Jcb_201408098 1..10

Alterations in nuclear shape and positioning are often associated with changes in the behavior of cells (Starr and Fridolfsson, 2010). Recent studies proposed that tissue stiffness affects the levels of the nuclear lamina protein lamin A/C, resulting in altered transcriptional regulation (Simon and Wilson, 2011; Burke and Stewart, 2013; Zwerger et al., 2013; Fedorchak et al., 2014; Swift and Discher, 2014). Muscles exhibit inherently altered biomechanical properties and therefore should develop a specific strategy to keep nuclear morphology intact. Myonuclei in striated muscles face variable cytoplasmic strain exerted by the tightly associated sarcolemma and the axial contractile forces. How the myonuclei resist these forces has yet to be elucidated. 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. However, a direct connection between LINC proteins and the MT network has yet to be established. In this work, we identify two additional cytoplasmic components, namely EB1 and Shot, that function together with MSP300 to maintain myonuclear morphology. Importantly, we show that the MSP300 protein exhibits elastic properties, helping the myonuclei to resist cytoplasmic strain. Finally, we demonstrate a link between the aberrant nuclear architecture detected in mutant muscles and altered levels of nuclear regulatory proteins.