How and why does β-actin mRNA target?

β-actin function in platelets and red blood cells can be performed by γ-actin and is therefore independent of actin isoform protein sequence

Actin is an essential component of the cytoskeleton in every eukaryotic cell. β-and γ-nonmuscle actin are over 99% identical to each other at the protein level but are encoded by different genes and play distinct roles in vivo. Blood cells, especially red blood cells (RBC), contain almost exclusively β-actin, and it has been generally assumed that this bias is dictated by the unique suitability of β-actin for RBC cytoskeleton function due to its specific amino acid sequence. Here we tested this assumption by analyzing the "β-coded γ-actin" (Actbcg) mouse model, in which the β-actin gene is edited by five-point mutations to produce γ-actin protein. Strikingly, despite lacking β-actin protein, Actbcg mice had no detectable phenotypes in RBCs, and no changes in the RBC shape, integrity, deformability, and molecular composition of their spectrin-based membrane skeleton. No actin-dependent changes were observed in platelets, another anucleate cell type enriched for β-actin. Our data show that, contrary to expectations, β-actin function in mature RBCs and platelets is independent of its protein sequence and therefore its enrichment in hematopoiesis and mature blood cells is likely driven entirely by its nucleotide-dependent functions.

Biological Roles of Local Protein Synthesis in Axons: A Journey of Discovery

The remit of this review is to give an autobiographical account of our discovery of the role of local protein synthesis in axon guidance. The paper reporting our initial findings was published in 2001. Here, I describe some of the work that led to this publication, the skepticism our findings initially received, and the subsequent exciting years of follow-up work that helped gradually to convince the neuroscience community of the existence and functional importance of local protein synthesis in multiple aspects of axon biology-guidance, branching, synaptogenesis, and maintenance. The journey has been an exhilarating one, taking me into a new field of RNA biology, with many unexpected twists and turns. In retelling it here, I have tried to recall the major influences on my thinking at the time rather than give a comprehensive review, and I apologize for any omissions due to my own ignorance during that era.

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.

Telocytes in the Luminal GI Tract

Telocytes are unique mesenchymal cells characterized by multiple remarkably long cytoplasmic extensions that extend hundreds of micron away from the cell body. Through these extensions, telocytes establish a 3-dimensional network by connecting with other telocytes and various cell types within the tissue. In the intestine, telocytes have emerged as an essential component of the stem cell niche, providing Wnt proteins that are critical for the proliferation of stem and progenitor cells. However, the analysis of single-cell RNA sequencing has revealed other stromal populations and mechanisms for niche organization, raising questions about the role of telocytes as a component of the stem cell niche. This review explores the current state-of-the-art, existing controversies, and potential future directions related to telocytes in the luminal gastrointestinal tract.

Synapse-specific burst coding sustained by local axonal translation

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

An Atlas of the Developing Drosophila Visual System Glia and Subcellular mRNA Localization of Transcripts in Single Cells

Glial cells are essential for proper nervous system development and function. To understand glial development and function, we comprehensively annotated glial cells in a single-cell mRNA-sequencing (scRNAseq) atlas of the developing Drosophila visual system. This allowed us to study their developmental trajectories, from larval to adult stages, and to understand how specific types of glia diversify during development. For example, neuropil glia that are initially transcriptionally similar in larvae, split into ensheathing and astrocyte-like glia during pupal stages. Other glial types, such as chiasm glia change gradually during development without splitting into two cell types. The analysis of scRNA-seq allowed us to discover that the transcriptome of glial cell bodies can be distinguished from that of their broken processes. The processes contain distinct enriched mRNAs that were validated in vivo. Therefore, we have identified most glial types in the developing optic lobe and devised a computational approach to identify mRNA species that are localized to cell bodies or cellular processes.

Local translation in microglial processes is required for efficient phagocytosis

Neurons, astrocytes and oligodendrocytes locally regulate protein translation within distal processes. Here, we tested whether there is regulated local translation within peripheral microglial processes (PeMPs) from mouse brain. We show that PeMPs contain ribosomes that engage in de novo protein synthesis, and these are associated with transcripts involved in pathogen defense, motility and phagocytosis. Using a live slice preparation, we further show that acute translation blockade impairs the formation of PeMP phagocytic cups, the localization of lysosomal proteins within them, and phagocytosis of apoptotic cells and pathogen-like particles. Finally, PeMPs severed from their somata exhibit and require de novo local protein synthesis to effectively surround pathogen-like particles. Collectively, these data argue for regulated local translation in PeMPs and indicate a need for new translation to support dynamic microglial functions.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

Visualizing and Quantifying mRNA Localization at the Invasive Front of 3D Cancer Spheroids

Localization of mRNAs at the front of migrating cells is a widely used mechanism that functionally supports efficient cell movement. It is observed in single cells on two-dimensional surfaces, as well as in multicellular three-dimensional (3D) structures and in tissue in vivo. 3D multicellular cultures can reveal how the topology of the extracellular matrix and cell-cell contacts influence subcellular mRNA distributions. Here we describe a method for mRNA imaging in an inducible system of collective cancer cell invasion. MDA-MB-231 cancer cell spheroids are embedded in Matrigel, induced to invade, and processed to image mRNAs with single-molecule sensitivity. An analysis algorithm is used to quantify and compare mRNA distributions at the front of invasive leader cells. The approach can be easily adapted and applied to analyze RNA distributions in additional settings where cells polarize along a linear axis.

Local mRNA translation and cytoskeletal reorganization: Mechanisms that tune neuronal responses

Neurons are highly polarized cells with significantly long axonal and dendritic extensions that can reach distances up to hundreds of centimeters away from the cell bodies in higher vertebrates. Their successful formation, maintenance, and proper function highly depend on the coordination of intricate molecular networks that allow axons and dendrites to quickly process information, and respond to a continuous and diverse cascade of environmental stimuli, often without enough time for communication with the soma. Two seemingly unrelated processes, essential for these rapid responses, and thus neuronal homeostasis and plasticity, are local mRNA translation and cytoskeletal reorganization. The axonal cytoskeleton is characterized by high stability and great plasticity; two contradictory attributes that emerge from the powerful cytoskeletal rearrangement dynamics. Cytoskeletal reorganization is crucial during nervous system development and in adulthood, ensuring the establishment of proper neuronal shape and polarity, as well as regulating intracellular transport and synaptic functions. Local mRNA translation is another mechanism with a well-established role in the developing and adult nervous system. It is pivotal for axonal guidance and arborization, synaptic formation, and function and seems to be a key player in processes activated after neuronal damage. Perturbations in the regulatory pathways of local translation and cytoskeletal reorganization contribute to various pathologies with diverse clinical manifestations, ranging from intellectual disabilities (ID) to autism spectrum disorders (ASD) and schizophrenia (SCZ). Despite the fact that both processes are essential for the orchestration of pathways critical for proper axonal and dendritic function, the interplay between them remains elusive. Here we review our current knowledge on the molecular mechanisms and specific interaction networks that regulate and potentially coordinate these interconnected processes.

Cytoplasmic Beta and Gamma Actin Isoforms Reorganization and Regulation in Tumor Cells in Culture and Tissue

The cytoplasmic actin isoforms (β- and γ-actins) contribute greatly to cellular processes such as cel-cell and cell-matrix interactions, as well as cell polarization, motility and division. Distinct isoforms modulations are linked to serious pathologies, so investigations of underlying mechanisms would be of major relevance not only for fundamental research but also for clinical applications. Therefore, the study of the relevant mechanisms of change in the isoform’s balance is important for basic research and for clinical studies. The disruption of actin cytoskeleton and intercellular adhesions contribute to the neoplastic transformation, as it is important for the tumor growth, invasiveness and metastasis. Cytoplasmic actins display the functional diversity: β-actin is responsible for contractility, whereas γ-actin participates in the submembrane flexible cortex organization and direction cell motility. The involvement of β- and γ-actin in cell architecture, motility, division, and adhesion junctions in normal cells is not equivalent, and the major question was following: whether isoform ratio and the distribution in the cell corresponds to pathological function. Significant data were obtained in the study of tumor and normal cells in culture, as well as on clinical material of human tissues, and via selective regulation of β- and γ-actin’s expression. Investigation of the actins’ diversity and function in cancers may help to choose the benefit treatment strategies, and to design new therapies.

DeepSpot: A deep neural network for RNA spot enhancement in single-molecule fluorescence in-situ hybridization microscopy images

Detection of RNA spots in single-molecule fluorescence in-situ hybridization microscopy images remains a difficult task, especially when applied to large volumes of data. The variable intensity of RNA spots combined with the high noise level of the images often requires manual adjustment of the spot detection thresholds for each image. In this work, we introduce DeepSpot, a Deep Learning-based tool specifically designed for RNA spot enhancement that enables spot detection without the need to resort to image per image parameter tuning. We show how our method can enable downstream accurate spot detection. DeepSpot's architecture is inspired by small object detection approaches. It incorporates dilated convolutions into a module specifically designed for context aggregation for small object and uses Residual Convolutions to propagate this information along the network. This enables DeepSpot to enhance all RNA spots to the same intensity, and thus circumvents the need for parameter tuning. We evaluated how easily spots can be detected in images enhanced with our method by testing DeepSpot on 20 simulated and 3 experimental datasets, and showed that accuracy of more than 97% is achieved. Moreover, comparison with alternative deep learning approaches for mRNA spot detection (deepBlink) indicated that DeepSpot provides more precise mRNA detection. In addition, we generated single-molecule fluorescence in-situ hybridization images of mouse fibroblasts in a wound healing assay to evaluate whether DeepSpot enhancement can enable seamless mRNA spot detection and thus streamline studies of localized mRNA expression in cells.

Contractile apparatus in CNS capillary pericytes

Significance: Whether or not capillary pericytes contribute to blood flow regulation in the brain and retina has long been debated. This was partly caused by failure of detecting the contractile protein α -smooth muscle actin ( α -SMA) in capillary pericytes. Aim: The aim of this review is to summarize recent developments in detecting α -SMA and contractility in capillary pericytes and the relevant literature on the biology of actin filaments. Results: Evidence suggests that for visualization of the small amounts of α -SMA in downstream mid-capillary pericytes, actin depolymerization must be prevented during tissue processing. Actin filaments turnover is mainly based on de/re-polymerization rather than transcription of the monomeric form, hence, small amounts of α -SMA mRNA may evade detection by transcriptomic studies. Similarly, transgenic mice expressing fluorescent reporters under the α -SMA promoter may yield low fluorescence due to limited transcriptional activity in mid-capillary pericytes. Recent studies show that pericytes including mid-capillary ones express several actin isoforms and myosin heavy chain type 11, the partner of α -SMA in mediating contraction. Emerging evidence also suggests that actin polymerization in pericytes may have a role in regulating the tone of downstream capillaries. Conclusions: With guidance of actin biology, innovative labeling and imaging techniques can reveal the molecular machinery of contraction in pericytes.

RNA localization in confined cells depends on cellular mechanical activity and contributes to confined migration

Cancer cells experience mechanical confining forces during metastasis and, consequently, can alter their migratory mechanisms. Localization of numerous mRNAs to cell protrusions contributes to cell polarization and migration and is controlled by proteins that can bind RNA and/or cytoskeletal elements, such as the adenomatous polyposis coli (APC). Here, we demonstrate that peripheral localization of APC-dependent RNAs in cells within confined microchannels is cell type dependent. This varying phenotype is determined by the levels of a detyrosinated tubulin network. We show that this network is regulated by mechanoactivity and that cells with mechanosensitive ion channels and increased myosin II activity direct peripheral localization of the RAB13 APC-dependent RNA. Through specific mislocalization of the RAB13 RNA, we show that peripheral RNA localization contributes to confined cell migration. Our results indicate that a cell’s mechanical activity determines its ability to peripherally target RNAs and utilize them for movement in confinement.

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Peripheral localization of APC-dependent RNAs in confinement depends on cell type

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RNA localization in confined cells is controlled by the mechanoactivity of cells

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RNA localization phenotype is influenced by the detyrosinated tubulin network

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Peripheral RNA accumulation functionally contributes to confined cell migration

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.

Ultrasensitive Trace Sample Proteomics Unraveled the Protein Remodeling during Mesenchymal-Amoeboid Transition

Deep mining the proteome of trace biological samples is critical for biomedical applications. However, it remains a challenge due to the loss of analytes caused by current sample preparation procedures. To address this, we recently developed a single-pot and miniaturized in-solution digestion (SMID) method for minute sample handling with three streamlined steps and completed within 3 h. The SMID approach outperformed the traditional workflow in substantially saving time, reducing sample loss, and exhibiting extensive applicability for 10-100 000 cell analysis. This user-friendly and high-sensitivity strategy enables ∼5300 proteins and 53 000 peptides to be confidently identified within 1 h of mass spectrometry (MS) time from a small amount of 1000 HeLa cells. In addition, we accurately and robustly detected proteomes in 10 mouse oocytes with excellent reproducibility. We further adopted SMID for the proteome analysis in cell migration under confinement, which induced cells to undergo a mesenchymal-amoeboid transition (MAT). During the MAT, a systematic quantitative proteome map of 1000 HeLa cells was constructed with seven expression profile clusters, which illustrated the application of SMID and provided a fundamental resource to investigate the mechanism of MAT.

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.

Focal pyroptosis-related genes AIM2 and ZBP1 are prognostic markers for triple-negative breast cancer with brain metastases

BACKGROUND

Extensive research has shown the role of pyroptosis in the occurrence, progression, and prognosis of breast cancer. The study sought to screen important pyroptosis-related genes and their role in the prognosis of triple-negative breast cancer (TNBC) patients with brain metastasis (BM).

METHODS

The Gene Expression Omnibus database was used to obtain transcriptome data from primary TNBC and from TNBC BM patients. Differentially expressed genes (DEGs) between the primary tumors and BMs were analyzed, and the expression, prognostic significance, immune infiltration, function, and drug sensitivity of the pyroptosis genes in the DEGs were analyzed.

RESULTS

In both data sets, 456 genes differed between primary TNBC and TNBC BM. Absent in melanoma 2 (AIM2) and Z-deoxyribonucleic acid-binding protein 1 (ZBP1) were found to be important pyroptosis genes in DEGs, and significant differences in their expression in primary lesions and BMs were observed. Patients with a high expression of AIM2 had a worse prognosis than low expression, while patients with a high expression of ZBP1 had a better prognosis than low expression. AIM2 and ZBP1 were positively correlated with the infiltration of most immune cells; however, AIM2 was negatively correlated with the infiltration of neural cell adhesion molecule 1 (CD56) bright natural killer cells and central memory cluster of differentiation 8 (CD8) T cells. Increased expression of ZBP1 is negatively correlated with high infiltration levels of central memory CD8 T cells and memory B cells.

CONCLUSIONS

Our findings suggest that AIM2 and ZBP1 increase immune cell infiltration and may be potential targets for predicting and treating TNBC BM.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

Proteins of the actin depolymerizing factor (ADF)/cofilin family are ubiquitous among eukaryotes and are essential regulators of actin dynamics and function. Mammalian neurons express cofilin-1 as the major isoform, but ADF and cofilin-2 are also expressed. All isoforms bind preferentially and cooperatively along ADP-subunits in F-actin, affecting the filament helical rotation, and when either alone or when enhanced by other proteins, promotes filament severing and subunit turnover. Although self-regulating cofilin-mediated actin dynamics can drive motility without post-translational regulation, cells utilize many mechanisms to locally control cofilin, including cooperation/competition with other proteins. Newly identified post-translational modifications function with or are independent from the well-established phosphorylation of serine 3 and provide unexplored avenues for isoform specific regulation. Cofilin modulates actin transport and function in the nucleus as well as actin organization associated with mitochondrial fission and mitophagy. Under neuronal stress conditions, cofilin-saturated F-actin fragments can undergo oxidative cross-linking and bundle together to form cofilin-actin rods. Rods form in abundance within neurons around brain ischemic lesions and can be rapidly induced in neurites of most hippocampal and cortical neurons through energy depletion or glutamate-induced excitotoxicity. In ~20% of rodent hippocampal neurons, rods form more slowly in a receptor-mediated process triggered by factors intimately connected to disease-related dementias, e.g., amyloid-β in Alzheimer’s disease. This rod-inducing pathway requires a cellular prion protein, NADPH oxidase, and G-protein coupled receptors, e.g., CXCR4 and CCR5. Here, we will review many aspects of cofilin regulation and its contribution to synaptic loss and pathology of neurodegenerative diseases.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Intracellular mRNA transport and localized translation

Fine-tuning cellular physiology in response to intracellular and environmental cues requires precise temporal and spatial control of gene expression. High-resolution imaging technologies to detect mRNAs and their translation state have revealed that all living organisms localize mRNAs in subcellular compartments and create translation hotspots, enabling cells to tune gene expression locally. Therefore, mRNA localization is a conserved and integral part of gene expression regulation from prokaryotic to eukaryotic cells. In this Review, we discuss the mechanisms of mRNA transport and local mRNA translation across the kingdoms of life and at organellar, subcellular and multicellular resolution. We also discuss the properties of messenger ribonucleoprotein and higher order RNA granules and how they may influence mRNA transport and local protein synthesis. Finally, we summarize the technological developments that allow us to study mRNA localization and local translation through the simultaneous detection of mRNAs and proteins in single cells, mRNA and nascent protein single-molecule imaging, and bulk RNA and protein detection methods.

Different translation dynamics of β- and γ-actin regulates cell migration

β- and γ-cytoplasmic actins are ubiquitously expressed in every cell type and are nearly identical at the amino acid level but play vastly different roles in vivo. Their essential roles in embryogenesis and mesenchymal cell migration critically depend on the nucleotide sequences of their genes, rather than their amino acid sequences; however, it is unclear which gene elements underlie this effect. Here we address the specific role of the coding sequence in β- and γ-cytoplasmic actins’ intracellular functions, using stable polyclonal populations of immortalized mouse embryonic fibroblasts with exogenously expressed actin isoforms and their ‘codon-switched’ variants. When targeted to the cell periphery using β-actin 3′UTR; β-actin and γ-actin have differential effects on cell migration. These effects directly depend on the coding sequence. Single-molecule measurements of actin isoform translation, combined with fluorescence recovery after photobleaching, demonstrate a pronounced difference in β- and γ-actins’ translation elongation rates in cells, leading to changes in their dynamics at focal adhesions, impairments in actin bundle formation, and reduced cell anchoring to the substrate during migration. Our results demonstrate that coding sequence-mediated differences in actin translation play a key role in cell migration.

Most mammalian cells make both β- and γ-actin, two proteins which shape the cell’s internal skeleton and its ability to migrate. The molecules share over 99% of their sequence, yet they play distinct roles. In fact, deleting the β-actin gene in mice causes death in the womb, while the animals can survive with comparatively milder issues without their γ-actin gene. How two similar proteins can have such different biological roles is a long-standing mystery.

A closer look could hold some clues: β- and γ-actin may contain the same blocks (or amino acids), but the genetic sequences that encode these proteins differ by about 13%. This is because different units of genetic information – known as synonymous codons – can encode the same amino acid. These ‘silent substitutions’ have no effect on the sequence of the proteins, yet a cell reads synonymous codons (and therefore produces proteins) at different speeds.

To find out the impact of silent substitutions, Vedula et al. swapped the codons for the two proteins, forcing mouse cells to produce β-actin using γ-actin codons, and vice versa. Cells with non-manipulated γ-actin and those with β-actin made using γ-actin codons could move much faster than cells with β-actin. This suggested that silent substitutions were indeed affecting the role of the protein.

Vedula et al. found that cells read γ-codons – and therefore made γ-actin – much more slowly than β-codons: this also affected how quickly the protein could be dispatched where it was needed in the cell. Slower production meant that bundles of γ-actin were shorter, which allowed cells to move faster by providing a weaker anchoring system. Overall, this work provides new links between silent substitutions and protein behavior, a relatively new research area which is likely to shed light on other protein families.

mRNA Targeting, Transport and Local Translation in Eukaryotic Cells: From the Classical View to a Diversity of New Concepts

Spatial organization of protein biosynthesis in the eukaryotic cell has been studied for more than fifty years, thus many facts have already been included in textbooks. According to the classical view, mRNA transcripts encoding secreted and transmembrane proteins are translated by ribosomes associated with endoplasmic reticulum membranes, while soluble cytoplasmic proteins are synthesized on free polysomes. However, in the last few years, new data has emerged, revealing selective translation of mRNA on mitochondria and plastids, in proximity to peroxisomes and endosomes, in various granules and at the cytoskeleton (actin network, vimentin intermediate filaments, microtubules and centrosomes). There are also long-standing debates about the possibility of protein synthesis in the nucleus. Localized translation can be determined by targeting signals in the synthesized protein, nucleotide sequences in the mRNA itself, or both. With RNA-binding proteins, many transcripts can be assembled into specific RNA condensates and form RNP particles, which may be transported by molecular motors to the sites of active translation, form granules and provoke liquid-liquid phase separation in the cytoplasm, both under normal conditions and during cell stress. The translation of some mRNAs occurs in specialized "translation factories," assemblysomes, transperons and other structures necessary for the correct folding of proteins, interaction with functional partners and formation of oligomeric complexes. Intracellular localization of mRNA has a significant impact on the efficiency of its translation and presumably determines its response to cellular stress. Compartmentalization of mRNAs and the translation machinery also plays an important role in viral infections. Many viruses provoke the formation of specific intracellular structures, virus factories, for the production of their proteins. Here we review the current concepts of the molecular mechanisms of transport, selective localization and local translation of cellular and viral mRNAs, their effects on protein targeting and topogenesis, and on the regulation of protein biosynthesis in different compartments of the eukaryotic cell. Special attention is paid to new systems biology approaches, providing new cues to the study of localized translation.

FMRP regulates STAT3 mRNA localization to cellular protrusions and local translation to promote hepatocellular carcinoma metastasis

Most hepatocellular carcinoma (HCC)-associated mortalities are related to the metastasis of cancer cells. The localization of mRNAs and their products to cell protrusions has been reported to play a crucial role in the metastasis. Our previous findings demonstrated that STAT3 mRNA accumulated in the protrusions of metastatic HCC cells. However, the underlying mechanism and functional significance of this localization of STAT3 mRNA has remained unexplored. Here we show that fragile X mental retardation protein (FMRP) modulates the localization and translation of STAT3 mRNA, accelerating HCC metastasis. The results of molecular analyses reveal that the 3′UTR of STAT3 mRNA is responsible for the localization of STAT3 mRNA to cell protrusions. FMRP is able to interact with the 3′UTR of STAT3 mRNA and facilitates its localization to protrusions. Importantly, FMRP could promote the IL-6-mediated translation of STAT3, and serine 114 of FMRP is identified as a potential phosphorylation site required for IL-6-mediated STAT3 translation. Furthermore, FMRP is highly expressed in HCC tissues and FMRP knockdown efficiently suppresses HCC metastasis in vitro and in vivo. Collectively, our findings provide further insights into the mechanism of HCC metastasis associated with the regulation of STAT3 mRNA localization and translation.

Shen et al. propose a mechanism for the metastasis of hepatocellular carcinoma (HCC) cells through the localization and translation modulation of the STAT3 oncogene by fragile X mental retardation protein (FMRP). To this end, the authors also find that FMRP knockdown efficiently suppresses HCC metastasis in vitro and in vivo.

Molecular similarity between the mechanisms of epithelial fusion and fetal wound healing during the closure of the caudal neural tube in mouse embryos

BACKGROUND

Neural tube (NT) closure is a complex developmental process that takes place in the early stages of embryogenesis and that is a key step in neurulation. In mammals, the process by which the neural plate generates the NT requires organized cell movements and tissue folding, and it terminates with the fusion of the apposed ends of the neural folds.

RESULTS

Here we describe how almost identical cellular and molecular machinery is used to fuse the spinal neural folds as that involved in the repair of epithelial injury in the same area of the embryo. For both natural and wound activated closure of caudal neural tissue, hyaluronic acid and platelet-derived growth factor signaling appear to be crucial for the final fusion step.

CONCLUSIONS

There seems to be no general wound healing machinery for all tissues but rather, a tissue-specific epithelial fusion machinery that embryos activate when necessary after abnormal epithelial opening.

The origin of the expressed retrotransposed gene ACTBL2 and its influence on human melanoma cells' motility and focal adhesion formation

We have recently found that β-actin-like protein 2 (actbl2) forms complexes with gelsolin in human melanoma cells and can polymerize. Phylogenetic and bioinformatic analyses showed that actbl2 has a common origin with two non-muscle actins, which share a separate history from the muscle actins. The actin groups' divergence started at the beginning of vertebrate evolution, and actbl2 actins are characterized by the largest number of non-conserved amino acid substitutions of all actins. We also discovered that ACTBL2 is expressed at a very low level in several melanoma cell lines, but a small subset of cells exhibited a high ACTBL2 expression. We found that clones with knocked-out ACTBL2 (CR-ACTBL2) or overexpressing actbl2 (OE-ACTBL2) differ from control cells in the invasion, focal adhesion formation, and actin polymerization ratio, as well as in the formation of lamellipodia and stress fibers. Thus, we postulate that actbl2 is the seventh actin isoform and is essential for cell motility.

The role of RNA-binding and ribosomal proteins as specific RNA translation regulators in cellular differentiation and carcinogenesis

Tight control of mRNA expression is required for cell differentiation; imbalanced regulation may lead to developmental disorders and cancer. The activity of the translational machinery (including ribosomes and translation factors) regulates the rate (slow or fast) of translation of encoded proteins, and the quality of these proteins highly depends on which mRNAs are available for translation. Specific RNA-binding and ribosomal proteins seem to play a key role in controlling gene expression to determine the differentiation fate of the cell. This demonstrates the important role of RNA-binding proteins, specific ribosome-binding proteins and microRNAs as key molecules in controlling the specific proteins required for the differentiation or dedifferentiation of cells. This delicate balance between specific proteins (in terms of quality and availability) and post-translational modifications occurring in the cytoplasm is crucial for cell differentiation, dedifferentiation and oncogenic potential. In this review, we report how defects in the regulation of mRNA translation can be dependent on specific proteins and can induce an imbalance between differentiation and dedifferentiation in cell fate determination.

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.

RAB13 mRNA compartmentalisation spatially orients tissue morphogenesis

Polarised targeting of diverse mRNAs to cellular protrusions is a hallmark of cell migration. Although a widespread phenomenon, definitive functions for endogenous targeted mRNAs and their relevance to modulation of in vivo tissue dynamics remain elusive. Here, using single-molecule analysis, gene editing and zebrafish live-cell imaging, we report that mRNA polarisation acts as a molecular compass that orients motile cell polarity and spatially directs tissue movement. Clustering of protrusion-derived RNAseq datasets defined a core 192-nt localisation element underpinning precise mRNA targeting to sites of filopodia formation. Such targeting of the small GTPase RAB13 generated tight spatial coupling of mRNA localisation, translation and protein activity, achieving precise subcellular compartmentalisation of RAB13 protein function to create a polarised domain of filopodia extension. Consequently, genomic excision of this localisation element and perturbation of RAB13 mRNA targeting-but not translation-depolarised filopodia dynamics in motile endothelial cells and induced mispatterning of blood vessels in zebrafish. Hence, mRNA polarisation, not expression, is the primary determinant of the site of RAB13 action, preventing ectopic functionality at inappropriate subcellular loci and orienting tissue morphogenesis.

Mechanisms and consequences of subcellular RNA localization across diverse cell types

Essentially all cells contain a variety of spatially restricted regions that are important for carrying out specialized functions. Often, these regions contain specialized transcriptomes that facilitate these functions by providing transcripts for localized translation. These transcripts play a functional role in maintaining cell physiology by enabling a quick response to changes in the cellular environment. Here, we review how RNA molecules are trafficked within cells, with a focus on the subcellular locations to which they are trafficked, mechanisms that regulate their transport and clinical disorders associated with misregulation of the process.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

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.

Bioorthogonal mRNA labeling at the poly(A) tail for imaging localization and dynamics in live zebrafish embryos

Live imaging of mRNA in cells and organisms is important for understanding the dynamic aspects underlying its function.

Live imaging of mRNA in cells and organisms is important for understanding the dynamic aspects underlying its function. Ideally, labeling of mRNA should not alter its structure or function, nor affect the biological system. However, most methods applied in vivo make use of genetically encoded tags and reporters that significantly enhance the size of the mRNA of interest. Alternately, we utilize the 3′ poly(A) tail as a non-coding repetitive hallmark to covalently label mRNAs via bioorthogonal chemistry with different fluorophores from a wide range of spectra without significantly changing the size. We demonstrate that the labeled mRNAs can be visualized in cells and zebrafish embryos, and that they are efficiently translated. Importantly, the labeled mRNAs acquired the proper subcellular localization in developing zebrafish embryos and their dynamics could be tracked in vivo.

Regulation of actin isoforms in cellular and developmental processes

Actin is one of the most abundant and essential intracellular proteins that mediates nearly every form of cellular movement and underlies such key processes as embryogenesis, tissue integrity, cell division and contractility of all types of muscle and non-muscle cells. In mammals, actin is represented by six isoforms, which are encoded by different genes but produce proteins that are 95-99 % identical to each other. The six actin genes have vastly different functions in vivo, and the small amino acid differences between the proteins they encode are rigorously maintained through evolution, but the underlying differences behind this distinction, as well as the importance of specific amino acid sequences for each actin isoform, are not well understood. This review summarizes different levels of actin isoform-specific regulation in cellular and developmental processes, starting with the nuclear actin's role in transcription, and covering the gene-level, mRNA-level, and protein-level regulation, with a special focus on mammalian actins in non-muscle cells.

Quantification of intracellular N-terminal β-actin arginylation

Actin is a ubiquitous, essential, and highly abundant protein in all eukaryotic cells that performs key roles in contractility, adhesion, migration, and leading edge dynamics. The two non-muscle actins, β- and γ-, are ubiquitously present in every cell type and are nearly identical to each other at the amino acid level, but play distinct intracellular roles. The mechanisms regulating this distinction have been the focus of recent interest in the field. Work from our lab has previously shown that β-, but not γ-, actin undergoes N-terminal arginylation on Asp3. While functional evidence suggest that this arginylation may be important to actin's function, progress in these studies so far has been hindered by difficulties in arginylated actin detection, precluding estimations of the abundance of arginylated actin in cells, and its occurrence in different tissues and cell types. The present study represents the first antibody-based quantification of the percentage of arginylated actin in migratory non-muscle cells under different physiological conditions, as well as in different cells and tissues. We find that while the steady-state level of arginylated actin is relatively low, it is consistently present in vivo, and is somewhat more prominent in migratory cells. Inhibition of N-terminal actin acetylation dramatically increases the intracellular actin arginylation level, suggesting that these two modifications may directly compete in vivo. These findings constitute an essential step in our understanding of actin regulation by arginylation, and in uncovering the dynamic interplay of actin's N-terminal modifications in vivo.

Reduction of mRNA export unmasks different tissue sensitivities to low mRNA levels during Caenorhabditis elegans development

Animal development requires the execution of specific transcriptional programs in different sets of cells to build tissues and functional organs. Transcripts are exported from the nucleus to the cytoplasm where they are translated into proteins that, ultimately, carry out the cellular functions. Here we show that in Caenorhabditis elegans, reduction of mRNA export strongly affects epithelial morphogenesis and germline proliferation while other tissues remain relatively unaffected. Epithelialization and gamete formation demand a large number of transcripts in the cytoplasm for the duration of these processes. In addition, our findings highlight the existence of a regulatory feedback mechanism that activates gene expression in response to low levels of cytoplasmic mRNA. We expand the genetic characterization of nuclear export factor NXF-1 to other members of the mRNA export pathway to model mRNA export and recycling of NXF-1 back to the nucleus. Our model explains how mutations in genes involved in general processes, such as mRNA export, may result in tissue-specific developmental phenotypes.

Translational regulation of protrusion-localized RNAs involves silencing and clustering after transport

Localization of RNAs to various subcellular destinations is a widely used mechanism that regulates a large proportion of transcripts in polarized cells. In many cases, such localized transcripts mediate spatial control of gene expression by being translationally silent while in transit and locally activated at their destination. Here, we investigate the translation of RNAs localized at dynamic cellular protrusions of human and mouse, migrating, mesenchymal cells. In contrast to the model described above, we find that protrusion-localized RNAs are not locally activated solely at protrusions, but can be translated with similar efficiency in both internal and peripheral locations. Interestingly, protrusion-localized RNAs are translated at extending protrusions, they become translationally silenced in retracting protrusions and this silencing is accompanied by coalescence of single RNAs into larger heterogeneous RNA clusters. This work describes a distinct mode of translational regulation of localized RNAs, which we propose is used to regulate protein activities during dynamic cellular responses.

Nanoparticle-based local translation reveals mRNA as a translation-coupled scaffold with anchoring function

The spatial regulation of messenger RNA (mRNA) translation is central to cellular functions and relies on numerous complex processes. Biomimetic approaches could bypass these endogenous complex processes, improve our comprehension of the regulation, and allow for controlling local translation regulations and functions. However, the causality between local translation and nascent protein function remains elusive. Here, we developed a nanoparticle (NP)-based strategy to magnetically control mRNA spatial patterns in mammalian cell extracts and investigate how local translation impacts nascent protein localization and function. By monitoring the translation of the magnetically localized mRNAs, we show that mRNA-NP complexes operate as a source for the continuous production of proteins from defined positions. By applying this approach to actin-binding proteins, we triggered the local formation of actin cytoskeletons and identified the minimal requirements for spatial control of the actin filament network. In addition, our bottom-up approach identified a role for mRNA as a translation-coupled scaffold for the function of nascent N-terminal protein domains. Our approach will serve as a platform for regulating mRNA localization and investigating the function of nascent protein domains during translation.

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.

High Rac1 activity is functionally translated into cytosolic structures with unique nanoscale cytoskeletal architecture

Rac1 activation is at the core of signaling pathways regulating polarized cell migration. So far, it has not been possible to directly explore the structural changes triggered by Rac1 activation at the molecular level. Here, through a multiscale imaging workflow that combines biosensor imaging of Rac1 dynamics with electron cryotomography, we identified, within the crowded environment of eukaryotic cells, a unique nanoscale architecture of a flexible, signal-dependent actin structure. In cell regions with high Rac1 activity, we found a structural regime that spans from the ventral membrane up to a height of ∼60 nm above that membrane, composed of directionally unaligned, densely packed actin filaments, most shorter than 150 nm. This unique Rac1-induced morphology is markedly different from the dendritic network architecture in which relatively short filaments emanate from existing, longer actin filaments. These Rac1-mediated scaffold assemblies are devoid of large macromolecules such as ribosomes or other filament types, which are abundant at the periphery and within the remainder of the imaged volumes. Cessation of Rac1 activity induces a complete and rapid structural transition, leading to the absence of detectable remnants of such structures within 150 s, providing direct structural evidence for rapid actin filament network turnover induced by GTPase signaling events. It is tempting to speculate that this highly dynamical nanoscaffold system is sensitive to local spatial cues, thus serving to support the formation of more complex actin filament architectures-such as those mandated by epithelial-mesenchymal transition, for example-or resetting the region by completely dissipating.

Hnrnpab regulates neural cell motility through transcription of Eps8

Cell migration requires a complicated network of structural and regulatory proteins. Changes in cellular motility can impact migration as a result of cell-type or developmental stage regulated expression of critical motility genes. Hnrnpab is a conserved RNA-binding protein found as two isoforms produced by alternative splicing. Its expression is enriched in the subventricular zone (SVZ) and the rostral migratory stream within the brain, suggesting possible support of the migration of neural progenitor cells in this region. Here we show that the migration of cells from the SVZ of developing Hnrnpab^-/- mouse brains is impaired. An RNA-seq analysis to identify Hnrnpab-dependent cell motility genes led us to Eps8, and in agreement with the change in cell motility, we show that Eps8 is decreased in Hnrnpab^-/- SVZ tissue. We scrutinized the motility of Hnrnpab^-/- cells and confirmed that the decreases in both cell motility and Eps8 are restored by ectopically coexpressing both alternatively spliced Hnrnpab isoforms, therefore these variants are surprisingly nonredundant for cell motility. Our results support a model where both Hnrnpab isoforms work in concert to regulate Eps8 transcription in the mouse SVZ to promote the normal migration of neural cells during CNS development.

Posttranscriptional and Posttranslational Regulation of Actin

Actin is one of the most abundant intracellular proteins, essential in every eukaryotic cell type. Actin plays key roles in tissue morphogenesis, cell adhesion, muscle contraction, and developmental reprogramming. Most actin studies have focused on its regulation at the protein level, either directly or through differential interactions with over a hundred intracellular binding partners. However, numerous studies emerging in recent years demonstrate specific types of nucleotide-level regulation that strongly affect non-muscle actins during cell migration and adhesion and are potentially applicable to other members of the actin family. This regulation involves zipcode-mediated actin mRNA targeting to the cell periphery, proposed to mediate local synthesis of actin at the cell leading edge, as well as the recently discovered N-terminal arginylation that specifically targets non-muscle β-actin via a nucleotide-dependent mechanism. Moreover, a study published this year suggests that actin's essential roles at the organismal level may be entirely nucleotide-dependent. This review summarizes the emerging data on actin's nucleotide-level regulation. Anat Rec, 301:1991-1998, 2018. © 2018 Wiley Periodicals, Inc.

Single-molecule analysis of endogenous β-actin mRNA trafficking reveals a mechanism for compartmentalized mRNA localization in axons

De novo protein synthesis in neuronal axons plays important roles in neural circuit formation, maintenance, and disease. Key to the selectivity of axonal protein synthesis is whether an mRNA is present at the right place to be translated, but the mechanisms behind axonal mRNA localization remain poorly understood. In this work, we quantitatively analyze the link between axonal β-actin mRNA trafficking and its localization patterns. By developing a single-molecule approach to live-image β-actin mRNAs in axons, we explore the biophysical drivers behind β-actin mRNA motion and uncover a mechanism for generating increased density at the axon tip by differences in motor protein-driven transport speeds. These results provide mechanistic insight into the control of local translation through mRNA trafficking.

During embryonic nervous system assembly, mRNA localization is precisely regulated in growing axons, affording subcellular autonomy by allowing controlled protein expression in space and time. Different sets of mRNAs exhibit different localization patterns across the axon. However, little is known about how mRNAs move in axons or how these patterns are generated. Here, we couple molecular beacon technology with highly inclined and laminated optical sheet microscopy to image single molecules of identified endogenous mRNA in growing axons. By combining quantitative single-molecule imaging with biophysical motion models, we show that β-actin mRNA travels mainly as single copies and exhibits different motion-type frequencies in different axonal subcompartments. We find that β-actin mRNA density is fourfold enriched in the growth cone central domain compared with the axon shaft and that a modicum of directed transport is vital for delivery of mRNA to the axon tip. Through mathematical modeling we further demonstrate that directional differences in motor-driven mRNA transport speeds are sufficient to generate β-actin mRNA enrichment at the growth cone. Our results provide insight into how mRNAs are trafficked in axons and a mechanism for generating different mRNA densities across axonal subcompartments.

The makings of the 'actin code': regulation of actin's biological function at the amino acid and nucleotide level

The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles in vivo We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.

IMP1 regulates UCA1-mediated cell invasion through facilitating UCA1 decay and decreasing the sponge effect of UCA1 for miR-122-5p

BACKGROUND

Long noncoding RNAs (LncRNAs) represent a class of widespread and diverse endogenous RNAs that can posttranscriptionally regulate gene expression through the interaction with RNA-binding proteins and micro RNAs (miRNAs). Here, we report that in breast carcinoma cells, the insulin-like growth factor 2 messenger RNA binding protein (IMP1) binds to lncRNA urethral carcinoma-associated 1 (UCA1) and suppresses the UCA1-induced invasive phenotype.

METHODS

RT-qPCR and RNA sequence assays were used to investigate the expression of UCA1 and miRNAs in breast cancer cells in response to IMP1 expression. The role of IMP1-UCA1 interaction in cell invasion was demonstrated by transwell analysis through loss-of-function and gain-of-function effects. RNA pull-down and RNA binding protein immunoprecipitation (RIP) were performed to confirm the molecular interactions of IMP1-UCA1 and UCA1-miR-122-5p involved in breast cancer cells.

RESULTS

In breast cancer cells, IMP1 interacts with UCA1 via the "ACACCC" motifs within UCA1 and destabilizes UCA1 through the recruitment of CCR4-NOT1 deadenylase complex. Meanwhile, binding of IMP1 prevents the association of miR-122-5p with UCA1, thereby shifting the availability of miR-122-5p from UCA1 to the target mRNAs and reducing the UCA1-mediated cell invasion. Accordingly, either IMP1 silencing or UCA1 overexpression resulted in reduced levels of free miR-122-5p within the cytoplasm, affecting miR-122-5p in regulating its target mRNAs.

CONCLUSIONS

Our study provides initial evidence that interaction between IMP1 and UCA1 enhances UCA1 decay and competes for miR-122-5p binding, leading to the liberation of miR-122-5p activity and the reduction of cell invasiveness.

Rapid and dynamic arginylation of the leading edge β-actin is required for cell migration

β-actin plays key roles in cell migration. Our previous work demonstrated that β-actin in migratory non-muscle cells is N-terminally arginylated and that this arginylation is required for normal lamellipodia extension. Here, we examined the function of β-actin arginylation in cell migration. We found that arginylated β-actin is concentrated at the leading edge of lamellipodia and that this enrichment is abolished after serum starvation as well as in contact-inhibited cells in confluent cultures, suggesting that arginylated β-actin at the cell leading edge is coupled to active migration. Arginylated actin levels exhibit dynamic changes in response to cell stimuli, lowered after serum starvation and dramatically elevating within minutes after cell stimulation by readdition of serum or lysophosphatidic acid. These dynamic changes require active translation and are not seen in confluent contact-inhibited cell cultures. Microinjection of arginylated actin antibodies into cells severely and specifically inhibits their migration rates. Together, these data strongly suggest that arginylation of β-actin is a tightly regulated dynamic process that occurs at the leading edge of locomoting cells in response to stimuli and is integral to the signaling network that regulates cell migration.

Lymphocyte-specific protein 1 regulates mechanosensory oscillation of podosomes and actin isoform-based actomyosin symmetry breaking

Subcellular fine-tuning of the actomyosin cytoskeleton is a prerequisite for polarized cell migration. We identify LSP (lymphocyte-specific protein) 1 as a critical regulator of actomyosin contractility in primary macrophages. LSP1 regulates adhesion and migration, including the parameters cell area and speed, and also podosome turnover, oscillation and protrusive force. LSP1 recruits myosin IIA and its regulators, including myosin light chain kinase and calmodulin, and competes with supervillin, a myosin hyperactivator, for myosin regulators, and for actin isoforms, notably β-actin. Actin isoforms are anisotropically distributed in myosin IIA-expressing macrophages, and contribute to the differential recruitment of LSP1 and supervillin, thus enabling an actomyosin symmetry break, analogous to the situation in cells expressing two myosin II isoforms. Collectively, these results show that the cellular pattern of actin isoforms builds the basis for the differential distribution of two actomyosin machineries with distinct properties, leading to the establishment of discrete zones of actomyosin contractility.

The actomyosin cytoskeleton plays an important role in polarised cell migration. Here the authors identify lymphocyte-specific protein (LSP)-1 as a regulator of actomyosin contractility in macrophages, by competing with supervillin for myosin IIA activators acting specifically on the β-actin isoform.

The relative composition of actin isoforms regulates cell surface biophysical features and cellular behaviors

BACKGROUND

Cell surface mechanics is able to physically and biomechanically affect cell shape and motility, vesicle trafficking and actin dynamics. The biophysical properties of cell surface are strongly influenced by cytoskeletal elements. In mammals, tissue-specific expression of six actin isoforms is thought to confer differential biomechanical properties. However, the relative contribution of actin isoforms to cell surface properties is not well understood. Here, we sought to investigate whether and how the composition of endogenous actin isoforms directly affects the biomechanical features of cell surface and cellular behavior.

METHODS

We used fibroblasts isolated from wild type (WT), heterozygous (HET) and from knockout (KO) mouse embryos where both β-actin alleles are not functional. We applied a combination of genome-wide analysis and biophysical methods such as RNA-seq and atomic force microscopy.

RESULTS

We found that endogenous β-actin levels are essential in controlling cell surface stiffness and pull-off force, which was not compensated by the up-regulation of other actin isoforms. The variations of surface biophysical features and actin contents were associated with distinct cell behaviors in 2D and 3D WT, HET and KO cell cultures. Since β-actin in WT cells and smooth muscle α-actin up-regulated in KO cells showed different organization patterns, our data support the differential localization and organization as a mechanism to regulate the biophysical properties of cell surface by actin isoforms.

CONCLUSIONS

We propose that variations in actin isoforms composition impact on the biophysical features of cell surface and cause the changes in cell behavior.