Academic literature on the topic 'Myosin mechanics'

Almost all animal cells maintain a thin layer of actin filaments and associated proteins underneath the cell membrane. The actomyosin cortex is subject to internal stress patterns which result from the spatiotemporally regulated activity of non-muscle myosin II motors in the actin network. We study how these active stresses drive changes in cell shape and flows within the cortical layer, and how these cytoskeletal deformations and flows govern processes such as cell migration, cell division and organelle transport. Following a continuum mechanics approach, we develop theoretical descriptions for three different cellular processes, to obtain - in collaboration with experimental groups - a detailed and quantitative understanding of the underlying cytoskeletal mechanics. We investigate the forces and cortex flows involved in adhesion-independent cell migration in confinement. Many types of cell migration rely on the extension of protrusions at the leading edge, where the cells attach to the substrate with specific focal adhesions, and pull themselves forward, exerting stresses in the kPa range. In confined environments however, cells exhibit migration modes which are independent of specific adhesions. Combining hydrodynamic theory, microfluidics and quantitative imaging of motile, non-adherent carcinosarcoma cells, we analyze the mechanical behavior of cells during adhesion-independent migration. We find that the accumulation of active myosin motors in the rear part of these cells results in a retrograde cortical flow as well as the contraction of the cell body in the rear and expansion in the front, and we describe how both processes contribute to the translocation of the cells, depending on the geometric and mechanical parameters of the system. Importantly, we find that the involved propulsive forces are several orders of magnitude lower than during adhesive motility while the achieved migration velocities are similar. Moreover, the distribution of forces on the substrate during non-adhesive migration is fundamentally different, giving rise to a positive force dipole. In contrast to adhesive migration modes, non-adhesive cells move by exerting pushing forces at the rear, acting to expand rather than contract their substrate as they move. These differences may strongly affect hydrodynamic and/or deformational interactions between collectively migrating cells. In addition to the work outlined above, we study contractile ring formation in the actin cytoskeleton before and during cell division. While in disordered actin networks, myosin motor activity gives rise to isotropic stresses, the alignment of actin filaments in the cortex during cell division introduces a preferred direction for motor-filament interactions, resulting in anisotropies in the cortical stress. Actin filaments align in myosin-dependent shear flows, resulting in possible feedback between motor activity, cortical flows and actin organization. We investigate how the mechanical interplay of these different cortical properties gives rise to the formation of a cleavage furrow during cell division, describing the level of actin filament alignment at different points on the cortex with a nematic order parameter, in analogy to liquid crystal physics. We show that cortical anisotropies arising from shear-flow induced alignment patterns are sufficient to drive the ingression of cellular furrows, even in the absence of localized biochemical myosin up-regulation. This mechanism explains the characteristic appearance of pseudocleavage furrows in polarizing cells. Finally, we study the characteristic nuclear movements in pseudostratified epithelia during development. These tissues consist of highly proliferative, tightly packed and elongated cells, with nuclei actively travelling to the apical side of the epithelium before each cell division. We explore how cytoskeletal properties act together with the mechanics of the surrounding tissue to control the shape of single cells embedded in the epithelium, and investigate potential mechanisms underlying the observed nuclear movements. These findings form a theoretical basis for a more detailed characterization of processes in pseudostratified epithelia. Taken together, we present a continuum mechanics description of the actomyosin cell cortex, and successfully apply it to several different cell biological processes. Combining our theory with experimental work from collaborating groups, we provide new insights into different aspects of cell mechanics.

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by the absence of dystrophin in the muscle cell membranes, rendering them susceptible to mechanical damage. DMD leads to respiratory or cardiac muscle failure and death. We hypothesized that alterations in contractile protein function contribute to DMD muscle weakness. We measured muscle strip stress, myosin heavy chain (MHC) isoform composition, velocity (ʋmax) of actin propulsion by myosin, and relative myosin force in control and mdx mouse (C57Bl/10) diaphragms. Stress was statistically smaller for mdx (0.23kg/cm2±0.11; mean±SE) than control (0.69kg/cm2±0.01) whereas the MHC isoform composition was not statistically different (type I: p=0.423, type IIa/IIx: p=0.804, type IIb: p=0.401). υmax of mdx myosin (1.24µm/s±0.07) was not statistically different from control (1.37µm/s±0.12; p=0.353). Relative myosin force was not statistically different between control and mdx myosin (p=0.932). Thus, alterations in myosin molecular function do not contribute to the weakness of the mdx mouse diaphragm.<br>La dystrophie musculaire de Duchenne (DMD) est une maladie génétique caracterisée par un manque de dystrophine dans la membrane des cellules musculaires, les rendant susceptibles au dommage mécanique. La mort survient suite à l'insuffisance des muscles cardiaques ou respiratoires. Notre hypothèse est que des altérations au niveau des protéines contractiles jouent un rôle dans la faiblesse musculaire de la DMD. Dans cette étude, nous avons mesuré le stress généré par des faisceaux musculaires, la composition des isoformes de la chaîne lourde de myosine (MHC), la vélocité (υmax) de propulsion de l'actine par la myosine, et la force relative de la myosine de diaphragme de souris control et mdx (C57Bl/10). Nous avons observé que le stress est statistiquement plus petit pour la souris mdx (0.23±0.11; moyenne±SE) que pour le control (0.69±0.01), mais que la composition de MHC n'est pas statistiquement différente (type I: p=0.423, type IIa/IIx: p=0.804, type IIb: p=0.401). υmax de la myosine mdx (1.24µm/s±0.07) n'est pas statistiquement différente du control (1.37µm/s±0.12; p=0.353). La force relative n'est pas statistiquement différente entre la myosine control et mdx (p=0.932). Donc des altérations de la fonction moléculaire de la myosine ne contribuent pas à la faiblesse du diaphragme de la souris mdx.

Two smooth muscle (SM) myosin heavy chain isoforms, generated by alternative mRNA splicing, differ by the presence (SM-B) or absence (SM-A) of a 7 amino acid insert in the motor domain. The rate of actin filament propulsion (nu max) of SM-B, as measured in the in vitro motility assay, is 2-fold greater than that of SM-A. I investigated the expression and function of these isoforms in healthy SM and in asthma. First, I determined the sequence of the SM-B isoform in human SM and quantified its expression at the mRNA and protein levels in several human organs. The SM-B isoform was mostly expressed in rapidly contracting phasic SM. I then purified myosin from multiple rat organs and found a rank correlation between SM-B content and numax.<br>I then quantified the expression of SM-B and several other contractile protein genes in endobronchial biopsies from normal and asthmatic subjects. SM-B, myosin light chain kinase (MLCK), which is responsible for myosin activation, and transgelin, a ubiquitously expressed actin binding protein but whose function is unknown, were overexpressed in the asthmatic biopsies. The increased SM-B expression and myosin activation, due to the increased MLCK expression, both contribute to the increased rate of shortening of the asthmatic airway SM. In addition, I showed that beyond its enzymatic effects, MLCK mechanically enhances numax. The binding of SM22 to actin, however, did not alter numax.<br>Finally, I addressed the mechanisms behind the unique capacity of SM to maintain force at low energy cost, namely the latch-state. This property is mostly observed in SM-A containing, tonic muscle. Using a laser trap, I measured the binding force of unphosphorylated (non-active) SM-A and SM-B myosin isoforms and found that they can both attach to actin and maintain force. I also measured numax at different MgADP concentrations and found that SM-A has a greater affinity for MgADP. Because MgADP must be released before myosin can detach from actin, these results suggest that the SMA isoform remains attached longer to actin, allowing it to get into the latch-state. These findings explain the greater propensity of tonic muscle to get into the latch-state.