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  • 1. Berger, Susanne
    et al.
    Schäfer, Gritt
    Kesper, Dörthe A
    Holz, Anne
    Eriksson, Therese
    Umeå University, Faculty of Science and Technology, Department of Molecular Biology (Faculty of Science and Technology).
    Palmer, Ruth H
    Umeå University, Faculty of Science and Technology, Department of Molecular Biology (Faculty of Science and Technology).
    Beck, Lothar
    Klämbt, Christian
    Renkawitz-Pohl, Renate
    Onel, Susanne-Filiz
    WASP and SCAR have distinct roles in activating the Arp2/3 complex during myoblast fusion2008In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 121, no Pt 8, p. 1303-1313Article in journal (Refereed)
    Abstract [en]

    Myoblast fusion takes place in two steps in mammals and in Drosophila. First, founder cells (FCs) and fusion-competent myoblasts (FCMs) fuse to form a trinucleated precursor, which then recruits further FCMs. This process depends on the formation of the fusion-restricted myogenic-adhesive structure (FuRMAS), which contains filamentous actin (F-actin) plugs at the sites of cell contact. Fusion relies on the HEM2 (NAP1) homolog Kette, as well as Blow and WASP, a member of the Wiskott-Aldrich-syndrome protein family. Here, we show the identification and characterization of schwächling--a new Arp3-null allele. Ultrastructural analyses demonstrate that Arp3 schwächling mutants can form a fusion pore, but fail to integrate the fusing FCM. Double-mutant experiments revealed that fusion is blocked completely in Arp3 and wasp double mutants, suggesting the involvement of a further F-actin regulator. Indeed, double-mutant analyses with scar/WAVE and with the WASP-interacting partner vrp1 (sltr, wip)/WIP show that the F-actin regulator scar also controls F-actin formation during myoblast fusion. Furthermore, the synergistic phenotype observed in Arp3 wasp and in scar vrp1 double mutants suggests that WASP and SCAR have distinct roles in controlling F-actin formation. From these findings we derived a new model for actin regulation during myoblast fusion.

  • 2.
    Carlsson, Sven R
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Simonsen, Anne
    Membrane dynamics in autophagosome biogenesis2015In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 128, no 2, p. 193-205Article in journal (Other academic)
    Abstract [en]

    Bilayered phospholipid membranes are vital to the organization of the living cell. Based on fundamental principles of polarity, membranes create borders allowing defined spaces to be encapsulated. This compartmentalization is a prerequisite for the complex functional design of the eukaryotic cell, yielding localities that can differ in composition and operation. During macroautophagy, cytoplasmic components become enclosed by a growing double bilayered membrane, which upon closure creates a separate compartment, the autophagosome. The autophagosome is then primed for fusion with endosomal and lysosomal compartments, leading to degradation of the captured material. A large number of proteins have been found to be essential for autophagy, but little is known about the specific lipids that constitute the autophagic membranes and the membrane modeling events that are responsible for regulation of autophagosome shape and size. In this Commentary, we review the recent progress in our understanding of the membrane shaping and remodeling events that are required at different steps of the autophagy pathway.

  • 3.
    Francis, Monika K.
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Holst, Mikkel R.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Vidal-Quadras, Maite
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Henriksson, Sara
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Department of Molecular Biology (Faculty of Medicine).
    Santarella-Mellwig, Rachel
    Sandblad, Linda
    Umeå University, Faculty of Medicine, Department of Molecular Biology (Faculty of Medicine).
    Lundmark, Richard
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Endocytic membrane turnover at the leading edge is driven by a transient interaction between Cdc42 and GRAF12015In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 128, no 22, p. 4183-4195Article in journal (Refereed)
    Abstract [en]

    Changes in cell morphology require coordination of plasma membrane turnover and cytoskeleton dynamics, processes that are regulated by Rho GTPases. Here, we describe how a direct interaction between the Rho GTPase Cdc42 and the GTPase activating protein (GAP) GRAF1, facilitate rapid cell surface turnover at the leading edge. Both Cdc42 and GRAF1 were required for fluid phase uptake and regulated the generation of transient GRAF1-coated endocytic carriers, distinct from clathrin coated vesicles. GRAF1 was found to transiently assemble at discrete Cdc42-enriched punctae at the plasma membrane resulting in a corresponding decrease in Cdc42 microdomain association. However, Cdc42 captured in its active state was, via a GAP domain mediated interaction, localised together with GRAF1 on accumulated internal structures derived from the cell surface. Correlative fluorescence and electron tomography microscopy revealed that these structures were clusters of small membrane carriers affected in their endosomal processing. We conclude that a transient interaction between Cdc42 and GRAF1 drives endocytic turnover and controls the transition essential for endosomal maturation of plasma membrane internalised by this mechanism.

  • 4.
    Gustavsson, Anna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Molecular Biology (Faculty of Science and Technology).
    Armulik, Annika
    Brakebusch, Cord
    Fässler, Reinhard
    Johansson, Staffan
    Fällman, Maria
    Umeå University, Faculty of Medicine, Department of Molecular Biology (Faculty of Medicine).
    Role of the β1-integrin cytoplasmic tail in mediating invasin-promoted internalization of Yersinia2002In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 115, no 13, p. 2669-2678Article in journal (Refereed)
    Abstract [en]

    Invasin of Yersinia pseudotuberculosis binds to beta1-integrins on host cells and triggers internalization of the bacterium. To elucidate the mechanism behind the beta1-integrin-mediated internalization of Yersinia, a beta1-integrin-deficient cell line, GD25, transfected with wild-type beta1A, beta1B or different mutants of the beta1A subunit was used. Both beta1A and beta1B bound to invasin-expressing bacteria, but only beta1A was able to mediate internalization of the bacteria. The cytoplasmic region of beta1A, differing from beta1B, contains two NPXY motifs surrounding a double threonine site. Exchanging the tyrosines of the two NPXYs to phenylalanines did not inhibit the uptake, whereas a marked reduction was seen when the first tyrosine (Y783) was exchanged to alanine. A similar reduction was seen when the two nearby threonines (TT788-9) were exchanged with alanines. It was also noted that cells affected in bacterial internalization exhibited reduced spreading capability when seeded onto invasin, suggesting a correlation between the internalization of invasin-expressing bacteria and invasin-induced spreading. Likewise, integrins defective in forming peripheral focal complex structures was unable to mediate uptake of invasin-expressing bacteria.

  • 5. Höög, Johanna L
    et al.
    Huisman, Stephen M
    Sebö-Lemke, Zsofia
    Sandblad, Linda
    European Mol Biol Labs, Cell Biol & Biophys Program, D-69117 Heidelberg, Germany .
    McIntosh, J Richard
    Antony, Claude
    Brunner, Damian
    Electron tomography reveals a flared morphology on growing microtubule ends2011In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 124, no Pt 5, p. 693-698Article in journal (Refereed)
    Abstract [en]

    Microtubules (MTs) exhibit dynamic instability, alternating between phases of growth and shortening, mostly at their uncapped plus ends. Based on results from cryo-electron microscopy it was proposed that growing MTs display mainly curved sheets and blunt ends; during depolymerisation curled 'ramshorns' predominate. Observations of MTs in mitotic cells have suggested that the situation in vivo differs from that in vitro, but so far, a clear comparison between in vivo and in vitro results has not been possible because MT end structures could not be correlated directly with the dynamic state of that particular MT. Here we combine light microscopy and electron tomography (ET) to show that growing MT plus ends in the fission yeast Schizosaccharomyces pombe display predominantly a flared morphology. This indicates that MT polymerisation in vivo and in vitro can follow different paths.

  • 6. Klinger, Stine C
    et al.
    Glerup, Simon
    Raarup, Merete K
    Mari, Muriel C
    Nyegaard, Mette
    Koster, Gerbrand
    Prabakaran, Thaneas
    Nilsson, Stefan K
    Umeå University, Faculty of Medicine, Department of Medical Biosciences, Physiological chemistry.
    Kjaergaard, Maj M
    Bakke, Oddmund
    Nykjær, Anders
    Olivecrona, Gunilla
    Umeå University, Faculty of Medicine, Department of Medical Biosciences, Physiological chemistry.
    Petersen, Claus Munck
    Nielsen, Morten S
    SorLA regulates the activity of lipoprotein lipase by intracellular trafficking2011In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 124, p. 1095-1105Article in journal (Refereed)
    Abstract [en]

    Many different tissues and cell types exhibit regulated secretion of lipoprotein lipase (LPL). However, the sorting of LPL in the trans Golgi network has not, hitherto, been understood in detail. Here, we characterize the role of SorLA (officially known as SorLA-1 or sortilin-related receptor) in the intracellular trafficking of LPL. We found that LPL bound to SorLA under neutral and acidic conditions, and in cells this binding mainly occurred in vesicular structures. SorLA expression changed the subcellular distribution of LPL so it became more concentrated in endosomes. From the endosomes, LPL was further routed to the lysosomes, which resulted in a degradation of newly synthesized LPL. Consequently, an 80% reduction of LPL activity was observed in cells that expressed SorLA. By analogy, SorLA regulated the vesicle-like localization of LPL in primary neuronal cells. Thus, LPL binds to SorLA in the biosynthetic pathway and is subsequently transported to endosomes. As a result of this SorLA mediated-transport, newly synthesized LPL can be routed into specialized vesicles and eventually sent to degradation, and its activity thereby regulated.

  • 7.
    Lundmark, Richard
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Carlsson, Sven R
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    SNX9 - a prelude to vesicle release2009In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 122, no 1, p. 5-11Article in journal (Refereed)
    Abstract [en]

    The sorting nexin SNX9 has, in the past few years, been singled out as an important protein that participates in fundamental cellular activities. SNX9 binds strongly to dynamin and is partly responsible for the recruitment of this GTPase to sites of endocytosis. SNX9 also has a high capacity for modulation of the membrane and might therefore participate in the formation of the narrow neck of endocytic vesicles before scission occurs. Once assembled on the membrane, SNX9 stimulates the GTPase activity of dynamin to facilitate the scission reaction. It has also become clear that SNX9 has the ability to activate the actin regulator N-WASP in a membrane-dependent manner to coordinate actin polymerization with vesicle release. In this Commentary, we summarize several aspects of SNX9 structure and function in the context of membrane remodeling, discuss its interplay with various interaction partners and present a model of how SNX9 might work in endocytosis.

  • 8.
    Mohan, Jagan
    et al.
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Molecular Infection Medicine Sweden (MIMS).
    Morén, Björn
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Molecular Infection Medicine Sweden (MIMS).
    Larsson, Elin
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Holst, Mikkel
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Lundmark, Richard
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics. Umeå University, Faculty of Medicine, Molecular Infection Medicine Sweden (MIMS). Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Cavin3 interacts with cavin1 and caveolin1 to increase surface dynamics of caveolae2015In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 128, no 5, p. 979-991Article in journal (Refereed)
    Abstract [en]

    Caveolae are invaginations of the cell surface thought to regulate membrane tension, signalling, adhesion and lipid homeostasis due to their dynamic behaviour ranging from stable surface association to dynamic rounds of fission and fusion with the plasma membrane. The caveolae coat is generated by oligomerisation of the membrane protein caveolin and the family of cavin proteins. Here, we show that cavin3 is targeted to caveolae by cavin1 where it interacts with the scaffolding domain of caveolin1 and promote caveolae dynamics. We found that the N-terminal region of cavin3 binds a trimer of the cavin1 N-terminus in competition with a homologous cavin2 region, showing that the cavins form distinct subcomplexes via their N-terminal regions. Our data shows that cavin3 is enriched at deeply invaginated caveolae and that loss of cavin3 in cells results in an increase of stable caveolae and a decrease of caveolae with short duration time at the membrane. We propose that cavin3 is recruited to the caveolae coat by cavin1 to interact with caveolin1 and regulate the duration time of caveolae at the plasma membrane.

  • 9.
    Nord, Hanna
    et al.
    Umeå University, Faculty of Medicine, Umeå Centre for Molecular Medicine (UCMM).
    Skalman, Lars Nygard
    Umeå University, Faculty of Medicine, Umeå Centre for Molecular Medicine (UCMM).
    von Hofsten, Jonas
    Umeå University, Faculty of Medicine, Umeå Centre for Molecular Medicine (UCMM).
    Six1 regulates proliferation of Pax7-positive muscle progenitors in zebrafish2013In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 126, no 8, p. 1868-1880Article in journal (Refereed)
    Abstract [en]

    In the embryonic zebrafish, skeletal muscle fibres are formed from muscle progenitors in the paraxial mesoderm. The embryonic myotome is mostly constituted of fast-twitch-specific fibres, which are formed from a fast-specific progenitor cell pool. The most lateral fraction of the fast domain in the myotome of zebrafish embryos derives from the Pax7-positive dermomyotome-like cells. In this study, we show that two genes, belonging to the sine oculus class 1 (six1) genes (six1a and six1b), are both essential for the regulation of Pax7(+) cell proliferation and, consequently, in their differentiation during the establishment of the zebrafish dermomyotome. In both six1a and six1b morphant embryos, Pax7(+) cells are initially formed but fail to proliferate, as detected by reduced levels of the proliferation marker phosphohistone3 and reduced brdU incorporation. In congruence, overexpression of six1a or six1b leads to increased Pax7(+) cell number and reduced or alternatively delayed fibre cell differentiation. Bone morphogenetic protein signalling has previously been suggested to inhibit differentiation of Pax7(+) cells in the dermomyotome. Here we show that the remaining Pax7(+) cells in six1a and six1b morphant embryos also have significantly reduced pSmad1/5/8 levels and propose that this leads to a reduced proliferative activity, which may result in a premature differentiation of Pax7(+) cells in the zebrafish dermomyotome. In summary, we show a mechanism for Six1a and Six1b in establishing the Pax7(+) cell derived part of the fast muscle and suggest new important roles for Six1 in the regulation of the Pax7(+) muscle cell population through pSmad1/5/8 signalling.

  • 10.
    Rilla, Kirsi
    et al.
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Lammi, Mikko
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Sironen, Reijo
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Törrönen, Kari
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Luukkonen, Merja
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Hascall, Vincent
    Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio, USA.
    Midura, Ronald
    Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio, USA.
    Hyttinen, Mika
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Pelkonen, Jukka
    Department of Clinical Microbiology, University of Kuopio, Kuopio, Finland.
    Tammi, Markku
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Tammi, Raija
    Department of Anatomy, University of Kuopio, Kuopio, Finland .
    Changed lamellipodial extension, adhesion plaques and migration in epidermal keratinocytes containing constitutively expressed sense and antisense hyaluronan synthase 2 (Has2) genes.2002In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 115, p. 3633-3643, article id 12186949Article in journal (Refereed)
    Abstract [en]

    Hyaluronan is a major component of the epidermal extracellular matrix, is actively synthesized by keratinocytes and shows fast matrix turnover in the stratified epithelium. We probed the importance of hyaluronan synthesis in keratinocytes by establishing cell lines carrying the exogenous hyaluronan synthase 2 (Has2) gene in sense and antisense orientations to increase and decrease their hyaluronan synthesis, respectively. Compared with cell lines transfected with the vector only, most clones containing the Has2 sense gene migrated faster in an in vitro wounding assay, whereas Has2 antisense cells migrated more slowly. Has2 antisense clones showed delayed entry into the S phase of cell cycle following plating, smaller lamellipodia and less spreading on the substratum. The decrease of hyaluronan on the undersurface of Has2 antisense cells was associated with an increased area of adhesion plaques containing vinculin. Exogenous hyaluronan added to the keratinocyte cultures had a minor stimulatory effect on migration after wounding but did not restore the reduced migratory ability of Has2 antisense cells. Hyaluronan decasaccharides that displace receptor bound hyaluronan in keratinocytes, and Streptomyces hyaluronidase sufficient to remove most cell surface hyaluronan had little effect on cell migration. The results suggest that the dynamic synthesis of hyaluronan directed by Has2, rather than the abundance of pericellular hyaluronan, controls keratinocyte migration, a cell function vital for the repair of squamous epithelia following wounding.

  • 11.
    Vidal-Quadras, Maite
    et al.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Holst, Mikkel R.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
    Francis, Monika K.
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB). Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Larsson, Elin
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB). Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Hachimi, Mariam
    Yau, Wai-Lok
    Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Peranen, Johan
    Martin-Belmonte, Fernando
    Lundmark, Richard
    Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB). Umeå University, Faculty of Medicine, Department of Medical Biochemistry and Biophysics.
    Endocytic turnover of Rab8 controls cell polarization2017In: Journal of Cell Science, ISSN 0021-9533, E-ISSN 1477-9137, Vol. 130, no 6, p. 1147-1157Article in journal (Refereed)
    Abstract [en]

    Adaptation of cell shape and polarization through the formation and retraction of cellular protrusions requires balancing of endocytosis and exocytosis combined with fine-tuning of the local activity of small GTPases like Rab8. Here, we show that endocytic turnover of the plasma membrane at protrusions is directly coupled to surface removal and inactivation of Rab8. Removal is induced by reduced membrane tension and mediated by the GTPase regulator associated with focal adhesion kinase-1 (GRAF1, also known as ARHGAP26), a regulator of clathrin-independent endocytosis. GRAF1-depleted cells were deficient in multi-directional spreading and displayed elevated levels of GTP-loaded Rab8, which was accumulated at the tips of static protrusions. Furthermore, GRAF1 depletion impaired lumen formation and spindle orientation in a 3D cell culture system, indicating that GRAF1 activity regulates polarity establishment. Our data suggest that GRAF1-mediated removal of Rab8 from the cell surface restricts its activity during protrusion formation, thereby facilitating dynamic adjustment of the polarity axis.

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