A Rho-dependent signaling pathway operating through myosin localizes beta-actin mRNA in fibroblasts

Unraveling Axonal Transcriptional Landscapes: Insights from iPSC-Derived Cortical Neurons and Implications for Motor Neuron Degeneration

Neuronal function and pathology are deeply influenced by the distinct molecular profiles of the axon and soma. Traditional studies have often overlooked these differences due to the technical challenges of compartment specific analysis. In this study, we employ a robust RNA-sequencing (RNA-seq) approach, using microfluidic devices, to generate high-quality axonal transcriptomes from iPSC-derived cortical neurons (CNs). We achieve high specificity of axonal fractions, ensuring sample purity without contamination. Comparative analysis revealed a unique and specific transcriptional landscape in axonal compartments, characterized by diverse transcript types, including protein-coding mRNAs, RNAs encoding ribosomal proteins (RPs), mitochondrial-encoded RNAs, and long non-coding RNAs (lncRNAs). Previous works have reported the existence of transcription factors (TFs) in the axon. Here, we detect a set of TFs specific to the axon and indicative of their active participation in transcriptional regulation. To investigate transcripts and pathways essential for central motor neuron (MN) degeneration and maintenance we analyzed KIF1C-knockout (KO) CNs, modeling hereditary spastic paraplegia (HSP), a disorder associated with prominent length-dependent degeneration of central MN axons. We found that several key factors crucial for survival and health were absent in KIF1C-KO axons, highlighting a possible role of these also in other neurodegenerative diseases. Taken together, this study underscores the utility of microfluidic devices in studying compartment-specific transcriptomics in human neuronal models and reveals complex molecular dynamics of axonal biology. The impact of KIF1C on the axonal transcriptome not only deepens our understanding of MN diseases but also presents a promising avenue for exploration of compartment specific disease mechanisms.

Simultaneous and Sensitive Sensing of Intracellular MicroRNA and mRNA for the Detection of the PI3K/AKT Signaling Pathway in Live Cells

Simultaneous detection of the concentration variations of microRNA-221 (miRNA-221) and PTEN mRNA molecules in the PI3K/AKT signaling pathway is of significance to elucidate cancer cell migration and invasion, which is useful for cancer diagnosis and therapy. In this work, we show the biodegradable MnO2 nanosheet-assisted and target-triggered DNAzyme recycling signal amplification cascaded approach for the specific detection of the PI3K/AKT signaling pathway in live cells via simultaneous and sensitive monitoring of the variation of intracellular miRNA-221 and PTEN mRNA. Our nanoprobes enable highly sensitive and multiplexed sensing of miRNA-221 and PTEN mRNA with low detection limits of 23.6 and 0.59 pM in vitro, respectively, due to the signal amplification cascades. Importantly, the nanoprobes can be readily delivered into cancer cells and the MnO2 nanosheets can be degraded by intracellular glutathione to release the Mn2+ cofactors to trigger multiple DNAzyme recycling cycles to show highly enhanced fluorescence at different wavelengths to realize sensitive and multiplexed imaging of PTEN mRNA and miRNA-221 for detecting the PI3K/AKT signaling pathway. Moreover, the regulation of PTEN mRNA expression by miRNA-221 upon stimulation by various drugs can also be verified by our method, indicating its promising potentials for both disease diagnosis and drug screening.

RNA localization to the mitotic spindle is essential for early development and is regulated by kinesin-1 and dynein

Mitosis is a fundamental and highly regulated process that acts to faithfully segregate chromosomes into two identical daughter cells. Localization of gene transcripts involved in mitosis to the mitotic spindle might be an evolutionarily conserved mechanism to ensure that mitosis occurs in a timely manner. We identified many RNA transcripts that encode proteins involved in mitosis localized at the mitotic spindles in dividing sea urchin embryos and mammalian cells. Disruption of microtubule polymerization, kinesin-1 or dynein results in lack of spindle localization of these transcripts in the sea urchin embryo. Furthermore, results indicate that the cytoplasmic polyadenylation element (CPE) within the 3'UTR of the Aurora B transcript, a recognition sequence for CPEB, is essential for RNA localization to the mitotic spindle in the sea urchin embryo. Blocking this sequence results in arrested development during early cleavage stages, suggesting that RNA localization to the mitotic spindle might be a regulatory mechanism of cell division that is important for early development.

Folylpolyglutamate synthetase mRNA G-quadruplexes regulate its cell protrusion localization and enhance a cancer cell invasive phenotype upon folate repletion

BACKGROUND

Folates are crucial for the biosynthesis of nucleotides and amino acids, essential for cell proliferation and development. Folate deficiency induces DNA damage, developmental defects, and tumorigenicity. The obligatory enzyme folylpolyglutamate synthetase (FPGS) mediates intracellular folate retention via cytosolic and mitochondrial folate polyglutamylation. Our previous paper demonstrated the association of the cytosolic FPGS (cFPGS) with the cytoskeleton and various cell protrusion proteins. Based on these recent findings, the aim of the current study was to investigate the potential role of cFPGS at cell protrusions.

RESULTS

Here we uncovered a central role for two G-quadruplex (GQ) motifs in the 3'UTR of FPGS mediating the localization of cFPGS mRNA and protein at cell protrusions. Using the MBSV6-loop reporter system and fluorescence microscopy, we demonstrate that following folate deprivation, cFPGS mRNA is retained in the endoplasmic reticulum, whereas upon 15 min of folate repletion, this mRNA is rapidly translocated to cell protrusions in a 3'UTR- and actin-dependent manner. The actin dependency of this folate-induced mRNA translocation is shown by treatment with Latrunculin B and inhibitors of the Ras homolog family member A (RhoA) pathway. Upon folate repletion, the FPGS 3'UTR GQs induce an amoeboid/mesenchymal hybrid cell phenotype during migration and invasion through a collagen gel matrix. Targeted disruption of the 3'UTR GQ motifs by introducing point mutations or masking them by antisense oligonucleotides abrogated cell protrusion targeting of cFPGS mRNA.

CONCLUSIONS

Collectively, the GQ motifs within the 3'UTR of FPGS regulate its transcript and protein localization at cell protrusions in response to a folate cue, inducing cancer cell invasive phenotype. These novel findings suggest that the 3'UTR GQ motifs of FPGS constitute an attractive druggable target aimed at inhibition of cancer invasion and metastasis.

mRNA Localization to the Endoplasmic Reticulum in Plant Endosperm Cells

Subcellular mRNA localization is an evolutionarily conserved mechanism to spatially and temporally drive local translation and, in turn, protein targeting. Hence, this mechanism achieves precise control of gene expression and establishes functional and structural networks during cell growth and development as well as during stimuli response. Since its discovery in ascidian eggs, mRNA localization has been extensively studied in animal and yeast cells. Although our knowledge of subcellular mRNA localization in plant cells lags considerably behind other biological systems, mRNA localization to the endoplasmic reticulum (ER) has also been well established since its discovery in cereal endosperm cells in the early 1990s. Storage protein mRNA targeting to distinct subdomains of the ER determines efficient accumulation of the corresponding proteins in different endosomal storage sites and, in turn, underlies storage organelle biogenesis in cereal grains. The targeting process requires the presence of RNA localization elements, also called zipcodes, and specific RNA-binding proteins that recognize and bind these zipcodes and recruit other factors to mediate active transport. Here, we review the current knowledge of the mechanisms and functions of mRNA localization to the ER in plant cells and address directions for future research.

Regulation and outcomes of localized RNA translation

Spatial segregation of mRNAs in the cytoplasm of cells is a well-known biological phenomenon that is widely observed in diverse species spanning different kingdoms of life. In mammalian cells, localization of mRNAs has been documented and studied quite extensively in highly polarized cells, most notably in neurons, where localized mRNAs function to direct protein production at sites that are quite distant from the soma. Recent studies have strikingly revealed that a large proportion of the cellular transcriptome exhibits polarized distributions even in cells that lack an obvious need for long-range transport, such as fibroblasts or epithelial cells. This review focuses on emerging concepts regarding the functional outcomes of mRNA targeting in the cytoplasm of such cells. We also discuss regulatory mechanisms controlling these events, with an emphasis on the role of cell mechanics and the organization of the cytoskeleton. This article is categorized under: Translation > Regulation RNA Export and Localization > RNA Localization.

The kinesin KIF1C transports APC-dependent mRNAs to cell protrusions

RNA localization and local translation are important for numerous cellular functions. In mammals, a class of mRNAs localize to cytoplasmic protrusions in an APC-dependent manner, with roles during cell migration. Here, we investigated this localization mechanism. We found that the KIF1C motor interacts with APC-dependent mRNAs and is required for their localization. Live cell imaging revealed rapid, active transport of single mRNAs over long distances that requires both microtubules and KIF1C. Two-color imaging directly revealed single mRNAs transported by single KIF1C motors, with the 3'UTR being sufficient to trigger KIF1C-dependent RNA transport and localization. Moreover, KIF1C remained associated with peripheral, multimeric RNA clusters and was required for their formation. These results reveal a widespread RNA transport pathway in mammalian cells, in which the KIF1C motor has a dual role in transporting RNAs and clustering them within cytoplasmic protrusions. Interestingly, KIF1C also transports its own mRNA, suggesting a possible feedback loop acting at the level of mRNA transport.

Applying Rho Pathway Inhibitors to Investigate Germ Plasm Localization

The correct assembly, migration, and segregation of the mRNAs of the germ plasm during the first cell divisions are intimately connected to the cytoskeleton and cytokinesis.RhoA is a key regulator of germ plasm localization during the first two cell division cycles in zebrafish embryos. Pharmacological inhibition of RhoA and his effector ROCK affected the correct assembly of microtubules in the cleavage furrow with the concomitant abnormal localization of germ plasm mRNAs. The inhibition of RhoA/ROCK pathway caused a significant decrease in the germ cell population later in development.

Collective cancer cell invasion requires RNA accumulation at the invasive front

Specific RNAs are enriched at protrusive regions of migrating cells. This localization is important for cell migration on 2D surfaces. However, in vivo, tumor cells navigate complex 3D environments often in collective groups. Here, we investigated protrusion-enriched RNAs during collective 3D invasion. We show that specific RNAs exhibit a striking accumulation at the front of invasive leader cells. We provide insights into the mechanism underlying RNA accumulation at the invasive front, and we further demonstrate that it is required for efficient 3D invasion of tumor cells. We additionally observe RNA enrichment at invasive sites of in vivo tumors, supporting the physiological relevance of this mechanism and suggesting a targeting opportunity for perturbing cancer cell invasion.

Localization of RNAs at protrusive regions of cells is important for single-cell migration on two-dimensional surfaces. Protrusion-enriched RNAs encode factors linked to cancer progression, such as the RAB13 GTPase and the NET1 guanine nucleotide exchange factor, and are regulated by the tumor-suppressor protein APC. However, tumor cells in vivo often do not move as single cells but rather utilize collective modes of invasion and dissemination. Here, we developed an inducible system of three-dimensional (3D) collective invasion to study the behavior and importance of protrusion-enriched RNAs. We find that, strikingly, both the RAB13 and NET1 RNAs are enriched specifically at the invasive front of leader cells in invasive cell strands. This localization requires microtubules and coincides with sites of high laminin concentration. Indeed, laminin association and integrin engagement are required for RNA accumulation at the invasive front. Importantly, perturbing RNA accumulation reduces collective 3D invasion. Examination of in vivo tumors reveals a similar localization of the RAB13 and NET1 RNAs at potential invasive sites, suggesting that this mechanism could provide a targeting opportunity for interfering with collective cancer cell invasion.

Sending messages in moving cells: mRNA localization and the regulation of cell migration

Cell migration is a fundamental biological process involved in tissue formation and homeostasis. The correct polarization of motile cells is critical to ensure directed movement, and is orchestrated by many intrinsic and extrinsic factors. Of these, the subcellular distribution of mRNAs and the consequent spatial control of translation are key modulators of cell polarity. mRNA transport is dependent on cis-regulatory elements within transcripts, which are recognized by trans-acting proteins that ensure the efficient delivery of certain messages to the leading edge of migrating cells. At their destination, translation of localized mRNAs then participates in regional cellular responses underlying cell motility. In this review, we summarize the key findings that established mRNA targetting as a critical driver of cell migration and how the characterization of polarized mRNAs in motile cells has been expanded from just a few species to hundreds of transcripts. We also describe the molecular control of mRNA trafficking, subsequent mechanisms of local protein synthesis and how these ultimately regulate cell polarity during migration.

Dual-Responsive DNA Nanodevice for the Available Imaging of an Apoptotic Signaling Pathway in Situ

Fluorescence imaging tools enable the in situ visualization of those molecules involved in various cell signaling pathways, but directly describing these pathways instead of the separate mediators in situ is much more meaningful and still full of challenges. In this work, a dual-responsive DNA nanodevice that allows the available imaging of an apoptotic signaling pathway in living cells has been developed. The nanodevice is constructed through assembling an elaborately designed Y-shaped DNA (Y-DNA) layer on gold nanoparticles (AuNPs). Only if an apoptotic signaling pathway involving the manganese superoxide dismutase (MnSOD) mRNA and downstream cytochrome c (Cyt c) is presented to serve as the input, the nanodevice can perform an "AND" logic gate operation, disassembling the Y-DNA from the AuNP surface and thereafter outputting a fluorescence signal. In comparison with the fluorescence imaging methods that target individual specific molecules, our strategy allows direct profiling of a specific signaling pathway by connected characterization of two correlative targets. The programmable feature of this strategy also shows the potential for the profiling of other signaling pathways. The concept of studying the line connecting two points will contribute to the systematic interrogation on the signaling networks in situ as the networks are composed of lines instead of individual points.

β-Actin mRNA interactome mapping by proximity biotinylation

The molecular function and fate of mRNAs are controlled by RNA-binding proteins (RBPs). Identification of the interacting proteome of a specific mRNA in vivo remains very challenging, however. Based on the widely used technique of RNA tagging with MS2 aptamers for RNA visualization, we developed a RNA proximity biotinylation (RNA-BioID) technique by tethering biotin ligase (BirA*) via MS2 coat protein at the 3' UTR of endogenous MS2-tagged β-actin mRNA in mouse embryonic fibroblasts. We demonstrate the dynamics of the β-actin mRNA interactome by characterizing its changes on serum-induced localization of the mRNA. Apart from the previously known interactors, we identified more than 60 additional β-actin-associated RBPs by RNA-BioID. Among these, the KH domain-containing protein FUBP3/MARTA2 has been shown to be required for β-actin mRNA localization. We found that FUBP3 binds to the 3' UTR of β-actin mRNA and is essential for β-actin mRNA localization, but does not interact with the characterized β-actin zipcode element. RNA-BioID provides a tool for identifying new mRNA interactors and studying the dynamic view of the interacting proteome of endogenous mRNAs in space and time.

RDI Calculator: An Analysis Tool to Assess RNA Distributions in Cells

Localization of RNAs to various subcellular destinations has emerged as a widely used mechanism that regulates a large proportion of transcripts in polarized cells. A number of methodologies have been developed that allow detection and imaging of RNAs at single-molecule resolution. However, methodologies to quantitatively describe RNA distributions are limited. Such approaches usually rely on the identification of cytoplasmic and nuclear boundaries which are used as reference points. Here, we describe an automated, interactive image analysis program that facilitates the accurate generation of cellular outlines from single cells and the subsequent calculation of metrics that quantify how a population of RNA molecules is distributed in the cell cytoplasm. We apply this analysis to mRNAs in mouse and human cells to demonstrate how these metrics can highlight differences in the distribution patterns of distinct RNA species. We further discuss considerations for the practical use of this tool. This program provides a way to facilitate and expedite the analysis of subcellular RNA localization for mechanistic and functional studies.

Extracellular matrix stiffness and cell contractility control RNA localization to promote cell migration

Numerous RNAs are enriched within cellular protrusions, but the underlying mechanisms are largely unknown. We had shown that the APC (adenomatous polyposis coli) protein controls localization of some RNAs at protrusions. Here, using protrusion-isolation schemes and RNA-Seq, we find that RNAs localized in protrusions of migrating fibroblasts can be distinguished in two groups, which are differentially enriched in distinct types of protrusions, and are additionally differentially dependent on APC. APC-dependent RNAs become enriched in high-contractility protrusions and, accordingly, their localization is promoted by increasing stiffness of the extracellular matrix. Dissecting the underlying mechanism, we show that actomyosin contractility activates a RhoA-mDia1 signaling pathway that leads to formation of a detyrosinated-microtubule network, which in turn is required for localization of APC-dependent RNAs. Importantly, a competition-based approach to specifically mislocalize APC-dependent RNAs suggests that localization of the APC-dependent RNA subgroup is functionally important for cell migration.Adenomatous polyposis coli (APC) regulates the localization of some mRNAs at cellular protrusions but the underlying mechanisms and functional roles are not known. Here the authors show that APC-dependent RNAs are enriched in contractile protrusions, via detyrosinated microtubules, and enhance cell migration.

Divergent roles of β- and γ-actin isoforms during spread of vaccinia virus

Actin is a major component of the cytoskeleton and is present as two isoforms in non‐muscle cells: β‐ and γ‐cytoplasmic actin. These isoforms are strikingly conserved, differing by only four N‐terminal amino acids. During spread from infected cells, vaccinia virus (VACV) particles induce localized actin nucleation that propel virus to surrounding cells and facilitate cell‐to‐cell spread of infection. Here we show that virus‐tipped actin comets are composed of β‐ and γ‐actin. We employed isoform‐specific siRNA knockdown to examine the role of the two isoforms in VACV‐induced actin comets. Despite the high level of similarity between the actin isoforms, and their colocalization, VACV‐induced actin nucleation was dependent exclusively on β‐actin. Knockdown of β‐actin led to a reduction in the release of virus from infected cells, a phenotype dependent on virus‐induced Arp2/3 complex activity. We suggest that local concentrations of actin isoforms may regulate the activity of cellular actin nucleator complexes.

RhoA/ROCK pathway activity is essential for the correct localization of the germ plasm mRNAs in zebrafish embryos

Zebrafish germ plasm is composed of mRNAs such as vasa and nanos and of proteins such as Bucky ball, all of which localize symmetrically in four aggregates at the distal region of the first two cleavage furrows. The coordination of actin microfilaments, microtubules and kinesin is essential for the correct localization of the germ plasm. Rho-GTPases, through their effectors, coordinate cytoskeletal dynamics. We address the participation of RhoA and its effector ROCK in germ plasm localization during the transition from two- to eight-cell embryos. We found that active RhoA is enriched along the cleavage furrow during the first two division cycles, whereas ROCK localizes at the distal region of the cleavage furrows in a similar pattern as the germ plasm mRNAs. Specific inhibition of RhoA and ROCK affected microtubules organization at the cleavage furrow; these caused the incorrect localization of the germ plasm mRNAs. The incorrect localization of the germ plasm led to a dramatic change in the number of germ cells during the blastula and 24hpf embryo stages without affecting any other developmental processes. We demonstrate that the Rho/ROCK pathway is intimately related to the determination of germ cells in zebrafish embryos.

Specific interaction of KIF11 with ZBP1 regulates the transport of β-actin mRNA and cell motility

ZBP1-modulated localization of β-actin mRNA enables a cell to establish polarity and structural asymmetry. Although the mechanism of β-actin mRNA localization has been well established, the underlying mechanism of how a specific molecular motor contributes to the transport of the ZBP1 (also known as IGF2BP1) complex in non-neuronal cells remains elusive. In this study, we report the isolation and identification of KIF11, a microtubule motor, which physically interacts with ZBP1 and is a component of β-actin messenger ribonucleoprotein particles (mRNPs). We show that KIF11 colocalizes with the β-actin mRNA, and the ability of KIF11 to transport β-actin mRNA is dependent on ZBP1. We characterize the corresponding regions of ZBP1 and KIF11 that mediate the interaction of the two proteins in vitro and in vivo. Disruption of the in vivo interaction of KIF11 with ZBP1 delocalizes β-actin mRNA and affects cell migration. Our study reveals a molecular mechanism by which a particular microtubule motor mediates the transport of an mRNP through direct interaction with an mRNA-binding protein.

In the right place at the right time: visualizing and understanding mRNA localization

The spatial regulation of protein translation is an efficient way to create functional and structural asymmetries in cells. Recent research has furthered our understanding of how individual cells spatially organize protein synthesis, by applying innovative technology to characterize the relationship between mRNAs and their regulatory proteins, single-mRNA trafficking dynamics, physiological effects of abrogating mRNA localization in vivo and for endogenous mRNA labelling. The implementation of new imaging technologies has yielded valuable information on mRNA localization, for example, by observing single molecules in tissues. The emerging movements and localization patterns of mRNAs in morphologically distinct unicellular organisms and in neurons have illuminated shared and specialized mechanisms of mRNA localization, and this information is complemented by transgenic and biochemical techniques that reveal the biological consequences of mRNA mislocalization.

Control of cell migration through mRNA localization and local translation

Cell migration plays an important role in many normal and pathological functions such as development, wound healing, immune defense, and tumor metastasis. Polarized migrating cells exhibit asymmetric distribution of many cytoskeletal proteins, which is believed to be critical for establishing and maintaining cell polarity and directional cell migration. To target these proteins to the site of function, cells use a variety of mechanisms such as protein transport and messenger RNA (mRNA) localization-mediated local protein synthesis. In contrast to the former which is intensively investigated and relatively well understood, the latter has been understudied and relatively poorly understood. However, recent advances in the study of mRNA localization and local translation have demonstrated that mRNA localization and local translation are specific and effective ways for protein localization and are crucial for embryo development, neuronal function, and many other cellular processes. There are excellent reviews on mRNA localization, transport, and translation during development and other cellular processes. This review will focus on mRNA localization-mediated local protein biogenesis and its impact on somatic cell migration.

The β-actin mRNA zipcode regulates epithelial adherens junction assembly but not maintenance

Epithelial cell-cell contact stimulates actin cytoskeleton remodeling to down-regulate branched filament polymerization-driven lamellar protrusion and subsequently to assemble linear actin filaments required for E-cadherin anchoring during adherens junction complex assembly. In this manuscript, we demonstrate that de novo protein synthesis, the β-actin 3' UTR, and the β-actin mRNA zipcode are required for epithelial adherens junction complex assembly but not maintenance. Specifically, we demonstrate that perturbing cell-cell contact-localized β-actin monomer synthesis causes epithelial adherens junction assembly defects. Consequently, inhibiting β-actin mRNA zipcode/ZBP1 interactions with β-actin mRNA zipcode antisense oligonucleotides, to intentionally delocalize β-actin monomer synthesis, is sufficient to perturb adherens junction assembly following epithelial cell-cell contact. Additionally, we demonstrate active RhoA, the signal required to drive zipcode-mediated β-actin mRNA targeting, is localized at epithelial cell-cell contact sites in a β-actin mRNA zipcode-dependent manner. Moreover, chemically inhibiting Src kinase activity prevents the local stimulation of β-actin monomer synthesis at cell-cell contact sites while inhibiting epithelial adherens junction assembly. Together, these data demonstrate that epithelial cell-cell contact stimulates β-actin mRNA zipcode-mediated monomer synthesis to spatially regulate actin filament remodeling, thereby controlling adherens junction assembly to modulate cell and tissue adhesion.

The functions and regulatory principles of mRNA intracellular trafficking

The subcellular localization of RNA molecules is a key step in the control of gene expression that impacts a broad array of biological processes in different organisms and cell types. Like other aspects of posttranscriptional gene regulation discussed in this collection of reviews, the intracellular trafficking of mRNAs is modulated by a complex regulatory code implicating specific cis-regulatory elements, RNA-binding proteins, and cofactors that function combinatorially to dictate precise localization mechanisms. In this review, we first discuss the functional benefits of transcript localization, the regulatory principles involved, and specific molecular mechanisms that have been described for a few well-characterized mRNAs. We also overview some of the emerging genomic and imaging technologies that have provided significant insights into this layer of gene regulation. Finally, we highlight examples of human diseases where defective transcript localization has been documented.

Immediate translation of Formin DIAPH1 mRNA after its exiting the nucleus is required for its perinuclear localization in fibroblasts

DIAPH1 is a formin protein which promotes actin polymerization, stabilizes microtubules and consequently is involved in cytoskeleton dynamics, cell migration and differentiation. In contrast to the relatively well-understood signaling cascades that regulate DIAPH1 activity, its spatial regulation of biogenesis is not understood. A recent report showed that synthesis of DIAPH1 is confined in the perinuclear ER compartment through translation-dependent mRNA targeting. However, the underlying mechanism of DIAPH1 local synthesis is yet to be elucidated. Here, we provide evidence to demonstrate that the 5'-cap-mediated immediate translation of DIAPH1 mRNA upon exiting nucleus is required for localizing the mRNA in the perinuclear ER compartment. This is supported by data: 1) Delayed translation of DIAPH1 mRNA resulted in loss of perinuclear localization of the mRNA; 2) Once delocalized, DIAPH1 mRNA could not be retargeted to the perinuclear region; and 3) The translation of DIAPH1 mRNA is 5'-cap dependent. These results provide new insights into the novel mechanism of DIAPH1 local synthesis. In addition, these findings have led to the development of new approaches for manipulating DIAPH1 mRNA localization and local protein synthesis in cells for functional studies. Furthermore, a correlation of DIAPH1 mRNA and DIAPH1 protein localization has been demonstrated using a new method to quantify the intracellular distribution of protein.

β-Actin mRNA compartmentalization enhances focal adhesion stability and directs cell migration

Directed cell motility is at the basis of biological phenomena such as development, wound healing, and metastasis. It has been shown that substrate attachments mediate motility by coupling the cell's cytoskeleton with force generation. However, it has been unclear how the persistence of cell directionality is facilitated. We show that mRNA localization plays an important role in this process, but the mechanism of action is still unknown. In this study, we show that the zipcode-binding protein 1 transports β-actin mRNA to the focal adhesion compartment, where it dwells for minutes, suggesting a means for associating its localization with motility through the formation of stable connections between adhesions and newly synthesized actin filaments. In order to demonstrate this, we developed an approach for assessing the functional consequences of β-actin mRNA and protein localization by tethering the mRNA to a specific location-in this case, the focal adhesion complex. This approach will have a significant impact on cell biology because it is now possible to forcibly direct any mRNA and its cognate protein to specific locations in the cell. This will reveal the importance of localized protein translation on various cellular processes.

mRNA localization: an orchestration of assembly, traffic and synthesis

Asymmetrical mRNA localization and subsequent local translation provide efficient mechanisms for protein sorting in polarized cells. Defects in mRNA localization have been linked to developmental abnormalities and neurological diseases. Thus, it is critical to understand the machineries mediating and mechanisms underlying the asymmetrical distribution of mRNA and its regulation. The goal of this review is to summarize recent advances in the understanding of mRNA transport and localization, including the assembly and sorting of transport messenger ribonucleic protein (mRNP) granules, molecular mechanisms of active mRNP transport, cytoskeletal interactions and regulation of these events by extracellular signals.

How and why does β-actin mRNA target?

beta-Actin mRNA is localized near the leading edge in several cell types where actin polymerization is actively promoting forward protrusion. The localization of the beta-actin mRNA near the leading edge is facilitated by a short sequence in the 3'UTR (untranslated region), the 'zipcode'. Localization of the mRNA at this region is important physiologically. Treatment of chicken embryo fibroblasts with antisense oligonucleotides complementary to the localization sequence (zipcode) in the 3'UTR leads to delocalization of beta-actin mRNA, alteration of cell phenotype and a decrease in cell motility. The dynamic image analysis system (DIAS) used to quantify movement of cells in the presence of sense and antisense oligonucleotides to the zipcode showed that net pathlength and average speed of antisense-treated cells were significantly lower than in sense-treated cells. This suggests that a decrease in persistence of direction of movement and not in velocity results from treatment of cells with zipcode-directed antisense oligonucleotides. We postulate that delocalization of beta-actin mRNA results in delocalization of nucleation sites and beta-actin protein from the leading edge followed by loss of cell polarity and directional movement. Hence the physiological consequences of beta-actin mRNA delocalization affect the stability of the cell phenotype.

MLCK regulates Schwann cell cytoskeletal organization, differentiation and myelination

Signaling through cyclic AMP (cAMP) has been implicated in the regulation of Schwann cell (SC) proliferation and differentiation. In quiescent SCs, elevation of cAMP promotes the expression of proteins associated with myelination such as Krox-20 and P0, and downregulation of markers associated with the non-myelinating SC phenotype. We have previously shown that the motor protein myosin II is required for the establishment of normal SC-axon interactions, differentiation and myelination, however, the mechanisms behind these effects are unknown. Here we report that the levels and activity of myosin light chain kinase (MLCK), an enzyme that regulates MLC phosphorylation in non-muscle cells, are dramatically downregulated in SCs after cAMP treatment, in a similar pattern to that of c-Jun, a known inhibitor of myelination. Knockdown of MLCK in SCs mimics the effect of cAMP elevation, inducing plasma membrane expansion and expression of Krox-20 and myelin proteins. Despite activation of myelin gene transcription these cells fail to make compact myelin when placed in contact with axons. Our data indicate that myosin II activity is differentially regulated at various stages during myelination and that in the absence of MLCK the processes of SC differentiation and compact myelin assembly are uncoupled.

Actin cytoskeleton in myofibroblast differentiation: ultrastructure defining form and driving function

Myofibroblasts are modified fibroblasts characterized by the presence of a well-developed contractile apparatus and the formation of robust actin stress fibers. These mechanically active cells are thought to orchestrate extracellular matrix remodeling during normal wound healing in response to tissue injury; these cells are found also in aberrant tissue remodeling in fibrosing disorders. This review surveys the understanding of the role of actin stress fibers in myofibroblast biology. Actin stress fibers are discussed as a defining ultrastructural and morphologic feature and well-accepted observations demonstrating its participation in contraction, focal adhesion maturation, and extracellular matrix reorganization are presented. Finally, more recent observations are reviewed, demonstrating its role in transducing mechanical force into biochemical signals, transcriptional control of genes involved in locomotion, contraction, and matrix reorganization, as well as the localized regulation of messenger RNA (mRNA) translation. This breadth of functionality of the actin stress fiber serves to reinforce and amplify its mechanical function, via induced expression of proteins that themselves augment contraction, focal adhesion formation, and matrix remodeling. In composite, the functions of the actin cytoskeleton are most often aligned, allowing for the integration and amplification of signals promoting both myofibroblast differentiation and matrix remodeling during fibrogenesis.

Functional analysis of the microtubule-interacting transcriptome

A combination of bioinformatic and RNA interference analysis of Xenopus tropicalis RNA-seq data shows that the identification of microtubule-associated (MT) mRNAs can be used for discovering novel factors in the processes of spindle pole organization and centrosome structure. MT-RNAs are likely to contribute to spindle-localized mitotic translation.

RNA localization is an important mechanism for achieving precise control of posttranscriptional gene expression. Previously, we demonstrated that a subset of cellular mRNAs copurify with mitotic microtubules in egg extracts of Xenopus laevis. Due to limited genomic sequence information available for X. laevis, we used RNA-seq to comprehensively identify the microtubule-interacting transcriptome of the related frog Xenopus tropicalis. We identified ∼450 mRNAs that showed significant enrichment on microtubules (MT-RNAs). In addition, we demonstrated that the MT-RNAs incenp, xrhamm, and tpx2 associate with spindle microtubules in vivo. MT-RNAs are enriched with transcripts associated with cell division, spindle formation, and chromosome function, demonstrating an overrepresentation of genes involved in mitotic regulation. To test whether uncharacterized MT-RNAs have a functional role in mitosis, we performed RNA interference and discovered that several MT-RNAs are required for normal spindle pole organization and γ-tubulin distribution. Together, these data demonstrate that microtubule association is one mechanism for compartmentalizing functionally related mRNAs within the nucleocytoplasmic space of mitotic cells and suggest that MT-RNAs are likely to contribute to spindle-localized mitotic translation.

Dynamics of native β-actin mRNA transport in the cytoplasm

Transport of messenger RNAs (mRNAs) in the cytoplasm is essential for localization to translation sites and for post-transcriptional regulation. Utilizing single-RNA sensitive probes and real-time fluorescence microscopy, we accurately quantified the dynamics of native, non-engineered, β-actin mRNAs within the cytoplasm of epithelial cells and fibroblasts for the first time. Using single-particle tracking and temporal analysis, we determined that native β-actin mRNAs, under physiologic conditions, exhibit bursts of intermittent, processive motion on microtubules, interspersed between time periods of diffusive motion, characterized by non-thermal enhanced diffusivity. When transport processes were perturbed via ATP depletion, temperature reduction, dynamitin overexpression and chemical inhibitors, processive motion was diminished or eliminated and diffusivity was reduced. These data support a model whereby processive, motor-driven motion is responsible for long-distance mRNA transport.

Characterizing mRNA interactions with RNA granules during translation initiation inhibition

When cells experience environmental stresses, global translational arrest is often accompanied by the formation of stress granules (SG) and an increase in the number of p-bodies (PBs), which are thought to play a crucial role in the regulation of eukaryotic gene expression through the control of mRNA translation and degradation. SGs and PBs have been extensively studied from the perspective of their protein content and dynamics but, to date, there have not been systematic studies on how they interact with native mRNA granules. Here, we demonstrate the use of live-cell hybridization assays with multiply-labeled tetravalent RNA imaging probes (MTRIPs) combined with immunofluorescence, as a tool to characterize the polyA+ and β-actin mRNA distributions within the cytoplasm of epithelial cell lines, and the changes in their colocalization with native RNA granules including SGs, PBs and the RNA exosome during the inhibition of translational initiation. Translation initiation inhibition was achieved via the induction of oxidative stress using sodium arsenite, as well as through the use of Pateamine A, puromycin and cycloheximide. This methodology represents a valuable tool for future studies of mRNA trafficking and regulation within living cells.

Molecular motors: directing traffic during RNA localization

RNA localization, the enrichment of RNA in a specific subcellular region, is a mechanism for the establishment and maintenance of cellular polarity in a variety of systems. Ultimately, this results in a universal method for spatially restricting gene expression. Although the consequences of RNA localization are well-appreciated, many of the mechanisms that are responsible for carrying out polarized transport remain elusive. Several recent studies have illuminated the roles that molecular motor proteins play in the process of RNA localization. These studies have revealed complex mechanisms in which the coordinated action of one or more motor proteins can act at different points in the localization process to direct RNAs to their final destination. In this review, we discuss recent findings from several different systems in an effort to clarify pathways and mechanisms that control the directed movement of RNA.

An RNA-zipcode-independent mechanism that localizes Dia1 mRNA to the perinuclear ER through interactions between Dia1 nascent peptide and Rho-GTP

Signal-peptide-mediated ER localization of mRNAs encoding for membrane and secreted proteins, and RNA-zipcode-mediated intracellular targeting of mRNAs encoding for cytosolic proteins are two well-known mechanisms for mRNA localization. Here, we report a previously unidentified mechanism by which mRNA encoding for Dia1, a cytosolic protein without the signal peptide, is localized to the perinuclear ER in an RNA-zipcode-independent manner in fibroblasts. Dia1 mRNA localization is also independent of the actin and microtubule cytoskeleton but requires translation and the association of Dia1 nascent peptide with the ribosome-mRNA complex. Sequence mapping suggests that interactions of the GTPase binding domain of Dia1 peptide with active Rho are important for Dia1 mRNA localization. This mechanism can override the β-actin RNA zipcode and redirect β-actin mRNA to the perinuclear region, providing a new way to manipulate intracellular mRNA localization.

RNA purification from tumor cell protrusions using porous polycarbonate filters

Actin-rich cellular protrusions or pseudopodia form via local actin filament polymerization and branching and represent a variety of polarized cellular domains including lamellipodia, filipodia, and neuronal growth cones. RNA localization and local protein translation in these domains are important for various cellular processes. RNA transport and local synthesis have been implicated in cell migration and tumor cell metastasis as well as in neuronal plasticity in neurons. Characterization of the mRNAs present in these domains is key to understanding the functional role of mRNA translocation and local protein translation in cellular processes. We describe here a method to segregate pseudopodia of metastatic cancer cells from the cell body using porous polycarbonate filters. This approach enables the purification and identification of RNAs and proteins in these protrusive cellular domains.

Microtubule-dependent mRNA transport in fungi

The localization and local translation of mRNAs constitute an important mechanism to promote the correct subcellular targeting of proteins. mRNA localization is mediated by the active transport of mRNPs, large assemblies consisting of mRNAs and associated factors such as RNA-binding proteins. Molecular motors move mRNPs along the actin or microtubule cytoskeleton for short-distance or long-distance trafficking, respectively. In filamentous fungi, microtubule-based long-distance transport of vesicles, which are involved in membrane and cell wall expansion, supports efficient hyphal growth. Recently, we discovered that the microtubule-mediated transport of mRNAs is essential for the fast polar growth of infectious filaments in the corn pathogen Ustilago maydis. Combining in vivo UV cross-linking and RNA live imaging revealed that the RNA-binding protein Rrm4, which constitutes an integral part of the mRNP transport machinery, mediates the transport of distinct mRNAs encoding polarity factors, protein synthesis factors, and mitochondrial proteins. Moreover, our results indicate that microtubule-dependent mRNA transport is evolutionarily conserved from fungi to higher eukaryotes. This raises the exciting possibility of U. maydis as a model system to uncover basic concepts of long-distance mRNA transport.

Microtubule-dependent localization of profilin I mRNA to actin polymerization sites in serum-stimulated cells

Specific localization of messenger RNA (mRNA) appears to be a general mechanism to accumulate certain proteins to subcellular compartments for participation in local processes, thereby maintaining cell polarity under strict spatiotemporal control. Transportation of mRNA with associated protein components (RNP granules) by the actin microfilament or the microtubule systems is one important mechanism to achieve this locally distributed protein production. Here we provide evidence for a microtubule-dependent localization of mRNA encoding the actin regulatory protein profilin to sites in mouse embryonic fibroblasts, which express enhanced actin polymerization.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Beta and gamma-cytoplasmic actins display distinct distribution and functional diversity

Using newly generated monoclonal antibodies, we have compared the distribution of beta- and gamma-cytoplasmic actin in fibroblastic and epithelial cells, in which they play crucial roles during various key cellular processes. Whereas beta-actin is preferentially localized in stress fibers, circular bundles and at cell-cell contacts, suggesting a role in cell attachment and contraction, gamma-actin displays a more versatile organization, according to cell activities. In moving cells, gamma-actin is mainly organized as a meshwork in cortical and lamellipodial structures, suggesting a role in cell motility; in stationary cells, gamma-actin is also recruited into stress fibers. beta-actin-depleted cells become highly spread, display broad protrusions and reduce their stress-fiber content; by contrast, gamma-actin-depleted cells acquire a contractile phenotype with thick actin bundles and shrinked lamellar and lamellipodial structures. Moreover, beta- and gamma-actin depleted fibroblasts exhibit distinct changes in motility compared with their controls, suggesting a specific role for each isoform in cell locomotion. Our results reveal new aspects of beta- and gamma-actin organization that support their functional diversity.

LPA-induced mutually exclusive subcellular localization of active RhoA and Arp2 mRNA revealed by sequential FRET and FISH

We previously demonstrated that mRNAs for the subunits of the Arp2/3 complex localize to protrusions in fibroblasts (Mingle et al. in J Cell Sci 118:2425-2433, 2005). However, the signaling pathway that regulates Arp2/3 complex mRNA localization remains unknown. In this study we have identified lysophosphatidic acid (LPA) as a potent inducer of Arp2 mRNA localization to protrusions in fibroblasts via the RhoA-ROCK pathway. As RhoA is known to be activated locally in the cells, we sought to understand how spatial activation of Rho affects Arp2 mRNA localization. By sequentially performing fluorescence resonance energy transfer (FRET) and fluorescence in situ hybridization (FISH), we have visualized active RhoA and Arp2 mRNA in the same cells. Upon LPA stimulation, approximately two times more cells than those in the serum-free medium showed mutually exclusive localization of active RhoA and Arp2 mRNA. These results demonstrate the importance of localized activation of Rho in Arp2 mRNA localization and provide new insights as to how Rho regulates Arp2/3 complex mRNA localization. To our best knowledge, this is the first report in which FRET and FISH are combined to detect localized protein activity and mRNA in the same cells. This method should be easily adopted for the detection of other fluorescence protein based biosensors and DNA/RNA in the same cells.

Genes involved in radiation therapy response in head and neck cancers

OBJECTIVES

This is a pilot study designed to identify gene expression profiles able to stratify head and neck squamous cell carcinoma (HNSCC) tumors that may or may not respond to chemoradiation or radiation therapy.

STUDY DESIGN

We prospectively evaluated 14 HNSCC specimens, arising from patients undergoing chemoradiotherapy or radiotherapy alone with curative intent. A complete response was assessed by clinical evaluation with no evidence of gross tumor after a 2-year follow-up period.

METHODS

Residual biopsy samples from eight complete responders (CR) and six nonresponders (NR) were evaluated by genome-wide gene expression profiling using HG-U133A 2.0 arrays. Univariate parametric t-tests with proportion of false discoveries controlled by multivariate permutation tests were used to identify genes with significantly different gene expression levels between CR and NR cases. Six different prediction algorithms were used to build gene predictor lists. Three representative genes showing 100% crossvalidation support after leave-one-out crossvalidation (LOOCV) were further validated using real-time QRT-PCR.

RESULTS

We identified 167 significant probe sets that discriminate between the two classes, which were used to build gene predictor lists. Thus, 142 probe sets showed an accuracy of prediction ranging from 93% to 100% across all six prediction algorithms. The genes represented by these 142 probe sets were further classified into different functional networks that included cellular development, cellular movement, and cancer.

CONCLUSIONS

The results presented herein offer encouraging preliminary data that may provide a basis for a more precise prognosis of HNSCC, as well as a molecular-based therapy decision for the management of these cancers.

RNA localization and polarity: from A(PC) to Z(BP)

Cell polarization relies on the asymmetric organization of cellular structures and activities, and is fundamentally important both during development and for the proper function of most somatic cells. Asymmetries in the distribution and activity of proteins can be achieved through localization of RNA molecules that usually give rise to proteins at specific subcellular sites. It is increasingly appreciated that this is a widely used mechanism affecting protein function at multiple levels. The description of a new RNA localization pathway involving the tumor-suppressor protein APC raises questions regarding coordination between distinct localization pathways and their effects on protein function and cell polarity.

Localized Rho GTPase activation regulates RNA dynamics and compartmentalization in tumor cell protrusions

mRNA trafficking and local protein translation are associated with protrusive cellular domains, such as neuronal growth cones, and deregulated control of protein translation is associated with tumor malignancy. We show here that activated RhoA, but not Rac1, is enriched in pseudopodia of MSV-MDCK-INV tumor cells and that Rho, Rho kinase (ROCK), and myosin II regulate the microtubule-independent targeting of RNA to these tumor cell domains. ROCK inhibition does not affect pseudopodial actin turnover but significantly reduces the dynamics of pseudopodial RNA turnover. Gene array analysis shows that 7.3% of the total genes analyzed exhibited a greater than 1.6-fold difference between the pseudopod and cell body fractions. Of these, only 13.2% (261 genes) are enriched in pseudopodia, suggesting that only a limited number of total cellular mRNAs are enriched in tumor cell protrusions. Comparison of the tumor pseudopod mRNA cohort and a cohort of mRNAs enriched in neuronal processes identified tumor pseudopod-specific signaling networks that were defined by expression of M-Ras and the Shp2 protein phosphatase. Pseudopod expression of M-Ras and Shp2 mRNA were diminished by ROCK inhibition linking pseudopodial Rho/ROCK activation to the localized expression of specific mRNAs. Pseudopodial enrichment for mRNAs involved in protein translation and signaling suggests that local mRNA translation regulates pseudopodial expression of less stable signaling molecules as well as the cellular machinery to translate these mRNAs. Pseudopodial Rho/ROCK activation may impact on tumor cell migration and metastasis by stimulating the pseudopodial translocation of mRNAs and thereby regulating the expression of local signaling cascades.

LC/MS identification of 12 intracellular cytoskeletal and inflammatory proteins from monocytes adherent on surface-adsorbed fibronectin-derived peptides

The extent and duration of the host response determines device efficacy, yet the mechanism is poorly understood. U937 promonocytic cells were cultured on peptide-adsorbed tissue-culture polystyrene to better understand surface-modulated intracellular events. Phosphotyrosine proteins were enriched by immunoprecipitation and analyzed by nanospray HPLC-coupled tandem mass spectrometry (LC/MS). Tyrosine-phosphorylated proteins were chosen based on physiological significance and previous densitometry results, which identified a set of proteins ranging from approximately 200 to approximately 23 kDa showing altered phosphorylation levels in response to various surface-adsorbed ligands and phosphorylation inhibitor AG18. Although LC/MS has been used for nearly a decade, its application to the field of biomaterials is relatively novel. Twelve intracellular proteins identified by nanospray LC/MS are potentially related to the host response. Eight of the twelve proteins are related to the cytoskeleton including: moesin, heat shock protein 90beta, alpha-tubulin, elongation factor 1alpha, beta actin, vimentin, plasminogen activator inhibitor 2, and heterogeneous ribonuclear protein A2. The remaining four proteins: high mobility group box 1, caspase recruitment domain 5, glycoprotein 96, and heterogeneous nuclear ribonucleoprotein D0 modulate inflammation. The specific effect each peptide has upon modulating the phosphorylation state of these proteins cannot be determined from this work; however, 12 viable targets have been identified for further investigation into the role each plays in the surface-mediated monocyte response.

Myosin-Va mediates RNA distribution in primary fibroblasts from multiple organs

Myosin-Va has been shown to have multiple functions in a variety of cell types, including a role in RNA transport in neurons. Using primary cultures of cells from organs of young dilute-lethal (Myo5a(d-l)/Myo5a(d-l)) null mutant mice and wild-type controls, we show that in some, but not all, tissues, RNA distribution is dramatically different in the homozygous null mutant cells. The dependence of RNA localization on myosin-Va correlates with the relative abundance of the brain-specific splicing pattern of the myosin-Va tail. We also show that myosin-Va is involved in RNA localization soon after synthesis, because the effects of its absence are diminished for RNAs that are more than 30 min old. Finally, we show that localization of beta-actin mRNA is significantly changed by the absence of myosin-Va. These results suggest that myosin-Va is involved in a transient transport or tethering function in the perinuclear region.

Mechanisms and cellular roles of local protein synthesis in mammalian cells

After the export from the nucleus it turns out that all mRNAs are not treated equally. Not only is mRNA subject to translation, but also through RNA-binding proteins and other trans-acting factors, eukaryotic cells interpret codes for spatial sorting within the mRNA sequence. These codes instruct the cytoskeleton and translation apparatus to make decisions about where to transport and when to translate the intended protein product. Signaling pathways decode extra-cellular cues and can modify transport and translation factors in the appropriate cytoplasmic space to achieve translation locally. Identifying regulatory sites on transport factors as well as novel physiological functions for well-known translation factors has provided significant advances in how spatially controlled translation impacts cell function.

A stem-loop structure in the wingless transcript defines a consensus motif for apical RNA transport

Although the subcellular localization of mRNA transcripts is a well-established mechanism for controlling protein localization, the basis for the recognition of mRNA localization elements is only now emerging. For example, although localization elements have been defined for many mRNAs that localize to apical cytoplasm in Drosophila embryos, no unifying properties have been identified within these elements. In this study, we identify and characterize an apical localization element in the 3'UTR of the Drosophila wingless mRNA. We show that this element, referred to as WLE3, is both necessary and sufficient for apical RNA transport. Full, unrestricted activity, however, requires the presence of one of several downstream potentiating elements. Comparison of WLE3 sequences within the Drosophila genus, and their predicted secondary structures, defines a highly conserved stem-loop structure. Despite these high levels of sequence and predicted structure conservation, however, mutagenesis shows significant leeway for both sequence and structure variation in the predicted stem-loop. Importantly, the features that emerge as crucial include an accessible distal helix sequence motif, which is also found in the predicted structures of other apical localization elements.

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.

ZBP2 facilitates binding of ZBP1 to beta-actin mRNA during transcription

Cytoplasmic mRNA localization regulates gene expression by spatially restricting protein translation. Recent evidence has shown that nuclear proteins (such as hnRNPs) are required to form mRNPs capable of cytoplasmic localization. ZBP1 and ZBP2, two hnRNP K homology domain-containing proteins, were previously identified by their binding to the zipcode, the sequence element necessary and sufficient for beta-actin mRNA localization. ZBP1 colocalizes with nascent beta-actin mRNA in the nucleus but is predominantly a cytoplasmic protein. ZBP2, in contrast, is predominantly nuclear. We hypothesized that the two proteins cooperate to localize beta-actin mRNA and sought to address where and how this might occur. We demonstrate that ZBP2, a homologue of the splicing factor KSRP, binds initially to nascent beta-actin transcripts and facilitates the subsequent binding of the shuttling ZBP1. ZBP1 then associates with the RNA throughout the nuclear export and cytoplasmic localization process.

Translocation of mRNAs by molecular motors: think complex?

In recent years, closer inspection of the dynamics of cytoplasmic mRNA transport processes has shed new light on the mechanisms by which transcripts are recognized by motor complexes and deposited at the correct site. Several studies have highlighted the significance of the motile properties of motor complexes in differential transcript localization. In yeast, mRNA cargoes may stimulate either the movement or anchorage of actin-based motors. In higher eukaryotes, emerging evidence suggests that mRNA cargoes can control their sorting by regulating the motility of motor complexes or their choice of subsets of cytoskeletal tracks. The transport machinery that is utilized by differentially localizing mRNAs appears to share some common motors and regulatory factors. A major challenge for the future is therefore to understand how motor complexes decode the information in mRNA sequences.