The first three minutes: smooth muscle contraction, cytoskeletal events, and soft glasses

Targeting cytoskeletal biomechanics to modulate airway smooth muscle contraction in asthma

To contract, to deform, and remodel, the airway smooth muscle cell relies on dynamic changes in the structure of its mechanical force-bearing cytoskeleton. These alternate between a "fluid-like" (relaxed) state characterized by weak contractile protein-protein interactions within the cytoskeletal apparatus and a "solid-like" (contractile) state promoted by strong and highly organized molecular interactions. In this review, we discuss the roles for actin, myosin, factors promoting actin polymerization and depolymerization, adhesome complexes, and cell-cell junctions in these dynamic processes. We describe the relationship between these cytoskeletal factors, extracellular matrix components of bronchial tissue, and mechanical stretch and other changes within the airway wall in the context of the physical mechanisms of cytoskeletal fluidization-resolidification. We also highlight studies that emphasize the distinct processes of cell shortening and force transmission in airway smooth muscle and previously unrecognized roles for actin in cytoskeletal dynamics. Finally, we discuss the implications of these discoveries for understanding and treating airway obstruction in asthma.

Sitting leg vasculopathy: potential adaptations beyond the endothelium

Increased sitting time, the most common form of sedentary behavior, is an independent risk factor for all-cause and cardiovascular disease mortality; however, the mechanisms linking sitting to cardiovascular risk remain largely elusive. Studies over the last decade have led to the concept that excessive time spent in the sitting position and the ensuing reduction in leg blood flow-induced shear stress cause endothelial dysfunction. This conclusion has been mainly supported by studies using flow-mediated dilation in the lower extremities as the measured outcome. In this review, we summarize evidence from classic studies and more recent ones that collectively support the notion that prolonged sitting-induced leg vascular dysfunction is likely also attributable to changes occurring in vascular smooth muscle cells (VSMCs). Indeed, we provide evidence that prolonged constriction of resistance arteries can lead to modifications in the structural characteristics of the vascular wall, including polymerization of actin filaments in VSMCs and inward remodeling, and that these changes manifest in a time frame that is consistent with the vascular changes observed with prolonged sitting. We expect this review will stimulate future studies with a focus on VSMC cytoskeletal remodeling as a potential target to prevent the detrimental vascular ramifications of too much sitting.

A life off the beaten track in biomechanics: Imperfect elasticity, cytoskeletal glassiness, and epithelial unjamming

Textbook descriptions of elasticity, viscosity, and viscoelasticity fail to account for certain mechanical behaviors that typify soft living matter. Here, we consider three examples. First, strong empirical evidence suggests that within lung parenchymal tissues, the frictional stresses expressed at the microscale are fundamentally not of viscous origin. Second, the cytoskeleton (CSK) of the airway smooth muscle cell, as well as that of all eukaryotic cells, is more solid-like than fluid-like, yet its elastic modulus is softer than the softest of soft rubbers by a factor of 104-105. Moreover, the eukaryotic CSK expresses power law rheology, innate malleability, and fluidization when sheared. For these reasons, taken together, the CSK of the living eukaryotic cell is reminiscent of the class of materials called soft glasses, thus likening it to inert materials such as clays, pastes slurries, emulsions, and foams. Third, the cellular collective comprising a confluent epithelial layer can become solid-like and jammed, fluid-like and unjammed, or something in between. Esoteric though each may seem, these discoveries are consequential insofar as they impact our understanding of bronchospasm and wound healing as well as cancer cell invasion and embryonic development. Moreover, there are reasons to suspect that certain of these phenomena first arose in the early protist as a result of evolutionary pressures exerted by the primordial microenvironment. We have hypothesized, further, that each then became passed down virtually unchanged to the present day as a conserved core process. These topics are addressed here not only because they are interesting but also because they track the journey of one laboratory along a path less traveled by.

Membrane adhesion junctions regulate airway smooth muscle phenotype and function

The local environment surrounding airway smooth muscle (ASM) cells has profound effects on the physiological and phenotypic properties of ASM tissues. ASM is continually subjected to the mechanical forces generated during breathing and to the constituents of its surrounding extracellular milieu. The smooth muscle cells within the airways continually modulate their properties to adapt to these changing environmental influences. Smooth muscle cells connect to the extracellular cell matrix (ECM) at membrane adhesion junctions that provide mechanical coupling between smooth muscle cells within the tissue. Membrane adhesion junctions also sense local environmental signals and transduce them to cytoplasmic and nuclear signaling pathways in the ASM cell. Adhesion junctions are composed of clusters of transmembrane integrin proteins that bind to ECM proteins outside the cell and to large multiprotein complexes in the submembranous cytoplasm. Physiological conditions and stimuli from the surrounding ECM are sensed by integrin proteins and transduced by submembranous adhesion complexes to signaling pathways to the cytoskeleton and nucleus. The transmission of information between the local environment of the cells and intracellular processes enables ASM cells to rapidly adapt their physiological properties to modulating influences in their extracellular environment: mechanical and physical forces that impinge on the cell, ECM constituents, local mediators, and metabolites. The structure and molecular organization of adhesion junction complexes and the actin cytoskeleton are dynamic and constantly changing in response to environmental influences. The ability of ASM to rapidly accommodate to the ever-changing conditions and fluctuating physical forces within its local environment is essential for its normal physiological function.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Exercise increases arterial stiffness independent of blood pressure in older Veterans

BACKGROUND

Exercise-induced changes in arterial function could contribute to a hypertensive response to exercise (HRE) in older individuals. We performed the present analysis to define the acute arterial stiffness response to exercise in ambulatory older adults.

METHODS

Thirty-nine Veterans (>60 years old), without known cardiovascular disease, participated in this study, including 19 Veterans who were hypertensive (70.8 ± 6.8 years, 53% women) and 20 Veterans who were normotensive (72.0 ± 9.3 years, 40% women). Arterial stiffness parameters were measured locally with carotid artery ultrasound and regionally with carotid-femoral pulse wave velocity (cfPWV) before and during the 10 min after participants performed a Balke maximal exercise treadmill stress test.

RESULTS

The arterial stiffness response to exercise was similar for control and hypertensive participants. At 6 min postexercise, cfPWV was significantly increased (Δ1.5 ± 1.9 m/s, P  = 0.004) despite mean blood pressure (BP) having returned to its baseline value (Δ1 ± 8 mmHg, P  = 0.79). Arterial mechanics modeling also showed BP-independent increases in arterial stiffness with exercise ( P  < 0.05). Postexercise cfPWV was correlated with postexercise SBP ( r  = 0.50, P  = 0.004) while baseline cfPWV ( r  = 0.13, P  = 1.00), and postexercise total peripheral resistance ( r  = -0.18, P  = 1.00) were not.

CONCLUSION

In older Veterans, exercise increases arterial stiffness independently of BP and the arterial stiffness increase with exercise is associated with increased postexercise SBP. BP-independent increases in arterial stiffness with exercise could contribute to a HRE in older adults.

Structural Features and Nonlinear Rheology of Self-Assembled Networks of Cross-Linked Semiflexible Polymers

Disordered networks of semiflexible filaments are common support structures in biology. Familiar examples include fibrous matrices in blood clots, bacterial biofilms, and essential components of cells and tissues of plants, animals, and fungi. Despite the ubiquity of these networks in biomaterials, we have only a limited understanding of the relationship between their structural features and their highly strain-sensitive mechanical properties. In this work, we perform simulations of three-dimensional networks produced by the irreversible formation of cross-links between linker-decorated semiflexible filaments. We characterize the structure of networks formed by a simple diffusion-dependent assembly process and measure their associated steady-state rheological features at finite temperature over a range of applied prestrains that encompass the strain-stiffening transition. We quantify the dependence of network connectivity on cross-linker availability and detail the associated connectivity dependence of both linear elasticity and nonlinear strain-stiffening behavior, drawing comparisons with prior experimental measurements of the cross-linker concentration-dependent elasticity of actin gels.

Airway and parenchymal tissue resistance and elastance in ex vivo sheep lungs: Effects of bronchochallenge and deep inspiration

Lung resistance (R_L) is determined by airway and parenchymal tissue resistance, as well as the degree of heterogeneity in airway constriction. Deep inspirations (DIs) are known to reverse experimentally induced increase in R_L, but the mechanism is not entirely clear. The first step towards understanding the effect of DI is to determine how each of the resistance components is affected by DI. In the present study, we measured R_L and apparent airway resistance (R_AW, which combines the effects of airway resistance and airway heterogeneity) simultaneously before and after a DI in acetylcholine (ACh)-challenged ex vivo sheep lungs. We found that at normal breathing frequency (0.25 Hz) ACh-challenge led to doubling of R_L, 80.3% of that increase was caused by an increase in R_AW; the increase in apparent tissue resistance (R_T) was insignificant. 57.7% of the increase in R_AW was abolished by a single DI. After subtracting R_AW from R_L, the remaining R_T was mostly independent of ACh-challenge and its reduction after a DI came mostly from the change in the mechanical properties of lung parenchyma. We conclude that at normal breathing frequency, R_L in an unchallenged lung is mostly composed of R_T, and the increase in R_L due to ACh-challenge stems mostly from the increase in R_AW and that both R_AW and R_T can be greatly reduced by a DI, likely due to a reduction in true airway resistance and heterogeneity, as well as parenchymal tissue hysteresis post DI.

AFM Stiffness Mapping in Human Aortic Smooth Muscle Cells

Aortic Smooth Muscle Cells (SMCs) play a vital role in maintaining mechanical homeostasis in the aorta. We recently found that SMCs of aneurysmal aortas apply larger traction forces than SMCs of healthy aortas. This result was explained by the significant increase of hypertrophic SMCs abundance in aneurysms. In the present study, we investigate whether the cytoskeleton stiffness of SMCs may also be altered in aneurysmal aortas. For that, we use Atomic Force Microscopy (AFM) nanoindentation with a specific mode that allows subcellular-resolution mapping of the local stiffness across a specified region of interest of the cell. Aortic SMCs from a commercial human lineage (AoSMCs, Lonza) and primary aneurysmal SMCs (AnevSMCs) are cultured in conditions promoting the development of their contractile apparatus, and seeded on hydrogels with stiffness properties of 12kPa and 25kPa. Results show that all SMC exhibit globally a lognormal stiffness distribution, with medians in the range 10-30 kPa. The mean of stiffness distributions is slightly higher in aneurysmal SMCs than in healthy cells (16 kPa versus 12 kPa) but the differences are not statistically significant due to the large dispersion of AFM nanoindentation stiffness. We conclude that the possible alterations previously found in aneurysmal SMCs do not affect significantly the AFM nanoindentation stiffness of their cytoskeleton.

Effects of TNFα on Dynamic Cytosolic Ca2 + and Force Responses to Muscarinic Stimulation in Airway Smooth Muscle

Previously, we reported that in airway smooth muscle (ASM), the cytosolic Ca²⁺ ([Ca²⁺]_cyt) and force response induced by acetyl choline (ACh) are increased by exposure to the pro-inflammatory cytokine tumor necrosis factor α (TNFα). The increase in ASM force induced by TNFα was not associated with an increase in regulatory myosin light chain (rMLC₂₀) phosphorylation but was associated with an increase in contractile protein (actin and myosin) concentration and an enhancement of Ca²⁺ dependent actin polymerization. The sensitivity of ASM force generation to elevated [Ca²⁺]_cyt (Ca²⁺ sensitivity) is dynamic involving both the shorter-term canonical calmodulin-myosin light chain kinase (MLCK) signaling cascade that regulates rMLC₂₀ phosphorylation and cross-bridge recruitment as well as the longer-term regulation of actin polymerization that regulates contractile unit recruitment and actin tethering to the cortical cytoskeleton. In this study, we simultaneously measured [Ca²⁺]_cyt and force responses to ACh and explored the impact of 24-h TNFα on the dynamic relationship between [Ca²⁺]_cyt and force responses. The temporal delay between the onset of [Ca²⁺]_cyt and force responses was not affected by TNFα. Similarly, the rates of rise of [Ca²⁺]_cyt and force responses were not affected by TNFα. The absence of an impact of TNFα on the short delay relationships between [Ca²⁺]_cyt and force was consistent with the absence of an effect of [Ca²⁺]_cyt and force on rMLC₂₀ phosphorylation. However, the integral of the phase-loop plot of [Ca²⁺]_cyt and force increased with TNFα, consistent with an impact on actin polymerization and, contractile unit recruitment and actin tethering to the cortical cytoskeleton.

Cytoskeletal remodeling slows cross-bridge cycling and ATP hydrolysis rates in airway smooth muscle

During isometric activation of airway smooth muscle (ASM), cross-bridge cycling and ATP hydrolysis rates decline across time even though isometric force is sustained. Thus, tension cost (i.e., ATP hydrolysis rate per unit of force during activation) decreases with time. The "latch-state" hypothesis attributes the dynamic change in cross-bridge cycling and ATP hydrolysis rates to changes in phosphorylation of the regulatory myosin light chain (rMLC20 ). However, we previously showed that in ASM, the extent of rMLC20 phosphorylation remains unchanged during sustained isometric force. As an alternative, we hypothesized that cytoskeletal remodeling within ASM cells results in increased internal loading of contractile proteins that slows cross-bridge cycling and ATP hydrolysis rates. To test this hypothesis, we simultaneously measured isometric force and ATP hydrolysis rate in permeabilized porcine ASM strips activated by Ca2+ (pCa 4.0). The extent of rMLC20 phosphorylation remained unchanged during isometric activation, even though ATP hydrolysis rate (tension cost) declined with time. The effect of cytoskeletal remodeling was assessed by inhibiting actin polymerization using Cytochalasin D (Cyto-D). In Cyto-D treated ASM, isometric force was reduced while ATP hydrolysis rate increased compared to untreated ASM strips. These results indicate that external transmission of force, cross-bridge cycling and ATP hydrolysis rates are affected by internal loading of contractile proteins.

Mechanopharmacology of Rho-kinase antagonism in airway smooth muscle and potential new therapy for asthma

The principle of mechanopharmacology of airway smooth muscle (ASM) is based on the premise that physical agitation, such as pressure oscillation applied to an airway, is able to induce bronchodilation by reducing contractility and softening the cytoskeleton of ASM. Although the underlying mechanism is not entirely clear, there is evidence to suggest that large-amplitude stretches are able to disrupt the actomyosin interaction in the crossbridge cycle and weaken the cytoskeleton in ASM cells. Rho-kinase is known to enhance force generation and strengthen structural integrity of the cytoskeleton during smooth muscle activation and plays a key role in the maintenance of force during prolonged muscle contractions. Synergy in relaxation has been observed when the muscle is subject to oscillatory length change while Rho-kinase is pharmacologically inhibited. In this review, inhibition of Rho-kinase coupled to therapeutic pressure oscillation applied to the airways is explored as a combination treatment for asthma.

Small-scale demixing in confluent biological tissues

Surface tension governed by differential adhesion can drive fluid particle mixtures to sort into separate regions, i.e., demix. Does the same phenomenon occur in confluent biological tissues? We begin to answer this question for epithelial monolayers with a combination of theory via a vertex model and experiments on keratinocyte monolayers. Vertex models are distinct from particle models in that the interactions between the cells are shape-based, as opposed to distance-dependent. We investigate whether a disparity in cell shape or size alone is sufficient to drive demixing in bidisperse vertex model fluid mixtures. Surprisingly, we observe that both types of bidisperse systems robustly mix on large lengthscales. On the other hand, shape disparity generates slight demixing over a few cell diameters, a phenomenon we term micro-demixing. This result can be understood by examining the differential energy barriers for neighbor exchanges (T1 transitions). Experiments with mixtures of wild-type and E-cadherin-deficient keratinocytes on a substrate are consistent with the predicted phenomenon of micro-demixing, which biology may exploit to create subtle patterning. The robustness of mixing at large scales, however, suggests that despite some differences in cell shape and size, progenitor cells can readily mix throughout a developing tissue until acquiring means of recognizing cells of different types.

The Huxley crossbridge model as the basic mechanism for airway smooth muscle contraction

The cyclic interaction between myosin crossbridges and actin filaments underlies smooth muscle contraction. Phosphorylation of the 20-kDa myosin light chain (MLC20) is a crucial step in activating the crossbridge cycle. Our current understanding of smooth muscle contraction is based on observed correlations among MLC20 phosphorylation, maximal shortening velocity (Vmax), and isometric force over the time course of contraction. However, during contraction there are changes in the extent of phosphorylation of many additional proteins as well as changes in activation of enzymes associated with the signaling pathways. As a consequence, the mechanical manifestation of muscle contraction is likely to change with time. To simplify the study of these relationships, we measured the mechanical properties of airway smooth muscle at different levels of MLC20 phosphorylation at a fixed time during contraction. A simple correlation emerged when time-dependent variables were fixed. MLC20 phosphorylation was found to be directly and linearly correlated with the active stress, stiffness, and power of the muscle; the observed weak dependence of Vmax on MLC20 phosphorylation could be explained by the presence of an internal load in the muscle preparation. These results can be entirely explained by the Huxley crossbridge model. We conclude that when the influence of time-dependent events during contraction is held constant, the basic crossbridge mechanism in smooth muscle is the same as that in striated muscle.

Saponins of Dioscorea Nipponicae Inhibits IL-17A-Induced Changes in Biomechanical Behaviors of In Vitro Cultured Human Airway Smooth Muscle Cells

Airway hyperresponsiveness (AHR) is one of the main pathologic features of bronchial asthma, which is largely attributable to enhanced contractile response of asthmatic airway smooth muscle. Although β2 adrenergic receptor agonists are commonly used to relax airway smooth muscle for treating AHR, there are side effects such as desensitization of long-term use. Therefore, it is desirable to develop alternative relaxant for airway smooth muscle, preferably based on natural products. One potential candidate is the inexpensive and widely available natural herb saponins of Dioscorea nipponicae (SDN), which has recently been reported to suppress the level of inflammatory factor IL-17A in ovalbumin-induced mice, thereby alleviating the inflammation symptoms of asthma. Here, we evaluated the biomechanical effect of SDN on IL-17A-mediated changes of cultured human airway smooth muscle cells (HASMCs) in vitro. The stiffness and traction force of the cells were measured by optical magnetic twisting cytometry (OMTC), and Fourier transform traction microscopy (FTTM), respectively. The cell proliferation was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetry, the cell migration was measured by cell scratch test, and the changes of cell cytoskeleton were assessed by laser confocal microscopy. We found that the stiffness and traction force of HASMCs were enhanced along with the increases of IL-17A concentration and exposure time, and SDN treatment dose-dependently reduced these IL-17A-induced changes in cell mechanical properties. Furthermore, SDN alleviated IL-17A-mediated effects on HASMCs proliferation, migration, and cytoskeleton remodeling. These results demonstrate that SDN could potentially be a novel drug candidate as bronchodilator for treating asthma-associated AHR.

Hyperresponsiveness: Relating the Intact Airway to the Whole Lung

We relate changes of the airway wall to the response of the intact airway and the whole lung. We address how mechanical conditions and specific structural changes for an airway contribute to hyperresponsiveness resistant to deep inspiration. This review conveys that the origins of hyperresponsiveness do not devolve into an abnormality at single structural level but require examination of the complex interplay of all the parts.

The actin regulator zyxin reinforces airway smooth muscle and accumulates in airways of fatal asthmatics

Bronchospasm induced in non-asthmatic human subjects can be easily reversed by a deep inspiration (DI) whereas bronchospasm that occurs spontaneously in asthmatic subjects cannot. This physiological effect of a DI has been attributed to the manner in which a DI causes airway smooth muscle (ASM) cells to stretch, but underlying molecular mechanisms-and their failure in asthma-remain obscure. Using cells and tissues from wild type and zyxin-/- mice we report responses to a transient stretch of physiologic magnitude and duration. At the level of the cytoskeleton, zyxin facilitated repair at sites of stress fiber fragmentation. At the level of the isolated ASM cell, zyxin facilitated recovery of contractile force. Finally, at the level of the small airway embedded with a precision cut lung slice, zyxin slowed airway dilation. Thus, at each level zyxin stabilized ASM structure and contractile properties at current muscle length. Furthermore, when we examined tissue samples from humans who died as the result of an asthma attack, we found increased accumulation of zyxin compared with non-asthmatics and asthmatics who died of other causes. Together, these data suggest a biophysical role for zyxin in fatal asthma.

Towards the understanding of cytoskeleton fluidisation-solidification regulation

The understanding of the self-regulation of the mechanical properties in non-sarcomeric cells, such as lung cells or cells during tissue development, remains an open research problem with many unresolved issues. Their behaviour is far from the image of the traditionally studied sarcomeric cells, since the crosstalk between the signalling pathways and the complexity of the mechanical properties creates an intriguing mechano-chemical coupling. In these situations, the inelastic effects dominate the cytoskeletal structure showing phenomena like fluidisation and subsequent solidification. Here, we proposes the inelastic contractile unit framework as an attempt to reconciles these effects. The model comprises a mechanical description of the nonlinear elasticity of the cytoskeleton incorporated into a continuum-mechanics framework using the eighth-chains model. In order to address the inelastic effect, we incorporate the dynamic of crosslinks, considering the [Formula: see text]-actinin and the active stress induced by the myosin molecular motors. Finally, we introduce a hypothesis that links the ability to fluidise and re-solidify as a consequence of the interaction between the active stress and the gelation state defined by the crosslinks. We validate the model with data obtained from experiments of drug-induced relaxation reported in the literature.

Smooth muscle function and myosin polymerization

Smooth muscle is able to function over a much broader length range than striated muscle. The ability to maintain contractility after a large length change is thought to be due to an adaptive process involving restructuring of the contractile apparatus to maximize overlap between the contractile filaments. The molecular mechanism for the length-adaptive behavior is largely unknown. In smooth muscle adapted to different lengths we quantified myosin monomers, basal and activation-induced myosin light chain (MLC) phosphorylation, shortening-velocity, power-output and active force. The muscle was able to generate a constant maximal force over a 2-fold length range when it was allowed to go through isometric contraction/relaxation cycles after each length change (length adaptation). In the relaxed state myosin monomer concentration and basal MLC phosphorylation decreased linearly, while in the activated state activation-induced MLC phosphorylation and shortening-velocity/power-output increased linearly with muscle length. The results suggest that recruitment of myosin monomers and oligomers into the actin filament lattice (where they form force-generating filaments) occurs during muscle adaptation to longer length with the opposite occurring during adaptation to shorter length.

Smooth muscle in human bronchi is disposed to resist airway distension

Studying airway smooth muscle (ASM) in conditions that emulate the in vivo environment within which the bronchi normally operate may provide important clues regarding its elusive physiological function. The present study examines the effect of lengthening and shortening of ASM on tension development in human bronchial segments. ASM from each bronchial segment was set at a length approximating in situ length (Linsitu). Bronchial tension was then measured during a slow cyclical strain (0.004Hz, from 0.7Linsitu to 1.3Linsitu) in the relaxed state and at graded levels of activation by methacholine. In all cases, tension was greater at longer ASM lengths, and greater during lengthening than shortening. The threshold of methacholine concentration that was required for ASM to account for bronchial tension across the entire range of ASM lengths tested was on average smaller by 2.8 logs during lengthening than during shortening. The length-dependency of ASM tension, together with this lower threshold of methacholine concentration during lengthening versus shortening, suggest that ASM has a greater ability to resist airway dilation during lung inflation than to narrow the airways during lung deflation. More than serving to narrow the airway, as has long been thought, these data suggest that the main function of ASM contraction is to limit airway wall distension during lung inflation.

Implementing cell contractility in filament-based cytoskeletal models

Cells are known to respond over time to mechanical stimuli, even actively generating force at longer times. In this paper, a microstructural filament-based cytoskeletal network model is extended to incorporate this active response, and a computational study to assess the influence on relaxation behaviour was performed. The incorporation of an active response was achieved by including a strain energy function of contractile activity from the cross-linked actin filaments. A four-state chemical model and strain energy function was adopted, and generalisation to three dimensions and the macroscopic deformation field was performed by integration over the unit sphere. Computational results in MATLAB and ABAQUS/Explicit indicated an active cellular response over various time-scales, dependent on contractile parameters. Important features such as force generation and increasing cell stiffness due to prestress are qualitatively predicted. The work in this paper can easily be extended to encompass other filament-based cytoskeletal models as well.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

Biphasic force response to iso-velocity stretch in airway smooth muscle

Airway smooth muscle (ASM) in vivo is constantly subjected to oscillatory strain due to tidal breathing and deep inspirations. ASM contractility is known to be adversely affected by strains, especially those of large amplitudes. Based on the cross-bridge model of contraction, it is likely that strain impairs force generation by disrupting actomyosin cross-bridge interaction. There is also evidence that strain modulates muscle stiffness and force through induction of cytoskeletal remodeling. However, the molecular mechanism by which strain alters smooth muscle function is not entirely clear. Here, we examine the response of ASM to iso-velocity stretches to probe the components within the muscle preparation that give rise to different features in the force response. We found in ASM that force response to a ramp stretch showed a biphasic feature, with the initial phase associated with greater muscle stiffness compared with that in the later phase, and that the transition between the phases occurred at a critical strain of ∼3.3%. Only strains with amplitudes greater than the critical strain could lead to reduction in force and stiffness of the muscle in the subsequent stretches. The initial-phase stiffness was found to be linearly related to the degree of muscle activation, suggesting that the stiffness stems mainly from attached cross bridges. Both phases were affected by the degree of muscle activation and by inhibitors of myosin light-chain kinase, PKC, and Rho-kinase. Different responses due to different interventions suggest that cross-bridge and cytoskeletal stiffness is regulated differently by the kinases.

The cell biology of asthma

The clinical manifestations of asthma are caused by obstruction of the conducting airways of the lung. Two airway cell types are critical for asthma pathogenesis: epithelial cells and smooth muscle cells. Airway epithelial cells, which are the first line of defense against inhaled pathogens and particles, initiate airway inflammation and produce mucus, an important contributor to airway obstruction. The other main cause of airway obstruction is contraction of airway smooth muscle. Complementary experimental approaches involving cultured cells, animal models, and human clinical studies have provided many insights into diverse mechanisms that contribute to airway epithelial and smooth muscle cell pathology in this complex disease.

Glass-like dynamics in the cell and in cellular collectives

Prominent fluctuations, heterogeneity, and cooperativity dominate the dynamics of the cytoskeleton as well as the dynamics of the cellular collective. Such systems are out of equilibrium, disordered, and remain poorly understood. To explain these findings, we consider a unifying mechanistic rubric that imagines these systems as comprising phases of soft condensed matter in proximity to a glass or jamming transition, with associated transitions between solid-like versus liquid-like phases. At the scale of the cytoskeleton, data suggest that intermittent dynamics, kinetic arrest, and dynamic heterogeneity represent mesoscale features of glassy protein-protein interactions that link underlying biochemical events to integrative cellular behaviors such as crawling, contraction, and remodeling. At the scale of the multicellular collective, jamming has the potential to unify diverse biological factors that previously had been considered mostly as acting separately and independently. Although a quantitative relationship between intra- and intercellular dynamics is still lacking, glassy dynamics and jamming offer insights linking the mechanobiology of cell to human physiology and pathophysiology.

Emergence of airway smooth muscle mechanical behavior through dynamic reorganization of contractile units and force transmission pathways

Airway hyperresponsiveness (AHR) in asthma remains poorly understood despite significant research effort to elucidate relevant underlying mechanisms. In particular, a significant body of experimental work has focused on the effect of tidal fluctuations on airway smooth muscle (ASM) cells, tissues, lung slices, and whole airways to understand the bronchodilating effect of tidal breathing and deep inspirations. These studies have motivated conceptual models that involve dynamic reorganization of both cytoskeletal components as well as contractile machinery. In this article, a biophysical model of the whole ASM cell is presented that combines 1) crossbridge cycling between actin and myosin; 2) actin-myosin disconnectivity, under imposed length changes, to allow dynamic reconfiguration of "force transmission pathways"; and 3) dynamic parallel-to-serial transitions of contractile units within these pathways that occur through a length fluctuation. Results of this theoretical model suggest that behavior characteristic of experimentally observed force-length loops of maximally activated ASM strips can be explained by interactions among the three mechanisms. Crucially, both sustained disconnectivity and parallel-to-serial transitions are necessary to explain the nature of hysteresis and strain stiffening observed experimentally. The results provide strong evidence that dynamic rearrangement of contractile machinery is a likely mechanism underlying many of the phenomena observed at timescales associated with tidal breathing. This theoretical cell-level model captures many of the salient features of mechanical behavior observed experimentally and should provide a useful starting block for a bottom-up approach to understanding tissue-level mechanical behavior.

An inverse power-law distribution of molecular bond lifetimes predicts fractional derivative viscoelasticity in biological tissue

Viscoelastic characteristics of many materials falling under the category of soft glassy substances, including biological tissue, often exhibit a mechanical complex modulus Y(ω) well described by a fractional derivative model: Y(ω) = E(iω/ϕ)k, where E = a generalized viscoelastic stiffness; i = (-1)1/2; ω = angular frequency; ϕ = scaling factor; and k = an exponent valued between 0 and 1. The term "fractional derivative" refers to the value of k: when k = 0 the viscoelastic response is purely elastic, and when k = 1 the response is purely viscous. We provide an analytical derivation of the fractional derivative complex modulus based on the hypothesis that the viscoelastic response arises from many intermittent molecular crosslinks, whose lifetimes longer than a critical threshold lifetime, tcrit, are distributed with an inverse power law proportional to t-(k+2). We demonstrate that E is proportional to the number and stiffness of crosslinks formed at any moment; the scaling factor ϕ is equivalent to reciprocal of tcrit; and the relative mean lifetime of the attached crosslinks is inversely proportional to the parameter k. To test whether electrostatic molecular bonds could be responsible for the fractional derivative viscoelasticity, we used chemically skinned human skeletal muscle as a one-dimensional model of a soft glassy substance. A reduction in ionic strength from 175 to 110 mEq resulted in a larger E with no change in k, consistent with a higher probability of interfilament molecular interactions. Thick to thin filament spacing was reduced by applying 4% w/v of the osmolyte Dextran T500, which also resulted in a larger E, indicating a greater probability of crosslink formation in proportion to proximity. A 10°C increase in temperature resulted in an increase in k, which corresponded to a decrease in cross-bridge attachment lifetime expected with higher temperatures. These theoretical and experimental results suggest that the fractional derivative viscoelasticity observed in some biological tissue arises as a mechanical consequence of electrostatic interactions, whose longest lifetimes are distributed with an inverse power law.

The role of smooth muscle cells in the pathophysiology of pelvic organ prolapse

Pelvic organ prolapse (POP) is a prevalent and disabling condition. The pathophysiology of prolapse is multifactorial, and no single mechanism adequately explains all aspects of its development. The pathophysiology of POP is complex and incompletely understood. Smooth muscle (SM), an integral part of the vaginal wall and endopelvic structures that support the pelvic viscera, has also been implicated in the pathophysiology of POP. In this article, we review the role of smooth muscle cells (SMC) in the pathophysiology of POP, also addressing the anatomy of SM in pelvic floor, morphometric analysis, biomechanical properties, and potential mechanisms.

Glassy dynamics, cell mechanics, and endothelial permeability

A key feature of all inflammatory processes is disruption of the vascular endothelial barrier. Such disruption is initiated in part through active contraction of the cytoskeleton of the endothelial cell (EC). Because contractile forces are propagated from cell to cell across a great many cell-cell junctions, this contractile process is strongly cooperative and highly nonlocal. We show here that the characteristic length scale of propagation is modulated by agonists and antagonists that impact permeability of the endothelial barrier. In the presence of agonists including thrombin, histamine, and H2O2, force correlation length increases, whereas in the presence of antagonists including sphingosine-1-phosphate, hepatocyte growth factor, and the rho kinase inhibitor, Y27632, force correlation length decreases. Intercellular force chains and force clusters are also evident, both of which are reminiscent of soft glassy materials approaching a glass transition.

Increased mechanical strain imposed on murine lungs during ventilation in vivo depresses airway responsiveness and activation of protein kinase Akt

Continuous positive airway pressure (CPAP) administered to tracheostomized rabbits and ferrets for 4 days or 2 wk suppresses bronchial reactivity in vivo and suppresses airway reactivity in lobes and tracheal segments isolated from these animals. In vitro studies of canine tracheal smooth muscle tissues indicate that mechanical loading suppresses the activation of the growth regulatory kinase, Akt, and that Akt is a negative regulator of smooth muscle differentiation. The transduction of mechanical signals in the tracheal tissues in vitro is mediated by integrin-associated adhesion complexes. To determine whether airway responsiveness and Akt activation are modulated by mechanical loads applied for short time periods to the airways of living animals in vivo, mice were mechanically ventilated for 2 h with high (5 cmH2O) or low (0-1 cmH2O) positive end-expiratory pressure (PEEP) and then ventilated at low PEEP for 30 min. Ventilation of mice with PEEP in vivo for 2 h depressed airway responsiveness to methacholine measured in vivo subsequent to the PEEP treatment. Airway narrowing in vitro in intraparenchymal airways in isolated lung slices and contractile responses of isolated tracheal segments in vitro were suppressed for at least 6 h subsequent to the in vivo exposure to PEEP. Tracheal segments isolated from high PEEP-treated mice exhibited significantly lower levels of Akt activation than tracheae from low PEEP-treated mice. The results indicate that mechanical loads imposed in vivo result in physiological and biochemical changes in the airway tissues after a relatively short 2-h period of in vivo loading.

Use of continuous positive airway pressure reduces airway reactivity in adults with asthma

Asthma is characterised by airway hyperreactivity, which is primarily treated with β-adrenergic bronchodilators and anti-inflammatory agents. However, mechanical strain during breathing is an important modulator of airway responsiveness and we have previously demonstrated in animal models that continuous positive airway pressure (CPAP) resulted in lower in vivo airway reactivity. We now evaluated whether using nocturnal CPAP decreased airway reactivity in clinically-stable adults with asthma. Adults with stable asthma and normal spirometry used nocturnal CPAP (8-10 cmH(2)O) or sham treatment (0-2 cmH(2)O) for 7 days. Spirometry and bronchial challenges were obtained before and after treatment. The primary outcome was the provocative concentration of methacholine causing a 20% fall in forced expiratory volume in 1 s (PC(20)). The CPAP group (n=16) had a significant decrease in airway reactivity (change in (Δ)logPC(20) 0.406, p<0.0017) while the sham group (n=9) had no significant change in airway reactivity (ΔlogPC(20) 0.003, p=0.9850). There was a significant difference in the change in airway reactivity for the CPAP versus the sham group (ΔlogPC(20) 0.41, p<0.043). Our findings indicate that chronic mechanical strain of the lungs produced using nocturnal CPAP for 7 days reduced airway reactivity in clinically stable asthmatics. Future studies of longer duration are required to determine whether CPAP can also decrease asthma symptoms and/or medication usage.

Passive stiffness of airway smooth muscle: the next target for improving airway distensibility and treatment for asthma?

Reduced airway distensibility due to increased airway stiffness is a characteristic of asthma. Airway stiffness is determined by the property and structural organization of the various elements of the airway wall, and is often divided into active and passive components. Active stiffness is thought to be associated with activation of muscle cells in the airway wall. This component of stiffness can be inhibited when active force produced by the muscle is abolished. Passive stiffness, on the other hand, is thought to stem from non-muscle component of the airway wall, especially the collagen/elastin fibrous network of the extracellular matrix within which the muscle cells are embedded. In this brief review, the notion that passive stiffness is exclusively extracellular in origin is challenged. Recent evidence suggests that a substantial portion of the passive stiffness of an in vitro preparation of tracheal smooth muscle is calcium sensitive and is regulated by Rho-kinase, although the underlying mechanism and the details of regulation for the development of this intracellular passive stiffness are still largely unknown. To reduce airway stiffness different lines of attack must be tailored to different components of the stiffness. The regulatable passive stiffness is distinct from the relatively permanent stiffness of the extracellular matrix and the stiffness associated with active muscle contraction. To improve airway distensibility during asthma exacerbation, a comprehensive approach to reduce overall airway stiffness should therefore include a strategy for targeting the regulatable passive stiffness.

Fluidization, resolidification, and reorientation of the endothelial cell in response to slow tidal stretches

Mechanical stretch plays an important role in regulating shape and orientation of the vascular endothelial cell. This morphological response to stretch is basic to angiogenesis, neovascularization, and vascular homeostasis, but mechanism remains unclear. To elucidate mechanisms, we used cell mapping rheometry to measure traction forces in primary human umbilical vein endothelial cells subjected to periodic uniaxial stretches. Onset of periodic stretch of 10% strain amplitude caused a fluidization response typified by attenuation of traction forces almost to zero. As periodic stretch continued, the prompt fluidization response was followed by a slow resolidification response typified by recovery of the traction forces, but now aligned along the axis perpendicular to the imposed stretch. Reorientation of the cell body lagged reorientation of the traction forces, however. Together, these observations demonstrate that cellular reorientation in response to periodic stretch is preceded by traction attenuation by means of cytoskeletal fluidization and subsequent traction recovery transverse to the stretch direction by means of cytoskeletal resolidification.

RhoA-induced cytoskeletal tension controls adaptive cellular remodeling to mechanical signaling

The ability to measure real-time mechanosensitive events at the subcellular level in response to discrete mechanical stimulation is a critical component in understanding mechanically-induced cellular remodeling. Vascular smooth muscle cells (VSMC) were transfected with RhoA constructs (wild type, dominant negative or constitutively active) or treated with ML-7 to induce specific cytoskeletal tension characteristics prior to mechanical stimulation. Tensile stress was applied to live VSMC using an atomic force microscope probe functionalized with extracellular matrix (ECM) proteins. The ECM induces selective integrin activation and focal adhesion formation, enabling direct manipulation of cortical actin through an active ECM-integrin-actin linkage. Therefore, locally induced mechanosensitive events triggered downstream activation of intracellular signaling pathways responsible for actin and focal adhesion remodeling throughout the cell. Integration of mechanical stimulation with simultaneous fluorescence imaging by spinning-disk confocal and total internal reflection fluorescence microscopy enabled visualization and quantification of molecular dynamic events at the sub-cellular level in real-time. Results provide evidence that the pre-existing cytoskeletal tension affects the actomyosin apparatus which in turn coordinates the ability of the cell to adapt to the externally applied stress. RhoA activation induced high cytoskeletal tension that correlated with increased stress fiber formation, cell stiffness, integrin activation and myosin phosphorylation. In contrast, blocking Rho-kinase or myosin function was characterized by low cytoskeletal tension with a decreased level of stress fiber formation, lower cell stiffness and integrin activation. Our findings show that VSMC sense and adapt to physical microenvironmental changes by a coordinated response of the actomyosin apparatus necessary to establish a new homeostatic state.

Cell-biomaterial mechanical interaction in the framework of tissue engineering: insights, computational modeling and perspectives

Tissue engineering is an emerging field of research which combines the use of cell-seeded biomaterials both in vitro and/or in vivo with the aim of promoting new tissue formation or regeneration. In this context, how cells colonize and interact with the biomaterial is critical in order to get a functional tissue engineering product. Cell-biomaterial interaction is referred to here as the phenomenon involved in adherent cells attachment to the biomaterial surface, and their related cell functions such as growth, differentiation, migration or apoptosis. This process is inherently complex in nature involving many physico-chemical events which take place at different scales ranging from molecular to cell body (organelle) levels. Moreover, it has been demonstrated that the mechanical environment at the cell-biomaterial location may play an important role in the subsequent cell function, which remains to be elucidated. In this paper, the state-of-the-art research in the physics and mechanics of cell-biomaterial interaction is reviewed with an emphasis on focal adhesions. The paper is focused on the different models developed at different scales available to simulate certain features of cell-biomaterial interaction. A proper understanding of cell-biomaterial interaction, as well as the development of predictive models in this sense, may add some light in tissue engineering and regenerative medicine fields.

Tissue length modulates "stimulated actin polymerization," force augmentation, and the rate of swine carotid arterial contraction

"Stimulated actin polymerization" has been proposed to be involved in force augmentation, in which prior submaximal activation of vascular smooth muscle increases the force of a subsequent maximal contraction by ∼15%. In this study, we altered stimulated actin polymerization by adjusting tissue length and then measured the effect on force augmentation. At optimal tissue length (1.0 L(o)), force augmentation was observed and was associated with increased prior stimulated actin polymerization, as evidenced by increased prior Y118 paxillin phosphorylation without changes in prior S3 cofilin or cross-bridge phosphorylation. Tissue length, per se, regulated Y118 paxillin, but not S3 cofilin, phosphorylation. At short tissue length (0.6 L(o)), force augmentation was observed and was associated with increased prior stimulated actin polymerization, as evidenced by reduced prior S3 cofilin phosphorylation without changes in Y118 paxillin or cross-bridge phosphorylation. At long tissue length (1.4 L(o)), force augmentation was not observed, and there were no prior changes in Y118 paxillin, S3 cofilin, or cross-bridge phosphorylation. There were no significant differences in the cross-bridge phosphorylation transients before and after the force augmentation protocol at all three lengths tested. Tissues contracted faster at longer tissue lengths; contractile rate correlated with prior Y118 paxillin phosphorylation. Total stress, per se, predicted Y118 paxillin phosphorylation. These data suggest that force augmentation is regulated by stimulated actin polymerization and that stimulated actin polymerization is regulated by total arterial stress. We suggest that K(+) depolarization first leads to cross-bridge phosphorylation and contraction, and the contraction-induced increase in mechanical strain increases Y118 paxillin phosphorylation, leading to stimulated actin polymerization, which further increases force, i.e., force augmentation and, possibly, latch.

Myosin filament polymerization and depolymerization in a model of partial length adaptation in airway smooth muscle

Length adaptation in airway smooth muscle (ASM) is attributed to reorganization of the cytoskeleton, and in particular the contractile elements. However, a constantly changing lung volume with tidal breathing (hence changing ASM length) is likely to restrict full adaptation of ASM for force generation. There is likely to be continuous length adaptation of ASM between states of incomplete or partial length adaption. We propose a new model that assimilates findings on myosin filament polymerization/depolymerization, partial length adaptation, isometric force, and shortening velocity to describe this continuous length adaptation process. In this model, the ASM adapts to an optimal force-generating capacity in a repeating cycle of events. Initially the myosin filament, shortened by prior length changes, associates with two longer actin filaments. The actin filaments are located adjacent to the myosin filaments, such that all myosin heads overlap with actin to permit maximal cross-bridge cycling. Since in this model the actin filaments are usually longer than myosin filaments, the excess length of the actin filament is located randomly with respect to the myosin filament. Once activated, the myosin filament elongates by polymerization along the actin filaments, with the growth limited by the overlap of the actin filaments. During relaxation, the myosin filaments dissociate from the actin filaments, and then the cycle repeats. This process causes a gradual adaptation of force and instantaneous adaptation of shortening velocity. Good agreement is found between model simulations and the experimental data depicting the relationship between force development, myosin filament density, or shortening velocity and length.

Stress transmission within the cell

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. During past decades, it has become evident that the deformable cytoskeleton (CSK), an intracellular network of various filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical responses. To understand how mechanical forces regulate cellular functions, it is necessary to first understand how the CSK develops mechanical stresses in response to applied forces, and how those stresses are propagated through the CSK where various signaling molecules are immobilized. New experimental techniques have been developed to quantify cytoskeletal mechanics, which together with new computational approaches have given rise to new theories and models for describing mechanics of living cells. In this article, we discuss current understanding of cell biomechanics by focusing on the biophysical mechanisms that are responsible for the development and transmission of mechanical stresses in the cell and their effect on cellular functions. We compare and contrast various theories and models of cytoskeletal mechanics, emphasizing common mechanisms that those theories are built upon, while not ignoring irreconcilable differences. We highlight most recent advances in the understanding of mechanotransduction in the cytoplasm of living cells and the central role of the cytoskeletal prestress in propagating mechanical forces along the cytoskeletal filaments to activate cytoplasmic enzymes. It is anticipated that advances in cell mechanics will help developing novel therapeutics to treat pulmonary diseases like asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Mechanical properties of cells and ageing

Mechanical properties are fundamental properties of the cells and tissues of living organisms. The mechanical properties of a single cell as a biocomposite are determined by the interdependent combination of cellular components mechanical properties. Quantitative estimate of the cell mechanical properties depends on a cell state, method of measurement, and used theoretical model. Predominant tendency for the majority of cells with ageing is an increase of cell stiffness and a decrease of cell ability to undergo reversible large deformations. The mechanical signal transduction in old cells becomes less effective than that in young cells, and with ageing, the cells lose the ability of the rapid functional rearrangements of cellular skeleton. The article reviews the theoretical and experimental facts touching the age-related changes of the mechanical properties of cellular components and cells in the certain systems of an organism (the blood, the vascular system, the musculoskeletal system, the lens, and the epithelium). In fact, the cell mechanical parameters (including elastic modulii) can be useful as specific markers of cell ageing.

Contractile response of human anterior vaginal muscularis in women with and without pelvic organ prolapse

The aim of this study was to compare the contractility of the anterior vaginal muscularis (AVM) from women with and without pelvic organ prolapse (POP). In vitro experiments were performed to measure the peak force generated in response to potassium chloride (KCl; 125 mmol/L) and phenylephrine by AVM tissue from women with and without POP. Cross-sectional areas and co-localization of α(1A) adrenergic receptor protein with smooth muscle α-actin in AVM strips were determined by histology and immunofluorescence, respectively. There were no differences in the mean amplitude of force generated in response to KCl normalized to either wet weight or muscle cross-sectional area (mN/mm(2)) between women with and without POP (P > .30). However, AVM from women with prolapse produced a significantly higher mean force to KCl normalized to total cross-sectional area compared to controls (P = .007). While the control samples demonstrated a consistent response to phenylephrine, there was no response to this stimulant generated by AVM tissue from women with POP. The proportion of co-localized α(1A) adrenergic receptors with smooth muscle α actin in AVM tissue was significantly less in women with POP compared to normal controls (P < .0001). Although there was significantly greater tissue stress generated by AVM from women with prolapse compared to controls, there were no differences in muscle stress. Absent response to phenylephrine by AVM from women with prolapse may be related to a lower expression of α(1A) adrenergic receptors in vaginal smooth muscle.

Pregnancy downregulates actin polymerization and pressure-dependent myogenic tone in ovine uterine arteries

Pregnancy is associated with significantly decreased uterine vascular tone and increased uterine blood flow. The present study tested the hypothesis that the downregulation of actin polymerization plays a key role in reduced vascular tone of uterine arteries in the pregnant state. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Activation of protein kinase C significantly increased the filamentous:globular actin ratio and contractions in the uterine arteries, which were inhibited by an actin polymerization inhibitor cytochalasin B. The basal levels of filamentous:globular actin were significantly higher in nonpregnant uterine arteries than those in near-term pregnant sheep. Prolonged treatment (48 hours) of nonpregnant sheep with 17β-estradiol (0.3 nmol/L) and progesterone (100.0 nmol/L) caused a significant decrease in the filamentous:globular actin. In accordance, the treatment of near-term pregnant sheep for 48 hours with an estrogen antagonist ICI 182 780 (10.0 μmol/L) and progesterone antagonist RU 486 (1.0 μmol/L) significantly increased the levels of filamentous:globular actin. Increased intraluminal pressure from 20 to 100 mm Hg resulted in an initial increase in uterine arterial diameter and vascular wall Ca(2+) concentrations, followed by a decrease in the diameter at a constant steady-state level of Ca(2+). Cytochalasin B blocked pressure-induced myogenic constrictions without effect on vascular wall Ca(2+) levels and eliminated the differences in pressure-dependent myogenic tone between nonpregnant sheep and near-term pregnant sheep. The results indicate a key role of actin polymerization in protein kinase C-induced myogenic contractions and suggest a novel mechanism of sex steroid hormone-mediated downregulation of actin polymerization underlying the decreased myogenic tone of uterine arteries in pregnancy.

Logarithmic superposition of force response with rapid length changes in relaxed porcine airway smooth muscle

We present a systematic quantitative analysis of power-law force relaxation and investigate logarithmic superposition of force response in relaxed porcine airway smooth muscle (ASM) strips in vitro. The term logarithmic superposition describes linear superposition on a logarithmic scale, which is equivalent to multiplication on a linear scale. Additionally, we examine whether the dynamic response of contracted and relaxed muscles is dominated by cross-bridge cycling or passive dynamics. The study shows the following main findings. For relaxed ASM, the force response to length steps of varying amplitude (0.25-4% of reference length, both lengthening and shortening) are well-fitted with power-law functions over several decades of time (10⁻² to 10³ s), and the force response after consecutive length changes is more accurately fitted assuming logarithmic superposition rather than linear superposition. Furthermore, for sinusoidal length oscillations in contracted and relaxed muscles, increasing the oscillation amplitude induces greater hysteresivity and asymmetry of force-length relationships, whereas increasing the frequency dampens hysteresivity but increases asymmetry. We conclude that logarithmic superposition is an important feature of relaxed ASM, which may facilitate a more accurate prediction of force responses in the continuous dynamic environment of the respiratory system. In addition, the single power-function response to length changes shows that the dynamics of cross-bridge cycling can be ignored in relaxed muscle. The similarity in response between relaxed and contracted states implies that the investigated passive dynamics play an important role in both states and should be taken into account.

Dense-body aggregates as plastic structures supporting tension in smooth muscle cells

The wall of hollow organs of vertebrates is a unique structure able to generate active tension and maintain a nearly constant passive stiffness over a large volume range. These properties are predominantly attributable to the smooth muscle cells that line the organ wall. Although smooth muscle is known to possess plasticity (i.e., the ability to adapt to large changes in cell length through structural remodeling of contractile apparatus and cytoskeleton), the detailed structural basis for the plasticity is largely unknown. Dense bodies, one of the most prominent structures in smooth muscle cells, have been regarded as the anchoring sites for actin filaments, similar to the Z-disks in striated muscle. Here, we show that the dense bodies and intermediate filaments formed cable-like structures inside airway smooth muscle cells and were able to adjust the cable length according to cell length and tension. Stretching the muscle cell bundle in the relaxed state caused the cables to straighten, indicating that these intracellular structures were connected to the extracellular matrix and could support passive tension. These plastic structures may be responsible for the ability of smooth muscle to maintain a nearly constant tensile stiffness over a large length range. The finding suggests that the structural plasticity of hollow organs may originate from the dense-body cables within the smooth muscle cells.

A sub-cellular viscoelastic model for cell population mechanics

Understanding the biomechanical properties and the effect of biomechanical force on epithelial cells is key to understanding how epithelial cells form uniquely shaped structures in two or three-dimensional space. Nevertheless, with the limitations and challenges posed by biological experiments at this scale, it becomes advantageous to use mathematical and 'in silico' (computational) models as an alternate solution. This paper introduces a single-cell-based model representing the cross section of a typical tissue. Each cell in this model is an individual unit containing several sub-cellular elements, such as the elastic plasma membrane, enclosed viscoelastic elements that play the role of cytoskeleton, and the viscoelastic elements of the cell nucleus. The cell membrane is divided into segments where each segment (or point) incorporates the cell's interaction and communication with other cells and its environment. The model is capable of simulating how cells cooperate and contribute to the overall structure and function of a particular tissue; it mimics many aspects of cellular behavior such as cell growth, division, apoptosis and polarization. The model allows for investigation of the biomechanical properties of cells, cell-cell interactions, effect of environment on cellular clusters, and how individual cells work together and contribute to the structure and function of a particular tissue. To evaluate the current approach in modeling different topologies of growing tissues in distinct biochemical conditions of the surrounding media, we model several key cellular phenomena, namely monolayer cell culture, effects of adhesion intensity, growth of epithelial cell through interaction with extra-cellular matrix (ECM), effects of a gap in the ECM, tensegrity and tissue morphogenesis and formation of hollow epithelial acini. The proposed computational model enables one to isolate the effects of biomechanical properties of individual cells and the communication between cells and their microenvironment while simultaneously allowing for the formation of clusters or sheets of cells that act together as one complex tissue.