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A cytogenetic feature of human centromeres is a long array of repetitive α-satellite DNA at the primary constriction of mitotic chromosomes. A combination of the centromere chromosome orientation FISH (Cen-CO-FISH) technique together with SIM was used to visualize centromere organization within a single chromatid. Collectively, the data indicate a structural organization for α-satellite repeats in a ring-like conformation, with CENP-A preferentially distributed on the lateral surface of the primary constriction, and showcase superresolution imaging methods that can be used for further understanding of centromere structures.

We developed a physics-informed deep neural network architecture able to achieve signal-to-noise ratio improvements starting from low-exposure noisy data. Our model is based on the nature of the photon detection process characterized by a Poisson probability distribution, an information which we included in the training loss function. Our approach surpasses previous algorithm performance for microscopy images; moreover, the generality of the physical concepts employed here, makes it readily exportable to any imaging context.

Recognition at the plasma membrane of danger signals (elicitors) belonging to the classes of the microbe/pathogen- and damage-associated molecular patterns is a key event in pathogen sensing by plants and is associated with a rapid activation of immune responses. Different cellular compartments, including plasma membrane, chloroplasts, nuclei and mitochondria, are involved in the immune cellular program. However, how pathogen sensing is transmitted throughout the cell remains largely to be uncovered. Arabidopsis NPK1-related Proteins (ANPs) are mitogen-activated protein kinase kinase kinases previously shown to have a role in immunity. In this paper, we studied the in vivo intracellular dynamics of ANP1- and ANP3-GFP fusions and found that under basal physiological conditions both proteins are present in the cytosol, while ANP3 is also localized in mitochondria. After elicitor perception, both proteins are present also in the plastids and nuclei, revealing a localization pattern that is so far unique. The N-terminal region of the protein kinases is responsible for their localization in mitochondria and plastids. Moreover, we found that the localization of ANPs coincides with the sites of elicitor-induced ROS accumulation and that plants lacking ANP function do not accumulate intracellular ROS. This article is protected by copyright. All rights reserved.

Microglia cells are active players in regulating synaptic development and plasticity in the brain. However, how they influence the normal functioning of synapses is largely unknown. In this study, we characterized the effects of pharmacological microglia depletion, achieved by administration of PLX5622, on hippocampal CA3-CA1 synapses of adult wild type mice. Following microglial depletion, we observed a reduction of spontaneous and evoked glutamatergic activity associated with a decrease of dendritic spine density. We also observed the appearance of immature synaptic features and higher levels of plasticity. Microglia depleted mice showed a deficit in the acquisition of the Novel Object Recognition task. These events were accompanied by hippocampal astrogliosis, although in the absence ofneuroinflammatory condition. PLX-induced synaptic changes were absent in Cx3cr1-/- mice, highlighting the role of CX3CL1/CX3CR1 axis in microglia control of synaptic functioning. Remarkably, microglia repopulation after PLX5622 withdrawal was associated with the recovery of hippocampal synapses and learning functions. Altogether, these data demonstrate that microglia contribute to normal synaptic functioning in the adult brain and that their removal induces reversible changes in organization and activity of glutamatergic synapses.

Mutations in the RNA-binding protein (RBP) FUS have been genetically associated with the motoneuron disease amyotrophic lateral sclerosis (ALS). Using both human induced pluripotent stem cells and mouse models, we found that FUS-ALS causative mutations affect the activity of two relevant RBPs with important roles in neuronal RNA metabolism: HuD/ELAVL4 and FMRP. Mechanistically, mutant FUS leads to upregulation of HuD protein levels through competition with FMRP for HuD mRNA 3’UTR binding. In turn, increased HuD levels overly stabilize the transcript levels of its targets, NRN1 and GAP43. As a consequence, mutant FUS motoneurons show increased axon branching and growth upon injury, which could be rescued by dampening NRN1 levels. Since similar phenotypes have been previously described in SOD1 and TDP-43 mutant models, increased axonal growth and branching might represent broad early events in the pathogenesis of ALS., Maria Giovanna Garone et al. use iPSC and mouse models to evaluate a mechanistic link between aberrant axonal phenotypes in ALS and the alteration of a cross-regulatory circuitry involving three RNA binding proteins: FUS, HuD and FMRP. Their results suggest NRN1 as a potential therapeutic target for ALS and provide further insight into the pathogenesis of this critical disorder.

Summary Amyotrophic lateral sclerosis (ALS) has been genetically linked to mutations in RNA-binding proteins (RBPs), including FUS. Here, we report the RNA interactome of wild-type and mutant FUS in human motor neurons (MNs). This analysis identified a number of RNA targets. Whereas the wild-type protein preferentially binds introns, the ALS mutation causes a shift toward 3′ UTRs. Neural ELAV-like RBPs are among mutant FUS targets. As a result, ELAVL4 protein levels are increased in mutant MNs. ELAVL4 and mutant FUS interact and co-localize in cytoplasmic speckles with altered biomechanical properties. Upon oxidative stress, ELAVL4 and mutant FUS are engaged in stress granules. In the spinal cord of FUS ALS patients, ELAVL4 represents a neural-specific component of FUS-positive cytoplasmic aggregates, whereas in sporadic patients it co-localizes with phosphorylated TDP-43-positive inclusions. We propose that pathological mutations in FUS trigger an aberrant crosstalk with ELAVL4 with implications for ALS., Graphical Abstract, Highlights • We report the RNA interactome of wild-type and mutant FUS in human motor neurons • Mutant FUS binds the mRNA 3′ UTR of other RNA-binding proteins, including ELAVL4 • ELAVL4, expressed at increased levels, interacts with mutant FUS in the cytoplasm • ELAVL4 proteinopathy occurs in both FUS ALS and in sporadic ALS patients, De Santis et al. show that the mutant RNA-binding protein FUS, linked to amyotrophic lateral sclerosis (ALS), targets other RNA-binding proteins, such as ELAVL4, in human motor neurons. This triggers aberrant crosstalk between mutant FUS and ELAVL4, which is found in pathological inclusions of ALS patients’ motor neurons.

Fragile X syndrome (FXS) is a neurodevelopmental disorder, characterized by intellectual disability and sensory deficits, caused by epigenetic silencing of the FMR1 gene and subsequent loss of its protein product, fragile X mental retardation protein (FMRP). Delays in synaptic and neuronal development in the cortex have been reported in FXS mouse models; however, the main goal of translating lab research into pharmacological treatments in clinical trials has been so far largely unsuccessful, leaving FXS a still incurable disease. Here, we generated 2D and 3D in vitro human FXS model systems based on isogenic FMR1 knock-out mutant and wild-type human induced pluripotent stem cell (hiPSC) lines. Phenotypical and functional characterization of cortical neurons derived from FMRP-deficient hiPSCs display altered gene expression and impaired differentiation when compared with the healthy counterpart. FXS cortical cultures show an increased number of GFAP positive cells, likely astrocytes, increased spontaneous network activity, and depolarizing GABAergic transmission. Cortical brain organoid models show an increased number of glial cells, and bigger organoid size. Our findings demonstrate that FMRP is required to correctly support neuronal and glial cell proliferation, and to set the correct excitation/inhibition ratio in human brain development.

Mutations in the RNA-binding protein (RBPs) FUS have been genetically associated with the motoneuron disease amyotrophic lateral sclerosis (ALS). Using both human induced pluripotent stem cells and mouse models, we found that FUS-ALS causative mutations affect the activity of two relevant RBPs with important roles in neuronal RNA metabolism: HuD/ELAVL4 and FMRP. Mechanistically, mutant FUS leads to upregulation of HuD protein levels through competition with FMRP for HuD mRNA 3’UTR binding. In turn, increased HuD levels overly stabilize the transcript levels of its targets, NRN1 and GAP43. As a consequence, mutant FUS motoneurons show increased axon branching and growth upon injury, which could be rescued by dampening NRN1 levels. Since similar phenotypes have been previously described in SOD1 and TDP-43 mutant models, increased axonal growth and branching might represent broad early events in the pathogenesis of ALS.

Mutations in mitochondrial transfer RNA (mt-tRNA) genes are responsible for a wide range of syndromes, for which no effective treatment is available. We previously reported that transfection of the nucleotide sequence encoding for the 16-residue β32_33 peptide from mitochondrial leucyl-tRNA synthetase ameliorates the cell phenotype caused by the mitochondrial tRNA mutations. In this work, we demonstrated that both the β32_33 peptide linked with the known (L)-Phe-(D)-Arg-(L)-Phe-(L)-Lys (FrFK) mitochondrial penetrating sequence and, strikingly, the β32_33 peptide per se, are able to penetrate both the plasma and mitochondrial membranes and exert the rescuing activity when exogenously administered to cells bearing the mutations m.3243A>G and m.8344A>G. These mutations are responsible for the most common and severe mt-tRNA-related diseases. In addition, we dissected the molecular determinants of constructs activity by showing that both the order of amino acids along the sequence and presence of positive charges are essential determinants of the peptide activity in cells and mt-tRNA structures stabilization in vitro. In view of future in vivo studies, this information may be required to design of β32_33 peptide-mimetic derivatives. The β32_33 and FrFK-β32_33 peptides are, therefore, promising molecules for the development of therapeutic agents against diseases caused by the mt-tRNA point mutations.

Human pluripotent stem cells (PSCs) are widely used for in vitro disease modeling. One of the challenges in the field is represented by the ability of converting human PSCs into specific disease-relevant cell types. The nervous system is composed of a wide variety of neuronal types with selective vulnerability in neurodegenerative diseases. This is particularly relevant for motor neuron diseases, in which different motor neurons populations show a different susceptibility to degeneration. Here we developed a fast and efficient method to convert human induced Pluripotent Stem Cells into cranial motor neurons of the branchiomotor and visceral motor subtype. These populations represent the motor neuron subgroup that is primarily affected by a severe form of amyotrophic lateral sclerosis with bulbar onset and worst prognosis. This goal was achieved by stable integration of an inducible vector, based on the piggyBac transposon, allowing controlled activation of Ngn2, Isl1 and Phox2a (NIP). The NIP module effectively produced electrophysiologically active cranial motor neurons. Our method can be easily extended to PSCs carrying disease-associated mutations, thus providing a useful tool to shed light on the cellular and molecular bases of selective motor neuron vulnerability in pathological conditions. Keywords: Spinal motor neuron, Cranial motor neuron, Induced pluripotent stem cells, Amyotrophic lateral sclerosis, Phox2a, piggyBac

Pseudomonas aeruginosa is a pathogenic bacterium known to cause serious human infections, especially in immune-compromised patients. This is due to its unique ability to transform from a drug-tolerant planktonic to a more dangerous and treatment-resistant sessile life form, called biofilm. Recently, two derivatives of the frog skin antimicrobial peptide esculentin-1a, i.e. Esc(1-21) and its D-amino acids containing diastereomer Esc(1-21)-1c, were characterized for their powerful anti-Pseudomonal activity against both forms. Prevention of biofilm formation already in its early stages could be even more advantageous for counteracting infections induced by this bacterium. In this work, we studied how the diastereomer Esc(1-21)-1c can inhibit Pseudomonas biofilm formation in comparison to the parent peptide and two clinically-used conventional antibiotics, i.e. colistin and aztreonam, when applied at dosages below the minimal growth inhibitory concentration. Biofilm prevention was correlated to the peptides' ability to inhibit Pseudomonas motility and to reduce the production of virulent metabolites, for example, pyoverdine and rhamnolipids. Furthermore, the molecular mechanism underlying these activities was evaluated by studying the peptides' effect on the expression of key genes involved in the virulence and motility of bacteria, as well as by monitoring the peptides' binding to the bacterial signaling nucleotide ppGpp. Our results demonstrate that the presence of only two D-amino acids in Esc(1-21)-1c is sufficient to downregulate ppGpp-mediated expression of biofilm-associated genes, presumably as a result of higher peptide stability and therefore prolonged interaction with the nucleotide. Overall, these studies should assist efficient design and optimization of new anti-infective agents with multiple pharmacologically beneficial properties.

Pseudomonas aeruginosa is a pathogenic bacterium known to cause serious human infections, especially in immune-compromised patients. This is due to its unique ability to transform from a drug-tolerant planktonic to a more dangerous and treatment-resistant sessile life form, called biofilm. Recently, two derivatives of the frog skin antimicrobial peptide esculentin-1a, i.e. Esc(1-21) and its D-amino acids containing diastereomer Esc(1-21)-1c, were characterized for their powerful anti-Pseudomonal activity against both forms. Prevention of biofilm formation already in its early stages could be even more advantageous for counteracting infections induced by this bacterium. In this work, we studied how the diastereomer Esc(1-21)-1c can inhibit Pseudomonas biofilm formation in comparison to the parent peptide and two clinically-used conventional antibiotics, i.e. colistin and aztreonam, when applied at dosages below the minimal growth inhibitory concentration. Biofilm prevention was correlated to the peptides' ability to inhibit Pseudomonas motility and to reduce the production of virulent metabolites, for example, pyoverdine and rhamnolipids. Furthermore, the molecular mechanism underlying these activities was evaluated by studying the peptides' effect on the expression of key genes involved in the virulence and motility of bacteria, as well as by monitoring the peptides' binding to the bacterial signaling nucleotide ppGpp. Our results demonstrate that the presence of only two D-amino acids in Esc(1-21)-1c is sufficient to downregulate ppGpp-mediated expression of biofilm-associated genes, presumably as a result of higher peptide stability and therefore prolonged interaction with the nucleotide. Overall, these studies should assist efficient design and optimization of new anti-infective agents with multiple pharmacologically beneficial properties.

Ferritin self-assembly has been widely exploited for the synthesis of a variety of nanoparticles for drug-delivery and diagnostic applications. However, despite the crucial role of ferritin self-assembly mechanism for probes encapsulation, little is known about the principles behind the oligomerization mechanism. In the present work, the novel “humanized” chimeric Archaeal ferritin HumAfFt, displaying the transferrin receptor-1 (TfR1) recognition motif typical of human H homopolymer and the unique salt-triggered oligomerization properties of Archaeoglobus fulgidus ferritin (AfFt), was site-selectively labeled with N-(1-pyrenyl)maleimide on a topologically selected cysteine residue inside the protein cavity, next to the dimer interface. Pyrene characteristic fluorescence features were exploited to investigate the transition from a dimeric to a cage-like 24-meric state and to visualize the protein in vitro by two photon fluorescence microscopy. Indeed, pyrene fluorescence changes upon ferritin self-assembly allowed to establish, for the first time, the kinetic and thermodynamic details of the archaeal ferritins oligomerization mechanism. In particular, the magnesium induced oligomerization proved to be faster than the monovalent cation-triggered process, highly cooperative, complete at low MgCl2 concentrations, and reversed by treatment with EDTA. Moreover, pyrene intense excimer fluorescence was successfully visualized in vitro by two photon fluorescence microscopy as pyrene-labeled HumAfFt was actively uptaken into HeLa cells by human transferrin receptor TfR1 recognition, thus representing a unique nano-device building block for two photon fluorescence cell imaging.

Altered cellular biomechanics have been implicated as key photogenic triggers in age-related diseases. An aberrant liquid-to-solid phase transition, observed in in vitro reconstituted droplets of FUS protein, has been recently proposed as a possible pathogenic mechanism for amyotrophic lateral sclerosis (ALS). Whether such transition occurs in cell environments is currently unknown as a consequence of the limited measuring capability of the existing techniques, which are invasive or lack of subcellular resolution. Here we developed a non-contact and label-free imaging method, named background-deflection Brillouin microscopy, to investigate the three-dimensional intracellular biomechanics at a sub-micron resolution. Our method exploits diffraction to achieve an unprecedented 10,000-fold enhancement in the spectral contrast of single-stage spectrometers, enabling, to the best of our knowledge, the first direct biomechanical analysis on intracellular stress granules containing ALS mutant FUS protein in fixed cells. Our findings provide fundamental insights on the critical aggregation step underlying the neurodegenerative ALS disease. Giuseppe Antonacci et al. present a method for non-contact and label-free imaging using Brillouin microscopy to examine intracellular biomechanics at the sub-micron level. They use this approach in an analysis of stress granules containing ALS-related FUS protein mutants.

Peritoneal fibrosis is a pathological alteration of the peritoneal membrane occurring in a variety of conditions including peritoneal dialysis (PD), post-surgery adhesions and peritoneal metastases. The acquisition of invasive and pro-fibrotic abilities by mesothelial cells (MCs) through induction of MMT, a cell-specific form of EMT, plays a main role in this process. Aim of this study was to evaluate possible effects of histone deacetylase (HDAC) inhibitors, key components of the epigenetic machinery, in counteracting MMT observed in MCs isolated from effluent of PD patients. HDAC inhibitors with different class/isoform selectivity have been used for pharmacological inhibition. While the effect of other inhibitors was limited to a partial E-cadherin re-expression, MS-275, a HDAC1-3 inhibitor, promoted: (i) downregulation of mesenchymal markers (MMP2, Col1A1, PAI-1, TGFβ1, TGFβRI) (ii) upregulation of epithelial markers (E-cadherin, Occludin), (iii) reacquisition of an epithelial-like morphology and (iv) marked reduction of cellular invasiveness. Results were confirmed by HDAC1 genetic silencing. Mechanistically, MS-275 causes: (i) increase of nuclear histone H3 acetylation (ii) rescue of the acetylation profile on E-cadherin promoter, (iii) Snail functional impairment. Overall, our study, pinpointing a role for HDAC1, revealed a new player in the regulation of peritoneal fibrosis, providing the rationale for future therapeutic opportunities.

Natural Killer cells are innate lymphocytes involved in tumor immunosurveillance. They express activating receptors able to recognize self-molecules poorly expressed on healthy cells but up-regulated upon stress conditions, including transformation. Regulation of ligand expression in tumor cells mainly relays on transcriptional mechanisms, while the involvement of ubiquitin or ubiquitin-like modifiers remains largely unexplored. Here, we focused on the SUMO pathway and demonstrated that the ligand of DNAM1 activating receptor, PVR, undergoes SUMOylation in multiple myeloma. Concurrently, we found that PVR is preferentially located in intracellular compartments in human multiple myeloma cell lines and malignant plasma cells and that inhibition of the SUMO pathway promotes its translocation to the cell surface, increasing tumor cell susceptibility to NK cell-mediated cytolysis. Our findings provide the first evidence of an innate immune activating ligand regulated by SUMOylation, and confer to this modification a novel role in impairing recognition and killing of tumor cells.

Summary The FUS gene has been linked to amyotrophic lateral sclerosis (ALS). FUS is a ubiquitous RNA-binding protein, and the mechanisms leading to selective motoneuron loss downstream of ALS-linked mutations are largely unknown. We report the transcriptome analysis of human purified motoneurons, obtained from FUS wild-type or mutant isogenic induced pluripotent stem cells (iPSCs). Gene ontology analysis of differentially expressed genes identified significant enrichment of pathways previously associated to sporadic ALS and other neurological diseases. Several microRNAs (miRNAs) were also deregulated in FUS mutant motoneurons, including miR-375, involved in motoneuron survival. We report that relevant targets of miR-375, including the neural RNA-binding protein ELAVL4 and apoptotic factors, are aberrantly increased in FUS mutant motoneurons. Characterization of transcriptome changes in the cell type primarily affected by the disease contributes to the definition of the pathogenic mechanisms of FUS-linked ALS., Highlights • We performed transcriptome analysis of hiPSC-derived ALS FUS mutant motoneurons • Bioinformatics analysis revealed cell adhesion pathways associated to sporadic ALS • miR-375 expression is affected by FUS mutation in motoneurons • Impairment of miR-375 alters RNA metabolism and increases pro-apoptotic factors, The FUS gene is associated to ALS. Rosa and colleagues identified alteration in the transcriptome of hiPSC-derived FUS mutant motoneurons. GO analysis pointed to pathways, such as cell adhesion, previously associated to neurodegenerative diseases. miR-375 impairment downstream of FUS mutation alters RNA metabolism and increases pro-apoptotic factors. Characterization of transcriptome changes in motoneurons contributes to the definition of ALS pathogenic mechanisms.

Protein conjugation with small ubiquitin-related modifier (SUMO) is a post-translational modification that modulates protein interactions and localisation. RANBP2 is a large nucleoporin endowed with SUMO E3 ligase and SUMO-stabilising activity, and is implicated in some cancer types. RANBP2 is part of a larger complex, consisting of SUMO-modified RANGAP1, the GTP-hydrolysis activating factor for the GTPase RAN. During mitosis, the RANBP2-SUMO-RANGAP1 complex localises to the mitotic spindle and to kinetochores after microtubule attachment. Here, we address the mechanisms that regulate this localisation and how they affect kinetochore functions. Using proximity ligation assays, we find that nuclear transport receptors importin-β and CRM1 play essential roles in localising the RANBP2-SUMO-RANGAP1 complex away from, or at kinetochores, respectively. Using newly generated inducible cell lines, we show that overexpression of nuclear transport receptors affects the timing of RANBP2 localisation in opposite ways. Concomitantly, kinetochore functions are also affected, including the accumulation of SUMO-conjugated topoisomerase-IIα and stability of kinetochore fibres. These results delineate a novel mechanism through which nuclear transport receptors govern the functional state of kinetochores by regulating the timely deposition of RANBP2.

Bioprinting techniques use bioinks made of biocompatible non-living materials and cells to build 3D constructs in a controlled manner and with micrometric resolution. 3D bioprinted structures representative of several human tissues have been recently produced using cells derived by differentiation of induced pluripotent stem cells (iPSCs). Human iPSCs can be differentiated in a wide range of neurons and glia, providing an ideal tool for modeling the human nervous system. Here we report a neural construct generated by 3D bioprinting of cortical neurons and glial precursors derived from human iPSCs. We show that the extrusion-based printing process does not impair cell viability in the short and long term. Bioprinted cells can be further differentiated within the construct and properly express neuronal and astrocytic markers. Functional analysis of 3D bioprinted cells highlights an early stage of maturation and the establishment of early network activity behaviors. This work lays the basis for generating more complex and faithful 3D models of the human nervous systems by bioprinting neural cells derived from iPSCs.

Live-cell single mRNA imaging is a powerful tool, but has been restricted in higher eukaryotes to artificial cell lines and reporter genes. We describe an approach that enables live-cell imaging of single endogenous labeled mRNA molecules transcribed in primary mammalian cells and tissue. We generated a knock-in mouse line in which an MS2 binding site (MBS) cassette was targeted to the 3′UTR of the essential β-actin gene. As β-actin-MBS was ubiquitously expressed, we were able to uniquely address endogenous mRNA regulation in any tissue or cell type. We simultaneously followed transcription from the β-actin alleles in real-time and observed transcriptional bursting in response to serum stimulation with precise temporal resolution. We performed tracking of single endogenous labeled mRNA particles being transported in primary hippocampal neurons. The MBS also provided a means for high sensitivity Fluorescence In Situ Hybridization (FISH), allowing detection and localization of single β-actin mRNA molecules in various mouse tissues.

The advent of new technologies for the imaging of living cells has made it possible to determine the properties of transcription, the kinetics of polymerase movement, the association of transcription factors, and the progression of the polymerase on the gene. We report here the current state of the field and the progress necessary to achieve a more complete understanding of the various steps in transcription. Our Consortium is dedicated to developing and implementing the technology to further this understanding.

Several pathogens have been described to enter host cells via cholesterol-enriched membrane lipid raft microdomains. We found that disruption of lipid rafts by the cholesterol-extracting agent methyl-β-cyclodextrin or by the cholesterol-binding antifungal drug Amphotericin B strongly impairs the uptake of the fungal pathogen Candida albicans by human monocytes, suggesting a role of raft microdomains in the phagocytosis of the fungus. Time lapse confocal imaging indicated that Dectin-1, the C-type lectin receptor that recognizes Candida albicans cell wall-associated β-glucan, is recruited to lipid rafts upon Candida albicans uptake by monocytes, supporting the notion that lipid rafts act as an entry platform. Interestingly disruption of lipid raft integrity and interference with fungus uptake do not alter cytokine production by monocytes in response to Candida albicans but drastically dampen fungus specific T cell response. In conclusion, these data suggest that monocyte lipid rafts play a crucial role in the innate and adaptive immune responses to Candida albicans in humans and highlight a new and unexpected immunomodulatory function of the antifungal drug Amphotericin B.

Patient-derived induced pluripotent stem cells (iPSCs) provide an opportunity to study human diseases mainly in those cases for which no suitable model systems are available. Here, we have taken advantage of in vitro iPSCs derived from patients affected by amyotrophic lateral sclerosis (ALS) and carrying mutations in the RNA-binding protein FUS to study the cellular behavior of the mutant proteins in the appropriate genetic background. Moreover, the ability to differentiate iPSCs into spinal cord neural cells provides an in vitro model mimicking the physiological conditions. iPSCs were derived from FUSR514S and FUSR521C patient fibroblasts, whereas in the case of the severe FUSP525L mutation, in which fibroblasts were not available, a heterozygous and a homozygous iPSC line were raised by TALEN-directed mutagenesis. We show that aberrant localization and recruitment of FUS into stress granules (SGs) is a prerogative of the FUS mutant proteins and occurs only upon induction of stress in both undifferentiated iPSCs and spinal cord neural cells. Moreover, we show that the incorporation into SGs is proportional to the amount of cytoplasmic FUS, strongly correlating with the cytoplasmic delocalization phenotype of the different mutants. Therefore, the available iPSCs represent a very powerful system for understanding the correlation between FUS mutations, the molecular mechanisms of SG formation and ALS ethiopathogenesis., Summary: Mutated FUS protein is aberrantly delocalized and recruited into stress granules in iPSC-derived motoneurons, which provide a new model system for amyotrophic lateral sclerosis.

Synthesis of mRNA in eukaryotes involves the coordinated action of many enzymatic processes, including initiation, elongation, splicing, and cleavage. Kinetic competition between these processes has been proposed to determine RNA fate, yet such coupling has never been observed in vivo on single transcripts. In this study, we use dual-color single-molecule RNA imaging in living human cells to construct a complete kinetic profile of transcription and splicing of the β-globin gene. We find that kinetic competition results in multiple competing pathways for pre-mRNA splicing. Splicing of the terminal intron occurs stochastically both before and after transcript release, indicating there is not a strict quality control checkpoint. The majority of pre-mRNAs are spliced after release, while diffusing away from the site of transcription. A single missense point mutation (S34F) in the essential splicing factor U2AF1 which occurs in human cancers perturbs this kinetic balance and defers splicing to occur entirely post-release. DOI: http://dx.doi.org/10.7554/eLife.03939.001, eLife digest To make a protein, part of a DNA sequence is copied to make a messenger RNA (or mRNA) molecule in a process known as transcription. The enzyme that builds an mRNA molecule first binds to a start point on a DNA strand, and then uses the DNA sequence to build a ‘pre-mRNA’ molecule until a stop signal is reached. To make the final mRNA molecule, sections called introns are removed from the pre-mRNA molecules, and the parts left behind—known as exons—are then joined together. This process is called splicing. However, it is not fully understood how the splicing process is coordinated with the other stages of transcription. For example, does splicing occur after the pre-mRNA molecule is completed or while it is still being built? And what controls the order in which these processes occur? One theory about how the different mRNA-making processes are coordinated is called kinetic competition. This theory states that the fastest process is the most likely to occur, even if the other processes use less energy and so might be expected to be preferred. Alternatively, the different steps may be started and stopped by ‘checkpoints’ that cause the different processes to follow on from each other in a set order. Coulon et al. used fluorescence microscopy to investigate how mRNA molecules are made during the transcription of a human gene that makes a hemoglobin protein. To make the RNA visible, two different fluorescent markers were introduced into the pre-mRNA that cause different regions of the mRNA to glow in different colors. Coulon et al. made the introns fluoresce red and the exons glow green. Unspliced pre-mRNA molecules contain both introns and exons and so fluoresce in both colors, whereas spliced mRNA molecules contain only exons and so only glow with a green color. By looking at both the red and green fluorescence signals at the same time, Coulon et al. could see when an intron was spliced out of the pre-mRNA. This revealed that in normal cells, splicing can occur either before or after the RNA is released from where it is transcribed. Thus, splicing and transcription does not follow a set pattern, suggesting that checkpoints do not control the sequence of events. Instead, the fact that a spliced mRNA molecule can be formed in different ways suggests kinetic competition controls the process. In some cancer cells, there are defects in the cellular machinery that controls splicing. When looking at cells with such a defect, Coulon et al. found that splicing only occurred after transcription was completed. This study thus provides insight into the complex workings of mRNA synthesis and establishes a blueprint for understanding how splicing is impaired in diseases such as cancer. DOI: http://dx.doi.org/10.7554/eLife.03939.002

The GTPase RAN, the regulators of its nucleotide-bound state, and its effectors represent a specialized network in the RAS GTPase superfamily and regulate the localization of macromolecules (RNAs and proteins) in subcellular compartments in interphase cells and at the mitotic apparatus when cells divide. Essential cell cycle processes, e.g., replication, repair, transcription, export of transcribed RNAs out of the nucleus, assembly of the mitotic apparatus, kinetochore function, chromosome segregation, nuclear reorganization, and rebuilding of the nuclear envelope and nuclear pores at mitotic exit, ultimately depend on RAN's ability to orchestrate localization of key target factors in space and time. To achieve this, RAN network members acquire themselves dynamic localization patterns. Biochemical fractionation protocols describe where the bulk of RAN network members localize. Immunofluorescence methods have revealed more subtle and complex patterns, with specific populations of RAN network components associating with cellular structures, or organelles, where they play crucial roles as spatial regulators for a large set of macromolecules. These localization studies are important to understand RAN modes of action and to identify new targets of RAN control. Here we describe methods for the visualization of RAN network members and effectors in mammalian cells.