See other articles in PMC that cite the published article. Abstract Cell migration plays an important role in many normal and pathological functions such as development, wound healing, immune defense and tumor metastasis. Cell migration and protein localization Cell migration plays an important role in many normal and pathological cellular functions such as development 1 — 5 , immune defense 6 — 8 , wound healing 9 — 11 and tumor metastasis 12 — Open in a separate window. Arp3 protein and mRNA are localized at the cell leading protrusions in wound tissue Day 4 rat skin wound tissue cryo-sections were processed for protein using immunofluorescence staining A or for mRNA using fluorescence in situ hybridization with tyramide signal amplification B.
A potential role of mRNA localization-mediated local protein synthesis on focal adhesion dynamics Focal adhesion dynamics plays a crucial role in cell migration — A potential role of cytoplasm streaming or force gradient in asymmetric mRNA localization in migrating cells As a common feature in leukocytes and amoebae, protoplasmic flow is responsible for driving cytoplasmic materials toward the pseudopods of a migrating cell — Manipulation of mRNA localization for mechanism and functional studies in cell migration In comparison to the studies for signaling and protein localization in cell migration, there are several unique challenges for the studies of mechanism and functions of mRNA-localization mediated local protein biogenesis.
Physiological significance of mRNA localization and local translation in cell migration and invasion Despite many reports on which the important role of mRNA localization during development and other cellular processes has been well demonstrated, there are still skeptical views regarding the importance of local protein synthesis in the cells as proteins diffuse quickly and mRNA localization is not always correlated with protein localization.
Prospective Significant progress has been made recently in the study of regulation of mRNA localization and local translation in motile cells. Collective cell migration in development. Ohata E, Takahashi Y. Locascio A, Nieto MA. Cell movements during vertebrate development: Curr Opin Genet Dev. The mechanical control of nervous system development. Mrass P, Weninger W. Immune cell migration as a means to control immune privilege: Migration, cell-cell interaction and adhesion in the immune system.
Ernst Schering Found Symp Proc. Targeting cells in motion: Li L, Jiang J. Regulatory factors of mesenchymal stem cell migration into injured tissues and their signal transduction mechanisms. Electrical fields in wound healing-An overriding signal that directs cell migration. Semin Cell Dev Biol. Cytoskeleton responses in wound repair. Cell Mol Life Sci. Becchetti A, Arcangeli A.
Integrins and ion channels in cell migration: Adv Exp Med Biol. Targeting tumor cell motility as a strategy against invasion and metastasis. Tumor cell migration in complex microenvironments. Kaverina I, Straube A. Regulation of cell migration by dynamic microtubules Semin.
Targeting tumor cell motility to prevent metastasis. Adv Drug Deliv Rev. Actin a, central player in cell shape and movement. Life at the leading edge. Actin dynamics at the leading edge: Directed cell invasion and migration during metastasis. Curr Opin Cell Biol. Rottner K, Stradal TE.
Actin dynamics and turnover in cell motility.
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Analysis of RNA localization and metabolism in single live bacterial cells: RNA localization in bacteria. The physiological significance of beta -actin mRNA localization in determining cell polarity and directional motility. Mis-localization of Arp2 mRNA impairs persistence of directional cell migration. Molecular insights into intracellular RNA localization. Int Rev Cell Mol Biol. The central dogma decentralized: Walter P, Blobel G. Translocation of proteins across the endoplasmic reticulum.
Signal recognition protein SRP mediates the selective binding to microsomal membranes of in-vitro-assembled polysomes synthesizing secretory protein. Blobel G, Dobberstein B. Transfer to proteins across membranes. Reconstitution of functional rough microsomes from heterologous components. Transfer of proteins across membranes. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma. Molecular biology of the cell. Garland Publishing, Inc; Partitioning and translation of mRNAs encoding soluble proteins on membrane-bound ribosomes.
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Localization of a subset of yeast mRNAs depends on inheritance of endoplasmic reticulum. Identification of a region within the placental alkaline phosphatase mRNA that mediates pdependent targeting to the Endoplasmic Reticulum. Hermesh O, Jansen RP. Take the RN A-train: Oleynikov Y, Singer RH. Why cells move messages: Czaplinski K, Singer RH. Pathways for mRNA localization in the cytoplasm. Localization of RNAs to the mitochondrial cloud in Xenopus oocytes through entrapment and association with endoplasmic reticulum.
Evidence for a transport-trap mode of Drosophila melanogaster gurken mRNA localization. Regulation and function of maternal mRNA destabilization during early Drosophila development. Remote control of gene function by local translation. Xing L, Bassell GJ. Identification of process-localized mRNAs from cultured rodent hippocampal neurons. Thomsen R, Lade Nielsen A. A Boyden chamber-based method for characterization of astrocyte protrusion localized RNA and protein. Sequences responsible for intracellular localization of beta-actin messenger RNA also affect cell phenotype.
Sorting of beta-actin mRNA and protein to neurites and growth cones in culture. Mili S, Macara IG. RNA localization and polarity: Neuronal RNA localization and the cytoskeleton. Results Probl Cell Differ. Beta-actin mRNA localization is regulated by signal transduction mechanisms. A Rho-dependent signaling pathway operating through myosin localizes beta-actin mRNA in fibroblasts. Characterization of a beta-actin mRNA zipcode-binding protein.
A predominantly nuclear protein affecting cytoplasmic localization of beta-actin mRNA in fibroblasts and neurons. Condeelis J, Singer RH. How and why does beta-actin mRNA target? Spatial regulation of beta-actin translation by Src-dependent phosphorylation of ZBP1. Interactions of elongation factor 1alpha with F-actin and beta-actin mRNA: The role of the cytoskeleton in eukaryotic protein synthesis. A minireview Cell Biol Int Rep. Efficient mammalian protein synthesis requires an intact F-actin system.
Visualization of mRNA translation in living cells. From birth to death: Jambhekar A, Derisi JL. Cis-acting determinants of asymmetric, cytoplasmic RNA transport. Lack of adenomatous polyposis coli protein correlates with a decrease in cell migration and overall changes in microtubule stability. Etienne-Manneville S, Hall A. Cdc42 regulates GSK-3beta and adenomatous polyposis coli to control cell polarity. Functions of integrins in cell adhesion and migration. Flux at focal adhesions: Huttenlocher A, Horwitz AR. Integrins in cell migration.
Cold Spring Harb Perspect Biol. Zhao X, Guan JL. Focal adhesion kinase and its signaling pathways in cell migration and angiogenesis. Asymmetric focal adhesion disassembly in motile cells. Assembly and disassembly of cell matrix adhesions. Force fluctuations within focal adhesions mediate ECM-rigidity sensing to guide directed cell migration.
Stehbens S, Wittmann T. Guiding cell migration by tugging.
The control of mRNA production, translation and turnover in suspended and reattached anchorage-dependent fibroblasts. Integrin binding and mechanical tension induce movement of mRNA and ribosomes to focal adhesions. RNA and RNA binding proteins participate in early stages of cell spreading through spreading initiation centers. Blocking beta-catenin binding to the ZBP1 promoter represses ZBP1 expression, leading to increased proliferation and migration of metastatic breast-cancer cells. Cytoplasmic streaming in amoeboid movement. Fourth, nascent proteins may have properties distinct from pre-existing copies, by virtue of post-translational modifications or through chaperone-aided folding pathways Lin and Holt, Lastly, a major advantage of mRNA targeting is that it allows fine-tuning of gene expression in both space and time.
Examples of this include targeting of different splice variants to distinct cellular compartments Baj et al. Three distinct mechanisms have been proposed to account for the asymmetric distribution of mRNAs within cells: Experimentally, distinguishing these mechanisms often requires combining analyses of RNA regulatory sequences with live imaging Box 1. Over the last decade, methods relying on RNA tagging and high-resolution microscopy have been developed to improve mRNA detection in living cells and organisms Armitage, A tethering approach, in which RNA recognition sequences bound by a specific RNA-binding protein are inserted into the transcript of interest, allows dynamic distribution of mRNA in living tissues to be monitored by co-expression of exogenous RNA-binding protein e.
Drawbacks of this technique include background fluorescence produced by unbound chimeric FPs, and the high number of RNA tags necessary for a strong signal, potentially altering mRNA behaviour. A recently developed technique uses RNA motifs aptamers that bind fluorogenic dyes. Spinach has been used for live imaging of abundant RNAs in cultured cells, but utility for in vivo mRNA imaging must yet be demonstrated.
Fluorescence imaging techniques have also been developed to follow endogenous mRNAs in live samples. These methods use molecular beacons, which are non-membrane-permeable hairpin-shaped molecules with an internally quenched fluorophore, fluorescence of which is restored upon binding to target RNAs C Bratu et al. Potential disadvantages include target accessibility and impairment of mRNA function upon binding of multiple beacons. Potential caveats here are that injected RNAs are not processed by endogenous machineries, and fluorophores might alter the function of labelled RNAs.
Three distinct mechanisms underlying mRNA localization. B mRNAs freely diffuse in the cytoplasm and are locally entrapped, at the cell cortex for example. C mRNAs destined for directional transport are recognized by specific trans-acting factors in the nucleus, where RNPs undergo different maturation steps. Upon export to the cytoplasm, RNP complexes are remodelled, and cytoplasmic factors ensuring coupling with molecular motors and transport along a polarized cytoskeleton are recruited. Once at the final destination, mRNAs are anchored and their translation is activated.
Generalized RNA degradation coupled to local protection has been described in Drosophila , where Hsp83 mRNA is uniformly distributed in early fertilized eggs, but restricted to the posteriorly localized germ plasm at later stages Ding et al. In the absence of the RNA degradation machinery, this selective accumulation is lost and Hsp83 transcripts are stabilized throughout the embryo Semotok et al. How the posterior pool of Hsp83 transcripts is protected from decay in wild-type conditions is, however, still unclear. In early Xenopus oocytes, localization of the germ plasm RNAs Xcat2 Nanos1 and Xdazl has also been proposed to rely on entrapment and association of freely diffusing RNA molecules with a densely packed endoplasmic reticulum ER concentrated in the vegetally localized mitochondrial cloud Chang et al.
Directed transport of transcripts along a polarized cytoskeletal network is a predominant mechanism to direct mRNA localization Bullock, that has been observed in a variety of cell types, including Drosophila oocytes and embryos, Xenopus oocytes, migrating mammalian cells and vertebrate neurons Gagnon and Mowry, A surprising finding from live-imaging analyses Box 1 was that active motion is not restricted to localizing mRNAs, as uniformly distributed transcripts also undergo cytoskeleton-dependent movements Fusco et al.
Rather, the unique feature of localized mRNA movement is that it is non-random, exhibiting a net bias explained by increased frequency and duration of directed transport events. How is this net transport achieved?
As described below, key features appear to involve specific recognition by trans-acting factors, assembly of localization-competent ribonucleoprotein RNP complexes, recruitment of molecular motors and transport along the cytoskeleton, as well as anchoring of the mRNA at the final destination. Finally, tight coupling with translational regulation is required to achieve spatially restricted protein synthesis. As revealed by proteomic analyses, mRNAs to be transported are packaged within complexes containing a large number of associated proteins Kanai et al. Some of these proteins bind to the mRNA upon transcription or splicing, rendering it competent for the future recruitment of the cytoplasmic transport machinery Marchand et al.
Consistent with a key influence of mRNA nuclear history, nuclear processing events such as splicing, transit through the nucleolus, deposition of key nuclear factors or translocation through specific nucleopore structures have been shown to be required for cytoplasmic targeting of localized mRNAs Giorgi and Moore, ; Marchand et al.
Following export to the cytoplasm, RNP complexes are remodelled, and cytoplasmic factors ensuring translational repression and specific coupling with molecular motors are recruited Lewis and Mowry, ; Besse and Ephrussi, By contrast, RNP complexes found in Drosophila embryos or the dendrites of mammalian neurons seem to contain a very limited number of RNA molecules, suggesting that mRNAs are transported independently of each other in these systems Mikl et al.
Notably, Egalitarian shows only a modest binding preference for its targets. However, association with BicD protein and formation of a ternary complex significantly enhances its affinity for localizing targets Dienstbier et al. A current limitation to our understanding of localized RNA recognition is the small number of well-characterized localization elements. Although genome-wide analyses have provided large datasets for the discovery of new zipcodes, it has proven difficult to identify common signatures shared by RNA molecules targeted to the same cellular sites.
This probably stems from the difficulty of predicting tertiary structures in silico, as well as from the fact that localized mRNAs can contain multiple localizing elements with redundant or complementary functions Gautreau et al. The nature and number of active molecular motors recruited to a target mRNA dictate the cytoskeletal tracks actin filaments or microtubules used for mRNA transport, the type of movement uni- or bidirectional , and the properties e. For example, the recruitment of several molecules of the myosin motor Myo4p by multiple localization elements increases the efficiency of ASH1 mRNA transport on actin filaments in yeast Chung and Takizawa, Furthermore, dendritically transported RNPs exhibit microtubule-dependent bidirectional movement, suggesting the recruitment and the activity of opposite polarity motors Doyle and Kiebler, Consistent with this view, the RNA-binding protein Fmrp has been shown to associate with dendritically localized transcripts, and to bind to KLC a component of the plus-end motor Kinesin-1 as well as to the dynein-interacting BicD protein Dictenberg et al.
A general trend emerging from live-imaging analyses is that bidirectional transport is commonly used in higher eukaryotes for mRNA targeting. This might allow RNPs to navigate around obstacles and ensure a constant reassessment and fine-tuning of directional transport. Once transported, mRNAs must be retained at their destinations. In cells with no static pre-localized anchor, this can be achieved via continuous rounds of short-range active transport, as shown for maintenance of bicoid mRNA localization at the anterior pole of late-stage Drosophila oocytes Weil et al.
In various cell types, including Drosophila oocytes, Xenopus oocytes and dividing yeast cells, static anchoring of mRNAs relies on cortical actin and actin-binding proteins King et al. Interestingly, alternative actin-independent mechanisms have been discovered: Local protein production requires translational repression of localized mRNAs during transport and subsequent activation at the final destination. Translational repressors have been shown to associate with transport RNPs by directly binding RNA regulatory sequences and blocking translation, largely at the initiation stage Besse and Ephrussi, Although the mechanisms regulating local activation of translation are much less clear, a theme emerging from studies in multiple systems is that phosphorylation of repressors at the destination either directly upon arrival or in response to external signals can decrease affinity for their target mRNAs, thereby relieving translational blockage.
Similarly, altering the phosphorylation status of the RNA-binding protein Fmrp Fmr1 — Mouse Genome Informatics appears to trigger a translational switch: Together, these mechanisms ensure tight localization of mRNAs, and subsequent protein production, to particular subcellular compartments. In the next sections, we will discuss the functional significance of such localization.
In many vertebrate and invertebrate organisms, localization of mRNA molecules in oocytes and eggs establishes a spatially restricted pattern of gene expression that acts to specify embryonic axes and cell fates reviewed by Kumano, Roles for asymmetric RNA localization in embryonic patterning are perhaps best studied in Drosophila , in which localized mRNAs underlie patterning along both the anteroposterior and dorsoventral axes Fig.
Localization of bicoid mRNA to the anterior of the developing oocyte establishes a gradient of transcription factor activity that specifies anterior cell fates Berleth et al. Localized maternal mRNAs in eggs and oocytes. A A stage 9 Drosophila oocyte is depicted, with accessory nurse cells shown on the left. The anteroposterior and dorsoventral axes are indicated on the left. Shown in the vegetal hemisphere are dazl blue and other vegetally localized mRNAs orange.
B-E The animal-vegetal axis is indicated on the left. Later in oogenesis, Vg1 and VegT mRNAs are transported to the vegetal cortex and are inherited during cleavage by the vegetal blastomeres Melton, ; Zhang and King, During oogenesis in zebrafish, many maternal mRNAs are differentially localized along the animal-vegetal axis Fig. Whereas several mRNAs, including dazl , are localized to the vegetal pole, pou2 pou5f1 — Zebrafish Information Network mRNA, which encodes a transcription factor that functions in endoderm specification, is localized to the animal pole Howley and Ho, ; Lunde et al.
The Wnt signalling pathway activated by these localized maternal mRNAs acts to establish the oral-aboral axis during embryogenesis Momose et al. This diverse group of mRNAs includes PEM1 , which encodes a protein of unknown function that plays a role in anterodorsal patterning Yoshida et al.
During cleavage, the PEM RNAs are segregated to the posterior blastomeres and act to specify cell fates and axial polarity during embryogenesis Sardet et al. As exemplified above, localization of maternal RNA determinants is a widely used strategy for establishing axial polarity during animal development. Although commonalities exist among the localization mechanisms, the molecular identities of localized mRNAs can, in some cases, be rapidly evolving.
Studies in the wasp Nasonia showed that anterior patterning is specified by anterior localization of orthodenticle mRNA Lynch et al.
Nonetheless, functional conservation is apparent, as the DNA-binding specificity of the homeodomain transcription factor encoded by orthodenticle is the same as that of bicoid Wilson et al. Asymmetric cell divisions produce daughter cells with distinct fates, and rely on the asymmetric segregation of key determinants, including localized mRNAs. Specific populations of mRNAs are localized for the first time in cleavage-stage embryos, where they are partitioned into only one daughter cell upon cell division.
During the early cleavage cycles of the Ilyanassa mollusc embryo, for example, a large fraction of mRNAs are localized to one of the two centrosomes and asymmetrically inherited Fig. As most of these mRNAs encode developmental patterning genes known for their regulatory functions in other organisms, it is likely that their differential segregation controls cell fate specification Kingsley et al.
Asymmetrically segregating mRNAs in dividing cells. A In Ilyanassa embryos, specific mRNAs blue localize to one of the two centrosomes of metaphasic cells at the 4-cell stage left. Upon division, these mRNAs are differentially inherited by daughter cells right. B Not mRNA yellow is delivered to one side of a Halocynthia embryo mesendoderm cell by nuclear migration. Not mRNA is inherited by the mesoderm daughter cell, but not the endoderm daughter cell.
Adapted from Takatori et al. C Drosophila embryo neuroblasts Nb divide asymmetrically to regenerate a Nb and produce a smaller cell, the ganglion mother cell GMC. Whereas inscuteable insc mRNA orange and Insc protein red localize at the apical side of interphasic Nb, prospero pros mRNA yellow and Pros protein green localize basally at anaphase, thus ensuring a differential inheritance of the two components.
A, anterior; Ani, animal; P, posterior; Veg, vegetal. In mesendoderm cells of the cell stage Halocynthia ascidian embryo, Not mRNA is delivered to the cytoplasm of the future mesoderm-forming pole after nuclear migration, and is asymmetrically partitioned into the mesoderm daughter cell through cytokinesis Fig. Not mRNA encodes a transcription factor that promotes mesoderm fate and suppresses endoderm fate, suggesting that this asymmetric partitioning is responsible for the segregation of germ layer fates Takatori et al.
The role of mRNA localization in lineage diversification has recently been extended to mammalian early embryogenesis. During early mouse development, the first cell fate decision is between the pluripotent inner cell mass and the external trophectoderm Jedrusik et al. In this context, the trophectoderm determinant-encoding cdx2 mRNA has been shown to concentrate at the apical side of 8- to cell blastomeres, and to be inherited exclusively by outside daughter cells upon division Jedrusik et al.
Although additional functional studies are required, these results suggest that unequal mRNA distribution might contribute to symmetry breaking between inside and outside cells. Asymmetric segregation of mRNAs encoding cell fate determinants has also been observed in the context of stem cell divisions. In Drosophila embryos, neural precursors neuroblasts divide unequally to generate a neuroblast and a smaller ganglion mother cell GMC.
In the absence of mRNA localization, Prospero protein still localizes basally, suggesting that prospero mRNA and protein are targeted independently. Functionally, these two processes appear to act redundantly to establish GMC-specific patterns of gene expression Broadus et al. Asymmetric mRNA inheritance requires segregation to be coupled with precise orientation of cell division.
In Drosophila neuroblasts, coupling is mediated by the adapter protein Inscuteable, and inscuteable mRNA is targeted apically in interphase neuroblasts Fig. Interestingly, mislocalization of inscuteable mRNA significantly reduces Inscuteable protein apical accumulation, and is associated with mitotic spindle misorientation Hughes et al. Surprisingly, the preferential inheritance of glide mRNA by the glioblast does not result in asymmetric inheritance of the Glide protein, but seems to contribute to a glioblast-specific positive autoregulatory loop Bernardoni et al.
Studies on specific transcripts suggest that this process is essential for epithelial cell polarization and functions. Establishment and maintenance of epithelial cell polarity rely on the asymmetric distribution of evolutionarily conserved protein complexes, such as the Crumbs complex. Remarkably, apical localization of sdt mRNA is developmentally regulated through an alternative splicing event and appears to contribute exclusively to the early phase of Sdt and Crb apical accumulation Fig.
Most recently, apical localization of the mRNA encoding the zonal occludens-1 ZO-1; also known as Tjp1 protein has been shown to control tight junction assembly and cell polarity in mammary epithelial cells Fig. Apicobasal mRNA targeting in epithelial cells. A In Drosophila follicular cells and embryonic epithelia, apical targeting of stardust sdt , orange mRNA is necessary to localize Crumbs Crb, blue and Sdt red proteins in young epithelial cells. At later stages, sdt mRNA is no longer apically distributed. B mRNA encoding zonal occludens-1 ZO-1 , purple is targeted apically in mammalian mammary epithelial cells, which is necessary for the localization of its protein product blue to apical tight junctions.
C wingless mRNA wg , orange accumulates at the apical pole of Drosophila embryonic epithelial cells, thereby promoting the secretion of Wg protein red stars. The left panel represents a lateral view of the follicular epithelium, and the right panel a basal view. Adapted from Schotman et al. Apical mRNA localization might also play crucial roles in the nucleation and positioning of cytoskeletal networks.
Whether targeting of these mRNAs is functionally relevant however remains to be studied. One means of optimizing secretion of signalling molecules in epithelia is to accumulate their transcripts near the site of secretion prior to translation.
Transcripts encoding Wingless Wg , a Drosophila member of the Wnt family, are apically localized within epithelial cells of the embryonic ectoderm Fig. Importantly, apical targeting of wg mRNA is crucial for autoregulation of Wg protein levels and apical secretion, thus ensuring robustness of the signalling activity. Apical targeting of upd mRNA is essential for efficient Upd secretion and signalling to adjacent epithelial cells, triggering their differentiation into migrating cells. Transcripts such as hairy, runt and even skipped , which encode pair-rule transcription factors responsible for Drosophila embryonic patterning, are also targeted apically in blastoderm embryos Fig.
This process is conserved through dipteran evolution and is essential for efficient transcription of patterning genes Bullock et al. By targeting translation near apically localized nuclei, mRNA transport could favour the nuclear uptake of encoded transcription factors. Cellular protrusions in migrating cells and growing neuronal processes accumulate proteins involved in sensing external cues, and regulating cell motility and directionality. Asymmetric targeting of these mRNAs is crucial for directional cell migration Condeelis and Singer, ; Liao et al.
Targeted mRNAs in migrating cells. Local synthesis of their corresponding proteins dark blue and dark green stars, respectively contributes to directional migration. Consistent with this, the tumour suppressor APC plays a crucial role in anchoring mRNAs specifically targeted to the protrusions of migrating fibroblasts Mili et al.
As revealed by studies performed on cultured Xenopus retinal neurons, axonal growth cone steering decisions require local translation of mRNAs targeted to growing axon tips Campbell and Holt, Consistent with this, axonal translation of proteins as diverse as polarity proteins involved in axon outgrowth Hengst et al. A strong link between axonal mRNA localization, local translation, and axon steering has been provided through studies performed in growing Xenopus and murine axons. Interestingly, the nature of locally translated proteins depends on the type of applied stimuli Lin and Holt, , and the repertoire of axonally localized mRNAs is regulated in response to external signals Willis et al.
In rat sensory neurons, for instance, mRNAs encoding cytoskeletal regulators and transport-related proteins are found in embryonic, but not adult axons Gumy et al. In addition to the ex vivo results described above, in vivo studies have shown that mRNA translation in axons is regulated during nervous system development.
For example, translation of EphA2 guidance receptor mRNA reporter constructs is activated in chick spinal cord axons only once commissural growth cones have crossed the midline Brittis et al. Also consistent with developmental regulation of axonal protein synthesis in vivo, local translation of mouse odorant receptor mRNAs is higher in immature than in adult olfactory bulbs Dubacq et al. Maturation of neuronal circuits involves remodelling of dendritic trees and refinement of synapse number and strength, two processes controlled by local translation of dendritically targeted mRNAs Sutton and Schuman, ; Doyle and Kiebler, The complexity of dendritic trees is a key parameter underlying neuronal activity and connectivity.
Strikingly, recent studies have revealed that the transport of specific mRNAs to dendrites is crucial for dendritic branching during development. Targeting of nanos mRNA to dendrites, for example, is required for proper branching of peripheral sensory neurons in Drosophila larvae Brechbiel and Gavis, In rat hippocampal neurons, differential localization of Bdnf splice variants along dendrites has been shown to lead to spatially restricted effects on dendritic architecture.
Whereas Bdnf transcripts restricted to the cell soma and proximal dendrites selectively affect proximal dendritic branching, transcripts with a distal dendritic localization affect peripheral dendrite complexity Baj et al. Formation of new synapses is crucial during early development of the nervous system, and is a multistep process involving initial assembly, maturation and stabilization. As shown in cultured Aplysia neurons, recruitment of the neuropeptide-encoding sensorin mRNA to nascent synapses is induced upon recognition of pre- and postsynaptic partners, and is required for further synaptic development and maturation Lyles et al.
These results illustrate that synaptogenesis not only involves recruitment of proteins or organelles, such as synaptic vesicles, but also requires mRNA targeting. In more mature neurons, local translation of dendritically localized mRNAs encoding proteins such as neurotransmitter receptors, ion channels and signal transduction enzymes is essential for the regulation of synaptic activity Sutton and Schuman, ; Doyle and Kiebler, Interestingly, specific populations of mRNAs are recruited to dendrites upon synaptic activity Steward et al.
Local translation of dendritic mRNAs in response to synaptic activity. Translation of dendritically targeted mRNAs is activated in response to synaptic activity and is essential for modulation of synaptic activity and dendritic spine morphogenesis. Strikingly, translation can be regulated at the synapse level, and thus represents an efficient way to individually tag activated synapses. Proteins synthesized locally in dendritic spines are represented in green. Although most of the aforementioned studies have been performed ex vivo, evidence has been provided for a functional requirement for dendritically localized protein synthesis in stable forms of synaptic plasticity and memory consolidation in vivo.
Moreover, mutations in genes involved in dendritic mRNA targeting or translation have been linked to several human neurological disorders, including the most common cause of inherited mental retardation Fragile X syndrome, consistent with a role for dendritically localized protein synthesis in the regulation of synaptic morphogenesis and plasticity Liu-Yesucevitz et al. The examples discussed above highlight the importance of mRNA localization, as well as its complexity. Given that hundreds of mRNAs can be targeted to diverse subcellular compartments and that their localization can be dynamic, it is clearly essential that the cell can tightly regulate complementary transport machineries in space and time.
For this, combinations of RNA-binding proteins selectively recognize different target mRNAs, ensuring the recruitment of specific molecular motors. In the Drosophila oocyte, for example, oskar and gurken mRNAs assemble into distinct RNP complexes, and their transport to the posterior and anterodorsal poles depends on the activity of kinesin and dynein, respectively Becalska and Gavis, How many mRNAs are transported by a given targeting machinery in a given cell type is still unclear, although individual RNA-binding proteins have the capacity to associate with hundreds of functionally related mRNAs Hogan et al.
Localizing various mRNAs to distinct cellular locations also implies that these compartments must be provided with a functional translation machinery. Whether components of the translation machinery are transported concomitantly with or independently of mRNAs is still unclear, although ribosomal constituents can be found in localizing RNP complexes Besse and Ephrussi, To modulate their repertoire of localized mRNAs, polarized cells must thus dynamically regulate the activity of their mRNA targeting machineries. In mouse hippocampal neurons, transport of the Fmrp cargo Camk2a mRNA to dendrites is increased upon stimulation of metabotropic glutamate mGluR receptors Dictenberg et al.
Although the underlying mechanisms are still unclear, an increased association between Fmrp and the Kif5 motor is observed after stimulation, suggesting that differential recruitment of molecular motors to RNPs could modulate mRNA trafficking in response to a stimulus. In migrating mammalian cells, two independent pathways controlled by the Zbp1 and Apc proteins can regulate the targeting of mRNAs to the leading edge Mili and Macara, ; their activity has been proposed be modulated in such a way that they are not simultaneously operational.
Intracellular mRNA targeting is now recognized as a central mechanism used in many if not all polarized cells and, as detailed here, plays a key role in multiple developmental contexts in a wide range of organisms. Strikingly, although the nature of transported mRNAs can vary greatly between cell types and species, mRNA targeting machineries appear to be conserved. Despite recent progress in the dissection of mRNA transport mechanisms and the dramatic proliferation of newly identified localized mRNAs, significant challenges remain.
First, how cells dynamically regulate transport machineries to control their repertoire of localized mRNAs during development is still unclear. Second, the relative contribution of mRNA and protein targeting must be systematically explored, as they may act in parallel either synergistically or redundantly in different contexts.
Finally, the extent to which non-coding RNAs are asymmetrically targeted is still largely unknown, although some intriguing studies have suggested that non-coding RNAs with specific distributions might have local regulatory or structural roles Tiedge et al. Interestingly, recent work has demonstrated a strong interplay between miRNA and proteins binding to localization elements Koebernick et al.
Answering these questions will require combining high-throughput biochemical and imaging approaches with functional analyses specifically addressing the role of mRNA transport in vivo. We are extremely grateful to Simon Bullock for helpful suggestions and critical reading of the manuscript. We thank the authors cited in the legend to Fig. Deposited in PMC for release after 12 months. National Center for Biotechnology Information , U. Find articles by Caroline Medioni. Find articles by Kimberly Mowry. Find articles by Florence Besse. This article has been cited by other articles in PMC.
Abstract Intracellular targeting of mRNAs has long been recognized as a means to produce proteins locally, but has only recently emerged as a prevalent mechanism used by a wide variety of polarized cell types. Introduction Establishment of cell polarity is crucial for the execution of developmental programmes governing key processes, including specification of cell fates, individual or collective cell movements and specialization of somatic cell types.
Open in a separate window. Examples of recent genome-wide screens for localized mRNAs. Why localize mRNAs rather than proteins? Proposed mechanisms for asymmetric mRNA localization Three distinct mechanisms have been proposed to account for the asymmetric distribution of mRNAs within cells: Live-imaging methods for visualizing mRNA localization. Cellular mechanisms underlying intracellular mRNA transport Assembly of transport-competent RNPs As revealed by proteomic analyses, mRNAs to be transported are packaged within complexes containing a large number of associated proteins Kanai et al.
Cis-regulatory elements and trans-acting factors Formation of transport-competent RNPs is initiated via the recognition of cis-regulatory elements present in RNA molecules by specific RNA-binding proteins. Recruitment of molecular motors and directed transport The nature and number of active molecular motors recruited to a target mRNA dictate the cytoskeletal tracks actin filaments or microtubules used for mRNA transport, the type of movement uni- or bidirectional , and the properties e. Translational regulation Local protein production requires translational repression of localized mRNAs during transport and subsequent activation at the final destination.
Establishment of embryonic polarity by localized maternal RNA determinants In many vertebrate and invertebrate organisms, localization of mRNA molecules in oocytes and eggs establishes a spatially restricted pattern of gene expression that acts to specify embryonic axes and cell fates reviewed by Kumano, Axis specification and positional identity during Drosophila oogenesis Roles for asymmetric RNA localization in embryonic patterning are perhaps best studied in Drosophila , in which localized mRNAs underlie patterning along both the anteroposterior and dorsoventral axes Fig.
Germ layer patterning and axis specification in vertebrate oocytes In Xenopus oocytes, maternal mRNAs localized to the vegetal cortex Fig. Evolutionary considerations As exemplified above, localization of maternal RNA determinants is a widely used strategy for establishing axial polarity during animal development. Specification of cell fate by asymmetric segregation of RNA determinants in dividing cells Asymmetric cell divisions produce daughter cells with distinct fates, and rely on the asymmetric segregation of key determinants, including localized mRNAs.
Asymmetric RNA inheritance in cleavage-stage embryos Specific populations of mRNAs are localized for the first time in cleavage-stage embryos, where they are partitioned into only one daughter cell upon cell division. Asymmetric RNA inheritance in stem cell divisions Asymmetric segregation of mRNAs encoding cell fate determinants has also been observed in the context of stem cell divisions.
Apically distributed mRNA encoding cell junction components and cytoskeletal regulators Establishment and maintenance of epithelial cell polarity rely on the asymmetric distribution of evolutionarily conserved protein complexes, such as the Crumbs complex. Functional signalling pathways and transcript localization along the apicobasal axis One means of optimizing secretion of signalling molecules in epithelia is to accumulate their transcripts near the site of secretion prior to translation.
Regulation of cell migration and guidance by mRNA targeting Cellular protrusions in migrating cells and growing neuronal processes accumulate proteins involved in sensing external cues, and regulating cell motility and directionality.