Article

Wilson, MH and Holzbaur, EL. Opposing microtubule motors drive robust nuclear dynamics in developing muscle cells. J Cell Sci 125(Pt 17): 4158-4169

Journal of Cell Science (Impact Factor: 5.43). 05/2012; 125(17). DOI: 10.1242/jcs.108688
Source: PubMed

ABSTRACT

Dynamic interactions with the cytoskeleton drive the movement and positioning of nuclei in many cell types. During muscle cell development, myoblasts fuse to form syncytial myofibers with nuclei positioned regularly along the length of the cell. Nuclear translocation in developing myotubes requires microtubules, but the mechanisms involved have not been elucidated. We find that as nuclei actively translocate through the cell, they rotate in three-dimensions. The nuclear envelope, nucleoli, and chromocenters within the nucleus rotate together as a unit. Both translocation and rotation require an intact microtubule cytoskeleton, which forms a dynamic bipolar network around nuclei. The plus- and minus-end directed microtubule motor proteins, kinesin-1 and dynein, localize to the nuclear envelope in myotubes. Kinesin-1 localization is mediated at least in part by interaction with klarsicht/ANC-1/Syne homology (KASH) proteins. Depletion of kinesin-1 abolishes nuclear rotation and significantly inhibits nuclear translocation, resulting in the abnormal aggregation of nuclei at the midline of the myotube. Dynein depletion also inhibits nuclear dynamics, but to a lesser extent, leading to altered spacing between adjacent nuclei. Thus, oppositely directed motors acting from the surface of the nucleus drive nuclear motility in myotubes. The variable dynamics observed for individual nuclei within a single myotube likely result from the stochastic activity of competing motors interacting with a complex bipolar microtubule cytoskeleton that is also continuously remodeled as the nuclei move. The three-dimensional rotation of myotube nuclei may facilitate their motility through the complex and crowded cellular environment of the developing muscle cell, allowing for proper myonuclear positioning.

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    • "Recent evidence has revealed the unique contribution of the microtubule (MT)-based cytoskeleton to nuclear shape and positioning in muscle fibers (Oddoux et al., 2013; Wilson and Holzbaur, 2015). In addition, various MT-associated proteins (MAPs), including dynein, kinesin, and MAP7/Esconsin, were shown to be essential for myonuclear positioning in striated muscles (Folker et al., 2012, 2014; Metzger et al., 2012; Wilson and Holzbaur, 2012). Furthermore, proteins of the linker of nucleoskeleton and cytoskeleton (LINC) complex, including the Klarsicht, ANC-1, Syne homology (KASH) proteins MSP300 and Klar and the Sad1p, UNC-84 (SUN) protein Klaroid (Elhanany-Tamir et al., 2012), are critical for maintenance of myonuclear positioning and shape. "
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    ABSTRACT: Muscle nuclei are exposed to variable cytoplasmic strain produced by muscle contraction and relaxation, but their morphology remains stable. Still, the mechanism responsible for maintaining myonuclear architecture, and its importance, is currently elusive. Herein, we uncovered a unique myonuclear scaffold in Drosophila melanogaster larval muscles, exhibiting both elastic features contributed by the stretching capacity of MSP300 (nesprin) and rigidity provided by a perinuclear network of microtubules stabilized by Shot (spectraplakin) and EB1. Together, they form a flexible perinuclear shield that protects myonuclei from intrinsic or extrinsic forces. The loss of this scaffold resulted in significantly aberrant nuclear morphology and subsequently reduced levels of essential nuclear factors such as lamin A/C, lamin B, and HP1. Overall, we propose a novel mechanism for maintaining myonuclear morphology and reveal its critical link to correct levels of nuclear factors in differentiated muscle fibers. These findings may shed light on the underlying mechanism of various muscular dystrophies. © 2015 Wang et al.
    Full-text · Article · May 2015 · The Journal of Cell Biology
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    • "The study of nuclear movement in muscle has revealed novel behaviors of moving nuclei (Wilson and Holzbaur, 2012; Folker et al., 2014), and has also identified proteins with novel roles in nuclear movement. MAP7/Ensconsin was long ago identified as a microtubule associated protein (Bulinski and Bossler, 1994), but a cellular role for this protein had not been identified. "
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    ABSTRACT: Muscle disease as a group is characterized by muscle weakness, muscle loss, and impaired muscle function. Although the phenotype is the same, the underlying cellular pathologies, and the molecular causes of these pathologies, are diverse. One common feature of many muscle disorders is the mispositioning of myonuclei. In unaffected individuals, myonuclei are spaced throughout the periphery of the muscle fiber such that the distance between nuclei is maximized. However, in diseased muscles, the nuclei are often clustered within the center of the muscle cell. Although this phenotype has been acknowledged for several decades, it is often ignored as a contributor to muscle weakness. Rather, these nuclei are taken only as a sign of muscle repair. Here we review the evidence that mispositioned myonuclei are not merely a symptom of muscle disease but also a cause. Additionally, we review the working models for how myonuclei move from two different perspectives: from that of the nuclei and from that of the cytoskeleton. We further compare and contrast these mechanisms with the mechanisms of nuclear movement in other cell types both to draw general themes for nuclear movement and to identify muscle-specific considerations. Finally, we focus on factors that can be linked to muscle disease and find that genes that regulate myonuclear movement and positioning have been linked to muscular dystrophy. Although the cause-effect relationship is largely speculative, recent data indicate that the position of nuclei should no longer be considered only a means to diagnose muscle disease.
    Full-text · Article · Dec 2013 · Frontiers in Physiology
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    • "Studies in mice have identified some of the factors necessary to position myonuclei (Lei et al., 2009; Puckelwartz et al., 2009; Puckelwartz et al., 2010); however, examination of the dynamics of myonuclear movement in vivo is difficult in mammalian systems. Conversely, cell culture systems have provided dynamic data, such as the speed and dynamics of nuclear movement and the organization of the cytoskeleton (Cadot et al., 2012; Wilson and Holzbaur, 2012; Zhang et al., 2009a), but the myonuclei in in vitro myotubes do not have defined end-point destinations in contrast to their in vivo counterparts. To assess the mechanisms of myonuclear movement in vivo, we utilized the developing Drosophila melanogaster embryo. "
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    ABSTRACT: Nuclei are precisely positioned within all cells, and mispositioned nuclei are a hallmark of many muscle diseases. Myonuclear positioning is dependent on Kinesin and Dynein, but interactions between these motor proteins and their mechanisms of action are unclear. We find that in developing Drosophila muscles, Dynein and Kinesin work together to move nuclei in a single direction by two separate mechanisms that are spatially segregated. First, the two motors work together in a sequential pathway that acts from the cell cortex at the muscle poles. This mechanism requires Kinesin-dependent localization of Dynein to cell cortex near the muscle pole. From this location Dynein can pull microtubule minus-ends and the attached myonuclei toward the muscle pole. Second, the motors exert forces directly on individual nuclei independently of the cortical pathway. However, the activities of the two motors on the nucleus are polarized relative to the direction of myonuclear translocation: Kinesin acts at the leading edge of the nucleus, whereas Dynein acts at the lagging edge of the nucleus. Consistent with the activities of Kinesin and Dynein being polarized on the nucleus, nuclei rarely change direction, and those that do, reorient to maintain the same leading edge. Conversely, nuclei in both Kinesin and Dynein mutant embryos change direction more often and do not maintain the same leading edge when changing directions. These data implicate Kinesin and Dynein in two distinct and independently regulated mechanisms of moving myonuclei, which together maximize the ability of myonuclei to achieve their proper localizations within the constraints imposed by embryonic development.
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