show that RPs synthesized in axons join axonal ribosomes in a nucleolus-independent fashion. Inhibition of axonal CUIC-regulated RP translation decreases local translation activity and reduces axon branching in the developing brain, revealing the physiological relevance of axonal RP synthesis in vivo. These results suggest that axonal translation supplies cytoplasmic RPs to maintain/modify local ribosomal function far from the nucleolus in neurons.
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•Axonal ribosomal protein (RP) synthesis is cue regulated via a 5′ UTR loop-forming motif•Axonal RP synthesis generates a free cytosolic pool of ribosomal proteins in axons•Axonally synthesized RPs join axonal ribosomes in a nucleolus-independent manner•Local RP synthesis is required for ribosome function and branch architecture
Local protein synthesis in axons supplies new ribosomal proteins far from the nucleolus, the known site of ribosome biogenesis. Shigeoka et al. provide evidence that axonally synthesized ribosomal proteins join pre-existing ribosomes and maintain translation activity in axons, which is required for axon terminal branching.
p62/SQSTM1 is known to act as a key mediator in the selective autophagy of protein aggregates, or aggrephagy, by steering ubiquitinated protein aggregates towards the autophagy pathway. Here, we use ...a yeast two-hybrid screen to identify the prefoldin-like chaperone UXT as an interacting protein of p62. We show that UXT can bind to protein aggregates as well as the LB domain of p62, and, possibly by forming an oligomer, increase p62 clustering for its efficient targeting to protein aggregates, thereby promoting the formation of the p62 body and clearance of its cargo via autophagy. We also find that ectopic expression of human UXT delays SOD1(A4V)-induced degeneration of motor neurons in a Xenopus model system, and that specific disruption of the interaction between UXT and p62 suppresses UXT-mediated protection. Together, these results indicate that UXT functions as an autophagy adaptor of p62-dependent aggrephagy. Furthermore, our study illustrates a cooperative relationship between molecular chaperones and the aggrephagy machinery that efficiently removes misfolded protein aggregates.
Chemokines and chemokine receptors are widely expressed by cells of the immune and nervous systems. This review focuses on our current knowledge concerning the role of chemokines in the ...pathophysiology of chronic pain syndromes. Injury- or disease-induced changes in the expression of diverse chemokines and their receptors have been demonstrated in the neural and nonneural elements of pain pathways. Under these circumstances, chemokines have been shown to modulate the electrical activity of neurons by multiple regulatory pathways including increases in neurotransmitter release through Ca-dependent mechanisms and transactivation of transient receptor channels. Either of these mechanisms alone, or in combination, may contribute to sustained excitability of primary afferent and secondary neurons within spinal pain pathways. Another manner in which chemokines may influence sustained neuronal excitability may be their ability to function as excitatory neurotransmitters within the peripheral and central nervous system. As is the case for traditional neurotransmitters, injury-induced up-regulated chemokines are found within synaptic vesicles. Chemokines released after depolarization of the cell membrane can then act on other chemokine receptor-bearing neurons, glia, or immune cells. Because up-regulation of chemokines and their receptors may be one of the mechanisms that directly or indirectly contribute to the development and maintenance of chronic pain, these molecules may then represent novel targets for therapeutic intervention in chronic pain states.
Local protein synthesis plays a key role in regulating stimulus-induced responses in dendrites and axons. Recent genome-wide studies have revealed that thousands of different transcripts reside in ...these distal neuronal compartments, but identifying those with functionally significant roles presents a challenge. We performed an unbiased screen to look for stimulus-induced, protein synthesis-dependent changes in the proteome ofXenopus retinal ganglion cell (RGC) axons. The intermediate filament protein lamin B2 (LB2), normally associated with the nuclear membrane, was identified as an unexpected major target. Axonal ribosome immunoprecipitation confirmed translation of lb2 mRNA in vivo. Inhibition of lb2 mRNA translation in axons in vivo does not affect guidance but causes axonal degeneration. Axonal LB2 associates with mitochondria, and LB2-deficient axons exhibit mitochondrial dysfunction and defects in axonal transport. Our results thus suggest that axonally synthesized lamin B plays a crucial role in axon maintenance by promoting mitochondrial function.
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► Unbiased proteomics approach to identify cue-induced changes in the axonal proteome ► Lamin B2 is locally synthesized in response to En-1 stimulation in retinal axons ► Loss of axonal lamin B2 leads to axon degeneration independently from the cell body ► Lamin B2 localizes to mitochondria, and its loss leads to mitochondrial dysfunction
An unexpected cytoplasmic role for a lamin protein in axon maintenance could offer insight into the cell type-specific nature of some laminopathies.
Recent technological advance in single-cell and single-nucleus transcriptomics has made it possible to generate an unprecedentedly detailed landscape of neuro-immune interactions in healthy and ...diseased brains. In this review, we overview the recent literature that catalogs single-cell-level gene expression in brains with signs of inflammation, focusing on maternal immune activation, viral infection, and auto-immune diseases. The literature also includes a series of papers that provide strong evidence for immunological contributions to neurodegenerative diseases, which, in a strict sense, are not considered neuroinflammatory. To help with the discussion, we present a diagram of experimental and analytical flows in the single-cell analysis of the brain. We also discuss the recurring themes of neuro-immune interactions and suggest future research directions.
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