Wiggert A van Cappellen

Erasmus MC, Rotterdam, South Holland, Netherlands

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Publications (74)326.08 Total impact

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    ABSTRACT: The postsynaptic apparatus of the neuromuscular junction (NMJ) traps and anchors acetylcholine receptors (AChRs) at high density at the synapse. We have previously shown that microtubule (MT) capture by CLASP2, a MT plus-end tracking protein (+TIP), increases the size and receptor density of AChR clusters at the NMJ through the delivery of AChRs, and that this is regulated by a pathway involving neuronal agrin and several postsynaptic kinases, including GSK3. Phosphorylation by GSK3 has been shown to cause CLASP2 dissociation from MT ends, and nine potential phosphorylation sites for GSK3 have been mapped on CLASP2. How CLASP2 phosphorylation regulates MT capture at the NMJ and how this controls the size of AChR clusters is not yet understood. To examine this we used myotubes cultured on agrin patches that induce AChR clustering in a two-dimensional manner. We show that expression of a CLASP2 mutant, in which the nine GSK3 target serines are mutated to alanine (CLASP2-9XS/A) and which is resistant to GSK3β-dependent phosphorylation, promotes MT capture at clusters and increases AChR cluster size, compared to myotubes that express similar levels of wild type CLASP2, or that are non-infected. Conversely, myotubes expressing a phosphomimetic form of CLASP2 (CLASP2-8XS/D) show enrichment of immobile mutant CLASP2 in clusters, but MT capture and AChR cluster size are reduced. Taken together our data suggest that both GSK3β-dependent phosphorylation and the level of CLASP2 play a role in the maintenance of AChR cluster size through the regulated capture and release of MT plus-ends.
    The Journal of biological chemistry. 09/2014;
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    ABSTRACT: Due to the tremendous progress in microscopic imaging of fluorescently labeled proteins in living cells, the insight into the highly dynamic behavior of transcription factors has rapidly increased over the past decade. However, a consistent quantitative scheme of their action is still lacking. Using the androgen receptor as a model system, we combined three different fluorescence microscopy assays: single-molecule microscopy, photobleaching and correlation spectroscopy, to provide a quantitative model of the action of this transcription factor. This approach enabled us to distinguish two types of AR-DNA binding: very brief interactions, in the order of a few hundred milliseconds, and hormone-induced longer-lasting interactions, with a characteristic binding time of several seconds. In addition, freely mobile ARs were slowed down in the presence of hormone, suggesting the formation of large AR-coregulator complexes in the nucleoplasm upon hormone activation. Our data suggest a model in which mobile, hormone-induced transcription factor-coregulator complexes probe DNA by briefly binding at random sites, and only forming relatively stable transcription initiation complexes when bound to specific recognition sequences.
    Journal of Cell Science 01/2014; · 5.88 Impact Factor
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    ABSTRACT: Chromatin remodeling is tightly linked to all DNA-transacting activities. To study chromatin remodeling during DNA repair, we established quantitative fluorescence imaging methods to measure the exchange of histones in chromatin in living cells. We show that particularly H2A and H2B are evicted and replaced at an accelerated pace at sites of UV-induced DNA damage. This accelerated exchange of H2A/H2B is facilitated by SPT16, one of the two subunits of the histone chaperone FACT (facilitates chromatin transcription) but largely independent of its partner SSRP1. Interestingly, SPT16 is targeted to sites of UV light-induced DNA damage-arrested transcription and is required for efficient restart of RNA synthesis upon damage removal. Together, our data uncover an important role for chromatin dynamics at the crossroads of transcription and the UV-induced DNA damage response.
    Molecular cell 08/2013; 51(4):469-79. · 14.61 Impact Factor
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    ABSTRACT: In mammalian meiotic prophase, the initial steps in repair of SPO11-induced DNA double-strand breaks (DSBs) are required to obtain stable homologous chromosome pairing and synapsis. The X and Y chromosomes pair and synapse only in the short pseudo-autosomal regions. The rest of the chromatin of the sex chromosomes remain unsynapsed, contains persistent meiotic DSBs, and the whole so-called XY body undergoes meiotic sex chromosome inactivation (MSCI). A more general mechanism, named meiotic silencing of unsynapsed chromatin (MSUC), is activated when autosomes fail to synapse. In the absence of SPO11, many chromosomal regions remain unsynapsed, but MSUC takes place only on part of the unsynapsed chromatin. We asked if spontaneous DSBs occur in meiocytes that lack a functional SPO11 protein, and if these might be involved in targeting the MSUC response to part of the unsynapsed chromatin. We generated mice carrying a point mutation that disrupts the predicted catalytic site of SPO11 (Spo11(YF/YF) ), and blocks its DSB-inducing activity. Interestingly, we observed foci of proteins involved in the processing of DNA damage, such as RAD51, DMC1, and RPA, both in Spo11(YF/YF) and Spo11 knockout meiocytes. These foci preferentially localized to the areas that undergo MSUC and form the so-called pseudo XY body. In SPO11-deficient oocytes, the number of repair foci increased during oocyte development, indicating the induction of S phase-independent, de novo DNA damage. In wild type pachytene oocytes we observed meiotic silencing in two types of pseudo XY bodies, one type containing DMC1 and RAD51 foci on unsynapsed axes, and another type containing only RAD51 foci, mainly on synapsed axes. Taken together, our results indicate that in addition to asynapsis, persistent SPO11-induced DSBs are important for the initiation of MSCI and MSUC, and that SPO11-independent DNA repair foci contribute to the MSUC response in oocytes.
    PLoS Genetics 06/2013; 9(6):e1003538. · 8.52 Impact Factor
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    ABSTRACT: LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.
    Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor
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    ABSTRACT: PURPOSE: To improve nanoliposomal-doxorubicin (DoxNL) delivery in tumor cells using liposome membrane-incorporated short-chain sphingolipids (SCS) with selective membrane-permeabilizing properties. DoxNL bilayers contained synthetic short-chain derivatives of known membrane microdomain-forming sphingolipids; C8-glucosylceramide (C8-GluCer), C8-galactosylceramide (C8-GalCer) or C8-lactosylceramide (C8-LacCer). METHODS: DoxNL enriched with C8-GluCer or C8-GalCer were developed, optimized and characterized with regard to size, stability and drug retention. In vitro cytotoxic activity was studied in a panel of human tumor cell lines and normal cells. Intracellular Dox delivery was measured by flow cytometry and visualized by fluorescence microscopy. For a further understanding of the involved drug delivery mechanism confocal microscopy studies addressed the cellular fate of the nanoliposomes, the SCS and Dox in living cells. RESULTS: C8-LacCer-DoxNL aggregated upon Dox loading. In tumor cell lines SCS-DoxNL with C8-GluCer or C8-GalCer demonstrated strongly increased Dox delivery and cytotoxicity compared to standard DoxNL. Surprisingly, this effect was much less pronounced in normal cells. Nanoliposomes were not internalized, SCS however transfered from the nanoliposomal bilayer to the cell membrane and preceded cellular uptake and subsequent nuclear localization of Dox. CONCLUSION: C8-GluCer or C8-GalCer incorporated in DoxNL selectively improved intracellular drug delivery upon transfer to tumor cell membranes by local enhancement of cell membrane permeability.
    Pharmaceutical Research 05/2013; · 4.74 Impact Factor
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    ABSTRACT: Polarization purity is one of the most stringent requirements for future radio telescopes. To evaluate polarization ratio levels of large phased arrays involving thousands of elements, a full wave simulation approach is very time consuming, when feasible. Commonly, large arrays are approximated by infinite array approach or studied by single (isolated) element analysis. Being simplified approaches, part of the effect of the array configuration is missed resulting in analysis error. A study of this effect for regular arrays based on the intrinsic cross-polarization ratio (IXR) is presented.
    Antennas and Propagation (EuCAP), 2013 7th European Conference on, Gothenburg, Sweden; 04/2013
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    ABSTRACT: Transcription steps are marked by different modifications of the C-terminal domain of RNA polymerase II (RNAPII). Phosphorylation of Ser5 and Ser7 by cyclin-dependent kinase 7 (CDK7) as part of TFIIH marks initiation, whereas phosphorylation of Ser2 by CDK9 marks elongation. These processes are thought to take place in localized transcription foci in the nucleus, known as "transcription factories," but it has been argued that the observed clusters/foci are mere fixation or labeling artifacts. We show that transcription factories exist in living cells as distinct foci by live-imaging fluorescently labeled CDK9, a kinase known to associate with active RNAPII. These foci were observed in different cell types derived from CDK9-mCherry knock-in mice. We show that these foci are very stable while highly dynamic in exchanging CDK9. Chromatin immunoprecipitation (ChIP) coupled with deep sequencing (ChIP-seq) data show that the genome-wide binding sites of CDK9 and initiating RNAPII overlap on transcribed genes. Immunostaining shows that CDK9-mCherry foci colocalize with RNAPII-Ser5P, much less with RNAPII-Ser2P, and not with CDK12 (a kinase reported to be involved in the Ser2 phosphorylation) or with splicing factor SC35. In conclusion, transcription factories exist in living cells, and initiation and elongation of transcripts takes place in different nuclear compartments.
    Genes & development 04/2013; 27(7):767-777. · 12.08 Impact Factor
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    ABSTRACT: Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15-80 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 10^-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band.
    Astronomy and Astrophysics 02/2013; 551(L11). · 5.08 Impact Factor
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    ABSTRACT: Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions: Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.
    Astronomy and Astrophysics 12/2012; 549(A11):1-16. · 5.08 Impact Factor
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    ABSTRACT: M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.
    Astronomy and Astrophysics 10/2012; 547(A56):1-20. · 5.08 Impact Factor
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    ABSTRACT: The chromatin architecture is constantly changing due to cellular processes such as cell proliferation, differentiation and changes in the expression profile such as gene activation or silencing. Unraveling the changes that occur in the chromatin structure during these processes has been a topic of interest for many years. It is known that gene activation of large gene loci is thought to occur by means of active looping mechanism. It was also shown for the β-globin locus that the gene's promotor interacts with an active chromatin hub by means of an active looping mechanism. This predicts that the locus changes in 3D nuclear volume occupation and chromatin shape. In search to visualize and measure these dynamic changes in chromatin structure of the β-globin locus, we used a 3D DNA-FISH method in combination with 3D image acquisition to volume render fluorescent signals into 3D objects. These 3D chromatin structures were geometrically analyzed and results prior to and after gene activation were quantitatively compared. Confocal and super-resolution imaging reveal that the inactive locus occurs in several different conformations. These conformations change in shape and surface structure upon cell differentiation into a more folded and rounded structure that has a substantially smaller size and volume. These physical measurements represent the first non-biochemical evidence that upon gene activation an actively transcribing chromatin hub is formed by means of additional chromatin looping.
    Journal of Cell Science 07/2012; · 5.88 Impact Factor
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    ABSTRACT: Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR Low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of -0.81 \pm 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last \sim 0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.
    Astronomy and Astrophysics 05/2012; 543(A2256):1-13. · 5.08 Impact Factor
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    ABSTRACT: Circularly polarized radiation is frequently used in satellite communication. This paper discusses how circular polarization can be generated by means of an array of linearly-polarized antenna elements by applying the sequential rotation technique. A calibration method is used to improve the axial ratio from ±30° to ±60° off bore sight by using a 4-bit accuracy for the active components. A measurement campaign was organized based on a demonstrator developed at ASTRON. The calibration of the axial ratio is applied to the measured element patterns of the ASTRON egg-crate structure antenna array operating at 7.14 GHz. The results prove that the axial ratio can be improved significantly. Finally the same technique will be applied to a linear array operating at 20 GHz with sequential rotation configuration in front of the reflector to improve the axial ratio in the boresight direction.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on, Prague, Czech Republic; 03/2012
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    ABSTRACT: Androgen-regulated gene expression is a highly coordinated dynamic process mediated by androgen receptor (AR) ligand binding and DNA binding, and by specific AR protein-protein interactions. The latter include DNA-binding domain (D-box) interactions in AR homodimers, and the interaction of the FQNLF motif in the AR N-terminal domain and the coactivator groove in the ligand-binding domain (N/C interaction). We have studied these interactions in AR homodimerization using quantitative imaging techniques. We found that the initial cytoplasmic intramolecular AR N/C interaction after ligand binding is followed by a D-box-dimerization-dependent transition to intermolecular N/C interaction in a proportion of nuclear ARs. The consecutive steps leading to homodimerization are initiated prior to DNA binding. Our data indicate the presence of nuclear pools of both AR homodimers and monomers. On the basis of AR-regulated reporter assays we propose specificity in regulation of gene expression by AR homodimers and monomers mediated by AR domain interactions. Moreover, our findings elucidate important steps in the spatiotemporal organization of AR intra- and inter-molecular interactions.
    Journal of Cell Science 02/2012; 125(Pt 8):1970-9. · 5.88 Impact Factor
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    ABSTRACT: When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to detect these pulses. In this work we propose an e?cient trigger implementation for LOFAR optimized for the observation of short radio pulses.
    Nuclear Instruments and Methods 02/2012; 664(1):171-185.
  • Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 02/2012; 664(1):171–185. · 1.14 Impact Factor
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    ABSTRACT: For molecular imaging using ultrasound contrast agents, targeted microbubbles are designed with specific ligands linked to the coated shell. Research is ongoing to determine the binding force of targeted microbubbles and to distinguish bound from unbound targeted microbubbles using ultrasound. For this, the actual surface of the targeted microbubbles that binds to a pathology is important. This study focuses on determining the surface contact of bound targeted microbubbles by fluorescence microscopy. Biotinylated lipid-coated microbubbles (3-7 μm in diameter) with either DSPC or DPPC as the main lipid were targeted to a streptavidin-coated surface. The binding area of targeted microbubbles was found to be 6 ± 4% of the total microbubble surface for microbubbles with DSPC as the main lipid (n=22) and 11 ± 4% for microbubbles with DPPC as the main lipid (n=24). The difference can be explained by the heterogeneous distribution of the ligand for DSPC microbubbles whereas the ligand is homogeneously distributed for DPPC microbubbles. These findings can be used to improve the binding of targeted microbubbles and for the ongoing research to distinguish bound from unbound microbubbles.
    Ultrasonics Symposium (IUS), 2012 IEEE International; 01/2012
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    W.A. van Cappellen, S.J. Wijnholds, L. Bakker
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    ABSTRACT: A novel method to form multiple high-sensitivity beams with a reflector based radio telescope is to use a phased array feed (PAF). To accurately reconstruct the polarization state of the incident radio waves, the beam patterns of the PAF are of crucial importance. The co- and cross-polar element patterns of an oversampled L-band PAF prototype system have been measured in an interferometric setup. It is demonstrated that, although the antenna elements sampling the two orthogonal polarizations of the focal field are not co-located, dual polarized beams can be formed by combining the element responses in a beamformer. The capability of the PAF to optimize its beams in terms of sensitivity, sidelobes and polarization characteristics is demonstrated.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012

Publication Stats

1k Citations
326.08 Total Impact Points

Institutions

  • 2005–2014
    • Erasmus MC
      • • Department of Reproduction and Development
      • • Department of Medical Informatics
      Rotterdam, South Holland, Netherlands
  • 2011–2013
    • Erasmus Universiteit Rotterdam
      • Department of Reproduction and Development
      Rotterdam, South Holland, Netherlands
  • 2009–2011
    • Netherlands Institute for Radio Astronomy
      Dwingelo, Drenthe, Netherlands
  • 2008
    • University of Groningen
      • Kapteyn Astronomical Institute
      Groningen, Province of Groningen, Netherlands
  • 2007
    • Chalmers University of Technology
      • Department of Signals and Systems
      Goeteborg, Västra Götaland, Sweden