Axonal mRNA localization and local protein synthesis in nervous system assembly, maintenance and repair

Abstract

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.

Figure 1. Growth cone turning regulated by differential mRNA translation

Gradients of protein synthesis-inducing guidance cues commonly activate global translational activity on the side of the growth cone nearest to the gradient by activating mammalian target of rapamycin (mTOR). However, the specific mRNA translated in response to the cue differs depending on whether it is an attractive or repulsive cue and determines the direction of growth cone turning. a | Stimulation by attractive cues, such as netrin 1 and brain-derived neurotrophic factor (BDNF), leads to asymmetric synthesis of β-actin on the side near to the source of the gradient, which is mediated by β-actin mRNA transport to this region by zipcode-binding protein 1 (ZBP1)^,. Spatially restricted synthesis of β-actin may lead to actin polymerization, cytoskeletal assembly and attractive turning of the growth cone. b | By contrast, repulsive cues, such as semaphorin 3A (SEMA3A) and SLIT2, activate the axonal translation of the actin-depolymerizing proteins RHOA and cofilin when uniformly applied in cell culture. A proposed model is shown, in which localized cytoskeletal disassembly may result in repulsive turning through polarized filopodial collapse. However, whether these molecules are translated asymmetrically in a repulsive gradient has not yet been tested.

Figure 2. Axon survival, maintenance and injury-induced responses regulated by local protein synthesis

a | Distal axons receive target-derived trophic factors. These target-derived cues activate local synthesis of mitochondrial, signalling or nuclear proteins required for axon maintenance and cell survival. Stimulation of axons with nerve growth factor (NGF) results in local synthesis of cyclic AMP responsive element-binding protein (CREB), which is then locally phosphorylated. This active form of axonal CREB is transported into the nucleus and is required for axon survival in cultured sensory neurons. NGF also stimulates the axonal synthesis of inositol monophosphatase 1 (IMPA1), an enzyme that regulates the inositol cycle. Axonally synthesized IMPA1 may regulate retrogradely transporting vesicles and is required for axon survival in cultured sympathetic neurons. Distal axons also synthesize nuclear-encoded mitochondrial proteins, and sustained local synthesis and mitochondrial import of such proteins are required for axon maintenance in cultured neurons. Lamin B2, a known nuclear envelope component, is also axonally synthesized and localized to mitochondria in distal axons, and sustained axonal synthesis of lamin B2 is required for axon maintenance in vitro and in vivo. b | Nerve injury generates a retrograde survival and/or repair signal, and local protein synthesis is required for its generation and relay to the nucleus. The retrograde dynein motor complex is inactivated by Ran GTPase in normal conditions. Nerve injury stimulates local synthesis of Ran-specific GTPase-activating protein (RANBP1), which displaces Ran from the dynein motor. Importin-β1 is also locally synthesized in response to nerve injury and binds to the Ran-free dynein motor complex, which results in the activated dynein motor complex. Transcription factors, such as signal transducer and activator of transcription 3 (STAT3), are also locally synthesized and activated in injured axons and bind to the activated dynein motor complex using the nuclear localization signal. A type III intermediate filament vimentin is also locally synthesized in injured axons and co-transported with other signalling molecules that bind to it.

Figure 3. Regulation of global translational activity through mTOR

Cues that induce and inhibit protein synthesis antagonistically regulate the activity of mammalian target of rapamycin (mTOR), which regulates cap-dependent mRNA translation by phosphorylating its two major targets: eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase (S6K). Protein synthesis-inducing cues, such as netrin 1 or brain-derived neurotrophic factor (BDNF), may increase mTOR activity through the AKT–mTOR pathway by activating phosphoinositide 3-kinase (PI3K) or by promoting ubiquitin–proteasome system (UPS)-mediated degradation of phosphatase and tensin homologue (PTEN), or through the mitogen-activated protein kinase (MAPK)–mTOR pathway by activating MAPK extracellular signal-regulated kinase 1 and 2 (ERK1/2)^, that inhibits the mTOR negative regulators tuberous sclerosis protein 1 (TSC1) and TSC2. Some cues, such as ephrin A, can inhibit protein synthesis by inhibiting ERK1/2 leading to TSC1/2 activation and mTOR inhibition. Spatiotemporal summation of cue-induced signals converging on mTOR might lead to asymmetric translational activity. DCC, deleted in colorectal carcinoma (netrin 1 receptor); PDK1, 3-phosphoinositide-dependent protein kinase 1; PIP2, phosphatidylinositol-4,5-bisphosphate (also known as PtdIns(4,5)P₂); PIP3, phosphatidylinositol-3,4,5-triphosphate (also known as PtdIns(3,4,5)P₃).

Figure 4. RNA-specific transport and translation

Axonal targeting of mRNAs is directed by cis-acting elements that are mainly localized to the 3′-untranslated regions (UTRs) of mRNAs. Retention of these axon-targeting cis-acting elements is commonly regulated by the use of different transcriptional termination sites^,. Extrinsic cues influence axonal mRNA repertoires by promoting transport of specific mRNAs. Axonally targeted mRNAs are recruited to RNA granules (transport ribonuceloproteins (RNPs)) by specific RNA-binding proteins (RBPs) and are transported along microtubules probably by kinesin motors. mRNAs remain translationally silent during transport. Extracellular signals activate the translation of specific mRNAs mainly by regulating RBPs. For example, neurotrophins and guidance cues activate the kinases SRC, calcium/calmodulin-dependent protein kinase II (CaMKII) and focal adhesion kinase (FAK), which phosphorylate the RBPs, zipcode binding protein 1 (ZBP1), cytoplasmic polyadenylation element binding protein (CPEB1), and growth factor receptor-bound protein 7 (GRB7), respectively. Cell surface receptors might regulate mRNA-specific translation by directly regulating ribosomes. For example, unstimulated netrin receptor DCC directly binds to ribosomes and inhibits translation, and ribosome composition influences mRNA selectivity. Different receptors may bind to ribosomes that are pre-tuned to specific mRNAs, and ligand stimulation might release such ribosomes and result in mRNA-specific translation. BDNF, brain-derived neurotrophic factor; KOR1, κ-type opioid receptor; NT3, neurotrophin 3; TRK, tyrosine kinase receptor.

Figure 5. Local mRNA translation as a mediator of stimulus-induced axonal responses

A proposed model for the function and mechanism of axonal mRNA translation. Neuronal axons contain a complex and dynamic transcriptome, and many mRNAs remain translationally silent. Various extrinsic cues stimulate translation of a distinct subset of mRNAs during development and in adulthood. For example, guidance cues induce local synthesis of cytoskeletal proteins in growing axons and regulate axon guidance and branching. Target-derived trophic factors promote local synthesis of proteins required for mitochondrial function and support the survival of distal axons. Nerve injury in adulthood stimulates local synthesis of nuclear factors that activate repair mechanisms.