Mechanical Activation of Cells Induces Chromatin Remodeling Preceding MKL Nuclear Transport

Abstract

For cells to adapt to different tissues and changes in tissue mechanics, they must be able to respond to mechanical cues by changing their gene expression patterns. Biochemical signaling pathways for these responses have been elucidated, and recent evidence points to the involvement of force-induced deformation of the nucleus. However, it is still unclear how physical cues received at the plasma membrane (PM) spatiotemporally integrate to the functional chromatin organization of the cell nucleus. To investigate this issue, we applied mechanical forces through magnetic particles adhered to the PM of single cells and mapped the accompanying changes in actin polymerization, nuclear morphology, chromatin remodeling, and nuclear transport of soluble signaling intermediates using high-resolution fluorescence anisotropy imaging. Using this approach, we show the timescales associated with force-induced polymerization of actin and changes in the F/G actin ratio resulting in nuclear translocation of the G-actin-associated transcriptional cofactor, megakaryoblastic acute leukemia factor-1 (MKL). Further, this method of measuring nuclear organization at high spatiotemporal resolution with simultaneous force application revealed the physical propagation of forces to the nucleus, resulting in changes to chromatin organization, followed by nuclear deformation. We also describe a quantitative model that incorporates active stresses and chemical kinetics to evaluate the observed timescales. Our work suggests that mechanical activation of cells is accompanied by distinct timescales involved in the reorganization of actin and chromatin assembly, followed by translocation of transcription cofactors from the cytoplasm to the nucleus.

Mechanical stimulation induces formin-dependent assembly of a perinuclear actin rim

“…Neither myosin IIA nor myosin IIB was found to be associated with the perinuclear actin rim, and inhibition of myosin II contractility did not affect formation of this actin structure. Although various studies have reported observations of actin reorganization on mechanical stimuli (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(14)(15)(16)(17), the transient perinuclear actin polymerization is revealed here for the first time to our knowledge.…”

“…Similarly, local application of force through fibronectin or collagen-coated beads trapped by optical or magnetic tweezers leads to the local reorganization of the actin cytoskeleton. This response is associated with reinforcement of bead attachment (11), recruitment of additional actin-associated proteins (12), and activation of a variety of signaling pathways (13)(14)(15)(16)(17). Most studies to date have explored the effects of force on actin structures directly associated with the sites of force application, such as focal adhesions and stress fibers.…”

Nuclear deformability and telomere dynamics are regulated by cell geometric constraints

“…Chromatin dynamics can be induced by nuclear ATPases such as polymerases, topoisomerases, and chromatin remodeling complexes at millisecond timescales (58-60). However, recent findings suggest that chromatin dynamics can also result from matrix remodeling (42,43). In addition, perturbation of cytoskeletal filaments via RNA interference affects histone diffusion timescales (7).…”

“…With the cytoskeleton physically linked to the nucleoskeleton, these extracellular mechanical signals can therefore be used to mediate changes in chromatin structure. Active cytoskeletal forces can mediate the mechanotransduction to the nucleus, to remodel 3D chromosome organization as well as permissivity to chromatin structure by regulatory molecules (42,43). Cytoskeletal to nuclear links are also essential to the maintenance of poised euchromatin and more repressive condensed chromatin, i.e., heterochromatin assembly (44).…”

Dynamic reciprocity: the role of annexin A2 in tissue integrity

“…While biochemical signals can alter gene expression by triggering intracellular signalling cascades, intercellular and cell-ECM biomechanical interactions control tensional forces which are conducted through the cytoskeleton to the cell nucleus (Chiquet, 1999). These forces control the shape of cells and the cell nucleus and have been shown to regulate chromatin organisation and hence gene expression (Iyer et al, 2012). The extracellular microenvironment is, therefore, crucial for regulating cell behaviour-it enables cells which contain identical genetic information to behave in a tissue-specific and synchronized manner (Bissell et al, 2005).…”