Actomyosin Pulls to Advance the Nucleus in a Migrating Tissue Cell

Abstract

The cytoskeletal forces involved in translocating the nucleus in a migrating tissue cell remain unresolved. Previous studies have variously implicated actomyosin-generated pushing or pulling forces on the nucleus, as well as pulling by nucleus-bound microtubule motors. We found that the nucleus in an isolated migrating cell can move forward without any trailing-edge detachment. When a new lamellipodium was triggered with photoactivation of Rac1, the nucleus moved toward the new lamellipodium. This forward motion required both nuclear-cytoskeletal linkages and myosin activity. Apical or basal actomyosin bundles were found not to translate with the nucleus. Although microtubules dampen fluctuations in nuclear position, they are not required for forward translocation of the nucleus during cell migration. Trailing-edge detachment and pulling with a microneedle produced motion and deformation of the nucleus suggestive of a mechanical coupling between the nucleus and the trailing edge. Significantly, decoupling the nucleus from the cytoskeleton with KASH overexpression greatly decreased the frequency of trailing-edge detachment. Collectively, these results explain how the nucleus is moved in a crawling fibroblast and raise the possibility that forces could be transmitted from the front to the back of the cell through the nucleus.

Tight coupling between nucleus and cell migration through the perinuclear actin cap

“…A recent study demonstrated that anterior actomyosin contraction was the driving force for the forward nuclear translocation, suggesting that the apical actin fibers (i.e. actin cap in our paper) were not necessary for this nuclear motion because nuclear translocation was observed while aligned apical fibers moved orthogonally or even while these fibers were stationary (Wu et al, 2014). Taking into account the dynamic assembly of LINC complexes that reorganize the actin cap in a highly dynamic manner, it may be possible to observe the lateral translation or stationary situation of actin-cap fibers, i.e.…”

The nucleus is an intracellular propagator of tensile forces in NIH 3T3 fibroblasts

“…Microtubule motors -like dynein and kinesin -generate shear forces on the nuclear surface that result in nuclear rotations in fibroblasts (Levy and Holzbaur, 2008;Wu et al, 2011), can cause nuclear translocations in epithelial cells (Wilson and Holzbaur, 2012) and are required for nuclear migration in Caenorhabditis elegans embryos Meyerzon et al, 2009). Actomyosin forces have been hypothesized to pull on the nucleus (Chancellor et al, 2010;Friedl et al, 2011;Sims et al, 1992;Wang et al, 2009;Wu et al, 2014), push on it (Friedl et al, 2011;Roth et al, 1995;Zhang et al, 2007) and shear it (Folker et al, 2011;Kim et al, 2014;Luxton et al, 2010). In addition, intermediate filaments can trap and resist the motion of the nucleus by forming a 'cage'-like structure around it (Ketema et al, 2007;Postel et al, 2011;Wilhelmsen et al, 2005), as well as transmit active forces from the actomyosin cytoskeleton to the nuclear surface .…”

“…In previous work (Wu et al, 2014), we used the Rac1 photoactivation assay (Machacek et al, 2009;Wang et al, 2010;Wu et al, 2009) to trigger local lamellipodia formation in serum-starved NIH 3T3 fibroblasts, which enabled selective perturbation to the nuclear force balance. The increased pulling force on the nucleus triggered by photoactivated lamellipodial formation suggested that a 'tug-of-war' between anterior and posterior pulling forces positions the nucleus in crawling NIH 3T3 fibroblasts (Wu et al, 2014); although it remained unclear whether these results apply to spontaneous lamellipodial formation in unstimulated migrating cells. In this paper, we perform traction-force microscopy analyses of intact and linker of nucleoskeleton-to-cytoskeleton (LINC)-complex-perturbed cells to show that efficient front-to-back transmission of tensile forces in the cell requires nucleus-cytoskeleton linkages.…”

Direct force probe reveals the mechanics of nuclear homeostasis in the mammalian cell

“…It is known that the migrating cell moves the nucleus by transferring cytoskeletal forces through connections between the cytoskeleton and the nuclear surface (1, 2). Even in a stationary cell, the nuclear shape and central position are stably maintained in mechanical homeostasis at defined locations in the cell, despite the fact that the dynamic cytoskeleton continues to generate constantly fluctuating forces on the nucleus (3, 4).The source of fluctuating forces on the nucleus includes nuclear-embedded microtubule motors such as dynein and kinesin (5-7) and actomyosin forces that push on and pull the nucleus (2,8,9). The nucleus is also exposed to extracellular forces, such as those applied to adhesion receptors, which can be transmitted through the cytoskeleton onto the nuclear surface (10-12).…”

“…The source of fluctuating forces on the nucleus includes nuclear-embedded microtubule motors such as dynein and kinesin (5-7) and actomyosin forces that push on and pull the nucleus (2,8,9). The nucleus is also exposed to extracellular forces, such as those applied to adhesion receptors, which can be transmitted through the cytoskeleton onto the nuclear surface (10-12).…”