Dynein motor contributes to stress granule dynamics in primary neurons

Friend or foe: The role of stress granule in neurodegenerative disease

Stress granules (SGs) are dynamic membraneless organelles that form in response to cellular stress. SGs are predominantly composed of RNA and RNA-binding proteins that assemble through liquid-liquid phase separation. Although the formation of SGs is considered a transient and protective response to cellular stress, their dysregulation or persistence may contribute to various neurodegenerative diseases. This review aims to provide a comprehensive overview of SG physiology and pathology. It covers the formation, composition, regulation, and functions of SGs, along with their crosstalk with other membrane-bound and membraneless organelles. Furthermore, this review discusses the dual roles of SGs as both friends and foes in neurodegenerative diseases and explores potential therapeutic approaches targeting SGs. The challenges and future perspectives in this field are also highlighted. A more profound comprehension of the intricate relationship between SGs and neurodegenerative diseases could inspire the development of innovative therapeutic interventions against these devastating diseases.

Using ALS to understand profilin 1's diverse roles in cellular physiology

Profilin is an actin monomer-binding protein whose role in actin polymerization has been studied for nearly 50 years. While its principal biochemical features are now well understood, many questions remain about how profilin controls diverse processes within the cell. Dysregulation of profilin has been implicated in a broad range of human diseases, including neurodegeneration, inflammatory disorders, cardiac disease, and cancer. For example, mutations in the profilin 1 gene (PFN1) can cause amyotrophic lateral sclerosis (ALS), although the precise mechanisms that drive neurodegeneration remain unclear. While initial work suggested proteostasis and actin cytoskeleton defects as the main pathological pathways, multiple novel functions for PFN1 have since been discovered that may also contribute to ALS, including the regulation of nucleocytoplasmic transport, stress granules, mitochondria, and microtubules. Here, we will review these newly discovered roles for PFN1, speculate on their contribution to ALS, and discuss how defects in actin can contribute to these processes. By understanding profilin 1's involvement in ALS pathogenesis, we hope to gain insight into this functionally complex protein with significant influence over cellular physiology.

Stress granule and P-body clearance: Seeking coherence in acts of disappearance

Stress granules and P-bodies are conserved cytoplasmic biomolecular condensates whose assembly and composition are well documented, but whose clearance mechanisms remain controversial or poorly described. Such understanding could provide new insight into how cells regulate biomolecular condensate formation and function, and identify therapeutic strategies in disease states where aberrant persistence of stress granules in particular is implicated. Here, I review and compare the contributions of chaperones, the cytoskeleton, post-translational modifications, RNA helicases, granulophagy and the proteasome to stress granule and P-body clearance. Additionally, I highlight the potentially vital role of RNA regulation, cellular energy, and changes in the interaction networks of stress granules and P-bodies as means of eliciting clearance. Finally, I discuss evidence for interplay of distinct clearance mechanisms, suggest future experimental directions, and suggest a simple working model of stress granule clearance.

Condensate functionalization with microtubule motors directs their nucleation in space and allows manipulating RNA localization

The localization of RNAs in cells is critical for many cellular processes. Whereas motor-driven transport of ribonucleoprotein (RNP) condensates plays a prominent role in RNA localization in cells, their study remains limited by the scarcity of available tools allowing to manipulate condensates in a spatial manner. To fill this gap, we reconstitute in cellula a minimal RNP transport system based on bioengineered condensates, which were functionalized with kinesins and dynein-like motors, allowing for their positioning at either the cell periphery or centrosomes. This targeting mostly occurs through the active transport of the condensate scaffolds, which leads to localized nucleation of phase-separated condensates. Then, programming the condensates to recruit specific mRNAs is able to shift the localization of these mRNAs toward the cell periphery or the centrosomes. Our method opens novel perspectives for examining the role of RNA localization as a driver of cellular functions.

Stress granule formation inhibits stress-induced apoptosis by selectively sequestering executioner caspases

Cytoplasmic stress granules (SGs) are phase-separated membrane-less organelles that form in response to various stress stimuli. SGs are mainly composed of non-canonical stalled 48S preinitiation complexes. In addition, many other proteins also accumulate into SGs, but the list is still incomplete. SG assembly suppresses apoptosis and promotes cell survival under stress. Furthermore, hyperformation of SGs is frequently observed in various human cancers and accelerates tumor development and progression by reducing stress-induced damage of cancer cells. Therefore, they are of clinical importance. However, the precise mechanism underlying SG-mediated inhibition of apoptosis remains ill-defined. Here, using a proximity-labeling proteomic approach, we comprehensively analyzed SG-resident proteins and identified the executioner caspases, caspase-3 and -7, as SG components. We demonstrate that accumulation of caspase-3/7 into SGs is mediated by evolutionarily conserved amino acid residues within their large catalytic domains and inhibits caspase activities and consequent apoptosis induced by various stresses. Expression of an SG-localization-deficient caspase-3 mutant in cells largely counteracted the anti-apoptotic effect of SGs, whereas enforced relocalization of the caspase-3 mutant to SGs restored it. Thus, SG-mediated sequestration of executioner caspases is a mechanism underlying the broad cytoprotective function of SGs. Furthermore, using a mouse xenograft tumor model, we show that this mechanism prevents cancer cells from apoptosis in tumor tissues, thereby promoting cancer progression. Our results reveal the functional crosstalk between SG-mediated cell survival and caspase-mediated cell death signaling pathways and delineate a molecular mechanism that dictates cell-fate decisions under stress and promotes tumorigenesis.

Non-specific adhesive forces between filaments and membraneless organelles

Many membraneless organelles are liquid-like domains that form inside the active, viscoelastic environment of living cells through phase separation. To investigate the potential coupling of phase separation with the cytoskeleton, we quantify the structural correlations of membraneless organelles (stress granules) and cytoskeletal filaments (microtubules) in a human-derived epithelial cell line. We find that microtubule networks are substantially denser in the vicinity of stress granules. When microtubules are depolymerized, the sub-units localize near the surface of the stress granules. We interpret these data using a thermodynamic model of partitioning of particles to the surface and bulk of the droplets. In this framework, our data are consistent with a weak (≲k B T) affinity of the microtubule sub-units for stress granule interfaces. As microtubules polymerize, their interfacial affinity increases, providing sufficient adhesion to deform droplets and/or the network. Our work suggests that proteins and other objects in the cell have a non-specific affinity for droplet interfaces that increases with the contact area and becomes most apparent when they have no preference for the interior of a droplet over the rest of the cytoplasm. We validate this basic physical phenomenon in vitro through the interaction of a simple protein-RNA condensate with microtubules.

RNA-binding proteins and post-transcriptional regulation in lens biology and cataract: Mediating spatiotemporal expression of key factors that control the cell cycle, transcription, cytoskeleton and transparency

Development of the ocular lens - a transparent tissue capable of sustaining frequent shape changes for optimal focusing power - pushes the boundaries of what cells can achieve using the molecular toolkit encoded by their genomes. The mammalian lens contains broadly two types of cells, the anteriorly located monolayer of epithelial cells which, at the equatorial region of the lens, initiate differentiation into fiber cells that contribute to the bulk of the tissue. This differentiation program involves massive upregulation of select fiber cell-expressed RNAs and their subsequent translation into high amounts of proteins, such as crystallins. But intriguingly, fiber cells achieve this while also simultaneously undergoing significant morphological changes such as elongation - involving about 1000-fold length-wise increase - and migration, which requires modulation of cytoskeletal and cell adhesion factors. Adding further to the challenges, these molecular and cellular events have to be coordinated as fiber cells progress toward loss of their nuclei and organelles, which irreversibly compromises their potential for harnessing genetically hardwired information. A long-standing question is how processes downstream of signaling and transcription, which may also participate in feedback regulation, contribute toward orchestrating these cellular differentiation events in the lens. It is now becoming clear from findings over the past decade that post-transcriptional gene expression regulatory mechanisms are critical in controlling cellular proteomes and coordinating key processes in lens development and fiber cell differentiation. Indeed, RNA-binding proteins (RBPs) such as Caprin2, Celf1, Rbm24 and Tdrd7 have now been described in mediating post-transcriptional control over key factors (e.g. Actn2, Cdkn1a (p21Cip1), Cdkn1b (p27Kip1), various crystallins, Dnase2b, Hspb1, Pax6, Prox1, Sox2) that are variously involved in cell cycle, transcription, cytoskeleton maintenance and differentiation in the lens. Furthermore, deficiencies of these RBPs have been shown to result in various eye and lens defects and/or cataract. Because fiber cell differentiation in the lens occurs throughout life, the underlying regulatory mechanisms operational in development are expected to also be recruited for the maintenance of transparency in aged lenses. Indeed, in support of this, TDRD7 and CAPRIN2 loci have been linked to age-related cataract in humans. Here, I will review the role of key RBPs in the lens and their importance in understanding the pathology of lens defects. I will discuss advances in RBP-based gene expression control, in general, and the important challenges that need to be addressed in the lens to define the mechanisms that determine the epithelial and fiber cell proteome. Finally, I will also discuss in detail several key future directions including the application of bioinformatics approaches such as iSyTE to study RBP-based post-transcriptional gene expression control in the aging lens and in the context of age-related cataract.

Regulation of spatially restricted gene expression: linking RNA localization and phase separation

Subcellular restriction of gene expression is crucial to the functioning of a wide variety of cell types. The cellular machinery driving spatially restricted gene expression has been studied for many years, but recent advances have highlighted novel mechanisms by which cells can generate subcellular microenvironments with specialized gene expression profiles. Particularly intriguing are recent findings that phase separation plays a role in certain RNA localization pathways. The burgeoning field of phase separation has revolutionized how we view cellular compartmentalization, revealing that, in addition to membrane-bound organelles, phase-separated cytoplasmic microenvironments - termed biomolecular condensates - are compositionally and functionally distinct from the surrounding cytoplasm, without the need for a lipid membrane. The coupling of phase separation and RNA localization allows for precise subcellular targeting, robust translational repression and dynamic recruitment of accessory proteins. Despite the growing interest in the intersection between RNA localization and phase separation, it remains to be seen how exactly components of the localization machinery, particularly motor proteins, are able to associate with these biomolecular condensates. Further studies of the formation, function, and transport of biomolecular condensates promise to provide a new mechanistic understanding of how cells restrict gene expression at a subcellular level.

Hyperosmotic Stress Induces a Specific Pattern for Stress Granule Formation in Human-Induced Pluripotent Stem Cells

Stress granules (SGs) are assemblies of selective messenger RNAs (mRNAs), translation factors, and RNA-binding proteins in small untranslated messenger ribonucleoprotein (mRNP) complexes in the cytoplasm. Evidence indicates that different types of cells have shown different mechanisms to respond to stress and the formation of SGs. In the present work, we investigated how human-induced pluripotent stem cells (hiPSCs/IMR90-1) overcome hyperosmotic stress compared to a cell line that does not harbor pluripotent characteristics (SH-SY5Y cell line). Gradient concentrations of NaCl showed a different pattern of SG formation between hiPSCs/IMR90-1 and the nonpluripotent cell line SH-SY5Y. Other pluripotent stem cell lines (hiPSCs/CRTD5 and hESCs/H9 (human embryonic stem cell line)) as well as nonpluripotent cell lines (BHK-21 and MCF-7) were used to confirm this phenomenon. Moreover, the formation of hyperosmotic SGs in hiPSCs/IMR90-1 was independent of eIF2α phosphorylation and was associated with low apoptosis levels. In addition, a comprehensive proteomics analysis was performed to identify proteins involved in regulating this specific pattern of hyperosmotic SG formation in hiPSCs/IMR90-1. We found possible implications of microtubule organization on the response to hyperosmotic stress in hiPSCs/IMR90-1. We have also unveiled a reduced expression of tubulin that may protect cells against hyperosmolarity stress while inhibiting SG formation without affecting stem cell self-renewal and pluripotency. Our observations may provide a possible cellular mechanism to better understand SG dynamics in pluripotent stem cells.

Response to stress in biological disorders: Implications of stress granule assembly and function

It is indispensable for cells to adapt and respond to environmental stresses, in order for organisms to survive. Stress granules (SGs) are condensed membrane‐less organelles dynamically formed in the cytoplasm of eukaryotes cells to cope with diverse intracellular or extracellular stress factors, with features of liquid‐liquid phase separation. They are composed of multiple constituents, including translationally stalled mRNAs, translation initiation factors, RNA‐binding proteins and also non‐RNA‐binding proteins. SG formation is triggered by stress stimuli, viral infection and signal transduction, while aberrant assembly of SGs may contribute to tissue degenerative diseases. Recently, a growing body of evidence has emerged on SG response mechanisms for cells facing high temperatures, oxidative stress and osmotic stress. In this review, we aim to summarize factors affecting SGs assembly, present the impact of SGs on germ cell development and other biological processes. We particularly emphasize the significance of recently reported RNA modifications in SG stress responses. In parallel, we also review all current perspectives on the roles of SGs in male germ cells, with a particular focus on the dynamics of SG assembly.

Complex Interactions Between Membrane-Bound Organelles, Biomolecular Condensates and the Cytoskeleton

Membrane-bound and membraneless organelles/biomolecular condensates ensure compartmentalization into functionally distinct units enabling proper organization of cellular processes. Membrane-bound organelles form dynamic contacts with each other to enable the exchange of molecules and to regulate organelle division and positioning in coordination with the cytoskeleton. Crosstalk between the cytoskeleton and dynamic membrane-bound organelles has more recently also been found to regulate cytoskeletal organization. Interestingly, recent work has revealed that, in addition, the cytoskeleton and membrane-bound organelles interact with cytoplasmic biomolecular condensates. The extent and relevance of these complex interactions are just beginning to emerge but may be important for cytoskeletal organization and organelle transport and remodeling. In this review, we highlight these emerging functions and emphasize the complex interplay of the cytoskeleton with these organelles. The crosstalk between membrane-bound organelles, biomolecular condensates and the cytoskeleton in highly polarized cells such as neurons could play essential roles in neuronal development, function and maintenance.

Microtubule-Driven Stress Granule Dynamics Regulate Inhibitory Immune Checkpoint Expression in T Cells

Despite the clinical success of blocking inhibitory immune checkpoint receptors such as programmed cell death-1 (PD-1) in cancer, the mechanisms controlling the expression of these receptors have not been fully elucidated. Here, we identify a post-transcriptional mechanism regulating PD-1 expression in T cells. Upon activation, the PDCD1 mRNA and ribonucleoprotein complexes coalesce into stress granules that require microtubules and the kinesin 1 molecular motor to proceed to translation. Hence, PD-1 expression is highly sensitive to microtubule or stress granule inhibitors targeting this pathway. Evidence from healthy donors and cancer patients reveals a common regulation for the translation of CTLA4, LAG3, TIM3, TIGIT, and BTLA but not of the stimulatory co-receptors OX40, GITR, and 4-1BB mRNAs. In patients, disproportionality analysis of immune-related adverse events for currently used microtubule drugs unveils a significantly higher risk of autoimmunity. Our findings reveal a fundamental mechanism of immunoregulation with great importance in cancer immunotherapy.

TDP-43 regulation of stress granule dynamics in neurodegenerative disease-relevant cell types

Stress granules (SGs) are cytoplasmic foci that form in response to various external stimuli and are essential to cell survival following stress. SGs are studied in several diseases, including ALS and FTD, which involve the degeneration of motor and cortical neurons, respectively, and are now realized to be linked pathogenically by TDP-43, originally discovered as a component of ubiquitin-positive aggregates within patients’ neurons and some glial cells. So far, studies to undercover the role of TDP-43 in SGs have used primarily transformed cell lines, and thus rely on the extrapolation of the mechanisms to cell types affected in ALS/FTD, potentially masking cell specific effects. Here, we investigate SG dynamics in primary motor and cortical neurons as well as astrocytes. Our data suggest a cell and stress specificity and demonstrate a requirement for TDP-43 for efficient SG dynamics. In addition, based on our in vitro approach, our data suggest that aging may be an important modifier of SG dynamics which could have relevance to the initiation and/or progression of age-related neurodegenerative diseases.

Life and Work of Stress Granules and Processing Bodies: New Insights into Their Formation and Function

The dynamic formation of stress granules (SGs), processing bodies (PBs), and related RNA organelles regulates diverse cellular processes, including the coordination of functionally connected messengers, the translational regulation at the synapse, and the control of viruses and retrotransposons. Recent studies have shown that pyruvate kinase and other enzymes localize in SGs and PBs, where they become protected from stress insults. These observations may have implications for enzyme regulation and metabolic control exerted by RNA-based organelles. The formation of these cellular bodies is governed by liquid-liquid phase separation (LLPS) processes, and it needs to be strictly controlled to prevent pathogenic aggregation. The intracellular concentration of key metabolites, such as ATP and sterol derivatives, may influence protein solubility, thus affecting the dynamics of liquid organelles. LLPS in vitro depends on the thermal diffusion of macromolecules, which is limited inside cells, where the condensation and dissolution of membrane-less organelles are helped by energy-driven processes. The active transport by the retrograde motor dynein helps SG assembly, whereas the anterograde motor kinesin mediates SG dissolution; a tug of war between these two molecular motors allows transient SG formation. There is evidence that the efficiency of dynein-mediated transport increases with the number of motor molecules associated with the cargo. The dynein-dependent transport may be influenced by cargo size as larger cargos can load a larger number of motors. We propose a model based on this emergent property of dynein motors, which would be collectively stronger during SG condensation and weaker during SG breakdown, thus allowing kinesin-mediated dispersion.

Interactions between the Translation Machinery and Microtubules

Microtubules are components of eukaryotic cytoskeleton that are involved in the transport of various components from the nucleus to the cell periphery and back. They also act as a platform for assembly of complex molecular ensembles. Ribonucleoprotein (RNP) complexes, such as ribosomes and mRNPs, are transported over significant distances (e.g. to neuronal processes) along microtubules. The association of RNPs with microtubules and their transport along these structures are essential for compartmentalization of protein biosynthesis in cells. Microtubules greatly facilitate assembly of stress RNP granules formed by accumulation of translation machinery components during cell stress response. Microtubules are necessary for the cytoplasm-to-nucleus transport of proteins, including ribosomal proteins. At the same time, ribosomal proteins and RNA-binding proteins can influence cell mobility and cytoplasm organization by regulating microtubule dynamics. The molecular mechanisms underlying the association between the translation machinery components and microtubules have not been studied systematically; the results of such studies are mostly fragmentary. In this review, we attempt to fill this gap by summarizing and discussing the data on protein and RNA components of the translation machinery that directly interact with microtubules or microtubule motor proteins.

Expanding Axonal Transcriptome Brings New Functions for Axonally Synthesized Proteins in Health and Disease

Intra-axonal protein synthesis has been shown to play critical roles in both development and repair of axons. Axons provide long-range connectivity in the nervous system, and disruption of their function and/or structure is seen in several neurological diseases and disorders. Axonally synthesized proteins or losses in axonally synthesized proteins contribute to neurodegenerative diseases, neuropathic pain, viral transport, and survival of axons. Increasing sensitivity of RNA detection and quantitation coupled with methods to isolate axons to purity has shown that a surprisingly complex transcriptome exists in axons. This extends across different species, neuronal populations, and physiological conditions. These studies have helped define the repertoire of neuronal mRNAs that can localize into axons and imply previously unrecognized functions for local translation in neurons. Here, we review the current state of transcriptomics studies of isolated axons, contrast axonal mRNA profiles between different neuronal types and growth states, and discuss how mRNA transport into and translation within axons contribute to neurological disorders.

Upstream Open Reading Frames Located in the Leader of Protein Kinase Mζ mRNA Regulate Its Translation

For protein synthesis that occurs locally in dendrites, the translational control mechanisms are much more important for neuronal functioning than the transcription levels. Here, we show that uORFs (upstream open reading frames) in the 5′ untranslated region (5′UTR) play a critical role in regulation of the translation of protein kinase Mζ (PKMζ). Elimination of these uORFs activates translation of the reporter protein in vitro and in primary cultures of rat hippocampal neurons. Using cell-free translation systems, we demonstrate that translational initiation complexes are formed only on uORFs. Further, we address the mechanism of translational repression of PKMζ translation, by uORFs. We observed an increase in translation of the reporter protein under the control of PKMζ leader in neuronal culture during non-specific activation by picrotoxin. We also show that such a mechanism is similar to the mechanism seen in cell stress, as application of sodium arsenite to neuron cultures induced translation of mRNA carrying PKMζ 5′UTR similarly to picrotoxin activation. Therefore, we suppose that phosphorylation of eIF2a, like in cell stress, is a main regulator of PKMζ translation. Altogether, our findings considerably extend our understanding of the role of uORF in regulation of PKMζ translation in activated neurons, important at early stages of LTP.

RNA-binding proteins in eye development and disease: implication of conserved RNA granule components

The molecular biology of metazoan eye development is an area of intense investigation. These efforts have led to the surprising recognition that although insect and vertebrate eyes have dramatically different structures, the orthologs or family members of several conserved transcription and signaling regulators such as Pax6, Six3, Prox1, and Bmp4 are commonly required for their development. In contrast, our understanding of posttranscriptional regulation in eye development and disease, particularly regarding the function of RNA-binding proteins (RBPs), is limited. We examine the present knowledge of RBPs in eye development in the insect model Drosophila as well as several vertebrate models such as fish, frog, chicken, and mouse. Interestingly, of the 42 RBPs that have been investigated for their expression or function in vertebrate eye development, 24 (~60%) are recognized in eukaryotic cells as components of RNA granules such as processing bodies, stress granules, or other specialized ribonucleoprotein (RNP) complexes. We discuss the distinct developmental and cellular events that may necessitate potential RBP/RNA granule-associated RNA regulon models to facilitate posttranscriptional control of gene expression in eye morphogenesis. In support of these hypotheses, three RBPs and RNP/RNA granule components Tdrd7, Caprin2, and Stau2 are linked to ocular developmental defects such as congenital cataract, Peters anomaly, and microphthalmia in human patients or animal models. We conclude by discussing the utility of interdisciplinary approaches such as the bioinformatics tool iSyTE (integrated Systems Tool for Eye gene discovery) to prioritize RBPs for deriving posttranscriptional regulatory networks in eye development and disease. WIREs RNA 2016, 7:527-557. doi: 10.1002/wrna.1355 For further resources related to this article, please visit the WIREs website.

Enterovirus Control of Translation and RNA Granule Stress Responses

Enteroviruses such as poliovirus (PV) and coxsackievirus B3 (CVB3) have evolved several parallel strategies to regulate cellular gene expression and stress responses to ensure efficient expression of the viral genome. Enteroviruses utilize their encoded proteinases to take over the cellular translation apparatus and direct ribosomes to viral mRNAs. In addition, viral proteinases are used to control and repress the two main types of cytoplasmic RNA granules, stress granules (SGs) and processing bodies (P-bodies, PBs), which are stress-responsive dynamic structures involved in repression of gene expression. This review discusses these processes and the current understanding of the underlying mechanisms with respect to enterovirus infections. In addition, the review discusses accumulating data suggesting linkage exists between RNA granule formation and innate immune sensing and activation.

Cytoplasmic dynein is required for the spatial organization of protein aggregates in filamentous fungi

Eukaryotes have evolved multiple strategies for maintaining cellular protein homeostasis. One such mechanism involves neutralization of deleterious protein aggregates via their defined spatial segregation. Here, using the molecular disaggregase Hsp104 as a marker for protein aggregation, we describe the spatial and temporal dynamics of protein aggregates in the filamentous fungus Aspergillus nidulans. Filamentous fungi, such as A. nidulans, are a diverse group of species of major health and economic importance and also serve as model systems for studying highly polarized eukaryotic cells. We find that microtubules promote the formation of Hsp104-positive aggregates, which coalesce into discrete subcellular structures in a process dependent on the microtubule-based motor cytoplasmic dynein. Finally, we find that impaired clearance of these inclusions negatively impacts retrograde trafficking of endosomes, a conventional dynein cargo, indicating that microtubule-based transport can be overwhelmed by chronic cellular stress.

Spatio-temporal Dynamics and Mechanisms of Stress Granule Assembly

Stress granules (SGs) are non-membranous cytoplasmic aggregates of mRNAs and related proteins, assembled in response to environmental stresses such as heat shock, hypoxia, endoplasmic reticulum (ER) stress, chemicals (e.g. arsenite), and viral infections. SGs are hypothesized as a loci of mRNA triage and/or maintenance of proper translation capacity ratio to the pool of mRNAs. In brain ischemia, hippocampal CA3 neurons, which are resilient to ischemia, assemble SGs. In contrast, CA1 neurons, which are vulnerable to ischemia, do not assemble SGs. These results suggest a critical role SG plays in regards to cell fate decisions. Thus SG assembly along with its dynamics should determine the cell fate. However, the process that exactly determines the SG assembly dynamics is largely unknown. In this paper, analyses of experimental data and computer simulations were used to approach this problem. SGs were assembled as a result of applying arsenite to HeLa cells. The number of SGs increased after a short latent period, reached a maximum, then decreased during the application of arsenite. At the same time, the size of SGs grew larger and became localized at the perinuclear region. A minimal mathematical model was constructed, and stochastic simulations were run to test the modeling. Since SGs are discrete entities as there are only several tens of them in a cell, commonly used deterministic simulations could not be employed. The stochastic simulations replicated observed dynamics of SG assembly. In addition, these stochastic simulations predicted a gamma distribution relative to the size of SGs. This same distribution was also found in our experimental data suggesting the existence of multiple fusion steps in the SG assembly. Furthermore, we found that the initial steps in the SG assembly process and microtubules were critical to the dynamics. Thus our experiments and stochastic simulations presented a possible mechanism regulating SG assembly.

Cells suffer from various environmental stresses such as heat shock and viral infection. In response to a stress, small non-membranous cytoplasmic aggregates, stress granules (SGs), are assembled. SGs contain mRNAs and related proteins. Hippocampal CA1 neurons located in the brain, which are vulnerable to ischemia, do not assemble SGs, while CA3 neurons, which are resilient to ischemia, assemble SGs. The dysfunction of SGs has been reported in human diseases including pathogenic viral infection. These observations led to a hypothesis that SGs play an important role in cell fate decisions, and the dynamics of SG assembly would regulate cell fate. However, the conditions that determine the number and distribution of SGs in a cell in response to a stress are largely unknown. We approached this problem by experiments and simulations. Our stochastic simulations replicated the observations. Furthermore, we found that initial steps in the SG assembly process were important to the dynamics of SG assembly, and that SG size resembled the gamma distribution both in simulations and experiments, suggesting the existence of multiple steps in the SG assembly process. To the best of our knowledge, this work was the first to show SG assembly in a whole cell by stochastic simulations.

Profilin 1 associates with stress granules and ALS-linked mutations alter stress granule dynamics

Mutations in the PFN1 gene encoding profilin 1 are a rare cause of familial amyotrophic lateral sclerosis (ALS). Profilin 1 is a well studied actin-binding protein but how PFN1 mutations cause ALS is unknown. The budding yeast, Saccharomyces cerevisiae, has one PFN1 ortholog. We expressed the ALS-linked profilin 1 mutant proteins in yeast, demonstrating a loss of protein stability and failure to restore growth to profilin mutant cells, without exhibiting gain-of-function toxicity. This model provides for simple and rapid screening of novel ALS-linked PFN1 variants. To gain insight into potential novel roles for profilin 1, we performed an unbiased, genome-wide synthetic lethal screen with yeast cells lacking profilin (pfy1Δ). Unexpectedly, deletion of several stress granule and processing body genes, including pbp1Δ, were found to be synthetic lethal with pfy1Δ. Mutations in ATXN2, the human ortholog of PBP1, are a known ALS genetic risk factor and ataxin 2 is a stress granule component in mammalian cells. Given this genetic interaction and recent evidence linking stress granule dynamics to ALS pathogenesis, we hypothesized that profilin 1 might also associate with stress granules. Here we report that profilin 1 and related protein profilin 2 are novel stress granule-associated proteins in mouse primary cortical neurons and in human cell lines and that ALS-linked mutations in profilin 1 alter stress granule dynamics, providing further evidence for the potential role of stress granules in ALS pathogenesis.

Osteopetrosis: genetics, treatment and new insights into osteoclast function

Osteopetrosis is a genetic condition of increased bone mass, which is caused by defects in osteoclast formation and function. Both autosomal recessive and autosomal dominant forms exist, but this Review focuses on autosomal recessive osteopetrosis (ARO), also known as malignant infantile osteopetrosis. The genetic basis of this disease is now largely uncovered: mutations in TCIRG1, CLCN7, OSTM1, SNX10 and PLEKHM1 lead to osteoclast-rich ARO (in which osteoclasts are abundant but have severely impaired resorptive function), whereas mutations in TNFSF11 and TNFRSF11A lead to osteoclast-poor ARO. In osteoclast-rich ARO, impaired endosomal and lysosomal vesicle trafficking results in defective osteoclast ruffled-border formation and, hence, the inability to resorb bone and mineralized cartilage. ARO presents soon after birth and can be fatal if left untreated. However, the disease is heterogeneous in clinical presentation and often misdiagnosed. This article describes the genetics of ARO and discusses the diagnostic role of next-generation sequencing methods. The management of affected patients, including guidelines for the indication of haematopoietic stem cell transplantation (which can provide a cure for many types of ARO), are outlined. Finally, novel treatments, including preclinical data on in utero stem cell treatment, RANKL replacement therapy and denosumab therapy for hypercalcaemia are also discussed.

Poliovirus infection induces the co-localization of cellular protein SRp20 with TIA-1, a cytoplasmic stress granule protein

Different types of environmental stress cause mammalian cells to form cytoplasmic foci, termed stress granules, which contain mRNPs that are translationally silenced. These foci are transient and dynamic, and contain components of the cellular translation machinery as well as certain mRNAs and RNA binding proteins. Stress granules are known to be induced by conditions such as hypoxia, nutrient deprivation, and oxidative stress, and a number of cellular factors have been identified that are commonly associated with these foci. More recently it was discovered that poliovirus infection also induces the formation of stress granules, although these cytoplasmic foci appear to be somewhat compositionally unique. Work described here examined the punctate pattern of SRp20 (a host cell mRNA splicing protein) localization in the cytoplasm of poliovirus-infected cells, demonstrating the partial co-localization of SRp20 with the stress granule marker protein TIA-1. We determined that SRp20 does not co-localize with TIA-1, however, under conditions of oxidative stress, indicating that the close association of these two proteins during poliovirus infection is not representative of a general response to cellular stress. We confirmed that the expression of a dominant negative version of TIA-1 (TIA-1-PRD) results in the dissociation of stress granules. Finally, we demonstrated that expression of wild type TIA-1 or dominant negative TIA-1-PRD in cells during poliovirus infection does not dramatically affect viral translation. Taken together, these studies provide a new example of the unique cytoplasmic foci that form during poliovirus infection.

A novel phenotype for the dynein heavy chain mutation Loa: altered dendritic morphology, organelle density, and reduced numbers of trigeminal motoneurons

Dynein, the retrograde motor protein, is essential for the transport of cargo along axons and proximal dendrites in neurons. The dynein heavy chain mutation Loa has been reported to cause degeneration of spinal motor neurons, as well as defects of spinal sensory proprioceptive neurons, but cranial nerve nuclei have received little attention. Here, we examined the number and morphology of neurons in cranial nerve nuclei of young, adult, and aged heterozygous Loa mice, with a focus on the trigeminal, facial, and trochlear motor nuclei, as well as the proprioceptive mesencephalic trigeminal nucleus. By using stereological counting techniques, we report a slowly progressive and significant reduction, to 75% of wild-type controls, in the number of large trigeminal motoneurons, whereas normal numbers were found for sensory mesencephalic trigeminal, facial, and trochlear motoneurons. The morphology of many surviving large trigeminal motoneurons was substantially altered, in particular the size and length of perpendicularly extending primary dendrites, but not those of facial or trochlear motoneurons. At the ultrastructural level, proximal dendrites of large trigeminal motoneurons, but not other neurons, were significantly depleted in organelle content such as polyribosomes and showed abnormal (vesiculated) mitochondria. These data indicate primary defects in trigeminal α-motoneurons more than γ-motoneurons. Our findings expand the Loa heterozygote phenotype in two important ways: we reveal dendritic in addition to axonal defects or abnormalities, and we identify the Loa mutation as a mouse model for mixed motor-sensory loss when the entire neuraxis is considered, rather than a model primarily for sensory loss.

Regulated protein aggregation: stress granules and neurodegeneration

The protein aggregation that occurs in neurodegenerative diseases is classically thought to occur as an undesirable, nonfunctional byproduct of protein misfolding. This model contrasts with the biology of RNA binding proteins, many of which are linked to neurodegenerative diseases. RNA binding proteins use protein aggregation as part of a normal regulated, physiological mechanism controlling protein synthesis. The process of regulated protein aggregation is most evident in formation of stress granules. Stress granules assemble when RNA binding proteins aggregate through their glycine rich domains. Stress granules function to sequester, silence and/or degrade RNA transcripts as part of a mechanism that adapts patterns of local RNA translation to facilitate the stress response. Aggregation of RNA binding proteins is reversible and is tightly regulated through pathways, such as phosphorylation of elongation initiation factor 2α. Microtubule associated protein tau also appears to regulate stress granule formation. Conversely, stress granule formation stimulates pathological changes associated with tau. In this review, I propose that the aggregation of many pathological, intracellular proteins, including TDP-43, FUS or tau, proceeds through the stress granule pathway. Mutations in genes coding for stress granule associated proteins or prolonged physiological stress, lead to enhanced stress granule formation, which accelerates the pathophysiology of protein aggregation in neurodegenerative diseases. Over-active stress granule formation could act to sequester functional RNA binding proteins and/or interfere with mRNA transport and translation, each of which might potentiate neurodegeneration. The reversibility of the stress granule pathway also offers novel opportunities to stimulate endogenous biochemical pathways to disaggregate these pathological stress granules, and perhaps delay the progression of disease.

Endogenous TDP-43, but not FUS, contributes to stress granule assembly via G3BP

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the selective loss of upper and lower motor neurons, a cell type that is intrinsically more vulnerable than other cell types to exogenous stress. The interplay between genetic susceptibility and environmental exposures to toxins has long been thought to be relevant to ALS. One cellular mechanism to overcome stress is the formation of small dense cytoplasmic domains called stress granules (SG) which contain translationally arrested mRNAs. TDP-43 (encoded by TARDBP) is an ALS-causative gene that we have previously implicated in the regulation of the core stress granule proteins G3BP and TIA-1. TIA-1 and G3BP localize to SG under nearly all stress conditions and are considered essential to SG formation. Here, we report that TDP-43 is required for proper SG dynamics, especially SG assembly as marked by the secondary aggregation of TIA-1. We also show that SG assembly, but not initiation, requires G3BP. Furthermore, G3BP can rescue defective SG assembly in cells depleted of endogenous TDP-43. We also demonstrate that endogenous TDP-43 and FUS do not have overlapping functions in this cellular process as SG initiation and assembly occur normally in the absence of FUS. Lastly, we observe that SG assembly is a contributing factor in the survival of neuronal-like cells responding to acute oxidative stress. These data raise the possibility that disruptions of normal stress granule dynamics by loss of nuclear TDP-43 function may contribute to neuronal vulnerability in ALS.

Contrasting pathology of the stress granule proteins TIA-1 and G3BP in tauopathies

Stress induces aggregation of RNA-binding proteins to form inclusions, termed stress granules (SGs). Recent evidence suggests that SG proteins also colocalize with neuropathological structures, but whether this occurs in Alzheimer's disease is unknown. We examined the relationship between SG proteins and neuropathology in brain tissue from P301L Tau transgenic mice, as well as in cases of Alzheimer's disease and FTDP-17. The pattern of SG pathology differs dramatically based on the RNA-binding protein examined. SGs positive for T-cell intracellular antigen-1 (TIA-1) or tristetraprolin (TTP) initially do not colocalize with tau pathology, but then merge with tau inclusions as disease severity increases. In contrast, G3BP (ras GAP-binding protein) identifies a novel type of molecular pathology that shows increasing accumulation in neurons with increasing disease severity, but often is not associated with classic markers of tau pathology. TIA-1 and TTP both bind phospho-tau, and TIA-1 overexpression induces formation of inclusions containing phospho-tau. These data suggest that SG formation might stimulate tau pathophysiology. Thus, study of RNA-binding proteins and SG biology highlights novel pathways interacting with the pathophysiology of AD, providing potentially new avenues for identifying diseased neurons and potentially novel mechanisms regulating tau biology.

Regulation of stress granules in virus systems

Virus infection initiates a number of cellular stress responses that modulate gene regulation and compartmentalization of RNA. Viruses must control host gene expression and the localization of viral RNAs to be successful parasites. RNA granules such as stress granules and processing bodies (PBs) contain translationally silenced messenger ribonucleoproteins (mRNPs) and serve as extensions of translation regulation in cells, storing transiently repressed mRNAs. New reports show a growing number of virus families modulate RNA granule function to maximize replication efficiency. This review summarizes recent advances in understanding the relationship between viruses and mRNA stress granules in animal cells and will discuss important questions that remain in this emerging field.

TDP-43 aggregation in neurodegeneration: are stress granules the key?

The RNA-binding protein TDP-43 is strongly linked to neurodegeneration. Not only are mutations in the gene encoding TDP-43 associated with ALS and FTLD, but this protein is also a major constituent of pathological intracellular inclusions in these diseases. Recent studies have significantly expanded our understanding of TDP-43 physiology. TDP-43 is now known to play important roles in neuronal RNA metabolism. It binds to and regulates the splicing and stability of numerous RNAs encoding proteins involved in neuronal development, synaptic function and neurodegeneration. Thus, a loss of these essential functions is an attractive hypothesis regarding the role of TDP-43 in neurodegeneration. Moreover, TDP-43 is an aggregation-prone protein and, given the role of toxic protein aggregates in neurodegeneration, a toxic gain-of-function mechanism is another rational hypothesis. Importantly, ALS related mutations modulate the propensity of TDP-43 to aggregate in cell culture. Several recent studies have documented that cytoplasmic TDP-43 aggregates co-localize with stress granule markers. Stress granules are cytoplasmic inclusions that repress translation of a subset of RNAs in times of cellular stress, and several proteins implicated in neurodegeneration (i.e. Ataxin-2 and SMN) interact with stress granules. Thus, understanding the interplay between TDP-43 aggregation, stress granules and the effect of ALS-associated TDP-43 mutations may be the key to understanding the role of TDP-43 in neurodegeneration. We propose two models of TDP-43 aggregate formation. The "independent model" stipulates that TDP-43 aggregation is independent of stress granule formation, in contrast to the "precursor model" which presents the idea that stress granule formation contributes to a TDP-43 aggregate "seed" and that chronic stress leads to concentration-dependent TDP-43 aggregation. This article is part of a Special Issue entitled: RNA-Binding Proteins.

Poliovirus unlinks TIA1 aggregation and mRNA stress granule formation

In response to environmental stress and viral infection, mammalian cells form foci containing translationally silenced mRNPs termed stress granules (SGs). As aggregates of stalled initiation complexes, SGs are defined by the presence of translation initiation machinery in addition to mRNA binding proteins. Here, we report that cells infected with poliovirus (PV) can form SGs early that contain T-cell-restricted intracellular antigen 1 (TIA1), translation initiation factors, RNA binding proteins, and mRNA. However, this response is blocked as infection progresses, and a type of pseudo-stress granule remains at late times postinfection and contains TIA but lacks translation initiation factors, mRNA binding proteins, and most polyadenylated mRNA. This result was observed using multiple stressors, including viral infection, oxidative stress, heat shock, and endoplasmic reticulum stress. Multiple proteins required for efficient viral internal ribosome entry site-dependent translation are localized to SGs under stress conditions, providing a potential rationale for the evolution and maintenance of the SG inhibition phenotype. Further, the expression of a noncleavable form of the RasGAP-SH3 domain binding protein in PV-infected cells enables SGs whose constituents are consistent with the presence of stalled 48S translation preinitiation complexes to persist throughout infection. These results indicate that in poliovirus-infected cells, the functions of TIA self-aggregation and aggregation of stalled translation initiation complexes into stress granules are severed, leading to novel foci that contain TIA1 but lack other stress granule-defining components.

The role of molecular microtubule motors and the microtubule cytoskeleton in stress granule dynamics

Stress granules (SGs) are cytoplasmic foci that appear in cells exposed to stress-induced translational inhibition. SGs function as a triage center, where mRNAs are sorted for storage, degradation, and translation reinitiation. The underlying mechanisms of SGs dynamics are still being characterized, although many key players have been identified. The main components of SGs are stalled 48S preinitiation complexes. To date, many other proteins have also been found to localize in SGs and are hypothesized to function in SG dynamics. Most recently, the microtubule cytoskeleton and associated motor proteins have been demonstrated to function in SG dynamics. In this paper, we will discuss current literature examining the function of microtubules and the molecular microtubule motors in SG assembly, coalescence, movement, composition, organization, and disassembly.

RNA granules: the good, the bad and the ugly

Processing bodies (PBs) and Stress Granules (SGs) are the founding members of a new class of RNA granules, known as mRNA silencing foci, as they harbour transcripts circumstantially excluded from the translationally active pool. PBs and SGs are able to release mRNAs thus allowing their translation. PBs are constitutive, but respond to stimuli that affect mRNA translation and decay, whereas SGs are specifically induced upon cellular stress, which triggers a global translational silencing by several pathways, including phosphorylation of the key translation initiation factor eIF2alpha, and tRNA cleavage among others. PBs and SGs with different compositions may coexist in a single cell. These macromolecular aggregates are highly conserved through evolution, from unicellular organisms to vertebrate neurons. Their dynamics is regulated by several signaling pathways, and depends on microfilaments and microtubules, and the cognate molecular motors myosin, dynein, and kinesin. SGs share features with aggresomes and related aggregates of unfolded proteins frequently present in neurodegenerative diseases, and may play a role in the pathology. Virus infections may induce or impair SG formation. Besides being important for mRNA regulation upon stress, SGs modulate the signaling balancing apoptosis and cell survival. Finally, the formation of Nuclear Stress Bodies (nSBs), which share components with SGs, and the assembly of additional cytosolic aggregates containing RNA -the UV granules and the Ire1 foci-, all of them induced by specific cell damage factors, contribute to cell survival.

The yeast ubiquitin-like domain protein Mdy2 is required for microtubule-directed nuclear migration and localizes to cytoplasmic granules in response to heat stress

MDY2 encodes a ubiquitin-like (UBL)-domain protein necessary for efficient mating in Saccharomyces cerevisiae. Unlike most UBL proteins, Mdy2 is apparently not subject to C-terminal processing and is localized predominantly in the nucleus. Deletion of MDY2 is associated with a five- to seven-fold reduction in mating efficiency, mainly due to defects in nuclear migration and karyogamy at the prezygotic stage. Here, we looked for two potential interacting partners of Mdy2, investigated the function of Mdy2 in nuclear movement, determined the increased heat sensitivity defects of mdy2Δ mutants, and inspected localization of Mdy2. Coprecipitation studies show that Mdy2 associates with α-tubulin and with the microtubule (MT)-associated dynactin subunit p150(Glued)/Nip100. nip100Δ mutants exhibit no defects in nuclear migration or in MT length or orientation during shmooing growth. Deletion of MDY2 display small nuclear migration phenotype during vegetative growth and seems to exacerbate the defects in mitotic nuclear migration seen in the nip100Δ strain. Deletion of MDY2 increased heat sensitivity of the cells and these strains accumulate mitotic nuclear migration defects and shortened MTs under these conditions. GFP-Mdy2 proteins which are localized predominantly in the nucleus at permissive temperature are localized to cytoplasmic foci during heat shock. Colocalization studies revealed that heat stress-induced enrichment of Mdy2 in cytoplasmic foci merged mainly with stress granules marker Pab1. During glucose deprivation a minority of Mdy2 foci overlapped with P-bodies marker Dcp2, while most Mdy2 foci and Pab1 foci overlap. Accordingly, we propose that Mdy2 plays a critical role in the MT-dependent processes of karyogamy and stress response.

The RNA superhighway: axonal RNA trafficking of kappa opioid receptor mRNA for neurite growth

Neurons are highly polarized cells with extensive sub-cellular compartmentalization to accommodate diverse local needs. Information flows between the pre-synaptic (axon) and post-synaptic (dendrite) compartments, as well as between the soma and the nerve termini. It is critical that a neuron controls efficient molecular transfer/transport through its axon. But this is extremely challenging to study because of the long distance molecules must travel through axons and the apparent contextual difference in the axons' various local environments, which should not be examined in isolation. Understanding the action in neurons of drug-responsive neurotransmitter receptors such as opioid receptors has been hindered by the lack of information on the control of molecular flow between such various sub-cellular neuron compartments. Recent studies have uncovered new transport systems other than the classical vesicle transport in neurons, particularly those utilizing various granules containing certain RNAs including protein-coding mRNAs. Through integrated approaches exploiting various experimental systems, tools, and methodologies, studies have provided solid evidence for functional roles of specific RNA granules in several biological processes crucial for the survival and function of neurons. These include neurons' transport of molecules/information, stress response, and local axonal translation. By using the kappa opioid receptor (KOR) as a model, studies have also revealed a novel physiological function of KOR in mediating growth factor-stimulated neurite outgrowth during a critical period of development, which requires specific KOR mRNA untranslated sequences that direct spatially and temporally specific expression of KOR.

Subcellular communication through RNA transport and localized protein synthesis

Interest in the mechanisms of subcellular localization of mRNAs and the effects of localized translation has increased over the last decade. Polarized eukaryotic cells transport mRNA-protein complexes to subcellular sites, where translation of the mRNAs can be regulated by physiological stimuli. The long distances separating distal neuronal processes from their cell body have made neurons a useful model system for dissecting mechanisms of mRNA trafficking. Both the dendritic and axonal processes of neurons have been shown to have protein synthetic capacity and the diversity of mRNAs discovered in these processes continues to increase. Localized translation of mRNAs requires a co-ordinated effort by the cell body to target both mRNAs and necessary translational machinery into distal sites, as well as temporal control of individual mRNA translation. In addition to altering protein composition locally at the site of translation, some of the proteins generated in injured nerves retrogradely signal to the cell body, providing both temporal and spatial information on events occurring at distant subcellular sites.

The role of RNA processing in the pathogenesis of motor neuron degeneration

Motor neurons are large, highly polarised cells with very long axons and a requirement for precise spatial and temporal gene expression. Neurodegenerative disorders characterised by selective motor neuron vulnerability include various forms of amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). A rapid expansion in knowledge on the pathophysiology of motor neuron degeneration has occurred in recent years, largely through the identification of genes leading to familial forms of ALS and SMA. The major emerging theme is that motor neuron degeneration can result from mutation in genes that encode factors important for ribonucleoprotein biogenesis and RNA processing, including splicing regulation, transcript stabilisation, translational repression and localisation of mRNA. Complete understanding of how these pathways interact and elucidation of specialised mechanisms for mRNA targeting and processing in motor neurons are likely to produce new targets for therapy in ALS and related disorders.

Identification of importin alpha1 as a novel constituent of RNA stress granules

Importin alpha is a nuclear transport receptor well established for its ability to mediate importin beta-mediated nuclear import of proteins that possess classical nuclear localization signal (cNLS). Previously, we reported that importin alpha rapidly accumulates to the nucleus in response to H2O2-induced oxidative stress, which implies a role for this protein in stress response. In this study, we show that importin alpha1 (also known as KPNA2 or Rch1), a major subtype of the importin alpha family, localizes to RNA stress granules (SGs), large cytoplasmic bodies that are thought to function as RNA triage sites during stress response. The recruitment of importin alpha1 to SGs was compatible with its nuclear accumulation during heat shock. Depletion of endogenous importin alpha1 using siRNA showed that importin alpha1 regulates the dynamics of SG assembly, and that it promotes cell survival in arsenite-treated cells. These data revealed, for the first time, the involvement of importin alpha in the assembly of RNA granules and its pro-survival role during stress response.

Dual action of epidermal growth factor: extracellular signal-stimulated nuclear-cytoplasmic export and coordinated translation of selected messenger RNA

We report the first example of a coordinated dual action of epidermal growth factor (EGF) in stimulating the nuclear-cytoplasmic export and translation of a select messenger RNA (mRNA). The effect of EGF is mediated by the RNA-binding protein Grb7 (growth factor receptor-bound protein 7), which serves as an adaptor for a specific mRNA-protein export complex and a translational regulator. Using the kappa-opioid receptor (OR [KOR]) as a model, we demonstrate that EGF activates nuclear SHP-2 (Src homology region 2-containing tyrosine phosphatase), which dephosphorylates Grb7 in the nucleus. Hypophosphorylated Grb7 binds to the KOR mRNA and recruits the Hu antigen R-exportin-1 (CRM1) complex to form a nuclear-cytoplasmic export complex that exports KOR mRNA. EGF also activates focal adhesion kinase in the cytoplasm to rephosphorylate Grb7, releasing KOR mRNA for active translation. In summary, this study uncovers a coordinated, dual activity of EGF in facilitating nuclear export of a specific mRNA-protein complex as well as translational activation of the exported mRNA.

Eukaryotic stress granules: the ins and outs of translation

The stress response in eukaryotic cells often inhibits translation initiation and leads to the formation of cytoplasmic RNA-protein complexes referred to as stress granules. Stress granules contain nontranslating mRNAs, translation initiation components, and many additional proteins affecting mRNA function. Stress granules have been proposed to affect mRNA translation and stability and have been linked to apoptosis and nuclear processes. Stress granules also interact with P-bodies, another cytoplasmic RNP granule containing nontranslating mRNA, translation repressors, and some mRNA degradation machinery. Together, stress granules and P-bodies reveal a dynamic cycle of distinct biochemical and compartmentalized mRNPs in the cytosol, with implications for the control of mRNA function.

Kappa opioid receptor contributes to EGF-stimulated neurite extension in development

Epidermal growth factor (EGF), a mitogen, also stimulates neurite extension during development, but the underlying mechanism is elusive. This study reveals a functional role for kappa opioid receptor (KOR) in EGF-stimulated neurite extension, and the underlying mechanism. EGF and activated EGF receptor (EGFR) levels are elevated in embryonic spinal cords during late gestation stages, with concurrent rise in protein levels of KOR and axon extension markers, growth-associated protein 43 (GAP43), and transient axonal glycoprotein-1 (TAG-1). Both GAP43 and TAG-1 levels are significantly lower in KOR-null (KOR(-/-)) spinal cords, and EGFR inhibitors effectively reduce the levels of KOR, GAP43, and TAG-1 in wild-type embryonic spinal cords. For KOR(-/-) or KOR-knockdown dorsal root ganglion (DRG) neurons, EGF can no longer effectively stimulate axon extension, which can be rescued by introducing a constitutive KOR expressing vector but not by a regulated KOR vector carrying its 5' untranslated region, which can be bound and repressed by growth factor receptor-bound protein 7 (Grb7). Furthermore, blocking KOR activation by application of anti-dynorphin, KOR antagonist, or EGFR inhibitor effectively reduces axon extension of DRG neurons. Thus, EGF-stimulated axon extension during development is mediated, at least partially, by specific elevation of KOR protein production at posttranscriptional level, as well as activation of KOR signaling. The result also reveals an action of EGF to augment posttranscriptional regulation of certain mRNAs during developmental stages.

The hsp90-FKBP52 complex links the mineralocorticoid receptor to motor proteins and persists bound to the receptor in early nuclear events

In this study, we demonstrate that the subcellular localization of the mineralocorticoid receptor (MR) is regulated by tetratricopeptide domain (TPR) proteins. The high-molecular-weight immunophilin (IMM) FKBP52 links the MR-hsp90 complex to dynein/dynactin motors favoring the cytoplasmic transport of MR to the nucleus. Replacement of this hsp90-binding IMM by FKBP51 or the TPR peptide favored the cytoplasmic localization of MR. The complete movement machinery, including dynein and tubulin, could be recovered from paclitaxel/GTP-stabilized cytosol and was fully reassembled on stripped MR immune pellets. The whole MR-hsp90-based heterocomplex was transiently recovered in the soluble fraction of the nucleus after 10 min of incubation with aldosterone. Moreover, cross-linked MR-hsp90 heterocomplexes accumulated in the nucleus in a hormone-dependent manner, demonstrating that the heterocomplex can pass undissociated through the nuclear pore. On the other hand, a peptide that comprises the DNA-binding domain of MR impaired the nuclear export of MR, suggesting the involvement of this domain in the process. This study represents the first report describing the entire molecular system that commands MR nucleocytoplasmic trafficking and proposes that the MR-hsp90-TPR protein heterocomplex is dissociated in the nucleus rather than in the cytoplasm.

Microtubules govern stress granule mobility and dynamics

Stress granules (SGs) are ribonucleoprotein (RNP)-containing assemblies that are formed in the cytoplasm in response to stress. Previously, we demonstrated that microtubule depolymerization inhibited SG formation. Here, we show that arsenate-induced SGs move throughout the cytoplasm in a microtubule-dependent manner, and microtubules are required for SG disassembly, but not for SG persistence. Analysis of SG movement revealed that SGs exhibited obstructed diffusion on an average, though sometimes SGs demonstrated rapid displacements. Microtubule depolymerization did not influence preformed SG number and size, but significantly reduced the average velocity of SG movement, the frequency of quick movement events, and the apparent diffusion coefficient of SGs. Actin filament disruption had no effect on the SG motility. In cycloheximide-treated cells SGs dissociated into constituent parts that then dissolved within the cytoplasm. Microtubule depolymerization inhibited cycloheximide-induced SG disassembly. However, microtubule depolymerization did not influence the dynamics of poly(A)-binding protein (PABP) in SGs, according to FRAP results. We suggest that the increase of SG size is facilitated by the transport of smaller SGs along microtubules with subsequent fusion of them. At least some protein components of SGs can exchange with the cytoplasmic pool independently of microtubules.

RhoA/ROCK1 signaling regulates stress granule formation and apoptosis

Cells form stress granules (SGs), in response to unfavorable environments, to avoid apoptosis, but it is unclear whether and how SG formation and cellular apoptosis are coordinately regulated. In this study we detected the small GTPase, Ras homolog gene family member A (RhoA), and its downstream kinase, Rho-associated, coiled-coil containing protein kinase 1 (ROCK1), in SG, and found that their stress-induced activities were important for SG formation and subsequent global translational repression. Importantly, only activated RhoA and ROCK1 were sequestered into SG. Sequestration of activated ROCK1 into SG prevented ROCK1 from interacting with JNK-interacting protein 3 (JIP-3) and its activation of c-Jun N-terminal kinase (JNK), a pathway triggering apoptosis, thereby protecting cells from apoptosis. This study identifies a specific signaling pathway, mediated by RhoA and ROCK1, which determines cell fate by promoting SG formation or initiating apoptosis during stress.

Regulation of translation by stress granules and processing bodies

Stress necessitates rapid reprogramming of translation in order to facilitate an adaptive response and promote survival. Cytoplasmic stress granules (SGs) and processing bodies (PBs) are dynamic structures that form in response to stress-induced translational arrest. PBs are linked to mRNA silencing and decay, while SGs are more closely linked to translation and the sorting of specific mRNAs for different fates. While they share some components and can interact physically, SGs and PBs are regulated independently, house separate functions, and contain unique markers. SG formation is associated with numerous disease states, and the expanding list of SG-associated proteins integrates SG formation with other processes such as transcription, splicing, and survival. Growing evidence suggests that SG assembly is initiated by translational arrest, and mediates cross talk with many other signaling pathways.

Role of microtubules in stress granule assembly: microtubule dynamical instability favors the formation of micrometric stress granules in cells

Following exposure to various stresses (arsenite, UV, hyperthermia, and hypoxia), mRNAs are assembled into large cytoplasmic bodies known as "stress granules," in which mRNAs and associated proteins may be processed by specific enzymes for different purposes like transient storing, sorting, silencing, or other still unknown processes. To limit mRNA damage during stress, the assembly of micrometric granules has to be rapid, and, indeed, it takes only approximately 10-20 min in living cells. However, such a rapid assembly breaks the rules of hindered diffusion in the cytoplasm, which states that large cytoplasmic bodies are almost immobile. In the present work, using HeLa cells and YB-1 protein as a stress granule marker, we studied three hypotheses to understand how cells overcome the limitation of hindered diffusion: shuttling of small messenger ribonucleoprotein particles from small to large stress granules, sliding of messenger ribonucleoprotein particles along microtubules, microtubule-mediated stirring of large stress granules. Our data favor the two last hypotheses and underline that microtubule dynamic instability favors the formation of micrometric stress granules.

Dynein and kinesin regulate stress-granule and P-body dynamics

Stress granules (SGs) and P-bodies (PBs) are related cytoplasmic structures harboring silenced mRNAs. SGs assemble transiently upon cellular stress, whereas PBs are constitutive and are further induced by stress. Both foci are highly dynamic, with messenger ribonucleoproteins (mRNPs) and proteins rapidly shuttling in and out. Here, we show that impairment of retrograde transport by knockdown of mammalian dynein heavy chain 1 (DHC1) or bicaudal D1 (BicD1) inhibits SG formation and PB growth upon stress, without affecting protein-synthesis blockage. Conversely, impairment of anterograde transport by knockdown of kinesin-1 heavy chain (KIF5B) or kinesin light chain 1 (KLC1) delayed SG dissolution. Strikingly, SG dissolution is not required to restore translation. Simultaneous knockdown of dynein and kinesin reverted the effect of single knockdowns on both SGs and PBs, suggesting that a balance between opposing movements driven by these molecular motors governs foci formation and dissolution. Finally, we found that regulation of SG dynamics by dynein and kinesin is conserved in Drosophila.

TDP-43 is recruited to stress granules in conditions of oxidative insult

Transactive response DNA-binding protein 43 (TDP-43) forms abnormal ubiquitinated and phosphorylated inclusions in brain tissues from patients with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration. TDP-43 is a DNA/RNA-binding protein involved in RNA processing, such as transcription, pre-mRNA splicing, mRNA stabilization and transport to dendrites. We found that in response to oxidative stress and to environmental insults of different types TDP-43 is capable to assemble into stress granules (SGs), ribonucleoprotein complexes where protein synthesis is temporarily arrested. We demonstrated that a specific aminoacidic interval (216-315) in the C-terminal region and the RNA-recognition motif 1 domain are both implicated in TDP-43 participation in SGs as their deletion prevented the recruitment of TDP-43 into SGs. Our data show that TDP-43 is a specific component of SGs and not of processing bodies, although we proved that TDP-43 is not necessary for SG formation, and its gene silencing does not impair cell survival during stress. The analysis of spinal cord tissue from ALS patients showed that SG markers are not entrapped in TDP-43 pathological inclusions. Although SGs were not evident in ALS brains, we speculate that an altered control of mRNA translation in stressful conditions may trigger motor neuron degeneration at early stages of the disease.