Non-muscle myosin II (NM II) is an actin-binding protein that has actin cross-linking and contractile properties and is regulated by the phosphorylation of its light and heavy chains. The three mammalian NM II isoforms have both overlapping and unique properties. Owing to its position downstream of convergent signalling pathways, NM II is central in the control of cell adhesion, cell migration and tissue architecture. Recent insight into the role of NM II in these processes has been gained from loss-of-function and mutant approaches, methods that quantitatively measure actin and adhesion dynamics and the discovery of NM II mutations that cause monogenic diseases.

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Non-muscle myosin II takes centre stage in cell adhesion and migration.

Directional migration is an important component of cell motility. Although the basic mechanisms of random cell movement are well characterized, no single model explains the complex regulation of directional migration. Multiple factors operate at each step of cell migration to stabilize lamellipodia and maintain directional migration. Factors such as the topography of the extracellular matrix, the cellular polarity machinery, receptor signalling, integrin trafficking, integrin co-receptors and actomyosin contraction converge on regulation of the Rho family of GTPases and the control of lamellipodial protrusions to promote directional migration.

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Random versus directionally persistent cell migration.

Directed cell migration is a physical process that requires dramatic changes in cell shape and adhesion to the extracellular matrix. For efficient movement, these processes must be spatiotemporally coordinated. To a large degree, the morphological changes and physical forces that occur during migration are generated by a dynamic filamentous actin (F-actin) cytoskeleton. Adhesion is regulated by dynamic assemblies of structural and signaling proteins that couple the F-actin cytoskeleton to the extracellular matrix. Here, we review current knowledge of the dynamic organization of the F-actin cytoskeleton in cell migration and the regulation of focal adhesion assembly and disassembly with an emphasis on how mechanical and biochemical signaling between these two systems regulate the coordination of physical processes in cell migration.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1437&context=cbe_pubs

Mechanical Integration of Actin and Adhesion Dynamics in Cell Migration

Glioblastoma multiforme (GBM) is a malignant astrocytoma of the central nervous system associated with a median survival time of 15 months, even with aggressive therapy. This rapid progression is due in part to diffuse infiltration of single tumor cells into the brain parenchyma, which is thought to involve aberrant interactions between tumor cells and the extracellular matrix (ECM). Here, we test the hypothesis that mechanical cues from the ECM contribute to key tumor cell properties relevant to invasion. We cultured a series of glioma cell lines (U373-MG, U87-MG, U251-MG, SNB19, C6) on fibronectin-coated polymeric ECM substrates of defined mechanical rigidity and investigated the role of ECM rigidity in regulating tumor cell structure, migration, and proliferation. On highly rigid ECMs, tumor cells spread extensively, form prominent stress fibers and mature focal adhesions, and migrate rapidly. As ECM rigidity is lowered to values comparable with normal brain tissue, tumor cells appear rounded and fail to productively migrate. Remarkably, cell proliferation is also strongly regulated by ECM rigidity, with cells dividing much more rapidly on rigid than on compliant ECMs. Pharmacologic inhibition of nonmuscle myosin II–based contractility blunts this rigidity-sensitivity and rescues cell motility on highly compliant substrates. Collectively, our results provide support for a novel model in which ECM rigidity provides a transformative, microenvironmental cue that acts through actomyosin contractility to regulate the invasive properties of GBM tumor cells. [Cancer Res 2009;69(10):4167–74]

https://cancerres.aacrjournals.org/content/canres/69/10/4167.full.pdf

The mechanical rigidity of the extracellular matrix regulates the structure, motility, and proliferation of glioma cells

Using two-colour imaging and high resolution TIRF microscopy, we investigated the assembly and maturation of nascent adhesions in migrating cells. We show that nascent adhesions assemble and are stable within the lamellipodium. The assembly is independent of myosin II but its rate is proportional to the protrusion rate and requires actin polymerization. At the lamellipodium back, the nascent adhesions either disassemble or mature through growth and elongation. Maturation occurs along an alpha-actinin-actin template that elongates centripetally from nascent adhesions. Alpha-Actinin mediates the formation of the template and organization of adhesions associated with actin filaments, suggesting that actin crosslinking has a major role in this process. Adhesion maturation also requires myosin II. Rescue of a myosin IIA knockdown with an actin-bound but motor-inhibited mutant of myosin IIA shows that the actin crosslinking function of myosin II mediates initial adhesion maturation. From these studies, we have developed a model for adhesion assembly that clarifies the relative contributions of myosin II and actin polymerization and organization.

Actin and alpha-actinin orchestrate the assembly and maturation of nascent adhesions in a myosin II motor-independent manner.

Although myosin II is known to play an important role in cell migration, little is known about its specific functions. We have addressed the function of one of the isoforms of myosin II, myosin IIB, by analyzing the movement and mechanical characteristics of fibroblasts where this protein has been ablated by gene disruption. Myosin IIB null cells displayed multiple unstable and disorganized protrusions, although they were still able to generate a large fraction of traction forces when cultured on flexible polyacrylamide substrates. However, the traction forces were highly disorganized relative to the direction of cell migration. Analysis of cell migration patterns indicated an increase in speed and decrease in persistence, which were likely responsible for the defects in directional movements as demonstrated with Boyden chambers. In addition, unlike control cells, mutant cells failed to respond to mechanical signals such as compressing forces and changes in substrate rigidity. Immunofluorescence staining indicated that myosin IIB was localized preferentially along stress fibers in the interior region of the cell. Our results suggest that myosin IIB is involved not in propelling but in directing the cell movement, by coordinating protrusive activities and stabilizing the cell polarity.

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Gregoxy C.H. Chua

Papers

Nonmuscle myosin IIb is involved in the guidance of fibroblast migration