BBA - Molecular Cell Research (v.1853, #11PB)

Editorial — Special issue on mechanobiology by Sarah Köster; Andreas Janshoff (2975-2976).

Mechanics of lipid bilayers: What do we learn from pore-spanning membranes? by Andreas Janshoff; Claudia Steinem (2977-2983).
The mechanical properties of biological membranes have become increasingly important not only from a biophysical viewpoint but also as they play a substantial role in the information transfer in cells and tissues. This minireview summarizes some of our recent understanding of the mechanical properties of artificial model membranes with particular emphasis on membranes suspending an array of pores, so called pore-spanning membranes. A theoretical description of the mechanical properties of these membranes might pave the way to biophysically describe and understand the complex behavior of native biological membranes. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Atomic force microscopy; Cell membrane fragments; GUVs; Model membranes; Pore-spanning membranes; Solid supported membranes;

Crowding of receptors induces ring-like adhesions in model membranes by Daniel Schmidt; Timo Bihr; Susanne Fenz; Rudolf Merkel; Udo Seifert; Kheya Sengupta; Ana-Sunčana Smith (2984-2991).
The dynamics of formation of macromolecular structures in adherent membranes is a key to a number of cellular processes. However, the interplay between protein reaction kinetics, diffusion and the morphology of the growing domains, governed by membrane mediated interactions, is still poorly understood. Here we show, experimentally and in simulations, that a rich phase diagram emerges from the competition between binding, cooperativity, molecular crowding and membrane spreading. In the cellular context, the spontaneously-occurring organization of adhesion domains in ring-like morphologies is particularly interesting. These are stabilized by the crowding of bulky proteins, and the membrane-transmitted correlations between bonds. Depending on the density of the receptors, this phase may be circumvented, and instead, the adhesions may grow homogeneously in the contact zone between two membranes. If the development of adhesion occurs simultaneously with membrane spreading, much higher accumulation of binders can be achieved depending on the velocity of spreading. The mechanisms identified here, in the context of our mimetic model, may shed light on the structuring of adhesions in the contact zones between two living cells. This article is part of a Special Issue entitled: Mechanobiology.Display Omitted
Keywords: Cell adhesion; Immunological synapse; Adhesion dynamics; Membrane transmitted correlations; Ligand-receptor bonds; Crowding effects; Membrane fluctuations; Diffusion–reaction systems;

Synthetic membrane systems, such as giant unilamellar vesicles and solid supported lipid bilayers, have widened our understanding of biological processes occurring at or through membranes. Artificial systems are particularly suited to study the inherent properties of membranes with regard to their components and characteristics. This review critically reflects the emerging molecular mechanism of lipid-driven endocytosis and the impact of model membrane systems in elucidating the complex interplay of biomolecules within this process. Lipid receptor clustering induced by binding of several toxins, viruses and bacteria to the plasma membrane leads to local membrane bending and formation of tubular membrane invaginations. Here, lipid shape, and protein structure and valency are the essential parameters in membrane deformation. Combining observations of complex cellular processes and their reconstitution on minimal systems seems to be a promising future approach to resolve basic underlying mechanisms. This article is part of a Special Issue entitled: Mechanobiology.Display Omitted
Keywords: Lipid clustering; Membrane invagination; Lectin; Model membrane system; Shiga toxin; Pathogen;

Reconstituting the actin cytoskeleton at or near surfaces in vitro by Rodrigo Cáceres; Majdouline Abou-Ghali; Julie Plastino (3006-3014).
Actin filament dynamics have been studied for decades in pure protein solutions or in cell extracts, but a breakthrough in the field occurred at the turn of the century when it became possible to reconstitute networks of actin filaments, growing in a controlled but physiological manner on surfaces, mimicking the actin assembly that occurs at the plasma membrane during cell protrusion and cell shape changes. The story begins with the bacteria Listeria monocytogenes, the study of which led to the reconstitution of cellular actin polymerization on a variety of supports including plastic beads. These studies made possible the development of liposome-type substrates for filament assembly and micropatterning of actin polymerization nucleation. Based on the accumulated expertise of the last 15 years, many exciting approaches are being developed, including the addition of myosin to biomimetic actin networks to study the interplay between actin structure and contractility. The field is now poised to make artificial cells with a physiological and dynamic actin cytoskeleton, and subsequently to put these cells together to make in vitro tissues. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Actin polymerization; Biomimetic systems; Actin-based motility; Contractility; Actin cortex; Lamellipodium;

Molecular control of stress transmission in the microtubule cytoskeleton by Benjamin J. Lopez; Megan T. Valentine (3015-3024).
In this article, we will summarize recent progress in understanding the mechanical origins of rigidity, strength, resiliency and stress transmission in the MT cytoskeleton using reconstituted networks formed from purified components. We focus on the role of network architecture, crosslinker compliance and dynamics, and molecular determinants of single filament elasticity, while highlighting open questions and future directions for this work.
Keywords: Microtubules; Elasticity; Rheology; Microtubule-associated protein;

Inelastic mechanics: A unifying principle in biomechanics by Matti Gralka; Klaus Kroy (3025-3037).
Many soft materials are classified as viscoelastic. They behave mechanically neither quite fluid-like nor quite solid-like — rather a bit of both. Biomaterials are often said to fall into this class. Here, we argue that this misses a crucial aspect, and that biomechanics is essentially damage mechanics, at heart. When deforming an animal cell or tissue, one can hardly avoid inducing the unfolding of protein domains, the unbinding of cytoskeletal crosslinkers, the breaking of weak sacrificial bonds, and the disruption of transient adhesions. We classify these activated structural changes as inelastic. They are often to a large degree reversible and are therefore not plastic in the proper sense, but they dissipate substantial amounts of elastic energy by structural damping. We review recent experiments involving biological materials on all scales, from single biopolymers over cells to model tissues, to illustrate the unifying power of this paradigm. A deliberately minimalistic yet phenomenologically very rich mathematical modeling framework for inelastic biomechanics is proposed. It transcends the conventional viscoelastic paradigm and suggests itself as a promising candidate for a unified description and interpretation of a wide range of experimental data. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Inelastic biomechanics; Rheological models; Glassy wormlike chain; Microrheology; Transient crosslinkers; Cytoskeleton; In-vitro biopolymer networks; Cell aggregates; Soft glassy rheology; Structural plasticity;

Mechanics and dynamics of reconstituted cytoskeletal systems by Mikkel H. Jensen; Eliza J. Morris; David A. Weitz (3038-3042).
The intracellular cytoskeleton is an active dynamic network of filaments and associated binding proteins that control key cellular properties, such as cell shape and mechanics. Due to the inherent complexity of the cell, reconstituted model systems have been successfully employed to gain an understanding of the fundamental physics governing cytoskeletal processes. Here, we review recent advances and key aspects of these reconstituted systems. We focus on the importance of assembly kinetics and dynamic arrest in determining network mechanics, and highlight novel emergent behavior occurring through interactions between cytoskeletal components in more complex networks incorporating multiple biopolymers and molecular motors.
Keywords: Biopolymer rheology; Actin; Cytoskeleton; Dynamic arrest; Composite networks; Active soft matter;

A guide to mechanobiology: Where biology and physics meet by Karin A. Jansen; Dominique M. Donato; Hayri E. Balcioglu; Thomas Schmidt; Erik H.J. Danen; Gijsje H. Koenderink (3043-3052).
Cells actively sense and process mechanical information that is provided by the extracellular environment to make decisions about growth, motility and differentiation. It is important to understand the underlying mechanisms given that deregulation of the mechanical properties of the extracellular matrix (ECM) is implicated in various diseases, such as cancer and fibrosis. Moreover, matrix mechanics can be exploited to program stem cell differentiation for organ-on-chip and regenerative medicine applications. Mechanobiology is an emerging multidisciplinary field that encompasses cell and developmental biology, bioengineering and biophysics. Here we provide an introductory overview of the key players important to cellular mechanobiology, taking a biophysical perspective and focusing on a comparison between flat versus three dimensional substrates. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Mechanosensing; Extracellular matrix; Cytoskeleton; Integrins; 2D vs 3D; Mechanotransduction;

Physical properties of cytoplasmic intermediate filaments by Johanna Block; Viktor Schroeder; Paul Pawelzyk; Norbert Willenbacher; Sarah Köster (3053-3064).
Intermediate filaments (IFs) constitute a sophisticated filament system in the cytoplasm of eukaryotes. They form bundles and networks with adapted viscoelastic properties and are strongly interconnected with the other filament types, microfilaments and microtubules. IFs are cell type specific and apart from biochemical functions, they act as mechanical entities to provide stability and resilience to cells and tissues. We review the physical properties of these abundant structural proteins including both in vitro studies and cell experiments. IFs are hierarchical structures and their physical properties seem to a large part be encoded in the very specific architecture of the biopolymers. Thus, we begin our review by presenting the assembly mechanism, followed by the mechanical properties of individual filaments, network and structure formation due to electrostatic interactions, and eventually the mechanics of in vitro and cellular networks. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Intermediate filament; Cell mechanics; Assembly; Persistence length; Polyelectrolyte; Network;

A biomechanical perspective on stress fiber structure and function by Elena Kassianidou; Sanjay Kumar (3065-3074).
Stress fibers are actomyosin-based bundles whose structural and contractile properties underlie numerous cellular processes including adhesion, motility and mechanosensing. Recent advances in high-resolution live-cell imaging and single-cell force measurement have dramatically sharpened our understanding of the assembly, connectivity, and evolution of various specialized stress fiber subpopulations. This in turn has motivated interest in understanding how individual stress fibers generate tension and support cellular structure and force generation. In this review, we discuss approaches for measuring the mechanical properties of single stress fibers. We begin by discussing studies conducted in cell-free settings, including strategies based on isolation of intact stress fibers and reconstitution of stress fiber-like structures from purified components. We then discuss measurements obtained in living cells based both on inference of stress fiber properties from whole-cell mechanical measurements (e.g., atomic force microscopy) and on direct interrogation of single stress fibers (e.g., subcellular laser nanosurgery). We conclude by reviewing various mathematical models of stress fiber function that have been developed based on these experimental measurements. An important future challenge in this area will be the integration of these sophisticated biophysical measurements with the field's increasingly detailed molecular understanding of stress fiber assembly, dynamics, and signal transduction. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Stress fiber; Biomechanical property; Mechanobiology; Subcellular laser ablation;

Elastic properties of epithelial cells probed by atomic force microscopy by Bastian R. Brückner; Andreas Janshoff (3075-3082).
Cellular mechanics plays a crucial role in many biological processes such as cell migration, cell growth, embryogenesis, and oncogenesis. Epithelia respond to environmental cues comprising biochemical and physical stimuli through defined changes in cell elasticity. For instance, cells can differentiate between certain properties such as viscoelasticity or topography of substrates by adapting their own elasticity and shape. A living cell is a complex viscoelastic body that not only exhibits a shell architecture composed of a membrane attached to a cytoskeleton cortex but also generates contractile forces through its actomyosin network. Here we review cellular mechanics of single cells in the context of epithelial cell layers responding to chemical and physical stimuli. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Indentation; Membrane tension; Cortical tension; Atomic force microscopy; Cytoskeleton;

Active cell mechanics: Measurement and theory by Wylie W. Ahmed; Étienne Fodor; Timo Betz (3083-3094).
Living cells are active mechanical systems that are able to generate forces. Their structure and shape are primarily determined by biopolymer filaments and molecular motors that form the cytoskeleton. Active force generation requires constant consumption of energy to maintain the nonequilibrium activity to drive organization and transport processes necessary for their function. To understand this activity it is necessary to develop new approaches to probe the underlying physical processes. Active cell mechanics incorporates active molecular-scale force generation into the traditional framework of mechanics of materials. This review highlights recent experimental and theoretical developments towards understanding active cell mechanics. We focus primarily on intracellular mechanical measurements and theoretical advances utilizing the Langevin framework. These developing approaches allow a quantitative understanding of nonequilibrium mechanical activity in living cells. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Cell mechanics; Nonequilibrium biophysics; Force measurement; Generalized Langevin Equation;

The measurement of cellular traction forces on soft elastic substrates has become a standard tool for many labs working on mechanobiology. Here we review the basic principles and different variants of this approach. In general, the extraction of the substrate displacement field from image data and the reconstruction procedure for the forces are closely linked to each other and limited by the presence of experimental noise. We discuss different strategies to reconstruct cellular forces as they follow from the foundations of elasticity theory, including two- versus three-dimensional, inverse versus direct and linear versus non-linear approaches. We also discuss how biophysical models can improve force reconstruction and comment on practical issues like substrate preparation, image processing and the availability of software for traction force microscopy. This article is part of a Special Issue entitled: Mechanobiology.Display Omitted
Keywords: Mechanobiology; Elasticity theory; Cellular biophysics; Cell forces; Traction force microscopy; Cell–matrix adhesion; Actin cytoskeleton;

Mechanotransduction in neutrophil activation and deactivation by Andrew E. Ekpenyong; Nicole Toepfner; Edwin R. Chilvers; Jochen Guck (3105-3116).
Mechanotransduction refers to the processes through which cells sense mechanical stimuli by converting them to biochemical signals and, thus, eliciting specific cellular responses. Cells sense mechanical stimuli from their 3D environment, including the extracellular matrix, neighboring cells and other mechanical forces. Incidentally, the emerging concept of mechanical homeostasis,long term or chronic regulation of mechanical properties, seems to apply to neutrophils in a peculiar manner, owing to neutrophils' ability to dynamically switch between the activated/primed and deactivated/deprimed states. While neutrophil activation has been known for over a century, its deactivation is a relatively recent discovery. Even more intriguing is the reversibility of neutrophil activation and deactivation. We review and critically evaluate recent findings that suggest physiological roles for neutrophil activation and deactivation and discuss possible mechanisms by which mechanical stimuli can drive the oscillation of neutrophils between the activated and resting states. We highlight several molecules that have been identified in neutrophil mechanotransduction, including cell adhesion and transmembrane receptors, cytoskeletal and ion channel molecules. The physiological and pathophysiological implications of such mechanically induced signal transduction in neutrophils are highlighted as a basis for future work. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Mechanosensitivity; Mechanical properties; Priming; Depriming; Immune response; Inflammation; Circulation; Migration; Activation; Deactivation; Mechanobiology; Mechanotransduction;

Distinct impact of targeted actin cytoskeleton reorganization on mechanical properties of normal and malignant cells by Yu.M. Efremov; A.A. Dokrunova; A.V. Efremenko; M.P. Kirpichnikov; K.V. Shaitan; O.S. Sokolova (3117-3125).
The actin cytoskeleton is substantially modified in cancer cells because of changes in actin-binding protein abundance and functional activity. As a consequence, cancer cells have distinctive motility and mechanical properties, which are important for many processes, including invasion and metastasis. Here, we studied the effects of actin cytoskeleton alterations induced by specific nucleation inhibitors (SMIFH2, CK-666), cytochalasin D, Y-27632 and detachment from the surface by trypsinization on the mechanical properties of normal Vero and prostate cancer cell line DU145. The Young's modulus of Vero cells was 1300 ± 900 Pa, while the prostate cancer cell line DU145 exhibited significantly lower Young's moduli (600 ± 400 Pa). The Young's moduli exhibited a log-normal distribution for both cell lines. Unlike normal cells, cancer cells demonstrated diverse viscoelastic behavior and different responses to actin cytoskeleton reorganization. They were more resistant to specific formin-dependent nucleation inhibition, and reinforced their cortical actin after detachment from the substrate. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: AFM; Force spectroscopy; Prostate cancer; Formin; Arp 2/3; Cortical actin;

Neuronal and metastatic cancer cells: Unlike brothers by Paul Heine; Allen Ehrlicher; Josef Käs (3126-3131).
During development neuronal cells traverse substantial distances across the developing tissue. In the mature organism, however, they are bound to the confines of the nervous system. Likewise metastatic cancer cells have the potential to establish auxiliary tumor sites in remote tissues or entirely different organs. The epithelial–mesenchymal transition is the transformation of proliferative cancer cells into a highly invasive state, which facilitates the crossing of tissue boundaries and migration across various environments. This review contributes a first look into the parallels and contrasts between physical aspects of neuronal and metastatic cancer cells.
Keywords: Cancer; Invasiveness; Neuronal; Motility; Cytoskeleton; Filopodia;

Cell–tissue–tissue interaction is determined by specific short range forces between cell adhesion molecules (CAMs) and ligands of the tissue, long range repulsion forces mediated by cell surface grafted macromolecules and adhesion-induced elastic stresses in the cell envelope. This interplay of forces triggers the rapid random clustering of tightly coupled linkers. By coupling of actin gel patches to the intracellular domains of the CAMs, these clusters can grow in a secondary process resulting in the formation of functional adhesion microdomains (ADs). The ADs can act as biochemical steering centers by recruiting and activating functional proteins, such as GTPases and associated regulating proteins, through electrostatic–hydrophobic forces with cationic lipid domains that act as attractive centers.First, I summarize physical concepts of cell adhesion revealed by studies of biomimetic systems. Then I describe the role of the adhesion domains as biochemical signaling platforms and force transmission centers promoting cellular protrusions, in terms of a shell string model of cells. Protrusion forces are generated by actin gelation triggered by molecular machines (focal adhesion kinase (FAK), Src-kinases and associated adaptors) which assemble around newly formed integrin clusters. They recruit and activate the GTPases Rac-1 and actin gelation promoters to charged membrane domains via electrostatic–hydrophobic forces. The cell front is pushed forward in a cyclic and stepwise manner and the step-width is determined by the dynamics antagonistic interplay between Rac-1 and RhoA. The global cell polarization in the direction of motion is mediated by the actin–microtubule (MT) crosstalk at adhesion domains. Supramolecular actin–MT assemblies at the front help to promote actin polymerization. At the rear they regulate the dismantling of the ADs through the Ca++-mediated activation of the protease calpain and trigger their disruption by RhoA mediated contraction via stress fibers. This article is part of a Special Issue entitled: Mechanobiology.Directed cell locomotion is driven by cyclic progression of the front and retraction of the end regulated by actin microtubule (MT) crosstalk. Adhesion domains (ADs) formed about integrin clusters act as biochemical steering and force generating centers. Their function is controlled in a Ca++- and force-dependent way by quaternary complexes of actin, IQGAP, calmodulin, and microtubule. The rhythmic cell progression is determined by the dynamic antagonism between the GTPases Rac-1 and Rho-A. The retraction is mediated by protease (P)-activated by Ca++ release from ER stores which mediates the dismantling and retraction of rearmost ADs by activation of RhoA and inhibition of Rac-1 in the cell–tissue adhesion zone.Display Omitted
Keywords: Physics of cell adhesion; Cell locomotion; Cell polarization; Actin–microtubule crosstalk; Cell adhesion as wetting process;

Phenomenological approaches to collective behavior in epithelial cell migration by Matthias L. Zorn; Anna-Kristina Marel; Felix J. Segerer; Joachim O. Rädler (3143-3152).
Collective cell migration in epithelial tissues resembles fluid-like behavior in time-lapse recordings. In the last years, hydrodynamic velocity fields in living matter have been studied intensely. The emergent properties were remarkably similar to phenomena known from active soft matter systems. Here, we review migration experiments of large cellular ensembles as well as of mesoscopic cohorts in micro-structured environments. Concepts such as diffusion, velocity correlations, swirl strength and polarization are metrics to quantify the cellular dynamics both in experiments as well as in computational simulations. We discuss challenges relating collective migration to single cell and oligocellular behavior as well as linking the phenotypic parameters to the underlying cytoskeleton dynamics and signaling networks. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Collective cell migration; Oligocellular assays; Phenomenological models; Active matter; Madin Darby canine kidney cells;

The (dys)functional extracellular matrix by Benjamin R. Freedman; Nathan D. Bade; Corinne N. Riggin; Sijia Zhang; Philip G. Haines; Katy L. Ong; Paul A. Janmey (3153-3164).
The extracellular matrix (ECM) is a major component of the biomechanical environment with which cells interact, and it plays important roles in both normal development and disease progression. Mechanical and biochemical factors alter the biomechanical properties of tissues by driving cellular remodeling of the ECM. This review provides an overview of the structural, compositional, and mechanical properties of the ECM that instruct cell behaviors. Case studies are reviewed that highlight mechanotransduction in the context of two distinct tissues: tendons and the heart. Although these two tissues demonstrate differences in relative cell–ECM composition and mechanical environment, they share similar mechanisms underlying ECM dysfunction and cell mechanotransduction. Together, these topics provide a framework for a fundamental understanding of the ECM and how it may vary across normal and diseased tissues in response to mechanical and biochemical cues. This article is part of a Special Issue entitled: Mechanobiology.
Keywords: Mechanotransduction; Cytoskeleton; Biomechanics; Cell mechanics; Tendinopathy; Diastolic dysfunction;