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In vivo imaging of labelled endogenous beta-actin mRNA during nucleocytoplasmic transport

Mon, 09/08/2014 - 9:47am

Export of messenger RNA occurs via nuclear pores, which are large nanomachines with diameters of roughly 120 nm that are the only link between the nucleus and cytoplasm. Hence, mRNA export occurs over distances smaller than the optical resolution of conventional light microscopes. There is extensive knowledge on the physical structure and composition of the nuclear pore complex, but transport selectivity and the dynamics of mRNA export at nuclear pores remain unknown. Here we developed a super-registration approach using fluorescence microscopy that can overcome the current limitations of co-localization by means of measuring intermolecular distances of chromatically different fluorescent molecules with nanometre precision. With this method we achieve 20-ms time-precision and at least 26-nm spatial precision, enabling the capture of highly transient interactions in living cells. Using this approach we were able to spatially resolve the kinetics of mRNA transport in mammalian cells and present a three-step model consisting of docking (80 ms), transport (5-20 ms) and release (80 ms), totalling 180 +/- 10 ms. Notably, the translocation through the channel was not the rate-limiting step, mRNAs can move bi-directionally in the pore complex and not all pores are equally active.

A perspective of the dynamic structure of the nucleus explored at the single-molecule level

Mon, 09/08/2014 - 9:47am

Cellular life can be described as a dynamic equilibrium of a highly complex network of interacting molecules. For this reason, it is no longer sufficient to "only" know the identity of the participants in a cellular process, but questions such as where, when, and for how long also have to be addressed to understand the mechanism being investigated. Additionally, ensemble measurements may not sufficiently describe individual steps of molecular mobility, spatial-temporal resolution, kinetic parameters, and geographical mapping. It is vital to investigate where individual steps exactly occur to enhance our understanding of the living cell. The nucleus, home too many highly complex multi-order processes, such as replication, transcription, splicing, etc., provides a complicated, heterogeneous landscape. Its dynamics were studied to a new level of detail by fluorescence correlation spectroscopy (FCS). Single-molecule tracking, while still in its infancy in cell biology, is becoming a more and more attractive method to deduce key elements of this organelle. Here we discuss the potential of tracking single RNAs and proteins in the nucleus. Their dynamics, localization, and interaction rates will be vital to our understanding of cellular life. To demonstrate this, we provide a review of the HIV life cycle, which is an extremely elegant balance of nuclear and cytoplasmic functions and provides an opportunity to study mechanisms deeply integrated within the structure of the nucleus. In summary, we aim to present a specific, dynamic view of nuclear cellular life based on single molecule and FCS data and provide a prospective for the future.

Nuclear physics: quantitative single-cell approaches to nuclear organization and gene expression

Mon, 09/08/2014 - 9:47am

The internal workings of the nucleus remain a mystery. A list of component parts exists, and in many cases their functional roles are known for events such as transcription, RNA processing, or nuclear export. Some of these components exhibit structural features in the nucleus, regions of concentration or bodies that have given rise to the concept of functional compartmentalization--that there are underlying organizational principles to be described. In contrast, a picture is emerging in which transcription appears to drive the assembly of the functional components required for gene expression, drawing from pools of excess factors. Unifying this seemingly dual nature requires a more rigorous approach, one in which components are tracked in time and space and correlated with onset of specific nuclear functions. In this chapter, we anticipate tools that will address these questions and provide the missing kinetics of nuclear function. These tools are based on analyzing the fluctuations inherent in the weak signals of endogenous nuclear processes and determining values for them. In this way, it will be possible eventually to provide a computational model describing the functional relationships of essential components.

Nuclear export dynamics of RNA-protein complexes

Mon, 09/08/2014 - 9:47am

The central dogma of molecular biology - DNA makes RNA makes proteins - is a flow of information that in eukaryotes encounters a physical barrier: the nuclear envelope, which encapsulates, organizes and protects the genome. Nuclear-pore complexes, embedded in the nuclear envelope, regulate the passage of molecules to and from the nucleus, including the poorly understood process of the export of RNAs from the nucleus. Recent imaging approaches focusing on single molecules have provided unexpected insight into this crucial step in the information flow. This review addresses the latest studies of RNA export and presents some models for how this complex process may work.

Multiscale dynamics in nucleocytoplasmic transport

Mon, 09/08/2014 - 9:47am

The nuclear pore complex (NPC) has long been viewed as a point-like entry and exit channel between the nucleus and the cytoplasm. New data support a different view whereby the complex displays distinct spatial dynamics of variable duration ranging from milliseconds to events spanning the entire cell cycle. Discrete interaction sites outside the central channel become apparent, and transport regulation at these sites seems to be of greater importance than currently thought. Nuclear pore components are highly active outside the NPC or impact the fate of cargo transport away from the nuclear pore. The NPC is a highly dynamic, crowded environment-constantly loaded with cargo while providing selectivity based on unfolded proteins. Taken together, this comprises a new paradigm in how we view import/export dynamics and emphasizes the multiscale nature of NPC-mediated cellular transport.

Nuclear pore component Nup98 is a potential tumor suppressor and regulates posttranscriptional expression of select p53 target genes

Mon, 09/08/2014 - 9:47am

The p53 tumor suppressor utilizes multiple mechanisms to selectively regulate its myriad target genes, which in turn mediate diverse cellular processes. Here, using conventional and single-molecule mRNA analyses, we demonstrate that the nucleoporin Nup98 is required for full expression of p21, a key effector of the p53 pathway, but not several other p53 target genes. Nup98 regulates p21 mRNA levels by a posttranscriptional mechanism in which a complex containing Nup98 and the p21 mRNA 3'UTR protects p21 mRNA from degradation by the exosome. An in silico approach revealed another p53 target (14-3-3sigma) to be similarly regulated by Nup98. The expression of Nup98 is reduced in murine and human hepatocellular carcinomas (HCCs) and correlates with p21 expression in HCC patients. Our study elucidates a previously unrecognized function of wild-type Nup98 in regulating select p53 target genes that is distinct from the well-characterized oncogenic properties of Nup98 fusion proteins.

Measuring image resolution in optical nanoscopy

Mon, 09/08/2014 - 9:47am

Resolution in optical nanoscopy (or super-resolution microscopy) depends on the localization uncertainty and density of single fluorescent labels and on the sample's spatial structure. Currently there is no integral, practical resolution measure that accounts for all factors. We introduce a measure based on Fourier ring correlation (FRC) that can be computed directly from an image. We demonstrate its validity and benefits on two-dimensional (2D) and 3D localization microscopy images of tubulin and actin filaments. Our FRC resolution method makes it possible to compare achieved resolutions in images taken with different nanoscopy methods, to optimize and rank different emitter localization and labeling strategies, to define a stopping criterion for data acquisition, to describe image anisotropy and heterogeneity, and even to estimate the average number of localizations per emitter. Our findings challenge the current focus on obtaining the best localization precision, showing instead how the best image resolution can be achieved as fast as possible.

Obesity, physical activity, and their interaction in incident atrial fibrillation in postmenopausal women

Tue, 08/26/2014 - 10:58am

BACKGROUND: Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with increased risk of stroke and death. Obesity is an independent risk factor for AF, but modifiers of this risk are not well known. We studied the roles of obesity, physical activity, and their interaction in conferring risk of incident AF.

METHODS AND RESULTS: The Women's Health Initiative (WHI) Observational Study was a prospective observational study of 93 676 postmenopausal women followed for an average of 11.5 years. Incident AF was identified using WHI-ascertained hospitalization records and diagnostic codes from Medicare claims. A multivariate Cox's hazard regression model adjusted for demographic and clinical risk factors was used to evaluate the interaction between obesity and physical activity and its association with incident AF. After exclusion of women with prevalent AF, incomplete data, or underweight body mass index (BMI), 9792 of the remaining 81 317 women developed AF. Women were, on average, 63.4 years old, 7.8% were African American, and 3.6% were Hispanic. Increased BMI (hazard ratio [HR], 1.12 per 5-kg/m(2) increase; 95% confidence interval [CI], 1.10 to 1.14) and reduced physical activity (>9 vs. 0 metabolic equivalent task hours per week; HR, 0.90; 95% CI, 0.85 to 0.96) were independently associated with higher rates of AF after multivariate adjustment. Higher levels of physical activity reduced the AF risk conferred by obesity (interaction P=0.033).

CONCLUSIONS: Greater physical activity is associated with lower rates of incident AF and modifies the association between obesity and incident AF.

Modulating artificial membrane morphology: pH-induced chromatic transition and nanostructural transformation of a bolaamphiphilic conjugated polymer from blue helical ribbons to red nanofibers.

Tue, 08/26/2014 - 8:17am

Design and characterization of helical ribbon assemblies of a bolaamphiphilic conjugated polymer and their color-coded transformation into nanofibers are described. An L-glutamic acid modified bolaamphiphilic diacetylene lipid was synthesized and self-assembled into right-handed helical ribbons with micron scale length and nano scale thickness under mild conditions. The ribbon structures were further stabilized by polymerizing well-aligned diacetylene units to form bisfunctional polydiacetylenes (PDAs). Transitions from flat sheets to helical ribbons and tubes were observed by transmission electron microscopy. The helical ribbons appear to originate from the rupture of flat sheets along domain edges and the peeling off between stacked lipid layers. These results point to the applicability of chiral packing theory in bolaamphiphilic supramolecular assemblies. Contact mode atomic force microscopy observations revealed that high order existed in the surface packing arrangement. Hexagonal and pseudorectangular packings were observed in flat and twisted regions of the ribbons, respectively, suggesting a correlation between microscopic morphologies and nanoscopic packing arrangements. The tricarboxylate functionalities of the bolaamphiphilic lipid provide a handle for the manipulation of the bisfunctional PDAs' morphology. Increasing solution pH caused the fraying of helical ribbons into nanofibers accompanied by a sharp blue-to-red chromatic transition. A dramatic change in circular dichroism spectra was observed during this process, suggesting the loss of chirality in packing. A model is proposed to account for the pH-induced morphological change and chromatic transition. The color-coded transition between two distinct microstructures would be useful in the design of sensors and other "smart" nanomaterials requiring defined molecular templates.

Cytocompatible poly(ethylene glycol)-co-polycarbonate hydrogels cross-linked by copper-free, strain-promoted click chemistry

Tue, 08/26/2014 - 8:17am

Strategies to encapsulate cells in cytocompatible three-dimensional hydrogels with tunable mechanical properties and degradability without harmful gelling conditions are highly desired for regenerative medicine applications. Here we reported a method for preparing poly(ethylene glycol)-co-polycarbonate hydrogels through copper-free, strain-promoted azide-alkyne cycloaddition (SPAAC) click chemistry. Hydrogels with varying mechanical properties were formed by "clicking" azido-functionalized poly(ethylene glycol)-co-polycarbonate macromers with dibenzocyclooctyne-functionalized poly(ethylene glycol) under physiological conditions within minutes. Bone marrow stromal cells encapsulated in these gels exhibited higher cellular viability than those encapsulated in photo-cross-linked poly(ethylene glycol) dimethacrylate. The precise control over the macromer compositions, cytocompatible SPAAC cross-linking, and the degradability of the polycarbonate segments make these hydrogels promising candidates for scaffold and stem cell assisted tissue repair and regeneration.

(alpha-NaYbF4:Tm(3+))/CaF2 core/shell nanoparticles with efficient near-infrared to near-infrared upconversion for high-contrast deep tissue bioimaging

Tue, 08/26/2014 - 8:17am

We describe the development of novel and biocompatible core/shell (alpha-NaYbF(4):Tm(3+))/CaF(2) nanoparticles that exhibit highly efficient NIR(in)-NIR(out) upconversion (UC) for high contrast and deep bioimaging. When excited at ~980 nm, these nanoparticles emit photoluminescence (PL) peaked at ~800 nm. The quantum yield of this UC PL under low power density excitation (~0.3 W/cm(2)) is 0.6 +/- 0.1%. This high UC PL efficiency is realized by suppressing surface quenching effects via heteroepitaxial growth of a biocompatible CaF(2) shell, which results in a 35-fold increase in the intensity of UC PL from the core. Small-animal whole-body UC PL imaging with exceptional contrast (signal-to-background ratio of 310) is shown using BALB/c mice intravenously injected with aqueously dispersed nanoparticles (700 pmol/kg). High-contrast UC PL imaging of deep tissues is also demonstrated, using a nanoparticle-loaded synthetic fibrous mesh wrapped around rat femoral bone and a cuvette with nanoparticle aqueous dispersion covered with a 3.2 cm thick animal tissue (pork).

Amphiphilic degradable polymers for immobilization and sustained delivery of sphingosine 1-phosphate

Tue, 08/26/2014 - 8:17am

Controlled delivery of the angiogenic factor sphingosine 1-phosphate (S1P) represents a promising strategy for promoting vascularization during tissue repair and regeneration. In this study, we developed an amphiphilic biodegradable polymer platform for the stable encapsulation and sustained release of S1P. Mimicking the interaction between amphiphilic S1P and its binding proteins, a series of polymers with hydrophilic poly(ethylene glycol) core and lipophilic flanking segments of polylactide and/or poly(alkylated lactide) with different alkyl chain lengths were synthesized. These polymers were electrospun into fibrous meshes, and loaded with S1P in generally high loading efficiencies (>90%). Sustained S1P release from these scaffolds could be tuned by adjusting the alkyl chain length, blockiness and lipophilic block length, achieving 35-55% and 45-80% accumulative releases in the first 8h and by 7 days, respectively. Furthermore, using endothelial cell tube formation assay and chicken chorioallantoic membrane assay, we showed that the different S1P loading doses and release kinetics translated into distinct pro-angiogenic outcomes. These results suggest that these amphiphilic polymers are effective delivery vehicles for S1P and may be explored as tissue engineering scaffolds where the delivery of lipophilic or amphiphilic bioactive factors is desired. reserved.

Bioorthogonally cross-linked hydrogel network with precisely controlled disintegration time over a broad range

Tue, 08/26/2014 - 8:17am

Hydrogels with predictable degradation are highly desired for biomedical applications where timely disintegration of the hydrogel (e.g., drug delivery, guided tissue regeneration) is required. However, precisely controlling hydrogel degradation over a broad range in a predictable manner is challenging due to limited intrinsic variability in the degradation rate of liable bonds and difficulties in modeling degradation kinetics for complex polymer networks. More often than not, empirical tuning of the degradation profile results in undesired changes in other properties. Here we report a simple but versatile hydrogel platform that allows us to formulate hydrogels with predictable disintegration time from 2 to >250 days yet comparable macroscopic physical properties. This platform is based on a well-defined network formed by two pairs of four-armed polyethylene glycol macromers terminated with azide and dibenzocyclooctyl groups, respectively, via labile or stable linkages. The high-fidelity bioorthogonal reaction between the symmetric hydrophilic macromers enables robust cross-linking in water, phosphate-buffered saline, and cell culture medium to afford tough hydrogels capable of withstanding >90% compressive strain. Strategic placement of labile ester linkages near the cross-linking site within this superhydrophilic network, accomplished by adjustments of the ratio of the macromers used, enables broad tuning of the disintegration rates precisely matching with the theoretical predictions based on first-order linkage cleavage kinetics. This platform can be exploited for applications where a precise degradation rate is targeted.

DRK/DOS/SOS Converge with Crk/Mbc/dCed-12 to Activate Rac1 during Glial Engulfment of Axonal Debris

Mon, 08/25/2014 - 3:23pm

Nervous system injury or disease leads to activation of glia, which govern postinjury responses in the nervous system. Axonal injury in Drosophila results in transcriptional up-regulation of the glial engulfment receptor Draper; there is extension of glial membranes to the injury site (termed activation), and then axonal debris is internalized and degraded. Loss of the small GTPase Rac1 from glia completely suppresses glial responses to injury, but upstream activators remain poorly defined. Loss of the Rac guanine nucleotide exchange factor (GEF) Crk/myoblast city (Mbc)/dCed-12has no effect on glial activation, but blocks internalization and degradation of debris. Here we show that the signaling molecules downstream of receptor kinase (DRK) and daughter of sevenless (DOS) (mammalian homologs, Grb2 and Gab2, respectively) and the GEF son of sevenless (SOS) (mammalian homolog, mSOS) are required for efficient activation of glia after axotomy and internalization/degradation of axonal debris. At the earliest steps of glial activation, DRK/DOS/SOS function in a partially redundant manner with Crk/Mbc/dCed-12, with blockade of both complexes strongly suppressing all glial responses, similar to loss of Rac1. This work identifies DRK/DOS/SOS as the upstream Rac GEF complex required for glialresponses to axonal injury, and demonstrates a critical requirement for multiple GEFs in efficient glial activation after injury and internalization/degradation of axonal debris.

Phosphatidic acid phospholipase A1 mediates ER-Golgi transit of a family of G protein-coupled receptors

Mon, 08/25/2014 - 2:35pm

The coat protein II (COPII)-coated vesicular system transports newly synthesized secretory and membrane proteins from the endoplasmic reticulum (ER) to the Golgi complex. Recruitment of cargo into COPII vesicles requires an interaction of COPII proteins either with the cargo molecules directly or with cargo receptors for anterograde trafficking. We show that cytosolic phosphatidic acid phospholipase A1 (PAPLA1) interacts with COPII protein family members and is required for the transport of Rh1 (rhodopsin 1), an N-glycosylated G protein-coupled receptor (GPCR), from the ER to the Golgi complex. In papla1 mutants, in the absence of transport to the Golgi, Rh1 is aberrantly glycosylated and is mislocalized. These defects lead to decreased levels of the protein and decreased sensitivity of the photoreceptors to light. Several GPCRs, including other rhodopsins and Bride of sevenless, are similarly affected. Our findings show that a cytosolic protein is necessary for transit of selective transmembrane receptor cargo by the COPII coat for anterograde trafficking.

A Large-Scale RNAi-Based Mouse Tumorigenesis Screen Identifies New Lung Cancer Tumor Suppressors that Repress FGFR Signaling

Mon, 08/25/2014 - 2:35pm

To discover new tumor suppressor genes (TSGs), we developed a functional genomics approach in which immortalized but non-tumorigenic cells were stably transduced with large-scale short hairpin RNA (shRNA) pools and tested for tumor formation in mice. Identification of shRNAs in resulting tumors revealed candidate TSGs, which were validated experimentally and by analyzing expression in human tumor samples. Using this approach, we identified 24 TSGs that were significantly down-regulated in human lung squamous cell carcinomas (hLSCCs). Amplification of fibroblast growth factor receptor 1 (FGFR1), which aberrantly increases FGFR signaling, is a common genetic alteration in hLSCCs. Remarkably, we found that 17 of the TSGs encode repressors of FGFR signaling. Knockdown of 14 of these TSGs transformed immortalized human bronchial epithelial cells and, in most cases, rendered them sensitive to FGFR inhibitors. Our results indicate that increased FGFR signaling promotes tumorigenesis in many hLSCCs that lack FGFR1 amplification or activating mutations.

High-resolution mapping of chromatin packaging in mouse embryonic stem cells and sperm

Mon, 08/25/2014 - 2:35pm

Mammalian embryonic stem cells (ESCs) and sperm exhibit unusual chromatin packaging that plays important roles in cellular function. Here, we extend a recently developed technique, based on deep paired-end sequencing of lightly digested chromatin, to assess footprints of nucleosomes and other DNA-binding proteins genome-wide in murine ESCs and sperm. In ESCs, we recover well-characterized features of chromatin such as promoter nucleosome depletion and further identify widespread footprints of sequence-specific DNA-binding proteins such as CTCF, which we validate in knockdown studies. We document global differences in nuclease accessibility between ESCs and sperm, finding that the majority of histone retention in sperm preferentially occurs in large gene-poor genomic regions, with only a small subset of nucleosomes being retained over promoters of developmental regulators. Finally, we describe evidence that CTCF remains associated with the genome in mature sperm, where it could play a role in organizing the sperm genome.

Drosophila Sirt2/mammalian SIRT3 deacetylates ATP synthase beta and regulates complex V activity

Mon, 08/25/2014 - 2:35pm

Adenosine triphosphate (ATP) synthase beta, the catalytic subunit of mitochondrial complex V, synthesizes ATP. We show that ATP synthase beta is deacetylated by a human nicotinamide adenine dinucleotide (NAD(+))-dependent protein deacetylase, sirtuin 3, and its Drosophila melanogaster homologue, dSirt2. dsirt2 mutant flies displayed increased acetylation of specific Lys residues in ATP synthase beta and decreased complex V activity. Overexpression of dSirt2 increased complex V activity. Substitution of Lys 259 and Lys 480 with Arg in human ATP synthase beta, mimicking deacetylation, increased complex V activity, whereas substitution with Gln, mimicking acetylation, decreased activity. Mass spectrometry and proteomic experiments from wild-type and dsirt2 mitochondria identified the Drosophila mitochondrial acetylome and revealed dSirt2 as an important regulator of mitochondrial energy metabolism. Additionally, we unravel a ceramide-NAD(+)-sirtuin axis wherein increased ceramide, a sphingolipid known to induce stress responses, resulted in depletion of NAD(+) and consequent decrease in sirtuin activity. These results provide insight into sirtuin-mediated regulation of complex V and reveal a novel link between ceramide and Drosophila acetylome.

A separable domain of the p150 subunit of human Chromatin Assembly Factor-1 promotes protein and chromosome associations with nucleoli

Mon, 08/25/2014 - 2:35pm

Chromatin Assembly Factor-1 (CAF-1) is a three-subunit protein complex conserved throughout eukaryotes that deposits histones during DNA synthesis. Here, we present a novel role for the human p150 subunit in regulating nucleolar macromolecular interactions. Acute depletion of p150 causes redistribution of multiple nucleolar proteins and reduces nucleolar association with several repetitive element-containing loci. Notably, a point mutation in a SUMO-interacting motif (SIM) within p150 abolishes nucleolar associations, whereas PCNA or HP1 interaction sites within p150 are not required for these interactions. Additionally, acute depletion of SUMO-2 or the SUMO E2 ligase Ubc9 reduces alpha-satellite DNA association with nucleoli. The nucleolar functions of p150 are separable from its interactions with the other subunits of the CAF-1 complex, because an N-terminal fragment of p150 (p150N) that cannot interact with other CAF-1 subunits is sufficient for maintaining nucleolar chromosome and protein associations. Therefore, these data define novel functions for a separable domain of the p150 protein, regulating protein and DNA interactions at the nucleolus.

Synergistic tumor suppression by combined inhibition of telomerase and CDKN1A

Mon, 08/25/2014 - 2:35pm

Tumor suppressor p53 plays an important role in mediating growth inhibition upon telomere dysfunction. Here, we show that loss of the p53 target gene cyclin-dependent kinase inhibitor 1A (CDKN1A, also known as p21WAF1/CIP1) increases apoptosis induction following telomerase inhibition in a variety of cancer cell lines and mouse xenografts. This effect is highly specific to p21, as loss of other checkpoint proteins and CDK inhibitors did not affect apoptosis. In telomerase, inhibited cell loss of p21 leads to E2F1- and p53-mediated transcriptional activation of p53-upregulated modulator of apoptosis, resulting in increased apoptosis. Combined genetic or pharmacological inhibition of telomerase and p21 synergistically suppresses tumor growth. Furthermore, we demonstrate that simultaneous inhibition of telomerase and p21 also suppresses growth of tumors containing mutant p53 following pharmacological restoration of p53 activity. Collectively, our results establish that inactivation of p21 leads to increased apoptosis upon telomerase inhibition and thus identify a genetic vulnerability that can be exploited to treat many human cancers containing either wild-type or mutant p53.